{"full_text": "The increase in atmospheric CO2 level has created a foremost environmental concern, as it is known to participate in the global climate change [1]. Sustainable solutions for the decreasing of atmospheric CO2 requires the use of fossil-free energy sources as well the development of new chemical technologies that can convert CO2 to useful chemicals, preferably with economic value [2\u20138]. For example, the catalytic coupling of CO2 with epoxides to form cyclic carbonates (Scheme\u00a01\n) has attracted considerable interest, especially due to its 100% atom economy. As a result a wide range of such catalytic systems have been developed [9].The active homogeneous systems include various metal-organic catalysts [10\u201319] as well as some organocatalysts [20\u201326]. The catalytic coupling reaction with metal-based catalysts follows several steps: (i) the activation of the epoxide by Lewis acidic metal upon coordination through the oxygen atom, (ii) epoxide ring opening by a nucleophile and concomitant formation of a M-OR bond, (iii) the insertion of CO2 into the metal-alkoxide bond and formation of the organic carbonate by cyclization, which is finally followed by (iv) the release of the cycloaddition products. In general, the catalytic performance of coupling catalysts is based on the cooperation of a Lewis acid centre with a nucleophile, e.g. a halide ion. For example, Miceli et\u00a0al. have used vanadium(V) aminotriphenolate complexes with an Bu4NI co-catalyst as archetypal examples on highly active catalyst system for the coupling of terminal and internal epoxides with CO2\n[10]. Several two-component organocatalysts based on phenols and Bu4NX (X\u00a0=\u00a0Br, I) have also been shown to couple CO2 and epoxides efficiently [27\u201329]. Recently, Hong et\u00a0al. synthesized an amine bisphenol carrying a quaternary ammonium/iodide ion pair in a pendant arm (Scheme\u00a02\n), and used it as a single-component organocatalyst for the coupling reaction of propylene oxide with CO2\n[26].We have previously used amine bisphenol ligands [30] to prepare high oxidation state metal complexes as bio-inspired model compounds and catalysts e.g. for epoxidation of alkenes [31\u201335]. Here we report the use of ammonium-functionalised amine bisphenol to prepare oxovanadium(V), dioxomolybdenum(VI) and dioxouranium(VI) complexes, which carry a cationic group in the ligand pendant arm with the aim of preparing active single-component catalysts that combine Lewis acidic metal centres and a nucleophile part necessary for the coupling reaction. The vanadium complex was studied as a catalyst for the coupling of CO2 with styrene oxide.The ligand precursor [H2L]I was prepared by the reaction between a tripodal amine bisphenol and iodomethane in acetonitrile applying a known procedure for corresponding compounds [26]. The reactions of [H2L]I and metal precursors VO(O-i-Pr)3, MoO2(acac)2 and UO2(OAc)2\u00b7H2O in methanol lead to the precipitation of the oxometal species [VO(OMe)(L)]I\u00b72MeOH (1), [MoO2(L)(H2O)]\u00b72MeOH (2) and [UO2(L)(OAc)]\u00b7[H2L]I\u00b74MeOH (3), respectively (Scheme\u00a03\n). Vanadium complex 1 crystallized as dark brown needles in a 63% yield. The solid compound is moderately stable under dry air, however, the crystals slowly deteriorate if kept in open atmosphere due to the loss of the solvate molecules. 1 is stable in dry organic solvents, but degrades gradually if wet solvents are used. The 1H NMR spectrum in CDCl3 indicates the presence of few isomers or conformations, as typical for pentacoordinated oxovanadium(V) aminophenolates in non-coordinating solvents [32,36]. However, in MeOH-d4, one major component (> 99%) is present in the 1H and 51V spectra. The 1H and 13C NMR spectra show anticipated chemical shifts for the deprotonated tridentate ligand. Interestingly, the methoxide ligand is not visible in the spectrum, probably due to the rapid interchange by deuterated solvent. Principally, phenols may act as redox-active, non-innocent ligands through the formation of phenyl radicals upon coordination. For 1, the 51V chemical shift, -470\u00a0ppm, is within the expected range for an oxidation state V(v), which indicates a redox-inactive behaviour [37]. The V=O stretch in the IR spectrum is seen at 948\u00a0cm\u22121, as characteristic for oxovanadium(V) aminophenolates [38].Complex 2 precipitated from the reaction mixtures as yellow plates, contaminated with some amount of solid impurities. The crystals as well as the contamination were moderately soluble in DMSO but practically insoluble in any other common solvents and therefore 2 could not be purified by washing or recrystallization. The IR spectrum show the \u03bd(MoO2)s and \u03bd(MoO2)a for cis-MoO2 as strong peaks at 914 and 900\u00a0cm\u22121\n[39]. The 1H and 13C NMR spectra of the crude product comprise typical chemical shifts for the tridentate amine bisphenolate ligand, e.g. benzylic methylene protons are seen as two two-proton doublets at 4.33 and 3.43\u00a0ppm, respectively. Similarly, 3 crystallised as brown crystals with some solid impurities and could not be further purified. The NMR spectra in DMSO-d6 show for coordinated amine bisphenolate ligand show chemical shifts ascribed for the coordinated ligand, e.g. doublets for the benzylic methylene protons at 5.02 and 4.00\u00a0ppm, as well as the for the free amine bisphenol molecule. In the IR spectrum, a strong peak was observed at 865\u00a0cm\u22121 due to \u03bd(UO2), as typical for the presence of a linear O=U=O group [40]. In addition, compounds 2 and 3 were successfully characterized by single-crystal XRD determination of selected crystals.In the solid state, 1 is formed of ion pairs, which crystallize with two molecules of solvate methanol in the asymmetric unit. In the complex part, the central V(V) ion is coordinated to the tridentate dianionic amine bisphenolate, one oxide anion and one methoxide to form a trigonal bipyramidal coordination sphere. Two phenolate oxygen atoms and an oxo ligand occupy the basal coordination sites while the nitrogen atom in the ligand backbone and a monodentate methoxide group occupy apical positions. The vanadium ion is located slightly above the plane formed by the O atoms. The V\u2013Ophenolate distances are 1.829(2) and 1.825(2)\u00a0\u00c5, respectively, whereas the V\u2013Omethoxide distance, 1.788(2)\u00a0\u00c5, is noticeably shorter. The V\u2013N bond in a trans position to the methoxide ligand is rather long, 2.303(3)\u00a0\u00c5. The terminal V=O bond length is 1.585(2)\u00a0\u00c5. In general, the structure and the coordination sphere around the metal centre are typical for the pentacoordinated amine bisphenolate V complexes [36,38,41,42]. The positive charge of the complex unit is located in the pendant ammonium cation, whereas the iodide anion and the solvent molecules are positioned in the cavities of the crystal lattice.In complex 2, the amine bisphenolate is coordinated to the dioxomolybdenum(VI) ion as a tridentate ligand through two phenolate oxygen atoms and the nitrogen donor in the ligand backbone. A water molecule is coordinated trans to the oxo group to complete the distorted octahedral coordination sphere. Two terminal oxides, two monoanionic phenolate groups and two neutral donors form a typical cis-oxo,trans-X,cis-L configuration around the metal centre [43]. The O=Mo=O angle, 103.8(2)\u00b0, the Ophenolate\u2013Mo\u2013Ophenolate angle, 152.8\u00b0, and the Mo=O distances, 1.687(5) and 1.715(4) \u00a0\u00c5, respectively, are typical for cis-dioxomolybdenum(VI) complexes with tridentate amine bisphenolate ligands [44,45]. The Mo-Ophenolate distances are 1.940(4) and 1.930(4) \u00c5, whereas the Mo-N and Mo-Owater distances, i.e. the bond lengths to the neutral donors, are 2.493(5) and 2.275(5) \u00c5, correspondingly. Again, the positive charge of the complex is located in the pendant-arm ammonium cation while the iodide anion and two solvent molecules reside in the crystal lattice.Compound 3 is a zwitterion where the anionic charge of the complex unit, formally the uranate anion, is balanced with the cationic charge in the pendant ammonium group. It crystallizes together with one ion pair of [H2L]I and four methanol molecules. The acetate anion is coordinated as the bidentate ligand and amine bisphenolate in a tridentate manner to the linear dioxouranium(VI) ion, which generates a distorted pentagonal bipyramidal geometry around the metal centre. The O=U=O angle is 179.2\u00b0 and the U=O bonds are 1.792(3) and 1.790(3)\u00a0\u00c5, respectively. The U\u2013Ophenolate distances are 2.208(4) and 2.196(4)\u00a0\u00c5, while the U\u2013Oacetate distances are noticeably longer, 2.464(4) and 2.483(4)\u00a0\u00c5, respectively. The U\u2013N distance is also rather long, namely 2.642(4)\u00a0\u00c5. The overall structure and the geometrical parameters of the complex unit resemble those found previously for UO2(HL\u2019)(NO3)]\u00b72CH3CN (H2L\u2019\u00a0=\u00a0(N,N-bis(2-hydroxy-3,5-dialkylbenzyl)-N\u2032,N\u2032-dimethylethylenediamine; alkyl\u00a0=\u00a0Me or tBu) [46].In all complexes, the tridentate ligand coordinates as an O,N,O donor to form two six-membered chelate rings, though the ligand conformation varies in different compounds. In pentacoordinated 1, the chelate rings have adopted half-chair conformations. The O1-V1-N8 and O2-V1-N8 bite angles are 79.4 and 79.8\u00b0, respectively, whereas the bite distances O1\u00b7\u00b7\u00b7N8 and O2\u00b7\u00b7\u00b7N8 are 2.665(4) and 2.673(4)\u00a0\u00c5. In the six-membered rings, the dihedral angle between the facing bonds O1-C1 and C7-N8 is 33.5\u00b0, whereas the dihedral angle between O2\u2013C15 and N8-C12 is -28.1\u00b0. The related bite angles in hexacoordinated 2 are of same magnitude, 81.0 and 79.2\u00b0 as in 1, while the bite distances are longer, 2.910(6) and 2.853(7)\u00a0\u00c5, as a result of the longer metal-donor distances. The dihedral angles O1-C1\u00b7\u00b7\u00b7C7-N8 and O2-C15\u00b7\u00b7\u00b7N8-C9 are remarkably larger than in 1, i.e. 52.2 and -51.3\u00b0, showing more puckered rings. The structures of the chelate rings bear resemblance to boat conformation. In heptacoordinated 3, the bite angles, 70.4 and 75.1\u00b0, are remarkably smaller than in 1 and 2 due to the larger central atom and longer metal-donor bonds. The bite distances, 2.818(6) and 2.968(5) \u00c5, are shorter compared to those in 2. Similarly to 1, the conformation of the chelate rings can be described as a half-chair.As vanadium complex 1 carries a Lewis acid centre and a nucleophilic iodide in a single, isolated compound, it presents as a potential catalyst for the coupling of CO2 with styrene oxide. In our reaction setup, 0.01\u00a0mmol of catalyst sample was mixed with 7\u00a0mmol of styrene oxide in an open vial and the reaction mixture was kept in an autoclave at 80\u00a0\u00b0C for five hours under a CO2 pressure of 10 bar. The reaction mixtures were subsequently analysed by 1H NMR (Table\u00a04). Along with 1, the ammonium iodide proligand [H2L]I as well as the known oxovanadium(V) complex [VO(OMe)(L\u2019)] (H2L\u2019 is N,N\u2032-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N\u2032,N\u2032-dimethylethylene-1,2-diamine), a neutral analogue of 1\n[47], were tested as references. A stoichiometric mixture of precursors [H2L]I and VO(OPr)3 was also tested as an in situ prepared analogue for 1. Under applied reaction conditions, compound 1 gave styrene carbonate in a 26% yield, the turn-over number (TON) being 182. Interestingly, the free proligand [H2L]I, isolated vanadium compound 1 and the VO(OPr)3/[H2L]I mixture gave all styrene carbonate in practically similar yields of which the in situ prepared catalyst displayed highest activity. On the contrary, [VO(OMe)(L\u2019)] did not show any catalytic activity in the absence of an additional nucleophile, but it could be activated by a Bu4NI co-catalyst. On the other hand, it is noteworthy that [VO(OMe)(L\u2019)] is hexacoordinated in the solid state, so the activation may require the dissociation of the pendant side-arm donor.We may suppose that the reactions catalysed by different catalyst systems apply different reaction mechanisms, as well. Specifically, the reaction involving [H2L]I as a catalyst most likely follows the organocatalytic mechanism proposed by Hong and co-workers [26]. In this reaction mechanism, a phenolic hydrogen bond donor activates the epoxide at the alpha carbon, then iodide nucleophile attacks leading to the formation of the ring-opened alkoxide intermediate. This intermediate subsequently reacts by a CO2 insertion leading finally to the formation of a cyclic product [26]. Conversely, complex 1 as well as the two-component systems VO(OPr)3/[H2L]I and [VO(OMe)(L\u2019)]/Bu4NI follow apparently the mechanism proposed by Licini and co-workers for the oxovanadium(v)aminotrisphenolate/ammoniumiodide system. In the suggested mechanism, the coordination of the epoxide to the metal through the oxygen donor is followed by either an internal (by the alkoxide/phenoxide oxygen) or an external (by the halide) nucleophilic attack, a CO2 insertion and a final cyclisation [10]. As [VO(OMe)(L\u2019)] did not show any activity without an external nucleophile, the internal nucleophiles, i.e. phenolate oxygens or coordinated nitrogen donor seem not to participate the reaction.In conclusion, an ammonium iodide -functionalised amine bisphenol reacts with V, Mo and U precursors as a tridentate O,N,O donor to form mononuclear oxovanadium(V), dioxomolybdenum(VI) and dioxouranium(VI) species, respectively. In the oxovanadium(V) and dioxomolybdenum(VI) complexes, the cationic charge in the pendant arm is balanced by iodide counter ion. By contrast, uranyl cation forms a zwitterionic complex, in which the anionic charge of uranate complex unit is compensated by the cationic pendant arm of the ligand. The oxovanadium(V) complex combines a Lewis acid metal centre and Lewis basic iodide moiety, which makes it the catalyst for the coupling of CO2 with styrene oxide. Study of the catalytic activity of the vanadium complex provided evidence on the importance of the ammonium moiety of the ligand since it serves the role of carrying the iodide nucleophile in the reaction.All syntheses and manipulations were carried out under ambient atmosphere. The solvents and chemicals purchased from commercial suppliers were used without further purifications. The IR spectra were measured with Bruker Optics, Vertex 70 device with a diamond ATR setup, whereas the NMR spectra were recorded with Bruker Avance 500 NMR (1H: 500\u00a0MHz, 51V: 132\u00a0MHz, 13C: 125\u00a0MHz) NMR spectrometer at 25\u00a0\u00b0C (298\u00a0K). The spectrometer was equipped with a broad-band observe probe (Bruker BBO-5\u00a0mm-Zgrad). The 0 ppm vanadium reference frequency was calculated from the TMS 1H frequency using the unified chemical shift scale by IUPAC (\u039e(51V, VOCl3)\u00a0=\u00a026.302948) [48]. Complex [VO(OMe)(L\u2019)] was prepared as previously reported [47]. The NMR spectra are given inThe ligand precursor was made applying a known procedure for corresponding compounds [26] 2.1 g (4\u00a0mmol) of N,N\u2032-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N\u2032,N\u2032-dimethylethylene-1,2-diamine [47] and 1.4\u00a0g of MeI (10\u00a0mmol) were mixed in 20\u00a0ml of acetonitrile and heated to the reflux temperature for three hours. The reaction mixture was allowed to cool to room temperature and 2.1\u00a0g (78%) of [H2L]I was isolated by filtration, washed with cold acetonitrile and dried in vacuum. 1H NMR (CDCl3): \u03b4 7.43 (s, 2H, ArOH), 7.26 (d, J\u00a0=\u00a02.2 Hz, 2H, ArH), 6.99 (d, J\u00a0=\u00a02.1 Hz, 2H, ArH.), 3.98 (t, J\u00a0=\u00a06.3 Hz, 2H, CH2\nNMe3), 3.83 (s, 4H, ArCH2\n), 3.22 (s, 9H, NMe3\n), 3.00 (t, J\u00a0=\u00a06.2 Hz, 2H, NCH2\n), 1.41 (s, 18H, t-Bu), 1.29 (s, 18H, t-Bu).134 mg (0.20\u00a0mmol) of [H2L]I was dissolved in 4\u00a0ml of MeOH and 50\u00a0\u00b5l (0.20\u00a0mmol) of VO(OPr)3 was added. The dark solution was kept at +4\u00a0\u00b0C for three days to obtain 105\u00a0mg (63%) of 1 as dark brown needles. A sample was kept in a vacuum desiccator for two days prior to the elemental and spectral analyses to remove the possible non-stoichiometric amount of the solvent of crystallisation. Found: C: 56.54; H: 7.81; N: 3.68. Calcd. for C36H60IN2O4V: C: 56.69; H: 7.93; N: 3.67. IR (cm\u22121) 2954w, 1438w, 1236m, 1168w, 1054s, 989w, 948m (V=O), 914m, 852m, 806w, 757m, 595s, 549w, 476w, 368w. ESI(+)-MS: m/z 635.4062 ([VO(OMe)(L)]+ calcd. m/z 635.3993). UV-Vis: 285 nm (\u03b5\u00a0=\u00a025\u00a0500 M\u22121cm\u22121), 360 nm (\u03b5\u00a0=\u00a010\u00a0500 M\u22121cm\u22121). 1H NMR (MeOH-d4): \u03b4 7.39 (d, J\u00a0=\u00a02.2 Hz, 2H, ArH), 7.30 (d, J\u00a0=\u00a02.2 Hz, 2H, ArH), 4.29 (d, J\u00a0=\u00a012.7 Hz, 2H, ArCH2\n), 3.64 (m, 2H, NCH2\n), 3.54 (d, J\u00a0=\u00a012.7 Hz, 2H, ArCH2\n), 3.12 (m, 2H, NCH2\n), 2.84 (s, 9H, NMe3\n), 1.55 (s, 18H, t-Bu), 1.34 (s, 18H, t-Bu). 51V NMR (MeOH-d4): -470 (major component, > 90%), -532 (minor), -553 (minor). 13C NMR (MeOH-d4): 165.4, 143.1, 137.2, 125.2, 125.1, 123.8, 60.4, 58.0, 54.2, 45.3, 36.3, 35.4, 32.3, 31.4.134 mg (0.20\u00a0mmol) of [H2L]I was dissolved in 4 ml of MeOH and 65\u00a0mg (0.20\u00a0mmol) of MoO2(acac)2 was added. The solution was kept at room temperature for three days to obtain 120 mg of 2 as yellow crystals together with a small amount colourless precipitate. The crystalline material is poorly soluble in common solvents and was not further purified. IR: 3421w(br), 3280w, 2958m, 2904m, 2869w, 1475vs, 1443m, 1413s, 1388m, 1360m, 1303m, 1254s, 1236s, 1203s, 1169s, 1128s, 1099w, 1022m, 1016s, 991w, 970w, 940s, 914s, 900vs, 843vs, 808w, 783w, 754s, 653w, 600w, 569s, 555s, 499m, 478m cm\u22121. 1H NMR (DMSO-d6,): \u03b4 7.46 (d, J\u00a0=\u00a02.2, 2H, ArH), 7.24 (d, J\u00a0=\u00a02.2 Hz, 2H, ArH), 4.33 (d, 2H, J\u00a0=\u00a011.5 Hz, ArCH2\n), 4.09 (q, J\u00a0=\u00a05.5 Hz, 2H, CH3\nOH), 3.64 (m, 2H, CH2\nNMe3), 3.43 (d, 2H, J\u00a0=\u00a011.5 Hz, ArCH2\n), 3.18 (d, J\u00a0=\u00a05Hz, 6H, CH3\nOH), 2.91 (m, 2H, NCH2\n), 2.78, (s, 9H, NMe3\n), 1.39 (s, 18H, t-Bu), 1.29 (s, 18H, t-Bu). 13C NMR (DMSO-d6): 159.7, 141.7, 135.8, 125.3, 123.4, 123.1, 55.0, 52.8, 48.6, 34.65, 34.2, 31.6, 30.1.67 mg (0.10 mmol) of [H2L]I was dissolved in 3 ml of MeOH and 42 mg (0.10\u00a0mmol) of UO2(CH3COO)2\u00b72H2O was added. The solution was kept at room temperature for three days to obtain 60 mg of 3 as brown crystals. The product contained a small amount of slightly coloured microcrystals, whereas attempts to purify the sample by washing or recrystallization failed. IR: 3344w(br), 2953m, 2906m, 2862m, 1543m, 1477vs, 1445vs, 1414s, 1387m, 1360m, 1308s, 1284m, 1271s, 1238s, 1205s, 1167m, 1130m, 1109m, 1049w, 989w, 969w, 914m, 865vs, 837s, 806m, 785m, 770m, 744m, 673m, 648m, 619s, 602s, 526s cm\u22121. 1H NMR (DMSO-d6): \u03b4 9.00 (s, 2H, ArOH), 7.36 (s, 2H, ArH), 7.34 (s, 2H, ArH), 7.15 (s, 2H, ArH), 7.04 (2H, ArH), 5.01 (d, 2H, J\u00a0=\u00a012.3 Hz, ArCH2\n), 4.00 (d, 2H, J\u00a0=\u00a012.3 Hz, ArCH2\n), 3.75 (s, 4H, ArCH2\n), 3.53 (t, J\u00a0=\u00a06.3 Hz, 2H, CH2\nNMe3), 3.44 (m, 2H, CH2\nNMe3), 3.25 (m, 2H, NCH2\n), 2.92 (s, 9H, NMe\n3), 2.85 (t, J\u00a0=\u00a06.3 Hz, 2H, NCH2\n), 2.53 (s, 9H, NMe\n3), 2.33 (s, 3H, OAc), 1.68 (s, 18H, t-Bu), 1.36 (s, 18H, t-Bu), 1.32 (s, 18H, t-Bu), 1.24 (s, 18H, t-Bu). 13C NMR (DMSO-d6): 184.3, 166.2, 152.4, 141.0, 137.4, 136.8, 136.2, 125.0, 124.9, 124.8, 123.0, 122.6, 122.5, 61.2, 61.0, 60.4, 54.5, 52.4, 52.3, 45.0, 42.6, 34.8, 34.6, 33.9, 33.6, 32.0, 31.4, 30.3, 29.6.In each experiment, the 0.01\u00a0mmol sample of the catalyst (1, [H2L]I, [H2L]I\u00a0+\u00a0VO(OPr)3, [VO(OMe)(L\u2019)] or [VO(OMe)(L\u2019)]\u00a0+\u00a0Bu4NI) was mixed in 0.8\u00a0ml (7\u00a0mmol) of styrene oxide and the reaction mixture was put in a stainless steel autoclave. The blank reaction was run without any catalyst. The reactor was then pressurized with CO2 to 10 bar at 80\u00b0C for five hours, whereas the reaction mixtures were subsequently analysed by 1H NMR by comparing the integrated intensities of aliphatic hydrogens in styrene oxide at 5.70, 4.83 and 4.37\u00a0ppm, respectively, to those chemical shifts of styrene carbonate at 3.88, 3.17 and 2.84\u00a0ppm.Data were collected on a Bruker-Nonius KappaCCD diffractometer with Apex II detector using Mo K\u03b1 radiation and the crystals kept at 170 K during data collection. For data collection, processing, and absorption correction the software packages COLLECT [49], DENZO-SMN [50] and SADABS [51] were, respectively used. The structure solving (direct methods) and refinement on F\n2 by full-matrix least-squares techniques were done within Olex2 [52] environment using SHELXS [53] and SHELXL [54] software packages, respectively. All non-hydrogen atoms were refined anisotropically whereas hydrogen atoms were refined using isotropic displacement parameters. O\u2013H hydrogen atoms were located from the difference density map when possible (all phenol groups and water molecules as well as some of the methanol solvent molecules) and refined using O\u2013H distance restraints. The remaining MeOH O\u2013H and all C\u2013H hydrogen atoms were refined with a riding atom model. The final refinement of structure of 2 was carried out using a HKLF5 file consisting of two domains in a ca. 6:4 ratio, which resulted in significant improvement of the refinement (Figs. 1\u20133\n\n\n, Tables 1\u20133\n\n\n\n, Table\u00a05\n).\nAnssi Peuronen: Investigation, Writing \u2013 review & editing. Esko Saloj\u00e4rvi: . Pasi Salonen: . Ari Lehtonen: Conceptualization, Supervision, Investigation, Writing \u2013 original draft.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Anssi Peuronen reports financial support was provided by Academy of Finland.We greatly acknowledge Mr. Qingan Wang for the number of catalyst tests. This work was supported by Academy of Finland (project no. 315911, for A.P.).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2022.132827.\n\n\nImage, application 1\n\n\n\n\n\nImage, application 2\n\n\n\n", "descript": "\n                  An amine bisphenol ligand with an ammonium iodide group in the pendant arm (H2L) reacts with V, Mo and U oxometal precursors to form oxovanadium(V), dioxomolybdenum(VI) and dioxouranium(VI) species, respectively. In methanol solutions, vanadium(V) and molybdenum(VI) form 1:1 complexes [VO(OMe)(L)]I\u00b72MeOH and [MoO2(L)(H2O)]I\u00b72MeOH, where the cationic charge in the pendant arm is counterbalanced by an iodide anion. Uranium(VI) forms a complex in which the anionic charge of uranate complex unit is compensated by the cationic pendant arm. The complex crystallises as a co-crystal containing a neutral ligand precursor, namely [UO2(L)(OAc)]\u00b7[H2L]I\u00b74MeOH. The oxovanadium(V) complex combines a Lewis acid, i.e. a pentacoordinated metal centre with a Lewis basic iodide moiety, which makes it a suitable catalyst for the coupling of CO2 with styrene oxide. The role of the ammonium moiety of the ligand is to carry the iodide nucleophile in the reaction.\n               "}
{"full_text": "Energy crisis and environmental pollution hinder the sustainable development of society. People have realized the significance of clean energy. Hydrogen energy is widely concerned and researched because of its high combustion efficiency, rich resources, clean and recyclable characteristics. Hydrogen energy is regarded as the most potential energy in the 21st century [1\u20135].Water electrolysis is an effective method to acquire hydrogen with the aid of electrocatalyst [6,7]. This method is simple, reliable, high conversion efficiency, which is able to achieve large-scale production. Platinum and palladium are the most common catalysts for hydrogen evolution reaction, which show good catalytic activity due to their low overpotential [8]. However, platinum and palladium are precious metals, which have low reserves on the earth and are expensive, which is not conducive to industrial production [9]. People began to study new materials to replace the precious metals in the hydrogen evolution reaction. It is found that Mo, W, Fe, Ni and Pd can be used as catalysts for hydrogen evolution reaction [10,11]. In the process people gradually realized that the binary alloy catalysts could not only reduce the cost, but also significantly reduce the overpotential and improve the stability of the material, which make the research on binary catalysts is attractive [12].For example, Paolo Salvi et al prepared porous Ni-Fe, Ni-Mo, Ni-Ti, Ni-Cr Alloys by chemical doping method [13]. The properties of Ni alloy doped with Mo did not significantly reduce after several hours of cathodic polarization, and its conductivity and electrode stability were improved.On the basis of binary catalyst, it is found that the surface roughness of the catalyst increases after adding the third element, which makes the specific surface area of the catalyst increase and further improves the activity and stability of the catalyst [14\u201316]. For example, Shervedani et al prepared Ni-Mo-P electrode and studied its cathodic polarization and alternative current (AC) impedance curve in 1\u00a0M NaOH, the test results show that the increase in electrode activity was due to increases in (i) surface roughness and (ii) intrinsic activity [17].Herein, PtAuFe/C ternary composite catalyst and Pt/C catalyst were prepared by direct reduction of sodium borohydride. PtAuFe/C and Pt/C catalysts were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The performance of PtAuFe/C composite catalyst was tested using the electrochemical workstation, and the mechanism of hydrogen evolution was clarified.All chemical reagents used in this experiment were of analytical grade and used without further purification, including carbon black (VulcanXC-72), chloroplatinic acid (H2PtCl6), chlorogold acid (HAuCl4\u00b74H2O), ferrous chloride (FeCl2), sodium borohydride (NaBH4), Nafion solution, ethanol (CH3CH2OH), isopropanol, alumina polishing powder, sulfuric acid.A certain amount of chloroplatinic acid solution (3.7\u00a0mg\u2022mL\u22121), chloroauric acid solution (4.78\u00a0mg\u2022mL\u22121) and ferrous chloride were mixed with 0.5\u00a0mol\u2022L\u22121 sodium borohydride solution. The acquired solution appeared black precipitation and bubbles. After standing for several hours, the solution was washed with anhydrous alcohol for three times and centrifuged using ultrasonic dispersion to remove impurity ions. Then PtAuFe powder with a mass ratio of 1:1:1 was obtained by drying in a 333\u00a0K vacuum oven for 6\u00a0h. Finally, PtAuFe/C (PtAuFe:C\u00a0=\u00a01:1) composite catalyst was prepared by ultrasonic dispersing of PtAuFe powder and carbon powder in isopropanol solution.Taking a certain amount of chloroplatinic acid solution (the theoretical loading is 50%) to prepare Pt/C catalyst. The preparation method is the same as above. Sung Mook Choi\u2019s group characterized the prepared Pt/C by TEM characterization, test results prove that uniformly-dispersed Pt nanoparticles are found on the carbon support [18].The catalysts were characterized by XRD, SEM and EDS. The crystal structure and composition of the catalyst were measured by XRD-6100. The working voltage is 40\u00a0kV, the working current is 30\u00a0mA, the scanning range is 20\u00b0\u201380\u00b0, and the scanning rate is 8\u02da/min. The surface morphology and composition of the catalyst were analyzed by SEM and EDS.All the electrochemical tests were carried out on the electrochemical workstation (CHI760e) in a three electrode system. The glass carbon electrode (GCE, the diameter is 3\u00a0mm) with catalyst drop (0.0757\u00a0mg/cm2), platinum wire and the saturated calomel electrode (SCE) are used as working electrode, counter electrode and reference electrode respectively. Taking 0.5\u00a0mg of prepared catalyst powder, 0.5\u00a0mg of carbon powder and 1.5\u00a0ml of isopropanol and mix them using ultrasonic dispersing for 30\u00a0min. 8\u00a0\u03bcl of prepared catalyst solution was dropped onto the working electrode surface, and then a drop of 0.5% Nafion solution was added onto the electrode to protect the catalyst.The XRD patterns of Pt/C and PtAuFe/C catalysts are shown in Fig. 1\na. Fig. 1a shows that the diffraction peaks of (1\u00a01\u00a01), (2\u00a00\u00a00) and (2\u00a02\u00a00) crystal planes of Pt are corresponding to the 2\u03b8 of 40.02\u00b0, 46.46\u00b0 and 67.76\u00b0 respectively. The diffraction peaks of (1\u00a01\u00a01), (2\u00a00\u00a00), (2\u00a02\u00a00) and (3\u00a01\u00a01) crystal planes of Au are corresponding to the 2\u03b8 of 38.32\u00b0, 47.02\u00b0, 64.82\u00b0 and 77.64\u00b0. The diffraction peak of Fe appears at 2\u03b8 of 44.62\u00b0, which corresponds to the (1\u00a01\u00a00) crystal plane of Fe. The results show that PtAuFe/C catalyst has been successfully synthesized.\nFig. 1b & c are the SEM images of Pt/C catalyst and PtAuFe/C composite catalyst. In Fig. 1b, Pt/C is aggregated in granular form and the aggregation was more serious. From Fig. 1c, we can find that adding Au and Fe changes the surface morphology. Compared with Pt/C catalyst, the agglomeration of PtAuFe/C composite catalyst is obviously reduced and the dispersion of catalyst is increased. Fig. 1d&e are the energy dispersive spectrum of Pt/C and PtAuFe/C composite catalysts respectively. Fig. 1d shows that the sample is mainly composed of Pt and carbon. Fig. 1e demonstrates that the composite sample is mainly composed of Pt, Au, Fe and carbon. The results are in accordance with those of XRD.\nFig. 2\na shows the hydrogen evolution reaction (HER) catalytic performance of PtAuFe/C composite catalyst and Pt/C catalyst at 298\u00a0K. The HER performance of electrocatalysts were tested by LSV in 0.5\u00a0M H2SO4. It can be seen that the initial hydrogen evolution potential of PtAuFe/C composite catalyst shifted positively about 10\u00a0mV compared with that of Pt/C catalyst, and the current density is 1.4 times of that of Pt/C catalyst at \u22120.4\u00a0V. It indicates that PtAuFe/C composite catalyst is of more positive initial hydrogen evolution potential and higher current density. The results show that the addition of Au and Fe improves the electrocatalytic hydrogen evolution performance of the catalyst.To understand the hydrogen evolution mechanism of the catalysts, Tafel curves of the prepared catalysts were calculated. Fig. 2b shows the Tafel curves of PtAuFe/C composite catalyst and Pt/C catalyst, and the corresponding electrochemical kinetic parameters are shown in Table 1\n. It can be seen from Fig. 2b that the kinetic characteristics of HER for the catalysts are in accordance with Tafel relationship. Table 1 shows that the Tafel slope b of Pt/C catalyst is 36\u00a0mV\u2022dec\u22121 demonstrating that the reaction mechanism of Pt/C catalyst is Volmer Heyrovsky reaction. The Tafel slope b of PtAuFe/C composite catalyst is 30\u00a0mV\u2022dec\u22121, and the reaction mechanism of PtAuFe/C composite catalyst is Volmer Tafel reaction. Compared with Pt/C catalyst, PtAuFe/C composite catalyst has a smaller Tafel slope b, which means that the current density of PtAuFe/C composite catalyst increases faster with the increase of overpotential and the faster hydrogen evolution rate was achieved.\nFig. 2c is the CV curves of PtAuFe/C and Pt/C. PtAuFe/C has a higher current density and a stronger redox peak than Pt/C under the same conditions. The electrochemical surface area (ECSA) of the samples are calculated according to the CV curves. It is found that the ECSA of Pt/C is 45.21\u00a0m2/g and that of PtAuFe/C is 57.96\u00a0m2/g. The higher the value of ECSA is, the higher the catalytic activity is. PtAuFe/C has a larger ECSA than Pt/C, indicating that PtAuFe/C has better catalytic activity, which is consistent with the results of LSV.Besides the activity, the stability of the catalysts is also an important factor to evaluate their performance. Fig. 2d&e show the LSV curves of Pt/C catalyst and PtAuFe/C composite catalyst. The stability of these two catalysts were judged by comparing the polarization current density after different scanning cycles. The current density corresponding to the LSV curve of Pt/C catalyst scanned by 1000 cycles of cyclic voltammetry (CV) at \u22120.4\u00a0V is 15% lower than that of the 1st cycle, while the current density corresponding to the LSV curve of PtAuFe/C composite catalyst scanned by 1000 circles of CV at \u22120.4\u00a0V is 14% higher than that of 1st cycle. This may be due to the emergence of new active sites in the catalyst after multi-cycle scanning. Therefore, PtAuFe/C composite catalyst has better stability than that of Pt/C.\nFig. 3\na is the LSV curves of PtAuFe/C composite catalyst at 298\u00a0K in different concentration of H2SO4 solutions. With the increase of the electrolyte concentration, the initial potential of hydrogen evolution has a significant positive shift and the current density has a significant increase. The initial potentials in 0.1\u00a0M, 0.5\u00a0M and 1\u00a0M of H2SO4 were 0.32\u00a0V, 0.29\u00a0V and 0.27\u00a0V respectively. The initial potentials in 0.5\u00a0M of H2SO4 were about 30\u00a0mV higher than that in 0.1\u00a0M of H2SO4, and the current density in 1\u00a0M of H2SO4 was 8.8 times higher than that in 0.1\u00a0M of H2SO4. It indicates that the concentration of H2SO4 has an obvious effect on the hydrogen evolution performance of PtAuFe/C. There are two ways of electrode polarization. One is the electrochemical polarization, which is caused by the slow reaction rate of a certain step in the process of electrolytic product precipitation (such as ion discharge, atom binding to molecules, bubble formation, etc.). The other is concentration polarization, which is caused by the difference between the concentration near the electrode and the concentration in the middle part of the solution due to the slow diffusion rate of the ions. By increasing the concentration of H2SO4, the ion concentration in solution will increase, which lead the increase of ion diffusion rate and the reduction of concentration polarization. For PtAuFe/C composite catalyst, the results show that the over potential of hydrogen evolution is reduced, the initial potential of hydrogen evolution is shifted positively, and the hydrogen evolution performance of the catalyst is improved.\nFig. 3b is the LSV curves of PtAuFe/C composite catalyst at different temperatures in the concentration of 0.5\u00a0M H2SO4 solution. Table 2\n shows the current density at \u22120.3\u00a0V for PtAuFe/C composite catalyst at different temperatures. It can be seen that temperature has an obvious effect on the hydrogen evolution reaction. As the temperature increase, the initial potential of hydrogen evolution moves forward and the overpotential of hydrogen evolution decreases. At the same potential, the current density increased gradually and the reaction rate of hydrogen evolution increased. The reason is that the temperature has an effect on the concentration polarization. When the temperature increases, the concentration polarization is reduced, so the hydrogen evolution overpotential is reduced and the hydrogen evolution performance of the catalyst is improved. In addition, with the increase of temperature, the chemical kinetic constant increases, the reaction speed is accelerated, and the hydrogen evolution performance of the catalyst is also improved.In this paper, Pt/C catalyst and PtAuFe/C composite catalyst were successfully prepared by direct reduction of sodium borohydride. XRD, SEM and EDS were used to characterize the crystal structure, morphology and composition. The hydrogen evolution mechanism of the two catalysts in H2SO4 and the effect of temperature and electrolyte concentration on the performance of PtAuFe/C composite catalyst were studied.In summary, Pt/C and PtAuFe/C composite catalysts have crystal structure. The prepared catalysts are relatively pure. Pt/C catalyst agglomerated seriously. With the addition of Au and Fe, the morphologies of the catalyst were changed and increased the specific surface area of the catalyst. PtAuFe/C composite catalyst is of more positive initial hydrogen evolution potential and higher current density. The reaction mechanism of Pt/C catalyst is probably Volmer Heyrovsky reaction, while PtAuFe/C composite catalyst is Volmer Tafel reaction. In addition, the PtAuFe/C composite catalyst has better stability. With the increase of the concentration of H2SO4, the initial potential of hydrogen evolution of PtAuFe/C composite catalyst shifted positively, the current density increased, and the hydrogen evolution performance is significantly improved. With the increase of temperature, the initial potential of PtAuFe/C composite catalyst for hydrogen evolution reaction gradually moves positively, the over potential of hydrogen evolution gradually decreases, the current density gradually increases at the same potential, and the activity of hydrogen evolution continuously increases.PtAuFe/C composite catalyst and Pt/C catalyst are prepared by sodium borohydride direct reduction method. The PtAuFe/C composite catalyst has larger specific surface area, smaller overpotential, higher polarization current density and better activity as well as stability than that of the Pt/C catalyst. The addition of non-precious metal Fe improves the hydrogen evolution property of PtAuFe/C composite catalyst.\nM. Nie: Writing - review & editing, Project administration. H. Sun: Data curation, Formal analysis. X.H. Tian: Supervision, Validation. J.M. Liao: Data curation. Z.H. Xue: Investigation, Methodology. Z.Z. Zhao: Software, Conceptualization. F. Xia: Validation, Formal analysis. J. Luo: Formal analysis, Visualization.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 Chongqing Key Laboratory for Advanced Materials & Technologies of Clean Energies (JJNY202002), Fundamental Research Funds for the Central Universities (XDJK2020B004) and Chongqing Graduate Research Innovation Project (CYS19106).", "descript": "\n                  In this paper, PtAuFe/C composite catalyst and Pt/C catalyst are prepared by sodium borohydride direct reduction method. The physical properties of the catalysts are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The electrocatalytic hydrogen evolution performance of the prepared catalysts is tested by linear scanning voltammetry (LSV) in H2SO4 solution. The results show that the PtAuFe/C composite catalyst has larger specific surface area, smaller overpotential, higher polarization current density and better stability than that of the Pt/C catalyst. The hydrogen evolution mechanism of PtAuFe/C composite catalyst is determined by Tafel curve, and the results show that PtAuFe/C composite catalyst is of composite desorption mechanism. In addition, the effect of temperature and electrolyte concentration on the performance of PtAuFe/C composite catalyst is also studied.\n               "}
{"full_text": "Hydrogen is a perfect way to store, transport, and convert energy for a comprehensive and clean energy concept. One of the main challenges that need to be overcome, before hydrogen economy is implemented worldwide, is safe and efficient hydrogen storage. Metal hydrides are promising materials to fulfil the target for hydrogen storage applications. To ensure the best hydrogen absorption/desorption properties, metal hydrides are often catalysed by carbon materials. It has been proven that pure carbon materials such as carbon nanotubes, graphite, porous carbon, and activated carbon can store hydrogen and the amount of physisorbed hydrogen is proportional to the specific surface area of carbon material [1]. Among recently studied metal hydride-based composites containing carbon structures one can find: Mg [2\u201356], Mg\u2013Ni elemental mixture [35,57], Mg\u2013Ni alloy [58\u201368], Ti [15,63,69,70], Ti\u2013Ni alloy [71\u201374], TiFe [63], LaNi5 [75], V [63], Ca [8,9], Na [8,9], NaAlH4 [76\u201378], NaMgH3 perovskite hydride [79], Li [8], LiBH4 [80\u201382], LiBH4\u2013MgH2 composite [83], Co\u2013Cu\u2013Si elemental mixture [84], C15 type Laves phase alloy [85] and body-centered-cubic (BCC) solid solution [2,86,87].It is clearly visible from the number of published papers that the influence of carbon addition on the hydrogen storage properties has been especially studied for Mg and Mg-based systems and the other groups of materials have not been studied in detail (see also Fig.\u00a01\nA). For instance, BCC alloys that are promising hydrogen storage materials and are known to absorb up to two atoms of hydrogen per atom of metal have been studied only in three papers. In these works, Yu et\u00a0al. have shown that carbon catalysts can improve the activation properties and hydrogen storage capacity of BCC alloy in solid-gas and electrochemical reaction, respectively [86,87]. Moreover, Ranjbar et\u00a0al. have indicated that carbon nanotubes can significantly lower the hydrogen absorption and desorption temperature of MgH2-BCC composite [2].Recently, Balcerzak has shown that Ti\u2013V BCC alloy absorbs up to 3.67\u00a0wt % of hydrogen at room temperature [88]. However, the application of this hydrogen storage material is limited mainly by the high temperature of hydride decomposition. Therefore, in this paper, we present our studies on the carbon materials' catalytic function on BCC alloys. Furthermore, these research findings are preceded and enriched by a short review of the carbon catalysts that have been used in the past to promote hydrogenation reaction of various metal-based systems.Several authors have shown that ball milling of graphite with metal hydrides (Mg, Mg2Ni, Ti-based composite) resulted in particle size refinement and an increase in the specific surface area of the composite [31,62,69]. Graphite acts here as a process control agent that prevents cold-welding and favours powder crushing process, which is essential for effective activation process and fast hydrogenation kinetics [63,89]. For example, Milanese et\u00a0al. have shown that addition of 5\u201315\u00a0wt % of graphite can highly reduce the number of activation cycles (from 15 to only 3) of Mg\u2013Ni elemental mixture [89]. Furthermore, graphite is known to give origin to carbon-based radicals that react with oxygen-containing species during milling, thus, preventing hydrogen storage material from oxidation [6,11] and hindering oxygen back diffusion from the bulk to the surface [64]. In another study, Borchers et\u00a0al. have shown that carbon atoms occupy the near-surface layer in the ball-milled composites, which leads to a radical decrease in the effective activation energy of the hydrogenation process [69].In other studies it has been shown that graphite is also an excellent addition to (i) promote Mg hydride formation during ball milling under reactive hydrogen atmosphere; (ii) reduce hydrogenation/dehydrogenation hysteresis; (iii) accelerate the hydrogen absorption and desorption kinetics [8,9,12,17]. The complementary studies revealed that the improvement is not related to any change in the hydrogen storage material structure or morphology and should be directly connected to the catalytic function of graphite [17]. It is noteworthy that the kinetics of the dehydrogenation process is one order of magnitude faster for graphite-containing composite than for commercial Mg. A similar performance has been also presented by Furster et\u00a0al., who have shown that Mg can reach the maximum capacity in a twice shorter time compared to pure magnesium [31]. However, it is important to note that Lototskyy et\u00a0al. have shown that the excess (more than 1\u00a0wt %) of expandable and thermally-expanded graphite in the Mg-based composite causes the deterioration of the hydrogenation kinetics. The authors have observed an incubation time during hydrogenation which as believed was required to delaminate the graphene layers from the bulk graphite [20]. The same group has observed that even a relatively modest addition of graphite (5\u00a0wt %) can greatly affect the cyclic stability during repeated hydrogen absorption/desorption (up to a hundred cycles) of Mg and Mg\u2013Ti hydrides at elevated temperatures (623\u00a0K) [15].To mitigate the disadvantages of the magnesium hydride, Jang et\u00a0al. have studied the influence of graphene on reactively milled Mg. The composite with 5\u00a0wt % of graphene showed the maximum capacity of 3.7, 5.1, and 5.7\u00a0wt % at 423, 523, and 623\u00a0K, respectively. Moreover, the hydrogen uptake at 423\u00a0K can be increased to 5.1\u00a0wt %, when the content of graphene is increased to 10\u00a0wt %. Graphene was found to play the role of absorbent to capture hydrogen, as well as a catalyst [53]. In another work, Liu et\u00a0al. have shown results for ball-milled MgH2 with highly crumpled graphene that was obtained by a thermal exfoliation method. The synthesized material exhibited improved hydrogen absorption capacity and kinetics allowing to capture 6.6\u00a0wt % of H2 within 1\u00a0min at 573\u00a0K [39].As stated by Cai et\u00a0al., carbon nanotubes (CNTs) can destabilize the metal-hydrogen bonding and, therefore, reduce the energy barrier for H2 nucleation [37]. It is probably related to the electron affinity of their curved surface. Therefore, to alter the charge distribution in the hydrides and weaken the interaction between hydride forming material and H atoms the contact between CNTs and metal hydride particles has to be ensured.Single-walled carbon nanotubes (SWCNTs) were used together with metallic catalysts to promote the hydrogenation properties of mechanically milled magnesium. Wu et\u00a0al. have shown that Mg-based composite with purified nanotubes absorbs 4.2\u00a0wt % and 6.0\u00a0wt % within an hour at 373 and 423\u00a0K (under an initial hydrogen pressure of 2\u00a0MPa), respectively [56]. Moreover, the MgH2-SWCNTs composite exhibits faster hydrogen absorption/desorption kinetics and decreased hydrogen desorption activation energy compared to pure magnesium hydride. Wu et\u00a0al. have proven that SWCNTs mechanically milled with MgH2 reduce the hydrogen absorption time by five times and hydrogen desorption temperature by 70\u00a0K compared to pure Mg [7,45,56]. SWCNTs have, however, only a small beneficial effect on the desorption temperature of LiBH4 and NaAlH4 \u2013 the dehydrogenation temperature is only 20\u00a0K lower compared to the unmodified material [80,81].Extraordinary hydrogenation kinetics have been observed for Mg ball milled with SWCNTs and 5\u00a0wt % of ZrO2 or FeTi [50,51]. The composite with SWCNTs and ZrO2 can absorb 6.75\u00a0wt % at 423\u00a0K in the first 100\u00a0s of the process and 4.0\u00a0wt % at room temperature within 700\u00a0s, while the composite with SWCNTs and FeTi captured 6.6\u00a0wt % in 60\u00a0s at 423\u00a0K [50,51]. Moreover, the study of Chen et\u00a0al. have revealed that ball milling of Mg with this carbon additive resulted in the formation of 50\u00a0nm magnesium grains [51].The research on metal hydrides revealed significant improvement of hydrogenation kinetics after the addition of multi-walled carbon nanotubes (MWCNTs). For example, a Mg-based material with 5\u00a0wt % of MWCNTs absorbs 90% of its total hydrogen storage capacity within 150\u00a0s at 573\u00a0K, while the pure Mg requires hours or days to reach the same storage capacity under the same conditions [2]. In another study, Aminorroaya et\u00a0al. have shown that the full hydrogen uptake can be reached in 2\u00a0min at 643\u00a0K under 2\u00a0MPa of H2 [35].The study of Laves phase alloy modified by MWCNTs has shown a decrease in the slope of hydrogenation plateau, which is extremely important for its applicability [85]. The versatility of this carbon additive has been also presented for Mg and Mg2Ni alloy with MWCNTs that were co-catalysed by TiF3, Pd, Al, and K2NbF7 according to their solid-gas or electrochemical hydrogen storage properties [34,43,44,57,61]. A study of Chen et\u00a0al. has revealed that 5\u00a0wt % of MWCNTs allowed keeping the maximum hydrogen capacity under cyclic hydrogen absorption/desorption of Mg hydride [33]. Moreover, Pukazhselvan et\u00a0al. who have studied the influence of MWCNTs on hydrogen storage properties of NaAlH4 for the first time, have shown good rehydrogenation characteristics with reversible capacity at 4.2\u00a0wt % level for this composite [77]. It is important to note that in some cases the improvement may be less pronounced for MWCNTs than for SWCNTs. Chuang et\u00a0al. have explained it by the various Young's modulus of these two carbon materials. SWCNTs have a higher modulus than MWCNTs but smaller strain and higher hardness and this can effectively affect the reduction of Mg crystallite size and thus hydrogen storage properties [48].Other studies prove that MWCNTs affect also the hydrogen desorption process. For example, Mg2Ni mixed with this carbon catalyst shows a 90% increase in hydrogen desorption rate in comparison to pure Mg2Ni [60]. Ranjbar et\u00a0al. have shown that 5\u00a0wt % of MWCNTs added to the Mg can reduce the MgH2 decomposition temperature by 125\u00a0K [2].MWCNTs were also co-mixed with graphene oxide to enhance the hydrogen storage properties of NaMgH3. In effect, the composite material was characterized by significantly reduced activation energy for both dehydrogenation steps. The synergetic effect of both carbon materials resulted in better dehydrogenation kinetics and lower dehydrogenation temperature [79].There are various proposed reasons for the positive impact of carbon nanotubes on hydrogen storage properties. One of them describes CNTs as channels for hydrogen diffusion to and from the interior of hydrogen storage material grains [2,44,60,66,86]. Their presence results in faster hydride formation and decomposition kinetics. Moreover, Yahya et\u00a0al. have stated that carbon nanotubes should also be considered as thermal bridges that facilitate the heat exchange process during the hydrogen absorption [44]. Cai et\u00a0al. have proposed that CNTs act as a three-dimensional framework that prevents the metal/metal hydride particles from sintering during hydriding/dehydriding cycles [40]. The sintering process is considered as one of the reasons for the deterioration of hydrogen storage properties.The influence of carbon nanofibres/graphite nanofibres (CNFs) on hydrogen storage properties of metal hydrides has not been studied so thoroughly as in the case of graphite or carbon nanotubes. However, the helical graphitic nanofibres were used to enhance the properties of NaAlH4 [78]. The authors have found that this carbon material possesses superior catalytic activity in improving the desorption kinetics and decreasing the hydrogen desorption temperature. Moreover, the NaAlH4 can be easily re-hydrogenated at moderate conditions, at 393\u00a0K, and under 9.12\u00a0MPa. It is worth mentioning that CNFs can greatly accelerate the kinetics of Mg hydrolysis. This composite releases 95% of the theoretical hydrogen generation yield within 4\u00a0min [16].As in the case of the above-mentioned carbon catalysts, also ultrafine diamonds have been used to promote the hydrogenation in Mg. The published studies showed that the addition of diamond powder can substantially increase the hydrogen absorption rate, strongly decrease the hydrogen decomposition temperature (by 100\u00a0K compared to the unmodified system) and significantly increase the storage capacity (by 25% compared to pure magnesium) for MgH2 [18,24].Activated carbon has also been used to catalyse hydrogenation reactions in different laboratories. Its co-milling with MgH2 decreased the onset and peak temperature of the hydrogen desorption process (compared to 693\u00a0K for as received Mg). The temperature depends on the content of carbon catalyst in the composite \u2013 for 1\u00a0wt % and 10\u00a0wt % of activated carbon is 622\u00a0K and 589\u00a0K, respectively. Moreover, these composites are able to absorb 6.5\u00a0wt % of H2 within 7\u00a0min at 573\u00a0K, 6.7\u00a0wt % of H2 within 2\u00a0h at 473\u00a0K and release 6.5\u00a0wt % of H2 within 30\u00a0min at 603\u00a0K [54]. The significant improvement of hydrogenation/dehydrogenation reactions (also kinetics) at even low activated carbon concentration has been also observed by Lototskyy et\u00a0al. and Wu et\u00a0al. [20,45]. Concluding, carbon acts as a carrier of the \u2018activated\u2019 hydrogen by a mechanism of spill-over.In a different study, Mandzhukova et\u00a0al. have studied the influence of activated carbon and 3d-metal-containing compounds on the hydrogenation properties of magnesium. The carbon-containing composites preserve their hydrogen storage capacity during prolonged cycling (at least up to 80 cycles, at 573\u00a0K, 1\u00a0MPa of H2). After 80 cycles the hydrogen absorption capacity of the composite was 6.9\u00a0wt %, which is close to the theoretical 7.2\u00a0wt % [90]. Scanning electron microscopy (SEM) images of the cycled composite showed very fine powder which favours the hydrogenation/dehydrogenation performance due to serious shortening of the hydrogen diffusion distance [55].So far, fullerene has been used to catalyse the hydrogenation reaction of Mg and Mg2Ni alloy. Alsabawi et\u00a0al. have systematically studied the catalytic effect of C60 addition to MgH2 with a concentration of fullerenes up to 10\u00a0wt %. The ball milling of MgH2 with 10\u00a0wt % of fullerenes for 10\u00a0h resulted in the reduction of hydrogen desorption temperature by nearly 35\u00a0K [49]. Moreover, the desorption of hydrogen from C60 containing composite is incomparably faster than from pure MgH2 (at 623\u00a0K) [45]. Fullerenes have been also used to create nanocrystalline Mg2Ni alloys with the increased surface area. In the work of Bouaricha et\u00a0al., the initially ball milled Mg2Ni/C60 composite was poured in toluene to dissolute the C60 and form an alloy with a highly reduced size of crystals and enlarged specific surface area [62]. As a result, C60 can at least twice accelerate the hydrogen desorption rate of Mg2Ni based composite [62].Only in a few studies, carbon black has been considered as a catalyst to improve the properties of metal hydrides [45,46]. The addition of carbon black to the Mg by ball milling considerably affects the kinetics of hydrogenation (at 423, 473, 573\u00a0K) and dehydrogenation (at 623\u00a0K) [45]. The fully hydrogenated composite with carbon black can desorb nearly 6\u00a0wt % within 15\u00a0min at 623\u00a0K, while the unmodified MgH2 needs an hour to release 4\u00a0wt % only.As demonstrated by Rud et\u00a0al., the addition of amorphous carbon can effectively improve the kinetics of the hydrogen absorption process and substantially increase the hydrogen storage capacity [18]. The composite material obtained via reactive ball milling (under initial pressure of 0.5\u20130.6\u00a0MPa of hydrogen) absorbs around 4.6\u00a0wt % of H2 after 25\u00a0h of milling, while the pure Mg reaches only 4.1\u00a0wt % of H2 in 100\u00a0h of milling [18]. In another study, Spassov et\u00a0al. have studied the hydrogen storage properties of Mg/amorphous carbon soot composite [24].The demineralized anthracite coal was used as a support in milling the Mg particles down to the nanoscale without cold welding. The studies on the influence of this carbon catalyst on hydrogen storage properties of MgH2 have shown that it improves the hydride decomposition kinetics, decreases the hydrogen desorption onset temperature, reduces the enthalpy change, and diminishes the dehydrogenation activation energy compared to bulk MgH2 [25\u201327].Although most of the mentioned research papers are focused on solid-gas reactions between metal and hydrogen, there are, however, some studies that deal with the influence of carbon addition on the performance of negative electrode in Ni-MHx secondary batteries [59,62,65,71\u201373,75,84,87]. For example, CNTs are beneficial to charge-transfer reactions improving the overall discharge capacity, strengthening high-rate dischargeability, reducing charge-transfer resistance, plateau voltage, influencing the hydrogen diffusion in the bulk of the electrode, and improving the cycle life of the Ni-MHx batteries [71,72,75]. The enhancement of Ni-MHx batteries performance is related not only to the electro-catalytic function of carbon nanotubes but also to carbon-induced changes in microstructure and morphology of electrode material. In particular, the reduction of particle size and simultaneous rise of the specific surface area provides a larger electrochemically accessible area and rapid channel for hydrogen transportation [84]. However, the improvement of properties is observed only when the addition of carbon material is relatively small \u2013 too high content of MWCNTs can cause a decrease in hydrogen storage properties due to the decrease in the active material density [75]. In another study, Guo et\u00a0al. have also observed a negative impact of carbon structures on the electrochemical properties of Mg\u2013Ni alloy. In this case, the carbon addition resulted in the decrease in initial discharge capacity, which was apparently caused by the fact that carbon atoms blocked the active sites for hydrogen storage [59].Despite that the above discussion shows rather unambiguously the desired effect of carbon materials on the hydrogen storage properties of metal hydrides, the properties of a specific composite strongly depend on the metal hydride and carbon catalyst used. Therefore, carbon catalyst impact studies should be undertaken each time a new composite is designed and synthesized. In our study, we pay attention to six different carbon catalysts. Two of them were the most studied carbon forms (taken as reference) \u2013 graphite and carbon nanotubes (see also Fig.\u00a01B). We supplemented this group with four relatively novel materials that were less tested in terms of hydrogenation catalysis \u2013 mesoporous carbon, carbon nanofibres, ultrafine diamonds, and fullerenes. In this paper, we extensively studied the effect of carbon catalysts on the hydrogen absorption/desorption properties of BCC alloy. We believe that these results can be used to design new BCC-based composites and a novel undeveloped group of materials \u2013 high-entropy alloys (HEA)-based composites for hydrogen storage.Ti1\n.\n\u00b7\n5V0.5 alloy was prepared from Ti and V elemental powders (Alfa Aesar,\u00a0\u2212325 mesh, 99.5% purity). This alloy was chosen, as it is one of the recently studied alloys from the Ti\u2013V system, that is characterized by high gravimetric hydrogen storage capacity but high hydrogen desorption temperature [88]. The alloy has been synthesized using the ball milling method under an argon atmosphere. The detailed synthesis procedure has been already published elsewhere [88].A series of carbon allotropes were used to prepare Ti1\n.\n\u00b7\n5V0.5-based composites: multi-walled carbon nanotubes \u2013 CNT (Sigma Aldrich, 6\u201313\u00a0nm outer diameter, 2\u20136\u00a0nm inner diameter, 2.5\u201320\u00a0\u03bcm long, 7\u201313 graphene layers, >98% purity), mesoporous carbon nanopowder - CM (Sigma Aldrich, 99.95% purity), carbon nanofibers - CNF (Sigma Aldrich, 100\u00a0nm diameter, 20\u2013200\u00a0\u03bcm long, \u226598% purity), diamond powder - UFD (Alfa Aesar, <1\u00a0\u03bcm, 99.9% purity), graphite - G (Alfa Aesar, \u223c45\u00a0\u03bcm, 99.9995% purity), fullerene \u2013 C60 (Alfa Aesar, 99% purity). The pre-synthesized alloy was mixed with 5\u00a0wt % of carbon structures using SPEX 8000\u00a0M shaker mill under an inert argon atmosphere. As several groups reported the destructive nature of ball milling on carbon structures we decided to mill alloy with carbon structures without milling balls \u2013 to reduce the energy during composite preparation [63]. This process lasted 30\u00a0min in each case. Additionally, various amounts of fullerenes (1, 5, 10\u00a0wt %) were milled with an alloy for 30\u00a0min and the obtained materials were tested to study the effect of amount of addition on hydrogen storage properties. Moreover, 5\u00a0wt % of fullerene was milled with an alloy for 10, 30 and 60\u00a0min, and the influence of milling time on the hydrogenation performance was determined. The nomenclature of the studied materials together with their Brunauer\u2013Emmett\u2013Teller (BET) specific surface areas (provided by suppliers) are listed in Table\u00a01\n. All the material handling was performed in a high-purity Ar atmosphere M-Braun glovebox (H2O and O2 levels below 1\u00a0ppm).The synthesized alloy and composite materials were characterized by powder X-ray diffraction (XRD) using a Panalytical Empyrean X-ray diffractometer with a Cu K\u03b1 radiation. The surface morphology was observed by SEM, using TESCAN MIRA 3 operating at 10\u00a0kV in secondary electrons (SE) and backscattered electrons (BSE) image modes. The microstructural characterization of carbon materials, alloy, and synthesized composites was acquired by employing a Hitachi Scanning Electron Microscope S3000\u00a0N (operated at 100\u00a0kV) and FEI TEM Titan3 G2 (operated at 80\u00a0kV). The Raman spectra of Ti1\n.\n5V0.5-based composites were recorded at room temperature to investigate the milling effect on the structure of carbon allotropes using Renishaw inVia Raman microscope equipped with a thermoelectrically (TE) - cooled CCD detector and an argon-ion laser working at 514.5\u00a0nm wavelengths. The Raman spectra were recorded in the spectral range 100\u20133200\u00a0cm\u22121 with a spectral resolution better than 2\u00a0cm\u22121. The exposure time of CCD detector was 300\u00a0s. To avoid sample overheating, the power of the laser beam was kept below 0.1\u00a0mW. The position of Raman peaks was calibrated before collecting the data using a crystalline silicon sample as an internal standard. The spectral parameters of bands were determined using the fitting package of Wire 3.4 software. The dehydrogenation behaviour was studied by differential scanning calorimetry (DSC) isochronal experiments that were performed utilizing the Netzsch DSC 404 apparatus with various heating rates q (5\u201340\u00a0K/min) and for temperatures from 323 to 1023\u00a0K. After initial equilibration, the continuous heating curves were measured in an argon flow of 100\u00a0ml/min. The effective activation energies E\na (of the dehydrogenation process) were calculated using the Kissinger formula from the ln (T\np\n2/q) vs. 1/T\np plots [91].Hydrogen absorption properties were determined using an automated Particulate Systems HPVA-200 Sievert's volumetric apparatus. Prior to the measurements, all of the as-prepared materials (alloy and composites) were evacuated at 303\u00a0K for 1\u00a0h under vacuum and activated with 3\u00a0MPa of H2 at room temperature. Around 1\u00a0g of material was placed in a stainless steel reactor for each measurement. The absorption kinetic measurements were performed at 303\u00a0K with an initial pressure of 3\u00a0MPa of H2.As written in the experimental section, a wide range of carbon allotropes was selected to improve the hydrogen storage properties of Ti1\n.\n5V0.5 BCC alloy. Table\u00a01 compares the results of physical absorption of N2 measurements (BET specific surface areas) that were performed by suppliers. It is obvious that carbon allotropes highly differ in terms of porosity, ranging from non-porous to highly porous structures (with BET up to 500\u00a0m2/g). The X-ray diffraction patterns and representative TEM/HRTEM micrographs of various carbon materials used as the catalyst for the present study are shown in Fig.\u00a02\n. All the materials are characterized by the well-defined crystal structures. The HRTEM image of a CNT shows the number of walls and large central hollow that provides an easy channel for the transport of hydrogen [2,3,60,61]. The CNTs present an inner diameter of about 5\u00a0nm, an external diameter of about 12\u00a0nm. Some of them were closed by a fullerene-shaped cap, while others were open at the ends. The removal of these caps provides an easy pathway for the diffusion of hydrogen through nanotubes\u2019 inner channels. The HRTEM images of some of the studied carbon materials show microporosity (pointed by arrows). It has been assumed in the past that the microporosity has an influence on the adsorption properties [1].\nFig.\u00a03\n shows the results of the detailed investigation of morphology and microstructure of Ti1\n.\n5V0.5 and studied composites. The SE SEM micrographs present particles characterized by irregular shapes and varying from a few to hundreds of micrometers in size. The micrograph obtained for Ti1\n.\n5V0.5_UFD_5_30 shows that the ultra-fine diamond particles form a tight layer on the alloy surface. Furthermore, the large carbon nanofibres are clearly visible on the alloy surface after milling. The BSE SEM observations were performed to evaluate the dispersion of carbon allotropes (black regions) on the Ti1\n.\n5V0.5 particles (bright regions). Ti1.5V0.5 alloy, which has been used as a reference, shows no presence of carbon structures. In contrast to the BCC alloy BSE SEM micrographs prove the dispersion of carbon structures on surfaces. However, the degree of dispersion varies with the type of carbon structure used. UFD and C60 are well distributed on the alloy particles. The other allotropes (G, CNT, CNF, CM) tend to concentrate in some material's areas, while the other areas remain completely devoid of them. The insets show BSE SEM micrographs of some of the materials obtained at higher magnification to show the undamaged carbon structures.TEM and HRTEM images of Ti1\n.\n5V0.5 show that the alloy consists of micrometer-sized particles (some nanoparticles were also observed) characterized by nanocrystalline and multicrystalline structure. TEM observation proved that carbon structures retained their structures after the synthesis of composites. However, it can be seen that the length of some CNTs is reduced from the submicrometer scale (Fig.\u00a02) to dozens or hundreds of nanometers (Fig.\u00a03). Some of the carbon nanofibres were also damaged or destroyed. For CNF-, G- and CNT-containing samples the connection of alloy nanoparticles with undamaged carbon structure was confirmed. Moreover, the TEM observation proved that UFDs are well attached to the alloy surface. It should also be mentioned that Lillo-Rodenas et\u00a0al. have stated that incorporation of carbon allotropes resulted in a reduction of metal-hydrides particles and stabilized the size of powder particles [4]. Notwithstanding, such a conclusion cannot be made based on the micrographs in Fig.\u00a03.The XRD patterns obtained from as-prepared alloy and composites are shown in Fig.\u00a04\nA. Ti1\n.\n5V0.5 alloy crystalized in BCC phase. A residual V-based minority BCC phase was also detected. According to the recent paper of Balcerzak [88], the fraction of the Ti\u2013V main BCC phase to the V-based BCC phase is 92.6%\u20137.4% (as obtained from Rietveld refinement). The lattice parameter of the main phase equals 3.239\u00a0\u00c5. The crystal structure of an alloy was not tacked after milling with carbon allotropes. This is contrary to the results obtained for ball-milled Mg2Ni-based composites for which a reduction of peaks intensity has been noticed after milling [62]. However, in these studies, balls were not used during milling and therefore much lower energies were generated during milling. It led to much lower deformations and strain formations in alloy material. Moreover, peaks related to specific carbon crystal structures are visible on some of the patterns. Ti1.5V0.5_G_5_30 pattern contains two peaks related to the graphite structure. The most intense one, at around 26.6 deg., corresponds to the interlayer distance between graphitic layers of 0.335\u00a0nm. The most intense peaks of fullerenes and UFD are also visible. The other carbon structures were not detected due to their low concentration or low crystallinity.Structural characteristics of carbon allotropes attached to the surface of particles can be inferred from Raman spectra, as presented in Fig.\u00a04B. For a better understanding of the milling effect on the carbon structures, the composites spectra are compared with spectra of original carbon allotropes structures. As the composites were prepared by milling without balls, no high impact mixing occurred. Therefore, the breakdown of carbon allotropes to form amorphous carbon was not allowed in such conditions and the characteristic Raman peaks were not altered. It confirms that the crystal structures of these carbon allotropes attached to the surface of alloy particles remained essentially unchanged. In carbon allotropes containing a mixture of sp2 and sp3 type carbon-carbon bonding, there are two important characteristic Raman bands at 1580\u00a0cm\u22121 and at 1350\u00a0cm\u22121, which were assigned to the in-plane vibrations of the C\u2013C bonds (G bands, stretching of all sp2 bonds, both in rings and chains) and vibration mode originated from the distorted hexagonal lattice of graphitic sp2 network near the crystal boundary (D bands), respectively. A closer look at the G peak shows in some cases (for composites with CNF and CNT) an asymmetry in the line shape that appeared at 1620\u00a0cm\u22121, which can be assigned to structural defects (distorted stacking order in c-axis direction) introduced to the structure with milling (D\u2019 bands). The analysis of Raman bands in the 1000-2000\u00a0cm\u22121 range is presented in detail in Fig.\u00a0S1. The insignificant peak at around 2460\u00a0cm\u22121 (D\u00a0+\u00a0D\u2033) is related to a combination of a D phonon and an acoustic longitudinal phonon D\u2033. At higher wavenumber for some of the carbon allotropes, additional G\u2032 (2650\u00a0cm\u22121) and D\u00a0+\u00a0G (2920\u00a0cm\u22121) peaks were observed. The structure of UFD and C60 differs from the above-described ones. Unmilled UFD exhibits one main band at 1332\u00a0cm\u22121\u00a0(T2g). However, for UFD-containing composite additional amorphous bands were observed (D and G), which stem from the milling of UFD with alloy material. Their presence in the spectrum indicates partial amorphisation of UFD. The Raman spectra of C60 and its composite show the sharp peak at 1460\u00a0cm\u22121 as indicative of C60 structure.It has been shown that the relative intensity ratio (ID/IG) of the D to the G band is an indicator of the perfection of the graphite layer surface [46]. The increase in the ID/IG ratio indicates an increase in the number of defects of carbon species and a simultaneous decrease in the degree of carbon graphitization in the composite. Table\u00a02\n shows the comparison of ID/IG ratio for CNT, CNF, G, and CM containing composites before and after milling (due to the different hybridization of carbon atoms a similar analysis is not possible for UFD and C60). It is clear that the level of graphitization is the lowest for Ti1\n.\n5V0.5_G_5_30, intermediate for Ti1\n.\n5V0.5_CNT_5_30, while the highest for Ti1\n.\n5V0.5_CNF_5_30 and Ti1\n.\n5V0.5_CM_5_30. Moreover, most of these carbon structures became more defected after milling (with an exception for mesoporous carbon). The ID/IG ratio can also be used to estimate a planar domain size La (also named as the size of graphene stacks) in carbon allotropes [92]. The calculated La values have been listed in Table\u00a02. Fig.\u00a0S2 shows that the dependence of the degree of graphitization (ID/IG) on the domain/crystallite size of tested carbon materials and its composites has a linear character. We can conclude here that the XRD, Raman spectroscopy, and microscopic observations prove that the carbon allotropes remained their structures after their synthesis process.The activated alloy and composites were hydrogenated at 303\u00a0K under initial H2 pressure of 3\u00a0MPa. The results of these studies were presented and summarized in Fig.\u00a04C and Table\u00a03\n. All of the studied materials absorb hydrogen without any incubation time. The Ti1\n.\n5V0.5 alloy absorbs nearly 95% of maximum hydrogen storage capacity within 48\u00a0min. In all the cases, the carbon-containing materials perform much better than the reference BCC alloy. The hydrogenation time was significantly reduced for carbon-containing composites. The fastest H2 uptake kinetic, with an uptake time of 12\u00a0min, was observed for Ti1\n.\n5V0.5_C60_5_30. Regarding this, it is interesting that studies on Mg2Ni-carbon structures composites have not shown any improvement of hydrogen absorption kinetics after milling with carbon structures (C60, graphite, Vulcan) [62]. One of the possible reasons for the different effects of carbon materials on the kinetics of hydrogen sorption is the use of ball milling [62], which led to the deformation of the crystalline structure of the carbon catalyst, but did not take place in this study. The hydrogen uptake of Ti1\n.\n5V0.5 alloy (3.68\u00a0wt %) was decreased to the level of 3.46\u20133.59\u00a0wt % for composite materials. The loss of the uptake is related to the addition of 5\u00a0wt % of carbon allotropes. Since carbon materials do not participate in hydrogen absorption dilution effect is observed, as the mass of active material is decreased and therefore the hydrogen uptake for composites decreases.The decomposition process of hydrogenated samples was measured by DSC. The most important outcomes from these studies can be seen in Fig.\u00a04D and Table\u00a03. It is clearly visible that the hydrogen desorption process of Ti1\n.\n5V0.5 alloy can be divided into three steps. The first one at Tp\u00a0=\u00a0750.3\u00a0K is related to the decomposition of dihydride to monohydride. The second one (814.1\u00a0K) corresponds to phase transformation of monohydride to hydrogenated BCC phase, while the last one (890.9\u00a0K) to desorption of hydrogen from hydrogenated BCC solid solution to the dehydrogenated BCC phase. The presence of carbon allotropes affects the temperature of the first decomposition process, while the position of the other two steps remains basically unchanged. The temperature of decomposition of dihydride phase is the most reduced for Ti1\n.\n5V0.5_C60_5_30, reaching 662.7\u00a0K. Comparing this temperature with the one observed for Ti1\n.\n5V0.5 alloy, the desorption temperature was decreased by 87.6\u00a0K. The reduction of hydride decomposition temperature was also observed for carbon-modified magnesium hydride [4]. DSC curves were measured for each studied sample with different heating rates (Fig.\u00a0S3-Fig.\u00a0S9). The obtained results were used to calculate the energy of activation (Ea) of the dehydrogenation process (utilizing Kissinger plots). Kissinger analysis was performed for the first endothermic effect, which is related to the decomposition of dihydride, as stated before. Nevertheless, one has to bear in mind that this is not a solitary process (overlapping peaks in heat flow curves) and can be affected to some degree by succeeding reactions. Table\u00a03 shows the Ea values calculated for all of the studied materials. In the best case, in terms of hydrogenation kinetics, the Ea is twice reduced compared to the Ti1\n.\n5V0.5 alloy. However, due to the measurement uncertainties, it is impossible to identify the composite with the lowest Ea value. Nevertheless, it is obvious that a fullerene-containing composite is characterized by a much lower activation energy of hydrogen desorption. The reduction of dehydrogenation Ea indicates that the energy barrier of hydrogen release from BCC alloy is reduced. As Ti1\n.\n5V0.5_C60_5_30 showed the best hydrogenation/dehydrogenation properties among all studied materials, further studies were focused on this fullerene-containing composite only. Surprisingly, in the previously published study on MgH2-type materials, the addition of fullerene negatively affected their absorption kinetics [49].As we did not observe any change in particle size after synthesis of composites (Fig.\u00a03) we exclude the influence of the carbon materials addition on the specific surface area of the BCC alloy and thus the improvement of hydrogen storage properties is with high probability connected to the catalytic function of each carbon allotrope. Moreover, we did not find any clear tendency of the BET area of carbon catalysts (provided by suppliers) on any specific hydrogenation/dehydrogenation property. Similarly, Lillo-Rodenas et\u00a0al. have shown that the addition of selected carbon materials considerably improves the hydride decomposition kinetics of MgH2 [4]. As in the previous case mentioned authors did not observe any relationship between the porosity and surface area of the carbon materials and hydride decomposition kinetics/temperature. Further detailed research is required to understand the mechanisms that led to improved properties in each studied composite.The hydrogen absorption and desorption measurements performed on the studied materials clearly showed that all carbon allotropes catalyse both hydrogenation and dehydrogenation reactions in BCC alloy. Since the best performance has been observed for composite with fullerene the further studies have been focused on this carbon-containing composite.To study the influence of milling time on hydrogenation/dehydrogenation properties of the fullerene-containing composite, 5\u00a0wt % of C60 was milled for 10, 30, and 60\u00a0min under the same milling conditions. Fig.\u00a05\n shows that the increase of the milling time did not affect the structure of alloy, but caused a gradual decrease of crystallinity of carbon structure (see the inset of Fig.\u00a05A). It has also been previously observed for MgNi2-based composite, that the diffraction peaks of C60 become broader and the intensity decreases with increased milling time, indicating grain refinement and introduction of strain into the structure [62]. However, for composites studied in this paper, the Raman spectra did not show any significant changes in the C60 structure (Fig.\u00a05B). We infer the presence of a small amount of C60 and alloy particles that get mechanically damaged in a contact with the jar wall during milling. The microstructure of the sample milled for 60\u00a0min (Ti1\n.\n5V0.5_C60_5_60) was studied by BSE SEM. A representative micrograph can be seen in Fig.\u00a05C. Even 60\u00a0min of milling was insufficient to disperse the carbon material over the entire surface area of the alloy and many uncovered Ti1\n.\n5V0.5 alloy areas can be easily found in Fig.\u00a05C (white regions). Hydrogen absorption measurements showed that extended milling time was detrimental to hydrogen storage properties. First of all, the hydrogen uptake is significantly reduced when the milling time increases: from around 3.5\u00a0wt % for samples milled for 10 and 30\u00a0min to only 2.8\u00a0wt % after 60\u00a0min. Secondly, the hydrogenation kinetics is lowered when the carbon allotrope is milled with an alloy material for a longer time (see Table\u00a03.). As shown by Alsabawi et\u00a0al. and Wu et\u00a0al., there is an optimal milling time that favours the hydrogen storage properties of MgH2 co-milled with C60 or SWCNTs [7,49]. As it has been shown in Ref.\u00a0[49], further milling leads to a serious degradation on these properties of the composite which are related to the destruction of carbon catalysts (proven by Raman spectra). However, in the present study Raman spectra did not prove any structural changes after milling. The only proof of carbon catalyst degradation has been observed by XRD.Most importantly, the Ti1\n.\n5V0.5_C60_5_10 composite can absorb nearly 3.3\u00a0wt % of hydrogen within a minute (at room temperature). This superfast absorption is almost fifty times faster than for Ti1\n.\n5V0.5 alloy. The DSC studies of hydrogenated fullerene-containing composites showed that dihydride decomposition temperature (648\u2013663\u00a0K) and Ea of dehydrogenation process (148\u2013157\u00a0kJ/mol) displayed comparable values and are independent of milling time (Figs.\u00a0S9\u201311). It is important to note, that even relatively low energy milling (without milling balls) can significantly affect the functionality of C60.Furthermore, the influence of various amounts of fullerene addition was also studied. For this reason 1, 5, and 10\u00a0wt % of C60 were milled with Ti1\n.\n5V0.5 for 30\u00a0min. The XRD patterns obtained for these composites exhibit no influence of the amount of carbon addition on the crystal structure of Ti1\n.\n5V0.5 alloy (Fig.\u00a06\nA). The inset of Fig.\u00a06A shows the angular region of the (311) C60 peak. The peak was not detected for the sample with only 1\u00a0wt % of fullerene, while for the other two composites it was clearly visible. The absence of the (311) peak in the Ti1\n.\n5V0.5_C60_1_30 pattern is probably related to the very low content of C60 in this particular sample, which is below the sensitivity threshold of XRD method. Especially, since Raman spectroscopy which is more sensitive to carbon materials showed no major changes in the structure after milling of samples with 1, 5, and 10\u00a0wt % of C60 (Fig.\u00a06B). The sample with the greatest content of fullerene was tested by BSE SEM (Fig.\u00a06C). Surprisingly, even a relatively large content of carbon allotrope does not guarantee the formation of a uniform C60 layer on the surface of alloy particles. As the size and morphology of particles remained the same changes in hydrogen storage properties should be connected with catalytic properties of carbon additive. Each of the samples was studied by Sieverts type apparatus to characterize its hydrogen storage properties. As can be seen in Table\u00a03 the decrease of H2 uptake is proportional to the amount of carbon allotrope in the composite structure. The more C60 that does not participate in hydrogenation reaction is used the less hydrogen storage capacity is. The same tendency has been observed by Milanese et\u00a0al. They have reported that the overall hydrogen uptake of Mg\u2013Ni elemental mixture decreased with increased graphite content in the composite (due to the inability of C to absorb H2) [89].We did not observe any clear relation between the amount of C60 and the kinetic properties of the alloy. It seems that 5\u00a0wt % of fullerene is an optimal concentration, as Ti1\n.\n5V0.5_C60_5_30 composite shows the fastest hydrogen absorption among materials with various amounts of C60. Furthermore, worse hydrogenation kinetic properties were observed for the Ti1\n.\n5V0.5_C60_10_30 composite. A very similar tendency has been observed in the study of Alsabawi et\u00a0al. They have found that for MgH2-based composite there exists an optimal amount of fullerene addition. When the C60 concentration exceeds 2\u00a0wt % in composite, the carbon addition harms the hydrogen storage properties [49], However, it should also be mentioned that in another work Dal Toe et\u00a0al. have not observed any significant improvement of hydrogenation kinetics when a larger amount of graphite was added to MgH2 [10].DSC studies of hydrogenated samples showed that the dihydride decomposition temperature is decreased, while the amount of C60 used in the composite increases (670.2\u00a0K for 1\u00a0wt %, 662.7\u00a0K for 5\u00a0wt %, and 651\u00a0K for 10\u00a0wt %). The same tendency was also observed for Ea (see also Figs.\u00a0S9, S12, S13). Ti1.5V0.5_C60_10_30 is the composite with the lowest dehydrogenation activation energy among all materials studied in this work (136\u00a0\u00b1\u00a05\u00a0kJ/mol).The presented results clearly show the beneficial effect of carbon allotropes on the hydrogen storage properties of BCC alloy. We believe that this complex study can help in the design of other BCC alloys and composites for hydrogen storage. They can be especially useful concerning the growing hydrogen community interest in the BCC HEA. The very recent studies on HEA have shown that these alloys can be considered as materials capable to reach the H/M ratio over 2, which is the upper limit for conventional BCC alloys [93]. Moreover, groups of Zepon and Zlotea considered HEAs as possible light-weight alloys for hydrogen storage [94,95]. The interest in HEAs for hydrogen storage has just begun and there are still many undeveloped areas in this topic. What is meaningful, there are no reports on the HEA-based composites synthesized with carbon catalysts. Therefore, we hope that this paper can serve as a guide for research on a new group of materials for hydrogen storage.Based on the presented and discussed studies, the following conclusions can be drawn:\n\n\u2022\nMilling of BCC alloy with carbon additives without milling balls is an effective way to disperse carbon catalysts on the hydrogen storage material surface without significant changes in both the alloy and the catalyst structures.\n\n\n\u2022\nAll of the proposed catalysts promote hydrogenation and dehydrogenation in studied BCC alloys.\n\n\n\u2022\nThe activation energy of the hydride decomposition process is at least twice reduced for composites with ultrafine diamonds, carbon mesoporous, or fullerenes in comparison to BCC alloy.\n\n\n\u2022\nThe temperature of the hydride decomposition process was decreased by nearly 100\u00a0K for fullerene-containing composites.\n\n\n\u2022\nThe extremely fast hydrogen absorption was observed for a composite containing 5\u00a0wt % of fullerene co-milled with BCC alloy for 10\u00a0min. The alloy absorbs 3.3\u00a0wt % of H2 within a minute at room temperature. The absorption is nearly fifty times faster compared to pure BCC alloy.\n\n\n\u2022\nProlonged milling of BCC alloy with C60 leads to serious degradation of hydrogen storage properties of a composite.\n\n\n\u2022\nIn terms of hydrogenation/dehydrogenation properties, the optimal concentration of fullerenes has been set at 5\u00a0wt %.\n\n\n\u2022\nSince milling with carbon allotropes did not cause any major microstructural and structural changes in the BCC alloy, the improvement of hydrogen storage properties is related to the catalytic function of added carbon materials.\n\n\nMilling of BCC alloy with carbon additives without milling balls is an effective way to disperse carbon catalysts on the hydrogen storage material surface without significant changes in both the alloy and the catalyst structures.All of the proposed catalysts promote hydrogenation and dehydrogenation in studied BCC alloys.The activation energy of the hydride decomposition process is at least twice reduced for composites with ultrafine diamonds, carbon mesoporous, or fullerenes in comparison to BCC alloy.The temperature of the hydride decomposition process was decreased by nearly 100\u00a0K for fullerene-containing composites.The extremely fast hydrogen absorption was observed for a composite containing 5\u00a0wt % of fullerene co-milled with BCC alloy for 10\u00a0min. The alloy absorbs 3.3\u00a0wt % of H2 within a minute at room temperature. The absorption is nearly fifty times faster compared to pure BCC alloy.Prolonged milling of BCC alloy with C60 leads to serious degradation of hydrogen storage properties of a composite.In terms of hydrogenation/dehydrogenation properties, the optimal concentration of fullerenes has been set at 5\u00a0wt %.Since milling with carbon allotropes did not cause any major microstructural and structural changes in the BCC alloy, the improvement of hydrogen storage properties is related to the catalytic function of added carbon materials.\nMateusz Balcerzak: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing \u2013 original draft, Visualization, Funding acquisition. Tomasz Runka: Formal analysis, Investigation, Writing \u2013 original draft. Zbigniew \u015aniadecki: Formal analysis, Investigation, 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.Financial assistance from National Science Centre, Poland (no. 2015/17/N/ST8/00271).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.2021.06.030.", "descript": "\n                  Body-centered cubic (BCC) alloys are considered as promising materials for hydrogen storage with high theoretical storage capacity (H/M ratio of 2). Nonetheless, they often suffer from sluggish kinetics of hydrogen absorption and high hydrogen desorption temperature. Carbon materials are efficient hydrogenation catalysts, however, their influence on the hydrogen storage properties of BCC alloy has not been comprehensively studied. Therefore, in this paper, composites obtained by milling of carbon catalysts (carbon nanotubes, mesoporous carbon, carbon nanofibers, diamond powder, graphite, fullerene) and BCC alloy (Ti1.5V0.5) were extensively studied in the non-hydrogenated and hydrogenated state. The structure and microstructure of the obtained materials were studied by scanning and transmission electron microscopes, X-ray diffraction (XRD), and Raman spectroscopy. XRD and Raman measurements showed that BCC alloy and carbon structures were in most cases intact after the composite synthesis. The hydrogenation/dehydrogenation studies showed that all of the used carbon catalysts significantly improve the hydrogenation kinetics, reduce the activation energy of the dehydrogenation process and decrease the dehydrogenation temperature (by nearly 100\u00a0K). The superior kinetic properties were measured for the composite with 5\u00a0wt % of fullerene that absorbs 3.3\u00a0wt % of hydrogen within 1\u00a0min at room temperature.\n               "}
{"full_text": "activated carboncarbon blackcarbon felt/graphite feltcarbon nanofibercarbon nanotubeelectrochemically exfoliated graphenehierarchical porous carbonsmesoporous carbonmultiwall carbon nanotubeordered mesoporous carbonreduced graphene oxidereticulated vitreous carbonreed straw activated carbonspectrographically pure graphitesingle-wall carbon nanotubebioelectrochemical systemdensity functional theoryelectrochemical advanced oxidation processhydrogen evolution reactionliquid crystal displaymagic angle spinningmicrobial electrolysis cellmicrobial fuel cellnuclear magnetic resonanceoxygen evolution reactionoxygen reduction reactionperoxi-coagulationpersistent organic pollutionrotating disk electroderotating ring-disk electrodestatistical Raman spectroscopyanthraquinonedisulphonateN-butyl-3-methylpyridinium dicyanamideCo-polypyrrolecobalt tetra-methoxy-phenyl porphyrindihydroxynaphthalene1-Ethyl-3-methylimidazolium dicyanamidehexaminepolyanilinepolyethyleniminePolypyrrolepolytetrafluoroethylenetert-butyl-anthraquinoneTriethylenetetramineHydrogen peroxide (H2O2) is a universal oxidizing agent which can be utilized either alone or in combination with other reagents for various purposes, such as pulp and textile bleaching [1], chemical synthesis [2], and wastewater treatment [3]. In the environmental field, H2O2 is used in many advanced oxidation processes (AOPs), such as H2O2/UV, H2O2/Fe2+, and H2O2/O3 [4]. In these H2O2-based AOPs, strong oxidant \u00b7OH radicals (E\u00b0(\u00b7OH/H2O)\u00a0=\u00a02.80\u00a0VSHE) are generated in situ. Afterwards, this free radical can non-selectively oxidize various contaminants at relatively high rate constants in the order of 106-1010\u00a0M\u22121\u00a0s\u22121 [5]. Moreover, the self-quenching of \u00b7OH radicals makes their lifetime in water as short as a few nanoseconds [5]. AOPs have been widely applied for the degradation of various pollutants such as antibiotics, herbicides, insecticides, endocrine-disrupting chemicals, pharmaceutical and personal care products (PPCPs), and effluent organic matter [6], showing their potential for water and wastewater treatment in the future [7,8].Currently, over 95% of commercially produced H2O2 worldwide is derived from the anthraquinone oxidation (AO) process (the alternate name is auto-oxidation process) [9]. Hydrogen, anthraquinone, and air are employed as raw materials in the AO process. The alkylanthraquinone precursor dissolved in an admixture of organic solvents is catalytically hydrogenated and then oxidized to obtain a diluted solution of H2O2 at 0.9\u20131.8% (wt). The following liquid-liquid extraction and distillation processes produce concentrated H2O2 solution at 35\u201350% (wt) (Fig.\u00a01\na). The major drawbacks of the AO process are (1) the use of large quantities of hazardous organic solvents, (2) highly concentrated H2O2 is explosive, which brings potential risks during transport and storage, (3) about 0.1% (wt) H2O2 at most is needed in the wastewater treatment process, which makes the concentration-dilution process a waste of cost and energy. These disadvantages and the decentralized requirements of users motivated the academic community and industry to develop other H2O2 synthesis methods and go beyond the AO process.An alternative method is the direct synthesis of H2O2 from H2 and O2. In this straightforward batch process [10], gaseous H2 and O2 are introduced into the liquid medium with catalysts. The proposed catalytic mechanism for direct synthesis is sequential hydrogenation of molecular oxygen. Firstly, the H2 molecule is dissociated into H atoms on the surface of the catalyst. Afterwards, an O2 molecule adsorbs onto the surface of the catalyst, followed by reacting with the H atom and thus forming the HOO\u2217 intermediate. H2O2 is finally obtained by hydrogenating the HOO\u2217 intermediate (Fig.\u00a01b) [11]. Although the first patent was granted in 1914, there is still no industrial process based on the direct synthesis for over 100 years because of the following three critical disadvantages [12,13]: (1) Safety - the direct synthesis avoids transportation of H2O2 to the site, but H2 is more explosive relative to H2O2. The H2 and O2 mixture gas has to be diluted by other \u201cinert\u201d gases (N2 or CO2) to operate below the lowest explosive limit, which also limits the productivity of H2O2; (2) Competing side reactions - the hydrogenation of O2 towards H2O2 (\u0394H\u00a0=\u00a0\u2212135.9\u00a0kJ\u00a0mol\u22121) is along with the direct formation of H2O (\u0394H\u00a0=\u00a0\u2212241.6\u00a0kJ\u00a0mol\u22121), the further reduction to H2O (\u0394H\u00a0=\u00a0\u2212105.8\u00a0kJ\u00a0mol\u22121) and H2O2 decomposition reaction (\u0394H\u00a0=\u00a0\u2212211.5\u00a0kJ\u00a0mol\u22121) (Fig.\u00a01b), which are all thermodynamically more favored than the desired main synthesis reaction; (3) Cost of catalyst - Although noble metal catalysts, such as Pd, Pd\u2013Au and Pd\u2013Sn, are proven to be effective, the rareness and high price of these materials make the direct synthesis hard for large-scale applications.Photo-catalysis through proton-coupled electron transfer is another alternative to generate H2O2. Briefly, in the heterogeneous photocatalytic process, an optical semiconductor is activated by irradiation of an appropriate light source to form photo-generated electron/hole (e\u2212/h+, Fig.\u00a01c) pairs, which under certain conditions induces the reduction of O2 to produce H2O2 [14]. Currently, it is accepted that H2O2 can be produced via either a one-step two-electron direct reduction or a two-step one-electron indirect reduction route [15]. Photo-catalysis has emerged as a promising alternative since it only requires an optical semiconductor, water, oxygen, as well as sufficient and renewable light as the driving force. As a hot topic in recent research, multiple photo-catalysts have been investigated and employed, including TiO2, graphite carbon nitride (g-C3N4), metal-organic compounds and their modification materials. However, photocatalytic H2O2 generation is still in its initial stage with several problems that need to be solved, such as poor selectivity toward 2 e\u2212 O2 reduction, relatively low response to sunlight, and high recombination rate of photo-generated species [15]. Moreover, the H2O2 can be further reduced with e\u2212 or decomposed by the irradiated UV light (\u03bb\u00a0<\u00a0400\u00a0nm), which causes the H2O2 production rate to be lower than the above two methods [14,16].The electrosynthesis of H2O2 via two-electron oxygen reduction reaction (ORR) is attracting growing attention. The electrosynthesis of H2O2 was first reported by Berl et\u00a0al., in 1939 by applying activated carbon as a cathode to achieve a 90% current efficiency [17]. Based on the two-electron ORR pathway, the Huron-Dow process was developed in the 1980s (by Dow and Huron Technologies, Inc.) to produce dilute alkaline H2O2 onsite (Eq. (1)) for pulp and paper bleaching (Fig.\u00a01d) [9]. Although it was successfully commercialized in 1991, the inherent disadvantages includes corrosion of the electrodes from the highly alkaline environment, carbonate formation from CO2 and the high Ohmic resistance in the system limiting its further development [12]. As a variant of the Huron-Dow process, the electro-Fenton (EF) process was first investigated and developed by Brilla's research group and Oturan's group in the 1990s [18,19]. The EF technology is based on the continuous H2O2 generation on a cathode (Eq. (2)) in an acidic electrolyte. \u00b7OH radicals are generated via Fenton's reaction (Eq. (3)) with the addition of a sufficient amount of Fe2+ ions. The homogeneous regeneration of Fe2+ on the cathode (Eq. (4)) makes the persistent organic pollution (POPs) continuously degraded in EF. Until now, EF has become the most popular electrochemical technology to degrade a variety of POPs, including pesticides [19], dyestuffs [20], PPCPs [21,22], and industrial pollutants [18]. Moreover, novel electrochemical advanced oxidation processes (EAOPs) based on the cathodic generation of H2O2 were developed for remediation of wastewater, such as photoelectro-Fenton [23], sono-electro-Fenton [24], peroxi-coagulation [25], and electro-peroxone processes [26].\n\n(1)\nH2O\u00a0+\u00a0O2\u00a0+ 2 e\u2212 \u2192 HO2\n\u2212\u00a0+\u00a0OH\u2212\n\n\n\n\n\n(2)\nO2\u00a0+\u00a02H+\u00a0+\u00a02 e\u2212 \u2192 H2O2 (E\u25e6\u00a0=\u00a00.68\u00a0VSHE)\n\n\n\n\n(3)\nFe2+\u00a0+\u00a0H2O2 \u2192 Fe3+\u00a0+\u00a0\u00b7OH\u00a0+\u00a0OH\u2212\n\n\n\n\n\n(4)\nFe3+\u00a0+\u00a0e\u2212 \u2192 Fe2+ (E\u25e6\u00a0=\u00a00.77\u00a0VSHE)\n\n\nHighly efficient cathode and the optimized system are two crucial prerequisites for the development of these EAOPs. Recently, substantial research has been devoted to the prediction and design of catalysts for electrochemical two-electron ORR and some of the progress has been previously presented in several reviews [12,27\u201335]. However, systematic and comprehensive reviews on carbonaceous two-electron ORR catalysts from the angle of mechanism and catalyst design to electrode fabrication have not been reported. As an emerging research field, it is necessary and indispensable to review and summarize the latest work on the development of these areas. Furthermore, from our point of view, the rapid development of electrode fabrication technology requires solid theoretical research in material science and catalytic science, as well as continuous optimization in engineering. In this review, we discuss the recently discovered mechanistic understanding of carbon materials catalysis and present important developments in carbon-based catalysts for two-electron ORR. Currently there are only a few comprehensive studies on two-electron ORR materials and the close correlation between two-electron ORR and four-electron ORR reactions. In the following sections we present some enlightening mechanisms and research results on four-electron ORR for the first time, including the predictive design by density functional theory (DFT) calculations and controllable doping/functionalization configurations as well as the construction of porous structures and defects to guide two-electron ORR design (Chapter 4). The remaining uncertainty on the real active sites for two-electron ORR are illustrated and discussed. Depending on the raw carbonaceous material and the operation mode, we also systematically summarize the preparation and modification methods for the formed carbon-based electrodes (Chapters 6, 7, and 8). Finally, we provide a detailed perspective on the challenges and opportunities in this rapidly developing field. We attempt to take full advantage of carbon-based materials in constructing highly efficient two-electron ORR catalysts and provide a thought for the amplification and application of electrocatalytic synthesis H2O2 with high efficiency and low cost. Additionally, based on the knowledge amassed from the references and our former work experience, we hope to provide guidance and suggestions for future research by summarizing the inconsistent or divergent experimental and computational methods. We encourage future studies to use more unified experimental methods and expressions, while avoiding the oversights from previous studies.The mechanism of electrochemical ORR is outlined in Fig.\u00a02\na. Generally, the ORR involves either a four-electron transfer pathway, which reduces O2 to H2O (Eq. (5)) and is attractive for fuel cells, or a two-electron pathway to produce H2O2 (Eq. (2)), which is desirable for environmental remediation [36]. Overall, the direct four-electron ORR involves multiple steps and intermediates (HOO\u2217, HO\u2217, O\u2217), which can be divided into the dissociative and associative way, depending on the oxygen dissociation barrier on the catalyst surface (Eqs. (6)\u2013(13)) [37,38].\n\n(5)\nO2\u00a0+\u00a04H+\u00a0+\u00a04 e\u2212 \u2192 2H2O\n\n\nDissociative pathway: the O\u2013O bond breaks into two O\u2217, which could be reduced to H2O as the final product.\n\n(6)\nO2\u00a0+ 2 \u2217 \u2192 2 O\u2217\n\n\n\n\n(7)\n2 O\u2217\u00a0+\u00a02H+\u00a0+\u00a02 e\u2212 \u2192 2 HO\u2217\n\n\n\n\n(8)\n2 HO\u2217\u00a0+\u00a02H+\u00a0+\u00a02 e\u2212 \u2192 2H2O\u00a0+\u00a02 \u2217\n\n\nAssociative pathway: the activated O2 molecule firstly couples the proton & electron to produce HOO\u2217, and then the O\u2013O bond of HOO\u2217 is cleaved and reduced to H2O.\n\n(9)\nO2\u00a0+\u00a0\u2217\u00a0+\u00a0H+\u00a0+\u00a0e\u2212 \u2192 HOO\u2217\n\n\n\n\n(10)\nHOO\u2217\u00a0+\u00a0H+\u00a0+\u00a0e\u2212 \u2192 O\u2217\u00a0+\u00a0H2O\n\n\n\n\n(11)\nO\u2217\u00a0+\u00a0H\u00a0+\u00a0e\u2212 \u2192 HO\u2217\n\n\n\n\n(12)\nHO\u2217\u00a0+\u00a0H+\u00a0+\u00a0e\u2212 \u2192 H2O\u00a0+\u00a0\u2217\n\n\n\n\n(13)\nHOO\u2217\u00a0+\u00a0\u2217 \u2192 O\u2217\u00a0+\u00a0HO\u2217\n\nWhere \u2217 denotes an unoccupied active site, and HOO\u2217, HO\u2217, O\u2217 represent the single adsorbed intermediates on the catalyst surface [36].Conversely, the two-electron pathway is comprised of two coupled electron & proton transfers together with one intermediate (HOO\u2217) (Eqs. (14) and (15)) [39].\n\n(14)\nO2\u00a0+\u00a0\u2217\u00a0+\u00a0H+\u00a0+\u00a0e\u2212 \u2192 HOO\u2217\n\n\n\n\n(15)\nHOO\u2217\u00a0+\u00a0H+\u00a0+\u00a0e\u2212 \u2192 H2O2\u00a0+\u00a0\u2217\n\n\nFrom the above reactions and the schematic diagram shown in Fig.\u00a02a, it is observed that breaking the O\u2013O bond (Eqs. (6) and (10)) is a necessary step in both dissociative and associative four-electron pathways. Therefore, preventing the O\u2013O bond from dissociation is critical in the selective catalysis for H2O2 synthesis [30]. Moreover, the obtained H2O2 could be further reduced to H2O, making the process an indirect four-electron ORR to reduce the H2O2 production (Eqs. (16)\u2013(19)). Consequently, shortening the H2O2 residence time on the catalyst surface is also critical to maintaining the stability of H2O2 during electro-synthesis. In summary, the ideal electrocatalyst with high activity and high selectivity toward two-electron ORR should have the property of minimizing the kinetic barriers for Eqs. 14 and 15 and maximizing the barrier for H2O2 reduction and HOO\u2217 dissociation to HO\u2217 and O\u2217 [36].\n\n(16)\nH2O2\u00a0+\u00a0\u2217\u00a0+\u00a0H\u00a0+\u00a0e\u2212 \u2192 HO\u2217\u00a0+\u00a0H2O\n\n\n\n\n(17)\nHO\u2217\u00a0+\u00a0H\u00a0+\u00a0e\u2212 \u2192 H2O\u00a0+\u00a0\u2217\n\n\n\n\n(18)\nH2O2\u00a0+\u00a02 \u2217 \u2192 2 HO\u2217\n\n\n\n\n(19)\n2 HO\u2217\u00a0+\u00a02H+\u00a0+\u00a02 e\u2212 \u2192 2H2O\u00a0+\u00a02 \u2217\n\n\nThe performance of the electrocatalyst depends on the binding energy of the reaction intermediates to the catalyst surface [36]. However, because of the existence of scaling relationships Eq. (20) between these intermediates, the activity is governed by a single parameter \u0394G\nOH\u2217 [40,41]. Benefiting from the development of DFT calculations on numerous close-packed metal surfaces, a volcano framework has established that the theoretical overpotential relates to the free energy of HO\u2217 for the ORR activity (Fig.\u00a02b) [42,43]. In brief, for the materials that bind HO\u2217 too weakly (i.e., to the right side of the peak), the ORR is limited by the activation of O2. The weak interaction with O\u2217 and HO\u2217 increases the selectivity toward two-electron ORR, but simultaneously lowers the ORR activity. For the materials that bind HO\u2217 too strongly (i.e., to the left side of the peak), the limiting step for the H2O2 and H2O production is associated with removing HOO\u2217 and HO\u2217, respectively. Considering the two-electron pathway is determined by only one intermediate, it is feasible to find an electrocatalyst with zero theoretical overpotential that has an optimal \u0394G\nHOO\u2217, binding HOO\u2217 neither too strongly nor weakly [43].\n\n(20)\n\u0394G\nHOO\u2217\u00a0=\u00a0\u0394G\nHO\u2217\u00a0+ 3.2\u00a0\u00b1\u00a00.2\u00a0eV\n\n\nThe electrocatalysts for two-electron ORR covers a wide range from noble metals (Pb, Au, etc.) and metal alloys to carbonaceous materials. The metal alloys, such as Pd-Au [44,45] and Pt\u2013Hg [36], were verified to have high selectivity toward H2O2 (70.8\u201392.5% under 0.1\u20130.3\u00a0V vs. RHE at pH 1). However, the wide application of these noble-metal catalysts is impacted by their scarcity and high cost [46]. Accordingly, metal-free and non-noble-metal catalysts are developed as sustainable alternatives. Carbon materials are a promising alternative for H2O2 electrosynthesis because of their high abundance, conductivity, activity, and lower price. Most importantly, with a variety of allotropes, the carbon materials have multiple morphologies and highly tunable electronic structures. This unique feature makes them an ideal platform for designing electrocatalysts at the atomic level [47].The characterization of electrocatalyst performance via the rotating ring disk electrode (RRDE) technique in a three-electrode system is necessary for evaluating ORR activity and H2O2 selectivity in the material design. RRDE is a convective electrode system containing a disk electrode, a coaxial Pt ring electrode together with a rotating shaft (Fig.\u00a03\n) [48]. In the RRDE, the ORR takes place on the disk electrode to generate both H2O2 and H2O. Thereafter, H2O2 is radially transferred to the coaxial Pt ring electrode by the forced convection resulting from the rotation of the electrode. Subsequently, H2O2 is oxidized back to O2 (Eq. (21)) at the ring electrode. The overall ORR activity and H2O2 selectivity could be quantified by analyzing the corresponding reduction disk current (i\n\nD\n) and oxidation ring currents (i\n\nR\n), respectively [49]. The selectivity of H2O2 is quantified by the Faradaic efficiency (\u03bb\n\nFaradaic\n) and the average transferred electrons number (n).\n\n(21)\nH2O2 \u2192 O2\u00a0+ 2H+\u00a0+ 2e\u2212\n\n\n\n\n\n(22)\n\n\n\n\u03bb\n\nF\na\nr\na\nd\na\ni\nc\n\n\n=\n\n\n\ni\nR\n\nN\n\n\ni\nD\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(23)\n\n\nn\n=\n\n\n4\n\n|\n\ni\nD\n\n|\n\n\n\n\n|\n\ni\nD\n\n|\n\n+\n\n\ni\nR\n\nN\n\n\n\n\n\n\nwhere N represents the collection efficiency of the RRDE, which means the fraction of product from the disk to the ring, based on the geometries of the ring and disk electrodes [30].Rotating disk electrode (RDE) is another tool for assessing the catalyst ORR activity. Compared with RRDE, the coaxial ring electrode is removed in the RDE system (Fig.\u00a03). The hydrodynamic and electrochemical properties of RDE are related to the Koutecky-Levich (K-L) equation [50].\n\n(24)\n\n\n\n1\nJ\n\n=\n\n1\n\nJ\nK\n\n\n+\n\n1\n\nJ\nL\n\n\n=\n\n1\n\nn\nF\nk\n\nC\n0\n\n\n\n+\n\n1\n\n0.62\nn\nF\n\nC\n0\n\n\nD\n0\n\n3\n2\n\n\n\nv\n\n\u2212\n\n1\n6\n\n\n\n\nw\n\n1\n2\n\n\n\n\n\n\n\nwhere j\n\nK\n and j\n\nL\n are the kinetic-limiting current density and diffusion-limiting current density (mA cm\u22122), n stands for the electron-transfer number, F is the Faraday constant (C mol\u22121), C\n\n0\n and D\n\n0\n are the bulk concentration (mol cm\u22123) and O2 diffusion coefficient (cm2 s\u22121) in the electrolyte, v is the kinematic viscosity of the electrolyte (cm2 s\u22121), \u03c9 represents the angular velocity (rad s\u22121), and k is the electron-transfer rate constant. n and k can be obtained from the slope and intercept of K-L equation, respectively.Typically the onset potential or overpotential (\u03b7) in (R)RDE tests are used to compare the activities among different electrocatalysts. And the selectivity of newly designed electrocatalysts toward H2O2 is mainly quantified in terms of n and \u03bb. In Tables\u00a01 and 2\n\n, the performance of newly developed electrocatalysts on (R)RDE are systematically summarized.Before discussing the carbon catalysts, it is vital to discuss the different types of obtained carbon materials. There are three allotropes as dictated by the carbon precursor [51]: graphite (sp\n\n2\n bonding), diamond (sp\n\n3\n), and amorphous carbon (disordered mixture of sp\n\n2\n and sp\n\n3\n). Due to the different combinations of carbon atom hybridization, carbon allotropes with different structures and properties are obtained. Until now, most of the two-electron ORR catalysts are graphite carbon and amorphous carbon.Graphite is formed by multilayered two-dimensional (2D) sheets of sp\n\n2\n hybridized carbon atoms with hexagonal lattice in the basal plane. The edges of its planes have terminations with carbon atoms arranged with zigzag or armchair configurations, and a van der Waals interaction along the transverse direction between the layers that is relatively weak. Apart from naturally existing graphite, the discovery of graphene and carbon nanotubes (CNTs) expanded the categories of sp\n\n2\n carbon materials. Graphene (a single layer form of graphite) is a 2D sheet honeycomb structure composed of sp\n\n2\n carbon atoms. Due to the remarkable electrical, thermal, physical, optical, and mechanical properties together with high specific surface area, it has received increasing attention [28,52]. Graphene can be prepared by a variety of processes, including mechanical cleavage [53], chemical vapor deposition growth [54], epitaxial growth [55], electrochemical exfoliation of graphite [56], and thermal/chemical/electrochemical reduction of graphene oxide (GO) [57]. Among those, the mechanical cleavage method results in high-quality graphene sheets but low yield, which cannot meet the large demand for graphene. The GO reduction method is considered to be one of the most promising routes for large-scale graphene production, which restores the essential properties. However, the production of graphene with minimum defects remains challenging. CNTs are tubular cylinders of carbon atoms that can be conceptually viewed as one or up to dozen graphene sheet(s) that are rolled up into a single-wall carbon nanotube (SWCNT) or a multiwall carbon nanotube (MWCNT) [51]. CNTs have been at the forefront of materials research in the last decade due to their high electrical conductivity (\u223c5000\u00a0S\u00a0cm\u22121) [58], high surface area (\u223c2630\u00a0m2\u00a0g\u22121) [59], high charge mobility (\u223c100,000\u00a0cm2\u00a0V\u22121\u00a0s\u22121) [60], as well as chemical stability, and significant mechanical strength. Currently, SWCNTs and MWCNTs are mainly produced by three techniques: arc-discharge [61], laser-ablation [62], and catalytic growth [63]. Among these methods, catalytic growth of nanotubes by the chemical vapor deposition (CVD) is the dominant mode of high-volume CNT production. In 2013, bulk purified MWCNTs were sold for less than 100 $ kg\u22121 [64]. The decrease in CNTs price increases their potential for application in various technological areas such as the chemical, medical, aerospace, energy, and automotive industries.Carbon black (CB) is an amorphous particle of nearly pure elemental carbon, consisting of grape-like aggregates of spherical primary particles, with the aggregates clustered into larger-sized agglomerates [65]. CB has relatively low quantities of extractable organic compounds and total inorganics (usually <1%) [66]. As a manufactured commercial product for over a century, it has plentiful applications as well as a variety of different trade names and physicochemical properties. A variety of CB grades with different properties (surface area, structure, aggregate size, abrasion resistance, etc.) are manufactured by controlling the conditions of the oil furnace production process. The most widely used CB material is Vulcan XC-72(R) (produced by Cabot Corporation, US) which is used in 80% of electrocatalysts [67], due to the large surface area (\u223c250\u00a0m2\u00a0g\u22121), high mesopore and macropore percentage (54%), and good electric conductivity (4.0\u00a0S\u00a0cm\u22121 at the packing fraction of 0.3 and 7.4\u00a0S\u00a0cm\u22121 at 0.4) [68,69]. In addition to Vulcan XC-72(R), there are some other commercial CB materials such as Printex L6, Black Pearls 2000, Acetylene Black, and Macsorb [70].Ordered mesoporous carbon (OMC) is a type of carbon material with regular arrays of uniform mesopores. The OMCs with different compositions vary from pure organic/inorganic frameworks to organic-inorganic hybrid frameworks have been widely investigated and reported in the past two decades. Normally, OMCs can be prepared by two different methods, the hard templating process (nano-casting strategy) by using mesoporous silicas and a soft templating process (direct synthesis) via self-assembly of block copolymers/surfactants and carbon precursors [71]. Except for high surface areas, OMCs also have outstanding special properties, including tunable pore sizes, alternative pore shapes, periodically arranged monodispersed mesopore space, and uniform nano-sized frameworks [72].Because of the above outstanding physical and chemical properties, tremendous investigations have been conducted to reveal the ORR catalytic characteristics of graphene, CNTs, CB, and OMCs. However, the electroactivity of pristine carbon catalysts still lags behind that of their metal counterparts because pristine carbon materials are inert to the adsorption and activation of O2 and ORR intermediates (Fig.\u00a02c) [36,44,73]. They always show high overpotentials and thus unpromising catalytic property toward ORR [74]. Therefore, the catalytic properties of carbon materials need to be improved by doping, functionalization, and structural regulation.Heteroatom doping refers to replacing part of carbon atoms in the carbon skeleton by other heteroatoms, including N, O, B, P, S, halogens, etc. (Fig.\u00a04\na) Because of the differences in atomic size, electronegativity, and binding states, heteroatom doping can regulate the spin and charge distribution, tune the absorption and activation of the ORR intermediates, and further change the catalytic performance of carbon [74,75]. For instance, B and P dopants tend to give electrons to carbon due to their lower electronegativity (B: 2.04; P: 2.19) compared with carbon (2.55), which creates a partial positive charge on the dopant atoms [76]. In contrast, nitrogen with higher electronegativity (N: 3.04) tends to rob electrons from carbon to generate a partial positive charge on the carbon atoms. Typically, the formation of partial positive/negative charges can both promote the interaction between the catalyst and O2, and the adsorption of O2 on the carbon materials. Though S doping does not disrupt the charge uniformity of carbon materials because of its similar electronegativity (2.58), larger size, and greater polarizability of the S atom enhance the spin density and charge delocalization on the neighboring carbon atoms to promote the ORR [77,78].As a convenient method to tune the electrochemical catalytic properties of carbon materials, doping has been utilized widely in developing ORR catalyst materials. Here, carbon materials with multiple heteroatoms doping for H2O2 production are reviewed.N-doping is the most extensively and promising form to modify sp\n\n2\n carbon for ORR for two reasons: one, N has a similar atomic radius as C, allowing it to easily replace C atoms without lattice mismatch; and two, the higher electron affinity of N makes the N dopant easy to change the atomic structures and electron arrangements of the carbon skeleton [46,74,78]. First synthesized by Gong et\u00a0al. [75], N-doped carbon materials for four-electron ORR have attracted much attention. Typically, there are two strategies for fabricating N-doped carbon catalysts. The first one is post-doping of porous carbons in the presence of nitrogen-containing precursors, which can effectually control the structures of the catalyst. However, the N-doping efficiency of post-doping is low and diverse. The other method is direct (in-situ) doping during the synthesis of catalysts to enhance the N-doping content and vary the N-doping structure. Nevertheless, the pore size and porosity of the catalyst are difficult to be precisely controlled in direct doping.Resulting from different synthetic precursors, catalysts, conditions, and procedures, there are various forms of N dopant that exist in the carbon skeleton, such as pyridinic N, pyrrolic N, graphitic N (or quaternary N), and pyridine-N-oxide (Fig.\u00a04b). However, not all N dopant-related dopants on the carbon materials constitute highly catalytic active species. There is debate on the real active sites in N-doped carbon for ORR. In general, planar pyridinic-N can enhance the electron-donating capability and weaken the O\u2013O bond due to its lone electron pair. As a result, pyridinic-N is thought to be the active site for four-electron ORR [79]. Guo et\u00a0al. [80]. Reported that carbon atoms adjacent to pyridinic-N are the real active sites for ORR in acidic media. Others have suggested that pyrrolic-N plays a key role in reducing O2 to H2O2, and pyridinic-N is the site for reducing H2O2 to H2O [81,82]. Conflictingly, Geng, et\u00a0al. [83] believed that graphitic-N instead of pyridinic-N might be responsible for the two-electron ORR. Kabir et\u00a0al. [84] claimed graphitic-N contributes significantly to peroxide generation in 0.5\u00a0M\u00a0H2SO4. The co-generation and co-existence of different types of N-doping species in the carbon materials are inevitable, making it difficult to distinguish their contributions. Moreover, the inter-transition of the different doping types induced by temperature makes the situation even more complex.In the recent 7\u20138 years, the N-doped carbon two-electron ORR catalysts have been systematically investigated to obtain different results on two-electron ORR active pores. N-doped OMC with a mean pore diameter of 13.2\u00a0nm was obtained from the ionic liquid N-butyl-3-methylpyridinium dicyanamide (BMP-dca) at 800\u00a0\u00b0C by hard-templating strategy using silica nanoparticles [85]. H2O2 production rate reached 0.17\u00a0g gcat\n\u22121 h\u22121 with a current efficiency of 65.2% at 0.1\u00a0V (vs. RHE). The RDE verified that the resultant material was highly active for the selective H2O2 generation. However, a comparable material synthesized at 1000\u00a0\u00b0C was favorable to the four-electron ORR [86]. The authors assumed the lower degree of delocalization, higher N content, and exposure of pyrrolic-N sites may favor the two-electron process.Sun et\u00a0al. [87] fabricated a series of N-doped OMC materials by pyrolysing the mixture of 1-Ethyl-3-methylimidazolium dicyanamide (EMIM-dca) and CMK-3 at different temperatures. Compared with six potentially suitable two-electron ORR carbon materials (Ketjen black EC 300J, Ketjen black EC 600JD, Black pearls 2000, Vulcan XC 72R, Graphene nano-plates, and CMK-3), the structural, compositional, and other physical properties were correlated with their catalytic performance. For six pristine carbon materials, large BET surface areas, positive zeta potentials, and high defect sites were all beneficial for H2O2 generation. Dissimilarly, the H2O2 selectivity of N-doped carbon catalyst is governed more by the N doping effect. The selectivity of the optimal N-doped material reached 95\u201398% in the potential range of 0.1\u20130.3\u00a0V (vs. RHE). In order to get a better understanding of the potentially related mechanistic roles of the different N species during the two-electron ORR, a novel N-doped OMC was prepared by annealing the mixture of CMK-3 and polyethylenimine (PEI) in the N2 atmosphere; achieving the highest H2O2 selectivity of 95\u201398.5% with the potential range of 0.1\u20130.4\u00a0V [88]. Analyzing chemical state trajectories of N species in the catalysts suggested pyridinic-N played a key role as an active site in acidic solution, while graphitic-N groups seemed to be active catalytic moieties in neutral and alkaline conditions.Other N-doped porous carbons were prepared from paraformaldehyde cross-linked collagen by sintering at various temperatures (400\u2013800\u00a0\u00b0C) [89]. Higher carbonization temperature brought more porous and sheet-like structures into the materials, and led to the formation of graphitic-N structure, the removal of oxygen-containing functional groups, and the decrease of N content, thus enhancing graphitic crystallinity. According to the electrochemical tests, N-doped carbon prepared at 400\u00a0\u00b0C showed excellent two-electron ORR with a selectivity of 93% over a wide potential range from 0.17 to 0.6\u00a0V (vs. RHE) due to the combination of pyridinic-N, pyrrolic-N, and the surface oxygen-containing functional groups.Since ORR generally proceeds on the surface of the catalysts, the unexposed active sites hidden in the catalyst body contribute little to the catalytic activity [74]. Therefore, the appropriate doping location and catalyst micro-configuration are more crucial than the gross doping content.To obtain a special carbon material with N-doping mainly at the surface, to provide active sites and high graphitic carbonaceous core to provide high electrical conductivity, N-doped graphitic carbon materials were fabricated by sequential pyrolysis of aniline and dihydroxynaphthalene (DHN) inside the SBA-15 hard silica template (Fig.\u00a05\na) [90]. To cover the template surface with monolayered aniline, the amount of aniline employed was determined based on the precise calculation of the molecular cross-sectional area of precursor and total pore volume of the template. The resultant materials displayed an ordered, hexagonal array of carbon rods, with a narrow pore size distribution centred at 4.3\u00a0nm, and specific surface area of 877\u00a0m2\u00a0g\u22121 (Fig.\u00a05b). The novel N-doped OMC exhibited outstanding performance with a transfer number of 2.1, and a H2O2 selectivity of 95% (Fig.\u00a05c). Which attributed to the high surface area, regular mesopores structure, a graphitic character, high content graphitic-N andpyridinic-N configurations.Except for the N species, other influence factors were also investigated. Park et\u00a0al. [91] studied the effect of the mesopores to discover that N-doped carbon materials with 3.4\u20134\u00a0nm well-ordered mesopores had high activity and selectivity (>90%) for H2O2 synthesis. In comparison, micropore-dominant N-doped activated carbon showed a higher onset potential than N-doped OMC, but a lower selectivity (56\u201360%). The excellent mass transfer of mesoporous structure enhanced the release of H2O2 within a relatively short contact time, which resulted in high selectivity toward H2O2 synthesis. Hasch\u00e9 et\u00a0al. [92] proposed that the electrochemical formation mechanisms for peroxide are dependent on the pH and the species of electrolyte, as well as the respective change of the peroxide species from H2O2 to hydroperoxide. In order to obtain a kinetic understanding of N-doped carbon catalysts in acidic media, a porous N-doped carbon with a surface area of 992\u00a0m2\u00a0g\u22121 was obtained by carbonization of polyimide nanoparticles through a two-step pyrolysis [93]. RRDE revealed that lower catalyst loading on the disk suppresses the further reduction of H2O2 in the catalyst matrix layer. When the catalyst loading density decreased to 30\u00a0\u03bcg\u00a0cm\u22122, the H2O2 selectivity was much higher than 80%. This study provided quantitative insight into the ORR mechanism over an N-doped carbon catalyst.Although N-doped carbon materials have been investigated extensively, carbon materials doped with B, P, S, and halogens have also been explored recently for their potential applications for electrocatalysis of ORR. However, almost all of the B/P/S doped carbon materials demonstrated an affinity for four-electron ORR instead of two-electron ORR for H2O2 generation [94\u201397].Zhang et\u00a0al. [98] studied the formation energy, electronic structures, transition states, and energy barriers of S-doped graphene clusters by DFT calculation to predict ORR activity of four types of S-doped graphene clusters (Fig.\u00a04c); including S atoms adsorbed on the graphene cluster surface (Type 1), S atom replacement at the graphene cluster armchair edge or zigzag (Type 2), SO2 substitution at the graphene edge (zigzag and armchair. Type 3), and two graphene clusters connected by an S ring (Type 4). Carbon atoms with high spin density or positive charge density are the active catalytic sites, which are often located at the zigzag edges or close to the SO2 doping structure. Two-electron ORR proceeds on the substitutional S atom with a high charge density, while four-electron ORR occurs simultaneously on the carbon atoms with a high positive spin or charge density.Considering the high electronegativity of Fluorine (3.98), the carbon electronic structure can be adjusted significantly by F atom doping. In addition, F-doping regulated the electron transfer properties by inducing polarization and changing the Fermi level [99,100]. Recently, a F-doped hierarchically porous carbon catalyst was developed from an aluminum-based metal-organic framework (MOF, MIL-53) precursor [101,102]. The selectivity of the ORR pathway strongly depended on the F doping species configurations. The covalent CF2 and CF3 facilitate the two-electron pathway because of the strong adsorption of O2 and the weak binding energy of the HOO\u2217 intermediate. Hence, the fabricated F-doped catalysts exhibited a high H2O2 yield of 113\u2013793\u00a0mmol\u00a0h\u22121\u00a0gcat\n\u22121, and selectivity reached 97.5\u201383% in the potential range of\u00a0\u22120.1\u00a0V to\u00a0\u22120.6 vs. RHE (pH 1).In 2021, Xia et\u00a0al. [103] systematically studied the effects of different dopants (B, N, P, S) in carbon material on its performance in 2e\u2212 ORR performance. Among all these dopants, B-doped carbon shows the highest activity and selectivity, with an onset potential of 0.773\u00a0V (vs. RHE) while maintaining over 85% selectivity across a broad potential window in 0.1\u00a0M KOH. BET, XPS, XAS, and Raman results excluded the possible morphological, structural, and electronic side effects on 2e\u2212 ORR. DFT calculations revealed the B-doped at single vacancy has nearly-zero overpotential, while molecule dynamics at constant potential indicates that the energy barrier for the 2e\u2212 pathway is lower than its 4e\u2212 counterpart.It has been recently proven that the co-doping of multiple types of heteroatom into carbon materials would increase the density of electrocatalytic active sites for two-electron or four-electron ORR processes [78]. N & S, N & B, and N & P co-doped carbon materials have been investigated for catalyzing two-electron ORR. The comparison of N-doped, S-doped, and N & S co-doped mesoporous carbons showed that a higher N content enhanced the catalytic activity while the effect of sulfur was opposite [104]. Though the N & S co-doped carbon showed a lower activity, the selectivity toward H2O2 (75%) was higher than N-doped samples (67\u201369%). In similar work, mesoporous carbons doped with either N, S, or both, were obtained by a one-pot molecular precursor auto-assembly followed by hydrothermal carbonization [105]. The dopant molecule was found to govern the ensuing structure and resulted in different average mesopore sizes (3.5\u00a0nm, 8.2\u00a0nm, 32\u00a0nm, and 34\u00a0nm corresponding to un-doped, N-doped, S-doped, and N & S co-doped carbons). The RDE test demonstrated that no beneficial effect was achieved by the co-doping of S & N. The best performance for two-electron ORR was achieved by N-doped catalyst with 4% (wt) N content and about 80% pore volume in the mesopore range. Dissimilarly, Zhu et\u00a0al. introduced N, S atoms into a carbon-based cathode [106]. Results showed the optimized N & S co-doped cathode presented over 42% improvement of H2O2 yield, which was higher than single N/S doping. Mechanism studies show that \u201cEnd-on\u201d O2 adsorption was achieved by adjusting electronic nature via N doping, while HOO\u2217 binding capability was tuned by spin density variation via S doping.Hybrid boron-carbon-nitrogen (BCN) materials have been tested for several catalytic applications [107,108]. To increase the selectivity toward H2O2 production, B & N co-doped carbons were prepared. BET surface area, together with the total content of B and N dopants were modulated by controlling the initial co-monomer precursor ratios [109]. Compared to solely N-doped carbon, the final loading of N by co-incorporating B with N increased significantly due to the formed isolated patches of h-BN, which provided higher activity and selectivity for the two-electron ORR. Moreover, systematic DFT calculations were performed to study the structures of different size h-BN domains doped into graphene, and different size C domains doped into an h-BN lattice [110,111]. The relationship between stimulated limiting potential and HO\u2217 adsorption energy is shown in Fig.\u00a05d. The results predicted 13% h-BN to have the best two-electron ORR performance.Li et\u00a0al. confirmed that N & P co-doping increases the two-electron ORR activity of cotton-stalk-derived activated carbon fibers significantly [112]. Co-doping N & P in the carbon lattice slightly changed the pore structures. Remarkably, (NH4)3PO4 treatment could not only embed N and P into the carbon skeleton but also introduced additional mesopores on the catalyst.Chemical functionalization is another powerful \u201cregulation screw\u201d to tailor the electron density and/or electron density distribution in the materials by introducing specific electrophilic/nucleophilic, ionic, or chiral sites. Oxygen functional groups (OFGs) are the most popular species modified onto carbon-based materials.Surface OFGs are often introduced into carbon materials by oxidation treatment. OFGs break the electrical neutrality of sp\n\n2\n carbon lattice to enhance the ORR activity. Zhong et\u00a0al. [113] discovered that the carboxyl group (OC\u2013OH) could weaken the CNF\u2013O bond more easily and exhibit the highest four-electron ORR activity. Moreover, all the OFGs on CNFs were found to be easily bonded with H2O2 to furtherly reduce H2O2 to H2O, thus making n of the resultant materials close to 4. Until now, this was the challenge to further pinpoint the active site to a specific group still remains.Recently, it was found reactivity of OFGs will change in different environments [114]. Kim et\u00a0al. [115] prepared a mildly reduced graphene oxide (mrGO) by heating purified GO at 100\u00a0\u00b0C flowing N2 overnight. The mrGO, which kept parts of the OFGs, showed stable peroxide formation activity together with highly selective at low overpotentials (about 0.01\u00a0V) in 0.1\u00a0M KOH solution. The experiments proved that carbonaceous catalysts with epoxy or ring ether groups situated either at plane edges or on their basal planes, exhibited remarkable two-electron reactivity, which was able to produce HO2\n\u2212 with nearly 100% selectivity and high stability (15\u00a0h at 0.45\u00a0V vs. RHE) in alkaline conditions. In other research with N-doped rGO [116], sp\n\n2\n carbon sites located next to oxide regions were identified as dominating the ORR activity by experimental and DFT calculation, which underlined the importance of OFGs rather than nitrogen functional groups (NFGs). These references suggested that the enhancement effect of OFGs on H2O2 production activity requires a synergistic contribution of the carbon lattice environments. Based on this assumption, Sun, et\u00a0al. [117] provided a novel idea into the coupling role of carbon cluster size and OFGs in H2O2 production. An activated coke electrocatalyst with size-tailored amorphous carbon clusters modified by OFGs yielded high activity (onset potential 0.83\u00a0V vs. RHE), high H2O2 selectivity (\u223c90%), and long-term stability. Based on this result and a series of control experiments, it was concluded that the size-reduced amorphous carbon lattices with abundant edges contributed to the high activity, while the basal carbon atoms in ether-modified small-size carbon planes are the most active sites towards H2O2 selectivity.Lu et\u00a0al. [118] demonstrated a facile approach to oxidize the raw CNTs by HNO3 to obtain O-CNTs. The O-CNTs drastically lowered the needed overpotential by \u223c130\u00a0mV at 0.2\u00a0mA compared with raw CNTs and increased the selectivity from \u223c60 to \u223c90%. Based on DFT calculations, ester groups (C\u2013O\u2013C) in the basal plane of the graphene and OC\u2013OH in the armchair edge were proved active and selective for H2O2 production.Zhang et\u00a0al. [119] introduced OFGs onto the Vulcan XC-72 CB by a simple calcination method at 200\u2013600\u00a0\u00b0C exposed to air. Characterization results showed both structural defects and OFGs content increased with the calcination temperature. Furthermore, many types of OFGs, such as C\u2013O\u2013C, C\u2013OH, CO, and OC\u2013OH, were successfully introduced onto the CB surface. With calcination at 600\u00a0\u00b0C, the RRDE onset potential increased from\u00a0\u22120.27\u00a0V to\u00a0\u22120.14\u00a0V (vs. Ag/AgCl) and the H2O2 selectivity increased slightly from 47.0-56.2% to 52.6\u201356.1% at\u00a0\u22120.35 to\u00a0\u22120.6\u00a0V (vs. Ag/AgCl).In order to reveal the nature and quantity of two-electron ORR active sites in the alkaline media, Lu, et\u00a0al. [120] synthesized various oxidized carbon black (OCB) with adjustable surface OFGs (CO, OC\u2013OH, -C-OH) by HNO3 treatment at 30\u2013120\u00a0\u00b0C. The OCB-120\u00a0\u00b0C had the most stable ring current and \u03bb of \u223c60% at 0.26\u20130.36\u00a0V (vs. RHE). It was also observed that the intrinsic activity of OC\u2013OH is much higher than that of CO.To investigate the synergistic influence of different N doping species and OFGs in carbon materials on the H2O2 production, N & O co-doped OMC was fabricated from HMT (hexamine), Pluronic F127, and resorcinol by a one-step hydrothermal method at 600\u2013900\u00a0\u00b0C [121]. The 700\u00a0\u00b0C carbonization sample had 443\u00a0m2\u00a0g\u22121 BET surface area, and COOH, CO, total N, graphitic N content together with the highest zeta potential and pyridinic N, C\u2013O\u2013C content (Fig.\u00a06\n). DFT calculations on the account of the adsorption energy of HOO\u2217 were applied to study the interactive effects between N species and OFGs (Fig.\u00a06i). Compared with the pure graphitic carbon (\u22120.608\u00a0eV), pyridinic N (\u22120.289\u00a0eV), graphitic N (\u22120.494\u00a0eV), COOH (\u22120.362\u00a0eV), C\u2013O\u2013C (\u22120.175\u00a0eV) doped carbon possessed a lower HOO\u2217 adsorption energy, which positively affects the production of H2O2. Among these, pyridinic N & C\u2013O\u2013C co-doped carbonaceous catalyst exhibited the lowest HOO\u2217 adsorption energy (\u22120.092\u00a0eV), which accelerated the HOO\u2217 protonation toward H2O2. Combined with the ideal dispersed performance, the 700\u00a0\u00b0C carbonization sample had the highest activity and selectivity (\u223c95%) at 0.4\u00a0V vs. RHE.OFGs can also be in-situ introduced onto the carbon-based electrode by physical/chemical/electrochemical methods to promote H2O2 yield. These will be described in detail in Chapters 6 and 7 based on the electrode type and the method.Non-noble metal (NPM)-based materials have been investigated as four-electron ORR electrocatalysts for more than a few decades. Recently, some researchers tried to load NPMs onto the carbon to test their catalytic performance for two-electron ORR. In 2011, series transition metal-carbon composite catalysts (M\u00a0=\u00a0V, Fe, Co, Ni, Cu, Zn, Sn, Ba, Ce) were obtained by heating the mixture of Vulcan XC-72 CB and metal nitrate salts at a high temperature of 900\u00a0\u00b0C in N2 [122]. As shown in Fig.\u00a07\na, it is clear that Co-activated samples have outstanding performance for the electrosynthesis of H2O2 than other samples in an acid medium. An optimized catalyst with 4% (wt) Co showed a high H2O2 selectivity of 80\u201390% at 0.1\u20130.4\u00a0V (vs. RHE). The selectivity of ORR is also related to the geometric arrangement of atoms on the surface of the catalyst [123]. HOO\u2217 normally binds onto atop sites, whereas O\u2217 binds onto hollow sites. Eliminating hollow sites will specifically destabilize O\u2217 without necessarily changing the activity. Therefore, Siahrostami, et\u00a0al. [36] predicted that catalysts such as Co-porphyrins that lack hollow sites might have high selectivity toward H2O2. Zhang et\u00a0al. [124] developed a Co-based catalyst with a negligible amount of onset overpotential and nearly 100% selectivity by modulating the oxygen functional groups near the atomically dispersed cobalt sites. It was revealed that the presence of epoxy groups near the Co\u2013N4 centers exceptionally enhanced H2O2 generation.Because of the incremental improvement witnessed in NPM electrocatalysts, various novel efficient NPM-based catalysts were developed. Among these NPM-based electrocatalysts, metal carbonitrides, including non-pyrolyzed transition metal macrocycles and pyrolyzed NPM-N-doped carbon (M-N-C) (M\u00a0=\u00a0Fe, Ni, Co, etc.) catalysts, have shown the most promising potential because of their efficient activity toward ORR.The investigation of non-pyrolyzed transition metal macrocycles on ORR dates back to the 1960s since Jasinski [125] first discovered the promoted ORR performance by cobalt phthalocyanine with a metal-N4 center. The electronic configuration of the metal centers is beneficial to bond with the O2 molecule and subsequent reduction of O2 [125]. Subsequently, multiple M\u00a0\u2212\u00a0N4 macrocycles, such as porphyrins, phthalocyanines, and tetraazaannulenes, have been widely investigated [126]. It was found that these catalysts are prone to catalyze two-electron reductions if they are adsorbed onto the surface of the electrode. The surface electrochemical behavior of adsorbed Co tetra-methoxy-phenyl porphyrin (CoTMPP) was investigated at different pH values [127]. The Co center one-electron redox process and the N4-ring two-electron redox process were recognized during the ORR. The adsorbed CoTMPP displayed strong activity for both O2 reduction and peroxide reduction. O2 can only be reduced to the stage of H2O2 in acidic conditions. In contrast, in neutral or alkaline solutions, the ORR was observed through a two-electron pathway in the low potential polarization range (0.13 to\u00a0\u22120.5\u00a0V vs. SCE) and the overall four-electron pathway to H2O in the high potential polarization range (about\u00a0\u22120.5 to\u00a0\u22121.5\u00a0V vs. SCE). However, the major problem of these non-pyrolyzed transition metal macrocycles is demetallation from the active sites resulting from the collapse of the macrocyclic structure caused by peroxide and superoxide intermediates during ORR [128,129].Enlightened by the high ORR performances of the transition metal macrocyclic complexes, pyrolyzed NPM-N-doped carbon (M-N-C), prepared via the thermal treatment of either metal N4-macrocyclic complexes or the mixture of metal salts, carbon and nitrogen precursors, have been extensively investigated. Until now, the role of the transition metals in the M-N-C catalysts is still controversial, and numerous types of active sites were inferred to be responsible for four-electron ORR activity [126,128,130], while the investigation on two-electron ORR was limited.Jaouen and Dodelet [131] have confirmed that Fe or Co, together with N, followed by a pyrolysis treatment, resulted in catalytic sites highly active for two-electron or four-electron ORR. Fe-activated carbons are active for H2O production, and Co-activated carbons are reported to be responsible for reducing O2 toward H2O2. However, according to the review by Bezerra et\u00a0al., 2008, both Fe\u2013N\u2013C and Co\u2013N\u2013C materials catalyze the ORR mainly through a four-electron process instead of a two-electron process [132]. Campos et\u00a0al. [133] reported that the electrocatalytic performance of Co catalysts obtained from nitrogen-ligands is greatly affected by heat treatment. Once the heat temperature exceeded 500\u00a0\u00b0C, a drop in H2O2 selectivity resulted from the progressive formation of metallic cobalt particles. H2O2 reduction was almost invisible without cobalt or when the cobalt is in the form of a complex. Olson et\u00a0al. [134] have studied the ORR mechanism of Co-polypyrrole-C (CoPPy/C) in alkaline media through structure-to-property analyze. Initially, two-electron ORR occurred on a Co-Nx type site to form HO2\n\u2212. The HO2\n\u2212 species further reacted either to form OH\u2212 via electrochemical reduction or to form OH\u2212 and O2 by chemical decomposition. It was speculated that decorating CoxOy/Co nanoparticles appears to be the site of HO2\n\u2212 destruction.In summary, the M-N-C catalysts (where M\u00a0=\u00a0Fe or Co) were thought to exhibit high activity towards the four-electron ORR following the peroxide formation-reduction pathway (O2 \u2013 H2O2 \u2013 H2O) in acidic media [135]. The active sites for H2O2 generation and reduction all exist in the catalysts. To obtain efficient M-N-C catalysts for H2O2 formation, the suppression of H2O2 further reduction is pivotal. Recently, researchers have tried to obtain highly active and selective M-N-C two-electron ORR catalysts by introducing other functional groups. Byeon et\u00a0al. [136] demonstrated that co-doping of MnO nanoparticles together with Mn-Nx moieties into carbon are efficient for peroxide production \u03bb of 74% at 0.2\u00a0V (vs. RHE). The favored two-electron ORR resulted from the increasing number of Mn-Nx sites inside the mesoporous N-doped carbon. Moreover, strong evidence showed that a further reduction of H2O2 was remarkably suppressed by adjacent MnO species. Li et\u00a0al. [137] pointed out that the atomic Co-Nx-C sites improve the ORR activity but lack the selectivity for H2O2 generation, while OFGs promote the selectivity for the two-electron ORR but exhibit limited kinetics for ORR. Therefore, a rational combination of Co-Nx-C sites and OFGs into 3D interconnected conductive hosts was prepared by heating the predesigned precursor prior to HNO3 treatment (noted as Co-POC-O). The Co-POC-O exhibited excellent catalytic performance in KOH (0.1\u00a0M) with a high onset potential (0.84\u00a0V vs. RHE) and selectivity of over 80%. Moreover, the synergy effect of atomic Co-Nx-C reactive sites and OFGs was identified by the control samples with only atomic Co-Nx-C reactive sites (Co-POC-R) or only OFGs (POC\u2013O) (Fig.\u00a07b\u2013f).Multiple metal oxides, especially group IV and V metals, are proven to be catalyst supports to replace carbon materials due to their abundant surface hydroxyl groups and chemical stability in acidic electrolytes [130,138]. However, their bulk form exhibit extremely low ORR activity resulting from the poor electrical conductivity and reduced reactive sites for oxygen species adsorption. Recently, alloying, forming highly dispersed nanoparticles, and reducing the crystalline sizes have been reported as an effective way to enhance the catalytic activities of metal oxides by increasing their exposed reactive sites, surface available defects, and electrical conductivity. Different carbon varieties were modified by various metal alloys or metal oxides with nanostructure to improve the two-electron ORR of carbonaceous electrocatalysts. Typically, the synthesized or purchased metal composites were supported onto the carbon by a modified polymeric precursor method [139] or sol-gel method [140]. Vulcan XC-72(R) with n\u00a0=\u00a03.1\u20132.5 and \u03bb\u00a0=\u00a041\u201373% was used frequently in Santos' team as the support to study the catalytic capacity of two-electron ORR. After being modified with V2O5, SnNi, WO2.72, MnO2, or W@Au, the n of metallic nanostructure modified CB decreased to 2\u20132.6, while \u03bb increased to 68\u201396% [141\u2013146]. Among them, a core-shell type W@Au nanostructures (1% W@Au/CB) presented the highest selectivity toward H2O2 with n of \u223c2 [147].Printex L6 CB (BET surface area of \u223c250\u00a0m2\u00a0g\u22121, primary particle size of 18\u00a0nm and density of 1.8\u00a0g\u00a0cm\u22123) with a \u03bb of 65.3\u201368% and n of 2.6\u20132.7 was another important carrier to develop metal compounds nanoparticles modified carbonaceous electrocatalysts for two-electron ORR [148,149]. After preparation optimization, 4% CeO2/CB specimen showed a \u03bb\u00a0=\u00a088% and n\u00a0=\u00a02.2\u00a0at\u00a0\u22120.4\u00a0V vs. Hg/HgO, while Ta2O5/CB (5% (w/w) Ta/C) exhibited a \u03bb\u00a0=\u00a083.2% and n\u00a0=\u00a02.3\u00a0at\u00a0\u22120.3 to\u00a0\u22120.5\u00a0V (vs. Ag/AgCl). A \u03bb of 1% Pd/CB was over 80% at about 0\u00a0V (vs. Ag/AgCl) [150]. In another work, the synthesized rGO with mean particle size of 5.7\u00a0\u00b1\u00a00.8\u00a0nm showed \u03bb\u00a0=\u00a073.7% and n\u00a0=\u00a02.52. With the Nb2O5 loading, Nb2O5/rGO composite (Nb/GO\u00a0=\u00a015% w/w) exhibited \u03bb\u00a0=\u00a085.3% and n\u00a0=\u00a02.28 (\u22120.20 to\u00a0\u22120.40\u00a0V vs. Ag/AgCl) [151]. In contrast, after loading 5% Fe3O4 nanoparticles, the \u03bb of Fe3O4/rGO was only 62% [152].Based on the above research, the conclusions were that the decoration of metallic nanostructures would bring more acidic oxygen species or special morphology to the surface of the carbons, resulting in a more acidic and hydrophilic surface, and thus improving H2O2 generation by enhancing oxygen adsorption or oxygen diffusion.Some researchers claimed the doped heteroatoms are the real ORR active sites in the carbon structures [81,82,84,87], however, another group of research recently proved that defects created by heteroatoms might be the actual active sites [153,154]. Some novel carbon-based electrocatalysts were developed with the guidance of this newly established defect-driven catalysis mechanism. In this section, we will not discuss this mechanism debate. Instead, we will provide a new insight on the mechanism for ORR and give specific examples on promoting two-electron ORR by creating different defective carbon materials.Perfect and defective graphene clusters are summarized and plotted in Fig.\u00a08\na. The Graphene with G585 divacancy defects (consisting of two pentagons and one octagonal) facilitated the O2 adsorption and lowered the following reaction energy barriers. DFT calculations showed that the point and line defects in graphene could tailor the local electronic structures and the distributions of nearby carbon atoms [155]. A pentagon ring located at the zigzag edge, the odd number of octagon ring, and fused pentagon ring line at the edge of the defective graphene are all proposed to be ORR active sites (Fig.\u00a08). Hu and co-workers proposed that pentagon and zigzag edge defects are more reactive in four-electron ORR [156]. Moreover, defective graphene fabricated by N doping following a removal approach was a trifunctional catalyst for the four-electron ORR, hydrogen evolution reaction, and oxygen evolution reaction [157]. DFT models predicted N-doped into the graphene is beneficial to lower the adsorption energy of O2 but unfavorable for the reduction.These results proved the versatility of the defect-driven catalysis for electrocatalysis. However, until now, insufficient effort has been placed on understanding what defective active sites selectively promote the two-electron ORR. Only a few research applied the defect-driven catalysis mechanism to design and explore two-electron ORR materials.Carbonization of MOF-5 under H2 will transform sp\n\n2\n-C bonds to sp\n\n3\n-C bonds [158]. The harvested hierarchical porous carbons (HPC) exhibited H2O2 selectivity of 80.9\u201395% in acid solution (pH 1 and 4), with both defects and sp\n\n3\n-C acting as active sites of two-electron ORR. This research certified the un-doped and un-functionalized defective carbon also has potential for the two-electron mechanism. However, later researchers presented opposing views about the active sites. For example, Tao, et\u00a0al. [159] confirmed that the defect sp\n\n3\n carbon atoms served as main active sites for four-electron ORR instead of two-electron ORR. Chen et\u00a0al. [160], Kim, et\u00a0al. [161] all stated the active sites of two-electron ORR are from sp\n\n2\n carbons.Chen et\u00a0al. [160] have experimentally and theoretically investigated the defect and pore size effect to the electrochemical H2O2 synthesis. Two porous carbon catalysts (predominantly microporous/mesoporous carbon, Micro C, and Meso C) were synthesized from similar precursors but different synthetic procedures. Characterizations showed the two materials had similar chemical identity and content of defects but different pore structures (surface area, pore size, and pore volume) (Fig.\u00a08b\u2013e). Electrochemical tests showed both carbons exhibit high activity with an onset potential of about 0.7\u00a0V (vs. RHE) and selectivity of >70% toward H2O2 (Fig.\u00a08f\u2013i). The better performance of the Meso C was attributed to the easier mass transfer in mesoporous structures. Spectroscopic analyses revealed that microporous/mesoporous carbon (Micro C and Meso C) contain sp\n\n2\n-type defects that might be the reactive sites for the two-electron ORR. DFT calculations indicated that the pentagon edge, single vacancies (SV), and 585 double vacancies (DV) in 2D graphene sheets are too reactive to contribute to ORR. While some of the defect configurations (555-6-777, 555\u2013777 line defect, and 555\u2013777 point defects) were identified as having comparable activity with PtHg4 (Fig.\u00a08j and k) (the ideal catalyst until now shown in Fig.\u00a02b and c) for the two-electron ORR. Kim et\u00a0al. [161] developed two N-rGO materials with different defect compositions. Based on the nuclear magnetic resonance technique and other X-ray-based tools, sp\n\n2\n carbon defects associated with epoxy or ether groups were identified to play a more critical role in promoting H2O2 formation than other functionalities, such as N defects or carboxylic acid edge sites.In recent research [162], the functionalized graphene sample with the largest electrochemical active surface area and the highest in-plane carbon defect density did not show the most efficient ORR activity and H2O2 selectivity, which was due to excess in-plane carbon defects that would lead to a conductivity decrease. In comparison, the graphene sample with the lowest in-plane carbon defect density had the highest H2O2 selectivity. These results emphasized that the optimization of graphene precursors defect site density is pivotal for adjusting the catalysts\u2019 catalytic activity and reaction pathway. Defect modulation is regarded as a burgeoning strategy to regulate the electronic structure of carbon-based materials.The Electrode is the key component of the electrochemical cell because it contacts the electrolyte and provides the reaction sites for the reactants. Ideal carbon-based electrodes must possess a large surface area, suitable porosity, internal channels, and low electronic resistance for high electrochemical activity. The following chapters mainly introduce the development of formed carbon-based electrodes, including general electrode preparation methods, various physical and chemical modification methods, and their applications. There are two kinds of modification methods for enhancing two-electron ORR activity of the electrodes: (1) modify the pristine electrodes (graphite plate & rod, graphite felt, reticulated vitreous carbon, and activated carbon fibers, Fig.\u00a09\na) by physical/chemical methods to tune the surface properties or to load effective two-electron ORR functional groups (N, O) onto the electrode surface; (2) use the pristine carbon electrodes as support and introduce other highly active & selective carbon materials, such as CNTs, CB, and graphene (Fig.\u00a010\na). For example, carbon felt and carbon paper are often used as soft current collectors for in situ construction of electroactive nano-carbon structures due to their simple handling and excellent conductivity.Various electrochemical testing techniques can be applied to investigate the electrochemical activity and interface properties of formed electrodes, including linear sweeping voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry (CA), and scanning electrochemical microscopy (SECM) in the three-electrode system [163]. The above test results can be used to support the demonstration of electrode performance and mechanism analysis. In our estimation, the most critical parameter is the H2O2 yield and the current efficiency of the electrodes. In order to contrast the performance of electrodes clearly and intuitively, the expression of H2O2 yield in this paper was uniformly transformed to mg h\u22121 unless specially noted. The current efficiency (CE, also known as Faradaic efficiency) of the cathodes was calculated from Eq. (25):\n\n(25)\n\n\nCE\n=\n\n\nn\nF\n\nC\n\n\nH\n2\n\n\nO\n2\n\n\n\nV\n\n\nI\nt\n\n\n\u00d7\n100\n%\n\n\n\nwhere n is the number of the transferred electrons from O2 to H2O2 (n\u00a0=\u00a02), F represents the Faraday constant (96,486 C mol\u22121), CH2\n\nO2\n\n represents the concentration of H2O2 (mol L\u22121), V stands for the bulk volume (L), I is the current (A), and t is the time (s). The performances of all mentioned cathodes are summarized in Table\u00a03\n, including the electrolyte conditions, operation mode, H2O2 yield, and CE.Graphite has excellent chemical stability and electrical conductivity. Furthermore, it is relatively easy to get at a low price. Early graphite-based electrodes were usually made of graphite rods or plates (Fig.\u00a09a) [164,165]. In 2010, a spectrographically pure graphite (SPG) rod was applied as a cathode in a microbial fuel cell (MFC) without any energy input to synthesize H2O2 with a yield of 0.46\u00a0mg\u00a0h\u22121 and the CE of 69.5% [166]. 3D graphite particle electrodes (GPE) prepared via extrusion-spheronization method from graphite-polytetrafluoroethylene (PTFE) were also used in MFCs to generate H2O2 [167]. Although anodic COD removal, electricity generation, and cathodic H2O2 production were realized in one single system, H2O2 yield was relatively low as 0.13\u00a0mg\u00a0h\u22121. Acidic pretreatment on raw graphite powder promoted the surface area and the OFGs content of GPE to increase the H2O2 yield of MFC by 41% (0.19\u00a0mg\u00a0h\u22121) [168]. Whereas the yield of H2O2 in the MFCs was still sparse due to the relatively low current density provided by the bio-anode.In the abiotic electrochemical system, the H2O2 yield mostly depends on the performance of the cathode. Thus research efforts aimed to improve H2O2 yield by modifying graphite electrodes.Polymer-modified electrocatalysts are very promising materials for ORR [169]. Consider quinonoid compounds were often used to modify electrodes because then can stop the ORR at the peroxide stage [170,171], Polypyrrole/anthraquinonedisulphonate (PPy/AQDS) composite film modified graphite produced H2O2 with the yield of 4.2\u00a0mg\u00a0h\u22121 and CE of 64\u201373% at\u00a0\u22120.65\u00a0V (vs. SCE) [172]. Quinonoid compounds were also employed to modify other kinds of carbon-based cathodes, and they will be discussed in the following chapters. In 2011, commercial MWCNTs (surface area 233\u00a0m2\u00a0g\u22121, outer diameters 8\u201315\u00a0nm, inside diameters 3\u20135\u00a0nm) were immobilized onto graphite surface by Khataee and co-workers [173]. 1.4\u00a0mg\u00a0h\u22121 of H2O2 was yielded while a CE of about 2.1% was achieved at 100\u00a0mA. Conducting polymers, such as Ppy and polyaniline (PANI), are widely used as catalysts or catalyst supports for ORR due to their stability, ease of electro-polymerization, and high conductivity [174]. Rabl et\u00a0al. [175] presented results that Ppy and PANI coating on carbon electrodes considerably improved the H2O2 selectivity by preventing undesired side or further reactions of H2O2 to H2O. Based on the above properties, Ppy/MWCNT and PANI/MWCNT nanocomposites were fabricated and electro-polymerized onto the graphite electrode or stainless steel [176,177]. ORR activity of different concentrations of MWCNT was investigated. The PANI/MWCNT nanocomposite modified stainless steel cathode with MWCNT content of 2% (wt) generated 1.1\u00a0mg\u00a0h\u22121 of H2O2 with a CE of about 42% at\u00a0\u22120.6\u00a0V (vs. SCE), 2.5 times higher than raw stainless steel, which was due to the superior electrocatalytic activity of MWCNT. Meanwhile, 2.5% w/w Ppy/MWCNT had the highest ORR electrocatalytic activity and H2O2 yield was increased by 70% to 3.4\u00a0mg\u00a0h\u22121 at\u00a0\u22120.55\u00a0V (vs. SCE). Recently, Chu, et\u00a0al. [178] fabricated multi-layer super-hydrophobic cathode by mixing graphite powder with CNT and PTFE. The hydrophobic property of carbon powder and heat treatment induced strong aerophily of cathodes, by which the cathode could adsorb more air bubbles under the air aeration than the hydrophilic cathode, and it exhibited an ideal performance for H2O2 generation at 37.6\u00a0mg\u00a0h\u22121 with an observed CE of 40% with 60\u00a0min of electrolysis.H2O2 yield on graphite-based cathodes is not usually satisfactory because of the small surface area of graphite rods, plates, or particles. The graphite cathodes could only be used under very low current density in the early prototype design. Although modification led to a significant improvement in the performance of ORR, the H2O2 yield and CE of graphite-based electrodes were still relatively low for application. Recently, 3D porous electrodes are becoming increasingly popular to counteract the low yield limitations of 2D electrodes in the electrochemical cells [179].Graphite felt (GF) or carbon felt (CF) are the most frequently used commercially available carbon materials. (GF is obtained from the graphitisation of the CF). As a typical 3D electrode (Fig.\u00a09b), GF has excellent features such as good electrical conductivity (370.4\u00a0S\u00a0m\u22121) [180], high volumetric surface area (22,100-22,700\u00a0m2\u00a0m\u22123), good mechanical integrity, high chemical resistance and stability, it is easy to fabricate and scale-up, and has a low cost [181]. It has been extensively used for H2O2 production in the field of EAOPs for wastewater treatment.Raw CFs (provided by Carbon-Lorraine Company) were applied as electrodes in EAOPs to electrogenerate H2O2 and fabricate an electro-Fenton system to successfully degrade 2, 4-dichlorophenoxyacetic acid, p-Nitrophenol, pentachlorophenol, methyl parathion, malachite green, and phenol [182\u2013187]. In early studies of EAOPs, pollutant degradation pathway and intermediates, optimal technical and economical degradation conditions, the kinetic mechanism of degradation were the major concern, rather than the performance of H2O2 production. Nevertheless, as demand continues to increase, researchers are demanding higher performance of GF/CF based cathode, which gave rise to surface modification or coating techniques.OFGs were verified to increase the hydrophilicity of carbon materials and thus promote the transfer of both electrons and dissolved oxygen [188]. Furthermore, OFGs or defects associated with OFGs are identified as two-electron reactive sites of carbon materials [115,118,161]. Therefore, increasing the number of OFGs on the electrode surface becomes the primary choice for electrode modification.Zhou et\u00a0al. [189,190] used hydrazine hydrate to increase O and N functional species content on the GF microfilaments surface. With the stronger hydrophilicity and faster electron transportation of modified GF, the H2O2 yield improved 160% to 11.5\u00a0mg\u00a0h\u22121 with CE of 82% at\u00a0\u22120.65\u00a0V (vs. SCE). Ou et\u00a0al. [191] successfully loaded O, N, and S-containing functional groups onto the GF by modification through the concentrated H2SO4, KMnO4, and NH3 activation. With better hydrophilicity and conductivity, the modified GF realized 47.9\u00a0mg\u00a0h\u22121 of H2O2 yield with CE of 11.8% at 640\u00a0mA, which was 73% higher than the raw GF. Wang et\u00a0al. [192] activated GF with KOH at a high temperature (900\u00a0\u00b0C) to harvest electrodes with higher surface area, higher hydrophilicity, and more OFGs for higher a H2O2 yield. The H2O2 yield of activated GF reached 40\u00a0mg\u00a0L\u22121\u00a0h\u22121 (volume unknown) at\u00a0\u22120.7\u00a0V (vs. SCE). After activating the GF cathode, the apparent rate constant of dimethyl phthalate degradation in the electro-Fenton system increased from 0.02 min\u22121 to 0.20 min\u22121. However, large amounts of energy were required due to the high temperature in this method. In 2020, Lai et\u00a0al. [193] demonstrated GF cathodes treated with NaOH at a lower temperature (400\u00a0\u00b0C) facilitated the OFGs loading and enhanced hydrophilicity. As a result, the modified GF realized about 30\u00a0mg\u00a0h\u22121 of H2O2 yield with CE of 94.6% at 50\u00a0mA. In order to develop simple methods to achieve large-scale modification, Jiang, et\u00a0al. [194] investigated the HNO3 and KOH reagents treatment under a much milder temperature (70\u00a0\u00b0C). Though both methods could increase the OFGs content, surface area, hydrophilicity, and HNO3 treated GF exhibited better performance to synthesis; resulting in 742.5\u00a0mg\u00a0h\u22121 of H2O2 at 20\u00a0V applied voltage, 8% and 69% higher than the KOH modified GF and unmodified GF, respectively. Acid pretreatment was also applied to activate GF via a low-cost and simple gaseous acetic acid activation [195]. The H2O2 yield of activated GF is enhanced by 6 fold\u201310.3\u00a0mg\u00a0h\u22121 with the CE of 75% at\u00a0\u22120.7\u00a0V (vs. SCE) due to higher contents of macropores, micropores, sp\n\n3\n carbon bonds, defects, and OFGs.Anodization was applied on GFs to increase the H2O2 yield of electrochemical modified GFs by 170% due to the generation of carbonyl, carboxyl, quinone, and ester groups [196]. However, CE of anodized GF decreased from 87% to 79%, indicating the anodizing modification encouraged both two-electron ORR and four-electron ORR. Electrode polarity reversal was also applied for situ anodically modification of GF to improve the hydrophilicity of the electrode surface and O2 mass transfer [197]. With high contents of carbonyl and hydroxyl groups, the H2O2 yield of GFs increased by 2.9 times, reaching 8.1\u00a0mg\u00a0h\u22121 at 100\u00a0mA.Comparing three oxidation modifications, Wang, et\u00a0al. [198] discovered that the H2O2 yield of electrochemically oxidized carbon fiber generated was 9\u00a0mg\u00a0h\u22121, 11.6 times that of raw CFs, and 16\u201398% higher than H2O2 oxidized CFs and Fenton (\u00b7OH) oxidized CFs. However, after 10-rounds of continuous runs, the H2O2 production of electrochemical modified GFs decreased by 42\u201361% due to the loss of OC\u2013OH species, and the destruction of the electrode structure. In the most recent research, the activity and selectivity of GF electrodes were improved for H2O2 electrogeneration by integrating chemical oxidation, electrochemical oxidation, and thermal treatment [199]. It was reported that HNO3 oxidation facilitated OFGs and defects formation, while electrochemical oxidation favored carboxyl removal and carbonyl groups formation. Moreover, the following thermal treatment engendered the rebounded hydrophilicity and thus enhanced the activity. The modified electrode (GF\u2013HNO3\u2013EC-N2) benefited from the above treatments, and exhibited a 5-fold higher H2O2 yield, 9\u00a0mg\u00a0h\u22121 with CE of 86% at 0\u00a0V (vs. RHE) than the pristine samples.Highly active and selective materials were also applied onto GFs to improve the H2O2 yield. For example, ZIF-8 was carbonizated under a N2 atmosphere to load N-doped porous carbon onto the GF [200]. The existing graphitic-N and sp\n\n2\n carbon promoted the electron transfer between catalyst surface and O2 molecules, as well as accelerating the ORR. With optimal condition, H2O2 yield increased 10 times and reached 6\u00a0mg\u00a0h\u22121 with CE of about 9% at 100\u00a0mA. Another N-doped carbon modified GF prepared by electro-deposition of PANI, carbonization, and activation was applied in electro-Fenton to generate and activate H2O2 to remove 85% of phenol in 180\u00a0min with a residual H2O2 accumulation of 4.6\u00a0mg\u00a0h\u22121 [201].Vulcan XC-72R CB was deposited on GF to increase the surface area as well as the pore volume of the electrode. This modification improved the H2O2 yield by about 10.7 times to reach H2O2 47.3\u00a0mg\u00a0h\u22121 with CE of 74.6% at 100\u00a0mA [202]. This was a milestone in developing a highly efficient modified GF electrode, because H2O2 yield was improved to tens of mg h\u22121 at 100\u00a0mA with CE far more than 10%. Vertical-flow electro-Fenton reactor, peroxide-coagulation system, and flow-through electro-peroxone systems were developed based on this CB modified GF electrode to realize different wastewater treatment functions [203\u2013205]. Most recently, an MWCNT & CB co-modified GF was fabricated [206]. Although the hydrophobicity was slightly increased (contact angle increased from 145.2\u00b0 to 154.9\u00b0), higher BET surface, pore volume, and OFGs content caused by modification still improved the ORR activity. As a result, a comparable H2O2 yield of 48\u00a0mg\u00a0h\u22121 with CE of 63% was obtained at 120\u00a0mA.In the past 7\u20138 years, graphene has been regarded as a promising material for H2O2 electrogeneration. In 2016, Mousset et\u00a0al. [207] tested three commercial pristine graphene materials (2D graphene monolayer, 2D graphene multilayer, and 3D graphene foam) as electrodes for H2O2 generation. Although 3D graphene foam exhibited the least hydrophilicity, it could surprisingly achieve the highest H2O2 generation,0.6\u00a0mg cmcat\n\u22123 at\u00a0\u22120.61\u00a0V (vs. Ag/AgCl), due to the contribution of higher surface area as well as superior electrical conductivity. However, compared with other carbon materials, the H2O2 electrogeneration from graphene was not satisfactory, indicating that pristine graphene itself was not the preferred electrode for two-electron ORR. Graphene was mostly used for coating various substrates to improve the surface area and the conductivity of raw electrodes and thus increase the catalytic performance.Le et\u00a0al. [208] loaded homogeneous dispersion of GO onto the CF and investigated the effect of electrochemical, chemical, and thermal reduction of GO on the electrodes performance. The reduction of GO was beneficial to H2O2 production because it enhanced hydrophilic characteristics and conductivity, as well as created more active sites. Though thermal reduction exhibited the highest electrochemical properties, electrochemical reduction had both high performance as well as low cost, which is regarded as the best modification method [209]. Analogously, GO was drop-casted onto a substrate disposed of liquid crystal display (LCD) glass, and then electrochemically reduced to form an ErGO-LCD electrode [210], which generated H2O2 with the yield of 2.3\u00a0mg\u00a0h\u22121 at\u00a0\u22121.5\u00a0V (vs. Ag/AgCl).Encouraged by the above results, electrochemically exfoliated graphene (EEGr) was utilized as the functional coating material to decorate carbon cloth (CC) [211] and carbon-fiber brush [212]. After the optimization of EEGr and Nafion concentration, the EEGr decorated cathodes increased by 40\u2013100% in H2O2 yield, and 26.3\u2013106% in phenol degradation rate in EF processes.Quinone-functionalized electrochemically exfoliated graphene (QEEGr) was coated on the CC electrode to generate 5\u00a0mg\u00a0h\u22121\u00a0H2O2, which was 9 times higher than the unmodified CC [213]. The presence of the quinone group was thought to facilitate two-electron ORR, thus initiating H2O2 generation without compromising the electrode electrical property. Moreover, QEEG-Fe3O4 coated CC composite electrode could continuously generate reactive oxygen species for complete degradation of Bisphenol A.EEGr and Vulcan XC-72R CB co-modified GF cathode were developed to generate 38.5\u00a0mg\u00a0h\u22121 of H2O2 at\u00a0\u22120.9\u00a0V (vs. SCE) in a neutral solution, which was 2 times that of the unmodified cathode [214]. N-doped graphene (N-EEGr) was derived by mixing the EEGr with ammonium nitrate followed by calcination under a N2 atmosphere to activate H2O2 to \u00b7OH for organics degradation, rather than increase the electrochemical generation of H2O2 [215]. A significantly different result was obtained with another N-EEGr modified electrode by loading the mixture of CB, Nx-EEGr, and PTFE on the GF [216]. The Nx-EEGr was prepared by annealing of melamine and graphene mixture under a N2 atmosphere, where x represented the mass ratio of melamine to graphene. The optimized N3-EEGr-CB-GF cathode improved H2O2 yield to 86\u00a0mg\u00a0h\u22121 due to the generated active graphite N and pyridinic N species and CC. Moreover, the presence of pyridinic N was able to catalyze H2O2 to produce \u00b7OH, which is beneficial to overcoming the effect of the initial pH on EF [201,217].RVC is a disordered glassy porous carbonaceous material with a solid foam network structure (Fig.\u00a09c). RVC has an exceptionally high surface area, high void volume, rigid structure, and low resistance to fluid flow [218]. These properties encouraged the applications of RVC in diverse areas such as sensors and monitors, chemical catalyst supports, and energy conversion [219]. Over the last 10 years, the corresponding research showed that the performance of RVC as cathodes for H2O2 synthesis are comparable or even better relative to the GF electrode.Coria et\u00a0al. [220] investigated the mass transport of GF, RVC and boron-doped diamond (BDD) cathodes during two-electron ORR in a filter-press electrolyzer to discover that the performances of porous GF and RVC with higher limiting currents were obviously better than the non-porous BDD cathode. Petrucci et\u00a0al. [221] confirmed that electrogeneration of H2O2 on a RVC electrode was 210% and 60% higher than that of graphite and CF, respectively, under 5\u00a0mA\u00a0cm\u22122. This observation was due to the better oxygen diffusion and larger reactive surface from a porous 3D structure.Recently a RVC electrode modified by anodic polarization was developed for drinking water disinfection. The H2O2 yield was 6.4\u00a0mg\u00a0h\u22121 with a CE of 43% at 24\u00a0mA, which was about 4 times of the unmodified RVC cathode [222]. The modification and application of RVC electrodes has gradually gained attention [223].ACFs are considered as a group of advanced porous materials with a fiber shape and a well-defined porous structure (Fig.\u00a09d) [224]. Except for the extremely large surface area (2000\u20132500\u00a0m2\u00a0g\u22121), the micropores of the ACFs are directly exposed to the surface, which reduces the mass transfer resistance and enhances the adsorption of various compounds. ACFs and their modification composites were extensively utilized in environmental remediation, such as the adsorption of organic and inorganic pollutants in water/air, capacitive deionization, the degradation of organic pollutants, and microbial decontamination [225]. However, only a few studies have focused on the catalytic production of H2O2 from ACF.Commercial ACF felt was utilized in an electro-Fenton system to degrade Acid Red 14 and levofloxacin [226,227]. Although almost 100% of Acid Red 14 or levofloxacin and 61\u201370% TOC were removed, the H2O2 yield property of the ACF cathode was only about 3.6\u00a0mg\u00a0h\u22121 at 500\u00a0mA with poor CE of 1.1% in the absence of Fe2+ during 180\u00a0min of electrolysis, which means most of the electricity was wasted. Similarly, a commercial ACF cathode was utilized in an electro-Fenton system for cationic red X-GRL degradation with a maximum H2O2 yield of 4.8\u00a0mg\u00a0h\u22121 and CE of about 1% [228]. Zhang et\u00a0al. [229] compared the electrocatalytic properties of two ACFs, which shared similar pore volumes as well as pore diameters but varied BET surface areas. Results showed that ACF with a higher surface area was correlated to faster H2O2 and \u00b7OH accumulation. However, the better ACF only had a H2O2 yield of 4.1\u00a0mg\u00a0h\u22121 and CE of about 1.8%.In 2018, Zhou et\u00a0al. [230] proposed an activated carbon-stainless steel mesh composite cathode (ACSS), which integrated H2O2 electrogeneration and activation together with pollutants adsorption. Although H2O2 yield was only 1.9\u00a0mg\u00a0h\u22121 at 100\u00a0mA with a CE of 3%, the ACSS enabled the iron-free electro-Fenton feasible under neutral pH to remove 61.5% of reactive blue after 90\u00a0min. Ren et\u00a0al. [231] successfully prepared a novel multilayer ACF-based composite cathode with rGO as the conducting layer and OMC as the oxygen diffusion channel. The electroactive surface area, oxygen diffusion rate and electron transport rate were all increased, and the H2O2 yield of ACF@rGO@OMC electrode reached 2.8\u00a0mg\u00a0h\u22121 with CE of 40.4% at\u00a0\u22120.7\u00a0V (vs. SCE).In summary, on a few activated carbon-based cathodes have been employed for H2O2 production and until its catalytic selectivity is substantially improved the potential for further applications will be limited [232].Generally, cathodes mentioned in the last chapter are immersed in the electrolyte. The gaseous O2 is dissolved into the electrolyte by aeration, and then the dissolved O2 diffuses with the electrolyte into the internal pores of the cathodes and reacts at active sites (Fig.\u00a010b). However, the immersed cathodes are usually unable to maintain high H2O2 yield and CE at large current densities (usually >10\u00a0mA\u00a0cm\u22122) because of the low solubility and inferior O2 mass transfer [164]. Lower CE of immersed cathodes results in the waste of the electricity and brings hidden safety trouble due to higher parasitic HER [233]. The birth of gas diffusion electrode (GDE) solved these challenges. As a kind of film electrode, GDE often consists of a reactive catalyst layer (CL), a gas diffusion layer (GDL), and an optional current collector. Applied for the electrogeneration of H2O2, the hydrophilic CL, which faces to the electrolyte, provides the reactive sites for the ORR, while the hydrophobic GDL facing to reactant gas provides a stable gas diffusion channel for the oxygen towards the catalyst layer and prevents electrolyte leakage. CLs are usually fabricated from the mixture of carbon-based catalyst powder and the binder, followed by being coated/rolled/painted/sprayed onto the gas diffusion layer. Based on the species of the main catalyst in the CL, the GDE can be sorted into carbon black-based-, carbon nanotube-based- and hybrid carbon GDE.The most widely utilized carbon materials for GL preparation are the Vulcan XC-72(R) and Printex L6 CB. GDE has been developed by E-TEK by painting Vulcan XC-72 CB and PTFE wet paste mixture uniformly onto a face of the carbon cloth. The H2O2 yield of Vulcan XC-72 CB-based GDE was 82.7\u00a0mg\u00a0h\u22121 with CE of about 29% at 450\u00a0mA. Based on this electrode, several different EAOP systems were developed, and numerous target POPs were successfully degraded [18,234\u2013240]. Although the advent of GDE substantially altered the mass transfer of O2, there is still much room for CE further improvement by promoting the CL catalytic activity and selectivity. Developing from Vulcan XC-72R CB and PTFE via a rolling method, the harvested GDE produced 158\u00a0mg\u00a0h\u22121\u00a0H2O2 at 520\u00a0mA with CE of 48%. Unlike traditional GDE, the CL of the electrode simultaneously acted as a GDL [241]. The GDE was also prepared by pressing and sintering a series of metal oxide modified Vulcan XC-72R CB mentioned in Chapter 4.3.2. With 0.2\u00a0bar pressurized pure O2 supply, H2O2 yield of W@Au/CB GDE, CeO2/CB GDE and MnO2/CB GDE were 21, 51, and 68\u00a0mg\u00a0h\u22121 at\u00a0\u22121.1\u00a0V (vs. Ag/AgCl) [141,145,147]. Meanwhile, 102\u00a0mg\u00a0h\u22121 of H2O2 was generated from WO2.72/CB GDE at\u00a0\u22121.3\u00a0V [144], and 102\u00a0mg\u00a0h\u22121 of H2O2 was generated from V2O5/CB GDE at\u00a0\u22121.5\u00a0V [142]. Nevertheless, CE was not calculated or mentioned in those studies, which makes it hard to compare those GDE with others\u2019 intuitively.Compared to Vulcan XC-72 CB, Printex L6 CB was demonstrated to be a better choice for H2O2 generation due to more oxygenated acid species content and higher hydrophilicity [242\u2013244]. When the aforementioned 5% Ta2O5/CB material was made into GDE, the H2O2 yield was 11\u00a0mg\u00a0h\u22121 at\u00a0\u22121.0\u00a0V (vs. Ag/AgCl) [148]. As quinones have been investigated as efficient catalysts for the improvement of two-electron ORR [245], GDEs were developed by modifying CB with different amounts of tert-butyl-anthraquinone (TBAQ) [246]. According to RRDE results, 1% TBAQ/CB showed the highest selectivity (89.6% with 2.2 electrons exchanged). The obtained GDE realized a H2O2 yield of 80\u00a0mg\u00a0h\u22121 at\u00a0\u22121\u00a0V (vs. SCE). Rocha et\u00a0al. [171] investigated the electro-activity of various quinone compounds (acenaphthoquinone (APQ; acenaphthylene-1,2-dione), menadione (MDA; 2-Methyl-1,4-naphthoquinone), and Alizarin Red S (ALZ; 1,2- dihydroxyanthraquinone)) modified CB for H2O2 production in an acid medium. The results showed that 1% of 1,2-dihydroxyanthraquinone was efficient for two-electron ORR and the resultant GDE realized a H2O2 yield of 77.1\u00a0mg\u00a0h\u22121 at 1900\u00a0mA (CE\u00a0=\u00a06.4%), which was 116% higher than the unmodified CB GDE.In 2009, Zarei et\u00a0al. [247] developed CNT-based GDE by bonding the ointment mixture of commercial MWCNT and PTFE to 50% PTFE-loaded carbon papers and calcined at 350\u00a0\u00b0C under N2 atmosphere. H2O2 yield was 24\u00a0mg\u00a0h\u22121 with CE of about 38% at 100\u00a0mA. The prepared GDE was utilized for C.I. Basic Yellow 2 removal via peroxi-coagulation, as well as C.I. Basic Red 46 degradation through the oxalate catalyzed photo-electro-Fenton [248,249].During the synthesis of modified GDE, the carbon materials also serve as a template or a platform that will disperse or adsorb the modifying reagents. The subsequent heat/hydrothermal treatment caused a uniform distribution of active sites associated with modified moieties. Therefore, the impact of carbon supports was different. Considering the contribution of quinone compounds toward two-electron ORR, Lu, et\u00a0al. [250] investigated the ORR activity of different TBAQ modified carbon materials (carbon aerogel, CNT, CB, graphene doped CB) to discover the TBAQ modified CNT exhibited the highest H2O2 yield (30.1\u00a0mg\u00a0h\u22121 at 50\u00a0mA), which was 27% higher than the unmodified CNT GDE and 9\u201356.4% higher than other TBAQ modified carbon GDEs. The characterization results showed more C\u2013C sp\n\n3\n carbon, and OFGs content together with the mesoporous structure resulting in the outstanding performance of the TBAQ modified CNT GDEs.According to the former results in Chapter 4, Co-based catalysts in the form of Co oxides, Co chalcogenides, or Co nanoparticles are the most efficient electrocatalysts for enhancing two-electron ORR in the acidic medium [133,251\u2013253]. The spraying of CoS2-MWCNTs was employed to manufacture GDE [254]. In the galvanostatic test, the H2O2 yield of CoS2-MWCNTs GDE reached 95\u00a0mg\u00a0h\u22121 with CE of about 50%. CoS2 particles were proven to play a significant role in enhancing the two-electron ORR as well as preventing H2O2 from further reduction to H2O to some extent. Enlightened by the above research, (Co, S, P)-decorated MWCNTs were prepared through a hydrothermal route [255]. The electrocatalyst was mixed with 2-propanol and Nafion. The mixture ink was sprayed onto a carbon cloth several times together with a carbon microporous layer to form the GDE. Compared with undecorated MWCNT GDE, (Co, S, P)-decorated MWCNTs GDE enhance the electrocatalytic H2O2 production to about 225\u00a0mg\u00a0h\u22121 with CE of 51\u201353% at 800\u00a0mA.Recently, GDE was fabricated by rolling, pressing, and calcining the mixture of P-doped CNTs and PTFE [256]. The successful doping of P increased the activity of CNT, which exhibited about 0.2\u00a0V more positive onset potential and 100% higher current density at\u00a0\u22120.8\u00a0V (vs. SCE). However, P-doping decreased the selectivity of two-electron ORR with n changing from 2.6 (CNTs) to 3.06 (P-CNTs). Although P-CNTs tended to four-electron pathway, the P-CNTs GDE still had excellent performance with a H2O2 yield of 207\u00a0mg\u00a0h\u22121 and CE of 88.5%, which was obviously higher than CNTs-GDE (67\u00a0mg\u00a0h\u22121 with CE of 64.7%). This result demonstrated the difference between the RRDE calculated selectivity and electrolysis calculated CE, which in fact was the difference between the theoretic selectivity of the material and the actual selectivity of the electrode in operation.Recently, researchers began to take advantage of the structural and physical properties of different carbon materials and started to investigate the mixture of different carbon materials as a catalyst. Carbon composite materials containing at least two kinds of carbons were reviewed in this section.Xu et\u00a0al. [257] investigated the performance of graphite-PTFE GDE and rGO & graphite-PTFE GDE to discover H2O2 yield of rGO & graphite-PTFE GDE (12.5\u00a0mg\u00a0h\u22121 with CE of about 40% at 24\u00a0mA) was nearly four times higher relative to graphite-PTFE GDE, due to the addition of rGO improved electrochemical conductivity and mesopores contents [258].Typically, rGO aggregation is an overlooked issue during the fabrication of electrodes. Because of the strong hydrophobic interactions between nano-sheets, rGO can aggregate easily in solution or in the drying process [115,259], which substantially reduces the accessibility of the reactants toward rGO basal planes on the electrodes [118]. CNTs stacked between rGO nano-sheets prevent the rGO from restacking to increase basal space and bridge the defects to enhance the electrical conductivity [260]. Liu et\u00a0al. [261] fabricated a novel N-rGO & CNT-PTFE GDE by doping N atoms in the rGO & CNT composite. RRDE results showed that the onset potential of N-rGO & CNT shifted positively in the range of 146\u2013363\u00a0mV with reference to bare rGO, CNT, and graphite. Moreover, N-rGO & CNT-PTFE GDE generated 1\u00a0mg\u00a0h\u22121\u00a0H2O2 at a relatively positive potential (\u22120.2\u00a0V vs. SCE), which was 2\u201310 times higher than the reference GDEs.Chen et\u00a0al. [262] reported that trace MWCNTs could construct \u201celectron-bridges\u201d interconnecting the CB particles and thus increase the conductivity and porosity of CB. Therefore H2O2 yield of CB & MWCNT-PTFE GDE was 41.3\u00a0mg\u00a0h\u22121 with a high CE of 65.1% at 100\u00a0mA. However, once poorly conductive AC embedding into the hybrid material, the CB particles would be huddled to some scattered aggregates to destroy the bridges, resulting in poor porous structure and conductivity. Furthermore, H2O2 is prone to be further reduced to H2O with the presence of AC.Zhu et\u00a0al. [263] mixed graphite powder with g-C3N4 to fabricated g-C3N4@GDE via the rolling method. Characterization results showed that hydrophilicity could be increased by a g-C3N4 modification, which could induce fast electrolyte penetration to the cathode surface. With moderate g-C3N4 mixing, g-C3N4@GDE generated the highest H2O2 (457.5\u00a0\u03bcM) compared with pure graphite GDE (328.2\u00a0\u03bcM) and pure g-C3N4 GDE (302.2\u00a0\u03bcM).In summary, multiple GDEs based on newly designed high-performance catalysts were developed, which could realize maximum H2O2 yield in the case of the excellent three-phase interface provided by GDE. Nevertheless, some technical issues remain, which require special attention, such as flooding issues caused by improper water management [264]. When the porous GDE was supersaturated, the electrolyte hindered the ability of oxygen to diffuse towards the active sites and thus destroyed the three-phase interfaces (TPIs) equilibrium and decreased the electrode performance. Characterizing, measuring, and solving flooding issues are still challenges for both two-electron and four-electron reactions [265,266].Compared with the immersed electrode, GDE greatly improved the utilization rate of oxygen during H2O2 production relative to the CE of the electrode [164]. However, the aforementioned GDE always needs pressurized air or pure oxygen gas, which increases the construction cost, reactor complexity, and safety. In order to reduce costs, while ensuring the satisfied cathodic performance in engineering applications, a novel sandwich-like air self-diffusion cathode manufactured via rolling method was developed by our group in 2015 [267], which was a deformation and expansion of our formerly developed AC-PTFE air cathodes for four-electron ORR [268\u2013272]. Instead of expensive and multistep-prepared CNTs, OMC or graphene, commercial carbon powder Vulcan XC-72R CB and graphite were used to fabricate CL. The mixture of carbon powders and PTFE was rolled onto a stainless steel mesh (SSM), while the CB-PTFE breathable waterproof GDL was rolled onto the other side. This air self-diffusion cathode is a practical design where oxygen in the air can actively diffuse through GDL to the internal interface of the CL, which needs no aeration or pressurized gas to generate H2O2. In order to distinguish our air cathode from GDE, the former is named \u201cair-breathing cathode\u201d (ABC).During the exploration of different proportions of CB and graphite in CL, it was found the pore area and volume of pure CB-PTFE electrode were 11.6 and 4 times of those of pure graphite-PTFE electrode, but the pore diameter of the former was only 31.6% of the latter. Both electrodes exhibited poor performance in electrogenerating H2O2 due to the indirect four-electron reaction or lack of active sites [267]. When two kinds of carbon materials were mixed together, the hybrid carbon-PTFE CL had moderate pore diameter, area, and volume. With the optimal mass ratio of CB to graphite (1:5), H2O2 yield reached 50\u00a0mg\u00a0h\u22121 (with CE of 92%) in an electrolysis cell at 86\u00a0mA. Aimed at improving the two-electron ORR activity for efficient H2O2 generation, Zhao et\u00a0al. tuned catalyst mesostructure and hydrophilicity/hydrophobicity by adjusting PTFE content in CB & graphite-PTFE CL and avoiding calcination under atmospheric conditions [273,274]. It was found that the electroactive area was more relevant to the specific surface area of the 3\u201310\u00a0nm mesopores rather than the total BET surface area, and the electroactive area decreased from 41\u00a0cm2 gcat\n\u22121 to 19\u00a0cm2 gcat\n\u22121 with PTFE increased from 0.57\u00a0g to 4.56\u00a0g. Higher PTFE content led to an excessive supply of H+ and induced the H2O2 decomposition and decreased the hydrophobicity to limit the amount of O2 diffused to catalytic sites. The ABC PTFE0.57 with the lowest PTFE content exhibited super-hydrophobic, highest H2O2 yield of 74.6\u00a0mg\u00a0h\u22121, and highest CE of 84% at 140\u00a0mA.These researchers emphasized the balance among the pore diameter, specific area, and volume, as well as the balance between hydrophilicity and hydrophobicity. In former studies, people devoted themselves to increasing the surface area together with the hydrophilicity of the electrode by numerous treatments [192,197,208,214,275]. However, conflicting results were obtained in our research. Compared with the coating or sparging method, the rolling method was an advanced method for catalyst layer fabrication because it could load more active sites onto the unit area [233]. On the one hand, higher active sites density improves the activity of the cathode. On the other hand, excessive active sites would further reduce H2O2 in the porous rolling cathode. Dissimilar to the immersed electrodes, our rolling ABC mainly utilized the O2 from the air rather than the dissolved O2 in the electrolyte by aeration. Consider the two-electron ORR need electron, proton, and O2 as reactants, and each of those is from solid, liquid, air, respectively, the solid-liquid-air three-phase equilibrium inside the porous catalyst layer is critical to the high performance of the electrode. Excessive pursuit of hydrophilicity in the cathode would induce the three-phase interfaces out of balance and engender the decrease of H2O2 production. Recently, the hydrophobicity property of the electrode has attracted a growing number of research, and H2O2 yield, as well as stability of the newly developed hydrophobic/super-hydrophobic/super-aerophilic electrode, have been significantly improved [276\u2013278]. Some scholars also preferred to use more hydrophobic materials as substrates for GDE [124].Based on our optimized highly efficient ABC, multiple electrochemical systems have been built for various wastewater (cyanobacterial boom water [279], formaldehyde-containing wastewater [280], phenolic wastewater [163], antiviral drug-containing wastewater [281]) treatments as well as sterilization and disinfection [274]. In conclusion, our series of research demonstrated highly efficient production of H2O2 via two-electron ORR can be realized by rolling hybrid carbon cathodes without using noble metals or other complex chemical promoters. The cheap commercial raw carbon materials combined with an easy manufacturing process makes the rolling ABC potential for large-scale application of in situ electrosynthesis of H2O2.In recent years, the active gas diffusion electrode has attracted growing attention, and multiple electrodes were developed with various names. rGO@graphite-based air diffusion cathode (rGO@GADC) was applied in an electro-UV/H2O2 system for penicillin sodium degradation to remove 91.1% of penicillin sodium with 56.8% of mineralization current efficiency [282]. A novel \u201cfloating cathode\u201d with half submerged inside the electrolyte synergistic utilized gaseous O2 from ambient air and the dissolved O2 in the electrolyte. With the formation of TPIs, the optimized floating GF electrode realized a H2O2 yield of 13.3\u00a0mg\u00a0h\u22121 with CE of 21% at 100\u00a0mA, which was 4.3 times of the submerged GF electrode [283]. A super-hydrophobic natural air diffusion electrode (NADE) was developed by coating CB onto CF followed by calcination. After the catalyst loading optimization, H2O2 yield reached 518.5\u00a0mg\u00a0h\u22121 with CE of 66.8% at 1200\u00a0mA [277,284]. In the catalysis materials fabrication field, researchers are also beginning to use self-diffusing substrates for material modifications to reduce electrode complexity [285,286].The electrosynthesis of H2O2 via two-electron ORR provides an alternative to the mature AO process or the emerging direct synthesis and photo-catalysis. The increased usage and decreased cost of renewable electricity will transform the chemicals industry. However, current developments of the cathode are still limited by at least four major challenges, including designing catalyst materials with high activity and selectivity, establishing theoretical calculation models closer to the actual experiments, fabricating materials and electrodes by simple and low-cost methods, maintaining stability over the long-term operation.\n\n1)\nDesigning catalyst materials with high activity and selectivity\n\n\nDesigning catalyst materials with high activity and selectivityTwo major challenges, namely improving activity and selectivity, need to be addressed before carbon-based electrocatalysts can compete with the current state-of-the-art. There is plenty of scope for the improvement of pristine catalyst materials via changing the substrate composition, architecture & defects, and surface property. However, the catalysis mechanism and critical active sites for ORR on carbonaceous electrocatalyst are still confusing and controversial [138,287]. There are remaining debates and discrepancies on the intrinsic properties of the catalytic sites, the effect of heteroatom/functional groups/defect, micro/mesoporous, hydrophilic/hydrophobic nature of the materials to the selectivity to the two-electron ORR process. This ambiguity in determining the true nature of two-electron ORR active sites in those carbon-based catalysts hinders the development of efficient catalysts. The current cutting-edge research is to identify the influence of a single factor on ORR catalytic activity and selectivity. Further development is still primarily based on trial-and-error approaches until now [288], and it is still difficult to realize the controllable synthesis of various defective or doping carbon materials. Meanwhile, new mechanisms are emerging continuously: In contrast to the ordinary ad-O2 mechanism, Chai, et\u00a0al. [39] found O2 adsorption is not required in the new mechanism. Instead, the H atom on carbon catalyst is abstracted by O2 molecule to generate either a HO2\n\u2212 ion or a HOO\u00b7 radical and thus generate H2O2.\n\n2)\nEstablish a theoretical calculation model closer to the actual experiments\n\n\nEstablish a theoretical calculation model closer to the actual experimentsIn most studies, researchers have reported to successfully develop ideal two-electron ORR catalysts based on the RRDE or RDE results when the electron transfer numbers of catalysts were close to 2. However, there is a significant leap from pure material testing to realistic electrode operation that is needed. The (R)RDE measurements are in O2 saturated, high electrolyte content solution together with almost unlimited mass transfer. These idealized systems only provide an upper boundary to the CE to H2O2 at most, while practical equipment tends to underperform. On the one hand, the internal channels and pore structures of the final fabricated electrode are relatively different from those of a pure carbon material or testing modes on the (R)RDE. On the other hand, even with pure O2 aeration and magnetic stirring, the O2 supply and mass transfer in 100\u20131000\u00a0mL level real reactors cannot compare with the conditions of RRDE. At this point, the reference value of the DFT model and (R)RDE results decrease. Such phenomena require modeling experiments in order to predict or represent the real equipment conditions more accurately.\n\n3)\nFabricate materials & electrodes by simple and low-cost methods\n\n\nFabricate materials & electrodes by simple and low-cost methodsIt should be noted that catalytic activity and the selectivity of novel catalysts have only exhibited a marginal increase while the preparation methods become more intricate, making the materials deviate from the target of more cost effective approaches. For many of the aforementioned high-performance catalysts, it is reported that a lower catalysts loading engenders the active sites sparse distribution, which decreases the probability of H2O2 further reduction. The selectivity toward H2O2 increases while decreasing the loading amount [87,93]. Typically, catalyst loading density on the RRDE was controlled at one hundred \u03bcg cm\u22122 level. Considering the mass transfer limitations in practical devices, catalyst loading density on the real electrode should be lower to keep high selectivity. This calls for the highly precise electrode fabrication method.In the future development, a simple fabrication method with low costs should be another critical criterion in designing catalysts and fabricating electrodes.\n\n4)\nMaintaining stability over the long-term operation\n\n\nMaintaining stability over the long-term operationExcept for catalytic activity and selectivity, the practical value of any material or electrode also relies on its long-term stability. Wang et\u00a0al. [198] found the performance of the OFGs modified cathode was reduced after 10-times continuous runs, which was ascribed to the cathode structure destruction and OFGs content decrease due to the H2O2 oxidation. This illustrated the poor stability of the cathode in the long-term operation [289]. Meanwhile, the stability tests in most previous research are far from enough. Presently, researchers often document the stability of new fabricated electrodes by showing negligible changes in current response or H2O2 yield after 5\u201320\u00a0h of operation [87,101,102,118,196]. These are not convincing results to obtain a stable performance conclusion. Active materials should not be proven stable until they are subjected to more rigorous electrosynthesis trials conducted over hundreds or even thousands of hours. In the most recent research, Cao, et\u00a0al. [290] presented a highly hydrophobic architecture GDE consisting of densely distributed N-doped carbon nano-polyhedra, thus enabling the 200\u00a0h durable electrolysis at 100\u00a0mA\u00a0cm\u22122. Li et\u00a0al. [291] evaluated the feasibility of electrochemical H2O2 production with CB-PTFE GDE. The results showed that the GDE could maintain high CE (>85%) as well as low energy consumption (<10\u00a0kWh per kg H2O2) for about 1000\u00a0h. This research suggests that electrochemical H2O2 production with GDE holds great promise for the development of compact treatment technologies. In the following exploratory research, the long-term stability and decay mechanism of materials, as well as electrodes in the EAOP systems, also need special attention. In our recent research [163], although the rolling ABC showed good stability under the condition of generating H2O2 (H2O2 yield decreased 17.8% after 200\u00a0h of operation), the electrodes decayed obviously when operated in the EAOP systems. It was found salt precipitation occurred due to the local alkalinization and enrichment of Na+, which would cause the block of the active sites and mass transfer channels. Meanwhile, the \u00b7OH generated in the EAOP system would cause damage to the carbonaceous electrode by adding defects and oxygen-containing functional groups onto the electrode during the non-selective oxidation. Four electrode performance decay factors were illustrated during the synthetic phenol wastewater degradation. For actual wastewater treatment, the operating life of the cathode in such conditions will only be shorter because the components of sewage are more complex and diverse. Thus, lifetime experiments under different conditions with longer testing times are suggested to analyze the correlations of physicochemical properties and catalyst/electrode performance decay. When the decay mechanism of the electrode performance in long-term operation is clear and definite, it can further guide the development and upgrading of the new long-life electrodes [292,293].The development of carbon-based materials should be combined with theoretical studies, regarded as a requisite aid for catalysts designed or electrode modification to tune ORR selectivity to H2O2. In particular, the models which can better reflect the performance of catalysts in realistic devices will be more popular. To achieve these ends, further studies of the fundamental principles is needed to fully understand the origin of activity enhancement.Although multiple theoretical simulations and physicochemical techniques have been applied to reveal the catalytic mechanism of various carbonaceous materials, it's still hard to distinguish the nature of active sites of two-electron & four-electron ORR at the current technology state. More advanced characterization techniques and sophisticated experimental design are needed to distinguish the active sites for two-electron and four-electron pathways. Pinpointing such sites or chemical motifs would have guiding significance for both two-electron and four-electron carbon-based catalyst designs in the future. Once the two-electron active sites are determined, these sites could be purposefully increased to enable more efficient two-electron ORR or be eliminated from catalysts where four-electron ORR is required.The ORR needs O2, protons and electrons with mole ratio of 1:4:4 (four-electron pathway) or 1:2:2 (two-electron pathway) as reactants, from liquid, gas, and electrode, respectively. The active sites that stay in liquid-gas-solid TPIs could efficiently catalyze the ORR [233,294,295]. The accessibility of the active sites to O2 molecules is critical, but usually deficient due to the low O2 solubility in the realistic aqueous solution. Therefore, series electrodes styles, such as 3D particle electrodes [167], \u201cfloating\u201d electrodes [283,296], and GDE, were invented to optimize the catalytic interface. Recently, scholars tried to accelerate the gas diffusion or tune water distribution to create more adequate TPIs inside the porous electrodes by porosity control and microarchitecture engineering. Therefore, superaerophilic CNT-array electrodes [294], superwetting electrodes [297], and breathing-mimicking electrodes [298] were invented to improve four-electron ORR activity. It could be observed from Eqs. (26) and (27) that the O2 demand of a two-electron pathway is twice that for a four-electron reaction at the same Faraday electron flux and proton supply, which means O2 supply is more vital. More simple surface/structure engineering techniques need to be developed in the future to enhance overall catalytic performances in H2O2 production.\n\n(26)\n4H+\u00a0+ 4 e\u2212\u00a0+ 1 O2 \u2192 2H2O\n\n\n\n\n(27)\n4H+\u00a0+\u00a04 e\u2212\u00a0+\u00a02 O2 \u2192 2H2O2\n\n\n\nProcess optimization and cost-efficiency are at the core of a suitable treatment strategy [299]. From the engineering perspective, reducing the cost to an acceptable level is a prerequisite for the application of this technology. According to Yang et\u00a0al.\u2018s calculation [12], the total costs per mole of H2O2 (C\ntotal) via two-electron ORR can be calculated and expressed as the sum of two parameters, electricity costs (C\nelectricity) and cathode costs (C\ncathode) (Eq. (28))\n\n\nC\ntotal\u00a0=\u00a0C\nelectricity\u00a0+\u00a0C\ncathode\n\n\n\n\n\n= p\nelectricity\nUIt\u00a0+\u00a0p\ncathode\nS\n\n\n\n\n\n(28)\n= p\nelectricity\nUnF/\u03bb\n\nFE\n\u00a0+\u00a0p\ncathode\nnF/jt\u03bb\n\nFE\n\n\n\nWhere, p\nelectricity represents the cost per unit energy of electricity ($ J\u22121), U stands for the cell potential (V), n\u00a0=\u00a02, which represents the electrons transfer number for generating H2O2, F stands for Faraday constant (96486\u00a0C\u00a0mol\u22121), and \u03bb\n\nFE\n is the Faradaic current efficiency. Meanwhile, p\ncathode is the capital cost per unit cathode area ($ cm\u22122), j represents the current density (A cm\u22122), and t is the total operating time of the cathode over its lifetime (s). As \u03bb\n\nFE\n stands for the denominator for electricity costs as well as cathode costs, it is clear that high CE plays a significant role in the performance of the electrode as well as the economic efficacy of the process. Meanwhile, in the cathode costs part, decreasing the numerator p\ncathode and increasing cathode lifetime t can also reduce the overall cost of the H2O2 synthesis, which highlights the relevance of costs of raw materials and preparation process together with electrode stability.Except for stability [163], longer-term goals for H2O2 electrosynthesis should focus on scalability: moving from benchtop experiments to syntheses on pilot or even industrially relevant scales [29]. Rapid H2O2 accumulation to desired concentration in large volume aqueous solution is the prerequisite of up-scaling [233]. Therefore, a more realistic aim would be to increase the current density on the electrodes in the H2O2 electrosynthesis without lowering selectivity. This would improve the synthesis efficiency and decrease the costs (Eq. (26)). Traditional carbon electrodes such as graphite rod/plate/particle, GF, and ACF are inefficient for H2O2 production in bi-dimensional configurations due to the limited dissolved O2 mass transfer in water [300]. In the presently available literature, most studies focus on selectivity on the premise of neglecting the current density. For those electrodes that can only withstand low current density, they have to be fabricated bigger and bigger to increase H2O2 production at higher current flux, which makes them impractical for applications. Recently, multiple GDEs have been reported for faster H2O2 productions with high current efficiencies. These GDEs could maintain the current efficiency over 70% at current densities >25\u00a0mA\u00a0cm\u22122 [301\u2013304]. Based on these electrodes, numerous contaminations were degraded in EAOP systems at pilot/pre-pilot plant scale (2.5\u2013100\u00a0L). Developing electrodes with efficient H2O2 production at large current flux is the inevitable trend in the future. Until now, the largest air cathode so far came from Zhang et\u00a0al. [305]. A 707\u00a0cm2 air cathode was utilized in a 3\u00a0L\u00a0EF reactor for efficient Rhodamine B degradation. The shape and operating conditions of film air cathode determine that it cannot be enlarged without limit. As a result, new electrode and auxiliary equipment structures need to be developed to accommodate modular assembly. Reactors applied for EAOP treatments of wastewater are also needed to be well developed to fulfill the optimum properties of the electrodes [306]. More effort should be dedicated to other aspects such as equipment, scale-up, engineering, and economic issues in applying EAOP technologies to real wastewaters at an industrial scale [300].The various applications of aqueous H2O2 will require a certain H2O2 concentration and may only tolerate a certain pH range [29]. According to RRDE & RDE measurements shown in Tables\u00a01 and 2, all the catalysts showed higher O2 reduction activity in alkaline media than in acid media. However, they were also less selective toward H2O2. In the practical application of H2O2, wastewater treated by AOPs is often in acid media, while pulp and paper bleaching is usually in an alkaline environment. Here it is recommended that researchers could first focus their efforts on H2O2 electrogenerating under acid conditions: AOP treatments only need 0.1% (wt) content of H2O2, which is two orders of magnitude below the demand for bleaching, making the application relatively easier. Moreover, Proton conducting polymeric membranes are much more technologically mature and cheaper than hydroxide conducting counterparts [307].In our review, there are multiple differences in systems, method, expression and, calculations, which make comparisons among the different research difficult. We suggest the following studies can use more unified experimental methods and expressions while avoiding mistakes described below.\n\n1)\nSystem: in the electrogeneration of H2O2 experiments or (R)RDE tests, except for pH value, different types and concentrations of electrolyte were used. Until now, the most commonly used electrolyte in the electrolysis is 50\u00a0mM Na2SO4 with a pH of 3 or 7, while O2 saturated 100\u00a0mM KOH or 500\u00a0mM\u00a0H2SO4 electrolyte is often employed in the (R)RDE tests.\n\n\n2)\nMethod: in general, the cathodic electrogeneration of H2O2 was conducted through galvanostatic mode in a two-electrode system connecting to a DC power or via chronoamperometry at constant potentials in a three-electrode system powered by a potentiostat. We found that a slight change in the distance between the reference electrode and the working electrode would dramatically change in H2O2 yield. Therefore, it is not easy to transversely compare the H2O2 yield at chronoamperometry because the researchers did not specify the distance between the electrodes. Moreover, as shown in Table\u00a03, CE of electrodes operated in chronoamperometry mode are usually unknown and even cannot be calculated by us due to the unpublished current flux. Considering the three-electrode system has many disadvantages, including the high cost of the equipment and the fuzzy parameters in the practical water treatment processes, the electrogeneration of H2O2 through galvanostatic mode in a two-electrode system is recommended in future investigations. As summarized in Tables\u00a01 and 2, in the RDE and RRDE tests, some parameters like onset potential, the definition of onset potential, and the potential range of calculated n were not given by authors. This would also bring troubles for readers, which should be avoided in the future.\n\n\n3)\nExpression: various expressions were utilized to describe the H2O2 yield in different studies, including mg L\u22121 h\u22121, mg h\u22121 cm\u22122, \u03bcM h\u22121, mmol h\u22121 gcat\n\u22121. The disunity of units makes direct comparison different. Although we convert the most H2O2 yield into mg h\u22121 in this review, there are still several results that cannot be normalized due to the incomplete system parameters (Table\u00a03). Furthermore, in some research, the newly fabricated or modified electrodes were directly utilized in the EAOP systems. The target pollutant removal efficiency/TOC removal efficiency was the only indicators to evaluate the electrodes, and the H2O2 yield of the cathodes were not mentioned. In our point of view, H2O2 yield expressed in the form of \u201cmg h\u22121\u201d could visualize the performance, and it would not be affected by solution volume and electrode area. Furthermore, the CE at a certain current is the most significant indicator of the cathode performance. We highly recommend the publishing of H2O2 yield together with the CE and current in the future papers. Meanwhile, with the development of materials, electrodes, and the continuous improvement of material requirements, more advanced and rational parameters or criteria are welcomed to be proposed in the future.\n\n\n4)\nCalculation: there is also divergence in the reactive area of the immersed cathodes. Some calculated the current density (mA cm\u22122) or H2O2 yield (in the form of mg h\u22121 cm\u22122) via dividing the projection area of the electrode. However, another group of researchers [193,200,201], including us [279], all believe the effective area should be at least twice the projected area because each side of the electrode is in contact with the electrolyte. This error in the arithmetic will double the calculation result and mislead the readers, which should be avoided in the future.\n\n\nSystem: in the electrogeneration of H2O2 experiments or (R)RDE tests, except for pH value, different types and concentrations of electrolyte were used. Until now, the most commonly used electrolyte in the electrolysis is 50\u00a0mM Na2SO4 with a pH of 3 or 7, while O2 saturated 100\u00a0mM KOH or 500\u00a0mM\u00a0H2SO4 electrolyte is often employed in the (R)RDE tests.Method: in general, the cathodic electrogeneration of H2O2 was conducted through galvanostatic mode in a two-electrode system connecting to a DC power or via chronoamperometry at constant potentials in a three-electrode system powered by a potentiostat. We found that a slight change in the distance between the reference electrode and the working electrode would dramatically change in H2O2 yield. Therefore, it is not easy to transversely compare the H2O2 yield at chronoamperometry because the researchers did not specify the distance between the electrodes. Moreover, as shown in Table\u00a03, CE of electrodes operated in chronoamperometry mode are usually unknown and even cannot be calculated by us due to the unpublished current flux. Considering the three-electrode system has many disadvantages, including the high cost of the equipment and the fuzzy parameters in the practical water treatment processes, the electrogeneration of H2O2 through galvanostatic mode in a two-electrode system is recommended in future investigations. As summarized in Tables\u00a01 and 2, in the RDE and RRDE tests, some parameters like onset potential, the definition of onset potential, and the potential range of calculated n were not given by authors. This would also bring troubles for readers, which should be avoided in the future.Expression: various expressions were utilized to describe the H2O2 yield in different studies, including mg L\u22121 h\u22121, mg h\u22121 cm\u22122, \u03bcM h\u22121, mmol h\u22121 gcat\n\u22121. The disunity of units makes direct comparison different. Although we convert the most H2O2 yield into mg h\u22121 in this review, there are still several results that cannot be normalized due to the incomplete system parameters (Table\u00a03). Furthermore, in some research, the newly fabricated or modified electrodes were directly utilized in the EAOP systems. The target pollutant removal efficiency/TOC removal efficiency was the only indicators to evaluate the electrodes, and the H2O2 yield of the cathodes were not mentioned. In our point of view, H2O2 yield expressed in the form of \u201cmg h\u22121\u201d could visualize the performance, and it would not be affected by solution volume and electrode area. Furthermore, the CE at a certain current is the most significant indicator of the cathode performance. We highly recommend the publishing of H2O2 yield together with the CE and current in the future papers. Meanwhile, with the development of materials, electrodes, and the continuous improvement of material requirements, more advanced and rational parameters or criteria are welcomed to be proposed in the future.Calculation: there is also divergence in the reactive area of the immersed cathodes. Some calculated the current density (mA cm\u22122) or H2O2 yield (in the form of mg h\u22121 cm\u22122) via dividing the projection area of the electrode. However, another group of researchers [193,200,201], including us [279], all believe the effective area should be at least twice the projected area because each side of the electrode is in contact with the electrolyte. This error in the arithmetic will double the calculation result and mislead the readers, which should be avoided in the future.In summary, with the comprehensive and critical review, we hope to attract scholars from different research fields and use their knowledge to push electrosynthesis of H2O2 to pilot or even industrially relevant scales.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 financially supported by the National Natural Science Foundation of China (No. 52070140), the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. HC202151) and the Postdoctoral Science Foundation of China (2021M702439). Jingkun An also thank the scholarship from the Shanghai Tongji Gao Tingyao Environmental Science & Technology Development Foundation (STGEF).", "descript": "\n                  Hydrogen peroxide (H2O2) is an efficient oxidant with multiple uses ranging from chemical synthesis to wastewater treatment. The in-situ H2O2 production via a two-electron oxygen reduction reaction (ORR) will bring H2O2 beyond its current applications. The development of carbon materials offers the hope for obtaining inexpensive and high-performance alternatives to substitute noble-metal catalysts in order to provide a full and comprehensive picture of the current state of the art treatments and inspire new research in this area. Herein, the most up-to-date findings in theoretical predictions, synthetic methodologies, and experimental investigations of carbon-based catalysts are systematically summarized. Various electrode fabrication and modification methods were also introduced and compared, along with our original research on the air-breathing cathode and three-phase interface theory inside a porous electrode. In addition, our current understanding of the challenges, future directions, and suggestions on the carbon-based catalyst designs and electrode fabrication are highlighted.\n               "}
{"full_text": "The industrial revolution and the mass consumption of coal, petroleum and natural gas causes the depletion of conventional fossil-based resources. The continuous consumption of these fossil fuels causes energy shortage and global warming [1\u20133]. To mitigate the reliance on these resources, many countries have been planning to develop alternative fuels. Biofuel becomes one of the promising options for clean energy and sustainable sources [4,5]. Besides its biodegradability, zero sulphur, a minimal amount of carbon dioxide emission and non-toxic [6,7], it also has a similar characteristic of fossil fuel, with better cetane number, flash point and calorific value [8,9]. Many researchers have been involved in developing green diesel production that can be derived from edible, non-edible oils and fats efficiently, economic, and eco-friendly processes [10]. The deoxygenation reaction can be the potential pathway to enhance the physicochemical properties of these edible and non-edible oils close to the diesel fuel quality standards [2,3]. The final product obtained from the reaction comprises gas, coke, organic liquid product and water [11,12]. This reaction process has advantages in great flexibility in the feedstock selection, characteristic of pyrolysis products associated with petroleum products, fast conversion gradient, and the probability to industrial scale-up for biofuel production [13,14].Nowadays, many heterogeneous solid catalysts have been examined for biofuel production via catalytic cracking such as ZrO2\n[2], MgO/Activated carbon [15], Mg-HZSM-5 [16], Ni-Mg/ZSM-5 [17], biochar [18], etc. Many selective solid catalysts become the best option for catalytic cracking of vegetable oils to improve the yield and reduce the cost of liquid fuels owing to the characteristics of these catalysts which are recyclable, regenerable and eco-friendly [1,2]. Zeolites are considered promising catalysts due to their greater cracking ability, unique porous structure, the presence of higher acidity and tremendous inherent stability [16,19]. However, numerous researchers are moved to ordered mesoporous materials due to their advantages such as high thermal stability, ease of surface modification, tolerable pore size, huge surface area and inert nature [20]. Moreover, the improvement on the physicochemical properties of the catalysts obtaining stability during a long period of time and outstanding catalytic conversion has been focused on. These might involve the combination of different transitions metals, the use of basic promoters of supports with oxygen storage capacity, and the optimization of preparation pathways of the catalysts.In biofuel production, supported heterogeneous catalysts are well known for their high activity and reusability. SiO2\n[21], Al2O3\n[22], TiO2\n[23], SnO2\n[24], zeolites [25] and other excellent supporting materials provide greater surface area, superior and long stability. Metal oxide catalysts have emerged as essential in most refining and petrochemical processes, as well as in the production of speciality chemicals and, more recently, in the improvement of environmental issues, particularly for depollution by increasing reaction selectivity to avoid unusable by-products [26]. Alkali or alkaline metal-supported solid catalysts are best known as solid base catalysts. Based on Hafriz et al.\n[27], the base catalyst is expected to obtain cracking oils with low acid values and good quality of oil in terms of good cold flow properties. Potassium will act as a chemical promoter which may increase the activity of the catalyst by an order of magnitude [28]. Besides that, potassium suppresses the production of methane and improves effective hydrocarbon selectivity [29]. A higher level of alkali oxide such as potassium oxide also has an adverse effect on hydrolytic stability when exposed to high humidity and temperature [30].Industrial players are concerned about catalyst deactivation since the cost for new catalyst and process shutdown require billions of dollars per year. The loss activity of the catalyst can be predicted for most processes and yet the drastic consequences must be avoided. Therefore, to ensure a good conversion process, the deactivation issues like the extent rate and reactivation must be determined. The deactivation is happened due to the formation of coke and blocking the micropore opening that is caused by the generation of polycyclic aromatic hydrocarbons and hence, lowers the catalytic activity [17]. A vital parameter to recover the catalytic activity is the capacity of the catalyst to be regenerated. Cleaning the coke deposit from the surface and the pores of the catalyst are some of the regeneration courses involved without destroying or modifying the structure of the catalyst [31]. From the economical aspect, the cost of regeneration is lower than by obtaining a fresh catalyst. From the environmental point of view, it is a spent catalyst that can form toxic metal compounds in the environment. Hence, it is an environmentally friendly option by regenerating the catalyst as an alternative to disposing of it as solid waste [12,32].The comparative study on the performance of K2O/SiO2 and dolomite catalysts, as well as reusability and regenerability study of the deoxygenation using K2O/SiO2 catalyst on deoxygenation reaction, has not yet been reported. Herein, the restrictions of the above mentioned have sparked the investigation on the comparative study, reusability and regenerability study of the K2O/SiO2 catalyst on deoxygenation of WCO. The final product from each cycle was analysed by using GC\u2013MS for chemical composition study.The potassium oxide supported silicon dioxide (K2O/SiO2 or denoted as KSi) catalyst with purity\u00a0>\u00a099 % was generously provided by Pakar Management Technology (M) Sdn. Bhd. Dolomite was purchased from Northern Dolomite Sdn. Bhd. (Perlis, Malaysia) [33] and was calcined at 900\u00a0\u2103 for 4\u00a0h [34]. The detailed characterization of calcined dolomite catalyst could be referred to Hafriz et al.\n[27,34]. Nitrogen gas (N2) was supplied from Biogas Sdn. Bhd. The standard for gas chromatography (GC) analysis namely liquid product alkane and alkene standard solution (C8-C20) with an internal standard of 1-bromohexane with purity (98%) were purchased from Sigma Aldrich. n-hexane for GC analysis (Merck) with purity\u00a0>\u00a098 % was utilized. The feedstock of waste palm cooking oil (WCO) with 82.8 % oxygenated compound and 17.2 % hydrocarbon compound (obtained from GC\u2013MS analysis) was collected from a cafeteria in Serdang, Selangor. The WCO was preheated at 100 \u2103 (30\u00a0min) to remove moisture content. However, as reported by Abdulkareem et al.\n[35], the moisture content in WCO will increase the decomposition of triglycerides into Free Fatty Acid (FFAs) and directly promote the oxygenation reaction under examination of the WCO deoxygenation under a series of FFAs (0\u201320 %) and water content (0.5\u201320\u00a0wt%). The main composition of WCO using GC\u2013MS analysis has been shown in Fig. 1\n. As tabulated in Fig. 1, the main groups of compounds present in WCO were carboxylic acid (59.13 %) and ester (14.58 %). There were three dominant peaks observed in WCO composition which is oleic acid (26.31 %), palmitic acid (20.64 %) and stearic acid glycidyl ester (9.12 %). As reported by Hafriz et al.\n[36], carboxylic acids were maintained even after deep frying as the boiling point of oleic acid is 358.85 \u2103 and for palmitic acid is 351 \u2103. These carboxylic acid and ester content are the major indicators for green diesel produced in terms of quality properties via catalytic deoxygenation.In order to identify the crystallography and dispersion states of the synthesized catalyst, an X-ray diffraction (XRD) analysis was performed. The XRD analysis was conducted using a Shimadzu diffractometer (model XRD-6000). A Brunauer-Emmett-Teller (BET) method was used to evaluate the specific surface area and pore distribution of the synthesized catalysts. The analysis was performed using Thermo-Finnigan Sorpmatic 1990 series with an N2 adsorption/desorption analyzer in a vacuum chamber at \u2212196\u00a0\u00b0C. The samples were degassed overnight at 150\u00a0\u2103 to remove moisture and foreign gases from the catalyst surfaces. Temperature-programmed desorption with CO2 as probe molecules has been used to investigate the basicity of the synthesized catalysts. The CO2-TPD analysis was carried out by using Thermo Finnigan TPD/R/O 1100 instrument equipped with a thermal conductivity detector (TCD). In the pretreatment step, 0.05\u00a0g of sample was pre-treated under N2 gas flow for 30\u00a0min at 250 \u2103, followed by CO2 gas at ambient temperature for 1\u00a0h to allow adsorption of CO2 onto the surfaces. The excess CO2 was subsequently flushed with N2 gas at a flow rate of 20\u00a0ml/min for 30\u00a0min. The desorption of CO2 from the basic sites of the catalyst was detected by TCD under helium gas flow (30\u00a0ml/min) from 50\u00a0\u2103 to 900\u00a0\u2103 for 30\u00a0min. The area under the graph (CO2 desorption peak) provided will determine the basicity of the synthesized catalyst. The morphological characteristics and elemental analysis of the catalysts were investigated by using Scanning Electron Microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX). The SEM images were observed through SEM LEO 1455 VP electron microscope with acceleration voltages of 30\u00a0kV. For the sample preparation, the catalyst powder was dispersed on an aluminium sample holder and glued by using double-sided tape. Then, it was coated with a thin layer of gold which is a type of conducting material using BIO-RAS Sputter Coater. Micrographs were recorded at various magnifications. The compositional analysis of the solid base catalyst was carried out by EDX that gives the flexibility of image control. The metal elements peaks were detected in the EDX spectra and the atomic percentages of the constituent elements presence in the fresh catalyst and regenerated catalyst were determined in the solid base catalyst.The deoxygenation reactions were performed in a fractionated cracking system as illustrated in Fig. 2\n. 150\u00a0g of WCO was preheated before being poured into the round bottom flask. 5\u00a0wt% catalysts were added to the reactor flask. The reaction was carried out under nitrogen (N2) flow with a flow rate of 150\u00a0cm3/min to eliminate the air inside the system. The sample was heated for 30\u00a0min until the temperature reaches 390\u00a0\u00b1\u00a05\u00a0\u00b0C with a heating rate of 100\u00a0\u00b0C/min. The liquid product was collected in the collecting flask. The catalytic performance of KSi as a deoxygenation catalyst was compared with thermal deoxygenation (without catalyst) and calcined dolomite in terms of conversion of WCO, the yield of pyrolysis oil as well as the selectivity of fuel range. Then the reusability and regeneration studies of KSi catalyst was also performed. The catalyst was reused for 5 runs of reactions without any treatment. Regenerability of catalyst took place after the completion of the reusability cycles of five runs. The catalyst was collected at the end of the cycle and regenerated to remove the coke deposited on the catalyst and activate the catalyst before being reused back for five consecutive runs by calcination in the furnace. After one complete reusability cycle (five consecutive runs), the remaining reactants at the bottom of the flask (coke\u00a0+\u00a0catalyst) were collected in the crucible and heated in the furnace at a temperature of 700\u00a0\u2103, a heating rate of 10 \u2103/min for 4\u00a0h with nitrogen flows continuously. After that, the catalyst was heated again at a temperature of 1000\u00a0\u2103 with a temperature rate of 10\u00a0\u2103/min for 4\u00a0h at open airflow, the catalyst was denoted as KSi-RG1. The reaction was continued with 5 runs of reactions by using KSi-RG1. The spent catalyst was again regenerated at the same conditions and denoted as KSi-RG2. The reaction was further continued for another 5 runs of reactions by using KSi-RG2. The product for each experiment was collected once the system cooled down at room temperature and analysed by using GC\u2013MS. The conversion of WCO was measured by using Equation\n\n(1)\n\n[16]:\n\n(1)\n\n\n%\nC\no\nn\nv\ne\nr\ns\ni\no\nn\n=\n\n\n\nm\na\ns\ns\no\nf\nW\nC\nO\n-\nm\na\ns\ns\no\nf\nc\no\nk\ne\n\n\n\nmassofWCO\n\n\nx\n100\n%\n\n\n\n\nThe liquid products were quantitatively analysed by Shimadzu GC-14B equipped with ZB-5MS column (30\u00a0m length\u00a0\u00d7\u00a00.25\u00a0mm inner diameter\u00a0\u00d7\u00a00.25\u00a0\u03bcm film thickness) in a split mode. The oven temperature was set to hold at 40\u00a0\u00b0C for 3\u00a0min, the ramping rate at 7\u00a0\u00b0C/min to reach 300\u00a0\u00b0C and holding temperature at 300\u00a0\u00b0C for 5\u00a0min. The injector temperature was set at 250\u00a0\u00b0C and He gas flow rate of 3.0\u00a0ml/min. The liquid product was dissolved with n-hexane. A different class of compounds especially heavy hydrocarbon (C21-C24) and oxygenated compounds obtained were recognized using the National Institute of Standards and Testing (NIST) library [37]. The GC\u2013MS results were measured using a peak area normalization method based on peak area percentages of the identified components. The hydrocarbon yield (Y) was calculated using Equation\n\n(2)\n\n[38\u201339].\n\n(2)\n\n\nY\n=\n\n\n\u2211\n\na\ni\n\n+\n\u2211\n\na\nj\n\n\n\n\u2211\n\na\nk\n\n\n\n\n\n\nWhere ai\n\u00a0=\u00a0Area of alkene (C8-C20), aj\n\u00a0=\u00a0Area of alkane (C8-C20), ak\n\u00a0=\u00a0Area of the product.The yield of chemical groups in the oxygenated compound (Z) was determined using Equation\n\n(3)\n\n[38].\n\n(3)\n\n\n\nZ\n\nproduct\n\n\n=\n\n\na\no\n\n\n\u2211\n\na\nt\n\n\n\nx\n100\n%\n\n\n\nWhere Zproduct\n\u00a0=\u00a0Yield of organic compound (%), ao\n\u00a0=\u00a0Area of the desired organic compound, and at\n\u00a0=\u00a0Total area of organic compounds.The percentage removal of the oxygenated compound was calculated by using Equation\n\n(4)\n\n[34,36].\n\n(4)\n\n\n\n\n\u03a3\nA\nr\ne\na\no\nf\no\nx\ny\ng\ne\nn\na\nt\ne\nd\nc\no\nm\np\no\nu\nn\nd\n(\nW\nC\nO\n)\n-\n\u03a3\nA\nr\ne\na\no\nf\no\nx\ny\ng\ne\nn\na\nt\ne\nd\nc\no\nm\np\no\nu\nn\nd\n(\nP\nO\n)\n\n\n\u03a3\nA\nr\ne\na\no\nf\no\nx\ny\ng\ne\nn\na\nt\ne\nd\nc\no\nm\np\no\nu\nn\nd\no\nf\nW\nC\nO\n\n\nx\n100\n%\n\n\n\n\nAs shown in Fig. 3\n, the different physical appearance of solid base catalyst of KSi in fresh, spent, and regenerated catalyst after first reusability cycles were observed. The fresh catalyst is shown in Fig. 3a was uncalcined as it can be directly used for the deoxygenation reaction of WCO. As mentioned in Table 1\n, the diameter pore size of the fresh catalyst was 15.93\u00a0nm which classified it as a mesoporous catalyst with high base properties. As shown in Fig. 3a solid base catalyst which was before use in the deoxygenation of WCO has white colour. As reported by Ooi et al.\n[40], silicon dioxide (SiO2) support catalyst is a refractory oxide with a white and colourless appearance. However, after the reaction took place, the colour of the solid base catalyst was changed into black as shown in Fig. 3b. A black coke produced during deoxygenation reaction was deposited on the surface of the catalyst causing them to be a deactivated catalyst. Thus, catalytic activity was reduced and conversion of WCO could not occur on the surface of the active site after 5 times reusability without any treatment. However, after regenerating at 1000\u00a0\u00b0C in open airflow, the colour was changed into grey as shown in Fig. 3c. Based on Bayramo\u011flu et al.\n[41], calcination at high temperatures is a more appropriate and simple method to regenerate the activity of catalysts with no waste solvent or solid waste product formation. The deposited coke, organic compound and adsorbed impurities on the spent catalyst were expected to be removed or diminished. This was directly restoring the activity of the catalyst for the next regeneration.\nTable 1 shows the physicochemical properties of the base solid catalyst of K2O/SiO2 or KSi and calcined dolomite (CD). The structure of the base solid catalyst could be observed based on the XRD pattern in terms of particles arrangement especially in internal structure. As shown in Table 1, the KSi catalyst was the amorphous structured catalyst with lacked an ordered internal structure of particles and were randomly arranged which is seen in the SEM image result (Fig. 4\na. The crystallize size of K2O was present at 2\u03b8\u00a0=\u00a028.27\u00b0 with JCPDS File:00\u2013025-0626. This showed that SiO2 was able to interact well with potassium by enhancing the crystallinity and stability of the synthesized catalyst. While calcined dolomite catalyst was the crystalline catalyst with the intense peak of CaO (54.39\u00b0) and MgO (62.39\u00b0). The particles of the crystalline catalyst were in an ordered structure of circular shape particles and a repeating pattern. The surface area of the CD catalyst was bigger than the KSi catalyst as mentioned in Table 1. This would provide a more active site for the substrate molecules to bind and undergo a chemical reaction onto a CD catalyst. Through the pore diameter mentioned in Table 1, it showed that KSi was a mesoporous catalyst (2\u201350\u00a0nm) and CD was a macroporous catalyst (>50\u00a0nm). Based on Hafriz et al.\n[42] and Asyikin-Mijan et al.\n[37] the mesoporous catalyst was an efficient catalyst to promote better decarboxylation (DCO2), decarbonylation (DCO) or hydrodeoxygenation (HDO) pathways. The basicity strength of the catalyst was measured by desorption of carbon dioxide in TPD-CO2 analysis. The CD catalyst desorbed 2872.73 \u03bc atom/g of CO2 at a temperature of 722 \u2103 as compared to the KSi catalyst, which desorbed 303.39 \u03bc atom/g of CO2 at the temperature of 656\u00a0\u2103. The CO2 desorption peak at temperature\u00a0>\u00a0500\u00a0\u2103 showed a relatively high basicity strength in TPD-CO2 analysis. The high basicity properties of catalysts should improve the quality of green fuel produced by lowering the oxygen content through a deoxygenation reaction [34].Scanning electron microscopy (SEM) analysis was performed to examine the morphology of the solid base catalyst (KSi) after 2 times of regeneration process with 15 times reusability and CD catalyst as shown in Fig. 4. Fresh KSi catalyst has gross and non-uniform particle structure as compared to calcined dolomite catalyst having a smooth spherical particle structure with the wide surface of catalyst as shown in Fig. 4\n(a & b). The CD catalyst has an obvious wide pore diameter, in line with XRD analysis which classified it as a macroporous catalyst. However, the macroporous structure catalyst should tend to undergo surface poisoning or pore filling of carbon in coke accumulation due to this big porosity of catalyst. The SEM-EDX result of fresh catalyst, regenerated KSi catalyst after first and second reusability cycle was shown in Fig. 4\n(a,c,d). It could be observed clearly in SEM images that the morphology of the fresh catalyst is different from the regenerated catalyst after the first and second reusability cycles. In a fresh catalyst, there are many tiny aggregated particles on the surface of the fresh catalyst than in a regenerated catalyst. The calcination at a high temperature not only influenced the morphology and structure of the regenerated catalysts, but it also effects on the porosity of the catalyst which became bigger as observed clearly on 1st regenerated (KSi-RG1) catalyst surface (\nFig. 4c). Coke formation and K2O deactivation after 5 times reusability cycle should occur on the catalyst surface resulted in a reduction in the performance of catalyst by lowering the yield of liquid product and affecting the selectivity of oil produced (fuel range). Based on Hafriz et al.\n[36]\n, coke accumulation might be occurred by two reaction pathways: polymerization of aromatic hydrocarbon (Equation 5) or condensation of WCO (Equation 6). While K2O deactivation could have resulted from the reaction of the K2O phase with H2O (Equation 7 and 8) or CO2 (Equation 9) to form potassium carbonate (K2CO3) which is H2O was generated from decarbonylation reaction and CO2 has been produced from decarboxylation reaction. The calcination at 1000 \u2103 should burn off the coke and unwanted impurities on the catalyst surface and recovered the K2O phase formation under calcination at 1000 \u2103.Coke accumulation:\n\n(5)\nPolymerization: Cn-Hn (Aromatics)\u00a0\u2192\u00a0Coke\n\n\n\n\n(6)\nCondensation: WCO\u00a0\u2192\u00a0Coke\n\n\nK2O deactivation:\n\n(7)\nK2O\u00a0+\u00a0H2O\u00a0\u2192\u00a02KOH\n\n\n\n\n(8)\nKOH\u00a0+\u00a0CO2\u00a0\u2192\u00a0K2CO3\u00a0+\u00a0H2O\n\n\n\n\n(9)\nK2O\u00a0+\u00a0CO2\u00a0\u2192\u00a0K2CO3\n\n\n\nBesides that, it also could be seen that the pore structure and pore shape of the regenerated catalyst were reduced even though the overall structure was almost identical. Thus, it slightly reduced the catalytic activity as the pore structure and pore shape decreased. These might be due to the destruction of brittle basicity catalyst structure morphology after 5 times reusability at the high calcination temperature. According to Fatimah et al.\n[43], the texture and surface design of catalyst is important to ensure catalytic activity.The spectrum of x-ray energy at each position on the samples versus counts is evaluated to determine the relative elemental concentration for each element presence present in the fresh, regenerated solid base catalyst (KSi) and calcined dolomite (CD) catalysts as shown in Table 2\n. Table 2 shows that the main element present in KSi was oxygen and silicon. It was in line with metal oxide composition analysis, which showed that SiO2 was the main composition of KSi which act as backbone or support catalyst. Based on Ooi et al. [40], SiO2 support is stable and has been used as good support for metal loaded catalyst in the catalytic reaction. With the advantages in low owing of coke deposition to the very weak acidity present in SiO2 and suitability to be used in hydrogen-free or low hydrogen pressure system, SiO2 support catalyst is more focused in these studies. While the element potassium, K presence showed that potassium oxide, K2O was successfully doped onto SiO2 surface and generated multiphase oxide catalyst with high base properties. K2O was expected to create a more active site, which plays a vital role in the deoxygenation of WCO to produce green diesel. In CD catalysts as shown in Table 2, CaO and MgO were the main composition presence after CaMg(CO3)2 was decomposed at calcination around 900 \u2103 [27]. Based on Lin et al.\n[44] and Asikin-Mijan et al.\n[37], CaO can be acted as a deoxygenation catalyst that could absorb more CO2 either in liquid or solid phase in order to remove oxygen compound via the decarboxylation-decarbonylation mechanism. While MgO in CD catalyst can enhance the strength of dolomite particle structure and at the same time, it plays the important role in increasing the oil quality by reducing oxygen levels [36,45]. As compared to fresh and regenerated KSi catalysts, the impurities element could be observed after 1st and 2nd regeneration of KSi-RG1 and KSi-RG2 catalysts due to the reusability. The elemental analysis in Table 2 revealed that in regenerated catalyst there are foreign elements (Fe, Mg and S) deposited on the surface of the catalyst. This shows that the loss of activity might be due to certain elements deposited on the solid base catalyst during repeated use in the deoxygenation process. Nevertheless, solid base catalyst still could be considered as an effective regenerate catalyst for the cracking of waste cooking oil.Based on Fig. 5\n, the conversion of WCO achieved the highest in the deoxygenation reaction using KSi (62.3 %) as a catalyst followed by thermal deoxygenation (without catalyst) (40.0 %) and CD catalyst (38.3 %). The highest conversion of WCO using KSi catalyst was due to lower accumulation of coke after reaction by measured using mass balance. The main composition of KSi catalyst which is SiO2 has been proven to play a bigger role in lowering the coke deposition due to the weak acidity surface present in SiO2. Besides that, many researchers have admitted that SiO2 as a supported catalyst is suitable for use in an H2-free reaction system. These advantages were also contributed to yielding the highest pyrolysis oil (51.8 %) when using KSi as a catalyst. The coke formation was found to be the highest (61.7 %) when using CD as a catalyst which might be due to macroporous structure catalyst resulted in increasing the possibility of coke deposition onto CD catalyst surface. Coke deposition caused active site coverage and pore-mouth blockage making the core pore network inaccessible to reactants (WCO). This will slightly reduce the catalytic activity of CD catalyst in deoxygenation reaction. Meanwhile, in thermal deoxygenation of WCO (without catalyst), the yield of pyrolysis oil was very low with only 1.9 %. Thus, this finding showed that the yield of pyrolysis oil from catalytic deoxygenation of WCO (with the presence of a catalyst) was enhanced as compared to thermal deoxygenation (without the presence of a catalyst). As reported by Faten et al.\n[46] and Hafriz et al.\n[27]\n, catalytic deoxygenation was faster and more selective than thermal deoxygenation which allows the reaction to happen under mild reaction conditions and hence maximizing the production of pyrolysis oil. As reported by Dewajani et al. [47], in catalytic cracking reactions, the oxygenated compounds which come from thermal cracking diffuse into the pores of catalysts and react with protons in the active site through several reaction pathways such as dehydration, decarboxylation, decarbonylation, and oligomerization. This will result in a low yield of gas products. Furthermore, the thermal deoxygenation of WCO was favoured to produce the highest gas yield as compared to catalytic deoxygenation in line with the result shown in Fig. 5. In order to increase the yield of pyrolysis oil, the catalyst presence is more favoured in the deoxygenation reaction due to the high surface area of the catalyst leading to increased active sites. The active site would provide more adsorption interaction of the reactant molecules into the surface of the catalyst. As reported by Horacek et al. [48], a high surface area of catalyst will improve the diffusion of reagent resulting in a higher deoxygenation degree at the same time it inhibits the breaking of the C\u2013C bonds. In addition, Ooi et al.\n[40] found out that the use of catalysts especially metal oxide catalysts with good chemical stability and high activity is the main support for deoxygenation reaction to occur.The amount of hydrocarbon and oxygenated compound presence in pyrolysis oil generated via deoxygenation of WCO using different catalysts are shown in Fig. 6\n. The higher yield of hydrocarbon compound generated in pyrolysis oil could be observed using calcined dolomite (CD) catalyst (88.3 %) followed by thermal deoxygenation (without catalyst) (82.2 %) and KSi catalyst (52.5 %). Calcined dolomite catalyst yielded high hydrocarbon compound was due to the presence of CaO and MgO, which can remove oxygen compounds by absorbing more CO2 and enhancing the strength of dolomite particle structure in deoxygenation reaction, respectively. Thus, the use of calcined dolomite was a favour to the formation of hydrocarbon which implied higher deoxygenation activity as in line with higher percentage removal of oxygenated compound (85.9 %) followed by in thermal deoxygenation (78.5 %) and KSi catalyst (42.6 %) as shown in Fig. 5. Chiam & Tye [49] and Kwon et al.\n[50] reported that the removal of oxygen contents inside used palm oil is required to improve biofuels quality: increase energy density, reduce viscosity, and stabilize biofuels. The undesired oxygen content especially insoluble impairs engine performance such as fouling of injector, plugging of the fuel filter, sticking of the ring, and formation of deposit in the engine. As reported by Sani et al.\n[51], at high temperatures, decomposition of oil to hydrocarbon is favoured without the presence of a catalyst which is in line with the result of hydrocarbon produced via thermal deoxygenation in Fig. 6. This is an indication that temperature plays a vital role in the production of hydrocarbon from waste cooking oil. However, in the deoxygenation of WCO using KSi catalyst, the hydrocarbon compound produced was lower due to the small pore diameter of the catalyst which inhibits the reaction pathway of deoxygenation to occur. As reported by Hafriz et al.\n[36], a catalyst with a larger pore structure promotes additional deoxygenation pathways including hydrodeoxygenation (HDO), decarbonylation (DCO), and decarboxylation (DCO2) in the deoxygenation of WCO. However, the largest porosity structure of catalyst (macroporous) contributed to the high deposition of coke formation in line with the result in Fig. 5 for calcined dolomite (coke; 61.7 %) and the mesoporous structure of catalyst would be the best catalyst for deoxygenation reaction pathway to occur.The composition profile of pyrolysis oil with different catalysts used was summarised as shown in Fig. 7\n. The figure shows that the main composition of hydrocarbon generated in pyrolysis oil was alkenes and alkanes with the same trend for thermal deoxygenation and different catalyst used. This proves that decarbonylation-decarboxylation of deoxygenation reaction was the main reaction pathway involved. Based on Hafriz et al.\n[36] and Faten et al.\n[46]\n, the decarbonylation reaction would remove the carbonyl group in WCO to produce alkenes by releasing CO and H2O (Equation 10) and the decarboxylation reaction would eliminate the carbonyl group by releasing CO2 to produce alkanes (Equation 11).Decarbonylation reaction:\n\n(10)\nWCO (triglyceride)\u00a0\u2192\u00a0Alkenes\u00a0+\u00a0CO\u00a0+\u00a0H2O\n\n\nDecarboxylation reaction:\n\n(11)\nWCO (triglyceride)\u00a0\u2192\u00a0Alkanes\u00a0+\u00a0CO2\n\n\n\nAs shown in Fig. 7, oxygenated intermediates compounds such as aldehyde, ketones, alcohol, and carboxylic acid were also detected, and this result is in agreement with the work found out by Azman et al.\n[39]. The carboxylic acid was found to be higher in pyrolysis oil generated using KSi catalyst and it could be due to the low basicity properties of KSi catalyst as compared to CD catalyst. The presence of high carboxylic acid in pyrolysis oil could increase the acid number of oil and it would affect cold-flow properties such as cold filter plugging point and freezing point as well as reduce the heating value of the fuel at the same time [52].\nFigure 8\n shows the proposed reaction pathways for deoxygenation of WCO (comprised of oleic acid, palmitic acid and stearic acid glycidyl ester) using KSi catalyst. The role of the catalyst will efficiently remove oxygen molecules while decrease carbon loss to retain the quality of the final fuel product which is the ideal catalytic deoxygenation pathway. The deoxygenation reaction pathways were established based on the distribution of hydrocarbon product using GC\u2013MS analysis with the majority of compound present in pyrolysis oil consisting of pentadecane, C15H32 \u2212 7.44 %, heptadecane, C17H34 \u2212 6.66 %, heptadecane, C17H36 \u2212 2.48 % and the rest were C8-C14 around 24.59 %. It showed that the KSi catalyst will provide a mild extent of deoxygenation efficiency and high selectivity towards the formation of C15 and C17 products. As shown in Fig. 8, the presence of catalyst will facilitate the oxygen removal pathway via the formation of stearic acid (C18:0) from the hydrogenation of oleic acid by the addition of in-situ hydrogen (from water gas shift reaction) and hydrolysis of the carboxylic ester. Carboxylic acid will undergo oxygen removal via deoxygenation reaction in two pathways which are decarboxylation and decarbonylation reaction. In decarboxylation of stearic acid, heptadecane with C17:0 will be formed by releasing CO2. Meanwhile, in the decarbonylation of stearic acid, n-heptadecene with C17:1 will be generated by releasing CO and producing H2O as by-products. Heptadecene could react with in-situ H2 in hydrogenation reaction to form thermodynamically stable heptadecane. This can be observed by intensified peak appearing in chromatography for pyrolysis oil using KSi catalyst. Besides that, palmitic acid presence in WCO composition will simultaneously undergo this deoxygenation reaction. The carbonylation of palmitic acid will produce pentadecene (C15:1) and H2O as a by-product by releasing CO. Pentadecane with C15:0 could be generated via hydrogenation reaction of pentadecane by addition of in-situ H2. Besides that, pentadecane could be formed through the decarboxylation of palmitic acid by releasing CO2. Furthermore, light components (C8-C14) with 24.59 % were also detected in the GC\u2013MS analysis due to mild cracking (C\u2013C cleavage) to the intermediate product n-heptadecene and pentadecene.The composition of green fuel was grouped by the carbon number of the gasoline, kerosene, and diesel fraction as similar to petroleum products and is presented in Fig. 9\n. The selectivity of hydrocarbon products was influenced by catalyst/without catalyst used. As shown in Fig. 9, the selectivity of hydrocarbon products generated using CD catalyst was towards gasoline carbon number which composes of light hydrocarbon with carbon number range C8-C12. The gasoline selectivity was due to the high basicity properties of CD catalyst, and this was in agreement with the finding by Asikin-Mijan et al.\n[37], Ca-based catalysed reaction with high basicity properties rendered higher selectivity towards gasoline products (C8-C12). While using thermal deoxygenation and KSi catalyst, the selectivity of hydrocarbon product generated was towards diesel range (C13-C24). The kerosene selectivity towards hydrocarbon fraction was composed of a mixture of alkanes, cycloalkanes, and aromatic compounds.The conversion of WCO is presented in Fig. 10\n. The percentage conversion was measured as a function of coke retaining amount. The trend for a conversion percentage of WCO was KSi\u00a0>\u00a0KSi-RG1\u00a0>\u00a0KSi-RG2. The conversion of WCO using KSi slightly decreased from 62.3 % in the 1st run to 41.8 % in the 5th run. This result was well correlated due to the loss of deoxygenation activity and deposition of coke formation on the active site of the catalyst. This was in line with the principle mentioned by Guisnet and Magnoux [53]; coke may affect catalyst activity in two ways: through active site coverage (poisoning) and pore blockages (active sites rendered inaccessible to reactants). After the first regeneration, the conversion of WCO in the first run was 58.6 %, a slight decrement in the conversion as compared to the 1st run using KSi catalyst. The conversion of WCO was continuously decreased as the KSi-RG1 was reused until the 5th run. The conversion obtained in the 1st run using KSi-RG2 was 53.0 %, lower as compared to the 1st run using KSi-RG1. The reduction in conversion of WCO using KSi-RG1 and KSi-RG2 was possibly due to the formation of coke that had not been totally removed from the surface of the catalyst in the regeneration process leading to the reduction of the active sites of the catalyst. As reported by Shao et al.\n[54], restoring the activity of a permanently deactivated catalyst with standard regeneration procedures is difficult. However, catalyst deactivation due to coke deposition is usually reversible, and the coke can easily be removed by oxidation with air (O2). Due to this, lesser conversion of WCO was produced using the regenerated catalyst as could be observed in Fig. 10.\nFigure 11\n shows the product distribution in terms of liquid and estimation gas measured based on mass balance via deoxygenation of WCO. The liquid product obtained from the deoxygenation process had two layers which are an aqueous phase and an oil phase. The aqueous phase contained primarily water and a few organic compounds whereas the oil phase contained primarily organic compounds such as acids, esters, phenols, aldehydes, alcohols, ketones, ethers and hydrocarbons [3]. The trend of liquid yield according to the reusability of KSi catalyst was 1st run\u00a0>\u00a02nd run\u00a0>\u00a03rd run\u00a0>\u00a04th run\u00a0>\u00a05th run for the first consecutive cycle. Massive loss in liquid product yield after the first cycle of KSi was possibly due to pore blockage resulted in unable to provide enough space for catalytic activity. The pores were blocked by deposition of coke, organic compound and adsorbed impurities which could be observed through the physical appearance of regenerated KSi catalyst change from white to grey colour (Fig. 3) and the coke accumulation coated on morphology structure of regenerated KSi catalyst as shown in Fig. 4 (c & d). In addition, the presence of other elements and metal oxides can be observed after regeneration as shown in Table 2, proving that the porosity of regenerated KSi catalyst has been blocked. Furthermore, the yield showed a slight decrement throughout the five consecutive runs owing to sufficient active site on the surface of the catalyst. The yield of liquid product was boosted after the first (46.0 %) and the second regeneration (40.3 %). The pattern for cycle performance was quite similar after each regeneration. The thermal regeneration step can treat the deactivated KSi catalyst under high temperature by restoring the catalytic activity of regenerated KSi catalyst as shown in pyrolysis oil yielded.\nFigure 12\n represents the value of hydrocarbon and oxygenated compounds in the liquid product. Hydrocarbon dominated over oxygenated compounds for each run and regeneration cycle. The highest yield of hydrocarbon (74.0 %) and the lowest oxygenated compound (26.1 %) were achieved during the 3rd run when using fresh KSi. However, the yield of hydrocarbon gave slightly improved after the first regeneration and achieved a high yield (74.9 %) during the 2nd run of KSi-RG1. On the contrary, the yield of hydrocarbon after the second regeneration (KSi-RG2) was reduced as compared to the hydrocarbon yield produced using KSi and KSi-RG1. The reduction in hydrocarbon yield could happen due to voids in the pores of the catalyst being partially filled with coke formation which contributes to the serious deactivation caused by difficult diffusion of reactants and products. The highest yield of hydrocarbon (67.5 %) and the lowest oxygenated compound (32.0 %) were attained during the 3rd run of KSi-RG2. This result was in agreement Li et al.\n[55] with that the composition and properties of hydrocracking oils obtained in this work were similar to the cracking and catalytic cracking oils obtained from vegetable oils in the presence of a basic catalyst. The minimum amount of oxygenated compound in a diesel engine, would lead to a good performance in power output and decrease fuel consumption [56].\nFigure 13\n shows the chemical constituents in the liquid product detected by GC\u2013MS analysis for KSi, KSi-RG1, and KSi-RG2. As shown in Fig. 13a, the main chemical group found were alkanes (49.31 %), carboxylic acid (35.70 %), alkenes (27.69 %), alcohol (10.12 %), and cycloalkane (7.42 %) with the occurrence of smaller concentration of diene, cycloalkane, aromatic, ketone, ester and, etc. GC\u2013MS analysis showed that oxygenated compounds that appear in the oil were mostly long-chain of carboxylic acid and ester. The significant amount of carboxylic acids in the liquid product was related to the high amount of fatty acids content in the waste cooking oil [14]. Nevertheless, the chemical group determined for KSi-RG1, and KSi-RG2 catalyst showed a similar trend. These findings are comparable with other researchers [57], where there were a high number of alkane groups in the pyrolysis product which used CaO as a catalyst for deoxygenation of WCO.As shown in Fig. 14\n, the main component in the liquid product was in the diesel range, followed by the kerosene and gasoline carbon range. The main component of the products was always hydrocarbon, non-polar oxygenates and organic acids [58]. The diesel produced using fresh KSi catalyst was in the range of 75.82 % to 81.02%. After the first regeneration, the highest biodiesel yield was attained (86.72 %) during the 5th run of the KSi-RG1 catalyst. Meanwhile, the highest diesel produced in the second regeneration was 84.5% during the 3rd run using KSi-RG2 catalyst. On the other hand, the yield of kerosene in the liquid product was also obtained as high as 89.05% in the 4th run using KSi, 75.49% in the 4th run using KSi-RG1 and 81.88% in the 5th run of KSi-RG2. Moreover, the highest yield of gasoline 24.0% was acquired during the 3rd run using KSi, 26.5% during the 4th run using KSi-RG1, and 26.7% during the 5th run using KSi-RG2 catalyst. From this finding, the selectivity of the diesel was improved after the first regeneration i.e., during the second cycle, possibly owing to the retaining coke that can modify the base property and pore dimension of the KSi catalyst. This finding was in agreement with Shao et al.\n[59],the retaining coke decorated the catalyst pore, which changed the product distribution and product selectivity of hydrocarbons in catalytic pyrolysis of biomass-derivates.\nTable 3\n shows the comparison studies in reusability and regenerability of various catalysts in the deoxygenation of WCO. The reusability and regenerability of catalysts are important steps as they can cut the cost of large-scale green diesel production. As reported by Shitao et al.\n[60], with the lengthening of the regeneration cycle of SBA-15@MgO@Zn catalyst to be used in deoxygenation of WCO, the production of liquid biofuels decreased while the yield of coke increased. This could be because, during the numerous catalytic cracking and regeneration cycles, the active sites of the SBA-15@MgO@Zn catalyst were gradually depleted. However, the average yield of hydrocarbon after 3 runs and 3 times of regeneration was still good which is around 68.5 %. As compared to a study conducted by Pacheco et al.\n[61], the yield of hydrocarbon products was low around 58.4 % but the selectivity of green diesel was higher which is 90 % by using thermal regeneration for Pd/SBA\u201115 catalyst. The authors found out that, the incomplete elimination of the oxygen can be explained by different factors that include catalyst deactivation due to impurities present in the WCO and the formation of carbonaceous deposits during the reaction also has a deactivating effect. It was parallel with finding based on this research through observation of foreign elements (Fe, Mg and S) and coke deposited on the surface of the regenerated KSi-RG1 and KSi-RG2 after thermal regeneration. However, KSi (K2O/SiO2) has shown good stability through 15 runs with 2 times of regeneration by giving 65.8 % in an average yield of hydrocarbon product with 79.4 % in average of green diesel selectivity. In the reusability and regeneration study of activated carbon (AC) based catalyst conducted by Alsultan et al.\n[38] and Wan et al.\n[62] as shown in Table 3, the spent catalyst has been reactivated by using the hexane washing technique. The regenerated AC catalyst was washed and reused for the next reaction cycle under the set-up reaction conditions. As reported by Wan et al.\n[62] through TGA analysis, the spent NiLa/AC catalyst exhibited the presence of hard coke around 26\u00a0wt%, which decomposed between 330 \u2103 to 750\u00a0\u2103; this showed that the catalyst surface was covered by coke formation after the deoxygenation reaction. This finding agreed with EDX analysis, which showed a remarkable increment in the carbon content in the NiLa/AC spent catalyst after 6 deoxygenation runs, hence, it strongly implies that the spent NiLa/AC catalyst was masked by the coke. However, based on Alsultan et al.\n[38], there is another factor that affects the deactivation of catalyst which is metal leaching. The leaching of the Ca2+ and La3+ ions from active sites of CaO-La2O3/AC catalyst into the reaction medium might cause a decrease in the catalytic activity throughout the 6th run. Through ICP-AES analysis results, the researchers found that the metal content in the liquid product from 1st to 6th runs catalyst was found to gradually increase from 0.27 to 0.70\u00a0ppm and 0.0 to 0.10\u00a0ppm for Ca2+ and La3+, respectively. The dissolution of Ca2+ and La3+ metal species in the liquid product during deoxygenation reaction simultaneously led to the reduction of deoxygenation reaction by decreasing the catalytic activity of CaO-La2O3/AC catalyst.This comparative study demonstrated that K2O/SiO2 (KSi) can be used as a catalyst in the deoxygenation of WCO producing a high yield of pyrolysis oil (51.8 %); however, a low amount of hydrocarbon compound (52.5 %) was obtained as compared to thermal deoxygenation and calcined dolomite catalyst. The dispersion of a high amount of K2O into SiO2 as a supported catalyst was recommended in a future study to improve the basic properties of synthesized K2O/SiO2 catalyst by eliminating more oxygen content in pyrolysis oil generated. The catalytic pyrolysis of WCO using KSi catalyst rendered higher selectivity towards diesel products (C13-C24) as contrasted to that of kerosene (mixture of alkanes, cycloalkanes and aromatic compounds). Reusability study indicated that KSi catalyst showed tremendous thermal stability and marvellous reactivity after 15 times of reusability and 2 times regeneration with the average yield of hydrocarbon and diesel yield of 32.49 % and 79.36 %, respectively. The gradual reduction shown in deoxygenation activity was mostly due to the coke depositions on the surface of the KSi catalyst.\nR.S.R.M. Hafriz: Conceptualization, Data curation, Methodology, Visualization, Writing \u2013 review & editing. I. Nor Shafizah: Conceptualization, Data curation, Methodology, Visualization, Writing \u2013 review & editing. N.A. Arifin: Validation, Writing \u2013 review & editing. A.H. Maisarah: Data curation, Formal analysis, Investigation, Methodology, Writing \u2013 original draft. A. Salmiaton: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Validation, Writing \u2013 review & editing. A.H. Shamsuddin: Project administration, Resource.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 support this research publication.", "descript": "\n                  In this work, the comparative study on the performance of K2O/SiO2 and calcined dolomite catalysts was conducted via deoxygenation of waste cooking oil (WCO). KSi catalyst has the potential as a deoxygenation catalyst due to mesoporous structure catalyst with high base properties which enhance oxygen removal. The result found that K2O/SiO2 catalyst could be used in the deoxygenation of WCO generating a high yield of pyrolysis oil as compared to thermal deoxygenation and calcined dolomite catalyst. Besides that, the reusability and regeneration of the K2O/SiO2 catalyst were evaluated in the deoxygenation reaction using WCO as a feedstock. Five consecutive runs of reusability test and three successive cycles with two regenerations were performed. The reusability and followed by regeneration tests were conducted at conditions: 30\u00a0min of reaction time, 390\u00a0\u00b1\u00a05\u00a0\u00b0C reaction temperature and 150\u00a0cm3/min of N2 flow rate. The liquid products obtained from each cycle were analyzed by GC\u2013MS. The deoxygenation of WCO using K2O/SiO2 catalyst rendered higher selectivity towards diesel products (C13-C24). The K2O/SiO2 catalyst presented a good performance in reusability and regenerability test with only \u223c19.3\u201322.4% dropped in pyrolysis oil yield after 5 consecutive runs and only \u223c11.20\u201313.23% drop in diesel yield after regeneration. The results showed that K2O/SiO2 catalyst has tremendous stability in the deoxygenation of WCO into green diesel and could be the alternative deoxygenation base catalyst for the deoxygenation process.\n               "}
{"full_text": "The research of biomass feedstock-derived chemicals being converted into fuels has received much interest in recent years [1\u20135]. It contributes to the establishing green and sustainable development in carbon\u2013neutral technology, which aids in alleviation of energy shortages and environmental pollution. Among these chemicals, renewable levulinic acid (LA) is considered a prominent platform chemical for biofuels and chemical synthesis [6]. LA is formed during the acid-catalyzed hydrolysis of lignocellulosic biomass. It is composed of a saturated keto and acid functional group that is selectively converted to valeric acid (VA) and \u03b3-valerolactone (GVL) [7\u20139]. These two chemicals are used as intermediates in the synthesis of valeric biofuels. In fine chemical synthesis, GVL could be used as a green solvent, food additive, and also precursor in the synthesis of gasoline fuels [10,11]. LA is converted into a variety of products, the most notable of which is the hydrogenation of LA to VA synthesis, which has received lot a of attention [12]. VA is identified as one of the most prominent chemical intermediates in fine chemical industries because of its outstanding physicochemical properties such as low toxicity and unique fuel characteristics. It is also widely used in cosmetics production, perfumes, ester-type lubricants in aviation turbine oils, vinyl stabilizers, refrigerants, fine chemicals, and pharmaceuticals [13\u201316]. Furthermore, VA is easily converted into 5-nonane, which can be used as a solvent in the paint and resin industries as well as a precursor in biofuel synthesis in the vapour-phase of traditional ketonization technology [16\u201318]. Valeric esters synthesized from VA, which is regarded as a promising constituent in advanced biofuels [19,20]. As a result of the wide range of applications for VA and its derivatives, a cost-performance competitive manufacturing process in biorefinery industries is required. Currently, in industry, VA is produced by combining syngas and 1-butene in an oxo process, followed by air oxidation of the aldehyde. VA markets are currently under intense pressure. This is primarily because Rhodium (Rh) based catalysts are expensive, homogeneous and soluble in the reaction mixture. The recovery and separation of the catalyst from reaction residues is the main disadvantage. In addition, side-products are forming, the majority of which are branched products, alcohols, and alkanes [21].LA and/or GVL to VA synthesis involves several reaction steps and formation of intermediates [12]. In the past few years, the challenges of different steps have been merged into a single-step and bifunctional catalysts have shown great interest in this reaction. It is known that multiple tandem reactions of LA to VA synthesis require metallic centres with a reducing property and active acid site [22\u201325]. Several recent research studies have recently focused on the noble and non-noble metal supported catalysts investigated and various reaction conditions in LA to VA synthesis (Table S1) [12]. Zhou et al. reported that palladium on carbon (Pd/C) and hafnium trifluoromethanesulfonate (Hf(OTf)4) bifunctional catalysts tested at 150\u00a0\u00b0C and 5.0\u00a0MPa of H2, it is showed 92% of VA selectivity with 100% LA conversion [26]. Furthermore, noble-metal supported catalysts are highly active for hydrogenation reactions [12,22,23]. Still, their cost is high, they have harsh reaction conditions, they deactivate quickly and they suffer from inactivity due to coke formation and deposition on their surface, which prevents their technical implementation in near future bio-refineries [22\u201328].Similarly, alternative catalysts discovered that low-cost non-noble bifunctional catalysts performed well in LA hydrogenation [24,25]. The long-term security of experiments necessitates the use of a fixed bed continuous flow reactor. Liu et al. conducted the conversion of GVL to valeric esters synthesis over Cu/ZrO2 catalyst. The catalyst showed GVL conversion of 85.4% and pentylvalerate selectivity of 98% obtained at 230\u00a0\u00b0C and 1.5\u00a0MPa H2\n[27]. Afterward, Chan-Thaw et al. examined the Cu/SiO2-ZrO2 catalyst for the synthesis of valeric esters from GVL. The catalyst showed 59% selectivity of ethyl valerate, and 69% of GVL conversion was obtained at 250\u00a0\u00b0C, 1\u00a0MPa H2\n[29]. Jiang et al. demonstrated that liquid-phase hydrogenation of GVL into ethyl valerate over Ni/La-Y in ethanol solvent at 200\u00a0\u00b0C under 30\u00a0bar H2 conditions [30]. These GVL-to-VA conversions were good; but,theyfrequently required severalstepsorstarted withaGVLresource.To simplify the procedure, converting LA into VA in a single-pot without isolating the GVL intermediate would be preferable.Karanwal et al. demonstrated that liquid-phase conversion of LA to VA synthesis by Nb-Cu/Zr on a silica catalyst at mild reaction conditions at 150\u00a0\u00b0C and 3\u00a0MPa H2 for 4\u00a0h in an aqueous medium, produced 99.8% of VA [24]. However, the drawbacks of liquid-phase hydrogenation of LA in terms of high-pressure demand, waste-discharge and leaching leads to both a decline in activity and pollution of the product purification. These are all anticipated to be alleviated by seeking the vapour-phase hydrogenation of LA under ambient and/or moderate reaction conditions. Even so, there aren\u2019t many reports on single-pot vapour-phase synthesis of VA from LA in a fixed bed continuous flow-reactor. Vapour-phase catalytic hydrogenation process is much simpler, more efficient and environmentally benign compared to the liquid-phase hydrogenation process [12].Due to its well-ordered mesoporous structure, wide surface area, thermal stability, and pore volume, Santa Barbara Amorphous-15 (SBA-15) supports metals and provides the necessary acidic sites for the conversion of LA. Weaker acid sites of SBA-15 prevent catalyst deactivation by coke formation, which may extend the catalyst lifetime and support acidity, as well as both Lewis and Br\u00f8nsted acidity promotes LA adsorption and VA production while also controlling metal leaching and carbonaceous species on the catalyst surface which are more important for developing a prominent, stable bifunctional catalysts [12]. In this work, we investigated the LA to VA synthesis in a single step with the design of non-noble metal-supported catalysts with different metal-doped mesoporous SBA-15 catalysts (M/mesoSBA-15; M\u00a0=\u00a0Zr, Nb, and Ti), which are mainly focused on Zr/mesoSBA-15, Nb/mesoSBA-15, and Ti/mesoSBA-15 catalysts with tunable Br\u00f8nsted/Lewis acidity. Based on the above results, we proposed a plausible reaction pathway for LA conversion to VA synthesis, the following steps are required [12,20]. These steps are follows: i) hydro-cyclization (intramolecular dehydration) of LA to GVL over a hydrogenation catalyst and acid sites via \u03b1-angilica lactone intermediate; ii) opening of the GVL ring on acid sites while GVL formed to pentenoic acid; and (iii) further reduction to produce VA, these as shown in Scheme 1\n. The catalysts were characterized in more detail to identify their excellent catalytic performance. The physicochemical parameters of the catalysts were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV\u2013vis diffuse reflectance spectroscopy (UV-DRS), temperature programmed desorptionof ammonia (NH3-TPD), and brunner-emmett-teller(BET) surface area, pyridine adsorption followed by FT-IR (Py-FTIR) analysis, etc. Finally, the enhanced catalytic activity studies have been tuned by the acidity and surface-active sites of the catalysts.Ammonium niobite (V) oxalate.hydrate was purchased from Alfa Aesar; levulinic acid, tetraethyl orthosilicate (TEOS), HCl, zirconium (IV) nitrate, titanium (IV) nitrate, cetyltrimethylammonium bromide (CTAB), Pluronic P123 were purchased from the sigma\u2013aldrich company. All chemicals are directly used as precursors.The SBA-15 materials were synthesized by a hydrothermal method from the previously reported publications [31,32]. In the first step, 2\u00a0g of P123 was dissolved in a well-mixed solution of 15\u00a0mL of H2O and 45\u00a0mL of 2\u00a0M HCl with continuous stirring, then 0.2\u00a0g of CTAB and 5.9\u00a0g of TEOS were added. The ratio would be 1 TEOS: 0.02 CTAB: 3.1 HCl: 115 H2O: 0.012: metal ratio of 15\u00a0wt% Polymer. The resultant solution was transferred into an autoclave and put in an oven at 100\u00a0\u00b0C for 24\u00a0h. Finally, the resulting precipitate was washed with deionized water and an ethanol solution. The obtained powder was calcined at 550\u00a0\u00b0C for 5\u00a0h.Furthermore, different metal-doped Nb/mesoSBA-15, Ti/mesoSBA-15, and Zr/mesoSBA-15 (M/Si ratio of 15\u00a0wt%) samples were synthesized by simple wetness impregnation method. The solid was then oven-dried (100\u00a0\u00b0C, 12\u00a0h) and calcined (500\u00a0\u00b0C, 5\u00a0h).The XRD experiments were conducted on a Rigaku miniflex diffractometer with nickel filtered Cu K\u03b1 radiation with 40\u00a0kV and 20\u00a0mA from 2\u03b8\u00a0=\u00a02 to 65\u00b0, the scanning rate is 2\u00b0min\u22121.The composition of the samples was performed on inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis by Perkin-Elmer Optima 3300 DV equipment. Prior to the analysis, the catalysts were dissolved in HCl, HF, H3PO4, and HNO3; these were well mixed for 2\u00a0h in a microwave oven. The subsequent solutions were diluted with demineralized water.The SBA-15 and various metal-doped SBA-15 materials textural properties were conducted by N2-adsorption\u2013desorption isotherms at 77\u00a0K, in a Micromeritics ASAP 2020 system. First the catalysts were evacuated at 200\u00a0\u00b0C for 3\u00a0h. The BET values are calculated by using the BET equation.A JEOL 2010 instrument was used for TEM analysis and conducting at 200\u00a0kV. The powder catalysts were dissolved in an EtOH solution through sonication for 10\u00a0min, then diffused on copper grids. The sample holder was put into the microscope column. The UV\u2013vis analysis was performed on a GBC UV\u2013Visible Cintra instrument.The acidity of the catalysts was investigated by NH3-TPD analysis using the 2920 Micromeritics device. The catalysts were pre-treated with pure He gas for 50\u00a0mL at 200\u00a0\u00b0C for 1\u00a0h and cooled to a lower temperature. The catalyst was in-situ reduced by 5% H2-Ar 40\u00a0mL at 250\u00a0\u00b0C/2h and then treated with pure He gas for 50\u00a0mL at the same temperature for 30\u00a0min. After that the NH3 absorption was carried out over 10% NH3-He gas for 75\u00a0mL at 80\u00a0\u00b0C/1h, then purged at 120\u00a0\u00b0C/2h for the physisorption of NH3. The TPD run was conducted from 120 to 750\u00a0\u00b0C and the outlet amount of NH3 was analyzed by thermal conductive detector (TCD). Py-FTIR analysis was performed at 250\u00a0\u00b0C for 2\u00a0h under air flow, then the reduction was conducted at the same temperature. Later, the pyridine adsorption was conducted at 110\u00a0\u00b0C, subsequently, the samples were cool down to room temperature. These samples were mixed with KBr and ground, and pressed into tablets. The spectra recorded by GC-FT-IR Nicolet 670 and KBr have taken the background spectrum.The vapor phase hydrogenation of LA to VA synthesis was tested in a continuous fixed bed with a stainless-steel tubular reactor (SSTR) at 265\u00a0\u00b0C with 0.2\u00a0MPa of H2 pressure. About 0.4\u00a0g of the catalyst was placed in the reactor. At 300\u00a0\u00b0C, the sample was first reduced with 50\u00a0mL of H2 flow. The reactant was pumped along with H2 flow at the desired reaction temperature. The catalytic performance of the catalyst was conducted at various reaction conditions. Hourly, the products are collected using an ice-water trap, and they are analyzed using GC with an DB-wax column. Before the GC analysis, the samples were diluted with methanol and analyzed by GC\u2013MS with an HP-1MS column. The amount of total carbon content was analyzed using the CHNS technique. The balance is about 98%. The below equation obtains the conversion of LA and selectivity.\n\n\n\nC\no\nn\nv\ne\nr\ns\ni\no\nn\n\n\n\n\n%\n\n\n\n=\n\n\nL\nA\n\nm\no\nl\ne\ns\n\n\n\n\ni\nn\n\n\n\n-\nm\no\nl\ne\ns\n\no\nf\n\nL\nA\n\n\n\n\no\nu\nt\n\n\n\n\n\n\nL\nA\n\nm\no\nl\ne\ns\n\n\n\n\ni\nn\n\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n\n\nS\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n\n\n\n\n%\n\n\n\n=\n\n\nm\no\nl\ne\ns\n\no\nf\n\no\nn\ne\n\np\nr\no\nd\nu\nc\nt\n\n\nm\no\nl\ne\ns\n\no\nf\n\na\nl\nl\n\np\nr\no\nd\nu\nc\nt\ns\n\n\n\u00d7\n100\n\n\n\n\nThe chemical composition of the different metal-doped SBA-15 catalysts was investigated by ICP-AES analysis and presented in Table 1\n. The chemical composition of the theoretical and experimental results is very close to\u00a0\u223c\u00a015\u00a0wt%. However, the actual doping of the metal content is lower than expected.The N2-physisorption isotherms and pore size distribution of SBA-15 of various metal-doped SBA-15 catalysts are shown in Fig. 1\na & b, and the textural parameters of the catalysts are presented in Table 1. All of the catalysts exhibited type IV isotherms with H1 hysteresis, which is assigned to the mesoporous nature owing to cylindrical pores and also a uniform pore size according to IUPAC classification [33]. These results revealed that all samples contained the hexagonal arrangement with a mesoporous structure even after doping the metals into the SBA-15. Parent SBA-15 showed a surface area value of 830 m2g\u22121 and a pore diameter of 6.3\u00a0nm, whereas the surface area of Zr, Ti, and Nb-doped mesoporous SBA-15 catalysts was decreased when compared to SBA-15. This is mainly due to increases in pore wall thickness and a decrease in pore volume, and also metal species could exist outside the skeleton, which led to a decrease in pore diameter, pore volume, and BET surface area values. These results are consistent with previous publications and also confirmed by XRD analysis [34,35]. Among those catalysts, the Zr/mesoSBA-15 catalyst exhibited a higher BET surface area and a 0.82 pore volume, while the Ti/mesoSBA-15 catalyst showed a lower surface area of 711\u00a0m2.g-1and a pore volume of about 1.03. The above results indicate that high surface area will be beneficial for the higher activity of LA to VA synthesis.The parent SBA-15 and metal-doped SBA-15 materials of XRD patterns are presented in Fig. 1c. The parent SBA-15 exhibited the peaks at 0.8\u00b0, 1.59o, and 1.8\u00b0 of 2\u03b8 values, which are assigned to the (100), (110), and (200) reflections, and the corresponding d spacing is (100)\u00a0=\u00a011\u00a0nm. These findings might be confirmed by the fact that SBA-15 showed a highly ordered 2D-hexagonal symmetry of the mesoporous nature of SBA-15 (p6mm), which can be assigned to the excellent long-range order formed within the SBA-15 [31,36]. Fig. 1c shows three characteristic peaks of hexagonal arrangements observed, but the intensity was decreased with the doping of Zr, Nb, and Ti. These results can also be co-related with the BET surface values, with the doping of metal promoters, the BET surface values decreased. These are presented in Table 1. The Ti/mesoSBA-15 catalyst only exhibited the plane (100) with lower intensity, this can be assigned to the support structure being lost, which can influence the BET surface area [37].The parent SBA-15 and those metal-doped SBA-15 materials of XRD patterns are presented in Fig. S1. The catalysts displayed the broad peak at 2\u03b8\u00a0=\u00a020-30\u00b0, were assigned to the amorphous silica. No diffraction peaks were observed from the metallic oxide species, indicating that the metallic phases had better dispersion on the external and internal surfaces of SBA-15, whose size should be below the XRD limits [38]. The mesoSBA-15 exhibited the peak at 2\u03b8\u00a0=\u00a0\u223c22\u00b0, ascribed to the amorphous-silica nature [39].TEM images evaluated the morphology of catalysts and the distribution of particle size of various metal-doped SBA-15 materials are presented in Fig. 2\n. The metal-doped SBA-15 samples were uniformly distributed on the mesoporous SBA-15. The average particle size of the catalyst was found to be 2.3 to 12\u00a0nm. According to the statistical results on more than 250 nanoparticles, the particle size was found to be 2.3\u00a0\u00b1\u00a01.0\u00a0nm for the Zr/mesoSBA-15 catalyst. The modification with Nb and Ti to form an inappropriate structure led to a slight increase in the particle size to 4 to 11\u00a0nm for the Nb/mesoSBA-15 and Ti/mesoSBA-15 catalyst (Fig. 2). Based on the above results reveal that well dispersion with the smaller particle size in the Zr/mesoSBA-15 catalyst is favourable for the higher catalytic activity.This spectra is useful for the identification of isolated transitional metal ions and clustered transition oxides with local coordination environments and electronic states. Here different metal-doped SBA-15 catalysts were investigated. The SBA-15 like silica materials showed that the absorption bands were found to be 200\u2013400\u00a0nm in Fig. 3\na and the adsorption bands gave more information about the metal atoms of the surrounding environment in the SBA-15. The band at 200\u2013240\u00a0nm is attributed to ligand\u2013metal charge transfer (L-M CT) transition that occurred. This band gave direct evidence of the metal-doped SBA-15 framework silica. The catalysts showed a sharp peak at 200\u00a0nm. The Ti/mesoSBA-15 catalysts sample exhibited two bands at high intensity, the band at 186\u2013230\u00a0nm, is ascribed to M\u2212O charge transfers in the TiO2 ion and the lower-intensity second band between 250 and 310\u00a0nm was assigned to the Ti atoms doped into the SBA-15 framework and occupied in the tetrahedral position. Here the bulk phase of isolated TiO2 might be negligible [40,41]. The Zr/mesoSBA-15 samples exhibited two bands at 206 and 318\u00a0nm. The 206\u00a0nm band is ascribed to the O2-Zr4+ (L-M CT) in tetrahedral coordination inside the SBA-15 [42,43]. The 318 band is assigned to the O-M CT cation in the octahedral coordination sphere. In the case of Nb/mesoSBA-15, it is also exhibited two types; a small intensity absorption band at 210\u2013240\u00a0nm observed, which is an O2\u2013-Nb+4 CT band and a second absorption band was observed at 240\u2013380\u00a0nm, this band indicates that the Nb species is in the form of tetrahedral coordination site [44\u201346]. Finally, UV-DRS spectra concluded that the metals are successfully doped into the mesoporous structure of SBA-15.The total amount of acidic sites on the catalyst's surface was determined using NH3-TPD analysis. The small size of the NH3 molecule has a strong basic nature and is stable at higher temperatures. NH3 is a probe molecule in the NH3-TPD analysis. The hydrogenation reaction is an acid catalyzed reaction, and it will proceed through the active and stable acid sites of the catalysts. The surface acidity plays a prominent role in this hydrogenation activity and selectivity. The amount of total acidity was determined and is presented in Table 2\n. From Fig. 3b, the catalysts exhibited three desorption peaks, at 100\u2013200\u00a0\u00b0C the characteristic nature of weak acid strength, and the peak in the range of 220\u2013400\u00a0\u00b0C is attributed to the moderate acidic sites, whereas the peak at\u00a0\u2265\u00a0400\u00a0\u00b0C is ascribed to the strong acidic sites [47\u201349]. The total amount of acidity was increased drastically after metal-doped on the SBA-15 support, which might be the replacement of protons by Zr (IV) ions, Ti (IV), and Nb (IV). Therefore, the metal ions interact with the acidic sites along with the SBA-15. Among those catalysts, the Zr/mesoSBA-15 catalyst exhibited a higher amount of acidic strength, these acidic sites are beneficial for the higher activity of LA to VA. As follows the order of the total acidity of the catalysts: Zr/mesoSBA-15\u00a0>\u00a0Ti/mesoSBA-15\u00a0>\u00a0Nb/mesoSBA-15\u00a0>\u00a0mesoSBA-15. The above results demonstrated that the acidic sites of materials were primarily attributed to the nature of metal-doped in mesoSBA-15 catalysts.From the NH3-TPD we could not be distinguished the Lewis and Br\u00f8nsted acid sites. Py-FTIR of SBA-15 and different metal-doped SBA-15 catalysts are presented in Fig. 3c. The Lewis and Br\u00f8nsted acid sites could be discovered with the help of this analysis. From the previous literature, pyridine molecule adsorbed via hydrogen-bonding interactions with surface OH groups and protons (H) might be combined with the Lewis (L) and Br\u00f8nsted (B) acidic surface sites. The bands observed at 1450 and 1610\u00a0cm\u22121 are assigned to L acidic sites [38,49]. The bands observed at 1540\u20131548\u00a0cm\u22121\n, are assigned to the B acidic sites [50]. The band at 1490\u20131500\u00a0cm\u22121, is ascribed to the combination of both B\u00a0+\u00a0L acidic sites. We observed that metal doping to SBA-15 increased the L acidic sites while slightly decreasing the B acidic sites. The metal (0) acts as an electron acceptor in reducing conditions, this can be beneficial for the L acidic site generation, including all the samples, the Zr/mesoSBA-15 catalyst showed the higher amount of L acidic sites and a lower amount of B acidic sites when compared to other catalysts and these results are presented in Table 3\n.We have examined the vapor phase synthesis of VA from LA over different metal-doped SBA-15 catalysts (Table 4\n). The parent SBA-15 exhibited very low selectivity of VA about 0.9% but it showed GVL with 18% selectivity, this is mainly due to the SBA-15 containing slightly Br\u00f8nsted and Lewis acidic sites. The Ti-doped SBA-15 showed 49% selectivity of VA and the Nb/SBA-15 sample exhibited 38% selectivity of VA. In contrast, the Zr-doped mesoporous SBA-15 catalyst showed 68% selectivity of VA. The best results can be obtained from the Zr-doped SBA-15 this might be because of the high diffusivity of Zr species on the support surface and the accessibility of the strong acidic sites, which are beneficial for the higher selectivity of VA [22]. Recently, several groups reported improvements to zirconia-supported catalysts, which can improve the acidity and accessibility of active sites on the catalyst surface, enhancing the activity in biofuel synthesis [51,52]. The characterization results revealed that the increase of VA selectivity with the doping of Zr species into SBA-15 can be improved by the formation of Lewis acid sites confirmed by Py-FTIR and NH3-TPD studies.In this reaction, the temperature will have a crucial impact on product selectivity\u2019s. The influence of temperature was studied over LA to VA synthesis on the Zr/mesoSBA-15 catalyst from 175 to 295\u00a0\u00b0C was studied and the obtained results are displayed in Fig. 4\na. It illustrates that the conversion of LA increased from 28% to 98% as the temperature rose from 175\u00a0\u00b0C to 295\u00a0\u00b0C, this is mainly ascribed to the increase in the number of active sites and these sites activate the reaction at higher temperatures. Whereas VA selectivity was increased to 68% at 275\u00a0\u00b0C when a further rise in temperature, the VA selectivity would be slightly decreased. It demonstrated that the reaction temperature above 275\u00a0\u00b0C does not improve the selectivity toward VA. Hence, the best reaction temperature for this reaction is 275\u00a0\u00b0C.The influence of WHSV on the performance of the Zr-doped SBA-15 catalyst for LA to VA synthesis is shown in Fig. S2. The pure LA solution, with a flow rate of 0.5 to 3.0\u00a0mL/h with WHSV\u00a0=\u00a01.425 to 8.55\u00a0h\u22121. With the increase of feed flow rate, the conversion of LA and VA selectivity decreased from 91% to 74% and 68% to 52%; the selectivity of GVL, alkyl levulinates (AL), and 2-methyltetrahydrofuran (MTHF) could be enhanced. The lower activity at higher WHSV is mainly attributed to the insufficient residence time in the reactor on the catalyst surface. The above results revealed that higher selectivity of VA was obtained at lower WHSV. This is primarily owing to the longer residence time on the catalyst surface. The calculation of WHSV is done by using the following formula.\n\n\n\nW\ne\ni\ng\nh\nt\n\nh\no\nu\nr\nl\ny\n\ns\np\na\nc\ne\n\nv\ne\nl\no\nc\ni\nt\ny\n\n\n(\nW\nH\nS\nV\n)\n\n=\n\n\nM\na\ns\ns\n\no\nf\n\nf\nl\no\nw\n\n\n\nS\na\nm\np\nl\ne\n\nw\ne\ni\ng\nh\nt\n\n(\ng\n)\n\n\n\n\n\n\nWhereas,.\n\nM\na\ns\ns\n\no\nf\n\nf\nl\no\nw\n=\n\n\nF\ne\ne\nd\n\nf\nl\no\nw\n\nr\na\nt\ne\n\n\n\nd\ne\nn\ns\ni\nt\ny\n\no\nf\n\nf\ne\ne\nd\n\nf\nl\no\nw\n\n\n\n\nThe influence of the catalyst weight on LA to VA synthesis over the Zr/mesoSBA-15 catalyst is shown in Fig. 4b. The weight of the catalyst ranged from 0.2 to 0.6\u00a0g, and the conversion of LA increased from 55% to 91%. As the amount of catalyst increased, the selectivity of VA reaches 68% at 0.6\u00a0g of catalyst weight. However, a further increase of catalyst amount (0.6 to 1\u00a0g) leads to a slight drop in VA selectivity. This might be due to the lower space velocity of active sites compared to reactants molecules. The optimal catalyst weight to perform the LA hydrogenation to VA synthesis is 0.6\u00a0g of catalyst.Zr/mesoSBA-15 catalyst was conducted for 52\u00a0h of TOS, and it is shown in Fig. 4c. An increase of reaction time from 1\u00a0h to 11\u00a0h, the catalyst showed 91% conversion of LA and the 68% selectivity toward VA. However, the LA conversion and the VA selectivity could be stable upto 17\u00a0h of reaction time and then decreased with TOS. These results reveal that the deactivation is mainly due to the blocking of the active acidic sites by coke formation on the catalyst surface, these can be proved by the CHNS technique as shown in Table 4.In summary, the different metal-doped mesoporous SBA-15 catalysts were designed successfully and then evaluated in a continuous fixed-bed vapor phase reaction of LA into VA synthesis. The different reaction parameters of the effect of temperature, WHSV, and catalysts weights were studied. The well distributed Zr species on SBA-15 will strengthen the strong acidic sites. Under the optimized reaction conditions, the Zr/mesoSBA-15 catalyst showed the highest 68% selectivity toward VA at 91% conversion of LA. The control of the acidity of strong acidic sites will be beneficial for the LA hydrogenation and GVL ring-opening to form VA. The catalyst also showed stable catalytic activity with a TOS of 52\u00a0h. VA selectivity has decreased mainly due to the coke formation over the catalyst's active sites. The results of XRD, TEM, NH3-TPD, and Py-FTIR characterizations are well evidenced for better catalytic activity.\nRamyakrishna Pothu: Conceptualization, Methodology, Investigation, Supervision, Writing \u2013 original draft. Harisekhar Mitta: Data curation. Rajender Boddula: Conceptualization, Methodology, Investigation, Supervision, Writing \u2013 original draft. Putrakumar Balla: Data curation. Raveendra Gundeboyina: Data curation. Vijayanand Perugopu: Data curation. Jianmin Ma: Conceptualization, Methodology, Investigation, 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.Ramyakrishna Pothu (Award number: 2017SLJ018367) acknowledges the China Scholarship Council (CSC), China, for the financial support.Supplementary data to this article can be found online at https://doi.org/10.1016/j.mset.2022.09.006.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Chemoselective hydrogenation of biomass platform molecules into value-added chemicals and fuels is essential for the exploitation of biomass, and SBA-15 based metal catalysts with hydrogenation centers and acid sites seem promising in this regard. Valeric acid (VA) is the most important platform molecule for valeric biofuels and value-added chemicals production. The main issue with using such bifunctional catalysts for biomass conversion is maintaining the catalyst's stability in the liquid phase under harsh conditions. In-addition, direct one-pot selective hydrogenation of levulinic acid (LA) into VA synthesis is challenging due to its complex reaction conditions involved. Herein, we design a bifunctional mesoporous catalysts (SBA-15 mesoporous material doped with various metals Nb, Ti, and Zr) investigated for this reaction under the vapour phase. Different instrumental approaches were used to examine the structure, phase composition, morphology, and surface elemental analyses of catalysts as-prepared. Among those catalysts, Zr-doped mesoporous SBA-15 catalyst showed the 91% conversion of LA and the 68% selectivity toward VA and promising stability in a 52\u00a0h time on-stream run. Metal dispersion inside the SBA-15 and their surface acidity (sufficient number of acid sites and surface-active metal oxide species) and higher surface area are beneficial for the selectivity of VA. This work offers a highly-efficient bifunctional catalyst for selective hydrogenation of biomass feedstocks.\n               "}
{"full_text": "With the depletion and increasing environmental impacts of the traditional fuels, such as coal and petroleum products, the emerging global challenge in both energy and environment fields has prompted intensive research on renewable energy-conversion and energy-storage systems, such as fuel cells, electrolyzers, and supercapacitors, as well as various batteries.\n1\n Electrocatalysts for oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR) are at the core of some of these energy-conversion and energy-storage systems. In the past decades, various materials, including noble metals, transition metals, and metal-free carbons, have been explored as electrocatalysts, aiming to achieve the high activity, durability, and selectivity for the reactions mentioned above. In general, catalysts are categorized into homogeneous and heterogeneous catalysts. Ninety percent of the current chemical industry processes use heterogeneous catalysts;\n2\n however, their low atom utilization efficiency brings crucial disadvantages in terms of activity and cost.\n3\n In comparison, homogeneous molecular catalysts offer uniform active sites, high atom utilization efficiency, and convenient structural tunability, but they have relatively low durability and recyclability. Therefore, it is desirable to develop novel catalysts that can combine the merits of both homogeneous and heterogeneous catalysts.Among various novel catalysts, single-atom catalysts (SACs) have aroused great interest due to their intriguing features, such as high atom utilization efficiency,\n4\u20136\n low-coordination environments of single-atom centers,\n7\n unique quantum size effects,\n8\n and tunable metal-support interactions.\n9\n For example, the atomic dispersion of metal atoms in SACs brings the maximum atom utilization efficiency for metals, and their quantum size effects create discrete energy-level distribution and distinctive HOMO-LUMO gaps.\n10\n Besides, strong interactions between active metal centers and adjacent coordinating atoms may enhance the catalytic activity, selectivity, and durability of metal centers.\n11\n Zhang and co-workers reported the first SAC in 2011 by anchoring Pt atoms on the surface of iron oxide nanocrystallites (denoted as Pt1/FeO\nx\n), which exhibits excellent stability and high catalytic activity for CO oxidation.\n12\n Their density functional theory (DFT) calculations and experimental data reveal that the partially vacant 5d orbitals of the positively charged, high-valent Pt atoms in Pt1/FeO\nx\n decrease the CO adsorption energy and the activation barrier of CO oxidation.Subsequently, different substrates have been used in SACs fabrication, including metal oxides, metal hydroxides, perovskites, zeolites, metal-organic frameworks (MOFs), and carbon materials. Among them, carbon materials attract researchers' intense attention because of their excellent conductivity and porous structures, which are beneficial for the electron/mass transfer of the reaction intermediates. Also, the structural diversity and designability of carbon materials (e.g., amorphous carbon, graphite, and diamond) enable the study of their precise structure-performance correlations, which is critical for both the in-depth understanding of the catalytic mechanism and the design of high-performance catalysts. In the last few years, many carbon-supported SACs (CS-SACs) have been reported, including Fe, Co, Ni, Ru, Ir, Au, Rh, Pd, and Ag supported on carbon nanotubes, graphene, carbon nanofibers, porous carbon, carbon nanosheets, and many other carbon nanomaterials. They have shown decent catalytic activities for many energy-conversion reactions including ORR, HER, OER, CO2RR, or NRR.\n13\u201316\n Some CS-SACs also show bifunctional or multi-functional catalytic activities, such as ORR and OER in metal-air batteries, HER, and OER in water electrolyzers.\n17\u201320\n Tremendous advances have been achieved, making it possible for CS-SACs to overtake traditional metal particles-based catalysts in the race to the renewable energy marketplace. Apart from the applications in electrocatalysis, the recent breakthroughs of CS-SACs also enable them to catalyze various vital reactions in energy-storage devices, such as supercapacitors, rechargeable lithium batteries, and sodium-sulfur batteries. A timely review of the rapidly growing field is highly desirable. This article aims to provide a concise and critical updated overview of recent progress by summarizing important work reported within the past 3 years on CS-SACs for essential reactions involved in energy conversion and storage and to present critical issues governing the fundamental understanding of reaction mechanisms and design strategies of CS-SACs. Through such a comprehensive review, we hope to increase the knowledge of CS-SACs significantly and attract a broad range of scientific communities to their future developments.Various CS-SACs have been reported in the last few years, and they have been used as electrocatalysts in different energy-conversion and energy-storage applications. However, the discussions of their advantages and disadvantages are scattered throughout the existing literature. Here, we provide a summary of their unique benefits and shortcomings related to material designs, superior catalytic activities, and practical applications.In general, CS-SACs are composed of carbon frameworks and metal dopants, which offer numerous opportunities to fabricate a variety of functionalized catalysts for satisfying the requirements in catalysis and energy-conversion devices. Carbon nanomaterials themselves formed by strong covalent bonding between carbon atoms possess the unique physicochemical properties, including controllable dimensions, ease of accessibility, high surface area, excellent conductivity, controllable porosity, and abundant defects. Take the structural controllability as an example: the wide variety of nanoscale physical dimensions, such as zero-dimensional (0D) graphene quantum dots, one-dimensional (1D) carbon nanotubes, two-dimensional (2D) graphene, and three-dimensional (3D) nano-diamond, provides an ideal platform to design high-performance catalysts. Also, their adjustable chemical compositions that may contain single, double, or ternary metal species, or no metal species, can be used for activity regulation. The high surface area, hierarchical pore structure, and excellent conductivity of carbon materials are also the essential preconditions for mass/electron transfer and accessibility between reactants and active sites, which are the incomparable superiorities for pure metal oxides, and hydroxides or perovskites. Moreover, the potential strong metal-support interactions between well-dispersed single metal atoms and carbon substrates can not only limit the aggregation of metal atoms but also tailor the geometric structures and electronic configurations of active catalytic sites. The activities of different coordination configurations of central atoms to a specific electrochemical reaction are of importance to the catalyst design, which may even change the reaction pathways via different electron transfer numbers.\n21\n For example, Yao et\u00a0al. unveiled the nature of M-N-C (M\u00a0= Mn, Fe, Co, Ni, and Cu) as catalytic centers that can change the ORR pathways spanning from 1e to 4e transfer processes.\n22\n DFT calculations show that the electronic structures of atomic Co can be finely tuned by bonding to diverse types of transition metals in the form of M-N4 motifs, resulting in different target reaction pathways in the ORR. N species can play two significant roles in boosting the intrinsic activities of Co-SACs while N coordinated with Co can manipulate the reactivity by modification of electronic distribution.\n23\n In another study, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption spectroscopy (XAS) have revealed that strong metal-S interactions on S-doped carbon substrates can effectively suppress the aggregation of metal atoms, resulting in the formation of single metal atom catalytic sites.\n24\n\n,\n\n25\n\nBesides, CS-SACs with homogeneously dispersed catalytic active sites and tunable physicochemical properties are the ideal candidates for the improvement of catalytic activity for many vital reactions (e.g., ORR, HER, and CO2RR) and related mechanistic understanding. For example, a series of transition-metal catalysts (i.e., Co, Cu, and Fe) with single atomic sites embedded in hollow N-doped carbon were synthesized to catalyze ORR.\n26\n In particular, the Co-based catalyst showed a catalytic activity similar to that of Pt/C catalyst in an acidic electrolyte. The experimental and computational analyses revealed that the superior catalytic activity for ORR could be ascribed to the dramatically enhanced hydrogenation of OH\u2217 species at single Co atom sites.\n27\n Besides, a series of single-atom transition metals (i.e., Co, Ni, and W) supported on N-doped graphene recently were fabricated as electrocatalysts for HER, exhibiting a low overpotential (E\n\nj\n\u00a0= 10 mA cm\u22122) and long-term durability.\n28\n Their DFT calculations and experimental data demonstrate a correlation between catalytic activity and the electronic structures of single atoms. The energy states of active valence d\n\nz\n\n2 orbitals and the corresponding antibonding states play a decisive role in the catalytic activity for HER. In particular, the Co catalyst has antibonding state orbitals neither empty nor fully filled, which offer optimum adsorption strength toward H adsorption than other catalysts.\n28\n Thanks to the tunability of active sites in CS-SACs, a series of Ni/Fe/bimetallic CS-SACs were successfully prepared by various synthetic strategies for some important electrochemical reactions.\n29\n\n,\n\n30\n For instance, the Ni porphyrin-based covalent triazine framework containing atomically dispersed NiN4 centers was recently fabricated by a pyrolysis process and further used for electrocatalytic CO2 reduction. The as-prepared Ni-based CS-SACs exhibit superior activities for CO2RR with high faradic efficiency (FE) of 97% for converting CO2 to CO at the potential of \u22120.9\u00a0V versus reversible hydrogen electrode (RHE).\n29\n\nApart from catalytic activities, CS-SACs have the potential to offer lower cost and better stability over many noble metal-based catalysts for practical applications.\n31\n CS-SACs would be more cost-effective because they provide the maximal atom utilization efficiency (100%) and potentially low manufacturing cost. For example, many recently developed CS-SACs have already shown superior catalytic performances to Pt/C in the fuel cells using alkaline electrolytes,\n32\n\n,\n\n33\n and some CS-SACs even outperform the state-of-the-art non-precious metal catalysts in acidic polymer electrolyte membrane fuel cells (PEMFCs).\n34\n\n,\n\n35\n Therefore, compared with conventional nanoparticles (NPs) or bulk catalysts, CS-SACs are expected to become a more efficient alternative for boosting the practical applications of advanced conversion reactions and renewable energy devices.Although CS-SACs have the potential to play essential roles in many energy-conversion and energy-storage applications, there are some critical challenges associated with their synthesis and stabilization. First, carbon-based substrates in M-N-C electrocatalysts would be electro-oxidized to CO2 and CO under high electrical potentials (e.g., >0.9 VRHE) during fuel cell tests, which results in carbon surface destruction and ruins active catalytic sites, such as FeN\nx\nC\ny\n species.\n20\n\n,\n\n36\n\n,\n\n37\n Second, a significant issue for the synthesis of CS-SACs is the difficulty in stabilizing isolated metal atoms on carbon supports without compromising their catalytic activities, especially at high-temperature conditions or under harsh reaction conditions. This can be ascribed to the higher mobility of individual metal atoms. They are more likely to aggregate into large particles because their surface energies are higher than corresponding metal clusters and NPs. To efficiently stabilize isolated metal atoms, the metal mass loading of current CS-SACs is kept low to minimize the agglomeration of metal atoms. Third, due to the diversity and complexity of their structures and components, it is challenging to pinpoint exact atom arrangements in CS-SACs and identify the origin of catalytic activity (see more discussion in the final section of this review). Therefore, various rational design strategies are needed for further development of CS-SACs.CS-SACs can offer many desirable benefits, as already described. However, due to the high specific surface energy of single atoms in CS-SACs, individual atoms may easily migrate and agglomerate into larger clusters. A key focus of the synthesis of CS-SACs is to achieve highly dispersed single atoms with high stability and density. In this section, the synthetic strategies of CS-SACs will be critically reviewed and classified into two types based on the integration mode of components, namely bottom-up and top-down methods.Several bottom-up methods have been used to synthesize CS-SACs, such as atomic layer deposition (ALD), wet chemistry synthesis, and ball milling.\n38\n\n,\n\n39\n They usually first create surface defects or heteroatom-induced coordination sites in carbon substrates, followed by absorption and reduction of metal precursors to obtain isolated single metal atoms. The modification of pristine carbon surfaces prevents the fast aggregation of metal precursors and metal atoms, which plays a vital role in CS-SAC preparation. Surface defects can change electronic structures of carbon surfaces, creating trapping centers for metal atoms. Furthermore, rationally designed coordinate sites using N, O, or S atoms can precisely anchor metal precursors via chelation.ALD allows precisely controlled deposition of diverse structures on carbon substrates, which enables well-dispersed single atoms by the self-limiting reaction mechanism. It is a typical bottom-up technique with excellent controllability in synthesizing CS-SACs. For example, functional groups on the graphene surface can react with a Pt precursor ligand, and the reaction ends spontaneously once all surface functional groups are occupied, resulting in a monoatomic Pt layer (Figure\u00a01\nA).\n5\n\n,\n\n40\n Furthermore, the mass loading of Pt and the size of Pt clusters can be controlled by altering ALD cycles.Subsequently, many efforts have been devoted to demonstrating that surface functional groups on carbon substrates are critical in obtaining high-performance CS-SACs.\n41\n Typically, pristine graphene will be oxidized in strong acid, followed by a thermal deoxygenation process to create anchoring sites or defects. For example, the single Pd atom catalysts were fabricated by using ALD and used for selective hydrogenation of 1,3-butadiene.\n5\n Pd metal precursors can be tightly anchored on surface phenol groups, forming -O-Pd-hafc surface species (Figures 1B and 1C), after which they are transformed into -O-Pd surface species (Figure\u00a01D). A transmission electron microscopy (TEM) image (Figure\u00a01E) shows that abundant single Pd atoms are well dispersed on the carbon substrate. Although many studies have demonstrated the excellent controllability of ALD in synthesizing CS-SACs, its employment in commercial catalyst production is often limited by its high cost and limited scalability.\n42\n\nIn comparison with ALD, wet chemistry synthesis methods (including co-precipitation and impregnation methods) are currently regarded as the most promising also widely adopted preparation routes because of their low cost, ease of operation, and excellent potential for mass production. They are also capable of providing high mass loading and excellent dispersity of single atoms through precisely controlling the nucleation and growth rate of metal species. In general, wet chemistry methods include three steps: adsorption of metal precursors, calcination, and activation.Two most common strategies used for the adsorption of metal precursors are the coordination strategy via heteroatoms (e.g., N, O, and S) in carbon substrates and an atom confinement strategy by designing structurally complex and highly defective carbon substrates.\n43\u201345\n For example, N-doped porous carbons were used as the substrates for synthesizing Au-based carbon-supported catalysts (denoted as AuSAs-NDPCs) by a coordination strategy (Figure\u00a02\nA). Compared with the N-free carbon, the loading amount and dispersity of single Au atoms are improved significantly. X-ray photoelectron spectroscopy (XPS) results (Figure\u00a02B) show that the electron density of N atoms in AuSAs-NDPCs is much lower, indicating the existence of strong interactions between Au and N.\n44\n Alternatively, a DFT calculation predicts that single atoms prefer to adsorb on the confined interfaces between reduced graphene oxide (rGO) and activated carbon (Figures 2E and 2F).\n39\n Inspired by the prediction, the atom confinement strategy can be used to synthesize a Pd-based SAC. Pd atoms are confined in a double-shelled hollow carbon substrate with rGO as the inner shell and amorphous carbon (AC) as the outer shell. TEM images show that abundant Pd single atoms with an average size of approximately 3\u20134\u00a0\u00c5 are well dispersed on the boundary of rGO@AC (Figures 2C and 2D).\n39\n However, due to the limited boundary area in rGO, the mass loading of Pd single atoms is low at 0.290 wt\u00a0%.Other than the above methods, several other approaches have also been used to increase the mass loading of single atoms. For example, Zhang et\u00a0al.\n47\n proposed a facile and inexpensive defect trapping strategy to synthesize a highly stable, atomically dispersed Ni catalyst on defective graphene (DG) (A-Ni@DG) with a high Ni loading (1.24 wt\u00a0%) by an incipient wetness impregnation method with subsequent acid leaching treatment. HAADF-STEM images demonstrate that the atomic Ni species (aNi) are uniformly trapped in the defects of graphene to form an aNi@defect catalyst. More recently, a universal three-step defect trapping approach was developed for preparing various Co-N4-x\nC\nx\n (x\u00a0= 0\u20134) catalysts with excellent controllability.\n23\n These studies demonstrate a promising defect trapping approach for producing highly active and stable CS-SACs with well-controlled coordination.Besides, the impregnation-adsorption method was developed to synthesize the Pt-based CS-SAC (denoted as Pt1/hCNC) with a higher Pt mass loading up to 2.92 wt\u00a0%.\n46\n The high mass loading is attributed to both the physical confinement of [PtCl6]2\u2212 precursors with a size of \u223c0.5\u00a0nm in micropores (\u223c0.6\u00a0nm) and strong interactions between [PtCl6]2\u2212 anions and N dopants. The adsorption energy of [PtCl6]2\u2212 on N-doped hCNC is 4.6 or 4.9 eV, indicating the formation of stable [C\nx\n(NH)2]2+[PtCl6]2\u2212 ion pairs via the strong electrostatic interaction (Figure\u00a02G). This strategy has also been extended to other precious metals, such as Pt, Au, and Ir. More recently, a photochemical solid-phase reduction strategy was used to synthesize well-isolated Pt atoms on N-doped porous carbon (denoted as Pt1/NPC).\n48\n In brief, the PtCl6\n2\u2212 ions are directly reduced under UV light irradiation and then preferentially deposited on NPC without additional treatments. Pt atoms are well dispersed on the carbon surface without clusters or NPs with a mass loading of 3.8 wt\u00a0%. Apart from the metal loading amount, the efficiency of this synthetic method is another concern. In this regard, microwave heating as an ultrafast method has an advantage for the synthesis of CS-SACs. By the microwave heating reduction of GO, a series of monodispersed atomic transition metals (for example, Co, Ni, and Cu) can be obtained within 2 s\n49\n Moreover, it not only reduces the reaction time but also largely suppresses side reactions, dramatically improving the synthesis efficiency and catalyst yield.Ball milling can cut and reconstruct chemical bonds of materials/molecules as a powerful technique to develop CS-SACs. The moving balls apply their kinetic energy to the materials, leading to single metal atoms embedded in the surface of carbon substrates. Recently, an Fe-based SAC was synthesized by ball-milling the mixture of iron phthalocyanine (FePc) and graphene nanosheets.\n50\n During the milling process, the external macrocyclic component of the FePc precursor could be destroyed and thus form FeN4 centers. Subsequently, the FeN4 centers are trapped on defect sites of graphene nanosheets while adjacent C atoms are reconstructed. The maximum mass loading of Fe is around \u223c2.7 wt\u00a0% without agglomeration. The method has been successfully extended to synthesize other single metal atom catalysts supported on graphene, including Mn, Co, Ni, and Cu.\n51\n\nTop-down methods offer several advantages, including simplification in the synthesis procedures, low cost, and environmental friendliness, in comparison with the bottom-up methods discussed above. They can effectively control different types of metal atoms and their loading rates by regulating synthesis parameters, such as temperature, concentration of metal precursors, and gas flow rate. Moreover, abundant defects and unsaturated sites can be created once bulk structures are downsized into clusters or even individual atoms, which is essential for improving the catalytic\u00a0performances of CS-SACs. We discuss the top-down methods in three subsections: pyrolysis of organic precursors, solid-state reactions, and electrochemical activation.Pyrolysis of organic precursors has long been considered as an efficient synthesis method for developing CS-SACs owing to its simplicity, high metal mass loading, and decent scalability. Also, it can be readily carried out across different laboratories without sophisticated instrumentation. The precursors include polymers, MOFs, and many other organic compounds containing metal species. Among them, various polymers are the most widely used precursors to form carbon substrates for CS-SACs by high-temperature pyrolysis because of the following reasons: (1) a variety of dopants, such as O, N, and S, can be precisely introduced into the framework of polymers, which lead to specific heteroatom dopants in the carbon substrates for metal ion adsorption; (2) the structures of carbon substrates, such as pore size, surface area, and bulk geometry, can be easily controlled by the polymer structures, solvents, and pyrolysis conditions; (3) polymers can also create carbon substrates with distinct atomic arrangements, which facilitates the study of structure-activity relationships in CS-SACs. For example, Zhao et\u00a0al.\n52\n polymerized dicyandiamide and Ni(II) acetylacetonate to synthesize a Ni-based SAC (denoted as NiSA-N-CNT) supported on tubular carbon structures for CO2RR, which has an ultrahigh Ni mass loading of 20.3 wt\u00a0%. Figure\u00a03\nA shows that the single Ni atoms coordinate with four nearest N atoms. The ratio of the Ni precursor and dicyandiamide can significantly affect the morphology of the obtained samples. The formation of tubular substrates is ascribed to the stress-introduced rolling of Ni-containing graphitic carbon nitride layers under high temperatures and electron beam irradiation conditions. In another study, Cheng et\u00a0al.\n53\n polymerized hemin porcine (HP) with Fe(III) acetylacetonate to synthesize an Fe-based SAC supported on graphene-like 2D carbon nanosheets for fuel cells in an acidic electrolyte. The HP precursors are assembled into 2D carbon nanosheets, resulting in a high surface area of 670.8 m2 g\u22121. To increase the defect concentration in the substrate, the dual (S and N)-doped polymers have been designed and used for the Fe-based SAC synthesis. The dual-doped polymers can create more anchoring sites and optimize the interactions between carbon substrates and metal atoms, leading to outstanding catalytic performances.\n33\n\n,\n\n54\n\nOther than using polymer precursors, MOFs, with controllable metal sites, periodic structural units, and adjustable pores, are regarded as ideal templates to produce carbon substrates by a one-step pyrolysis. The unique structural characteristics of MOFs provide advantages over simple mixtures of metal ions and organic precursors. First, MOFs can generate carbon substrates, heteroatoms, and metal atoms simultaneously by a one-step pyrolysis. Organic ligands and metal atoms in MOFs play different roles in resulting CS-SACs. Second, organic linkers in MOFs can serve as coordination sites to adsorb targeted metal ions. Alternatively, MOFs can encapsulate metal precursors into their porous framework by spatial confinement.\n58\n Both coordination sites and pore structures of MOFs can be tailored to increase the density of active catalytic sites. In addition, steric repulsion forces related to pore structures of MOFs can also be tuned to influence the adsorption of reaction intermediates.\n59\n For example, bimetallic (Co and Zn) MOFs were used to synthesize a Co-based CS-SAC supported on N-doped carbon (denoted as CoSA/N-C).\n55\n Zn2+ sites in MOFs could be considered as \u201cdiluting sites\u201d to prevent the agglomeration of Co2+ during pyrolysis (Figure\u00a03B). X-ray absorption fine structure (XAFS) results show that there is no Co\u2013Co bond, suggesting that there are only isolated Co atoms. Similar but different, Fe-TCPP (TCPP\u00a0= tetrakis(4-carboxyphenyl) porphyrin) with a 3D network was used to synthesize an Fe-based SAC supported N-doped carbon for ORR. H2-TCPP ligands effectively prolong the distance between Fe3+ sites, thereby suppressing the agglomeration of Fe atoms during pyrolysis. Also, 3D network structures can be partially preserved, which are beneficial for the mass transfer of O species during ORR (Figure\u00a03C).\n56\n\nFurthermore, various fillers and soft and hard templates have been used in pyrolysis to optimize the pore structures of carbon substrates. For example, Zhu et\u00a0al.\n60\n pyrolyzed MIL-101-NH2 encapsulated with the fillers (dicyandiamide and FeCl3) to synthesize an Fe-based SAC supported on hierarchical carbon substrate for ORR. The hierarchically porous architecture is achieved owing to the internal stress and stains during the thermal decomposition of the fillers. One the other hand, Yang et\u00a0al. pyrolyzed ZIF-8 to synthesize a Zn-based SAC supported on hollow N-doped porous carbon with a high mass loading of 11.3 wt\u00a0%. A hollow structure can be obtained by using polystyrene (PS)/SiO2 particles as a hard template (Figure\u00a03D).\n57\n\n,\n\n61\n\nCS-SACs can also be synthesized by high-temperature solid-state reactions, which are simple, generic, and potentially suitable for large-scale production. However, the rapid nucleation and growth of solid-state products during solution-phase synthesis are regarded as the main challenge for the formation of single atoms. To improve the yield of single atoms, Wu and co-workers proposed an in situ thermal atomization approach to convert carbon-supported Ni NPs into surface-bounded single Ni atoms at 900\u00b0C in Ar atmosphere (Figure\u00a04\nA).\n62\n Ni NPs not only acted as a metal precursor but also served as a catalyst to break C\u2013C bonds on the carbon surface. In situ environmental TEM observation showed that Ni NPs first agglomerated and diffused into carbon matrix as the pyrolytic temperature increases. Thereafter, Ni NPs were eroded at a higher temperature (\u223c900\u00b0C), and vaporized single Ni atoms were anchored on carbon surfaces by forming Ni-N coordination sites at N-rich defects. In another study, bulk Cu powder and ZIF-8 were heated in a tube furnace under NH3 atmosphere to synthesize a Cu-based SAC supported on N-doped porous carbon (Figures 4B and 4C). It was proposed that NH3 molecules served as a \u201ctransporter\u201d to haul out Cu atoms to form volatile Cu(NH3)\nx\n. Afterward, single Cu atoms were bonded to volatilized Zn nodes in ZIF-8. Furthermore, SACs based on other metals, such as Co and Ni, have also been synthesized by similar solid-state reactions.\n63\n\nAs an effective top-down synthesis method for CS-SACs, electrochemical activation plays an essential role because of its simplicity, low cost, and environmental friendliness. Creating defects and vacancies on a carbon substrate surface is beneficial to capture moving transition-metal atoms. Moreover, CS-SACs fabricated by this method provide some advantages for in situ studies of correlations between structures of performances of catalysts. For example, Yao et\u00a0al. synthesized atomically isolated Ni atoms embedded in graphitized carbon (denoted as A-Ni-C) for efficient HER via the electrochemical activation method.\n64\n The as-prepared Ni-MOF as a precursor was carbonized at 700\u00b0C in N2 to obtain Ni@C. After an acid (HCl) leaching treatment, the electrocatalyst was activated by constant potential and cyclic voltammetry treatment. Subsequently, in situ formed single Ni atoms were homogeneously dispersed on the graphitized carbon substrate. Besides monometal CS-SACs, bimetallic Co-Pt C/N based single-atom catalysts (denoted as A-CoPt-NC) were also synthesized by a facile electrochemical activation strategy.\n65\n\n,\n\n66\n Specifically, Co cores were removed from stable Co/C core-shell structures, producing N-doped defective carbons for anchoring atomic metal species. HAADF-STEM and X-ray absorption near-edge structure (XANES) analysis showed that isolated Co/Pt atoms are trapped in a vacancy-type defect in the shell of carbon capsules, thereby forming atomic Co-Pt-N-C coordination structures as active centers. Although the metal loading amount in the resultant A-CoPt-NC catalyst is low (\u223c1.72 wt\u00a0% and \u223c0.16\u00a0wt\u00a0% for Co and Pt, respectively), the catalyst displays excellent activity and robust stability for ORR.Due to their excellent catalytic performances, CS-SACs have been explored in various critical energy-conversion and energy-storage applications. Many studies have been devoted to this research area in the last 3 years. Here, we summarize representative studies in three subsections. The first subsection focuses on the applications related to several essential reactions for energy conversion, including ORR, HER, OER, CO2RR, and NRR. Studies on bi-, tri-, even multi-functional catalysts for multiple reactions simultaneously are also included. The second and third subsections concentrate on the emerging applications of CS-SACs in supercapacitors and rechargeable batteries, respectively.The cathodic ORR is at the heart of many energy-conversion devices, including fuel cells and metal-air batteries.\n67\n\n,\n\n68\n However, the large overpotential required by ORR severely hinders the efficiency and practical application of these devices. Currently, carbon-supported Pt NPs are the commonly used catalyst with the best catalytic activity for ORR. The scarcity and high cost of Pt make it challenging for the wide adaption of Pt/C catalysts. Moreover, the poor stability of Pt catalysts caused by methanol crossover or CO poisoning severely affects their service life. Considerable efforts have been devoted to developing CS-SACs with low cost and high catalytic activities for ORR.A common strategy is to stabilize isolated atomic metal species on porous carbon substrates to obtain high catalytic activities for ORR. For instance, Li et\u00a0al. synthesized an Fe-based SAC supported on carbon nanospheres. Specifically, 3D Fe and N co-doped hollow carbon nanospheres were formed by polymerizing aniline and pyrrole in the presence of carbon nanotubes (denoted as CNT-Fe/NHCNS) (Figures 5A\u20135C).\n69\n This porous structure inhibited the agglomeration of Fe, leading to the formation of abundant atomic Fe-N\nx\n sites. The CNT-Fe/NHCNS (with a mass loading of 0.2\u00a0mg cm\u22122) displayed a half-wave potential (E\n1/2) of \u223c0.84\u00a0V and a high limiting current density of \u223c5.40 mA cm\u22122 in an O2-saturated 0.1\u00a0M HClO4 electrolyte. Another Fe-based SAC with similar structures was constructed by dispersing Fe atoms on N-doped carbon nanospheres (denoted as Fe-N-C HNSs). Atomically dispersed Fe atoms were anchored on carbon nanospheres produced from a biomaterial (histidine) with SiO2 NPs as templates.\n70\n The Fe-N-C HNSs exhibited an E\n1/2 of 0.87\u00a0V and a limiting current density of 5.80 mA cm\u22122 in an O2-saturated 0.1\u00a0M KOH electrolyte.Besides their superior activities in alkaline electrolytes, CS-SACs also demonstrate high ORR performances in acidic electrolytes.\n35\n\n,\n\n71\n For example, Liu et\u00a0al. synthesized a Pt-based SAC by maximizing the utilization efficiency of Pt on a defective carbon substrate (denoted as Pt1.1/BPdefect).\n34\n Computational calculations combining with experimental data revealed that single Pt atoms were anchored by four C atoms neighboring C divacancies. The formed Pt-C4 moieties are usually considered as the main active catalytic centers for ORR. As a result, the limiting current density (5.50 mA cm\u22122) and power density (520 mW cm\u22122) of an acidic fuel cell using Pt1.1/BPdefect in its cathode is comparable with fuel cells using commercial Pt catalysts (Figure\u00a05D). Moreover, the atom utilization in Pt1.1/BPdefect is as high as 0.09 gPt kW\u22121, which can reduce the cost of catalysts in fuel cells.\n34\n In another study, an Fe-based SAC supported on N-doped carbon was produced by pyrolyzing hollow ZIF-8 with ferric acetylacetonate and g-C3N4.\n71\n A high-density Fe(II)-N4-H2O moiety (4.5\u00a0\u00d7 1013 sites cm\u22122) is anchored on the porous carbon. Of note, the catalyst has the E\n1/2 of 0.780\u00a0V and 0.845\u00a0V in 0.1\u00a0M HClO4 and 0.1\u00a0M KOH electrolytes, respectively (Figures 5E and 5F). When it was used in the cathode of an H2/O2 PEMFC, the device delivered a current density of 400 mA cm\u22122 at 0.7\u00a0V or 133 mA cm\u22122 at 0.8\u00a0V, as well as a maximum power density of 628 mW cm\u22122. Most recently, Li et\u00a0al. used a secondary-atom-assisted method to synthesize an Fe-based SAC supported on 1D porous N-doped carbon nanowires (denoted as Fe-NCNWs).\n72\n Due to its unique geometric structure and high Fe mass loading, Fe-NCNWs yielded an E\n1/2 of 0.91\u00a0V and average kinetic current density (J\nK) of 6.0 mA cm\u22122 at 0.9\u00a0V in an alkaline electrolyte, and a satisfactory E\n1/2 of 0.82\u00a0V and average J\nK of 8.0 mA cm\u22122 at 0.8\u00a0V in an acidic electrolyte, respectively. Furthermore, Fe-NCNWs also displayed superior long-term stability and methanol toleration in both alkaline and acidic electrolytes.HER, together with OER, determine H2 production by electrocatalytic water splitting using energy generated from sustainable sources.\n73\n Although Pt-based catalysts are one of the best catalysts for HER, the scarcity and high cost of Pt call for novel catalysts. Furthermore, the utilization of Pt in conventional Pt-based catalysts is very low.\n43\n A large number of CS-SACs, containing metal atoms, such as Co, Fe, Ru, Pt, W, and Mo, have recently been reported to address the above issues.\n74\u201376\n Moreover, some CS-SACs offer additional benefits, such as high electrical conductivity, tunable porous structures, and long-term stability in both acidic and alkaline electrolytes.Liu et\u00a0al.\n74\n synthesized a Pt-based SAC for HER in an acidic electrolyte by anchoring Pt atoms on onion-like carbon nanospheres (denoted Pt1/OLC). Pt1/OLC with 0.27\u00a0wt\u00a0% displayed a low overpotential (\u03b7\n10) of \u223c38\u00a0mV at the current density of 10\u00a0mA cm\u22122. Theoretical calculations suggested that a tip-enhanced local electric field at Pt sites on a curved carbon surface would boost the reaction kinetics of HER. In another study, a Pt-based SAC was synthesized by anchoring single Pt atoms on aniline-stacked graphene (denoted as Pt SASs/AG) using a facile microwave reduction method.\n32\n Extended X-ray absorption fine structure (EXAFS) results revealed that d-electron structures of Pt atoms are optimized by coordinating Pt with N in aniline molecules. Impressively, the as-fabricated Pt SASs/AG catalyst delivered a \u03b7\n10 of 12\u00a0mV and a mass current density of 22,400 AgPt\n\u22121 under 50\u00a0mV. It should be stressed that the utilization of Pt in Pt SASs/AG is 46 times higher than that in commercial 20 wt\u00a0% Pt/C catalyst. Furthermore, computational calculations were used to explain the correlation between catalytic activities and atomic structures of Pt SASs/AG. The considered models included Pt(111), single Pt atom adsorbed on graphene (Ptad/G), and Pt SASs/AG (Figures 6A\u20136C). Their corresponding partial densities of states (PDOSs) of 5d orbitals of Pt atoms are quite different (Figures 6D\u20136F). Pt atoms in Pt SASs/AG can retain atomic orbital characteristics of isolated Pt atoms via their adjacent aniline molecules. The Gibbs free energy (\u0394G\nH\u2217) diagram (Figure\u00a06G) indicates that the \u0394G\nH\u2217 of Pt SASs/AG is \u22120.127 eV, which is comparable with that of Pt(111) facet (\u22120.121 eV). Thus, it was proposed that d-electron structures of Pt atoms and the hydrogen adsorption energy are optimized by the coordination between atomically isolated Pt and N in aniline molecules, leading to the improved catalytic activities for HER.Beyond CS-SACs supported on porous carbon substrates, CS-SACs with core-shell structures have also attracted significant interest for HER. Recently, Zhang et\u00a0al. reported a Pt-based SAC by isolating Pt atoms in N-doped porous carbon core-shell structures (denoted as Pt@PCM).\n77\n Significantly, Pt@PCM exhibited low \u03b7\n10 of 105 and 139\u00a0mV in 0.5\u00a0M H2SO4 and 1.0\u00a0M KOH electrolytes, respectively (Figures 7A\u20137C). It also showed long-term durability in both acidic and alkaline electrolytes. EXAFS results and DFT calculations suggested that the active catalytic sites are lattice-confined Pt centers and activated C/N atoms adjacent to Pt, which should be contributable to the active origin of the superior performances.Compared with electrochemical HER in acidic media, it is more challenging to catalyze HER in alkaline electrolytes due to additional requirements on water adsorption and activation.\n78\n Several theoretical studies have suggested that HER in acidic electrolytes is related to H adsorption (H\nad) at active catalytic centers, whereas in alkaline electrolytes it is governed by the delicate balance among three descriptors: (1) the H\nad on the catalyst surface, (2) the prevention from hydroxyl adsorption (OHad) referred as the poisoning of active sites, and (3) the energy required to dissociate water molecules.\n79\u201381\n In general, the electron transfer kinetics of HER on Pt surfaces in alkaline electrolytes are approximately two orders of magnitude lower than those in acidic electrolytes.\n82\n Recently, Lu et\u00a0al. synthesized a Ru-based CS-SAC by co-doping Ru and N on carbon nanowires.\n75\n The catalyst showed \u03b7\n10 of 12\u00a0mV in a 1\u00a0M KOH electrolyte and 47\u00a0mV in a 0.1\u00a0M KOH electrolyte. The theoretical calculations revealed that RuC\nx\nN\ny\n moieties in the catalyst have a very low hydrogen binding energy, which lowers the kinetic barrier for water dissociation. Most recently, a W-based CS-SAC (denoted as W-SAC) was synthesized by pyrolyzing an MOF (Figures 7D and 7E).\n58\n The W-SAC showed a \u03b7\n10 of 85\u00a0mV and a small Tafel slope of 53\u00a0mV dec\u22121 in 0.1\u00a0M KOH electrolyte. HAADF-STEM and XAFS analysis results suggest that the W1N1C3 moiety (Figure\u00a07F) is the favored local structure of W species. Furthermore, DFT calculations indicated that the W1N1C3 moiety can significantly reduce H adsorption energy, thereby lowering the overpotential of HER.OER is a four-electron-proton coupled reaction, which requires high energy to overcome its kinetic barrier.\n83\n In the past decades, various novel catalysts in forms of clusters, NPs, and nanosheets have been explored to reduce the overpotential of OER. However, developing high-performance OER catalysts with low cost, high stability, and high activities remains difficult. CS-SACs with the ultimate metal utilization efficiency may deliver better catalytic performances for OER.\n17\n Several CS-SACs have been applied to OER.Recently, a Ni-based CS-SAC (denoted as HCM@Ni-N) was synthesized by pyrolyzing a core-shell structure composed of silica nanospheres coated with resorcinol formaldehyde and methylimidazole-Ni (MI) (Figure\u00a08\nA).\n84\n Theoretical calculations indicated that Ni and N atoms can greatly reduce the reaction energy barrier for OER and accelerate its catalytic kinetics (Figures 8B and 8C). Consequently, HCM@Ni-N exhibited a \u03b7\n10 of 304\u00a0mV, which is lower than that of RuO2 (393\u00a0mV), HCM@Ni (without N) (489\u00a0mV), and HCM@N (without Ni) (502\u00a0mV) (Figure\u00a08D). Besides, the corresponding Tafel slope (Figure\u00a08E) of HCM@Ni-N (76\u00a0mV dec\u22121) is considerably smaller than that of HCM@Ni (187\u00a0mV dec\u22121) and HCM@N (203\u00a0mV dec\u22121). For pristine HCM (hollow carbon matrix) and HCM@N, the conversion of OOH\u2217 to O2\u2217 is the rate-determining step (RDS) with the largest reaction free energy of 4.14 and 3.6 eV, respectively. The effective polarization between isolated Ni and coordinated N atoms exerts a significant impact on the RDS in the catalytic process. For HCM@Ni, a lower limiting barrier of 2.36 eV is obtained with the formation of O2\u2217 as the RDS. Moreover, the RDS for HCM@Ni-N is completely changed to the oxidation of OH\u2217 to O\u2217 with the smallest limiting barrier of 1.83 eV, indicating the much easier dissociation of H2O on the HCM@Ni-N and resulting in the promoted reaction kinetics and modified reaction mechanism. Specifically, the unsaturated Ni atoms should be the dominant active centers, and the electronic coupling between Ni and surrounding N atoms can induce electronic redistribution and the lower electron density near the E\nFermi, leading to a significant enhancement of OER activity. More recently, a Ni-based CS-SAC (denoted as A-Ni@DG) was synthesized by incipient wetness impregnation of atomically dispersed Ni (1.24 wt\u00a0%) on DG.\n47\n A-Ni@DG showed a \u03b7\n10 of 270\u00a0mV for OER in a 1.0\u00a0M KOH electrolyte, which is lower than that of Ir/C (320\u00a0mV), DG (340\u00a0mV), and Ni@DG (310\u00a0mV). Furthermore, A-Ni@DG exhibited a Tafel slope of 47\u00a0mV dec\u22121, indicating excellent OER kinetics. Theoretical calculations unveiled that the excellent catalytic activities originate from the tuned local electronic structures of the atomic Ni.To systematically study the relationship among catalytic activity and the interaction of metal centers and carbon, Chen et\u00a0al. constructed two types of N on the surface of graphene (graphitic N and pyridine-like N).\n85\n DFT calculation results suggested that pristine graphene would exhibit negligible catalytic activities for OER. However, the incorporation of graphitic and pyridine-like N in graphene can modulate the electronic structure of supported single Co atoms. The calculated free energy change diagrams imply that pyridine-like N can effectively induce local positive charges on the supported isolated Co atoms, which is beneficial for the adsorption of O-containing species. However, the local charge density of single Co atoms coordinated with more N atoms are likely to become too positive, which is unfavorable for conversing O intermediates. Hence, the authors concluded that moderate positive charge density on single Co atoms is desirable for efficient adsorption and transformation of O intermediates, which may be realized via suitable structure modification of the graphene surface.The excessive CO2 in the atmosphere from the intensive consumption of fossil fuels may cause significant changes in our ecosystems, for example, ocean acidification and global warming.\n86\n\n,\n\n87\n Consequently, exploring methods to convert CO2 to fuels or other value-added chemicals is important for addressing our environmental and energy challenges.\n87\u201389\n Among different methods, electrochemical CO2RR is promising because it usually can take place at room temperature and under ambient pressure, and it can produce a variety of useful gas and liquid substances, such as CO, CH3OH, CH4, and HCOOH. For example, CO can be directly used in gas-to-liquid conversion reactions to produce methanol by hydrogenation or generate liquid hydrocarbon fuels by the Fischer-Tropsch process. Nevertheless, CO2RR suffers from sluggish kinetics because of the low local concentration of CO2 and the low density of active sites on catalyst surfaces. In the past few years, various materials have been explored as catalysts for CO2RR, including metals, metal oxides, chalcogenides, and molecular metal compounds, as well as carbon nanomaterials.\n90\u201392\n Recent studies show that CS-SACs outperform many catalysts based on metal NPs.\n93\u201395\n\nFor example, Zheng et\u00a0al.\n96\n reported a Ni-based SAC (denoted as Ni-NCB) by depositing Ni on low-cost N-doped carbon black and applied it for CO2RR toward CO. A TEM image (Figure\u00a09\nA) shows that Ni-NCB has onion-like, defective graphene layers, serving as substrates for single Ni atoms. Ni-NCB also displayed an excellent performance for CO2RR when tested in a traditional H cell under 0.55\u00a0V overpotential in a 0.5\u00a0M KHCO3 aqueous electrolyte. A plateau of faradic efficiency toward CO (FECO) above 95% was observed over a broad potential range from \u22120.6 to \u22120.84\u00a0V versus RHE (Figure\u00a09B). Ni-NCB delivered a current density above 100 mA cm\u22122 with a nearly 100% selectivity to CO, as well as excellent stability (Figures 9C and 9D). Ni-NCB outperformed several previously reported noble metal-based catalysts.\n97\n\n,\n\n98\n When Ni-NCB was integrated into a 10\u00a0\u00d7 10\u00a0cm\u22122 modular cell, the CO evolution current can reach 8.3 A with a high FECO of 98.4% and a CO production rate of 3.34\u00a0L h\u22121 per unit cell. The superior catalytic performance was attributed to the high Ni mass loading, the maximum utilization efficiency of Ni atoms, and the gas diffusion layer.In another study, atomically dispersed FeN5 single-atom sites were embedded in N-doped graphene (denoted as FeN5) and further used for electrocatalytic reduction of CO2.\n99\n FeN5 exhibited a high FE toward CO at 97.0% under an overpotential of 0.35\u00a0V (Figure\u00a010\nA), which is superior to many catalysts, including N-doped carbon nanomaterials\n100\n\n,\n\n101\n and CS-SACs containing Fe, Co, Ni, or Zn.\n93\n\n,\n\n102\n\n,\n\n103\n\n,\n\n94\n FeN5 also possessed excellent stability with FECO above 97% under \u22120.46\u00a0V versus RHE over 24\u00a0h (Figure\u00a010B). DFT calculation results (Figure\u00a010C) demonstrate that the free energy change of a key step (CO2 \u2192 \u2217COOH) is only 0.77 eV on FeN5, which is much lower than that on FeN4 at 1.35 eV. Moreover, it is proposed that the axial pyrrolic N ligand in FeN5 can diminish the electron density of Fe 3d orbitals and reduce the Fe-CO \u03c0 back-donation, thus achieving rapid desorption of CO (Figures 10D\u201310G).Apart from CO product, a Cu-based CS-SAC (denoted as CuSAs/TCNFs) was synthesized by anchoring Cu atoms on carbon nanofibers for electrocatalyzing CO2 into methanol.\n95\n Brunauer-Emmett-Teller measurement showed that the as-prepared CuSAs/TCNFs possesses a high specific surface area of 618 m2 g\u22121 with uniform pores of \u223c100\u00a0nm, benefiting the mass transfer during electrocatalysis. Impressively, the C1 production selectivity of the as-prepared catalysts was nearly 100% under an overpotential of \u22120.9 V, including CH3OH (44%) and CO (56%), respectively. DFT calculations reveal ed that the \u2217CO intermediates adsorbed on the active sites prefer to be converted into CH3OH rather than being released from the catalyst surface as CO due to the relatively high binding energy between single Cu atoms and \u2217CO.\n95\n Furthermore, the C\u2013C coupling (dimerization) pathway of \u2217CO intermediates was substantially obstructed because of the synergetic effect of porous carbon structures and atomically distributed Cu active sites, resulting in the high selectivity to CH3OH. Therefore, it is critical to better understand the correlation between the material design and catalytic selectivity toward different products of CS-SACs, which offers an excellent platform for developing highly efficient catalysts with improved CO2RR performances (Table 1\n).Ammonia (NH3) is an essential precursor for the synthesis of fertilizers and other biological compounds. It has also been considered as an emerging fuel.\n107\n The current Haber-Bosch process used in NH3 synthesis is energy intensive and consumes 1\u20133% of the world's annual energy usage. Furthermore, the thermodynamically limited conversion is only \u223c15%. The Haber-Bosch process also requires H2, which is currently produced by the steam reforming of natural gas with substantial CO2 emission (\u223c2 tonCO2 tonNH3\n\u22121).\n108\n Therefore, it is highly desirable to develop more green and sustainable synthesis methods for NH3. Different catalysts have been explored for NH3 synthesis, including biological nitrogenases, photocatalysts, thermal catalysts, and electrocatalysts. Among them, NRR by electrocatalysts has attracted significant interest\n109\n because NH3 can be produced from H2O and atmospheric N2 without H2 at room temperature.\n110\n In comparison with ORR, HER, and OER, it is much more challenging to split N2 with the incredibly sturdy N\u2261N bond into free N radicals. Moreover, the adsorption of N2 on catalyst surfaces is often unsatisfactory, which adversely affects the formation of reaction intermediates, limiting the selectivity and yield of NH3.\n111\n Although many metal-based catalysts have been studied for NRR, both the NH3 yield and FE are still far from the requirements of practical applications. It should be noted that the bonding between most metals and N2 is too weak to enable efficient N2 adsorption and activation, which is often considered as the rate-limiting step for NRR. Besides, d-orbital electrons in transition metals are more favorable for the formation of metal H bonds, resulting in the adverse HER at 0 V, lower than that of NRR at 0.057\u00a0V versus standard hydrogen electrode (SHE), which compromises the FE for NRR. CS-SACs have some unique advantages to serve as electrocatalysts for NRR: (1) abundant exposed catalytic active sites providing high catalytic activities, especially for the adsorption of N2; (2) porous and conductive carbon substrates facilitating rapid mass transport and electron transfer; and (3) optimized surface properties with tunable hydrophobicity and hydrophilicity, enabling efficient three-phase contacts among solid catalysts, liquid electrolytes, and gaseous reactants.Recently, an Fe-based CS-SAC (denoted as FeSA-N-C) was synthesized by modulating polypyrrole-iron coordination complexes, yielding atomically dispersed Fe atoms on N-doped graphene-like structures (Figures 11A and 11B).\n112\n FeSA-N-C and N-doped carbon (N-C) were studied as catalysts for NRR in an N2-saturated 0.1\u00a0M KOH solution under different applied potentials. FeSA-N-C is more active for NRR, with a more positive onset potential of 0.193\u00a0V versus RHE compared with that of N-C (Figure\u00a011C). The FE of FeSA-N-C is up to 56.55%, with an NH3 yield of 7.48\u00a0\u03bcg h\u22121 mg\u22121 at 0\u00a0V versus RHE (Figure\u00a011D). In contrast, N-C shows a much lower FE of 9.34% with extremely low NH3 yield (Figures 11E and 11F). DFT calculation results suggested that FeSA-N-C can effectively boost the access of N2 with a low energy barrier of 2.38\u2009kJ\u2009mol\u22121. The localized high concentration of N2 around Fe sites can facilitate N2 adsorption with a low binding Gibbs free energy of \u22120.28\u2009eV.Apart from Fe-based CS-SACs, several other CS-SACs also show excellent performances for NRR. Qin et\u00a0al.\n44\n reported an Au-based CS-SAC (denoted as AuSAs-NDPCs) by decorating single Au atoms on the surface of N-doped porous carbon for NRR. AuSAs-NDPCs showed a stable NH3 yield of 2.32\u00a0\u03bcg h\u22121 mg\u22121 at \u22120.2 V. Besides noble metals, a Mo-based CS-SAC (denoted as SA-Mo/NPC) was synthesized by anchoring Mo atoms on N-doped porous carbon. Due to the high density of MoN\nx\n active sites on hierarchically porous carbon frameworks, SA-Mo/NPC delivers an NH3 yield of 34.0\u00a0\u00b1 3.6\u00a0\u03bcg h\u22121mg\u22121 with a high FE of 14.6%\u00a0\u00b1 1.6% in a 0.1\u00a0M KOH electrolyte at room temperature.\n113\n Moreover, no obvious current drop was observed after a long-term test of 50,000 s. Geng et\u00a0al. synthesized a Ru-based CS-SAC (denoted as Ru SAs/N-C) by pyrolyzing a Ru-containing derivative of zeolitic imidazolate frameworks (ZIF-8).\n114\n Besides well-dispersed single Ru atoms, Raman spectra demonstrate the existence of defective structures in the N-doped carbon substrate. XANES and EXAFS results revealed that N atoms are well coordinated with Ru atoms. Due to its unique structure, Ru SAs/N-C exhibited an NH3 yield of 120.9\u00a0\u03bcg h\u22121mg\u22121 in an N2-saturated 0.05\u00a0M H2SO4 electrolyte, which is 1\u20132 orders higher than that of traditional metal-based electrocatalysts\n115\n\n,\n\n116\n or metal-free carbon catalysts.\n117\n\n,\n\n118\n Some critical test methods, such as the isotopic (15N2) labeling experiment, are necessary to unambiguously demonstrate that the NH3 obtained in NRR experiments is substantially produced by electrochemical reduction of N2 rather than from other exogenous sources.\n119\u2013121\n The isotopic labeling experiment can show a distinguishable chemical shift of doublet coupling in 1H nuclear magnetic resonance spectra, which can be attributed to 15N in 15NH4+, thus confirming the origin of the obtained NH3.Bifunctional electrocatalysts for ORR and OER, HER and ORR, or HER and ORR are desirable for rechargeable metal-air batteries, water electrolyzers, and regenerative fuel cells. They are capable of catalyzing two reactions; thus, it may help to simplify devices that would otherwise be required to accommodate two different types of catalysts.In general, rechargeable Zn-air batteries require catalysts in their cathodes for both ORR and OER during discharging and charging, respectively. To this end, a bimetallic Co-Ni-based CS-SAC (denoted as CoNi-NPs/NC) supported on N-doped hollow carbon nanocubes (NC) was synthesized (Figures 12A and 12B).\n17\n As shown in Figure\u00a012D, CoNi-SAs/NC displays a high catalytic activity for ORR with an onset potential of 0.88 V, an E\n1/2 of 0.76 V, and a large limiting current density of 4.95 mA cm\u22122 in an O2-saturated 0.1\u00a0M KOH electrolyte, outperforming N-doped hollow carbon nanocubes, CoNi NPs supported on NC (CoNi-NPs/NC). Also, CoNi-NPs/NC exhibited a superior activity for OER with a \u03b7\n10 of 340\u00a0mV and a Tafel slope of 58.7\u00a0mV dec\u22121 (Figures 12E and 12F). The rechargeable Zn-air battery was assembled using CoNi-NPs/NC in its cathode for both ORR and OER. Worthy of note, this battery exhibited excellent stability over 40 discharge-charge cycles (Figure\u00a012G). Multiple Zn-air batteries were connected to power many red light-emitting diodes (LEDs) (Figure\u00a012C).Both ORR and HER are required for water electrolyzers. One example for both ORR and HER is a Co-based CS-SAC (denoted as CoSAs/PTFs) synthesized using porous porphyrinic triazine-based frameworks.\n18\n The combination of HAADF-STEM image and XAS analysis confirmed that isolated single Co atoms in the Co-N4 moiety are homogeneously embedded in hierarchically porous substrates.\n18\n As a result, CoSAs/PTFs showed an excellent catalytic activity for ORR with a high E\n1/2 of 0.808\u00a0V and a large limiting current density of 6.14 mA cm\u22122 in an O2-saturated 0.1\u00a0M KOH electrolyte. For HER, CoSAs/PTFs displayed a \u03b7\n10 of 94\u00a0mV and a Tafel slope of 50\u00a0mV dec\u22121. DFT calculations unveiled that the electrocatalytic activity is related to the synergistic interplay between hierarchically porous carbon substrates and Co-N4.Pt-based catalysts usually show high activities for ORR and HER; however, their activities for OER are relatively unsatisfactory.\n122\n\n,\n\n123\n Several metal oxides, such as RuO2 and IrO2, are efficient catalysts for OER, whereas their activities for HER are poor.\n124\n\n,\n\n125\n Some researchers have explored tri- or multi-functional electrocatalysts, which can catalyze multiple reactions.\n126\u2013128\n For example, Co-based CS-SACs are catalytically active for ORR, OER, and HER in separate studies;\n113\n\n,\n\n129\n\n,\n\n130\n while Ni-based CS-SACs are capable of catalyzing OER, HER, and CO2RR in other studies.\n64\n\n,\n\n86\n\n,\n\n131\n This research suggests that CS-SACs may serve as tri- or multi-functional electrocatalysts.A recent demonstration involved an Fe-based CS-SAC (denoted as Fe-N4 SAs/NPC) synthesized by a polymerization-pyrolysis-evaporation method, which contains atomically dispersed Fe-N4 active sites embedded in N-doped porous carbon.\n132\n Fe-N4 SAs/NPC exhibits excellent activities for ORR, OER, and HER (Figures 13A\u201313C). Its onset potential and E\n1/2 for ORR were \u223c0.972\u00a0V and 0.885 V, respectively, and its \u03b7\n10 for OER was 0.43 V, outperforming commercial 20 wt\u00a0% Pt/C and RuO2 catalysts. Furthermore, Fe-N4 SAs/NPC showed a \u03b7\n10 of 0.202\u00a0V and a Tafel slope of 123\u00a0mV\u2009dec\u22121 for HER, comparable with those of 20\u00a0wt\u00a0% Pt/C. Importantly, Fe-N4 SAs/NPC was successfully applied in a water electrolyzer and a Zn-air battery (Figures 13D and 13E). The electrolyzer using electrodes containing Fe-N4 SAs/NPC required a low \u03b7\n10 of 1.67\u00a0V (Figure\u00a013F). Encouragingly, the Zn-air battery containing Fe-N4 SAs/NPC has a lower charge-discharge voltage gap (1.45\u00a0V at 50\u00a0mA\u2009cm\u22122) and a larger power density (232 mW\u2009cm\u22122) than those of the battery based on Pt/C and Ir/C catalysts (1.59\u00a0V at 50 mA\u2009cm\u22122 and 52.8 mW\u2009cm\u22122) (Figure\u00a013G). DFT calculations attributed the trifunctional activity for ORR-OER-HER to the coupling effects between Fe-N4 active centers and porous carbon frameworks.Supercapacitors are electrochemical energy-storage devices, which can store electrical energy based on electrical double-layer capacitance (EDLC) or pseudocapacitance resulting from fast surface redox reactions or ion insertions on electrode surfaces. They can deliver higher power with long cycling life but lower energy density than conventional batteries.\n133\n\n,\n\n134\n The efficient adsorption/desorption of electrolyte ions on electrode surfaces of supercapacitors is critical for their energy storage.\n135\n Current supercapacitor electrodes are mostly based on porous carbon materials with large surface area to deliver EDLC. Emerging applications in consumer electronics, hybrid electric vehicles, and industrial electric utilities require supercapacitors with higher energy-storage density.\n134\n\n,\n\n136\n CS-SACs may be incorporated into carbon electrodes. It has been proposed that single-atom sites may catalyze some surface redox reactions or play other beneficial roles, which increase pseudocapacitance, leading to higher energy-storage density.\n137\u2013140\n The following studies have explored this idea and demonstrate its feasibility to some extent.Yu et\u00a0al. embedded multi-components, including Ni, P, N, and O, into a carbon substrate by one-step pyrolysis of a pre-designed MOF at different temperatures. The resulting hierarchical Ni/P/N/C composite was directly used as supercapacitor electrodes.\n137\n The composite contained Ni single atoms uniformly dispersed in microporous carbon substrates. The Ni/P/N/C composite pyrolyzed at 500\u00b0C has a high specific capacitance of 2879\u00a0F g\u22121 at the current density of 1 A g\u22121 (Figure\u00a014\nA). In another study, a Co-based CS-SAC (denoted as Co-POM/rGO) was successfully synthesized by depositing polyoxometalate (POM) on the surface of rGO aerogel at a mild temperature.\n138\n This material has a relatively small specific surface area of \u223c173.3 m2 g\u22121, and the electrode made of Co-POM/rGO has a specific capacitance of 211.3\u00a0F g\u22121 at 0.5 A g\u22121 based on galvanostatic charge/discharge (GCD) measurement (Figure\u00a014B). A solid-state asymmetric supercapacitor was assembled using Co-POM/rGO with an energy density of 37.6 Wh kg\u22121 at the power density of 500\u00a0W kg\u22121. The supercapacitor has reasonably good cycling stability with capacitance retention of 95.2% after 5,000 charge-discharge cycles at 2 A g\u22121 (Figure\u00a014C). More recently, Shan et\u00a0al.\n140\n incorporated K or Na single atoms on 2D g-C3N4 decorated with MnO2 (denoted as CNM). Figure\u00a014D shows that K-CNM has a specific capacitance of 373.5\u00a0F g\u22121 at 0.2 A g\u22121, which is approximately 4 times higher than that of g-C3N4@MnO2 with K atoms, and also higher than that of Na-CNM at 294.7\u00a0F g\u22121. Also, K-CNM has some cycling stability with 93.7% capacitance retention after 1,000 charge-discharge cycles at 1 A g\u22121 (Figure\u00a014E). The authors proposed that doping single metal atoms improved the electrical conductivity of electrodes and enhanced the mass transfer of electrolyte ions, contributing to the observed higher capacitance.Many new batteries are currently being explored due to the strong demand for more efficient energy-storage solutions.\n141\u2013143\n Rechargeable metal-air batteries, sodium-sulfur (Na-S) batteries, and lithium-sulfur (Li-S) batteries have attracted significant interest because of their potentially low cost, high energy-storage capacity, and prolonged service life.\n144\u2013147\n It has been proposed that adding CS-SACs in battery electrode materials may improve the energy density and rate performance of these emerging batteries. CS-SACs play essential roles in Zn-air batteries, as discussed earlier in the Bifunctional Electrocatalysis section. In this section, we focus on recent studies of the application of CS-SACs in Na-S and Li-S batteries.\n148\u2013152\n\nSingle Co atoms were distributed in hollow carbon nanospheres to serve as S host (denoted as S@Con-HC) in Na-S batteries.\n148\n Na-S batteries using S@Con-HC electrodes delivered a capacity of 220.3 mAh g\u22121 at 5 A g\u22121 and displayed an enhanced cycling stability with a capacity of 508 mAh g\u22121 after 600 cycles at 0.1 A g\u22121 (Figures 15A and 15B). Combining operando Raman spectroscopy analysis, XRD, and computational calculations, the authors proposed that S@Con-HC can improve the reactivity of S and alleviate the \u201cshuttle effect.\u201d\n148\n The adsorption energy of Na2S2 on Co sites is \u22127.85\u2009eV whereas that of Na2S is \u221210.67\u2009eV. The binding of Na2S2 and Na2S clusters is much stronger on Co sites than on pristine carbon surface, which alleviates the dissolution of S and impedes the shuttle effect. This work connects the fields of batteries and electrocatalysts, providing a new exploration direction to enhance the performance of Na-S batteries.Several other studies have explored the application of CS-SACs in Li-S batteries. Wang et\u00a0al.\n149\n synthesized a nanostructured Li2S cathode composed of uniformly dispersed single Fe atoms anchored on a porous N-rich carbon substrate. The assembled Li-S battery demonstrated a high rate performance of 588 mAh g\u22121 at 12 C and a long cycling life with a capacity decay rate of 0.06% per cycle for 1,000 cycles at 5 C. A working mechanism was proposed and is presented in Figure\u00a015C. The Fe-based CS-SAC reduces the energy barrier, leading to a low activation voltage of Li2S. In another study, single Ni atoms were confined in N-doped graphene (denoted as Ni@NG) as separators in Li-S batteries.\n150\n XANES results (Figure\u00a015D) suggest that active Ni centers in Ni-N4 can chemically trap Li polysulfides (LiPS) and form strong S\nx\n\n2\u2212\u00b7\u00b7\u00b7Ni\u2013N bonds. The charge transfer between LiPS and oxidized Ni sites provides low energy barriers, leading to fast LiPS conversions during charging/discharging. Li-S batteries assembled with Ni@NG displayed a stable cycling life with 0.06% capacity decay per cycle after 500 cycles. Most recently, Xie et\u00a0al. synthesized a Co-based CS-SAC (denoted as SC-Co) as interlayers for Li-S batteries.\n151\n Homogeneous dispersion of Co atoms was obtained, which catalyzed redox reactions of S (Figure\u00a015E). Inspiringly, Li-S batteries with the SC-Co interlayer exhibited a high initial capacity of 1,130 mAh gS\n\u22121 at 0.5 C (1 C\u00a0= 1,672 mAh gS\n\u22121) and a final capacity of 837 mAh gS\n\u22121 after 300 cycles, corresponding to a capacity retention of 74.1% and a low fading rate of 0.086% per cycle (Figure\u00a015F). Such batteries may find applications in electric vehicles.The development of efficient and cost-effective catalysts is of great importance for many energy-conversion and energy-storage applications. CS-SACs, as a new class of catalysts, have the potential to replace or complement current noble metal-based catalysts for a range of electrochemical reactions, including ORR, HER, OER, CO2RR, and NRR. They are also useful in improving the performance of electrochemical energy-storage devices such as supercapacitors and rechargeable batteries. Presently, CS-SACs are fabricated using both bottom-up and top-down methods, such as ALD, wet chemistry synthesis, high-energy ball milling, pyrolysis of organic precursors, and high-temperature solid-state reactions, as well as electrochemical activation. Considering the quality of resulting CS-SACs and scalability, current synthesis methods still face various challenges for practical applications. For the characterization of CS-SACs, high-resolution TEM (HRTEM) and XAS analysis involving EXAFS and XANES are usually required to identify geometric and electronic structures. In particular, XAS analysis has been extensively applied to determine the oxidation state of metal centers, bond length, short-range disorder, coordination number, and local geometry. Furthermore, computational methods have often been used to understand the mechanisms related to catalytic activity and stability. Although tremendous progress has been made in the past few years by the strategy that combines advanced synthetic approaches and characterizations, there remain many challenges to be addressed. We provide our views on several key challenges below.Currently it is still challenging to prepare CS-SACs with high metal mass loadings. For example, the pyrolysis of organic precursors is a high-temperature process, which often inevitably leads to the aggregation and sintering of metal atoms into nanoclusters or NPs if strong interactions between metal atoms and substrates are absent. High energy input is required in ball-milling-based methods, which also would cause the agglomeration of single atoms. This decreases the total number of catalytic sites and changes their electronic structures, thereby reducing the catalytic activities of CS-SACs. However, high-temperature treatment is often beneficial for the graphitizing of carbon materials to obtain better electrical conductivity in carbon substrates. Therefore, novel synthesis methods, which can minimize exposure to high temperatures or high energy inputs while producing carbon substrates with sufficient electrical conductivity, are desirable to obtain CS-SACs with high metal mass loadings.Certain advances in the rational design of high-performance CS-SACs have been made by developing various synergistic approaches in the past years. On the foundation of such design lies the materials' atomistic and electronic structure that dictates the intrinsic activity trends and related catalytic mechanism. However, the unambiguous identification of the atomistic structure of the CS-SACs represents a severe challenge for their activity origin study. Several characterization techniques, such as STEM, XRD, M\u00f6ssbauer spectroscopy, and electron paramagnetic resonance spectroscopy, have been used to probe the atomistic or electronic structures of SACs. The inherent characteristics of CS-SACs, such as the non-crystallographic ordering of the metal atoms and the heterogeneity in structure and composition, suggest that sophisticated techniques should be employed. Recently, many publications have proved synchrotron XAS to be a well-suited and powerful technique for characterizing CS-SACs because it can provide valuable information about the coordination environment and the chemical state of the probed atom in an element-selective way. Moreover, XAS allows the study of the dynamic process of electrochemical reactions under operando conditions, which is critical for establishing a precise structure-activity relationship and understanding the catalytic mechanism on the three-phase interface. Nonetheless, unambiguously extracting the exact geometric and electronic structure by XAS remains neither trivial nor straightforward, and the complementary use of different techniques is often necessary. Therefore, methods employing multiple technologies combined, such as HRTEM-XAS-XPS, are highly feasible for genuinely identifying the geometric and electronic structure of CS-SACs.In general, chemical reactions would take place at active catalytic sites that contain unsaturated, distorted, or edge atoms. Understanding the correction between the structures of active sites and their catalytic properties is critical to developing better catalysts. The difficulty of gaining such understanding can be attributed to the structural ambiguity of many potential active sites in a catalyst as well as the poor applicability of transferring information gained from one catalyst to others. Thus, it is desirable to develop model CS-SACs, in which their active catalytic sites are well defined and applicable to different catalysts. Furthermore, an important task linked to this is the precise quantification or imaging of active sites in CS-SACs, especially in reaction conditions. Advanced characterization tools, such as XAS, TEM, atomic force microscopy, and various spectroscopies, are critical to resolving many puzzles.Stability or durability is another crucial factor in practical applications of CS-SACs. The degradation of active catalytic sites is a common phenomenon in many catalysts. For example, Pt NPs in catalysts used in fuel cells would be oxidized from Pt to Pt2+ and aggregate into larger particles due to Ostwald ripening. The dissolved Pt2+ ions can diffuse at the micrometer scale and cause the degradation of fuel cells over time. Thus, there is generally a trade-off between the activity and long-term stability of catalysts. Most of CS-SACs reported so far demonstrating superior activities were tested in research lab settings. There is still a lack of reliable experimental data related to their long-term stability in practical devices. It is essential first to obtain such data to evaluate the performance of CS-SACs in realistic application conditions. If they face a trade-off between activity and stability similar to that in other catalysts (most likely they do), novel methods are needed to increase their stability while retaining their high activities.Many current studies of CS-SACs have already used various computational studies to explain their observed catalytic properties. For example, theoretical calculations are quite successful in predicting atomic structures of CS-SACs, which have been verified by spectroscopies experimentally. The experimental results further stimulate the development of new theories to understand the reaction mechanism. The feedback loop between computational and experimental studies has driven the in-depth understanding and development of CS-SACs for various potential energy-storage and energy-conversion applications. However, many computational studies are based on ideal models with assumptions, simplification, and multiple tunable parameters. It is necessary to avoid the temptation of jumping into simple conclusions by artificially matching experimental data with computational results. We believe the successful feedback loop between theory and experiments will be essential to an in-depth understanding of the structure, mechanism, and kinetics of the catalytic centers. Also, it will guide the design and development of CS-SACs with a desirable activity for specific reactions crucial in energy conversion and storage, as well as for large-scale chemical synthesis, environmental monitoring, and energy devices.This work was supported by the Australian Research Council under the Future Fellowships scheme (FT160100107), Discovery Project (DP180102210), and ARC Discovery Early Career Researcher Award (DE200101669). S.Z. thanks financial support from the F.H. Loxton fellowship.Conceptualization, S.Z. and Y.C.; Writing \u2013 Original Draft, Yongchao Yang and Yuwei Yang; Writing \u2013 Review & Editing, Z.P., K.-H.W., C.T., H.W., L.W., A.M., C.Y., J.D., S.Z., and Y.C.; Supervision, S.Z. and Y.C.", "descript": "\n                  Single-atom catalysts (SACs) have the advantages of both homogeneous and heterogeneous catalysts, which show promising application potentials in many renewable energy-conversion technologies and critical industrial processes. In particular, carbon-supported SACs (CS-SACs) are of great interest because of their maximal atom utilization (\u223c100%), unique physicochemical structure, and beneficial synergistic effects between active catalytic sites and carbon substrates. In this review, we offer a critical overview of the unique advantages of CS-SACs related to their material designs, catalytic activities, and potential application areas. The state-of-the-art design and synthesis of CS-SACs are described under the framework of bottom-up and top-down approaches. We also comprehensively summarize recent advances in developing CS-SACs for important electrochemical reactions, i.e., oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, CO2 reduction reaction, nitrogen reduction reaction, serving as bi-/multi-functional electrocatalysts, and usages in supercapacitors and batteries. Lastly, the critical challenges and future opportunities in this emerging field are highlighted.\n               "}
{"full_text": "Because of the diverse magnetic domain configurations from superparamagnetism, single domain, vortex domain to multi-domain, magnetic functional materials with different frequency spectrum characteristics are widely used in the field of medical treatment, information record/storage, electronic device, and electromagnetic wave absorption [1\u20133]. Due to the lack of effective synthesis strategy and direct visible characterization methods, the responding behaviors of magnetic vortex domain at nanoscale are not clear when interacting with electromagnetic (EM) wave at gigahertz (GHz) frequency band.Recently, metal-organic frameworks (MOFs) derivatives have been widely popular in EM wave dissipation materials and functional devices because of their unique three-dimensional periodic structure and adjustable electromagnetic properties [4\u20136]. Firstly, size, morphology, and micro-nano structure of MOFs-derived composites can be maintained from targeted precursors, providing the confined component distribution. Secondly, MOFs-derived EM wave absorbers show unique magnetic-carbon interfaces design, electronic migration routes, and EM balance features, encouraging the dielectric dissipation. More importantly, the cation host can be converted into different magnetic components, including Fe/Co/Ni metal, FeNi/Ni1-x\nCo\nx\n alloy, Fe3O4 oxide, Fe3C nitride, and NiFe2O4 ferrite with different magnetic structure [7,8]. These magnetic particles offer huge space to regulate the magnetic domain, saturation magnetization, and magnetic respond behaviors, dominating the final consumption ability with incident EM energy. To fabricate the wide-frequency EM wave absorbers, many MOFs precursors and carbonized derivatives were designed such as Co-ZIF-67, Co-MOF-74, Fe-MOF-5, Fe-MIL-88, Co\u2013Zn-ZIF-67 and so on [9\u201314]. Those EM wave absorbers exhibit magnetic-carbon synergy effect and strong absorption intensity. However, the MOFs-derived absorbers still face some challenges for modern practical application, including wide frequency absorption, low adding mass, and tunable absorption frequency. Meanwhile, the magnetic domain motion, magnetic coupling effect, and absorption mechanism is unclear. So, the further investigation is urgently needed, which still face huge challenge.Herein, a nanoscale magnetic vortex domain is firstly observed in the MOFs-derived Ni particles wrapped by graphitized carbon shell. Because of the high symmetry spheres boundary exposed by carbon shell, Nickel magnetic domain tends to be arranged to form the special magnetic vortex conformation according to the easy magnetization surface. Magnetic-carbon microsphere was successfully assembled by abundant of core-shell Ni@C units using Ni-MOFs as a precursor, which not only constructs balanced magnetic-dielectric distribution but also forms directional electronic migration routes. As EM wave absorbers, MOFs-derived Ni@C microspheres exhibit outstanding energy absorption ability and wide frequency respond regions. The minimum reflection loss (RLmin) values of Ni@C\u2013H, Ni@C\u2013S, and Ni@C\u2013V microspheres can reach to \u221247.2\u00a0\u200bdB, \u221248.5\u00a0\u200bdB, and \u221254.6\u00a0\u200bdB, respectively. Benefited from the confined magnetic vortex motion under high-frequency EM field, Ni@C\u2013V microsphere possess the widest efficient absorption bandwidth (EAB) to 5.0\u00a0\u200bGHz at only 2.0\u00a0\u200bmm and 25% mass adding. Simultaneously, both reversion motion and core polarity change of the magnetic vortex together contribute to the enhanced EM energy dissipation and expanded absorption frequency. MOFs-derived magnetic-dielectric microspheres with special magnetic domain structure project a new idea to fabricate excellent EM wave absorption candidates.The synthetic schematic of MOF-derived Ni@C microspheres is displayed in Fig.\u00a01\n. By adjusting the organic ligands, various Ni-MOFs precursors are firstly synthesized in the mixed EtOH/DMF/H2O solvent under undergoing a solvothermal reaction. Due to the different functional groups on the benzene ring structure, changeable coordination and periodic structure can be built between the organic ligand (H3BTC, TA and ATA) and metal Nickel (Ni2+) host, dominating the final size and morphology. In Fig.\u00a0S1, it can be found the diversity microsphere morphology among Ni-MOFs-H, Ni-MOFs-S, and Ni-MOFs-V precursors. Secondly, obtained Ni-MOFs powders are further annealed in an Ar condition. Nickel ions are reduced to nickel metal particles/clusters acting as a catalyst. Finally, adjacent organic ligand is transformed into graphitized carbon shell wrapping the Ni core, which form a basic core-shell Ni@C unit. Finally, MOFs-derived Ni@C\u2013H, Ni@C\u2013S, and Ni@C\u2013V microspheres are successfully fabricated with different Ni-MOFs precursors.As displayed in the XRD spectrum (Fig.\u00a02\na), the typical diffraction peaks located at 2\u03b8\u00a0\u200b=\u00a0\u200b44.5\u00b0, 519\u00b0, and 76.5\u00b0 are belonged to the Ni metal (JCPDS# 83\u20134000) and the weak diffraction peak of 2\u03b8\u00a0\u200b=\u00a0\u200b26\u00b0 are contributed to the carbon component. To further the evaluate the degree of graphitization, Raman spectrum of MOFs-derived Ni@C microspheres are further obtained (Fig.\u00a02b). Generally, the intensity ratio of D-band and G-band (I\nD/I\nG) is a key indicator, dominating the catalytic ability of reduced Ni NPs. The I\nD/I\nG ratio of MOFs-derived Ni@C microspheres keep similar value of 1.02, implying the equal capacity to promote the graphitization of organic ligands. Meanwhile, due to the presence of magnetic Ni NPs, MOFs-derived Ni@C microspheres exhibit unique magnetic responding capacity, reflecting by the hysteresis loop curves (Fig.\u00a02c). The saturation magnetization (Ms) values of Ni@C\u2013H, Ni@C\u2013S, and Ni@C\u2013V microspheres are 50.2 emu/g, 58.8 emu/g, and 53.1 emu/g, respectively. As results, MOFs-derived Ni@C microspheres display similar component, graphitization degree, and magnetic property.Due to the diversity of the coordination mode between the host nickel ion and organic ligand, MOFs-derived Ni@C microspheres exhibit different microstructures from the SEM and TEM images (Figs.\u00a02 and 3\n). Using H3BTC as ligand, the size of Ni@C\u2013H microspheres is 1.5\u20132.0\u00a0\u200b\u03bcm, and spindle-shaped nanoparticles randomly decorate on the surface of the rough microsphere (Fig.\u00a02d\u2013f, Fig.\u00a03a1-a3). When the ligand is TA, obtained Ni@C\u2013S microspheres show a smooth surface and a perfect spherical shape at 2.0\u20134.0\u00a0\u200b\u03bcm (Fig.\u00a02g\u2013i, Fig.\u00a03b1\u2013b3). Different with above-mentioned Ni@C composites, MOFs-derived Ni@C\u2013V microsphere has a unique wrinkled surface and the particle size is 1.5\u20132.0\u00a0\u200b\u03bcm when the ligand is ATA (Fig.\u00a02j-l, Fig. 2c1-c3). In the HRTEM images, highly graphitized carbon shells wrap the metallic nickel particle constructing unique core-shell structure (Fig.\u00a03a4, 3b4, 3c4). Because of the ligand transformation and brooked MOFs frameworks, there are lots of porous and gaps in those Ni@C microspheres (Fig.\u00a02d\u2013l). Due to the confinement reduction effect at nanoscale, carbonized Ni-MOFs microspheres indicate the uniform element mapping distribution at micrometer scale (Fig.\u00a0S2). As results, magnetic-carbon Ni@C composites with various nano-micro structure are successfully prepared using Ni-MOFs as templates.To evaluate the EM wave dissipation capacity, MOFs-derived Ni@C microspheres are characterized by the vector network analyzer (VNA) at 2\u201318\u00a0\u200bGHz. Generally, EM parameters are composed of complex permittivity (\u03b5\nr\u00a0\u200b=\u00a0\u200b\u03b5\u2032-j\u03b5\u2033) and complex permeability (\u03bc\nr\u00a0\u200b=\u00a0\u200b\u03bc\u2032-j\u03bc\u2033) [15\u201317]. They can not only reflect the intrinsic storage properties (\u03b5\u2032, \u03bc\u2032), but also determine the final energy absorption ability (\u03b5\u2033, \u03bc\u2033) of EM wave conversion materials. As shown in Fig.\u00a04\na\u2013c, EM parameters of three MOFs-derived Ni@C microspheres all show classical frequency-dependency feature. Remarkably, the real part (\u03b5\u2032) value of the Ni@C\u2013H, Ni@C\u2013S, and Ni@C\u2013 Ni@C\u2013V decrease from the initial 9.1 to 5.4, 11.5 to 8.1, and 12.6 to 7.4, respectively. As the frequency increases, the imaginary part (\u03b5\u2033) data of the complex permittivity show a similar declining trend. The \u03b5\u2033 values decrease from 4.1 to 2.4 for Ni@C\u2013H, from 4.8 to 2.6 for Ni@C\u2013S, and 7.0 to 2.8 for Ni@C\u2013V, respectively. Due to the permeability dispersion characteristics, the real part (\u03bc\u2032) and imaginary part (\u03bc\u2033) values of MOFs-derived Ni@C microspheres are relatively similar without huge difference, which maintain at 1.15\u20131.0 and 0.1\u20130.02, respectively. Higher EM parameters values in the Ni@C\u2013V microspheres mean the stronger energy absorption [18]. Compared with other Ni@C microspheres, MOFs-derived Ni@C\u2013V microspheres hold the higher attenuation constant (Fig.\u00a0S3) and loss tangent values (Fig.\u00a0S4), also meaning the better EM wave absorption capacity.Based on the obtained EM parameters, the reflection loss (RL) values of MOFs-derived Ni@C powders are calculated as shown in Fig.\u00a04d\u2013f. Due to the well impedance matching (Z\u00a0\u200b=\u00a0\u200bZ\nin/Z\n0) values (Fig.\u00a0S5) and synergy EM wave absorption ability, MOFs-derived three Ni@C microsphere all display high-performance EM absorption. Changing the thickness from 1.5\u00a0\u200bmm to 4.0\u00a0\u200bmm, the RLmin values of the Ni@C\u2013H are \u22127.8\u00a0\u200bdB, \u221232.4\u00a0\u200bdB, \u221247.2\u00a0\u200bdB, \u221231.6\u00a0\u200bdB, \u221230.2\u00a0\u200bdB, and \u221230.7\u00a0\u200bdB (Fig.\u00a04g). When the thickness is 3.0\u00a0\u200bmm, Ni@C\u2013S microsphere displays the strongest RL value of \u221248.5\u00a0\u200bdB at 8.6\u00a0\u200bGHz (Fig.\u00a04h). For the Ni@C\u2013V microspheres, the RLmin value can reach \u221254.6\u00a0\u200bdB at 10.6\u00a0\u200bGHz, and the efficient absorption (RL\u00a0\u200b\u2264\u00a0\u200b\u221210\u00a0\u200bdB) regions up to 5.0\u00a0\u200bGHz at only 2.0\u00a0\u200bmm (Fig.\u00a04i). In addition, the influence of ratio of Ni/C, the annealing temperature, and the filling ratio on the EM absorption of MOFs-derived Ni@C powder are also discussed in detail (Table\u00a0S1, Table\u00a0S2, Table\u00a0S3, Fig.\u00a0S6, Fig.\u00a0S7, Fig.\u00a0S8). It can be concluded that the Ni@C\u2013V microspheres with special conditions (1.5\u00a0\u200bmmol Ni2+ adding mass, 600\u00a0\u200b\u00b0C annealing, 25% filling ratio) can exhibit outstanding EM energy conversion behavior. Surprisingly, MOFs-derived Ni@C\u2013V microspheres exhibit outstanding EM wave conversion ability and tuning efficient absorption frequency (C-band, X-band, Ku-band), which dominate the huge potential as lightweight and efficient EM wave absorbers. Definitely, the EM wave dissipation ability and the absorption bandwidth of MOFs-derived Ni@C\u2013V microspheres get enhanced significantly compared with the Ni@C\u2013H and Ni@C\u2013S microspheres, indicating that the magnetic vortex could provide more contributions. The associated EM wave absorption mechanisms are discussed as following aspects.\n\ni)\n\n3D magnetic coupling network enhanced electromagnetic wave consumption. Due to the presence of metallic Ni NPs, MOFs-derived magnetic Ni@C microspheres exhibit unique magnetic domain structure and responding properties. Clearly, the metal content of Nickel in the Ni@C\u2013H, Ni@C\u2013S and Ni@C\u2013V is 89.4%, 67.4%, and 77.1%, respectively (Fig.\u00a0S9). With the support of electronic holography technology, the reconstructed phase hologram is used to display the intrinsic magnetic field line distribution of Ni@C microspheres (Fig.\u00a05\nb, e, 5h). It can provide strong evidence for understanding the magnetic loss mechanism toward EM wave energy [4,5,19\u201322]. Clearly, all the Ni@C samples can radiate out high-density magnetic field line surrounding the microspheres surface (Fig.\u00a05a, d, 5g). Meanwhile, the space scope of emitted magnetic flux lines greatly exceeds the size of the Ni@C sphere itself. Zooming in the surface region of Ni@C\u2013V microsphere (Fig.\u00a05j and k), shape-dependence magnetic lines distribution is directly observed from the reconstructed holography images (Fig.\u00a05l). The shared magnetic flux lines among those Ni@C microspheres dominates the enhanced magnetic coupling effect (Fig.\u00a05c, f, 5i). The unique interaction can boost the magnetic responding intensity and expand the responding space [23\u201325]. As a result, constructing unique 3D magnetic coupling network improve the complex permeability and the magnetic consumption ability toward incident EM wave energy.\n\n\nii)\n\nHeterojunction Ni\u2013C interfaces and connected graphited carbon shells boosted dielectric absorption. Intrinsic dielectric properties of Ni@C microspheres are particularly important, which determines the dielectric absorption ability. According to the Debye theory and dielectric dissipation mechanism, the main contribution is represented by the conduction loss and interfacial polarization [26\u201329]. The relationship between the complex permittivity (\u03b5\u2032, \u03b5\u2033) and intrinsic conductivity (\u03c3) can be explained as follow formulas:\n\n\n\n\n(1)\n\n\u03b5\n\u2032\n=\n\n\u03b5\n\u221e\n\n+\n\n\n\n\u03b5\ns\n\n\u2212\n\n\u03b5\n\u221e\n\n\n\n1\n+\n\n\n\n\u03c9\n\u03c4\n\n\n2\n\n\n\n\n\n\n\n\n(2)\n\n\n\n\u03b5\n\u2033\n\n=\n\n\n\n\u03b5\ns\n\n+\n\n\u03b5\n\u221e\n\n\n\n1\n+\n\n\n(\n\n\u03c9\n\u03c4\n\n)\n\n2\n\n\n\n\u03c9\n\u03c4\n+\n\n\u03c3\n\n\u03c9\n\n\u03b5\n0\n\n\n\n\n\n\nwhere \u03b5\u2032 is the real part of complex permittivity, \u03b5\u2033 is the imaginary part of complex permittivity, \u03b5\ns is the static permittivity, \u03b5\n\u221e is relative dielectric permittivity at the high-frequency limit, \u03c9 is angular frequency, \u03c4 is polarization relaxation time and \u03c3 is conductivity, and \u03b5\n0 is vacuum dielectric permittivity.\n3D magnetic coupling network enhanced electromagnetic wave consumption. Due to the presence of metallic Ni NPs, MOFs-derived magnetic Ni@C microspheres exhibit unique magnetic domain structure and responding properties. Clearly, the metal content of Nickel in the Ni@C\u2013H, Ni@C\u2013S and Ni@C\u2013V is 89.4%, 67.4%, and 77.1%, respectively (Fig.\u00a0S9). With the support of electronic holography technology, the reconstructed phase hologram is used to display the intrinsic magnetic field line distribution of Ni@C microspheres (Fig.\u00a05\nb, e, 5h). It can provide strong evidence for understanding the magnetic loss mechanism toward EM wave energy [4,5,19\u201322]. Clearly, all the Ni@C samples can radiate out high-density magnetic field line surrounding the microspheres surface (Fig.\u00a05a, d, 5g). Meanwhile, the space scope of emitted magnetic flux lines greatly exceeds the size of the Ni@C sphere itself. Zooming in the surface region of Ni@C\u2013V microsphere (Fig.\u00a05j and k), shape-dependence magnetic lines distribution is directly observed from the reconstructed holography images (Fig.\u00a05l). The shared magnetic flux lines among those Ni@C microspheres dominates the enhanced magnetic coupling effect (Fig.\u00a05c, f, 5i). The unique interaction can boost the magnetic responding intensity and expand the responding space [23\u201325]. As a result, constructing unique 3D magnetic coupling network improve the complex permeability and the magnetic consumption ability toward incident EM wave energy.\nHeterojunction Ni\u2013C interfaces and connected graphited carbon shells boosted dielectric absorption. Intrinsic dielectric properties of Ni@C microspheres are particularly important, which determines the dielectric absorption ability. According to the Debye theory and dielectric dissipation mechanism, the main contribution is represented by the conduction loss and interfacial polarization [26\u201329]. The relationship between the complex permittivity (\u03b5\u2032, \u03b5\u2033) and intrinsic conductivity (\u03c3) can be explained as follow formulas:Based on the equations, the imaginary part (\u03b5\u2033) of complex permittivity is proportional to the electronic conductivity (\u03c3), which means that the higher electronic conductivity is benefit to the enhanced dielectric dissipation ability [30\u201335]. Undergoing the carbothermal reduction process, carbon-containing organic ligand were finally converted into the connected carbon shells. Catalyzed by the inner nickel core, graphitized carbon shells connect to each other, forming an electron transport network. Under the action of high-frequency microwave field, the rapid electron migration ability improves the conduction loss in the Ni@C\u2013V material by generating the microwave energy into Joule heat [36,37]. Meanwhile, MOFs-derived Ni@C microsphere is assembled by plentiful connected magnetic@carbon nanoparticles. Reduced Ni NPs are wrapped by the graphitized carbon shell, constructing heterojunction Ni\u2013C interfaces and fast electronic migration routes.Visually, the information of charge density distribution surrounding at Ni\u2013C interfaces is observed via high-resolution electron holography images (Fig.\u00a06\na). The Ni\u2013C interfaces are clearly distinguished by different featuring colors between nickel core and carbon shell (Fig.\u00a06b and c). Focusing on the carbon layers, graphitized carbon tightly bridges magnetic Ni core, building rich heterejunction Ni\u2013C interfaces regions (Fig.\u00a06d). Because the metallic nickel core is wrapped by the graphitized carbon layer, a mutation of charge density distribution is generated at the Ni\u2013C core-shell interfaces (Fig.\u00a06e). Due to the difference in the electrical properties between Ni and carbon, the metallic Ni tends to accumulate more negative charges, while the position of the carbon layer gathers more positive charges (Fig.\u00a06f). The similar situation also occurs in the two connected Ni@C\u2013V units, indicating that the charge distribution can be effectively modulated in the heterejunction region (Fig.\u00a06g and h). Therefore, MOFs-derived Ni@C\u2013V microspheres provide the high-density interfacial polarization regions, contributing to the enhanced dielectric loss.\n\niii)\n\nConfined magnetic vortex reversal expanded the efficient absorption frequency. By adjusting the organic ligand into ATA, MOFs-derived Ni@C\u2013V microsphere are assembled by plentiful core-shelleyi@C unit. Compared with other reported MOFs-derived Ni@C EM wave absorption materials, special magnetic vortex structure was firstly observed in the Ni core at nanoscale derived from Ni-MOF-V precursor. Benefited from the chemistry environment and coordination structure from ATA ligand, host nickel ion (Ni2+) was reduced into magnetic NPs, which further promoted the graphitization of organic ligands. In turn, formed carbon shell on the surface of Ni core will limited the expand space with the growth of magnetic Ni NPs. With high symmetry spheres and boundary restriction of graphited carbon shell, confined magnetic vortex structure is generated in the Ni@C\u2013V microspheres powders. However, the Ni@C\u2013H, Ni@C\u2013V and other reported Ni@C magnetic-carbon composites do not show the magnetic vortex structure. The other important factor is the size of magnetic NPs, which is necessary to provide enough space to generate magnetic vortex core and domains. According to EM wave absorption theory, the natural resonance of magnetic Ni@C\u2013V microsphere becomes the main dissipation mechanism below 8\u00a0\u200bGHz. Increased ferromagnetic resonance behavior contributes to the magnetic loss at 10\u201318\u00a0\u200bGHz [38\u201342]. Surprisingly, unique magnetic vortex domain is firstly observed in the soft magnetic Ni NPs at nanoscale (Fig.\u00a07a). Benefited from the easy magnetization axis and high symmetry of confined sphere spaces, magnetic Ni NPs with vortex domain structure are confined by the carbon shell. With the assistance of electron holography technology, the vortex domain is visually observed in magnetic Ni NPs (Fig.\u00a07b). Surrounded by carbon shell, dispersed magnetic Ni NPs hold the whole magnetic domain structure with the vortex core region about 10\u00a0\u200bnm, and two connected magnetic Ni NPs display similar vortex domain, which marked by the purple circular shape (Fig.\u00a07c).\n\n\n\nConfined magnetic vortex reversal expanded the efficient absorption frequency. By adjusting the organic ligand into ATA, MOFs-derived Ni@C\u2013V microsphere are assembled by plentiful core-shelleyi@C unit. Compared with other reported MOFs-derived Ni@C EM wave absorption materials, special magnetic vortex structure was firstly observed in the Ni core at nanoscale derived from Ni-MOF-V precursor. Benefited from the chemistry environment and coordination structure from ATA ligand, host nickel ion (Ni2+) was reduced into magnetic NPs, which further promoted the graphitization of organic ligands. In turn, formed carbon shell on the surface of Ni core will limited the expand space with the growth of magnetic Ni NPs. With high symmetry spheres and boundary restriction of graphited carbon shell, confined magnetic vortex structure is generated in the Ni@C\u2013V microspheres powders. However, the Ni@C\u2013H, Ni@C\u2013V and other reported Ni@C magnetic-carbon composites do not show the magnetic vortex structure. The other important factor is the size of magnetic NPs, which is necessary to provide enough space to generate magnetic vortex core and domains. According to EM wave absorption theory, the natural resonance of magnetic Ni@C\u2013V microsphere becomes the main dissipation mechanism below 8\u00a0\u200bGHz. Increased ferromagnetic resonance behavior contributes to the magnetic loss at 10\u201318\u00a0\u200bGHz [38\u201342]. Surprisingly, unique magnetic vortex domain is firstly observed in the soft magnetic Ni NPs at nanoscale (Fig.\u00a07a). Benefited from the easy magnetization axis and high symmetry of confined sphere spaces, magnetic Ni NPs with vortex domain structure are confined by the carbon shell. With the assistance of electron holography technology, the vortex domain is visually observed in magnetic Ni NPs (Fig.\u00a07b). Surrounded by carbon shell, dispersed magnetic Ni NPs hold the whole magnetic domain structure with the vortex core region about 10\u00a0\u200bnm, and two connected magnetic Ni NPs display similar vortex domain, which marked by the purple circular shape (Fig.\u00a07c).In order to determine the magnetic vortex domain conformation, the final magnetic structure is reconstructed with phase information images (Fig.\u00a07d\u2013f). Nanoscale Ni cores possess various magnetic vortex configuration with different spin (C\u00a0\u200b=\u00a0\u200b\u00b11) and polarity (p\u00a0\u200b=\u00a0\u200b\u00b11), respectively (Fig.\u00a07g\u2013i). The moment and respond behaviors in vortex domain of Ni particles can build a new EM wave dissipation mechanism. In the results of micromagnetic simulation (Fig.\u00a08\n), when the magnetic Ni NPs are magnetized under a high-frequency magnetic field, initial magnetic vortex state quickly changes to other configurations (Fig.\u00a08a-l, Movie S1, Supporting information). Limited by the geometry shape, the magnetized vortex state in spherical magnetic Ni core will change to the \u201cC shape\u201d state as the external magnetic field changes. Incident EM energy can be converted and dissipated in the domain evolution process from the energy lowest point to the unstable energy configuration [43]. Magnetic coupling effect and vortex-domain moment behaviors of magnetic Ni core provide an innovative insight and discussion about the magnetic absorption mechanism [44\u201346]. Compared with reported Ni-based MOFs-dervied EM absorption materials, MOFs-derived Ni@C\u2013V exhibits outstanding EM energy conversion with the advantages of lightweight, low filling ratio, strong absorption and wide absorption frequency, especially in the X-band (Table\u00a0S4). As results, constructing vortex domain structure in the MOFs-derived functional materials provides a new adjustment strategy to boost electromagnetic wave energy dissipation.Supplementary data related to this article can be found at https://doi.org/10.1016/j.apmate.2023.100111.The following is the supplementary data related to this article:\n\nMultimedia component 2\nMultimedia component 2\n\n\n\nIn summary, a unique magnetic vortex domain structure is firstly observed in the MOFs-derived Ni NPs. Limited by the symmetry spheres and boundary condition, reduced Ni core is confined in the graphited carbon shell, constructing the basic electromagnetic wave dissipation unit. Benefited from the magnetic-dielectric synergy effect, MOFs-derived Ni@C\u2013V microsphere exhibits outstanding EM absorption ability. The RLmin value of Ni@C\u2013V microspheres can reach \u221254.6\u00a0\u200bdB at 2.5\u00a0\u200bmm. More important, the magnetic coupling network and vortex-domain reversal in the Ni@C\u2013V microsphere together contribute to the expanded efficient absorption frequency up to 5.0\u00a0\u200bGHz at only 2.0\u00a0\u200bmm. Meanwhile, core-shell Ni@C\u2013V units construct plentiful heterojunction Ni\u2013C interfaces and build connected electronic migration routes, which encourages the interfacial polarization and conduction loss. With unique magnetic vortex domain structure and synergy dissipation mechanism, MOF-derived functional microspheres show great application prospects in electromagnetic wave absorption filed.All the chemicals used were of analytical grade without further puri\ufb01cation and were purchased from Sinopharm Chemical Reagent Co, Ltd. Typically, 1.5\u00a0\u200bmmol nickel nitrate hexahydrate (0.436\u00a0\u200bg) and 1.5\u00a0\u200bg PVP K-30 are poured into the mixed solution with 10\u00a0\u200bmL distilled water (H2O), 10\u00a0\u200bmL ethanol (EtOH) and 10\u00a0\u200bmL\u00a0\u200bN, N-dimethylformamide (DMF). The mixed solution is magnetic stirred for 10\u00a0\u200bmin. Then, 0.15\u00a0\u200bg trimellitic acid (H3BTC), 0.15\u00a0\u200bg terephthalic acid (TA), and 0.15\u00a0\u200bg diaminoterephthalic acid (ATA) as the ligand are added into above-mentioned solution, respectively. After another magnetic stirred for 10\u00a0\u200bmin, the three solutions are transferred to a 50\u00a0\u200bmL autoclave and keep at 150\u00a0\u200b\u00b0C for continuous heating for 12\u00a0\u200bh. The products are taken out and washed with ethanol and distilled water three times, respectively. The solid powders are placed in a vacuum drying oven at 60\u00a0\u200b\u00b0C for 12\u00a0\u200bh, and products are marked as Ni-MOFs-H, Ni-MOFs-S, and Ni-MOFs-V, respectively.Obtained Ni-MOFs precursors are annealed in a tube furnace. The calcination conditions are 600\u00a0\u200b\u00b0C for 5\u00a0\u200bh in an Ar atmosphere, and the heating rate is 2\u00a0\u200b\u00b0C/min. Due to the different ligands, the final black MOFs-derived product magnetic Ni@C powders are marked as Ni@C\u2013H, Ni@C\u2013S, and Ni@C\u2013V, respectively.The crystal structure and components of as-synthesized Ni@C microspheres are characterized by the X-ray diffractometer (XRD, Bruker, D8-Advance, Germany). Raman data is obtained by the Ramoscope (inVia, Renishaw, United Kingdom). The magnetic prosperities and hysteresis loop of MOFs-derived Ni@C powders are tested by vibrating sample magnetometer (VSM, Quantum Design, United States). The morphology and microstructure of the Ni@C microspheres are examined by the scanning electron microscope (FESEM, S-4800, Japan) and transmission electron microscope (TEM, JEM-2100\u00a0\u200bF, Japan). Those Ni@C microspheres are prepared with paraffin matrix with mass ratios 1:4. Then, mixed sample are compacted into a coaxial ring of 7.00\u00a0\u200bmm outer diameter and 3.04\u00a0\u200bmm inner diameter. Related electromagnetic parameters were measured via a vector network analyzer (VNA, N5230C, Agilent, United States) in 2\u201318\u00a0\u200bGHz range. The reflection loss (RL) values were calculated by the following formula:\n\n(3)\n\nZ\n=\n\n|\n\nZ\n\ni\nn\n\n\n/\n\nZ\n0\n\n|\n\n=\n\n\n\u03bc\nr\n\n/\n\n\u03b5\nr\n\n\ntan\n\nh\n\n\nj\n\n\n\n2\n\u03c0\nf\nd\n\nc\n\n\n\n\n\u03bc\nr\n\n\n\u03b5\nr\n\n\n\n\n\n\n\n\n\n(4)\n\n\nR\nL\n=\n20\n\nlog\n\n|\n\n\n(\n\n\nZ\n\ni\nn\n\n\n\u2212\n\nZ\n0\n\n\n)\n\n/\n\n\nZ\n\ni\nn\n\n\n+\n\nZ\n0\n\n\n\n|\n\n\n\n\nwhere Z\nin is the normalized input impedance of absorber, Z\n0 is the impedance of free space, \u03b5\nr is the complex permittivity, \u03bc\nr is the complex permeability, \u0192 is the frequency, c is the light velocity, and d is the thickness of the absorber, 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.Lei Wang and Mengqiu Huang contributed equally to this work. This work was supported by the National Natural Science Foundation of China (52231007, 51725101, 11727807, 52271167, 22088101), the Ministry of Science and Technology of China (973 Project Nos. 2021YFA1200600 and 2018YFA0209100), the Shanghai Excellent Academic Leaders Program (19XD1400400).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.apmate.2023.100111.", "descript": "\n                  Magnetic domain structure plays an important role in regulating the electromagnetic properties, which dominates the magnetic response behaviors. Herein, unique magnetic vortex domain is firstly obtained in the Ni nanoparticles (NPs) reduced from the Ni-based metal-organic frameworks (MOFs) precursor. Due to both the high symmetry spheres and boundary restriction of graphited carbon shell, confined magnetic vortex structure is generated in the nanoscale Ni core during the annealing process. Meanwhile, MOFs-derived Ni@C assembly powders construct special magnetic flux distribution and electron migration routes. MOFs-derived Ni@C microspheres exhibit outstanding electromagnetic (EM) wave absorption performance. The minimum reflection loss value of Ni@C\u2013V microspheres with vortex domain can reach \u221254.6\u00a0\u200bdB at only 2.5\u00a0\u200bmm thickness, and the efficient absorption bandwidth up to 5.0\u00a0\u200bGHz at only 2.0\u00a0\u200bmm. Significantly, configuration evolution of magnetic vortex driven by the orientation and reversion of polarity core boosts EM wave energy dissipation. Magnetic coupling effect among neighboring Ni@C microspheres significantly enhances the magnetic reaction intensity. Graphitized carbon matrix and heterojunction Ni\u2013C interfaces further offer the conduction loss and interfacial polarization. As result, MOFs-derived Ni@C\u2013V powders display unique magnetic vortex, electronic migration network, and high-performance EM wave energy dissipation.\n               "}
{"full_text": "Catalytic decomposition of methane (CDM) is recognized as a promising approach for the co-production of CO2-free H2 and high value-added carbon nanomaterials (CNMs) [1]. Conventionally, steam methane reforming (SMR) followed by the water-gas shift reaction is one of the most developed processes for large-scale hydrogen generation. Despite the optimizations, SMR is associated with high emissions of CO2 (ca. 12 t CO2/t H2), high capital and operating costs [2]. CDM has the advantage of producing H2 and high value-added carbon nanostructures in a single step without generating greenhouse gases (Reaction 1). In this regard, the CDM process becomes increasingly cost-competitive when public policies support free taxes or negative costs related to CO2 yield and the solid carbon has commercial value [3]. The carbon nanostructures formed in this process include mainly carbon nanotubes [4] or nanofibers [5] and in some cases few-layered graphene or graphite nanosheets [6].\n\n(1)\n\n\n\nCH\n4\n\n\n(\ng\n)\n\n\u2192\nC\n\n(\ns\n)\n\n\n\n+\n2\nH\n\n2\n\n\n(\ng\n)\n\n+\n\u0394\n\nH\n\n25\n\u00b0\nC\n\n0\n\n=\n75.6\n.\nk\nJ\n/\nmol\n\n\n\n\nCatalysts typically used in the CDM reactions are based on Ni and Fe, with operating temperatures between 500 and 900\u00a0\u00b0C [7\u20139]. Although Ni-based catalysts are the most active and stable at temperatures between 500 and 700\u00a0\u00b0C, it rapidly deactivates with increasing temperature [10,11]. On the other hand, Fe-based catalysts are cheaper and require higher temperatures (700\u2013900\u00a0\u00b0C) [8,12,13]. This latter range of temperatures provides a positive shift of the thermodynamic equilibrium of the CDM reaction, and thus higher methane conversion may potentially be obtained, as well as an improved structural order in the obtained graphitic nanomaterials [14].The main steps involved in the CDM reaction are: (1) methane cracking, (2) dissolution and diffusion of carbon through the metal particle, and (3) the supersaturation and subsequent precipitation of carbon for the formation of nanostructured carbon [5]. Although the stages of the formation and growth of carbon for Ni- and Fe-based catalysts are certainly related to common factors, there are some differences. The production of as-grown carbon by CDM for Ni catalysts occurs through facet mechanism [15], while for Fe catalysts it is through a complex system of different active phases composed of metallic Fe structures and Fe\u2013C alloys, i.e., Fe3C, \u03b1-Fe, \u03b3-Fe, and their alloys [16]. To the best of our knowledge, no studies have been concerned with the influence of each of these components on the results of catalytic reaction, which is an important aspect of the use of iron-based materials in the CDM reaction [17]. A possible reason for this remains in the difficulty to rationalize the factors that lead to the formation of iron phases and metaphase observed under different reaction conditions and catalysts.Catalyst deactivation by carbon encapsulation and sintering is the prime challenge found in the CDM process [18]. To promote a longer catalyst lifetime, different metal loadings [19], supports [20], synthesis methods [21], reactor configurations and conditions [22] have been studied. For example, Inaba et\u00a0al. [23] investigated Fe-supported alumina catalyst at different temperatures, CH4 flow rates, and CO2 concentrations for the production of carbon nanotubes. CH4 conversion achieved 60% at temperatures higher than 700\u00a0\u00b0C. They stated that it was possible to increase stability by decreasing gas velocity. By adding CO2 in the feedstock, higher temperatures and longer catalytic lifetimes can be obtained. Besides this, the prereduction of iron oxides using H2 is not mandatory and their reduction can proceed during the CH4 stream at temperatures higher than 680\u00a0\u00b0C to provide a sufficiently high and stable conversion.Expensive catalysts and synthesis methods can compromise the viability of CDM [2]. Depending on the application of the carbon, it is necessary to purify the obtained carbon by removing the metal with acid treatment [4]. An alternative approach is to regenerate the spent catalyst from the CDM by oxidation to reuse the catalyst [24]. In both contexts, Fe-based catalyst is considered suitable for CDM because of its price. Table 1\n presents a literature survey on Fe-based catalysts for CDM with their respective reaction conditions and main catalytic results. Table 1 shows that the studies related to CDM mostly use H2 in the catalyst activation stage; however, from an industrial standpoint, it is desirable to operate with CH4 in the reduction stage to minimize costs. Enakonda et\u00a0al. [25] studied supported Fe\u2013Al materials for CDM evaluating the reducing atmosphere with CH4 and H2. Interestingly, the catalytic activity using CH4 activation was higher than H2 activation (Table 1). The authors suggested that part of spinel FeAl2O4 was reduced by H2, which may result in the sintering of Fe0 and the lowering of surface area. In contrast, the effect of CH4 and H2 gases as a reducer agent on non-supported iron-based material is still poorly known, thus it was thoroughly investigated in this work.Recently, iron ores have been identified as a promising unconventional catalyst for the CDM reaction to minimize costs [18]. Here, this work explores the use of two different iron ores as catalysts in the production of hydrogen and nanostructured carbon materials via CDM. The iron ores (Tierga and Ilmenite) were chosen because of their low price, wide availability, non-toxicity and catalytic activity in other reactions involving CH4 [26,27]. Both were subjected to CDM reaction for the first time. Tierga iron ore is mainly composed of Fe2O3, and Ilmenite ore contains species of iron and titanium. After the selection of the most active ore (Tierga), several aspects were studied such as reducing atmosphere, reaction temperature and weight hourly space velocity (WHSV) to find an optimal reaction condition providing high catalytic activity and stability. The reduction with CH4 had a positive impact on the structure of Tierga and the yield of as-deposited nanocarbon mainly at more moderate temperatures. We observed that various types of carbon nanostructures such as graphite-like nanosheets and tubular carbon structures with a high degree of graphitization were obtained over Tierga.The iron ore that has iron oxide as its main component was named according to its place of origin, Tierga. The other ore containing iron and titanium was called Ilmenite. Tierga was supplied by PROMINDSA (Tierga, Spain) and the Ilmenite by Titania A/S (Sokndal, Norway). The materials were sieved to 200\u2013300\u00a0\u03bcm, and then used as a catalyst for the CDM reaction without further treatment.The crystalline structures of the materials were characterized by X-ray diffraction using a diffractometer Bruker D8 Advance Series 2. The powder XRD patterns were further processed for quantitative and qualitative analysis by applying the Rietveld refinement method (see Supplementary materials). The existence of impurities was determined by inductively coupled plasma optical emission spectrometry (ICP-OES; Ametek Spectroblue). Temperature programmed reduction (TPR-H2) tests were performed using an AutoChem Analyzer II 2920. TPR-H2 profiles were acquired using 250\u00a0mg of fresh catalyst, under a hydrogen-argon mixture (10% H2) with a flow rate of 50\u00a0mL/min from room temperature to 950\u00a0\u00b0C using a heating rate of 10\u00a0\u00b0C/min. N2 physisorption experiments were analyzed in a Micromeritics Tristar apparatus. The adsorption and desorption of N2 were determined at \u2212196\u00a0\u00b0C. Thermogravimetric analysis (TGA) was carried out in a NETZSCH TG 209 F1 Libra thermobalance coupled with the mass spectrometer (MS), OmniStar TM. The sample (ca. 30\u00a0mg) was heated from room temperature to 900\u00a0\u00b0C in a total flow rate of 50\u00a0mL/min of methane or hydrogen diluted in argon (10% CH4 or 10% H2) using a heat rate of 10\u00a0\u00b0C/min. Temperature programmed oxidation (TPO) profiles of the carbon were obtained in the same apparatus from room temperature to 900\u00a0\u00b0C using a heating rate of 10\u00a0\u00b0C/min, under an air/nitrogen flow rate of 50\u00a0mL/min (25:75\u00a0vol:vol). The microstructure of the samples was investigated by transmission electron microscopy (TEM, JEOL-2000 FXII). Raman spectra were measured in a Horiba Jobin-Yvon LabRAM HR800 UV spectrometer equipped with a charge-coupled detector. The degree of graphitization of carbon was measured using the Raman and XRD results. From the characteristic peaks of carbon from XRD data, it was possible to obtain the interplanar distance (d\n\n002\n) between graphene layers of diffraction peak (002) using the Bragg equation. The graphitization index, \n\ng\n\n, was calculated using Equation (2) [54]. The layer thickness (\n\n\nL\nc\n\n\n) of carbon was calculated by Equation (3), where \u03bb is the X-ray wavelength, B is the angular width of the (002) diffraction peak at half-maximum intensity (radians) and \u03b8 is the Bragg angle for reflection (002). The number of graphene layers (\n\n\nn\nL\n\n\n) was estimated using Equation (4).\n\n(2)\n\n\ng\n=\n\n\n\n0.3440\n\u2212\n\n\nd\n002\n\n\n\n0.3440\n\u2212\n0.3354\n\n\n\n\n\n\n\n\n(3)\n\n\n\nL\nc\n\n=\n\n\n0.89\n\n\u03bb\n\n\nB\n\ncos\n\n\u03b8\n\n\n\n\n\n\n\n\n(4)\n\n\n\nn\nL\n\n=\n\n(\n\nL\nc\n\n/\n\nd\n002\n\n)\n\n+\n1\n\n\n\n\nThe catalytic tests were performed in a fixed bed reactor at different pretreatment and reaction conditions. In a typical run, 600\u00a0mg of fresh catalyst was reduced from room temperature to 900\u00a0\u00b0C for 1\u00a0h under H2 or CH4 flow rate of 1.2\u00a0L/h. Then, the CDM reaction was carried out using a pure CH4 flow rate of 1.2\u00a0L/h, at 800, 850, or 900\u00a0\u00b0C for 3\u00a0h. The samples after the reaction were named according to the reducing atmosphere and the reaction temperature. For example, the sample Tierga reduced with H2 at 900\u00a0\u00b0C for 1\u00a0h was named Tierga-H2, and after CDM at 850\u00a0\u00b0C it was named Tierga-H2850. The composition of the exhausted gases was determined by gas chromatography (see Supplementary materials). The CH4 conversion [XCH4(%)] is given by Equation (5), where \n\n\nC\n\nH\n2\n\n\n=\n\nF\n\nH\n2\n\n\n/\n\nF\nT\n\n\u00d7\n100\n\n is referred to the percentages of the hydrogen content in the exhausted gases and \n\n\nF\n\nH\n2\n\n\n\n and \n\n\nF\nT\n\n\n are the H2 molar flow rate and total molar flow rate in the reactor output, respectively. The amount of carbon deposited on the catalyst (gc/gcat) was estimated using Equation (6), where M\n\nc\n is the carbon molar mass (12.0107\u00a0g/mol), V\n\nm\n is the CH4 molar volume (22.4\u00a0L/mol), QCH4 is the volumetric CH4 flow rate fed to the reactor (1.2\u00a0L/h), and t is the run time (h).\n\n(5)\n\n\n\nX\n\nC\n\nH\n4\n\n\n\n\n(\n%\n)\n\n=\n\n\n\nC\n\nH\n2\n\n\n\n200\n\u2212\n\n\nC\n\nH\n2\n\n\n\n\n\n\u00d7\n\n100\n\n\n\n\n\n\n(6)\n\n\n\ng\nc\n\n=\n\n\nM\nc\n\n\nV\nm\n\n\n\n\n\n\u222b\n\n0\nt\n\n\nQ\n\nC\n\nH\n4\n\n\n\n\nX\n\nC\n\nH\n4\n\n\n\nd\nt\n\n\n\n\nTierga and Ilmenite presented a non-porous structure with surface area of 5.2 and 0.8\u00a0m2/g, respectively. The results of XRD and ICP can be seen in Table 2\n, Tierga consisted mainly of iron (III) oxide (\u03b1-Fe2O3; hematite) and Ilmenite of pseudobrookite (Fe2TiO5).The reducibility of these iron ores was studied by TPR-H2, and its profile is shown in Fig.\u00a01\n. The main peaks were observed in the profile at 440, 690, and 860\u00a0\u00b0C for Tierga. TPR profiles observed in the literature for unsupported \u03b1-Fe2O3 materials were analogous to those observed for Tierga [32], which suggested the following global reduction mechanism: \u03b1-Fe2O3 \u2192 Fe3O4 \u2192 FeO \u2192 \u03b1-Fe [32]. In Fig.\u00a01, the first peak close to 400\u00a0\u00b0C was related to the transformation of hematite to magnetite, Fe2O3 \u2192 Fe3O4. The existence of peaks above 570\u00a0\u00b0C in these conditions implied the occurrence of the intermediate FeO phase [32]. After that, the transformation of the Fe3O4 phase to metallic Fe between 500 and 900\u00a0\u00b0C occurred in a two-step magnetite reduction pathway, Fe3O4 \u2192 FeO \u2192 Fe [33].Four main peaks centered at 430, 620, 920 and 945\u00a0\u00b0C were observed for Ilmenite in Fig.\u00a01. The peak at 430\u00a0\u00b0C corresponded to the transformation of \u03b1-Fe2O3 \u2192 Fe3O4, followed by the stepwise reduction process previously described. The other stages of the reduction of \u03b1-Fe2O3 were overlapped by the changes of Fe\u2013Ti\u2013O. The peak between 500 and 650\u00a0\u00b0C was attributed to the reduction of: Fe3O4 \u2192 FeO, Fe2TiO5 \u2192 FeTiO3 (Reaction 7) and Ilmenite-Fe3+ \u2192 Ilmenite-Fe2+ [34,35]. Peaks above 900\u00a0\u00b0C were ascribed to the reduction of FeO \u2192 Fe and Ilmenite-Fe2+ \u2192 Ilmenite-Fe0 [35]. The H2 consumption of Tierga was five times higher than Ilmenite, 314 and 56\u00a0cm3/g, respectively.\n\n(7)\nFe2TiO5(s)\u00a0+\u00a0TiO2(s)\u00a0+\u00a0H2(g) \u2194 2FeTiO3(s)\u00a0+\u00a0H2O(v)\n\n\nThe steps of in situ activation with H2 or CH4 are the same for iron oxide: \u03b1-Fe2O3 \u2192 Fe3O4 \u2192 FeO \u2192 \u03b1-Fe [25]. However, the in situ reduction of the catalyst with CH4 may differ from that with H2 in the formation of gaseous byproducts throughout the reduction process. By the reduction with CH4, the formation of traces of COx gases is motivated by the reaction between CH4 and the oxygen of the catalyst from the metal oxide and support [25]. Regarding the properties of the catalyst, it had been pointed out that the reducing agent can modify the type of as-grown carbon [36]. Given these aspects, it is expected that the catalyst undergoes different transformations in particle size, sintering and carbon deposit when reduced with CH4. The study of the reducing atmosphere effect was conducted with Tierga because it has a greater amount of active phase.The evaluation of byproduct formation during the reduction step of Tierga was carried out in a thermobalance using 30\u00a0mg and a flow rate of 50\u00a0mL/min containing 10% of CH4 or 10% of H2 in Ar. The gases evolved were analyzed by mass spectrometry. Fig.\u00a02\n shows the variation of each gas during the experiment, as well as the sample mass variation and temperature. The appearance of CO and CO2 mainly occurred at approximately 700\u00a0\u00b0C for both atmospheres (CH4 and H2) due to the decomposition of the dolomite phase that takes place at that temperature [37]. The profile of CO and CO2 occurred differently between the two pretreatments because an additional formation of COx gas was expected from the interaction between CH4 and catalyst between 600 and 900\u00a0\u00b0C as aforementioned. Simultaneously, the reduction with CH4 can lead to other interactions between CH4 and the byproducts formed during the reduction (H2O, CO2, CO, H2), i.e., the gas-water shift reaction, steam and dry reforming of CH4. This can be evidenced by the diverse water vapor profiles between CH4 and H2 reduction pretreatments. For Tierga pretreated with H2 (Fig.\u00a02-b), the water vapor profile had maximum peaks at 490, 710, and 810\u00a0\u00b0C, similar to that observed in the TPR-H2 (Fig.\u00a01). However, the water vapor profile for CH4 reduction pretreatment material had more discrete peaks with maximum peaks at 740 and 880\u00a0\u00b0C (Fig.\u00a02-a), suggesting a CH4 reforming reaction along the reduction stage as also indicated in another study [38]. Additionally, it is worth mentioning that neither CO nor CO2 was found at 900\u00a0\u00b0C for Tierga-H2 or Tierga-CH4, in good agreement with the previously reported results [25].When the reduction with CH4 was carried out in a fixed bed reactor, the onset of the CH4 decomposition reaction and the formation of byproducts became more evident (Fig.\u00a03\n). The CH4 decomposition started after 30\u00a0min of reduction at 900\u00a0\u00b0C as the CH4 conversion increased abruptly. The profiles of CO and CO2 were similar to those seen in the experiments using thermobalance (Fig.\u00a02-a). The same experiment was not carried out with H2 at fixed bed because the analysis of gaseous byproducts during the reduction with H2 had already been verified in Fig.\u00a02-b and the CDM reaction proceeds only by contacting methane. According to these reducibility tests, the reduction step for both atmospheres was established up to 900\u00a0\u00b0C after 1\u00a0h. The reduction stage for 1\u00a0h at 900\u00a0\u00b0C is called the activation step hereafter.XRD patterns for Tierga pretreated with H2 or CH4 at 900\u00a0\u00b0C for 1\u00a0h are shown in Fig.\u00a04\n. Both materials presented characteristic peaks of the \u03b1-Fe phase (ICSD 64998), SiO2 (ICSD 42498) and new peaks regarding CaO (ICSD 673084) and MgO (ICSD 88058) phases resulting from the decomposition of dolomite. The absence of iron oxides peaks indicated the complete reduction to \u03b1-Fe. Fe3C (ICSD 064689) and graphite (ICSD 76767) were also identified in Tierga-CH4, suggesting that the CDM reaction started during the reduction step with CH4. In fact, Zhou et\u00a0al. [33] demonstrated that CDM starts with the formation of Fe3C and graphite simultaneously on the surface of \u03b1-Fe through the reaction between Fe and CH4 (Equation (8)). As soon as Fe3C is formed, it acts as a catalyst and promotes the methane decomposition into H2 and carbon [33]. The carbon diffuses into Fe3C to form supersaturated Fe3C1+x, which is unstable and immediately decomposes back to stoichiometric Fe3C and graphite carbon [33].\n\n(8)\n3Fe (s)\u00a0+\u00a02CH4 (g) \u2194 Fe3C (s)\u00a0+\u00a0C (s)\u00a0+\u00a04H2 (g)\n\n\nThe concentration and mean crystallite size of \u03b1-Fe depended on the pretreatment performed. The percentage of the \u03b1-Fe phase in Tierga-H2 and Tierga-CH4 catalysts were 85 and 73\u00a0wt%, respectively. The lower concentration of this active phase in Tierga-CH4 was explained by the transformation of this phase into Fe3C. Moreover, the mean crystallite size of \u03b1-Fe in the Tierga treated with H2 (80\u00a0nm) was bigger than the iron ore activated by CH4 (46\u00a0nm), which indicated the fragmentation of the \u03b1-Fe phase in Tierga-CH4 into smaller crystals by the adjacent formation of iron carbide and graphite as previously reported in other catalysts [39,40]. Another possibility involved the effect of Fe3C and carbon on the catalyst activity. From literature, the iron carbide and carbon can act as textural promoters and prevent the sintering of \u03b1-Fe particles, positively impacting in the catalytic activity, as already reported for Ni-based materials during CDM above 500\u00a0\u00b0C [41]. To confirm this hypothesis and to probe in more detail the ability of these structures to act as promoters in Tierga, different temperatures were used in the reaction (see Section Effect of temperature). Other works also report that the reduction with H2 is more severe and leads to larger crystallite sizes by sintering [42]. Once the catalyst is reduced along the activation stage, the reaction step proceeds.After pure H2 prereduction at 900\u00a0\u00b0C for 1\u00a0h, Tierga and Ilmenite were subjected to a reaction with pure CH4 at 800\u00a0\u00b0C. Fig.\u00a05\n shows the profiles of the samples in terms of H2 production (left y axis) and CH4 conversion (right y axis) during the reaction. Only H2 and CH4 gases were detected during the reaction. The catalytic activities of both declined with time on stream, and after 1\u00a0h it increased. A more detailed discussion of this behavior was made in Section Effect of temperature. Due to the lower Fe loading and higher reduction temperature of the Fe\u2013Ti\u2013O structures, Ilmenite exhibited worse catalytic activity than Tierga in terms of H2 concentration ranged from 15 to 18%, than those of Tierga with 46\u201347%. The CH4 conversion ranged from 8 to 10% for Ilmenite, and 30\u201332% for Tierga. Tierga produced a high carbon yield at 800\u00a0\u00b0C (0.82 gc/gcat and 1.6 gc/gFe), indicating that is a promising natural catalyst to be used in CDM and, consequently, it was conducted to further experiments.The influence of the space velocity at 850\u00a0\u00b0C, with CH4 activation, and WHSV ranging from 2 to 6\u00a0L/(gcat\u2219h) was evaluated and the corresponding H2 concentration and CH4 conversion evolutions over Tierga catalyst are shown in Fig.\u00a0S1. With decreasing the space velocity, there was a gain in contact time and consequently, an increase in conversion. The H2 content profile slightly rose for a WHSV of 2\u00a0L/(gcat\u2219h). When increasing the WHSV to 4 and 6\u00a0L/(gcat\u2219h), the catalyst underwent a deactivation process after 1\u00a0h of reaction. The trend of the WHSV of 2\u00a0L/(gcat\u2219h) will be described in detail in the following section.The effect of the operating temperature on Tierga-CH4 and Tierga-H2 activities was evaluated at 800, 850 and 900\u00a0\u00b0C using WHSV\u00a0=\u00a02\u00a0L/(gcat\u2219h). H2 concentration and CH4 conversion changes are shown in Fig.\u00a06\n-a. A significant increase in the amount of produced H2 was obtained with rising temperature for both catalysts: Tierga-H2 and Tierga-CH4. According to literature, the amount of produced H2 by CDM increases as the temperature increases and the pressure falls [7]. High H2 concentration (70%) and no deactivation were observed for Tierga-H2 and Tierga-CH4 at 850\u00a0\u00b0C. At 800\u00a0\u00b0C stable conversion was observed for the catalyst treated with H2 during about 100\u00a0min, followed by slowly rose to 32%, while the CH4 conversion increased from 24 to 40% after 3\u00a0h of reaction for the catalyst treated only with CH4. At 900\u00a0\u00b0C, although it exhibited the highest initial catalytic activity, there was a slight deactivation after the first hour of reaction for both Tierga-CH4 and Tierga-H2 (ca. 10% H2 decay). The decrease in the catalytic activity of Tierga-CH4900 and Tierga-H2900 after 1\u00a0h can be primarily assigned to the encapsulation of the active phase.The early period of catalytic activity, immediately before the period of constant carbon growth, is commonly named the induction period. This step in CDM is usually associated with carbon migration and saturation in catalysts, and metal reconstruction [15]. In Figs. 5 and 6-a, the samples Ilmenite-H2800 and Tierga-H2 (at 800, 850 and 900\u00a0\u00b0C) showed an initial drop of H2 production and CH4 conversion between 10 and 50\u00a0min. This fall may be related to the period necessary for carbon supersaturation of \u03b1-Fe and Fe3C to take place. Such induction period tended to decrease with rising temperature over Tierga (Fig.\u00a06-a). After carbon supersaturation, carbon precipitation occurs. Regarding Tierga-CH4, the active structures were already partially saturated and therefore had an increasing trend of catalytic activity. The low initial concentrations of Fe3C and graphite were not sufficient to make these catalysts act as a structural promoter at the beginning of the reaction. As there was an increase in the concentration of Fe3C and carbon, they could act as support and possibly explain the high stability at 800 and 850\u00a0\u00b0C.\nFig.\u00a06-b summarizes the amount of formed H2 and the conversion of CH4 after 3\u00a0h of reaction as a function of temperature. The final conversion of CH4 at 800\u00a0\u00b0C was about 35% for Tierga-H2 and Tierga-CH4. At higher temperatures, the conversion was close to 56% and it was independent on the treatment of Tierga. This result revealed that the initial fragmentation and previous saturation with carbon observed in the XRD pattern (Fig.\u00a04) had a positive impact on the catalytic results mainly at 800 and 850\u00a0\u00b0C after 3\u00a0h of reaction. Such initial catalyst fragmentation with CH4 may have brought about the inhibition of agglomeration and sintering of iron-based materials. This disaggregation likely led to greater exposure of the active phase which resulted in higher catalytic activity for Tierga-CH4 catalyst, as other authors previously reported [43].The amount of deposited carbon from these CDM experiments is shown in Table 3\n. Despite the slight difference between the results for the same temperature, the carbon formation was favored with CH4 as the reducing agent and with rising temperature. Comparing the carbon yield of Tierga with data taken from the literature (Table 1) is a non-trivial task owing to the diversity of experimental systems. In some cases, Tierga has superior performance than iron-based synthetic catalysts (e.g., 100% Fe2O3), which contribute to boosting the competitiveness of Tierga iron ore to reach a commercial level. On the other hand, Tierga material displayed inferior carbon yield than other ones possibly due to an absence of support and a small number of alkaline impurities such as potassium and sodium (Table 2) as previously reported [20]. The experimental conditions used in this work and the results obtained for Tierga without H2 pretreatment could be considered as a good advantage for industrial application. In addition, Tierga presents other advantages such as low-cost and high Fe loading.\nFig.\u00a0S2 shows the diffractograms of the Tierga catalysts after the reaction. The spent catalysts were composed mostly of \u03b1-Fe (ICSD 64998), \u03b3-Fe (ICSD 185721), Fe3C (ICSD 064689) and graphite (ICSD 76767) phases in all samples except for Tierga-H2800 sample that did not have the pattern of \u03b3-Fe. The as-deposited carbon presented d002 values between 0.3376 and 0.3366\u00a0nm and g\n\np\n between 0.74 and 0.86, respectively (Table 3), i.e., parameters close to the perfect single crystal of graphite structure, which is 0.3354\u00a0nm and g\n\np\n close to 1. The characterization of carbon by XRD indicated the formation of graphite-like materials with \n\n\nL\nc\n\n\n between 17 and 21\u00a0nm and a number of graphene layers (\n\n\nn\nL\n\n\n) between 52 and 64. Due to the low carbon formation over Ilmenite (0.3 gc/gcat), only Tierga catalysts were characterized after CDM.Most samples after the reaction were composed of the \u03b3-Fe structure. This phase is less characterized experimentally due to its instability at temperatures below the boiling point (727\u00a0\u00b0C). \u03b3-Fe can be an intermediate phase in the production of Fe3C and graphite at high temperatures [44]. The \u03b1-Fe (body-centered cubic system) and \u03b3-Fe (face-centered cubic system) phases have a great affinity with carbon, which allows the dissolution of carbon atoms in the network of these metals, reaching a maximum of 0.022% wt. of C at 740\u00a0\u00b0C for \u03b1-Fe, and 2.14% wt. of C at 1150\u00a0\u00b0C for \u03b3-Fe. The diffractogram of \u03b3-Fe without carbon saturation found in the literature (ICSD 41506) had peaks at 2\u03b8\u00a0=\u00a045.8, 53.4 and 78.9\u00b0; however, the peaks at 2\u03b8\u00a0=\u00a043.8, 50.9, 74.9\u00b0 presented in Fig.\u00a0S2 can be attributed to \u03b3-Fe saturated with carbon (ICSD 185721) [45,46]. This is because \u03b3-Fe allows the insertion of carbon in the interstices of the crystalline network. The rearrangement decreases part of the associated metal-metal energy and changes the diffraction lines of \u03b3-Fe metal to lower angles, as observed for Tierga. Similar results have been reported in earlier publications [45,46]. The carbon-saturated \u03b3-Fe phase was observed only after reaction (Fig.\u00a0S2), and not in the initial activation step (Fig.\u00a04), which suggests that enough carbon was formed during the reaction to protect and stabilize this intermediate phase.Based on the XRD data, the iron-based phases were quantified by Rietveld refinement (Fig.\u00a07\n). The amount of Fe3C decreased with increasing temperature, while iron species increased. The most striking variation in the final composition of the iron phase between the materials took place at 800\u00a0\u00b0C: Fe3C was the major product in Tierga-H2800, while \u03b1-Fe and \u03b3-Fe become dominant in Tierga-CH4800. However, this difference between the catalysts gradually decreased up to 900\u00a0\u00b0C. These results indicated that the characteristics of the catalyst after diverse activation atmospheres led to distinct reaction mechanisms at moderate temperatures motivated by the generation of iron phases with distinct crystal systems and fractions.The most widely reported reaction mechanism is based on the transformation of \u03b1-Fe into Fe3C and graphite (Equation (8)). In contrast, previous studies have shown that the mechanism of carbon formation from \u03b1-Fe can vary according to the concentration [47] and crystallite size [45] of the \u03b1-Fe phase in reactions performed at the same temperature. Wirth et\u00a0al. [47] revealed that depending on the concentration of \u03b1-Fe in a temperature range close to the eutectic temperature (700\u2013800\u00a0\u00b0C), \u03b3-Fe or Fe3C can be obtained, the latter would give rise to carbon. While for Takenaka et\u00a0al. [45], the \u03b1-Fe structure was transformed into Fe3C or \u03b3-Fe depending on the crystallite size of iron oxide. The supported Fe2O3 crystallites with smaller sizes were transformed into Fe3C, while larger ones were transformed into \u03b3-Fe saturated with carbon atoms [45]. Based on these studies, it became evident that for Tierga with reaction taking place at 800\u00a0\u00b0C (close to the eutectic point), Fe3C nucleation was favored when the active phase of the catalyst was mainly composed of \u03b1-Fe with larger crystallite size, i.e., Tierga-H2 catalysts. Yet at 800\u00a0\u00b0C, the \u03b3-Fe phase was preferably promoted by a system with a lower concentration of \u03b1-Fe and smaller average crystallite size (Tierga-CH4 catalysts). As the reaction temperature overpassed the eutectic point towards higher temperatures for other catalysts (Tierga-CH4850, Tierga-H2850, Tierga-CH4900 and Tierga-H2900), there was a higher tendency to promote the nucleation of \u03b3-Fe [47]. Once \u03b1-Fe, \u03b3-Fe, or Fe3C appeared, the carbon dissolution begins to happen and when it reaches the supersaturation of carbon in the metal and/or carbide, the precipitation and growth of carbon occur.Correlating the XRD results with the catalytic tests for Tierga-CH4800 and Tierga-H2800 it was possible to evaluate the effect of the different active phases (\u03b1-Fe, \u03b3-Fe and Fe3C) on the conversion and H2 production. The concentration of 55% H2 (v/v) was obtained in CDM after 3\u00a0h for Tierga-CH4800. As Tierga-CH4800 was composed mainly of \u03b1- and \u03b3-Fe at the end of the reaction, it seems to indicate that \u03b1- and \u03b3-Fe phases were more effective catalysts than Fe3C. A possible explanation for these results may be that the carbide requires a higher amount of carbon for supersaturation than the metal, maximum of 6.67% wt. of C for Fe3C [17]. While \u03b1-Fe and \u03b3-Fe require a lower amount of carbon for graphite precipitation to occur, usually less than 3% wt. of C [17]. In addition, the carbide bulk diffusion coefficient is lower than that of the metals \u03b3-Fe and \u03b1-Fe, implying a higher difficulty in precipitating graphite using carbide [48]. Thus, the mitigation of carbide formation resulted in greater activity of the catalyst, and it was achieved by changing the activation atmosphere to CH4. As seen in the XRD results, the activation with CH4 led to the initial fragmentation of the \u03b1-Fe phase and inhibition of large amounts of Fe3C.\nFig.\u00a08\n shows the TEM images of the as-grown carbon from Tierga with different pretreatments and CDM reaction conditions. TEM images confirmed as-deposited carbon in all spent Tierga in the form of carbon nanomaterials (CNMs) with a high degree of graphitization (d\n\n002\n\u00a0=\u00a03.35\u00a0\u00c5), including multi-layered graphene, graphite nanosheets (GNSs) and carbon nanofilaments. The nanofilaments were multi-walled carbon nanotubes and chain-type carbon nanofibers.In all samples, the GNSs structures (marked with white dotted rectangles) appeared in higher quantities. Generally, they were transparent, rippled graphene/graphite layers, and disengaged from the metallic particles (Fig.\u00a08-a, c). Tubular structures (marked with black dotted rectangles) were sparser and shorter, without (Fig.\u00a08-f) and with (Fig.\u00a08-g) encapsulated iron-based nanoparticles. The chain-type carbon nanofibers (Fig.\u00a08-g) had multiple graphite walls around the metal, similar to those observed in previous works with Fe [23]. The metallic particles in the images were round and covered with a thin layer of graphite (Fig.\u00a08-c). As the temperature increased, the agglomeration of the metal particles increased (Fig.\u00a08-e) as well as the number of metal particles within the chains (Fig.\u00a08-f).The nanocarbon structures such as GNSs and carbon nanofilaments observed in this work can be explained by the quasi-liquid state theory [16,20]. According to some studies [50,51], iron species in the quasi-liquid state combined with the absence of support can produce GNS. Iron-based species with low dispersion and large particles when in quasi-liquid state elongate and expand to form a thin film composed of metal and carbide metal [51]. This film is capable of allowing the dissolution, precipitation of carbon and growth of graphene or graphite sheets on its surface [51]. The formation and growth of short carbon nanotubes and chain-type carbon nanofibers observed in the TEM images may have happened analogously. We can infer that the segregation of the active phase during the reaction enabled the formation and growth of these carbons, as also noticed in some previous works [51,52]. The carbon was precipitated out from the smallest metallic iron particle supersaturated with carbon. The growth occurs in a cylindrical shape and extends to maintain the void inside the tube [7,20,33]. With its growth, the interface between the carbon and metal walls decreases and the insertion of the metallic particle into the tube or chain may occur during this process, thus forming the carbon nanotube or chain-type carbon nanofibers [7,20,33].Raman spectra of the as-deposited carbon nanostructures over the Tierga catalysts are presented in Fig.\u00a0S3. In the Raman first-order spectra (1100 and 1700\u00a0cm\u22121) of the materials, it is possible to observe the characteristic peaks of disordered graphite including D, G, D\u2032 at 1350, 1580 and 1620\u00a0cm\u22121 respectively. The second-order (2500\u20133300\u00a0cm\u22121) is the result of overtones and combinations of the bands in the first order, and for the studied materials, peaks were observed in approximately 2450, 2720 and 3240\u00a0cm\u22121, which are attributed to the first overtone of bands at 1220, 1350, 1620\u00a0cm\u22121, and the band 2950\u00a0cm\u22121 is a combination of band G and D. The 2D band (~2700\u00a0cm\u22121) is characteristic of structures with few and multiple layers of graphene and graphite. Analogous spectra are found in the literature for multilayer graphene and graphite [53].The integral intensity ratio I\n\nD\n\n/I\n\nG\n is widely used to express the degree of graphitization for the carbon, i.e. the lower I\n\nD\n\n/I\n\nG\n ratio, the higher crystalline order of the carbon species. The average parameters of the spectra are shown in Table 3. The I\n\nD\n\n/I\n\nG\n values of all samples were all below 1 (Table 3), which means that the carbon is ordered, with minor contributions from disordered particles.The low I\n\nD\n\n/I\n\nG\n value corroborates TEM images, showing that the carbon nanostructures were predominantly composed of multilayer graphene or graphite nanosheets and small quantities of nanofilaments. The results presented in this work agreed well with other studies in which low I\n\nD\n\n/I\n\nG\n was favored when the final product was multilayer graphene flakes, high temperatures, and flows of pure methane [36,54,55]. Compared to synthetic pure Fe2O3 reported in the literature [14], Tierga generated hybrid carbon with fewer defects and a higher amount of carbon. The materials showed I\n\nD\n\n/I\n\nG\n results close at the same reaction temperature (Table 3), however, the most significant difference was between the materials Tierga-CH4800 and Tierga-H2800. The I\n\nD\n\n/I\n\nG\n value was 0.19 for Tierga-CH4800 and 0.25 for Tierga-H2800, which means that the material Tierga-CH4800 was nanostructured with fewer defects than Tierga-H2800. These same materials showed the greatest difference in carbon yield, 1.05 gc/gcat for Tierga-CH4800 and 0.82 gc/gcat Tierga-H2800. This result is in concordance with previous studies [56] which suggested that one of the conditions for carbon growth is preventing disordered the carbon formation. The quality of produced carbon depended on the catalyst treatment and interestingly the results were better with the treatment of the catalyst with CH4 which is an advantage in the development of the CDM industrial process.Finally, TPO was performed to evaluate the thermal stability of the spent Tierga catalysts (Fig.\u00a09\n). In all profiles, it is first observed that there was a slight gain in mass close to 450\u00a0\u00b0C, which may be related to the oxidation of the metallic iron and iron carbide phases located on the surface, followed by a sharp decay in mass between 600 and 630\u00a0\u00b0C. The higher the reaction temperature, the greater the displacement of the oxidation temperature to higher temperatures. This result is consistent with what was observed in Raman and TEM, which indicates a highly ordered crystal structures (500\u2013700\u00a0\u00b0C) with the absence of amorphous carbon (~400\u00a0\u00b0C).Tierga and Ilmenite were confirmed as an active catalyst in the production of CO2-free H2 and carbon. However, Tierga showed significantly higher catalytic results than Ilmenite and was therefore further investigated. The methane conversion and hydrogen concentration over Tierga were 56% and 70%, respectively, after 3\u00a0h of reaction. Tierga reduced with CH4 demonstrated superior performance with greater activity and stability than Tierga pretreated with H2 at moderate temperatures. CH4 activation has contributed to the fragmentation of the active phase \u03b1-Fe which led to smaller crystallites preventing agglomeration and sintering. Such characteristics also promoted the formation of \u03b3-Fe rather than Fe3C. The high stability of Tierga can be primarily associated with a high degree of graphitization. At 900\u00a0\u00b0C, there were no significant differences between the Tierga materials in terms of the conversion and reaction mechanism, however, the deactivation started after a certain time, which is related to the encapsulation of chain-like carbon nanofibers. XRD, TEM and Raman revealed the production of structures with nanosheets of graphite and carbon nanotube structures with a high degree of graphitization. WHSV and reaction temperature play a central role in the stability of this material as well, in which the optimal conditions were 2\u00a0L/(gcat\u2219h) and 850\u00a0\u00b0C. The use of iron ore as a natural and low-cost catalyst in the production of nanocarbon structures can contribute as an alternative to assessing the practical use of the CDM 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.Brazilian funding to support this work was provided by CNPq [Process 141308/2018-4], FAPESP [Process 2018/01258-5], CAPES [Finance Code 001]. Spanish funding was provided by the European Regional Development Fund and the Spanish Economy and Competitiveness Ministry (MINECO) [ENE2017-83854-R]. Authors would like to acknowledge the use of \u201cServicio General de Apoyo a la Investigaci\u00f3n-SAI, Universidad de Zaragoza\u201d. The authors also thank PROMINDSA and Titania A/S for providing the iron ore used in this work.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.2021.08.065.", "descript": "\n                  Tierga and Ilmenite Fe-based ores are studied for the first time in the catalytic decomposition of methane (CDM) for the production of carbon dioxide-free hydrogen and carbon nanomaterials. Tierga exhibits superior catalytic performance at 800\u00a0\u00b0C. The effect of the reaction temperature, space velocity and reducing atmosphere in the catalytic decomposition of methane is evaluated using Tierga. The highest stability and activity (70\u00a0vol% hydrogen concentration) is obtained at 850\u00a0\u00b0C using methane as a reducing agent. Reduction with methane causes the fragmentation of the iron active phase and inhibits the formation of iron carbide, improving its activity and stability in the CDM. Hybrid nanomaterials composed of graphite sheets and carbon nanotubes with a high degree of graphitization are obtained. Considering its catalytic activity, the carbon quality, and the low cost of the material, Tierga has a competitive performance against synthetic iron-catalysts for carbon dioxide-free hydrogen and solid carbon generation.\n               "}
{"full_text": "With the rapid development of social economy, the environmental pollution caused by the over-utilization of traditional energy is becoming more and more serious. In order to reduce the environment damage caused by fossil fuels, it is urgently needed to develop the green sustainable energy [1\u20134]. Due to the advantages of direct methanol fuel cell (DMFC) with low environmental pollution, high energy conversion efficiency, easy storage and transportation, it is expected to become an effective clean energy that can be widely used in electric vehicles or portable electronic devices [5\u20137]. However, the slow kinetics of methanol oxidation reaction (MOR) is the main challenge that hinders its further commercialization, and quite a few attempts have been made to solve this problem. At present, the most widely used catalyst for MOR in DMFC on the market is Pt-based catalysts [8\u20139], while there are still many problems to be solved. For example, they can be easily poisoned by intermediates such as CO produced during the MOR process, which reduces the efficiency of the catalyst [10\u201312]. Moreover, the high cost of Pt is another shortcoming which limits its further development [13\u201315]. It is very important to find low-cost, high catalytic performance and stable catalysts to replace precious metal catalysts [16]. It is reported that some transition metal and its oxide catalysts shows enhanced reaction kinetics and stronger anti-poisoning ability for MOR. At present, Ni/NiO [17\u201319] and Cu/CuO [20] catalysts are widely used in alkaline DMFC, which show excellent catalytic performance for the electrooxidation of methanol. For these catalysts, the oxygen-containing functional groups adsorbed on transition metal oxides (NiO or CuO) can effectively remove reaction intermediates, thereby improving anti-toxicity and obtaining higher stability [21\u201323]. In addition, compared to other expensive transition metals, Cu and Ni, as metal precursors, are inexpensive and have a wide range of sources.Another successful strategy to improve the performance of electrocatalysts is to load heterogeneous metal nanoparticles on the catalyst to form the composite [24\u201325]. One widely employed example is Au nanoparticles (AuNPs). The surface of AuNPs can be treated as a highly negative electron absorber to promote the oxidation of transition metal cations to higher oxidation states [26]. Anchoring AuNPs on the surface of nanomaterials hopefully facilitates electron transfer rate, thereby improving the electrochemical catalytic efficiency for MOR. AuNPs have been used to form the nanocomposite with metal oxides such as SnO2\n[27], ZnNb2O6\n[28], CeO2\n[29] so as to improve the catalytic activity.In this work, via solvothermal method, nanoplates composed of copper and nickel oxides with different valence states were synthesized. AuNPs were absorbed onto the nanoplates by introducing them into the Au colloid. And the catalystic activity towards MOR was studied in potassium hydroxide electrolyte. XRD and XPS characterizations prove that the Cu and Ni in the nanoplates are multivalent state. TEM and EDS-mapping characterizations show that a large number of AuNPs are uniformly absorbed onto the ultra-thin multivalent Cu-Ni oxide nanoplate. Electrochemical tests show that the prepared electrocatalyst has high electrochemical activity and excellent stability towards the catalytic performance for MOR.Chloroauric acid tetrahydrate (HAuCl4\u00b74H2O), nickel (II) nitrate hexahydrate (Ni(NO3)2\u00b76H2O, 99%), potassium hydroxide (KOH, 99%), copper nitrate hexahydrate (Cu(NO3)2\u00b76H2O 99%) and hexamethylenetetramine (C6H12N4, 99%) were all purchased from Aladdin (Shanghai, China). In the experiment, ultra-pure deionized water (18.2 M\u03a9) was used. All reagents are analytical reagent grade, no further purification is required.Energy dispersive X-ray spectroscopy (EDS) system (Oxford X-Max, UK), X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Escalab 250Xi), transmission electron microscopy (TEM, Tecnai G2-20, American FEI company), X-Ray diffractometry (XRD, Rigakuultima iv, Japan). The XRD spectrums were recorded at 2\u03b8 values ranging from 30\u00b0 to 80\u00b0.Dissolve 44.6\u00a0mg of nickel (II) nitrate hexahydrate, 53.8\u00a0mg of copper nitrate hexahydrate and 95.3\u00a0mg of hexamethylenetetramine in 68\u00a0mL of methanol to form a homogeneous dispersion. The resulting dispersion was then transferred into a 100\u00a0mL autoclave and heated to 120\u00a0\u00b0C for 9\u00a0h. After cooling, the sample was filtered out and washed three times with mixed solution of ethanol and water (volume ratio 1:1) by centrifugation under 8000\u00a0rpm for 10\u00a0min to remove residual impurities, and dried in a vacuum drying oven at 60\u00a0\u00b0C for 12\u00a0h. The obtained products were put into a muffle furnace, heated to 300\u00a0\u00b0C (5\u00a0\u00b0C per minutes) and kept at 300\u00a0\u00b0C for 4\u00a0h to prepare multi-valent Cu-Ni oxide nanoplates.Au colloids were prepared according to the literature [30]. For the AuNPs decoration, 40\u00a0mg of m-v oxide was dispersed in the Au colloid and stirred for 2\u00a0h. The products were then collected and washed in ultrapure water to remove impurities. AuNPs decorated multi-valent Cu-Ni oxide (AuNPs/m-v oxide) was finally dried at 60\u00a0\u00b0C for further use.For modification of the working electrode, the GCE (glassy carbon electrode) was firstly polished with alumina powder and cleaned before use. 4\u00a0mg of AuNPs/m-v oxide was dispersed in 1\u00a0mL of deionized water and ultrasonic for 10\u00a0min to get uniform dispersion. 3\u00a0\u00b5L of the dispersion was dropped onto the surface of the pre-treated GCE to get the AuNPs/m-v oxide/GCE. Finally, 5\u00a0\u00b5L Nafion solution (1%) was covered on the electrode surface for seal. For comparison, M\u2212v oxide/GCE was also fabricated with the similar procedure by replacing AuNPs/m-v oxide with m-v oxide. All electrochemical measurements were performed on CHI 660D with a three-electrode electrochemical system. The MOR tests were carried out in 1\u00a0M potassium hydroxide with or without methanol. Platinum foil and calomel electrode (with saturated potassium chloride) were served as counter electrode and the reference electrode.Transmission electron microscope was used to characterize the morphology of the prepared samples. Fig. 1\nA is the TEM image of m-v oxide. It is found that a certain number of thin nanoplates are overlapped with each other. Fig. 1B is the high resolution TEM image of m-v oxide, it shows a couple of well-resolved and interlaced fringes with interplanar distances of 0.233, 0.276, 0.213, 0.230\u00a0nm, which are assigned to the lattice distance for the (111) planes of CuO, the (110) planes of NiO, the (200) planes of Cu2O, the (102) planes of Ni2O3 respectively.After the obtained nanoplates are dropped into the Au colloid and stirred for 2\u00a0h, plenty of dark dots are emerged on the surface of the nanoplates, as shown in Fig. 1C. This proves that the AuNPs are successfully decorated onto the oxide. In addition, the HRTEM image of AuNPs/m-v oxide (Fig. 1D shows the atom lattice fringe for the dark dot is 0.235\u00a0nm, which corresponds to the Au (111) plane. This can be further inferred that the AuNPs/m-v oxide composite was successfully prepared.\nFig. 2\n is the XRD patterns of the prepared AuNPs/m-v oxide (curve a) and m-v oxide (curve b), as well as the standard spectrums of Au (curve c), NiO (curve d, PDF#47\u20131049), CuO (curve e, PDF#48\u20131548), Ni2O3 (curve f, PDF#14\u20130481) and Cu2O (curve g, PDF#34\u20131354). As compared to the standard spectrums, both the characteristic peaks for NiO, CuO, Ni2O3 and Cu2O are all appeared in the XRD spectrums of AuNPs/m-v oxide (curve a) and m-v oxide (curve b). It is worth noting that for the AuNPs/m-v oxide, the diffraction peaks of Au (38\u00b0, 44.3\u00b0, 64.5\u00b0, 77.5\u00b0) are partially overlapped with the diffraction peaks of m-v oxide, indicating that the deposited Au nanoparticles are small and highly dispersed on the surface of m-v oxide [31].In order to understand the elemental composition and valence state information of the composite material, the XPS measurement was carried out and the corresponding results are shown in Fig. 3\n. Fig. 3A shows the whole spectrum of AuNPs/m-v oxide (curve a) and m-v oxide (curve b). Fig. 3B-3E is the amplified binding energy spectrums for C 1\u00a0s, Au 4f, Ni 2p and Cu 2p. Binding energy in all spectra is calibrated based on the carbon standard binding energy of 284.6\u00a0eV. In Fig. 3B for C 1\u00a0s, beside the standard C 1\u00a0s binding energy peak at 284.6\u00a0eV from adventitious reference carbon, the binding energy of 287.6\u00a0eV, whichis observed both in AuNPs/m-v oxide (curve a) and m-v oxide (curve b), corresponds to the signals of C-O and OH-C\u00a0=\u00a0O bonding[13,32]. The peaks of Au 4f XPS spectrum (curve a in Fig. 3C at 84.0\u00a0eV and 87.7\u00a0eV are allocated to Au 4f 5/2 and 7/2, which is the typical Au0 valence state [33]. However, in the XPS spectrum of m-v oxide (curve b in Fig. 3C, these is just a smooth baseline in these energy range. This difference confirms the formation of AuNPs for the AuNPs/m-v oxide. In Fig. 3D for Ni 2p, there are two accompanying satellite peaks at 861.2\u00a0eV and 879.5\u00a0eV, and the other four peaks. These peaks are related to Ni 2p 3/2 and 1/2 [34], and confirms the existence of Ni2+ and Ni3+\n[35\u201337]. In Fig. 3E for Cu 2p, the binding energy at 932.5\u00a0eV and 952.6\u00a0eV are the peak of the Cu+ 2p 3/2 and 1/2 orbital, and the peaks at 934.4\u00a0eV and 954.6\u00a0eV are related to the 2p 3/2 and 1/2 orbital peaks of Cu2+. These peaks prove the co-existence of Cu+ and Cu2+ in the oxide [38]. The XPS results are consistent with the XRD results and further prove that there is multiple valence states for Cu and Ni in the prepared nanomaterials, which is contribute to the oxidation process to obtain better catalytic performance.The highly dispersed structure of nanoparticles in the catalyst helps to improve the catalytic performance for methanol oxidation. Fig. 4\n shows SEM elemental mapping images for C (A), O (B), Ni (C), Au (D) and Cu (E) in AuNPs/m-v oxide.It can be observed that C, O, Ni, Au and Cu signals are all detected in the scanning area, and the dispersion in the sample are very uniform. Combining the above XRD and TEM results, it can be fully proved that the AuNPs/m-v oxide is successfully synthesized with good crystallinity.The anodic oxidation peak current density is usually used to evaluate the catalytic ability of catalyst. Herein, the electrocatalytic performance of two nanomaterials on MOR is evaluated by comparing their electrochemical performance. Fig. 5\n is the electrochemical response of the AuNPs/m-v oxide and m-v oxide modified electrodes in 1\u00a0M potassium hydroxide electrolyte containing 0.5\u00a0M methanol. In the blank 1\u00a0M potassium hydroxide electrolyte, the AuNPs/m-v oxide (curve c) and m-v oxide (curve d) modified electrode show no obvious redox peak. After the addition of 0.5\u00a0M methanol, both the AuNPs/m-v oxide (curve a) and m-v oxide (curve b) modified electrode exhibit obvious oxidation current at potential of 1.20\u00a0V, and the oxidation peak current density (21.1\u00a0mA/cm2) for AuNPs/m-v oxide/GCE is 3.4 times bigger than m-v oxide/GCE (6.2\u00a0mA/cm2). This confirms that the m-v oxide contributes to the electrochemical catalysis of MOR, and the existence of AuNPs further promotes this process. In addition to the catalytic current density, the onset potential is another important parameter for the electrochemical catalyst. The onset potential for MOR on the AuNPs/m-v oxide/GCE is about 0.5\u00a0V, which is much lower than the m-v oxide/GCE (0.7\u00a0V). The enhanced electrocatalytic performance of AuNPs/m-v oxide may be attributed to the factor that the well-distributed AuNPs can act as electron absorbers to promote the oxidation of Cu and Ni cations [25,39\u201340], making metal ions reach higher oxidation state. The higher oxidation state stimulates the rapid charge transfer on the electrode/electrolyte interface and improves the catalytic activity in MOR.To study the kinetics property, the relationship between the peak current density and scan rate of AuNPs/m-v oxide/GCE and m-v oxide/GCE was investigated, which is shown in Fig. 6\n. The CV response of different modified electrodes in 0.1\u00a0M potassium chloride electrolyte with 2.0\u00a0mM Ferri/Ferro-Cyanide at different scan rates (from 30 to 100\u00a0mV/s) was recorded. It can be seen from the results that both for the AuNPs/m-v oxide/GCE (Fig. 6A and m-v oxide/GCE (Fig. 6B, the peak current is continuously enlarged as the scanning rate increases. For AuNPs/m-v oxide/GCE (Fig. 6C, the linear equation between the anode peak current density value and the square root of scan rate is j (mA/cm2)\u00a0=\u00a00.1526 v\n1/2 \u20130.66, and the correlation coefficient R2\u00a0=\u00a00.9923. The linear equation between the cathode peak current density and the square root of scan rate is j (mA/cm2) = \u2212 0.1305 v\n1/2\u00a0+\u00a00.4397 with the correlation coefficient R2\u00a0=\u00a00.9952. For m-v oxide/GCE, the linear equation between the anode peak current density and the square root of scan rate is j (mA/cm2)\u00a0=\u00a00.0558 v\n1/2 \u20130.0938, the correlation coefficient R2\u00a0=\u00a00.9981. The linear equation for the cathode peak current density is j (mA/cm2) = \u2212 0.0366 v\n1/2 \u20130.0855, the correlation coefficient R2\u00a0=\u00a00.9833. The relationship between the peak current density and the scan rate indicates that both for the AuNPs/m-v oxide/GCE and m-v oxide/GCE, the kinetics process is diffusion control process in the reaction [41].The CV response of AuNPs/m-v oxide/GCE and m-v oxide/GCE in 1\u00a0M potassium hydroxide electrolyte with different concentrations of methanol (0.3\u00a0\u223c\u00a01.0\u00a0M) were tested, as shown in Fig. 7\nA and 7B. It can be observed that for the two modified electrodes, the peak current density increases as the methanol concentration increases. As the methanol concentration increases from 0.3 to 1.0\u00a0M, the anodization peak current density for m-v oxide/GCE is increased from 3.7 to 7.4\u00a0mA/cm2, while this value is increased from 17.0 to 30.5\u00a0mA/cm2 for AuNPs/m-v oxide/GCE. Fig. 7C shows the linear relationship between the oxidation peak current density of the two catalysts and the methanol concentration, the slope values of AuNPs/m-v oxide and m-v oxide are 19.5 and 5.1 respectively. A larger slope value indicates a higher electron transfer efficiency of the catalyst [41]. The bigger peak current density and larger slope value proves the electrochemical catalytic effect of AuNPs/m-v oxide is better than that of m-v oxide, which is coincident with the above results. This phenomenon can be attributed to the following reasons: First, via the bifunctional mechanism, the mixing multivalent metal oxides can effectively adsorb substances such as hydroxyl groups, and can convert CO-like intermediates into CO2, thereby improving electrocatalytic performance [42]. In addition, with the excellent electronic conductivity, the existence the AuNPs can accelerate the electron transfer rate between metal oxides and electrolyte. Third, the AuNPs absorbed on the surface of the metal oxide can collect negative electrons to a large extent, which promotes the oxidation of Cu and Ni cations to higher chemical valence state, so as to improve catalytic efficiency in MOR [25].In order to evaluate the catalytic performance of nanocomposite, the comparison between this work and other published catalysts is shown in Table 1\n. Comparing the experimental results for AuNPs/mv oxide, it shows higher current density than other catalyst reported in Table 1 in alkaline conditions, confirming the higher electrochemical catalytic performance for methanol oxidation reaction in alkaline conditions.The reproducibility of the AuNPs/m-v oxide catalyst was studied by investigating the oxidation peak current of five AuNPs/m-v oxide/GCE in 1\u00a0M potassium hydroxide containing 0.5\u00a0M methanol. These electrodes were modified under the same condition for the CV test. The relative standard deviation (RSD) of peak current density for five electrodes was calculated to be 2.75%, which means a remarkable reproducibility.In practical applications, the stability of the catalyst is also an important parameter for its commercial application, therefore, the long-term stability of the catalyst is tested by monitoring the i-t curve of AuNPs/m-v oxide/GCE and m-v oxide/GCE in potassium hydroxide (1\u00a0M) electrolyte containing 0.5\u00a0M methanol at 1.20\u00a0V for 7200\u00a0s. Comparing the current curves obtained from two modified electrodes in Fig. 8A, a sharp drop in current density is clearly observed in the first period of time, it may be due to the formation of double-layer capacitors, which is a kinetic process. As the intermediates continue to accumulate and occupy the active sites of the catalyst during the methanol oxidation process, the current slowly decays, and finally reaches a steady state. The current density of AuNPs/m-v oxide stabilizes at 1.016\u00a0mA/cm2 after 6000\u00a0s, which is 1.24 times higher than the steady current density of m-v oxide (0.823\u00a0mA/cm2). The higher steady current density of AuNPs/m-v oxide indicates the stronger anti-poisoning ability in the electro-oxidation process. Fig. 8\nB shows the slopes of the Tafel plots of AuNPs/m-v oxide and m-v oxide, the slopes of the two catalysts are 41.87 and 81.81\u00a0mV/dec respectively. All these results show that AuNPs/m-v oxide nanocomposite has excellent catalytic activity and anti-poisoning performance against various toxic intermediates [20,52].Cu-Ni oxide with the decoration of AuNPs was effectively synthesized through traditional solvothermal method and surface absorption. The morphology, structure, chemical valence and element composition of the nanomaterial were characterized by a series of techniques like TEM, XRD, XPS and EDS. XRD and XPS characterizations prove that the Cu and Ni are existent with multi-valent state in the oxide. Different nanomaterial modified electrodes of Cu-Ni oxide/GCE and AuNPs/Cu-Ni oxide/GCE were fabricated in the procedure and employed to investigate the electrochemical catalytic effect in MOR. Electrochemical experiments confirmed that AuNPs/m-v oxide has better electrochemical performance than m-v oxide. Via the bifunctional mechanism, together with the excellent electronic conductivity of AuNPs, the AuNPs/m-v oxide exhibits outstanding electrochemical catalysis effect in MOR process. It shows a high oxidation peak current density of 21.1\u00a0mA/cm2, good tolerance and stability to toxic intermediates. Further works are still carrying on improving the performance by optimizing the component dose and surface modification of the nanocomposites. What\u2019s more, considering the easy fabrication and high catalytic efficiency, it provides a promising alternation of MOR electrochemical catalyst for further catalytic 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.Thanks for financial support from the National Natural Science Foundation of China (Grant Nos. 21861018), Natural Science Foundation of Jiangxi Province (20202ACBL214012, 20182BCB22010), Jiangxi Provincial Key Laboratory of Functional Molecular Materials Chemistry (20212BCD42018), the Youth Jinggang Scholars Program in Jiangxi Province, Key Laboratory of Testing and Tracing of Rare Earth Products for State Market Regulation and Qingjiang Excellent Young Talents Program of Jiangxi University of Science and Technology.", "descript": "\n                  Cu-Ni oxide nanocomposite was synthesized via solvothermal method with nickel (II) nitrate and copper nitrate as Ni and Cu resource respectively. AuNPs/oxide nanocomposite was obtained by dispersing the oxide into the pre-synthesized Au colloid. Transmission electron microscope (TEM) shows that the synthesized oxides are nanoplate morphology. Interplanar distance from the high resolution TEM (HRTEM) image shows that the Ni and Cu in the oxide are multivalent state. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterization further proves that the Ni and Cu are multivalent state, and the Au is Au0 form in the nanocomposte. EDS-mapping images confirm large numbers of AuNPs are uniformly absorbed onto the ultra-thin multivalent Cu-Ni oxide nanoplate. The electrochemical properties of multivalent oxide (m-v oxide) with or without AuNPs decoration are investigated in 0.1\u00a0M potassium chloride electrolyte with 2.0\u00a0mM Ferri/Ferro-Cyanide. When employed for the electrochemical catalysis in methanol oxidation reaction (MOR), the AuNPs/m-v oxide exhibits better electrochemistry performance than m-v oxide. The current density of AuNPs/m-v oxide (21.1\u00a0mA/cm2) is 3.4 times bigger than that of m-v oxide/GCE (6.2\u00a0mA/cm2). What\u2019s more, it shows good electrochemical stability and lower slope of the Tafel plot (41.87\u00a0mV/dec).\n               "}
{"full_text": "There is a broad variety of possible applications for dopant transition metal oxides with different morphological structures. The possibility of tailoring their electronic properties by doping and quantum containment of carriers is one main feature that has made these materials special. The semiconductor materials have drawn significant focus due to their possible applications for instance catalysts, and sensors for preventing environmental problems [1\u20135].Among many semiconductor materials, Mn3O4 (hausmannite) has gained research focus due to its unique structural properties together with intriguing physicochemical properties that are of high significance for battery applications [6]. It is usually prepared by hydrothermal pathway, carburization technique, chemical decomposition, ball milling, sun-freezing, self-combustion, co-precipitation, chemical reduction [7\u201313]. Furthermore, Mn3O4 nanostructures are not only utilised for potential application, but also play a significant part in electromagnetic applications.Microwave-assisted combustion techniques have increased control over the size and shape of the synthesized nanomaterials and have created similar and mono-dispersed nanomaterials. Increased control over the scale and shape of the end Ni-doped Mn3O4 is the advantage of this approach over other approaches. The combustion method is very facile and only takes a few minutes, and it has been widely applied to the preparation of various nano-scale oxide materials. This synthetic technique makes use of the heat energy liberated by the redox exothermic reaction at a relative low igniting temperature between the metal nitrates and urea or other fuels. Furthermore, the combustion method is also safe, instantaneous and energy saving. The advantage of microwave combustion method is that it leads to a highly exothermic reaction, which in turns led to a direct formation of spherical particles. The microwave heating causes the uniform distribution of temperature between the surface and the bulk material, and there by leading to the fast formation of nanoparticles [14\u201317].The aim of the present research is to focus on the preparation of Ni-doped Mn3O4 by a user-friendly microwave combustion method and in addition the examination of magnetic, optical, morphological, and structural properties.In a standard synthesis process, 6.0\u00a0ml of Mn(NO3)2 solution (0.4\u00a0M) is added to Ni(NO3)2 solution. In addition to that, 8.0\u00a0ml of urea (0.4\u00a0M) was introduced to Mn(NO3)2, and then the contents continuously stirred. Manganese precursor alongside nickel nitrate and urea (fuel) was put within a domestic-type convection microwave oven (Make - IFB; Design \u2212 20SC2) and subsequently exposed to microwave energy for ten minutes with a power of 1200 Watt together with 2450\u00a0MHz microwave frequency. The uniform resulting substance initially began to boil followed by vaporization occurred as well as the releasing of the gases whilst the microwave combustion. If the substance of chemical sources is prolonged to the automated stage of combustion, the substance will evaporate and right away produce a solid. The solid material obtained was properly washed with ethanol, after that dried at 80\u00a0\u00b0C for 2\u00a0h, and lastly, the obtained materials labeled as Mn3O4 (a) Ni-doped at 0.01(b), 0.02(c) and 0.03(d).The nickel doped and pristine Mn3O4 samples X-ray diffraction patterns were scanned with the Rigaku X-ray diffraction meter utilizing CuK radiation. High-resolution scanning electron microscopy (HR-SEM) pictures were obtained by the Philips XL30 ESEM, that is installed together with energy-dispersive X-ray spectroscopy. High-resolution transmission electron microscopy (HR-TEM) pictures had been captured through the Philips EM 208 transmission electron microscope by applying an accelerated voltage of 200\u00a0kV. The UV\u2013visible (Cary100) and also the Cary Eclipse Fluorescence Spectrophotometers had been utilized for recording diffuse reflectance spectra as well as optical properties for the Mn3O4 nanostructures prepared by present approach.XRD was tested for finding the crystalline structure of Mn3O4 and Ni-doped Mn3O4 and in addition, the doping behavior of Ni 2+ in the different samples as shown in Fig.\u00a01\n(a). The sharp peaks having 2-theta values 32.32, 36.09, 38.03, 44.39, 50.71, 58.45, 63.84, and 67.65 can be assigned to the (101), (112), (103), (211), (004), (220), (105), (321), (224) and (400) planes of Ni-doped Mn3O4. Diffraction peaks corresponding to the structure of the Mn3O4 hausmannite could also be distinguished (JCPDS No. 24\u20130734). However, the characteristic peaks attributed to Mn3O4 are mainly sharp and broad which may be due to the crystalline structure of synthesized Mn3O4. It is also noted that the most intense peak is gradually shifted to a higher angle and is widened by increasing the Ni content as clearly shown in Fig.\u00a01(b) of this. This widening and shifting of the diffraction peak with Ni doping strongly recommend that Ni ions have successfully replaced the Ni ion with the Mn3O4 lattice.There are a number of reasons for this feature, such as inversion of the cation distribution (Mn3+ into tetrahedral interstices and Ni2+ into octahedral interstices). When the doped amount of Ni was increased to oxide peak at about intensity peaks 48\u00b0 (marked by asterisk) was detected. The crystal structure and their physical properties depend on the Mn site symmetry. Mn3+ ion has an electronic configuration of t2g\n3 eg0. It usually occupies the intensity peaks octahedral sites in spinel structure which theoretically has no orbital angular momentum. Similarly, Mn ion has a zero orbital momentum at the B site with electronic configuration of t2g3 eg2 in the spinel structure. One would expect that of intensity peaks Ni substitution and these opposite trivalent spins are found to destroy the structure. However, Ni substitutions can induce intensity peaks variations of the exchange coupling into this mixed spinel system. In other words, one understands that Mn valence is reduced in the Td (i.e. from trivalent to divalent Mn) site with a complete reaction at dopant becomes a normal spinel-like (intermediate spinel) structure [18,19].In addition, the observed line widening of the diffraction peaks indicates that the synthesized materials are within the nanometer range. The full width at the half-maximum (FWHM) of the major peaks increases with an increase in doping concentrations, which can be attributed to a decrease in crystalline size. The peak position and the FWHM are obtained by fitting the measured peaks with two Gaussian curves in order to find the true peak position and width corresponding to monochromatic Cu Ka radiation. Since the systematic error decreases as the Bragg angle increases, the values of the average grain size and the lattice parameter of the samples were calculated for the reflection peak which possesses both higher angle and reasonable intensity. As the particle size decreases, the crystal lattice becomes less aligned leads to broadening of the XRD pattern. Hence, there is an inverse relation between NPs size and the sharpness of the XRD peaks [20]\n.\nThe mean crystallite for all samples was measured by employing the Scherrer equation [21].\n\n\n\nL\n=\n\n\n0.89\n\u03bb\n\n\n\u03b2\ncos\n\u03b8\n\n\n\n\n\n\nWherein, the mean crystal size (\u00c5) and X-ray source wavelength (1.5404 \u00c5) are denoted as L and \u03bb correspondingly. The full width at half maximum (FWHM) and diffraction angle of the respective peaks (radians) are labelled as \u03b2 and \u03b8 respectively. A possible factor for the trend of decreasing crystallite size is that the Ni concentration increasing of the continuation for the ordering process among crystallite size is currently observed in Fig. 1c\n\n. The crystal size of all the samples is in the order of a nanometer. This confirms that the Ni-doped Mn3O4samples are nanomaterials and that the size of the crystals varies depending on the doping.The explanation for the smaller sizes of the Ni-doped samples is assumed to be the fact that Ni, which is added within the device, is settled within the lattice of Mn3O4 and thus forms bonds with the unstable oxygen atoms of Mn3O4 or due to oxygen desorption. This means that Ni doping can lower the nucleation rate of Mn3O4 and in turn, doping atoms can influence the size of the particles. Thus with Ni doping, Ni-doped Mn3O4 samples are smaller in size than the undoped Mn3O4\n[22]. This difference due to the strain, crystallite size, and lattice parameters that can induce a greater broadening in the diffraction peak (microstructural and microstrain) for the broadening peaks Fig.\u00a01d. Diffraction studies can be helpful when it is important to understand the state of the chemical mixture of the elements involved or the steps in which they exist. The advantage of the diffraction method over traditional chemical analysis is that it is simpler, requires only a relatively small amount of sample, and is therefore non-destructive.UV\u2013visible absorption technique is an effective non-destructive instrument for the analysis of optical properties. Absorption spectra of undoped and Ni-doped Mn3O4 were recorded in the 200\u2013800\u00a0nm wavelength range as shown in Fig. 2\n. Absorption spectra depend on many factors, such as oxygen vacancy, morphology, bandgap, and impurity. The cut-off wavelength region is 400\u2013600\u00a0nm, indicating the photo-excitation of electrons from the valence band to the conduction band. It is also observed that the absorption edge has continuously changed to a higher wavelength (yellow-green shift) as the Ni content rises to the host Mn3O4 and as a result, the size is reduced [23,24].In general, the PL properties of Ni-doped Mn3O4 are highly determined by their defect states (i.e. defect speeds, defect load states, defect concentrations, etc.). The PL emission spectra and the findings of the samples are shown in Fig. 3a\n\n. As seen in the diagram.\n3b\n, the emission peak of the Ni-doped Mn3O4 is shifted to a higher wavelength and its emission peak varies from 520 to 550\u00a0nm. However, the intensity of this function is very poor and size-independent [25,26]. On the other hand, the nano-sized Ni-doped Mn3O4 allows for a further degree of independence and size reliance on extreme PL emissions. High band-edge luminescence is consistent with a low dislocation density as well as a low surface defect density, as these defects appear to quench the band-edge radiative recombination (\nFig. 3c\n\n). Since the size-related PL spectra intensity of nano semiconductors can be quantified, it is possible to calculate an optical particle size using the intensity shift measured from the emission spectrum. The crystallite size from PL spectra was calculated from line broadening of the with two Gaussian curves in order to find the true peak position line by the using the Scherer equation (\nFig. 3d\n\n). The shift in the PL peaks could be assigned because of the restriction of size and intensity by the Ni. PL peaks to many interdependent factors, for instance, lattice dislocation, electron\u2013phonon coupling, localization of charge carriers because of the point defects, and interface effects [27\u201330]. The weakness of the deep-level emissions also indicates that these nanoparticles have a stoichiometric structure, likely having a low density of point defects. The findings from PL studies are compatible with the SEM/TEM and XRD.On the other hand, once location oxygen vacancies had been presented upon the decreased metal oxides, various superficial oxygen-vacancy levels surface preceding and partially overlapping by way of the VB of dopant metal oxides (mainly O2\n\np\n and M3\n\nd\n, with a few places. Additionally, the increase of the VB to CB can easily likewise direct result in the widening concerning and the size of the VB. This kind of theoretical function suggested that oxygen vacancies may perform the function of changing the valence band of metal oxides and dopant oxides. It is also noticed that both emission peaks are gradually moved to longer wavelengthswith Ni content growth. This redshiftwas well correlated with the bandgap narrowing, as demonstrated by the UV absorption spectra. The green emission mechanism can be understood as follows. From the above case, PL spectra have a thin, wider emission band and are mainly used for the application of luminescence as they cover a wide range of the spectral area [31].The morphology and structure of the Ni-doped Mn3O4 NPs are described by a high-resolution electron microscopy (HR-TEM) study. A distinctive HR-TEM image is shown in Fig. 4\n\n. HR-TEM images show nanoparticles with a length of 100\u2013110\u00a0nm and a width of 3\u20135\u00a0nm. The nanoparticles observed are formed by aggregation of NPs and the samples are also agglomerated due to the magnetic nature of the materials. For the nanopowder doped material produced according to this method, this tendency toward different aggregation/agglomeration levels and the creation of small agglomerates from a few particles is also noted. The mean particle size value (TEM size, 39\u201323\u00a0nm) is nearer to the average crystallite size (XRD size, 31\u201327\u00a0nm), which is calculated from the XRD data in accordance with the Debye-Scherrer formula (\nFig. 5\n\n). Minor differences are caused by the measurement or calculation errors. This difference in grain size is explained by the fact that in TEM, only a few grains are examined and measurements are conducted manually, but in XRD, grains are regarded as a whole as well as calculations are performed by applying specific formulas [32]. The overall examination of TEM micrographs shows that certain latex compounds create a covering around the particles, preventing them from agglomerating and therefore contributing to the stability of the nanoparticles. The observed slight difference in particle size value as estimated from the two different techniques (XRD and HR-SEM) may be due to some structural disorder and strain in the lattice resulted from different ionic radii and/or clustering of the nanomaterials [33]\n. An in-depth high-resolution electron microscopy analysis was performed to examine the nanoscale fine structure of the Ni-doped Mn3O4 device. The high-resolution TEM (HR TEM) image of a single nanoparticle has been taken and is shown in Fig. 6\n. It is apparent from the figure that the nanoparticle is single crystalline in nature, with transparent lattice fringes that can be seen on the whole particle. The bulk of the particles are observed to be mainly faceted to crystalline.M\u2212H hysteresis cycles have been reported using a vibrating sample magnetic scale operated at a applied magnetic field of 10\u00a0k Oe at room temperature to estimate the impact of Ni conversion on residual magnetization, compulsion and saturation magnetization. Fig. 7\n Magnetic parameter values such as saturation magnetization (MS), coercivity (HC), and remanence (Mr) are seen in Table 1\n\n. Factors such as cation replacement, grain size, and A-B exchange interactions have been shown to have a significant effect on the magnetic properties of oxide materials. Increased grain size and decreased A-B superexchange interaction cause canting spins on the surface of nanoparticles that minimize the magnetic characteristics of the samples in question. The magnetic parameters indicated suggest the lack of the soft magnetic nature of Mn3O4 by the replacement of nickel [34].The electrical conductivity (\u03c3) was determined through the electrode area (A) and also from the sample thickness (t) of the sample by employing the following equation.\n\n\n\n\u03c3\n=\n\nt\n\nAR\n\n\n\n\n\n\nIn which R is the calculated resistance. The electrical conductivity was determined to be in the order of 10\u22122 S/ cm, which revealed a progressive tendency with the dopant concentration until 0.03%. The incorporation of Ni on pristine Mn3O4 samples lattice was observed to be beneficial in order to increasing the electrical conductivity on the whole temperature range of measurement.When 0.03% Ni is doped, a decrease in conductivity is observed. The maximum conductivity value of 2.13\u00a0\u00d7\u00a010\u20132 S/cm observed for Mn3O4 undoped and Ni doped Mn3O4 samples at 500\u00a0\u00b0C was found to be around two times higher than that of pure Mn3O4 sample (0.98\u00a0\u00d7\u00a010\u22122 S/cm).The activation energy was calculated using the Arrhenius' equation given below:\n\n\n\n\u03c3\n=\n\n\u03c3\no\n\nexp\n\n\n\n\n-\n\n\nE\na\n\n\nkT\n\n\n\n\n\n\n\u2192\n\n\n\n\nWherein Ea is the activation energy of electrical conduction, k can be called Boltzmann constant, T is generally denoted for temperature and \u03c3o is the pre-exponential factor. The activation energy needed for the movement of the charge carriers was determined through the Arrhenius plot which is displayed in Table 2\n\n. The activation energy reduces by increasing in temperature because of the thermally initiated mobility of charge carriers from one atomic site to another according to hopping conduction mechanism [35]. It could be noticed that the activation energy of pristine Mn3O4 and Ni doped Mn3O4 structure reduced until 0.03% which showed the minimum activation energy of 0.69\u00a0eV when compared to pristine Mn3O4 (0.87\u00a0eV). Because of the spinel structure's porosity, Ni ions may be dispersed over the nanoparticle\u2019s surface (as demonstrated by the TEM and XRD studies in the present work). Even if an attempt is made to include a very high dopant concentration in such nanoparticles, the local energy of the dopant is greater than the energy in the larger part of the semiconductor. Thermodynamically, it may be disadvantageous for the dopant to stay in the nanoparticle as a dopant and be excluded from the \u201cquality\u201d of the semiconductor on the surface of the particle [36,37].The catalytic test of pure Mn3O4 synthesized by and Ni-doped Mn3O4 is proved as follows. In 100\u00a0ml beakers (five sets), 0.1\u00a0mmol\u22121 concentration of 4-nitrophenol aqueous solution (50\u00a0ml) was injected concurrently with 0.529\u00a0mol/dm3 concentration of H2O2 (made at the time for 50\u00a0ml). To the above mixture, synthesized different Mn3O4 catalysts (0.001\u00a0mol) were added into the different beaker. For comparison, one beaker does not contain any catalyst. The above mixture were continueously stirred. For each of the sets, the periods necessary for full decolorization from yellow colour were recorded and in Table 3\n the values were summarized. It has been found that 4-nitrophenol color changes earlier in the presence of a catalyst than in the absence of a catalyst. Among them, Ni-doped Mn3O4 (0.03%) showed the greatest catalytic activity (decolorization takes place in 15\u00a0min) in comparison to the pure and other Ni-doped Mn3O4 samples. It is significant to point out that the concentration of Ni has direct impact on the size, DRS, VSM and PL emission spectra. It has been pointed out that among all the factors, the surface chemical state is the most significant factor since it is related to the electron transfer and separation efficiency at the interface. The presence of an appropriate amount of Ni ions on the catalyst surface plays an important role on improving catalytic performance [38\u201340]. Though, it is significant to assume that despite the doping concentrations used in the present work, there are some interactions between them. Regardless of the dopant concentration applied in this research, it is crucial to expect that they interact.Ultra-fine Mn3O4 Ni-doped nanoparticles were effectively synthesized through microwave techniques. Ni substitution has resulted in a remarkable increase in the structural, optical, and magnetic properties of Ni-doped Mn3O4. Structural, phase, purity, particle size, was studied by powder XRD, TEM, HR-TEM, and VSM spectral analysis, respectively. The Crystalline size of the synthesized nanoparticles was measured using the Debye-Scherer equation, which decreased from 31\u00a0nm to 27\u00a0nm. PL spectra intensity structural defects in the lattice, which including oxygen vacancy, replacement, and interstitial atoms, which cause localized chargelosses. The effect of doping leads to a decrease in crystallite size. The size of the crystal grain depending on certain doping-induced nucleation centers and the growth stress disruption as a result ofthe difference in the atomic radius among Mn and Ni dopants.The author P. Tamizhdurai have affiliation with university madras direct or indirect financial interest. The authors extend their appreciation to the Deanship of Scientific Research, This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project Number (PNURSP2022R19), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. Also the author Dr Abualnaja appreciated Taif University Researchers Supporting Project number (TURSP-2020/267), Taif University, Taif, Saudi Arabia. We further confirm that the order of authors listed in the manuscript has been approved by all of us.We wish to confirm that there are no known conflicts of interest associated with this publication.This statement is signed by all the authors to indicate agreement that the above information is true and correct.This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project Number (PNURSP2022R19), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. Also the author Dr Abualnaja appreciated Taif University Researchers Supporting Project number (TURSP-2020/267), Taif University, Taif, Saudi Arabia.", "descript": "\n                  Ni-doped Mn3O4 nanoparticles (NPs) were synthesized by a simple one-pot microwave combustion procedure utilizing urea as a fuel. X-ray diffraction, transmission electron microscopy (TEM), diffuse reflectance spectroscopy, Photoluminescence spectra, and vibrating sample magnetometer. The particle size and the crystalline size measured from the HR-TEM monographs and XRD study suggest the similarity of the data collected from these two measurements. Photoluminescence (PL) spectra demonstrated increased luminescence amplitude with increased Ni concentration. Thus, the present study determines the time required for 4-nitrophenol yellow to colorless by Ni-doped Mn3O4 and Mn3O4 samples.\n               "}
{"full_text": "The increasing request for energy, chemicals, and materials globally due to megatrends needs an intervention to ensure that these supplies are produced efficiently using carbon-neutral processes and renewable feedstocks. Under this emerging socio-economic picture, biomass exploitation allows for a broad range of biofuels and value-added biochemicals (Attard et al., 2020; Clark, 2019). A desirable and practically unused candidate to obtain these commodities is almond hulls, i.e., an unavoidable food waste produced in large amounts in the almond processing industry. Almond production has augmented noticeably recently, e.g. global production has risen from 0.7 Mt. in 2007 to 1.5 Mt. in 2020 (Driedfruit.net, January 2022; Factfish.com, January 2022). This lignocellulosic material is also known as almond green shell cover and accounts for more than half of the total almond weight (Esfahlan et al., 2010). It comprises carbohydrate matter (cellulose, hemicellulose and pectin), along with lignin, tannin-like poly-phenolic compounds and ash (Esfahlan et al., 2010).One way to improve food security is by decreasing food waste to reduce environmental impacts; developing sustainable processes to efficiently manage this unavoidable food waste by-product is necessary (Dutta et al., 2022; Mak et al., 2020). However, little work has been conducted, and the publications reported to date primarily covered the extraction of some constituents (saccharides, bioproducts and/or phenolic species) rather than addressing strategies to valorise the entire material (Rem\u00f3n et al., 2021b). For example, Pinelo et al. (2004), Sfahlan et al. (2009) and Sfahlan et al. (2009) extracted phenols from almond hulls to produce antioxidants. Concerning the production of carbohydrates, Offeman et al. (2015) conducted hot water extraction experiments to recover the carbohydrate content of hulls obtained from different varieties of almonds. Ferrandez-Villena et al. (2019) used almond hulls to manufacture renewable particleboards, while Gonz\u00e1lez et al. (2005) analysed the combustion of different almond hulls. These works provide valuable insights to recover some almond hulls constituents, but they do not provide alternative solutions for integral valorisation within a biorefinery strategy. Therefore, it is still required to develop new routes to achieve complete and more sustainable management of the whole material. Among the different approaches for biomass valorisation, hydrothermal hydrogenation might be an attractive route to transform it into a plethora of valuable liquids for the energy sector and pharmaceutical, cosmetic and textile industries. These include water-soluble oligomers, oligosaccharides, saccharides, sugar alcohols, polyhydric alcohols, carboxylic acids, furans and nitrogen and phenolic compounds (Ribeiro et al., 2021).Hydrothermal hydrogenation uses water at hydrothermal conditions. It is conducted at moderate temperatures (150\u2013300 \u00b0C) and relatively low H2 pressures (20\u201380 bar) in the presence of a catalyst (Ribeiro et al., 2021). Therefore, not only hydrogenation reactions take place. In contrast, a series of cascade, \u2018one-pot\u2019 transformations, mainly comprising hydrolysis, hydrogenation, and hydrogenolysis reactions co-occur. This \u2018one-pot\u2019 strategy aids in shortening the reaction time, diminishing unwanted by-products formation, and helps reduce the consumption of energy, solvents and reagents. Concerning the catalyst, metal (Fe, Co, Ni, Pd, Pt, Ru, Rh, Ir, Ag and Au) supported catalysts have frequently been used (Ribeiro et al., 2021). Ru is considered one of the most promising active phases when balancing crucial factors such as activity, selectivity, deactivation resistance and price. In particular, Ru-based catalysts are very active in hydrogenation reactions. They are economically more competitive than other metals such as Pt and Au and suffer from lesser deactivation than Ni-based catalysts. Carbonaceous materials have commonly been used as supports due to their outstanding chemical immovability in non-oxidising media and bespoke surfaces combined with stunning textural properties (Cardoso et al., 2021; Cardoso et al., 2018; Ochoa et al., 2018; Rem\u00f3n et al., 2016a; Zhu et al., 2013). Besides, carbon-neutral processes can be used for their synthesis (Pinilla et al., 2017; Rem\u00f3n et al., 2021a), linking very well with the emerging biorefinery and circular economy ideologies (Dutta et al., 2022; Mak et al., 2020). Active metals supported on carbon materials have been used as catalysts for the hydrogenation of biomass-derived compounds.Notwithstanding these excellent features, work addressing the use of catalysts based on Ru supported on carbonaceous materials for the hydrothermal hydrogenation of biomass is scarce. For example, Matsagar et al. (2020) used a commercial Ru (5 wt%)/C catalyst for the hydrogenation of furfural to tetrahydrofurfuryl alcohol at mild reaction conditions. In another work, Dutta et al. (2019) critically reviewed the work conducted on the hydrogenation of levulinic acid to produce \u03b3-valerolactone (GVL) over noble metal catalysts. This review also shows that the use of Ru-based catalysts is scarce. Even though these and other works in the literature have addressed the hydrothermal hydrogenation of biomass structural compounds, it is essential to note that the reactivity of biomass could be different from that of these carbohydrates alone. This accounts for several interactions between species taking place in the upgrading process. For example, Ribeiro et al. (Ribeiro et al., 2016; Ribeiro et al., 2017b) found synergetic interactions between cellulose and hemicellulose during their hydrothermal hydrogenation over a 0.4 wt% Ru/CNT catalyst. Remarkably high sorbitol (74%) and xylitol (76%) yields were achieved during the co-valorisation of both carbohydrates through a two-step procedure, firstly 2 h at 170 \u00b0C, followed by 4 h at 205 \u00b0C. Besides, it is difficult to predict the behavior of actual biomass based on its structural composition due to different factors affecting its recalcitrance (Pu et al., 2013). This information suggests that it is vital to address the hydrothermal hydrogenation of actual biomass. Still, work conducted on the hydrothermal hydrogenation of actual biomass using carbon-supported metal catalysts is not very well reported. Most publications have focused on producing sugar alcohols and glycols from forestry and agricultural residues at some fixed processing conditions.For sugar alcohols, Palkovits et al. (2010) addressed the hydrothermal hydrogenation of spruce wood chips over a Ru supported on carbon (Ru/C) catalyst at 160 \u00b0C and 50 bar H2 pressure, achieving a biomass conversion of 59% with a sorbitol yield of 36%. Guha et al. (2011) used a 2 wt% Ru supported on activated carbon (Ru/AC) catalyst for converting beet fibre. They reported an arabitol yield as high as 83% when the process was conducted at 155 \u00b0C for 24 h, using 50 bar of H2. K\u00e4ldstr\u00f6m et al. (2011) hydrothermally hydrogenated bleached birch kraft pulp over a (Ru/C) catalyst at 185 \u00b0C and 20 bar of H2 for up to 30 h, leading to the production of xylose (up to 0.12 mol/mol biomass) and glucose (up to 0.03 mol/mol biomass). Zhou et al. (2015) transformed Jerusalem artichoke tube into hexitols (32% mannitol and 61% sorbitol) by hydrothermal hydrogenation over a 3 wt% Ru/AC-SO3H catalyst, conducting the process at 100 \u00b0C and 60 bar H2 for 5 h. Yamaguchi et al. (2016) studied the hydrothermal hydrogenation of different feedstocks (Japanese cedar, eucalyptus, bagasse, empty fruit bunch and rice straw) over a 3 wt% Ru - 1 wt% Pt/C catalyst, achieving a total sugar yield as high as 55%. Ribeiro et al. (2017a) used cotton wool, cotton textile, tissue paper and printing paper to produce sugar alcohols at 205 \u00b0C and 50 bar of H2 with a 0.4 wt% Ru supported on carbon nanotubes (Ru/CNT) catalyst. Except for printing paper, complete substrate conversion was attained with all materials, with sorbitol yields ranging from 51 to 56% in all the cases. Li et al. (2018) achieved a high polyols production during the hydrothermal hydrogenation of cornstalk (25% sorbitol, 12% xylitol and 5% ethylene glycol) and beechwood (18% sorbitol, 13% xylitol and 4% ethylene glycol) over Ru/C at 200 \u00b0C and 30 bar of H2 for 8 h.Regarding biomass conversion into glycols, Ribeiro et al. (2021) converted different waste materials (Eucalyptus wood, corncob, cotton wool and tissue paper) into ethylene glycol, using a 0.4 wt% Ru/CNT catalyst at 205 \u00b0C, 50 bar H2 for 5 h. The biomass conversion and ethylene glycol yields were as follows: eucalyptus wood (X = 74%, Y = 25%), corncob (X = 91%, Y = 14%), cotton wool (X = 100%, Y = 42%) and tissue paper (X = 100%, Y = 34%). In another work, Pang et al. (2018) used a 5 wt% Ru/AC catalyst for glycols production from ball-milled Miscanthus at 245 \u00b0C and 50 bar H2 for 6 h. Under such conditions, the whole material was converted into a mix of valuable liquid products, including ethylene glycol (34.6%), 1,2-propylene glycol (8.1%), glycerol (8.8%), 1,2-butanediol (9.7%) and sorbitol (4.3%). Li et al. (2018) converted cornstalk into polyalcohols and alkyl cyclohexanes over different Ru/C catalysts. The Ru/C catalyst reduced at 300 \u00b0C showed the best performance when the process was conducted at 200 \u00b0C and 30 bar H2 for 8 h. The molar yield of alkyl cyclohexanes was 97.2%, with a total polyalcohol yield of 52.7% (24.5% sorbitol, 12.2% xylitol, and 16.0% C2\u2013C4 polyols).These publications afford valuable information on the hydrothermal hydrogenation of biomass using Ru catalysts supported on different carbonaceous materials. Although some work has been conducted on the synthesis of Ru/CNF catalysts, these were not employed to this end. For example, Yang et al. (2016) synthesized several Ru/CNF catalysts with different Ru loadings (0.09\u20130.64 wt%) by the one-pot conversion of Ru-functionalised metal-organic framework fibres. These were tested in the hydrogenation of lactic acid (Lac-Ac) to \u03b3-valerolactone (GVL), with the best results (96% Lac-Ac conversion with 95% GVL yield) being achieved using the 0.27 wt% Ru/CNF catalyst. Nevertheless, to the best of our knowledge, carbon nanofibres (CNF) have never been employed to synthesise Ru-based catalysts to this end. Also, the effect of the processing conditions is not yet well understood using actual biomass. In previous work in our research group, Frecha et al. (2019) used a 0.4 wt% Ru/CNF catalyst for the hydrothermal hydrogenation of cellobiose (a cellulose model compound). The effect of the processing time (0\u22123 h) was analysed on the cellobiose conversion and reaction products distribution at 180 \u00b0C employing an initial H2 pressure of 4 bar. Complete cellobiose conversion was attained within the first 30 min of reaction, with a sorbitol yield progressively increasing from 5 to 46% over the course of the reaction, showing the promising properties of this catalyst for the hydrothermal hydrogenation of biomass due to its excellent activity in hydrolysis/depolymerisation and hydrogenation reactions.Given this background, this work explores, for the first time, the hydrothermal hydrogenation of almond hulls (a lignocellulosic unavoidable food waste) over a carbon-neutral Ru/CNF catalyst, with a very low (0.4 wt%) Ru content. Initially, the effects of the processing conditions on the distribution of the products (gas, liquid and solid) and the detailed chemical composition of the liquid phase have been thoroughly analysed. Additionally, a possible reaction pathway has been developed to explain the formation of the most representative liquid products. Then, the process has been optimised for the selective production of value-added chemicals, including oligomers, carboxylic acids, sugar alcohols and polyhydric alcohols. Finally, the energetic aspects of this process have been analysed and discussed. Therefore, considering the lack of publications exploring the use of Ru/CNF as a catalyst, along with the negligible literature covering the impact of the processing conditions on the hydrothermal hydrogenation of biomass in general and almond hulls in particular, this work symbolises a step forward in this area, providing novel information on the chemical, catalytical and energetic aspects of this process.A ruthenium supported on carbon nanofibers (Ru/CNF) catalyst, previously used to hydrolytically hydrogenate cellobiose (Frecha et al., 2019), was employed in this work. It consists of Ru nanoparticles of around 1.2 nm supported on CNF, resembling a fishbone structure (observed by Transmission Electron Microscopy), with a Ru content of 0.4 wt% (calculated by Inductive Coupled Plasma). The synthesis comprises two key stages: the synthesis of the fibres and the subsequent deposition of Ru onto the CNF by incipient wetness impregnation, utilising RuCl3 as the Ru precursor. The detailed procedure is provided in the supporting information. The support (CNF) contribution to biomass depolymerisation was addressed in previous work using cellulose as the substrate. Due to their low acidity, the contribution of the CNF did not substantially improve that of water at hydrothermal conditions (Frecha et al., 2021). Besides, the performance of the catalyst was compared to that of the CNF alone in previous publications. The experiments were performed at similar conditions (180 \u00b0C, 40 bar H2, 3 h) to those used in this work (Frecha et al., 2019). When CNF were used as a catalyst, cellobiose was depolymerised to glucose and fructose, with levulinic acid, HMF and humins being produced as main by-products. This indicated that hydrogenation reactions did not occur to a substantial step. In contrast, when the experiments were conducted in the presence of Ru/CNF, the liquid phase was made up of sorbitol, xylitol, cellobitol and glucose, thus highlighting the hydrogenation properties of the Ru/CNF catalyst in comparison to the original CNF. Besides, in another work (Rem\u00f3n et al., 2019b), the same CNF were tested for the hydrodeoxygenation of guaiacol (a bio-oil model compound) and their activity was compared to that of a Mo2C/CNF catalyst. Again, the CNF showed little activity for hydrogenation. Additionally, for the hydrothermal hydrogenation of almond hulls, previous work conducted revealed that hydrolysis and depolymerisation reactions are promoted by the acidity provided by water at hydrothermal conditions (Rem\u00f3n et al., 2021b) and the CNF, while Ru species mostly catalyse hydrogenations (Frecha et al., 2019; Rem\u00f3n et al., 2019b).The hydrothermal hydrogenation tests were conducted in a small batch, high-pressure reactor (Berghof Products, BR-40 series, 45 mL). Before the reaction, the almond hulls were mixed with the required amount of catalyst (Ru/CNF) in a planetary miller (PM 100 CM, Retsch, Germany), comprising a zirconia vessel (50 mL) and 10 zirconia balls of 10 mm each. This mix-milling step was conducted at room temperature and a rotation speed of 600 rpm for 30 min to diminish mass transfer limitations between the catalyst and almond hulls. The solid mixture (almond hulls and catalyst) was then loaded into the reactor, along with 20 mL of deionised water. The reactor was closed and filled in with N2 to achieve a pressure higher than that used at the reaction conditions to confirm its airtightness. Subsequently, the reactor was purged with H2 and filled in with the required amount of H2 to achieve the initial H2 pressures used in the experiments, i.e., 20, 35 and 50 bar. For these initial pressures, the final H2 pressure achieved at the reaction conditions were 26, 39 and 50 bar at 150 \u00b0C; 33, 50 and 66 bar at 190 \u00b0C; and 40, 60 and 80 bar at 230 \u00b0C. Due to the excess of H2 used, minimal pressure variations took place during the experiments. A ramp time (from room temperature to the reaction conditions) of around 35\u201345 min (depending on the reaction conditions) and a rotation speed of 1000 rpm were used for all the experiments. Once the reaction terminated, the reactor was quenched with cold water to achieve initial conditions as speedily as possible. A gas sample was then collected and analysed. The reactor was opened, and its content recovered as a final step. Subsequently, a solid-liquid extraction was accomplished in a funnel. The solid was dried at 105 \u00b0C for 24 h and quantified gravimetrically, while the aqueous stream was kept for further analysis.The effects of the temperature (150\u2013230 \u00b0C), initial H2 pressure (20\u201350 bar), reaction time (20\u2013360 min) and catalyst/biomass ratio (0.25\u20131 g/g) were analysed using as the response variables the distribution of the overall reaction products (gas, liquid and solid yields) and the chemical composition of the liquid phase. These intervals are commonly used for the hydrothermal hydrogenation of biomass or related structural model compounds. Besides, it must be borne in mind that the Ru loading in the catalyst is as low as 0.4 wt% and that the support consists of renewable-based carbon nanofibers produced from biomass. These values account for a Ru/biomass loading as low as 0.1 to 0.4 g Ru/g biomass. In previous work, we addressed the hydrothermal treatment of almond hulls for biofuels production in the absence of hydrogen and catalyst. We reported that low biomass/water ratios favour hydrolysis and depolymerisation reactions. On the contrary, high biomass/water ratios promote pyrolysis and thermal decomposition reactions, favouring gas production. This latter accounts for the lower amount of water present in the reaction medium. The present work is directed towards producing value-added liquid products; therefore, a low biomass/water ratio (5 wt%) was used.The calculations and analytical methodologies employed for their determination are listed in Table 1\n. The composition of the gas phase was determined using a micro gas chromatograph. The chemical composition of the liquid phase was determined by Gas and High-Performance Liquid Chromatography (GC and HPLC). Elemental analyses were conducted using a Carlo Erba EA1108 Elemental Analyser using the Channiwala and Parikh (2002) empirical formulae to determine the HHV of the spent solids. Thermogravimetric analyses were conducted on a NETZSCH TG 209F1 Libra TGA209F1D-0277-L apparatus, using air as the carrier gas (50 mL STP min\u22121), increasing the temperature from 25 to 1000 \u00b0C (10 \u00b0C min\u22121). Detailed information regarding these calculations is included in the supporting information. Besides, details covering the experimental is provided as supplementary material and reported elsewhere (Frecha et al., 2019; Frecha et al., 2021; Rem\u00f3n et al., 2021b; Rem\u00f3n et al., 2019a; Rem\u00f3n et al., 2018c).The experiments to address the influence of the processing parameters were planned following a 24 Box-Wilson Central Composite Face Centred (CCF, \u03b1: \u00b11) design. The data were then analysed through a 95% confidence (p-value = 0.05) ANOVA (to determine significance) coupled with a cause-effect Pareto test (to calculate relative importance). For both tests, codec variables (between \u22121 and +1) were utilised, thus making the factors directly comparable. To analyse the results, it is essential to note that the codec formulae obtained from the ANOVA of the 28 runs were used to develop interaction plots showing the main effects and interactions detected. These figures were used to address the impact of the processing conditions and interactions on the process. In addition, in these figures, when possible, some experimental points were added to graphically show that the lack of fit is not significant. This detailed methodology warrants a clear and accurate analysis of the experimental data (Rem\u00f3n et al., 2018a). More information on this methodology is given as supporting information.The almond hulls used in this work were from Marcona almonds harvested in Spain. The complete preparation procedure and characterisation results of the material are fully reported in our previous publication (Rem\u00f3n et al., 2021b). Very briefly, almond hulls were dried at 60 \u00b0C overnight to prevent moulds formation during storage. Then, they were knife milled and sieved to a particle size of ca. 100\u2013200 \u03bcm. The dried hulls were characterised by proximate and ultimate, elemental and fibre analyses. The proximate analysis showed that almond hulls were made up of 6.72 \u00b1 2.87 wt% moisture, 62.72 \u00b1 1.93 wt% volatiles, 18.77 \u00b1 0.57 wt% fixed carbon and 11.80 \u00b1 0.37 wt% ash (primarily K, Mg and Ca). Structurally, they comprised 12.60 \u00b1 0.77 wt% cellulose, 19.40 \u00b1 1.17 wt% hemicellulose, 25.10 \u00b1 2.47 wt% lignin, 7.81 \u00b1 0.23 wt% proteins, 11.80 \u00b1 0.87 wt% ash and 16.57 \u00b1 1.27 wt% others (mainly waxes and lipids). Overall, the material consists of 44.23 \u00b1 1.38 wt% C, 4.65 \u00b1 0.21 wt% H, 49.88 \u00b1 1.52 wt% O and 1.25 \u00b1 0.07 wt% N and has a HHV of 15.74 \u00b1 0.50 MJ/kg. These values are in line with those reported in the literature (Aktas et al., 2015; Gonz\u00e1lez et al., 2005).\nTable 2\n outlines the experimental hydrothermal hydrogenation conditions used and the experimental results attained. These include the overall distribution of reaction products (yields to gas, aqueous and solid), along with the detailed chemical composition of the aqueous stream. Table S1 lists the detailed chemical composition of the liquid phase. The influences of the reaction parameters on these results according to the ANOVA and cause-effect Pareto principle analyses (contemplating all runs conducted) are included in Table S2.The hydrothermal hydrogenation of almond hulls over our Ru/CNF catalyst leads to the formation of three main reaction products: a gas stream, an aqueous fraction and a spent solid product. The yields to these fractions depend on the reaction conditions and vary by 0\u20135%,49\u201382% and 13\u201351%, respectively. The gas stream primarily consists of CO2, along with unreacted H2. Depending on the processing conditions, the spent solid product essentially comprises unreacted biomass, together with reacted biomass from depolymerisation and repolymerisation reactions. The aqueous phase includes value-added liquid chemicals of different nature. CO2 as the only product in the gas suggests that its formation primarily occurs by decarboxylation, decarbonylation, reforming and thermal cracking reactions (Cheng et al., 2017; Wang et al., 2012; Xu et al., 2013). In the scope of this publication, the influence of the processing conditions on the properties of these two fractions (gas and solid) has not been discussed in full. The cause-effect Pareto test (Table S2) reveals the gas yield is affected mainly by the temperature and pressure, both individually and combined, while the liquid yield depends on the temperature and two binary interactions: temperature-catalyst and pressure-time loading. At the same time, this latter interaction and the temperature are the variables exerting the most significant influence on the solid yield. The detailed impact of the effects of the processing conditions and most meaningful interactions on these yields are summarised in Fig. 1\n.The impact of the temperature on the overall distribution of the reaction products is directed by the catalyst loading. When a low catalyst/biomass ratio (0.25 g/g) is used, its influence relies on the initial H2 pressure (Fig. 1 a/b, e/f and i/j). For a low initial H2 pressure (20 bar), without regard to the reaction time, an increase in the temperature leads to increases in the gas (especially between 160 and 200 \u00b0C) and liquid yields at the expense of the solid yield; with these changes being more marked for a longer than a short reaction time. These developments suggest the beneficial impact of the temperature on the process, which allows almond hulls conversion into gas and liquid species via hydrolysis, depolymerisation, deamination, reforming and thermal cracking reactions (Dimitriadis and Bezergianni, 2017; Gollakota et al., 2018; Kumar et al., 2018; Thiruvenkadam et al., 2015).Spreading the H2 pressure from 20 to 50 bar not only significantly modifies the overall distribution of these products but also the influences of the temperature and reaction time, in some cases. In particular, such an increase in the H2 pressure drops the gas yield. This pressure upturn facilitates the diffusion of water into almond hulls, leading to a more efficient solid-water interaction (Rem\u00f3n et al., 2021b; Rem\u00f3n et al., 2021c). The amount of protons in the reaction medium also increases, owing to a more significant water dissociation at high pressure, promoting acid-catalyzed reactions (Schienbein and Marx, 2020). At the same time, such a pressure spread also augments the H2 availability in the reaction media, which promotes hydrogenation and hydrogenolysis reactions (Cheng et al., 2017), favouring liquid and solid formation (Kumar et al., 2018; Thiruvenkadam et al., 2015).Furthermore, the impacts of this pressure spread on the liquid and solid yields rely on the reaction time. On the one side, for a speedy process (20 min), the liquid yield increases and the solid yield drops. This suggests that hydrolysis, depolymerisation, hydrogenation and hydrogenolysis reactions, boosting water-soluble liquid species, take place to a more substantial extent. On the other, for a lengthy treatment (360 min), the effect of the pressure depends on the temperature. While at low temperature (160\u2013190 \u00b0C), increasing the pressure does not alter the liquid and solid yields, both rise when higher temperatures are used (between 190 and 230 \u00b0C). The competition between different reactions might account for these differences. On the one side, an increase in the reaction temperature leads to a lesser spread of hydrogenation and hydrolysis reactions. This latter accounts for both the exothermic character of these transformations (Xu et al., 2013) and the lesser amount of H2 available in the liquid phase owing to the lower H2 solubility in water at a high than low temperature (Wang et al., 2012). On the other, increasing the reaction time promotes secondary decomposition reactions, thus favouring the transformation of some liquid products, such as furan compounds, into solids (Xu et al., 2013).Therefore, these results indicate that there is compensation between the positive effect of augmenting the initial H2 pressure and the negative impact of using long reaction times at a low temperature. Conversely, such compensation does not take place at high temperatures for long reaction times. As a result of these variances, different outcomes are perceived for the liquid and solid yields with the temperature when a high initial H2 pressure (50 bar) is used. Regardless of the reaction time, increasing the temperature increases the liquid yield and drops the solid yield between 180 and 195 \u00b0C by reason of the positive kinetic influence of the temperature on the process (Rem\u00f3n et al., 2021b; Rem\u00f3n et al., 2021c). With further increased up to 230 \u00b0C, the liquid yield decreases and the solid yield increases on account of the transformation of liquids into solid species.Increasing the catalyst loading also modifies the reaction products distribution. These changes depend on the initial H2 pressure and are more critical for a longer than a short process duration. On the one hand, for a speedy process (20 min), at 20 bar (Fig. 1 a/e/i vs c/g/k), increasing the catalyst/biomass ratio from 0.25 to 1 g/g significantly decreases the gas yield and increases the solid yield without substantially modifying the liquid yield. An increase in the catalyst loading favours biomass depolymerisation (via hydrolysis, hydrogenation and hydrogenolysis reactions, yielding water-soluble liquids) over the direct thermal decomposition of the solid material to gases (Lam and Luong, 2014). At the same time, an upsurge in the catalyst amount also endorses the subsequent decomposition of these liquids formed. These can evolve towards solid species, such as humins, via dehydration and repolymerisation. Conversely, at 50 bar, the liquid yield increases at the expense of the solid yield, especially at temperatures higher than 200 \u00b0C, due to the pressure promoting liquid production (Rem\u00f3n et al., 2021b; Rem\u00f3n et al., 2021c). On the other hand, when a long reaction time is used (360 min), increasing the catalyst amount rises the gas and liquid yields at a high temperature at the expense of solid formation, without regard to the initial H2 pressure. These transformations take place to a more significant extent, given the more prolonged exposure of the material to hydrothermal conditions (Rem\u00f3n et al., 2021b; Rem\u00f3n et al., 2021c). As a result, for a high catalyst/biomass ratio (1 g/g), the temperature depicts two distinctive influences depending on the initial H2 pressure (Fig. 1 c/d, g/h and k/l). At 20 initial bar H2, an increase from 160 to 230 \u00b0C progressively upturns the gas and liquid yields and diminishes the solid yield regardless of the reaction time. These variations are particularly more marked for a longer than a short reaction time as described earlier (Prado et al., 2016).An increment in the initial H2 pressure leads to a decrease in the gas yield, as commented for a low catalyst/biomass ratio. For a short reaction time (20 min), such an increase in pressure significantly upsurges the liquid yield and drops the solid yield, regardless of the temperature. These variations result from the beneficial kinetic influence of the pressure when a short reaction time is applied, thus promoting biomass decomposition into liquid species but preventing secondary reactions from occurring substantially. Contrariwise, when a long reaction time is used (360 min), the effect of the pressure is only significant at a high temperature (190\u2013230 \u00b0C). An increase in pressure leads to a slight decrease and increase in the liquid and solid yields, respectively, due to the transformation of liquid species into solid products by dehydration, condensation and repolymerisation (Xu et al., 2013). This pressure influence also modifies the effect of the temperature. Thus, at 50 bar, between 160 and 190 \u00b0C, the gas yield is meagre, while the liquid yield increases at the expense of the solid yield. These variations account for depolymerisation, hydrogenation and hydrogenolysis reactions favoured at high pressure and low temperature in the presence of a catalyst (Wang et al., 2012; Xu et al., 2013). Conversely, a further increase up to 230 \u00b0C increases the gas yield very sharply at the expense of the liquid yield, with the solid yield being practically unaffected due to the transformation of some liquid species into gases using high temperatures for long processing times (Dimitriadis and Bezergianni, 2017; Gollakota et al., 2018).Owing to these phenomena, the impact of the reaction time is directed by the catalyst/biomass ratio and initial H2 pressure. In particular, the reaction time significantly influences the liquid and solid yields when a low catalyst loading is used (0.25 g/g), while its impact on the gas yield is not significant from a practical point of view (Fig. 1 a/e/i vs b/f/j). At a low initial H2 pressure (20 bar), lengthening the process leads to an increase in the liquid yield at the expense of the solid yield. Conversely, at 50 bar, such an increase in the process duration diminishes the liquid yield and increases the solid yield. Rising the pressure promotes a greater spread of the reactions occurring in the liquid phase, favouring liquids production and their subsequent transformation into less polymerised products. These can then evolve towards solid species by dehydration, condensation, and repolymerisation (Xu et al., 2013), as described above. For a high catalyst loading (1 g/g), the influence of the reaction time is significant at low pressure (20 bar). Under such conditions, lengthening the process substantially increases the gas and liquid yields and diminishes the solid yield. Besides, the higher the pressure, the lesser is the impact of the reaction time, as the positive kinetic influence of the former can mask the effect of the latter. This development accounts for the beneficial impact of the catalyst on the liquid species transformation into gaseous products via cracking, deoxygenation, deamination (Dimitriadis and Bezergianni, 2017; Gollakota et al., 2018), and solid species through dehydration, condensation, and repolymerisation (Xu et al., 2013).The liquid phase comprises a complex pool of different species whose composition depends on the reaction parameters. It includes oligomers (46\u201381 wt%), saccharides (2\u20137 wt%), sugar alcohols (2\u201315 wt%), polyhydric alcohols (1\u20138 wt%), carboxylic acids (7\u201331 wt%), furans (0\u20133 wt%), nitrogen-containing species (0\u20131 wt%) and phenolic compounds (0\u20132 wt%). Oligomers are produced mostly from the depolymerisation of cellulose and hemicellulose, leading to the presence of cello-oligosaccharides and xylo-oligosaccharides in the liquid product. Ligno-oligomers resulting from the depolymerisation of lignin are also present in this fraction. Saccharides result from the hydrolysis and depolymerisation of cellulose and hemicellulose. The hydrogenation of these latter fractions leads to the formation of sugar alcohols and polyhydric alcohols, while carboxylic acids and furans are mostly produced via dehydrogenation reactions. Phenolic compounds are produced from the depolymerisation of ligno-oligomers, while nitrogen-containing species result from the degradation of the proteins present in the original biomass. Table S1 lists the detailed chemical composition of the liquid phase, while Fig. 2\n shows a reaction pathway covering the detailed formation of liquid products from the structural components (cellulose, hemicellulose, lignin and proteins) in almond hulls.Cellulose decomposition occurs via a first hydrolysis step, yielding cello-oligosaccharides (Jiang et al., 2020; Jiang et al., 2019; Rem\u00f3n et al., 2020; Rem\u00f3n et al., 2018b; Rem\u00f3n et al., 2018c). These can progressively depolymerise to glucose (Davila et al., 2019; Jiang et al., 2020; Jiang et al., 2019), which can be subsequently transformed into fructose via isomerisation (Li et al., 2018). When a hydrogen-rich atmosphere is achieved at hydrothermal conditions, these species can undergo different transformations (Li et al., 2018). These include hydrogenations yielding sugar alcohols, such as sorbitol and mannitol (Manaenkov et al., 2019; Sun and Liu, 2011), dehydrogenations to produce gluconic acid, and/or dehydration, generating 5-hydroxymethylfurfural (5-HMF) (Rem\u00f3n et al., 2018c). This latter can be subsequently decomposed into levulinic and formic acids by hydrolysis. Simultaneously, glucose and fructose can be further decomposed into small oxygenates via the retro-aldol reaction. On the one hand, the former can evolve towards erythrose and 2-hydroxy acetaldehyde (Manaenkov et al., 2019), while the latter can decompose into 2,3-dihydroxypropanal and 1,3-dihydroxypropan-2-one. Erythrose may lead to the formation of erythritol and/or 1,2-butanol via hydrolytic dehydration (Li et al., 2018). Additionally, 2-butanol can be subsequently produced from both species and evolve to butane-2-one. In addition, ethane-1,2-diol can be formed from 2-hydroxy-acetaldehyde (Manaenkov et al., 2019), whose subsequent decomposition might lead to the formation of ethanol and/or acetaldehyde, both of which can subsequently evolve towards acetic acid (Rem\u00f3n et al., 2018d).On the other hand, 2,3-dihydroxypropanal and 1,3-dihydroxypropan-2-one can be produced from fructose via the retro aldol reaction. Both species can be decarboxylated, yielding ethane-1,2-diol, which can evolve towards acetaldehyde via dehydration, and/or 2-hydroxy-acetaldehyde by dehydrogenation. These chemicals can be decomposed into ethane-1,2-diol through decarbonylation (King et al., 2010; Lin, 2013). Acetaldehyde and 2-hydroxyacetaldehyde can be produced by dehydration and dehydrogenation, respectively (King et al., 2010; Lin, 2013; Wawrzetz et al., 2010). The former can lead to ethanol and acetic acid, while the latter can be decomposed into methanol (King et al., 2010; Lin, 2013; Wawrzetz et al., 2010). In addition, glycerol can also be produced by the hydrogenation of 2,3-dihydroxypropanal and 1,3-dihydroxypropan-2-one, both of which can be dehydrogenated yielding 1-hydroxypropan-2-one (Rem\u00f3n et al., 2016c; Rem\u00f3n et al., 2016d; Xu et al., 2022). This can undergo further hydrogenation towards propane-1,2-diol (Gandarias et al., 2010; King et al., 2010; Lin, 2013; Wawrzetz et al., 2010; Zhang et al., 2012). Additionally, lactic acid can be produced from 1,3-dihydroxypropan-2-one via rearrangement (dehydration/hydration) (Onda et al., 2008; Xu et al., 2021). At the same time, propane-1,2-diol can subsequently be dehydrated to form propane-2-one and/or propionaldehyde, which can be hydrogenated to propane-2-ol and propane-1-ol, respectively (Gandarias et al., 2010). Afterwards, ethanol might be produced from propane-2-ol by cracking and hydrogenation (Lin, 2013).Hemicellulose depolymerises yielding hemi-(xylo)-oligosaccharides, which progressively evolve towards xylose formation (Jiang et al., 2018; Rem\u00f3n et al., 2019a; Rem\u00f3n et al., 2018c). This saccharide can be subsequently hydrogenated to produce arabitol, xylitol and/or threitol, and/or dehydrated yielding furfural, which can be decomposed into formic acid (Putro et al., 2016). At the same time, xylose can lead to 2-hydroxyacetaldehyde and 2,3-dihydroxypropanal/1,3-dihydroxypropan-2-one via the retro aldol reaction (Putro et al., 2016; Sun and Liu, 2011). Ethane-1,2-diol can be produced from both species via hydrogenation and decarboxylation, respectively, while glycerol can also be attained from the hydrogenation of 2,3-dihydroxypropanal/1,3-dihydroxypropan-2-one (Putro et al., 2016; Sun and Liu, 2011). Then, both alcohols can evolve towards forming the same species as described above for cellulose.The reactivity of lignin and proteins at the processing conditions of this work is lower than that of the carbohydrate fraction, attending to the number and amounts of products formed. At the conditions tested, lignin depolymerisation leads to the formation of ligno-oligomers that progressively depolymerise and decompose towards different phenolic compounds: phenol-2-methoxy-4-propyl, phenol,2-6-dimethoxy, phenol,2-methoxy and phenol (Li et al., 2012; Li et al., 2018; Madsen et al., 2017; Madsen and Glasius, 2019; Yang et al., 2018). Proteins can be hydrolysed, yielding amino acids, which can be further decarboxylated, decarbonylated and deaminated (Kumar et al., 2018; Yang et al., 2015), leading to the formation of amides (propenamide), amines (2-pentanamine) and ammonia, respectively. Additionally, pyridines, such as 3-pyridinol, can be produced from the reaction between ammonia and the furans produced from the carbohydrate content (Madsen et al., 2017).The influence of the processing conditions according to the Pareto test is listed in Table S2. For the most abundant species, this analysis reveals that the proportion of oligomers in the liquid is mainly affected by the temperature and reaction time, along with the initial H2 pressure and catalyst loading. The proportion of saccharides relies on the temperature and catalyst loading, while the relative amount of sugar alcohols mainly depends on the reaction temperature and its interaction with the reaction time. The reaction time and its interaction with the pressure and catalyst loading are responsible for the changes observed in the proportions of polyhydric alcohols. Carboxylic acids are primarily affected by the reaction temperature and some interactions with the time and pressure, while the proportion of ketones is substantially influenced by the catalyst loading and reaction time. The detailed influence (from the ANOVA of all runs) of these effects and interactions on the proportions of the most abundant liquid species are plotted in Fig. 3\n.Different outcomes are observed when a low catalyst/biomass ratio (0.25 g/g) is used (Fig. 3 a/e/i/m/q/u and b/f/j/n/r/v for 20 and 360 min, respectively). The reaction temperature does not significantly alter the chemical composition for a quick treatment (20 min) and low initial H2 pressure (20 bar). An increase from 180 to 230 \u00b0C leads to moderate decreases in the relative amounts of oligomers and saccharides. It increases the concentration of sugar alcohols, with the proportions of polyhydric alcohols, carboxylic acids and ketones being unaffected practically. These transformations suggest an initial conversion of oligomers (mostly cello-oligosaccharides and hemi/xylo-oligosaccharides) into saccharides via hydrolysis (Jiang et al., 2020; Jiang et al., 2019; Rem\u00f3n et al., 2020; Rem\u00f3n et al., 2018b; Rem\u00f3n et al., 2018c), and the subsequent transformation of these species into sugar alcohols by hydrogenation (Manaenkov et al., 2019; Putro et al., 2016; Sun and Liu, 2011).An increase in the initial H2 pressure from 20 to 50 bar modifies the composition of the liquid effluent. At a low temperature (160\u2013200 \u00b0C), such an increase augments the proportions of oligomers at the expenses of the concentrations of saccharides, sugar alcohols and carboxylic acids. This accounts for the rise in the liquid yield occurring when the pressure of the system increases, which favours water penetration and promotes the decomposition of almond hulls into water-soluble oligomers (Rem\u00f3n et al., 2021b; Rem\u00f3n et al., 2021c; Schienbein and Marx, 2020). However, under such conditions, these species do not evolve towards less-depolymerised species, and consequently, their relative amounts in the liquid phase increase. This seems to indicate that the subsequent conversion of these water-soluble macromolecules is the rate-determining step. On the contrary, at a high temperature (210\u2013230 \u00b0C), the concentration of carboxylic acids increases and the proportions of oligomers and saccharides diminish when the H2 pressure increases. This pressure spreads not only promotes the formation of oligomers but also their subsequent transformation into other products (Rem\u00f3n et al., 2021b).As a result of these different outcomes, when an elevated initial H2 pressure (50 bar) is applied, the proportion of oligomers decreases between 160 and 230 \u00b0C, while the relative amounts of saccharides, sugar alcohols and carboxylic acids increase. The progressive transformation of saccharides accounts for this, as their conversion into small oxygenated species, such as carboxylic acids, is promoted by the positive kinetic influence of the pressure and temperature.(Jiang et al., 2020; Jiang et al., 2019; Rem\u00f3n et al., 2020). However, these alterations are particularly important between 160 and 190 \u00b0C for sugar alcohols and from 190 to 230 \u00b0C in the case of carboxylic acids. The solubility of H2 in water diminishes with augmenting the reaction temperature (Wawrzetz et al., 2010) and hydrogenation reactions are also less favoured at a high temperature due to their exothermic nature (Cheng et al., 2017; Wang et al., 2012; Xu et al., 2013). Thus, the beneficial kinetic impact of the pressure boots the transformation of oligomers differently depending on the temperature, with hydrogenations being favoured at a low temperature and dehydration reactions occurring to a more substantial extent at a high temperature.Additionally, the effect of the reaction time is ruled by the initial H2 pressure and reaction temperature. At 20 bar, lengthening the duration from 20 to 360 min rises the proportions of sugar alcohols and polyhydric alcohols at a low temperature (160\u2013190 \u00b0C). These variations are accompanied by diminishments in the relative amounts of oligomers, carboxylic acids and ketones. Increasing the reaction time promotes the gradual conversion of oligomers into less polymerised species, mostly saccharides, along with their hydrogenation to sugar alcohols and polyhydric alcohols. These transformations are favoured at a low temperature due to the endothermicity of hydrogenation reactions (Cheng et al., 2017; Wang et al., 2012; Xu et al., 2013) and the greater amount of H2 dissolved in the liquid phase (Wawrzetz et al., 2010). Conversely, at a high temperature (190\u2013230 \u00b0C), such an increase in the reaction time increases the proportions of oligomers at the expenses of the relative amounts of sugar alcohols and carboxylic acids. Prolonging the reaction time at a high temperature promotes the initial biomass conversion into water-soluble oligomers and the transformation of some low molecular mass oxygenates, such as carboxylic acids, into gases (Lorente et al., 2019; Rem\u00f3n et al., 2021b; Rem\u00f3n et al., 2019c; Rem\u00f3n et al., 2021c). These developments account for the increments observed in the proportions of oligomers and the diminishments occurring for sugar alcohols and carboxylic acids. At 50 bar, the effect of the reaction time is more marked.Regardless of the temperature, an increase in the reaction time leads to a sharp decrease in the proportion of oligomers, accompanied by increases in the concentration of saccharides, sugar alcohols and polyhydric alcohols. The concentration of carboxylic acids increases at a low temperature and decreases at a high temperature, while the relative amount of ketones is unaffected. These outcomes are thought to result from the positive influence of the reaction time on the process, which favours the transformation of oligomers into saccharides via hydrolysis (Lorente et al., 2019; Rem\u00f3n et al., 2021b; Rem\u00f3n et al., 2019c; Rem\u00f3n et al., 2021c) and their subsequent transformation into sugar alcohols and polyhydric alcohols (Zhou et al., 2012). In this case, the thermodynamic H2 limitation at a high temperature is less detrimental, as it can be compensated by a more prolonged exposure at hydrothermal conditions along with the greater H2 pressure applied.Owing to these differences, diverse outcomes are observed with varying the temperature and initial H2 pressure for a reaction time of 360 min. At 20 bar, increasing the temperature increases the proportion of oligomers and decreases the relative amount of saccharides. The former is associated with the increase observed in the liquid yield with increasing the temperature, while the latter suggests converting saccharides into other species; with these transformations being boosted when a high temperature is applied for a long reaction time. For example, the proportion of polyhydric alcohols initially decreases between 150 and 190 \u00b0C and then stabilises with a further increase in the temperature. Contrarily, a trend-off is observed for the relative amount of sugar alcohols between 160 and 190 \u00b0C, followed by a sharp decrease with a further temperature increment to 230 \u00b0C. These transformations suggest that hydrogenations resulting in sugar alcohols occur more significantly than the transformation of saccharides into sugars alcohols via a first retro-aldol reaction and subsequent hydrogenation. Conversely, at a high temperature, hydrogenations are less predominant due to the lower H2 solubility in water (Wawrzetz et al., 2010), which favours the conversion of saccharides, first by the retro-aldol reaction, and then by a series of dehydration, hydration and decarboxylation reactions, increasing the concentrations of carboxylic acids and ketones.For a long reaction time (360 min), the effect of the initial H2 pressure is different. Notably, a pressure spread from 20 to 50 bar H2 increases the proportion of saccharides and carboxylic acids at the expense of the relative amount of oligomers, regardless of the temperature, due to the positive influence of the pressure. Besides, the concentrations of sugar alcohols and polyhydric alcohols diminish at a low temperature and increase at a high temperature, while the proportion of ketones is unaffected. At a low temperature, these diminishments are related to the substantial increase occurring in the proportion of carboxylic acids, primarily acetic acid, which denotes that the beneficial pressure kinetic impact helps shift the conversion of saccharides into carboxylic acids via a tandem of consecutive transformations, including the retro-aldol reaction coupled with hydrogenations and dehydrations as described in the reaction pathway (Jiang et al., 2020; Jiang et al., 2019; Rem\u00f3n et al., 2020). As a result, the effect of the temperature is less critical, probably due to the higher pressure and longer reaction time used. Notably, the proportions of oligomers, saccharides and ketones are barely affected by the temperature. On the contrary, the proportion of polyhydric alcohols increases linearly. Simultaneously, a maximum and a minimum occur for the relative amounts of sugar alcohols and carboxylic acids, respectively. An initial augment between 150 and 190 \u00b0C increases the proportion of the former at the expense of the relative amount of the latter. In contrast, a subsequent increase leads to an opposite outcome. These trends suggest that the hydrogenation of saccharides occurs to a substantial extent at a low temperature, accounted for by the positive effect of the pressure and reaction time, combined with an appropriate H2 solubility in water (Wawrzetz et al., 2010) and an appropriate reaction temperature promoting hydrogenation reactions (Cheng et al., 2017; Wang et al., 2012; Xu et al., 2013). Conversely, an increase in the temperature exerts a positive kinetic influence on the retro-aldol conversion of saccharides. Thus, the subsequent evolution of these species towards carboxylic acids is preferential over the direct hydrogenation.Increasing the catalyst loading modifies not only the liquid phase chemical composition but also the effects that the reaction temperature and initial H2 pressure have on the process (Fig. 3 a/e/i/m/q/u vs c/g/k/o/s/w and b/f/j/n/r/v vs d/h/l/p/t/x for 20 and 360 min, respectively). For a catalyst/biomass ratio of 1 g/g, the influence of the reaction temperature does not depend on the initial H2 pressure or reaction time for the most abundant species in the liquid effluent. An increment in the temperature between 150 and 200 \u00b0C leads to a decrease in the proportion of oligomers at the expense of the relative amounts of sugar alcohols and polyhydric alcohols. This results from the beneficial impact of the temperature on hydrolysis and depolymerisation combined with a temperature range wherein hydrogenations are thermodynamically promoted. On the contrary, a subsequent increment up to 230 \u00b0C increases the concentration of oligomers and drops the proportions of sugar alcohols and polyhydric alcohols. These differences might be a consequence of the kinetic and thermodynamic influence of the reaction temperature. An initial increase in the temperature favours the conversion of biomass into water-soluble oligomers, promoting their subsequent transformation into saccharides, sugar alcohols and polyhydric alcohols by hydrogenation in the presence of a high amount of catalyst. These latter transformations are favoured at a low temperature due to the more significant H2 dissolved in the aqueous medium and the endothermic character of hydrogenation reactions (Cheng et al., 2017; Wang et al., 2012; Xu et al., 2013). However, a prolonged increment in the temperature hinders these transformations. Consequently, the relative amount of oligomers increases due to the positive influence of the temperature on biomass hydrolysis and the limited extension of hydrogenation reaction at a high temperature. These phenomena are more marked when a high catalyst loading is used, as hydrogenations occur to a greater extent.On the contrary, the proportions of saccharides, carboxylic acids and ketones are influenced by the initial H2 pressure and reaction time. For a 20 min reaction time, the concentration of saccharides is not directed by the reaction temperature; a trend-off is observed without regard to the initial H2 pressure. Conversely, the relative amount of carboxylic acids increases with rising the temperature from 150 to 230 \u00b0C, irrespective of the initial H2 pressure; yet, a sharper increase is observed for a low than high H2 pressure. The beneficial impact of the temperature on saccharides decomposition via the retro-aldol reaction and the subsequent promoting effect of the temperature on dehydration, decarboxylation and hydration reactions, leading to the formation of carboxylic acids, might be responsible for such developments (Rem\u00f3n et al., 2016b). Concurrently, the relative amount of ketones diminishes between 150 and 230 \u00b0C when the process is conducted at 20 bar due to the sharp upturn occurring for carboxylic acids, with the effect of the temperature being less and less important as the initial H2 pressure increases. In this case, greater H2 pressure can compensate for the lower solubility of H2 at a high temperature.In addition, the initial H2 pressure also directs the chemical composition. For a speedy treatment (20 min), different outcomes occur. On the one side, an increase from 20 to 50 bar H2 increases the relative amount of polyhydric alcohols at the expenses of the proportions of saccharides and sugar alcohols. These developments suggest that the retro-aldol reaction might be quicker than the direct hydrogenation of saccharides to sugar alcohols, favouring the formation of polyhydric alcohols over sugar alcohols under such conditions. In this case, spreading the H2 pressure kinetically boots these transformations, owing to the rise in the total pressure of the system. On the other, the pressure for the proportions of oligomers, carboxylic acids and ketones relies on the temperature. While the concentration of carboxylic acids increases at a low temperature (150\u2013190 \u00b0C) due to decreases occurring in the relative amounts of oligomers and ketones, the opposite trend is observed at a high temperature (190\u2013230 \u00b0C); i.e., the relative amounts of oligomers and ketones increase at the expense of the carboxylic acids concentration, when the initial H2 pressure rises. An increase in the pressure positively influences hydrolysis and hydrogenation reactions at a low temperature, thus facilitating the first conversion of biomass into water-soluble oligomers and their transformation into less depolymerised species via hydrolysis and hydrogenation. Conversely, hydrolysis and depolymerisation occur significantly at a high temperature, with the subsequent hydrogenation of sugars being the limiting step. This can hinder the conversion of saccharides and increases the proportion of oligomers in the liquid.Furthermore, an increase in the reaction time modifies the effect of the pressure. Augmenting the H2 pressure from 20 to 50 bar, using a reaction time of 360 min, leads to a substantial spread in the relative amount of saccharides, especially between 150 and 190 \u00b0C, temperatures at which the relative amount of polyhydric alcohols also increases. Such increases occur along with a depletion in the concentration of oligomers. Under such conditions, the positive influence of the processing time and the H2 pressure promote hydrolysis and hydrogenation reactions combined with the higher amount of H2 dissolved at a low temperature (Wawrzetz et al., 2010) and the exothermicity character of hydrogenations (Cheng et al., 2017; Wang et al., 2012; Xu et al., 2013). Besides, at higher temperatures (190\u2013230 \u00b0C), the same spread in the H2 pressure upsurges the relative amount of oligomers. It diminishes the proportion of polyhydric alcohols due to the positive and negative impacts of the initial H2 pressure at a high temperature on hydrolysis and hydrogenation reactions.Owing to these distinctive effects exerted by the temperature and initial H2 pressure, the impact of the reaction time relies on these two variables. At 20 bar, it is particularly important at a high temperature (190\u2013230 \u00b0C). Within this range, lengthening the process (from 20 to 360 min) increases the relative amounts of oligomers and polyhydric alcohols, leading to decreased concentrations of saccharides, sugar alcohols and ketones. This might be accounted for by hydrolysis and depolymerisation reactions, yielding saccharides, being faster than the following transformation of these species into other chemicals. At 50 bar H2, when a temperature between 150 and 190 \u00b0C is used, lengthening the process increases the proportions of saccharides, polyhydric alcohols and carboxylic acids and diminishes the relative amounts of oligomers, sugar alcohols and ketones. In this case, an increase in the reaction time promotes oligomers depolymerisation, and also it shifts the decomposition of saccharides via the retro-aldol reaction. Conversely, when the process is conducted at a high temperature (190\u2013230 \u00b0C), such an increase in the reaction time increases the relative amounts of oligomers and sugar alcohols at the expenses of the concentrations of carboxylic acids and ketones. At a high temperature, these latter species can be easily converted into gaseous products (Lorente et al., 2019; Rem\u00f3n et al., 2020; Rem\u00f3n et al., 2021b; Rem\u00f3n et al., 2019a), which connects very well with the proliferation seen in the gas production at a high temperature and using a long reaction time.Five likely optimisation scenarios were sought to transform almond hulls into valuable liquids, using the formulae obtained from the ANOVA of the experimental results (Table 3\n). The lack of fit of the models developed in this work is not significant with 99% confidence (p-value > 0.01). Besides, the predicted R2 of all these models were higher than 0.95, allowing their use for prediction purposes. This proves validation for the optima obtained in this work. In this regard, it must be borne in mind that these optima serve as the starting point for future process commercialisation and scale-up of the process as they may depend on the biomass and type of reactor. The gas and solid yields were minimised in these optima, while the liquid yield was maximised to ensure the selective conversion of the material into liquid species. The first optimisation considers the transformation of almond hulls into water-soluble oligomers, while the second minimises their production to obtain a liquid product containing less polymerised species (oxygenates), i.e., primarily saccharides, sugar alcohols, polyhydric alcohols and carboxylic acids. The third and the fourth aim at transforming almond hulls into a liquid product containing high amounts of sugar alcohols and carboxylic acids, respectively. Additionally, the fifth includes the concurrent production of alcohols (sugar alcohols and polyhydric alcohols) and carboxylic acids. All restrictions have been assigned with different importance (from least important, 1, to most important, 5) for these case scenarios so that operating conditions satisfying all the criteria could be sought. The overall yield has been provided with a relative importance of 3, while 5 has been given to the chemical composition to ensure that quality (purity) prevails over quantity.Opt. 1 shows that 91% of almond hulls can be converted into an oligomer-rich (74 wt%) liquid mixture conducting the process at 230 \u00b0C and 35 bar H2 for 360 min using 1 g cat/g biomass. On the contrary, Opt. 2 reveals that the production of small liquid oxygenated compounds (58 wt%) prevails over the formation of oligomers (42 wt%), conducting the process at 206 \u00b0C and 39 bar H2 for 20 min using 1 g cat/g biomass. Nano- and ultra-filtration can resolve the separation of oligomers from saccharides and other liquid oxygenates (Feng et al., 2009; Vegas et al., 2006). This would allow the selective transformation of almond hulls into oligomers (primarily cello- and xylo-oligomers) in high yield and purity. Opt. 1 would convert up to 67% of the original material into oligomers assuming an ideal separation. Cello- and xylo-oligomers are widely used as natural, renewable-based prebiotic materials owing to their excellent physicochemical and physiological properties (Bian et al., 2014; Carvalho et al., 2013; Lin et al., 2017; Miguez et al., 2018).Concerning the production of valuable liquid oxygenates, sugar alcohols and carboxylic acids are the most abundant species in the liquid. The former (Opt. 3) production can be maximised (61% liquid yield, containing 19 wt% sugar alcohols), conducting the process at 150 \u00b0C and 32 bar H2 for 20 min using 0.25 g cat/g biomass. Likewise, oligomers can be easily removed from the aqueous effluent, which would result in the transformation of up to 30% of the original material into oligosaccharides and the other 30% into an aqueous product containing up to 38 wt% of sugar alcohols. Considering the total amount of carbohydrates (cellulose and hemicellulose) in almond hulls, this equals a sugar alcohols yield of 36% with respect to the entire carbohydrate content. This represents an improvement in yield and time reduction compared to the data reported by Palkovits et al. (2010) and Li et al. (2018) for other lignocellulosic (wood chips, cornstalk and beechwood) materials. On the contrary, Opt. 4 demonstrates that the latter are maximised (72% liquid yield, containing 26 wt% carboxylic acids) when the hydrothermal reaction is conducted at 198 \u00b0C and 42 bar H2 for 20 min using 1 g cat/g biomass. After oligomers separation, this represents the conversion of up to 37% of the almond hulls into an aqueous mixture comprising up to 50 wt% of carboxylic acids. This equals 58% of the carbohydrate content of the biomass converted into carboxylic acids, and it is one of the best results reported in the literature so far.Additionally, Opt. 5 suggests that alcohols and carboxylic acids can be maximised concurrently (68% liquid yield, containing 15 wt% sugar alcohols, 8 wt% polyhydric alcohols and 25 wt% carboxylic acids) at 187 \u00b0C and 35 bar H2 for 360 min using 1 g cat/g biomass. The difference between Opt. 1 and Opt. 5 agrees with the more significant H2 dissolved in the aqueous medium and the endothermic character of hydrogenation reactions, as discussed previously. As such, 37% of the raw biomass could be converted into an upgraded aqueous fraction containing 28 wt% of sugar alcohols and 46 wt% of carboxylic acids. These fractions can be easily fractionated by distillation considering the standard boiling point of the sugar alcohols (216\u2013230 \u00b0C), polyhydric alcohols (188\u2013290 \u00b0C) and carboxylic acids (101\u2013122 \u00b0C) produced. Fig. 4\n shows a schematic flow diagram to selectively transform almond hulls into oligomers, alcohols and carboxylic acids. These species are renewable-based substitutes for petroleum-based chemicals of paramount interest for the energy sector and pharmaceutical, cosmetic and textile industries. As such, these promising results, combined with the environmental friendliness and holistic features of this hydrothermal process, exemplify a landmark step change to managing and valorising unavoidable food waste.Nonetheless, the bottleneck of this process might be the separation and reusability of the catalyst. This accounts for a ball-milling step before the reaction to increase the biomass catalyst effective contact. Such a pretreatment substantially increases the reactivity of the biomass, diminishing solid-solid mass transfer limitations, but hampers the subsequent separation of the spent biomass from the spent catalyst. Given this, a possible solution to improve the sustainable and economic aspects of this process when it comes to addressing future scale-up and possible commercialisation might be the separation and recovery of Ru (Swain et al., 2013) from the spent solid (spent almond hulls and carbon nanofibers used as a support). Then, the recovered Ru could be used for preparing a new fresh catalyst, while the solid carbonaceous material could be subjected to a hydrothermal treatment to produce liquid and solid biofuels concurrently (Rem\u00f3n et al., 2021b), and/or it might be pyrolysed, gasified or combusted to produce energy. This would close the loop and allow the total usage of the original almond hulls and catalyst within a biorefinery unit.A theoretical energy assessment has been conducted to address the viability of the process and provide added value to the spent solid produced. Table 4 shows the elemental analyses and HHVs of the spent solids produced at optimum conditions. This characterisation reveals that the remaining solid is an energy-dense material (26\u201333 MJ/kg), which combustion could provide the required energy for the process. As the CNF and almond hulls are renewable, carbon-neutral materials, CO2 emissions will be environmentally neutral. According to the thermogravimetric analysis conducted in an oxygen atmosphere, such decomposition occurs at two specific intervals: i.) 200\u2013380 \u00b0C and ii.) 400\u2013620 \u00b0C (Fig. S.2). The decomposition of the fresh catalyst reveals that this takes place between 420 and 620 \u00b0C, which indicates that the former interval accounts for the combustion of the spent biomass (almond hulls), while the latter corresponds to the decomposition of the carbon nanofibres. Table 4 shows the amount of material decomposed at a temperature lower than 380 \u00b0C, which correlates well with the catalyst/biomass ratio. Given these data, combusting the spent solid using a temperature lower than 400 \u00b0C would be a plausible strategy to recover the spent catalyst from the solid mixture. The theoretical energy for the process could be approximated as that required to heat the reaction mixture (biomass, catalyst, water and H2) from room to the reaction temperature, assuming an ideal adiabatic reactor without heat losses. Thus, considering that most of this energy corresponds to the energy required for heating 20 mL of water (due to the significant excess of water compared to the solid material and H2), Table 4 shows that the theoretical energy for the process shifts between 11 and 17 KJ. Besides, the combustion of the spent solid material provides between 18 and 40 KJ. This corresponds to around 40\u201360% more energy than theoretically required. Consequently, the controlled combustion of the spent solid material could also serve as a suitable strategy to recover the spent catalyst and provide the energy required for the process.Since the combustion of the spent solid takes place within two intervals, different options arise for process intensification. One might be recovering the Ru from the spent solid and proceeding with the combustion of the whole material. Another could comprise the combustion of the whole material and recover the Ru within the ash content. Then, the Ru recovered could be impregnated onto fresh CNF to prepare more catalyst. Besides, temperature-controlled combustion might also be plausible. This would be intended to selectively combust the biomass content at a temperature lower than 400 \u00b0C to separate the biomass from the spent catalyst and provide energy for the process. This would leave a solid material comprising the Ru/CNF catalyst and biomass ashes, which could be recovered and re-used. However, the catalytic properties of this catalyst might have been altered. Therefore, all these options must be carefully compared and studied experimentally in future work.This work has explored the hydrothermal hydrogenation of almond hulls over a carbon neutral Ru/CNF catalyst for the first time. The influence of the operating conditions has been thoroughly addressed, and the process has been optimised for the selective production of value-added liquid species. The processing conditions exerted a significant influence on the hydrothermal hydrogenation of almond hulls, controlling the yields to gas (0\u20135%), liquid (49\u201382%) and solid (13\u201351%) and the chemical composition of the aqueous product. This stream primarily comprised oligomers (46\u201381 wt%), saccharides (2\u20137 wt%), sugar alcohols (2\u201315 wt%), polyhydric alcohols (1\u20138 wt%) and carboxylic acids (7\u201331 wt%). The temperature and reaction time influenced the extension of hydrolysis, depolymerisation, deamination, hydrolysis, hydrogenation and dehydration reactions. Additionally, the initial H2 pressure and catalyst loading kinetically promoted these transformations, facilitating the production of different liquid species depending on the other processing conditions. The extensions of these reactions are ruled by the amount of H2 effectively dissolved in the reaction medium and the prevalence of hydrogenations over dehydration/decarboxylation reactions or vice versa. Process optimisation revealed that up to 65\u201367% of the original biomass could be converted into i.) high-purity (mostly, cello- and xylo-) oligomers alone, and/or ii.) oligomers (31 wt%) and small oxygenates (17 wt% carboxylic acids, 11 wt% sugar alcohols and 6 wt% polyhydric alcohols) concurrently. Combined with the environmentally friendly and holistic features of this hydrothermal process, these promising results exemplify a landmark step change to managing and valorising unavoidable food waste. The energy assessment revealed that the combustion of the spent solid takes place at different temperatures and could provide the energy required for the process. As a result, Ru could be recovered from the spent solid mixture prior to/after its combustion, with the energy produced exceeding the theoretical energy for the process. Alternatively, the temperature-controlled combustion at temperatures lower than 400 \u00b0C would also provide energy for the process, leaving a resultant solid material mainly comprising the spent Ru/CNF catalyst. Future experimental work should be directed towards experimentally addressing and exploring all these options, which would close the loop and allow the total usage of the original almond hulls and catalyst within a biorefinery unit.Javier Rem\u00f3n: Conceptualisation, Methodology, Validation, Formal analysis, Investigation, Data curation, Writing - Original Draft, Writing - Review & Editing, Supervision, Project administration, Funding acquisition.Raquel Sevillla-Gasca: Validation, Formal analysis, Investigation, Data curation.Esther Frecha: Methodology, Validation, Formal analysis, Investigation, Data curation.Jos\u00e9 Luis Pinilla: Conceptualisation, Writing - Review & Editing, Resources, Supervision, Project administration, Funding acquisition.Isabel Suelves: Conceptualisation, Writing - Review & Editing, Resources, 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 wish to express their gratefulness to FEDER and the Spanish Ministry of Science, Innovation and Universities (Grant Number ENE2017-83854-R) for providing financial support. Besides, Javier Rem\u00f3n is grateful to the Spanish Ministry of Science, Innovation and Universities for the Juan de la Cierva (JdC) fellowships (Grant Numbers FJCI-2016-30847 and IJC2018-037110-I) awarded.\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.2022.154044.", "descript": "\n                  The almond industry leaves behind substantial amounts of by-products, with almond hulls being the primary residue generated. Given that one way to improve food security is by decreasing waste to reduce environmental impacts, developing sustainable processes to manage this by-product is necessary. Herein, we report on the hydrothermal hydrogenation of almond hulls over a carbon-neutral Ru supported on carbon nanofibres (Ru/CNF) catalyst, addressing the temperature, H2 pressure, time and catalyst loading. These variables controlled the distribution of the reaction products: gas (0\u20135%), liquid (49\u201382%) and solid (13\u201351%), and ruled the composition of the liquid effluent. This aqueous fraction comprised oligomers (46\u201381 wt%), saccharides (2\u20137 wt%), sugar alcohols (2\u201315 wt%), polyhydric alcohols (1\u20138 wt%) and carboxylic acids (7\u201331 wt%). The temperature and reaction time influenced the extension of hydrolysis, depolymerisation, deamination, hydrolysis, hydrogenation and dehydration reactions. Additionally, the initial H2 pressure and catalyst loading kinetically promoted these transformations, whose extensions were ruled by the amount of H2 effectively dissolved in the reaction medium and the prevalence of hydrogenations over dehydration/decarboxylation reactions or vice versa depending on the catalyst loading.\n                  Process optimisation revealed that it is feasible to convert up to 67% of almond hulls into merchantable oligomers at 230 \u00b0C, 35 bar initial H2, using 1 g catalyst/g biomass (0.4 g Ru/g biomass) for 360 min. Additionally, decreasing the temperature to 187 \u00b0C without modifying the other parameters could convert this material into oligomers (31 wt%) and small oxygenates (17 wt% carboxylic acids, 11 wt% sugar alcohols and 6 wt% polyhydric alcohols) concurrently. The theoretical energy assessment revealed that the total and partial combustion of the spent solid material could provide the required energy for the process and allow catalyst recovery and reutilisation. This environmental friendliness and holistic features exemplify a landmark step-change to valorising unavoidable food waste.\n               "}
{"full_text": "Electrochemically produced hydrogen is considered an essential energy storage technology in the process of decarbonization, connecting intermittent renewable energy sources with our everyday life in a CO2-neutral energy economy. In the temperature range below 100\u00a0\u00b0C, alkaline water electrolyzers (AWE) which employ liquid alkaline solutions as the electrolyte and proton exchange membrane water electrolyzers (PEMWE) based on proton conductive solid electrolytes mainly govern the market [1,2].Anion exchange membrane water electrolysis (AEMWE) is considered one viable solution to combine the high current densities and (asymmetric) pressure operation of membrane-based electrolyzers with the benefits of the alkaline environment. These advantages feature stable low-cost platinum group metal (PGM)-free catalysts, transport media and bipolar plates made of nickel or stainless steel. In the past years, anion exchange ionomers have reached higher technological maturity concerning hydroxide conductivity [3,4] and chemical stability [5,6] in an alkaline environment. Nevertheless, the durability of state-of-the-art membrane materials is expected to be higher in water than in alkaline solution. Common degradation pathways are, e.g. via nucleophilic attacks of hydroxide on the cationic functionalities in the ionomer [7].Hydroxide conductivity is commonly equated with a high pH environment established by the ionomer and in particular the associated hydroxide ions balancing its fixed positive charges. With such materials present in membrane and catalyst layer, it is expected that the operation of AEMWE systems in pure water is sufficient to thermodynamically stabilize PGM-free catalysts [8]. In the field of fuel cells, this could be shown in multiple studies [9,10]. For water electrolysis, on the other hand, long-term stable operation was not possible so far when using PGM-free catalysts in pure water. Instead, Ir and Pt are often employed for oxygen evolution- (OER) and hydrogen evolution (HER) reactions to achieve stable long-term operation of water-fed AEMWE [11,12], which misrepresents the original intention to replace costly materials. For operation with PGM-free catalysts, additional electrolyte (e.g., KOH or K2CO3) is commonly used to externally establish a high pH environment around the catalyst. Furthermore, these supporting electrolytes increase the system's conductivity allowing to achieve higher current densities [13].In the past, notable AEMWE cell performances were reported for CuCoOx-based catalyst, formerly commercially available as Acta 3030 from Acta S.p.a., curr. known as Enapter. Fig.\u00a01\n illustrates the best performing cells from a literature review on previously realized AEMWE performances employing CuCoOx as the OER catalyst in different electrolytes. For further information on the individual works and additional data, see SI. It has to be noted that the compared performance data was reported for different AEM materials, manufacturing strategies, and operating temperatures. Also, the preconditioning for the MEAs varied between the different reports. Nevertheless, this survey clearly shows that operation in KOH solutions resulted in the highest performances reported so far. However, durability has only been investigated by Faraj et\u00a0al. for 500\u00a0h in 1\u00a0wt% K2CO3 solution with a low-density polyethylene-based AEM [14]. To the best of our knowledge, no similar reports on long-term stability in water can be found in the literature.Cobalt oxides are widely discussed in literature as potential OER catalysts [15] \u2013 even in acidic environments [16]. On the other hand, rather noble Cu has been shown to electrochemically dissolve in various pH and potential ranges [17,18], which renders thermodynamic stability of this catalyst an open question.In the present study, we investigate catalyst- and catalyst layer stability for an AEMWE system with CuCoOx-based anode in different electrolytes aiming to establish a generic understanding of durability determining factors for AEMWE. For this purpose, we focus on the anode only, as this electrode features the most significant potential for cost reduction. We specifically investigate the impact of the liquid feed (pure water and 0.1\u00a0M KOH) on the electrolyzer performance and relate it to electrochemical and mechanical stability descriptors representative of the catalyst and membrane employed.CuCoOx as the oxygen evolution catalyst was prepared according to a literature-reported procedure by dissolving 4.5\u00a0g CuSO4(H2O)5 and 10\u00a0g CoCl2(H2O)6 in 200\u00a0mL deionized water and adding 150\u00a0mL 0.75\u00a0M Na2CO3 solution while stirring. The purple precipitate was filtered off, washed thoroughly with deionized water, and dried under vacuum for 12\u00a0h and ground before calcination at 400\u00a0\u00b0C for 5\u00a0h [30]. Product control of the black powder was performed via X-ray diffractometry on a Bruker D8 Advance instrument using a Cu X-ray source. An associated diffractogram is depicted in Figure\u00a0S1. From energy-dispersive X-ray spectroscopy (EDX), a stoichiometric composition of Cu0.5Co2.5O4 was determined, which is in line with other literature reports [29].Cathode electrodes (cPTEs) were fabricated via a spray coating procedure employing an ExactaCoat device (SonoTek), as reported elsewhere [31]. H24C5 gas diffusion layers with a microporous layer (Freudenberg) were coated with an ink comprised of 1\u00a0wt% solids consisting of 10\u00a0wt% AP1-HNN8-00 Aemion binder (Ionomr Innovations) and 90\u00a0wt% HiSPEC 9100 (Johnson Matthey) in a 1:4 solvent mixture of 1-propanol in deionized water. To obtain the final ink, the catalyst was weighed into a glass bottle and the full amount of water was added. Subsequently, the Aemion binder dissolved in the 1-propanol proportion of the ink solvent was added to the bottle. Homogenization was conducted by immersing an ultrasonic horn (UP200St, Hielscher) for 30\u00a0min into the ink while stirring and cooling the bottle with ice. This procedure was constant throughout all electrode batches to ensure a comparable effect of the homogenization procedure on the ionomer [32] as well as potential contaminations [33] from the ultrasonic horn. For spray coating, the gas diffusion layers were fixed on the heating plate associated to the spray coater at 80\u00a0\u00b0C. The spray path was meander shaped with a pitch of 1.5\u00a0mm and an offset of 0.5\u00a0mm to ensure a high level of homogeneity of the catalyst layers. The loading was monitored via the weight increase of a reference sample with fixed area, which was also placed on the spray coater. The final platinum loading was 0.5\u00a0mg\u00a0cm\u22122 in all prepared samples.Anode electrodes (aPTEs) were fabricated by spray coating CuCoOx-inks with a solid content of 1\u00a0wt% and different binder:catalyst ratios onto 220\u00a0\u03bcm thick nickel felts (fiber diameter: 14\u00a0\u03bcm, 85% porosity, Bekaert). In contrast to the Pt/C based cathode inks but in accordance to previous reports on IrO2-based aPTE systems [31,34], slow particle precipitation of the oxide in the spray coater's syringe during the spray coating process resulted in a higher binder content in the final catalyst layer than in the original ink, even for optimized compositions. Therefore, the final catalyst layer composition for the anodes was determined via thermogravimetric analysis (TGA) of the catalyst layers scratched off the spray coating masks. For this purpose, samples were heated in air from 30 to 1000\u00a0\u00b0C with a heating rate of 10\u00a0K\u00a0min\u22121 in a Setsys CS Evo TG-DTA device (SETARAM). A binder content of 2 and 8\u00a0wt% in the ink's solid content resulted in approximately 10 and 30\u00a0wt% polymer in the final catalyst layer, which was found to be reproducible for the material combination employed in this study. Ink preparation was performed in a similar fashion as for the cathodes employing the same solvent ratio. For the anodes, the ink was prepared the day before electrode fabrication, ultrasonicated for 30\u00a0min, stirred overnight and ultrasonicated again right before the spray coating procedure following a similar spray pattern as described above. The nickel felt material was cut into squares of 5\u00a0cm2\nvia laser cutting, deburred and cleaned with 2-propanol and water before fixing them into PTFE frames on the spray coater at 120\u00a0\u00b0C.For electrode heat treatment, samples were heated to 220\u00a0\u00b0C in an N2-purged furnace (OTF-1200X-S-II, MTI) within 30\u00a0min and kept at this temperature for another 30\u00a0min before leaving it to cool down to ambient temperature.Membranes (AP1-HNN8-50 Aemion, Ionomr Innovations) and electrodes were soaked in 1\u00a0M KOH solution (prepared from potassium hydroxide Emsure, Merck) for 48\u00a0h and 12\u00a0h, respectively, before MEA assembly to activate the materials. For electrolysis testing in water, the materials were immersed in pure water before cell testing to remove residual KOH.The cell fixture for electrolysis tests was an in-house setup, comprised of aluminum endplates on either side with inlets for heating elements (HPL, T\u00fcrk\u00a0+\u00a0Hillinger GmbH) and corresponding in- and outlets for the electrolyte. To prevent any contact of the electrolyte and the housing, PTFE tubes were led through the endplates to the flow fields and fixed using a thermoelectric flanging tool (Bola). The MEA was clamped between two identical flow fields made of titanium with a 5\u00a0\u03bcm gold layer coating. A straight flow design similar to the one reported by B\u00fchler et\u00a0al. [34] was used in this study. MEA compression was adjusted using reinforced PTFE foils (HighTechFlon) with a thickness of 0.18\u00a0mm at the cathode and two individual gaskets at the anode with thicknesses of 0.075 and 0.150\u00a0mm to prevent electrolyte leakage. The cell temperature was monitored using thermocouples (Type T, Omega) inserted into the flow fields.In this study, all electrolysis measurements were performed on a self-constructed test bench (Figure\u00a0S2) employing a peristaltic pump (Masterflex L/S, Cole Parmer) to circulate the electrolyte in PTFE tubing through a heating bath (Aqualine AL25, Lauda) before entering the electrolysis cell with a flow rate of 40\u00a0mL\u00a0min\u22121. Two 2\u00a0L PTFE bottles (VWR) purged with a 100\u00a0mL\u00a0min\u22121 nitrogen stream were employed as electrolyte reservoirs and gas-liquid separators. An Octostat (Ivium) was used to control electrochemical experiments.After stabilizing the cell temperature, a standardized test protocol was employed. First, a short break-in procedure was performed with subsequent constant voltage holds going in three steps from 1.8\u00a0V to 2.0\u00a0V and 2.1\u00a0V with a holding time of 200\u00a0s each. Following this, two polarization curves were recorded with a hold time of 30\u00a0s and varying current step sizes as a trade-off between high resolution in the activation region and reasonable measurement time. In the low-current density region from 1 to 10\u00a0mA\u00a0cm\u22122 the step size was 1\u00a0mA\u00a0cm\u22122, which was increased to 10\u00a0mA\u00a0cm\u22122 in the range of 10\u2013100\u00a0mA\u00a0cm\u22122. Above current densities of 100\u00a0mA\u00a0cm\u22122, the step width of 100\u00a0mA\u00a0cm\u22122 was kept constant. Every single current step was followed by a galvanostatic impedance scan between 100\u00a0kHz and 100\u00a0Hz to determine the high-frequency resistance (HFR) from the Nyquist plot's x-intersection [35]. After a constant voltage hold at 1.9\u00a0V, further polarization curves were recorded. A VMP-300 potentiostat (BioLogic) with three 10 A booster boards was employed for long-term measurements after the previous experiments.All electron microscopy experiments were carried out on a Zeiss Crossbeam 540 focussed ion beam scanning electron microscope (FIB SEM). For this purpose, the electrodes were attached to aluminum sample stubs using carbon tabs. The images were recorded with a beam current of 2\u00a0nA at an acceleration voltage of 5\u00a0kV. A four-quadrant backscatter detector was used for image acquisition.Catalyst spots on glassy carbon (Sigradur, HTW GmbH) were prepared via drop-casting 10\u00a0\u03bcL of individual inks of CuCoOx and 10, and 30 w% AP1-HNN8-00 binder with a solid content of 5\u00a0wt% in 1-propanol. A custom-made three-electrode electrochemical scanning flow cell (SFC) depicted in Figure\u00a0S3 was used for activity measurements of the catalyst spots. The Ar-purged electrolyte feed (0.05\u00a0M KOH or 0.05\u00a0M NaH2PO4 buffer) passes a C counter electrode and into the SFC. The catalyst spots on the working electrode can be automatically contacted with the electrolyte of the SFC. A capillary channel inside the SFC connects the reference electrode (Ag/AgCl, Metrohm). The electrolyte outlet is connected to the sample introduction system of an ICP-MS (Nexion 350x, PerkinElmer), which allows online quantification of dissolved species from the catalyst. The ICP-MS sensitivity was calibrated daily using Ge (40\u00a0\u03bcg\u00a0L\u22121) as an internal standard, with a 4-point calibration line mixed from standard solutions (Cu, Co 1000\u00a0mg\u00a0L\u22121 Merck Centripur). The electrolyte flow rate was measured to be 3.6\u00a0\u03bcL\u00a0s\u22121. For detailed information about the experimental setup, measurement, and calibration procedures, please refer to our previous works [18,36,37].Annealing of SFC samples was performed by heating the glassy carbon plate in an N2 atmosphere in a furnace (GHA 300, Carbolite Gero) to 220\u00a0\u00b0C in 30\u00a0min, keeping this temperature constant for another 30\u00a0min and leaving it to cool down to room temperature in an inert atmosphere.Trace elemental analysis of the liquid electrolyte after electrochemical MEA characterization was performed by acidifying 2.5\u00a0mL of the anode compartment (total amount: 1\u00a0L) with 5\u00a0mL 0.1\u00a0M HNO3 and injecting it directly into the ICP-MS employing the same calibration line.For swelling experiments, samples of 25\u00a0\u03bcm thick AP1-HNN8-50 Aemion membranes (Ionomr Innovations) were cut into rectangular pieces of 10\u00a0\u00d7\u00a050\u00a0mm and dried overnight at 50\u00a0\u00b0C in vacuum. The initial sample size was determined before immersing the membrane pieces into the respective solutions of 0.05\u00a0M KOH, 0.1\u00a0M KOH, or 1\u00a0M KOH for 24\u00a0h at either room temperature or 70\u00a0\u00b0C. The samples for swelling in deionized water were subjected to ion exchange in 1\u00a0M KOH solution for 48\u00a0h, rinsed, and soaked thoroughly with deionized water to prevent any additional KOH contamination in the system. Dimensional swelling was determined by measuring the resulting sample size according to eq. (1):\n\n(1)\n\n\nd\ni\nm\n.\n\ns\nw\ne\nl\nl\ni\nn\ng\n=\n\n\n\n(\n\n\nV\n\nf\ni\nn\na\nl\n\n\n\u2212\n\nV\n\ni\nn\ni\nt\ni\na\nl\n\n\n\n)\n\n\nV\n\ni\nn\ni\nt\ni\na\nl\n\n\n\n\u22c5\n100\n%\n\n\n\n\nMechanical properties were investigated using the samples from the dimensional swelling experiment after equilibration at room temperature. A Shimadzu EZ-SX universal testing machine with a 100\u00a0N load cell was used to perform uniaxial tensile tests. Elastic moduli were determined from stress-strain curves measured at ambient conditions with a constant crosshead speed of 5\u00a0mm\u00a0min\u22121. At least three samples of each type were measured and the previously measured dimensions of the swollen samples were employed for data fitting.Replacement of costly Ti-based current collectors and precious metal catalysts such as IrO2 represents the most considerable potential for cost-reduction in AEMWE compared to PEMWE. For the anodes in our study, CuCoOx catalyst layers with 10\u00a0wt% or 30\u00a0wt% Aemion binder were applied to porous nickel felts via spray coating to prepare porous transport electrodes (PTE). Cathodes were based on the same stable Pt/C gas diffusion electrodes employing 10\u00a0wt% Aemion [31] in the catalyst layer to exclude chemical cathode dissolution effects. The general MEA design is depicted in Fig.\u00a02\n a, whereas Fig.\u00a02 b and c feature scanning electron microscope (SEM) images of the resulting anodic catalyst layer structures with a CuCoOx loading of 2\u00a0mg\u00a0cm\u22122.For PEMWE MEAs, an optimum (Nafion) binder content of roughly 10\u00a0wt% in the final catalyst layer was reported previously by B\u00fchler et\u00a0al. for PTE-based MEA designs [34] based on Ti porous transport layers with a similar structure. This value was used as a starting point for our investigations anticipating similar behavior of AEMWE and PEMWE operated in pure water. As shown in Fig.\u00a02 d, the cell in pure water feed with 10\u00a0wt% binder showed a poor performance reaching only 20\u00a0mA\u00a0cm\u22122 and decayed to a negligible minimum in less than 1\u00a0h of constant voltage operation (data not shown). In an attempt to increase the amount of \u201calkaline\u201d functionalities around the catalyst, the binder content in the anodic catalyst layers was accordingly increased to 30\u00a0wt%. Under the same operating conditions, the onset potential was in a similar range of 1.6\u00a0V and the current density could be doubled to 40\u00a0mA\u00a0cm\u22122 at 1.8\u00a0V, still far too low to compete with any existing technology. Thus, similar electrodes were tested with a 0.1\u00a0M KOH electrolyte feed. It was found that even by the addition of these low amounts of KOH the performance could be increased by a factor of 20\u201345\u00a0at 1.8\u00a0V and lower the onset potential to 1.5\u00a0V (Fig.\u00a02 d), which is in good agreement with recent literature [11].An interesting outcome of the above experiment was the different performance trends concerning binder content in the anode. While the cell performance in pure water still improved with increasing the polymer content from 10 to 30\u00a0wt%, the opposite was found for alkaline solution. The transition from operation in pure water to dilute KOH entails significant changes in the membrane- and contact resistances due to different swelling behavior of the materials in the respective electrolyte. As the nickel-based porous transport layer is expected to contribute to the catalytic activity of the catalyst (see Figure\u00a0S4), a simple model system was required.Therefore, a scanning flow cell (SFC) setup (Figure\u00a0S3) was employed to investigate the effect of different binder contents in a CuCoOx catalyst layer on activity in alkaline- (0.05\u00a0M KOH solution) and neutral pH (0.05\u00a0M phosphate buffer solution). Surprisingly, the catalyst's activity (Fig.\u00a02 e) did not follow the same trend observed in the full cell setup concerning binder content. Overpotentials determined for a current densitiy of 0.1\u00a0mA\u00a0cm\u22122 were 1.55\u00a0V at pH 12.7 and 1.67\u00a0V at pH 7 for the samples with 10\u00a0wt% ionomer, whereas for the 30\u00a0wt% ionomer samples 1.62\u00a0V (pH 12.7) and 1.77\u00a0V (pH 7) were observed. In both investigated pH environments, activity is higher for the sample with the lower binder content. In terms of electrolyte pH, even mild KOH concentrations are sufficient to increase the catalyst's OER activity drastically compared to pH 7.The combination of full cell and three electrode measurements suggests that Aemion binder alone is not enough to supply sufficient OH\u2212 species to PGM-free catalysts. Only in presence of an alkaline electrolyte (feed), OER performance of CuCoOx was sufficient. This result follows recent doubts in the literature that alkaline pH environments are merely established by the presence of anion exchange ionomers in catalyst layers while feeding pure water [38,39]. Cao et\u00a0al. even observed that the local pH in an Aemion membrane does not rise above 9.3, where OER occurs, and hydroxide is consequently consumed [40]. However, as discussed below, the binder's properties change with temperature and electrolyte type, which needs to be considered in data interpretation.In three-electrode SFC experiments, it becomes apparent that the binder content lowers the activity of CuCoOx, potentially due to reduced accessibility of the catalytic sites. Similar observations can be found in literature for LSV analysis also for other anion exchange ionomers [41,42]. In our experiments, this binder influence is more pronounced in alkaline solution than at pH 7. Similarly, the higher binder content in the KOH-operated full cell exhibits a slightly worse performance. In this system, the overall resistance is lower due to the electrolyte's high conductivity; the catalyst and membrane connectivity through the binder is no longer vital (primary OH\u2212 ion source). Thus, the role of electrode binders in AEMWE differs significantly depending on the feed electrolyte used.After evaluating the effect of pH and binder content on the catalyst's OER activity, the critical question of stability needs to be addressed. The presented findings suggest that the AEM cannot supply enough \u201calkaline\u201d functionalities in a neutral electrolyte feed, which draws attention to the PGM-free catalyst's thermodynamic limitations. While Co is expected to form stable oxides in alkaline environments and at high potentials, neutral conditions favor the dissolved Cu2+ and Co2+ species [8,15,18]. To better understand the interplay of electrolyte pH, binder content, and catalyst stability, we performed online inductively coupled plasma mass spectrometry (ICP-MS) measurements that track Co, Cu, and I dissolution from the catalyst layer at open circuit potential (OCP).Firstly, it should be mentioned that the iodine leaching rate was used to monitor the ion-exchange process of the ionomer from its iodide form during contact with the liquid electrolyte of the SFC (Fig.\u00a03\n). It dissolves immediately upon contact (marked by the vertical line and an asterisk, \u2217) and declines over the first 100\u00a0s for both electrolytes, indicating a completed ion-exchange process. The iodine leaching rate further scales with the amount of ionomer in the catalyst layer.It is noteworthy that even at OCP, the catalyst's dissolution differs drastically between the different pH regimes. A first intense dissolution peak is immediately observed for both Cu and Co in a neutral environment upon contacting the catalyst at OCP (Fig.\u00a03 a), indicating the thermodynamic instability at this pH. Afterward, the initially high Cu contact dissolution peaks reside back to the baseline. Such a decrease of Cu leaching over time indicates the dealloying of the catalyst's surface, leaving behind a mostly stable CoOx surface layer. Similar dealloying behavior of thermodynamically unstable metals is often reported in Pt-based PEM fuel cell binary catalyst systems [43\u201346]. Interestingly, the metals' dissolution behavior in alkaline media is more complex and severely impacted by the polymer activation process. Directly at contact (marked with an asterisk, \u2217 Fig.\u00a03 b), the catalyst is significantly more stable in an alkaline environment than in neutral conditions. Co exhibits a negligible dissolution rate within the first 50\u00a0s, while Cu shows slight dissolution. However, once most iodide ions are exchanged from the ionomer, the catalyst film suffers from severe particle detachment indicated by the highly noisy dissolution signal (indicated with a pound, #, Fig.\u00a03 b). On the other hand, the severe particle detachment in an alkaline environment points toward a loss of the ionomer's mechanical strength and its ability to guarantee adhesion of catalyst particles in its hydroxide form in an alkaline environment.From swelling experiments with 25\u00a0\u03bcm membranes made of high IEC Aemion (Fig.\u00a04\n a), we observed a significant electrolyte uptake already at ambient temperature, which was very pronounced for the SFC relevant KOH concentrations and pure water. Fortin et\u00a0al. observed previously that the relationship between swelling and film thickness is not linear for Aemion materials [47]. Therefore, thin films as featured in a catalyst layer (Fig.\u00a02 b and c) are thus expected to expand even more than the membranes employed in our swelling experiments, which could explain the particle detachment in SFC measurements.For ionomers, which have been investigated more thoroughly already, e.g., Nafion, there are quite a few reports in the literature on the effect of heat treatment on mechanical thin film properties [48]. Reduced binder swelling is expected to be favorable for durable catalyst layers [49]. Fig.\u00a04 a suggests that after heat treating Aemion for 30\u00a0min at 220\u00a0\u00b0C, swelling in the different electrolyte concentrations becomes negligible and does not change significantly even at elevated temperatures. Simultaneously, the thermal treatment results in a more than 1000-fold increase in the elastic modulus of Aemion (Fig.\u00a04 b).Considering long-term AEMWE operation, catalyst stability during OER is an essential requirement. Fig.\u00a05\n a and b show the dissolution of Co and Cu during a standard galvanostatic hold at 5\u00a0mA\u00a0cm\u22122 for 3\u00a0min in neutral and alkaline electrolyte. Here, the fundamental importance of pH for non-PGM materials becomes apparent. At pH 7 (Fig.\u00a05 a), both Co and Cu dissolve during the OER, which is expected according to thermodynamics. Initially, Cu exhibits an up to ten times higher dissolution rate than Co but declines due to Cu depletion at the catalyst surface. The relatively lower dissolution rate with higher amounts of ionomer is suspected to originate from mass transport of the dissolved ions in the ionomer itself. Similar observations were made for Nafion ionomers in fuel cell environments previously [50]. With the dissolution data at hand, operation in deionized water was no longer considered in this study due to a lack of electrochemical stability of CuCoOx under these conditions. In the alkaline region (Fig.\u00a05 b), on the other hand, both metal concentrations remain below the detection limit of the ICP-MS, while only particle detachment represented by the spikes in the dissolution signal is still observed due to the above-described mechanical phenomenon.Last, we could confirm experimentally that heat treatment of the catalyst layers at 220\u00a0\u00b0C before the measurements is an efficient way to improve their mechanical stability in SFC measurement. This facile pretreatment step resulted in minimized particle detachment during OER in an alkaline environment, as indicated by the absence of dissolution signal spikes in Fig.\u00a05 c.Some previous works in the literature (overview see supplementary information) report catalyst loadings in the range of up to 30\u00a0mg\u00a0cm\u22122 for CuCoOx-based anodes [3], while more recent works even implement CuCoOx-based thin films as catalyst layers without any addition of ionomer binder [25,26]. Thus, we aimed to elucidate the influence of the absolute catalyst loading on our AEMWE system's performance operated in a 0.1\u00a0M KOH solution. Samples in the range of 1\u20134\u00a0mg\u00a0cm\u22122 CuCoOx loading were fabricated. While the overall cell performance improved slightly with higher loadings, the electrolyte reservoir was significantly colored after the measurements with non-heat treated electrodes, which is indicative for catalyst particle detachment. The electrode's heat treatment allowed to significantly improve the cell's performance with higher loadings. Fig.\u00a06\n a suggests that the onset potential could be significantly decreased from 1.51\u00a0V for the 1\u00a0mg\u00a0cm\u22122 sample to 1.49\u00a0V with 4\u00a0mg\u00a0cm\u22122 CuCoOx in the anode. As featured in Figure\u00a0S5, the catalyst layer structure did not exhibit visible changes after the heat treatment, and catalyst layer activity is not expected to be adversely affected (Fig.\u00a04 c).It has to be noted that the electrode heat treatment resulted in a substantial increase in the cell's HFR (in the range of 50\u00a0m\u03a9\u00a0cm2, see Figure\u00a0S6) compared to the untreated electrode. However, particle detachment from the catalyst layer during operation could be mitigated by this approach resulting in a clear electrolyte solution even after a 70\u00a0h durability experiment for the 4\u00a0mg\u00a0cm\u22122 samples. After operating the MEA with 4\u00a0mg\u00a0cm\u22122 CuCoOx loading without additional thermal treatment in 0.1\u00a0M KOH at 70\u00a0\u00b0C for roughly 10\u00a0h, the amount of Co and Cu dissolved was found to be 56\u00a0\u03bcg\u00a0L\u22121 and 263\u00a0\u03bcg\u00a0L\u22121, respectively. For a similar sample with additional heat treatment, the amount of Co dissolved in the electrolyte feed was only 3\u00a0\u03bcg\u00a0L\u22121 after the long-term measurement depicted in Fig.\u00a06 b. The amount of dissolved Cu for this sample was 263\u00a0\u03bcg\u00a0L\u22121, which again supports the surface leaching effect also observed for the SFC measurements in an alkaline pH environment (Fig.\u00a05 b and c). Our study, therefore, highlights the positive effect on performance and mechanical properties through heat treatment of Aemion binders in catalyst layers. A better understanding of the chemical structure changes through possible cross-linking as well as optimization of the process remain a challenge for future studies.Fortin et\u00a0al. reported a similar effect of catalyst layer degradation in MEAs employing Ir black as OER catalyst and the same Aemion binder (7\u00a0wt% in the catalyst layer) resulting in degradation rates in the range of \u223c3 mV h\u22121 in 0.1\u00a0M KOH at 50\u00a0\u00b0C and a constant current of 500\u00a0mA\u00a0cm\u22122 [47]. With our PGM-free anode featuring 10\u00a0wt% Aemion in the catalyst layer, it was possible to achieve lower degradation rates of 0.87\u00a0mV\u00a0h\u22121, even at a higher operating temperature. Moreover, the absolute cell performance is among the best reported for a CuCoOx-based anode (see Figure\u00a0S7).These results are promising; however, future studies would require longer runtimes to fully evaluate the stability of employed binders for comparison to greater lifetimes as recently shown by Motealleh et\u00a0al. for AEMWE systems with PGM-free electrodes employing perfluorosulfonic acid (PFSA) binders [51]. Particularly in an alkaline electrolyte feed, the high chemical stability of PFSA binders was found to be favorable for the performance of AEMWE cells [52]. Here, the polymer itself acts as a purely mechanical binder (such as PTFE in the study by Pavel et\u00a0al. [21]), whereas the hydroxide supply is established by the electrolyte base [53].This study on CuCoOx and Aemion based anodes in AEMWE revealed that the electrode binder's role and behavior differs with changing electrolyte feed. As stability of CuCoOx in a feed of neutral electrolyte could not be enhanced by increased AEI binder content in the catalyst layer, cell operation in pure water was found unfavorable.MEA operation in dilute KOH solution overall resulted in drastically improved cell performances. From the studied cases it could be elucidated that the system does not solely rely on the binder's hydroxide conductivity under these conditions. Further, the high pH environment, essential for thermodynamically stable PGM-free catalysts, is sufficiently established by the external KOH feed. However, for stable operation, a thorough understanding of the ionomer's mechanical properties \u2013 particularly in its active form \u2013 is vital to guarantee the catalyst's electrochemical stability and a durable adhesion between PTLs, catalyst layers, and membrane. In the case of Aemion, it was found that heat treatment at 220\u00a0\u00b0C drastically increased the electrode lifetime and should be considered for stable AEMWE operation with PGM-free electrodes. A thorough investigation of the changes within the ionomer structure caused by the heat treatment will be helpful to tailor this process for different kinds of PGM-free OER catalysts.Supporting Information \u2013 PDF-file containing additional data and a literature survey on CuCoOx-based AEMWE devices.\nB. M.: Conceptualization, Formal Analysis, Methodology, Investigation, Visualization, Writing \u2013 Original Draft (lead) F. S.: Conceptualization, Formal Analysis, Investigation, Methodology, Visualization, Writing \u2013 Original Draft (supporting), M. H.: Investigation, Writing \u2013 Reviewing and Editing (supporting), M. B.: Resources, Writing \u2013 Reviewing and Editing (supporting), D. A.: Investigation, Writing \u2013 Reviewing and Editing (supporting), D. M.: Investigation, Writing \u2013 Reviewing and Editing (supporting), S. C.: Supervision, Writing \u2013 Reviewing and Editing (supporting), S. T.: Funding Acquisition (lead), Supervision, Writing \u2013 Reviewing and Editing (supporting), R. P.: Supervision, Writing \u2013 Reviewing and Editing (lead).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 financial support from the Bavarian Ministry of Economic Affairs, Regional Development, and Energy. Furthermore, the fruitful discussions on test bench design with Marco Bonnano, and the help of Dominik Kraus with TGA measurements and Dirk D\u00f6hler with XRD.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.ijhydene.2021.11.083.", "descript": "\n                  Anion exchange membrane (AEM) water electrolysis is considered a promising solution to future cost reduction of electrochemically produced hydrogen. We present an AEM water electrolyzer with CuCoOx as the anode catalyst and Aemion as membrane and electrode binder. Full cell experiments in pure water and 0.1\u00a0M KOH revealed that the optimum binder content depended on the type of electrolyte employed. Online dissolution measurements suggested that Aemion alone was not sufficient to establish an alkaline environment for thermodynamically stabilizing the synthesized CuCoOx in a neutral electrolyte feed. A feed of base is thus indispensable to ensure the thermodynamic stability of such non-noble catalyst materials. Particle loss and delamination of the catalyst layer during MEA operation could be reduced by employing a heat treatment step after electrode fabrication. This work summarizes possible degradation pathways for low-cost anodes in AEMWE, and mitigation strategies for enhanced system durability and performance.\n               "}
{"full_text": "No data was used for the research described in the article.While the cement sector is critical for sustainable growth in many countries, it generates large volumes of toxic waste on a daily basis, including CO2 emissions and cement kiln dust (CKD). Toxic waste, such as CKD, may be very harmful to human life, animals, and plants, regardless of whether it reaches the ground, streams, or even air. The dangers associated with CKD arise from its high alkalinity and high concentration of heavy metals [1]. Daily, massive amounts of CKD are generated at two cement plants located in the vicinity of Qena city (Upper Egypt). Additionally, for every ton of cement clinker produced [2], around 54\u2013200\u00a0kg of CKD is generated, and 600\u2013700\u00a0kg of CO2 gas is released into the environment [3]. Herein, we attempt to suggest successful solutions and safe ways for eliminating this type of industrial waste (CKD) that contributes to environmental pollution while maintaining a balance between the environment and natural resources through waste recycling and mitigating the cement industry's negative environmental impact. Additionally, examining the economic benefits of converting CKD, a potentially harmful byproduct, into a useful material such as impurity-free hydroxyapatite (HAPT). During our literature survey, only one published article was found explaining the formation of HAPT from CKD [4], but their product was found to be mixed with several elements and SiO2.Numerous recently published articles detailed the methods for preparing HAPT from a variety of natural sources, including eggshell [5,6], catfish bones and animal bones [7], cuttlebone and bovine bone waste [8,9], pigeon bone waste [10], porcine byproducts [11], cockle shells or snail shell waste [12,13], turtle shell [14] and natural phosphate rocks [15]. On the other hand, several articles are concerned with the preparation methods of hydroxyapatite, including solid-state and wet chemical precipitation methods [5], hydrothermal porous hydroxyapatite preparation [16], the microemulsion method [17] and replica method [18]. Finally, HAPT with angiogenic-osteogenic properties was prepared for biomedical applications in bone repair via direct refluxing of the cuttlebone with (NH4)2HPO4\n[9] or via HAPT modification with gum tragacanth [19].Apart from its biomedical applications, HAPT has a variety of industrial applications as an active compound or catalyst, including catalytic oxidation of volatile organic compounds [20], solventless self-aldol condensation of butyraldehyde to 2-ethylhexenal [21], CO-oxidation reaction [22] and ethanol conversion [23]. Furthermore, HAPT is widely used as catalyst support in many reactions, such as: dry reforming of methane using HAPT-supported nickel catalysts [24], N-oxidation of tertiary amines with H2O2 by W/HAPT [25], and steam reforming of ethanol over cobalt-supported HAPT [26]. Based on our tracing about the different uses of HAPT, as a heterogeneous catalyst, during the conversion of sec-butanol to trans-2-butene or any other products. We were unable to locate any published article that examined this reaction in conjunction with HAPT under any conditions. As a result, our research herein is regarded as the first successful scientific attempt to produce trans-2-butene directly from sec-butanol at relatively low temperatures in a single step. We converted several industrial wastes into pure catalysts early on, such as aluminum dross tailings (ADT) to high surface area \u03b3-Al2O3\n[27\u201329], via \u03b3-AlOOH boehmite, using a variety of precipitation methods and techniques. Additionally, we prepared a series of 1\u201310\u00a0wt% FeOx/Al2O3 catalysts, based on \u03b3-AlOOH recovered from (ADT) and another waste (SPW) steel-pickling chemical waste [30]. These catalysts (1\u201310\u00a0wt% FeOx/Al2O3) calcined at 600\u00a0\u00b0C demonstrated high activity in ethanol dehydration to diethyl ether and ethene production [30].This ground-breaking work exemplifies a contemporary and relevant set of points, which include the following:(1) For the first time, hydroxyapatite (HAPT) was prepared easily and directly from cement kiln dust (CKD) as industrial waste at 500\u00a0\u00b0C. As a result, our work herein aims to identify the economic benefits associated with CKD as a harmful byproduct through its conversion into a value-added material.(2) Our simple method results in a HAPT with a hierarchical mesopore structure composed of thin sheets and flakes. Additionally, it is characterized by a dense population of surface acidic sites of varying strength that is uniformly generated and distributed across its surface.(3) This wide range of mesopores and the strong surface acidity of HAPT enabled it to be used as a potent catalyst for the first time during the conversion of sec-butanol (SB). The catalyst showed superior activity in converting SB to t-2-butene with a selectivity of 99% at a low-temperature range of 200\u2013300\u00a0\u00b0C.A CKD sample was obtained from Misr Qena Cement Plant- Qeft at Qena governorate - Egypt, with a greyish-green colour, where its chemical composition was analyzed by the producer and presented in \nTable 1. The total wt% of analyzed items in (Table 1) was equal to 92.26%. TG analysis of an original sample of CKD from RT up to 500\u00a0\u00b0C showed three successive steps with about 7.4% mass loss, Fig. S1 (supplementary information). This could be related to the removal of moisture as well as the desorption of CO2 that is linked to the sample's surface during the storage of CKD in the air for a long time. TG-curve and FT-IR spectrum of original CKD, each one interprets the other and both match well together; see Fig. S1. Furthermore, the clinker in the cement industry contains SO3 and Cl in the form of K2SO4, while chloride form stable compounds with alkalies such as K2O but more volatile than sulphate.Most of the chemicals herein were purchased as analytical grade pure chemicals from El Nasr Pharmaceutical Chemicals Co. Egypt-ADWIC, including nitric acid (HNO3, 50\u201355%), oxalic acid [H2C2O4.2\u00a0H2O], ammonia solution (25%) and diammonium hydrogen phosphate [(NH4)2HPO4].Before HAPT preparation, an estimation of the extent of Ca-ions that would be recovered using the locally delivered CKD was performed. 5\u00a0g of CKD sample was added to 50\u00a0mL deionized water with continuous stirring using a magnetic stirrer (1500 RPM at room temperature). Then, 12.5\u00a0mL of (50\u201355%) HNO3 was added dropwise until effervescences ceased. The resulting mixture was filtered off through crucible Gooch (Sintered Glass-G3), and the solid residue was washed with deionized water several times. The dissolved portion of CKD in the filtrate was calculated as 4.57\u00a0g concerning the weighed undissolved part after the complete dryness of the crucible. The filtrate was kept for use in the next step. Following that, 100\u00a0mL of 4% oxalic acid was added to the filtrate, followed by diluted aqueous ammonia solution until the pH reached 8.5\u20139.0 to precipitate calcium as CaC2O4\u00b7H2O. The precipitate boiled, settled, and filtered immediately through ashless filter paper. Finally, after complete dryness of the ashless filter paper, the precipitate was calcined at 500\u00a0\u00b0C for 2\u00a0h in a muffle furnace, then cooled and weighed as =\u00a04.463\u00a0g of CaCO3. According to the obtained gravimetric analysis results, we recovered approximately 54.7% of CaO from the dissolved portion, while CaO was calculated to be 50% of the original CKD sample. According to these findings, Ca-hydroxyapatite (HAPT) sample was prepared in a new experiment from cement kiln dust as follows: 10\u00a0g of CKD was dissolved in the required volume of nitric acid (H2O: HNO3; 4:1) with continuous stirring and gentle heating of the mixture at 60\u00a0\u00b0C. To eliminate the undissolved part (UDP) of CKD from the clear solution of Ca-ions, filtration was applied through crucible Gooch Sintered Glass-G3. The calculated amount of (NH4)2HPO4 was dissolved in deionized water and added to Ca-solution in a 250\u00a0mL beaker. The resulting mixture was heated at 60\u00a0\u00b0C over a hot-plate, and then aqueous ammonia solution was added from a burette with continuous stirring until the complete precipitation of Ca-HAPT in a strong basic medium [31] as shown in (Eq.1).\n\n(1)\n10Ca(NO3)2\u00b74\u2009H2O + 6(NH4)2HPO4 + 8NH4OH \u2192Ca10(PO4)6(OH)2 + 20NH4NO3+ 46\u2009H2O\n\n\nThe white precipitate of HAPT was washed several times with deionized water, dried in an oven at 120\u00a0\u00b0C for 12\u00a0h, then ground in a mortar and stored. The obtained HAPT was calcined at 500\u00a0\u00b0C for 3\u00a0h in a 100\u00a0mL.min\u22121 oxygen flow.Thermal analyses of HAPT as prepared were carried out using thermogravimetric (TG) and differential scanning calorimetric (DSC) techniques. These experiments were performed at a 10\u00a0\u00b0C.min\u22121 heating rate in 40\u00a0mL.min\u22121 N2-gas flow rate, using a 50\u00a0H Shimadzu thermal analyzer-Japan. The instrument is equipped with data acquisition and handling system (TA-50WSI), and highly sintered \u03b1\u2013Al2O3 was applied as reference material in DSC experiments.X-ray Diffraction analysis (XRD) of the calcined samples at 500\u00a0\u00b0C, as well as the original CKD sample, were analyzed by X-ray powder diffraction (XRD) using a Brucker AXS-D8 Advance diffractometer (Germany), equipped with a copper anode generating Ni-filtered CuKa radiation (k\u00a0=\u00a01.5406 \u00c5) from a generator operating at 40\u00a0kV and 40\u00a0mA, in the 2\u03b8 range between 20\u00b0\u2212\u00a080\u00b0. The instrument is supported by interfaces of DIFFRAC\n\n\nplus\n\n SEARCH and DIFFRAC\n\n\nplus\n\n EVA to facilitate an automatic search and match of the crystalline phases for identification purposes with the COD crystallographic database.The FTIR spectra were recorded using a Magna-FTIR 500 (USA), between 4000 and 250\u00a0cm\u22121, operating Nicolet Omnic software and adopting the KBr disk technique.Surface textural properties of HAPT samples that were calcined at 500\u00a0\u00b0C (viz. specific surface area, pore volume, and mean pore radius) were calculated from nitrogen adsorption-desorption isotherms recorded at liquid nitrogen temperature (i.e.\u2013196\u00a0\u00b0C) using automatic Micromeritics ASAP2010 (U.S.) equipped with online data acquisition and handling system operating BET and BJH analytical software. All samples were degassed at 200\u00a0\u00b0C and 10\u22125 Torr for 2\u00a0h before measurements (1\u00a0Torr\u00a0=\u00a0133.3\u00a0Pa).The morphological characterization of the prepared samples was performed by Scanning Electron Microscopy (SEM) using a FEI Quanta 250 FEG MKII with a high-resolution environmental microscope (ESEM) using XT microscope Control software. EDX dot mapping analysis combined with FESEM was used to determine the homogeneous dispersion of sample constituents. Scanning Electron Microscopy (SEM) images of samples were carried out on a FEI Quanta 250 FEG MKII with a high-resolution environmental microscope (ESEM) using XT Microscope Control software linked to an Electron Dispersive X-ray (EDX) detector. The EDX used was a 10 mm2 SDD Detector-x-act from Oxford Instruments, which utilizes Aztec\u00ae EDX analysis software.The total number of acidic sites (sites.g\u22121) over HAPT at 500\u00a0\u00b0C was measured using the temperature-programmed desorption (TPD) of tetrahydrofuran (THF, 99\u00a0+%, stabilized with 0.025% butylated hydroxytoluene-Sigma) condensed phase, as a probe molecule. The experimental details can be explained as follows [32,33]. 50\u00a0mg of sample preheated at 350\u00a0\u00b0C for 1\u00a0h in the air before the probe molecule is exposed. 20\u00a0\u00b1\u00a02\u00a0mg covered sample with THF were subjected to thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analyses at a heating rate of (10\u00a0\u00b0C.min\u22121) in dry N2 flowing (40\u00a0mL.min\u22121), using a 50\u00a0H Shimadzu thermal analyzer (Japan). The thermal analyzer is equipped with a data acquisition and handling system (TA-50WSI). \u03b1\u2013Al2O3 was used as the reference material in DSC measurements. The mass loss due to desorption of THF during TG experiments from the acidic sites was determined to measure the total surface acidity as sites.g\u22121. The following (Eq.2) is used to estimate the total number of surface acidic sites [34].\n\n(2)\n\n\nTotal\n\nNo\n\n.\n\n\nof\n\nsurface\n\nacidic\n\nsites\n\n(\nsites\n/\n\ng\n\n)\n=\n\n\nmoles\n\nof\n\ndesorbed\n\nTHF\n\n\nx\n\n\nAvogadro\n\u2032\n\ns\n\n\nnumber\n\n(\nsites\n/\nmol\n)\n\n\nweight\n\nof\n\nTG\n\nsample\n\n(\n\ng\n\n)\n\n\n\n\n\n\nCatalyst activity experiments were performed at atmospheric pressure in a conventional fixed bed U-shaped quartz reactor. The catalytic activity and selectivity of catalyst samples for decomposing sec-butanol (SB) to products, mostly in the temperature range of 200\u2013300\u2009\u00b0C, were investigated. In each experiment, 0.2\u2009g of catalyst sample was preheated at 400\u2009\u00b0C inside a fixed-bed continuous flow reactor for 1\u2009h in an airflow (100\u2009mL.min\u22121) before measurements; then, the temperature was gradually decreased to 200\u2009\u00b0C. Sec-butanol (SB) (Fluka, \u226599%) vapors were generated by passing a stream of air (100\u2009mL.min\u22121) through the liquid SB in a glass saturator thermostatically stabilized at 5, 10 and 15\u2009\u00b0C. The gas hourly space velocity of 30\u2009L.gcat\n\u22121.h\u22121 was used in all the experiments. The expected reaction products as cis-2-butene (c-2-butene), trans-2-butene (t-2-butene), and methyl ethyl ketone (MEK) were analyzed and detected using an online gas chromatograph (Shimadzu GC-14), equipped with a data processor model Shimadzu Chromatopac C-R4AD. An automatic sampling was continuously performed using a heated gas sampling cock, type HGS-2, at 140\u2009\u00b0C, using a hydrogen flame ionization detector (FID) and a stainless-steel column (PEG20 M 20% on Chrmosorb W, 60/80 mesh, 3\u2009m\u2009\u00d7\u20093\u2009mm) at 75\u2009\u00b0C. The % conversion of SB and % selectivity of products were calculated [35] using the following (Eqs.3 and 4):-\n\n(3)\n\n\n\n%\nSB\n\nconversion\n=\n\n\n\n\n[\n\n\nNo\n\n.\n\n\nmoles\n\n\n\nSB\n\n\n]\n\n\n\ni\n\n\nn\n\n\n\n\u2212\n\n\n[\n\n\nNo\n\n.\n\n\nmoles\n\n\n\nSB\n\n\n]\n\n\n\no\n\n\nu\n\n\nt\n\n\n\n\n\n\n\n[\n\n\nNo\n\n.\n\n\nmoles\n\n\n\nSB\n\n\n]\n\n\n\ni\n\n\nn\n\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n(4)\n\n\nProduct\n\nselectivity\n(\n%\n)\n=\n\n\n\n\n[\n\nNo\n\n.\n\n\nmoles\n\n\n\nof\n\n\n\nproduct\n\n]\n\n\n[\n\nTotal\n\n\n\nno\n\n.\n\n\nmoles\n\n\n\nof\n\n\n\nproducts\n\n]\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n[No. moles SB]in and [No. moles SB]out = the number of SB moles in the feed and outlet streams, respectively.The thermal decomposition behavior of precipitated HAPT was investigated using TG and DSC techniques, as presented in (\nFig. 1(a)). Starting from RT and increasing to 500\u2009\u00b0C, the TG profile exhibited consecutive steps with % mass loss equal to 24.89%. Additionally, a very small weight loss step between 500 and 600\u2009\u00b0C was monitored, associated with a 1.75% weight loss. The corresponding DSC profile recorded for HAPT revealed a series of endothermic peaks in the temperature range of RT-179\u2009\u00b0C, due to the slow dehydration step of the precipitated HAPT, followed by a strong endothermic peak associated with the main thermal decomposition process of the prepared HAPT. This enormous step may be ascribed to the decomposition of NH4NO3\n[36] that formed between HAPT particles, as presented in (Eq.1), during the precipitation process. Besides, there are two small broad endothermic peaks at 329 and 474\u2009\u00b0C. These peaks may be due to the decomposition of the residual traces of NH4NO3 in depth inside the bulk of HAPT particles. Thermal analyses results of precipitated HAPT indicate that calcination at 500\u2009\u00b0C for 3\u2009h in oxygen is suitable for preparing well-defined HAPT, as will be discussed later by XRD and FT-IR analyses.To compare the original CKD and the purified obtained HAPT sample, XRD analysis of both samples was employed, as shown in Fig. 1(b). The primary constituent of CKD was tricalcium silicate (C3S) [Ca3(SiO4)O], as confirmed by the most intense Miller index (312) at 2\u03b8 =\u200929.2\u00b0, besides other six planes at (013), (203), (101), (204), (313) and (431) which correspond to 2\u03b8 =\u200922.8, 26.3, 31.2, 32.2, 33.8 and 39.1\u00b0, respectively (COD 9014362). The second compound in CKD was identified as dicalcium silicate (C2S) [Ca2SiO4] based on a group of diffraction peaks at 2\u03b8 =\u200932.8\u00b0 (111), 35.7\u00b0 (122), 41.0\u00b0 (221), 43.0\u00b0 (230), 48.3\u00b0 (240), 56.2\u00b0 (062), 57.0\u00b0 (223), 60.6\u00b0 (170), 64.5\u00b0 (243) and 72.8\u00b0 (115) (COD 1546025). The last five diffraction peaks are extremely faint, see Fig. 1(b)-CKD. These results are consistent with data published recently [37,38]. Finally, two diffraction peaks associated with Ca(OH)2 were observed in the XRD pattern of CKD at 2\u03b8 =\u200946.9\u00b0 (102) and at 50.7\u00b0 (110) [39]. The X-ray diffraction pattern of HAPT at 500\u2009\u00b0C clearly revealed 26 diffraction peaks (Fig. 1(b)), six of which are the most intense peaks for a highly crystalline mixture of both Ca10(PO4)6(OH)2 (COD 1100066) and Ca5(PO4)3(OH) (COD 9002213) at 2\u03b8 =\u200925.2, 26.6, 28.1, 28.9, 42.4 and 45.3\u00b0 which correspond to (201), (002), (012), (210), (302) and (023), respectively. Additionally, two additional Ca-compounds were identified in the XRD pattern of HAPT prepared at 500\u2009\u00b0C, i.e. Ca3(PO4)2 with two diffraction peaks at 30.1\u00b0 (012) and 33.4\u00b0 (110) (COD 1521426) as well as a single diffraction peak related to the presence of Ca(OH)2 at 36.6\u00b0 (002) (COD 9009098). The XRD analysis (Fig. 1(b)) demonstrated that the prepared sample of HAPT was free from any compounds of other elements detected in the XRF of the original CKD, as shown in Table 1. The most intense diffraction line (002) at 26.6\u00b0, in the case of HAPT, was used to calculate the crystallite size to be 23.6\u2009nm using the Scherrer equation [40]. Our XRD analysis of HAPT prepared at 500\u2009\u00b0C is consistent with results from recent publications [41\u201343].According to many authors [44\u201346], the XRD patterns of the prepared HAPT mostly exhibited three main planes (002), (211), and (300) as the main growth planes of HAPT crystals. The plane (211) is greatly sensitive and influenced by calcination temperature as well as the Ca-source used to prepare HAPT [46]. An interesting paper should be considered carefully [46] that recently explained the stepwise growth of the different planes of HAPT, especially plane (211), during heating HAPT with different heating rates, i.e. 3, 6, and 9\u2009\u00b0C.min\u22121. and in-situ recording X-ray diffractograms in the temperature range of 400\u2013900\u2009\u00b0C. Therefore, with enlargement of the area in the range of 2\u03b8 =\u200924\u201335\u00b0, in a new window (\nFig. 2), the XRD pattern of HAPT at 600\u2009\u00b0C prepared from CKD (sample not included) showed a strong (211) plane, in comparison with the faint very weak plane in case of HAPT at 500\u2009\u00b0C (the subject of this paper). On the other side, both (002) and (300) planes sharply appeared in diffractogram of HAPT at 500\u2009\u00b0C.Furthermore, to prove that Ca-source plays an important role in the type and structure of the resulting HAPT, we used the same applied precipitating agent, i.e. (NH4)2HPO4 diammonium hydrogen phosphate, and different sources. The other sample of HAPT-NO3 at 500\u2009\u00b0C we used as a comparative catalyst from a pure Ca-source (Ca(NO3)2) (in the section of decomposition of sec-butanol) exhibited a completely different XRD pattern from that of HAPT-CKD at 500\u2009\u00b0C, see Fig. 2. An attention should be made to the following observations: i) HAPT-NO3 at 500\u2009\u00b0C was characterized essentially by the polymorph hydroxyapatite structure [47] with formula Ca5(PO4)3(OH) according to (COD- 9002213). ii) The three main planes (002), (211) and (300) sharply and easily distinguished in XRD pattern of HAPT-NO3. iii) at 500\u2009\u00b0C, at the same calcination temperature, the XRD pattern of the corresponding HAPT-CKD gave a very faint plane at (211), as explained earlier, with two sharp planes at (002) and (300). Our prepared HAPT-CKD at 500\u2009\u00b0C was identified by matching the recorded diffractogram with standard card (COD-1100066) and was mainly composed of Ca10(PO4)6(OH)2 with little traces of polymorph Ca5(PO4)3(OH). The positions of the recorded planes in the two diffractograms are quite different due to matching each one with a different standard card, i.e. (COD- 9002213) and (COD-1100066).\n\nFig. 3(a) exhibits the FT-IR analysis of the original CKD, as delivered, the precipitated HAPT and HAPT calcined at 500\u2009\u00b0C. The spectrum of CKD is completely different from the spectra of HAPT samples. Furthermore, the three spectra have a complex group of bands, especially in the range of 500\u20131700\u2009cm\u22121, as shown in Fig. 3(a). In the case of CKD, its spectrum contains a group of bands in the range of 800\u20131200\u2009cm\u22121, related to the asymmetric and symmetric stretching vibrations of Si-O bonds [48], due to the presence of both C2S and C3S, as confirmed by XRD analysis (Fig. 1(b)). The bands at 3645, 3432 and 1644\u2009cm\u22121 are associated with the stretching and bending vibrations of O-H due to adsorbed water and the presence of Ca(OH)2 in CKD [48,49]. A strong absorption band at 1419\u2009cm\u22121 corresponds to the asymmetric stretching vibration of V\n\n3\n C-O due to the formation of CaCO3 in CKD, which is present in atmospheric CO2\n[48,50,51]. Moreover, a sharp and small absorption band at 1385\u2009cm\u22121 could be assigned to the vibration of nitrate [49] that is still adsorbed between CKD particles. Finally, an absorption band appeared at 517\u2009cm\u22121 that was assigned to Ca-O symmetric stretching vibrations [52] in the spectrum of CKD. The spectra of HAPT as prepared and after calcination at 500\u2009\u00b0C for 3\u2009h in oxygen are quite similar and have the same features. Typical absorption bands of phosphate groups were observed in spectra of HAPT in the range of 1112\u2013985\u2009cm\u22121 that were assigned to \u03bd(PO4\n3-) [53], while bands at 662 and 579\u2009cm\u22121 related to the \u03bd4 bending vibrations of P-O mode in the crystalline HAPT network and the band at 916\u2009cm\u22121 resulted from the \u03bd1 symmetric P-O stretching vibration [54]. These bands are assigned to the stretching vibrations, symmetric bending and asymmetric bending of vibrations of PO4\n3\n-\n[55]. In addition, the broad bands centered at 3445 and 1632\u2009cm\u22121 indicate the presence of adsorbed water, especially in the fresh precipitated HAPT [56]. Moreover, a broad band at about 1300\u2009cm\u22121 due to \u03b4(NH4\n+) appeared in spectra of HAPT at 500\u2009\u00b0C [53], besides a sharp band at 1386\u2009cm\u22121 that ascribed to the vibration of nitrate [49] as traces that formed during the preparation of HAPT using HNO3. Finally, an absorption band located at 754\u2009cm\u22121 could be assigned to (P2O7\n2-) [57]\n, and another band related to the symmetric stretching vibrations of Ca-O [58] is shifted to 530\u2009cm\u22121. Based on the XRD and FT-IR analysis results discussed above, one can conclude that the prepared HAPT at 500\u2009\u00b0C is quite an impurity-free sample, as no silicate or other elemental compounds were detected.To get a deep look at the absorption bands, those reflect the structure of the resulting HAPT from CKD, which is not clearly observed in the spectra presented in Fig. 3(a). IR-spectrum enlargement of HAPT-CKD at 500\u2009\u00b0C, in the range of 1250\u2013500\u2009cm\u22121, presented in Fig. 3(b). The expanded spectrum showed a group of stretching vibrations of the phosphate group in HAPT-CKD at 1112 and 1059\u2009cm\u22121\n[53], at 1030 and 983\u2009cm\u22121 for \u03bd3 vibrational modes of (PO4\n3-) [59,60], as well as \u03bd4-mode at 535\u2009cm\u22121\n[59]. Furthermore, the bending vibration modes (\u03bd4) of PO4\n3- functional groups were recorded at 662, 603 and 579\u2009cm\u22121\n[31,54]. The stretching vibration of P=O and the bending vibration of P-O are located at 1044, 916 and 559\u2009cm\u22121, respectively [54,61].The surface characteristics of the HAPT at 500\u2009\u00b0C, such as surface area and porosity, were studied using nitrogen adsorption/desorption isotherm, as shown in Fig. 3(c). The figure shows a type-IV isotherm with an H3-hysteresis loop (p/p\u00b0 >0.9) [62,63]. This clearly indicates the formation of secondary slit-shaped pores of increased size in nanoparticle aggregates [62]. The corresponding pore size distribution calculated using the BJH method from the desorption branch of the isotherm clearly demonstrated the presence of major pores in the range of 2.4 \u2013 6.8\u2009nm with a few pores slightly larger than 11\u2009nm, indicating the evident hierarchical mesoporous characteristics of HAPT, see Fig. 3(c). However, the calculated surface area of HAPT prepared at 500\u2009\u00b0C from CKD was 3\u2009m2.g\u22121. As fundamentals, the abundant mesopores and the hierarchical structure of the HAPT sample would facilitate effective contact of active sites across its surface as a catalyst, as well as mass transport during the catalytic reaction that will be discussed later. The calculated surface area (SBET) of our sample HAPT at 500\u2009\u00b0C agrees with recently published values for various HAPT samples prepared from eggshells [64].\nFig. 3(e) exhibits the SEM images of HAPT at 500\u2009\u00b0C with different magnifications. At low magnification, the SEM image reveals that the HAPT surface is composed of brittle particles; however, as the magnification increases, more details become visible. Furthermore, the microstructure of HAPT contains a diverse array of mesopores with different pore diameters (as indicated by the yellow arrows in Fig. 3(e). In addition, HAPT particles have a microstructure of transparent thin sheets and flakes (as indicated by red arrows). Each sheet or flake is self-assembled from a large number of loosely packed nanoparticles that connect to form a layered structure, see Fig. 3(e). This distinctive morphology of HAPT particles, as determined from SEM images in Fig. 3(e), offers additional support to the reaction's expected high catalytic properties. The corresponding EDX spectrum obtained from the surface of HAPT confirms the presence of only the Ca, P and O elements, as illustrated in Fig. 3(d). In addition, these results indicated the formation of Ca-deficient hydroxyapatite. Moreover, the calculated weight ratio and a molar ratio of Ca: P elements were 0.78 and 0.61, respectively, compared to the stoichiometric HAPT\u2019s (Ca/P) molar ratio [65] of 1.67. It is worth noting that EDX is a surface technique that reveals only a 10\u2009nm depth below the catalyst's surface, resulting in the discrepancy between the measure and stoichiometric calculation.In order to confirm further the purity of the prepared HAPT from CKD, by a simple method, depending on the dissolution of Ca2+ in a diluted HNO3 followed by separation of this clear solution of Ca(NO3)2 and precipitating HAPT using (NH4)2HPO4 with adjusting the pH of the medium. In addition to the EDX-spectrum (see Fig. 3(d)) that revealed the purity of HAPT-CKD. We performed another accurate technique for the HAPT-CKD, i.e., elemental mapping, as presented in \nFig. 4. As an accurate and modern analysis technique, the obtained results of the elemental mapping strongly supported the formation of a pure sample of hydroxyapatite (HAPT-CKD at 500\u2009\u00b0C). The whole spectrum revealed that only P, Ca, and O were recorded as only constituents in HAPT at 500\u2009\u00b0C (see Fig. 4).Over the last two decades, considerable research has been conducted on the use of HAPT as an active catalyst, either pure or as a catalyst support, in a variety of chemical reactions [20\u201326]. While none was conducted on the decomposition reactions of sec-butanol (SB), as a result, the work herein represents the first successful attempt to use HAPT as an acidic catalyst to convert SB to trans-2-butene (t-2-butene) via single-step alcohol dehydration. Whether considering the catalyst preparation or the catalytic activity findings, a very high degree of reproducibility has been observed, and the carbon mass balance was almost 100% in all catalytic tests, with no carbon coke formation observed in the spent catalysts. To economically produce t-2-butene from SB in a single step, the following experiments were conducted:\n\n(1)\nThe effect of air and N2-gas as carriers\n\n\nThe effect of air and N2-gas as carriersTo compare the effect of a costless carrier, such as air and a known-cost carrier, such as N2-gas, on the dehydration reaction. The conversion of SB was studied using 0.2\u2009g of HAPT and Vp of SB of 0.93 kPa in the temperature range of 200\u2013300\u2009\u00b0C, using 100\u2009mL.min\u22121 of air or N2-gas as carriers in two separate experiments with gas hourly space velocity (GHSV) =\u200930\u2009L.gcat\n\u22121.h\u22121, as shown in Fig. 5(a). At all reaction temperatures, using air as a carrier had a beneficial effect on SB conversion. It increases by approximately 5.2\u20138.7% with the increasing reaction temperature from 200 to 300\u00a0\u00b0C, see Fig. 5(a).Furthermore, at a reaction temperature of 200\u2009\u00b0C, the % conversion was 40%, then gradually and steadily increased to 91.4\u2009\u00b1\u20091.8% at 300\u2009\u00b0C using an air carrier. As a result, air was used as a costless and effective carrier in all experiments herein. Fig. 5(b) presents the distribution of all products produced during the conversion of SB at the corresponding reaction temperature. The obtained results indicated that the major product was t-2-butene, which had a higher and constant % selectivity of 99%, whereas the other products, c-2-butene and MEK, had a selectivity of less than 1%. Finally, N2-gas, as an inert carrier in catalysis, sometimes do not activate the surface sites over the catalyst responsible for producing the desired product. Therefore, using air as a carrier reduces the reaction costs and initiates the active sites to perform high alcohol conversion at a low temperature, as we studied in Fig. 5(a). Air is superior to nitrogen as a carrier gas in this reaction due to the dissimilar properties of the two gases. Unlike air, which is a mixture of inert and active gases, N2 is an inert gas, and its inertness is due to the covalent bonds formed by the three lone pairs of electrons in its molecule [66]. Thus, the potential of air to initiate the active sites of the catalyst and regenerate the used active sites responsible for producing the desired product is greater than that of nitrogen. Furthermore, GC analysis identified only the product trans-2-butene and unreacted SB in each injection during the reaction in the temperature range of 200\u2013300\u2009\u00b0C (see \n\n\n\n\nFigs. 5,6,9).\n\n(2)\nEffect of the reactant space velocities on the catalyst reactivity\n\n\nEffect of the reactant space velocities on the catalyst reactivityThree different reactant space velocities were used during the dehydration of SB over HAPT, namely 20, 30 and 60\u2009L.gcat\n\u22121.h\u22121, in the temperature range of 200\u2013300\u2009\u00b0C. As shown in Fig. 5(c), increasing the space velocity resulted in a gradual and significant decrease in % of the conversion of SB at all reaction temperatures. This decrease in the % conversion is readily apparent when WHSV is increased from 30 to 60\u2009L.gcat\n\u22121.h\u22121. This phenomenon obviously indicates that at a higher GHSV value (60\u2009L.gcat\n\u22121.h\u22121), insufficient time was allowed for the reacting molecules of SB to convert to products [67]. Furthermore, the WHSV has a significant effect on reaction kinetics, and the high value of WHSV results in a rate-limited conversion [68]. Therefore, we utilized the intermediate value (30\u2009L.gcat\n\u22121.h\u22121) in all experiment, which produced favorable results.\n\n(3)\nComparison of HAPT-CKD derived from CKD and HAPT-NO3 derived from Ca(NO3)2\n\n\n\nComparison of HAPT-CKD derived from CKD and HAPT-NO3 derived from Ca(NO3)2\nTo demonstrate the validity of CKD as a Ca-rich source for preparing HAPT, as a benefit of that huge industrial waste-CKD, we prepared another sample of HAPT-NO3 under the same conditions using pure Ca(NO3)2 (BDH-analytical grade). A comparison of the catalytic activities of both samples was performed during the conversion of SB, in the temperature range of 200\u2013300\u2009\u00b0C, using WHSV =\u200930\u2009L.gcat\n\u22121.h\u22121, as shown in Fig. 6. At all reaction temperatures, HAPT-CKD exhibited incomparable activity towards the conversion of SB to t-2-butene. The % conversion of SB using HAPT-CKD, in the temperature range of 200\u2013250\u2009\u00b0C, was 9.2\u20139.8 times that of HAPT-NO3, see Fig. 6. As the reaction temperature increased gradually to 275 and 300\u2009\u00b0C, a significant activation occurred for HAPT-NO3. This could be attributed to the distribution of different acidic sites over both samples, as discussed in the following paragraph, as well as the crystallite size of both samples that were determined by XRD analysis (HAPT-NO3 XRD is not shown). The crystallite size of HAPT-CKD and HAPT-NO3 samples were 23.64 and 32.09\u2009nm, respectively. As a result, the crystallite size of HAPT-CKD is smaller than that of HAPT-NO3.To obtain information about the acidic site's capacity and the distribution of these sites over each sample, temperature-programmed desorption (TPD) of tetrahydrofuran (THF) was performed using TG and DSC techniques [34], as shown in Fig. 7. The use of tetrahydrofuran as acidity-probe molecule instead of the known acidity probe molecules such as pyridine and ammonia is due the low surface area of the prepared HAPT catalysts. According to our recent publication [34], the accurate measurement of surface acidity of the catalysts with limited values of surface area requires a relatively moderate basic molecule as THF with pKb =\u200916.08, rather than strong basic molecules such as NH3 (pKb = 4.75) or pyridine (pKb = 8.77). In the case of HAPT-CKD, the % mass loss due to desorption of THF from the total acidic sites, in the temperature range of 125\u2013500\u2009\u00b0C, was 39.43% see Fig. 7(a). This is approximately 1.77 times the calculated value for HAPT-NO3. This clearly indicates that the acidic sites density is higher in the case of HAPT-CKD, with a total no. of acidic sites of 3.29\u2009\u00d7\u20091021 sites.g\u22121, while it was 1.84\u2009\u00d7\u20091021 sites.g\u22121 for HAPT-NO3. The DSC-TPD analysis of THF from both samples revealed important information about the distribution of these acidic sites over their surfaces. Fig. 7(c), the DSC-TPD profile of HAPT-CKD showed the presence of weak acidic sites maximized at 148\u2009\u00b0C, a big peak of the moderate acidic sites from 163\u2009\u00b0C up to 350\u2009\u00b0C, and a small peak due to strong sites maximized at 395\u2009\u00b0C. On the other hand, DSC-TPD profile of HAPT-NO3 exhibited a very faint peak maximized at 154\u2009\u00b0C related to weak acidic sites, a strong peak maximized at 263\u2009\u00b0C associated with the presence of moderate acidic sites, followed by a weak peak at 368\u2009\u00b0C ascribed for strong sites, as shown in Fig. 7(c). This broad range of acidic sites over the surface of HAPT, including weak, moderate and strong sites, has been previously observed [22] using the NH3-TPD technique. Consideration should be given to the big peak of the moderate acidic sites in both DSC-TPD profiles of the two samples, as shown in Fig. 7(c). In the case of HAPT-CKD, this massive peak can be divided into two sub-acidic sites using Gaussian line profiles, see Fig. 7(b). Furthermore, the calculated no. of moderately acidic sites under this peak was 2.77\u2009\u00d7\u20091021 sites.g\u22121 in the case of HAPT-CKD, while it is estimated to be 1.53\u2009\u00d7\u20091021 sites.g\u22121 in the case of HAPT-NO3. As previously explained and illustrated in Fig. 6, this obvious difference in the number of moderately acidic sites present in each sample will significantly affect the catalytic activity of each sample during the conversion of SB to t-2-butene.It is known that the main adsorption sites of HAPT particles are Ca2+ ions as Lewis acidic sites and O atoms of PO4\n3- as Lewis basic sites, and the existence of these sites in different proportions is the dominant factor regulating the catalytic activity and selectivity of HAPT [22\u201324,69]. Bittencourt et al. [69] studied the adsorption properties of a wide range of probe molecules, including CO, CO2, C2H2, CH4, H2, H2O, NH3, SO2, and BCl3, on the surface of HAPT. They found that all the selected probe molecules are adsorbed preferentially close to the most exposed Ca2+ ions. Thus, we believe that the acidic sites involved in the adsorption of THF molecules and in the dehydration of sec-butanol are mainly the Lewis acidic sites (i.e. Ca2+ ions). As a result of the preceding discussion, THF can accurately and quantitatively count the surface acidic sites [34] over HAPT molecules as we proposed based on the published chemical structure of HAPT [70], see Fig. 8(a). The vapour molecules of THF will directly interact with Lewis acidic Ca2+ cationic centers that are abundantly available over its surface, see Fig. 8(b).Our proposed mechanism is quite similar to one recently published [71] for pyridine adsorption over H2-treated CeO2-MeOx samples. As proposed in Fig. 8(c), these acidic sites over the prepared HAPT-CKD catalyst are responsible for the dehydration of SB to t-2-butene [72], with superior activity and selectivity starting from 200\u2009\u00b0C up to 300\u2009\u00b0C, see Fig. 5(b). Additionally, the perfect distribution of surface moderate acidic sites on HAPT-CKD makes it easy to snipe and firmly bond water molecules over these sites, see Fig. 8(c). This facilitates the dehydration reaction, and the catalyst became more active as the reaction temperature increased from 200\u00b0 to 300\u00b0C. The produced t-2-butene exhibits superior properties to c-2-butene [73], with known values such as kinetic diameter =\u20094.31\u2009\u00c5, dipole moment (D) =\u20090.00 and polarizability equal to 81.8\u2009\u00d7\u200910\u221225 cm\u22121. Likewise, it was established in a recent study that c-2-butene is consistently less stable than t-2-butene [74] by 0.9\u20131.4\u2009kcal.mol\u22121. Soto et al. [75] and Bedia et al. [76] stated that sec-butanol could attack the active sites over the catalyst surface from different directions, giving multiple adsorption complexes. Therefore, the formation of 1-butene, cis-2-butene and trans-2-butene and their abundances depend on the acid-base properties of each catalyst [75,76]. Furthermore, they calculated the equilibrium (cis/trans) ratio in the homogeneous phase to be approximately 0.6. This value showed that trans-isomer formation is favored because it is thermodynamically more stable [75].Numerous articles examined the kinetics of t-2-butene ignition in a jet-stirred reactor and a combustion bomb [77,78], while others investigated and studied the isomeric flames of butene isomers [79] using in situ molecular-beam mass spectroscopy and gas chromatography techniques. Furthermore, t-2-butene and c-2-butene showed promising thermal performance in the solar organic Rankine cycle (ORC) [80]. A recent article also studied the catalytic isomerization and hydroformylation of butenes [81]. Due to the widespread use of 2-butene isomers as chemical intermediates, the global 2-butene market is expected to expand significantly [82] between 2022 and 2028. As a result, the advancement of numerous industries necessitates additional research into producing more low-cost, high-productivity t-2-butene. This is the major merit of the work herein.\n\n(4)\nStability and reusability of the catalyst HAPT-CKD\n\n\nStability and reusability of the catalyst HAPT-CKDTwo distinct experiments were conducted to determine the stability and reusability of the catalyst HAPT-CKD at 500\u2009\u2103 during the conversion of SB to t-2-butene:(i) A fresh sample was tested under the same conditions, as previously described using an air-carrier and demonstrated conversion of SB ranging from 40\u2009\u00b1\u20090.8 to 91.4\u2009\u00b1\u20091.8% in the temperature range of 200\u2013300\u2009\u00b0C, see Fig. 9(a). The % selectivity for t-2-butene was always 99%. Following that, the same sample was regenerated for 1\u2009hr by heating at 350\u2013400\u2009\u00b0C in air flow (100\u2009mL.min\u22121). The temperature is then reduced, and the experiment is repeated using the regenerated sample under the same conditions. The used catalyst after regeneration exhibited increased activity towards the conversion of SB at all reaction temperatures, see Fig. 9(a), while the % selectivity toward t-2-butene remained constant. This phenomenon could be explained by increased activation of acidic sites over the catalyst surface in the range of 200\u2013300\u2009\u00b0C. (ii) To confirm the catalyst's robustness and stability, a subsequent four-cycle experiment was performed on the same sample under the same conditions, as shown in Fig. 9(b), in the temperature range of 200\u2013300\u2009\u00b0C. In the second cycle, the results showed that the sample attained its maximum catalytic activity at all reaction temperatures. After that, the activity of HAPT gradually decreased in the range of 200\u2013250\u2009\u00b0C before slightly increasing again. The trend in catalytic activity observed in Fig. 9(b) over the course of the four cycles is likely the result of two opposing effects on the catalyst surface. The first is the effect of exposing the catalyst to air as an active carrier gas, which activates the acidic sites on the surface of the catalyst and increases its catalytic activity. The second factor is exposure to the reaction feed, which exhausts acidic sites and reduces catalytic activity. In cycle 2, the combination of the two effects appears optimal. The primary objective of these successive cycles is to avoid changes in any of the reaction conditions [83], including a) the catalyst weight, b) the distribution of the sample on the catalyst bed, and c) the exact position at which the catalyst is inside the reactor in the vertical furnace during the reaction. Finally, these experiments demonstrated the durability and reusability of the prepared HAPT-CKD, at 500\u2009\u00b0C, as a costless catalyst during the conversion of SB to t-2-butene with a high % selectivity of 99%.Furthermore, the stability of any catalyst can be observed during the applied catalytic reaction. The catalyst loses its reactivity due to coke deposition during the applied reaction or poisoning of the active sites by poisoning molecules. In our case, the stability of our catalyst was tested by a simple experiment. Thermogravimetric analysis of the used catalyst, after four cycles of SB-reaction over it, was done twice compared to a fresh sample (see Fig. S2). Due to the strong dehydration activity of the catalyst (HAPT-CKD at 500\u2009\u00b0C), the sample lost about 2.3% of its weight on average, while the fresh sample lost only 0.4% of its weight. This is considered good evidence for the stability and constant reactivity of HAPT-CKD at 500\u2009\u00b0C, where no coke is deposited over the catalyst's surface.Hydroxyapatite (HAPT) was successfully prepared as a pure nanocrystalline compound at 500\u2009\u00b0C from cement kiln dust (CKD) as industrial waste using a simple and cost-effective method. According to EDX, elemental mapping, and SEM analyses, the resulting HAPT at 500\u2009\u00b0C is a Ca-deficient structural sample. Moreover, its surface morphology is characterized by thin sheets and flakes, as well as a significant hierarchical distribution of mesopores. This distinguished structure enhances the catalytic activity of HAPT when used as an active catalyst in the conversion of sec-butanol (SB) to t-2-butene at relatively low temperatures in the temperature range of 200\u2013300\u2009\u00b0C. Air was utilized as a carrier rather than N2-gas during the conversion of SB to t-2-butene over the HAPT catalyst. The catalyst demonstrated remarkable activity at relatively low temperatures with an extremely high selectivity of 99% towards t-2-butene. The perfect distribution of acidic sites positively reflected on the catalytic activity of HAPT in comparison with another sample prepared from Ca(NO3)2 under the same conditions. We strongly recommend that HAPT be prepared from CKD as a cost-effective and calcium-rich source. Additionally, as demonstrated by SEM micrographs, due to its distinct surface morphology. We invite our colleagues and researchers to utilize our HAPT-CKD sample in biomedical applications involving bone repair.\nMahmoud Nasr: Visualization, Conceptualization, Formal analysis, Investigation, Methodology, Software, Writing \u2013 original draft, Writing \u2013 review & editing. Samih A. Halawy: Writing \u2013 review & editing, Formal analysis, Validation Project administration, Supervision. Safaa El-Nahas: Writing \u2013 review & editing, Software, Project administration, Supervision. Adel Abdelkader: Writing \u2013 review & editing, Software, Project administration, Supervision. Ahmed I. Osman: Writing \u2013 review & editing, Conceptualization, Methodology, Proofreading, 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 wish to dedicate this work to the spirit of the distinguished Egyptian professor Dr Samih A. Halawy who passed away on the 2nd of September 2022. The authors wish 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). The authors thank Dr Charlie Farrell for proofreading the revised manuscript.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcata.2023.119039.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  This research is regarded as the first successful attempt at directly producing highly pure nanocomposite hydroxyapatite (HAPT) from cement kiln dust (CKD) using a cost-effective preparation method. The crystallite size of HAPT was 23.6\u00a0nm and showed a hierarchical mesoporous unit with a Ca-deficient structure according to SEM-EDX analyses. HAPT exhibits a high population of different acidic sites, i.e. weak, moderate and strong acidic sites, as determined by TG and DSC-TPD experiments using tetrahydrofuran as a probe molecule. The wide range of acidic sites over HAPT is clearly and positively enhanced its catalytic activity during the conversion of sec-butanol to trans-2-butene. In addition, our prepared HAPT demonstrated greater catalytic activity when sec-butanol was converted using air as a carrier rather than N2-gas. A comparison between the catalytic activity of HAPT prepared from the waste CKD and pure Ca(NO3)2 was also conducted, showing HAPT derived from waste streams with higher catalytic activity.\n               "}
{"full_text": "Sulfur compound contents before reaction, \u03bcg/gSulfur compound contents at reaction time t, \u03bcg/gSulfur removal, %The emission of sulfur oxides (SOX) in vehicle exhaust can cause serious environmental problems and threaten human health. Thus, countries around the world have set up strict limits on the sulfide content in fuels, and it is very urgent to develop efficient deep desulfurization technology. In recent years, hydrodesulfurization (HDS) has been widely used to remove organic sulfur compounds, which is conducted at harsh reaction conditions (300\u2013350\u00a0\u00b0C, 2\u201310\u00a0MPa) with expensive hydrogenation catalysts. HDS can effectively remove aliphatic sulfur compounds (Sampieri et\u00a0al., 2005). Furthermore, removal of aromatic sulfur compounds is difficult because steric hindrance. Some technologies have been developed to overcoming technical defects of HDS, these desulfurization technologies are adsorption desulfurization (ADS) (Luo et\u00a0al., 2021), extractive desulfurization (EDS) (Li et\u00a0al., 2013) and oxidative desulfurization (ODS) (Wang et\u00a0al., 2018), etc. Among them, the ODS technology has been widely researched due to its advantages such as mild operating conditions, low energy consumption, and high removal efficiency of aromatic sulfides. In the ODS process, organic sulfides can be oxidized into sulfones with strong polarity, and then removed by extraction or adsorption (Zhen et\u00a0al., 2019). Various oxidants, including H2O2, oxygen and organic peroxides have been applied to the ODS process. H2O2 is widely used due to its lower cost, higher oxidation performance and environmental protection. However, the production and storage of hydrogen peroxide may cause safety and cost issues (Liu et\u00a0al., 2021). Oxygen is not only environmentally friendly, moreover it is easy to get. Thus, oxygen as the oxidant in ODS, it has attracted the attention of researchers (Wang et\u00a0al., 2021).In ODS, the reaction between oxygen and sulfide is difficult to be carried out under mild conditions. Therefore, it is very important to develop high activity catalysts. In recent years, some catalysts, such as inert metal-based material (Nakagawa et\u00a0al., 2019), metal organic frameworks (MOFs) (Tang et\u00a0al., 2020), polyoxometalate (POMs) (Chi et\u00a0al., 2019), transition metal oxide (TMO) (Wang et\u00a0al., 2017) etc., are applied to the field of aerobic oxidative desulfurization (AODS). TMO has been widely concerned in ODS due to their higher activity and lower cost (Wang et\u00a0al., 2020). For example, Wang et\u00a0al. (Wang et\u00a0al., 2020) synthesized V2O5 nanosheets with oxygen vacancies by rapid gas drive stripping method. The sulfur removal can reach 99.7% under the optimum reaction conditions. Shi et\u00a0al., (2016) proposed a simple sol-gel method to prepare Ce\u2013Mo\u2013O catalysts. The catalyst can completely remove BT and 4, 6-DMDBT, the removal rate of BT is 97%, and the oxidation process does not need to add sacrificial agent. Dong (Dong et\u00a0al., 2019) et\u00a0al. has developed ultra-thin a-CO(OH)2 nanosheets with molybdate intercalation, its derived Co\u2013Mo\u2013O mixed metal oxide has shown excellent sulfur removal performance. Wu et\u00a0al., (2020) found that the interaction of strong metal edges between Pt and h-BN can improve the aerobic oxygen oxidation performance in fuel oil. Liu et\u00a0al., (2020) established Co\u2013Ni\u2013Mo\u2013O mixed metal oxide nanotubes with a hollow structure preferred to application of AODS. The preparation of the above catalysts often requires harsh preparation conditions such as calcination or the addition of organic reagents to control the structure of the catalyst. These defects are not conducive to the industrialization process of aerobic oxidative desulfurization.Cerium molybdate has been widely applied in the fields of inorganic pigments (Dargahi et\u00a0al., 2020) and photocatalysts (Xing et\u00a0al., 2016). At present, there is no report of cerium molybdate used as a catalyst for AODS. In this paper, Ce2(MoO4)3 was synthesized and characterized by FT-IR, XRD, SEM, XPS. Compared with the previous TMO, the synthesis of the Ce2(MoO4)3 can be carried out under mild reaction conditions (lower temperature and shorter reaction time), and the reaction process does not need to add structure promoter and high-temperature calcination. The oxidative desulfurization of model oil was determined by using Ce2(MoO4)3 as the catalyst and oxygen as the oxidant. Furthermore, based on the experimental conclusion, the functions of Ce and MoO4 are illustrated in ODS, and proposed possible reaction mechanism.Ammonium molybdate (98\u00a0wt%), Cerium Nitrate Hexahydrate (AR), Decahydronaphthalene (AR), Dibenzothiophene (AR), 4,6-Dimethyldibenzothiophene (AR), Benzothiophene (AR), All above reagents were purchased from Aladdin Reagent Co., Ltd.. Oxygen (99\u00a0wt%, Hubei Guangao Biological Technology Co., Ltd.).FT-IR of the synthesized catalysts was characterized by a Nicolet FT-IR spectrophotometer (Nexus 470, Thermo Electron Corporation) with spectral range of 4000-400cm-1 and resolution better than 1.5\u00a0cm-1. XRD graphics of the Ce2(MoO4)3 sample were obtained using the Philips diffractometer utilizing high-intensity Cu K\u03b1 radiation in X\u2019Pert MPD model (40\u00a0kV; 100\u00a0mA; 1.5406\u00a0\u00c5), and the step scan technique at 2 theta angles range between 10\u00b0 and 70\u00b0. Surface morphology of the Ce2(MoO4)3 was characterized by SEM (ZEISS Gemini SEM 500, Germany). The elemental composition of the Ce2(MoO4)3 was studied by energy-dispersive X-ray spectroscopy (EDS). XPS (PHI5000 Versaprobe II, Japan) was used to survey and evaluate the elemental composition and surface chemical state of Ce2(MoO4)3.In this work, the Ce2(MoO4)3 catalyst was synthesized by the reflux method. 0.5296g ammonium molybdate was dissolved in 40\u00a0mL distilled water, marked as solution-A, then, solution-A was added to a triangular flask with a cooling condenser. After that, aqueous solution (40\u00a0mL) of 0.8744g cerium nitrate hexahydrate was added dropwise to Solution-A. The triangular flask containing the mixed solution was transferred to an oil bath and stirred at 80\u00a0\u00b0C for 1\u00a0h. The pale yellow precipitation was obtained. The products after centrifugal separation were washed three times with absolute ethanol and distilled water, and dried at 90\u00a0\u00b0C for 5\u00a0h.The model oil with S-content of 250\u00a0\u03bcg/g was prepared by dissolving 0.718g dibenzothiophene in 500\u00a0mL decahydronaphthalene. The AODS reaction was carried out in a three-neck flask. Firstly, 20\u00a0mL simulated oil and a certain amount of Ce2(MoO4)3 were added to three-necked flask with reflux device. Then, the three-neck flask was placed in a preheated oil bath. oxygen was injected at a flow rate of 0.2L/min. The AODS reaction begins at a certain temperature and agitation speed. A small amount of upper oil phase is taken as sample every 20\u00a0min and the sulfur content of the sample was measured by WK-2D microcoulomb analyzer, then calculation of sulfur removal by formula (1). The reaction device is shown in Fig.\u00a01\n.\n\n(1)\n\n\n\u03b7\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\n\nThe FT-IR and XRD characterization results of the as-synthesized Ce2(MoO4)3 are revealed in Fig.\u00a02\n. As shown in Fig.\u00a02(a), the infrared absorption peaks at 3382\u00a0cm-1 and 1616\u00a0cm-1 are attributed to stretching and bending mode of O\u2013H from water adsorbed on the surface of samples (Xing et\u00a0al., 2016), the smaller peak at 1384\u00a0cm-1 corresponds to the bending vibration of the Ce\u2013O\u2013H bond. The narrow peaks between 1070 and 1150\u00a0cm-1, the peaks around 870, 712 and 634\u00a0cm-1 associate to the stretching vibration peaks of the Mo\u2013O bond (Yousefi et\u00a0al., 2012). Crystallinity of Ce2(MoO4)3 was determined by XRD analysis. The XRD patterns are shown in Fig.\u00a02(b). From the results, typical characteristic peaks of samples associate to Ce2(MoO4)3 crystals (JCPDS: 00-057-0952) and the structure is amorphous nanocrystal (Kartsonakis and Kordas, 2010). The individual peaks at 2\u03b8 angles are obtained to 21.19\u00b0, 24.47\u00b0, 27.61\u00b0, 28.33\u00b0, 29.99\u00b0, 36.39\u00b0and 46.34\u00b0, which are assigned to (112), (004), (200), (204), (220), (116), (312) planes of monoclinic Ce2(MoO4)3, respectively. No peak of other crystal phases is observed in the spectrum.The morphology of the as-prepared Ce2(MoO4)3 was investigated by SEM. As shown in Fig.\u00a03\n(a), the sample is composed of nanocrystals with rod-like structure and particles. The particle size of nanocrystals is uneven due to lower reaction temperature and shorter reaction time. Moreover, nanocrystals were revealed high dispersibility and low packing density. According to relevant reports (Xing et\u00a0al., 2016), the morphology of Ce2(MoO4)3 can be changed according to different synthesis conditions. The EDS measurement of the sample is shown in Fig.\u00a03 (b), indicates that the sample is composed of Ce, Mo and O elements, and its molar ratio is approximately n(Ce): n(Mo): n(O)\u00a0=\u00a02:3:12. It is consistent with the composition of cerium molybdate.For the benefit of obtaining the composition and chemical state of catalyst, XPS analysis of Ce2(MoO4)3 catalysts was performed. As shown in Fig.\u00a04\n(a), the survey curves of catalysts further demonstrate the coexistence of Ce, Mo and O elements, each element has a spin-orbit core energy level. Among them, the spin-orbit energy spectrum of Ce elements can be divided into two multiple energy levels (U and V), they correspond to the core-level spin-orbit splitting of Ce 3d3/2 and Ce 3d5/2 (Kanai et\u00a0al., 2017). The high-resolution XPS spectrum of Ce 3d was shown in Fig.\u00a04(b), the U0, V0, U1, V1 peaks corresponding to Ce3+ state and the remaining 5 peaks corresponding to Ce4+ state (Sakthivel et\u00a0al., 2015). The presence of Ce3+ and Ce4+ indicates that the Ce2(MoO4)3 catalyst has redox properties.The high-resolution XPS spectrum of the spin-orbit core energy level of O1s was displayed Fig.\u00a04(c). The sharp peak observed at 530.51\u00a0eV assigned to lattice oxygen in the Mo\u2013O bond. And the side peak at 532.55\u00a0eV can be corresponded the O2 was adsorbed on Ce2(MoO4)3 surface (Sakthivel et\u00a0al., 2015).The XPS spectrum of the spin-orbit core energy levels of Mo 3d5/2 and Mo 3d3/2, the binding energy at 232.68\u00a0eV related to the Mo6+ oxidation state of the Mo 3d5/2 spin-orbit core energy level in Fig.\u00a04 (d). The other peak appears at 235.87\u00a0eV is the Mo 3d3/2 spin-orbit core energy level of Mo6+ oxidation state, it can be attributed to the existence of Mo=O or Mo\u2013O bond, but no additional peaks of Mo4+ or Mo5+ are observed. These peaks are consistent with the reported XPS spectra of Ce 3d and Mo 3d (Karthik et\u00a0al., 2017).In order to study the effect of catalyst structure in ADOS, oxidative desulfurization activity of different catalysts such as Ce2(MoO4)3, CeVO4, Ce2(WO4)3 and Na2MoO4 were investigated under the same experimental conditions. As shown in Fig.\u00a05\n, The MoO4\n2- has the highest sulfur removal in these catalysts owning the same cation. Efficiencies change follows the sequence MoO4\n2-\u00a0>\u00a0VO4\n3-\u00a0>\u00a0WO4\n2-. Similarly, based on the fact that the sulfur removal performance of Ce2(MoO4)3 is much higher than that of Na2MoO4. It can conclude that Ce3+ also play a significant role in ODS. The above results indicate that the synergistic effect between Ce3+ and MoO4\n2- leads to higher sulfur removal.The reaction temperature is an important factor affecting the desulfurization rate. In AODS, the catalyst can show high sulfur removal at high temperature (>100\u00b0C). The oxidative desulfurization activity of Ce2(MoO4)3\u00a0at different reaction temperatures was shown the Fig.\u00a06\n, sulfur removal increases from 10.8% at 80\u00b0C to 99.6% at 100\u00b0C in 120\u00a0min. This is because increased the temperature can accelerate the collision rate between reactant molecules (Zhao et\u00a0al., 2007). When the reaction temperature was increased from 100\u00b0C to 110\u00b0C, the sulfur removal rate increased to 99.6% in 80min. However, high temperature will also cause side reactions of hydrocarbons oxidation and the increase of energy consumption (Eseva et\u00a0al., 2021). In conclusion, 100\u00b0C as the optimum reaction temperature.The catalyst dosage is one of the most important parameters for the industrialization of ODS. The effect of Ce2(MoO4)3 dosage on sulfur removal was investigated. The results are shown in Fig.\u00a07\n, catalyst dosage increased from 0.02g to 0.05g results in the sulfur removal of DBT increasing from 58.8% to 99.6%. Nevertheless, the catalyst dosage was increased from 0.05g to 0.06g, the sulfur removal decreased from 99.6% to 90.8%. The results show that increasing the amount of catalyst is beneficial to increasing the number of active sites (Qiu et\u00a0al., 2016). However, excessive catalyst will cause agglomeration and limit the contact area with DBT, which will affect the diffusion of reactants and products, so reduce the sulfur removal (Eseva et\u00a0al., 2021). Therefore, the optimal catalyst dosage of 0.05g was selected to oxidative desulfurization process.In this experiment, the Ce2(MoO4)3 has a high oxidation desulfurization activity on DBT in AODS. However, it is very necessary to study sulfur removal performance of catalysts for other sulfides such as BT and 4,6-DMDBT due to the diversity of sulfides in actual fuel. As shown in Fig.\u00a08\n, the removal rate of 4,6-DMDBT and BT are 94% and 26%, they are lower than that of DBT. According to the literature (Zhao et\u00a0al., 2017), the difference of removal efficiency can be attributed to the electron cloud density of sulfur atoms and steric hindrance effect. The methyl groups in 4,6-DMDBT would hinder process of oxidative desulfurization reaction, resulting in a lower removal rate of DBT. The higher electron cloud density results the higher the oxidation desulfurization capacity. The electron densities of the S atom in DBT, 4,6-DMDBT and BT are 5.758, 5.760 and 5.739, respectively (Mao et\u00a0al., 2017). Therefore, sulfur removal of DBT, 4,6-DMDBT and BT are affected by the steric hindrance and electron density (Otsuki et\u00a0al., 2000).In ODS process, the presence of olefins and aromatics will affect the removal of organic sulfide. Here, toluene and cyclohexene are selected as the models of aromatics and olefins, effects of their addition on sulfur removal are explored in Fig.\u00a09\n. Under optimal reaction conditions, when 5\u00a0wt%-toluene and 5\u00a0wt%-cyclohexene were added to 20\u00a0mL model oil, removal efficiency slightly decreased to 97.6% and 94.1%, respectively. It can be seen that both cyclohexene and toluene have a certain influence on the sulfur removal, which may be caused by the competitive reaction among toluene, cyclohexene and DBT (Liu et\u00a0al., 2021). The results show that the sulfur removal of DBT in the AODS system is less affected by olefins and aromatics.After the AODS reaction, the catalyst was filtered from the reaction system, and then washed with CCl4 under a magnetic stirrer at 25\u00b0C for 30min. Finally, the catalyst was dried at 80\u00a0\u00b0C for 8\u00a0h. Thereafter, the recovered Ce2(MoO4)3 catalyst was used in the next AODS under the optimal conditions. The exhibited results in Fig.\u00a010\n illustrates that the sulfur removal of DBT reduced to 94.7% after the five cycles. The recovered Ce2(MoO4)3 catalyst was analyzed by FT-IR analysis measurement. The FT-IR spectrum was shown in Fig.\u00a011\n. It can be seen that the fresh catalyst and the recovered catalyst have similar absorption peaks, Therefore, the catalyst has high stability.In order to investigate aerobic oxidation desulfurization mechanism, a free radical capture experiment was designed. P-benzoquinone (\u00b7O2\n- trapping agents) and isopropanol (\u00b7OH trapping agents) were added to AODS system, respectively. The experiment results are shown in Fig.\u00a012\n. Sulfur removal after adding isopropanol can reach 97.2%, while the sulfur removal after adding p-benzoquinone is only 12%, this indicates that a large number of superoxide radicals [\u00b7O2\n-] generated during the reaction are captured by p-benzoquinone, leads to reduce sulfur removal. The results show that [\u00b7O2\n-] radical is the intermediate activated product of oxidation reaction.After oxidative desulfurization reaction, The Ce2(MoO4)3 was washed by CCl4. The CCl4 solution was evaporated by rotary evaporator to obtain oxidation products of the sulfur compounds, FT-IR characterization results of oxidation products are shown in Fig.\u00a013\n(a). Two infrared absorption peaks at 1292 and 1166\u00a0cm-1, which correspond to the characteristic absorption peaks of dibenzothiophene sulfone (DBTO2). In addition, the obtained CCl4 solution was analyzed by GC-MS measurement. The results are shown in Fig.\u00a013 (b), the strong peak corresponding to DBTO2 (m/z\u00a0=\u00a0216.0) at 17.604\u00a0min was found. The FT-IR and GC-MS analysis proved that DBT was oxidized to DBTO2.In the research of aerobic oxidation of molybdenum-based catalysts (L\u00fc et\u00a0al., 2013; Ma et\u00a0al., 2020; Xun et\u00a0al., 2019), molybdenum sites have mixed valence states, the conversion between different valence states leads to the production of active molybdenum species. This experiment is different from these literatures. Through the XPS characterization, it can be seen that there is only Mo6+ in the Ce2(MoO4)3 catalyst. Mo peroxides may be formed in the presence of oxygen. This phenomenon is very common in the oxidative desulfurization system with molybdenum-based catalyst (Jiang et\u00a0al., 2019; Zhang et\u00a0al., 2019). Refer to previous research (Shi et\u00a0al., 2016, Zhang et\u00a0al., 2019), the mechanism of oxidative desulfurization was shown in Fig.\u00a014\n. First, oxygen molecules are adsorbed on the Ce3+ site of the catalyst to form Ce3+-O2, and some of the Ce3+-O2 further formation Ce4+-[\u00b7O2\n-] superoxide, then these Ce4+-[\u00b7O2\n-] interact with Mo sites on the catalyst to produce some active molybdenum species to further oxidize DBT compounds to DBTO2 (Shi et\u00a0al., 2016; Zhu et\u00a0al., 2007).The aerobic desulfurization performance of Ce2(MoO4)3 for diesel fuel with S-content of 150\u00a0\u03bcg/g was also investigated. It can be seen from Fig.\u00a015\n, under optimal conditions, the sulfur removal of 21.1% was obtained. Compared with the model oil, the sulfur removal is too low due to the complexity of components in actual diesel. After the AODS, 1\u00a0mL of acetonitrile was added to the AODS system to extraction of oxidation products of sulfur, removal of sulfur compounds from diesel was reached 74.8%. When extraction desulfurization in diesel fuel was carried out by acetonitrile as extractant, sulfur removal is only 8.2%. The experimental results show that the AODS-extraction desulfurization system under the action of Ce2(MoO4)3 can still remove most of the sulfide in diesel fuel.Ce2(MoO4)3 was synthesized by a simple reflux method using ammonium molybdate and cerium nitrate hexahydrate as raw materials. Ce2(MoO4)3 was used as the catalyst in AODS systems. Sulfur removal of 99.6% for DBT, 94% for 4,6-DMDBT and 26% for BT were obtained under the optimized conditions of 20\u00a0mL model oil, catalyst dosage of 0.05g, oxygen flow rate of 0.2L/min, 100\u00b0C, respectively. The introduction of both olefin and aromatic hydrocarbon cannot significantly change the oxidative desulfurization activity of the catalyst. The superoxide radical generated by oxygen under the action of catalyst is the key factor for the oxidation of sulfide. The Ce2(MoO4)3 has a strong regenerative capacity and the sulfur removal reached 94.7% after five cycles.The authors also acknowledge the financial support of the Natural Science Foundation of Liaoning Province (2019-ZD-0064); Doctoral Fund of Liaoning Province (201501105).", "descript": "\n                  Ce2(MoO4)3 was synthesized by a simple reflux method using cerium nitrate hexahydrate and ammonium molybdate as reactants. The as-prepared Ce2(MoO4)3 was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS). The removal of dibenzothiophene (DBT) in model oil was studied using Ce2(MoO4)3 as catalyst and oxygen as oxidant. The reaction factors such as reaction temperature, amount of catalyst, and sulfide type on sulfur removal were researched. The results prove that both Ce3+ and MoO4\n                     2- play significant role in the conversion from DBT to DBTO2. The Ce2(MoO4)3 catalyst has an excellent performance for the sulfur removal of DBT. Under the optimum reaction conditions, sulfur removal of 99.6% was obtained. After recycling five times, no significant loss in catalyst activity of Ce2(MoO4)3. Mechanism of aerobic oxidative desulfurization was proposed based on the experiment of free radical capture and infrared characterization.\n               "}
{"full_text": "Data will be made available on request.Power-to-gas processes such as the catalytic hydrogenation of CO2 to CH4 are promising strategies to store renewable energy and accommodate fluctuations in energy consumption and production. Efficient CO2 methanation requires the development of active, selective, and durable catalysts[1,2].Solid base metal oxides such as Al2O3, CeO2, TiO2, MgO, and SiO2 are the most applied supports in the methanation reaction as they help facilitate the adsorption of CO2, but a deep understanding of the role of the support, the active catalyst, and the reciprocity between them is still missing in most of the studies, therefore in this paper, we will specifically focus upon Ru and Ni supported on MgO.Studies have shown that different key factors of design catalysts, such as active metal, metal-support interaction, and promotors influence selectivity to methane [3]. Ni and Ru have been some of the most used metals dispersed on different solid supports with high surface area in CO2 methanation [4\u201311]. Although researchers have primarily focused on Ni-based catalysts due to their relatively low cost and availability. Noble metal catalysts such as Ru-based are prone to less carbon deposition and show higher catalytic performance at lower temperatures [12]. Notably, lowering the reaction temperature results in a thermodynamic hampering of CO formation and hindering the deactivation of catalysts caused by sintering [13\u201316]. The reaction is performed on large scales thereby, activity and selectivity improvements and a deeper understanding of the reaction and the associated catalysts can have a significant impact the on viability of the technology.There have been considerable investigation and discussion on the mechanism of CO2 methanation, known as the Sabatier reaction, and there are two possible pathways proposed[17\u201319]. The two proposed overall mechanisms include 1) direct conversion of CO2 to CH4 where formate species are the primary intermediates; 2) conversion of CO2 to CO via reverse water gas shift reaction followed by hydrogenation of CO to CH4\n[20].Multiple studies have been dedicated to investigating the underlying mechanism, which demonstrated that the initial reaction that occurs is the dissociative adsorption of CO2 to form adsorbed CO and O (CO2 \u2192 CO* + O*). It is found that the rate-limiting step is the cleavage of the C-O bond of the adsorbed species to adsorbed C and O (CO* \u2192 C* + O*). This dissociation can take place by either the H-assisted paths with formate or carbonyl hydride as intermediates or by direct dissociation of C-O to its components, C* and O*[21\u201327]. To complete the catalytic cycle, it is required that C* is hydrogenated by four dissociated H* and desorbed as CH4. Generally, the activity and selectivity of the catalyst are determined by the active metal bond strength to CO and H, which directly dictate the coverage of the surface. Hence, the support plays an integral part in dictating the bond strength of CO* and H* and can help facilitate the desired selectivity. For example, CeO2\u2212x can utilize the oxygen vacancies to directly associate CO2 to CO*\u00a0. In contrast, MgO reacts with CO2 to form MgOCO2 (or as a normal carbonate, MgCO3) and is hydrogenated by spillover hydrogen provided from Ru which could function as a bifunctional reaction mechanism[28]. However, the mechanism is still being investigated and discussed.Recently, J. Tan et al.[29] improved the catalytic activity of Ni/ZrO2 catalyst using MgO as a dopant to confine Ni active sites. However, the catalytic improvement was limited, and they demonstrated that MgO had no role in the intrinsic activity. Cimino et al.[30] promoted the Ru/Al2O3 catalyst by alkali metals as the base to enhance CO2 capture from flue gas and subsequent methane formation. The study demonstrated that CO2 capture capacity at room temperature improves in the alkali promoted catalysts which resulted in the most active catalyst for CO2 conversion giving site time yield (STY) of 444\u00a0molCH4 molRu\n\u22121 h\u22121 at temperatures of 375\u00a0\u00b0C. Therefore, we propose a catalytic system consisting of a MgO support with basic properties to enhance CO2 adsorption due to the basic-acid interaction and Ru as active sites. Furthermore, applying MgO as the support is demonstrated to reduce catalyst deactivation caused by sintering and carbon deposition[22\u201324]. To the best of our knowledge, there is no research that has specifically targeted catalytic activity of MgO-supported Ru catalyst for CO2 hydrogenation.In this paper, we show higher activity, methane selectivity, and stability have been achieved through developments of catalysts, including introducing novel synthesis methods, changing morphologies of the support, optimizing the metal dispersion, and enhancing metal-support interaction[29,31\u201335]. It is notable from the literature that researchers have been showing increasing interest in solid base metal oxide supports in different industries, such as methane to syngas, biomass to fuels, and CO2\n[36\u201344]. Specifically, we focused on synthesizing a high surface area nano MgO support to disperse active metal of Ru and Ni for catalytic conversion of CO2 to CH4. The synthesized materials were characterized by XRD, nitrogen physisorption, SEM/SEM-EDS, in-situ DRIFT, and TEM. For evaluating the catalytic performance of Ru-based catalysts, different loadings of Ru on high surface area MgO support were tested at different temperatures, and the optimum Ru catalyst was compared to its Ni-containing counterpart. The results show that 5\u00a0wt\u00a0% Ru/MgO catalyst results in the highest yield at 375\u00a0\u00b0C with some initial activity down to 250\u00a0\u00b0C. At 375\u00a0\u00b0C, the catalyst also showed high stability over 50\u00a0h on stream of conversion.First, oxalic acid (Sigma Aldrich, \u226599.5\u00a0%) was dissolved in distilled water and heated to the boiling point, followed by mixing it with bulk low surface area magnesium oxide (MgO) powder (Sigma Aldrich, 97\u00a0%) to precipitate magnesium oxalate (MgC2O4). The solid was separated by filtration, washed with distilled water, dried at 80\u00a0\u00b0C overnight, and then calcined at 500\u00a0\u00b0C (5\u00a0\u00b0C/min ramp) for 4\u00a0h yielding a high surface area MgO.An adequate amount of the as-synthesized MgO was taken and impregnated via incipient wetness impregnation method with an aqueous solution of either Ru(NH3)6Cl3 (Sigma Aldrich, 98\u00a0%) or Ni(NO3)2\u00b76\u00a0H2O (Sigma Aldrich, 98\u00a0%). Prior to characterization, the Ru and Ni-containing catalysts were reduced to metallic form under a constant flow of Formier gas (10\u00a0% H2 in N2) for 2\u00a0h at 450\u00a0\u00b0C and 500\u00a0\u00b0C (5\u00a0\u00b0C/min ramp), respectively. Three different Ru concentrations of 3, 5 and 7\u00a0wt\u00a0% supported on MgO were named as 3Ru/MgO, 5Ru/MgO and 7Ru/MgO, respectively. Similarly, 5\u00a0wt\u00a0% Ni on MgO was named as 5Ni/MgO.N2 physisorption was performed at 77\u00a0K on a Micromeritics 3Flex surface area and porosimetry analyzer. Samples were outgassed under vacuum at 400\u00a0\u00b0C overnight before measurement. The specific surface area (SBET) was calculated from the N2 adsorption data by the BET method in the relative pressure range of 0.05\u20130.3 (P/P0). Micropore volumes (Vmicro) and total pore volumes (Vtotal) were determined using the t-plot method and from a single-point read at a relative pressure of P/P0 =\u00a00.95, respectively.The particle sizes and morphologies were investigated by scanning electron microscopy (SEM) using a Quanta 200 ESEM FEG operated at 20\u00a0kV and by transmission electron microscopy (TEM) on a FEI Tecnai T20 G2 microscope operated at 200\u00a0kV. All samples were coated with gold for 1\u00a0min under 20\u00a0mA current prior to SEM or dispersed directly on a holey carbon grid for the TEM analysis. Energy-dispersive X-ray spectroscopy (EDS) was performed using SEM-EDS elemental mapping by studying the sample with electron scanning microscope (Quanta 200 ESEM FEG) operated at 20\u00a0keV and equipped with an Oxford Instruments X-Max 50\u00a0mm2 EDS analyzer using Aztec 3.3 Service Pack 1 software for data analysis.All synthesized catalysts were characterized with powder X-ray diffraction at ambient atmosphere and temperature with a HUBER G670 Guinier camera in transmission mode using a CuK\u03b1 radiation from a focusing quartz monochromator. The data was recorded from 2\u03b8 of 5\u201390\u00b0 over 1\u00a0h.Hydrogen temperature programmed reduction (H2-TPR) was performed upon the incipient wetness impregnated sample and carbon dioxide temperature programmed desorption (CO2-TPD) were performed upon the reduced catalysts and was carried out on a Micromeritics AutoChem II 2920 chemisorption analyzer to study the reducibility of MgO-supported metal catalysts and basicity of MgO, respectively. For H2-TPR analysis, the samples were heated under 5\u00a0% H2 in He to 600\u00a0\u00b0C with heating ramp of 5\u00a0\u00b0C/min while recording the TCD signal. CO2-TPD was carried out by first heating the sample to 500\u00a0\u00b0C for 60\u00a0min under He atmosphere, followed by treating the sample with pure CO2 flow at 40\u00a0\u00b0C for 30\u00a0min. The last step was heating the sample under He to 500\u00a0\u00b0C (5\u00a0\u00b0C/min ramp) to desorb CO2 while recording the TCD signal.\nIn-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFT) was carried out using Thermo Nicolet 6700 FTIR equipped with a reactor and Praying Mantis diffuse reflectance accessory from Harrick Scientific Products. The experiments were carried out by reducing the sample in a constant flow of Formier gas followed by cooling it to room temperature in Formier gas (10\u00a0% H2 in N2). Hereafter, the mercury cadmium telluride (MCT) detector was cooled to 77\u00a0K, and the atmosphere was changed to N2 and heated to 400\u00a0\u00b0C for at least 30\u00a0min, and a background spectrum was recorded. Hereafter the atmosphere changed to 25\u00a0ml/min CO2, and spectra were recorded every third minute for an hour. After an hour, the atmosphere was changed to 25\u00a0ml/min of Formier gas and spectra were recorded every third minute for an hour. Lastly, the atmosphere was changed to reaction mixture consisting of a flow of 80\u00a0ml/min Formier gas (10\u00a0% H2 in N2) and 2\u00a0ml/min CO2.CO2 methanation reaction was carried out in a stainless steel fixed-bed reactor with a diameter of 5.1\u00a0mm (PID Eng&Tech, Microactivity Effi reactor). The set-up was equipped with a thermocouple in contact with the catalyst bed to control the reaction temperature, an automatic liquid-gas separator and mass flow controllers for N2, CO2 and H2. The reactor was loaded with 100\u00a0mg of catalyst powder (fraction size 180\u2013355\u00a0\u00b5m) diluted with 600\u00a0mg quartz (fraction size 180\u2013355\u00a0\u00b5m) and fixed with quartz wool. The catalysts were then reduced at 450\u00a0\u00b0C or 500\u00a0\u00b0C (in case of Ni supported catalyst) for 2\u00a0h with a heating ramp of 5\u00a0\u00b0C/min under mix flow of 5\u00a0ml/min H2 and 45\u00a0ml/min N2. The catalytic tests were performed at atmospheric pressure and temperature range of 200\u2013500\u00a0\u00b0C using gas composition of 80\u00a0ml/min H2, 20\u00a0ml/min CO2, and 20\u00a0ml/min N2 corresponding to gas hourly space velocity (GHSV) of 60000\u00a0ml/gcatalyst/h. The system was maintained for 60\u00a0min at each temperature setpoint to reach a steady state. The reaction products were analyzed with an online GC (Agilent 7820\u00a0A) equipped with a TCD and FID detector. The experimental error was calculated \u00b1\u00a05\u00a0%.\n\nFig. 1 shows the adsorption-desorption isotherm for the synthesized MgO sample that exhibits a typical IV(a) isotherm[45]. This isotherm has a H3 hysteresis loop, which may originate from the interparticle voids between aggregated nanoparticles of MgO. The BJH analysis of desorption isotherm for the pore size distribution (PSD) shows a broad peak at around 8\u00a0nm. In addition, Table S1. compares textural properties of the synthesized sample and bulk low surface area MgO, which reports considerably higher specific surface area (SBET) and total pore volume (Vtot) for the synthesized MgO sample. Therefore, the results confirm the successful synthesis of a support with high surface. The surface area and pore volume have been increased tenfold by the synthesis, see table S1, where the bulk MgO has a surface area of 22\u00a0m2/g and a pore volume of 0.06\u00a0cm3/g and the nano MgO has a surface area of 200\u00a0m2/g and pore volume of 0.06\u00a0cm3/g.Interestingly, it was demonstrated in previous reports[46,47] that size and shape of MgO crystals are important in the population of basic sites, which results in different activity and selectivity in catalysis. Specifically, Coluccia et al.\n[48] proposed that surface defects in MgO provide under-coordinated O2- atoms that act as strong basic sites. Therefore, we performed CO2-TPD to compare the CO2 adsorption capacity of synthesized and bulk MgO samples.As expected, the XRD analysis of the prepared support showed the typical diffraction pattern for pure FCC MgO[49], see \nFig. 2. In the diffractogram for the catalysts containing 3 and 5\u2009wt\u00a0% Ru, no Ru could be observed due to the low loading. But at the 7\u2009wt\u00a0% Ru/MgO, metallic Ru could be observed, which goes along with the JCPDS card. These Ru peaks are located at 2\u03b8 of 38.4, 44.0, 58.2, and 69.6\u00b0, corresponding to planes (100), (101), (102), and (110), respectively[50]. For the corresponding 5Ni/MgO catalyst, the most intense peak from Ni(111) at around 2\u03b8 of 43.5\u00b0 overlaps with the MgO(200) peak at 43.1\u00b0. The average particle size of MgO was calculated to be about 32\u2009nm using the Scherrer equation[51] and FWHM for the MgO peak position at 2\u03b8=\u200943.1\u00b0.We investigated the morphology of the high surface area MgO nanoparticles by electron microscopy. The SEM images in Fig. S1 show a rough surface of the relatively large MgO particles. At larger magnification, the SEM analysis confirms that these large MgO particles consist of small, agglomerated nanoparticles. Moreover, SEM-EDS results for 5Ru/MgO show a high distribution of Ru on the support (\nFig. 3e). SEM-EDS of 3Ru/MgO and 7Ru/MgO alongside with 5Ni/MgO samples are provided in Fig. S2 which also show a high dispersion of metals on these catalysts. As the active metals have been impregnated via incipient wetness impregnation, it was expected to have a high dispersion of metal particles. The small particle size calculated with the Scherrer equation supports the good dispersion of metal particles.The TEM images in Fig. 3a) and Fig. 3b) confirm the results from the SEM analysis and show that distinct MgO nanoparticles are agglomerated in larger particles. In addition, atomic fringes are present in Fig. 3b), which is another evidence and confirmation of the crystallinity of synthesized MgO material. The average particle size as measured from 150 particles in the TEM was around 35\u2009nm, which is in good agreement with the particle size of MgO as estimated from the XRD analysis using the Scherrer equation (32\u2009nm).\nFig. 3c exhibits the high dispersion of Ru nanoparticles on 5Ru/MgO after reduction at 450\u2009\u00b0C for 2\u2009h. The histogram indicates that most Ru nanoparticles are between 2 and 4\u2009nm in size (Fig. 3d). The average particle sizes of Ru in 3Ru/MgO and 7Ru/MgO are 2\u20134 and 4\u20136\u2009nm, respectively. The corresponding particle size histograms are given in the supporting information (see Fig. S3).Results in \nFig. 4a show that synthesized MgO has a larger CO2 adsorption capacity which could be correlated to its higher surface area and population of basic sites as reported previously[52]. Furthermore, different desorption temperatures indicate different strength of CO2 bond to MgO. This observation determines that CO2 is adsorbed partially at temperatures below 180\u2009\u00b0C in the synthesized MgO which is likely favorable for the catalytic conversion of CO2 at low temperatures.\nFig. 4b-e shows the H2-TPR results of the fresh catalysts after incipient wetness impregnation with the metal precursors and drying at 80\u2009\u00b0C for >\u200924\u2009h. The three samples with increasing Ru loadings (Fig. 4b-d) share two main peaks around 220\u2009\u00b0C and 305\u2009\u00b0C. We assign these peaks to the stepwise reduction of Ru3+ to Ru0\n[53,54]. Some differences in the reduction profiles indicate some complex speciation related to the precursor loading. Nevertheless, all samples were fully reduced at temperatures above 425\u2009\u00b0C. Based on these results, we decided to reduce all the Ru-based catalysts at 450\u2009\u00b0C. Fig. 4e) shows that the complete reduction of the Ni-based catalyst occurs at around 366\u2009\u00b0C corresponding to Ni2+ \u2192 Ni0\n[55].\n\nFig. 5 shows the catalytic performance of different synthesized catalysts tested for conversion of CO2 to CH4 at temperature range of 200\u2013500\u2009\u00b0C and GHSV of 60000\u2009h\u22121. As expected, the conversion in all catalysts increases with the temperature until it gets limited by thermodynamic equilibrium. Both 5Ni/MgO and 7Ru/MgO have some activity at 275\u2009\u00b0C while 3Ru/MgO needs to be at 300\u2009\u00b0C to activate. Impressively, the 5Ru/MgO catalyst already converts CO2 to CH4 at temperatures down to 250\u2009\u00b0C. The catalysts also achieved the highest conversion and selectivity at different temperatures. Among the Ru-containing catalysts, the 5Ru/MgO catalyst achieved the highest yield of CH4 (54\u00a0% conversion and 98\u00a0% selectivity) at 375\u2009\u00b0C. This may be explained by the higher metal loading compared to 3Ru/MgO and the higher metal dispersion compared to 7Ru/MgO. The catalytic tests also show that 5Ru/MgO is more active and selective than 5Ni/MgO. The 5Ni/MgO catalyst resulted in 45\u00a0% conversion and 95\u00a0% selectivity at 450\u2009\u00b0C. Under the given reaction conditions, this corresponds to a site time yield (STY) of 263\u2009molCH4 molNi\n\u22121 h\u22121. For comparison, the 5Ru/MgO catalysts resulted in a STY of 520\u2009molCH4 molmetal\n\u22121 h\u22121.Further catalytic test results show that catalytic conversion of CO2 to CH4 is impossible over pure MgO supports (Fig. S4), which suggests that metal active sites are required to boost activity and selectivity. Furthermore, higher catalytic performance of 5Ru/MgO compared to 5\u2009wt\u00a0% Ru on bulk MgO with low surface area (Fig. S5) demonstrates the important role of high surface area in synthesized MgO from two perspectives. Firstly, the higher surface area of MgO provides higher metal dispersion and smaller metal particle size that result in improved catalytic performance as already discussed. Secondly, the population of basic sites is higher in high surface area MgO as confirmed from CO2-TPD, which persuades higher CO2 adsorption and activation.\nTable S2 in the supporting information compiles the performance of the synthesized catalysts in this work and recently reported Ni and Ru-based catalysts tested under similar catalytic conditions. These data show that the 5Ru/MgO catalyst presented here has higher STY compared to the recently reported catalysts [5,8,29,30].\nIn-situ DRIFTS studies were conducted to further study the difference between catalytic performances of Ru on bulk and Ru on synthesized nano MgO supports by mapping the species present at the surface of MgO supports under different gas compositions. \nFig. 6a and Fig. 6c present results for Ru supported on bulk MgO and nano MgO, respectively, at 400\u2009\u00b0C under 100\u00a0% CO2 over 60\u2009min. Comparing the two spectra in Fig. 6a and Fig. 6c, there are certain similarities in the spectra range of 1025\u20131100\u2009cm\u22121, which include the peaks that are assigned to monodentate carbonates, the peaks located at 1300\u20131600\u2009cm\u22121 assigned to magnesium carbonate, MgCO3, and additionally the peak at 2078\u2009cm\u22121 designated to the Ru-CO*\u2009carbonyl peak. The presence of the surface carbonyl peaks indicates dissociative addition mechanism of CO2 on CO. The peak at 2094\u2009cm\u22121 has previously been attributed as COad which is at a ruthenium-oxide interface, or CO which is co-adsorbed with Oad. Interestingly, the peak located at 1980\u2009cm\u22121, which is likely a COad species is either adsorbed to Ru with lower coordination, very small Ru nanoparticles, or MgO [56,57]. Furthermore, the concentration of the peak at 1980\u2009cm\u22121 decreases slowly and steadily with time for the nano MgO (Fig. 6c), whereas in the case of bulk MgO (Fig. 6a) the peak only appears in the beginning[56,58\u201360]. The peak which solely appears on nano MgO is a shoulder at 1681\u2009cm\u22121, which is assigned to a bidentate carbonate. A decrease in signal at 1862\u2009cm\u22121 is seen when Ru on nano MgO is exposed to CO2, and this indicates potential catalyst oxidation, which may be a result of an interaction between the lattice O and the ruthenium nanoparticles [56].In Fig. 6b) and d), 10\u00a0% H2 in N2 is introduced and there is an immediate large decrease in the concentration for all the surface species and clear formation of methane (peak range of 2900\u20133100\u2009cm\u22121). Methane formation stops after 4\u2009min for both nano MgO and bulk MgO. Noticeably, the peak occurring at around 1980\u2009cm\u22121 decreases at the same rate as the methane formation for both samples. Furthermore, the Ru-CO* peaks at 2094\u2009cm\u22121 disappear immediately. On the other hand, the carbonate species located at 1300\u20131600\u2009cm\u22121 decrease slowly and this diminish is faster in nano MgO sample (Fig. 6d) than in bulk MgO (Fig. 6b). This observation is likely an indication of more readily available carbonates due to the smaller MgO particle size [61,62]. In general, the IR-study shows that the main catalytic pathway follows the bifunctional mechanism as suggested by McFarland et al.[28]. We assign the peaks between 1300 and 1600\u2009cm\u22121 to the formation of large amounts of MgCO3 on the catalyst's surface.Under H2, the decrease in MgCO3 and transient increase in adsorbed carbonyl species between 1980 and 2100\u2009cm\u22121 indicate that the carbonates are involved in the reduction of CO2 to CO intermediates. Finally, these intermediates are hydrogenated into the methylene groups that appear as a small shoulder at 1300\u20131305\u2009cm\u22121 before further reduction and methane desorption.Under reaction conditions with both CO2 and H2, Fig. S6, shows clear peaks from MgCO3 at 1300\u20131600\u2009cm\u22121, the CO intermediates at 1980\u2009cm\u22121, and the methyl intermediates at 1305\u2009cm\u22121. In contrast, the peaks from Ru-CO* between 2078 and 2094\u2009cm\u22121 disappear. This indicates that the Ru species are short-lived [24] and that the conversion of the carbonyl intermediate may be the rate-determining step[63].We reproduced the results by repeating the test on 5Ru/MgO fresh catalyst, which confirms the repeatability of obtained results for 5Ru/MgO (Fig. S7). In addition, we investigated the stability of 5Ru/MgO and 5Ni/MgO for 50\u2009h at 375\u2009\u00b0C and 450\u2009\u00b0C, respectively, with GHSV of 60,000\u2009h\u22121. \nFig. 7 shows high stability of 5Ru/MgO catalyst over 50\u2009h time on stream compared to 5Ni/MgO catalyst, which suffers from deactivation and a decrease of both the conversion and selectivity. Since 5Ru/MgO catalyst performs better at a lower temperature, the active sites are less prone to sintering and deactivation, while Ni containing catalyst requires higher performing temperature that leads to faster deactivation. Thus, we tested the 5Ni/MgO catalyst at low temperature of 375\u2009\u00b0C for comparison. Moreover, the GHSV was decreased to 7500\u2009h\u22121 to achieve almost as high conversion as for 5Ru/MgO at the same temperature. As Fig. S8 presents, both catalysts exhibit high stability and selectivity toward CH4 at 375\u2009\u00b0C. However, the resulted STY for 5Ni/MgO is about 30\u2009molCH4 molmetal\n\u22121 h\u22121 which is significantly lower than of for 5Ru/MgO (520\u2009molCH4 molmetal\n\u22121 h\u22121) performed at the same temperature. Overall, 5Ru/MgO catalyst seems to be the best catalyst with the highest catalytic performance amongst the tested catalysts for CO2 hydrogenation to CH4. The samples were also characterized after the stability test. The XRD of the spent sample (Fig. S10), does not show any significant change, however, the isotherms and pore size distribution changed significantly (Fig. S11). The isotherm of 5Ru/MgO appears similar to the fresh sample. The pore volume has decreased due to the impregnation of Ru. However, the 5Ni/MgO has changed significantly, though the composition hasn\u2019t changed. This could be a sign of the pore structure is not stable at the higher temperature, which is required for the Ni catalyzed methanation.This work introduced a simple method to synthesize high surface area nano MgO with high crystallinity. The high surface area MgO was used to support highly dispersed Ru and Ni nanoparticles and tested for CO2 methanation. TEM and SEM-EDS showed high dispersion of 5\u2009wt\u00a0% Ru on high surface area MgO support. The CO2-TPD results showed that high surface area MgO has higher CO2 adsorption capacity than low surface area MgO. The catalytic results show that the catalysts are highly active, and the 5\u2009wt\u00a0% loading of Ru on MgO catalyst has the highest activity at lower temperatures resulting in STY of 520\u2009molCH4 molmetal\n\u22121 h\u22121 at 375\u2009\u00b0C. This catalyst outperformed Ni-containing MgO catalyst (STY of 30\u2009molCH4 molmetal\n\u22121 h\u22121 at 375\u2009\u00b0C) even though both show high catalytic stability at 375\u2009\u00b0C.Our FT-IR study shows that both MgO and Ru play a decisive role in the reaction mechanism and indicates that the catalyst facilitates the bifunctional reaction mechanism. Furthermore, the study suggests that the conversion of carbonyl intermediates is the rate-determining step.Based on this, we believe that the introduced synthesis method is a facile approach to make high surface area MgO, which is an efficient support to disperse active metal sites in addition to chemisorbing and activating CO2 for catalytic conversion of CO2 to CH4 process.\nFarnoosh Goodazi: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing \u2013 original draft, Writing \u2013 review & editing, Visualization. Mikkel Kock: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing \u2013 original draft, Writing \u2013 review & editing, Visualization Jerrik Mielby: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Writing \u2013 original draft, Writing \u2013 review & editing. Visualization, Supervision. S\u00f8ren Kegn\u00e6s: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Writing \u2013 original draft, 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 supported by Independent Research Fund Denmark (grant no. 6111-00237 and 0217-00146B), Haldor Tops\u00f8e A/S, Villum fonden (Grant No. 13158) and the European Union\u2019s Horizon 2020 research and innovation program under grant agreement No 872102. The authors gratefully acknowledge the Department of Chemistry, Technical University of Denmark (DTU) for the support of the project.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2023.102396.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  This work shows that Ru nanoparticles supported on high surface area nano MgO is a highly active and selective catalyst for CO2 methanation, which is a promising method to store renewable energy and limit the emission of greenhouse gasses. We studied the effect of the Ru loading on MgO supports with different surface areas and compared the results to the corresponding Ni-based catalyst. Our results show that high surface area MgO containing 5\u00a0wt\u00a0% Ru has the highest activity. This catalyst was stable for more than 50\u00a0h and resulted in 54\u00a0% conversion at 375\u00a0\u00b0C, which, under the given reaction conditions, corresponds to a site time yield of 520\u00a0molCH4 molRu\n                     \u22121 h\u22121. For comparison, the Ni-based catalyst only resulted in 45\u00a0% conversion at 450\u00a0\u00b0C with a low selectivity to CH4 (STY=263\u00a0molCH4 molNi\n                     \u22121 h\u22121). Furthermore, Ru on high surface area MgO catalyst was already active at low temperature of 250\u00a0\u00b0C due to chemisorption and activation of CO2 on the MgO support, which is promising for low-temperature CO2 methanation.\n               "}
{"full_text": "Renewable energy sources, that minimize environmental impact, have been the basis of several studies in the last few years to find aiming an alternative to substitute the use of fossil fuels. Low-cost materials have been used for biofuel production, which contribute to reducing the damage caused by fossil fuels (Sammah and Ghiaci, 2018). Confirming this trend, the global biodiesel production in 2020 was estimated at around 36.9 million metric tons (Huang et al., 2020). Furthermore several studies have shown a market recovery in 2021, reaching the average production of 46 billion liters between 2023 and 2025 (International Energy Agency, 2020).Biodiesel is a sustainable alternative to the demand of diesel usage which enables of decrease pollutants emission such as CO2 (Mostafa and El-Gendy, 2017). This product can be classified as first-generation (derived from edible vegetable oils), second-generation (non-edible raw material or waste), and third-generation (algae biomass) biodiesel (Ramos et al., 2019). Some studies have been developed to optimize the synthesis process of biofuel, which normally proceeds by triacylglyceride transesterification with methanol or ethanol in the presence of catalysts that may have basic (NaOH, KOH, CH3ONa and CH3OK) or acid (H2SO4, H3PO4, HCl, and R \u2500 SO3H) characteristics. However, these catalysts are more difficult to be separated and reused, in addition to being able to cause secondary reactions such as saponification (Farias et al., 2020; Manr\u00edquez-Ram\u00edrez et al., 2013).In this context, heterogeneous catalysts have been applied for the biodiesel production, because they minimize the problems arising from the use of homogeneous catalysts, enabling a more efficient purification process (Luo et al., 2017; Niju et al., 2016). Several feedstock have been used in this catalysts production, such as plant and animal-derived compounds, metallurgical/mining industrial residues and natural clays. (Rizwanul Fattah et al., 2020). A few heterogeneous catalysts used to produce biodiesel are sodium silicate for soybean biodiesel (Guo et al., 2012), silica coated on Fe3O4 magnetic nanoparticles (Thangara et al., 2019), ground alkaline metals for palm biodiesel (Salamatinia et al., 2013), K2O/NaX and Na2O/NaX for Safflower biodiesel (Muci\u00f1o et al., 2014), and alkaline metal catalyst (Li, Na and K) supported on rice husk silica for WCO biodiesel (Hindryawati et al., 2014).Search for greater economic viability and reduction in the environmental impact of the biodiesel production process has motivated the usage of waste biomass (Gollakota et al., 2019). Thereby, the WCO may be a good choice since it is cheaper than the refined oil (Vela et al., 2020), non edible and it can reduce the environmental impact caused by its inappropriate discarding (Gollakota et al., 2019), as around 5 million tons of refined oil are consumed per year around the world (Tan et al., 2019). Meanwhile, WCO normally presents relatively high free fatty acids that require pretreatment by esterification, before the transesterification, in order to avoid the soap formation (Ding et al., 2012).This work contributes to the development of novel heterogeneous catalysts (SPS, sodium potassium silicates) for producing biodiesel from WCO. Moreover, the alternative MPI silica was applied for the catalysts production after its modification with alkali hydroxides. Despite being a waste material, there was no need of WCO pre-treatment (acid esterification) and the direct transesterification was accomplished using SPS. In this process, it was possible to use the catalyst for five reaction cycles. The use of waste raw material (WCO) for biodiesel production and a natural source (MPI silica) for the catalyst preparation are relevant factors that ensure low-cost and environmentally friendly biodiesel production process.Waste cooking soybean oil was obtained from Brazilian restaurant. The other reagents were: Silica beach sand (MPI silica); Hydrochloric acid (HCl, Synth, 36.5%); Sodium hydroxide (NaOH, Vetec, 99%); Potassium hydroxide (KOH, Synth PA); Ethanol (C2H5OH, Dynamic PA, 96%); Sodium chloride (NaCl, Dynamic, 99%); Methanol (CH3OH, Vetec, 99.8%); Sodium sulfate (Na2SO4, Vetec, 99%); Benzoic acid (C7H6O2, Dynamic, 99.5%); Phenolphthalein (C20H14O4, Vetec PA ACS) and Distilled water. The chemicals obtained were used as received.Amorphous silica MPI was synthesized from beach sand using a methodology developed in this research group and described by de Carvalho et al. (2015). In this work, the SPS catalysts were obtained by calcination of mixtures containing alkaline hydroxides (NaOH and KOH) and MPI silica, at 450\u00a0\u00b0C in a muffle furnace for 4\u00a0h. Different molar ratios of NaOH:KOH:MPI silica were used (1:1:1; 2:1:1; 1:2:1, and 1:1:2), to produce the catalysts named SPS, SPS-1, SPS-2, and SPS-3 respectively. The obtained catalysts were applied in preliminary tests for the biodiesel production, using the following synthesis conditions: 3.5% (w/w, catalyst/WCO), 9:1 (molar ratio A/O), 2\u00a0h, and 70\u00a0\u00b0C.The catalysts were characterized by various techniques. X-ray diffraction analyses (XRD) were performed using a Bruker D2 Phaser device (Bruker AXS, Madison, WI, USA) with CuK\u03b1 radiation (\u03bb\u00a0=\u00a01.5406\u00a0\u00c5), 30\u00a0kV filament, 10\u00a0mA current, Ni filter and a LYNXEYE detector in the range from (2\u03b8) 5 to 50\u00b0 for MPI silica, and 5 to 70\u00b0 for the catalysts. Elemental analysis was conducted on a Bruker S2 Ranger X-ray fluorescence (XRF) Spectrometer (Bruker AXS, Madison, WI, USA) using Pd or Ag radiation (max. power 50\u00a0W, max. voltage 50\u00a0kV, max. current 2\u00a0mA, XFlash\u00ae Silicon Drift Detector). Fourier transform infrared spectroscopy (FTIR) was accomplished in a Shimadzu IRAffinity-1 spectrometer (Columbia, MD, USA) with attenuated total reflectance (ATR). Spectrum analysis variation was in the range of 600\u20134000\u00a0cm\u22121 with a resolution of 4\u00a0cm\u22121 and 32 scans. The thermal analysis was performed in a thermo microbalance (TG-209-F1-Libra, Netzsch, Selb, Germany) using an alumina crucible for measuring 10\u00a0mg of the samples with a continuous heating rate of 10\u00a0\u00b0C\u00a0min\u22121 in nitrogen (N2(g)) purge gas at a flow rate of 20\u00a0mL\u00a0min\u22121.The morphologies and chemical composition of catalysts were obtained using a field emission scanning electron microscope (FESEM, Auriga, Carl Zeiss, Oberkochen, BW, Germany) and energy dispersive X-ray spectroscopy (EDX, XFlash Detector 410-M, Madison, WI, USA), respectively.The analysis of CO2 desorption at programmed temperature (CO2-TPD) consisted of weighing a 200\u00a0mg of the SPS catalyst. Next, the pre-treatment was performed with heating at 200\u00a0\u00b0C under a N2 flow of 16\u00a0mL\u00a0min\u22121 for 1\u00a0h. At the end of this period, the temperature was reduced to 60\u00a0\u00b0C and the CO2 flow (16\u00a0mL\u00a0min\u22121) was inserted into the reaction line to start the adsorptive process for 30\u00a0min. The analysis was subsequently started in a He (g) atmosphere in the temperature range of 40\u00a0\u00b0C to 500\u00a0\u00b0C with a heating rate of 10\u00a0\u00b0C\u00a0min\u22121 min under He (g) flow (16\u00a0mL\u00a0min\u22121). The desorbed CO2 was then quantified by a thermal conductivity detector.The deconvolution methodology was applied to the CO2-TPD, and TG/DTG of catalyst and biodiesel using PeakFit 4.12 software (Systat Software, Inc., Berkshire, UK) by applying Gauss\u00a0+\u00a0Lorentz for better curve adjustment, Savitzky-Golay smoothing filter (<20%) and linear baseline.Hammett\u2019s basicity test was performed using the titration method with acid-base indicators. A phenolphthalein indicator (H_= 9.3) and a 0.01\u00a0mol/L methanolic benzoic acid solution were used as the titrant for the experimental procedure. The Hammett basicity test consisted of stirring 0.15\u00a0g of the catalyst with 2\u00a0mL of methanolic indicator solution at a concentration of 0.1\u00a0mg\u00a0mL\u22121 for 30\u00a0min at 300\u00a0rpm. The obtained data were then applied in Eq. (S1) and (S2) (all figures, tables and equations indicated with S are in the supplementary material) to calculate the number of basic sites from the basicity calculation.Raman spectra were obtained using a confocal Raman microscope (LabHAM HR Evolution, HORIBA Scientific), with laser wavelength of 532\u00a0nm, grade: 600 gr mm\u22121, laser power of 1% and scanning range 400\u20134000\u00a0cm\u22121. N2 adsorption\u2013desorption isotherms at 77\u00a0K were used to determine the textural parameters of the SPS catalyst in a Micromeritics ASAP 2020 apparatus (Norcross, GA, USA). The specific surface area (SBET) was calculated using the Brunauer \u2013 Emmett \u2013 Teller (BET) equation.Biodiesel was synthesized in three consecutive stages: (i) previous filtration of the WCO in a separating funnel to remove impurities; (ii) transesterification reaction at 70\u00a0\u00b0C with methanol and catalyst in a reflux reactor; and (iii) purification according to the methodology described in the literature (Fernandes et al., 2012), with adaptations. The time, catalyst concentration and molar ratio parameters were varied for synthesis optimization.Hydrogen Nuclear Magnetic Resonance (NMR 1H) and Carbon-13 (NMR 13C) one-dimensional analyses were obtained by a Bruker Avance III HD NMR SPECT. 300 Spectrometer operating at frequencies of 300.13\u00a0MHz for hydrogen (1H) and 75.47\u00a0MHz for carbon (13C) respectively. The WCO and biodiesel were dissolved in deuterated chloroform (CDCl3) in the proportion of 20\u00a0mg of sample to 0.5\u00a0mL of solvent. The chemical shifts (\u03b4) were expressed in parts per million (ppm) and Tetramethylsilane (TMS) was used as an internal standard. Eq. (S3) was used to calculate the conversion of esters (Gohain et al., 2017).The thermal analysis (TG/DTG) was accomplished in a thermo microbalance (TG-209-F1-Libra, Netzsch, Selb, Germany) using nitrogen (N2(g)) as purge gas at a flow rate of 20\u00a0mL\u00a0min\u22121, alumina crucible, heating rate of 10\u00a0\u00b0C\u00a0min\u22121, 10\u00a0mg of sample and final temperature was 600\u00a0\u00b0C. The deconvolution was performed using the Peakfit v.4.12 software.The physical\u2013chemical properties were obtained under conditions for density at 20\u00a0\u00b0C and kinematic viscosity at 40\u00a0\u00b0C according to the American Society for Testing and Materials (ASTM) D4052 and D7042, respectively. The yields of the transesterification reactions were calculated using Eq. (S4) (Yang et al., 2016). The acidity index was obtained applying Eq. (S5) (AOCS, 2009).The regeneration was performed in three stages: (1) washing with mixture of hexane and ethanol (50\u00a0mL, 1:1 v/v) under stirring for 1\u00a0h, (2) soaking in a new mixture of hexane and ethanol (50\u00a0mL, 1:1 v/v) for 4\u00a0h to remove excess glycerol and waste cooking oil that could remain on its surface and (3) oven drying for 2\u00a0h at 150\u00a0\u00b0C. This procedure was repeated for five cycles.The overall methodology employed in this research is presented in a simplified flow diagram depicted in Fig. 1\n.The catalysts syntheses were performed with different proportions of MPI silica, NaOH, and KOH in order to evaluate the reaction yield under the reaction conditions described in Table S1. (S indicates supplementary material). The selected SPS catalyst was obtained with lower proportion (1:1:1) of the reagents, since the others did not significantly affect the reaction yield for WCO biodiesel production, Table S2. However, the concentrations of the species obtained by XRF that correspond to the oxides of the components showed differences related to the feed molar ratio (Do Nascimento-Dias et al., 2017), which probably occurred due to the utilization of sodium and potassium hydroxides that reacted with silica forming the metal silicates and producing the catalyst, with the possibility of varying the concentrations. In addition, it was possible to verify that pure silica did not have the ability to catalyse the reaction, indicating the need to modify its structure and composition.The XRD pattern for MPI silica (Fig. 2\na) exhibited a broad peak centered at 2\u03b8 angle 22.8\u00b0, characteristic of an amorphous silica structure with the presence of short-range order in atomic clusters (Salakhum et al., 2018; Stanishevsky and Tchernov, 2019), the crystalline plane phases of the SPS catalyst corresponding to Na2O (2\u03b8\u00a0=\u00a032.23\u00b0, ICSD 060435), KCl (2\u03b8\u00a0=\u00a040.72\u00b0, ICSD 044281), K2(Si2O5) (2\u03b8\u00a0=\u00a028.57\u00b0, 31.77\u00b0 and 58.87\u00b0, ICSD 280480), K2O (2\u03b8\u00a0=\u00a041.50\u00b0 and 66.53\u00b0, ICSD 060489), SiO2 (2\u03b8\u00a0=\u00a013.10\u00b0 and 25.87\u00b0, ICSD 065497), K2CO3 (2\u03b8\u00a0=\u00a029.87\u00b0, 31.75\u00b0, 31.77\u00b0 and 66.47\u00b0, ICSD 000662) and Na2(Si2O5) (2\u03b8\u00a0=\u00a032.80\u00b0 and 50.47\u00b0, ICSD 080378) were shown in Fig. 2b indicating that the modification of the MPI silica to obtain catalytic sites was successful.The XRF results for the SPS catalyst and MPI silica were described in Table\u00a01\nand allowed to confirm the elements presence of found in the XRD phases, revealing the presence of alkali oxides as the main components of the produced SPS. These compounds have the ability to promote the catalytic activity of the transesterification reaction (Chouhan and Sarma, 2013), since mixed metal oxides are present as an interesting class of solid heterogeneous catalysts, allowing the association of the various oxide phases that promote appropriate characteristics for the reaction process (Lee et al., 2016).FTIR spectra of MPI silica and SPS were presented in Fig. S1 and Table S3. For both materials the band at 1417\u00a0cm\u22121 indicates the existence of Na2CO3 formed by the reaction of sodium hydroxide with atmospheric CO2 (Belmokhtar et al., 2016; Simanjuntak et al., 2014). The broadband in the region of 2500 and 3750\u00a0cm\u22121 may be attributed to OH groups from silanol and adsorbed water (Hindryawati et al., 2014). For MPI silica the band at 1314\u00a0cm\u22121 is associated with asymmetrical stretching of the siloxane (Si-O-Si) bonds (Hindryawati et al., 2014). Additional description of the SPS FTIR spectrum were provided in Table S3.A comparison of both spectra, allowed observing a decrease in the intensity of the bands in the range of 3526 to 2451\u00a0cm\u22121 in MPI silica, probably due to moisture loss during the silica calcination to produce the SPS. Additionally, the decrease of a signal at 1606\u00a0cm\u22121 and disappearance of the band at 1314\u00a0cm\u22121, in the SPS catalyst might have occurred due to a reaction of the silane\u2019s groups polycondensation during SPS preparation. The increase in intensity of the band at 1427\u00a0cm\u22121 is attributed to the carbonates formed after the treatment of MPI silica to produce SPS (Peyne et al., 2017). Moreover an enlargement of the peak at 1000\u00a0cm\u22121 (Si-O-Si group in the MPI silica) in the range of 1000\u2013800\u00a0cm\u22121 is noted, which could be probably due to the overlap of some peaks present in the SPS. The changes in the silicon bands would be caused by depolymerization of the silica network by the potassium ions, which are network-modifying agents and may affect the number of atoms in the first coordination sphere of the atomic silicon (Puligilla et al., 2018).The results of the thermogravimetric analysis for the MPI silica exhibited three main events characteristics of amorphous silica (Fig. 3\na). The first event of mass loss (4.8%) occurred in the range of 27.39\u00a0\u00b0C to 190\u00a0\u00b0C, due to the removal of physisorbed water on the silica surface. The second mass loss event occurs over a wide temperature range of 190 to about 632.86\u00a0\u00b0C (mass loss of 2.22%), probably due to the condensation of less stable silanols and of silane into siloxane. The third event (9.31% of mass loss) starting at 632.86\u00a0\u00b0C may be attributed to the more stable silanols were dehydroxylated and condensed (De Carvalho et al., 2015; Kin et al., 2009).The results of the thermal analysis for SPS, Fig. 3b, showed (I) a 5.75% of mass loss attributed to the water molecules adsorbed on the material (34\u2013190\u00a0\u00b0C) and (II) a second mass loss event of 1.35% attributed to the silanols and silane into siloxane condensation, of the 190\u00a0\u00b0C to 604.96\u00a0\u00b0C, and (III) third mass loss event of 10.38%, release of water by condensation/polymerization of the Si-OH groups above 604.96 (He et al., 2010; Kin et al., 2009). The DTG deconvolution result of the SPS catalyst, Fig. 3c and Table S4 confirmed the three mass loss events verified in Fig. 3b. This shows the stability of the material after removing the water by heating for activating the catalyst.The FESEM micrograph for the MPI silica revealed the presence of irregularly distributed particles (Fig. 4\na). The EDX maps showed a homogeneous distribution of the elements Si, Al, O, and Na on the surface of the support (Fig. 4b-e). FESEM micrograph for SPS (Fig. 4f) indicated larger irregular structures in relation to the image obtained for the MPI silica (Fig. 4a). Upon modification of MPI silica with alkali treatment and calcination the surface heterogeneity (with steps and kinks) and particle agglomeration are enhanced. The mapping images (Fig. 4g-j) demonstrated that there is good dispersion of the elements Si, Na, K, and O over the catalyst. The morphological changes in the silica MPI which acts as a support presented formation of the SPS catalyst by adding alkali metals and the adopted synthesis methodology. The EDX maps and spectra for both materials, SPS (Fig. 4k) and MPI silica (Fig. S2) are in agreement with the XRD (Fig. 2) and XRF results (Table 1).The CO2-TPD results of SPS (Fig. 5\na) exhibited several desorption peaks, indicating the presence of with weak (100.67\u2013149.71\u00a0\u00b0C), medium (149.71\u2013273\u00a0\u00b0C) and strong (273\u2013405\u00a0\u00b0C) basic sites, demonstrating the heterogeneity of the material (Eom et al., 2015).The deconvolution of the CO2-TPD (Fig. 5b and Table S5) enabled to verify that the largest proportion of basic sites are strong characteristic, composing about 51.9%, with 31.6% corresponding to sites of medium strength.The results of the Hammett basicity test were positive to identify that SPS has basic sites in the range of 9.3\u00a0<\u00a0H_ <15 of phenolphthalein (pKb\u00a0=\u00a09.8), corresponding to 3.2\u00a0mmol\u00a0g\u22121 of the indicator, which denote the presence of a substantial quantity of active sites at alkaline pH in the SPS (Okoye et al., 2019). According to the specialized literature, sodium and potassium silicates have a basic strength above the phenolphthalein range (range 15\u00a0<\u00a0H_ <18.4) (Hindryawati et al., 2014), justifying the catalytic capacity of the SPS catalyst (this study). The obtained results of basicity are consistent with those of CO2-TPD.In the Raman spectrum of SPS catalyst, Fig. 6\n, there were peaks in the range of 500\u2013700\u00a0cm\u22121 that could be attributed to vibrational stretching and bend of the Si-O-Si (Santos et al., 2019; Partyka and Le\u015bniak, 2016). At 965.20\u00a0cm\u22121 there was a peak that corresponds to the antisymmetric stretching of the Si-O bond (Zhu et al., 2019). At 1059.84\u00a0cm\u22121, a peak of strong intensity was attributed to the symmetrical stretching of the C-O of the K2CO3 group (Ma et al., 2021). A peak at 1074.96\u00a0cm\u22121 was assigned to stretching CO3\n2\u2013 of the MgCO3 (Williams et al., 1992).The N2 adsorption\u2013desorption isotherms of SPS (Fig. S3) were classified as type III at relative pressures of 0.1\u00a0<\u00a0P/Po\u00a0<\u00a00.6, and type IV(a) at 0.6\u00a0<\u00a0P/Po\u00a0<\u00a00.98, with an H3 hysteresis cycle (Thommes et al., 2015). The specific surface area (SBET) obtained for the SPS catalyst is 0.710\u00a0m2 g\u22121 and the pore volume is 0.00421\u00a0cm3 g\u22121. This demonstrates a sharp drop when compared to the MPI silica that presented SBET of 33.54\u00a0m2 g\u22121 and pore volume of 0.18\u00a0cm3 g\u22121, according to Carvalho et al. (2015). This decrease may be associated with the addition of K+ and Na+ and also due to the particle agglomeration upon calcination as seen in FESEM images. Strong basic sites that may occlude the catalyst pores (Farias et al., 2011).In accordance with the obtained XRD, XRF, and Raman results, three species make up the SPS catalyst: potassium carbonate, alkali metal oxides, and sodium and potassium silicates. In this work, a mechanism was proposed for biodiesel synthesis by transesterification reaction, adapted from Guo et al. (2012). This proposal is based on the silicate interactions (Fig. 7\n), in which the methanol approaches the catalyst surface favoring the ion exchange between the metal silicate (Na or K) and the hydrogen from the alcohol forming the methoxide. There is a subsequent nucleophilic attack of the methoxide on the carbonyl of the triglyceride forming the tetrahedral intermediate that after rearrangement results in the fatty acid methyl esters (FAME). Lastly, an intramolecular rearrangement of protons in the diglyceride occurs to stabilize the charge.The reaction yield (%) for the biodiesel synthesis (calculated by Eq. S4) was assessed at different conditions of time, molar ratio of alcohol to oil (A/O), SPS catalyst concentration and its reuse (Fig. 8\n). It was observed that the yield progressively increased with increase in time (Fig. 8a), catalyst concentration, and molar ratio of A/O (Fig. 8b-c), as determined through Eq. (S4). The tests for the SPS reuse revealed a good catalyst performance as the reaction yield was approximately the same during in the first four cycles, with a decrease only in the fifth cycle (Fig. 8d). The factors that most influenced the yield were time and A/O molar ratio from 12:1 to 15:1.The 1H NMR spectra for the waste cooking oil (WCO) (Fig. 9\na) and biodiesel (Fig. 9b) were obtained with the purpose of evaluating the biodiesel purity and the conversion to esters in the SPS-catalysed transesterification reaction. The conversion may be confirmed by the disappearance of the peaks at 4.1\u20134.3\u00a0ppm, attributed to hydrogen from triglycerides (WCO) (Ruschel et al., 2016), and the appearing of a signal at 3.7\u00a0ppm due to hydrogen from methoxy groups of methyl esters (biodiesel) (Gohain et al., 2017), resulting in a conversion of around 93.89% (Eq.(S3)). In addition, absence of contaminants is observed.In the 13C NMR spectra of WCO and biodiesel (Fig. S4), carboxyl ester group (signals at 174.3 and 54.43\u00a0ppm) and olefinic groups (unsaturated methyl ester, signals in the range of 130.18\u2013129.73\u00a0ppm) respectively were observed (Fig. S4a). Furthermore, triglycerides peaks (CH-O and CH2-O) are found between 68.88 and 61.77\u00a0ppm (Fig. S4a) (Tariq et al., 2011). A peak referring to methyl carbon appears at 14.9\u00a0ppm, while carbons of methylene groups (hydrocarbon chain) were observed in the range of 34.9\u201322.48\u00a0ppm (Fig. S4b) (Gohain et al., 2017). Comparing the results for WCO and biodiesel, it is possible to observe the disappearance of triglyceride peaks and the appearance of ester carbonyl signal (C-O) at 51.43\u00a0ppm in the 13C NMR of biodiesel (Fig. S4b) (Tariq et al., 2011).Thermal analysis of WCO and biodiesel produced from WCO (Fig. 10\na and 10b) also confirmed the conversion and quality of biodiesel produced with the SPS catalyst, in agreement with the 1H NMR analysis (Fig. 9b). The DTG deconvolution method was performed on the DTG curve of biodiesel (Fig. S5 and Table S6) to evaluate its components. A mass loss event for WCO is observed in the range of 347\u2013475\u00a0\u00b0C, (Fig. 10a) which corresponds to the decomposition of triglycerides while for the biodiesel the mass loss (Fig. 10b) occurs at lower temperatures in the region of 163 to 266\u00a0\u00b0C due to the decomposition of smaller molecules, corresponding to monoalkyl ester (Dantas et al., 2007; Misutsu et al., 2015). The SPS showed similar behaviour as other catalysts studied in the specific literature, which are described in Table 2\n.It was possible to observe four components (Fig. S5) which corresponds to the observed mass loss in the TGA curve of biodiesel. Of these, three are attributed to monoalkyl ester, corresponding to 98% of the integrated area of DTG, and the other peak corresponds to oxidation products formed (about 2%) (D\u00edaz-Ballote, 2018). It is possible to prove the synthesis occurrence via the thermal evaluation of biodiesel, which corroborates the 1H and 13C NMR results. The values calculated for the physicochemical properties such as density at 20\u00a0\u00b0C (0.890\u00a0g\u00a0cm\u22123), kinematic viscosity at 40\u00a0\u00b0C (5.061\u00a0mm2 s\u22121) and acidity index (between 0.355 and 0.155\u00a0mg KOH g\u22121) were within the specifications of National Agency of Petroleum Natural Gas and Biofuels (ANP N\u00b0 45, 2014),. Characteristic bands of biodiesel were observed in the FTIR spectrum of biodiesel (Fig. S6). The band at 1736\u00a0cm\u22121, was attributed to the elongation of the carbonyl bond; absorptions in the 1425\u20131447\u00a0cm\u22121 correspond to the asymmetric CH3 flexion, and the bands in the range of 1188\u20131200\u00a0cm\u22121 correspond to O-CH3 stretching (Mumtaz et al., 2012, De Morais et al., 2013).The sodium potassium silicate (SPS) catalyst obtained in this work has a simple preparation method and may be considered of low-cost since it was produced from MPI silica derived from beach sand. In addition, exhibited high catalytic activity for biodiesel production for the conversion of waste oil (WCO) without previous of free fatty acids esterification. The efficiency of the catalyst synthesis method was verified by the XRD and XRF results, as well as the deconvolution method using the CO2-TPD result to evaluate the strength of basic catalyst sites. The biodiesel production process involving heterogenous catalysis showed some advantage as the purification steps are more efficient than the homogeneous one, reducing mainly the amount of waste water. On the other hand, the reuse cycles of the catalyst indicate the possibility of its application in industrial scale. The reaction time was the most decisive parameter for the biodiesel yield (92%) and its high conversion (93.89%) can be verified by the 1H NMR results. The spectroscopic, thermal analysis, and physicochemical data suggest the biodiesel from WCO was suitable for use as fuel.This research was funded by Higher Education Improvement of Coordination Personnel - Brazil (CAPES) - Funding Code 001.\nKeverson G. de Oliveira: Writing \u2013 original draft, Conceptualization, Investigation, Formal analysis. Ramoni R.S. de Lima: Conceptualization, Investigation, Writing \u2013 review & editing, Formal analysis. Clenildo de Longe: Investigation, Writing \u2013 review & editing. Tatiana de C. Bicudo: Investigation, Writing \u2013 review & editing. Rafael V. Sales: Investigation. Luciene S. de Carvalho: 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 authors acknowledge the support provided by the Post-Graduate Chemistry Program (PPGQ/UFRN), the Energetic Technologies Research Group (GPTEN), and the Central Analitica (IQ/UFRN). This study was partly financed by the Coordena\u00e7\u00e3o de Aperfei\u00e7oamento de Pessoal de N\u00edvel Superior \u2013 Brasil (CAPES) \u2013 Finance Code 001.", "descript": "\n                  Heterogeneous catalysts, named SPS (sodium potassium silicates), were synthesized with an alternative silica (MPI silica) obtained from beach sand. In this work, the MPI was modified with NaOH and KOH producing silicate-based catalyst for biodiesel synthesis from waste cooking oil (WCO). The obtained catalyst was characterized by XRD, CO2-TPD, the Hammett basicity test, XRF, FESEM, EDX, FTIR and TG/DTG. The results confirmed the presence of K2O/Na2O oxides and their silicates, the main active sites responsible for the catalytic action. CO2-TPD and the Hammett basicity data suggested the presence of weak, medium and strong basic sites. Biodiesel yield was about 92% and the SPS catalyst was reused for five cycles. The biodiesel conversion by NMR 1H was about 93.89%. The DTG deconvolution revealed the decomposition of four typical biodiesel compounds (R2\u00a0=\u00a00.9987). The method applied for the WCO biodiesel production using SPS catalyst represents an environmentally friendly process, based on low-cost material and reuse of waste biomass.\n               "}
{"full_text": "Catalysts play a key role in today\u2019s chemical industry. About 75% of chemical processes are based on catalytic reactions and over 90% of new processes developed in recent years use catalysts (Hagen,\u00a02015). Catalysts may enhance reaction kinetics by orders of magnitude or provide the selectivity required for economic production of chemicals. Consequently, the selection of a suitable catalyst is a key step in chemical process design. Today, catalysts are usually selected based on experimental methods such as high-throughput experimental screenings or combinatorial chemistry. However, these methods are usually not target-oriented and may lead to a huge experimental effort. Moreover, the chemical design space of possible catalyst molecules is vast (Fink et\u00a0al., 2005; Reymond, 2015). Therefore, it is not feasible to test all potential catalysts experimentally and the full potential of the chemical design space is thus likely not exploited. Consequently, it is highly desirable to develop in silico-methods to explore the chemical design space and to suggest the most promising candidates for experimental testing.To identify the most promising molecules in large design spaces efficiently, Computer-Aided Molecular Design (CAMD) (Austin\u00a0et\u00a0al., 2016) methods have been developed. CAMD methods explore the chemical design space in silico, typically based on optimization algorithms that are employed to find the best candidates. As described in our recent review article (Gertig\u00a0et\u00a0al., 2020c), CAMD methods comprise of 3 building blocks:\n\n1.\nAn algorithm is required to explore the chemical design space and to suggest and optimize molecular structures (Papadopoulos\u00a0et\u00a0al., 2018). Typically, the design spaces are defined by a set of functional groups or molecule fragments. Structures of candidate molecules are generated from these groups or fragments. The optimization of these structures can be based on deterministic or stochastic optimization (Papadopoulos\u00a0et\u00a0al., 2018).\n\n\n2.\nCAMD requires a sound prediction of such unknown properties, e.g., molecular and thermodynamic quantities as well as chemical properties such as reaction kinetics. The reason for the need to predict properties in CAMD is that CAMD methods are supposed to not only examine already known molecular structures, but also to suggest new candidate molecules with unknown properties. Thermodynamic properties are commonly predicted in CAMD based on group-contribution (GC) methods (Papadopoulos et\u00a0al., 2018; Gmehling, 2009). GC methods offer the advantage of a straightforward implementation and computational efficiency (Gani,\u00a02019). However, the chemical design space accessible with these methods is limited to the functional groups a GC method was trained for. Moreover, the prediction of different properties often requires several GC methods. In contrast, the computationally more demanding quantum chemical (QC) (Atkins\u00a0and Friedman,\u00a02011) methods are not limited to certain functional groups. In conjunction with thermochemistry (Paulechka and Kazakov, 2017; Umer and Leonhard, 2013), QC methods provide consistent predictions of molecular and thermodynamic properties as well as kinetics of chemical reactions (Vereecken\u00a0et\u00a0al., 2015). Due to the availability of increased computational capacities, even the use of advanced QC methods in CAMD has become feasible in recent years (Gertig\u00a0et\u00a0al., 2020c).\n\n\n3.\nThe performance of the designed molecules needs to be evaluated during CAMD based on a chosen objective. Commonly, CAMD methods use simple performance indicators as objective function defined based on predicted molecular, thermodynamic or chemical properties (Papadopoulos\u00a0et\u00a0al., 2018). However, such indicators may not capture all aspects and trade-offs relevant for the intended use of the designed structures. Thus, CAMD using simple performance indicators likely results in the design of sub-optimal molecules (Adjiman\u00a0et\u00a0al., 2014). Preferentially, CAMD should evaluate all candidate molecules directly based on their intended application (Gertig\u00a0et\u00a0al., 2020c). In the context of process design, the designed molecules are applied in processes e.g., as working fluids, solvents or catalysts (Papadopoulos\u00a0et\u00a0al., 2018). Thus, each candidate molecule should be evaluated using process optimizations to determine the process performance that can be reached using the candidate. The integration of process optimizations into the design procedure corresponds to the extension of CAMD to integrated Computer-Aided Molecular and Process Design (CAMPD) (Papadopoulos\u00a0et\u00a0al., 2018).\n\n\nAn algorithm is required to explore the chemical design space and to suggest and optimize molecular structures (Papadopoulos\u00a0et\u00a0al., 2018). Typically, the design spaces are defined by a set of functional groups or molecule fragments. Structures of candidate molecules are generated from these groups or fragments. The optimization of these structures can be based on deterministic or stochastic optimization (Papadopoulos\u00a0et\u00a0al., 2018).CAMD requires a sound prediction of such unknown properties, e.g., molecular and thermodynamic quantities as well as chemical properties such as reaction kinetics. The reason for the need to predict properties in CAMD is that CAMD methods are supposed to not only examine already known molecular structures, but also to suggest new candidate molecules with unknown properties. Thermodynamic properties are commonly predicted in CAMD based on group-contribution (GC) methods (Papadopoulos et\u00a0al., 2018; Gmehling, 2009). GC methods offer the advantage of a straightforward implementation and computational efficiency (Gani,\u00a02019). However, the chemical design space accessible with these methods is limited to the functional groups a GC method was trained for. Moreover, the prediction of different properties often requires several GC methods. In contrast, the computationally more demanding quantum chemical (QC) (Atkins\u00a0and Friedman,\u00a02011) methods are not limited to certain functional groups. In conjunction with thermochemistry (Paulechka and Kazakov, 2017; Umer and Leonhard, 2013), QC methods provide consistent predictions of molecular and thermodynamic properties as well as kinetics of chemical reactions (Vereecken\u00a0et\u00a0al., 2015). Due to the availability of increased computational capacities, even the use of advanced QC methods in CAMD has become feasible in recent years (Gertig\u00a0et\u00a0al., 2020c).The performance of the designed molecules needs to be evaluated during CAMD based on a chosen objective. Commonly, CAMD methods use simple performance indicators as objective function defined based on predicted molecular, thermodynamic or chemical properties (Papadopoulos\u00a0et\u00a0al., 2018). However, such indicators may not capture all aspects and trade-offs relevant for the intended use of the designed structures. Thus, CAMD using simple performance indicators likely results in the design of sub-optimal molecules (Adjiman\u00a0et\u00a0al., 2014). Preferentially, CAMD should evaluate all candidate molecules directly based on their intended application (Gertig\u00a0et\u00a0al., 2020c). In the context of process design, the designed molecules are applied in processes e.g., as working fluids, solvents or catalysts (Papadopoulos\u00a0et\u00a0al., 2018). Thus, each candidate molecule should be evaluated using process optimizations to determine the process performance that can be reached using the candidate. The integration of process optimizations into the design procedure corresponds to the extension of CAMD to integrated Computer-Aided Molecular and Process Design (CAMPD) (Papadopoulos\u00a0et\u00a0al., 2018).CAMPD methods have already been used extensively for integrated in silico design of molecules and processes, e.g., working fluids and Organic Rankine Cycles (Schilling et\u00a0al., 2017; 2020; Linke et\u00a0al., 2015) or extraction solvents and processes (Austin et\u00a0al., 2017; Papadopoulos and Linke, 2009; Scheffczyk et\u00a0al., 2018). A very comprehensive review of CAM(P)D applications was recently given by Papadopoulos\u00a0et\u00a0al.\u00a0(2018). For reactive chemical processes, CAMD methods have been used to design reaction solvents that accelerate reaction kinetics (Gertig et\u00a0al., 2019a; Struebing et\u00a0al., 2013; 2017; Liu et\u00a0al., 2019a; 2019b). Moreover, CAMPD methods have been developed for integrated design of reaction solvents and processes (Zhou et\u00a0al., 2015; Gertig et\u00a0al., 2020b; Zhang et\u00a0al., 2020).The main difference of CAMPD methods for non-reactive and for reactive processes lies in the building block property prediction: CAMPD methods for non-reactive processes are commonly based on GC methods for property prediction (Papadopoulos\u00a0et\u00a0al., 2018). In contrast, the prediction of reaction kinetics usually requires quantum chemistry. Quantum chemical methods in conjunction with thermochemistry and transition state theory (TST) (Vereecken et\u00a0al., 2015; Eyring, 1935) have proven to be suited to predict reaction kinetics in CAM(P)D of reaction solvents and processes (Gertig\u00a0et\u00a0al., 2020c).CAMPD methods for the integrated design of molecules and non-reactive processes have gained a high level of maturity and several CAM(P)D methods have also been developed for the design of reaction solvents. The use of computational methods has as well gained importance in the search for new catalysts during the past years as shown by several recent review articles. The review by Ahn\u00a0et\u00a0al.\u00a0(2019) introduces organic and metalorganic catalysis as well as strategies for the use of computational methods in the search for new catalysts. Foscato and Jensen\u00a0(2020) present an elaborated review of computational methods for the development of homogeneous catalysts including large-scale in silico screenings. Freeze\u00a0et\u00a0al.\u00a0(2019) provide an extensive review of catalyst development strategies that especially includes heterogeneous catalysts. These reviews show that computational methods are nowadays used extensively to shed light on catalytic mechanisms, to explain experimental findings or to evaluate new catalysts before going to experiments. Moreover, various systematic approaches for the use of in silico methods in catalyst development have been proposed. Nevertheless, designing molecular catalysts in silico is still regarded as one of the \u201choly grails in chemistry\u201d (Poree\u00a0and Schoenebeck, 2017) and only few approaches to automated CAMD of catalyst molecules have been shown so far.For in silico studies of catalysis, some authors have proposed to employ an abstract catalytic environment. In an early approach called \u201cTheozymes\u201d, Tantillo\u00a0et\u00a0al.\u00a0(1998) determine the transition state (TS) of chemical reactions using quantum chemical methods. Subsequently, functional groups are chosen to represent the catalyst and placed around the transition state structure. The spatial positions of these functional groups are optimized in order to stabilize the TS. The stabilization of the TS reduces the activation barrier of the chemical reaction and accelerates reaction kinetics. Thus, the best possible catalytic activity is determined for the chosen functional groups. Recent studies (Hare\u00a0et\u00a0al., 2017) are still based on the original work of Tantillo et\u00a0al. In a recent approach, Dittner and Hartke (2018, 2020) employ an abstract environment that is optimized in order to maximize the catalytic effect of electrostatic interactions. Approaches based on abstract representation of catalysts are well suited to study catalytic effects and provide a theoretical optimum of catalytic activity. However, no real catalyst structures are designed directly.Few approaches have been proposed in literature to design catalyst molecular structures directly. Lin\u00a0et\u00a0al.\u00a0(2005) design transition metal catalysts based on selected functional groups. To optimize the molecular structure of the catalysts, a suitable tabu search algorithm (Chavali\u00a0et\u00a0al., 2004) is employed. The properties of designed catalysts are predicted with Quantitative Structure-Property Relationships (QSPR) that are fitted to experimental data. Targets for properties such as density and toxicity are used as design objectives. Consequently, the method directly suggests catalyst structures, but does not optimize these structures based on the achieved catalytic performance. Chu\u00a0et\u00a0al.\u00a0(2012) employ a quantitative structure-activity relationship (QSAR) model in catalyst design. This QSAR model relates the catalytic activity of Ruthenium catalyst complexes for olefin metathesis to descriptors such as bond distances, angles and partial charges (Occhipinti\u00a0et\u00a0al., 2006). New catalyst complexes are constructed from a fixed metal core, a list of ligand scaffolds and a variety of molecule fragments that can be attached to the ligand scaffolds. An evolutionary algorithm optimizes the structure of the catalyst. Krausbeck\u00a0et\u00a0al.\u00a0(2017) propose a design method called \u201cMolecular Scaffold Design\u201d based on the idea that unstable, distorted structures occur during chemical reactions and that catalysts need to stabilize such structures to enhance reaction kinetics. The design starts with an unstable fragment with frozen geometry that includes distorted reactants, the core of the catalyst and additional binding sites. A list of atoms that may be added at binding sites is specified. Subsequently, a set of candidates is generated by enumerating the different combinations of atoms that can be added at the binding sites and saturating the resulting structures with hydrogen atoms. These additional hydrogen atoms are replaced by binding sites in the subsequent iteration. The structures are scored with a measure of forces on the nuclei of the unstable fragment computed with quantum mechanical density functional theory (DFT). In an iterative procedure, new \u201conion shells\u201d (Krausbeck et\u00a0al., 2017) of atoms are constructed around promising candidates from previous steps until a structure is obtained where the forces on the nuclei in the unstable fragment vanish. More recently, Chang\u00a0et\u00a0al.\u00a0(2018) designed Ni catalyst complexes for a catalytic CO/CO\n\n\n2\n\n conversion. In their design approach, selected groups of the ligands of the Ni complexes are optimized with the objective to minimize the activation energy of the rate-limiting reaction step. During the design procedure, the activation energies are predicted using the tight binding linear combination of atomic potentials (TB-LCAP) (Xiao\u00a0et\u00a0al., 2008) method. Promising candidates from the design are subsequently investigated in more detail using DFT.The design approaches discussed above can be regarded as pioneering work towards in silico design of molecular catalysts. However, two important building blocks of a reliable, direct in silico design of molecular catalyst structures have not been completed, yet. First, a reliable prediction method is needed for the catalytic performance of candidate catalysts, i.e., the acceleration of the reaction kinetics by the designed catalysts. To be reliable, the prediction should employ high-level QC methods already during the design procedure. Second, the ultimate objective of chemical process design is maximum process performance rather than the acceleration of the chemical reactions. Thus, a process-based evaluation of each candidate catalyst is desired. For this purpose, process optimizations have to be integrated into the in silico design of molecular catalysts. In this work, we propose a CAMPD method called CAT-COSMO-CAMPD that integrates the discussed building blocks into the in silico design of molecular catalysts. The prediction of catalytic effects is based on TST and advanced QC methods such as DLPNO-CCSD(T) (Riplinger\u00a0and Neese,\u00a02013) and COSMO-RS (Klamt\u00a0et\u00a0al., 2010). This prediction is broadly applicable and not limited to certain functional groups. Thereby, large and diverse chemical design spaces can be explored. Optimal catalyst structures and process conditions are determined by a hybrid optimization scheme: The genetic optimization algorithm LEA3D (Douguet\u00a0et\u00a0al., 2005) generates and optimizes catalyst structures based on a library of 3D molecule fragments. Deterministic process optimizations maximize the performance of processes for each molecular catalyst considered during the design procedure. Thus, the desired process-based evaluation is ensured for all candidate catalysts. Currently, CAT-COSMO-CAMPD is applicable to the integrated design of homogeneous molecular catalysts and chemical processes involving gaseous and liquid phases. Potential extensions to other classes of catalysts are discussed in Section\u00a04.The proposed CAT-COSMO-CAMPD method is explained in detail in Section\u00a02 of this article. Next, the application of CAT-COSMO-CAMPD to the case study of a catalytic carbamate-cleavage process is presented (Section\u00a03). Subsequently, current limitations and future prospects of CAT-COSMO-CAMPD are discussed and conclusions are drawn (Section\u00a04).The integrated catalyst and process design problem is formulated as optimization problem specifying the generic CAMD problem discussed by Gani\u00a0(2004):\n\n(1)\n\n\n\n\n\n\nmax\n\nx\n,\ny\n\n\n\n\n\n\n\nf\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n\n\n\n\n\nprocess-based\n\nobjective\n\n\n\n\n\n\n\n\ns\n.\nt\n.\n\n\n\n\n\n\nk\n=\n\nh\n1\n\n\n(\nx\n,\n\n\u0398\n\n)\n\n\n\n\n\n\n\n\nkinetic\n\nmodel\n\n\n\n\n\n\n\n\n\n\u0398\n\n=\n\nh\n2\n\n\n(\nx\n,\ny\n)\n\n\n\n\n\n\n\n\nthermodynamic\n\nproperty\n\nmodel\n\n\n\n\n\n\n\n\n0\n=\n\nh\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n\n\n\n\n\n\nprocess\n\nmodel\n\n\n\n\n\n\n\n\n\ng\n1\n\n\n(\ny\n)\n\n=\n0\n\n\n\n\n\n\n\nchemical\n\nfeasibility\n\n\n\n\n\n\n\n\n\ng\n2\n\n\n(\ny\n)\n\n\u2264\n0\n\n\n\n\n\n\n\nchemical\n\nfeasibility\n\n\n\n\n\n\n\n\n\nc\n1\n\n\n(\n\n\u0398\n\n)\n\n\u2264\n0\n\n\n\n\n\n\n\nconstraints\n\non\n\nthermodynamic\n\n\n\n\n\n\n\n\n\n\n\nproperties\n\n\n\n\n\n\n\n\nc\n2\n\n\n(\ny\n)\n\n\u2264\n0\n\n\n\n\n\n\n\nconstraints\n\non\n\nmolecular\n\nproperties\n\n\n\n\n\n\n\n\n\nc\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\u2264\n0\n\n\n\n\n\n\n\nconstraints\n\non\n\nthe\n\nprocess\n\n\n\n\n\n\n\n\nx\n\u2208\nX\n\n\n\n\n\n\n\nvariable\n\nprocess\n\nconditions\n\n\n\n\n\n\n\n\ny\n\u2208\nY\n\n\n\n\n\n\n\nmolecular\n\nstructure\n\nof\n\ncatalyst\n\n\n\n\n\n\n\nIn Problem (1), \n\nf\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n represents the process-based objective (e.g., conversion or yield) that may depend on the variable process conditions \nx\n, on thermodynamic equilibrium properties \n\n\u0398\n\n and on the reaction kinetics determined by reaction rate constants \nk\n. The objective is maximized by optimizing the variable process conditions \nx\n and the molecular structure of the catalyst molecule \ny\n. Rate constants \nk\n themselves also depend on \nx\n and \n\n\u0398\n\n and are calculated using a kinetic model \n\n\nh\n1\n\n\n(\nx\n,\n\n\u0398\n\n)\n\n\n. The thermodynamic equilibrium properties \n\n\u0398\n\n depend on the variable process conditions \nx\n as well as on the molecular structure of the catalyst molecule \ny\n and are calculated using a thermodynamic property model \n\n\nh\n2\n\n\n(\nx\n,\ny\n)\n\n\n. The equations of the process model are represented by \n\n\nh\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n. Equality constraints \n\n\ng\n1\n\n\n(\ny\n)\n\n\n and inequality constraints \n\n\ng\n2\n\n\n(\ny\n)\n\n\n ensure chemical feasibility of the catalyst molecules, e.g., correct valency of all atoms in the molecule. Additionally, constraints on thermodynamic properties \n\n\nc\n1\n\n\n(\n\n\u0398\n\n)\n\n\n (e.g., minimal boiling point of the catalyst molecule), on molecular properties \n\n\nc\n2\n\n\n(\ny\n)\n\n\n (e.g., number of atoms in the molecule or restrictions on the combination of functional groups) and on the process \n\n\nc\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n may be used. The variable process conditions \nx\n contained in a range of allowed process conditions \nX\n represent the process-related degrees of freedom. Besides quantities like pressures and temperatures, these process-related degrees of freedom may e.g., also include vessel sizes or compositions of mixtures fed to the process. The molecular structure of the catalyst molecule \ny\n is contained in a chemical design space \nY\n.It should be noted that the rate constants \n\nk\n=\n\nh\n1\n\n\n(\nx\n,\n\n\u0398\n\n)\n\n\n in Problem (1) do not directly depend on the molecular structure of the catalyst molecule \ny\n. However, this missing direct dependence does not mean that catalysts do not impact the rate constants but rather reflects the way rate constants are calculated. Catalyst molecules influence rate constants by reducing so-called activation barriers that reactions need to overcome to take place. We regard these activation barriers as quantities associated with thermodynamic pseudo-equilibria and therefore include them in the thermodynamic equilibrium properties \n\n\u0398\n\n. The calculation of reaction rate constants is explained in more detail in Section\u00a02.1. Subsequently, the prediction of thermodynamic equilibrium properties (Section\u00a02.2) and process modeling (Section\u00a02.3) are described, before the solution approach to the optimization Problem (1) is presented (Section\u00a02.4).The kinetics of catalytic reactions are described by reaction rate constants \nk\n that indirectly depend on the structure of the used catalyst molecule \ny\n as explained above. The methods we use to predict rate constants were described in detail in earlier work (Gertig et\u00a0al., 2019, 2021; Kr\u00f6ger et\u00a0al., 2017). An overview is given in the following without detailed derivations of all equations.The rate constants \nk\n of elementary reactions are calculated based on conventional transition state theory (TST) (Vereecken et\u00a0al., 2015) and the so-called Eyring Equation (Eyring,\u00a01935):\n\n(2)\n\n\nk\n=\n\n\n\nk\n\nB\n\n\nT\n\nh\n\n\n\n(\n\nV\n\nm\n\n\n)\n\n\n(\nn\n\u2212\n1\n)\n\n\nexp\n\n(\n\u2212\n\n\n\n\u0394\n\n\nG\n\u2021\n\n\n\nR\nT\n\n\n)\n\n.\n\n\n\nIn Eq.\u00a0(2), \n\nk\n\nB\n\n\n is the Boltzmann constant, \nT\n is the temperature at which the reaction takes place, \nh\n is Planck\u2019s constant and \nR\n is the gas constant. The molar volume of the reaction phase is represented by \n\nV\n\nm\n\n\n and \nn\n denotes the reaction order defined as the sum of the stoichiometric coefficients of all reactants. The activation barrier \n\nG\n\u2021\n\n is the difference in molar Gibbs free energy between the state of the reactants and a so-called transition state (TS). According to conventional TST, this transition state is a first-order saddle point in energy that is passed along the reaction path and can be determined using quantum chemical methods (Foresman\u00a0and Frisch,\u00a02015). The TS is an unstable state in pseudo-equilibrium with the reactant state.\n1\n\n\n1\nThe term \u201cpseudo-equilibrium\u201d implies that the reactants are not in equilibrium with the products at the same time, although the back-reaction might proceed via the same transition state.\n\nIn case the rate constant \nk\n is calculated for a reaction taking place in a liquid phase, the activation barrier \n\nG\n\u2021\n\n is split in different contributions:\n\n(3)\n\n\n\n\n\nk\n=\n\n\n\nk\n\nB\n\n\nT\n\nh\n\n\n(\n\nV\n\n\nm\n\n\n\ni\n.\nG\n.\n\n\n)\n\n\n\n\n\n\n\n\n(\nn\n\u2212\n1\n)\n\n\nexp\n\n(\n\u2212\n\n\n\n\u0394\n\n\nG\n\n\u2021\n,\n\ni\n.\nG\n.\n\n\n\n\n\nR\nT\n\n\n)\n\n\n\n\n\n\n\n\n*\n\n\n\n\u220f\ni\n\n\n\u03b3\ni\n\nuN\n\n\n\n\n\u03b3\n\u2021\n\nuN\n\n\n\nexp\n\n(\n\u2212\n\n\n\n\u0394\n\n\n\n\nG\n\n\u02dc\n\n\u2021\n\nsolv\n\n\n\u2212\n\n\u2211\ni\n\n\n\u0394\n\n\n\n\nG\n\n\u02dc\n\ni\n\nsolv\n\n\n\n\nR\nT\n\n\n)\n\n\n\n\n\n\n\n\n=\n\nk\n\ni\n.\nG\n.\n\n\n\n\n\n\u220f\ni\n\n\n\u03b3\ni\n\nuN\n\n\n\n\n\u03b3\n\u2021\n\nuN\n\n\n\n\n\n\n\nexp\n\n(\n\u2212\n\n\n\n\u0394\n\n\n\n\nG\n\n\u02dc\n\n\u2021\n\nsolv\n\n\n\u2212\n\n\u2211\ni\n\n\n\u0394\n\n\n\n\nG\n\n\u02dc\n\ni\n\nsolv\n\n\n\n\nR\nT\n\n\n)\n\n.\n\n\n\n\n\n\nThis splitting into different contributions offers the advantage that the most appropriate methods can be chosen for the calculation of each contribution (Deglmann et\u00a0al., 2009; Peters et\u00a0al., 2008; Hellweg and Eckert, 2017; Coote, 2009). In Eq.\u00a0(3), \n\nV\n\n\nm\n\n\n\ni\n.\nG\n.\n\n\n is the molar volume of the reaction phase in the used ideal gas reference state and may be calculated using the ideal gas law. The activation barrier in the ideal gas reference state is denoted by \n\n\n\u0394\n\n\nG\n\n\u2021\n,\n\ni\n.\nG\n.\n\n\n\n\n. The first 3 terms shown in the upper line of Eq.\u00a0(3) determine a reaction rate constant \n\nk\n\ni\n.\nG\n.\n\n\n in the ideal gas reference state:\n\n(4)\n\n\n\nk\n\ni\n.\nG\n.\n\n\n=\n\n\n\nk\n\nB\n\n\nT\n\nh\n\n\n\n(\n\nV\n\n\nm\n\n\n\ni\n.\nG\n.\n\n\n)\n\n\n(\nn\n\u2212\n1\n)\n\n\nexp\n\n(\n\u2212\n\n\n\n\u0394\n\n\nG\n\n\u2021\n,\n\ni\n.\nG\n.\n\n\n\n\n\nR\nT\n\n\n)\n\n.\n\n\n\nSolvation effects represent the non-ideal effects that the environment of the reacting species has on the rate constant \nk\n. These solvation effects are accounted for by two terms in Eq.\u00a0(3): First, the ratio of the product of the unsymmetrically normalized activity coefficients \n\n\u03b3\ni\n\nuN\n\n\n of all reactants \ni\n to the unsymmetrically normalized activity coefficient \n\n\u03b3\n\u2021\n\nuN\n\n\n of the transition state. Second, an exponential term containing the different Gibbs free energies of solvation \n\n\n\nG\n\n\u02dc\n\n\nsolv\n\n\n of the reactants \ni\n and the transition state \n\u2021\n, respectively. The exponential term accounts for the difference between the ideal gas reference state and a liquid reference state. The term with the unsymmetrically normalized activity coefficients \n\n\u03b3\n\nuN\n\n\n in turn accounts for the difference between the liquid reference state and the actual reaction mixture composition. This latter term is sometimes neglected, which corresponds to approximating the rate constant \nk\n by the rate constant at infinite dilution. The Gibbs free energies of solvation \n\n\n\nG\n\n\u02dc\n\n\nsolv\n\n\n are computed based on molar reference states. A suitable choice of the ideal gas reference state is the reaction temperature \nT\n and a reference pressure of \n\n\np\n0\n\n=\n1\n\n bar \n\n\u2248\n1\n\n atm. Details about the use of reference states as well as a derivation of Eq.\u00a0(3) can be found in our earlier work (Gertig et\u00a0al., 2019a; 2020b).To determine all quantities necessary to evaluate Eq.\u00a0(3), the following computation scheme is applied:\n\n1.\nOptimized geometries of reactants, catalysts and transition states are determined using the quantum-mechanical density functional theory (DFT) method B3LYP (Stephens\u00a0et\u00a0al., 1994) with empirical dispersion correction (Grimme\u00a0et\u00a0al., 2010) (B3LYP-D3) and TZVP basis set. Vibrational analysis is carried out subsequently. The B3LYP method is known for good accuracy in geometry optimization and frequency analysis in spite of the rather moderate computational resources required (Zheng et\u00a0al., 2009; Gottschalk et\u00a0al., 2018). The rigid rotor harmonic oscillator (RRHO) (Atkins\u00a0and Friedman,\u00a02011) model is used in the frequency analysis. The software Gaussian 09 (Frisch\u00a0et\u00a0al., 2013) is employed for both geometry optimization and frequency analysis.\n\n\n2.\nTo obtain accurate electronic energies, single point (SP) calculations are performed with the post-Hartree-Fock method DLPNO-CCSD(T) (Riplinger and Neese, 2013; Riplinger et\u00a0al., 2013) with aug-cc-pVTZ basis set and TightPNO settings. The software ORCA (Neese,\u00a02018) is used for these SP calculations.\n\n\n3.\nActivation barriers \n\n\n\u0394\n\n\nG\n\n\u2021\n,\n\ni\n.\nG\n.\n\n\n\n\n in the ideal gas reference state are determined by thermochemical calculations with GoodVibes (Funes-Ardoiz\u00a0and Paton,\u00a02018) based on the RRHO approximation. As the RRHO approximation can cause significant errors in calculated entropies in case of low frequencies, Grimme\u2019s quasi-harmonic treatment (Grimme,\u00a02012) is employed to reduce these errors.\n\n\n4.\nNext, the reaction rate constants \n\nk\n\ni\n.\nG\n.\n\n\n in the ideal gas reference state are computed using Eq.\u00a0(4).\n\n\n5.\nOptionally, tunneling corrections to \n\nk\n\ni\n.\nG\n.\n\n\n may be computed in case significant impact of tunneling on the reaction rate is expected. Tunneling corrections based on Eckart\u00a0(1930) computed with the TAMkin package (Ghysels\u00a0et\u00a0al., 2010) have been found to be a reasonable choice.\n\n\n6.\nThe advanced solvation model COSMO-RS (Klamt et\u00a0al., 2010; Klamt, 1995; Klamt et\u00a0al., 1998) is used to calculate Gibbs free energies of solvation \n\n\n\nG\n\n\u02dc\n\n\nsolv\n\n\n of reactants, catalysts and transition states. The software turbomole (Ahlrichs et\u00a0al., 1989; TURBOMOLE, 2015) is employed for COSMO (Klamt\u00a0and Sch\u00fc\u00fcrmann,\u00a01993) calculations with BP86 (Becke, 1988; Perdew, 1986a; 1986b) and def2-TZVP basis set. For this purpose, geometries are optimized with BP86/def2-TZVP in vacuum and the actual COSMO calculations are performed as SP calculations. The COSMO-RS calculations (COSMOtherm; Eckert and Klamt, 2002) to obtain the \n\n\n\nG\n\n\u02dc\n\n\nsolv\n\n\n values are performed subsequently.\n\n\n7.\nTo calculate the required unsymmetrically normalized activity coefficients \n\n\u03b3\n\nuN\n\n\n, we either employ COSMO-RS directly or a suitable surrogate model fitted to data from COSMO-RS.\n\n\n8.\nFinally, reaction rate constants \nk\n in liquid phase are calculated using Eq.\u00a0(3).\n\n\nOptimized geometries of reactants, catalysts and transition states are determined using the quantum-mechanical density functional theory (DFT) method B3LYP (Stephens\u00a0et\u00a0al., 1994) with empirical dispersion correction (Grimme\u00a0et\u00a0al., 2010) (B3LYP-D3) and TZVP basis set. Vibrational analysis is carried out subsequently. The B3LYP method is known for good accuracy in geometry optimization and frequency analysis in spite of the rather moderate computational resources required (Zheng et\u00a0al., 2009; Gottschalk et\u00a0al., 2018). The rigid rotor harmonic oscillator (RRHO) (Atkins\u00a0and Friedman,\u00a02011) model is used in the frequency analysis. The software Gaussian 09 (Frisch\u00a0et\u00a0al., 2013) is employed for both geometry optimization and frequency analysis.To obtain accurate electronic energies, single point (SP) calculations are performed with the post-Hartree-Fock method DLPNO-CCSD(T) (Riplinger and Neese, 2013; Riplinger et\u00a0al., 2013) with aug-cc-pVTZ basis set and TightPNO settings. The software ORCA (Neese,\u00a02018) is used for these SP calculations.Activation barriers \n\n\n\u0394\n\n\nG\n\n\u2021\n,\n\ni\n.\nG\n.\n\n\n\n\n in the ideal gas reference state are determined by thermochemical calculations with GoodVibes (Funes-Ardoiz\u00a0and Paton,\u00a02018) based on the RRHO approximation. As the RRHO approximation can cause significant errors in calculated entropies in case of low frequencies, Grimme\u2019s quasi-harmonic treatment (Grimme,\u00a02012) is employed to reduce these errors.Next, the reaction rate constants \n\nk\n\ni\n.\nG\n.\n\n\n in the ideal gas reference state are computed using Eq.\u00a0(4).Optionally, tunneling corrections to \n\nk\n\ni\n.\nG\n.\n\n\n may be computed in case significant impact of tunneling on the reaction rate is expected. Tunneling corrections based on Eckart\u00a0(1930) computed with the TAMkin package (Ghysels\u00a0et\u00a0al., 2010) have been found to be a reasonable choice.The advanced solvation model COSMO-RS (Klamt et\u00a0al., 2010; Klamt, 1995; Klamt et\u00a0al., 1998) is used to calculate Gibbs free energies of solvation \n\n\n\nG\n\n\u02dc\n\n\nsolv\n\n\n of reactants, catalysts and transition states. The software turbomole (Ahlrichs et\u00a0al., 1989; TURBOMOLE, 2015) is employed for COSMO (Klamt\u00a0and Sch\u00fc\u00fcrmann,\u00a01993) calculations with BP86 (Becke, 1988; Perdew, 1986a; 1986b) and def2-TZVP basis set. For this purpose, geometries are optimized with BP86/def2-TZVP in vacuum and the actual COSMO calculations are performed as SP calculations. The COSMO-RS calculations (COSMOtherm; Eckert and Klamt, 2002) to obtain the \n\n\n\nG\n\n\u02dc\n\n\nsolv\n\n\n values are performed subsequently.To calculate the required unsymmetrically normalized activity coefficients \n\n\u03b3\n\nuN\n\n\n, we either employ COSMO-RS directly or a suitable surrogate model fitted to data from COSMO-RS.Finally, reaction rate constants \nk\n in liquid phase are calculated using Eq.\u00a0(3).As discussed in our previous work (Gertig\u00a0et\u00a0al., 2021), it may be important to search for conformers of reacting species and transition states to obtain accurate reaction rate constants \nk\n. We perform this conformer search using rotor scans (Foresman\u00a0and Frisch,\u00a02015). For the integrated catalyst and process design with CAT-COSMO-CAMPD, we assume that it is sufficient to find the conformer with the lowest energy for all species by means of such scans. This assumption is considered a compromise between prediction accuracy and the effort and complexity of the computations performed during the integrated design. The rotor scans to search the most stable conformers are performed for reactants, products and solvents in advance of the actual design. As catalysts and transition states change during the design, selected automated rotor scans are performed as explained in Section\u00a02.4.The expected uncertainty in the rate constants \nk\n computed with the methods described above was discussed in detail in previous work (Gertig et\u00a0al., 2019, 2021; Kr\u00f6ger et\u00a0al., 2017) and is thus only mentioned briefly here. Generally, we expect that the predicted rate constants \nk\n should agree with experimentally determined rate constants \n\nk\n\nexp\n\n\n within one order of magnitude at a temperature of 25\n\n\n\u2218\n\nC. The uncertainty is expected to decrease with increasing temperature. Furthermore, some errors cancel if the prediction is used for comparison of different candidate catalysts, which is advantageous in CAM(P)D where absolute values are less important than rankings and trends.All thermodynamic equilibrium properties \n\n\u0398\n\n required to solve Problem (1) are computed based on the COSMO-RS (Klamt\u00a0et\u00a0al., 2010) model. These quantities typically include Gibbs free energies of solvation \n\n\n\nG\n\n\u02dc\n\n\nsolv\n\n\n, pure component vapor pressures \n\np\ni\n\nS\n\n\n as well as activity coefficients \n\u03b3\n and unsymmetrically normalized activity coefficients \n\n\u03b3\n\nuN\n\n\n.We compute the Gibbs free energies of solvation \n\n\n\nG\n\n\u02dc\n\n\nsolv\n\n\n at all temperatures of interest directly using COSMO-RS. This is practical because the \n\n\n\nG\n\n\u02dc\n\n\nsolv\n\n\n are calculated for defined reference states and thus do not change with changing reaction mixture composition during process simulations and optimizations.Pure component vapor pressures \n\np\ni\n\nS\n\n\n are calculated using the Antoine equation (Pfennig,\u00a02004). The required Antoine parameters for each component are computed with COSMO-RS.In contrast to \n\n\n\nG\n\n\u02dc\n\n\nsolv\n\n\n and \n\np\ni\n\nS\n\n\n, the activity coefficients \n\u03b3\n and unsymmetrically normalized activity coefficients \n\n\u03b3\n\nuN\n\n\n depend on the reaction mixture composition. Therefore, these quantities have to be re-evaluated frequently in case mixture compositions change. These re-evaluations would consume too much time if COSMO-RS was used directly during process simulations and optimizations. This time-consumption is not only caused by the solution of the COSMO-RS equations themselves, but also by required software interfacing. Thus, the NRTL (Renon\u00a0and Prausnitz,\u00a01968) activity coefficient model is used as surrogate model. NRTL parameters are automatically fitted to activity coefficient data generated for every system under consideration using COSMO-RS.Process models are formulated based on balance equations and equations accounting for phase equilibria. Moreover, power laws (Levenspiel,\u00a01999) are employed in conjunction with the predicted reaction rate constants \nk\n to describe the rates of elementary reactions. The predicted property data allows to formulate a wide range of process models. In the case study presented below, we consider semi-batch operation. In this case, the resulting set of equations is a differential-algebraic system of equations (DAE). The DAE system used in the case study is discussed in the supporting information. The process models are implemented in MATLAB\u00a0(2019) and solved with the ode15s solver.In the following, the solution approach of the proposed CAT-COSMO-CAMPD method to the integrated catalyst and process design Problem (1) is explained. The solution approach of CAT-COSMO-CAMPD follows our quantum chemistry-based design methods for solvents (Scheffczyk et\u00a0al., 2018; Gertig et\u00a0al., 2019a; 2020b; Scheffczyk et\u00a0al., 2017; Fleitmann et\u00a0al., 2018). A preliminary and brief presentation of CAT-COSMO-CAMPD was already given at the conference corresponding to this special issue (Gertig\u00a0et\u00a0al., 2020a). CAT-COSMO-CAMPD employs a hybrid optimization scheme. The genetic optimization algorithm LEA3D (Douguet\u00a0et\u00a0al., 2005) is used to identify the structure \n\ny\n*\n\n of the optimal catalyst molecule, whereas gradient-based process optimization is used to determine optimal values of the variable process conditions \n\nx\n*\n\n for each considered catalyst.The LEA3D algorithm generates 3D molecular structures based on a pre-defined library of 3D molecule fragments. Fragments are combined randomly in the initial step of the optimization to obtain a first generation of candidate molecules. Based on the performance of these initial candidates, LEA3D uses genetic operations to alter the structures and to suggest the next generation of candidates. New generations are iteratively suggested until a specified maximum number of generations is reached. By this procedure, the space of candidate molecules is systematically explored to determine the optimal structure \n\ny\n*\n\n. Other convergence criteria suitable for genetic algorithms could be employed (Safe\u00a0et\u00a0al., 2004). Still, the used genetic algorithm is stochastic such that convergence to the global optimum cannot be guaranteed.Working with 3D molecular structures in the optimization of catalyst molecules offers the major advantage that quantum chemistry-based property prediction can be employed in a straightforward way: QC methods generally require 3D starting geometries as input. QC-based property prediction offers several advantages (see Section\u00a01: Quantum chemical methods and thermochemistry consistently predict a broad range of molecular, thermodynamic and chemical properties including reaction rate constants. In contrast to e.g., group-contribution methods, QC methods are not limited to previously fitted functional groups. Thus, a wide variety of 3D molecule fragments may be chosen when setting up the chemical design space.Currently, CAT-COSMO-CAMPD requires the user to specify a so-called scaffold fragment among the other 3D fragments. This scaffold fragment contains the reactants as well as the catalytically active group contained in all catalysts designed in one CAT-COSMO-CAMPD run. Thus, during one design run, the mechanism of catalysis does not change. The scaffold fragment is employed to construct starting geometries for the search of transition states of the catalytic reactions with the designed catalysts. Further information about scaffold fragments is given in the subsequent description of the design procedure and an example is discussed in Section\u00a03.1.The complete CAT-COSMO-CAMPD procedure to solve the integrated design Problem (1) is shown in the flowchart in Fig.\u00a01\n and explained in the following:\n\n1.\nFirst, several specifications have to be made:\n\n\u2022\nThe reaction network and the process under consideration have to be specified.\n\n\n\u2022\nThe catalytically active group is chosen. The designed catalysts are based on this group.\n\n\n\n\n\n2.\nQuantum chemical and thermochemical calculations are performed for reactants and products in the specified reaction network as well as for solvents used in the process. These QC and thermochemical calculations are discussed in Section\u00a02.1 (Steps 1\u20133 and 6). Moreover, a so-called reference system is defined (Fig.\u00a01) that corresponds to a reaction system including a typical molecule with the catalytically active group as catalyst. QC calculations are employed to determine the transition state geometry for the reference system. From this TS geometry, the scaffold fragment is constructed (for an example, see Fig.\u00a04). As already described above, the scaffold fragment contains the reactants and the catalytically active group of the catalyst. All atoms of the reactants and the catalytically active group are positioned in space as in the determined TS geometry of the reference system. The scaffold fragment is used in Step 3 when the chemical design space \nY\n is defined.\n\n\n3.\nDesign specifications have to be made for the integrated catalyst and process design:\n\n\u2022\nA fragment library is provided that contains various 3D molecule fragments including the catalyst scaffold fragment. From these 3D fragments, catalyst structures are constructed. The choice of fragments in the library determines the chemical design space \nY\n for CAMD of catalysts. It is important to note that the resulting design space does not correspond to a set of possible catalysts, but to a set of possible transition states of the catalytic reaction under consideration. The reason is that the scaffold fragment contains not only the catalytically active group of the catalyst, but also the reactants. Consequently, the transition states are designed directly instead of the catalyst molecules. The direct design of the transition states is advantageous because starting geometries for the optimization of the transition state structures have to be provided. Obtaining suitable starting geometries of transition states can be considered the critical aspect of the automated prediction of rate constants \nk\n of catalytic reactions.\n\n\n\u2022\nThe user may choose rotor scans and additional pre-optimizations performed in Step 5.\n\n\n\u2022\nSettings are required for the genetic algorithm LEA3D used for the optimization of catalyst molecular structures. These settings include the maximum number of generations, the number of candidate catalyst molecules per generation as well as the probabilities for genetic operations such as mutation and cross-over of candidates.\n\n\n\u2022\nThe process model \n\n\nh\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n is specified.\n\n\n\u2022\nThe process-based objective \n\nf\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n of the design is chosen.\n\n\n\u2022\nThe values of constant process parameters are assigned and process degrees of freedom \nx\n are selected.\n\n\n\u2022\nThe allowed range of operating conditions \nX\n is defined.\n\n\n\u2022\nConstraints are set including constraints \n\n\nc\n1\n\n\n(\n\n\u0398\n\n)\n\n\n on thermodynamic properties, \n\n\nc\n2\n\n\n(\ny\n)\n\n\n on molecular properties and \n\n\nc\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n on the process.\n\n\n\nLEA3D inherently respects the constraints \n\n\ng\n1\n\n\n(\ny\n)\n\n\n and \n\n\ng\n2\n\n\n(\ny\n)\n\n\n to ensure chemical feasibility of designed molecules. Moreover, LEA3D ensures that each candidate catalyst molecule contains one scaffold fragment and thus one catalytically active group.\n\n\n4.\nLEA3D suggests a generation of 3D molecular structures of transition states with candidate catalysts. The initial generation is created by random combination of the scaffold fragment with further molecule fragments from the fragment library. Candidates of subsequent generations are obtained from genetic operations such as mutation and cross-over that are applied to promising candidates of the preceding generation. The generated structures are passed to the automated property prediction and gradient-based process optimization.\n\n\n5.\nSuitable starting geometries for the optimization of catalysts and transition states are determined. It is important to note that by the term \u201cgeometry\u201d of a molecule or TS, we here understand the set of spatial positions of all atoms comprising the molecule or TS. In contrast, \u201cmolecular structures\u201d include the connectivity of atoms as shown in structural formulas of molecules. In principle, 3D geometries of transition states are already provided by LEA3D in Step 4. However, it happens occasionally that these structures are not accurate enough for the subsequently used QC methods to work properly. Thus, a first pre-optimization improves the TS geometries employing the force field method MMFF94 (Halgren, 1996a; 1996b; 1996c) available in the chemical toolbox Open Babel (O\u2019Boyle et\u00a0al., 2011; Open Babel). As this first pre-optimization is not suited to handle transition states, the scaffold fragment is substituted by a dummy atom. Afterwards, the dummy is re-substituted by the scaffold fragment using translation and rotation operations in 3D cartesian coordinate space. Next, further pre-optimization is performed using Gaussian 09. Selected rotor scans are performed using the QC method AM1 (Dewar\u00a0et\u00a0al., 1985) to ensure that the minimum energy conformer of each TS is identified. The selection of rotor scans to perform is made by the user (Step 3). Optionally, a geometry optimization with the DFT method B3LYP that minimizes energy may follow the rotor scans. During rotor scans and energy minimization with Gaussian 09, it has to be ensured that no atomic distances are changed that correspond to bonds that break or form during the considered reaction. For this purpose, the \u201cmodredundant\u201d option (Foresman\u00a0and Frisch,\u00a02015) is employed. The TS geometries obtained from the described pre-optimizations are used as starting geometries for subsequent QC calculations. Starting geometries of the catalyst molecules are extracted as a subset of the TS starting geometries.\n\n\n6.\nTo predict reaction kinetics of the catalytic reactions, reaction rate constants \nk\n are computed as explained in Section\u00a02.1. Calculations for reactants, products and solvents performed in Step 2 are not repeated here.\n\n\n7.\nRequired thermodynamic equilibrium properties \n\n\u0398\n\n are predicted as described in Section\u00a02.2.\n\n\n8.\nGradient-based process optimizations are performed for processes with each candidate catalyst. For these processes, the process optimizations determine the optimal process performance \n\n\nf\n*\n\n\n(\n\nx\n*\n\n,\n\n\u0398\n\n,\nk\n)\n\n\n as well as the corresponding optimal values of variable process conditions \n\nx\n*\n\n. The interior point algorithm available in the built-in function fmincon in MATLAB is employed for process optimizations. The evaluations of the process model \n\n\nh\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n required during process optimization are performed with the solver ode15s as already mentioned in Section\u00a02.3.\n\n\nFirst, several specifications have to be made:\n\n\u2022\nThe reaction network and the process under consideration have to be specified.\n\n\n\u2022\nThe catalytically active group is chosen. The designed catalysts are based on this group.\n\n\nThe reaction network and the process under consideration have to be specified.The catalytically active group is chosen. The designed catalysts are based on this group.Quantum chemical and thermochemical calculations are performed for reactants and products in the specified reaction network as well as for solvents used in the process. These QC and thermochemical calculations are discussed in Section\u00a02.1 (Steps 1\u20133 and 6). Moreover, a so-called reference system is defined (Fig.\u00a01) that corresponds to a reaction system including a typical molecule with the catalytically active group as catalyst. QC calculations are employed to determine the transition state geometry for the reference system. From this TS geometry, the scaffold fragment is constructed (for an example, see Fig.\u00a04). As already described above, the scaffold fragment contains the reactants and the catalytically active group of the catalyst. All atoms of the reactants and the catalytically active group are positioned in space as in the determined TS geometry of the reference system. The scaffold fragment is used in Step 3 when the chemical design space \nY\n is defined.Design specifications have to be made for the integrated catalyst and process design:\n\n\u2022\nA fragment library is provided that contains various 3D molecule fragments including the catalyst scaffold fragment. From these 3D fragments, catalyst structures are constructed. The choice of fragments in the library determines the chemical design space \nY\n for CAMD of catalysts. It is important to note that the resulting design space does not correspond to a set of possible catalysts, but to a set of possible transition states of the catalytic reaction under consideration. The reason is that the scaffold fragment contains not only the catalytically active group of the catalyst, but also the reactants. Consequently, the transition states are designed directly instead of the catalyst molecules. The direct design of the transition states is advantageous because starting geometries for the optimization of the transition state structures have to be provided. Obtaining suitable starting geometries of transition states can be considered the critical aspect of the automated prediction of rate constants \nk\n of catalytic reactions.\n\n\n\u2022\nThe user may choose rotor scans and additional pre-optimizations performed in Step 5.\n\n\n\u2022\nSettings are required for the genetic algorithm LEA3D used for the optimization of catalyst molecular structures. These settings include the maximum number of generations, the number of candidate catalyst molecules per generation as well as the probabilities for genetic operations such as mutation and cross-over of candidates.\n\n\n\u2022\nThe process model \n\n\nh\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n is specified.\n\n\n\u2022\nThe process-based objective \n\nf\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n of the design is chosen.\n\n\n\u2022\nThe values of constant process parameters are assigned and process degrees of freedom \nx\n are selected.\n\n\n\u2022\nThe allowed range of operating conditions \nX\n is defined.\n\n\n\u2022\nConstraints are set including constraints \n\n\nc\n1\n\n\n(\n\n\u0398\n\n)\n\n\n on thermodynamic properties, \n\n\nc\n2\n\n\n(\ny\n)\n\n\n on molecular properties and \n\n\nc\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n on the process.\n\n\nA fragment library is provided that contains various 3D molecule fragments including the catalyst scaffold fragment. From these 3D fragments, catalyst structures are constructed. The choice of fragments in the library determines the chemical design space \nY\n for CAMD of catalysts. It is important to note that the resulting design space does not correspond to a set of possible catalysts, but to a set of possible transition states of the catalytic reaction under consideration. The reason is that the scaffold fragment contains not only the catalytically active group of the catalyst, but also the reactants. Consequently, the transition states are designed directly instead of the catalyst molecules. The direct design of the transition states is advantageous because starting geometries for the optimization of the transition state structures have to be provided. Obtaining suitable starting geometries of transition states can be considered the critical aspect of the automated prediction of rate constants \nk\n of catalytic reactions.The user may choose rotor scans and additional pre-optimizations performed in Step 5.Settings are required for the genetic algorithm LEA3D used for the optimization of catalyst molecular structures. These settings include the maximum number of generations, the number of candidate catalyst molecules per generation as well as the probabilities for genetic operations such as mutation and cross-over of candidates.The process model \n\n\nh\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n is specified.The process-based objective \n\nf\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n of the design is chosen.The values of constant process parameters are assigned and process degrees of freedom \nx\n are selected.The allowed range of operating conditions \nX\n is defined.Constraints are set including constraints \n\n\nc\n1\n\n\n(\n\n\u0398\n\n)\n\n\n on thermodynamic properties, \n\n\nc\n2\n\n\n(\ny\n)\n\n\n on molecular properties and \n\n\nc\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n on the process.LEA3D inherently respects the constraints \n\n\ng\n1\n\n\n(\ny\n)\n\n\n and \n\n\ng\n2\n\n\n(\ny\n)\n\n\n to ensure chemical feasibility of designed molecules. Moreover, LEA3D ensures that each candidate catalyst molecule contains one scaffold fragment and thus one catalytically active group.LEA3D suggests a generation of 3D molecular structures of transition states with candidate catalysts. The initial generation is created by random combination of the scaffold fragment with further molecule fragments from the fragment library. Candidates of subsequent generations are obtained from genetic operations such as mutation and cross-over that are applied to promising candidates of the preceding generation. The generated structures are passed to the automated property prediction and gradient-based process optimization.Suitable starting geometries for the optimization of catalysts and transition states are determined. It is important to note that by the term \u201cgeometry\u201d of a molecule or TS, we here understand the set of spatial positions of all atoms comprising the molecule or TS. In contrast, \u201cmolecular structures\u201d include the connectivity of atoms as shown in structural formulas of molecules. In principle, 3D geometries of transition states are already provided by LEA3D in Step 4. However, it happens occasionally that these structures are not accurate enough for the subsequently used QC methods to work properly. Thus, a first pre-optimization improves the TS geometries employing the force field method MMFF94 (Halgren, 1996a; 1996b; 1996c) available in the chemical toolbox Open Babel (O\u2019Boyle et\u00a0al., 2011; Open Babel). As this first pre-optimization is not suited to handle transition states, the scaffold fragment is substituted by a dummy atom. Afterwards, the dummy is re-substituted by the scaffold fragment using translation and rotation operations in 3D cartesian coordinate space. Next, further pre-optimization is performed using Gaussian 09. Selected rotor scans are performed using the QC method AM1 (Dewar\u00a0et\u00a0al., 1985) to ensure that the minimum energy conformer of each TS is identified. The selection of rotor scans to perform is made by the user (Step 3). Optionally, a geometry optimization with the DFT method B3LYP that minimizes energy may follow the rotor scans. During rotor scans and energy minimization with Gaussian 09, it has to be ensured that no atomic distances are changed that correspond to bonds that break or form during the considered reaction. For this purpose, the \u201cmodredundant\u201d option (Foresman\u00a0and Frisch,\u00a02015) is employed. The TS geometries obtained from the described pre-optimizations are used as starting geometries for subsequent QC calculations. Starting geometries of the catalyst molecules are extracted as a subset of the TS starting geometries.To predict reaction kinetics of the catalytic reactions, reaction rate constants \nk\n are computed as explained in Section\u00a02.1. Calculations for reactants, products and solvents performed in Step 2 are not repeated here.Required thermodynamic equilibrium properties \n\n\u0398\n\n are predicted as described in Section\u00a02.2.Gradient-based process optimizations are performed for processes with each candidate catalyst. For these processes, the process optimizations determine the optimal process performance \n\n\nf\n*\n\n\n(\n\nx\n*\n\n,\n\n\u0398\n\n,\nk\n)\n\n\n as well as the corresponding optimal values of variable process conditions \n\nx\n*\n\n. The interior point algorithm available in the built-in function fmincon in MATLAB is employed for process optimizations. The evaluations of the process model \n\n\nh\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n required during process optimization are performed with the solver ode15s as already mentioned in Section\u00a02.3.As the quantum chemical calculations performed in Steps 6 and 7 may be computationally demanding, the output files of these calculations are stored in dedicated QC file databases. If structures are suggested in Step 4 that were already considered previously, the demanding QC calculations can be skipped. It is important to note here that the prediction of reaction kinetics (Step 6) already requires some thermodynamic equilibrium properties \n\n\u0398\n\n. Moreover, certain equilibrium properties need to be re-evaluated when conditions such as reaction mixture composition change during dynamic processes or when variable process conditions \nx\n are changed during process optimizations (Step 8). Thus, Steps 6 to 8 are not performed strictly in the sequence displayed in Section\u00a02.4.\n\n9.\nThe candidate catalysts of the current generation are ranked based on the determined values of the optimal process performance \n\n\nf\n*\n\n\n(\n\nx\n*\n\n,\n\n\u0398\n\n,\nk\n)\n\n\n.\n\n\n10.\nA new generation of candidate structures is generated by LEA3D based on the previous generation using genetic operations (Step 4). The probabilities that certain candidates of the previous generation are selected as parents for candidates of the new generation are related to the process performance \n\n\nf\n*\n\n\n(\n\nx\n*\n\n,\n\n\u0398\n\n,\nk\n)\n\n\n. Steps 4 to 10 are repeated until the maximum number of generations specified in Step 3 is reached.\n\n\n11.\nAs output of the integrated catalyst and process design with CAT-COSMO-CAMPD, a ranked list of catalyst structures \ny\n including the corresponding values of \n\n\nf\n*\n\n\n(\n\nx\n*\n\n,\n\n\u0398\n\n,\nk\n)\n\n\n and \n\nx\n*\n\n is assembled.\n\n\nThe candidate catalysts of the current generation are ranked based on the determined values of the optimal process performance \n\n\nf\n*\n\n\n(\n\nx\n*\n\n,\n\n\u0398\n\n,\nk\n)\n\n\n.A new generation of candidate structures is generated by LEA3D based on the previous generation using genetic operations (Step 4). The probabilities that certain candidates of the previous generation are selected as parents for candidates of the new generation are related to the process performance \n\n\nf\n*\n\n\n(\n\nx\n*\n\n,\n\n\u0398\n\n,\nk\n)\n\n\n. Steps 4 to 10 are repeated until the maximum number of generations specified in Step 3 is reached.As output of the integrated catalyst and process design with CAT-COSMO-CAMPD, a ranked list of catalyst structures \ny\n including the corresponding values of \n\n\nf\n*\n\n\n(\n\nx\n*\n\n,\n\n\u0398\n\n,\nk\n)\n\n\n and \n\nx\n*\n\n is assembled.Obtaining a ranked list instead of a single optimal catalyst as output of the design offers an important advantage: The user may choose candidates for experimental testing among several near-optimal candidates from the design. Thus, further criteria such as ease of synthesis, commercial availability or toxicity that were not considered in the design may be taken into account in the final choice.To demonstrate the application of CAT-COSMO-CAMPD, an integrated catalyst and process design is performed for a catalytic carbamate-cleavage process (Wang\u00a0et\u00a0al., 2017) of methyl phenyl carbamate (MPC) to phenyl isocyanate and methanol (Fig.\u00a02\n). Carbamate-cleavage reactions represent challenging steps in possible production routes to industrially important isocyanates (Six\u00a0and Richter,\u00a02003). One such production route that aims at CO\n\n\n2\n\n-based isocyanate production (Kaiser\u00a0et\u00a0al., 2018) has been investigated in the research project (Carbon2Chem). The design of carbamate-cleavage processes is challenging because the cleavage reactions are strongly endothermic. Moreover, reaction equilibria strongly favor carbamate formation and are thus very unfavorable for carbamate-cleavage processes (Leitner\u00a0et\u00a0al., 2018). Typically, reaction temperatures of \n\n\nT\n\nR\n\n\n>\n\n\n200\n\n\u2218\n\n\nC\n\n\n are required to drive the reaction. To avoid fast back-reactions, continuous removal of the alcohol produced as by-product is required during carbamate-cleavage. In case of volatile alcohols such as methanol, this continuous removal can be ensured using stripping with inert nitrogen gas (Cao\u00a0et\u00a0al., 2015). A suitable process flowsheet already introduced in previous studies (Gertig et\u00a0al., 2020b; 2020a; 2019b) is shown in Fig.\u00a03\n. A semi-batch reactor is employed to carry out the catalytic cleavage reaction that takes place in the liquid phase using di-phenyl ether as solvent. Nitrogen is used for stripping to carry the formed volatile methanol out of the reactor. A flash is used to condense and recycle unintentionally removed isocyanate, carbamate, solvent and catalyst. Ideally, only nitrogen and methanol leave the semi-batch process with the top product stream of the flash. A process model \n\n\nh\n3\n\n\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n\n for this process was developed as explained in Section\u00a02.3 and is discussed in more detail in the supporting information. Required rate constants \nk\n of the catalytic cleavage reaction as well as thermodynamic equilibrium properties are computed as described in Sections\u00a02.1 and 2.2. It was shown in previous work (Gertig\u00a0et\u00a0al., 2021) that the employed methods are suited to carbamate-cleavage reactions. Due to the challenging nature of carbamate-cleavage, it is expected to be difficult to obtain satisfying isocyanate yields in the considered semi-batch cleavage process, even when using a catalyst. Thus, the isocyanate yield is a good choice for the objective of the integrated catalyst and carbamate-cleavage process design. The specifications of the according design with CAT-COSMO-CAMPD are given in the following section.The reaction and process under consideration are specified as described above. As catalytically active group, the carboxyl group is chosen that is known to have catalytic properties (Satchell\u00a0and Satchell,\u00a01975). The constructed scaffold fragment is shown in Fig.\u00a04\n. As can be seen, the scaffold fragment contains the reactant MPC in a partially cleaved state as well as the carboxyl group of the catalyst that is designed. The carboxyl group acts as both proton acceptor and proton donor in a concerted reaction: The proton originally bound to the nitrogen of the carbamate (atom 2 in Fig.\u00a04) is accepted and simultaneously, the proton initially contained in the carboxyl group (atom 6 in Fig.\u00a04) is donated to the methoxy group of the carbamate to form the by-product methanol. The violet dot marks the anchor point where the scaffold fragment is connected to other 3D molecule fragments in the catalyst design.No significant influence of tunneling is expected for the catalytic carbamate-cleavage. Thus, no tunneling corrections are computed. One rotor scan is performed for each candidate around the bond between the carboxyl group and the rest of the catalyst (atom 4 and violet dot in Fig.\u00a04; see step 5 in Section\u00a02.4).The yield used as objective \n\nf\n(\nx\n,\n\n\u0398\n\n,\nk\n)\n\n of the integrated design is defined as the final moles of isocyanate present in the reactor divided by the initial moles of carbamate provided. This objective is maximized solving the integrated design Problem (1) in order to identify the optimal catalyst structure \n\ny\n*\n\n and corresponding optimal values of variable process conditions \n\nx\n*\n\n. The temperature \n\nT\n\nF\n\n\n in the flash and the volume flow \n\n\nV\n\u02d9\n\n\n\nN\n\n2\n\n\n of nitrogen fed to the reactor are chosen as variable process conditions for process optimizations. The reaction temperature \n\nT\n\nR\n\n\n in the reactor is fixed to 473.15\u00a0K. This reaction temperature lies in the typical range of reaction conditions for carbamate-cleavage (Gertig et\u00a0al., 2021; Wang et\u00a0al., 2017). The reaction temperature is lower compared to our previous study of auto-catalytic carbamate-cleavage (Gertig\u00a0et\u00a0al., 2020b) to take into account that the designed catalysts accelerate the cleavage reaction. Higher temperatures are expected to accelerate the cleavage reaction, but may also lead to undesired side reactions. The pressure of the process is set to \n\n\np\n\nset\n\n\n=\n4\n\n\nbar\n\n\n. The semi-batch process is allowed to run for 12\u00a0h. A reactor volume of \n\n\nV\n\nR\n\n\n=\n1\n\n\n\nm\n\n3\n\n\n as well as initial fractions of 15 mass-% MPC and 5 mass-% catalyst in the reaction mixture are chosen.The constraints \n\n\ng\n1\n\n\n(\ny\n)\n\n\n and \n\n\ng\n2\n\n\n(\ny\n)\n\n\n (see Problem (1)) to ensure chemical feasibility of designed catalyst molecules are inherently respected by the LEA3D algorithm as already mentioned in Section\u00a02.4. Moreover, constraints \n\n\nc\n2\n\n\n(\ny\n)\n\n\n ensure that each designed catalyst contains exactly one scaffold fragment and limit the number of non-hydrogen atoms in the designed catalysts to a maximum of 13. The operating range \nX\n is set to allow flash temperatures of \n\n280\n\n\nK\n\n<\n\nT\n\nF\n\n\n<\n380\n\n\nK\n\n\n as well as nitrogen volume flows of \n\n\n5\n\u00d7\n\n10\n\n\u2212\n5\n\n\n\n\nm\n\n3\n\n\n\ns\n\n\n\u2212\n1\n\n\n\n<\n\n\nV\n\u02d9\n\n\n\nN\n\n2\n\n\n<\n\n1.5\n\u00d7\n\n10\n\n\u2212\n1\n\n\n\n\nm\n\n3\n\n\n\ns\n\n\n\u2212\n1\n\n\n\n\n. The temperature range used for \n\nT\n\nF\n\n\n likely allows for the use of cooling water. The optimal nitrogen volume flow \n\n\nV\n\u02d9\n\n\n\nN\n\n2\n\n*\n\n is sought between values close to zero and an upper bound that is expected to lie well above favorable values. To define the design space \nY\n for catalyst design, various 3D alkyl, aryl, ether, ester, keto, nitrile, halide, sulfene, sulfide, and imine fragments are provided. The full list of fragments is given in the supporting information. After the initial generation of candidate catalyst molecules designed by random combination of fragments, the integrated catalyst and carbamate-cleavage process design is run for 6 further generations of candidates using a number of 12 candidates per generation.The integrated catalyst and carbamate-cleavage process design with CAT-COSMO-CAMPD results in the optimized catalyst molecular structure \n\ny\n*\n\n with SMILES (Weininger,\u00a01988) code ClCOC(C(=O)O)C1CCCCC1. The corresponding 2D structural formula is shown in Fig.\u00a05\n. The achieved objective function value \n\n\nf\n*\n\n\n(\nx\n*\n,\n\n\u0398\n\n,\nk\n)\n\n\n amounts to a yield of 21%. The corresponding optimal values \n\nx\n*\n\n of the variable process conditions are a flash temperature of \n\n\nT\n\nF\n\n\n=\n\n281.1\n\n\n\nK\n\n\n and a nitrogen volume flow of \n\n\n\nV\n\u02d9\n\n\n\nN\n\n2\n\n\n=\n\n5.4\n\u00d7\n\n10\n\n\u2212\n2\n\n\n\n\nm\n\n3\n\n\n\ns\n\n\n\u2212\n1\n\n\n\n\n. The top catalyst molecule enables a considerably higher process performance compared to common carboxylic acids such as acetic acid (9% yield, also displayed in Fig.\u00a05). In total, 33 candidate catalysts from the design shown in Fig.\u00a05 meet all constraints. The full list of SMILES codes of these catalyst molecules can be found in the supporting information.The obtained results show that CAT-COSMO-CAMPD successfully identifies molecular structures of catalysts that considerably improve the predicted process performance compared to common reference molecules. However, it is not clear at this point whether the whole integrated catalyst and process design was required to achieve this result or whether it would be sufficient to perform a less complex catalyst design optimizing the rate constant \nk\n of the catalytic cleavage reaction. To shed light on this question, a catalyst design that is not integrated with process optimization is presented in the following.Performing a less complex catalyst design without integrated process optimization corresponds to reducing the Computer-Aided Molecular and Process Design (CAMPD) to a Computer-Aided Molecular Design (CAMD). Thus, Problem (1) (see Section\u00a02) simplifies: The objective function \n\nf\n(\n\n\u0398\n\n,\nk\n)\n\n does not depend on any variable process conditions \nx\n and no process model \n\nh\n3\n\n is required any more. Consequently, also the possibility to set process constraints \n\nc\n3\n\n as well as an operation range \nX\n are removed. Except any specifications that are not relevant for the resulting CAMD problem, the catalyst design without integrated process optimization is specified as the integrated design (Section\u00a03.1).The catalyst design to maximize the rate constant \nk\n of the catalytic carbamate-cleavage reaction results in brominated formic acid with SMILES code OC(=O)Br as optimal catalyst molecule. The 2D structural formula is shown in Fig.\u00a06\n. The corresponding predicted rate constant at 200\n\n\n\u2218\n\nC amounts to \n\nk\n=\n\n4.43\n\u00d7\n\n10\n\n\u2212\n6\n\n\n\n\n\u00a0m\n\n\n6\n\nmol\n\n\n\n\u2212\n2\n\n\ns\n\n\n\n\u2212\n1\n\n\n. In total, 26 catalysts from the design meet all constraints (Fig.\u00a06). While we suspect brominated formic acid to be unstable at reaction conditions, the results of the design with \nk\n as objective demonstrate the problems associated with simple performance measures: Using a catalyst that enables a high rate constant \nk\n may still lead to a poor process performance. Brominated formic acid as catalyst leads to the highest predicted rate constant \nk\n, but the process using this catalyst achieves only a relatively low yield of 13% after 12\u00a0h (see Fig.\u00a05). The underlying reason for the poor performance of simple design objectives is that besides a high reaction rate constant, further aspects are important to reach maximum process performance. For example, the volatility of the catalyst influences catalyst loss due to stripping with nitrogen. Highly polar catalysts increase the overall polarity of the reaction mixture, which has adverse effects on the reaction rate depending on the amount of catalyst used. Moreover, the catalyst molecule influences activities of other substances in the mixture and thus the vapor-liquid equilibria (VLE) in the process. Consequently, for example, the catalyst impacts the activity coefficient of methanol in the flash and can lead to unfavorable methanol recycling. In summary, several criteria need to be accounted for and trade-offs between these criteria have to be made in order to reach optimum process performance. Therefore, an integrated catalyst and process design is required that combines molecular design with process optimization. Still, evaluation criteria used to identify promising catalyst molecules might sometimes even go less far as to calculate reaction rate constants. For many reactions, simpler, heuristic criteria could be thought of. In the subsequent section, we discuss why such criteria may not only fail to identify catalysts enabling optimal process performance, but even fail to find catalysts that enable optimal rate constants.In many cases, heuristic criteria for catalyst design can be defined that allow assessing candidates with less effort than computing reaction kinetics or using process-based evaluation such as our design method CAT-COSMO-CAMPD. For the catalytic carbamate-cleavage reaction considered in the present case study, we discuss three possible examples of heuristic evaluation criteria:\n\n\u2022\nThe barrier in electronic energy \n\n\n\u0394\n\n\nE\n\nel\n\n\n\n computed in vacuum often represents the largest contribution to the overall activation barrier \n\n\n\u0394\n\n\nG\n\u2021\n\n\n in Gibbs free energy that the reaction has to overcome. Thus, a heuristic design objective for catalyst design could be to minimize \n\n\n\u0394\n\n\nE\n\nel\n\n\n\n of the reaction.\n\n\n\u2022\nThe catalyst and the carbamate form a ring for concerted proton transport in the transition state of the catalytic carbamate-cleavage reaction (see Fig.\u00a04, atoms 1\u20138). Thus, it could be supposed that the proton donor and/or acceptor properties of the carboxyl group determine catalytic activity. These donor/acceptor properties are related e.g., to the partial charges of the oxygen atoms of the carboxyl group of the catalyst. Therefore, the partial charges should be either strong or weak, depending on whether proton acceptance or donation is critical, or a certain value represents the optimal trade-off. Thus, another objective for catalyst design could be to maximize or minimize partial charges of the carboxyl oxygen atoms or to match a certain target value.\n\n\n\u2022\nThere are assumptions mentioned in literature (Satchell\u00a0and Satchell,\u00a01975) that a nucleophilic character of the oxygen atom of the reactant\u2019s methoxy group stabilizes the transition state of the reaction. Thus, it is supposed that an important function of the catalyst is to increase this nucleophilic character. A high nucleophilicity should be associated with a high negative partial charge of the oxygen atom and should be observable in the transition state of the catalytic reaction.\n\n\nThe barrier in electronic energy \n\n\n\u0394\n\n\nE\n\nel\n\n\n\n computed in vacuum often represents the largest contribution to the overall activation barrier \n\n\n\u0394\n\n\nG\n\u2021\n\n\n in Gibbs free energy that the reaction has to overcome. Thus, a heuristic design objective for catalyst design could be to minimize \n\n\n\u0394\n\n\nE\n\nel\n\n\n\n of the reaction.The catalyst and the carbamate form a ring for concerted proton transport in the transition state of the catalytic carbamate-cleavage reaction (see Fig.\u00a04, atoms 1\u20138). Thus, it could be supposed that the proton donor and/or acceptor properties of the carboxyl group determine catalytic activity. These donor/acceptor properties are related e.g., to the partial charges of the oxygen atoms of the carboxyl group of the catalyst. Therefore, the partial charges should be either strong or weak, depending on whether proton acceptance or donation is critical, or a certain value represents the optimal trade-off. Thus, another objective for catalyst design could be to maximize or minimize partial charges of the carboxyl oxygen atoms or to match a certain target value.There are assumptions mentioned in literature (Satchell\u00a0and Satchell,\u00a01975) that a nucleophilic character of the oxygen atom of the reactant\u2019s methoxy group stabilizes the transition state of the reaction. Thus, it is supposed that an important function of the catalyst is to increase this nucleophilic character. A high nucleophilicity should be associated with a high negative partial charge of the oxygen atom and should be observable in the transition state of the catalytic reaction.The 3 criteria discussed above were evaluated for the catalysts designed in the design run presented in Section\u00a03.4. All required quantities were extracted from the output of the DLPNO-CCSD(T) calculations (see Step 2 of the computation scheme explained in Section\u00a02.1). Mulliken charges (Mulliken,\u00a01955) are used to approximate the partial charges of atoms. Fig.\u00a07\n plots the criteria versus the logarithm of the reaction rate constant \nk\n achieved with the respective catalysts. For reasons discussed in the preceding sections, it cannot be expected that the heuristic criteria reflect the achievable process performance. Still, one could argue that using one of the heuristic criteria should at least result in the design of catalysts that optimize the reaction rate constant \nk\n of the catalytic carbamate-cleavage. However, if this was the case, the chosen criterion should correlate well with \nk\n.As can be seen in the upper part of Fig.\u00a07, there is a general trend of increasing rate constants \nk\n with decreasing barriers in electronic energy \n\n\n\u0394\n\n\nE\n\nel\n\n\n\n as expected. However, the correlation of \nk\n with \n\n\n\u0394\n\n\nE\n\nel\n\n\n\n is clearly not good enough for \n\n\n\u0394\n\n\nE\n\nel\n\n\n\n to serve as design objective. In particular, the catalyst leading to the lowest \n\n\n\u0394\n\n\nE\n\nel\n\n\n\n enables only a moderate rate constant \nk\n. The reason for the insufficient correlation is that important effects on \nk\n are not reflected by \n\n\n\u0394\n\n\nE\n\nel\n\n\n\n. Although representing a major contribution to the activation barrier \n\n\n\u0394\n\n\nG\n\u2021\n\n\n that in turn determines the rate constant, evaluation of catalysts based on \n\n\n\u0394\n\n\nE\n\nel\n\n\n\n completely neglects entropic and solvation effects.The partial charges of the oxygen atoms in the carboxyl group of the catalyst are shown in the middle part of Fig.\u00a07. The term \u201cdonor O-atom\u201d hereby refers to the oxygen atom initially bonded to a hydrogen atom. The correlation of these partial charges with \nk\n is not satisfactory. Indeed, there is a trend that the absolute values of the charges of the oxygen atoms decrease with increasing rate constant \nk\n, which could be related to proton donor and acceptor properties of the catalysts. In fact, most of the highly ranked catalysts contain electron-withdrawing groups like halogens or nitrile groups, which explains the small absolute charge of the oxygen atoms in the carboxyl group. However, it can be seen in Fig.\u00a07 that the trend is only weak and there is strong scattering.The third suggested heuristic criterion, the nucleophilic character of the oxygen atom of the methoxy group, is evaluated based on the Mulliken charge of the O-atom in the TS. Interestingly, some of the catalysts that enable very high rate constants strongly increase the absolute charge of the O-atom, relating to an increased nucleophilicity. However, no general trend is observed over the whole range of \nk\n values. Thus, the third heuristic criterion is also no reliable design objective.The above discussion shows why choosing heuristic criteria as design objectives is not reliable: Important effects are likely neglected by such criteria. The correlation of heuristic criteria with the ultimate goal of a catalyst design may be weak and there may be strong scattering.In summary, two important conclusions are drawn from the presented results: First, a full computation of reaction kinetics is required in catalyst design. Heuristic criteria do not guarantee finding the catalyst structures that enable the largest catalytic effects. Second, various other effects besides accelerating reaction kinetics are important to achieve optimal process performance. Consequently, a process-based evaluation of candidate catalysts as used by CAT-COSMO-CAMPD is strongly recommended.Here, we present CAT-COSMO-CAMPD as a method for integrated in silico design of catalysts and processes. The integrated design is formulated as optimization problem and a hybrid optimization scheme is employed to solve the design problem: The genetic optimization algorithm LEA3D is used to explore the chemical design space and to identify the most promising candidate catalyst molecules. Process conditions are optimized in gradient-based process optimizations. The LEA3D algorithm works with 3D structures and designs molecules based on a library of 3D molecule fragments. Thus, 3D structures of all considered molecules are available throughout the design and the direct use of quantum chemical methods for property prediction is facilitated. Consequently, CAT-COSMO-CAMPD avoids simplified property prediction methods and the need for extensive experimental data. Reaction rate constants are predicted based on transition state theory and thermodynamic properties of mixtures are computed using the advanced solvation model COSMO-RS.The application of CAT-COSMO-CAMPD for integrated catalyst and process design is demonstrated for a catalytic carbamate-cleavage process. The results show that promising catalyst molecules are identified and processes are optimized successfully. Moreover, we show that the integration of molecular design with process optimization is required to achieve optimal process performance. In contrast, selection of catalysts based on predicted reaction rate constants or heuristic criteria likely fails to find the optimal catalyst structures.So far, we have applied CAT-COSMO-CAMPD only to organic molecular catalysts. More sophisticated homogeneous catalysts such as catalyst complexes may be designed with the same approach, although the QC-based prediction of the catalytic activity of the complexes will be more demanding. First, larger system sizes will either require more computer power or using less sophisticated QC methods. Second, converging geometry optimizations during automated design may become more challenging for larger catalyst structures. Third, different possible spin multiplicities will have to be considered for some catalyst complexes.The design of heterogeneous catalysts is not within the current scope of our method. For such catalysts, dedicated approaches are required to obtain candidate catalysts and to predict catalytic activity. For a review of such approaches, the reader is referred to Freeze\u00a0et\u00a0al.\u00a0(2019). However, the integration of catalyst design with process optimization using a hybrid optimization scheme as introduced with CAT-COSMO-CAMPD may also be applied in the design of heterogeneous catalysts.Possible extensions to CAT-COSMO-CAMPD also include more sophisticated process simulation and optimization. For the demonstration of CAT-COSMO-CAMPD in the case study presented in the previous section, a process model was implemented that contains the 2-phase reactor and a flash. Considering additional separation steps or even the complete downstream processing is straightforward in the sense that the additional steps may be implemented in the same process model (Scheffczyk\u00a0et\u00a0al., 2018). Of course, the process optimization is expected to become increasingly challenging with an increasing number of degrees of freedom.In principle, it is also possible to call an external software for rigorous process simulation. In this case, the user could benefit from available libraries with detailed unit operation models and dedicated solution algorithms. In turn, the use of external programs requires suitable interfacing and thus needs to consider how data is transferred to and from the process simulation software. In addition, it must be ensured that no convergence problems occur that would require manual overrides and thus hinder the automation of the design. However, promising solutions for this approach have been presented (Scheffczyk et\u00a0al., 2018; L\u00f3pez et\u00a0al., 2018; Navarro-Amor\u00f3s et\u00a0al., 2014). As an alternative to integrating external software into the design, sophisticated process simulations and optimizations could be used to examine promising candidates after the actual integrated design.Current limitations of CAT-COSMO-CAMPD in the design of homogeneous catalysts are the accuracy and computational requirements of QC methods used as well as the need to know the mechanism of catalysis and the active group in advance. However, in silico design methods such as CAT-COSMO-CAMPD will benefit from the growth of computational capacity and the ongoing development of efficient but accurate QC methods. Thus, we expect that the design of increasingly complex systems will become possible. There are also promising developments in the field of automated generation of reaction networks (D\u00f6ntgen et\u00a0al., 2015; 2018; Simm et\u00a0al., 2018; Dewyer et\u00a0al., 2018) that may serve to overcome the need of CAT-COSMO-CAMPD to know the mechanism of catalysis in advance. Moreover, such an automated determination of reaction mechanisms could ease the consideration of multiple catalytically active groups and undesired side reactions. At present, none of the available methods seems to be accurate and efficient enough for a broad range of systems. However, improving such methods is subject of ongoing research.Surrogate models (e.g., neural networks) may complement QC methods in property prediction during design. For the design of reaction solvents, a hybrid approach using both QC-based prediction of reaction rates and a surrogate model was already introduced by Struebing et\u00a0al. (2013, 2017). In their work, only part of the candidate molecules are investigated using the full QC-based treatment. Most candidates are evaluated with a surrogate model that is constantly refined during the design procedure using the results from QC. However, one should be aware that the advantages of surrogates regarding computation time and computational capacity usually come with the disadvantage of a lower accuracy compared to the original model. Still, depending on the computational requirements and the number of candidates that need to be evaluated, also integrated catalyst and process design might benefit from such a hybrid approach. Exploiting the recent advances in machine learning for catalyst design (dos\u00a0Passos\u00a0Gomes et\u00a0al., 2021) could therefore be promising.Despite the use of high-level QC methods, there is no guarantee that the prediction of catalytic activity does not overestimate the performance of candidates. Therefore, we recommend integrating in silico design with selected experiments in the future. A first step would be testing the top candidates obtained with CAT-COSMO-CAMPD experimentally as already shown for our approach to integrated design of solvents and separation processes (Scheffczyk\u00a0et\u00a0al., 2018). The experimental results could be used to confirm the catalytic activity and, if required, to modify the used combination of QC methods for a second design run with CAT-COSMO-CAMPD.In summary, this article shows the promise of methods for integrated in silico design of catalysts and processes. The proposed CAT-COSMO-CAMPD method was demonstrated successfully in the presented case study and is generally applicable to various other systems. As computational design of molecular catalysts is still a quite unexplored field of research, we expect major developments in the coming years.\nChristoph Gertig: Conceptualization, Methodology, Software, Formal analysis, Visualization, Writing \u2013 original draft, Writing \u2013 review & editing. Lorenz Fleitmann: Methodology, Software, Writing \u2013 review & editing. Carl Hemprich: Methodology, Software, Formal analysis, Visualization, Writing \u2013 review & editing. Janik Hense: Methodology, Formal analysis, Writing \u2013 review & editing. Andr\u00e9 Bardow: Conceptualization, Funding acquisition, Writing \u2013 review & editing, Supervision. Kai Leonhard: Conceptualization, Funding acquisition, Writing \u2013 review & editing, Supervision.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.C.G., A.B. and K.L. gratefully acknowledge funding from the German Federal Ministry of Education and Research (BMBF) within the project Carbon2Polymers (03EK30442C). L.F. thanks the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for funding under Germany\u2019s Excellence Strategy - Cluster of Excellence 2186 \"The Fuel Science Center\u201d - ID: 390919832. Furthermore, the authors are grateful to J. Langanke, M. Leven and E. Erdkamp for valuable discussions. Simulations were performed with computing resources granted by RWTH Aachen University under projects rwth0284 and rwth0478.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.compchemeng.2021.107438.\n\n\nSupplementary Data S1\n\nSupplementary Raw Research Data. This is open data under the CC BY license http://creativecommons.org/licenses/by/4.0/\n\n\nSupplementary Data S1\n\n\n\n", "descript": "\n                  Catalysts are of paramount importance as most chemical processes would be uneconomical without suitable catalysts. Consequently, the identification of appropriate catalysts is a key step in chemical process design. However, the number of potential catalysts is usually vast. To suggest promising candidates for experimental testing, in silico catalyst design methods are highly desirable. Still, such computational methods are in their infancy. Moreover, simple performance indicators are commonly employed as design objective instead of evaluating the actual process performance enabled by considered catalysts. Here, we present the CAT-COSMO-CAMPD method for integrated in silico design of homogeneous molecular catalysts and processes. CAT-COSMO-CAMPD integrates design of molecular catalysts with process optimization, enabling a process-based evaluation of every designed candidate catalyst. Reaction kinetics of catalytic reactions are predicted by advanced quantum chemical methods. We demonstrate for a catalytic carbamate-cleavage process that CAT-COSMO-CAMPD successfully identifies catalyst molecules maximizing the predicted process performance.\n               "}
{"full_text": "The alarming increase of anthropogenic emissions of greenhouse gases (GHG) is encouraging extensive research to mitigate the impact of these emissions. The transition to low-carbon societies demands strategies to reduce GHG emissions, consisted mainly of CO2 and CH4. Most of the CO2 emissions come from the consumption of fossil fuel for energy (International Energy Agency, 2018). Nevertheless, it is less known the important contribution of intensive livestock and other organic waste to GHG emissions, which come from anaerobic digestion (IPCC Fourth Assessment Report, 2014). These emissions, mainly in the form of biogas (a mixture primarily formed by CO2 and CH4), were regarded a waste rather than a value, but this trend is changing. Biogas industry business model is moving to two different scenarios: (i) upgrading to biomethane in order to produce biofuels to generate energy and (ii) chemical valorisation via reforming to produce syngas, primarily formed by H2 and CO, which is an interesting platform chemical in chemical synthesis. Indeed, syngas is widely used as a precursor to synthetise fuels and other hydrocarbons via Fischer-Tropsch process, or other value-added products, such as methanol or acetic acid (Mart\u00edn-Espejo et al., 2022). Biogas upgrading via chemical valorisation is deemed an emerging trend which may provide many opportunities to the chemical industry to tackle GHG emissions (Baena-Moreno et al., 2021). In this context, thermocatalytic biogas upgrading to syngas can be addressed via dry reforming of methane (DRM, Eq. (1)), representing an attractive way to convert CO2 and CH4.\n\n(1)\n\n\nC\n\nO\n2\n\n+\nC\n\nH\n4\n\n\u2192\n2\nCO\n+\n2\n\nH\n2\n\n\n\n\n\nTypically, conventional catalysts for DRM are formed by a metal active phase which is dispersed over a support structure. Noble metals, like Ru, Rh or Pt, are reported to display great performance. Nevertheless, due their scarcity and high market prices, research focus is switching into the use of transition metals, such as Ni and Co (Jang et al., 2019; Sharifianjazi et al., 2021). Thermodynamically, DRM is a very endothermic reaction which requires high temperatures and energy inputs. At such temperatures, conventional catalysts are prone to deactivation due to sintering of the active phase and formation of coke deposits coming from side reactions such as Boudouard reaction (Eq. (2)), CH4 decomposition (Eq. (3)) and CO and CO2 reduction (Eqs. (4) and (5)) (Nikoo and Amin, 2011). Extensive investigation has been conducted in order to find cost-effective catalysts which avoid carbon deposition and sintering of the active phase while exhibiting acceptable catalytic performance.\n\n(2)\n\n\n2\n\nCO\n\n\n\u2192\n\n\nC\n\n+\nC\n\nO\n2\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\n\n\n\n\n(4)\n\n\nCO\n+\n\nH\n2\n\n\u2192\n\n\nC\n\n+\n\nH\n2\n\nO\n\n\n\n\n\n\n(5)\n\n\nC\n\nO\n2\n\n+\n2\n\nH\n2\n\n\u2192\n\n\nC\n\n+\n2\n\nH\n2\n\nO\n\n\n\n\nNickel-based catalysts are frequently chosen among other transition metals owing to their commendable activity, low price and availability. However, these catalysts suffer from coking and sintering, which leads to rapid deactivation. Multiple strategies have been applied to deal with the deactivation of these catalysts. Recent literature review reports highlight the most promising advances to design nickel-based DRM catalysts resistant to deactivation (Huang et al., 2022; Le sach\u00e9 and Reina, 2022; Yentekakis et al., 2021). The use of bimetallic formulations, the addition of promoters or the combination of different support structures are typical strategies used to enhance the performance of Ni-based catalysts, tuning the nature of the catalysts. In general, chemical and structural properties (e.g., redox, acid/base, oxygen mobility) are tuned using these strategies, making an impact on the stability and reaction mechanism. Besides, dispersion and particle size can be influenced by these parameters. As a promising alternative, the use of inorganic complex structures is aimed at stabilising the active phase in the structure while remaining active and accessible. Spinels, sandwich, tubular or mesoporous structures, hydroxyapatite, hexaaluminate, hydrotalcite, perovskites and pyrochlores have been investigated to improve the performance of DRM (Bhattar et al., 2021; Le Sach\u00e9 and Reina, 2022). Pyrochlores, with the formula A2B2O7, and perovskites, ABO3 and A2BO4, are mixed oxide materials in which A-site is typically substituted by a large rare-earth trivalent metal whereas B-site is substituted by a tetravalent transition metal of smaller diameter. These materials are highly crystalline, possess great thermal stability and oxygen mobility, which makes then suitable for high temperature and coke-prone processes, such as dry reforming of methane (Xu et al., 2020). For these very reasons, these mixed oxide materials have been previously studied for biogas reforming. For instance, Bhattar et al. (2020) studied the effect of the addition of Sr- and Ca- to Ni-substituted lanthanum zirconate catalysts. It was found that the addition of small amount of Sr improved the performance of the catalyst as well as an increase of the resistance of the catalyst to deactivation from carbon deposition. On the other hand, La2Ce2O7 and LaNiO3 were synthetised by Ramon et al. (2022) in order to elucidate the catalytic activity of this catalyst in DRM reaction, comparing two different synthesis methods. In another study by Ma et al. (2014), they investigated the effect of nickel-supported La2Zr2O7 pyrochlore-like materials for steam reforming of methane, showing an excellent catalytic behaviour since coking resistance was highly improved. Two studies by Le Sach\u00e9 et al. (2018a, 2020) successfully proved the incorporation of Ni into a La2Zr2-xNixO7-\u03b4 pyrochlore structure for dry and bi-reforming of methane. According to the XRD results, the formation of a perovskite-type La2NiZrO6 is responsible for the great catalytic performance. Bai et al. (2022) have studied the effect of the substitution of nickel over both cerium and zirconium on B-site La2(CeZrNi)2O7 for dry reforming of methane. In this study, it is believed that the exceptional oxygen vacancies and the interaction of the exsolved Ni with the support were key properties for the outstanding performance of the catalysts. Nevertheless, the incorporation of just cerium and zirconium to form a complex inorganic structure has not been tried for dry reforming despite cerium\u00b4s excellent oxygen storage/release ability (Teh et al., 2021). Attempts have been tried for Ce2Zr2O7 materials in photocatalysis for organic pollutants abatement, as Jayaraman and Mani (2020) have studied over a g-C3N4 support structure. Ce2Zr2O7 has also been studied on PbS in order to study the electrochemical properties as a supercapacitor electrode (Bibi et al., 2019). As of today, no attempts have been made to study the catalytic activity of Ni-substituted Ce2Zr2O7 for DRM.In this scenario, this work addresses the design of inorganic complex structures to stabilise nickel, leading to robust catalysts for DRM. Under this premise, the utilisation of a thermally stable cerium zirconate oxide structure is studied, inserting and stabilising nickel within the structure (Ce2Zr2-xNixO7-\u03b4). This study is focused on the synthesis, catalytic activity, pre- and post-characterisation of Ni-substituted cerium zirconate for DRM. Specifically, different loading of Ni, from 0 to 15 wt.%, were incorporated to the structure, substituting on the B-site of the mixed oxide structure. Our work showcases an effective strategy to design robust and economically viable gas-phase CO2 conversion catalysts with potential applications in reforming units and biogas plants.The catalysts were prepared through a modified version of the original Pechini method (Pechini, 1967), which is described elsewhere (Gaur et al., 2011; Kumar et al., 2016; Tietz et al., 2001). This method was chosen since it is reported to produce uniform substituted and non-susbtituted catalyst crystals (Haynes et al., 2008). Cerium nitrate (Ce(NO3)3\u00b76H2O), zirconyl nitrate (ZrO(NO3)2\u00b76H2O), provided by Sigma-Aldrich, and nickel nitrate (Ni(NO3)2\u00b76H2O), provided by Alfa Aesar, were used as precursors. The necessary amount of each precursor was separately dissolved in deionised water and then mixed together. Citric acid (CA) was dissolved in deionised water and incorporated to the mixture of precursors while stirring at room temperature. The amount of citric acid added was molar ratio of CA:metal of 1.2:1. The solution was heated while stirring to 90\u00b0C to ensure metal complexation and ethylene glycol (EG) was added to the solution drop-wise, using a molar ratio of EG:CA 1:1. The solution was then continuously stirred and concentrated due to evaporation of the water until the appearance of a viscous gel. The stirring was then stopped and the dense gel was left at 90-100\u00b0C to promote the polyesterification reaction between the citric acid and ethylene glycol. The decomposition of the nitrate precursors led to large plumes of NOx gas. Once the toxic gas is released, the solid was dried at 100\u00b0C overnight. The resulting compound was powdered manually in an agate mortar and then calcined in a crucible at 1000\u00b0C during 8 h, using a heating rate of 7.5\u00b0C min\u22121, to ensure phase transition. To simplify, a special notation is chosen and the catalysts will be referred as CZ, CZN5, CZN10, CZN12 and CZN15 for 0, 5, 10, 12.5 and 15 wt.% of Ni, respectively.The textural properties of the samples were characterised by nitrogen adsorption-desorption measurements at liquid nitrogen temperature (-195.8\u00b0C) in a Micromeritics Tristar II apparatus. Before analysis, the samples were out-gassed under vacuum conditions at 250 \u00b0C for 4 h. The specific surface area (SBET) was calculated using the Brunauer-Emmet-Teller (BET) method. The average pore volume was determined as the ratio of the pore volume and the specific surface area. This was then normalised using a coefficient which depends on the pores shape.The metal content of Ni was measured by inductively coupled plasma spectroscopy (ICP-MS) using iCAP 7200 ICP-OES Duo (ThermoFisher Scientific) spectrometer previous microwave digestion in an ETHOS EASY (Milestone) microwave digestion platform.X-Ray Diffraction (XRD) measurements were carried out on X'Pert Pro PANalytic diffractometer with Cu-K\u03b1 anode at room temperature, working at a voltage of 45 kV and a current of 40 mA. The diffractograms were registered between 20 and 90\u00b0 (2\u03b8) with a step size of 0.05\u00b0 and a step time of 300 s. The structural determination was done by comparison with PDF2 ICDD2000 (Powder Diffraction File 2 International Center for Diffraction Data, 2000) database.Temperature-programmed reduction (TPR) with H2 was carried out on the calcined catalysts in a conventional U-shaped quartz reactor connected with a thermal conductivity detector (TCD) using a flow of 50 mL min\u22121 of 5% H2 (v/v) diluted in Ar. TPR measurements were performed using 100 mg of each catalyst and a heating rate of 10\u00b0C min\u22121 from room temperature to 900\u00b0C, using a CO2 (s)/acetone cold trap to condense the water formed during the process.Scanning electron microscopy (SEM) was carried out on the calcined catalysts under vacuum using a Hitachi S4800 SEM-FEG 0.5-30 kV voltage microscope using a cold cathode field emission gun of 1 nm resolution and equipped with a Bruker-X Flash-4010 EDS analyser.Transmission electron microscopy (TEM) of the samples were performed on a JEOL 2100Plus (200 kV) microscope. It was equipped with an Energy Dispersive X-Ray analysis system (EDX X-Max 80T, Oxford Instruments) and a CCD camera for image recording.The catalytic performance of the prepared samples for DRM reaction was evaluated under atmospheric pressure in a tubular, continuous flow fixed-bed reactor (Hastelloy reactor) with an internal diameter of 9 mm in an automatised Microactivity Reference apparatus from PID Eng&Tech. Stability tests were performed in a tubular fixed bed quartz reactor with an inside diameter of 10 mm.The catalysts were sieved and the 100-200 \u00b5m fraction was placed in the reactor over a quartz wool bed. Prior to the activity test, the catalysts were in situ reduced in a flow of 50 mL min\u22121 40% H2 (v/v) in He, at 800\u00b0C for 1 h using a heating rate of 7.5\u00b0C min\u22121. The reaction was performed passing a reactant feed flow of 100 mL min\u22121 and molar ratio of N2:CH4:CO2 2:1:1, every 50\u00b0C from 500 to 800\u00b0C until achieving the steady state on each step. The WHSV (Weight Hourly Space Velocity) was fixed at 30 L gcat\n\u22121 h\u22121. Furthermore, stability tests were conducted at the same conditions, at 600\u202f\u00b0C and 800\u00b0C, during 100\u202fh.The composition of the product gas stream was monitored using an on-line gas chromatography (Agilent Technologies) equipped with a HayeSep Q and Mol sieve 5A column. An ABB AO2020 on-line gas analyser was used to determine the composition of the product gas stream in the stability tests. The spent samples were recovered for post-reaction characterisation. In all the cases, carbon balance was closed \u00b15%.The conversion (Xi) of the reactants (Eqs. (6) and (7)) and the H2/CO molar ratio (Eq. (8)) was calculated in order to evaluate the catalytic behaviour. The conversion was calculated as follows:\n\n(6)\n\n\n\nX\n\nC\n\nH\n4\n\n\n\n\n\n(\n%\n)\n\n=\n\n\n\nF\n\nC\n\nH\n4\n\n,\nin\n\n\n\u2212\n\nF\n\nC\n\nH\n4\n\n,\nout\n\n\n\n\nF\n\nC\n\nH\n4\n\n,\nin\n\n\n\n\u00b7\n100\n\n\n\n\n\n\n(7)\n\n\n\nX\n\nC\n\nO\n2\n\n\n\n\n\n(\n%\n)\n\n=\n\n\n\nF\n\nC\n\nO\n2\n\n,\nin\n\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\u00b7\n100\n\n\n\n\n\n\n(8)\n\n\n\nH\n2\n\n/\nCO\n=\n\n\nF\n\n\nH\n2\n\n,\nout\n\n\n\nF\n\nCO\n,\nout\n\n\n\n\n\n\nwhere F is the molar flow of CH4, CO2, H2, and CO, respectively, and the subscripts in or out correspond to either the inlet and the outlet reactor flow.Chemical composition of nickel and textural properties of the prepared samples are listed in Table 1\n. The metal loading of the catalysts is close to the nominal values of 5, 10, 12.5 and 15 wt.% of Ni, witnessing the successful preparation method to carefully adjust the desired active phase loading. Still, nickel amounts of the high-Ni containing samples (CZN12 and CZN15) are slightly lower than the intended values which might indicate a threshold on optimal Ni uptake.Regarding the textural properties of the samples, we observe the noteworthy low surface area of all of the synthesised materials in contrast to benchmark supported catalysts, which have much higher surface area (e.g., Ni-doped Al2O3-CeO2 with SBET\u202f=\u202f208 m2 g\u22121 (Marinho et al., 2021)). Therefore, this factor is important to consider since the engineered materials herein are considered to be active for DRM despite their low surface area. In other words, the DRM reaction does not necessarily require high-surface area catalysts to reach high performances as we will evidence further down in this work. Furthermore, a slight reduction of the surface area is observed when the Ni loading is increased. This reduction is more evident in the pore volume which indicates a certain degree of Ni particles blockage of the catalysts\u00b4 pores.In order to confirm the formation of the different crystalline inorganic structures, XRD analysis of the calcined catalysts was performed. The resulting normalised diffractograms of the Ni-doped cerium zirconate oxide structures with 0, 5, 10, 12.5 and 15 wt.% are presented in Fig. 1\n. No characteristic diffraction peaks of individual CeO2 oxide phases are observed, which may suggest the incorporation of Ce into the inorganic lattice structure. CZ undoped catalyst pattern presents the characteristic diffraction features of Ce0.5Zr0.5O2 tetragonal structure with space group P42/nmc (ICDD Card No. 00-038-1436) and Ce2Zr2O7 pyrochlore cubic structure Fm\n\n3\n\u00af\n\nm (ICDD Card No. 00-008-0221). Interestingly, when Ni loading is increased, a slight shift in the diffraction peaks towards lower angles is observed between the undoped material (CZ) and the doped catalysts (CZNX). Therefore, the partial substitution of Ni on the B-site affects the crystalline structure. This shift can be observed in 29.4, 33.9, 48.9 and 58.1\u00b0 2\u03b8 of the characteristic diffraction features of Ce0.5Zr0.5O2. The incorporation of Ni using the synthesis method produces a change in the final structure of the cerium zirconate oxide, incrementing the lattice parameter of the material, as reported when doping these structures (Haynes et al., 2008; Le Sach\u00e9 et al., 2018b; Pakhare et al., 2013). On the other hand, it is observed that Zr is not completely incorporated into the inorganic complex structure in CZN5 and CZN10, appearing a diffraction line around 30\u00b0 2\u03b8 which may correspond to ZrO2 tetragonal structure with space group P42/nmc (ICDD Card No. 00-024-1164).For CZN5, an interesting effect is observed. Diffraction lines abovementioned, associated to the inorganic mixed oxide, start to unfolding, distinctly observing two peaks with similar intensity instead of one, as emphasised in the inset of Fig. 1. It seems that two crystalline phases are clearly formed, appearing the characteristic features of Ce0.5Zr0.5O2 tetragonal structure (ICDD Card No. 00-038-1436) and the separate Ce2Zr2O7 pyrochlore cubic structure (ICDD Card No. 00-008-0221). For Ce2Zr2O7 pyrochlore cubic structure, the diffraction lines at 2\u03b8 values of 28.9, 33.6, 48.1 57.1, 59.9 and 70.4\u00b0 are attributed to the (222), (400), (440), (622), (444) and (800) crystal planes. Considering the radius ratio rA/rB, which is relevant factor for A2B2O7-\u03b4 inorganic complex structures (Bai et al., 2022; Xu et al., 2020), the substitution of Ni on B-site within the structure produces a decrease of this parameter since Ni radius is smaller than that of Zr, which is associated with a less ordered structure. When the loading of Ni is further increased, the unfolding phenomenon vanished, appearing mainly one diffraction pattern corresponding again to Ce2Zr2O7 cubic structure (ICDD Card No. 00-008-0221). Therefore, it is proved that the incorporation of Ni produces variations in the inorganic oxides formed in this material.Regarding the active phase, it is observed that part of the Ni is inserted in the inorganic structure. Cubic perovskite CeNiO3 phase with space group Pm\n\n3\n\u00af\n\nm (Material project Card No. mp-866095) can be identified in all the doped materials at 33.5, 48.1 and 59.8\u00b0. Besides, the presence of some NiO domains can be observed in the diffractogram. Diffraction lines 37.3, 43.3, 62.9 and 75.4\u00b0 2\u03b8 are attributed to rhombohedral NiO (ICDD Card No. 00-022-1189). In general, the intensity of the diffraction line attributed to NiO species remains the same as the Ni content increases. Interestingly, in the most intense diffraction line of NiO, it is observed a slight displacement of the peak towards higher degrees as the nickel content is increased, reaching 43.5\u00b0 2\u03b8 for CZN15. This increase is associated to NiO1-x species, where x is higher as the increase of this diffraction line angle. Indeed, the ratio Ni/O is closer to orthorhombic Ni4O3 (mp-656887) than NiO in CZN15. This is closely related to the diffraction lines shift previously reported of the mixed oxide structure to lower angles. Therefore, it may be concluded that Ni is distributed in the lattice structure, interacting differently with the oxide species resulting of the formation of this material.Further insights on the redox behaviour of our mixed-oxide systems were gathered by temperature-programmed reduction (TPR) experiments. The resulting H2 consumption profiles are depicted in Fig. 2\n. The composition of the samples, interactions among the active species of the calcined catalysts and the conditions necessary for the pre-treatment step were also analysed in light of the TPR data. The undoped material CZ has been found to have small reducibility. A small H2-consumption signal appears due to the possible interaction with the Ce species inserted in the catalyst. This signal appears between 400 and 540\u00b0C. Nevertheless, this reduction event is small. Overall, the observed H2 consumption in the Ni-based materials is mainly due to the reducibility of Ni species upon its incorporation in the catalysts. In the profiles herein presented of the doped materials, three main reduction regions are worth considering. The first region, with a temperature peak between 365 and 375\u00b0C, can be attributed to the reduction of NiO1-x species which are interacting with the Ce0.5Zr0.5O2 phase. The presence of this phase is presented mainly in CZN5 XRD in Fig. 1, which is consistent with the results. TPR signal is reduced for CZN10, CZN12 and CZN15, since this phase is disappearing when the Ni content is increased in the samples. A medium temperature region is observed between 400 and 435\u00b0C. This phase can be attributed to the interaction of NiO1-x species with the Ce2Zr2O7 pyrochlore phase. In this case, it is observed that the peak is displaced to higher temperatures as the Ni content is increased, which may indicate a stronger interaction with the phase. Besides, the intensity of the signal increases with the Ni loading, which may indicate that higher Ni loadings may lead to larger proportions of NiO1-x. Finally, the most intense signal, between 535 and 580\u00b0C, is attributed to the partial reduction of CeNiO3 phase, which is more difficult to reduce. This signal, in general, decreases as the Ni content is increased.XRD analysis was also performed on all the samples reduced at 800\u00b0C for 1 h. NiO1-x species were reduced to Ni0 since the characteristic peaks of NiO1-x shifted to higher diffraction line angles after the H2 treatment. Characteristic peaks of Ni0 at 44.6 and 50.0\u00b0 2\u03b8 are observed (ICDD Card No. 01-087-0712). Nevertheless, CeNiO3 phase still appears, which may indicate that this phase is just partially and/or superficially reduced but bulk crystalline domains remained upon the selected pre-reduction treatment.In order to calculate the size of Ni crystallite, Scherrer equation is used for all the samples in which Ni is incorporated, using the most intense peak, 44.6\u00b0 2\u03b8. The results can be observed in Table 2\n. In general, there is homogeneity in the size of Ni crystallite. No increments are observed as the Ni load is increased in the catalysts. Ni particle size is expected to be no bigger than 35 nm and well-dispersed on the surface of the inorganic structure despite the low surface area of these catalysts.To visualise the morphology of the catalysts, SEM analysis was performed in the reduced catalysts. As can be observed in Fig. 3\na, corresponding to the undoped inorganic structure CZ, a homogeneous, dense structure with small cavities is observed throughout the sample. No granular or spherical shape is observed. CZN5, on the other hand, presents two different structures. In Fig. 3b, a similar porous structure is observed, which may correspond to the Ce0.5Zr0.5O2 phase, whereas Fig. 3c structure revealed spherical shape of Ce2Zr2O7, as reported elsewhere (Bibi et al., 2019). Fig. 3d corresponds to CZN10, which still present zones with some spherical shape and other complex structures. Fig. 3e presents element mapping distribution from EDX, showcasing the homogenous distribution of our active phases.The reduced catalysts were tested under the above DRM reaction conditions. The effect of the temperature in DRM was studied at a temperature range from 500 to 800\u00b0C and atmospheric pressure, using a reactant gases molar ratio of CO2:CH4 1:1. As can be observed in Fig. 4\n, CO2 conversion is greater than CH4 conversion for all the catalytic systems at all temperatures. This may be due to the higher activation energy of CH4 than CO2 (i.e., the energy barrier to active C-H cleave bond is higher than CO2 dissociation), requiring higher temperatures in agreement with DFT results reported elsewhere (Niu et al., 2020; Zhu et al., 2009). Besides, the possible occurrence of the reverse water-gas shift (RWGS, Eq. (9)) reaction, which competes with DRM, may contribute to the higher CO2 conversion due this parallel route consuming CO2 simultaneously.\n\n(9)\n\n\nC\n\nO\n2\n\n+\n\nH\n2\n\n\n\u2192\n\n\nCO\n\n+\n\nH\n2\n\nO\n\n\n\n\nDue to the endothermic nature of the reaction, both CO2 and CH4 conversion increase with temperature. At low temperatures, CH4 conversion is low and far from the equilibrium values but, as the temperature rises, it gets better, reaching a conversion of 45% at 800\u00b0C for CZN10. As mentioned before, CH4 needs higher temperatures to overcome the energy barrier necessary for its activation. Like CH4, CO2 conversion is lower at low temperature range, but it becomes higher as the temperature increases. At 800\u00b0C, the conversion reached a value of 60% for CZN10, which is closer to equilibrium conditions. These commendable catalytic results are achieved despite of the low specific surface area reported. Focusing on the two best results, CZN5 and CZN10, the conversion gap between them become closer as the temperature is increased for both CH4 and CO2, even surpassing CO2 conversion offered by CZN10 catalysts if compared to CZN5 at 800\u00b0C.The un-doped CZ catalyst does not show activity in DRM, as can be predicted due to the lack of active metallic Ni phase in the solid. As the metal loading is increased to 10 wt.%, the conversion of both CH4 and CO2 rises due to the higher Ni concentration presented in the sample. Interestingly, CZN5 presents conversion values close but lower to CZN10 despite having half of Ni metal content. This can be related to the accessibility of Ni active sites to the reactant gases due to the interaction with the phases presented in the catalyst. The presence mixed phases in CZN5 leads to remarkable activity results which are close to the those exhibited by the CZN10, which contains twice the Ni content. Particularly, this might be related to the presence of Ce0.5Zr0.5O2 phase, which interacts more closely with Ni active centres, offering a more active catalyst according to TPR results. In any case, CZN10 shows the highest catalytic performance among the studied series. When the metal loading is further increased to 15 wt.% Ni, a decrement of the conversion can be observed despite having more Ni in the samples. This can be related again to the accessibility of Ni active centres. The lower presence of Ce0.5Zr0.5O2 phase as the Ni content is increased might be responsible for the decrease of conversion when compared to CZN5 and CZN10. Indeed, it appears to be an optimum amount of Ni which maximise the conversion in DRM, being close to 10 wt.%. Despite the more insertion of Ni in the structure on B-site, substituting Zr, the activity did not improve.In terms of H2/CO molar ratio, the tendency is to increase the ratio as the temperature increases, as observed in Fig. 5\n. This increment might be related to a higher CH4 conversion, leading to better H2 production and thus higher ratio. The tendency for all the doped samples is similar, which may indicate the presence of parallel competing reactions. RWGS leads to the opposite effect in the ratio since it consumes H2, whereas methane decomposition generates H2. In fact, among the parallel reactions affecting DRM, RWGS and CH4 decomposition have a remarkable influence in the reactant conversion and H2/CO molar ratio, as previously reported in literature (Bradford and Vannice, 1999; Le Sach\u00e9 et al., 2018a, 2018b). These results may suggest that the contribution of CH4 decomposition side reaction is favoured at higher temperature, thus yielding a higher H2/CO ratio.In order to further explore the catalytic performance, stability tests were performed to study how efficient the catalyst is over long-term reaction run. CZN10 was chosen to conduct the stability tests to check its behaviour over a period of 100 h at 600\u00b0C and 800\u00b0C. The results at 600\u00b0C can be observed in Fig. 6\na. This catalyst displays good stability over time. It is observed a first period of stabilisation to reach the steady state and, after that, it shows signs of small deactivation starting with conversions of CO2 and CH4 of 24% and 18% and achieving conversions of 18% and 12% after 100 h on-stream, respectively. Again, the conversion of CO2 is slightly larger than that of CH4. Besides, a decrease in the molar H2/CO ratio from 0.6 to 0.45 is noted. It is estimated a declination rate of 0.0758 and 0.0701% h\u22121 for CH4 and CO2, respectively. CH4 is acknowledged to be activated by Ni active centres whereas CO2 by the support. Therefore, it is reasonable that, if deactivation is occurring due to coke deposition or sintering of the active sites, CH4 is reported to be slightly more sensitive to deactivation than CO2.At 800\u00b0C (Fig. 6b), results are slightly different and very promising. The conversion displays stable conversion levels over time. CO2 conversions sets around 49-51% whereas CH4 conversion is slightly lower. CH4 conversion is again reported to be more sensitive to a small deactivation than CO2 conversion. The molar H2/CO ratio is, in this case, quite stable and over 0.66 which be a useful syngas for some industrial applications such as hydroformylation reactions (Le Sach\u00e9 and Reina, 2022).For a broad picture to place our catalysts within the DRM scenario, Table 3\n offers a comparison of our results with relevant studies available in literature. Herein, we must emphasise the low surface area of the catalysts presented in this work when compared reference catalysts. Very interestingly our work demonstrates that the DRM reaction does not necessarily require high-surface area catalysts to reach high performances. In other words, the DRM reaction is not a surface-area sensitive process. In addition, we shall remark that reaction time herein reported for our stability tests is considerably higher than standard experimental data from literature, providing stronger evidence of the stability and resistance of the materials. Actually, most of the reports included in the table test the catalysts for very short time to be considered realistic stability tests and we want to draw the readers attentions to this matter since long-term stability test of 100 h and beyond are needed as solid proof-of-concept for the catalyst\u00b4s resistance towards deactivation in a process like DRM.Overall, our CZN10 is deemed as a fairly stable catalyst when tested at 800\u00b0C delivering noticeable levels of conversion and a commendable H2/CO molar ratio in continuous operation during 100 h. As a result, our best formulation leads to a valuable syngas composition with potential interest for the chemical industry. Furthermore, it is interesting to remark that operational troubles with catalyst\u00b4s stability observed at 600\u00b0C can easily be overcome by rising the reaction temperature to 800\u00b0C. This is still a low temperature regime when it comes to industrial reformers which typically run on the 900-1000\u00b0C range, opening further opportunities for our catalytic formulation. Additionally, we shall highlight that the excellent catalytic performance displayed by our samples is achieved at high space velocities (i.e., 30 L g\u22121 h\u22121). Again, industrial reformers are operated at significantly lower space velocities which means that the implementation of our catalysts in a potential realistic application could lead to considerable reduction of the reforming reactor volume; or in other words, significant CAPEX savings.Deactivation of the catalyst is mainly caused by carbon deposition and/or sintering of the active phase. In order to elucidate this, the best performing catalyst was analysed after reaction by XRD in order to detect any structural changes after the different treatments it underwent. In Fig. 7\n, the XRD pattern of CZN10 calcined, after the reduction treatment, after DRM reaction conditions at 600\u00b0C and 800\u00b0C for 100 h are shown. As it can be observed, the inorganic crystalline structure of the sample remains intact since there are no differences in the diffraction characteristics between the calcined and the post-reaction catalysts. This demonstrates the high thermal stability of the catalyst despite the reaction conditions. In addition, it can be observed a slight displacement of the characteristic peaks 44.4 and 51.7\u00b0 2\u03b8, corresponding to the characteristic planes of (111) and (200) of metallic Ni0 to lower angles after 100 h at 600\u00b0C, indicating that part of the surface Ni0 is oxidising again to form NiO1-x species. On the other hand, the XRD pattern after 100 h at 800\u00b0C shows that NiO is appearing very likely due surface oxidation when transferring the sample from the reactor to the XRD chamber.Small growth of Ni particles is observed after reaction conditions. It was estimated a growth of the particle size from 33.7 to 39.8 and 37.3 nm at 600\u00b0C and 800\u00b0C, respectively. This small change occurred after 100 h of reaction, which affirmed the stability and robustness of the catalyst after the substitution of Ni on the complex oxide structure, preventing partially the sintering of the particles. It is thus estimated that a small proportion of the Ni inserted within the inorganic structure is exsolved, producing this increase in the particle size. Exsolution is actually considered a smart strategy to design efficient catalysts for energy applications when the metal has a good capacity to exsolve under reaction environments (Carrillo and Serra, 2021; Kousi et al., 2021; Kwon et al., 2017; Zhang et al., 2020).Carbon deposition is also studied as deactivation cause. The formation of carbon is observed in XRD pattern after 100 h at 600\u00b0C, where a peak attributed to graphitic carbon is detected. This peak corresponds to the graphite lattice plane (002) of carbon nanotubes at 26\u00b0 2\u03b8. At 800\u00b0C, no carbon structures are observed. Carbon formation is hard to avoid due to the intricate reaction, since C-H activation of CH4 involves the formation of carbon species, widely studied elsewhere (Guharoy et al., 2019). Besides, the reaction temperature has an important influence on carbon deposition. It must be emphasised that carbon deposition is thermodynamically more favoured at lower temperatures, between 600 and 750 \u00b0C (Nikoo and Amin, 2011). Besides, carbon deposition is closely related to Ni particle size, since the larger the clusters the more favoured the carbon deposition is. This may be another reason of carbon deposition due to the increment of Ni crystallite size.TEM images of the (a) reduced, (b) after 48 h reaction and (c) after 100 h at 600 \u00b0C reaction are shown in Fig. 8\n. Fig. 8a shows a particle of the reduced catalyst. After 48 h at 600 \u00b0C, some carbon nanotubes (CNT) are formed, as observed in Fig. 8b. Nevertheless, the amount is negligible. After 100 h at 600 \u00b0C, significant amount of CNTs appear. These CNTs start growing from the interface of the active metal phase and the support structure, \u201cpulling out\u201d part of the Ni particle from the surface. Nevertheless, despite the formation of carbon deposits, the catalyst remains acceptable since this carbon is partially covering the active sites of Ni, being the rest of the Ni atoms accessible for the reaction. A similar behaviour was reported for a Ni-substitute La pyrochlore (Le Sach\u00e9 et al., 2018a). The main causes of catalyst deactivation at 600 \u00b0C are the carbon deposits around Ni particles and the increase of Ni clusters. This situation is overcome when the reaction is run at 800 \u00b0C where our post-stability XRD pattern shows a carbon-free sample which is explain the excellent conversion levels at this reaction conditions for a continuous 100 h test.This work addresses the preparation, characterisation and testing of a series of Ni-promoted cerium zirconate oxide structures for gas-phase CO2 valorisation via DRM. Structural analysis revealed the presence of different inorganic mixed oxide structures depending on the amount of nickel incorporated. Nickel is believed to be incorporated within the lattice structure, remaining part of the nickel in the surface interacting with the support structure resulting in a complex structure with different types of Ni active species as evidenced by XRD and TPR experiments.CZN10 showed the best catalytic performance for DRM. Nevertheless, CZN5, having half of the nickel content, presented commendable conversion values, indicating that the crystalline Ce0.25Zr0.25O2 phase, which is only presented in CZN5, is a relevant specie that makes Ni more accessible enhancing the DRM behaviour. Stability test of CZN10 over 100 h demonstrated the long-term thermal stability of the catalysts, showing small deactivation in the low-temperature range (600 \u00b0C) and excellent stability at 800 \u00b0C. Such deactivation at 600 \u00b0C is ascribed to graphitic carbon deposition as evidenced by TEM. XRD analysis after 100 h hours revealed that the crystalline structure of the catalyst remained intact and part of the nickel was exsolved, slightly increasing the particle size of the Ni clusters, which are responsible for the activity of the catalyst. Potential regeneration of spent catalyst in the low-temperature operation range (600 \u00b0C) should be further investigated as smart strategy to reuse the catalysts in realistic applications.All in all, this work showcases a strategy to design thermally stable catalysts based on nickel promoted cerium-zirconium mixed oxides where nickel is incorporated within the structure, being able to withstand DRM conditions and deliver high quality syngas for long-term operations. Very importantly, the remarkable performance herein demonstrated by this Ni-engineered catalysts is achieved at relatively high space velocities when compared to industrial reformers which means that they can be instrumental for CAPEX savings in realistic applications while also paving the way to design compact DRM units that might fit very well in the biogas processing industry.Authors declare no conflict of interest.Financial support for this work was gathered from grant 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.", "descript": "\n                  The increasing anthropogenic emissions of greenhouse gases (GHG) is encouraging extensive research in CO2 utilisation. Dry reforming of methane (DRM) depicts a viable strategy to convert both CO2 and CH4 into syngas, a worthwhile chemical intermediate. Among the different active phases for DRM, the use of nickel as catalyst is economically favourable, but typically deactivates due to sintering and carbon deposition. The stabilisation of Ni at different loadings in cerium zirconate inorganic complex structures is investigated in this work as strategy to develop robust Ni-based DRM catalysts. XRD and TPR-H2 analyses confirmed the existence of different phases according to the Ni loading in these materials. Besides, superficial Ni is observed as well as the existence of a CeNiO3 perovskite structure. The catalytic activity was tested, proving that 10 wt.% Ni loading is the optimum which maximises conversion. This catalyst was also tested in long-term stability experiments at 600 and 800\u00b0C in order to study the potential deactivation issues at two different temperatures. At 600\u00b0C, carbon formation is the main cause of catalytic deactivation, whereas a robust stability is shown at 800\u00b0C, observing no sintering of the active phase evidencing the success of this strategy rendering a new family of economically appealing CO2 and biogas mixtures upgrading catalysts.\n               "}
{"full_text": "Metal nanoparticles are commonly used to catalyze many chemical processes [1]. Since catalytic reactions occur at the metal surface, the high surface-area-to-volume-ratio of a nanoparticle provides an effective number of active sites per weight of metal in the overall catalyst. One of the biggest challenges for any catalytic system is to maintain this maximum amount of active sites throughout the lifespan of a solid catalyst [2]. Particularly in the case of metal nanoparticles, their growth is a prevailing phenomenon in which the efficient utilization of the metal in a catalyst is compromised, usually leading to a detriment in catalytic performance [3\u20135]. In order to stabilize the nanoparticles and prevent their growth, these are typically dispersed over a support material. The nature of the support is crucial in delivering this stability and offers an opportunity to develop improved solid catalysts.Reducible oxides used as support material display characteristic interactions with metal nanoparticles. During reductive conditions, one effect that arises is coverage of the nanoparticles by in-situ generated suboxides from the support [6,7], the so called Strong Metal-Support Interaction (SMSI) [8,9]. This effect can modify the available metal surface area, the electronic state of the metal and the particle shape [10\u201312]. An interesting example is the substantial change in reactivity of nickel during carbon monoxide hydrogenation: nickel supported on non-reducible oxides (e.g. Al2O3, SiO2) selectively hydrogenates carbon monoxide to methane, whereas when supported on reducible oxides like TiO2 or Nb2O5 the product distribution shifts towards heavier hydrocarbons [13\u201317]. Furthermore, previous reports have suggested that reducible supports can also deliver unique stability to nickel-based catalysts [13,18].The interest to achieve stable nickel-based systems in the presence of carbon monoxide arises from the extensive utilization of nickel catalysts in reactions involving carbon monoxide as reactant, intermediate or product [19\u201323] and the poor stability of nickel in the presence of carbon monoxide at low temperatures [24,25]. Deactivation of nickel-based catalysts during carbon monoxide hydrogenation proceeds most often through particle growth by the formation and diffusion of volatile nickel carbonyl [25\u201327]. This phenomenon is a classic example of Ostwald ripening, where species containing metal atoms, in this case nickel carbonyl, diffuse from smaller towards larger nanoparticles leading to metal sintering [28,29]. In order to prevent this, the reaction is typically operated at high temperatures and low CO pressures, since these conditions disfavor the formation of nickel carbonyl [24,26,30]. However, such conditions compromise the product selectivity mainly towards methane and therefore hamper the application of nickel catalysts for the synthesis of more commercially attractive products, such as long-chain hydrocarbons (C5+) or olefins [31,32]. Alternative strategies to inhibit the formation of nickel carbonyl in these catalysts have been explored in literature, for instance, by alloying nickel with copper [33,34] or by depositing nickel on titania, a reducible support [13].Here, we studied the effect of SMSI in nickel supported on niobia for the hydrogenation of carbon monoxide. For this, different reduction temperatures (250\u2013450\u2009\u00b0C) on NiO/Nb2O5 were used prior to H2-chemisorption, in order to determine the extent of SMSI, and prior to CO hydrogenation. H2-uptake suppression was observed when increasing the reduction temperature which is characteristic of the SMSI effect. Simultaneously, an increase in reduction temperature led to a decrease in nickel-based catalytic activity, however stable catalytic performance was gained in return with high selectivity for long-chain hydrocarbons. Ni/Nb2O5 showed higher turnover frequency and C5+ selectivity compared to nickel supported on a non-reducible support (\u03b1-Al2O3). The overall results obtained pointed out to an inhibition of nickel carbonyl formation by SMSI in Ni/Nb2O5, leading to a stable supported nickel catalyst for CO hydrogenation.Niobium oxide (Nb2O5) was used as support and obtained by crystallization of niobium oxide hydrate (Nb2O5\u2022nH2O, HY-340, AD/4465), which was provided by Companhia Brasileira de Metalurgia e Minera\u00e7\u00e3o \u2013 CBMM. The crystallization was carried out in stagnant air at 600\u2009\u00b0C during 4\u2009h with a ramp of 5\u2009\u00b0C\u2009min\u22121. The obtained Nb2O5 had a pseudo-hexagonal TT-phase, a specific surface area of 9\u2009m2\u2009g\u22121 and a specific mesopore volume of 0.05\u2009cm3\u2009g\u22121.A nickel supported on niobia catalyst was prepared using the incipient wetness impregnation method. Prior to impregnation the support (75\u2013150\u2009\u03bcm grains) was dried under vacuum at 80\u2009\u00b0C for 1\u2009h, thereafter the impregnation was performed at room temperature with a 4.2\u2009M aqueous solution of Ni(NO3)2\u20226H2O (Acros, 99%) for a 6\u2009wt.% Ni. In the next step, the catalyst was dried for 1\u2009h at 60\u2009\u00b0C in a fixed bed reactor under N2 flow and subsequently in the same reactor and gas flow calcined for 2\u2009h at 350\u2009\u00b0C (3\u2009\u00b0C\u2009min\u22121). Nickel supported on \u03b1-alumina (BASF) was prepared in the same way. Metal loadings were defined as the mass of metallic Ni per gram of reduced catalyst.Temperature programmed reduction (TPR) analyses were performed using a Micromeritics Autochem 2990 instrument, where 100\u2009mg sample was dried at 120\u2009\u00b0C for 1\u2009h in Ar flow followed by reduction from room temperature up to 700 or 1000\u2009\u00b0C (5\u2009\u00b0C\u2009min\u22121) in a 5\u2009vol% H2/Ar flow. Powder X-ray diffractograms were measured using a Bruker-AXS D2 Phaser X-ray diffractometer, Co-K\u03b1 radiation (\u03bb\u2009=\u20091.789\u2009\u00c5). Bright field transmission electron microscopy (TEM) and Scanning transmission electron microscopy (STEM-EDX) images were acquired with a Philips Tecnai-20 FEG (200\u2009kV) microscope equipped with an energy dispersive X-ray (EDX) and high-angle annular dark-field (HAADF) detector. The reduced and subsequently passivated samples for the microscopy analysis were prepared by suspending the catalysts in 2-propanol (>99.9%, Sigma-Aldrich) using sonication and dropcasting the suspension on a carbon-coated Cu grid (200 mesh). The nickel particle size was determined using the iTEM software by analyzing at least 500 particles. Particle surface average diameters or Sauter mean (D[3,2]) were then calculated and corrected for a 2\u2009nm NiO shell [35]. H2-chemisorption was measured on a Micromeritics ASAP 2020C using \u223c100\u2009mg of sample. Prior to the measurement, the calcined catalyst was reduced in H2 flow at different temperatures during 2\u2009h (5\u2009\u00b0C\u2009min\u22121). The sample was then evacuated, cooled to 150\u2009\u00b0C and H2-chemisorption was measured at that temperature. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was performed on a SPECTRO ARCOS in order to establish the nickel content before and after catalysis; samples were extracted using aqua regia.Fourier-transform Infrared (FT-IR) spectroscopy measurements were carried out in a Specac \u201cHigh Temperature High Pressure\u201d transmission FT-IR cell. A self-supported catalyst wafer was prepared by applying on a sample a force of 4000\u2009kg for 20\u2009s, yielding a wafer of 16\u2009mm diameter, and < 1\u2009mm thickness. Catalyst wafers were reduced in-situ, each at different temperatures of 250, 350 and 450\u2009\u00b0C (N2/H2\u2009=\u20092 v/v; both Linde, 5.0). Subsequently, a sample was cooled down to 230\u2009\u00b0C flushed with 5.0 purity N2 for 10\u2009min, after which flowing CO (Linde, 5.0) was added with a ratio N2/CO\u2009=\u20092 v/v at 1\u2009bar total pressure. Due to low photon-transmittance of the Nb2O5-supported Ni catalyst, 256 spectra were averaged to improve signal-to-noise. Spectra were recorded with a resolution of 4\u2009cm\u22121.Catalytic performance was carried out in a quartz glass plug-flow reactor, loaded with 15\u201320\u2009mg catalyst (38\u2013150 \u03bcm) diluted with \u223c200\u2009mg SiC. Catalysts were reduced in situ at 250, 350 or 450\u2009\u00b0C (5\u2009\u00b0C\u2009min\u22121, 2\u2009h) in an Ar/H2\u2009=\u20092.0 v/v flow (GHSV\u2009=\u2009190 000\u2009h\u22121). After reduction, CO hydrogenation was performed at 230\u2009\u00b0C, 1\u2009bar, H2/CO\u2009=\u20092.0 v/v, GHSV\u2009=\u200928 000\u2009h\u22121 and CO conversion < 5%. Reaction was carried out for 90\u2009h. Finally, the CO flow was stopped and the H2 flow and temperature were kept for one hour in order to remove remaining hydrocarbons for further analysis of the catalyst. C1-C18 products were analyzed by online gas chromatography (Varian 430 GC, CP sil-5 column).A nickel supported on niobia catalyst was synthetized by incipient wetness impregnation method. After subsequent drying and calcination, the nickel content was determined by ICP-OES, being 5.6\u2009\u00b1\u20090.1\u2009wt.%. Nickel supported on \u03b1-alumina was also synthetized as comparative system, with a nickel content of 5.5\u2009\u00b1\u20090.1\u2009wt.% as determined by ICP-OES. Temperature programed reduction was carried out on the support (Nb2O5), the calcined NiO/Nb2O5 and NiO/\u03b1-Al2O3 samples. The corresponding reduction profiles for the niobia-based samples are shown in Fig. 1\n. Nb2O5 showed a gradual consumption of hydrogen starting at 600\u2009\u00b0C and a maximum consumption rate at 940\u2009\u00b0C. The hydrogen consumption was assigned to the reduction of the Nb2O5 surface to NbO2, along with the change in color of the sample to deep indigo, characteristic of Nb4+ ions [36,37]. The reduction profile for the NiO/Nb2O5 sample showed a small hydrogen consumption signal at 200\u2009\u00b0C which might be attributed to the reduction of Ni3+. The main hydrogen consumption between 230 and 430\u2009\u00b0C was attributed to the reduction of NiO to metallic Ni [38,39]. Hydrogen consumption continued above 700\u2009\u00b0C related to reduction of Nb2O5, catalyzed by the metallic nickel [40]. The consumption peak at 780\u2009\u00b0C might correspond to the initial reduction of the support surface (Nb2O5 to NbO2) and further consumption above 810\u2009\u00b0C to the reduction of bulk Nb2O5 and possibly subsequent reduction of NbO2 to Nb2O3. Nickel oxide supported on \u03b1-Al2O3 showed a similar reduction profile to the niobia-based sample (Figure S1), with a small hydrogen consumption signal at 220\u2009\u00b0C ascribed to Ni3+ reduction and a main signal between 250 and 450\u2009\u00b0C for NiO reduction to Ni. For both Nb2O5- and \u03b1-Al2O3-supported samples, the total hydrogen consumption below 450\u2009\u00b0C corresponded to the complete reduction of all nickel oxide to metallic nickel.Based on the NiO/Nb2O5 TPR profile, four different reduction temperatures, namely 250, 350 and 450\u2009\u00b0C, were chosen to study their effect on CO hydrogenation. The degree of reduction for the low temperatures (250 and 350\u2009\u00b0C) was calculated by measuring TPR of NiO/Nb2O5 with an additional dwell step of 2\u2009h at 250 or at 350\u2009\u00b0C. The isothermal step at 250\u2009\u00b0C resulted in two main distinctive signals in hydrogen uptake for the reduction of nickel oxide (Figure S2, A). The first one was observed by reaching 250\u2009\u00b0C with a sharp increase in hydrogen uptake which gradually decreased back to the baseline throughout the 2\u2009h at 250\u2009\u00b0C. Based on the hydrogen uptake the degree of reduction at this temperature was 58%. The second main hydrogen uptake signal was observed after the isothermal step with a maximum at 360\u2009\u00b0C (t\u2009=\u2009200\u2009min), this indicates that temperatures higher than 250\u2009\u00b0C are necessary to completely reduce the nickel oxide to metallic nickel. The hydrogen uptake observed at 360\u2009\u00b0C might relate to the observed shoulder at the same temperature in Fig. 1, which might correspond to the reduction of nickel oxide species with a stronger interaction with the support. The TPR profile with an isotherm step at 350\u2009\u00b0C (Figure S2, B) showed a main hydrogen uptake signal which corresponded to a degree of reduction of 96%, indicating that most of the nickel oxide is reduced to metallic nickel at 350\u2009\u00b0C.\nTable 1\n shows the hydrogen uptake determined by H2-chemisorption for Ni/Nb2O5 and Ni/\u03b1-Al2O3 after reduction at different temperatures. An increase of the reduction temperature resulted in a decrease in hydrogen uptake for Ni/Nb2O5, resulting in an apparent increase in the derived particle size. This suppression of hydrogen chemisorption by reducible oxidic supports, the so called strong metal-support interaction (SMSI) effect, is a well-documented phenomenon attributed to coverage of the metal nanoparticles by suboxides from the support upon reductive conditions [6,8]. The degree of coverage by the suboxides is a temperature-dependent phenomenon, in which higher reduction temperatures enhance the mobility of these species and coverage of the nanoparticles [41]. Powder X-ray diffraction (Figure S3) neither showed the formation of new crystalline species (e.g. nickel niobates) nor provided indications of SMSI after reduction of Ni/Nb2O5. Reduction at 250\u2009\u00b0C showed a substantial hydrogen uptake even though nickel oxide was not completely reduced at 250\u2009\u00b0C as shown by the TPR results, indicating that most of the particles\u2019 surface consisted of metallic nickel. The Ni/\u03b1-Al2O3 sample showed also a decreased in the hydrogen uptake upon increasing the reduction temperature, however this decrease was not as severe as the one observed for Ni/Nb2O5. After the chemisorption measurement and exposure to air at room temperature, the samples were analyzed by TEM (Fig. 2\n and Figure S4). TEM images showed for all Ni/Nb2O5 samples a uniform distribution of nickel nanoparticles over the niobia. Furthermore, a similar nickel particle size (\u223c 12\u2009nm) was determined based on TEM as shown in Table 1, indicating no significant effect of the reduction temperature on the nickel particle size and confirming that the suppressed hydrogen chemisorption results related to the SMSI effect. In the case of the Ni/\u03b1-Al2O3 sample, TEM images (Figure S4) revealed a slight increase in particle size upon increasing the reduction temperature, in line with the results obtained from hydrogen chemisorption. The discrepancy observed here between the experimental and theoretical H2-uptake can be explained by the more significant impact of larger particles when determining the D[3,2] value, a surface-based diameter. Since a considerable amount of very small nanoparticles would not be detected by TEM.The catalytic performance of the Ni/Nb2O5 and Ni/\u03b1-Al2O3 catalysts was evaluated by varying the reduction temperatures similar to the H2-chemisorption experiments. The results are shown in Fig. 3\n where nickel-normalized catalytic activity (Nickel Time Yield, NTY) is plotted against time-on-stream (TOS) up to 90\u2009h, furthermore a summary of the catalytic performance is shown in Table 2\n. The initial NTY (TOS\u2009=\u20090) showed consistency with the H2-chemisorption results, i.e. reduction at low temperatures for the niobia-supported sample displayed high H2-uptake along with markedly high initial NTY whereas an increase of the reduction temperature led to a suppression of the H2-uptake and a decrease in the initial NTY. However, the decrease in initial NTY is not proportional to the decrease in H2-uptake for unknown reasons. The stability throughout time significantly varied for each reduction temperature. Reduction at 250\u2009\u00b0C led to severe deactivation, down to 70% loss in NTY at TOS\u2009=\u200990\u2009h. A less pronounced deactivation was observed when the reduction temperature was increased to 350\u2009\u00b0C with only 40% loss in NTY, however the catalyst did not reach steady state during the experiment due to continuous deactivation. In stark contrast, reduction at 450\u2009\u00b0C showed a catalytic performance, with a low initial NTY which increased during the first 30\u2009h of the reaction followed by a stable conversion until the end of the experiment. This might indicate a partial recover of the available metallic surface area during reaction conditions, which has been associated in literature to re-oxidation of the suboxides (e.g. NbOx) by water produced during reaction, hence modifying the SMSI effect [42]. Contrary to the Ni/Nb2O5 sample, the reduction temperature had a minor effect on the catalytic performance of the Ni/\u03b1-Al2O3 as shown in Fig. 3. Reduction at 350\u2009\u00b0C led to a small increase in NTY than when reduced at 450\u2009\u00b0C at the beginning of the experiment, this difference originated from their different initial particle size as revealed by their same initial turnover frequencies (TOF). Their prevalent decrease in NTY during the experiment resulted in almost similar NTY values at TOS\u2009=\u200990\u2009h. The niobia-supported sample showed independently of the reduction temperature higher NTY values than the alumina-supported sample at the end of the experiment. The nickel content was determined after catalysis by ICP-OES showing no metal loss during the experiment for all samples.The promotional effect of niobia was maintained in all cases as shown by the TOFs compared to Ni/\u03b1-Al2O3, determined either by particle size distribution from TEM or H2-chemisorption (Table 2). Initial TOFs based on TEM particle size distributions showed the highest values for Ni/Nb2O5 reduced at low temperatures (250 and 350\u2009\u00b0C) and decreased when increasing the reduction temperature to 450\u2009\u00b0C. The inverse trend was observed for the apparent initial TOFs based on chemisorption results (TOFapp, Table 2); an increase in reduction temperature led to a substantial increase in TOFapp due to the hydrogen chemisorption suppression by SMSI (vide supra). However, the nickel surface under reaction conditions is expected to change and therefore these TOFs are indicated as \u2018apparent\u2019. Interestingly, Ni/Nb2O5 reduced at 250\u2009\u00b0C shows consistent values for both initial TOFs indicating that coverage of the nickel nanoparticles by suboxides from the support has not taken place at this temperature. Consequently, the resulting high TOF might originate from the interphase of the nickel nanoparticles and the support. For Ni/\u03b1-Al2O3, the apparent TOFs and TOFs based on TEM have the same values and SMSI does not play a role in this catalyst system.The change in reduction temperature additionally influenced the selectivity of the nioba-supported catalyst, as shown with the Anderson\u2013Schulz\u2013Flory (ASF) product distribution plot in Fig. 4\n and in Table 2. For all reduction temperatures, niobia-supported catalysts showed higher selectivity towards long-chain hydrocarbons when compared to the alumina-supported catalyst. At TOS\u2009=\u200990\u2009h, the catalyst reduced at 350\u2009\u00b0C had the highest \u03b1 value, with the highest selectivity to C5+ products, followed by the reduction temperature at 250\u2009\u00b0C. Reduction at 450\u2009\u00b0C led to a shift in product distribution to shorter hydrocarbons and therefore a smaller \u03b1 value. Suppressed C2H4/C2H6 was observed for the catalyst reduced at 250 or 350\u2009\u00b0C, which has been attributed in the case of cobalt-based catalysts to re-adsorption of olefins to the metal surface to further increase chain-growth [43,44]. However, reduction at 450\u2009\u00b0C did not show this behavior, instead a slight increase in the olefin selectivity was observed, as shown in Table 2. Re-adsorption of olefins could be hindered on the metal surface, shifting the selectivity to shorter hydrocarbons. On the other hand, Ni/\u03b1-Al2O3 showed the lowest \u03b1 value, a high selectivity for methane and to a lesser extent for C2 to C10 products for both reduction temperatures. The formation of C2+ products in this case might be due to some small nickel metal nanoparticles (< 3\u2009nm) found in this catalyst (Figure S4), which agrees with previous research reports [45].The Ni/Nb2O5 samples after catalysis were analyzed by TEM and the results are shown in Fig. 2. Significant changes for the nickel nanoparticles were observed for the spent catalyst reduced at 250\u2009\u00b0C: broadened particle size distribution and increased average particle size (12\u2009nm to 27\u2009nm) were observed. Likewise, large particles were observed for the spent catalyst after reduction at 350\u2009\u00b0C leading to a particle mean size of 18\u2009nm. The observed nickel particle growth agreed with the stability of the catalyst during reaction; where reduction at 250\u2009\u00b0C led to the severest deactivation and the most pronounced particle growth, increase of the reduction temperature to 350\u2009\u00b0C attenuated the particle growth and diminished the deactivation rate. The resulting particle sizes and catalytic activity at TOS\u2009=\u200990\u2009h led to similar TOFs for both reduction temperatures when compared to initial TOFs (Table 2). This is an indication that the decrease in NTY was mainly due to particle growth. The slight decrease in TOF might relate to carbon deposition over the nickel surface. In a similar way, TEM of the spent Ni/\u03b1-Al2O3 catalysts revealed a substantial increase in nickel particle size (Figure S5), indicating that the decrease in NTY originated mainly from particle growth. Particle growth for nickel-based catalysts under these reaction conditions most likely occurs via Ostwald ripening by the formation of Ni(CO)4 [25\u201327]. In contrast, the nickel particles remained well distributed over the support for the Ni/Nb2O5 sample reduced at 450\u2009\u00b0C. No significant change in particle size was observed after catalysis, with a final mean particle size of 12\u2009nm. Therefore, the increase in CO conversion during the first hours of the experiment means that sites more active became available and thus the TOF almost doubled (Table 2). These results suggest that SMSI inhibited the formation of Ni(CO)4 on a Nb2O5 support leading to a more stable catalyst. Ni(CO)4 formation rate has been reported to depend on the nickel surface morphology with particularly low coordinated Ni atoms readily reacting to form carbonyls [46,47], thus NbOx species might be responsible for blocking or modifying the electron density of these sites.Fourier-Transform Infrared (FT-IR) spectroscopy was used to study the differences for the sample reduced at 250, 350 and 450\u2009\u00b0C in their tendencies to form nickel carbonyl. In-situ reduction of the wafer was carried out at these three different temperatures. Thereafter FT-IR spectra were recorded at 230\u2009\u00b0C under atmospheric pressure of CO/N2\u2009=\u20092 v/v flow (Fig. 5\n). Interestingly, a pronounced sharp band at 2080\u2009cm\u22121 can be observed when the sample was reduced at 250\u2009\u00b0C. This band is ascribed to subcarbonyl Ni(CO)x (x\u2009=\u20092, 3) species, precursors of Ni(CO)4, in accordance with literature [47\u201349]. These species were also detected for the catalyst reduced at 350\u2009\u00b0C and in both cases the band disappeared after flushing with N2. In contrast, reduction at 450\u2009\u00b0C did not give rise to this subcarbonyl Ni(CO)x band. These results show that there is a reduction temperature dependency in nickel carbonyl formation. Lower reduction temperatures thus most likely led to rapid deactivation during CO hydrogenation due to Ni particle growth via the formation and diffusion of Ni(CO)4 originated from the detected Ni(CO)x species. High reduction temperature showed stable catalytic activity (Fig. 3), suggesting that the SMSI effect suppressed the formation of Ni(CO)x species and hence Ni(CO)4, avoiding the diffusion of nickel over the support. Furthermore, the FT-IR spectra plotted in Fig. 5 show that the degree of CO activation in the adsorbed state is affected by the reduction temperature. That is, a reduction at 250\u2009\u00b0C shows a large contribution of Ni(CO)x at 2090\u2009cm\u22121 corresponding to the strongest carbon-oxygen bond based on the relatively high wavenumber of this band. A band at 1580\u2009cm-1 is also observed which is attributed to carboxylate-type species, these may have originated from the oxidation of CO by the remaining NiO in this sample as shown by the TPR results. For a slightly higher reduction temperature (350\u2009\u00b0C) besides the band at 2090 cm\u22121, a broad peak at around 1960\u2009cm\u22121 is observed, which is ascribed to CO adsorbed in a 2-fold bridge position [50,51]. At the highest reduction temperature (i.e., 450\u2009\u00b0C) a small peak at \u223c 1266 cm\u22121 appears, which can be ascribed to the weakest CO bond [52], or lowest wavenumber observed in this set of experiments.Two effects of the SMSI can explain the inhibition of nickel carbonyl formation. On one hand, NbOx suboxides might physically block the more reactive low-coordinated Ni atoms on the surface of the nanoparticles, preventing the formation of subcarbonyl Ni(CO)x species. A similar effect has been shown in literature by addition of alkali metals or sulfur to nickel-based catalysts [53,54]. On the other hand, the suboxides partially covering the nickel nanoparticle\u2019s surface are capable of transferring electrons to the nickel [55\u201357]. In this case, Nb4+ or Nb3+ in the NbOx suboxides might transfer electron density to the metallic nickel, resulting in electron-rich Ni\u03b4\u2212 atoms at the surface. Upon CO chemisorption at the nickel surface, Ni\u03b4\u2212 increases the back-donation to the CO 2\u03c0* antibonding orbital weakening the C\u2013O bond, as suggested by FT-IR, and thus avoiding the formation of Ni(CO)x species. Furthermore, an electron-rich metallic surface could hinder the re-adsorption of electron-rich molecules, explaining the increased olefin to paraffin ratios when the catalyst was reduced at high temperatures.The effect of different reduction temperatures was studied for nickel nanoparticles supported on niobia. An increase of the reduction temperature led to H2-chemisorption suppression, a typical phenomenon caused by reducible oxidic supports in which suboxides from the support cover partially the metal nanoparticles. The initial nickel-based catalytic activity was in line with the chemisorption results where high H2-uptake corresponded to high initial CO conversion. However, low reduction temperatures turned into a fast deactivation due to nickel particle growth as shown by TEM, whereas a high reduction temperature led to stable catalytic performance and no significant particle growth. Interestingly, reduction of the niobia-supported catalyst at high temperature brought about an activation period during the first hours under reaction conditions followed by stable nickel-based activity. FT-IR measurements of CO adsorbed on Ni/Nb2O5 showed that nickel subcarbonyls readily formed after low but not after high reduction temperature. This could explain the particle growth involving the formation and diffusion of nickel tetracarbonyl, which formed from the detected nickel subcarbonyls. The inhibition of nickel tetracarbonyl formation after high temperature reduction is associated to the presence of suboxide species over the nickel surface, by either physically blocking exposed low-coordination nickel atoms, or by enhancing the electron density on the nickel surface and facilitating C\u2013O bond rupture instead of nickel tetracarbonyl formation. The reduction treatment had a strong influence in the product distribution, where the highest selectivity towards C5+ was obtained after reduction at 350\u2009\u00b0C, while a further increase of the reduction temperature shifted the product distribution towards lighter products. Finally, the promotional effect of reducible oxides, such as niobia, in CO hydrogenation was clearly shown since independently of the reduction temperature nickel supported on niobia showed higher nickel-based activity, TOF and C5+ selectivity compared to nickel supported on \u03b1-alumina, a non-reducible support. We have shown that niobia used as support material offers the possibility to make stable nickel-based catalysts for CO hydrogenation with tunable product spectrum.\nCompanhia Brasileira de Metalurgia e Minera\u00e7\u00e3o (CBMM) is thanked for financial support of this research. Dr. Robson Monteiro and Mr. Rog\u00e9rio Ribas (CBMM) are acknowledged for useful discussions and supplying the niobia support. Mr. Wouter Lamme (Utrecht University, UU), Mrs. Petra Keijzer (UU) and Mrs. Savannah Turner (UU) are acknowledged for performing TEM measurements. KPdJ acknowledges the European Research Council (ERC) for a EU FP7 ERC Advanced Grant no. 338,846.Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cattod.2018.11.036.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  Stability of metal nanoparticles under reaction conditions is crucial in many catalytic processes. Nickel-based catalysts often encounter severe particle growth in the presence of carbon monoxide due to the formation and migration of nickel carbonyl. In this research, we showed that the reduction temperature of nickel oxide supported on niobia (Nb2O5) influenced the stability of the resulting nickel catalyst during subsequent carbon monoxide hydrogenation. Low reduction temperatures resulted in high initial nickel-normalized activity towards long-chain hydrocarbons (C5+), but fast deactivation throughout the experiment. High reduction temperatures led to a shift in product distribution towards shorter hydrocarbons and a decreased initial nickel-normalized activity, while during the first hours of the experiment an increase in turnover frequency and nickel-normalized activity was observed, resulting eventually in a stable catalytic performance. Electron microscopy analysis revealed extensive particle growth after catalysis when the catalyst had been reduced at low temperatures and no significant changes in particle size when reduced at high temperatures. By use of in-situ FT-IR spectroscopy, nickel subcarbonyl species which are precursors of volatile nickel tetracarbonyl were detected on Ni/Nb2O5 after low temperature reduction and exposure to CO, but not after high temperature reduction. Hence, particle growth is explained by the formation and diffusion of nickel carbonyl and subsequent Ostwald ripening, that leads to larger nickel particles with concomitant decrease in nickel-normalized activity. The stability of the catalyst reduced at high temperature was linked to the formation of niobium suboxides and their partial coverage of the nickel particles limiting the formation of nickel carbonyl and slowing down particle growth.\n               "}
{"full_text": "Owing to paucity of fossil fuels, it is critical to transform renewable sources into value added compounds and fuel additives [1]. Biodiesel manufacturing has sparked a lot of interest in this direction. Transesterification of vegetable oil and animal fat yields biodiesel. Glycerol is a 10% byproduct of the overall biodiesel synthesis process [2]. Biodiesel production is estimated to hit 41.4 billion litres in 2025, while glycerol production will hit 4.14 billion litres [3]. Glycerol conversion to various chemical such as glycerol carbonate, acrolein, glyceric acid, mono and di-glycerid, propylene glycol (1,2-PDO) and trimethylene glycol has been reported in recent literature [4]. Furthermore, selective transformation of biomass derived glycerine to 1,2-PDO by hydrogenolysis has gain noteworthy importance because of great marketable value of 1,2-PDO [5]. Also, plastics, polymers, agriculture adjuvants, solvents, the tobacco industry, detergents, and numerous functional fluids have all employed 1,2-PDO as a basic ingredient [6]. Presently, propylene glycol is formed from fossil fuel resources through hydration of 1,2-Epoxypropane via chlorohydrin, hydroperoxide processes and ethane epoxidation process [7,8]. As a result, making propylene glycol from bio-glycerol is an environmentally friendly procedure [9].Several noble metal and transition metal catalyst were synthesized, and their performance has been documented in literature [5,10,11]. Catalysts made of noble metals have been reported to be effective in this process [12]. Nevertheless, the generation of degradation products by means of excessive hydrogenolysis is one of the primary drawbacks of noble metal catalyst [13]. Because of their high activity and selectivity for the breakdown of the C\u2013O bond, Cu-based catalysts supported on oxides with acidic sites were studied for hydrogenolysis of glycerol [4,14,15]. As previously stated, hydrogenolysis of glycerol to propylene glycol is a multistep reaction involving dehydration of glycerol to hydroxyacetone and then hydrogenation of hydroxyacetone to propylene glycol [16]. However, it is observed that all previous investigations reported in literature were mainly dedicated to catalyst synthesis and characterizations along with experimental reaction parameter optimization. Only a few studies have been conducted on kinetic model construction and kinetic parameter estimates for this reaction [15\u201321].Lahr et\u00a0al. [17] investigated the hydrogenolysis of glycerol on a 5\u00a0\u200bmol% Ru/C catalyst and suggested a Langmuir-Hinshelwood-Hougen-Watson (LHHW) rate model that took into account the 1,2-PDO and EG formation in addition to the impact of competitive adsorption of above compound and glycerine over catalyst's site. Torres et\u00a0al., described power-law model with activation energy of 54.19\u00a0\u200bkJ/mol [18]. The kinetic study over LDH catalyst showed energy of activation as 65.5\u00a0\u200bkJ/mol with power law model [9]. Sharma et\u00a0al. [19] investigated the kinetics of glycerine hydrogenolysis in the presence of a Cu:Zn:Cr:Zr catalyst. The Langmuir-Hinshelwood model was modified to pseudo first order reaction by assuming the adsorption of glycerol and hydrogen did not inhibit the reaction. The calculated energy of activation was stated as 31.7\u00a0\u200bkcal\u00a0\u200bmol\u22121. The kinetics of hydrogenolysis of glycerol using a Ni\u2013Cu/Al2O3 material were described by Gandarias et\u00a0al. [20], and LHHW model was built to define direct transformation of glycerine into 1,2- PDO, which then hydrogenolysis to 1-propanol. Kinetics of glycerol over Raney Ni catalyst achieved reaction activation energy of 60\u00a0\u200bkJ/mol using LHHW model [21]. A Cu\u2013ZnO\u2013Al2O3 catalyst with a 57.8\u00a0\u200bkJ/mol activation energy was also employed for the hydrogenolysis of glycerol [22]. Dossin el.al., reported a kinetic study of Eley-Rideal (ER) type model for transesterification reaction using batch slurry-reactor [23].The aim of present study was to develop a suitable reaction kinetic model for hydrogenolysis of glycerol in presence of bi-functional layered double hydroxide (LDH) catalyst. The modified power law and Eley-Rideal type model were developed to fit obtained experimental outcomes. Surface adsorption, reaction, and desorption stages of reactants and products molecules on numerous active sites present on surface of catalyst were studied in the model development. The obtained equations were solved in MATLAB by means of ode45 and ode23s. Finally, the best values for the reaction kinetic parameters were found by minimizing the residual sum of squares between the experimental and model predicted values for the experiments that were carried out. The results showed that the ER model was able to successfully relate the model anticipated and experimental reactant and product amounts.The data required for the kinetic study of glycerol to 1,2- PDO using bi-functional layered double hydroxide (LDH) catalyst are generated by performing the experiments using an autoclave reactor (Amar equipment). All the details such as catalyst preparation, catalyst activity test and changes with reaction conditions on selectivity and conversion are already published in our previous work [24,25].It is essential to grasp best promising reaction transformations for glycerol hydrogenolysis over the Cu0.45Zn0.15Mg5.4Al2O9 catalyst. The reaction mechanisms for glycerol conversion to 1,2-PDO was reported in previous studies [15,26\u201328]. There are two main reaction pathways for glycerine hydrogenolysis to propylene glycol over noble and transition metal catalysts in the literature [4]. In a two-step process, dehydration of glycerol over the acidic/basic sites of the catalyst produces metastable acetol as an intermediate product, which is followed by hydrogen addition to acetol over the active metallic sites of the catalyst to produce 1,2-PDO [9,13].The obtained predominant reaction product in this investigation was 1,2-PDO. EG was also been formed as a second major product obtained with traces of undesirable compounds such as methyl alcohol, ethyl alcohol, hydroxyacetone, 1- PO, 2- PO. So as to find out reaction path, intermediate and final reaction products i.e. 1,2-PDO, EG and acetol were taken as the feed instead of glycerol and reaction was carried out under optimized reaction conditions (210\u00a0\u200b\u00b0C temperature, 4.5\u00a0\u200bMPa pressure, 800\u00a0\u200brpm, 20\u00a0\u200bwt% of 100\u00a0\u200bg glycerol solution, 1.6\u00a0\u200bg catalyst). The obtained experimental data for the reaction mechanism are shown in Table\u00a01\n.Glycerol as reactant achieved 100% conversion with \u223c94% selectivity to1,2-PDO. While acetol achieved 99.7% conversion with \u223c94% selectivity to 1,2- PDO. Some traces (<5%) of over hydrogenolysis products were also obtained. The conversion of 1,2-PDO was less than 4%, and the reaction products were methyl alcohol, ethyl alcohol, 1-PO, and 2-PO. In the case of EG as feed, the conversion was around 8%, and the end reaction products were methyl alcohol and ethyl alcohol. The obtained results from the experiments are shown in Table\u00a01. On the basis of results obtained from reaction mechanism experiments, the plausible scheme of chemical transformations is shown in Fig.\u00a01\n. Similar kind of reaction pathway was reported for other non-noble metal catalyst [27].The experimental data utilized are shown in Fig.\u00a0S1. The detailed information about influence of intra particle diffusion and external mass-transfer were explained in supporting information (Fig.\u00a0S2). Reaction were elementary and reaction rate will depend upon the concentration of the reactant of the corresponding equation.The reaction can be written as: -\n\n(1)\n\n\nGlycerol\u00a0\u200b\n\n\u2192\ncatalyst\n\n\u00a0\u200b\n1\n,\n2\n-\nPDO\n\n\n\n\n\n\n(2)\n\n\nGlycerol\u00a0\u200b\n\n\u2192\ncatalyst\n\n\u00a0\u200bEG\n\n\n\n\n\n\n(3)\n\n\nEG\u00a0\u200b\n\n\u2192\ncatalyst\n\n\u00a0\u200bEthanol\n\n\n\n\nRate equations for reaction can be written as: -For Glycerol,\n\n(4)\n\n\n-\n\n\ndC\nG\n\ndt\n\n\u00a0\u200b\n=\n\u00a0\u200b\n\n[\n\n\nwk\n1\n\n\nH\n\nH\n2\n\n\n\n]\n\n\nC\nG\n\n\nP\n\nH\n2\n\n\n+\n\n[\n\n\nwk\n2\n\n\nH\n\nH\n2\n\n\n\n]\n\n\nC\nG\n\n\nP\n\nH\n2\n\n\n\n\n\n\nFor 1,2-Propanediol,\n\n(5)\n\n\n-\n\n\ndC\n\n1\n,\n2\n-\nPDO\n\n\ndt\n\n\u00a0\u200b\n=\n\u00a0\u200b\n\n[\n\n\nwk\n1\n\n\nH\n\nH\n2\n\n\n\n]\n\n\nC\nG\n\n\nP\n\nH\n2\n\n\n\n\n\n\nFor Ethylene glycol,\n\n(6)\n\n\n-\n\n\ndC\nEG\n\ndt\n\n\u00a0\u200b\n=\n\u00a0\u200b\n\n[\n\n\nwk\n2\n\n\nH\n\nH\n2\n\n\n\n]\n\n\nC\nG\n\n\nP\n\nH\n2\n\n\n-\n\n[\n\n\nwk\n3\n\n\nH\n\nH\n2\n\n\n\n]\n\n\nC\nEG\n\n\nP\n\nH\n2\n\n\n\n\n\nwhere, CG, C1,2-PDO, CEG are glycerol concentration, 1,2-PDO and EG correspondingly at any time \u2018t\u2019, k1, k2, k3 are specific reaction rate constant, \n\n\nP\n\nH\n2\n\n\n\n is hydrogen's partial pressure and HH2 is henry's constant. Also, w stands for concentration (kg/m3(liquid)) of the catalyst.For the estimation of the parameters of the reaction kinetics and predicted time dependent concentration data, rate equations were elucidated mathematically in MATLAB via ode23s by fitting the data obtained experimentally. The optimum kinetic parameter values were found by reducing the residual sum of squares among simulated and experimental glycerine amounts across experiments. Parity plots are used to compare the observed and model estimated concentration of glycerine, 1,2- PDO, and EG, as illustrated in Fig.\u00a02\n. The excellent match between the experimental and modelled concentrations was established by these data. Using the Arrhenius equation, the impact of changing the temperature of reaction on rate constant was utilized to determine the activation energy. Fig.\u00a03\n shows the plot of ln k vs 1/T. Table\u00a02\n shows the pre-exponential factor and activation energy for the production of 1,2-PDO and EG using a modified power law model. Activation energy for formation of 1,2-PDO was found to be 52.6\u00a0\u200bkJ/mol with pre-exponential factor 7.1\u00a0\u200b\u00d7\u00a0\u200b106\u00a0\u200bmol/gcat.h and 58.6\u00a0\u200bkJ/mol and 3.2\u00a0\u200b\u00d7\u00a0\u200b106\u00a0\u200bmol/gcat.h for the formation of EG by using modified power-law model.To calculate the preliminary reaction rate parameters, a modified power law model was employed. The power law model, on the other hand, has a significant flaw: it does not account for all of the factors that affect heterogeneous reactions, such as adsorption, surface reaction and desorption on a catalyst site. While, for heterogeneous processes, the Eley-Rideal model is a regularly used realistic way to obtain the rate expression. For solid catalyzed reactions, the Eley-Rideal model is favoured because it incorporates a rate equation derived from reaction mechanism that includes actual surface phenomena throughout reaction. This approach considers adsorption, surface reaction and desorption steps on catalyst active site when calculating reaction rate. As a result, Eley-Rideal type model was developed to better explain reaction kinetics.In this model, initially the glycerol molecule adsorbed on catalyst site undergoes dehydration to form water and acetol. In next step, the adsorbed acetol reacts with hydrogen present in the reactor to form adsorbed propylene glycol. The desorption of 1,2-PDO, acetol and water from catalyst surface take place in the final step with regeneration of the active centers.The following reaction processes were used to get the rate equations:Step 1: Adsorption of glycerol (G) on catalyst's active site ($):\n\n(7)\n\n\nG\n\n+\n\n$\n\n\n\u21c4\n\nk\n\n\u2212\n1\n\n\n\nk\n1\n\n\n\n\nG\n\n.\n\n$\n\n\n\n\nStep 2: Adsorbed glycerol is dehydrated to adsorbed acetol, and then the surface interaction between adsorbed acetol (A.$) and hydrogen molecule (H2) occurs:\n\n(8)\n\n\nG\n\n.\n\n$\n\n\n\n\u21c4\n\nk\n\n\u2212\n2\n\n\n\nk\n2\n\n\n\n\nA\n.\n$\n+\nW\n\n\n\n\n\n\n(9)\n\n\nA\n.\n\n$\n\n+\n\n\nH\n2\n\n\n\n\u21c4\n\nk\n\n\u2212\n3\n\n\n\nk\n3\n\n\n\n\nP\n\n.\n\n$\n\n\n\n\n1,2- PDO, acetol, and water are represented by P, A, W, correspondingly.Step 3: Desorption of adsorbed acetol and 1,2- PDO from surface of catalyst, as well as active site regeneration:\n\n(10)\n\n\nA\n.\n\n$\n\n\n\u21c4\n\nk\n\n\u2212\n4\n\n\n\nk\n4\n\n\n\nA\n\n+\n\n$\n\n\n\n\n\n\n(11)\n\n\nP\n.\n\n$\n\n\n\u21c4\n\nk\n\n\u2212\n5\n\n\n\nk\n5\n\n\n\nP\n\n+\n\n$\n\n\n\n\nIndividual rate equations can be expressed as follows using Eqs. (7) - (11):\n\n(12)\n\n\n\n(\n\u2212\n\n\nr\n1\n\n)\n\n=\n\n\nk\n1\n\n\n(\n\nC\nG\n\n\nC\n$\n\n\u2212\n\n\n\nC\n\nG\n\n.\n\n$\n\n\n\nK\n1\n\n\n)\n\n\n;\n\n\nK\n1\n\n=\n\n\nk\n1\n\n\nk\n\n\u2212\n1\n\n\n\n\n\n\n\n\n\n(13)\n\n\n\n(\n\u2212\n\n\nr\n2\n\n)\n\n=\n\n\nk\n2\n\n\n(\n\nC\n\nG\n.\n$\n\n\n\u2212\n\n\n\n\nC\n\nA\n.\n$\n\n\n\nC\nW\n\n\n\nK\n2\n\n\n)\n\n\n;\n\n\nK\n2\n\n=\n\n\nk\n2\n\n\nk\n\n\u2212\n2\n\n\n\n\n\n\n\n\n\n(14)\n\n\n\n(\n\u2212\n\n\nr\n3\n\n)\n\n=\n\n\nk\n3\n\n\n(\n\nC\n\nA\n.\n\n$\n\n\n\nP\n\nH\n2\n\n\n\u2212\n\n\n\nC\n\nP\n.\n\n$\n\n\n\nK\n3\n\n\n)\n\n\n;\n\n\nK\n3\n\n=\n\n\nk\n3\n\n\nk\n\n\u2212\n3\n\n\n\n\n\n\n\n\n\n(15)\n\n\n\n(\n\u2212\n\n\nr\n4\n\n)\n\n=\n\n\nk\n4\n\n\n(\n\nC\n\nA\n.\n\n$\n\n\n\u2212\n\n\n\n\nC\nA\n\n\nC\n$\n\n\n\nK\n4\n\n\n)\n\n;\n\n\nK\n4\n\n=\n\n\nk\n4\n\n\nk\n\n\u2212\n4\n\n\n\n\n\n\n\n\n\n(16)\n\n\n\n(\n\u2212\n\n\nr\n5\n\n)\n\n=\n\n\nk\n5\n\n\n(\n\nC\n\nP\n.\n\n$\n\n\n\u2212\n\n\n\n\nC\nP\n\n\nC\n$\n\n\n\nK\n5\n\n\n)\n\n;\n\n\nK\n5\n\n=\n\n\nk\n5\n\n\nk\n\n\u2212\n5\n\n\n\n\n\n\n\nThe equilibrium constants for the corresponding reactions are K1, K2, K3, K4, K5.Adsorption, surface reaction, and desorption are all rate limiting stages, hence three alternative rate equations were generated. It was expected that rate limiting step would be surface reaction. Thus, rate of surface reactions can be written as follows:The resulting rate equations are reduced as follows:From Eq. (13),\n\n(17)\n\n\n\n(\n\u2212\n\n\nr\n2\n\n)\n\n=\n\n\nk\n2\n\n\n\nC\n\nG\n.\n\n$\n\n\n\n\n\n\nfrom Eq. (14),\n\n(18)\n\n\n\n(\n\u2212\n\n\nr\n3\n\n)\n\n=\n\n\nk\n3\n\n\n\nC\n\nA\n.\n\n$\n\n\n\n\nP\n\nH\n2\n\n\n\n\n\n\nPost considering the adsorption and desorption steps as significantly faster than surface reaction [29], we have.From Eq. (12), \n\n\n\nr\n1\n\n\nk\n1\n\n\n\n=\n\n0\n\n, then\n\n(19)\n\n\n\nC\n\nG\n.\n\n$\n\n\n\n=\n\n\nK\n1\n\n\u00a0\u200b\n\nC\nG\n\n\u00a0\u200b\n\nC\n$\n\n\n\n\n\nSimilarly, from Eq. (15), \n\n\n\nr\n4\n\n\nk\n4\n\n\n\n=\n\n0\n\n, then\n\n(20)\n\n\n\nC\n\nA\n.\n\n$\n\n\n\n=\n\n\n\n\nC\nA\n\n\u00a0\u200b\n\nC\n$\n\n\n\nK\n4\n\n\n\n\n\n\nSimilarly, from Eq. (16), \n\n\n\nr\n5\n\n\nk\n5\n\n\n\n=\n\n0\n\n, then\n\n(21)\n\n\n\nC\n\nP\n.\n\n$\n\n\n\n=\n\n\n\n\nC\nP\n\n\u00a0\u200b\n\nC\n$\n\n\n\nK\n5\n\n\n\n\n\n\nSubstitution of Eqs. (20) and (21) into the surface reaction rate Eqs. (17) and (18) subsequently results to:\n\n(22)\n\n\n\n(\n\u2212\n\n\nr\n2\n\n)\n\n\n=\n\n\nk\n2\n\n\n\nK\n1\n\n\n\nC\nG\n\n\n\nC\n$\n\n\n\n\n\n\n\n(23)\n\n\n\n(\n\u2212\n\n\nr\n3\n\n)\n\n\n=\n\n\n\n\nk\n3\n\n\nC\nA\n\n\nP\n\nH\n2\n\n\n\nC\n$\n\n\n\nK\n4\n\n\n\n\n\n\nFrom the catalyst site's overall balance, we have\n\n(24)\nCT$\u00a0\u200b=\u00a0\u200bC$\u00a0\u200b+\u00a0\u200bCG.$\u00a0\u200b+\u00a0\u200bCA.$\u00a0\u200b+\u00a0\u200bCP.$\n\n\n\nWhen the values of adsorbed species concentrations are substituted in total site balance (Eqn. (24)) the following results are obtained:\n\n(25)\n\n\n\n\n\n\n\nC\n\nT\n$\n\n\n=\n\nC\n$\n\n+\n\nK\n1\n\n\nC\nG\n\n\nC\n$\n\n+\n\n\n\nC\nA\n\n\u00a0\u200b\n\nC\n$\n\n\n\nK\n4\n\n\n+\n\n\n\nC\nP\n\n\u00a0\u200b\n\nC\n$\n\n\n\nK\n5\n\n\n\n\n\n\n\n\n\nC\n$\n\n\n=\n\n\n\nC\n\nT\n$\n\n\n\n(\n\n1\n+\n\n\nK\n1\n\n\n\nC\nG\n\n+\n\n\n\nC\nA\n\n\nK\n4\n\n\n+\n\n\n\nC\nP\n\n\nK\n5\n\n\n\n)\n\n\n\n\n\n\n\n\n\n\n\n\n\nSubstituting Eq. (25) into Eqs. (21) and (23) yields\n\n(26)\n\n\n\n(\n\u2212\n\n\nr\n2\n\n)\n\n\n=\n\n\n\n\nk\n2\n\n\n\nK\n1\n\n\nC\nG\n\n\nC\n\nT\n$\n\n\n\n\n[\n\n1\n+\n\n\nK\n1\n\n\n\nC\nG\n\n+\n\n\n\nC\nA\n\n\nK\n4\n\n\n+\n\n\nC\nP\n\n\nK\n5\n\n\n\n]\n\n\n\n=\n\n\n\n\nk\n2\n\u2032\n\n\n\nK\n1\n\n\nC\nG\n\n\n\n[\n\n1\n+\n\n\nK\n1\n\n\n\nC\nG\n\n+\n\n\n\nC\nA\n\n\nK\n4\n\n\n+\n\n\nC\nP\n\n\nK\n5\n\n\n\n]\n\n\n\n\n\nwhere \n\n\nk\n2\n\u2032\n\n=\n\nk\n2\n\n\nC\n\nT\n$\n\n\n\n, glycerol to acetol apparent reaction rate constant.\n\n(27)\n\n\n\n(\n\u2212\n\n\nr\n3\n\n)\n\n\n=\n\n\n\n\nk\n3\n\n\nC\nA\n\n\nP\n\nH\n2\n\n\n\nC\n\nT\n$\n\n\n\n\n\nK\n4\n\n\n[\n\n1\n+\n\n\nK\n1\n\n\n\nC\nG\n\n+\n\n\n\nC\nA\n\n\nK\n4\n\n\n+\n\n\nC\nP\n\n\nK\n5\n\n\n\n]\n\n\n\n\n=\n\n\n\n\nk\n3\n\u2032\n\n\nC\nA\n\n\nP\n\nH\n2\n\n\n\n\n\nK\n4\n\n\n[\n\n1\n+\n\n\nK\n1\n\n\n\nC\nG\n\n+\n\n\n\nC\nA\n\n\nK\n4\n\n\n+\n\n\nC\nP\n\n\nK\n5\n\n\n\n]\n\n\n\n\n\n\nhere \n\n\nk\n3\n\u2032\n\n=\n\nk\n3\n\n\nC\n\nT\n$\n\n\n\n is apparent reaction rate constant for acetol to 1,2-PDO. Eqs. (26) and (27) reflect the concluding rate expressions for two step reaction, glycerol dehydration to acetol subsequently acetol hydrogenation to 1,2-PDO.The mole balance of individual species i at any instant in time t for the jth reaction may be stated as follows to develop the model:\n\n(28)\n\n\n\n\nd\n\n\nC\ni\n\n\n\nd\n\nt\n\n\n\n=\n\n\n\u2211\nj\n\n\nn\n\ni\nj\n\n\n\nr\nj\n\n\n\n\n\nThe steady state concentrations of glycerol, acetol, and 1,2-PDO, respectively, are CG, CA, and CP and nij represents stociometry coefficient of ith species in jth reaction. rj: rate of jth reaction.The set of ordinary differential equation (Eqn. (28)) produced were numerically solved using MATLAB's ode23s function coupled with genetic algorithm (GA) optimization for stiff systems to estimate unknown parameters in the rate equations. The residual sum of squares, f (fitness function), among observed 1,2-PDO, glycerol, and acetol concentrations and estimated concentration values generated from model equations was minimized using GA.As follows is the expression of the Objective function:\n\n(29)\n\n\nf\n=\n\n\u2211\n\ni\n=\n\n1\n\nN\n\n\n\n\n[\n\n\n\n(\n\nC\n\n\nG\n,\nexp\n\n\n\ni\n\n\n\u2212\n\nC\n\n\nG\n,\ns\ni\nm\n\n\n\ni\n\n\n)\n\n2\n\n\n+\n\n\n\n(\n\nC\n\n\nA\n,\nexp\n\n\n\ni\n\n\n\u2212\n\nC\n\n\nA\n,\ns\ni\nm\n\n\n\ni\n\n\n)\n\n2\n\n+\n\n\n\n(\n\nC\n\n\nP\n,\nexp\n\n\n\ni\n\n\n\u2212\n\nC\n\n\nP\n,\ns\ni\nm\n\n\n\ni\n\n\n)\n\n2\n\n\n]\n\n\n\n\n\nwhere, N represents experimental runs and \n\n\nC\n\nG\n,\nexp\n\ni\n\n\n, \n\n\nC\n\nA\n,\nexp\n\ni\n\n\n, and \n\n\nC\n\nP\n,\nexp\n\ni\n\n\n are experimental concentration of glycerine, hydroxyacetone and 1,2-PDO, while The corresponding anticipated concentrations derived by solving the model equations are \n\n\nC\n\nG\n,\ns\ni\nm\n\ni\n\n\n , \n\n\nC\n\nA\n,\ns\ni\nm\n\ni\n\n\n and \n\n\nC\n\nP\n,\ns\ni\nm\n\ni\n\n\n.For the estimate of kinetic parameters, a simple genetic algorithm code was constructed in this work. The best answer is determined by the size of the population, the probability of the genetic operators (crossover and mutation), and the seed values used. The objective function was minimized and the kinetic parameters were estimated using a population of 1000. For crossover and mutation probability, optimal values of genetic operators of 0.9 and 0.1 were used, respectively. Using equation below, pre-exponential factors and activation energy were calculated via assessed value for rate constants and adsorption constants acquired at various temperature [22].\n\n(30)\nki\u00a0\u200b=\u00a0\u200bki\no exp [-Ei/(RT)]\n\n\nThen all equilibrium constants are described by the equation given below [22].\n\n(31)\nKj\u00a0\u200b=\u00a0\u200bKj\no exp[Ej/(RT)]\n\n\nThe obtained values are summarized in the Table\u00a03\n.Parity plots are used to compare the observed and model simulated concentrations of glycerol and propylene glycol, as illustrated in Fig.\u00a04\n. Arrhenius plot to evaluate the energy of activation of adsorption of glycerol, formation of 1,2-PDO and desorption of 1,2-PDO are shown in Fig.\u00a05\n. It was discovered that activation energy for production of propylene glycol is 41.42\u00a0\u200bkJ/mol. The value estimated using the modified power law model (52.6\u00a0\u200bkJ/mol) was found to be compatible with this finding. The obtained activation was compared with previously reported studies in Table\u00a0S1. The results showed that the suggested Eley-Rideal model suited the experimental and simulated data quite well. The obtained activation energy differs for both the models owing to the assumptions utilized for model development.Kinetic study was performed over Bi-functional layered double hydroxide (LDH) catalyst. The effect of different reaction parameter was utilized to obtain the kinetic parameters of the hydrogenolysis of glycerol. Based on reaction products distribution plausible reaction mechanism was proposed. Two different type of kinetic models i.e. To suit the experimental data, modified power law and Eley-Rideal were tested. To simulate experimentally obtained concentration time data, a set of differential equations was created and numerically solved using ode23s in MATLAB in conjunction with the genetic algorithm optimization tool. The kinetic parameters were obtained by minimizing the residual sum of squares between the predicted and experimental concentrations of glycerol, propylene glycol, and EG. By using modified power law model, energy of activation and pre-exponential factor were estimated as 52.6\u00a0\u200bkJ\u00a0\u200bmol\u22121 and 7.1\u00a0\u200b\u00d7\u00a0\u200b106\u00a0\u200bmol/gcat.h for formation of 1,2-PDO and 58.6\u00a0\u200bkJ/mol and 3.2\u00a0\u200b\u00d7\u00a0\u200b106\u00a0\u200bmol/gcat.h for the formation of EG, separately. Furthermore, the kinetic parameters were determined using a more realistic Eley-Rideal model, and the activation energy for the synthesis of 1,2-PDO was compared to the modified power law model. The activation energy obtained by using the Eley-Rideal model (41.42\u00a0\u200bkJ/mol) for the surface reaction step of glycerol was comparable with the activation energy obtained by using the modified power law model (52.6\u00a0\u200bkJ/mol). The results revealed that the suggested modified power law and Eley-Rideal model successfully linked the rate data, as well as the actual and predicted concentrations of glycerol and 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.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.crgsc.2022.100289.", "descript": "\n                  The performance of a highly efficient Cu0.45Zn0.15Mg5.4Al2O9 catalyst was investigated using a high-pressure autoclave reactor by adjusting several reaction conditions. The involvement of intermediates (Hydroxyacetone, Propylene glycol, Ethylene glycol) was explored to better understand the reaction pathways. The hydrogenolysis reaction was shown to be a multistep process, including the dehydration of glycerine to hydroxyacetone and then the hydrogenation of hydroxyacetone to 1,2-PDO. Finally, several kinetic models were established, and experimental outcomes were fitted to these models. The reaction kinetic parameters were calculated in MATLAB using ode45 and ode23s combined with optimization strategies to solve the resultant ordinary differential equations. The modified Power law model and the Eley-Rideal (E-R) models adequately correlated the experimental outcomes for two step hydrogenolysis reaction, according to the findings. The modified power law model revealed a pseudo-first order reaction in case of glycerol, and the energy of activation was calculated as 52.6\u00a0\u200bkJ/mol.\n               "}
{"full_text": "Metal-air batteries, such as Zn-air batteries, Mg-air batteries, Al-air batteries are a class of safe, reliable, and efficient energy storage devices have attracted increasing attention [1]. Research has proven they have much higher theoretical energy density than that of state-of-the-art Li-ion battery by producing electric energy through a redox reaction between metal and oxygen [2,3]. Among them, Al-air batteries possess great potential for large scale application due to the high specific capacity (2.98 Ah g\u22121) and energy density (8100 Wh kg\u22121), abundant resources of aluminum, environmentally friendly nature with high recyclability etc [4,5]. However, the sluggish kinetics of oxygen reduction reaction (ORR) normally resulting in serious cathode polarization and low energy efficiency is one of the serious issues hindering the wide commercialization of Al-air batteries [6,7].Platinum (Pt) nanoparticles (NP) dispersed on active carbon materials (Pt/C) has been commonly used to effectively prompt the ORR process, however, it suffers from high cost, low utilization efficiency and poor durability [8,9]. Tremendous efforts have been devoted on the development of low or non-Pt ORR catalysts [10], and the incorporation of other transition metals was reported to simultaneously enhance the ORR activity and durability [11,12]. It has been reported that transition metal (M) such as, Ag, Pd, Cu, Fe, Ni etc. are introduced to form Pt-M alloy or bimetallic catalysts to reduce Pt loading, increase utilization efficiency, high activity and stability. Among the transition metals, gold (Au) is a special candidate in view of its higher oxidation potential than Pt, which encourage the combination of Au and Pt to be a stable catalyst [13,14].The alloying of Pt with Au is a direct way of incorporation, which was reported to exert apparent effect on the electronic structure owing to the strong coupling between Pt and Au atoms, resulted in attractive ORR catalytic activity [12,15-17]. The problem is that Pt and Au are not always miscible in a whole range of concentrations and phase segregation can be expected, which influence the stability of catalyst seriously [18]. As an alternative choice, forming core (Au) -shell (Pt) structure has been reported to suppress the degradations of Pt nanoparticles (NPs) by up-shifting the dissolution potential of Pt and thereby pledging good long-term stability [19-24]. Shi et al. prepared Au-Pt core\u2013shell catalyst in size of 30\u00a0~\u00a075\u00a0nm aided by ionic liquid, which effectively improved the ORR catalytic activity and stability in comparison to the Pt/C catalyst, because of the high utilization of Pt and the protection of Pt active sites by Au [23]. Further increasing the stability of Au-Pt core\u2013shell can be achieved by doped Au core with titanium oxide at vertex and edges, which restricted to much Au segregation on to the Pt at surface facets, as reported by Hu et al [21].In a reverse way to fabricate Au-Pt core\u2013shell catalyst, decorating Pt surface with Au atoms to protect the vulnerable sites at edges and corners was also reported [25-27]. Kodama et al. deposited Au atoms on step sites of Pt single-crystal surface, which raised the ORR activity by 70% and also improved the durability of Pt [27]. Moreover, Takahashi et al., modified the edges and corners of Pt nanoparticles with arc-plasma deposition, the stability as well as the activity of Pt catalysts was improved significantly [25,26]. With this physical vapour deposition technique, the deposited amounts and deposition site of Au on Pt catalysts is easy to control.Recent attempts are addressed to develop bimetallic AuPt nanosize catalysts, which has been proved that Au clusters confer stability by raising the Pt oxidation potential and stabilizing Pt against dissolution under harsh work environment [28,29]. Zheng et al. synthesized smaller AuPt NPs (d \u2248 5\u00a0nm) in form of popcorn-like aggregates clusters (in size of ca. 36\u00a0nm), which only exhibited better ORR catalytic activity than 10\u00a0wt% Pt/C, poorer than 20\u00a0wt% Pt/C possibly due to the aggregated structure [29]. It is known that increase the particles size can improve the stability, but sacrifice the activate surface area therefore the catalytic activity. Further investigation to synthesis AuPt catalysts with low metal loading, high activity and stability still need further investigation.Following this context, we synthesized a series of AuxPt/MWNTs catalysts (x\u00a0=\u00a00.25, 0.67, 1.68 and 4.55 of atom ratio) by a simple one-pot reduction of chloroauric acid and chloroplatinic acid with the tris(hydroxylmethyl)phosphine oxide (THPO) in presence of MWNTs. The synthesized AuxPt NPs are highly dispersed with an average diameter of ca. 3.0\u00a0nm. The Au0.67Pt/MWNTs catalyst with metal loading of 10.2\u00a0wt% (Au:4.1\u00a0wt%, Pt:6.1\u00a0wt%) exhibited a competitive ORR catalytic activity and durability to 20\u00a0wt% Pt/C catalyst. The Au1.68Pt/MWNTs by properly increasing Au loading to 8.95\u00a0wt% (Pt:5.3\u00a0wt%) as the Al-air battery cathode showed larger capacity and power density, superior durability than 20\u00a0wt% Pt/C cathode.Commercial platinum catalyst (Pt/C, 20\u00a0wt% and 10\u00a0wt%, Alfa Aesar), chloroauric acid (HAuCl4, 99.999%, Shanghai Titan Scientific Co. Ltd., China), tetrakis(hydroxylmethyl) phosphonium chloride (THPC, 80% aqueous solution, Sigma-Aldrich), Nafion solution (5\u00a0wt%, Sigma-Aldrich) and multi-walled carbon nanotubes (MWNTs, diameter\u00a0=\u00a010\u00a0~\u00a020\u00a0nm, length\u00a0=\u00a010\u00a0~\u00a030\u00a0mm) were used directly without further treatment. Chloroplatinic acid (H2PtCl6, 37%), potassium hydroxide (KOH, AR), sodium hydroxide (NaOH, AR) and hydrogen peroxide (H2O2, AR, 30% aqueous solution) are purchased from Sinopharm Chemical Reagent Co., Ltd.The MWNTs was pretreated using a moderate surface oxidation to increase water affinity [13,30]. Typically, 200\u00a0mg of MWNTs was added in a gas-proof Erlenmeyer flask with a separating funnel in connection to a vacuum pump. The flask was vacuumed to a pressure of 0.01\u00a0MPa for 10\u00a0min, then 40\u00a0mL of deionized water and H2O2 mixture was added. The suspension was then sonicated for 10\u00a0min followed mixing for 2\u00a0h using magnetic stirrer, then kept still overnight. The pre-treated MWNTs were separated from the suspension by centrifuging at 8000\u00a0rpm, then dried in an oven at 80\u00a0\u00b0C overnight.43\u00a0mg of the treated MWNTs were added into 95\u00a0mL of deionized water at 75\u00a0\u00b0C and mixed using ultrasonic for 5 mins, and then 1\u00a0mL of 24.3\u00a0mM HAuCl4 and 1.25\u00a0mL of 20\u00a0mM H2PtCl6 were added, followed by addition of 600\u00a0\u03bcL of 1\u00a0M NaOH and 2\u00a0mL of 50\u00a0mM THPC. It was kept stirring for further 3\u00a0h at 75\u00a0\u00b0C to make uniform suspension, and then transferred to ice bath and stood overnight. The product was rinsed with deionized water till pH neutral, and using a freeze-dryer. the obtained hybrid was named as Au1.68Pt/MWNTs. Three other hybrids were synthesized using the same procedure with different volumes content of HAuCl4 and H2PtCl6 solution, specifically, with 0.333\u00a0mL of HAuCl4 and 2.083\u00a0mL of H2PtCl6, 0.5\u00a0mL of HAuCl4 and 1.875\u00a0mL of H2PtCl6, 1.5\u00a0mL of HAuCl4 and 0.625\u00a0mL of H2PtCl6, the catalysts are marked as Au0.25Pt/MWNTs, Au0.67Pt/MWNTs and Au4.55Pt/MWNTs, respectively.Catalyst morphology and elemental analyses were carried out using a spherical aberration corrected field emission transmission electron microscope (TEM, Titan G2 60\u2013300) operated at 200\u00a0kV. The structure of the catalysts was characterized by an X-ray diffractometer (XRD, PANalytical) equipped with Cu k\u03b1 radiation. The chemical component of the catalysts was investigated using an X-ray photoelectron spectroscope (XPS, ESCALAB 250Xi, Thermo Fisher Scientific) using Al k\u03b1 radiation. The metal loading in the catalysts was examined by an inductively coupled plasma mass spectrometer (ICP-MS, X-Series \u2161, Thermo Fisher Scientific), where the hybrids were calcinated at 400\u00a0\u00b0C for 2\u00a0h and then 500\u00a0\u00b0C for 5\u00a0h in air to burn up the MWNTs substrate, after cooling to 200\u00a0\u00b0C the residuals were treated with aqua regia. The ICP-MS technique was used to determine the metal content in the solution, where each sample was tested for three times, taking the average value as the loading amount of Au and Pt for each sample.4\u00a0mg of a AuxPt/MWNTs catalyst, 100\u00a0\u03bcL of Nafion solution, 200\u00a0\u03bcL ethanol and 800\u00a0\u03bcL deionized water are used to prepare the catalyst ink, the slurry was mixed using an ultrasonic sound bath for 30 mins. Cyclic voltammetry (CV) analysis was performed by using an electrochemical workstation (CHI660E, CH Instruments) at a scan rate of 20\u00a0mV\u00a0s\u22121 in N2 or O2 saturated 0.1\u00a0M KOH solutions. The working, counter and reference electrodes are glassy carbon electrode (GCE, d\u00a0=\u00a04\u00a0mm, S\u00a0=\u00a00.126\u00a0cm2), platinum wire and Ag/AgCl electrode, respectively. 8\u00a0\u03bcL of the catalyst slurry was dropped on the GCE, it was dried in ambient temperature to obtain a smooth coverage on the electrode with catalyst loading of 0.23\u00a0mg\u00a0cm\u22122. All potential values were given with the respective to reversible hydrogen electrode (RHE) scale, the potentials were converted from Ag/AgCl electrode by using \n\n\n\u03c6\n\ntestvsRHE\n\n\n=\n\n\u03c6\n\ntestvsAg\n/\nA\ng\nC\nl\n\n\n+\n0.209\n+\n0.059\np\nH\n\n,where \n\n\n\u03c6\n\ntestvsRHE\n\n\n\nand \n\n\n\u03c6\n\ntestvsAg\n/\nA\ng\nC\nl\n\n\n\nis the testing potential verse RHE and Ag/AgCl reference electrode, respectively, 0.209 is the standard potential of Ag/AgCl electrode. The relationship between \n\n\n\u03c6\n\nRHE\n\n\n\nand pH are showed in Fig.S1.The ORR kinetics of the AuxPt/MWNTs hybrid was examined using linear scan voltammetry (LSV) method. ORR kinetics was investigated using rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) technologies in O2 saturated 0.1\u00a0M KOH. The rotating speed were 400\u00a0rpm, 625\u00a0rpm, 900\u00a0rpm, 1225\u00a0rpm and 1600\u00a0rpm with the scan rate at 5\u00a0mV\u00a0s\u22121 for RDE. The RRDE equipped with a glassy carbon disk electrode (d\u00a0=\u00a04\u00a0mm, S\u00a0=\u00a00.126\u00a0cm2) and a Pt ring electrode (S\u00a0=\u00a00.189\u00a0cm2) and it was performed at scan rate of 1600\u00a0rpm only. In the experiment, the disk potential scanned from 1.0 to 0.2\u00a0V at a rate of 5\u00a0mV\u00a0s\u22121, and the ring potential was fixed at 1.8\u00a0V. Prior to testing, 5\u00a0\u03bcL and 8\u00a0\u03bcL of the catalyst slurry were coated and dried on the RDE and RRDE with catalyst loading of 0.25 and 0.23\u00a0mg\u00a0cm\u22122, respectively.Al-air battery performance were measured in a homemade testing cell fabricated with Al foil anode (99.99%, 4.5\u00a0cm2), 4\u00a0M KOH electrolyte and air cathode. The air cathode comprises of a current collector (Ni foam) and a carbon paper (1\u00a0cm2) coated with 2\u00a0mg catalyst layer. The discharge polarization curves were carried out at 1\u00a0mV\u00a0s\u22121 between the potential widow of 1.8\u20130\u00a0V vs. Al. the specific capacity was recorded at 100\u00a0mA\u00a0cm\u22122, the dynamic galvanostatic measurement were performed between 1\u00a0mA\u00a0cm\u22122 and 200\u00a0mA\u00a0cm\u22122, the durability of air electrode was tested by discharging five cycles by replacing Al anode and electrolyte after each discharge.As shown in Table 1\n, the overall Au and Pt loading amounts of the Au4.55Pt/MWNTs, Au1.68Pt/MWNTs, Au0.67Pt/MWNTs and Au0.25Pt/MWNTs catalysts are measured as 15.05\u00a0wt%, 14.25\u00a0wt%, 10.2\u00a0wt% and 9.5\u00a0wt% by the ICP-MS analysis, from which the exact Au/Pt ratios of the catalyst are determined as 4.55, 1.68, 0.67 and 0.25, respectively.\nFig. 1\n a1, b1 and c1 illustrate the TEM images of Au4.55Pt/MWNTs, Au1.68Pt/MWNTs and Au0.67Pt/MWNTs catalysts, respectively, along with the corresponding ones in higher magnification shown in Fig.1 a2, b2 and c2. All AuxPt NPs are deposited uniformly on the MWNTs substrates. The average particle size of the Au4.55Pt on MWNTs is measured in the picture as 3.02\u00a0nm, the Au1.68Pt as 2.98\u00a0nm, and the Au0.67Pt/MWNTs as 2.96\u00a0nm, suggesting that the variation of Au/Pt ratios did not influence much on the size of the bimetallic AuxPt NPs. In the high-resolution TEM (HRTEM) images in Fig. 1 a3, b3 and c3, two d-spacing values of 0.236\u00a0nm and 0.225\u00a0nm are measured, assigned to the Au (1\u00a01\u00a01) and Pt (1\u00a01\u00a01) facets, respectively [31]. The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images in the Fig. 1 a4, b4 and c4 display that the Au and Pt NPs stay overlapped but do not grow together. The energy dispersive X-ray (EDX) analyses in Fig. 1 a5, b5 and c5 also display that Au (green color) and Pt (red color) NPs are very close to each other, implying the possible interaction between the Au and Pt NPs.XRD patterns of three AuxPt/MWNTs catalysts in Fig. 2\n present the same feature. The peak at 26.6\u00b0 can be assigned to the (0\u00a00\u00a02) facet of MWNTs (JCPDF No. 25\u20130284), and the peaks at 38.1\u00b0, 44.3\u00b0, 64.5\u00b0 and 77.5\u00b0 are corresponding to (1\u00a01\u00a01), (2\u00a00\u00a00), (2\u00a02\u00a00) and (3\u00a01\u00a01) facets of Au (JCPDF No. 04\u20130784). The peaks at 39.5\u00b0, 45.9\u00b0, 67.0\u00b0 are attributed to (1\u00a01\u00a01), (2\u00a00\u00a00) and (2\u00a02\u00a00) facets of Pt (JCPDF No. 87\u20130636), where the diffractions of Pt are enhanced with the decrease of the Au/Pt ratio. No signal assigned to AuPt alloy is seen in the XRD patterns.\nFig. 3\n displays the XPS analysis results for AuxPt/MWNTs catalysts. The high-resolution signals of C1s for the three hybrids (column a) are fitted with three peaks at 284.8\u00a0eV, 285.2\u00a0eV and 286\u00a0eV in correspondence to C-H/C-H, C-P-O and C-OH groups, respectively[32]. The P2p signals (column b) present two fitting peaks at 133.8\u00a0eV and 134.7\u00a0eV in each curve, assigned to P2p3/2 and P2p1/2 groups [32]. The resolved Au4f signals (column c) manifest a doublet at 84.3\u00a0eV and 87.9\u00a0eV, attributed to the 4f7/2 and 4f5/2 of metallic Au [13]. The Pt4f signal (column d) can be fitted into a doublet at 71.4\u00a0eV and 74.7\u00a0eV associated with metallic Pt, and the peaks at 72.5\u00a0eV and 75.8\u00a0eV are attributed to the divalent state of Pt (Pt2+) [23,24]. The results indicate of triphenylphosphine oxide (THPO) as the capping molecule on the AuxPt NPs, which is normally generated from the cleavage of THPC in alkaline solutions [30].The CV plots of the AuxPt/MWNTs catalysts and 20\u00a0wt% Pt/C catalyst are recorded in both O2 and N2 saturated 0.1\u00a0M KOH ranging from 1.2\u00a0V to 0\u00a0V at a scanning rate of 20\u00a0mV\u00a0s\u22121. In hydrogen underpotential deposition (HUPD) region, peaks observed between 0\u00a0V and 0.4\u00a0V attributed to hydrogen adsorption and desorption. For the Au4.55Pt/MWNTs hybrid, Fig. 4\na exhibits an oxygen reduction peak at 0.86\u00a0V with the current density of 0.86\u00a0mA\u00a0cm\u22122 in O2 saturated 0.1\u00a0M KOH solution, in contrast to the curve in N2 saturated electrolyte. With the decrease of the Au/Pt ratio, the reduction peak potential of the Au1.68Pt/MWNTs and Au0.67Pt/MWNTs catalysts shifts positively to 0.866\u00a0V and 0.87\u00a0V, respectively, along with larger peak current densities of 0.93\u00a0mA\u00a0cm\u22122 and 0.96\u00a0mA\u00a0cm\u22122. Further decreasing the Au/Pt ratio, the reduction peak potential of Au0.25Pt/MWNTs shift positively to 0.876\u00a0V, however, the peak current decays to 0.64\u00a0mA\u00a0cm\u22122 as showed in Fig.S2. In comparison, the 20\u00a0wt% Pt/C catalyst exhibits an oxygen reduction peak at 0.886\u00a0V with the current density of 0.44\u00a0mA\u00a0cm\u22122. The CV plots of MWNTs were also measured (see Fig.S3), which presented a reduction peak at 0.736\u00a0V with the current density of 0.241\u00a0mA\u00a0cm\u22122 in O2 saturated 0.1\u00a0M KOH solution, demonstrating that MWNTs has weak catalytic activity towards oxygen reduction reaction, decorating with AuPt NPs improve the ORR activity significantly. It is seen that all the AuxPt/MWNTs catalysts present larger reaction current densities than the 20\u00a0wt% Pt/C catalyst for oxygen reduction.The RDE experiment was also used to characterize the ORR performance of the AuxPt/MWNTs catalysts in O2-saturated 0.1\u00a0M KOH solutions. The onset potential (E\nonset) and half-wave potential (E\n1/2) are used to characterize the ORR catalytic activity, defined as the potentials at 5% and 50% of the diffusion-limited current density, respectively. The RDE polarization curves of the AuxPt/MWNTs catalysts, MWNTs and commercial Pt/C can be found in Fig. 5\na and Fig.S4 and Fig.S5. Compare to MWNTs, the diffusion-limited current density of AuxPt/MWNTs increase significantly. It is found that with Au/Pt ratio changes from 4.55 to 1.68 and 0.67, the diffusion-limited current density increases, it then decays when the Au/Pt ratio further decrease to 0.25, thus the Au1.68Pt/MWNTs and Au0.67Pt/MWNTs exhibits the largest diffusion-limited current density. The loading mass of Pt for all AuxPt/MWNTs is less than 10\u00a0wt%, but the diffusion-limited current density is larger than that of 10\u00a0wt% Pt/C.The mechanism of ORR process can be studied by using the Koutecky-Levich (K-L) plots (Fig. 5b and Fig.S4 and S5), with indication of the relationship between the inverse square root of the rotating rate (\u03c9\n-1/2) and the reciprocal of current density (J\n\u22121), and the following equations are used to calculated the overall electron transfer number (n):\n\n(1)\n\n\n\n\nJ\n\n\n-\n1\n\n\n=\n\nJ\n\nk\n\n\n-\n1\n\n\n+\n\nJ\n\nL\n\n\n-\n1\n\n\n=\n\nJ\n\nk\n\n\n-\n1\n\n\n+\n\n\n(\nB\n\n\n\u03c9\n\n\n1\n/\n2\n\n\n)\n\n\n-\n1\n\n\n\n\n\n\n\n\n(2)\n\n\nB\n=\n0.2\nn\nF\n\nC\n\nO\n2\n\n\n\n\n(\n\nD\n\nO\n2\n\n\n)\n\n\n2\n/\n3\n\n\n\n\n\u03c5\n\n\n-\n1\n/\n6\n\n\n\n\n\n\nwhere J\nk is the kinetic current density, J\nL is the diffusion-limited current density, \u03c9 is the angular velocity (rpm), B\n-1 is the slope of K-L plot, F is the Faraday constant, C\nO2 (1.2\u00a0\u00d7\u00a010-6 mol cm\u22123) and D\nO2 (1.9\u00a0\u00d7\u00a010-6 cm s\u22121) are the bulk concentration and diffusion coefficient of dissolved oxygen, and \u03bd (0.01\u00a0cm2 s\u22121) is the viscosity coefficient. The n values are determined from the K-L plots as 3.9\u00a0~\u00a04.1 for the Au4.55Pt/MWNTs (Fig.S4b), 3.8\u00a0~\u00a04.0 for the Au1.68Pt/MWNTs(Fig. 5b), 3.9\u00a0~\u00a04.1 for the Au0.67Pt/MWNTs (Fig.S4d), 3.8\u00a0~\u00a04.1 for the Au0.25Pt/MWNTs (Fig.S4f), 3.6\u00a0~\u00a03.9 for the 10\u00a0wt% Pt/C (Fig.S5b) and 4.1\u00a0~\u00a04.3 for the 20\u00a0wt% Pt/C (Fig.S5d), demonstrating the four-electron pathway towards ORR in alkaline medium. However, the n value for the MWNTs is determined as 1.6\u00a0~\u00a01.9 (Fig.S4h), suggesting a two-electron ORR process in connection with the generation of H2O2. Hence, the MWNTs substrate can catalyze oxygen reduction in alkaline medium but show little effect on the ORR performance of the AuxPt/MWNTs catalysts.The RDE polarization curves at 1600\u00a0rpm of the AuxPt/MWNTs catalysts are further studied. As displayed in Fig. 5c, the E\nonset and E\n1/2 values are measured as 1.158\u00a0V and 0.890\u00a0V for the Au4.55Pt/MWNTs, 1.159\u00a0V and 0.892\u00a0V for the Au1.68Pt/MWNTs, 1.150\u00a0V and 0.894\u00a0V for the Au0.67Pt/MWNTs, 1.159\u00a0V and 0.895\u00a0V for Au0.25Pt/MWNTs, 1.151\u00a0V and 0.895\u00a0V for the 20\u00a0wt% Pt/C catalyst, respectively. Compared with the 20\u00a0wt% Pt/C, the Au0.67Pt/MWNTs and Au1.68Pt/MWNTs catalysts manifest comparable values of E\n1/2 and diffusion-limited current. Tafel plots converted from polarization curves shown in Fig. 5d are also used to analyze the ORR kinetics, where the slopes for the Au4.55Pt/MWNTs, Au1.68Pt/MWNTs, Au0.67Pt/MWNTs and Au0.25Pt/MWNTs catalysts are determined as 77\u00a0mV dec-1, 74\u00a0mV dec-1 , 72\u00a0mV dec-1 and 94\u00a0mV dec-1, the one for the 20\u00a0wt% Pt/C catalyst is 73\u00a0mV dec-1, demonstrating that the ORR kinetic of Au4.55Pt/MWNTs, Au1.68Pt/MWNTs, Au0.67Pt/MWNTs is similar to the one of 20\u00a0wt% Pt/C catalyst.Notably, AuxPt/MWNTs catalysts show significant advantages when compared with the Pt/C catalyst in view of specific activity and mass activity. The electrochemical active surface areas (ECSA) of Pt was measured according to a method reported by Shao-Horn and co-workers [33,34](See ref. Fig. S6), the resulting ECSA of Pt for Au4.55Pt/MWNTs, Au1.68Pt/MWNTs, Au0.67Pt/MWNTs, Au0.25Pt/MWNTs and 20\u00a0wt% Pt/C is 184.5\u00a0m2 g-1\nPt, 204.8\u00a0m2 g-1\nPt, 87.9\u00a0m2 g-1\nPt, 33.8\u00a0m2 g-1\nPt, 90.9\u00a0m2 g-1\nPt, and the values of specific activity based on Pt are determined as 0.16\u00a0mA\u00a0cm\u22122, 0.08\u00a0mA\u00a0cm\u22122, 0.174\u00a0mA\u00a0cm\u22122, 0.128\u00a0mA\u00a0cm\u22122, 0.054\u00a0mA\u00a0cm\u22122 at 0.9\u00a0V(Fig. 5e), respectively. The mass loading of Pt was measured using an ICP-MS, results in Table 1, show the Pt contents are 2.7\u00a0wt%, 5.3\u00a0wt%, 6.1\u00a0wt% and 7.6\u00a0wt% for Au4.55Pt/MWNTs, Au1.68Pt/MWNTs, Au0.67Pt/MWNTs, Au0.25Pt/MWNTs, respectively. The Pt mass activity values are 295\u00a0mA\u00a0mg\u22121, 164\u00a0mA\u00a0mg-1and 153\u00a0mA\u00a0mg\u22121 and 112\u00a0mA\u00a0mg\u22121 for Au4.55Pt/MWNTs, Au1.68Pt/MWNTs, Au0.67Pt/MWNTs and Au0.25Pt/MWNTs, which are generally two to six times higher than that of 20\u00a0wt% Pt/C (50\u00a0mA\u00a0mg\u22121) as displayed in Fig. 5f. the metal mass activity while both Au and Pt included are 55\u00a0mA\u00a0mg\u22121, 61\u00a0mA\u00a0mg\u22121, 91\u00a0mA\u00a0mg\u22121, and 89\u00a0mA\u00a0mg\u22121 for Au4.55Pt/MWNTs, Au1.68Pt/MWNTs, Au0.67Pt/MWNTs and Au0.25Pt/MWNTs, respectively, higher than the value for the 20\u00a0wt% Pt/C catalyst (50\u00a0mA\u00a0mg\u22121), indicating that the ORR activity increased by decorating Pt with Au cluster. possibly because the interaction between Pt and Au.RRDE measurement was employed to further investigate the oxygen reduction mechanism of the AuxPt/MWNTs catalysts in 0.1\u00a0M KOH. O2 is reduced on the glassy carbon disk electrode, and the ORR intermediate H2O2 is oxidized on the Pt ring electrode. The following equations are used to calculated the overall electron transfer number (n) and the corresponding H2O2:\n\n(3)\n\n\nn\n=\n\n\n4\n\u00d7\n\nI\nd\n\n\n\n\nI\nd\n\n+\n\nI\nr\n\n/\nN\n\n\n\n\n\n\n\n\n(4)\n\n\n\nH\n2\n\n\nO\n2\n\n%\n=\n200\n\u00d7\n\n\n\nI\nr\n\n/\nN\n\n\n\nI\nd\n\n+\n\nI\nr\n\n/\nN\n\n\n%\n\n\n\n\nwhere I\nd represents the disk current (A), I\nr is the ring current (A), N (collection efficiency) is taken as 44% according to our previous literature [13,30]. Fig. 6\na, 6c and 6e illustrate the disk currents of the AuxPt/MWNTs catalysts increase with the potential scan of disk electrode in the range from 1.2\u00a0V to 0.2\u00a0V, but the ring current approaches to zero for the AuxPt/MWNTs catalysts. Fig. 6b, 6d and 6f further illustrate that the H2O2 yields are close to zero and the total electron transfer numbers are determined as about 4 for all AuxPt/MWNTs catalysts, similar as 20wt.%Pt/C (Fig. S8), which again identifies the little contribution of the MWNTs substrate to the catalytic performance of the AuxPt/MWNTs catalysts.Apart from the ORR performance, the durability and methanol tolerance are also measured, which are carried out by the current versus time (i-t) chonoamperometry. In the durability tests, the potential was fixed at half-wave potential making the oxygen reduction to take place on the catalysts continuously, and the current was recorded. As shown in Fig. 7\na, the reaction currents for all catalysts decrease at the initial stage and then reached plateau. After 30000\u00a0s, about 84.6%, 87.5% and 87.8% of initial reaction current is observed for Au4.55Pt/MWNTs, Au0.67Pt/MWNTs and 20\u00a0wt% Pt/C, respectively. In contrast, Au1.68Pt/MWNTs exhibits higher stability with 91.6% of current retention, suggesting the superiority in practical application.In order to investigate the methanol tolerance of AuxPt/MWNTs catalysts, the I-t curves at 0.85\u00a0V vs RHE in O2-saturated 0.1\u00a0M KOH solution with the addition of 3\u00a0M methanol were recorded. As shown in Fig. 7b, apparent current decay is seen for the 20\u00a0wt% Pt/C catalyst soon after a current fluctuation in response to the addition of methanol, leaving only 44% of the initial value at 1400\u00a0s. In contrast, the current retention values are about 91%, 88% and 80% for the Au4.55Pt/MWNTs, Au1.68Pt/MWNTs and Au0.67Pt/MWNTs catalysts. It is noteworthy that the superior methanol tolerance of AuxPt/MWNTs catalysts to the Pt/C catalyst associates with the incorporation of Au NPs, in other words, the Au NPs serve to protect the Pt NPs from poisoning to some extent via particular interaction. The superior methanol tolerance of AuxPt/MWNTs catalyst also endow the application in direct methanol fuel cell.The AuxPt/MWNTs catalysts are then investigated as cathode in a home-made cell Al-air cell illustrated in Fig. 8\na. The cell consists an Al anode, air cathode and 4\u00a0M KOH electrolyte. For comparison propose, the battery performance of 20\u00a0wt% Pt/C was also tested. As showed in Fig. 8b, the cell with Au4.55Pt/MWNTs presents the lowest open circuit potential (OCP), which starts at 1.69\u00a0V but decays quickly to 1.43\u00a0V in half an hour followed by further gradual decrease to 1.36\u00a0V during 5\u00a0h. The OCP of the cell with Au1.68Pt/MWNTs starts at 1.78\u00a0V, then declines slightly to 1.67\u00a0V and maintained stable till the end of the testing. The cell with Au0.67Pt/MWNTs also has a starting OCP of 1.78\u00a0V, it declines to 1.55\u00a0V slowly during the 5\u00a0h testing. Although the OCP of the battery with 20\u00a0wt% Pt/C starts at 1.88\u00a0V, it decreases to below 1.63\u00a0V after 5\u00a0h. Fig. 8c exhibits the discharge behavior of Al-air batteries at the current density of 100\u00a0mA\u00a0cm\u22122. The battery with Au4.55Pt/MWNTs has a discharge capacity as large as 939 mAh g\u22121, however, exhibits the discharge potential lower than 0.8\u00a0V. The other three batteries present the discharge potential above 0.9\u00a0V, the discharge capacity is 921, 898 and 886 mAh g\u22121 for Au1.68Pt/MWNTs, Au0.67Pt/MWNTs and 20\u00a0wt% Pt/C, respectively. In order to further investigate the performance of hybrid catalysts, the discharge polarization curves and the corresponding powder density curves are recorded as shown in Fig. 8d. The potential decrease sharply for the battery with Au4.55Pt/MWNTs, resulting in a maximum power density (Pmax\n) of 72.7 mW cm\u22122. In contrast, the potential of the batteries with Au1.68Pt/MWNTs, Au0.67Pt/MWNTs and 20\u00a0wt% Pt/C cathodes decrease much slower, with the corresponding Pmax\n of 146.8 mW cm\u22122, 143.1 mW cm\u22122 and 144.2 mW cm\u22122, respectively. The assembled Al-air battery with as Au1.68Pt/MWNTs cathode can drive a fun working for at least 6\u00a0h, (Fig. S9a), and also can drive fan and hygrometer in series running for at least 3\u00a0h due to the high powder density (Fig. S9b).Apart from the above performance, the durability of catalysts is another critical factor to determine the service life of Al-air batteries. Fig. 8e displays the dynamic galvanostatic measurements of the Al-air batteries with Au1.68Pt/MWNTs, Au0.67Pt/MWNTs and 20\u00a0wt% Pt/C cathode, which are tested at the constant current density between 1 and 200\u00a0mA\u00a0cm\u22122(60\u00a0min for each discharge plateau), accordingly, the discharge potential plateau decreases with increasing current density. There is no potential drops observed at each potential plateau with Au1.68Pt/MWNTs as cathode. In contrast, with Au0.67Pt/MWNTs and 20\u00a0wt% Pt/C cathode have potential drop happened often especially at higher current densities. To investigate the long-term stability of the catalysts, potential variations of Al-air batteries are recorded for five cycles which is operated by replacing the Al foil and electrolyte at the end of each cycle, the cathode is reused during these cycles. The potential of all cathodes is dropping at the initial stage and then the discharge plateau occurs. It is obvious that the discharge potential with Au1.68Pt/MWNTs cathode is stable for the five cycles, but it drops at the fourth cycle for the ones with Au0.67Pt/MWNTs and 20\u00a0wt% Pt/C cathode. In order to study the stability of Au1.68Pt/MWNTs after long-term operation, the morphology was characterized using TEM and HAADF as showed in Fig.S10. TEM image shows that the nanoparticles aggregated slightly, the HAADF and elemental mapping reveal that Au and Pt nanoparticles are still existed in the formation of bimetal particles, which endows the high stability of Au1.68Pt/MWNTs. These above results demonstrate that compared with 20 wt.%Pt/C, the AuxPt/MWNTs catalysts combines the advantages of high catalytic activity, superior durability, and low cost.AuxPt/MWNTs catalysts were synthesized by a facile one-pot method, where the ultrafine AuxPt NPs capped with THPO were uniformly deposited on the MWNTs substrate in an average size of\u00a0~\u00a03.0\u00a0nm. The AuxPt/MWNTs catalysts perform four-electron pathway towards ORR, and exhibit superior catalytic activity in terms of specific activity and mass activity. Amount which, the Au1.68Pt/MWNTs catalyst exhibits higher powder density, higher specific capacity and better durability than 20\u00a0wt% Pt/C when used as Al-air cathode. The above results demonstrate the incorporation of Pt and Au NPs enhanced the catalytic performance towards ORR. The excellent catalytic performance and stability of the bimetallic AuxPt/MWNTs catalysts allow prospective applications as efficient and stable catalysts on Al-air battery and fuel cells at lower Pt usage.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 Natural Science Foundation of China (No. 51874051), Guangxi Natural Science Foundation (No. 2018GXNSFAA281184, 2019GXNSFAA245046), Guangxi Key Laboratory of Optical and Electronic Materials and Devices (No. 20KF-4, 20AA-18) and Bagui Scholar Program of Guangxi Province. The authors are also grateful for the assistance from Mr. Xiaobin Zhou from Shiyanjia Lab (www.shiyanjia.com) on materials characterizations.Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2021.150474.The following are the Supplementary data to this article:\n\nSupplementary material 1\n\n\n\n", "descript": "\n                  A series of AuPt nanoparticles supported on multi-walled carbon nanotubes (AuxPt/MWNTs) catalysts with ultrafine distribution (d \u2248 3.0\u00a0nm) were synthesized for Al-air battery cathode to enhance the oxygen reduction reaction. Among them, Au0.67Pt/MWNTs catalyst with metal loading of 10.2\u00a0wt% (Au:4.1\u00a0wt%, Pt:6.1\u00a0wt%) exhibited a superior ORR catalytic activity and competitive durability to 20\u00a0wt% Pt/C catalyst. When applied as Al-air battery, appropriate increasing Au loading encourage better battery performance. Au1.68Pt/MWNTs with 8.95\u00a0wt% of Au and as little as 5.3\u00a0wt% Pt content exhibit larger specific capacity (921 mAh g\u22121) and power density (146.8 mW cm\u22122) as well as better durability than 20\u00a0wt% Pt/C catalyst when it is assembled as cathode in Al-air battery.\n               "}
{"full_text": "", "descript": "\n                  The Ni\u201325%X (X=Fe, Co, Cu, molar fraction) solid solutions were prepared and then doped into MgH2 through high-energy ball milling. The initial dehydrogenation temperatures of MgH2/Ni\u201325%X composites are all decreased by about 90 \u00b0C relative to the as-milled pristine MgH2. The Ni\u201325%Co solid solution exhibits the most excellent catalytic effect, and the milled MgH2/Ni\u201325%Co composite can release 5.19 wt.% hydrogen within 10 min at 300 \u00b0C, while the as-milled pristine MgH2 can only release 1.78 wt.% hydrogen. More importantly, the dehydrogenated MgH2/Ni\u201325%Co composite can absorb 5.39 wt.% hydrogen at 275 \u00b0C within 3 min. The superior hydrogen sorption kinetics of MgH2/Ni\u201325%Co can be ascribed to the actual catalytic effect of in-situ formed Mg2Ni(Co) compounds. First-principles calculations show that the hydrogen absorption/desorption energy barriers of Mg/MgH2 systems decrease significantly after doping with transition metal atoms, which interprets well the improved hydrogen sorption properties of MgH2 catalyzed by Ni-based solid solutions.\n               "}
{"full_text": "Oxidation of alcohols to their corresponding aldehydes or acids is a fundamentally important reaction both academically and industrially [1]. Although the stoichiometric oxidant, such as KMnO4, Cr-based reagents, iodine compounds [2], are efficient and versatile in catalytic oxidation of alcohols, they are neither environmentally friendly nor economically scalable. The oxidation of alcohols over heterogeneous catalysts by using O2 as an oxidant offers a safer, greener, and cheaper alternative to the stoichiometric oxidizing [1,3]. However, a base additive is required to eliminate \u03b2-H in most cases of heterogeneous catalysis, which is widely regarded as costly and non-environmentally friendly, also complicating product purification [1,3-5]. Therefore, from the standpoint of green and sustainable chemistry, solid base supports, which could facilitate alkali-free alcohols oxidation, are very attractive.Among various solid base supports, hydrotalcite (HTC), also known as layer double hydroxide, has been considered as an excellent catalyst support due to its tunable basicity /acidity properties [5\u20139]. By taking advantage of Br\u00f8nsted-base sites arising from HTC support, several groups have synthesized HTC-supported metal nanoparticles/nanoclusters catalysts with good catalytic activity in alkali-free alcohol oxidation [5\u201314]. Mitsudome et\u00a0al. [6] synthesized Au/Mg-Al HTC, which could catalyze the oxidation of various alcohols like benzylic alcohols and cyclohexanols without extra additives. Wang et\u00a0al. [10] prepared Au/Mg-Al HTC with smaller particle size (1\u20135\u00a0nm) of Au nanoparticles, which showed superior catalytic properties in aerobic alcohol oxidation. Li et\u00a0al. [11] synthesized Au nanoclusters (1.5\u00a0nm) deposited on Mg-Al HTC (Au NCs/Mg-Al HTC), and its catalytic performance was higher than previously reported Au/Mg-Al HTC catalysts. HT-supported Pd nanoparticles catalysts also exhibited excellent catalytic activity in oxidation of alcohols [8,9,14]. The TOF for benzyl alcohol oxidation of Pd/Mg-Al HT [9] were higher than most of Au NPs/HT or Au NCs/HT catalysts [6,10,11], which could achieve up to 11,590\u00a0h\u00a0\u2212\u00a01 under higher reaction temperature (140 \u00baC). In our previous study [14], we observed that the flower-like structure of Ni-Al HTC increased the catalytic activity of Pd/HTC catalyst under base-free condition, presumably due to improved support basicity and synergetic interactions between Ni and Pd atoms in individual nanoparticles.Numerous works have demonstrated that the Pd precursors could affect the catalytic performance of supported Pd catalysts [15\u201318]. For example, Ali et\u00a0al. [18] investigated the influence of various supports and Pd precursors on their activity in the CO hydrogenation, and showed that the catalyst prepared by PdCl2 presented higher activity than Pd(NO3)2 as precursor due to the Cl\u2212from the PdCl2 increasing the number of intermediates/sites of CO hydrogenation. Zhang et\u00a0al. [17] found that the activity of Pd/C catalysts in phenol hydrogenation was strongly affected by Pd precursors. In addition, the reduction reagents type and the presence of another transition metal also affect activity of supported Pd catalysts [15].Thus, this work is built on the earlier observations that as-synthesized flower-like Pd/HTC (10\u00a0wt.% Pd loading) catalyst could efficiently catalyze alkali-free oxidation of benzyl alcohols [5,14], and we wished to dig further into whether control synthesis of the flower-like Pd/HTC catalysts from various Pd precursors, reduction reagents, and Pd loading amount would result in further enhanced activities and product selectivity for alcohol oxidation. Herein, a series of flower-like Pd/HTC catalysts with three of palladium precursors (Na2PdCl4, K2PdCl4, PdCl2), two of reducing reagents (NaBH4, N2H4), and different Pd loading dose (1, 2, 3, 5, 7, 10\u00a0wt.%) were synthesized and characterized to determine possible effects of above parameters on structural properties and catalytic performance. Benzyl alcohol was chosen as a typical substrate in the reaction to investigate the structure-performance relationship. Moreover, the XPS and FTIR studies were used to unveil the possible mechanism reasons for the different catalytic behaviors over synthesized Pd/HTC catalysts.Potassium tetrachloropalladate (II) (K2PdCl4), sodium tetrachloropalladate (II) (Na2PdCl4), palladium (II) chloride (PdCl2), hydrazine hydrate (N2H4) and sodium borohydride (NaBH4), acetonitrile-d3 (CD3CN) were purchased from Sigma-Aldrich Denmark A/S (S\u00f8borg, Denmark). All chemicals were of analytical grade and used without further purification.Ni-Al hydrotalcite (HTC) was synthesized by urea decomposition method with the assistance of F\n+ to control the morphology [14]. In a typical procedure, 0.24\u00a0mmol of Ni(NO3)2\u20226H2O, 0.08\u00a0mmol of Al(NO3)3\u20229H2O, 1\u00a0mmol of urea and 0.64\u00a0mmol of NH4F were dispersed in 200\u00a0mL of deionized water, and homogeneously mixed. The resulting solution was then transferred into a sealed Teflon autoclave at 130 \u00b0C for 24\u00a0h, followed by centrifugation and washing for several times with distilled water. The green powder of Ni-Al HTC was obtained by drying in the oven at 70 \u00b0C for overnight.0.2\u00a0g of Ni-Al HTC was dispersed in 50\u00a0mL of distilled water (solution A), at the same time, 0.031\u00a0g of K2PdCl4 (0.094\u00a0mmol) was dissolved in 5\u00a0ml of aqueous solution (solution B). And then two solutions were mixed together, followed by stirring at room temperature under a nitrogen atmosphere for 3\u00a0h. 0.056\u00a0g of hydrazine hydrate (1.76\u00a0mmol) was introduced to the solution for reduction of Pd2+. The mixtures were continuously agitated for another 2\u00a0h under a nitrogen atmosphere. The black powders were obtained by centrifugation, and then subjected to washing with distilled water for several times. The as-obtained powder sample was dried at 70\u00a0\u00b0C overnight and denoted as K-H. The samples of Na-H and Pd-H were obtained by following a similar process except for replacing K2PdCl4 with 0.028\u00a0g of Na2PdCl4 (0.094\u00a0mmol), and 0.017\u00a0g of PdCl2 (0.094\u00a0mmol), respectively. When PdCl2 was used as Pd precursor, the aqueous solution was mildly acidified with diluted HCl to pH 3.To examine the effect of thermal treatment, the synthesized Pd/HTC catalysts were pretreated at 200\u00a0\u00b0C for 1\u00a0h in air and the samples were named as K-H-200, Na-H-200 and Pd-H-200, respectively.In a typical reaction, 0.028\u00a0g of Na2PdCl4 (0.094\u00a0mmol) was stirred with PVA solution (1 wt%, PVA/Pd\u00a0=\u00a02\u00a0wt./1\u00a0wt.), and the solution was denoted as solution C. The solutions A and C were mixing together under N2 atmosphere at room temperature for 3\u00a0h, and then a freshly prepared NaBH4 solution (0.1\u00a0M, NaBH4/Pd\u00a0=\u00a05\u00a0mol/mol) was introduced into the mixtures to allow reaction proceed for another 2\u00a0h. The resulting sample was obtained by centrifugation, washing and drying in the oven at 70\u00a0\u00b0C overnight and named as Na-Na. The K-Na and Pd-Na were synthesized using a similar procedure except for changing the Pd precursors from Na2PdCl4 to K2PdCl4 and PdCl2, respectively.The morphologies and chemical elements were characterized by Hitachi TM3030 SEM (Krefeld, Germany) equipped with energy-dispersive spectrometer (EDS) of Quantax 70 system (Bruker, Berlin, Germany) and FEI Tecnai F20TEM (Hillsboro, OR, USA). Powder X-ray diffraction (XRD) patterns were collected in standard programs of 20\u00a0min on an Aeris Panalytical XRD with a Cu K\u03b11 source over a range from 5 to 90\u00ba, operating at 40\u00a0kV and 15\u00a0mA, at room temperature. IR spectra were recorded on a FTIR spectrometer (PIKE, Madison, WI; Bruker, Ettlingen, Germany) with a resolution of 4\u00a0cm \u22121. The powder samples were heated in an oven at 80 \u00baC for 4\u00a0h before the IR investigations, and 2\u00a0mg of powder was used for IR measurement. Acetonitrile-d3 was adsorbed on the samples at room temperature and pressure. The difference spectra in all figures were obtained by subtracting the spectra of the samples before the admission of the adsorbate. The chemical states of metal in samples were characterized by X-ray photoelectron spectroscopy (Specs XR 50 X-ray source +PHOIBOS 100 Analyzer) with an Al K\u03b11 source. The use of 1H and 13C NMR spectroscopy on a Bruker Avance III spectrometer at 400\u00a0MHz identified the structures of products from alcohols oxidation reaction.All oxidation reactions of alcohols were carried out in 15\u00a0ml glass tubes placed in a shaking incubator (IKA Incubator shaker KS 4000 i control) with a temperature control at 70\u00a0\u00b0C and a rotation speed of 250\u00a0rpm. Typically, 1\u00a0mmol of benzyl alcohol (BA) and 20\u00a0mg of as-synthesized Pd/HTC catalysts were added in 5\u00a0ml of xylene, which is acting as the reaction medium. All experiments were conducted at least in triplicates and the results are reported as means\n\u00b1\nstandard deviations. The aliquots from the reaction mixture were analyzed by using a gas chromatography (Bruker, Billerica, MA, USA) system equipped with a flame ionization detector (FID) and a ZB-FFAP column (30\u00a0m length, 0.25\u00a0mm I.D., 0.25\u00a0\u00b5m film thickness; Phenomenex, V\u00e6rl\u00f8se, Denmark). The temperature of both injector and detector of GC-FID were set at 250 \u00baC. The temperature program for oven was as follows: holding at initial temperature of 90 \u00baC for 1\u00a0min, followed by increasing up to 170 \u00baC at the rate of 20 \u00baC /min, and then increasing further to 220 \u00baC at the rate of 10 \u00baC /min, and finally keeping at 220 \u00baC for 10\u00a0min. The column flow was maintained at a constant pressure of 22 Psi throughout the analysis with helium as the carrier gas. The conversion and yield were quantified using an area normalization method. The standard reference compounds were used to identify product and side product based their respective retention time. Based on GC analysis, conversion of BA, and yield of benzaldehyde (BD), and selectivity of BD were estimated as following equations:\n\n\n\nT\nh\ne\n\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n=\n\n(\n\n1\n\u2212\n\n\n\nT\nh\ne\n\na\nr\ne\na\n\no\nf\n\ns\nu\nb\ns\nt\nr\na\nt\ne\n\np\ne\na\nk\n\n\nT\nh\ne\n\nt\no\nt\na\nl\n\np\ne\na\nk\n\na\nr\ne\na\ns\n\no\nf\n\nb\no\nt\nh\n\ns\nu\nb\ns\nt\nr\na\nt\ne\n\na\nn\nd\n\np\nr\no\nd\nu\nc\nt\ns\n\n\n\n)\n\n*\n100\n%\n\n\n\n\n\n\n\n\nT\nh\ne\n\ny\ni\ne\nl\nd\n\no\nf\n\nB\nD\n=\n\n(\n\n\n\nT\nh\ne\n\na\nr\ne\na\n\no\nf\n\nB\nD\n\np\ne\na\nk\n\n\nT\nh\ne\n\nt\no\nt\na\nl\n\np\ne\na\nk\n\na\nr\ne\na\ns\n\no\nf\n\nb\no\nt\nh\n\ns\nu\nb\ns\nt\nr\na\nt\ne\n\na\nn\nd\n\np\nr\no\nd\nu\nc\nt\ns\n\n\n)\n\n*\n100\n%\n\n\n\n\n\n\n\n\nT\nh\ne\n\nS\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n\no\nf\n\nB\nD\n=\n\n(\n\n\n\nT\nh\ne\n\na\nr\ne\na\n\no\nf\n\nB\nD\n\np\ne\na\nk\n\n\nT\nh\ne\n\nt\no\nt\na\nl\n\np\ne\na\nk\n\na\nr\ne\na\ns\n\no\nf\n\na\nl\nl\n\np\nr\no\nd\nu\nc\nt\ns\n\n\n)\n\n*\n100\n%\n\n\n\n\nAfter the reaction was completed, the solid catalyst was separated through precipitation by centrifugation. Then, the liquid reaction mixture was dried over anhydrous Na2SO4, filtered, and evaporated by a rotary evaporator to 0.5\u00a0mL for purification of aldehyde products on silica gel by preparative TLC plates (L\u00a0\u00d7\u00a0W, 20\u2005cm\u00d720\u2005cm, Merck). The solvent path was 19\u2005cm and the development system was chloroform /methanol (3\u2009:\u20091, v/v). A UV light (265\u00a0nm) was used to identify the products on TLC plates. The bands containing aldehyde product, were scraped and extracted with ethyl acetate and evaporated by the rotary evaporator. The weight of dried products was measured for calculation of isolated yield. The quantification of the isolated yield was based on the ratio of the purified product to the theoretical yield of expected product based on the conversion of substrate. Chemical structure identification of aldehyde products by NMR was recorded on a Bruker Avance III spectrometer (400\u2005MHz).Data processing was applied in IBM SPSS statistics 21. One-way analysis of variance (one-way ANOVA) and independent-Samples T Test were performed to identify significant differences between groups (P<0.05).The formation of Pd nanoparticles requires the ligand dissociation of Pd salts followed by reduction of Pd 2+ cation. Hydrazine hydrate and sodium borohydride are commonly used as reducing reagents; and their reduction mechanisms are suggested as following equations [19,20]:\n\n(1)\n\n\n\nHydrazine\nHydrate\n:\n\n\n\n[\n\nPdC\n\nl\n4\n\n\n]\n\n\n2\n\u2212\n\n\n+\n\nN\n2\n\n\nH\n4\n\n\n+\n2\nO\n\n\n\nH\n\n\u2212\n\n\u2192\n\nPd\n+\n\n\nN\n2\n\n\n+\n4\nC\n\n\n\nl\n\n\u2212\n\n\n+\n2\n\n\nH\n2\n\nO\n\n\n\n\n\n\n(2)\n\n\n\nSodium\nborohydride\n:\n\n\n\n\n\n[\n\nPdC\n\nl\n4\n\n\n]\n\n2\n\n\n\u2212\n\n\n+\nNaB\n\n\nH\n4\n\n\u2192\n\nPd\n+\nB\n\n\nH\n3\n\n\n+\n4\nC\n\n\n\nl\n\n\u2212\n\n\n+\nN\n\n\n\na\n\n+\n\n+\n\n\nH\n\n+\n\n\n\n\nIt has been proved that preparation and pretreatment of catalysts affect their catalytic behavior [5,15-17]. To investigate the effect of precursor and reducing reagents on structure-activity relationship of Pd/HTC catalysts, three kinds of Pd salts (namely Na2PdCl4, K2PdCl4, and PdCl2) and two types of reduction reagents (NaBH4 and N2H4), were used to for the syntheses of Pd/HTC catalysts.The crystal structures of as-synthesized Pd/HTC catalysts were characterized by XRD (Fig.\u00a01\n). It could be seen that the diffraction peaks of all samples exhibited the pure hydrotalcites-like phase without other detectable impurities [5]. No additional peaks assignable to any Pd phase could be observed in the Fig.\u00a01 due to the detection limit of the instrument. A closer inspection and comparison of reflections corresponding to d\n003 (the inset figure in Fig.\u00a01) showed a reduced peak intensity in order of Na-Na < K-Na < Na-H < K-H. The real reason for this remains unknown. However, Han et\u00a0al. [21] and Hao et\u00a0al. [22] suggested that the introduction of metal species on the surface of support materials resulted in the decrease of XRD peak intensity of support materials. In our case, the amount of Pd loading is same for all prepared Pd/hydrotalcite samples, the XRD peak intensity of hydrotalcite can reflect the dispersion of metal particles to some extent. The lower XRD intensity of HTC support means that the more even dispersion of Pd nanoparticles on the surface of supports.To observe the dispersion of Pd nanoparticles more intuitively, typical TEM micrographs for as-obtained samples by using three kinds of Pd salts as precursors and reduced with NaBH4/N2H4 are exhibited in Fig.\u00a02\n, and their particle size distributions derived from the TEM images by counting at least 100 particles in each case are also presented. The representative high-resolution transmission electron microscopy (HR-TEM) image of as-obtained samples in Figure S1 showed that the d-spacing of d111\u00a0=\u00a00.23\u00a0nm corresponded well with that of the (111) plane of Pd [23], which confirmed successful formation of Pd nanoparticles on the surface of Ni-Al HTC.As shown in Fig.\u00a02, using N2H4 as reducing reagent (Eq.\u00a0(1)), the average size of resulting Pd nanoparticles in the samples (Pd-H, Na-H, K-H) were generally bigger than that in the samples (Pd-Na, Na-Na, K-Na) using NaBH4 as reducing reagent (Eq.\u00a0(2)]. The shape and distribution of Pd nanoparticles are more regular and uniform in the Na-Na, and K-Na than those in the corresponding Na-H, and K-H, which is consistent with XRD analysis results. The phenomenon is in line with expected results because in the process of NaBH4 reduction, PVA worked as stabilizers to protect the Pd nanoparticles from agglomeration and crystal growth, while the lacking of stabilizers in the N2H4 reduction leads to a disorder gathering of Pd nanoparticles.The distribution of Pd nanoparticles in the samples of Pd-Na, K-Na and Na-Na are carefully inspected and compared with NaBH4 as reducing reagent, as shown in Fig.\u00a03\n (a, c, e). It could be seen that the distribution of Pd nanoparticles is more even in K-Na and Na-Na than that in Pd-Na. The average size of Pd nanoparticles obeyed the order that Pd-Na (3.86\u00a0nm) > Na-Na (3.14\u00a0nm) > K-Na (2.76\u00a0nm). It seems that the cations Na+ or K\n+ in precursors affected the Pd nanoparticle size and distribution. The possible explanation could be that the alkali metal ions of Na+ and K\n+ in the sol-gel precursor enforced the interaction between Pd species and Ni-Al HTC support, which decreased the mobility of Pd species, therefore the rate of nuclei growth would be slower, resulting in the formation of smaller Pd nanoparticles. The K\n+ has larger ionic radii that engenders much stronger interaction than Na+, thus induces stronger transport inhibition of Pd species on the surface of Ni-Al HTC, leading to smaller Pd nanoparticles. This is an interesting phenomenon which is similar to the Hofmeister effect [24\u201326].When N2H4 is used as a reduction reagent, there is no significant difference in Pd nanoparticles size and distribution for K-H (3.92\u00a0nm) and Na-H (3.82\u00a0nm), which were produced by K2PdCl4 and Na2PdCl4 as precursors, respectively. This is because by using N2H4 as a reducing reagent, the precursor is not colloid, so the Hofmeister effect is not existing. However, by using the PdCl2 as Pd source in precursor still cause bigger Pd nanoparticles (Pd-H, 4.66\u00a0nm), indicating that the presence of alkali metal ions in precursors can help to reduce the aggregate of Pd nanoparticles when N2H4 is used as reducing reagent.To further investigate the structure-activity relationship resulted from different Pd precursors and reducing reagents, benzyl alcohol oxidation was used as a typical reaction to examine the catalytic activity of the as-synthesized catalysts, and the results are presented in Fig.\u00a03 (a-c). In Fig.\u00a03c, ANOVA analysis showed that there has no significant difference in BA conversion among those Pd/HTC catalysts that synthesized from different precursors and reducing reagents. However, the product selectivity was affected by the reduction reagents (Fig.\u00a03b).For a better comparison, the conversion of BA and the selectivity of BD against the mean size of Pd nanoparticles are plotted in Fig.\u00a03a and 3b, respectively. As shown in Fig.\u00a03a, the average BA conversion displayed an increasing mode for NaBH4-reduced Pd nanoparticles but a declining mode for N2H4 as reduced catalyst against the increasing mean size of resulting Pd nanoparticles. However, the ANOVA analysis showed that the difference of BA conversion for each sample is not significant. As depicted in Fig.\u00a03b, the selectivity of BD for those samples which were obtained from by using N2H4 as reduction reagent (94\u201396%) is obviously higher than corresponding catalysts produced from by using NaBH4 as a reduction reagent (88\u201393%). It suggested that N2H4 reduced Pd nanoparticles might have a favorable catalytic structure and property towards selective oxidation of benzyl alcohol into benzaldehyde over NaBH4 reduced product. The reasons accounting for that will be exploited in the following section.Our previous work has disclosed the possible catalytic mechanism of Pd/HTC catalysts [14]. In this work, we also attempted to reveal the mechanism reasons accounting for sorts of similarity (e.g. in activity) or difference (e.g. in selectivity) in catalytic behaviors of Pd/HTC catalysts, which were obtained from different Pd salts and reduction reagents. Chemical changes of the surface caused by different Pd precursors and reduction agents was further studied by the XPS technique. The XPS spectra of synthesized samples were recorded (Fig.\u00a04\n), and the relative fractions of species in Ni 2p3/2 and Pd 3d5/2 are listed in Table S1. As shown in Fig.\u00a04, the peaks of Pd 0, Pd 2+ in Pd 3d5/2 are distinguished among samples of Na-H, Na-Na, K-H and K-Na. The deconvolution of the Pd 3d spectra for all samples showed two components with binding energies (BE) of the Pd 3d5/2 electrons at about 335.3 and 336.7\u00a0eV, which can be assigned to Pd 0 and Pd 2+ species, respectively (Fig.\u00a04) [27]. From the Table S1, the deconvolution results showed that the oxidized state of Pd species in those samples is not significantly different, reflected in catalytic activity, the BA conversions over those Pd/HTC catalysts are similar (Fig.\u00a03a). Furthermore, the binding energy (BE) value of the Pd 3d5/2 electron in Na-Na and K-Na are slightly higher than that in the Na-H and K-H, indicating that there is a slight electron transfer from Pd NPs to surrounding species when using NaBH4 as reduction reagents.The acid properties of the catalysts can affect their catalytic behavior, especially for product selectivity [28]. The comparison of the FTIR spectra of acetonitrile-d3 adsorbed at room temperature on the catalysts was employed for the determination of the acidity of the samples [29,30]. Kotrla et\u00a0al. [29] reported that if CN in CD3CN is combined with Br\u00f8nsted-acid (B-acid) sites in solid catalysts, the stretching vibration of the CN bond will shift to the lower wavenumber direction, furthermore, the stronger the acidity gives a greater displacement of the vibration of CN bond. Therefore, we examined the acidities of some typical samples by FTIR with alkaline probe molecule (CD3CN). Al2O3 is known as a solid acid, whereas MgO is a typical basic oxide. We choose those three chemicals as the references to comparison the shift of CN bond vibration when acetonitrile-d3 adsorbed at room temperature on the catalysts (Fig.\u00a05\n).As shown in Fig.\u00a05, all as-synthesized Pd/HTC catalysts show similar FTIR spectra. A broad band that centered at around 3450 cm\u22121 is attributed to the stretching mode of OH\u2212 group from the brucite-like layers, and the strong adsorption at around 1357 cm\u22121 is associated with the carbonate anions in the interlayers. In addition, an intense peak at around 2262 cm\u22121 was displayed in all FTIR spectra, which is assigned to the stretching vibration of the CN bond for acetonitrile-d3. The red dotted line in Fig.\u00a05b represents the position of the CN bond vibration when acetonitrile-d3 adsorbed on SiO2 and MgO (the wavenumber at 2263 cm\u22121), while the black dotted line denotes the position for CN bond combined with the B-acid sites in Al2O3 (the wavenumber at 2261 cm\u22121). As shown in Fig.\u00a05b, the blue shift of the CN bond for acetonitrile-d3 was observed from all Pd/HTC catalysts, which indicates that acidic sites exist on the surface of the as-obtained samples. From the shifting distance of the CN bond vibration peak (Fig.\u00a05c), it can be seen that Pd-H possesses relatively strong acidity (the wavenumber at 2260 cm\u22121). Indeed, the Pd-H exhibited the highest BD selectivity among those as-synthesized Pd/HTC catalysts (Fig.\u00a03b). This is similar to a previous observation, Fang et\u00a0al. [28] proved that the acidity of support affected the product selectivity in alcohol dehydrogenation, the stronger acidity of support gave higher product selectivity.From the XPS and FTIR studies, we plausibly postulate that the use the NaBH4 as reduction reagent, the stabilizer protected Pd nanoparticles from aggregate, so the Pd nanoparticles evenly distributed, however, the stabilizer also takes up some B-acid site, leading to a lower product selectivity.The XRD patterns of Pd/HTC catalysts with varied Pd loading amounts were displayed in Figure S5. All diffraction peaks of the XRD patterns (Figure S5) could be assigned to characteristic peaks of hydrotalcite without any impurities, and the typical peaks of Pd were not observed in all cases, likely due to the detection limit of the instrument. The Pd nanoparticles distribution of those Pd/HTC catalysts were investigated by TEM images (Fig.\u00a06\n and Figure S6). With the increasing of Pd loading amount, the mean sizes of Pd nanoparticles increased from 2.48\u00a0nm (1\u00a0wt.%) to 3.65\u00a0nm (3\u00a0wt.%) and then slightly decreased to 3.14\u00a0nm (5\u00a0wt.%), finally kept at around 3\u00a0nm even the Pd loading increase to 10\u00a0wt.%. Du et\u00a0al. [5] reported that metal nanoparticles are more inclined to load on the edge of pedals because the edges of the \u201cflower-pedals\u201d in Ni-Al HTC possess abundant coordinatively unsaturated metal sites (CUS) which result in higher cohesive energy, so it is more conducive to the deposit and grow metal nanoparticles. At low amount of Pd loading, there is relatively sufficient CUS on the edges of pedals, therefore, the Pd particle size climbed from 2.48\u00a0nm to 3.65\u00a0nm with the increase of Pd loading due to the aggregation (Figure S6(a) and S6(b)). While under the situation of high Pd loading amount, because of all CUS were occupied, the Pd nanoparticles were gradually loaded onto other area of pedals, leading to the slight decreasing of average particle size (Figure S6(c) and S6(d)).The catalytic performances of those Pd/HTC catalysts with varied Pd loading amount were investigated by the BA oxidation and the results are presented in Fig.\u00a07\n. As shown in Fig.\u00a07, the BD selectivity continuously declined with the increase of Pd loading amount because the side reaction like disproportion and over-oxidation (Scheme\u00a01\n) are more readily occurred along with high Pd amount. Furthermore, when the load of Pd exceeds 5%, the BA conversion and BD yield are descended with the increase of Pd loading amount (Fig.\u00a07). It had been proved that the basic sites on the surface of Ni-Al HTC also plays an important role in the oxidation of alcohol [14]. Therefore, although Pd nanoparticles are actually active components, the high amount of Pd loading might occupy the basic sites on the surface of Ni-Al HTC, leading to a decreased catalytic activity. A balance point between Pd loading and basic sites on the surface of Ni-Al HTC support could be found. It can be seen that the selectivity of BD and conversion of BA for the Pd/HTC catalysts with 3\u00a0wt.% and 5\u00a0wt.% Pd loading are obviously higher than others with the low loading or high loading (Fig.\u00a07).The pretreated process of catalyst was also investigated in this work, as shown in Figure S2, T-test analysis of the catalytic activities of Pd/HTC samples and its corresponding pretreated samples demonstrated that the pretreated Pd/HTC catalysts were not exhibiting enhanced activity comparing to the original ones, and the catalytic performances of the Pd-Na and Na-H were even dropping after pretreatment. This indicated that the ameliorating effect of pretreatment for Au/HT catalysts [5] cannot be generalized to Pd/HTC catalysts likely due to the different redox potentials of each metal.The general applicability of Pd/HTC catalyst for aerobic oxidation of alcohols was further evaluated by extending the substrate scope (Table\u00a01\n). The product structure was confirmed by NMR analysis of isolated product, and the data are in the supporting information. As shown in Table\u00a01, the synthesized Pd/HTC catalyst shows a higher catalytic activity for primary aromatic alcohols excluding those having highly electron-withdrawing substituents such as (2-(trifluoromethyl)phenyl)methanol, p-bromobenzyl alcohol and p-nitrobenzyl alcohol (Entries 1\u201310). Notably, the Pd/HTC catalyst could catalyze the benzyl alcohol oxidation in the solvent-free conditions, the TOF can reached 1182\u00a0h\u00a0\u2212\u00a01 (Table\u00a01, Entry 2), which is higher than previously reported Pd/HTC catalyst [8]. Primary allylic alcohols like cinnamyl alcohol also exhibited good reactivity (Entries 11), however, the side product of hydrogenation compound could be generated simultaneously. The selectivity is significantly lower for allylic alcohols (Entries 11 and 12). In addition, the synthesized Pd/HTC catalyst has excellent regioselectivity and preferred to catalyze the terminal hydroxyl group in benzylic alcohols. For examples, in the oxidation of 1-phenylethane-1,2-diol, only the product of 2\u2011hydroxy-2-phenylacetaldehyde was found in the reaction mixture after 7\u00a0h (Entry 13); and the secondary benzylic alcohols give very low yield of carbonyl product compared to primary alcohols (Entries 14\u201317). Moreover, the synthesized Pd/HTC catalyst also could catalyze the oxidation of cyclohexanols under mild reaction conditions, although it might need longer reaction time to achieve a good yield (Entry 18).In this work, we prepared a series of flower-like HTC-supported Pd nanoparticles catalysts using three kinds of Pd salts (PdCl2, Na2PdCl4 and K2PdCl4), two types of reduction reagents (NaHB4 and N2H4), and various Pd loading amount ((1, 2, 3, 5, 7 and 10\u00a0wt.%). These catalysts were characterized by SEM-EDX, TEM, and XRD. It was shown that the presence of alkali metal ions in precursors assists to reduce the aggregation of Pd nanoparticles; the Pd nanoparticles distributed more evenly when NaBH4 was used as reduction reagent compared to using N2H4 as reducing reagent, and Hofmeister effect was observed in the former case. The benzyl alcohol oxidation was chosen as a model reaction to elucidate the effect of Pd salts and reduction reagents on structure-activity relationship of Pd/HTC catalysts. Although the mean size of Pd nanoparticles synthesized by using N2H4 as reduction reagent was bigger than corresponding Pd precursor reduced by NaHB4, the benzyl alcohol conversion was neither affected by the Pd precursor nor the reduction reagents. However, higher benzaldehyde selectivity was achieved when N2H4 is used as reduction reagent. The XPS and FTIR studies elucidated that the oxidation states of Pd were not significantly changed by using different Pd salts and reduction reagents, but the Br\u00f8nsted-acid sites on the surface support was affected by reduction reagents, thereby affecting product selectivity. Furthermore, although the Pd nanoparticles are real active component in Pd/HTC catalysts, higher Pd loading amount over the optimal dosage resulted in a decreasing catalytic performance due to the decrease of basic sites. The optimal Pd loading was in the range of 3\u00a0wt.% - 5\u00a0wt.%. In the last, the synthesized flower-like Pd/HTC catalyst exhibited general applicability to extended substrate scope of aromatic and aliphatic alcohols. However, high catalytic activity is not extendable to the aromatic alcohols with electron-withdrawing substituents and second alcohols; and regioselectivity is also largely limited to oxidation of primary alcohols when both types of alcohols are present simultaneously.\nRongrong Dai: Methodology, Data curation, Writing \u2013 original draft. Zheng Guo: Conceptualization, Funding acquisition, 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.Financial supports from Novo Nordisk Foundation (Grant no. NNF16OC0021740), AUFF-NOVA, Aarhus Universitets Forskningsfond (AUFF-E-2015-FLS-9\u201312) and Danmarks Frie Forskningsfond | Teknologi og Produktion (0136\u201300206B) are gratefully acknowledged. R. Dai thanks for the financial support from the China Scholarship Council (CSC).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.mcat.2022.112403.\n\n\nImage, application 1\n\n\n\n\n\nImage, application 2\n\n\n\n", "descript": "\n                  Benzyl alcohol was chosen as a model substrate for Pd/hydrotalcite mediated selective oxidation of alcohols to investigate the roles of palladium precursors (Na2PdCl4, K2PdCl4, PdCl2), reduction reagents (N2H4 and NaBH4), and Pd loading amount in determining structure-activity relationship of flower-like nano-catalyst. XRD analyses show typical characteristic peaks of all hydrotalcites regardless of palladium precursors; whereas TEM studies show a larger mean size of Pd nanoparticles obtained with reducing reagent N2H4 over NaBH4. However, the difference in particle sizes didn't result in a significant difference in benzyl alcohol conversion, but extended to catalytic selectivity where N2H4-reduced Pd/hydrotalcite achieved 94%-96% selectivity over 88%-93% by NaBH4. FTIR-CD3CN surface acidity study suggested that a stronger acidity of N2H4 reduced Pd/hydrotalcite may account for its better selectivity. 3% Pd/hydrotalcite with N2H4 as reducing reagent demonstrated the best catalytic performance; and was successfully extended to oxidation of 17 aromatic and aliphatic alcohols. The conversion of alcohols is strongly dependent on individual alcohol molecular structure and electronic effect; but all reactions showed medium-to-excellent selectivity.\n               "}
{"full_text": "CO2 conversion to high value-added chemicals such as methanol and ethylene glycol (EG), which are both important bulk chemicals, had been paid great efforts since it could solve the problem of reliance on the non-renewable fossil fuels [1\u20136]. However, direct hydrogenation of CO2 is limited by the high activation energy for cleavage of the CO bonds of CO2. Therefore, studying the indirect transformation route of CO2 can effectively avoid the limitation of thermodynamics and kinetics [7]. Owing to the exothermic reaction of methanol synthesis, low reaction temperature is favorable. Ding and his coworkers [1] proposed the selective hydrogenation of ethylene carbonate (EC) derived from CO2 to simultaneously synthesize methanol and EG with 100% atom economy over the homogeneous Ru-based catalyst. Whereas, the difficult separation processes and the high-cost of Ru-based homogeneous catalysts imposed the high manufacture cost. Therefore, to overcome these limitations, researchers have paid much attention to develop relatively inexpensive [8,9] and heterogeneous catalysts such as Cu\u2013SiO2-PG and Cu/MCM-41 [10,11] with high product selectivity in EC hydrogenation in recent years. Obviously, using renewable carbon source CO2 or CO2-derived EC to synthesize methanol and EG is an important research topic related to the sustainable development of resources, energy, and economic society [12\u201314]. Especially methanol, which is a perfect carrier of hydrogen energy, could be used as basic material to produce olefins that is in great domestic demand via methanol-to-olefins (MTO). Therefore the production of methanol from CO2 could realize carbon fixation and the emission reduction, and is of great significance for realizing carbon neutrality [15]. At present, Cu-based catalysts have good catalytic performance in catalyzing ester hydrogenation, but they are prone to agglomeration, and the loss of copper species and change of valence state during use could reduce the activity. However, the size and morphology of the active species of the carbon modified Cu-based catalyst were altered, hence the catalytic activity and stability of the catalyst were improved. For example, Cu@C catalyst prepared by Xiao et\u00a0al. [16] using Cu-BTC as the precursor system has smaller metal particle size and larger specific surface area than catalyst prepared from traditional precipitation method, and the porous carbon matrix can inhibit the agglomeration of metal particles in the catalyst. Li et\u00a0al. [17] showed that after modification with glucose, the highest Cu+/(Cu0\u00a0\u200b+\u00a0\u200bCu+) molar ratio of Cu8G1/SiO2-AE catalyst was detected. In addition, the glucose modification of the Cu/SiO2-AE catalyst can also alleviate the deactivation of the catalyst distinctly. Li et\u00a0al. [18] modified the Cu@SiO2 catalysts with \u03b2-cyclodextrin. The results revealed that the involvement of \u03b2-cyclodextrin improved the Cu dispersion and facilitated the exposure of more copper active sites, which also indicated that the confined catalyst inhibited the sintering of copper particles. Meanwhile, the stability of the attained carbon modified catalyst was superior. Therefore, to improve the potential application of Cu-based catalysts in industry for EC hydrogenation, it is highly desired to promote the catalytic activity and stability of Cu-based catalysts, as well as study the stabilization mechanism [18].Recently, metal-organic framework derived (MOF-derived) catalysts attract increasing attention due to their tunable structures and designable pore surfaces, and the core-shell structure was found to be able to prevent the migration and aggregation of metal particles. In general, the organic ligands in MOFs facilitates the dispersion of metal copper and clusters, which could form highly dispersed nanoparticles in MOF-derived materials [19\u201323]. In comparison with the conventional catalysts, MOF-derived Co2P/CN\nx\n [24], Ni/C [25], Pd@Co/CNT [26] catalysts exhibited superior performances and stability in hydrogenation reactions. Moreover, it is known that the nitrogen-doped carbon-based catalyst could lead to undesirable transesterification of EC and methanol, resulting in the decrease of methanol productivity [27]. Therein, the new class of porous regularly crystalline materials Cu3(BTC) with large ordered pores and remarkable surface area was considered as the precursors to prepare highly dispersed Cu-based catalysts with core-shell structures (Scheme 1\n). Moreover, the as-prepared catalysts were extensively characterized by N2 physisorption, TGA measurement, FT-IR, XRD, TEM, SAED, XPS, and XAES to analyze the microstructure and the physicochemical properties. The catalytic performances, the influence of reaction conditions and the durability of the as-prepared catalyst were studied. Furthermore, the synergistic mechanism to catalytic performance between different valence of copper, and the influence of graphite oxide (GO) on stability of copper species was also investigated.\nSynthesis of Cu@GO. Cu@GO catalyst was synthesized by ultrasonic precipitation (UP) method derived from metal-organic framework. Certain amount of Cu(NO3)2\u00b73H2O, NaOH, and 1,3,5-benzenetricarboxylic acid were dissolved in 100\u202fmL deionized water, 110\u202fmL deionized water, and 250\u202fmL ethanol with stirring, respectively. Afterwards, the attained aqueous alkali was added into the copper ion liquid dropwise around 40\u202fmin under stirring. Then, 1,3,5-benzenetricarboxylic acid solution was dropped at the same drop speed. Thereby the attained bluish violet turbid liquid was dealt by ultrasonic at 313\u202fK for 3\u202fh to obtain copper MOFs. Subsequently, the suspension was separated by centrifuge and washed by ethanol and deionized water for several times. Then, the as-prepared solid was dried at 373\u202fK overnight. After drying the solid was calcined under nitrogen atmosphere at 723\u202fK for 4\u202fh. Before being used for catalysis, the solid was reduced by 10\u202fvol% H2/N2 at 623\u202fK for 4\u202fh, and the finally obtained catalysts were named as Cu@GO.\nSynthesis of Cu-catalyst. The Cu-catalyst was synthesized by the following procedure. The desired amounts of Cu(NO3)2\u00b73H2O and NaOH were weighted and dissolved in 100\u202fmL and 110\u202fmL deionized water, respectively. After stirring for 10\u202fmin, the aqueous alkali was added into the solution of Cu2+ dropwise in around 60\u202fmin to attain Cu complex precipitation. After aging for 3\u202fh at ambient temperature, the attained suspension was separated via filtration and washed carefully by deionized water for several times. The subsequent step was the same as the Cu@GO catalyst and the finally obtained catalyst was named as Cu-catalyst.\nCharacterization. On Quantachrome Autosorb-1, N2 physisorption analysis was performed. The measurement started at liquid nitrogen temperature (77\u202fK), then the sample was outgassed in vacuum at 573\u202fK for 3\u202fh. Through Brunauer-Emmett-Teller (BET) method, the special surface area (S\nBET) was obtained. Meanwhile, the total pore volume (V\npore) was determined from the absorbed volume of nitrogen with a relative pressure of 0.99. Based on the desorption branch of the isotherm, pore size distribution was estimated by the Barrett, Joyner, and Halenda (BJH) method. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) on PerkinElmer optima 5300DV was used to determine the Cu content of catalysts. TGA measurement was realized on Diamond TG-DTA6300 thermo-gravimetry analyzer in the purity N2 atmosphere with a heating rate of 5\u202fK min-1. X-ray diffraction (XRD) patterns of samples were obtained through a PANalytical Empyrean diffractometer with Cu K\u03b1 radiation (\u03bb\u202f=\u202f0.15406\u202fnm) over 2\u03b8 range of 10o\u201390o. Fourier-transform infrared spectra (FT-IR) was measured on a Bruker Tensor 27 spectrometer. The catalysts were ground and uniformly dispersed in KBr to obtain pellets suitable for FT-IR characterization. Raman spectra was attained on an apparatus pfLabRam HR800. The powder catalyst was dispersed on a glass slide for Raman characterization. H2 temperature-programmed reduction (H2-TPR) was executed on a Quantachrome Chembet pulsar TPR/TPD instrument. Before reducing it by hydrogen, He gas flow was used to pretreat the calcined catalysts at 473\u202fK for around 1\u202fhour. Then it was cooling down to ambient temperature. Subsequently, the TPR program was executed with an increasing temperature program from 323\u202fK to 803\u202fK at the rate of 10\u202fK\u202fmin-1 under 10\u202fvol% H2/N2. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were attained by field-emission transmission electron microscopy (JEOL, JEM-2100F), which was operated at an acceleration voltage of 200\u202fkV to characterize the morphologies and the crystal structures of the Cu@GO. Before scanning the element distribution, the powder was sonicated in ethanol for around half hour. Then, the suspension was carefully drop-casted on copper grids, which were supported by holey carbon films. The test was obtained after the sample drying with EDX scanning mode on JEOL. In order to get the oxidation state of catalyst and to know more surface physical properties, X-ray photoelectron spectroscopy (XPS) and X-ray Auger electron spectroscopy (XAES) were reached. All measurements were achieved under an ultra high vacuum via an ESCALAB 259Xi spectrometer with Al K\u03b1 radiation (1486.6\u202feV) and a multichannel detector. Then the binding energy with C 1s at 284.6\u202feV with accuracy of \n\n\u00b1\n\n0.2\u202feV was calibrated.\nComputational methods and models [28\u201330]. During the molecular simulations, periodic boundary conditions were implemented along the Cu basal plane, and the velocity Verlet algorithm with a time step of 1 fs was used. All the MD simulations were conducted in the NVT ensemble. The Langevin thermostat were employed to keep the constant temperature of the substrate. The interatomic interaction of C atoms could be described by airebo pair style, the interaction between Cu atoms could be reflected by the EAM potential, and the Cu\u2013C interaction could be reflected by the Lennard-Jones potential (r\u202f=\u202f3.0825\u202f\u00c5, e\u202f=\u202f0.02578\u202feV). Fig.\u00a01\n illustrated the MD simulation model, where Cu1 represented the first layer and Cu2 represented the second layer.The first-principles [25] were employed to perform all spin-polarization density functional theory (DFT) calculations [36] within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) formula. The projected augmented wave (PAW) potentials [19,20] were chosen to describe the ionic cores and used a plane wave basis set with a kinetic energy cutoff of 400\u202feV to consider the valence electrons. The Gaussian smearing method allowed partial occupancies of the Kohn\u2212Sham orbitals with a width of 0.05\u202feV. When the energy change was smaller than 10-6\u202feV, the electronic energy was considered self-consistent. When the energy change was smaller than 0.05\u202feV\u202f\u00c5\u22121, a geometry optimization was considered convergent. The vacuum spacing perpendicular to the plane of the structure is 15\u202f\u00c5. The Brillouin zone integration is performed on the structure using 2\u202f\u00d7\u202f2\u202f\u00d7\u202f1 Monkhorst-Pack k-point sampling. Finally, the adsorption energies (E\nads) were calculated as E\nads\u202f=\u202fE\nad/sub - E\nad - E\nsub, wherein E\nad/sub, E\nad, and E\nsub are the total energies of the optimized adsorbate/substrate system, the adsorbate in the structure, and the clean substrate, respectively. The free energy was calculated using the equation:\n\n(1)\n\n\nG\n=\nE\n+\nZ\nP\nE\n\u2212\nT\nS\n\n\n\nwhere G, E, ZPE, and TS are the free energy, total energy from DFT calculations, zero point energy, and entropic contributions, respectively. In the calculation, U correction had been set as 3.68 for Cu atoms and the bottom layers had been fixed in this system.Figure\u00a0S1 showed the pyrolysis process of the dried Cu@GO catalyst in N2 atmosphere. Four weight losses from room temperature to 400\u202f\u00b0C in TGA curve can be seen. The first loss of 4.6% is deemed as the loss of the absorbed guest molecules within the porous structure of the dried Cu@GO catalyst [31]. The second and third losses of 7.5% and 1.9% corresponded to the loss of bound water under the temperature lower than 250\u202f\u00b0C. The last weight loss of 38.7% around 300\u202f\u00b0C could be considered to be the decomposition of the dried precursor of Cu@GO catalyst Cu3(BTC)2 [32,33].The specific surface areas and pore size distribution of Cu@GO and Cu-catalyst were studied by N2 adsorption-desorption, and the details were illustrated in Fig.\u00a0S2 and Table\u00a01\n. The S\nBET of Cu@GO and Cu-catalyst were 96.9 and 1.6\u202fm2 g-1 and the average pore sizes of these two catalysts were 3.0 and 20.4\u202fnm with the pore volume of 0.07 and 0.01\u202fcm3 g-1, respectively. In comparison with Cu-catalyst, Cu@GO catalyst possessed a larger specific surface area, which was expected to provide more accessible active sites [34].Accordingly, the pore size of Cu@GO catalyst was quite lower than that of Cu-catalyst, implying the smaller Cu particles formed in Cu@GO catalyst promoted the accessible active sites. As shown in Fig.\u00a0S2a, according to IUPAC classification, N2 isothermal physisorption curve of Cu@GO catalyst can be classified as type IV with H1-type hysteresis loops, indicating the wedge hole of the pore [2,35]. While the physisorption curve of Cu-catalyst was type III without hysteresis loop, suggesting the accumulation of cooper species [18]. Fig.\u00a0S2b showed that the pore size distribution of Cu@GO catalyst was relatively narrow and sharp with a range of 3\u201310\u202fnm, promoting the formation of relatively smaller Cu particles as well as implying the pore size and pore structure of the as-prepared Cu@GO catalyst were highly random [24]. Meanwhile, ICP was used to determine the Cu contents of the attained catalysts collected in Table\u00a01, and the results indicated that the Cu contents in Cu@GO and Cu-catalyst were 76.2% and 93.4%, respectively.As shown in Fig.\u00a02\na, the XRD spectrum of the dried precursor of Cu@GO catalyst could match well with Cu3(BTC)2 crystal that the diffraction peaks 2\u03b8 of 9.9o, 10.9o, 13.4o, 21.3o, 26.1o, and 28.6o could be assigned to (220), (400), (440), (600), (731), and (751) crystal planes of Cu3(BTC)2. The result proved that the Cu MOF of Cu3(BTC)2 could be synthesized via the ultrasonic method [27,37-40]. As for the reduced Cu@GO catalyst, the (111), (200), and (220) crystal planes of Cu2O at the positions of 36.5o, 42.6o, and 61.7o were observed and the (111), (200), and (220) crystal planes of Cu at the positions of 2\u03b8 of 43.5o, 50.6o, and 74.2o were also discovered in the attained catalyst [33,41], and there was no characteristic peaks of CuO residual were observed, thereby it can be speculated that all of the Cu2+ were reduced during the calcination and reduction process. Accordingly, the foundation of graphite oxide in Fig.\u00a02a verified the formation of carbon film which promoted the stability of Cu@GO catalyst to a large extent [42]. For the sake of exploring the differences between the contrastive Cu-catalyst and Cu@GO catalyst, XRD analysis of Cu-catalyst was also performed, as illustrated in Fig.\u00a02b. The main diffraction peaks at 2\u03b8 values of 35.5o, 37.7o, 38.8o, 48.8o, 53.4o, 61.6o, 66.2o, 68.1o, and 72.3o could be assigned to (110), (111), (200), (202), (020), (022), (311), (220), and (311) planes of CuO, and there was no any other phase could be detected [43,44], implying the dried precursor of Cu-catalyst was the purity CuO after drying overnight. For the reduced Cu-catalyst, the observed peaks can be divided into two parts, 2\u03b8 values of 36.5o, 42.6o, and 61.7o were assigned to (111), (200), and (220) planes of Cu2O and 2\u03b8 values of 43.5o, 50.6o, and 74.2o were assigned to (111), (200), and (220) planes of Cu, respectively. Whereas, the planes of CuO (110), (111), and (200) corresponded to 2\u03b8 values of 32.4o, 35.5o, and 38.8o still existed in the reduced Cu-catalyst [45], which could be ascribed to the incomplete reduction or oxidized by oxygen in the atmosphere when it was exposed to the air atmosphere. The as-prepared Cu-catalyst possessed similar crystal planes with Cu@GO catalyst. It could be speculated that the exit of residual carbon could obviously prevent the oxidization of Cu@GO catalyst in the air atmosphere.In order to analyze the functional groups of the dried precursor of Cu@GO and the reduced Cu@GO catalysts, FT-IR was also employed, and the results were shown in Fig.\u00a03\n. For the precursor of Cu@GO catalyst, it was found that there was an obvious adsorption peak of 474\u202fcm-1, which could be attributed to the Cu\u2013O stretching vibration of pure CuO with a monoclinic structure [46,47]. Furthermore, the observation of 739\u202fcm-1 could be attributed to the out-of-plane bending vibration of C\u2013H group [48]. The characteristic band of Cu3(BTC)2 located at 1377\u202fcm-1 standing for the stretching vibration of aromatic ring. Another characteristic band at 1104\u202fcm-1 can be identified as the stretching bond of C\u2013C on the aromatic ring as well [29,35]. The strong vibrational bands at 1627\u202fcm-1 and the strong broad and bands between 3180 and 3510\u202fcm-1 could be pointed out as \u2013COOH of H3BTC which was used as a linker in Cu-BTC [49]. Through the existence of these six distinct absorption peaks, it was not hard to speculate that the successful formation of Cu3(BTC)2 which was the dried precursor of Cu@GO catalyst. For the curve of the reduced Cu@GO catalyst, the peak at 739\u202fcm-1 could be assigned to the C\u2013H group out-of-plane bending vibration. The intensity declined remarkably, implying that the main C\u2013H structure was destroyed during the pyrolysis process in the N2 atmosphere while there was still some carbon residual of C\u2013H structure. The bands of 1055\u202fcm-1, 1107\u202fcm-1, and 1242\u202fcm-1, which could be assigned to the characteristic C\u2013C stretching vibration, CO\u2013Cu stretching, and epoxy groups C\u2013O\u2013C stretching vibration of the carboxyl group [44], indicated the existence of carboxyl groups, the result of which further proved the structure of graphite oxide. Therefore, it could be speculated that the graphite oxide was formed over the copper particles. Furthermore, the strong absorption bands of 630\u202fcm-1 could be assigned to the inorganic network Cu\u2013O of Cu2O phase and this bond shifted from 474\u202fcm-1 of the dried precursor Cu3(BTC)2 during the reduced process, implying that Cu+ was formed [47].Raman test was performed to study the shape of carbon in Cu@GO catalyst. It was shown in Fig.\u00a04\n that the G-band (\u223c1598\u202fcm-1) existed in as-prepared catalyst, which was ascribed to the stretching vibration of the sp2-hybridized carbon atoms in graphite oxide. The existence of sp2 hybridization was favorable to the interaction between Cu species and graphite oxide film [50,51]. There was a weak D-band (\u223c1327\u202fcm-1), suggesting that there was less disordered density of carbon atoms at the plane edges in the reduced Cu@GO catalyst. The presence of D-band and G-band verified that carbon and copper exist simultaneously in Cu@GO catalysts, which was in accordance with the results of XRD and FT-IR.The H2-TPR profiles of Cu@GO and Cu-catalysts were depicted in Fig.\u00a05\n. It could be found that both reduction peaks from Cu@GO and Cu-catalyst are symmetrical. Thereby the onset reduction temperature of Cu@GO catalyst was 475\u202fK, ascribed to the well dispersed CuO particles [52]. Whereas, the onset reduction temperature of Cu-catalyst was 518\u202fK, presumably ascribed to the larger CuO\nx\n particle size with deteriorated dispersion. In this situation, it could be concluded that after calcination, Cu@GO catalyst was reduced to both Cu+ and Cu0 species selectively according to its reduction temperature. While it also could be deduced that the reduction process of the precursor calcinated Cu-catalysts was incomplete, which further verified the results from FT-IR and XRD.\nFig.\u00a06\na-b is the SEM images of the dried precursors of Cu@GO. It is seen that the precursors of Cu@GO and Cu3(BTC)2 were formed into octahedral structures which were consistent with the previous observation for cubic MOF crystal [40]. The whole morphology of Cu@GO catalyst was illustrated in Fig.\u00a06c-d, and it was clearly seen that the metallic Cu particles was embedded in the porous carbon matrix uniformly with the range of Cu particles sizes from 20 to 130\u202fnm. It could be also found that the abundant and disordered nanoporous was formed in the attained Cu@GO catalyst and was connected via a network of channels which was consistent with the result of pore size distribution from N2 physisorption. Meanwhile, high-resolution was further performed to study the microstructure of Cu@GO catalyst. The structure of Fig.\u00a06e revealed that there was a film of carbon which sealed the Cu nanoparticles in the graphite oxide layer. The lattice spacing of Cu@GO catalyst shown in Fig.\u00a06f was also measured afforded by HRTEM with 0.21\u202fnm of (111) crystal plane of metallic Cu and 0.27\u202fnm of (111) crystal plane of Cu2O [53,54], respectively, coinciding with the identified crystal planes from XRD.As shown in Fig.\u00a06g, selected-area electron diffraction (SAED) was adopted to probe the diffraction planes of the attained catalyst. There were a series of rings in SAED pattern, which could be assigned to the characteristic diffraction planes of (111), (200), and (220) of metallic Cu and the Cu2O planes of (111), (200), and (220), which was consistent with the results from XRD [40,55,56]. As for the comparison of Cu-catalyst, the Cu particles distributed irregularly, as shown in Fig.\u00a06h-i.\nFig.\u00a07\n showed that the elements of C, Cu, and O were uniformly dispersed in Cu@GO catalyst. It is seen from Fig.\u00a07b that there were a series of vacancies in C-mapping, and the residual carbon from Cu3(BTC)2 existed by connecting network channels. Fig.\u00a07c illustrated the uniform distribution of Cu is in the form of plenty of in Cu nanoparticles which match well with the vacancy in C-mapping, indicating Cu nanoparticles were encapsulated in carbon film equably which coincided with the results from N2 physisorption, XRD, FT-IR, and Raman. Moreover, the graphite oxide film could encapsulate the copper particles. Furthermore, Fig.\u00a07d of O-mapping revealed the uniform co-existence of C, Cu, and O indicating the energy bonds between these three elements. In summary of the TEM image, FT-IR, and EDX mapping, a conclusion could be drawn that the ultrasonic precipitation was an excellent pathway for producing the Cu-based catalyst covered by carbon which promoted the catalytic activity and stability simultaneously.The high angle annular dark-field scanning/transmission electron microscope was executed to verify the core-shell structure of Cu@GO catalyst. The cross-sectional compositional line profile was carried out and illustrated in Fig.\u00a08\n. Fig.\u00a08b exhibited that Cu and C co-existed in each part, while the Cu content went up from the margins to centers of the particles. Meanwhile, the contents of carbon around the margins of the particles were much higher than the content of Cu, and the contents of carbon were lower in the center of catalyst particles. Therefore, it could be speculated that the nanoparticles Cu cores and carbon shells were successfully prepared via the as-mentioned method which were accordant with the results from TEM, XRD, and EDX.In Fig.\u00a09\na, the XPS survey spectra showed three different elements, C, O, and Cu in Cu@GO catalyst, where the element of C was derived from the residual carbon during the pyrolysis process. Three peaks in C 1s high-resolution spectrum of Cu@GO catalyst, centering at 284.6, 285.4, and 286.5\u202feV which can be assigned to C\u2212C/CC, C\u2212O, and CO in Fig.\u00a09b, respectively [57,58]. These carbon peaks were the characteristic peaks of graphite carbon derived graphite oxide corresponding to the results of XRD and FT-IR, and further verified the formation of carbon film over Cu core. The binding energy at 933.8\u202feV from Cu 2p high-resolution spectrum in Fig.\u00a09c was ascribed to Cu in Cu0 and Cu\u202f+\u202fstate [59,60], hinting that both Cu2O and Cu phases were detected in Cu@GO and Cu-catalyst. The intensive broad peak of 943.0\u202feV, which was attributed to Cu2+ in Cu-catalyst [61], revealed that the residual CuO was oxidized again when it was exposed to the air atmosphere, which was in accordance with the XRD result. Whereas, the Cu2+ appeared in Cu-catalyst was considered to limit the catalytic performance to some extent, because the atom efficiency of Cu for the formation of active Cu species inevitably decreased. Therefore, it could be speculated that the existence of carbon inhibited the oxygenation of catalyst, facilitated the distribution of Cu nanoparticles. XAES was further employed to differentiate Cu0 and Cu\u202f+\u202fspecies in catalysts, and the asymmetric peaks were divided into two overlapping Cu LMM Auger kinetic energy around 913.0 and 917.0 eV correspond with Cu+ and Cu0 shown in Fig.\u00a09d [62], respectively. There was an interesting phenomenon that the broad peak of Cu catalyst could be deconvoluted to three different peaks thereby the Cu2+ was observed at kinetic energy of 918.2\u202feV [63], suggesting the incomplete reduction of Cu-catalyst which was also confirmed by XRD and XPS results. As calculated in Table\u00a02\n, the ratio of Cu+/(Cu+\u00a0\u200b+\u00a0\u200bCu0) was 0.50 for Cu@GO catalyst which was appropriate, while the ratio of Cu+/(Cu+\u00a0\u200b+ Cu0) in Cu-catalyst was 0.61 as\u00a0a favorable catalyst. Whereas, the Cu2+ appeared in Cu-catalyst limited the catalytic performance. In comparison, the ratio of Cu+ in Cu@GO catalyst attained a balance level which was also elevated by the residual graphite oxide [64]. The results certified the co-existence of Cu+ and Cu0 in the as-prepared catalysts, and the synergistic effect on the surfaces of catalysts.The performance of Cu@GO and Cu-catalyst in the hydrogenation of EC was operated under the temperature of 473\u202fK, time of 4\u202fh and the hydrogen pressure of 5\u202fMPa. It is seen in Table\u00a03\n that, the conversion of the EC attained 71.2% with 69.2% methanol and 98.2% EG selectivity, showing excellent catalytic activity in EC hydrogenation by applying Cu@GO catalyst. As for Cu-catalyst, EC conversion was only 44.9% with methanol and EG selectivity were 21.1% and 80.7%, respectively, the results of which was extremely lower than Cu@GO catalyst although the Cu content of Cu-catalysts (93.4%) is higher than Cu content of Cu@GO catalyst (76.2%). Therefore, it is speculated that high distribution of Cu species in Cu@GO catalyst assured the higher catalytic activity thereby the disordered carbon nanoporous facilitated the distribution of Cu nanoparticles as well as inhibited the oxidizing process of low valence Cu species. These results implied that the structure of Cu@GO catalyst with Cu core and graphitic oxide shell derived from Cu3(BTC)2 enhanced the catalytic activity effectively, synchronously increased the selectivity of co-product methanol and EG.The effect of reaction time and temperature on hydrogenation of EC was studied to find the optimized reaction conditions. As illustrated in Fig.\u00a010\na, the reaction temperatures ranging from 453 to 493\u202fK were tested. EC conversion increased with the reaction temperatures all the time, while the selectivity of EG declined slightly from 99.9% at 453 to 94.6% at 493\u202fK. The selectivity of methanol increased with temperature and reached to the highest of 69.0% at the temperature of 473\u202fK, then kept unchanged with further increase the temperature to 493\u202fK. Moreover, the high TOF of 1526 mgEC gcat\n\u22121 h\u22121 could be attained in the hydrogenation of EC at 493\u202fK.The result of the temperature effect indicated that the temperature is of great importance in the catalytic activity thereby higher temperature is benefited to the transformation of EC, but the co-product methanol and EG will be deteriorated if the reaction temperature was too high. The effect of reaction time was also studied, which was illustrated in Fig.\u00a010b. Conversion of EC increased gradually with the reaction time and attained complete conversion at 493\u202fK for 6\u202fh with 67.1% methanol and 91.7% EG selectivity. Noteworthy to say that, EG selectivity decreased from 94.6% to 91.7% with reaction time prolonged to 6\u202fh, implying the reaction carried out at high temperature for a long time accelerated the deterioration of EG. The attained results verified that Cu@GO catalysts exhibited superior catalytic activity for EC hydrogenation.The stability of catalysts plays a significant role in its application in chemical industry, and the present unstable Cu-based catalyst or the unclear stabilization mechanism is desired to be developed. The reusability of Cu@GO catalyst was shown in Fig.\u00a011\n. Compared with the first run, EC conversion increased to 75.0% with selectivity of methanol and EG increased to 83.3% and 99.9% in the second run, respectively, assigned to the excitation of Cu@GO catalytic sites in the first run. EC conversion declined to 62.8% in the third run and maintained at a relative balance value around 65.0% in the subsequent cycles. The methanol selectivity also decreased in the subsequent cycles and reached to 67.3% after six runs, showing excellent stability. Furthermore, EG selectivity kept at 99.9% without any change after the second run. In summary, 64.3% EC conversion, 67.3% methanol, and 99.9% EG selectivity could be reached after 6 runs. The reason could be speculated that the graphitic oxide inhibited the aggregation of Cu, thus enhanced the stability of Cu-based catalysts, simultaneously promoted the distribution of Cu species to improve the catalytic activity.In order to further study the stabilization mechanism of graphite oxide film of Cu-based catalyst, characterizations of XRD, TEM, and XPS of the reused Cu@GO catalyst were employed. As depicted in Fig.\u00a012\na, the same planes of metallic Cu and Cu2O could be observed in the reused Cu@GO catalyst, and it could be speculated that the formation of graphite oxide film inhibited the microstructure variation of Cu-based catalyst effectively. The TEM image shown in Fig.\u00a012b reflected that there was hardly any changes of the nanoporous channel by connecting network, thereby the pore size of the reused catalyst also maintained the same level compared with the fresh Cu@GO catalyst. Herein, the formation of graphite oxide film inhibited the volume diffusion to restrain the aggregation of Cu particles. Meanwhile, XAES was performed for analyzing the distribution of Cu species, and results showed that Cu+ accounts for 51% of (Cu+\u00a0\u200b+\u00a0\u200bCu0), the value of which was essentially preserved in comparison with the fresh Cu@GO catalyst. This phenomenon implied the Cu species of the as-prepared catalyst was not obviously influenced under the high temperature reaction. In combination of the characterization result of the reused and fresh Cu@GO catalyst, it can be deduced that the existence of graphite oxide film is beneficial to the catalyst stability. Furthermore, it can be concluded the higher balance of Cu species should be contributed to the excellent catalytic activity.Molecular simulation calculations were carried out to study the effect of C on the stability of catalyst, and its radial distribution function was shown in Fig.\u00a0S3. It is shown that the maximum radial distribution probability of Cu1 and Cu2 in the first and second layers of the two models is 2.5\u202f\u00c5. However, in the Cu\u2013C model, the distribution range of Cu1 is narrower and the maximum probability value is higher, indicating that the relaxation range of Cu1 is smaller than that of Cu2, Cu atoms on the surface are more stable, which decrease the aggregation of Cu particles effectively. The molecular simulation results indicated that graphite oxide can promote the stability of Cu-based catalyst, which is consistent with the experimental results.The adsorption and dissociation behaviors of EC and H2 on Cu with different valence states were simulated. The adsorption of EC and H2 on Cu/C and Cu2O/C was simulated to study the effect of graphite oxide membrane on the adsorption of EC and H2. The optimized geometric structure of DFT is shown in Fig.\u00a0S4. It is seen that the initial distance between cyclic oxygen and Cu+\u202fsite of EC molecule is 1.908\u202f\u00c5, and the initial distance between carbonyl oxygen and Cu+ site is 2.454\u202f\u00c5. The cyclic oxygen in EC molecule is closer to Cu+ site than its carbonyl oxygen, the adsorption energy of EC molecules on Cu0 and Cu+\u202fare -0.49 and -0.64\u202feV, respectively. These results indicate that EC molecule is stably adsorbed to Cu0 and Cu+\u202fsites, and the interaction between carbonyl oxygen and Cu+\u202fsites is dominant. Fig.\u00a0S4c and d showed the adsorption of H2 on Cu0 and Cu+. The adsorption energy of H2 on Cu0 and Cu+ is -0.35 and -0.40\u202feV, respectively. This indicated that the presence of graphite oxide film is conducive to the adsorption of H2 molecules on Cu0 and Cu+\u202fsites.\nFig.\u00a013\n showed the dissociation process of H2 on Cu0 and Cu+, respectively. As shown in Fig.\u00a013a, the initial distance between hydrogen molecule and Cu0 site is 1.985\u202f\u00c5, and the initial distance between the two H atoms and Cu0 site is 1.228\u202f\u00c5. After the dissociation, the distance between the two H atoms and Cu0 site becomes 1.991\u202f\u00c5 and 1.834\u202f\u00c5, the distance between the two H atoms becomes 2.689\u202f\u00c5. Similarly, as shown in Fig.\u00a013b, the initial distance between hydrogen molecule and Cu+ site is 1.987\u202f\u00c5, and the initial distance between the two H atoms is 1.267\u202f\u00c5. After the dissociation, the distance between the two H atoms and Cu+ site becomes 1.800\u202f\u00c5 and 1.766\u202f\u00c5. As shown in Fig.\u00a014\n, the dissociation energies of H2 on Cu0 and Cu+\u202fare 31.08 and 23.16\u202fkcal\u00b7mol-1, respectively. The results showed that H2 adsorbed on the catalyst is stable and easily dissociated through the synergistic action of Cu0 and Cu+\u202fdue to the presence of graphite oxide.\nFig.\u00a015\n showed the differential charge density of H2 and EC adsorbed by Cu/C and Cu2O/C structures, where yellow is the area of electron accumulation and green is the area of electron dissipation. The results showed that Cu0 and Cu+\u202fexhibited special electronic states when the catalyst adsorbs H2 and EC. At the same time, the C layer also has electron transfer, which further explained the role of graphite oxide.In this work, Cu@GO catalyst with carbon film derived from MOF was prepared by ultrasonic precipitation method, which was then used in the hydrogenation of EC derived from CO2 to produce methanol and EG simultaneously. The Cu particles were encapsulated in graphite oxide film with random nanoporous channel connected via network. The results of the catalytic performance proved that the residual carbon of Cu@GO catalyst promoted the catalytic activity. Meanwhile, the ratio and the synergistic effect between the copper species of Cu+ and Cu0 contribute to the superior catalytic activity. A high TOF of 1526 mgEC gcat\n-1 h-1 was attained in the hydrogenation of EC at 493\u202fK. Moreover, the reusability study of Cu@GO catalyst showed excellent stability of it. DFT calculation results indicated that the graphite oxide film inhibited the aggregation of Cu particles, therefore promoted the stability of Cu-based catalyst effectively. In conclusion, a facile method was provided for preparing uniformly distributed and stable Cu@GO catalyst via ultrasonic precipitation method derived from MOF, which showed excellent catalytic performance towards the hydrogenation of CO2-derived EC to synthesize methanol, as well as 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.We are grateful to the National Natural Science Foundation of China (21576272), \u201cTransformational Technologies for Clean Energy and Demonstration\u201d, Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA 21030600) and Science and Technology Service Network Initiative, Chinese Academy of Sciences (KFJ-STS-QYZD-138), the National Key Research and Development Program of China (2019YFC1906701) for the financial supports.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.gce.2021.12.004.", "descript": "\n                  The synthesis of sustainable methanol and ethylene glycol (EG) via hydrogenation of ethylene carbonate (EC) has caught researchers\u2019 growing interests on account of the indirect chemical utilization of CO2. Core-shell Cu@GO catalysts with random nanoporous network of graphite oxide (GO) were synthesized via a simple method of ultrasonic precipitation. Cu@GO catalysts were analyzed systematically by N2 physisorption, TGA measurement, XRD, FT-IR, Raman, TEM, SEM, and XPS (XAES). In particular, the mentioned method was confirmed to be effective to fabricate the high dispersity core-shell Cu@GO catalysts through promoting the specific surface area. The as-prepared Cu@GO catalyst was then successfully applied in the hydrogenation of CO2-derived EC to produce methanol and EG. A high TOF of 1526 mgEC gcat\n                     -1 h-1 could be attained in EC hydrogenation at the reaction temperature of 493\u202fK. Accordingly, the correlation of catalytic structure and performance disclosed that the synergistic effect between Cu+ and Cu0 was responsible for achieving high activity of the catalyst. In addition, the reusability of Cu@GO catalyst suggested that graphite oxide shell structure could decrease the aggregation of Cu particles, thus enhance the stability of Cu-based catalysts. DFT calculation results suggested that the involvement of carbon film on Cu was favorable for the stabilization of the active sites. This study is helpful for developing new and stable catalytic system for indirect chemical utilization of CO2 to synthesize commodity methanol and EG.\n               "}
{"full_text": "Accumulation of degradation-resistant pollutants such as nitroaromatics and azo dyes in water bodies poses a threat to the aquatic ecosystem as well as human health. The high water solubility of certain nitroaromatics like 4-nitrophenol (4-NP) and azo dyes can make their removal quite challenging. This triggered the development of methods based on efficient adsorbents (Yagub et al., 2014; Gupta et al., 2013; Dias and Petit, 2015; Parida et al., 2021), photocatalytic degradation (Dias and Petit, 2015), bio-degradation (Marvin-Sikkema and de Bont, 1994; Ju and Parales, 2010), and catalytic conversion (Fu et al., 2019; Zeng et al., 2013) as viable purification strategy to maintain good water quality. Among these options, catalytic conversion is the preferred method as it offers a possibility to convert the pollutants to valuable products and less harmful counterparts with high efficiency. For example, upon hydrogenation, carcinogenic 4-NP can be converted to 4-aminophenol (4-AP), which is an intermediate for corrosion inhibitors (Thenmozhi et al., 2014; Guenbour et al., 2000), pharmaceutical molecules, and dyes (Buschmann, 2007). Transforming nitro pollutants to useful amino compounds is a green and commercially beneficial approach to get rid of pollutants.Among various methods, noble metal nanoparticles (NMPs) catalyzed hydrogenation continues to draw considerable attention owing to their high activity and oxidative stability (Fu et al., 2019; Zheng and Zhang, 2007; Qin et al., 2019; Lu et al., 2006; So\u011fuk\u00f6mero\u011fullar\u0131 et al., 2019; K\u00e4stner and Th\u00fcnemann, 2016). With the rise in demand for active catalysts, the use of noble metals like Au, Pt, and Pd-based catalysts have grown recently (Fu et al., 2019; So\u011fuk\u00f6mero\u011fullar\u0131 et al., 2019; Ansar and Kitchens, 2016; Nguyen et al., 2019; Sun et al., 2014; Johnson et al., 2013; Goepel et al., 2014; Fu et al., 2019). However, the high cost of these metals is a major drawback and thus triggered the development of supports for enhanced recoverability. Supports containing magnetic particle is one such approach for easy recovery and reuse of expensive NMP catalysts (Zeng et al., 2013; Yang et al., 2020; Xu et al., 2020). The development of bimetallic nanoparticle catalysts is another approach to reduce the cost along with improvement in the activity compared to their monolithic counterparts (Qin et al., 2019; Fu et al., 2018). Despite these attempts, the overall price of such catalysts remains higher than catalysts prepared from metals like Ni, Cu, and Ag.In this context, MNPs of moderately active and low-cost metals (Cu and Ag) are still attractive choices as catalysts (Li et al., 2015; Zhou et al., 2020; Das et al., 2019; Dong et al., 2014; Qian et al., 2020; Bhaduri and Polubesova, 2020; Sudhakar and Soni, 2018; Budi et al., 2021). To improve the activity of silver nanoparticles (AgNPs), supports have been designed to boost the catalytic activity via increasing the available surface area for catalysis. Using this principle, nanosheets (Li et al., 2015; Qian et al., 2020; Mao et al., 2018), conductive polymers (Das et al., 2019), and fibrous silica (Dong et al., 2014) were employed for the successful enhancement of the catalytic activity of AgNPs. On the other hand, porous supports prepared from carbon and organic polymers are also known to have a positive effect on the catalytic activity of NMPs (Zhou et al., 2020; Bhaduri and Polubesova, 2020; Gong et al., 2019; Xia et al., 2016; Budi et al., 2020). In the case of these supports, confined space reaction and enrichment of micro-environment abound NMPs by adsorption of substrate molecules enhance the activity (Qin et al., 2019; Gong et al., 2019; C\u00e1rdenas-Lizana et al., 2013). Moreover, fast electron transfer from support like carbon black to NMPs is also known to improve catalytic efficiency (Qin et al., 2019). Although these supports are known to enhance the catalytic activity of NMP, a portion of the NMP surface is shielded by the support, thus, resulting in their underutilization.To counter the underutilization of NMPs, relatively mobile cross-linked polymeric networks or hydrogels are investigated as supports (Lu et al., 2006; Li et al., 2010, 2011; Wang et al., 2010; Irene, 2018; Begum et al., 2019). These supports allow easy access to the NMP surface along with substrate and product exchange. Such polymeric supports also facilitate tuning the catalytic activity via an external stimulus such as temperature, pH, and salt concentration (Lu et al., 2006; Li et al., 2010; Li et al., 2011; Wang et al., 2010; Irene, 2018). However, complicated preparation methods of such stimuli-responsive catalysts make them unattractive (Li et al., 2010; Li et al., 2011). Lack of complete control over activity also hinders the wide acceptability of such responsive catalysts (Lu et al., 2006; K\u00e4stner and Th\u00fcnemann, 2016; Li et al., 2011; Irene, 2018). Considering the potentials of responsive supports, a simplified method needs to be developed to improve the catalytic activity, controllability, and selectivity of embedded NMPs.Herein, a simple water-in-oil emulsion route is reported for the preparation of AgNP embedded responsive hydrogel microsphere catalyst. Such catalyst was prepared by the emulsification of an aqueous solution of Trivinylphosphine oxide (TVPO), Piperazine, and AgNO3 followed by simultaneous Michael addition crosslinking of TVPO-Piperazine and simultaneous in-situ formation of AgNPs within the emulsified micro-droplets. The composite microspheres were characterized by X-Ray diffraction (XRD), Scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Dynamic light scattering (DLS) analysis confirmed the pH-responsive behavior of the composite hydrogel-microsphere. pH-responsive swelling-deswelling of the composite microsphere was utilized to control the access to AgNPs and modulate their catalytic activity (\nFig. 1). This feature was investigated using 4-NP, Congo red (CR), and Methylene blue (MB) as model pollutants. Switching ON-OFF of the catalytic activity was demonstrated by changing the pH of the reaction medium. Substrate selectivity of the novel catalyst was also investigated using a mixture of 4-NP and MB. Finally, the reusability of the catalysts was demonstrated to highlight their practical application potential.AgNO3 (\u2265 99.0%), Sodium borohydride (NaBH4), MB, CR, 4-NP, Phosphoryl trichloride, vinyl magnesium bromide (1\u2009M in THF), dry THF, piperazine, and Span 80 were purchased from Sigma-Aldrich and used as received. Trivinylphosphine oxide (TVPO) was synthesized by a reported procedure and confirmed by NMR analysis (Nazir et al., 2020b).Hydrogel was prepared by Michael addition of TVPO (64.0\u2009mg, 0.50\u2009mmol) and piperazine (64.6\u2009mg, 0.75\u2009mmol) in 2.5\u2009mL of water (40\u2009\u00b0C for 16\u2009h) (Nazir et al., 2020b). pH-responsive swelling of the transparent hydrogel was determined by measuring the swelling ratio (SR) by the procedure reported in our previous publication (Nazir et al., 2020b) and details can also be found in Sec. S1.1.2, 5, and 10.5\u2009mol% of AgNO3 was added to 2.5\u2009mL aqueous solution of TVPO (64.0\u2009mg) and piperazine (64.6\u2009mg) kept in an ice bath. The solutions were then transferred to cuvettes, sealed, and transferred to an oven (at 40\u2009\u00b0C). All operations were carried out in dark and separate cuvettes were used for each duration. UV\u2013vis spectra of solutions were recorded at intervals.For the preparation of the responsive catalyst, the process was developed to prevent the AgNP formation on the surface of microspheres. Therefore, AgNO3 was not premixed with the Michael adducts, rather it was added after the initiation of gelation. As shown in \nFig. 2a, 0.5\u2009g of Span 80 was mixed with 20\u2009mL of cyclohexane using Ultraturax (19,000\u2009rpm, 5\u2009min). Then, a freshly prepared 5\u2009mL aqueous solution of TVPO (128\u2009mg, 1\u2009mmol) and piperazine (129.2\u2009mg, 1.5\u2009mmol) was added dropwise to this mixture under stirring (19,000\u2009rpm) to obtain a milky emulsion. Then, the centrifuge tube was covered with aluminum foil and transferred to a water bath under stirring (40\u2009\u00b0C, 500\u2009rpm). After 15\u2009min, 0.1\u2009mL aqueous solution of AgNO3 (48\u2009mg/mL) was added to the emulsion and stirred for 16\u2009h to obtain a light brown emulsion (Fig. S2). Then, the emulsion was dialyzed (24\u2009h) using ethanol as dialysate and 15 KD RC dialysis tubes. The dialysate was replaced 3 times followed by dialysis in deionized water (24\u2009h) to replace ethanol with water. This sample is named E1 (Fig. 2a) and the emulsion prepared without the addition of AgNO3 is named E0.Emulsion of TVPO (128\u2009mg, 1\u2009mmol) and piperazine (129.2\u2009mg, 1.5\u2009mmol) was prepared in a pressure reactor by the procedure as described in Section 2.3.1, and the pressure reactor was transferred to a water bath at 40\u2009\u00b0C under stirring (15\u2009min). 0.1\u2009mL aqueous solution of AgNO3 (48\u2009mg/mL) was added to the emulsion and stirred for 2\u2009h (Fig. 2b). Then, the reactor was pressurized with H2 (~2.5\u2009bar) and the stirring was continued for 14\u2009h at 40\u2009\u00b0C. The emulsion was then purified by dialysis as reported in Method 1 and the sample was named E2.UV\u2013vis spectra for in-situ AgNP formation and catalytic reaction were recorded using a Varian Cary 50 UV\u2013Vis Spectrophotometer. Catalytic reduction of 4-NP, MB, and CR were analyzed by recording the UV\u2013vis spectra of the reaction solution as a function of time and the residual concentration was determined using their corresponding UV\u2013vis calibration curves.NMR analysis was carried out Bruker AV-III 400 NMR spectrometer (Bruker Biospin AG, F\u00e4llanden, Switzerland). The 1H, 13C, and 31P NMR spectra were recorded using Bruker standard pulse on a 5\u2009mm CryoProbe\u2122 equipped with z-gradient employing 90\u00b0 pulse lengths of 11.4\u2009\u00b5s (1H), 10.0\u2009\u00b5s (13C), and 16.0\u2009\u00b5s (31P).ICP-OES analysis was used to determine the silver content (Ag-content) of samples. ICP-OES 5110 (Agilent, Basel, Switzerland) apparatus was used for these experiments. Samples preparation for ICP-OES consisted of mixing 10\u2009mg of the sample with 3\u2009mL HNO3, followed by the digestion at 250\u2009\u00b0C for 30\u2009min using microwave heating.XRD analysis was carried out in a Stoe IPDS-II instrument, operating at a voltage of 50\u2009kV and a current of 40\u2009mA with Mo Kalpha radiation (\u03bb\u2009=\u20090.71073\u2009\u00c5) at an angular range (2\u03b8) of 5\u201350\u00b0. The instrumental contribution was taken into consideration by measuring the diffraction pattern of LaB6 as the reference material and used within Topas software (Coelho, 2018).DLS analysis was carried out in a Malvern Zetasizer Nano ZS90 at 25\u2009\u00b0C to determine the particle size of hydrogel-microspheres. Before analysis, the pH of the dispersion was adjusted to the desired value and kept for 1\u2009h at 25\u2009\u00b0C to achieve an equilibrium swelling. The same samples were used to measure the \u03b6-potential using folded capillary Zeta Cell DTS1070.SEM analysis was carried out in a Hitachi S-4800 SEM equipment operating in scanning and transmission mode (30\u2009kV). For SEM images, samples were prepared by putting a drop of emulsion on a silicon wafer and evaporating the water at room temperature for 16\u2009h followed by 7\u2009nm Au/Pd coating. For transmission images, the sample was prepared by putting a drop of emulsion on a lacey carbon grid and evaporating the water over 16\u2009h at room temperature.XPS analysis was carried out on a Physical Electronics (PHI) Quantum 2000 X-ray photoelectron spectrometer equipped with a monochromatized AlK\u03b1 source (at 15\u2009kV, 28.8\u2009W), and a hemispherical electron energy analyzer fitted with a channel plate and a position-sensitive detector. The sample was analyzed with an electron take-off angle of 45\u00b0 and spectra were recorded with constant pass energy mode (46.95\u2009eV and energy resolution of 0.95\u2009eV). Spectra were processed with PHI MultiPak.14.4\u2009mg of 4-NP (0.1\u2009mmol) and 35.4\u2009mg of NaBH4 (0.94\u2009mmol) were dissolved in 12.0\u2009mL water (40\u2009\u00b0C) maintained at different pH. The reduction was initiated by adding 1.5\u2009mL of E2 (at the same pH) to the freshly prepared 4-NP-NaBH4 solution under stirring (500\u2009rpm). The reduction of 4-NP was monitored by measuring the UV\u2013vis intensity of 4-NP centered around 400\u2009nm after required dilution. Separate samples were prepared for each duration. TOF of the reaction was calculated as the moles of 4-NP reduced by a mole of Ag-atom in an hour.For MB hydrogenation, 60\u2009\u03bcL of MB solution (10.0\u2009g/L) and NaBH4 (35.4\u2009mg, 0.94\u2009mmol) were added to 12.7\u2009mL of water at desired pH. Then, 0.75\u2009mL of E2 was added to the solution and the reaction was monitored by UV\u2013vis spectroscopy after required dilution.For CR reduction, 60\u2009\u03bcL of CR solution (20.0\u2009g/L) and NaBH4 (35.4\u2009mg, 0.936\u2009mmol) were added to 12.75\u2009mL of water at desired pH followed by the addition of E2 (0.75\u2009mL) under stirring. Then the catalytic reduction of CR was monitored by UV\u2013vis spectroscopy. Both MB and CR reduction were carried out at 40\u2009\u00b0C and separate samples were prepared for each duration.The hydrogels were synthesized via previously reported Michael addition crosslinking of TVPO and piperazine (at 40\u2009\u00b0C) in presence of 2, 5, and 10.5\u2009mol% AgNO3 (Nazir et al., 2020a, 2020b). Colorless solutions turned dark red with time, indicating the formation of AgNPs (\nFig. 3 and S1). The appearance of a UV\u2013vis band of nanosilver at \u03bbmax 380\u2009nm just after 5\u2009min confirmed the formation of Ag0 nuclei at the early stage (Fig. S1d). The redshift of the band with time indicated the growth of nuclei to AgNPs (Agnihotri et al., 2014; Zhao et al., 2013). The presence of AgNO3 led to faster gelation (Fig. S1a-c) and the solution containing 10.5\u2009mol% of AgNO3 formed the gel within 45\u2009min compared to 16\u2009h for pristine hydrogel (at 40\u2009\u00b0C). Fast gelation in the case of precursor solution containing AgNO3 can be attributed to physical reasons. In-situ nanoparticle formation (Mishra et al., 2014; Hoogesteijn von Reitzenstein et al., 2016) and crosslinking of precursors lead to a rapid rise in viscosity compared to pristine precursor solution. A combination of these factors reduces the flow behavior and the solutions behave like a hydrogel.As explained earlier (Fig. 1), the swelling-deswelling behavior of the composite hydrogel is important to control the catalytic activity. As a measure of responsiveness, the swelling ratio (SR) of AgNP-hydrogel composites was determined (Eq.S1, Sec. S1.1). From Fig. 3b it can be seen that a slight decrease in deswelling was observed in composites prepared with 1.0 and 2.0\u2009mol% of AgNO3. However, composite prepared with 10.5\u2009mol% AgNO3 displayed only limited deswelling, which can be attributed to the formation of the rigid matrix due to the presence of a large number of AgNPs. Finally, gel-phase NMR analysis of composite hydrogel (Fig. S2) confirmed the absence of any side reaction in presence of AgNO3. Details of NMR analysis can be found in Sec. S2.Direct synthesis of AgNP-hydrogel composite from the precursors (TVPO and piperazine, silver salts) has simplified the catalyst preparation. The use of bulk composite as a catalyst can lead to poor catalytic activity due to diffusion limitations. To overcome such drawbacks and facilitate easy access to catalytic sites, the AgNP-hydrogel composite was prepared in the form of microspheres by an emulsion process (Fig. 2). Considering moderate Ag-content and minimum loss in swelling behavior, hydrogel-microspheres were prepared using 2.0\u2009mol% of AgNO3.The absence of AgNPs on the surface of the microsphere is important to achieve complete control over the catalytic activity of AgNPs by swelling-deswelling. Therefore, the method was developed to prevent the formation of AgNPs on the surface of hydrogel-microspheres. Particularly, premixing of AgNO3 with the Michael addition precursors was avoided, rather AgNO3 was added to the emulsion after initiation of gelation within droplets to facilitate the formation of AgNPs a few nanometers below the surface. Hydrogel-microspheres without AgNPs (E0) and with AgNPs (E1) were prepared by this method (Section 2.3.1). The light brown color of E1 indicates the presence of AgNPs within microspheres (Fig. S3). The solid content of the purified E1 in water was found to be 0.9% with an Ag-content of ~0.35\u2009wt% on dried E1 (Table S1), which is significantly lower than the calculated Ag-content of 1.1%. This can be attributed to the fast gelation of Michael precursors within the droplets. This leads to the formation of tertiary amines before the complete reduction of AgNO3. The reduction potential of tertiary amines is known to be lower than the secondary amines (Piao et al., 2011). As a result, unreduced AgNO3 is removed from E1 during the subsequent purification step.To enhance the Ag-content in the hydrogel-microspheres, H2 was introduced in the reactor (~2.5\u2009bar) 2\u2009h after the addition of AgNO3 (E2,\nFig. 2b), and the emulsion was stirred for a further 14\u2009h. H2-assisted method (E2) resulted in a much darker emulsion than E1 (Fig. S3), indicating higher Ag-content in E2. UV\u2013vis spectra of purified E0, E1, and E2 were recorded after equal dilution in water (\nFig. 4a). In the case of E0, UV-absorption was observed only below 320\u2009nm, which is consistent with our previous report (Nazir et al., 2020b). This makes easy detection of embedded AgNPs within E1 and E2 hydrogel microspheres. The absorption band of AgNPs was visible at ~415\u2009nm (Parida et al., 2020; Sirohi et al., 2019), owing to the transparency of the hydrogel matrix within this wavelength range. The higher intensity of the band at 415\u2009nm for E2 indicates a higher concentration of AgNPs within microspheres.The formation of AgNPs within microspheres was also confirmed by powder XRD of dried E1 and E2 (Fig. 4b). Diffused diffraction pattern of hydrogel (E0) was present in the XRD patterns of both E1 and E2. Diffraction peaks at 2\u03b8 of 17.7\u00b0, 20.4\u00b0, 29.0\u00b0 and 33.5\u00b0 in case of E1 and E2 can be assigned to (111), (200), (220), and (310) reflections of a face-centered cubic Ag0 crystals (Parida et al., 2020). The size of AgNPs determined using 111-plane was found to be ~8\u2009nm for both E1 and E2. Then, Ag-content in dried E2 was found to be 0.7\u2009\u00b1\u20090.04\u2009wt%, which is close to the calculated value (i.e. 1.1%, Table S1). Therefore, only E2 was selected for further characterization and catalytic study. From the solid content and ICP-OES analysis (Table S1), the Ag-content of E2 was calculated to be 0.059\u2009mg/mL.The pH responsiveness of E2 was determined by DLS analysis at different pH values. At pH 4, the particle size was 980\u2009nm and \u03b6-potential of +\u200934\u2009mV indicates its good dispersion stability at this pH (\nFig. 5a, \nTable 1). A decrease in the particle size (807\u2009nm) and \u03b6-potential (+13\u2009mV) was observed with an increase in the pH to 7. This led to poor dispersion stability of E2 and precipitation after ~2\u2009h (Fig. S4). To compare the swelling behavior of E2 with the bulk composite hydrogel, the ratio of particle volume at pH 4 and pH 7 (VpH4/VpH7) was calculated using the average particle size obtained in DLS experiments. VpH4/VpH7 of E2 was found to be 1.8, which is in agreement with the ratio of SR of the bulk hydrogel at pH 4 and pH 7 (SRpH4/SRpH7 =1.9).Increasing the pH to 10 resulted in a decrease of the \u03b6-potential to \u2212\u20096\u2009mV (Table 1), indicating the unstable nature of the dispersion (Fig. S4), and two populations of particles were observed during DLS analysis (Fig. 5a). Although the decrease in particle size of E2 is expected at pH 10, the larger particle size observed at this pH is due to the aggregation of smaller particles. Two particle populations make it difficult to compare the change in volume at pH 10 (Fig. 5a). Further increasing the \u2265\u2009pH 12, an extensive aggregation was observed with unreliable DLS results.SEM analysis of E2 was carried out both in scanning and transmission mode to determine the location of AgNPs in microspheres. As expected, no sign of AgNPs was observed during SEM analysis of E0 (Fig. 5b). SEM images of E2 showed the presence of AgNPs a few nanometers below the surface of the microsphere (Fig. 5c). Transmission images also confirmed the presence of AgNPs (13\u2009\u00b1\u20093\u2009nm, Fig. 5d, and e) few nanometers within the microsphere. The size of AgNPs determined by transmission images was found to agree with XRD analysis (~8\u2009nm). It is worth mentioning here that, absence of AgNPs on the surface of microspheres prevents any catalytic activity at its deswollen state, which offers excellent control over the activity just by swelling and deswelling of the E2.XPS analysis was carried out to determine the oxidation state of the silver in E2 (Sirohi et al., 2019; Ru\u00edz-Baltazar et al., 2018; Cheng et al., 2015). The high-resolution scan in Fig. 5f shows the presence of two peaks between 365 and 377\u2009eV due to spin-orbital splitting to Ag 3d5/2 and Ag 3d3/2 core levels. Both the peaks can be resolved into two components, with major peaks at 368.2 (3d5/2) and 374.2\u2009eV (3d3/2) assigned to Ag0. The small peaks at 369.6 and 375.3\u2009eV were assigned to oxidized silver (Ag+). Based on the peak area, the Ag0 component was found to be ~95%. Analyzing the N1s spectra, an overall shift towards lower binding energy was observed in E2 as compared to E0 (Fig. 5g). This can be attributed to an increased number of secondary amines in E2 than E0, which is in agreement with the NMR analysis. Analysis of N1s spectra confirms the absence of any AgNPs-hydrogel chemical interactions, and AgNPs are physically stabilized within the hydrogel.To study the possible reduction of 4-NP in the absence, aqueous solutions of 4-NP and NaBH4 at different pH were prepared (Section 2.5.1) and the solutions were monitored by UV\u2013vis spectroscopy (Fig. S6a). No change in the intensity of 4-NP absorption peak (at 400\u2009nm) was observed over time. This indicates a lack of 4-NP reduction in absence of E2 at all pH ranges (Table S2). Adding the required quantity of E2 to the reaction solution, the yellow color started to disappear (\nFig. 6a) along with a decrease in the intensity of the 4-NP UV-absorption peak and the appearance of a new band at ~300\u2009nm corresponding to 4-AP (Fig. 6b). The conversion and rate of reaction were determined from the intensity of the UV-absorption peak at 400\u2009nm and given in Fig. S6b and Fig. 6c. C\n\n0\n and C are the initial and residual concentrations of 4-NP at a given time, respectively.Linear correlation between \u2212ln(C/C\n\n0\n) vs. time indicates the pseudo-first-order reduction of 4-NP at all pH values (Fig. 6c), with a visible effect of pH on the rate of hydrogenation of 4-NP (Fig. 6c). The highest k value at pH 4 (0.11\u2009min\u22121) followed by a decrease with an increase in pH (Fig. 6c) indicates the effect of deswelling of hydrogel matrix surrounding the AgNPs. Increasing the pH \u2265\u200912, a very low k value (0.0006\u2009min\u20131) was observed, which signifies the lack of 4-NP hydrogenation (Fig. 6c and S6b). These observations, highlight the ability to control the activity of AgNPs by controlling the swelling of the microspheres. TOF of the 4-NP hydrogenation (at pH 4) was determined at different reaction duration (Fig. S6c) and was found to decrease with time, which is in agreement with earlier reports (Kozuch and Martin, 2012). TOF at full conversion was found to be ~170\u2009h\u22121 and ~100\u2009h\u22121 for pH 4 and 7 respectively (\nTable 2). TOFs achieved by E2 at full conversion are higher than the recently reported state of art Au (Qin et al., 2019; Fu et al., 2018) and Ag (Zhou et al., 2020; Mao et al., 2018) based catalysts.Based on the swelling behavior of composite microsphere and catalytic activity at different pH, a mechanism to explain the tunable behavior is proposed in \nFig. 7. As observed during DLS analysis, E2 reached a swollen state around pH 4, which coincides with the pH of the highest catalytic activity. This signifies the easy accessibility of AgNPs for the hydrogenation of 4-NP. Positive \u03b6-potential of E2 at this pH (Table 1) also favors the adsorption of 4-NP and BH4\n- on the catalyst. As a result, the local concentration of 4-NP and BH4\n- around the micro-environment of AgNPs remains high and facilitates high catalytic activity. A similar strategy has been used to enhance the catalytic activity of Palladium/MOF catalysts for styrene hydrogenation (Huang et al., 2016). Additionally, pores of the hydrogel can also act like nano-reactors to enhance the activity via the well-known confined space reaction (Fig. 7) (Gong et al., 2019; C\u00e1rdenas-Lizana et al., 2013). A combination of these factors led to fast hydrogenation of 4-NP. Increasing the pH of the reaction medium decreased the access to AgNPs due to the deswelling of the E2 (Fig. 7a). The decrease in \u03b6-potential (Table 1) with pH also reduces the local concentration of 4-NP. As a result, the catalytic reduction of 4-NP slowed down. Complete deswelling along with negative \u03b6-potential of E2 above pH \u2265\u200912 block the access to AgNPs and, prevents any catalytic reduction (Figs. 6c, 7a).\nE2 was also used for the reduction of pollutants of anionic (Congo red) and cationic (methylene blue) nature. The reduction pathway of carcinogenic azo dye like CR by AgNPs in the presence of NaBH4 and AgNPs is well established and shown in Fig. S7a (Rajesh et al., 2014; Nasrollahzadeh et al., 2020). The residual CR in water is quantifiable by recording the UV\u2013vis intensity of CR solution at different times (Fig S7b). Hydrogenation of CR under acidic medium (pH 4) was fast (k\u2009=\u20091.8\u2009min\u20131) with a TOF of ~124\u2009h\u20131 (\nFig. 8a, Table 2) highlighting the excellent activity of E2. The activity decreased with an increase in the pH and activity was turned OFF at pH 13 (Fig. 8a, S7c). Such behavior resembles the reduction of 4-NP and can be explained by Fig. 7a. In our previous study, we have shown that under an acidic medium this hydrogel favors adsorption of anionic dyes (Nazir et al., 2020b). Changing the pH to alkaline results in the repulsion of dye molecules from the hydrogel matrix (Nazir et al., 2020b). This behavior favors the adsorption of CR and faster degradation under an acidic medium. Repulsion between CR and hydrogel microsphere along with deswelling of E2 prevents any degradation above pH 12.During MB reduction, the highest catalytic activity was observed at pH 4 (Fig. 8b, S8) with a k value of 0.52\u2009min\u20131, which is lower than the k-value observed during the reduction of 4-NP and CR. It is due to the decreased adsorption of cationic MB by the positively charged E2 (Fig. 7b). As a result, the TOF of 27\u2009h\u20131 was observed during MB reduction at pH 4 (Table 2). On the contrary to 4-NP and CR hydrogenation, it was not possible to completely stop the hydrogenation of MB at pH 13. This can be explained as the adsorption and slow diffusion of MB molecules to reach AgNPs for catalysis owing to the negative surface charge of E2 at pH 13 (Figs. 7b and 8b).Differential catalytic reduction of cationic and anionic molecules by E2 at pH \u2265\u200912 prompted the investigation of the substrate selective activity of the novel catalyst. A mixture of MB to 4-NP (3\u2009\u00d7\u200910\u20134 mmol) maintained at pH 13 was subjected to hydrogenation. The green color of the solution slowly turned yellow (\nFig. 9c), suggesting the selective reduction of only MB. UV\u2013vis spectra of the solution also confirmed the hydrogenation of only MB (Fig. 9a, b). Interestingly, the concentration of 4-NP remained unchanged even after 3\u2009h. This can be attributed to the selective and competitive adsorption of cationic MB molecules on E2, due to the negative surface charge at this pH.To demonstrate the practical applicability, the catalyst E2 was subjected to reuse cycles using 4-NP as a substrate. E2 maintained its activity even after the fifth cycle (Fig. 9d) and activity could be turned OFF at the third cycle by raising the pH to 12. E2 recovered its activity in the fourth cycle (at pH 4.5), highlighting the reversibility of its activity. No loss in catalytic activity after the fifth reuse cycle indicates easy recovery of E2 and low loss of silver. Ag-content of 0.65\u2009\u00b1\u20090.06% determined by ICP-OES analysis after the fifth cycle confirmed a very low Ag loss (Table S1) and minimizes the risk of secondary pollution due to silver leaching from E2. Transmission images indicate hydrogel matrix successfully prevents agglomeration of AgNPs (Fig. S9), although some degree of agglomeration of hydrogel microspheres can be observed in Fig. S9. Agglomeration of microspheres can take place during sample preparation on a TEM grid or during catalyst recovery. From the catalyst reuse study, it is clear that such agglomeration is temporary and has no effect on its catalytic activity.TOF of the novel catalyst (E2) calculated after 4-NP reduction (at pH 4) is superior to most of the recently reported AgNP-catalysts (\nTable 3, Entry 2\u20136). In the case of entry 7, excellent catalytic activity was reported for AgNP based catalysts, which is attributed to their ultrafine particle size and porous support. The catalyst prepared from an alloy of gold and silver also displayed lower TOF than E2 (entry 8). AuNP based catalyst reported in entries 9 and 10 displayed significantly higher activity than E2, because of porous support that provides easy access to AuNPs and inherently high activity of AuNPs. Interestingly, E2 requires a significantly lower quantity of NaBH4 to achieve such high TOF compared to highly active catalysts listed in Table 3. Additionally, ease of controlling the activity and catalytic selectivity are distinct advantages of E2 compared to the reported catalysts.In summary, this work demonstrates a simple method to prepare AgNP based pH-responsive catalysts with excellent catalytic reduction of pollutants like 4-NP and dyes (CR and MB). The pH-responsive hydrogel support offers the possibility to control the access to AgNPs and the micro-environment around them, thereby tuning the catalytic activity. At its swollen state, AgNP-hydrogel-microsphere (E2) displayed an excellent catalytic reduction with TOF of 170\u2009h\u20131 and 124\u2009h\u20131 for 4-NP and CR respectively. Increasing the pH of the reaction media resulted in a decrease in activity and was turned OFF once the pH was above 12. The composite catalyst also displayed a selective reduction of only MB at pH 13 from a mixture of MB and 4-NP, owing to the selective adsorption of MB (cationic pollutant) due to negative \u03b6-potential at this pH. Successful recyclability of this novel catalyst makes it suitable for various practical applications. Additionally, the substrate selectivity of this catalytic system can be utilized for further development of selective catalysts using other metals.\nDambarudhar Parida: Conceptualization; Methodology; Investigation; Data curation; Formal analysis; Supervision; Validation; Visualization; Writing - original draft; Writing - review & editing. Eva Moreau: Investigation; Data curation; Formal analysis. Rashid Nazir: Investigation. Khalifah A. Salmeia: Formal analysis; Writing - review & editing. Ruggero Frison: Investigation. Ruohan Zhao: Investigation. Sandro Lehner: Investigation. Milijana Jovic: Investigation. Sabyasachi Gaan: Methodology; Project administration; Resources; 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.The authors thank Dr. Daniel Rentsch for the NMR analysis. We appreciate the support of Mr. Ton Markaj for ICP-OES analysis and the support of Dr. Roland Hauert for XPS analysis. The NMR hardware was partially granted by the Swiss National Science Foundation (SNSF, grant no. 206021_150638/1). The manuscript was written through the contribution of all and all authors have approved the final version of the manuscript.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jhazmat.2021.126237.\n\n\n\nSupplementary material.\n\n\n\n.", "descript": "\n                  A simple method is reported for the preparation of silver nanoparticle (AgNP) embedded pH-responsive hydrogel microparticle catalyst via Michael addition gelation and in-situ silver nitrate (AgNO3) reduction. The AgNP-hydrogel microsphere exhibited an efficient reduction of pollutants like 4-Nitrophenol (4-NP) and Congo red (CR) under acidic medium with turn over frequency (TOF) of ~170\u00a0h\u20131 and ~124\u00a0h\u20131 respectively. Interestingly, the activity of the catalysts was turned-OFF under a basic medium (\u2265 pH 12) due to the deswelling pH-responsive matrix surrounding the AgNPs. On the contrary, turning-OFF the hydrogenation of a cationic pollutant like methylene blue (MB) using high pH (\u2265 12) was not possible, due to ionic interaction of MB molecules with the negatively charged catalyst at this pH. This feature was used to demonstrate selective hydrogenation of only MB from a mixture of 4-NP and MB. Finally, five recycling steps confirmed the reusability and practical application potential of the catalyst.\n               "}
{"full_text": "Hydrocracking of long-chain paraffins into transportation fuels constitutes one of the most versatile processes in the petrochemical industry. It involves the cracking and isomerization of hydrocarbons and is primarily used to obtain high-quality middle distillates. Hydroisomerization refers to processes in which the branching occurs with very limited cracking. It is used to improve the cold-flow properties of diesel fuel and to obtain high-octane gasoline blending components and lube oils with good cold-flow properties. These hydroconversion processes follow a bifunctional mechanism in which a dehydrogenation/hydrogenation (metal or metal sulphide sites) function is combined with acid-catalyzed isomerization and cracking functions [1].The hydrocarbon product distribution depends amongst other on the balance and (spatial) intimacy of these two functions, giving rise to \u201cideal\u201d vs. \u201cnon-ideal\u201d hydrocracking. Shape selectivity refers to the influence of the pore size in which the cracking reactions take place [2\u20134]. In the classical interpretation of the hydrocracking mechanism [5,6], n-alkane is dehydrogenated on a metal site to the corresponding n-alkene, which desorbs and diffuses to Br\u00f8nsted acid sites where it further reacts via carbenium ion chemistry. This includes skeletal rearrangements and \u03b2-scission (carbon\u2011carbon bond cleavage) reactions. The rate of the \u00df-scission step depends on the degree of isomerization previously attained, i.e., alkylcarbenium intermediates with a larger number of branches will crack faster. The product of the primary \u03b2-scission reaction can undergo additional transformations, including a secondary \u00df-scission event if diffusion of the intermediate to hydrogenation sites is too slow. Thus, desorption, diffusion and the relative location of the hydrogenation to the acid function play a role in the distribution of the cracked products. Typical hydrogenation functions include metals such as Pt, Pd and Ni, because they provide sufficient dehydrogenation activity, although with a tendency to paraffin hydrogenolysis in the case of Ni and Pt [7,8]. The acidic component is usually provided by a zeolite or amorphous silica-alumina. While these solids can also serve as a support for the hydrogenation function, composite catalysts may also contain other supports such as \u03b3-alumina on which the metal phase can be dispersed. Strongly acidic zeolites are favoured for hydrocracking purposes, while supports with a limited acidity are more selective towards isomerization. Zeolites contain acid sites located in micropores with dimensions in the order of the size of hydrocarbons to be converted, inducing specific shape selectivity. Amorphous silica-alumina typically has larger mesopores, which are arranged in a random manner compared to the micropore topology of zeolites. Ordered mesoporous silicas such as MCM-41, which are acidic by introducing aluminum in the amorphous silica network, have also been used for hydrocracking purposes to establish the effect of a uniform pore size and ordered pore arrangement on cracking reactions [9]. The industrial relevance of such ordered mesoporous materials is, however, limited because of the high cost associated with the organic templates used in their preparation as well as their low hydrothermal stability. While MCM-41 presents a tubular pore system, MCM-48 exhibits a three-dimensional system of mesopores. SBA-15 is another ordered mesoporous silica with a similar hexagonal structure as MCM-41 but with larger pores, thicker silica walls and, therefore, higher hydrothermal stability [10,11].In the present work, we compared the performance of different ordered mesoporous silicas (SBA-15, MCM-41, MCM-48) and amorphous silica-alumina (ASA), which contains disordered mesopores, as acidic supports for the hydroconversion of n-hexadecane (n-C16) in a trickle-bed microflow reactor. The aim was to understand the influence of the size and order of the (meso)pores on the product distribution. Acidity was introduced into the ordered mesoporous silicas by aluminum either in the synthesis step or by post-synthesis alumination of the calcined silica materials. The acidic, textural and morphological properties of the porous materials were characterized by elemental analysis, X-ray diffraction (XRD), N2 porosimetry, transmission electron microscopy (TEM), solid state NMR spectroscopy and IR spectroscopy. The Pd metal phase was characterized by H2 chemisorption and IR spectroscopy of adsorbed CO.A range of SBA-15 samples with a target Si/Al ratio of 20 were synthesized under acidic conditions, employing HCl solutions with pH\u00a01.0, 1.5, 1.6, 1.7, 2.0, and 2.5. In a typical procedure based on a literature recipe [12], a solution A was prepared by dissolving 2\u00a0g Pluronic P123 in 70\u00a0ml in the acid solution followed by stirring at 313\u00a0K for 6\u00a0h. A second solution B was obtained by dissolving 0.22\u00a0g aluminum triisopropoxide and 3.2\u00a0ml tetramethylorthosilicate (TMOS) in 5\u00a0ml of the acid solution, followed by stirring at room temperature for 2\u00a0h. Solution B was then added to solution A, followed by further stirring at 313\u00a0K for 20\u00a0h. The resulting suspension was transferred to a Teflon-lined stainless-steel autoclave, which was then sealed and heated at 373\u00a0K for 48\u00a0h. The resulting materials are denoted according to the pH of the starting solution as P1.0, P1.5, P1.6, P1.7, P2.0 and P2.5. An siliceous MCM-41 sample was prepared according to literature [13], by dissolving 3.8\u00a0g tetramethyl ammonium hydroxide solution (TMAOH, 25% wt in water) and 4.6\u00a0g cetyltrimethyl ammonium bromide (CTAB) in 34.1\u00a0g water. After stirring at 308\u00a0K for 1\u00a0h, 3.0\u00a0g fumed silica was added and the gel was further stirred at room temperature for 20\u00a0h. The gel was transferred to a Teflon-lined stainless-steel autoclave and heated at 423\u00a0K for 48\u00a0h. A siliceous MCM-48 sample prepared according to Ref. [14] was synthesized by dissolving 2.14\u00a0g fumed silica in 30\u00a0g a 10\u00a0wt% cetyltrimethylammonium hydroxide solution in water (CTAOH). After stirring at room temperature for 2\u00a0h, the gel was transferred to a Teflon-lined stainless-steel autoclave and heated at 408\u00a0K for 24\u00a0h. After the hydrothermal synthesis step, the solids were filtrated and washed with deionized water. All these samples were dried overnight at 373\u00a0K and the template was removed in a next step by calcination at 773\u00a0K for 10\u00a0h (SBA-15 samples) and 823\u00a0K for 6\u00a0h (MCM-41 and MCM-48 samples).The siliceous MCM-41 and MCM-48 samples were aluminated by a dry alumination method [15,16]. For this purpose, 2.0\u00a0g calcined MCM-41 was dispersed in 50\u00a0ml of dry n-hexane. Solutions containing 0.17\u00a0g (0.11\u00a0g) aluminum isopropoxide in 150\u00a0ml of n-hexane were prepared to obtain materials with Si/Al ratios of 40 (60) (denoted hereafter as M41\u201340 and M41\u201360). The MCM-41 suspension was added to the aluminum-containing solution and the mixture was stirred at room temperature for 24\u00a0h. For MCM-48, 1\u00a0g silica was directly added to a solution containing 0.06\u00a0g of aluminum isopropoxide in 50\u00a0ml of n-hexane and the resulting dispersion was further stirred at room temperature for 24\u00a0h to obtain a Si/Al ratio of 60 (sample M48\u201360). The solids were recovered by filtration and washed with n-hexane. The samples were dried overnight at 373\u00a0K and subsequently calcined at 823\u00a0K for 4\u00a0h. A commercial ASA (amorphous silica alumina 75/25 w/w from supplier JGC) was used as received.The elemental composition of the solids was determined by ICP-OES (Spectro CirosCCD ICP optical emission spectrometer). X-ray diffraction (XRD) patterns were recorded on a Bruker D2 Endeavor diffractometer using Cu K\u03b1 radiation with a scanning speed of 0.02 o/s in the 2\u03b8 range of 0\u201310 o. N2 adsorption and desorption isotherms were measured at 77\u00a0K on a Micromeritics TriStar II 3020 instrument. For this purpose, 100\u00a0mg of the sample were transferred in a glass sample tube, followed by drying at 393\u00a0K in a N2 flow overnight.IR spectra were recorded in the range of 4000\u2013400\u00a0cm\u22121 using a Bruker Vertex V70v Fourier-transform infrared spectrometer. CO IR spectroscopy was used to probe the acid and metal sites. To evaluate the acidity, the sample was cooled to ~90\u00a0K and CO was introduced into the cell via a sample loop connected to a six-port valve. After each CO dosage, a spectrum was recorded. For metal site determination, a similar procedure was followed with the difference that the samples were reduced before IR characterization carried out at 303\u00a0K. Reduction was performed in pure hydrogen by heating to 673\u00a0K at a rate of 3\u00a0K/min, followed by a dwelling time of 1\u00a0h. The sample was evacuated until a residual pressure of 1\u00a0\u00d7\u00a010\u22125\u00a0mbar was reached. The sample was cooled to 303\u00a0K and CO was introduced into the cell.For IR spectroscopy of adsorbed pyridine, the probe molecule was introduced from an ampoule at its vapor pressure at room temperature. After exposure of the dehydrated sample to pyridine for 10\u00a0min, the cell was evacuated and a spectrum was recorded. Further spectra were recorded after outgassing for 1\u00a0h at 423\u00a0K, 573\u00a0K and 773\u00a0K. The acidic properties were also characterized by H/D exchange of the hydroxyl groups with deuterated benzene (C6D6) followed by IR spectroscopy. In a typical experiment based on literature [17], C6D6 was dosed to the dehydrated sample compartment from a manifold until a pressure of 10\u00a0mbar is reached, which corresponds to a total amount of 0.33\u00a0mmol (sample density 7.5\u00a0mg/cm2). The sample was exposed to C6D6 for 10\u00a0s, followed by evacuation for 1\u00a0h. The final pressure was lower than 10\u22126\u00a0mbar. Then, a spectrum of the partially exchanged sample was recorded. This sequence was automatically repeated with exposure times of 30\u00a0s, 5\u00a0min, 10\u00a0min, 20\u00a0min, and 30\u00a0min at 303\u00a0K, 30\u00a0min at 323\u00a0K, 30\u00a0min at 373\u00a0K, 30\u00a0min at 423\u00a0K and 30\u00a0min at 523\u00a0K.\n27Al nuclear magnetic resonance (NMR) spectra were recorded on a 11.7\u00a0T Bruker DMX500 NMR spectrometer operating at 132\u00a0MHz. Transmission electron microscopy (TEM) images were taken using a FEI Tecnai 20 at an acceleration voltage of 200\u00a0kV. The samples were suspended in ethanol and dispersed over a holey Cu grid coated with carbon film.H2 uptake measurements were used to determine the metal surface area in reduced catalysts. Typically, 50\u00a0mg sample was loaded in a quartz reactor. Prior to dosing, samples were reduced in flowing H2 (1\u00a0h, 673\u00a0K, 3\u00a0K/min), evacuated at 723\u00a0K for 1\u00a0h to remove chemisorbed hydrogen and cooled to 353\u00a0K under vacuum. Chemisorption analysis was then carried out at 353\u00a0K.The solid acids were loaded with 1\u00a0wt% Pd by wet impregnation with an aqueous Pd(NH3)4(NO3)2 solution. The resulting samples were calcined at 723\u00a0K in flowing air for 4\u00a0h. In order to perform n-C16 hydroconversion activity measurements, the catalyst was dried in the reactor at 1\u00a0bar and 473\u00a0K for 1\u00a0h in a He flow and subsequently reduced at 60\u00a0bar in a H2 flow. During reduction, the temperature was increased at a rate of 3\u00a0K/min to 673\u00a0K followed by an isothermal period of 1\u00a0h. Then, the temperature of the catalyst bed was lowered to 473\u00a0K and the packed bed was wetted by maintaining a liquid flow rate of 1\u00a0ml/min for 10\u00a0min. The reactor was operated at a H2/n-C16 molar ratio of 20 and a weight hourly space velocity (WHSV) of 10 gn-C16 gcat\n\u22121\u00a0h\u22121. The reaction temperature was increased stepwise and the reaction was equilibrated for 3\u00a0h before product sampling. The reactor effluent was analyzed by gas chromatograph equipped with an RTX-1 column and a flame ionization detector. The identification of isomers and cracked fractions was done in accordance to the elution sequence reported in the literature [18]. Due to the large number of products observed, monobranced (ex. 3-methylpentadecane or 4-methylpentadecane) and multibranched (ex. 2,13-dimethyltetradecane or trimethyltridecanes) C16 isomers were lumped as \u2018C16 isomers\u2019, while the fractions including normal paraffins from methane to n-pentadecane and their corresponding isomers were lumped as \u2018cracked products\u2019.The composition of the calcined materials as determined by ICP analysis is presented in Table 1\n. The Si/Al ratios of the SBA-15 samples are lower with increasing pH of the synthesis solution. Small-angle XRD patterns (Fig. 1\n, left) show that SBA-15 samples with a low Al content obtained at a pH below 2 exhibit three well-resolved peaks related to 100, 110 and 200 planes, characteristic of the p6mm hexagonal symmetry of SBA-15 [11]. From the a\n0 value of about 12\u00a0nm and the pore size distribution derived from porosimetry, a wall thickness between 2.4\u00a0nm and 4\u00a0nm was determined for these samples [19]. The absence of low-angle features in the silica materials prepared at pH\u00a02 and higher show that the samples with a higher Al content do not contain ordered mesopores. The isoelectric point of silica is around 2. This means that, when SBA-15 is prepared at a pH of 2 or higher, there are not enough positively charged protonated hydroxyl groups that can interact with the poly(ethylene oxide) groups of the Pluronic P123 mesoporogen. As a consequence, mainly disordered silica is obtained as discussed before [20,21]. In general, a pH lower than 1.5 is needed to obtain a sufficiently charged silica surface to assemble a well-ordered silica-polymer mesophase using Pluronic P123 [22].The MCM-41 sample with a low Al content (M41\u201360) shows the relevant diffraction peaks related to 100, 110, 200 and 210 planes of the p6mm hexagonal symmetry [13,23]. At higher Al content (sample M41\u201340), the intensity of these peaks is lower, indicating a lower order of mesopores. This phenomenon, which has been mentioned in the literature [24,25], is likely related to a decrease in the long-range order at higher Al loading, although the local hexagonal order of the material is largely maintained. The XRD pattern of M48\u201360 contains the typical diffraction peaks associated with 211, 220 and 420 planes, which is due to the Ia3d cubic structure [16]. As expected, the walls of the MCM-41 (1.3\u00a0nm) and MCM-48 (1.2\u00a0nm) samples are thinner than those of SBA-15. The physisorption isotherms of these samples were of type IV, typical of mesoporous materials (Fig. 2\n). The presence of sharp pore filling/emptying steps within a narrow p/p\n0 range for SBA-15 and MCM-41 materials is indicative of uniform cylindrical pores, presenting an H1 hysteresis loop [12,23]. The MCM-41 samples prepared using CTAB as structure-directing agent normally have pores in the range of 2\u20135\u00a0nm with relatively closed hysteresis loops in comparison to the ones observed in SBA-15 materials in which capillary condensation is more pronounced due to the larger pore size [26,27].The NL-DFT method was used to calculate the pore size distribution from the adsorption branch of the isotherms (Fig. S1 of supporting information). Increasing pore sizes were obtained for SBA-15 samples containing more Al. For example, the Si-SBA-15 sample (P1.0) has an average pore size of 7.9\u00a0nm. Addition of more Al to the gel to arrive 0.25\u00a0wt% (P1.5) caused the pores to swell to a size of 8.8\u00a0nm. A higher Al content (i.e., comparing 0.48\u00a0wt% and 0.80\u00a0wt%, samples P1.6 and P1.7) led to pores larger than 9\u00a0nm. This behavior has been observed before [19] and can be explained by the swelling properties of the micellar arrays by isopropanol obtained by hydrolysis of the Al precursor. A comparison of the textural properties of M41\u201340 and M41\u201360 (Table 1, Fig. S1) shows that alumination by the dry-grafting method causes the pores to shrink, decreasing the pore volume and surface area [28]. The MCM-48 sample shows a sharp pore filling step in the p/p\n0 range from 0.20 to 0.35, suggesting a good mesostructural order with uniform pore channels and a relatively narrow pore size distribution [29]. A weak hysteresis loop between p/p\n0 values of 0.40 and 0.80 points to capillary condensation in secondary mesopores arising from interparticle voids [30,31]. ASA, on the other hand, exhibits an isotherm with a broad pore filling profile and a hysteresis loop in the p/p\n0 range from 0.43 and 1.00, which points to pores with broad size distribution as might be expected for this type of material. The pore size distribution ranges between 2\u00a0nm and 60\u00a0nm. The much wider pores result in lower surface area and pore volume in comparison to the ordered mesoporous samples.\nFig. 3\n shows representative TEM images of the mesoporous materials. The images of samples P1.5, P1.6 and P1.7 confirm a morphology consisting of regular arrays of long cylindrical channels with a uniform hexagonal arrangement along the 100 plane [12,19,32]. The mesopores are not always running in a straight way through the matrix, but they can be curved to some extent. It is also evident that sample P1.7 (Fig. 4\n-f) exhibits regions where the characteristic features of SBA-15 are not present, indicating a lower order of the mesopores. In spite of the absence of low-angle features in the XRD patterns, samples P2.0 and P2.5 still preserve areas with uniform pores, which are consistent with the shapes of the adsorption isotherms presented in Fig. 2 and the uniform pore size distribution observed in Fig. S1. Nevertheless, the ordered hexagonal pore system is not present to the same extent as in the other SBA-15 materials prepared at lower pH. The M41\u201360 sample (Fig. 3-i) displays mesopores arranged in a hexagonal honeycomb-like structure, separated by thin amorphous silica pore walls [9,33].The image of M41\u201340 shown in Fig. 3-j reveals that an irregular pore arrangement of cylindrical pores can still be observed, making apparent that the incorporation of aluminum into the framework affects the long-range order of the mesopores without affecting the mesoporous nature of the material [34]. MCM-48 (Fig. 3 k-l) has been reported to present uninterrupted channels along the 100 and 111 planes, but due to changes in the curvature of the material, no channels are observed in other directions such as the 110 plane [35].The coordination environment of aluminum in the samples was investigated by 27Al MAS NMR spectroscopy (Fig. 4). The spectra are characterized by a strong peak at 55\u00a0ppm due to tetrahedrally coordinated aluminum (AlIV), indicating that a significant fraction of Al atoms has been grafted as tetrahedral species on the silica of the mesoporous materials. Part of these Al species may also be in the silica network due to the calcination procedure. An additional weak feature at 0\u00a0ppm can be associated to Al species in octahedral coordination (AlVI) [33]. The Al speciation for the investigated materials is given in Table 1. The AlIV fraction of all the samples lies between 0.54 and 0.66. As expected, no Al was incorporated in the SBA-15 samples synthesized at pH\u00a01 [36].IR spectroscopy of adsorbed CO at 90\u00a0K was used to determine the density and strength of Br\u00f8nsted acid sites (BAS). The carbonyl stretching region of IR spectra for samples M48\u201360 (left) and P1.7 (right) is displayed in Fig. 5\n. The band at 2138\u00a0cm\u22121 belongs to physisorbed CO and the one at 2156\u00a0cm\u22121 to CO coordinating to silanol groups. Br\u00f8nsted acid sites (BAS) are represented by the band at 2174\u00a0cm\u22121. Previous work has shown that this feature is a composite of bands due to a small amount of strong BAS at 2178\u00a0cm\u22121 and a larger amount of relatively weak BAS at 2172\u00a0cm\u22121 [37]. The weak feature observed at 2190\u00a0cm\u22121 is due to weak Lewis acid sites (LAS).Deconvolution of the peak related to BAS according to a procedure described before [38] provides an estimate of the population of these two types of BAS [39,40]. Spectra of sample P1.7 at different CO coverages are given in the supporting information. Table 2\n shows that the concentration of strong BAS increases with the Al content for the SBA-15 samples. The concentrations are however very low, explaining why CO-perturbed bridging OH groups at 3300\u00a0cm\u22121 cannot observed as a distinct feature for these samples [41]. IR spectra of adsorbed pyridine followed by evacuation at 423\u00a0K, 573\u00a0K and 773\u00a0K for the SBA-15 samples are presented in Fig. 6\n. The bands at 1545\u00a0cm\u22121 and 1455\u00a0cm\u22121 are assigned to pyridine adsorbed on, respectively, BAS and LAS, while the band at 1490\u00a0cm\u22121 can be associated with both types of adsorbed pyridine [41,42]. The acid site concentrations derived from pyridine IR are listed in Table 2. In general, the SBA-15 samples contain small amounts of BAS in accordance with the results obtained with CO IR spectroscopy. The absence of a feature related to BAS in these spectra recorded after evacuation at 773\u00a0K shows that only minor amounts of strong BAS are present in these samples.To determine the presence of small amounts of strong BAS in a semi-quantitative manner, H/D exchange of OH groups with deuterated benzene was followed by IR spectroscopy. The OD region for samples M48\u201360 (left), P1.7 (center) and the OH region for P1.7 (right) are shown in Fig. 7\n. All the samples present a clear peak at 2683\u00a0cm\u22121, corresponding to selective H/D exchange of bridging hydroxyl groups, even at short exposure times (10\u201330\u00a0s) for materials with high aluminum content.These samples also display an additional feature at 2632\u00a0cm\u22121, related with the exchange of bridging OH groups exhibiting additional electrostatic interactions to adjacent oxygens [43] or bridging OH interacting instead with hydrogen from residual water [17], which become predominant at increasing exposure temperatures for all the samples. The H/D exchange of weak silanol groups was also observed as a band developing at 2752\u00a0cm\u22121. The analogous bands of bridging OH groups located in the OH region at 3638\u00a0cm\u22121 and 3570\u00a0cm\u22121 [44] cannot be observed in the IR spectra because of the broad OH band at 3746\u00a0cm\u22121 (Fig. 6 top, right). The concentration of strong BAS was determined from the spectra after H/D exchange at 323\u00a0K for 30\u00a0min (Table 2), using a molar extinction coefficient of 2\u00a0\u00d7\u00a0106\u00a0cm/mol [17].For n-C16 hydrocracking activity, the samples were loaded with ~1.0\u00a0wt% Pd (Table 2). The atomic Pd/H+ ratios were between 1.6 and 82.2, indicating that there should be enough metal function to ensure an appropriate supply of olefins to the acid sites and to establish acid-catalyzed isomerization and cracking reactions as the rate-limiting steps [45,46]. The dispersion of the Pd metal phase was determined by H2 chemisorption (i.e., the ratio between irreversibly adsorbed hydrogen (Hirr) and the total amount of Pd [47]). We assumed a Hirr/Pds\u00a0=\u00a01 stoichiometry for the Pd nanoparticles [48]. The Pd dispersion is below 0.5 for all samples and Pd/ASA exhibits the lowest Pd dispersion. The Pd particle size distribution was determined in more detail by TEM for the SBA-15 and MCM-41/MCM-48 samples (Fig. S2). Relatively uniformly sized Pd particles were observed for these samples with average sizes ranging from 3.0\u00a0\u00b1\u00a00.7\u00a0nm for sample P1.7 to 11.3\u00a0\u00b1\u00a03.4\u00a0nm for P1.6. Based on the average pore size determined by Ar porosimetry, we estimate the fraction of Pd particles located inside the mesopores. For SBA-15, the majority of Pd particles have sizes smaller than the pore size (Table S1), except for P1.5 and P1.6 for which only, respectively, 15% and 30% of the Pd particles are smaller than the pore size. On the contrary, the Pd particles in the M41\u201340, M41\u201360 and M48\u201360 sample are mostly larger than the mesopores, indicating that most of the Pd particles are located on the external surface of the M41S silicas.We also studied by IR spectroscopy the adsorption of CO on metallic Pd at 303\u00a0K. Before CO dosing, the samples were reduced at 673\u00a0K and evacuated to a pressure lower than 10\u22125\u00a0mbar. An example spectrum of CO adsorbed on sample P1.7 (Fig. S4) shows a broad low-frequency signal between 1700 and 2000\u00a0cm\u22121 and a high-frequency signal between 2000\u00a0cm\u22121 and 2150\u00a0cm\u22121, generally assigned to CO bridged and linear chemisorbed to Pd, respectively [49]. Deconvolution of the IR bands was carried out following literature [50]. The peak at 1895\u00a0cm\u22121 has been attributed to \u03bc2-bridge-bonded CO on the (100) plane of Pd [51]. The 1930\u00a0cm\u22121 peak is due to \u03bc2-bridge-bonded CO on (111) planes and the ones at 1961\u00a0cm\u22121 and 1976\u00a0cm\u22121 relates to bridge-bonded CO on Pd particle edges and steps [52,53]. The signal located at 2074\u00a0cm\u22121 is assigned to linear CO bound to (111)/(111) and (111)/(100_ edges sites and the one at 2091\u00a0cm\u22121 is due to CO residing on corner Pd atoms [50]. Bridge-to-linear adsorbed CO molar ratios (B/L) calculated after saturation with CO are presented in Table 2 using molar extinction coefficients of 4.1\u00a0\u00d7\u00a0106\u00a0cm/mol (bridge species) and 0.36\u00a0\u00d7\u00a0106\u00a0cm/mol from literature [54]. B/L values are in line with the dispersion trend following from H2 chemisorption, i.e., samples with high dispersion present a lower amount of CO bound on bridge sites.The hydroconversion of n-C16 was carried out at a weight hourly velocity of 10 gn-C16 gcat\n\u22121\u00a0h\u22121, a total pressure of 60\u00a0bar and a H2/n-C16 ratio of 20. The n-C16 conversion for the investigated samples is shown as a function of the temperature in Fig. 8\n. For all sample families with the exception of M41\u201340, the activity correlates with the Al content.The apparent activation energies fall in the range of 169\u2013220\u00a0kJ/mol, which is higher than typical values for catalysts containing amorphous silica-alumina as the acidic component [55]. Both skeletal isomers and cracked alkanes were obtained as reaction products (Fig. S5). Overall, the highest isomers yield was obtained for the ASA-based catalyst. For the SBA-15 catalysts, the isomers yields were very similar as a function of conversion with the exception of P1.7, which presented a much lower isomers yields (more cracking). The other samples presented lower isomers yields in the order M41\u201340\u00a0\u2248\u00a0M41\u201360\u00a0>\u00a0M48\u201360.\nFig. 9\n (Fig. S5) shows the molar ratio between cracked (C) and total (T) products extrapolated to zero n-C16 conversion [56,57], denoted by the parameter (C/T)x\u21920, as a function of Al content. In ideal hydrocracking and at low conversion, monobranched isomers should be dominant products without formation of cracking products, i.e., (C/T)x\u21920 should approach zero. There is a strong relation between the initial formation of cracked products and the structure of the materials. (C/T)x\u21920 decreases with increasing Al content for the SBA-15 samples. This is counterintuitive, because hydrocracking is expected to be ideal for low acidity materials. The observed trend cannot be directly related to the Pd dispersion (cf. Table 2). Instead, we attribute the decreasing cracking tendency with increasing Al content to the loss of order of mesopores in these samples. The lowest (C/T)x\u21920 values are obtained for the disordered samples prepared at a pH of 2 or higher. A similar trend with respect to pore ordering can be observed for the MCM-41/MCM-48 catalysts. Especially, the catalyst presenting long cylindrical pores (M41\u201360) exhibits higher (C/T)x\u21920 than M48\u201360 with interconnected channels. This behavior, which is likely enhanced by the low Pd dispersion of the latter sample, relates to the longer residence of olefinic intermediate in one-dimensional ordered pore systems, resulting in enhanced cracking. Partial disorder of the mesopores will decrease the diffusion length inside these pores. We further investigated whether improving the metal function would affect the product distribution. For this purpose, we added 0.5\u00a0wt% Pt to the P1.7 sample by a follow-up impregnation procedure using an aqueous H2PtCl6*6H2O solution and calcination at 723\u00a0K in flowing air for 4\u00a0h. This catalyst is denoted as P1.7(+Pt) in Fig. 10\n. For this sample, the (C/T)x\u21920 ratio decreased to the same values obtained for the disordered materials prepared at higher pH, indicating that in the original P1.7 sample the metal function was not strong enough relative to the diffusion lengths.\nFig. 10 shows the n-C16 conversion as a function of temperature for the P1.7 and P1.7(+Pt) samples. It can be observed that the addition of Pt results in a significantly higher n-C16 conversion. The apparent activation energy was lowered from 220\u00a0kJ/mol to 154\u00a0kJ/mol, approaching the values reported in the literature [55]. Together with the much higher isomers yields observed for P1.7(+Pt), we conclude that the performance of P1.7 was limited by the metal function. We also estimated the average number of acid-catalyzed steps involved in n-C16 hydroconversion extrapolated to zero conversion (nas, x\u21920, Table S1), considering the selectivity of monobranched/multibranched isomers and cracked products. It takes one acid-catalyzed step to form monobranched isomers, 2 and 3 for dibranched and tribranched isomers, respectively, and typically 4 for cracking. Therefore, nas, x\u21920 would approach unity for an ideal hydrocracking catalyst. Conversely, a catalyst operating outside this regime, i.e. with a poor balance of hydrogenation and cracking sites or presenting severe shape selectivity effects will present higher nas, x\u21920 values, indicating consecutive isomerization and cracking of the more reactive multibranched isomers already at low conversion [57]. Values of nas, x\u21920 around 1.4 and 1.6 seem to indicate that the samples do not suffer from shape selectivity or a poor metal/acid balance. Nevertheless, the values are slightly higher for the samples with ordered mesopores.We also observed that the distribution of monobranched isomers at low and 50% n-C16 conversion is very similar for all samples (Fig. S6). At both conversion levels, the product distribution is dominated by 7-methylpentadecane (7-meC15) and 8-methylpentadecane (8-meC15) with lower amounts of other isomers. At a n-C16 conversion of 50%, nonetheless, the amounts of 7-meC15 and 8-meC15 are less pronounced and the concentration of other isomers increases.The product distribution of cracked hydrocarbons is plotted in Fig. 11\n. Samples supported on supports with a well-ordered mesopore structure present higher cracking selectivity than samples with a disordered mesopore structure (i.e., ASA, M41\u201340, P2.0 and P2.5). In general, all Pd-loaded catalysts exhibit an asymmetric distribution of cracked products shifted to lighter products with C4-C6 being the dominant products. Cracking is more substantial for samples with ordered mesopores in comparison with samples having disordered pore systems. Adding Pt to P1.7 also leads to a lower degree of cracking and a cracked product distribution corresponding more to the one expected for ideal hydrocracking. These differences suggest that some degree of secondary cracking takes place, which associated with an increased residence time of the olefinic intermediates as discussed previously [58]. It is evident from the results that, in the case the metal hydrogenation function is not strong enough, the product distribution is influenced not only by the order of the pores, but also by the concentration of acid sites. The occurrence of secondary cracking suggests that the diffusion of hydrocarbons plays an important role in the reaction mechanism, especially inside the long cylindrical pores of SBA-15 and MCM-41 based catalysts [59].It appears also that the addition of Pt as a stronger hydrogenation function can also impact the product distribution. The observation that Pt addition also increases the overall n-C16 conversion implies that the dehydrogenation function of P1.7 is not strong enough. This may be because Pt presents a stronger hydrogenation activity or Pt is more readily dispersed than Pd. While this may be due to the low Pd dispersion in the P1.7 sample, this result shows that a too low hydrogenation function leads to overcracking in 1-dimensional mesopores. Wang et al. earlier showed the sensitivity of the product distribution on the location, size and shape of Pt nanoparticles [60].In this work we obtained insight into the role of size and order of the pores of (ordered) mesoporous silica (SBA-15, M41S and ASA) on the bifunctional hydrocracking of n-hexadecane. A series of Al-modified ordered mesoporous SBA-15, MCM-41 and MCM-48 and disordered ASA were employed as the acid component for the bifunctional catalysts, which were also loaded with Pd as the hydrogenation component. SBA-15 materials were prepared at different pH, which was found to simultaneously influence the Si/Al ratio and order of mesopores. Al was introduced in MCM-41 and MCM-48 by a post-synthesis grafting method. All materials including ASA exhibited low acidity compared to zeolites. A general trend is that increasing Al incorporation (alumination for M41S, higher pH synthesis for SBA-15) led to a loss of ordering of the mesopores. The Pd metal phase was Pd sites homogeneously dispersed as ~10\u00a0nm particles. In n-C16 hydroconversion, it was observed that secondary cracking is more pronounced for catalysts containing long one-dimensional cylindrical pores (SBA-15 and MCM-41) than for catalysts containing a three-dimensional ordered or disordered mesoporous texture, which can be attributed to the difference in residence time of intermediates in the mesopores. From the observation that secondary cracking increased for lower Pd dispersion, it is inferred that the distance of acid sites in the mesopores and the metal phase mainly located outside these pores also plays a role. This was further corroborated by adding Pt to a mesoporous Pd/SBA-15 sample, which improved the overall catalytic activity and decreased secondary cracking reactions. Ideal hydrocracking operation is approached for ASA, MCM-48, and SBA-15 prepared at a high pH with a disordered mesopore system.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 financially supported by Shell Global Solutions International B.V. The authors thank A.M. Elemans-Mehring for ICP-OES analysis and the Soft Matter Cryo-TEM Research Unit of Eindhoven University of Technology for access to TEM facilities. The authors thank Arno van Hoof, Tobias Kimpel and Jiadong Zhu for TEM analyses.\n\n\n\nSupporting information\n\nImage 1\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.fuproc.2022.107259.", "descript": "\n                  The catalytic performance in n-hexadecane hydrocracking of a set of bifunctional catalysts composed of acidic mesoporous silicas, i.e., SBA-15, MCM-41, MCM-48, and amorphous silica-alumina (ASA), and Pd as acid and (de)hydrogenation components, respectively, was investigated. The selectivity to cracked products and the occurrence of secondary cracking depended on the pore topology, acidity, and Pd dispersion. The Si/Al ratio and the mesopore order of SBA-15 were modified by changing the pH in the synthesis step. Al was introduced in the M41S materials by post-synthesis grafting. All materials including ASA exhibited low acidity compared to crystalline zeolites. Increasing Al content led to a decrease of the order of mesopores. Secondary cracking of n-hexadecane was more pronounced for catalysts containing long one-dimensional cylindrical pores (SBA-15 and MCM-41) in comparison with catalysts containing three-dimensional ordered (MCM-48) or disordered (ASA) mesopores. The selectivity difference is attributed to differences in residence time of intermediates in the mesopores. The distance between acid sites located in mesopores and Pd nanoparticles primarily located outside these pores also influences the product distribution. Ideal hydrocracking operation is approached for ASA, MCM-48, and SBA-15 prepared at a high pH containing disordered mesopores.\n               "}
{"full_text": "Data will be made available on request.Supramolecular chemistry paves a bright avenue for constructing numerous functional materials with advanced applications in adsorption/separation, catalysis, sensing, biomedical technologies etc. [1,2]. For porous materials, such as zeolites [3], mesoporous silicas [4] and mesoporous organosilica nanoparticles [1,5], supramolecular host-guest systems show a particular potential.Relatively novel crystalline hybrid materials [6], metal-organic frameworks (MOFs) or porous coordination polymers, which are composed of organic molecules (linkers) and metal ions, can be regarded themselves as supramolecular systems. Under selected conditions, their self-assembly from organic and inorganic building blocks, which are polytopic ligands (linkers) and metal ions, leads to a unique MOF structure in a predictable way [7]. Furthermore, MOF modular building principle assists in constructing a large diversity of topologies, cavity sizes, and functionalities. One of most exciting MOF features is the infinity of ligands that can be used to generate them [8,9].Alongside their inorganic counterparts, zeolites, MOF matrices show a potential in the development of functional (nano)materials including supramolecular systems. To date, the significant research efforts have been directed towards design and development of host-guest structures using MOF host matrices due to their exciting properties, such as extremely high porosity, large pore volume, open porous structure and versatile chemical composition. In this way, a number of functional molecules and species can be encapsulated in the MOF host matrix, which is regarded as a porous scaffold for their accommodation [10]. These molecules can impart some novel functions in MOF materials or enhance/optimize existing intrinsic properties [11]. This approach holds great potential for catalytic applications of MOFs. Following this strategy, new catalytic properties can be introduced in the MOF porous host.The MOF-based host-guest materials show efficiency in number of catalytic applications from fine chemical synthesis to photocatalysis [12,13]. Nowadays, there are three main established approaches for the immobilization of functional molecules or nanoscale objects in MOFs, known as \u2018\u2018ship in a bottle\u2019\u2019 approach [14\u201316], \u2018\u2018bottle around a ship\u2019\u2019 approach [15,17,18], and one-step synthesis approach (\nScheme 1) [14\u201318]. In all cases, guest molecules are bound by weak host-guest interactions with MOF walls. The \u2018\u2018ship in a bottle\u2019\u2019 strategy involves the encapsulation of guest molecules in the cavities of MOFs, followed by further treatment leading to the desired functional structure. The second \u201cbottle around a ship\u201d strategy involves introducing functional guest molecules in the reaction mixture containing reagents for MOF synthesis [17,19]. Supramolecular host-guest composites based on MOF host matrices have been successfully prepared with different functional inorganic [20\u201322] (Table S1, Supporting Information (SI)) and organic guests (\nTable 1) [23\u201330]. Most of these composites were effective catalysts for photocatalytic reactions, hydrogenation of \u03b1,\u03b2-unsaturated carbonyl compounds, synthesis of 1,5-benzodiazepines from 1,2-phenylenediamine and ketones etc.Propylene carbonate (PC) as one of the cyclic carbonates is an important chemical, which is widely applied as a solvent, in plasticizers, lithium battery electrolytes, monomer in the preparation of polycarbonates, and the intermediate in the manufacture of fine chemicals [31]. Nowadays, the cycloaddition reaction of propylene oxide (PO) with CO2 provides an industrial route to PC using CO2 in place of phosgene with both economic and environmental benefits (\nScheme 2).This reaction can be catalyzed by homogeneous catalysts, but the problem of recyclability is inherent to homogeneous catalysts. Heterogeneous catalytic systems also were used to catalyze this reaction. Heterogeneous catalysts include tetraalkylammonium salts of transition-metal-substituted polyoxometalates, such as [(n-C7H15)4N]6[\u03b1-SiW11O39Co] and [(n-C7H15)4N]6[\u03b1-SiW11O39Mn] [32], zeolites, such as SSZ-13 [33], Ga-TS-1 [34], M/H-ZSM-5 (M - Zn2+, Fe3+, Co2+, Ni2+) [35] etc. Metal-organic frameworks (MOFs) also were demonstrated to be effective heterogeneous catalysts that can capture and convert CO2 to cyclic carbonates under mild conditions (Table S2\n, SI) [36\u201340]. The accessible and unsaturated metal cations in the framework of MOFs are important for the high activity because of their function as Lewis acid sites (LAS) to activate the epoxide.Metalloporphyrins as ligands in MOF structures can serve as the catalytic sites for the reaction with CO2. Thus, Gao and co-workers explored a metalloporphyrin-based MOF, denoted as MMPF-9, for heterogeneous catalysis in the synthesis of PC from PO and CO2. MMPF-9 exhibited excellent catalytic performance under mild conditions (1\u2009atm of CO2 at room temperature) [41]. The yield of PC was 87.4\u00a0% after 48\u2009h of the reaction. Note that the activity of MMPF-9 was significantly higher than the catalytic activity of HKUST-1 (49\u00a0%). Functionalization of the linker also allows improving catalytic properties of MOFs. Thus, Ma and co-workers [42] prepared two MOFs functionalized by quaternary ammonium or phosphorus bromide ionic liquid, MIL-101-N(n-Bu)3Br and MIL-101-P(n-Bu)3Br, by post-synthetic modification of the parent MIL-101-NO2. Such approach allows the process to be carried out without a co-catalyst, despite the fact that a higher temperature and CO2 pressure are required.In our investigation we would like to draw attention to the synthesis of a novel supramolecular composite (MIL/K-OH) based on calix[4]arene with hydroxyl groups in the arene \u201cbowl\u201d (K-OH, \nFig. 1) as a functional guest molecule and NH2-MIL-101(Al) as a porous host and investigation of its catalytic potential in synthesis of PC from PO and CO2.This study is focused on three main issues: (a) evaluation of the effect of the synthesis method, i.e., MW-assisted technique and the solvothermal procedure, in respect of structural, morphological, and textural properties of the produced MIL/K-OH composite, (b) the understanding of the role of the functional calix[4]arene molecule on the catalytic behavior of this host-guest system in the synthesis of PC, and (c) assessment of its potential for further applications in acid-base catalysis.Propylene oxide (> 98\u00a0%, Acros Organics), tetra-n-butylammonium bromide (TBABr) (Sigma-Aldrich), AlCl30.6\u2009H2O (Aldrich), 2-aminobenzene-1,4-dicarboxylic acid (ABDC, 99\u2009+ %, Acros Organics), 1,3,5-benzenetricarboxylic acid (H3BTC) and 1,3,5-trimethyl-benzenetricarboxylate (Me3-BTC, 98\u00a0%, Aldrich), ortho-phosphoric acid (H3PO4, 85\u2009wt\u00a0%, Merck), sodium hydroxide (NaOH, 4\u2009M), nitric acid (HNO3, 60\u2009wt\u00a0%), N,N-dimethylformamide (DMF, Aldrich) were used without any further purification.\nNH\n\n2\n\n-MIL-101(Al) sample was prepared by MW-assisted synthesis under atmospheric pressure according to [43]. 0.51\u2009g of AlCl30.6\u2009H2O, 0.56\u2009g of 2-aminobenzene-1,4-dicarboxylic acid, and 40\u2009ml of DMF were transferred into a glass ampoule and heated at atmospheric pressure in a chamber of a MW oven \u201cVigor\u201d (200\u2009W, 20\u2009min, 130\u2009\u00b0\u0421). The formed solid was separated on a centrifuge and washed with DMF (3\u2009\u00d710\u2009ml) and acetone (3\u2009\u00d710\u2009ml). Then the crystalline product was treated with boiling methanol (20\u2009ml) under stirring (24\u2009h), isolated on a centrifuge, and evacuated at 130\u2009\u00b0\u0421 for 7\u2009h.\nCalix[4]arene K-OH (25,26,27,28-tetrahydroxycalix[4]arene) was synthesized according to [44]. Anhydrous aluminum chloride (21.0\u2009g, 158\u2009mmol) was added gradually to a mixture of tert-butylcalix [4]arene (20\u2009g, 0.03\u2009mmol), phenol (13.56\u2009g, 0.144\u2009mmol) and toluene (125\u2009ml). The resulted mixture was stirred (RT, 1\u2009h), then was poured in water (25\u2009ml) acidified with conc. H\u0421l (5\u2009ml). The organic layer was separated, and the solvent was removed by distillation. Methanol was added to the residue. The obtained K-OH material was recrystallized from a chloroform-methanol mixture. The product yield was 7.40\u2009g (56\u00a0%). 1HMR (CHCl3, 300 MGz) 3.57 (s, 4\u2009H, CH2), 4.27 (s, 4\u2009H, CH2), 6.75 (t, 4\u2009H, ArH), 7.07 (d, 8\u2009H, ArH), 10.22 (s, 4\u2009H, ArOH).\nMIL/K-OH(MW) composite. A solution of AlCl30.6\u2009H2O (1.02\u2009g, 4.22\u2009mmol), 2-aminobenzene-1,4-dicarboxylic acid (1.12\u2009g, 6.18\u2009mmol) and K-OH (0.873\u2009g, 2.04\u2009mmol) in DMF (70\u2009ml) was transferred into a glass ampoule and heated at atmospheric pressure in a chamber of a MW oven \u201cVigor\u201d (200\u2009W, 20\u2009min, 130\u2009\u00b0\u0421). The crude product was rinsed with DMF (3\u2009\u00d710\u2009ml) and acetone (3\u2009\u00d710\u2009ml), then dried at 130\u2009\u00b0C for 6\u2009h under a vacuum. The chemical composition of the MIL/Ks-OH(MW) sample is shown in \nTable 2.\n\nMIL/K-OH(Solv) composite.\n A solution of AlCl30.6\u2009H2O (1.02\u2009g, 4.22\u2009mmol), 2-aminobenzene-1,4-dicarboxylic acid (1.12\u2009g, 6.18\u2009mmol) and K-OH (0.873\u2009g, 2.04\u2009mmol) in DMF (70\u2009ml) was transferred into a Teflon autoclave and heated in a thermostated oven (130\u2009\u00b0\u0421, 72\u2009h). The crude product was rinsed with DMF (3\u2009\u00d710\u2009ml) and acetone (3\u2009\u00d710\u2009ml), then dried at 130\u2009\u00b0C for 6\u2009h under a vacuum. The chemical composition of the MIL/K-OH(Solv) sample is shown in Table 2.C, H, N, and S analyses of the synthesized materials were performed using a Euro EA Elemental Analyzer.The porous structures of samples were determined from the adsorption isotherm of N2 at \u2212\u2009196\u2009\u00b0C using an \u201cASAP 2020 Plus\u201d (\u201cMicromeritics\u201d) instrument. The specific surface area (SBET) was calculated from the adsorption data over the relative pressure range between 0.05 and 0.20.X-ray powder diffraction (XRD) data were collected in a reflection mode using a Panalytical EMPYREAN instrument with a linear X\u2032celerator detector and non-monochromated Cu K\u03b1 radiation (\u03bb\u2009=\u20091.5418\u2009\u00c5), measurement parameters: tube voltage/current 40\u2009kV / 35\u2009mA, divergence slits of 1/16 and 1/8\u00b0, 2\u03b8 range 2\u201330\u00b0, speed 0.1\u00b0 min\u22121. High-resolution synchrotron measurements (\u03bb\u2009=\u20090.45085\u2009\u00c5) were carried out at room temperature at beam line ID22 of the European Synchrotron Radiation Facility (ESRF, Grenoble, France).DRIFT spectra were recorded using a Shimadzu FTIR-8300S spectrometer with a DRS-8000 diffuse reflectance cell in the range between 400 and 6000\u2009cm\u20131 with a resolution of 4\u2009cm\u22121. All spectra are presented in F(R) Kubelka-Munk scale: \n\nF\n(\nR\n)\n=\n\n\n\n\n(\n1\n\u2212\nR\n)\n\n2\n\n\n\n2\nR\n\n\n\n, where R is the reflection coefficient.The basicity of ZIFs was determined by DRIFT spectroscopy with CDCl3 as a probe molecule using the technique reported in Ref. [45] (SI).Electron microscopy investigation (TEM) was performed using a JEM-2200FS (JEOL, Tokyo, Japan) electron microscope operating at 200\u2009kV with a lattice resolution of 0.1\u2009nm.Before the reaction, all catalysts were activated at 150\u2009\u00b0C for 2\u2009h in air in order to remove adsorbed water. The standard procedure was as follows: the cycloaddition of CO2 to propylene oxide was carried out in a 30\u2009ml stainless-steel autoclave. Typically, 24\u2009mmol of PO, 50\u2009mg (0.85\u2009mmol based on PO) of a catalyst, 50\u2009mg (0.85\u2009mmol based on PO) of TBABr were loaded into the autoclave. The autoclave was purged 3 times with CO2 to remove air and was charged with CO2 up to 0.8\u2009MPa. Then the autoclave was heated to 80 \u25e6C (the heating time was 15\u2009min). The mixture was kept under stirring for 5\u2009h. After the reaction completion, the autoclave was cooled, and excess CO2 was released. The product was diluted with toluene and analyzed with a gas chromatograph (Agilent Technologies 7820\u2009A) equipped with a capillary column (Agilent HP-5).The chemical composition of the NH2-MIL-101(Al) sample and MIL/K-OH composites is shown in Table 2. It can be seen from these data that experimental and theoretical weight amounts of C, H, N elements in the NH2-MIL-101(Al) are close. The composite synthesis method dictates remarkably the K-OH content in the NH2-MIL-101(Al) matrix. So, using MW-assisted synthesis, rather negligible loading (3.1\u2009wt\u00a0%) of K-OH in the porous host is achieved. This phenomenon is explained by a very short reaction time (\u223c 30\u2009min) under conditions of fast MW-synthesis, which is insufficient for the ideal accommodation of K-OH molecules in the NH2-MIL-101(Al) pores. On the contrary, the solvothermal procedure realized at a prolonged reaction time (72\u2009h) results in a larger K-OH loading (16.9\u2009wt\u00a0%) in the NH2-MIL-101(Al) host matrix.IR spectra of NH2-MIL-101(Al) and MIL/K-OH samples are shown in \nFig. 2\n. There are bands at 552 and 1398\u2009cm\u22121 from the benzene ring in the spectrum of NH2-MIL-101(Al), as well as the bands at 897, 1581 and 1675\u2009cm\u22121 from carboxylic groups, at 1261, 1338, 1625 and in the region of 3100\u20133500\u2009cm\u22121 from -NH2 groups [46\u201349]. Two bands at 3301 and 3383\u2009cm\u22121 can be assigned to antisymmetric (\u03bdasym(NH2)) and symmetric (\u03bdsym(NH2)) stretching vibrations of the -NH2 groups, respectively. This may indicate the existence of two types of amine groups in the framework [48,49]. The main types of characteristic bands in the FTIR spectrum of NH2-MIL-101(Al) are shown in \nTable 3.All these bands are also observed in the spectrum of MIL/K-OH composites. Unfortunately, the bands of K-OH overlap with those of the NH2-MIL-101(Al) material; however, some differences can be observed. First of all, the bands assigned to the C-H bending vibrations of the benzene ring and stretching vibrations of Al-O shift from 552 to 544\u2009cm\u22121 and from 470 to 459\u2009cm\u22121, respectively. The red shift is observed for the band at 3501\u2009cm\u22121 assigned to the antisymmetric stretching vibrations of \u2013NH2 groups (\u03bd\nsym(NH2)). These changes can be caused by (a) the interaction between the \u2013NH2 groups of the NH2-MIL-101(Al) framework and \u2013OH groups of K-OH, and (b) interaction between \u2013OH groups of K-OH and Lewis acid sites formed by Al3+ ions. Interaction between functional groups of K-OH and NH2-MIL-101(Al) is confirmed by the disappearance of the band at 3145\u2009cm\u22121 attributed to the -OH stretching hydrogen bonds, i.e., cooperative \u2022\u2022OH \u2022\u2022 OH \u2022\u2022 OH chains of the framework [50] in the spectrum of K-OH (Fig. 2).Secondly, the characteristic band at 897\u2009cm\u22121 corresponding to the vibrations of the substituted aromatic ring (C-H bending and CC-H stretching) is not observed in spectra of MIL/K-OH samples. This disappearance can arise from \u03c0-\u03c0 stacking interaction between the aromatic rings of K-OH and the aromatic structure of NH2-MIL-101(Al).Another feature in the spectra of the samples is the dependence of the position of the band from the \u2013NH2 groups (\u03bdsym(NH2)) on the method of the composite synthesis. In the spectrum of MIL/K-OH(Solv), the shift is larger (\u0394\u2009=\u200926\u2009cm\u22121) than that in the spectrum of the MIL/K-OH(MW) material (\u0394\u2009=\u200916\u2009cm\u22121). This phenomenon can be provoked by the difference in the strength of the interaction between K-OH molecules and the framework of NH2-MIL-101(Al).as a C-H acid probe molecule (DRIFT-CDCl3) also point to the interaction between \u2013OH groups of K-OH and basic sites of NH2-MIL-101(Al). The spectrum of CDCl3 adsorbed on the NH2-MIL-101(Al) material is shown in \nFig. 3\n. The interaction of CDCl3 with basic sites of samples leads to the appearance of two bands at 2252 and 2212\u2009cm\u22121 that can be assigned to basic sites formed by \u2013NH2 and Al-OH groups, respectively. This suggestion follows from our early investigations of MIL-100(Al) and NH2-UiO-66(Zr) (Table S3, SI) [51]. The band at 2212\u2009cm\u22121 is not observed in spectra of MIL/K-OH composites, which can indicate that basic sites formed by -NH2 groups are absent due to the interaction with \u2013OH groups of K-OH.XRD patterns of NH2-MIL-101(Al) and MIL/K-OH composites are shown in \nFig. 4. The main diffraction peaks of samples are similar to those of the previously reported patterns of the NH2-MIL-101 family, which has a cubic unit cell [52\u201354]. The values of the unit cell parameter of NH2-MIL-101(Al) is 89.71\u2009\u00c5, which is lower in comparison with MIL/K-OH(MW) and MIL/K-OH(Solv) (Table S4, SI). The refined unit cell parameter is equal to a =\u200987.860(2) \u00c5 according to synchrotron examinations (Figs. S1-S2\n, SI).The coherent scattering region, DXRD was calculated using the Debye-Scherrer's formula (Eq. 1) [55,56]:\n\n(1)\n\n\n\n\nD\n\n\nXRD\n\n\n=\n\n\n0.9\n\u22c5\n\u03bb\n\n\n\u03b2\n\u22c5\n\ncos\n\n\u03b8\n\n\n\n\n\nwhere DXRD is the coherent scattering region (nm), \u03b2 is the full-width at half maximum of the peak (radian), \u0398 is the Bragg angle of a diffraction peak (grad), and \u03bb is the X-ray wavelength of CuK\u03b1 (0.1542\u2009nm). DXRD values for MIL/K-OH(MW) and MIL/K-OH(Solv) (23.6\u2009\u00b1\u20091.0\u2009nm) are also larger than those for NH2-MIL-101(Al) (Table S4, SI). Moreover, there are noticeable differences in the ratio of the heights of the low-angle peaks. Thus, the increase in the ratios of integral intensities of I(333)/I(442) and I(662)/I(664) reflections is observed in XRD patterns of MIL/K-OH composites (Table S4, SI). There is also a noticeable difference in the heights of the (222) reflection in XRD patterns of MIL/K-OH(MW) and MIL/K-OH(Solv) materials (Fig. 4). The XRD pattern of the MIL/K-OH(MW) composite differs less from the XRD pattern of the NH2-MIL-101(Al) sample, unlike the XRD pattern of the MIL/K-OH(Solv) material. The lower intensity of the (222) reflection and the higher I(333)/I(442) and I(662)/I(664) ratios in the XRD pattern of MIL/K-OH(Solv) as compared to the XRD pattern of the pristine NH2-MIL-101(Al) material can indicate a partial pore blockage by calix[4]arene guest molecules. Note that K-OH reflexes (Fig. S1-S3, SI) are almost not observed in the synchrotron X-ray powder diffraction patterns of the MIL/K-OH(MW) and MIL/K-OH(Solv) composites. Probably, this phenomenon could be explained both by the location of the K-OH molecules inside the MIL porous host and a low K-OH content in the MIL/K-OH(MW) composite.TEM images of the MIL/K-OH are presented in \nFig. 5. The MIL/K-OH(MW) sample is composed of nanocrystals with a highly-ordered mesoporous structure. The outside surface of MIL/K-OH(MW) is relatively dense with roughness and has nano-sized finger-like channels fitted for the K-OH encapsulation. The morphology of the MIL/K-OH(Solv) nanomaterial is similar to that of MIL/K-OH(MW) nanocrystals (Fig. 5). At the same time, the size of K-OH particles is larger in MIL/K-OH(Solv) in comparison with MIL-K-OH(MW). In the TEM image of MIL/K-OH(Solv) (Fig. 5), the particles with the interplanar spacing of the crystallites around 0.24\u2009nm can be revealed, which corresponds to the lattice spacing of K-OH [57]. The differences in the guest K-OH location in the host structure affects the textural properties of the MIL/K-OH composites.were studied by N2 low-temperature adsorption. The corresponding results are shown in \nTable 4. The introduction of K-OH into the NH2-MIL-101(Al) matrix leads to the decreasing specific surface area and total pore volume, and this reduction is dependent on the preparation method of the composite. The effect of the MW-assisted procedure on the specific surface area and total pore volume expressed as a decrease of these parameter values as compared with the NH2-MIL-101(Al) reference sample is lower in comparison with the solvothermal method.[a] V\u03a3 was estimated from the adsorption value at p/p\n\no\n =\u20090.99; [b] \n\n\n\nV\n\n\n\u03bc\n\n\n=\n\n\nV\n\n\n\u03a3\n\n\n\u2212\n\n\nV\n\n\nmeso\n\n\n\n; [c] Cumulative mesopore volume calculated from the desorption branch of the isotherm by the BJH method and the standard thickness of the adsorption film.Mesoporosity of MIL/K-OH(MW) is higher (Vmeso/V\u03a3 = 0.39) than that of MIL/K-OH(Solv) (Vmeso/V\u03a3 = 0.26) due to the different filling of the framework with guest K-OH molecules. Actually, according to the elemental analysis data (Table 2), the MIL/K-OH(Solv) material contains much more K-OH molecules than its MIL/K-OH(MW) counterpart.In order to evaluate the application prospective of the synthesized MIL/K-OH composites, they have been tested for their catalytic activity in the chemical fixation of CO2 gas through the conversion of CO2 and propylene oxide (PO) to propylene carbonate (PO) (Scheme 2\n). Catalytic properties of MIL/K-OH composites were investigated at 80\u2009\u00b0C under 0.8\u2009MPa of CO2. The main results are shown in \nTable 5. According to the test results and GC-MS analysis, PC was the major product with about 99\u00a0% selectivity in all cases. A blank experiment indicated that the conversion of PO was <\u2009< 1\u00a0% after 5\u2009h. In the presence of K-OH, the conversion of PO for 5\u2009h was less than 3\u00a0% (Table 5, run 2). The addition of tetra-n-butylammonium bromide (TBABr) as a co-catalyst leads to the increasing conversion of PO to 35\u00a0% (Table 5, run 3). A similar effect of TBABr was observed for NH2-MIL-101(Al) and NH2-UiO-66(Zr) (Table 5, runs 7\u20138 and 12\u201313). Therefore, a binary system was used for the investigation of catalytic properties of MIL/K-OH composites.Conversions of PO in the presence of MIL/K-OH(Solv) and MIL/K-OH(MW) were 74\u00a0% and 82\u00a0%, respectively (Table 5, runs 4\u20135). These results show that the activities of the MIL/K-OH composites depend on their preparation method. The MW assisted procedure favors the higher activity of the composite than the traditional solvothermal method. The difference in catalytic activities of MIL/K-OH(Solv) and MIL/K-OH(MW) can be explained by several reasons, which mainly are based on the difference in textural properties of samples and particle size of K-OH. First of all, the higher activity of MIL/K-OH(MW) in comparison with MIL/K-OH(Solv) can be a result of the higher specific surface area and mesoporosity (Vmeso/V\u03a3) (Table 4) that can affect the accessibility of active sites for reagents. Another reason can be related with the difference in the particle size of K-OH that can affect both the number and reactivity of active sites [58].The reaction mechanism of cycloaddition of CO2 to epoxide to produce cyclic carbonates is ascribed to the action of the \u201cLewis acid site and basic site\u201d pair (LAS-BS) [37\u201340]. Physicochemical investigations and catalytic data point that the structure of active sites in the NH2-MIL-101(Al) material and MIL/K-OH composites are different (\nScheme 3). Thus, LAS and BS are formed by Al3+ ion and -NH2 group of the linker in the NH2-MIL-101(Al) sample. The coordination of oxygen atom of the propylene oxide ring to LAS favors the electron density redistribution and, therefore, activation of propylene oxide, while interaction of CO2 with BS leads to its activation. In MOF/TBABr binary systems, the main role of the co-catalyst is to increase the rate of ring-opening of propylene oxide. Bromine ion attacks the least hindered carbon atom of propylene oxide with formation of reactive oxygen anion. Subsequently, CO2 reacts with the oxygen anion of the opened propylene oxide to form an intermediate, and finally, bromine ion is eliminated by a ring-closing step to produce propylene carbonate from the intermediate while regenerating the catalyst. In contrast to NH2-MIL-101(Al), -NH2 groups in the MIL/K-OH composite are blocked by K-OH due to the interaction of -OH groups of K-OH with the amine groups (Scheme 3). It can be suggested that the other three -OH groups can interact with TBABr to form an ion pair. In this case, the bromine ion is more mobile and can accelerate the ring-opening step due to the nucleophilic attack to C1 atom of epoxide to produce an intermediate. Probably, for this reason, the activity of the MIL/K-OH(MW) composite is higher than that of the pristine NH2-MIL-101(Al) material.[a] 24\u2009mmol of propylene oxide; [b] Catalyst was used in second cycle; [c] [Zn(EIM)2], EIM \u2013 2-ethylimidazole; [d] [Zn(cbIM)2], cbIM - 5- chlorobenzimidazole; [e] - [Zn(abIM)2], abIM - 4-Azabenzimidazole.Stability of MIL/K-OH(Solv) was investigated in a cyclic test at 24\u2009mmol of PO, catalyst/TBABr of 0.85/0.85\u2009mmol/mmol, 0.8\u2009MPa of CO2, 80\u2009\u00b0C for 5\u2009h. After the first cycle, the catalyst was separated from the reaction mixture by filtration, washed with toluene, dried in air, calcined at 150\u2009\u00b0C in air for 3\u2009h and used in the next cycle. These tests revealed a slight activity decrease from 74\u00a0% (first cycle) to 67\u00a0% (second cycle) (Table 5\n). Comparison of the IR spectra of the MIL/K-OH(Solv) before and after catalysis clearly shows that no changes are observed in the IR spectra of the sample after the first and second catalytic cycles when compared to the pristine material (\nFig. 6). The decreasing activity is due to the blocking of active sites with reaction products as it follows from the appearance of new bands in the region of 850\u20131200\u2009cm\u22121.The comparison of the efficiency of the most active sample, i.e. MIL/K-OH(MW), with the activity of binary systems based on metal-organic frameworks with NH2-groups in the structure of the linker, such as NH2-MIL-101(Al), NH2-MIL-53(Al) and NH2-UiO-66(Zr), is shown in Table 5 (runs 5, 7, 10 and 12). Results indicate that activities of all NH2-containing MOFs are lower in comparison with that of MIL/K-OH(MW). The activity of MIL/K-OH(MW) also was compared with activities of binary systems based on zeolitic imidazolate frameworks (ZIFs) reported in the literature [58\u201360]. Data shown in Table 5 and Table S2 (SI) indicate that MIL/K-OH(MW)/TBABr is definitely a promising catalytic system for the cycloaddition of CO2 to PO. Moreover, this binary system allows producing PC under relatively mild reaction conditions. The conversion of PO was 77\u00a0% at 50\u2009\u00b0C for 24\u2009h.For the first time, novel host-guest composite materials based on calix[4]arene with hydroxyl groups in the arene \u201cbowl\u201d (K-OH) and the aluminum(III) 2-aminoterephthalate metal-organic framework (NH2-MIL-101(Al), MIL) have been synthesized according to one-step in situ or \u201cbottle-around-the-ship\u201d approaches. The effect of the synthesis method, i.e., MW-assisted technique (MW) and the solvothermal procedure (Solv), on the structural, morphological, textural and catalytic properties of MIL/K-OH composites was demonstrated.The possibility of the interaction via the strong hydrogen bond between protons of \u2013OH groups of K-OH and \u2013NH2 groups of the NH2-MIL-101(Al) framework has been proven by spectroscopic investigations. According to IR spectroscopy, the interaction between the functional groups of K-OH and NH2-MIL-101(Al) was stronger in MIL/K-OH(Solv). According to SEM study, both MIL/K-OH(MW) and MIL/K-OH(Solv) composites have a highly-ordered mesoporous structure with a partial pore blockage by K-OH guest molecules. However, the particle size of K-OH is larger in MIL/K-OH(Solv) in comparison with MIL-K-OH(MW) as confirmed by TEM data. The effect of the MW-assisted procedure on the specific surface area and total pore volume is lower in comparison with the solvothermal method due to the different filling of the framework with guest K-OH molecules.The catalytic performance of the novel MIL/K-OH(MW) and MIL/K-OH(Solv) composite materials was investigated in solvent-free coupling of CO2 and propylene oxide (PO) to produce propylene carbonate (PC) at 0.8\u2009MPa of CO2 and 80\u2009\u00b0C and compared with that of the individual components. K-OH guest molecules modify the catalytic properties of the NH2-MIL-101(Al) host, e.g., the activity of the MIL/K-OH(MW) composite is higher than that of the pristine NH2-MIL-101(Al) material. In turn, the use of the MIL/K-OH(MW) catalyst results in the increased PO conversion in comparison with the MIL/K-OH(Solv) material, which is related to the difference in the textural properties of both composites. The binary system MIL/K-OH(MW)/[n-Bu4N]Br was demonstrated to be a promising catalytic system for the cycloaddition of CO2 to PO. In the presence of this binary system, the conversion of PO was 77\u00a0% with 99\u00a0% selectivity towards PC at 1.2\u2009MPa of CO2, 50\u2009\u00b0C for 24\u2009h.\nLeonid M. Kustov: Conceptualization, Writing \u2013 review & editing. Vera I. Isaeva: Supervision, Writing \u2013 review & editing. Maria N. Timofeeva: Supervision, Writing\u00a0\u2013 review & editing. Ivan A. Lukoyanov: Conceptualization, Formal analysis, Investigation, Methodology, Visualization, Writing \u2013 original draft. Valentina N. Panchenko: Conceptualization, Formal analysis, Investigation, Writing \u2013 original draft. Evgeny Y. Gerasimov: Formal analysis, Investigation, Writing \u2013 original draft. Vladimir V. Chernyshev \u2013 Formal analysis, Investigation. Lev M. Glukhov: Formal analysis, 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.This work was supported by the Ministry of Science and Higher Education of the Russian Federation within the governmental order for Boreskov Institute of Catalysis (\u0410\u0410\u0410\u0410-\u041021-121011390055-8) and project no. 075-15-2021-591 for N.D. Zelinsky Institute of Organic Chemistry. The authors acknowledge the Novosibirsk State University Shared Equipment Center \u201cApplied Physics\u201d and \u201cVTAN\u201d, and ESRF for providing access to the ID22 station (experiment MA-4527).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2022.102262.\n\n\n\nSupplementary material.\n\n\n\n.", "descript": "\n                  In this study, novel composite materials based on calix[4]arene with hydroxyl groups in the arene \u201cbowl\u201d (K-OH) and the aluminum(III) 2-aminoterephthalate metal organic framework (NH2-MIL-101(Al), MIL) have been synthesized according to in situ or \u201cbottle around ship\u201d strategy. The effect of the synthesis method, i.e., MW-assisted technique (MW) and the solvothermal procedure (Solv), on the structural, morphological, textural and catalytic properties of MIL/K-OH composites was demonstrated. Catalytic performance of the MIL/K-OH(MW) and MIL/K-OH(Solv) composites was studied in solvent-free coupling of CO2 and propylene oxide (PO) to produce propylene carbonate (PC) at 0.8\u00a0MPa of CO2 and 80\u00a0\u00b0C. The activity of the MIL/K-OH(Solv) catalyst was higher in comparison with MIL/K-OH(MW) material as a result of the differences in the textural properties. The binary system MIL/K-OH(MW)/[n-Bu4N]Br was demonstrated to be a promising catalyst for cycloaddition of CO2 to PO. In its presence, the conversion of PO was 77\u00a0% with 99\u00a0% selectivity towards PC at 1.2\u00a0MPa of CO2, 50\u00a0\u00b0C for 24\u00a0h.\n               "}
{"full_text": "Enantioselective metal-catalysis is one of the most important synthetic approaches for preparing enantioenriched compounds, which occupy a central position in areas ranging from medicinal chemistry to chiral materials. The performance of chiral metal-catalysts depends, mainly, on the correct choice of the chiral ligand [1]. Among the large amounts and variety of ligands built up, only some have a broad applicability. A large reaction, substrate and/or reagent scope are worthwhile to minimize the time devoted to ligand finding and preparation. The most efficient, called \u201cprivileged chiral ligands\u201d, derive from a few core structures [2]. Surprisingly, most of them possess C2 symmetry (e.g. BINOL, BINAP, TADDOL \u2026). The reason for initially selecting bidentate ligands with C2-symmetry was to reduce the number of catalyst-substrate arrangements and transition states, helping mechanistic studies and therefore facilitating the understanding of the correlation between ligand architecture and catalytic results crucial to find the optimal catalyst. However, the intermediates that take place in the catalytic cycle may not be symmetric and in these cases the desymmetrization of the ligand has proven to allow a better enantiocontrol in certain reactions. An easy and suitable way to desymmetrize a ligand is to introduce in the ligand design two different donor atoms. In the last decades, heterodonor ligands have therefore increased their use in catalysis, by being able to facilitate the stereocontrol thanks to the different electronic and steric properties of two distinct coordination groups [3]. Moreover, the presence of the two different donor groups facilitates the discovery of highly effective ligands for a given reaction since it is easy to independently tune both donor groups. Among heterodonor ligands, those containing both P- and N-donor groups have a predominant position, with phosphorus-oxazoline ligands the most commonly investigated due to their ready accessibility and modular construction (see Section 2). During the last decade the oxazoline group has been substituted for other more robust N-donor groups (e.g. imines, amines, oxazoles \u2026) that in some cases have allowed further catalytic improvement (see Section 3). Over the last years, there has been a new renaissance in the use of chiral P,O- and P,S- ligands in asymmetric catalysis, which have led to very interesting new successful applications (see Section 4). We here offer a critical review on the design and application of the most successful bidentate heterodonor P-oxazoline, P-other N, P-O and P-S ligand families in asymmetric processes. In addition to the comprehensive contents, for each group of ligands this review also offers a new perspective of presenting such ligands. While other reviews present the ligands by time or asymmetric reaction, for the first time we grouped them by their relationship between their structure and catalytic performance, which helps to associate the structural characteristics of a set of catalysts with their catalytic capacity and will facilitate further research on the field. Finally, the representative applications of these heterodonor family of ligands in total synthesis are collected at the end of this review (Section 6).Most of P-oxazoline ligands are prepared from easily obtainable chiral amino alcohols in short and efficient synthetic sequences [3a-e]. The beginning of P-oxazoline ligands can be found in 1993, with the preparation of the phosphine-oxazoline PHOX ligands 1 (Fig. 1\n) [4] that have been applied with huge success to a large variety of asymmetric transformations (e.g. allylic substitution and decarboxylative allylation reactions, hydrogenation, inter- and intramolecular Heck reactions, conjugate additions to enones, Diels-Alder and aza-Diels-Alder reactions, etc.) [2]. Despite being reported >25\u00a0years ago, PHOX ligands maintain their success in new enantioselective transformations, emphasizing their category as privileged ligand [5].Since then, many new P-oxazoline ligands have been synthesized by varying either the ligand skeleton or the steric/electronic properties of the phosphine group or by swapping the phosphine moiety by other P-donor groups (e.g. phosphoroamidite, phosphinite) [3a-e]. These modifications allowed to improve enantioselectivities in some specific reactions. However, a few of them have been used with success to different asymmetric reactions and have shown a wide substrate and/or reagent scope. Next, we collect the families of P-oxazoline ligands that have successfully showed a large range of reaction and/or substrate scope and the relationship between their structural design and their catalytic performance. We will center on recent works and a quick overview of previous reports will also be incorporated.Some successful modifications of the PHOX ligands are electronics by introducing withdrawing groups in the phenyl backbone ring or/and in the phosphine moiety. In this respect, it can be highlighted the recent synthesis of ligands L1 developed by Stoltz\u2019s group (Scheme 1\n). The synthesis of these ligands relies in the Cu-catalyzed Ullmann-type coupling which allows the modular synthesis of ligands L1 even in demanding steric and electronic cases, avoiding the discrete synthesis of anionic reagents (Scheme 1) [6a].Stoltz\u2019s group found that L1 (X\u00a0=\u00a0CF3 and R\u00a0=\u00a04-CF3-C6H4) was highly favorable in the Pd-catalyzed decarboxylative allylation of cyclic allyl carbonates, providing higher catalytic performance than PHOX ligands (Scheme\u00a02\na) [6]. This methodology has facilitated the transformation of substituted cyclic allyl enol carbonates into a range of natural products (e. g. (+)-hamigeran, (\u2212)-cephalotaxine and elatol; see Section 6) [7]. The same group has recently expanded this methodology to acyclic enol carbonates, achieving also excellent enantioselectivities (Scheme\u00a02b) [8]. Interestingly, the same catalytic system was also able to achieve excellent enantiocontrol in the decarboxylative allylation of bench-stable \u03b2-keto allyl esters. This latter finding has facilitated the synthesis of many quaternary cyclobutanones and cyclopentanones as well as heterocyclic compounds (e.g. piperazines, diazepanones, etc., Scheme\u00a02c) [9]. This methodology has again enabled the total synthesis of several natural products (e.g. (+)-sibirinine, (\u2212)-goniomitine, (+)-limaspermidine, nigelladine A, etc; see Section 6) [10].The Stolz\u2019s group also demonstrated the benefits of ligand L1 in the Pd-catalyzed enolate alkylation cascade procedure that allows the installation of vicinal quaternary and tertiary stereocentres at the \u03b1-carbon of a ketone [11]. This multiple bond-forming procedure takes place by conjugate addition of the chiral Pd/L1-enolate, generated in situ from \u03b2-keto allyl esters, to malononitriles (Scheme\u00a03\na). More recently, You\u2019s group disclosed the usefulness of Pd/L1 catalyst in the highly diastereo- and enantioselective synthesis of tetrahydrofurobenzofurans and tetrahydrobenzothienofurans through a Pd-catalyzed dearomative [3\u00a0+\u00a02] cycloaddition of nitrobenzofurans (Scheme\u00a03b) [12].Guiry\u2019s group also disclosed the usefulness of ligand L1 (X\u00a0=\u00a0CF3 and R\u00a0=\u00a04-CF3-C6H4) in the preparation of sterically demanding tertiary \u03b1-aryl ketones, such as isoflavones and \u03b1-aryl-1-indanones, via Pd-catalyzed decarboxylative protonation of \u03b1-aryl-\u03b2-keto allyl esters [13]. Interestingly, they also showed that the nature of the proton source has an impact on enantioselectivity, which clearly indicates that it is involved in the enantioselective determining step. By switching the proton source form Meldrum\u2019s acid to formic acid both enantiomers of the \u03b1-aryl-1-indanones can be accessed (Scheme 4\n).Another notable modification of the PHOX ligands was to add a biaryl phosphite functionality (a\u2013e) instead of the phosphine moiety, which increases the \u03c0-acceptor character of the ligand [14]. Ligands L2a\u2013e (R\u00a0=\u00a0tBu, iPr, Et, Ph; Scheme 5\n) have the advantage to be air-stable solids, which are easily prepared by attaching several amino alcohols and phosphorochloridites to the 2-hydroxyphenyl cyanide scaffold (Scheme 5) [15].Ligands L2 were initially designed to increase the substrate range of PHOX ligands in Pd-catalyzed allylic substitutions [14,16]. In Pd-catalyzed allylic substitution reactions, ligands able to induce high enantioselectivities in a large range of substrates' type and nucleophiles are exceptional [1c,17]. Improving Pd-PHOX catalysts, which gave excellent enantioselectivities with rac-(E)-1,3-diarylallyl substrates but low-to-moderate ee\u2019s for cyclic and 1,3-dialkylallyl substrates, respectively [2a,3], Pd/L2 catalysts provided an excellent catalytic performance in the allylic substitution for all of them [14]. Thus, higher activities (TOFs\u00a0>\u00a02400\u00a0mol substrate\u00a0\u00d7\u00a0(mol Pd\u00a0\u00d7\u00a0h)\u22121) than with PHOX ligands were achieved due to the \u03c0-acceptor character of the phosphite moiety. In addition, the high enantioselectivities (up to 99%\u00a0ee) were reached not only for disubstituted substrates, but also for tri- and monosubstituted ones (Scheme 6\n) [15]. The highest enantioselectivities for the benchmark substrate, rac-1,3-diphenylallyl acetate, were reached with the simple tropoisomeric ligand L2b independent of the oxazoline substituent (R\u00a0=\u00a0Ph, Et, iPr or tBu). Pd/L2b was also tolerant with the variation of the type of nucleophile (Scheme 6). In this respect, excellent enantioselectivities were reached with butenyl-, pentenyl-, propargyl and allyl-substituted malonates, fluorobis(phenylsulfonyl)methane (a fluoromethide synthon) and non-aromatic alcohols (ee's up to >99%, Scheme 6), whose resulting substitution products have proved valuable in the preparation of more complex chiral products [18]. Pd/L2b was also effectively applied to symmetrical 1,3-diaryl- and 1,3-dialkylallyl acetates with different electronic and steric demands with a large variety of C-nucleophiles (Scheme 6). For cyclic and monosubstituted substrates, the highest\u00a0ee\u2019s (up to >99% and 92%, respectively; Scheme 6) were achieved using ligands L2b and L2e. It should be noted the high regioselectivities in favor to the branched chiral product attained in the allylic alkylation of monosubstituted substrates, since most of the Pd-catalyst led to the preferential formation of the achiral linear isomer [16,19]. The phosphite moiety of the ligand is key to understand the high regioselectivities achieved towards the branched isomer. Thus, the phosphite group favors the nucleophilic attack at the most substituted allylic terminal carbon atom thanks to the trans-influence [14]. Excellent results were also reached in alkylation of 1,3,3\u2032-trisubstituted allylic acetates (Scheme 6).The broad substrate scope of the Pd/L2b catalyst system was rationalized by NMR studies and DFT calculations of their Pd-\u03b72-olefin and Pd-\u03b73-allyl intermediates complexes [14b]. These studies indicated that: (a) the tropoisomeric biphenyl phosphite group in ligand L2b adopts an (S)-configuration in the Pd-\u03b73-allyl intermediates with hindered as well as unhindered substrates (Fig. 2\n); and (b) the Pd/L2 catalysts are able to readjust the binding chiral pocket's size to the substrate requirements, which explains the high\u00a0ee\u2019s achieved in a very diverse set of substrates. This latter feature is crucial to explain the success of Pd/L2 catalytic system in other asymmetric transformations, such as hydrogenation of unfunctionalized olefins [20], intermolecular Heck reactions, with results comparable to Pd/PHOX catalyst, using ligand L2b (R\u00a0=\u00a0Ph) [21], and in the hydroboration of olefins [22]. Interestingly, for the latter transformation Ir/L2b (R\u00a0=\u00a0iPr) catalyst proved to be of an exceptional effectiveness, attaining higher\u00a0ee\u2019s (up to 94%) than phosphine-oxazoline PHOX ligands [23]. These results are specially remarkable because achieving high selectivities in the hydroboration of 1,1\u2032-disubstiuted alkenes is difficult, due to face selectivity issues and the difficulties in controlling the regiospecific boration in the terminal \u03b2-position [24]. Particularly, Pd/L2b is the only catalytic system able to hydroborate \u03b1-tert-butylstyrenes, thus complementing Cu-NHC catalysts, the only other system able to hydroborate \u03b1-alkyl styrenes with high\u00a0ee\u2019s [25].A final benefit of the new phosphite-oxazoline L2 ligands compared to the PHOX ligands is that the most efficient ligand in all of the catalytic asymmetric reactions discussed above are derived from affordable (S)-phenylglycinol or (S)-valinol (R\u00a0=\u00a0Ph or iPr) instead of the more costly (S)-tert-leucinol found in PHOX ligands.Other modifications of the PHOX ligands are on the oxazoline group, by attaching the phenyl backbone ring to the stereogenic center next to the oxazoline (ligands L3; Fig. 3\n) [26], or introducing other oxazoline substituents such as, ferrocene, tricyclic and sugar oxazoline groups (e.g. ligands L4 and L5; Fig. 3) [27]. However, in any case the enantioselectivities and the substrate scope improved those attained with the PHOX ligands. Thus, ligands L3 (R1\u00a0=\u00a0Cy or Ph and R2\u00a0=\u00a0tBu) led to high enantioselectivities in the Ir-catalyzed hydrogenation of unfunctionalized olefins but only in the reduction of some methylstilbenes (ee's up to 99%\u00a0ee) and a range of \u03b2-methylcinnamic esters with enantioselectivities up to 99% [26]. Ligand L3 (R1\u00a0=\u00a0Ph and R2\u00a0=\u00a0tBu) has also been used with success in the Pd-catalyzed allylic alkylation of benchmark substrate (ee's of 98%) and in the intermolecular Heck reaction of 2,3-dihydrofuran with the phenyl triflate (94%\u00a0ee) [28]. Ligands L4 and L5 followed a similar trend than the PHOX ligands in the Pd-catalyzed allylic alkylation. Thus, they provided high enantioselectivities with rac-(E)-1,3-diarylallyl substrates, but low for cyclic ones. Ligand L4 provided also high enantiocontrol in the Pd-catalyzed Heck reaction of 2,3-dihydrofuran using various aryl triflates (ee\u2019s up to 98%) [29].Another modification into the oxazoline ring was to introduce substituents in the 5 and/or 5\u2032 positions (e.g. ligands L6 and L7; Fig. 4\n) [30] providing similar levels of enantioinduction than the usually most effective PHOX ligand, the tBu-PHOX, but with the advantage of being readily accessible as both enantiomers from either the (S) or (R)-valine rather than from expensive tert-leucinol enantiomers.Besides these modifications on the phosphine and oxazoline moieties and in phenyl backbone ring, many changes on the ligand backbone have been studied. One of these modifications includes a methylene spacer linking the phenyl ring of the ligand backbone and the oxazoline ring (ligands L8), forming with the metal a higher seven-membered chelate ring (phosphine-oxazoline ligands L8, R1\u00a0=\u00a0Me, H; R2\u00a0=\u00a0Me, iPr, tBu; Scheme 7\n) [31]. The phosphine moiety in ligands L8 has also been exchanged for a biaryl phosphite group (ligands L9; Scheme 7) [32]. Ligands L8 and L9 were synthesized in few steps from easily available starting material, as illustrated in Scheme 7.Zhou and coworkers used Pd/L8 catalytic systems in the intermolecular Heck reaction (Scheme 8\n) [31]. The intermolecular asymmetric Heck reaction is less developed than the intramolecular version due to regioselectivity issues, which hampers its application for the synthesis of more complex molecules [33]. Pfaltz early demonstrated that the use of tBu-PHOX ligand can overcome the regioselectivity issue, although it requires from 3 to 7\u00a0days for full conversion [34]. Ligands L8 (R1\u00a0=\u00a0Me, H and R2\u00a0=\u00a0tBu) provided high regio- and enantioselectivities (up to 95%\u00a0ee, Scheme 8), with results comparable to PHOX ligands, in the reaction of 2,3-dihydrofuran and various aryl triflates [31]. From these results it should be highlighted that ligands that contained hydrogens in the benzylic position provided the R-enantiomer while ligands with methyl substituents at R1 provided the S-enantiomers (Scheme 8).More recently our group decided to replace the phosphine group in ligands L8 by several \u03c0-acceptor biaryl phosphite moieties (ligands L9b, e, f\u2013h; R1\u00a0=\u00a0Me, H; R2\u00a0=\u00a0iPr, tBu, Ph; Scheme 7) [32]. This change increases the activity, because the presence of the phosphite group favors the migratory insertion, which come up to be the rate-determining step. At the same time the substrate scope could be extended to other heterocyclic and carbocyclic olefins and to other triflates including non-aromatic ones (ee's up to 98% and regioselectivities up to 99%). The best results were obtained with the ligand that had biaryl phosphite groups b, e and h, a hydrogen in R1 positions and an iPr oxazoline substituent, avoiding the use of the costly tBu substituent required in the analogous phosphine-oxazoline L8 and PHOX ligands [32].Advantageously, the same family of P-oxazoline ligands L8\u2013L9 also provided an excellent catalytic performance in the reduction of unfunctionalized olefins or olefins with poorly coordinative groups. This was a relevant finding because the reduction of these type of substrates is underdeveloped compared with the asymmetric hydrogenation of alkenes containing coordinative groups [35]. This is because catalysts able to hydrogenate unfunctionalized olefins are very sensitive to changes in the olefin geometry and to changes in the substitution pattern. Thus, for instance, most of the catalysts perform well for trisubstituted E-unfunctionalized alkenes. Only very recently have appeared catalysts able to reduce Z-trisubstituted and 1,1\u2032-disubstituted. The asymmetric reduction of tetrasubstituted unfunctionalized olefins still remains a challenge. This substrate-dependent behavior was already displayed with the pioneering Pfaltz\u2019s design of [Ir(PHOX)(cod)]BArF (cod\u00a0=\u00a01,5-cyclooctadiene and BArF\u00a0=\u00a03,5-(F3C)2-C6H3)4B) catalyst precursors [36], which mainly provides high\u00a0ee\u2019s in the hydrogenation of a small group of E-trisubstituted olefins [37]. The authors first prepared the catalyst precursors [Ir(cod)(L8\u2013L9)]BArF by reaction of the corresponding ligand with [Ir(\u03bc-Cl)(cod)]2 and subsequent Cl/BArF anion exchange to give air stable red\u2013orange solids in high yields (Scheme 9\n). The VT-NMR spectra (from +35 to \u221285\u00a0\u00b0C) showed one single isomer in solution [20,38].They found that the use of [Ir(cod)(L8)]BArF (L8; R1\u00a0=\u00a0H and R2\u00a0=\u00a0iPr) allowed to extend the range of E-trisubstituted olefins to include allylic alcohols and \u03b1,\u03b2-unsaturated ketones and esters (ee\u2019s up to 98%) [38]. Replacing the phosphine moiety in ligands L8 by several \u03c0-acceptor biaryl phosphite groups (ligands L9) further extended the array of substrates successfully hydrogenated, including more challenging 1,1\u2032-disubstituted olefins [20]. The highest enantioselectivities were obtained with [Ir(cod)(L9)]BArF containing the ligand with the less expensive Ph or iPr oxazoline substituents and hydrogen atoms in the benzylic position. The phosphite group depended on the substrate to be hydrogenated (a summary of the reduction of 55 olefins with Ir/L9 are shown in Fig. 5\n). Interestingly, environmentally friendly solvent 1,2-propylene carbonate (PC) could be used instead of the commonly used dichloromethane without any deleterious effect on enantioselectivity. High enantioselectivities were therefore attained in the reduction of trisubstituted olefins including the more challenging triarylsubstituted substrates ones and those containing several poorly coordinative groups such as \u03b1,\u03b2-unsaturated ketones, amide, lactones, lactams, alkenyl boronic esters and enol phosphinates (ee\u2019s up to >99%). Even thought, highly enantioselective hydrogenation catalysts for 1,1\u2032-disubstituted substrates are very scarce [39], it was gratifying to obtain\u00a0ee\u2019s up to 98% in a large number of tert-butyl-aryl 1,1\u2032-disubstituted alkenes (Fig. 5) which differs in the steric and electronic characteristics of the aryl substituent. Decreasing the bulkiness of the alkyl substituent on these \u03b1-alkyl-styrenes results in slightly lower enantioselectivities (ee\u2019s from 83% to 91%), due to a competing isomerization pathway as it was disclosed by means of deuterium labeling experiments. Similar values of enantioselectivities were found in the hydrogenation of 1,1\u2032-disubstituted alkenyl boronic esters and enol phosphinates (Fig. 5).In addition, [Ir(cod)(L9)]BArF were also successfully applied in the reduction of an additional challenging class of substrate; the cyclic \u03b2-enamides (Scheme 10\n) [40]. Despite, there is an important number of therapeutic agents (e.g. robalzotan, rotigotine, terutroban and alnespirone) [41] than can be accessed via their hydrogenation, there are only few catalysts able to hydrogenate such substrate class with high\u00a0ee\u2019s, being the majority based on rhodium and ruthenium [42]. In 2016, Verdaguer\u2019s and Riera\u2019s group demonstrated that Ir-PN catalyst can also be used, exceeding the scope of Ru/Rh-catalyst [43]. Then, our group decided also to study the application of Ir/L9\n[40]. Enantioselectivities were high for many cyclic \u03b2-enamides derived from, 2-tetralones and 3-chromanones when using Ir/L9f (R1\u00a0=\u00a0H; R2\u00a0=\u00a0iPr) catalyst (Scheme 10). To note, the high enantioselectivity achieved in the reduction of N-(5-methoxy-3,4-dihydronaphthalen-2-yl)acetamide, which provides a crucial intermediate for the synthesis of rotigotine [40]. We also found that both enantiomers of the products can be accessed by exchange iridium to rhodium. Again, the use of PC has not effect on the enantioselectivities.Many of the backbone changes in the PHOX ligands also includes the replacement of phenyl backbone ring of PHOX ligand by other moieties (Fig. 6\n), such as ferro- and ruthenocene groups (e.g., ligands L10\u2013L14) [44], biphenyl or binaphthyl groups (e.g., ligands L15) [45], several heterocyclic backbones (e. g., ligands L16\u2013L19) [46], an alkyl chain (e.g., ligands L20\u2013L28) [47] and bicyclic, sugar, and spiro backbones (e.g., ligands L29\u2013L34). In many of these latter backbones modifications the phosphine group has also been replaced by a phosphinite, a phosphite, an aminophosphine and an stereogenic P groups. Among them those having two carbons linking the two donor functionalities have been employed with great success to several enantioselective reactions.In this respect, we can point up the families of phosphinite/phosphite-oxazoline ligands L22\u201323 and L24\u201327 (Fig. 6), where the ortho-phenylene tether of the PHOX has been changed by an alkyl chain. They were initially designed to provide a wider substrate capacity in the asymmetric reduction of unfunctionalized olefins. Starting from different carboxylic acid derivatives, chiral serine or threonine methyl esters, and Grignard reagents a range of hydroxyl-oxazolines were easily attained, which after treatment with the corresponding chlorophosphine or phosphorochloridite gave access to ligands L22\u2013L23 (Scheme 11\n) [48].The phosphinite\u2013oxazolines L22, developed by Pfaltz, (Fig. 6 and Scheme 11) are one of the most successful ligands for the Ir-catalyzed reduction of unfunctionalized olefins [24]. Unlike the PHOX ligands, the phosphorus unit is bonded to the stereogenic center next to the oxazoline.The Ir-catalyst precursors were synthesized using the same process described for previous [Ir(cod)(L8\u2013L9)]BArF complexes, obtaining air-stable orange powders that needed to be purified by column chromatography on silica gel. With [Ir(cod)(L22)]BArF, they reached excellent enantioselectivities for the first time in the reduction of E- and Z-2-aryl-2-butenes (Fig. 7\n) [48a,49]. The author optimized the enantioselectivity for each substrate by systematic modifications of the substituents at the oxazoline ring and ligand skeleton. The best enantioselectivities were achieved with [Ir(cod)(L22)]BArF, containing diphenylphosphinite ligands L22 (R1\u00a0=\u00a0Ph) with a methyl group at R3 and a benzyl at the alkyl chain (R4), although, the correct choice of the oxazoline R2 substituent and the configuration of the carbon of R3 is determined by olefin geometry. Thus, for E-trisubstituted olefins, \u00a0ee\u2019s are highest with a 3,5-Me2-Ph or a Ph oxazoline R2 substituent and an S-configuration for R3, while for Z-olefins they are best with a Ph oxazoline substituent and an R-configuration into the ligand. Further optimization of ligand parameters allowed for the first time to reduce some more challenging terminal olefins and 1,1\u2032-disubstituted enamines (ee's up to 99%; Fig. 7) with the ligand that contains a methyl and a benzyl group at R3 and at R4, respectively but a cyclohexenyl at R1\n[49b,c]. More recently, Ir-L22 also allowed the hydrogenation of \u03b1,\u03b2-unsaturated nitriles (ee's up to 98%, Fig. 7) [49d]. These catalysts also perform well in 1,2-propylene carbonate, a green solvent, allowing the catalysts to be recycled several times [50].Then our group synthetized the phosphite-based analogues of ligands L22, which broadened the number of 1,1\u2032-disubstituted substrates to be reduced with success [51]. By using [Ir(cod)(L23j)]BArF and [Ir(cod)(L23b)]BArF (R2\u00a0=\u00a0Ph, R3\u00a0=\u00a0H and R4\u00a0=\u00a0Me) catalysts high\u00a0ee\u2019s (up to >99%) were therefore attained in the hydrogenation of a range of (het)aryl\u2013alkyl disubstituted olefins, allylic alcohols and allylic silanes (29 compounds; Fig. 8\n), surpassing previous successfully Ir/L9 and Ir/L22 catalysts [39]. It should point up that Ir/L23j (R2\u00a0=\u00a0Ph, R3\u00a0=\u00a0H and R4\u00a0=\u00a0Me) also attained high\u00a0ee\u2019s for trisubstituted olefins and \u03b1,\u03b2-unsaturated esters. For Z-trisubstituted olefins and allylic alcohols the highest catalytic performance was attained with Ir/23b (R2\u00a0=\u00a0Ph, R3\u00a0=\u00a0H and R4\u00a0=\u00a0Me) [51]. Advantageously, the use of PC enabled the recycle of the catalysts until five times.Useful, ligands L23 were also used in the allylic substitution of many mono, di- and trisubstituted linear hindered and unhindered linear allylic acetates (up to 99%\u00a0ee) with C- and N-nucleophiles [48b]. Moreover, reversing the configuration of the alkyl chain or reversing the configuration of the phosphite group led to both enantiomers of the products. The results surpass PHOX ligands and are similar to the best accounted with the previous family of phosphite-oxazoline ligands L2 except for cyclic substrates (ee's up to 83%). To increase the enantioselectivity in the cyclic substrates the oxazoline group was changed by a thiazoline group (see Section 3) [18c]. With this simple modification the enantioselectivities improved significantly (94%\u00a0ee). Both families of ligands (phosphite-oxazoline/thiazoline) are complementary. The study of the Pd-\u03c0 allyl intermediates allowed to explain the catalytic performance. Thus, 1,3-diphenyl allyl and 1,3-cyclohexenyl allyl Pd-complexes showed that the substitutents at the backbone ligand chain and at the oxazoline group have to be correctly combined to give the isomer that reacts faster and to avoid complexes with the ligand coordinated monodentated. However, for the unhindered lineal substrates, the enantioselectivity is explained throught a late transition state were the substituent at the alkyl chain favored to reach a specific Pd-olefin complex [48b].Ligands L23 also provided high catalytic performance in the Pd-catalyzed Heck reaction. High regio- and enantioselectivity could be achieved using many substrates and triflate sources, with ligands L23b (R2\u00a0=\u00a0p-CH3-Ph or tBu, R3\u00a0=\u00a0H, R4\u00a0=\u00a0CH3, Fig. 6) [21]. Interestingly, the reaction times were considerably reduced by using microwave-irradiation conditions (from the 24\u00a0h with PHOX ligands to 10\u00a0min with ligand L23b) and regio- and enantioselectivities were still high (ee's up to 99%; Scheme 12\n).Another modification of ligands L22 was the development of ligands L24 (Fig. 6, R1\u00a0=\u00a0o-Tol, Ph and R2\u00a0=\u00a0tBu, iPr,), with the alkyl chain linked to the C-2 of the oxazoline group as in PHOX ligands [52]. Albeit the substrate range in asymmetric reduction of unfunctionalized olefins is reduced than with Ir/L22, they are complementary. Thus, high enantioselectivities were attained in the hydrogenation of allylic alcohols, alkenes with heteroaromatic substituents and the cyclic substrate 6-methoxy-1-methyl-3,4-dihydronaphthalene. Advantageous, Harmata and Hong have also used Ir/L24 catalyst in the total synthesis of pseudopteroxazole, a natural antitubercular agent (see Section 6). The catalysts was able to hydrogenate the internal double bond, and not the exocyclic CC bond, with high regioselectivity in 90% yield [53].More recently, new families of P-oxazolines (ligands L25\u2013L27; Fig. 6) analogous to Pfaltz ones, still with two carbon atoms between the P- and N-donor functionalities, have been developed. The phosphite/phosphinite-oxazoline ligands L25\u2013L27 were used in the enantioselective Ir-catalyzed hydrogenation and Pd-catalyzed allylic substitutions. Ligands L26 and L27 were prepared in a similar manner than ligand L24 and L25 (Scheme 13\n). Condensation of readily available chiral \u03b1-acetoxy acids with a range of chiral aminoalcohols followed by oxazoline formation using diethylaminosulfurtrifluoride (DAST) and subsequent alcohol deprotection yielded a range of hydroxyl-oxazolines. The later were then treated with the corresponding chlorophosphine or phosphorochloridite to give access to ligands L26\u2013L27 (Scheme 13) [40].About the hydrogenation, air stable orange-solids [Ir(cod)(L)]BArF (L\u00a0=\u00a0L26 and L27b, f\u2013h, k complexes were the first catalysts able to successfully hydrogenate di-, tri- and tetrasubstituted unfunctionalized olefins (ee's up to 99% in 62 examples, Fig. 9\n) [40]. As early stated, the asymmetric hydrogenation of tetrasubstituted olefins is still a challenge [35f,54]. Thus, there are a very limited number of catalyst able to hydrogenate them and those that do shows poor versatility, except for the recent publication with Ir/L21 catalyst [55] (Fig. 6) [56]. Improving previous reports, high enantioselectivities (up to 98%) were attained in the hydrogenation of several indenes, 1,2-dihydro-napthalene and a broad scope of acyclic tetrasubstituted olefins under mild reaction conditions using Ir-phosphinite-oxazoline L26 catalysts (Fig. 9) [40]. Significantly, it was also found that the phosphinite-oxazoline ligand L26 to be used depend on the olefin to be reduced. In this respect, while the highest enantioselectivity in the reduction of the more bulky cyclic indene substrates is obtained with the less bulky phosphinite group (Ph), for the less bulky indenes and acyclic substrates phosphinite ligands with bulkier substituents are needed (o-tolyl and cylcohexyl groups, respectively) to reach the highest enantioselectivity. Finally, by simple replacing the phosphinite by the right phosphite moiety (ligands L27) the same family of catalysts could also effectively hydrogenated a range of unfunctionalized tri- and disubstituted substrates (Fig. 9). The catalysts could also effectively reduce a variety of olefins with different functional groups from those poorly coordinative (e.g.enones, lactames and vinyl boronates) to highly coordinative ones (e.g. \u03b2-enamides) [40].Compared to Pd/L2 catalyst Pd/L27g (R2\u00a0=\u00a0R3\u00a0=\u00a0Ph) also gave higher activities (TOF up to 8000\u00a0h\u22121) and high enantioselectivities in the allylic substitution of a wide number of substrates (ee\u2019s up to >99%, 74 examples in total, Fig. 10\n) [57]. Symmetrically disubstituted linear allylic acetates, containing alkyl or aryl substituents, with a variety of C-nucleophiles, including \u03b1-substituted malonates, malononitrile, diketones, 2-cyanoacetates and pyrroles were successfully alkylated with Pd/27\u00a0g. High enantioselectivities were reached with: i) both alkyl and aryl amines, ii) benzylic, allylic and iii) silanols. By introducing in ligand L27g a second methyl group at the alkyl chain, \u00a0ee\u2019s could be improved up to >99% in the alkylation of cyclic substrates (Fig. 10). In addition, the Pd/L27g catalyst is one of the few catalyst that can deracemize unsymmetrically disubstituted substrates such as 1,1,1-trifluoro-4-phenylbut-3-en-2-yl acetate via dynamic kinetic asymmetric transformation with several malonates (yield\u2019s up to 72% and\u00a0ee\u2019s up to 80%). Regioselectivities up to 90% and\u00a0ee\u2019s up to 98% were offered in the alkylation of 1-arylallyl acetates with malonates. Nevertheless, the regioselectivity into the branched product decreased with \u03b1-substituted malonates (e.g. it dropped from 83% using dimethyl malonate to 60% using dimethyl 2-methylmalonate). NMR and DFT studies showed an early TS, in which the enantioselectivity is guided by the ratio of the Pd-\u03b73-allyl compounds and the relative electrophilicity of the allylic terminal carbon atoms. It was also found that the population of these intermediates is affected by the ligand parameters. Thus, whereas for cyclic substrates the configuration of the phosphite functionality together with the substituents in the alkyl chain guide the population of exo and endo isomers, for linear substrates its ratio is also affected by the oxazoline group [57].Most heterodonor P-oxazoline ligands developed have the chirality in the stereogenic carbon centers located on the oxazoline ring and/or in the carbon backbone (Fig. 6). We have also showed ligands that combines a chiral oxazoline or/and chiral carbon backbone with a phosphite with axial chirality (Fig. 6). However, few P-oxazoline ligands with a P-sterogenic center have been published, mainly due by the complexity of preparing bulky P-stereogenic phosphines in optically pure form (e.g. ligands L18\n[46b], L19\n[46c], L20\n[58] and L28\n[59]; Fig. 6). Verdaguer and Riera\u2019s have recently reported a simpler protocol for the synthesis of P-stereogenic aminophosphine-oxazoline ligands L28 (MaxPHOX ; R2\u00a0=\u00a0Ph, iPr, tBu; R3\u00a0=\u00a0iPr; Scheme 14\n). The synthesis of L28 relies in the fact that upon activation enantioenriched tert-butylphenyl phosphinous acid borane undergoes stereospecific nucleophilic substitution with a range of amino-oxazoline compounds (Scheme 14) [59b]. Reaction of L28 with [Ir(\u03bc-Cl)(cod)]2 and NaBArF using the above mentioned standard protocol led to [Ir(cod)(L28)]BArF catalyst precursors [59b]. Actually, ligands L28 only differ from ligands L26 and L27, in the replacement of phosphinite/phosphite groups by an aminophosphine group, so ligands L28 still have two carbon atoms linking the two donor groups.Usefully, these catalysts were able to efficiently hydrogenate a variety of tetrasubstituted olefins: indenes and 1,2-dihydro-napthalene derivatives (ee\u2019s up to 96%) and also acyclic tetrasubstituted olefins (ee\u2019s up to 99%; Fig. 11\n) [59a]. These excellent results were also attained in the reduction of tetrasubstituted vinyl fluorides (dr\u2019s >99% and\u00a0ee\u2019s up to 98%) [59a]. Catalysts Ir/L28 (R1\u00a0=\u00a0(S)-iPr and R2\u00a0=\u00a0(R)-tBu or (R)-iPr), which have the oxazoline substituent and the bulky group at the P-center cis to each other, also reached excellent results (>99%\u00a0ee) for cyclic \u03b2-enamides (10 examples; Fig. 11) using only 3\u00a0bar of hydrogen pressure [43]. The process could also be performed with greener solvents such as ethyl acetate and methanol. Ir/L28 (R1\u00a0=\u00a0(R)-iPr and R2\u00a0=\u00a0(S)-iPr) catalyst also provided comparable results than the best Ir\u2013P,N systems reported in the hydrogenation of challenging N-aryl imines (Fig. 11) [60]. The reaction was performed with a balloon of H2 at \u221220\u00a0\u00b0C, achieving up to 96%\u00a0ee. The authors found that the configuration at the P*-center had almost no effect on the enantioselectivity of the catalyst. Useful, the authors were able to isolate the active specie, complex 2, which was efficiently used to the direct hydrogenation of N-methyl ketimines (Fig. 11) [61]. Both, N-methyl imines and N-alkyl imines were hydrogenated with\u00a0ee\u2019s up to 94% using only 1\u00a0mol % of catalyst and 3\u00a0bar of H2, at 0\u00a0\u00b0C. The effective hydrogenation of this class of substrates had not yet been achieved, maybe due to the higher basicity of N-methyl amines than the N-aryl amines, which may lead to catalyst deactivation [62]. Ir/L28 (R1\u00a0=\u00a0(R)-iPr and R2\u00a0=\u00a0(R)-Ph) catalyst was also used with effectiveness in the enantioselective isomerization of N-allyl amides to enamides (Fig. 11), which allowed to shorter the reported synthetic route for obtaining the antibiotic R-sarkomycin methyl ester (See Section 6) [63].Andersson group was one of the few pioneering researchers in designing suitable ligands for the challenging Ir-catalyzed enantioselective reduction of unfunctionalized olefins. They synthesized an aminophosphine-oxazoline family of ligands L29 (R1\u00a0=\u00a0Cy, o-Tol, Ph; R2\u00a0=\u00a0tBu, H, Ph; R3\u00a0=\u00a0Ph, H, Fig. 6), with a rigid bicyclic backbone, to overcome the limited substrate scope in this process [64]. Ligands L29 have also two carbons atoms linking the two donor functionalities. Ligands L29 were prepared in a multigram scale from (1S,3R,4R)-2-((benzyloxy)carbonyl)-2-azabicyclo[2.2.1]heptane-3-carboxylic acid or (1S,3R,4R)-2-(((4-nitrobenzyl)oxy)carbonyl)-2-azabicyclo[2.2.1]heptane-3-carboxylic acid, which are accessible via stereoselective aza-Diels Alder reaction [65] followed by Cbz- or p-NO2-Cbz-protection of the free amine. The oxazoline moiety was introduced via amide coupling with the desired 1,2-aminoalcohol followed by addition of mesyl chloride and base. Amine deprotection by hydrogenolysis using Pd/C followed by reaction with the appropriate chlorophosphine completes the synthesis of ligands L29 (Scheme 15\n). The Ir-catalyst precursors [Ir(cod)(L29)]BArF were then prepared as previously described (see Scheme 9). For first time, Ir/L29 catalysts furnished high enantioselectivities in the reduction of enol phosphinates [64b,c], vinyl silanes [64d], vinyl boronates [64f], fluorinated olefins [64e], \u03b1,\u03b2-unsaturated lactones [64g], \u03b1,\u03b2-unsaturated acryclic esters [64g] and \u03b3,\u03b3-di- and \u03b2,\u03b3-disubstituted allylic alcohols [64h] (Fig. 12\n). The related aminophosphite-oxazoline/thiazole ligands extended the number of substrates that could be reduced with selectivities similar to the best published (see Section 3) [66]. They successfully reduced E- and Z-tri- and disubstituted olefins (ee\u2019s up to 99%), including those with poorly coordinative groups (e.g. alkenylboronic esters, enones, vinylsilanes \u2026) [66].Another notable family of P-oxazoline ligands with two carbon atoms linking the two donor functionalities, are the pyranoside ligands L30 (R\u00a0=\u00a0Me, iPr, tBu, Ph, Bn; Fig. 6). As with ligands L22\u2013L23 the P is bonded to the stereogenic center next to the oxazoline nitrogen atom but differs in the presence of a more rigid sugar backbone. Ligands L30 were efficiently synthesized from d-glucosamine, an inexpensive natural feedstock (Scheme 16\n). d-glucosamine was first treated with the desired acid chloride or anhydride to form the corresponding amide [67]. Then, the hydroxyls groups at C-4 and C-5 position were protected with a benzaldehyde, to provide more rigidity to the backbone, and the rest of alcohols were acetylated. Formation of the oxazoline group was then achieved in the presence of anhydrous SnCl4. Deacetylation followed by treatment with the corresponding phosphorochloridite led to ligands L30. Ligand L30 provided high catalytic performance in the hydrogenation of unfunctionalized olefins, allylic substitutions and intermolecular Heck reactions [68]. In fact, [Ir(cod)(L30)]BArF were the first fruitful use of catalyst precursors with biaryl-based phosphite ligand in the hydrogenation of unfunctionalized alkenes [68a]. These Ir catalyst precursors were prepared straightforward as a single isomer following the previously described methodology (Scheme 9) as orange air-stable solids [68a,b]. The use of [Ir(cod)(L30b)]BArF and [Ir(cod)(L30l)]BArF (R\u00a0=\u00a0Ph) complexes containing bulky groups in the biaryl phosphite functionality led to high enantioselectivities (ee\u2019s between 91% and >99%) in trisubstituted olefins (until 25 examples, Fig. 13\n), including triarylsubstituted substrates, \u03b1,\u03b2-unsaturated ketones and esters and vinylboronates among other type of olefins. High enantioselectivities were also reached in a range of terminal olefins (19 examples, Fig. 13) including heteroaromatic ones (ee\u2019s up to 99%). Note that lower\u00a0ee\u2019s were achieved when using the phosphinite-oxazoline analogues.A computational study showed that the reaction proceed through an IrIII/IrV catalytic cycle where the migratory insertion of the hydride is the step that control the selectivity [68b]. From the calculated TSs structures, a quadrant model describing the ligand-substrate interactions was developed. The occupance in this quadrant model suited perfectly for olefins containing E-geometry (Fig. 14\n). Calculations also shown that by varying the biaryl-phosphite substituents it is possible to modulate the occupance of the semihindered quadrant allowing the coordination of Z-olefins as well.Ligands L30l (R\u00a0=\u00a0Me) and L30b (R\u00a0=\u00a0Ph) were also used in the intermolecular Heck reaction [68e,f]. They provided high activities (full conversions in minutes with microwave irradiation) and enantio- and regioselectivities (up to 99%) for a range of carbo- and heterocyclic olefins and triflate sources. In contrast to PHOX ligands, having non-bulky substituents at the oxazoline has a positive effect on both, selectivities and activities. Finally, good activities and high enantioselectivities (up to 99%) have also been reached in the substitution of tri- di- and monosubstituted linear substrates and cyclic substrates [68c,d]. For hindered linear substrate ligand L30b (R\u00a0=\u00a0Ph) provided the best results, while for unhindered linear substrates ligand L30g (R\u00a0=\u00a0Me) provided the best results, and ligand L30f (R\u00a0=\u00a0iPr) was the best for cyclic substrates. The related phosphinite-oxazoline analogues reached lower enantioselectivities [67]. The elucidation of NMR of the Pd-\u03b73 allyl intermediates allowed the rationalization of the experimental catalytic results. They showed that the substituents at both the oxazoline and the phosphite moieties are crucial for high\u00a0ee\u2019s by enhancing the amount of the faster Pd-\u03b73 allyl isomer and, at the same time, eluding the presence of species with the ligand coordinated as monodentated. They also corroborated that the nucleophile attacks the allylic terminal carbon which is trans to the phosphite functionality [68].Since the pioneering work of Chan and coworkers, the spiro backbone has been identified as a privileged arrangement for ligand families and catalysts [69,70]. We can highlight four main types of spiro phosphine-oxazoline ligands (Fig. 6): the SpinPHOX [71] (L31) developed by Ding, SIPHOX [72] (L32) reported by Zhou, HMSI-PHOX [73] (L33) developed by Lin and SMIPHOX [74] (L34) by Teng. The SMIPHOX was developed having in mind some distinct features compared with the other three, such as an spiro indane-based P,N ligand with non-C2-symmetric skeleton and higher rigidity and only one chiral center avoiding the complex sterochemistry. Among them, we can highlight the work of Zhou and coworkers with the spiro phosphine-oxazoline (SIPHOX) Ir-catalysts. SIPHOX ligands were prepared from enantiopure 1,1\u2032-spirobiindane-7,7\u2032-diol (SPINOL), which are prepared from 3-methoxybenzaldehyde followed by resolution with N-benzylcinchonidinium chloride [75]. SPINOL was then transformed to the corresponding phosphine-triflate compounds by ditriflation of the diol, monophosphinylation with the desired diarylphosphine oxide in the presence of Pd(OAc)2 followed by reduction with trichlorosilane. Phosphine-triflates were then transformed to the phosphine-acids by Pd-catalyzed carbonylation followed by hydrolysis of the formed esters. Amide formation with the desired 1,2-aminoalcohol in the presence of DCC (N,N\u2032-dicyclohexylcarbodiimide) and HOBT (1-hydroxylbenzotriazole) followed by treatment with mesyl chloride and base led to spirocyclic phosphine-oxazolines L32 (Scheme 17\n). They could efficiently reduce imines and represented the first Ir-catalyst precursors [Ir(cod)(SIPHOX)]BArF (Fig. 15\n) able to reduce under basic reaction conditions a broad range of unsaturated carboxylic acids [72b,c], getting over the constraints of Rh- and Ru-catalysts, which are mainly restricted to acrylic and cinnamic acids (Fig. 15) [76]. The base is required to form the carboxylate anion that coordinates to iridium. Thus, Ir/L32, containing a bulky phosphine-aryl group (R1\u00a0=\u00a03,5-tBu2Ph), proved to be highly active (TONs up to 10.000) and enantioselective (ee\u2019s >99%) with a group of \u03b1-aryloxy and \u03b1-alkyloxy substituted \u03b1,\u03b2-unsaturated acids (Fig. 15). Nevertheless, Ir/SIPHOX catalysts do not perform well for \u03b1,\u03b2-unsaturated esters [77]. The same authors demonstrated the potential of Ir/L32 catalyst with the synthesis of a crucial intermediate in the preparation of rupintrivir (a rhinovirus protease inhibitor, see Section 6) [72c]. It should be noted that the key in the high activity of this catalyst can be found in the ligand\u2019s steric constraints which prevents the formation of inactive trimeric species, which are formed for most of the Ir/P-N catalyst [78].Over the years, Zhou\u2019s group have extensively studied the hydrogenation of several different classes of \u03b1,\u03b2-unsaturated carboxyxlic acids [77,79]. Thus, for example, excellent\u00a0ee\u2019s have been attained in the asymmetric hydrogenation of \u03b1-aryl- and \u03b1-oxymethyl-substituted cinnamic acids using Ir-SIPHOX catalysts (Fig. 16\n). This transformation were used in the preparation of (S)-(+)-homoisoflavone, a natural product with antibacterial activity (see Section 6) [80]. Latter, several heterocyclic olefins containing a carboxylic acid group were also successfully hydrogenated (ee's up to 99%\u00a0ee, Fig. 16). Again Zhou\u2019s group made use of this finding for the synthesis of the GABA uptake inhibitors (R)-tiagabine and (R)-nipecotic acid (see Section 6) [72e]. Interestingly, Ir/SIPHOX is also able to efficiently reduced a large number of tetrasubstituted acrylic acids (ee's up to 99%; Fig. 16), which have subsequently be used in the synthesis of the pyrethroid insecticide Fenvalerate and the antihypertensive drug Mibefradil (see Section 6) [81]. Interestingly, Ir/SIPHOX catalyst maintained its efficient when the carboxylic is moved away from the olefin. Thus, a range of \u03b2,\u03b3-unsaturated acids [72d] and terminal \u03b3,\u03b4-unsaturated acids [72f,82] were successfully hydrogenated (ee\u2019s up to 99%; Fig. 16) using Ir/SIPHOX (Ar\u00a0=\u00a0Xyl and R\u00a0=\u00a02-naphthylmethyl) catalyst. Again, the total synthesis of several natural products (e.g. (R)-xanthorrhizol, (R)-aristelegone-A \u2026) were attained (see Section 6). Finally, Ir/SIPHOX (Ar\u00a0=\u00a0Xyl and R\u00a0=\u00a0H) demonstrated its usefulness in the hydrogenation of terminal olefins containing a benzoic acid group (ee\u2019s up to >99%; Fig. 16), leading compounds with a benzylmethyl stereocenter like those found in the natural sesquiterpene phenols (S)-curcudiol and (S)-curcumene (see Section 6) [82]. Very recently, Zhang\u2019s group have accounted an oxa-spirocyclic version of L32 that has demonstrated their usefulness in the hydrogenation of terminal methylene-tetrahydro-benzo[d]azepin-2-ones [83].The mechanistic study, including DFT calculations, agrees with an Ir(III)/Ir(V) catalytic pathway [84]. More important, using sodium 2-methyl-3-phenylacrylate as a benchmark substrate they were able to isolate migratory insertion Ir(III) intermediate 3, with the carboxylate coordinated to iridium (Fig. 17\n). They also managed to characterize by X-Ray diffraction dimeric hydrido complexes 4 and 5 (Fig. 16), which further support the coordination of the carboxylate to iridium.In this section, a collection of the developed heterodonor P,N-ligands with a N-donor group other than an oxazoline ring is presented. Besides the common tuneable properties of chiral ligands, e.g phosphorus group, backbone and source of chirality, P,N-other ligands allow variation on the hybridization of the N-atom. The tuning on the N-donor group leads to a wide array of P,N-other ligands. Ligands with amino N-donors (N-sp3), imino N-donors (N-sp2), cyclic imino N-donors (N-sp2) and pyridino N-donors (N-sp2) have been synthesized and used in several metal-catalyzed asymmetric reactions [3d,85].The first reports on chiral P,N-ligands came out back in the 70\u2032s with the work of Hayashi and Kumada, where they showed that chiral aminophosphines were promising ligands for asymmetric catalysis [86]. In their early reports they developed the first P,N-ligands bearing planar chirality, the (aminoalkylferrocenyl)phosphines PPFA and MPFA (L35 and L36, respectively, Scheme 18\n). PPFA and MPFA were synthesized by introducing a phosphino group into \u03b1-dimethylaminoethylferrocene (6) through stereoselective lithiation [86a]. The overall synthesis starts with the transformation of ferrocene to intermediate 6 in 6 steps, via the formation of formylferrocene (Scheme 18) [87]. Both ligands were initially applied to the Rh-catalyzed hydrosilylation of ketones with high yields (up to 89%) albeit with 49%\u00a0ee [86a]. PPFA was also used in the Ni-catalyzed asymmetric Grignard cross-coupling with (1-phenylethyl)magnesium bromide and vinyl bromide providing an enantioselectivity of 63%\u00a0ee [86b]. Later, the same synthetic strategy was used to prepare the more constrained PTFA ligand L37 reported by Weissensteiner, but ligand L37 was prepared from intermediate 8 through formation of the corresponding imine (Scheme 18) [88]. The decrease flexibility on ligand L37 was beneficial for the enantioselectivity in the Ni-catalyzed asymmetric Grignard cross-coupling, affording an\u00a0ee of 79% [89]. In 2006, Jin\u2019s group synthesized L38 with a cyclic amine as nitrogen donor. L38 was synthesized from formylferrocene via diphenylphosphinoferrocenecarboxaldehyde obtained through Kagan\u2019s method (Scheme 18) [90]. This modification was found to be beneficial for the asymmetric induction, which was illustrated with the excellent enantioselectivities obtained in the Pd-catalyzed allylic substitution with dimethylmalonate (99.6%\u00a0ee) [91], which were comparable to the obtained with its P,N-oxazoline analogue [92].Ferrocenes with planar chirality attracted considerable attention in asymmetric catalysis, especially PPFA, which has been widely used for other researchers in many asymmetric transformations (Scheme 19\n). For example, Wang accounted the use of PPFA to the Cu-catalyzed addition of diethylzinc to imines, affording high enantioselectivities (up to 97%\u00a0ee; Scheme\u00a019a) [93]. Later, Sestelo and Sarandeses employed PPFA as the chiral ligand for obtaining 1,1\u2032-binaphthyls by Pd-catalyzed cross coupling reactions of triorganoindium reagents, with\u00a0ee\u2019s up to 86% (Scheme\u00a019b) [94]. Very recently, Guo has shown the use of PPFA in the Ag(I)-catalyzed tandem [3\u00a0+\u00a02] cycloaddition/1,4-addition between aza-o-quinone methides (ao-QMs) and azomethine ylides, yielding imidazolidine derivatives with excellent diastereo- and enantioselectivities (up to 20:1 dr and 98%\u00a0ee; Scheme\u00a019c) [95]. The strategy has been extended to the addition of arynes, generated in situ from o-silylaryl triflates (Scheme\u00a019d) [96]. Finally, PPFA has also showed utility in the Ir-catalyzed ring-opening of low-activity azabenzonorbornadiene with various aliphatic and aromatic amines, providing the corresponding chiral vicinal 1,2-diamine scaffolds in high yields and enantioselectivities (up to 97%\u00a0ee; Scheme\u00a019e) [97].Hayashi described the transformation of (S,R)-PPFA (L35; Scheme 18) to P-imine ligands L39 in three steps (Scheme 20\n). To achieve new ligands L39 the dimethylamino group on (S,R)-L35 was replaced by an amino moiety via acetate intermediate 10. Next, condensation of amino intermediate 11 with benzaldehyde furnished P-imine ligands (S,R)-L39\n[98]. Ligands with electron-withdrawing groups at the aryl moiety provided higher enantioselectivities than its N-sp3 counterpart PFFA (L35) (up to 90%\u00a0ee vs 16%\u00a0ee) in the Rh-catalyzed asymmetric hydrosilylation of acetophenones [98]. Ligands L39 were also screened in the Pd-catalyzed allylic alkylations and again, the best selectivities were offered with a ligand having an electrodeficient aryl group. Using dimethylmalonate as nucleophile, the substitution of diphenylallyl acetate and pivalate and some cyclic substrates furnished promising enantioselectivities (up to 96%\u00a0ee and 91%\u00a0ee, respectively) [99]. Since then, other ferrocenylphosphino-imine ligands have been prepared from ferrocenylphosphino-amine compound 11 and the corresponding aldehyde (ligands L40\u2013L41; Scheme 20) [100]. In contrast, ligand L42 was prepared by mixing 11 with imidate 13, which was prepared from the desired 2-methylbenzonitrile (12) in 4 steps (Scheme 20) [101].In general, ligands L40\u2013L42 attained also high enantioselectivities in the benchmark Pd-allylic alkylation [100,101]. In addition, ligands L41 containing quaternary ammonium salts were also tested in the substitution of the benchmark substrate with different carbon nucleophiles and benzyl amine, providing enantioselectivities up to 94%\u00a0ee (L41, X\u00a0=\u00a0I, n\u00a0=\u00a00) [100b]. This ligand was also employed in allylic etherification reactions with a range of benzyl alcohols, providing enantioselectivities up to 91%\u00a0ee, but moderate yields (up to 74%) [102]. Ligands L42 (R\u00a0=\u00a05-Cl) bearing an imidate moiety showed even higher enantioselectivities in the allylic substitution of 1,3-diphenylallyl acetate using different malonate nucleophiles (up to 99% yield and >99%\u00a0ee) [101]. In addition, the alkylation of cyclic substrates and the unhindered substrate rac-1,3-dimethyl-3-acetoxyprop-1-ene also resulted in good enantioselectivities (ee\u2019s ranging from 75 to 90%). Phosphino-imidate ligands L42 were also applied in the Ir-catalyzed hydrogenation of di-, tri- and tetrasubstituted olefins, providing moderate to good enantioselectivities (45\u201391%\u00a0ee) [103].Similarly, \u03b1-phosphino-\u03b2-imine ligands L43\u2013L45 were prepared from the corresponding aldehyde and amine. Phosphine-hydrazone ligands L43 were obtained through condensation of intermediate 7 (also used for the synthesis of L38; Scheme 18) and the corresponding pyrrolidine 14 (Scheme 21\n). These ligands were screened in the Pd-catalyzed allylic substitution of the benchmark linear substrate with dimethyl malonate (L43, R\u00a0=\u00a0H; 96%\u00a0ee) and benzylamine (L43, R\u00a0=\u00a0Me; 96%\u00a0ee) [104]. Ligands L44, derived from condensation of 1-ferrocenylalkylamine 15 and 2-(diphenylphosphino)benzaldehyde 16 (Scheme 21), provided a better enantioselectivity of 97%\u00a0ee in the benchmark reaction [105]. Ligands L45, having a phenyl-chromium tricarbonyl motif as planar chirality source (Scheme 21), gave even a higher enantioselectivity (>98%\u00a0ee) than L43 and L44 when tested in the benchmark allylic substitution reaction [106]. Studies showed that for these ligands the enantioinduction is predominantly controlled by the planar chiral element and increases with the bulkiness of the N-substituents of the imine.Recently, (S,R)-PPFA ligand (Scheme 18) has been modified to provide ferrocene ligands L46 with a benzimidazole moiety (Scheme\u00a022\na). As for ligands L39\u2013L41, their synthesis proceeds through acetate intermediate 10 but in this case, amine 18 was subjected to reaction with benzimidazole 19 to yield the corresponding ligand [107]. Ligand L46 was key to achieve the highly enantioselective Pd-catalyzed [3\u00a0+\u00a02] cycloaddition of propargylic esters with \u03b2-ketoesters, providing high yields and enantioselectivities (up to 98%\u00a0ee). The reaction gave access to a range of chiral 2,3-dihydrofurans with an exocyclic double bond that remain unavailable with the known synthetic methods (Scheme\u00a022b).P,N-ligands with axial chirality have found place in a broad range of applications in asymmetric catalysis [85e,g]. The advances of BINAP ligand in Ru-catalyzed asymmetric hydrogenations [108] together with the contemporary success of ferrocene-based P,N-ligands by Hayashi and Kumada [86a-b,d], led to the development of QUINAP (L47), which can be considered as the first highly efficient axially chiral P,N-ligand [109]. The crucial step in the synthesis of the racemic ligand was achieved via Pd-catalyzed cross-coupling of aryl chloride 20 and boronic acid 21 (Scheme\u00a023\na). After cleavage of the methyl ether group, the phosphine group was introduced through conventional chemistry. At this point, it was required a final diastereomeric resolution of the corresponding palladium salts obtained through reaction of rac-22 and palladium complex 23. Diastereomers 24 could then be decomplexed by reaction with a 1,2-bis-(diphenylphosphino)ethane (dppe) to furnish the enantiomerically pure (R)- and (S)-L47\n[109,110]. However, this methodology implied two main drawbacks. First, stoichiometric amounts of chiral palladium complex 23 were required, and second, the introduction of the phosphine group had to be done prior to the resolution step. This implied a resolution for every single ligand and thus, limiting the access to ligand diversity. These limitations have prompted to the search of more straightforward synthetic routes for the synthesis of enantiopure QUINAP and related ligands, by many research groups [111]. A novel way for the synthesis of QUINAP came in 2013 by Stoltz et al [111f]. The method consisted in the Pd-catalyzed asymmetric phosphination of aryl triflate 25 via dynamic kinetic resolution using Pd/26 catalytic system (Scheme\u00a023b). Recrystallization further increased the\u00a0ee from 90% to up to >99.5%. In 2016, Lassaletta reported a new methodology for the synthesis of QUINAP via Pd-catalyzed dynamic kinetic C\u2013P cross-coupling between triflate 25 and a trimethylsilylphosphine, with the use of a Josiphos-type ligand 27 (\nScheme\u00a023c) [111g]. This method gave access to QUINAP with 91.5%\u00a0ee as well as several other potential P,N-ligands with axial chirality.Over the years it has been demonstrated that QUINAP is among the most outstanding axially chiral P,N-ligands with applications in many enantioselective transformations [85e,g]. The initial work with QUINAP was focused on Rh-catalyzed hydroboration of aryl alkenes and Pd-catalyzed allylic alkylation. Later it has also showed its utility in several other asymmetric transformations. Brown et al. early demonstrated the value of QUINAP in the Rh-catalyzed hydroboration of vinylarenes, which after oxidation led to variety of secondary alcohols with\u00a0ee\u2019s up to 96% (Scheme\u00a024\na) [112]. This methodology was also used in the synthesis of primary and secondary chiral amines with good to high enantioselectivities (77\u201398%\u00a0ee, Scheme\u00a024b) [113]. In this case, the obtained chiral catecholboronate esters were transformed to the desired amines through alkylation with MeMgCl or ZnEt2, followed by conventional electrophilic amination with NH2OSO3H (for primary amines) or R\u2032NHCl (for secondary amines).Morken and co-workers found that QUINAP was an excellent ligand also for the enantioselective Rh-catalyzed diboration of alkenes with dicathecol diboron [114]. It efficiently catalyzed the reaction of alkenes with dicatechol\u2013diborane, to yield the syn-addition products. Subsequent oxidation yielded the corresponding enantiopure diols (Scheme\u00a024c). The system showed a big scope for trans-disubstituted olefins, and unlike the Rh/Quinap-catalyzed hydroboration, the reaction occurred also with purely aliphatic alkenes. For trisubstituted alkenes, the enantioselectivities were also very high although yields were somewhat lower, while monosubstituted and cis-substituted alkenes reacted with lower enantioselection. Finally, Morken\u2019s system also allowed the Rh-catalyzed tandem diboration/Suzuki/oxidation reaction to provide several chiral 1-aryl-2-ols in one-pot and in an operationally simple way (Scheme\u00a024d) [114b].As stated above, the early work with QUINAP showed that it was also useful in the Pd-catalyzed alkylation of rac-1,3-diphenylallyl acetate and dimethyl malonate (98%\u00a0ee, Scheme\u00a025\na) [115]. Recently, Lassaletta and co-workers accounted the use of Pd/QUINAP catalyst in the dynamic kinetic asymmetric Buchwald-Hartwig amination and alkynylation reactions (Scheme\u00a025b,c) [116]. Thus, a variety of enantiopure amino- and alkynyl-heterobiaryls were attained in high yields and\u00a0ee 's up to 93% and 98%, respectively. Both processes were used to access to different axially chiral ligands, such as the IAN-type N,N-ligands [116b]\nKnochel and co-workers were the first in showing the utility of QUINAP in Cu-catalyzed coupling reactions. Initially, the Cu(I)/QUINAP catalyst was applied to a range of enamines with terminal alkynes providing the corresponding propargylamines in up to 90%\u00a0ee (Scheme\u00a026\na) [117]. Later, it was found that the same system allowed the three-component reaction between aldehydes, amines, and alkynes (A3 coupling). A wide range of propargylic amines could be afforded with good yields and enantioselectivities without the need to preform sensitive enamines (Scheme\u00a026b) [118]. QUINAP was also screened in the Cu-catalyzed \u03b2-borylation of \u03b1,\u03b2-unsaturated esters, but while excellent conversions were achieved, the enantioselectivities didn\u2019t surpass 79%\u00a0ee (Scheme\u00a026c) [119]. Schreiber applied the Cu/QUINAP system to the alkynylation of different isoquinoline iminium salts providing chiral 1-alkynyl tetrahydroisoquinoline derivatives (ee\u2019s up to 99%; Scheme\u00a026d) [120]. More recently, the Cu/QUINAP has been combined with a photo redox catalytic system for the cross-dehydrogenative-coupling of alkynes to N-aryl tetrahydroquinolines (Scheme\u00a026e) [121]. This strategy allows the direct use of tetrahydroquinolines without preformation of the iminium salt maintaining the high\u00a0ee\u2019s (up to 96%), albeit with moderate to high yields (up to 90%).The QUINAP ligand was found to be an excellent candidate for the Ag-catalyzed [3\u00a0+\u00a02] cycloaddition reaction of tert-butyl acrylate with azomethine ylides. The reaction gives access to pyrrolidines with multiple stereocentres, with an endo:exo ratio of >20:1 and up to 96%\u00a0ee (Scheme\u00a027\na) [122]. In 2013, Reisman expanded this methodology to the preparation of pyrrolizidines with up to 6 stereogenic centers in one flask with up to 94%\u00a0ee (Scheme\u00a027b) [123].In 2010, Murakami reported the Ni-catalyzed allene cycloaddition reaction of 1,2,3,4-benzothiatriazine-1,1(2H)-dioxides and allenes using QUINAP (Scheme 28\n) [124]. The reaction showed a broad scope with enantioselectivities up to 97%\u00a0ee.The success with QUINAP pushed other researcher to explore other related atropisomeric P,N-ligands L48\u2013L52 (Scheme 29\n\n) in asymmetric catalysis. QUINAZOLINAP (L48) [125] and PyPHOS (L49) [126] were prepared following a similar route than the used originally for QUINAP. These ligands also required to be resolved for which stoichiometric amounts of the chiral Pd complex 23 were needed. In addition, in the case of QUINAZOLINAP, the resolution procedure had to be further modified depending on the steric bulk of the substituent at the 2-position. This was not the case for PINAP ligands (L50\u2013L52), which were designed to overcome these drawbacks. For these ligands, the diastereoisomers can therefore be separated by crystallization or column chromatography (Scheme 29) [127]. Thus, the racemic backbone was first easily prepared by selective oxidative Friedel\u2013Crafts coupling of the dichlorophthalazine with 2-naphthol. Then, heteroaryl chloride 28 could react with (R)-phenylethanol (29) followed by triflation to provide 30, or being first subjected to triflation and then react with the desired chiral amine 31 to provide 32. Finally, using the same methodology as in the synthesis of QUINAZOLINAP, 30 and 32 were phosphinated to furnish PINAP type ligands L50\u2013L52\n[128]. Separation of diastereomeric (R,Sax\n)- and (R,Rax\n)-mixtures were then easily done by column chromatography [128].The application of ligands L48\u2013L52, showed in some cases better results and even broader applicability than QUINAP. For instance QUINAZOLINAP ligands exhibited even slightly higher enantioselectivities in the Rh-hydroboration of a broad selection of vinylarenes, thanks to the tunability of the substituent at the 2-position of the atropoisomeric backbone [125b,h]. Concretely, the use of 2-methyl QUINAZOLINAP (L48; R\u00a0=\u00a0Me) provided excellent enantioselectivities (up to 99.5%\u00a0ee). Note that the high\u00a0ee\u2019s are maintained when using tri- and tetrasubstituted vinyl arenes (e.g. indene, stilbene and 1,2-dihydronaphthalene), which usually proceeded with low enantioselectivities. L48 were also applied to Pd-catalyzed allylic alkylations. The enantioselectivity was dependent on the 2-position of the quinazoline backbone, being ligands with a 2-iPr substituent the most enantioselective (up to 94%\u00a0ee) [125b,f,g]. PyPHOS (L49) ligands also proved to be useful in the Rh-catalyzed hydroboration of vinylarenes.As previously commented, PINAP (L50\u2013L52) ligands have the advantage over QUINAP and related ligands of not requiring chiral Pd salts for their resolution, and therefore they are easily accessed. Moreover, Carreira showed that O-PINAP ligand (L50) gave comparable\u00a0ee\u2019s than QUINAP in the hydroboration of styrenes (up to 94%\u00a0ee) and the cycloaddition of azomethine ylides and acrylates (up to 95%\u00a0ee) [127a]. More important is the excellent performance achieved in Cu-catalysis by the PINAP ligand family. N-PINAP (L51) exhibited even higher enantioselectivities in the Cu-catalyzed A3 coupling for the preparation of propargyl amines (90\u201399%\u00a0ee) [127a]. Later, the scope of the reaction has been further extended by Carreira and Ma. For instance, Carreira reported the preparation of propargylic amines bearing the more labile group 4-piperidone, which allowed easy deprotection to afford propargylic primary amines 33 in high yields (up to 92%) and up to 96%\u00a0ee (Fig. 18\n) [129]. Ma developed a highly enantioselective A3 coupling of pyrrolidine, 2-methylbut-3-yn-2-ol and several aromatic aldehydes, which previously had shown lower enantioselectivities than aliphatic aldehydes (Fig. 18, compounds 34). A range of propargylic amines were obtained in 91\u2212>99%\u00a0ee [130]. The Cu/PINAP three-component coupling of propargylic alcohols, aldehydes and pyrrolidine was also used for the synthesis of chiral allenols 35 (Fig. 18) [131]. The reaction proceeds through formation of the corresponding propargylic amine and posterior Zn-mediated deamination. More recently, Ma has disclosed the A3 coupling of terminal alkynes, aldehydes and 3-pyrroline or isoindoline and subsequent [1,5]-hydride transfer catalyzed by CuBr to provide the (E)-N-allyl pyrroles with high yields. By using N-PINAP ligands (L51) it was possible to achieve the chiral (E)-N-allyl pyrroles in 97%\u00a0ee and the four possible diastereoisomers of 37 in >99%\u00a0ee (Fig. 18) [132]. The synthetic applicability of the A3 coupling using the Cu/N-PINAP system has been recently shown by Oguri, who used this methodology to prepare anti-malarial 6-aza-artemisinins in only four steps [133]. It has been also found that the use of tetrahydroisoquinoline as an amine source in A3 couplings affording tetrahydroisoquinoline-alkaloid derivatives (38, Fig. 18) [134]. The corresponding products were furnished with excellent yields and high enantioselectivities (up to 95%\u00a0ee). This strategy has been fruitfully applied in the total synthesis of several natural products, such as (+)-crispine A and (+)-dysoxyline with an excellent\u00a0ee of 98% [134], and various naturally occurring alkaloids (see Section 6) [135].Another important application of Cu/PINAP catalytic systems is the alkyne conjugate addition to Meldrum\u2019s acid derivatives. The initial catalyst screening showed that while phosphine ligands (e.g. Josiphos, BINAP and Monophos) and N,N-ligands gave low\u00a0ee\u2019s (up to 25%), the first generation of PINAP ligands (L50\u2013L52) gave moderate yields (up to 58%) and enantioselectivities (up to 80%\u00a0ee). The incorporation of amino alcohols derived from amino acids in the 2-position of the PINAP scaffold, lead to more enantioselective ligands for this transformation (Scheme 30\n, ligand L52). In addition, methoxy-substituted ligands (L52) catalyzed the reaction faster. With the optimized ligand, the addition of phenylacetetylene to various Meldrum\u2019s acids in aqueous media proceeded smoothly and with enantioselectivities of 82\u201397%\u00a0ee [136].Finally, Gu et al. showed a different application for O-PINAP (L50) ligands. Thus, a range of quinilinoferrocenes with a planar chirality were attained via Pd-catalyzed asymmetric intramolecular CpH bond functionalization/cyclization reaction of 2-halophenyl ferrocenecarboxylic amides [137]. However, only moderate enantioselectivities were achieved (up to 67%\u00a0ee, Scheme 31\n).Another important family of axially chiral ligands are the nitrogen analogues of Hayashi\u2019s MOP ligands [138], the MAP ligand family L53 (Scheme 32\n) [139]. Ko\u010dovsk\u00fd and co-workers synthesized for the first time the MAP ligands from the known biaryl amino alcohol NOBIN. First, alkylation of the amine group takes place, followed by the introduction of the diphenylphosphine group on triflate intermediate 39 trough Pd-catalyzed coupling with R2P(O)H. Successive reduction of intermediate 40 affords the desired MAP ligand. Although designed to act as bidentate P,N-donor ligands, NMR studies of the PdCl2/MAP complex showed a mixture of three species, where the major one was a cyclometallated complex [140].MAP ligand (R\u00a0=\u00a0Ph) was applied to various Pd-catalyzed asymmetric transformations, namely asymmetric allylic alkylation, Hartwig-Buchwald aminations and Suzuki cross-couplings (ee's up to 73%) [141]. Next, related L53 with R\u00a0=\u00a0Cy allowed for the first time the preparation of enantiopure chiral biaryls with up to 92% enantiomeric excess through asymmetric Suzuki-Miyaura (Fig. 19\n\n[142]). Later, the substrate scope of boronic acids and aryl halides was expanded, including also axially chiral heteroaromatic and biphenyl compounds (Fig. 19) [143].Following the same synthetic strategy than for MAP-ligands, Ding prepared the octahydro analogues H8-MAP (H8-L53, Fig. 20\n) which gave higher enantioselectivities than L53 in the Pd-catalyzed alkylation of (E)-1,3-diphenylallyl acetate (83%\u00a0ee vs. 73%\u00a0ee) [144]. The higher enantioselectivity obtained was attributed to the larger bite angle of the H8-L53 ligands [145].Axially chiral P,N-ligands L54\u2013L57 with a rigid amide linker are also derived from NOBIN (Scheme 33\n). To synthesize L54, NOBIN is first transformed to amino-phosphine compound 41 in 3 steps, which then reacts with 2-picolinic acid 42. Ligands L54 were screened in the Cu-catalyzed 1,4-addition of diethylzinc to different linear enones. A ligand with a 2-Me-pyridine moiety (L54, R\u00a0=\u00a0Me) provided the highest enantioselectivities (up to 98%\u00a0ee). A promising enantioselectivity (up to 98\u00a0ee) was obtained also for the purely aliphatic enone (E)-5-methylhex-3-en-2-one [146]. More recently, the presence of a phenyl group in the ortho-position of the pyridyl moiety (L54, R\u00a0=\u00a0Ph) allowed the conjugate addition of various aldehydes with good yields (78\u201390%) and good to high enantioselectivities (75\u201398%\u00a0ee) [147].Later, Hu and co-workers described the analogues phosphinite ligands L55\u2013L57 (Scheme 33). The synthesis of (S)-L55 is shown is Scheme 33. In this case the pyridine-carboxylate moiety was installed prior to the insertion of the phosphorus group. Next, intermediate 43 was mixed with (S)-Feringa\u2019s phosphoroamidite (S)-44 to afford the desired ligand. (S)-L55 (R\u00a0=\u00a0Me) afforded enantioselectivities up to 97%\u00a0ee in the Cu-catalyzed conjugate addition of diethylzinc to linear enones (uo to 97%\u00a0ee) [148]. H8-NOBIN-derivatives L56 and L57, which were obtained in a similar manner than L55, showed also an excellent catalytic performance [149]. However, ligands L55 as well as L54 failed for cyclic enones, with\u00a0ee\u2019s not higher than 53%\u00a0ee.Ligands (R)-L54 and (S,S)-L55 (R\u00a0=\u00a0Me) have been efficiently used in different asymmetric tandem Cu-catalyzed Michael/Mannich reactions, furnishing excellent enantioselectivities (Scheme 34\n). The procedure allowed the synthesis of a broad range of chiral functionalized products with multiple stereocenters, which could be used to prepare valuable compounds, such as pyrrolidines, isoindolinones and azetidines [150].The more electron-rich chiral biphenyl backbone 45 has been also used to prepare atropoisomeric P,N-ligands (Scheme 35\n). Phosphine- and phosphite-pyridyl ligands L58 gave excellent enantioselectivities in the Cu-catalyzed conjugate addition of diethylzinc to linear enones (ee\u2019s up to 96%) [151].Atroposiomeric ligands L59 with a 2-pyridyl moiety to a binepine scaffold have shown excellent results in asymmetric Pd-catalysis. Besides axial chirality, these ligands contain two elements of central chirality, one in the benzylic position and the other at the phosphorus atom (Scheme 36\n). The synthesis of ligands L59 starts from dilithiated binaphthyl compound 46, which is transformed to borane-protected binepine 47 in 4 steps. Then, the benzylpyridine moiety is incorporated in the presence of BuLi, providing 48 as a single diastereoisomers except in the cases where the phosphorus center was a phenyl group [152]. Finally, borane-protected compounds 48 were deprotected by refluxing them in an excess of diethylamine for 4\u00a0days, yielding ligands L59 as off-white crystalline with good to excellent yields.. Ligands L59 were first tested in the Pd-catalyzed intramolecular \u03b1-arylation of \u03b1-branched aldehydes, surpassing the results achieved with QUINAP, PINAP or PHOX. The ligand screening revealed that a ligand with a more electronrich aromatic substituent in the P-group and a non-substituted pyridyl moiety (L59a) showed the best catalytic performance (Scheme\u00a037\na). A selection of aldehydes could be used furnishing the corresponding cyclic products with excellent enantioselectivities (up to >99%\u00a0ee) and good yields [152] It was also successfully used in the Pd-catalyzed Heck reaction between 2-substituted furans and aryl triflates to yield functionalized 2,5-dihydrofurans with fully saturated C2 stereocenters. In this case the optimal ligand contained a tert-butyl moiety at the phosphorus group (L59b), showing moderate yields but enantioselectivities up to 94%\u00a0ee for a range of substrates (Scheme\u00a037b) [153]. To gain information about the catalytic species formed during catalysis, the authors performed the complexation of ligands L59 to [PdCl2(CH3CN)2], leading to air-stable complexes [(L59)PdCl2]. NMR and IR spectroscopy proved the bidentate coordination of the ligand, which was also corroborated with the molecular geometry obtained by single-crystal X-ray analysis. It was also found that the ligand adopts a nearly ideal square-planar geometry and that the P-donor has a stronger trans effect than the N-donor atom [152].The first developed generation of axially chiral ligands were based on 6-membered heterocyclic motifs. All of them were built on a binaphthalene or a biphenyl backbones with an element in the ortho-position that hinders the rotation about the biaryl bond [108c-e]. The replacement of one of the naphthalene rings by a 5-membered ring was already attempted by Brown et al., with the synthesis of the indole-based ligand L60 (Scheme 38\n). However, the new ligand turned to be not configurationally stable. In 2003, Aponick designed the StackPhos ligand bearing an imidazole group (L61, R\u00a0=\u00a0Ph, Scheme 38), in which the \u03c0\u00a0\u2212\u00a0\u03c0 interaction between the electron-rich naphthalene ring and the electron-poor pentafluorophenyl group on the non-coordinating nitrogen is crucial to prevent tropoisomerism [154]. The synthesis of racemic L61 was achieved in 6 straightforward steps starting from 2-hydroxy-1-naphthaldehyde 49, in which the imidazole ring and the diphenylphosphino group were readily introduced (Scheme 38). In contrast to QUINAP and its derivatives, the resolution of L61 was achieved through deracemization instead of resolution. Reaction of racemic ligand with chiral Pd-salt 23 resulted in a single diastereoisomer, which was then treated with dppe to release ligand L61 in high yield and 98%\u00a0ee. Later, related phosphinoimidazoline ligands L62\u2013L63 (Scheme 38) were independently developed by Guiry (UCD-Phim) [155] and Aponick (StackPhim) [156]. The idea behind them was to circumvent the use of stoichiometric amounts of expensive chiral Pd-amine complex 23 to access the enantiopure ligands. Similarly to Carreira\u2019s PINAP, these ligands bear an element with central chirality that allows the separation of the diastereomeric mixture through recrystallization or column chromatography. Note that StackPhim (L62) and UCD-Phim (L62) are diastereomers.The first application of the StackPhos ligand (L61, R\u00a0=\u00a0Ph) was in the Cu-catalyzed A3-coupling between dibenzylamine, trimethylsilylacetylene and a range of aldehydes, including the more challenging aromatic ones. The corresponding propargylamines 50 were yielded in high yields and\u00a0ee\u2019s up to 97% (Fig. 21\na) [154]. The protocol was extended to the synthesis of amino skipped diynes 51 (up to 96%\u00a0ee), a class of chiral molecules with minimal differences in two of the substituents rendering them chiral (Fig. 21a) [157]. The Cu/StackPhos system has also allowed the enantioselective copper-catalyzed alkynylation of quinolinium salts and chromanones, delivering the desired products 52 in high yields and enantioselectivities (up to 98%\u00a0ee; Fig. 21a) [158]. The potential of the reaction was demonstrated in the syntheses of the tetrahydroquinoline alkaloids [158a] (+)-galipinine, (+)-cuspareine, and (\u2212)-angustureinem, as well as (\u2212)-martinellic acid (see Section 6) [159]. More recently, a set of StackPhos ligands bearing different substituents in the imidazole ring have been applied to the alkyne conjugate addition to Meldrum\u2019s acid derivatives. A ligand with a methyl group (L61, R\u00a0=\u00a0Me) in combination with Cu(OAc)2 exhibited the best catalytic performance (up to 92% yield and 98%\u00a0ee) [160]. The transformation gives \u03b2-alkynyl Meldrum\u2019s acid building blocks (compounds 53, Fig. 21a), which could be used in the asymmetric synthesis of the vasopressin V2-receptor agonist OPC 51,803 (see Section 6). More recently, Aponick\u2019s group used L61 for the synthesis of chiral \u03b4-lactones via a tandem acetylide addition/alkyne heterofuntionalization process catalyzed by Cu and Ag, respectively (Fig. 21b) [161].The newer UCD-Phim ligands L62, developed by Guiry and coworkers, showed excellent results in the Cu-catalyzed A3-coupling reaction of aliphatic aldehydes, showing in some of the cases greater enantioselectivities than StackPhos ligands (up to 98%\u00a0ee) [155]. In 2019, the scope was extended to aromatic, alkenylic and alkynylic aldehydes, as well as secondary cyclic amines, achieving up to 99%\u00a0ee [162]. The StackPhim ligand L63, developed by Aponick, has been used to prepare C2-aminoalkyl five-membered heterocycle motifs (up to 94%\u00a0ee, Scheme 39\n). The strategy used consists in a convergent alkynylation/cyclization sequence [156].Amino-phosphine ligands with a spiro center (L64\u2013L66, Scheme 40\n) have proved to be highly effective in the hydrogenation of \u03b1,\u03b2-unsaturated ketones as well as alkenes bearing nitro or carboxylic acid groups when using Ir-catalysts. The first ligands of this class were the SpiroAP ligands (L64) developed by Zhou et al. [163]. The introduction of a CH2-group before the primary amino moiety, and later a CMe2-group, afforded chiral spiro benzylamino-phosphine SpiroBAP [164] and SpiroBAP-R [165] ligands (L65\u2013L66). The synthesis of amino-phosphine spiro ligands L64\u2013L66 starts with the transformation of commercially available SPINOL into diarylphosphine/triflate intermediate 54 (Scheme 40). To obtain SPiroAP ligands (L64), 54 is converted to the dimethyl ester derivative by Pd-catalyzed carbonylation, followed by subsequent basic hydrolysis to provide carboxylic acid derivative 55. In the case of SpiroBAP and SpiroBAP-R ligands (L65\u2013L66), the synthesis proceeds through Pd-catalyzed cyanation of 54. Next, reduction of cyanate intermediate 56 with LiAlH4 or with MeLi afforded SpiroBAP and SpiroBAP-R ligands, respectively.SpiroAP ligands (L64) were screened in the Ir-hydrogenation of exocyclic \u03b1,\u03b2-unsaturated enones to afford exo-cyclic allylic alcohols and \u03b2-arylmethyl cyclic alcohols [163]. The corresponding Ir-complexes were formed in situ during catalysis. The best ligand contained a bulky phosphine group (Ar\u00a0=\u00a03,5-di-tert-butylphenyl), giving excellent enantioselectivities and yields (Scheme 41\n). The potential of this methodology was demonstrated with the synthesis of a crucial intermediate in the preparation loxoprofen, a nonsteroidal anti-inflammatory drug (see Section 6).Prompted by the excellent results obtained in the hydrogenation of ketones, Zhou and co-workers prepared spiro benzylamino-phosphine SpiroBAP (L65) [164]. In this case, Ir-complexes were synthesized prior to catalysis, following the same procedure used for preparing [Ir(cod)(L8)]BArF. The new complexes were stable to air and and could be stored without degradation for a few months. X-ray diffraction analysis showed that L65 (Ar\u00a0=\u00a0Ph) acts as a chelating P,N ligand and creates a rigid chiral pocket around the iridium center. Again, the presence of a bulky aryl phosphine (L65, Ar\u00a0=\u00a03,5-tBu2-C6H3) exhibited the best catalytic results in the hydrogenation of \u03b1-aryl- and \u03b1-alkyl acrylic acids (Scheme 42\n). A range of chiral carboxylic acids, including naproxen and related anti-inflammatory drugs, were attained in excellent enantioselectivities (up to 98%) and TOFs (up to 6000\u00a0h\u22121).The presence of a dimethyl group at the benzylic position of the amine group on SpiroBAP-R ligands allowed the enantioconvergent hydrogenation of \u03b2-aryl-\u03b2-methyl-nitroalkenes (91\u201398%\u00a0ee values) and \u03b2-alkyl-\u03b2-methyl-nitroalkenes (77\u201395%\u00a0ee values; Scheme 43\n) [165]. [Ir(cod)(L66)]BArF was able to hydrogenate diastereomeric mixtures of E- and Z-nitroalkenes, thus avoiding tedious isolation of the substrate isomers.Very recently, Jiao and co-workers have published the air-stable ligands L67 bearing a rigid spiro[indane-1,2\u2032-pyrrolidine] backbone (Scheme 44\n) [166]. In contrast to SPINOL-derived ligands L64\u2013L66, the spiral center must be created during the synthesis of ligands L67 (Scheme 44). Thus, the key step to build the spiral center is accomplished through AlCl3-mediated intramolecular Friedel\u2013Crafts-type reaction of 58, which is obtained first from 57 after 5 steps. After three recrystallizations of the diastereomeric mixture of 59a\u2013b, pure diastereoisomer 59a was afforded. Next, compound 60 is obtained via multiple steps. Demethylation of 60 followed by triflation of the phenolic hydroxy group, coupling with diphenylphosphine oxide and two subsequent reduction steps gave ligand L67. Ligands L67 were applied in the Pd-catalyzed asymmetric allylic substitution of benchmark substrate with dimethylmalonate but also with several alcohols and amines as nucleophiles with moderate to high yields and enantioselectivities (60\u201399% yield and 61\u201397%\u00a0ee). The authors were able to obtain a crystal structure of [Pd(II)(\u03b73-1,3-diphenylallyl)(L67, Ar\u00a0=\u00a03,5-tBu2Ph)]PF6, which gave information about the transition states of the substitution reactions.Many new P,N-ligands bearing central chirality has also been developed. Few years later of the discovery of PPFA ligands (L35), Hayashi and Kumada designed a new library of ligands L68 from natural \u03b1-amino acids (Scheme 45\n). Phosphine-amino ligands L68 catalyzed the Ni- Grignard cross-coupling of (1-phenylethyl)magnesium bromide and vinyl bromide, for which the bulkiest tert-Leuphos ligand L68 (R1\u00a0=\u00a0tBu) provided the highest\u00a0ee value (up to 94%\u00a0ee) [86c]. The asymmetric induction was thought to result from the hemilability of these ligands. Several analogues of these chiral \u03b2-aminophosphine, have been developed throughout the years, because of their stability, low toxicity and ease handling. These ligands can be readily synthesized through nucleophilic phosphide substitution of derivatized amino alcohols 62 containing a leaving group (LG) (Scheme 45). Besides, this scaffold has been also used in the design of other P,N-ligands containing an imine, amide or pyridine as N-donor group. A review about \u03b2-aminophosphine derivative has been recently reported [85e]. Another example of \u03b2-aminophosphine ligands are compounds L69 derived from L-valine (Scheme 45), which were applied in the palladium catalyzed allylic substitution of (E)-1,3-diphenylallyl acetate with dimethyl malonate. The nitrogen substitution constituted a key factor in the stereochemical outcome of the reaction, with enantioselectivities that ranged from 56%\u00a0ee (R) to 92%\u00a0ee (S) [167].Air-stable phosphite-amino ligands L70 (Scheme 46\n) were prepared in two steps also from commercial 1,2-amino alcohols. These ligands were tested in enantioselective Pd-catalyzed allylic substitution reactions. A mechanistic study allowed the optimization of the ligand parameters from a full ligand library, identifying ligands L70a\u2013b as the best. High enantioselectivities were achieved for a linear and cyclic substrate with several C-, N-, and O-nucleophiles (32 examples, \u00a0ee values up to 99%) [168]. Ligands L70a\u2013b were easily obtained by methylation of intermediates 63 with formic acid and formaldehyde resulted in dimethylated amino alcohols 64. Subsequent reaction with the desired phosphorochloridite led to ligands L70a\u2013b. Studies on the Pd-\u03c0-allyl intermediates provided insights about the effect of the ligand parameters on the origin of the enantioselectivity. It was found that the higher enantioselectivities obtained with ligands containing a hydrogen as the R2 substituent (L70), compared with ligands with a R2\u00a0=\u00a0Me, were mainly due to a higher electronic differentiation between the more electrophilic allylic terminal C atoms, making the major Pd-\u03b73 allyl isomer more reactive [168].\u03b2-Aminophosphine ligands derived from starting material other than amino acids have been also found to be efficient in Pd-allylic substitutions. For instance, phosphite-amino ligand L71 with a protected pyrrolidine-3,4-diol moiety has been recently prepared from cheap D-mannose (Scheme 47\n) [169]. N-Boc protected aminoalcohol 65, which was obtained from D-mannose [170], was subjected to Boc deprotection followed by reaction with the appropriate phosphorochloridite. The optimized ligand L71 was employed in the Pd-catalyzed enantioselective allylic substitution of linear and cyclic substrates. Enantioselectivities ranging from 80 to 91%\u00a0ee were obtained using various C- and N-nucleophiles. In the case of cyclic substrates both enantiomers of the final products could be attained by switching the chirality of the biaryl phosphite group. A study of the Pd\u2013\u03c0-allyl intermediates showed that to achieve high enantioselectivities in the substitution of cyclic substrates, the ligand components need to be appropriately chosen to either enhance the difference in the ratio of the Pd\u2013allyl isomers formed or to enhance the reactivity of the nucleophile towards each Pd\u2013allyl isomer. In contrast, the key of success when using linear substrates is to avoid the formation of Pd\u2013allyl complexes with monodentate coordinated ligands. The study also indicated that the sugar backbone is able to control the configuration of the amino group upon coordination [169]. Note that L71 contains an amino group that is part of a cyclic backbone. Most of the P-amino ligands that exhibited remarkable results in asymmetric catalysis contain a non-cyclic amino group [85b,d]. Only few P,N-ligands bearing a cyclic amine have provided high enantioselectivities, mostly achieved only in the benchmark substrate [97,171].In 2011, a new family of cinchona-derived phosphino-amine ligands (L72\u2013L73) was developed by Dixon and co-workers [172]. These ligands consist on a cinchona backbone bearing three different sites that allow a cooperative catalysis: a Br\u00f8nsted (N) and a Lewis base (P) and a H-bond-donor group (NH). Ligands L72 and L73 were readily prepared from commercially available ortho-diphenylphosphino benzoic acids and the desired 9-amino(9-deoxy)epicinchona alkaloids (66a\u2013b; Scheme 48\n).Phosphine-amino ligands L72 and L73 (R1\u00a0=\u00a0Et; R2\u00a0=\u00a0H; Ar\u00a0=\u00a0Ph) were mixed with Ag2O to yield cooperative Ag(I)-based Br\u00f8nsted base/Lewis acid catalysts that effectively promoted the asymmetric aldol reaction of isocyanoacetate nucleophiles and aldehydes (Scheme 49\n) [172]. Both aromatic and branched aliphatic aldehydes could be used to provide oxazolines with high diastereo- and enantioselectivities (up to 98%) by using ligand L73 (R1\u00a0=\u00a0Et; R2\u00a0=\u00a0H; Ar\u00a0=\u00a0Ph). However, linear aliphatic aldehydes showed lower enantioselectivities.After its first application, Ag(I)/L72 catalyst has been efficiently used in the enantioselective catalytic addition of isocyanides to many other electrophilic compound such as aldehydes [173], aldimines [174], ketones [175] ketimines [176], allenoates [177], alkynyl ketones [178], other carbon\u2013carbon double bond containing electron withdrawing groups (EWG) [179], and p-quinone methides (p-QMs) [180]. Besides isocyanide chemistry, quinine-derived ligands L72 have recently showed impressive results in asymmetric Cu-catalyzed cross-coupling reactions (Scheme 50\n) [181]. Ligand L72a (R1\u00a0=\u00a0vinyl; R2\u00a0=\u00a0OMe; Ar\u00a0=\u00a03,5-tBu2-C6H3) allowed the largely unexplored asymmetric Cu-catalyzed Sonogashira Csp3-Csp cross-coupling between a range ofalkyl halides and alkynes (>120 examples, up to 99%\u00a0ee; Scheme\u00a050a). To show the utility of this transformation, they performed the asymmetric Sonogashira C(sp3)C(sp) cross-coupling reaction of a mesogenic compound with the core structures of several bioactive molecules, such as estrone, biotin etc (see Section 6). L72b (R1\u00a0=\u00a0vinyl; R2\u00a0=\u00a0OMe; Ar\u00a0=\u00a0Ph) has also allowed the radical asymmetric oxidative C(sp3)C(sp) cross-coupling of unactivated C(sp3)H bonds on N-fluorocarboxamides with terminal alkynes (Scheme\u00a050b) [182]. A range of chiral alkynyl amides were afforded in a highly regio-, chemo-, and enantioselective manner (up to 97%\u00a0ee).Very recently, chinchona-derived ligands L72c\u2013d\n(R1\u00a0=\u00a0vinyl; R2\u00a0=\u00a0OMe; Ar\u00a0=\u00a02,6-Me2-C6H3 or 9-phenanthryl) have been used in the Cu-catalyzed enantioconvergent radical Suzuki\u00a0\u2212\u00a0Miyaura C(sp3)\u00a0\u2212\u00a0C(sp2) cross-coupling of racemic alkyl halides with B(mac)-derived boronate esters (Scheme\u00a050c) [183]. The reaction showed a broad scope regarding both coupling partners, including aryl- and heteroarylboronate esters, as well as benzyl-, heterobenzyl-, and propargyl bromides and chlorides furnishing high enantioselectivities.As ferrocene-based P-imine ligands L43\u2013L45 (Scheme 21), the phosphine-imino ligands L74\u2013L77 bearing central chirality (Scheme 51\n) were initially applied in Pd-allylic substitutions. All ligands could be easily prepared by mixing (diphenylphosphino)benzaldehyde 67 with a desired chiral amino scaffold (for ligands L74\u2013L76) or a sulfinamide (for ligand L77).Thus, ligands L74, prepared from commercially available SAMP as the chiral amine, provided somewhat lower enantioselectivity (up to 92%\u00a0ee) than related ligand L43 (up to 96%\u00a0ee) [104] in the Pd-allylic substitution reactions [184]. Using available peptides as chiral amines, Hoveyda et al. prepared a large library of peptide-based ligands L75 and L76. Over the years, its modular nature has allowed to achieve excellent enantioselectivities in the Cu-catalyzed conjugate addition of a broad range of \u03b1,\u03b2-unsaturated substrates (Fig. 22\n) [185] For example, L75a (R1\u00a0=\u00a0iPr, R2\u00a0=\u00a0Bn, R3\u00a0=\u00a0NHnBu) exhibited excellent enantioselectivities (up to 98%\u00a0ee) in the conjugate addition of different alkylzincs to cyclic enones [185a,h], while L75b (R1\u00a0=\u00a0iPr, R2\u00a0=\u00a0p-OtBu-Bn, R3\u00a0=\u00a0NHnBu) was preferred for the linear ones (Fig. 22) [185b,e]. Both ligands have been very useful also with unsaturated furanones, with\u00a0ee\u2019s up to >98% (L75a) and up to 97%\u00a0ee (L75b) (Fig. 22) [185f]. When nitroalkenes were used as Michael acceptors, the best ligands were found to be L75c (R1\u00a0=\u00a0tBu, R2\u00a0=\u00a0p-OBn-Bn, R3\u00a0=\u00a0NHnBu) for cyclic nitroalkenes (up to 96%\u00a0ee) [185c], and L75d (R1\u00a0=\u00a0tBu, R2\u00a0=\u00a0p-OBn-Bn, R3\u00a0=\u00a0NEt2) for trisubstituted linear nitroalkenes (up to 98%\u00a0ee; Fig. 22) [185g,186]. The Cu/L75d was used by Carreira in the total synthesis of (+)-Daphmanidin E (see Section 6) [187]. The same ligand also allowed the tandem conjugate addition\u2013nitro-Mannich reaction for the preparation of anti- and syn-\u03b2-nitroamines with three contiguous stereocenters (up to 96%\u00a0ee; Fig. 22) [188].A ligand bearing only a peptidic fragment has been also efficiently used in the Cu-catalyzed conjugate addition of cyclic enones (up to >98%\u00a0ee) [185d] and lactones (up to 96%\u00a0ee) [185f]. Ligands L76 have been found to induce high enantioselection in a wide range of asymmetric CN bond forming transformations such as the aza-Diels-Alder [189] and Mannich type reactions [189b,190].P,N-sulfinyl imine ligands L77, in which the chirality is found in the sulphur atom, were obtained via Ti-mediated condensation of the corresponding sulfinamide with compound 67 (Scheme 51). With a tert-butyl substituent attached to the imine an enantioselectivity up to 96%\u00a0ee in the allylic alkylation of the benchmark substrate was attained [191]. Ligands L77 were also used to the Ir-catalyzed hydrogenation of trisubstituted olefins, but with only moderate enantioselectivities [192].Other ligands bearing the chirality at the sulphur centre are the P-sulfoximine ligands L78\u2013L81 (Scheme 52\n), which have been efficiently used in Ir-catalyzed hydrogenation reactions. Bolm et al. developed phosphinosulfoximine ligands L78 and L79 in few steps. The key step consists in the Cu-mediated coupling N-arylation of sulfoximines with bromo-aryl phosphine oxide 68. Reductive deoxygenation of 69 with trichlorosilane gave the corresponding ligands as solid, air-stable products in good yields Scheme 52\n[193].Ligands L78 were screened to the enantioselective Ir-catalyzed hydrogenation of N-aryl imines, using iodine as a promoter. A ligand containing an isobutyl and a phenyl N-substituent provided the best catalytic performance. With the optimal ligand it was possible to reduce a range of imines with\u00a0ee\u2019s over 90%\u00a0ee for most of the substrates [193]. Later, analogous bicyclic ligands L79 were prepared following a similar synthetic strategy starting from 1,8-diiodonaphthalene. These ligands showed 92%\u00a0ee in the hydrogenation of 2-methylquinoline, albeit moderate enantioselectivities were obtained for other quinolone derivatives (55\u201387%\u00a0ee) [194]. More important are the results attained in the olefin hydrogenation of \u03b2,\u03b2\u2019-disubstituted enones [195]. Prior to this, the existing methods for preparing valuable optically pure ketones were mainly non-catalytic and with a limited substrate scope [196]. A range of enones could be reduced with enantioselectivities up to 97%\u00a0ee (Scheme 53\n). However, the effectiveness of the catalyst is affected by the substitution pattern of the enone and the steric constraints of the olefin substituents. For instance, a low enantioselectivity was observed for \u03b1,\u03b2-disubstituted enones (55%\u00a0ee for (E)-3-methyl-4-phenyl-3-buten-2-one) [197].To increase the scope of P-sulfoximine ligands in Ir-hydrogenation reactions, the phosphine group on L78 has been recently substituted by biaryl phosphite moieties, leading to ligands L80. The more rigid benzothiazine derivative L81 was also synthesized [198]. In contrast to L78\u2013L79, the sulfoximine group was inserted prior to the P-group. The synthesis of L80\u2013L81 starts with alcohol protection of the corresponding 1-Br-phenols 70 and 73 with methoxymethyl chloride. Then, MOM-protected intermediates were coupled with (S)-S-methyl-S-phenylsulfoximine (76), which upon deprotection of the MOM group in acid media, reacted with the desired phosphorochloridite. Finally, Ir-complexes [Ir(cod)(L80-L81)]BArF were prepared using the same methodology than for [Ir(cod)(L8)]BArF\n[198]. The authors found that Ir-complexes containing ligands L80 were obtained as a mixture of isomers. In contrast, complexes containing ligands L81 with a more rigid backbone were present as a single isomer, suggesting that in the case of ligands L80 two different stable conformations for the six-membered chelate ring are possible. One more time, having a biaryl phosphite moiety on the ligand scaffold improved the substrate versatility of the hydrogenation reaction. Thus, [Ir(cod)(L80\u2013L81)]BArF complexes increased the scope attained in the reduction of \u03b1,\u03b2-unsaturated enones (95\u201397%\u00a0ee), including \u03b1,\u03b2-disubstituted enones and with an exocyclic double bond. Furthermore, other olefins bearing poorly coordinative groups, such as lactones, diphenyl alkenylboronic esters among others, were also hydrogenated with\u00a0ee\u2019s up to 99% (Fig. 23\n).As mentioned previously (see Section 2) the phosphino-oxazoline ligands (PHOX) made a breakthrough in asymmetric catalysis due to their synthetic versatility and broad catalytic applicability [85c,199]. With the aim of exploring other five-membered nitrogen heterocycles, P,N-ligands bearing aromatic heterocycles such as oxazoles, thiazoles and imidazoles, as well as other non-aromatic rings (e.g. thiazolines and imidazolines) have been developed. The resulting P,N ligands have shown excellent results in asymmetric catalysis, especially in Ir-catalyzed hydrogenations and in Pd-catalyzed reactions.This field has been pioneered by Andersson's group with the aim to enlarge the substrate versatility in the challenging hydrogenation of unfunctionalized olefins. They started by rational design of the bicyclic oxazole-based P,N-ligands L82 (Scheme 54\n) [200]. Their synthesis starts with the transformation of diazodimedone in presence of a catalyst and benzonitrile. Catalytic enantioselective reduction of the obtained keto oxazole with (R)-Me-CBS-borane provided (S)-alcohol derivative, in which the desired phosphinite group was installed through conventional chemistry. These ligands met the criteria established from a computational study made by the same authors about the hydrogenation of (E)-1,2-diphenyl-1-propene with the Pfaltz Ir/PHOX-catalyst [201]. Thus, ligands L82 combines the presence of a P- and a N-donor atom, to achieve a significant trans effect, with a rigid bicycle to reduce conformational flexibility, and a six-membered chelate ring is generated upon complexation to Ir. All this resulted in Ir/L82 complexes that generate an appropriate chiral environment for asymmetric induction to the substrate (Scheme 54). The outstanding enantioselectivities (93\u201399%\u00a0ee) achieved by the Ir/L82 complexes in the hydrogenation of 1,2-disubstituted styrenes corroborated the computationally derived selectivity model.Then, by systematic modification of L82 by replacing either the oxazole by a thiazole group or the phosphinite group by a N-phosphine moiety, phosphine-thiazole L83\n[202] and N-phosphine-thiazole ligands L84\n[64e] were obtained (Scheme 55\n). In the case of L83, 5- and 6- and 7- fused-rings were studied. Both ligand families are derived from ketoesters\u00b8 which were transformed into thiazole esters 77 through condensation with benzothioamide. Next, for the preparation of phosphine ligands L83, the corresponding thiazole ester 77 was converted to alcohol (rac)-78. In contrast, when the target was the aminophosphine ligand L84, ester 77 was first converted to the amide 79 and then it was reduced to amine (rac)-80. Both, alcohol and amine derivatives were obtained as racemates and resolved by preparative chiral HPLC. Finally, they were converted to the corresponding phosphine or aminophosphine ligands through already reported chemistry.The catalyst precursors [Ir(cod)(L83-L84)]BArF were prepared using the same methodology than for [Ir(cod)(L8)]BArF, and were tested in asymmetric hydrogenation reactions. By selecting the appropriate ligand L82\u2013L84 it was possible to increase the substrate scope considerable. For instance, phosphine-thiazole ligands L83 showed to be more suitable than the oxazole counterpart L82 in the Ir-hydrogenation of \u03b1,\u03b2-unsaturated esters. To note that the six-membered ring backbone showed the best catalytic performance. With a di-o-tolyl phosphine moiety and the appropriated substituent on the thiazole ring, derivatives of \u03b1- and \u03b2-methyl cinnamic acid ethyl esters were reduced with high enantioselectivities (80\u201398%\u00a0ee for (E)-olefins and 95%\u00a0ee for a (Z)-olefin) (Fig. 24\n) [202].Instead, for the reduction of vinyl fluorides the best enantioselectivities was achieved with [Ir(cod)(L84)]BArF\n[64e]. At this time, the hydrogenation of fluorine-containing olefins was little explored, probably, due to the ability of vinylic fluorine to be cleaved off [203]. In contrast, the Ir/L84 catalyst showed little defluorination. Although the substrate scope was limited, it was possible to hydrogenate a trisubstituted fluoroolefin bearing a hydroxy and an acetate group in 99% and >99%\u00a0ee, respectively (Fig. 24). Ir-catalyst bearing ligand L84 was also very useful in the hydrogenation of 1,1\u2032-disubstituted vinylphosphonates and carboxyethylvinylphosphonates (Fig. 24) [204]. It should be noted that for this last type of substrates, E- and Z-isomers as well as their mixtures could be hydrogenated in excellent enantioselectivities (up to >99%\u00a0ee). Finally, with the use of both thiazole ligands L83 (R\u00a0=\u00a0Ar\u00a0=\u00a0Ph, n\u00a0=\u00a01) and L84, it was possible to hydrogenate a range of 1,1\u2032-diaryl substituted olefins in excellent enantioselectivities (up to >99%\u00a0ee) [205].The corresponding phosphite-oxazole and thiazole-based ligands L85\u2013L86 were also prepared and applied in Ir-catalyzed hydrogenation of unfunctionalized olefins or with poorly coordinative groups (Fig. 25\n). Thiazole-based ligands L86 provided the highest enantioselectivities. The introduction of the phosphite moiety allows to extend the substrate scope. Ir/L86 catalytic system allowed the hydrogenation of both E- and Z-trisubstituted olefins along with 1,1\u2032-disubstituted terminal alkenes, furnishing excellent enantioselectivities (ee\u2019s up to 99%) [206]. The catalytic system also tolerated the presence in the olefin of some neighboring polar groups (e.g. esters, alcohols, phosphinates \u2026) for which\u00a0ee values up to 99% has been also attained.Useful, ligands L85 and L86 were also screened in the Pd-allylic substitution reaction [207]. After ligand screening and in contrast to the hydrogenation, it was found that oxazole ligands (L85) exhibited in general higher enantioselectivities than L86. With the proper selection of each ligand parameter it was possible to achieve high enantio- and regioselectivities (ee up to 96%) for a variety of cyclic and tri-, di- and monosubstituted linear substrates (Fig. 26\n). The study of Pd-allyl intermediates by NMR and DFT aided to understand the influence of the ligands parameters on the ratio of the Pd-allyl species and the electrophilicity of the allylic terminal carbon atoms. Ligands L85 provided also high enantio- and regioselectivities in Pd-catalyzed intermolecular Heck reactions of 2,3-dihydrofuran with several aryl triflates [208].To study the effect of the backbone in the catalytic activity, Andersson\u2019s group developed the other two ligand families L87\n[209] and L88\n[210] (Scheme 56\n), in which the backbone was modified while keeping the thiazole unit. Ligands L87, which feature an open-chain-backbone, were synthesized from inexpensive methyl 3-oxobutanoate (Scheme\u00a056a). The synthesis starts with the introduction of the thiazole ring. Next, Oppolzer sultam (81) was introduced in order to incorporate a chiral alkyl chain by using diverse alkyl halides in the presence of lithium hexamethyldisilazide (LHMDS). Each chiral ligand precursor 82 was obtained with high diastereomeric purities. After reduction with LiAlH4, the corresponding alcohols were transformed to the final phosphino-thiazole ligands L87. Unfortunately, the new ligands L87 were slightly less successful than the their more rigid counterparts L83 in the Ir-catalyzed hydrogenation of unfunctionalized trisubstituted olefins, allylic alcohols and imines [209].In contrast, the introduction of a bicyclic amine into the ligand resulted beneficial in terms of scope for the Ir-hydrogenation of poorly coordinative olefins [64f,h,210,211]. In addition, their synthesis was achieved in fewer steps than L87 (Scheme\u00a056b). Ligands L88 were synthesized from (1S,3R,4R)-2-azabicyclo[2.2.1]heptane-3-car-boxylic acid [212]. This intermediate was transformed to N-Boc-protected thioamide in 3 steps, which was then cyclized with an \u03b1-bromoketone to provide the N-protected thiazole. After removal of the protecting group, the phosphorus fragment was incorporated to ligands L88. The resulting Ir-complexes [Ir(cod)(L88)]BArF were efficiently used in the hydrogenation of olefins bearing a variety of poorly coordinating groups, showing comparable results to those obtained with phosphine-thiazole L84\n[210]. Moreover, the use of Ir/L88 was beneficial, widening the scope of the hydrogenated olefins (Fig. 27\na). Excellent enantioselectivities were obtained for vinyl boronates (up to 98%\u00a0ee) [64f], vinylic, allylic and homoallylic sulfones (up to 99%\u00a0ee) [211a], \u03b3-substituted cinnamyl alcohols (up to 99%\u00a0ee) [64h] and very recently to \u03b1,\u03b2-unsaturated \u03b1-fluoro aryl and alkyl ketones (up to >99%\u00a0ee) [211c]. Importantly, the use of an Ir-complex containing the enantiomer of L88 (R\u00a0=\u00a0Ph, Ar\u00a0=\u00a0o-Tol) allowed the cooperative dynamic kinetic asymmetric hydrogenation of allylic alcohols (Fig. 27b), to produce a broad range of chiral alcohols containing two stereogenic centres with excellent diastereoselectivities (up to 95:5) and enantioselectivities (up to 99%) [211b]. Mechanistic studies supported that racemization of the substrate is achieved by cleavage and reforming of the oxygen\u2013carbon bond.The combination of a biaryl phosphite and the nitrogenated bicyclic backbone of L88, lead to the family of ligands L89 (Fig. 28\n) [66]. [Ir(cod)(L89)]BArF were synthesized following the same methodology than for [Ir(cod)(L8)]BArF. Ir-complexes with a ligand bearing an (S)ax-binaphthol moiety helped to expand the substrate scope of the previous successful N-phosphane ligands L88 on the Ir-hydrogenation of unfunctionalized olefins. For example, E- and Z-tri- and 1,1\u2032-disubstituted substrates, \u03b1,\u03b2-unsaturated enones, alkenylboronic esters and disubstituted olefins were reduced with excellent enantioselectivities (up to 99%\u00a0ee) [66]\nFrom the results obtained with the oxazole- and thiazole-based ligands developed by Andersson's group it can been concluded that they showed a different substrate scope in the hydrogenation of olefins. Having this in mind, the group developed a new set of ligands with an imidazole ring (L90; Scheme 57\n), to investigate even further the substrate scope [213]. The imidazole moiety was formed through condensation of ester 83 (obtained from esterification of 2-aminonicotinic acid) with the corresponding \u03b1-bromoacetophenone. Regioselective hydrogenation of 84 with Pd/C in TFA and subsequent reduction of the ester moiety yielded the corresponding alcohol. After HPLC resolution, chiral alcohol was transformed to the phosphine-imidazole ligands L90.Following the same methodology than for [Ir(cod)(L8)]BArF, ligands L90 were complexed to [IrCl(cod)]2 and mixed with NaBArF to give the corresponding iridium/BArF complexes. Ir-complexes of imidazole ligands L90 showed a similar catalytic performance to its oxazole and thiazole analogues in the asymmetric hydrogenation of olefins with poorly-coordinative groups (up to 98%\u00a0ee). In addition, imidazole ligands L90 showed good results in the Ir-catalyzed hydrogenation of the demanding vinyl fluorides. It was proposed that the reason was the decreased basicity of the imidazole ring, resulting in less defluorination. The Ir/L90 catalyst (R\u00a0=\u00a0Ph, Ar\u00a0=\u00a0Ph or 3,5-Me2-C6H3) were able to reduce various vinyl fluorides with up to 86%\u00a0ee, including unsaturated esters that the previously commented ligands 84 could not hydrogenate (Fig. 29\n) [213].Interestingly, the tuning of the substituent on the imidazole ring of ligand L90 led to excellent ligands for the highly regio- and enantioselective reduction of 1,4-cyclohexadienes (Scheme\u00a058\na-d) [214]. They showed an impressive substrate scope, providing excellent results for substrates having little functionality, but also for others bearing strongly coordinating substituents and heterocycles fused rings (up to 99%\u00a0ee). Finally, ligand L90d has been recently used in the asymmetric hydrogenation of allylsilanes (ee\u2019s up to 99%; Scheme\u00a058e) [215]. The compounds were further subjected to the Hosomi-Sakurai allylation yielding the corresponding homoallylic alcohols with three stereogenic centers in excellent diastereoselectivities.Recently, Riera, Verdaguer and co-workers presented the synthesis of a novel class of P-chirogenic aminophosphine-imidazole ligands L91\u2013L92\n[216], (Scheme 59\n) which are analogues the MaxPHOX family of ligands [43]. The new ligands were prepared from N-Boc valine, as MaxPHOX ligands, but the imidazole moiety was introduced through condensation with ortho-phenylenediamine (ligand L91) or with phenacylbromide, followed by cyclization with ammonium acetate (ligands L92). Removal of the borane protecting group with neat pyrrolidine, followed by treatment with [IrCl(cod)]2 and counterion exchange with NaBArF yielded the corresponding MaxPHOX/Ir-complexes. A catalyst precursor based on benzoimidazole ligand (L91) was found to be superior than its imidazole counterpart in the hydrogenation of the model cyclic \u03b2-enamide (89% vs 72%\u00a0ee). However, it showed a lower catalytic activity and enantioselectivity than its oxazoline analogue.Phosphite-thiazoline ligands L93\n[18c] (Scheme 60\n) were designed to expand the substrate scope of successful phosphite-oxazoline ligand family L23, which despite their versatility there was still room for improvement in the Pd-catalyzed allylic substitution of cyclic substrates [48b]. To this aim, the oxazoline was replaced with a thiazoline (ligands L93), in order to reduce the chiral pocket created by the ligands and make it more appropriate for cyclic substrates. Ligands L93 were synthesized from (R)-cysteine methyl ester hydrochloride in only 3 steps (Scheme 60) [18c]. Then, the thiazoline group was formed by coupling with ethyl benzimidate hydrochloride. Next, the formed thiazoline ester was reduced with MeMgBr to afford hydroxyl thiazoline in 40%\u00a0ee. Semipreparative chiral HPLC was used to give access to both enantiomers of 86. Finally, the phosphite group was introduced to provide ligands L93.As predicted, phosphite\u2013thiazoline ligands L93 improved considerably the enantioselectivities of unhindered cyclic substrates (up to 94%\u00a0ee; Fig. 30\na), while oxazoline analogues L23 worked better for the rest of substrates. The introduction of a thiazoline ring was also advantageous for the Ir-catalyzed hydrogenation of olefins [217]. Ir/BArF complexes bearing thiazoline ligands L93 allowed to increase the number of substrates efficiently reduced, including Z-trisubstituted olefins, trifluoromethylated olefins and enones (Fig. 30b).Busacca and co-workers patented the phosphine-imidazolines analogues of PHOX ligands, the so called BIPI (L94) (Boehringer-Ingelheim phosphinoimidazolines) [218]. The additional nitrogen atom provides an extra tuning site in the ligand scaffold by modifying the N-substituent group. Ligands L94 were synthesized in a modular way as shown in Scheme 61\n\n[219]. The imidazoline ring was built by condensation of o-haloimidates with the desired chiral diamine, furnishing the haloimidazolines. Then, the phosphine group was installed via the aromatic SNAr reaction with phosphide nucleophiles. Finally, reaction of the resulting phosphine-imidazolines with the corresponding alkyl halides furnished BIPI ligands L94 (Scheme 61). In the cases where R2 was an electron-rich aryl group (e.g., R2\u00a0=\u00a0p-OMe-C6H4) or with electron-deficient aryl groups on the phosphine (e.g. Ar\u00a0=\u00a03,5-F2-C6H3), the N-alkyl substituent was incorporated previous to the phosphine moiety.BIPI ligands (L94) were screened in Pd-catalyzed intramolecular Heck reactions that involve the formation of a chiral quaternary centre [219,220]. It was shown that the enantioselectivity increased with the use of more electron deficient phosphines (e.g. Ar\u00a0=\u00a03,5-F2-C6H3), which allowed to achieve the highest enantioselectivities reported at that time with BINAP and PHOX for challenging substrates (ee's up to 87%; Scheme 62\n). Over the years, BIPI ligands have also been used in the Rh- and Ir-catalyzed hydrogenation of imines [221], unsaturated ureas [222], urea esters, Boc-, Cbz-enecarbamates and enamides [223], and unfunctionalized olefins [56b]. The BIPI ligand was also used in the synthesis of a cathepsin S inhibitor (see Section 6) [224].A year later of the development of BIPI ligands, Pfaltz and co-workers reported a similar ligand family called PHIM ligands (L95, Fig. 31\n). In this case, easily accessible \u03b1-hydroxyamides were used instead of diamines, so one of the alkyl substituents on the resulting imidazoline ring was exchanged by a hydrogen atom. The correpsonding Ir-complexes were prepared by using the same protocol used for [Ir(cod)(L8)]BArF complexes and tested in the hydrogenation of various unfunctionalized olefins, exhibiting in some cases higher enantioselectivities than its PHOX analogues [225]. Later, Ir-complexes containing PHIM ligands were efficiently applied in the asymmetric hydrogenation of the poorly studied vinylsilanes [226].Then, Pfaltz also patented the SimplePHIM ligands (L96), which are a simplified version of PHIM ligands (Scheme 63\n) [227]. These ligands were obtained from oxalyl chloride, which was coupled with the corresponding aminoalcohols to yield intermediate 87. After formation of the oxazoline ring using standard procedures, the phosphine group was installed to afford ligands L96. [Ir(cod)(L96)]BArF complexes were evaluated in the hydrogenation of acetophenone N-phenylimine [228] and vinylsilanes [226]. Although they provided high enantioselectivities (up to 95% and 88%\u00a0ee, respectively), they didn\u2019t showed the best catalytic performance among all ligands tested. In contrast, they turned to be very efficient in the asymmetric hydrogenation of terminal boronic esters (up to 96%\u00a0ee, Scheme 63) [52b].Heterodonor P,N-ligands containing a pyridine group have turn into popular alternatives to P-oxazoline ligands because of the pyridine robustness and their straightforward synthesis. However, few P-pyridine ligands have provided outstanding results in terms of reaction scope. The success of QUINAP and PHOX ligands pushed Katsuki and co-workers to develop phosphine-pyridine ligands L97 and apply them in Pd-catalyzed asymmetric allylic substitutions [229,230]. Next, other similar bicyclic P-pyridine ligands have been synthesized and used in many asymmetric transformations (L98\u2013L104; Scheme\u00a064\na). The synthesis of L97 starts with chloropyrindine or chlorotetrahydroquinoline compounds, which were transformed into the corresponding chiral intermediates 88 in 5 steps, via epoxide formation and its subsequent opening [231]. Next, Suzuki cross-coupling of compounds 88 with 2-hydroxyphenylboronic acid gave the corresponding pyridylphenols, which can be then converted into the desired 2-(phosphinoaryl)pyridines in a conventional manner (Scheme\u00a064b) [229].A ligand with a 5-membered fused ring and an isopropyl substituent (L97, n\u00a0=\u00a01, R\u00a0=\u00a0iPr), provided the best enantioselectivities in the Pd-catalyzed allylic alkylation of linear substrates with dimethyl malonate (up to 98%\u00a0ee) [229,230]. This ligand was also used in Pd-catalyzed intramolecular allylic amination [232], Baeyer-Villiger reactions [233] and tandem allylic substitution reactions for the formation of heterocycles [234], providing moderate to good enantioselectivities.A similar ligand bearing a pinene moiety was developed by Chelucci\u2019s and Malkov-Ko\u010dovsk\u00fd\u2019s groups, simultaneously (L98, Scheme 64). This ligand could be synthesized in fewer steps than L97 since it is derived from chiral and inexpensive (\u2212)-\u03b2-pinene [235]. Ligands L98 were used in the Pd-catalyzed allylic alkylation of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate (50%\u00a0ee) [235a] and in the Pd-catalyzed Heck reaction of dihydrofuran and phenyl triflate (88%\u00a0ee) [235b]. The use of ent-L98 with an isopropyl substituent on the pinene moiety increased the scope and enantioselectivity provided by L97 in the Pd-catalyzed Baeyer-Villiger reaction of cyclobutanones [236]\nFollowing the same synthetic methodology, Andersson reported phosphine and phosphinite ligands L99 and L100 with the phosphorous donor atom attached to the pinene chiral motif. The cis phosphine ligand L99 gave the highest enantioselectivities (up to 97%; Fig. 32\n) in the Ir-catalyzed hydrogenation of different trisubstituted olefins (e.g. methylstilbene derivatives, \u03b1,\u03b2-unsaturated esters \u2026) [237]. However, activities were poor even at 100\u00a0bar of hydrogen pressure. Later, the same group developed ligands L101, in which the pinene element has been removed. The resulting ligands have indeed the same backbone than oxazole, thiazole and imidazole ligands L83, L84 and L90, but with a pyridine N-donor group. The correpsonding Ir-complexes were obtained using the same protocol described for [Ir(cod)(L8)]BArF. The new Ir-catalysts showed better activities and enantioselectivities than the ones with pinene-containing ligands L99 in the hydrogenation of several trisubstituted olefins (up to 99%\u00a0ee; Fig. 32) [238]. In most of the cases, complexes bearing five-membered-ring ligands (n\u00a0=\u00a01) were better catalysts than six membered-ring, which is the opposite trend observed in the case of their oxazole, thiazole and imidazole counterparts.Ligands L102 are also analogues of L98 but with the phosphine group directly attached to the pinene ring instead of being at the phenyl scaffold. [Ir(cod)(L102)]BArF were applied to the reduction of different olefins although they only proved to be successful for some 1,1\u2032-disubstituted enol phosphonates, providing 70\u201390%\u00a0ee (L102, R\u00a0=\u00a0p-MeO-C6H4; Fig. 32) [239].Later, the pinene moiety was replaced by a (+)-camphor moiety to give ligands L103 and L104. [Ir(cod)(L103)]BArF was screened in the asymmetric hydrogenation of various substrates, although it only gave a high enantioselectivity in the reduction of trans-\u03b1-methylstilbene (94%\u00a0ee, Fig. 32) [240]. [Ir(cod)(L104)]BArF were also applied in the Ir-catalyzed hydrogenation of trans-\u03b1-methylstilbenes. A complex containing a ligand without any substituent on the pyridyl moiety (L104, R1\u00a0=\u00a0R2\u00a0=\u00a0H) provided the best enantioselectivities (up to 96%\u00a0ee, Fig. 32) [241]. However, the hydrogenation of trisubstituted alkenes functionalized with alcohol, acetate and ester groups only attained moderate to good enantioselectivities (58\u201380%\u00a0ee). Notably, the same ligand allowed the hydrogenation of methyl (Z)-2-acetamido-3-phenylacrylate in 97%\u00a0ee (Fig. 32), which made it the first ligand that showed a good performance in the Ir-hydrogenation of dehydroamino acids that have been traditionally studied using Rh and Ru catalysts [242].The Pfaltz\u2019s group has prepared several phosphine- and phosphinite-pyridine ligands (L105\u2013L108) specially designed for the Ir-catalyzed hydrogenation of olefins. Phosphine- and phosphinite-pyridine ligands L105\u2013L106 (Scheme 65\n) were designed to be sterically similar to PHOX [243]. Phosphine ligands L105 were prepared from cheap ethyl picolinate, which was readily alkylated by lithiated BH3-protected methyldiphenylphosphane to yield ketone (Scheme 65). This intermediate was reduced enantioselectively by (\u2212)-chlorodiisopinocampheyl borane [(\u2212)-Ipc2BCl]. As the resulting pyridyl-alcohol was an oil, it couldn\u2019t be recrystallized so the hydroxyl group was protected with tBuMe2SiOTf. After recrystallization, the hydroxyl group was deprotected and protected again with the desired silyl group. Finally, the phosphine was deprotected to give ligands L105. In the case of phosphinite ligands, the synthesis was even more straightforward (Scheme 65). Pyridyl-alcohols were synthesized by ketone reduction or by alkylation of aldehydes with 2-lithiopyridine. After HPLC resolution, the introduction of the phosphinite group lead to ligands L106.An extensive ligand screening of [Ir(cod)(L105-L106)]BArF catalyst-precursors in the hydrogenation of trans-\u03b1-methylstilbene, indicated that complexes bearing phosphinites were superior to those containing phosphines (L105) (up to 97%\u00a0ee vs. 88%\u00a0ee). High enantioselectivities (up to 96%\u00a0ee) were also afforded with other interesting olefins. Among them, it should be highlighted that [Ir(cod)(L106)]BArF (R1\u00a0=\u00a0R2\u00a0=\u00a0tBu) allowed the hydrogenation of the acyclic tetrasubstituted alkene shown in Scheme 66\n in 81%\u00a0ee and >99% conversion, which has been usually hydrogenated with low conversion and poor enantioselectivity.Bicyclic phosphinite-pyridines L107 (Scheme\u00a067\na) were designed to study whether the more rigid conformation due to an additional ring could increased their enantioselectivities than the provided with L105\u2013L106\n[244]. Later, related disubstituted pyridine ligands with a bulky aryl group on the 2-position of the pyridyl scaffold were also developed (L108, Scheme\u00a067b). Ligands L107 are accessible from simple, available starting materials throught the pyridyl alcohols (Scheme\u00a067a). The key step is the oxidation of the trisubstituted pyridines, which are prepared in three steps from the corresponding ketone, to the corresponding N-oxides. N-oxides were then subjected to a Boekelheide rearrangement to yield pyridyl alcohols, which could be resolved by preparative HPLC. Recently, the same group demonstrated that pyridyl alcohols can be easily resolved via enzymatic kinetic resolution [245]. Enantiopure pyridyl alcohols were transformed towards the desired phosphinite ligands using the same methodology than for previous ligand L105\u2013L106. The next generation of bicyclic ligands (L108) were prepared from chiral silyl-protected chloropyridine precursor (Scheme\u00a067b) [246]. The introduction of the bulky group on the 2-pyridyl scaffold was introduced through Suzuki\u2013Miyaura cross-coupling using commercially available boronic acids.Ligands L107 and L108 improved the catalyst performance of previous L105\u2013L106 in the Ir-catalyzed hydrogenation of many olefins, becoming one of the few privileged ligand family for this transformation (Fig. 33\n). Generally, 2-phenyl substituted ligands with a bulky tBu and o-Tol phosphinite group provided the highest enantioselectivities [244]. Ligand L107b was initially found to be successful in the reduction of dihydronaphthalenes, allylic alcohols and \u03b1,\u03b2-conjugated esters (Fig. 33). This ligand was also successfully employed in the hydrogenation of the dihydronaphthalene core of antitumoral (+)-mutisianthol in 90%\u00a0ee (see Section 6) [247]. A ligand from the L107-family (R1\u00a0=\u00a0Ph, R2\u00a0=\u00a0Ph) allowed the total synthesis of (+)-torrubiellone C [248] and (\u2212)-pyridovericin [249], by catalyzing the hydrogenation of the appropriate \u03b1,\u03b2-unsaturated ester motif enantioselectively (see Section 6). (S)-L107b was also found to promote the asymmetric hydrogenation of furans with unprecedented high enantioselectivities (Fig. 33) [244]. The scope of these substrates was later extended, furnishing excellent enantioselectivities for a range of monosubstituted furans with a 3-alkyl or 3-aryl group and for benzofurans with an alkyl substituent at the 2- or 3-position [250]. Other heterocycles such as 2- and 3-substituted indoles or benzo[b]thiophene 1,1-dioxides could be also hydrogenated with enantioselectivities up to >99%\u00a0ee (Fig. 33) [251]. Another type of substrates where ligand L107b proved to be successful are pinacol derived boronic esters [52b]. While the previously mentioned SimplePHIM ligands were good for terminal boronic esters, L107b was the optimal choice for trisubstituted substrates (Fig. 33), providing up to >99%\u00a0ee. More recently, it has been found that ligands L107 provided higher enantioselectivities than the related PHOX family in the hydrogenation of trisubstituted vinylsilanes (ee\u2019s up to >99%\u00a0ee) [246]. Besides olefins with poorly coordinating groups, it was also found that phosphinite-pyridine ligands (S)-L107b,e are able to hydrogenate purely alkyl-substituted olefins with outstanding enantioselectivities (up to 99%\u00a0ee, Fig. 33) [252]. Its hydrogenation has been used to prepare different biologically relevant products, such as \u03b1- and \u03b3-tocopheryl acetates, precursors of main components of vitamin E (see Section 6) [252b,253]. Other relevant natural products, such as macrocidin A and long-chain polydeoxypropionaes have been synthesized through hydrogenation of long-chain molecules with the ligand family L107\n[254].Later, it was found that the addition of bulkier substituents at the 2-pyridino position (e.g. R1\u00a0=\u00a09-Anth) led to even more enantioselective ligands for the hydrogenation of dihydronaphthalene substrates (with (S)-L107d) [246]. It was also found that disubstituted ligands (S)-L108 exhibited excellent enantioselectivities in the hydrogenation of some challenging substrates. In particular, a ligand with a 9-anthracene group ((S)-L108a) showed an excellent enantioselectivity of >99%\u00a0ee and 95.2:4.8 dr in the hydrogenation of (E,E)-farnesol (Fig. 33), even higher than the afforded with ligands L107 (99%\u00a0ee). Furthermore, unprecedented enantioselectivities (96\u201399%\u00a0ee) were attained in the hydrogenation of \u03b1-substituted \u03b1,\u03b2-unsaturated esters ((S)-L108b) [246,255]. \u03b2-Methyl-substituted esters (Fig. 33), which resulted to be more problematic than expected, could be hydrogenated in also high enantioselectivities (up to 98%\u00a0ee) [254]. The reduction of a broad range of maleic and fumaric acid diesters was also achieved with the use of ligands (S)-L108c\n[256]. Finally, it has been showed once again the applicability of ligands L107-108 by the successful hydrogenation (ee's up to 98%\u00a0ee and TON\u00a0>\u00a09300) of a dihydroquinoline core of agrochemical importance [257].As for other P,N-ligands, Di\u00e9guez's group replaced the phosphinite fragment of ligands L106 with a biaryl phosphite moiety, yielding the library of ligands L109\u2013L110 (Fig. 34\n) [18a]. Note that despite the broad substrate scope of P-pyridine ligands developed by Pfaltz et al., the catalysts were suitable mainly for trisubstituted olefins. With the incorporation of a flexible biaryl phosphite moiety (L109\u2013L110), it was possible to increase the scope of the hydrogenated substrates to 1,1\u2032-disubstituted terminal alkenes, successfully furnishing both enantiomers in up to 99%\u00a0ee. Moreover, the catalytic performance was preserved for a range of E- and Z-trisubstituted olefins (Fig. 34). The system showed high tolerance to neighboring functional groups (such as alcohols, esters, silanes \u2026), leading as well to excellent enantioselectivities (up to >99%\u00a0ee) [258].The new phosphite-pyridine ligands were also successfully applied in the Pd-allylic substitution of tri- and disubstituted allylic substrates with C-, O- and N-nucleophiles (Fig. 35\n) [18a]. The system was highly efficient in the substitution of trisubstituted substrates (up to >99%\u00a0ee). A range of carbocyclic compounds were easily attained by combining in a sequential manner the Pd-allylic alkylation with Ru-catalyzed ring closing metathesis. Furthermore, the reaction could be performed in environmentally friendly solvents, concretely 1,2-propylene carbonate and ionic liquids (1-butyl-3-methyl imidazolium hexafluorophosphate and N-butyl-N-methyl pyrrolidinium bis(trifluoromethylsulfonylamide). To note is that catalyst reuse could be fulfilled up to 5 runs by using the latter ionic liquids, while maintaining the excellent enantioselectivities. Studies of the Pd-1,3-diphenyl, 1,3-dimethyl and 1,3-cyclohexenyl allyl intermediates by NMR spectroscopy showed that in general, for enantioselectivities to be high the ligand parameters need to be correctly combined so that the isomer that reacts faster with the nucleophile is predominantly formed [18a].Qu and co-workers prepared a series of air-stable P-chiral pyridyl-dihydrobenzooxaphosphole ligands, which were called BoQPhos (L111). These ligands could be readily obtained by a diastereoselective nucleophilic aromatic substitution of sulfonyl pyridines with P-stereogenic intermediates (Fig. 36\n) [259]. Sulfonyl pyridines were obtained from 2-chloropyridine or 2,6-dichloropyridine, while the P-stereogenic fragment was prepared in 8 steps from methyldichlorophosphine [260].[Ir(cod)(L111)]BArF complexes were tested in the hydrogenation of some tri- and tetrasubstituted unfunctionalized alkenes. Although the\u00a0ee 's obtained were moderately good (76\u201390%\u00a0ee), it should be highlighted that it was possible to hydrogenate the more challenging tetrasubstituted indene and dihydronaphthalene in high conversions (90\u2013>99%) and promising enantioselectivities (76\u201380%\u00a0ee) with [Ir(cod)(L11a)]BArF (Scheme 68\n) [259]. More recently, it has been showed the utility of methoxy-substituted L111b for the Ir-catalyzed hydrogenation of pyridinium salts [261]. Ir/L11b allowed the preparation of piperidines bearing 2-alkyl and 2-aryl substituents of different nature, with enantioselectivities up to 86% and 98% respectively (Scheme 68).Finally, Fan and co-workers have accounted the most recent family of P-pyridine ligands, consisting in aminophosphine-pyridine ligands L112 (Scheme 69\n). These ligands contain 2-(pyridin-2-yl)-substituted 1,2,3,4-tetrahydroquinoline backbone that were achieved via Ru-catalyzed asymmetric hydrogenation of 2-(pyridin-2-yl)quinolines (Scheme\u00a069a) [262]. Ligands L112 were screened in the Ir-catalyzed hydrogenation of the model substrates trans-\u03b2-methylcinnamate and E-methylstilbene, which were reduced in >99%\u00a0ee. More important are the high enantioselectivities obtained in the reduction of challenging 7-membered ring imines, concretely benzazepines and benzodiazepines. A ligand with a 2-isopropyl-pyridine moiety (L112a) proved to be the most active and enantioselective ligand, showing good diastereomeric ratios and\u00a0ee\u2019s up to 99% for both types of substrates (Scheme\u00a069b). It should be noted that the dihydrogenation of benzazepines was performed as a two-step one-pot process, otherwise only partially reduction was observed, being the CC bond the most reactive one.Chiral P,O-ligands have traditionally played a less important role than P,N-ligands in enantioselective catalysis. The hemilability of the P,O-ligands, owing to the occurrence of both a hard (O) and a soft (P) bases on the same metal center, facilitates several transformations at the metal center, such as oxidative addition, ligand exchange, isomerization, etc., that often has a positive effect on catalytic activity. However, this hemilability, can at the same time cause a detrimental effect on enantioselectivity, since the ligand can be coordinated in a monodentate fashion in the enantiodiscriminating transition state. MOP ligands constitute one of the early examples in which a P,O-ligand acts as monodentate ligand (Fig. 37\n) [263,264]. Despite this, MOP-ligands have been used with great effectiveness in various asymmetric reactions [263]. In all these cases, the ligand acts as a chiral monophosphine with the ether group that produces a secondary interaction with the incoming nucleophile/reagent. Such secondary interaction, as early demonstrated by Hayashi and coworkers, is key to maximize enantioselectivities.Heterodonor phosphine-phosphine oxide ligands have been applied with great effectiveness in different asymmetric transformations (Fig. 38\n) [265]. These ligands can be prepared via Pd-catalyzed monooxidation of the corresponding bidentate phosphines [266].In this respect, Faller and coworkers found that BINAP(O) ligand provided high enantioselectivities in the Ru-catalyzed Diels-Alder reactions (Scheme\u00a070\na) [267]. More recently, the groups of Oestreich [268] and Hou [269] independently published the application of BINAP(O) in the Pd-catalyzed asymmetric intermolecular Heck reaction (Scheme\u00a070b). They found an important change in the regio- and enantioselectivity of the arylation of cyclic alkenes when BINAP(O) ligand was used instead of BINAP. In this respect, the arylation of 2,3-dihydrofuran behaves as perfectly regiodivergent; while the use of BINAP favors the formation of the thermodynamically more stable 2-aryl-2,3-dihydrofuran, the use of BINAP(O) led to the preferential formation of 2-aryl-2,5-dihydrofuran. In addition, little alkene migration was observed with BINAP(O) [268]. The effectiveness of Pd-BINAP(O) in asymmetric Heck reaction was also demonstrated in the efficient kinetic resolution of 2-substituted-dihydrofurans providing optically enriched trans-2,5-disubstituted-dihydrofurans and 2-substituted dihydrofurans in high yield and\u00a0ee\u2019s (S factor of up to 70; Scheme\u00a070b) [269]. Zhou\u2019s group also reported the application of BINAP(O) in the preparation of chiral fused carbo- and heterocycles through a domino reaction involving an asymmetric intermolecular Heck reaction followed by a diastereoselective cyclization (Scheme\u00a070c) [270].Another relevant example of heterodonor phosphine-phosphine oxide ligands can be found in the work of Charette\u2019s group that reported the application of BozPHOS, a monoxide version of the Me-Duphos (Fig. 38), in the Cu-catalyzed 1,2-addition of diorganozinc reagents to N-phosphinoylarylaldimines (ee\u2019s up to 99%; Scheme 71\n) [271]. However, its application was in part hampered by the accessibility and stability of the N-phosphinoylalkylaldimines. To solve this, they demonstrated that the reaction also worked well using the sulfinic adduct of N-phosphinoylimines (Scheme 71) [272].The latest heterodonor phosphine-phosphine oxide design with an outstanding applicability can be found in the work of Zhou's group, that reported the successfully application of spirocyclic SDP(O) (Fig. 38) in Pd-catalyzed intermolecular Heck reactions (Scheme 72\n). Useful, SDP(O) ligands not only allowed the arylation of standard substrates such as 2,3-dihydrofuran or cyclopentene [273], but also of 5 substituted-2,3-dihydrofurans [274] that led to the preparation of a chiral quaternary carbon center (Scheme\u00a072a). More interestingly, the use of SDP(O) ligands was further extended to the desymmetrization of 4-substituted-cyclopent-1-enes and other bicyclic olefins via asymmetric Heck reaction and hydroarylation, respectively (Scheme\u00a072b) [275]. The use of spirocyclic SDP(O) ligands also allowed the unique asymmetric intermolecular Heck reaction with aryl halides as coupling partners (Scheme\u00a072c) [276]. A wide range of aryl bromides and chlorides, including examples with heteroaromatic groups, were efficiently introduced in the Heck reaction of various cyclic olefins (35 examples with\u00a0ee\u2019s typically >95%).Hemilabile amido-phosphine ligands have also shown their effectiveness in asymmetric catalysis. Tomioka and coworkers early demonstrated that the effectiveness of this type of ligands was mainly consequence of the coordination of the amide carbonyl oxygen to the metal [277]. Among all the amido-phosphine ligands, we should highlight ligands L113\u2013L119 (Fig. 39\n).The synthesis of such ligands turned to be quite straightforward (Scheme 73\n). Thus, proline-based ligands were easily attained from (S)-tert-butyl-2-(bromomethyl)pyrrolidine-1-carboxylate through a nucleophilic substitution with a range of metallated phosphines (Scheme\u00a073a). Elimination of the N-Boc protecting group, led the corresponding free amine, which reacted with the desired acetyl chlorides, isocyanates or carbamoyl chlorides. Oxygen-sensitive phosphine moieties, such as di-tert-butyl- or dicyclohexylphosphines, should be protected as borane adducts to avoid oxidation through the ligand synthesis. The synthesis of non-biaryl atropoisomeric ligands L118 and L119 starts from the corresponding aromatic tertiary amide, which is transformed to the corresponding amido-phosphine compound by lithiation followed by reaction with ClPPh2 (Scheme\u00a073b). For ligand L118, the racemic amido-phosphine was chemically resolved by using (\u2013)-camphanic chloride, which allowed the separation by flash chromatography of the corresponding diastereoisomers. Alkaline saponification, followed by etherification gave ligand L118. For ligand L119, the 2-(diphenylphosphaneyl)-N,N-diisopropylbenzamide was then formylated to give the corresponding aldehyde, which allowed the introduction of the chiral N-tert-butanesulfinyl imine, by condensation with (R)-tert-butanesulfinamide and subsequent 1,2-addition of PhMgBr. The diastereomeric amido-phosphines were then separated by flash chromatography.The simple proline-based ligand L113, prepared by Tomioka and coworkers was applied with success in several asymmetric transformation: the Rh-catalyzed 1,4-addition of arylboronic acids to cycloalkenones (ee\u2019s up to 97%) [277], the Cu-catalyzed conjugate addition of dialkylzinc reagents to nitroalkenes (ee\u2019s up to 80%) [278] and in the highly regio- and enantioselective (regio\u2019s up to >99% and\u00a0ee\u2019s up to 91%) allylic substitution of Grignard reagents to cinnamyl-type allylic bromides [279] (Scheme 74\n).Later, they also studied the introduction of an extra stereogenic center at the N-Boc amido group of ligand L113 (ligands L114). The use of N-Boc-L-valine-connected amido-phosphine ligands L114 allowed to extended the applicability of ligand L113 to the Rh-catalyzed arylation of N-tosyl- and N-phosphinoyl aldimines (ee\u2019s up to 99%; Scheme 75\n) [280] and in the Cu-catalyzed conjugate addition of dialkylzincs to \u03b2-aryl-\u03b1,\u03b2-unsaturated sulfonylaldimines [281] (ee\u2019s up to 91%; Scheme 75).Later, they further studied other peptidic modifications of ligand L113 and found that ligand L115, involving a small D-Phe-D-Val dipeptide, was useful in the asymmetric conjugate addition of organozinc reagents to cycloalkenones (ee\u2019s up to 98%; Scheme 76\n) [282]. Advantageously, the Cu/L115 catalyst was also applied in the kinetic resolution of (rac)-5-substituted cycloalkenones to yield trans-3,5-disubstituted alkanones with excellent\u00a0ee\u2019s (up to 90%) and excellent trans/cis ratios (up to >98/2 dr; Scheme 76) [283]. Then, they also reported the useful of Cu/L115 system in the conjugate addition of 6-substituted cyclohexenones to give disubstituted cyclohexanones, albeit with marginally selectivity (cis/trans ratio close to 1). Epimerization of these mixtures with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) favored the formation of the most stable trans-cyclohexenones as the major product in high yields and\u00a0ee\u2019s (up to 96%; Scheme 76) [284].Later, the Pfaltz\u2019s group further modified Tomioka\u2019s proline-based ligand L113 to include several dialkyl and dialkyl phosphino groups as well as urea groups and bulky amide at the pyrrolidine N-atom (ligands L116; Fig. 39) [285,286]. Ligands L116 containing bulky phosphine moieties (R1\u00a0=\u00a0Cy or tBu) shown its effectiveness in the Ir-hydrogenation of trans-methylstilbene olefins and olefins with poorly coordinative groups (e.g. \u03b1,\u03b2-unsaturated ketones and carboxylic esters; Fig. 40\na). These results were pretty unexpected having in mind the lability of the Ir-O bond, and unambiguously demonstrated that ligands L116 remain coordinated in a bidentate fashion during the catalytic cycle, most likely due to the highly acidic character of the IrIII/IrV intermediates.More recently, related pyrrolidine-based P,O ligands derived from cheap carbohydrates (D-ribose, D-mannose and D-arabinose; ligands L117) were also screened in the Ir-catalyzed hydrogenation of olefins [287]. The presence of a rigid bicyclic skeleton in ligands L117 had a positive effect on enantioselectivity, enabling to successful hydrogenate also 1,1\u2032-disubstituted allylic acetates (Fig. 40b). As a further advantage and in contrast to L116, ligand L117 does not require the presence of less stable, bulkier phosphine substituents for optimal performance.Another relevant class of amido-phosphine ligands are the non-biaryl atropoisomeric ligands L118 and L119 (Fig. 39). Ligand L118 provided an enantioselectivity of 95% in the Pd-catalyzed allylic substitution reaction of 1,3-diphenylallyl acetate with dimethyl malonate as nucleophile [288]. The Pd/L118 catalyst was applied in the asymmetric Heck reaction, albeit with low\u00a0ee\u2019s (up to 55%) [289]. More recently, Cia and Xu groups applied the amidophosphine ligand L119, which proved to be highly efficient in Ag-catalyzed [3\u00a0+\u00a02] cycloaddition reactions. Catalyst Ag/L119 mediated the [3\u00a0+\u00a02] cycloaddition of aldiminoesters with nitroalkenes to yield optically enriched nitrosubstituted pyrrolidines (dr\u2019s up to >99:1 and\u00a0ee\u2019s up to 99%; Scheme 77\n) [290]. The same catalyst was also used in the preparation of imide-containing pyrrolidines by reaction of iminoesters with maleimides (dr\u2019s up to >98:2 and\u00a0ee\u2019s up to 99%; Scheme 77) [291].Sulfinamido-phosphines L120\u2013L123 are another type of heterodonor P,O ligands that has recently shown its applicability in asymmetric catalysis (Fig. 41\n) [292,293]. Their synthesis is again fairly straightforward, as can be seen in the short synthesis of ligands L120 (Scheme 78\n). Thus, condensation of (R)-tert-butanesulfinamide with the corresponding 2-phosphinobenzaldehyde led to imino-phosphine intermediates. Stereoselective addition of R2Li followed by reaction with R3Cl (for R3\u00a0\u2260\u00a0H) yielded L120 ligands.Ligands L120 (R1\u00a0=\u00a0Ph, Cy, Ad; R2\u00a0=\u00a0Ar, Me, tBu; R3\u00a0=\u00a0H, Me, CH2-9-Anth) have proved to be useful in several asymmetric transformation, such as the Cu-catalyzed [3\u00a0+\u00a02] cycloaddition of azomethine ylides with a range of \u03b2-trifluoromethyl \u03b2,\u03b2-disubstituted enones and \u03b1-trifluoromethyl \u03b1,\u03b2-unsaturated esters [292a,b], the Pd-catalyzed Suzuki reaction for the preparation of axially chiral phosphonates and phosphine oxides [292c], the Pd-catalyzed intramolecular Heck reaction of allyl aryl ethers [292d] and the Pd-catalyzed intermolecular Heck reaction of alkynes [292e] (Scheme 79\n).Ligands L121\u2013L123 have also shown its usefulness but due to their latest development the range of reactions where they have been applied is still limited. Thus, the Xantphos-inspired ligand L121 has been applied with success in the arylation of sulfenate anions (ee\u2019s up 99%) [293a], and ligands L122 and L123 in the boroacylation of 1,1\u2032-disubstituted allenes [293b] and 1,3-dipolar cycloadditions [293c-d], respectively.The application of anionic P,O ligands is less developed than its neutral analogous despite of the early successful application of phosphine-carboxylate ligands L124 and L125 (Scheme\u00a080\na) [294]. These ligands attained high enantioselectivities in the Pd-catalyzed allylic substitution reactions (ee\u2019s up to >99%; Scheme\u00a080a). More recently, phosphine-sulfonate ligands L126 allowed the asymmetric copolymerization of polar vinyl monomers with carbon monoxide to yield highly head-to-tail isotactic \u03b3-polyketone polymers (Scheme\u00a080b) [295]. In 2017, Zhou\u2019s group accounted a series phosphine-carboxylate ligands (SpiroCAP) related to phosphine-oxazoline ligands L32, which demostrated to be efficient in the hydrogenation of terminal unsaturated carboxylic acids (Scheme\u00a080c) [79].P-thioether ligands have a strong preponderancy among the P,S-ligands [3f,g,296]. In the early 90\u00a0s, researchers realized that P\u2013thioether compounds could be of great use in asymmetric catalysis with the growth of a quite important number of P-thioether ligands. Even so, they found that the presence of diastereoisomeric mixtures of catalytically active species that arise from the sulfur coordination, which becomes an stereogenic center after coordination, made difficult to achieve high enantioselectivities. Thus, very few of the early developed P-thioether ligands had found an important impact in asymmetric catalysis. In the last decade, many efforts have been made to understand how to control the sulfur coordination. For this, mechanistic investigations had played a relevant role and they have revealed that sulfur coordination can be controlled, which have led to a new push on the use of P-thioether ligands in asymmetric catalysis. This section focuses on these new P-S ligands and the correlation between their ligand architecture and catalytic results.BINAP and ferrocene-based ligands have shown they widely prominent useful in asymmetric catalysis. The development of their heterodonor versions was therefore an expected step. In 1994, Gladiali and coworkers used BINAP-based phosphine-thioether ligands L127 (R\u00a0=\u00a0Me and \ni\nPr; Fig. 42\n) in hydroformylation and transfer hydrogenation. Albeit its moderate success, this pioneering work spread the way for the use of P-S ligands in asymmetric catalysis [297]. Then, other groups reported the application of ligands L127 (R\u00a0=\u00a0Me, Ph, 2-iPr-C6H4, 2-Naph, 3,5-Xyl and Cy) in the Pd-catalyzed allylic alkylation using as nucleophiles the model dimethyl malonate and the less studied indoles (R\u00a0=\u00a02-iPr-C6H4;\nFig. 42) with more success (96%\u00a0ee), although the high\u00a0ee's are limited to the benchmark linear substrate rac-1,3-diphenylallyl acetate [298]. More recently, a chiral biphenyl version of ligands L127 has been developed, providing similar high\u00a0ee\u2019s in the Pd-catalyzed allylic substitution with indoles [299].In contrast to binaphtyl- and biphenyl-based P-thioether ligands, since the early development of ferrocene-based P-thioether ligands in 1996 by the groups of Pregosin [300] and Togni [301], a broad range of this type of P-thioethers have been developed [3f]. Among them, it should be underlined: the phosphine-thioether Fesulphos ligands L128, the N\u2013phosphine-thioether FerroNPS-type ligands L129\u2013L130, and the phosphine-thioether ThioClick-Ferrophos L131 (Fig. 43\n). The synthesis of these ligands can be achieved in few steps (Scheme 81\n). Fesulphos ligands L128 were prepared from R-tert-butylsulfenylferrocene in two steps. The first step involves the diastereoselective introduction of the desired phosphine moiety via ortho-lithiation. The second consists in the reduction of the sulfoxide to the thioether group with HSiCl3\u00b7NEt3. Ligands L129\u2013L130 were achieved from Ugi\u2019s amine, which facilitates the diastereoselective introduction of the thioether group. The tertiary amine was then transformed to the secondary amine or to the benzimidazole. Finally, treatment with ClPPh2 led to ligands L129\u2013L130. ThioClick-Ferrophos ligand L131 was synthesized following an analogous method starting from (S,R\np)-ortho-bromo-(1-azidoethyl)ferrocene.Fesulphos ligands L128 (R\u00a0=\u00a0Ph, 4-FPh, 4-CF3Ph, 2-furyl, Cy, o-Tol, 1-Naph; Fig. 43), developed by Carretero's group, have become one of the most useful P\u2013S families in asymmetric catalysis (Scheme 82\n). They have been used with great success in various CC bond forming reactions, in combination with both Pd and Cu catalyst precursors [302]. Thus, in their first report they demonstrated the usefulness of Pd/L128 catalyst in the Pd-catalyzed allylic substitution of the benchmark substrate rac-1,3-diphenylallyl acetate using several malonates and amines as nucleophiles (ee\u2019s up to 98%; Scheme 82) [302a,b]. Pd/L128 also demonstrated to be efficient in the asymmetric ring opening of heterobicyclic alkenes with diorganozinc reagents (ee\u2019s up to >99%; Scheme 82) [302a,c,f]. Since then, Carretero and coworkers have also demonstrated the usefulness of Cu/L128 catalyst precursors in a range of other metal mediatedreactions, such as Mannich-type reactions of N-sulfonylimines with several electrophiles [302h,l], (aza)-Diels-Alder reactions of electron rich alkenes with aldimines [302d,g] and 1,3-cycloaddition reactions of azomethine ylides with an extensive variety of activated olefins [302e,i,k,m,o-r] (ee\u2019s up to >99%; Scheme 82).Later, Chan\u2019s group developed ligands L129\n[303] and L130\n[304] (Fig. 43), which combines both planar and central chirality, and demonstrated their versatility in Pd-catalyzed allylic substitutions. Thus, Pd/L129 was successfully applied in the allylic substitution of benchmark rac-1,3-diphenylallyl acetate with dimethyl malonate, a range of amines and several aliphatic alcohols (ee\u2019s up to 98%; Scheme 83\n) [303a,b], being one of the first successful examples of allylic etherifications with non-aromatic alcohols. More recently, the use of ligand L130 containing a benzimidazole unit, extended the nucleophile scope to indoles (ee\u2019s up to 96%; Scheme 83) [304a] and to the alkylation of some cyclic substrates with\u00a0ee\u2019s up to 87% [304b].ThioClick-ferrophos ligand L131 (Fig. 43), developed by Fukuzawa\u2019s group, was screened in the Ag-catalyzed Mannich reactions of N-tosylimines with a glycine Schiff base [305a] with moderate diastereoselectivities and high enantioselectivities (dr\u2019s up to 7:3 and\u00a0ee\u2019s up to 98%) (Scheme 84\n). Ag/L131 also efficiently catalyzed the 1,3-cycloadditions of azomethine ylides using a variety of activated alkenes providing similar enantioselectivities than those achieved with the Cu/Fesulphos L128 catalyst (ee\u2019s up to 99%) [305b-g]. Later, the same group also found that Ag/L131 catalyzed the conjugate addition of several types of Michael acceptors to different imino esters, oxazoline-esters and related substrates (ee\u2019s up to >99%; Scheme 84) [305i-l]. Finally, by switching the positions of the thioether and the phosphine moieties high enantioselectivities can also be reached in Pd-catalyzed allylic substitution of rac-1,3-diphenylallyl acetate with dimethyl malonate, benzylamine and some benzylic alcohols (ee\u2019s up to 90%) [305m].An additional group of powerful P\u2013S ligands are those having the two donor functionalities linked by two carbon atoms [296,306]. One of the first successful examples of such a group of ligands can be found with phosphinite-thioether ligands L132, with a very simple ligand backbone (Scheme 85\n) [307] These ligands are prepared from the corresponding chiral epoxide via epoxide ring opening with the corresponding thiol or from the corresponding Evan\u2019s N-acyl carboximide in few steps to provide the corresponding thioether-alcohol. Treatment of the later compounds with the desired chlorophosphine gives access to ligands L132 (Scheme 85).The Evans' group demonstrated that by optimizing the different ligand parameters (thioether and phosphinite substituents as well as the ligand backbone) it is possible to control the thioether coordination to the metal. As a result, ligands L132 constitutes one of the early-developed P\u2013thioether ligands that provided excellent results to several asymmetric reaction, such as the Rh-catalyzed hydrosilylation of ketones and hydrogenation of alkenes [307c] and the Pd-catalyzed allylic substitutions [307a,b] (Scheme 86\n). However, the substrate/reagents scope was still low.Much later our group decided to replace the phosphinite moiety in the Evans' ligands L132 by the benefits of biaryl phosphite groups (ligand L133; R1\u00a0=\u00a0tBu, 2,6-Me2Ph; Fig. 44\na). Air stable ligands L133 were fruitfully applied in the hydrogenation of unfunctionalized alkenes, including terminal olefins and olefins with poorly coordinative groups (ee\u2019s up to 99%; Fig. 44b) [308]. The catalysis were carried out using the preformed catalyst precursors [Ir(cod)(L133)]BArF, prepared using the same methodology than for [Ir(cod)(L8)]BArF. One single isomer was found except for complexes containing ligands with a flexible biphenyl moiety, due to the tropoisomerization of these units. The X-Ray structure confirms the coordination of the ligand throught the P and S atoms to the metal, whith a twist-boat conformation of the chelate ring and a pseudoaxial disposition of the thioether moiety. It should be highlighted that the use of Ir/L133 allowed the pioneering reduction of 1,1\u2032-disubstituted aryl-substituted boronic esters. Interestingly, the enantioselectivity is mainly determined by the thioether substituent and the configuration of the phosphite group. The replacement of the phosphite moiety by several phosphinite groups provided lower\u00a0ee\u2019s. The study of the reaction intermediates by HPNMR (high pressure NMR) spectroscopy and DFT calculations allowed to found the active Ir-dihydride alkene species, which follows the classical Halpern-mechanism, in which the minor species are the most active ones [309]. In addition such mechanistic study provided helpful insights to understand the influence of the different ligand elements on enantioselectivity.Other relevant families of P\u2013S ligands having a two carbon atoms linker are the families of P-thioethers ligands L134 and L135 and the phosphoroamidite-thioether ligands L136\u2013L138 (Fig. 45\n). They resemble very much to the Evans' ligands in the simplicity of the ligand backbone, which aids the recognition of key intermediates by NMR as well as speeds up the ligand optimization by DFT calculations. Ligands were prepared from the corresponding epoxides (ligands L134 and L135) or aziridine (compounds L136\u2013L138) following the same synthetic strategy as for the synthesis of Evan\u2019s ligands.The arylglycidol-based phosphinite-thioether ligands L134 (R1\u00a0=\u00a0Ar, tBu, Ad, Cy; R2\u00a0=\u00a0Ph, Tol, Cy, Mes, R3\u00a0=\u00a0Me, Tr, Bn) (Fig. 45) have found to be useful in allylic substitutions and hydrogenation reactions [310]. A practical advantage offered by L134 is the fact that they are made in three steps from accessible arylglycidols [310a]. In addition, both enantiomers of these P,S-ligands can be reached by simple selection of the tartrate ester used in the Sharpless epoxidation leading to the arylglycidol. Pd/L134 catalytic systems provided similar high enantioselectivities than Evans' ligands L132, working under milder reactions conditions, room temperature and shorter reactions times, in the allylic substitution of di- and trisubstituted linear allylic acetates with a range of malonate-type nucleophiles and amines, and also extended the nucleophile scope to the less studied aliphatic alcohols [310a]. Ligands L134 were also used in the Rh-catalyzed hydrogenation of dehydroamino acids, albeit with lower success than related ligands L132\n[310b]. Much more notable are the results reached in the Ir-catalyzed hydrogenation of unfunctionalized olefins [310c], in spite of the known difficulties of enantiocontrol associated to substrates lacking metal-coordinating functionalities. [Ir(cod)(L134)]BArF complexes were therefore able to reduce a large number of olefins, with similar\u00a0ee\u2019s than the best ones attained with Ir\u2013P,N catalysts (43 examples, \u00a0ee 's up to 99%; Fig. 46\n). Unlike ligands L133 with a cyclohexane-based backbone, the use of phosphite analogues led to lower enantioselectivities than with phosphinite-thioether ligands L134. The crystal structures of these Ir-catalyst precursors showed that while ligands with a phosphite moiety had the thioether group in equatorial, in the related phosphinites the thioether was in axial. This contrasts with the pseudoaxial arrangement of the thioether substituent in Ir-structures with cyclohexane-based phosphite\u2013thioether commented above, that also form a six-membered chelate ring. This behaviour seems to show that the disposition of the thioether substituent (in this case, axial disposition) is important to obtain high enantioselectivity. DFT calculations indicated that the reaction proceeds via an Ir(III)/Ir(V) catalytic system in which the enantioselectivity-determining step is the migration of a hydride to the coordinated alkene. In addition, the analysis of the transition states allowed to develop a quadrant model system that facilitates rationalization of the catalytic results. These DFT studies were also crucial to guide the ligand optimization process towards high enantioselectivities. They indicated the need of ligands with a mesityl group at the carbon next to the thioether group (Ar\u00a0=\u00a02,4,6-Me3-C6H2) and a bulky aromatic thioether groups (2,6-dimethylphenyl or 1-naphthyl moieties, depending on the substrate). The application of mesityl-containing ligands L134 are therefore crucial to achieve the highest\u00a0ee\u2019s for a range of olefins including examples containing poorly coordinative groups and terminal alkenes (ee\u2019s up to >99%; Fig. 46). Remarkably, the catalytic systems could be also recycle up to 3 times with 1,2-propylene carbonate.Recently, phosphite\u2013thioether ligands L135 (R1\u00a0=\u00a0iPr, nPr, tBu, Ph, 2,6-Me2Ph, 4-CF3Ph, 4-MeOPh, 9-Anth) (Fig. 45), prepared in three steps from indene, were designed to maximize the substrate range in Pd-catalyzed allylic substitution reactions [18d]. The simple indene backbone facilitated both DFT and NMR studies of Pd-allyl key intermediates, which were used to optimize the thioether and phosphite substituents in the search of the best catalyst. As a result, catalyst Pd/L135b (R1\u00a0=\u00a09-Anth) is one of the very few catalysts able to afford excellent enantioselectivities (typically >95%\u00a0ee) for a large number of unhindered and hindered allylic acetates with an array of C, N and O nucleophiles (Scheme 87\n; 40 compounds in total). Notably, the excellent performance of L135b was maintained using 1,2-propylene carbonate as solvent. Mechanistic investigations provided an elucidation into the exceptionally rare wide substrate scope. Enantioselectivity is therefore controlled by the relative stability of the Pd-\u03b73-allyl intermediates and the electrophilicity of the allylic terminal carbons. More concretely, Pd/L135b catalytic system not only favors the preferential formation of one of the possible Pd-allyl intermediates, but also speeds up the nucleophilic addition at the terminal allylic carbon atom trans to the phosphite moiety of most stable Pd-allyl intermediate. In addition, the authors took advantage of the great diversity of the allylic substitution products arising from the introduction of malonates having allyl and propargylic groups for the preparation of chiral functionalized carbo- and heterocycles as well as polycarbocyles. The former compounds were prepared by means of ring-closing metathesis, while the latter were prepared via Pauson-Khand reaction (Scheme 88\n).Phosphoroamidite-thioether ligands L136\u2013L138 (Fig. 45), easily prepared in three steps from (2S,3S)-2,3-diphenylaziridine, have also been effectively applied in many asymmetric transformations [311]. Thus, ligand L136 been fruitfully applied in the Pd-catalyzed allylic substitution of 1,3-diarylallyl acetates with a collection of indoles and hydrazones (ee\u2019s up to 98; Scheme\u00a089\na) [311a,b]. Similar high\u00a0ee\u2019s were also achieved in the allylic substitution of rac-1,3-diphenylallyl acetate with benzyl amine and benzyl alcohol [311a]. Ligand L136 also evidenced to be highly competent in both Cu- and Pd-catalyzed cycloaddition reactions. Thus, for instance, catalyst Cu/L136 afforded a range of polysubstituted endo pyrroles in high diastereo- and enantioselectivities via 1,3-cycloaddition of azomethine ylides and nitroalkenes (Scheme\u00a089b) [311c]. Interestingly, the use of related H8-Binol-derived ligand L137 (Fig. 45) led to the formation of the exo pyrroles (Scheme\u00a089b) [311c. Catalyst Pd/ L136 was successfully used in inverse-electron demand decarboxylative [4\u00a0+\u00a02] cycloaddition reactions. Thus, highly functionalized dihydroquinol-2-ones were produced with excellent selectivities (d.r.\u00a0>\u00a020:1 and\u00a0ee\u2019s up to 95%; Scheme\u00a089c) [311d]. Pd/L136 has recently found to be beneficial in the visible-light-driven [5\u00a0+\u00a02] cycloaddition of vinylcyclopropanes with \u03b1-diazoketones. This new methodology provides facile access to highly functionalized 7-membered ring lactones (d.r.\u2019s up to 16:1 and\u00a0ee\u2019s up to 96%; Scheme\u00a089d) [311e]. Similarly, a series of quinolinones were synthesized via Pd-catalyzed light-driven decarboxylate [4\u00a0+\u00a02] cycloaddition of tosylated vinyl carbamates with in situ generated ketenes (ee\u2019s up to 96%\u00a0ee; Scheme\u00a089e) [311g]. It should be mentioned that some Pd-catalyzed decarboxylative cycloaddition reactions do not requires the presence of a chiral biaryl phosphoroamidite moiety (ligand L138; Fig. 45). Thus, a range of tetrahydroquinolines bearing three contiguous stereocenters were efficiently prepared using Pd/L138 by reaction of benzoxazinanones with activated alkenes (d.r.\u2019s typically >95:5 and\u00a0ee\u2019s up to 98%\u00a0ee; Scheme\u00a089f) [311f].Carbohydrates have also been used as platforms for preparing P-thioether ligands. The use of carbohydrates is advantageous since they are cheap and readily available. Moreover, they have a well-established chemistry and they are highly functionalized, which favor the synthesis of highly modular ligand libraries and enable an easy ligand optimization for each particular substrate and reaction [312]. Khiar\u2019s group were the first to apply carbohydrate P\u2013thioether ligands in catalysis. They used pyranoside ligands L139 and L140 (Fig. 47\n) in the Rh-catalyzed hydrogenation of some enamides (ee's up to 98%) and in the Pd-catalyzed allylic substitution of benchmark 1,3-diphenylallyl acetate (ee's up to 96%) [313]. The use of pseudo-enantiomeric ligands L139 and L140 allowed the preparation of both isomers of the products, without having to prepare the enantiomeric ligands from the expensive L-sugar series.Furanoside phosphite-thioether ligands L141 and L142 (R\u00a0=\u00a0Ph, Me, iPr, tBu, 4-MePh, 4-CF3Ph, 2,6-Me2Ph) (Fig. 47) were prepared from D-xylose in multigram scale. Treatment of D-xylose with I2 in acetone followed by deprotection of the more reactive isopropylidene group led to 1,2-O-isopropylidene-\u03b1-D-xylofuranose (key for the synthesis of ligands L141), which was easily transformed to the ribofuranoside anologue (key for the synthesis of L142). From both xylo- and ribofuranoside diols, ligands L141 and L142 were prepared by introducing the thioether group at the primary alcohol, via an SN2 reaction, followed by treatment with the desired phosphorochloridite (Scheme 90\n).Phosphite-thioether ligands L141 and L142 (Fig. 47) represented the first use of P,S-ligands in the asymmetric hydrogenation of unfunctionalized olefins or with poorly coordinative groups [314]. The catalysis were carried out using the preformed catalyst precursors [Ir(cod)(L141\u2013L142)]BArF, prepared using the same methodology than for [Ir(cod)(L8)]BArF. The X-Ray analysis indicated that in contrast to previously commented Ir/P-S complexes, such as [Ir(cod)(L133)]BArF, the thioether substituent adopts an equatorial disposition. In this context, the use of ribofuranoside ligand L142k (R\u00a0=\u00a02,6-Me2-C6H3) provided high\u00a0ee 's in the hydrogenation of methyl stilbene-type olefins, Z-trisubstituted olefins and triarylsubstituted olefins (Fig. 48\n). The latter provides a feasible entry point to valuable compounds containing diarylmethine chiral centers. Ir/L142k catalytic system also led to high\u00a0ee 's in the reduction of many 1,1\u2032-disubstituted olefins. In addition, both enantiomers of the reduced products can be easily attained by changing the configuration of the biaryl phosphite moiety. Another interesting feature of this ligand is that the furanoside scaffold allowed to restrict efficiently the tropoisomerization of conformationally flexible biphenyl phosphite moieties. For most of the substrates studied, similar high enantioselectivities have therefore been reached with the cheap achiral bulky biphenyl phosphite moiety (ligand L142b). Again, these catalysts work well in 1,2-propylene carbonate, helping the catalysts to be recycled several times. Finally, the use of phosphinite or phosphine analogues led to lower enantioselectivities.When ligands L141 and L142 were tested in allylic substitution of linear hindered 1,3-disubstituted allylic acetates with a range of C-nucleophiles (e.g. \u03b1-substituted malonates, diketones, cyano esters \u2026) and a selection of O- and N-nucleophiles, high enantioselectivities were attained using Pd/L142k (R\u00a0=\u00a02,6-Me2-C6H3) catalyst (ee\u2019s up to >99%; Scheme 91\n) [18b,315]. To achieve high enantioselectivities for more demanding cyclic and unhindered linear substrates, the use of xylofuranoside ligand L141h (R\u00a0=\u00a01-Naph) was required (ee\u2019s up to >99%; Scheme 91). This feature was rationalized with the aid of NMR studies of the Pd-allyl intermediates and DFT calculations of the TSs using cyclohex-2-en-1-yl acetate as model substrate. These studies demonstrated how the size of the chiral pocket in the catalytic species is affected by the configuration at C-3 of the furanoside backbone. Thus, by using catalyst Pd/L141h, only one of the two possible syn/syn diastereomer Pd-1,3-cyclohexenyl-allyl intermediate is predominantly formed (dr\u2019s\u00a0>\u00a020:1) [315].Finally, Taddol-type phosphite-thioether ligands were made from L-tartaric acid (ligands L143) and D-mannitol (ligands L144) and screened in several asymmetric transformations (Fig. 47) [316]. Several positions of the ligands (R1\u00a0=\u00a0Me, 1-Ad, Ph, tBu, 2,6-Me2Ph, 1-Naph, 2-Naph; R2\u00a0=\u00a0Me, H, Ph; R3\u00a0=\u00a0Me, H, Ph) can be easily varied through highly efficient methods. This methods also allowed to generate at will new stereogenic centers next to the donor functionalities (R4\u00a0=\u00a0H, Me, CH2OTBDMS, CH2OTBDPS, CH2OTIPS, CH2OTr; R5\u00a0=\u00a0H, Me). [Ir(cod)(L144f)]BArF (R1\u00a0=\u00a01-Naph, R4\u00a0=\u00a0(R)-CH2OTBDMS and R5\u00a0=\u00a0H) provided\u00a0ee\u2019s up to 95% in the hydrogenation of trans-methylstilbene-type substrates, \u03b2,\u03b2\u2019-disubstituted unsaturated esters, \u03b1,\u03b2-disubstituted enones, lactones and lactams bearing an exocylic double bond [316b]. Note that for most of these substrates the selenoether version of the ligands attained slightly higher enantioselectivities than the thioether analogues [316b]. The use of catalyst [Ir(cod)(L143h)]BArF (R1\u00a0=\u00a01-Naph; R2\u00a0=\u00a0R3\u00a0=\u00a0H) was necessary to maximize\u00a0ee\u2019s in the reduction of terminal olefins (ee\u2019s up to 99%) [316b]. Interestingly, by using [Ir(cod)(L144g)]BArF (R1\u00a0=\u00a01-Naph, R4\u00a0=\u00a0(S)-CH2OTBDMS and R5\u00a0=\u00a0H) enantioselectivities up to 99% were attained in the hydrogenation of cyclic \u03b2-enamides (Scheme\u00a092\na). Interestingly, the exchange of the metal from Ir to Rh led to the preferential formation of the opposite enantiomer (Scheme\u00a092a) [316a]. This is one of the rare examples of enantioswitchable metal-catalyzed transformation. This allows, for instance, access to both enantiomers of the precursors for the synthesis of rotigotine (used in the treatment of Parkinson's disease) [41a] and alnespirone (a selective 5-HT1A receptor full agonist) [41d]. In addition, the use of [Rh(cod)(L144f)]BF4 (R1\u00a0=\u00a01-Naph, R4\u00a0=\u00a0(R)-CH2OTBDMS and R5\u00a0=\u00a0H) and [Rh(cod)(L144g)]BF4 (R1\u00a0=\u00a01-Naph, R4\u00a0=\u00a0(S)-CH2OTBDMS and R5\u00a0=\u00a0H) catalyst proved also be useful in the asymmetric hydrogenation of functionalized olefins, such as dehydroamino acids (Scheme\u00a092b) [316b]. This is again a quite unique feature, since the reduction of both unfunctionalized and functionalized alkenes follows very different catalytic cycles, and each type of substrate has been shown to require a particular catalyst type (Rh/PP-catalysts for functionalized and Ir/PN-catalysts for unfunctionalized) for optimal results [35e].Interestingly, the use of ligand L144g (R1\u00a0=\u00a01-Naph, R4\u00a0=\u00a0(S)-CH2OTBDMS and R5\u00a0=\u00a0H) and its derivatives containing different silylated protecting groups (R4\u00a0=\u00a0(S)-CH2OTBPMS and (S)-CH2OTIPS) also furnished high enantioselectivities in the Pd-catalyzed allylic substitutions (ee\u2019s up to 99%) [316c]. Interestingly, for cyclic substrates it is possible to select the enantiomeric series of the substitution product, as in the case of the hydrogenation of terminal alkenes, by swapping the configuration of the biaryl phosphite moiety.Another strategy to overcome the problem of controlling the configuration of the S-thioether group after coordination to the metal center is the exchange of the thioether group by a chiral sulfoxide. Fig. 49\n shows the most successful P-sulfoxide ligands developed to date. Ligands L145 and L146 were prepared from the corresponding bromo and 1,3-dibromobenzenes, which reacts with (R)-tert-butyl tert-buthanethiosulfinate to yield the corresponding sulfoxides. From the latter, the desired phosphine moiety was diastereoselectively introduced via ortho-lithiation (Scheme 93\n). Similarly, reaction of two equivalents of (R)-(tert-butylsulfinyl)benzene with 1-(dichlorophosphaneyl)piperidine led to ligand L148. Ligand L147 was efficiently prepared from (2S, 3S)-2,3-diphenyl aziridine. Aziridine ring opening with 4-Br-thiophenol followed by diastereoselective oxidation led to the corresponding amino-sulfoxide compound. Reaction of the latter with 2-(diphenylphosphanyl)benzoic acid gives access to ligands L147 (Scheme 93).Ligands L145 were the first successful application of P-sulfoxide ligands to several asymmetric reactions [317]. Thus, Rh/L145 provided high enantioselectivities in the Rh-catalyzed 1,4-addition of arylboronic acids to a large number of electron-deficient olefins (up to 98%\u00a0ee; Scheme\u00a094\na) [317a]. The presence of a second sulfoxide moiety at the other ortho position of the phosphine group (ligand L146; R\u00a0=\u00a0Ph) allowed the construction of chiral \u03b3,\u03b3-diarylsubstituted carbonyl compounds via the same reaction, that led to the preparation of bioactive compounds such as sertraline (ee\u2019s up to >99%; Scheme\u00a094b and Section 6) [318]. Ligands L145 also allowed the first Cu-catalyzed formation of \u03b1-aryl-\u03b2-borylstannanes by means of a three-component borylstannation of aryl-substituted alkenes (Scheme\u00a094c) [317b]. Such transformation relies in the efficiency in controlling the stereochemistry of B-Cu addition as well as its ability to facilitate the transmetallation of enantioenriched alkyl-Cu species with retention of configuration. More recently, an efficient cooperative Cu/Pd-catalyzed asymmetric allylboration of alkenes has been reported (Scheme\u00a094d) [317c]. The application of CuOAc/L145 (R1\u00a0=\u00a0OiPr, R2\u00a0=\u00a0iPr) in combination with Pd(dppf)Cl2 catalysts allowed the three-component reaction of styrenes, B2(pin)2 and allyl carbonates to be done with\u00a0ee\u2019s as high as 97%.Phosphine-sulfoxide ligand L147 (Fig. 49) was fruitfully applied (ee\u2019s up to 99%) in the Pd-catalyzed allylic substitution with a very broad nucleophile scope (various malonates, including examples with different functionalities at the \u03b1-position, as well as ketoesters, amines, alcohols and indoles (Scheme 95\n) [319].The N-phosphine-bis(sulfoxide) ligand L148 (Fig. 49) have recently been used in the allylic etherification and amination of 1,3-diarylallyl acetates with benzylic alcohols and amines (ee\u2019s up to 99%; Scheme\u00a096\na) [320a]. Even more interesting are the excellent results achieved in the allylic alkylation of puzzling unsymmetrically 1,3-disubstituted allylic acetates. Pd/L148 catalyzed the dynamic kinetic resolution of this class of substrates with indoles (up to 84% yield with up to 95%\u00a0ee; Scheme\u00a096b) [320b]. The bifunctional character of this ligand is responsible for this favorable stereocontrol. The authors postulate that the two sulfoxide moieties play a different role, while one of them tightly coordinates to Pd, the other one directs the nucleophilic attack via a hydrogen bond interaction. Finally, ligand L148 also provided excellent enantioselectivities in the Rh-catalyzed 1,4-addition of arylboronic acids to cyclic enones (up to 98%\u00a0ee) and of sodium tetraarylborates to chromenones (up to >99%\u00a0ee; Scheme\u00a096c) [320c].A ligand acquires an even greater value if, in addition to showing a high reaction and substrate applicability, it can be used in the total synthesis of relevant chiral compounds. We here compile some applications of previously discussed heterodonor ligands to total synthesis (Table 1\n). Different P-oxazoline/pyridine/amine/imine/thiazole/imidazoline ligands, five families of P,O and three families of P-thioether/sulfoxide ligands have found applications. The examples clearly illustrate the potential of these ligands in total synthesis. However, as can be see below, only a small set of the ligands commented has found suitable application to date. Among these we have phosphine-oxazoline L1, with electronic withdrawing groups in the phenyl ring and the phosphine moiety of the PHOX ligand, and the spiro based-SIPHOX ligand L32 with many applications each. The other three groups of ligands are the phosphine-amine L75, the phosphinite-pyridine L107, developed by Pfaltz, and the family of N-phosphine-thiazole ligands L84 and L88 developed by Andersson's group. The low application or no application for the rest of ligands may be due, in part, to the fact that research has focused mainly on finding the right ligand/s for specific asymmetric catalytic reactions. Specifically, in finding the correlation between the structure of the ligand and the catalytic capacity. Another factor to take into account is the availability of a chiral ligand. Still many of these ligands are prepared through multi-step syntheses, from costly starting material and/or toxic reagents and with low yields, which reduces their commercial interest. The progress made in the recent years to obtain ligands that are made in fewer steps and that can be manipulated in the air will hopefully widen the spectrum of chiral ligands that are commercially available.The success of phosphine-oxazoline ligands (PHOX) inspired the development of many new P-oxazoline ligand families that cover modifications in the ligand scaffold and/or the steric /electronic characteristics of the phosphine group to the change of the phosphine group with a phosphinite or phosphite group. New development arrived also by replacing the oxazoline group by several other N-donor groups and O- and S-donor groups. The structures of these chiral heterodonor ligands have gained in diversity and new families of very efficient ligands have emerged, which have allowed to improve catalytic performance in some asymmetric transformations. In addition, the majority of these ligands maintain the short and efficient synthetic route developed with PHOX ligands. The utility of the most successful heterodonor ligand families has been described in this review. We have shown how a suitable ligand design, aided by mechanistic studies, these ligands have become versatile ligands for metal-mediated asymmetric reactions, with superior catalytic performance in many reactions than the best C2-symmetric N,N and P,P-ligands reported so far. Their excellent results together with its easy synthesis and tailor-made modularity spread the way to new generations of heterodonor ligands and to further enlarge the range of processes catalyzed by them. This will help the progress and will therefore drive the growth of asymmetric catalysis as a vital element to achieve the sustainable production of enantiopure compounds in the coming years.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 financial support from the Ministerio de Econom\u00eda y Competitividad (CTQ2016-74878-P), Ministerio de Ciencia e Innovaci\u00f3n (PID2019-104904GB-I00), European Regional Development Fund (AEI/FEDER, UE), the Catalan Government (2017SGR1472) and the ICREA Foundation (ICREA Academia award to Montserrat Di\u00e9guez).", "descript": "\n                  The success of phosphine-oxazoline ligands (PHOX) inspired the progress in P-oxazoline ligand families by modifying either the ligand backbone, the electronic and/or steric properties of the phosphine group or by exchanging the phosphine to a phosphinite or a phosphite group. In this respect, the structures of chiral P-oxazoline ligands have become more diverse and new families of very efficient ligands have emerged, which have improved catalytic performance in some asymmetric transformations, with an increased versatility, both in the range of reactions and in the range of substrates/reagents. In addition, most of ligands are synthesized from easily accessible chiral amino alcohols, maintaining the short and efficient synthetic route developed for PHOX ligands. New ligands have been developed by replacing the oxazoline functionality with several other N-donor groups, e.g. imidazole, thiazole, oxazole, pyridine, etc., and O- and S-groups. This review offers a critical overview of the utility of these most successful bidentate heterodonor P-N, P-O and P-S ligand families applied in metal-mediated processes. We illustrate how, through proper ligand design, these heterodonor bidentate ligand families can be an excellent source of ligands, with superior catalytic performance in many asymmetric reactions than the best C2-symmetric N,N and P,P-ligands reported so far.\n               "}
{"full_text": "Data will be made available on request.Modern society is heavily dependent on the chemical industries for most of its needs. This dependency comes with a huge environmental impact as a result of the constant emission of greenhouse gases from their chemical processes. In this regard, the development and integration of green technologies, such as the solid oxide electrolyzer cell (SOEC) in the industries, will play a big role in reducing these emissions. For example, as advanced high-temperature electrochemical devices, SOECs can produce green chemical intermediates (such as CO) from industrial gaseous waste like CO2 through the use of renewable energy sources [1,2]. In addition to providing chemical intermediates, the difficulty in the storage of renewable energy at peak seasons can be solved by utilizing the renewable energy in SOEC operations (Power-to-X) [1\u20133]. Green hydrogen, for example, can be produced from renewable energy sources through electrochemical water splitting (Equation (1)). Furthermore, synthesis gas (CO\u00a0+\u00a0H2), an important synthetic fuel in downstream processes, can also be produced through the co-electrolysis of H2O and CO2 [4\u20136]. From thermodynamics perspective, the simultaneous electrolysis of CO2 and steam is possible as the reduction of CO2 during co-electrolysis is facilitated by the reverse water gas shift reaction (RWGS) [3,7]. Eqs. (1) and (2) represent the separate steam and CO2 electrolysis reactions with their energy demands. The reactions during co-electrolysis operations involve a much more complex process due to the catalytic RWGS reaction (Eq. (3)) taking place at the same time with the steam and CO2 electrolysis [4,8].\n\n[1]\n\n\nH\n2\n\nO\n\u2192\n\nH\n2\n\n+\n\n1\n2\n\n\nO\n2\n\n,\n\n\u0394\n\n\nH\nr\n\n\n(\n900\n\u00b0\nC\n)\n\n\n=\n249\n\nk\nJ\n\u2027\nm\no\n\nl\n\n-\n1\n\n\n.\n\n\n\n\n\n[2]\n\nC\n\nO\n2\n\n\u2192\nC\nO\n+\n\n1\n2\n\n\nO\n2\n\n,\n\n\u0394\n\n\nH\nr\n\n\n(\n900\n\u00b0\nC\n)\n\n\n=\n282\n\nk\nJ\n\u2027\nm\no\n\nl\n\n-\n1\n\n\n.\n\n\n\nRWGS\n\n[3]\n\n\nH\n2\n\n\nC\n\nO\n2\n\n\u2192\n\nH\n2\n\nO\n+\nCO\n,\n\n\u0394\n\n\nH\nr\n\n\n(\n900\n\u00b0\nC\n)\n\n\n=\n33\n\nk\nJ\n\u2027\nm\no\n\nl\n\n-\n1\n\n\n.\n\n\n\nThe high operating temperatures of SOEC (700\u2013900\u00a0\u00b0C) eliminates the need for a noble-metal catalyst as well as allows for higher efficiency and increased production rate [9,10]. However, the thermodynamic advantage offered by the high-temperature operation presents a challenge for electrode material selection, especially for the fuel electrode which is in contact with different types of fuel. So far, Ni-YSZ (yttria stabilized zirconia) cermet has been the conventional electrode for the fuel electrode [11,12]. Ni acts as an electron conductor as well as an electro-catalyst for the reduction of fuel gases. YSZ, on the other hand, provides the pathway for ionic conductivity [13]. Extensive research in the electrochemical characteristics of Ni-YSZ electrode has already been performed, indicating its good electro-catalytic activity under different operating conditions [11\u201315].However, during long term operation, a significant degradation is observed especially under electrolysis mode [16,17]. Microstructural degradation as a result of Ni-migration and Ni coarsening was identified as significant factors causing the degradation at the Ni-YSZ fuel electrode [16]. Furthermore, different microstructural optimization techniques have been applied to reduce the Ni-migration [18,19]. For example, Ovtar et al. [18] studied the influence of GDC infiltration on the long term durability of Ni-YSZ cermet in steam electrolysis conditions. They reported that the infiltration of GDC nanoparticles into the Ni-YSZ cermet significantly decreased the voltage degradation rate by more than 900%. Recently, efforts to completely replace the YSZ oxide phase with the GDC have also been made [20\u201322]. Unlike YSZ, GDC is a mixed ionic and electronic conducting material under reducing conditions. In a reducing atmosphere, the Ce4+ is reduced to Ce3+ which generates the electronic property in addition to the ionic property. A positive consequence of this reduction is the extension of the triple phase boundary (TPB) beyond the electrolyte/electrode/gas interface to almost the entire surface area of the electrode exposed to the gas phase. Hence, the kinetics of the electrochemically active species increases leading to a lower polarization [23,24]. Also, the catalytic activity of ceria-based materials has been linked to the reversible transition between Ce4+ and Ce3+ [25\u201327].With regards to the electrochemical processes and performance, most literature [28\u201331] has reported the presence of two dominant arcs in the Nyquist plot of the Ni-GDC electrode obtained in fuel cell mode by electrochemical impedance spectroscopy (EIS). For example, Fu et al. [29] have reported the presence of two arcs in the Nyquist plot of a Ni-GDC fuel electrode and LSCF-GDC oxygen electrode, a dominant low frequency arc and high frequency arc. Lomberg et al. [30] and Macedo et al. [28] have also reported two dominant arcs from Ni-GDC electrodes cells fabricated with a commercial Ni-GDC powder and a Ni infiltrated GDC cell respectively. The two dominant Nyquist arcs were attributed to a diffusional process in the low frequency and a charge transfer process in the high frequency. A Ni-GDC symmetrical cell was also analyzed by Aravind et al. [32] in humidified hydrogen (40:60, H2:N2), and they identified three processes at 1123\u00a0K while Sumi et al. [33,34] identify about 5 processes through DRT deconvolution of the spectra for the single cell containing Ni-GDC fuel electrode. In steam electrolysis conditions, Athanasiou et al. [25] applied the analysis method of differences in the deconvolution of their impedance spectra obtained from the Ni-GDC fuel electrode and LSM oxygen electrode. They identified four different electrode processes; two low frequency processes (0.04\u20130.20\u00a0Hz) followed by two intermediate frequency processes (5\u201330\u00a0Hz).Following the inconclusive literature reports on the electrochemical processes in SOEC mode, this study aims to shed more light on the electrochemical processes occurring in Ni-GDC fuel electrodes as well as to investigate the long-term stability behaviour in both steam and co-electrolysis conditions. To this aim electrolyte-supported single cells (Ni-GDC//8YSZ//GDC//LSCF) were fabricated, followed by electrochemical characterization using electrochemical impedance spectroscopy (EIS) at different temperatures. The impedance data were also obtained at different partial pressures of steam, CO2 and oxygen at OCV. Furthermore, measurements under polarization (0.7\u20131.4\u00a0V) were recorded in order to investigate the charge transfer process. Equivalent circuit models, as well as distribution of relaxation times (DRT), were used to investigate the dominant electrochemical processes present in single cells.Electrolyte supported single cells were prepared for the electrochemical measurements. The fuel electrode consists of commercial NiO\u2013Ce0.9Gd0.1O0.95 (GDC) powder from Marion Technologies (NiO:GDC, 65:35) while the LSCF (La0.58Sr0.4Co0.2Fe0.8O3-\u03b4) oxygen electrode powder was synthesized through the modified Pecheni method [35]. The fuel electrode paste was made by mixing NiO-GDC in 6% ethyl cellulose (binder) and \u03b1-terpineol. Afterward, the slurry was mixed using a planetary vacuum mixer (THINKY Mixer ARV-310) and further homogenized by roll milling for about 30\u00a0min. The button cells were fabricated by using dense 8YSZ electrolyte supports from Kerafol\u00ae (d\u00a0=\u00a020\u00a0mm, thickness\u00a0\u223c\u00a0250\u00a0\u03bcm). A thin layer (\u223c 4\u20135\u00a0\u03bcm) of GDC was screen printed (EKRA screen printing Technologies) on one side of 8YSZ substrates as a barrier layer (sintered at 1350\u00a0\u00b0C for 1\u00a0h under air). The fuel electrodes (15\u201318\u00a0\u03bcm) were afterward screen printed on the opposite side of the electrolyte. Four different sintering temperatures i.e. 1200, 1250, 1300 and 1350\u00a0\u00b0C for 2\u00a0h (with 2\u00a0\u00b0C/min ramp) were considered. Among them, 1200\u00a0\u00b0C for 2\u00a0h was selected as an optimized sintering condition based on the polarization resistance (Supplementary Information Fig. S1) and good adhesion. The pure LSCF layer was screen printed on the side containing the GDC barrier layer (sintered at 1080\u00a0\u00b0C for 3\u00a0h). Finally, a thin NiO layer (\u223c 8 \u03bcm) was also screen printed on the fuel electrode side as a current collector. The single cell configuration before reduction of the NiO in the electrode is represented as NiO-GDC//8YSZ//GDC//LSCF, with an effective area of 0.785\u00a0cm2.The single cells were electrochemically characterized using a NorEcs Probostat\u2122 set-up. For the electrochemical measurements, the cell was heated up to 900\u00a0\u00b0C (at 1\u00b0/min) under N2. After reaching 900\u00a0\u00b0C, the Nickel oxide in the fuel electrode was gradually reduced to Nickel metal by systematically replacing N2 with H2 as described here [36], with a total flow rate of 9 Nl\u2027h\u22121. On the oxygen electrode side, a flow rate of 9 Nl\u2027h\u22121 of compressed air was used. After the reduction, the voltage against current density (I\u2013V) characteristics and electrochemical impedance measurements were recorded using an IviumStat potentiostat/galvanostat. Three different measurements were carried out; the first is impedance measurement as a function of temperature from 900 to 750\u00a0\u00b0C with 25\u00a0\u00b0C steps at the open-circuit voltage (OCV). The frequency range during the measurement was from 0.11\u00a0Hz to 110\u00a0kHz with an AC amplitude of 50\u00a0mV and 21 frequencies per decade. The second set of impedance measurements was carried out at a different gas composition (psteam, and pCO2) as well as different partial pressures of oxygen at OCV conditions. Each of the measurements was carried out in both steam (H2O:H2, 50:50) and co-electrolysis (H2O:CO2:H2, 40:40:20) conditions. Finally, the long-term stability tests of the single cells were carried out under steam electrolysis (H2O:H2, 50:50, 3.6% fuel utilization) and co-electrolysis conditions (H2O:CO2:H2, 40:40:20, 2.3% fuel utilization) at 900\u00a0\u00b0C with \u22120.5 A\u2027cm\u22122 current density for 500\u00a0h.The EIS was utilized in obtaining the impedance spectra while the complex nonlinear least-square (CNLS) fit analysis procedure, as well as the distribution of relaxation times (DRT) procedure, were applied to analyze the acquired impedance spectra. For the CNLS analysis, an equivalent circuit model (ECM) which best describes the impedance spectra was proposed. The ECM was subsequently fitted to the measured data by using a commercially available CNLS-fit program (RelaxIS\u00ae software, RHD-Instruments).The DRT analysis technique was applied on the measured impedance spectra [37\u201340]. Common electrochemical equivalent circuit elements; RQ, Warburg and Gerisher elements have their corresponding DRT analytical forms. The RQ represents a constant phase element (CPE) in parallel to a resistance, wherein the CPE models the behaviour of an imperfect capacitor. In the DRT transformation, each RQ-equivalent circuit element gives a peak at specific relaxation frequencies in the DRT plot. On the other hand, asymmetric equivalent circuit elements, such as the finite length Warburg element and the Gerisher element, have complex analytical equations, thereby exhibiting more than one peak over a range of frequencies; one large peak at a characteristic frequency followed by additional smaller peaks at higher frequencies [37,38,41]. As a consequence, the additional peaks can overlap with other electrochemical processes and impede effective deconvolution of the measured spectra. Also, it is worthy to note that the presence of noise in the measured impedance spectra can generate additional artificial peaks in the DRT [42].The microstructure of the measured cells was analyzed with Quanta FEG 650 (FEI\u00a9) scanning electron microscope. The samples were immersed in resin. After polishing, a copper tape was looped around and the sample was sputtered with gold in order to reduce the charging effect. A detailed description of the sample preparation is found here [43].Electrochemical measurements of single cells were performed in the temperature range of 750\u2013900\u00a0\u00b0C, after the reduction of the fuel electrode. The current-voltage characteristics, as well as polarization resistance (Rp), were then obtained as previously mentioned [44]. Experimental open circuit voltages of the cells under each measurement condition were well within 15\u00a0mV of the theoretical Nernst potential which implies adequate cell sealing. The OCV of the co-electrolysis conditions is seen to be slightly lower than those of steam electrolysis which agrees with the theoretical calculations from the Nernst equation (Supplementary table T1).\nFig. 1\na and Fig. 1b illustrate the I\u2013V curve of the cells under both steam (50:50, H2:H2O) and co-electrolysis conditions (40:40:20, H2O:CO2:CO). In general, decreasing the operating temperatures resulted in a decrease in current density and an increase in polarization resistance. For example, in the steam electrolysis mode, the current density reached at 1.5\u00a0V decreased from 1.31\u00a0A\u2027cm\u22122 at 900\u00a0\u00b0C to 0.41\u00a0A\u2027cm\u22122 at 750\u00a0\u00b0C while the Rp increased from 0.061 to 0.31\u00a0\u03a9\u2027cm2. A similar trend was observed for the co-electrolysis conditions. The decrease in electrochemical performance is attributed to a decrease in the cell kinetic activities with decreasing temperatures. Furthermore, the I\u2013V curves in both electrolysis modes illustrate good continuity across the OCV implying that these Ni-GDC cells can work in reversible SOCs in fuel cell mode [11]. The effect of the difference in the percentage of the fuel gas composition (50% steam and 80% H2O/CO2) can be seen in the I\u2013V curves. For example, in EC mode (at 1.5\u00a0V, 900\u00a0\u00b0C), the steam electrolysis conditions, despite having a lower fuel percentage, exhibits a similar current density (1.31\u00a0A\u2027cm\u22122) to that of co-electrolysis (1.37\u00a0A\u2027cm\u22122).\nFig. 1c and d compares the Rp as well as the Arrhenius plots of ohmic resistance (Rs) and polarization resistance (Rp) under steam and co-electrolysis conditions. The Rs corresponds to the high frequency intercept with the real axis on the Nyquist plot while Rp represents the difference between the low and high frequency intercept with the real axis in the Nyquist plot. The steam electrolysis, even with a lower fuel percentage (50% steam and 80% H2O/CO2 at 9 Nl\u2027h\u22121), shows lower Rp values, especially at higher operating temperatures, than the co-electrolysis condition. For example, at 900\u00a0\u00b0C, the steam electrolysis shows a Rp value of 0.061\u00a0\u03a9\u2027cm2 while 0.089\u00a0\u03a9\u2027cm2 is obtained in co-electrolysis mode, thereby implying a higher activity towards H2O reduction than CO2/H2O reduction. At lower temperatures (750\u00a0\u00b0C), the steam and co-electrolysis modes exhibit similar Rp values of 0.319\u00a0\u03a9\u2027cm2 and 0.313\u00a0\u03a9\u2027cm2 respectively. This could be attributed to the domination of the electrode processes and resistances by lower electrode conductivities at lower operating temperatures. Furthermore, no difference is observed in the ohmic resistance of both electrolysis modes. This suggests that the ohmic resistance contribution is not affected by the difference in the fuel gas. The activation energies for the resistance contribution Rs and Rp were calculated from the slope of the Arrhenius plot, according to the equation [4].\n\n[4]\n\n\nln\n\nR\n=\n\u2212\nln\n\n\n\u03c3\n0\n\n+\n\n\nE\nA\n\n\n\nR\ng\n\nT\n\n\n\n\n\n\nThe steam and co-electrolysis conditions show similar activation energies of 77\u00a0\u00b1\u00a04\u00a0kJ\u2027mol\u22121 and 75\u00a0\u00b1\u00a04\u00a0kJ\u2027mol\u22121 respectively. To explain the similar activation energies; in the presence of sufficient steam percentage in co-electrolysis conditions, the CO2 is converted to CO through the reverse water gas shift [8,45], hence the electrochemistry of both conditions tend to show similar behaviour. A similar activation energy of 90.54\u00a0kJ\u2027mol\u22121 was obtained by Grosselindermann et al. [46] for the symmetrical cell of Ni-GDC under steam electrolysis conditions. A separate analysis of the LSCF symmetrical cell showed that the oxygen electrode has only a little contribution to the total Rp, indicating that the fuel electrode is the major contributor to the Rp [46].For the ohmic resistance, an activation energy of 69 kJ\u2027mol\u22121 (0.71\u00a0eV) is obtained, which is close to values reported for the ionic conductivity in YSZ electrolytes in literature [47,48]. This indicates that the ohmic resistance is controlled by the resistance of the thick electrolyte and therefore the same for steam- and co-electrolysis in this study.The quality of the measured impedance data was investigated by applying the Kramers-Kronig transformations test, as a mathematical validation tool [49]. For the impedance evaluation, the measured spectra data were evaluated with both DRT and CNLS fitting. The DRT analysis allowed for the deconvolution of physical processes while also highlighting the frequency ranges associated with each process. To accurately identify the electrode processes, measurements were taken under varying conditions; temperature variations allow the identification of relaxation frequencies of thermally activated processes, and measurement under varied oxygen partial pressure and fuel composition allows for the identification of oxygen and fuel electrode processes respectively. As a consequence of extensive DRT analysis, an equivalent circuit model (Fig. 2\na) consisting of three time constants (RQ) in series with a finite length diffusion (Warburg short element) was developed to evaluate the impedance spectra via a CNLS fit procedure. P1, P2 and P3 in the DRT plot (Fig. 2b) are modelled as 3 RQ-element, while P4 corresponds to a finite length Warburg short element. P4a peak is attributed to a possible satellite peak of the Warburg short element. The inductive effect in the high-frequency region of the Nyquist plot, originating from the wiring, is accounted for by an inductance (I) element, and the ohmic losses are accounted for by the ohmic resistor (Rs) in series with the R//CPE and Ws.To evaluate the quality of the fit and the chosen ECM, a simulated impedance spectrum and DRT plot were generated from the proposed ECM model. Fig. 2a shows an exemplary comparison between the experimental and simulated Nyquist plot as well as the DRT (with a lambda value of 10\u22126) for the steam electrolysis at OCV and 900\u00a0\u00b0C. The plots show a qualitative agreement between the chosen model and experimental results based on the impedance features and characteristics relaxation frequencies. The small peak at the low frequency region (0.1\u00a0Hz) is regarded as a possible artifact in the spectrum from the experimental measurement set-up. Furthermore, the residuals obtained from the fitting (Supplementary info S2) are uniformly distributed around the frequency axis with a relative error value of around 2%. This is an indication that the applied ECM can adequately reproduce the measured impedance spectral over the recorded frequency range [50]. However, since residuals are only a mathematical quantity, the physical correctness of the proposed model in comparison to the measured impedance must be validated by additional experiments by measuring impedance spectra under different operating conditions.To investigate thermally activated processes in the electrode resistance, the cells were measured under different operating temperatures from 750 to 900\u00a0\u00b0C at OCV and constant fuel gas composition (50:50, H2O:H2 and 40:40:20, H2O:CO2:CO). Fig. 3\na and b illustrate the Nyquist plot (without the ohmic resistance) of the impedance spectra in both steam- and co-electrolysis conditions as a function of temperature. A similar trend in temperature dependence was observed in both electrolysis mode, i.e., a shift in the spectra towards higher resistance values as well as an increase in the magnitude of the arcs with a decrease in temperature. Furthermore, the obtained impedance spectra exhibit two distinct arcs; a low-frequency arc and a high-frequency arc. The high-frequency arc demonstrates a significant temperature dependence, shifting towards higher values with a decrease in temperature. At lower temperature values of 775\u00a0\u00b0C and 750\u00a0\u00b0C, this arc dominates the Nyquist plot. On the other hand, the low-frequency arc shows little or negligible dependence on temperature. Similar features were also observed in the literature [25,46].\nFig. 3c and d show the DRT plots of the impedance spectra as a function of temperature. In general, an increase in the peak area, which corresponds to an increase in resistance, is observed with a decrease in temperature. In both electrolysis conditions, the high frequency P1, P2 and P3 processes show strong temperature dependence, indicating they are thermally activated processes. The P1 doesn't show any shift in the frequency while peaks P2 and P3 shift towards lower frequencies with a decrease in the temperature. At lower temperatures, the peak P3 dominates and overlaps with suspected satellite peaks of a finite length Warburg process. Such overlap limits effective and complete deconvolution of the processes. The P4 peak, which occurs within the frequency range of 1\u00a0Hz\u201310\u00a0Hz, is found to show the least dependence on temperature. This is particularly true in steam electrolysis. In co-electrolysis, an inconsistent temperature dependence is observed in the DRT.\nFig. 3e and f show the Arrhenius plots of the individual resistances obtained from the fitting of the ECM. It is observed that the high frequency resistances R1 and R2, corresponding to process P1 and P2 in the DRT plot respectively, have the most pronounced dependencies on temperature. Calculation of the activation energies from the Arrhenius plot shows that R1 has an activation energy of 127 (1.32\u00a0eV) and 130\u00a0kJ\u2027mol\u22121 (1.35\u00a0eV) for steam and co-electrolysis respectively. Such a high frequency process with high activation energy could be attributed to charge transfer processes in the bulk/TPB of the electrodes [42,51,52]. The fit also illustrates that the low-frequency Ws element exhibits very small activation energies, especially in steam electrolysis. In the co-electrolysis, the inconsistency observed at the low frequency of the DRT appears to show a slight negative activation energy in the fitting. However, the trend illustrates a virtually independent temperature variation. Such a small activation energy is usually attributed to gas diffusion process [11,51,53]. In general, the Arrhenius plot, which shows good agreement with the DRT, highlights that the high frequency regime exhibits significant temperature dependence while the low-frequency regime is almost temperature independent. The notable remark is in the mid-frequency regime (R3), which shows low activation energy in the fit while the DRT plot suggests a thermally activated process. This discrepancy is attributed to the limitation in effective deconvolution of a possible temperature-dependent process due to the presence of satellite peaks of the Ws, occurring within the same frequency range.The impedance measurement under different partial pressures and fuel gas compositions was also performed to identify the impedance contributions from the fuel electrode. In steam electrolysis, the steam content was systematically reduced from 50% to 10% steam, while in co-electrolysis, the composition of steam and CO2 was simultaneously varied from 10% to 50%. Fig. 4\na and b show the variation of impedance spectra as a function of change in the fuel gas at 900\u00a0\u00b0C. In both electrolysis modes, a decrease in resistance with an increase in the steam partial pressure is observed. In the co-electrolysis mode, this implies that the cell shows higher kinetics towards H2O reduction than CO2 reduction [11]. The Nyquist plots exhibit the characteristics high and low frequency arcs. The high-frequency arc shows a mild increase in magnitude while the low-frequency arc exhibits a remarkable increase in the magnitude with a decrease in the steam partial pressure. This indicates a pronounced dependence of the low-frequency arc on steam partial pressure.\nFig. 4c and d show the DRT spectra of the impedance spectra as a function of partial pressure and composition. The four distinct peaks, P4, P3, P2 and P1, observed in the DRT plots correspond to the Ws, R3, R2 and R1 of the equivalent circuit fitting, respectively. In both electrolysis modes, P4, P3 and P2 exhibit an observable dependence on the partial pressure variation, increasing in magnitude with a decrease in steam partial pressure. The P1 peak exhibits only a slight dependency to fuel partial pressure variation. The low-frequency peak (P4) shows the most drastic dependence on the fuel gas composition variation in both electrolysis modes. However, in co-electrolysis mode, the P3 peak is virtually unchanged by the change in gas composition. Fig. 4e and f show the plots of resistances R1, R2, R3 and Ws as a function of H2O partial pressure in steam and co-electrolysis modes. It can be clearly seen that Ws has the most significant dependencies in steam partial pressures. R3, on the other hand, illustrates mixed dependence, showing slight dependence in steam electrolysis and virtually unaffected in co-electrolysis. Hence it is uncertain if this process is partly a fuel electrode or exclusively an oxygen electrode process. R1 and R2 exhibit observable dependence on the partial pressure variation.The characteristics behaviour of the low-frequency process, both in steam and temperature variation, is usually attributed to a diffusion process [53\u201355]. The observed diffusion process appeared to be dominant in the obtained impedance spectra. In such a case, Bessler et al. [54,55] proposed the term, gas concentration impedance for the impedance caused by gas-phase activities. Generally, the impedance contribution from the gas diffusion process in electrolyte supported cells are considered to be reduced due to the smaller thickness of the fuel electrode in comparison to the thicker substrate in anode supported cells. However, the fuel electrode fabrication method, the fuel gas channel and the contact mesh also contribute significantly to the gas diffusion impedance [46,54,55]. Also, surface diffusion of species could add some contribution to the low frequency impedance peak. Unlike the EIS of Ni-YSZ [44,51], the low frequency process of Ni-GDC electrodes has been majorly associated with either a finite-length Warburg or finite-length Gerischer impedance behaviour rather than the conventional RQ time constant [25,30,46]. This is possibly due to the interplay between gas diffusion processes and other surface exchange processes associated with MIEC electrodes [25,46]. However, this low frequency process exhibits an almost independent temperature variation with a low activation energy. This indicates that this process is most likely dominated by gas diffusion process.Also, the electrochemical activities of GDC-containing electrodes have been reported to exhibit a low frequency peak due to the chemical capacitance of the GDC phase [30,46,56]. For example, in the analysis of Ni-infiltrated GDC electrodes, Lomberg et al. [30] reported a possible coupling of the gas diffusion process and chemical capacitance at the low frequency regime. The chemical capacitance of a specimen refers to its ability to store chemical energy via oxygen vacancies in response to changes in the local oxygen chemical potential. In the Ni-GDC electrode, the chemical capacitance is suspected to originate from the variation of the oxygen nonstoichiometry of the GDC phase [30,56]. However, chemical capacitance is reported to show pronounced dependence on temperature variation [30,46,56]. On the contrary, there is almost no temperature dependency in the low frequency process in our analysis, especially in steam electrolysis. This is in agreement with the results obtained by Watanabe et al. [57], wherein they observed insignificant temperature dependence for the low frequency process. They, however, attributed the low frequency process to a gas conversion impedance that has a non-stoichiometry capacitance. To further clarify this, an analysis of the DRT plot (Supplementary Information Fig. S3) from an electrolyte supported Ni-YSZ on the same test rig show the occurrence of the characteristic low frequency peak within the same relaxation frequency range. This suggests that the low frequency process is rather dominated by the gas diffusion process, with possible additional contributions from the chemical capacitance due to the non-stoichiometry of the GDC phase. Similar results were observed by Aravind et al. [32]. In their study of symmetrical Ni-GDC cells, the low frequency process, fitted with a Ws, was assigned to a gas phase-diffusion process. Grosselindenmann et al. [46] attributed the low frequency process to a coupled process of gas diffusion and activation polarization.In general, the controversy in the attribution of the low frequency process could be attributed to the different fabrication methods, composition (Ni:GDC ratio) as well as cell configurations employed by different groups. For example, a 50:50 ratio of Ni:GDC was used by Grosslindenman et al. [46] for a cell with a configuration Ni-GDC//GDC//YSZ//GDC//LSCF. Meanwhile, in our work, a 65:35 Ni: GDC ratio was used without a GDC sandwich between the fuel electrode and the electrolyte. While the percentage of the GDC in the cermet may have played a little role, the presence of a GDC layer between the fuel electrode and the electrolyte may have significantly affected the electrochemistry at the electrolyte/electrode interface. To further shed more light to this, single cells consisting of GDC fuel electrode and LSCF oxygen electrode was analyzed using impedance spectroscopy. Preliminary results of the impedance spectra indicates that the low frequency region shows significant temperature dependence, which points to the chemical capacitance of the oxygen nonstoichiometry of the GDC phase [30,46,56]. However, the details of this result as well as a comprehensive study of the influence of the Ni-GDC composition, microstructure and cell configuration would be the scope of another paper.The P2 (R2) process shows significant temperature and pH2O dependency. On the other hand, P3 (R3) only exhibited slight steam partial pressure dependence, especially in steam electrolysis while also showing pronounced temperature dependence. This means that these two processes are fully (or partly) fuel electrode processes. Investigations of the response of surface oxygen vacancies, electrons and adsorbates on the ceria surfaces-gas interface confirm that the desorption/adsorption, as well as the electron transfer between the species, is a rate-determining step in the water-splitting reaction [58,59]. Generally, molecular water adsorption on a bare ceria surface is energetically more favourable than complete dissociative adsorption. However, the presence of oxygen vacancies on reduced ceria greatly enhances water dissociation over molecular adsorption [60]. This causes the electrochemical reduction of the fuel gas on the GDC electrodes to be dominated by surface reactions [24,60]. The detailed mechanistic steps for water splitting reaction on ceria electrodes is still unclear, however, it has been confirmed that the OH\u2212 is an existing specie on doped ceria and that it participates as an active intermediates species for both water-splitting reaction and hydrogen oxidation reaction [25,58,60]. Hence, the mid frequency processes are therefore, associated with the surface electrode reaction, most likely desorption/adsorption of species of the fuel gas species in addition to a charge transfer process.To investigate the oxygen electrode contribution to the impedance spectra, the impedance measurements were performed on the single cells by varying the oxygen partial pressure from 0.1 to 1\u00a0atm. Fig. 5\na shows the DRT plot of the impedance spectra as a function of oxygen partial pressure at 900\u00a0\u00b0C. The fuel gas composition and partial pressure of steam (H2O:H2, 50:50) were kept constant. From the plot, the low and mid-frequency peaks of P4, P3 and P2 exhibit a partial increase in peak values with a decrease in pO2. This indicates that these peaks are partly oxygen electrode processes. Similar trends were also obtained for the resistance and Warburg elements from the fitting of the impedance with the equivalent circuit in Fig. 5b with R2 exhibiting the most pronounced dependence. The R1 and R3 resistances on the other hand suggest a slight dependency on pO2 variation. A notable observation from the electrochemical investigation is that the high frequency resistance R1 exhibits a similar (slight) dependence in both pH2O (Fig. 4e and f) and pO2 (Fig. 5b). However, the DRT (Fig. 5a) shows that the P1 peak is unaffected by pO2. Therefore, considering the contradictory trend between the P1 peaks of the DRT (Fig. 5a) and the R1 of the ECM fitting (Fig. 5b), it is, therefore, arguable to assign this process as partly an oxygen electrode process. To resolve this difficulty, a symmetrical LSCF cell was fabricated and measured in a two-electrode measurement in air at 900\u00a0\u00b0C at 0\u00a0V. Fig. 5c illustrates the DRT of the impedance spectrum obtained from symmetrical LSCF electrodes as compared to those of the single cells.The DRT obtained from the symmetrical cell was normalized by dividing the resistance contribution by two. Four major peaks are observed in the DRT of the LSCF oxygen electrode, a low frequency peak (P4), two intermediate frequency peaks (P3 and P2) and a high frequency region (P1). The electrochemical behaviour of LSCF has been extensively studied, hence the electrochemical processes were inferred from literature [51,52,61]. Literature reports on similar LSCF electrodes attributed the small low frequency peak (P4) to diffusion on the oxygen electrode, while the intermediate frequency processes of P3 and P2 are assigned to oxygen surface exchange kinetics and oxide ion diffusion respectively. Lastly, the P1 process is attributed to a charge transfer process [42,51,61,62].The comparison of the DRT plot between the symmetrical cell and single cell indicates that the high frequency P1 peak is a contribution from both the oxygen electrode and fuel electrode, most likely a charge transfer process at the oxygen electrode and the charge transfer process at the TPB of the fuel electrode. Since GDC is a MIEC material, charge transfer reaction can occur both at the DPB (of GDC and gas) and the TPB (of Ni, GDC and gas). The intermediate frequency peaks of P2 and P3 are an interplay between the oxygen electrode and the fuel electrode. The P2 peak is mainly a fuel electrode process while P3 is mainly an oxygen electrode process. The low frequency oxygen diffusion process is seen to be shifted to a lower frequency from the diffusion process of the fuel electrode. This illustrates that the little peak around 1\u00a0Hz\u00a0(P4a in Fig. 5a) is part of the gas diffusion process of the oxygen electrode. A separate time constant was not assigned to this process since it overlaps with the fuel electrode contribution. Attempts to fit the data with an added time constant to account for this DRT peak did not result in significant improvement in the fit results.In summary, the electrochemical analysis of the impedance spectra highlights that the electrochemical activities of single cells of Ni-GDC fuel electrodes and LSCF oxygen electrode are dominated by four electrode processes; a low frequency peak, two intermediate peaks and a high frequency peak. Table 1\n summarizes the possible electrochemical reaction steps of the Ni-GDC with the LSCF oxygen electrode.To investigate the long-term stability of the cell, degradation measurements were carried out for 500\u00a0h at 900\u00a0\u00b0C and \u22120.5\u00a0A\u2027cm\u22122 in both electrolysis modes. Two different cells were measured for each electrolysis mode. The first cell was operated without any intermediate measurements during the degradation while for the second cells, I\u2013V and impedance measurements at OCV as well as under polarization (from 0.7\u00a0V to 1.4\u00a0V) were taken every one hundred hours to investigate the evolution of the degradation. Fig. 6\na shows the degradation behaviour in steam (50% H2O) and co-electrolysis conditions (80% fuel) in the cell operated without interruptions. In steam electrolysis conditions, two different degradation regions were observed; a low degradation region observed within the first 150\u00a0h followed by a progressive degradation region. A smaller degradation rate is obtained in the co-electrolysis conditions than in steam electrolysis. The co-electrolysis exhibits a degradation rate of 308\u00a0mV\u2027kh\u22121 while 499\u00a0mV\u2027kh\u22121 is observed in the steam electrolysis. Direct comparison of the obtained degradation rates with those of Ni-YSZ is challenging due to the different operating conditions (fuel gas mixture, operating temperature and current density) used in the literature [11,17,63]. For the second cell operated with intermediate I\u2013V and impedance measurements, a steady decrease in the current density is observed with degradation time. For example, in steam electrolysis, the current density at 1.5\u00a0V decreased from 1.11\u00a0A\u2027cm\u22122 at 100\u00a0h to 0.41\u00a0A\u2027cm\u22122 after 500\u00a0h (Supplementary Fig. S4). A similar trend was observed for the co-electrolysis mode, but to a lesser extent.Analysis of the degradation behaviour using the impedance spectra recorded every 100\u00a0h (Fig. 6b and c) shows that the ohmic resistance plays the most significant role in the degradation of the cell. Table 2\n summarizes the obtained ohmic and polarization resistances as a function of degradation. In steam electrolysis, for example, the ohmic resistance increased from 0.47\u00a0\u03a9\u2027cm2 at the start of degradation to 1.14\u00a0\u03a9\u2027cm2 after 500\u00a0h. Similarly, in co-electrolysis, the ohmic resistance increased from 0.53\u00a0\u03a9\u2027cm2 at the start of the degradation to 1.05\u00a0\u03a9\u2027cm2 after 500\u00a0h. Comparing the ohmic resistance degradation between steam and co-electrolysis, it is observed that the steam electrolysis (0.67\u00a0\u03a9\u2027cm2) showed a higher ohmic degradation than co-electrolysis conditions (0.51\u00a0\u03a9\u2027cm2). In literature, the increase in ohmic resistance is attributed to the formation of the resistive SrZrO3 phase at the GDC/electrolyte interface [64,65]. Also, a gradual surface oxidation of the Ni mesh has been reported [20,66] to cause an increase in the ohmic resistance. Such current collector oxidation increases the contact resistance of the electrode and the currect collector. Furthermore, the higher ohmic resistance in the steam electrolysis could also be attributed to increased pore formation near the electrode/electrolyte interface due to a possible increase in Ni migration away from the electrolyte [16]. In addition to the ohmic resistance, the Nyquist plots illustrate that the high/mid-frequency regime exhibits a pronounced contribution to the degradation in both electrolysis modes. However, this is more pronounced in steam electrolysis. Evaluation of the Rp with degradation time indicates that in steam electrolysis, the Rp increased from 0.07\u00a0\u03a9\u2027cm2 at the start of degradation to 0.28\u00a0\u03a9\u2027cm2 after 500\u00a0h (300% increase). While in co-electrolysis, the Rp increased from 0.10 to 0.16\u00a0\u03a9\u2027cm2 (60% increase). This implies that, in addition to the ohmic contribution, the Rp also shows a pronounced contribution to the degradation rate, especially in steam electrolysis, while in co-electrolysis the Rp contribution is less severe.\nFig. 6d and e shows the variation of the individual resistances as a function of degradation time. In both electrolysis modes, R1 and R2 exhibit the most significant effect on the degradation rate. However, the contribution of these two resistances as a function of degradation time is more pronounced in steam electrolysis than in co-electrolysis. The higher contribution of these resistances in steam electrolysis degradation could be attributed to greater Ni particle growth and depletion due to higher p(H2O) in steam electrolysis [16,67]. Furthermore, the impedance spectra under polarization obtained during the degradation time shows that the high frequency region is also mostly affected by degradation (Supplementary Fig. S5).For the LSCF oxygen electrode, R1 and R2 which are partly oxygen electrode processes, significantly contributed to the degradation. Reported FIB-SEM and ICP-OES studies on aged LSCF electrodes illustrate that the degradation phenomena is not due to any microstructural changes but rather due to the presence of Sr-rich surface phases after cell aging [68\u201371]. Hence, the contribution of the oxygen electrode (majorly in R1 and R2) can be attributed to Sr surface segregation on the LSCF oxygen electrode (Supplementary Fig. S6). The low frequency processes were less affected by the degradation time. Overall, the observed degradation is attributed to both the fuel and oxygen electrodes.The microstructure of the degraded cells was analyzed with a scanning electron microscope and compared with a reduced cell that was not electrochemically operated. SEM images were obtained with both the secondary and backscattered electron detectors. ImageJ\u00ae software was used to analyze the approximate particle size distribution of the fuel electrode. A comparison of the microstructure of the degraded cells under different electrolysis modes was performed. Fig. 7\na\u2013c shows the microstructure of the reduced fuel electrode while Fig. 7e\u2013g and Fig. 7i\u2013k represent the microstructure of the cell after degradation in steam and co-electrolysis conditions respectively. Fig. 7(d, h and l) represents the microstructure of the corresponding oxygen electrode. Firstly, the microstructures illustrate an increase in the Ni particle size of the degraded fuel electrode as compared to the reduced cell. Based on the fuel electrode SEM images covering a cross section of 13\u00a0\u03bcm\u00a0at 3 different locations on the electrode, an approximate mean particle size was determined. The reduced cell has an average Ni particle size of 1.37\u00a0\u03bcm while 2.19\u00a0\u03bcm (indicating a 62% increase) and 2.86\u00a0\u03bcm (exhibiting a 109% increase) were obtained for the co-electrolysis and steam electrolysis mode respectively. The particle size distribution is shown in Fig. 8\n. This points to depletion and agglomeration of Ni particles during cell operation. A similar Ni particle growth of 140%/100\u00a0h was observed by Holzer et al. [72] for a Ni-GDC electrode operated in humid atmosphere (60% H2O, 40% N2/H2). The Ni particle agglomeration is, however, observed to be more severe in steam electrolysis than in co-electrolysis. The 47% higher Ni particle growth observed in the steam electrolysis as compared to the co-electrolysis conditions could be attributed to the difference in p(H2O). The higher steam partial pressure could have facilitated more Ni particle growth [72\u201374] as well as depletion of volatile Ni species, most notably Ni(OH)2 [16,67]. Such Ni particle growth will invariably reduce the surface area for electrochemical reaction. Furthermore, the backscattered electron images of the reduced cell show an even distribution of the GDC particles as compared to the degraded cells, which show an uneven GDC particle distribution. This points to a loss of GDC percolation in the measured cells. The increased Ni particle growth and the higher loss of GDC percolation in steam electrolysis as compared to the co-electrolysis could have contributed to the significant impact of the R2 process on the degradation test in steam electrolysis when compared to the co-electrolysis conditions (Fig. 7b and c). The GDC is observed to cover parts of the Ni particles as could be seen in Fig. 7(g and k). The extent to which such coverage impacts the electrochemical performance of the electrode is still unclear, however, the loss of GDC percolation and the coverage of Ni particles could further reduce the electrochemical reaction zones leading to an increase in the resistance of the underlying process. Overall, the post-test analysis of the cells highlights three observations; Ni depletion and agglomeration, loss of GDC percolation and lastly, coverage of the Ni particles with GDC [75]. These observations could be (partly) responsible for the observed reduction in cell performance during operation. Loss of GDC percolation and Ni depletion could lead to loss of connectivity between the Ni and the oxide phase leading to decrease in the electrochemical reaction zone. With regards to the microstructural changes observed and reported in literature [16,63,67] for Ni-YSZ after degradation, pronounced Ni coarsening and Ni depletion at the electrode/electrolyte interface have been reported as a major degradation phenomenon. However, a direct comparison of the extent of the Ni coarsening reported in the literature with those of the Ni-GDC could not be made due to the different operating conditions (temperature, gas pressure, test rig design) used by different working groups.For the oxygen electrode, no significant change is observed in the microstructure of the three cells. However, as the GDC barrier layer is not completely dense, migration of volatile SrO to the electrolyte interface cannot be entirely hindered. As a consequence, the SrO reacts with zirconia to form the insulating SrZrO3 at the electrolyte interface (Supplementary Fig. S6). Furthermore, Sr segregation and formation of cobalt oxide particles have also been reported to cause LSCF degradation [68\u201371,76].In this study, a comprehensive electrochemical impedance analysis of electrolyte-supported single cells comprising Ni-GDC fuel electrode and LSCF oxygen electrode was performed by CNLS fitting to an Equivalent Circuit Model (ECM) and Distribution of Relaxation Times (DRT), both under steam and co-electrolysis conditions. The impedance spectra were obtained in the 750\u2013900\u00a0\u00b0C temperature range. Further measurements were also carried out at different steam compositions as well as different H2O/CO2/CO compositions for co-electrolysis at OCV. The observed processes were further modelled using 3\u00a0R//CPE elements in series with a finite length diffusion element (Ws). The low frequency process, modelled with a finite length Warburg short, is attributed to diffusion and surface processes, the two intermediate frequency processes are attributed to an overlap of (surface exchange and ion diffusion processes in the) oxygen electrode and (surface electrode reactions with charge transfer in the) fuel electrode processes. The high frequency process corresponds to a charge transfer at both electrodes. Long-term stability tests of the single cells were carried out under steam electrolysis (H2O: H2, 50:50) and co-electrolysis (H2O:CO2:CO, 40:40:20) conditions at 900\u00a0\u00b0C with \u22120.5 A\u2027cm\u22122 current density for 500\u00a0h. Steam electrolysis conditions exhibit the highest degradation rate of 499\u00a0mV\u2027kh\u22121, while a lower degradation rate of 308\u00a0mV\u2027kh\u22121 is observed under co-electrolysis conditions. The post-test analysis of the operated cell shows Ni depletion and agglomeration, loss of GDC percolation as well as coverage of the Ni particles with GDC.\nIfeanyichukwu D. Unachukwu: Methodology, Investigation, Formal analysis, Validation, Conceptualization, Data curation, Software, Visualization, Writing \u2013 original draft, Writing \u2013 review & editing. Vaibhav Vibhu: Methodology, Formal analysis, Validation, Conceptualization, Software, Supervision, Visualization, Writing \u2013 review & editing. Izaak C. Vinke: Methodology, Supervision, Validation, Project administration, Conceptualization, Resources, Software, Visualization. R\u00fcdiger-A. Eichel: Supervision, Funding acquisition, Project administration, Resources. L.G.J. (Bert) de Haart: Methodology, Supervision, Validation, Funding acquisition, Project administration, Conceptualization, Resources, Software, Visualization.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 from the German Federal Ministry of Education and Research (BMBF) through the iNEW 2.0 Project: incubator for sustainable and renewable value chains, under grant agreement number 03SF0627A.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.jpowsour.2022.232436.", "descript": "\n                  The present study investigates the electrochemical performance and degradation behaviour of a Nickel - Gd2O3 doped CeO2 (Ni-GDC) electrode containing single cell under steam electrolysis and co-electrolysis modes. The cell consists of the Ni-GDC fuel electrode, an 8\u00a0mol% Y2O3 stabilized ZrO2 (8YSZ) electrolyte layer, a GDC barrier layer and a (La,Sr)(Co,Fe)O3 (LSCF) oxygen electrode. Firstly, the electrolyte-supported single cells were fabricated and characterized using DC- and AC-techniques in the 750\u2013900\u00a0\u00b0C temperature range. Distribution of relaxation times (DRT) analysis was employed to resolve frequency-dependent electrode processes. The observed processes were further modelled using an equivalent circuit model (ECM) with 3\u00a0R//CPE (resistor//constant phase element) in series with a finite length diffusion element (Warburg short - Ws). Long-term stability tests of the single cells were carried out under steam electrolysis (H2O:H2, 50:50) and co-electrolysis (H2O:CO2:CO, 40:40:20) conditions at 900\u00a0\u00b0C with \u22120.5 A\u2027cm\u22122 current density for 500\u00a0h. Steam electrolysis conditions exhibit the highest degradation rate of 499\u00a0mV\u2027kh\u22121, while a lower degradation rate of 308\u00a0mV\u2027kh\u22121 is observed under co-electrolysis conditions. The post-test analysis of the operated cell shows increased Ni particles size, suggesting Ni agglomeration in both electrolysis modes.\n               "}
{"full_text": "We, as humans, consume energy in different forms in our day to day life. Most of it is in the form of electrical energy that comes from the burning of finite fossil fuels. Combustion of fossil fuels harms our environment due to large scale carbon emission and alternative solutions with clean and sustainable energy are required to overcome our dependency on fossil fuels. Solar energy and wind energy are the most exploited among renewable energies. The amount of solar energy reaching the surface of the earth is several folds greater than that required for human progress so even a low conversion performance would be satisfactory to fulfill the energy requirements. [1] However, wind and solar energy generation are not constant and need efficient and economical storage facilities and conversion to better fuels. Energy conversion to hydrogen releases no carbon dioxide (CO2) and it can also generate clean discharge (water) on combustion. [2,3] This has attracted much attention due to its green nature and high energy density. Water splitting is amongst the many promising technologies that enable the production of hydrogen and oxygen, which converts sustainable renewable energy into ideal chemical energy using electricity or sunlight. [4,5] This energy conversion technology has amassed worldwide attention owing to its high energy conversion efficiency, a potentially wide range of applications, and negligible environmental pollution. [6\u20138] Integrated solar water-splitting systems that integrate light-capturing semiconductors with electrocatalysts to effectively split water display specific promise as a means of direct sunlight to fuel production. [9] Thus generated fuels can be used in various applications to reduce carbon emission dramatically. For example, the hydrogen used in a hydrogen driven engine does not burn the fuel like in the conventional internal combustion engine. Instead, it fuses with the oxygen from the air to form H2O instead of CO2. No primary wastes are produced in the electrochemical splitting of water when the electricity is used from solar cells. As a result, it is considered to be a clean process for producing H2 and O2 gases.Electrochemical water splitting can be classified into two half-reactions and they are OER and HER. These reactions are kinetically inactive and require an overpotential to occur at a practical rate. The overpotential loss due to OER is generally much greater than HER. Therefore, OER is generally viewed as the bottleneck of water splitting. [9,10] As a consequence, water electrolysis requires effective catalysts to expedite these reactions smoothly and unhindered. Presently, carbon-based platinum is the most efficient catalyst for HER [11\u201314] while the standard catalysts for OER are ruthenium- and iridium- based oxides. [15] The first reported work on the OER catalyst was by Meyer and co-workers [16] on the so-called \u201cblue dimer\u201d a binuclear oxo-bridged ruthenium complex. Even though these are excellent electrocatalysts, scarcity in nature and high cost make it difficult to administer them into large scale applications. This gave rise to the widespread study of different non-precious earth-abundant catalysts that have high efficiency for both OER and HER. Considerable progress has been made in using transition metals for better efficiency in OER. Intellectual studies of Co, Ni, Fe, and Mn-based oxides in OER dates back to more than half a century ago. [17\u201320] Among these metals, special mention goes to cobalt-based compounds for OER and HER reactions. Although cobalt has no biological relevance in water splitting, and although it is significantly less abundant than Fe, Mn, or Ni, it is now emerging as a fascinating metal due to its catalytic power for OER and HER. [21] Cobalt compounds, either as molecular species or as three-dimensional materials, seem to be attractive multi-electron catalysts for both HER and OER in the water-splitting process.Since OER is considered to be kinetically sluggish, researchers have done extensive studies to overcome this drawback in overall water splitting. Cobalt seems to be an effective catalyst for enhanced and fast OER. Herein, we discuss recent developments regarding characterization, design, and evaluation in the field of cobalt-based electrocatalyst based on noble metals, noble metal alloys, transition metal compounds, perovskite, and functional nanocarbon structures. Further, we share the challenges faced and our outlook on this topic. We also consider future aspects as well.A classic electrolyzer consists of three components, i.e. anode, cathode, and an electrolyte. The electrocatalysts are coated on the electrodes for a speedy activity. In such a system the electrocatalysts used are different on the anode and the cathode which facilitates OER and HER, respectively. But recent studies have come up with new electrocatalysts which are bifunctional for both OER and HER. Ruthenium- and iridium- based compounds had been considered as the novel catalysts for OER due to their high performance in a wide range of pH. [15] Since these precious metals are scarce and expensive, earth-abundant transition metal catalysts are studied to replace them. Generally, electrolyzers operate under high conductive media, i.e. either acidic or alkaline. One drawback of these transition metals is that they degrade in an acidic condition due to high oxidative potential which, in turn, hinders the activity of the catalyst. So, to overcome this, most of the transition metal-based OER catalysts are studied under alkaline medium.We can find many proposed mechanisms for OER in literature. [22\u201325] Let us denote \u201cM\u201d to be the active site of a catalyst. In the first step, a hydroxyl radical (OH\u2212) is adsorbed onto the active site (M) to give M\u2013OH by an electron (e\u2212) oxidation on OH\u2212. Then another OH\u2212 reacts with M\u2013OH to give M\u2013O by removing a proton and electron pair. M\u2013OH and M\u2013O are common intermediates in most of the proposed mechanisms. The dissimilarities start after the formation of M\u2013O. Generally, there are two reaction mechanisms to form oxygen gas (O2) from M\u2013O. In the first (green pathway), it is the direct combination of two M\u2013O intermediates to produce oxygen gas and free active site M. In the second (white pathway), there is a formation of M\u2013OOH intermediate (hydroperoxide intermediate) by the nucleophilic attack of OH\u2212 on M\u2013O, paired with an e\u2212 oxidation. A further proton paired electron transfer results in the decomposition of M\u2013OOH into O2(g) and the free active site M. This forms the basis for the majority of the mechanisms proposed with the change in the number of electron transfer in individual steps (single/multiple electron reaction). The bonding interaction within the intermediates (M\u2013O, M\u2013OH, and M\u2013OOH) is decisive for the overall electrocatalytic ability. [26] A brief schematic explanation is given in Fig. 1\n.The mechanisms are proposed based on certain electrocatalytic kinetic parameters like overpotential (\u03b7) and Tafel slope (b). These are also used to evaluate the electrocatalyst\u2019s catalytic performance.Overpotential is defined as, the difference between the potential required to practically run the reaction and the theoretically found out equilibrium potential of the reaction. It is one of the important factors for assessing the performance of an OER catalyst. But, it is difficult to get the exact value of the overpotential. So, the potential value at a constant current density is taken as the overpotential. Normally, a 10\u00a0mA\u00a0cm\u22122 current density value is set to find the overpotential of the target reaction.For OER, the overpotential is calculated as the potential difference between the potential reaching a current density (i) of 10\u00a0mA\u00a0cm\u22122 and the equilibrium potential of 1.23\u00a0V. Generally, an electrocatalyst with an overpotential in the range of 0.3\u20130.4\u00a0V is considered to be of excellent catalytic activity for OER.Tafel slope is usually drawn to evaluate the reaction kinetics and mechanism. It is also used to contrast the catalytic activity of different catalysts. As stated by the Butler-Volmer equation,\n\n(1)\n\n\ni\n=\n\ni\nO\n\n\n\ne\nx\np\n\n\n\n\n\n\u03b1\na\n\nn\nF\nE\n\n\nRT\n\n\n\n\n+\nexp\n\n\n\n\n\n\u03b1\nc\n\nn\nF\nE\n\n\nRT\n\n\n\n\n\n\n\n\n\nwhere \u201ci\u201d is the current density, \u201cio\u201d is the exchange current density, \u201c\u03b1a\u201d the anodic charge transfer coefficient, \u201c\u03b1c\u201d the cathodic charge transfer coefficient, \u201cR\u201d the universal gas constant, \u201cF\u201d the Faraday\u2019s constant (96485C mol\u22121), \u201cn\u201d the number of electrons involved in the electrode reaction, \u201cE\u201d applied potential, and \u201cT\u201d absolute temperature (K).When there is very high overpotential for the anodic electrode, in the above equation the overall current is largely due to the anodic electrode. Therefore the equation can be simplified as,\n\n(2)\n\n\ni\n\u2248\n\ni\nO\n\ne\nx\np\n\n\n\n\u03b1\na\n\nF\n\u03b7\n\n\nRT\n\n\n\n\n\n\nThe above equation is known as the Tafel equation. [27] This can be further reduced as,\n\n(3)\n\n\nlog\n\n(\ni\n)\n\n=\nlog\n\ni\nO\n\n+\n\n\u03b7\nb\n\n\n\n\n\n\n\n(4)\n\n\nb\n=\n\n\n\u03c3\n\u03b7\n\n\n\u03c3\nl\no\ng\n(\ni\n)\n\n\n=\n\n\n2.303\nR\nT\n\n\n\u03b1\nF\n\n\n\n\n\n\n\nTafel slope (b) defines how swiftly the current increases with the overpotential applied. It also helps in finding the rate-determining step (RDS) and formulating a mechanism for the reaction. In an OER, the RDS can be either a single electron reaction or multiple electron reaction. In a single electron reaction mechanism, the transfer coefficient, designated by \u201c\u03b1\u201d, becomes the symmetry factor (\u03b2) which is given by the equation below,\n\n(5)\n\n\n\u03b1\n=\n\u03b2\n=\n\n1\n2\n\n+\n\n\u03b7\n\u03bb\n\n\n\n\nwhere \u03bb is the re-organization energy. From this, the Tafel slope shows a value of 120\u00a0mV dec\u22121. In other words, if the Tafel slope of an electrocatalyst is 120\u00a0mV dec\u22121 then the RDS is a single electron reaction and accordingly, a mechanism can be proposed. Whereas in multiple electron reaction, according to Bockris and Reddy, the transfer coefficient is formulated as\n\n(6)\n\n\n\n\u03b1\na\n\n=\n\n\n\u03b7\nb\n\n\u03bd\n\n+\n\n\u03b7\nr\n\n\u03b2\n\n\n\nwhere \n\n\u03b7\nb\n\n is the number of electrons that transfer back to the electrode before RDS, \n\u03bd\n is number of RDS that have occurred in the overall reaction and \n\n\u03b7\nr\n\nis the number of electrons that are involved in the RDS. The Tafel slope and transfer coefficient are therefore related to the number of electrons participating in the reaction. Therefore, different Tafel slope defines different RDS and hence different mechanisms for the reaction. In order for an OER catalyst to be considered good in catalytic activity, it should possess a low Tafel slope.To obtain a low Tafel slope, various cobalt-based catalysts have been reported in the literature by different teams of researchers. Mainly on noble metals, transition metals, perovskites, and carbon-based cobalt compounds. We will discuss the recent developments in the above-mentioned categories.In an electrolyzer, the OER reaction is kinetically sluggish and requires higher overpotential than the HER reaction. Therefore, catalysts with enhanced activity are required to overcome this drawback. Noble metals are among the catalysts that show high activity as well as durability in a vast range of pH for OER reactions. Iridium, ruthenium, gold, and silver are some commonly used noble metals for this purpose. However, because these elements are scarce and expensive, researchers have tried to use them optimally or replacing them in many ways by synthesizing catalysts consisting of cost-effective alternatives while maintaining equal or better performance. Here we discuss mainly the cobalt-based noble metals used for improved OER activity. Since cobalt has high activity for OER, it can be doped with another active catalyst to better the performance.Iridium has been used as a novel catalyst for OER due to its high stability and activity in both acidic and alkaline medium. Cobalt linked with iridium for OER has been reported by many research groups. Eunju Lee Tae et al. have studied crystalline cobalt oxides nanoparticles (nc-CoOx) on ITO (indium tin oxide) glass substrate doped with\u00a0~\u00a05\u00a0mol % crystalline iridium oxide nanoparticles (nc-IrOx). [28] It displayed a much lower overpotential (\u03b7) and Tafel slope (b) in comparison to the nc-CoOx electrode and nc-IrOx electrode. Under a buffer solution of 0.1\u00a0M phosphate, the \u03b7 at 1\u00a0mA\u00a0cm\u22122 values of [nc-CoOx]ITO were 0.37\u00a0V at pH 7 and 0.34\u00a0V at pH 13, whereas for [nc-IrOx/nc-CoOx]ITO were 0.22\u00a0V at pH 7 and 0.19\u00a0V at pH 13. A drop of 0.15\u00a0V can be seen regardless of the pH condition. The Tafel slope for [nc-IrOx/nc-CoOx]ITO was 29 to 34\u00a0mV dec\u22121 and for [ncIrOx]RDC (rotating disc carbon electrode) was 38 to 44\u00a0mV dec\u22121. A work performed by Youkui Zhang et al. saw the development of a self-assembled 3-dimensional Cobalt-Iridium (CoIr-x) hierarchical structures using a one-step reduction path with NaBH4 as a reducing agent. [29] The Ir species were incorporated into the irregular surfaced cobalt-based hydroxide nanosheets (3D CoIr-x) as clusters and single atoms. It showed enhanced activity in the neutral and alkaline medium compared to the commercially available IrO2 electrocatalyst. [29] In the neutral media (1.0\u00a0M phosphatic buffer solution, pH@7), the CoIr-0.2 (0.2 is the molar ratio of Ir to Co in precursor or CoIr with 9.7\u00a0wt% Ir content) showed a \u03b7 of 0.373\u00a0V@10\u00a0mA\u00a0cm\u22122 current density, which is relatively lower when compared to the IrO2 (0.431\u00a0V@10\u00a0mA\u00a0cm\u22122). Regarding the Tafel slope, CoIr-0.2 exhibited a value of 117.5\u00a0mV dec\u22121, which is also below the value for IrO2 of 132.1\u00a0mV dec\u22121. Whereas in the basic media (1.0\u00a0M potassium hydroxide (KOH), pH@14), CoIr-0.2 needed an \u03b7 of 0.235\u00a0V to achieve 10\u00a0mA\u00a0cm\u22122 and Tafel slope of 70.2\u00a0mV dec\u22121. From the above-mentioned electrocatalyst studies, we can understand that the OER activity is favored more by the alkaline medium. This is possible because the rate-determining step of the OER reaction is the accelerated OH\u2212 discharge process for the aforementioned catalysts. [30] Recent studies indicate that by enabling electrochemical oxidation of metallic electrocatalysts in basic media one can considerably enhance the behavior of OER in neutral electrolytes as compared to the direct activation in neutral media without pre-oxidation of metallic electrodes. [31] Taehyun Kwon and co-workers fabricated hollow octahedral nanocages of Co-doped on IrCu alloy (Co-IrCu ONC/C) on carbon support [32], which showed excellent OER activity in addition to prolonged stability in acidic medium. The catalyst exhibited an \u03b7 value of 0.293\u00a0V at a current density of 10\u00a0mA\u00a0cm\u22122, as compared to Ir/C catalyst that showed an \u03b7 of 0.315\u00a0V. A Tafel slope value of 50\u00a0mV dec\u22121 was reported for Co-IrCu ONC/C, and the catalyst also demonstrated high durability with a reduction of only 3 percent after 2000 cycles of CV analysis. In comparison, the Ir/C catalyst dramatically deactivated with a 50% decrease in current density. The outstanding performance of Co-IrCuONC/Cis credited tothe 3Dintegratedstructure. In another work, Waqas Qamar Zaman and the team constructed a multimetallic IrO2 catalyst by co-doping with two different 3d transition metals (nickel and cobalt) to atomically replace 50% of the precious metal. [33] It was developed by the hydrothermal method to make sure composite homogeneity is achieved and later crystallized at 400\u00a0\u00b0C. The lower crystal formation energy (calculated using DFT (density functional theory) studies) for co-doping was the main factor for the significant dopant penetration. The synthesized co-doped IrO2 (Ir-NC-50) demonstrated \u03b7 at 10\u00a0mA\u00a0cm\u22122 current density of 0.285\u00a0V which was significantly lower than individually doped IrO2 by cobalt and nickel. The Tafel data of Ir-NC-50 was 53\u00a0mV dec\u22121 lower than the classic IrO2 (65\u00a0mV dec\u22121). Therein, they also establish a linear correlation between the decreasing onset potential and the decreasing iridium concentration. This helps in reducing the iridium doping by replacing it with transition metals without any decrease inactivity. Stability studies were carried out by mounting the prepared material (Ir-NC-50) on the Ti plate in an acidic medium at a constant current of 10\u00a0mA\u00a0cm\u22122 for 5 hrs. It showed negligible differences in activity, pre and post chronopotentiometry tests. Areum Yu et al. fabricated a crystalline one-dimensional (1D) tubular nanocomposite of iridium and cobalt (IrxCo1-xOy) through electrospinning and successive calcination. [34] The Nanocomposite with various Ir to Co ratios was created to test the efficiency and find the optimum loading of Ir to maximize the activity for OER reaction. Ir0.46Co0.54Oy nanotubes demonstrated the best activity for OER as well as high stability in alkaline medium. Scanning electron microscopy (SEM) images showed that Co3O4 had a smooth tubular structure and IrO2 a fiber morphology. The simple creation of Co3O4 shell in IrxCo1\u2212xOy probably offers a prototype of the tubular structure, promotes the development of IrO2 by combining precursors with Ir, and ultimately creates the mixed IrCo oxide nanotubes. To assess the electrochemical properties of the optimum catalyst (Ir0.46Co0.54Oy), cyclic voltammetry (CV) was carried out in 1\u00a0M KNO3 aqueous solution. The CV curves possessed a rectangular shape, showing the swift charging and discharging processes. Electrocatalytic measurements were conducted on a rotating disc electrode (RDE) voltammetry in an aqueous solution of 1\u00a0M NaOH at 1600\u00a0rpm electrode rotation speed. Ir0.46Co0.54Oy nanotubes had a \u03b7 value of 0.310\u00a0V at a current density of 10\u00a0mA\u00a0cm\u22122 and a Tafel slope of 58.6\u00a0mV dec\u22121, which was lower compared to IrOy nanofibers and Co3O4 nanotubes. The Stability test did not show much change in the potential used to generate 10\u00a0mA\u00a0cm\u22122 after 1000 repetitive scans. Transition-metal oxides could demonstrate greater stability during OER when compared to metals, possibly due to metal oxides being already present at higher oxidation state and further oxidation shifts is less likely to take place. [35] In a recent work by Yingjun Sun et al., they reported a new material of Pt-rich PtCo/Ir-rich IrCo trimetallic fishbone like nanowires, denoted as PtCo/Ir FBNWs. [36] The optimized Pt62Co23/Ir15 FBNWs only needed an overpotential of 0.308\u00a0V, much lower than the commercial Ir/C (0.380\u00a0V), to reach a current density of 10\u00a0mA\u00a0cm\u22122. The catalyst also displayed great activity in a broad pH range. DFT calculations reported that the catalyst\u2019s high activity was because of the modulation of highly electron active Ir-5d orbital on the Pt-based hetero-d-band-junction. It also acts as an outstanding trifunctional (ORR, OER, and HER) catalyst in a wide range of pH levels. Alloys containing five or more metal elements in a single phase are categorized as high entropy alloys (HEAs) [37], which provides ample possibilities to alter alloys\u2019 catalytic activities and surface electronic properties. [38\u201341] Recently, Zeyu Jin and coworkers worked on nanoporous HEA (np-HEA) of AlNiCoIrMo alloy, which showed high activity towards both OER and HER in acidic medium. [42] They were synthesized by incorporating Ir with other four metals into one single nanostructure phase of de-alloyed Al-based precursor alloys with merely 20 atomic % of Ir. The composition effect showed that the as-synthesized np-HEA had a nano ligament size of about 2\u00a0nm. An overpotential value of 0.233\u00a0V was needed to attain the current density of 10\u00a0mA\u00a0cm\u22122 and a mass current density of about 115\u00a0mA\u00a0mg\u22121 in an acidic medium. Seung Woo Lee et al. achieved a highly active and stable 3D mesoporous binary oxide of Ir and Ru (MS- IrO2/RuO2) with an optimum Ir to Ru molar ratio of 1:10 via nano-replication followed by Adams method. [43] When compared with conventional IrO2/RuO2 (0.340\u00a0V), MS- IrO2/RuO2 had a lower overpotential of 0.300\u00a0V at 10\u00a0mA\u00a0cm\u22122. After the accelerated stability test for over 2\u00a0h, the increase in overpotential was found to be 0.022\u00a0V for MS- IrO2/RuO2, whereas 0.044\u00a0V for IrO2/RuO2, demonstrating a better stability for MS- IrO2/RuO2 (Fig. 2\n).A ternary compound with iridium, ruthenium, and cobalt was studied by J.L. Corona-Guinto and co-workers. [44] They developed a RuIrCoOx powder by chemical reduction method, followed by a thermal oxidation process. Cyclic and linear voltammetry at 20\u00a0mV\u00a0s\u22121 scan rate and 900\u00a0rpm in 0.5\u00a0M H2SO4, was utilized to analyze the electrochemical properties of the electrocatalysts. The RuIrCoOx specimens revealed a Tafel slope value of 70\u00a0mV dec\u22121 at low current densities and 108\u00a0mV dec\u22121 at high current densities. When compared to RuIrOx, Tafel slopes of RuIrCoOx were lower in both the high and the low current densities. Whereas, the overpotential of RuIrCoOx was found to have a value of 0.410\u00a0V at a current density of 18\u00a0mA\u00a0cm\u22122. Chronopotentiometry experiments were performed in a current pulse of 0.25\u00a0mA\u00a0cm\u22122 to 75\u00a0mA\u00a0cm\u22122 in 300\u00a0s. The pulse length of the currents was found to be long enough to keep the voltage constant. This result shows that the use of mixed metals can contribute to synergetic effects that may increase the stability and selectivity of OER kinetics. Lei Wang et al. reported an active hollow Ru-modulated CoxP polyhedral structure (Ru-RuPx-CoxP) by adopting a facile solid\u2013liquid-phase chemical method. [45] Catalytic activity measured the value of \u03b7@10\u00a0mA\u00a0cm\u22122 as 0.291\u00a0V and a Tafel slope of 85.4\u00a0mV dec\u22121 in an electrolyte of 0.1\u00a0M KOH. The obtained value was low compared to RuO2 (\u03b710\u00a0=\u00a00.312\u00a0V) and IrO2 (\u03b710\u00a0=\u00a00.393\u00a0V). They found that Ru modulation can cause unstable surface termination, improve the electron transfer and promote the reaction kinetics by enhancing the density of states at the Fermi level to improve the electron transfer which further reduces the adsorption energy gap between the intermediates. Youngmin Kim et al. synthesized RuO2/Co3O4 nanowires by electrospinning process [46]\n, which showed an \u03b7 of 0.410\u00a0V at 10\u00a0mA\u00a0cm\u22122 when subjected to 1600\u00a0rpm in 0.1\u00a0M KOH solution saturated with oxygen. In comparison to Co3O4 and Ketjenblack (KB), RuO2/Co3O4 NWs showed a relatively greater OER current density and lower onset and overpotentials. The study established that the addition of highly active and conductive RuO2 onto the 1-D Co3O4 nanowire enhances the bifunctional performance of catalyst to a larger extent when compared to pure 1-D Co3O4. Caiyan Gao et al. reported ruthenium-cobalt nanoalloys encapsulated in carbon layers doped with nitrogen fabricated through incipient wetness impregnation pyrolysis process (RuCo@NC-750\u00a0\u00b0C, 1.56\u00a0wt% Ru). [47] It showed great stability and activity towards OER. ICP-AES test of RuCo@NC-750\u00a0\u00b0C after 10\u00a0h of CA test showed unchanged Ru and Co content compared to the initial sample. It exhibited an \u03b7 value at 10\u00a0mA\u00a0cm\u22122 of 0.308\u00a0V and a TOF value of 0.35\u00a0s\u22121 at \u03b7300 for OER. They related the enhanced activity to the introduction of Ru into the Co lattice matrix, which can significantly increase the transfer of an electron from the alloys to the carbon surface and increase the defects on the carbon surface. In a recent work by Pengsong Li et al., a monoatomic Ru attached on the surface of CoFe-LDH (Ru/CoFe-LDH), with an optimized wt% of 0.45 Ru, displayed an excellent OER activity with a \u03b7@10\u00a0mA\u00a0cm\u22122 as low as 0.198\u00a0V, significantly lower Tafel slope of 39\u00a0mV dec\u22121 and high stability in basic solution when compared to CoFe-LDH and commercial RuO2 catalysts. [48] In-situ and operando XAS measurements along with DFT\u00a0+\u00a0U calculation indicated an electronic coupling that is strong between LDH and Ru support, which acts as a cocatalyst to reduce the kinetic energy barrier to form *OOH from *O intermediate and avoided the formation of the high oxidation state of Ru. A comparison of catalytic activity of different transition metal LDH as cocatalysts revealed a trend as \u03b710 (Ru/CoFe-LDHs) (~0.198\u00a0V)\u00a0<\u00a0\u03b710 (Ru/NiFe-LDHs) (~0.220\u00a0V)\u00a0<\u00a0\u03b710 (Ru/NiCo-LDHs) (~0.240\u00a0V)\u00a0<\u00a0\u03b710 (Ru/MgAl-LDHs) (~0.290\u00a0V). Shaoyun Hao and coworkers modified the electronic properties of NiCo-LDH by incorporating Ru cations by one-step chlorine (Cl\u22121) corrosion of Ni foam (NiCoRu-LDH@NF). [49] Due to the high adsorption energy and improved active sites, the catalyst showed \u03b7@100\u00a0mA\u00a0cm\u22122 of 0.270\u00a0V, a low Tafel slope of 40\u00a0mV dec\u22121 (Fig. 3\n), and continued stability (55 Hours at 100\u00a0mA\u00a0cm\u22122) in basic solution. As in the previous results, here also, Ru reduced the energy barriers from *OH to *OOH which accelerated the reaction kinetics of OER. Juan Wang and the team fabricated a Co-doped RuO2 NWs (molar ratio, Ru:Co\u00a0=\u00a019:1) by combining a facile wet-chemical process and post-annealing treatment. [50] They demonstrated an \u03b7@10\u00a0mA\u00a0cm\u22122 as low as 0.2\u00a0V under acidic medium. First-principle calculations estimating the adsorption free energy of intermediates indicate a modulation in d-band center after metal doping, which is thought to be the cause of the enhancement in the OER activity. Jieqiong Shan et al. performed the generation of an alloy of RuIr nanocrystal with Co metal (Co-RuIr). [51] Addition of Co gave way to an enhancement of electronic structure of Co-RuIr alloy resulting in 0.235\u00a0V overpotential at 10\u00a0mA\u00a0cm\u22122 and a Tafel slope value of 66.9\u00a0mV dec\u22121 in 0.1\u00a0M HClO4 media. This improvement was due to the expected Co leaching which results in increased concentration of O2\u2212 species, which further favors OER catalytic activity.Ruohao Dong et al. designed a 2-D layered double hydroxide (LDH) structure comprising of silver (Ag) decorated Co(OH)2 nanosheets (Ag@Co(OH)2) via a selective reduction\u2013oxidation technique from metal nitrates. [52] In the field of electrocatalysis, cobalt compounds are granted close attention, among the 2-D materials community. Ag@Co(OH)2 showed excellent performance for OER and a yield of gram-scale, possibly because the near-surface oxygen vacancies in the Co(OH)2 nanosheets have the capacity to improve the electrophilic ability of O being absorbed, promote the adsorption of \u2013OH on active sites, and shape the \u2013OOH adsorbed species. [53\u201354] In addition, the Co(OH)2 interlayer spacing is greater than the typical transition metal hydroxides, resulting in a high ion transfer tendency and improved OER kinetic capability. [55] The electrochemical measurements were performed in an electrochemical workstation that has a three-electrode system in a solution of 1\u00a0M KOH (pH\u00a0~\u00a013.85) at room temperature. CV measurements were performed by sweeping the potential from a value of 1.3 to 1.7\u00a0V (vs RHE) at 10\u00a0mV\u00a0s\u22121 sweep rate and LSV with a 5\u00a0mV\u00a0s\u22121 scan rate. From the LSV curves \u03b7@10\u00a0mA\u00a0cm\u22122 of Ag@Co(OH)2 was found to be as low as 0.270\u00a0V, whereas a pure Co(OH)2, synthesized using the same technique showed an \u03b7@10\u00a0mA\u00a0cm\u22122 of 0.350\u00a0V. [56\u201357] They also prepared ternary materials comprising of cobalt, silver, and other transition metal elements (Ag@Co-Ni LDH and Ag@Co-Fe LDH), which showed better performance than the original Co LDH. This explained how silver doping on the LDH can enhance its efficiency by intensifying the catalyst\u2019s electrical conductivity. During the chronopotentiometry analysis (about 8 hrs.), the catalytic performance of Ag@Co(OH)2 faded only faintly with time. Ag@Co(OH)2 showed the greatest value of current density of 37.6\u00a0mA\u00a0cm\u22122 @ 0.350\u00a0V that was threefold more than that of pure Co(OH)2 nanosheets. In the case of Tafel slope Ag@Co(OH)2 exhibited a value of 67\u00a0mV dec\u22121. Kai-Li Yan et al. synthesized an Ag-doped Co3O4 nanowire array reinforced FTO (Ag-Co/FTO) via a facile electrodeposition-hydrothermal process in acidic media (0.5 H2SO4), which presented 1.91\u00a0V vs RHE onset potential and 0.680\u00a0V overpotential. [58] The results indicate that the vertical growth of Co3O4 nanowire was a result of Ag film deposition on the FTO substrate, and the formation of the mesoporous nanostructure can be attributed to Ag2O in Ag-Co hydroxide precursors. In a similar work, B. Jansi Rani et al. reported Ag-doped Co3O4 nanorods via hydrothermal method at different temperatures (90, 120, 150, and 180\u00a0\u00b0C). [59] The sample synthesized at 150\u00a0\u00b0C showed a good activity towards OER with 0.344\u00a0V overpotential to achieve a current density of 9.45\u00a0mA\u00a0cm\u22122. Conducting an EIS study and following the Nyquist plot, they concluded that the product synthesized at 150\u00a0\u00b0C had a low resistance of 13\u00a0\u03a9 and a narrow arc radius indicating quicker interfacial electron transfer. The enhancement in activity could have been due to the incorporation of Ag into Co3O4 crystal lattice which enhances the electrical conductivity and reduces the catalyst\u2019s internal resistance. It is also reported that Ag metal enhances the kinetic reversibility, specific capacitance, and redox reaction of pristine Co3O4. [60] In a different work, Xiaoyun Li et al. fabricated Ag loaded Fe-Co-S embedded on nitrogen-doped carbon composite (CISC-Ag) via calcination method and subsequent simple dipping route (Ag). [61] When they compared the electrocatalytic activity with pristine CISC, they found that CISC-Ag-3% (0.329\u00a0V) exhibited a lower overpotential at 10\u00a0mA\u00a0cm\u22122 than pristine CISC (0.366\u00a0V). The measured photocurrent density of Hematite photoanode covered with CISC-Ag-3% (with catalyst loading mass of 0.06\u00a0mg\u00a0cm\u22122) was found to be 0.527\u00a0mA\u00a0cm\u22122 that was 7 times that the bare hematite photoanode (0.075\u00a0mA\u00a0cm\u22122) at a bias voltage of 1.23\u00a0V vs. RHE. They elucidated that this activity was the result of the presence of metallic Ag in the catalyst, which suppresses the surface electron-hole recombination and endorses electron-hole formation. The electrical conductivity of Ag is mainly attributed to the loosely bonded outer electrons of Ag atoms. Therefore, adding Ag to any catalyst increases its electrical conductivity. Xu Zhao and coworkers worked on engineering the electrical conductivity of lamellar Ag-CoSe2 nanobelts (width 300\u2013500\u00a0nm) via partial cation exchange method. [62] About 1% of Ag+ cations enhanced the catalyst\u2019s OER stability and activity when compared to CoSe2 nanobelts. 1% Ag-CoSe2 demonstrated 0.320\u00a0V overpotential at 10\u00a0mA\u00a0cm\u22122, 22.36\u00a0mA\u00a0cm\u22122 current density at an \u03b7 of 0.350\u00a0V and low Tafel slope of 56\u00a0mV dec\u22121. They interpreted that the introduction of Ag+ had two effects on the catalyst. Firstly, they found that Ag+ caused a 5.6% decrease in the electrochemical active surface area (ECSA) which hinders the activity; and secondly, due to the improved electron transport, there was an increase in Co4+ sites which promotes the OER activity. The small decrement in ECSA was compensated by a more active generation of Co4+ active sites.Aolin Lu and team worked on a core\u2013shell structure of gold (Au)-cobalt nanoparticles, with the core being Au nanoparticle and shell being Co3O4 supported by carbon (AuCo/C). [63] They developed the structure by two different methods i.e. a facile one-pot synthesis and an injection synthesis. Due to the synergistic interaction between the shell and the core, core\u2013shell nanocrystals (NCs) exhibit exceptional catalytic activity in comparison to single-component NCs. [64\u201365] Oxygen and nitrogen saturated solutions were used for OER activity measurements. Electrochemical polarization curves were analyzed at 5\u00a0mV\u00a0s\u22121 scan rate after the CV test with the working electrode subjected to a rotation of 2000\u00a0rpm. They found that the optimal ratio of Au:Co to be 2:3 (Ag40Co60/C), which showed the highest value of current density of 428.6 and 366.9\u00a0mA\u00a0mg\u22121 respectively, at 1.67\u00a0V, for OER in N2 and O2 saturated solutions when prepared by one-pot synthesis. A Tafel slope value of 65\u00a0mV dec\u22121 was exhibited by Au40Co60/C, which was lower than bulk Au and bulk Co3O4. Boon Siang Yeo et al. reported cobalt oxide (CoOx) deposited onto the surface of rough gold (Au) substrate as monolayers for better performance for OER reaction. [66] They reported the OER activity of \n\n0.4 monolayer (ML) of CoOx on Au to be approximately 3 times greater than that of bulk iridium and 40 times greater than that of bulk CoOx under the same electrochemical conditions. The turn over frequency (TOF) of \n\n0.4 ML CoOx on Au was found to be \n\n1.8\u00a0s\u22121. This was mainly due to the growth in the surface CoIV population confirmed by Raman spectroscopy. Xunyu Lu et al. reported Au nanoparticles-doped mesoporous Co3O4 structure (Au/mCo3O4) synthesized via a nano casting process using mesoporous silica KIT-6 as a hard template. [67] An enhanced OER activity was obtained with 2.5% Au doped mCo3O4 which displayed a lower onset potential of 1.53\u00a0V (vs. RHE) and overpotential of \u03b710\u00a0=\u00a00.440\u00a0V when compared to mCo3O4 (onset potential\u00a0=\u00a01.56\u00a0V (vs. RHE), \u03b710\u00a0=\u00a00.520\u00a0V). This superior performance was ascribed to the doping of electronegative Au nanoparticles, highly exposed active sites, and large Brunauer\u2013Emmett\u2013Teller (BET) surface area. In a different work, Xu Zhao and associates developed CoSe2 nanobelts decorated with a trace amount (0.1\u00a0wt%) of isolated Au atom (Au1-CoSe2). [68] They probed from the study that the trace amounts of isolated Au atoms enhanced the exposure of cobalt active sites, limited the use of Au, shifted up the d-band center, and thereby reduced the H2O adsorption energy of active sites. An \u03b7 of 0.303\u00a0V at 10\u00a0mA\u00a0cm\u22122 and a Tafel slope of 42\u00a0mV dec\u22121 was calculated from the LSV curve of Au1-CoSe2 in O2 saturated alkaline medium (Fig. 4\n).To elevate a catalyst\u2019s activity, synthesis of specific catalyst morphologies and nanostructures has been practiced for a long time. [69] Based on Mott\u2013Schottky's effect, Zhong-Hua Xue and his team created a Janus particle based on cobalt. [30] A Janus Co/CoP nanoparticle was created by a controllable vacuum-diffusion method for continuous phosphidation of carbon-coated metallic cobalt nanoparticle. In Janus particle, one plane was metallic (Co) and the other was semi-conductive (CoP). Co/CoP had a high ECSA than that of pure Co sample which resulted in an enhanced activity for Co/CoP. Co/CoP-x showed OER activity in a wide range of pH which makes it different from other conventional electrocatalysts. Overpotential of Co/CoP-5 (\u03b7\u00a0=\u00a00.340\u00a0V, where 5 indicates the weight ratio of NaH2PO2 and Co elements) @ 10\u00a0mA\u00a0cm\u22122 was significantly less than Co (0.430\u00a0V) and CoP (0.380\u00a0V) species in 1.0\u00a0M KOH basic media. In the neutral media (1.0\u00a0M PBS), Co/CoP-5 attained a current density of 2.6\u00a0mA\u00a0cm\u22122 at a potential of 1.8\u00a0V (vs. RHE) and in acidic media (0.5\u00a0M H2SO4) it achieved a current density of 1.3\u00a0mA\u00a0cm\u22122 at 1.8\u00a0V (vs. RHE) potential. The preparation of aligned cobalt-based Co@CoOx nanostructures was performed by Qi and team that was achieved by the pyrolysis of cobalt oxalate precursors. [70] The 2-dimensional alignment of the derived cobalt nanoparticles was guided by the precursor\u2019s 2-dimensional morphology. To obtain a current density of 10\u00a0mA\u00a0cm\u22122 the resulting electrocatalyst needs a significantly low overpotential value of 0.298\u00a0V. They performed the OER experiment in an aqueous solution of 1\u00a0M KOH on a simple glassy carbon electrode. The nanoparticles\u2019 compact alignment as well as the metallic nature of the bulk of the catalyst is capable of assisting with the inner- and inter-particulate transfer of charge. In addition, the particles\u2019 2-dimensional alignment can, in general, intercept the dissolution and ripening of cobalt metal nanoparticles, as compared with isolated nanoparticles that is produced via traditional techniques. [71] The resulting electrocatalyst could also demonstrated a superior stability for electrolysis of 24\u00a0h at 1.55\u00a0V vs RHE. Jing et al. performed the synthesis of an oriented assembly constructed by hexagonal Co(OH)2 nanosheets. [72] A solvothermal technique in a mixture of methanol, water, and ethanol were first carried out to achieve single-crystalline CoC2O4 micro rods having ultra-high aspect ratio. The CoC2O4 was then converted into Co(OH)2 from anion-exchange by immersing the CoC2O4 precursor in a solution of alkali. The 1-dimensional microrod single crystals were converted into 2-dimensional nanosheets assembly and 3-dimensional structural voids were generated in the oriented assembly. The resulting materials combined the qualities of both 2-dimensional and 3-dimensional arrangements, displaying improved water oxidation activity in comparison to the freely isolated nanosheets. The assembly\u2019s magnified performance is primarily due to two reasons. The first is that the anisotropic 2-dimensional nanosheet substructure possesses intrinsic high charge transfer capacity. [73\u201374] The fabricated ultra-long structure provides a directed pathway for the transfer of charge. The second reason is that the 3-dimensional structural voids elevate the accessible surface area and assist with mass transport. [75] The assembled arrangement can also prevent the aggregation and ripening that takes place during electrolysis for enhanced durability. [71] Relative to the isolated nanosheet counterpart the nanosheet assembly demonstrated a greater activity for OER at similar mass loading. The nanosheet assembly could achieve 10\u00a0mA\u00a0cm\u22122 current density at an overpotential of 0.359\u00a0V, and to achieve the same current density, the isolated nanosheet displayed an overpotential of 0.394\u00a0V. In addition, the nanosheet assembly has a Tafel slope of 76\u00a0mV dec\u22121 when determined by LSV at 5\u00a0mV\u00a0s\u22121. This value is comparatively smaller than that shown by the isolated nanosheets (95\u00a0mV dec\u22121). Wu\u2019s group carried out a facile preparation of cobalt metal thin films through physical vapor deposition (PVD) on different nonconductive substrates. [76] These substrates include polyimide, mica sheet, regular and quartz glass, and polyethylene terephthalate (PET). When the group performed surface electrochemical modification using cyclic voltammetry, the films became active for electrocatalytic water oxidation. This activity was demonstrated by the film as it achieved 10\u00a0mA\u00a0cm\u22122 current density at a low overpotential value of 0.330\u00a0V in a solution of 1\u00a0M KOH. This is also the value required for a photovoltaic equipment to achieve 12.3% solar-to-hydrogen efficiency. [77] The electrodes were also sturdy, and their activity remained unchanged during chronopotentiometry analysis of long durations. Furthermore, no other energy or time consuming treatments (e.g., annealing, aging, or anodic oxidation, are necessary to reach the achieved activity with the resulting catalyst film. Wan and team developed a new electrocatalyst of Co(OH)F for OER. [78] The 3-dimensional Co(OH)F microspheres were constructed by building blocks of 2-dimensional nanoflake, which are then woven by 1-dimensional nanorod foundations. Weaving and constructing the substructures of 1-dimensional nanorods and 2-dimensional nanoflakes could deliver high structural void porosity with abundant interior space in the 3-dimensional material synthesized. The Co(OH)F material\u2019s hierarchical structure merges the merits of all material dimensions in heterogeneous catalysis. The advantages being possessed by the anisotropic low-dimensional (1-dimensional and 2-dimensional) substructures include swift charge transport and high surface-to-volume ratio. Furthermore, the nanorods\u2019 interconnectivity is valuable for charge transport. The 3-dimensional arrangement generates adequate number of active sites per surface area and is useful for efficient mass diffusion during catalysis. With the synthesized material a low overpotential value of 0.313\u00a0V was required to drive an OER current density of 10\u00a0mA\u00a0cm\u22122 in 1\u00a0M aqueous solution of KOH. Guo\u2019s team employed a facile 2-phase protocol to synthesize an \u03b1-Co(OH)2 by utilizing sodium oleate as a phase-transfer surfactant. [79] The team regulated the structure and crystallinity of the \u03b1-Co(OH)2 by heat treatments toward improved electrocatalytic OER activity. Calcination of the synthesized \u03b1-Co(OH)2 at a temperature of 200\u00a0\u00b0C produced a networked and well-dispersed nanoparticles of CoO (Co-200). The CoO sample synthesized displayed an OER current density of 10\u00a0mA\u00a0cm\u22122 under a low overpotential value of 0.312\u00a0V in a 1\u00a0M KOH aqueous solution. The improved activity could be described by the presence of ultra-small particle size and ample open spaces, both of which can deliver many surface catalytic sites. In addition, the onset potential for OER was 0.290\u00a0V and the Tafel slope was 75\u00a0mV dec\u22121. Zhao et al. successfully produced a unique hollow and porous CoO tetragonal prism-like structure through performing a facile and efficient co-precipitation technique. [80] With this technique, Co3(OAc)5OH particles of uniform size were synthesized using cobalt acetate in presence of polyvinylpyrrolidone (PVP K30). PVP itself possesses a strong coordination capability to metal ions through the N and/or CO functional groups. The Co3(OAc)5OH being produced has a highly uniform and discrete tetragonal prism-like system. The produced material was then calcinated at a temperature of 200\u00a0\u00b0C for a duration 3\u00a0h, in the presence of argon, and at a heating rate of 2\u00a0\u00b0C/min to obtain the porous CoO structure (CoO-200). High activity as well as high stability could be demonstrated by the porous and hollow CoO microprisms in 1\u00a0M KOH solution. A low overpotential of 0.280\u00a0V was needed to achieve 10\u00a0mA\u00a0cm\u22122 current density. A Tafel slop of 70\u00a0mV dec\u22121 was also displayed by the Co-based catalyst that indicates a fast water oxidation kinetics. The high performance observed could be due to the synergistic effect that exists between 2 different but finely-distributed CoO crystalline phases, ameliorative crystallinity, uniform particle size, low mass transfer resistance, and high surface area exploited from the unique porous arrangement. Liang\u2019s research team synthesized \u03b2-Co(OH)2/Co(OH)F hierarchical hexagrams with a six-fold symmetrical arrangement. [81] During the synthesis, hexagonal \u03b2-Co(OH)2 plates were first produced that behave as templates for the growth of Co(OH)F nanorods. An intermediate of \u03b2-Co(OH)2/Co(OH)F hybrid was then generated that consists of plate-like \u03b2-Co(OH)2 hexagonal cores appended with 6 rod-like CO(OH)F branches. Long reaction durations could lead to the complete conversion of \u03b2-Co(OH)2 hexagons that resulted in the formation of authentic six-branched Co(OH)F nanorods. As a result, nanorods of Co(OH)F were ordered into a six-fold symmetry. Another point to note is that along \u03b2-Co(OH)2 hexagon edges the growth of Co(OH)F nanorods could be observed as lateral branches in place of perpendicular to hexagons. The unusual epitaxial growth mechanism is regarded to be because of the matching between a-axis of \u03b2-Co(OH)2 crystals and the b-axis of Co(OH)F crystals, which is advantageous for electrocatalysis. Relative to pure Co(OH)F and \u03b2-Co(OH)2, the hybrid material could demonstrate enhanced water oxidation activity such as lower overpotential of 0.329\u00a0V to deliver 10\u00a0mA\u00a0cm\u22122 current density. Liang et al. performed a polyvinylpyrrolidone (PVP)-assisted pyrolysis to carry out the transformation of ZIF-67 into meso/microporous cobalt-embedded nitrogen-enriched carbon (Co-NC) material for both OER and ORR. [82] During pyrolysis, PVP was enclosed within ZIF-67 in one-pot and remained in it. With this technique, the breakdown of the porous structure at low temperatures could be avoided. The group chose PVP for a number of reasons. The first one was that PVP could be encapsulated because of the strong coordination interaction between the CO groups in PVP and the metal ion sites in metal\u2013organic frameworks. [83] The second reason was due to the findings by Lai et al. in which they found that a PVP derived carbon//ZIF derived carbon interfacial structure could be generated in PVP/ZIF nanocomposites. [84\u201385] The interfacial arrangement may lead to the improved electrocatalytic activities. Therefore, the group proposed that the meso/microporous Co-NC material could modify the pyrolysis functioning of ZIF-67, producing large electrochemical surface area. In addition, the presence of PVP could cause an increase in N content and the generation of the interfacial structure that could further contribute to the enhanced OER and ORR electrocatalytic activities. The group synthesized a number of materials and amongst them P-Co-NC-4 (4 being the synthetic PVP/Co2+ molar ratio) demonstrated the best activity. Analysis of the sample using LSV showed that it displayed an onset potential of 0.90\u00a0V. Its overpotential value at 10\u00a0mA\u00a0cm\u22122 current density was 0.315\u00a0V and it displayed a Tafel slope of 75.7\u00a0mV dec\u22121. Liang and group prepared quasi-single-crystalline CoO hexagrams that was characterized by structural long-range ordering and plentiful oxygen vacancies as defects. [86] The material was synthesized at \u03b2-Co(OH)2/Co(OH)F hexagrams\u2019 critical phase transition point. The matching between the a-axis of \u03b2-Co(OH)2 crystals and the b-axis of Co(OH)F crystals is vital for the generation of CoO hexagram single crystals. The resulting material, specifically P-400 (400\u00a0=\u00a0pyrolysis temperature, P\u00a0=\u00a0pyrolysis step), possessing abundant defects were very efficient for the oxidation of water as it demonstrated a low overpotential value of 0.269\u00a0V to deliver a current density of 10\u00a0mA\u00a0cm\u22122 in 1\u00a0M KOH aqueous solution. Liang and team used a simple preparation technique to produce 2-dimensional ultrathin \u03b1-Co(OH)2 nanosheets. [87] The technique involved mixing cobalt salt aqueous solution with a methanolic solution of 2-methylimidazole at room temperature. The products synthesized were of nanosheets form that were micrometer in size and possessed an average thickness of approximately 2.5\u00a0nm. The ultrathin structure provided the \u03b1-Co(OH)2 nanosheet with the ability to perform greatly for OER. An overpotential value 0.267\u00a0V was displayed by the resulting material at j\u00a0=\u00a010\u00a0mA\u00a0cm\u22122. With this strategy it is also possible to synthesize other 2-dimensional cobalt-based layered double or triple hydroxides. Furthermore, the \u03b1-Co(OH)2 nanosheets demonstrated a Tafel slope of 64.9\u00a0mV dec\u22121. This value is comparatively lower than that of commercial RuO2 (78.7\u00a0mV dec\u22121) and hexagonal \u03b1-Co(OH)2 plates (81.2\u00a0mV dec\u22121). Quentin Daniel et al. established that when cobalt porphyrins were deposited on FTO glasses (via spin coating), they decompose into thin film of CoOx on the surface during electrochemical water oxidation under borate buffer (pH 9.2, 0.1\u00a0M). [88] The thin film was only detectable by XPES using low photon energies (1000\u00a0eV). The newly formed catalyst showed advanced activity for OER with a high TOF value of the order of 10\u00a0s\u22121. Huiling Sun et al. reports the synthesis of four Cobalt corroles attached with different acid/base pendants in neutral aqueous solution. [89] The working electrode used was catalyst loaded on FTO glass. Complex LCH [2]PO(OH) [2] \u2013Co showcased higher performance for both OER and HER compared to other complexes. The LCH [2]PO(OH)2 \u2013Co complex showed an overpotential of 0.45\u00a0V. Samaneh Sohrabi and team worked on a composite 3D porous coordination network (PCN) with 3D nanochannels via solvothermal method with the help of Zr6 clusters and tetrakis (4-carboxyphenyl) porphyrin cobalt (MOF). [90] Because of the presence of porphyrins and ultrastable Zr6 clusters on the backbone of the MOF, it was an easy access for the reactants. The catalyst developed showcased an overpotential of 0.4\u00a0V.Bangan Lu and co-workers synthesized a nanowire array of nickel and cobalt oxides freely standing on nickel foam substrate (NixCo3-xO4-1:1). [91] They reported that Ni doping on the Co3O4 increased the roughness and in turn increased the activity of the catalyst. The Ni doped Co3O4 (Ni:Co\u00a0=\u00a01:1) and pure Co3O4 had the same nanowire array structure when SEM image analysis was conducted. When the atomic ratio of Ni:Co was larger than 1:1, a change in morphology into nano flake structure was seen. Electrochemical studies showed an overpotential value at 5\u00a0mA\u00a0cm\u22122 mg\u22121 of 0.56\u00a0V and 0.65\u00a0V for NixCo3-xO4-1:1 and Co3O4 respectively. At 0.6\u00a0V the current density of NixCo3-xO4-1:1 was about 5 times in magnitude than that of pure Co3O4. There was also a 5-fold difference in the roughness factor between NixCo3-xO4-1:1 and Co3O4, which proves that an increase in roughness increases the activity of the catalyst. Ni doping is assumed to increase the electrocatalytic activity of Co3O4, either by increasing its roughness factor and surface area (geometrical effect) or by enhancing its conductivity (electronic effect) or both. [92] The stability test studied showed a negligible change in potential after 10\u00a0h in an alkaline medium. Small Tafel slopes, lower overpotential, and high current density are due to the presence of large active electrochemical surface area (ECSA) as well as 1-D morphology for better charge conduction. Siwen Li et al. had a similar approach to develop a Co-Ni based 1-D nanotube with adjustable Co:Ni ratios by a cation-exchange method to build hydroxides (CoNi(OH)x) grown on a conductive Cu substrate for OER. [93] The nanotube morphology of the catalyst aided in forming a conductive structure with a large surface area, and therefore producing ample catalytic reaction sites. It is also reported that the Co2+ at octahedral sites (Oh) gives a better result for OER than Co2+ at tetrahedral sites (Th). [94\u201395] When X-ray absorption spectroscopy (XAS) was carried out, a peak at 781.1\u00a0eV in the Co l-edge XAS spectra correlates to the characteristic peak of Co2+ ions at Oh. [96] The LSV showed an onset potential value of 1.48\u00a0V (vs. RHE) and an \u03b7 as low as 0.280\u00a0V at 10\u00a0mA\u00a0cm\u22122 with a Tafel slope of \n\u2248\n77\u00a0mV dec\u22121. EIS analysis revealed that CoNi(OH)x nanotube retains a considerably lower charge transfer resistivity which is due to the reaction between the O* intermediate and different hydroxides on the surface of the catalyst. [97] Xuehui Gao et al. synthesized hierarchical NiCo2O4 hollow micro cuboids constructed by 1-D porous nanowire subunits. [98] It exhibited a small onset potential of 1.46\u00a0V (vs. RHE) to reach 1\u00a0mA\u00a0cm\u22122, 1.52\u00a0V (vs. RHE) at 10\u00a0mA\u00a0cm\u22122, and a Tafel slope of 53\u00a0mV dec\u22121. OER activity of NiCo2O4 is accredited to its unique hollow mesoporous structure composed of 1-D nanowires, which provides easy access for electrolytes to the active sites. Substituting a second metal into monometallic phosphides could efficiently alter the electronic structure of the parent compounds and further enhance the OER activity. Lei Han et al. prepared Ni-Co mixed oxide nanocages from Ni-Co Prussian blue analog (PBA) cubes metal\u2013organic framework (MOF) precursors through an anisotropic chemical etching route. [99] Due to their complex 3-D cage-like hollow structure in addition to the high surface area to volume ratio, they exhibited low overpotential (0.380\u00a0V@10\u00a0mA\u00a0cm\u22122) and Tafel slope (50\u00a0mV dec\u22121) under basic medium. Wook Ahn and coworkers fabricated a multivoid nanocuboidal MOF catalyst with multiple mesosized and microsized pores synthesized from a ternary Ni-Co-Fe MOF (NCF-MOF) by a facile co-precipitation and post heat treatment method. [100] Altering ion exchange rates of the transition metals in the MOF are used to produce heteroatom doping, interconnected internal voids, and favorably tuned electronic structure by combining the outer electrons of active Co and Fe metal ions, which leads to reduced adsorption strength with the intermediates. All these aids in bringing about enhanced activity of OER with low overpotential (\u03b710\u00a0=\u00a00.320\u00a0V) and Tafel slope (49\u00a0mV dec\u22121) for the catalyst. Bocheng Qiu et al. reported Ni-Co bimetallic phosphide nanocages (NiCoP) with constant dispersion of Ni and Co atoms by using Cu2O cubes as sacrificial templates. [101] Ni0.6Co1.4P nanocages derived from Ni0.6Co1.4(OH)2 nanocages exhibited notable activity towards OER (\u03b710\u00a0=\u00a00.3\u00a0V, 80\u00a0mV dec\u22121) when compared to Ni2P(\u03b710\u00a0=\u00a00.420\u00a0V, 128\u00a0mV dec\u22121) and CoP (\u03b710\u00a0=\u00a00.370\u00a0V, 100\u00a0mV dec\u22121). The stability test at 1.53\u00a0V for 10\u00a0h proved that Ni0.6Co1.4P had the least current density loss (10%) compared to CoP(20%) and Ni2P(30%) nanocages. The authors elucidated that appropriate doping of Co atoms can significantly lower the activation barrier of the catalyst and increase the density of states (DOS) at the Fermi level, resulting in low intermediate adsorption energy and high charge carrier density. Enlai Hu et al. presented a template-assisted strategy to organize 2-D nanosheets of Ni-Co precursors into an oriented stacking of 3-D anisotropic Ag2WO4 cuboid particles. [102] After successive heating, etching, and phosphorization treatments, Ni-Co precursors are converted to open and hierarchical Ni-Co\u2013P hollow nano bricks (HNBs). Overpotential value of 0.270\u00a0V to achieve 10\u00a0mA\u00a0cm\u22122 current density and a Tafel slope of 76\u00a0mV dec\u22121 was observed. The extended stability was tested by a CA measurement and only about 6.5% of the initial current was lost in 20\u00a0h time period. Micropores and mesopores among the oriented stacking and macropores due to the open and hollow interior promote exposure of active sites as well as penetration of electrolytes into the catalyst which further eases the OER activity. [98,103] Xin Liang and coworkers formed Ni2P-CoP bimetallic phosphides via low-temperature phosphorization of Ni-Co organic frameworks. [104] Enhanced catalytic activity was achieved by controlled formation of interfaces of Ni2P-CoP, which reduced the bandgap and promoted faster electron transport. LSV curves showed onset potential (1.50\u00a0V (vs. RHE)) and overpotential (\u03b710\u00a0=\u00a00.320\u00a0V) to be lower than Ni2P and CoP. Jiayuan Li et al. came up with a facile synthesis of single-phase ternary Ni2-xCoxP (x\n\u2264\n1.0) rGO hybrids with well-regulated Co doping concentration. [105] It is noted that the presence of rGO increases the number of surface-active sites and enhances the hybrid electrodes\u2019 activity. Co doping controls the active sites\u2019 catalytic activity and accelerates the charge transfer process of the catalyst. Onset potential of 0.251\u00a0V (vs. RHE), \u03b7 of 0.270\u00a0V at 10\u00a0mA\u00a0cm\u22122, and a small Tafel slope of 65.7\u00a0mV dec\u22121 were reported for the NiCoP/rGO hybrids (x\u00a0=\u00a01) in an electrolyte of 1.0\u00a0M KOH. Long-term catalytic stability proved stable OER current density of 50\u00a0mA\u00a0cm\u22122 at 0.360\u00a0V overpotential for 18\u00a0h. Hanfeng Liang and coworkers fabricated ternary a NiCoP nanostructure from hydrothermally formed NiCo hydroxides via PH3 plasma-assisted approach, supported on nickel foam, for the first time (NiCoP/NF). [106] The plasma-assisted process promoted low-temperature reaction and fast preparation of the catalyst. From energy-dispersive X-ray spectroscopy (EDS) mapping, they found the ratio of Ni:Co:P to be 1.106:1:1.138, which was close to NiCoP. Electrocatalytic studies measured an \u03b7 of 0.28\u00a0V to obtain a current density of 10\u00a0mA\u00a0cm\u22122, which was lower than Ni2P/NF (0.34\u00a0V) and NiCo-OH/NF (0.404\u00a0V). They attributed the enhanced OER activity to the Co addition which lowered the activation barrier, altered the electronic structure, and synergistic effect between Ni and Co. Junyuan Xu et al. worked on tri-metallic equimolar FeCoNiP on carbon nanofiber (CNF) pre-catalyst prepared by chemical reduction followed by phosphorization treatment. [107] Overpotential as small as 0.2\u00a0V at 10\u00a0mA\u00a0cm\u22122 current density and a high TOF of 0.94\u00a0s\u22121 at an \u03b7 of 0.35\u00a0V was measured from the LSV data under alkaline medium. Also, a greater mass activity of 5000\u00a0mA\u00a0mg\u22121 was obtained at \u03b7\u00a0=\u00a00.330\u00a0V. CNFs aided in the improved charge transfer and increased the nucleation sites during wet chemical reduction. The authors interpreted that Co helps in reducing the overpotential in the low potential region and Ni boosts the anodic current in the high potential region. Jingchao Zhang et al. fabricated mesoporous Ni-Co sulfide nanotubes via template-free solvothermal method followed by anion-exchange process. [108] Due to the synergetic effect between Ni and CO, altered electronic structure, and increased surface area, Ni0.13Co0.87S1.097 nanotube exhibited improved performance for OER with lower onset potential (0.262\u00a0V at 1\u00a0mA\u00a0cm\u22122), overpotential (0.316\u00a0V at 10\u00a0mA\u00a0cm\u22122) and Tafel slope (54.72\u00a0mV dec\u22121) in comparison to CoS1.097 (\u03b71\u00a0=\u00a00.280\u00a0V, \u03b710\u00a0=\u00a00.331\u00a0V and Tafel slope\u00a0=\u00a055.54\u00a0mV dec\u22121). A 3-D structure promotes OER activity by contributing to the high specific surface area, more defects as exposed active sites, accelerated H2O adsorption, and easy gas permeability. Chengzhou Zhu et al. designed a 3D bimetallic Ni-Co oxide hollow nanosponges (HNS) by a sodium borohydride reduction strategy (Ni-Co2-O HNS). [109] Due to the hollow structure, synergetic effect between Ni-Co and high specific surface area, the catalyst exhibited a superior OER activity. Ni-Co2-O HNS had a porous and interconnected network and an ultra-low density of around 0.08\u00a0g\u00a0cm\u22123. LSV curve showed an onset potential of as low as 1.501\u00a0V (vs. RHE) and an \u03b7 of 0.362\u00a0V in 0.1\u00a0M O2 saturated KOH solution. In a different work, Seok-Hu Bae and co-workers shaped a 3D conductive carbon-shelled Ni-Co nanowire structure (CCS Ni-Co NWs) (Fig. 5\n). [110] The Ni-Co nanowires grown on the carbon fiber woven fabric (hydrothermal method) were coated with conductive carbon shell via glucose carbonization followed by annealing processes. The granular and porous structure of Ni-Co nanowires aids in the rapid release of O2 and provides an enlargement in the number active of sites. Whereas the carbon shell aids in fast electron transmission from the active site to the current collector (carbon fiber fabric) and avoids the dispersion of catalytic particles during active O2 evolution. These properties of the structure helps in enhancing the catalyst\u2019s OER activity. A 0.302\u00a0V overpotential @10\u00a0mA\u00a0cm\u22122 with a Tafel slope of 43.6\u00a0mV dec\u22121 in KOH solution of 1\u00a0M concentration was reported. When compared with Ni-Co NWs, the charge transfer resistance of CCS Ni-Co NWs was lower, implying increased current access and enhanced charge transport efficiency. [111] Another notable merit of this catalyst is that it acts as a catalytic electrode, which can be deposited directly on the working electrode without any binders. Cheng Du et al. reported a continuous hydrothermal, oxidation, and phosphidation process using NaH2PO2 to synthesize a 3-D nest-like ternary NiCoP supported on carbon cloth (CC) electrocatalyst. [112] In alkaline medium, \u03b7 of 0.242\u00a0V @ 10\u00a0mA\u00a0cm\u22122 with a Tafel slope of 64.2\u00a0mV dec\u22121 was reported. They concluded that the addition of urea, carbon cloth, and the coexistence of Ni and Co precursors to be the main reason for the 3-D nest-like structure. Furthermore, the carbon cloth acts as a current collector to improve the conductivity and charge transferability. [105]\nLinzhou Zhuang and coworkers reported Fe-Co oxide nanosheet (FexCoy-ONS, x/y indicates the molar ratio of Fe/Co) synthesized by a solution reduction process using NaBH4 reducing agent to improve oxygen vacancies and the catalyst\u2019s active sites. [113] The optimized Fe1Co1-ONS had a high specific surface area of about 261\u00a0m2 g\u22121 which resulted in an \u03b7 of only 0.308\u00a0V@10\u00a0mA\u00a0cm\u22122 and Tafel slope as low as 36.8\u00a0mV dec\u22121 in an alkaline solution (0.1\u00a0M KOH). The results obtained were superior to those of commercial RuO2. At \u03b7\u00a0=\u00a00.350\u00a0V, Fe1Co1-ONS showed a current density of 54.9 A g\u22121, which is 5.8 times larger than that of RuO2 available commercially. A detailed characterization using X-ray photoelectron spectroscopy (XPS) and Photoluminescence spectroscopy confirmed that the excellent OER performance of the catalyst was due to abundant oxygen vacancies which result in an easy excitation of the delocalized electrons into the conduction band near the oxygen-deficient sites. Jin-Xian Feng et al. designed a FeOOH sandwiched cobalt hybrid nanotube arrays supported on nickel foam (FeOOH/Co/FeOOH HNTAs-NF). [114] Co and FeOOH were loaded as \n\n0.28\u00a0mg\u00a0cm\u22122 and \n\n0.22\u00a0mg\u00a0cm\u22122, respectively. EIS studies confirmed that FeOOH/Co/FeOOH HNTAs-NF has a significantly smaller electronic resistance when compared to FeOOH NTAs-NF. This result confirms that the Co metal layer enhances electron transmission because of its high electrical conductivity and Ni foam acts as a current collector which together overcomes the poor electric conductivity of FeOOH. The optimum thickness of 25\u00a0nm for FeOOH showed the highest OER activity.FeOOH/Co/FeOOH HNTAs-NF showed an overpotential of just 0.250\u00a0V (Fig. 5) to reach 20\u00a0mA\u00a0cm\u22122 current density and a Tafel slope as low as \n\n32\u00a0mV dec\u22121 in an alkaline medium. Chronopotentiometric studies performed for 50\u00a0h showed no negligible change in overpotential to maintain the current density values of 20, 50, 100, and 200\u00a0mA\u00a0cm\u22122. Theoretical studies suggest that the energy of OER intermediates can be modulated with the inclusion of metal elements for a given metal oxide. Bo Zhang et al. fabricated a gelled FeCoW (oxy)hydroxides (G-FeCoW) using a sol\u2013gel procedure that would include incorporation of W6+ into FeCo (oxy) hydroxides, which is hydrolyzed at a controlled rate to achieve atomic homogeneity. [115]) Here, tungsten (W) modulated 3d metal oxide (CoOOH), provided excellent adsorption energies for OER intermediates which in turn enhanced catalytic activity. When the G-FeCoW catalyst underwent electrocatalytic studies, it exhibited surprising results compared to the FeCo LDH and NiFe LDH. It presented an \u03b7 of 0.191\u00a0V at 10\u00a0mA\u00a0cm\u22122 current density when deposited on a gold-plated Ni foam. This is significantly lower than the precious metal-based electrocatalyst used for OER previously. The stability test showed no appreciable increase in potential under 30\u00a0mA\u00a0cm\u22122 current density for 550\u00a0h. When the catalytic measurements were conducted on glassy carbon (GC) electrode, the catalyst showed an \u03b7 of 0.223\u00a0V at 10\u00a0mA\u00a0cm\u22122 current density, a TOF of 0.46\u00a0s\u22121 and mass activity of 1175 A g\u22121. Similar work has been reported by Peng Fei Liu et al. where they used molybdenum (Mo6+) to modulate 3d metal (oxy) hydroxides (FeCoMo) to attain better adsorption energy for the OER intermediates and provide rich active sites for OER. [116] FeCoMo displayed an \u03b7 of 0.277\u00a0V at 10\u00a0mA\u00a0cm\u22122 current density on GC and no evidence of degradation was reported for about 40\u00a0h at constant 10\u00a0mA\u00a0cm\u22122 current density. It exhibited a bulk mass activity of 177.35 A g\u22121 at 0.3\u00a0V overpotential, which is approximately 7 times larger in comparison to IrO2. Harshad A. Bandal and coworkers prepared a composite electrode of high activity for water splitting by placing ordered spinel Fe-Co oxide (50\u00a0nm thickness) on the surface of Ni foam (FeCoO-NF). [117] When compared to CoO-NF (\u03b710\u00a0=\u00a00.268\u00a0V), FeCoO-NF (\u03b710\u00a0=\u00a00.244\u00a0V) revealed advanced performance for OER activity. Tafel slope of FeCoO-NF (57\u00a0mV dec\u22121) was also lower than the compared CoO-NF (67\u00a0mV dec\u22121), which indicates that the incorporation of iron into Co3O4 has a positive effect on enhancing the OER activity. When compared to conventional RuO2, FeCoO-NF required comparatively less overpotential to reach the 50 and 100\u00a0mA\u00a0cm\u22122 current density marks. The 3-D white fungus-like structure of FeCoO-NF aided in the effective transport of electrons between the catalyst and electrolyte, easy dissipation of O2, and reduced solution and charge transfer resistance. Wei Liu et al. fabricated an amorphous Co-Fe hydroxide (CoFe-OH) nanosheets (20\u201330\u00a0nm thickness) via facile electrodeposition for 20\u00a0min grown homogeneously on a graphite substrate\u2019s surface. [118] Due to their hierarchical network formed by the nanosheets, it resulted in a high electrochemically active surface area which exhibited a low \u03b7 (0.280\u00a0V at 10\u00a0mA\u00a0cm\u22122) and low Tafel slope (28\u00a0mV dec\u22121when compared to Fe-OH and Co-OH samples in alkaline medium. In a different work; Hui Xu, Jingjing Wei, and coworkers created a 2-D CoFe oxyhydroxide nanosheet doped with phosphorous (2D-CoFeP NS) in alkaline medium (1\u00a0M KOH), which delivered 0.305\u00a0V overpotential at 10\u00a0mA\u00a0cm\u22122 current density with a low Tafel slope of 49.6\u00a0mV dec\u22121. [119] 2D-CoFeP NSs electrode maintained long term stability with a negligible decrease in potential at the 10\u00a0mA\u00a0cm\u22122 current density for 24\u00a0h. The doped phosphorus played a critical part in modifying the surface-active sites of the catalyst. To add to the phosphorization, and synergetic effect between Co and Fe, the unique structure provides a great surface area and ample interlinked channels for O2 release and mass transport. In a similar work, Xiao Zhang and team introduced a novel CoFeP multi-void nanocages (CoFeP-NC) that were derived from CoFe-PBA (Prussian blue analog) nanocubes via a self-template phosphorization process with uniform size ranging from 250\u00a0nm to 350\u00a0nm. [120] From the XPS spectra, it was clear that both Co and Fe acted as active sites in the catalyst, and the synergism induced between them improved the electronic structure as well. In alkaline medium, CoFeP-NC showed an \u03b7 as low as 0.180\u00a0V at 10\u00a0mA\u00a0cm\u22122 current density, superior stability, and a turnover frequency of 0.93\u00a0s\u22121 at 0.270\u00a0V overpotential. This enhanced OER activity could be explained by the porous hollow system with large surface area, high effective active sites, and reduced charge transfer distance. The pyridinic N doped in the CoFeP catalyst provided an added synergistic effect to OER activity. The catalyst exhibited a fast-current density increase within a small change in overpotential (\u03b7100\u00a0=\u00a00.280\u00a0V). Yuan et al. developed a hierarchical hollow nanocube structure that was based on ultrathin CoFe-layered double hydroxide (CoFe-LDH) nanosheets. [121] The group first prepared Cu2O nanocubes as the self-sacrificing template. They employed a template-assisted route for the production of hollow nanocubes based on CoFe-LDH nanosheets through coordinating etching. The performance of the resulting material was demonstrated when it displayed a low overpotential of 0.270\u00a0V for 10\u00a0mA\u00a0cm\u22122 current density for water oxidation. A low Tafel slope value of 58.3\u00a0mV dec\u22121 as well as a long-term stability was also displayed in an aqueous solution of 1\u00a0M KOH. DFT study was also performed by the research group and the analysis revealed that Fe addition provided a metallic identity with Co(OH)2, assisting in electron transfer. Qian Zhou et al. reported a facile cation-exchange process for creating iron-doped Co(OH)2 nanosheets with the augmented active site. [122] Iron-doped Co(OH)2 nanosheets showed lower Tafel slope and overpotential (53\u00a0mV dec\u22121, \u03b710\u00a0=\u00a00.320\u00a0V) when compared to pristine Co(OH)2 nanosheets (69\u00a0mV dec\u22121, \u03b710\u00a0=\u00a00.370\u00a0V). After the cation exchange process, Fe3+/Co2+, the Fe-doped Co(OH)2 nanosheets had substantial grain boundaries, rougher surface, improved hydrophilicity, and enhanced electronic properties which resulted in an enhanced activity for OER. In recent work, Lei Zhong and coworkers produced Fe doped CoTe (Fe-CoTe) by a one-step solvothermal process, which showed excellent activity and stability without any activation process. [123] A 0.300\u00a0V overpotential at 10\u00a0mA\u00a0cm\u22122 current density and 45\u00a0mV dec\u22121 Tafel slope value were observed from the electrocatalytic measurements. The authors elucidated that Fe-CoTe had the maximum Fe-Co synergy and the catalytic performance was due to intrinsic properties of Fe-CoTe, and not from the Fe impurity adsorbed from the electrolyte. The improved amount of lattice oxygen also aided to the enhanced OER activity. The charge transfer resistance of Fe-CoTe was only 1/6th of the pristine CoTe catalyst, representing the Fe-doping effect. Sheng-Hua Ye et al. fabricated Fe substituted CoOOH porous nanosheets arrays developed on a cloth of carbon fiber (FexCo1-xOOH PNSAs/CFC, \n\n0\n\u2264\nx\n\u2264\n0.33\n\n) with 3-D structures via in-situ anodic oxidation of \u03b1-Co(OH)2 NSAs/CFC. [124] Fe0.33Co.0.67OOH PNSAs/CFC showed enhanced activity towards OER with a low \u03b710 of 0.266\u00a0V and a Tafel slope value of 30\u00a0mV dec\u22121. X-ray absorption fine spectra (XAFS) studies indicated a partial substitution of CoO6 octahedral structures in CoOOH by FeO6 octahedral during the conversion from \u03b1-Co(OH)2 to FexCo1-xOOH. Detailed DFT calculations indicated that such substitution can reduce the energy levels of the intermediates and products as FeO6 octahedron is a highly active site for OER. Li-Ming Cao and team proposed a concrete pathway for the hierarchical fabrication of a novel self-supporting 3-D porous sulphur-doped NiCoFe LDH nanosheets (S-NiCoFe LDH) on carbon cloth. [125] The EIS measurements revealed that the charge transfer resistance (RCT) value of S-NiCoFe LDH was smaller than those of undoped LDH (NiCoFe LDH and NiFe LDH), which indicates that sulphur doping helped in improving the catalyst\u2019s electrical conductivity. A low \u03b7 of 0.206\u00a0V at 10\u00a0mA\u00a0cm\u22122 current density as well as a Tafel slope of 46\u00a0mV dec\u22121 was reported from the electrocatalytic measurements. The XPS results supported that the Co-S bonds and Ni-S bonds were altered into Ni/Co oxyhydroxides that further enhanced the OER activity.Jingrui Han et al. developed an amorphous Mn-Co-P layer on MnCo2O4 supported on a titanium mesh (Mn-Co-P@MnCo2O4/Ti) through a cathodic polarization in NaPO2H2 solution. [126] Under alkaline medium, the catalyst demonstrated an \u03b7 of 0.269\u00a0V at a current density of 10\u00a0mA\u00a0cm\u22122, Tafel slope of 102\u00a0mV dec\u22121 which was lower compared to MnCo2O4/Ti (\u03b710\u00a0=\u00a00.362\u00a0V, 210\u00a0mV dec\u22121). XPS results revealed that the Mn-Co-P layer was produced on the surface of the MnCo2O4 as a shell that boosts the OER activity. Xijun Liu et al. reported hierarchial ZnxCo3-xO4 nanoarrays which had secondary nanoneedles grown on primary rhombus-shaped pillar arrays supported on titanium (Ti) foil prepared by the co-deposition of zinc and cobalt precursors followed by calcination in air. [127] A 0.320\u00a0V overpotential @ 10\u00a0mA\u00a0cm\u22122 was detected for ZnxCo3-xO4-1:3 (1:3 is the ratio of Zn and Co precursor used) nanoarrays with a low Tafel slope value of 51\u00a0mV dec\u22121. ZnxCo3-xO4 can be directly used as electrodes for OER. [128] The close contact of the 3-D porous structure to Ti foil ensured long term stability and gave way for the conduction of electrons. [127,129] The enhanced performance of ZnxCo3-xO4 is attributed to the unique hierarchical 3-D nanostructure which brings about high porosity, large surface area, more active sites, increased roughness factor, and improved gas permeability. Jianfeng Ping and team prepared a 3-D porous CoAl-layered double hydroxide (LDH) nanosheets (CoAl-NS) onto a 3D graphene network (3DGN) by electrostatic self-assembly (3DGN/CoAl-NS). [130] Here to obtain the CoAl-NSs, the CoAl-LDH (NO3\u2212) crystal with the largest interlayer area was made use for exfoliation. The electrochemical activity was studied in 1\u00a0M KOH with a loading mass of CoAl-NSs on 3DGN as about 0.05\n\u00b1\n0.01\u00a0mg\u00a0cm\u22122. The results revealed an \u03b7 at 10\u00a0mA\u00a0cm\u22122 current density to be 0.252\u00a0V and a 36\u00a0mV dec\u22121 Tafel slope value. At \u03b7\u00a0=\u00a00.300 and 0.350\u00a0V, the current densities values were 45.37 and 91.74\u00a0mA\u00a0cm\u22122, respectively. Stability tests confirmed a nearly constant current density for 18\u00a0h at \u03b7\u00a0=\u00a00.250 and 0.280\u00a0V. The exposed active edge sites of the CoAl-NSs made it easy for the proton paired electron transfer process during OER. [131\u2013132] Also a constant coating of single layer CoAl-NSs on the 3DGN by electrostatic self-assembly is an effective way by which it can expedite the reaction kinetics and accelerate the electron transfer. [133\u2013134] Furthermore, the unique structure of 3DGN aids for the access of ions to the catalysts and prevents the restacking of CoAl-NSs [135\u2013137].Perovskites exhibit an ABO3 type of empirical formula with A generally being a rare-earth or alkaline earth metal while B being commonly a transition metal. It has been stated that doping of A- or B- site cations in the perovskites structures is an effective way to improve OER activity. [138\u2013139].Denis Kuznetsov et al. introduced a high electronegative (Bi3+) element into the A-site of the strontium cobalt perovskites to sustain high Co-O covalency through the inductive effect. [140] An exceptionally low Tafel slope of 25\u00a0mV dec\u22121 was obtained for the bismuth substituted strontium cobalt perovskites. This was attributed to the potentially increased hydroxide kinship on the catalyst\u2019s surface by the introduction of Bi3+ ions. Xi Cheng et al. analyzed the influence of Sr substitution into the A- site of LaCoO3 perovskites. [141] The surface composition, bulk electronic structure, electrochemical activity, and conductivity for the La1-xSrxCoO3 perovskite series (\n\n0\n\u2264\nx\n\u2264\n1.0\n)\n\n were investigated experimentally and theoretically. A phase transition from rhombohedral (LaCoO3) to cubic structure (La1-xSrxCoO3) was observed after the gradual replacement of La by Sr. They found that Sr substitution has the effect of aligning along the Co-O-Co axis, straightening the octahedral cage, and rising the average oxidation state of Co ions. DFT calculations proved that the above merits improve the overlap between the unoccupied Co 3d conduction bands and the occupied O 2p valence bands which further improved the catalyst\u2019s OER activity.To understand the influence of B-site substitution in perovskites, Maria A. Abreu-Sepulveda et al. investigated an organized substitution of Co by Fe in La0.6Ca0.4CoO3 perovskites (La0.6Ca0.4Co1-xFexO3) under alkaline medium via a facile glycine-nitrate synthesis. [142] A rise in the surface concentration of different Co oxidation states by the incorporation of Fe was showcased by the XPS results. Specific activity trend of the substitution followed the trend: Fe0.9\u00a0>\u00a0Fe0.8\u00a0>\u00a0Fe0\u00a0>\u00a0Fe0.1\u00a0>\u00a0Fe0.2\u00a0>\u00a0Fe0.5\u00a0>\u00a0Fe1.0. Iron incorporation decreased the barrier for electron transfer and facilitated the generation of cobalt-hydroxides. A Tafel slope value of 49\u00a0mV dec\u22121 was determined for La0.6Ca0.4Co0.1Fe0.9O3 (x\u00a0=\u00a00.9). They found that Fe complexes are significant for OER by enabling the interaction of Co-OH bond and CoOOH are responsible for the electronic conductivity. Under alkaline medium layered double hydroxide perovskites PrBaCo2O6-\n\n\u03b4\n (PBC) has been found to be very active. The addition of Fe further enhances the activity. Xiaomin Xu et al. reported BaCo0.9-xFexSn0.1O3-\n\n\u03b4\n\n(BCFSn) perovskites oxides through doping Fe and Sn in BaCoO3-\u03c3 parent oxide via solid-state reaction under alkaline medium. [143] BCSFsn-721 (x\u00a0=\u00a00.2) displayed a low value of onset potential (\n\u2248\n1.53\u00a0V vs. RHE), overpotential (\n\u2248\n0.420\u00a0V at 10\u00a0mA\u00a0cm\u22122 current density) and a Tafel slope value of 69\u00a0mV dec\u22121. They established that the catalyst\u2019s OER activity can be tuned by simply altering the concentration of Fe and Sn. The mass activity of the catalyst can be further enriched by reducing their particle size or creating pore structures with a large surface area. [144] Bae-Jung Kim et al. doped iron into the B-site of PBC with different ratios to synthesize PrBaCo2(1-x)Fe2xO6-\u03b4 (x is 0.2 or 0.5; designated as PBCF82 and PBCF55, respectively) nanoparticles in the size range of 5\u201330\u00a0nm. [145] PBCF82 and PBCF55 exhibited the same Tafel slope value of 50\u00a0mV dec [1] which was lower than PBC (72\u00a0mV dec\u22121) and greater current densities at 1.55\u00a0V vs. RHE (17.1 and 19.7 A g\u22121) (Fig. 6\n, A-E). When stability tests were conducted, PBCF55 lost only 32% of its starting current density, while PBC lost approximately 74% of its initial current density. They elucidated from their studies that Fe incorporation stabilizes cobalt in the lower oxidation state by delivering finer distribution of charge, encouraging the emergence of oxygen vacancies, and improving the structural stability of the layered double perovskites catalyst by supporting the formation of oxy(hydroxide) layer. Yinlong Zhu and coworkers fabricated SrNb0.1Co0.7Fe0.2O3-\n\n\u03b4\n\n(SNCF) under alkaline medium. [146] SNCF was ball milled to increase its surface area, which further increases the OER activity. Advanced OER ability with low onset potential (1.49\u00a0V vs. RHE), overpotential (\u03b710\u00a0=\u00a00.420\u00a0V), and Tafel slope of 76\u00a0mV dec\u22121 was observed due to excellent ionic and charge-transfer capabilities along with optimized eg orbital filling, and high O2 desorption and OH\u2212 adsorption abilities. It also exhibited good stability for the long term due to the incorporation of Nb5+ cations on the B-site of the catalyst. The catalytic performance of conventional Ba0.5Sr0.5Co0.8Fe0.2O3\u2212\n\n\u03b4\n\nperovskites (BSCF) is limited by a low specific surface area (0.5\u00a0m2 g\u22121). Yisu Yang et al. developed porous BSCF perovskites with ordered pore structure (3\u201310\u00a0nm) via a novel in-situ tetraethoxysilane (TEOS) template technique to increase the specific surface area (reaching a value of 32.1\u00a0m2 g\u22121) of the conventional BSCF perovskites. [147] This method increased the specific surface area of the nonporous BSCF by 60 times. An optimum ratio of 3.4 for TEOS to BSCF was found to have the highest performance for OER. Under alkaline conditions, BSCF-3.4 exhibited the highest current density of 35 A g\u22121 at 1.63\u00a0V vs. RHE (\u03b7 of 0.4\u00a0V) (Fig. 6, f-j), which was 5.3 times higher than nonporous BSCF (6.6 A g\u22121), and a Tafel slope value of 62\u00a0mV dec\u22121. They concluded from their studies that silica-containing impurities reduce the conductivity of the electrodes and the enhanced activity was strictly related to the microstructural properties of the catalyst.Chao Su et al. synthesized perovskites oxides with the composition of SrM0.9Ti0.1O3-\u03b4 (M\u00a0=\u00a0Co, Fe) via the sol\u2013gel method. [148] SrCo0.9Ti0.1O3-\u03b4 (SCT) showed better functioning stability in comparison to SrFe0.9Ti0.1O3-\u03b4 (SFT), BSCF, and IrO2. Such OER activity could be accredited to the low average bond energy of Co-O, optimal eg electron filling, and good charge transferability. Xiaoming Ge et al. designed a novel La(Co0.55Mn0.45)0.99O3-\u03b4 (LCMO) nanorods (diameter of 45\u201355\u00a0nm and aspect ratio of 3\u201310) using a hydrothermal method followed by heat treatment. [149] The 1% B-site lattice vacancy offers an added advantage for good OER activity of oxides. [150] The further synergetic covalent coupling that exists between LCMO and reduced graphene oxide doped with nitrogen (NrGO) exhibited exceptional bifunctional activity for OER and ORR. The OER onset potential of LCMO/NrGO was about 0.45\u00a0V vs RHE and the potential to reach 10\u00a0mA\u00a0cm\u22122 was 0.787\u00a0V vs. RHE, which was lower compared to Ir/C. The coupling between the NrGO and LCMO, NrGO\u2019s permeating electrical conduction, and the intrinsic activity of LCMO perovskites resulted in the advanced OER activity. Anchu Ashok et al. investigated on lanthanum based electrocatalytically active LaMO3 (M\u00a0=\u00a0Cr, Mn, Fe, Co, Ni) perovskites produced through a single-step solution combustion method. [151] Results from the study showed enhanced OER for LaCoO3 that is because of the optimum stabilization of reaction intermediates present in the RDS of OER. The stability test proved LaCoO3 to be the most stable among the perovskites studied in the report. Taking inspiration from a previous work by Mohamed A. Ghanem [152] on the mixed anion perovskites (ABOxXy, X is a non-oxygen anion), Yuto Miyahara and co-workers studied the bi-functionality, for OER and ORR, of layered cobalt perovskite oxychlorides, namely Sr2CoO3Cl and Sr2Co2O5Cl2, synthesized through a solid-state reaction that utilizes Sr2Co2O5 as a precursor. [153] The catalyst was found to be highly active, which was because of the upshift of the O p-band center compared to the Fermi level caused by the incorporation of Cl\u2212 into the oxygen sites. The onset potential of the oxychlorides was exhibited to outperform the state-of-the-art BSCF perovskites. Tafel slope also reveals the same outcome about the activity of oxychlorides (60 and 62\u00a0mV dec\u22121, respectively) when compared to BSCF perovskites (72\u00a0mV dec\u22121).Over the past few decades, carbon materials have received substantial attention as a support in various electrocatalyst due to their high thermal stability, environmental friendliness, good conductivity, chemical inertness, high specific surface area, corrosion-resistant, tunable surface function, and higher stability in both acidic and alkaline medium. [154\u2013155] Based on the crystal structure, carbon atoms can be of various allotrope forms with distinct and unique physical and chemical properties. Various carbonaceous nanomaterials such as carbon nanofiber (CNF), carbon nano coil (CNC), nano carbon black (CB), single/multi-walled carbon nanotubes (SWCNT/MWCNTs), carbon mesoporous (CMS) and graphene/graphene oxides (G/GOs) are possibly incorporated with the cobalt-based catalyst in order to enhance the electronic conductivity and electrochemical performance. [156] Here we discuss various cobalt-based catalysts supported on carbon materials towards oxygen evolution reaction.Carbon nanotubes (CNTs) have received significant attention in the area of fuel cells as effective support because of the large surface area, high electronic conductivity, thermal stability, and durability that they offer. CNTs are rolled-up sheets of single (SWCNTs) or multi-layer (MWCNTs) carbon atoms (graphene) in cylindrical form.Lu and Zaho prepared crystalline cobalt oxide nanoparticle of ~6\u00a0nm size incorporated with mildly oxidized multiwalled carbon nanotubes (Co3O4/mMWCNT) and used it as an effective catalyst for H2O oxidation. They studied the correlation between various other carbon structures such as single-walled CNTs (SWCNTs), graphene, and multi-walled CNTs (MWCNTs) with different oxidation states in terms of charge transport and surface functionalization towards water oxidation reaction. The results showed that the hybrid mildly oxidized MWCNTs (Co3O4/mMWCNT) with a 0.390\u00a0V overpotential value (at 10\u00a0mA\u00a0cm\u22122) and 1.51\u00a0V vs. RHE onset potential that can sustain the electrochemical reaction even under harsh environment with minimum carbon corrosion acting as a promising electrocatalyst towards OER [157]. Zeng et al. reported the bifunctional cobalt (II/III) oxides strongly anchored onto a lightweight,conductive, and crosslinked aerogel film of carbon nanotubes (CNTs) as a free-standing air electrode. The LSV profile showed improved performance of crosslinked aerogel film of carbon nanotubes (CNTs) when compared with pristine CNT aerogel, N\u2010CNT aerogel, and pure Co3O4 in terms of onset potential (1.45\u00a0V) and potential (1.7\u00a0V vs RHE at a current density of 10\u00a0mA\u00a0cm\u22122) (overpotential of 0.47\u00a0V) [158]. Shuo and coworkers followed the pyrolysis of metal\u2013organic framework (MOF) encapsulated Co3O4 for the successful generation of Co-embedded N-doped CNTs with porous carbon (PC) that showed prolonged stability and excellent activity in alkaline solution. The polarization curve for Co\u2013CNT/PC exhibited lower onset potential than Co-doped over porous carbon (Co-PC) that could be attributed to the improvement in the electrical conductivity for CNT. [159] Zhang and coworkers reported a superior activity and remarkable stability for cobalt carbonatehydroxidehydrate(CCHH)nanosheets strongly adhered on the mildly oxidized MWCNTs in presence of diethylenetriamine(DETA). They found that the presence of DETA greatly influences the structure and morphology of the CCHH/MWCNT composite and thereby enhanced the resulting OER performance. Thus, prepared hybrid CCHH/MWCNT exhibited lower onset potential (approximately 1.47\u00a0V vs RHE) (overpotential of 0.285\u00a0V @ 10\u00a0mA\u00a0cm\u22122) and good kinetics that was clear from the Tafel slope analysis. [160] In 2014, the same group reported a study on Co3O4\nnanorod\u2013multi-walled carbon nanotube hybrid (Co3O4@MWCNT) that exhibited 0.309\u00a0V overpotential at 10\u00a0mA\u00a0cm\u22122\ncurrent density in an alkaline medium that also possess superior activity and stability. [161] Fang et al. analyzed the synergistic influence of Co(II), organic ligands, and CNTs that offered excellent activity and durability to sustain in a harsh environment without any carbon corrosion. The hierarchical 3-D unique system with a large surface area improved the transportation of electrons and secured the anchoring of the catalyst\u2019s active sites to the CNTs. Co-MOF@CNTs offered impressive durability and activity when compared to 20\u00a0wt% Pt/C and RuO2 catalysts. They studied the influence of the OER performance on the amount of CNT in the overall catalyst and found that the overpotential followed an inverted volcano type trend with CNT weight percentage with increasing order of 5% < 1% < 10% <15% of CNTs. [162] In order to improve the surface defect, chemically active sites, and the surface defects of CNTs; hetero atom doping with boron, phosphorous and nitrogen are widely used. Dicobalt phosphides (Co2P) are another category of Co-based catalyst that recently achieved wide attention owing to their catalytic and magnetic properties. Various reports are available on Co2P anchored CNTs delivering excellent electrochemical performance. Hui et al. utilized N, P co-doped CNTs for the anchoring of CoP/CoP2 nanoparticles that exhibited low overpotential, higher current density and excellent stability over 100\u00a0h. Moreover, they conducted density functional theory calculations and molecular dynamics simulations that concluded the synergetic effects of CoP and CoP2 improved the electrocatalytic performance; also the heteroatom-doped CNTs readily diffuse out the generated O2 molecule to help in improving the electrocatalytic oxygen evolution reaction. [163] Das and co-workers followed a novel one pot synthesis of phosphine free (PH3) Co2P anchored over N, P dual doped carbon nanotubes without any external carbon additive. The average diameter of prepared Co2P was found to be 55\u00a0nm and for NPCNTs the range was between 80 and 250\u00a0nm. The hybrid Co2P/NPCNT displayed a small onset potential value of 1.293\u00a0V and an \u03b7 of 0.370\u00a0V (at 10\u00a0mA\u00a0cm\u22122) that was expected for a solar water-splitting device with 10% efficiency. [164] Guo and team utilized a combination of dicobalt phosphide (Co2P)\u2013cobalt nitride (CoN) core\u2013shell nanoparticles synthesized using direct pyrolysis method as double active sites for the incorporation with N doped CNTs that showed excellent trifunctional performance. The interface between CoN and N\u2010doped CNTs was the active site for OER that has attracted applications in flexible and rechargeable Zn-air batteries. [165] Cobalt sulfides,including CoS2, CoS4, Co3S4, and Co9S8, have been found to be attractive and are novel electrocatalysts for the storage of energy as well as conversion applications because of the unique chemical and physical properties. [166\u2013167] The performance of cobalt sulfideswere improved by further optimization of electrode surface with carbon-based materials. Wang and co-workers prepared an integrated 3-D model of carbon-paper/carbon-tubes/cobalt-sulfide array that displayed impressively high performance towards OER. The unique hybrid structure possibly enhanced the accessibility and availability of active sites, the capacity to transport electron, and improved the release of product gases. They studied the catalyst without any CNT incorporation and found that CP/CTs/Co-S have excellent behavior in terms of potential and current density when compared to CP/Co-SN. [168] Xinwei and his team used atomic layer deposition (ALD)for the successful deposition of the thin layer of Co9S8 (~7 nm) onto the CNT network scaffold with a high surface area that showed remarkably great performance for rechargeable Zn-air batteries [169].The electrical conductivity of oxidized-CNTs is lower than non-oxidized CNTs that limit their catalytic performance, nonetheless, this limitation, in some cases can be overcome by using a mildly oxidized graphene/CNTs. [170] Ting Ma et al. performed an in-situ synthesis of ultra-small Co\u2013Mn\u2013O spinel nanoparticles that have an average nanoparticle size of 4.4\u00a0nm reinforced over the non-oxidized CNTs that enabled strong coupling to aid with the transfer of electron and enhanced the activity. CMO@CNTs exhibited a lower onset potential of 2.558\u00a0V and a Tafel slope of 81.1\u00a0mV dec\u22121 that showed superior performance when compared with CMO@rGO, and CMO@Vulcan, CMO@oxCNTs, and CMO\u00a0+\u00a0CNTs. [171]\nFig. 7\n shows the synthesis of non-spinel MnCo oxide, Co\nx\nMn1\u2212\n\nx\nO (Mn2+, Co2+)anchored over N-doped CNTs that showed much higher OER activity than commercial IrO2. Co2+ was regarded as the active site for the evolution of oxygen because NCNT/MnO exhibited inferior performance than NCNT/CoO as reported by Liu\u2019s group. [172] Kunpeng and his team followed a two-step gas phase process for the fabrication of hierarchical hybrid Co3O4\u2013MnO2\u2013CNT. The prepared spinel Mn-Co mixed oxide triggered the growth of multi-walled carbon nanotube and the active metal particles remained on the CNT surface were greatly influenced by the growth time. An HNO3 vapor treatment was used to convert the active metals to their higher oxidation state, where MnO was oxidized to MnO2, and Co was converted to Co3O4. Subsequently, a small amount of oxygen functional group was created on the surface of the catalyst that facilitated the release of gases during the reaction. [173] Liu\u2019s group synthesized morphology-controlled La2O3/Co3O4/MnO2\u2013CNTs hybrid nanocomposites which showed excellent durability and activity when compared to the commercial 20% Pt/C catalyst. The oxygen evolution reaction onset potential for La2O3/Co3O4/MnO2\u2013CNTs, CNT, MnO2, La2O3/Co3O4/MnO2, La2O3/Co3O4\u2013CNTs, Co3O4/MnO2\u2013CNTs and 20% Pt/Cwas found to be1.42, 1.69, 1.51, 1.70, 1.52, 1.52 and 1.67\u00a0V, respectively. This indicates that the powerful coupling effect that is present between La2O3\nnanorod and MnO2\nnanotubes, Co3O4\nand CNTs produces a synergy for the catalytic performance. [174] Another catalyst, Ni, showed enhanced performance in OER when alloyed with Co to form bimetallic Ni-Co. Many works have been reported on the encapsulation of NiCo with conducting CNTs in order to improve the electronic conductivity. Jie et al. fabricated a 3D network of NiCo encapsulated with nitrogen-doped CNTs (NiCo@NCNTs) as shown in Fig. 7e -hthat showed superior activity than the bimetallic composite (Co@NCNTs and Ni@NCNTs) owing to the synergy between cobalt and nickel. They conducted a detailed study on the effect of coupling on NiCo@NCNTs in comparison with physically mixed NiCo and CNTs. The superiority of encapsulated NiCo@NCNTs caused the transfer of electrons from NiCo alloy to the walls of carbon nanotubes that reduced the local work function on the carbon surface. Also, the wrapping of CNTs over the active NiCo alloy effectively resisted the etching in harsh environment and made the catalyst active and stable for long duration. [175] Yang and co-workers fabricated ultra-small NixCo3\u2212xO4 nanocrystals (~5 nm) decorated over pristine MWCNTs using the solvothermal method without destroying the CNTs. Pristine MWNTs showed an efficient electron transfer network and its incorporation with well-constructed spinel NixCo3\u2212xO4 led to an outstanding electrochemical performance [176]. Numerous cobalt-carbon based catalysts were reported showing extremely high activity and performance due to the hierarchical structure and synergetic effect including Co-N/CNT [177], Co\u2010NRCNTs [178], Co(OH)x-NCNT [179], Ni foam\u2010supported N\u2010CNT@Co3O4 [180], CNTs-Au@Co3O4\n\n[181], CoHCF/CNT [182], Co-CNT/Ti3C2 [183], CoFe/Co8FeS8/CNT. [184]\nExceptional properties of graphene, such as great electrical conductivity, large surface area, and fine chemical and mechanical stability has led to extensive research activities, particularly utilizing them as a catalyst substrate. [185] As a result, it has also been implemented in electrical devices such as fuel cells and lithium batteries with enhanced electrochemical operation. [186\u2013190] In terms of the use of graphene as a material for catalyst production, many graphene-based composite catalysts have been synthesized in the recent past. The resulting composites are applied on substrate electrodes, normally, using drop-casting techniques [191\u2013192].A composite of graphene and Co3O4 (G-Co3O4 composite) has been reported by Zhao\u2019s research team, that possesses a unique sandwich-architecture as shown in Fig. 8\na. [193] Analysis of the composite using TEM and FESEM has shown that there is a homogeneous distribution of Co3O4 on the 2 sides of graphene nanosheet (Fig. 8, b\u2013e). A superior catalytic behavior towards OER in an alkaline solution of 1\u00a0M KOH has been observed (Fig. 8, f\u2013i). An onset potential of 1.454\u00a0V vs. RHE was exhibited by the composite. Furthermore, within the same alkaline solution, the achievement of 10\u00a0mA\u00a0cm\u22122 current density was observed at an \u03b7 of 0.313\u00a0V that is more superior than that of the mesoporous Co3O4 catalyst (0.525\u00a0V) and Co3O4/SWNTs (0.593\u00a0V). In terms of stability, the composite is expected to exhibit long-term stability as it demonstrated no clear decay in current density during testing in alkaline solution after 10\u00a0h as well as an undisturbed morphology. The extraordinary behavior could be due to the synergistic influence arising from the combination of both Co3O4 and graphene that include swift electron transfer rate, large electroactive surface area, and better chemical and electrical coupling of the composite. In another study performed by Zhao et al., a catalyst of CoO nanoparticles wrapped by porous graphene sheets was synthesized using 1-D silica nanorods as a template to prepare the porous graphene. [194] The catalyst possessed great specific surface area and porosity and showed rapid charge transport kinetics. An improvement in catalytic activity was also seen for OER that includes large current density and a low onset potential. When the performance of the catalyst was studied in a KOH solution of 0.1\u00a0M concentration via LSVs, the PGE-CoO hybrid demonstrated a small onset potential of 1.4934\u00a0V vs. RHE that is considerably lower than GE-CoO (1.5494\u00a0V vs. RHE) and CoO (1.5594\u00a0V vs. RHE) itself. Moreover, at 10\u00a0mA\u00a0cm\u22122 current density, the composite exhibited a low overpotential of 0.348\u00a0V. To measure the efficiency of the PGE-CoO catalyst Tafel plots were obtained from the LSVs and a Tafel slope value of 79\u00a0mV dec\u22121 was determined. In comparison to the value obtained for GE-CoO and CoO this value is way smaller (GE-CoO showed 192\u00a0mV dec\u22121 and CoO showed 354\u00a0mV dec\u22121). The improvement in performance could be accredited to the presence of large electroactive surface area, porous structure, and a strong chemical coupling between both CoO NPs and graphene. In addition, the catalyst could maintain fine stability towards OER in an alkaline solution, possibly due to CoO NPs corrosion prevention characteristic introduced by the wrapped structure.Wang\u2019s group produced a series of electrocatalyst in which graphene and cobalt oxide NPs nano-hybrids (Co-N/G) are doped with nitrogen via a one-pot hydrothermal method. [195] A nitrogen precursor is known as 2, 4, 6-Triaminopyrimidine was also utilized to anchor cobalt oxides NPs onto the graphene oxide surface. The composites synthesized consisted of cobalt oxides in the form of Co3O4 and CoO as well as a high content of doped nitrogen (~6 at. %) comprising of pyrrolic, pyridinic, and graphitic types. The resulting synergistic effect generated from the coupling between Co NPs and nitrogen-doped graphene allowed the composite samples to be used as catalysts for both OER and ORR. The as-synthesized Co-N/G 600 (sample carbonized under N2 atmosphere at 600\u00a0\u00b0C) showed the highest potential for application in reversible electrochemical energy conversion fuel cells and metal-air batteries. This is because Co-N/G 600 demonstrated an excellent bifunctional catalytic activity with high efficiency in which high activities for oxygen evolution reaction and oxygen reduction were observed at a potential of 0.76\u00a0V (1.554\u00a0V onset potential vs RHE) and \u22120.2\u00a0V (0.855\u00a0V onset potential vs RHE), respectively. In addition, the Co-N/G 600 catalyst showed both fine stability and durability for both the type of reactions. Graphene-based materials doped with nitrogen have been used by Hou et al., where nitrogen-doped graphene was combined with a Co-embedded porous carbon polyhedron to form N/Co-doped PCP//NRGO. [196] A simple pyrolysis of graphene oxide (GO) and zeolitic imidazolate-framework (ZIF), ZIF-67, was implemented in the preparation of the new novel hybrid electrocatalyst after which metallic cobalt was partially etched away. The utilization of ZIF-67 was performed to take advantage of the plentiful Co-N moieties and the unique dodecahedral morphologies available with ZIF-67. [197] With these properties, ZIF-67 may be a fitting precursor for the generation of N/Co-doped PCP. The as-synthesized hybrid catalyst showed excellent performance, including great stability, not only for OER but also for ORR and HER. Such enhancement could be associated with the dual-active-site mechanisms that emerge from the synergetic influences between NRGO sheets and PCP doped with N/Co. The hybrid electrocatalyst also demonstrated a four-electron pathway, great durability, and high tolerance towards methanol. Furthermore, during the performance analysis of N/Co-doped PCP//NRGO for oxygen evolution reaction, only a small \u03b7 value of 1.66\u00a0V was noticed at a current density of 10\u00a0mA\u00a0cm\u22122. Qiao\u2019s group co-doped graphene with both Co and nitrogen and inserted carbon nanospheres into the graphene sheets interlayers. [198] The carbon nanospheres behaved as \u201cspacers\u201d that enlarged the accessible surface area of graphene and provided many electrolyte channels. These two unique properties helped in promoting the diffusion of reaction species to the active sites. Enhanced conductivity could also be guaranteed as the carbon nanospheres could further act as \u201cshortcuts\u201d for interplanar electron transport. The synthesized catalyst possessed bifunctional stability and catalytic activity for both ORR and OER in a basic medium. When compared to Pt/C catalysts the overall oxygen electrode activity parameter (\u0394E) of the bifunctional Co-N-GCI electrocatalyst was relatively lower (0.807\u00a0V). The overpotential for Co-N-GCI catalyst at 10\u00a0mA\u00a0cm\u22122 current density was determined to be 0.426\u00a0V, that was much lower than those obtained for Co-N-G hybrid (not intercalated with conductive carbon nanospheres, 0.472\u00a0V), and commercial Pt/C (0.621\u00a0V), at the same current density. Moreover, an excellent intrinsic OER kinetic of Co-N-GCI was confirmed by a relatively lower Tafel slope value of about 69\u00a0mV dec\u22121 in comparison to Co-N-G (~78\u00a0mV dec\u22121), IrO2/C (~83\u00a0mV dec\u22121) and Pt/C (~168\u00a0mV dec\u22121). A strongly coupled hybrid electrocatalyst of CoOx NPs grown on B, N-decorated graphene (CoOx NPs/BNG) was produced by Tong et al. that is suitable for catalyzing both ORR and OER. [199] An abundant presence of oxygen vacancies and strong CoNC bridging bonds were identified in the hybrid using advanced spectroscopic techniques. These qualities promote the enhancement inability to transfer electron, a greater number of active sites, and a strong synergetic coupled effect. Towards OER in a solution of KOH with 0.1\u00a0M concentration, the hybrid electrocatalyst functioned with high efficiency by demonstrating a low \u03b7 of 0.295\u00a0V (at 10\u00a0mA\u00a0cm\u22122 current density) and a Tafel slope of 57\u00a0mV dec\u22121. These values are significantly lower than that for NG (0.500\u00a0V overpotential, 110\u00a0mV dec\u22121 Tafel slope) and Co-BG (0.320\u00a0V overpotential, 70\u00a0mV dec\u22121 Tafel slope) catalysts. Synthesis of N- and B-doped graphene hollow spheres coated with Co3O4 (Co3O4/NBGHSs) was reported by Jiang\u2019s team for use as a potential catalyst for both ORR and OER. [200] The resulting catalyst had the ability to perform with comparatively higher activities and durability for the two reactions than RuO2/C and Pt/C. The coupling between NBGHSs and Co3O4, high electrical conductivity, the strong interaction with O2 being adsorbed, and the specific hollow design were the contributing factors in the improved performance of the catalyst. Using LSV studies in an alkaline 0.1\u00a0M solution of KOH, a value of onset potential of about 1.6\u00a0V was recorded, which is more negative than those found for pure Co3O4 hollow microspheres, Co3O4/BGHSs, Co3O4/NGHSs, Co3O4/GHSs, NBGHSs, and Pt/C. However, in comparison to the conventional RuO2/C catalyst, the OER onset potential of Co3O4/NBGHSs is greater. The \u03b7 for attaining a current density of 10\u00a0mA\u00a0cm\u22122 was determined to be approximately 0.47\u00a0V for Co3O4/NBGHSs, which is less than that of RuO2/C with a potential of about 0.52\u00a0V. Lu et al. produced N- doped Co3O4 nanocrystals combined with core\u2013shell structured carbon nanotube-graphene nanoribbon (N-csCNT-GNR) scaffolds. [201] A high loading of Co3O4 was achieved during the synthesis by utilizing a microwave-assisted controlled unzipping of MWCNTs. The high surface area of carbon nanomaterials, as well as excellent electrical conductivity, could both be achieved as the csCNT-GNR structures possess an interlinked unzipped graphene nanoribbon and an intact MWCNT core. [202\u2013204] The composite catalysts were also proven to be incredibly active towards both OER and ORR. OER investigation results obtained from the study in 0.1\u00a0M KOH have shown that Co3O4/N-csCNT-GNR could perform very actively by exhibiting an onset potential value of 1.51\u00a0V. An \u03b7 of 0.360\u00a0V (iR corrected polarization curve) was also observed to obtain 10\u00a0mA\u00a0cm\u22122 current density. The remarkable activities demonstrated by the synthesized composite was found to be more superior in comparison to Ir/C catalyst for OER [205\u2013206].Ganesan\u2019s research team prepared a bifunctional hybrid electrocatalyst for ORR and OER in which cobalt sulfide NPs are grown on a nitrogen and sulfur co-doped graphene oxide surface through a solid-state thermolysis technique. [207] During the synthesis process, the size, and phase of the particle could be controlled by altering the treatment temperature. Three different treatment temperatures of 400\u00a0\u00b0C, 500\u00a0\u00b0C, and 600\u00a0\u00b0C were employed in addition to the use of cobalt thiourea and graphene oxide to successfully disperse cobalt sulfide NPs onto graphene oxide. Analysis performed using X-ray diffraction has shown that the hybrids produced at 400\u00a0\u00b0C and 500\u00a0\u00b0C consisted of pure CoS2 phase while that synthesized at 600\u00a0\u00b0C contained Co9S8 phase. A simultaneous co-doping of both nitrogen and sulfur on graphene oxide was confirmed via X-ray photoelectron spectroscopy that acts as sites to strongly anchor CoS2 NPs onto the GO surface. Amongst the catalysts synthesized CoS2(400)/N, S-GO displayed an excellent electrode performance. It exhibited a potential of approximately 0.82\u00a0V vs. RHE in basic medium (Fig. 8n), that was far superior compared to Ir/C (0.92\u00a0V), Ru/C (1.01\u00a0V), and Pt/C (1.16\u00a0V).In addition to the graphene-based OER electrocatalysts mentioned earlier, many other catalysts have been reported which have the potential to catalyze OER with finer electrocatalytic activity and stability. Some of these catalysts include Fe3O4@Co9S8/rGO-2 [166], Cu@GDY/Co [208], N-CG-CoO [209], and Co-Bi NS/G. [210]\nMany of the recent research studies that focused on the replacement of platinum-based catalysts with hybrids consisting of non-precious metal have employed mesoporous carbon in addition to graphene and nanotubes. These carbon materials are generally doped with heteroatoms (e.g., nitrogen) before they are introduced into transition metals such as cobalt, iron, manganese, and their complexes. [211\u2013213] Doping with heteroatoms can help modify the surface electronic structure as well as develop surface hydrophilicity to adsorb O2 species particularly in ORR. [214] Non-precious metal-supported carbon materials doped with nitrogen commonly have excellent performance for both OER and ORR when they contain cobalt oxides. This can be because of their ability of cobalt to change their valence states and maintain a steady activity [215].Liu et al. prepared an efficient OER electrocatalyst in the form of 3-D mesoporous carbon-framework-encapsulated CoTe2 nanocrystals from a metal\u2013organic framework (MOF) precursor. [216] They also implemented tellurization and carbonization processes that aided in yielding nanocomposites of CoTe2 and graphitic carbon doped with nitrogen (CoTe2@N-GC) immediately from ZIF-67. The resultant catalyst demonstrated a much greater performance towards OER by exhibiting an \u03b7 value of 0.300\u00a0V (at 10\u00a0mA\u00a0cm\u22122 current density) and value of Tafel slope of 90\u00a0mV dec\u22121 in comparison to porous N-doped graphitic carbon powder and pristine CoTe2. The presence of N-doped graphitic carbon matrix support provides an interaction with the confined CoTe2 nanocrystals to enhance OER in addition to offering fully accessible active sites and better electrical conductivity. A mesoporous carbon material doped with nitrogen has previously been utilized by Hu\u2019s group along with cobalt oxide NPs enclosed in graphitic layers as a promising non-noble metal oxygen electrode catalyst. [217] A series of catalysts were produced through a facile one-pot synthesis technique that involved polymerization, centrifugation washing, and pyrolysis. Several bifunctional catalysts were developed that possess incredible performance through the adjustment of the carbonization temperatures. Analysis performed on the optimal and as-produced Co-N/C 800 (800\u00a0\u00b0C carbonization temperature) catalyst showed that the catalyst presented a small reversible \u03b7 value of 0.96\u00a0V between OER and ORR. This value recorded is even greater than those offered by 20\u00a0wt% Pt/C (0.270\u00a0V), RuO2 (0.390\u00a0V), and IrO2 (0.460\u00a0V) catalysts, and indicates that the catalyst can act as a top performance non-noble metal bifunctional catalyst for reactions involving reversible oxygen electrode. A facile soft-template mediated technique was employed by Shen\u2019s research team that assisted in fabricating nanostructured Co-Fe double sulphides that are covalently enclosed within N-doped mesoporous graphitic carbon (Co0.5Fe0.5S@N-MC). [218] Characterization methods such as X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction were conducted during the study to unravel the connection between the structural characteristics and the composite\u2019s catalytic behavior. Based on the analysis, there was a moderate substitution as well as a fine distribution of Fe in bimetallic sulfide composites that were suspected to generate a beneficial influence on both the activation and adsorption of species containing oxygen. As a result, a unique catalyst with enhanced performance towards OER and ORR was produced that is far better than the monometallic counterparts. In addition, a covalent bridge exists between the mesoporous carbon shells and the active sulfide particles that create easy pathways for the transport of mass and electron. The features possessed by the Co0.5Fe0.5S@N-MC catalyst resulted in an early onset potential value of around 1.57\u00a0V and 10\u00a0mA\u00a0cm\u22122 current density at a low \u03b7 of 0.41\u00a0V. A relatively lower Tafel slope value of 159\u00a0mV dec\u22121 than IrO2 (267\u00a0mV dec\u22121) was also observed. Wang\u2019s group also described the use of mesoporous carbon to produce electrocatalysts that have the bifunctional ability in the form of Co/Co3O4/Co(OH)2/N-doped mesoporous carbon (Co-NC) through a one-pot synthesis process. [219] The remarkably superior Co-NC catalyst synthesized was Co-NC 750 (750\u00a0\u00b0C carbonization temperature) that had the highest quantities of pyridinic nitrogen as well as an optimized ratio of three cobalt species. The advantage that was achieved from the presence of the strong enclosure influence between N-graphitic shell of Co-NC and Co/Co3O4/Co(OH)2 core included a reduced reversible overvoltage value of 1.02\u00a0V between both OER and ORR in an alkaline medium. Table 1\n presents some of the important parameters related to the carbon-supported cobalt catalysts for OER in a conveniently accessible manner.Various other forms of carbon supports have also been combined with cobalt-based materials to create stable and active electrocatalysts for OER. Carbon supports that include carbon cloth, carbon nanodiamond, and carbon black were some of the unique carbon-based materials that have shown improvement in reactions involving oxygen evolution. The following sections will provide reviews on some of the contemporary research works incorporating the special type of carbon supports for OER.Wang and co-workers made use of carbon cloth as support for cobalt phosphide nanoarrays that could efficiently catalyze OER and hydrogen evolution reaction (HER) in basic media. [220] An overall potential of 1.61\u20131.63\u00a0V (\u03b7overall\u00a0=\u00a00.380\u20130.400\u00a0V) was necessary for water splitting in a 2-electrode configuration at a current density of 10\u00a0mA\u00a0cm\u22122 over 72\u00a0h. It was found that a layer of CoOx, the active species, covered the CoP catalyst surface during electrolysis, but the improved activity was mostly due to the presence of CoP core and the nanoarray morphology. 10\u00a0mA\u00a0cm\u22122 current density was obtained at an \u03b7 value of 0.281\u00a0V when the synthesized catalyst was utilized for OER. The development of a bifunctional electrocatalyst for OER and HER using Ni promoted Co disulfide nanowire array and carbon cloth support (Ni2.3%-CoS2/CC) was performed in the past by Fang et al.\n[221] A simple hydrothermal method was used for the preparation of Ni2.3%-CoS2/CC that allows carbon cloth to be uniformly coated with Ni2.3%-CoS2 nanowires of 50 to 100\u00a0nm in diameter and length of several micrometers. The OER activity of Ni2.3%-CoS2/CC was studied in a basic 1\u00a0M KOH solution and 0.270\u00a0V overpotential was needed to obtain 10\u00a0mA\u00a0cm\u22122 current density. This value is far more superior than previously reported catalysts that include CoMn LDH (0.324\u00a0V), Co-P (0.345\u00a0V), NiCo LDH (0.367\u00a0V), and Ni-doped Co3O4 (0.530\u00a0V) [131,228\u2013230] In addition, a Tafel slope of 119\u00a0mV dec\u22121 was recorded and Ni2.3%-CoS2/CC retained 91% of its current density after 12\u00a0h of fixed overpotential electrolysis. Another example of electrocatalyst previously synthesized that implemented carbon support in the form of carbon cloth was Co(OH)2@Ni(OH)2/CC which was produced by Wang\u2019s research team. [222] In comparison to Ni(OH)2/CC, Co(OH)2/CC, and commercial RuO2 catalyst, the newly synthesized hybrid catalyst demonstrated a relatively better OER performance by showing approximately 0.330\u00a0V overpotential at 10\u00a0mA\u00a0cm\u22122 current density. Furthermore, the catalyst exhibited prolonged durability even after 10\u00a0h of operation. These enhanced features could be accredited to the distinctive 3-D hierarchical core\u2013shell system present as well as the synergistic influence between Ni(OH)2 and Co(OH)2. Wang and co-workers supported cobalt carbonate hydroxide (CCH), a cobalt-based mineral salt, on carbon black to form a resultant catalyst, indicated as CCH/C that can catalyze OER, as well as ORR.223 Investigations on phase-dependent electrochemical characteristics performed during the research work, showed that extending the time of hydrothermal reaction can considerably modify the CCH\u2019s crystalline phase in CCH/C. This alteration could further influence the activity of the catalyst towards both ORR and OER. Two types of catalyst, denoted as CCH-2/C and CCH-16/C, were generated by applying thermal treatment at 170\u00a0\u00b0C for 2\u00a0h and 16\u00a0h respectively. Excellent activity and stability were observed for CCH/C in an alkaline media for ORR in comparison to a Pt/C catalyst (Vulcan XC-72 supporting 40\u00a0wt% platinum) available commercially. With regards to OER, a small \u03b7 of 0.509\u00a0V was recorded for CCH-2/C to obtain 10\u00a0mA\u00a0cm\u22122 current density. This value is less positive than that of Pt/C and it is 0.065\u00a0V less active than that found for Ir/C catalyst. These findings provided an evidence that CCH-2/C could be a promising catalyst when utilized as a cathode material for OER. Fan\u2019s research team produced a composite catalyst of Co-OBA/C (OBA\u00a0=\u00a04,4\u2032-Oxybis (benzoic acid)) involving carbon black. [224] The synthesis process included an integration of a metal\u2013organic framework of Co-OBA with black carbon through a hydrothermal process. The composite was evaluated for ORR and OER using linear sweep voltammetry (LSV) in an alkaline medium, and results indicate a potential of 0.553\u00a0V for Co-OBA/C at 10\u00a0mA\u00a0cm\u22122, relatively smaller than the value obtained from Co-OBA (0.758\u00a0V) and Co-OBA\u00a0+\u00a0C (0.691\u00a0V). In addition, the Tafel slope of Co-OBA/C was the lowest, with a value of 85.7\u00a0mV dec\u22121, when compared to Co-OBA (110.9\u00a0mV dec\u22121) and carbon black (178.4\u00a0mV dec\u22121).A special form of carbon support in the form of nanodiamond (ND) was used by Wu et al. to synthesize a Co-embedded nitrogen-doped graphitized carbon shell that covers an ND core (CoNC/ND). [231] The final catalyst synthesized had a bifunctional property that can improve both ORR and OER. CoNC/ND showed an onset potential of 1.285\u00a0V (vs. RHE) for OER and better durability relative to CoNC catalyst obtained from carbon black. The synergistic effect of the Co-N moieties in the carbon shell is expected to have helped improve the catalytic performance while the ND core plays a critical part in maintaining the high stability of CoNC/ND catalyst. In addition to the electrocatalysts described earlier, various research teams have made use of other forms of carbon supports in their studies, and some of these include CoTPP/C [232], NiCoP/C [225], Co2P@NPC [226], and CS-Co/Cs. [227] Moreover these discussed catalysts, there were more advancement in the Co-based catalyst and the electrochemical parameters were represented in Table 2\n.Water splitting is one of the most effective and green way to easily convert sustainable energies (solar, wind, and blue energies) into useful high purity fuels (H2 and O2). Among the two half-reactions of water splitting (HER and OER), water oxidation reaction (OER) is a kinetically sluggish reaction which hinders the easy conversion of water into H2 and O2, and considered a bottleneck for large scale applications. So, it is vital to develop a potent catalyst that can demonstrate both prolonged stability and low overpotential, which can further improve the OER activity and display a better overall faradaic efficiency. However, there are some challenges that we must first overcome to create such an effective electrocatalyst:The atomic rearrangement and reaction mechanism is still not well understood due to rapid changes and multiple possibilities between the steps of the OER process. Without understanding the mechanism, we cannot predict the rate-determining step (RDS) of a reaction, and without knowing the RDS we cannot pinpoint the phenomenon regulating the activity for the catalyst, which makes it difficult to further improve it. In the case of multi-metal compound catalysts, the exact recognition of catalytically active sites is important to improve the OER activity. Because of the rapid transformation in OER process, chemical changes and restructuring of the catalyst is difficult to detect and requires advanced tools e.g. combination of in-situ spectrometric methods, electrochemical techniques, microscopy techniques and theoretical calculations for finding out the critical factors affecting a reaction and precisely determining the catalyst active sites.Most of the precious metal-based catalysts for OER (such as Ir- and Ru-based) work efficiently under acidic medium, whereas transition metal-based catalysts work best under alkaline medium. We have only very few catalysts that show excellent behavior within a broad pH range. The search for a versatile catalyst working in a wide range of industrial electrolysis conditions (strongly acidic to strongly basic conditions) is underway. Perhaps, even a better option is to have a catalyst that operates best in the neutral media to avoid corrosion issues and increase the durability of the catalyst compared to alkaline and acidic condition systems.The use of carbon-based support (e.g. carbon black) has a possibility of carbon corrosion and electrochemical oxidation that produces CO or CO2 at high potentials for long-time use. This effect hinders the performance of the catalyst which is due to a reduction in the reactive surface area as well as the dissolution of the support into the electrolyte. Dilution of the doped material into the electrolyte can also disturb the accurate measurement of the activity.In the case of soluble active catalysts, binders are needed to immobilize the catalyst onto the solid surfaces (e.g. GC). The polymer binders hinder effective charge transfer between the catalyst and electrolyte and disrupt the gas permeation from/to the catalyst. So a need exists to convert such catalysts into electrode materials by efficient and economical methods for grafting them onto a solid surface without the use of any binders.The experimental results obtained and the DFT results vary often because the practical catalysts do not comprise of a perfect single surface as the theoretical model frequently use. DFT helps in finding the active sites of a catalyst. As the catalysts get complicated (multimetal doping or addition of support), it is more difficult to identify the exact active site of the catalyst using computational tools.A good electrocatalyst should possess good electrical conductivity, large number of active sites, and resistance to corrosion under high anodic potential. The concept of using different metals to meet individual functional requirements in extended lattices may be useful in constructing catalysts that can follow and satisfy multiple criteria simultaneously. Integrating these high-performance catalysts on to solar cells for fuel production is admired by many as they offer clean and sustainable solutions for energy requirements. Formulation of a bifunctional or trifunctional catalyst that can work in all media is still a dream to be achieved. Creative modeling and production of exclusive nanostructures to enhance the performance for OER is possible with advanced facilities. A catalyst design approach is required following systemic steps based on descriptors rather than the trial-and-error method to create good catalysts. Effective studies should focus on a comprehensive evaluation of reaction mechanism, online monitoring of chemical changes, analysis of structural transformation by applying operando, and in-situ techniques to decipher the catalyst structure that transitions into active sites during the reaction conditions. This knowledge with advanced synthesis tools could help in designing versatile catalysts that can perform outstandingly under varying industrial conditions.World energy consumption has been increasing at a drastic rate. The energy produced from the burning of the limited fossil fuels is not sustainable and affects the environment adversely. So, the need for an energy transition towards a more sustainable and renewable form is of paramount importance for our future generations. Electrolysis of water using renewable energies has received much attention due to its clean method to produce chemical fuels that have the capability to substitute the existing carbon-emitting fossil fuels. In this review, we discussed various cobalt-based electrocatalysts for the OER. It has the capacity for large scale applications by replacing the precious-metal-based catalysts that are scarce and costly. We have discussed cobalt in presence of noble metal (Ir, Ru, Au, and Ag) as catalysts with high activity towards OER in acidic medium with a low amount of noble metals present in the electrocatalyst. Largely the review focuses on catalysts where cobalt is present with other transition metal (such Fe, Ni, Co, and Mn) in a bimetallic or tri-metallic form, which show outstanding OER activity in alkaline media. Furthermore, we have discussed some of the cobalt-based perovskite oxides with partial doping of A-site and B-site with other elements and anion substitution, which aided in the high activity of the perovskite catalyst. As support, we have included the effect of carbon-based compounds that enhance the OER activity in presence of cobalt catalysts. The review also includes a discussion on the mechanism of OER along with comparing the performance of OER catalysts with the help of measurement standards like the overpotential and Tafel slope.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 NPRP grant (NPRP8-145-2-066) from the Qatar National Research Fund (a member of the Qatar Foundation). The statements made herein are solely the responsibility of the authors. The author(s) would also like to acknowledge the support from Qatar University's internal grant QUCG-CENG-19/20-7.", "descript": "\n                  The future of the world energy lies in clean and renewable energy sources. Many technologies, such as solar cells, wind turbines, etc., have been developed to harness renewable energies in different forms of fuel. Amongst them, electrolysis of water to produce oxygen and hydrogen is one of the paramount developments towards achieving clean energy, which has attained significant attention due to its green and simple method for the production of fuels. In electrolysis of water, the half-reaction containing the oxygen evolution reaction (OER) is a reaction that is kinetically sluggish, which requires higher overpotential to produce O2, when compared to the other half-reaction, i.e. hydrogen evolution reaction (HER). Many electrocatalysts are studied extensively to be used in the OER process to get an economical yield out of it. Noble metal-based catalysts are the state-of-the-art catalyst used for OER currently. But due to their high cost and scarcity, they cannot be applied in a large-scale manner to be used in the future. The non-noble metals (transition metals and perovskites) are gaining interest by exhibiting on par or better OER performance compared to the noble metal used. Due to their low cost, ample resources, and several metals available, they have opened up a variety of areas with a different combination of metals to be used as a catalyst for OER. Amongst these metals, cobalt has received massive appreciation for performing as an excellent OER catalyst. Multi metals, multimetal mixed oxides, multimetal phosphides, perovskites, and carbon-supported catalysts containing cobalt have shown low overpotential with high long-term stability. Therefore, in this review, we go through different cobalt-based electrocatalysts for OER, the general mechanism governing the OER process, the challenges that we are facing today to enhance the catalytic performance, and future aspects to overcome such challenges.\n               "}
{"full_text": "The hydrogenation of aromatic rings has been widely concerned due to its valuable applications in industry [1\u20133]. The hydrogenation of toluene to methylcyclohexane (MCH) is an important way of hydrogen storage [4\u20137]. The saturation of toxic aromatic compounds is desired because they are the main source of air pollutions [8\u201310]. Benzene [11,12] and toluene were usually used as probe molecules to investigate the hydrogenation of aromatic rings, in which toluene is of relatively low toxicity [13]. Supported Pd and Ru catalysts have been widely studied for the hydrogenation of aromatic rings [14\u201316].It is known that solvents affect the catalytic activities due to different solvent polarity, H2 solubility, hydrogen transfer ability, solvent-reactant interactions and solvent-catalyst interactions. Up to now, quite a few studies were published concerning the effects of hydrogen solubility and solvent polarity on the hydrogenation of aromatic rings [17\u201319]. However, the interactions among reactants, solvents and catalyst surfaces are complicated and to understand such interactions needs massive efforts of studies. In fact, such studies were relatively few.In this paper, we present a preliminary study on how the pre-adsorbed solvents affected the strengths of adsorption of reactants on the catalysts and thus changed the activity for the hydrogenation of toluene. Specifically, the Pd/SiO2 and Ru/SiO2 (5%wt) were prepared for the hydrogenation of toluene in n-hexane, isopropanol (IPA), tetrahydrofuran (THF) and methanol. The microcalorimetric adsorption was employed to measure the interactions of solvents with the catalysts, as well as their effects on the adsorption of toluene. The hydrogen transfer from IPA to toluene on the surfaces was observed by IR, which accounted for the promotion effect of IPA for the hydrogenation of toluene over the supported Pd and Ru catalysts.The preparation, characterization and catalytic tests of catalysts were described in detail in the Supporting Information (SI).A calculation proved that the reaction rates were not affected by the mass transport limitation (see Tables S2 and S3 in SI).The physical properties of the support and supported catalysts were studied previously [15]. The surface areas of the SiO2, Pd/SiO2 and Ru/SiO2 were 877, 634 and 821 m2/g, with the average pore sizes of 4.5, 3.4 and 4.1\u00a0nm, respectively. The averaged metal particle sizes were estimated to be approximately 2.1 and 6.5\u00a0nm, respectively, in the Pd/SiO2 and Ru/SiO2, by TEM images. In addition, the H2 uptakes were about 204 and 21\u00a0\u03bcmol/g for the Pd/SiO2 and Ru/SiO2, respectively, as measured by the microcalorimetric adsorption of H2. The physical properties of the samples were described in detail in SI.The conversion and turnover frequencies (TOF) of toluene at different weight hourly space velocities (WHSV) on the Ru/SiO2 and Pd/SiO2 in n-hexane, IPA, THF and methanol are presented in Fig. 1\n. MCH was the only product detected (Fig. S4). The conversion of toluene decreased with the increase of WHSV. The conversion of toluene was different in different solvents over the catalysts. The results indicated that the activities for the reaction in the solvents followed the order of n-hexane> IPA\u00a0>\u00a0THF\u00a0>\u00a0methanol over the two catalysts, revealing that aromatic rings were easier to be hydrogenated in n-hexane and IPA than in THF and methanol. In addition, the conversions of toluene were higher on the Ru/SiO2 than on the Pd/SiO2 in each of these solvents, demonstrating that Ru was more active than Pd for the hydrogenation of aromatic rings.The turnover frequency (TOF) of toluene was calculated according to the number of converted toluene molecules per second divided by the number of surface active metal sites determined by the microcalorimetric adsorption of H2 (considering that the low H2 pressures might minimize the extents of absorption of H2 in Pd [20]). The results are given in Fig. 1. The conversion of toluene decreased, while the TOF of toluene increased with WHSV, until the constant values were reached, which represented the intrinsic activity of catalysts for the reaction [21\u201323]. It was meaningless to compare the TOF values for the Pd/SiO2 and Ru/SiO2 owing to the absorption of H2 in Pd. However, it was meaningful to compare the TOF values of the same catalyst for the hydrogenation of toluene in different solvents. The maximum TOF values of toluene were about 0.062, 0.029, 0.010 and 0.002\u00a0s\u22121 on the Pd/SiO2 in n-hexane, IPA, THF and methanol, respectively, while they were about 1.9, 1.6, 0.28 and 0.037\u00a0s\u22121, respectively, on the Ru/SiO2. The intrinsic activity with the solvents followed the order of n-hexane> IPA\u00a0>\u00a0THF\u00a0>\u00a0methanol on the Pd/SiO2, while the order was n-hexane\u2248 IPA>\u00a0>\u00a0THF\u00a0>\u00a0methanol on the Ru/SiO2.The dielectric constants are 1.88, 19.9, 7.58 and 32.7 (Table S4) for n-hexene, IPA, THF and methanol, respectively. No clear relationship was found for the catalytic activity and dielectric constants of solvents, consistent with the results reported [17,24].IPA, THF and methanol are hydrophilic while n-hexane is hydrophobic. SiO2 is also hydrophilic so that it adsorbed n-hexane weakly, which might be a reason why it was a good solvent for the hydrogenation of toluene over the Pd/SiO2 and Ru/SiO2 catalysts.Previously, we found that the hydrogenation of diisopropylimine was significantly promoted by IPA on the Ni catalysts due to the HTR (hydrogenation transfer reaction) between diisopropylimine and IPA [25]. A similar effect must occur here with IPA as a solvent. To confirm the effect, toluene and IPA were co-fed onto the Pd/SiO2 and Ru/SiO2 under N2 atmosphere. The results are given in Table S5. The data showed that some IPA was dehydrogenated to acetone while some toluene was simultaneously hydrogenated to MCH. The equivalent molar ratio for the dehydrogenation of IPA and the hydrogenation of toluene was 3/1. The ratio was found to be 3.3 on the Pd/SiO2 and 5.0 on the Ru/SiO2, indicating that not all the H atoms dehydrogenated from IPA were used to hydrogenate toluene (they might be also combined to H2). With such effect, IPA promoted the activity for the hydrogenation of toluene, as compared to the solvents THF and methanol. In addition, the HTR effect was stronger on the Ru/SiO2 than on the Pd/SiO2, owing to the higher activity for the dehydrogenation of IPA to acetone on the Ru/SiO2 than on the Pd/SiO2.Fig. S5 (A) shows the results for the microcalorimetric adsorption of solvents on the SiO2. The initial heats for the adsorption of n-hexane, IPA, THF and methanol on the SiO2 were 46, 70, 79 and 59\u00a0kJ/mol, respectively, with the coverages of about 585, 2379, 2412 and 2635\u00a0\u03bcmol/g. Thus, the interaction was weak between SiO2 and n-hexane, while that was quite strong between SiO2 and IPA or THF. The initial heat for the adsorption of methanol was significantly higher than that of n-hexane, but lower than those of IPA and THF. In addition, the heats for the adsorption of methanol decreased significantly slower with coverage than those of IPA and THF, probably owing to that the methanol molecule is smaller than those of IPA and THF so that more molecules of methanol could be adsorbed on the same surface area of SiO2.\nFig. 2\n shows the adsorption of solvents on the Ru/SiO2. The initial heats for the adsorption of n-hexane, IPA, THF and methanol on the Ru/SiO2 were 52, 74, 82 and 66\u00a0kJ/mol, respectively, with the coverages of about 637, 2933, 2441 and 2591\u00a0\u03bcmol/g, while those on the Pd/SiO2 were 80, 90, 95 and 76\u00a0kJ/mol, respectively, with the coverages of 598, 2578, 2170 and 2702\u00a0\u03bcmol/g (see Fig. S5 (B)). Thus, the adsorption heats of these solvents were higher on the metals than on the SiO2, indicating the stronger interactions of solvents with the metals than with the support. In addition, the adsorption heats of these solvents were significantly higher on Pd than on Ru, revealing the weaker interactions of solvents with Ru than with Pd, which might be one of the reasons why the Ru/SiO2 was less affected than Pd/SiO2 by the solvents and why the Ru/SiO2 was more active than Pd/SiO2 for the hydrogenation of toluene in these solvents.The coverages of each of the following solvents n-hexane, THF and methanol on the catalysts were close to those on the support, while the coverages of IPA on the catalysts (2933 and 2578\u00a0\u03bcmol/g) were significantly higher than that on the support (2379\u00a0\u03bcmol/g), indicating the dehydrogenation of IPA on the catalysts.The adsorption heats of solvents could be correlated with the conversion of toluene. Specifically, the weaker adsorption of solvents favored the conversion of toluene. For example, n-hexane was weakly adsorbed and the conversion of toluene was high on the catalysts with n-hexane as a solvent, while THF and methanol were quite strongly adsorbed on the catalysts and inhibited the conversion of toluene. However, this rule was not applicable to IPA as a solvent, owing to the HTR effect which promoted the conversion of toluene in another way, although IPA was more strongly adsorbed than methanol on the catalysts.\nFig. 3\n shows the adsorption heats and coverages of toluene over the Ru/SiO2 before and after the pre-adsorption of solvents. The initial heats of toluene on the clean SiO2, Pd/SiO2 and Ru/SiO2 were 72, 77 and 85\u00a0kJ/mol, respectively, with the coverages of 2521, 2502 and 2569\u00a0\u03bcmol/g [15] (Table S6), indicating that toluene adsorbed more strongly on metals than on SiO2 and more weakly on Pd than on Ru which could explain the higher conversion of toluene on Ru than on Pd.After the pre-adsorption of n-hexane, IPA, THF and methanol, the initial heats of toluene on the Ru/SiO2 decreased by 8, 13, 21 and 22%, respectively, with the decreases of coverages by 23, 18, 24 and 29%. Similar results were obtained on the Pd/SiO2 (Fig. S6), indicating that the pre-adsorbed solvents inhibited the adsorption of toluene on Pd and Ru. After the pre-adsorption of solvents, the adsorption heats of toluene on the Pd/SiO2 and Ru/SiO2 catalysts followed the order of n-hexane> IPA\u00a0>\u00a0THF\u00a0>\u00a0methanol (Table S6), in consistence with their activity orders for the hydrogenation of toluene in these solvents. The results demonstrated that the strength of adsorption of toluene with the solvents on the catalysts was a key factor affecting the activity for the hydrogenation of toluene. In addition, toluene was more strongly adsorbed on the Ru/SiO2 than on the Pd/SiO2 with and without the solvents, which might be an important reason why the Ru/SiO2 was significantly more active than the Pd/SiO2 for the hydrogenation of toluene.Although the adsorption heat of THF was higher than that of methanol, the coverage of methanol was significantly higher than that of THF, on the catalysts, leading to the more decreased coverage of toluene and thus the more decreased activity in methanol than in THF on the catalysts for the hydrogenation of toluene (Scheme S1).\nFig. 4\n shows the IR spectra of adsorbed IPA on the support and catalysts. For the adsorbed IPA on SiO2, the bands at 2978, 2937 and 2893\u00a0cm\u22121 were assigned to the vibrations of \u03bd(-CH3) in IPA, while the bands at 1466 and 1387\u00a0cm\u22121 were attributed to the vibrations of \u03b4as(-CH3) and \u03b4s(-CH3) [26\u201329]. The band at 1345\u00a0cm\u22121 belonged to the vibration of \u03b4(\u03b1-C-H) [27,30]. Since O atoms in IPA may interact strongly with surface cations, we suggested the surface structures of adsorbed IPA on SiO2 as in Scheme S2 (a) and (b).The IR spectra of IPA adsorbed on the Pd/SiO2 and Ru/SiO2 were mostly similar to that on SiO2, except for the two newly appeared bands at 1692 and 1292\u00a0cm\u22121. Thus, the same surface structures might be formed for the adsorption of IPA on the metals as on SiO2 (Scheme S2 (c) and (d)). The new bands at 1692 and 1292\u00a0cm\u22121 could be assigned to the vibrations of \u03bd(C=O) [29,31] and \u03bd(C-C-C) in acetone [32,33], respectively, indicating the formation of surface acetone (Scheme S2 (e)) resulted from the dehydrogenation of IPA on the metals. It should be mentioned that the intensity of the band at 1692\u00a0cm\u22121 was significantly stronger on the Ru/SiO2 than on the Pd/SiO2, indicating that the dehydrogenation of IPA was easier on Ru than on Pd.\nFig. 5\n shows the FTIR spectra of co-adsorbed IPA and toluene on the Ru/SiO2, as well as those of adsorbed toluene and IPA only on the Ru/SiO2 for comparison. The IR spectra for the co-adsorbed IPA and toluene on the Pd/SiO2 were not shown since they were similar to those on the Ru/SiO2.It is seen that the bands of adsorbed toluene did not change with the pre-adsorbed IPA, but the intensities of bands of the skeletal vibrations of aromatic rings (1603 and 1496\u00a0cm\u22121) [15,34,35] were weakened, indicating that the pre-adsorbed IPA did not change the surface structure of adsorbed toluene, but reduced the amount of toluene adsorbed on Ru.The intensities of the characteristic bands at 1603 and 1496\u00a0cm\u22121 in the spectra of IPA/toluene almost disappeared. Meanwhile, the bands belonging to \u03bd(-CH3) in IPA at 2978, 2937 and 2893\u00a0cm\u22121 were weakened while the band for \u03bd(-C=O) at 1692\u00a0cm\u22121 (surface acetone) was slightly enhanced. These results suggested the surface reactions occurred between adsorbed toluene and IPA which might be schematically expressed in Scheme 1\n.Adsorption heats are the good measures of interactions of solvents with catalysts. The weaker adsorption of a solvent resulted in the stronger interaction of toluene with the catalysts. The strength of adsorption of toluene with the solvents on the catalysts was a key factor affecting the activity for the hydrogenation of toluene. Normal hexane was a good solvent for the hydrogenation of toluene on the Pd/SiO2 and Ru/SiO2 since it adsorbed weakly on these catalysts.The solvents studied adsorbed more weakly on Ru than on Pd so that toluene was more strongly adsorbed on Ru than on Pd with pre-adsorbed solvents, which might be a reason why Ru was more active than Pd here.Although IPA adsorbed strongly on the metals and inhibited the adsorption of toluene significantly, it promoted the activity for the hydrogenation of toluene by the HTR effect. Such effect was more profound on Ru than on Pd, leading IPA to be a good solvent (as good as n-hexane) for the hydrogenation of toluene on Ru.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.2021.106330.", "descript": "\n                  The effects of solvents (n-hexane, isopropanol (IPA), tetrahydrofuran (THF) and methanol) on the hydrogenation of toluene over the Pd/SiO2 and Ru/SiO2 catalysts were studied. Microcalorimetric adsorption and IR spectroscopy were employed to understand the effects. It was found that n-hexane adsorbed weakly on the catalysts and thus affected less the hydrogenation of toluene, while THF and methanol adsorbed strongly on the catalysts and inhibited the activity of hydrogenation of toluene significantly. IPA also adsorbed strongly on the catalysts, but it exhibited a hydrogen transfer effect on the surfaces that promoted the conversion of toluene.\n               "}
{"full_text": "The need to diminish fossil fuel dependence in the chemical industry has led to the intensification of bio-based alternative products. Bio-based chemicals, products that are derived from biomass and other biological materials, are pivotal to the development of a more innovative and low-emissions economy while ensuring biodiversity and environmental protection [1]. The use of bio-based chemicals have raised as an alternative to standard chemicals since their environmental footprint is limited compared to their traditional counterparts. From a technical point of view, almost all the fuels and chemicals normally produced from fossil feedstocks can be produced from biomass in the so-called biorefinery [2]. Thus, bio-chemicals could drive a new economy model based on more thoughtful uses of natural resources. Hence, the European Commission aims to accelerate the market uptake of bio-chemicals, and estimates an annual growth rate of around 3.6% per year from 2018 to 2025, what represents an important expanding sector [1]. At present, researchers and entrepreneurs are involved in the development and commercialization not only of biofuels, but also of bio-derived replacements for basic chemicals. The path to a more sustainable industry will inevitably depend on the development of new high value-added chemicals, via catalytic process, from renewable sources [3].A very attractive platform chemical from biomass is glycerol, which was identified as a top 12 bio-based chemical by the US Department of Energy [4]. Production of chemicals from raw glycerol is a good alternative to absorb the glycerol surplus generated from biodiesel industries. Several products can be obtained from glycerol, either gaseous products (i.e. hydrogen, syngas, alkanes, etc.), as well as high value-added liquid products (i.e. hydroxyacetone, pyruvaldehyde, acrolein, propanal, lactic acid, glyceric acid, acetaldehyde, etc.) [5\u201310].Another valuable chemical that can be produced from hydrogenolysis of glycerol is 1,2-propanediol. It is widely used in the synthesis of unsaturated polyester resins, as coolant and also in pharmaceutical, food, and cosmetics formulations [11]. Currently, 1,2-propanediol is produced through the hydration of propylene oxide, which in turn comes from the selective oxidation of propylene, a fossil resource [12].Conversion of glycerol to 1,2-propanediol involves the removal of an oxygen atom by the addition of hydrogen (C-O bond cleavage with simultaneous hydrogen addition), a catalytic reaction known as hydrodeoxygenation (HDO). The conventional way to carry out this process requires an external H2 supply, usually derived from fossil fuels, which increases the cost and can generate safety issues due to the high diffusivity and flammability of molecular hydrogen [13]. In addition, hydrogen can block metal centers causing an inhibitory effect on catalytic activity [14]. Other explored alternative is the catalytic transfer hydrogenation (CTH) by using a hydrogen donor molecule such as methanol, 2-propanol, or 2-butanol [15]. Nevertheless, the formation of by-products and the cost of acquisition and handling of the hydrogen donor can significantly increase process costs [13]. A suitable alternative to circumvent the use of external and non-renewable sources of hydrogen is to carry out glycerol HDO with in-situ produced H2 via aqueous-phase reforming (APR) of the substrate, in a one-pot reaction.The present investigation aims to develop a catalytic system that could drive the in-situ generation of hydrogen and its immediate consumption, in the reaction medium, in the HDO reaction. Therefore, a metal-acid bifunctional catalyst is required. In a simple way, the acid function is pivotal for the dehydration reaction whereas the metal function catalyses the hydrogenation reaction [16]. \nScheme 1 shows the widely accepted reaction path for the glycerol HDO in liquid phase.Non-noble metals are less expensive than noble metals and present a greater selectivity towards C-O bond cleavage, and therefore, they are more efficient and economical for 1,2-propanediol production [17]. Transition metals and, more specifically, cobalt-based catalysts, have proved effective in both the production of hydrogen by aqueous-phase reforming (APR) and for glycerol HDO reaction depending on metal\u2013acid/base characteristics [18\u201320].Promoters can increase the metal-support interaction, the dispersion of the active metal and can also tune the acid/base characteristics of the catalyst [21]. The addition of a promoter is an economical and time-efficient strategy that can enhance activity [22]. Cerium has been used by several researchers to improve catalyst activity and stability in liquid phase reactions. Its positive effect has been mainly attributed to the existence of oxygen vacancies in the cerium oxide (CeO2) lattice, which may promote WGS reaction by the activation of the H2O molecule [23\u201326]. In addition, its high oxygen mobility can improve hydrothermal stability and avoid the formation of coke precursors [27,28].In our previous work [29], cobalt-based catalysts were synthesized from cobalt aluminate spinels with different Co/Al molar ratio. These catalysts showed remarkable glycerol conversion and considerable efficiency for C-O cleavage where catalyst with CoAl =\u20090.625 molar ratio exhibited the most promising performance. Based on that result and known the notable catalytic properties of cerium, in the present work, Ce-modified cobalt aluminate (Co/Al molar ratio = 0.625) catalysts are investigated for the glycerol HDO. In this way, we seek to modify the physico-chemical properties of the pristine catalyst to produce value-added liquid products from glycerol HDO. As far as we know, this type of one-pot synthesized catalysts have not yet been studied in glycerol HDO. The catalytic experiments were carried out in a continuous reactor, where hydrogen was in-situ generated via APR. The synthesized catalysts were thoroughly characterized by several techniques, either in the fresh and reduced form. The various physicochemical properties were correlated with the catalytic performance. In addition, exhausted catalysts were characterized to identify the main deactivation causes.A series of catalysts based on cobalt aluminate (nominal Co/Al molar ratio = 0.625) doped with cerium were one-pot synthesized by coprecipitation method. In a typical synthesis, an aqueous solution containing Co(NO3)2.6\u2009H2O (99.999% trace metal basis, Sigma Aldrich), Al(NO3)3.9\u2009H2O (98% trace metal basis, Fluka) and Ce(NO3)3.6\u2009H2O (99.999% trace metal basis, Sigma Aldrich) was dropwise added into a beaker containing an aqueous solution of Na2CO3 (99.8%, Fluka), under stirring. The synthesis was carried out at room temperature at pH 10, adjusted with NaOH solution (2\u2009M). The resulting suspension was aged at room temperature for 24\u2009h, filtered, washed several times with de-ionized water, dried overnight at 110\u2009\u00b0C in an oven and calcined in a muffle furnace at 500\u2009\u00b0C for 5\u2009h (heating at 5\u2009\u00b0C/min), in static air atmosphere. The obtained solids were abbreviated xCeCoAl (x: 0, 0.3, 2.1), where x denotes the weight percentage of Ce. As a reference, bare CeO2 and Co3O4 were also synthetized by precipitation, following the same protocol.Bulk chemical composition of the solids and metal leaching in catalytic runs were analysed by inductively coupled plasma optical emission spectroscopy (ICP-OES). Textural properties of the solids were obtained from nitrogen adsorption-desorption isotherms at 77\u2009K in a Micromeritics TRISTAR II 3020 equipment. Prior to the adsorption, the samples were outgassed at 300 \u00baC for 10\u2009h for removing moisture and adsorbed gases. The specific surface area and the main pore size were determined with the BET and BJH (desorption-branch) methods.XRD spectra were obtained in a PANalytical Xpert PRO diffractometer with CuK\u03b1 radiation (\u03bb\u2009=\u20091.5418\u2009\u00c5). X-ray diffracted radiation was recorded from (2\u03b8 values) 20\u201380\u00ba for the powder samples. The identification of the crystal phases was carried by comparison with International Centre of Diffraction Data (ICDD) database. Crystallite size was calculated from the X-ray line broadening analysis using Debye-Scherrer formalism. The lattice parameter (a) of cubic crystal structure was calculated by Eq. 1:\n\n(1)\n\n\na\n=\n\n\n\u03bb\n\n\n2\n\nsin\n\n\u03b8\n\n\n\n\n\n\nh\n\n2\n\n+\n\n\nk\n\n2\n\n+\n\n\nl\n\n2\n\n\n\n\n\n\nwere; \u03b8 is the diffraction angle; and h, k and l are the Miller indices.STEM images were obtained in a FEI Titan Cubed G2 60\u2013300 electron microscope with a high-brightness and a Super-X EDX system under HAADF detector for Z contrast imaging (camera length of 185\u2009mm). The samples were dispersed in ethanol and kept in an ultrasonic bath for 15\u2009min. Afterwards, a drop of suspension was spread onto a TEM copper grid (300 Mesh) covered by a holey carbon film and dryed under vacuum. Particle size distribution was obtained from the statistical analysis of at least 300 particles. The average size of the nanoparticles was calculated from volume to surface ratio (Eq. 2), using ImageJ software:\n\n(2)\n\n\n<\nd\n>\n=\n\n\n\u03a3\n\n\nn\n\n\ni\n\n\n\n\nd\n\n\ni\n\n\n3\n\n\n\n\n\u03a3\n\n\nn\n\n\ni\n\n\n\n\nd\n\n\ni\n\n\n2\n\n\n\n\n\n\n\nbeing di the diameter of ni particles.The oxidation state of the surface elements was analyzed by X-ray photoelectron spectroscopy (XPS) on a SPECS spectrometer with Phoibos 150 1DDLD device, using monochromatic Al K\u03b1 (1486.7\u2009eV) X-ray source with 30\u2009eV pass energy at 0.05\u2009eV steps. Samples, previously degassed, were introduced to the ultra-high vacuum analysis chamber (10-6 Pa) where the detailed analyses of the elements were performed (time 0.1\u2009s and step energy 30\u2009eV) with an exit angle of 90\u00b0. Samples were reduced in-situ, when required. The spectrometer was previously calibrated with Ag (Ag 3d5/2, 368.26\u2009eV). The BE were calibrated by taking C 1\u2009s peak (284.6\u2009eV) of adventitious carbon as reference. Peaks were deconvoluted after Shirley background subtraction, using a mixed Gaussian Lorentzian function (CASA XPS software).The UV\u2013vis\u2013NIR DRS spectra were recorded in a Cary 5000 equipment coupled to Diffuse Reflectance Internal (Varian). The reflectance data were converted into absorption by the Kubelka-Munk transformation. The Tauc plots were used to evaluate the difference in the energy of inner electron transitions of the solids. For the calculation, \n\n\n\n(\n\n\u03b1\nh\n\u03bd\n\n)\n\n2\n\nv\ns\n\nh\n\u03bd\n\nplot was used, where \u03b1 is the absorption coefficient, \u03bd is the frequency of light and h is the Planck\u2019s constant.\n27Al Solid State NMR measurements were performed on a 9.4\u2009T Bruker AVANCE III 400 spectrometer operating at resonance frequencies of 104.26\u2009MHz for 27Al. AlCl3 aqueous solution was used as a reference. The spectra were acquired at a spinning frequency of 60\u2009kHz employing a PH MASDVT400W BL 1.3\u2009mm ultrafast probe head. A single pulse of 0.3\u2009\u03bcs duration was applied (recycle delay 0.2\u2009s, 36,000 scans).The reducibility of the calcined samples was analysed by H2-TPR, carried out in a Micromeritics AutoChem 2920 apparatus. The solid was initially heated in He stream at 550\u2009\u00b0C for 1\u2009h to desorb impurities, and then cooled down to room temperature. Then, 5% H2-Ar flow was passed through the bed containing the sample while temperature was increased up to 950 \u00baC (heating rate 10 \u00baC/min) and hold for 1\u2009h. A cold trap was used to prevent water generated by reduction, and reactor exhaust was analysed by Thermal Conductivity Detector (TCD).Both the hydrogenation and dehydrogenation reactions require metallic function. The number of accessible metallic cobalt atoms was measured by H2 pulse chemisorption, carried out in a Micromeritics AutoChem 2920 apparatus, at 35 \u00baC. Samples were previously reduced at 600\u2009\u00b0C for 30\u2009min. A chemisorption stoichiometry HCo =\u20091/1 [30] and a cross-sectional area of 0.0662\u2009nm2/atCo were assumed [31]. Further information on the metallic function of the catalysts was obtained by measuring their activity in cyclohexane dehydrogenation, which will mainly depend on the metal accessibility [32]. The cyclohexane dehydrogenation was performed over 20\u2009mg of reduced catalysts, in a fixed-bed reactor at 250 \u00baC and atmospheric pressure, feeding a mixture of anhydrous cyclohexane and hydrogen (1:3000\u2009mol ratio). The gas product (benzene and cyclohexane) were online analysed by GC (column Al2O3-KCl, HP) coupled to a flame ionization detector (FID).The amount and strength of surface acid sites of the reduced catalysts were measured by means of NH3-TPD and isomerization of 3,3-dimethyl-but-1-ene (33DM1B) model reaction. For ammonia chemisorption/desorption experiments (Micromeritics AutoChem 2920 equipment) a series of 10% NH3-He pulses were introduced at 90 \u00baC, until saturation. Subsequently, the sample was exposed to He flow for 60\u2009min to remove reversibly bound NH3. Finally, the temperature was raised to 950\u2009\u00b0C (heating rate 5\u2009\u00b0C/min) with continuous ammonia monitoring. The total acidity was calculated from the integration of the pulses, and the strength of the acid sites was evaluated from the corresponding TPD profile. The model reaction of skeletal isomerization of 33DM1B was used to characterize the Br\u00f8nsted acid sites, since Lewis acid centers are not involved in this reaction [33]. The catalyst (100\u2009mg) was in-situ reduced, and cooled down to the reaction temperature (300\u2009\u00b0C) under inert flow. The 33DM1B partial pressure and flow rate were set at 20 kPa and 15.2\u2009mmol/h, respectively. The obtained products were online analysed by GC-FID on a RTx-1 (Restek) column.The characterization of the carbonaceous deposits in the spent catalysts was carried out by Raman spectroscopy (Renishaw InVia Raman spectrometer, Leica DMLM microscope) using 514\u2009nm laser. The power density of the laser beam was reduced in order to avoid the photo-decomposition of the samples. In order to improve the signal to noise ratio, 40\u2009s were used for each spectrum and 10 scans were accumulated at 10% of the maximum power of the 514\u2009nm laser, in the 1000\u20132000\u2009cm\u22121 spectral window.Carbonaceous deposits on spent catalysts were quantified by Temperature Programmed Hydrogenation (TPH) in a Setaram Setsys Evolution thermobalance coupled to a Mass Spectrometer (Pffeifer Vacuum OmniStar) following the evolution of m/z\u2009=\u200915 (CH4) signal. First, sample was cleaned under a He flow, at 550\u2009\u00b0C for 1\u2009h in order to remove absorbed organics. After cooled down to 40\u2009\u00b0C, 5%H2/Ar flow was passed through the sample heated at 10\u2009\u00b0C/min up to 900\u2009\u00b0C.Catalytic performance was evaluated in a bench-scale \ufb01xed-bed up-flow reactor (Microactivity Effi, PID Eng&TEch) with synthetic aqueous solution of glycerol (10\u2009wt% glycerol) at 260 \u00baC/50\u2009bar, operating at WHSV=\u200924.5\u2009h-1. About 0.5\u2009g of catalyst, particle size of 0.04\u20130.16\u2009mm, were mixed with deactivated quartz wool and in-situ reduced under 10% H2/He flow at 600 \u00baC and atmospheric pressure, for 1\u2009h. The reactor was pressurised with He and when the desired pressure was achieved, the He flow was switched to bypass, and the glycerol solution (0.2\u2009mL/min) was fed to the reactor while the temperature was progressively raised up to the reaction temperature, at 5 \u00baC/min. Catalytic performance was measured at 3\u2009h TOS (time on stream). Zero time was taken when reactants reached the catalyst bed, once the reaction temperature was reached. Gaseous and liquid phases were separated at 5 \u00baC in a Peltier device. The gaseous products were swept away with a He flow (40\u2009mL/min) applied immediately after the backpressure regulator. Product distribution was online analysed by a \u03bcGC (Agilent) equipped with four columns (Al2O3-KCl, PPQ and MS5A columns that used He as a carrier, and MS5A column which used Ar as a carrier). The liquid product was collected every hour in 2\u2009mL glass vials and off-line analysed by GC-FID (Agilent, 6890N) and HPLC-RI (Waters, Hi-Plex H column). The quantification of liquid compounds was performed by external calibration. Total organic carbon (TOC) in the liquid phase was measured on a Shimadzu TOC L apparatus. The carbon balance was 96%\u2009\u00b1\u20095 for all the experiments. After catalytic test, spent catalyst was recovered and characterized by a sort of techniques.The catalytic performance was calculated according to the following indices. The total glycerol conversion (XGly) was calculated as:\n\n(3)\n\n\n\n\nX\n\n\nG\nl\ny\n\n\n\n(\n%\n)\n\n=\n100\n\u00d7\n\n\n\n\nF\n\n\nG\nl\ny\n\n\ni\nn\n\n\n\u2212\n\n\nF\n\n\nG\nl\ny\n\n\no\nu\nt\n\n\n\n\n\n\nF\n\n\nG\nl\ny\n\n\ni\nn\n\n\n\n\n\n\n\nwhere \n\n\nF\n\n\nG\nl\ny\n\n\ni\nn\n\n\n and \n\n\nF\n\n\nG\nl\ny\n\n\no\nu\nt\n\n\n are the glycerol molar flow at the reactor inlet and outlet, respectively.The carbon yield to liquid (Xliq) is the ratio of the total moles of carbon in the liquid products to the moles of carbon fed:\n\n(4)\n\n\n\n\nX\n\n\nl\ni\nq\n\n\n\n(\n%\n)\n\n=\n100\n\u00d7\n\n\n\n\u2211\n\ni\n=\nm\n\nn\n\n\n\n\nF\n\n\nC\natoms\n,\nliq\n\n\no\nu\nt\n\n\n\n\n\n3\n\n\nF\n\n\nGly\n\n\ni\nn\n\n\n\n\n\n\n\nyield (Yi) and selectivity (Si) of liquid product i were calculated on the basis of carbon molar flow of i product in liquid phase, as follows:\n\n(5)\n\n\n\n\nY\n\n\ni\n\n\n\n(\n%\n)\n\n=\n100\n\u00d7\n\n\n\n\nF\n\n\nC\natoms\n,\ni\n\n\no\nu\nt\n\n\n\n\n3\n\n\nF\n\n\nGly\n\n\ni\nn\n\n\n\n\n\n\n\n\n\n\n(6)\n\n\n\n\nS\n\n\ni\n\n\n\n(\n%\n)\n\n=\n100\n\u00d7\n\n\n\n\nF\n\n\nC\natoms\n,\ni\n\n\no\nu\nt\n\n\n\n\n3\n\n\nF\n\n\nGly\n\n\ni\nn\n\n\n\u00b7\n\n\nX\n\n\nG\nl\ny\n\n\n\n\n\n\n\n\nSelectivity to alkanes (Salk) was calculated on the carbon basis, as follows:\n\n(7)\n\n\n\n\nS\n\n\na\nl\nk\n\n\n\n(\n%\n)\n\n=\n100\n\u00d7\n\n\n\n\u2211\n\nn\n=\n1\n\n3\n\n\nn\n\u00b7\n\n\nF\n\n\n\n\nC\n\n\nn\n\n\n\n\nH\n\n\n2\nn\n+\n2\n\n\n\n\no\nu\nt\n\n\n\n\n\n3\n\n\nF\n\n\nGly\n\n\ni\nn\n\n\n\u00b7\n\n\nX\n\n\nG\nl\ny\n\n\n\n\n\n\n\n\nWhile the overall selectivity to liquid products accounts the total C atoms in liquid phase, excluding unreacted glycerol.\n\n(8)\n\n\n\n\nS\n\n\nl\ni\nq\n\n\n\n(\n%\n)\n\n=\n100\n\u00d7\n\n\n\n\u2211\n\ni\n=\nm\n\nn\n\n\n\n\nF\n\n\nC\natoms\n,\ni\n\n\no\nu\nt\n\n\n\n\n\n3\n\n\nF\n\n\nGly\n\n\ni\nn\n\n\n\u00b7\n\n\nX\n\n\nG\nl\ny\n\n\n\n\n\n\n\n\nHydrogen yield (YH2) is the ratio between the molar flow of hydrogen produced and the theoretical one according to the glycerol fed to the reactor.\n\n(9)\n\n\n\n\nY\n\n\nH\n2\n\n\n\n(\n%\n)\n\n=\n100\n\u00d7\n\n\n\n\nF\n\n\nH\n2\n\n\no\nu\nt\n\n\n\n\n7\n\u00b7\n\n\nF\n\n\nGly\n\n\ni\nn\n\n\n\n\n\n\n\n\nThe degree of oxygen removal (DOR) was defined as the ratio of the removed oxygen atoms in the liquid phase to the initial molar flow of oxygen.\n\n(10)\n\n\nD\nO\nR\n\n(\n%\n)\n\n=\n100\n\u00d7\n\n\n\n\nF\n\n\nOatoms\n\n\ni\nn\n\n\n\u2212\n\n\u2211\n\ni\n=\nm\n\nn\n\n\n\n\nF\n\n\nO\natoms\n,\ni\n\n\no\nu\nt\n\n\n\n\n\n\n\nF\n\n\nOatoms\n\n\ni\nn\n\n\n\n\n\n\n\n\n\n\nTable 1 lists the bulk chemical composition and textural properties of the synthesized solids. Actual metal content measured by ICP-OES revealed a good agreement with the nominal Ce content (variation less than 3%) and Co/Al ratio (slightly higher than nominal).The isotherms of all the calcined solids (\nFig. 1A) were IV type, which were characteristic of mesoporous materials, with the P/P0 position of the inflection point (around 0.5) corresponding to a mesoporous range diameter. The hysteresis loop shape suggested randomly distributed mesopores formed by nanoparticle assembles. Regarding the BJH pore size distribution of the solids, it varied with Ce content (Fig. 1B). While pristine 0CeCoAl and 0.3CeCoAl presented a unimodal PSD with maximum at 6.8\u2009nm sized pore, sample 2.1CeCoAl presented a bimodal distribution with pores at 7.8\u2009nm and 13\u2009nm, which would anticipate a certain ceria segregation in the later assay. The surface area, pore volume and average pore size of the solids are shown in Table 1. Pristine 0CeCoAl showed SBET =\u2009125.3\u2009m2/g, which was slightly lower than stoichiometric CoAl2O4 (about 150\u2009m2/g) [29]. Recall that 0CeCoAl solid had more Co than stoichiometric (CoAl = 0.625 vs. 0.5), in form of Co3O4, characterized by its low surface area [34]. Coprecipitation of Ce together with Co and Al leaded to an increase by 16\u201318% in the SBET as compared to 0CeCoAl counterpart. Likewise, an increase in pore volume was observed (between 22% and 33% increase) for Ce-doped solids.After reduction (at 600\u2009\u00b0C, 2\u2009h), the isotherms were also of IV type. The specific surface area of Ce-doped solids slightly dropped (by 7\u201310%) as compared to calcined counterparts, with a concomitant increase in the mean pore size values (increase by 14\u201318%). It should be noted that this increase was much less pronounced than that revealed by 0CeCoAl sample (41%). Decrease in the SBET in the reduced solids was attributed to the dilution effect by metallic cobalt in the solid surface. Compared to 0CeCoAl assay, SBET of xCeCoAl reduced samples was 30\u201336% higher. Therefore, up to this point, we can conclude that the addition of Ce notably improved the textural properties of the solid in both calcined and reduced conditions, acting as structural promoter.XRD diffractograms of Ce-doped and pristine cobalt aluminate solids, in their calcined and reduced forms, are displayed in \nFig. 2. All the calcined samples exhibited the characteristic peaks for spinel phase. It should be noted that the Co/Al ratio in our catalyst was 0.64 which is around 30% larger than the stoichiometric ratio in CoAl2O3 (i.e. CoAl = 0.5). Thus, the segregation of part of Co atoms to form Co3O4 was expected to occur. The formation of CoAl2O4 spinel phase occurs due to the counter-diffusion of Co\u03b4+ and Al3+ ions at the interface between Al2O3 and Co3O4 oxides, both isotopic crystal structures, formed at the early stages of the thermal treatment [35]. Unfortunately, it was difficult to differentiate between Co3O4 and CoAl2O4 by XRD analyses due to the similarities in the crystal structure and d-spacing (PDF 00\u2013042\u20131467 and PDF 00\u2013044\u20130160, respectively). Diffraction patterns of Ce-doped solids were almost the same as that of pristine 0CeCoCAl, indicating that the solid retained the same crystal structure upon Ce doping. Moreover, the spinel peaks became narrower upon Ce addition (inset in Fig. 2A), indicating increase in the crystallite size. Only the catalysts with the highest Ce content showed diffraction peaks at 28.7\u00ba and 48.3\u00b0 (2\u03b8) attributed to CeO2 in its cerianite-phase structure (PDF 00\u2013034\u20130394), indicating segregation of Ce. The broadness of its characteristic XRD peaks suggested small size of the ceria crystallites. This peak was not observed for sample 0.3CeCoAl, which confirmed that for that solid, Ce ions were substituted in the spinel lattice. According to literature, the cerium solubility into spinel phase is CeCo =\u20090.03 molar ratio [36]. In our 2.1CeCoAl solid, the actual Ce/Co ratio was 0.025, close to the solubility limit. A careful inspection of (311) peak of the spinel phase showed a slight downshift in its position (inset of the Fig. 2), indicating that the addition of Ce caused a distortion in the spinel structure. Accordingly, Ce-doping generated a growth in the spinel lattice parameter (\nTable 2), which was attributed to the larger ionic radius of Ce4+ ions (101\u2009pm) as compared to Co2+ (79\u2009pm), Co3+ (69\u2009pm) and Al3+ (67.5\u2009pm) host ions. Similar features were obtained by others [37].The intensity ratio of the (220) to (440) diffraction planes, indicative of the cobalt ions in tetrahedral to octahedral sites [38], increased as Ce-content augmented (Table 2), indicating a preferential arrangement of cobalt ions in tetrahedral coordination [39]. The mean crystallite size of spinel phase (without distinction between CoAl2O4 and Co3O4) increased between 28% and 40% with Ce doping. The average crystallite sizes of segregated ceria phase could not be determined due to its weak signal. Undoubtedly, lattice changes would affect spinel structure.After reduction, diffraction peaks of spinel phase remained (Fig. 2B), indicating that the reduction temperature was not enough for complete reduction of all cobalt species. Upon reduction, diffraction peaks assignable to metallic Co emerged in all samples, whereas in sample 2.1CeCoAl, the formation of an alloy-like Ce5Co19 rhombohedral phase (PDF 026\u20131084) more stable structure was observed. The formation of this stable phase would explain the shift at higher temperatures of peaks III and IV observed in the H2-TPR study (see below). The mean crystallite size of metallic Co of Ce-doped catalysts (Table 2) were notably smaller than that of the pristine 0CeCoAl assay. It could be concluded that, Ce insertion in the cobalt aluminate spinel structure, strengthens the metal-support interaction in the reduced catalyst and contributes to a decrease of Co0 particles size.The morphology of the reduced materials was analysed by STEM micrographs. The obtained micrographs and the resulting particle size distribution histograms are depicted in \nFig. 3. The chemical composition of the nanoparticles was also confirmed by EDX analysis. Cobalt nanoparticles (mixture of cube and cuboid shapes) in Ce-doped catalysts were homogenously distributed, as were the tiny cerium nanoparticles that were found completely scattered throughout the samples. Ce-rich domains were observed for 2.1CeCoAl assay, indicating some Ce-rich phase. Co0 particle size was measured by imageJ software, and a similar mean diameter (ca. 20\u2009nm) was measured for all catalysts, suggesting they are composed by agglomerates of smaller crystallites. Regarding the particle size distribution, particle size ranged between 10 and 26\u2009nm while 75% of the nanoparticles were smaller than 20\u2009nm. Ce-modified solids showed a broader distribution curve, in line with the Co0 crystallite sizes evaluated by XRD (0.3CeCoAl: 9.8\u2009nm; 2.1CeCoAl: 7.2\u2009nm).XPS spectroscopy was employed to examine the oxidation state and the surface composition of xCeCoAl solids. The BE of the photoelectron peaks for both calcined and reduced samples are summarised in \nTable 3. The high-resolution XPS spectra for Co, Ce and Al in the calcined solids are shown in Fig. S1 (Supporting Information). It can be seen that the spin-orbit splitting (\u0394BE) between Co 2p3/2 and Co 2p1/2 peaks was 15.21\u2009eV for 0CeCoAl, which increased to 15.67\u2009eV at the lowest Ce loading (0.3CeCoAl). Further increase in Ce doping decreased the \u0394BE (i.e. 15.35\u2009eV for sample 2.1CeCoAl). The satellite peak of Co 2p3/2 is located at approximately 4\u20135\u2009eV higher BE than the main Co 2p3/2 peak, which is characteristic of Co2+ in CoAl2O4 framework [40,41]. In the Al 2p region, both the calcined and reduced samples show peaks at around 74\u2009eV which can be assigned to Al3+ cations bonded with oxygen. The peaks could be deconvoluted into octahedral and tetrahedral Al3+ contributions, being the former at higher BE [42]. All the solids had mainly octahedral Al3+. In addition, in the fresh catalysts Co 2p3/2 band appeared shifted to lower binding energies as Ce loading increased, what suggested a stronger interaction between elements in Ce-doped solids, in agreement with H2-TPR results.Ce 3d peaks of the calcined solids (Fig. S1C, Supporting information) showed a 3d3/2 and 3d5/2 separation of around 18.2\u2009eV, in good agreement with literature [43\u201345]. Unambiguously, no characteristic peaks of tetravalent Ce4+ were detected. Instead, characteristic peaks of Ce3+ were observed at around 902.7\u2009\u00b1\u20090.5\u2009eV and 885.2\u2009\u00b1\u20090.7\u2009eV. The latter result of Ce 3d94f1 O 2p6 final-state [43].The Co 2p XPS spectra of the reduced solids are displayed in \nFig. 4. The most striking difference was the signal from metallic cobalt, absent for calcined solids. It appeared at 778.0\u2009eV for 0CeCoAl catalyst and shifted to higher BE for samples xCeCoAl (\u0394E = 0.7\u20131.1\u2009eV), in agreement with their lower band gap energies. In addition, for the reduced xCeCoAl, deconvolved Co0 peak were less intense than for 0CeCoAl catalyst, in agreement with H2-TPR results.Two typical bands could be observed in the O 1s XPS spectrum of the reduced samples (Fig. S1D, Supporting Information). The peak at 530.9\u2013531.6\u2009eV corresponded to adsorbed oxygen (Oads) species and the other, located at lower BE coincided with surface lattice oxygen (Olatt) [46]. Oads species has greater mobility than Olatt, and are considered responsible for maintaining the charge balance in the structure [47,48]. The ratio Oads/Olatt significantly decreased after Ce loading (1.07\u20131.51 vs. 3.34) what implied a decrease in oxygen vacancies.A quantitative evaluation of the chemical composition of Co and Al at solid surface was done for both the calcined and reduced samples (Table 3). It is interesting to note that surface Co/Al ratio was lower than the bulk ratio measured by ICP-OES which was around 0.63 for all catalyst. This enrichment in Al could be attributed to its lower surface energy in comparison to Co [49], which generated cobalt-deficient aluminium-rich phase on the surface. Also, note that upon reduction, the Co/Al ratio decreased by half for the non-doped reference catalyst, whereas it remained constant for Ce-doped samples. The smaller metallic Co particle sizes of the later could be involved in preserving the Co/Al ratio at the surface.\n\nFig. 5 shows the UV\u2013vis DRS spectra of the calcined catalysts. Bare CeO2 sample, used as reference, showed two absorbance bands below 400\u2009nm. Bands at 257\u2013278\u2009nm can be attributed to O2--Ce4+ charge transfer transitions involving Ce3+ (\u2248255\u2009nm) and Ce4+ ions (\u2248278\u2009nm) with different coordination numbers, whilst a characteristic vibration of interband transitions at 328\u2009nm was observed [50]. The spectrum of 0CeCoAl sample was very similar to bare Co3O4, with a small blueshift of the bands corresponding to tetrahedrally coordinated Co2+ ions, indicative of Co-Al interaction [29,51]. Thereby, 0CeCoAl shows features from tetrahedral Co2+ (d\u2013d transition bands at 1210, 1330 and 1500\u2009nm; d\u2013d absorption bands at 558, 585 and 621\u2009nm) [52]. On the other hand, Ce-doped solids exhibited two broad bands in the UV region centred at around 420 and 635\u2013644\u2009nm and characteristic cobalt aluminate bands in the NIR region (1200\u20131500\u2009nm). These bands were found to be less intense than those shown by pristine 0CeCoAl assay with a slight band shift from 1420 to 1460\u2009nm, indicative of Ce incorporation to spinel.The direct band gap (Eg) for all samples was estimated by Tauc plot (Fig. S2, Supporting Information). Typically, Eg corresponds to a ligand-to-metal charge transfer excitation energy, and shows the tendency of adjacent transition-metal centers to gain electron density [53]. A band gap of 1.38\u2009eV was estimated for pristine 0CeCoAl solid and decreased to 0.81 and 0.48\u2009eV for samples with 0.3% and 2.1% of cerium, respectively. The lowering of Eg for Ce-doped samples could be attributed to the incorporation of metal cations into the framework of spinel crystal [54]. The distortion caused in the cell could act as trapping-centers to capture electrons [55].\nFig. S3 (Supporting information) shows the 27Al NMR spectra obtained from the reduced solids. Catalyst 0CeCoAl shows an asymmetric single peak, at 2\u2009ppm, corresponding to octahedral aluminium. A subtle signal at around 60\u2009ppm, corresponding to tetrahedral aluminium, could be also deduced [56]. Ce-doping caused a slight shift of main peak to the right, and the appearance of a bulge in the 25\u201340\u2009ppm range, which can be ascribed to five-coordinated aluminium. Both features indicated an increase in the disorder degree in the Ce-containing solids [57], in agreement with XRD results.\n\nFig. 6 displays the H2-TPR profiles of xCeCoAl solids. Those of bulk Co3O4 and CeO2 (5 times magnified) are also displayed as a reference. The reduction profile of bulk Co3O4 comprised two well defined reduction bands with maximum at 300 \u00baC and 425 \u00baC, and ascribed to Co3+ \u2192 Co2+ and Co2+ \u2192 Co0 reduction steps, respectively [58]. Bare CeO2 also showed two main reduction bands: the low temperature peak, centred at 496\u2009\u00b0C, can be ascribed to the reduction of surface caps of ceria whereas the high temperature peak, centred at 845\u2009\u00b0C, can be ascribed to the reduction of the innermost layers (bulk) of the ceria [59].The reduction of the pristine 0CeCoAl [29] started at around 150 \u00baC. Four reduction peaks could be identified. Peak I (at 292 \u00baC) was ascribed to Co3+ \u2192 Co2+ reduction of the surface cobalt cations without any interaction with the alumina or cobalt aluminate phases. Peak II (at 413 \u00baC) was also attributed to the reduction of Co3+ species though in close interaction with alumina or cobalt aluminate [29,60]. The reduction peak at 594 \u00baC (peak III) was assigned to Co2+ \u2192 Co0. Finally, peak IV (at 783 \u00baC) was assigned to the reduction of cobalt ions in the stoichiometric cobalt aluminate (CoAl2O4) phase [29,61]. The XRD signals from Co3O4 and CoAl2O4 are consistent with the peak assignment in H2-TPR measurement. It is interesting to note that the strong interaction between the cobalt ions and the support (mixture of alumina and cobalt aluminate) notably hindered both Co3+ \u2192 Co2+ and Co2+ \u2192 Co0 reduction stages. Therefore, peak III from 0CeCoCe assay significantly upshifted (by 159 \u00baC) as compared with bare Co3O4. This behaviour reflects the lower reducibility of Co\u03b4+ species in the Ce doped assays and is in line with XPS data where BE of Co 2p3/2 decreased with Ce loading in the calcined series.In the Ce-doped samples, due to the low amount of CeO2 added, its reduction peaks were overlapped by the most intense cobalt reduction peaks. As for catalyst 0CeCoAl, Ce-doped samples showed four reduction peaks. After addition of a small amount of Ce (0.3\u2009at%), peaks I and II, ascribed to Co3+ \u2192 Co2+, shifted to lower temperatures indicating a promotional effect of ceria on the partial reduction of both kinds of Co3+. However, such promotional effect was not observed in catalyst 2.1CeCoAl (see \nTable 4). Peak III (reduction of Co2+ to Co0) appeared at higher temperatures than those exhibited by 0CeCoAl, probably due to the hardening of Co-O-Al bonds because of the presence of electron trapping sites. Finally, the reduction of cobalt species in CoAl2O4 phase (peak IV), was again favoured at small Ce doping whereas in 2.1CeCoAl it took place at the highest temperature (813 \u00baC, 30 \u00baC higher than that of 0CeCoAl). This higher temperature requirement led to a lower degree of reduction of this sample after the activation process carried out before the catalytic experiments (reduction at 600 \u00baC). Overall, Ce-modified solids would achieve a smaller reduction than pristine 0CeCoAl, especially 2.1CeCoAl, worsened by the formation of Ce5Co19.\nTable 4 shows the results of H2-TPR analysis. Strictly speaking, H2 consumption due to ceria reduction should be subtracted. However, the maximum theoretical H2 consumption expected from ceria reduction in sample 2.1CeCoAl was 0.076\u2009mmolH2\u00b7g-1, what represented 1% of the total H2 uptake. Thus, the total H2 consumption is given.The amount of exposed metal, calculated by isothermal H2 pulse chemisorption, is gathered in \nTable 5. Consistent with the reducibility results discussed above, Ce addition remarkably decreased the available cobalt. This depletion was around 36% at the lowest Ce loading, and reached up to 44% at 2.1% Ce.The activity of catalysts in the cyclohexane dehydrogenation model reaction was measured in terms of TOF values (Table 5). Measured values clearly indicate a higher activity of surface atoms in Ce-doped samples. The activity in cyclohexane dehydrogenation, expressed as TOFdehyd, increased between 42% and 52% for 0.3CeCoAl and 2.1CeCoAl catalysts respect to non-doped catalyst. UV\u2013vis DRS analysis revealed differences in the inner electron transitions of the solids. Also, Ce-doped catalysts had smaller Co0 crystallites than pristine 0CeCoAl. Both features modified the metal-surroundings interaction, which could affect the intrinsic activity of cobalt in cyclohexane dehydrogenation. Moreover, the higher intrinsic activity of Ce-doped catalysts is also a desirable characteristic for activating H2 molecule in hydrogenolysis reactions.The surface acid site density of the reduced solids is summarised in Table 5, and the strength distribution, according to the desorbed NH3 profiles, is displayed in Fig. S4 (Supporting Information). The acid site strength were categorized according to their desorption temperatures, as follows: weak (desorption temperature range 90\u2013300\u2009\u00b0C), medium (300\u2013650\u2009\u00b0C) and strong (> 650\u2009\u00b0C), while the percentage contribution of each strength site was estimated from the area under ammonia desorption profile. Ce-doping led to an increase in the density of surface acid sites (increase by 55\u201372%) and a change in the distribution of the acid strength. Ce-doped solids exhibited a higher density of medium and strong acid centers compared to their counterpart 0CeCoAl assay. In percentage terms, strong acid sites increased from 8.1% for sample 0CeCoAl to 26% for both Ce-containing samples. Morterra et al. [62] found that Ce-doped alumina had more acidity than parent \u03b3-alumina, and ascribed this to the modified environment of Al3+ cations by cerium. In fact, 27Al NMR and UV\u2013vis DRS results pointed to this.Cobalt aluminate is characterized by having Lewis acidity rather than Br\u00f8nsted acidity [63]. Thus, it is expected the Lewis acid centers to prevail in Ce-doped solids. In glycerol APR conditions, dehydration of primary or secondary hydroxyl from glycerol depends on the abundance of Lewis or Br\u00f8nsted acid sites [64]. Therefore, it was considered useful to evaluate the Br\u00f8nsted acidity of the catalysts. Evaluation of Br\u00f8nsted acidity was done in terms of the activity in the skeletal isomerization of 33DM1B. Table 5 showed large differences between 0CeCoAl and Ce-doped catalysts. Both Ce-containing catalysts were 70\u201390 times more active for skeletal isomerization of 33DM1B, indicating their acid sites had more acid Br\u00f8nsted characteristics than 0CeCoAl, probably linked to their higher penta-coordinated Al content. In conclusion, the total acid sites density increased with Ce, which is a key feature to protonate the hydroxyl groups before oxygen loss as water.The Weisz-Prater and Mears criteria (Table S1, Supporting Information) was used to confirm the absence of mass-transfer limitations in catalytic experiments. Blank tests carried out with the reactor bed filled solely with quartz wool (used to support the catalyst inside the reactor) showed no glycerol conversion, which suggested that homogeneous APR had no contribution to the catalytic conversion of glycerol. Similarly, HDO reaction with calcined 0CeCoAl catalyst resulted in almost null conversion, which indicated that metallic function was required for the HDO reaction. Furthermore, catalytic experiments with reduced cobalt oxide (Co3O4) resulted in low liquid yield [29], indicating the acid sites are involved in the production of liquids.The catalytic performance in the glycerol HDO without external addition of hydrogen was studied in a continuous fixed bed reactor feeding a solution of 10\u2009wt% glycerol/water, at WHSV of 24.5\u2009h-1, during 3\u2009h TOS. The results expressed as glycerol conversion, carbon yield and selectivity to liquids are shown in \nFig. 7.Catalyst 0CeCoAl [29] exhibited higher glycerol conversion (95.7%) than Ce-doped catalysts. Among Ce-modified catalysts, that with the lowest cerium load showed slightly higher glycerol conversion (Xgly= 60.0% for 0.3CeCoAl vs 55.0% for 2.1CeCoAl). The low glycerol conversions obtained with Ce-containing catalysts might be related to the stabilizing effect of cerium on the Co2+ ion, delaying its reduction to Co0, as seen by H2-TPR analyses. Furthermore, and related to the above, available metallic area was lowered for Ce-doped catalyst.Carbon yield to liquid (Xliq) was also markedly lowered for Ce-containing catalysts, and decreased with increasing dopant loading. This trend is intrinsically linked with the glycerol conversion presented for these samples. Nevertheless, selectivity towards liquid products improved by 34.5% with the addition of 0.3% Ce and by 27.1% on 2.1% Ce added, evidencing that the selectivity to liquid phase products was undoubtedly favoured by doping with Ce.\n\nFig. 8 A displays the yield of the main liquid products obtained. For all the catalysts, 1,2-propanediol was the foremost product generated, which indicated that H2 was indeed produced by APR. Hydroxyacetone, the major intermediate product in 1,2-propanediol formation, was the second leading liquid product. The high yields to both compounds, in addition to acetone (dehydration product of 1,2-propanediol), demonstrated high activity of the synthesized catalysts for the glycerol hydrodeoxygenation (HDO). Catalyst 0.3CeCoAl presented the maximum yield to 1,2-propanediol (34%) while 0CeCoAl and 2.1CeCoAl exhibited 28% and 25%, respectively. 0.3CeCoAl catalyst also manifested the highest yield to hydroxyacetone (22.6%). However, the utmost yield towards ethanol was achieved with pristine 0CeCoAl catalyst. The density and distribution of the acid sites is of important consideration in the production of by-products [65]. Ce-doped catalysts presented a higher density of medium and strong acid centers than pristine 0CeCoAl, which makes them better catalysts for dehydration reactions compared to bare cobalt aluminate catalyst that is conducive to dehydrogenation reactions.Besides the liquid products displayed in Fig. 8A, 1-propanol was detected for all the catalysts. Concerning 0CeCoAl catalyst, a bunch of products was detected: propionic acid, acetic acid and very low amounts of 2-propanol, acetaldehyde and methanol. Obtaining this great variety of liquid products could be explained by the highest metal availability, and therefore a high reactivity, of the 0CeCoAl catalyst, which could facilitate the concatenation of successive dehydration/hydrogenation reactions. These results showed the relevance of the Ce-doped catalysts for 1,2-propanediol production from glycerol HDO with in-situ generated H2. No liquids produced by Br\u00f8nsted acid sites (acrolein, or 1,3-propanediol) were detected, indicating prevalence of the Lewis acidity.Catalyst activity for bond cleavage was also analysed. From data in Fig. 8B it is shown that carbon selectivity to primary products (products from single C\u2013C or C\u2013O bond scission, such as ethylene glycol, hydroxyacetone and propylene glycol) was remarkable high for Ce-doped catalyst, 0.3CeCoAl and 2.1CeCoAl (82.5% and 79.6%, respectively). In turn, catalyst 0CeCoAl achieved 44.1% selectivity towards primary products while its selectivity to secondary ones (products obtained after cleavage of additional C-C or C-O bonds) was 24%, which is three-fold higher than that of Ce-doped catalysts. Pristine cobalt aluminate catalyst also attained a considerable selectivity to gas products (24.6%) in accordance with its high metallic surface area. Among the Ce-doped samples, catalyst 2.1CeCoAl displayed slightly lower selectivity to primary and secondary products than catalyst with 0.3% of cerium.A summary of the reaction products is shown for the third hour TOS (\nTable 6). Carbon selectivity to ethylene glycol (EG) was around 5% for both Ce-doped catalysts and 3.3% for pristine 0CeCoAl. Carbon selectivity to 1,2-propanediol (1,2-PD) step up in the following order: 0CeCoAl (28.9%) <\u20092.1CeCoAl (42.4) <\u20090.3CeCoAl (46.5). Ce-containing catalysts also presented a higher selectivity towards hydroxyacetone (HA) (31\u201332%) compared to 0CeCoAl (11.9%). According to these results, Ce-doped catalysts can balance between acid centers (necessary for obtaining hydroxyacetone via glycerol dehydration) and metal availability for the further hydrogenation of hydroxyacetone into 1,2-propanediol. Both Ce-doped catalysts have less available Co0, but of higher intrinsic activity as deduced from the cyclohexane dehydrogenation activities (Table 5). Ce-doped catalysts still have scope for improvement to decrease hydroxyacetone production in favour of 1,2-propanediol.The low selectivity to hydroxyacetone and 1,2-propanediol presented by 0CeCoAl catalyst was due to the formation of other liquid compounds and its proficiency to break C-C bonds. In this sense, the formation of light alcohol and short chain alkanes is an inconvenience to be solved for the commercial development of this process for primary products [13].The oxygen removal degree (DOR) reflects the ability of the catalysts to generate deoxygenated liquid compounds. Ce-doping enhanced DOR, as it passed from 60.7% for 0CeCoAl to 63.5\u201368.3% for 0.3 and 2.1CeCoAl. Liquid products were clustered into two main group. Products obtained by C\u2013C bond cleavage only (ethylene glycol, methanol) and those classified as C\u2013O bond scission products. The selectivity to both types of cleavage are given in Table 6. For all the catalysts, selectivity to C-O cleavage products (SC-O) was considerable high (i.e. up to 86.9% for catalyst 0.3CeCoAl). In the same vein, 0CeCoAl catalyst presented the lowest value (SC-O = 64.5%). In contrast, selectivity to SC-C barely reached 5% (0.3CeCoAl). We can affirm that these catalysts, and particularly Ce-containing samples, exhibited excellent C\u2013O bond scission function. Thus, the presence of cerium favoured the dehydration pathway, by slight increase of surface acid sites density, even under conditions in which cobalt aluminate based-catalysts have shown to favour dehydrogenation route [29].\nTable 6 also gathered hydrogen yield (YH2) and selectivity to alkanes (Salk). Regarding YH2, it follows the same trend as the liquid yield. It indicated that the obtained H2 readily reacts to yield liquid products. Pristine 0CeCoAl showed the highest YH2 and the lowest value was obtained for catalyst 2.1CeCoAl (10.2%). This catalyst, instead, displayed the maximum selectivity to alkanes (54%, mainly methane). In brief, selectivity towards alkanes increased with cerium content, which was evidenced by the decrease in YH2. It should be stressed that YH2 reflects the hydrogen consumed in side reactions for the formation of both liquid and gaseous compounds while Salk was calculated taking into account the flow of carbon atoms in alkanes with respect to the total carbon atoms only in gaseous products. These data lead us to conclude that, for catalyst 0CeCoAl, hydrogen produced in-situ is consumed mainly in the formation of diverse liquid products, while Ce-containing catalyst were more prone to the formation of alkanes.A slight decrease in XGly and Xliq was observed in the course of reaction (Fig. S5, Supplementary Information), more pronounced for Ce-doped catalysts. From 2\u2009h to 3\u2009h TOS, XGly decreased by 9\u201314% for Ce-containing catalysts vs 2% for 0CeCoAl assay. In the same period, conversion to liquid decreased by 4\u20139% for the doped catalysts, while for the non-doped assay increased by 4%, which was due to the decrease in the production of gaseous compounds while the conversion of glycerol remains more stable. In order to investigate the potential changes in the physico-chemical properties underwent by catalysts in the reaction, the characterization of spent catalysts was carried out, and the results obtained are shown in \nTable 7.N2 adsorption-desorption isotherms and BJH pore size distribution of the spent catalysts are shown in Fig. 1. The form of the isotherms of the spent catalysts resembled those of fresh reduced solids, indicating the pore structure was preserved. All the spent catalysts adsorbed more nitrogen than the parent fresh reduced solid, which, in turn, was reflected in the increase in SBET. Spent 0CeCoAl catalysts showed 75% increment in SBET, while Ce-doped catalysts incremented by 26\u201357%. Reynoso el at. observed that hydrated alumina species were involved in the specific surface area gain [29]. Under hydrothermal conditions \u03b3-alumina could be hydrated to gibbsite or boehmite, which in turn, could be leached-off and re-deposited on the catalyst surface, generating additional porosity in the solid [66]. Differences between reduced and spent catalysts were appreciated in the BJH pore size distribution. Spent catalysts had bimodal distributions, with maxima at approximately 3\u20134\u2009nm and 8\u201310\u2009nm pores. The peak from smaller pores suggested the presence of new phases not observed in the fresh reduced form (e.g. gibbsite or cerium hydroxycarbonate). The former narrow peak was more pronounced for solids with higher amount of cerium. Many phenomena could explain this evolution of the textural properties, such as hydration of the alumina [67], deposits of carbonaceous materials [68] or the formation of new phases [69].Crystalline phases in the exhausted catalysts were identified by XRD (\nFig. 9A). The obtained diffractograms still showed the well-known characteristic peaks of spinel (either cobalt oxide and cobalt aluminate). XRD reflexions from metallic Co were still visible for all catalysts. The poor resolution of the spectra from those peaks prevented us to calculate the crystallite size. We could hypothesize that coalescense of metallic cobalt was not significant, despite of the high temperature used. A new peak at around 34\u00ba (2\u03b8) emerged for all spent catalysts, which corresponded to cubic CoO phase (PDF 048-1719). This fact indicated that cobalt could be oxidized in the aqueous media of the reaction [70,71]. It could be concluded that the oxidation of cobalt was limited to the outmost caps of the particles, since XRD signal from bulk Co0 remained. None of the spent catalysts showed diffraction peaks from hydrated alumina (boehmite or gibbsite), which could be due to its amorphous form. For Ce-doped catalysts, new XRD peaks arose and became significant for 2.1CeCoAl catalyst. Those peaks centred at 24.6\u00ba and 30.6\u00ba were attributable to the hexagonal phase of cerium hydroxycarbonate (Ce(CO3)OH) (PDF 32-0189). This new phase was formed by carbonation of ceria by the CO formed, promoted by the harsh hydrothermal conditions [72]. Cerium hydroxycarbonate has been also detected in the spent Ni/CeO2 catalysts after usage in the aqueous-phase reforming of methanol [73]. The sharpness of these Ce-containing new phase peaks indicated that the product was well crystallized. All the Ce-containing spent catalysts showed peaks from Ce5Co19 phase structure, though weakened with respect the reduced form.Leaching of catalyst components contribute to the deactivation of heterogeneous catalysts in the liquid-phase reactions [22]. Accordingly, the catalysts metal constituents Al, Ce and Co in the reaction liquid product were analysed by ICP-MS. All spent catalysts experienced leaching of the three metals, to a different extent. At first glance, Ce-containing catalysts underwent greater leaching of all metals. Cobalt was the most leached metal and the percentage of leaching in the Ce-containing samples was 2\u20133 times higher than for catalysts 0CeCoAl. This behaviour could be due to the smaller size of metallic Co in the Ce-doped catalysts, which are more prone to oxidation and subsequent leaching [74]. Co was thermodynamically prone to be oxidized (preliminary stage of leaching), also the phase transition during alumina hydration could promote metal particle detachment. Due to the re-deposition of aluminium hydroxyde on the catalyst surface, leaching of aluminium was 1\u20132\u2009less intense than cobalt. Meanwhile, the amount of Ce leached out from the catalysts increased in proportion to the amount of Ce in the samples.The spent catalysts were also subjected to H2-TPR analysis (Fig. S56, Supplementary Information) and the hydrogen uptake values are reported in Table 7. The hydrogen consumption was higher than the required for the reduction of solely cobalt species (deduced through the reducibility degree). Indeed, all samples showed hydrogen consumption below 600\u2009\u00b0C, which indicated the catalysts were oxidized in the course of the reaction. It is also noteworthy the intense H2 consumption at temperatures above 600 \u00b0C. Actually, the used catalysts showed negligible consumption below 500 \u00b0C. Other authors [75] have investigated reduction temperature range after and before reactions, concluding that spent catalysts show a shift towards higher temperatures compared to fresh one due to the aggregation or sintering of metal clusters.Overall, the metallic availability of all catalysts was seriously compromised, as was also confirmed by H2 chemisorption that revealed a decrease of about 64% for catalyst 2.1CeCoAl and up to 96% for monometallic 0CeCoAl. One plausible explanation is the re-deposition of aluminium on the catalyst surface, covering the Co0 sites by hydrated alumina [76,77]. Two other phenomena that cannot be ignored are the oxidation of cobalt and the preferential leaching of the smallest particles, thus remaining the largest particles, consequently reducing accessible metal atoms [78,79].The results obtained by TPH, displayed in Table 7, present a very low deposition of carbonaceous material, which decreased with the Ce-content. The recorded Raman spectra of the used catalysts (Fig. 9B) exhibited the typical peaks from carbon, named D and G bands, at ca. 1350 and 1590\u20131600\u2009cm-1, respectively. The shape of the spectra was similar for all catalysts with a broad and less intense D-band and a more intense G-band. The shoulder observed at 1710\u2009cm-1 in the sample 2.1CeCoAl could be attributed to an overtone of an M point phonon [80]. The degree of graphitization of carbonaceous materials was estimated by the relative intensity ratio of D and G bands (ID/IG inset in Fig. 9B). The ID/IG values obtained were in the 0.22\u20130.38 range and did not show correlation with the composition of the samples. Raman and TPH agree with the absence of any peak from coke in the XRD pattern. It seems that the principal deactivation issues are those related to the loss of metal surface area, due to the encapsulation by alumina, and the oxidation of cobalt species. Cobalt oxidation could lead to metal leaching, another phenomenon responsible for the decrease in the metal surface.The effect of Ce-coprecipitation to improve the activity and selectivity of cobalt aluminate-based catalysts for glycerol HDO was discussed. Properties of catalysts based on cobalt aluminate spinels were tuned through Ce doping, a simple, fast and cost-effective synthesis method. Ce-modification of the 0CeCoAl catalyst increased SBET (by around 16\u201330%) and also the spinel crystallite size. For the reduced sample, in addition to cobalt aluminate and metallic cobalt species, formation of Ce5Co19 phase was identified at the highest Ce-content. Also, Ce-doping increased total acidity through a prominent increment of medium-strong acid centers.Ce-doped catalysts exhibited lower glycerol conversion than their counterpart, 0CeCoAl. Nevertheless, their higher acid sites density have influenced the activity of these catalysts, presenting a higher selectivity towards deoxygenated liquid products. Catalyst testing in a fixed-bed reactor at 260\u2009\u00b0C and 50\u2009bar without external hydrogen source revealed that: (i) H2 is produced; and (ii) it is in part consumed hydrogenating liquid intermediates, which makes these catalysts to have advantageous catalytic properties for glycerol HDO in a sustainable way. The main products of glycerol conversion were 1,2-propanediol and hydroxyacetone. Selectivity to liquid products of catalyst 0.3CeCoAl was 92% and selectivity to 1,2-propanediol was over 46%, indicating that Ce-doped catalysts are compelling HDO catalysts.Future research should focus on stability issues since post-reaction characterization verified a decrease in cobalt availability that can be attributed to re-deposition of hydrated alumina on the catalyst surface. This phenomenon, in conjunction with the deposition of carbonaceous material, could be responsible for the increment in SBET and the change in the pore size distribution. Further confirmed fact was that despite the addition of cerium, the catalysts continued to exhibit metal oxidation and leaching phenomena.\nA.J. Reynoso: Investigation, Writing \u2013 original draft. U. Iriarte-Velasco: Formal analysis, Writing \u2013 review & editing. M.A. Guti\u00e9rrez-Ortiz: Resources, Funding acquisition, Supervision. J.L. Ayastuy: Funding acquisition, 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.This research was supported by grant PID2019-106692EB-I00 funded by MCIN/AEI/10.13039/501100011033. The authors thank for technical support provided by SGIker of UPV/EHU and European funding (ERDF and ESF). Cyclohexane dehydrogenation and isomerization of 33DM1B model reaction were conducted at the Institut de Chimie des Milieux et des Mat\u00e9riaux, Universit\u00e9 de Poitiers, (CNRS UMR 7285 IC2MP). The authors acknowledge the kind assistance of Dr. Catherine Especel and Dr. Laurence Vivier.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2022.107612.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  In this study, the hydrodeoxygenation (HDO) of glycerol with in-situ produced H2\n                     , via aqueous-phase reforming, was investigated over Ce-doped CoAl2O4 catalysts synthesized by coprecipitation. The catalytic runs were performed at 50\u00a0bar/260 \u00baC for 3\u00a0h TOS in a fixed bed reactor. The synthesized catalysts were extensively characterized to better understand the effect of physicochemical and surface characteristics on the catalytic performance. The results revealed that doping with Ce increases the population and strength of acid centers, which results in a higher selectivity towards deoxygenated liquid products. Ce-doped catalysts exhibited higher yield to hydroxyacetone and 1,2-propanediol. Post-reaction characterization revealed a decrease of the metallic surface area, mainly due to alumina coating, and to a lesser extent, due to the oxidation and leaching of the cobalt.\n               "}
{"full_text": "With the increasing demand for energy and the need to decrease the use of conventional energy sources, the development and utilization of renewable energy becomes extremely important. Another aspect is the environmental pollution due to the use of traditional energy sources, so it is urgent to develop clean and efficient energy systems. Hydrogen is one kind of efficient and clean energy carriers, which has attracted extensive worldwide attention in recent years [1\u20133]. One of the key technologies for hydrogen energy development and utilization is how to storage hydrogen safely. Magnesium hydride (MgH2) is one of the most suitable candidates for hydrogen storage materials because of its high gravimetric energy density. Unfortunately, the relatively high operating temperature and the slow de/hydrogenation rate hinder its extensive industrial application. Therefore, the hydrogen storage properties of MgH2 need to be further improved to overcome these drawbacks.One of the effective ways for reducing the de/hydrogenation temperature and improving the de/hydrogenation rate of MgH2 is the introduction of catalysts, such as transition metals [4\u20139], intermetallic compounds [10\u201312], transition metals oxides [13\u201318], and carbon materials [19\u201326]. Zhang et\u00a0al. [4] introduced Fe into MgH2 by wet-chemical ball milling and found that the onset temperature of desorption and the apparent activation energy of dehydrogenation of MgH2\u00a0+\u00a05\u00a0wt.% Fe were 182.1\u00a0\u00b0C and 40.7\u00a0\u00b1\u00a01.0\u00a0kJ/mol, respectively, thus being far lower than the values for pure MgH2. Korablov et\u00a0al. [6] reported that the activation energy of Ti-doped MgH2 is 53.6\u00a0kJ/mol and the investigated 0.75Mg-0.25Ti composite can absorb hydrogen at room temperature. Yang et\u00a0al. [10] used FeCo nanosheets to enhance the hydrogen storage properties of MgH2 and observed that FeCo-containing MgH2 can rapidly uptake 6.7\u00a0wt.% of H2 within one minute at 300\u00a0\u00b0C. The hydrogenation and dehydrogenation activation energies of FeCo-containing MgH2 were reduced to 65.3\u00a0\u00b1\u00a04.7\u00a0kJ/mol and 53.4\u00a0\u00b1\u00a01.0\u00a0kJ/mol, respectively. Transition metal oxides have also been proved to be promising alternatives for improving the hydrogen storage properties of MgH2. Wang et\u00a0al. [14] doped 9.0\u00a0wt.% of V2O3@C in MgH2 by mechanical milling and demonstrated that the presence of V weakens the strength of Mg\u2013H bonds in MgH2; thus, the hydrogenation and dehydrogenation temperatures of V-catalyzed MgH2 are strongly reduced. Valentoni et\u00a0al. [17] reported that VNbO5-doped MgH2 can adsorb more than 5\u00a0wt.% of H2 within five minutes at 160\u00a0\u00b0C and its hydrogen storage capacity does not decline even after 70 hydrogen absorption-desorption cycles. It has also been shown that carbon materials can significantly improve the thermodynamic and kinetic properties of MgH2. For example, Liu et\u00a0al. [21] introduced one-dimensional bamboo-shaped carbon nanotubes (BCNTs) with a high specific surface area to improve the integrated hydrogen storage properties of MgH2 and found that the dehydrogenation activation energy (97.97\u00a0 kJ/mol) and enthalpy (68.92\u00a0kJ/mol) of MgH2@BCNTs were reduced by 111.24\u00a0kJ/mol and 6.07\u00a0kJ/mol as compared to those of pristine MgH2, respectively. Zhang et\u00a0al. [25] noticed that the hydrogen absorption/desorption rates of TiO2@C-containing MgH2 are significantly faster than those of pristine MgH2 and the Mg-H bond strength weakens under the catalytic action of TiO2@C. The above results imply that although the operating temperature and hydrogen storage kinetics of MgH2 can be significantly improved by adding catalysts, this approach still cannot meet the requirements of practical applications.In recent years, two-dimensional (2D) materials were used as catalysts to improve the hydrogen storage properties of MgH2. Liu et\u00a0al. [27] used two-dimensional Nb4C3T\nx\n to enhance the hydrogen storage properties of MgH2 and asserted that the hydrogen storage kinetics and thermodynamics of MgH2 are improved by the unique layered structure of in-situ-formed NbH\nx\n. Wang et\u00a0al. [28] prepared NbTi nanocrystals using a NbTiC solid-solution as precursor and revealed that NbTi-containing MgH2 starts to release H2 at 195\u00a0\u00b0C and desorbs about 5.8\u00a0wt.% of H2 within 30\u00a0min at 250\u00a0\u00b0C. Liu et\u00a0al. [29] found that 5\u00a0wt.%Ti3C2-containing MgH2 has excellent hydrogen storage kinetics and can uptake 6.1\u00a0wt.% of H2 within 30\u00a0s at 150\u00a0\u00b0C. Furthermore, the hydrogenated samples can absorb hydrogen at room temperature. In the periodic table, V and Ti are neighboring elements; thus, they have similar electronic structures and also manifest some similar catalytic effects on the hydrogen storage performance of MgH2. Theoretical calculations have revealed that the heat of formation for dehydrogenated V-containing MgH2 is \u221243.42\u00a0kJ/mol, which is 7.85\u00a0kJ/mol lower than that of Ti-containing MgH2\n[30]. da Concei\u00e7\u00e3o et.al. [31] studied results indicated that VC could enhance the hydrogen absorption and desorption kinetics properties of Mg, and a desorption rate of 1.0\u00a0\u00a0\u00d7\u00a0\u00a010\u22122\u00a0wt.%\u00a0s\u22121 at 300\u00a0\u00b0C was obtained for VC-catalyzed Mg system. In our previous study [32], it was found that the hydrogen storage properties of V-doped Mg-Al alloys were better than those of the Ti-doped samples. It was also reported that the catalytic effect of V-based compounds on the hydrogen storage performance of Mg-based alloys is superior to that of single V [33]. Therefore, it can be speculated that V-based 2D materials might have some unexpected catalytic effects on the hydrogen storage properties of MgH2. Hence, in this research, VC was synthesized using V4AlC3 as a precursor and employed to improve the hydrogen storage performance of MgH2.V4AlC3 (1.5\u00a0g) (purity\u226598%, Beijing Beke New Material Technology Co., LTD) was slowly poured into 40\u00a0ml of 40% HF (purity\u226599%, Aladdin) under magnetic stirring for 96\u00a0h at 55\u00a0\u00b0C. The resulting product was washed with deionized water more than three times until the pH value became \u22656. Finally, VC was obtained by drying the washed sample for 24\u00a0h in a frozen drying oven. Subsequently, 10\u00a0wt.% of VC was incorporated into commercial MgH2 (purity\u226598%, Langfang Bede Trading Co., LTD) by milling a mixture of MgH2 and 10\u00a0wt.% of VC for five hours at a milling speed of 500\u00a0rpm (ball-to-power weight ratio\u00a0=\u00a040:1) in an Ar atmosphere; the product was named MgH2-VC.The phase compositions of the samples were characterized by X-ray diffraction (XRD; Miniflex 600, Rigaku) under Cu-K\u03b1 radiation at 40\u00a0kV and 200\u00a0mA with a scanning step size of 5\u00b0/min. The micromorphologies of the samples were examined by field-emission scanning electron microscopy (FE-SEM; SU8020, HITACHI) and transmission electron microscopy (TEM; FEI Tecnai G2, f20 s-twin 200\u00a0kV). The distributions of V and C in the VC-doped samples were characterized by energy-dispersive X-ray spectrometry (EDS) coupled with SEM and TEM. The surface chemical bonding structures of the samples were characterized by an X-ray photoelectron spectroscope (XPS; ESCALAB 250Xi Microprobe), and binding energy spectra were fitted via XPSPEAK41 software. The de/hydrogenation performances of the samples were analyzed by a Sievert-type device. The activated samples were heated from room temperature to 380\u00a0\u00b0C at a rate of 1\u00a0\u00b0C\u00b7min\u22121 under a hydrogen pressure of 6\u00a0MPa during hydrogenation and at a rate of 0.5\u00a0\u00b0C\u00b7min\u22121 under a hydrogen pressure of 0.01\u00a0MPa during dehydrogenation. The dehydrogenation and hydrogenation kinetics of the samples were determined at different temperatures under a hydrogen pressure of 6\u00a0MPa and a vacuum pressure of 0.01\u00a0bar, respectively.Dehydrogenation simulations were performed with the Vienna Ab Initio Simulation Package (VASP) [34\u201336]. The interaction between electrons and ions and the exchange-correlation effect were analyzed using the projector augmented wave (PAW) method of Bl\u00f6chl [37] and the Perdew-Burke-Ernzerhof (PBE) functional [38] under generalized gradient approximation (GGA) [39], respectively. The cutoff energy of the plane wave basis set was set to 500\u00a0eV. The convergence criteria for the Hellmann\u2013Feynman force and the total energy were 1\u2005\u00a0\u00d7\u00a010\u22122eV/\u00c5 and 1\u2005\u00a0\u00d7\u00a010\u22124 eV/atom, respectively. The dipole correction along the surface normal was also considered. The Monkhorst-Pack method with 5\u2005\u00a0\u00d7\u00a0\u20055\u2005\u00a0\u00d7\u00a0\u20051 k-point meshes was employed for the dehydrogenation of Mg4H8 clusters on the VC (100) surface.\nFig.\u00a01\n displays a XRD pattern and a SEM image of VC, and SEM-EDS images of VC-doped MgH2. It is noticeable that Al in V4AlC3 was corroded in 40% HF (Fig.\u00a01a), implying the successful synthesis of VC. The sharp diffraction peaks at 2\u03b8\u00a0=\u00a037.4\u00b0, 43.4\u00b0, 63.1\u00b0, 75.6\u00b0, and 79.7\u00b0 (Fig.\u00a01a) correspond to the (111), (200), (220), (311), and (222) lattice planes of VC (JCPDS card no. 74\u20131220). An additional diffraction peak appears at low-angles stemming from the (002) lattice plane of the graphite nitrate (JCSDS, card No. 742330). The high-resolution V 2p and C 1\u00a0s XPS spectra of VC are presented in Fig.\u00a01b and 1c, respectively. The binding energies of 520.7\u00a0eV (V 2p3/2) and 513.3\u00a0eV (V 2p1/2) with an energy gap of 7.4\u00a0eV can be assigned to V-C bonds, and the peak at 522.4\u00a0eV (V 2p1/2) and 514.9\u00a0eV (V 2p3/2) correspond to the V-T\nx\n bond (T\nx\n= \u2013O, OH, and \u2013F) [40,41]. The peaks at 282.3\u00a0eV, 284.8\u00a0eV, 286.2\u00a0eV, and 288.6\u00a0eV are due to C-V, C-C, C-O and O-C=O bonds (Fig.\u00a01d), respectively [41,42]. The microtopography of VC is presented in Fig.\u00a01(d\u2013f). Apparently, VC possesses an accordion-like layered structure. These results suggest that VC was successfully synthesized using V4AlC3 as a precursor. After VC was introduced into MgH2 sample, numerous small particles attached to the large particles in VC-doped MgH2 composite (Fig.\u00a01g), and EDS mapping image indicates that VC had been dispersed in MgH2 after ball milling (Fig.\u00a01h).The microstructure of 2D VC was further examined by TEM, HRTEM, SAED, and EDS. The TEM image in Fig.\u00a02\na confirms the synthesis of nanoscale VC. The distance between crystal faces in VC was calculated as 0.208\u00a0nm (Fig.\u00a02b), which corresponds to the (200) lattice plane of VC. The SAED pattern in Fig.\u00a02c reveals diffraction rings from the (111), (200), (220), (311), and (222) lattice planes of VC. The EDS mapping image indicates that V and C are homogeneously distributed in VC. The above results suggest that VC was successfully prepared by the etching method using V4AlC3 as a precursor. Moreover, Fig.\u00a02g reveals that many small particles clustered around the large particles in VC-doped MgH2 composites, which is consistent with the observation from Fig.\u00a01g. In Fig.\u00a01g, the size of the large particles is in the range of 1.0um\u223c10.0um, the small ones are at the submicron level. In Fig.\u00a02g, the grains are about 200\u00a0nm in size, and the small ones are about 50\u00a0nm. The distances between crystal faces of 0.219\u00a0nm and 0.210\u00a0nm can be assigned to the (111) plane of MgH2 and the (200) plane of VC, respectively. In addition, diffraction rings associated with the (002), (101), (212) planes of MgH2 and the (200) plane of VC are visible in Fig.\u00a02i. Therefore, the SEM-EDS, HRTEM and SAED results are in good agreement with the XRD observations, suggesting that VC and MgH2 were well mixed during the preparation process.\nFig.\u00a03\n(a, b) display the isothermal and non-isothermal hydrogenation and dehydrogenation curves of MgH2 and VC-doped MgH2. MgH2 and VC-doped MgH2 can absorb approximately 6.98\u00a0wt.% and 6.42\u00a0wt.% of hydrogen, respectively, when heated from room temperature to 350\u00a0\u00b0C. Undoped MgH2 can hardly absorb hydrogen until the temperature reaches 125\u00a0\u00b0C. However, dehydrogenated VC-doped MgH2 can absorb hydrogen even at room temperature, and its hydrogen absorption capacity is about 5.0\u00a0wt.% when the temperature reaches 150\u00a0\u00b0C. It was found that undoped MgH2 and VC-doped MgH2 release about 6.95\u00a0wt.% and 5.75\u00a0wt.% of hydrogen, respectively, when heated from room temperature to 400\u00a0\u00b0C. However, the initial dehydrogenation temperatures of these two samples are different. Undoped MgH2 cannot release hydrogen until the temperature reaches 320\u00a0\u00b0C. The onset dehydrogenation temperature of VC-doped MgH2 significantly decreases after the incorporation of VC, and it starts to release hydrogen at 170\u00a0\u00b0C, which is 150\u00a0\u00b0C lower than for undoped MgH2. The operating temperature of VC-doped MgH2 is also found to be lower than those of TiO2@C-doped MgH2\n[43], FeB@CNTs-doped MgH2\n[44], SrTiO3-doped MgH2\n[45], and other materials [46,47]. To investigate the catalytic effect of VC on the hydrogen absorption/desorption kinetics of MgH2, the isothermal hydrogen absorption/desorption kinetics of undoped and VC-doped MgH2 were analyzed at various temperatures (Fig.\u00a03(c\u2013f)). Undoped MgH2 can absorb approximately 4.0\u00a0wt.% of H2 within 180\u00a0min at 175\u00a0\u00b0C and 6.0\u00a0wt.% of H2 within 180\u00a0min at 200\u00a0\u00b0C, respectively, and its hydrogen absorption capacity increases to about 6.2\u00a0wt.% for temperatures beyond 225\u00a0\u00b0C. In contrast, VC-doped MgH2 exhibits excellent hydrogenation properties and can absorb about 3.0\u00a0wt.% of H2 at 25\u00a0\u00b0C within 180\u00a0min, and its hydrogen absorption capacity increases as the temperature increases. It can absorb approximately 4.0\u00a0wt.% of H2 within 180\u00a0min at 50\u00a0\u00b0C, and the hydrogen absorption capacity increases to about 5.5\u00a0wt.% when the temperature increases above 100\u00a0\u00b0C. In particular, VC-doped MgH2 can absorb 5.0\u00a0wt.% of H2 within 9.8\u00a0min at 100\u00a0\u00b0C. Hence, the catalytic effect of VC on the hydrogen absorption performance of MgH2 is superior to that of Ni-V [48], NiS [49], Ni@rGO [5], Fe3S4\n[50], and Co@C [51].For example, Mg-Ni-V [48], Mg-5\u00a0wt.%NiS [49], and MgH2\nCo@C [51] can only absorb 1.0\u00a0wt.%, 3.5\u00a0wt.%, and 2.71\u00a0wt.% of hydrogen within 10\u00a0min at 100 \u00b0C, respectively. During dehydrogenation, undoped MgH2 can desorb approximately 7.0\u00a0wt.% of hydrogen at 375\u00a0\u00b0C at a hydrogen desorption rate of 1.06\u00a0\u00b1\u00a00.02\u00a0wt.%/min, and the hydrogen desorption rate decreases to 0.42\u00a0\u00b1\u00a00.04\u00a0wt.%/min and 0.13\u00a0\u00b1\u00a00.01\u00a0wt.%/min at 350\u00a0\u00b0C and 325\u00a0\u00b0C, respectively. VC-doped MgH2 can release approximately 6.0\u00a0wt.% of hydrogen at 325\u00a0\u00b0C at a hydrogen desorption rate of 2.18\u00b1 0.03\u00a0wt.%/min, which is 16.8 times faster than what was found for undoped MgH2 under the same conditions. The dehydrogenation rates of VC-doped MgH2 at 300\u00a0\u00b0C, 275\u00a0\u00b0C, and 250\u00a0\u00b0C are 1.32\u00a0\u00b1\u00a00.02\u00a0wt.%/min, 0.54\u00a0\u00b1\u00a00.02\u00a0wt.%/min, and 0.26\u00a0\u00b1\u00a00.04\u00a0wt.%/min, respectively; thus, desorption proceeds even faster than for undoped MgH2 at 325\u00a0\u00b0C (0.13\u00a0\u00b1\u00a00.01\u00a0wt.%/min). At 300\u00a0\u00b0C, VC-doped MgH2 can desorb 5.0 wt% of H2 within only 3.2\u00a0min. The dehydrogenation kinetics of VC-doped MgH2 are also better than those of Mg-Ni-V [48], Mg-5\u00a0wt.%NiS [49], and MgH2-Co@C [51]. Therefore, both the hydrogenation/dehydrogenation properties and hydrogen absorption/desorption kinetics of MgH2 are significantly improved after the addition of VC.The apparent activation energy is an important parameter to evaluate the hydrogen adsorption and desorption kinetics of materials. It can be calculated by the Johnson-Mehl-Avrami (JMA) equation (ln [\u2212ln(1\u2212\u03b1)]\u00a0=\u00a0nlnk\u00a0+\u00a0nlnt, where \u03b1, k, t and n are the phase transformation fraction, a temperature-dependent kinetic parameter, the reaction time and the order of the reaction, respectively [52]). According to the slopes of the \n\nln\nk\n\u223c\n\n1000\n/\nT\n\n\n plots shown in Fig.\u00a03(g, h), the hydrogenation apparent activation energy (E\nabs) values of VC-doped MgH2 and undoped MgH2 were calculated as 42.4\u00a0\u00b1\u00a01.4\u00a0kJ/mol and 77.3\u00a0\u00b1\u00a03.0\u00a0kJ/mol, respectively. In addition, the dehydrogenation apparent activation energy (E\ndes) values of VC-doped MgH2 (89.3\u00a0\u00b1\u00a02.8\u00a0kJ/mol) and undoped MgH2 (138.5\u00a0\u00b1\u00a02.4\u00a0kJ/mol) were also obtained from the linear relationship of the lnk versus 1000/T plots, revealing that the apparent activation energy for hydrogenation and dehydrogenation of MgH2 is significantly reduced after the addition of VC. Thus, VC is an efficient catalyst to improve the hydrogen absorption/desorption properties of MgH2. Table\u00a01\n represents the empirical dehydrogenation apparent activation energies of catalyzed common MgH2 systems. It can be seen from the table that the dehydrogenation activation energy of oxides or Nb-based compounds as catalysts was higher than that of VC as catalysts. For example, the dehydrogenation activation energy of VNbO5-catalyzed (99.0\u00a0kJ/mol) [17], KNbO3-catalyzed (93.6\u00a0kJ/mol) [54], NbN-catalyzed (113.9\u00a0kJ/mol) [55], MnFe2O4-catalyzed (108.4\u00a0kJ/mol) [59] MgH2 was 9.7\u00a0kJ/mol, 4.3\u00a0kJ/mol, 24.6\u00a0kJ/mol and 19.1\u00a0kJ/mol lower than that of VC-catalyzed MgH2. In addition, the dehydrogenation activation energy of VC-catalyzed MgH2 system also lower than those of K2SiF6-catalyzed, Ni@pCNF-catalyzed, HfCl4- catalyzed and some others catalyzed MgH2 systems listing in Table\u00a01. Obviously, the introduction of VC into MgH2 remarkably lowered the hydrogen desorption energy barrier.To further analyze the hydrogen storage properties of MgH2 and VC-doped MgH2, pressure-composition-isotherm (PCI) measurements were performed at various temperatures (Fig.\u00a04\na and c). The hydrogen absorption and desorption plateau increases with the increasing temperature. Undoped MgH2 causes completely reversible hydrogen absorption/desorption at 350, 375, and 400\u00a0\u00b0C; however, it cannot undergo a reversible hydrogen storage process at 325\u00a0\u00b0C. The reversible hydrogen storage capacity of undoped MgH2 is approximately 6.7 wt%. After doping with 10\u00a0wt.% of VC, although the reversible hydrogen storage capacity of VC-doped MgH2 is reduced to about 5.8\u00a0wt.%, completely reversible hydrogenation/dehydrogenation is observed at 300, 325, 350, and 375\u00a0\u00b0C. Moreover, the reversible hydrogenation/dehydrogenation temperatures for MgH2 remarkably decrease after the addition of VC. The hydrogenation/dehydrogenation enthalpies of undoped and VC-doped MgH2 samples were estimated by the Van't Hoff equation (\n\nln\nP\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, where P is the hydrogenation/dehydrogenation plateau pressure, R is the universal gas constant, \u0394H is the reaction enthalpy,\u2005\u0394S is the reaction entropy, and T is the hydrogenation/dehydrogenation temperature). The linear relationships between lnP and 1/T for the hydrogenation and dehydrogenation processes are plotted in Fig.\u00a04b and 4d. The hydrogenation and dehydrogenation enthalpies of VC-doped MgH2 were calculated as 71.6\u00a0\u00b1\u00a02.8\u00a0kJ/mol H2 and 74.7\u00a0\u00b1\u00a00.8\u00a0kJ/mol H2, respectively. These values are about 2.1\u00a0kJ/mol and 2.4\u00a0kJ/mol lower than those of undoped MgH2 (73.7\u00a0\u00b1\u00a02.7\u00a0kJ/mol and 77.1\u00a0\u00b1\u00a05.3\u00a0kJ/mol), respectively. The decrease of reaction enthalpy is responsible for the decrease of the hydrogenation and dehydrogenation temperature of MgH2, indicating that the addition of VC dramatically improves the hydrogen storage properties of MgH2.To investigate the role of VC in the improvement of the hydrogen storage performance of MgH2, XRD characterization of hydrogenated/dehydrogenated VC-doped MgH2 specimens was performed. For comparison, the XRD characterization results for VC-doped MgH2 and undoped MgH2 are also presented in Fig.\u00a05\n. The diffraction peaks of rehydrogenated MgH2 can be assigned to MgH2 (Fig.\u00a05a). The crystal structure of VC does not change during milling and hydrogen absorption and desorption processes, implying that VC remains stable and only acts as a catalyst during these processes (Fig.\u00a05(b\u2013d)). In addition, the diffraction peaks of VC-doped MgH2 became sharp after dehydrogenation, indicating an increment in the crystallization.It has been proved that VC acts as a catalyst during the dehydrogenation of MgH2. Moreover, the reflections of the VC (200) lattice planes can be clearly detected from its XRD Fig.\u00a01a) and SAED (Fig.\u00a02b) patterns. Hence, in the present study, the VC (100) surface consisting of 80 atoms with five atomic layers was constructed and a vacuum layer of 15\u00a0\u00c5 was used between the slabs. An adsorption model of an Mg4H8 cluster on the VC (100) surface was built to simulate the role of VC in the dehydrogenation process of MgH2 (Fig.\u00a06\n). The geometries of the adsorbate and VC layers were completely relaxed, except for the bottom three layers, during DFT calculations. To reveal the dehydrogenation behavior of VC-doped MgH2, the dehydrogenation enthalpy and electronic structure of MgH2 on the VC (100) surface were studied. It should be noted that dehydrogenation energy affects the dehydrogenation rate and dehydrogenation temperature of hydrogen storage materials. To analyze the catalytic effect of VC on the dehydrogenation properties of MgH2, the dehydrogenation energy of the pure Mg4H8 cluster and a Mg4H8 cluster on the VC (100) surface were calculated by Eqs.\u00a0(1) and ((2), respectively. In the Mg4H8 cluster, Mg atoms were arranged in a tetrahedron structure, that is, two kinds of H atoms bonded with one Mg atom and three Mg atoms each, which are labeled as Htop and Hface (Fig.\u00a01), respectively:\n\n(1)\n\n\n\n\u0394\n\n\nH\n\nd\ne\n\n\n\n(\n\nM\n\ng\n4\n\n\nH\n8\n\n\n)\n\n=\nE\n\n(\n\nM\n\ng\n4\n\n\nH\n\n8\n\u2212\nx\n\n\n\n)\n\n+\n\nx\n2\n\nE\n\n(\n\nH\n2\n\n)\n\n\u2212\nE\n\n(\n\nM\n\ng\n4\n\n\nH\n8\n\n\n)\n\n\n\n\n\n\n\n(2)\n\n\n\n\n\n\n\u0394\n\n\nH\n\nd\ne\n\n\n\n(\n\nVC\n+\nM\n\ng\n4\n\n\nH\n8\n\n\n)\n\n\n\n\n=\n\n\n\nE\n\n(\n\nVC\n+\nM\n\ng\n4\n\n\nH\n\n8\n\u2212\nx\n\n\n\n)\n\n\n\n\n\n\n\n\n\n+\n\n\nx\n2\n\nE\n\n(\n\nH\n2\n\n)\n\n\u2212\nE\n\n(\n\nVC\n+\nM\n\ng\n4\n\n\nH\n8\n\n\n)\n\n\n\n\n\n\n\n\nHere,\u2005E(Mg4H8) and E(VC\u00a0+\u00a0Mg4H8) are the total energies of the Mg4H8 cluster and of the Mg4H8 cluster on the VC (100) surface, respectively, E(Mg4H8\u00a0\u2212\u00a0x\n) and E(VC\u00a0+\u00a0Mg4H8\u00a0\u2212\u00a0x\n) are the total energies of the Mg4H8 cluster and of the Mg4H8 cluster on the VC (100) surface with the desorption of x number (x\u00a0=\u00a01, 2, 8) of H atoms, respectively, and E(H2) is the total energy of gaseous H2.\nFig.\u00a07\n presents the dehydrogenation energy for the desorption of one H atom (Htop or Hface), two H atoms, and eight H atoms in the Mg4H8 cluster and the Mg4H8 cluster on the VC (100) surface. It was found that in all cases, the dehydrogenation energy of Mg4H8 is improved by VC, causing a decrease in the dehydrogenation temperature of VC-doped MgH2. The catalytic effects of the Mg4H8 cluster on the VC (100) surface were also revealed. For the desorption of one H atom, Htop atoms require a lower dehydrogenation energy than Hface atoms. For the desorption of two H atoms, Hface and Htop atoms require the lowest dehydrogenation energy. This finding confirmed that the addition of VC is beneficial to weaken interaction between Mg and H. Further, to reveal the interactions between MgH2 and the VC (100) surface, the total density of state (DOS), the partial density of states (PDOS), and the electron density difference of Mg4H8 on the VC (100) surface were calculated (Fig.\u00a07). It is evident from the DOS shown in Fig.\u00a07a that the s orbitals of Mg are located at \u22124.0\u00a0eV below the Fermi level and are hybridized with the d orbitals of V atoms and the p orbitals of C, indicating a strong interaction between Mg and VC. The yellow (blue) areas in Fig.\u00a08\n(b) indicate the increase (decrease) of electron density. It is discernible from Fig.\u00a08(b) that mass charge depletion areas appear around Mg atoms, whereas charge accumulation areas are found between Mg4H8 and the VC (100) surface, indicating that electrons of the s and p orbitals of Mg are transferred to the VC (100) surface, weakening the bonding between Mg and H atoms. The lengths of Mg-Htop and Mg-Hface bonds in Mg4H8 on the VC (100) surface are longer than those in Mg4H8. In the pure Mg4H8 cluster, the lengths of Mg-Htop and Mg-Hface bonds are 1.70\u00a0\u00c5 and 1.99\u00a0\u00c5, respectively, and the corresponding values for Mg4H8 on the VC (100) surface increase to 1.82\u00a0\u00c5 and 2.07\u00a0\u00c5, respectively. These findings indicate that the elongation of Mg-H bonds promotes the dehydrogenation of MgH2.VC was successfully synthesized by an etching method and employed to improve the de/rehydrogenation of MgH2. VC imparts superior catalytic effects on the hydrogen storage thermodynamics and kinetics of MgH2. VC-doped MgH2 can absorb hydrogen at room temperature and release hydrogen at 170\u00a0\u00b0C. Non-isothermal hydrogenation tests revealed that undoped MgH2 can hardly absorb hydrogen until the temperature reaches 125\u00a0\u00b0C, which is far higher than for the VC-doped samples. Isothermal re/hydrogenation measurements indicate that VC-doped MgH2 can absorb 5.0\u00a0wt.% of H2 within 9.8\u00a0min at 100\u00a0\u00b0C and desorb 5.0\u00a0wt.% of H2 within 3.2\u00a0min at 300\u00a0\u00b0C. At 325\u00a0\u00b0C, VC-doped MgH2 can release approximately 6.0\u00a0wt.% of H2 with a hydrogen desorption rate of 2.18\u00a0\u00b1\u00a00.03\u00a0wt.%/min, which is 16.8 times faster than for undoped MgH2 under the same conditions. The\u2005Ea\n of VC-doped MgH2 is 89.3\u00a0\u00b1\u00a02.8\u00a0kJ/mol, which is about 49.2\u00a0kJ/mol lower than that of undoped MgH2. Mass charge depletion areas were detected around Mg atoms, whereas charge accumulation areas were found between Mg4H8 and the VC (100) surface, weakening the bonding between Mg and H atoms. The Mg-H bond length on the VC (100) surface is significantly longer than that in MgH2. The elongation of Mg-H bonds promotes the dehydrogenation of MgH2; thus, the hydrogen storage properties of MgH2 are remarkable improved through addition of VC.This work was supported by the National Natural Science Foundation of China (Grant Nos. 52261038 and 51861002), the Natural Science Foundation of Guangxi Province (Grant No. 2018GXNSFAA294125), and the Innovation-driven Development Foundation of Guangxi Province (Grant No. AA17204063). J.E. acknowledges additional support by the Ministry of Science and Higher Education of the Russian Federation in the framework of the Increase Competitiveness Program of NUST \"MISiS\" (grant number K2-2020-046). We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.", "descript": "\n                  Hydrogen is considered one of the most ideal future energy carriers. The safe storage and convenient transportation of hydrogen are key factors for the utilization of hydrogen energy. In the current investigation, two-dimensional vanadium carbide (VC) was prepared by an etching method using V4AlC3 as a precursor and then employed to enhance the hydrogen storage properties of MgH2. The studied results indicate that VC-doped MgH2 can absorb hydrogen at room temperature and release hydrogen at 170\u00a0\u00b0C. Moreover, it absorbs 5.0\u00a0wt.% of H2 within 9.8\u00a0min at 100\u00a0\u00b0C and desorbs 5.0\u00a0wt.% of H2 within 3.2\u00a0min at 300\u00a0\u00b0C. The dehydrogenation apparent activation energy of VC-doped MgH2 is 89.3\u00a0\u00b1\u00a02.8\u00a0kJ/mol, which is far lower than that of additive-free MgH2 (138.5\u00a0\u00b1\u00a02.4\u00a0kJ/mol), respectively. Ab-initio simulations showed that VC can stretch Mg-H bonds and make the Mg-H bonds easier to break, which is responsible for the decrease of dehydrogenation temperature and conducive to accelerating the diffusion rate of hydrogen atoms, thus, the hydrogen storage properties of MgH2 are remarkable improved through addition of VC.\n               "}
{"full_text": "Until date, the majority of automobile transportation uses traditional oil fuels and releases a significant amount of CO2 polluting the environment significantly. Thus, the depletion of fossil fuels (including petroleum, coal, and natural gas) and increasing energy demand are the essential challenging problems that cause for the efficient designing of energy storage devices and discovering of alternative earth abundant energies [1\u201312], while several renewable energy production technologies being developed as alternative energy resources, such as solar, wind, hydrothermal and other renewable energy sources [13]. However, there is a contrast between renewable energy power production and storage technology. The European Union has installed renewable energy storage capacity of 50 GW, which corresponds to only about 5% of our daily generation [14]. The US department of energy launched the \u2018\u2018EV Everywhere Grand Challenge\u2019\u2019 (EV: Electric vehicle) as an initiative for improved batteries with dramatically reduced cost and weight, aimed at producing EVs that are as affordable as today\u2019s gasoline-powered vehicles [15]. Therefore, the need to store energy is increasing, and energy storage devices are one way to do this. Different types of storage devices have been used based on the storage requirements, where long-term usage needs batteries, while short time delivery requires supercapacitors. For the last couple of decades, with pioneering work on lithium-ion batteries (LIB) [16\u201319], there has been a dramatic increase in portable electronic devices. The emergence of electric and hybrid vehicles demands the high-energy storage systems due to the limitations of state-of-the-art LIB. Even the development of high-energy storage systems is essential to save extra power produced from the renewable energy storage and deliver to where it is demanding. Thus, there is a big interest in developing a new type of batteries beyond Li-ion battery [7\u20139,13,14,20\u201322].Among the electrochemical batteries, the metal-air batteries have been given considerable re-attention due to their outstanding higher energy density. In contrast to the LIB, the cathode breathing oxygen from the atmosphere serves as a fuel during the cycling process in the metal-air battery system. Further, it has an infinite source of the reactant on both anode (metallic sheet) and cathode (atmospheric oxygen), which results in high theoretical energy [23\u201325]. Table 1\n shows the theoretical energy density of various metal-air batteries [26]. Among the various metal-air batteries, metals such as Ca, Al, Fe, and Zn are suitable for the aqueous system with open-air friendly, whereas Li and Mg are mainly suitable for the non-aqueous system, where their stability is limited at atmospheric condition [24]. Within this, Li-oxygen and Zn-air batteries (ZAB) have been identified as next-generation energy storage devices. Although Li-oxygen batteries have a much higher energy density, the ZAB system could reach commercialization sooner [27\u201330]. ZAB has various benefits such as low cost, abundance, low equilibrium potential, environmental benignity, a flat discharge voltage, and a long shelf life. The most important merit of ZAB is that the assembly and working of batteries can be performed under ambient conditions (Fig. 1\n) [26,31,32]. During the last decades, the primary ZAB have been commercialized for various application such as hearing aids in medical applications, telecommunication, and electronic devices with long operation time in remote places [32].For rechargeable ZAB, a number of ways have been proposed, such as hydraulic, mechanical, and electrical recharging. Each method of recharging has its own merits and demerits. Working of hydraulic recharging in ZAB is like fuel cell; there will be a continuous supply of reactants to the electrodes. Secondly, the mechanical recharging is similar to the hydraulic method, whereas it needs a continuous replacement of electrolyte and metallic anode in the system. However, these two methods require special requirement and special setup for regeneration. On the other hand, electrical recharging is like other rechargeable batteries, where the ZAB recharge by running a current through it without changing the system [23]. A rechargeable zinc-air battery is composed of three compartments such as zinc metal as an anode, an air-breathing electrode as cathode, and an alkaline solution as an electrolyte. The cathode compartments were named as gas-diffusion electrode (GDE), which further divided into a gas diffusion layer (GDL) and a catalytic active layer (CL). The GDL is a porous structure made by weaving carbon fibers into a carbon cloth or by pressing carbon fibers together into a carbon paper. The active catalyst plays critical roles in a typical ZAB application, and it is able to catalyze both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of the oxygen during the cycling. The GDE was prepared by a uniform coating of active catalyst into the GDL by brush coating or spray coating technique. For a rechargeable ZAB system, during the cycling process, the Zn/Zn2+ redox reaction occurs at the anode side and the ORR/OER occurs at the cathode side, simultaneously. In a discharge conditions, the oxygen from the atmosphere diffuses into the GDE due to the oxygen pressure difference between the outside and inside cell, while the catalyst facilitates the reduction of oxygen to hydroxyl ions (OH\u2212) in alkaline electrolyte with the help of electrons generated from the oxidation of zinc metal as the anode reaction. The OH\u2212 ions then moved into anode compartment through the electrolyte, and then it react with Zn2+ to form a zincate (Zn(OH)4\n2\u2212) ion (Fig. 1). During the continuous cycling, the soluble zincate ions reaches its saturation limit, and then it is converted into solid ZnO, where the ZnO precipitates on the surface of the anode as thistle-like structure [26,33]. In view of thermodynamics, charging is a reversible chemical reaction of discharging, but this is not easy due to the precipitation of solid ZnO on the anode. On the other hand, OER is also a reversal of ORR reaction, where necessarily more O\u2013O bonds are built by breaking O\u2013H bonds [34,35]. Based on these reactions, GDE is a vital component of ZAB, and it is called a three-phase reaction spot, where the solid electrode interfaces with liquid electrolyte, while the reactant is in the gas form. In recent years, ZAB is advancing, whereas a number of well-defined challenges with the rechargeable ZAB exist in the laboratory level, which need to be addressed to provide superior performance in practical applications. Moreover, the precipitation of insoluble white ZnO with thistle-like structure on the surface of the Zn anode, and shape change and dendrite growth of the Zn anode during repeated charge\u2013discharge processes could be detrimental to the stability of ZAB. To avoid structural deformation of metallic zinc, researchers developed high surface area anode with different morphology such as zinc particles, spheres, flakes, ribbons, fibers, and foams for enhancing the reaction mechanism [26,36].There are several potential problems associated with the cathode compartment such as (i) low efficiency due to sluggish oxygen reduction/-evolution reactions kinetics; (ii) atmospheric CO2 possibly react with hydroxyl ions to form carbonates; (iii) formation of insoluble ZnO discharge product, which deposit on the surface of GDE; (iv) inadequate understanding of catalysts effect, and (v) mechanical breakdown generated on the catalyst from the surface of the GDL due to the oxygen evolution during charging (Fig. 1) [26,37]. Further, the use of electrocatalysts in the GDE needs to be investigated for their ability to lower the over-potential for charge and discharge reactions and enhance the cycle life. To date, several kinds of precious and non-precious catalysts have been explored for ZAB, including metal oxides, perovskites, chalcogenide, allotropes of carbon-based materials, noble metals, and so on (Fig. 2\n\nb) [34,38\u201345 46\u201380]. So far noble metals (Pt, Ru, Ir) and their alloys (Pt-Ru/C) are used in commercial rechargeable ZAB, whereas they are too expensive to be viable for large-scale commercial applications [81]. Despite these challenges, recent experimental and computational work has provided much-desired information for understanding new catalyst design and synthesis. These cathode materials essentially improve the cycling stability of ZAB to some extent; in addition, the cost of the material should not be compromised. Current challenges in electrocatalyst research not restricted to materials processing by programmable design and assembly, by understanding and predicting defects across time and length scales as well as functionalizing defects for unprecedented properties, and by the discovery of multilateral systems of extreme environments. Thus, it is anticipated that these challenges can only be overcome by enhancing basic understanding of electrocatalyst, and that will ultimately enable advancement in ZAB system (Fig. 2\na) [82].With the Nobel Prize-winning work \u201cfor groundbreaking experiments regarding the two-dimensional material graphene\u201d in 2010, the development of innovative graphene-based materials is a key element to design sustainable and resource efficient industries of modern society [83]. The fundamental understanding of graphene materials with regard to significant structure\u2013property correlations on different spatial and time scales, the future of direct manufacturing and production processes and efficient modelling and simulation methods are derisive prerequisites innovation in graphene-based engineering. From the socio-ecological, technical and economic point of view, graphene-based materials today have the extremely important function of an \u201cenabling material\u201d. The high-tech system in energy and environmental technology, automotive and aerospace engineering, medical sciences, information and communication technology, electrical engineering and process engineering are impossible without key enabling materials and components based on graphene [84\u201386]. Graphene, an allotrope of carbon having a monolayer of sp2 hybridized carbon atoms tightly packed into a two dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all other dimensionalities. Graphene has attracted great interest in energy application due to its band overlap between the valences bands to the conduction band with almost zero bandgaps [83]. Graphene is better described as a mother of all graphitic forms and it can be wrapped up into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphite. Graphene is one of the versatile materials for various applications due to its peculiar properties such as specific surface area (2630 m2g\u22121), intrinsic mobility (200,000\u00a0cm2 v\u22121s\u22121), Young\u2019s modulus (1.0 TPa), thermal conductivity (5000 Wm\u22121K\u22121), optical transmittance (97.7%), and good electrical conductivity (103 Sm\u22121). In order to produce high quality and quantity of graphene without losing the properties, various techniques have been established. They were broadly classified into two kinds: top-down and bottom-up approaches. The method such as chemical exfoliation, mechanical exfoliation, and chemical synthesis fit into the top-down approach, whereas the method such as pyrolysis, epitaxial growth, and thermal chemical vapor deposition (CVD) method fit into the bottom-up approach (Fig. 3\n). For the preparation of graphene based-air catalysts, the usage of pure graphene is almost restricted due to the fact that graphene is almost not soluble, while it cannot be dispersed in water or any organic solvent [87,88].Different from graphene, graphene oxide (GO) almost contains a variety of oxygen functional groups, like hydroxyl and epoxy group on its basal plane, and carboxyl at its edge. The GO with its outstanding water solubility, control over the functionalization and ease in preparation make them the most popular precursor of graphene based-composites. From the literature, GO is synthesized frequently via chemical oxidation of natural graphite using Hummers and Offeman method in which NaNO3 and KMnO4 dissolved in concentrated H2SO4 was used to oxidize graphite into graphite oxide [87,89,90]. In Hummers\u2019 method, first reduces the interlayer van der Waals forces of the graphitic layer to increase the interlayer spacing. Then it exfoliates graphene with a single to few layers by rapid heating or sonication. To solve the problem associated with Hummers\u2019 method, various modified Hummers\u2019 method has been developed for better usage. Various research work have been reported to improve the electrochemical performance of the catalyst such as non-metals, noble metals, non-noble metals, metal oxides, perovskites, nitrides, sulfides, and carbides, where the synergistic coupling between catalysts with graphene-based materials is a promising approach to create more active sites, and that can improve the electrical conductivity, chemical stability due to an interaction between graphene structures with the electrocatalyst. The graphene structure possesses the strongest in-plane C\u2013C bond, while \u03c0 bond in the out-of-plane contributes to a delocalized network of electrons, which is responsible for electron conduction of graphene, and that affords weak interaction among graphene layers or between graphene and catalyst. Further, the electrical and chemical properties of graphene-based materials are easily tuned by substituting with heteroatoms, such as N, P, B, and S, which tailor their electron-donor properties of graphene [91]. The effect of the dopants on the electrocatalytic properties of the graphene-based materials is mainly associated with three features of the dopant element: the number of electrons in the external shell, the electronegativity, and the size [92\u201395]. These materials generally facilitate the charge transfer, which could be mostly due to the difference in electronegativity between the graphene and the dopant. If the electronegativity of the dopant is larger, then the charge difference on the adjacent carbon atoms can be higher, and hence electrocatalytic activity can be greater. Recently, a large number of the heteroatom-doped (N, S, B, P, and halogens) graphene-based materials are focused for metal-air battery application due to their high activity and long-term stability. To further enhance the activity of heteroatom-doped graphene, research is also conducted on various catalysts such as integration of graphene with non-metals, noble metals, non-noble metals, metal oxides, nitrides, sulfides, carbides, and other carbon composites, where an overall enhancement in the bi-functional activity in metal-air battery can be achieved due to the synergistic effect exerted by the dopants and catalyst [84,85]. In this respect, the potential graphene-based air catalysts such as graphene with heteroatoms, non-metals, noble metals, non-noble metals, metal oxides, perovskites, nitrides, sulfides, carbides, and other carbon composites have been reviewed in the present paper in-light-of-their high oxygen reduction reaction/ oxygen evolution reaction activity and zinc-air battery performance for the development of zinc-air batteries. Moreover, this review further extend the recent progress on the zinc-air batteries including the strategies used to improve the high cycling-performance (stable even up-to 394 cycles), capacity (even up-to 873\u00a0mA\u00a0h\u00a0g\u22121), power density (even up-to 350 mW cm\u22122), and energy density (even up-to 904\u00a0W\u00a0h\u00a0kg\u22121).\nTable 2\n depicts the ORR/ OER activity and zinc-air battery performance of various kinds of reported graphene with non-metals based air catalysts [96\u2013106]. Creating defects on the graphene can enhance the ORR/OER activity and ZAB performance. Jia et. al. [101] have observed that DG exhibits high ORR/OER activity with high ZAB performance. It was prepared by the following steps: At first, NG was obtained by annealing the graphene with melamine for 2\u00a0h at 700\u00a0\u00b0C under N2 atmosphere; Finally, DG was prepared by annealing the NG for 2\u00a0h at 1150\u00a0\u00b0C under N2 atmosphere. NG contains pyridinic, pyrrolic, and graphitic N whereas DG contains absence of N, and DG is composed of defect graphene with I\nD/I\nG ratio of 1.13, and it contains holes, while it possesses various structural defects (Fig. 4\n\nc) such as pentagons, heptagons, and octagons at the edge of holes, and it exhibits high wettability (contact angle: 44.3\u00b0), and that can possibly enhance its ZAB performance with ORR/OER activity. They observed that DG exhibits higher ORR/OER activity than NG. It exhibits high OER (EJ=10: 1.6\u00a0V) and ORR activity (E1/2: 0.76\u00a0V; n: 3.87) with low \u0394E of 0.84\u00a0V. ZAB with DG affords 100\u00a0mA\u00a0mg\u22121 at 1\u00a0V and high power density of 154 mW mg\u22121 at 195\u00a0mA\u00a0mg\u22121. ZAB with DG (Fig. 4\na) exhibits initial polarization voltage of ~0.76\u00a0V with negligible increased polarization voltage of ~0.03\u00a0V (Fig. 4\nb) after 95 cycles at 10\u00a0mA\u00a0mg\u22121 with Zn foil as anode and 6\u00a0M KOH with 0.2\u00a0M Zn(O2CCH3)2 as electrolyte, which indicates its much high performance.Doping N with graphene can enhance the ORR activity and ZAB performance. Tian et. al. [97] have observed that NG exhibits high ORR activity with high ZAB performance. It was prepared by the following steps: At first, a powder was obtained by dissolving dicyandiamide and monohydrate glucose in water followed by drying; Finally, NG was prepared by heating the powder at 580\u00a0\u00b0C for 4\u00a0h followed by 850\u00a0\u00b0C for 6\u00a0h under Ar atmosphere. It is composed of N-doped graphene, and it possesses wrinkled sheet structure, and it exhibits some disordered areas in the graphene layer, and it contains O\u2013C, OC, and O\u2013CO bonds, and it contains pyridinic N at 398.2\u00a0eV, pyrrolic N at 400.2\u00a0eV, and graphitic N at 400.9\u00a0eV, where the N-doped graphene can provide spontaneous adsorption and fast solid-state diffusion of oxygen on ultra-large graphene surface, and that can possibly enhance its ZAB performance with ORR activity. It exhibits high ORR activity (E1/2: \u22120.18\u00a0V (vs Ag/AgCl)). ZAB with NG affords high capacity of 793\u00a0mA\u00a0h\u00a0g\u22121 at 100\u00a0mA\u00a0cm\u22122 and power density of 218 mW cm\u22122.N-doped graphene obtained through carbonization of natural rice can create edge effects and topological defects, and that can enhance the ORR/OER activity. Tang et. al. [100] have observed that NG exhibits high ORR/OER activity. It was prepared by direct carbonization (at 950\u00a0\u00b0C for 1.5\u00a0h under Ar atmosphere) of sticky rice as carbon precursor, Mg(OH)2 as a template, and melamine as a nitrogen source. It is composed of N-doped graphene, and it contains pyridinic N, pyrrolic N, and quaternary N, and it exhibits I\nD/I\nG ratio of 1.24, which suggests the existence of holes and edge defects, and it exhibits high surface area (1100\u00a0m2 g\u22121), and it possesses mesoporous structure, and it contains sp2 hybridized N\u2013C bonds, and it contains nanosized holes all over the plane, and it contains C and N, which are uniformly distributed, and that can enhance its ORR/OER activity. It exhibits high OER (EJ=10: 1.67\u00a0V) and ORR activity (E1/2: 0.77\u00a0V; n: ~3.8) with low \u0394E of 0.9\u00a0V. ZAB with NG exhibits OCP of 1.42\u00a0V and power density of 3 mW cm\u22122.Preparing N-doped graphene with electron-withdrawing pyridinic N and electron-donating quaternary N can serve as active sites for ORR and OER, respectively, and that can enhance the ORR/OER activity and ZAB performance. Yang et. al. [102] have observed that NG exhibits high ORR/OER activity with high ZAB performance. It was prepared by the following steps: At first, fine powder mixture was obtained by grinding the melamine and L-cysteine at a mass ratio of 4:1; Finally, NG was prepared by pyrolysis of the mixture at 600\u00a0\u00b0C for 2\u00a0h followed by carbonization at 1000\u00a0\u00b0C for 2\u00a0h under Ar atmosphere. It is composed of N-doped graphene, and it possesses 3D structured nano-ribbon networks with entangled and crumpled wrinkle-like structures, and it exhibits high surface area (~530\u00a0m2 g\u22121), and it contains mesopores and, it exhibits a pore volume of ~2.9\u00a0cm3 g\u22121, and it contains pyridinic N (1.45 atomic %), pyrrolic N (0.95 atomic %) and quaternary N (2.8 atomic %), where electron-withdrawing pyridinic N moieties (p-type domain, Fig. 4\nd) can act as active sites for OER, while electron-donating quaternary N (n-type domain, Fig. 4\nd) can serve as active sites for ORR, and it exhibits the I\nD/I\nG ratio of 3.34, which suggests its high disordered carbon structure, and it possesses small mean average crystallite size of the sp2 domains (1.3\u00a0nm), while it exhibits low sp2/sp3 ratio (0.36), which indicates its high edge sites, which can enhance electron transfer rate and electrocatalytic activity, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER (EJ=10: 1.66\u00a0V) and ORR activity (E1/2: 0.84\u00a0V; n: ~3.95) with low \u0394E of 0.82\u00a0V. ZAB with NG exhibits OCP of 1.46\u00a0V, 20\u00a0mA\u00a0cm\u22122 at 1.09\u00a0V, capacity of 873\u00a0mA\u00a0h\u00a0g\u22121 and power density of 65 mW cm\u22122. ZAB with NG affords initial polarization voltage of ~0.8\u00a0V with an increased polarization voltage of ~0.2\u00a0V after\u00a0>\u00a0150 cycles at 2\u00a0mA\u00a0cm\u22122 with Zn foil as an anode and 6\u00a0M KOH with 0.2\u00a0M ZnCl2 as the electrolyte, which indicates its high performance.Preparing N-doped exfoliated graphene can enhance the ZAB performance. Lee et. al. [106] have observed that N-ex-G exhibits high ZAB performance. It was obtained by one-step thermal reduction and NH3 treatment at 1100\u00a0\u00b0C under an extreme heating rate (>150\u00a0\u00b0C sec\u22121). It is composed of N-doped exfoliated graphene, and it possesses wrinkled surfaces and wavy edges, and it exhibits hexagonal symmetry, which suggests the existence of symmetrical three-fold sp2 bonding of carbon atoms, and it exhibits I\nD/I\nG ratio of 1.14, and it contains pyridinic N, pyrrolic N, and quaternary N, and that can possibly enhance its ZAB performance. ZAB with N-ex-G affords 47.6\u00a0mA\u00a0cm\u22122 at 0.8\u00a0V and power density of 42.4 mW cm\u22122.S-doped graphene foam prepared from food (idly) can enhance ORR/OER activity and ZAB performance. Patra et. al. [103] have observed that S-doped graphene foam exhibits high ORR/OER activity with high ZAB performance. It was prepared by the following steps: At first, S-doped graphene-idli was obtained using S-doped graphene and rice flour through microwave treatment; Finally, S-doped graphene foam was prepared by calcination of the S-doped graphene-idli for 1\u00a0h at 300\u00a0\u00b0C under air atmosphere, where the furnace had been pre-heated at 80\u00a0\u00b0C prior to calcination. It is composed of S-doped graphene foam, and it contains\u2013C\u2013S\u2013C\u2013at 163.7 and 164.3\u00a0eV, and\u2013C\u2013SOx\u2013C\u2013at 169.8\u00a0eV (Fig. 4\ne), and it possesses porous, rough, crumpled, and sponge-like structure, and it exhibits high surface area (499\u00a0m2 g\u22121) with a pore volume of 0.522\u00a0cm3 g\u22121, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high ORR and OER activity with low \u0394E of 0.65\u00a0V. ZAB with S-doped graphene foam exhibits high OCP of ~1.40\u00a0V, ~ 250\u00a0mA\u00a0cm\u22122 at 0.6\u00a0V, and power density of ~300 mW cm\u22122. ZAB with S-doped graphene foam affords negligible increased polarization voltage after cycled for 125\u00a0h at 1\u00a0mA\u00a0cm\u22122 with carbon tape coated Zn powder as an anode and O2-saturated 6\u00a0M KOH with 0.2\u00a0M ZnCl2 as the electrolyte, which suggests its high performance.Preparing S, N co-doped graphene-like electrocatalyst can enhance the ORR activity and ZAB performance. Zhang et. al. [105] have observed that SN-G exhibits high ORR activity with high ZAB performance. It was prepared from keratin as precursor along with potassium hydroxide activation, followed by high-temperature graphitization and NH3 treatment. It is composed of S, N co-doped graphene-like nanobubble and nanosheet hybrids, and it exhibits much high surface area (1799\u00a0m2 g\u22121) with a pore volume of 1.01\u00a0cm3 g\u22121. Further it exhibits I\nD/I\nG ratio of 1.04, and characteristic peak of graphene (Raman spectra: ~2700\u00a0cm\u22121), and it contains predominant C-sp2 (73%) with C-sp3, C\u2013N, C\u2013O and CO. The presence of pyridinic-N, pyrrolic-N, graphitic-N, thiophene-S, and oxidized-S, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity (E1/2: 0.842\u00a0V; n: 3.98). ZAB with SN-G exhibits high capacity of 767\u00a0mA\u00a0h\u00a0g\u22121 and power density of 201 mW cm\u22122.Preparing defect enriched S, N co-doped graphene-like carbon can enhance the ORR activity and ZAB performance. Zhang et. al. [98] have observed that Def-SN-GLC exhibits high ORR activity with high ZAB performance. It was obtained from cystine as a precursor through KOH activation and NH3 injection at high temperature. It is composed of defect enriched S, N co-doped graphene-like carbon, and it contains carbon nanosheets with sub-transparent and wrinkled lamellar properties, which possesses similar morphology of graphene. The high surface area (1309\u00a0m2 g\u22121) with pore volume of 0.86\u00a0cm3 g\u22121, and it exhibits I\nD\n/I\nG ratio of 1.09, and it exhibits characteristic peak of graphene (Raman spectra: ~2700\u00a0cm\u22121). Further, it contains pyridinic-N, pyrrolic-N, graphitic-N, thiophene-S, and oxidized-S, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity (E1/2: 0.849\u00a0V; n: 3.96). ZAB with Def-SN-GLC affords high OCP of 1.5\u00a0V, 20\u00a0mA\u00a0cm\u22122 at 1.24\u00a0V and power density of 252 mW cm\u22122.B, N co-doped graphene with graphitic N and BC3 can facilitate the ORR activity. Qin et. al. [96] have observed that B, N-pG-O catalyst exhibits high activity for ORR. It was prepared by the following steps: At first, N-pG-O was obtained by hydrothermal treatment of N-pG with 8\u00a0M HNO3 for 12\u00a0h at 60\u00a0\u00b0C; Finally, B, N-pG-O was obtained by annealing N-pG-O with boric acid for 15\u00a0min at 1000\u00a0\u00b0C. It is composed of B, N co-doped porous graphene, and it possesses honeycomb-like porous structure, and it exhibits high surface area (1303\u00a0m2 g\u22121), and it contains graphitic N and BC3, and it shows I\nD/I\nG ratios of 1.05, and that can possibly enhance its ORR activity. They observed through density functional theory (DFT) that the synergistic effect between BC3 and graphitic N can enhance the reduction of oxygen. It exhibits high ORR activity (E1/2: 0.86\u00a0V; n: 3.84 to 3.95). ZAB with B, N-pG-O affords open circuit potential (OCP) of ~1.39\u00a0V and power density of 30.43 mW cm\u22122. Moreover, ZAB with B, N-pG-O air\u2013cathode exhibits increased polarization voltage of ~0.55\u00a0V after 30 cycles at 1\u00a0mA\u00a0cm\u22122 with Zn plate as an anode and 6\u00a0M KOH as the electrolyte, which indicates its reasonable performance.Integrating the holey graphene framework with CNTs can enhance the ORR activity and ZAB performance. Cheng et. al. [104] have observed that N-CNTs-HGF (HGF: Holey graphene framework) exhibits high ORR activity with high ZAB performance. It was prepared by the following steps: At first, GO was obtained by oxidation of natural graphite powder through a modified Hummers' method; Then, Fe2O3/HGF was obtained by hydrothermal treatment of GO with FeCl3 and KOH/H2O/ethylene glycol at 180\u00a0\u00b0C for 6\u00a0h followed by heat treatment at 850\u00a0\u00b0C for 2\u00a0h under Ar atmosphere; Finally, N-CNTs-HGF was obtained by annealing of Fe2O3/HGF with melamine at 800\u00a0\u00b0C for 2\u00a0h under Ar/H2 atmosphere followed by removal of Fe with concentrated HCl. It is composed of crooked bamboo-like N-doped CNTs, which are anchored on the holey graphene framework. The structure contains pyridinic-N, pyrrolic-N, and graphitic-N, while it shows C\u2013N\u2013C at 285.2\u00a0eV, and it contains C\u2013C at 284.8\u00a0eV, C\u2013O at 286.6\u00a0eV, O\u2013CO at 289\u00a0eV, and CO at 288.3\u00a0eV, which are attributed to the graphene and CNT based materials, and it contains mesopores, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity (E1/2: 0.85\u00a0V; n: 3.9). All-solid ZAB with N-CNTs-HGF exhibits high OCP of 1.43\u00a0V, 10\u00a0mA\u00a0cm\u22122 at ~0.55\u00a0V, the capacity of 625\u00a0mA\u00a0h\u00a0g\u22121, an energy density of 614\u00a0W\u00a0h\u00a0kg\u22121, and power density of ~8.5 mW cm\u22122. All-solid ZAB with N-CNTs-HGF is bendable, which indicates its flexibility, and it affords increased polarization of ~0.17\u00a0V after\u00a0>\u00a060 cycles at 5\u00a0mA\u00a0cm\u22122 with Zn foil as an anode and solid electrolyte gel as the electrolyte, which indicates its high performance.Preparing N, P co-doped carbon framework through pyrolysis of a supermolecular aggregate of self-assembled phytic acid, melamine, and graphene oxide can enhance the ORR activity and ZAB performance. Zhang et. al. [99] have observed that MPSA-GO (MPSA: Melamine\u2013phytic acid supermolecular aggregate) exhibits high ORR activity with high ZAB performance. It was prepared by self-assembling melamine and phytic acid into MPSA in the presence of graphene oxide, followed by pyrolysis at 1000\u00a0\u00b0C for 1\u00a0h under argon atmosphere. It possesses 3D porous carbon networks, which are co-doped with nitrogen and phosphorus, and it contains graphitic sp2 carbon at 284.6\u00a0eV, and it exhibits C\u2013N and/or CN at 285.6\u00a0eV, P\u2013C bond at ~131.6\u00a0eV and P\u2013O bond at ~133.2\u00a0eV, and it contains pyridinic N at 398.3\u00a0eV, pyrrolic N at 400.1\u00a0eV, graphitic N at 401.2\u00a0eV, and oxidized N at 403.7\u00a0eV, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity. ZAB with MPSA-GO exhibits high power density of 310\u00a0W\u00a0g\u22121.Thus, various strategies including creating defects on the graphene [101], doping N with graphene [97], N-doped graphene obtained through carbonization of natural rice [100], preparing N-doped graphene with electron-withdrawing pyridinic N and electron-donating quaternary N [102], preparing N-doped exfoliated graphene [106], S-doped graphene foam prepared from food (idly) [103], preparing S, N co-doped graphene-like electrocatalyst [105], preparing defect enriched S, N co-doped graphene-like carbon [98], B, N co-doped graphene with graphitic N and BC3\n[96], integrating holey graphene framework with CNTs [104], and preparing N, P co-doped carbon framework [99] enhanced the ORR/OER activity and ZAB performance.\nTable 3\n depicts the ORR/OER activity and zinc-air battery performance of various kinds of reported graphene with non-noble metals based air catalysts [107\u2013116]. Integrating Fe/Fe3C@C nanoparticles with graphene framework can facilitate the electron/charge transport and that can enhance the ORR/OER activity and ZAB performance. Wang et. al. [107] have observed that Fe/Fe3C@C-NG/NCNTs exhibits high ORR/OER activity with high ZAB performance. It was prepared by the following steps: At first, GO was obtained by modified Hummer\u2019s method; Then, a powder was obtained by sonicating the SBA-15 with GO suspension for 2\u00a0h followed by freeze-drying for 48\u00a0h; Later, black powder was obtained by heating the powder with FeCl3\u00b76H2O and dicyandiamide at 550\u00a0\u00b0C for 4\u00a0h followed by heating at 800\u00a0\u00b0C for 1\u00a0h under N2 atmosphere; Finally, Fe/Fe3C@C-NG/NCNTs was obtained by washing the black powder with 10% HF for 24\u00a0h followed by drying. It is composed of \u03b1\u2013Fe/Fe3C@C nanoparticles, which are enwrapped in 3D N-doped graphene and bamboo-like CNTs. It exhibits I\nD/I\nG ratio of 0.7, which indicates the existence of such high degree of graphitization, which can be ascribed to the co-existence of graphene and bamboo-like CNTs. It possesses mesopores and macropores, while it exhibits high surface area (117.6\u00a0m2 g\u22121), which can enhance the mass transport and afford abundant active sites, and it contains pyrrolic-N, pyridinic-N/Fe-N, graphitic-N, oxidized-N, Fe0, Fe2+, and Fe3+, where Fe\u2013Nx can be formed when the pyridinic-N coordinate with Fe, which could facilitate its ORR activity, and it exhibits high turnover frequency of 3.06 e site\u22121 s\u22121 on the basis of Fe\u2013Nx sites, and it contains CO, C\u2013OH, and C\u2013O\u2013C. The existence of these hydrophilic oxygen-containing groups can enhance the three-phase contact among the electrode, electrolyte, and reactants, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER (EJ=10: 1.68\u00a0V) and ORR activity (E1/2: 0.84\u00a0V; n: 3.90 to 4.00) with low \u0394E of 0.84\u00a0V. ZAB with Fe/Fe3C@C-NG/NCNTs exhibits high capacity of 682.6\u00a0mA\u00a0h\u00a0g\u22121, energy density of 764.5\u00a0W\u00a0h\u00a0kg\u22121, power density of 101.2 mW cm\u22122, OCP of 1.37, and 10\u00a0mA\u00a0cm\u22122 at 1.12\u00a0V for 40\u00a0h. ZAB with Fe/Fe3C@C-NG/NCNTs affords initial polarization voltage of 0.89\u00a0V with an increased polarization voltage of 0.13\u00a0V after 297 cycles at 10\u00a0mA\u00a0cm\u22122 with Zn plate as anode and 6\u00a0M KOH with 0.2\u00a0M Zn(O2CCH3)2 as electrolyte, which indicates its high performance.Preparing Fe\u2013N active sites, and integrating CNTs and graphene with MOF can enhance its activity and electronic conductivity, and that can enhance the ORR/OER activity and ZAB performance. Yang et. al. [110] have observed that Fe-MOF@CNTs-G exhibits high ORR/OER activity with high ZAB performance. It was prepared by the following steps: At first, MIL-53(Fe) was obtained by hydrothermal treatment at 150\u00a0\u00b0C for 6\u00a0h; Then, a mixture was obtained by ultrasonic treatment of MIL-53, (NH4)2S2O8, and melamine, while the mixture was frozen overnight; Finally, Fe-MOF@CNTs-G was obtained by heating the above product at 240\u00a0\u00b0C for 2\u00a0h and 900\u00a0\u00b0C for 1\u00a0h under N2 atmosphere. It contains graphite carbon and Fe3C phase, and it is composed of graphene-like structure, which is formed around the MOFs, while short CNTs also emerges on the surface of MOFs, where homogenous Fe3C nanoparticles are enwrapped at the end of the CNTs. The structure contains C, N, O, and S, which are uniformly distributed, and it possesses a high surface area (90\u00a0m2 g\u22121), and it contains CC, C\u2013C, C\u2013S, C\u2013N\u2013C/CO, pyridinic N (24.8%), pyrrolic N (24.8%), Fe\u2013N (24.8%), and graphitic N (25.6%), and it exhibits the I\nD/I\nG ratio of 0.93, which indicate the existence of defects, and it exhibits high electrochemically active surface area (8.03 mF cm\u22122). The high surface area within this material can afford abundant Fe\u2013N active sites and facilitate the transfer of the reactants, while CNTs and graphene can enhance the electronic conductivity of the MOF substrate, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER (EJ=10: 1.646\u00a0V) and ORR activity (E1/2: 0.873\u00a0V; n: 3.92 to 3.99) with low \u0394E of 0.773\u00a0V. ZAB with Fe-MOF@CNTs-G exhibits high capacity of 637.4\u00a0mA\u00a0h\u00a0g\u22121, energy density of 734.1\u00a0W\u00a0h\u00a0kg\u22121, power density of 95.3 mW cm\u22122, and OCP of 1.414\u00a0V. ZAB with Fe-MOF@CNTs-G affords initial polarization voltage of 1.01\u00a0V with negligible increased polarization voltage of 0.04\u00a0V after 100 cycles at 10\u00a0mA\u00a0cm\u22122 with Zn plate as anode and 6\u00a0M KOH with 0.2\u00a0M Zn(O2CCH3)2 as electrolyte, which indicates its much high performance.Integrating NiFe nanoparticles with graphene can alter the electronic modulation and that can enhance the ORR/OER activity and ZAB performance. Zhu et. al. [115] have observed that NiFe@NCX exhibits high ORR/OER activity with high ZAB performance. It was prepared by the following steps (Fig. 5\n\na): At first, NiFe-MIL was obtained by hydrothermal treatment at 100\u00a0\u00b0C for 15\u00a0h; Then, a mixture was obtained by stirring the suspension containing NiFe-MIL and melamine in ethanol followed by evaporation; Later, the mixture was pyrolyzed at 600\u00a0\u00b0C for 1\u00a0h and 800\u00a0\u00b0C for 1\u00a0h under N2 atmosphere; Finally, NiFe@NCX was prepared by pickling the pyrolysis product with 1\u00a0M HCl for 8\u00a0h at 80\u00a0\u00b0C to remove the unstable metal species. It possesses 3D flake-like structure, and it is composed of ultra-fine NiFe nanoparticles (cubic NiFe2 phase), which are encapsulated by N-doped thin graphene nanosheets; It contains Fe, Ni, C and N, which are uniformly distributed, and it exhibits high surface area (350\u00a0m2 g\u22121), and it is mesoporous, and it exhibits I\nD/I\nG ratio of\u00a0<\u00a01, which indicates the existence of highly ordered graphitic structure, which can enhance its electrical conductivity, and it contains pyridinic N at 398.9\u00a0eV and quaternary N at 400.9\u00a0eV, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER (EJ=10: 1.555\u00a0V) and ORR activity (E1/2: 0.86\u00a0V; n: 4.1) with low \u0394E of 0.695\u00a0V. ZAB with NiFe@NCX exhibits high capacity of 583.7\u00a0mA\u00a0h\u00a0g\u22121, and energy density of 732.3\u00a0W\u00a0h\u00a0kg\u22121 (Fig. 5). ZAB with NiFe@NCX affords initial polarization voltage of 0.39\u00a0V with increased polarization voltage of 0.29\u00a0V after 205 cycles (Fig. 5\ne) at 10\u00a0mA\u00a0cm\u22122 with Zn plate as anode and 6\u00a0M KOH as electrolyte, which indicates its high performance.Integrating Vulcan carbon with CoFe-N-rGO can spatially separate the CoFe-N-rGO layers and can improve its conductivity, and that can enhance the ORR activity and ZAB performance. Kashyap et. al. [116] have observed that CoFe-N-rGO-Vulcan exhibits high ORR activity with high ZAB performance. It was prepared by the following steps: At first, CoFe-N-rGO was obtained by hydrothermal treatment at 130\u00a0\u00b0C for 12\u00a0h; Then, CoFe-N-rGO was annealed at 150\u00a0\u00b0C for 12\u00a0h; Finally, CoFe-N-rGO-Vulcan was obtained by mixing the 2:8 ratio of CoFe-N-rGO and Vulcan carbon. It is composed of cobalt ferrite nanoparticles, which are homogeneously distributed on the N-rGO, while CoFe-N-rGO layers are spatially separated by Vulcan carbon (Fig. 6\n\na), and it exhibits high surface area (187\u00a0m2 g\u22121), and it contains micro/mesopores, and it contains pyridinic N and pyrrolic N, and it contains Co2+, Co3+, Fe2+, and Fe3+, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity (E1/2: \u22120.133\u00a0V (vs Hg/HgO); n: 3.7). ZAB with CoFe-N-rGO-Vulcan exhibits high capacity of ~630\u00a0mA\u00a0h\u00a0g\u22121, the power density of 155 mW cm\u22122, and 30\u00a0mA\u00a0cm\u22122 at 1.0\u00a0V.Preparing Co nanoclusters distributed on N-doped carbon can enhance the ORR activity and ZAB performance. Gao et. al. [108] have observed that Co-N-rGO exhibits high ORR activity with high ZAB performance. It was prepared by the following steps: At first, Co-MOF/GO was obtained by hydrothermal treatment at 120\u00a0\u00b0C for 36\u00a0h; Then, the above product was pyrolyzed at 850\u00a0\u00b0C for 2\u00a0h under N2 atmosphere; Finally, Co-N-rGO was obtained by pickling the pyrolysis product with 3.0\u00a0M HCl for 12\u00a0h. It is composed of Co nanoclusters (Diameter: \u2248 1\u20132\u00a0nm), which are uniformly distributed on N-doped carbon, and it contains Pyridinic-N, Pyrrole-N, Graphitic-N, an N-oxide, and it contains graphitic carbon, C\u2013O, and CO bonds, and it exhibits I\nD/I\nG ratio of 0.95, which suggests the existence of a high degree of graphitized carbon, and it exhibits high surface area (179\u00a0m2 g\u22121), and it contains mesopores, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity (E1/2: 0.831\u00a0V; n: 3.75). ZAB with Co-N-rGO exhibits high capacity of 795\u00a0mA\u00a0h\u00a0g\u22121, a power density of 175 mW cm\u22122, and 300\u00a0mA\u00a0cm\u22122 at 0.65\u00a0V.Preparing Co/N/O tri-doped graphene with intrinsic structural defects and atomically dispersed Co\u2013N\nx\n\u2013C active sites can enhance the ORR/OER activity and ZAB performance. Tang et. al. [111] have observed that Co/N/O-G exhibits high ORR/OER activity with high ZAB performance. It was prepared by the carbonization of a powdery mixture at 950\u00a0\u00b0C for 1.5\u00a0h under Ar atmosphere, where the mixture was composed of gelatinized amylopectin, in situ generated Mg(OH)2 nanoflakes, melamine, and cobalt nitrate. It is composed of Co/N/O tri-doped graphene (Fig. 6\nb), and it exhibits much high surface area (541.5\u00a0m2 g\u22121), and it contains micro/mesopores. It exhibits I\nD/I\nG ratio of 1.32, which suggests the existence of much high defects, and it contains pyridinic N, Co\u2013Nx, pyrrolic N, quaternary N, oxidized N and chemisorbed N, where the pyridinic N bound to Co is up-shifted with \u2248 1\u00a0eV from pristine pyridinic N (\u2248 398.4\u00a0eV), while C\u2013N shoulder noticeably shifts to higher binding energy, which can be ascribed to the strong electron-withdrawing effect of cobalt coordinated with the nitrogen in Co\u2013N\nx\n\u2013C moieties. The decreased electron density in adjacent C atoms can enhance the adsorption of ORR/OER intermediates, that can facilitate the electron transfer, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high ORR/OER activity with low \u0394E of 0.95\u00a0V. ZAB with Co/N/O-G exhibits high capacity of 750\u00a0mA\u00a0h\u00a0g\u22121, energy density of 840\u00a0W\u00a0h\u00a0kg\u22121, power density of 152 mW cm\u22122, and OCP of 1.44\u00a0V. ZAB with Co/N/O-G affords initial polarization voltage of ~1.00\u00a0V with increased polarization voltage of ~0.12\u00a0V after 180 cycles at 2\u00a0mA\u00a0cm\u22122 with Zn foil as anode and 6\u00a0M KOH with 0.2\u00a0M ZnCl2 as electrolyte. Moreover, solid ZAB is constructed (Fig. 6\nc), where Co/N/O-G, Zn foil, and poly(vinyl alcohol) gel are used as cathode, anode, and electrolyte, respectively, and it is flexible, while it delivers stable potential during charge/discharge cycling even at different bent state.Preparing cobalt nanoparticles encapsulated in N-enriched graphene shells with hollow graphene spheres can afford Co-N-C active sites, and that can enhance the ORR activity and ZAB performance. Zeng et. al. [113] have observed that Co@NG-acid exhibits high ORR activity with high ZAB performance. It was prepared by the following steps: At first, cobalt analogue of Prussian blue (PB-Co) was obtained by co-precipitation method; Then, Co@NG was prepared by annealing the PB-Co at 600\u00a0\u00b0C for 1\u00a0h under Ar atmosphere; Finally, Co@NG-acid was obtained by treating the Co@NG with 1\u00a0M HCl for overnight. It is composed of metallic cobalt nanoparticles (Fig. 6\nd), which are enwrapped in N-enriched graphene shells, while it possesses hollow graphene spheres due to the leaching of Co by acid treatment. It contains \u2248 10\u00a0wt% of Co in both Co2+, Co3+, while it contains pyridinic N, pyrrolic N, and quaternary N, where Co-N-C active sites could be formed, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity (E1/2: 0.83\u00a0V; n: >3.8). ZAB with Co@NG-acid exhibits a high power density of 350 mW cm\u22122 and 255\u00a0mA\u00a0cm\u22122 at 1\u00a0V.Preparing Co, N-co doped CNT/graphene heterostructure can generate pyridinic N-C and Co-N active sites, and that can enhance the ORR activity and ZAB performance. Yang et. al. [109] have observed that Co, N-CNT-NG exhibits high ORR activity with high ZAB performance. It was prepared by the following steps: At first, dried precursor was obtained by stirring the solution containing 0.2\u00a0g of g-C3N4 and 0.17\u00a0M CoCl2\u00b76H2O for 24\u00a0h followed by evaporation; Finally, Co, N-CNT-NG was prepared by pyrolyzing the above precursor at 550\u00a0\u00b0C for 2\u00a0h and then at 800\u00a0\u00b0C for 2\u00a0h under N2 atmosphere. It is composed of Co, N-co doped CNT/graphene heterostructure, where Co nanoparticles are enwrapped by CNT/graphene heterostructure, and it contains C, N, and Co, which are uniformly distributed. It contains pyridinic N, pyrrolic N/Co-N, quaternary N, oxidised pyridinic N, where pyridinic N-C and Co-N species can act as highly electroactive sites, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity (E1/2: 0.85\u00a0V; n: 3.96), while ZAB with Co, N-CNT-NG exhibits a power density of 88 mW cm\u22122.Preparing the graphene matrix with Cu(I)-N active sites can enhance the ORR activity and ZAB performance. Wu et. al. [114] have observed that Cu\u2013N@G exhibits high ORR activity with high ZAB performance. It was prepared by pyrolysis of solid-phase precursors containing copper phthalocyanine and dicyandiamide at 800\u00a0\u00b0C for 2\u00a0h under Ar atmosphere, followed by acid treatment (0.5\u00a0M H2SO4) for 12\u00a0h at 70\u00a0\u00b0C to eliminate unstable Cu species. It is composed of Cu atoms, which are embedded in the graphene matrix, it contains Cu, N and C, which are uniformly distributed, and it contains pyridinic N, pyrrolic N, graphitic N, and oxidized N. It possesses much high surface area (333.877\u00a0m2 g\u22121), and it contains mesopores, while their electrochemical and theoretical studies reveal that Cu(I)-N is considered as the active site for catalyzing the ORR, where O atoms prefer to adsorb on Cu atoms in Cu-N2, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity (E1/2: 0.80\u00a0V; n: 3.96). ZAB with Cu\u2013N@G exhibits high power density of ~210 mW cm\u22122 and 142\u00a0mA\u00a0cm\u22122 at 1\u00a0V, which indicates its high performance.Integrating Ag NW with graphene aerogel can inhibit restacking of graphene sheets and facilitate electronic conductivity, and that can enhance the ORR activity and ZAB performance. Hu et. al. [112] have observed that Ag NW-GA exhibits high ORR activity with high ZAB performance. It was prepared by mixing the Ag NW and GO suspensions followed by hydrothermal self-assembly for 3\u00a0h at 90\u00a0\u00b0C. It is composed of Ag NW, which are tightly attached to the graphene sheets, while 0D Ag nanocrystals are uniformly distributed on the graphene sheets (Fig. 6\ne). Ag NW-graphene aerogel possesses an open sponge structure, where Ag NW inhibits the restacking of graphene sheets, and facilitate electronic conductivity, and act as an Ag source for the ultrasmall Ag nanocrystals deposition, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity. ZAB with Ag NW-GA exhibits high capacity of 637.3\u00a0mA\u00a0h\u00a0g\u22121, the energy density of 794.5\u00a0W\u00a0h\u00a0kg\u22121, the power density of 331 mW cm\u22122, OCP of 1.48\u00a0V, and 206\u00a0mA\u00a0cm\u22122 at 1.0\u00a0V, which indicates its high performance.Thus, several strategies including integrating Fe/Fe3C@C nanoparticles with graphene framework [107], integrating NiFe nanoparticles with graphene [115], integrating Vulcan carbon with CoFe-N-rGO\n[116], preparing Co nanoclusters on N-doped carbon [108], preparing Co/N/O tri-doped graphene [111], preparing cobalt nanoparticles encapsulated in N-enriched graphene shells with hollow graphene spheres [113], preparing Co, N-co-doped CNT/graphene heterostructure [109], preparing graphene matrix with Cu(I)-N active sites [114], integrating Ag NW with graphene aerogel [112], and preparing Fe\u2013N active sites and integrating CNTs and graphene with MOF [110] improved the ORR/OER activity and ZAB performance.\nTable 4\n depicts the ORR/OER activity and zinc-air battery performance of various kinds of reported graphene with metal oxides based air catalysts [37,117\u2013126]. Preparing MnO2 nanofilm on N-doped hollow graphene can enhance the ORR activity and ZAB performance. Yu et. al. [120] have observed that MnO2-NG exhibits high ORR activity with high ZAB performance. It was prepared by template method and mild oxidation process through the following steps: At first, SiO2@rGO was obtained by drying the silica/graphene oxide suspension for overnight at 80\u00a0\u00b0C followed by heating at 850\u00a0\u00b0C for 2\u00a0h under N2 atmosphere; Then, hollow graphene spheres was obtained by removal of SiO2 templates by etching with 10\u00a0wt% of HF for 12\u00a0h at room temperature; Later, NG was prepared by hydrothermal treatment of the above hollow graphene with NH4OH at 180\u00a0\u00b0C for 8\u00a0h followed by annealing at 800\u00a0\u00b0C for 2\u00a0h under N2 atmosphere; Finally, MnO2-NG was obtained by treating NG with 0.012\u00a0mol L\u22121 of KMnO4 at 60\u00a0\u00b0C for 2\u00a0h followed by annealing at 800\u00a0\u00b0C for 2\u00a0h under N2 atmosphere. It is composed of MnO2 nanofilms, which are uniformly anchored on the transparent N-doped hollow graphene spheres, and it exhibits high surface area (302\u00a0m2 g\u22121) with pore volume of 1.8\u00a0cm3 g\u22121. It contains C, N, O and Mn, which are homogenously distributed, while it contains 84.9 at % of C, 2.3 at % of N, 8.7 at % of O and 4.1 at % of Mn; It contains graphitic N, pyridinic N, pyrrolic N and N-oxides, where graphitic N and pyridinic N can enhance much ORR activity, and it contains C\u2013O and Mn4+, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity (E1/2: 0.84\u00a0V; n: 3.65 to 3.85). ZAB with MnO2-NG exhibits high capacity of 744\u00a0mA\u00a0h\u00a0g\u22121 at 10\u00a0mA\u00a0cm\u22122, a power density of 82 mW cm\u22122, OCP of 1.48\u00a0V, and discharge voltage of 1.14\u00a0V at 25\u00a0mA\u00a0cm\u22122, which indicates its high performance.Ionic liquid (IL) moiety can increase the conductivity and electrocatalytic activity of rGO while integrating Mn3O4 with rGO-IL can enhance the ORR activity and ZAB performance. Lee et. al. [125] have observed that Mn3O4@rGO-IL exhibits high ORR activity with high ZAB performance. It was prepared by facile solution-based growth mechanism. They observed that rGO-IL exhibits higher ORR activity than rGO, while Mn3O4@rGO-IL exhibits highest ORR activity than rGO-IL. It is composed of crystalline Mn3O4 nanoparticles, which are anchored on the rGO-IL nanosheet, where ionic liquid moiety not only increases the conductivity but also increases the electrocatalytic activity compared with pristine rGO, while the synergic effect between Mn3O4 and rGO-IL can facilitate its catalytic activity, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity, while ZAB with Mn3O4@rGO-IL exhibits high power density of 120 mW cm\u22122.Integrating CoMn2O4 with NrGO can enhance the ORR/OER activity and ZAB performance. Prabu et. al. [124] have observed that CoMn2O4/NrGO exhibits high ORR/OER activity with high ZAB performance. It was prepared by hydrothermal treatment at 150\u00a0\u00b0C for 3\u00a0h. It is composed of CoMn2O4 nanoparticles (~20\u00a0nm), which are dispersed on N-doped rGO (Fig. 7\n\na), where CoMn2O4 possesses tetragonal structure, and it contains mesopores, and it contains pyridinic-N, pyrrolic-N and quaternary-N, where the synergistic effect between the spinel cobalt manganese oxide and nitrogen-doped graphene sheets can facilitate its electrocatalytic activity, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER (EJ=10: 1.66\u00a0V) and ORR activity (E1/2: 0.75\u00a0V; n: 4) with low \u0394E of 0.91\u00a0V. ZAB with CoMn2O4/NrGO affords initial polarization voltage of 0.7\u00a0V with increased polarization voltage of 0.16\u00a0V after 100 cycles at 20\u00a0mA\u00a0cm\u22122 with Zn plate as anode and 6\u00a0M KOH as electrolyte, which indicates its high performance. Moreover, Prabu et. al. [37] have observed that CoMn2O4/NrGO exhibits high ZAB performance by consuming oxygen from the atmosphere. ZAB with CoMn2O4/NrGO exhibits high capacity of 610\u00a0mA\u00a0h\u00a0g\u22121 and OCP of 1.15\u00a0V. ZAB with CoMn2O4/NrGO (Fig. 7\nb) affords initial polarization voltage of 0.7\u00a0V with increased polarization voltage of 0.36\u00a0V after 200 cycles at 20\u00a0mA\u00a0cm\u22122 with Zn plate as anode and 6\u00a0M KOH as electrolyte, which indicates its high performance.Integrating MnCoFeO4 with N-rGO can enhance the ORR/OER activity and ZAB performance. Zhan et. al. [126] have observed that MnCoFeO4-N-rGO exhibits high ORR/OER activity with high ZAB performance. It was prepared by hydrothermal treatment at 150\u00a0\u00b0C for 3\u00a0h. It is composed of MnCoFeO4 nanoparticles (~5 nm), which are uniformly dispersed on N-rGO nanosheets, where MnCoFeO4 is comprised of Mn and Co atoms, which are displaced some Fe atoms from the Fe3O4 lattice. It contains pyridinic N at 399.1\u00a0eV and graphitic N at 400.5\u00a0eV, and Mn3+, Co2+ and Fe3+, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER (EJ=10: 1.71\u00a0V) and ORR activity (E1/2: 0.78\u00a0V; n: 3.8) with low \u0394E of 0.93\u00a0V. ZAB with MnCoFeO4-N-rGO exhibits OCP of 1.46\u00a0V. ZAB with MnCoFeO4-N-rGO affords initial polarization voltage of ~1.25\u00a0V with negligible increased polarization voltage after 75 cycles at 10\u00a0mA\u00a0cm\u22122 with Zn plate as an anode and 6\u00a0M KOH as an electrolyte.Functionalizing graphene oxide with 1-hexyl-3-methylimidazolium chloride molecules can enhance the ionic conductivity and mechanical properties of the solid electrolyte, and that can enhance the ZAB performance. Zarrin et. al. [121] have observed that Co3O4\n5-HMIM-GO exhibits high ZAB performance. 5-HMIM-GO was prepared by the following steps (Fig. 7\nc): At first, GO nanosheets were functionalized with HMIM in an aqueous solution containing KOH; Finally, 5-HMIM-GO was obtained by vacuum filtration method. 5-HMIM-GO solid electrolyte is composed of graphene oxide, which is functionalized with 1-hexyl-3-methylimidazolium chloride molecules through both covalent and non-covalent bonds, where covalent bond induced by esterification reactions, while non-covalent bond induced by electrostatic \u03c0cation\u2013\u03c0stacking. It exhibits high hydroxide conductivity at 30% RH and room-temperature, and it is free-standing and flexible membrane, and it possesses high mechanical properties (Tensile strength: 35.49\u00a0MPa; Young\u2019s Modulus: 1.7 GPa; Elongation at break: 2.13%; Toughness: 0.37\u00a0MPa), where the toughness of 5-HMIM-GO is boosted to ~62% when compared to that of bare GO. It contains C, O, and N, which are uniformly distributed, and it exhibits the I\nD/I\nG ratio of 1.6, which suggests the existence of high defects, and it contains 4.89 atomic % of N, 37.67% of O, and 57.43% of C. It contains highly intense C\u2013C/C\u2013H peak at 284.99\u00a0eV along with\u2013C\u2013N, CO, OC\u2013OH, N\u2013C\u2013N, and OC\u2013N\u2212, and that can possibly enhance its ZAB performance. ZAB with Co3O4\n5-HMIM-GO exhibits OCP of 1.25\u00a0V. ZAB with Co3O4\n5-HMIM-GO affords initial polarization voltage of ~0.9\u00a0V with negligible increased polarization voltage after 60 cycles at 200\u00a0mA\u00a0g\u22121 with zinc pellet as anode and 5-HMIM-GO as a solid electrolyte, which indicates its much high performance.Integrating Co3O4 with N doped graphene can enhance the ORR activity and ZAB performance. Singh et. al. [122] have observed that Co3O4-NG exhibits high ORR activity with high ZAB performance. It was prepared by hydrothermal treatment at 120\u00a0\u00b0C for 24\u00a0h. It is composed of spinel Co3O4 spherical nanoparticles (~60\u00a0nm), which are well dispersed on N doped graphene (Fig. 7\nd). It contains pyridinic-N, pyrrolic-N, and quaternary-N, and it exhibits the I\nD/I\nG ratio of 1.39, which suggests the existence of high defects, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity (E1/2: \u22120.11\u00a0V (vs Hg/HgO); n: 3.7). ZAB with Co3O4-NG exhibits high capacity of ~590\u00a0mA\u00a0h\u00a0g\u22121, the energy density of ~840\u00a0W\u00a0h\u00a0kg\u22121, the power density of ~190 mW cm\u22122, and OCP of 1.52\u00a0V.Integrating Co3O4 nano-rods with reduced graphene oxide can enhance the conductivity and defects, and that can enhance the ZAB performance. Shen et. al. [118] have observed that rGO-Co3O4 exhibits high ZAB performance. It was prepared by hydrothermal treatment at 120\u00a0\u00b0C for 5\u00a0h followed by annealing at 300\u00a0\u00b0C for 2\u00a0h. They observed that it exhibits higher conductivity than Co3O4. It is composed of crystalline Co3O4 nano-rods, which are dispersed on the surfaces of the reduced graphene oxide, and it exhibits I\nD/I\nG ratio of 1.043, which suggests the existence of defects, and it exhibits high conductivity, and that can possibly enhance its ZAB performance. ZAB with rGO-Co3O4 exhibits 47.2\u00a0mA\u00a0cm\u22122 at 0.8\u00a0V. ZAB with rGO-Co3O4 affords initial polarization voltage of ~1.5\u00a0V with an increased polarization voltage of ~0.25\u00a0V after 100 cycles at 50\u00a0mA\u00a0cm\u22122 with Zn plate as the anode and 6\u00a0M KOH as an electrolyte, which indicates its high performance.Preparing predominant metallic Co with small fraction of its oxides anchored on N-doped reduced graphene oxide can enhance the ORR activity and ZAB performance. Liu et. al. [123] have observed that Co-CoO/NrGO exhibits high ORR/OER activity with high ZAB performance. It was prepared by hydrothermal treatment followed by pyrolysis. They observed that Co-CoO/NrGO exhibits higher ORR activity than CoO/NrGO (Fig. 8\n\na). It is composed of crystalline Co-CoO nanorods, which are enwrapped with N-rGO, where Co-CoO is comprised of predominant metallic Co with minor CoO. It contains Co0 and Co2+, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER (EJ=10: 1.47\u00a0V) and ORR activity (E1/2: 0.78\u00a0V; n: 3.7 to 3.9) with low \u0394E of 0.69\u00a0V. ZAB with Co-CoO/NrGO exhibits 35\u00a0mA\u00a0cm\u22122 at 1.08\u00a0V. ZAB with Co-CoO/NrGO affords polarization voltage of 1.26\u00a0V at 50\u00a0mA\u00a0cm\u22122 with zinc plate as an anode and 6.0\u00a0M KOH as the electrolyte.Integrating amorphous bimetallic oxide with N-doped reduced graphene oxide can enhance the conductivity, electrochemically active surface area, and create oxygen deficiency, and that can enhance the ORR/OER activity and ZAB performance. Wei et. al. [117] have observed that Fe0.5Co0.5Ox-NrGO exhibits high ORR/OER activity with high ZAB performance. It was prepared from Prussian blue analog nanocrystals by low-temperature (at 300\u00a0\u00b0C) oxidative decomposition of the Fe\na\nCo1\u2212\n\na\n/polyethylenimine/GO hybrid (Fig. 8\nb). It is composed of amorphous bimetallic oxide (Fe0.5Co0.5Ox) nanoparticles, which are dispersed on N-doped reduced graphene oxide, it exhibits high conductivity, and it exhibits high oxygen deficiency (23.9%). It exhibits the I\nD/I\nG ratio of 1.06, which suggests the existence of defects, and it exhibits pyridinic N (18.24%), pyrrolic N (18.75%), graphitic N (47.12%), and oxidized N (15.89%), where the existence of abundance graphitic N can facilitate the ORR activity, and it affords high electrochemically active surface area (6\u00a0cm2 at 0.1\u00a0mg), and that can possibly enhance its ORR/OER activity and ZAB performance. It shows the better OER and ORR activity with low \u0394E of 0.74\u00a0V. ZAB with Fe0.5Co0.5Ox-NrGO exhibits the high capacity of 756\u00a0mA\u00a0h\u00a0g\u22121 at 10\u00a0mA\u00a0cm\u22122, the energy density of 904\u00a0W\u00a0h\u00a0kg\u22121, the power density of 82 mW cm\u22122, and OCP of 1.43 to 1.44\u00a0V. ZAB with Fe0.5Co0.5Ox-NrGO affords initial polarization voltage of 0.79\u00a0V (Fig. 8\nc) with increased polarization voltage of 0.1\u00a0V after 60 cycles at 10\u00a0mA\u00a0cm\u22122 with Zn plate as the anode and 6\u00a0M KOH with 0.2\u00a0M ZnCl2 as an electrolyte.Integrating Fe-doped NiOOH with graphene-encapsulated FeNi3 can enhance the OER activity and ZAB performance. Wang et. al. [119] have observed that FeNi3@G@Fe-NiOOH exhibits high OER activity with high ZAB performance. It was prepared by arc discharging method followed by electrochemical activation. They observed that FeNi3@G@Fe-NiOOH exhibits higher OER activity than FeNi3@Fe-NiOOH, Fe-NiOOH@G, and Fe-NiOOH. It is composed of Fe-doped NiOOH, which are grown on graphene-encapsulated FeNi3 nanodots. It contains Ni0, Ni3+ and Fe3+, and that can possibly enhance its OER activity and ZAB performance. It exhibits high OER activity (EJ=10: 1.52\u00a0V). ZAB with FeNi3@G@Fe-NiOOH affords initial polarization voltage of ~0.525\u00a0V with an increased polarization voltage of ~0.275\u00a0V after 360 cycles at 1\u00a0mA\u00a0cm\u22122 with Zn foil as the anode and 6\u00a0M KOH with 0.2\u00a0M Zn(O2CCH3)2 as the electrolyte.Thus, various strategies including preparing MnO2 nanofilm on N-doped hollow graphene [120], integrating Mn3O4 with rGO-IL [125], integrating CoMn2O4 with NrGO\n[124], integrating MnCoFeO4 with N-rGO\n[126], functionalizing graphene oxide with 1-hexyl-3-methylimidazolium chloride molecules [121], integrating Co3O4 with N doped graphene [122], integrating Co3O4 nano-rods with reduced graphene oxide [118], preparing predominant metallic Co with small fraction of its oxides anchored on N-doped reduced graphene oxide [123], integrating amorphous bimetallic oxide with N-doped reduced graphene oxide [117], and integrating Fe-doped NiOOH with graphene-encapsulated FeNi3\n[119] enhanced the ORR/OER activity and ZAB performance.\nTable 5\n depicts the ORR/OER activity and zinc-air battery performance of various kinds of reported graphene with nitrides, sulfides, carbides, and other carbon composites based air catalysts [127\u2013133]. Integrating Ni3FeN with NrGO can enhance its conductivity, and that can enhance the ORR/OER activity and ZAB performance. Fan et. al. [128] have observed that Ni3FeN-NrGO exhibits high ORR/OER activity with high ZAB performance. It was prepared by heat treatment at 700\u00a0\u00b0C for 2\u00a0h under NH3 atmosphere. They observed that Ni3FeN-NrGO exhibits higher conductivity than Ni3FeN, Ni3N/rGO and Ni3N. It is composed of 2D Ni-Fe nitride nanoplates (~8 nm), which are strongly anchored with the N-doped reduced graphene oxide, and it exhibits high conductivity, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER (EJ=10: 1.63\u00a0V) and ORR activity (E1/2: ~ 0.7\u00a0V; n: 3.8) with low \u0394E of ~0.93\u00a0V. ZAB with Ni3FeN-NrGO affords initial polarization voltage of 0.77\u00a0V (Fig. 9\n\na) with negligible increased polarization voltage of 0.05\u00a0V after 180 cycles at 10\u00a0mA\u00a0cm\u22122 with Zn as the anode and 4\u00a0M KOH as the electrolyte.Preparing CoSx@C3N4 integrated with reduced graphene oxide having porous structure can afford sufficiently exposed active sites, and that can enhance the ORR/OER activity and ZAB performance. Niu et. al. [127] have observed that CoSx@C3N4/rGO exhibits high ORR/OER activity with high ZAB performance. It was prepared by the following steps: At first, porous g-C3N4 was obtained by controlled pyrolysis of Co2+/melamine networks at 500\u00a0\u00b0C for 1\u00a0h under air atmosphere; Finally, CoSx@C3N4/rGO was prepared by mixing porous g-C3N4 with GO followed by heating with sulfur powder at 400\u00a0\u00b0C for 0.5\u00a0h under N2 atmosphere. It is composed of CoSx@C3N4 integrated with reduced graphene oxide, and it possesses a porous structure. It contains \u2013C\u2013O\u2212, \u2212C\u2013C\u2212, \u2212C\u2013C\u2212 (sp3) (Fig. 9\nb), \u2212C\u2013S\u2212, \u2212NC\u2013N\u2212, Co2+, Co3+, S2\u2212, polymeric S2\n2\u2212, \u2212CS\u2212, and\u2013 C\u2013S\nn\n\u2013C\u2212, where the highly porous morphology with sufficiently exposed active sites can facilitate its catalytic activity, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER (EJ=10: 1.57\u00a0V) and ORR activity (E1/2: 0.78\u00a0V; n: 3.96) with low \u0394E of 0.79\u00a0V. ZAB with CoSx@C3N4/rGO exhibits OCP of 1.38\u00a0V. ZAB with CoSx@C3N4/rGO affords initial polarization voltage of ~1.5\u00a0V with negligible increased polarization voltage after 394 cycles at 50\u00a0mA with Zn plate as the anode and 6\u00a0M KOH with 0.2\u00a0M Zn(O2CCH3)2\u00b76H2O as the electrolyte. Thus, various strategies including integrating Ni3FeN with NrGO\n[128], and preparing CoSx@C3N4 integrated with reduced graphene oxide [127], improved the ORR/OER activity and ZAB performance.Integrating CoS2 with N, S co-doped graphene oxide can enhance the ORR/OER activity and ZAB performance. Ganesan et. al. [130] have observed that CoS2/N, S-GO exhibits high ORR/OER activity with high ZAB performance. It was prepared by solid-state thermolysis approach at 400\u00a0\u00b0C for 2\u00a0h using cobalt thiourea and graphene oxide. It is composed of crystalline cobalt sulfide nanoparticles, which are anchored on N, S co-doped graphene oxide, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER (EJ=10: 1.62\u00a0V) and ORR activity (E1/2: 0.79\u00a0V) with low \u0394E of 0.83\u00a0V. ZAB with CoS2/N, S-GO exhibits high capacity of 767\u00a0mA\u00a0h\u00a0g\u22121. ZAB with CoS2/N, S-GO affords initial polarization voltage of 0.78\u00a0V with negligible increased polarization voltage after 70 cycles at 10\u00a0mA\u00a0cm\u22122 with Zn plate as the anode and 6\u00a0M KOH as an electrolyte.Integrating CoSx with N, S co-doped graphene can enhance the ORR/OER activity and ZAB performance. Geng et. al. [131] have observed that CoSx@N, S-G exhibits high ORR/OER activity with high ZAB performance. It was prepared by hydrothermal treatment. It is composed of CoSx nanoparticles (~50\u00a0nm), which are anchored on N, S co-doped graphene, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER and ORR activity. ZAB with CoSx@N, S-G affords initial polarization voltage of ~0.95\u00a0V with negligible increased polarization voltage after 50 cycles at 1.25\u00a0mA\u00a0cm\u22122, which indicates its high performance.Integrating NiCo2S4 with ultrathin S-doped graphene having high specific surface area and the electrochemically active surface area can enhance the ORR/OER activity and ZAB performance. Liu et. al. [129] have observed that S-G/NiCo2S4 exhibits high ORR/OER activity with high ZAB performance. It was prepared by hydrothermal treatment at 160\u00a0\u00b0C followed by sulfurization process at 300\u00a0\u00b0C for 2\u00a0h. They observed that S-G/NiCo2S4 exhibits higher specific surface area and electrochemically active surface area than NiCo2S4. It is composed of urchin-like NiCo2S4 microsphere, which is encapsulated by the ultrathin S-doped graphene (Fig. 9\nc). The porous 3D-interconnected network contains Ni, Co, S, and C, which are uniformly distributed with high surface area (227\u00a0m2 g\u22121), and high electrochemically active surface area (14.1 mF cm\u22122). It exhibits I\nD/I\nG ratio of 0.91, which is higher than 0.79 of bare G, which suggests the generation of structural distortion and defects, and it contains Co2+, Co3+, Ni2+, Ni3+, C\u2013C/CC, C\u2013O, C\u2013S, CO/OC\u2013O, and C\u2013S\u2013C, and that can possibly enhance its ORR/OER activity and ZAB performance. It exhibits high OER (EJ=10: ~1.56\u00a0V) and ORR activity (E1/2: 0.88\u00a0V; n: 3.86 to 3.96) with low \u0394E of 0.69\u00a0V. ZAB with S-G/NiCo2S4 exhibits high power density of 216.3 mW cm\u22122. ZAB with S-G/NiCo2S4 affords initial polarization voltage of 0.8\u00a0V with negligible increased polarization voltage of about 0\u00a0V after 150 cycles at 10\u00a0mA\u00a0cm\u22122 with Zn plate as the anode and 6\u00a0M KOH with 0.2\u00a0M Zn(O2CCH3)2 as the electrolyte, while the ZAB powers a mini-fan (Fig. 9\nd), which indicates its much high performance. Thus, several strategies including integrating CoS2 with N, S co-doped graphene oxide [130], integrating CoSx with N, S co-doped graphene [131], and integrating NiCo2S4 with ultrathin S-doped graphene [129] enhanced the ORR/OER activity and ZAB performance.Preparing Fe3C(Fe) with N-doped graphitic layers as shell and N-doped graphene as an interface layer can enhance the ORR activity and ZAB performance. Yang et. al. [132] have observed that Fe3C(Fe)@N-GrapLy exhibits high ORR activity with high ZAB performance. It was prepared by pyrolyzing a mixture of Prussian blue (PB) and glucose at 850\u00a0\u00b0C for 6\u00a0h under Ar atmosphere. It is composed of metallic Fe-cluster embedded in crystalline Fe3C nanoparticles, which are enwrapped in N-doped graphitic layers (~5 nm), where the interface layer between Fe3C(Fe) and graphitic layers could be the N-doped graphene. It contains pyridinic N, pyrrolic N, graphitic N and oxidized N, where pyridinic N and graphitic N can facilitate the activity for ORR, and it possesses high surface area (418\u00a0m2 g\u22121). The mesopores and macropores also can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity. ZAB with Fe3C(Fe)@N-GrapLy exhibits the high capacity of 790\u00a0mA\u00a0h\u00a0g\u22121, the power density of 186 mW cm\u22122, and OCP of 1.53\u00a0V. Thus, preparing Fe3C(Fe) with N-doped graphitic layers as shell and N-doped graphene as interface layer [132] improved the ORR/OER activity and ZAB performance.Preparing N-doped vertically aligned carbon nanotubes on graphene foam can enhance the ORR activity and ZAB performance. Cai et. al. [133] have observed that N-CNTs-G exhibits high ORR activity with high ZAB performance. It was prepared by the following steps: At first, graphene was obtained on Ni foam by chemical vapor deposition; Then, CNTs was grown on G/Ni foam by plasma enhanced chemical vapor deposition, and the Ni template was removed by submerging CNTs-G/Ni foam in 1\u00a0M/1M FeCl3/HCl solution for overnight, while CNTs-G was activated by HNO3 solution; Finally, N-CNTs-G was obtained by coaxially polymerizing the polyaniline on the sidewalls of the CNTs-G followed by carbonization at 800\u00a0\u00b0C for 2\u00a0h under N2 atmosphere. It is composed of N-doped vertically aligned carbon nanotubes, which are supported by graphene foam, and it exhibits the I\nD/I\nG ratio of 0.60, and it exhibits high surface area (101.1\u00a0m2 g\u22121). It contains 91.64 atomic % of C, 2.11 atomic % of N and 6.24 atomic % of O; It contains pyridinic N, pyrrolic N, graphitic N and oxidized N, where pyridinic N and graphitic N can facilitate the activity for ORR, and that can possibly enhance its ORR activity and ZAB performance. It exhibits high ORR activity. ZAB with N-CNTs-G exhibits 58\u00a0mA\u00a0cm\u22122 at 0.8\u00a0V. ZAB with N-CNTs-G affords initial polarization voltage of 0.87\u00a0V with negligible increased polarization voltage of ~0.05\u00a0V after 240 cycles at 10\u00a0mA\u00a0cm\u22122 with Zn plate as the anode and 10\u00a0M KOH with 0.2\u00a0M Zn(O2CCH3)2 as the electrolyte.Fabrication of nitrogen doped graphene nanotube complexes can enhance the ORR activity and ZAB performance. Kong et. al. [134] have observed that N doped graphene nanotube complexes exhibits high ORR activity with high ZAB performance. It was prepared based on the hydrothermal treatment and annealing process, where urea is used as the nitrogen source and boric acid (H3BO3) is used as the splicing agent. It is composed of few-layer graphene and carbon nanotubes, which have been randomly mixed; It possesses 3D porous nanostructure; It contains interconnected macropores; It contains abundant defects; It possesses ultrahigh specific surface area and pore volume; It contains high density of pyridinic-N; It exhibits low charge transfer resistance; Thus, it could modify the electronic structure, afford optimal adsorption with intermediates, expose abundant active sites, facilitate the charge transfer process, enhance the conductivity, and that could enhance the ORR performance, and that could improve the ZAB performance. It exhibits high ORR activity (E1/2\u00a0=\u00a00.89\u00a0V). ZAB with N doped graphene nanotube complexes exhibits high open circuit potential of 1.54\u00a0V, large power density of 149 mW cm\u22122, high specific capacitance of 873 mAh gZn\n\u22121, and high stability.Preparing heteroatoms doped CNT-graphene hybrids can enhance the ORR activity and ZAB performance. Huang et. al. [135] have observed that N, S co-doped CNT-graphene hybrids exhibits high ORR activity with high ZAB performance. It was derived from biomolecule (guanine), where it was obtained by pyrolysis of the guanine-sulfate and OCNT. It is composed of N, S co-doped CNT-graphene hybrids; It possesses 3D hierarchically porous structure; Thus, it could modify the electronic structure, afford optimal adsorption with intermediates, expose abundant active sites, facilitate the charge transfer, enhance the conductivity, and that could enhance the ORR performance, and that could improve the ZAB performance. It exhibits high ORR activity (E1/2\u00a0=\u00a00.87\u00a0V). ZAB with N, S co-doped CNT-graphene hybrids exhibits high open circuit potential of 1.48\u00a0V, large power density of 188 mW cm\u22122, high specific capacitance of 800 mAh gZn\n\u22121, and high stability.Preparing porous carbon materials comprising of few-layer graphene sheets and carbon nanotubes can enhance the ORR activity and ZAB performance. Kong et. al. [136] have observed that carbon-tube-graphene complexes exhibits high ORR activity with high ZAB performance. It was prepared based on the hydrothermal and pyrolysis treatments, where graphene nanosheets and carbon nanotubes are used as building block, while boric acid (H3BO3) is used as splicing agent. It is composed of few-layer graphene sheets and carbon nanotubes, which have been self-assembled; It contains hierarchical porous carbon materials having sponge-like architecture; It contains interconnected macropores; It contains defects; It possesses high surface area and pore volume; It exhibits low charge transfer resistance; Thus, it could expose abundant active sites, facilitate the charge transfer, enhance the conductivity, and that could enhance the ORR performance, and that could improve the ZAB performance. It exhibits high ORR activity (E1/2\u00a0=\u00a00.841\u00a0V). ZAB with carbon-tube-graphene complexes exhibits high open circuit potential of 1.38\u00a0V, large power density of 65 mW cm\u22122, and high stability.Preparing reduced graphene oxide modified heteroatom-doped ultra-thin hollow carbon spheres can enhance the ORR activity and ZAB performance. Sheng et. al. [137] have observed that rGO@N doped hollow carbon sphere composites exhibits high ORR activity with high ZAB performance. It was prepared based on the self assembly followed by carbonization followed by etching. It is composed of reduced graphene oxide modified nitrogen-doped ultra-thin hollow carbon spheres; It contains defects; It contains high density of pyridinic-N; Thus, it could modify the electronic structure, afford optimal adsorption with intermediates, expose abundant active sites, facilitate the charge transfer, enhance the conductivity, and that could enhance the ORR performance, and that could improve the ZAB performance. It exhibits high ORR activity. ZAB with N doped graphene nanotube complexes exhibits high open circuit potential of 1.54\u00a0V, large power density of 142 mW cm\u22122, high specific capacitance, and high stability.Preparing defect-rich carbon fiber having porous graphene skin can enhance the ORR activity and ZAB performance. Wang et. al. [138] have observed that defect-rich carbon fiber with porous graphene skin exhibits high ORR activity with high ZAB performance. It was prepared using high-temperature H2 etching approach. It is composed of defect-rich carbon fiber with porous graphene skin; Thus, it could modify the electronic structure, afford optimal adsorption with intermediates, expose abundant active sites, facilitate the charge transfer, enhance the conductivity, and that could enhance the ORR performance, and that could improve the ZAB performance. It exhibits high ORR activity. ZAB with N doped graphene nanotube complexes exhibits high open circuit potential of 1.54\u00a0V, large power density of 91.4 mW cm\u22122, high specific capacitance of 707 mAh g\u22121, and high stability.Thus, various strategies including preparing N-doped vertically aligned carbon nanotubes on graphene foam [133] fabrication of nitrogen doped graphene nanotube complexes [134], preparing heteroatoms doped CNT-graphene hybrids [135], preparing porous carbon materials comprising of few-layer graphene sheets and carbon nanotubes [136], preparing reduced graphene oxide modified heteroatom-doped ultra-thin hollow carbon spheres [137], and preparing defect-rich carbon fiber having porous graphene skin [138] enhanced the ORR/OER activity and ZAB performance.The sluggish kinetics of ORR and OER are often considered as the bottleneck of a variety of electrochemical energy conversion technologies, including water electrolyzers, metal-air batteries, and fuel cells. Hence, development of cheap and efficient ORR and OER catalysts are essential, which not only be the alternative for the expensive noble metal catalysts (ORR: Pt, OER: IrO2 or RuO2, and ORR/-OER: Pt-Ru/C) but also bring the electrochemical energy systems nearer to their theoretical limits. The ORR/OER activity and zinc-air battery performance of several types of graphene-based air catalysts such as graphene with non-metals, non-noble metals, metal oxides, nitrides, sulfides, carbides, and other carbon composites have been identified to develop promising ZABs.Various strategies including creating defects on the graphene [101], doping N with graphene [97], N-doped graphene obtained through carbonization of natural rice [100], preparing N-doped graphene with electron-withdrawing pyridinic N and electron-donating quaternary N [102], preparing N-doped exfoliated graphene [106], S-doped graphene foam prepared from food (idly) [103], preparing S, N co-doped graphene-like electrocatalyst [105], preparing defect enriched S, N co-doped graphene-like carbon [98], B, N co-doped graphene with graphitic N and BC3\n[96], integrating holey graphene framework with CNTs [104], and preparing N, P co-doped carbon framework [99] enhanced the ORR/OER activity and ZAB performance.Moreover, several strategies including integrating Fe/Fe3C@C nanoparticles with graphene framework [107], integrating NiFe nanoparticles with graphene [115], integrating Vulcan carbon with CoFe-N-rGO\n[116], preparing Co nanoclusters on N-doped carbon [108], preparing Co/N/O tri-doped graphene [111], preparing cobalt nanoparticles encapsulated in N-enriched graphene shells with hollow graphene spheres [113], preparing Co, N-co-doped CNT/graphene heterostructure [109], preparing graphene matrix with Cu(I)-N active sites [114], integrating Ag NW with graphene aerogel [112], and preparing Fe\u2013N active sites and integrating CNTs and graphene with MOF [110] improved the ORR/OER activity and ZAB performance.In addition, various strategies including preparing MnO2 nanofilm on N-doped hollow graphene [120], integrating Mn3O4 with rGO-IL [125], integrating CoMn2O4 with NrGO\n[124], integrating MnCoFeO4 with N-rGO\n[126], functionalizing graphene oxide with 1-hexyl-3-methylimidazolium chloride molecules [121], integrating Co3O4 with N doped graphene [122], integrating Co3O4 nano-rods with reduced graphene oxide [118], preparing predominant metallic Co with small fraction of its oxides anchored on N-doped reduced graphene oxide [123], integrating amorphous bimetallic oxide with N-doped reduced graphene oxide [117], and integrating Fe-doped NiOOH with graphene-encapsulated FeNi3\n[119] enhanced the ORR/OER activity and ZAB performance.Moreover, various strategies including integrating Ni3FeN with NrGO\n[128], preparing CoSx@C3N4 integrated with reduced graphene oxide [127], integrating CoS2 with N, S co-doped graphene oxide [130], integrating CoSx with N, S co-doped graphene [131], integrating NiCo2S4 with ultrathin S-doped graphene [129], preparing Fe3C(Fe) with N-doped graphitic layers as shell and N-doped graphene as interface layer [132], preparing N-doped vertically aligned carbon nanotubes on graphene foam [133] fabrication of nitrogen doped graphene nanotube complexes [134], preparing heteroatoms doped CNT-graphene hybrids [135], preparing porous carbon materials comprising of few-layer graphene sheets and carbon nanotubes [136], preparing reduced graphene oxide modified heteroatom-doped ultra-thin hollow carbon spheres [137], and preparing defect-rich carbon fiber having porous graphene skin [138] enhanced the ORR/OER activity and ZAB performance.The vital factors governing the performance of ZAB should be considered in future research to provide superior performance in practical applications:\n\n1.\nRecently significant efforts have been done to fabricate several kinds of high performance air catalysts for ZAB but the high performance air catalysts are very limited because of the poor electrical conductivity, poor thermal stabilities, inferior cycling stability, restricted specific capacity, and slower kinetic diffusion. Hence, additional progresses are indeed necessary by fabricating high performance graphene-based air catalysts including graphene with non-metals, non-noble metals, metal oxides, nitrides, sulfides, carbides, and other carbon composites.\n\n\n2.\nRecently, several kinds of high performance graphene-based air catalysts for ZAB have been reported. Nevertheless, very limited facile synthesis route have been explored for the fabrication of graphene-based air catalysts. Therefore, additional efforts are obviously needed by exploring facile and green synthesis route to diminish or alleviate the use or generation of hazardous substances for the fabrication of high performance graphene-based air catalysts for ZAB.\n\n\nRecently significant efforts have been done to fabricate several kinds of high performance air catalysts for ZAB but the high performance air catalysts are very limited because of the poor electrical conductivity, poor thermal stabilities, inferior cycling stability, restricted specific capacity, and slower kinetic diffusion. Hence, additional progresses are indeed necessary by fabricating high performance graphene-based air catalysts including graphene with non-metals, non-noble metals, metal oxides, nitrides, sulfides, carbides, and other carbon composites.Recently, several kinds of high performance graphene-based air catalysts for ZAB have been reported. Nevertheless, very limited facile synthesis route have been explored for the fabrication of graphene-based air catalysts. Therefore, additional efforts are obviously needed by exploring facile and green synthesis route to diminish or alleviate the use or generation of hazardous substances for the fabrication of high performance graphene-based air catalysts for ZAB.\nMohammed-Ibrahim Jamesh: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Software, Validation, Visualization, Writing - original draft, Writing - review & editing. Prabu Moni: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Software, Validation, Visualization, Writing - original draft, Writing - review & editing. A.S. Prakash: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Software, Validation, Visualization, Writing - review & editing. Moussab Harb: 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.One of the authors (Dr. M.I.J) thanks to the Science and Engineering Research Board (SERB), Department of Science and Technology, Government of India, for funding under National Post-Doctoral Fellowship scheme with the reference no. PDF/2017/000015. One of the authors, Dr. Prabu Moni grateful to the Department of Science and Technology (DST), New Delhi, India for awarding INSPIRE Faculty Award (DST/INSPIRE/04/2016/000530). One of the authors, Dr. MOUSSAB Harb thanks to the King Abdullah University of Science and Technology (KAUST).", "descript": "\n                  The development of cheap and efficient oxygen reduction and evolution reaction catalysts are important, which not only push the electrochemical energy systems including water electrolyzers, metal-air batteries, and fuel cells nearer to their theoretical limits but also become the substitute for the expensive noble metal catalysts (Pt/C, IrO2 or RuO2 and Pt-Ru/C). In this review, the recently reported potential graphene-based air catalysts such as graphene with non-metals, non-noble metals, metal oxides, nitrides, sulfides, carbides, and other carbon composites are identified in-light-of-their high oxygen reduction reaction/oxygen evolution reaction activity and zinc-air battery performance for the development of high energy density metal-air batteries. Further, the recent progress on the zinc-air batteries including the strategies used to improve the high cycling-performance (stable even up-to 394 cycles), capacity (even up-to 873\u00a0mAh g\u22121), power density (even up-to 350\u00a0mW cm\u22122), and energy density (even up-to 904\u00a0W\u00a0h\u00a0kg\u22121) are reviewed. The scientific and engineering knowledge acquired on zinc-air batteries provide conceivable development for practical application in near future.\n               "}
{"full_text": "Electrochemical water splitting is a well-known promising technology for the sustainable production of high-purity hydrogen [1]. The development of efficient electrocatalysts primarily consisting of earth-abundant elements [2,3] for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) as well, is of great importance for large-scale hydrogen production by water splitting [4,5]. During the last decades, transition-metal-based materials have been and are still considered as alternative HER/OER electrocatalysts due to their earth-abundance, low-cost, and promising activity [6]. Specifically, transition metal alloys [7], nitrides [8], phosphides [9], metal oxides [10], metal hydroxides [11], chalcogenides [12], and other compounds have been proved as active materials for HER/OER electrocatalysis [13]. Furthermore, transition-metal-based materials appear as possible efficient bifunctional electrocatalysts for both HER and OER, which are cost-effective and with higher efficiency in practical applications [14,15]. Since the different reaction mechanisms of HER and OER requires different structural and electronic properties for the electrocatalysts, bimetallic and multi-metallic compounds-based materials appear as more promising candidates for the overall water splitting [16,17]. For example, Chen et al. [18] fabricated porous Fe-Mo oxide hybrid nanorods on nickel foam (NF) as efficient bifunctional electrocatalysts for water splitting. Li et al. [19] synthesized a porous amorphous Ni/Ni-Fe-Mo suboxide nanoplates array on NF. As well known, NF is a popular substrate that can rivet and disperse catalytic components, thus resulting in high loadings of active catalytic components with the consequence in providing abundant catalytic sites [4,20]. Moreover, NF can be used as a substrate and be directly transformed into one of the multiple active components of the catalyst [21]. Fei et al. [22] synthesized ultrathin Fe-doped Ni3S2 arrays on NF for efficient water splitting, which transformed NF into Ni3S2 via a Na2S-induced chemical etching process. Although previous works have made prominent progress, developing low-cost, highly efficient, and durable bifunctional electrocatalysts by combining both the advantages of multimetallic nanomaterials and the utilization of NF is still significant and challengeable [23,24].Herein, we report the development of an efficient bifunctional Ni-Fe/NiMoNx electrocatalyst deposited on NF, using a hydrothermal method followed by NH3 treatment. During the hydrothermal process, Fe and Mo elements were introduced, while partial NF substrate was transformed into Ni(OH)2. The following NH3 treatment resulted in the formation of Ni-Fe/NiMoNx, which showed high-efficiency electrocatalysis toward both HER and OER in alkaline medium. As a consequence, the prepared Ni-Fe/NiMoNx electrocatalyst showed efficient HER and OER performance at low overpotentials of 49 and 260\u00a0mV at 20\u00a0mA\u00a0cm\u22122, respectively. The overall water splitting of Ni-Fe/NiMoNx couple electrodes required a low cell voltage of 1.54\u00a0V for 10\u00a0mA\u00a0cm\u22122.The Nickel foam (NF) was pretreated by ultrasonication in 3.0\u00a0M HCl solution, ethanol, and deionized water. Fe(NO3)3\u00b79H2O (1.5\u00a0mmol Fe) and (NH4)6Mo7O24\u00b74H2O (1.5\u00a0mmol Mo) were dissolved in 60\u00a0mL deionized water. After magnetic stirring for 1\u00a0h, the solution was transferred into a 100\u00a0mL Teflon-lined stainless-steel autoclave, and then one piece of pretreated NF (2\u00a0\u00d7\u00a05\u00a0cm) was immersed into the solution. The autoclave was sealed and maintained at 150\u00a0\u00b0C for 6\u00a0h. After cooled down at room temperature, the obtained NiMoFe-Pre was taken out and cleaned with deionized water and ethanol.The prepared NiMoFe-Pre was transferred into a tubular furnace with NH3 atmosphere flow and annealed at 400\u00a0\u00b0C for 2\u00a0h, and then Ni-Fe/NiMoNx was obtained after natural cooling at room T. The NiMoFe-H400 was prepared in the same tubular furnace at the same temperature but treated in 10\u00a0vol% H2/Ar gas atmosphere.Powder X-ray diffraction (XRD) patterns were obtained on Rigaku Smartlab equipment. Scanning electron microscopy (SEM) experiments were performed on a Zeiss Sigma 500 microscope. Transmission electron microscopy (TEM) images were acquired on an FEI Tecnai G2 F30 microscope. X-ray photoelectron spectroscopy (XPS) tests were performed on a Thermos Scientific spectrometer.The electrochemical testing was performed on a VSP-300 (BioLogic, France) electrochemical workstation. A three-electrode system in 1.0\u00a0M KOH was used, where the prepared NF-based electrode (0.5\u00a0\u00d7\u00a01.0\u00a0cm) was used as working electrode, Hg/HgO as a reference electrode, and graphite rod and platinum wire were used as counter electrodes for HER and OER, respectively. The iR correction was applied to all the LSV curves, and all potentials were converted into the RHE (E\n\nRHE\n\u00a0=\u00a0E\n\nHg/HgO\n\u00a0+\u00a00.098\u00a0V\u00a0+\u00a00.0592pH). Electrochemical impedance spectroscopy (EIS) tests were recorded in the frequency range between 50 mHz and 100\u00a0kHz with an amplitude of 5\u00a0mV, and tested at the voltages of \u22120.022\u00a0V for HER and 1.480\u00a0V for OER.The electrode was prepared by the hydrothermal method followed by NH3 gas treatment (Fig. 1a). The NiMoFe-Pre nanosheets with smooth surfaces were first grown on the nickel foam using the hydrothermal method. During the hydrothermal process, Mo and Fe elements were introduced, and the substrate NF was partially corroded to become Ni precursor due to the presence of H+ from ammonium hydrolysis and the excess of NO3\n\u2212 in solution [25]. The powder XRD patterns showed that the formed NiMoFe-Pre precursor was mainly composed of Ni(OH)2, Fe2O3, and MoO3 (Fig. S1), which proved the successful loading of Fe, Mo, and the utilization of NF. To obtain Ni-Fe/NiMoNx, the NiMoFe-Pre precursor nanosheets were further treated in NH3 gas atmosphere. SEM image showed that numerous nanoparticles are formed on the surface of nanosheets after being annealed in NH3 gas atmosphere (Fig. 1a). The presence of nanoparticles should be due to phase separation of different components that are formed by ammonia gas treatment, beneficial to the increase of the specific surface area and to the exposure of active sites.Analysis of TEM images further exhibited that Ni-Fe/NiMoNx were composed of ultrathin nanosheets anchoring with nanoparticles (Fig. 1b). The high-resolution TEM (HRTEM) image of Ni-Fe/NiMoNx showed typical lattice spacings of 0.174 and 0.203\u00a0nm on nanoparticles, corresponding to the (200) and (111) planes, respectively, of Ni (Fig. 1c), indicating the formation of Ni nanoparticles on nanosheets surfaces during nitridation. After fast Fourier transformation (FFT) and inverse FFT, the lattice spacing was clearly observed, where 0.215\u00a0nm correspond to the (002) plane of Ni3N, and 0.244\u00a0nm correspond to the (110) plane of Mo5N6, indicating the formation of nitrides by nitridation. In the selected area electron diffraction (SAED) image, the (200), (111), (110), and (311) planes of Ni and (111) plane of Ni3N were displayed (Fig. 1d). The energy-dispersive X-ray spectroscopy (EDS) images showed that Ni element mainly exist in the nanoparticles and partially on the nanosheets, while Fe, Mo, N, and O elements were uniformly distributed on the nanosheets (Fig. 1e). Combining with HRTEM and EDS results, the Ni nanoparticles and the main elements of nanosheets are preliminarily clarified.The compositions of Ni-Fe/NiMoNx were further investigated by powder XRD patterns (Fig. 2a). They show the existence of Ni, Ni3N, NiO, and Mo5N6. There are no distinct characteristic peaks of the Fe element, which is probably due to the coverage by the adjacent strong characteristic peaks of nickel. XPS analyses were conducted to study the surface chemical states in the Ni-Fe/NiMoNx. The XPS survey scan confirmed the presence of Ni, Mo, Fe, O and N elements in the catalyst (Fig. 2b). The high-resolution spectra of Ni 2p (Fig. 2b) evidenced the presence of Ni2+ and Ni0 due to NiO and Ni phases, whereas Ni1+ appeared is due to the Ni3N phase. The coexistence of NiO, Ni, and Ni3N indicates the incomplete nitridation after the NH3 gas treatment.In the case of Fe 2p XPS spectra, the Fe0 signal proved the existence of metallic Fe (Fig. 2d). The presence of Fe0 and Fe2+ further proved the incomplete nitridation process by NH3 gas treatment. In the case of Mo 3d spectra, peaks of Mo6+, Mo4+, and Mo3+ were located in the regions of Mo 3d3/2 and Mo 3d5/2 (Fig. 2e). Besides, there is another peak which is located in the region of Mo 3p (Fig. 2f). Characteristic peaks of metal-N and NH appeared in the region of N 1\u00a0s. The metal-N bonding is obviously from Ni3N and Mo5N6. The formation of the NH bonding might be due to the hydrogen adsorption properties of metal nitrides. Specifically, binding with N atoms alters the d-band structure of the host metal, thereby contracting the d-band of the metal [8]. This alteration changes the coupling state between the adsorbed hydrogen s-band and the metal d-band, thus the adsorption of hydrogen tends to the HER process [26,27]. All these XPS results combined with HRTEM and powder XRD data showed the incomplete reduction and nitridation occurred during NH3 gas treatment, and which resulted in the formation of Ni-Fe/NiMoNx hybrid nanosheets.The HER performance of Ni-Fe/NiMoNx and other samples was characterized in 1.0\u00a0M KOH solution. As can be observed from the linear sweep voltammetry (LSV) curves (Fig. 3a), Ni-Fe/NiMoNx shows low overpotentials of 49 and 107\u00a0mV to achieve the hydrogen evolving currents of 20 and 100\u00a0mA\u00a0cm\u22122, respectively (Fig. 3a), which is comparable to Pt/C/NF (\u03b720\u00a0=\u00a026\u00a0mV) and much lower than NiMoFe-H400 (\u03b720\u00a0=\u00a0139\u00a0mV), NiMoFe-Pre (\u03b720\u00a0=\u00a0190\u00a0mV), and NF (\u03b720\u00a0=\u00a0280\u00a0mV). Ni-Fe/NiMoNx has also a low Tafel slope at 70.74\u00a0mV dec\u22121 (Fig. 3b), better than NiMoFe-H400, NiMoFe-Pre, and NF.The HER performance is comparable and even better than recently reported HER electrocatalysts (Table S1, Supporting Information). Nyquist plots were obtained from the EIS tests (Fig. 3c). On the basis of equivalent circuit model, the charge transfer resistance increases in the order: Ni-Fe/NiMoNx (Rct\u00a0=\u00a01.7\u00a0\u03a9)\u00a0<\u00a0NiMoFe-H400 (Rct\u00a0=\u00a011.8\u00a0\u03a9)\u00a0<\u00a0NF (Rct\u00a0=\u00a057.55\u00a0\u03a9)\u00a0<\u00a0NiMoFe-Pre (Rct\u00a0=\u00a0220.9\u00a0\u03a9). The lower impedance value means a higher charge transfer rate and faster electrode kinetics for HER. Previous works have proved that nitrides exhibit low electrical resistance and bind exceptionally to both water molecules and hydrogen atoms [28,29]. Furthermore, the nitrides and metallic Ni and Fe are capable to permit rapid electron transfer between the active surface sites of the catalyst and the NF current conductor [10]. In order to highlight the importance of nitrides for HER, NiMoFe-H400 was prepared with 10\u00a0vol% H2/Ar gas treatment instead of NH3. Their powder XRD and XPS results proved the presence of similar components such as Ni, NiO, and Fe, which is due to the incomplete reduction, except the case of nitrides (Figs. S2 and S3, Supporting Information). The Ni-Fe/NiMoNx possesses lower overpotential, Tafel slope, and impedance than the NiMoFe-H400 solid. This indicates the presence of Ni3N and Mo5N6 which can significantly improve charge transfer rates, thus highlighting the HER enhancement after NH3 gas pre-treatment.NH3 gas treatment at different temperatures exhibited different HER performance. SEM and powder XRD patterns indicated different degrees of nitridation at different temperatures, ca. 300, 400, and 500\u00a0\u00b0C (Figs. S4 and S5, Supporting Information). Compared with Ni-Fe/NiMoNx (N400), the powder XRD patterns of NiMoFe-N300 and NiMoFe-N500 showed significant diffraction peaks of NiO and Mo5N6, respectively. This indicates that higher temperatures of NH3 gas treatment can lead to a higher degree of nitridation. As shown in Fig. S6 (Supporting Information), Ni-Fe/NiMoNx (N400) showed lower overpotential and lower impedance than NiMoFe-N300 (\u03b720\u00a0=\u00a0245\u00a0mV, Rct\u00a0=\u00a0113.2\u00a0\u03a9) and NiMoFe-N500 (\u03b720\u00a0=\u00a056\u00a0mV, Rct\u00a0=\u00a04.92\u00a0\u03a9). Comparative analyses of these catalysts led us to conclude that Ni-Fe/NiMoNx has the best HER performance (Fig. 3d). Furthermore, to calculate the double-layer capacitance (Cdl), the cyclic voltammograms at various scan rates for samples in the non-faradaic capacitance current range were obtained (Fig. S7, Supporting Information). The Cdl of Ni-Fe/NiMoNx was 22.03 mF cm\u22122, which is much higher than those obtained over NiMoFe-H400, NiMoFe-Pre, NF, and NiMoFe-N300 (Fig. 3e and Fig. S6, Supporting Information). Since Cdl is positively associated with electrochemical surface area (ECSA), it is concluded that Ni-Fe/NiMoNx has higher ECSA and thus more exposed electrocatalytic active sites.Stability is another critical parameter for the electrocatalytic performance evaluation of the solid. In Fig. 3f, there is no significant difference in LSV curves before and after 1000\u00a0cycles of voltammetry scanning. The electrocatalyst also exhibits durability at the current density of \u2212100\u00a0mA\u00a0cm\u22122 for 40\u00a0h with a decline of only 15\u00a0mV. The SEM image after long-term HER showed well-defined nanosheet geometry, further confirming the good structural stability of electrocatalyst (Fig. S8, Supporting Information).Ni-Fe/NiMoNx achieved the oxygen-evolving current of 20 and 100\u00a0mA\u00a0cm\u22122 at overpotentials of 260 and 292\u00a0mV, respectively (Fig. 4a), with Tafel slope of 39.26\u00a0mV dec\u22121 (Fig. 4b). The overpotential is significantly lower than that of NiMoFe-Pre (\u03b720\u00a0=\u00a0280\u00a0mV) and NF (\u03b720\u00a0=\u00a0356\u00a0mV). Notably, NiMoFe-H400 exhibits similar overpotential (\u03b720\u00a0=\u00a0262\u00a0mV) and Tafel slope (39.26\u00a0mV dec\u22121) for Ni-Fe/NiMoNx. The reason behind this is the similar chemical states and components formed by the incomplete reduction, and the collective effect of these multiple components makes the easy formation of metal (oxy)hydroxide active sites for OER, and the consequent adsorption of *O, *OH, and *OOH intermediates neither too strong nor too weak. Thus, it enhanced the rate of processes of both adsorption of intermediates and evolution of O2, resulting in rapid OER kinetics with obviously lower Tafel slopes after the incomplete nitridation/reduction [30,31]. Both the low overpotential and Tafel slope indicate the good electrocatalytic activity of Ni-Fe/NiMoNx for OER, which outperforms the recently reported non-precious metal electrodes (Table S1, Supporting Information). EIS tests show the high charge transfer capability of Ni-Fe/NiMoNx (Rct\u00a0=\u00a07.9\u00a0\u03a9), which is smaller than that in other samples.The OER performances of NiMoFe-N300 and NiMoFe-N500 were also tested to investigate the effect of the annealing temperature (Fig. S9, Supporting Information). It was found that calcination temperature in the NH3 gas atmosphere can affect the degree of nitridation and alter the OER performance. Comparative analysis of the OER performance of different catalysts showed that Ni-Fe/NiMoNx has the best OER performance (Fig. 4d). Besides, Ni-Fe/NiMoNx has excellent stability in OER. The LSV curves before and after 1000\u00a0cycles of voltammetry scanning are almost coincident as seen in Fig. 4e. After 40\u00a0h long-term testing at 100\u00a0mA\u00a0cm\u22122, the potential has a small fluctuation of only 9\u00a0mV (Fig. 4f) [31]. After long-term OER testing, the SEM image was obtained which confirms the good structural stability (Fig. S10, Supporting Information).The overall water splitting performance was performed in an alkaline electrolyzer with a two-electrode configuration, utilizing two Ni-Fe/NiMoNx electrodes as the anode and cathode. The polarization curves showed that Ni-Fe/NiMoNx-based electrolyzer requires a cell voltage of only 1.54\u00a0V at a current density of 10\u00a0mA\u00a0cm\u22122, whereas 1.83\u00a0V is required for NF||NF. The water splitting performance of the present catalyst appears better than many reported electrocatalysts, such as P-Co3O4/NF (\u03b710\u00a0=\u00a01.63\u00a0V) [32], ZnCo2S4/NF (\u03b710\u00a0=\u00a01.66\u00a0V) [33], and NiCo2S4/NF (\u03b710\u00a0=\u00a01.61\u00a0V) [34]. The LSV curves before and after 1000\u00a0cycles of voltammetry scanning were almost coincident, as shown in Fig. 5c. The durability test showed a stable potential at 100\u00a0mA\u00a0cm\u22122 for over 40\u00a0h, both exhibiting remarkable stability for water splitting (Fig. 5d). Therefore, the above results and discussion led us to confirm that Ni-Fe/NiMoNx is a highly efficient and durable catalyst for water splitting.An efficient and durable HER and OER bifunctional Ni-Fe/NiMoNx electrocatalyst was successfully designed and prepared. The Ni-Fe/NiMoNx showed low overpotentials of 49 and 260\u00a0mV for HER and OER at a current density of 20\u00a0mA\u00a0cm\u22122, respectively, with Tafel slopes of 70.74 and 39.26\u00a0mV dec\u22121. For overall water splitting, Ni-Fe/NiMoNx-based electrolyzer requires low cells voltage, ca. 1.54\u00a0V for a current density of 10\u00a0mA\u00a0cm\u22122 in 1.0\u00a0M KOH, which is durable at a constant current density of 100\u00a0mA\u00a0cm\u22122 for 40\u00a0h. Enhanced HER and OER performance is due to the combined effect of the presence of multiple active electrocatalytic components, after NH3 gas pre-treatment. Besides, this work highlights the effect of NF and provides a new strategy for the rational design of bifunctional water electrolysis catalysts.\nYu Qiu: Methodology, Formal analysis, Investigation, Data curation, Writing \u2013 original draft. Mengxiao Sun: Methodology, Formal analysis, Investigation, Writing \u2013 original draft. Jia Cheng: Formal analysis, Investigation. Junwei Sun: Formal analysis, Investigation. Deshuai Sun: Methodology, Formal analysis, Resources, Writing \u2013 review & editing, Supervision. Lixue Zhang: Conceptualization, Methodology, Formal analysis, 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 study was financially supported by the National Natural Science Foundation of China (No. 22075159), Taishan Scholar Program (No. tsqn202103058), and the Youth Innovation Team Project of Shandong Provincial Education Department (No. 2019KJC023).\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.2022.106426.", "descript": "\n                  Fabricating bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is challengeable. Herein, an efficient and durable bifunctional Ni-Fe/NiMoNx electrocatalyst has been synthesized on nickel foam. This electrocatalytic system exhibits significant HER and OER performances with overpotentials of 49 and 260\u00a0mV at 20\u00a0mA\u00a0cm\u22122, respectively. For overall water splitting, Ni-Fe/NiMoNx electrodes require only 1.54\u00a0V for 10\u00a0mA\u00a0cm\u22122. This enhanced HER and OER electrocatalytic performance is due to the combined effect of multiple active components formed after NH3 treatment. This work provides a new strategy for the rational design of bifunctional electrocatalysts for overall water splitting.\n               "}
{"full_text": "Ever since the beginning of the twentieth century, the demand for energy and technological advances have been intercorrelated with the rapid development of new processes for the conversion of different forms of energy. The development and expansion of the car generated the increased demand for the liquid instead of previously dominant solid fuels. Most fractions of crude oil, the only abundant resource of liquid energy carriers, however, are not suitable for combustion in engines. Following the demand of lower molecular weight products, thermal cracking, the decomposition of longer into shorter hydrocarbons was developed. If a solid acid catalyst is added, the reaction can occur at significantly lower temperatures, and the selectivity to fuels with a higher octane number is increased. Catalysts, therefore, act by facilitating the overall reaction as well as favoring certain products, resulting in the reduced energy consumption of the process. This and other closely related catalytic processes nowadays are used to produce almost all available petrol, jet, and diesel fuels. Beyond hydrocarbon cracking, catalytic materials have been used for a variety of reactions such as the Haber-Bosch process for the fixation of atmospheric nitrogen, the production of hydrogen from natural gas, the synthesis of plastics, and commodity and fine chemicals\u2014most of which rely on the use of metallic transition metals on metal oxides.\n1\n\nAs shown in Figure\u00a01\nA, the energy consumption based on nuclear and coal resources are not expected to increase any further in the next few decades. The demand for petroleum and other liquid fuels will continue to increase in the upcoming decades while renewable resources and natural gas, both of which are much more directly related to catalysis than nuclear energy and coal, are projected to play a dominant role by 2050. It becomes apparent that the industrial sector is currently responsible for more than half of the global energy consumption, with a projected slight decrease of 4.2% in 2050 (Figure\u00a01B). Similarly, the transportation sector accounts for around another quarter of the global energy consumption and will moderately rise. The commercial and residential energy consumption makes up less than 25% in 2012, and will continue to be less than a quarter by 2050. When taking a closer look at the energy consumption of industrial subsectors (Figure\u00a01C), catalysis-related processes such as the production of basic chemicals will witness an overall growth in significance until 2040. The same trend is obvious for the transportation subsectors with an obvious increase in the use of natural gas and jet fuels whereas the consumption of diesel and motor gas are projected to plummet.\n2\u20134\n Despite its already high significance in current energy-related industries, we expect catalytic processes to grow in importance in the future for the utilization of both conventional and unconventional energy resources. In particular, a number of above-mentioned chemical transformations have already become active research topics in SAC.Reports on atomically dispersed ionic transition metals have been reported earlier.\n5\u201311\n Then the term \u201csingle-atom catalysis\u201d (SAC) was coined in 2011 by Zhang, Li, Liu, and colleagues, and has spurred tremendous developments. The evolution of related concepts and name conventions have been detailed recently,\n12\n and herein, we will focus the scope of this review predominantly on energy-related applications published in the recent decade.\n13\n Comprehensive introduction on the synthetic strategies of SACs,\n14\u201316\n specific types of support materials,\n17\u201319\n electrocatalysis,\n16,18,20\n and photocatalysis\n21,22\n could be found in excellent reviews that were recently published. In contrast to nanoparticle-based catalysts, where a significant fraction of metal atoms is buried below the surface and is thus inaccessible to reactants, SACs offer the maximum possible atom economy and maximized metal-support interactions. SACs exhibit electronic states ranging from positively to negatively charged and carry the absence of neighboring metallic atoms, significantly different from metallic nanoparticles. Furthermore, nanoparticles tune properties through d-band, while SACs display unique tunability because of their homogeneous-like ability to control the frontier orbital geometry and energy of the active sites. Meanwhile, the geometric structures of SACs are also different from that of homogeneous catalysts.\n23\n The metal species of homogeneous catalysts are coordinated by ligands or substrates. The geometric structures are flexible and the coordination environment can be changed easily during the reaction conditions. For single-atom catalysts, the geometric structures are partially restricted by the support, which could be regarded as a rigid, bulky ligand to coordinate single-atom metals. Those unparalleled properties give rise to unique reactivity patterns and have thus enabled their potential use in a range of energy applications.\n12,16,24\n\n\nFigure\u00a02\n depicts the conversion of energy carriers, in particular, fossil resources such as natural gas, crude oil, and coal as well as renewable energy such as solar, wind, hydro, and biomass over SACs. The products of those reactions might be the final products, or represent energy carriers that are used for grid-scale or transportation-scale energy storage including hydrocarbon, oxygenate, hydrogen, and ammonia fuels as well as a variety of chemicals and electrochemical energy storage systems. Each of those sectors will be reviewed and assessed individually in the context of SAC, and a general future perspective for the field of SAC for energy applications will be provided.Hydrocarbon fuels are currently produced from fossil fuels (coal, petroleum, and natural gas).\n25\n They are likely to remain as the principal sources of transportation fuels for the next decades. According to the International Energy Outlook 2017 released by the U.S. Energy Information Administration, the world primary fossil fuels consumption reached 514 quadrillion Btu in 2017, accounting for 82% of the global primary energy consumption. The global fossil fuels consumption is projected to reach 739 quadrillion Btu in 2040. Due to the diminishing fossil fuel reserves and continuous increase in CO2 emission from human activity, it becomes desirable to develop efficient catalytic systems to produce hydrocarbon fuels from sustainable resources such as biomass and CO2.\n26\n\nAs the most abundant renewable organic carbon source, biomass is an attractive alternative feedstock for fuels. In many developing countries, a large percentage of fuels consumed is derived from biomass.\n27\n The transformation of oxygen-rich biomass to hydrocarbon fuels requires oxygen removal reactions such as hydrodeoxygenation (HDO) to form molecules that have desirable properties for upgrading. The traditional HDO catalysts generally suffer from poor catalytic performance and fast deactivation at high temperature. To improve the activity and stability of HDO catalysts, Tsang and co-workers developed a single-atom Co/MoS2 catalyst, in which isolated Co atoms were anchored covalently to sulfur vacancies of MoS2 monolayers.\n28\n The single Co atoms were observed in the HAADF-STEM images (Figure\u00a03\nA), and DFT calculation was carried out to simulate the geometric structure of Co (Figure\u00a03B). The prepared Co/MoS2 catalyst with a large number of Co-S-Mo active sites exhibited excellent performance in the selective HDO of lignin-derived 4-methylphenol to toluene. The high catalytic performance of Co/MoS2 allowed the reaction to proceed at 180\u00b0C, which normally requires 300\u00b0C to occur. The lower operation temperature triggers energy saving, as heating is one of the main costs in large-scale industry applications, potentially pushing the hydrocarbon fuels production from biomass to commercial viability. Later, Tsang and co-workers designed a bifunctional catalyst consisting of atomically dispersed Pd and ultrasmall molybdenum phosphate nanoparticles for the conversion of phenolic monomers as well as wood and bark-derived oligomers into liquid hydrocarbons.\n29\n The prepared catalyst showed almost complete conversion of phenol to cyclohexane at 383 K. It was proposed that atomically dispersed Pd species promoted the hydrogenation of phenol to cyclohexanol, which was dehydrated to cyclohexene by Br\u00f8nsted and Lewis acid sites on MoO3-P2O5 nanoparticles. Finally, the formed cyclohexene was hydrogenated to cyclohexane. The bifunctional catalyst also displayed state-of-the-art activity for the production of hydrocarbon fuels from water-insoluble bio-oil with a yield of 29.6 wt % under mild conditions.The production of hydrocarbon fuels from CO2 reduction may both mitigate global climate change and secure energy supply, but one shall keep in mind that every step for the transformation of CO2 needs energy, including capture, storage, and conversion. The cost of capturing CO2 from the air was estimated to range from $94 to $232 per ton of CO2.\n30\n The advancement of techniques is likely to further bring down the cost to make CO2 a viable carbon source. In parallel, a cheap and renewable energy source such as electricity or H2 is needed to upgrade CO2. Another critical issue is to develop highly active catalysts that are able to lower the reaction temperature and pressure without compromising activity, thus decreasing the energy consumption. Besides, catalysts with excellent selectivity would avoid the energy-intensive separation and purification process. Recent studies suggest that SACs are promising catalysts for CO2 transformation into hydrocarbons.The desirable CO2 reduction pathway is the direct one-step conversion into hydrocarbons and oxygenates (vide infra). Most of the currently available catalysts, however, can only reduce CO2 to CO, which is the first step towards the production of hydrocarbon fuels,\n31\n but recent advances suggest SACs might be exceptional. SACs showed tunable selectivity towards CO\n32\u201334\n and hydrocarbons.\n35\n Single atoms Pt/Pd supported on g-C3N4 for the photocatalytic reduction of CO2 was studied using DFT calculation by Du and co-workers.\n36\n In the presence of Pt or Pd single atoms, the absorption edge of g-C3N4 was extended from 2.7 to 0.2 eV as a result of the electron excitation from d band of the metal to the conduction band of g-C3N4. For Pd1/g-C3N4, HCOOH is the favored product with an activation barrier of 0.66 eV; however, Pt1/g-C3N4 preferred the formation of CH4 with a barrier of 1.16 eV. Head-Gordon and co-workers further examined in\u00a0silico the electrocatalytic reduction of CO2 to hydrocarbons on 28 single-atom alloys (SAAs), in which single-atom M species (M\u00a0= Cu, Ni, Pd, Pt, Co, Rh, and Ir) were dispersed on the host Au or Ag.\n37\n The SAAs with M\u00a0= Co, Rh and Ir, showed desirable performance in the conversion of CO2 to methane. It was calculated that the host metals Au or Ag were responsible for the reduction of CO2 to CO, which was captured and further converted to methane by the nearby single-atom species. The performance of transition metals in CO2 electrocatalytic reduction was limited by the scaling relationship.\n38\n To break it, Jung and co-workers studied the catalytic performance of TiC as well as single metal atoms doped-TiC during CO2 hydrogenation by DFT.\n39\n As shown in Figures 3C and 3D, the binding energy of both *COOH and *CHO on pure metals depends linearly on the binding energy of *CO; however, a nonlinear relationship was observed on TiC-supported SACs. These calculations offer encouraging motivation to develop and characterize SACs for CO2 conversion. The theoretical calculation predicted that single-atom Cu dispersed on the CeO2 (110) surface could induce the formation of three oxygen vacancies in each neighboring Cu atom, and promote the conversion of CO2 to methane.\n40\n Experimental results were consistent with the prediction that an excellent selectivity towards CH4 was achieved on mesoporous CeO2 nanorod supported single-atom Cu species.In 2016, Ye and co-workers reported an efficient single-atom Co catalyst supported on porphyrin-based MOF for the conversion of CO2 to CH4 under visible light, mimicking the photosynthetic process in nature.\n35\n Both experiment and theory demonstrated that atomically dispersed Co atoms improved the electron-hole separation efficiency in the metal-organic framework (MOF). The photogenerated electrons were transferred efficiently from MOFs to single Co atoms. Compared with the MOF without Co, the incorporation of single-atom Co to MOF increased the formation rate of CH4 by up to 6 times.Methane is the main component of natural gas. According to the U.S. Energy Information Administration, the global methane consumption was 130 quadrillion Btu in 2017, and is projected to rise to 218 quadrillion Btu in 2050.\n2\u20134\n Around 90% of global natural gas production is used in the direct combustion for electricity generation and industrial heating,\n41\n and less than 1% natural gas is being used as transportation fuels in the U.S.\n42\n Industry is interested in the transformation of natural gas to easily transportable vehicle fuels such as aromatics and higher-value compounds to reduce the need for coal and petroleum.\n43\n Both indirect and direct strategies for the conversion of methane to higher hydrocarbon fuels have been explored.\n44\n The indirect methods require the formation of syngas (a mixture of H2 and CO), which is then transformed to hydrocarbons by Fischer-Tropsch synthesis. Syngas production is an energy-intensive process, accounting for over 60% of the total capital cost of gas to liquid plants.\n45\n The direct transformation of methane circumvents the low efficiency and high capital cost syngas production step, although it suffers from severe thermodynamics limitations at low temperature.\n46,47\n\nIn 2014, Bao and co-workers reported the direct methane conversion to higher hydrocarbon products in oxygen-free conditions at high temperature.\n48\n Single-atom Fe embedded in the silica matrix (Fe/SiO2) exhibited 48.1% methane conversion, and 99% total hydrocarbon (such as ethylene and aromatics) selectivity at 1,090\u00b0C in a single pass experiment. No coke deposition was detected due to the absence of adjacent Fe species, which are considered necessary for oligomerization. In 2018, the nonoxidative conversion of methane to higher hydrocarbons was also reported on single-atom Pt/CeO2 catalysts at 900\u20131,000\u00b0C,\n49\n and the onset temperature of methane activation (less than 900\u00b0C) was a bit lower than that over Fe/SiO2 (less than 950\u00b0C). It was also demonstrated that, under mild conditions, single-atom Rh/ZrO2 was efficient for the conversion of methane to ethane by O2 in the gas phase, while only CO2 was formed over Rh nanoparticles.\n50\n The stabilization of CH3 intermediates over single-atom Rh was crucial for methanol and ethane formation, while the C\u2013H bonds of adsorbed CH3 species were successively dissociated on Rh nanoparticles. Although the conversion of methane was limited, this work opens up a new way for the production of value-added fuels from methane under mild condition.Apart from making higher hydrocarbons, the conversion of methane in fuel cells using SACs has been demonstrated. Compared to the conventional combustion-based technologies, fuel cells are able to convert the chemical energy of the fuels to electricity with less pollution and higher efficiency. Although H2 is the most ideal energy source in fuel cells, the cheaper and more readily available hydrocarbon fuels such as methane are more attractive in the immediate future. The potential of using hydrocarbon fuels in the solid oxide fuel cells (SOFCs) has been investigated, either via the direct electrochemical oxidation of hydrocarbons or via formation of H2 and CO as the first step.\n51,52\n However, the extremely high operating temperature (800\u20131,000\u00b0C) hampered the practical application of hydrocarbon fuel-cells.\n53\n It is desirable to develop hydrocarbon fuel cells working at intermediate temperature.Along this line, Liu and co-workers reported a robust methane fuel cell, in which a catalyst with atomically dispersed Ru and Ni on CeO2 was coated on the anode.\n54\n The fuel cells enabled the direct electrochemical oxidation of methane containing 3.5% H2O at 500\u00b0C with no apparent coke formation after operation for 550 h. The authors attributed the outstanding activity and stability to the synergistic effect of single-atom Ni and Ru species in activating methane and H2O. As shown in Figures 3E and 3F, DFT simulations showed that single-atom Ni is responsible for the activation of C\u2013H bond in methane. To form CO, an oxygen atom was removed from CeO2, thus creating an oxygen vacancy, together with which single-atom Ru species participated in the activation of H2O. Currently, there are very few publications available on single-atom catalyzed methane oxidation in the fuel cell, and much more work needs to be done in this area.Compared with traditional gasoline fuel, oxygenates have lower toxicity, reduced CO2 emissions, higher octane rating, and are more environmentally friendly and sustainable.\n55,56\n For example, the blending of 20% ethanol (one of the most common oxygenated fuels) in gasoline decreased the emission of CO, hydrocarbons, and NOx by 60%, 40%, and 20%, respectively.\n57\n It was also reported that every 10% addition of ethanol to gasoline could increase the octane number by 5 units.\n58\n According to the Renewable Fuels Association (RFA), the world ethanol production increased more than 100% from 13,123 million gallons in 2007 to 27,050 million gallons in 2017.Currently, ethanol is mostly produced via fermentation of sugars derived glucose. The production of ethanol from lignocellulosic biomass is receiving major research attention due to its easy availability, low cost, and minimal competition with food production.\n59\n Zhang and co-workers proposed a two-step transformations of lignocellulose to ethanol\n60\n: lignocellulose was firstly converted to methyl glycolate (MG), which then underwent hydrogenation to form ethanol on copper-based catalysts. To further improve the efficiency of the second step, a single-atom Pt@Cu alloy catalyst was employed. The single-atom Pt species improved Cu dispersion, promoted H2 activation, and minimized C\u2013C bond cleavage, thus enhancing the activity and selectivity of ethanol. Li and co-workers developed a single-atom Ru/C3N4 catalyst, which showed temperature-dependent selectivity in the conversion of biomass-derived vanillin\u2014a typical lignin model compound as a precursor for fuel additives.\n61\n The selective hydrodeoxygenation of vanillin to 2-methoxy-p-cresol is a widely studied model reaction for the much more complicated real biomass deoxygenation to fuel additives. Removal of oxygen is a critical issue for biomass upgrading, as the high oxygen content is the cause for low energy density, instability, corrosiveness, and high viscosity. Lower reaction temperature facilitated the hydrogenation of vanillin to form vanillyl alcohol, while higher temperature promoted the production of 2-methoxy-p-cresol via hydrodeoxygenation.The direct conversion of methane to oxygenated products using H2O2 as an oxidant in the aqueous phase was performed on SACs. Tao and co-workers designed a single-atom Pd catalyst with 2.0% CuO on ZSM-5, which showed a TOF of 2.78 s\u22121 with 86% selectivity toward methanol at 95\u00b0C.\n62\n Lee and co-workers reported five times recycling of single-atom Rh/ZrO2 catalyst in the oxidation of methane to methanol by\u00a0H2O2 at 70\u00b0C without significant deactivation.\n50\n The graphene-confined single iron atoms were even active under ambient temperature for the direct conversion of methane to oxygenated fuels following a radical pathway.\n63\n Among various transition metals evaluated, only single-atom Fe was active due to the unique O\u2013FeN4\u2013O structure (Figure\u00a04\nA). To understand the origin of activity, the methane activation rate as a function of formation energy (Gf) of O-MN4-O (M\u00a0= Cr, Mn, Co, Ni, Cu) was determined (Figure\u00a04B). Compared with other O-MN4-O species, O-FeN4-O has a moderate free energy of formation (Gf) and shows the best ability to compromise all the energy barriers, and thus exhibited the best performance for methane activation. Methane was also converted to methanol and other oxygenates using molecular O2, despite of the lower activity. In 2017, Flytzani-Stephanopoulos and co-workers reported that single Rh atoms dispersed on TiO2 and ZSM-5 showed high performance in catalyzing the formation of acetic acid and methanol from methane using O2 and CO under mild conditions.\n64\n After 3\u00a0h of reaction at 150\u00b0C, 21,295 micromoles of acetic acid and 230 micromoles of methanol per gram of catalysts were produced on Rh/ZSM-5 and Rh/TiO2, respectively. Tao and co-workers also reported similar results that single-atom Rh anchored on the wall of microporous ZSM-5 transferred methane to methanol, formic acid, and acetic acid through the coupling of methane, O2, and CO.\n65\n\nSACs catalyzed CO2 hydrogenation into oxygenates such as methanol,\n66\u201369\n and formic acid\n36,70,71\n has been studied. The catalytic performance and reaction pathway of Pt single atoms Pt1@MIL and nanoparticle Ptn@MIL (MIL is a typical MOF which consists of \u03bc3-oxo bridged Cr(III)trimers cross-linked by terephthalic acid) were compared during CO2 hydrogenation to methanol.\n68\n The TOF and selectivity for methanol over Pt1@MIL was 5.6 times and 6.8 times higher, respectively, than that over Ptn@MIL. A significant amount of byproduct CO was produced over Ptn@MIL (Figure\u00a04C). The reaction mechanism showed that COOH* was the main intermediate on the Ptn@MIL catalyst, while on Pt1@MIL the key intermediate was identified as HCOO*. The distinct pathway over Pt1@MIL offered a lower energy barrier as well as the high selectivity for methanol production. The synergetic interaction between neighboring single metal atoms affects the reaction pathway as well as the activation energy in CO2 hydrogenation.\n72\n Individual single-atom Pt preferred the hydrogenation of CO2 into methanol, while both methanol and formic acid were formed over neighboring single Pt atoms. Zhang and co-workers designed a porous organic polymer (POP) with aminopyridine functionalities to anchor single-atom Ir for the conversion of CO2 to formate (Figure\u00a04D).\n71\n The formed polymeric framework (denoted as AP-POP) with electron-donating aminopyridine functional groups was used as support to disperse single-atom Ir via wet impregnation followed by reduction. The fabricated single-atom Ir/AP-POP with an analogous structure to that of the homogeneous catalyst showed a turnover number (TON) of 25,135, representing one of the most active heterogeneous catalysts so far for formate synthesis from CO2 hydrogenation.Formic acid electrocatalytic oxidation as the anodic reaction in the fuel cell has attracted intensive research activities. The formic acid oxidation reaction follows two pathways: in the direct pathway formic acid is converted to CO2 (Equation\u00a01); while in the indirect pathway CO is generated (Equation\u00a02), leading to the deactivation of catalyst due to the strong affinity of CO to metal.\n73\n\n\n\n(Equation\u00a01)\nHCOOH \u2192 CO2\u00a0+ 2H+\u00a0+ 2e\u2212\n\n\n\n\n\n(Equation\u00a02)\nHCOOH \u2192 COads\u00a0+ H2O \u2192 CO2\u00a0+ 2H+\u00a0+ 2e\u2212\n\n\n\nIn 2013, Lee and co-workers finely controlled the amount of Pt on Au nano-octahedra from single Pt atoms to Pt overlayers, and the performance of different Pt species in formic acid electrocatalytic oxidation was compared.\n74\n The single-atom Pt showed a mass activity of 62.6 A/mgPt, which was almost 10 times higher than that over Pt overlayers. The single-atom Pt preferred the reaction pathway towards direct oxidation due to the absence of Pt nanoparticles and the bifunctional effects of Pt\u2212Au sites. In 2018, a series of bimetallic PtAu nanoparticles with various Pt loading from 4% to 96% for formic acid oxidation were reported.\n75\n A similar conclusion was reached that single-atom Pt showed a higher resistance to CO poisoning, and exhibited orders of magnitude improvement in the oxidation of formic acid compared with Pt nanoparticles. DFT analysis indicated that the CO adsorption on single-atom Pt is weaker as a result of the electronic effects induced by the Pt-Au binding interaction as well as the discrete Pt active sites. To meet the requirements of practical application of single metal atoms catalysts in formic acid fuel cell, the loading of Pt was increased to 8 wt % while maintaining the atomic dispersion.\n76\n Single Pt atoms anchored on antimony-doped tin oxide (Pt1/ATO) maintained superior formic acid oxidation activity to the conventional Pt/C catalyst even after 1,800 cycles.H2 is regarded as an ideal fuel for the future. The weight energy density of H2 is 122\u00a0kJ/g, 2.75 times higher than that of hydrocarbon fuels.\n77\n According to the \u201cHydrogen Generation-Global Market Outlook (2017\u20132026)\u201d report, the global H2 production market is projected to reach $207.48 billion by 2026 at an annual growth rate of 8.1% from the starting point of $103.20 billion in 2017. It is predicted that 1 in 12 cars in South Korea, California, Japan, and Germany may be powered by hydrogen by 2030.\n78\n Hydrogen element is abundant in nature in the form of H2O, hydrocarbons, and biomass. However, a separate energy source such as electricity, light or heat is needed to extract H out of these sources in the form of H2. The main commercial H2 production relies on steam reforming, oil reforming, coal gasification, and water electrolysis, accounting for 50%, 30%, 18%, and 2% respectively.\n79\n The overall challenge of using H2 as fuel compounds comes from the ability to produce H2 efficiently at low cost. Besides, compression energy for H2 storage accounts for 10%\u201315% of the H2 energy content. Single-atom metal catalysts make efficient use of the noble metal atoms and have found applications in H2 production from methane or methanol reforming, water-gas shift reaction (WGSR), hydrogen evolution reaction (HER) and photocatalysis.Steam methane reforming is the most commonly used method to produce H2 in\u00a0a\u00a0large scale. One ton of H2 production forms 9\u201312 tons of CO2 in the process,\n80\n\u00a0making it a significant contributor to CO2 emission on earth.\n81\n McFarland and co-workers designed a stable molten Ni-Bi metal alloy catalyst, the active sites of which were atomically dispersed for the conversion of methane to H2 and carbon without CO2 and other byproduct formation at 1,065\u00b0C (Figures 5A\u20135D).\n82\n The previously used solid catalyst suffered severe deactivation due to carbon deposition, while the molten metal alloy showed stable performance in seven days of continuous operation. The formed carbon floated to the surface of the molten metal alloy where it was skimmed off easily. The H2 production from methane at 1,100\u00b0C was also catalyzed by a single atom Fe@SiO2 catalyst.\n48\n The concentration of H2 in the effluent varied from 10.9% to 51.2% with the generation of value-added hydrocarbons (ethylene and aromatics) as by-products.\u00a0The H2 fuel was generated from methanol steam or aqueous-phase reforming. Ma and co-workers reported that atomically dispersed Pt on \u03b1-MoC exhibited superior low-temperature H2 production activity as well as stability in aqueous phase methanol reforming, with an average activity of 18,046 mole of H2 per moles of Pt per hour.\n83\n Due to the strong interaction between Pt and \u03b1-MoC, electron-deficient single Pt atoms were highly dispersed on the support, facilitating the adsorption and activation of methanol. Besides, the \u03b1-MoC promoted the dissociation of H2O to form abundant surface-bound hydroxyls, which benefited the reforming of active intermediates at the interfaces between \u03b1-MoC and single-atom Pt.Water-gas shift reaction (WGSR), an important industrial reaction to produce H2 for\u00a0energy application and to remove CO impurity in H2 fuel cell, has been intensively studied on single-atom Au,\n8,84\u201386\n Pt,\n8,87\u201390\n Ir,\n91\n and Pd catalysts.\n92\n Flytzani-Stephanopoulos and co-workers proposed that positively charged Au and Pt SACs were active in WGS, while Au or Pt metal nanoparticles did not contribute significantly to the reaction as the removal of the particles by cyanide did not affect\u00a0the activity.\n8,84\n Later, they reported that the addition of alkali ions (Na, K) helps to stabilize mononuclear Au or Pt atoms on zeolite KLTL and MCM-41 for low-temperature WGSR.\n85,90\n Besides, the activation energy of single-atom Au in WGS was independent of supports, regardless of inert supports such KLTL and\u00a0MCM-41 or reducible supports including TiO2, CeO2, and Fe2O3 (Figure\u00a05\nF).\u00a0Zhang and co-workers also reported that single-atom Ir/FeOx catalyst showed more than 10 times activity of the Ir nanoparticles in WGSR.\n91\n The single-atom Ir improved the reducibility of FeOx and promoted the formation of oxygen vacancies, resulting in the excellent catalytic activity of single-atom Ir/FeOx catalyst.The hydrogen evolution reaction (HER), the cathodic half-reaction of water splitting, offers a reliable solution for the sustainable H2 production. HER occurs through the reduction of protons (Equation\u00a03) in acidic electrolytes or the reduction of water (Equation\u00a04) in alkaline electrolytes.\n93\n\n\n\n(Equation\u00a03)\n2H+\u00a0+ 2e- \u2192 H2\n\n\n\n\n\n(Equation\u00a04)\n2H2O\u00a0+ 2e- \u2192 H2\u00a0+ 2OH-\n\n\n\nUnder standard temperature and pressure conditions, the enthalpy change for the formation of H2 is 286\u00a0kJ/mol, which corresponds to a voltage of 1.23\u00a0V for the reversible electrolysis cell.\n94\n Under ideal conditions, an external potential of 1.23\u00a0V should be sufficient to drive the HER reaction. However, applying an overpotential is required to overcome the activation barriers and drive the electrochemical reaction. Efficient catalysts, mostly based on noble metals, can lower the overpotential.\n95,96\n To maximize the noble metal utilization efficiency, lower the catalyst cost, and improve the activity, selectivity as well as stability, the HER reaction has been carried out on single-atom noble metals,\n97\n such as Pt,\n98\u2013100\n Pd,\n101\n Ru.\n102,103\n Besides, the HER reaction is also effectively promoted by non-noble metal SACs including Co\n104\n and Ni.\n105,106\n\nAmong the noble metal SACs in H2 production from HER, single Pt atoms are the most widely studied. Single Pt atoms dispersed on nitrogen-doped graphene nanosheets (NGNs) by atomic layer deposition (ALD) technique exhibited as much as 37 times higher activity than the commercial Pt/C in HER, due to the partially unoccupied 5d states of single Pt atoms.\n107\n Wu and co-workers designed an ultra-low temperature (\u221260\u00b0C) ultraviolet photochemical method to prepare single-atom Pt and suppress the nucleation process of Pt atoms.\n108\n The prepared single-atom Pt catalyst exhibited lower overpotential (55\u00a0mV at 100 mA cm\u22122) and excellent stability in 5,000 cyclic voltammetry cycles. To understand the relationship between the coordination environment of single metal atoms and their catalytic performance, single-atom Pt catalysts with tunable coordination environment were dispersed on graphdiyne (GDY).\n109\n The four-coordinated Pt species (Pt-GDY1) were 3.3 times more active than five-coordinated Pt species (Pt-GDY2) in HER, due to the higher total unoccupied density of states of Pt 5d orbital and near to zero hydrogen adsorption Gibbs free energy on Pt-GDY2. Sun and co-workers reported that the electronic structure of single Pt atoms was modified by the coordination of nitrogen in aniline, showing superior HER performance and stability.\n110\n CO is often regarded as a poison ligand for Pt in heterogeneous catalysis, but Hyuck Choi and co-workers observed an unexpected improvement effect of CO on the performance of single-atom Pt in HER reaction.\n111\n The CO-ligation on the single-atom Pt promoted the dissociation of water to form Hads on Pt, thus enhancing the HER performance. Single Pt atoms were also used as co-catalysts to improve the HER performance. Two dimensional (2D) MoS2, as a potential alternative to Pt, has been studied in HER reaction. However, the performance of 2D-MoS2 needs to be improved as only the edge sites of the 2D-MoS2 contribute to the reaction while most sites at in-plane positions are inactive. Bao and co-workers doped single Pt atoms into the 2D MoS2 via the substitution of Mo sites to trigger the HER activity of MoS2.\n112\n The doped single-atom Pt tuned the H atoms adsorption behavior on the neighboring S atoms, leading to a significant improvement of HER activity on MoS2.Although noble metal SACs have shown excellent performance in H2 production from HER, it is not undesirable to replace noble metals with non-noble metals to make H2 a competitive energy carrier. The single Ni atoms dispersed on defective graphene showed a TOF of 0.3 s\u22121.\n113\n Although this value is almost one order of magnitude lower than that of the commercial Pt/C (2.30 s\u22121) at 50\u00a0mV overpotential,\n114\n platinum is four orders of magnitude more expensive than nickel.\n115\n The defective graphene offered a high density of anchoring sites through the efficient electron transfer between single-atom Ni and the 2\u03c0 antibonding state of the adjacent carbon atoms.\n113\n Chen and co-workers dispersed single Ni atoms on nanoporous graphene for HER.\n116\n The unique sp-d orbital charge transfer between single atom Ni and the neighboring carbon atom resulted in a low overpotential around 50\u00a0mV. The dynamic structure of single-atom Co under alkaline HER condition was studied using operando X-ray absorption spectroscopy (Figure\u00a05G).\n117\n The adsorption edge of the single-atom Co under open-circuit conditions was shifted towards higher energy side in comparison to ex situ samples; besides, a further shift was observed when the potentials of \u22120.04 and \u22120.1\u00a0V were applied (Figure\u00a05H), indicating the oxidation state change of single-atom Co under working conditions. The operando EXAFS of single-atom Co showed different oscillation frequencies compared with the ex situ sample, and the intensity of Co\u2013O/N peaks also changed when potentials were applied, suggesting a structural change of the single-atom Co under working conditions.The photocatalytic H2 production under light irradiation is considered as a type of artificial photosynthesis.\n118\n Generally, the photocatalytic H2 production involved the adsorption of light to generate electron-hole pairs, followed by charge separation and surface reaction. The overall photocatalytic performance is determined by both the thermodynamics and kinetics of the above steps.\n119\n Photocatalysts usually suffer from lower efficiency and selectivity towards H2 evolution under solar energy due to the high probability of charge-hole recombination events. The utilization of single metal atoms, as a new form of co-catalyst, can suppress electron-hole recombination, thus increasing the H2 photocatalytic production efficiency.\n120\u2013123\n It was reported that single-atom Pt dispersed on C3N4 dramatically enhanced the photocatalytic H2 formation.\n124,125\n Ultrafast transient absorption spectroscopy indicated that the change of the intrinsic surface trap states in the support induced by the single Pt atoms contributed to the performance enhancement.\n125\n Due to the improved hydrogen binding energy, single-atom Pt confined into the metal-organic framework (MOF) exhibited a TOF 30 times greater than Pt nanoparticles.\n126\n Single-atom Pd/g-CN was reported to show a TOF of 417 h\u22121, which was much better than that of benchmark Pt/g-CN (76 h\u22121) for photocatalytic H2 evolution reaction.\n101\n Hyeon and co-workers reported the design of highly active hollow TiO2 photocatalyst with single-atom Cu anchored in the Ti vacancies.\n127\n The oxidation state change of single-atom Cu induced by the atomic localization of photogenerated electrons promoted the activation of neighboring TiO2, and improved the H2 production performance dramatically.H2 fuel cells convert H2 and O2 to electricity with H2O and heat as the by-products. In comparison to other energy converters such as internal combustion engines and power plants, H2 fuel cells are free of emissions besides H2O. Even if H2 is produced by existing technology from non-renewable natural gas, the overall pollutant emission will be decreased by 30% for cars and trucks driven by H2 in comparison to gasoline-powered counterparts.\n128\n Development of cost-effective and high-performance catalysts for the electrocatalytic oxygen reduction reaction (ORR) is key to realizing the large-scale application of H2 fuel cells. The ORR occurring at the cathode of electrochemical energy devices proceeds via either a two-electron (2e\u2212) or four-electron (4e\u2212) pathway. The four-electron pathway that reduces oxygen directly into the water is highly preferred for batteries because of the high energy-conversion efficiency. The catalysts are the \u201cheart\u201d of the H2 fuel cells, and noble metals, particularly Pt are the essential elements for the ORR catalysts. Increasing the utilization efficiency of noble metals by making them atomically dispersed, while not compromising the catalytic performance, have become a potential solution.\n129\u2013131\n There is also considerable incentive to develop non-precious metal catalysts, such as Fe,\n132\u2013134\n Co,\n135,136\n Mn,\n137,138\n Cu,\n139\n and Zn\n140\n to replace Pt-based ORR catalysts.\n141\n\nThe single Au atoms dispersed on TiC were fabricated for the ORR.\n114\n The TOF of single-atom Au/TiC (1.57 s\u22121) was almost 3 times higher than that of Au nanoparticles supported on TiC (0.54 s\u22121) during ORR in acidic solution at 0.2 V. A quasi-Pt-allotrope ORR catalyst consisting of hollow Pt3Co nanosphere as the core and N-doped carbon with single-atom Pt as the shell exhibited stable 4e\u2212 ORR over 10,000 cycles.\n142\n Single Ru atoms dispersed on N-doped graphene by forming Ru-N4 moieties offered better resistance toward methanol, and CO poisoning than commercial Pt/C catalyst.\n143\n\nAmong the earth-abundant transition metal SACs, metal-nitrogen-carbon (M\u00a0= Fe or Co) based catalysts have been regarded as one of the most promising candidates in ORR.\n136,144\u2013147\n Li and co-workers reported single-atom Co catalysts, in which cobalt atoms were anchored in hierarchically porous N-doped carbon\n148\n and hollow N-doped carbon spheres.\n149\n A half-wave potential of 0.892 V\u201453\u00a0mV more positive than that of commercial Pt/C\u2014was obtained on the designed single Co atoms. The promotional effect was attributed to the synergistic contribution from both isolated Co atoms and the unique 3D hierarchical porous structure of the carbon support.\n148\n Lin and co-workers compared the ORR performance of hierarchically porous Co\u2013N\u2013C and Fe\u2013N\u2013C SACs.\n150\n The Fe\u2013N\u2013C offered a half-wave potential of 0.972 V, which was 49\u00a0mV higher than that on Co\u2013N\u2013C, as the single-atom Fe\u2013N\u2013C promoted the release of OH* intermediate, thus improving the ORR performance.Fe-based catalysts have attracted great interest for ORR. Single-atom Fe species were anchored on graphene hollow nanospheres using SiO2 as the template and Fe phthalocyanine as the precursor.\n151\n The rigid planar macrocycle structure of Fe precursor and the strong \u03c0\u2013\u03c0 interaction between Fe precursor and graphene oxide were beneficial for the dispersion of Fe. The atomically dispersed Fe species showed excellent activity, stability for ORR and tolerance toward methanol, NOx, and SO2 poisoning. The incorporation of S to single-atom Fe dispersed on nitrogen-doped carbon further improved the ORR activity due to the formation of thiophene-like structure (C\u2013S\u2013C) that decreased the electron localization of single-atom Fe.\n152\n The properties of the supports also have a great influence on the activity of the single-atom Fe catalysts.\n153\n Fe anchored on nitrogen-doped graphene with identical FeN4C12 moieties were prepared by the pyrolysis in Ar or NH3. The ORR performance over NH3-pyrolyzed catalyst was much higher than that over Ar-pyrolyzed one, due to the formed basic N-groups in the NH3 pyrolysis process. While some studies referred to the formation of Fe- pyrrolic-N structures as the origin of high performance of single-atom Fe catalyst confined in carbon supports,\n154\n another study suggested that the size of supports was also critical for the ORR activity.\n155\n In the range of 20 to 1000\u00a0nm, the best ORR activity was achieved at a particle size of 50\u00a0nm. Single-atom Fe bonded to graphdiyne through the formation of Fe\u2013C also showed comparable activity as commercial Pt/C during ORR, in which single-atom Fe species promoted the reduction of oxygen directly into water while suppressing the formation of H2O2.\n156\n\nBatteries have been regarded as promising candidates for sustainable energy storage and conversion because of the high energy density and low cost. ORR is a relevant process for both fuel cells and batteries. The rate-determining step of SACs in ORR is complicated and still under debate. While O2 adsorption was identified as the rate-limiting step for single Zn atoms,\n140\n the reduction of adsorbed O2 to OOH*,\n143\n and the desorption of OH were proposed as the slowest step on single-atom Ru and Pt, respectively.\n157\n The SACs showing exceptional performance in fuel-cell might also have the potential to be applied in batteries.\n158,159\n For example, single Fe atoms dispersed on the hollow carbon polyhedron containing N, P, and S as dopants were tested in H2 fuel cell and Zn-air battery.\n159\n The designed catalysts delivered a superior current density of 400 mW/cm2 at 0.40 eV in the H2-air-fuel test, comparable to that of commercial Pt/C catalyst. Moreover, the single-atom Pt, as the air cathode for the Zn-air battery, showed negligible voltage change after 500 cycle tests with 200,000 s; whereas a significant voltage decreased was observed on commercial Pt/C catalyst. In this section, we touch upon recent progress made on the utilization of SACs in batteries.Single-atom Fe\n160\u2013164\n and Co\n165\n have been integrated in Zn-air batteries in lab-scale and showed better stability than commercial Pt/C catalyst. Deng and co-workers reported that in comparison to nanoclusters and nanoparticles, single-atom Co showed the best activity, durability, and reversibility in Zn-air batteries.\n166\n Single-atom FeN4 species dispersed on open-mesoporous N-doped-carbon nanofibers were used as the electrode in Mg-air batteries.\n167\n The prepared electrode offered high open-circuit voltage, long operating life, and excellent flexibility, which showed a potential application in wearable and bio-adaptable Mg-air batteries. Single-atom Co embedded on N-doped graphene was applied as a cathode in Zn-air batteries.\n168\n The formed Co\u2013N\u2013C moieties promoted the formation of Li2S in discharge process as well as the decomposition in the charge process.The development of the Haber-Bosch process\u2014the catalytic hydrogenation of N2 into ammonia\u2014has enabled the strong population growth worldwide since the beginning of the twentieth century. For the first time, fertilizer could be produced synthetically on a large scale. For the last 100 years, the Haber-Bosch process has essentially not witnessed any major improvements and continues to constitute a major climate change driver consuming 1%\u20132% of global energy, predominantly because of the production of H2 as the reducing agent. Mainly due to slow reaction kinetics, high temperatures (400\u2013500\u00b0C) and pressure (150\u2013250 bar) must be employed, and the reaction mixture must be passed through catalyst beds multiple times to achieve favorable conversions. As the equilibrium conversion is higher at low temperatures but the activation of the strong N\u2013N triple bond is challenging, the development of efficient catalysts could significantly impact the ecological footprint of fertilizer production. SACs based on bimetallic catalyst structures have been proposed for this daunting task, and the development could further improve our knowledge on the active site structure during reaction conditions. Beyond thermal catalysis, more recent advances in photo- and electrocatalysis with SACs could prove promising as ambient pressure and temperatures are sufficient to achieve reasonable reaction rates for the fixation of nitrogen gas. Due to its high gravimetric hydrogen content of 17.8%, ammonia is often regarded as a potential hydrogen storage compound. Ammonia can be compressed under much lower pressure (10\u00a0bar) and temperature (\u221233\u00b0C) compared to hydrogen (\u2212240\u00b0C). Furthermore, the concentration of N2 in the air is 78.1% while that of CO2 is 0.04% by volume rendering nitrogen-containing hydrogen storage compounds more straight-forward. Both the decomposition of ammonia into carbon-free hydrogen for hydrogen fuel cells and the use of direct ammonia fuel cells are envisioned.\n169,170\n Especially, energy storage on a small scale based on the conversion of stranded energy resources into a chemical storage compound may rely on the electrocatalytic production of ammonia.\n171\n\nBimetallic alloys where one metal is atomically dispersed in a solid \u2018solution\u2019 of another metal are popular catalysts. On the very small side of this approach are bimetallic single-cluster catalysts like Rh1/Co3 supported on cobalt oxide. Although this has been realized experimentally for the thermal reduction of NO to N2 and N2O, it was recently proposed based on DFT calculations that among other single-cluster catalysts, Rh1/Co3 would also be capable of reducing nitrogen into ammonia.\n172\n The capacity of the metal surrounding the atomically dispersed element to buffer charges and contribute to the catalytic reaction synergistically are believed to mainly contribute to the predicted catalytic performance (Figure\u00a06\nA).Different experimental studies have validated the use of SACs for the electrocatalytic nitrogen reduction reaction (NRR)\u2014mostly based on Fe, Mo, and Ru. A nitrogen-doped carbon nanotube-supported Fe-based SAC synthesized by the pyrolysis of an iron-containing MOF reduced nitrogen to ammonia at \u22120.2\u00a0V versus RHE with Faradaic efficiencies of 9.28%, and a production rate of 34.83\u00a0\u03bcg h\u22121 mgcat.\n\u2212\n1 An iron species surrounded by 3 nitrogen atoms was proposed as the main active species and poisoning with thiocyanate salts showed that the nitrogen reduction activity was prohibited by the presence of Lewis bases. The mechanism was proposed to follow a distal pathway where dinitrogen binds in an end-on fashion on the Fe atoms, and the first ammonia production reaction occurs on the nitrogen atom more distant to Fe.\n173\n A Mo-based SAC supported on nitrogen-doped carbon material was synthesized in a similar pyrolysis procedure to the above-mentioned Fe SAC. Faradaic efficiencies of 14.6%\u00a0\u00b1 1.6% with an ammonia formation rate of 34.0\u00a0\u00b1 3.6\u00a0\u03bcg h\u22121 mgcat\n\u22121 were achieved at \u22120.3\u00a0V versus RHE in 0.1\u00a0mol L\u22121 KOH solution. With around 10 wt % Mo loading, the catalyst does not exhibit a significant decrease in activity in 14\u00a0h and no formation of Mo clusters after the reaction was observed by HAADF-STEM.\n22\n Similar to the thermal catalytic reduction of nitrogen, Ru exhibits the best reaction rates achieved so far for the NRR using SACs. Using a MOF-pyrolysis procedure (Figure\u00a06B), atomically dispersed Ru supported on nitrogen-doped carbon achieves Faradaic efficiencies of 29.6% at \u22120.2\u00a0V versus RHE with reaction rates of 120.9\u00a0\u03bcg h\u22121 mgcat\n\u22121 in 0.05\u00a0mol L\u22121 sulfuric acid. Again, the triple nitrogen-coordinated structure was proposed to be the active site.\n174\n The addition of ZrO2 to Ru SACs supported on nitrogen-doped carbon was found to be sufficient to suppress the competing hydrogen evolution reaction, and ammonia Faradaic efficiencies of 21% were achieved at \u22120.21\u00a0V versus RHE with maximum ammonia formation rates of 3.67\u00a0mg h\u22121 mgRu\n\u22121. A duration test over 60\u00a0h indicated high stability of the SAC under reaction conditions (Figure\u00a06C). Plausibly, the presence of nitrogen in the carbon material and the addition of ZrO2 are essential to not only enhance the stability and activity for electrochemical reactions but also improve the Faradaic efficiencies for ammonia production.\n175\n\nDue to the importance of alternative ways to convert nitrogen gas into ammonia and the scarcity of experimentally reported catalysts, many DFT-based studies have been conducted to guide the rational catalyst design and screening. Similar to the best experimental systems, isolated Ru atoms supported on different nanoporous carbon materials have been predicted to be stable and active for the NRR, although the competing HER increases the necessary overpotential.\n176\n A systematic study of different transition metals on nitrogen-doped carbon employing three defined properties including stability, the competitive adsorption of dinitrogen against dihydrogen molecules, and the competition of the first dinitrogen protonation against hydrogen adsorption on metal sites. Based on this analysis, Co- and Cr-containing SACs are predicted to yield the highest activity and selectivity for ammonia production at low overpotentials.\n177\n Besides carbon-based materials, boron has been proposed as a powerful support and even active site for NRR. Upon binding of dinitrogen to sp3-hybridized boron atoms, the B-to-N \u03c0-back bonding populates N\u2013N \u03c0* orbitals and thus activates the notoriously strong N\u2013N bond (Figure\u00a06D). Depending on the support for isolated boron atoms, the NRR activity can be improved while the HER activity can be suppressed (Figure\u00a06E). Mo SACs supported on defective boron nitride with a boron monovacancy were calculated to surpass equivalent noble metal-based catalysts which were assigned to the unique ability of Mo to stabilize N2H* and destabilize NH2* species (Figure. 6F).\n178\n One of the challenges of utmost importance is the suppression of the HER under reaction conditions relevant for NRR which has been addressed recently by the computational comparison of 120 transition metal SACs supported on different nitrogen and carbon-containing scaffolds. The authors found that Ti and V have the strongest ability to activate dinitrogen as well as the lowest free energy barriers for the NRR while exhibiting little predicted HER activity.\n179\n\nThe direct reduction of nitrogen into ammonia using sunlight as the sole energy source would be favorable but the development of efficient catalysts is a major challenge. Atomically dispersed copper on carbon nitride were shown to generate ammonia under the illumination of visible light (420\u00a0nm) with quantum efficiencies of around 1% and reaction rates of 186\u00a0\u03bcg h\u22121 gcat\n\u22121 around 7 times higher than pure carbon nitride.\n65\n Doping isolated low-valent Mo atoms into W18O49 nanowires was sufficient to enhance the catalytic activity by around 7 times compared to the un-doped material and can achieve ammonia formation rates of 195.5\u00a0\u03bcg h\u22121 gcat\n\u22121 with an apparent quantum efficiency of 0.028% under simulated AM 1.5 light irradiation. The interface between W and Mo was calculated to be the active site, and the reason for the enhanced catalytic performance (Figure\u00a06G).\n180\n Similar to the NRR, boron atoms have been predicted to efficiently convert dinitrogen into ammonia on a semiconductor material such as carbon nitride. Besides the activation of dinitrogen molecules, boron atoms can enhance the visible light absorption of carbon nitride and thus are expected to improve the photocatalytic nitrogen reduction.\n181\n\nBeyond the above-mentioned categories, the production of commodity and fine chemicals is closely connected to energy-consumption, providing access to agrochemicals, pharmaceuticals, polymers, fragrances, food additives, adhesives, lubricants, among others. According to the process intensification workshop held by U.S. Department of Energy in 2015, the overall US manufacturing sector in 2010 reached 19.24 quadrillion British thermal units (quads), where the chemical production processed consumed 1.15 quads.\n182\n Developing more efficient chemical process will reduce the energy consumption of the chemical sector and greenhouse gas emission. It becomes increasingly difficult to achieve full conversion while maintaining high selectivity for more complex chemicals, and therefore, laborious post-treatment becomes inevitable. Based on estimates by the Oak Ridge National Laboratory, separation processes account for around 15% of the total annual US energy consumption and for approximately 40%\u201350% of the total energy consumption in chemical processes.\n183,184\n More challenging separations such as in the pharmaceutical industries caused by very rigorous purity requirements and complex separation tasks such as the resolution of enantiomers would increase the energy consumption more significantly. Improving the selectivity of chemical reactions as well as replacing homogenous catalysts with suitable recyclable catalysts are thus imperative if the energy for separation is to be decreased. Besides offering the opportunity to conduct chemical reactions under milder and thus less energy-intensive conditions, SACs are also able to improve the reaction selectivity. For selective hydrogenation reactions where isomeric products or mixtures of alkynes, alkenes, and alkanes are particularly difficult to separate, SACs have proven to be excellent selective catalysts surpassing their nanoparticle counterparts. On the bridge between homogeneous and heterogeneous catalysis, SACs have been shown to combine activity and selectivity for certain coupling and hydrofunctionalization reactions well beyond other heterogeneous catalysts while they are easily removed from the reaction solution by filtration. Besides, the catalysts can be used continuously when fixed bed reactor is applied.Due to the absence of adjacent metal atoms, the activation of hydrogen and the subsequent hydrogenation reaction will occur much more selectively. One such example is the selective hydrogenation of acetylene \u2013 a major impurity hampering the ethylene polymerization reaction \u2013 to ethylene without promoting the complete hydrogenation to ethane. Similarly, the hydrogenation of butadiene, which is a strong poison for alkene polymerization catalysts, into the butene isomers requires the development of highly selective catalysts. Both positively charged SACs, as well as SAAs, have been used based on Pd,\n185\u2013190\n Pt,\n191\u2013194\n and Au\n9,10,195\n all of which showed selectivities for the semi-hydrogenation products far exceeding nanoparticle-based catalysts. Several other selective hydrogenation reactions have also been achieved, such as the chemoselective conversion of nitroaromatics to amines\n159,191,196,197\n and azo compounds\n198,199\n or the semi-hydrogenation of quinoline.\n200\n Compared to nanoparticle-catalysts, several SACs have been proven to be CO-tolerant hydrogenation catalysts probably due to the weak adsorption of CO on positively charged noble metals,\n201,202\n allowing the direct use of industrial-grade hydrogen gas as feedstock. Of note, the CO adsorption strength on single-atom Pt is still under debate. While some reports suggest CO adsorption on Pt1 is much weaker than that on Pt nanoparticles,\n203\n other studies provide evidence for the strong adsorption of CO on single-atom Pt.\n204,205\n\nAdditionally, a Pt1/\u03b1-MoC catalyst offers high activity in the water-gas shift reaction so that water can be used as hydrogen source (Figure\u00a07\nA).\n201\n\nSACs have been shown to show great promise for several hydrofunctionalizations reactions such as hydroformylation, hydrosilylation, and hydrochlorination reactions\u2014all industrially relevant reactions where nanoparticle-based heterogeneous catalysts are inferior compared to homogeneous catalysts. Rh SACs supported on ZnO\n206\n or CoO\n207\n show high activity and simultaneously high selectivity of up to 95% toward a certain isomeric aldehyde in stark contrast to Rh clusters of higher nuclearity and most homogenous catalysts (Figure\u00a07B).\n207\n The authors ascribe this enhanced reactivity to the dynamics of Rh atoms on the CoO support or the charge transfer from Zn to Rh on ZnO yielding almost metallic atomically dispersed Rh. Another application for atomically dispersed positively charged Pt atoms is the alkene hydrosilylation reaction, the arguably most important industrial application for homogenous Pt catalysts. Several Pt SACs and an SAA have been demonstrated for the hydrosilylation of different alkenes.\n208\u2013211\n The high activity is normally attributed to either the high valence and thus the facile insertion of Pt into the C\u2013H bond\n210\n or the charge transfer of Pd to Au in dilute Pd-Au alloys.\n211\n Recycling studies revealed that the SACs could be used up to 5 times without significant loss of activity with TONs of up to 105. For the production of polyvinylchloride, the production of its monomer\u2014vinylchloride\u2014by the hydrochlorination reaction of acetylene is inevitable. The conventional heterogeneous industrial catalyst is based on toxic Hg, but recently single-site gold catalysts have been identified as a viable alternative. The reaction mechanism on carbon-supported Au has been experimentally proven to be based on an Au(I)-Au(III) redox cycle (Figure\u00a07C).\n212,213\n In contrast, CeO2-supported Au catalysts follow an Au(0)-Au(I) redox cycle because of electronic coupling with a Ce(IV)/Ce(III) cycle. The authors also show that catalytically inactive Au nanoparticles can decompose into isolated Au atoms when exposed to a C2H2/HCl mixture under reaction conditions.\n214\n\nCoupling reactions belong to the most important reactions in the synthesis of complex chemicals such as those in the pharmaceutical industries. Traditionally, heterogenous nanoparticle catalysts are neither particularly active nor selective, and thus the sector mostly relies on the use of homogeneous catalysts which are inherently difficult and energetically expensive to recycle. Recently, different SACs have been shown to be active in the Ullmann,\n215\n Sonogashira,\n216,217\n Heck,\n216\n and Suzuki\n216,218,219\n couplings. Besides the positive charge of noble metals in SACs resembling homogeneous metal complexes, the mobility of metal ions in supports such as carbon nitride was reported to be the reason for the catalytic activity sometimes surpassing homogeneous complexes. Recycling and flow reactor stability studies reveal that the SACs sustained coupling reactions over a long-time period in stark contrast to the stability of homogeneous complexes (Figure\u00a07D).\n219\n\nLight olefins belong to the most crucial building blocks in the chemical industries. The recent exploitation of shale gas deposits spurred interest in the dehydrogenation reaction of light paraffins such as propane. Harsh reaction conditions resulting in catalyst stability issues as well as the formation of coke and other side products plague the development of suitable SACs. Pt SACs on CeO2 are stable under propane dehydrogenation reaction conditions, but the selectivity towards propylene was negligible. This was assigned to the facile C\u2013C bond cleavage on Pt1 sites.\n220\n More recent calculations, however, indicated that SAAs with Pt diluted in more abundant metals such as Cu seem to combine both the excellent C\u2013H activation capabilities of the noble metal and the low first dehydrogenation reaction barrier but prevent the further dehydrogenation of propylene to undesired side products. In fact, Pt/Cu SAAs are capable of breaking the scaling relationships between the propane dehydrogenation activity and selectivity commonly observed for single metal and alloy catalysts.\n221\n Similar turnover frequency (TOF) values of 0.72 s\u22121 for Pt nanoparticles and 0.56 s\u22121 for Pt/C SAAs at 520\u00b0C under otherwise identical reaction conditions were observed. Of note, the propylene selectivity was around 3.2 times higher for the SAA (90%) indicating a significantly better performance of SAA catalysts.\n221\n It was predicted based on DFT calculations that Pd/Cu SAAs would also exhibit favorable performance in the dehydrogenation of propane.\n222\n\nAs shown in the sections above and summarized in Table 1\n, SACs have been reported to show superior catalytic activity to their nanoparticle counterparts in a wide range of catalytic applications, including hydroformylation to selective hydrogenation,\n185\u2013194\n dehydrogenation,\n221,222\n water-gas shift reaction,\n8,84,91\n and hydrogen evolution reaction.\n98,107,108\n It is not unreasonable to propose, as a rule of thumb, that SACs may be superior to NPs in the reactions that are conventionally more successful using metal complexes as catalysts. Likewise, the design of SACs should learn from the wisdom in homogeneous catalysis to fine-tune the frontier orbital geometry and energy of the active sites.Meanwhile, SACs are not as active as nanoparticles in some other reactions. SACs might even be completely inactive in case that two or more neighboring metal atoms are required to activate a reactant. It is well-known that the electrooxidation of methanol in fuel cell dominantly involves three or four Pt atoms to accommodate the formed CHxO intermediate.\n20\n The single-atom Pt dispersed on thiolated multiwalled carbon nanotubes (S-MWNTs) was almost inactive, while Pt nanoparticles were favorable for the methanol oxidation.\n223\n Similarly, Pt nanoclusters Pt4 and Pt10 anchored on indium tin oxide (ITO) showed excellent ethanol oxidation performance, while single-atom Pt1/ITO was much less efficient.\n224\n Similarly, Pd ensembles rather than single-atom were proposed to be responsible for the ethanol oxidation reaction.\n225\n SACs could also be inferior to their nanoparticle counterparts in terms of reaction selectivity. For example, Pt SACs on CeO2, when employed in propane dehydrogenation, exhibited negligible selectivity towards propylene due to facile C\u2013C bond cleavage.\n220\n\nAlthough SACs have received intensive research activities, deep understanding of the working mechanism of SACs is still under development. The debate remains regarding whether SACs are active or not in certain reactions, and if yes, whether they are more active than nanoparticle counterparts. Examples include CO oxidation, methane activation, and N2 hydrogenation. In the case of methane oxidation, Lee and co-workers argued that Rh single-atoms promoted the conversion of CH4 to methanol using O2 in the gas phase or H2O2 in the aqueous solution,\n50\n while single Pd atoms were reported to be inactive for the same reaction.\n226\n For N2 hydrogenation, both experiment, and DFT simulation indicated that N2 dissociation on Ru(0001) was dominantly determined by the step sites,\n227\n whereas N2 reduction to ammonium was predicted to be feasible on single-atom Ru where step sites were not available.\n172,174,175\n\nSingle-atom catalysis emerges from the in-depth study of supported metal nanoparticle catalysts that already found wide industrial applications in oil refining, coal transformation, fertilizer production, and many more. Thanks to the technological advances in the spatial and temporal resolution of analytical tools, within merely a few years\u2019 time hundreds of reports generated in labs around the world authenticated the existence of the isolated single-atom species on various supports and their active participation in catalytic reactions. This does not only fundamentally change the way we view the structure and function of metal-based catalysts, but also provides grand opportunities for a more efficient usage of fossil resources, less energy-intensive processes for chemicals production, more effective energy storage and the novel transformations of alternative energy sources.The development of efficient, selective, and stable catalysts with low cost is crucial for energy-related applications. Due to the maximized atom utilization efficiency and unparalleled electronic and geometric features, SACs have exhibited exciting technological, and fundamental significance in nearly every field of energy transformation and storage. In this review, the recent advances of SACs in the transformation of hydrocarbons, oxygenates, H2 fuel, batteries, ammonia, and fine chemicals have been summarized. Particular attention was paid to structure-performance relationship and the advantages of SACs in comparison to traditional nanoparticle or commercial catalysts in energy-related catalytic reactions. The prospect of using SACs in energy application looks promising, and enormous advances have been achieved to date. However, future research should be devoted to the following aspects to foster further growth of the area, and potentially push the SACs for practical energy application.Noble metal-based single atoms catalysts account for about two-thirds of published articles in the past five years.\n228\n Nevertheless, the non-precious SACs exhibiting comparable activity as noble-atom SACs are more attractive, given that performance and cost of catalysts are two important factors affecting the energy conversion. As discussed in the review, 3-d metal-based SACs exhibited comparable or even superior performances to noble-metal catalysts in several photocatalytic and electrocatalytic reactions. Future work should be directed to nitride or carbide supports combined with non-noble single metal atoms, which may offer unique electronic interactions with the metals generating improved performance.Considering that many energy-related applications require harsh operation condition, the development of industrial-scale manufacturing methods that offer stable and high metal loading SACs at affordable cost is essential. Although several strategies have been reported along this line, these methods rely on strong anchoring sites on particular supports and therefore to a certain extent suffer from a lack of general applicability. Universal stabilization strategies for the synthesis of a wide range of SACs are pressingly needed. A possible approach is to learn from strategies to make stable colloidal nanoparticles,\n228\n such as electrostatic interaction and steric hindrance, which have been well studied and even quantitatively described in the past decades. On another note, despite that more than 80% of all the heterogeneous catalysts are fabricated by wet-chemical impregnation or precipitation,\n229\n it may not be ideal for every type of SACs synthesis since low loadings are necessary to keep the metals atomically dispersed. Very recently, a facile shockwave method was developed to synthesize thermally highly stable SACs.\n230\n Solid-state syntheses such as this one and other less conventional methods may find unique advantages in making SACs in the future.At present, the identification of single atoms is mainly achieved by the combined use of HAADF-STEM, CO-DRIFT-IR adsorption, and XAFS. While these techniques generate a clear picture of the structure of dominant metal species in a catalyst, none provides accurate electronic structure and coordination environment of the single metal atoms with spatial resolution under the working state. Therefore, the current understanding of the structures of active sites in complex heterogeneous SACs and their working mechanism in catalytic reactions are largely based on postulations derived from statistically averaged properties. A potential solution\u00a0to the problem is the development of single-atom electron spectroscopy, which would enable structural identification of individual metal species under a\u00a0microscope. This emerging technique has been successfully applied in the revealing localized electronic structure of single atoms,\n231\n but its usefulness to help understand the catalytic function of isolated metal atoms remains to be explored.Another challenging and critical task in SACs is to develop a technique that does not only microscopically or spectroscopically image various metal species, but also differentiates which ones are active in catalysis and which ones are not. Often, various sized metal species coexist in a working heterogeneous catalyst. The contributions of all these species in catalysis are hard to disentangle. Even for single-atom species, their structure and catalytic property are likely to be non-identical. Considering most catalytic reactions are associated with heat effect, active sites will induce a significant change of the local temperature. As such, we envisage sub-nanometer resolution thermometry combined with atomic resolution electron microscopy would offer a powerful tool to contrast metal species that are more active from the ones that are less active or completely inert. Recently, plasmons\n232\n and phonons\n233\n have been used to probe the temperature of nano-objects in the electron microscope. Leveraging on these advances, a thermometry-microscopy system for the above-mentioned application may become a reality.Although SACs have been intensively studied in various energy transformations, more research work should be devoted to expanding the application of SACs in even broader areas. We propose several reactions where SACs deserve further exploration: (1) Fischer-Tropsch synthesis. In industry, cobalt and iron nanoparticles are widely used. Metallic single-atom cobalt, iron or ruthenium alloys might offer unique selectivity in Fischer-Tropsch reaction; (2) Hydrocracking of heavy oil. Heavy oil hydrocracking is currently promoted by Pt nanoparticles supported on zeolites. A few single-atom Pt alloy catalysts have been successfully prepared in the literature.\n60,234\n It would be interesting to test the performance of these catalysts in hydrocracking despite their stability potentially representing an issue. (3) C-H activation. While methane activation has been realized by SACs, C\u2013H activation of larger molecules such as benzene derivatives has rarely been reported. Considering that the Palladium complex is widely used in the C\u2013H activation,\n235\n it deserves more effort to expand the application of SACs in C\u2013H functionalization of more complicated substrates. (4) N\u2261N activation. Several DFT simulations for the hydrogenation of N2 to NH3 have been performed on SACs. It was predicted that SACs are promising for the conversion of N2 to NH3. However, experimental validation of these reports is rare at the moment. Provided low temperature, pressure ammonia synthesis become viable, one could envisage decentralized facilities for NH3 production and point distribution.Along with the catalytic application of SACs getting increasingly broad, standardized operation protocols should be established for various reactions using SACs. When preparing this review, we realized that the performance of catalysts in most cases is measured under various conditions, making the comparison of catalytic behavior of SACs challenging. A good practice is surfacing. For instance, McCrory and co-workers developed a benchmarking protocol, using the potential increase after 2\u00a0h of galvanostatic polarization at 10 mA/cm2 per geometric surface area to test the stability of OER catalysts.\n236,237\n More such efforts should be spent\u00a0for the rational comparison of SACs in a broad range of energy-related applications.The past few decades have witnessed a growing synergy between theoretical simulation and experimental investigations in catalysis. Traditionally, the DFT calculations were carried out within the concept of potential energy surface, in which a simplified model under idealized conditions (ultra-high vacuum and \u2212273\u00b0C) was considered.\n238\n The fast development of hardware and software makes it possible to simulate the catalytic reactions under realistic conditions. A deeper understanding of the reaction mechanism and structure-performance relationship under realistic conditions will benefit the rational design of single-atom catalysts for the specific energy transformation process.The development of data science has enabled the big data strategies to discover the underlying correlations and making predictions. Machine learning for data analysis is spreading rapidly in catalysis,\n238\n and it is mainly focused on two aspects in heterogeneous catalysis: (1) the direct prediction of catalytic performance and (2) developing a model to estimate the reaction rate indirectly. Very recently, single-atom transition metals anchored on graphdiyne with outstanding electron transfer ability were identified using a deep-learning algorithm and big-data technique.\n239\n We anticipate growing employment of machine leaning in guiding the design of particular SACs for energy transformation. Ideally, it is combined with fast synthetic platforms and high throughput performance screening techniques that have already been commercialized.While hundreds of papers are available for SACs, only a handful of cases have been reported for dinuclear and multi-nuclear species without organic ligands as active sites.\n240\u2013242\n There is a clear gap between SACs and well-studied nanoparticle catalysts. An atom-by-atom approach to synthesize active sites ranging from single-atoms to atomically precise metal clusters on the same support is highly desirable. In this regard, the concept of single-atom catalyst has been recently extended to single-cluster catalyst (SCC),\n172\n i.e., each catalyst bears only one type of Mx (x \u2265 1) species with a specific number of x. In this manner, the nuclearity effect in heterogeneous catalysis could be systematically studied, understood, and rationalized. The well-known B5 sites of Ru(0001) and the more recently proposed Fe3 sites on \u03b8-Al2O3(010)\n243\n for ammonia synthesis could both be considered as multi-nuclear metal sites. We expect research along this line will provide important new discovery in structure-activity correlations, which will ultimately benefit the identification of the best catalyst in each energy application.We thank the National University of Singapore Flagship Green Energy Program (R-279-000-553-646 and R-279-000-553-731) for financial support.N.Y. and J.P.-R. conceived and supervised the preparation of the review. S.D., M.J.H., and N.Y. collected references and wrote the manuscript. N.Y. and J.P.-R. revised and finalized the manuscript. All authors approved the final version of the manuscript.", "descript": "\n                  Almost a quarter of the energy consumed globally is directly or indirectly related to the use of a catalytic process. Conventional nanoparticle-based catalysts recently witnessed the dawn of its potential successor\u2014heterogeneous single-atom catalysts (SACs), which allow the maximum possible dispersion of metal atoms on the catalyst surface, possess unparalleled electronic structure and geometric configuration, and exhibit, otherwise, exceptional performance in a range of energy-related applications. Herein, we critically review the use of heterogeneous SACs in the generation and conversion of hydrocarbons, oxygenates, H2 fuel, ammonia, commodity and fine chemicals, and the electrochemical energy storage in and release from batteries. We describe the importance of those energy-related compounds in the current energy infrastructure and discuss how catalysis\u2014in particular, single-atom catalysis\u2014can be used more effectively in each application. At last, general guidelines and trends guiding the future design of stable and efficient single-atom catalysts for sustainable energy transformations are provided.\n               "}
{"full_text": "The constant growth in global energy demand has entailed a massive consumption and excessive depletion of fossil fuels, leading to an energy crisis as well as a health crisis due to greenhouse gas emissions, responsible for climate change and global temperature increase. The atmospheric CO2 level (one of the main greenhouse gases contributors) has also increased, reaching a value of 416.47\u00a0ppm (May 2020) [1] and is forecast to reach up to 570\u00a0ppm by 2100 [2], while the safety limit is estimated at 350\u00a0ppm [3]. As a result, there is an urgent need to control and mitigate these emissions by pursuing low-carbon alternatives including CO2 capture, sequestration, and utilization [1]. Indeed, research efforts are focused on the use of CO2 as carbon pool, given its potential to become genuine feedstock to produce value-added products hence turning a problem into a virtue.Nonetheless, conversion of CO2 is an arduous task given its high thermodynamic stability due to the two strong equivalent CO linear bonds; bonds that possess a much higher bonding energy (750\u00a0kJ\u00a0mol\u22121) than that of other carbon bonds (C\u2013H, 411\u00a0kJ\u00a0mol\u22121; C\u2013C, 336\u00a0kJ\u00a0mol\u22121; C\u2013O, 327\u00a0kJ\u00a0mol\u22121) [4]. Therefore, a suitable CO2 transformation route must overcome these energy barriers requiring high energy input, preferably coming from carbon-neutral sources, as well as the use of an active catalyst or high pressures and temperatures [5\u20137]. Among all the possible techniques to convert CO2, including thermochemical, electrochemical, photochemical and biological processes [1,7\u20139], electrochemical reduction of CO2 has attained a growing interest due to its multiple possible uses in the energy sector and chemical industries, producing value-added fuels and chemicals at mild conditions and in a carbon-neutral way [10\u201312].Electrochemical reduction of CO2 allows tuning the selectivity of the value-added products obtained [13]. Traditionally, research on electrocatalytic reduction of CO2 has mainly focused on the formation of C1 products (such as CO, formate and methanol) since these simple 2e\u2212 transfer reactions are kinetically more favourable [14,15]. However, the production of C2+ species would be rather interesting from the application perspective [16]. C2+ alcohols contain higher energy densities, lower toxicity and corrosiveness compared to methanol, being more suitable as blending or even pure fuels in existing internal combustion engines; and short-chain alkanes can be directly injected into gas-distribution grids enhancing the calorific value of natural gas or biogas. They can also be regarded as entry platform chemicals for current value chains, e.g. light olefins for the production of polymers [9,17]. Moreover, and taking acetate as an example, its direct production from the electrochemical reduction of CO2 is a more efficient and effective process in terms of energy and steps compared to the traditional industrial multistep process from fossil fuels, besides more environmentally friendly from a CO2 emissions point of view [14].However, C2+ products reaction pathways are complex and strongly influenced by the catalyst surface, electrode materials, reaction medium (buffer strength and electrolyte solution), design of electrochemical cell or working conditions such as temperature, pH or pressure and concentration of CO2 [18,19]. The high C\u2013C coupling activation energy required joint to its bond formation competition with C\u2013H and C\u2013O bond formations limit the efficiency towards C2+ products. The later along with the overpotential gap between the essential CO intermediates formation and that of C2+ species are the main impediments in the practical application of CO2 reduction to C2+ species in commercial electrolysers. Indeed, these aspects are regarded as the major bottlenecks accounting for the low energy efficiency of C2+ products compared to C1 counterparts [19,20]. Still the commercial appetite and their extraordinary added value makes necessary to strengthen the research efforts towards direct CO2 to C2+ products.In order to achieve the practical viability and implementation of this technology, higher efficiency and energy requirements as well as lower operational costs must be met. Electricity plays a key role in the profitability of the CO2 reduction reaction (CO2RR), being the main factor in the operational costs [21,22]. Nonetheless, one of the advantages of the electrochemical reduction of CO2 is that it can be approached as an efficient way to store all excess electrical green energy (generated from unpredictable and intermittent sources such as wind and solar) as transportable chemicals and fuels [23]. Regarding the efficiency requirements, commercial electrolysers require current densities above 200\u00a0mA\u00a0cm\u22122 as well as long-term durability catalysts [21,24,25]. For these reasons, research on the development of efficient catalysts is encouraged.Recently, several efforts have been made to obtain suitable electrocatalysts, able to improve catalytic activity and selectivity by controlling their chemical states, size, morphology, surface defects, crystal facets, porosity or by creating heterostructures [6,20,26\u201328]. Generally, catalysts explored for the electrochemical reduction of CO2 are based on metals. While formic acid is the main product using Sn, Pb, In, Hg or Bi as catalysts; Zn, Ag, Au and Pd have been found to be selective towards CO [10,23,29]. In the case of C2+ products, Cu has demonstrated a unique ability to facilitate C\u2013C coupling, although it is not a selective catalyst since numerous C1\u2013C3 hydrocarbon and oxygenate products have been observed on Cu surface [19,30].In this scenario and given the research gaps and motivation for this appealing CO2 conversion route, herein we analyse the recent advances and efforts in the electrochemical CO2 reduction to C2+ products. Beyond summarising the fundamental aspects for the development of a suitable catalyst as well as the mechanistic pathways of the most industrially desired C2+ species, this review makes emphasis on key aspects to design highly selective electrocatalysts. Additionally, as a very important aspect not frequently addressed in electrocatalysis literature, we bring techno-economical requirements into discussion targeting potential industrial implementation.Electrocatalytic CO2 reduction is especially appealing due to the wide variety of important products derived from it and their multiple advantages. The process can be carried out in neutral pH, at room temperature and atmospheric pressure and it can be controlled by the reaction temperature and the electrode potential. The chemical consumption can be minimised since electrolytes can be completely recycled and the intermittent renewable energy can be converted into stable chemical energy. Moreover, the system is modular, compact and easy to scale-up [8,9,13,31]. However, despite all the advantages, some drawbacks must be overcome for the technology to fully take off at commercial level. The activation of the very stable CO2 molecule is the first step in its electrochemical reduction and requires high overpotential. In fact, the formation via single-electron transfer of CO2*- radical intermediate (E\u00a0=\u00a0\u22121.9\u00a0V vs. SHE) is considered as the rate-determining step on most transition metal-based catalysts [3,32]. In general, heterogeneous electrocatalytic reactions take place at the catalyst surface-electrolyte interface [33], so the main CO2 electroreduction process could be simplified as follows: CO2 chemisorption and bond forming interaction on the catalyst surface, electron and/or proton transfers leading to C\u2013H, C\u2013O bonds or C\u2013C coupling; and product species rearrangement and desorption from the catalyst surface into the electrolyte [3,20,34], While formate and CO generation from C\u2013O and C\u2013H has been further studied and can be achieved upon implementing the right electrocatalyst at low overpotential [32], hydrocarbon or alcohol production is a more complex task. A summary of the most accepted activation routes for the electrochemical CO2 reduction to C2+ products is given in Fig. 1\n [12,19,20,32].Due to the multiple reaction pathways that can be conducted in parallel and competitive ways as well as more complex pathways that are not elucidated yet, a wide-ranging product distribution is generally obtained. The reaction pathways and, consequently, product distributions strongly depend on the electrocatalyst. Noble metals (Ag, Au and Pd) have been found as high selective catalysts for C1 products like CO and formic acid, while activity towards more than 2e\u2212 products such as methanol and methane has been obtained with Cu-based catalysts [12]. Besides the choice of a selective electrocatalyst, product distribution can be also adjusted by tuning external parameters such as the electrode characteristics (surface, morphology, facets, \u2026), operation conditions (applied overpotential, electrolyte, anodic reaction) or the cell design itself.In general, electrolytes should provide stable pH at bulk, good ionic conductivity and moderate to high CO2 solubility [22]. The type, concentration and composition of electrolytes have been found to affect CO2 electrochemical reduction. One of the most common and favourable electrolytes is alkaline aqueous solution because it stands lower overpotential than its neutral counterpart [22,35], helps to supress the hydrogen evolution reaction (Eq. (1)) and provides a high conductivity which reduces ohmic losses [35]. Hydrogen evolution reaction (HER) (equation (1)) is the main competing reaction to CO2 reduction. Due to the proton insufficiency on the catalyst surface, a basic media helps to supress this undesired reaction boosting CO2 electrochemical reduction products [35,36].\n\n(1)\n\n\n2\n\nH\n+\n\n+\n\n2\n\ne\n\u2212\n\n\n\u2192\n\n\nH\n2\n\n\n\n\n\nIn the study of Verma et al. [37] it was observed with different alkaline aqueous solutions higher current densities as increasing the solutions concentrations and through electrochemical impedance spectroscopies demonstrated the decrease of the cell resistance due to the ionic conductivity increase while increased the concentrations. Moreover, as Gabardo et al. reported [35], a 240-mV positive shift of the onset potential was achieved while using a 10\u00a0M KOH solution as electrolyte instead of a 1\u00a0M KOH one.The CO2 solubility can be increased using organic solvents instead of water solutions as electrolytes. Although CO2 is a nonpolar molecule, it possesses an appreciable polarizability and an ability to accept hydrogen bonds from suitable donor solvent [38]. Most of the organic electrolytes are polar solvents and allow the electrochemical CO2 reduction in a wider potential window [20]. The use of an aprotic solvent (such as acetonitrile or dimethylformamide) enables the *CO\u2013CO dimerization while CH4 production is favoured in a protic solvent. Nonetheless, electrochemical CO2 reduction can be tuned by adding protic compounds to aprotic solvents in order to facilitate the generation of H-containing products [20]. Some research on the use of ionic liquids (added to aqueous or organic solvents) has been made looking for a higher conductivity and hence, a decrease in overpotential [39\u201341]. Ionic liquids (ILs) could be proper electrolytes since they have good thermal stability, high CO2 solubility, wide electrochemical window, low vapor pressure and partially HER inhibition [20,42]. However, ILs also present some drawbacks concerning their high cost, the environmental toxicity embedded in their production, liquid products extraction struggling [43] or cathodic corrosion [44]. The latest works where ILs has been used as electrolyte are summarised in Table 1\n. Cathodic catalysts can suffer from deactivation due to the presence of trace impurities (metal ions or organics) coming not only from noble metal anodic catalysts dissolved because of operating conditions [45], but from the electrolyte, being the main deactivation cause in Cu, Ag and Au catalysts [46]. However, deactivation due to impurities can be avoided if a pre-electrolysis with a sacrificial electrode or an irreversible coordination between metal ions and a chelating agent (e.g. ethylenediaminetetraacetic acid) are conducted [46].As depicted in Fig. 1, the formation of C2+ products through different reaction pathways depends on the protonation process. The protonation transfer can affect the product distribution in electrochemical CO2 reduction, therefore, pH near the catalyst electrode surface, bulk pH, and local pH were found to affect the final product distribution [6].Due to the consumption of protons near the electrode surface as a result of proton and water reduction, a local pH (usually more alkaline than bulk pH) can be generated [32]. This difference in pH is caused by mass transport limitations [61] and depends on the operation conditions: current density, bulk pH, types of cations or anions, and on the electrode morphology [6,32,61]. Providing the adequate electrolyte with the right pH is a key factor to control the reaction pathway and the stability of the reaction intermediates [19,62]. As an example, it has been reported that selectivity towards ethylene on Cu electrocatalysts can be fine-tuned by modifying the buffering capacity. At high local pH, ethylene production through *CO\u2013CO dimerization pathway is both kinetically and thermodynamically favourable, whereas the C1 pathway is suppressed [63,64].In accordance with pH effect, H+ and OH\u2212 concentration has been shown to influence both, the activity and selectivity of the electrochemical CO2 reduction. Comparing several electrolytes containing potassium precursors (KCl, K2SO4, KClO4), phosphate buffer and KHCO3 aqueous solution diluted and concentrated, it was found that diluted KHCO3 KCl, K2SO4, KClO4 solutions favoured the formation of C2H4 and alcohols [65,66]. The presence of OH\u2212 anions produced from electrochemical reactions provokes a pH increase in the electrocatalyst surface enhancing C2 selectivity by suppressing HER [63,67]. Concentrated electrolytes (concentrated KHCO3 and phosphate buffer) are capable of neutralising this OH\u2212 anions. The local pH does not differ from the bulk pH and CH4 formation is favoured [65].Besides H+ and OH\u2212 ions, the electrolyte cations and anions nature was also reported to have a significant effect on product selectivity. Increasing size of alkali cation from Li+ to Cs+ usually leads to higher electrochemical CO2 reduction rates and higher C2/C1 products. This behaviour was attributed to differences in the local pH and changes in the electrochemical potential in the outer Helmholtz plane (OHP) due to the cation size [68]. The presence of hydrated alkali metal cations located at the edge of the Helmholtz plane stabilises the adsorption of surface intermediates such as *CO2, which is the intermediate precursor to the formation of C2 products through C\u2013C coupling. Slighter cations are strongly hydrated, hindering cation-specific adsorption on the electrode surface [12]. Larger cations are more energetically favoured at the OHP than smaller ones, hence an increase in cation size typically leads to a large cations coverage [68].The influence of halide anions on electrochemical CO2 reduction has also been studied. The halide adsorption on the catalyst surface affects to the activity, selectivity, and the electronic structure, depending on the halide size and its concentration [69,70]. It has been reported that adsorbed halide anions could limit the proton adsorption, inhibiting HER [71]. While using Cu as electrocatalyst, halides are able to stabilize the *CO intermediate through the formation of a covalent Cu-halide interaction [69]. Due to this increase in *CO population on the catalyst surface, adding Cl\u2212, Br\u2212 and I\u2212 to the electrolyte can lead to an increase in CO selectivity and hence a large methane production [69], or a lower overpotential and an increase in CO2 reduction rate while keeping C2\u2013C3 faradaic efficiencies [70]. Among the mentioned halides, KI electrolyte led to the highest *CO formation on the catalyst surface and then, the highest C2 selectivity [72].Working at higher pressures encompasses the possibility of operating at higher temperatures and an increase in the CO2 solubility according to Henry's law [73]. This increase favours the adsorption of CO2 species on the catalyst surface due to the higher CO2 concentration in the electrolyte [22], and some works have reported higher current densities by increasing CO2 partial pressure [73,74]. Therefore, changing the CO2 partial pressure can tune the relative surface coverage and hence, the stability of CO2 reduction intermediates and the product selectivity [35]. The combination of both, an increase in the reaction pressure and a highly alkaline environment could improve CO selectivity and energy efficiency reaching industry relevant current density values [35]. Nevertheless, working at high pressures imposes some drawbacks since an unbalanced pressure could result in drying the catalyst surface or flooding of the catholyte causing operational issues [22].Regarding the temperature effect, it has rarely been studied and most studies were conducted at ambient temperature. An increase of temperature implies a lower CO2 solubility, so the undesired HER will prevail over the CO2 reduction process. Moreover, the temperature increase is also limited by the possible membrane degradation, the operational window is rather narrow, although it could be widened if coupled with a pressurised system [22]. In general, higher temperatures lead to higher currents since ionic conductivity increases and the diffusion coefficient also rises, thus the CO2 transport is more efficient [75]. The CO2 solubility problem could be overcome while supplying CO2 and water vapor in a gas phase electrolyser provided with a proper gas diffusion electrode (GDE). That way, the reaction kinetics would be enhanced due to the higher temperature and would compensate the poor solubility [22,75]. Nonetheless, further studies of temperature effects should be conducted in order to achieve the optimal operation conditions and assess its economic viability.The effects of pressure and temperature strongly depend on the cell design, i.e., their permissible and adequate values are certainly disparate if a gas phase system or a liquid phase system is used, and additional studies are needed. However, and considering the limited available studies, it could be stated that slightly higher than ambient conditions of pressure and temperature would be favourable in terms of product selectivity and reaction rate [22].The energy barrier between the onset potential and the standard reduction potential, the overpotential, is highly dependent on the working electrode. The different ranges of the applied overpotential can affect the preferable pathways to form C\u2013C coupling. The *CO\u2013COH coupling is dominant at high overpotentials, while the *CO\u2013CO dimerization is favoured at low overpotentials [6,19].Up-to-date, CO2 cathodic reaction has usually been coupled with anodic water oxidation since water electrolysis is a well-known process without mass transport limitations, and all accomplishments and knowledge achieved in water electrolysers can be adapted to CO2 electrolysers [76]. However, oxygen evolution reaction (OER) requires a high overpotential that limits the energy efficiency of the whole electrolyser (OER can consume up to 90% electricity in CO2RR [77]) and oxygen gas could corrode and oxidise metallic catalysts or cell components [22]. Replacing this reaction for another with lower cell voltage requirements and providing high-value anodic products while maintaining a free of emissions process could be of interest. The work of Vass et al. [76] reviews and summarises different interesting alternatives to the widely used OER, including the CO2RR coupled with an already existing technology, the use of industrial waste as sacrificial agents or the production of raw and fine chemicals [78\u201380]. In the case of Li et al. [77], possible anodic oxidation reactions have been summarised and classified attending to the desired cathodic and/or anodic products. Some of the valued-added anode processes that have been lately paired up with CO2RR are summarised in Table 2\n\n. Moreover, first technoeconomic analysis have shown promising results concerning CO2RR coupled with organic compounds oxidation [81], achieving an electricity consumption save up to 53% in the case of the glycerol oxidation [78]. Notwithstanding, the complexity of coupling CO2 cathodic reaction with these anodic reactions in terms of operation conditions, cell design and structure, product separation is still a challenge that requires further research (see Table 3).Significant efforts and improvements have been made on developing a suitable electrocatalytic reaction system that enhances the electrochemical CO2 reduction reaction [13,22,25]. As represented in Fig. 2\n, electrochemical reaction systems can be categorised into H-cell systems, flow cell systems and microfluidic reactors.H-type cells are widely used because of their simple assembly, operation and products separation, their versatile configurations and low cost. In this system, working and reference electrodes are fixed in the cathodic reaction chamber while counter reaction is fixed in the anodic one. Both chambers are prefilled with the electrolyte and work without recycling. Their main disadvantage is the low CO2 solubility in the liquid electrolyte, which limits the current density [98], as well as the large distance between electrodes [99]. Moreover, selectivity towards C2+ products has always been diminished in favour of HER [13]. For all mentioned above, H-type cells are a suitable batch reactor for studying and comparing different electrocatalysts and products in lab-scale, although they do not fit in further industrial applications.The main component of the flow cell systems is the membrane-electrode assembly (MEA), where the electrodes are assembled to the solid polymer electrolyte or membrane, which acts a separation barrier of the chambers and is in charge of ion transportation between them. Depending on the type of ion transported, membranes can be cation exchange membrane (CEM), i.e., protons are transported from the anolyte to the cathodic chamber; anion exchange membrane (AEM), where hydroxide ions are transported from the anode to the cathode or the combination of both, bipolar membrane (BPM).According to the way of feeding the CO2 into the cell, two cell systems can be distinguished: gas phase and liquid phase electrolyser. In the former type, a humidified gaseous CO2 stream can be directly used as feedstock at cathode, enhancing this way mass diffusion and production rates [13]. The use of this cell design has also been reported to increase CO selectivity [100], to improve partial current density and stability for formate production [101], and to selectively generate C2+ products [14,102].In the liquid phase electrolysers, liquid electrolyte is in both electrodes. For that reason, the system can be pressurised and CO2 can be fed without further humidification [35]. In general, these flow cells have shown a better electrochemical CO2 reduction performance compared with H-type cells, mainly due to the enhanced CO2 diffusion and the local gas-electrolyte-catalyst interface [13].Microfluidic cells are an attractive alternative electrolyser configuration where, in some cases, the membrane is replaced by a thin gap between the electrodes filled with flowing electrolyte stream. The CO2 molecules are easily diffused into the electrode-electrolyte interface. Besides avoiding issues related to membrane degradation and cost, its absence allows wider pH and reaction temperature windows [103]. However, the existence of crossover could result in the products oxidation at some extent, although a multichannel design, a nanoporous separator [104] or a dual-electrolyte system [105] could solve this problem.The electrode surface and morphology has been reported to strongly affect in the activity and selectivity on the electrochemical CO2 reduction. Formation of defect sites, nano or complex structures, subsurface oxygen or crystal facets (schemed in Fig. 3\n) are the dependant factors [13,32].The formation of defect sites from vacancies or grain boundaries has been pointed out as the responsible of increasing activity and selectivity in electrochemical CO2 reduction [106,107]. Grain boundaries can alter the surfer properties of particles, generating active sites and inducing a large surface catalytic area [26]. These modifications of the particles and the catalyst surface can lead to a lower energy barrier for CO2 reduction or to an enhancement of CO adsorption and the C\u2013C bond formation with a consequent stabilization of C2 intermediates and higher selectivity for this C2+ products [107,108].Several efforts have been made to create nanostructured catalysts more active and selective than bulk or foil electrocatalysts. It was found that roughened Cu electrodes could provide a high local pH, which promotes C2H4 generation over CH4 [109,110]. Catalyst structure is capable of controlling C2+ products selectivity by tuning the pore size and depth of nanostructures such as nanowire arrays, nanofoams, nanoparticles or nanocubes [111\u2013113]. Smaller and deeper pores increase ion concentration and intermediates residence times, enhancing C2+ products selectivity. Moreover, intermediates can be confined into the nanostructure generating long chain molecules [26,114].Besides physical factors, the intrinsic catalyst properties could also affect the activity and selectivity [19] and several studies have proved the strong facet dependence on electrochemical CO2 reduction product selectivity [6,19]. As an example, it has been observed in Cu catalysts that Cu(100) favours selectivity towards ethylene and the C\u2013C coupling improving C2+ products selectivity while Cu(111) favours the methane and formate formation [67,115,116].The electrocatalyst performance could also be adjusted by tuning the oxidation states via oxide-derived materials. Using again the copper catalysts as an example, it was observed that Cu oxidation states can easily change during electrochemical reaction conditions and oxide-derived catalysts enhance the activity and selectivity towards C2+ products [6,19]. The presence of residual oxygen, subsurface oxygen or oxidized cooper states favours the stabilization of the intermediates thus enhancing C\u2013C coupling [117,118].As mentioned before, due to the multiple possible reaction pathways, numerous products can be originated. However, product distribution can be tuned by designing an active and selective electrocatalyst. The complex formation of C2+ products is gaining attention due to their higher energy density and economic value [119], and several research projects are currently focused on their production [120\u2013122]. The numerous and assorted applications, where these products can be valuable, are summarised in Fig. 4\n. The search of a catalyst with adequate electronic properties to be selective for the C\u2013C coupling is vital to generate species with one or more C\u2013C bonds. Among metals, copper has been found particularly active for the formation of C2+ compounds, although a few non copper-based catalysts such as heteroatom-doped carbon materials [123], NiP [124], NiGa [125] or PdAu [126] have shown significant formation of C2+ species. In the following subsections we will discuss the catalysts design to successfully facilitate the production of key C2 and C2+ products which are highly appealing for the chemical industry and whose electrochemical production might result in a game-changing approach.3.1.1. Oxalate. Oxalate or oxalic acid is the simplest dicarboxylic acid widely used in the chemical industry in applications such as pharmaceuticals and textiles manufacturing, rare earth extraction, oil refining or metal processing [127\u2013130]. Oxalate is a precursor to glycols, which are in turn precursors to valuable synthetic materials [131,132]. Some research has been done concerning the beneficial effect of its application to improve quality of some vegetable and fruits and to extend their storage time and prevent its early degradation [133\u2013135] and its role in the recovery of valuable metal ions from lithium-ion batteries [136,137]. Regarding current investigations, oxalic acid effect is being subject of study in a treatment to a honeybee's disease [138].The electrochemical CO2 reduction products strongly depend on both the chemical nature of the electrode and the reaction medium. In a protic solvent as water, formate would be the main product [139,140]. Since the solubility of CO2 in aqueous solutions is really low (about 30\u00a0mM), water molecules would be available nearby the electrode surface and HER would be favoured. In fact, only hydrogen is yielded while water is the only component of the electrolyte, the charge transfer proceeds favourable to water [141]. Using aprotic solutions that provide a higher solubility (about 240\u00a0mM in acetonitrile as an example [141]) as solvents would lead to CO2*- intermediate disproportionation to carbon monoxide and carbonate or dimerization to oxalate ions [139]. Besides their higher CO2 solubility, aprotic nonaqueous solvents such as acetonitrile, dimethyl formamide, dimethyl sulfoxide or propylene carbonate have been widely used because of their large cathodic window and the inhibition of HER [130,142].In this aprotic medium, CO and carbonate were reported as the major products using metals such as Pt, Pd, In, Zn, Sn or Au as catalysts, while oxalic has been reported as the major product using a Pb, Sn or Hg electrode [143,144].It is known that the first and primary step on the electrochemical CO2 reduction is the formation of the intermediate radical ion CO2*-, a change and mass-transfer controlled reaction that must be conducted at a sufficiently cathodically polarized electrode. In a nonaqueous aprotic medium, two main competitive pathways have been repeatedly described in literature: nucleophilic coupling of CO2*- with nearby CO2, that could produce CO and carbonate, or oxalate through ECE mechanism; and the purely dimerization of two radical ions following an EC mechanism, which would also lead to an oxalate anion production [131,140,141,143,145\u2013147]. Dimerization reaction is more favourable at high concentrations of CO2*- radical anions while the reductive disproportionation of CO2 could be conducted at low CO2*- concentrations [148]. The reaction mechanism for oxalate production is depicted in Fig. 5\n.Lead has traditionally been chosen as the working cathode due to its high HER overpotential and its low cost, which favours CO2 reduction [143,144,147,149]. In nonaqueous medium, oxalate has been reported as the main product and oxalate faradaic efficiencies (FEs) exceeding 80% were registered from the very early studies [143,144]. The work of Eneau-Innocent et al. studied the role of Pb electrode in a propylene carbonate electrolyte [142]. Propylene carbonate large electrochemical window, CO2 solubility and relative low toxicity made this aprotic solvent suitable for the electrochemical reduction. It was found that Pb electrodes are very selective towards oxalate via direct dimerization route. In situ IR spectroscopy measurements revealed that unstable intermediate CO2*- radical ions evolve quickly to adsorbed Pb\u2013CO2*-, which dimerizes, and oxalate is finally desorbed.Some studies have also explored Pb or stainless steel electrodes coupled with sacrificial anodic electrodes in order to easily remove oxalate product from the electrochemical system [130,147,148]. Zinc has been widely used as a sacrificial anode because it produces zinc cation which fast reacts with oxalate anion towards insoluble zinc oxalate, preventing the further reduction or oxidation reaction of oxalate anion [147]. Lv et al. achieved a 96.8% FE for producing oxalate in 0.1\u00a0M tetrabutylammonium perchlorate in acetonitrile with a lead cathode and a zinc sacrificial anode [147]. Such a high efficiency was a result of a combination of two factors: water total elimination and low temperature (5\u00a0\u00b0C), as shown in Fig. 6\n. As observed in the reaction mechanism (Fig. 5), the presence of water in the electrochemical CO2 reduction negatively affects to oxalate selectivity (see Fig. 6) [60]. Therefore, in this study [147], the electrochemical system was subjected to a pre-electrolysis and CO2 desiccation process in order to remove possible trace of water from the electrolyte and from CO2. It was found a decrease up to 40% in oxalate FE where 1.25 v/v% of water was added to the electrolyte solution, confirming the interferences of water in electrochemical CO2 reduction. In the case of temperature effect, solubility of CO2 increases while decreasing the temperature, enhancing CO2 availability on the electrode surface for its reduction.The advantages of incorporating ILs as electrolytes were also explored for oxalate production in Pb electrodes. A novel aromatic ester anion functionalized IL of 4-(methoxycarbonyl) phenol tetraethylammonium ([TEA][4-MF-PhO]) was designed based on catalytic and stability properties of aromatic esters and quaternary ammonium salts, respectively [60]. DFT calculations were performed in order to investigate the interaction between aromatic ester anion of [4-MF-PhO]- and CO2 revealing the ability of the anion with phenoxy and ester double active sites to bend the stable CO2 molecule to CO2*-. H+ cations provided by anolyte were combined with the anion-radical generating [4-MF-PhO-COOH]- that eventually dimerized to oxalic acid (Fig. 6). High partial current densities (9.03\u00a0mA\u00a0cm\u22122) and acceptable FEs (86%) obtained on Pb electrode in [TEA][4-MF-PhO]-acetonitrile electrolyte assert the integration of IL in this aprotic electroreduction systems for oxalate production and open a potential path of research and improvement.Besides all progress related to Pb or stainless steel electrodes, cathodic materials such as metal organic frameworks (MOFs), metal-complexes or carbon supported metals have also been explored to perform the electrochemical CO2 reduction to oxalate [132,150\u2013152]. Metal organic frameworks are porous materials with a crystalline ordered structure capable of storing CO2, which have also presented attractive features such as high porosity or thermal stability and adjustable chemical functionality [153\u2013155]. They are indeed an emerging family showing promising features for electro and thermal catalysis [156] and their careful implementation in CO2 to oxalate process merits further studies.Shentil Kumar et al. coated Cu-BTC on a glassy carbon electrode to prove it as an efficient electrocatalyst for the oxalate production from CO2 [151]. The electrochemically synthesized MOF was capable of sequestrating and electro reducing CO2 simultaneously. The electrochemical reduction can be conducted inside the pores through heterogeneous electron transfer between the MOFs and the previously adsorbed and compressed CO2 molecules, generating a 90% pure oxalic acid with a FE of 51%.Silver was also tested as cathodic catalyst in a carbon-silver hybrid configuration [150]. Babassu coconut, an important Brazilian vegetation that has been previously used for nanocarbon production [157], has been hydrothermally carbonised in the presence of silver nitrate solution, obtaining the Ag@C catalytic system (see Fig. 7\n). Comparing the results obtained with this system with those in absence of silver nanoparticles, it can be concluded that silver nanoparticles enabled the charge transfer for the CO2 reduction besides enhancing the electrocatalytic activity. Moreover, synthesis with longer residence times resulted in larger deposits of silver particles that influenced the size of the carbon spheres and increased oxalic acid production.As opposed to the previous works, Paris et al. reported the use of a Cr\u2013Ga electrode supported on glassy carbon to produce oxalate in a KCl aqueous solution electrolyte [158]. This catalytic system consisted of a Cr2O3\u2013Ga2O3 thin alloy film and, whereas neither Cr2O3 nor Ga2O3 films alone could produce oxalate, the alloy of the two oxides led to oxalate FE of 59% at a pH of 4.1. Organic solvent or those with low proton availability reported the production of the oxalate dianion; however, in this aqueous system, a combination of monoanionic and dianionic species was generated. The Cr\u2013Ga system originates a more appealing product distribution since protonated oxalates are more desired species from an industrial point of view [158].Oxalate was obtained at a more positive potential than that required for CO2*- intermediate generation, i.e., this process provides a CO2*- independent and lower energetic pathway than those previously reported (Fig. 5). This low energetic reaction pathway could involve a C1 product as oxalate precursor, and experiments feeding CO, formate, methanol or their combinations instead of CO2 were conducted. It was observed that CO-methanol combination did lead to oxalate production. Labelling experiments showed that carbon atoms oxalated derived from CO, although both methanol and CO are required for its production. Results obtained from this report encourage additional studies to advance in oxalate production in aqueous solutions [158].3.1.2. Acetate. As already mentioned, the traditional synthesis of acetic acid involves a three-step energy-intensive process (methane or coal to syngas, syngas to methanol and methanol carbonylation to acetic acid) using methane or coal as feedstocks [159,160]. Therefore, an efficient direct production of acetic acid can revolutionise the production of this key chemical. Indeed, acetate is not only an important end-product, but a versatile platform chemical capable of producing medium-chain fatty acids, alcohols and bioplastics, e.g., vinyl acetate monomers, esters and acetic anhydride [161,162]. Further, some industrial processes like denitrification or biological phosphorus removal also use acetate as carbon substrate [163].Unfortunately, acetate has not received a lot of attention as product of electrochemical CO2 reduction. The reaction pathway for its formation remains certainly unclear, although several possible routes have been suggested (as depicted in Fig. 8\n). At high potentials and high local pH, ethanol and acetate could be directly produced from acetaldehyde via Cannizzaro-type reaction [164]. Garza et al. pointed out that the ratio ethanol/acetate did not follow the proposed reaction [165]. They suggested the existence of additional pathways, such as the acetate production as a by-product on the ethylene pathway through *OCH2COH dimerization to a three-membered ring compound and further reduction.In general, the formation of CO2*- radical anion was accepted as the first step in the electrochemical CO2 reduction [16,123]. Genovese et al. provided experimental evidence for the acetic acid formation from a nucleophilic attack of *CH3 adsorbed species by this intermediate radical anion [16]. Once the radical anion was adsorbed, it can be reduced at the electrode surface until a \u2013CH2OH specie is originated, and then generating methanol or proceeding with reduction until \u2013CH3 species generation. In the case of conducting the electrochemical reduction on a N-doped nanodiamond/Si rod array electrocatalysts, the in situ infrared spectroscopy identified OOC-COO as intermediate [123]. Based on the assumption that kinetics C\u2013C coupling is faster than CO2*- protonation [166], the pathway proposed involves the combination of two radical anions to generate the OOC-COO intermediate that would be further protonated and reduced producing acetate. Following the same assumption and while using Cu(I)/C- doped boron nitride as catalyst in the presence of 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4)\u2013LiI solution, Sun et al. proposed the formation of a [Emim\u2013CO2]+ complex that reduces the electron transfer barrier to CO adsorption [160]. CO adsorbed is then reduced and protonated until methanol formation (detected product) that would couple with CO2*- to generate acetate. This process is promoted by the presence of the Lewis acidic cation Li+ and the strong nucleophilicity of I\u2212, although a more detailed mechanism involving these species needs further investigation.Alternatives to the above describe route has also been proposed ruling out CO2*- generation. This is the case of the works of Panglipur et al. [89] and Grace et al. [167] In the former one, acetic acid is believed to be generated by the formation and adsorption of CO and H+ species, which would generate CH2 that, in turn, could react with CO and be hydrated until acetic acid generation [89]. In the second work, due to the ANH groups of polyaniline catalyst and the use of methanol as electrolyte, H adatoms are generated on the electrode surface and then transferred to the CO2 molecules generating formic acid that could be attacked by the methanol and form acetic acid [167].Although copper-based catalysts are the most suitable for C2+ products from CO2, bulk copper catalysts face limitations (poor selectivity, high energy input, low activity) that must be overcome tuning these copper catalysts. More selective catalysts can be generated with smaller nanoparticles, defect sites, core-shell structures, oxidation states or heteroatomic doping [168]. In that sense, several tuned copper-based catalysts have reported high faradaic efficiencies toward acetic acid or acetate.The choice of a suitable support could be a key to improve bulk copper performance. The incorporation of conducting polymers such as polyaniline have been proved to decrease the electrochemical CO2 reduction overpotential [167]. Cu2O nanoparticles, with oxygen species to easily adsorb CO2 and Cu\u00a0+\u00a0ions promoting CO2 reduction as active sites, were disposed in a polyaniline matrix. This configuration led to a synergistic effect which allows the generation of acetic acid as the main product with an efficiency of 63% [167].Boron-doped diamond (BDD) has been widely used in electrochemical application due to its adequate characteristics for those purposes [169]. Although it is able to produce formaldehyde on its own under ambient conditions [89], as support of copper particles, it resulted in a selective electrode to acetic acid generation. Acetic acid was not observed while using copper or BDD as electrodes separately [89]. Same behaviour was observed in an electrocatalyst based on copper nanoparticles supported on carbon nanotubes (CNTs), achieving a production of acetic acid with a selectivity close to 60% [16].Pure titanium oxide does not present activity in the electrochemical CO2 reduction process itself [170], but its ability to control the local pH and its good conductivity make nanotubular TiO2 array (TNA) a suitable support. In the work of Zang et al. TNA was applied as the support of a modified polyoxometalate Cu catalyst [168]. Polyoxometalates (a type of transition metal oxygen anion clusters) were induced to modify the local electronic and protonic environment of Cu nanoparticles conforming a Mo8 modified cubic fragmented Cu submicron particles on TNA (Mo8@Cu/TNA) catalytic system. The skeleton structure and SEM images of the catalytic system can be observed in Fig. 9\n.Acetate is originated following the single carbon coupling pathway (schemed in Fig. 8), i.e., CO2 molecules were coupled with formed *CH3. Experiments on mechanism investigation showed the synergistic effects of Cu planes and polyoxometalate cluster promoting the generation of the intermediate *CH3 tuning the selectivity towards acetate. Moreover, Mo oxide clusters modified Cu catalyst obtaining a fragmented Cu surface rich on grain boundaries. Ag was selected as metal to substitute Cu, but in view of selectivity and FE obtained while using Ag instead of Cu, these control experiments just served to highlight the synergistic effect between Cu surface and Mo oxides.Wang et al. also incorporated Ag in the electrocatalyst, but combined with Cu, forming clusters (\u223c 6\u00a0nm) of Cu and Ag on the surfaces of electropolymerized films, (Cu)m, (Ag)n/polymer/GCE [171]. The increase in the acetate formation as Ag was added to the nanoparticles evidenced the important role of surface metal composition. As observed in Fig. 10\n, it is believed that the presence of Ag could modified reaction mechanism although is still uncertain. Ag catalysts are able to promote CO2 reduction to CO [172], so Ag sites could facilitate the CO formation that would be further captured on neighbouring Cu sites for C\u2013C bond formation and later acetate production (Fig. 10).N-doped systems also represent an appealing family of materials for electrochemical CO2 conversion. Indeed, they are chemically and active stable compounds that present an overpotential for HER higher than most reported electrocatalysts [173]. This way electrochemical CO2 reduction could be conducted suppressing H2O reduction and hence, enhancing the selectivity for the desired product [174]. Boron nitride (BN) promotes the CO2 chemisorption in the presence of electrons, thus the role of a C-doped BN as support of a Cu(I) complex Cu(I)/BN-C for acetic acid production was evaluated achieving a FE of 80.3% [160]. The support and the catalyst complex behaviours were evaluated separately, finding that bulk BN is not active for CO2 reduction, but BN-C can generate large amounts of formic acid since N-doped carbon materials can absorb CO2 and stabilize CO2*- [175]. The high selectivity to acetic acid is then related to a synergistic effect between catalyst complex and BN-C. CO2 is first adsorbed and converted into CO2*- due to BN-C and then protonated and reduced into methanol that would generate the desired acetic acid via C\u2013C coupling with the help of the Cu metal center [160].Due to the observed proper characteristics of N-doped materials, N-doped nanodiamond supported on a Si rod array (NDD/Si RA) (schemed in Fig. 11\n) was tested for electrochemical CO2 reduction showing high efficiency, fast kinetics and good selectivity towards acetate [123]. As observed in Fig. 11, an increase in the N content led to an enhance in acetate and formate production rates due to the defects sites presence and the carbon atoms polarized by the N doping [176]. Defects sites and polarized carbon atoms may promote CO2 adsorption and CO2*- stabilization and rod array structure provides a large surface area that facilitates electron transfer. The combination of all these effects would explain the promising results obtained [123].As mentioned before, Cu-based catalysts have shown excellent performance on electrochemical CO2 reduction and high selectivity towards C2+ products, although they are not the only ones and, as in the case of the N-doped nanodiamond catalyst, modified Fe, Mn, Au, Pd or In metals have demonstrated its activity and selectivity towards acetic acid [15,177\u2013180].Fe oxyhydroxide nanostructures (Ferihydrite-like clusters, Fh-FeOOH) were also supported on N-doped graphitic materials (Fe/N\u2013C), and FE values were up to 97% with a high selectivity to acetic acid [177]. Results supporting the same clusters on O-doped carbon were not so encouraging, revealing the favourable Fe\u2013N interaction. N-dopants allowed the stabilization of Fe(II) species instead of Fe0 (as in the case of using O-dopants) that promotes HER. Acetic acid formation was attributed to the presence of these Fe(II) species adjacent to N sites, while Fe(II) species are believed to reduce HCO3\n\u2212 species, N sites enable the C\u2013C coupling.Indium metal favours the formation of C1 products, although modifying its electronic properties inducing transitions metal oxygen anion clusters such as polyoxometalate (which has also reported beneficial effects modifying Cu electrodes) [168] can tune its selectivity towards C2+ products. Li et al. [180] and Zha et al. [15] works have incorporated polyoxometalates as electrolyte to assist In for the electrochemical CO2 reduction. As expected, the electrocatalytic performance of In resulted in the formation of hydrogen and formic acid as the only products. The addition of SiW11Mn to the electrolyte gave a result the appearance of acetic acid as well as hydrogen and formic acid, according to ion chromatography, gas chromatography and mass spectrometry analyses [180]. The presence of SiW11Mn reduced the overpotential and enriched In0 species since it enabled the re-reduction of elemental In.In the case of adding SiW9V3 to the electrolyte, the reaction mechanism was investigated through IR monitoring experiments and gas chromatography [15]. Control experiments (electrolyte without adding SiW9V3, and SiW9V3 electrolyte without In metal as catalyst) reported that SiW9V3 was necessary for the acetic acid formation (as in the previous case using SiW11Mn) and In for the CO2 reduction since the first step on the mechanism was the In-facilitated formation of In\u2013CO3\n- (instead of CO2*- intermediate). Besides, XPS spectra indicated that the V-center of SiW9V3 participates in the electron transfer process decreasing the overpotential, i.e., V-center efficiently catalyses the reduction of CO2. Faradaic efficiencies in this system reached values up to 96% with nearly 96% selectivity towards acetic acid.The synthesis of C2+ products from CO2, such as ethylene or ethanol, is of high importance due to the essential role of these products in the chemical and energy industry [119,181\u2013183]. Ethylene is widely used as a building block in the production of many raw materials such as polyethylene, ethylene oxide, vinyl acetate, and ethylene glycol. Usually, ethylene is obtained from steam cracking of naphtha under harsh production conditions (800\u2013900\u00a0\u00b0C). Although steam cracking is the industry standard for ethylene production, it presents different disadvantages. This process is non-catalytic and non-selective and is high energy and capital intensive, yielding into many by-products which require extensive separations and purification [184,185]. In recent years, the ethylene electrochemical synthesis from CO2RR is gaining attention since this approach offers mild conditions and an environmental pathway for ethylene production [186,187]. However, the use of CO2RR for the highly selective production of compounds of economic interest such as ethylene is still a challenge [186]. Concerning the catalyst, in the electrochemical environment of this reaction, i.e., abundant protons and negative electrode polarization, different catalytic behaviours have been observed: Ni, Fe, Pt, and Ti cathodes preferentially produce H2 over CO/CO2 production. Post-transition metals such as Pb, In, Sn, and Tl mostly produce formates. Additionally, Ag, Au, Pd and Zn reduce CO2 only to CO. On the other hand, Cu possess the outstanding ability to reduce CO2 or CO to CH4, C2H4, C2H5OH, and a variety of products [188\u2013190].Copper catalysts have been extensively explored in the electrocatalytic synthesis of ethylene. It has been proved that tuning the intermediates' stabilities can favour a desirable reaction pathway and by consequence, improve the selectivity [190\u2013192] However, the mechanistic pathway for the formation of ethylene over Cu catalyst is still under academic discussion [19]. While the C\u2013C coupling of two carbenes (*CH2) has been proposed as the determinant step for the synthesis of the C2 product from CO2RR over Cu catalyst [193], more recent, studies concluded that C2 product formations are more attributed to CO dimerization.Different theoretical and experimental studies have supported that the CO dimerization pathway is the limiting step for ethylene production [192]. As shown in Fig. 12\n, in the CO dimerization, *CO\u00a0+\u00a0*CO (Fig. 12a), *C2O2\n\u2212 intermediate can exist either from carbon and oxygen atoms from *C2O2\n\u2212 intermediate, which are bounded to the catalyst surface in a bridging mode (Fig. 12b) or by the *C2O2\n\u2212 intermediate which is bounded to the surface by two C atoms (Fig. 12c). However, both ways proceed to further protonation of the *C2O2\n\u2212 intermediate giving place to more stable intermediates such as *CO\u2013COH (Fig. 12d) of *COCHO [194]. Alternatively, Goodpaster et al. proposed that while at low overpotential, the formation of the C\u2013C bond takes place in a reaction of two *CO bounded to the surface, at higher overpotential, the reaction between *CO and *CHO (Fig. 12e and f) is favoured as a result of the larger activation barrier to the formation of the CO dimer [195]. This intermediate is followed by the reaction with *CO to form *COCHO (Fig. 12g) and finally ethylene formation [164,165].Copper has been selected as an ideal catalyst in the electrochemical conversion of CO2 due to the thereof mentioned characteristics. Its reaction performance has been widely explored in CO2RR by controlling morphology [113], grain boundaries [166], facets [196], oxidation states [197], molecule decoration [188\u2013190], and dopants [198]. Among these methods, the design of Cu metallic nanostructures for C2+ products seems more promising due to the simple synthesis and the easy study of the structure-activity relationship of the catalyst. For instance, Roberts et al. showed that modifying the structure of the copper surface is a novel pathway to improve ethylene efficiency and selectivity. These authors obtained nanotubes-covered copper (CuCube) surface with high selectivity and low overpotential to ethylene formation, but also probed the effect of (100) sites in the C\u2013C coupling as a strategy to target multicarbon products [199].Recently Zhang et al. reported the design of Cu nanosheets as an electrocatalyst for ethylene synthesis from CO2RR. These nanosheets (Fig. 13\na) present defects in the size of 2\u201314\u00a0nm, which were observed to be strengths in the adsorption, enrichment, and confinement for reaction intermediates and hydroxyl ions on the catalyst. As shown in Fig. 13b, the maximum FE reached with these copper nanosheets was up to 83.2%, which is the highest value among all the studied electrocatalysts to date [200].Loiudice et al. explored the effect of shape and size of Cu nanocrystals (NCs) in ethylene production. They explored crystal cube and spheres of Cu NCs, observing an increase in the activity as the crystal size decreased [116].The modification of oxidative copper states has also been proved to be an alternative to enhance the selectivity and efficiency of C2+ products [197,201]. Anodized-copper was investigated in CO2RR as a tool to improve ethylene selectivity. It was observed that compared with a Cu foil, this anodized-Cu catalyst presented a two-fold improvement [202]. But most importantly, the selectivity continued stable over 40\u00a0h of the experiment. As it is observed in Fig. 14\na, the catalyst performance remained stable, retaining an average of 38.1% FE for ethylene production. In this experiment, it was also observed the electrochemical treatment of the as-synthesized catalyst as a critical parameter in the ethylene selectivity (Fig. 14b). When the material was exposed to electrochemical reduction treatment at mild biased potential, the species were observed to suffer a reduction, quickly decreasing the ethylene selectivity. Meanwhile, when the treatment was performed at highly biased harsh conditions, the catalyst presents much extended durability for selectivity C\u2013C coupling. These observations give evidence of the relationship between Cu\u2013O-containing surface states and the durability of ethylene production, which are of high importance to develop new strategies for controlling selectivity and durability of O\u2013Cu catalyst for electrochemical synthesis of ethylene [202].The Cu-based alloys such as Ag\u2013Cu, Au\u2013Cu, Au\u2013Pd, and Cu\u2013Pt have been demonstrated to present a high efficiency in the C1 production via stabilization of the intermediates [203,204]. However, the effect of these alloys in the production of C2+ hydrocarbons seem to be more complex [205]. For instance, Chang et al. reported an atomically dispersed Cu\u2013Ag bimetallic catalyst where it was observed that Cu-rich zones preferred the production of hydrocarbons. In contrast, Ag-rich zones are dominated by CO. These experiments highlighted the importance of the atomic ratio for CO2RR electrocatalysts [206]. Similar studies using Cu\u2013Ag alloys from additive-controlled electrodeposition showed that Ag sites were believed to act as a promoter for CO formation during the electrocatalytic CO2 reduction, helping the C\u2013C coupling in the neighbouring Cu due to the availability of CO intermediate [207].Molecular distribution in Pd\u2013Cu catalyst has also proved to play an important role in improving the selectivity of C2+ compounds. For instance, Ma et al. demonstrated that a phase-separated Pd\u2013Cu catalyst offered an ethylene selectivity up to 50% compared with its disordered and ordered counterparts (Fig. 15\na and b) [208]. Since according to the d-band theory, typically, lower d-band center transition metals show weaker binding on the in-situ generated intermediates on the metal surface [209]. The Pd\u2013Cu alloy exhibited a similar catalytic activity and selectivity for CO that the ones obtained with Cu nanoparticles (NPs). It offered a wide d-band difference (Fig. 15c) which helps to conclude that the geometry and structure effect may play a more important role than the electronic effect for enhancing the selectivity of hydrocarbons in phase-separated Cu\u2013Pd alloys case [208].The nature of the electrocatalytic CO2 reduction imposes the need of catalysts with specific active sites for the reactants adsorption and the further transformation of multiple intermediates, thus makes primary importance the increasing of specific active sites in the design of catalyst. As discussed in the oxalate section, MOFs have emerged as an alternative to act as both, as electrocatalysts or precursors to derive in different heterostructured catalysts due to their extraordinary properties and could be useful for electrochemical ethylene production from CO2. In fact, from an electrochemical point of view, the permanent porosity, ultrahigh surface area, coordinatively unsaturated metal sites, adjustable pore size, and active sites homogeneous dispersion makes them highly attractive for this kind of reactions [210].Post-synthesis modification of MOFs is a strategy of high relevance in the design of electrocatalysts since it could assemble the functional metal fractions such as metal porphyrin and metal complex that would enhance the CO2 reduction in MOFs, particularly in those with coordinatively unsaturated metal sites, which interact with CO2 working as Lewis acid [29,211,212]. Cu-BTC (HKUST-1), which presents open metal sites, has been widely studied in the CO2RR due to its structural features that enhance the catalytic performance. Nam et al. reported the strategy involving the MOF-regulated Cu cluster formation (Fig. 16\na) as a tool to optimize the selectivity of ethylene in comparison with MOF-based active carbon. They obtained an efficiency of up to 45% (Fig. 16b) [213].MOFs derivates such as metal oxides, porous carbons, carbides, phosphides, and nitrides have appeared to overcome the main limitations of MOFs related to the poor stability and conductivity in electrochemical reactions [214\u2013217]. For instance, Zheng et al. studied N-doped nanoporous carbon obtained from ZIF-8 for ERC, emphasizing the calcination temperature in the reaction and the mechanism. According to the catalytic performance, higher pyrolysis temperature resulted in a higher activity with a maximum FE for CO formation of 95.4% [218].The design of atomically dispersed metals in activated carbon (AC) has also been a powerful tool in the electrochemical conversion of CO2. As shown in Fig. 17\n, Guan et al. boosted the CO2 electroreduction to CH4 and C2H4 by tuning the neighbouring single-copper sites in a dopped-AC showing the Cu-coping concentration as a tool to direct the selectivity to the desired product. For instance, they showed that at a Cu high concentration, the distance between Cu-Nx species was close enough to enable C\u2013C coupling and, by consequence, produce C2H4. In contrast, a Cu concentration lower than 2.4% mol, the distance between species was larger to the formation of CH4 was favoured [219].The electrocatalytic conversion of CO2 to higher-value hydrocarbons beyond the C1 products is an area of extraordinary importance for applications such as transportation, fuels, energy storage, and the chemical industry [220]. With a worldwide production of ca. 88.5\u00a0Mt/y and a projection of US$105.2 billion by 2025, ethanol is an important organic chemical for the biofuel and food industries [221,222]. For instance, approximately 80% of global ethanol produced is used as fuel, followed by food, pharmaceutical, and cosmetics applications [223].In recent years, electrochemical production of ethanol has been proposed as a green route to obtain this high-value chemical. As observed in Fig. 12, the electrochemical production of ethanol takes place in a similar pathway to ethylene, where the C\u2013C coupling activity is considered a critical step in ethanol production, similarly to other C2+ products. Garza et al. proposed an ethanol pathway over Cu catalyst where it was observed that *COCHO intermediate (Fig. 12g) is the key to determinate the selectivity between ethylene and ethanol [165]. The glyoxal can be reduced to acetaldehyde and ethanol (Fig. 12h), followed by a further reduction to glycolaldehyde (Fig. 12i) and ethylene glycol/vinyl alcohol (Fig. 12j) or acetaldehyde, and finally to ethanol [164,189].The main goal for CO2RR to ethanol is to improve selectivity at high conversion rates, which are influenced by the catalyst and the process conditions. By far, Cu-based catalysts have been the most studied to catalyse CO2RR for ethanol production [201,224,225]. In this sense and similarly to ethylene, different strategies have been studied to enhance ethanol selectivity in Cu-based catalysts, such as nanoparticle morphology [226,227], oxidation states [228], and Cu-based alloys [208,229]. For instance, Duan et al. studied the crystallinity of Cu nanoparticles as a driven parameter in the ethanol selectivity, observing that amorphous nanoparticles enhance the adsorption of CO2 at room temperature, which plays a key role in the CO2RR [230].Several studies have demonstrated the strong relationship between product selectivity of CO2RR and the crystal facets of Cu [231]. For example, Cu(100) increases the selectivity for ethylene, while in Cu(111) catalyst, methane is the main hydrocarbon product [67,115,116,232]. Jiang et al. developed and studied an efficient nanocube-shaped catalyst with significant improvement in C2+ selectivity. It was reported that Cu(100) and (211) facets favour ethanol and other C2+ products over Cu(111) via dimerization of *CO to form *OCCO and the subsequent proton-electron transfer and surface hydrogenation to form *OCCHO [196].The oxidation states of copper can vary among Cu0, Cu+, and Cu2+, and their states change reversibly during electrochemical reaction conditions. Copper oxide-based catalysts improve the activity and selectivity of C2+ products. Generally, copper oxide-based catalysts are synthesized by the growing of Cu2O from Cu-based precursors followed by high-temperature treatment and consequently reducing Cu2O to form Cu0 sites. However, the elucidation of the pathways of this enhancement toward C2 formation is still under debate [19]. Tentatively, the enhancement of the selectivity for ethanol of such catalyst could be attributed to (i) the presence of residual oxygen atoms close to the surface that favour the modification of the electronic structure of Cu atoms and the increase of the CO binding energy, which by consequence promotes C\u2013C couplings [233]; (ii) the strong adsorption of H2O molecules due to the presence of residual Cu\u00a0+\u00a0which may work synergically with Cu0 sites. These H2O molecules favour the CO2 conversion to CO [117,118]. For instance, Handoko et al. reported a mechanistic study electroreduction of CO2 to ethanol and other C2+ hydrocarbons on Cu2O-derived films. They synthesized Cu2O films with five different morphologies and proved a relationship between the crystal size and the selectivity to C2 products being able to reach a FE of c. a. 20% in middle-sized particles [234].A wide variety of morphologies of copper oxide-based materials has been reported as a determinant factor to enhance ethanol production. For instance, Daiyan et al. reported the synthesis of nanowires of Cu2O/CuO/Cu foam (Fig. 18\na) that exhibited a FE for ethanol formation of 31% [235]. Also, 3D dendritic Cu\u2013Cu2O/Cu catalyst (Fig. 18b) were synthesized and proved a FE up to 39.2% attributed to the high density of exposed active sites and favourable Cu2O:Cu ratio [220].Copper alloys have attracted considerable attention as a strategy to change the composition of Cu single crystal catalysts via combining it with a secondary metal able to produce H2 (i.e., Fe, Ni, Ag, Au, and Pd) and to tune the activity and selectivity through optimization of the binding strength of the key intermediates on the surface of the catalyst [6]. For instance, Shen et al. described the synthesis of submicron arrays based on the alloy CuAu reaching a FE up to 29%. It showed the critical role of Cu:Au ratio content in the selectivity ethanol/ethylene [236]. Interestingly, in the same way as metal ratios, the phase distribution of the metal involved in the alloys has been proved to be a determinant factor in directing the selectivity to ethylene or ethanol. As observed in Fig. 19\na, which shows the ethanol/ethylene selectivity in an Cu/Ag catalyst, the selectivity is three times higher in the catalyst with phase blended, probably related to the effect of the dopant in the surrounding Cu atoms but also to the effect of Ag\u2013Cu biface boundaries that suppresses the HER and favours the formation of mobile CO on Ag sites and its further reaction to a residual intermediate on Cu sites (Fig. 19b and c) [225].In the last decade, MOFs, and especially those formed by Cu metal ions or clusters, have served as active electrocatalyst in CO2RR but also as precursors for highly dispersed Cu over N-doped carbon catalysts [237,238]. For instance, Zhao et al. synthesized a porous Cu/C catalyst consisting of Cu2O and metal Cu particles embedded in a porous carbon matrix through the pyrolysis of Cu-based MOF (HKUST-1) (Fig. 20\n). A maximum FE for ethanol production of 34.8% at a potential of \u22120.5\u00a0V was obtained using this selective catalyst [239].The electrochemical reduction of CO2 to C3+ products remains a challenge. Among the C1\u2013C3 alcohols, n-propanol possesses the highest energy-mass density (30.94\u00a0kJ/g) and an octane number up to 118 [240,241]. These relevant properties make the efficient and green production of n-propanol a target point nowadays.The n-propanol production by a CO2 reduction reaction has been only observed in Cu-based catalysts. It has been hypothesized the pathways for n-propanol production as a result of the transformation of acetaldehyde into vinyl alcohol by a tautomerization equilibrium [227,242]. As observed in Fig. 21\n, the adjacent intermediates CO and *CH2 are inserted into *CH3\u2013CH in a similar way. Then, the reduction to propionaldehyde (CH3\u2013CH2\u2013CHO) and, consequently, to n-propanol takes place [20].Ren et al. reported a mechanistic study to n-propanol electrosynthesis via the design of nanocrystals agglomerates with unprecedented catalytic activity. The defect sites generated in the catalyst surface was responsible for the improved activity of the Cu nanocrystals and, hence, the n-propanol formation [227].In contrast with the previous sections, in which each product has been reviewed separately, here we give a general overview of the market aspects and techno-economic analysis carried out to date. The market size of the targeted products is critical from both a commercial perspective and CO2 utilization potential. In this vein, Fig. 22\n presents the approximate market size of the C2-products here considered [243\u2013249]. As shown, for example, propanol and acetic acid market sizes are very low in comparison with other potential CO2 utilization routes. For instance, other relevant CO2 utilization alternatives such as CaCO3 production, have a market sizes one hundred times higher than propanol and acetic acid (116 Mton/a market size) [250]. Nonetheless, their production from CO2 should not be dismissed as the production capacities may fit with the capacities of small-medium CO2 emitters. A fair example of these emitters is biogas upgrading, that could achieve the category of negative CO2 emissions technology if propanol or acetic acid are produced from the CO2 contained in this bio-resource. Oxalic acid is a very interesting C2 product from a utilization perspective since it is demanded by pharmaceuticals, textiles manufacturing, rare earth extraction, oil refining or metal processing industries. Nonetheless, worldwide oxalic acid production is above 0.230 Mton/a [249], which places oxalic acid in the same range than propanol and acetic acid. Therefore, this option cannot provide an alternative for CO2 utilization of large emitters. Ethylene and ethanol present market sizes considerably greater, and as explained in previous sections, the production of these chemicals is crucial for the end-products consumed by our society nowadays.On the other hand, Fig. 22 also presents the market prices for the products studied. From a market price point of view, ethanol, ethylene and propanol are the most attractive products to be achieved via CO2 electrocatalytic reduction. Although rapidly booming, CO2 utilization technologies are currently far from being competitive with traditional technologies, hence focusing on products with elevated market prices could be a good strategy to balance economic performance. Following this reasoning, the production of oxalic and acetic acid from CO2 would be difficult to become economically profitable with the current market prices in the short term. However, the growing projection for CO2 emission taxes along with the ongoing commitments to pursue a net-zero emissions opens the possibility for the viable production of all these chemicals using electrochemical routes.Concerning techno-economic works carried out to date, only a few studies are reported in literature. Economic assessment of CO2 electrocatalytic reduction are scarce probably because of the complexity of the analysis and the lack of reliable commercial scale data. In this sense, this criticism aims to be an encouraging call for experts in techno-economic and profitability evaluations. Focusing on the works performed, Table 1 gathers the most important characteristics and findings of the techno-economic analysis available in the literature. Perhaps the most impacting study carried out to date was performed by Jouny et al. [243] In this work, authors present the end-of-life net present value (NPV) of a 100 ton/day plant for various CO2 reduced products: propanol, formic acid, carbon monoxide, ethanol, ethylene and methanol. From the products targeted in their study, here we focus on propanol, ethanol and ethylene. Under the current conditions, authors conclude that the production of these C2-products from CO2 is not profitable. Nevertheless, the result could be reverted if reasonable electrocatalytic performance benchmarks are achieved. In agreement with these authors, these performances must be 300\u00a0mA\u00a0cm\u22122 and 0.5\u00a0V overpotential at 70% FE. Another very important work was presented by Kibria et al. [244]. In their work, a techno-economic analysis was carried out with the aim of ensuring the economic viability of the process by the identification of profitable CO2 electrocatalytic reduced products as well as the performance targets that should be met to achieve it. As in the previous discussed work, this study includes parameters such as current density, energy and faradaic efficiencies, and stability. The most interesting point of this work is the prices predicted for ethanol and ethylene to achieve profitable scenarios. According to these authors analysis, ethanol price should achieve 1400 $/ton to get a net present value equal to zero (revenues equal to costs at the end of the plant life). In comparison with the current market price for ethanol (around 1000 $/ton, see Fig. 22), the difference is 40%. This fact shows the great technological challenge that we face to make electrocatalysis economically attractive in the context of direct CO2 conversion to C2 products. In the same study, a price of 1700 $/ton is predicted for ethylene to be a profitable alternative. Again, in comparison with current market prices (approximately 1300 $/ton), the threshold needed is remarkably higher. Interestingly, the authors present a sensitivity analysis based on a tornado plot, revealing that current density is the main parameter with room for improvement for all the products considered.The transition towards sustainable modern societies relies on the implementation of disruptive technologies for CO2 utilization. Among these technologies, electrocatalytic CO2 conversion to added value products will play a major role given its advantages compared to traditional thermal catalysis. In particular, the fact the electrochemical reactions can take place at very mild conditions represents a major bright side of this approach. So far, most the academic works are focused on the conversion of CO2 to C1 products since the reaction is \u201celectrochemically cheaper\u201d. Nevertheless, the production of C2 and C2+ is more appealing for the chemical industry given the broad market applications of these advanced products. Herein, challenges on the catalysts design to achieve high faradaic efficiencies and end-product selectivity are identified as the main bottlenecks for the electrochemical CO2 conversion to C2 and C2+ compounds. Among the different studied heterogenous catalysts, Cu-based formulations outstand showcasing the best activity/selectivity balance. However, in many cases their performance is still below the threshold to be considered as commercially viable options. In this sense, new formulations of advanced materials including multi-alloy systems, N-doped catalysts, optimized porous MOFs structures among many others are under development showing promising results. In this sense, our review provides an end-product guided perspective of the progress within catalyst design to deliver C2 and C2+ from electrochemical CO2 conversion routes. Beyond offering an illustrative analysis for experts and a straightforward starting point to newcomers in the field, our work would also like to emphasize the need to strengthen the research efforts within the catalysis and energy communities in the conversion of CO2 to advanced products. More precisely we advocate for a C2 and C2+ production using a new generation of advanced electrocatalytic materials.Along with the catalysts design and electrochemical processes considerations, market studies are crucial to ascertain the viability of the CO2 electrochemical conversion routes. Our analysis indicates that products with a broad market such as ethanol, ethylene and propanol are worth exploring in the short term while electrochemical production of other key products such as oxalic and acetic acids are not yet economically appealing. Still, progress on catalyst design pushing forward products selectivity and overall CO2 conversion will certainly help to make these options also viable. In any case, the production of advanced products using CO2 as carbon pool and electrocatalysts design seem to share a common destiny whose convergence will result in a remarkable contribution to decarbonise the chemical industry, opening new routes for the desired low-carbon future.The authors equally contributed to 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.Financial support for this work was gathered from Spanish Ministry of Science and Spanish Ministry of Science and Innovation through the projects RTI2018-096294-B-C33 and RYC2018-024387-I. This work was also partially funded by the University of Seville via the VI PPIT grant scheme for talented researchers. Support from CO2Chem UK through the EPSRC grant EP/P026435/1 is also acknowledged. Financial support from the European Commission through the H2020-MSCA-RISE-2020 BIOALL project (Grant Agreement: 101008058) is also acknowledge.", "descript": "\n                  The energy crisis caused by the incessant growth in global energy demand joint to its associated greenhouse emissions motivates the urgent need to control and mitigate atmospheric CO2 levels. Leveraging CO2 as carbon pool to produce value-added products represents a cornerstone of the circular economy. Among the CO2 utilization strategies, electrochemical reduction of CO2 conversion to produce fuels and chemicals is booming due to its versatility and end-product flexibility. Herein most of the studies focused on C1 products although C2 and C2+ compounds are chemically and economically more appealing targets requiring advanced catalytic materials. Still, despite the complex pathways for C2+ products formation, their multiple and assorted applications have motivated the search of suitable electrocatalysts. In this review, we gather and analyse in a comprehensive manner the progress made regarding C2+ products considering not only the catalyst design and the electrochemistry features but also techno-economic aspects in order to envisage the most profitable scenarios. This state-of-the-art analysis showcases that electrochemical reduction of CO2 to C2 products will play a key role in the decarbonisation of the chemical industry paving the way towards a low-carbon future.\n               "}
{"full_text": "Efficient and sustainable electrocatalysts are needed to increase the competitivity of hydrogen energy over conventional, polluting power generation. Especially in acidic environments, such as proton exchange membrane fuel cells (PEMFC), where the stability of the catalyst is critical, material selection and design is very important\u00a0[1]. Since Pt is the most active but also expensive catalyst, efforts are made in order to find a compromise between cost and performance of the catalyst\u00a0[2]. Alternatively, water electrolysis and fuel cells can operate in alkaline media as a less aggressive environment for the catalyst\u00a0[3].Apart from the selection of a catalyst material, the structure of catalysts is optimised in order to maximise their surface area while minimising the amount of material\u00a0[4]. This can be achieved by synthesising materials with high surface-to-volume ratios such as porous structures, nanoparticles, or nanotubes. Thin films, in spite of their lower surface area, typically show higher specific electrochemical activity compared to nanoparticles\u00a0[5].Ni is a good candidate to partially replace Pt via alloying. Several features related to the Gibbs free energy, electronegativity and lattice mismatch point to a good electrocatalytic activity of Ni-Pt alloys towards HER\u00a0[6]. Ni is commonly used for the protection of electrical contacts, and electrodeposited Ni coatings are often used for corrosion protection\u00a0[7,8]. For example, electrodeposited Ni-based Ni-Co-B coatings have shown to improve the corrosion resistance in a fuel cell environment compared to uncoated stainless steel and Al\u00a06061 alloy\u00a0[9]. Many Ni-based compounds have been investigated for HER, however almost exclusively in alkaline media\u00a0[10\u201313].A critical issue for Ni-rich alloys is hydrogen embrittlement, which can occur especially in environments containing H2S, and even during the hydrogen evolution reaction\u00a0[14]. Electrodeposited Ni films are especially susceptible to hydrogen embrittlement due to interstitial, monoatomic hydrogen dissolved in the crystal lattice\u00a0[15]. However, this form of hydrogen embrittlement is reversible\u00a0[16].Single-phase mesoporous Ni-Pt thin films exploit both the increase of surface area provided by the porous structure, as well as a reduced usage of Pt via alloying with Ni. In a previous study, Ni-rich films were produced by electrodeposition and thoroughly characterised, showing excellent performance at HER in 0.5\u00a0M H2SO4\u00a0[17]. A good stability over 200 cycles of HER was reported for Ni-Pt films with different compositions, ranging from 99\u00a0at% Ni (1\u00a0at% Pt) to 61\u00a0at% Ni (39\u00a0at% Pt). Yet, their electrochemical behaviour in acidic and alkaline media remains to be explored. In view of their potential integration in PEMFCs, their long-term stability must be further assessed by corrosion studies to determine if the mesoporous Ni-Pt thin films can be safely used in acidic media, or if an alkaline electrolyte is more favourable for long-term stability instead.Also, the effect of the composition on the electrochemical behaviour must be investigated. The selective dissolution in acidic media, also known as leaching, is a common issue for Pt alloyed with a transition metal\u00a0[18], but may be hindered in single-phase alloys, where the noble metal can protect the transition metal atoms if its content is sufficiently high. Other strategies with the intent to minimise the leaching of Ni include the synthesis of core-shell structures with a Pt-rich surface protecting the Ni-rich core\u00a0[19]. The corrosion resistance of Ni-Pt alloys is expected to increase with the Pt content. However, any effect of the mesoporosity on the electrochemical behaviour needs to be investigated. For instance, the porosity may lead to crevice corrosion due to differential aeration inside the pores\u00a0[20].This work focusses on the effect of composition (Ni and Pt contents) and mesoporosity on the observed electrochemical behaviour, corrosion properties, and long-term stability of Ni-Pt alloy thin films. In order to assess the electrochemical properties of Ni-rich Ni-Pt alloy thin films, the behaviour is studied both in acidic (0.5\u00a0M H2SO4) and alkaline (1\u00a0M NaOH) electrolytes, two media which are commonly used to study materials for acidic and alkaline fuel cells. The studies were conducted by cyclic voltammetry (CV) and with the use of the electrochemical microcell technique (EMT), which allows for multiple measurements on a single sample, with the aim to identify the oxidation and reduction reactions occurring. In addition, electrochemical impedance spectroscopy (EIS) was used to provide an understanding of the corrosion resistance of the Ni-Pt films in acidic media. Althoughcathodic potentials, which are usually not critical in terms of corrosion, are applied during HER, the electrode potential can rise well into the range of oxidising potentials when not in operation\u00a0[21].All electrochemical tests were performed on single-phase, nanocrystalline Ni-Pt thin films with varying composition, both with and without mesoporosity, which had been structurally characterised elsewhere\u00a0[17,22]. A series of dense and mesoporous Ni-Pt films, ranging from 61\u00a0at% to 99\u00a0at% Ni, are investigated in the present study (Table\u00a01\n).The samples consisted of a Si wafer sputter-deposited with a Ti adhesion layer and a Cu seed layer, onto which Ni-Pt films were grown (Fig.\u00a01\n). The films were potentiostatically deposited from an aqueous electrolyte, using a micelle-forming surfactant in the case of the mesoporous films\u00a0[17]. TEM analyses were performed on a Jeol JEM-2011 at 200\u00a0kV acceleration voltage. Sample preparation was done by grinding, polishing and Ar ion milling (for mesoporous Ni92Pt8) as well as by cutting with a focused ion beam (FIB, for dense Ni91Pt9).An Autolab 302N potentiostat/galvanostat was used to perform all electrochemical tests. Initial CVs were performed in a conventional three-electrode set-up using an Ag|AgCl reference electrode (RE) and a platinum spiral as counter electrode (CE). HER in 0.5\u00a0M H2SO4 was investigated by linear sweep voltammetry (LSV), sweeping the potential from \u20130.15\u00a0V to \u20130.5\u00a0V vs. Ag|AgCl in an identical set-up but with the use of a graphite rod as CE. For evaluation of the long-term stability at HER in 0.5\u00a0M H2SO4, a 24\u00a0h long electrolysis was performed at a geometric current density of \u201310\u00a0mA/cm2. Results obtained from these methods, which deal with the characterisation of the whole films (as opposed to local measurements) are hereafter denoted as global scale tests.The EMT was used to study the electrochemical behaviour of the Ni-Pt thin films in detail. Contrarily to its usual scope to study single inclusions, grains, phases etc.\u00a0[23], and due to the homogeneous, single-phase and nanocrystalline character of the Ni-Pt thin films, this method was used here to study the overall representative behaviour of the films locally, and, moreover, allowed to perform multiple measurements on the same sample. Results from this method are further denoted as local technique, local scale or EMT.The set-up consists of an optical microscope, into which the electrochemical cell is mounted, and the sample functioning as working electrode (WE) is placed on a conductive sample holder, all placed inside a Faraday cage (Fig.\u00a02\n). The samples\u2019 surfaces were electrically connected to the sample holder via copper tape. The electrochemical microcell is filled with the electrolyte, a Pt wire is used as CE and an Ag|AgCl electrode as RE. The electrolyte is connected to the WE through a glass microcapillary with a tip diameter of 50\u2013200\u00a0\u00b5m. The tip is covered with a silicone gasket to protect the capillary and facilitate the contact with the WE\u00a0[24]. Using the objectives of the optical microscope, the area to be measured was checked before each measurement in order to confirm that it was homogeneous and free of defects or surface pollution.The EMT was used in the following studies:\n\n\u2022\nCVs\n\n\u2022\nin NaOH from \u20130.5\u00a0V to 0.6\u00a0V and 1.5\u00a0V vs. Ag|AgCl\n\n\n\u2022\nin H2SO4 from \u20130.5\u00a0V to 0.5\u00a0V vs. Ag|AgCl\n\n\n\n\n\n\u2022\nEIS in H2SO4\n\n\n\nCVs\n\n\u2022\nin NaOH from \u20130.5\u00a0V to 0.6\u00a0V and 1.5\u00a0V vs. Ag|AgCl\n\n\n\u2022\nin H2SO4 from \u20130.5\u00a0V to 0.5\u00a0V vs. Ag|AgCl\n\n\nin NaOH from \u20130.5\u00a0V to 0.6\u00a0V and 1.5\u00a0V vs. Ag|AgClin H2SO4 from \u20130.5\u00a0V to 0.5\u00a0V vs. Ag|AgClEIS in H2SO4\nEIS in NaOH did not yield any valid data since the currents were below the detection limit. EIS was conducted in a frequency range from 100\u00a0kHz to 3\u00a0mHz, after an equilibration time of 5\u00a0min at open circuit potential (OCP), with an amplitude of 10\u00a0mV.The thin films were analysed before and after electrochemical measurements by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX) on a Zeiss Merlin electron microscope to study the effects of exposure to acidic and alkaline media on the microstructure and composition. Imaging was performed using the InLens detector with an acceleration voltage of 1\u20132\u00a0kV, while an acceleration voltage of 20\u00a0kV was used for EDX.For quantification of dissolution or leaching of the mesoporous Pt-Ni thin films in sulfuric acid, the surfaces of Ni95Pt5, Ni92Pt8, and Ni84Pt16 films were immersed in 0.5\u00a0M H2SO4 at OCP for 10\u00a0min and the solution was then analysed by inductively coupled plasma mass spectrometry (ICP-MS) using an Agilent 7500ce spectrometer to determine the amount of dissolved Ni and Pt.The TEM cross-sections of the mesoporous Ni92Pt8 (Fig.\u00a03\na) and dense Ni91Pt9 (Fig.\u00a03b) films are shown for comparison. While the latter shows its fully dense appearance, a homogeneous distribution of pores can be observed in the mesoporous counterpart. Film thickness lies between 200\u00a0nm and 300\u00a0nm, which holds for all compositions.At high resolution, the nanocrystallinity of the films is revealed (Fig.\u00a03c) and further confirmed by the selected area electron diffraction (SAED) pattern, where the existence of arbitrary crystal orientations of the single-phase fcc Ni-Pt alloy is shown (Fig.\u00a03d).The performance of the Ni-rich Ni-Pt thin films at HER in 0.5\u00a0M H2SO4 (Eq.\u00a0(1)) shows that hydrogen production is highly reproducible, with very little changes up to 200 sweeps. Representatively for all compositions, the Ni84Pt16 alloy film shows the HER during LSV, reaching current densities of up to 150\u00a0mA/cm2 at \u20130.3\u00a0V vs. reversible hydrogen electrode (RHE, Fig.\u00a04\na). A certain deviation between individual sweeps is always obtained due to the temporary blocking of the surface by hydrogen bubbles being formed\u00a0[17]. Comparable activity at HER in the same media has been reported for mesoporous Pt-rich Fe-Pt films\u00a0[25]. Thus, the Ni-Pt films reported here are able to achieve the same activity at HER with a significantly lower Pt content.\n\n(1)\n\n\n2\n\n\nH\n\n+\n\n+\n2\n\n\ne\n\n\u2212\n\n\n\u27f6\nNi-Pt\n\n\nH\n2\n\n\n\n\n\nLong-term electrolysis experiments indicate that the recorded potential changes over time. Specifically, at constant operation at \u201310\u00a0mA/cm2, it is observed that an increasingly cathodic potential is needed to maintain the HER current (Fig.\u00a04b). However, irrespective of composition or the presence of porosity in the alloy, this potential stabilises over time and indicates that no further degradation in performance is expected. The higher the Pt content in the films, the lower is the resulting change in potential. An increase in the overpotential required for HER and oxygen evolution reaction (OER) at constant current density in macroporous electrodeposited Ni has also been observed in alkaline media\u00a0[26]. Contrarily, the constant operation of Ni foam at HER in 0.5\u00a0M H2SO4 at a fixed potential for 15\u00a0h revealed a shift of the current density towards more negative values, which was related to a surface roughening of the Ni foam\u00a0[27].The microstructure of the films after 200 sweeps of HER appears unaltered compared to the as-deposited state (Fig.\u00a05\na, b). Upon closer examination, the size of the mesopores appears slightly bigger after HER experiments, suggesting the removal of material such as oxides, hydroxides or other contaminants from the surface. A comparison of EDX spectra before and after HER did not reveal significant differences in the Ni/Pt ratios.After the 24\u00a0h long electrolysis, cracking was observed in dense and mesoporous Ni-Pt films (Fig.\u00a05c). In all cases, a slight decrease in Ni content after the electrolysis was observed, in addition to particles containing nickel and sulfur formed on the surface (possibly nickel sulfate), visible as white particles in Fig.\u00a05c. Both these effects are likely to be the origin of the potential increase observed during electrolysis.Although the Ni-Pt films are targeted for application in acidic media, application in alkaline media is a possible alternative. Since alkaline media is far less aggressive, it allows for non-destructive electrochemical characterisation, and for validation of the set-up and measurement parameters before the behaviour of the Ni-Pt films in acidic media is studied. As aforementioned, a period of inoperation of electrolysers may transiently cause the application of oxidising potentials to the cathode and it is therefore important to evaluate the electrochemical response of the catalyst in the anodic range\u00a0[21].In the CVs obtained from both global scale and local measurements in NaOH, a redox reaction is observed within the potential window from \u20130.5\u00a0V and 0.6\u00a0V (Fig.\u00a06\na). The oxidation and reduction peaks correspond with the redox reaction between Ni(OH)2 and NiOOH (Eq.\u00a0(2))\u00a0[28].\n\n(2)\n\n\nNi\n\n\n(\nOH\n)\n\n2\n\n+\n\n\nOH\n\n\u2212\n\n\u21cc\nNiOOH\n+\n\nH\n2\n\nO\n+\n\n\ne\n\n\u2212\n\n\n\n\n\nBoth methods reveal the same redox reaction at similar current densities, confirming that\u2014due to the homogeneity of the Ni-Pt thin films\u2014the EMT technique is able to capture the films\u2019 global electrochemical properties. Using the local technique, the oxidation and reduction peaks are wider with respect to the global technique, and the peaks are more separated. This observation is made for all film compositions, and applies especially at higher scanning speeds. This is probably due to the very small volume of the electrochemical microcell, where the availability of species such as \n\n\nOH\n\n\u2212\n\n is reduced and the kinetics are thus slowed and limited by the diffusion of those species through the capillary as a very constrained path. This effect is more significant at high scanning speeds, where the depletion of species available at the electrode is even faster.At low scan rates with the local technique (i.e. 10\u00a0mV/s and 20\u00a0mV/s), progressive widening of the peaks makes it possible to discriminate between two oxidation peaks which were not resolved on the global scale (Fig.\u00a06b). The split into two oxidation peaks can be explained by both the oxidation of \u03b1- and \u03b2-Ni(OH)2 at different potentials\u00a0[29] as well as the formation of NiOOH with different crystallographic structures\u00a0[30]. Indeed, a shoulder to the right of the main peak was already observed using EMT at 100\u00a0mV/s (Fig.\u00a06a).The behaviour of the Ni-Pt films was studied in the potential range between \u22120.5\u00a0V and 1.5\u00a0V vs Ag|AgCl. When the anodic potential was as high as 1.5\u00a0V, a large oxidation peak centred around 1\u00a0V was observed for all samples (Fig.\u00a07\n). After this large oxidation peak was recorded, no further major oxidation or reduction currents were observed within the entire potential window in the subsequent cycles. However, microscopic analyses confirmed that the Ni-Pt film was still intact and no dissolution had taken place. Therefore, the large oxidation peak observed in the first cycle can be attributed to a passivation of the film\u2019s surface which could not be reversed within the applied potential window and may be related to an irreversible oxidation of Pt \u00a0[31,32]. In order to avoid the effect of passivation on the redox reaction Ni(OH)2\n\n\u21cc\n NiOOH, the anodic limit was set to 0.6\u00a0V, resulting in a stabilisation of the redox reaction (Fig.\u00a08\n).The characteristics of the redox reaction (Eq.\u00a0(2)) resolved by the CVs show dependencies on the film composition, porosity, and the scan rate. Although the redox reaction is determined to involve the formation of Ni hydroxides/oxyhydroxides, the reaction is enhanced by the incorporation of higher amounts of Pt in the films (Fig.\u00a09\n). This may be related to the fact that Pt as an excellent electrocatalyst is able to enhance electrochemical reactions on neighbouring Ni atoms due to synergistic effects. In this way, the reaction enhances while increasing the Pt content while the amount of Ni atoms available on the surface decreases. As a result, there should be a certain Pt content for which the maximum effect is obtained.In absence of porosity, the measured currents are significantly lower. This difference is not as high as might be expected from the difference in surface area between mesoporous and dense films, but correlates well with the fact that the ECSA determined in a previous study did not reveal significant differences between the different film morphologies\u00a0[17].For the mesoporous Ni-Pt films with Ni contents of 84% and lower (i.e. 61%, 76% and 84% Ni), a square root dependency is appreciated between the peak oxidation (and reduction) current density and the scan rate (Fig.\u00a010\n). The peak current densities were taken from the peaks corresponding to Eq.\u00a0(2) at approx. 0.4\u00a0V (oxidation peak) and 0.3\u00a0V (reduction peak). In those cases where the relationship is linear, the redox reaction is diffusion-controlled by the diffusion of \n\n\nOH\n\n\u2212\n\n in solution (cf. Eq.\u00a0(2)). The highest activity is observed for 76%\u00a0Ni, the films containing 84% and 61%\u00a0Ni follow with similar activities. This observation consolidates the assumption that Pt acts as an electrocatalyst for the reaction, and thus there is a certain Pt content at which the electrochemical activity of the surface towards the observed redox reaction between Ni(OH)2 and NiOOH is the most active.For higher Ni contents, the trend is not linear. Above a certain scan rate of about 80\u2013100\u00a0mV/s, the current stagnates, indicating that the kinetics of the redox reaction becomes limited by another factor. The peak reduction currents are generally lower than the corresponding oxidation currents due to the generally wider reduction peaks (cf. Fig.\u00a09). Nevertheless, the trends observed on the reduction part of the reaction are equal to those on the oxidation.The observed characteristics of the Ni-Pt films may be exploited in application as an electrochemical supercapacitor. Although the current densities are not comparable to those reported for Ni-based electrochemical supercapacitors\u00a0[33,34], an appropriate anodic oxidation treatment may achieve superior performance, taking advantage of the enhancement provided by the addition of Pt.Due to the high Ni contents in all films, a dissolution of Ni takes place in sulfuric acid when anodic potentials are applied. In the CVs of the dense films, this dissolution is predominant in the first anodic sweep (Fig.\u00a011\n). A similar observation had been made during LSV experiments of electrodeposited Ni-P\u00a0[21]. The observed oxidation is most likely not exclusively the dissolution of Ni (Eq.\u00a0(3)), which is attributed to the highest oxidation peak for all compositions, but also that of the underlying Cu, which has its standard potential in this potential range and may thus be assigned to the smaller oxidation peak which appears as a shoulder at 0.48\u00a0V for 93%\u00a0Ni and at 0.66\u00a0V for 98%\u00a0Ni\u00a0[35]. The third, most anodic oxidation peak can be referred to the oxidation or dissolution of Pt\u00a0[35]. At the very beginning of the anodic sweep, negative currents related to proton reduction (i.e. HER) are clearly observed, suggesting that hydrogen adatoms form on the surface and are subsequently oxidised (Eq.\u00a0(4)), thus contributing to the oxidation current as well, especially at very low anodic potentials.\n\n(3)\n\n\nNi\n\u27f6\n\n\nNi\n\n\n2\n+\n\n\n+\n2\n\n\ne\n\n\u2212\n\n\n\n\n\n\n\n(4)\n\n\n\nH\nad\n\n\u27f6\n\n\nH\n\n+\n\n+\n\n\ne\n\n\u2212\n\n\n\n\n\nInterestingly, the oxidation waves shift to more anodic potentials with increasing Ni content of the films. Hence, the alloying of higher amounts of Pt with Ni increases the surface activity for electrochemical reactions, as seen before, but also accelerates the dissolution of the material in acidic media under anodic polarisation. For the mesoporous films, the dissolution processes were faster and immediately exposed the Cu seed layer which was dissolved simultaneously. This prevented a detailed analysis of the anodic processes taking place in sulfuric acid. In any case, an anodic polarisation of the Ni-Pt films in sulfuric acid leads to dissolution and failure of the films and must be avoided in application.\nFig.\u00a012\n shows the Nyquist plots for both mesoporous and dense Ni-Pt films after EIS at OCP in acidic media. In general, the impedance increases with the Pt content.The fitting for the EIS spectra was done by simulating an electrical equivalent circuit with the solution resistance Rs, in series with a parallel circuit of the charge-transfer resistance Rct and the double-layer capacitance Cdl, whose behaviour is modelled by a constant phase element (CPE), and represents a simplified Randles circuit (Fig.\u00a013\n)\u00a0[36]. This model was used for the fitting of the spectra acquired in 0.5\u00a0M H2SO4 at OCP of all mesoporous and dense Ni-Pt films.The results of the fitting show that, expectably, the solution resistance is similar in all cases, except for Ni98Pt2 (Table\u00a02\n). The determined charge-transfer resistance is generally higher for the mesoporous films, indicating that those are more resistant to corrosion than the dense films. This is a counter-intuitive result, suggesting that an increase in the surface area does not lower the corrosion resistance of the Ni-Pt thin films. The highest charge-transfer resistances is found for mesoporous Ni76Pt24 and Ni61Pt39. The double-layer capacitance, which depends mainly on the surface area and the surface composition, indicates an increase with the Pt content and is generally higher for mesoporous the films, although this difference is relatively low and may be related to the minor differences in ECSA observed\u00a0[17].The Bode plots show that the general behaviour of the mesoporous Ni-Pt films is very similar, the main differences lying in the magnitude of both impedance and phase (Fig.\u00a014\n).At high frequencies, the impedance follows a composition dependence, increasing with Ni content. The frequency dependence of the phase angle shows a shift of the maximum phase angle towards lower frequencies with the Pt content, indicating an increase in capacitance.Metiko\u0161-Hukovi\u0107 et\u00a0al. compared sputter-deposited nanocrystalline Ni with electrodeposited Ni by EIS during HER in alkaline media, finding that the electrodeposited Ni films exhibited higher Faradaic resistance and significantly lower double-layer capacitance in the order of 1\u00a0\u00b5F/cm2\u00a0[37]. Krstaji\u0107 et\u00a0al. investigated the impedance of Ni at HER in NaOH and found that the impedance decreased when the HER potential was made more negative, reaching impedance values much lower than those reported here, and thus indicate that the impedance at potentials where HER takes place is significantly lower than at corrosion potential. The observed Cdl was in the order of 100\u00a0\u00b5F/cm2\u00a0[38]. Perez et\u00a0al. conducted a similar study when investigating the oxygen reduction reaction (ORR) on Pt both in NaOH and H2SO4. A minimum of the impedance at a certain ORR potential was observed, again showing that impedance is lower when the material is employed in its function as a catalyst\u00a0[39]. Juskowiak-Brenska et\u00a0al. found that for electrodeposited Ni coatings the charge-transfer resistance in acidic media decreased significantly when the coating thickness was decreased, yielding 20\u00a0\u03a9\u00a0cm2 for thickness of 1\u00a0\u00b5m\u00a0[40]. In comparison, the mesoporous Ni-Pt films presented here show superior Rct considering their thickness of 200\u2013300\u00a0nm.SEM/EDX analyses after EIS show that the Ni/Pt ratio decreased, i.e. a leaching of Ni has taken place in all Ni-Pt films, and indicate that the electrochemical process observed in EIS is dominated by the dissolution of Ni. A good stability of the Ni-Pt films in absence of external polarisation can therefore not be guaranteed, i.e. the Ni-Pt films may only be able to serve reliably at HER in 0.5\u00a0M H2SO4 as long as it is cathodically polarised, i.e. under cathodic protection.Indeed, a parallel experiment which consisted in an incubation of the mesoporous Ni-Pt films in 0.5\u00a0M H2SO4 resulted in the leaching of Ni, which was less pronounced when the Pt content was higher. For Ni95Pt5, the leaching of Ni was 23\u00a0\u00b1\u00a03% in mass with respect to the total mass of the Ni-Pt film, while this value amounted to 17\u00a0\u00b1\u00a03% for both Ni92Pt8 and Ni84Pt16. Alia et\u00a0al. found that Ni-Pt nanowires did not present any leaching of Ni when the Pt content was above 75% in mass, corresponding to about 47\u00a0at%\u00a0[41].The investigated electrodeposited Ni-Pt is a multifunctional alloy which may serve both in alkaline and acidic environments, provided that a reducing potential is applied when working in acidic conditions.Due to the observed reversible redox reaction Ni(OH)2\n\n\u21cc\n NiOOH in NaOH, the Ni-Pt thin films with Ni contents of 84% and lower may be used in the charge and discharge of batteries as well as supercapacitors, where electrical charge can be stored in the form of chemical bonds, and very high currents can be retrieved\u00a0[33,34,42].On the other hand, it was observed that the HER in 0.5\u00a0M H2SO4 is stable and reproducible up to 200 LSV cycles, however, anodic polarisation as well as an absence of polarisation at OCP will lead to leaching of Ni into the sulfuric acid. In long-term HER operation at \u201310\u00a0mA/cm2, an increase in potential is recorded, although stabilising over time. The higher the Pt content of the alloy, the lower was the resulting increase in potential. Interestingly, the mesoporous Ni-Pt films seem more resistant to corrosion, the corrosion resistance increasing roughly with the Pt content. The films showing the highest resistance to corrosion in sulfuric acid, and at the same time a very high activity at the redox reaction Ni(OH)2\n\n\u21cc\n NiOOH, are mesoporous Ni76Pt24 and Ni61Pt39.Since in an application such as a PEMFC, the leaching of Ni will most likely lead to a degradation in the fuel cell performance, two approaches may be used to overcome this phenomenon: using a higher Pt-content alloy to minimise the dealloying effect, or, secondly, a dual-layer structure of the catalyst with a Pt-Ni alloy catalyst layer stacked onto a Pt catalyst layer\u00a0[43].\nKonrad Eiler: Conceptualization, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Halina Krawiec: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision, Funding acquisition. Iryna Kozina: Conceptualization, Validation, Resources. Jordi Sort: Resources, Writing - review & editing, Supervision, Funding acquisition. Eva Pellicer: Conceptualization, Methodology, Validation, Resources, 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 has received funding from the European Union\u2019s 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 Ministerio de Econom\u00eda, Industria y Competitividad (MAT 2017-86357-C3-1-R and associated FEDER Project).", "descript": "\n                  Reliability and long-term performance are the key features of modern energy storage and conversion devices. The long-term stability depends entirely on the electrochemical and corrosion properties of device components. Single-phase mesoporous Ni-rich Ni-Pt thin films have shown to be a promising electrocatalyst for hydrogen evolution reaction (HER) and therefore with potential application in fuel cells, water electrolysers or similar devices. The HER activity of the mesoporous Ni-Pt films is reliable and stable in 0.5\u00a0M H2SO4 up to 200 linear sweep voltammetry cycles, however leaching of Ni occurs in absence of cathodic polarisation. Long-term electrolysis measurements at a HER current of \u201310\u00a0mA/cm2 reveal an increase in potential over time, which is minimised when the Pt content is increased. In 1\u00a0M NaOH, the material is stable up to an applied anodic limit of 1.5\u00a0V vs. Ag|AgCl although surface passivation takes place at 1.0\u00a0V. If the anodic limit does not exceed 0.6\u00a0V vs. Ag|AgCl, a fully reversible redox reaction is observed by cyclic voltammetry, with redox charges increasing with Pt content and scan speed. In addition, significantly higher current densities are recorded for mesoporous films compared to dense counterparts. This charge/discharge behaviour of the redox reaction indicates that the mesoporous Ni-Pt films may as well be used as an electrochemical supercapacitor. As a HER catalyst, the material is safely applicable in alkaline media.\n               "}
{"full_text": "In recent years, the contradiction between environment and human development has intensified with the acceleration of the pace of human development, thus causing various problems [1\u20133]. Especially in the manufacturing industry, such as steel mill, metallurgic industry, electron plating, printing, leather and pharmaceuticals, various toxic organic compounds and heavy metal ions are discharged from these industries. They have caused serious environmental pollution and destroyed the aquatic environment [4\u20136]. Colored dyes in natural water bodies, on the one hand, affect the photosynthesis of aquatic organisms and, on the other hand, destroy human senses of scenery. But it has more serious potential threats. Toxic organic compounds have the characteristic of a difficult self-decomposition in natural environments [7,8]. For example, 4-nitrophenol will accumulate in water ecological food chains and, eventually, enter the human body. 4-Nitrophenol and its derivatives can also cause damage to the human central nervous system, liver, kidney and blood as well [9,10]. Not only organic pollutants, but also hexavalent chromium ions in water cause serious harm to the environment and the human body. When hexavalent chromium accumulates in the human body, it will cause anemia, nephritis, neuritis and other diseases. After long-term contact, hexavalent chromium will cause lung cancer and nasopharyngeal carcinoma [11,12]. Therefore, removing toxic organic compounds and heavy metal chromium ions from water is an inevitable and important task for researchers.In the past decades, researchers have studied the reduction/degradation of toxic organic compounds and heavy metals by various methods such as photocatalytic degradation, desorption, membrane flocculation and filtration [13,14]. Among them, bimetallic catalysts have been investigated by researchers due to their high efficiency and unique characteristics. In Han's experiment, the Cu\u2013Fe bimetal was used and tested for RhB [15]. Wen et\u00a0al. also prepared iron and cerium bimetal oxides and used them to oxidize arsenite [16]. Moghadam et\u00a0al. also prepared Fe/Zn bimetal nanoparticles to treat petroleum wastewater [17]. \u0160uligoj et\u00a0al. prepared silica-supported Cu\u2013Mn and tested it for dye degradation [18]. Our research group also synthesized Cu based catalysts for which a strong catalytic reduction performance was observed towards different pollutants [19\u201323]. Based on the above consideration, the noble MoSrOS bimetal catalyst was synthesized by a simple method.In this study, a novel wool-coiled molybdenum-based bimetallic sulfur oxide MoSrOS catalyst with a varying amount of Sr was synthesized. The catalysts characterizations were performed by using XRD, XPS, SEM, FTIR, DRS, EIS, and S\nBET. The catalytic reduction efficiencies were also tested with MO, 4-NP, MB, RhB, and Cr(VI) pollutants. It is expected that the wool-coiled molybdenum-based bimetallic oxysulfide MoSrOS catalyst could be used in wastewater treatment.Under magnetic stirring, 20\u00a0mmol of ammonium molybdate ((NH4)6Mo7O24\u00b74H2O) and 10\u00a0mmol of strontium nitrate (Sr(NO3)2) were added into 800\u00a0mL distilled water. Then, after 20\u00a0min, 40\u00a0mmol thioacetamide (CH3CSNH2) was dropped into the mixture solution and reacted for 30\u00a0min. The mixture solution was heated to 90\u00a0\u00b0C and reacted for 2\u00a0h. In order to determine the effect of strontium nitrate on the catalyst activity, 4.0, 10.0, 20.0, and 40\u00a0mmol of strontium nitrate were added to the preparation process. The final solutions are abbreviated as MoSrOS-1, MoSrOS-2, MoSrOS-3 and MoSrOS-4 respectively. The resulting samples were washed, and finally dried by a rotary evaporator.The MoSrOS catalysts were characterized by a Rigaku X-ray diffractometer. The PHI5700 photoelectron spectrometer was used for an XPS analysis. Ultraviolet\u2013visible diffuse reflectance and absorption spectra were performed by the ultraviolet\u2013visible spectrophotometer (TU1901). The field emission scanning electron microscopy (HITACHI SU-8010 microscope) was used for a morphology analysis. The N2 adsorption\u2013desorption was done by an ASAP 2020 porosity and specific surface area analyzer. The electrochemical impedance (EIS) measurement was checked by a SP-300 Biologic Science together with a three electrodes system of platinum plate, Ag/AgCl/KCl, and glassy carbon electrodes in the 0.1M KCl electrolyte solution.MoSrOS catalysts catalytic reduction of pollution was performed as follows. Firstly, 15\u00a0mg of sodium borohydride was added in 20\u00a0ppm\u00a0MB aqueous solution. Subsequently, 10\u00a0mg of the catalyst was added into the MB aqueous solution. By specified intervals in time, 2\u00a0mL mixture was taken and analyzed by the ultraviolet spectrophotometer. Other pollutants were tested according to the same procedure and time intervals adjusted, depending on the activity.\nFig.\u00a01\na shows the survey XPS spectrum of MoSrOS-2. The Mo, Sr, S, O, and C elements were obverted in the spectrum, the C1s peak belongs to a trace amount of foreign carbon. Fig.\u00a01b indicates the high resolution Mo 3d XPS spectrum of MoSrOS-2. The peaks for Mo 3d5/2 and Mo 3d3/2 with binding energies of 232.6 and 235.8\u00a0eV, respectively, indicate the presence of a Mo6+ state in the MoSrOS-2 solution [24]. The peaks for Mo 3d5/2 and Mo 3d3/2\u00a0at 230.3 and 233.5\u00a0eV, respectively, are attributed to Mo4+ [24]. Fig.\u00a01c also illustrates the XPS spectra of Sr 3d in the MoSrOS-2 solution. The peaks for Sr 3d5/2 and Sr 3d3/2 located at 133.1\u00a0eV and 134.9\u00a0eV respectively, with a separation of 1.8\u00a0eV, show the presence of Sr2+ in the MoSrOS-2 solution [25]. Moreover, Fig.\u00a01d indicates the S 2p XPS spectra in MoSrOS-2. The peaks at 161.6 and 162.8\u00a0eV corresponded to S 2p3/2 and S 2p1/2 orbitals [26], respectively, and belonged to S2\u2212. Fig.\u00a01e illustrates the O1s XPS spectrum in the MoSrOS-2 sample. The peaks shown at 529.8, 530.5, and 531.4\u00a0eV corresponded to O\nLattice\n, O\nVacancy\n, and hydroxy oxygen, respectively [27,28].\nFig.\u00a02\n demonstrates the XRD diffraction patterns for MoSrOS and the standard SrMoO4 (PDF #85\u20130809). The XRD peaks for MoSrOS correspond to the structure of tetragonal SrMoO4. The peaks indicated at 27.680\u00b0, 29.713\u00b0, 33.190\u00b0, 38.014\u00b0, 45.140\u00b0, 47.645\u00b0, 51.493\u00b0 and 55.994\u00b0 were related to the (112), (004), (200), (211), (204), (220), (116), and (312) crystal planes, respectively. It is observed that the diffraction peak positions of MoSrOS-1, MoSrOS-2, MoSrOS-3 and MoSrOS-4 were similar, which indicated that the strontium nitrate contents did not affect the peak position in the XRD. The peak intensities of the MoSrOS samples decreased with increasing strontium nitrate contents. The average crystal sizes of MoSrOS prepared with different strontium nitrate content were calculated by the Scherrer formula. The average crystal sizes of MoSrOS-1, MoSrOS-2, MoSrOS-3 and MoSrOS-4 are 43.3, 47.9, 46.4 and 39.1\u00a0nm, respectively.The morphology of the samples specified from SEM images is indicated in Fig.\u00a03\n. The MoSrOS-3 catalyst granules looked as regular spherical-like knitting wool balls (Fig.\u00a03a). As we checked from Fig.\u00a03b, the ball were composed of weeny nanorods with several microns in size. The nanorods are straight and uniform. Fig.\u00a03c\u2013f also indicate the EDX elemental mapping of Mo, O, S, and Sr and an uniform distribution of the elements was observed.\nFig.\u00a04\n illustrates the FTIR spectra of MoSrOS, prepared with different amounts of strontium nitrate. The broad absorption band situated at 3460\u00a0cm\u22121 and 1637\u00a0cm\u22121 are typical characteristics of O\u2013H stretching vibrations and bending vibration due to the absorbed water on the sample surface, respectively [29]. The peaks located at 866 and 813\u00a0cm\u22121 are due to stretching vibrations of isolated molybdenum-oxygen tetrahedral molybdate ions [30]. The 1390\u00a0cm\u22121 peak is due to S\u2013O stretching vibrations in the MoSrOS catalysts [31]. Moreover, the 941\u00a0cm\u22121 peak is due to the Sr\u2013O stretching vibration [25]. It was observed that the peak intensities at 1390\u00a0cm\u22121 and 813\u00a0cm\u22121 decreased with the increased amount of strontium nitrate. While the peaks at 941\u00a0cm\u22121 increased with the increased amount of strontium nitrate.The nitrogen adsorption and desorption isotherm of the MoSrOS-2 catalyst is shown in Fig.\u00a0S1a. It was obviously observed that the curves were consistent with the type IV isotherm of a hysteresis loop [32]. The BJH pore size distribution curve is shown in Fig.\u00a0S1b. It was observed that the pore size distributions of MoSrOS-2 are between 10\u00a0nm and 80\u00a0nm. The S\nBET, average pore diameter, and total pore volume are listed in the Table\u00a0S1. The S\nBET, average pore diameter, and total pore volume of catalysts were 17.1~20.2\u00a0m2/g, 4.18~4.67\u00a0nm, and 0.018~0.025\u00a0cm3/g, respectively. These three parameters also decrease with the increase of Sr content.The DRS and the absorption spectra of different Sr content catalysts are shown in Fig.\u00a05\na,b. As it is clear from Fig.\u00a05b, the samples have a very long wavelength absorption range including the ultraviolet and infrared regions and show that there were more absorption states or defects in the sample band. The classic Tauc approach, \n\n\n\n(\n\n\u03b1\nh\nv\n\n)\n\n\n1\nn\n\n\n=\nk\n\n(\n\nh\nv\n\u2212\n\nE\ng\n\n\n)\n\n\n, (where \u03b1\u00a0=\u00a0absorbance coefficient, h\u00a0= Planck constant, k\u00a0=\u00a0absorption constant for a direct transition, h\u03bd\u00a0=\u00a0absorption energy, and E\ng\u00a0=\u00a0band gap) was used to calculate the bandgap of the samples [33,34]. The molybdates of the scheelite type could have a tetragonal structure for which electronic transitions are allowed [35]. The charges present in the valence band (maximum energy level) will be transferred to the minimum energy level after the absorption process [36]. Hence, n\u00a0=\u00a01/2 was adopted in the equation and \u03b1 was replaced by the absorption constant k. The band gaps of MoSrOS-1, MoSrOS-2, MoSrOS-3, MoSrOS-4, were obtained as 2.97, 2.39, 2.30, and 2.28\u00a0eV, respectively (Fig.\u00a05c).\nFig.\u00a05d shows the EIS of MoSrOS prepared with varying amounts of Sr for which a Randles fitting was performed in order to estimate the electron transfer resistance in the electrochemical analysis. The CPE, R1, and R2 symbols were the double layer capacitance and the electrolyte and electron transfer resistances, respectively. Based on the Randles fitting, the electron transfer resistance values of MoSrOS-1, MoSrOS-2, MoSrOS-3, and MoSrOS-4 were 1604\u00a0\u03a9, 470\u00a0\u03a9, 1006\u00a0\u03a9 and 2215\u00a0\u03a9, respectively. The MoSrOS-2 sample had the lowest electron transfer resistance, indicating that for this sample the most efficient electron transfer could be achieved. Based on the EIS measurement, MoSrOS-2 was expected to show the highest activity for the reduction reaction.\nFig.\u00a06\na,b show the catalytic efficiencies after adding only sodium borohydride and/or the catalyst on 4-NP. The results show that adding only one of them has no significant effect on the 4-NP reduction. After NaBH4 was added, the 4-NP absorption peak at 317\u00a0nm shifted to 400\u00a0nm, which is indicative for the 4-nitrophenolate ions formation in the alkaline solution [37] by which the color of the solution was changed from light yellow to dark yellow. Fig.\u00a06c also shows the reduction of 4-NP when sodium borohydride and MoSrOS-2 were added simultaneously. As shown from Fig.\u00a06c, 4-NP was completely reduced within 20\u00a0min. Simultaneously, a peak representing 4-AP appears at 300\u00a0nm, which indicates that 4-NP was reduced to 4-AP. Fig.\u00a06d indicates the reduction activities of all the MoSrOS-based catalysts on 4-NP. It can be seen that MoSrOS-2 and NaBH4 have the best reduction efficiency on 4-NP. According to the kinetic analysis, it was found that the pseudo first-order kinetic rate constant (k) of the 4-NP reduction is shown as follows: MoSrOS-2 (k\u00a0=\u00a00.25494 min\u22121)\u00a0>\u00a0MoSrOS-3 (k\u00a0=\u00a00.02688 min\u22121)\u00a0>\u00a0MoSrOS-4 (k\u00a0=\u00a00.01668 min\u22121)\u00a0>\u00a0MoSrOS-1 (k\u00a0=\u00a00.00294 min\u22121). Table\u00a0S2 shows the performance of our MoSrOS catalyst and its comparisons with data reported in literature on the catalytic reduction of 4-NP. From Table\u00a0S2, Ni-PVAm/SBA catalyst can perform well, with a short reaction time and a high rate constant, but a catalyst of 120\u00a0mg has to be used, which amount is about 12 times higher than we used for MoSrOS. Our MoSrOS at 10\u00a0mg can not only save the cost but also improve the separation problem. While comparing with the g-C3N4/CuS catalyst, our kinetic rate constant of MoSrOS-2 catalyst is almost 3.6 times higher than that of the g-C3N4/CuS catalyst at the same dosage. Overall, the advantages of the MoSrOS catalyst are evident in the dark reduction of 4-NP.\nFig.\u00a07\na shows the MB reduction with MoSrOS-2 in the presence of NaBH4. The MB (100\u00a0mL, 20\u00a0ppm) was reduced completely within 6\u00a0min in the presence of MoSrOS-2 catalyst and NaBH4. Fig.\u00a07b shows a control experiment in which only NaBH4 was added. When only NaBH4 was added into the MB solution, the curve of MB degradation had almost no change. Similarly, the degradation curve of the MB solution in the presence of the MoSrOS-2 catalyst did not change as well (Fig.\u00a07c). This indicates that MoSrOS or NaBH4 alone had little effect on the MB reduction. Fig.\u00a07d shows the MB reduction efficiencies of the catalysts with varying Sr contents in MoSrOS with NaBH4. It can be seen from the pattern that the reduction efficiencies of catalysts with different Sr content were different and MoSrOS-2 had the best reduction performance on MB. It was found that the pseudo first order kinetic rate constant (k) of the MB reduction were: MoSrOS-2 (0.3654\u00a0min\u22121)\u00a0>\u00a0MoSrOS-3 (0.0529\u00a0min\u22121)\u00a0>\u00a0MoSrOS-1 (0.0070\u00a0min\u22121)\u00a0>\u00a0MoSrOS-4 (0.0031\u00a0min\u22121).The MoSrOS catalysts were also used to evaluate their catalytic reduction activities for RhB and MO, as shown in Figs.\u00a0S2 and S3, respectively. It can be seen that 100\u00a0mL (20\u00a0ppm) RhB and 100\u00a0mL (50\u00a0ppm) of MO aqueous solutions were completely reduced within 20\u00a0min, respectively. Figs.\u00a0S2(b,c) S3(b, c) were the control experiments with only MoSrOS-2 and with only NaBH4 for the reduction of RhB and MO, respectively. It can be seen that RhB and MO didn't reduce by with the catalyst only or by only NaBH4. Figs.\u00a0S2d and S3d also indicate the performance of the MoSrOS catalysts prepared with different Sr contents towards the reduction of RhB and MO. It can be seen by the reduction effect that MoSrOS-2 was the best catalyst for both RhB and MO dyes. According to the kinetic analysis, it was found that the pseudo first-order kinetic rate constant (k) of the 4-NP reduction is of the following order: MoSrOS-2 (k\u00a0=\u00a00.25494 min\u22121)\u00a0>\u00a0MoSrOS-3 (k\u00a0=\u00a00.02688 min\u22121)\u00a0>\u00a0MoSrOS-4 (k\u00a0=\u00a00.01668 min\u22121)\u00a0>\u00a0MoSrOS-1 (k\u00a0=\u00a00.00294 min\u22121), while the k constant of the MO reduction is in the order: MoSrOS-2 (0.1118\u00a0min\u22121)\u00a0>\u00a0MoSrOS-3 (0.0187\u00a0min\u22121)\u00a0>\u00a0MoSrOS-4 (0.0181\u00a0min\u22121)\u00a0>\u00a0MoSrOS-1 (0.0078\u00a0min\u22121).\nFig.\u00a08\na,b show the catalytic effect of adding only sodium borohydride or catalyst into the potassium dichromate aqueous solution. The results also show that adding only one of them has no effect on the Cr(VI) reduction. Fig.\u00a08c indicates the reduction of a potassium dichromate solution with the simultaneous addition of MoSrOS-2 and sodium borohydride. As shown in Fig.\u00a08c, the 20\u00a0ppm potassium dichromate aqueous solution was completely reduced within 30\u00a0min. Fig.\u00a08d also shows the reduction of potassium dichromate in the presence of sodium borohydride by catalysts with different amounts of strontium. The catalytic efficiency with an appropriate molybdenum strontium ratio is far stronger. The kinetic pseudo first-order rate constant (k) of the Cr(VI) reduction were: MoSrOS-2 (0.0856\u00a0min\u22121)\u00a0>\u00a0MoSrOS-3 (0.0545\u00a0min\u22121)\u00a0>\u00a0MoSrOS-4 (0.0236\u00a0min\u22121)\u00a0>\u00a0MoSrOS-1 (0.0145\u00a0min\u22121). In conclusion, MoSrOS-2, MoSrOS-3, MoSrOS-1 and MoSrOS-4 are the catalysts with the best pollutant reduction ability. It can be seen from the reduction rate constants of 4-NP that the reaction rate constants of the catalyst MoSrOS-2 are 9.48, 15.28 and 86.71 times that of the catalyst MoSrOS-3, MoSrOS-1 and MoSrOS-4, respectively. It also has excellent reduction performance for other pollutants.To check the MoSrOS stability, the MoSrOS-2 catalyst was run six times towards the 4-NP reduction and the results are shown in Fig.\u00a0S4a. At the 6th run, the MoSrOS-2 still maintained to reduce 93.1% 4-NP. After the 6th run, XRD and XPS characterizations of the MoSrOS-2 sample were performed. Fig.\u00a0S4b shows the XRD diffraction patterns of MoSrOS-2 after 4-NP reduction. It was obvious that the diffraction peak positions of MoSrOS-2 after reduction of 4-NP are similar with those of MoSrOS-2 before the reduction of 4-NP. Fig.\u00a0S4c shows the Mo3d XPS spectrum of MoSrOS-2 after the 4-NP reduction. The peak positions at 232.6\u00a0eV and 235.7\u00a0eV are due to Mo6+3d5/2 and Mo6+3d3/2 orbits, respectively [24]. The peaks at 230.3\u00a0eV and 233.4\u00a0eV are due to Mo4+3d5/2 and Mo4+3d5/2, respectively [24]. Fig.\u00a0S4d shows the Sr 3d XPS spectrum in the MoSrOS-2 sample after the 4-NP reduction. The peaks at 133.1\u00a0eV and 134.9\u00a0eV correspond to Sr2+3d5/2 and Sr2+3d3/2, respectively [25]. Fig.\u00a0S4e indicates the S2p XPS spectrum of MoSrOS-2. The peaks located at 161.6\u00a0eV and 162.8\u00a0eV originated from S2\u2212 2p3/2 and S2\u2212 2p1/2, respectively [26]. Fig.\u00a0S4f also illustrates the O1s XPS spectrum in the MoSrOS-2 sample after the 4-NP reduction. The peaks shown at 529.8\u00a0eV, 530.5\u00a0eV and 531.5\u00a0eV correspond to O\nLattice\n, O\nVacancy\n, and hydroxy oxygen, respectively. The result further indicates the MoSrOS-2 catalyst stability after the reduction reaction. For a comparative purpose, other MoSrOS samples were also tested for their stability, as shown in Fig.\u00a0S5. This table indicates the importance of composition control in achieving a good catalyst.After the exploration of the one-pot synthesis of forming MoSrOS, this compound can be identified as the S-doped and Mo4+-existing SrMoO4 phase. Rare earth-doped SrMoO4 was studied for its photoluminescence [38]. SrMoO4/MoS2 has been studied for its photoreduction of Cr(VI) [39]. The single SrMoO4 phase of MoSrOS performs with an excellent catalytic reduction as a few reports show. The S doping in the SrMoO4 lattice can distort the MoSrOS structure and increase the catalyst activity. The existence of Mo4+ in MoSrOS, which can be viewed as the Mo6+ ion attached with two electrons, enhances its electron transport ability. With this improved transport property, MoSrOS can initiate the catalytic reduction with NaBH4, as described below.\nFig.\u00a09\n shows the proposed reduction mechanism of pollutants in the presence of the MoSrOS catalyst and NaBH4. Firstly, NaBH4 is hydrolyzed to generate borohydride ions and sodium ions immediately in water. Then, borohydride ions react with water to discharge the hydride ion on the catalyst surface and borate ions are simultaneously released [9,40,41]. Afterwards, pollutants adsorbed on the catalyst particles react with hydrogen ions to convert 4-NP into 4-AP [42]. When electrons and hydrogen ions are transferred from borohydride ions to pollutions, Mo4+ transfers two electrons to become Mo6+ in helping this reaction [43]. The catalyst's functions provide an interaction site for orientation-controlled electrophilic nucleophilic reactions. Mo4+ and Mo6+ in the MoSrOS structure provide the electron hopping Mo4+ \u2192 Mo6+, whereas NaBH4 is the hydrogen ions and electrons supply source. Mo4+ and Mo6+ in MoSrOS are also expected to generate the anion vacancy.A series of wool-coiled MoSrOS catalysts were synthesized via a simple hydrothermal method. In this experiment, the contents of strontium nitrate were changed to control the size, shape and physical properties of the catalysts. The catalysts were tested towards the reduction of organic and inorganic pollutants. The reduction test proved that the MoSrOS catalyst with a suitable amount of strontium nitrate has a great reduction activity for MB, RhB, MO, 4-NP, and Cr(VI) with NaBH4 as a reducing agent. In this experiment, 100\u00a0mL (20\u00a0ppm) of MB, 100\u00a0mL (50\u00a0ppm) of RhB, 100\u00a0mL (50\u00a0ppm) of MO, and 100\u00a0mL (50\u00a0ppm) of Cr(VI) were completely reduced within 6, 20, 40, and 30\u00a0min, respectively. The results suggest that MoSrOS based catalysts have great potential in 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 supported by the National Natural Science Foundation of China under the grant No. 31000269, the China Postdoctoral Science Foundation under the grant No. 2018M632562, the Strait Postdoctoral Science Foundation under the grant No. 1323H0005, and Innovation Foundation of Fujian Agriculture and Forestry University under the grant No. CXZX2020129B.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.jsamd.2021.07.008.", "descript": "\n                  Wool-coiled MoSrOS bimetal oxysulfide catalysts were synthesized by a simple method. The catalysts were characterized by different instruments. The MoSrOS reduction activities were also investigated by the reduction of Cr(VI), Rhodamine-B (RhB), Methyl orange (MO), Methylene blue (MB) and 4-nitrophenol (4-NP). The results showed that 100\u00a0mL (20\u00a0ppm) of 4-NP was completely reduced into 4-aminophenol (4-AP) in 20\u00a0min. Moreover, 100\u00a0mL (20\u00a0ppm) of MB, 100\u00a0mL (50\u00a0ppm) of RhB, 100\u00a0mL (50\u00a0ppm) of MO and 100\u00a0mL (50\u00a0ppm) of Cr(VI) were completely reduced within 6, 20, 40, and 30\u00a0min, respectively, by MoSrOS-2 bimetal oxysulfide catalyst. Hence, the wool-coiled MoSrOS bimetal oxysulfide catalysts could be used in the detoxification of toxic organic and inorganic pollutants.\n               "}
{"full_text": "concentration, mol/m3\nmass fraction, ppm or wt%activation energy of i reaction in l reactor, kJ/molpre-exponential factor of rate constant for reaction i in l reactor, h\u22121\u00b7(mg/kg)-n\nspace time, hreaction rate constant for reaction i in l reactorheat of reaction i in l reactor, kJgas-law constant, J/(mol\u00b7K)the reaction temperature, Kreaction pressure, MPaorder of desulfurization for the 4 lumps, respectivelyorder of desulfurization concerning H2/oil volume ratio for the HDS, respectivelyinhibition factor of H2S for the HDS, respectivelythe superficial velocity of the gas, m/sthe superficial velocity of the liquid, m/sgas\u2013liquid interfacial, cm\u22121\nliquid\u2013solid interfacial area, cm\u22121\npartial pressures, MPagas\u2013liquid mass-transfer coefficient mass-transfer coefficient, cm/sliquid\u2013solid mass-transfer coefficient mass-transfer coefficient, cm/sthe liquid-phase concentrations of H2S at the catalyst surface, mol/cm3\nthe liquid-phase concentrations of sulfur composition, mol/cm3\nsum of square errorsthe number of experimentsthe output variablesthe values calculated by the modelthe experimental dataaxial position along the catalyst bed, cmreaction rate of sulfur components, mol/(kg\u00b7h)the apparent rate constanttotal sulfur concentration, mol/m3\nCalculatedExperimentalTriA-aromaticDi-aromaticMono-aromaticCycloalkaneParaffinthe kind of componentthe number of reactors, the 1st reactor or the 2nd reactorSulfur compoundsthe first lump in desulfurization at the 1st reactorthe second lump in desulfurization at the 1st reactorthe third lump in desulfurization at the 1st reactorthe third lump in desulfurization at the 2nd reactorthe third lump in desulfurization at the 2nd reactorthe concentration of TriA-aromatic at the l reactorthe concentration of TriA-aromatic at the l reactorthe concentration of Di-aromatic at the l reactorthe concentration of Cycloalkane at the l reactorthe concentration of Paraffin at the l reactorWith the environmental legislation increasing stringent[1], it is imperative to produce ultra-low sulfur content (\uff1c10\u00a0ppm) and low polycyclic aromatics hydrocarbons content (PAHs) (\uff1c3\u00a0wt%) diesel oil [2,3]. Hydrotreating (HDT) is fundamental to removing sulfur, PAHs, and other impurities [4]. As consequence, improved hydrotreating processes have to be developed.The high and low temperature dual reaction zone RTS process was developed by Sinopec Research Institute of Petroleum Processing Co., Ltd (RIPP) [5]. The RTS technology was characterized by the removal of most sulfides, PAHs, and almost all nitrogen compounds in the first reactor at high temperature and low space velocity. The second reactor was primarily responsible for aromatics hydrogenation and ultra-deep HDS under the condition of low temperature and high space velocity [6,7].For a new diesel hydrotreating unit, the construction and confirmation of kinetic models are necessary for optimizing the operation parameters [8]. The multi-lump kinetic models established in the published journals can describe deep HDS well with sulfur content reduced to 300\u00a0\u223c\u00a0500\u00a0ppm[9\u201311]. However, The HDS kinetics performance will change after sulfur content lower than 10\u00a0ppm and the kinetic behaviors of the reactions had not received considerable attentions in the present literatures.Hydrogen sulphide (H2S), which is produced as a by-product of sulphur removal, was reported to significantly inhibit hydrogenation of sulfur compounds by many scholars [12\u201314]. The existing researches were carried out by changing the concentration of H2S in the inlet of the reactor to determine the effect of H2S on HDS activity. However, the actual adsorption concentration of H2S in the catalytic active center was not considered in their kinetic model. Therefore, exploring the concentration distribution of H2S on the catalyst surface is crucial for optimizing the HDS kinetic model.Most of the aromatic hydrosaturation (AHS) kinetic models account for one sum reaction and very few were developed to accommodate three lumps of reacting aromatic compounds including mono-, di-, and tri-aromatics [15,16]. To improve the accuracy of kinetic model, Wu et al [17] proposed L-H kinetic models to describe the process of AHS, the aromatics were lumped into four groups including tri-aromatics, di-aromatics, mono-aromatics and non-aromatics. However, with the deep saturation of aromatics, the ring opening of cycloalkanes that occur in hydrotreating to form paraffin cannot be ignored[18]. As a result, a more accurate kinetic model for AHS in the RTS process which have scant attention in previous research in the above condition is deserved to be established to fit the real reaction in the industry.In this work, the main focus is on developing optimal kinetic models for simulating the reactions of HDS, HDN, and AHS of diesel from a refinery in the RTS process, specifically at sulfur content and PAHs content below 10\u00a0ppm and 3\u00a0wt%, respectively. For HDS, a hydrodesulfurization (HDS) kinetic model was developed based on a fundamental and comprehensive understanding of the different HDS reactivities of various sulfur compounds in middle distillates and the inhibition effects of the coexisting H2S compounds. The calculation of the H2S concentration on the catalyst surface was based on the three-film theory [19]. For deep AHS, the hydrocarbons in diesel are classified into five lumps to make up for the incompleteness of the four-lumped kinetic model. Finally, the developed models were used to predict sulfur and hydrocarbon concentrations for the determination of reaction conditions.A new generation NiMo/\u03b3-Al2O3 catalyst (RS-2100) with strengthened metal-support interaction was used to replace the noble metal catalyst, by inventing the assembly technology of high\u2010performance active phase and the stabilization technology of well\u2010dispersion active phase [6]. The diameter (dp), length (dL\n), surface area (Sp\n), pore volume (Vp\n), bulk density (\u03c1b\n) and average pore diameter (r) of the NiMo/\u03b3-Al2O3 catalyst were 1.6\u00a0mm, 2.0\u00a0mm, 176.7\u00a0m2/g, 0.28\u00a0cm3/g, 0.826\u00a0g/cm3, and 6.3\u00a0nm, respectively.. Here, Mo was the catalytic active component, Ni was the promotor and \u03b3-Al2O3 was the support. The oxidized catalyst had the following composition: NiO: 4.4\u00a0wt%, MoO: 26.6\u00a0wt%, P2O5: 7.1\u00a0wt%; Al2O3: 61.9\u00a0wt%.\nTable 1\n\u00a0presents the feedstock parameters of the 1st and 2nd reactors in the RTS process, respectively. Feedstock II was the product in the first reactor of the two-stage process by Feedstock I at the following conditions: the temperature\u00a0=\u00a0340\u00a0\u00b0C, LHSV\u00a0=\u00a01.5\u00a0h\u22121, H2/oil volume ratio\u00a0=\u00a0300 and pressure\u00a0=\u00a06.4\u00a0MPa. Feedstock I is composed of 80 % straight-run diesel and 20 % light catalytic cycle oil, which are provided by Tsingtao Refining & Chemical Company.The pilot-scale experiments were carried out in a continuous TBR system. A schematic diagram of the apparatus was depicted in Fig. 1\n. The whole reaction system consists of the feed section, the reactor section, the product separation section and the collection section. In the reactor unit, the reactor is designed as a tube with a length of 160\u00a0cm and an inside diameter of 2.4\u00a0cm. The length of the reactor is subdivided into three sections. The first part, having a length of 60\u00a0cm, was packed with SiC particles which were used to heat the feedstock and to provide a uniform distribution of gas, liquid and hydrogen saturation of the feed. The second section with a length of 50\u00a0cm contained a packing of 60\u00a0g catalyst and SiC particles. The last section was packed with SiC particles of nearly the same size as the catalyst in the other reactor sections. At the center line of the reactor, there is a thermo-well containing three thermocouples used to control the axial temperature profile within the reactor. The greatest deviation from the desired temperature value was about 1\u00a0\u00b0C.In the initial stage of the diesel hydrotreating process, the catalysts were presulfurized by 2nd atmospheric side-stream solution containing 2 % CS2 at 320\u00a0\u00b0C for 10\u00a0h [20]. Following the presulfurization, the diesel feedstock was switched to the kinetic experiments, where the running time on stream of each experiment was 48\u00a0h to keep a stable catalyst activity. Experimental group 1 using feedstock I was designed to collect accurate hydrotreating kinetic data for the 1st reactor of the RTS process at a pressure of 6.4\u00a0MPa, temperature, LHSV, and H2/oil volume ratio were in the range of 340\u2013360\u00a0\u00b0C, 1.5\u20134.5\u00a0h\u22121, 300\u2013800 (NPT, v/v) ratio, respectively. Experimental group 2 was conducted by Feedstock II to obtain reliable hydrotreating kinetic data for the 2nd reactor of the RTS process at a pressure of 6.4\u00a0MPa, temperature, LHSV, and H2/oil volume ratio were varied from 330 to 350\u00a0\u00b0C, 2.25\u20134.5\u00a0h\u22121, and 300\u2013800 (NPT, v/v), respectively. Fig. 2\n describes the general flow pattern of the RTS process.The qualitative and quantitative analyses of sulfur compounds were accomplished with the aid of the Agilent 7890B HP gas chromatographic-sulfur chemiluminescence detector (GC-SCD), using an HP-5 (30\u00a0m\u00a0\u00d7\u00a00.32\u00a0mm\u00a0\u00d7\u00a00.25\u00a0\u03bcm) capillary GC column. The total sulfur, nitrogen, and aromatic concentrations were measured by the ASTM D-5453, ASTM D-4629, and ASTM D-2425 methods, respectively.To model the RTS hydrotreating process, sulfur compounds were divided into three lumps according to their reactivity. Hydrocarbons were divided into five categories according to chemical structure. In particular, nitrogen compounds could be rapidly eliminated in the 1st reactor of the RTS process, so that the HDN model could be neglected in the 2nd reactor.The model equations could be established with the following assumptions.\n\n(1)\nThe pilot reactors were operated isothermally and isobarically.\n\n\n(2)\nNo catalyst deactivation happened during the hydrogenation reaction.\n\n\n(3)\nOn account of the high H2/oil volume ratio, the fluctuation of hydrogen partial pressure was negligible.\n\n\n(4)\nVaporization and condensation could be neglected.\n\n\n(5)\nThe catalyst surface was completely and uniformly wetted.\n\n\nThe pilot reactors were operated isothermally and isobarically.No catalyst deactivation happened during the hydrogenation reaction.On account of the high H2/oil volume ratio, the fluctuation of hydrogen partial pressure was negligible.Vaporization and condensation could be neglected.The catalyst surface was completely and uniformly wetted.To estimate a kinetic model that accurately describes the ultra-deep HDS for the RTS process in the sulfur concentration range from more than 10000\u00a0ppm to 10\u00a0ppm, a clear understanding of the types of sulfur compounds present in diesel feed and hydrotreated product oils and their reactivity is very important [21].\nFig. 3\n illustrated sulfur concentration and sulfur-type variation along the catalyst bed in the RTS process. To monitor the fluctuation of sulfur species, the results indicate that diesel feeds contain a large number of individual sulfur compounds which can be classified into three lumps. The GC-SCD chromatograph was first separated into three regions according to the retention time as follows: region 1 with a retention time ranging from 42.0 to 54.5\u00a0min, region 2 from 54.5 to 63.4\u00a0min, and region 3 from 63.4 to 75.0\u00a0min. Lump 1 contained all the sulfur compounds in region 1 except 4-MDBT, which represented the majority of the benzothiophene-type compounds. Lump 2 represented the major dibenzothiophenes without any alkyl substituent at the 4- or 6-position in region 2. Group 3 included 4,6-DMDBT, 2,4,6-TMDBT, and all sulfur compounds in region 3. Accordingly, the different numbers and distributions of groups in diesel would determine their different HDS kinetic behaviors, and the simplified reaction network of lumps is shown in Fig. 4\n.Most of the works have studied the inhibiting effect of the hydrogen sulfide on the hydrodesulfurization rates of the S-compounds [22]. However, the actual adsorption concentration of H2S in the catalytic active center was not considered in their kinetic model. Therefore, exploring the concentration distribution of H2S on the catalyst surface is crucial for optimizing the kinetic model. The Langmuir\u2013Hinshelwood reaction equation of each lump for the RTS process can be described as follows:\n\n(1)\n\n\n\n\n\n\n\n\n\ndw\n\n\nS\n\n1\n,\n1\n\n\n\n\ndt\n\n\n=\n\n\n\nk\n\n1\n,\n1\n\n\n\nw\n\nS\n\n1\n,\n1\n\n\n\nn\n1\n\n\n\n\n\n\n\n\nH\n2\n\n\nO\ni\nl\n\n\n\n\n\n\na\n1\n\n\n\n\n1\n+\n\n\u03b3\n1\n\n\nw\n\n\nH\n2\n\nS\n\n\n\n\n#\n\n\n\n\n\n\n\n\n\n\n(2)\n\n\n\n\n\n\n\n\n\ndw\n\n\nS\n\n2\n,\n1\n\n\n\n\ndt\n\n\n=\n\n\n\nk\n\n2\n,\n1\n\n\n\nw\n\nS\n\n2\n,\n1\n\n\n\nn\n2\n\n\n\n\n\n\n\n\nH\n2\n\n\nO\ni\nl\n\n\n\n\n\n\na\n2\n\n\n\n\n1\n+\n\n\u03b3\n2\n\n\nw\n\n\nH\n2\n\nS\n\n\n\n\n#\n\n\n\n\n\n\n\n\n\n\n(3)\n\n\n\n\n\n\n\n\n\ndw\n\n\nS\n\n3\n,\n1\n\n\n\n\ndt\n\n\n=\n\n\n\nk\n\n3\n,\n1\n\n\n\nw\n\nS\n\n3\n,\n1\n\n\n\nn\n3\n\n\n\n\n\n\n\n\nH\n2\n\n\nO\ni\nl\n\n\n\n\n\n\na\n3\n\n\n\n\n1\n+\n\n\u03b3\n3\n\n\nw\n\n\nH\n2\n\nS\n\n\n\n\n#\n\n\n\n\n\n\n\n\n\n\n(4)\n\n\n\n\n\n\n\n\n\ndw\n\n\nS\n\n2\n,\n2\n\n\n\n\ndt\n\n\n=\n\n\n\nk\n\n2\n,\n2\n\n\n\nw\n\nS\n\n2\n,\n2\n\n\n\nn\n4\n\n\n\n\n\n\n\n\nH\n2\n\n\nO\ni\nl\n\n\n\n\n\n\na\n4\n\n\n\n\n1\n+\n\n\u03b3\n4\n\n\nw\n\n\nH\n2\n\nS\n\n\n\n\n#\n\n\n\n\n\n\n\n\n\n\n(5)\n\n\n\n\n\n\n\n\n\ndw\n\n\nS\n\n3\n,\n2\n\n\n\n\ndt\n\n\n=\n\n\n\nk\n\n3\n,\n2\n\n\n\nw\n\nS\n\n3\n,\n2\n\n\n\nn\n5\n\n\n\n\n\n\n\n\nH\n2\n\n\nO\ni\nl\n\n\n\n\n\n\na\n5\n\n\n\n\n1\n+\n\n\u03b3\n5\n\n\nw\n\n\nH\n2\n\nS\n\n\n\n\n#\n\n\n\n\n\n\n\n\nH2S was produced at the sulfided NiMo sites of the bifunctional catalyst, then transported into the liquid phase, and finally, diverted from the liquid phase into the gas phase via mass transfer [23\u201325]. Therefore, the concentration of H2S did not always increase with the reactor length on the catalyst surface. For reliable estimation and scale-up of pilot plant data, a three-phase reactor model was necessary. The calculation of the H2S concentration on the catalyst surface was based on the three-film theory as shown in Fig. 5\n. The differential mass-balance Eqs. (6) - (11) had been obtained for the concentration of H2S at different phases as follows [14,26,27].Gas to liquid interface mass transfer equation:\n\n(6)\n\n\n\n\n\n\n\n\nu\nG\n\n\nRT\n\n\n\n\nd\n\nP\n\n\nH\n2\n\nS\n\nG\n\n\n\ndz\n\n\n+\n\nk\n\n\nH\n2\n\nS\n\nL\n\n\na\nL\n\n\n\n\n\nP\n\n\nH\n2\n\nS\n\nG\n\nH\n\n-\n\nc\n\n\nH\n2\n\nS\n\nL\n\n\n\n=\n0\n#\n\n\n\n\n\n\n\n\nLiquid to gas interface mass transfer equation:\n\n(7)\n\n\n\n\n\n\n\nu\nL\n\n\n\nd\n\nP\n\n\nH\n2\n\nS\n\nG\n\n\n\ndz\n\n\n-\n\nk\n\n\nH\n2\n\nS\n\nL\n\n\na\nL\n\n\n\n\n\nP\n\n\nH\n2\n\nS\n\nG\n\nH\n\n-\n\nc\n\n\nH\n2\n\nS\n\nL\n\n\n\n+\n\nk\n\n\nH\n2\n\nS\n\nS\n\n\na\nS\n\n\n\n\nc\n\n\nH\n2\n\nS\n\nL\n\n-\n\nc\n\n\nH\n2\n\nS\n\nS\n\n\n\n=\n0\n#\n\n\n\n\n\n\n\n\nSolid to liquid interface mass transfer equations:\n\n(8)\n\n\n\n\n\n\n\n1\n\nu\nL\n\n\n\nk\n\n\nH\n2\n\nS\n\nS\n\n\na\nS\n\n\n\n\nc\n\n\nH\n2\n\nS\n\nL\n\n-\n\nc\n\n\nH\n2\n\nS\n\nS\n\n\n\n=\n\n\nd\n\nc\n\n\nH\n2\n\nS\n\n\n\n\ndz\n\n\n#\n\n\n\n\n\n\n\n\nliquid to solid interface mass transfer equations:\n\n(9)\n\n\n\n\n\n\n\nk\n\n\nH\n2\n\nS\n\nS\n\n\na\nS\n\n\n\n\nc\n\n\nH\n2\n\nS\n\nL\n\n-\n\nc\n\n\nH\n2\n\nS\n\nS\n\n\n\n=\n-\n\nr\n\n\nH\n2\n\nS\n\n\n#\n\n\n\n\n\n\n\n\n\n\n(10)\n\n\n\n\n\n\n\nr\n\n\nH\n2\n\nS\n\n\n=\n\nk\n\napp\n\n\n\n\n\n\n\nc\n\nS\n\nL\n\n\n\n\n\n1.5\n\n\n#\n\n\n\n\n\n\n\n\nAssuming the organic sulfur compound had the same molecular weight as the whole sample, its concentration could be estimated by using the weight fraction wS\n.\n\n(11)\n\n\n\n\n\n\n\nc\nS\n\n=\n\n\u03c1\nM\n\n\nw\nS\n\n#\n\n\n\n\n\n\n\n\nThe first reactor of RTS was rapidly denitrifying in a high-temperature environment. The nitrogen molecules were considered as a single lump, and the HDN process was described with the pseudo-first-order kinetic power-law model [17]. The kinetic equation was shown in Eq. (12).\n\n(12)\n\n\n\n\n\n\n-\n\n\nd\n\nw\nN\n\n\n\ndt\n\n\n=\n\nk\n\n4\n,\n1\n\n\n\nw\nN\n\n#\n\n\n\n\n\n\n\n\nThe network diagram of the aromatic hydrosaturation reaction in diesel fuel was shown in Eq. (13), which indicated the aromatic hydrosaturation reaction processes ring-by-ring reversibly [28]. To get a better understanding of reaction rules during diesel hydrogenation, the aromatics are classified into five lumps namely tri-aromatics (TriA), di-aromatics (DiA), mono-aromatics (MA), cycloalkanes (CA), and paraffin (PA). Kinetic models for the AHS reaction in a gas/oil system mainly assume that hydrogenation and dehydrogenation reactions occur according to the Langmuir\u2013Hinshelwood mechanisms and the HDA reaction is represented as a first-order reversible reaction [29,30]. In Eqs. (14)-(18) the forward reaction and the backward reaction are assigned in first order for the aromatics to obtain the rate equations as follows:\n\n(13)\n\n\n\n\n\n(14)\n\n\n\n\n\n\n-\n\n\nd\n\nw\n\nT\nr\ni\nA\n,\nl\n\n\n\n\ndt\n\n\n=\n\nk\n\n5\n,\nl\n\n\n\nw\n\nT\nr\ni\nA\n,\nl\n\n\n-\n\nk\n\n6\n,\nl\n\n\n\nw\n\nD\ni\nA\n,\nl\n\n\n#\n\n\n\n\n\n\n\n\n\n\n(15)\n\n\n\n\n\n\n-\n\n\nd\n\nw\n\nD\ni\nA\n,\nl\n\n\n\n\ndt\n\n\n=\n\n\n-\nk\n\n\n5\n,\nl\n\n\n\nw\n\nT\nr\ni\nA\n,\nl\n\n\n+\n\nk\n\n6\n,\nl\n\n\n\nw\n\nD\ni\nA\n,\nl\n\n\n\n\n+\nk\n\n\n7\n,\nl\n\n\n\nw\n\nD\ni\nA\n,\nl\n\n\n-\n\nk\n\n8\n,\nl\n\n\n\nw\n\nM\nA\n,\nl\n\n\n#\n\n\n\n\n\n\n\n\n\n\n(16)\n\n\n\n\n\n\n-\n\n\nd\n\nw\n\nM\nA\n,\nl\n\n\n\n\ndt\n\n\n=\n\nk\n\n9\n,\nl\n\n\n\nw\n\nM\nA\n,\nl\n\n\n-\n\nk\n\n10\n,\nl\n\n\n\nw\n\nC\nA\n,\nl\n\n\n\n\n-\nk\n\n\n7\n,\nl\n\n\n\nw\n\nD\ni\nA\n,\nl\n\n\n+\n\nk\n\n8\n,\nl\n\n\n\nw\n\nM\nA\n,\nl\n\n\n#\n\n\n\n\n\n\n\n\n\n\n(17)\n\n\n\n\n\n\n-\n\n\nd\n\nw\n\nC\nA\n,\nl\n\n\n\n\ndt\n\n\n=\n\n\n-\nk\n\n\n9\n,\nl\n\n\n\nw\n\nM\nA\n,\nl\n\n\n+\n\nk\n\n10\n,\nl\n\n\n\nw\n\nC\nA\n,\nl\n\n\n\n\n+\nk\n\n\n11\n,\nl\n\n\n\nw\n\nC\nA\n,\nl\n\n\n#\n\n\n\n\n\n\n\n\n\n\n(18)\n\n\n\n\n\n\n-\n\n\nd\n\nw\n\nP\nA\n,\nl\n\n\n\n\ndt\n\n\n=\n\n\n-\nk\n\n\n11\n,\nl\n\n\n\nw\n\nC\nA\n,\nl\n\n\n#\n\n\n\n\n\n\n\n\nThe constants of reaction rate and the adsorption equilibrium in equations satisfied the Arrhenius equation and Van\u2019t Hoff equation [31], such as Eq. (19) and Eq. (20):\n\n(19)\n\n\n\n\n\n\n\nk\n\ni\n,\nl\n\n\n=\n\nA\n\ni\n,\nl\n\n\nexp\n\n\n-\n\n\n\nEa\n\n\ni\n,\nl\n\n\n\nRT\n\n\n\n\ni\n=\n1\n-\n11\n;\nl\n=\n1\n,\n2\n#\n\n\n\n\n\n\n\n\n\n\n(20)\n\n\n\n\n\n\n\n\nd\nl\nn\n\nk\n\ni\n,\nl\n\n\n\n\ndT\n\n\n=\n-\n\n\n\u0394\n\nH\n\ni\n,\nl\n\n\n\n\nRT\n\n\n#\n\n\n\n\n\n\n\n\nThe model parameters were optimized with a multivariate function Levenberg\u00a0\u2212\u00a0Marquardt, and the ordinary differential equations were solved by a variable step-size adaptive Runge-Kutta method [32]. A program utilizing self-coded MATLAB language and the weighted least absolute error terms in Eq. (21) were used to estimate the kinetic parameters in the proposed lump model.\n\n(21)\n\n\n\n\n\n\nSSE\n=\n\n\n\n\u2211\n\ni\n=\n1\n\n\nNE\n\n\n\n(\n\n\u0393\n\ni\n,\nl\n\n\nCal\n\n\n-\n\n\u0393\n\ni\n,\nl\n\n\nEx\n\n\n)\n\n\n2\n\n\n\n\n\n\n\n\n\n\nFig. 6\n shows the distribution of H2S in the multi-phases along with the reactors in the RTS process. The H2S concentration in the liquid phase and solid phase increased initially due to the high reaction rate at the beginning. As the HDS reaction was processed, the H2S concentration in the liquid phase gradually decreased along with the mass transfer gradient prevailing rather than the chemical reaction. After entering the second reactor of the RTS process, the concentration of H2S in the gas\u2013liquid-solid three-phase reached equilibrium and the content of H2S was 25\u00a0ppm on the solid surface at this reaction condition. The H2S concentration distribution along the reactor axis in the HDS process is in a reasonable range, which is similar to the reported values for several distillate cuts [8,27,33].To reflect the effect of H2S concentration on the HDS reaction, the H2S concentration distribution equation on the catalyst surface along the axial direction of the reactor was obtained by the BiHill simulating method [34]. Based on that the H2S concentration distribution equation in Eq. (22) was used to estimate the kinetic parameters of HDS in the proposed lump model.\n\n(22)\n\n\n\n\n\n\n\nw\n\n\nH\n2\n\nS\n\n\n=\n1841.20\n\u00c3\n\u00b7\n\n(\n1\n+\n\n\n(\n\n\n0.25\n\nz\n\n)\n\n\n3.49\n\n\n)\n\n\u00c3\n\u00b7\n\n(\n1\n+\n\n\n(\n\nz\n\n1.06\n\n\n)\n\n\n2.02\n\n\n)\n\n\n\n\n\n\n\n\n\nwhere the relationship between the axial position along the reactor and the residence time is shown in Eq. (23).\n\n(23)\n\n\n\n\n\n\nz\n=\n42\n\nt\n\n0.667\n\n\n#\n\n\n\n\n\n\n\n\nThe distribution equation of H2S adsorption concentration in the first reactor and the equilibrium adsorption value in the second reactor on the catalyst surface are brought into Eqs. (1)-(5), respectively, to obtain Eqs (24)-(28).\n\n(24)\n\n\n\n\n\n\n\n\nd\n\nw\n\nS\n\n1\n,\n1\n\n\n\n\n\ndt\n\n\n=\n\n\n\nk\n\n1\n,\n1\n\n\n\nw\n\nS\n\n1\n,\n1\n\n\n\nn\n1\n\n\n\n\n(\n\n\nH\n2\n\n\nOil\n\n\n)\n\n\na\n1\n\n\n\n\n1\n+\n\n\u03b3\n1\n\n1841.20\n\u00c3\n\u00b7\n\n(\n1\n+\n\n\n(\n\n\n0.25\n\n\n42\n\nt\n\n0.667\n\n\n\n\n)\n\n\n3.49\n\n\n)\n\n\u00c3\n\u00b7\n\n(\n1\n+\n\n\n(\n\n\n42\n\nt\n\n0.667\n\n\n\n\n1.06\n\n\n)\n\n\n2.02\n\n\n)\n\n\n\n\n\n\n\n\n\n\n\n\n\n(25)\n\n\n\n\n\n\n\n\nd\n\nw\n\nS\n\n2\n,\n1\n\n\n\n\n\ndt\n\n\n=\n\n\n\nk\n\n2\n,\n1\n\n\n\nw\n\nS\n\n2\n,\n1\n\n\n\nn\n2\n\n\n\n\n(\n\n\nH\n2\n\n\nOil\n\n\n)\n\n\na\n2\n\n\n\n\n1\n+\n\n\u03b3\n2\n\n1841.20\n\u00c3\n\u00b7\n\n(\n1\n+\n\n\n(\n\n\n0.25\n\n\n42\n\nt\n\n0.667\n\n\n\n\n)\n\n\n3.49\n\n\n)\n\n\u00c3\n\u00b7\n\n(\n1\n+\n\n\n(\n\n\n42\n\nt\n\n0.667\n\n\n\n\n1.06\n\n\n)\n\n\n2.02\n\n\n)\n\n\n\n\n\n\n\n\n\n\n\n\n\n(26)\n\n\n\n\n\n\n\n\n\ndw\n\n\nS\n\n3\n,\n1\n\n\n\n\ndt\n\n\n=\n\n\n\nk\n\n3\n,\n1\n\n\n\nw\n\nS\n\n3\n,\n1\n\n\n\nn\n3\n\n\n\n\n\n\n\n\nH\n2\n\n\nO\ni\nl\n\n\n\n\n\n\na\n3\n\n\n\n\n1\n+\n1841.20\n\u00c3\n\u00b7\n\n\n1\n+\n\n\n\n\n\n\n0.25\n\n\n42\n\nt\n\n0.667\n\n\n\n\n\n\n\n\n3.49\n\n\n\n\n\u00c3\n\u00b7\n\n\n1\n+\n\n\n\n\n\n\n42\n\nt\n\n0.667\n\n\n\n\n1.06\n\n\n\n\n\n\n2.02\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n(27)\n\n\n\n\n\n\n\n\n\ndw\n\n\nS\n\n2\n,\n2\n\n\n\n\ndt\n\n\n=\n\n\n\nk\n\n2\n,\n2\n\n\n\nw\n\nS\n\n2\n,\n2\n\n\n\nn\n4\n\n\n\n\n\n\n\n\nH\n2\n\n\nO\ni\nl\n\n\n\n\n\n\na\n4\n\n\n\n\n1\n+\n25\n\n\u03b3\n4\n\n\n\n\n\n\n\n\n\n\n\n\n\n(28)\n\n\n\n\n\n\n\n\n\ndw\n\n\nS\n\n3\n,\n2\n\n\n\n\ndt\n\n\n=\n\n\n\nk\n\n3\n,\n2\n\n\n\nw\n\nS\n\n3\n,\n2\n\n\n\nn\n5\n\n\n\n\n\n\n\n\nH\n2\n\n\nO\ni\nl\n\n\n\n\n\n\na\n5\n\n\n\n\n1\n+\n\n\n25\n\u03b3\n\n5\n\n\n\n\n\n\n\n\n\n\n\nThe obtained kinetic parameters for the first and second reactors are presented in Table 2\n. It is observed that the orders of reaction vary between 1.0 and 2.0. This is a common observation for lumped kinetics, as lumping large spectral compounds with a broad reactivity distribution can give any apparent reaction order [35]; It has been reported that the hydrotreating reaction of diesel follow half to second order kinetics [20,32]. The order of the inhibition effect of H2S was Lump1 \uff1e Lump2\uff1eLump3, from those results we conclude that the inhibition effect of H2S monotonically decreases with the increasing number of substituents bonded to the thiophene ring. It can be inferred from the activation energy values that MA have stable resonance structures, which makes them the most difficult group of aromatic hydrocarbons to hydrogenate compared to DiA and TriA [29,30]. Although the activation energy and pre-exponential factor are dependent on the feedstock and catalyst utilization, our estimates for HDS, HDN, and AHS are within an acceptable range and are comparable to reported values [36]. However, mutual effects between sulfur compounds, nitrogen compounds and aromatic occur simultaneously in the HDT process were not considered in the proposed kinetic model. Further studies are necessary to include the inhibition of sulfur, nitrogen and aromatics under more severe conditions in the HDT kinetic models.As shown in Fig. 7\n, the satisfactory linearity between the logarithm of the reaction rate constants and the reciprocal of temperature in HDS, HDN and AHS indicated that the rate constants fitted the Arrhenius equation well. The temperature dependence of the inhibition factor of nitrogen compounds described in Fig. 7(a) proved that the HDN reaction strongly depends on temperature, which was in accordance with the van\u2019t Hoff equation perfectly [37].The derived kinetic parameters were incorporated into the kinetic equation and the RTS process under experimental circumstances could be simulated. Figs. 8 and 9\n\n illustrate the excellent agreement between experimental and simulation results. The relative error between the predicted and experimental values is less than 10.7 % in the RTS process of HDS, HDN, and AHS, indicating that the kinetic model significantly predicts the effect of HDS, HDN, and AHS.The change in product concentration with the space time for sulfur, hydrocarbon, and nitrogen compounds at different reaction temperatures are presented in Figs. 10-13\n\n\n\n. The simulated results proved that kinetic models were suitable for diesel HDS, HDN, and AHS in the RTS process, even if the sulfur and PAHs concentrations are lower than 10\u00a0ppm and 3\u00a0wt%, respectively. However, it should be noted that additional parameters such as internal diffusion of catalyst, axial and radial dispersion, energy and mass balance, and so on need further investigations when the set of rate laws is applied to larger scale reactors.\nFig. 10 showed the effect of the reaction temperature acting on the amount of residual S and N in the 1st reactor. It indicates that N is removed significantly when the temperature reaches 350\u00a0\u00b0C. The total N content is almost removed at the residence time of 0.4\u00a0h. However, the residence time for deep desulfurization is longer than 0.67\u00a0h, indicating that it will perhaps be an obvious choice to add an extra reactor to the 1st reactor. Moreover, different lumps of S compounds showed significant variations for HDS. Lump 1 showed the highest reaction rate and improved rapidly with increasing temperature. The reaction rate of lump 2 was low below 350\u00a0\u00b0C. Catalytic activity improved rapidly at a temperature over 350\u00a0\u00b0C, resulting in higher conversion. The trend of lump 3 was similar to that of lump 2; the only difference was that conversion increased rapidly when the temperature reached 360\u00a0\u00b0C.\nFig. 11 showed the effect of the reaction temperature acting on the amount of residual Sulfur in the 2nd reactor. It indicates that the relative content of these virtually refractory compounds in total sulfur increases over space time, the reaction rate gets slower and slower. Therefore, it is critical to developing catalysts with a higher activity that can convert these refractory compounds.\nFigs. 12-13 summarize pilot plant data demonstrating how the aromatic species change as a function of the residence time in the RTS process. For AHS, the two-ring aromatic is converted to the mono-aromatic relatively quickly as shown by a steep decline in PAHs concentration as a function of residence time below 0.2\u00a0h. At longer residence times, which represent space velocities of about 1\u00a0h\u22121 or lower, there is very little change due to equilibrium constraints. For mono-ringed aromatic saturation, there is a steady increase in conversion as the residence time is increased, and eventually, the mono-ringed concentration begins to decrease indicating that mono-ring saturation starts to increase as the residence time is increased. These data show that PAHs saturation occurs fairly readily under typical hydrotreating conditions, but the saturation of mono rings aromatics is much more difficult and is aided by higher residence time or improved catalyst kinetic ability.Since the conversion of the mono-aromatic compounds provides the most significant boost in product volume, hydrotreaters with very short residence time will have difficulty achieving higher volume swell due to the much slower rate of saturating the final aromatic ring. These units will require a higher temperature to drive the reaction's kinetic saturation portion. This can have some negative effects on expected cycle time due to the higher start of run temperature and the increased fouling rate associated with higher temperature. Therefore, considering the various factors, the optimal operating conditions in the 1st and 2nd reactors are 350\u00a0\u00b0C and 340\u00a0\u00b0C, respectively, at a constant pressure of 6.4\u00a0MPa and H2/oil volume ratio of 300 v/v.The hydrogenation efficiency of RTS and conventional HDT process on Feedstock I were tested at different LHSV under the reaction condition of pressure of 6.4 Mpa, H2/oil volume ratio of 300 v/v. The results are summarized in Table 3\n. It is clear that the volume space velocity of RTS process is twice that of traditional HDT process at same HDS efficiency and the product color is close to water white in the RTS process. The test results show that RTS technology is a better choice than conventional HDT technology for ultra-deep hydrodesulfurization.The RTS process has been applied in SINOPEC Changling Refining & Chemical Company. The feedstock was composed of 75% SRGO and 25% LCO. The contents of sulfur and PAHs in feedstock and the refined oils during the long-term operation of the device are shown in Fig. 14\n and Fig. 15\n, respectively. It can be seen that the sulfur contents and PAHs contents of refined oil were less than 10\u00a0ppm and 5\u00a0wt% during the entire operation period of 1000\u00a0days. Therefore, the RTS process with high and low temperature dual reaction zone processes a high HDT efficiency and stability.A three-lumping L-H kinetics model, based on the molecular structures and the retention times of the sulfur compounds in GC-SCD chromatographs, is developed to accurately describes the trend of sulfur content decreasing from more than 10000\u00a0ppm to less than 10\u00a0ppm. The actual adsorption concentration of H2S in the catalytic active center is calculated by the three-film theory and the inhibiting effect of H2S on the hydrodesulfurization rates of S-compounds was studied. The results show that the inhibition effect of H2S monotonically decreases with the increasing number of substituents bonded to the thiophene ring.Based on proper division method, a new five lumping model for the AHS of diesel is established. The model includes tri-aromatics, di-aromatics, mono-aromatics, cycloalkanes, and paraffin as lumps, which can describe the trend of PAHs concentration decreasing from 19.7\u00a0wt% to less than 3.0\u00a0wt%.In addition, comparisons between the experimental data and predictions using the lumping kinetic models showed agree well, with average deviation lower than 10.7% at different operating conditions. Therefore, these models can serve as an effective guide for diesel hydrotreatment, and the calculated results indicate that the optimal operating conditions in the 1st and 2nd reactors are 350\u00a0\u00b0C and 340\u00a0\u00b0C, respectively, at a constant pressure of 6.4\u00a0MPa and H2/oil volume ratio of 300 v/v.We thank the financial support of China Petrochemical Corporation (Sinopec Group, 120051-1) for financial support.", "descript": "\n                  The high and low temperature dual reaction zone RTS (removing trace sulfur) technology is a novel hydrotreating process but lack of in-depth understanding of its kinetics. Three-lump and five-lump kinetic models were developed based on the diesel hydrogenation experimental data which were carried out in the RTS process under various operating conditions to predict the concentrations of ultra-level sulfur and aromatics in hydrotreated oil samples, respectively. Moreover, the inhibiting effect of the hydrogen sulfide (H2S) on the hydrodesulfurization rates of S-compounds has been studied by performing calculations with the actual adsorption concentration of H2S in the catalytic active center. The proposed models were able to reproduce the RTS process with good adjustment and accuracy, and relative deviations below 10.7 % at sulfur content below 10\u00a0ppm and polycyclic aromatics hydrocarbons content below 3\u00a0wt%. Therefore, these models can serve as an effective guide for diesel hydrotreatment, and the calculated results indicate that the optimal operating conditions in the 1st and 2nd reactors are 350\u00a0\u00b0C and 340\u00a0\u00b0C, respectively, at a constant pressure of 6.4\u00a0MPa and H2/oil volume ratio of 300 v/v.\n               "}
{"full_text": "As an important intermediate in the manufacture of bulk commodity, such as nylon 6 and nylon 66, the production of cyclohexanone on a commercial scale is very intriguing[1]. Hydrogenation of phenol is thought to be a better alternative compared to the oxidation route of cyclohexane which requires harsh reaction conditions with one-way conversion less than 10% and generates complex byproducts difficult to separate[2\u20135]. Generally, hydrogenation of phenol involves either a \u201cone-step\u201d or \u201ctwo-step\u201d process[6]. The \u201cone-step\u201d process, i.e. hydrogenation of phenol straight forward to cyclohexanone, is considered to be a greener route compared with the \u201ctwo-step\u201d one as it avoids the endothermic back-dehydrogenation step towards cyclohexanol, which consumes additional energy[7]. The ultimate challenge for \u201cone-step\u201d process, which would limit the possibility for industrial application, lies in holding a high selectivity (>95%) when reaching the full conversion for an efficient catalyst[8\u201310].Supported noble metal catalysts have been verified as very efficient for hydrogenation[8,11\u201315]. However, the selectivity to a certain substrate such as phenol varies to a great extent among different noble metals[8,11,16\u201319]. Though the selectivity can be affected by reaction conditions [15,21,23\u201326] and supports of different properties[9,15,27\u201329], the general trend can still be informed by studies currently available, which reflects the inner factors controlled by the active components (i.e. noble metals) in nature[8,11,15,16,19,20,30,31]. Generally, the selectivity to cyclohexanone follows the order: Pd\u00a0>\u00a0Pt\u00a0>\u00a0Rh, Ru and Ni for the most studied active metal components[14,32]. Due to the superior performance, Pd catalysts are the most studied catalysts. They usually give a selectivity in the range of 80\u2013100% at full conversion ranging different supports. Pt catalysts, yet, always afford a moderate selectivity in the range of about 50\u201385% without modification of the second metal like Cr[16,33,34]. In contrast, Rh, Ru and Ni catalysts often offer a much lower selectivity of only 5\u201360%[17,35,36]. Therein identifying the fatal factors determining the inner selectivity of noble metal catalysts to phenol hydrogenation, which would guide the direction for catalyst screening[37], is the key to efficient rational catalyst design.A deep investigation to the reaction mechanism is certainly helpful in that the difference along the reaction pathways will give valuable information causing the distinction of such apparent catalytic behaviors as selectivity, activity and deactivation. Previously, it is generally agreed that phenol is hydrogenated to an unsaturated cyclohexenol firstly. Then, quick tautomerization of cyclohexenol leads to the desired product cyclohexanone, which may undergo further hydrogenation to cyclohexanol[11,38,39]. However, it is noteworthy that tautomerization of cyclohexenol most likely occurs under the catalysis of noble metals [40,41] in that the barrier for this process will be much lower than that in the gas phase or under the water mediated condition [14,42] without the catalysis of metals. On the other hand, surface chemistry investigations revealed that the OH bond (in phenol) scission could readily occur on various metals upon phenol adsorption[43\u201347], which may greatly influence the reaction pathways[40,48,49]. In fact, this OH bond scission will cause tautomerization of phenol before the hydrogenation process[40,50]. To what extent the OH bond scission would affect the reaction pathways and further the product distribution under certain conditions is to be assessed considering that the selectivity to cyclohexanone varies a lot over sorts of noble metals. Whether the factors determining the selectivity remain the same among various noble metals is also to be addressed since recent work suggest that the strong interaction between alkali metal cation and the carbonyl group in cyclohexanone (Mn+-OC-) would also suppress the further hydrogenation of cyclohexanone[51,52].In this work, first-principles studies on the reaction pathways for phenol hydrogenation on noble metals (Pt, Pd and Ru) are performed with OH bond scission under consideration. To support our results, experiments for aqueous hydrogenation of phenol are conducted accordingly on Pt/SiO2, Pd/SiO2 and Ru/SiO2 catalysts. Pt, Pd and R are chosen because of their typical discrepancy of selectivity to cyclohexanone. SiO2 is chosen as the support since it is normally considered as inert in most reactions [22,27,50,53]. The conversion of phenol is deliberately controlled at a low value to capture the initial selectivity evolution at the very beginning. The solvation effects haven\u2019t been included in this theoretical calculation since it has been discussed in detail elsewhere and summed up into four points [8,42,54\u201360]: different solubility of hydrogen in reaction solution, competitive adsorption between solvent molecules and reactants/products on the active sites of the catalyst, inducing agglomeration of supported metal nano-particles, and non-covalent interactions between solvent molecules and or reactants/products with the solvent. Recently, our work has also revealed the promotion of tautomerization by water [14]. Our main results show that no matter whether the OH bond scission occurs before or during the aromatic hydrogenation process, different reaction pathways always result in the formation of cyclohexanone at first under mild conditions. Cyclohexanol is produced by the over hydrogenation of cyclohexanone. The remarkable low selectivity to cyclohexanone on Ru is ascribed to stabilization for metastable adsorption of cyclohexanone, which enhances the chance for sequential hydrogenation, and the co-catalysis by H2O, which promotes the hydrogenation of the CO bond in cyclohexanone to a great deal.Na2PdCl4 (Pd, 40%), H2PtCl6\u2022xH2O (Pt, \u226537.5%), RuCl3\u2022xH2O (Ru, 40%), fumed SiO2 were used as received from Aladdin Chemistry Co with a specific surface area of 600\u00a0m2/g. Active carbon (HPC) was home-made by a \u201cbread leavening\u201d method [61].Catalysts were prepared with the classical impregnation method. In a typical process, Na2PdCl4 precursor was dissolved in deionized water and impregnated on fumed SiO2 to produce a catalyst of 1\u00a0wt% Pd over fumed SiO2. After the impregnation, the catalysts were dried at 50\u00a0\u00b0C overnight and then reduced in a 30\u00a0mL/min H2 flow at 80\u00a0\u00b0C for 1\u00a0h with a heating rate of 5\u00a0\u00b0C/min. The obtained catalyst was denoted as Pd/SiO2. As for Pt and Ru, the reduction temperatures were settled as 80 and 160\u00a0\u00b0C, respectively.X-ray diffraction (XRD) data were collected on an Ultima TV X-ray diffractometer with Cu K\u03b1 radiation (1.54\u00a0\u00c5). Transmission electronic microscopy (TEM) measurements were taken on a Hitachi HT-7700 microscope instrument at 100\u00a0kV. The detailed results and discussion for the as-prepared catalyst were shown in Figs.\u00a0S1 and S2 in the supporting information.For atmospheric reaction, in a typical process, 0.585\u00a0mmol phenol, 25\u00a0mg\u00a0Pd/SiO2 and 5\u00a0mL deionized water were put into a three-neck flask. The reaction was carried out at a temperature of 65\u00a0\u00b0C with magnetic stirring at a speed of 1000\u00a0rpm. Before reaction, a balloon filled with hydrogen was connected to the flask to replace the air. For catalytic reaction operated beyond atmospheric pressure, the hydrogenation was carried out in a 50\u00a0mL stainless steel high-pressure batch reactor. Firstly, certain amounts of substrate, catalyst, and solvent were put into the autoclave. Then, the reactor was purged three times with pure H2 to remove residual air. After that, the reactor was charged with H2 of required pressure and the reaction mixture was stirred at 65\u00a0\u00b0C with magnetic stirring at a speed of 1000\u00a0rpm. After reaction, the reactor was cooled to room temperature with water bath and then the remaining H2 was vented. The contents of products and substrate were determined by GC-FID and the products were identified by GC-MS.The calculations are performed by using periodic, spin-polarized DFT as implemented in Vienna ab initio program package (VASP) [62,63]. The electron-ion interactions are described by the projector augmented wave (PAW) method proposed by Bl\u00f6chl [64] and implemented by Kresse [65]. RPBE functional [66] is used as exchange-correlation functional approximation. A plane wave basis set with an energy cutoff of 400\u00a0eV is used. A p (5\u00a0\u00d7\u00a05) surpercell containing a four-layer slab with 100 atoms was modeled as catalyst and (111) plane is considered as the active surface. For Pd and Ru, only gamma point is used for the Brillouin zone sampling when energy barrier is calculated, while a (2\u00a0\u00d7\u00a02\u00a0\u00d7\u00a01) k-point grid is used for Pt and Ir owing to the deep d band. For phenol and cyclohexanone adsorption, a (2\u00a0\u00d7\u00a02\u00a0\u00d7\u00a01) k-point grid is used on all the catalysts. The results of adsorption energy for k-point test are listed in Table\u00a0S1 And (3\u00a0\u00d7\u00a03\u00a0\u00d7\u00a01) k-point grids were used for the Brillouin zone sampling for bader charge analysis [67]. The periodic condition is employed along the x and y direction. The vacuum space along the z direction was set to be 13\u00a0\u00c5. The upper two layers are allowed to relax during the structure optimization, while the bottom two layers of atoms are fixed. The relaxation is stopped when the force residue on the atom is smaller than 0.02\u00a0eV/\u00c5. The transition states are calculated by using the climbing image nudged elastic band (CI-NEB) method [68].The adsorption energy for molecule chemisorption is defined respectively as:\n\nEb\u00a0=\u00a0Etot \u2013 Eslab \u2013 Emol\n\n\nwhere Etot is the total energy after a molecule adsorption on catalysts; Eslab is the energy of the clean catalyst alone; Emol is the energy of the molecule in the gas phase.The conversion and cyclohexanone selectivity are plotted versus time during the hydrogenation of phenol at 65\u00a0\u00b0C as shown in Fig.\u00a01\n. To minimize the impact of over hydrogenation, mild conditions are chosen and low conversions (<5%) are controlled deliberately. Only cyclohexanone is formed when the conversion is below 1% on Pt/SiO2. Then the selectivity decreases when extending the time, indicating that cyclohexanol may well be obtained by the sequential hydrogenation of cyclohexanone since the main product can well be expected to be cyclohexanol on Pt catalysts given the full conversion of phenol [16,19]. The selectivity remains 100% under the reaction time on Pd/SiO2, which is consistent with many other reports showing good selectivity. However, severe over-hydrogenation occurs on Ru/SiO2 since the beginning, with the selectivity of cyclohexanone expected to be lower than 50% given the conversion higher than 5%. Despite the distinct selectivity, one thing in common is that the path to cyclohexanone is prior even if the path direct to cyclohexanol exists since the main.Product is cyclohexanone at low conversions. Similar results can also be found for noble metals supported on active carbon as seen in Fig.\u00a0S3. Note that the alkali metal cation induced by the Pd precursor might modify the selectivity to cyclohexanone. The Pd/SiO2 catalysts made with PdCl2 precursor is also tested. The results show that the selectivity to cyclohexanone also keeps to 100% at low conversions of phenol (see Fig.\u00a0S4 in the supporting information), indicating that the selectivity regulating ability occurs only when the alkali metal cations reach a certain concentration [52].To unravel the hydrogenation mechanism and factors resulting in the selectivity distinction over different metal catalysts, the reaction pathways were conducted in first principle. According to Yoon et\u00a0al. the first hydrogen (H) addition on Ni (111) and Pt (111) has the highest energy barrier [42]. And Li et\u00a0al. suggested that the dissociation of phenol group (OH) before phenyl ring hydrogenation would lead to a different product distribution or selectivity [40]. Therein the first hydrogenation step seems to be rather important and is investigated on Pt, Pd, and Ru firstly. Both direct hydrogenation and dissociative hydrogenation (hydrogenation after OH dissociation, i.e. the tautomerization of phenol) are considered.For phenol adsorption, the most stable geometries on Pt and Pd are the same (see top view in Fig.\u00a02\n and side view in Fig.\u00a0S5), which is consistent with previous reports [40], with their adsorption energies being \u22120.60 and \u22120.49\u00a0eV respectively. However, this is not the case on Ru, with C1, C3, and C5 bonded to the corresponding Ru atoms. The adsorption energy on Ru (\u22120.71\u00a0eV) is much bigger than that on Pt and.Pd, indicating a stronger adsorption.Then the first hydrogenation steps were calculated (see Fig.\u00a03\n). On Pt, the dissociation barrier of OH along the dissociative hydrogenation path is only 0.50\u00a0eV, which is in consistent with reported work [22,40,49], while the sequential phenoxy hydrogenation barrier reaches 0.96\u00a0eV. Hence the overall barrier along the dissociative hydrogenation path is generally determined by phenoxy hydrogenation. The energy barrier for the direct hydrogenation path is 1.06\u00a0eV, much similar to that for the sequential phenoxy hydrogenation. Together with the similar reaction energy for each step of the two paths (about 0.22\u00a0eV), it is reasonable to conclude that both the direct and dissociative hydrogenation paths may exist in the reaction under mild conditions. In fact, numerous phenol adsorption studies on Pt (111) [45], Pd (110) [47], and other metal surfaces [43,44,46] suggested that phenol can dissociate to phenoxy at rather low temperatures. When reaction occurs on Pd, the situation is rather similar with those on Pt, except that the overall energy barrier is determined by OH scission (0.69\u00a0eV) along the dissociative path, making the trend along this path relatively smaller than that on Pt.However, the situation on Ru is different. OH scission on Ru is rather exothermic [22,69], together with a lowest barrier of only 0.41\u00a0eV compared with that on Pt and Pd. Meanwhile, the carbon atom in aromatic ring to be hydrogenated is different on Ru (C3) compared to that on Pt and Pd (C1) along the dissociative path. Moreover, the hydrogenation energy barrier along the dissociative hydrogenation path is still lower than the one along the direct hydrogenation path. So the dominant reaction may well undergo the dissociative hydrogenation path on Ru [22].To figure out the OH scission tendency on different metals, i.e. the favor of the dissociative hydrogenation path before the aromatic hydrogenation process, OH scission on the transition metal catalysts which are commonly used as hydrogenation or hydrodeoxygenation catalysts is calculated. A well-defined BEP relationship is shown in Fig.\u00a04\n (a) except for Pt, with a slope of 0.30 and intercept of 0.64. The small slope value (<0.5) indicates an early transition state during the OH scission and a less thermodynamic driving process [70\u201373], which is consistent with the experimental facts at low temperatures mentioned above. In general, OH dissociates exothermically on Fe, Co and Ru, while endothermically on Ni, Pt, Pd and other noble metals, with most and least favorable scission on Fe and Ni respectively. The BEP relationship suggests that more oxophilic catalysts, such as Fe, Co and Ru, tend to dissociate OH more easily and in turn the reaction more likely follows the dissociative path from the beginning, and in reverse. The reason giving rise to the deviation of Pt contributes to the strong repulsion of ketonic group (CO) after OH scission [40,48]. As shown in the DIS state in Figure.3, CO of phenoxy titled away from the Pt surface, while the O atom bind to the surface for the other metals (see Fig.\u00a04 (c) and (d)). The repulsion increases the dissociation energy of phenol on Pt, making the datum of Pt deviating to the right from the normal line.To figure out the inner reason for the deviation, the electronic structure analysis was applied. As seen in Fig.\u00a05\n, the density of states (DOS) of C atom in the ketonic group on metal surfaces along the normal axis, like Pd, Ir and Ru, locate deeper than that on Pt. The strong interaction between the ketonic group (through the aromatic ring) and the metal surface makes sure the anti-bond between the O atom of CO and metal.Surface being lifted above the Fermi level (see violet dashed lines). One would suspect that whether the anaerobism of Pt leads to the repulsion of CO since Pt is noblest among the studied subjects. This possibility can be excluded as we compare the DOS\u2019s for stable and meta-stable adsorption of phenoxy on Pt respectively, as shown in Fig.\u00a05 d and e. At meta-stable state, the O atom in CO binds to the Pt surface and the main bonding DOS of the carbon atom in CO locates between \u22128 and \u22124.5\u00a0eV. However, the binding breaks when phenoxy adjusts to a stable state and the location of the main bonding DOS of the C atom shifts up forward by 0.5\u00a0eV correspondingly. It clearly illustrates the local energy sacrifice of the O\u2013Pt binding so as to achieve a global optimization of phenoxy, while it is not the case on Pd and Ir as shown in Fig.\u00a0S6.The similarity of the first hydrogenation step on Pt and Pd cannot tell the difference on the selectivity of cyclohexanone. Then further hydrogenation steps are investigated. Note that OH scission may happen in the aromatic hydrogenation process [49]. The OH scission possibility during the aromatic hydrogenation process is valued (see Fig.\u00a04b). Since the OH group titled more away from the surface when the first two H atoms are added to the phenyl ring (see Fig.\u00a04 e and f), the scission barriers are expected to be rather high. Hence the OH dissociation energies were calculated after 0, 3, 4 and 5\u00a0H atoms were added to the phenyl ring (see Fig.\u00a04b). Results suggest that the OH scission becomes thermodynamically favorable after the phenyl ring was hydrogenated by 4\u00a0H atoms. Combining the BEP relationship, the scission barriers would be lower than 0.65\u00a0eV on Pd, so further hydrogenation calculations started after 4\u00a0H atoms addition before OH scission. Both direct and dissociative paths were considered on Pt and Pd, while only dissociative path was calculated on Ru since the dissociative path dominates the hydrogenation of phenol on Ru catalyst (see Fig.\u00a04a).The intermediate with 4\u00a0H atoms being hydrogenated are denoted as 4H. For 4H, OH remains on Pt and Pd, forming adsorbed cyclohexenol, while it has dissociated at the beginning on Ru. The energy profiles for 4H sequential hydrogenation on these catalysts are shown in Fig.\u00a06\n. For 4H hydrogenation on Pt along the dissociative hydrogenation path, OH dissociates at first, overcoming 0.46\u00a0eV in the formation of IM3. Chemisorbed cyclohexanone forms when one H was added to IM3 with an energy barrier of 0.77\u00a0eV. Then it takes only 0.17\u00a0eV for cyclohexanone to desorb. In view of the over hydrogenation fact on Pt, the sequential hydrogenation of.Chemisorbed cyclohexanone is considered. However, the hydrogenation of C1 in cyclohexanone is calculated to be rather difficult, within a barrier up to 1.67\u00a0eV, much higher than the first H addition step. Thereafter cyclohexanol is not likely to be formed along this path. Along the direct hydrogenation path, a barrier of only 0.15\u00a0eV is required in the fifth hydrogenation step ending with IM1. When IM1 reacts forward continually, it has to desorb first, forming suspended and meta-stable IM2. The path splits into two branches then. One branch leads to the OH scission in IM2, forming the desired cyclohexanone without any barrier. Another is that IM2 overcomes a small energy barrier of 0.30\u00a0eV undergoing sequential hydrogenation of C1 to form cyclohexanol. Apparently, the first branch (forming molecular cyclohexanone) dominates along the direct path. The second branch unseals given that cyclohexanone assembled on the surface to a concentration enough to make the equilibrium of the first branch reversed. Comparing with the two paths, the direct path encounters lower barriers and seems more favorable from a kinetic point of view. However, the intrinsic selectivity to cyclohexanone is determined by none of them since both the two paths prefer to form cyclohexanone firstly in principle, while cyclohexanol is formed subsequently from molecular cyclohexanone hydrogenation. Note that all the calculation is performed at a low hydrogen surface coverage without considering hydrogen partial pressure, aiming to simulate the experiment conditions. When increasing the hydrogen partial pressure, OH scission in IM2 is expected to be inhibited or even reversed owing to the high hydrogen coverage or increased reduction potential. As a result, hydrogenation of IM2 or resulted cyclohexanone to cyclohexanol becomes favorable. This helps explain the reason why phenol hydrogenation on Pt gives a high selectivity to cyclohexanone at both low hydrogen pressure and conversion (see Fig.\u00a01a), while the selectivity decreases when hydrogen pressure rises [20,74\u201376]. Note that 4H and/or IM2 would also desorb and undergo the tautomerization by water to form cyclohexanone [14]. Since the process on the metal surface is more energy favorable, it is not included this time.In the case of the hydrogenation process on Pd, the general trend is similar with that on Pt except for some concrete values. It is essential to claim that the direct path is still not responsible to the formation of cyclohexanol since it is easier to form cyclohexanone through OH scission with a barrier of only 0.11\u00a0eV than to form cyclohexanol through sequential hydrogenation with a barrier up to 0.58\u00a0eV (this value increase to 0.67\u00a0eV when same k-points sampling was used on Pt). The desorption energy of cyclohexanone is 0.20\u00a0eV, similar with that on Pt. Compared with Pt, the selectivity difference to cyclohexanone may be well caused by one single reason, namely, the ease of over-hydrogenation. The lower barrier on Pt (0.30\u00a0eV) makes cyclohexanone easier to be over hydrogenated than that on Pd (0.58 or even 0.67\u00a0eV), thus a lower selectivity to cyclohexanone is well expected. To confirm the hypothesis, cyclohexanone was hydrogenated under the same conditions as phenol (see Fig.\u00a07\na). Results show that cyclohexanone is easy to be converted to cyclohexanol on Pt, while it is not the case on Pd, which testifies the hypothesis above.As discussed above, only dissociative path is considered for phenol hydrogenation on Ru in that phenol prefers to undergo OH scission before the aromatic hydrogenation.Hence the final state is actually IM3 after 4\u00a0H atoms are added, which naturally serves as the initial state in Fig.\u00a06c for Ru. The energy barrier (0.70\u00a0eV) for the fifth H addition is similar with the case on Pt, forming chemisorbed cyclohexanone. Then the desired product formed after overcoming a desorption energy as large as 0.46\u00a0eV. For the over hydrogenation of cyclohexanone to cyclohexanol, two possible paths are studied as discussed above on Pt and Pd. The first path starts from desorbed cyclohexanone. The first H is added at the O atom of CO in cyclohexanone, absorbing 0.84\u00a0eV energy. Then the second is added at the C atom of CO (C1), with an energy barrier of 0.44\u00a0eV and a great heat release. The second path starts from chemisorbed cyclohexanone. Note that there is a metastable chemisorption state (IM6, endothermic by 0.40\u00a0eV) of cyclohexanone on Ru which is not found on Pt and Pd, with both the C and O atoms of CO binding to the surface. Though this state makes no contribution to cyclohexanone over hydrogenation at this moment (in gas phase), it does be responsible when solvent of H2O is considered as you will see below. The origin of this meta-stable state can be attributed to the radicalization of C1 on Ru compared with that on Pt and Pd as seen in Fig.\u00a07b for chemisorbed ketone (IM4). Bader charge analysis reveals that C1 on Ru gains the most electrons. Interestingly, only a barrier of 0.71\u00a0eV and an energy gain of 0.10\u00a0eV are required when the first H is added to C1 and the second to the O atom of CO with a barrier of 0.76\u00a0eV, more favorable than the first path, which is opposite to that on Pt and Pd. Specifically, It is easier to add H atom to the O atom of CO first on Pt and Pd, while it is preferential to C1 first on Ru. The big difference is expected to originate from this radicalization.Comparing the over-hydrogenation situation with that on Pt and Pd, the selectivity to cyclohexanone on Ru is expected to be rather high. However, the experimental results in Fig.\u00a01 are incompatible to this expectation. In fact, the lowest selectivity on Ru was achieved. This inconsistence may well be caused by the promotion of the solvent (H2O), since H2O is proved to be a very efficient co-catalyst for ketone hydrogenation on Ru catalyst, while it is not on Pt and Pd [77\u201379]. Then the effect of solvent is considered, i.e. a chemisorbed H2O near cyclohexanone was implemented concretely to model the effect of H2O as reported [77,80]. Note that the solvation effect (dielectric constant) of H2O is out of the scope in this work in that it has been investigated in detail elsewhere and its impacts to noble metals are considered to similar [42,54]. Fig.\u00a08\n shows the energy profiles along the preferred over-hydrogenation path of cyclohexanone in water. The co-adsorption of cyclohexanone and H2O (IM6\u2019) gives a lower energy (stabilization energy) of 0.25\u00a0eV due to the hydrogen bond stabilization between the O of CO and the H of H2O, compared to their separate adsorption, which suggests that H2O induced co-adsorption will increase the coverage of cyclohexanone on the surface of Ru and accordingly the possibility of further hydrogenation of cyclohexanone. Consequently, the following hydrogenation steps are more likely to advance in the presence of H2O nearby [77]. In contrast to the situation of no solvent being considered in Fig.\u00a06. The barrier of H addition to the C1 decreases by 0.15\u00a0eV, and reaction energy changes to be rather exothermic. The barrier of the following H addition decreases by surprising 0.56\u00a0eV when using the H atom of co-adsorbed H2O as H source rather than H2, forming chemisorbed cyclohexanol. At last, the resulted OH is hydrogenated by the dissociated H atom of H2 with a barrier of 0.94\u00a0eV. It is obvious that H2O as a co-catalyst can greatly accelerate the over hydrogenation process of cyclohexanone on Ru. This can be testified by changing the solvent of H2O to ethanol for cyclohexanone hydrogenation as seen in Fig.\u00a07a. When.Hydrogenating in ethanol, the conversion of cyclohexanone on Ru/SiO2 is only 9%, lower than that on Pt/SiO2 (12%). Instead, nearly full conversion is achieved on Ru/SiO2 in H2O, while it is only 68% on Pt/SiO2. The obvious increase of the activity for cyclohexanone hydrogenation and the shifts in activity order verify the co-catalyzing effect of H2O. Note that the regeneration of H2O is the rate-determining step for over hydrogenation in H2O. The high barrier of this step seems to be paradoxical to the promotion effect from H2O. In fact, water molecules exist as clusters or slides on the surface in experimental conditions [81\u201384]. The positions of H affect the regeneration activity of H2O. Two typical positions of H are then explored in a water cluster as examples [84], as seen in Fig.\u00a08. The barriers and energy profiles for water regeneration both decrease to a certain extent when H is located at the edge and in the center of the water cluster respectively. This indicates the regeneration of water may be much easier in real conditions. In fact, the regeneration of water requires only 0.28\u00a0eV when the solvation effect is considered [85].For Pt and Pd, this path is not feasible in that the chemisorbed cyclohexanone desorbs from the metal surface and stays with chemisorbed H2O by hydrogen bond interaction (see Fig.\u00a0S7). Hence the path with C0 being preferentially hydrogenated is interrupted. For another path started from IM1, though the energy barrier for the formation of cyclohexanone rises from 0.16 to 0.36\u00a0eV on Pt, the meta-stable intermediate IM2 vanished, which indicates the inhibition of IM2 to cyclohexanol. In contrast, the transformation from IM1 to cyclohexanone becomes barrierless on Pd (see Fig.\u00a0S7). To sum up, phenol is more likely to be hydrogenated to cyclohexanol on Ru, while cyclohexanone is preferred on Pt and Pd in the presence of H2O.Then how to make an integrated description to predict the trend of cyclohexanone selectivity for phenol hydrogenation based on the intrinsic character of noble metal active sites, eliminating the co-catalyzing effect of the solvent? Generally, two factors must be taken into account, namely, the intrinsic activity to cyclohexanone hydrogenation and the competitive chemisorption between phenol and cyclohexanone which affects the proportion of the active sites available for cyclohexanone hydrogenation.After carefully examining the structures along the favorable hydrogenation pathways of cyclohexanone, we suspect that the binding strength of H atom (Eb(H)) on metal surface may be vital to the activity. As seen in Fig.\u00a06, the hydrogenation of IM2 (Pd and Pt) or IM5 to cyclohexanol actually involves the hydrogenation of a radical. In this process, the radical is not binding to the surface of the catalysts. In other words, the catalysts will not affect the radical directly. Since the radical is same to all the catalysts, then the barriers for hydrogenation are only determined by the activity of the H atoms on the metal surfaces. The stronger binding of H atom on the metal surface, the harder release of H atom from the metal surface, the less activity for the hydrogenation of the radical. Then the H atom binding energy against the corresponding energy barrier of cyclohexanol formation on various noble metal catalysts is plotted as shown in Fig.\u00a09\na. As expected, a clear linear relationship was obtained, indicating weak binding of H atom on the surface is favorable for cyclohexanone hydrogenation. Since Eb(H) is charged by the d band center (see Fig.\u00a09b), which has been proved by previous work [86], the intrinsic activity for cyclohexanone hydrogenation is then actually controlled by the d band center of the noble metal catalyst as shown in Fig.\u00a09c. According to the Sabatier principle, the activity is roughly proportional to the binding energy of the reactant or intermediate for each branch. Then the activity should be proportional to 1/Ea owing to the linear relationship between Ea and Eb(H).As for the second aspect, we simply assume the proportion of the active sites available for cyclohexanone hydrogenation is proportional to the binding energy ratio (Eb (one/pl)) of cyclohexanone to phenol. Combining with the first aspect, we propose the value of Eb (one/pl)/Ea should be a reasonable rough descriptor to predict the trend of cyclohexanone selectivity for phenol hydrogenation. To testify our hypothesis, Eb (ketone/phenol)/Ea is correlated to the yield ratio of cyclohexanol to cyclohexanone (Rationol/one) when ethanol was used as solvent. As shown in Fig.\u00a09d, Rationol/one increases along with Eb (one/pl)/Ea, meaning Eb (ketone/phenol)/Ea can be used as a reasonable rough descriptor for rational catalyst design about phenol selective hydrogenation. And with this relationship, we can predict that the value of Eb (one/pl)/Ea should be no higher than 0.6, which is the value for Pd, if high cyclohexanone selectivity were expected to be achieved on this catalyst. It is noteworthy that we failed to correlate Eb (one/pl) to a single electronic structure descriptor intending to build a full-electronic-structure descriptor in that the d band center failed to describe the binding strength trend of phenol and cyclohexanone with multiple adsorption sites (see Fig.\u00a0S8).Both experimental and theoretical studies suggest that phenol is intrinsically inclined to be hydrogenated to cyclohexanone at first among the studied noble catalysts, independent of the direct or dissociative paths. Cyclohexanol is mainly produced by over hydrogenation. Generally, two main reasons are responsible for the over hydrogenation for most of the studied noble metal catalysts, i.e. the discrepancy of the over hydrogenation barrier and the competitive chemisorption between phenol and cyclohexanone, based on which a quantitative descriptor, Eb (one/pl)/Ea, is firstly proposed to theoretically evaluate and predict the selectivity to cyclohexanone for rational catalyst design. For a special case, H2O can serve as an efficient co-catalyst for phenol over hydrogenation on Ru, which results in the extremely low cyclohexanone selectivity in aqueous solution.The authors declare no competing financial interest.We sincerely appreciate the fruitful discussion with Prof. Qingfeng Ge and Dr. Quanxi Zhu.This work was supported by Financial support from the National Natural Science Foundation of China (21908189, 21872121), the National Key R&D Program of China (2016YFA0202900), the Key Program supported by the Natural Science Foundation of Zhejiang Province, China (LZ18B060002), and the Key R&D Project of Zhejiang Province (2020C01133).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.nanoms.2020.11.002.", "descript": "\n                  Selective hydrogenation of phenol to cyclohexanone is intriguing in chemical industry. Though a few catalysts with promising performances have been developed in recent years, the basic principle for catalyst design is still missing owing to the unclear catalytic mechanism. This work tries to unravel the mechanism of phenol hydrogenation and the reasons causing the selectivity discrepancy on noble metal catalysts under mild conditions. Results show that different reaction pathways always firstly converge to the formation of cyclohexanone under mild conditions. The selectivity discrepancy mainly depends on the activity for cyclohexanone sequential hydrogenation, in which two factors are found to be responsible, i.e. the hydrogenation energy barrier and the competitive chemisorption between phenol and cyclohexanone, if the specific co-catalyzing effect of H2O on Ru is not considered. Based on the above results, a quantitative descriptor, Eb(one/pl)/Ea, in which Ea can be further correlated to the d band center of the noble metal catalyst, is proposed by the first time to roughly evaluate and predict the selectivity to cyclohexanone for catalyst screening.\n               "}
{"full_text": "Data will be made available on request.To cope with the climate crisis we are facing today due to greenhouse gases from fossil fuel use, the transition to renewable energy is essential. Hydrogen is a key factor in increasing the share of renewable energy because it is an efficient, clean energy carrier: it can be produced from renewable energy and later be used as a fuel without greenhouse gas emissions. As hydrogen is a gas at room temperature, finding a means to more efficiently store hydrogen in a solid-state hydrogen storage material with enhanced accessibility remains a challenge.TiFe-based metal hydrides are one type of promising solid-state hydrogen storage materials [1,2]. They have a reasonable hydrogen storage capacity of 1.9\u00a0wt% when a dihydride (TiFeH\u223c2) is formed [3], and are cost effective for scalable deployment. However, initial hydrogenation of TiFe-based alloys\u2014the so-called activation process\u2014is difficult [4\u20136]. In part, this is thought to be due to the formation of native protective surface oxides or hydroxides by reacting with air, which prohibit successive reaction with hydrogen. A number of studies have been conducted to mitigate such difficulty and have found that the activation must typically be achieved through repeated heat treatment and/or hydrogen absorption/desorption cycling that induces surface cracking due to volume changes. For example, Reilly et al. activated TiFe by heating to 673\u00a0K in vacuum and cooling to room temperature in a hydrogen atmosphere; this cycle was repeated until hydrogenation occurred [3,7]. Besides time- and energy-intensive thermal treatment processes, research efforts have been made primarily along two lines [2] to achieve activation at room temperature: mechanical treatment and alloying. Mechanical treatment is usually done by high-pressure torsion [8,9], ball-milling [10,11] and rolling [12]. These mechanical treatments produce nanostructured TiFe that remains active even after prolonged exposure to air, probably due to enhanced hydrogen diffusion both at the surface and inside the bulk. In the alloying strategy, Mn was first used to improve the activation properties [13]. Other elements such as Cr, Zr, Ni, etc. are also known to be effective for room-temperature activation [14\u201316].Recently, Kobayashi et al. reported that nano-sized TiFe powders were difficult to activate because the oxide layer is more stable than that of bulk [17], which contrasts well with some previous reports showing comparatively easy activation of TiFe powders composed of nano-sized crystallites [10]. Their studies highlight that activation is closely linked to the characteristic of surface oxide layer, and accordingly, identifying the surface catalysts that may promote activation has been an issue. Many studies have argued that Fe clusters serve as catalysts. Fe clusters on the activated surface were first observed by Bl\u00e4sius et al. [7], and Schlapbach et al. further observed the formation of Fe and TiO2 after the heating process for activation [18]. Selective oxidation of Ti at the surface supported the idea of metallic Fe catalyzing the dissociation of hydrogen molecules on the surface [19\u201321]. Surface characterization utilizing X-ray photoelectron spectroscopy (XPS) also revealed the presence of metallic Fe after vacuum heat treatment [5], after temperature cycling between 223 and 573\u00a0K under 3\u00a0MPa of hydrogen [22], or after repeated absorption/desorption at 673\u00a0K [23]. Nevertheless, there have also been reports that other types of compounds instead contribute to activation. Hiebl et al. [24] claimed that Ti2FeO\nx\n may help absorb hydrogen, and Mintz et al. [25] also found it during heat treatment of TiFe. Schober pointed out that the oxidation of TiFe mainly produces TiFe2 instead of Fe clusters and argued that the catalyst for activation should be oxides such as FeTiO\nx\n, Ti\nn\nO2\n\nn\n\n-1, etc. [26]. Therefore, identifying how Ti and Fe are distributed in the oxide layer is crucial to understanding initial activation mechanisms and promoting the utilization of TiFe-based alloys as hydrogen storage materials.In addition to the attempts to activate pure TiFe, efforts have been devoted to elucidating the role of alloying elements in modifying the surface oxide layer since alloying elements critically affect the activation kinetics. Seiler et al. observed more Fe particles on the surface of a Ti(Fe, Mn) alloy than pure TiFe by means of Auger electron spectroscopy, XPS and magnetization measurements. During 25 hydrogenation (5\u00a0MPa) and dehydrogenation (0.1\u00a0MPa) cycles at room temperature, Fe segregation was found to be much more pronounced on the surface of Ti(Fe, Mn) than on TiFe [27]. This fact was used to explain the relatively easier activation of Mn-doped TiFe. Likewise, alloying elements such as Zr, Cr, Mn, and V, have led to different activation kinetics [16,28,29]. According to Park et al., TiFe alloys in which Fe is partially substituted with Mn or Cr were activated without heat treatment, whereas TiFe alloyed with Ni, Co, or Cu were not activated under the same conditions [30]. Part of the reason was the existence of an easy-to-activate secondary phase formed by Mn or Cr [31\u201333]; however, other studies reported that even single-phase TiFe can exhibit improved activation kinetics depending on the composition [29,34]. Kim et al. [35] attributed the varying performance of alloying elements to their oxidizing power compared to that of Fe. Based on the hypothesis that preferential sites for hydrogen chemisorption on TiFe surfaces are Fe clusters formed during heat treatment, the retarded activation of alloys mixed with Cu and Ni powders was understood in terms of the less stable Cu and Ni oxides versus Fe oxides. Conversely, mixing with Al, Si, Mn, and Mg powders led to easier activation because they are stronger oxide formers than Fe and they sacrificially form oxides, protecting Fe clusters from oxygen contamination.The effect of alloying elements is not limited to modification of the surface oxide layer. It was well established, both by experiments and calculations, that the thermodynamics of hydride formation is also affected by the alloying elements [28,36\u201338]. Interestingly, the dissolution of hydrogen in pure TiFe is endothermic and the solubility of hydrogen at room temperature is very low [37,39]. This is an uncommon characteristic in interstitial metal hydrides and indicates that there is an energy penalty for TiFeH formation prior to further hydrogenation, which can make the initial stages of hydrogenation very difficult. Most alloying elements sitting on the Fe sublattice tend to stabilize the monohydride, TiFeH\u2014in other words, the formation enthalpy of TiFeH from TiFe and H2 becomes more exothermic upon alloying [40]. This effectively reduces or eliminates the thermodynamic penalty, thereby accelerating the activation process.Systematic study of the activation kinetics of TiFe alloys alongside analysis of the oxide layer and the thermodynamics of the hydride formation will help to further understand the activation mechanism. Herein, we characterized TiFe mixed with alloying elements with different electron affinities. In each alloy, 10 at% Fe was replaced by M (M\u00a0=\u00a0V, Cr, Co and Ni), giving a general formula TiFe1\u2212\n\nx\nM\nx\n (x\u00a0=\u00a00.1). The alloying elements V and Cr have higher oxide stability than Fe, whereas Co and Ni have lower oxide stability; pure TiFe was also investigated as a reference. Activation experiments were conducted for each alloy, and the oxidation states of the elements in the oxide layer were analyzed by XPS. The spatial distribution of the atoms in the oxide layer was also observed on an atomic scale using atom probe tomography (APT). These surface analysis results were combined with density functional theory calculations, whereby the energetics of hydride nucleation were investigated and correlated with the activation characteristics.As starting materials, Ti (99.995\u00a0% purity, RND Korea), Fe (99.9\u00a0% purity, RND Korea), V (99.99\u00a0% purity, KRT), Cr (99.95\u00a0% purity, RND Korea), Co (99.95\u00a0% purity, KRT) and Ni (99.99\u00a0% purity, KRT) were used. Alloys were arc-melted in an Ar atmosphere. All samples weighed approximately 20\u00a0g (\u00b10.005\u00a0g). Ti getter was used to minimize the absorption of oxygen during melting. Alloy buttons were turned over five times to ensure compositional homogeneity. The weight loss of the samples after the arc-melting was <1\u00a0%. Heat treatment was performed after arc-melting to eliminate the secondary phases. The samples were annealed at 1273\u00a0K for three weeks in vacuum-sealed quartz tubes and then water quenched right after annealing.A home-made Sieverts-type apparatus was used to carry out the first-time hydrogenation, i.e., activation. For the activation process, a stainless steel reactor (2\u00a0cm3) was charged with approximately 300\u00a0mg of the samples (ground to a particle size of 2\u20133\u00a0mm in air). The reactor was evacuated to a rough vacuum of 0.1\u00a0Pa for 60\u00a0min at 323\u00a0K (or 473\u00a0K). After evacuation, 5\u00a0MPa H2 (99.9999\u00a0% purity) was applied to the sample at 323\u00a0K.X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer, Cu K\u03b1 radiation, \u03bb\u00a0=\u00a01.5418\u00a0\u00c5) was employed for the phase analysis. About 1\u00a0g of sample was hand-crushed, sieved under 100\u00a0\u03bcm and loaded. The 2\u03b8 range for the diffraction studies was from 20 to 115\u00b0, with a step size of 0.03\u00b0 and a duration time of 15\u00a0s step\u22121. The phase analysis was carried out using the Rietveld refinement method (TOPAS software ver. 5, Bruker AXS GmbH).The microstructure of the alloys was characterized by scanning electron microscope (SEM, INSPECT F, FEI Company). The chemical composition of the annealed samples was analyzed using energy dispersive X-ray spectroscopy (EDS). The specimens were mounted and mechanically polished with 1\u00a0\u03bcm diamond compound for the SEM observation.The oxidation states of the elements at the surface layer were characterized by XPS (NEXSA, Thermo Fisher Scientific) depth profiling. The X-ray source is a microfocus monochromatic Al K\u03b1 (1486.6\u00a0eV) with a spot size of 100\u00a0\u03bcm\u00a0\u00d7\u00a0100\u00a0\u03bcm. The specimens were mechanically polished and handled in air before the XPS measurement. For depth profiling, sputter etching of the alloy surface was performed using Ar+ at 1\u00a0kV under the condition optimized to the sputter rate of 4.2\u00a0nm\u00a0min\u22121 for SiO2. The depth profile passing energy was 58.70\u00a0eV and the detection limit was 0.5 at%.Needle-shaped specimens for APT analysis were prepared using a dual-beam focused ion beam (FIB, Nova NanoLab 600, FEI Company) with the site-specific \u201clift-out\u201d method. The specimens were analyzed in a local electrode atom probe (LEAP 4000\u00a0X\u00a0HR, AMETEK) by applying a 10\u00a0ps and 50 pJ of ultraviolet laser pulses (\u03bb\u00a0=\u00a0355\u00a0nm) with a pulse repetition rate of 200\u00a0kHz. The detection rate is 3 ions per 100 pulses on average. The base temperature is 54\u00a0K, and the ion flight path is 382\u00a0mm. The detection efficiency is limited to 38\u00a0% due to the open area of the microchannel plate. The APT data were processed using the commercial software package (IVAS 3.8.8, CAMECA). To visualize the distribution of the elements, raw files containing the number of counts of each element in the 1\u00a0nm3 cubic grid (3D grid-voxel) were used.To simulate the energy change upon TiFeH nucleation in pure and alloyed TiFe, density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) [41]. The exchange\u2013correlation energies were evaluated within the Perdew\u2013Burke\u2013Ernzerhof generalized gradient approximation [42], and a projector-augmented wave (PAW) potential [43] was used. The PAW potentials employed are H, Ti_pv, V_pv, Cr_pv, Fe, Co, and Ni in the VASP pseudopotential library.A 4\u00a0\u00d7\u00a04\u00a0\u00d7\u00a04 supercell of TiFe unit cell containing 128 metal atoms and a 6\u00a0\u00d7\u00a06\u00a0\u00d7\u00a06 supercell containing 432 metal atoms were employed to compare the nuclei-size dependent total energy of the systems. For the 4\u00a0\u00d7\u00a04\u00a0\u00d7\u00a04 supercell, 3\u00a0\u00d7\u00a03\u00a0\u00d7\u00a03 and 4\u00a0\u00d7\u00a04\u00a0\u00d7\u00a04\u00a0k-point grids were used for the structure optimization and the final total energy calculation, respectively. For the 6\u00a0\u00d7\u00a06\u00a0\u00d7\u00a06 supercell, a 2\u00a0\u00d7\u00a02\u00a0\u00d7\u00a02\u00a0k-point grid was used for all processes. The Methfessel\u2013Paxton smearing scheme [44] with a smearing energy of 0.2\u00a0eV was applied with a plane-wave cutoff energy of 500\u00a0eV. Structure optimization was performed until the energy of the system converged within\u00a0<\u00a010\u22124 eV, and we confirmed that the same trends for TiFe1\u2212\n\nx\nM\nx\n alloys were reproduced when computed using tighter energy and force convergence criteria of\u00a0<\u00a010\u22125 eV and\u00a0<\u00a00.005\u00a0eV/\u00c5, respectively, in the 4\u00a0\u00d7\u00a04\u00a0\u00d7\u00a04 supercell. Our calculations were done without spin polarization and Hubbard U correction, which best matches with the enthalpy of reaction from TiFe + \u00bd H2 to TiFeH as measured by Guo et al. [45]. As a reference state for hydrogen, an isolated H2 molecule was simulated in a cubic cell of 30\u00a0\u00c53 in size. The optimized H\u2013H distance was 0.750\u00a0\u00c5.When the composition of TiFe alloy is Ti-rich as in the case of TiFe1\u2212\n\nx\nM\nx\n, the DFT study by Ko et al. [37] predicted that all M elements (M\u00a0=\u00a0V, Cr, Co and Ni) prefer to occupy the Fe site. Therefore, we consider configurations in which only the Fe atoms are replaced by M atoms. Four and eight M atoms were introduced in the 4\u00a0\u00d7\u00a04\u00a0\u00d7\u00a04 supercell, corresponding to\u00a0x\u00a0=\u00a00.0625 and 0.125, respectively. The sublattice composed of the M atoms takes either a face-centered cubic (4\u00a0M atoms) or a diamond (8\u00a0M atoms) structure where the M atoms are equally spaced by 8.3 and 5.1\u00a0\u00c5, respectively, before relaxation. On the other hand, 24\u00a0M atoms were introduced in the 6\u00a0\u00d7\u00a06\u00a0\u00d7\u00a06 supercell (x\u00a0=\u00a00.111), where they form multiple {111} planes. The distance between the M atoms is 7.3\u00a0\u00c5 on {111} planes, and the M-substituted {111} planes are spaced by 5.1\u00a0\u00c5 along the [111] directions. The initial positions of the M atoms are summarized in Table S1. Within the supercell, hydrogen atoms are arranged to have a local TiFeH structure, and the cell parameters and atomic positions were optimized. Zero-point energy was not included because we are interested only in the relative energy change before and after alloying in order to extract trends.The motivation of this study is to understand the mechanistic effects on initial hydrogenation of Fe substitution with other elements. To exclude the effect of secondary phases such as TiFe2\u2212\n\ny\nM\ny\n, which strongly changes the activation kinetics [15,34,46], it is important to make a single-phase alloy. For this purpose, each sample was heat treated at 1273\u00a0K for three weeks. XRD and SEM analyses were performed to identify the phase composition. Fig. 1\n shows the XRD results for the five samples. Diffraction peaks from the typical secondary phases such as TiFe2\u2212\n\ny\nM\ny\n or Ti4Fe2O\nz\n were very small, and the proportion of the major phase TiFe0.9M0.1 was higher than 96\u00a0wt% in all samples. The phase composition and the lattice parameters of TiFe0.9M0.1 are summarized in Table S2. In accordance with the XRD analysis result, SEM images in Fig. S1 shows that regions having different contrast, i.e., secondary phases, are very minor. The overall sample composition in the entire area shown in Fig. S1 was obtained by EDS and listed in Table 1\n, confirming that the compositions are close to the target compositions and that M alloying elements preferentially replace Fe atoms.The initial hydrogenation of these samples was tested at a hydrogen pressure of 5\u00a0MPa at 323\u00a0K. Two sets of samples were prepared by vacuum-treating at 323 or 473\u00a0K for 1\u00a0h before applying hydrogen. The time variation of the absorbed hydrogen of these samples is presented in Fig. 2\n. The performance of vacuum-treated samples at 323\u00a0K is presented in Fig. 2a, indicating that samples with M\u00a0=\u00a0V and Cr are activated within a few hours, whereas those with M\u00a0=\u00a0Fe, Co, and Ni remain inactivated even after 400\u00a0h of hydrogen exposure. This result is similar to the study by Kim et al. [35] showing that mixing with metal powders that form more stable oxides than iron oxide promoted activation. One important difference is that we directly alloyed TiFe with other metal elements, while the metal powders were simply mixed with TiFe in their study; nevertheless, the trend appears the same. However, since the activation condition in Fig. 2a could not distinguish among the kinetics of the M\u00a0=\u00a0Fe, Co, and Ni samples, we increased the temperature of the vacuum treatment from 323 to 473\u00a0K and performed another set of activation experiments. High-temperature vacuum treatment generally promotes activation [7] by removing surface impurities, especially oxygen [47]. Fig. 2b shows that the heat treatment at higher temperature did not significantly change the kinetics for M\u00a0=\u00a0Fe, but it did improve the kinetics for M\u00a0=\u00a0Co and Ni (M\u00a0=\u00a0V and Cr retained the faster activation kinetics observed for the lower-temperature treatment). The result is similar to the report by Lee et al. [40] showing that single-phase TiFe1\u2212\n\nx\nM\nx\n (M\u00a0=\u00a0Co, Ni, and Al) is more easily activated than pure TiFe. We note that slight fluctuations in the absorbed hydrogen, including occasional drops below zero, are artifacts of temperature fluctuations as only the temperature of the reactor containing the alloy was controlled, while other parts were exposed to the varying temperature of the room.To probe the effect of alloying on the chemical composition of the native surface oxide layer, we first identified the oxidation states of its constituent elements by XPS analysis. The samples were polished under ambient conditions prior to the XPS experiment, and the XPS depth-profiling was conducted without pre-cleaning so as not to lose the outermost oxide layer. Fig. 3\n illustrates the XPS depth profile results. To better visualize the depth dependence of oxidation states, 36 profiles were plotted in a single graph with color-coded intensities. Depth in the y-axis value, i.e., the distance from the surface, was estimated based on the sputtering rate of SiO2 under the same condition. The outermost surface is located at y\u00a0=\u00a00\u00a0nm, and the y-value decreases toward the oxide/alloy interface. Among the five alloys, M\u00a0=\u00a0V, Fe, and Ni were chosen as representative cases for analysis and the results are presented in Fig. 3a, b, and c, respectively. In the outermost oxide layer, all metal elements exist in oxidized states as follows: Ti 2p3/2 peak at 459\u00a0eV from Ti(IV) oxide, Fe 2p3/2 peak at 710\u00a0eV from Fe(III) oxide, and V 2p3/2 peak at 515\u2013516\u00a0eV from V(IV) or V(III) oxide [48\u201350]. In contrast, no distinct Ni(II) peak is observed in the oxide layer based on the Ni binding energy (Fig. 3c). It seems that only a minor amount of nickel oxide, if any, is formed. In terms of the penetration depth and peak height, the degree of oxidation is more pronounced in the order of Ti(IV)\u00a0>\u00a0V(IV or III)\u00a0>\u00a0Fe(III)\u00a0>\u00a0Ni(0), which is consistent with the oxygen affinity of the metallic elements [51]. Moving towards the bulk alloy (negative y-values in Fig. 3), the peaks shift to lower binding energies, indicating the transition to a metallic state. The Ti 2p3/2 peak at 454.2\u00a0eV and Fe 2p3/2 peak at 707.1\u00a0eV can be assigned to Ti(0) and Fe(0), respectively [52]. The alloying elements also become metallic: V 2p3/2 peak at 512.6\u00a0eV and Ni 2p3/2 peak at 853.1\u00a0eV correspond to V(0) and Ni(0) state, respectively [48\u201350] (the next strongest peaks on the graphs correspond to 2p1/2 peaks of the respective metal element). One conclusion from the XPS analysis is that the oxidation of V is more prominent than Fe, which may indicate sacrificial oxidation of V and protection of Fe from oxidation, as discussed previously in the context of Mn-substituted TiFe [22,27,53]. On the other hand, Ni is apparently more inert to oxidation than Fe despite the better activation kinetics of TiFe0.9Ni0.1 over pure TiFe, implying the elemental oxyphilic tendency alone, as derived from the XPS results, cannot fully explain the roles of the alloying elements in the TiFe activation mechanism.Beyond its overall chemical composition, the atomistic distribution of elements in the oxide layer can also be varied upon introduction of the alloying species. To visualize this distribution within the surface oxide, atom probe tomography was conducted. For the APT analysis, each alloy was mechanically polished under ambient conditions as was done for the XPS analysis. The surface was coated with Ni (approximately 100\u00a0nm thickness) using an e-beam evaporator to protect the surface during FIB milling, and finally a needle-shaped specimen was prepared by FIB-SEM as shown in Fig. S2. Fig. 4\n displays the APT reconstruction of the five samples, resembling the shape of the tips in the lower panel of Fig. S2. In Fig. 4, the green spheres at the top are Ni atoms from the Ni-cap coating on the specimen, and the blue ones are TiO or TiO2 species; the maps indicate that the oxide layer is mainly composed of Ti oxides. Since the reconstruction images are drawn at the same scale for five samples, the thickness of the oxide layer can be directly compared, based on the z-values approximately perpendicular to the surface.\nFig. 5\n shows a proximity histogram measuring the depth-dependent atomic percent of elements, computed by taking the planar average of the three-dimensional (3D) information from Fig. 4. Here, zero distance refers to the location of the isoconcentration surface of 30 at% Ni, with negative values penetrating into the bulk interior; note that this distance definition is different from the z-value in Fig. 4. The difference among the samples is quite noticeable. First, in pure TiFe (Fig. 5c), the Ti:Fe ratio is almost 1:1 both in the oxide layer and in the bulk matrix (distance below\u00a0\u223c\u00a0\u00a0\u221210\u00a0nm). Only the increased oxygen concentration distinguishes the oxide layer. On the contrary, the relative concentration between Ti and Fe deviates for each of the alloyed specimens. In the case of M\u00a0=\u00a0Co and Ni (Fig. 5d and e), Ti-oxide with an overall stoichiometry of TiO\u223c1 (a mixture of TiO2, TiO, and Ti) is formed at the outermost part of the oxide layer (distance between\u00a0\u22123 and 0\u00a0nm). Below this Ti-oxide layer towards the bulk interior, an Fe-rich region with significantly lower oxygen concentration appears, which indicates partial oxidation of Fe. Notably, the co-existence or segregation of Co, which is more inert element than Fe, is suppressed in the outermost Ti-oxide layer. Ni appears to behave similarly to Co, although the overlap with the Ni coating hinders unambiguous interpretation of the histogram profile. Nevertheless, the Ni concentration slightly increased right below the Ti-oxide layer (3\u20135\u00a0nm from the surface), which is consistently found in the XPS result in Fig. 3c indicating that Ni remained in the matrix not participating in the oxide formation.Different tendencies are seen for M\u00a0=\u00a0V and Cr, for which the formation of Ti-oxide layer is much less pronounced. For M\u00a0=\u00a0V, an Fe-rich region stretches from the outermost surface to distance\u00a0=\u00a0\u00a0\u22125\u00a0nm, and the concentration of Ti and O is quite low even in the Ti-oxide layer. For M\u00a0=\u00a0Cr, although the Ti-oxide layer develops as in the M\u00a0=\u00a0Co and Ni samples, the concentrations of Ti and O are lower than those for M\u00a0=\u00a0Co and Ni, and the layer is quite thin. If we define an oxygen threshold of 20 at% or more as the boundary for the Ti-oxide layer, its thickness follows the order Fe (9\u00a0nm)\u00a0>\u00a0Ni (3.1\u00a0nm)\u00a0>\u00a0Co (2.6\u00a0nm)\u00a0>\u00a0Cr (1\u00a0nm)\u00a0>\u00a0V (0\u00a0nm). This trend agrees very well with the activation behavior in Fig. 3: the thinner the Ti-oxide layer, the easier the activation.In addition to the depth-profiles in Fig. 5 integrated over the xy-plane, we use the APT 3D reconstruction data in Fig. 4 to investigate the atomic distribution within the xy-plane. To determine appropriate z-values for cross-section image analysis, we first plotted in Fig. 6\na the integrated oxygen content at each z-value (normalized to its maximum). In each case, a z-value that lies slightly closer to the surface than the layer with maximum oxygen content was selected to draw a representative contour map for the outermost Ti-oxide layer. The contour maps in Fig. 6b and 6c show the concentrations of Ti and Fe, respectively. It is clear that the concentration of Ti in the oxide layer of the M\u00a0=\u00a0Co and Ni samples is higher than that of Fe. Moreover, Ti and Fe are segregated within the xy-plane, indicating phase separation between Ti and Fe within the oxide. Conversely, for M\u00a0=\u00a0V, the concentration of Fe is significantly higher than that of Ti, although separation of Ti-containing and Fe-containing phases is again observed. The segregation of Ti and Fe is not obvious in pure TiFe or for M\u00a0=\u00a0Cr. These characteristics are summarized in Table 2\n. Contour maps for some other z-values are provided in Figs. S3\u2013S7 and show similar trends.The APT analysis reveals that thinner Ti-oxide and higher Fe concentration in the oxide layer promotes activation, as expected given the known protective nature of Ti oxides compared to Fe oxides. From the surface oxide characterization, we could successfully uncover how the alloying elements could accelerate the activation process. These alloying elements induce selective oxidation and segregation of Ti and Fe within the oxide layer [53], and the redistribution of the major elements suppresses the formation of the thick passivating Ti oxide found in pure TiFe: the overall effect is enhanced activation kinetics and the degree to which this kinetics is promoted depends on the oxygen affinity of the alloying elements. Although the detailed surface analyses provide meaningful information on the characteristics of the surface oxides, they are not complete enough to fully explain the observed activation behavior of the alloys in this study. Therefore, we proceeded further to study the hydride nucleation as described in the following section.In addition to the consequences for the passivating surface oxide layer, we looked into another potential result of alloying elements on the activation kinetics of TiFe\u2014namely, the nucleation of TiFeH from the TiFe matrix. A peculiar characteristic of hydrogen dissolution in TiFe is that the reaction to TiFeH initially proceeds endothermically, with the solubility increasing with temperature prior to eventual exothermic formation of the fully hydrided product [39]. It contrasts with other interstitial metal hydrides such as LaNi5, for which hydrogen dissolution is strictly exothermic [54]. Hence, in addition to the surface oxide composition, the alloying element may also affect the hydrogen dissolution energy, which in turn governs the kinetics of nucleation. Because this possibility is difficult to probe experimentally, we invoked DFT calculations to estimate the energy change for the following hydride nuclei formation reaction:\n\n(1)\nTi\nn\nFe\nn\n\n\u2212\n\np\nM\np\n\u00a0+\u00a0q/2 H2\u00a0\u2194\u00a0Ti\nn\nFe\nn\n\n\u2212\n\np\nM\np\nH\nq\n, \u0394Enucl\n\n\n\n\nwhere M stands for the alloying elements. To mimic experimentally relevant compositions, we chose three representative cases for study. Specifically, we used n\u00a0=\u00a064 and 216 for the 4\u00a0\u00d7\u00a04\u00a0\u00d7\u00a04 and 6\u00a0\u00d7\u00a06\u00a0\u00d7\u00a06 supercells, respectively. The number of alloying atoms per supercell, p, was 4 or 8 in the 4\u00a0\u00d7\u00a04\u00a0\u00d7\u00a04 supercell and 24 in the 6\u00a0\u00d7\u00a06\u00a0\u00d7\u00a06 supercell, corresponding to\u00a0x\u00a0=\u00a00.0625, 0.125, and 0.111, respectively, in TiFe1\u2212\n\nx\nM\nx\n (compared with\u00a0x\u00a0=\u00a00.1 experimentally).The hydride nucleus formation energy, \u0394Enucl\n, is plotted as a function of the number of hydrogen atoms (q) in Fig. 7\n. Because hydrogen incorporates interstitially in TiFe without significant lattice rearrangement, H atoms were simply inserted into the would-be H lattice sites of TiFeH around the central Fe or M atoms within each TiFe simulation cell. To minimize spurious interactions across periodic supercell images, the maximum number of H atoms in each case was set to 24, which corresponds to a\u00a0\u223c\u00a02\u00a0\u00d7\u00a02\u00a0\u00d7\u00a02 TiFe supercell for the dimension of the TiFeH nuclei, as illustrated by the shaded areas in the insets of Fig. 7a-c. As expected, the computed hydrogen absorption reaction in pure TiFe is endothermic, which is evident from the positive \u0394Enucl\n regardless of the number of inserted H atoms. For the 4\u00a0\u00d7\u00a04\u00a0\u00d7\u00a04 supercell in Fig. 7a and b, \u0394Enucl\n reaches a maximum at q\u00a0=\u00a016 corresponding to a radius for TiFeH nuclei of\u00a0\u223c\u00a03.8\u00a0\u00c5 and an apparent barrier for homogeneous nucleation of\u00a0\u223c\u00a01.5\u00a0eV. However, determining the exact barrier and critical nucleus size is challenging within DFT due to long-ranged strain relaxation effects, which demand large supercell sizes. As a step in this direction, we expanded the size of TiFe matrix to 6\u00a0\u00d7\u00a06\u00a0\u00d7\u00a06, inserted the same number of H atoms, and examined the energy evolution in Fig. 7c. The energy of system monotonically increases up to 24H atoms, placing a lower bound on both the critical nucleus radius (which should be greater than\u00a0\u223c\u00a04.8\u00a0\u00c5 corresponding to the limit of 24H atoms explored) and on the nucleation barrier (which should be greater than\u00a0\u223c\u00a02.0\u00a0eV).Overall, we can conclude that a sizeable barrier must be overcome to nucleate TiFeH within the TiFe matrix. Although endothermic TiFeH formation is well documented, the energetics of the initial process has not been fully appreciated in relation to the nucleation barrier and the difficulty of activation in TiFe. Nevertheless, we caution against overinterpretation of our computed values, as the model considers only homogeneous nucleation within constrained, perfect single-crystal TiFe, and even for this simple case, exact quantities of critical nucleation parameters are difficult to discern. Instead, we are primarily concerned with how trends change with addition of alloying elements.\nFig. 7 further demonstrates that the alloying elements strongly affect the energetics of hydrogen dissolution and hydride nucleus formation. At the lowest concentration, x\n=\u00a00.0625 in Fig. 7a, which is closest to pure TiFe, all alloying elements M exhibit an energy barrier when 16H atoms are inserted in the 4\u00a0\u00d7\u00a04\u00a0\u00d7\u00a04 simulation cell. However, V and Cr dramatically decrease the nucleation barrier from 1.5\u00a0eV (pure TiFe) to 0.04 and 0.4\u00a0eV (for V and Cr, respectively); eventually, these barriers disappear altogether at the higher alloying concentration of\u00a0x\u00a0=\u00a00.125 (Fig. 7b). Co and Ni also help decrease the nucleation barriers, but not as powerfully as V and Cr, as demonstrated by the retention of barriers at\u00a0x\u00a0=\u00a00.125. Trends are similar for the larger 6\u00a0\u00d7\u00a06\u00a0\u00d7\u00a06 supercell (Fig. 7c), which can more effectively capture the large elastic strain fields. In this case, Co and Ni show overall increasing energy profiles up to 24H atoms for\u00a0x\u00a0=\u00a00.111, but hydrogen incorporation remains consistently more favorable than for pure TiFe. Meanwhile, alloying with V or Cr removes the nucleation barriers and converts endothermic hydrogen absorption to an exothermic reaction, in agreement with the results in the smaller supercell for\u00a0x\u00a0=\u00a00.125. Therefore, we conclude that the chemical effect brought by the alloying elements shows the sequential trend of V\u00a0>\u00a0Cr\u00a0>\u00a0Ni\u00a0>\u00a0Co\u00a0>\u00a0Fe, matching well with the experimentally measured time to initiate activation.While our simulations more directly studied the energetics of hydride nucleus formation, previous DFT studies proposed that the bulk TiFeH formation energy can also inform changes in nucleation behavior upon alloying [36,37]. The stability of hydride can affect activation kinetics by adjusting thermodynamic driving force. For instance, Mn addition resulted in more stable hydride and allowed hydrogenation under lower pressure [8]. The bulk hydride formation energy is defined according to the following reaction:\n\n(2)\nTiFe1\u2212\n\nx\nM\nx\n + \u00bd H2\u00a0\u2194\u00a0TiFe1\u2212\n\nx\nM\nx\nH. \u0394Eform\n\n\n\n\n\nFig. 8\na shows that the formation energy follows a volcano plot with the atomic numbers (Z) of the alloying elements (V (Z\u00a0=\u00a023), Cr (Z\u00a0=\u00a024), Fe (Z\u00a0=\u00a026), Co (Z\u00a0=\u00a027) and Ni (Z\u00a0=\u00a028)), with the highest value centered at Fe. The largely symmetric nature of the plot demonstrates that bulk thermodynamics alone cannot explain the observed activation trends (V\u00a0>\u00a0Cr\u00a0>\u00a0Ni\u00a0>\u00a0Co\u00a0>\u00a0Fe) upon alloying. Values of \u0394Enucl\n for the largest simulated Ti216Fe192M24H24 nuclei in Fig. 7c are co-plotted as solid triangles in Fig. 8a. Similar to \u0394Eform\n, pure TiFe shows the least favorable \u0394Enucl\n; however, it is interesting to see that incorporation of electron donors with smaller electronegativity (V and Cr) are far more effective at promoting hydride nucleation compared to the electron acceptors with larger electronegativity (Co and Ni), even rendering its formation energetically favorable.In order to identify the origin of the chemical trends among the M elements, we proceeded to perform further structural and electronic analysis. First of all, based on the optimized cell volume of TiFe1\u2212\n\nx\nM\nx\n prior to hydrogen absorption, we found that the electron donors (V and Cr) expand the TiFe lattice more than the electron acceptors (Co and Ni). Fig. 8b shows that at\u00a0x\u00a0=\u00a00.111, incorporation of V increases the molar volume of TiFe by 1.70\u00a0%, whereas Ni expands the lattice by only 0.66\u00a0%, in good agreement with the experimental volume expansion in Table S2. As the TiFeH phase requires larger molar volume than TiFe, the lattice expansion upon alloying may benefit nucleation kinetics, especially for V and Cr. Besides the volumetric effect, V and Cr atoms also have lower electronegativity than Fe, Co and Ni (Fig.\u00a0S8), resulting in stronger reduction of the H atoms in the TiFeH nuclei. This is reflected in the higher average Bader charges in Fig. 8b. Both ground-state molar volume and electronegativity are important descriptors determining the hydrogen absorption properties in intermetallic alloys [56]. Elements with larger volume and lower electronegativity tend to form more stable hydrides, as demonstrated by V and Cr in the current investigation.In addition, it seems that electron-donating nature of V and Cr leads to the migration of nearest-neighbor octahedral H atoms to tetrahedral interstitial sites, as displayed in Fig.\u00a0S9. Larger Bader charges on H imply a larger effective volume, and hence H atoms that are nearby V or Cr may prefer the more spacious tetrahedral sites. These differences in charge state and tetrahedral interstitial occupancy of H atoms can further impact the hydrogen absorption-induced volume expansion of TiFe. When 24H atoms were inserted to form TiFeH nuclei, the M\u2212H bonds were found to be longer than Fe\u2212H bonds for all M atoms (Fig.\u00a0S10a), even though Co and Ni have similar metallic radii to Fe. On the other hand, Fe\u2212H bonds within the TiFeH nuclei are shortened only when V or Cr atoms are nearby, implying that the H atoms at tetrahedral interstitial sites can help mitigate the elastic energy of TiFeH nuclei while facilitating stronger Fe\u2212H bond formation. Indeed, the least degree of volume expansion upon TiFeH nucleus formation was observed for M\u00a0=\u00a0V and Cr (Fig.\u00a0S10b).In summary of the calculation results, the reducing power of V and Cr and their accommodation of lattice expansion play a synergistic role in changing the local chemical environment and electronic state of H atoms in the TiFeH nuclei region and reducing the elastic energy penalty for the nucleation of TiFeH. These impacts are realized by the conversion of endothermic to exothermic hydrogen absorption. Although some lattice expansion is observed for M\u00a0=\u00a0Co and Ni, their electronegativity is insufficient to further reduce the inserted H atoms beyond pure TiFe, somewhat muting their impact. The penalties for hydride nucleation in alloys with M\u00a0=\u00a0Co and Ni are lower than in pure TiFe, but the endothermic nature of hydrogen absorption remains, explaining the asymmetric behavior of \u0394Enucl\n with the atomic number of the alloying element observed in Fig. 8a.In this study, TiFe0.9M0.1 (M\u00a0=\u00a0V, Cr, Fe, Co and Ni) alloys were investigated from two different perspectives to explain the experimentally observed variation in activation kinetics: (i) elemental distribution and valence state inside surface oxide revealed by XPS and APT analyses and (ii) energetics of hydride nucleation predicted by DFT calculations. M\u00a0=\u00a0V and Cr alloys were easily activated at 323\u00a0K, whereas M\u00a0=\u00a0Co and Ni alloys were activated only after vacuum pretreatment at 473\u00a0K; pure TiFe was not activated at all. Overall, our findings point to a combined effect of passivating oxide modification and altered hydride nucleation energetics as key elements for promoting activation.Several specific factors were identified in our analysis. XPS indicated that the oxidation of V is more pronounced than that of Fe, which suggests sacrificial oxidation of V and protection of Fe from oxidation. APT analysis further suggested that thinner Ti oxide and higher Fe concentration in the surface oxide layer may promote activation. In addition, the introduction of a foreign element with a different oxygen affinity appears to induce selective oxidation and phase separation of Ti and Fe in the oxide layer. We propose that this redistribution of major elements inhibits the formation of the thick, continuous passivating oxide layers found on pure TiFe, thus accelerating the activation process.From a nucleation point of view, the reducing power and lattice expansion tendency for M\u00a0=\u00a0V and Cr act synergistically to alter the local chemical and strain environment and reduce the energy penalty associated with TiFeH nucleation. Their effect is realized by converting the endothermic hydrogen absorption into an exothermic absorption. Meanwhile, the effect of M\u00a0=\u00a0Co and Ni is limited to a more modest decrease in the nucleation penalty, while retaining endothermic absorption. The trend in hydride nucleus formation energetics follows that of the activation kinetics (V\u00a0>\u00a0Cr\u00a0>\u00a0Ni\u00a0>\u00a0Co\u00a0>\u00a0Fe), highlighting the additional importance of understanding nucleation to interpret the activation behavior of TiFe alloys.\nHayoung Kim: Investigation, Formal analysis, Writing \u2013 original draft, Visualization. ShinYoung Kang: Methodology, Investigation, Formal analysis, Writing \u2013 original draft, Visualization. Ji Yeong Lee: Investigation. Tae Wook Heo: Resources, Project administration. Brandon C. Wood: Funding acquisition, Writing \u2013 review & editing. Jae-Hyeok Shim: Resources, Writing \u2013 review & editing. Young Whan Cho: Resources, Writing \u2013 review & editing. Do Hyang Kim: Supervision. Jin-Yoo Suh: Writing \u2013 original draft, Project administration, Funding acquisition. Young-Su Lee: Conceptualization, Methodology, Investigation, Writing \u2013 original draft, 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 research was supported by the Korea Institute of Science and Technology [grant numbers 2E31851, 2E31858] and by the National Research Foundation of Korea [grant number NRF-2020M1A2A2080881]. Part of this study was performed under the auspices of the DOE by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344, with support from the Hydrogen Materials Advanced Research Consortium (HyMARC), established as part of the Energy Materials Network by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, under Contract No. DE-AC52-07NA27344. Computational resources were sponsored by the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program and the DOE's Office of EERE, located at the Argonne National Laboratory and National Renewable Energy Laboratory.Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2022.155443.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Despite the promise of TiFe-based alloys as low-cost solid-state hydrogen storage materials with mild operating conditions and reasonable hydrogen capacity, their initial hydrogenation process is difficult, hindering broad utilization. The effect of alloying element on the initial hydrogenation kinetics of TiFe alloys, TiFe0.9M0.1 (M\u00a0=\u00a0V, Cr, Fe, Co and Ni), was evaluated by analyzing changes to the passivating surface oxide layer that inhibits hydrogen permeation, as well as the ease of initial-stage hydrogen absorption into the underlying matrix. X-ray photoelectron spectroscopy and atom probe tomography revealed key variations in surface oxide compositions and thinning of the passivating oxide layer compared to pure TiFe, which suggests suppressed oxide growth by alloying-induced elemental redistribution. At the same time, density functional theory calculations predicted exothermic formation of hydride nuclei when alloying with V or Cr, as well as a reduced nucleation barrier when alloying with Co or Ni. Overall, these results are consistent with the observed experimental trend of the activation kinetics. We propose that improvements in activation kinetics of TiFe with alloying arises from the combined effect of reduced passivating oxide thickness and easier hydride nucleation, offering a starting point for alloy design strategies towards further improvement.\n               "}
{"full_text": "Compared with conventional fossil fuels, hydrogen fuel\u00a0(Nikolaidis and Poullikkas, 2017; Luo et al., 2018; Holladay et al., 2009; Dawood et al., 2020) has attracted significant attention for its unique features, such as high flame propagation velocity, high heating value of combustion, and zero carbon emissions. However, the storage and transportation problems of hydrogen fuel impede its practical applications\u00a0(Dincer, 2012; Kalinci et al., 2015; Granovskii et al., 2007, 2006; Acar and Dincer, 2014; Hu and Ruckenstein, 2003). New technology of energy carrier for the storage and transportation of hydrogen gradually becomes more urgent and essential. Ammonia (NH3), as an alternative energy carrier of hydrogen, has drawn extensive attention and research\u00a0(Xue et al., 2019; Miura and Tezuka, 2014; Elishav et al., 2017) due to the advantages of ammonia mainly including easy liquefaction, low cost of storage and transportation, and no carbon emission.The methods of hydrogen production from ammonia decomposition can be classed as: catalyst\u00a0(Xiao et al., 2011; Walkosz et al., 2020; Wang et al., 2019, 2017; Jolaoso et al., 2018; Podila et al., 2020, 2016, 2017; Zaman et al., 2018), plasma\u00a0(Soucy and Boulos, 1995; Hayakawa et al., 2020; Akiyama et al., 2018; Qiu et al., 2004; Hsu and Graves, 2005, 2003; Zhao et al., 2014), and catalyst combining plasma\u00a0(Zhao et al., 2013; Yi et al., 2019; Hayakawa et al., 2019; El-Shafie et al., 2020). Decomposition of ammonia by catalyst was a thermal decomposition method, which usually needs relatively large space for the sake of catalyst storage and heating elements. In addition, the start-up time of ammonia decomposition by thermal catalyst usually cannot satisfy the requirement of engine, which needs the instantaneous ammonia decomposition. Owing to high-energy electron and enough reactive species produced in a short time scale (less than micro-seconds), plasma technology was a promising alternative for instantaneous ammonia decomposition. Various types of plasma reactor have been designed and investigated, including thermal plasma, e.g.\u00a0RF discharge\u00a0(Soucy and Boulos, 1995), cold plasma, e.g.\u00a0dielectric barrier discharge (DBD)\u00a0(Hayakawa et al., 2020; Akiyama et al., 2018) and micro-hollow cathode discharge\u00a0(Qiu et al., 2004; Hsu and Graves, 2005, 2003), and warm plasma, e.g.\u00a0non-thermal arc plasma\u00a0(Zhao et al., 2014, 2013). In thermal plasma, the thermal effect on the ammonia decomposition is dominant. For instance,\u00a0Soucy and Boulos (1995) has developed RF thermal plasma reactor to produce hydrogen from ammonia, and achieved a high ammonia conversion rate of 98% under the condition of 13\u00a0kW discharge power and 28 SLM NH3 gas flow rate. However, relatively high heat loss by conduction leads to low energy efficiency, which would impede its practical application. In cold plasma, high-energy electron and reactive species play an important role in ammonia decomposition.\u00a0Hayakawa et al. (2020) has designed a DBD reactor with a hydrogen separation membrane to produce hydrogen from ammonia. High pure hydrogen was produced, but hydrogen generation rate was only 20\u00a0ml/min because of low gas flow rate for the sake of increasing the residence time in discharge zone. Warm plasma can maintain moderate gas temperature (usually 1500\n\u223c\n4000 K) and enough reactive species, which would be more suitable for the ammonia decomposition. In general, non-thermal arc plasma (NTAP) was one of fundamental methods for warm-plasma generation.\u00a0Zhao et al. (2014) has designed a NTAP reactor by two tube electrodes driven by alternating current. The increase of effective plasma volume and gas temperature results in high energy efficiency (EE) of hydrogen production (330.1 L/kW \n\u22c5\nh). However, the gas flow rate in the NTAP reactor was still in the order of milliliter, and the absolute hydrogen production rate is limited. Besides, the heat generated from NTAP needs to be utilized more efficiently, such as heating the catalyst. In previous study combining with catalyst, only DBD plasma source was investigated\u00a0(Yi et al., 2019; Hayakawa et al., 2019). The catalyst was placed in the discharge zone of DBD plasma, which may lead to the spark discharge and damages to the plasma reactor. In hence, taken together, requirement of a new NTAP reactor combining with catalyst for hydrogen production from ammonia becomes urgent and essential.In the present work, a novel NTAP reactor combined with NiO/Al2O3 catalyst has been developed to dissociate ammonia instantaneously. The moderate gas temperature in this NTAP reactor is very suitable for the high performance of ammonia decomposition. Large processing capacity and no cooling system contributes to the higher energy utilization efficiency. To utilize the extra heat of NTAP and further increase the ammonia conversion rate, NiO/Al2O3 catalyst was added at the nozzle exit of NTAP reactor to avoid the interference between catalyst and plasma, as mentioned in above DBD plasma combining with catalyst. The effects of gas flow rate and discharge power on the gas temperature, electron density of arc plasma, and hydrogen production rate are investigated through optical emission spectrometer, thermal couple and hydrogen detector, respectively.\nFig.\u00a01 presented the schematic diagram of the experimental setup for hydrogen production by NTAP. The experimental apparatus mainly includes a NTAP reactor, NiO/Al2O3 catalyst, a high-frequency high-voltage power supply (driving frequency: 23.8\u00a0kHz), a feed gas system, a cooling condenser, hydrogen detector, and a diagnostic system. The NTAP reactor mainly consists of a rod-type high-voltage electrode, concentric ground electrode, and a swirl gas ring. The commercial NiO/Al2O3 catalyst (weight: 200 g, model: AD-946, company: JIANGXI HUIHUA TECHNOLOGHY CO., LTD) was placed in the region with a distance of 6\u00a0mm away from the plasma generator exit. The mass fraction of NiO in NiO/Al2O3 catalyst is about 15%. The crystal structure of NiO is hexagonal, and the crystal parameters of NiO is 2.9552\u00a0\u00c5 \n\u00d7\n 2.9552\u00a0\u00c5 \n\u00d7\n 7.2275\u00a0\u00c5. The average crystalline size of NiO is 23.975\u00a0nm. The crystal structure of Al2O3 is hexagonal, and the crystal parameters of Al2O3 is 4.758\u00a0\u00c5 \n\u00d7\n 4.758\u00a0\u00c5 \n\u00d7\n 12.991\u00a0\u00c5. The average crystalline size of Al2O3 is 77.82\u00a0nm. The NTAP reactor was powered by a high-frequency high-voltage power supply with adjustable output power ranging from 0 to 700 W. The pure ammonia (99.999%) was injected into the swirl gas ring to act as the discharge gas, and its gas flow rate was controlled by a mass flow controller (0\n\u223c\n50 SLM). The discharge current \n\n\nI\n\n\na\nr\nc\n\n\n was obtained by measuring the current of a resistor (\n\nR\n=\n1\n\n\u03a9\n\n) in series with the NTAP reactor. The discharge voltage \n\n\nV\n\n\na\nr\nc\n\n\n and current \n\n\nI\n\n\na\nr\nc\n\n\n were measured by a four-channel oscilloscope (KEYSIGHT DSOX2024A) with a high-voltage probe (Tektronix P6021A) and normal voltage probe (KEYSIGHT N2862B), respectively. The discharge power can be obtained by discharge voltage \n\n\nV\n\n\na\nr\nc\n\n\n and current \n\n\nI\n\n\na\nr\nc\n\n\n as follows: \n\n(1)\n\n\nP\n=\n\n\n1\n\n\nT\n\n\n\n\n\u222b\n\n\n0\n\n\nT\n\n\n\n\nV\n\n\na\nr\nc\n\n\n\u00d7\n\n\nI\n\n\na\nr\nc\n\n\nd\nt\n,\n\n\n\nWhere \nT\n represented the one cycle of discharge voltage waveform. The effective value of discharge voltage \n\n\nV\n\n\nR\nM\nS\n\n\n and \n\n\nI\n\n\nR\nM\nS\n\n\n were calculated by the following equations: \n\n\n(2)\n\n\n\n\nV\n\n\nR\nM\nS\n\n\n=\n\n\n1\n\n\nT\n\n\n\n\n\n\n\u222b\n\n\n0\n\n\nT\n\n\n\n\nV\n\n\na\nr\nc\n\n\n2\n\n\nd\nt\n\n\n,\n\n\n\n\n(3)\n\n\n\n\nI\n\n\nR\nM\nS\n\n\n=\n\n\n1\n\n\nT\n\n\n\n\n\n\n\u222b\n\n\n0\n\n\nT\n\n\n\n\nI\n\n\na\nr\nc\n\n\n2\n\n\nd\nt\n\n\n,\n\n\n\n\n\nThe optical emission spectrum was obtained by a spectrometer (AvanSpec-2048). The gas temperature at the nozzle exit was measured by a thermal couple. Ammonia was decomposed by NTAP according to the following reaction: \n\n(4)\n\n\n2\n\n\nNH\n\n\n3\n\n\n\n\n\u27f6\n\n\nplasma\n\n\n\n\nN\n\n\n2\n\n\n+\n3\n\n\nH\n\n\n2\n\n\n,\n\n\n\nThe products of ammonia decomposition were NH3, \n\n\nH\n\n\n2\n\n\n, and \n\n\nN\n\n\n2\n\n\n. The hydrogen content was analyzed by a hydrogen detector (SKY600-XYH2, Shanghai Xiyu Instrument Equipment Co., Ltd). The energy efficiency EE for hydrogen production can be defined as follows: \n\n(5)\n\n\nEE\n=\n\n\nv\no\nl\nu\nm\ne\n\no\nf\n\n\n\nH\n\n\n2\n\n\n\np\nr\no\nd\nu\nc\ne\nd\n\n\n[\nL\n]\n\n\n\nD\ni\ns\nc\nh\na\nr\ng\ne\n\np\no\nw\ne\nr\n\n\n[\nkW\n\u22c5\nh\n]\n\n\n\n,\n\n\n\n\n\n\n\nFig.\u00a02 shows the typical discharge voltage and current evolution of NTAP reactor at a time span of 1000\u00a0\n\u03bc\ns and 50\u00a0\n\u03bc\ns. It can be seen that the shape for the envelope of discharge voltage was sawtooth-like with a time scale of hundreds of microseconds as shown in Fig.\u00a02(a). The magnitude of discharge voltage waveform varied from about 1.8\u00a0kV to 3\u00a0kV. The detailed voltage and current waveforms were shown in Fig.\u00a02(b). The waveform of discharge voltage has characteristics of the sinusoid-like shape with the amplitude of around 1.9\u00a0kV. The frequency of discharge voltage was about 23.8\u00a0kHz. The discharge current can be divided into two parts: the spike component and the sine-like component. The spike component and the amplitude of sine-like component of current reached to around 4 A and 430\u00a0mA, respectively. The current\u2013voltage characteristics of NTAP was shown in Fig.\u00a03. The voltage\u2013current characteristics of NTAP presented negative slope. Both the variable region of \n\n\nV\n\n\nR\nM\nS\n\n\n and \n\n\nI\n\n\nR\nM\nS\n\n\n and the absolute value of the slope increased with the increasing of gas flow rate as shown in Fig.\u00a03, which indicates that the NTAP reactor presented in this paper is more controllable for operating at high ammonia gas flow rate.\n\n\n\nFig.\u00a04 presents the typical optical emission spectrum of NTAP at the gas flow rate of 30 SLM and discharge power of 700 W. The emission spectrum of NTAP was dominated by the emission bands of NH*(A3\n\n\u03a0\n\n\n\u2192\nX3\n\n\n\n\u03a3\n\n\n\u2212\n\n\n), \n\n\nN\n\n\n2\n\n\n*(C3\n\n\u03a0\n\n\n\n\n\nu\n\n\n\n\n\u2192\nB3\n\n\u03a0\n\n\n\n\n\ng\n\n\n), \n\n\nH\n\n\n2\n\n\n*, NH2*(\n\n\n\n\n\nA\n\n\n\u0303\n\n\n\n\n2\n\n\n\n\nA\n\n\n1\n\n\n\n\n\n\u2192\n\n\n\n\n\n\n\nX\n\n\n\u0303\n\n\n\n\n2\n\n\n\n\nB\n\n\n1\n\n\n\n), and the atom spectrums of \n\n\nH\n\n\n\u03b1\n\n\n (656.3\u00a0nm), \n\n\nH\n\n\n\u03b2\n\n\n (486.1\u00a0nm), and Cr (422.7\u00a0nm, 425.3\u00a0nm, 427.4\u00a0nm, 428.9\u00a0nm, 588.9\u00a0nm, 589.5\u00a0nm). These reactive species, such as NH*, \n\n\nN\n\n\n2\n\n\n* and \n\n\nH\n\n\n2\n\n\n*, indicate that ammonia has been effectively dissociated by NTAP. These intermediate and final products was formed by following reaction process\u00a0(Fateev et al., 2005; van\u00a0den Oever et al., 2005; Yang et al., 2002; Yasui et al., 2003; Arakoni et al., 2007):Energy exchange: \n\n\n(6)\n\n\ne\n+\n\n\nNH\n\n\n3\n\n\n\u2192\n2\ne\n+\n\n\nNH\n\n\n3\n\n\n+\n\n\n,\n\n\n\n\n(7)\n\n\ne\n+\n\n\nNH\n\n\n3\n\n\n+\n\n\n\u2192\n\n\nNH\n\n\n2\n\n\n+\nH\n,\n\n\n\n\n(8)\n\n\ne\n+\n\n\nNH\n\n\n3\n\n\n+\n\n\n\u2192\nNH\n+\np\n\n\nH\n\n\n2\n\n\n+\n\n\n2\n\u2212\n2\np\n\n\nH\n\n\n\n\n(\np\n=\n0\n,\n1\n)\n\n,\n\n\n\n\n(9)\n\n\ne\n+\n\n\nNH\n\n\n3\n\n\n+\n\n\n\u2192\nN\n+\nq\n\n\nH\n\n\n2\n\n\n+\n\n\n3\n\u2212\n2\nq\n\n\nH\n\n\n\n\n(\nq\n=\n0\n,\n1\n)\n\n,\n\n\n\n\n(10)\n\n\ne\n+\n\n\nNH\n\n\n3\n\n\n\u2192\n\n\nNH\n\n\n2\n\n\n\u2212\n\n\n+\nH\n,\n\n\n\n\n(11)\n\n\ne\n+\n\n\nNH\n\n\n3\n\n\n\u2192\n\n\nNH\n\n\n2\n\n\n+\n\n\nH\n\n\n\u2212\n\n\n,\n\n\n\n\n Binary processes: \n\n\n(12)\n\n\n\n\nNH\n\n\n2\n\n\n+\nN\n\u2192\n\n\nN\n\n\n2\n\n\n+\n2\nH\n,\n\n\n\n\n(13)\n\n\n\n\nNH\n\n\n2\n\n\n+\nH\n\u2192\n\n\nH\n\n\n2\n\n\n+\nNH\n,\n\n\n\n\n(14)\n\n\n\n\nNH\n\n\n3\n\n\n+\nH\n\u2192\n\n\nH\n\n\n2\n\n\n+\n\n\nNH\n\n\n2\n\n\n,\n\n\n\n\n(15)\n\n\n\n\nNH\n\n\n2\n\n\n+\nNH\n\u2192\n\n\nN\n\n\n2\n\n\n\n\nH\n\n\n3\n\n\n,\n\n\n\n\n(16)\n\n\nNH\n+\nH\n\u2192\n\n\nH\n\n\n2\n\n\n+\nN\n,\n\n\n\n\n(17)\n\n\nNH\n+\nNH\n\u2192\n\n\nN\n\n\n2\n\n\n+\n2\nH\n,\n\n\n\n\n(18)\n\n\n\n\nN\n\n\n2\n\n\n\n\nH\n\n\n3\n\n\n+\nH\n\u2192\n\n\nN\n\n\n2\n\n\n+\n2\n\n\nH\n\n\n2\n\n\n,\n\n\n\n\n It can be seen that high-energy electron is crucial to exchange energy with atom and molecules, and form the intermediate products. Besides, in the binary processes, most of these reaction rate coefficients rely on the gas temperature, such as reaction (13) and (14)\u00a0(Arakoni et al., 2007). Therefore, the parameters of electron, such as electron density, and gas temperature are the key factors that influence the performance of ammonia decomposition.\n\n\nFig.\u00a05 presents the gas temperature (\n\n\nT\n\n\ng\n\n\n) at the exit of NTAP reactor under different gas flow rate and discharge power. It can be seen that the gas temperature increased with the discharge power when the gas flow rate was kept a constant. The lowest value of gas temperature was 673.2 K at the discharge power of 210 W and gas flow rate of 30 SLM, and the highest value was 1116.2 K at the discharge power of 700 W and gas flow rate of 20 SLM. According to precious literature\u00a0Lin et al. (2018), the gas temperature inside the nozzle channel was much higher than that at the exit of NTAP reactor. The thermal pyrolysis effect of NTAP on the ammonia decomposition may be remarkable inside the nozzle channel, because some key endothermic reactions of producing hydrogen becomes more significant with the increasing of gas temperature. For instance, the abstraction of H from NH2 and NH3 by H atoms can produce \n\n\nH\n\n\n2\n\n\n by following reactions as mentioned in above section\u00a0(Arakoni et al., 2007): \n\n\n(19)\n\n\n\n\nNH\n\n\n2\n\n\n+\nH\n\u2192\n\n\nH\n\n\n2\n\n\n+\nNH\n,\n\nk\n=\n1\n.\n1\n\u00d7\n1\n\n\n0\n\n\n\u2212\n10\n\n\n\n\ne\n\n\n\u2212\n4451\n/\n\n\nT\n\n\ng\n\n\n\n\n\n\n\n\n(20)\n\n\n\n\nNH\n\n\n3\n\n\n+\nH\n\u2192\n\n\nH\n\n\n2\n\n\n+\n\n\nNH\n\n\n2\n\n\n,\n\nk\n=\n6\n.\n5\n\u00d7\n1\n\n\n0\n\n\n\u2212\n13\n\n\n\n\n\n(\n\n\nT\n\n\ng\n\n\n/\n300\n)\n\n\n\n2\n.\n76\n\n\n\n\ne\n\n\n\u2212\n5135\n/\n\n\nT\n\n\ng\n\n\n\n\n\n\n\n\n The rate coefficients for reaction (13) and (14) at 300 K are 3.96*10\u221217 cm 6 s\u22121 and 2.39*10\u221220 cm 6 s\u22121, respectively, while these values at 1000 K are 1.28*10\u221212 cm 6 s\u22121 and 1.06*10\u221213\u00a0cm 6 s\u22121, respectively. This phenomenon suggests that increasing the gas temperature is beneficial for the production of hydrogen. In hence, the gas temperature plays an important role in the ammonia decomposition by NTAP. In addition, the gas temperature in NTAP is higher than that of dielectric barrier discharge, so the effect of thermal decomposition of ammonia becomes more significant in NTAP.\nThe electron density is an important parameter in the energy change and the formation of intermediate products. The electron density can be calculated from the Stark broadening of the \n\n\nH\n\n\n\u03b2\n\n\n Balmer line \u00a0(Kelleher et al., 1993; Wiese et al., 1975). The typical Voigt fit of the recorded \n\n\nH\n\n\n\u03b2\n\n\n line and the electron density as a function of discharge power and gas flow rate were shown in Fig.\u00a06. It can be seen that the electron density was in the order of 1018 m\u22123, and it increased by enhancing the discharge power and decreasing the gas flow rate. Hence, there is enough number of high-energy electron in NTAP to participate in the energy change and the formation of intermediate products.\n\nThe effects of gas flow rate, discharge power, and catalyst on the hydrogen production performance of NTAP were investigated and presented in Fig.\u00a07. It can be seen that increasing the discharge power and decreasing gas flow rate contributed to the enhancement of hydrogen production. On the one hand, increasing gas flow rate results in the decreasing of residence time of ammonia in the discharge zone, which leads to reducing the chance of collision probability between ammonia molecule and high-energy electrons, and further the decreasing of hydrogen content. On the other hand, increasing gas flow rate contributes to the increasing of the volume of ammonia processed by NTAP in unit time. Obviously, in our experiment condition (gas flow rate of NH3: 10 slm\n\u223c\n30 slm), increasing the gas flow rate is beneficial for the absolute value for the production of hydrogen. Enhancing the discharge power can result in the higher average energy obtained by each ammonia molecule in the discharge zone. Therefore, more ammonia molecules have enough energy to participate in the ammonia decomposition. In addition, the NTAP combining with catalyst would effectively enhance the hydrogen production. The highest hydrogen content in the production of ammonia decomposition reached about 34.8% as shown in Fig.\u00a07(a). Although the highest hydrogen production rate was achieved at the gas flow rate of 20 SLM, the highest energy efficiency 1080.0 L/kW\n\u22c5\nh was obtained at the gas flow rate of 30 SLM as shown in Fig.\u00a07(b). Owing to the catalyst heated by the plasma jet automatically, the length of plasma jet can influence the size of the space loading catalyst. The length of plasma jet can be elongated with the increasing of gas flow rate, which suggests that higher gas flow rate helps to more catalyst heated by plasma jet. Therefore, high gas flow rate contributes to the homogeneous heating of catalyst, and the higher absolute hydrogen production performance can be obtained by combination of plasma and catalyst. Besides, the hydrogen production rate is closely associated with the ammonia decomposition rate. Higher hydrogen production rate can be realized by increasing the discharge power, which means that higher ammonia decomposition rate can be obtained at high discharge power.The hydrogen production performances of NTAP in this paper compared with that of other plasma types were shown in Table\u00a01. The largest absolute hydrogen production rate and highest energy efficiency were simultaneously obtained, which were difficult for other plasma sources due to the limitation of structure and plasma techniques. The high performance of ammonia decomposition may be related to the discharge features of NTAP. In comparison with that of cold plasma reactor, relative higher gas temperature and higher electron density of NTAP as mentioned in above section can generate more radical species facilitating chemical reaction rate. The plasma reactor of this paper has higher energy utilization efficiency due to no cooling systems, compared with that of thermal plasma reactor. In addition, compared with that of the NTAP reactor in literature\u00a0Zhao et al. (2014), the higher performance of ammonia decomposition in this paper may be associated with the gas injection method of plasma reactor. The swirl flow in NTAP reactor of this paper is favorable for the energy exchange between ammonia molecules and plasma, and thus the ammonia decomposition, while the straight flow is utilized in literature\u00a0Zhao et al. (2014).Besides, according to experiment, the hydrogen content of 5\n\u223c\n20% in hydrogen/ammonia combustion can efficiently enhance the combustion stability and flame propagation speed. Therefore, the shortcoming of ammonia fuel may be overcome by means of on-line hydrogen production from ammonia by NTAP. In addition, in order to evaluate the potential of ammonia decomposition by NTAP, the NH3/H2 mixture after ammonia decomposition was injected to the engine. In our engine (NH3\nH2 fuel) under development, the ratio of the power of electric generator to the output power of engine that we expected is less than 2%. The rated output power of this engine is about 5\u00a0kW, which means that ideally the discharge power of plasma must be less than 100 W. However, in order to maintain the steady operation of engine under present conditions, the discharge power of plasma needs to be larger than 500 W, which suggests that the energy efficiency must be enhanced by five times at least.\n\n\nIn summary, an instantaneous, high-efficiency and large-scale hydrogen production device from ammonia by NTAP combined with NiO/Al2O3 catalyst has been developed. The effects of gas flow rate, discharge power on the gas temperature, electron density, and the hydrogen production performance were investigated. The main experimental results and conclusions were listed as follows:With the increase of gas flow rate, the variable region and slope of discharge voltage and current were larger, which indicates that the NTAP reactor is more suitable for operating at high gas flow rate for the sake of easy output adjustment.Owing to the emission spectrum of NTAP dominated by NH*(A3\n\n\u03a0\n\n\n\u2192\nX3\n\n\n\n\u03a3\n\n\n\u2212\n\n\n), \n\n\nN\n\n\n2\n\n\n*(C3\n\n\u03a0\n\n\n\n\n\nu\n\n\n\n\n\u2192\nB3\n\n\u03a0\n\n\n\n\n\ng\n\n\n), \n\n\nH\n\n\n2\n\n\n*, and the atom spectrums of \n\n\nH\n\n\n\u03b1\n\n\n, \n\n\nH\n\n\n\u03b2\n\n\n, it can be observed that ammonia can be effectively dissociated by the NTAP. By measuring the gas temperature, the thermal decomposition effect of NTAP would be more remarkable compared with some other plasma sources, e.g.\u00a0dielectric barrier discharge.The hydrogen content in the production of ammonia decomposition varied in a large region (5.3%\n\u223c\n34.8%) by adjusting the gas flow rate, discharge power, and adding catalyst or not. In the case of NTAP, the highest energy efficiency was 783.4 L/kW\n\u22c5\nh at the discharge power of 700 W and gas flow rate of 30 SLM, while the highest energy efficiency of 1080.0 L/kW\n\u22c5\nh was obtained by means of NTAP combining with catalyst.\nQ.F. Lin: Investigation, Writing - original draft. Y.M. Jiang: Data curation. C.Z. Liu: Resources. L.W. Chen: Writing - review & editing, Conceptualization. W.J. Zhang: Validation. J. Ding: Project administration. J.G. Li: 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 the National Natural Science Foundation of China (11575252, 11775270), and Institute of Energy of Hefei comprehensive National Science Center, China\n (19KZS206, 21KZS201).", "descript": "\n                  Owing to the storage and transportation problems of hydrogen fuel, exploring new methods of the real-time hydrogen production from ammonia becomes attractive. In this paper, non-thermal arc plasma (NTAP) combining with NiO/Al2O3 catalyst is developed to produce hydrogen from ammonia with high efficiency and large scale. The effects of ammonia gas flow rate and discharge power on the gas temperature, electron density, the hydrogen production rate, and energy efficiency were investigated. Experimental results show that the optical emission spectrum of NTAP working with pure ammonia medium was dominated by the atom spectrum of H\n                        \n                           \n                           \n                              \u03b1\n                           \n                        \n                     , H\n                        \n                           \n                           \n                              \u03b2\n                           \n                        \n                     , and molecular spectrum of NH component. Under the optimum experimental condition of plasma discharge, the highest energy efficiency of hydrogen production reached 783.4 L/kW\n                        \u22c5\n                     h at NH3 gas flow rate of 30 SLM. When the catalyst was added, and heated by the NTAP simultaneously, the energy efficiency further increased to 1080.0 L/kW\n                        \u22c5\n                     h.\n               "}
{"full_text": "Climate change caused by the burning of fossil fuels is one of the major global problems. The development for clean and renewable energy resources to ensure the sustainable development of the world is in urgent need [1,2]. Proton exchange membrane fuel cells (PEMFCs) that can convert the chemical energy (from clean and renewable fuels such as hydrogen) directly into electrical power have attracted tremendous attentions [3]. In the fuel cell system, oxygen reduction reaction (ORR) proceeds at the cathode while hydrogen oxidation reaction (HOR) occurs at the anode. The catalysts are essential to lower the activation energy, thus making these reactions kinetically-activated [4,5]. Moreover, the kinetics of ORR is five-orders-of magnitude slower than that of HOR, which leads to more catalyst usage in the cathode. Therefore, developing highly efficient cathodic ORR electrocatalysts is highly desired for high-performance and low-cost fuel cells.ORR can be catalytically achieved through two different mechanisms: (i) four-electron (4e\u2212) pathway to directly produce water in acid media or hydroxide anion in alkaline media; (ii) two-electron (2e\u2212) pathway to create the intermediate compound (hydrogen peroxide) in acid media or peroxide anion in alkaline media [6]. Compared with 2e\u2212 pathway, the 4e\u2212 process is more attractive because it could ensure higher operating potentials and current efficiency in fuel cells. Currently, the carbon black supported platinum (Pt/C) catalysts are the state-of-the-art 4 e\u2212 catalysts for ORR in PEMFCs. Due to the scarcity of Pt, finding alternative catalysts with less cost is one of the central tasks for the large-scale commercial implementation of the fuel cell industry. Low-Pt catalysts, non-precious metal catalysts, and metal-free catalysts have been widely investigated as substitutes for the commercial Pt/C [7\u20139].Graphene is a 2D layer with sp2-bonded carbon atoms, which has attracted worldwide attention in both metal-free catalysts and supporting materials for ORR. Inspired by the successful development of graphene, a variety of studies of using other types of 2D materials as cost-effective ORR catalysts have started to appear, such as transition metal dichalcogenides (TMDs), phosphorene, hexagonal boron nitride (h-BN) and graphitic carbon nitride [10\u201312]. Besides, the 2D graphene can be split into smaller particles such as 0D quantum dots and 1D nanoribbons [13]. With this revelation, low-dimensional materials have recently emerged as new types of electrocatalysts [14\u201317].When the materials are smaller than 100\u00a0nm in at least one dimension, their electronic structure and surface properties change drastically, resulting in unique physical and chemical properties due to their quantum size effect and large surface area. In low-dimensional materials, the motion of electrons is restricted in zero, one, and two dimensions. As shown in Fig. 1\n, the polts of the density of states (DOS) versus energy in low dimensional structures are distinct from each other. It's a staircase for 2D materials, a fence for 1D material, and a pack of discrete lines for 0D materials. In 0D materials, such as nanoparticles or quantum dots, the electrons are confined in all three directions (x, y, z-axis) and cannot move anywhere [15]. The 0D material shows abundant edges and low coordinated sites. In 1D materials, such as nanotubes, nanorods, and nanowires, the electrons are restricted in two directions and can only move in one direction. The 1D material displays a high length-to-width ratio with preferred facets. In 2D materials, such as thin nanofilms and nanosheets, only one direction is restricted for the movement of the electron. And they exhibit high surface area and edge effects. Dimensionality plays a vital role in depicting the fundamental properties of a material. Due to the quantum mechanical effects, the properties of these low-dimensional materials are significantly different from those of bulky materials.Low-dimensional materials with designated thickness, size, and morphology show many advantages for catalysis [15]. On the one hand, the high surface area could increase the number of active sites on the surface, which enhances the atom utilization. On the other hand, their unique electronic structure and low coordinated environment benefit the interactions between catalysts and reactants. According to the Sabatier principle, the adsorption energy of the reactants and intermediates on the catalyst surface should be neither too small nor too large. The interactions between catalysts and reactants are largely dependent on the electronic structure of the catalyst, affecting the catalytic activity and stability of the catalyst. Therefore, the controlled synthesis process of low-dimensional catalyst materials with desirable electronic structures is the key to the promotion of catalytic performance.The synthetic strategies of low-dimensional materials can be divided into two main categories: (i) top-down and (ii) bottom-up, as shown in Fig. 2\n. Briefly, the top-down approach is to reduce the bulk material to the nanoscale by liquid-phase exfoliation or physical methods. For example, graphene can be obtained from graphite by breaking the van der Waals forces between the graphite layers using adhesive tape [18]. The general idea of the bottom-up strategy is using small building blocks made of atoms or molecules to synthesize nanomaterials. Chemical synthesis methods such as chemical vapor deposition (CVD) and hydro/solvothermal methods are typical examples [19]. In general, top-down approaches are simple and easy for scale-ups but lack the precise control of the morphology. The bottom-up method is good at synthesizing nanomaterials with precisely controlled size, shape, and morphology. The synthetic approaches of low-dimensional materials have been well summarized in many recent papers [20,21]. This review will focus on the relationship between the structure and the ORR activity of the low low-dimensional materials.2D materials have emerged as the promising catalysts for ORR since the 2D-graphene was explored experimentally in 2004 [23]. As shown in Fig. 3\n, a wide range of 2D materials such as graphitic carbon nitride (g-C3N4), metal dichalcogenides (TMDs, e.g. MoS2, MoSe2), layered metal, layered metal-organic frameworks (MOFs), MXenes (transition metal carbides, nitrides, and carbon nitrides), layered transition metal oxides (TMDs, e.g. MnO2), and hexagonal boron nitride (h-BN) have also been successfully prepared and applied in electrochemistry in recent years. These 2D materials are of single or a few layers in thickness. This distinct structure along with the extraordinary physical and chemical properties such as mechanical flexibility and high specific surface area make 2D nanomaterials promising as the catalyst support in electrochemical energy conversion. Moreover, the electrocatalytic activity of 2D material can be tailored by inducing intramolecular charge transfer with heteroatom-doping or defect engineering. The unique electronic structure and increased active sites render 2D material appealing candidates for the ORR catalyst [11]. However, the active sites of these 2D materials differ significantly, which will be discussed in the following section.Graphene-based catalysts have attracted great attention for ORR [24\u201326]. Graphene exhibits many excellent chemical and physical properties as well as the unique graphitic basal plane structure that could supply numerous active sites and facilitate the electron transport. Unfortunately, the ORR activity of pristine graphene is very poor due to the lack of free electrons for the reaction and a limited number of active sites [27]. The electroneutrality of pristine graphene should be broken down to create charged sites that are favorable for O2 adsorption. Introducing dopants and generating defects on graphene could modulate the surface energy, the chemisorption energy of O2, as well as the local electronic properties and thus enhance the catalytic properties [28].Among various possible dopants, N-doped graphene is by far the most investigated material. The atomic radius of N is close to C. So it is relatively easy to incorporate N into the backbone of the graphene material. The carbon atoms that are adjacent to the more electronegative nitrogen atoms are supposed to be the active sites for ORR [30]. The interaction with oxygen molecules will be facilitated because of the favorably changed charge profile. As shown in Fig. 4\n, there are three types of nitrogen dopants (pyridinic, pyrrolic, and graphitic). Each type of these nitrogen dopants influence the catalytic property differently and the exact role of the specific N specie in ORR is still under debate [31,32]. Pyridinic nitrogen is believed to be the most active site for ORR [33,34]. The lone electron pair of pyridinic N could donate to the \u03c0-bond of the carbon matrix, making pyridinic N atom itself electron-attractive and catalytically active. Satoshi Yasuda et al. [35] used temperature-induced surface polymerization of nitrogen-containing aromatic molecules to get two types of N-doped graphene: one is mainly composed of pyridinic nitrogen and the other is graphitic nitrogen. It was revealed that pyridinic nitrogen could catalyze the ORR via the 4e\u2212 process, whereas graphitic nitrogen reduced oxygen via a 2e\u2212 pathway. However, different conclusions were proposed by other researchers. [36,37], Luo et al. [38] employed a pyrolysis method using methane (CH4) and ammonia (NH3) as carbon and nitrogen sources, and the graphene doped with nearly 100% pyridinic N was obtained on the surface of a Cu substrate. The results showed that the pyridinic N doped graphene prefers a 2e\u2212 pathway for ORR, indicating that the pyridinic N couldn't effectively promote ORR by a 4e\u2212 dominated process. This result may play a vital role in tailoring the electronic properties for the improvement of the ORR performance of the pyridinic N. Ruoff et al. [37] prepared the N-doped graphene with controlled N doping species by selectively annealing graphene oxide (GO) with different N-containing precursors. Higher ORR activity and larger limiting current density could be obtained by graphitic N-dominated catalysts. In contrast, more positive onset potential of ORR could be found in the pyridinic N-dominated catalysts. However, the catalytic ability of doped graphene regarding the ORR depends on many complicated factors of the coordination environment including surface area, morphology, crystallographic structure, oxidation state, and chemical composition. It cannot be simply summarized over a few studies. The comprehensive and profound research for ORR is needed to discuss the real mechanism.Meanwhile, as shown in Fig. 5\n, the dopping of heteroatoms in graphene lattice can cause a charge density redistribution because of their distinct electronegativity. In practice, the atomic radius of heteroatoms should be considered. Large atoms such as Br, I, and Se cannot be incorporated into the carbon lattice. Boron (B), sulfur (S), and phosphorus (P) have been widely used for the heteroatom-doped graphene [30,39,40]. The catalytic activities will be enhanced because of the spatial distortion and charge redistributions. Since the electronegativity of B atoms is less than that of carbon atoms, when they are introduced into graphene, the p-type conductivity can be induced. The B-doped graphene is believed to be a good catalyst for promoting the 4e\u2212 process [41]. As for S, the electronegativity of S is close to that of carbon. However, the size of the S atom is larger than that of the C atom. Spatial distortion of S-doped graphene is a key factor to enhance the ORR activity. The carbon atoms near doped sulfur atoms and the zigzag edges are the catalytic active sites in S-doped graphene [42]. In terms of P doping, the P3p and C2p orbital are hybridized by sp [3]-orbital configuration, which is shown as a pyramidal structure. These structures are easily oxidized to generate C\u2013P\u2013O bonds. The active sites of P-doped graphene are positively charged carbon atoms because the O atoms in the C\u2013P\u2013O structure can polarize the P and the adjacent C atoms. Graphene doped with two or more kinds of heteroatoms have also been proven to be interesting because of the increased number of dopant heteroatoms and the synergistic effects between the dopants. B\u2013N co-doped, P\u2013N co-doped, S\u2013N co-doped graphenes are the most widely studied materials for ORR [43,44]. The intermediate adsorption on the surface of doped graphene can be enhanced by tailoring the electron-donor properties with controlled dopant types and their amounts. Elucidating the underlying ORR mechanism of the doped graphene will be one of the top priorities for future research.Transition-metal dichalcogenide (TMD or TMDC) monolayers are thin-layer materials and their general chemical formula is MX2. The M represents transition-metal atoms (e.g. Mo, W, etc.) whereas the X represents chalcogen atoms (e.g. S, Se, Te). In the MX2 catalysts, a single layer of transition metal atoms is situated between two layers of chalcogen atoms where a strong covalent bond between the M and X atoms is observed [45]. The 2D TMDs show adjustable bandgaps that are decided by the stacking order between layers and the different coordination models between M and X atoms, which will lead to different surface properties and electronic structures and thus varied catalytic activities will be observed [46].Among the known sixty TMDCs thus far, 2D molybdenum disulfide (MoS2) has attracted much attention for the ORR catalyst. Their active sites are the edges and corner atoms with low-coordination. In particular, Mo atoms passivated by S atoms that are found at the edges are determined to be the most active sites because of their sulfide rich and unsaturated coordinated environment as well as their dangling bonds [47]. However, the pristine MoS2 shows poor electron transport properties and limited active sites. Various attempts have been made to expose more active edge sites and improve conductivity. For example, heteroatom doping can be used to activate the atoms on the basal plane. Similar to the doping process of the graphene, B, N, P, and O were introduced to the MoS2 [48,49]. Moreover, metallic atoms such as Fe, Co, Ni are proved to be effective for improving the ORR activity as well [50,51]. In addition, conducting nanomaterials, such as graphene, are used as the support to improve the electron transport properties of the catalysts. As shown in Fig. 6\n, the N\u2013MoS2/C composite materials were obtained by thermal treatment with the mixture of ammonium molybdate, thiourea, melamine, and Pluronic F127 [52]. The ORR activity was enhanced as a result of the abundant active sites that were derived from rich-defects on the N-doped MoS2/carbon materials and the high electron conductivity.Transition-metal oxides (TMOs) consist of oxygen atoms with transition-metals from the d-block of the periodic table (such as Mn, Fe, Co, Ni, Ti, etc) [53]. The 2D layered structures have extra advantages such as high surface area and edge defects. Quasi-2D sheets of the TMOs are stacked together by van der Waals interactions. Transition metal atom possesses unpaired electrons and dangling bonds, leading to strong surface polarization of the TMOs [5,54]. The 2D TMOs also show multiple valence states, various metal oxidation states, and hence a tunable ORR activity. Since the thickness of the 2D TMOs is limited (always less than 2\u00a0nm), most of the low-coordinated transition-metal atoms are exposed on the surface and serve as the active sites [55,56].However, due to the poor conductivity, TMOs exhibit unsatisfactory ORR performance. Highly conductive support material with large surface areas such as graphene is used to form a 2D graphene/TMO heterostructure to address the conductivity issue [58]. There are interphase ligand effects and excellent interfacial interactions between TMOs and graphene due to oxygen-containing functional groups found on graphene. Thus, the use of graphene can improve the conductivity while the TMOs on the surface could maximize electrochemically accessible surface area. Therefore, this synergistic effect provides graphene/TMO heterostructure with a better ORR performance. As shown in Figs. 7 and 2\nD manganese oxide nanosheets/graphene composite was synthesized by heating the graphene oxide with KMnO4 [57]. The nanopores were then introduced into the composites by heating with S powder. The improved ORR activity of the nanoporous MnO2 nanosheets was related to the active Mn3+/4+ sites, the large surface area, and oxygen vacancies, which reduced the kinetic barriers and facilitated the oxygen adsorption.Among different crystal structures of TMOs, the perovskite structures with a general formula ABO3 have attracted great attention recently because of their non-stoichiometric chemistry and variable crystal structures [60,61]. The 2D perovskite oxide consists of stacked layers with edge-sharing octahedra and shows a higher surface. As shown in Fig. 8\n, three mechanisms are proposed to explain the active sites in 2D perovskite oxide for ORR. The oxygen can be absorbed on the catalyst surface by three configurations: (a) end-on adsorption, (b) side-on adsorption, and (c) bidentate adsorption. For both mechanisms in Fig. 8a and b, the metal sites are the active sites for ORR. However, the mechanisms in Fig. 8c show that the metal sites and oxygen vacancies can both serve as the active sites for ORR. In alkaline media, the OH group near the oxygen vacancies is displaced by one atom of O2 while the other atom of O2 fills the oxygen vacancy. In this case, the O\u2013O bond is weakened, facilitating the further reaction to form OH\u2212. The oxygen vacancies can also change the electronic structures of perovskite oxides, leading to the presence of redox couples and electron holes, benefiting the ORR by promoting charge transfer and increasing the electrical conductivity. Therefore, the ORR performance of perovskite oxides is largely related to oxygen vacancies.The 1D electrocatalysts are the ideal candidates with high catalytic activity for ORR, owing to their unique structural superiorities such as rapid electron and mass transport, large surface area, low vulnerability to aggregation, and dissolution. The catalytic activity of 1D electrocatalysts is not only dependent on their composition but also their geometry such as the number of walls, length, diameter, and chirality. Nanotubes, nanoribbons, and nanowires are the most common 1D electrocatalysts that have been widely used for boosting the ORR activity.Nanotubes are made of various materials that take the shape of tubes with their diameters measured in nanometers. Nanotubes are frequently used as ORR catalysts because of their inherent morphologic features such as large surface area and rapid mass transport as shown in Fig. 9\n. Moreover, nanotubes show high ORR stability because they can inhibit carbon corrosion, Ostwald ripening, and aggregation under fuel cell operating conditions [62].Carbon nanotubes (CNTs) can be directly used as the metal-free ORR catalyst or as the catalyst support. However, the electrons in pristine CNTs are too inert for ORR. Surface engineering is essential to modulate the chemisorption energy of O2 and increase active sites thereby enhancing the ORR activity. Similar to graphene, the doping of heteroatoms is frequently used to change the surface properties. In particular, the N-doped CNTs are attracting much more attention [63,64]. This pioneering work was achieved by L. Dai's group in 2009 [6]. Vertically aligned nitrogen-containing carbon nanotubes were employed to be the ORR catalyst with improved catalytic activity and high stability compared to commercial Pt/C catalysts. The carbon atoms adjacent to N dopants show high positive charge density and thus serve as active sites. Moreover, N-doped CNTs are used to replace carbon black to build Pt nanoparticles supported by N-doped CNTs material [65,66]. Uniform Pt nanoparticles with an ultra-small size can be prepared on the N-doped CNTs and show improved ORR activity and high stability when compared with the carbon black and pure CNTs.To avoid the carbon-corrosion problem, self-supported Pt nanotubes were developed to address the activity and long-term durability of ORR [67,68]. These unique Pt nanotubes show preferential exposure of highly active crystal facets, easy electron transport, and inherent high stability. Also, the built of 1D Pt-based alloy and core/shell nanotubes structure has attracted remarkable attention [69,70]. Self-supported Pt-based nanotubes can be served as an alternative to carbon-supported materials and offer a broad materials library of noble metal structures.Nanoribbons are strips of 2D materials with a narrow width (less than 50\u00a0nm) in the plane. Graphene nanoribbons (GNRs) can be regarded as the unzipped CNTs. Compared with CNTs, GNRs show an open structure where the abundant edges are visible. The desirable catalytic activity towards ORR can be obtained by controlling the edge structures and chemical terminations [71]. However, the synthesis of the GNRs is not simple with opening the CNTs being one of the most promising approaches. As shown in Fig. 10\n, the longitudinally unzipping of CNTs was used to get GNRs, and after an in-situ polymerization and thermal treatment process, nitrogen doping was achieved [71]. The TEM image showed the unzipped CNTs and the as-prepared GNRs demonstrate a thin elongated morphology. The N-doped GNRs showed an enhanced ORR activity with the dominant 4e- pathway. The active sites were the C atoms near the graphitic N and the pyridinic N at the edges.Compared to the CNTs and GNRs, the nanowires are relatively easier to be synthesized, especially for metal and metal oxide nanowires, such as MnO2, Co3O4 [62,72,73]. In particular, the building of the nanowire structure has been proved to be an efficient strategy to enhance the mass activity of Pt-based catalyst [74,75]. The morphology of 1D nanowires is beneficial to maintain high Pt-surface utilization and thus increases the electrochemical surface area (ECSA). As shown in Fig. 11\n, because of the uncoordinated configuration on the surface, jagged Pt-based nanowires show the highest mass activity compared with other Pt nanostructures such as nanoparticles, nanoframes, etc. Moreover, the Pt-based nanowires display enhanced durability. The unique 1D nature of nanowires can be anchored on the support by multiple points. Therefore, Ostwald ripening and particle agglomeration are not prone to occur. Also, the Pt-based nanowire shows fewer defective sites and lattice boundaries vulnerable to Pt dissolution [76].The 0D materials are materials having all three dimensions on the nanometer scale. Various materials, such as nanoparticles and quantum dots, are the typical example of 0D materials. Usually, 0D electrocatalysts for ORR can be classified into metal nanoparticles, metal alloy nanoparticle, and metal complexes. Compared with bulk materials, the 0D nanostructures always show high surface free energy and low-coordinated atoms, and thus considerably improving their ORR activity [77\u201379]. However, 0D nanoparticles always suffer from aggregation. Therefore, the use of the appropriate support material is an efficient way to enhance their stability. In low dimensional materials, the van der Waals forces of the 1D and 2D materials allow them to strongly interact with 0D nanoparticles. Therefore, the 0D/1D or 0D/2D heterostructures are formed and show a synergistic effect on enhanced catalytic activity for ORR [80,81].When the size of the catalyst reaches the limitation (single atom) it can be considered as the 0D material as well. Single-atom catalysts (SACs) can maximize the atomic utilization, which indicates that ideally, every active single atom can serve as an active site [83\u201390]. Moreover, SACs show excellent activity and exclusive selectivity for ORR because of the unsaturated coordination environments and their unique electronic structure. As shown in Fig. 12\n, a carbon\u2010supported defect\u2010anchored Pt SACs were synthesized and showed high ORR performance with high onset potential and the 4e\u2212 ORR process [82]. In an acidic H2/O2 fuel cell, the Pt SACs with carbon defeat showed super-high platinum utilization of 0.09 gPtkW\u22121. The individual Pt atom was anchored by the carbon defects and the Pt dispersion was improved. The Pt atoms anchored by four carbon atoms (Pt/C4) was supposed to be the active sites. Furthermore, the Pt alloy and the other metal SACs were also prepared [89,91,92]. The SACs catalyst should be strongly coupled to the support to enhance stability. Furthermore, the catalytical activity is highly related to the coordination enlivenment of the metal atoms [93]. So the support effect should be considered when using the SACs.M-N-C (M\u00a0=\u00a0Fe, Co, Cu, Ni, Mn, etc.) based ORR catalysts is another unique class of SACs. In 2009, breakthroughs were achieved by the Dodelet group at INRS that the activity of the Fe\u2013N\u2013C catalyst reached to the level of Pt/C catalyst [94]. Since then, M-N-C catalysts have attracted extensive research interests. When the active species of M-N-C catalysts are downsizing into single atoms level, it can maximum atom-utilization and thus enhance the intrinsic activity of catalysts [95,96,97]. As shown in Fig. 13\n, similar to noble metal SACs, the catalytic performances of these atomically dispersed non-precious metal catalysts are also affected by their coordination environments and geometric configurations. To further elucidate the synthesis\u2212structure\u2212property correlations, more work should be done in the combination of precisely controlled synthetic process, in-situ structural characterizations, simulations, and intrinsic catalytic performance in the future.The different categories of low dimensional materials used in ORR have been reviewed. When the size of the materials is reduced to the nanometer scale (at least one of their dimensions), their electronic structure and their surface properties change drastically, resulting in unique physical and chemical properties due to the quantum size effect and large surface area. The electrocatalytic activity of low dimension materials is highly related to their electronic properties and nanostructures. The reaction mechanisms of low dimension materials are discussed by the combination of computational and experimental results. By reviewing the past progress, we expect to extend in-depth research on PEMFC catalysts of different dimensions. The low dimensional material can be used as both the catalysts and the supporting materials. Low-dimensional materials not only show a promising way to enhance the ORR activity but also provide new fundamental insight into ORR.Despite much progress, great efforts are still needed to further develop the low dimensional catalysts for ORR. For example, more theoretical and experimental studies are required to develop efficient strategies for alloying, hybridizing, doping (bi-doping and multi-doping), and combining different dimensional materials to further improve the thermodynamics and the kinetics of the catalysts. More systematical structure-property studies should be done to quantify the active sites and clarify the corresponding reaction mechanism of ORR. This calls for the employment of advanced characterization techniques, especially in-situ ones (such as XRD, XAS, IR, Raman), to investigate the ORR process based on low-dimensional catalysts in half cells and real fuel cell environments.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 the support from the Doctor's Initial Funding of Guizhou Normal University, China (GZNUD[2019]25, GZNUD[2019]29), International Science and Technology Corporation Program of Guizhou Province, China (G[2013]7022), Youth Science and Technology Talent Development Project from Guizhou Provincial Department of Education, China (KY[2016]063), Fonds de Recherche du Qu\u00e9bec-Nature et Technologies(FRQNT), the Natural Sciences and Engineering Research Council of Canada (NSERC), Institut National de la Recherche Scientifique (INRS), and Centre Qu\u00e9b\u00e9cois sur les Mat\u00e9riaux Fonctionnels (CQMF).", "descript": "\n                  Developing highly efficient electrocatalysts to facilitate the sluggish cathodic oxygen reduction reaction (ORR) is a key challenge for high-performance fuel cells. Low-dimensional materials have attracted great attention recently because of their unique structure and properties. In this review, the application of zero-dimensional (0D), one\u2010dimensional (1D), and two\u2010dimensional (2D) materials in ORR are discussed and particular attention is given to the relationship between their structure and the ORR activity. Graphene-based materials, transition metal dichalcogenides, transition metal oxide, nanotubes, nanoribbons, nanowires, and single-atom ORR catalysts are introduced and classified by their geometric dimension.\n               "}
{"full_text": "Scientific consensus on the dangers of anthropogenic climate change has become near irrefutable in recent years, prompting a drive to reduce greenhouse gas emissions from a number of industries. Hydrogen gas is an important bulk chemical that is used primarily in the production of fertilizers and refining of petroleum crudes. It is also considered a strategic, clean alternative to fossil fuel for transport and grid electricity. The industrial production of \n\n\nH\n\n\n2\n\n\n as a bulk chemical is currently dominated by steam reforming (SR) of natural gas; a large scale, carbon intensive process that requires centralized production.Chemical looping steam reforming (CLSR) is a \n\n\nH\n\n\n2\n\n\n production technology designed to improve the provision of heat and reduce the burden of \n\n\nH\n\n\n2\n\n\n separation in the SR process, thereby reducing carbon dioxide emissions, improving efficiency and potentially allowing decentralized generation and distribution\u00a0(Protasova and Snijkers, 2016; Adanez et al., 2012; Dueso et al., 2012). Decentralization of \n\n\nH\n\n\n2\n\n\n production via smaller scale generation processes can facilitate the use of widely available renewable biomass and waste feedstocks such as biogas, biodiesel, pyrolysis oils, ethanol and glycerol\u00a0(Cheng et al., 2017). The CLSR process relies on the cyclical reduction and re-oxidation of a metal oxide, which is known as an Oxygen Carrier (OC). The OC is cyclically exposed to a reducing fuel/steam atmosphere and an oxidizing air atmosphere in one of two ways: (1) by alternating the fuel/steam mix and air feeds to a single fixed bed reactor or (2) by physically moving the OC between two fluidized bed reactors, each one fed with a continuous stream of fuel/steam or air. In the steam reforming half cycle, the OC is reduced to a catalytically active state by a hydrocarbon fuel/steam flow and serves as the catalyst for the steam reforming of the methane feed (SR, Eq.\u00a0(1)) and the water gas shift reaction (WGS, Eq.\u00a0(2)). \n\n(1)\n\n\n\nCH\n\n\n4(g)\n\n\n+\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n(g)\n\n\n\u21c6\n\n\n3H\n\n\n2(g)\n\n\n+\n\n\nCO\n\n\n(g)\n\n\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n228\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\n\n\n(2)\n\n\n\nCO\n\n\n(g)\n\n\n+\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n(g)\n\n\n\u21c6\n\n\nCO\n\n\n2(g)\n\n\n+\n\n\nH\n\n\n2(g)\n\n\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n\u2212\n33\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\nCarbon may be formed during this process causing deactivation of the catalyst. Carbon is produced by: thermal cracking (Eq.\u00a0(3)); CO disproportionation (Eq.\u00a0(4)); or reduction of CO (Eq.\u00a0(5)). \n\n(3)\n\n\n\nCH\n\n\n4(g)\n\n\n\u21c6\n\n\n3H\n\n\n2(g)\n\n\n+\n\n\nC\n\n\n(s)\n\n\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n75\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\n\n\n(4)\n\n2\n\n\nCO\n\n\n(g)\n\n\n\u21c6\n\n\nCO\n\n\n2(g)\n\n\n+\n\n\nC\n\n\n(s)\n\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n\u2212\n172\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\n\n\n(5)\n\n\n\nCO\n\n\n(g)\n\n\n+\n\n\nH\n\n\n2(g)\n\n\n\u21c6\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n(g)\n\n\n+\n\n\nC\n\n\n(s)\n\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n\u2212\n131\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\nUnder the oxidation half-cycle the OC, and any carbon deposited during steam reforming, is oxidized by an air flow in highly exothermic reactions. These reactions provide heat throughout the reactor bed for the subsequent endothermic SR reaction\u00a0(Protasova and Snijkers, 2016; Adanez et al., 2012).The choice of OC is therefore integral to the design of the CLSR process. An OC must present high reactivity for the reduction and oxidation reactions involved and maintain that reactivity over extended cycling. It must exhibit high oxygen transfer capacity and favourable thermodynamic properties. The OC should also be readily reduced by a number of fuel mixtures, and act as an effective catalyst for the SR reactions involved in the process. Resistance to attrition and agglomeration is important if the OC is to be used in a fluidized bed reactor, and the cost and complexity of synthesis of the OC must be kept to a minimum\u00a0(Adanez et al., 2012; Dueso et al., 2012; Ryd\u00e9n and Ramos, 2012; Tang et al., 2015; Noorman et al., 2007; Gay\u00e1n et al., 2009; Quddus et al., 2013). Moreover resistance to sintering induced degradation is important; low density fibrous mats with minimal numbers of contact points between the fibres may offer advantages in this context.Many carriers have been tested for use in chemical looping applications but supported transition metals have attracted the most interest\u00a0(Adanez et al., 2012; Quddus et al., 2013). Metal oxides of iron, nickel, manganese, copper and cobalt have all been used, supported on alumina, zirconia, silica, titania, or bentonite\u00a0(Adanez et al., 2012; Gay\u00e1n et al., 2009; Ryu et al., 2001). Nickel when supported on alumina (Ni/\n\u03b1\n-Al2O3 or Ni/\n\u03b3\n-Al2O3) is by far the most widely researched OC for syngas and \n\n\nH\n\n\n2\n\n\n production. This is thanks to strong catalytic activity for the SR reactions, acceptable oxygen transfer capacity and high redox reaction rates at the temperatures encountered in CLSR\u00a0(Adanez et al., 2012; Tang et al., 2015; Noorman et al., 2007; Quddus et al., 2013; Zafar et al., 2005, 2006; Noorman et al., 2010; de\u00a0Diego et al., 2008).The synthesis method used to produce an alumina supported OC must achieve close control of the distribution of the deposited metal oxide and minimize chemical reactions that degrade the catalytic activity\u00a0(Dueso et al., 2012; Gay\u00e1n et al., 2009, 2008; Mattisson et al., 2006). In the case of nickel oxygen carriers the formation of spinel nickel aluminate (NiAl2O4) reduces the amount of NiO\u00a0(Gay\u00e1n et al., 2009; Quddus et al., 2013; Mattisson et al., 2006; Dueso et al., 2010): NiO is the more reactive species in redox and in the SR and WGS reactions (Eqs. (1) and (2))\u00a0(Noorman et al., 2007; Mattisson et al., 2006; Dueso et al., 2010). It is thus imperative to devise a synthesis procedure that minimizes the formation of NiAl2O4 and therefore maximizes the proportion of Ni in the form of NiO.Metal support interactions dictate whether NiO or NiAl2O\n\n\n\n4\n\n\n is formed during synthesis; a strong chemical interaction will produce NiAl2O\n\n\n\n4\n\n\n whereas a weak interaction will produce NiO\u00a0(Quddus et al., 2013; Zafar et al., 2006). Bolt et\u00a0al. suggested that there are likely to be two major factors that affect these interactions\u00a0(Bolt et al., 1998); the crystalline phase of alumina used and the temperature to which the OC\u2019s are exposed. Bolt hypothesized that due to its \u201cdefect spinel structure\u201d \n\u03b3\n-Al2O\n\n\n\n3\n\n\n facilitates the entry of metal cations (e.g.\u00a0Ni\n\n\n\n2\n+\n\n\n) into its crystal lattice, and therefore \n\u03b3\n-Al2O\n\n\n\n3\n\n\nwould interact more strongly with the metal species and at a more rapid rate than \n\u03b1\n-Al2O3 during synthesis\u00a0(Bolt et al., 1998). Additionally if exposed to temperatures above 900\u00a0\u00b0C, the \n\u03b3\n-Al2O3 would undergo phase transformation to \n\u03b8\n-Al2O3 and subsequently \n\u03b1\n-Al2O3 thereby promoting chemical reactions between metal and support\u00a0(Bolt et al., 1998). These hypotheses were confirmed by a number of studies. Cheng et\u00a0al. and Matisson et\u00a0al. found that this strong interaction begins to occur when a Ni \n\u03b3\n-Al2O3 catalyst is exposed to temperatures exceeding 600\u00a0\u00b0C and becomes increasingly prevalent above 800\u00a0\u00b0C\u00a0(Mattisson et al., 2006; Cheng et al., 1996). In a series of investigations, Dueso et al. (2012) and Dueso et al. (2010). investigated the metal-support reactions between two nickel OC\u2019s involving \n\u03b3\n-Al2O3 and \n\u03b1\n-Al2O3 supports; the \n\u03b1\n-Al2O\n\n\n\n3\n\n\nOC produced a higher proportion of NiO than the \n\u03b3\n-Al2O3 OC. This increased proportion of NiO when using \n\u03b1\n-Al2O3 was corroborated by the work by You et al. (2014).The addition of cobalt has been found to improve the performance of Ni/Al2O3 catalysts\u00a0(Bolt et al., 1998; Hossain et al., 2007; Hossain and de\u00a0Lasa, 2007). This has been attributed to preferential formation of CoAl2O4 over NiAl2O4 spinel due to faster kinetics of the CoO\u2013Al2O3 reaction\u00a0(Bolt et al., 1998). This results in a greater proportion of \u2018free\u2019 NiO, and therefore improved OC performance.These OC\u2019s also offer advantages such as excellent redox cycling durability, strong suppression of carbon formation and stable SR performance attributed to the interactions between the two active metals\u00a0(You et al., 2014; Hossain et al., 2007; Hossain and de\u00a0Lasa, 2007; Jin et al., 1998).Although metal-support interactions have been widely reported, the effect of synthesis method upon homogeneity of metal dispersion is less well discussed. In many of the papers mentioned above, traditional incipient wetness impregnation techniques were used to deposit an active metal upon the support. In this technique, a metal salt solution is introduced to the support and the mixture then dried in order to bring about the deposition of the salt inside the pores of the support. A critical stage in this technique is drying; as the solvent evaporates, the salt is gradually concentrated and deposited. As a result a non-uniform deposition may be expected\u00a0(Dueso et al., 2010; Zhao et al., 2000). Urea-decomposition is an alternative deposition route which offers superior control of the distribution of catalyst over a substrate. The method relies on the slow decomposition of urea above 90\u00a0\u00b0C in aqueous media which causes a uniform increase in pH throughout the solution\u00a0(Tang et al., 2015; Gay\u00e1n et al., 2009).In the present work low-density mats of Saffil fibre are investigated as a novel Ni/Co substrate material\u00a0for CLSR applications. Urea decomposition and wet impregnation deposition routes have been examined. Saffil (catalytic grade) is a fibrous crystalline material consisting of \n\u03b3\n-Al2O3 (95%) and SiO\n\n\n\n2\n\n\n (5%). The Saffil supported OC\u2019s have been evaluated in terms of hydrogen yield and purity, and methane conversion in a fixed bed CLSR process over several redox cycles. The results demonstrate a significant difference in morphology of the deposited metal oxide coating and the corresponding steam reforming performance for OCs manufactured by controlled precipitation reactions as opposed to evaporative wet impregnation.The motivation for the work was to investigate the feasibility of alternative OCs in CLSR. If high activity in both steam reforming and oxygen transfer, retention of large specific surface area and thermal stability under chemical looping conditions can be demonstrated, the high void spaces in a loose assembly of Saffil supported OCs offers the potential to deliver sufficient residence times with low reactor load. Consequently new small and medium scale hydrogen production processes could be realized in future, catering to distributed feedstocks such as unconventional gases (e.g.\u00a0shale wells) and biomass products (e.g.\u00a0large farms, anaerobic digestion plants, biorefineries).The Saffil-supported OC\u2019s were synthesized via three methods: Wet Impregnation (WI), Urea Decomposition Precipitation (DP) and Hydrothermal Synthesis (HT). Nickel and cobalt were used in varying quantities providing three different nickel to cobalt ratios. A total of nine OC\u2019s were made (summarized in Table\u00a01). A conventional pelletized 18 wt% NiO/\n\u03b1\n-Al2O3 (18 wt% NiO, crystallite size 45\u00a0nm\u00a0(Cheng et al., 2017)) was lightly ground into \n\u223c\n\n\n200\n\n\u03bc\nm\n granules which could be accommodated in the bench top reactor.\n\nStarting reagents were Ni(NO3)2.6H2O (10.8756 g), and for mixed catalysts, Co(NO3)2.6H2O (0.2981 g or 0.9052 g) of purity 99.9% (Fischer Scientific). The cobalt loadings were either 0 wt%, 0.6 wt% or 1.8 wt%. The reagents were dissolved in 250 ml of distilled water, into which 10 g Saffil mat, cut into approximately 5\u00a0mm3 shreds, was introduced. The mixture was placed in a drying oven held at 100\u00a0\u00b0C for 6\u00a0h under an air atmosphere to impregnate the fibres with Ni/Co salts. The resultant material was then calcined at 600\u00a0\u00b0C for 4\u00a0h under an air atmosphere to decompose the Ni/Co nitrates to oxides.Experimental conditions for deposition of the catalyst on Saffil were based on those reported in the literature to be optimal for urea based homogeneous precipitation onto porous Al2O3 substrates\u00a0(Zhao et al., 2000). The Ni and Co precursors in the same amounts as in WI were added to water, 250 ml, containing dissolved urea, CO(NH2)2, (Fischer Scientific) and Saffil to give a molar ratio of 1.7 Ni/Co:Urea. This mixture was placed in a beaker covered with a watch glass and placed in an oven; the temperature was raised from room-temperature at 10\u00a0\u00b0C min\u22121 to 95\u00a0\u00b0C for a dwell time of 24\u00a0h under an air atmosphere. This allowed for the degradation of urea to raise pH and precipitate Ni/Co hydroxides as per the reaction: \n\n(6)\n\nCO\n\n\n\n(\n\n\nNH\n\n\n2\n\n\n)\n\n\n\n2\n\n\n+\n\n\n3H\n\n\n2\n\n\nO\n\u2192\n\n\n2OH\n\n\n\u2212\n\n\n+\n\n\n2NH\n\n\n\n\n4\n\n\n+\n\n\n\n\n+\n\n\nCO\n\n\n2\n\n\n\u2191\n\n[42]\n\n\n\nThe mixture was then placed in a drying oven for 6\u00a0h under an air atmosphere at 100\u00a0\u00b0C to evaporate any remaining solution, and then calcined at 600\u00a0\u00b0C for 4\u00a0h.Starting reagents in the same proportions as for WI and DP but in smaller quantities (4.3502 g of Ni(NO3)2.6H2O; 0.1191 g or 0.3621 g of Co(NO3)2.6H2O) and a proportionate amount of Saffil were added to 100 ml water into which urea (CO(NH2)2,) was also added at a molar ratio of 1.7 Ni/Co:Urea. The mixture was transferred to a 125 ml Teflon lined autoclave (Parr Scientific) and placed in a furnace heated at 10\u00a0\u00b0C min\u22121 to 95\u00a0\u00b0C and held at this temperature for 24 h. The mixture was transferred from the hydrothermal reactor to a drying oven and heated at 100\u00a0\u00b0C for 6\u00a0h to evaporate any remaining solution, followed by calcination at 600\u00a0\u00b0C for 4\u00a0h.The morphology and microstructure of the OC\u2019s were analysed using a Hitachi SU8230 high performance cold field emission scanning electron microscope (SEM). The samples were affixed to 10\u00a0mm aluminium stubs via carbon tape and all samples were carbon coated (thickness 2\u20133\u00a0nm) prior to SEM analysis. The materials heavily charged under the electron beam; therefore low voltage (1\u20132\u00a0kV) and current (0.9 \n\u03bc\nA) settings were used to reduce this.The phase composition of the raw support and synthesized OC\u2019s were analysed using powder X-ray diffraction (XRD). This was carried out using a Bruker D8 diffractometer with a Cu X-ray tube (\n\u03bb\n=\n1\n.\n5406\n \u00c5). A step size of 0.025\u00b0, scan speed of 2 s per step and a range of 10\u201375\u00b0 2\n\u03b8\n were used. Background determination, peak identification and phase identification were conducted using XPert HighScore Plus analysis software. All samples were ground by pestle and mortar to form a powder prior to analysis.The nanostructures of the OC\u2019s were analysed using an FEI Titan3 Themis (scanning) transmission electron microscope (TEM) operating at 300 kV and fitted with a high angle annular dark field (HAADF) STEM detector, a Super-X 4-detector energy dispersive X-ray (EDX) analysis system and a Gatan One-View CCD. The EDX analysis was performed using Bruker Espirit software (version 1.9.4). All samples were ground in ethanol and added by pipette onto 400 mesh holey carbon coated Cu TEM grids (Agar Scientific).Quantitative elemental analysis was conducted with the use of a Varian 240\u00a0s Atomic Adsorption Spectrophotometer (AAS). Samples were digested in 10 ml of 1:1 HCl and heated under reflux for 30 min. The solution was then diluted to 250 ml with distilled water. Nickel content was analysed using a wavelength of 352.4\u00a0nm and a slit width of 0.5\u00a0nm; cobalt was analysed at a 240.7\u00a0nm wavelength and a slit width of 0.2\u00a0nm.A Quantachrome Instruments NOVA 2200e was used to determine specific surface area (SSA) via the Brunauer\u2013Emmett\u2013Teller (BET) method. All samples were outgassed at 200\u00a0\u00b0C for three hours and analysed using \n\n\nN\n\n\n2\n\n\n as the adsorbate gas at a temperature in the region of 77 K.Chemical looping steam reforming experiments assessed the performance of the Saffil based OC\u2019s in comparison to the conventional SR catalyst (granulated) in the CLSR process. The experiments consisted of a SR half-cycle and an oxidation half-cycle. The SR half-cycle exposed the various OC\u2019s to CH4 and \n\n\nH\n\n\n2\n\n\nO under \n\n\nN\n\n\n2\n\n\n carrier gas, chemically reducing the OC\u2019s as well as performing steam reforming. The carrier gas was used to enable elemental balances and to reach the minimum flow rates required by the analyser for accurate measurements. Oxidation was induced in the half-cycle of air-exposure.Appropriate conditions for chemical looping experiments were chosen by calculating chemical equilibrium compositions at assigned temperatures and pressures using the computer program CEA (Chemical Equilibrium with Application) developed by NASA Lewis Research Centre\u00a0(McBride and Gordon, 1994). Conditions of 700\u00a0\u00b0C and a molar steam to carbon ratio (S:C) of 3 were found to maximize the equilibrium methane conversion and provide high hydrogen yield and purity while minimizing solid carbon deposition and steam flows.The reactor set-up used in the chemical looping experiments is shown in Fig.\u00a01. CH4, \n\n\nH\n\n\n2\n\n\n and \n\n\nN\n\n\n2\n\n\n (BOC, purity 99.995%) and on-site compressed air were used as reactant gases. Mass flow controllers (Bronkhorst EL-FLOW, range 0.1\u2013180 sccm) set the flow rate of CH4 and \n\n\nH\n\n\n2\n\n\n whereas \n\n\nN\n\n\n2\n\n\n and air were controlled by electric rotameters (Bronkhorst MASS-VIEW, range 0\u20132000 sccm). A programmable syringe pump was used to introduce distilled water into the heated zone of the reactor to satisfy the S:C required for each experiment. The 316 stainless steel (SS) reactor had an inside diameter of 13.2\u00a0mm and a length of 750\u00a0mm of which the bottom 500\u00a0mm was heated using a vertical tube furnace (Elite Thermal systems TSVH12/30/450) controlled by a temperature controller (Eurotherm 3216). Additionally a second K-type thermocouple was placed adjacent to the bottom basket and was used to monitor temperature in the catalyst bed to ensure the desired temperature was reached in each experiment.\nIn each experiment the reactor was loaded with 2 g of OC. The bed volume was 12.5\u00a0cm3 and consisted of 10 stainless steel mesh baskets in which the OC\u2019s were held. These baskets were placed in the reactor by resting on a small steel bar welded across the tube. Given the difference in bulk density between the two catalysts (conventional 18 wt% NiO was \n\u223c\n1 g cm\u22123, the Saffil OC\u2019s were \n\u223c\n0.2 g cm\u22123) the conventional SR catalyst used for comparison purposes was diluted with silica sand to increase the bed volume to that of the Saffil catalyst.As the analysers used in the apparatus were highly sensitive to water, a condenser (jacketed heat exchanger using a cooling fluid consisting of 1:1 mix of water and ethylene glycol chilled to 2\u00a0\u00b0C) and a moisture trap (silica gel) were used to remove water prior to gas analysis. The outlet composition of the remaining gases was recorded in vol% every 5 s by an ABB Advanced Optima analyser using three modules; Uras 14 (CO, CO2 and CH4 measured by infrared adsorption), Caldos 15 (\n\n\nH\n\n\n2\n\n\n via thermal conductivity), and Magnos 106 (\n\n\nO\n\n\n2\n\n\n via paramagnetic analysis).All experiments were conducted at 700\u00a0\u00b0C with S:C = 3, a \n\n\nN\n\n\n2\n\n\n flow rate of 1000 sccm, CH4 flow rate of 111 sccm and liquid \n\n\nH\n\n\n2\n\n\nO flow rate of 0.25 sccm. The experiments were conducted via the following procedure. (1) The reactor was heated at 10\u00a0\u00b0C/min to 700\u00a0\u00b0C under a flow of 1000 sccm \n\n\nN\n\n\n2\n\n\n. (2) A mixture of 5 vol% H2 in N2 flowed through the catalyst bed at 1000 sccm to reduce the catalyst: reduction was inferred from a rise in the outlet concentration of H2 to 5 vol%). This ensured that the catalyst was in the reactive metal form rather than the non-catalytically active oxide phase. (3) The reactor was purged with \n\n\nN\n\n\n2\n\n\n until \n\n\nH\n\n\n2\n\n\n was no longer present in the outlet. (4) The initial SR cycle was then performed by switching on the water feed to pump water into the reactor. When water contacts the reduced catalyst as steam, \n\n\nH\n\n\n2\n\n\n is formed via oxidation of the catalyst (water splitting); when this was detected by the analysers, a flow of 111 sccm CH4 was added to the \n\n\nN\n\n\n2\n\n\n flow resulting in a total flow of 1111 sccm of reactant gas at 10 vol% CH4. This SR regime was continued for 20 min. (5) The CH4 and water flows were switched off and the reactor purged with \n\n\nN\n\n\n2\n\n\n for 5 min. (6) The N2 flow was switched off and the Air flow set to 1000 sccm for 5 min; in order to oxidize the catalyst. (7) The reactor was then purged for 5 min under 1000 sccm \n\n\nN\n\n\n2.\n\n\n (8) The water and CH4 flows were switched on again and reduction of the catalyst occurred: the reduced catalyst then promoted SR and steps 5\u20138 were repeated until the catalyst had been reduced and oxidized 7 times.The outputs of the chemical looping experiments were defined by the following equations: \n\n(7)\n\n\n\nH\n\n\n2\n\n\nY\ni\ne\nl\nd\n=\n\n\n\n\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\n\n\nH\n\n\n2\n\n\n,\no\nu\nt\n\n\n\u2217\n\n\n\n\nW\n\n\u00af\n\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\n\n\nCH\n\n\n4\n\n\n,\ni\nn\n\n\n\u2217\n\n\n\n\nW\n\n\u00af\n\n\n\n\n\nCH\n\n\n4\n\n\n\n\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n(8)\n\n\n\nH\n\n\n2\n\n\nP\nu\nr\ni\nt\ny\n=\n\n\n\n\n\n\nx\n\n\n\n\nH\n\n\n2\n\n\n,\no\nu\nt\n\n\n\n\n\n\nx\n\n\nC\nO\n,\no\nu\nt\n\n\n+\n\n\nx\n\n\n\n\nCO\n\n\n2\n\n\n,\no\nu\nt\n\n\n+\n\n\nx\n\n\n\n\nCH\n\n\n4\n\n\n,\no\nu\nt\n\n\n+\n\n\nx\n\n\n\n\nH\n\n\n2\n\n\n,\no\nu\nt\n\n\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n(9)\n\n\n\nCH\n\n\n4\n\n\n\nC\no\nn\nv\ne\nr\ns\ni\no\nn\n,\n\n\nX\n\n\n\n\nCH\n\n\n4\n\n\n\n\n=\n\n\n\n\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\n\n\nCH\n\n\n4\n\n\n,\ni\nn\n\n\n\u2212\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\n\n\nCH\n\n\n4\n\n\n,\no\nu\nt\n\n\n\n\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\n\n\nCH\n\n\n4\n\n\n,\no\nu\nt\n\n\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n(10)\n\n\n\nH\n\n\n2\n\n\nO\n\nC\no\nn\nv\ne\nr\ns\ni\no\nn\n,\n\n\nX\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n=\n\n\n\n\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\n\n\nH\n\n\n2\n\n\nO\n,\ni\nn\n\n\n\u2212\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\n\n\nH\n\n\n2\n\n\nO\n,\no\nu\nt\n\n\n\n\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\n\n\nH\n\n\n2\n\n\nO\n,\no\nu\nt\n\n\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n(11)\n\n\n\nr\n\n\nr\ne\nd\n\n\n,\nO\n\nC\n=\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\nd\nr\ny\n,\no\nu\nt\n\n\n\n\n\n\nx\n\n\nC\nO\n,\no\nu\nt\n\n\n+\n\n\nx\n\n\n\n\nCO\n\n\n2\n\n\n,\no\nu\nt\n\n\n+\n\n\nx\n\n\n\n\nO\n\n\n2\n\n\n,\no\nu\nt\n\n\n\n\n\u2212\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\n\n\nH\n\n\n2\n\n\nO\n,\ni\nn\n\n\n\n\nX\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\nUnder both half cycles, molar flow rate of gas component i (\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\ni\n,\no\nu\nt\n\n\n) was defined as the dry gas mol fraction in the gas products \n\n\nx\n\n\ni\n\n\n measured online, multiplied by \n\n\n\n\nn\n\n\n\u0307\n\n\n\n\nd\nr\ny\n,\no\nu\nt\n\n\n, the molar flow rate of total dry outlet gas from the nitrogen balance. (\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\nd\nr\ny\n,\no\nu\nt\n\n\n) was estimated using a nitrogen balance. During the methane/steam feed half cycle, methane conversion and the carbon balance error (where 100% results in a perfect balance) were estimated using a carbon balance. Water conversion was then estimated using a hydrogen balance. Finally, the rate of OC reduction (\n\n\nr\n\n\nr\ne\nd\n\n\n,\nO\nC\n) was estimated with the use of an additional oxygen balance. Total moles of metal oxide reduced over a given time were obtained from a time integration of the rate formula over the duration of the methane/steam feed. In-depth discussion of the elemental balances can be found elsewhere for a generic \n\n\nC\n\n\nn\n\n\nH\n\n\n\nm\n\n\nO\n\n\n\nk\n\n\n fuel\u00a0(Pimenidou et al., 2010). For the air feed half cycle, a carbon balance would have determined if any carbon was present on the OC, as it would have oxidized to CO or CO2, but it will be seen that neither gases were detected during the air feeds. An oxygen balance then determined the rate of Ni oxidation, and from its integration over time, the moles of Ni oxidized during air feed.\n\n\n\nThe nominal and chemically analysed metal loadings of the OC\u2019s were generally in good agreement, Table\u00a01, confirming the validity of the synthesis methods employed. X-ray powder diffraction showed NiO peaks and broad peaks of \n\u03b3\n-Al2O3 (ICDD 00-010-0425) and SiO\n\n\n\n2\n\n\n(ICDD 01-080-6157) from the Saffil support, Fig.\u00a02\u00a0(Peng et al., 2001). No CoO was detected in the mixed catalyst samples, as its concentration fell below the XRD detection limit. Important in the context of OC performance, any NiAl2O4 was also below XRD detection limits (< 5%).Scanning electron microscopy of uncoated Saffil indicated fibre diameters in the range 2\u2013\n5\n\n\u03bc\nm\n, Fig.\u00a03a. Examination of the coated fibres showed the WI method was the least effective deposition route, giving erratic distributions of the metal oxide phase, Fig.\u00a04a\u2013b; some fibres were completely covered in a layer of densely packed metal oxide particles and there were examples of micron sized agglomerates on top of some regions of the coating. In terms of crystallite morphology, the WI route led to coatings composed of round, nodular crystallites with a wide size distribution, including some up to \n\u223c\n 100\u00a0nm in size, Fig.\u00a04c. This variability is typical of a wet impregnation method as during drying a range of precipitation and growth conditions exist leading to non-uniformity in coating integrity and particle sizes\u00a0(Neimark et al., 1981).The DP and HT synthesis routes each gave more uniform distributions of the deposited material with little or no evidence of uncoated areas. Fig.\u00a04d and e show SEM images for DP. Micrographs of HT samples appear in Fig.\u00a04g\u2013i. Both coatings appeared as a series of folded layers, possibly representing a series of interconnected platelet crystallites, with edges normal to the surface of the fibres. The network of folded \u2018ridges\u2019 with solid walls up to \n\u223c\n100\u00a0nm in width framed void spaces with lateral dimensions of up to \n\u223c\n 400\u00a0nm for DP and to 500\u00a0nm for the less densely folded HT coatings.The addition of Co seemed to have little effect on the morphology of the catalysts produced by any of the synthesis routes. Fig.\u00a05a and b show typical SEM cross sections of a DP sample revealing a metal oxide layer thickness of \n\u223c\n 500\u00a0nm. The ridge structure extends from the outer surface to the fibre-interface indicating a progressive growth of the metal oxide layer perpendicular to the fibre surface.The BET specific surface areas of the uncoated Saffil fibre and the synthesized OC\u2019s are shown in Table\u00a01. The as-received Saffil fibres had a SSA of 106 m2 g\u22121. The deposition of NiO by the WI method reduced the SSA compared to the Saffil substrate. By contrast, the HT and DP OC\u2019s generally exhibited a higher SSA, with one exception, the 18Ni DP sample (102 m2 g\u22121). The OC\u2019s via the HT route produced the most consistent SSA results, with all HT OC\u2019s showing an SSA in excess of 115 m2 g\u22121 and varying in SSA by only \n\u223c\n5 m2 g\u22121. There appeared to be no effect on SSA from cobalt doping. The increase in measured SSA from DP and HT synthesis routes, relative to WI, is consistent with the porous coatings disclosed by SEM (Fig.\u00a03).Transmission electron microscopy of uncoated Saffil fibres had indicated a crystallite size of approximately 4\u20136\u00a0nm, Fig.\u00a03b. TEM of the 18Ni WI sample, Fig.\u00a06a and b, revealed rounded particle profiles consistent with the nodular structures observed by SEM (Fig.\u00a04b and c). The estimated average size of these crystals was 73\u00a0nm, with a range from 20\u00a0nm to 150\u00a0nm (based on 50 crystals measured). TEM images of the coatings in DP and HT samples revealed that the folds (wall thickness \n\u223c\n 100\u00a0nm) were composed of a sub structure of sub 10\u00a0nm crystallites, Fig.\u00a06c\u2013e. Some detached fibril-like nanoparticles were also evident. Corresponding HAADF STEM images and EDX maps are shown in Fig.\u00a07. The location of the interface between fibre-and coating was located from EDX analysis (white and black arrows inset); from this the depth of Ni coating could be evaluated. The 18Ni WI sample indicated a deposited layer of < 200\u00a0nm whilst the coatings in the DP and HT samples were thicker at \n\u223c\n 300\u00a0nm and \n\u223c\n 400\u00a0nm respectively, consistent with the thickness estimated from SEM section (Fig.\u00a05a). Analysis of electron diffraction patterns yielded d-spacings in agreement with the XRD analysis conducted earlier that was consistent with a NiO coating (refer to Figure S1 and Table S1).Aluminium was detected from TEM-EDX co-existing with Ni (and Co) even at the outermost surface of the metal oxide coating (Area 1, Fig.\u00a07, Table\u00a02 and Figure S2). Table\u00a02, shows the semi quantitative analysis of Areas 1 and 2 in Fig.\u00a07a\u2013i. The quantity of Al at the surface of the deposited layer (Area 1) is least in the WI sample (15 at. %) and highest in the HT sample (35 at. %). Corresponding EDX spectra are shown in Figures S3\u2013S5.\n\n\n\nThe presence of Al at the surface of the deposited layer may arise from partial dissolution of Saffil during the chemical treatments involved in synthesizing the OC\u2019s. It has been reported that addition of nickel and cobalt nitrate to water can cause some dissolution of pure powdered \n\u03b3\n-Al2O3 supports due to the change in pH\u00a0(Espinose et al., 1995). In the present work, the Al detected by TEM-EDX in the catalyst phase suggests some co-precipitation of Ni\n\n\n\n2\n+\n\n\n, Co\n\n\n\n2\n+\n\n\n and Al3\uff0b species ions occurred. This sequence may account for the very unusual \u2018honeycomb\u2019 structure to the DP and HT coatings on Saffil, as identified by SEM (Fig.\u00a04). The Saffil fibres were only in contact with the precursor solution for a limited amount of time in the WI method (i.e.\u00a0until the precursor solution had evaporated), thereby severely limiting any Saffil dissolution. The chemical conditions in DP, and more so HT synthesis methods are expected to promote increased Saffil dissolution.We tentatively ascribe the morphology of the coatings from DP and HT methods to a formation mechanism involving layered intermediate phases such as double layer hydroxides, DLHs. The soluble Al species and the presence of carbonate ions from urea decomposition, along with Ni/Co ions would provide a solution environment from which DLHs, [M\n\n\n\n1\n\n\n\u2212\n\n\nx\n\n\n\n\n2\n+\n\n\nM\n\n\n\nx\n\n\n3\n+\n\n\n(OH)2][CO\n\n\n\n\n\n3\n\n\nx\n\n\n\u22152\n\n\n]\n\u22c5\n\nmH2O could form\u00a0(Li et al., 2012; Feng et al., 2009; Xu et al., 2015; Christensen et al., 2006).\n\nThe reactions may be represented as follows in Eqs. (12)\u2013(15). \n\n(12)\n\n\n\nAl\n\n\n2\n\n\n\n\nO\n\n\n3\n\n\n+\n\n\n3H\n\n\n2\n\n\nO\n+\n\n\n2OH\n\n\n\u2212\n\n\n\u2192\n2\n\n\n\n[\n\n\nAl(OH)\n\n\n4\n\n\n]\n\n\n\n\u2212\n\n\n\n\n\n\n\n(13)\n\n\n\nNi\n\n\n2\n+\n\n\n+\n2\nO\n\n\nH\n\n\n\u2212\n\n\n\u2192\n\n\nNi(OH)\n\n\n2\n\n\n\n\n\n\n\n(14)\n\n\n\nCO\n\n\n2\n\n\n+\n\n\nH\n\n\n2\n\n\nO\n\u2192\n\n\nCO\n\n\n3\n\n\n2\n\u2212\n\n\n+\n2\n\n\nH\n\n\n+\n\n\n\n\n\n\n\n\n(15)\n\n\n(\n1\n\u2212\nx\n)\n\n\n\nNi(OH)\n\n\n2\n\n\n+\n\n\nxAl(OH)\n\n\n\n\n4\n\n\n\u2212\n\n\n\n\n+\n\n\nx\n\u2215\n2\n\n\nCO\n\n\n3\n\n\n\n\n2\n\u2212\n\n\n\u2192\n\n\n\n[\n\n\nNi\n\n\n1\u2212x\n\n\n\n\nAl\n\n\n3\n+\n\n\n\n\n\nx\n\n\n\n\n(OH)\n\n\n2\n\n\n]\n\n\n\nx+\n\n\n\n\n\n(\n\n\n\n\nCO\n\n\n3\n\n\n\n\n\u2212\n2\n\n\n)\n\n\n\nx\u22152\n\n\n+\n\n\n2xOH\n\n\n\u2212\n\n\n\n\n\n\nDouble layer hydroxides, formed in this case from Ni and Al ions (the latter from dissolution of Saffil fibres) as represented by Eqs. (12)\u2013(15), typically crystallize with a characteristic platelet morphology due to their hexagonal crystal structure and low surface energy of ab crystal planes. For example Li et\u00a0al. report platelet DLH crystals from urea derived synthesis of Ni/Mg catalysts on alumina particles that have broad similarities to the DP and HT product morphologies\u00a0(Li et al., 2012). A high lateral growth rate of anisotropic LDH crystals, perpendicular to the Saffil support may in part explain the ridged, folded structure observed in DP and HT samples. However a future dedicated crystal growth study would be required to fully understand the growth mechanism.The outlet composition for the Saffil OC\u2019s during the SR and the oxidation half cycle is shown in Fig.\u00a08 for a DP sample, and is representative of all the OC\u2019s tested. The inset graph in Fig.\u00a08 shows the CH4 and \n\n\nH\n\n\n2\n\n\nO conversions and the rate of NiO reduction (\n\n\nr\n\n\nred\n\n\n,OC) derived from Eqs. (9)\u2013(11) during the first 100 s of the SR half-cycle.The onset of OC reduction is clearly shown in the inset figure: \n\n\nr\n\n\nred\n\n\n,OC increased after 10 s and peaked at 25 s coinciding with a rapidly decreasing negative \n\n\nH\n\n\n2\n\n\nO conversion, and an increasingly positive CH4 conversion. Negative \n\n\nH\n\n\n2\n\n\nO conversion signifies a production of \n\n\nH\n\n\n2\n\n\nO, this coupled with an increase in CH4 conversion implies that the global reduction of the OC through complete combustion (Eq.\u00a0(16)) was favoured over other likely reduction reactions (Eqs. (17), (18) or (19)). After 25 s, an abundance of the reduced OC caused the initiation of the SR and WGS reactions (Eqs.\u00a0(1) and (2)) thereby consuming \n\n\nH\n\n\n2\n\n\nO and thus increasing the \n\n\nH\n\n\n2\n\n\nO conversion.\n\nAs the reduction neared completion, \n\n\nr\n\n\nred\n\n\n,OC decreased, while \n\n\nH\n\n\n2\n\n\nO conversion increased and reached steady state as the SR and WGS reactions dominated. \n\n\n(16)\n\n\n\nCH\n\n\n4(g)\n\n\n+\n\n\n4NiO\n\n\n(s)\n\n\n\u21c6\n\n\n4Ni\n\n\n(s)\n\n\n+\n\n\nCO\n\n\n2(g)\n\n\n+\n2\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n(g)\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n135\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\n\n\n\n(17)\n\n\n\nCH\n\n\n4(g)\n\n\n+\n\n\nNiO\n\n\n(s)\n\n\n\u21c6\n\n\nNi\n\n\n(s)\n\n\n+\n\n\nCO\n\n\n(g)\n\n\n+\n2\n\n\nH\n\n\n2(g)\n\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n213\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\n\n\n(18)\n\n\n\nCO\n\n\n(g)\n\n\n+\n\n\nNiO\n\n\n(s)\n\n\n\u21c6\n\n\nNi\n\n\n(s)\n\n\n+\n\n\nCO\n\n\n2(g)\n\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n\u2212\n48\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\n\n\n(19)\n\n\n\nH\n\n\n2(g)\n\n\n+\n\n\nNiO\n\n\n(s)\n\n\n\u21c6\n\n\nNi\n\n\n(s)\n\n\n+\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n(g)\n\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n\u2212\n15\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\n\nFig.\u00a08 shows no CO2 or CO peaks during the oxidation half cycle, a result shown across all the OC\u2019s\u2019 tested. This suggests the absence of complete or partial oxidation of solid carbon(Eqs.\u00a0(20) and (21)), and implies a lack of carbon deposition during reduction and steam reforming, thus oxidation of the OC was favoured (Eq.\u00a0(22)). \n\n(20)\n\n\n\nC\n\n\n(s)\n\n\n+\n\n\nO\n\n\n2(g)\n\n\n\u21c6\n\n\nCO\n\n\n2(g)\n\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n\u2212\n393\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\n\n\n(21)\n\n\n\nC\n\n\n(s)\n\n\n+\n0\n.\n5\n\n\nO\n\n\n2(g)\n\n\n\u21c6\n\n\nCO\n\n\n(g)\n\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n\u2212\n110\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\n\n\n(22)\n\n\n\n2Ni\n\n\n(s)\n\n\n+\n\n\nO\n\n\n2(g)\n\n\n\u21c6\n2\n\n\nNiO\n\n\n(s)\n\n\n\n\u0394\n\n\nH\n\n\n298K\n\n\n=\n\u2212\n468\n\n\n\u00a0kJ\u00a0mol\n\n\n\u2212\n1\n\n\n\n\n\n\nTable\u00a03 shows the maximum rate of OC reduction (maximum \n\n\nr\n\n\nred\n\n\n,OC). The synthesis method did not have a pronounced effect on the maximum rate of reduction (Table\u00a03), however the maximum \n\n\nr\n\n\nred\n\n\n,OC increased slightly with decreasing Ni:Co ratio. All of the equations used to calculate these factors are available elsewhere\u00a0(Pimenidou et al., 2010).The average \n\n\nH\n\n\n2\n\n\n purity and yield and CH\n\n\n\n4\n\n\nconversion over 7 CLSR cycles for all 9 OC\u2019s is compared to the standard 18 wt% NiO catalyst and equilibrium values shows an error < 5% in the carbon balance between the molar flows of carbon in and out of the reactor across all experiments, indicating accuracy in the measurements and elemental balance analysis.All of the Saffil OC\u2019s presented an improvement in terms of average CH4 conversion and \n\n\nH\n\n\n2\n\n\n yield over the conventional 18 wt% NiO SR catalyst, Fig.\u00a09, Table\u00a04. The 18Ni HT OC was the most effective, returning a 9.9% improvement in CH4 conversion and a 4.6% improvement in \n\n\nH\n\n\n2\n\n\nyield. The other Saffil OC\u2019s improved CH4 conversion by between 3.4% and 8.5% while \n\n\nH\n\n\n2\n\n\nyield was improved by 3.1% to 4.2%. \n\n\nH\n\n\n2\n\n\n purity was largely unchanged.\nThe CH4 conversion per cycle over the 7 CLSR test cycles is shown in Fig.\u00a010 and indicates that the performance of the Saffil OC\u2019s was stable and consistently superior to the conventional 18 wt% NiO catalyst (in granulated form), with HT having optimum performance, Table\u00a04. The \n\n\nH\n\n\n2\n\n\n yield and purity performance over 7 cycles are shown in Figure S6 and show similar enhancements in the Saffil OC\u2019s.There was no measurable difference in performance of OCs modified by cobalt. Additions of Co to Ni/Al2O3 have been reported by others to improve steam reforming performance by suppressing NiAl2O4 spinel formation\u00a0(Hossain and de\u00a0Lasa, 2007). A NiAl2O4 phase is normally formed during high temperature heat-treatments, thereby reducing the amount of available Ni catalyst. The absence of any effect of Co on steam reforming performance in the present work may relate to the milder heat-treatment schedules for OC synthesis and chemical looping which minimizes spinel formation\u2014indeed no spinel phase was identified by X-ray or electron diffraction in any OC.In summary, the synthesis of Saffil supported OC\u2019s via decomposition of urea (DP and HT) rather by simple liquid evaporation (WI) gives superior CLSR performance. The differences in the \n\n\nH\n\n\n2\n\n\n yield and CH4 conversion can be explained by the lower crystallite size, improved coating uniformity and more open texture. These factors are known to be important in conventional non-fibrous Ni/\n\u03b3\n-Al2O3 catalysts\u00a0(Christensen et al., 2006; Ashok et al., 2008; Lucr\u00e9dio and Assaf, 2006; Song et al., 2013). Reduced particle size results in a lower diffusional resistance to mass transfer between products and reactants in heterogeneous catalysis; the porous structure of the coating identified by SEM will aid gas diffusion, as will the relatively lose packing of the fibres. The role of any incorporated Al on the performance of the OC\u2019s is uncertain at this stage.The low density Saffil OC\u2019s developed in this project represent a different type of fibre-based catalyst to the densely packed fibre beds traditionally used for catalytic combustion of methane in space heating using platinum group metals\u00a0(Radcliffe and Hickman, 1975; Trimm and Lam, 1980b,a) which are currently receiving widespread interest for used in microchannel reactors for the rapid production of \n\n\nH\n\n\n2\n\n\n for use in hydrogen fuel cells\u00a0(Reichelt et al., 2014; Zhou et al., 2015).The performance comparisons highlight that for a given catalyst mass and volume, Ni/Co fibrous OC\u2019s offer significant advantages over traditional catalysts in fixed bed CLSR. Furthermore the low density fibrous OC\u2019s may provide other benefits for fixed bed processes. The low-density Saffil OC\u2019s present a high surface area to volume ratio and high void fraction in the catalyst bed leading to an excellent compromise between mass transfer for the catalytic reactions and pressure drop through the bed. Additionally they are easily manipulated into various shapes and have a high thermal stability\u00a0(Reichelt et al., 2014; Zhou et al., 2015; Reichelt and Jahn, 2017; Sadamori, 1999). These factors may prove beneficial to fixed bed CLSR processes, a research field in which fibre catalysts have not been applied. The lower thermal inertia offered by a less dense catalyst in conjunction with the reduction in diffusional resistance potentially allows fast and homogeneous heat provision during oxidation for the endothermic SR reactions allowing for efficient production of \n\n\nH\n\n\n2\n\n\n through the SR reactions. Further research is required to establish if these promising features can indeed facilitate a change in CLSR reactor geometry suited to small and medium scale hydrogen production.Ni/Co fibrous oxygen carriers (OC\u2019s) have been fabricated utilizing low density mats of a polycrystalline alumina/silica fibre support (Saffil). The performance of oxygen carriers deposited on Saffil by urea homogeneous precipitation in chemical looping steam reforming was equal or better than the commercial 18 wt% NiO catalyst (in granulated form). The Saffil based OC\u2019s synthesized in this work showed improved average values of methane conversion and hydrogen yield over the tested seven redox cycles. All of the OC\u2019s were reduced by the fuel steam mixture and produced no solid carbon during reforming. Moreover the process advantages of fibre catalysts including high malleability, thermal stability, high surface to volume ratio and void fraction indicate that fibrous catalysts are a promising alternative to conventional catalysts in fixed bed chemical looping steam reforming. Future work will explore the long-term stability and durability of these fibrous OC and explore the kinetics and mass transfer properties associated with reduction and oxidation to further examine their suitability for CLSR in comparison to commercial pelletized catalysts.\n\n\n\n\n\n\n\nNomenclature\n\n\n\n\nOC\nOxygen carrier\n\n\nSR\nSteam reforming\n\n\nCLSR\nChemical looping steam reforming\n\n\nWI\nWet Impregnation\n\n\nDP\nUrea decomposition-precipitation\n\n\nHT\nHydrothermal DP\n\n\nLDH\nLayered double hydroxide\n\n\nSEM\nScanning electron microscopy\n\n\nTEM\nTransmission electron microscopy\n\n\nEDX\nEnergy dispersive X-ray analysis\n\n\nBET\nBrunauer\u2013Emmett\u2013Teller\n\n\nSSA\nSpecific surface area\n\n\nXRD\nX-ray diffraction\n\n\nHAADF\nHigh angle annular dark field\n\n\nAAS\nAtomic adsorption Spectrophotometry\n\n\nS:C\nMolar steam to carbon ratio\n\n\nSS\nStainless steel\n\n\n\n\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\ni\n,\no\nu\nt\n\n\n\n\ndry moles in the reactor outlet of species i\n\n\n\n\n\n\n\n\n\nn\n\n\n\u0307\n\n\n\n\ni\n,\ni\nn\n\n\n\n\ndry moles in the reactor inlet of species i\n\n\n\n\n\n\n\nx\n\n\ni\n,\no\nu\nt\n\n\n\n\nvol % in the reactor outlet of species i\n\n\n\n\n\n\n\nx\n\n\ni\n,\ni\nn\n\n\n\n\nvol % in the reactor inlet of species i\n\n\n\n\n\n\n\n\n\nW\n\n\u00af\n\n\n\ni\n\n\n\n\nMolar mass of species i\n\n\n\n\n\n\n\nX\n\n\ni\n\n\n\n\nConversion of species i\n\n\n\n\n\n\n\nr\n\n\nr\ne\nd\n\n\n,\nOC\n\n\nRate of OC reduction (mol s\u22121)\n\n\n\n\n\nThanks to Jenny Forrester for her assistance with XRD analysis and to Stuart Micklethwaite for his assistance during SEM imaging. This work was supported by the EPSRC via the Low Carbon Technologies Doctoral Training Centre (EP/G036608/1) and the UKCCSRC via the Call 2 Capture Projects\n (UKCCSRC-C2-181). Further thanks must be extended to Jonathan Cross and Unifrax Ltd for supplying the CG Saffil material and for their input into this project.None.Supplementary material related to this article can be found online at https://doi.org/10.1016/j.egyr.2018.10.008.The following is the Supplementary material related to this article. \n\nMMC S1\n\n\n\n", "descript": "\n                  Motivated by possible future applications in low pressure drop reactors for hydrogen production by fixed bed chemical looping steam reforming (CLSR), novel high porosity fibrous mats of aluminosilicate fibres have been investigated as a substrate for Ni/Co oxygen carriers (OC\u2019s). When compared to granules of a conventional 18 wt% NiO steam reforming catalyst tested over seven redox cycles of CLSR, the fibrous OCs produced by homogeneous chemical precipitation routes increased the average methane conversion by up to 10% and hydrogen yield by up to 5%. All of the OC\u2019s could be reduced by a CH\n                        \n                           \n                           \n                              4\n                           \n                        \n                        \u2215\n                     H\n                        \n                           \n                           \n                              2\n                           \n                        \n                     O mixture and produced no solid carbon during reforming.\n               "}
{"full_text": "Anion exchange membrane fuel cells (AEMFCs) have been investigated as a low-cost fuel cell alternative to proton exchange membrane fuel cells (PEMFCs) due to the potential use of non-platinum group metal (PGM) catalysts and the enhanced oxygen reduction kinetics on non-PGM catalysts under alkaline conditions [1\u20139]. The technical challenge for AEMFCs is that there is still much room for improvement in both performance and durability compared to PEMFCs. To obtain high performance and durability for AEMFCs, the chemical/mechanical stability and anion conductivity of the anion exchange membrane (AEMs) continue to be improved [10\u201318]. In recent years, various strategies have been devised to overcome these problems, such as microphase separation, cross-linking, and organic-inorganic composites [3,18,19]. This means that AEMs can be manufactured with higher anion conductivity while maintaining the same ion exchange capacity (IEC) [13,20]. In addition, Mandal et al. reported a high performance AEMFC with an anionic conductivity of 212\u00a0mS\u00a0cm\u22121, cell performance of 3.5\u00a0W\u00a0cm\u22122, and cell durability of more than 545\u00a0h at 80\u00a0\u00b0C due to the introduction of cross-linking and long alkyl spacers [21].Other important factors are the development of effective non-PGM catalysts. For anodes, mainly Ni-based non-PGM catalysts have been reported [4,6,22]. For cathodes, mainly non-PGM catalysts based on Fe, Co and other transition metals have been reported [7,23\u201329]. Among these, Hossen et al. reported the remarkable result that an Fe\u2013N\u2013C catalyst had the same performance as that of platinum-supported carbon (Pt/C), by combining the N\u2013C materials used in the synthesis of catalysts and optimizing the ionomer content of the cathode catalyst layers (CLs) [26].In addition to the above components, water management at both the anode and cathode is also an important factor in AEMFCs. This is because water is produced at the anode by the hydrogen oxidation reaction (HOR) and consumed at the cathode by the oxygen reduction reaction (ORR) of AEMFCs. In other words, the AEMFC must provide sufficient water to maintain hydration of the AEMs and electrodes, without flooding the anode or drying the cathode. Also, water moves from the cathode to the anode, due to the electroosmotic drag associated with the movement of OH\u2212, and moves from the anode to the cathode due to back-diffusing water [30\u201343]. Continuing from our previous paper [44], the present paper focuses on effective water management in the AEMFC. In the previous research, we discovered the current density-voltage (I\u2013V) hysteresis phenomenon that accompanies an increase or decrease in the current density (CD) of a cell that uses a Fe\u2013N\u2013C catalyst as a cathode catalyst. This suggested that the hysteresis phenomenon resulted from the difference in the absorption capacity of liquid water in the cathode CL and affected the water supply at the reaction sites of the cathode. By more detailed Tafel slope component analysis, this I\u2013V behavior can be characterized as a direct transition from kinetic control to combined gas-ion-water transport control combination, with a unique 8\u00a0\u00d7\u00a0slope behavior, i.e., the intrinsic kinetic slope is multiplied by 8. These results also supported the importance of back-diffusing water.In the case of Fe\u2013N\u2013C being used in the cathode catalyst layer, one of the methods for improving water management performance, i.e., improving the flux of back-diffusing water utilized from the anode, is the use of a thin electrolyte membrane. It has been reported that thinning the membrane shortens the distance of anion conduction and increases the flux of back-diffusing water, improving AEMFC performance [2,3,5,9].In addition, Dekel et al. and Yassin et al. reported that by reducing the membrane thickness from 28\u00a0\u03bcm to 10\u00a0\u03bcm and increasing the water diffusion coefficient of the membrane, not only the power generation performance was improved, but also the cell life was extended, based on model calculations [45,46]. Also, Jiang et al. reported that the cell performance was improved by hydrophilizing the surface of the membrane via the creation of hydrophilic functional groups using a plasma [47].In addition, for using Fe\u2013N\u2013C catalysts, the active reaction sites of the cathode must have a balanced hydrophilicity/hydrophobicity in order to be accessed by both oxygen gas and water for the ORR. To achieve high ORR performance for Fe\u2013N\u2013C catalysts, it is important to have a hierarchical pore structure with macro/mesopores for reactant access to the Fe\u2013N\u2013C active sites present in the micropores [48,49]. We reported that Fe\u2013N\u2013C catalyst particles synthesized using carbon with a high specific surface area carbon, specifically, refluxing Black Pearls (BP-2000, Asian-Pacific Specialty Chemicals Kuala Lumpur) with nitric acid increased the number of mesopores, and show ORR activity equivalent to that of a Pt/C [28]. In addition to the catalyst layer structure, the interfacial interaction between the ionomer and the active sites of the catalyst is also an important factor. Using nanoparticle silver catalysts and quaternary ammonium functionalized triblock copolymer ionomers, Buggy et al. reported that ionomer-catalyst interactions may also have a significant effect on ionomer water uptake [50]. Santori et al. also reported, by means of Fe\u2013N\u2013C catalysts and RDE evaluations, that low ionomer gas permeability has a negative impact on catalyst bed performance because of the increased reactant consumption (O2) per active site for catalysts with low site densities [51].In this paper, we report two approaches for the improvement of the water management ability in order to suppress the I\u2013V hysteresis phenomenon [44]. To fully understand the effect of AEMs on suppressing this AEMFC performance hysteresis phenomenon, it is necessary to test AEMs with various structures such as hydrocarbon-based membranes, cross-linking, and microphase separation. In this study, an electrolyte membrane (quaternized poly(arylene perfluoroalkylene), QPAF-4), which was developed by the University of Yamanashi and Takahata Precision Co., Ltd. [13], was used as the electrolyte membrane and binder. The QPAF-4 AEM with the molecular structure shown in Fig. S1 is suitable for this study because of its excellent microphase molecular structure, high gas permeability, high alkaline stability and high membrane mechanical strength. QPAF-4 is also soluble in methanol, which is highly volatile and minimizes the effect of the catalyst layer preparation. The QPAF-4 membrane was also compared with a cell using an A201 membrane (Tokuyama Corp.), which is widely used in AEMFCs. First, we sought to increase the flux of back-diffusion water from the anode by both thinning and hydrophilizing a QPAF-4 AEM, and subjecting this membrane to hydrophilic treatment. This study led us to realize again the importance of water transport at the interface between the membrane and the catalyst layer. Second, to develop a high ORR activity catalyst layer with a macro/mesopores layer accessible to both oxygen gas and water, we sought to improve the water supply to the reaction active sites by suppressing the absorption of water at the cathode by using a specially developed Fe\u2013N\u2013C catalyst provided by the Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences [28]. This Fe\u2013N\u2013C is a catalyst with a high specific surface area of 1200\u20131500\u00a0m2\u00a0g\u22121 in Brunauer-Emmett-Teller (BET), and 30\u00a0nm Fe nanoparticles are uniformly present, based on transmission electron microscope (TEM) images. In addition, the I\u2013V results reported for high-power AEMFCs, as shown in many reports, have typically been obtained for high gas flow rates, e.g., 1\u00a0L\u00a0min\u22121. This is quite low in terms of hydrogen and oxygen flow rate utilization, 3% for hydrogen (stoichiometric ratio\u00a0=\u00a033.3) and 1.5% (stoichiometric ratio\u00a0=\u00a066.6) for oxygen at 1\u00a0A\u00a0cm\u22122 [21,37,38], which are difficult to be implemented in practical fuel cell systems. Our cells have been operated at the low flow rate of 0.1\u00a0L\u00a0min\u22121 (30% hydrogen utilization and 15% oxygen utilization), which is close to the actual operating conditions of an AEMFC. Chen et al. have proposed a new normalized efficiency metric of W cm\u22122 divided by the flow rate, W s cm\u22122 L\u22121 [52]. The cell using QPAF-4 in our previous paper achieved values of 120\u2013198\u00a0W\u00a0s\u00a0cm\u22122 L\u22121, which is competitive with the performance reported in many AEMFC papers [21,37,38]. On the other hand, the importance of water balance is not new, as it has been mentioned in many previous reports [30\u201343]. However, we demonstrate and propose that the water absorption of the catalyst and the impediments to water transport at the membrane catalyst layer interface make this important water management challenge more pronounced, and thus a more serious problem, at the lower flow rates and lower pressure drops that are easier to achieve in practical systems.QAPF-4, which was used as an electrolyte membrane and binder, was synthesized based on the synthesis procedure of Ono et al. [13]. The catalyst inks for the anodes were prepared with Pt catalyst supported on carbon black (Pt/CB: TEC10E50E, Tanaka Kikinzoku Kogyo, K. K.), methanol and pure water by stirring for 30\u00a0min and use of a planetary ball mill containing 20 zirconia beads with a diameter of 5\u00a0mm. Subsequently, 5\u00a0wt% QPAF-4-MeOH (IEC\u00a0=\u00a02.0 meq g\u22121) binder solution was added to the slurry, and the mixture was further stirred with a planetary ball mill for 30\u00a0min. The weight ratio of QPAF-4 binder to support carbon was 0.8. In the same way, the catalyst inks for the cathodes were prepared with the Fe\u2013N-Cc catalyst (synthesized and supplied by the CIAC from Black Pearls (BP-2000, Asian-Pacific Specialty Chemicals Kuala Lumpur)) and Fe\u2013N-Cp catalyst (PMF-011904, supplied by Pajarito Powder), 5\u00a0wt% QPAF-4-MeOH binder solution (IEC\u00a0=\u00a02.0 meq g\u22121), methanol and pure water by use of a planetary ball mill. The weight ratio of QPAF-4 binder to support catalyst was set to 0.43. These catalyst inks were directly sprayed onto the microporous layers (MPLs) of the gas diffusion layers (GDLs) as the anode (W1S1010, Cetech Co., Ltd.) and cathode (29BC, SGL Carbon Group Co., Ltd.) by the pulse-swirl-spray (PSS, Nordson Co., Ltd.) technique to prepare the gas diffusion electrodes (GDEs). The electrode areas were 4.41\u00a0cm2, the Pt loading of the CL was 0.20\u00a0\u00b1\u00a00.02 mgPt cm\u22122, and these Fe\u2013N\u2013C loading of CLs were 0.50\u00a0\u00b1\u00a00.05 mgcat. cm\u22122. The prepared GDEs were immersed in 1\u00a0M KOH 80\u00a0\u00b0C for 2 days before measurement to ion-exchange to the OH\u207b form. Similarly, the QPAF-4 electrolyte membranes (IEC\u00a0=\u00a02.0 meq g\u22121, ca. 10 and ca. 30\u00a0\u03bcm) were also immersed in 1\u00a0M KOH aqueous solution at 80\u00a0\u00b0C for 2 days before measurement. To remove excess KOH aqueous solution, the GDEs and the electrolyte membranes were sandwiched between Kim Towels (Nippon Paper Cresia Co., Ltd.). Next, the GDEs and the electrolyte membranes were immersed in ultrapure water for approximately on hour, being constrained so as not to float, and then sandwiched between Kim Towels again to remove the ultrapure water. After the KOH was thoroughly removed, each set of GDEs and QPAF-4 membrane was pressed together in-cell to form the membrane electrode assembly (MEA) without hot pressing. The MEAs were sandwiched between two single serpentine flow graphite plates and 200\u00a0\u03bcm silicone/poly(ethyl benzene-1, 4-dicarboxylat/silicone gaskets (SB50A1P, Maxell Kureha Co., Ltd.) and were fastened at 10 kgf cm\u22122 with four springs. For the reversible hydrogen electrode (RHE), a 5\u00a0mm diameter disk was cut from the Pt/CB 29BC GDE prepared above and applied to the membrane on the cathode side. The hydrogen source for the RHE was the anode outlet, supplied through a heated (90\u00a0\u00b0C) gas line. The CL surface of the RHE was in contact with the electrolyte membrane in the cell, and the GDL surface was in contact with gold wire, which was connected to the anode and cathode by terminals through a multi-input data logger (NR-500, KEYENCE Corp.) and a high voltage measurement unit (NR-HV04, KEYENCE Corp.), respectively, and the polarizations of the anode and cathode were measured. Fig. S2 shows an overview of the cell with reference electrode.The cell voltages (V) as a function of current density (I) were measured with hydrogen and oxygen at 60\u00a0\u00b0C at various pressures. Hydrogen and oxygen gases were supplied to the anode and the cathode at a flow rate of 100\u00a0mL\u00a0min\u22121. The \ufb02ow rates of all gases were controlled by mass \ufb02ow controllers. These gases were humidi\ufb01ed at 100% relative humidity (RH) by bubbling through a hot water reservoir. The I\u2013V curves were galvanostatically measured under steady-state operation by use of an electronic load (PLZ664WA and KFM2150, Kikusui Electronics Corp.) controlled by a measurement system (fuel cell characteristic evaluation device, Netsuden Kogyo Corp.). The measurement times in the direction of increasing current were 1\u00a0min up to 0.02\u00a0A\u00a0cm\u22122, 3\u00a0min up to 0.1\u00a0A\u00a0cm\u22122, 5\u00a0min up to 0.2\u00a0A\u00a0cm\u22122, 7\u00a0min up to 0.3\u00a0A\u00a0cm\u22122, and 10\u00a0min up to 1.0\u00a0A\u00a0cm\u22122. The measurement times in the direction of decreasing current were just half those used for increasing current. Also, since resistances are difficult to measure with alternating current (AC) impedance at current densities below 0.1\u00a0A\u00a0cm\u22122\u00a0(KFM2150, Kikusui Electronics Corp.), they were measured with a 1\u00a0kHz external resistance meter (MODEL 3566, Tsuruga Electric Corp.) For current densities of 0.1\u00a0A\u00a0cm\u22122 or more, the membrane resistance was measured by AC impedance.An ozone/UV surface treatment device (EKBIO-1100, EBARA JITSUGYO Co., Ltd.) was used to hydrophilize the QPAF-4 electrolyte membrane. Ozone was generated by UV lamps (wavelengths of 245\u00a0nm and 185\u00a0nm) installed at the top of the chamber. The chamber temperature and water temperature were set at 40\u00a0\u00b0C, and the QPAF-4 membrane was placed 90\u00a0mm below the chamber ceiling. In an air atmosphere for 10\u00a0min, the back and front sides were turned over and hydrophilized twice in total. The ozone concentration in the chamber became steady after 2\u00a0min from the starting hydrophilization and showed a level of about 50\u00a0ppm.For the surface-conduction analysis by current-sensing atomic force microscopy (CS-AFM) on the QPAF-4 membranes of ca. 10\u00a0\u03bcm thickness, the GDE was first prepared by spraying a catalyst ink containing the Pt/CB and QPAF-4 binders (IEC\u00a0=\u00a02.0 meq g\u22121) as a binder on the 29BC GDL using the PSS technique in the same manner as described above. The weight ratio of QPAF-4 binder to support catalyst was adjusted to 0.8. The Pt loading of the electrodes was 0.2\u00a0\u00b1\u00a00.02 mgPt cm\u22122. The GDE was subsequently immersed in 1\u00a0M KOH aqueous solution at 80\u00a0\u00b0C for 2 days and then immersed in a saturated aqueous solution of NaHCO3 aqueous solution at 40\u00a0\u00b0C for 2 days, and then dried to ion-exchange it to the HCO3\n\u2212 ion form. The membrane and the GDE in the HCO3\n\u2212 ion form were pressed at 10 kgf cm\u22122 and 140\u00a0\u00b0C for 3\u00a0min. The ozone/UV surface treatment was carried out on the QPAF-4 membrane surface attached to the GDE.The CS-AFM setup was prepared according to prior literature [53\u201359] making use of a commercial AFM system (SPM-5500, Agilent) equipped with a home-made environmental chamber under a purified (CO2-free) air atmosphere at 40\u00a0\u00b0C and 70% RH. A Pt/Ir-coated silicon tip (NanoWorld) was used for the CS-AFM measurements. The morphological and current images were obtained in the contact mode, with a contact force of 5\u00a0nN on the membrane surfaces and a tip bias voltage of \u22122.0\u00a0V [59]. Before measurements, humidified air was supplied to the environmental chamber (dead volume\u00a0=\u00a0500\u00a0mL) at 100\u00a0mL\u00a0min\u22121 for 2\u00a0h. During the AFM measurements, the flow rate was reduced to 10\u00a0mL\u00a0min\u22121.To ensure that there was no tip damage of the surface and that it was free of airborne impurities such as dust, we obtained the first and second scanned images at the same position for each CS-AFM measurement. The tip bias voltage was kept at \u22122.0\u00a0V during the image acquisition.The electrochemical measurements were carried out with an Automatic Polarization System (HZ-5000, Hokuto Denko Co.) using the rotating disk electrode (RDE) method. A Pt mesh was used as the counter electrode and the RHE was used as the reference electrode. The working electrode was prepared as follows: the catalyst ink was prepared by mixing 1\u00a0mg of each catalyst with 0.025\u00a0mL of ultrapure water and 4.975\u00a0mL of ethanol via ultrasonication. A 1\u00a0\u03bcL droplet of this ink was deposited onto a glassy carbon electrode (GC area\u00a0=\u00a00.283 and 0.196\u00a0cm2, Naito Rika Co. Ltd.) using a 1-\u03bcL syringe. The amounts of the Pt/CB ((4 \u03bcgPt cm\u22122, TEC10E50E), Fe\u2013N-Cp (11\u00a0\u03bcg\u00a0cm\u22122) and Fe\u2013N-Cc (11\u00a0\u03bcg\u00a0cm\u22122) catalysts was controlled by the number of drops. Subsequently, Nafion diluent obtained by diluting a 5\u00a0wt% Nafion solution with 75\u00a0vol% of ethanol was pipetted on the electrode surface, yielding an average thickness of 0.03\u00a0\u03bcm. The electrolyte solution, 0.1\u00a0M KOH, was prepared from reagent grade chemicals and ultrapure water. Polarization curves for the ORR were recorded in O2-saturated electrolyte at 5\u00a0mV\u00a0s\u22121 (positive potential scan) and several rotation rates from 1000 to 2500\u00a0rpm.The wettability of QPAF-4 membranes with/without hydrophilization and CL surfaces were investigated by contact angle measurement (DM-501, Kyowa Interface Science Co., Ltd.). Reagents (wetting tension test mixture, Kanto Chemical Co., Inc.) having different surface tensions of 30, 40, 50, and 73\u00a0mN\u00a0m\u22121 were pipetted on membranes and CL surfaces, and the contact angles were measured. The contact angle was evaluated by analysis software (FAMAS, Kyowa Interface Science Co., Ltd.) that can be used for the sessile drop method and a half-angle method. To reduce the measurement error, the measured value was calculated by averaging the values of 6 times measured the front and the back sides.We applied N2 adsorption to investigate the pore structures of the CLs. The N2 physisorption experiments were measured at 77\u00a0K by use of an automated gas sorption analyzer (Autosorb iQ, Anton-Paar GmbH). All the samples (0.1\u00a0g or more) were degassed at 60\u00a0\u00b0C for 24\u00a0h in an onboard degassing port, prior to the adsorption experiments. The N2 adsorption measurements were conducted in the P/P0 range 0.025\u20130.997, where P represents the gas pressure and P0 the saturation pressure. The specific surface areas and pore volume distributions were calculated by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. To obtain precise measurements of the values of the CLs and avoid the influence of the values of GDLs, catalyst-coated membranes (CCMs) were prepared by coating the catalysts on the QPAF-4 electrolyte membrane by the PSS method. The CCMs (6\u00a0cm\u00a0\u00d7\u00a06\u00a0cm) were divided into three parts and placed in the measurement cell. The specific surface area and pore size distribution were calculated from the obtained adsorption isotherm curves.We also applied water vapor adsorption to investigate the pore structures of the CLs. The experiments of water vapor physisorption were measured at 60\u00a0\u00b0C with water vapor sorption analyzers (Vstar, Anton-Paar GmbH). All the samples (0.1\u00a0g or more) were degassed at 60\u00a0\u00b0C for 24\u00a0h in an onboard degassing port prior to the adsorption experiments. The values of water vapor adsorption were measured in the P/P0 range 0.05\u20130.95. In the case of the CLs, these were formed on a PP film by the PSS method and were removed and filled into the cell.In our previous studies [44], it has been clarified that the supply of water to the catalytic active sites has a great influence on the hysteresis of the power generation performance of the cathode in the AEMFC. Therefore, in order to increase the supply of generated water to the reaction active site of the cathode, we sought to reduce the membrane thickness and increase the flux of back-diffusing of water.In Fig. 1\n(a\u2013e), the changes in the cell polarization curve, ohmic resistance, and anode cathode polarization curve using Pt/CB CL as the cathode CL are shown. As the thickness of the electrolyte membrane decreased from 33\u00a0\u03bcm to 11\u00a0\u03bcm by comparisons between circles and triangles in Fig. 1(a) and (b), contrary to our expectations, there was a large difference in the potential between increasing and decreasing current, which was the phenomenon we have termed I\u2013V hysteresis. These I\u2013V hysteresis tended to be larger for thinner membranes, as shown in Fig. S3. In addition, this hysteresis was not a tendency peculiar to the QPAF-4 membrane and was also observed in cells using the A201 membrane (Tokuyama Corporation) (Fig. S4). Based on the comparison in Fig. 1(c), the ohmic resistance decreased from 0.088\u00a0\u03a9\u00a0cm2 to 0.066\u00a0\u03a9\u00a0cm2 at 1.0\u00a0A\u00a0cm\u22122 as the membrane became thinner.We hypothesized that one of the causes of I\u2013V hysteresis was water transport at the interface between the membrane and the cathode, and therefore, to improve the transport, we hydrophilized the surface of the 11\u00a0\u03bcm membrane. In Fig. 1(a) and (b), the hysteresis observed for the hydrophilized 11\u00a0\u03bcm membrane decreased, and the ohmic resistance of the hydrophilized membrane in Fig. 1(c) was 0.080\u00a0\u03a9\u00a0cm2 at 1.0\u00a0A\u00a0cm\u22122, which was higher than that for the unmodified 11\u00a0\u03bcm membrane and less than that for the 33\u00a0\u03bcm membrane.In order to investigate the cause of the I\u2013V hysteresis in detail, the polarization contributions of the anode and cathode were measured independently. As shown in Fig. 1(d and e), this I\u2013V hysteresis occurred predominantly on the anode side and disappeared after hydrophilizing the membrane. The increase of I\u2013V hysteresis for the 11\u00a0\u03bcm membrane with increasing CD is considered to be due to flooding by generated water in the anode.\nFig. 1(f\u2013l) show the results of investigating whether the effect of the hydrophilization is effective even in the case of cells using the Fe\u2013N-Cp cathode catalyst. The cell using the Fe\u2013N-Cp catalyst and the 11\u00a0\u03bcm QPAF-4 membrane showed hysteresis at both anode and cathode. However, as seen in Fig. 1(f) and (g), the I\u2013V hysteresis decreased even in the cell using the Fe\u2013N\u2013C CL when the hydrophilized 11\u00a0\u03bcm QAPF-4 membrane was used. Regarding the ohmic resistance in Fig. 1(h), the resistance of the hydrophilized 11\u00a0\u03bcm membrane was 10% (increasing CD) and 20% (decreasing CD) smaller than that of the 33\u00a0\u03bcm membrane at 0.5\u00a0A\u00a0cm\u22122 and increased significantly with increasing CD, in contrast with the case of the Pt/CB cathode in Fig. 1(c). These phenomena can be explained by the fact that, in the case of the Fe\u2013N-Cp cathode, water absorption was larger than that for Pt/CB and this led to the increasing ohmic resistance. In Fig. 1(i), I-V hysteresis did not occur in the anodic polarization, and, in Fig. 1(j), another instance of I\u2013V hysteresis, in this case, for the Fe\u2013N-Cp cathode polarization with the 33\u00a0\u03bcm membrane, was suppressed by use of the hydrophilized 11\u00a0\u03bcm membrane. Therefore, it was found that the improvement of the water transport at the interface between the membrane and the cathode by use of the hydrophilization treatment on a thin electrolyte membrane is effective against the hysteresis phenomenon of both the anode and the cathode.In Fig. 2\n and Fig. S5, the change in contact angle for various reagents with different surface tensions used for each MeOSO3\n- form membrane showed that the hydrophilicity of the membrane surface depended on the thickness, and those for the hydrophilized 11\u00a0\u03bcm QPAF-4 membrane were an average of 21\u00b0 lower than those for the non-hydrophilized QPAF4 membrane. As for the reason for the change in contact angle depending on the thickness of the AEM, when the membranes were dried in the same casting method on a substrate, the hydrophobic substructure would be more stable and thus more prevalent at the membrane/vapor interface, and the top surface were more hydrophobic. In a review of proton-based PFSA membranes, Kusoglu and Weber reported that at low hydration levels, the membrane/vapor interface layer was highly hydrophobic and resistive, and the fluorocarbon chains were oriented parallel to the membrane/vapor interface, which in some cases limits the mass transport of water and ions [63]. When imagined with a minimal molecular structure, i.e., two molecular chains, the ionic groups of the QPAF-4 AEM, which are hydrophilic, face inward, and the main chain structure, which is hydrophobic, faces outward. We estimate that the smaller the thickness of the bulk membrane, the more the hydrophobization would be enhanced. This result is also in agreement with the results of the OH\u207b-form membrane, which is the same counter ion as that for the fuel cell evaluation. As shown in Fig. S5, the contact angle also increased with decreasing membrane thickness, but the contact angle decreased for all membrane thicknesses after the membranes were subjected to hydrophilization. Hydrophilization of the membrane surface reduces impediments to water movement at the membrane-cathode interface. These results support the result of promoting the movement of water by the hydrophilization of the membrane surface and suppressing the hysteresis of the I\u2013V performance.\nFig. 3\n shows the results of CS-AFM analysis to investigate the effect of the hydrophilization on the 10\u00a0\u03bcm OPAF-4 membrane on the current distribution at the interface between the membrane and the cathode. There were no clear differences of more or less than 10\u00a0nm in the topography shown at the upper part of Fig. 3(a), but in the current image in the lower part, it was found that the hydrophilization treatment clearly improved the uniformity of the current distribution. As shown in Fig. S6, similar microscopic results were confirmed in other parts of the membrane surface. These results can also be clearly confirmed in the difference in the current profile on the lines shown in Fig. 3(b). The results of the effect of the hydrophilization of the membrane surface on the osmotic pressure in liquid water showed similar changes in Fig. S7. Therefore, from these analysis results, we determined that hydrophilization does not improve the diffusivity of water inside the membrane but contributes to the improvement of the surface anion conduction and water pathways. The results of the membrane improvement approach show that hydrophilization of the QPAF-4 membrane surface was more effective than thinning in increasing the amount of back-diffusing water produced at the anode. We can conclude that a membrane with a hydrophilic surface can also function with the effect of reducing the membrane thickness. This effect was confirmed in the power generation performance results shown in Fig. 1, even for the Fe\u2013N-Cp catalyst.As a second approach to suppress the I\u2013V hysteresis, we investigated the use of a new non-PGM catalyst that would have improved performance in comparison with that of the Fe\u2013N-Cp material, used in the previous research [44], by using the Fe\u2013N-Cc catalyst developed at the CIAC Laboratory.\nFig. 4\n shows the pore size distributions calculated by the BJH method and N2 adsorption isotherms of these Fe\u2013N\u2013C catalysts and Pt/CB catalysts, respectively. In Fig. 4, in the Fe\u2013N-Cc CL, the hysteresis of the nitrogen adsorption/desorption isotherm is smaller than that for the Fe\u2013N-Cp CL, suggesting that the pore volumes of the ink bottle structures [60,61] were also lower. In addition, the isotherm on the low-pressure side near P\u00a0=\u00a00 had a larger slope than those for the other CLs, and the cumulative pore distribution and the log differential pore volume distributions calculated from the adsorption/desorption isotherms indicated many pores of \u223c5 and 40\u00a0nm. The Fe\u2013N-Cc is synthesized from the high specific surface area carbon Black Pearls [28], and it is clear that the characteristics of the carbon material also affect the gas adsorption of the catalyst layer.\nFig. 5\n shows water vapor adsorption isotherms and contact angle changes of these Fe\u2013N\u2013C catalysts and Pt/CB catalysts, respectively. Comparing the water vapor adsorption isotherms of all of the CLs in Fig. 5(a), on the low-pressure side near P\u00a0=\u00a00, where the first layer of water molecules was adsorbed, the amount of water vapor adsorbed by the Fe\u2013N-Cc CL was as low as Pt/CB CL. On the high-pressure side near P\u00a0=\u00a01, the Fe\u2013N-Cc CL had a similarly large adsorption amount as the Fe\u2013N-Cp CL, and the water adsorbed value increased with increasing pressure applied. These results indicate that water was not able to enter into the pores of the Fe\u2013N-Cc CL despite the larger pore volume than that of the Pajarito CL at low pressure, close to a practical fuel cell condition. Fig. 5(b) shows that the Fe\u2013N-Cc CL was more hydrophobic than the Fe\u2013N-Cp CL from the contact angle measurements of the CLs and the hydrophobicity was comparable to that of the Pt/CB CL. The high hydrophobicity of the Fe\u2013N-Cc material is thought to be attributed to that of Black Pearls, a furnace black carbon similar to Kejten Black, the carbon support material for Pt/CB.Polarization performances for the Fe\u2013N-Cc (11\u00a0\u03bcg\u00a0cm\u22122), Fe\u2013N-Cp (11\u00a0\u03bcg\u00a0cm\u22122) and Pt/CB ((4 \u03bcgPt cm\u22122, TEC10E50E) catalysts using the RDE technique are shown in Fig. 6\n and S8. The amounts of catalysts for the RDE were prepared at the same ratio used for the cathodes of the MEAs. The plateau currents for the Fe\u2013N\u2013C catalysts did not reach the limiting current density for the 4-electron ORR, because the amounts of the catalysts were very small, and thus there was a limitation due to mass transport between catalyst agglomerates on the electrode surface [62]. Nevertheless, the ORR activity can be compared in terms of the half-wave potentials: the Fe\u2013N-Cc catalyst exhibited 0.81\u00a0V at 0.05\u00a0mA\u00a0cm\u22122, which was 0.18\u00a0V higher than that of the Pajarito catalyst but was 0.084\u00a0V lower than that of Pt/CB with the same catalyst ratio. Fig. 7\n shows the cell performance using each cathode CL with a 30\u00a0\u03bcm QPAF-4 membrane. Table 1\n shows the voltages at three different current densities, 0.2, 0.5 and 0.8\u00a0A\u00a0cm\u22122. The voltage differences of the I\u2013V hysteresis at 0.2\u00a0A\u00a0cm\u22122 and 0.5\u00a0A\u00a0cm\u22122 for the cell using the Fe\u2013N-Cc CL were 0.05\u00a0V and 0.03\u00a0V, respectively, which were smaller than the corresponding values, 0.20\u00a0V and 0.15\u00a0V for the cell using the Fe\u2013N-Cp CL, and the performance at current densities below 0.5\u00a0A\u00a0cm\u22122 was comparable to that for the Pt/CB CL. In addition, the ohmic resistance of the Fe\u2013N-Cc cell was 0.090\u00a0\u03a9\u00a0cm2 at 0.5\u00a0A\u00a0cm\u22122, which was similar to that of the Pt/CB CL.The reason why I\u2013V hysteresis was suppressed in the Fe\u2013N-Cc CL can be rationalized as being due to its high hydrophobicity (explained in Section 3.2.1), so that the volume of water absorbed by the catalyst is reduced, and the water required for the reaction is secured. In addition, the reason why the ohmic resistance of the Fe\u2013N-Cc cell was low is considered to be that there was little water absorbed by the catalyst, and the water content of the membrane increased. However, in the higher current density region, for example, 0.8\u00a0A\u00a0cm\u22122, the voltages of cell using the Fe\u2013N-Cc CL decreased to 0.32\u00a0V compared with the case of 0.43\u00a0V using the Pt/CB CL and were nearly the same as those using the Pajarito CL (during decreasing current density). The reason for the performance decay at high current density is considered to be due to the fact that the catalyst layer was 3 times thicker than that of the Pt/CB, which was nearly the same as that of the Pajarito catalyst (Fig. S9), and thus the diffusion overvoltages for oxygen and water required for the cathode reaction increased. On the other hand, with a thinner CL, there are fewer Fe active sites, and more oxygen is needed to penetrate the catalyst layer through the ionomer [51]. This may affect the performance and the use of ionomers with high gas permeability should be considered.The insights gained from the two parts of the present study are summarized as schematic images of water management to suppress I\u2013V hysteresis by use of the surface-hydrophilized electrolyte membrane in Fig. 8\n(a) (Section 3.1) and by the use of the Fe\u2013N\u2013C catalyst with low water absorption in Fig. 8(b) (Section 3.2). Fig. 8(a) shows that we hydrophilized the surface of the QPAF-4 membrane, and this approach led to an improvement in performance due to the increased amount of water available for the ORR. By making the electrolyte membrane surface hydrophilic to improve the diffusivity of water at the surface rather than in the interior, it was possible to suppress the I\u2013V hysteresis, even for the cell using the Fe\u2013N-Cp catalyst, which has high water absorption [44]. Therefore, increasing the utilization of back-diffusing water from the anode is essential in the supply of water to reaction sites of the cathode [32,38]. These results indicate that the water diffusivity, not only in the interior of both the membrane and CL, but also at the interface between membrane and cathode, is important.\nFig. 8(b) shows diagrammatically the effect of changing the cathode catalyst from the Fe\u2013N-Cp to the CIAC-developed counterpart. The Fe\u2013N-Cc material exhibited lower water absorbability than the Pajarito material. The use of this material led to decreased absorption of the back-diffusing water into the interior of the carbon in the CL and an increased volume of water supplied to reaction sites on the surface of the carbon, which are those with the shortest oxygen diffusion distance. These improvements demonstrate that water transport is the main limitation responsible for the previously reported I\u2013V hysteresis [44] and provide strategies to achieve higher performance AEMFCs through proper water management and formation of water transport pathways.In previous studies, we clarified that the supply of water to the catalytic active sites was responsible for the severe voltage drop observed as the current density was increased and thus had a great influence on the hysteresis of the I\u2013V performance of the cathode in AEMFCs. In the present work, we examined two approaches for the improvement of the water management ability, with the aim of suppressing the I\u2013V hysteresis phenomenon.First, to increase the supply of generated water to the reaction active sites of the cathode, we decreased the membrane thickness and hydrophilized the membrane surface in order to increase the flux of back-diffusing water. These improvements of the water transport at the interface between the membrane and the cathode by use of the hydrophilization on a thin electrolyte membrane were effective in eliminating the I\u2013V hysteresis phenomenon at both the anode and the cathode. Based on various types of membrane characterization, including CS-AFM, contact angle and osmotic pressure, we also clarified that hydrophilization does not improve the diffusivity of water in the interior but contributes to the improvement of surface anion conduction and water transport pathways. This effect was confirmed in the power generation performance measurements, even with the Fe\u2013N\u2013C catalyst with high water uptake that was associated with the initial observation of the I\u2013V hysteresis.Second, from the viewpoint of the catalyst layer, to suppress the I\u2013V hysteresis, we sought to improve the performance in comparison with that obtained for the Fe\u2013N-Cp catalyst used in the previous research by using the recently developed Fe\u2013N-Cc catalyst. The nitrogen adsorption results indicated that water was not able to enter into the pores of the Fe\u2013N-Cc CL despite the larger pore volume than that of the Fe\u2013N-Cp CL at low pressures, close to the practical fuel cell condition. From water vapor adsorption and contact angle evaluation, the high hydrophobicity of the Fe\u2013N-Cc catalyst was assigned to the hydrophobicity of Black Pearls, the furnace black carbon, which is similar in its characteristics to Kejten Black, which is the carbon support material of Pt/CB. From results, the reason for the I\u2013V hysteresis suppression by the Fe\u2013N-Cc CL was able to be assigned to its high hydrophobicity, so that the volume of water absorbed by the catalyst was decreased, and the water required for the reaction was secured. In addition, the reason for the low ohmic resistance of the Fe\u2013N-Cc-based cell was considered to be that the volume of water absorbed by the catalyst was suppressed, and thus the water content of the membrane increased.These improvements have demonstrated that water transport is the main limitation responsible for the voltage drop observed as the current density is increased, with the resulting I\u2013V hysteresis, and thus have provided strategies for achieving higher performance AEMFCs through proper water management and formation of water transport pathways.\nKanji Otsuji: Conceptualization, Formal analysis, Investigation, Writing \u2013 original draft. Yuto Shirase: Investigation. Takayuki Asakawa: Investigation. Naoki Yokota: Resources. Katsuya Nagase: Resources. Weilin Xu: Resources. Ping Song: Resources. Shuanjin Wang: Resources. Donald A. Tryk: Writing \u2013 review & editing. Katsuyoshi Kakinuma: Conceptualization, Validation. Junji Inukai: Conceptualization, Validation. Kenji Miyatake: Conceptualization, Validation, Resources, Funding acquisition. Makoto Uchida: Conceptualization, Methodology, Validation, 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 was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) Japan through funds for the \u201cAdvanced Research Program for Energy and Environmental Technologies,\u201d by the Japan Society for the Promotion of Science (JSPS) and the Swiss National Science Foundation (SNSF) under the Joint Research Projects (JRPs) program, and by the Japan Science and Technology (JST) through Strategic International Collaborative Research Program (SICORP).We are deeply grateful to Pajarito Powder supplying the Fe\u2013N-Cp catalyst.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.jpowsour.2022.230997.", "descript": "\n                  Anion exchange membrane fuel cells (AEMFCs) are vulnerable to water management problems, since water is produced at the anode and consumed at the cathode. Previously we found severe voltage losses when increasing the current density in an AEMFC with a commercial Fe\u2013N\u2013C cathode catalyst. In the present work, we have clearly identified the problem as being related to water management and developed two approaches to alleviating the problem: by use of a thin hydrophilized membrane, the diffusivity of water at the surface was improved, and the severe I\u2013V hysteresis was suppressed, despite the cell using an Fe\u2013N\u2013C cathode catalyst with a high water absorption rate. The voltage loss was also alleviated by the use of a recently developed Fe\u2013N\u2013C catalyst with higher hydrophobicity, which decreased the absorption of back-diffusing water into the catalyst layer and increased the amount of water supplied to the reaction sites These improvements have demonstrated that water transport is the main limitation for the previously reported hysteresis and provide strategies to achieve higher performance AEMFCs through proper water management and formation of water transport pathways.\n               "}
{"full_text": "Volatile organic compounds (VOCs) are defined by the WHO as a group of organic compounds whose melting point is lower than room temperature and boiling point is between 50 and 260\u202f\u00b0C, including alkanes, olefins, aromatics, and so on. It has been proved that VOCs are primary pollutants of PM2.5 and photochemical smog [1], and most of VOCs are harmful to human health, such as benzene, toluene, so more and more research on VOCs removal are been carried out. At present, the main methods of VOCs removal include adsorption, catalytic combustion, thermal combustion, biodegradation, and so on [2]. The catalytic combustion is proved the most efficient technology for the removal of VOCs because of their lower-temperature operation, high degradation activity and less secondary pollution [3], and the core of catalytic combustion is to design and prepare stable and high efficient catalysts [4]. Many scholars pay attention on research of perovskite with ABO3 when the B site was occupied by Mn element because it could apply abundant lattice defects and higher oxygen mobility [5\u20137] compared with other transition metal elements [8,9]. When the B element is fixed, changing the A site element of the perovskite can increase the surface oxygen species concentration, specific surface area and low temperature reduction performance of the catalyst, thereby changing the catalytic activity of the catalyst [10\u201312]. Using of three-dimensionally ordered macroporous (3DOM) catalyst in the catalytic combustion of VOCs not only helps to increase the effective active specific surface area of the catalyst, but also facilitates the diffusion of reactants and products in the catalyst pores and enriches the macroporous structure [13,14]. Designing perovskite into 3DOM structure has very important basic research significance and practical significance for catalytic combustion of VOCs [15\u201318]. Three-dimensional (3D) ordered mesoporous manganese dioxide with high specific surface area and cubic symmetry prepared using the hard template method can completely oxidize formaldehyde at 130\u202f\u00b0C [19]. Jiang et al. [20] supported Mn3O4-Au nanoparticles on 3DOM La0.6Sr0.4CoO3, which made the catalyst possess higher specific surface area, adsorbed oxygen concentration and good low temperature reducibility.The work was focused on discussing the effect of A-site element on catalytic performance of manganese-based perovskite catalyst. All the prepared samples were characterized by the Brunauer\u2013Emmett\u2013Teller method (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS) and the activity of the materials for eliminating toluene was investigated by catalytic combustion technology to obtain (3DOM AMnO3 catalysts with the best performance.All chemicals used were A.R. grade without further purification. Methyl methacrylate (MMA) was obtained from Macklin. Cerium (III) nitrate hexahydrate, Nickel Nitrate, Lanthanum nitrate, Manganous nitrate, Polyethylene glycol-400, methanol, Potassium persulfate, P-Hydroxybenzoic acid, were purchased from Sinopharm Chemical Reagent Co.,Ltd.The polymethyl methacrylate (PMMA) colloidal crystal microspheres were synthesized using emulsifier-free emulsion polymerization approach [17,21,22]. A three-necked round-bottomed alaskite reactor (1000\u202fmL) filled with 650\u202fmL of deionized water equipped with a magnetic stirrer was heated by a hot water bath. In order to remove the air in the reactor, a pipet for pure N2 introduction was also connected to the vessel. Under constant stirring (350\u202frpm) and with N2 bubbling, the water was kept at 70\u202f\u00b0C for 30\u202fmin and then 57\u202fmL of methyl methacrylate monomer inhibited with ca. 0.03% p-hydroxyl benzoic acid was poured into the reactor through the third opening which was otherwise closed with a stopper. After further stirring and N2 bubbling at 70\u202f\u00b0C for 15\u202fmin, a solution of potassium persulfate initiator (0.20 g dissolved in 20\u202fmL of deionized water) preheated to 70\u202f\u00b0C was added. With N2 bubbling and stirring, the reaction was allowed to run at 70\u202f\u00b0C for 45\u202fmin, and then the emulsion was cooled to room temperature and mixed with 1300\u202fmL deionized water. The PMMA colloidal crystal microspheres were left suspended in the liquid medium.PMMA- template was prepared by constant temperature suspension film forming method [23]. The emulsion was centrifuged at 7000\u202frpm for 40\u202fmin. After mixing the solid layer and deionized water into a homogeneous emulsion, it was dried at 80\u202f\u00b0C by a hot water bath and the PMMA hard-templating with surface gloss and orderly accumulation was obtained.The 3DOM AMnO3 support was prepared using the PMMA-templating strategy [24\u201326]. In a typical method. Nitrate salts Ce(NO3)3 6H2O, La(NO3)3 6H2O, Ni(NO3)2: Mn(NO3)2 according to the molar ratio of 1:1, were dissolved in 20.9\u202fmL of methanol (MeOH) and 3.0\u202fmL of polyethylene glycol-400 adjust the total metal concentration of the precursor solution to 2\u202fmol/L. At room temperature (RT) under stirring for 2\u202fh to obtain a transparent solution. 2.0\u202fg of the PMMA template was soaked in the above pre-cursor solution for 4\u202fh. After the mixture was filtered, the obtained wet PMMA template was dried in air at RT for 48\u202fh, The thermal treatment process was divided into two steps: (i) the dried PMMA was first claimed in a RT flow at a ramp of 1\u202f\u00b0C/min from RT to 300\u202f\u00b0C and kept at this temperature for 4\u202fh, and (ii) the sample obtained after step (i) was claimed in an air flow at a ramp of 1\u202f\u00b0C/min from RT to 600\u202f\u00b0C and maintained this temperature for 5\u202fh, thus obtaining the 3DOM AMnO3 support.The morphology and size of samples were observed using S-4800 field emission scanning electron microscope (SEM, Hitachi Co; Japan) and JEM-2100UHR transmission electron microscope (TEM, JEOL; Japan). The Brunauer-Emmett-Teller (BET) surface area, pore volumes and pore diameters were performed with N2 adsorption/desorption isotherms on Micromeritics ASAP2020M analyzer. The crystal structure of samples was determined using X\u2019Pert PRO MPD using Cu K\u03b1 radiation (\u03bb\u202f=\u202f0.1541\u202fnm) from 10\u00b0 to 80\u00b0. X-ray photoelectron spectroscopy (XPS) was measured on a Thermo ESCALAB 250Xi XPS system from a monochromatic aluminum anode X-ray source with K\u03b1 radiation (1486.6\u202feV), and the spectra were calibrated with the C1s peak at 284.6\u202feV as an internal standard. Hydrogen temperature-programmed reduction (H2-TPR) experiments were carried out on a chemical adsorption analyzer (PAC-1200).The catalytic activities of the samples were evaluated in a continuous fixed bed reactor with a diameter of 28\u202fmm and a length of 500\u202fmm. To minimize the effect of hot spots, 5\u202fmL of quartz sands (40\u201360 mesh) was used to dilute 1\u202fmL of the sample (40\u201360 mesh). The reactant was com-posed of 300\u202f\u00b1\u202f50\u202fmg/m3 toluene. Reactants and products were analyzed online by a gas chromatograph (Agilent 7890B) equipped with a flameionization detector (FID) and a TCD. When the reaction temperature was below 350\u202f\u00b0C, toluene showed less signs of conversion, indicating that the self-decomposition of toluene at high temperature could be excluded in this experiment.The XRD results of the catalysts were shown in Fig. 1\n, It was found that all three catalysts (3DOM LaMnO3, 3DOM CeMnO3, 3DOM NiMnO3) reveal the peak value at 24.8\u00b0, 33.1\u00b0, 36.7\u00b0, 47.6\u00b0, 50.8\u00b0, 55.2\u00b0, 59.2\u00b0 and 69.5\u00b0. Previous studies have shown that the peaks correspond to the characteristic peaks of the perovskite crystal phase, which indicates that the study has successfully synthesized a catalyst with a perovskite crystal phase. Small differences exist in these diffraction patterns, especially for the sample of 3DOM LaMnO3 with unclear peak of perovskite, meanwhile, the diffraction peaks of the 3DOM CeMnO3 are sharp and symmetrical with increasing half width, and moving to the higher angle. In addition to the characteristic diffraction peaks of perovskite. The sample of 3DOM CeMnO3 with additional peaks of the CeO2 (2\u03b8\u202f=\u202f28.6\u00b0, 56.2\u00b0) and MnO2 (2\u03b8\u202f=\u202f65.8\u00b0). Phases indicates that formation of CeMnO3 may lead to small amount of residual CeO2 and MnO2. Because Ce4+ is the constant valence state of Ce and higher than the conventional perovskite A-site element [27], the perovskite structure of the CeMnO3 catalyst is unstable. Some Ce and Mn elements are retained, resulting in the formation of CeO2 and MnOx. The presence of oxides makes the catalyst have more lattice oxygen and surface oxygen vacancies [28], which is conducive to the catalytic combustion of toluene.The presence of macropores in 3DOM AMnO3 catalysts is further assessed by SEM in Fig. 2\n. 3DOM CeMnO3 shows high quality three-dimensional ordered macropore (Fig. 2a, b). The samples exhibit the macroporous materials contain a skeleton surrounding uniform periodic arrange voidswith an average diameter of 200\u202f\u00b1\u202f50\u202fnm and a wall thickness of 16\u201318\u202fnm [17]. However, the pore structure of 3DOM LaMnO3 (Fig. 2c, d) has low porosity, poor permeability and irregular macroporous structure. 3DOM architecture agrees well with TEM analysis (Fig. 3\n). It can be seen that the macroporous structure formed by 3DOM CeMnO3 was the most complete and the SEM of NiMnO3 showed that the catalyst didn\u2019t form macroporous structure. The result also agree well with BET. In Fig. 3, (a) and (c) represent 3DOM CeMnO3, and (b) and (d) represent 3DOM LaMnO3. From the HRTEM characterization diagram of 3DOM CeMnO3, the (2\u202f0\u202f0) and (1\u202f1\u202f2) crystal planes of orthorhombic perovskite and (1\u202f1\u202f1), (2\u202f0\u202f0) crystal plane and (2\u202f1\u202f1) crystal plane of Mn3O4. Moreover, the arrangement of each crystal plane is intricate and shows a small amount of lattice distortion. It can be inferred that the 3DOM CeMnO3 catalyst has more surface oxygen defects, which is consistent with the results of XRD and XPS characterization.\nFigs. 4 and 5\n\n shows the N2 adsorption-desorption isotherms and pore size distribution of catalysts prepared. Except 3DOM NiMnO3 catalyst, all adsorption-desorption isotherms display a type II isotherm with type H3 hysteresis loop (P/P0\u202f=\u202f0.7\u20131.00), indicating the existences of mesopores or macropores. In the catalysts [29], which is consistent with the SEM images in Fig. 2. At the same time, it can be seen that the adsorption amount of the 3DOM CeMnO3 catalyst was faster than the other sample (Fig. 1A), which corresponds to the specific surface area shown in Table 1\n (3DOM CeMnO3 is 48.8\u202fm2/g, 3DOM LaMnO3 is 35.6\u202fm2/g and 3DOM NiMnO3 is 11.05\u202fm2/g.).When the specific surface area is much larger, the adsorption and transfer rate of toluene on the surface of the 3DOM catalyst is greatly increased and more active sites are exposed to increase the catalytic reaction rate. Fig. 5 shows the pore size distribution for all catalysts where clearly defined peaks around 0\u20135\u202fnm are observed and confirms the existence of mesoporous as is mentioned above. It is worthy to mention [30] the size of the specific surface area is related to the integrity of the pore structure. The results show that the A-site element of perovskite has a great influence on the apparent structure.The oxidation state of manganese and the properties of oxygen species in catalysts can be analyzed by XPS. The composition of surface elements of catalysts can be discussed. The corresponding results were shown in Fig. 6\n and Table 2\n. Among all the catalysts investigated, the Mn4+/Mn3+, and Oads/Olatt ratios over 3DOM CeMnO3 were the highest showing a strong mutual effect among manganese, oxygen, and Cerium. As shown in Fig. 6 (a), the Mn 2p3/2 signal peaks of the catalysts can be decomposed into two components: binding energies at 641\u202feV and 642.7\u202feV can be respectively classified as surface Mn3+ species and satellite peaks of surface Mn4+ species, indicating that Mn3+ and Mn4+ coexist in all samples. The O1s signal peaks of the catalysts can be decomposed into two components. The binding energies of the peaks are 529.2\u202feV and 531.4\u202feV, respectively. They belong to the species of surface lattice oxygen (Olatt) and the species of surface adsorbed oxygen Oads (O, O2\u2013 or O2\n2\u2013) [31]. NiMnO3 catalyst has a peak of binding energy at 532.8 ev, which belongs to the carbonate species [32\u201334]. This peak indicates that PMMA template is prone to carbonization in the process of calcination of the catalyst, so it is not conducive to the formation of macroporous structure, which is also consistent with the porous structure shown in the SEM diagram of 3DOM NiMnO3 catalyst (Table 3\n).The oxygen species can be strongly adsorbed in oxygen defects on surfaces of perovskite catalysts [34] and high concentrations of adsorbed oxygen might promoted the catalytic activity [21]. Because of the stronger Ce\u2013O and Mn\u2013O interactions, the 3DOM CeMnO3 catalyst exhibited the highest Oads/Olatt ratio. The large amount of active absorbed oxygen improved the catalytic performance for the oxidation reactions. Interestingly, the peak of lattice oxygen in the 3DOM CeMnO3 catalyst shifted to a higher BE by 0.5\u202feV. This denoted the enhancement in the O2\u2013 mobility of the catalyst, which facilitated the combustion of toluene.During catalytic combustion reaction, the oxygen adsorbed on the catalysts will be converted to lattice oxygen, while the surface Mn4+ will be reduced to Mn3+. At the same time, some lattice oxygen on the surface will be consumed. Therefore, the catalyst has high concentration of Mn4+ and high surface adsorbed oxygen can improve the performance of catalytic oxidation of VOCs. The formation of perovskite by cerium-manganese catalyst is unstable, which results in more non-stoichiometric perovskite structure in the catalyst. This will make perovskite crystal have more oxygen defects [35], which is more conducive to the adsorption of oxygen species on the surface, and can greatly promote the catalytic combustion of toluene. As shown in Table 2, the adsorption oxygen and Mn4+ concentration of 3DOM CeMnO3 catalyst are the highest and the catalytic performance of this catalyst is the best, which is consistent with the results of H2-TPR.H2-TPR was used to characterize the redox ability of the catalyst, the characterization results were shown in Fig. 7\n. The peaks can be attributed to the reduction of Mn species in B-site of perovskite-type catalysts [25]. The low-temperature reduction peaks of 3DOM CeMnO3, 3DOM 3DOM NiMnO3 and 3DOM LaMnO3 catalysts occur at 425\u202f\u00b0C, 440\u202f\u00b0C and 475\u202f\u00b0C respectively, which are mainly attributed to the reduction of Mn4+ species (Mn4+\u202f\u2192\u202fMn3+) transformation in B-site of perovskite-type catalysts. The high-temperature reduction peaks appeared at 500\u202f\u00b0C, 530\u202f\u00b0C and 570\u202f\u00b0C respectively, which can be attributed to the single electron reduction of Mn3+ which is in coordination unsaturation Translate into Mn2+. For manganese-based catalysts, Mn4+ is the main substance to promote the catalytic reaction [36,37]. After changing the A-site element to Cerium, the reduction peak becomes wider and the reduction temperature moves to a lower temperature. Obviously, the intensity of reduction peak of Mn3+ in 3DOM CeMnO3 catalyst is higher than that of the other two catalysts, and there is almost no peak [27] in 3DOM NiMnO3. This means that 3DOM CeMnO3 has better redox performance at low temperature and is more conducive to the removal of toluene [38].\nFig. 8\n showed the effect of the A site element on the catalytic combustion of toluene. Although the removal rate of toluene hardly changed at lower temperatures, as the reaction temperature increased, obvious differences gradually appeared. When the conversion of toluene is 50%, the order of removal efficiency of toluene is 3DOM CeMnO3\u202f>\u202f3DOM LaMnO3\u202f>\u202f3DOM NiMnO3. When the conversion of toluene is 90%, the activity of catalyst is 3DOM CeMnO3\u202f>\u202f3DOM NiMnO3\u202f>\u202f3DOM LaMnO3. The results show that although all the samples have highly catalytic activity, the catalytic activity of different elements still varies greatly. According to Table 1, Positive correlation between catalytic activity and specific surface area of the sample. The 3DOM CeMnO3 exhibited the highest catalytic activity, the ignition temperature (T50%) and complete conversion temperature (T90%) were 100\u202f\u00b0C and 172\u202f\u00b0C, respectively. For the 3DOM NiMnO3 catalyst, T50% is lower than the 3DOM LaMnO3 catalyst. In contrast, T90% is higher than the 3DOM LaMnO3 catalyst. This indicates that 3DOM NiMnO3 has better catalytic activity at high temperature, which can be consistent with the results of H2-TPR and XPS.Studies have shown that the catalytic combustion process of toluene belongs to the first order kinetics. In the presence of excess oxygen, there is a relationship between the concentration of VOCs (c) and other parameters, as shown in formula (1-1), where the parameters r, k, A, and Ea represent the reaction rate (mol\u00b7s\u22121), Reaction rate constant (s\u22121), antecedent factor (s\u22121) and apparent activation energy (kJ\u00b7mol\u22121).\n\n(1-1)\n\n\nr\n\n=\n\n-\n\nk\nc\n\n=\n\n\n\n\n\n-\n\nA\n\ne\nx\np\n\n\n\n\n-\nE\na\n/\nR\nT\n\n\n\n\n\n\n\n\nc\n\n\n\n\n\nFig. 9\n show Arrhenius curves of samples at different firing temperatures (obtained before the toluene conversion is below about 20%). As can be seen from the figure, there is a good linear relationship between Lnk and 1 / T. R2 is 0.9693 (3DOM CeMnO3), 0.936 (3DOM LaMnO3), and 0.891 (3DOM NiMnO3). The 3DOM CeMnO3 catalyst with the best pore structure and the largest specific surface area (38.8\u202fm2\u00b7g\u22121) also showed the lowest apparent activation energy (34.51\u202fkJ\u00b7mol\u22121, SV\u202f=\u202f15,000\u202fh\u22121).Three kinds of Mn-based perovskite structure catalysts, 3DOM CeMnO3, 3DOM LaMnO3 and 3DOM NiMnO3, prepared by the PMMA Hard-Templating- Excessive impregnation method were employed as the catalyst for simultaneous toluene removal from air. The research results demonstrated that 3DOM CeMnO3 has the most complete macroporous structure, regular channels, strong permeability and high porosity. Its specific surface area reaches 48.8\u202fm2/g\u22121, which is much larger than that of 3DOM NiMnO3 (11.1\u202fm2/g\u22121), thus providing more adsorption sites for toluene molecule. 3DOM CeMnO3 had the best toluene removal performance among the catalysts. The superior catalytic performance of 3DOM CeMnO3 resulted mainly from the abundance of oxygen vacancies and the strong interaction between CeO2 and MnOx formed during calcination. The 3DOM CeMnO3 sample showed lower apparent activation energy (34.51\u202fkJ\u00b7mol\u22121, SV\u202f=\u202f15,000\u202fh\u22121) and the best catalytic activity for toluene combustion, with the reaction temperatures (T50%, and T90%) required for achieving toluene conversions of 50%, and 90% being 100\u202f\u00b0C, 172\u202f\u00b0C. Toluene conversion of 3DOM CeMnO3 could be at least 90% in the air atmosphere at 160-180\u202f\u00b0C, which was the optimal temperature range for the toluene removal, exhibiting prominent low-temperature activity. Therefore, 3DOM CeMnO3 perovskite oxide has broad prospects and can be used as an active catalyst in a natural carrier such as cordierite to form a monolith catalyst for industrial production.The author is grateful for the financial support of the Natural Science Foundation of Shandong Province [ZR2019MEE112].", "descript": "\n                  Three-dimensionally ordered macroporous manganese-based perovskite catalyst (3DOM AMnO3, A\u202f=\u202fCe, La, Ni) were synthesized by PMMA hard-templating and impregnation method. Physicochemical properties of the samples were characterized by means of various techniques including XRD, BET, SEM, TEM, XPS and H2-TPR, and their catalytic activities were evaluated by toluene combustion. It was found that the 3DOM AMnO3 in each of the samples was perovskite in crystal structure, and only the samples possessed a good quality 3DOM architecture with a surface area of 48.8\u202fm2/g. Due to the highest adsorbed oxygen species concentration (Oads/Olatt\u202f=\u202f2.330), the best low-temperature reducibility (The low-temperature reduction peaks of 3DOM CeMnO3 catalysts occur at 425\u202f\u00b0C) and the strong interaction between CeO2 and MnOx formed during calcination. The 3DOM CeMnO3 sample showed lower apparent activation energy (34.51\u202fkJ\u00b7mol\u22121, SV\u202f=\u202f15,000\u202fh\u22121) and the best catalytic activity for toluene combustion, with the reaction temperatures (T50%, and T90%) required for achieving toluene conversions of 50%, and 90% being 100\u202f\u00b0C, 172\u202f\u00b0C at SV\u202f=\u202f15,000\u202fh\u22121, respectively.\n               "}
{"full_text": "In catalysis science field, transition-metal heterogeneous catalysts can be considered one of the most important and far-reaching scientific developments up to now, since they promote the development of energy storage & conversion, chemicals manufacture, as well as prevention & treatment of environment pollution. The development of chemical industry field requires identification of improved transition metal catalysts with improved efficiency, selectivity and durability for each reaction processes, and made from earth-abundant elements. However, the trial-and-error approaches are still a common practice to search for new catalysts, largely due to the lack of deep insight into the fundamental relationship between structure feature of reactions site and its catalytic performance [1,2]. In heterogeneous catalysis, the atomic arrangement around the active sites is of significant influence on the processes of reaction cycle. Therefore, an important challenge in achieving a rational design of optimal transition metal heterogeneous catalysts, is to unveil the structural stability of the active centers and to explore how their configuration transform upon exposure to the realistic environments, attracting increasing research effort [3].Since two decades ago, first-principles-based or density functional theory (DFT) calculation tools have been used to tackle this challenge and take advantage of a predictive and analytical ability without the help of experiment [4]. They base on quantum chemical theory to reach a quantitative description of the catalytic process in present of ultra-high vacuum and 0\u00a0K. However, aiming to address issues of catalysis, studies with consideration of the impact of reactant partial pressures and temperature, cannot be solely finished by first-principles calculations. In order to involve the impact of realistic environments, DFT calculations are accompanied with concepts of chemical potential, which is an important concept in thermodynamics, since all of the thermodynamic properties of a material at a given temperature and pressure can be obtained from its chemical potential. Chemical potential-based model can construct a frame to assess intrinsic thermodynamic tendency and criteria for certain chemical process and map out the influence of external environment. In the meantime, the energetics for specific system or object in the thermodynamic model would be calculated precisely and conveniently by DFT calculations. Notably, first-principles calculation tools and thermodynamic theory have been coupling, and have been firmly adopted in the conceptual toolbox of theoretical surface catalysis [5]. The first-principles-aided thermodynamic models provide access to determine the stability of catalyst and the surface structure reconstruction under given thermodynamic condition, which is regarded as a prominent hallmark.Here, we briefly review recent theoretical works on transition metal heterogeneous catalysts by means of first-principles computation-aided thermodynamic models. According to the common concerns on transition metal heterogeneous catalysts, first, we will outline the whole thermodynamic framework to obtain the chemical potential of specific catalysts and analyze their thermodynamic stability. Then, we will demonstrate representative examples showing how to identify structure evolution of catalyst under operando reaction conditions.To be more feasible to interact with reactants, the size of transition metal catalysts usually disperse in the form of micrometer, nanometers or even atomic level scale. In the solid\u2013gas thermocatalysis, the metal species tend to disintegration and/or sinter due to high temperature and adsorbed reactant, either through migration and agglomerate of metal atoms and nanoparticles, or through Ostwald ripening [6\u20138]. In the end, the lack of active surface area leads to the decrease and deactivation of overall activity of the metal species. As for solid\u2013liquid electrocatalysis, especially in acid media, the catalytic activity fading of metal nanoparticles is caused by various processes, such as dissolution, agglomeration, and detachment [9,10]. These factors are tangled and often happen simultaneously. For example, the dissolved metal species from small particles may be redeposited on large nanoparticles. These mechanisms result in the loss of electrochemical surface and decayed performance. A thermodynamic understanding or evaluation of the structural destabilization tendency at atomic level would be helpful to propose an efficient method for suppressing deactivation processes and increasing the durability of transition metal catalysts. Extensive studies have employed first-principles computation-aided thermodynamic models, and provided a thermodynamically quantitative description of structural stability of transition metal with consideration of the effect of temperatures, pressures and the presence of reactants.An insight into Ostwald ripening and disintegration of metal nanoparticles under reaction conditions is regarded as the central issue on the thermal stability of metal nanoparticles heterogeneous catalysts for durable practical implementation. To reach a precise analysis on thermal stability of metal nanoparticles, Li et\u00a0al. proposed a first-principles computation-aided thermodynamic model to quantitatively describe Ostwald ripening and disintegration processes with consideration of the reactant, particle size and morphology [11,12]. This theory model contains chemical potential of supported metal nanoparticles and formation energy of monomers on supports, as well as corresponding sintering thermodynamic tendency, in the presence of given temperature and reactant partial pressure. As display in Fig.\u00a01\na, a supported nanoparticle is modeled by a spherical segment, the chemical potential of which as a function of curvature radius R is derived by Gibbs\u2013Thomson (G\u2013T) formula [13].\n\n(1)\n\n\n\n\u03bc\n\nN\nP\n\n\n=\n\n\n2\n\u03a9\n\n\u03b3\n\nN\nP\n\n\n\n/\nR\n\n\n\n\nwhere \u03a9 is the molar volume of bulk atom and \u03b3\n\nNP\n is the weighted surface energy of the nanoparticles.Taking into account that nanoparticle expose various facets i with area ratio \n\n\nf\ni\n\n\n and corresponding surface energy \n\n\n\u03b3\ni\n\n\n, the weighted surface energy of nanoparticles \n\n\n\u03b3\n\nN\nP\n\n\n\n could be, \n\n(2)\n\n\n\n\u03b3\n\nN\nP\n\n\n=\n\n\u2211\ni\n\n\nf\ni\n\n\n\u03b3\ni\n\n\n\n\n\nOstwald ripening of nanoparticles proceed through the decomposition of small particles to form monomers (mono-atom or multi-atom complexes), as well as the diffusion and the attachment to larger particles of monomers on the support. Therefore, as shown in Fig.\u00a01b, in addition to determining the thermodynamic priority of detachment/attachment of the monomers from/toward the nanoparticles, the formation energy of the metal monomers (n atoms, \u0394Ef\nMO) has an impact on the concentration of monomers, which is expressed by the relative chemical potential of monomers to nanoparticle.\n\n(3)\n\n\n\u0394\n\nE\n\nM\nO\n\nf\n\n\n(\nR\n)\n\n\n=\n\n\n\u03bc\n\nM\nO\n\n\n\u2212\n\n\u03bc\n\nN\nP\n\n\n\n\n\n\n\n\n(4)\n\n\n\n\u03bc\n\nM\nO\n\n\n=\n\n\n\nE\n\nM\nO\n@\ns\nu\np\np\no\nr\nt\n\n\n\u2212\n\nE\n\ns\nu\np\np\no\nr\nt\n\n\n\u2212\nn\n\nE\n\nb\nu\nl\nk\n\n\n\nn\n\n\n\n\nwhere \u03bc\n\nMO\n is the chemical potential of the metal monomers with relative to bulk, EMO@support\n is the total energy of supported monomers, Esupport\n is the energy of the substrate and Ebulk\n is the energy per bulk atom.The concentration of the metal monomers with respect to nanoparticles of curvature radius R is defined by, \n\n(5)\n\n\n\nc\n\nM\nO\n\n\n\n(\nR\n)\n\n=\n\n\nexp\n\n(\n\n\u2212\n\n\n\u0394\n\nE\n\nM\nO\n\nf\n\n\n(\nR\n)\n\n\n\n\nk\nB\n\nT\n\n\n\n)\n\n\n\n\n\n\na\n0\n\n\n2\n\n\n\n=\n\nc\n\nM\nO\n\n\ne\nq\n\n\n\nexp\n\n(\n\n\u2212\n\n\n\n\u03bc\n\nN\nP\n\n\n\n\n\nk\nB\n\nT\n\n\n\n)\n\n\n\n\nwhere a\n0 is the lateral lattice constant of support, and \n\n\nc\n\nM\nO\n\n\ne\nq\n\n\n=\n\n\nexp\n\n(\n\n\u2212\n\n\n\n\u03bc\n\nM\nO\n\n\n\n(\nR\n)\n\n\n\n\nk\nB\n\nT\n\n\n\n)\n\n\n\n\n\n\na\n0\n\n\n2\n\n\n\n\n is the concentration of the monomers in equilibrium relative to the bulk. Hensen et\u00a0al. carried out first-principles-aided thermodynamic models to study the dependence of ripening mechanism of CeO2(111)-supported Pd nanoparticles on size [16]. Particle coalescence is possible only for nanoparticles with less than 5\u00a0Pd atoms, while Ostwald ripening is the dominant sintering mechanism for larger cluster. Since metal-support interaction plays an important role in stabilizing catalysts, first-principles-aided thermodynamic modeling has been applied to reveal influence of facets and crystal phases of substrate on structural stability of supported metal nanoparticles. For example, Li et\u00a0al. studied thermodynamic tendency of Pt nanoparticle ripening on various pristine TiO2 surfaces of both anatase and rutile phases [17].Under reaction conditions, reactants adsorbing on supported nanoparticles would decrease the surface tension and stabilize the nanoparticles, which affects subsequent Ostwald ripening and disintegration (Fig.\u00a01b). The correct of surface tension on the facet i at given environment temperature and reactant partial pressure is expressed by,\n\n(6)\n\n\n\u0394\n\n\u03b3\ni\n\n\n(\n\nT\n,\nP\n\n)\n\n=\n\n\u03b8\ni\n\n\n\n\n[\n\n\nE\n\nr\ne\na\nc\nt\na\nn\nt\n\n\na\nd\n\n\n\n(\n\n\n\u03b8\ni\n\n\n)\n\n\u2212\n\n\u03bc\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n(\n\nT\n,\nP\n\n)\n\n\n]\n\n\n/\n\n\nA\ni\n\n\n\n\n\n\nwhere \u03b8 is the coverage of adsorbate, Ai\n is unit area of the surface i, and \n\n\n\u03bc\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n(\nT\n,\nP\n)\n\n\n is chemical potential of free reactant molecules. The coverage dependence \n\n\nE\n\nr\ne\na\nc\nt\na\nn\nt\n\n\na\nd\n\n\n\n(\n\n\n\u03b8\ni\n\n\n)\n\n\n (average adsorption energy of reactant) can be evaluated by first-principles theory calculation and fitted with polynomial functions. The coverage \u03b8i at specific T and P would be derived from the differential adsorption energy of reactants [15] (Fig.\u00a01c\u2013d).\n\n(7)\n\n\n\nE\n\nr\ne\na\nc\nt\na\nn\nt\n\n\nd\ni\nf\n\n\n\n(\n\n\n\u03b8\ni\n\n\n)\n\n=\n\n\nd\n\n[\n\n\n\u03b8\ni\n\n\u00d7\n\nE\n\nr\ne\na\nc\nt\na\nn\nt\n\n\na\nd\n\n\n\n(\n\n\n\u03b8\ni\n\n\n)\n\n\n]\n\n\n\nd\n\n\u03b8\ni\n\n\n\n=\n\n\u03bc\n\nC\nO\n\n\n\n(\n\nT\n,\nP\n\n)\n\n\n\n\n\nBy substituting the revised surface energy of supported nanoparticles with adsorbates, chemical potential of nanoparticles turn to be a function of T and P, which has impacts on the Ostwald ripening and disintegration processes.\n\n(8)\n\n\n\n\u03bc\n\nN\nP\n\n(\nr\ne\na\nc\nt\na\nn\nt\n)\n\n\n\n=\n\n\n2\n\u03a9\n\n\u03b3\n\nN\nP\n\n(\nr\ne\na\nc\nt\na\nn\nt\n)\n\n\n\n\nR\n\n=\n\n\n2\n\u03a9\n\n\u2211\ni\n\n\nf\ni\n\n\n[\n\n\n\u03b3\ni\n\n+\n\u0394\n\n\u03b3\ni\n\n\n(\nT\n,\nP\n)\n\n\n]\n\n\nR\n\n\n\n\n\n\nFig.\u00a01e\u2013f show contour plot of \u03bcNP(CO) correlating with curvature radius, temperature, and pressure [14]. In general, the smaller radius, higher temperature, and lower pressure will bring about a higher \u03bcNP(CO), while the thermal stability of Au nanoparticles is only sensitive to particle size when it grows larger.Apart from reducing the surface energy and stabilize the nanoparticles, reactants would stabilize metal monomers detached from nanoparticles by forming metal-reactant complexes presented in Fig.\u00a01b, which assist Ostwald ripening behavior. Likewise, first-principles-aided thermodynamic model can be applied to put insight into the effect of the adsorption of reactants on Ostwald ripening. The Gibbs free energy of adsorption \n\n\u0394\n\nG\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n would be corrected.\n\n(9)\n\n\n\u0394\n\nG\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n=\n\nE\n\na\nd\ns\n\n\n\n(\n\nr\ne\na\nc\nt\na\nn\nt\n\n)\n\n\u2212\n\n\n\u03bc\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n(\n\nT\n,\nP\n\n)\n\n\n\n\n\nTo satisfy exothermic adsorption of reactants on metal monomers (\n\n\u0394\n\nG\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n< 0), \n\n\n\u03bc\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n(\n\nT\n,\nP\n\n)\n\n\n is required to meet the following criteria.\n\n(10)\n\n\n\n\u03bc\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n(\n\nT\n,\nP\n\n)\n\n>\n\nE\n\na\nd\ns\n\n\n\n(\n\nr\ne\na\nc\nt\na\nn\nt\n\n)\n\n\n\n\n\nThe chemical potential of monomers with reactant adsorbed, \u03bcMO\n(reactant), is expressed by, \n\n(11)\n\n\n\n\u03bc\n\nM\nO\n\n(\n\nr\ne\na\nc\nt\na\nn\nt\n\n)\n\n\n\n=\n\n\u03bc\n\nM\nO\n\n\n+\n\u0394\n\nG\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n\n\nwhere Eads\n (reactant) is reactant adsorption energy on metal monomers, \u03bc\n\nreactant\n (T, P) is the chemical potential of free reactant molecule as a function of pressure and temperature. The chemical potential of monomers upon reactant adsorption is lowered by \n\n\u0394\n\nG\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n. The concentration of monomers upon reactant adsorption is, \n\n(12)\n\n\n\nc\n\nM\nO\n\n(\n\nr\ne\na\nc\nt\na\nn\nt\n\n)\n\n\n\n\n(\nR\n)\n\n=\n\nc\n\nM\nO\n\n\ne\nq\n\n\n\n\nexp\n\n(\n\n\u2212\n\n\n\n\u03bc\n\nN\nP\n(\nr\ne\na\nc\nt\na\nn\nt\n)\n\n\n+\n\u0394\n\nG\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n\n\nk\nB\n\nT\n\n\n\n)\n\n\n\n\n\n\na\n0\n\n\n2\n\n\n\n\n\n\n\nApart from accelerating Ostwald ripening, reactants could decompose supported nanoparticles into the metal-reactant complexes distributing on substrate. Reactant-assisted disintegration of metal particles is usually used to regenerate the sintered catalysts. Corresponding first principle-involved thermodynamic study has also been developed. In order to assess whether reactant-induced decomposition of nanoparticle into the metal-reactant complexes is a thermodynamically spontaneous process, the thermodynamic tendency is defined by, \n\n(13)\n\n\n\u0394\n\nG\n\nN\nP\n\n\nd\ni\ns\n\n\n\n(\n\nR\n,\nT\n,\nP\n\n)\n\n=\n\n\nE\n\nc\no\nm\n\nf\n\n\u2212\n\n\u03bc\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n(\n\nT\n,\nP\n\n)\n\n\u2212\n\n\u03bc\n\nN\nP\n\n(\n\nr\ne\na\nc\nt\na\nn\nt\n\n)\n\n\n\n\u2212\nT\nS\n\n\n\nwhere S is the configurational entropy of metal-reactant complexes, and \n\n\nE\n\nc\no\nm\n\nf\n\n=\n\n\u03bc\n\nM\nO\n\n\n+\n\n\nE\n\na\nd\ns\n\n\n\n(\n\nr\ne\na\nc\nt\na\nn\nt\n\n)\n\n\n is the formation energy of the metal-reactant monomers on substrate relative to the bulk and reactants in gas phase. Since the value of \n\n\u0394\n\nG\n\nN\nP\n\n\nd\ni\ns\n\n\n\n is controlled by the formation energy of the complexes, the chemical potential of nanoparticle and free reactant molecule, and the configuration entropy due to the decomposition, these factors have the following influence and implication. The composition of reactant gas should be varied according to diverse supported nanoparticle systems, which guarantee reactant can interacts strongly with metal monomers for desired disintegration, since \n\n\nE\n\nc\no\nm\n\nf\n\n\n is dominated by the interaction among reactant, metal and support. As for specific catalysts and supports, the calcination in corresponding oxidizing conditions had been adopted to improve the dispersal of metal catalysts in experiment. Besides, \n\n\u0394\n\nG\n\nN\nP\n\n\nd\ni\ns\n\n\n\n is determined by the reaction environment and the radius of nanoparticles. Therefore, chemical potential of reactants should be reached for thermodynamically spontaneous disintegration (\n\n\u0394\n\nG\n\nN\nP\n\n\nd\ni\ns\n\n\n\n) is, \n\n(14)\n\n\n\n\u03bc\n\nr\ne\na\nc\nt\na\nn\nt\n\n\nd\ni\ns\n\n\n\u2265\n\n\nE\n\nc\no\nm\n\nf\n\n\u2212\n\n\u03bc\n\nN\nP\n\n(\n\nr\ne\na\nc\nt\na\nn\nt\n\n)\n\n\n\n\u2212\nT\nS\n\n\n\n\nAs for a specific T and P, the nanoparticles of the radius less than R(dis) will be disintegrated.\n\n(15)\n\n\nR\n\n(\n\nd\ni\ns\n\n)\n\n\u2264\n\n\n\n2\n\u03a9\n\n\u03b3\n\nN\nP\n\n(\nr\ne\na\nc\nt\na\nn\nt\n)\n\n\n\n\n\n\nE\n\nc\no\nm\n\nf\n\n\u2212\n\n\u03bc\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n(\n\nT\n,\nP\n\n)\n\n\u2212\nT\nS\n\n\n\n\n\n\nThe DFT-based thermodynamic theory developed above was employed to study CO surrounded TiO2(110) supported Rh nanoparticles, of which sintering and disintegration behavior coincide with observation in experiments, and demonstrates how the metal-carbonyl monomers affect Ostwald ripening and disintegration of supported nanoparticles [11]. The dependence of \n\n\u0394\n\nG\n\nN\nP\n\n\nd\ni\ns\n\n\n\n for both Rh(CO) and Rh(CO)2 as metal-reactant monomers on T at experimental P\u00a0=\u00a010\u22121\u00a0mbar are shown in Fig.\u00a02\na. Rh(CO) and Rh(CO)2 will become the main complexes during 750\u2013770 and 370\u2013750\u00a0K, respectively. When T\u00a0\u2264\u00a0370\u00a0K, the \n\n\u0394\n\nG\n\nN\nP\n\n\nd\ni\ns\n\n\n\n become negative, Rh nanoparticles disintegrated to individual Rh(CO)2 become thermodynamically spontaneous process. The impact of pressure on agglomeration and disintegration at T\u00a0=\u00a0300\u00a0K is plotted in Fig.\u00a02b. Rh(CO) becomes the dominant monomers at most experimental conditions [10\u221225, 10\u221224] mbar, where Rh(CO)2 becomes thermodynamically favorable monomers in the range of [10\u221224, 10\u22124] mbar. When P\u00a0>\u00a010\u22124\u00a0mbar, Rh nanoparticles of d\u00a0=\u00a020\u00a0\u00c5 tend to decompose into Rh(CO)2 fragments in thermodynamics. At 300\u00a0K and 10\u22121\u00a0mbar, the corresponding \n\n\u0394\n\nG\n\nN\nP\n\n\nd\ni\ns\n\n\n\n of nanoparticles into Rh(CO)2 monomers versus diameter is shown in Fig.\u00a02c, which indicates that Rh nanoparticles decompose into Rh(CO)2 spontaneously when the diameter is smaller than 60\u00a0\u00c5. Similar theoretical works investigated the detachment of Cu monomers from CeO2(111)-supported Cu nanoparticles onto the substrate with and without CO adsorption, which show that the adsorption of CO reduces detachment energy and promote the formation of metal monomers species on ceria [18]. First-principles-aided thermodynamic models have also been employed to understand the stability of TiO2-supported Rh, Pd, and Pt nanoparticles in NO or CO atmosphere [19], by studying the thermodynamic tendency for disintegration of nanoparticles into metal-reactant complexes. NO is found to be a more efficient reactant for nanoparticles disintegration and redispersion than CO. And Rh nanoparticles are found to be most sensitive to either NO- or CO-induced disintegration. The study of reactant-involved disintegration and redispersion process of FeO/Pt (111) supported Au particles in CO atmosphere is another example [20]. It is found that CO stabilizes the ripening Au monomer by forming Au carbonyls according to phase diagram at a wide range of temperatures and CO pressures. CO decrease the onset temperature of ripening by a few hundred kelvins.As a frontier in heterogeneous catalyst field, the single-atom catalysts (SACs) has brought about much interest, due to the efficient utilization of expensive metals, as well as excellent selectivity and high activity for specific reactions compared to supported nanoparticles [21]. The precondition of advantages of SACs is the thermal stability of isolated metal atom against aggregation under catalytic reaction conditions [22]. Therefore, the binding strength between isolated metal atom and substrate is significant for sinter-resistant SACs. In thermodynamic aspect, the support-induced lower chemical potential of single atoms compared to nanoparticles lead to the thermal stability of SAC against sintering, since single atoms can't disperse and form nanoparticles spontaneously. Similar to the thermodynamic framework of Ostwald ripening of nanoparticle, a quantitative description on thermal stability of SACs has been achieved by combining first-principle calculation and chemical potential-based thermodynamic model, considering the environment condition, substrate, and metal-reactant interaction [14]. According to first-principle-aided thermodynamic model, the chemical potential of supported isolated atom is approximately expressed by the difference between formation energy of supported single atoms and the bulk (\u03bcbulk\u00a0=\u00a00):\n\n(16)\n\n\u03bcSA\n\u00a0=\u00a0(ESA@support\n\u00a0\u2212\u00a0Esupoort\n\u00a0\u2212\u00a0Efree atom\n)\u00a0\u2013\u00a0(Ebulk\n\u00a0\u2212\u00a0Efree atom\n)\u00a0=\u00a0ESA@support\n\u00a0\u2212\u00a0Esupoort\n\u00a0\u2212\u00a0Ebulk\n\n\n\nwhere ESA@support\n is the total energy of supported single atoms, Esupport\n is the energy of the support, Efree atom\n is the energy of free-standing atom and Ebulk\n is the energy per bulk atom. The energy change for accreting a metal single atoms into metal nanoparticles is determined by the chemical potentials of nanoparticles relative to single atoms:\n\n(17)\n\n\u0394Ef\nSA\n\u00a0=\u00a0\u03bcNP\n\u00a0\u2212\u00a0\u03bcSA\n\n\n\n\nHere, \u0394Ef\nSA\n(R) reflects the thermal stability of supported metal single atoms against aggregation. A larger value (especially positive value) generally implies that metal single atoms thermodynamically tend to resist aggregation. The concentration of metal single atoms with respect to nanoparticle of radius R is defined as, \n\n(18)\n\n\n\nc\n\nS\nA\n\n\n=\n\n\nexp\n\n(\n\n\u2212\n\n\n\u0394\n\nE\n\nS\nA\n\nf\n\n\n\n\nk\nB\n\nT\n\n\n\n)\n\n\n\n\n\n\na\n0\n\n\n2\n\n\n\n\n\n\nwhere a\n0 is the lateral lattice constant of support. Based on the expressions above, the thermal stability of metal single atoms is associated with temperature, nanoparticle size, and embedding location of single atoms.Li et\u00a0al. applied the first principle-aided thermodynamic model to design thermally stable Au SACs on series of oxide support under CO oxidation reaction [14]. It is shown in Fig.\u00a03\na, that the thermal stability of Au SACs can be adjusted by diverse factors including temperature, pressure, nanoparticle size, and the reducibility of the substrate, according to the chemical potentials of Au single atoms with respect to Au nanoparticles. Gao et\u00a0al. reported the effect of introducing nitrogen atoms into carbon-based support on the thermodynamic stabilities of single-atom iron catalysts by first principle calculations. The combination of chemical potential calculation and electronic structure analysis indicates that the doped N promotes charge transfer and, accordingly, improves thermodynamic stabilities [23]. Likewise, Li et\u00a0al. found that a defect site of carbon support makes the chemical potential of Au single atoms much lowered, which unveils the great impact of defects on binding and stabilizing the Au single atoms [24]. Senftle and his group recently revealed that the degree of metal single atoms anchored to supports is governed by the chemical potential of binding process [25]. Metal atoms with strong exothermic adsorption on a support possess higher thermal stability and are hindered to diffuse or agglomerate. Based on first principle-aided thermodynamic model, Wei et\u00a0al. proposed that the atomization and agglomeration of metal species depend on binding strength between metal-support (chemical potential of single atoms) and metal\u2013metal (chemical potential of nanoparticles) [26]. So they explored the origin of thermally stable single atoms supported on N-doped carbon derived from noble metals nanoparticles.Similar to nanoparticles, the chemical potential of single atoms is influenced by reaction conditions. The chemical potential of single atoms with reactant adsorption, \u03bcSA (reactant), is expressed by, \n\n(19)\n\n\n\n\u03bc\n\nS\nA\n\n(\n\nr\ne\na\nc\nt\na\nn\nt\n\n)\n\n\n\n=\n\n\u03bc\n\nS\nA\n\n\n+\n\nE\n\na\nd\ns\n\n\n\n(\n\nr\ne\na\nc\nt\na\nn\nt\n\n)\n\n\u2212\n\n\u03bc\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\n(\n\nT\n,\nP\n\n)\n\n\n\n\nwhere Eads\n (reactant) is reactant adsorption energy on single atoms; \u03bcreactant\n (T, P) is the chemical potential of free reactant molecule as a function of pressure and temperature. The reaction Gibbs free energy of metal single atoms aggregation upon reactant adsorption, \u0394Gagg\n, can be estimated by, \n\n(20)\n\n\n\u0394\n\nG\n\na\ng\ng\n\n\n=\n\n\u03bc\n\nN\nP\n\n(\n\nr\ne\na\nc\nt\na\nn\nt\n\n)\n\n\n\n\u2212\n\n\u03bc\n\nS\nA\n\n(\n\nr\ne\na\nc\nt\na\nn\nt\n\n)\n\n\n\n\n\n\n\n\u0394Gagg is also an index that represents the thermal stability of supported metal single atoms against aggregation. A larger value (especially positive value) suggests that single atom is a more thermodynamic preferable form for metal species than nanoparticle upon reactant adsorption. Fig.\u00a03b shows the tendency of \u03bcSA(CO) and \u03bcNP(CO) (R) (R\u00a0=\u00a020\u00a0\u00c5) with varying temperature at given CO partial pressure, for Au single atoms supported on various oxide [14]. It is shown that \u03bcSA(CO) on MgO(100) is always 1\u00a0eV higher than \u03bcNP(CO) (R) at all studied temperature range, which suggests that the Au single atoms highly tend to cluster. The weak CO binding to Au atom at the ceria oxygen vacancy resulted in the similar between \u03bcSA(CO) and \u03bcSA. On the contrary, with the presence of CO adsorption, Au single atoms on CeO2(111) and ceria steps becomes more stable and resist sintering below 200\u2013300\u00a0K, due to a positive \u0394Gagg. As shown in Fig.\u00a04\n, Hensen et\u00a0al. applied \u0394Gagg to determine the threshold of CO partial pressure and temperature, where isolated Pt atoms are thermally stable against sintering on CeO2(111), and the threshold size of Pt nanoparticles with thermodynamic stability relative to isolated Pt single atoms [15].Comprehension of the electrochemical stability, especially dissolution of transition metal catalysts is crucial in solid\u2013liquid heterogeneous electrocatalysis. Previous theoretical works mainly focus on transition metal catalysts used in acid oxygen reduction reactions (ORR) of proton exchange membrane fuel cells (PEMFCs). One of the most critical issues on electrochemical stability is the acid dissolution of metal species on fuel cells working potential. The first-principles-aided thermodynamic approach has been employed to understanding the electrochemical stability and predict the dissolution potential of transition metal electrocatalyst [27,28]. The formula of metal bulk dissolution is expressed by,\n\n\nM\u2194M\n2+\u00a0+\u00a02e\n\u2212\n\n\n\nThe dissolution process is investigated for an n-atom nanoparticle by applying the same method,\n\n\nM\n\nn\n\u2194M\nn-1\u00a0+\u00a0M\n2+\u00a0+\u00a02e\n\u2212\n\n\n\nThus, the reaction free energies at a given potential U is defined by\n\n(21)\n\u0394G\u00a0=\u00a0G(M\n2+,aq)\u00a0\u2212\u00a02eU\u00a0\u2212\u00a0G(M,s)\n\n\n\n\n\n(22)\n\u0394G\u00a0=\u00a0G(M\n\nn-1)\u00a0+\u00a0G(M\n2+,aq)\u00a0\u2212\u00a02eU\u00a0\u2212\u00a0G(Mn\n)\n\n\nThe chemical potential of cations G (M2+,aq) is derived from both experimental standard redox potentials U0 and calculated chemical potential of bulk metals G (M,s)\n\n(23)\n\nG(M\n2+,aq)\u00a0=\u00a0G(M,s)\u00a0+\u00a02eU\n0\n\n\n\nBased on Nernst equation, U\u00a0=\u00a0U\n0\u00a0\u2013\u00a0RT/zF\u00a0\u00d7\u00a0log ([M\n2+]), reduction potentials U can be modified according to different concentration of M2+(aq) in the electrolyte. It requires \u0394G\u00a0<\u00a00 for the dissolution proceeding spontaneously. Therefore, the equilibrium potential for the dissolution of catalyst, named by the dissolution potential, can be estimated in the following, \n\n(24)\n\nU\n\nbulk\n\u00a0=\u00a01/2e [G(M\n2+,aq)\u00a0\u2212\u00a0G(M,s)]\n\n\n\n\n(25)\n\nU\n\nn\n\u00a0=\u00a01/2e [G(M\n\nn-1)\u00a0+\u00a0G(M\n2+,aq)\u00a0\u2212\u00a0G(M\n\nn\n)]\u00a0=\u00a0U\n\nbulk\n\u00a0+\u00a01/2e [G(M\n\nn-1)\u00a0+\u00a0G(M,s)\u00a0\u2212\u00a0G(M\n\nn\n)]\n\n\nAs seen in Fig.\u00a05\na, Jinnouchi et\u00a0al. reported the site-dependent dissolution potentials of the Pt particles by first-principles-aided thermodynamic approach, which found that Pt atoms at edges with higher d-band centers dissolve more facilely than those at flat surfaces [27]. Pt atoms attaching to the carbons have lower redox potential. Seo et\u00a0al. demonstrated the obvious size-dependent dissolution process for Pt nanoparticles with diameters smaller than 3\u00a0nm (Fig.\u00a05b) [29]. The dissolution process begins in edges and vertices and makes more (111) facets exposed. These results are in line with the consensus that the catalytic performance of Pt in nanoscale is restricted under the PEMFC environment due to their inferior electrochemical stability. In Fig.\u00a05c, Ceder et\u00a0al. explored the origin of lower dissolution potential of Pt nanoparticles relative to Pt bulk, since Pt nanoparticle dissolve through a different mechanism from that of bulk Pt, where dissolution of Pt nanoparticle takes place through electro-oxidation of Pt oxide to Pt2+ cations [28]. Li et\u00a0al. applied first-principles-aided thermodynamic models to explore the factors influencing the dissolution of the Pt nanoparticles [30]. The oxygen chemisorption can hinder the dissolution by decreasing the surface energy and increasing the standard redox potential of dissolution/deposition. In addition, the dissolution is accelerated obviously at higher electrode potential, while it is impeded by large particle size.The dissolution of Pt-based alloy was likewise studied based on first-principles-aided thermodynamic approach. A DFT study on Pt\u2013Co nanoparticle showed that the Pt shell-Co core structure possess a stronger anti-dissolution ability than pure Pt nanoparticles [31]. Another theoretical work on the Pt\u2013Au nanoparticle concluded that the Pt dissolutions is hindered by substitution of edge Pt by Au atoms [32]. Balbuena et\u00a0al. showed that the impact of Pt on the anti-dissolution ability of the second metal component in the alloy has the same order as that in pure metal, with PtIr and Ir possessing the highest dissolution potential [33]. Remarkably, based on a thermodynamic analysis on electrochemical stability by predicting the threshold potential of dissolution for a series of supported transition metal single atoms or clusters, Li et\u00a0al. predicted a method for synthesizing high-purity and high-loading single atoms or clusters, shown in Fig.\u00a06\n [34]. They demonstrated the applicability of an electrochemical potential window, by which any metal species beyond electrochemical potential window are corroded away from substrate, while the high stability against dissolution of single atoms remains on the support.More and more significant findings emerging from experimental studies in situ and operando conditions indicate a dynamic nature of the catalyst surface under the catalytic reaction conditions. The catalyst structure may constantly vary before, during and after the catalytic process attributed to the interaction with reactant species. For example, ultra-thin oxide layers would form and cover the transition metal particles in an oxidizing environment [35]. The surface metal component of alloy would change upon exposure to reaction environments due to surface segregation, resulting in the enrichment of one metal on the shell and the other in the core [36]. The presence of reactants may adjust the ratio of exposed crystal facet so that the dynamic morphology reshaping of the metal particle could occur under reaction conditions [37]. All structural evolution of catalyst mentioned above would provide different active sites from those in absence of reaction condition, which could not be described precisely by 0\u00a0K/Ultra High Vacuum (UHV) surface science experiments or theoretical simulation. An insight into the relationship between configuration reconstruction of the transition metal catalyst and the reaction environment is crucial for a deep comprehension of catalysis and for a rational design of catalysts. Nowadays, theoretical methods are moving from 0\u00a0K/UHV models to operando investigations, including first-principles computation-aided thermodynamic framework, which precisely predicts the reconstructed configuration of transition metal catalysts with thermodynamic tendency upon the reaction environment.In a given reaction environment, the reactant gas may oxide or reduce the catalyst surface, and reactant molecules may combine with catalyst surface and lead to surface termination phase transition, such as thin oxide-like structures on exposure to high oxygen pressures [38]. The influence of the reactant partial pressures and temperatures on the reactant-involved surface phase transition have been taken into account by employing first-principles-aided thermodynamic model proposed by Reuter [5,39,40]. The first-principles-based thermodynamic framework is briefly demonstrated below. The Gibbs ensemble is suitable to identify the most stable system geometry and composition at given reaction condition by the minimum surface free energy of unit area A:\n\n(26)\n\n\n\u03b3\n\n(\n\nT\n,\nP\n\n)\n\n=\n\n1\nA\n\n\n[\n\nG\n\n(\n\nT\n,\n\nP\ni\n\n,\n\nN\ni\n\n,\n\nN\nj\n\n\n)\n\n\u2212\n\nN\ni\n\n\n\u03bc\ni\n\n\n(\nT\n,\nP\n)\n\n,\n\nN\nj\n\n\n\u03bc\nj\n\n\n(\nT\n,\nP\n)\n\n\n]\n\n\n\n\nwhere \n\nG\n\n(\n\nT\n,\n\nP\ni\n\n,\n\nN\ni\n\n,\n\nN\nj\n\n\n)\n\n\n is the Gibbs free energy of a specific surface models containing \n\n\nN\ni\n\n\n species i and \n\n\nN\nj\n\n\n species j, \n\n\n\u03bc\ni\n\n\n and \n\n\n\u03bc\nj\n\n\n are the chemical potentials of individual reservoirs of each components. It represents the energy needed to create certain surface termination for taking all atoms out of corresponding reservoirs. After calculating \n\n\u03b3\n\n(\n\nT\n,\nP\n\n)\n\n\n of possible reactant-involved surface termination configuration, the one which possesses the lowest \n\n\u03b3\n\n(\n\nT\n,\nP\n\n)\n\n\n is the thermodynamically most preferable configuration at given reaction conditions. Therefore, the excess energy relative to suitable reference is used to evaluate various surface configurations:\n\n(27)\n\n\n\u03b3\n\n(\n\nT\n,\nP\n\n)\n\n\u2212\n\n\u03b3\n0\n\n\n(\n\nT\n,\nP\n\n\n)\n\n=\n\n1\nA\n\n\n[\n\nG\n\n(\n\nT\n,\n\nP\ni\n\n,\n\nN\ni\n\n,\n\nN\nj\n\n\n)\n\n\u2212\n\nG\n0\n\n\n(\n\nT\n,\n\nP\ni\n\n,\n\nN\ni\n'\n\n,\n\nN\nj\n'\n\n\n)\n\n\u2212\n\u0394\n\nN\ni\n\n\n\u03bc\ni\n\n\n(\n\nT\n,\nP\n\n)\n\n\u2212\n\u0394\n\nN\nj\n\n\n\u03bc\nj\n\n\n(\nT\n,\nP\n)\n\n\n]\n\n\n\n\n\nThe Gibbs free energies is approximated by total DFT energies accompanied with vibrational contribution, which makes it simplified to determine thermodynamically stable surface phase, by only evaluating DFT energy difference.\n\n(28)\n\n\n\u03b3\n\n(\n\nT\n,\nP\n\n)\n\n\u2212\n\n\u03b3\n0\n\n\n(\n\nT\n,\nP\n\n)\n\n\u2248\n\n1\nA\n\n\n[\n\n\u0394\n\nE\n\nt\no\nt\n\n\n\u2212\n\u0394\n\nN\ni\n\n\u0394\n\n\u03bc\ni\n\n\n(\n\nT\n,\nP\n\n)\n\n\n]\n\n\n\n\nwhere \n\n\nE\n\ns\ne\ng\n\u2212\nn\n\u2212\nm\n\n\n\n the vibrational contribution is contained in the free energy part \n\n\u0394\n\nN\ni\n\n\u0394\n\n\u03bc\ni\n\n\n(\n\nT\n,\nP\n\n)\n\n\n.\n\n(29)\n\n\n\u0394\n\nE\n\nt\no\nt\n\n\n=\n\n\nE\n\nt\no\nt\n\n\n\n(\n\n\nN\ni\n\n,\n\nN\nj\n\n\n)\n\n\u2212\n\nE\n0\n\nt\no\nt\n\n\n\n(\n\n\nN\ni\n'\n\n,\n\nN\nj\n'\n\n\n)\n\n\u2212\n\u0394\n\nN\ni\n\n\nE\n\nt\no\nt\n\n\n\n(\ni\n)\n\n\u2212\n\u0394\n\nN\nj\n\n\nE\n\nt\no\nt\n\n\n\n(\nj\n)\n\n\n\n\n\nThe first-principles-aided thermodynamic model has been applied to confirm the thermodynamically preferable atom arrangement of the surfaces termination under different reaction environments and it is routinely used to describe the surface phase transition of complex multicomponent systems. Since transition metals act as common catalysts for many thermal catalytic oxidation reaction, previous studies mainly focus on the surface phase transition of close-packed transition metal slabs under oxygen atmosphere. The growth of ultra-thin oxide covering transition metal, which bears a little resemblance to the bulk oxides, have been displayed, including Ag (111) [41,42], Pd (111) [43], Pd (100) [44], Rh (100) [45], Rh (110) [46], Pt (110) [47], Rh (111) [48], Ni(110) [49], Cu(111) [50], Cu(100) [51] and Au (111) [52]. These researches explored the surface atomic arrangement that tend to exist thermodynamically, and how they are determined by the reactant pressure and temperature. Li et\u00a0al. employed DFT calculations on possible oxygen-induced reconstructed configurations at Ag (111) [53,54]. Through ab initio-based thermodynamic model, the surface free energies of Ag surface structures with various O coverage as a function of oxygen chemical potential were plotted (Fig.\u00a07\n), by which some thermodynamic steady oxygen-involved reconstructed Ag surface, such as the p (4\u00a0\u00d7\u00a04) phases, have been identified by theoretically and experimentally confirmed in scanning tunneling microscopy STM studies. Analogous theoretical models were applied on electrocatalytic oxygen reduction reaction of Pt nanoparticle, which indicated that hydroxide radical formation at (100)-edges at 0.5\u00a0V (RHE) would be substituted by atomic oxygen at 0.75\u00a0V, and the atomic oxygen aggregate to form PtO2 chains sinking into the subsurface with increasing electrode potential till 1.18\u00a0V [31].Apart from pure transition metal, first-principles-aided thermodynamic model has also achieved a deep understand on surface phase transition of bimetallic alloy under reaction condition [55]. Scheffler et\u00a0al. plotted the surface phase diagram of the (111) crystal facets of Ag\u2013Cu alloy as a function of oxygen chemical potential depending on temperature and pressure, as well Cu surface content (Fig.\u00a08\na) [56,57]. According to the surface phase diagram, the most thermodynamic stable at given temperature and pressure as a function of surface element composition can be determined. For example, copper impurities in silver host prefer to stay in subsurface in the absence of oxygen, while in the oxygen atmosphere typically used in ethylene epoxidation, surface phase suggests that, with different the copper surface concentration, clean Ag (111), thin copper oxide layers, and thick oxide-like structures can coexist. In a multicomponent atmosphere, the presence of different reactant gas plays an individual role in surface termination reconstruction of catalyst, so that chemical potential of involved reactants need to be accounted for in order to assess the precise surface configurations. A typical case is a study by Scheffler et\u00a0al. to identify preferable surface structures of Ag\u2013Cu alloy determined by chemical potential of both ethylene and oxygen during ethylene epoxidation reaction, which indicates a dynamical coexistence of CuO layer and AgO\u2013CuO shell (Fig.\u00a08b) [58]. Another example is the surface phase diagram of the Pd (100) slab under a reactive condition of CO oxidation. The stable surface configurations in CO oxidation condition locates at the boundary between phases reactant binding to surface oxide and reactant attaching on the metallic slab (Fig.\u00a08c) [59].In addition to oxidizing environment, first-principles-aided thermodynamic model have been successfully applied to describe surface phase transition under the other atmosphere. Scheffler et\u00a0al. showed the surface phase diagram of rutile RuO2(110) under a humid environment containing oxygen and water vapor [60]. Catherine Stampfl et\u00a0al. demonstrated the effect of N chemisorption on the reconstruction of Cu (111) (100) and (110) surfaces in N2 atmosphere [61,62]. A peculiar atom arrangement for 0.75\u00a0ML coverage of N atoms is predicted, namely a metastable \u2018\u2018N-trimer cluster\u2019\u2019 on the metal slab. A surface nitride-like configuration is found to be energetically favored in surface phase diagram, and exists within a narrow scope of nitrogen chemical potential until the growth of bulk Cu3N. The atomic morphology change of NiCr alloy surface induced by fluorine chemisorption is investigated by first-principles-aided thermodynamic models to explore the early-stage corrosion processes of nickel-based alloys in strong oxidizing environment [63].A synergistic effect of transition metal alloys is strongly related with the surface composition, which may promote or hinder desirable and undesirable chemical reactions. The surface element content would change upon exposure to certain reaction environments, where one of the metal components enrich the surface region and the other stay in core, known as surface segregation [64]. Thermodynamically, surface segregation is derived from disparity of surface energy among diverse metals, so that the reaction environment would cause the transition of surface composition by tuning the surface energy. First-principles-aided thermodynamic model have been developed well to determine thermodynamic steady surface composition of alloy by defining segregation energy (Eseg), which is defined as the energy needed to overcome for moving solute metal from the interior to the surface layer of the host metal and calculated by the following equation:\n\n(30)\n\n\n\nE\n\ns\ne\ng\n\u2212\nn\n\u2212\nm\n\n\n=\n\nE\n\n\nn\n\nt\nh\n\n\n\n\n\u2212\n\nE\n\n\nm\n\nt\nh\n\n\n\n\n\n\n\n\n\n\n(31)\n\n\n\nE\n\ns\ne\ng\n\u2212\nn\n\u2212\nb\nu\nl\nk\n\n\n=\n\nE\n\np\nu\nr\ne\n\nb\nu\nl\nk\n\n\n+\n\nE\n\n\nn\n\nt\nh\n\n\n\n\n\u2212\n\nE\n\ni\nm\np\nu\nr\ni\nt\ny\n\ni\nn\n\nb\nu\nl\nk\n\n\n\u2212\n\nE\n\np\nu\nr\ne\n\ns\nu\nr\nf\na\nc\ne\n\n\n\n\n\nwhere \n\n\nE\n\ns\ne\ng\n\u2212\nn\n\u2212\nm\n\n\n\n is the total energy difference of alloy surface model with solute metal atoms in the nth layer and mth layer. \n\n\nE\n\ns\ne\ng\n\u2212\nn\n\u2212\nb\nu\nl\nk\n\n\n\n, is the total energy difference between solute metal atoms in the nth layer and bulk. Negative Eseg\n indicates solute atoms in bulk prefer diffusing to the surface, while a positive Eseg implies solute atoms tend to stay in the bulk interior. Depending on the segregation energy, element-dependent segregation behavior of transition metal alloys for extended surface model and cluster model were transformed to a color-coded matrix [65,66].The thermodynamic steady surface composition would vary due to the environmental atmosphere. O2 and H2 tend to drive different metal elements to migrate to the surface when alloy catalysts are exposed to an O2 and H2 atmosphere, respectively, which is very meaningful for the applications of Pt-based alloy systems on PEMFC electrocatalysis. DFT calculation results have predicted a reversible surface segregation behavior of Pt\u2013Ni nanoparticles upon alternating H2 and O2 environments [67\u201369]. Ni surface segregation is not obvious under a reducing atmosphere (such as H2) resulting from the weak bonding between H and Ni, while Ni prefer segregating onto the surface in the form of NiO under O2 atmosphere. Byungchan Han et\u00a0al. found that a high surface coverage of oxygen accounts for the Co segregation of thermodynamically stable Co core-Pt shell nanoparticles under the working condition in fuel cells (Fig.\u00a09\na) [32]. Balbuena and his co-worker investigated surface component transition of Pt-based alloy catalysts upon acidic oxygen reduction reaction conditions, and discovered surface segregation adjust metal dissolution through tuning oxidation state of the subsurface atoms [70]. They also explored surface segregation of Pt3M (M\u00a0=\u00a0Fe, Co, and Ni) alloys in oxidizing condition, which suggested that both the Pt-segregated and M-segregated slabs only become stable than the original one when O coverage reaches over 1/4 monolayer [71]. Scheffler et\u00a0al. combined DFT calculation and thermodynamic method to describe Ag3Pd(111) surface under an oxygen atmosphere, analyzed profiles of segregation, adsorption, and surface free energies in increasingly oxygen-rich environments [72].Apart from O2 and H2 atmosphere, transition metal tends to have strong interaction with other reactant gas, such as CO, which causes surface segregation. For example, as for Cu\u2013Pt bimetallic catalysts under CO atmosphere, the strong affinity with CO accounts for Pt pulled out to the top surface and formation of a Cu\u2013Pt near-surface alloy thermodynamic steady state (Fig.\u00a09b) [73]. Donna A. Chen et\u00a0al. combined DFT calculations and thermodynamic model for Ni1Au121 clusters with the Ni atoms in the center and confirmed that the most stable structure under CO atmosphere belongs to CO binding to a surface segregated Ni atom. CO-adsorption-induced Pd surface segregation, dynamics of PdAu swapping, and Pd clustering in AuPd bimetallic surfaces are demonstrated by Graeme Henkelman's group [74].Recent in situ experimental characterizations have observed the reversible reshaping behavior of transition metal nanoparticles in the presence of reactant environment [75]. The shape of nanoparticles can affect the number of active sites and, further, the catalytic reactivity. The crystal facet stability will change under reactant atmosphere, where more open facets are exposed at the expense of close-packed surfaces, increasing the number of coordination-unsaturated sites. A proper theoretical modeling is an effective way to reveal the physical insight behind the geometry reconstruction and to help achieve shape control for higher catalysis performance. Recently, a first-principles-aided thermodynamic model was proposed by Gao and his group, containing the Wulff construction theory, adsorption isotherms theory, and DFT calculations [37]. This model achieved great success in quantitatively describing the precise equilibrium shape of nanoparticles at different temperatures and gas pressures, such as the reshaping of Cu nanoparticles in the water vapor condition (Fig.\u00a010\na) [76] and the shape evolution of Ru, Pt, Pd, Cu and Au nanoparticles under CO and NO environment (Fig.\u00a010b) [77,78].Based on Wulff construction, the optimal geometry of the crystal with the lowest total surface free energy is built according to the surface free energy of every crystal surface. Under a given reactant condition, the surface free energy of a clean surface \n\n\n\u03b3\n\nh\nk\nl\n\n\n\n should be revised by considering the interface tension from reactant adsorption:\n\n(32)\n\n\n\n\u03b3\n\nh\nk\nl\n\n\ni\nn\nt\n\n\n=\n\n\u03b3\n\nh\nk\nl\n\n\n+\n\n\n\u03b8\n\n(\nT\n,\nP\n)\n\n\nE\n\na\nd\ns\n\n\n\n\n\nA\n\na\nt\n\n\n\n\n\n\n\nwhere E\n\nads\n and A\n\nat\n are average adsorption energy and surface area per atom, respectively. (T, P) is the reactant molecules coverage on surface determined by the temperature and reactant partial pressure, expressed by the Langmuir adsorption isotherms.\n\n(33)\n\n\n\n\u03b8\n\n1\n\u2212\n\u03b8\n\n\n=\nP\nK\n\n\n\nwhere K is the adsorption equilibrium constant, expressed by:\n\n(34)\n\n\nK\n=\nexp\n\n(\n\n\u2212\n\n\n\u0394\nG\n\n\nR\nT\n\n\n\n)\n\n=\nexp\n\n(\n\n\u2212\n\n\n\nE\n\na\nd\ns\n\n\n\u2212\nT\n\n(\n\n\nS\n\na\nd\ns\n\n\n\u2212\n\nS\n\ng\na\ns\n\n\n\n)\n\n\n\nR\nT\n\n\n\n)\n\n\n\n\nwhere S\n\ngas\n is the entropy of the gas in the gas phase and S\n\nads\n is the adsorption entropy.\n\n(35)\n\n\n\n\u03b3\n\nh\nk\nl\n\n\ni\nn\nt\n\n\n=\n\n\u03b3\n\nh\nk\nl\n\n\n+\n\n\n\u03b8\n\n(\n\nT\n,\nP\n\n)\n\n\n(\n\n\nE\n\na\nd\ns\n\n\n\u2212\nz\nw\n\u03b8\n\n)\n\n\n\n\nA\n\na\nt\n\n\n\n\n\n\n\n\n\n\n(36)\n\n\n\n\u03b8\n\n1\n\u2212\n\u03b8\n\n\n=\nP\nK\n\ne\nx\np\n\n(\n\n\u2212\n\n\nz\nw\n\n\nR\nT\n\n\n\u03b8\n\n)\n\n\n\n\nwhere z is the number of first-neighboring molecules of adsorbates at 1\u00a0ML adsorption coverage, w is interaction energy between two nearest adsorbed molecules. After obtaining corrected surface tension of each crystal facet, the Wulff theorem is applied to construct the equilibrium geometry of nanoparticles at a given T and P.In addition to mono-component reactant gas, the thermodynamic model is feasible for the environments including multi-component atmosphere [79]. First-principles-aided thermodynamic model has made great success in reproducing in situ experimental characterization on nanoparticle exposed to H2 and O2 conditions (representative reductive and oxidative gases). The simulated shape evolution of Pd nanoparticles up to ambient conditions reached perfect agreement between the in situ environmental transmission electron Microscopy (ETEM) observation at the same T and P [80]. According to first-principles calculations, the surface tension of the Pd (110) and (100) surface is stabilized under O2 atmosphere, while the surface free energy of the (111) facet changes to the most unstable surface, which leads to surface area proportion transition of low-coordination sites up to 82%. Likewise, the prediction of thermodynamic model shows that the facet proportion of Pd (100), (110) and (111) at the equilibrium of state for Pd nanoparticles in H2 atmosphere, are 22%, 48% and 30%, respectively, which coincide with the experimental observation. As for the shape reconstruction of the Au nanoparticles caused by O2 atmosphere, first-principles-aided thermodynamic model described the equilibrium geometry evolution of Au nanoparticles upon different temperature [81]. The reshaping of Au nanoparticles with truncated octahedron began with the corner when decreasing the temperature, and eventually became a round shape, which is in line with the observation of in situ TEM. On the contrary, the morphology evolution of the Au nanoparticles in H2 atmosphere was not obvious. In addition to strong reductive and oxidative gases, N2, known as an inert gas, was also predicted to affect the equilibrium geometry of nanoparticles at ambient conditions, matching the experimental results. The coverage of N2 on the Pd (110) surface is much higher than that (111) and (100) surfaces, which makes the surface free energy of the (110) slab decreased and induces an increase of facet proportion [82]. First-principles-aided thermodynamic model were also reported to describe the reshaping of multi-metallic nanoparticles. The H2 environment-induced morphological evolution of PdCu alloy nanoparticles transforming from spheres to truncated cubes was visualized by theoretically calculating the surface free energy of PdCu alloy surface with or without hydrogen adsorption (Fig.\u00a010c) [83]. In addition to transition metal, the equilibrium morphology of transition metal oxide under realistic reaction conditions has been successful predicted by first-principles-aided thermodynamic models [84].Heterogeneous catalysts are generally supported on a high-surface-area substrate. Supports can alter morphology of nanoparticle, and furthermore the perimeter of the interface between the nanoparticle and the substrate providing active centers. It is helpful to theoretically predict the reshaping of interface between metal particle and substrate under reaction conditions. The contact-surface tension between the nanoparticles and the substrate for supported nanoparticles system, can be evaluated by the Wulff-Kaischew theorem.\n\n(37)\n\n\n\n\u03b3\n\nc\n\u2212\ns\n\nE\n\n=\n\n\u03b3\nA\n\n\u2212\n\nE\n\na\nd\nh\n\n\n\u2212\n\n(\n\n\n\n\n\u03b8\nB\n\n\nE\n\na\nd\ns\n\nB\n\n\n\n\nA\n\na\nt\n\nB\n\n\n\n\n)\n\n\n\n\nwhere \u03b3\n\nA\n is the surface tension of the facet of nanoparticles adhering support, E\n\nadh\n is the binding energy between nanoparticles and support,\u03b8\n\nB\n is the coverage of reactant on the substrate, \n\n\nE\n\na\nd\ns\n\nB\n\n\n is the adsorption energy of the reactant on the substrate, and \n\n\nA\n\na\nt\n\nB\n\n\n is the surface area of the substrate.Pt@SrTiO3, as a model system of supported nanoparticles, was studied by first-principles-aided thermodynamic model under a H2 environment [85]. Theoretical prediction show that Pt nanoparticle display a clear wetting on a SrTiO3(110) surface in absence of H2, consistent with TEM images. The H2 adsorption-involved structural transformation was modelled in Fig.\u00a010d. Since H2 molecules compete with the Pt nanoparticle for attachment on the SrTiO3 substrate, which results in the dewetting of the supported Pt nanoparticle. As a result, the shape of the contact surface and the number of atoms at interface perimeter are both adjusted by the gas environment.First-principles-aided thermodynamic model has taken a dramatic development and established a computational strategy in evaluating structural stability and exploring structural reconstruction of transition metal catalysts under reaction condition, which was briefly summarized. First-principles computation-aided thermodynamic framework provides thermodynamic understanding or evaluation of the structural destabilization tendency, which is helpful to propose measures for suppressing deactivation processes and increasing the durability of transition metal catalysts. In addition, it belongs to computational operando investigations, precisely predicts the reconstructed configuration of transition metal catalysts with thermodynamic tendency in the reaction environment.A new computational approach, which can manage big database of accumulated by DFT calculations, is suggested to essentially develop in the future, by which energetics parameters in thermodynamic model can be correlated with readily available physical properties of the reactant, metal and the support, and identify key parameters known as descriptor. The descriptor influencing reactant-metal, metal\u2013metal and metal\u2013support interactions is promising to be used in conjunction with Machine learning to develop a predictive model for screening high-stability catalysts and understanding their structure evolution in future rational catalyst design.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 that could be construed as influencing the position presented in, or the review of, the manuscript entitled \u201cFirst-principles-aided thermodynamic modeling of transition-metal heterogeneous catalysts: A review\u201d.This work is supported by the National Natural Science Foundation of China (Grant Nos. 21822801), China Postdoctoral Science Foundation (2019TQ0021) and the Fundamental Research Funds for the Central Universities (XK1802-1 and XK180301).", "descript": "\n                  Over the past decade, the first-principles-aided thermodynamic models have become standard theoretical tools in research on structural stability and evolution of transition-metal heterogeneous catalysts under reaction environment. Advances in first-principles-aided thermodynamic models mean it is now possible to enable the operando computational modeling, which provides a deep insight into mechanism behind structural stability and evolution, and paves the way for high-through screening for promising transition-metal heterogeneous catalysts. Here, we briefly review the framework and foundation of first-principles-aided thermodynamic models and highlight its contribution to stability analysis on catalysts and identification of reaction-induced structural evolution of catalyst under reaction environment. The present review is helpful for understanding the ongoing developments of first-principles-aided thermodynamic models, which can be employed to screen high-stability catalysts and predict their structural reconstruction in future rational catalyst design.\n               "}
{"full_text": "The selective oxidation of sulfides into the corresponding sulfoxides is one of the most significant procedures within both industrial and laboratory settings [1,2]. However, among a large number of traditional oxidants that have been widely used for this selective oxidation, many of them are dangerous to use or toxic [3\u20135]. Moreover, sulfoxides can often undergo over-oxidation to their corresponding sulfones during relatively harsh or harmful reaction conditions [6,3,7,8], and therefore, to overcome these limitations and considering the eco-sustainability, green chemistry and atom economy, a considerable amount of research has been focused toward the development of new and effective catalytic systems based on the use of aqueous H2O2 as a green oxidant [9\u201312]. The tetrazole unit continues to arise great attention in both industry and academia. Tetrazoles are of interest in pharmaceuticals, synthetic organic chemistry, coordination chemistry, catalysis technology, the photographic industry, and organometallic chemistry. Also, tetrazoles and their derivatives have been reported as analgesic, antiviral, anti\u2010inflammatory, anti\u2010proliferative, antibacterial, potential anti-HIV drug candidate, antifungal, herbicidal and anticancer agents. [13\u201315] Most notably, the 5-substituted 1H-tetrazole is frequently used as a carboxylic acid isostere in medicinal chemistry, [16,17]. Numerous active pharmaceutical ingredients containing a tetrazole moiety are currently on the pharmaceutical market. Later methods to the synthesis of tetrazoles in the literature generally involve the costly and poisonous metal, suffer from intense water reactivity, or utilization of hydrazoic acid, which is very poisonous, unstable, and flammable. [18\u201321] Also, there is a growing demand for safe, energetically efficient, and environmentally friendly procedures. Homogeneous catalysts have several advantages such as better performance, high turnover numbers, and high selectivity [22-24,10]. But significant weaknesses are obvious: separating homogeneous catalysts from the reaction medium is tedious and requires several expensive and particular methods [25\u201327]. Some difficulties frequently related to the homogeneous catalysts can be easily overcome by heterogenization of their counterpart on the surfaces of both organic and inorganic solids [28,29]. Magnetic nanoparticles have attractive physical and chemical properties . Superparamagnetism, high magnetic susceptibility, and low curie temperature are some unique magnetic properties of MNPs [30]. Fe3O4 nanoparticles are a good candidate as a support material for heterogeneous catalysts because of their great properties such as the abundance of unique activities, low toxicity and price, simple synthesis and functionalization, large surface area, and easy separation with magnetic field [31,32]. Fe3O4 has a cubic inverse spinel structure. Magnetic nanocatalysts are used to accelerate various organic reactions [33]. In this paper, we studied the selective oxidation of sulfides and the synthesis of 1H-tetrazole in green media with a green catalyst. The new synthesis proposes a suitable, and eco-friendly synthetic method. The novel Ag-containing creatinine functionalized Fe3O4 catalyst was designed by attaching Ag onto the surface Fe3O4. The catalysts have been used in the selective oxidation of sulfides and synthesis of 1H-tetrazole in green media with the classical and ultrasonic methods. At first, creatinine functionalized Fe3O4 has been prepared through the surface functionalization of Fe3O4 to offer Fe3O4@Creatinine sample, which on the treatment of AgNO3 results in the development of novel Fe3O4@Creatinine@Ag catalyst (Fig.\u00a01\n).Today, many efforts are made to create conditions for the manufacturing of chemicals that meet most of the principles of green chemistry. One way to achieve this goal is to use catalysts that, while taking advantage of their benefits, do not impose any restrictions on the principles of green chemistry. The design and use of eco-friendly catalysts achieve the mutual goals of protecting the environment and promoting economic benefits simultaneously. The\u00a0objective\u00a0of this study was to produce a new eco-friendly nanocatalyst with high potential, high mechanical and thermal stability, high contact surface with the selective performance of healthy and eco-friendly materials and solvents, and cost-effective to selective oxidation of sulfides and prepare tetrazole derivatives in a green solvent. Most tetrazoles are obtained in toxic solvents at high temperatures with low efficiencies. Therefore, due to the significant industry demand for these products, materials based on magnetic nanoparticles can be chemically modified to design catalysts containing environmentally friendly active centers. In the synthesis of this catalyst, the environmentally friendly ligand is used to stabilize the eco-friendly active site on the desired support. What distinguishes this catalyst and this research is its high eco-friendliness in several respects. The structure of this catalyst and also, the reaction are based on green chemistry under mild reaction conditions. At a relatively low temperature compared to the reported work, tetrazole derivatives are prepared in water solvent with good efficiency. Other features of this designed catalyst include high activity, stability, easy recovery, and low cost.A mixture of FeCl3\u20226H2O (5.2\u00a0g) and FeCl2\u20224H2O (2.0\u00a0g) was introduced to deoxygenated water (25\u00a0mL) containing few drops of conc. HCl. Subsequently, 250\u00a0mL of NaOH (1.5\u00a0M) solution was added dropwise. The whole mixture was stirred vigorously at 60\u00a0\u00b0C. Then, brown-colored Fe3O4 NPs were isolated using a magnetic stick. It was rinsed using distilled water and dried at 40\u00a0\u00b0C.The Fe3O4 (1\u00a0g) was decorated by the reaction with 3-chloropropyltrimethoxysilane (CPTMS) (1.5\u00a0mL) under refluxing toluene (24\u00a0h). In the next step, the obtained Fe3O4\nCl was washed thoroughly with n-hexane and dried at 40\u00a0\u00b0C (Scheme 1, Supporting information).Creatinine incorporation into Fe3O4 was obtained by the following method: Triethylamine (3\u00a0mL) was added to the suspension of 1\u00a0g of the Fe3O4\nCl and 2\u00a0mmol of creatinine (0.22\u00a0g) in 30\u00a0mL of toluene. Under the stirring condition, the mixture was refluxed (48\u00a0h). The obtained product was separated and rinsed using deionized water. Then, the Fe3O4@Creatinine was dried at 40\u00a0\u00b0C (Scheme 2, Supporting information).Afterward, the obtained Fe3O4@Creatinine (1\u00a0g) was decorated by the reaction with AgNO3 (2.5\u00a0mmol) under refluxing ethanol (15\u00a0h). In the next step, the obtained Fe3O4@Creatinine@Ag was separated, and rinsed with ethanol, and dried at 40\u00a0\u00b0C (Scheme 3, Supporting information).Fe3O4@Creatinine@Ag was added (70\u00a0mg) to a mixture of nitrile (1\u00a0mmol), NaN3 (1.2\u00a0mmol), and H2O (3\u00a0ml) at 90\u00a0\u00b0C. At the end of the reaction (checked by TLC), the Fe3O4@Creatinine@Ag was separated using magnetic decantation and washed with EtOAc and distilled water. The organic layer was preserved with 10\u00a0mL of 5\u00a0N HCl. Then the organic was washed with water and dried over anhydrous Na2SO4 to produce the wanted products (Scheme 4, Supporting information).Fe3O4@Creatinine@Ag was added (60\u00a0mg) to a mixture of sulfoxide (1\u00a0mmol), H2O2 (0.6\u00a0mL) as oxidant, and EtOH (3\u00a0ml) at room temperature. After completion of the reaction, checked by TLC, the Fe3O4@Creatinine@Ag was collected using magnetic decantation and rinsed with ethanol and distilled water. Then, the product was extracted with diethyl ether and rinsed using distilled water, and dried over anhydrous Na2SO4. The product was purified by a plate to give the wanted product (Scheme 4, Supporting information).(Table 2, Entry 1) Ethyl phenyl sulfoxide\n1HNMR (400\u00a0MHz, CDCl3, ppm): \u03b4 3.01 (s, 3H), 3.11\u20133.17 (m, 2H), 7.29 (s, 2H), 7.57\u20137.70 (m, 1H), 7.92\u20137.94 (m, 2H).(Table 2, Entry 8) Dibenzyl Sulfoxide\n1HNMR (400\u00a0MHz, CDCl3, ppm): \u03b4 4.22 (s, 4H), 7.40\u20137. 45 (m, 10H).(Table 2, Entry 9) Benzyl phenyl sulfoxide\n1HNMR (400\u00a0MHz, DMSO, ppm): \u03b4 4.70 (s, 2H), 7.14\u20137.15 (m, 2H), 7.30\u20137.34 (m, 3H) 7.56\u20137.60 (m, 2H) 7.71\u20137.75 (m, 3H).(Table 5, Entry 1) 5-(3-Nitrophenyl)\u22121H-tetrazole. 1HNMR (400\u00a0MHz, DMSO, ppm): \u03b4 7.92 (t, 2H), 8.40\u20138.50 (m, 2H), 8.86 (s, 1H.)(Table 5, Entry 2) 5-(4-Nitrophenyl)\u22121H-tetrazole\n1HNMR (400\u00a0MHz, DMSO, ppm): \u03b4 8.30\u20138.33 (d, 2H), 8.45\u20138.47 (d, 2H).FT-IR spectra of Fe3O4 MNPs (a), Fe3O4\nCl (b), Fe3O4@Creatinine (c), and Fe3O4@Creatinine@Ag were recorded (Fig.\u00a02\n). The band at around 600 cm\u22121 is the typical band of the vibration of metal-oxygen (Fe-O) in the nanomagnetic compounds. The proper grafting 3-chloropropyltrimethoxysilane linker to pure Fe3O4 is recognized by the presence of the stretching vibrations of the CH2 group at 2958 cm\u22121 and in stretching vibration modes of Si-O at 1040 cm\u22121 in the FT-IR spectra of Fe3O4\nCl (Fig.\u00a02b) [34]. In addition, the FTIR spectra for Fe3O4@Creatinine demonstrated new bands at 1680 and 1475 cm\u22121 which are related to stretching vibration modes of C\u00a0=\u00a0O and C\u00a0=\u00a0N bonds of creatinine respectively [35], which is an indication that creatinine has been attached successfully onto the Fe3O4. FT-IR spectrum d in Fig.\u00a02 shows the final catalyst after adsorption of silver metal onto Fe3O4@Creatinine.Thermogravimetric curves of Fe3O4 and Fe3O4@Creatinine@Ag are shown in Fig.\u00a03\n. Three major mass loss processes can be detected for the Fe3O4@Creatinine@Ag sample. The mass loss between 150 and 450\u00a0\u00b0C (11.87%), 450\u2013600\u00a0\u00b0C (4.18%), and 600\u2013800\u00a0\u00b0C (4.22%) is assigned to the decomposition of organic molecules in the Fe3O4@Creatinine hybrid material [36].As seen in Fig.\u00a04\n, the XRD analysis patterns of the Fe3O4, Fe3O4@Creatinine@Ag were recorded by X-ray powder diffraction (XRD). XRD pattern of Fe3O4 NPs (Fig.\u00a04a) has been confirmed by the bands at 2\u04e8 (34.9\u00b0, 41.20\u00b0, 50.24\u00b0, 63.10\u00b0, 67.40\u00b0 and 74.27\u00b0), corresponding to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) reflections. These spectra reflect the inverse spinel crystal structure of Fe3O4 NPS [37], indicating that a pure and highly crystalline product was obtained. Fig.\u00a04b shows a representative XRD spectrum of the Fe3O4@Creatinine@Ag nanocomposite. The same set of characteristic peaks were observed for Fe3O4@Creatinine@Ag after surface modification, indicating that the functionalization of Fe3O4 did not significantly disturb the Fe3O4 phase. Besides the characteristic diffraction peaks of Fe3O4 reflections, the diffraction peaks at 37.562, 44.546, 64.665, and 76.584 can be indexed to (111), (200), (220), and (311) planes of silver with the face-centered cubic (fcc) structure (space group: Fm3 m), respectively (JCPDS, No. 04\u20130783) [38,39].Using a VSM (vibrating sample magnetometer), the magnetic properties of the samples Fe3O4 MNPs, and Fe3O4@Creatinine@Ag were investigated (Fig.\u00a05\n). As illustrated in Fig.\u00a05, the saturation magnetization of the Fe3O4 MNPs and Fe3O4@Creatinine@Ag is 62.82, and 32.6 emu/g, respectively. The reduction of saturation magnetization of Fe3O4@Creatinine@Ag nanoparticles is because of the coating of Fe3O4 with the non-magnetic layers (SiO2, Creatinine, Silver). All MNPs reveal superparamagnetic behavior [40].The particle shape and surface texture were evaluated by scanning electron microscopy (SEM). SEM images of Fe3O4, Fe3O4@Creatinine@Ag nanoparticles are shown in Fig\u00a06\na, and 6b, respectively. The shapes of Fe3O4 and Fe3O4@Creatinine@Ag nanoparticles are spherical. There were no considerable changes in the morphology after immobilization of the silver complex on the magnetite surface.The chemical composition of the Fe3O4@Creatinine@Ag nano-catalysts was determined by the EDS analysis (Fig.\u00a07\n). Using EDS spectroscopy, the presence of carbon, iron, nitrogen, oxygen silicon, and silver elements was confirmed in the structure of the catalysts. The exact content of silver that immobilized on the Fe3O4 nanoparticle was measured by inductively coupled plasma optical emission spectrometry method (ICP-OES). This amount was 0.17\u00a0mmol g\u00a0\u2212\u00a01 for Fe3O4@Creatinine@Ag.The size and morphology of the samples were examined by TEM analysis (Fig.\u00a08\n). The TEM and SEM images of as-synthesized Fe3O4@Creatinine@Ag demonstrate that the sample consists of almost spherical nanoparticles with sizes ranging from ca. 20 to 50\u00a0nm.\nFig.\u00a09\n shows the UV\u2013vis spectroscopic results for the Fe3O4@Creatinine@Ag composite spheres catalyst. Surface plasmon resonance was identified and the appearance of distinctive bands in the metal nanocomposite (Fe3O4@Creatinine@Ag) was noted, exhibiting a specific surface plasmon peak around 390\u00a0nm depicts the synthesis of silver nanoparticles [41,42].Khan et.al observed a specific surface plasmon peak for Ag/Fe2O3 at 410\u00a0nm. The average size of the nanocomposite was to be 25\u2212 35\u00a0nm [41].The redshifts of SPR were attributed to the increase in NP size due to p-p interactions between the NPS and dye molecules, while bathochromic SPR shifts were attributed to the modest aggregation of NPs in hybrids, the very low surface curvature, and large particle size, which facilitated orbital overlap to afford a stiffer plasmon surface and induced the damping of electrons of NPs with high-energy resonance [43]. We estimate that the particles size of the Fe3O4@Creatinine@Ag obtained by uv-vis is according to the dimensions obtained from SEM and TEM.We studied the catalytic performance of the Fe3O4@Creatinine@Ag in the oxidation of sulfide at room temperature. In continuation of our studies, we compared ultrasonic and classical methods in the oxidation reaction. To optimize the experimental conditions, the oxidative of methyl phenyl sulfide using H2O2 at room temperature was chosen as a model reaction. The influence of the different experimental parameters like the amount of the catalyst and H2O2, and the nature of the solvent were investigated and the results are shown in Table\u00a01\n.The amount of the catalyst was evaluated on the direct synthesis of sulfoxide from sulfide in the range of 0 to 6\u00a0mg (Table\u00a01). When the amount of the catalyst is raised to 6\u00a0mg, the sulfoxide yield is increased from trace to 95%, resulting probably from the presence of more catalytic active sites [44]. As expected, this factor has a positive effect on the sulfide conversion, since an increase in the catalyst dosage produces an increase in the conversion of sulfide. For further increase in catalyst dose up to 8\u00a0mg, conversion of sulfide was increased but was not a significant quantity. It was found that 60\u00a0mg of the catalyst is an appropriate amount of catalyst.To gain more understanding, the effect of oxidant (H2O2) was studied. Experiments were performed with different amounts of H2O2 in the range of 0.3\u00a0mL to 0.9\u00a0mL using model reaction. Table\u00a01 depicts that the conversion of sulfide rose with an increment in the H2O2 amount. In subsequent studies, 0.6\u00a0mL of H2O2 was chosen as the appropriate quantity.We next explored the effect of solvents on the catalytic performance by the Fe3O4@Creatinine@Ag and H2O2. Different solvents e.g., ethyl acetate, ETOH, acetonitrile, and solvent-free condition was studied. The results in Table\u00a01 show that the yields of methyl phenyl sulfoxide were markedly increased in ethanol. It should be also noted that it has been demonstrated previously that ethanol binds to the metal of the catalysis as ligands, and one of the effects of the alcohol solvents on the catalyst activity results from the coordination of alcohols as ligands [45].The protic solvent may act by decreasing the negative charge density on oxygen by hydrogen bonding, thus promoting nucleophilic attack by the second sulfide. In aprotic solvents, the intermolecular reaction is less efficient and the intermediate either decomposes, giving ground-state oxygen (favored at room temperature) or rearranges to sulfone (at low temperatures). The effects of Protic solvent suggest that intermediates are stabilized not only by hydrogen bonding with protic solvents but also by coordination with solvents [46].The effect of active metal (mmol/g of Ag on Fe3O4@Creatinine@Ag) on catalytic performance was investigated with different loading ratios of Ag (0.10, 0.17, 0.25, and 0.43\u00a0mmol/g) on Fe3O4@Creatinine@Ag performance. It was found that with an increase in active metal, the sulfide conversion was increased i.e., 88%, 95%, 96%, and 97% of sulfoxide yield was obtained. Sulfide oxidation efficiency up to 0.43\u00a0mmol/g shows a positive response due to an increase in the availability of active metal species.On having the optimized experimental conditions, we then recognized the scope of an overview of the Fe3O4@Creatinine@Ag to the selective oxidation of a range of sulfides to the related sulfoxide. As shown in Tables 2 and 3, a wide variety of sulfides including aryl, diaryl, alkyl, and dialkyl sulfides furnished the corresponding sulfoxide in excellent yields with classical (Table 2, Supporting information) and ultrasonic (Table 3, Supporting information) methods. The results show that ultrasonic is an appropriate method for the oxidation of sulfides to the related sulfoxide.Based on the previously reported mechanism for the oxidation of sulfides into sulfoxides using hydrogen peroxide in the presence of a catalyst, one explanation for this process is the complex formation between the hydrogen peroxide and the M\n+ (intermediate A). After the formation of [M\n+\u2212OOH], it may form oxo metal species as the active oxidant (intermediate B). This active species can oxidize organic sulfides by the formation of the oxidant-substrate complex (intermediate C) and the oxygen transfer to the organic substrate (Scheme 5 and 6\n\n) [47\u201350].The oxidation of different compounds with H2O2 on the Ag-containing catalyst shows the crucial role of decomposition of hydrogen peroxide. There is no doubt that the decomposition of hydrogen peroxide plays a crucial role in the oxidation of organic compounds.The catalytic performance of the new Fe3O4@Creatinine@Ag was also examined in the synthesis of 5-substituted 1H-tetrazoles. To optimize the experimental conditions, benzyl cyanide with NaN3 was chosen as a representative model. The effect of the different experimental parameters like temperature, solvent, and catalyst amount was studied and the results are shown in Table\u00a04\n.To optimize the best solvent needed for the synthesis of 5-substituted 1H-tetrazoles, we have carried out this reaction in the various solvent over Fe3O4@Creatinine@Ag as the catalyst. The results are listed in Table\u00a04. All the reactions have been performed under refluxing conditions considering the different solvents. Ethanol is a polar protic solvent failed to produce the desired 1H-tetrazole in good yield (entries 1). When polar aprotic solvents like DMF and DMSO are used, 43% and 83% yield of products are obtained, respectively in long reaction time (entries 2, 3). It was found that water is the best solvent for reaction yield (entry 5). This effect can be attributed to the strong hydrogen bond interaction at the organic-water interface, which stabilizes the reaction intermediate [51]. Also due to the partial solubility of some azide derivatives in water, it is the best solvent for reaction yield.As expected, an increase in the reaction temperature promotes a faster transformation of nitrile. However, the productivity increased on raising the temperature. The increase of reaction temperature will accelerate the thermal movement of molecules and increase the probability of intermolecular collision; thus, the rate of the reaction is improved. Different research groups established that using high temperature is necessary for cycloaddition reactions [52], so it seems reasonable to think that increasing the reaction temperature would be a good choice for improvement in the yield. The best result for this reaction was obtained at 90\u00a0\u00b0C and conversion is practically total. (Table\u00a04, entry 5).The effect of catalyst dose is a significant parameter in the synthesis of 5-substituted 1H-tetrazoles. The effect of catalyst dose on the synthesis of 5-substituted 1H-tetrazoles was studied with the dose range of 0\u201390\u00a0mg. As seen in Table\u00a04, the synthesis of 5-substituted 1H-tetrazoles rate rose significantly as catalyst dose increased from 0\u00a0mg to 70\u00a0mg. For further increase in catalyst dose up to 90\u00a0mg, the yield was increased but was not a significant quantity. On increasing the catalyst dose from 0\u00a0mg to 70\u00a0mg, the synthesis of 5-substituted 1H-tetrazoles rate was enhanced from trace to 96% since higher doses provide more active sites, which in turn provides more chances for nitrile molecules to come into contact with a catalyst. Nevertheless, by further increment in catalyst amount from 70\u00a0mg to 90\u00a0mg, the synthesis of 5-substituted 1H-tetrazoles enhanced gradually since nitrile and active sites present on the catalyst reaches equilibrium. From this study, the authors conclude that a catalyst dose of 60\u00a0mg chosen was the best dose and was used for further work.The effect of active metal (mmol/g of Ag on Fe3O4@Creatinine@Ag) on catalytic performance was investigated with different loading ratios of Ag (0.10, 0.17, 0.25, and 0.43\u00a0mmol/g) on Fe3O4@Creatinine@Ag performance. It was found that with an increase in active metal, the nitrile conversion was increased i.e., 78%, 96%, 97%, and 99% of 1H-tetrazole yield was obtained.Based on our attained experimental results, the experimental conditions have been recognized as 90\u00a0\u00b0C in H2O with 70\u00a0mg catalyst. To recognize the scope and overview of Fe3O4@Creatinine@Ag catalyzed synthesis of 1H-tetrazole, different benzonitriles were used as substrates under our optimal conditions, as presented in Table 5 (Supporting information). The 1H-tetrazole was separated in short reaction times and high yields.In the study of azide-nitrile cycloaddition catalyzed by Ag+, the coordination of the nitrile substrate to the Lewis acidic silver is the source of the catalysis in the formation of 1H-tetrazoles. Ag+ coordinated to the nitrile, and this is the dominant factor influencing [2\u00a0+\u00a03] cycloaddition. Subsequent neucleophilic attack by azide)intermediate A\u00a0+\u00a0B includes mechanism A [53] and B [54]) followed by hydrolysis produces tetrazole as the end product, with Ag+ catalyst being released for the next cycle of reactions (Scheme\u00a07\n) [55].We were also interested in comparing the catalytic performance of our catalyst with that reported in the literature. At this point, selective oxidation of methyl phenyl sulfide and synthesis of 5-(4-chlorophenyl)\u22121H-tetrazole was chosen (Table\u00a06\n). As can be seen, the catalytic performance of our procedure is superior to other catalysts.The recyclability of Fe3O4@Creatinine@Ag was studied for the synthesis of methyl phenyl sulfoxide and 5-(4-chlorophenyl)\u22121H-tetrazole under optimized reaction conditions. After completion of the reaction, the catalyst could be easily separated by using an external magnetic field, then it was rinsed with ethyl acetate and subsequently dried and reused directly for the next run. The results are presented in Fig.\u00a010\n. We could employ the catalyst for 6 runs for the synthesis of methyl phenyl sulfoxide and 5 runs for the synthesis of 5-(4-chlorophenyl)\u22121H-tetrazole without an appreciable decrease in catalytic performance. Fig\u00a011\n. shows the image of the Fe3O4@Creatinine@Ag nanocatalyst suspension without and with an external magnetic field.In summary, we have reported the synthesis of silver supported on the surface of magnetic Fe3O4 nanoparticles (Fe3O4@Creatinine@Ag) through a facile method. Both the structure and the chemical nature of the catalyst were confirmed using XRD, ICP-OES, FT-IR, SEM, EDX, VSM, and TGA methods. This sustainable magnetic nanocatalyst leads to the efficient oxidation of the sulfides and the synthesis of 5-substituted 1H-tetrazoles with good yields and selectivity under mild conditions. Furthermore, applying ultrasound irradiation in the oxidation of sulfides leads to increase yields, and decrease reaction times. The nanocatalyst could be effectively recovered under a magnetic field without a remarkable decrease in the catalytic performance.The authors report declaration of interest in this work.The authors are deeply grateful to the University of Kurdistan for the financial support and also Erfan Ghadermazi for their help on this research project.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.apsadv.2021.100192.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  The surface of the Fe3O4 has been functionalized with 3-chloropropyltrimethoxysilane, which undergoes an SN2 substitution reaction of the chloro group with the nitrogen of ligand (creatinine), offering the Fe3O4@Creatinine. A new recyclable Ag attached Fe3O4 MNPs (Fe3O4 @Creatinine@Ag) has been produced through a post-synthetic method. FT-IR (Fourier Transforms Infrared), TGA (Thermogravimetric Analysis), XRD (X-ray Diffraction), EDS (Energy-Dispersive X-ray Spectroscopy) and VSM (Vibrating Sample Magnetometer) confirm the effectiveness of the performed chemical modification to synthesize the catalyst. This catalyst displays high catalytic performance in the synthesis of 5-substituted 1H-tetrazoles in water and the selective oxidation of sulfides. In the presence of nano- Fe3O4 @Creatinine@Ag as an efficient heterogeneous nano-catalyst, the corresponding 5-substituted 1H-tetrazoles and sulfoxide were afforded under the mild condition in good to excellent yields. A wide variety of sulfides furnished the corresponding sulfoxide with classical and ultrasonic methods. The results show that ultrasonic is an appropriate method for the oxidation of sulfides to the related sulfoxide. The catalyst can be separated by simple recovery and reused for several periods without any remarkable decrease in the catalysis activity and selectivity.\n                  In the synthesis of this catalyst, the environmentally friendly ligand is used to stabilize the eco-friendly active site on the desired support. What distinguishes this catalyst and this research is its high eco-friendliness in several respects. The structure of this catalyst and also, the reaction are based on green chemistry. At a relatively low temperature compared to the reported work, tetrazole derivatives are prepared in water solvent with good efficiency.\n               "}
{"full_text": "Renewable biomass-based feedstocks are hydrogen deficient and often require the use of external hydrogen to generate green fuels/blends that are compatible with the current fossil fuels [1,2]. Aqueous-phase reforming (APR) is a promising catalytic route to generate hydrogen from dilute aqueous streams containing organic molecules [3,4]. Byproduct and waste streams from food industries or biorefineries [5] often contain dissolved organics usually in the range of 5\u201320 wt.%. One typical example is the aqueous phase of pyrolysis oil which contains a variety of oxygenates such as acids, aldehydes, alcohols, sugars to name a few [2,4].APR is a challenging process for a catalyst due to the drastic hydrothermal conditions (e.g., 225\u2013275 \u00b0C and 35\u201390 bar) used and complex feedstocks utilized, requiring an active and particularly stable catalyst [3]. Typically APR is carried out over supported metal catalysts, e.g., Ni [6] and Pt [7] based catalysts. Critical issues for supports and active metals (e.g., textural properties and phase changes, leaching, and sintering) [8] were often reported for APR catalysts. Recent developments have shown that Pt/C is a promising candidate for the APR of a variety of organic components [9\u201313].Pt is, however, an expensive noble metal [14] and its loading on catalyst should be minimized for commercial application. This is generally achieved by altering Pt size (e.g. high dispersion [15]) and distribution (e.g., egg-shell [16]) employing different supports. Several effective and controllable means, such as varying Pt loading [17], applying various preparation, calcination, and reduction protocols [18\u201320], have been reported.Changing Pt size influences catalyst characteristics, which in turn, affects catalytic performance for APR. Lehnert et al. [21] studied APR of glycerol over Pt/Al2O3 catalyst and suggested that C-C cleavage in oxygenates (promoting the formation of C1 species which can be steam reformed to yield H2 [2]) occurs preferentially on face Pt atoms, which increased with Pt particle size. Kirilin et al. observed a similar trend in turnover frequency (TOF) for different carbon-supported Pt catalysts for APR of xylitol [22]. However, Wawrzetz et al. [17] and Barbelli et al. [18] observed only a slightly increased TOF for Pt/Al2O3 catalysts with Pt size increase from 1.1 to 2.6 nm for APR of glycerol, relating it to the enhanced and simultaneous hydro-deoxygenation reactions consuming hydrogen. Ciftci et al. [23] studied Pt size domain of 1.2\u20134 nm and obtained an optimized performance for Pt size of ca. 2 nm for Pt/C catalysts for APR of glycerol. Chen et al. [24] screened an even wider Pt size range of 1.6\u20135.7 nm for Pt/Al2O3 catalyst for APR of low boiling point fraction of bio-oil and reported an optimized Pt size of 2.6 nm for H2 production. These results are somehow contradictory. Nevertheless, it needs to be noted that in general, Pt size for the fresh catalysts was applied to correlate with catalyst performance.As compared with the widely-investigated Pt size effect [24], the influence of distribution (uniform and egg-shell) of Pt with varied sizes on APR performance has not been reported yet to the best of our knowledge. In order to comprehensively study the effects of Pt size and also Pt distribution, we have applied different preparation protocols (vide infra) to make a variety of Pt catalysts with distinguishable Pt characteristics. Ethylene glycol (EG) was used as a model reactant to evaluate catalyst performance since it is a simple oxygenate with both carbon atoms connected to OH-groups. This allows to estimate the catalyst preference for CC and CO cleavage [25]. Besides, a carbon material was used as the catalyst support in this study, considering the variety of support materials (e.g., Al2O3, SiO2, ZrO2, and TiO2) that been extensively studied for supported Pt catalysts for APR of EG (Table 1\n). Pt/C catalysts show appreciable turnover frequency for H2 production (TOF-H2) compared with the state-of-the-art catalysts, namely Pt/AlO(OH) and Pt/SiO2 (Table 1), both of which have stability problems. Therefore, the development of Pt/C catalyst is a valid argument and should aim to maximize TOF-H2. Since a Sibunit carbon-supported Pt catalyst showed better H2 productivity than other types of carbon materials supported Pt catalysts for APR of xylitol [22], it was used in this study to prepare the Pt/C catalysts. In total, four representative Pt/C catalysts with distinctive Pt characteristics such as small and agglomerated Pt particles, in a uniform fashion or with concentrated Pt particles on the surface in an egg-shell structure, are reported in this contribution. Moreover, the properties of the spent catalysts after 7-h APR of EG were correlated with the catalytic behavior.Sibunit carbon with a particle size of 100\u2212200 \u03bcm was supplied by Boreskov Institute of Catalysis, Russia. H2PtCl6 was supplied by OAO Aurat, Russia. Pt-PVP colloid was prepared by a method published in Ref [33]. Analytical grade Na2CO3, formic acid, and ethylene glycol (EG, >99 %) were supplied by Sigma-Aldrich.Four Pt/C catalysts were prepared by variable methods, which are summarized in Table 2\n. Pt/C-IM and Pt/C-OX catalysts were prepared via incipient wetness impregnation with H2PtCl6 followed by drying in air overnight at 100 \u00b0C. Afterwards, the dried sample was reduced in H2 at 320 \u00b0C for 6 h to produce the Pt/C-IM catalyst. Alternatively, the dried sample was further calcined at 420 \u00b0C for 6 h followed by a reduction in H2 at 700 \u00b0C for 5 h to make the Pt/C-OX catalyst. Pt/C-PR catalyst was prepared by precipitation of H2PtCl6 with Na2CO3 followed by a reduction in formic acid. After drying in air, the dried sample was reduced in H2 at 700 \u00b0C for 5 h. Pt/C-CL catalyst was prepared using a Pt-PVP colloid via wet impregnation. After drying in air, the sample was loaded to the APR reactor (vide infra) and treated in a hot compressed water stream (HCW, 2 mL/min) at 225 \u00b0C and 35 bar for 1 h, in order to remove PVP from the catalyst [20]. Afterwards, the sample was unloaded from the reactor and dried in air.Pt loading was semi-quantitatively analyzed by wavelength dispersive X-ray fluorescence (WDXRF) spectroscopy on S8 Tiger (Bruker) with the powder pellet method. An undiluted sample (ca. 0.5 g) was milled and loaded in a 29-mm die. Specific surface area (SBET) was determined from N2 physisorption measurement at -19,615 \u00b0C on Tristar 3000 (Micromeritics) according to the Brunauer-Emmett-Teller (BET) method [34]. Pt surface area and dispersion were determined by pulse CO chemisorption on ChemiSorb 2750 (Micromeritics). The catalyst was pretreated in He at 200 \u00b0C (5 \u00b0C min\u22121) for 1 h, followed by pulse chemisorption of CO at room temperature. Pt dispersion was calculated by assuming that the adsorbed CO to Pt ratio is 1 [35]. Pt size was measured by high-resolution transmission electron microscopy (TEM) using a CM300ST-FEG (Philips) operated at 300 kV acceleration voltage. The catalyst was ultrasonicated in ethanol, followed by deposition on a carbon-coated copper grid. Approximately 250 particles across 10 spots were counted. The same transmission electron microscope was also used to record TEM images in cross-section (XTEM) images of catalyst grains to measure the size of agglomerated Pt particles. The catalyst particles were embedded in a resin (the details are shown in Supplementary Information, SI), which allows the observation in cross-section in order to locate the Pt nano-particles concentrated on the catalyst surface. Approximately 200 Pt particles for the Pt/C-PR and Pt/C-CL catalysts, and 50 Pt particles for the Pt/C-IM catalyst were counted for analyzing the mean size of the agglomerated Pt particles. Pt content on the catalyst surface was analyzed by X-ray photoelectron spectroscopy (XPS) in a Quantera Scanning X-ray Microprobe (PHI) equipped with an AlK\u03b1 monochromatic X-ray source (1486.6 eV). The catalysts with two different particle sizes of 100\u2013250 \u03bcm (for grains as-prepared) and 20\u221240 \u03bcm (for powder after grinding) were analyzed.Aqueous-phase reforming of ethylene glycol solution (2.5 wt.% in water, feeding rate of 2 mL min\u22121) over the Pt/C catalysts (loading of 1 g) was carried out on a bench-scale continuous-flow fixed bed reactor setup (Fig. 1\n) at 225 \u00b0C and 35 bar for a time on stream (TOS) of 7 h. The details of the experimental setup and procedure, and of the product analyses are given in SI. Catalyst performance was defined and calculated by using Eqs. 1\u20138.\n\n(1)\n\nC\no\nn\nv\ne\nr\ns\ni\no\nn\n\u2009\no\nf\n\u2009\nE\nG\n\u2009\n\n%\n\n=\n(\n1\n-\n\u2009\n\n\nm\no\nl\n\u2009\no\nf\n\u2009\nE\nG\n\u2009\ni\nn\n\u2009\np\nr\no\nd\nu\nc\nt\n\n\nm\no\nl\n\u2009\no\nf\n\u2009\nE\nG\n\u2009\ni\nn\n\u2009\nf\ne\ne\nd\n\n\n)\n\u2009\n\u00d7\n\u2009\n100\n\n\n\n\n\n(2)\n\nC\na\nr\nb\no\nn\n\u2009\ny\ni\ne\nl\nd\n\u2009\no\nf\n\u2009\np\nr\no\nd\nu\nc\nt\n\u2009\n\n%\n\n=\n\n\nm\no\nl\n\u2009\no\nf\n\u2009\nc\na\nr\nb\no\nn\n\u2009\ni\nn\n\u2009\nl\ni\nq\nu\ni\nd\n\u2009\no\nr\n\u2009\ng\na\ns\ne\no\nu\ns\n\u2009\np\nr\no\nd\nu\nc\nt\n\n\nm\no\nl\n\u2009\no\nf\n\u2009\nc\na\nr\nb\no\nn\n\u2009\ni\nn\n\u2009\ni\nn\n\u2009\nf\ne\ne\nd\n\n\n\u2009\n\u00d7\n\u2009\n100\n\n\n\n\n\n(3)\n\nY\ni\ne\nl\nd\n\u2009\no\nf\n\u2009\n\nH\n2\n\n\u2009\n\n%\n\n=\n\u2009\n\n\nm\no\nl\n\u2009\n\nH\n2\n\n\u2009\np\nr\no\nd\nu\nc\ne\nd\n\n\nm\no\nl\n\u2009\nc\na\nr\nb\no\nn\n\u2009\nc\no\nn\nv\ne\nr\nt\ne\nd\n\n\n\u2009\n\u00d7\n\u2009\n\n1\n\nR\nR\n\n\n\u2009\n\u00d7\n\u2009\nX\n\u2009\n(\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n)\n\u2009\n\u00d7\n\u2009\n100\n\n\n\n\n\n(4)\n\nE\nG\n\u2009\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n\u2009\nr\na\nt\ne\n\u2009\n\n\n\n\n\u03bc\nm\no\nl\n\n\nE\nG\n\n\n\u2009\n\nA\n\nP\nt\n\n\n-\n1\n\n\n\u2009\n\n\nm\ni\nn\n\n\n-\n1\n\n\n\n\n=\n\n\n\u03bc\nm\no\nl\n\u2009\no\nf\n\u2009\nE\nG\n\u2009\nc\no\nn\nv\ne\nr\nt\ne\nd\n\n\ns\nu\nr\nf\na\nc\ne\n\u2009\na\nr\ne\na\n\u2009\no\nf\n\u2009\nP\nt\n\u2009\n\u00d7\n\u2009\nT\nO\nS\n\u2009\no\nf\n\u2009\n30\n\u2009\nm\ni\nn\n\n\n\n\n\n\n\n(5)\n\n\nH\n2\n\n\u2009\np\nr\no\nd\nu\nc\nt\ni\no\nn\n\u2009\nr\na\nt\ne\n\u2009\n\n\n\n\n\u03bc\nm\no\nl\n\n\n\nH\n2\n\n\n\n\u2009\n\nA\n\nP\nt\n\n\n-\n1\n\n\n\u2009\n\n\nm\ni\nn\n\n\n-\n1\n\n\n\n\n=\n\n\n\u03bc\nm\no\nl\n\u2009\no\nf\n\u2009\n\nH\n2\n\n\u2009\np\nr\no\nd\nu\nc\ne\nd\n\n\ns\nu\nr\nf\na\nc\ne\n\u2009\na\nr\ne\na\n\u2009\no\nf\n\u2009\nP\nt\n\u2009\n\u00d7\n\u2009\nT\nO\nS\n\u2009\no\nf\n\u2009\n30\n\u2009\nm\ni\nn\n\n\n\n\n\n\n\n(6)\n\nT\nO\nF\n\u2009\nf\no\nr\n\u2009\n\nH\n2\n\n\u2009\np\nr\no\nd\nu\nc\nt\ni\no\nn\n\u2009\n\n\n\n\nm\no\nl\n\n\n\nH\n2\n\n\n\n\u2009\n\n\nm\no\nl\n\n\nP\nt\n\n\n-\n1\n\n\n\u2009\n\n\nm\ni\nn\n\n\n-\n1\n\n\n\n\n=\n\n\nm\no\nl\n\u2009\no\nf\n\u2009\n\nH\n2\n\n\u2009\np\nr\no\nd\nu\nc\ne\nd\n\n\nm\no\nl\n\u2009\no\nf\n\u2009\nP\nt\n\u2009\n\u00d7\n\u2009\nT\nO\nS\n\u2009\no\nf\n\u2009\n30\n\u2009\nm\ni\nn\n\n\n\u2009\n\n\n\n\n\n(7)\n\nS\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n\u2009\nf\no\nr\n\u2009\nc\na\nr\nb\no\nn\n\u2009\ns\np\ne\nc\ni\ne\ns\n=\n\n\nm\no\nl\n\u2009\no\nf\n\u2009\nc\na\nr\nb\no\nn\n\u2009\ni\nn\n\u2009\nl\ni\nq\nu\ni\nd\n\u2009\no\nr\n\u2009\ng\na\ns\ne\no\nu\ns\n\u2009\np\nr\no\nd\nu\nc\nt\n\n\nm\no\nl\n\u2009\no\nf\n\u2009\nc\na\nr\nb\no\nn\n\u2009\ni\nn\n\u2009\ni\nn\n\u2009\nf\ne\ne\nd\n\u2009\n*\n\u2009\nc\no\nn\nv\ne\nr\ns\ni\no\nn\n\u2009\no\nf\n\u2009\nE\nG\n\n\n\u2009\n\u00d7\n\u2009\n100\n\n\n\n\n\n(8)\n\nS\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n\u2009\nf\no\nr\n\u2009\n\nH\n2\n\n=\n\n\nm\no\nl\n\u2009\n\nH\n2\n\n\u2009\np\nr\no\nd\nu\nc\ne\nd\n\n\nm\no\nl\n\u2009\nc\na\nr\nb\no\nn\n\u2009\nc\no\nn\nv\ne\nr\nt\ne\nd\n\n\n\u2009\n\u00d7\n\u2009\n\n1\n\nR\nR\n\n\n\n\n\nVarious methods (Section 2.2 and Table 2) have been applied to prepare the four Pt/C catalysts with distinguishable Pt particle sizes and distributions (viz., with concentrated Pt particles on the surface or in a homogeneous fashion). BET surface areas (Table 2) of fresh Pt/C-IM, Pt/C-OX, and Pt/C-PR catalysts (340 - 372 m2 g\u22121) are relatively close to that of the Sibunit carbon support (350 m2 g\u22121). However, a decreased SBET was observed on the fresh Pt/C-CL catalyst (296 m2 g\u22121), indicating that wet impregnation with the Pt-PVP colloid influenced textural property of the Sibunit carbon though a low amount of Pt (0.7 wt.%, Table 2) was loaded on the Pt/C-CL catalyst. This is most likely related to Pt concentrated on the catalyst surface (vide infra), due to the lower penetration of the Pt colloid into the pores of the support.XPS analyses (Table 2, the corresponding spectra are shown in Figs. S1-S4) of the as-prepared catalyst grains (100\u2013250 \u03bcm) and the after-ground powder (20\u221240 \u03bcm) show that the Pt concentration on the outer shell is higher than in the inner core of the Pt/C-IM, Pt/C-PR, and Pt/C-CL catalysts. Particularly, the difference is extremely large for the latter two catalysts, showing that the Pt concentrations on the outer surface are approximately 53 % (Pt/C-PR catalyst) and 4 times (Pt/C-CL catalyst) higher than the inner ones. Comparatively, the Pt/C-OX catalyst, which was prepared by incipient wetness impregnation followed by calcination and reduction at high temperatures, shows a similar Pt concentration on the outer surface and in the inner core. This might indicate that Pt was relatively homogeneously distributed in the Pt/C-OX catalyst, while Pt was more concentrated on the surface of the Pt/C-PR and Pt/C-CL catalysts.The speculation about concentrated Pt particles on the surface of the Pt/C-PR and Pt/C-CL catalysts is further confirmed by XTEM images of the catalysts as prepared, showing that the Pt particles are more visible on the catalyst grain edge than in the core (Fig. 2\nC and D). XTEM images of the grain cores (Fig. 2C and D, right) display the fairly even distributed small Pt particles (< 3 nm). Comparatively, more concentrated small Pt particles are observed on the grain edges (Fig. 2C and D, left). Besides, agglomerated Pt particles with mean sizes of 11 nm and 24 nm (Table 2) are also present on the grain edges (with an approximate depth of 500 nm) of Pt/C-PR and Pt/C-CL catalysts. These are totally different from the XTEM images of the Pt/C-OX catalyst (Fig. 2B), showing that small Pt particles were evenly distributed on both the edge and in the core. No agglomerated Pt particles are observed on the Pt/C-OX catalyst, confirming the homogeneity of the Pt particles on the catalyst.The uniformly distributed Pt with a small particle size on the Pt/C-OX catalyst is also evidenced by the sharp Pt particle distribution of 1\u20133 nm (Fig. 3B) analyzed by TEM. The broader Pt particle size distributions for the Pt/C-PR (1\u20139 nm, Fig. 3C) and Pt/C-CL (1\u20136 nm, Fig. 3D) catalysts are most likely related to the larger Pt particles present in the outer shell of the Pt/C-PR and Pt/C-CL catalysts.The mean Pt particle sizes analyzed by CO chemisorption (Table 2) and TEM (Table S2) indicate that the Pt/C-IM and Pt/C-OX catalysts have smaller Pt particle sizes compared with the Pt/C-PR and Pt/C-CL catalysts. It needs to be noted that the Pt particle sizes estimated from CO chemisorption and TEM differ significantly, considering that only limited particles were counted from the TEM images (e.g., 200\u2013400, Fig. 3) and CO chemisorption might over-estimate (Pt\u2212CO stoichiometry) Pt surface area. Nevertheless, the above trends in the Pt particle sizes for the four Pt/C catalysts are similar according to these two analyses.As shown above, the catalysts prepared by the incipient wetness impregnation method, viz Pt/C-IM, and Pt/C-OX catalysts, have uniformly distributed small-size Pt particles. This is different from the catalysts prepared by precipitation (viz., Pt/C-PR catalyst) and wet impregnation with Pt colloid (viz., Pt/C-CL catalyst), which have small and also large Pt particles concentrated on the outer shell of the catalyst grains. According to the semi-quantified Pt content (by XPS, Table 2) and the visual Pt distribution (by XTEM, Fig. 2) on the catalyst edge and in the catalyst core for the four Pt/C catalysts investigated, the Pt/C-CL catalyst has the highest degree of Pt concentration on the catalyst surface, followed by the Pt/C-PR and Pt/C-IM catalysts (Table 2). Pt on the Pt/C-OX catalyst is distributed in a more homogeneous fashion as compared with that on the Pt/C-IM catalyst, indicating that high-temperature calcination and reduction enhanced the homogeneity of Pt on the Pt/C catalysts [36].Aqueous-phase reforming of ethylene glycol over the above four Pt/C catalysts were continuously performed on a fixed bed reactor at 225 \u00b0C and 35 bar for 7 h. Catalyst performance over TOS is shown in Fig.4\nA in terms of EG conversion. In general, the initial EG conversion is comparable (e.g., 37.5\u201339.4 %) among the Pt/C catalysts investigated, except for the Pt/C-OX catalyst which shows a relatively lower EG conversion of 26.7 %. All the Pt/C catalysts exhibited excellent stability, evidenced by only a slight drop (ca. 4\u20135 %) in EG conversion after TOS of 3.5 h. Negligible deactivation occurred afterwards, indicating a steady state of the Pt/C catalysts for EG conversion. Accordingly, the products during TOS of 3.5\u20137 h were averaged to evaluate the representative products from APR of EG over the Pt/C catalysts.The excellent total carbon balance closures (e.g., 97\u2013101 %) indicate negligible coke formation during APR of EG over the Pt/C catalysts. The selectivity\u2019s to various products are shown in Table 3\n. The major carbon-related products are gases, which consist of CO, CO2 and CH4 (Fig. 4B). A very small amount of EG was converted to liquid phase products (Fig. 4C) such as methanol, ethanol, acetic acid, glycolaldehyde and larger polyol (e.g., glycerol).The yield of the most interesting product, viz., H2 (Fig. 4B), differs dramatically with the Pt/C catalysts. Pt/C-PR catalyst has the highest H2 yield (26.4 %), followed by Pt/C-IM (24.1 %), Pt/C-CL (20.4 %) and Pt/C-OX (13.7 %) catalysts. This points to the different characteristics of active Pt sites on the four Pt/C catalysts.As discussed above, the Pt/C catalysts evolved to the steady-state after a TOS of 3.5 h (Fig. 4A). To correlate the catalyst characteristics with the catalytic performance during the 3.5\u20137 h TOS period (Fig. 4B and C), the used Pt/C catalysts after continuous-flow APR of EG for 7 h were characterized. The four used Pt/C catalysts showed comparable BET areas (326 - 343 m2 g\u22121, Table 2) with the fresh ones, indicating insignificant changes in catalyst pore structure after 7-h TOS. In addition, the Pt loadings on the fresh and used Pt/C catalysts (Table 2) are similar, showing a negligible loss of Pt under the severe APR reaction conditions.However, Pt particle sizes are larger on the used Pt/C catalysts than on the fresh ones according to both CO chemisorption and TEM analyses (Tables 2 and S2, and Fig. 3). The growth of Pt particles during APR reactions was often observed on supported Pt catalysts, e.g., Pt/C [13] and Pt/Al2O3 [8]. As a consequence, the Pt particle size distributions for the used Pt/C catalysts were broadened (Fig. 3), which is particularly significant for the Pt/C-PR catalyst (Fig. 3C). The mean Pt particle size on the Pt/C-PR catalyst was dramatically increased from 3.2 to 8.3 nm as measured by TEM (Table S2), and from 4.2 to 10.7 nm as measured by CO chemisorption (Table 2). Comparatively, the Pt/C-OX and Pt/C-CL catalysts show smaller changes on the Pt particle size. For the latter catalyst, the high-degree Pt concentration on catalyst surface with agglomerated Pt particles (Section 3.1) might have resistance to a further Pt agglomeration [37], which is reflected by the slightly increased Pt particle size (Table 2) from 24 nm for the fresh Pt/C-CL catalyst (Fig. 2D) to 30 nm for the used one (Fig. 5\nC). Besides, the preparation method for the Pt/C-CL catalyst also has influence on the stability, e.g., by hydrothermal treatment to remove PVP and to stabilize the nanoparticles on the support [20]. Whereas for the Pt/C-OX catalyst, the stability of the Pt particle size might be related to the high-temperature calcination and the reduction enhancing Pt and C interaction [36]. As such, a further check of the presence of the agglomerated Pt particles on the used Pt/C-OX catalyst by XTEM was not carried out, considering that no agglomerated Pt particles presented on the fresh catalyst (Fig. 2B) as well.XTEM images of the used Pt/C-PR catalyst (Fig. 5B) show larger Pt particles (e.g., 20\u201340 nm) on the catalyst edge as compared with the fresh catalyst (Fig. 2C), resulting in a nearly doubled Pt particle size (Table 2). This is in good agreement with the change on mean Pt particle size (by TEM (Table S2) and CO chemisorption (Table 2)) on the Pt/C-PR catalyst after the APR reaction. Similarly, agglomerated Pt particles with a mean size of 17 nm (Table 2) were also formed on the used Pt/C-IM catalyst edge (Fig. 5A), in line with the increased mean Pt particle size from 2.7 nm (for the fresh catalyst, by CO chemisorption, Table 2) to 8.8 nm (for the used catalyst, Table 2). It needs to be noted here that no agglomerated Pt particles are observed in the core of the catalyst (Fig. 5-right), indicating the Pt agglomeration mainly took place on the surface of the Pt/C catalyst under APR conditions.It was demonstrated above that a stable catalytic performance in APR of EG in terms of EG conversion and H2 production was obtained over Pt/C catalysts, which were prepared by a different method in order to alter Pt size and Pt distribution on a Sibunit carbon support. In this contribution, we have used the diluted solution to investigate the relationship between APR performance and catalyst characteristics. For such a diluted stream, the industrial implementation of APR should be further considered, e.g., the economic feature related to the energy consumption for heating the H2O.To recall, the Pt/C catalysts prepared by a general method as incipient wetness impregnation (viz., Pt/C-IM and Pt/C-OX catalysts), have evenly distributed Pt particles with small sizes. A high-temperature treatment, e.g. calcination followed by reduction, was applied to strengthen the interaction between Pt and carbon support. As a consequence, the Pt/C-OX catalyst showed much better stability on Pt size and Pt distribution under the severe APR conditions than the Pt/C-IM catalyst having agglomerated Pt particles on catalyst surface after 7-h APR of EG. Alternatively, the Pt/C catalysts prepared by precipitation method (Pt/C-PR catalyst) and a more novel method of impregnation of pre-prepared Pt colloid (Pt/C-CL catalyst) obtained small Pt particles, as well as agglomerated Pt particles concentrated on the catalyst grain edge. It seems that the inhomogeneous Pt distribution formed on the surface of Pt/C-PR and Pt/C-CL catalysts, and the degree of Pt concentration on the surface of the latter is higher than that of the former. Compared with the Pt/C-CL catalyst, the fresh Pt/C-PR catalyst has more amount of agglomerated Pt particles with a smaller size, resulting in a bigger mean Pt particle size (Table 2). However, these small Pt particles on the Pt/C-PR catalyst grew faster than those large Pt particles on the Pt/C-CL catalyst under the APR reaction conditions. As such, the mean Pt particle size for the Pt/C-PR catalyst increased remarkably, while only a slightly increased mean Pt particle size was observed for the Pt/C-CL catalyst.Since all the Pt/C catalysts were prepared using the same Sibunit carbon support, any difference observed in the chemistry, e.g., product distribution, over different Pt/C catalysts (Fig. 4) should be related to Pt characteristics, e.g., Pt particle size and its distribution. In order to properly correlate the catalyst performance with the catalyst characteristics, the in-situ characterizations of the catalyst during APR is required, e.g., by an in-situ attenuated total reflectance Fourier transform infrared (ATR-IR) technique [26]. However, this is very challenging for Pt/C catalysts, due to the fact that the refractive index of carbon and the internal reflection element (ZnSe) is too similar to obtain ATR-IR spectra for carbon-supported catalysts. The fresh catalyst might change greatly under APR conditions even after a short TOS [38], leading to an inappropriate relationship between initial catalyst performance with fresh catalyst characteristics. Considering that the Pt/C catalysts evolved to a relatively steady state after TOS of 3.5 h (Section 3.2), it might be assumed that Pt/C catalyst characteristics remain stable during TOS of 3.5\u20137 h. Therefore, the averaged EG conversion (Fig. 4A) and H2 production (Fig. 4B) during TOS of 3.5\u20137 h, and the characteristics of Pt on the used Pt/C catalysts (Table 2) after TOS of 7 h were used to calculate reaction rates by using Eqs. 4 \u2013 6. In addition, Pt can be taken as metallic Pt during the APR reactions, considering that the pre-reduction of the Pt/C catalysts was performed at temperatures higher than the reduction temperature of PtOx for Sibunit carbon supported Pt catalysts (e.g., Tmax of 125 \u00b0C [22]). Even though there might be a very small fraction of PtOx species due to the partial oxidation during the storage and loading to the reactor [22], they would probably be reduced by the H2 formed during APR at a reaction temperature of 225 \u00b0C. In order to study the effect of Pt size on catalyst performance, EG conversion and H2 production rates based on the available Pt surface area (\u03bcmolEG(or H2) APt\n\u22121 min\u22121) are shown in Fig. 6\nA. The mean Pt particle size (Fig. 6A) and the mean size for the agglomerated Pt particles (Fig. 6B) were analyzed by CO chemisorption and XTEM, separately.It is interesting to observe that the rates for both EG conversion and H2 production increased linearly with the increased Pt particle sizes. Comparatively, the sensitivity to Pt particle size for H2 production rate is higher than for an EG conversion rate, as indicated by the slopes of the fitted lines in Fig. 6A. This result is consistent with that reported by Lehnert et al. [21], who also observed a higher H2 production from APR of glycerol over Pt/Al2O3 catalysts with a bigger Pt particle size. This is most likely related to the enhanced C-C cleavage of oxygenates on more Pt surface forming H2, in turn competing for C-O cleavage reaction yielding low hydrocarbons [2]. The extremely low yields of C1 and C2 hydrocarbons (Fig. 4B) also confirm this.It needs to be highlighted here that the mean Pt particle sizes on the Pt/C catalysts in this study are quite big (e.g., 3\u201311 nm in Fig. 6A), related to the presence of large Pt particles. The correlations between Pt particle size and the rates for EG conversion and H2 production (Fig. 6-B) suggest that a mean size for agglomerated Pt particles of ca. 20.7 nm is the most suitable. There is a trade-off of the size of the agglomerated Pt particles for an optimal H2 production rate over Pt/C catalysts, due to the fact that the number of exposed surface Pt atoms continues to decrease as the size of agglomerated Pt particles increases. As a consequence, the Pt/C-PR catalyst, which has a number of Pt particles with small size concentrated on the catalyst grain edge, has the highest TOF for H2 production of 248 molH2 molPt\n\u22121 min\u22121 (Fig. 7\n). Comparatively, the Pt/C-CL catalyst of which the level of Pt concentration on the surface is the highest has a much lower TOF-H2 (100 molH2 molPt\n\u22121 min\u22121, Fig. 7), due to the presence of Pt particles with a large size. For the Pt/C-IM catalyst, which has the Pt particles relatively homogeneously distributed both on the grain edge and in the core of the catalyst, has a much higher TOF-H2 (78 molH2 molPt\n\u22121 min\u22121, Fig. 7) than the Pt/C-OX catalyst (18 molH2 molPt\n\u22121 min\u22121, Fig. 7). This is obviously related to the bigger mean Pt particle size for the Pt/C-IM catalyst compared with the Pt/C-OX catalyst.Of great interest is that the highest TOF-H2 (248 molH2 molPt\n\u22121 min\u22121) obtained on the Pt/C-PR catalyst in this study is much higher than those reported Pt/C catalysts (Table 1) by Shabaker et al. (7.5 molH2 molPt\n\u22121 min\u22121) [28] and Kim et al. (103 molH2 molPt\n\u22121 min\u22121) [32], representing the best performance of Pt/C for APR of EG to bio-H2. Furthermore, what is significant is that the TOF-H2 of Pt/C-PR is close to the top two catalysts (viz., Pt/AlO(OH) catalyst with a TOF of 300 molH2 molPt\n\u22121 min\u22121 [26] and Pt/SiO2 catalyst with a TOF of 275 molH2 molPt\n\u22121 min\u22121 [27]) developed for APR of EG so far.Stability of Pt/SiO2 catalyst, related to the leaching of silica under APR conditions, is a critical issue for a long-term practical application [39]. Pt/AlO(OH) catalyst might have a high tendency for coke formation during APR [40] due to the acidity of AlO(OH) support [41]. It has been demonstrated in this study that the Sibunit carbon is stable under APR conditions and coking on Pt/C catalyst is negligible during a continuous 7-h APR of EG (Section 3.2). Besides, using carbon as a carrier for supported Pt catalysts ensures that it is easy to harvest Pt for recycling after usage by burning [42]. Having a high intrinsic activity for hydrogen production and an excellent stability for long-term operation, Pt/C catalyst could definitely be an excellent catalyst for APR of oxygenates for bio-H2 production.Pt concentrated on the catalyst surface with small-size Pt particles on Pt/C catalyst is advantageous for APR of a small molecule (viz., EG), which might also be significant for larger oxygenates. Further exploitation of Pt/C catalyst for APR of the aqueous phase of pyrolysis oil or other waste aqueous oxygenate streams is thus recommended. On the other hand, a large amount of CO (e.g., carbon yield of 18\u201325 %, Fig. 4B) were also formed during APR of EG over Pt/C catalysts. This indicates an inefficient water-gas shift (WGS, CO + H2O \u2192 H2 + CO2) reaction, in line with the low yield of CO2 (Fig. 4B). Therefore, bio-H2 production over Pt/C catalysts via APR could be further improved, e.g., by adding a second metal such as Ni to enhance WGS reaction (e.g., Pt-Ni/Al2O3 catalyst for APR of EG [43]).Catalyst preparation protocols, including the incorporation of the metal precursor (e.g., incipient wetness impregnation, precipitation, and impregnation of Pt colloid) and further treatment (e.g., high-temperature calcination and reduction), affect Pt size and Pt distribution (homogeneous Pt distribution and with concentrated Pt particles on the surface).Pt/C catalysts showed excellent H2 yields (up to 24.1 %) for aqueous-phase reforming of ethylene glycol and excellent catalyst stabilities with a slight drop (ca. 4\u20135 %) in EG conversion (ca. 37.5\u201339.4 %) after 3.5-h TOS. The characteristics of the used catalysts after 7-h APR of EG, which were in a steady-state, were used to correlate the catalyst performance. The linear relationships between mean Pt particle size (in a range of 3\u201311 nm investigated) and the rates for EG conversion and H2 production were observed.Pt/C-PR catalyst, which was prepared by the precipitation method, had small Pt particles distributed in the catalyst as well as large Pt particles concentrated on the catalyst grain edge after TOS of 7 h. Pt/C-PR catalyst showed the highest turnover frequency for H2 production (TOF-H2 of 248 molH2 molPt\n\u22121 min\u22121) among the four Pt/C catalysts investigated. This was attributed to the preferred Pt particles concentrated on the catalyst surface with the biggest mean Pt particle size (ca. 10.7 nm) and the appropriate mean size of agglomerated Pt particles (ca. 21 nm). This superb TOF-H2 and the excellent stability of the Pt/C catalyst make it promising for APR of EG as compared with the state-of-the-art Pt catalysts, viz., Pt/AlO(OH) (TOF-H2 of 300 molH2 molPt\n\u22121 min\u22121) and Pt/SiO2 (TOF-H2 of 275 molH2 molPt\n\u22121 min\u22121) catalysts. Pt/C catalysts are therefore recommended for APR of other model oxygenates (e.g., hydroxyacetone) present in waste streams and also APR of real waste streams (e.g., the aqueous phase of pyrolysis oil) to make renewable and green H2.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.\nA.K.K. Vikla: Investigation, Conceptualization, Methodology, Validation, Writing - original draft, Writing - review & editing. I. Simakova: Resources, Validation, Supervision, Writing - review & editing. Y. Demidova: Investigation, Resources, Validation. E.G. Keim: Investigation, Resources, Validation, Writing - review & editing. L. Calvo: Investigation, Resources, Validation. M.A. Gilarranz: Resources, Supervision, Writing - review & editing. Songbo He: Conceptualization, Writing - original draft, Supervision, Writing - review & editing. K. Seshan: Supervision, Writing - review & editing, Funding acquisition.The research was funded by European Union Seventh Framework Programme (FP7/2007-2013) within the project SusFuelCat under grant agreement No. 310490. Dr. Vikla would like to thank Ing. B. Geerdink for his technical and emotional support when carrying out the APR experiments, and also Ing. Benno Knaken for his expertise in maintaining the high-pressure reactors. Prof. L. Lefferts is thanked for preliminary discussions. Besides, Mrs. K. Altena-Schildkamp is thanked for BET and CO chemisorption measurements. Mr. Tom Velthuizen is thanked for XRF characterization, and Mr. Gerard Kip for XPS analysis at MESA + NanoLab. IS acknowledges with support from Ministry of Science and Higher Education of the Russian Federation.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2020.117963.The following are Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  Pt/C catalysts with varied Pt sizes and distributions were investigated for aqueous-phase reforming (APR) of ethylene glycol (EG) to H2. APR experiments were performed on a continuous-flow fixed bed reactor with a catalyst loading of 1 g and EG feeding of 120 mL h\u22121 at 225 \u00b0C and 35 bar for 7 h. The fresh and used Pt/C catalysts were characterized by XRF, BET, CO chemisorption, TEM, XTEM, and XPS. Catalyst preparation protocols changed Pt characteristics on Pt/C catalysts, leading to a distinguishable H2 production. The rates for EG conversion and H2 production increased linearly with mean Pt size (3\u201311 nm), while having a volcano relationship with the mean size of agglomerated Pt particles (17\u201330 nm). Pt with concentrated Pt particles on surface of Pt/C catalysts was more preferable for APR of EG than the homogeneously distributed in catalysts. Optimal performance was obtained over a Pt/C-PR catalyst, which was prepared by precipitation method, showing a superb turnover frequency of 248 molH2 molPt\n                     \u22121 min\u22121 for H2 production from EG in APR. Besides, Pt/C catalysts also showed excellent stability. These results have shown the promise of Pt/C catalyst for APR of EG, which can be extended for bio-H2 production via APR of biomass-derived oxygenates in waste streams.\n               "}
{"full_text": "Chemical looping gasification (CLG) is recognized as a novel technology that has advantages in energy utilization and syngas quality improvement [1,2]. Compared with conventional gasification, CLG replaced gaseous oxygen with an oxygen carrier (OC). The OCcommonly consists of a metal oxide, thus, the product, syngas, will not be diluted in a nitrogen source from air. On the other hand, reduced metals, such as Ni and Fe, have function in tar cracking catalysis. A lower reaction temperature also reduces the generation of NOx from SO2\n[3]. Therefore, CLG is considered more appropriate for organic solid waste management or other raw materials with complicated components [4].Wast pulp (WP) is the main solid waste from the paper industry, and the reuse of WP is an important problem for the optimization of the whole industry chain of the paper industry. The main constituent of WP is cellulose, it possesses rich resource and energy potential. However, the plentiful volatiles cause considerable tar generation in the gasification process. Therefore, CLG is a flexible and efficient technology for WP management.In the CLG process, the OC transfers oxygen and heat while catalysing the pyrolysis of raw materials. An ideal OC should have good activity in redox reactions, good mechanical strength and high thermodynamic stability. Cost is also the important factor to be considered. Owing to the complicated components of WP or other organic solid waste, the inactivation of the OC source from sintering and inert component generation caused by ash and other pollutants must be considered seriously. On the other hand, cheap ore and solid waste, such as slag and sludge, are rich in transition metal oxides and hence have the potential for use as low-cost OCs in CLG processes [5,6]. Guo et al. [7] tested the performance of natural copper ore and haematite as an OC for biomass CLG, and the results indicated that the natural ore has better mechanical behaviour and thermal stability than pure CuO or Fe2O3. Schmitz et al. [8,9] used manganese ore as an OC to assist the oxidation of biomass char in the chemical looping process. The results showed that manganese ore has excellent redox behaviour, but the mechanical strength needs to be strengthened. Deng et al. [10] explored the behaviour of copper slag in municipal sludge CLG and found that alkali and alkaline earth elements such as K and Ca can improve CLG activity. Chen et al. [11] investigated bauxite residual for H2-rich syngas production via the CLG process and proved that the bauxite residual could be a high-reactivity OC in the CLG process. Moghtaderi et al. [12] compared concrete waste and CaO behaviours in syngas upgrading in CLG of biomass and revealed that concrete waste has a longer cycle life than pure CaO. The research of Yang et al. [13] revealed the inactivity mechanism of phosphogypsum OC in the CLG process, in which the generation of transition metal silicate gradually reduced the activity of the OC. Although the studies listed above showed the potential of natural minerals in CLG, the low redox activity and cycle performance restrict the practical application of low-cost natural minerals. Introducing other ingredients to modify natural minerals has been recognized as an efficiency solution. Ryden et al. [14] used NiO as an additive to modify waste products from the steel industry and proved that only a small amount of NiO can improve the activity for Fe-based OCs. Sun et al. [15] explored the behaviour of manganese-modified ilmenite as an OC in biomass CLG and found that the generation of Fe-Mn oxide would improve the activity of the OC. The research of Bao et al. [16,17] revealed that heterogeneous ions such as K+ and Ca2+ promoted the redox reaction of iron ore. Tian et al. [5] used cement to modify copper ore, which not only promoted the gasification activity of copper ore but also improved the thermal stability and mechanical behaviour. Otomo et al. [18] explored the effect of calcium nitrate melt infiltration for biomass chemical looping process and found that the calcium salt improve the behaviour of ilmenite obviously. The research of Yan et al. [19] revealed that heterogeneous ions enhance fuel conversion in chemical looping process with red mud as OC. In summary, an economic effect and high performance can be achieved by introducing a small amount of artificial ingredients.Aiming at cellulose solid waste, we developed multimetal composite OCs [20\u201322]. The results revealed that the Ca-based material has an obvious catalytic function in the reforming of H2O and tar and char decomposition. Base on Ni-based material was active in tar cracking catalysis, the Ca-Fe OC modified by NiO achieved complete conversion of cellulose to syngas at 850\u00a0\u00b0C [21]. Therefore, the complexes of Ca, Fe and Ni should be considered as potential OCs for WP management. Ni-containing electroplating sludge (NES) is an important solid waste for the electroplating industry. The abundant heavy metal content gives it a greater potential to become an high-efficiency OC. Our previous research [23] explored the reactivity of NES as an OC in dyeing sludge CLG. A carbon conversion efficiency of 81\u00a0% and 0.37 Nm3/kg syngas could be obtained at 850\u00a0\u00b0C. Considering that NES is rich in Ni, Fe, and Ca, NES should be a potential OC for WP gasification. However, the relatively lower carbon conversion efficiency and H2 yield revealed that NES might be incompetent for tar cracking and inappropriate for raw materials with high volatility, such as cellulose or WP. The Ni in NES is combined with other metals and forming a complexed compound such as NixFeyOz\n[24,25]. It has more reduced steps and makes it more difficult for the Ni element to form as Ni0 to exert its catalytic function. Hence, NiO was introduced to modify NES to promote tar cracking and syngas generation in this study. Ni-based materials should be placed on the outside of OC particles to easily contact reactants and performed as catalysts for tar cracking and H2 release [26,27]. The immersion method has been proven to be an effective method to realize the above assumptions [21]. Thereby, the NES modified by NiO via the immersion method, i.e., NiO-modified NES (NNES), was used as the OC in WP CLG in this research. The reactivity was explored under different working conditions in a fixed bed, and a few characterizations were conducted on the OC to reveal the reaction characteristics of the OC.WP was collected from a paper factory in Guangzhou, and the received WP was predried. The sample was dried at 105\u00a0\u00b0C for 20\u00a0h to obtain a usable material. Table 1\n shows the proximate and ultimate analyses of the usable material. The high content of volatiles and the molar ratio of C/H/O (\u223c6:11:5.5) were similar tocellulose (C6H10O5), itcorroborated that the main composition of WP was cellulose. Under ideal circumstances, all C should be formed as CO and H should be formed as H2 after gasification, therefore the ideal syngas yield should be 1.51 Nm3/kg. Notably, small amounts of ash and N could be found in WP, it reveals that other compositions should exist and may influence the pyrolysis and gasification behaviour of WP, resulting in different characteristics from pure cellulose. Compared with pure cellulose [20], the fixed carbon increase from 5\u00a0% to 7% to about 10\u00a0%, and the content of H2 in thermolysis gas was much higher than pure cellulose. Noticeably, the small amount of nitrogen may come from the biomass components such as proteins. In CLG process, the raw material is mainly under reducing atmosphere, and the generation of pollution such as NOx could be avoided.NES was collected from an electroplating factory in Guangzhou. The raw NES was dried at 105\u00a0\u00b0C for more than 48\u00a0h, calcined at 950\u00a0\u00b0C for 6\u00a0h, ground and sieved to 100 mesh as a useable sample. Table 2\n shows the element composition of the NES (measured by X-ray fluorescence (XRF)). It can be seen that the main composition of NES is oxides of Fe, Ni and Ca, and the mole ratio of Fe/Ni/Ca is approximately 2.8:2:1. The content of S is approximately 3.42\u00a0wt%, and it may derved from SO4\n2-. Notably, the content of Mg is also higher than 1\u00a0wt%. The existence of Mg2+ will promote the generation of syngas and reduce the activation energy of pyrolysis [28], and benefit for WP gasification.NNES was synthesised by the immersion method as shown in Fig. 1\n. Briefly, NES was placed into a nickel nitrate (Ni(NO3)2\u00b76H2O, AR) solution at the set concentration (in completed NNES, the mass ratio of NiO and NES was 0, 0.1, 0.5, and 1.0) with vigorous stirring, and the as-formed samples were aged for 24\u00a0h, dried at 105\u00a0\u00b0C for 20\u00a0h and calcined at 950\u00a0\u00b0C for 6\u00a0h to obtain NNES.The WP CLG test was conducted in a fixed bed experimental device as shown in Fig. 2\n. The fixed bed experimental device consisted of a quartz tube (length: 800\u00a0mm and inner diameter: 17\u00a0mm), temperature control device, gas control device, water injection pump, tar/water filter devices and gas collection bag. The tar/water filter devices were three scrubbers in series. The first two scrubbers were filled with ethylene glycol (C2H6O2, AR) and placed in an ice water bath. The third scrubber was filled with allochroic silica gel. In a typical experiment, the mixed OC and WP were placed in a quartz hanging basket (height: 50\u00a0mm and inner diameter: 3\u00a0mm) and hung in a nonheated area in advance. Then, 20\u00a0mL/min nitrogen was introduced into the quartz tube, and the heated area was heated to the designated temperature. Deionized water was injected into the quartz tube early to create a steam atmosphere. With the start of the experiment, the hanging basket was sent to the centre of the heat area of the quartz tube. Meanwhile, gas bags were used as collection devices to collect the gas flowing through the fixed bed device. Each experiment lasted 1\u00a0h. In the cycle performance experiment after the gasification process, 100\u00a0mL/min air was introduced into the reactor, which lasted 30\u00a0min.The gas was analysed by refinery gas chromatography (Agilent 7890A), and the yield of gaseous products was calculated as follows:\n\n(1)\n\n\n\nY\ni\n\n=\n\n\n60\n\n\n\nq\n\nN\n2\n\n\nC\n\ni\n\n\n\n\nC\n\nN\n2\n\n\n\nm\n\nC\nE\n\n\n\n\n\n\n\nwhere \n\nY\ni\n\n represents the yield of the gaseous product (Nm3/kg), \ni\n represents gaseous products such as CO, CO2, H2, CH4, and C2H4, \n\nC\ni\n\n represents the concentration of gaseous products, \n\nC\n\nN\n2\n\n\n represents the concentration of nitrogen, \n\nq\n\nN\n2\n\n\n represents the flow of nitrogen (Nm3/min), and \n\nm\n\nC\nE\n\n\n represents the mass of CE (kg).The total syngas yield was calculated as follows:\n\n(2)\n\n\n\nY\n\nt\no\nt\na\nl\n\n\n=\n\u2211\n\nY\ni\n\n\n\n\nwhere \n\nY\n\nt\no\nt\na\nl\n\n\n represents the total gas yield (Nm3/kg).The effective syngas yield is the yield of flammable gas and is calculated as follows:\n\n(3)\n\n\n\nY\n\ne\nf\nf\ne\nc\nt\ni\nv\ne\n\n\n=\n\nY\n\nt\no\nt\na\nl\n\n\n-\n\nY\n\n\nC\nO\n\n2\n\n\n\n\n\nwhere \n\nY\n\ne\nf\nf\ne\nc\nt\ni\nv\ne\n\n\n represents the effective syngas yield and \n\nY\n\n\nC\nO\n\n2\n\n\n represents the CO2 yield.The carbon conversion efficiency was calculated as follows:\n\n(4)\n\n\n\n\u03b7\nc\n\n=\n\n\n12\n\u2211\n\nj\ni\n\n\nY\ni\n\n\n\n\nV\nm\n\n\n\nm\n\ncarbon\n\n\n\nm\n\nCE\n\n\n\n\n\n\u00b7\n100\n%\n\n\n\nwhere \n\n\u03b7\nc\n\n represents the carbon conversion efficiency, \n\nj\ni\n\n represents the number of carbon atoms in molecule \ni\n, \n\nV\nm\n\n is the gas molar volume (Nm3/mol), \n\nm\n\nc\na\nr\nb\no\nn\n\n\n represents the mass of carbon in CE (kg), and \n\nm\n\nC\nE\n\n\n represents the mass of cellulose.The lower heating value (LHV, MJ/Nm3) was calculated as follows:\n\n(5)\n\n\nLHV\n=\n\n\n10.82\n\nY\n\nH\n2\n\n\n+\n12.54\n\nY\n\nC\nO\n\n\n+\n35.88\n\nY\n\n\nC\nH\n\n4\n\n\n+\n59.44\n\nY\n\n\n\nC\n2\n\nH\n\n4\n\n\n\n\nY\n\nt\no\nt\na\nl\n\n\n\n\n\n\n\nThe OC was characterized with X-ray diffraction (XRD, Panalytical X\u2019pert Pro diffractometer) and scanning electron microscopy (SEM, Hitachi SU70 instrument) with energy dispersive spectroscopy to reveal the change in NNES in the CLG process.Considering the environmental toxicity and cost, the addition of NiO should be as little as possible. Based on this, we explored the effect of the addition contents of NiO in the WP CLG process, and the results are shown in Fig. 1. With the addition of NiO, the H2 yield was obviously improved, which caused the growth of the total and effective syngas yields and H2/CO. Especially for the sample with mN/mNES\u00a0=\u00a00.1, the carbon conversion efficiency achieved is approximately 85\u00a0% at 850\u00a0\u00b0C, the total syngas yield reached 1.19 Nm3/kg (79\u00a0% of ideal syngas yield) and H2/CO was 1.2 at 850\u00a0\u00b0C without steam addition. Compared with the NES, the total syngas yield increased by approximately 29\u00a0%, and the H2 yield increased by approximately 112\u00a0%, which shows the excellent activity of NiO addition on H2 release and tar or char pyrolysis. However, the carbon conversion efficiency did not increase with NiO addition. This phenomenon indicates that NES already has excellent tar and char gasification activity, and the independent NiO shows no obvious enhancement in tar or char cracking compared to the Ni-Fe complex oxide [29\u201333]. Noticeably, as mN/m NES increased to 0.5 or higher, the syngas yield, carbon conversion efficiency and H2/CO markedly decreased. The results revealed that NiO might cover the surface of OC grains and hinder the synergy of NiO and NES. Therefore, the small amount of NiO addition is also based noon the consideration of OC activity maximization. According to Fig. 3\n, when the best syngas yield and H2/CO were obtained, the negative impact of NiO on carbon conversion efficiency can be ignored. Hence, mN/mNES\u00a0=\u00a00.1 was used as the standard in further experiments, and NNES refers to samples with mN/mNES\u00a0=\u00a00.1 in the following unless otherwise specified.The effect of the added amount of OC is shown in Fig. 4\n. Compared with WP pyrolysis in N2 atmosphere, the addition of NNES effectively promotes gasification. The total syngas yield improved by approximately 60\u00a0% from 0.75 Nm3/kg (50\u00a0% of ideal syngas yield) to 1.19 Nm3/kg, the effective syngas yield increased approximately 42\u00a0% from 0.68 Nm3/kg (45\u00a0% of ideal syngas yield) to 0.96 Nm3/kg (64\u00a0% of ideal syngas yield), and the carbon conversion efficiency increased from 72\u00a0% to 84\u00a0%. The increase was mainly from the increase in CO2 and H2, which proved the activity of NNES on oxygen release and volatile reforming assistance. Markably, when mOC/mWP was higher than 1.0, the syngas yield decreased obviously. It can be due to the decrease in the H2 and CO yield. However, the CO2 yield increased slightly. The result reveals that the excessive addition of NNES would cause deeper oxidation of syngas and H2 and CO would be further oxidized to H2O and CO2, causing a decline in the syngas yield. Therefore, mOC/mWP\u00a0=\u00a01 should be the appropriate ratio for WP CLG.Steam is universally used as an assistant gasification agent in gasification processes [34] to improve the carbon conversion efficiency and H2 yield. In the CLG process, the synergy of steam and OC is the focus for researchers. With steam addition, the process of organic compound gasification can follow the following path:\n\n(R1)\n\n\n\nC\nn\n\n\nH\nm\n\n\nO\nl\n\n+\n\nH\n2\n\nO\n\u2192\n\n\nC\nO\n\nk\n\n+\n\nH\n2\n\n\n\n\n\n\n\n(R2)\n\n\n\nC\nn\n\n\nH\nm\n\n\nO\nl\n\n+\nO\nC\n\n(\no\nx\ni\nd\ni\nz\ne\nd\n)\n\n\u2192\n\n\nC\nO\n\nk\n\n+\nO\nC\n\n(\nr\ne\nd\nu\nc\ne\nd\n)\n\n\n\n\n\n\n\n(R3)\n\n\n\nC\nn\n\n\nH\nm\n\n\nO\nl\n\n\u2192\n\n\na\nC\nO\n\nk\n\n+\n\n\nb\nH\n\n2\n\n+\n\nC\n\nn\n-\na\n\n\n\nH\n\nm\n-\n2\nb\n\n\n\nO\n\nl\n-\na\nk\n\n\n\n\n\n\n\n\n(R4)\n\n\n\nH\n2\n\nO\n+\nC\nO\n\u2192\n\nH\n2\n\n+\n\n\nC\nO\n\n2\n\n\n\n\n\n\n\n(R5)\n\n\n\nH\n2\n\nO\n+\nO\nC\n\n(\nr\ne\nd\nu\nc\ne\nd\n)\n\n\u2192\n\nH\n2\n\n+\nO\nC\n\n(\no\nx\ni\nd\ni\nz\ne\nd\n)\n\n\n\n\n\n\n\n(R6)\n\n\nC\nO\n+\nO\nC\n\n\no\nx\ni\nd\ni\nz\ne\nd\n\n\n\u2192\n\n\nC\nO\n\n2\n\n+\n\nO\nC\n\n(\nr\ne\nd\nu\nc\ne\nd\n)\n\n\n\n\nwhere a, b, and l\u00a0=\u00a00,1,2,3,\u2026\u2026; n and m\u00a0=\u00a01,2,3,\u2026\u2026; \n\n\nC\nn\n\n\nH\nm\n\n\nO\nl\n\n\n refers to the raw material, i.e., tar or char; and k\u00a0=\u00a01,2.As shown in R1-R6, the addition of H2O improves the generation of CO2 and H2. The phenomenon is exhibited in Fig. 5\n. The yield of CO2 and H2 increased obviously with the amount of added water; but the CO yield decreased, which corresponds to R4-R6. The carbon conversion efficiency increased from 84\u00a0% to 90\u00a0% with steam addition, which indicates that the synergy of H2O and OC promotes the gasification of tar or char. Notably, H2/CO increased from 1.2 to 3.6 gradually when the water flow increased from 0 to 2.4\u00a0mL/g(WP) but remained at 3.6 when the water addition flow was higher than 2.4\u00a0mL/g(WP). The results reveal that H2/CO of produced syngas can be adjusted by injecting water at below 2.4\u00a0mL/g(WP); however, excess steam addition has no noticeable effect on the products. Considering the energy and resource savings, 2.4\u00a0mL/g(WP) should be the proper water addition flow for the generation of hydrogen-rich syngas.\nFig. 6\n shows the effect of temperature. Fig. 6 (a) and (b) show that when the temperature increased from 800\u00a0\u00b0C to 850\u00a0\u00b0C, the total and effective syngas yields and carbon conversion efficiency increased sharply, which might be due to oxygen release from the Fe-based material [35]. When the temperature was higher than 850\u00a0\u00b0C, the temperature has little effect on the carbon conversion efficiency. However, the H2 yield decreased sharply when the temperature exceeded 850\u00a0\u00b0C, which might be due to further oxidization of H2. Noticeably, the variation of H2/CO at high temperature indicates that the temperature should be the essential factor for syngas ingredient adjustment.The performance of the NNES in a steam atmosphere was different from that in a dry atmosphere. According to Fig. 6 (c) and (d), the total and effective syngas yields increased slightly with increasing temperature. The carbon conversion efficiency was similar at different temperatures in a steam atmosphere. However, a higher temperature is beneficial to H2O reforming (R4-R6). Thereby, H2/CO improved obviously as the temperature increased. Based on product quality and energy savings, 850\u00a0\u00b0C should be the appropriate temperature for WP CLG with NNES as the OC.The redox cycling performance of the NNES in a steam atmosphere at 850\u00a0\u00b0C is shown in Fig. 7\n. According to the curves of the total and effective syngas yields, the total syngas yield fluctuates around approximately 1.45 Nm3/kg, and the effective syngas yield fluctuates approximately 1.20 Nm3/kg. The activity of NNES for WP gasification does not decline after 10 redox cycles, which reveals the excellent cyclic behaviour of NNES, especially for H2O reforming. However, the carbon conversion efficiency declined by approximately 10\u00a0% from approximately 95\u00a0% to 85\u00a0%, which indicates that the tar cracking activity of NNES decreased gradually after the redox cycles. The inactivation of NNES might be due to agglomeration and subsidence of surface-active components, the OC characterization presented in Section 3.2 will corroborate it. Notably, the decline in carbon conversion efficiency indicates a decrease in CO yield, and results in the growth of H2/CO after redox cycles.Overall, a small amount of NiO will effectively improve the gasification activity of NES by enhancing H2 generation and tar cracking. For WP CLG, 850\u00a0\u00b0C and mOC/mWP\u00a0=\u00a01 should be the appropriate conditions for syngas generation; with steam injection, the H2 yield will be increased sharply, and H2/CO could be adjusted from 1.2 to 3.6 with NNES as the OC. After 10 redox cycles, NNES maintains a favourable activity on syngas generation and volatile reforming. NNES should be a potentially low-cost OC for cellulose solid waste CLG.To further explore the reaction process of the NNES, XRD and SEM combined with EDS were conducted, and the results are shown in Fig. 6 and Fig. 7. Fig. 6 shows the XRD patterns of the OCs. It can be found that in NES, Ni and Fe were comparable to NiFe2O4, an efficient OC for biomass CLG [36]. Ca is formed as Ca2Fe2O5, which is a high-activity compound for H2O reforming and H2 generation [37,38]. The results revealed that NES is a natural OC with high activity for WP CLG. The crystal composition of NNES is similar to that of NES. However, in the process of NNES preparation, the samples underwent multiple calcination cycles, and the same elements were more prone to agglomerate and cause crystal phase separation; therefore, Fe2O3 and SiO2 could be observed in the XRD pattern of NNES. After reaction with WP at 850\u00a0\u00b0C with 2.4\u00a0mL/g(WP) water injection, although the main composition of NNES was still NiFe2O4, the characteristic peaks of Ni0 or Fe0 can be observed in the XRD pattern. The results indicates the good oxygen releasing activity of NNES. On the other hand, the appearance of Ni0/Fe0 would promote tar cracking and H2 generation. Notably, Ca2Fe2O5 can be found in the XRD pattern more obviously, which reveals that with the reduction of Ni, Ca tended to combine with Fe and form Ca2Fe2O5. According to previous studies [21,39], Ca2Fe2O5 is an excellent intermediary to realize the reforming of H2O and reducing substances by an inert redox cycle as follows:\n\n(R7)\n\n\n\n\n3\nH\n\n2\n\nO\n+\n2\nC\na\nO\n+\n2\nF\ne\n\u2192\n3\n\nH\n2\n\n+\n\n\nC\na\n\n2\n\n\n\nF\ne\n\n2\n\n\nO\n5\n\n\n\n\n\n\n\n(R8)\n\n\n\nC\nn\n\n\nH\nm\n\n\nO\nl\n\n+\n\n\nC\na\n\n2\n\n\n\nF\ne\n\n2\n\n\nO\n5\n\n\u2192\n2\nC\na\nO\n+\n2\nF\ne\n+\n\nm\n2\n\n\nH\n2\n\n+\nk\nC\nO\n+\n\n(\nn\n-\nk\n)\n\nC\n\nO\n2\n\n\n\n\n\nThe generation of Ca2Fe2O5 explained the higher H2 yield with steam addition. NNES supplied a platform for H2O absorption and reforming [20], thereby maintaining a high effective syngas yield with water injection.The XRD pattern of regenerated NNES shows the crystal composition of NNES after a redox cycle. This result was similar to that of Fresh-NNES, which shows the excellent regeneration performance of NNES. However, the peaks of SiO2 are more obvious, showing that crystal phase separation still appeared after the redox reaction. Noticeably, CaSO4, a potential effective OC for biomass CLG [40], can be found in the regenerated OC. The sulfate in NNES is also an active ingredient and it enhance the oxygen-carrying capacity of NNES. In fresh OC, sulfate might exist as other forms and can\u2019t be observed in XRD pattern. In the redox cycles, Ca would combine with other anions such as SO4 and form as CaSO4. However, CaSO4 would release oxygen and might not be regenerated in such complexed compound as NNES. Therefore, small amount of SO2 may be generation in multiple redox cycles. The further study of migration of hazardous elements in NNES is necessary.After 10 redox cycles, though NiFe2O4 is still main crystal composition of NNES, the characteristic peak of silicate is manifest in XRD pattern. Inert compound such as CaFeSi2O6 can be found in NNES, it reveals that multiple redox reaction would cause active cation combining with SiO2 and reduce the active ingredient. The generation of silicate results in the declining of oxygen-carrying capacity of NNES, and explains the decrease of carbon conversion efficiency after 10 redox cycles.The morphological changes are provided by SEM images shown in Fig. 9. As the Fig. 9 (a) shown, the NNES was composited by grains of tens to hundreds of nm in diameter. However, with the reduced of active composition, the size of grains increased obviously in Fig. 9 (b), reveals the agglomeration happened in gasification reaction. The sintering can be also observed by comparing Fig. 9 (a) and Fig. 9 (c). The regenerated NNES was consisted of grains whose diameter was over 250\u00a0nm, which much bigger than that in fresh NNES. After 10 redox cycles, there\u2019s no obviously grains can be observed on surface of NNES, it indicates the serious agglomeration has been happened. The agglomeration decrease the surface area of NNES, impede the contact of active composition and reactant, thereby cause the inactivity of NNES.The EDS results are shown in Table 3\n. The important elements are listed separately in the table, and the elements with less than 1\u00a0% content, i.e., Mg, Al, Mn, P, etc., were aggregated in others. Compared with the results of the XRF analysis of the NES in Table. 2, it can be summarized that Fe and Ni tend to distribute on the surface in the NNES. The loading of Ni by the immersion method is prone to distribute Ni on the outside of the NES grains and attracts Fe atoms to form NiFe2O4 on the surface of the NNES. After reduction, Ni0/Fe0 will be prone to stay on the surface and exert a better catalytic effect. For the reduced OC, the O and Ca contents on the surface increased sharply, which maybe because the reduction of NiFe2O4 exposed inert oxides such as CaO, and Ni/Fe tended to agglomerate. The content of Ni, Fe or Cr decreased in regenerated NNES compared with fresh NNES, which proves the agglomeration of transition metal elements in NNES. Notably, the S content increased in the regenerated sample, which corresponds to the generation of sulfate, as observed in Fig. 8\n\n. In the sample after 10 redox cycles, the content of metal elements decreased further. However, might be due to the generation of inert silicate, the content of Si increased substantially. Noticeably, the content of S dramatically decreases, it confirms the presence and consumption of active sulfates such as CaSO4.In total, the characterization of the NNES explained the inactivity of the OC after multiple redox cycles. The agglomeration of the active element hinders the contact of the catalyst and reactant. Crystal phase separation due to agglomeration inhibits the synergy of different compositions [21]. Meanwhile, redox reactions promote the combination of metal positive ions with Si and the generation of inert silicate, and reduces the oxygen carrying capacity and reaction activity of NNES.A small amount of NiO was introduced to modify the NES, and greatly improved activity for WP CLG. 10\u00a0wt% NiO was considered the most appropriate addition amount for NES modification. At 850\u00a0\u00b0C, when mOC/mWP\u00a0=\u00a01 with 2.4\u00a0mL/g(WP) water injection, 1.73 Nm3/kg syngas with an LHV of 11.9\u00a0MJ/Nm3 and H2/CO of 3.63 was obtained from WP CLG and the carbon conversion efficiency was over 90\u00a0%. The above reaction conditions are considered the most appropriate work conditions for H2-rich syngas produced. H2/CO can be adjusted easily by steam addition and temperature control. After 10 redox cycles with the above conditions, the syngas yield shows no obvious decline. However, the decreased carbon conversion efficiency reveals the slow inactivation of NNES.XRD and SEM combined with EDS was conducted to reveal the reaction mode and inactivation mechanism of NNES. The results indicate that the redox reaction promotes the agglomeration and weakens the synergy of different compositions and catalyst contact areas. Multiple redox cycles also cause inert silicates generation and reduce the oxygen carrying capacity and reaction activity of NNES.Overall, NNES should be a potential OC with high efficiency and low cost for cellulose solid waste CLG such as WP CLG.\nGenyang Tang: Methodology, Investigation, Writing \u2013 original draft. Jing Gu: Conceptualization, Data curation, Writing \u2013 review & editing. Guoqiang Wei: Data curation, Writing \u2013 review & editing. Haoran Yuan: Resources, Funding acquisition, Writing \u2013 review & editing. Yong Chen: 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 the Key-Area Research and Development Program of GuangDong Province [2020B1111380001]\uff0cGuangdong Basic and Applied Basic Research Foundation [2021B1515020068].", "descript": "\n                  Waste pulp (WP) is a typical byproduct of paper industry, and Chemical-looping gasification (CLG) as a recently developed technology is highly suited to dispose high-volatile wastes like WP. In order to make a high-efficiency oxygen carrier (OC) for CLG of WP, the Ni-containing electroplating sludge (NES) was used as the matrix and NiO modification was performed to enhance the hydrogen production in CLG. These resulted in a potentially high-efficiency OC denoted as NNES. Testing CLG of WP was in a fixed-bed reactor at 850\u00a0\u00b0C by adopting NNES as the OC, injecting 2.4\u00a0mL/g(WP) water, and keeping a mass ratio of 1.0 between OC to WP. It produced 1.73 Nm3/kg syngas that has an LHV of 11.9\u00a0MJ/Nm3 and a H2/CO ratio of 3.63. In 10 redox cycles, the syngas yield did not have obvious decrease, but a certain reduction in the activity of NNES was observed. Characterization of the spent NNES revealed that it is the Ni agglomeration and inert silicate generation which reduced the activity of NNES.\n               "}
{"full_text": "Hydrogen (H2), with the merits of high enthalpy, zero-emission utilization and abundant natural resources, has been considered as renewable and clean energy carrier allowing a replacement of conventional fossil fuels [1,2]. Water electrolysis as a promising environment-friendly technology for producing hydrogen, has attracted enormous attention in recent years [3,4]. In water splitting, Hydrogen evolution reaction (HER) heavily depends on state-of-the-art Pt or Pd/Rh based catalysts owing to their lower overpotential and small values of Tafel slope [5,6]. Most notably, the high-cost and low-abundance of precious metals severely limited their wide applications. Up to now, numerous efforts have been made towards development of earth-abundant and low-cost electrocatalysts for high-efficiency and stable hydrogen evolution reaction. In this respect, transition metals (i.e., Ni, Co and Fe) based electrocatalysts, including metal oxides/hydroxides [7\u20139], chalcogenides [10\u201313], nitrides [14,15], borides [16,17], phosphides [18\u201320], and alloys [21,22], have attracted most attention as the substitutes for noble metal catalysts.Among them, transition metal phosphides (TMPs) are among the most promising electrocatalysts as the negatively charged P in TMPs can provide superior activity by capturing positively charged protons in HER process [23,24]. Furthermore, the amorphous materials endow excellent catalytic activity due to their unique structure and abundant unsaturated sites. Recently, Xu and co-workers reported that the fabrication of amorphous phosphides can regulate the P content of phosphides by breaking the limitation of ordered stoichiometric crystal structures [24]. Therefore, it is expected that exploiting amorphous phosphides with the merits of amorphous materials would overcome high catalytic reaction barriers and reach higher activities for HER. The Ni\u2013Fe\u2013P and Co\u2013P materials meet the above criteria and deserve further design and development. Moreover, recent reports demonstrated that the Ni\u2013Fe\u2013P materials exhibit superior catalyst activity owing to its bimetallic system [25,26]. Some reports found that microstructures of Co\u2013P materials determine its catalyst activity [27,28].In an electrocatalytic process, the catalyst activity is strongly related to the following three key steps [29,30]: (i) mass diffusion; (ii) electron transfer; (iii) surface/interface reactions. It has been widely recognized that electronic structure adjustment and surface/interface optimization are the reliable strategies. So, higher catalytic reactivity has been achieved though judiciously engineering electrocatalysts through the use of electronic and structural engineering [31,32], strain engineering [33] and defects engineering [34]. However, most of strategies utilize energy-intensive, time-consuming and complicated preparation processes, which largely hampered their extensive commercial applications. Combining different catalyst interfaces to get more efficient electrocatalysts is a simple and effective strategy [35], such as Ni3S2/Co9S8 [10], Pt-NC/Ni-MOF [36], CoO/CoP [37].The electroless plating is a controllable autocatalytic chemical reduction process for metal deposition. In the process of electroless plating, non-metallic atoms (i.e., P, B, S) are co-deposited with metal atoms, resulting in the formation of amorphous alloys. Therefore, transition metals (i.e., Ni, Co, Fe, Mo) with electrocatalytic activity could be deposited easily as electrocatalysts. Moreover, since the alloys prepared by industrialized electroless plating are deposited directly on the substrate without using additional binder, electroless plating is quite suitable for the fabrication of electrocatalytic electrodes suitable applications [38,39].Inspired by these aspects, we constructed layer-by-layer alternately stacked Ni\u2013Fe\u2013P and Co\u2013P films deposited on nickel foam (marked as Co\u2013P/Ni\u2013Fe\u2013P/NF), via facile electroless plating and de-alloying process. Notably, there is a significant interaction between different layers, instead of a simple stacking of the Ni\u2013Fe\u2013P and Co\u2013P films. Using scanning electron microscope (SEM) study, we demonstrated that Ni\u2013Fe\u2013P films induced and promoted the specific growth of following films. Furthermore, charge transfer between metal and P atom of different layers was observed using X-ray photoelectron spectroscopy (XPS) characterization, implying the regulation of interfacial electronic structures. Aiming at these phenomena, in-depth and detailed research has been carried out in this paper. Most importantly, the as-prepared Ni\u2013Fe\u2013P/Co\u2013P/NF electrocatalysts exhibit remarkable HER performance (a low overpotential of 43.4\u00a0mV at 10\u00a0mA\u00a0cm\u22122), as well as a lower Tafel slope of 56.5\u00a0mV dec\u22121 and outstanding durability throughout 72\u00a0h in an alkaline medium. The innovative strategy of this work would promote the meticulous design and will create a better understanding of the electrocatalysts.Commercial nickel foam (1\u00a0\u00d7\u00a03\u00a0cm2) was placed in 200\u00a0mL beaker containing sodium carbonate anhydrous (3\u00a0g), sodium hydroxide (2\u00a0g), trisodium phosphate dodecahydrate (0.5\u00a0g), and deionized water (100\u00a0mL). Boil for 3\u00a0min to remove greasy dirt from the surface of nickel foam (NF). Then, after deionized water washing, NF was cleaned thoroughly with 3\u00a0mol L\u22121 HCl 30\u00a0min for eliminating surface oxides.The chemical deposition of Co\u2013P alloys on NF was performed in the solution (denoted as solution A) at 90\u00a0\u00b0C, containing 25\u00a0g\u00a0L\u22121 cobalt sulfate heptahydrate, 20\u00a0g\u00a0L\u22121 trisodium citrate dihydrate as complexing agent, 30\u00a0g\u00a0L\u22121 ammonium fluoride as stabilizer, 40\u00a0g\u00a0L\u22121 sodium hypophosphite monohydrate as reducing agent. The pH values were regulated to 9 through adding appropriate amount of ammonia. Bubbles were slowly generated around NF during electroless plating. After 30\u00a0min of deposition, the Co\u2013P/NF electrode cleaned with deionized water and dried for 8\u00a0h at 60\u00a0\u00b0C.The chemical deposition of Ni\u2013Fe\u2013P alloys on NF was performed in the solution (denoted as solution B) at 90\u00a0\u00b0C, containing 7.5\u00a0g\u00a0L\u22121 nickel sulfate, 17.5\u00a0g\u00a0L\u22121 ammonium ferrous sulfate, 20\u00a0g\u00a0L\u22121 trisodium citrate dihydrate as complexing agent, 30\u00a0g\u00a0L\u22121 ammonium fluoride as stabilizer, 40\u00a0g\u00a0L\u22121 sodium hypophosphite monohydrate as reducing agent. The pH of the electroless plating solution was adjusted and controlled to 9 with ammonia water. Bubbles were slowly generated around NF during electroless plating. After 60\u00a0min of deposition, Ni\u2013Fe\u2013P/NF electrodes were taken out from the solution, cleaned with deionized water. Then, the sample was dealloyed in the 5% HCl solution for 30\u00a0s at RT, after repeated cleaning dried for 8\u00a0h at 60\u00a0\u00b0C.The preparation process of Co\u2013P/Ni\u2013Fe\u2013P/NF combined two steps mentioned above. Step one, Co\u2013P films were deposited on NF for 10\u00a0min in solution A. Step two, Ni\u2013Fe\u2013P films were deposited for 30\u00a0min in solution B based on the samples obtained from the previous step; then, the samples were dealloyed in the 5% HCl solution for 30\u00a0s at RT. Generally, the multilayered Co\u2013P/Ni\u2013Fe\u2013P/NF electrodes were obtained by cyclic execution of steps one and two, marked as 2\u00a0L (2 Layers), 2\u00a0L\u2019, 3\u00a0L, 4\u00a0L, 5\u00a0L and so on, wherein the 2\u00a0L\u2019 represents the electrodes obtained by changing the order of steps one and two. Finally, the last samples were dried at 60\u00a0\u00b0C for 8\u00a0h. Furthermore, to investigate the effect of dealloying time, different dealloying time (0, 30, 60 and 90 s) was performed for preparing 5\u00a0L samples. According to the controllability of deposition time to coating thickness, different deposition time of Co\u2013P (5, 10 and 15\u00a0min) and Ni\u2013Fe\u2013P (15, 30 and 45\u00a0min) in 5\u00a0L samples were performed to investigate the effect of coating thickness, marked as X/Y, wherein the X and Y represent the deposition time of Co\u2013P and Ni\u2013Fe\u2013P, respectively.The electrochemical measurements were performed in a standard three-electrode system on an electrochemical work station with a CHI 760\u00a0E (Shanghai Chenhua, China) in 1\u00a0mol\u00a0L\u22121 KOH solution. For the HER tests, the as-prepared electrodes (1\u00a0\u00d7\u00a01\u00a0cm2), a carbon rod and a Hg/HgO (1\u00a0mol\u00a0L\u22121 KOH) electrode were used as the working electrode, the counter electrode and the reference electrode, respectively. The linear sweep voltammetry (LSV) was carried out at a sweeping rate 1\u00a0mV\u00a0s\u22121 with 90% iR-compensation. The LSV was run five times for obtaining more accurate and stable polarization curves. Electrochemical impedance spectroscopy (EIS) was performed to measure the solution resistance (Rs) and charge transfer resistance (Rct). Electrochemical impedance spectroscopy (EIS) was performed at \u22120.1\u00a0V (vs. RHE) in the frequency range from 100\u00a0kHz to 0.01\u00a0Hz. And the equivalent circuit of the EIS data was fitted with Z-View software. The electrochemical active surface area (ECSA) was evaluated from the electro-chemical double-layer capacitance (Cdl), through collecting cyclic voltammograms (CVs) in the potential range non-faradaic processes at various scan rates from 2 to 10\u00a0mV\u00a0s\u22121 in the potential range from \u22120.9 to \u22120.98\u00a0V versus Hg/HgO. The potential was converted to the reversible hydrogen electrode (RHE) according to the Nernst equation: E(RHE)\u00a0=\u00a0E(vs. Hg/HgO) + 0.9268\u00a0V. Chronopotentiometry (CP, 10\u00a0mA cm\u22122) was performed for 72\u00a0h in 1.0\u00a0mol L\u22121 KOH solution to evaluate the long-term stability of the Co\u2013P/Ni\u2013Fe\u2013P/NF electrodes. Multi-current steps test from 20 to 200\u00a0mA\u00a0cm\u22122 with an increment of 20\u00a0mA\u00a0cm\u22122 per step for Co\u2013P/Ni\u2013Fe\u2013P/NF electrodes. The chronopotentiometry and multi-current process were carried out without iR-compensation.The fabrication process of Co\u2013P/Ni\u2013Fe\u2013P/NF electrodes with hierarchical structure was illustrated in Fig.\u00a01\n, including repeated deposition Co\u2013P and Ni\u2013Fe\u2013P coating and the de-alloying process for Ni\u2013Fe\u2013P alloys. The nickel foam with unique 3D network and tortuous skeleton structure was used as both a current collector and substrate.The phase structures of different samples were studied by X-ray diffraction (XRD) patterns. As shown in Fig.\u00a02\n, the inconspicuous diffraction peaks of Co\u2013P/NF can be assigned to pure hexagonal-phase metallic Co (JCPDS card 05\u20130727) and the Co\u2013P/NF exhibit a broad peak of an amorphous alloy around 25\u00b0, which indicate the samples mixed amorphous and nano-crystalline structure features. Moreover, the obvious peaks in XRD patterns of Co\u2013P/Ni\u2013Fe\u2013P/NF and Ni\u2013Fe\u2013P/NF attribute to Ni foam (JCPDS card 04\u20130850), as the incomplete separation of the coatings and nickel foam. The broad peaks at 25\u00b0 and 45\u00b0 of Co\u2013P/Ni\u2013Fe\u2013P/NF and Ni\u2013Fe\u2013P/NF suggest the existence form of amorphous phase of the alloys, indicating that the crystal phase of Co\u2013P alloys transformed into amorphous phase after interface engineering.To detect the structure morphology of different electrodes, scanning electron microscope (SEM) has been employed to explore the details. Cross-sectional SEM images (Fig.\u00a03\na, Fig.\u00a0S3a and S3c in the Supporting Information), elemental mapping images (Fig.\u00a03b and Fig.\u00a0S3b), and Line-scan SEM-EDS elemental distribution curves (Fig.\u00a0S3d) unambiguously showed that Co\u2013P/Ni\u2013Fe\u2013P/NF-5L electrodes have multilayer structure comprised of Co\u2013P and Ni\u2013Fe\u2013P films. There are varied surface morphologies in different layers of Co\u2013P/Ni\u2013Fe\u2013P/NF electrodes. As Fig.\u00a03a and Fig, S1 displayed, the Co\u2013P alloys that first deposited are densely grown on the Ni foam. The Co\u2013P alloy is distributed in blocks on the nickel foam (Fig.\u00a04\na), which is observed from the surface image. The Ni\u2013Fe\u2013P alloys grown on the Co\u2013P film or Ni foam are both uniformly and regularly deposited on the.Substrate (Fig.\u00a03a and Fig.\u00a0S2). After 30 s de-alloying process, the Ni\u2013Fe\u2013P film surface has distinct nano-micropores structure (Fig.\u00a04b). As revealed in Figs. 3c, 3d and 4c, Co\u2013P alloys in third layer of Co\u2013P/Ni\u2013Fe\u2013P/NF electrodes are nanosheets structure, which is different with the Co\u2013P alloys grown on Ni foam. Moreover, unlike the layer two and Ni\u2013Fe\u2013P/NF, the Ni\u2013Fe\u2013P alloys in fourth layer have cracked structure (Figs. 3e and 4d). The Co\u2013P alloys in layer five show another new structure, which is randomly distributed on the layer four in particles form (Fig.\u00a04e and f). It is because of combining these surfaces with various structures that the active sites are increased tremendously as well as the diffusion and mass transfer are accelerated greatly [40].In order to investigate the reason and mechanism for the above phenomenon, a series of experiments were carried out. In the course of experiment, whether de-alloying treatment for Ni\u2013Fe\u2013P film was performed or not, which had a significant influence on depositing Co\u2013P and the following films. A large number of bubbles were rapid produced in intense reaction when Co\u2013P film was deposited on Ni\u2013Fe\u2013P film that had been de-alloying treated. The experimental steps are as follows: First, one layer Ni\u2013Fe\u2013P film was deposited on the Ni foam. Then, regarding the de-alloying step performed or not as a variable. At last, one layer Co\u2013P film was deposited on Ni\u2013Fe\u2013P film. As shown in Fig.\u00a0S5, discrete lamellar structure was formed in the Co\u2013P coating sections. When the Ni\u2013Fe\u2013P coatings were not performed de-alloying process, there is no discrete layered structure in Co\u2013P film (Fig.\u00a0S6). It is observed from the Fig.S6c that the Ni\u2013Fe\u2013P coatings without performing de-alloying process have smoother surface (Fig.\u00a04b) compared to the coatings after de-alloying process. In alkaline solution, the electroless plating including two steps: the oxidation of hypophosphite and the co-deposition of phosphorus with the metals (Ni & Fe or Co) [41], respectively as:\n\nH2PO2\n- + 2OH- = H2PO3\n- + H2O + 2e (1)\n\n\n\n\n(2)\n4H2PO2\n\u2212\u00a0=\u00a02OH\u2212\u00a0+\u00a0H2\u00a0+\u00a02P\u00a0+\u00a02H2PO3\n\u2212\n\n\n\nThe electroless plating initiates a metal deposition process by means of auto-catalytically active centers on the surface of the substrate. In combination with the experimental phenomena, the rapid release of bubbles means that step (2) reacts violently. The de-alloying process activated the Ni\u2013Fe\u2013P film, which maybe form abundant reaction sites of auto-catalytically on the surface of the Ni\u2013Fe\u2013P film. And it was also further activated Co\u2013P film. In brief, dispersed growth Co\u2013P film is formed due to the co-deposition of phosphorus with the metals is accompanied by rapid release of bubbles. In addition, the fracture of Ni\u2013Fe\u2013P film in layer four owe to the dispersed structure of Co\u2013P film. As for the various dispersed morphologies of Co\u2013P in different layers and samples (Figs. 3d, 3e, 4c, 4e, and Fig.\u00a0S5c), which attributes to the concentration of solution A decreases. In summary, the de-alloying process activated the coatings, which endowed the electroless plating process more abundant autocatalytic active center and thus further promoted the dispersive growth of these films.The surface chemical composition of Co\u2013P/Ni\u2013Fe\u2013P/NF-5L was characterized by x-ray photoelectron spectroscopy (XPS). In Fig.\u00a05\na, the fitting peaks of Ni 2p at 852.25 and 869.45\u00a0eV coincide with Ni 2p3/2 and Ni 2p1/2 for Ni0 [42,43]. The other two peaks emerged at 855.95 and 873.65\u00a0eV should be assigned to Ni 2p3/2 and Ni 2p1/2 according to the spin\u2013orbit characteristics of Ni2+, accompanying with two palpable satellite peaks (denoted as \u201csat.\u201d) [44,45]. The fitted broad Fe 2p3/2 envelope in Fig.\u00a05b is resolved into two peaks at 712.6\u00a0eV and 716.6\u00a0eV (sat.), which corresponds to FeOX species when exposed to air [46,47]. It is also observed that a very weak peak with the binding energies of \u223c706\u00a0eV in Fe 2p3/2 is in agreement with.The characteristics of Fe0 [43], indicating the existence of relatively little metallic Fe in the catalysts. For Co 2p in Fig.\u00a05c, the peaks emerged at 2p1/2 781.15\u00a0eV and 2p3/2 796.95 eV originate from high valence state cobalt and metallic cobalt [48\u201350]. Note that the peak located at 777.60\u00a0eV is in agreement with the Co0 signal [51], demonstrating that the presence of metallic cobalt in the catalysts. The metallic characteristics have a series of advantages including superior electron transportation capacity, high hardness and high tensile strength, which is eligible to be efficient and robust electrocatalys. Moreover, compared to red phosphorus (130.0\u00a0eV), P 2p has a lower binding energies of 129.60 eV (Fig.\u00a05d), which is assigned to reduced phosphorus in the form of metal phosphides [52]. This demonstrates that electron density transfer from the metallic (mainly Ni and Co) state with positive charge (\u03b4+) to P with negative charge (\u03b4-), which illustrates the P with the electronegativity effect of trapping positively charged protons in electrocatalys [50,52]. Another broad peak located at 133.20\u00a0eV is attributed to phosphate species (P5+) and the P species arising from superficial oxidation of the alloys exposed to air [53,54]. In addition, Supplementary Table S1 compares the XPS data of our previous work Co\u2013P/NF, Ni\u2013Fe\u2013P/NF and of this work Co\u2013P/Ni\u2013Fe\u2013P/NF in detail. Note that the Ni 2p XPS region of Co\u2013P/Ni\u2013Fe\u2013P/NF shows slightly negative shift than that of Ni\u2013Fe\u2013P/NF, with a negative shift of 0.15\u00a0eV at Ni 2p peaks. Similar with Ni 2p, the Co 2p XPS region of Co\u2013P/Ni\u2013Fe\u2013P/NF is also observed the slightly negative shift of 0.15\u00a0eV than that of Co\u2013P/NF at Co 2p peaks. Interestingly, the P 2p XPS region of Co\u2013P/Ni\u2013Fe\u2013P/NF exactly shows positive shift of about 0.30\u00a0eV than that of Ni\u2013Fe\u2013P/NF and Co\u2013P/NF. This indicates that the electron transfer from P to Co and Ni in the multilayer films. The transfer of electrons proves that the electronic structure of the layered materials is tuned when the interface is constructed [55]. Research shows that there are strong electronic interactions between deposited Co\u2013P alloys and Ni\u2013Fe\u2013P alloys, after performing interface engineering, which created ample metal active sites and provided more electron transfer access to promote the electrocatalytic in the alloys.To evaluate HER activity, the Co\u2013P/Ni\u2013Fe\u2013P/NF electrodes were subjected as an cathode to three electrode chemical cell in 1\u00a0mol L\u22121 KOH electrolyte. For comparison, Ni\u2013Fe\u2013P/NF, Co\u2013P/NF, bare NF and 20% Pt/C coated on NF (Pt/C on NF, 20% Pt/C loading: 5\u00a0mg\u00a0cm\u22122) were also measured at the same conditions. As expected, the Co\u2013P/Ni\u2013Fe\u2013P/NF electrodes after multilayer interface engineering exhibit prominent catalytic activity for HER with a lower overpotential of 43.4\u00a0mV at 10\u00a0mA\u00a0cm\u22122 (Fig.\u00a06\na). Even when compared with the benchmarking Pt/C, the Co\u2013P/Ni\u2013Fe\u2013P/NF only requires an additional 26\u00a0mV to reach 10\u00a0mA\u00a0cm\u22122. It obviously outperforms the corresponding single-layer electrodes of Co\u2013P/NF (\u03b710\u00a0=\u00a0177.5\u00a0mV) and Ni\u2013Fe\u2013P/NF (\u03b710\u00a0=\u00a0106.9\u00a0mV). Even at high current density, the Co\u2013P/Ni\u2013Fe\u2013P/NF electrodes still keep comparatively outstanding performance (Fig.\u00a06b). Moreover, according the loading mass table in Fig.\u00a06a, the Co\u2013P/Ni\u2013Fe\u2013P catalyst displays a high mass activity of 7.311\u00a0mA\u00a0mg\u22121 (Fig.\u00a0S7) at an overpotential of 200\u00a0mV by normalization with the loading mass, which is significantly higher than the catalysts of Co\u2013P (0.395\u00a0mA\u00a0mg\u22121) and Ni\u2013Fe\u2013P (2.029\u00a0mA\u00a0mg\u22121). Calculation details of mass activity (mA g\u22121) are seen in the Supporting Information. This indicates that the superior performance of multilayer Co\u2013P/Ni\u2013Fe\u2013P/NF is originated from the rational regulation of coating interface by interface engineering and the synergistic effect of Co\u2013P and Ni\u2013Fe\u2013P alloys, instead of the simple increaseing of loading mass. To further investigate the effect of layer number, dealloying time, and coating thickness, the different Co\u2013P/Ni\u2013Fe\u2013P/NF samples were also fabricated by changing deposition layers number, dealloying time and deposition time, as shown in Figs. S9, S10 and S11. It is observed from the Fig.\u00a0S9a that there is an increasing trend towards HER activity for all the multilayer Co\u2013P/Ni\u2013Fe\u2013P/NF samples with increasing layers number till five layers, demonstrating that the superposition of coating layer could accelerate the hydrogen evolution of electrocatalyst. Furthermore, the de-alloying process has immense impact on the HER catalytic activity. The Co\u2013P/Ni\u2013Fe\u2013P/NF sample without performing de-alloying process shows poor performance, in that the unactivated Ni\u2013Fe\u2013P and Co\u2013P alloys can't form the favorable nanostructures. However, the prolonged de-alloying treatment led to severe corrosive for the alloys in acidic media, and thus the metal atom ratio and the stable structure were destroyed. Consequently, the Co\u2013P/Ni\u2013Fe\u2013P/NF sample fabricated at 30 s de-alloying time displays excellent electrocatalytic activity than at de-alloying time (Fig.\u00a0S10a). Similarly, the appropriate deposition thickness of Co\u2013P films and Ni\u2013Fe\u2013P films could sufficiently exert electrocatalytic effect. The result reveals that the sample shows an optimal HER catalytic activity when the deposition time of Co\u2013P and Ni\u2013Fe\u2013P are 10\u00a0min and 30\u00a0min, respectively (Fig.\u00a0S11a). As observed in SEM image (Fig.\u00a03a), the thickness of Co\u2013P films and Ni\u2013Fe\u2013P films are about 1\u00a0\u03bcm and 2\u00a0\u03bcm, respectively. These results demonstrate that more layers of deposition, suitable dealloying time and appropriate deposition thickness would enhance the HER activity.The evaluation of the amount of active sites for HER obtained through calculating the electrochemical active surface area (ECSA). There is a positively correlation between ECSA and the electrochemical double-layer capacitance (Cdl), according to the equation ECSA\u00a0=\u00a0Cdl/Cs where Cs is the specific capacitance with a fixed value under identical electrolyte conditions [56]. And the electrochemical double-layer capacitance (Cdl) value is measured via cyclic voltammetry (CV) test (Fig.\u00a0S8). Evidently, the multilayer Co\u2013P/Ni\u2013Fe\u2013P/NF shows a higher Cdl (445.4\u00a0mF\u00a0cm\u22122) than Co\u2013P/NF (17.7\u00a0mF\u00a0cm\u22122) and Ni\u2013Fe\u2013P/NF (390.2\u00a0mF\u00a0cm\u22122), suggesting more catalytically active sites on the multilayered alloys. Notably, combining with the XRD patterns (Fig.\u00a02), it is apparent that the Ni\u2013Fe\u2013P/NF and Co\u2013P/Ni\u2013Fe\u2013P/NF with amorphous phase have higher Cdl value than Co\u2013P/NF with crystal phase. This result indicates that the enhanced performance of Co\u2013P/Ni\u2013Fe\u2013P/NF more likely resulted from the phase transformation of Co\u2013P alloys, meaning that amorphous electrocatalysts could provide more active sites in HER process, which is consist with reported previously [21,57].To figure out the reaction kinetics, linear fitting Tafel plots were further conducted to further characterize the electrocatalysts. As widely accepted, the various values of Tafel slopes correspond to the different determining rate steps, including Volmer, Heyrovsky and Tafel steps. In this work, the multilayer Co\u2013P/Ni\u2013Fe\u2013P/NF sample has a low Tafel slope of 56.5\u00a0mV dec\u22121 (Fig.\u00a06c) which is lower than that of Co\u2013P/NF (83.6\u00a0mV dec\u22121) and Ni\u2013Fe\u2013P/NF (78.3\u00a0mV dec\u22121), indicating that the Heyrovsky step is the mainly rate-determining step [58]. Furthermore, the Co\u2013P/Ni\u2013Fe\u2013P/NF-3L has a small Tafel slope of 54.4\u00a0mV dec\u22121 (Fig.\u00a0S9b), which is even smaller than that of Co\u2013P/Ni\u2013Fe\u2013P/NF-5L (56.5\u00a0mV dec\u22121), revealing the nanosheets Co\u2013P films in layer three endow the electrochemical HER with favorable kinetics. In addition, the Tafel slope of Co\u2013P/Ni\u2013Fe\u2013P/NF samples fabricated at de-alloying time of 0\u00a0s and 30\u00a0s have significantly difference, confirming that nanostructures alloys formed after activation have rapid HER reaction kinetics (Fig.\u00a0S10b).The electrode\u2013electrolyte interfaces reaction kinetics were studied by electrochemical impedance spectroscopy (EIS) analysis. The charge transfer resistance (Rct) of the interface between the electrolyte and the catalysts is usually considered to be determined by the semicircle diameter of the Nyquist plots [45]. And the Rct is closely related to the reaction situation between the electrolyte and the catalysts. Fig.\u00a06d exhibits the Nyquist plots of Co\u2013P/NF, Ni\u2013Fe\u2013P/NF and Co\u2013P/Ni\u2013Fe\u2013P/NF, implying a faster electron transfer for the multilayer Co\u2013P/Ni\u2013Fe\u2013P/NF. Besides, the values of the Rct and the solution resistance (Rs) extracted from Nyquist plots decrease with the increasing of the layer number of films (Fig.\u00a0S9d), indicating that the interface engineering effectively promoted the charge transfer efficiency for HER. In addition, the as-prepared samples after de-alloying process exhibit small impedance (Fig.\u00a0S10c), which reveals that the favorable structures of the multilayer alloys after activation provided abundant and efficient transfer pathways for electrons and ions in the catalysts.Finally, in order to further evaluate the long-term stability of the Co\u2013P/Ni\u2013Fe\u2013P/NF electrode, the Co\u2013P/Ni\u2013Fe\u2013P/NF was tested in 1\u00a0mol L\u22121 KOH solution employing a three-electrode system. There was no obvious increasing for overpotential of Co\u2013P/Ni\u2013Fe\u2013P/NF electrode at the current density of 10\u00a0mV\u00a0cm\u22122 after running at least 72\u00a0h (Fig.\u00a07\na). Moreover, multi-current steps test from 20 to 200\u00a0mA\u00a0cm\u22122 with an increment of 20\u00a0mA\u00a0cm\u22122 per step was performed to assess the electrocatalytic durability of the Co\u2013P/Ni\u2013Fe\u2013P/NF electrode. The potential immediately reach a stable state at the initial current of 20\u00a0mA\u00a0cm\u22122 and keep stable for 1000s, and continue to remain stable at the later steps (Fig.\u00a07b). In addition, after long-term stability test, the XRD pattern (Fig.\u00a0S12a), SEM images (Fig.\u00a0S12b) and elemental mapping (Fig.\u00a0S12c) of the post-HER Co\u2013P/Ni\u2013Fe\u2013P/NF sample have no obvious change compared with fresh samples. These results reveal that the Co\u2013P/Ni\u2013Fe\u2013P/NF electrode has superior long-term stability, mechanical robustness, conductivity and mass transportation property [50,59], which is promising and available electrocatalysts for HER.Combining with the above research results, the reasons for the excellent performance of Co\u2013P/Ni\u2013Fe\u2013P/NF electrode are explained from the following three points: (i) source of activity, (ii) hydrogen evolution reaction kinetics, and (iii) electronic transfer and mass transportation capability. Based on the results of the SEM images (Figs. 3, 4 and Fig.\u00a0S4), LSV curves (Fig.\u00a06a, S9a, S10a and S11a), mass activity data (Fig.\u00a0S7) and XPS analysis, after rational design and preparation processes, the Co\u2013P/Ni\u2013Fe\u2013P/NF electrode with favorable multilayer structures and prominent electrocatalytic performance for HER was successfully fabricated via the interface engineering. The Co\u2013P/Ni\u2013Fe\u2013P/NF electrode with best hydrogen evolution performance has five layers films alternately stacked by Co\u2013P and Ni\u2013Fe\u2013P films, and has a lower overpotential of 43\u00a0mV at 10\u00a0mA\u00a0cm\u22122. Compared with recent advanced electrocatalysts for HER in 1\u00a0mol L\u22121 KOH solution, the Co\u2013P/Ni\u2013Fe\u2013P/NF still maintains a higher advantage (shown in Table S2). These results powerfully certified the success of multilayer strategy in this study.\n\n(i)\nSource of activity. According to the previous research, Yang et\u00a0al. believed that the crystalline materials, like Co\u2013P alloys in our work, have limited active sites existing on the edges or surface of crystalline materials. As for amorphous materials, like Ni\u2013Fe\u2013P and Co\u2013P/Ni\u2013Fe\u2013P alloys in our work, their short-range ordered and long-range disordered structures provide rich defect sites to act as active centers in the HER process. Therefore, the whole amorphous electrocatalysts could adequately provide active sites to boost the interfacial catalytic reactions [21,60]. In addition, the P element has important function in electrocatalysts for HER. Liu and Rodriguez [61] discovered that the Ni2P-(001) behaves somewhat like the hydrogenase, and the P atoms with electronegativity attract the electrons from metal atoms. In HER process, the negatively charged P atoms in phosphides can trap proton to enhance the catalytic efficiency. In this work, according to the results of XPS, there are similar situation in Co\u2013P/Ni\u2013Fe\u2013P/NF. Moreover, the amorphous structure of Co\u2013P/Ni\u2013Fe\u2013P/NF could break through the limitation of crystal lattice and thus facilitate the catalytic reaction. Our work proves the superiority of amorphous materials in the field of electrocatalysis once again.\n\n\n(ii)\nHydrogen evolution reaction kinetics. In alkaline electrolytes, the HER proceed with the following steps: Water dissociation and H\u2217 (adsorbed hydrogen) generation step (120\u00a0mV dec\u22121)\n\n\n\n\nH2O + e- \u2192 H\u2217 + OH- (Volmer step) (1)\n\n\nSource of activity. According to the previous research, Yang et\u00a0al. believed that the crystalline materials, like Co\u2013P alloys in our work, have limited active sites existing on the edges or surface of crystalline materials. As for amorphous materials, like Ni\u2013Fe\u2013P and Co\u2013P/Ni\u2013Fe\u2013P alloys in our work, their short-range ordered and long-range disordered structures provide rich defect sites to act as active centers in the HER process. Therefore, the whole amorphous electrocatalysts could adequately provide active sites to boost the interfacial catalytic reactions [21,60]. In addition, the P element has important function in electrocatalysts for HER. Liu and Rodriguez [61] discovered that the Ni2P-(001) behaves somewhat like the hydrogenase, and the P atoms with electronegativity attract the electrons from metal atoms. In HER process, the negatively charged P atoms in phosphides can trap proton to enhance the catalytic efficiency. In this work, according to the results of XPS, there are similar situation in Co\u2013P/Ni\u2013Fe\u2013P/NF. Moreover, the amorphous structure of Co\u2013P/Ni\u2013Fe\u2013P/NF could break through the limitation of crystal lattice and thus facilitate the catalytic reaction. Our work proves the superiority of amorphous materials in the field of electrocatalysis once again.Hydrogen evolution reaction kinetics. In alkaline electrolytes, the HER proceed with the following steps: Water dissociation and H\u2217 (adsorbed hydrogen) generation step (120\u00a0mV dec\u22121)Electrochemical desorption step (40\u00a0mV dec\u22121)\n\nH\u2217 + H2O + e- \u2192 H2 + OH- (Heyrovsky step) (2)\n\n\nChemical desorption step (30\u00a0mV dec\u22121).\n\n2H\u2217 \u2192 H2 (tafel step) (3)\n\n\nThe HER process in alkaline solution follows the Volmer\u2013Heyrovsky process or the Volmer\u2013Tafel process, depending on the Tafel slope given in above equations [62]. In this work, as revealed in Tafel plots (Fig.\u00a06c), the Tafel slope value of Co\u2013P/NF, Ni\u2013Fe\u2013P/NF and Co\u2013P/Ni\u2013Fe\u2013P/NF are 83.6\u00a0mV dec\u22121, 78.3\u00a0mV dec\u22121 and 56.5\u00a0mV dec\u22121, respectively. Accordingly, the Volmer\u2013Heyrovsky process determined HER process on the Co\u2013P/Ni\u2013Fe\u2013P/NF. Moreover, different from the HER in acidic electrolytes, H\u2217 generation in alkaline medium requires to break covalent H\u2013O\u2013H bond, which requires extra energy [63]. According to the changed Tafel slope (Fig.\u00a06c) after interface engineering, this reveals that the water dissociation and H\u2217 generation kinetics are significantly improved. The XPS analysis (shown in Table S1) after interface engineering reveals the amelioration of electronic structures of Co\u2013P/Ni\u2013Fe\u2013P/NF, which probably decreased Gibbs free energy of intermediates generation and benefited the electrochemical desorption step.\n\n(iii)\nElectronic transfer and mass transportation capability. On the one hand, the catalysts were directly deposited on the nickel foam with 3D porous structure via electroless plating and didn't require extra binder, which is conducive to sufficient contact between the substrate and the catalysts. And the intrinsic metallic properties of alloys endow the electrocatalysts with higher electrical conductivity. On the other hand, multilayer strategy effectively reduced the charge transfer resistance and the solution resistance on electrode (Fig.\u00a0S9d), demonstrating the efficient interface reaction between solution and electrocatalysts. Furthermore, various morphologies alloys (shown in Figs. 3, 4 and Fig.\u00a0S3) in Co\u2013P/Ni\u2013Fe\u2013P/NF with hierarchical and highly interconnected structure are not only quite beneficial for the diffusion of ions, but also meet the requirements of different reaction situation in solution.\n\n\nElectronic transfer and mass transportation capability. On the one hand, the catalysts were directly deposited on the nickel foam with 3D porous structure via electroless plating and didn't require extra binder, which is conducive to sufficient contact between the substrate and the catalysts. And the intrinsic metallic properties of alloys endow the electrocatalysts with higher electrical conductivity. On the other hand, multilayer strategy effectively reduced the charge transfer resistance and the solution resistance on electrode (Fig.\u00a0S9d), demonstrating the efficient interface reaction between solution and electrocatalysts. Furthermore, various morphologies alloys (shown in Figs. 3, 4 and Fig.\u00a0S3) in Co\u2013P/Ni\u2013Fe\u2013P/NF with hierarchical and highly interconnected structure are not only quite beneficial for the diffusion of ions, but also meet the requirements of different reaction situation in solution.To summarize, we have successfully constructed the interface engineering based on the Ni\u2013Fe\u2013P and Co\u2013P films by utilizing the characteristics of autocatalytic reaction in electroless plating process. The de-alloying process activated the Ni\u2013Fe\u2013P films to induce subsequent films dispersion growth, which formed various morphologies alloys. Notably, the as-prepared Co\u2013P/Ni\u2013Fe\u2013P/NF electrode shows impressive catalytic performance and long-term durability in 1\u00a0mol L\u22121 KOM solution, which only requires an ultralow overpotential of 43.4\u00a0mV and 113.6\u00a0mV at the current density of 10\u00a0mA\u00a0cm\u22122 and 100\u00a0mA\u00a0cm\u22122, respectively. And the Co\u2013P/Ni\u2013Fe\u2013P/NF electrode also has a low Tafel slope of 56.5\u00a0mV dec\u22121. Such superior catalytic activity should attribute to: (1) the amorphous Co\u2013P/Ni\u2013Fe\u2013P alloys with unique chemical structure supplied rich active sites. (2) The interface engineering improved the kinetics of interfacial reaction. (3) Multilayer strategy effectively reduced the charge transfer resistance and the solution resistance. (4) The various morphologies alloys enhanced the ion transportation capability. (5) The interface coupling of layered alloys led to the improvement of electronic structure. The innovative strategy of this research may play a guidance role in design and development of catalysts for HER.There are no conflicts to declare.This work was financially supported by the Taishan scholar foundation of Shandong (ts201712046) and the National Natural Science Foundation of China (Grant No. 51672145).The following is the Supplementary data to this article:\n\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2020.07.023.", "descript": "\n                  Judiciously engineering the electrocatalysts is attractive and challenging to exploit materials with high electrocatalytic performance for hydrogen evolution reaction. Herein, we successfully perform the interface engineering by alternately depositing Co\u2013P and Ni\u2013Fe\u2013P films on nickel foam, via facile electroless plating and de-alloying process. This work shows that there is a significant effect of de-alloying process on alloy growth. The electronic structure of layered alloys is improved by interface engineering. The multilayer strategy significantly promotes the charge transfer. Importantly, the Co\u2013P/Ni\u2013Fe\u2013P/NF electrode fabricated by interface engineering exhibits excellent electrocatalytic hydrogen evolution activity with an overpotential of 43.4\u00a0mV at 10\u00a0mA\u00a0cm\u22122 and long-term durability for 72\u00a0h in alkaline medium (1\u00a0mol L\u22121 KOH). The innovative strategy of this work may aid further development of commercial electrocatalysts.\n               "}
{"full_text": "An imminent decarbonization of the energy sector is needed in order to reduce the environmental damage caused by fossil fuel. As a result, clean, sustainable and renewable energy sources appear to be potential alternatives. In fact, the development of hydrogen-based technologies can help to reduce or eliminate greenhouse gas emission. In this sense, hydrogen (H2) possesses the highest specific energy content among all conventional fuels and, it can be used as a green energy carrier using fuel cells and internal combustion engines by releasing only nontoxic by-products such as water [1]. However, the main drawback related to this compound is its low volumetric energy density which increases storage and transport costs [2]. An alternative to remove these issues is the use of hydrogen carrier compounds.In this respect, liquid fuels generated from hydrogen (ammonia, methanol, metal amine salts, etc.) might be easily stored and transported to be in situ decomposed to produce clean hydrogen through suitable conversion processes [3,4]. Ammonia (NH3) is a promising hydrogen carrier because of its high volumetric energy density and high hydrogen content, well-known technology for production and distribution and relatively low cost [5]. Moreover, its decomposition only produces hydrogen and nitrogen. Therefore, ammonia is an exceptional carbon-free hydrogen vector. However, fuel cells which are very sensitive to ammonia concentration (<1\u00a0ppm), demand high purity hydrogen [6]. Thus, almost complete ammonia conversion is required at relatively low temperature (<500\u00a0\u00b0C). For that purpose, hydrogen production from ammonia decomposition requires efficient and low-cost catalysts to reduce the reaction temperature and the energy cost of the process.Promising results of ammonia decomposition at low temperatures are achieved with ruthenium (Ru) catalysts [6\u20138]. Nevertheless, catalytic activity is very affected by other factors such as the size of metal particles since it is a structure-sensitive reaction [7,8]. Consequently, hydrogen production from ammonia has been widely studied using different types of active phases (Ni, Co, Rh, Pd, Pt, etc.) and supports such as Al2O3, SiC, ZrO2, CeO2, La2O3, MgO and carbonaceous materials [6\u201313]. Moreover, different types of promoters have been investigated to enhance the catalytic activity by adding alkaline species to the active phase [11\u201314]. In fact, these promoters increase the electron-donation to the active metals and stabilize the binding energy between metal and N atoms, favouring ammonia decomposition reaction.In recent years, a novel route for adding promoters to heterogeneous catalyst has been developed through the phenomenon of Electrochemical Promotion of Catalysis (EPOC). This phenomenon discovered by Stoukides and Vayenas in 1981 [15] is a promising alternative way to explore the in-situ addition of electronic promoters to a heterogeneous catalyst and hence, to enhance catalytic reaction rates [16\u201318]. This phenomenon is based on the electrochemical supply of promoter ions from a solid electrolyte material (support) to a metal catalyst (working electrode) by the application of electrical currents or potentials. This electrochemical activation of the catalyst by using a solid electrolyte support allows the in-situ electrochemical addition or removal of a wide variety of promoters, anionic (O2\u2212) or cationic (Na+, K+, H+) to different kinds of catalysts in a wide range of catalytic reactions [16,17,19\u201322].The EPOC phenomenon has been widely investigated for different kinds of hydrogen production reactions, e.g., catalytic reforming of methane [20], water gas shift reaction [23\u201325] and reforming or partial oxidation of alcohols such as methanol and ethanol [16,26,27]. However, one can find in literature a unique previous study of EPOC in the catalytic decomposition of ammonia [17]. In this previous study an iron catalyst film deposited on both K2YZr(PO4)3 (K+ ionic conductor) and, CaZr0.9In0.1O3-\u03b1 (H+ ionic conductor material), were electrochemically activated. Although very promising results were obtained, a high temperature range (500\u2013600 \u00b0C) was explored probably due to the requirements for ionic conductivity of the solid electrolyte used.In this work it has been explored for the first time in the literature, the effect of the electrochemical promotion for low temperature catalytic decomposition of ammonia (250\u2013350 \u00b0C). For that purpose, a ruthenium catalyst and an alkaline solid electrolyte (Na-\u03b2Al2O3 and K-\u03b2Al2O3) have been used on the catalytic reaction. Very promising results have been obtained in a lower temperature range (250\u2013350 \u00b0C) which have been discussed in terms of the EPOC rules and the mechanism of ammonia decomposition reaction. Hence, relevant findings are reported of great interest for the general catalysis field which could serve for the design of future novel catalyst formulations.Na-\u03b2Al2O3 and K-\u03b2Al2O3 (20\u00a0mm diameter and 1\u00a0mm thickness from Ionotec company) solid electrolyte discs were used as supports. First, thin coatings of gold paste (Gwent Electronic Materials) were deposited on the one side of the solid electrolyte disk as Au counter (CE) and reference (RE) electrodes, followed by calcination steps at 300 \u00b0C for 1\u00a0h (5 \u00b0C\u00b7min\u22121) and 800 \u00b0C for 2\u00a0h (5 \u00b0C\u00b7min\u22121). Blank experiments demonstrated the catalytically inactive properties of the prepared gold counter and reference electrodes for ammonia catalytic decomposition reaction. Then, an electrically continuous ruthenium catalyst film-working electrode (WE) (geometric area of 2.01 cm2) was deposited on the other side of the disk as schematically shown in Figure S1 of the supporting information. An impregnation method described in detail elsewhere was used [28] by using a RuCl3 solution. The precursor salt, RuCl3\n\u00b73H2O (Sigma Aldrich) was dissolved in 1:1 (volume) water: 2-propanol (Sigma Aldrich, 99.9% purity) solution, followed by a calcination step at 500 \u00b0C for 1\u00a0h (5 \u00b0C\u00b7min\u22121). Both obtained electrochemical catalysts (Ru/Na-\u03b2Al2O3 and Ru/K-\u03b2Al2O3) showed a similar final metal loading of 1.3 mgRu\n\u00b7cm\u22122. Before the catalytic activity measurements, the metal catalyst film was reduced under 5 v/v% H2/He gas mixture (100 mL\u00b7min\u22121) at 400 \u00b0C for 1\u00a0h (10 \u00b0C\u00b7min\u22121), in order to ensure the complete reduction of the active ruthenium particles, achieving a metal catalyst film with an in plane electrical resistance around 30 \u00a0\u03a9.For the ex-situ characterization of the ruthenium catalyst film, X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques were used. XRD diffractograms were obtained on a Philips X'Pert MPD with co-filtered Cu-Ka radiation (\u03bb=1.54056\u00a0\u00c5), after reduction under 5 v/v% H2/Ar and for used catalyst. The XRD pattern were recorded from 20<2\u03b8<80\u00b0 with a scan rate of 0.02\u00b0 step size and acquisition time of 4\u00a0s per step. The crystal size was determined by the Debye-Scherrer Eq.\u00a0(1):\n\n(1)\n\n\n\nd\n=\n\n\n\nK\n\u00b7\n\u03bb\n\n\n\u03b2\n\u00b7\ncos\n\u03b8\n\n\n\n\n\nwhere d is the average particles size (nm), assuming particles are spherical, K\u00a0=\u00a00.9, \u03bb=1.54056\u00a0\u00c5, \u03b2 is the full width at half the diffracted peak and \u03b8 is the Bragg angle.SEM images of the ruthenium films were performed by ZEISS GeminiSEM 500 FE-SEM with a PIN-diode BSE detector. This instrument was equipped with an energy-dispersive X-ray spectroscopy (EDX) analyzer to verify the composition of the samples.Catalytic tests were carried out in a single chamber solid electrolyte cell reactor configuration (Figure S2 of the supporting information). The electrochemical catalyst was suspended in the reactor by using gold wires (Alfa Aesar, 99.95% purity) which also allow the electrical connections of the three electrodes (Counter (CE), Reference (RE) and Working electrode (WE)) with the potentiostat-galvanostat (Voltalab PGZ 301, Radiometer Analytical). The EPOC phenomenon was investigated by applying different electrical potentials between the WE and CE and measured between the WE and RE electrodes (VWR), according to the technique generally used in conventional three-electrode electrochemical cells [29]. Reaction gasses (Air Liquide) were certified standards of helium (99.999% purity) and ammonia (5011.50\u00a0ppm) and the gas flows were controlled by a set of calibrated mass flow meters (Brooks 5850 E).The ammonia decomposition reaction was carried out at atmospheric pressure operating at a temperature range from 220 to 350 \u00b0C, using an inlet composition of 1250\u00a0ppm ammonia with an overall gas flow rate of 200 mL\u00b7min\u22121 (He balance). Gas effluents were analysed on-line with a dispersive IR Rosemount X-STREAM Enhanced XEGP continuous gas analyzer (EMERSON) for ammonia detection. The hydrogen formation rate (mmol H2\n\u00b7min\u22121\n\u00b7gcat\n\u22121) was calculated from balance of the ammonia content in the outgas stream, while conversion of ammonia (\n\n\nX\n\n\n\nN\n\n\n\nH\n\n3\n\n\n\n) was calculated as follows:\n\n(2)\n\n\n\nX\n\nN\n\nH\n3\n\n\n\n\n(\n%\n)\n\n=\n\n\n\nF\n\nN\n\n\n\nH\n3\n\n\nin\n\n\n\n\u2212\n\nF\n\nN\n\n\n\nH\n3\n\n\nout\n\n\n\n\n\nF\n\nN\n\n\n\nH\n3\n\n\nin\n\n\n\n\n\n.\n\n100\n\n\n\n\n\nWhere, \n\nF\n\nN\n\n\n\nH\n3\n\n\nin\n\n\n\n and \n\nF\n\nN\n\n\n\nH\n3\n\n\nout\n\n\n\n referred to the inlet and outlet ammonia molar flows (mmol gas\u00b7min\u22121), respectively. Furthermore, the apparent activation energy of the synthesized catalysts was calculated from the Arrhenius plot at low conversion values (<10%) in order to operate into differential conditions.The electrochemical catalyst was also in-situ characterized in the single chamber cell reactor by cyclic voltammetry experiments. A cyclic voltammetry experiment with simultaneous ammonia recording was performed under ammonia catalytic decomposition reaction (1250\u00a0ppm ammonia and a gas flow of 200 mL\u00b7min\u22121) at 280 \u00b0C and a scan rate of 5 mV\u00b7s\u2212\n1.Firstly, the catalytic activity of the ruthenium catalyst film was tested from 200 to 450 \u00b0C in the ammonia decomposition reaction under open circuit conditions (O.C.) for Ru/Na-\u03b2Al2O3 electrocatalyst (Figure S3 of the supporting information). This initial experiment demonstrated that the prepared metal catalyst film was active for the ammonia catalytic decomposition reaction under the explored reaction conditions. It shows a typical trend of ammonia conversion increasing with the reaction temperature due to the increase in the reaction kinetics [3]. However, it can be observed that full conversion of ammonia was never reached (maximum conversion of 74% at 450 \u00b0C) due to certain bypass of the gas reactants flow to the ruthenium catalyst film. This behavior is typically observed with those kinds of solid electrolyte cells reactors and have been reported in previous studies [30].The crystalline structure of the ruthenium catalyst film in the Ru/Na-\u03b2Al2O3 electrocatalyst after hydrogen reduction and after ammonia decomposition reaction was examined by XRD (Fig.\u00a01\n). The reduced electrocatalyst showed the main diffraction peaks corresponding to hexagonal phase of Na-\u03b2Al2O3 (JCPDS: 19\u20131173) associated with the electrolyte support [31] and the hexagonal phase of metallic ruthenium (JCPDS: 06\u20130663) [7]. After catalytic activity measurements, the post-reaction ruthenium catalyst film showed the same crystalline phases than the reduced sample (metallic ruthenium and Na-\u03b2Al2O3) not showing crystalline phases derived from the EPOC experiments. Using the Scherrer equation at peak 2\u03b8=44\u00b0, the average particles size of ruthenium for both electrocatalyst, reduced and post reaction, was 15.7 and 21.4\u00a0nm, respectively. These results demonstrated certain sintering of the metal particles in the catalyst film during the catalytic activity measurements. This kind of sintering of metal particles are typical for conductive catalyst films and have been reported in previous studies of EPOC [28].\nFig.\u00a02\n shows SEM micrographs and EDX analysis of the metal film on reduced catalyst (a, b and c) and after ammonia decomposition reaction (d, e and f). Typically, the ruthenium film after reduction treatment was porous and continuous, which suggested that the preparation technique was adequate to synthesize ruthenium films for EPOC experiments [32]. It was observed from the EDX of the reduced sample (Fig.\u00a02b), large concentrations of sodium (in blue) which were attributed to the thermal migration of alkaline ions from the solid electrolyte to the ruthenium catalyst film during the catalyst preparation procedure and reduction step [16,21]. From the SEM analysis of the post reaction sample, in agreement with the XRD, a sintering of the metal particles in the catalyst film could be observed. Furthermore, some cracks in the ruthenium catalyst film after catalysis were observed at microscopy level. However, these cracks did not affect the electrical conductivity of the ruthenium catalyst film as verified along the surface with a multimeter. In any case, this sintering process of the metal particles shown in the sample after catalytic reaction probably occurred at the beginning of the experimental tests (initial experiment under open circuit conditions shown in Figure S3), since reproducible and stable catalytic activity values were obtained during EPOC experiments (as will be shown below).\nFig.\u00a03\n shows the variation of the hydrogen production rate vs. time at different applied potentials between 2 and \u22122\u00a0V at 300 \u00b0C. Each polarization was applied for 30\u00a0min in order to achieve a steady state catalytic behavior.In agreement with previous EPOC studies and considering that the initial polarization at VWR= 2\u00a0V allowed to obtain a metal catalyst film free of any sodium ions (reference state, i.e., un-promoted catalytic activity), the subsequent decrease in the applied potential led to the electrochemical supply of sodium ions to the catalyst surface. It was confirmed by the obtained measured negative electrical currents (not shown here) of higher magnitude as the applied potential decreased to more negative values, corresponding to a higher electrochemical supply of electropositive alkali sodium ions. This direct correlation has been recently confirmed by an Operando Near Ambient Pressure Photoemission Spectroscopy performed on Ni/K-\u03b2Al2O3 in a previous work of the research team [33]. Concerning the variation of the catalytic activity with the different applied polarizations, according to previous studies [22,24,25,34\u201337], one may rationalize the observed variation in the catalytic activity on the basis of the catalyst-electrode work function modification upon applied potential. A decrease in the catalyst potential below the open circuit conditions, and therefore, in the catalyst work function, led to an increase of the electronic density of ruthenium catalyst with a concomitant spillover of Na+ ions from the electrolyte onto the metal catalyst surface. This decreased work function, weakened the ruthenium chemical bond with electron-donor adsorbates and strengthened those with electron acceptors ones. In this work, the strong promotional effect, observed when lowering the catalyst potential below the open circuit conditions (50\u00a0mV), can be explained on the basis of a strengthening of the chemisorptive bond of weakly adsorbed N surface species, which stabilizes N on the catalyst surface. It facilitates the ammonia decomposition reaction, in good agreement with a previous EPOC study in the catalytic ammonia decomposition reaction on iron deposited on K2YZr(PO4)3, a K+ conductor solid electrolyte [17]. This experimental observation is also in good agreement with the mechanism proposed in the literature for the catalytic ammonia decomposition reaction, which follows these six consecutive steps [38]:\n\n(3)\n\n\nStep\n\n1\n:\n\n\nN\n\n\n\nH\n\n3\n\n\n(\n\ng\n\n)\n\n\n\u2194\n\n\nN\n\n\n\nH\n\n3\n\n\n(\n\na\n\n)\n\n\n\n\n\n\n\n(4)\n\n\nStep\n\n2\n:\n\n\nN\n\n\n\nH\n\n3\n\n\n(\n\na\n\n)\n\n\n+\n\n\ns\n\n\n\u2194\n\n\nN\n\n\n\nH\n\n2\n\n\n(\n\na\n\n)\n\n+\n\nH\n\n\n(\n\na\n\n)\n\n\n\n\n\n\n\n(5)\n\n\nStep\n\n3\n:\n\n\nN\n\n\n\nH\n\n2\n\n\n(\n\na\n\n)\n\n\n+\n\n\ns\n\n\n\u2194\n\nNH\n\n(\n\na\n\n)\n\n+\n\nH\n\n\n(\n\na\n\n)\n\n\n\n\n\n\n\n(6)\n\n\nStep\n\n4\n:\n\nNH\n\n(\n\na\n\n)\n\n\n+\n\n\ns\n\n\n\u2194\n\n\nN\n\n\n(\n\na\n\n)\n\n+\n\nH\n\n\n(\n\na\n\n)\n\n\n\n\n\n\n\n(7)\n\n\n\nStep\n\n5\n:\n\n2\nN\n\n\n(\na\n)\n\n\n\u2192\n\n\nN\n2\n\n\n(\ng\n)\n\n\n+\n2\ns\n\n\n\n\n\n\n\n(8)\n\n\nStep\n\n6\n:\n\n2\n\nH\n\n\n(\n\na\n\n)\n\n\n\u2194\n\n\n\nH\n\n2\n\n\n(\n\ng\n\n)\n\n+\n2\n\ns\n\n\n\n\nwhere s represents a vacant site of the catalyst surface, (g) stands for gas and (a) stands for adsorbed molecules. Hence, a previous detailed kinetics analysis [8] has demonstrated that for the case of a ruthenium based catalyst under similar reaction conditions than our work, step 2 is the main rate determining step in the overall reaction mechanism. Then, the stabilization of NH2(a) adsorbed species on the catalyst surface when the potential is decreased, might increase the kinetics of the dehydrogenation reactions (steps 2\u20134), leading to an overall activation of the process. On the other hand, the increase in the value of the applied potential above that of the open circuit conditions (50\u00a0mV), enhanced the binding strength of the electron donor ammonia molecules (facilitating the ammonia adsorption, step 1 of the mechanism) [17]. It led to a slight increase in the catalytic activity, as can be observed under application of 2\u00a0V in Fig.\u00a03, but much less important considering that a higher effect is produced when the electronic effect increases the kinetics of the rate determining step. Finally, it can also be observed that the final application of a potential of 2\u00a0V allowed to recover the initial un-promoted catalytic activity achieved at the beginning of the experiment, leading to a reversible EPOC phenomenon [16,25]. It implied that the same amount of sodium ions, initially transferred from the solid electrolyte to the catalyst during the previous polarizations, were returned from the catalyst to the solid electrolyte after the final application of 2\u00a0V. It also demonstrated the stability of the catalyst film under EPOC reaction conditions. Therefore, the sintering of the metal particles observed in Figs.\u00a01 and 2 might occurred during the preliminary reaction experiment under O.C. (Figure S3), stabilizing the ruthenium catalyst film for the subsequent EPOC experiments.In order to study the influence of the reaction temperature, the same experiment performed at 300 \u00b0C was repeated at other reaction temperatures, i.e. 250 \u00b0C, 320 \u00b0C and, 350 \u00b0C. The steady state variation of the ammonia conversion vs. the applied polarization at the different reaction temperatures is shown in Fig.\u00a04\n. Also, to quantify the magnitude of the EPOC phenomenon, Fig.\u00a04 shows the maximum value calculated of the rate enhancement ratio (\u03c1), obtained by the following equation and typically used in EPOC studies [24,39]:\n\n(9)\n\n\n\n\u03c1\n\n=\n\n\n\nr\n\nr\n0\n\n\n\n\n\nwhere r0 is the un-promoted catalytic reaction rate (VWR=2\u00a0V) and r is the promoted catalytic reaction rate at the explored potential.Firstly, it could be observed a similar overall EPOC behavior, at the explored reaction temperatures, to the one already described in Fig.\u00a03, with an increase in the catalytic reaction rate as the applied potential decreases from the open circuit potential values (typically around 50\u00a0mV). As typically found in previous EPOC studies with alkaline ion conductors [40], the highest value of the rate enhancement ratio was achieved at an intermediate reaction temperature, i.e. 300 \u00b0C, increasing the ammonia conversion 1.4 times vs. the un-promoted conditions (2\u00a0V). Hence, at low temperatures (i.e. 250 \u00b0C) the EPOC phenomenon is limited by the low ionic conductivity of the solid electrolyte which decreases the amount of electrochemically supplied ions. However, above certain reaction temperatures (e.g. 320 \u00b0C), the relative increase induced by the EPOC phenomenon, measured by \u03c1, is limited by the initial higher value of the un-promoted catalytic activity (i.e. 30% at 320 \u00b0C). On the other hand, it is also interesting to note that, a poisoning effect can be observed at higher temperatures (above 320 \u00b0C), which led to an optimal value of the applied potential which maximized the catalytic activity at VWR= \u22121\u00a0V at 320 \u00b0C and VWR= \u22120,5\u00a0V at 350 \u00b0C. This behavior typically found in previous studies of alkaline EPOC can be attributed to an excess of promoting species on the catalyst surface which block the catalytic active sites [41]. In the present study, as already mentioned, the application of negative potentials decreased the relative coverage of electron donor molecules (ammonia) (step 1), causing a poisoning effect observed above certain temperatures and below certain applied potentials. Hence, above certain temperatures and above certain alkali coverage on the catalyst, the mechanism could be limited by the ammonia adsorption step, which may also justify the higher inhibiting effect observed at 350 \u00b0C vs. 320 \u00b0C at high negative potential (close to \u22122\u00a0V). On the other hand, it could be observed that higher optimal applied potential values (which maximized the catalytic activity) were obtained at higher reaction temperatures. It can be attributed to an increase in the solid electrolyte ionic conductivity with temperature leading to a higher supply of sodium ions for the same potential. Obtained results clearly demonstrated the interest of the EPOC phenomenon to electrochemically supply the optimal promoter amount at different reaction conditions (e.g. temperature), which is not possible with conventional heterogeneous catalyst doped with alkali, where a fix amount of promoter is added to the catalyst during the preparation step. This is one of the most interesting findings of EPOC which has been analysed in detail in a previous reviewed manuscript [42].In order to evaluate the electro-promotional effect of other type of alkali ions, the EPOC phenomenon was investigated on a Ru/K-\u03b2Al2O3 electrochemical catalyst. \nFig.\u00a05\n\n shows the variation of the hydrogen production rate vs. time at different applied potentials (between VWR= 2\u00a0V and \u22122\u00a0V) and reaction temperatures (300 and 320 \u00b0C). Each polarization was again applied for 30\u00a0min until a steady state catalytic behavior is achieved.In good agreement with the results obtained with Ru/Na-\u03b2Al2O3, a strong activation effect in the catalytic activity was observed under conditions of electrochemically supply of alkali ions (K+). The observed promotional effect can be explained considering the influence of the alkali ions on the binding strength of chemisorbed reactants and intermediate molecules, as previously discussed for Ru/Na-\u03b2Al2O3. The electrochemical supply of potassium ions enhanced the chemisorption of electron acceptor molecules (weakly adsorbed N surface species) facilitating ammonia decomposition reactions, increasing the rate of the three consecutive steps 2, 3 and 4, on the previously mentioned reaction mechanism. In this case, it can be observed that an optimum applied potential of VWR= 0\u00a0V and VWR=0.5\u00a0V was obtained at 300 \u00b0C and 320 \u00b0C, respectively. These optimal VWR values were higher than the ones obtained for Ru/Na-\u03b2Al2O3, which can be justified considering the higher electronic effect on the catalyst of K+vs. Na+ions, attributed to the different ionic size of both cations (Na+= 0.10\u00a0nm and K+=0.13\u00a0nm). In addition, some authors suggested that the higher dipole moment of potassium ion (~14 Debye) could increase the promotional effect of K+ vs. Na+ (whose dipole moment was ~6 Debye) and H+ ions into the CO2 hydrogenation reaction on Ru catalysts [43]. Hence, Lang et\u00a0al. [44] clearly showed that the larger the alkali cation was, the greater the electric field suffered by co-adsorbed species located at an adjacent site. These higher promotional effect of K+ vs. Na+ ions have been observed in different chemical reactions through Electrochemical Promotion [35,45,46]. On the other hand, in a previous study of a conventional heterogeneous Ru catalyst doping with K+ and Na+ led to an increase in the ammonia conversion with respect un-doped catalyst at lower temperature, being the Ru-K the highest ammonia conversion due to the lower electronegative of potassium [47].Thus, K+ should perturb the Ru-NH3 bond more strongly than Na+, leading to a higher activation effect. In fact, considering the lower ionic conductivity of K-\u03b2Al2O3 (5.5 (ohm.m)\u22121) vs. Na-\u03b2Al2O3 (23.8 (ohm.m)\u22121) at 300 \u00b0C provided by the supplier (Ionotec), lower values of potassium vs. sodium ions coverages on the metal catalyst surface would be attained for the same explored potential range (at the same temperature). Then, the higher effect on the reactants and intermediates chemisorption induced by K+ vs. Na+ can also explain the lower optimal potential values obtained, which maximized the hydrogen production rate. As already mentioned, the presence of an optimal VWR value, and hence, an optimal alkali coverage on the catalyst, is probably due to an excessive decrease on the adsorption of electron acceptor ammonia molecules induced by potassium, which limits step 1 in the overall reaction mechanism. Results are in good agreement with a previous work of EPOC on catalytic ammonia decomposition reaction, in which moderate potassium coverages electrochemically supplied at VWR=1.3\u00a0V were found to optimize the catalytic activity of an iron catalyst [17]. On the other hand, it is also interesting to note that a slight permanent EPOC effect was observed at 300 \u00b0C (the initial un-promoted catalytic rate was not reached after the final polarization of 2\u00a0V). This permanent effect is clearer at 320 \u00b0C in which larger differences between the initial and final polarization at VWR=2\u00a0V were obtained. This kind of permanent EPOC effect has been also observed in previous studies above certain reaction temperatures working with K+ ions conductor solid electrolytes [35]. It might be explained considering the possible formation of different kinds of promoting phases with higher stability on the catalyst surface, which cannot be electrochemical decomposed under the explored reaction conditions (temperature and electrical potential) [26]. Likely, an applied potential value higher than 2\u00a0V would be required for the decomposition of such promotional phases in order to reach the initial un-promoted state. At this point, it should be mentioned that under the explored reaction conditions, K+ ions may also form different kinds of surface species (promotional phases) as a result of their reaction with the chemisorbed reactant and intermediates molecules. These species such as ammonia and nitrogen adsorbed molecules react with K+ ions by distinct charge transfer electrochemical reactions (e.g., potassium nitrites, nitrates or nitrides among others) occurring at the different applied VWR values. In fact, various kinds of promotional phases have been already shown in other EPOC studies with potassium ions conductor electrolyte, verified by ex-situ FTIR [35] and XPS measurements [48]. This in-situ formation of different kinds of adsorbed species will be analysed with more detail in the next section by cyclic voltammetry measurements. Thus, the presence of distinct kind of promotional phases could also confirmed the presence of two local maximum on the hydrogen production rates observed at 320\u00a0\u00b0C at VWR=0.5\u00a0V and \u22121.5\u00a0V. The formation and nature of different kind of promotional phases causing two optimal applied potential values have been reported in a previous work of hydrogen production via partial oxidation of methanol on platinum catalyst film deposited on K-\u03b2Al2O3\n[26]. Thus, the catalytic rates would be affected by the formation of several kinds of promoter phases as will be also lately confirmed by cyclic voltammetry. The higher promotional effect of potassium vs. sodium ions can be clearly observed also on Figure S4 of the supporting information, which compares the potentiostatic variation of \u03c1 vs. VWR at the two common explored reaction temperatures (300 and 320 \u00b0C). It can be observed that in both cases higher \u03c1 values were obtained for Ru/K-\u03b2Al2O3 vs. Ru/Na-\u03b2Al2O3 at lower VWR values. Hence, in the case of Ru/K-\u03b2Al2O3 the catalytic hydrogen production rate via ammonia decomposition under optimal promotor coverage is multiplied by a factor close to 231.8% at 300 \u00b0C, which is one of the most important findings of the present work. It led to an overall higher ammonia conversion on the Ru/K-\u03b2Al2O3 electrochemical catalyst vs. Ru/Na-\u03b2Al2O3 as can be observed on Figure S5 of the supporting information. Considering the similar nature of both kinds of ruthenium catalyst films deposited on both, Na-\u03b2Al2O3 and K-\u03b2Al2O3 (same preparation procedure and same geometric area of the catalyst working-electrode), the observed difference in the ammonia conversion is clearly due to a higher promotional effect of potassium. Therefore, considering that the most interesting results were obtained by the Ru/K-\u03b2Al2O3 electrochemical catalyst, this sample was selected for further reaction and characterization experiments as will be shown below.\nFig.\u00a06\n shows the ammonia conversion on Ru/K-\u03b2Al2O3 electrochemical catalyst through temperature programmed reaction experiments (2 \u00b0C\u00b7min\u22121) under application of three different potentials (VWR=2, 0 and \u22121\u00a0V).In first place it can be observed that under the three explored VWR values, ammonia conversion increased with the reaction temperature due to an increase in the reaction kinetics and the endothermic nature of the reaction. In good agreement with previous results, it can be observed that the application of a mild potential value (VWR= 0\u00a0V) enhanced ammonia conversion at the whole explored temperature range vs. the un-promoted potential value (2\u00a0V). Under these conditions, a moderate amount of potassium ions was supplied to the catalyst leading to an electrochemical activation of the metal catalyst film. This catalyst potential (VWR= 0\u00a0V) was selected from the results obtained in Fig.\u00a05 in order to improve the catalytic activity in a wide temperature range (220\u2013320 \u00b0C). However, it can be observed again that, a decrease in the potential to VWR=\u22121\u00a0V decreased ammonia conversion in the whole temperature range. It was related to a strong increase on the K+ coverage, which weakens the binding strength of ammonia, decreasing the reaction rate. Moreover, the excess of promoter over catalyst surface, induced by a negative potential, led to the blockage of ruthenium active sites causing electrochemical poisoning, similarly to a previous EPOC study on ammonia decomposition using an Fe electrochemical catalyst under potassium coverages [17]. This kind of poisoning behavior has been also observed in previous studies of catalytic ammonia decomposition on ruthenium catalysts chemically doped with potassium [47] and cesium [49].In any case, results shown in Fig.\u00a06 demonstrate the interest of EPOC phenomenon to activate the catalyst at lower reaction temperatures, which may be of great interest for energy saving, especially for endothermic reactions as the one explored here. In this sense, Figure S6 of the supporting information shows the reaction temperature necessary to achieve a specific ammonia conversion (5 and 10%) as a function of the three explored potentials (VWR=2, 0 and \u22121\u00a0V) used in the experiment of Fig.\u00a06. It can be observed an activation by means of a reduction of 20 \u00b0C to achieve 5% of ammonia conversion and 30 \u00b0C for 10% ammonia conversion induced by EPOC under VWR= 0\u00a0V. At this point it should be mentioned that a further optimization could be performed (out of the scope of the present study), applying the optimal potential value at each explored temperature to maximize the hydrogen production rate.The apparent activation energy values (Ea) were calculated from these experiments via the Arrhenius plot at each applied potentials (VWR=2, 0 and \u22121\u00a0V) as shown in Figure S7 of the supporting information. The obtained values, which were in the range of previously obtained ones with ruthenium catalysts [7,50], decrease from Ea\u00a0=\u00a080.9 kJ\u00b7mol\u22121 at VWR= 2\u00a0V to Ea\u00a0=\u00a074.6 kJ\u00b7mol\u22121 at VWR= 0\u00a0V. This decrease in the Ea induced under electrochemical activation conditions is in good agreement with the promotional mechanism explained above. Hence, the optimal potassium coverages achieved at VWR=0\u00a0V allowed an activation of the chemisorption of electron acceptor molecules (N adsorbed molecules), which promotes the rate determining step of ammonia decomposition reaction, decreasing the Ea\n[17]. In addition, a similar decrease in the apparent activation energy of 5\u201320 kJ\u00b7mol\u22121 was observed for ruthenium catalysts chemically doped with potassium species on ammonia decomposition reaction [47].\nFig.\u00a07\n shows the cyclic voltammetry experiment (CV) of Ru/K-\u03b2Al2O3 with the simultaneous recording of ammonia signal under ammonia catalytic decomposition reaction at 280 \u00b0C, from VWR=2\u00a0V to \u22122\u00a0V and a scan rate of 5 mV\u00b7s\u2212\n1. Before the cyclic voltammetry experiment, the electrochemical catalysts were kept at 2\u00a0V for 30\u00a0min in order to define an initial reference state.Staring from 2\u00a0V, during the initial forward scan from 2\u00a0V to \u22122\u00a0V, it can be observed negative current values attributed to the electrochemical supply of K+ ions from the solid electrolyte to the metal catalyst film with the simultaneous formation of surface promotional phases between the K+ ions and the chemisorbed reactant and product species [51]. According to the obtained CV curve one could envisage different kinds of charge transfer reactions evidenced by the different obtained cathodic peaks vs. other CV curves obtained in EPOC systems where single cathodic peaks were typically observed [51]. It demonstrates the formation of different kinds of promotional phases between the K+ ions and chemisorbed molecules, in good agreement with the different observed promotional effects already discussed in Fig.\u00a05. During the forward CV scan, an increase in the hydrogen production rate was also observed due to the electrochemical supply of K+ ions. In good agreement with the previous results shown in Fig.\u00a05, an optimal potential value was attained corresponding to an optimal alkali coverage on the catalyst surface. During the positive scan from \u22122\u00a0V to 2\u00a0V (backward scan) the different promotional phases previously formed on the metal catalyst film were electrochemically decomposed, returning the K+ ions to the solid electrolyte and leading to the appearance of different anodic current peaks. It is interesting to note that during this positive scan a new promotional state is again reached at VWR= \u22120.5\u00a0V maximizing the hydrogen production rate. In addition, during the backward scan a second local maximum in the hydrogen production rate was also observed at VWR= 1\u00a0V. Considering the dynamic character of the experiment, these results again confirmed the presence of different kinds of promotional phases which locally optimize the hydrogen production rate at different potentials. Some of these species were likely formed from the partial electrochemical decomposition of the promotional phases initially formed during the forward scan. The existence of a permanent EPOC effect is also clear at the end of the cyclic voltammetry experiment at 2\u00a0V which showed a higher hydrogen production rate in the backward vs. the forward scan. At this point (VWR= 2\u00a0V), positive current values were obtained showing that certain promotional phases were still present on the catalyst surface and were not completely removed (via electrochemistry). It supports the origin of the permanent EPOC phenomenon observed in Fig.\u00a05 and previously discussed in the case of the Ru/K-\u03b2Al2O3 electrochemical catalyst.The electrochemical supply of alkaline ions (Na+ and K+) to a ruthenium catalyst film activates the hydrogen production rate via catalytic ammonia decomposition reaction. The promotional effect is due to the strengthening of the chemisorptive bond of weakly adsorbed N surface molecules which stabilizes N adsorbed species on the ruthenium catalyst surface. This stabilization increases the kinetics of the ammonia decomposition reaction leading to the overall activation of the process.A higher promotional effect of K+ ions was found vs. Na+ ions. It is attributed to the higher electronic effect induced by K+which allows to increase the hydrogen production rate above 230% under optimal potential conditions. However, a large amount of alkali ions supplied to the catalyst at very negative potentials, led to a poisoning effect on the catalytic activity. It is associated with a strong decrease in the chemisorption of the electron donor molecules (ammonia).Temperature programmed reaction experiments show the interest of EPOC for the electrochemical activation of the catalyst at lower temperatures, which may contribute to decrease on the overall energy requirements of the process.A permanent EPOC effect was also found for the case of Ru/K-\u03b2Al2O3 which led to a permanent activation of the catalyst under the explored conditions. It was attributed to a higher stability of some promotional phases formed during the negative polarization as supported by cyclic voltammetry.\nM. Pinz\u00f3n: Investigation, Methodology, Validation, Visualization, Writing \u2013 original draft. E. Ruiz-L\u00f3pez: Investigation, Methodology. A. Romero: Conceptualization, Visualization, Writing \u2013 review & editing, Supervision, Funding acquisition. A.R. de la Osa: Conceptualization, Visualization, Writing \u2013 review & editing, Supervision, Funding acquisition. P. S\u00e1nchez: Conceptualization, Visualization, Writing \u2013 review & editing, Supervision, Funding acquisition. A. de Lucas-Consuegra: Conceptualization, Visualization, 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 the Regional Government of Castilla-La Mancha and the European Union [FEDER funds SBPLY/180501/000281].Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.mcat.2021.111721.\n\n\nImage, image 1\n\n\n\n", "descript": "\n                  This study reports the electrochemical activation (EPOC) of ruthenium catalyst film with alkaline ion conductors for hydrogen production via catalytic decomposition of ammonia. Two electrocatalysts, Ru/Na-\u03b2Al2O3 and Ru/K-\u03b2Al2O3 have been prepared, characterized, and tested under low temperature reaction conditions (250\u2013350 \u00b0C). The electrochemically supply of moderate amounts of alkaline ions (Na+ and K+) from the solid electrolyte support to the ruthenium catalyst film, activated the hydrogen production rate. The promotional effect has been attributed to a strengthening of the chemisorptive bond of weakly adsorbed N surface species, which stabilizes N adsorbed molecules on the ruthenium catalyst surface and thus facilitating the ammonia decomposition reaction. Among the two alkali ions, the effect of potassium was stronger, increasing the hydrogen production rate above 230% at 300 \u00b0C under optimally conditions. Temperature programmed reaction experiments also confirms the interest of EPOC for the activation of the catalyst at low temperatures.\n               "}
{"full_text": "The intensive consumption of fossil fuels along with excessive emission of carbon dioxide (CO2) acceleratingly exacerbate global environmental problems, which severely limit the potential of a sustainable progress of human civilization.\n1,2\n Developing clean energy conversion technologies becomes extremely urgent to circumvent these challenges. Electrochemical CO2 reduction reaction (CO2RR) under ambient conditions, coupled with renewable electricity sources, represents a promising approach to curb CO2 emissions while generating value-added fuels and chemicals.\n3\u201313\n In a variety of CO2RR pathways such as C1 (CO, formate, methane, etc.),\n14\u201321\n C2 (ethylene, ethanol, etc.),\n22\u201328\n or C3 (n-propanol, etc.),\n29,30\n the reduction of CO2 to CO is currently one of the most promising practices due to its relatively high selectivity and large current density, as well as the facile separation of gas product from liquid water. More importantly, CO as a fundamental chemical feedstock such as the component of syngas, holds a large market compatibility and a wide range of applications in bulk chemicals manufacturing, medicine, and so on. Despite recent breakthroughs on exploiting various selective catalysts for reduction of CO2 to CO, the ultimate practical viability of this technology, however, is contingent upon the scaling up of CO2RR process, which is still in its infancy with challenges in catalyst cost, product selectivity, scalable activity, as well as long-term stability.On the way of scaling up CO2RR for practical CO2 electrolysis, mass production of\u00a0high-performance catalysts with cost efficiency is the cornerstone and first step.\u00a0However, there are only a few known catalysts to date, including Au and Ag noble metals,\n31\u201334\n developed to deliver a significant selectivity toward CO evolution. As a cost-effective substitute and for the continuous efforts in our group,\n35,36\n earth-abundant single-atom catalysts (SACs) provide an intriguing paradigm for CO2-to-CO conversion, with projected high atomic efficiency, superior activity, and selectivity.\n37\u201341\n The Ni single atoms coordinated in graphene vacancies, with/without neighboring N coordination, have been demonstrated to be highly selective to CO.\n42\u201345\n Nevertheless, the commonly pursued strategies for preparing SACs,\n46\n e.g., core-shell strategy, confined pyrolysis strategy, and polymer encapsulation strategy are not as straightforward to scale up, and sometimes lack general applicability: most of the carbon precursors, including graphene oxides,\n36,45\n carbon nanotubes,\n47\n and metal organic frameworks (MOFs),\n43\n are either not economically viable for large-scale production, or involve relatively complicated preparation steps; in addition, some of the carbon matrix with nanosheet structures suffer from gas diffusion limit when piled up layer by layer on the electrode, greatly hindering the reduction current density for practical implementation. In this sense, developing a facile process for massive production of SACs becomes an important stepping-stone for practical CO2 electrolysis.Another critical challenge that goes beyond the nature of the electrocatalysts revolves around the low current density needed to maintain a high CO selectivity. In a traditional H-cell device where the catalysts were immerged in liquid water, the maximal CO evolution current was limited by the following two factors: (1) the solubility of CO2 in water is relatively low, and beyond some point the CO2RR current density will be dominated not by the reaction kinetics but by the mass diffusion limitation, and (2) due to the concentrated water molecules around the catalyst surface, once the overpotential is gradually increased for larger current density, the hydrogen evolution side reaction (HER) can take off and eventually dominate the reaction as observed in previous studies.\n43,44,48\n Fuel cell technology emerges as a platform for maximizing the throughput of CO2RR as reflected in the current and selectivity boost, via preventing the catalyst from direct contact with liquid water, as well as facilitating CO2 gas diffusion.\n36,49\u201354\n In addition, the compact design of cell and membrane electrode assembly (MEA) can further boost a practical CO2 electrolyzer system with scalable stacks and gas flow system.\n55\n\nHerein, we report the synthesis of high-performance Ni SACs with commercial carbon black particles as the support via a simple and scalable method. The Ni single-atomic sites exhibit excellent performance for CO2RR in a traditional H-cell, with a CO faradic efficiency (FECO) of \u223c99% at \u22120.681\u00a0V in 0.5\u00a0M KHCO3 aqueous electrolyte. More importantly, large current densities above 100 mA cm\u22122 with nearly 100% CO generation, which are \u223c10-fold higher than the current densities in H-cell, were demonstrated on an anion MEA. An ultra-high CO/H2 ratio of 114, which we define as the \u201crelative selectivity\u201d when the CO selectivity is close to 100% and H2 below 1% by gas chromatography (GC), was achieved while maintaining a significant current of 74 mA cm\u22122. In addition, after 20\u00a0hr continuous operation with an average current density of \u223c85 mA cm\u22122, the CO formation FE was still maintained around 100%, while H2 below 1%. When the Ni SACs were further integrated into a 10\u00a0\u00d7 10-cm2 modular cell, the CO evolution current in one unit cell can be scaled up to as high as 8.3 A with an FECO of 98.4%, representing a large CO generation rate of 3.34\u00a0L hr\u22121 per unit cell.Instead of starting with well-defined graphene matrix or precursors such as polymers or MOFs,\n35,36,43,56\n we used commercially available carbon blacks with activated surface to trap Ni single atoms and thus form a similar coordination environment and active sites for CO2-to CO-conversion. Compared with that of graphene nanosheets where the layer-by-layer stacking could block the gas diffusion pathways,\n36\n the nanoparticulate morphology of the carbon black support further facilitates the CO2 diffusions across the gas diffusion layer to ensure a high local concentration of reactants. An illustration of the synthetic process for the catalyst is shown in Figure\u00a01\n. In a typical preparation (see Experimental Procedures), 1\u00a0g of activated carbon blacks was well dispersed in water, followed with drop-by-drop addition of Ni2+ solution under vigorous stirring. Due to the presence of defects and oxygen-containing functional groups on the surface as well as the high surface areas, the activated carbon black possesses a high adsorption capacity to metal cations in aqueous solution. To ensure a full, but not excess, adsorption of Ni2+ on the carbon black, the solution was stirred overnight and then centrifuged to collect the products denoted as Ni2+-adsorbed carbon black (Ni2+-CB). Subsequently, the Ni2+-CB was mixed with certain amount of urea as the N source and annealed at elevated temperatures (800\u00b0C) in Ar for 1\u00a0hr, with gram scale catalysts (denoted as Ni-NCB) produced.The high-resolution transmission electron microscopy (HRTEM) image of Ni-NCB in Figure\u00a02\nA shows the onion-like, defective graphene layers in CB particles, which can serve well as the coordination matrix for Ni single atoms. The corresponding aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image reveals the individually and uniformly dispersed Ni atoms as bright spots on the CB nanoparticle (Figure\u00a02B). The individual Ni atoms were well separated from each other and were relatively stable under electron beam irradiation, suggesting strong anchoring (Figure\u00a02C). In supplementation, a large area TEM image confirms that no Ni nanoparticles or clusters were formed on the CBs (Figure\u00a0S1). Elemental mapping by energy-dispersive X-ray spectroscopy (EDS) demonstrates that Ni and N species are homogeneously distributed throughout the carbon framework (Figure\u00a0S2). The mass loading of Ni was determined to be \u223c0.27 wt % by inductively coupled plasma atomic emission spectrometry (Experimental Procedures). X-ray photoelectron spectroscopy (XPS) characterization of Ni-NCB was performed to further elucidate the profile of elemental composition and related chemical states (Figure\u00a0S3). The Ni 2p spectrum of Ni-NCB shows a positive Ni 2p3/2 binding energy (854.9 eV) relative to Ni metal\u00a0(852.6 eV), indicating the positive oxidation states of Ni single atoms (Figure\u00a02D). The XPS N 1s spectrum deconvoluted into pyridinic (\u223c398.3 eV), Ni-N (\u223c399.5 eV), pyrrolic (\u223c400.5 eV), quaternary (\u223c401.3 eV), and oxidized (\u223c403.0 eV)-like N species (Figure\u00a0S4).\n45,57\n The atomic concentrations of Ni and N in Ni-NCB determined by XPS is 0.28 at % and 1.81 at %, respectively. Synchrotron-based X-ray absorption near-edge spectroscopy (XANES) and extended X-ray fine structure (EXAFS) were used to determine the electronic and local coordination of the single-atomic sites in Ni-NCB (Experimental Procedures). The Ni K-edge XANES profiles in Figure\u00a02E indicate that Ni species in Ni-NCB were in a higher oxidation state than Ni foil and lower than NiO, according to the near-edge position, which is consistent with the XPS results. As shown in the EXAFS results in R space (Figure\u00a02F), Ni-NCB exhibits prominent peaks at 1.4 and 1.9\u00a0\u00c5 arising from the first shell Ni-N or Ni-C coordination.\n58\n No other typical peaks for Ni-Ni contribution at longer distances (2.2\u00a0\u00c5) were observed. Thus, Ni atoms were atomically dispersed throughout the N-doped carbon blacks. Although different Ni-N and Ni-C structures have been proposed in literatures,\n35,36,59\n the explicit coordination environment of Ni is still not clear and awaits further exploring.The CO2 electrocatalytic reduction activity and selectivity of Ni-NCB were first evaluated in a standard three-electrode H-cell configuration with CO2-saturated 0.5\u00a0M KHCO3 as the electrolyte. In control, an N-doped carbon black (denoted as N-CB) and a Ni-doped carbon black (denoted as Ni-CB) were also prepared for comparison (Figures S5 and S6). As revealed by linear sweep voltammetry in Figure\u00a0S7, Ni-NCB shows a much higher current density in CO2-saturated electrolyte than that of N2, indicating the participation of CO2 gas in the reaction. Steady-state chronoamperometry of CO2 electrolysis was recorded under different potentials between \u22120.3 and \u22121\u00a0V versus reversible hydrogen electrode (vsRHE). The FE of gas products were analyzed by online GC (Figures 3\nA and S8; Supplemental Information).\n35,60\n In CO2-saturated 0.5\u00a0M KHCO3, Ni-NCB exhibits current densities significantly higher than those of Ni-CB and N-CB (Figure\u00a0S8). H2 and CO are the major gas products in all these three samples. For Ni-NCB, CO signals were detectable at \u22120.41\u00a0V vs RHE, suggesting that the onset overpotential of CO2 to CO is at least lower than 290\u00a0mV. It is noted that the overall FE under this potential is far less than 100%, which is possibly due to the instrumental detection limit. As the potential becomes more negative, the FE of CO increases, while that of H2 decreases correspondingly. A high plateau of CO FEs over 95% was retained under a broad potential range from \u22120.6 to \u22120.84\u00a0V vs RHE, with a maximum CO selectivity of above 99% at \u22120.68\u00a0V vs RHE while the competitive HER suppressed to 2%. No other liquid products were detected by 1H nuclear magnetic resonance (NMR) (Figure\u00a0S9). In sharp contrast, NCB exhibits a faint activity for CO generation, indicating that Ni single atoms play a critical role in activating CO2 to produce CO (Figure\u00a0S8). In addition, Ni-CB only shows a maximum FECO of 29%, which is presumably attributed to the poor dispersion of Ni atoms on the CBs in the absence of nitrogen, as demonstrated in our previous study.\n35,36\n The partial current shown in Figure\u00a03B demonstrates that the activity of the Ni-NCB is better than, or comparable with, most of the noble-metal-based catalysts reported to date.\n32,33,61\n Moreover, Ni-NCB exhibits a high intrinsic CO2 reduction activity, reaching a specific CO current of 111 A g\u22121. Besides, a CO2-to-CO Tafel slope of 101\u00a0mV/decade on Ni-NCB (Figure\u00a0S10) suggests that the first electron transfer process generating surface adsorbed *COOH species is possibly the rate-determining step for CO evolution.\n45\n To further testify the intrinsic activity of Ni-NCB for CO2 reduction, the CO production turnover frequency (TOF) per Ni single-atomic site is calculated based on the total mass loaded on the electrode, as a minimum value of estimation, as well as the electrochemical double layer capacity (EDLC), as the effective surface area normalization (Figure\u00a0S11). As shown in Figure\u00a03C, the TOF of Ni-NCB normalized by the mass and electrochemical active surface area (ECSA) for CO production was calculated to be 3.67 and 9.66 s\u22121, respectively, at an overpotential of 0.56 V, which is better than, or comparable with, those of metal porphyrins or noble metal catalysts in aqueous solutions.\n7,8,31,62\n Furthermore, to elucidate the influence of Ni content, N doping, as well as annealing temperature, a series of control samples were prepared and tested for CO2 reduction (Figures S12\u2013S18). It shows that both the partial current density and CO FE of Ni-NCB annealed under NH3 atmosphere are slightly lower than those with urea as N precursor. This could be due to the different vapor pressures of the N dopants, or different radicals from N2H4 and NH3 under high temperature. It also reveals that increasing the Ni loading leads to the generation of Ni clusters, which impairs the overall performance for CO2RR (Figure\u00a0S14). Besides, appropriate temperature and amount of N doping are required to gain the optimal performance of Ni-NCB. More importantly, the CO2-to-CO reduction performance of Ni-NCB is extremely stable, retaining 99% of the initial current for CO formation (\u223c23 mA cm\u22122) after 24\u00a0hr of continuous operation, with FECO remaining above 95%. Post-catalysis HAADF-STEM imaging and EXAFS (Figure\u00a0S19) show that those Ni species still maintain the feature of well-dispersed single atoms, reiterating the excellent chemical stability of the Ni atomic sites in Ni-NCB.The scaling up of CO generation rate in a traditional H-cell is limited by the following two factors: (1) a larger overpotential is usually required to deliver a higher kinetic current, which, however, can promote strong HER competition due to the contact between catalyst and liquid water, and (2) the reduced CO2 gas reactant in an H-cell configuration is that dissolved in liquid water, therefore the reaction rate beyond a certain point is limited by CO2 mass diffusion. To circumvent this issue and inspired by fuel cell reaction mechanisms, an anion MEA was adopted in a gas-phase electrochemical reactor to greatly boost the current density while maintaining high CO selectivity (Experimental Procedures).\n36\n On the cathode side, humidified CO2 gas was supplied. This high concentration of CO2 and low concentration of H2O vapor can block the direct contact between catalyst and liquid water and prevent limiting of reactant diffusion. On the anode side, 0.1\u00a0M KHCO3 solution was circulated whereby the water oxidation is taking place (Figure\u00a0S20). As shown in Figure\u00a03E, the CO2 conversion increases rapidly above 2.1\u00a0V cell voltage and reaches a significantly high current density of 130 mA cm\u22122 at only 2.7\u00a0V without iR compensations. Notably, the catalyst maintains nearly 100% FE for CO formation across a broad range of current densities from 30 to 130\u00a0mA cm\u22122, while the FE of H2 was suppressed to a minimum of 0.9% (Figures 3F and S21). It is important to mention here that, due to the experimental errors introduced by GC detection, the measured CO selectivity could sometimes be slightly higher than 100%, especially when H2\u00a0was suppressed to below 1%. In this case, we propose to define the CO/H2 ratio,\u00a0which we denote as relative selectivity, as an additional criterion to more accurately evaluate the high selectivity toward CO evolution. As shown in Figure\u00a03G, with the gradual increase of cell voltage, the CO/H2 ratio increases accordingly and reaches a maximum value of 113.8, with a high CO2RR current density of 74 mA cm\u22122. This is to our knowledge the highest ratio of CO/H2 under a significant current density compared with the most active catalysts reported to date (Table S1). An impressive stability of the catalyst in this gas-phase electrochemical reactor is also presented in Figure\u00a03H, with an average current density of 85 mA cm\u22122 over 20\u00a0hr continuous electrolysis, while maintaining CO formation FEs \u223c100% and H2 below 1%. The slight degradation of the current density was probably attributed to several factors including the deactivation of catalysts, the corrosion of the gas diffusion layer, as well as membrane degradation (Figures S22\u2013S24). Overall, this high performance of the Ni-NCB catalyst in the gas-phase electrochemical reactor opens up great opportunities in scaling up highly selective CO2 reduction.Motivated by the superior activity of Ni-NCB and its facile synthesis process, it is expected that, by increasing the catalyst loading, extending the size of the gas diffusion layer, as well as alternatively stacking anodes and cathodes, the Ni-NCB integrated gas-phase electrochemical reactor can be further scaled up to produce large CO generation currents for potential practical applications. Here we\u00a0customized one unit cell with a 10\u00a0\u00d7 10-cm2 anion MEA as a preliminary demonstration to justify this application possibility in the future (Figures 4A\u20134C). Considering the high CO2 flow rate needed to ensure sufficient reactants, gas collecting bags were\u00a0used to collect the gas products which were later analyzed by GC under different cell voltages (Experimental Procedures). As shown in Figures 4D\u20134F, a record-high CO2RR current of 8.3 A was achieved with a high CO selectivity approximating to 99% and H2 about 1%. Delivering an average current of \u223c8 A for stability test, our device maintained a stable CO selectivity of more than 90% for over 6\u00a0hr continuous electrolysis with a total volume of 20.4\u00a0L CO generated (Figure\u00a04G). This represents a CO generation rate of 3.42\u00a0L hr\u22121 or 0.14\u00a0mol hr\u22121 and a conversion rate of 11.33%.It is anticipated that this work can be further pushed forward toward commercialized CO2 electrolysis by optimizing several technological aspects according to industry standards. First, stability is one of the major concerns, which suffers from the corrosion of both anode and cathode, as well as membrane and electrode decay, which require tremendous efforts to overcome. In addition, CO2 feed recycling can be set up by separating CO2 from gas products to achieve a sustained CO2 supply. The cost of the anode should also be taken into consideration, which can be greatly reduced by replacing IrO2 with efficient transition metal-based materials. Meanwhile, one circumstance should be paid attention to, where the metal leaching happens from the anode to be deposited onto the cathode, which will hamper the CO2RR by encouraging competitive HER.In conclusion, a highly efficient transition metal-based SAC was synthesized via an economic and scalable protocol, and applied in CO2 electrolysis for large-scale production of CO. The results demonstrate that it is promising to replace noble metal catalysts, such as Au or Ag, with earth-abundant materials with remarkable CO evolution performance approaching practical expectations, which opens an avenue for future renewable energy infrastructures and achieves a significant progress in closing the anthropogenic carbon cycle for global sustainability.The carbon blacks were activated by dispersing 2\u00a0g carbon blacks in 100\u00a0mL of 9\u00a0M nitric acid solution followed with refluxing at 90\u00b0C for 3\u00a0hr. The Ni-NCB catalyst was prepared via a facile ion adsorption process followed with further pyrolysis. Typically, a 3-mg/mL nickel nitrate stock solution was first prepared by dissolving Ni(NO3)2\u22c56H2O (Puriss, Sigma-Aldrich) into Millipore water (18.2 MW\u22c5cm). A carbon black suspension was prepared by mixing 1\u00a0g activated carbon blacks (Vulcan XC-72, purchased from Fuel Cell Store and activated in acid bath) with 400\u00a0mL of Millipore water, and tip sonicated (Branson Digital Sonifier) for 30\u00a0min until a homogeneous dispersion was achieved. Then 40\u00a0mL of Ni2+ solution was dropwise added into carbon black solution under vigorous stirring overnight and then centrifuged to collect the products (Ni2+-CB). The as-prepared Ni2+-CB powder was mixed with urea with a mass ratio of 1:10, and then heated up in a tube furnace to 800\u00b0C under a gas flow of 80 standard cubic centimeters per minute (sccm) Ar (UHP, Airgas) and maintained for 1\u00a0hr, obtaining the final products. NCB and Ni-CB were prepared in a similar way but with the absence of Ni precursor and urea, respectively. Ni-NCB-NH3 was prepared by annealing the as-prepared Ni2+-CB powder at 800\u00b0C under a gas flow of 80 sccm NH3. Ni-NCB-1:5 and Ni-NCB-1:20 were prepared in the same way as Ni-NCB, except by varying the mass ratio of Ni2+-CB powder and urea to 1:5 and 1:20. Ni-NCB-600 and Ni-NCB-1000 were prepared by just varying the annealing temperature to 600\u00b0C and 1,000\u00b0C. Ni excess was prepared with a modified strategy reported before.\n36\n\nThe electrochemical measurements were run at 25\u00b0C in a customized gastight H-type glass cell separated by Nafion 117 membrane (Fuel Cell Store). A BioLogic VMP3 work station was employed to record the electrochemical response. The set-up of the three-electrode test system can be found in our earlier reports.\n35,36\n Typically, 5\u00a0mg of as-prepared catalyst was mixed with 1\u00a0mL of ethanol and 100\u00a0\u03bcL of Nafion 117 solution (5%, Sigma-Aldrich), and sonicated for 20\u00a0min to get a homogeneous catalyst ink. Ink (80\u00a0\u03bcL) was pipetted onto a 2-cm2 glassy carbon surface (0.2\u00a0mg/cm2 mass loading). For the stability test, 500\u00a0\u03bcL of the ink was air-brushed onto a carbon fiber paper gas diffusion layer toward a mass loading of 1.25\u00a0mg/cm2, and then vacuum dried prior to use. All potentials measured against a saturated calomel electrode were converted to the RHE scale in this work using E (vs RHE)\u00a0= E (vs SCE)\u00a0+ 0.244\u00a0V\u00a0+ 0.0591*pH, where pH values of electrolytes were determined by an Orion 320 PerpHecT LogR Meter (Thermo Scientific). Solution resistance (Ru) was determined by potentiostatic electrochemical impedance spectroscopy at frequencies ranging from 0.1\u00a0Hz to 200 kHz, and manually compensated as E (iR corrected versus RHE)\u00a0= E (vs RHE) \u2212 Ru *I (amps of average current).For the anion MEA test (or scale-up fuel cell test), 1.25\u00a0mg/cm2 Ni-NG and IrO2 was air-brushed onto two 2\u00a0\u00d7 2-cm2 (or 10\u00a0\u00d7 10-cm2) Sigracet 35 BC gas diffusion layer electrodes as a CO2RR cathode and an oxygen evolution reaction anode, respectively. A PSMIM anion-exchange membrane (Dioxide Materials) was sandwiched by the two gas diffusion layer electrodes to separate the chambers. On the cathode side, a titanium gas flow channel supplied 50 sccm (or 500\u00a0sccm) humidified CO2 while the anode was circulated with 0.1\u00a0M KHCO3 electrolyte at 2\u00a0mL min\u22121 (or\u00a010\u00a0mL min\u22121) flow rate. The cell voltages in Figures 3E\u20133H were recorded without iR correction. The 10\u00a0\u00d7 10-cm2 MEA response was recorded by a Sorensen DCS 33-33 power supply and is shown in Figure\u00a04 without iR correction.During electrolysis, CO2 gas (99.995%, Airgas) was delivered into the cathodic compartment containing CO2-saturated electrolyte at a rate of 50.0 sccm (monitored by an Alicat Scientific mass flow controller) and vented into a Shimadzu GC-2014 GC equipped with a combination of molecular sieve 5A, Hayesep Q, Hayesep T, and Hayesep N columns.\n35,60\n A thermal conductivity detector was mainly used to quantify H2 concentration, and a flame ionization detector with a methanizer was used to quantitative analysis CO content and/or any other alkane species. The detectors are calibrated by three different concentrations (H2: 100, 1,042, and 49,830 ppm; CO: 100, 496.7, and 9,754 ppm) of standard gases. The gas products were sampled after a continuous electrolysis of \u223c15\u00a0min under each potential. The partial current density for a given gas product was calculated as below:\n\n\n\n\nj\ni\n\n=\n\nx\ni\n\n\u00d7\nv\n\u00d7\n\n\n\nn\ni\n\nF\n\nP\n0\n\n\n\nRT\n\n\n\u00d7\n\n\n\n(\n\ne\nl\ne\nc\nt\nr\no\nd\ne\n\na\nr\ne\na\n\n)\n\n\n\n\u2212\n1\n\n\n\n\n\nwhere x\n\ni\n\u00a0is the volume fraction of certain product determined by online GC referenced to calibration curves from three standard gas samples, v\u00a0is the flow rate, n\n\ni\n is the number of electrons involved, p\n0\u00a0= 101.3\u00a0kPa, F is the Faraday constant, and R is the gas constant. The corresponding FE at each potential is calculated by\n\n\n\nFE\n=\n\n\n\nj\ni\n\n\n\n\nj\n\ntotal\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\nFor a 10\u00a0\u00d7 10-cm2 MEA, the FEs of H2 and CO were tested ex situ and calculated based on the concentration normalization.1D 1H NMR spectra were collected on an Agilent DD2 600 MHz spectrometer to test if any liquid products present during the CO2 reduction (Figure\u00a0S9). Typically, 600\u00a0\u03bcL of electrolyte after electrolysis was mixed with 100\u00a0\u03bcL of D2O (Sigma-Aldrich, 99.9 at % D) and 0.05\u00a0\u03bcL DMSO(Sigma-Aldrich, 99.9%) as internal standard.Calculation of TOF by mass loading normalization: catalyst loading on glass carbon electrode is 0.2\u00a0mg cm\u22122. The content of Ni in Ni-NCB is 0.27 wt %. The moles of active sites per cm2:\n\n\n\nN\n=\n\n\n0.2\n\u00d7\n\n\n10\n\n\n\u2212\n3\n\n\n\u00d7\n0.27\n\u00d7\n\n\n10\n\n\n\u2212\n2\n\n\n\n\n58\n\n\n=\n9.31\n\u00d7\n\n\n10\n\n\n\u2212\n9\n\n\n\nm\no\nl\ne\n\nc\n\nm\n\n\u2212\n2\n\n\n\n\n\n\n\n\n\n\nTOF\n\n(\n\n\ns\n\n\u2212\n1\n\n\n\n)\n\n=\n\n\nJ\n\u00d7\nF\n\nE\n\nCO\n\n\n\u00d7\n0.965\n\n\n2\n\u00d7\n96485.3\n\u00d7\n9.31\n\u00d7\n\n\n10\n\n\n\u2212\n9\n\n\n\n\n\n\n\n\nCalculation of TOF by ECSA normalization: according to the reported EDLC value of graphene \u223c21\u00a0\u03bcF/cm2(36), the electrochemical surface area of graphene layers in Ni-NCB was calculated to be 390.5\u00a0cm2, given the 8.2 mF/cm2 EDLC value of Ni-NCB. The moles of carbon atoms on the electrochemical surface can be calculated to be 390.5\u00a0\u00d7 10\u22124/2,600\u00a0\u00d7 12\u00a0= 1.25\u00a0\u00d7 10\u22126 mol, where 2,600 m2 g\u22121 is the theoretical specific surface area of graphene. Taken together the Ni atomic content in Ni-NG was determined to be 0.28% by XPS (Figure\u00a0S1), and the number of Ni sites in the surface was N\u00a0= 3.5\u00a0\u00d7 10\u22129 mol. Accordingly, \n\nTOF\n\n(\n\ns\n\n\u2212\n1\n\n\n)\n\n=\n\n\nJ\n\u00d7\nF\n\nE\n\nCO\n\n\n\u00d7\n0.965\n\n\n2\n\u00d7\n96485.3\n\u00d7\n3.5\n\u00d7\n\n\n10\n\n\n\u2212\n9\n\n\n\n\n\n.The STEM characterization in Figure\u00a01A was carried out using a JEOL ARM200F aberration-corrected scanning transmission electron microscope at 200 kV with an image resolution of \u223c0.08\u00a0nm. All other TEM images were obtained by using a JEOL 2100 transmission electron microscope operated under 200 kV. EDS analysis was performed at 300 kV using Super-X EDS system in a Probe-corrected FEI Titan Themis 300 S/TEM. Drift correction was applied during acquisition. XPS was obtained with a Thermo Scientific K-Alpha ESCA spectrometer, using a monochromatic Al K\u03b1 radiation (1,486.6 eV) and a low energy flood gun as neutralizer. The binding energy of the C 1s peak at 284.6 eV was used as reference. Thermo Avantage V5 program was employed for surface componential content analysis as well as peaks fitting for selected elemental scans. XAS spectra on Ni K-edge was acquired using the SXRMB beamline of Canadian Light Source. The SXRMB beamline used an Si(111) double-crystal monochromator to cover an energy range of 2\u201310 keV with a resolving power of 10,000. The XAS measurement was performed in fluorescence mode using a four-element Si(Li) drift detector in a vacuum chamber. The powder sample was spread onto double-sided, conducting carbon tape. Ni foil was used to calibrate the beamline energy.This work was supported by the Rowland Fellows Program at Rowland Institute, Harvard University. The Center for Nanoscale Systems (CNS) is part of Harvard University. This research used resources of the Canadian Light Source, which is supported by NSERC, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan. J.L. and N.T. were supported by the National Science Foundation under CHE-1465057, and gratefully acknowledge the use of facilities within the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University. T.Z. and N.T. acknowledge funding from the China Scholarship Council (CSC) (201706340152 and 201704910441, respectively). J.Z. acknowledges support from MOST of China (2014CB932700) and NSFC (21573206). This work was performed in part at the CNS, a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award no. ECS-0335765. H.W. acknowledges support from Rice University.H.W. designed the studies. T.Z. conducted the synthesis and catalytic tests of catalysts. K.J. performed the characterization of catalysts. N.T. and J.L. conducted HRTEM characterization. Y.H. performed XAFS measurements. J.Z. provided suggestions on the work. T.Z. and H.W. wrote the manuscript. All authors discussed the results and commented on the manuscript.H.W. has submitted a patent application (US 62/486,148, 2017) regarding the transition single-atom catalyzed carbon dioxide conversion technology.Supplemental Information includes 24 figures and 1 table and can be found with this article online at https://doi.org/10.1016/j.joule.2018.10.015.\n\n\nDocument S1. Figures S1\u2013S24 and Table S1\n\n\n\n\n\nDocument S2. Article plus Supplemental Information\n\n\n\n", "descript": "\n                  The scaling up of electrocatalytic CO2 reduction for practical applications is still hindered by a few challenges: low selectivity, small current density to maintain a reasonable selectivity, and the cost of the catalytic materials. Here we report a facile synthesis of earth-abundant Ni single-atom catalysts on commercial carbon black, which were further employed in a gas-phase electrocatalytic reactor under ambient conditions. As a result, those single-atomic sites exhibit an extraordinary performance in reducing CO2 to CO, yielding a current density above 100 mA cm\u22122, with nearly 100% selectivity for CO and around 1% toward the hydrogen evolution side reaction. By further scaling up the electrode into a 10\u00a0\u00d7 10-cm2 modular cell, the overall current in one unit cell can easily ramp up to more than 8 A while maintaining an exclusive CO evolution with a generation rate of 3.34\u00a0L hr\u22121 per unit cell.\n               "}
{"full_text": "With recent changes in the energy-consuming sector's advancements and depleting fossil fuels, significant attention has been drawn to transform renewable resources into transportation fuels and fine chemicals [1,2]. Selective chemical conversion of lignocellulosic biomass, especially cellulose and hemicellulose, provides platform chemicals such as 5-hydroxymethylfurfural, levulinic acid (LA), and furfural, which can be further down streamed to versatile alkyl levulinates [3,4]. Alkyl levulinates possess high lubricity, low toxicity, stable flashpoint, and moderate flow properties, making them suitable gasoline and diesel blends. Moreover, alkyl levulinates have numerous applications as a solvent, plasticizer, and precursor to obtaining \u03b3-valerolacotone [5]. Among the alkyl levulinates, butyl levulinate (BL), an oxygenate fuel additive with high octane number, rich oxygen content, and low solubility in water makes it a better fuel blend than ethyl levulinate and an alternative for the water-soluble carcinogen methyl tert-butyl ether (MTBE) [6].The esterification of LA with alcohols and alcoholysis of furfuryl alcohol (FAL) in the acid-catalyzed environments are the general methods to produce the alkyl levulinates. However, the production of butyl levulinate using LA as a raw material is an expensive process, with product water inhibit the reaction to progress effectively [7,8]. The other route via FAL's alcoholysis has gained much attention due to its smooth and cost-effective pathway for the BL synthesis. The BL production was reported using various homogeneous acid catalysts such as H2SO4, AlCl3\n[9], and a double SO3-H functionalized ionic liquids, [10], which surmises that the butanolysis of FAL is a function of strong acidity. To avoid the major drawbacks of homogeneous systems comprehended by potential reactor corrosion, recovery, and recyclability, several heterogeneous catalyst systems such as ion exchange resins [11], zeolites, SBA-16 [6], metal oxides [12,13], mesoporous aluminosilicates [14], and zinc exchanged heteropoly acids [15] catalysts have been successfully employed for the alkyl levulinates production.To accommodate butanolysis reaction, a facile, highly stable, and easily regenerable tungstated zirconia (WZr) catalyst [16] was first employed to test its activity. Hino and Arata first reported the WZr catalyst's synthesis to cope with the challenges posed by the desorption of active SO4\n2- ions from SO4\n2-/ZrO2 catalysts [17]. W-O-Zr bonds' presence makes it strongly stable, which overcomes the detachment of active WOx species from ZrO2 support. WZr catalysts efficiently enhanced the alkanes isomerization [18], hydrogenolysis [19], dehydration [20], aqueous phase hydrolysis [21], selective catalytic NOx reduction [22], cyclohexane hydration [23], liquid phase Beckmann rearrangement [24], esterification and transesterification reactions [25]. Besides, LA's esterification with butanol was carried out to obtain BL with 97% selectivity [26]. Owing to the excellent catalytic activity of WZr, this study was interested in reporting the WZr activity for the butanolysis of FAL with lower initial butanol to FAL mole ratios.Upon the WZr catalyst activity for butanolysis of FAL, with barely around 28% of the BL yield, the reaction was carried out using sulfonated carbon catalyst. Carbon-based catalysts with low synthesis cost, high catalytic activity, and abundant carbon sources make them suitable heterogeneous catalytic systems [27]. The partially pyrolyzed carbon source is rich in oxygen content composed of several surface functional groups such as carboxylic acid, aromatic hydrocarbons, and phenolic hydroxyl groups [28]. Thus, obtained carbon black can easily be functionalized with -SO3H groups by sulfonation with various -SO3H group sources, for instance, conc. H2SO4, fuming H2SO4, and 4-Benzenediazoniumsulfonate. Literature reports were available to find the excellent catalytic activity of sulfonated carbon catalysts for a wide range of reactions like fatty acids (FFA) esterification [29,30], cellulose hydrolysis [31], transesterification of vegetable oils [32], glycerol conversion [33], and esterification reactions [34\u201336].Most of the studies reported for the butanolysis of FAL were with high initial 1-butanol to FAL molar ratios to avoid the formation of FAL polymers. Bringu\u00e9 et al. performed the butanolysis of FAL over sulfonated polystyrene-divinylbenzene (PS-DVB) resins at low initial 1-butanol to FAL molar ratios (8:1) with a maximum of 63% BL yield at 110\u00a0\u00b0C. This study also reported that 2-BMF was still unconverted to BL at the end of the reaction time owing to the reaction temperature. The Amberlyst catalysts are thermally challengeable above 120\u00a0\u00b0C and the lower reaction temperature influenced to obtain the maximum yield [11]. Similarly, Enamula et al. also employed a high initial mole ratio of 16 at high reaction temperature (180\u00a0\u00b0C) to obtain 63\u00a0mol% of BL yield using Al2O3/SBA-15 catalyst. The same catalyst at 110\u00a0\u00b0C resulted in 91\u00a0mol% of BL yield but at the initial mole ratio of 65 [13]. Yang et al. reported the activity of a magnetic carbonaceous solid acid (SMWP) catalyst with 91\u00a0mol% of BL yield, operating at an initial mole ratio of 40 (BtOH: FAL) [8]. Thus, the present study focuses on increasing the BL yield to a maximum level at a possible lower initial molar (BtOH: FAL) ratio.Pluronic P-123, zirconium (IV) butoxide solution (80 wt.% in 1-butanol), ammonium metatungstate hydrate, Conc. HNO3 (65%), n-Butanol (purity\u00a0>\u00a099.0%), carbon tetrachloride (CCl4), and methanol were acquired from Sigma-Aldrich. Ethanol, furfuryl alcohol, sucrose, and sulfuric acid were obtained from Fuji film Wako chemicals Ltd. n-Butyl levulinate and dibutyl ether were purchased from TCI chemicals. All the chemicals were used as received.The catalyst synthesis was carried out using the evaporation-induced self-assembly (EISA) method mentioned elsewhere to obtain a mesoporous structured catalyst [37]. Ammonium meta tungstate hydrate (0.05074\u00a0mmol) and zirconium (IV) butoxide solution (80 wt.% in 1-butanol) (68.982\u00a0mmol) were used as precursors for WO3 and ZrO2, and pluronic P123 (1.724\u00a0mmol) was used as a structure directive agent. In a typical synthesis, Pluronic P123, tungsten, and zirconia precursors were dissolved in 250\u00a0ml of ethanol. To this solution, 2\u00a0ml of H2O and 8\u00a0ml of HNO3 were added to promote the condensation during the synthesis and to maintain the pH of the solution below the electrostatic point of the tungsten and zirconia. The solution was aged for 12 hours with continuous stirring of 300\u00a0rpm. After that, the homogeneous mixture was dried in a hot air oven at 40\u00a0\u00b0C for 48 hours to facilitate the slow evaporation and then completely dried further at 70\u00a0\u00b0C for 12 hours. Finally, thus obtained solid was calcined in air at 800\u00a0\u00b0C for 6 hours with a ramping rate of 1\u00a0\u00b0C/min (25\u00a0\u2192\u00a0200\u00a0\u00b0C, 1-hour stay at 200\u00a0\u00b0C, 200\u00a0\u2192\u00a0400\u00a0\u00b0C, 1-hour stay at 400\u00a0\u00b0C, 400\u00a0\u2192\u00a0800\u00a0\u00b0C, 6 hours stay 800\u00a0\u00b0C). The prepared catalysts were termed xWZrT (where\u00a0\u00d7\u00a0-wt.% of WO3 & T-calcination temperature, \u2070C). The promoted catalyst was prepared by the wet impregnation method with noble and novel metals and further calcined at respective reduction temperatures of the metals. The notation yM15WZr800 (M- Metal, y-wt.% of the metal) was used to represent the promoted catalysts.The catalyst was synthesized by incomplete carbonization of the sucrose, followed by sulfonation at the designated temperature for a particular time under inert conditions [38\u201340]. The catalyst preparation was done in two steps. In the first step, 5\u00a0g of sucrose was partially carbonized at 400\u00a0\u00b0C for 15 hours under constant N2 flow (200\u00a0ml/min) to obtain a black solid. Thus, obtained carbon black was grounded to a fine powder. In the second step, 2\u00a0g of carbon black was sulfonated using 40\u00a0ml of conc. H2SO4(>98%) at 80\u00a0\u00b0C for 10 hours under inert conditions. After the sulfonation, the black mixture was washed with hot distilled water and then vacuum filtered until the neutral pH of the water was observed. Finally, the catalyst was dried at 100\u00a0\u00b0C for 12 hours before the direct use. The prepared catalyst was denoted by SO3H_C80S (Carbon black sulfonated at 80\u00a0\u00b0C) and the spent catalyst after 3 cycles by SO3H_C80S Spent.The specific surface area and pore size distribution of the prepared catalysts were analyzed using nitrogen adsorption/desorption isotherms data obtained at 77\u00a0K using the BELSORP-MiniX analyzer. The catalyst samples were first degassed under vacuum (10\u22125 torr) conditions at 573\u00a0K (Tungstated zirconia catalysts) and at 393\u00a0K (carbon catalysts) for 4 hours to remove the surface adsorbed species and moisture on the catalysts. The specific surface area was determined by the adsorption isotherm of nitrogen in the relative pressure range of 0.05\u00a0<\u00a0p/p0\u00a0<\u00a00.3 using the BET equation. The pore size distribution and pore volume were determined by the BJH desorption method using desorption isotherm.Fourier-transform infrared spectroscopy (FTIR) was done using a Bruker ALPHA II. The samples were mixed with spectroscopic grade potassium bromide (KBr, 100\u00a0mg) and pressed to acquire a circular transparent disk with a hydraulic press. The spectra were collected from 4000 to 400\u00a0cm\u22121 with a resolution of 4\u00a0cm\u22121 for 16 scans using KBr disks.The pyridine probed FTIR was performed to distinguish the acidic sites present on the catalyst. Pyridine was added to the catalyst samples and allowed pyridine to adsorb on the samples' surface for 2 hours at room temperature. The unadsorbed pyridine was removed by keeping the samples in the oven at 383\u00a0K for 2 hours. 100\u00a0mg of KBr was added to the samples, and the transparent pellets were made using the hydraulic press. The IR spectra were recorded against the KBr background.The acidic properties of the tungstaed zirconia and metal promoted catalysts were estimated by the NH3 adsorption and temperature-programmed desorption (NH3-TPD) technique using BEL-CAT (MicrotracBEL corp.) automated chemisorption analyzer with a TCD detector. The samples were first pretreated with helium gas (50\u00a0ml/min) at 250 \u00b0C for 1 hour and cooled down to 100 \u00b0C. After that, ammonia adsorption was carried out using a 5% NH3 gas mixed with helium (95%) for 30 minutes at 100 \u00b0C. After completing ammonia adsorption, the samples were purged with pure He for 30 minutes and allowed the TCD stabilization. Finally, the ammonia desorption spectra were obtained by gradually increasing the temperature with 10 \u00b0C/min until the final temperature, followed by the calibration with a 5% NH3-He mixture. The amount of ammonia adsorbed in mmol/g was automatically calculated by the ChemMaster software using the calibration curve and the amount of ammonia taken. The ammonia adsorption technique was not performed for carbon catalysts as the detachment of -SO3H progressed at higher temperatures (>230 \u00b0C) during the analysis (TGA analysis, SFig.2.).The acidic sites of the carbon catalysts were measured by the acid-base back titrations using an aqueous ion-exchange method using NaHCO3 base solution followed by the titration against aqueous HCl solution [41]. In a typical process, 30\u00a0mg of catalyst was dispersed in 0.005\u00a0N NaHCO3 solution and continuously stirred for 24 hours. After that, the resulting mixture was filtered, and the filtrate was titrated against 0.005\u00a0N HCl solution using a methyl orange indicator. The quantification of the acid sites on the catalysts was calculated by the amount of NaHCO3 consumed.The catalytic activity was tested for the butanolysis of FAL in a 100\u00a0ml high-pressure batch reactor (Parr Instruments). In a typical experiment, specified amounts of reactants and catalyst were charged into the reactor. The reactor was purged with nitrogen gas several times to ensure the inert environment and then pressurized. The reaction temperature and agitation speed were fixed, and the reaction was carried for the specified times after reaching the set temperature. After completing the reaction, the reactor was cooled down to room temperature, and the sample was collected. The liquid samples were centrifuged to remove the catalyst traces and diluted with internal calibration solvent carbon tetrachloride and dilutant methanol before analyzing with GC-FID (flame ionization detector) and GC\u2013MS (mass spectroscopy). The following equations were used for the quantification calculations [33].\n\n\n\nC\no\nn\nv\ne\nr\ns\ni\no\nn\n\n\n(\nm\no\nl\n%\n)\n\n=\n\n\n\n\nI\nn\ni\nt\ni\na\nl\n\nm\no\nl\ne\ns\n-\nF\ni\nn\na\nl\n\nm\no\nl\ne\ns\n\n\n\nof\n\nF\nA\nL\n\n\nI\nn\ni\nt\ni\na\nl\n\nm\no\nl\ne\ns\n\nof\n\nF\nA\nL\n\n\n\u00d7\n100\n\n\n\n\n\n\n\n\nS\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n\n\n(\nm\no\nl\n%\n)\n\n=\n\n\nm\no\nl\ne\ns\n\no\nf\n\na\n\np\nr\no\nd\nu\nc\nt\n\no\nb\nt\na\ni\nn\ne\nd\n\n\nt\no\nt\na\nl\n\nm\no\nl\ne\ns\n\no\nf\n\nthe\n\np\nr\no\nd\nu\nc\nt\n\n\n\u00d7\n100\n\n\n\n\n\n\n\n\nY\ni\ne\nl\nd\n\n\n(\nm\no\nl\n%\n)\n\n=\n\n\nm\no\nl\ne\ns\n\no\nf\n\nB\nL\n\nf\no\nr\nm\ne\nd\n\n\nt\nh\ne\no\nr\ne\nt\ni\nc\na\nl\n\nm\no\nl\ne\ns\n\no\nf\n\nB\nL\ne\nx\np\ne\nc\nt\ne\nd\n\n\n\u00d7\n100\n\n\n\n\nThe reuse test was conducted to determine the number of cycles that the catalyst can be used without requiring regeneration. Thus, three reaction cycles of the best performing catalyst were performed for the spent analysis. After completing every reaction, the catalyst was separated from the reaction mixture by vacuum filtration and several times washings with methanol followed by ethanol to remove the adsorbed organic compounds. After that, the catalyst was dried at 100 \u2070C for 12 hours before use.The N2 physisorption illustrates the physical properties of the prepared WZr catalysts. The typical isotherms correspond to type IV isotherm represents the mesoporous nature of the catalyst. Fig. 1\na and 1b further showed that for a 15 wt.% WO3 loading, the incipient of the hysteresis loop shifted towards higher relative pressure with the increase in calcination temperature. On the contrary, for a fixed calcination temperature of 800 \u00b0C, the beginning of the hysteresis loop was observed at P/P0 of 0.55 for 10WZr800 and shifted towards higher relative pressure with the increase in the WO3 content in tungstated zirconia catalysts. These results exhibited the expansion and contraction of the pores with the increase in calcination temperature for fixed WO3 content and WO3 loading for a fixed calcination temperature, respectively. The pure zirconia's surface area was also quite low, representing its non-porous nature at such a high calcination temperature. The pure zirconia and pure WO3 have a poor surface area (Table 1\n, entries 1,2), which explains the integration of W-O-Zr bonds needed to stabilize this catalyst, which is responsible for its strong thermal and mechanical strength [42]. An increase in calcination temperature at a constant metal oxide loading increases the surface density of the metal on the support oxide, which triggers the mobility of support metal atoms [43]. In the case of tungstated zirconia catalyst, calcination at higher temperature promoted Zr atoms mobility, which triggered the sintering of Zr atoms along with augmentation of pore size [26]. These phenomena were observed for 15WZr900. The specific surface area of the catalyst was reduced to 17.5\u00a0m2/g with widened pores (Table 1, entry 5). The surface density of the W dispersion on the ZrO2 support was calculated by the following equation, which is a measure of the tungsten monolayer coverage [44].\n\n\n\nSurface\n\ndensity\n\nof\n\nW\n\n=\n\n\n\n\n\n(\n\n\nWO\n\n\n3\n\n\n\nLoading\n\n(\nwt\n.\n%\n)\n\n/\n100\n)\n\n\u00d7\n6\n.\n023\n\u00d7\n\n\n10\n\n\n23\n\n\n\n\n231\n.\n8\n\n\n(\nformula\n\nweight\n\nof\n\n\n\nWO\n\n\n3\n\n\n)\n\n\u00d7\nBET\n\nSurface\n\narea\n\n(\n\n\nm\n\n\n2\n\n\n\n\ng\n\n\n-\n1\n\n\n)\n\n\u00d7\n\n\n10\n\n\n18\n\n\n\n\n\n\n\n\nAn increase in WO3 loading increased the surface area to 79\u00a0m2/g for 15 wt.% calcined at 800 \u00b0C (Table 1, entry 4) for which the surface density was around 4.9\u00a0W-atom/nm2, which is in the range for a typical value of the surface monolayer coverage [45,46]. The pore size distribution measured by the BJH desorption method was also represented in Fig. 1b. The 15WZr800 catalyst has a pore diameter of 10.6\u00a0nm compared to 8.3\u00a0nm of 10WZr800, and the depth of the pores for the 15 Wt.% is longer than 10 Wt.%, which was clearly shown by the increased pore volume for the 15 Wt.% loading (Table 1, entries 3,4). The further increase in the calcination temperature to 900 \u00b0C profoundly affected the pore structure of the catalyst, resulting in the widening of the pores caused by the sintering of the Zr atoms (Table 1, entry 5) [16].The specific surface area of the prepared carbon-based catalysts measured by the N2 adsorption technique at 77\u00a0K was reported in the following Table 2\n. The specific surface area of the catalysts is relatively influenced by the carbonation temperature and sulfonation temperatures [47]. The carbon catalysts have the specific surface area\u00a0<\u00a01\u00a0m2/g (Table 2, entries 1\u20133), mostly because of the formation of amorphous carbon by incomplete carbonization of the sucrose, which resulted in the dispersion of large phenol hydrophilic and carboxylic acid functional groups [29,36]. There was no significant change in the surface area of the carbon black after the sulfonation with conc. H2SO4 indicates that the \u2013SO3H groups were incorporated into the carbon structure by bonding with the existing functional groups. The pore volume and pore diameter of the catalysts were very low, caused by no surface area development as well as the pore structure. N2 adsorption isotherms and micropore analysis are shown in the Supporting information (SFig.8a &8b).NH3-TPD study was implemented to understand the surface acidic properties of the tungstated zirconia catalysts and the metal promoted catalysts. The NH3-TPD spectra are shown in Fig. 2\na and b. The total surface acidity and the peak temperatures are summarized in Table 3\n. The ammonia desorption was obtained in a broad range from 100 \u00b0C to 800 \u00b0C to understand the acid strength of the catalysts. To comprehend and quantify the surface acidity, the peak temperatures below 350 \u00b0C were assigned to the weaker acidic sites. The peaks above 350 \u00b0C correspond to the stronger acidic sites [25,26]. However, the intensity of stronger acidic strength peaks was very low compared to the lower temperature peaks.The surface acidity was clearly affected by WO3 loading and the calcination temperature (Table 3, entries 1,2 &3). An increase in WO3 loading from 10 wt.% to 15 wt.% induced a hike in the surface acidity from 0.143\u00a0mmol/g to 0.201\u00a0mmol/g (Table 3, entries 1,2). For 15 wt.% WO3 loading, the surface density was 4.9\u00a0W-atoms/nm2 (Table 1, entry 4), which generally corresponds to the monolayer coverage of the W-atoms, thereby increasing the surface acidity [46]. The collapse of the pore structure (BET results, Fig. 1b) at such a higher calcination temperature (900 \u00b0C) probably caused the drop in the total acidity to 0.122\u00a0mmol/g.The peak corresponding to weaker acidic strength was observed in the range of 175\u2013198 \u00b0C for all the catalysts, which indicates that the metal promotion influenced the strong acidic sites upon interaction with the W atoms. The surface acidity of all metal promoted catalysts constrained in a narrow range from 0.199 to 0.209\u00a0mmol/g (Table 3, entries 4\u20139). The yield of BL was also restricted to a narrow range of 10.7\u00a0mol% to 14.2\u00a0mol% (Table 3, entries 4\u20139). Even though the enhancement in terms of the acidity of the tungstated catalyst was achieved very slightly by the metal promotion, it has not led to a significant difference in the activity towards increasing the BL yield.\nFig. 3\n. demonstrates the FTIR spectra of sulfonated carbon catalysts and carbon black at 298\u00a0K in the range of 4000 to 400\u00a0cm\u22121, which indicates the functional groups present on the surface of the catalyst and their interaction. The band at 1058\u00a0cm\u22121 correlated to the symmetrical stretching of O\u00a0=\u00a0S\u00a0=\u00a0O in SO2 bonding and the band at 1162\u00a0cm\u22121 to the asymmetrical stretching of SO2, which were not observed in the carbon black compared to sulfonated catalysts [40,41,48]. These functional groups clearly indicate the incorporation of -SO3H groups onto the carbon black. The reduction in the intensity of the peaks at 1162\u00a0cm\u22121 and 1058\u00a0cm\u22121, which corresponds to asymmetrical and symmetrical stretching of -SO3H, supports the desorption of -SO3H groups after the reaction. The polyaromatic hydrocarbon of C\u00a0=\u00a0C stretching was due to the vibration band at 1606\u00a0cm\u22121\n[34,41]. The carboxylic acid groups, one of the major functional groups formed by the partial carbonization of the carbon source, were attributed to the presence of a vibration band at 1700\u00a0cm\u22121, which represents the stretching of a C\u00a0=\u00a0O of a \u2013COOH group [32]. The band at 2930\u00a0cm\u22121 illustrates aromatic methoxyl groups, which were subsequently suppressed during the sulfonation process. Finally, the band at 3600\u00a0cm\u22121 corresponds to the \u2013OH stretch phenolic functional groups [49]. Thus, FTIR analysis provided insights into the surface functional groups of the carbon catalysts and the desorption of -SO3H groups.To distinguish the nature of the surface acidity of the tungstated zirconia catalyst, Pyridine-FTIR analysis was studied and represented by Fig. 4\n. and the peak areas were reported in Table 4\n. The band at 1440\u00a0cm\u22121 corresponds to the interaction of pyridine Bronsted molecules with H+ electron-accepting molecules corresponding to Lewis acidity. The band at 1540\u00a0cm\u22121 attributed to the presence of Bronsted acidic molecules. The band at 1490\u00a0cm\u22121 represents the combined acidic strength of Lewis and Bronsted. The intensity of the bands progressed with WO3 loading from 10 wt.% to 15 wt.% (Table, entries 1 & 2). Increasing the WO3 content led to the development of polytungstate species and the formation of Zr-WOx clusters [49,50]. At 15 wt.% of WO3 loading, the monolayer coverage was accomplished by progressing the condensation of monotungstate to polytungstate species by enhancing the surface acidity. The tungsten atom in the polytungstate has the ability to delocalize the adjacent zirconia electrons, thereby generating the proton development to compensate the delocalized electrons, which enhances the growth of the Bronsted acidity [51,52]. At a higher calcination temperature of 900 \u00b0C, the band intensity corresponding to Lewis acidity weakened, and other peak intensities reduced considerably. The basic reason attributes to the pore structure collapse at elevated temperatures, accompanied by the Zr atoms sintering [16,53].As described in the NH3-TPD analysis section, the metal promotion slightly altered the surface acidic properties of the catalysts. An incorporation of Pt and Pd metals resulted in close Bronsted to Lewis acidic sites that of neat tungstated zirconia (Table 4, entries 2, 4 & 5). The BL yield obtained for these catalysts also in the close range from 13.5 to 14.42\u00a0mol% (Table 4, entries 2, 4 & 5). The Ni and Cu upgradation remarkably increased the intensity of the Lewis acidity compared to other novel metals (Table 4, entries 7 & 8). This change probably surmised to be the interaction between metals (Ni, Cu) and ZrO2 surface, thereby revoking the Zr+4O2- activation, which induces the Lewis acidity and simultaneously making unavailability of electrons for polytungstate to develop the Bronsted acidic sites. The modification with Fe and Co metals also induced in the hike of Lewis acidity compared to the neat and noble (Pt, Pd) metal incorporation (Table 4, entries 6,9). Despite all these structural and physicochemical alterations by metal incorporation of the tungstated catalysts, the yield of BL was not improved significantly. Thus, making this catalyst low selective for the butanolysis of FAL.\nFig. 5\n and Table 5\n. represent the Pyridine-FTIR spectrum and the normalized peak areas corresponding to the acidic sites present on the catalyst, as well as the amount of acidity possessed by the catalyst. The strong Bronsted acidic band was due to SO3H groups and some other functional groups such as phenolic and carboxylic acid groups present on the carbon black represented by the FTIR analysis. The incorporation of -SO3H groups onto carbon black enhanced the Lewis acid groups and combination of Lewis\u00a0+\u00a0Bronsted acidic groups (Table 5, entry 2). The reason being that the sulfonation oxidizes the functional groups present on the carbon network, especially carboxyl and methoxyl groups (Bands at 1703\u00a0cm\u22121 & 2930\u00a0cm\u22121 in Fig. 3). The decrease in the spent catalyst's acidic nature was ascribed to the detachment of -SO3H groups that are weakly bonded to polycyclic aromatic carbon network [30,54]. Konwar et al. reviewed the biodiesel production using various carbon-based catalysts, in which the sulfonated carbon-based catalysts exhibited the leaching of -SO3H groups during the reaction [27]. Due to the surface and textural properties of the carbon catalysts observed from the BET results, the pyridine FTIR spectrums were different compared to tungstated zirconia catalysts. Table 5. Compares the presence of Lewis and Bronsted acidic sites of the carbon catalysts. For the spent catalyst (Table 5, entry 3), the Lewis acidic sites were reduced. This was surmised to be the detachment of weakly bonded functional groups caused by the affinity between the reactants and the hydroxyl groups of the carbon network [55]. The total surface acidity values of the catalysts were measured by the acid-base back titration (Table 5, entry 1,2,3) and are in alignment with the reported literature [30,34,53]. The carbon black showed an acidity of 1.12\u00a0mmol/g composed of the multiple surface functional groups. The sulfonated catalyst enhanced by SO3H groups, which were building blocks for the strong acidity, displayed 2.357\u00a0mmol/g of surface acidity. The spent catalyst after 3 cycles of use and the leaching of the surface functional groups possessed 1.658\u00a0mmol/g of acidity contributed by the strong -SO3H groups. The acidity and activity for butanolysis of FAL were in the order of SO3H/C\u00a0>\u00a0SO3H/C spent\u00a0>\u00a0Carbon black, which complied with the pyridine FTIR results.The butanolysis of FAL was carried out using the tungstated zirconia and sulfonated carbon catalysts. Scheme.1\n represents the conversion of FAL to BL through the reaction intermediate 2-BMF over the solid acid catalysts [7,56,57]. The hydroxyl groups of FAL molecule protonated by the catalyst and then attack of 1-butanol to this conjugated FAL molecule to form the reaction intermediate was the initial step of butanolysis of FAL. The reaction intermediate was identified through the GC\u2013MS (GCMS-QP2020 NX) analysis. Further conversion of 2-BMF to BL was a prolonged step, which can be regarded as the rate-determining step of butanolysis of FAL [6,8,58]. The polymerization of FAL molecules in the acidic media is a significant concern to use the lower initial BtOH: FAL ratios and the dehydration of n-butanol to the di-butyl ether was non consuming FAL byproduct of this reaction system.\nTable 6\n shows the activity of the tungstated zirconia catalysts for FAL butanolysis, which indicates the selectivity and yield of 2-BMF & BL. The metal oxide loading and calcination temperature parameters were applied for this catalyst with 10&15 wt.% of WO3, and 800 \u00b0C &900 \u00b0C calcination temperature. An increase in the WO3 loading enhanced its activity in terms of the FAL conversion from 77% to 95\u00a0mol% with 5.5 % to 14\u00a0mol% BL yield (Table 6, entries 1,2). As discussed in the NH3-TPD analysis (Fig. 2a, Table 3, entries 1,2), the increase in the acidity of the catalyst enhanced the FAL conversion and the BL yield. Further increase in the calcination temperature for 15 wt.% from 800 to 900 \u00b0C resulted in the collapse of the pore structure with widened pores and drop in the acidity caused in decreasing the catalytic activity to 50.5\u00a0mol% FAL conversion (Table 6, entry 3). The catalyst activity for this reaction was minimal as only around 14.43\u00a0mol% of BL yield was observed after 6 hours of the reaction (Table 6, entries 2). In contrast, the same catalyst showed excellent catalytic activity for other reactions mentioned in the introduction section. The same catalyst was tested for dehydration 1-butanol resulted in 72.5\u00a0mol% 1-butanol conversion with 35\u00a0mol% of di-butyl ether yield after 1 hour of the reaction (data not shown). The incorporation of metals onto the tungstated zirconia catalyst enhanced its catalytic activity for various reactions [22,59,60]. The promotion with noble and novel metals such as Pt, Pd & Ni, Fe, Cu, and Co also resulted in the similar catalytic activity of fresh catalyst (Table 6, entries: 4\u20139). The acidity of the catalysts slightly changed and more or less remained in the order of neat 15WZr800 catalyst (Table 3, entry 3). The FAL conversion has reached a maximum for all the catalysts. In contrast, the BL yield was varied in the range of 10\u201314\u00a0mol% with a 25\u201332% yield range of 2-BMF, indicating that the FAL molecules polymerization and unconverted intermediates were progressed. The reaction temperature was also studied for this catalyst in the range of 130\u2013240 \u00b0C with an interval of 20 \u00b0C, and a maximum of 28\u00a0mol% BL yield was observed at 240 \u2070C after 2 hours of the reaction (SFig. 1). The reaction was also studied with a high initial BtOH to FAL mole ratio, such as 60:1, which resulted in only 11.2\u00a0mol% of BL yield with 26\u00a0mol% of 2-BMF yield (Table 6, entry10). Due to the lack of sufficient acidity (0.201\u00a0mmol/g) of the catalyst, the conversion of 2-BMF to BL was not progressed to obtain higher yields.To study the effect of reaction temperature, the butanolysis reaction was carried in the range of 130\u2013210 \u00b0C. Fig. 6\n. explains the impact of reaction temperature on FAL butanolysis for sulfonated carbon catalyst on the selectivity and yield of reaction products. All the previous studies reported that the reaction temperature favors the butanolysis reaction by converting the reaction intermediate 2-BMF to BL via furan ring-opening mechanism [6,8,12,15]. The FAL oligomers tend to form FAL polymers in acidic conditions at higher reaction temperatures, which is the primary concern for high-temperature reactions. A reaction with neat FAL in the tetralin solvent at 190 \u00b0C is also conducted to understand the formation of FAL polymers in acidic conditions. The GC-FID chromatogram of neat FAL reaction was shown in the Supporting information (SFig. 5). However, Milan et al. reported that the activation energies for forming FAL polymers are identical to that of the primary reaction concluding that high reaction temperatures can be favorable under optimized conditions [61]. At the reaction temperature of 130 \u00b0C, though the FAL conversion was near completion, the yield of BL was 49\u00a0mol%, accompanied by the unconverted reaction intermediates after 6 hours of the reaction. Further increase in the temperature to 150 \u00b0C, there was a slight increase in the BL yield with a reduced yield of others, including FAL polymers. As the reaction temperature progressed, there was a significant change in the BL yield, indicating that higher reaction temperatures are needed for this catalyst system to convert the reaction intermediate 2-BMF to BL. Almost a similar BL yield of around 80\u00a0mol% was observed for 190 \u00b0C & 210 \u00b0C, indicating that the reaction temperature reached its threshold value, thus optimizing the reaction temperature at 190 \u00b0C. The selectivity of the BL was steadily increasing with reaction temperature, whereas the 2-BMF selectivity was gradually decreased, and there was no 2-BMF at 210 \u00b0C left to convert to BL. The yield of others, including FAL polymers, was on a declining trend with the reaction temperature and kept constant for 190 \u00b0C and 210 \u00b0C temperatures. Designating that higher reaction temperatures favor the product formation. The total surface acidity of the sulfonated carbon catalyst was way higher (2.357\u00a0mmol/g) than the 15WZr800 (0.201\u00a0mmol/g) catalyst, which facilitated the transformation of 2-BMF to BL towards higher yields.The butanolysis of FAL was studied at 190 \u00b0C with sulfonated carbon catalyst for the reaction time profile with 8.5:1 butanol to FAL mole ratio. Fig. 7\n. represents selectivity and yield of BL along with the yield of 2-BMF as a function of time. FAL conversion was proceeded rapidly and almost completely converted after 1 hour while 54\u00a0mol% BL yield and 26\u00a0mol% of 2-BMF were observed. The temperature has a significant effect on this reaction, which accelerated the conversion of 2-BMF to BL via furan ring-opening (Fig. 6). As the time progressed, the BL yield increased sharply to 82\u00a0mol% after 6 hours. The selectivity of the BL also kept on increasing with time and reached a maximum of 98\u00a0mol% after 6 hours. Meanwhile, the yield of 2-BMF was decreased over time and completely converted to BL after 6 hours. However, after the complete conversion of 2-BMF, the BL yield has slightly reduced to 80\u00a0mol% at 7 hours, which implies the beginning of the BL degradation. Though the complete conversion of FAL was observed after one hour, the formation of BL progressed up on time to achieve the maximum value and settled at 6 hours, thereby optimizing the reaction time to 6 hours.As described in the introduction, most of the studies for butanolysis of FAL were reported at a higher initial molar ratio of butanol to FAL (>30) with higher selectivities. This study intended to reduce the initial mole ratio to a maximum achievable range. The effect of initial FAL concentration was studied in the range of 30:1 to 4:1 at 190 \u2070C for 6 hours using sulfonated carbon catalyst, and results are reported in Fig. 8\n. The results depicted that increase in the initial FAL concentration resulted in the decreasing trend of BL yield, and increasing the FAL polymers yield replicates that the formation of FAL polymers was favored upon high FAL concentration. The yield of BL reduced from 84\u00a0mol% to 57\u00a0mol% from 30:1 to 4:1 initial molar ratio of BtOH to FAL. The complete conversion was obtained even at lower mole ratios because of the high activity of the catalyst, but the respective yields of BL were decreased. Almost no intermediates are left to convert at such high temperatures, as evident from the reaction temperature optimization. Moreover, a decrease in the BL yield was obvious because of the fact that unavailability of FAL conjugates due to FAL polymerization. Higher initial concentrations of FAL led to the polymerization under acidic conditions, thereby decreasing the product formation [11\u201313]. A maximum of 80\u00a0mol% of BL yield was achieved at a much lower initial molar ratio of 8.5:1, whereas for 6:1 initial molar ratio, the BL yield was limited to 68\u00a0mol% only. The yield of FAL polymers inclined with increasing initial FAL concentration and relatively increased to 33\u00a0mol% for 4:1 initial molar ratio. Therefore, feasible and economic studies are needed to find suitable initial conditions for this reaction to obtain maximum yields of BL.\nFig. 9\n. exemplifies the variation of BL yield with respect to the amount of catalyst loaded at a reaction temperature of 190 \u00b0C for 6 hours. The reaction was first performed without the catalyst, which resulted in no conversion of FAL, indicating that a minimum amount of a catalyst is needed to enhance the FAL conversion. Starting with a 4.5 wt.% of catalyst loading (0.25\u00a0g), 94% of FAL conversion was achieved with most of the unconverted reaction intermediates (around 20\u00a0mol% of 2-BMF) left to convert to BL. With an increase in the catalyst amount from 0.25 to 0.5\u00a0g, the complete conversion of FAL was achieved along with a significant increase in BL yield from 57 to 80\u00a0mol%. An increase in the catalyst amount provided more acidity in the reaction, which boosted the FAL conversion and transformed the intermediate 2-BMF to BL completely. Upon further increase in the catalyst loading to 0.75\u00a0g, a slight decrease in the BL yield, around 77\u00a0mol%, was observed. The yield of FAL polymers increased from 15% to 23\u00a0mol% with the catalyst amount. This was bounded to happen in acidic conditions at higher temperatures and probably caused by the presence of the more active sites, which promoted the initial conversion of FAL molecules to polymerize, thereby increasing the yield of FAL polymers [61]. Thus, a minimum of 9 wt.% of catalyst loading was sufficient for the complete conversion of FAL with a maximum of 80\u00a0mol% BL yield.The sulfonated carbon catalyst activity was compared with some typical previously reported solid acid catalysts, metal salts such as AlCl3 and dilute H2SO4. The results are summarized in Table 7\n. The reaction was performed at 190 \u2070C using a very low concentrated H2SO4 to mild H2SO4 (1\u00a0M) to study the effect of acid concentration (SFig. 3). The reaction with AlCl3.6H2O at 123 \u00b0C yielded 92\u00a0mol% of BL at a very high butanol to FAL mole ratio (Table 7, entry 1). This study depicts that the BL formation not only correlated to Bronsted acidic sites of Al salts but also with the Lewis acidic sites from Al3+ ions. An optimal combination of both Bronstedic and Lewis sites would proceed with a unique selectivity towards the BL. In the present work, the reaction was performed with anhydrous AlCl3 at 190 \u00b0C for 2 hours at a lower initial mole ratio of 8.5:1, and 73\u00a0mol% of BL yield was observed with 6\u00a0mol% of 2-BMF yield (Table 7, entry 8). The reaction was carried out only for 2 hours because of the fact that chloride ion corrodes the reactor at such high temperature and might affect the reaction results [62\u201364]. Alumina supported SBA-15 catalyst showed 91% of BL yield for a period of 6 hours at 180 \u2070C but at the initial mole ratio of 65 remarking the FAL polymerization to 9% at high temperature even at a high initial mole ratio of butanol to FAL (Table 7, entry2). Waste paper derived magnetic carbanaceous (SMWP) catalyst also showed 91 % BL yield at a reduced initial mole ratio of 41 with consistent activity (Table 7, entry 3). Sulfonated SBA-15 catalyst at a much reduced initial molar ratio of 16 gave 63 % of BL after 4 hours but at a much lower temperature of 100 \u00b0C (Table 7, entry 4). The Amberlyst 39 catalyst displayed 63 % BL yield at 110 \u00b0C after 6 hours (Table 7, entry 5). The authors have mentioned that unconverted intermediates such as 2-BMF and 4,5,5-tributoxy-2-pentanone at low-temperature reaction caused the 63% yield of BL. Moreover, the ionic resin Amberlyst catalysts are thermally unstable above 120 \u00b0C. The reaction with 1\u00a0M H2SO4 showed a maximum yield of 85% after 6 hours of reaction (Table 7, entry 7).The carbon black showed the optimal activity for the reaction with 34\u00a0mol% of BL yield (Fig. 10\n). The selectivity and the yield of both 2-BMF & BL were identical to each other and settled at 48\u00a0mol% selectivity and 34\u00a0mol% yield. The carboxylic acid and phenolic groups formed during the incomplete carbonization of sucrose were responsible for the carbon black's optimal activity. The fig. illustrates the reaction results with the carbon black. Upon the results with 0.5\u00a0g of catalyst, the loading was increased to 1\u00a0g, which resulted in a similar outcome. The product distribution indicated that a maximum of 0.5\u00a0g of catalyst loading was enough to convert the FAL to the products.The deactivation study (Fig. 11\n) for the sulfonated carbon catalyst was performed for three recycles. After every reaction, the catalyst was recovered by vacuum filtration and dried for 12 hours at 120 \u00b0C for the next use. The heterogenity of the reaction was reported in SFig. 9. which indicated a slight leaching of the -SO3H groups. Moreover, based on the heterogeneity test results, the reaction was mainly catalysed by the acid sites on thesurface of sulfonated carbon catalyst instead of the leached -SO3H groups.. After three recycles, the BL yield was reduced to 49\u00a0mol%, indicating that the -SO3H groups bonded to weakly functional groups of the carbon network were desorbed during the reaction. Though the FAL was fully converted after 6 hours for all the recycles, the 2-BMF left unconverted to BL, attributing the need for acidity for the conversion. This problem can be overcome by regenerating the activity of the catalyst by sulfonation. The catalyst after 3 cycles was regenerated by sulfonation at 80 \u00b0C under inert conditions, and it replicated the results as that of the fresh catalyst with 78\u00a0mol% of BL yield compared to 80\u00a0mol% with fresh catalyst.In conclusion, the butanolysis of FAL at a lower initial mole ratio was conducted using two different catalysts. The tungstated zirconia catalyst and the metal promoted catalysts resulted in a maximum of 28\u00a0mol% of BL yield, signifying the need for high catalytic activity for butanolysis reaction. The sulfonated carbon catalyst (surface acidity 2.357\u00a0mmol/g) resulting in 80\u00a0mol% of BL yield showed superior activity caused by the strong Bronstedic \u2013SO3H groups and the aided acidic carbon groups. The partially carbonized sucrose to carbon black showed better catalytic activity than the tungstated zirconia catalyst caused by the presence of phenolic, hydroxyl, and carboxylic acid functional groups. Thus, this work demonstrated that the high BL yields as high as more than 80\u00a0mol% with sulfonated carbon catalyst even at low butanol: FAL ratio as low as 8.5. Further decreasing the initial mole ratio resulted in decreasing the BL yield accompanied by FAL polymerization. The deactivation study reveals that the weakly bonded \u2013SO3H groups were detached from the carbon network resulted in the catalytic activity to 49\u00a0mol% of BL yield after 3 recycles with 100% FAL conversion and regained the activity to 78\u00a0mol% BL yield upon regeneration.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 in part by Japan Science and Technology Agency Strategic International Collaborative Research Program (JST SICORP) Grant Number JPMJSC18H1, Japan. U.R. Thuppati acknowledges the financial support by JICA IITH-FRIENDSHIP (D1956755) scholarship for suppoeting this studySupplementary data to this article can be found online at https://doi.org/10.1016/j.crcon.2021.03.003.The following are the Supplementary data to this article:\n\nSupplementary Data 1\n\n\n\n", "descript": "\n                  This work presents the formation of butyl levulinate, a potential fuel additive, and an excellent renewable chemical obtained by the butanolysis of furfuryl alcohol (FAL) over a solid acid catalyst. The butanolysis of furfuryl alcohol reaction is a strong function of acidity for which tungstated zirconia (WO3-ZrO2), a robust solid acid catalyst, and a sulfonated carbon catalyst were employed to produce high yields of butyl levulinate targeting a lower initial molar ratio of butanol to FAL. A maximum of 28\u00a0mol% yield of butyl levulinate was obtained with tungstated zirconia catalyst. Easily prepared sulfonated carbon catalyst at high reaction temperatures facilitated the complete conversion of reaction intermediate, 2-butoxymethylfuran (2-BMF) through which butyl levulinate was formed, and as high as 80\u00a0mol% of butyl levulinate yield was produced at an initial mole ratio of 8.5:1 of butanol to FAL. The better results of sulfonated carbon catalyst could be attributed to the presence of -SO3H, carboxylic acid, and phenolic OH groups on the carbon surface.\n               "}
{"full_text": "The authors are unable or have chosen not to specify which data has been used.Cobalt is widely used in metallurgy, mainly in the manufacturing of alloys which are corrosion and wear resistant while cobalt superalloys are also heat resistant [1]. Moreover, cobalt is also used to make magnets and high-speed tool steels. In non-metallurgical applications, cobalt is used as a catalyst in the petroleum and chemical industries, in batteries, and as a component of drying agents for various paints and inks [1]. Thus, high purity cobalt is required in most of its industrial applications and an effective cobalt extraction process is essential [2]. However, Co(II) and Ni(II) are often present in ores and their separation is challenging since both metals have very similar physicochemical properties due to their adjacent positions in the periodic table [3].Solvent extraction (SX) is the method often chosen by industry for Co(II) and Ni(II) separation due to its high separation efficiency [3]. Acidic organophosphorus extractants such as di-(2-ethylhexyl)phosphoric acid (D2EHPA) [4], 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC 88A) [5], and bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272) [6] have been applied to the SX of Co(II). However, D2EHPA exhibits poor separation of Co(II) from Ni(II). PC 88A offers a better selectivity over D2EHPA, although both these extractants also extract Ca(II). Cyanex 272, on the other hand, provides good separation of Co(II) from both Ni(II) and Ca(II) [6\u20138]. Other extractants for Co(II), namely bis(2,4,4-trimethylpentyl)dithiophosphinic acid (Cyanex 301) and bis(2,4,4-trimethylpentyl)monothiophosphinic acid (Cyanex 302), have been reported to offer even better Co(II) and Ni(II) separation than Cyanex 272 [9].However, all the extractants mentioned above are acidic, and thus the extraction process is strongly dependent on the pH of the aqueous phase. Over the past few years, ionic liquids (ILs) have been found to be promising extractants in SX [10].ILs are salts composed of a cation and an anion, which exist as liquids even at low temperatures, including room temperature. ILs have several advantageous properties over common organic solvents, such as negligible vapour pressure, good thermal stability and high intrinsic conductivity [10]. ILs are also known as designer solvents since their properties can be tuned to particular applications (e.g., ILs can be tuned to have low solubility in water, thus limiting their loss to the aqueous phase during the extraction process [11]). Due to these features, ILs have been widely studied as extractants for metal ions. Trialkylmethylammonium chloride (the main component of Aliquat 336) is an example of a quaternary alkylammonium-based IL, which has been studied for the Co(II) and Ni(II) separation.Co(II) and Ni(II) have different coordination preferences in aqueous media. In concentrated electrolyte solutions, Co(II) exhibits a tendency to form tetrahedral complexes which are less pronounced with Ni(II) [3]. Co(II) and Ni(II) separation with Aliquat 336 is based on this difference, i.e., Co(II) is extracted by anion exchange as an anionic tetrahedral chlorocomplex, either as the tetrachlorocobaltate(II) ion (CoCl4\n2\u2013) or the hydrogen tetrachlorocobaltate(II) ion (HCoCl4\n\u2013) depending on the solution pH [12,13]. Quaternary phosphonium-based ILs, such as trihexyl(tetradecyl)phosphonium chloride ([P66614][Cl]) and trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate ([P66614][C272]), are also able to separate Co(II) from Ni(II). The Co(II) extraction mechanism using [P66614][Cl] or [P66614][C272] in the presence of a high concentration of HCl or NaCl is the same as that involving Aliquat 336 (i.e., ion-exchange extraction) [14\u201317]. However, when using [P66614][C272] in the absence of HCl, it is suggested that Co(II) forms a neutral complex with the IL's phosphinate anion and its counter anion is extracted by the IL's cation to preserve solution electroneutrality [14]. It has also been reported that [P66614][Cl] can achieve high separation of Co(II) from Ni(II) in sulfate media via an anion-exchange mechanism as described by Onghena et al. [18]. In this case it is still the tetrachlorocobaltate(II) anion that is extracted, although the extraction process is more complex since it only occurs in the presence of sulfuric acid in the aqueous phase. It is suggested that the HSO4\n\u2212 ion firstly replaces the chloride ion in the IL which then complexes with Co(II) in the aqueous phase to form the tetrachlorocobaltate(II) anion. This anion is then transferred to the organic phase by anion-exchange with the HSO4\n\u2212 ion of the IL in its HSO4 form. However, in most cases, the extraction of Co(II) from chloride solutions involving anion-exchange requires the use of hydrochloric acid and/or lithium chloride solutions with concentrations as high as 7\u00a0M [19,20] which can present some difficulties for large scale industrial applications.Larsson and Binnemans [21] described the conversion of [P66614][Cl] into a form which contains a lipophilic and metal complexing anion, such as thiocyanate, rather than the more hydrophilic chloride anion (i.e., [P66614][SCN]). This IL can then be used to extract a target cation from the aqueous phase by direct complexation rather than by the formation of a complex anion in the aqueous phase first as in the conventional extraction by anion-exchange. Hence, the anion of such ILs can strongly complex with the target metal cation from the aqueous phase to form an anionic complex, while the cation of the IL forms two ion-pairs: one with the newly formed and extracted into the organic phase anionic metal complex, and the other with the counter anion of the target metal ion (which is only extracted into the organic phase in order to preserve electroneutrality in both phases). Larsson and Binnemans [21] have termed this process as \u2018split-anion\u2019 extraction. We are of the opinion that the term \u2018bifurcated extraction\u2019 better depicts this process, and it will thus be used throughout this manuscript.The bifurcated extraction mechanism using [P66614][SCN] has not only been used for the separation of Co(II) from Ni(II) in sulfate media [18], but also for the separation of transition metal ions (including Co(II)) from rare earth ions in aqueous nitrate or chloride solutions [22]. This extraction mechanism has also been explored with quaternary ammonium-based ILs with different anions for the separation of cobalt from samarium [23]. However, the stoichiometry proposed in the above-mentioned cases has not been determined experimentally.Since bifurcated extraction does not require a high chloride concentration in the aqueous phase and is independent of the aqueous solution pH, it has the potential to provide an efficient way for the separation of Co(II) and Ni(II) in industry. Thus, this paper describes a detailed study of the extraction process associated with the use of [P66614][C272] and [P66614][SCN], both dissolved in toluene, for extracting and separating Co(II) without the need for high concentrations of chloride in the aqueous phase or adjustment of the pH.All the chemicals used in this study were AR grade unless stated otherwise. [P66614][Cl] (commercially known as Cyphos\u00ae IL 101, >95.0%, Aldrich) and [P66614][C272], also known as Cyphos\u00ae IL 104 (>95.0%, Strem Chemicals), were used in toluene (99.5%, Ajax) as the diluent. Solutions for the SX studies were prepared from Co(II) stock solutions in deionised water (\u226518.2 M\u03a9 cm, Synergy 185, Millipore) using, CoCl2\u00b76H2O (99.7%, J.T. Baker), Co(SCN)2 (99.9%, Aldrich), Co(NO3)2\u00b76H2O (BDH), and CoSO4\u00b77H2O (BDH). KSCN (VWR Chemicals) was used to convert [P66614][Cl] to its SCN\u2212 form. Deionised water, 0.5\u00a0M HCl (32\u00a0wt%, Ajax), 0.5\u00a0M HNO3 (70\u00a0wt%, Ajax) and ethylenediaminetetraacetic acid disodium salt (EDTA) (Chem-Supply) were used for the preparation of aqueous solutions for back-extraction. The pH of the EDTA solution was adjusted to 7\u20138 by the addition of NaOH pellets (Chem-Supply). The pH was measured using a smart Chem-Lab Multi-Parameter Laboratory Analyser (TPS).For the study of the extraction of other metal ions, the following chemicals were used to prepare stock solutions containing 1000\u00a0mg L\u22121 of the associated cations: NaCl, NaNO3, CaCl2\u00b72H2O, Ca(NO3)2\u00b74H2O (all from Chem-Supply), Ni(NO3)2\u00b76H2O, CdCl2\u00b72.5H2O, CuCl2\u00b72H2O, Cu(NO3)2\u00b72.5H2O (all Ajax), Cd(NO3)2\u00b74H2O, MgCl2\u00b76H2O, Mg(NO3)2\u00b76H2O (all BDH), Zn(NO3)2\u00b76H2O (98%), NiCl2\u00b76H2O (both Sigma-Aldrich), ZnCl2 (UniLab). The above-mentioned salts were dissolved in deionised water.Acidic impurities in [P66614][Cl] were determined by potentiometric titration in ethanol solution using 0.01\u00a0M NaOH and monitoring the pH with a Chem-Lab Multi-Parameter Laboratory Analyser (TPS).Co(II) concentrations in aqueous solutions were determined by atomic absorption spectrometry (AAS) (Z-2000 Series Polarized Zeeman AAS, Hitachi) using the following conditions: acetylene flow \u2212 1.8 L min\u22121, acetylene pressure \u2212 160\u00a0kPa and air flow \u2212 15.0 L min\u22121, hollow cathode lamp (Hitachi) current and wavelength \u2013 15\u00a0mA and 240.7\u00a0nm, respectively.Other metal ion concentrations were determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 4300 DV, Perkin Elmer) using the following conditions: RF power \u2212 1300\u00a0W, plasma flow \u2212 15.0L min\u22121, auxiliary flow \u2212 0.2L min\u22121, nebuliser gas flow \u2212 0.7L min\u22121.UV\u2013visible spectroscopic studies of the organic phases (Libra S12, Biochrom) were carried out using toluene as the reference.Fourier transform infrared measurements (FTIR, Tensor 27 IR, Bruker) were conducted on KBr crystal disks to examine the [P66614][SCN] organic phase before and after extracting Co(II) from a 20\u00a0g L\u22121 Co(II) solution as well as to assess the presence of water in the organic phases containing [P66614][C272] or [P66614][SCN] loaded with Co(II).Electrospray ionisation mass spectrometry (ESI-MS) (Agilent 6520 Quadrupole Time of Flight (Q-ToF) mass spectrometer, USA, coupled to an Agilent 1100 autosampler) was used in the negative mode under the following conditions: drying gas flow rate, 7L min\u22121; nebuliser pressure, 40 psi; drying gas temperature, 300 \u02daC; capillary voltage, 4000\u00a0V; skimmer voltage, 65\u00a0V; scan range acquired, 50\u2013500\u00a0m/z. Each analysis involved the injection of dissolved in acetonitrile (HPLC grade, Sigma, USA) 0.005\u00a0M [P66614][C272] samples (1\u00a0\u03bcL), collected before and after extraction of Co(NO3)2, into the carrier solvent stream of 70:30 (v/v) acetonitrile (HPLC grade, Sigma, USA): 0.1% formic acid (HPLC grade, Sigma, USA), flowing at 0.3\u00a0mL\u00a0min\u22121.The procedure used to convert [P66614][Cl] to its thiocyanate form was similar to that described by Rout and Binnemans [22]. This was carried out by shaking 100\u00a0mL of 0.1\u00a0M [P66614][Cl] dissolved in toluene with 100\u00a0mL of 3\u00a0M KSCN solution in a separation funnel for 5\u00a0min. After separation, an orange-coloured organic phase was obtained, which was washed twice with 100\u00a0mL of 3\u00a0M KSCN and four times with 100\u00a0mL of deionised water. After each washing with deionised water, the orange colour diminished to yield a faint yellow organic phase.SX studies using both [P66614][C272] and [P66614][SCN] were carried out by shaking 20\u00a0mL of 0.1\u00a0M [P66614][C272] or [P66614][SCN] in toluene with 20\u00a0mL of 100\u00a0mg L\u22121 cobalt(II) chloride, thiocyanate, nitrate or sulfate in 125\u00a0mL glass jars for 1\u00a0h using an orbital shaker set to 200\u00a0rpm (Platform Mixer OM06, Ratek) or by mixing with a magnetic stirrer set to 750\u00a0rpm (magnetic multistirrer, VELP Scientifica) for 2\u00a0h. After phase separation, 0.5\u00a0mL of the aqueous phase was removed, diluted 20 times and the Co(II) concentration was determined by AAS.The organic phase (15\u00a0mL) after the Co(II) extraction experiments was back-extracted using 15\u00a0mL of deionised water, 0.1\u00a0M EDTA, 0.2\u00a0M EDTA, 0.5\u00a0M HNO3 (for [P66614][C272]) or 0.5\u00a0M HCl (for [P66614][SCN]) with contact times of 1\u00a0h for [P66614][C272] and 3\u00a0h for [P66614][SCN]. Solutions were stirred using a magnetic stirrer and, after phase separation, 0.5\u00a0mL of the aqueous phase was withdrawn for Co(II) determination.The percentage of extraction (%E) and back-extraction (%BE) were calculated using Eqs. (3) and (4), respectively:\n\n(3)\n\n\n%\nE\n=\n\n\n\n\n\n\n\nM\n\n\n\nIF\n\n\n-\n\n\n\nM\n\n\n\nFF\n\n\n\n\n\n\nM\n\n\n\nIF\n\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(4)\n\n\n%\nB\nE\n=\n\n\n\n\n\n\nM\n\n\n\nFR\n\n\n\n\n\n\nM\n\n\n\nIF\n\n\n-\n\n\n\nM\n\n\n\nFF\n\n\n\n\n\n\n\u00d7\n100\n\n\n\nwhere \n\n\n\nM\n\n\n\nIF\n\n\n is the initial concentration of metal in the aqueous (feed) solution, \n\n\n\nM\n\n\n\nFF\n\n\n is the concentration of metal in the aqueous solution after extraction, and \n\n\n\nM\n\n\n\nFR\n\n\n is the concentration of metal in the back-extraction solution.In order to investigate the stoichiometry for the extraction of Co(II) by [P66614][C272] and [P66614][SCN], a 20-mL aliquot of 0.1\u00a0M of extractant in toluene was mixed using a magnetic stirrer with 20\u00a0mL of increasingly high concentrations (up to 20,000\u00a0mg L\u22121) of a Co(II) solution (nitrate for [P66614][C272] and chloride for [P66614][SCN]) for 3\u00a0h for each concentration until a plateau was reached to signify that the organic phase had been fully loaded with Co(II). After phase separation, 15\u00a0mL of the organic phase were collected and back-extracted with an equal volume of 0.5\u00a0M HNO3 for [P66614][C272] and 0.5\u00a0M EDTA for [P66614][SCN]. Complete back-extraction was achieved in 5\u00a0h for [P66614][C272] and 3\u20134\u00a0days for [P66614][SCN]. The back-extraction solutions were diluted with deionised water and the Co(II) concentration was determined by AAS.For [P66614][C272] only, an extraction experiment with a lower concentration of extractant (0.005\u00a0M) was also conducted in order to be able to analyse the Co(II) concentration directly in the aqueous feed phase. Aqueous feed solutions with increasing concentrations of Co(II) of up to 600\u00a0mg L\u22121 (with nitrate as the counter ion) were used, following the same procedure as described above.Studies were carried out using the same extraction procedure as described in Section 2.4 with aqueous feed solutions containing a mixture of Co(II), Ni(II), Cd(II), Cu(II), Zn(II), Na(I), Mg(II) and Ca(II) (1.7\u00a0mM each).As mentioned in Section 2.3, an orange colour appeared in the toluene organic phase during the conversion of [P66614][Cl] to its thiocyanate form (i.e., [P66614][SCN]). The UV\u2013visible spectrum of the organic phase is shown in Fig. 1\n along with that of the original [P66614][Cl].There is an absorption peak at 480\u00a0nm for [P66614][SCN] which is absent in the spectrum of the original [P66614][Cl]. This peak is characteristic for the [FeSCN]2+ species [24] and demonstrates that [P66614][Cl], as received from the supplier, contained a small amount of Fe3+ as an impurity. This was confirmed by the fact that the spectrum of [P66614][Cl] showed a peak at 360\u00a0nm, characteristic of the FeCl4\n\u2212 species [25].Another important observation made when initially using [P66614][SCN] to extract Co(II) from neutral solutions was the fact that the pH of the aqueous solution decreased unexpectedly by approximately 3 pH units after extraction (from pH 5.1\u00a0\u00b1\u00a00.2 to 2.2\u00a0\u00b1\u00a00.1, n\u00a0=\u00a03). Bradaric et al. [26] have described the synthesis of trihexyl(tetradecyl)phosphonium chloride using trihexylphosphine and 1-chlorotetradecane as starting materials yielding 98\u00a0wt% purity, of which 93.9% corresponded to the IL, and the remaining were impurities (e.g., 4.4% trihexylphosphonium hydrochloride, 0.3% HCl, <0.3% tetradecane isomers, <0.7% secondary alkylphosphines). Potentiometric titration of commercial [P66614][Cl] (Section 2.2) with a standard solution of NaOH confirmed that the commercial product, as supplied, contained 58.4\u00a0mmol L\u22121 of a monobasic acid (presumably HCl) which after dilution with toluene was sufficient to decrease the aqueous solution pH by 3 pH units as observed in the preliminary extraction experiments using unwashed IL. It is interesting to note that the hydrogen ion associated with trihexylphosphonium hydrochloride was not removed during the process of its conversion to the thiocyanate form but only when the thiocyanate form of the IL was used to extract Co(II). This is possibly due to the complexation of trihexylphosphine with CoCl2.Extraction experiments were initially conducted with 0.1\u00a0M [P66614][C272] or [P66614][SCN] in toluene as the organic phase, and cobalt(II) nitrate or cobalt(II) chloride in the aqueous phase, respectively (both 100\u00a0mg L\u22121 Co(II), Fig. S1, Supplementary Material). After the extraction, different aqueous phases were investigated for their ability to back-extract Co(II), namely deionised water and solutions of EDTA, and HNO3 for [P66614][C272], and deionised water and solutions of EDTA and HCl for [P66614][SCN]. For [P66614][C272], \u2265 90% back-extraction of Co(II) was achieved within 10\u00a0min using 0.2\u00a0M EDTA and 0.5\u00a0M HNO3 solutions, but only 20% back-extraction was achieved in 40\u00a0min with deionised water (Fig. S2, Supplementary Material). For [P66614][SCN], only the EDTA solution was able to back-extract Co(II) due to the high stability of the corresponding Co(II)-EDTA complex (log K\nCo(II)-EDTA\u00a0=\u00a016.26 [27]). Complete back-extraction was achieved in 1\u00a0h for 0.2\u00a0M and 2\u00a0h for 0.1\u00a0M EDTA solutions (Fig. S3, Supplementary Material). Back-extraction of Co(II) in the case of [P66614][SCN] was significantly slower than that for [P66614][C272] because of the high stability of the cobalt(II)-thiocyanate complex.The intense blue colour of the organic phase after Co(II) extraction for each of the extractants suggested tetrahedral coordination of the ligands around the Co(II) ion. This was confirmed on examination of the UV\u2013visible spectrum of the organic phase as shown in Fig. 2\n for each extractant.The spectrum of [P66614][C272] organic phase after extraction (Fig. 2A) shows three absorption peaks consistent with a Co(II) complex with asymmetric tetrahedral coordination [27,28]. Xun and Golding [29] obtained a similar spectrum for the Co(II)-Cyanex 272 complex, however, in their case the extractant existed in a dimeric form and so a direct comparison with [P66614][C272] should be made with caution, even though both extractants have the same anion.The spectrum of the [P66614][SCN] organic phase after extraction of Co(II) (Fig. 2B) shows a single absorbance peak at 625\u00a0nm. According to Bjerrum [30], this peak is characteristic of a tetrahedral tetrathiocyanatocobaltate(II) complex anion.In order to elucidate the Co(II) extraction stoichiometry in the cases of [P66614][C272] and [P66614][SCN], studies were carried out to fully load the organic phase with Co(II) as described in Section 2.5 to determine the mole ratio of IL to Co(II). The results obtained for [P66614][C272] (Fig. 3\n) indicated that on fully loading of the organic phase with Co(II), the mole ratio of [P66614][C272] to Co(II) was equal to 2.8:1 thus suggesting that 3 molecules of [P66614][C272] were required to extract one Co(II) ion. However, since the extracted Co(II) species had a tetrahedral configuration, it was virtually impossible to form a complex composed of one cobalt ion and three phosphinate anions. This experiment was repeated with a lower concentration of IL in the organic phase (where the Co(II) concentration after extraction was measured in the feed solution, with no need to do back-extraction) and the same ratio was obtained (Fig. 3). Moreover, it should be noted that the pH before and after solvent extraction was not significantly different (e.g., 6.0 vs 6.3, respectively). It has been reported for the extraction of Co(II) using Cyanex 272 by SX that oligomeric species with 2 or even 3 cobalt centres are formed at\u00a0>42% Co(II) loading of the organic phase [31,32]. Since the ratio [P66614][C272]/Co(II) determined here experimentally was 2.8 and the organic phase was fully saturated, it was suggested that an oligomeric species, consisting of 3 cobalt centres and 8 phosphinate anions, similar to that proposed by Best et al. for Cyanex 272 [32], was present. Due to the formation of such oligomeric species, the viscosity of the organic phase increased and for that reason longer equilibration times were used to guarantee that equilibrium was reached.The extraction mechanism is thus described by Eq. (1) (Table 1\n) which is clearly an example of bifurcated extraction in which the nitrate ion (presence in the organic phase confirmed by ESI-MS in negative mode, Fig. S4, Supplementary Material) only serves the purpose of preserving electroneutrality and the complexing anion is that originally associated with the IL.It should be noted that Rybka and Regel-Rosocka [14] proposed an alternative stoichiometry (Eq. (2), Table 1) suggesting a 2:1 ratio of [P66614][C272] to Co(II). However, this was not based on the experimentally determined mole ratio of IL to Co(II) and required each phosphinate anion to act as a bidentate ligand in order to explain the tetrahedral structure of the complex. As discussed by Carson et al. [31] the phosphinate ligands are insufficiently flexible to allow for O-Co-O angles to be close to the ideal tetrahedral angle when acting in their bidentate coordination mode. The asymmetric tetrahedral structure, as evidenced by the UV\u2013Visible spectrum of the Co(II)/[P66614][C272] complex, shown in Fig. 2A, is consistent with the proposed oligomeric complex.Co(II) back-extraction in the present work with 0.5\u00a0M HNO3 solution can be described by Eq.\n(3) (Table 1). In this back-extraction stoichiometry it is suggested that the H+ ion protonates the bis(2,4,4-trimethylpentyl)phosphinate anion thus forming Cyanex 272 (i.e., bis(2,4,4-trimethylpentyl)phosphinic acid) [33]. This means that the IL does not fully return to its original form. However, washing the back-extracted organic phase with a low concentration of sodium hydroxide solution is expected to allow the complete regeneration of [P66614][C272], and consequently its reusability [33]. On the other hand, back-extraction with EDTA is expected to return the IL to its original form as described by Eq. (4) (Table 1).The results of the Co(II) extraction stoichiometry study in the case of [P66614][SCN] are shown in Fig. 4\n. It should be noted that longer equilibration times were implemented to guarantee that equilibrium was reached even in the presence of very high Co(II) concentrations (Fig. S5, Supplementary Material). Moreover, no significant pH change was observed before and after the solvent extraction (e.g., pH 5.3 vs 5.0, respectively). The results obtained suggest a [P66614][SCN] to Co(II) mole ratio of 2.6:1.An extraction stoichiometry of 2.6:1 is in contrast to the 4:1 stoichiometry suggested by Rout and Binnemans [22] as discussed earlier, however, their proposal was not based on an exhaustive study of the stoichiometry as presented above. An extraction stoichiometry that is in agreement with a [P66614][SCN] to Co(II) mole ratio of 2.6:1 can be described by Eq. (5) (Table 2\n) where the mole ratio is 2.5:1.In order to explain the tetrahedral coordination associated with the Co(II) ion and the 2.5:1 stoichiometry, we propose a structure in which two Co(II) ions are bridged by a thiocyanate ion as shown in Fig. 5\n.Support for this structure arises from the fact that the thiocyanate anion has a linear structure and is ambidentate since both the S and N atoms can act as electron pair donors in the formation of transition metal complexes. Transition metal complexes are known, where the thiocyanate anion acts as a bridging group, such as that suggested in Fig. 5, which can be confirmed by FTIR [34,35]. Hence, the organic phase containing [P66614][SCN] was analysed by FTIR before and after saturation with Co(II), and the results are presented in Fig. 6\n.The spectral band at 2052\u00a0cm\u22121 observed for the organic phase before extraction corresponds to the thiocyanate anion of the IL (CN stretching frequency), which shifts to 2071\u00a0cm\u22121 with a shoulder at 2088\u00a0cm\u22121 after loading the organic phase with Co(II). It has been reported that, pronounced \u03c5C\u2013N bands within the 2020\u20132096\u00a0cm\u22121 range indicate the presence of M\u2212SCN\u2212M coordination of the thiocyanate group (M standing for a metal ion). Moreover, the shoulder is indicative of the presence of SCN\u2212 ions in bridging and terminal positions [35]. Hence, the FTIR spectra, shown in Fig. 6, provide the evidence required to support the structure presented in Fig. 5.Equation 7 (Table 2) represents the back-extraction of Co(II) with EDTA and, as for the case with [P66614][C272], EDTA returns the IL back to its original form.The presence of water in the organic phases containing [P66614][C272] or [P66614][SCN] loaded with Co(II) was also assessed by FTIR (Fig. S6, Supplementary Material). Within the range 3500\u20133300\u00a0cm\u22121, no broad peak was observed which indicated the absence of any O\u2013H stretch vibration. Hence, it can be concluded that water was not present in the organic phases, thus having no influence on the extraction process.The results from the Co(II) SX studies using different aqueous phase counter anions in the cases of both [P66614][C272] and [P66614][SCN] are shown in Fig. 7\n. The organic phase acquired an intense blue colour after extraction of each of the four counter anions and the extent of extraction was similar for both extractants. It can be seen that 100% of the Co(II) was extracted with thiocyanate and nitrate as the counter anions while lower degree of extraction was obtained when the counter anions were chloride and sulfate. The order of extraction clearly follows that predicted from the Hofmeister series [36] for anions in which the less hydrated, and hence more lipophilic anions (i.e., thiocyanate and nitrate), are more readily extracted than the more hydrated and less lipophilic anions (i.e., chloride and sulfate).The extraction of other metal ions using [P66614][C272] and [P66614][SCN] was studied for Ca(II), Mg(II), Na(I), Cd(II), Ni(II), Cu(II), or Zn(II) ions using their chloride or nitrate solutions and the results are presented in Figs. 8 and 9\n\n.It can be seen in Fig. 8 that, except for Na(I), [P66614][C272] extracts all the other cations studied from their aqueous solutions, which is not surprising since the phosphinate anion of [P66614][C272] is a strong complexing agent, particularly for transition metal cations. It should be noted that the extraction percentage from nitrate solutions, with the exception of Cu(II), is higher than that from chloride solutions due to the higher lipophilicity of nitrate.In the case of [P66614][SCN], Fig. 9 shows that Mg(II), Ca(II) and Na(I) are not extracted because these cations do not form stable complexes with thiocyanate unlike Cd(II), Cu(II), Co(II), and Zn(II) which form stable complexes and are successfully extracted. The most important result in this experiment is that Ni(II) is not extracted from both its chloride or nitrate solutions and this leads to the conclusion that [P66614][SCN] presents itself as a promising extractant for the separation of Co(II) and Ni(II).The results of the present study have demonstrated that both [P66614][C272] and [P66614][SCN] in toluene as diluent can act as bifurcated extractants for Co(II) from nitrate or chloride solutions in the absence of strong acids or high concentrations of salts. This further supports the argument that ILs can extract the target species by ion-exchange or ion-pair extraction depending on the system being studied.The UV\u2013visible spectral studies have shown that the Co(II) coordination sphere geometry in the organic phase for both extractants is tetrahedral which is also supported by the intense blue colour of the organic phase. A study of the Co(II) extraction stoichiometry has determined that the [P66614][C272] to Co(II) stoichiometric mole ratio is 2.8:1 (with saturated organic phase), which along with the fact that the structure is tetrahedral suggests eight phosphinate anions bridging three cobalt centres. However, it should be noted that if the Co(II) loading in the organic phase is low, a different stoichiometric mole ratio would be expected (i.e., 4:1) [31]. In the case of [P66614][SCN], the extraction stoichiometry, also confirmed by FTIR analysis, has been found to consist of two tetrahedral Co(II)-thiocyanate complex anions bridged by a thiocyanate ion. In the case of [P66614][SCN], an EDTA solution has been found to be able to completely back-extract Co(II), while regenerating the organic phase to its original form. In the case of [P66614][C272], Co(II) could be completely back-extracted with a solution of HNO3, although the IL likely did not return to its original form due to protonation and formation of Cyanex 272. Therefore, it can be expected that washing the back-extracted organic phase with a low concentration of sodium hydroxide solution or using EDTA instead of HNO3 as receiving phase could potentially regenerate the organic phase to its original form.It is also shown that the extraction of Co(II) is enhanced if its counter anion in the aqueous phase is more lipophilic (e.g., nitrate, thiocyanate). As expected for [P66614][C272], limited discrimination has been found in its ability to extract cations other than Co(II) due to the strong complexing ability of the phosphinate anion and the fact that neutral aqueous solutions have been used. On the other hand, [P66614][SCN] has not extracted Ni(II) from nitrate or chloride solutions thus proving to be a useful extractant for the separation of not only Co(II) and Ni(II) without the use of acidic solutions or high concentrations of chloride, but also for the separation of Co(II) from Ca(II), Mg(II) and Na(I).\nSyane A. Satyawirawan: Validation, Formal analysis, Investigation, Writing \u2013 original draft. Robert W. Cattrall: Conceptualization, Writing \u2013 review & editing, Supervision. Spas D. Kolev: Conceptualization, Resources, Writing \u2013 review & editing, Supervision. M. In\u00eas G.S. Almeida: Conceptualization, Investigation, 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.Supplementary data to this article can be found online at https://doi.org/10.1016/j.molliq.2023.121764.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The solvent extraction of Co(II) from its aqueous solutions free of acids or chloride salts at high concentration by the ionic liquids trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate ([P66614][C272]) and trihexyl(tetradecyl)phosphonium thiocyanate ([P66614][SCN]) was characterised at saturation of the organic phase with Co(II) using toluene as diluent. The ionic liquids\u2019 cation participated in the formation of two ion-pairs in the organic phase, i.e., one with the complex anion containing cobalt, and the other with the cobalt counter anion, thus preserving electroneutrality in both the organic and aqueous phases. This was considered to be an example of bifurcated extraction.\n                  UV\u2013visible spectral studies of the intensively blue organic phase after Co(II) extraction demonstrated that the Co(II) coordination sphere geometry in the case of both extractants was tetrahedral. The Co(II) extraction stoichiometry was experimentally determined for the first time by saturating the organic phase with Co(II). The [P66614][C272] to Co(II) stoichiometric mole ratio in the extracted adduct was found to be 2.8:1, suggesting that eight phosphinate anions acted as bridging ligands around three cobalt centres. For the [P66614][SCN], the stoichiometric mole ratio was found to be 2.5:1 with the Co(II)-thiocyanate adduct consisting of two Co(II)-thiocyanate tetrahedra linked by a bridging thiocyanate ion.\n                  The Co(II) extraction was enhanced when the aqueous phase contained an anion with high lipophilicity (e.g., nitrate, thiocyanate). [P66614][SCN] did not extract Ni(II) from nitrate or chloride solutions, thus demonstrating its potential for the separation of Co(II) from Ni(II) without requiring the addition of high concentrations of acids or chloride salts to the aqueous phase.\n               "}
{"full_text": "Industrial waste management is beneficial for the energy security of Poland and development of local economy. The by-products of food industry are abundant sources of biomass which can be thermally converted into fuel, chemicals, or used for environmental applications [1\u20133]. One of the methods to utilize the by-products of food industry is pyrolysis. Fast pyrolysis is an economical method to convert biomass waste into oil, which is characterized by a high energy density. However, oil obtained by fast pyrolysis has undesirable properties such as low chemical stability and high acidity [4]. The main disadvantage of oils obtained by fast pyrolysis is the high oxygen content; therefore, deoxygenation and aromatization of oils have been investigated in many studies [5\u20138]. Zeolites have been effectively used to decrease the oxygen content in oils. Zeolite Socony Mobil #5, abbreviated ZSM-5, and Zeolite Y are commonly used to deoxygenate the biomass pyrolysis vapors [4,5]. Silica\u2013alumina ratio (SAR) significantly affects the acidity and reactivity of a catalyst during biomass pyrolysis [6]. The catalyst acidity increases with decreasing SAR values; zeolites with low SAR values exhibit better cracking capabilities [7,8]. Moreover, zeolites with large pore sizes allow easier access to more long-chain compounds at the acidic sites for decarbonization and dehydration reactions [9,10]. Therefore, the catalytic performance can be improved by the addition of hydrogen donors during pyrolysis [11,12]. Zeolites with low SARs afford relatively lighter oils [13]. One of the major technical barriers inhibiting the commercialization of the catalytic pyrolysis technology is rapid catalyst deactivation [14,15]. Despite promising advances in catalytic fast pyrolysis, the issue of coke deposition on the catalyst surface persists [16,17]. Catalyst deactivation involves active site poisoning, pore blockage, mineral deposition, and is inversely proportional to the SAR [18,19]. A catalyst with the high aluminum content (SAR < 30) promotes the aromatization reaction, which corresponds to a high amount of acid sites. Aromatic compounds decrease with increasing SAR ratio. When increasing the SAR ratio of the catalyst, its deactivation was considerably as a result of the lower rate of coke precursor condensation reactions [20]. As a result, the significantly prolonged catalytic lifetime increases with an increase in the SAR value [21]. Heracleous et al. [22] reported catalyst regeneration under air oxidation at 500\u00a0\u00b0C, without adversely affecting the catalyst structure and strength of the acidic sites; catalyst activity was maintained even after seven catalyst regeneration cycles [23]. Yung et al. [24] reported that 89% of coke could be removed from the catalyst during 120\u00a0min of regeneration at 550\u00a0\u00b0C. Interestingly, the coke deposition rate was higher in the fluidized bed method than in the fixed bed method [25]. Another significant aspect that determines coke deposition on the catalyst is the catalyst-to-biomass (C/B) ratio [15,26\u201328]. Excess catalyst increases the reaction probability of the pyrolysis products with the acidic sites on the catalysts [15]. An increase in the C/B ratio from 0.5 to 10 resulted in an increase in the hydrocarbon vapor content in the range of 6\u201315% liquids [26]. Additionally, the catalytic activity could be lost when the C/B ratio was 1/10. With an increase in the C/B ratio from 0 to 15, the yields of the monocyclic and polycyclic aromatics increased, whereas those of the oxygenates progressively decreased [27].Compared to fast pyrolysis, intermediate pyrolysis is a relatively new method of thermal conversion, and the first pilot-scale reactor for intermediate pyrolysis was developed in 2009 [29]. In addition to char and gas, the products of intermediate pyrolysis include two-phase liquids (aqueous phase + bio-oil). Fast pyrolysis is often selected owing to the maximum liquid yield obtained; however, it is not useful for industrial residue or agricultural biomass [29]. In terms of agricultural products like straw, switch grass, husks, or miscanthus - fast pyrolysis produces a low yield of bio-oil, from 35% for wheat straw to 58\u00a0wt% for miscanthus [30]. Additionally, high water content in agriculture biomass is disadvantages in fast pyrolysis. The formation of high-molecular-weight tars and dry and brittle char is a critical issue, rendering intermediate pyrolysis more attractive than fast pyrolysis. Intermediate pyrolysis is characterized by longer vapor and feedstocks residence times than those in fast pyrolysis, as shown at \nTable 1.Although the yield of bio-oil obtained by intermediate pyrolysis is lower than that obtained by fast pyrolysis, the bio-oil is more stable and contains less oxygen [33]. Research on intermediate pyrolysis has mainly focused on the effects of the type of feedstock and process temperature on the yield and product quality [34\u201339]. High pyrolysis temperatures (500\u00a0\u00b0C) favor fragmentation reactions and result in a better quality oil with a low oxygen mass fraction [34]. Oils produced during the intermediate pyrolysis of waste sludge have satisfactory characteristics for use as diesel engine fuel [35]. Interestingly, even an aqueous phase with a low heating value and high water content can be effectively used. The anaerobic digestion of the water phase with the addition of char allows fuel (methane) production [36]. According to Primaz et al. [37], the char yield decreased with an increase in temperature from 400\u00b0 to 600\u00b0C. Higher pyrolysis temperatures resulted in secondary char cracking and increased the gas product yield. Pyrolysis gas can be recycled to the reactor as a carrier gas which consists of nitrogen >\u00a0carbon dioxide >\u00a0carbon monoxide >\u00a0methane >\u00a0hydrogen [38]. Recent studies have suggested the risk of clogging pipes by tars formed during the intermediate pyrolysis of biomass [40]. Therefore, use of a catalyst is advisable to minimize tar formation, but only a few studies have investigated catalytic intermediate pyrolysis [34,41\u201344]. The commercial Ru/C catalyst is better suited for oil deoxidation than NiCu/Al2O3. In addition, more aqueous phase was formed when Ru/C was used instead of NiCu /Al2O3\n[34]. Mohammed et al. [41] determined the effects of ZSM-5 and Zeolite A (zinc-exchanged) on the intermediate pyrolysis of Napier grass, where the yield of biochar did not change after introducing the ZSM-5 catalyst into the reactor. Furthermore, a low proportion of ZSM-5 catalyst (up to 1.0% by weight of feedstock) had no effect on the oil yield [41]. Catalytic biomass pyrolysis with and without a steam reformer was the aim of the study reported by Mahmood et al. [42]. The addition of steam increased the calorific value of the gas from 3 to 10.8 MJ/m3 at 500\u00a0\u00b0C, mainly due to an increase in the hydrogen content. Intermediate biomass pyrolysis with thermocatalytic reforming promoted the formation of phenol in oil (30.88% mass) and hydrogen in gas (19.4\u00a0vol%) [43]. Recent thermogravimetric analysis\u2013Fourier-transform infrared spectroscopy (TGA\u2013FTIR) studies have shown that zinc-containing nanopowders affect the pyrolysis yield. The bio-oil yield increased in the presence of ZnWO4, ZnAl2O4, and Mn\u2013Zn ferrite, while the highest non-condensable gas yield was obtained with the addition of 2% Ag/ZnO catalyst [44].Only a few reports on catalytic intermediate pyrolysis are currently available, which is the main motivation to investigate this subject area. It is well-known that the application of catalysts with low SAR values of <\u00a030 in fast pyrolysis is advantageous, but this effect is not as prominent in intermediate pyrolysis. Therefore, the effect of catalyst acidity (Zeolite Y: SAR = 26 and ZSM-5: SAR = 352) on biomass pyrolysis was investigated. In this study, the physical and chemical properties of the biomass and catalysts before and after pyrolysis were examined. Catalytic pyrolysis with 2-, 4-, 6-, 8-, and 10-fold usage of the same catalyst (ZSM-5 or Zeolite Y) was performed, and the coke contents of the catalysts, trace elements in the catalysts, and compositions of the pyrolysis products were analyzed. The novelty of this work includes the determining the effect of coked catalysts after 2-, 4-, 6-, 8-, and 10-fold usage in biomass pyrolysis. The effect of the multiple usage of the catalysts on impurity deposition was also investigated. Moreover, the catalysts that were reused 10 times during pyrolysis were regenerated, and impurity removal was examined. To the best of our knowledge, such studies have not been previously reported in the literature. Additionally, it was studied the amount of coke after the regeneration of ZSM-5 and Zeolite Y under intermediate pyrolysis conditions. This is significantly important to define the kind of coke e.g., hard (Cn) or light coke (-CnH2n).Post-extraction rapeseed meal (abbreviated as RM), a by-product of oil production, was obtained from an industrial oil-pressing plant located in Poland. In 2020, rapeseed harvest in Poland was 2.7 million tons [45], from which approximately 1.6 million tons of meal was produced. In the 2020/21 season, global rapeseed harvest forecasts will amount to about 63.0 million tonnes. In the group of key producers, crops increased in Canada, India, China and Australia [45]. RM is traditionally used for feeding animals because of its high protein content. However, many recent studies have reported new methods of waste management, which can allow the expansion of its application potential [23,46,47]. Xia et al. [23] demonstrated that porous carbon derived from RM could be successfully used for various energy storage applications such as in lithium-ion batteries and supercapacitors. Zhang et al. [46] proposed the synthesis of a catalyst based on carbon derived from RM with nitrogen and sulfur doping. The new catalyst was an excellent replacement for the commercial Pt/C catalyst used for energy conversion in fuel cells. Poskrobko and Kr\u00f3l [47] reported the significant potential of RM for use in the co-gasification of wood biomass and RM. The addition of RM to the wood biomass contributed to an increase in the calorific value of the synthesized gas.The catalysts used in this study were the hydrogenated forms of ZSM-5 and Zeolite Y (ZY) that were purchased from Acros Organics and Alfa Aesar, respectively. The catalysts were in the form of gray (ZSM-5) and white (ZY) powders.A Truspec CHNS 628 Leco (USA) analyzer was employed for the ultimate analysis (carbon (C), hydrogen (H), nitrogen (N) and sulfur (S)) of RM, catalysts, char, bio-oil, and aqueous phase. TGA of the catalysts under air and nitrogen atmospheres (constant flow rate of 50\u00a0mL/min, approximately 4\u00a0mg sample, and heating rate of 10\u00a0\u00b0C/min) was performed using a Mettler Toledo TGA/SDTA 851 system (Switzerland). The elemental contents of the catalysts were evaluated using the X-ray fluorescence (XRF) method (ZSX Primus II Rigaku spectrometer, USA). The morphology and structures of the chars were determined using scanning electron microscopy (SEM; Inspect S50 apparatus, FEI, the Netherlands). SEM images were collected using a secondary electron detector in the high-vacuum mode, and the applied acceleration voltage was 3\u00a0keV. The specific surface areas and average pore diameters of the fresh, used, and regenerated catalysts were determined using the Brunauer\u2013Emmett\u2013Teller (BET) method. The total pore volume was determined using the Barrett\u2013Joyner\u2013Halenda (BJH) method. The analysis was done by apparatus of Micromeritics, USA. FTIR spectroscopy was employed to identify the functional groups of the organic compounds in the studied samples using a Bruker Alpha II system (Bruker Optics Inc., USA). The infrared absorption frequency was in the 400\u20134000\u00a0cm\u22121 range. The bio-oil composition was determined by gas chromatography-mass spectrometry (GC-MS; Agilent GC 7890 B equipped with an MS 5977\u00a0A mass spectrometer and a flame ionizer detector, Agilent Technologies, USA) technique. The gas phase was analyzed by GC (Agilent Technology 7890\u00a0A, Agilent Technologies, USA).A simplified diagram of the experimental set-up employed to investigate the pyrolysis process is shown in \nFig. 1. Preparation of RM for the experiments included drying under ambient conditions and sieving of the particles (300\u2013750\u00a0\u00b5m). Each experiment began by weighing 1.5\u00a0g of the catalyst and placing it in a reactor. Next, the reactor was electrically heated to 500\u00a0\u00b0C and maintained at a constant temperature for 1\u00a0h. The RM sample (1.5\u00a0g) was placed on a boat into the zone of the water cooler, and the reactor was purged with nitrogen at a flow rate of 100\u00a0mL/min for 5\u00a0min. The main process of intermediate pyrolysis started with the insertion of the boat with the RM into the heated furnace. The residence time of the sample was 7\u00a0min. During this time, the temperatures of the sample and reactor were monitored using K-type thermocouples. \nFig. 2 shows the temperature profiles of the sample and reactor. One RM sample was investigated for 30\u00a0min, including 5\u00a0min of nitrogen purge, 7\u00a0min of pyrolysis, and 18\u00a0min of sample cooling. After starting the pyrolysis, that is, from the fifth minute of investigation, the temperature of the sample increased rapidly and reached 412\u00a0\u00b0C after 2\u00a0min\u00a0of pyrolysis. Then, the temperature of the sample increased slowly. After the pyrolysis was completed, the sample boat was moved back to the water-cooler zone. The pyrolysis vapors flowed to the ice tank, where the aqueous and bio-oil phases were condensed. A gravimetric settler was used to separate the aqueous phases and bio-oil. The non-condensed dried gases were collected in a Tedlar bag for GC analysis.Five series of measurements were performed for the investigated catalysts used for the pyrolysis of 2, 4, 6, 8, and 10 RM samples; here, 10 RM sample indicates that the catalyst was held in the reactor for 10 pyrolysis cycles of the new biomass sample. After each series of measurements, the catalyst was removed from the pyrolysis reactor, and the composition of the pyrolysis gas was determined for each series of measurements. Liquid phases (bio-oil and aqueous phase) were not collected for each sample series (2, 4, 6, 8, and 10) because of the need to clean the system and difficulty in collecting an appropriate amount for analysis. After 10-fold usage of ZSM-5 and Zeolite Y catalysts, they were regenerated in muffle furnace. Regeneration took place in an air atmosphere at a temperature of 500\u2009\u00b0C for 5\u2009h.\n\nTable 2 shows proximate, ultimate and compositional analysis of rapeseed meal (RM). The volatile matter (74% mass) is comparable to that of wood, and the moisture content (5.66%) is low. The low moisture content is an advantage for RM as it prevents biological degradation during storage. However, the ash content is relatively higher compared to those in other biomass wastes [9,43]. Based on the ultimate analysis, the effective hydrogen to carbon molar ratio (H/C\n\neff\n) was calculated using Eq. (1).\n\n(1)\n\n\n\nH\n\n/\n\n\n\nC\n\n\neff\n\n\n\n\n=\n(\n\n\nH\n\u2212\n2\nO\n)\n\n/\n\nC\n\n\n\n\n\nThe H/C\n\neff\n ratio of the biomass commonly ranges from 0 to 0.35 [11,27,28]. The high amount of oxygen with moderate carbon and hydrogen contents in the biomass results in an H/C\n\neff\n ratio of 0.22 for the RM. Biomass with a low H/C\n\neff\n ratio tends to produce aromatics during pyrolysis [27]. In RM, the cellulose, hemicellulose, and lignin contents are comparable (24.23%). The RM composition is dominated by other biopolymers such as protein, fat, and polysaccharides [48].Additionally, RM was ashed at 550\u2009\u00b0C to identify the components present in the ash. The ash-forming elements in the biomass may play important roles during pyrolysis. Elements from the alkali metal group, mainly potassium, can promote primary dehydration reactions [49]. Furthermore, a significant amount of ash may accumulate on the catalyst surface during subsequent pyrolysis cycles. The chemical analysis data for ash is presented in \nTable 3, indicating that ash is phosphorus-rich and contains relatively large amounts of potassium, calcium, and magnesium. The presence of potassium and calcium increase the intrinsic catalytic reactivity of RM.\n\nFig. 3 shows the FTIR spectrum of RM, where the stretching and bending vibrations are marked. The wavenumber ranges for different functional groups are listed in \nTable 4, based on the research equipment database. The exception is the class of compounds corresponding to the wavenumber below 700\u2009cm\u22121, which was identified based on the literature data [50,51], because of a lack of data in the system database. N\u2013H, C\u2013N, and CO bonds of the proteins present in RM (protein content is 35% of dry mass [48]) were identified in the FTIR spectrum.Two catalysts, ZSM-5 and ZY, were used for pyrolysis. Hereafter, the following representations are used to describe the catalyst samples: ZSM-5_0 and ZY_0 indicate fresh catalysts before pyrolysis, ZSM-5_10 and ZY_10 represent the catalysts used 10 times for pyrolysis of the RM samples, and ZSM-5_reg and ZY_reg are the catalysts after regeneration at 500\u2009\u00b0C for 5\u2009h. \nFig. 4 shows the SEM images of the catalysts, where significant differences are observed in the shapes and structures of ZSM-5 and ZY. Particles of the ZSM-5 catalyst are larger than the ZY particles, and are crystals with round shapes and different sizes, which are conglomerated. No noticeable changes in the main crystal structure and morphology are observed after the use and regeneration of ZSM-5, but the particles appear to be less dense. The ZY catalyst has smaller particles with a regular cubic shape compared to ZSM-5. The particles have rough edges, and slight agglomeration of particles is observed. The regeneration of ZY does not affect the morphology and structure of this zeolite.FTIR was used to examine the structures of the ZSM-5 and ZY samples. The structure of the zeolite can be considered as a set of interconnected TO4 (T\u2009=\u2009silica or alumina) units of SiO4 and AlO4 tetrahedra. \nFig. 5 shows the FTIR spectra of the catalysts, where the peaks are observed only in the mid-infrared region (1250\u2013400\u2009cm\u20131). The adsorption bands are observed at 1224, 1070, 797, 546, and 437\u2009cm\u22121 for the ZSM-5 catalyst, and 1205, 1053, 832, 607, 527, and 455\u2009cm\u22121 for the ZY catalyst. The adsorption bands at 1224\u2009cm\u22121 (external asymmetric stretching), 1070 and 1053\u2009cm\u22121 (internal asymmetric stretching), 797\u2009cm\u22121 (external asymmetric stretching), and 455\u2009cm\u22121 (T\u2013O band) correspond to highly siliceous materials [52]. The representative signals, but with lower intensities at 546 and 547\u2009cm\u22121, are attributed to the bending vibrations of the five-membered rings of O\u2013T\u2013O in the zeolite topological structure. Wavenumber ranges are provided in accordance with the literature reports [53].The fresh (0), used (10), and regenerated (reg) catalysts were characterized using the nitrogen physisorption method to determine the effects of deactivation and regeneration on the physicochemical properties of the catalyst (\nTable 5). BET analysis was performed to determine the specific surface area (SBET). A comparison of ZSM-5 and ZY showed that the SBET value of ZY was twice that of ZSM-5. The 10-fold usage of ZY during pyrolysis resulted in a significant decrease in the surface area (up to 34%), whereas the change was small (only 7%) for ZSM-5. After heating at 500\u2009\u00b0C, ZY was regenerated, attaining its original or higher surface area. For ZY_reg, the surface of the micropores increased, which caused an increase in the BET surface. Regeneration can change the physical structure of the catalyst [54]. As it was reported by Brito et al. [55] an increase in BET surface area can be observed during the first catalyst regeneration, and subsequent regeneration can lead to SBET drop. The increase in BET surface area is attributed to the dehydrogenation of the coke resulting in the formation of additional mesoporosity at the regeneration temperature [24]. After regeneration of ZSM-5, the surface area value did not return to that of the fresh catalyst, indicating that a temperature of 500\u2009\u00b0C was very low to clean the catalyst; this observation was consistent with that described in a prior literature report [24]. The total pore volume (Vtot) for ZSM-5 was relatively lower and decreased after usage and regeneration, whereas the volume was 4-fold higher for ZY and did not change after pyrolysis. In terms of the porosities of these zeolites, the diameter of micropores (<2\u2009nm) and mesopores (2\u2013100\u2009nm) were almost equal in ZY, and thus the surface area of the micropores was approximately two times the area of the mesopores. In ZSM, both the volume and surface area of the mesopores were higher than those of the micropores.To determine the rate of coke deposition on the catalysts, ultimate analysis (carbon, hydrogen and nitrogen) was performed for the ZSM-5 and ZY samples. As shown in \nFig. 6a, the carbon content in ZSM-5 increases from 2% to 4.16% (two-fold) after 10 uses, while it increases from 1.21% to 20.63% (17-fold) in ZY (Fig. 6b). Carbon deposited faster on ZY than on ZSM-5, and the carbon content in ZY_4 after fourth use was the same as that in ZSM-5_10 after 10 uses. The coke deposited on the analyzed catalysts is classified as hard Cn, because the increase of hydrogen content was negligible compared to carbon. As reported in the literature [56], removal of hard coke is actually more difficult than light coke (-CnH2n). Moreover, elemental analysis revealed that the ZY_10 catalyst contained more hydrogen and nitrogen than ZSM-5_10. Carbon deposition reduces the porosity of the catalyst and hinders the pyrolysis active site reaction [15]. As reported by Heracleous et al. [22], coke deposition depends on the duration of pyrolysis. The ZSM-5 catalyst (SAR = 45) contained 4.8% carbon and 6.9% carbon after 20 and 40\u2009min, respectively.TGA data of the investigated catalysts are presented in \nFig. 7a-f. The mass losses of the catalysts upon heating to 900\u2009\u00b0C under nitrogen and air atmospheres are shown in Figs. 7a-b and 7c-d, respectively. Under nitrogen atmosphere, the mass losses for ZSM-5_0 and ZY_0 are 4.2% and 12.8%, respectively. In contrast, the catalyst samples after pyrolysis under nitrogen atmosphere are characterized by lower mass losses of 3.7% and 6.7% for ZSM-5_10 and ZY_10, respectively. As shown in Fig. 7c-d, ZSM-5_10 and ZY_10 display two distinct mass loss regions. The first mass loss is observed at 25\u2013300\u2009\u00b0C, which is attributed to the desorption of water and volatile species (i.e., reactants, products, and reaction intermediates) adsorbed on the catalyst surface [57]. The second mass loss is observed at 300\u2013900\u2009\u00b0C, which corresponds to the decomposition of coke species deposited on the catalyst surface. The mass losses for the ZSM-5_10 and ZY_10 catalysts in air atmosphere at 300\u2013900\u2009\u00b0C show good agreement with the results of the coke content analysis shown in Fig. 6a-b. Catalyst regeneration was performed in a muffle furnace in air. The mass loss trends for the ZSM-5_reg and ZY_reg catalysts are shown in Figs. 7e and 7f, respectively. Analyzing the results of the thermogravimetric investigations, the increase in the specific surface area is due to the large weight loss (12%) of the fresh catalyst when heated to a pyrolysis temperature of 500\u2009\u00b0C. The loss in weight of the catalyst leads to an increase in SBET. However, as mentioned earlier, it was done only after the first regeneration. It was proved that it is more difficult to regenerate the catalyst used for intermediate pyrolysis than for fast pyrolysis [58].Elemental analysis of the regenerated catalysts was performed, which afforded the following results: 1.36% C, 1.15% H, and 0.33\u2009N in the ZSM-5_reg catalyst, and 1.09% C, 0.54% H, and 0.26% N in ZY_reg. However, more carbon and hydrogen remained in ZSM-5_reg than in ZY_reg, but less coke was deposited on ZSM-5_10. This result indicated that the complete removal of coke from the catalyst during regeneration was very difficult. As a result of biomass pyrolysis, other elements were also deposited on the catalysts [16]. \nTable 6 summarizes the elements detected in fresh, used, and regenerated catalysts. Increases in most of the trace elements (except Na, Zr, and Mg) in ZY_10 are observed in comparison to the contents in the ZY_0 sample. Interestingly, the most abundant elements in RM ash, such as phosphorus and potassium, were not deposited on the ZSM-5_10 and ZY_10 catalysts. The regeneration resulted in the removal of coke, but also a significant loss in sulphur deposited on the catalysts. The sodium content of the catalysts increased after regeneration.After the pyrolysis of each 1.5\u2009g sample of RM, the boat retained 0.446\u2009g of char (29.7% by weight). The type of catalyst used did not affect the char composition. Elemental analysis data of char are listed in \nTable 7. As expected, char has a significantly higher content of carbon and lower percentages of hydrogen and oxygen compared to those of the feedstock. The second most abundant element in the char is potassium. Potassium is a desirable element for pyrolysis because it increases char production [59]. Moreover, potassium has a better impact on the pyrolysis product yields than phosphorus [60].\n\nFig. 8 shows the morphologies of raw biomass and char obtained after pyrolysis using ZSM-5 (as an example). The data for the char obtained using ZY is not presented because the catalyst types does not significantly affect the char properties and morphology. As shown in Fig. 8, raw post-extraction RM mainly has a fibrous texture with a few spherical cavities, and the surface is coherent. Analysis of char shows the effect of pyrolysis on the material structure. The char has many openings and a tubular-shaped structure, which can be compared to a honeycomb structure, where the diameters of the honeycomb pores are <10\u2009\u00b5m. The obtained honeycomb structure of char can play an important role in oxygen reduction during catalytic processes [61]. Notably, some char particles do not have open pores, suggesting that a pyrolysis temperature of 500\u2009\u00b0C is not sufficiently high to remove all volatile matter.One of the most important characteristics of char is its functional groups, corresponding to the wavenumber range of 1700\u2013400\u2009cm\u22121, as shown in \nFig. 9. The FTIR spectra of the chars obtained by pyrolysis using ZSM-5 and ZY were similar because the catalyst was only in contact with the released vapors of the heated RM samples. Two functional groups in the broadest wavenumber ranges, corresponding to the aromatics (1700\u20131500\u2009cm\u22121) and alkanes (1200\u2013980\u2009cm\u22121), were notable.The aqueous phase is produced because of the presence of residual moisture within the RM and during dehydration reactions in the pyrolysis process. The total weight of the liquid phase (aqueous phase + bio-oil) after pyrolysis of 10 RM samples (1.5\u2009g per sample) did not depend on the type of the catalyst used and was 8.0\u2009g +/- 0.5\u2009g. However, the weight percentage of bio-oil in the liquid phase was higher when ZSM-5 was used instead of ZY, i.e., 21% and 19%, respectively. Unfortunately, during the pyrolysis variants of eight or less samples, it was more difficult to obtain a repeatable mass of the liquid. Hereafter, the combined aqueous phase and bio-oil samples from all variants (2, 4, 6, 8 and 10 RM samples) were analyzed. As shown in \nTable 8, the aqueous phase formed using ZY has 2.51% more carbon than the aqueous phase formed using ZSM-5, which contains 2.5% more oxygen. In contrast, more carbon and less hydrogen percentages are observed in the bio-oil collected after pyrolysis using ZSM-5 instead of ZY.Aqueous phases and bio-oils were also characterized using FTIR, and the data are shown in \nFig. 10. The broad peak at 3400\u20133200\u2009cm\u22121 implies that the aqueous phase and bio-oil samples contain compounds with O-H groups, such as water, alcohols, and phenols. The sharp bands at approximately 2956, 2925, and 2853\u2009cm\u22121 are attributed to the C-Hx vibrations, indicating the presence of hydrocarbon molecules [62]. The vibrations observed between 1780\u2009cm\u22121 and 1680\u2009cm\u22121 are attributed to the presence of the CO bond, which indicates the presence of aldehydes, ketones, and carboxylic acids. A few low-intensity peaks observed at 1200\u2013970\u2009cm\u22121 correspond to the presence of alcohols and phenols.To better understand the effects of the two different catalysts on the RM intermediate pyrolysis, GC-MS analysis of the bio-oils was performed. \nTable 9 shows the detected and identified compounds in the bio-oil and their peak area percentages. The bio-oil contains a complex mixture of low- to intermediate-molecular-weight hydrocarbon chains with C4 to C24 units, which is consistent with the results reported by Mahmood et al. [42]. The bio-oils obtained by pyrolysis were categorized into alkanes, alkenes, alkynes, alcohols, acids, ketones, single-ring and polycyclic aromatic hydrocarbons, phenols, N-containing aromatic compounds, nitriles, amines, and other oxygenated compounds. Aromatic compounds, ketones, acids, and oxygenates were the dominant components in the bio-oils obtained by catalytic pyrolysis. Phenols, cyclopentanes, and furans are classified as flammable organics [43]. The SAR value of the catalyst (26 for ZY_0 and 352 for ZSM-5_0) in intermediate pyrolysis slightly affected the contents of carboxylic acids, ketones, polycyclic aromatic hydrocarbons, phenols, and N-containing aromatic compounds in the bio-oils. Interestingly, the total content of the oxygenates in the bio-oil (alcohols + ketones + phenols + other oxygenates) was lower with the use of ZSM-5 (26.51%) than that of ZY (29.57%). This result is in contrast to that of fast pyrolysis [27]. This may be because of different vapor residence times in the reactor for the aforementioned types of pyrolysis. In the present study on intermediate pyrolysis, the residence time was 20\u2009s, while in fast pyrolysis this time was approximately 1.5\u2009s [8].The concentrations of the detected pyro-gas components, converted to nitrogen-free composition, are shown in \nFig. 11a-j. The main component of pyro-gas was CO2, whose concentration increased with the reuse of catalysts. The highest concentrations of carbon monoxide from the catalytic pyrolysis process were obtained with the first use of catalysts (both ZSM-5 and Zeolite Y). After 4-, 6-, 8-, and 10-fold uses of the catalysts (ZSM-5 or Zeolite Y) a reduction in the concentration of carbon monoxide in the pyro-gas was observed, which may be attributed to the weakening of the decarbonylation and decarboxylation reactions [41]. The hydrocarbons consisted of alkanes, alkenes, and alkynes. The pyro-gas obtained using the ZSM-5_0 and ZY_0 catalysts was characterized by the lowest carbon dioxide and highest hydrogen concentrations. The use of ZY_2 afforded higher concentrations of hydrogen, ethene, and ethyne compared to those obtained using ZSM-5_0. In particular, for the ZY catalyst, increases in the percentages of hydrogen and ethyne in the pyro-gas were accompanied by a low methane content. This phenomenon may be represented by reaction (2).\n\n(2)\n\n\n2\nC\n\n\nH\n\n\n4\n\n\n\u2192\n3\n\n\nH\n\n\n2\n\n\n+\n\n\n\nC\n\n\n2\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\nThe concentration of methane in obtained pyro-gas in the presence of ZSM-5 catalyst was lower and constant during the reused of the catalyst. This means that the catalyst promotes the conversion of methane precursors to aromatic hydrocarbons [41,63]. In the case of Zeolite Y, the concentration of methane increased significantly from its eighth use. The concentrations of methane, acetylene, propane, and butane were higher for each increase in the reusage cycle of the ZY catalyst compared to those observed with ZSM-5.For the catalytic intermediate pyrolysis of rapeseed meal, mass and carbon balances of the products were also calculated and shown in \nFig. 12. Due to the inability to analyze samples of oils and aqueous phases after 2- and 4- fold usage of catalysts, the mass and carbon balances were based on average values from pyrolysis of 10 samples. The catalyst samples were not weighed after pyrolysis and they were not included. When analyzing the mass balance (Fig. 12a-b)), the dominant product is the aqueous phase: 42% for ZSM-5% and 43% for Zeolite Y. This result was confirmed by other literature studies [42,43]. The char accounted for nearly 30% of the mass of products, and it did not depend on the type of catalyst used. This conclusion was expected because the catalyst was not in direct contact with the char. Compared to the intermediate pyrolysis of wood (see Table 1), lower yield char and pyro-gas weight percentages were obtained. On the other hand, from the carbon balance shown in Fig. 12 c-d\u00a0resulting that char contains 41.4% carbon. A similar percentage of carbon from this balance was reported by Funke et al. [33] for four types of feedstocks. The second pyrolysis product in terms of carbon content was pyro-gas. Interestingly, the carbon percentage in pyro-gas formed in the presence of the ZSM-5 catalyst was 5% higher than in the case of Zeolite Y. It is worth noting that the Zeolite Y catalyst was responsible for 5% by mass of carbon in the balance.Catalytic intermediate pyrolysis of RM with 2-, 4-, 6-, 8-, and 10-fold usage of the catalyst (ZSM-5 or Zeolite Y) was performed. The repeated use of less acidic ZSM-5 catalyst (SAR = 352) was better than that of ZY (SAR\u2009=\u200926) for bio-oil production with a low oxygen content. The type of catalyst did not significantly affect the yield and char composition because the catalyst was used downstream to the pyrolysed sample. Comparable masses of the liquid phases were obtained for both ZSM-5_10 and ZY_10 catalysts. The main component of the liquid phase was the aqueous phase (approximately 81% for ZY and 79% for ZSM-5). The carbon and hydrogen contents in the bio-oils obtained using both catalysts were similar. GC-MS analysis allowed the identification of the main components in bio-oils, which included carboxylic acids, ketones, phenols, and compounds containing nitrogen and oxygen. In the bio-oils obtained using ZSM-5, a lower total oxygenate content (alcohols + ketones + phenols + other oxygenates) was observed compared to the bio-oils obtained using ZY. The elemental analysis data confirmed the higher oxygen content (greater by 0.7%) of the bio-oil obtained using the ZY catalyst compared to that obtained using the ZSM-5 catalyst.Notably, repeated use of the same catalyst significantly affected the composition of the produced pyro-gas, particularly methane and hydrogen contents. With an increase in the catalyst usage, the contents of hydrogen and carbon monoxide decreased, while carbon dioxide concentration in the pyro-gas increased. The 10-fold usage of the catalyst resulted in significantly higher carbon deposition on ZY than on ZSM-5. In contrast, after the regeneration of the catalysts (500\u2009\u00b0C and 5\u2009h), more coke remained on ZSM-5 than on ZY.\nWojciech Jerzak: Conceptualization, Methodology, Writing \u2013 original draft, Data Curation, Investigation. Aneta Magdziarz: Project administration, Funding acquisition, Writing\u00a0\u2013 review & editing, Investigation. Ningbo Gao: Writing\u00a0\u2013\u00a0review & editing, Investigation. Izabela Kalemba-Rec: Writing\u00a0\u2013\u00a0review & editing, Investigation.The authors declare no known competing financial interests or personal relationships that can influence the work reported in this paper.The research project was supported by the program \"Excellence initiative \u2013 research university\" of the AGH University of Science and Technology, Poland (Grant AGH No. 501.696.7996), the Key Program for China EU International Cooperation in Science and Technology Innovation, China (Grant No. 2018YFE0117300), and Shaanxi Provincial Natural Science Foundation Research Program-Shaanxi Coal Joint Funding, China (2019JLZ-12).", "descript": "\n                  This study describes an experimental investigation comparing the effects of two catalysts (ZSM-5 and Zeolite Y) on intermediate pyrolysis. The catalysts are characterized by different acidities, expressed by silica-to-alumina ratios of 352 (ZSM-5) and 26 (Zeolite Y). The post-extraction rapeseed meal was pyrolysed in a fixed-bed furnace at a temperature of 500\u00a0\u00b0C, and the pyrolysis products (char, liquid phase, and pyro-gas) were characterized in detail. The catalysts were assessed based on their reusage capability. Five-fold more carbon was deposited on Zeolite Y than on ZSM-5 after 10-fold use during the pyrolysis of rapeseed meal. Moreover, the ultimate analysis of the catalysts showed increases in the hydrogen and nitrogen contents, which were significantly higher for Zeolite Y than those for ZSM-5. The catalysts showed different effects on the properties of the products. Better-quality pyro-gas was obtained with Zeolite Y, but reusage of this catalyst resulted in decreases in the hydrogen and carbon oxide concentrations. Compared to Zeolite Y, ZSM-5 afforded bio-oil with a lower oxygen content. Phenols and ketones were dominant compounds in both bio-oils. Regeneration of Zeolite Y caused to increase of specific surface area.\n               "}
{"full_text": "No data was used for the research described in the article.Dibenzyl phosphateDimethylglyoximePolyethylene glycolNickel-metal hydride batteryNiMHB anodeIonic liquidRare earth elementLight rare earth elementHeavy rare earth elementRare earth element oxideLanthanidesLayered double hydroxideAqueous biphasic systemExtraction efficiencyMetal\u2212organic frameworkNanoporous grapheneA pluronic triblock copolymerPr and NdSodium rare earth double sulfate (NaRE(SO4)2)Polyethylene oxide polymer with an average molar mass of 1500\u00a0g\u00a0mol\u22121\nTributyl phosphateTri-n-octyl phosphine oxideDi-(2-ethylhexyl) phosphoric acid1-methylimidazolium hydrogen sulfateTrialkylphosphine oxideBis (2,4,4-trimethylpentyl) phosphinic acidBis(2,4,4-trimethylpentyl) dithiophosphinic acidBis [2, 4, 4-trimethyl pentyl] mono thiophosphinic acidTrioctyldecylamine chloride2-ethylhexylphosphonic acid mono-2-ethylhexyl esterTricaprylmethylammonium chloride (dihexyl diglycolamate)\nTrihexyl(tetradecyl)phosphonium chloride (nitrate)\nTrihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl) phosphinate1-ethyl-3-methylimidazolium chloride[Trihexyl(tetradecyl) phosphonium]2[2,2' -(1,2-phenylenebis(oxy))dioctanoate]Trihexyl(tetradecyl)phosphonium thiocyanate (bis(trifluoromethanesulfonyl)imide)\nTetrabutylammonium nitrateEthylenediaminetetraacetic acidDiethylenetriaminepentaacetic acidEthylenedinitrilo tetra acetic acid disodium salt dihydrateSec-octyl phenoxy acetic acidBenzyltributylammonium myristic acetateBenzyltributylammonium dodecanedioic acetate2-thenoyltrifluoroacetone1-butyl-3- methylimidazolium bis(trifluoromethanesulfonyl)imide (bis(perfluoroethanesulfonyl)imide) (di(2-ethylhexyl)-oxamate)\nTetraoctylammonium dioctyl-diglycolamate (di(2-ethylhexyl)-oxamate) (oleate)\nMethyltrioctylammonium naphthenic acid (Peanut oil) (Rapeseed oil) (Sunflower seed oil) (Flaxseed oil)\nTrioctyl(2-ethoxy-2-oxoethyl)ammonium dihexyl diglycolamateN,N,N,N-tetrabutyl-3-oxapentane-diamideN,N,N,N-tetraoctyl-3-oxapentane-diamide1,4-diisopropylbenzene1-methyl-1-butylpyrrolidinium bis(trifluoromethanesulfonyl)imideTributyl-methyl ammonium nitrate (bis(trifluoromethylsulfonyl)imide)\n(2,4-dimethylheptyl) (2,4,4\u2032-trimethylpentyl) phosphinicRare earth elements (REEs) are a group of metals comprising 15 lanthanides, as well as yttrium and scandium [1]. These elements are naturally present in various minerals, including silicates, carbonates, and phosphates. Due to their similar ionic radii, REEs exhibit interchangeable properties within a given mineralogy, resulting in difficulties in their separation during mineral processing [2]. The increase of the atomic number in REEs adds electrons to their inner incomplete orbital (4f) rather than the outer orbital, resulting a decrease in the ionic radius of the trivalent lanthanides from La3+ to Lu3+, a phenomenon known as \u201clanthanide contraction\u201d [3]. As a result of lanthanide contraction, it is possible to separate different rare earth elements individually [4].REEs are inputs to a range of critical applications from the electronics and energy industry to defense and military technologies, accounting for more than 10% of the US$75 trillion worth of the global economy [4,5]. Due to the strategic role of these elements, REE-bearing minerals were categorized as critical minerals in 2018 in the United States [6]. Fig. 1\n illustrates the utilization of REEs in various applications in the United States (2018), the European Union (2019), and globally (2019). The major industrial applications of individual REEs are also featured in Table 1\n. REEs are primarily utilized in the catalysts, glass, and ceramics industries in the United States and the European Union. However, affected by China's REEs industry, the production of magnets is the main end-use application of these elements globally.Although the term \u201crare\u201d is used for rare earth elements, they are not particularly rare in terms of average crustal abundance [4,9]. The scarcity of REEs is relevant to their low concentration in most of their deposits [4]. Deposits containing a sufficient concentration of REEs to support mining operations are not frequently found [4,9]. This can make the mining and extraction of REEs more challenging and costly. Carbonate igneous rocks, known as carbonatites, are the most significant REEs deposits in the world [4]. Alkaline igneous rocks, placer deposits with monazite-xenotime mineralization, and ion-adsorption clay deposits are other principal economic REEs sources [9]. The major secondary resources of REEs are permanent magnets, batteries, fluorescent lamps, catalysts, phosphogypsum, bauxite residue (red mud), mine tailings, metallurgical slags, and fly ash [5]. According to the United States Geological Survey (USGS) 2021 report, world economic reserves of REEs in 2020 totaled 120,000\u00a0kt REEs oxides (except for yttrium), and more than 500\u00a0kt for Y2O3. Based on the report, as shown in Fig. 2\n, the world mine production of REEs was estimated to be 240\u00a0kt of REEs oxides (REOs) in 2020. China is the leading country in both reserves and mine production, with 44,000\u00a0kt and 140\u00a0kt, respectively [10].REEs as strategic high-tech metals currently play a critical role in the global technological advancement. As the world is moving towards a cleaner and greener future, meeting the growing demand for REEs is becoming increasingly challenging due to the limited availability of economically viable resources and the monopoly of a few countries on their production and supply. Primary and secondary resources are the two main options for securing demand for REEs. Factors such as the relative distribution of light (LREEs) and heavy (HREEs) REEs within a deposit, the complexity of processing, and the environmental impacts associated with REEs mining and processing greatly affect the economic viability of REEs prospecting. The co-occurrence of several LREEs (e.g., Nd, La, and Ce) in a single deposit is often accompanied by simultaneous extraction of the others, which makes the overall process more cost-effective. However, HREEs are typically found in lower concentrations within deposits, which raises concerns about potential shortages in the near future due to limited production volume [2].Electronic waste (e-waste) is considered one of the main secondary sources of REEs that can potentially secure a significant part of their demand. Fluorescent materials, battery alloys, and permanent magnets are the major sources of REEs. The total amount of e-waste generated worldwide in 2019 was estimated to be 53.6\u00a0Mt, and it is projected to reach 74.7\u00a0Mt by 2030 [11]. Only 17.4% of the generated e-waste in 2019 was recovered, which means about $57 billion worth of recoverable materials were mostly dumped or incinerated [11]. Currently, less than 1% of the REEs in end-of-life products are recycled globally [12], indicating a significant potential for utilizing e-waste as a source of REEs. Studies have recently focused on replacing REEs in critical technologies with alternative materials. Hitachi, Tesla, Renault, and BMW are among the leading companies actively engaged in the minimization or substitution of REEs in the compartments of their products [2].A variety of physical, chemical, and thermal processes have been developed to extract REEs from primary or secondary resources. Physical processes are mainly utilized to concentrate REEs from gangue materials in raw ore (e.g., through froth flotation), and to liberate REEs-containing materials from other components in secondary resources. Despite being simple and environmentally friendly, they often require further chemical or thermal treatment to remove trace impurities. Pyrometallurgical recovery of REEs, which involves the separation of REEs in oxide form (slag) via a high-temperature process, is mainly used on a large scale for commercial operations. The resulting REEs slag is then subjected to hydrometallurgical processing to remove various impurities. Alternatively, hydrometallurgy can be used as the sole recovery technique for the recovery and refining of REEs from different sources. The process usually begins with leaching the raw material using acidic or alkaline solutions. Depending on the composition and chemical structure of the raw material, a roasting step may be incorporated into the process in order to remove volatile components and enhance the water-solubility of the solid phase. The resulting aqueous solution (pregnant leach solution, PLS) is then used in various hydrometallurgical processes, such as precipitation, solvent extraction, or adsorption, for group or individual separation of REEs. Due to the chemical similarities of REEs, efficient separation and purification of individual REEs require the development of new technologies to reduce the cost and environmental impact associated with the process.Due to the hydrogen-absorbing properties of nickel-lanthanide alloys, REEs have been utilized in energy storage since the early \u201990s, leading to their extensive use in nickel-metal hydride batteries (NiMHBs). NiMHBs were commercialized in 1991 and have since found applications in electric vehicles and rechargeable products [5]. The components of NiMHBs include an anode composed of hydrogen-absorbing alloys (MH), a cathode made of nickel hydroxide, a separator between the electrodes, and a potassium hydroxide electrolyte. The anode is an AB5-type alloy (A is a mixture of La, Ce, Nd, and Pr; and B is Ni, Co, Mn, and Al) containing about 33\u00a0wt% REEs, mainly La with about 21\u00a0wt%, and Ce, Nd, and Pr with 6.5\u00a0wt%, 3\u00a0wt%, and 2\u00a0wt%, respectively. Fig. 3\n depicts a schematic representation of a NiMHB cell, along with the average chemical composition of its anode and cathode.This review paper aims to investigate various methods proposed for recovering REEs from spent Ni-MH batteries and study the available techniques for individual separation of the elements. The current state-of-the-science and an in-depth understanding of hydrometallurgical, pyrometallurgical, and combined methods for recovering REEs from the spent batteries will be introduced and discussed. To complete the recovery cycle of REEs from NiMHBs, the current and emerging techniques for individual separation of these elements are investigated, and their challenges and potential future directions are highlighted. The study is a comprehensive examination of the available techniques, which combines both narrative and systematic reviews to assess the potential of each method for sustainable recycling of NiMHBs.The preliminary processing of NiMHBs usually includes discharging (to avoid short circuits), opening of casings, liberation of seals and separators, shredding, and separation of different fractions (fluff, metals, and black mass). The black mass fraction, which mainly contains anode and cathode, is processed for REEs recovery. Fig. 4\n is a process flow diagram illustrating the various stages involved in the recovery and individual separation of REEs from NiMHBs. The recovery of REEs from spent NiMHBs can be achieved through either hydrometallurgical or pyrometallurgical processes. However, the purification and individual separation of the elements is typically performed via hydrometallurgical methods. The methods are further explained in the following sections.Electroslag refining [13,14], liquid metal extraction [13,15\u201319], glass slag process [20\u201326], direct melting [27\u201330], and gas-phase extraction [28,31\u201337] are the most common pyrometallurgical techniques developed for the recovery of REEs from different sources [14]. Among the mentioned processes, molten slag extraction is the most favorable technique for the recovery of REEs from spent NiMHBs, due to the efficient recovery of both REEs and Ni. The method is suitable for group separation of REEs oxides in the form of a slag phase, which requires further hydrometallurgical processing for purification and individual separation of the elements. During the smelting process, REEs present in the waste batteries are oxidized and isolated in the slag phase, while Ni, Co, and Fe are transferred to the metallic phase. The properties of the slag, such as viscosity, melting point, and vapor pressure, are directly related to its composition. Effective separation of the REEs in the slag phase can be achieved through the adjustment of the slag composition by the addition of fluxing agents. Fluxing agents are substances introduced to the smelting system to improve the fluidity and to effectively isolate unwanted impurities in the form of a slag [38]. Limestone, silica, dolomite, lime, borax, and fluorite are the most common fluxes used in this technique [38]. Both the metal and slag phases are valuable products in the molten slag extraction process. However, in the context of recovering REEs from batteries, selecting the appropriate slag system is a crucial step in the process. The slag system selected must meet the criteria outlined in Table 2\n.As previously stated, the batteries undergo pre-processing, and the separated black mass is utilized as the raw material for the molten slag extraction process. The black mass is introduced into the smelting furnace in conjunction with fluxing agents and reductant materials, either directly or after a pre-oxidation step. As a result, smelting can be divided into two types of direct smelting and oxidized smelting.During direct smelting of NiMHB, metallic REEs act as reducing agents due to their high affinity with oxygen. Since the Gibbs free energy of the REEs oxide is lower than the other oxides in the system, the following reactions are expected to take place during the smelting process [39]:\n\n(1)\n\n\n3\nS\ni\n\nO\n2\n\n\n\n(\ns\n)\n\n+\n4\nR\nE\n\n\n(\nl\n)\n\n\u2192\n3\nS\ni\n\n\n(\nl\n)\n\n+\n2\nR\n\nE\n2\n\n\nO\n3\n\n\n\n(\n\ns\nl\na\ng\n\n)\n\n\n\n\n\n\n\n(2)\n\n\n3\nM\ng\nO\n\n\n(\ns\n)\n\n+\n2\nR\nE\n\n\n(\nl\n)\n\n\u2192\n3\nM\ng\n\n\n(\ng\n)\n\n+\nR\n\nE\n2\n\n\nO\n3\n\n\n\n(\n\ns\nl\na\ng\n\n)\n\n\n\n\n\n\n\n(3)\n\n\nC\na\nO\n\n\n(\ns\n)\n\n+\nR\nE\n\n\n(\nl\n)\n\n\u2192\nC\na\n\n\n(\ng\n)\n\n+\nR\n\nE\n2\n\n\nO\n3\n\n\n\n(\n\ns\nl\na\ng\n\n)\n\n\n\n(\n\n\n\nT\n>\n1700\n\no\n\nC\n\n)\n\n\n\n\n\n\n\n(4)\n\n\n3\nN\ni\nO\n\n\n\n(\ns\n)\n\n\nC\na\nt\nh\no\nd\ne\n\n\n+\n2\nR\nE\n\n\n(\nl\n)\n\n\u2192\n3\nN\ni\n+\nR\n\nE\n2\n\n\nO\n3\n\n\n\n\n\nBy reduction of SiO2 according to (Eq. (1)), Si either dissolves in the Ni\u2013Co alloy or forms intermetallic compounds. MgO is reduced to metallic Mg (Eq. (2)); however, it will eventually evaporate at \u223c1090\u00a0\u00b0C, implying that magnesia is not a suitable refractory for this process. The same type of reaction is valid for Al2O3; however, metallic Al may be re-oxidized due to its high reactivity with oxygen, or it may form intermetallic compounds. M\u00fcller and Friedrich [39] investigated the effectiveness of CaO\u2013SiO2 and CaO\u2013CaF2 fluxing systems to recover REEs from NiMHBs. According to the study, the CaO\u2013SiO2 system shows poor metal-slag separation due to high slag viscosity, leading to Si enrichment in the metal phase. In contrast, the CaO\u2013CaF2 system exhibits excellent metal-slag separation, highly efficient REEs separation, and a near-complete transformation of Ni and Co to the metal phase. Maroufi and colleagues [40,41] suggested an alternative approach in which REEs are oxidized via iron oxide (hematite) during smelting. The process involves heating a 1:1 mixture of MHA-hematite in a graphite crucible, where the CO2 released from the reaction of hematite with graphite oxidizes the REEs, resulting in the formation of a REEs-rich slag phase.By heating the battery's black mass in air atmosphere, nickel hydroxide is first decomposed into NiO and water at a temperature in the range of 250\u2013300\u00a0\u00b0C [42], and by increasing the temperatures, all the constituent elements eventually transform into their oxide forms. Maroufi and colleagues employed pure iron as an alloying solvent and reducing agent to separate Ni and Co from REEs in MHA. A metallic alloy and a REEs-rich slag were produced by heating the mixture of oxidized MHA and pure iron at 1550\u00a0\u00b0C in argon atmosphere. The presence of carbon in this process is essential as it contributes to the metallization of Ni, Co, and Fe and improves phase separation by reduction of the metallic phase viscosity [40,43].An alternative method for selective reduction of oxidized Ni, Co, and Fe is to introduce a hydrogen gas atmosphere, eliminating the need for adding reducing agents to the charge. Deng and colleagues studied the recovery of REEs from NiMHBs employing the thermal isolation method. A pre-oxidized mixture of anode-cathode was reduced by hydrogen at 800\u00a0\u00b0C. The product was mixed with Al2O3 and SiO2 as fluxing agents, and the mixture was smelted at 1550\u00a0\u00b0C in argon atmosphere, resulting in a slag phase rich in REOs, Al oxide, and Mn oxide, and a Ni-based alloy. The slag was water-quenched and then crushed to facilitate the acid leaching process [44]. The SiO2\u2013CaO-based slag is another system that can efficiently absorb REEs oxides from pre-oxidized NiMHB. Despite Tang and colleagues reporting high efficiencies for REEs recovery and metal-slag separation through this system, their results demonstrated a significant amount of metallic Ni and Co remaining trapped in the slag phase [45].The Ellingham diagram in Fig. 5\n illustrates the temperature-dependent stability of the various oxides present in the NiMHB smelting process. Since REEs\u2019 lines are positioned below other elements (except for Ca), REOs are more stable than other oxides, which means that metallic REEs can easily reduce the oxides of the elements above their lines. In the diagram, La represents other LREEs as their lines have similar positions. The reduction of NiO and CoO by REEs is thermodynamically favorable due to the significant difference in Gibbs free energy between REOs and Ni\u2013Co-oxides. However, the limited amount of metallic REEs in the anode is not sufficient for complete metallization of NiO and CoO. Therefore, the use of external reducing agents such as C or Fe is necessary in both direct and oxidized battery smelting methods to achieve full metallization.To understand the reaction mechanism of direct and oxidized battery smelting, MHA is considered as the raw material, in which LaNi5, NdCo5, and CeCo5 are the dominant phases. The products of both processes are a slag phase rich in REEs and a Ni-based alloy.The battery (and additives, e.g., fluxing agents) is first heated in the smelting furnace (T\u00a0~\u00a01600\u00a0\u00b0C [39]) under reducing conditions, where the anode phases are decomposed based on Eqs. (5)\u2013(7).\n\n(5)\n\n\nL\na\nN\n\ni\n5\n\n\n\n(\ns\n)\n\n\n\u2192\n\u0394\n\nL\na\n\n\n(\ns\n)\n\n+\n5\nN\ni\n\n\n(\ns\n)\n\n\n\n\n\n\n\n(6)\n\n\nC\ne\nC\n\no\n5\n\n\n\n(\ns\n)\n\n\n\u2192\n\u0394\n\nC\ne\n\n\n(\ns\n)\n\n+\n5\nC\no\n\n\n(\ns\n)\n\n\n\n\n\n\n\n(7)\n\n\nN\nd\nC\n\no\n5\n\n\n\n(\ns\n)\n\n\n\u2192\n\u0394\n\nN\nd\n\n\n(\ns\n)\n\n+\n5\nC\no\n\n\n(\ns\n)\n\n\n\n\n\nNi and Co form an alloy bath at the bottom of the furnace. Owing to the high affinity of REEs with oxygen, they may either reduce other oxides present in the system or be oxidized by the purged oxygen (Eq. (8)). The difference in the Gibbs free energy of the oxides is the highest between LREEs and Ni and Co, which means thermodynamically, reduction of their oxides by LREEs is favorable. Due to their low density, oxidized REEs form a slag layer placed on top of the alloy bath.\n\n(8)\n\n\nR\nE\n\n\n(\nl\n)\n\n+\n\nO\n2\n\n\n\n(\ng\n)\n\n\u2192\nR\n\nE\n2\n\n\nO\n3\n\n\n\n(\n\ns\nl\na\ng\n\n)\n\n\n\n\n\nThe oxidized battery smelting process involves a more complex reaction mechanism, requiring a pre-oxidation step that leads to the formation of various intermetallic compounds. This technique involves the oxidation of Ni and Co prior to smelting, which is achieved through an oxidation roasting step at around 1000\u00a0\u00b0C, as shown in Eqs. (9)\u2013(13) [40,46].\n\n(9)\n\n\n2\nL\na\nN\n\ni\n5\n\n\n\n(\ns\n)\n\n+\n7\n\nO\n2\n\n\n\n(\ng\n)\n\n\u2192\n2\nL\na\nN\ni\n\nO\n3\n\n\n\n(\ns\n)\n\n+\n8\nN\ni\nO\n\n\n(\ns\n)\n\n\n\n\n\n\n\n(10)\n\n\n2\nL\na\nN\ni\n\nO\n3\n\n\n\n(\ns\n)\n\n\u2192\nL\n\na\n2\n\nN\ni\n\nO\n4\n\n\n\n(\ns\n)\n\n+\nN\ni\nO\n\n\n(\ns\n)\n\n+\n0.5\n\nO\n2\n\n\n\n\n\n\n\n(11)\n\n\nL\n\na\n2\n\nN\ni\n\nO\n4\n\n\n\n(\ns\n)\n\n\u2192\nL\n\na\n2\n\n\nO\n3\n\n\n\n(\ns\n)\n\n+\nN\ni\nO\n\n\n(\ns\n)\n\n\n\n\n\n\n\n(12)\n\n\nN\ni\n\n\n(\ns\n)\n\n+\n0.5\n\nO\n2\n\n\n\n(\ng\n)\n\n\u2192\nN\ni\nO\n\n\n(\ns\n)\n\n\n\n\n\n\n\n(13)\n\n\nC\no\n\n\n(\ns\n)\n\n+\n0.5\n\nO\n2\n\n\n\n(\ng\n)\n\n\u2192\nC\no\nO\n\n\n(\ns\n)\n\n\n\n\n\nDuring the oxidation stage, REEs present in the raw material, or those that may have formed as per Eqs. (5)\u2013(7), also undergo oxidation and generate intermetallic oxides, as shown in Eqs. (14) and (15).\n\n(14)\n\n\nN\nd\n\n\n(\ns\n)\n\n+\n4\nC\ne\n\n\n(\ns\n)\n\n+\n4.75\n\nO\n2\n\n\n\n(\ng\n)\n\n\u2192\n5\nN\n\nd\n0.2\n\nC\n\ne\n0.8\n\n\nO\n1.9\n\n\n\n(\ns\n)\n\n\n\n\n\n\n\n(15)\n\n\nL\na\n\n\n(\ns\n)\n\n+\n2\nN\ni\n\n\n(\ns\n)\n\n+\n2\n\nO\n2\n\n\n\n(\ng\n)\n\n\u2192\nL\na\nN\ni\n\nO\n3\n\n\n\n(\ns\n)\n\n+\nN\ni\nO\n\n\n(\ns\n)\n\n\n\n\n\nThe Ni and Co oxides are then reduced to their metallic forms. This reduction can be accomplished through a separate solid-state gas reduction process or by incorporating a reducing agent into the oxide product before it is added to the smelting furnace, as shown in Eqs. (16) and (17). As previously mentioned, carbon-based materials or pure iron can be utilized as reductants in this process. Fig. 6\n illustrates a schematic of the slag/metal interface in the smelting furnace, as well as the key reactions between MHA, the slag system, and refractory materials.\n\n(16)\n\n\nN\ni\nO\n\n(\ns\n)\n\n+\n\n[\nreductant\n]\n\n\u2192\nN\ni\n\n(\ns\n)\n\n+\n\n[\n\nreductant\n\noxide\n\n]\n\n\n\n\n\n\n\n(17)\n\n\nC\no\nO\n\n(\ns\n)\n\n+\n\n[\nreductant\n]\n\n\u2192\nC\no\n\n(\ns\n)\n\n+\n\n[\n\nreductant\n\noxide\n\n]\n\n\n\n\n\n\nTable 3\n provides an overview of the various slag systems compositions used for recovery of REEs from NiMHBs via molten slag extraction, along with their respective advantages and disadvantages.In the molten slag extraction method, the REEs isolated in the slag phase are typically in high concentrations (50\u201360\u00a0wt% REEs) [47]. In most cases, the slag undergoes hydrometallurgical processes for further purification or individual separation of the elements [48]. It could also be used as the raw material for molten salt electrolysis. As an example of industrial application of molten slag extraction, Umicore Group, a Belgian mining company, and Rhodia Group (Solvay) in France have jointly developed a process based on Umicore's ultra-high-temperature (UHT) smelting technology for recycling NiMHBs [49,50]. The spent batteries, coke, iron, and fluxes are melted via plasma heating in a shaft furnace. This results in the production of a (Ni\u2013Co\u2013Fe)-based alloy and a REEs-rich slag phase. The slag is then sent to Rhodia's plant for impurity removal and further refinement of the REEs [51].While pyrometallurgical routes have demonstrated promising potential for concentrating REEs from various sources, hydrometallurgical techniques are widely employed for REEs extraction and purification. Precipitation and solvent extraction are the most commonly used hydrometallurgical methods for recovering REEs from end-of-life NiMHBs. Recently, techniques such as the use of ionic liquids and supercritical fluid extraction have also been developed.Like most transition metals, the REEs exist in aqueous solutions as cations, with solubility generally decreasing as pH increases [52]. The REEs cations typically have a charge of +3, but under reducing conditions (and generally high-temperature), europium may have a charge of +2, and under oxidizing conditions, cerium may have a charge +4 [4]. This difference in ionic charge has been used as a basis for separating Ce and Eu from other REEs in multiple studies [53\u201356]. Common inorganic ligands that REEs tend to combine with include sulfate, carbonate, fluoride, hydroxide, and phosphate [57]. Factors such as pH, temperature, redox conditions, and thermodynamic stability of phases play a role in determining the tendency of REEs to partition between solid (either by adsorption or (co)precipitation) and aqueous phases [4].The hydrometallurgical techniques for recycling NiMHBs typically involve an initial acid leaching step, in which the crushed battery (or electrode material) is dissolved in an acidic solution. The type and concentration of the acid, solution temperature, and agitation time all play a crucial role in determining the efficiency of the leaching process. Mineral acids are widely used in the leaching process of NiMHBs. The most common leaching agents reported in the literature are H2SO4 [58\u201360], HCl [61\u201363], and HNO3 [64,65]. There are also reports on the use of organic acids, such as oxalic acid and formic acid as leaching agents [66\u201368]. However, it should be noted that during this process, the REEs tend to precipitate in the form of organic salts. Although the leaching efficiency and leaching kinetics of mineral acids are significantly higher, organic acids can be more environmentally friendly for greener production [69,70]. Table 4\n provides a summary of various leaching techniques and their respective outcomes for extracting REEs from NiMHBs electrode materials.As previously noted, REEs tend to bind with inorganic ligands such as sulfates, fluorides, and phosphates, with oxalates being the most commonly reported among organic ligands [63,68,83]. Fig. 7\n illustrates the chemical structures of anhydrous REE double salt, oxalate, and sulfate.NaRE(SO4)2, known as REE double salt, form in the presence of Na+ and SO4\n2\u2212 in the solution, typically at a pH greater than 1, as shown in Eq. (18) [74\u201376,84\u201386]. Under these conditions, the LREEs present in NiMHB leaching solution (La, Ce, Nd, Pr) eventually form mixed crystal REE double salts [74]. This advantage has made double salt precipitation the most commonly reported technique for group separation of REEs. The resulting double salt can be mixed with NaOH solution to produce rare earth mixed hydroxides (Eq. (19)) [74]. These mixed hydroxides can then decompose into REEs mixed oxides through a calcination step.\n\n(18)\n\n\nR\n\nE\n\n3\n+\n\n\n\n(\n\na\nq\n\n)\n\n+\nN\n\na\n+\n\n\n(\n\na\nq\n\n)\n\n+\n2\nS\n\nO\n4\n\n2\n\u2212\n\n\n\n(\n\na\nq\n\n)\n\n+\nx\n\nH\n2\n\nO\n\n(\nl\n)\n\n\u2192\nR\nE\n.\nN\na\n\n\n(\n\nS\n\nO\n4\n\n\n)\n\n2\n\n.\nx\n\nH\n2\n\nO\n\n(\ns\n)\n\n\n\n\n\n\n\n(19)\n\n\nN\na\nR\nE\n\n\n(\n\nS\n\nO\n4\n\n\n)\n\n2\n\n.\nx\n\nH\n2\n\nO\n\n(\ns\n)\n\n+\n3\nN\na\nO\nH\n\n(\n\na\nq\n\n)\n\n\u2192\nR\nE\n\n\n(\n\nO\nH\n\n)\n\n3\n\n\n(\ns\n)\n\n+\n2\nN\n\na\n2\n\nS\n\nO\n4\n\n\n(\n\na\nq\n\n)\n\n+\n\n(\n\nx\n+\n3\n\n)\n\n\nH\n2\n\nO\n\n(\nl\n)\n\n\n\n\n\nThe data presented in Fig. 8\n demonstrates the effectiveness of the precipitation method for extracting La, Nd, Pr, and Ce from a sulfate leachate of NiMHB black mass in the form of double salts. The results indicate a high level of efficiency, with almost complete precipitation of La, Nd, and Ce, and about 90% efficiency for Pr, within 1\u00a0h of the reaction.Oxalic acid (C2H2O4) is an effective agent for precipitation of REEs from different acidic solutions. It has also been reported to be an effective leaching agent for MHA [68]. However, it has a disadvantage of co-precipitation of Ni, Co, and Cu with REEs, which requires further purification steps [63]. Additionally, to achieve efficient separation, the amount of oxalic acid added to the system must exceed its stoichiometric amount with the REEs significantly [83]. The reaction of REEs with oxalate ion is shown in Eq. (20). The resulting product, REEs mixed oxalate, is usually subjected to calcination to convert it into mixed oxides.\n\n(20)\n\n\n2\nR\n\nE\n\n3\n+\n\n\n\n(\n\na\nq\n\n)\n\n+\n\nC\n2\n\n\nO\n4\n\n2\n\u2212\n\n\n\n(\n\na\nq\n\n)\n\n+\nx\n\nH\n2\n\nO\n\n(\nl\n)\n\n\u2192\nR\n\nE\n2\n\n\n(\n\n\nC\n2\n\n\nO\n4\n\n\n)\n\n.\nx\n\nH\n2\n\nO\n\n(\ns\n)\n\n\n\n\n\nLiquid antisolvent precipitation is a technique based on the addition of a water-miscible organic solvent to an aqueous solution, which creates a supersaturated solution by changing the solubility of the solute. The supersaturation forces the solute to precipitate in the form of a salt [88,89]. In a study, Korkmaz and colleagues [90] achieved a total recovery of up to 86% for REEs from a MHA leachate with negligible co-precipitation of other elements. REEs were precipitated as mixed hydrated sulfates when ethanol and 2-propanol were added as antisolvents, as Korkmaz reported. Although REEs recovery yield was high, a significant volume of alcohol was required to effectively precipitate REEs [90]. The literature on the precipitation of REEs from the leaching solutions of NiMHBs electrode materials is summarized in Table 5\n.The mixed REEs compounds or solutions obtained from various processes in most cases require purification or individual separation before being used in high-tech applications. While several techniques such as ion exchange, fractional crystallization, extraction chromatography, and chemical deposition have been reported in the literature, solvent extraction is one of the most commonly used methods for individual separation of REEs [5]. In addition to its high efficiency and cost-effectiveness, solvent extraction has a high potential for scaling up. Nevertheless, there are concerns regarding its environmental impacts, as it requires the use of toxic organic solvents. The basic formulas for solvent extraction are presented in Eqs. (21)\u2013(24).\n\n(21)\n\n\nD\ni\ns\nt\nr\ni\nb\nu\nt\ni\no\nn\n\nr\na\nt\ni\no\n:\n\nD\nM\n\n=\n\n\n\n[\nM\n]\n\n\nO\nr\ng\na\nn\ni\nc\n\n\n\n\n[\nM\n]\n\n\nA\nq\nu\ne\no\nu\ns\n\n\n\n\n\n\n\n\n\n(22)\n\n\nS\ne\np\na\nr\na\nt\ni\no\nn\n\nf\na\nc\nt\no\nr\n:\n\n\u03b2\n\nA\n/\nB\n\n\n=\n\n\nD\nA\n\n\nD\nB\n\n\n\n\n\n\n\n\n(23)\n\n\nO\n/\nA\n=\n\n\nO\nr\ng\na\nn\ni\nc\n\np\nh\na\ns\ne\n\nv\no\nl\nu\nm\ne\n\n\nA\nq\nu\ne\no\nu\ns\n\np\nh\na\ns\ne\n\nv\no\nl\nu\nm\ne\n\n\n\n\n\n\n\n\n(24)\n\n\nS\n/\nL\n=\n\n\nS\no\nl\ni\nd\n\np\nh\na\ns\ne\n\nm\na\ns\ns\n\n\nL\ni\nq\nu\ni\nd\n\np\nh\na\ns\ne\n\nm\na\ns\ns\n\n\n(\n\nv\no\nl\nu\nm\ne\n\n)\n\n\n\n\n\n[\n\ng\n.\n\ng\n\n\u2212\n1\n\n\n\n]\n\n\no\nr\n\n\n[\n\ng\n.\nm\n\nl\n\n\u2212\n1\n\n\n\n]\n\n\n\n\n\nThe use of cation exchangers is a common method for extracting REEs due to their positive charge in aqueous solutions. The cation exchanger replaces H+ with the REE cation, forming a complex that can be dissolved into the organic phase. The extraction reaction is shown in Eq. (25).\n\n(25)\n\n\nR\n\nE\n\nn\n+\n\n\n+\n\n\nn\nH\nA\n\n\u203e\n\n\u21cc\n\n\nR\nE\n\nA\nn\n\n\n\u203e\n\n+\nn\n\nH\n+\n\n\n\n\n\nNeutral extractant's molecule create a neutral complex with the anion of an acid and a REE cation. Eq. (26) represents the complexation of REE3+ with nitrate ions and tributyl phosphate as a neutral extractant [92].\n\n(26)\n\n\nR\n\nE\n\n3\n+\n\n\n+\n3\nN\n\nO\n3\n\u2212\n\n+\n3\n\n\nT\nB\nP\n\n\u203e\n\n\u21cc\n\n\nR\nE\n\n\n(\n\nN\n\nO\n3\n\n\n)\n\n3\n\n.\n3\nT\nB\nP\n\n\u203e\n\n\n\n\n\nA variety of amines can be utilized to extract REE ions from acidic solutions. Research indicates that amines are primarily effective in extracting REEs from sulfate-based aqueous solutions [92,93]. The extraction of REEs form a sulfuric acid solution by a primary amine can be explained through two steps: 1) acid extraction by the amine (Eqs. (27) and (29)), and 2) complexation of the amine-acid molecule with the REE cation (Eqs. (28) and (30)) [92].At low H2SO4 concentration:\n\n(27)\n\n\n2\n\n\nR\nN\n\nH\n2\n\n\n\u203e\n\n+\n2\n\nH\n+\n\n+\nS\n\nO\n4\n\n2\n\u2212\n\n\n\u21cc\n\n\n\n\n(\n\nR\nN\n\nH\n3\n\n\n)\n\n2\n\nS\n\nO\n4\n\n\n\u203e\n\n\n\n\n\n\n\n(28)\n\n\nR\n\nE\n\n3\n+\n\n\n+\n1.5\n\nS\n\nO\n4\n\n2\n\u2212\n\n\n+\n1.5\n\n\n\n\n(\n\nR\nN\n\nH\n3\n\n\n)\n\n2\n\nS\n\nO\n4\n\n\n\u203e\n\n\u21cc\n\n\n\n\n(\n\nR\nN\n\nH\n3\n\n\n)\n\n3\n\nR\nE\n\n\n(\n\nS\n\nO\n4\n\n\n)\n\n3\n\n\n\u203e\n\n\n\n\n\nAt high H2SO4 concentration:\n\n(29)\n\n\n\n\nR\nN\n\nH\n2\n\n\n\u203e\n\n+\n\nH\n+\n\n+\nH\nS\n\nO\n4\n\u2212\n\n\u21cc\n\n\nR\nN\n\nH\n3\n\n.\nH\nS\n\nO\n4\n\n\n\u203e\n\n\n\n\n\n\n\n(30)\n\n\nR\n\nE\n\n3\n+\n\n\n+\n1.5\n\nS\n\nO\n4\n\n2\n\u2212\n\n\n+\n1.5\n\n\n\n(\n\nR\nN\n\nH\n3\n\n.\nH\nS\n\nO\n4\n\n\n)\n\n2\n\n\u203e\n\n\u21cc\n\n\n\n\n(\n\nR\nN\n\nH\n3\n\n\n)\n\n3\n\nR\nE\n\n\n(\n\nS\n\nO\n4\n\n\n)\n\n3\n\n\n\u203e\n\n+\n1.5\n\nH\n2\n\nS\n\nO\n4\n\n\n\n\n\nSolvent extraction is an effective method for separating target elements or impurities from aqueous solutions [94]. Table 6\n summarizes recent studies on the solvent extraction recovery of REEs from leaching solutions of NiMHBs electrode materials. Petranikova and colleagues [78] successfully extracted Fe and Zn from HCl leachate of MHA with a mixture of Cyanex 923 and TBP, and then extracted REEs using a Cyanex 923-TBP-Decanol system. Similarly, Fernandes and colleagues [62] purified the HCl leachate of NiMHB active material from Zn and Fe using a pure TBP system, and separated Co using a 10 v.% Alamine 336 organic phase. In this work, two different approaches were proposed for recovering lanthanides from the raffinate: solvent extraction with PC 88\u00a0A or precipitation using (NH4)2C2O4, among which the solvent extraction yielded a product of high purity.Another approach reported in literature is the group extraction of elements, followed by selective scrubbing of impurities from the loaded organic phase [95,96]. Larsson and colleagues [95] used a mixed Cyanex 923-TBP organic phase to extract REEs, Al, Co, and Mn from the HCl leachate of MHA, while most of the Ni content remained in the raffinate. They selectively scrubbed Co and Mn from the loaded organic phase using a NaNO3 solution, and then stripped the REEs and Al from the organic phase using an HCl solution [95]. In a study by Zhang and colleagues [96], D2EHPA was used for recovering REEs from NiMHB sulfuric acid leachate. The presence of Ni and Co in the stripping solution can negatively impact the process of recovering REEs via precipitation of REEs oxalate, as they may co-precipitate with the oxalate ions. To address this, the co-extracted Co and Ni were scrubbed from the loaded organic phase using a diluted H2SO4 solution prior to stripping [96]. It is important to note that the stripping process of trivalent iron from D2EHPA typically requires concentrated acidic solutions [94]. Therefore, it may be more efficient to use D2EHPA for REEs recovery from MHA leaching solutions, as MHA does not contain iron.Ionic liquids (ILs) are liquids composed of only ions, with a melting point below 100\u00a0\u00b0C [98]. They have several desirable properties, such as low vapor pressure, broad liquid range, low flammability, high thermal stability, good solubility for both inorganic and organic compounds, as well as controllable hydrophobicity [92,99]. These properties make them suitable for a variety of applications, including in chemical industry, electrochemistry, and separations. The use of ILs for the separation of REEs is a novel approach, in which ILs can be used in solvent extraction as extractant, diluent, or both [92]. The extraction mechanism of ILs is similar to molecular solvents, and the metal cation forms an extractable neutral complex with the IL. Moreover, the approaches mentioned for organic extractants, e. i., group or individual separation, scrubbing, and selective stripping techniques, can be applied to ionic liquids.An article by Prusty and colleagues [100] compares the extraction mechanisms of REEs using ILs and molecular solvents. The unique properties of ILs make them potential candidates for optimizing REEs solvent extraction, as they can potentially be replaced with conventional organic solvents and diluents [101,102]. Furthermore, ILs have a reduced environmental impact compared to typical organic solvents.There are limited reports on REEs extraction from NiMHBs using ionic liquids. Table 7\n summarizes recent studies on REEs extraction from NiMHBs electrode materials leaching solutions using ionic liquids. Larsson and Binnemans [103] investigated the separation of REEs from a synthetic chloride solution derived from dissolved NiMHBs electrode materials. The extractant employed was Cyanex 923 dissolved in the nitrated form of Aliquat 336 IL, [A336][NO3]. Although the proposed system was not able to separate REEs from other elements, it demonstrated significant potential for selective scrubbing and stripping. More than 98% of the co-extracted Ni was scrubbed from the extract by an MgCl2 solution. In a stepwise stripping process, Co and Mn were stripped from the extractant using a NaNO3 solution, followed by selective stripping of REEs using a HCl solution. In another study, Larsson and Binnemans [104] employed Aliquat 336 IL to separate Co, Fe, Mn, and Zn, from the synthetic chloride leachate of NiMHBs. REEs were extracted using Cyanex 923 diluted in the nitrated form of Cyphos IL 101. The only impurity, Ni, was scrubbed from the extract and REEs were then stripped from the IL as a group. The nitrated form of Cyphos IL 101 was also used by Hoogerstraete and Binnemans [105] to separate La from a Ni-based solution. The aforementioned studies leveraged the so-called split-anion extraction process, which is well-adaptable to ILs. Nitrate anion has a strong affinity for coordination with the organic phase, while chlorine anions show higher affinity for aqueous phases. Since REEs form extractable nitrate complexes, the substitution of chloride for nitrate in Aliquat 336 and Cyphos 101 molecules allows the extraction of REEs coordinated with the nitrate ligands of the ILs [106,107].Supercritical fluids have high solvating power, low viscosity (during the process), and low surface tension, making them highly efficient extraction media. The low viscosity and high diffusivity of supercritical fluids result in superior mass transport compared to traditional liquids, leading to improved extraction yield and rate when compared to other extraction methods [112]. CO2 is particularly suitable as a solvent for extracting REEs from various resources, as it is cost-effective, has a moderate critical pressure, and low critical temperature [48]. Yao and colleagues [113] utilized supercritical CO2 as a solvent and TBP-HNO3 complex as a chelating agent to extract REEs from MHA. The proposed system involves leaching out REEs from the raw material using supercritical CO2, followed by the complexation of REEs cations with TBP and nitrate ions. High recovery yields for REEs were reported in this study under optimal experimental conditions. One limitation of supercritical fluid extraction is the dependency on high-temperature and high-pressure reactors. Despite this, it is considered a more environmentally friendly process in terms of hazardous waste generation when compared to solvent extraction.The application of adsorption in the separation of REEs is less common compared to solvent extraction and precipitation techniques, due to its lower extraction capacity for REEs and lower selectivity in separating individual REEs [114]. However, there have been a limited number of studies on the use of adsorbents in the recovery of REEs from NiMHBs. Table 8\n presents a summary of recent studies on the application of various adsorbents in the recovery of REEs from NiMHBs electrode materials. In a study by Araucz and colleagues [115], Purolite S975 and Diphonix resins were employed to separate La(III) from a synthetic nitrate solution containing La(III) and Ni(II) in the presence of citric acid. The results indicated that while the resins had a high adsorption efficiency, they were not effective for separating the elements individually [115]. Fila and colleagues [116] reported a similar outcome for Diphonix resin, as well. Gasser and Aly [117] suggested a novel synthetic adsorbent named Mg\u2013Fe-LDH-Cyanex 272 for the separation of La(III) and Nd(III) from the sulfate leachate of spent NiMHBs. The new adsorbent displayed superior adsorption capacities compared to Purolite S975 and Diphonix resins in previous literature [115\u2013117]. The adsorbent was stripped using diluted HCl, but its uptake efficiency for La dropped from 87% to 40% after ten cycles of use [117]. Zhi and colleagues [118] developed a new extraction-precipitation method using dibenzyl phosphate (DBP) to recover REEs from waste NiMHBs. By adding DBP to the sulfuric acid leachate of the battery, nearly complete precipitation of all the REEs was achieved, while co-precipitation of Ni, Co, and Mn was less than 1.75%. This method has the advantage of generating a larger particle size of the precipitate compared to other conventional techniques, which facilitates solid-liquid separation. Additionally, the loaded DBP is recyclable and reusable through a simple stripping process, making this technique more environmentally friendly.The aqueous biphasic systems (ABS) are ternary systems composed of water and two water-soluble solutes with distinct hydration entropies, which enable the reversible separation of a mixed-phase into two aqueous-rich phases within a specific composition range [120]. Vargas and colleagues [64] proposed the ABS for the recovery of La, Ce, and Ni from MHA using pluronic triblock copolymer (L35) and dimethylglyoxime (DMG) as the ABS. After a sequential extraction process, Ni, Ce, and La were separated with relatively high purities, demonstrating the potential of this method for the individual separation of these elements. De Oliveira and colleagues [61] suggested the use of ABS for the separation of La(III) from NiMHBs leachate utilizing a combination of PEO1500-Li2SO4-water in the presence of 1,10-phenanthroline (extractant agent) as the ABS. This work reported a high separation efficiency for La(III) over other REEs after three extraction steps.Korkmaz and colleagues [121] proposed a method for the recovery of REEs from MHA based on sulfation of raw material, selective roasting, and water leaching, which resulted in a total REEs recovery of 96%. The anode material was combined with concentrated sulfuric acid and subsequently dried. The sulfated mixture was then roasted at a high temperature, followed by leaching with water. The REEs were efficiently recovered with minimal contamination of Ni and Co. The solid residue of the leaching process is a mixture of nickel and cobalt oxides with trace impurities that may be subjected to further processing.Honda Motor Co., Ltd., in collaboration with Japan Metals & Chemicals Co., developed a hydrometallurgical process to recover up to 80% of the REEs present in NiMHBs [122]. The process involves the acid leaching of the active material and the recovery of REEs in the forms of oxides. The REEs are then metallized through a molten salt electrolysis process and subsequently reutilized to manufacture NiMH battery anode [84].In a patent by Smith and Swoffer [123], the batteries are hammer-milled under a water spray to produce a slurry. Following several physical separation processes, including screening and filtration, the majority of the non-metallic fraction is removed and an intermediate product rich in Fe and Ni is obtained. The product is then passed through a magnetic stripping device to separate Fe and Ni, followed by processing in a froth or foam floatation setup. The sediment of the floatation process is the metallic AB2 or AB5 alloy, which is filtered and recovered. In another patent by Burlingame and Burlingame [124], after battery dismantling, the active material is oxidized at 1000\u00a0\u00b0C, converting it to NiO and a REEs oxide-nickel oxide compound. The resulting oxide is then mixed with ammonium sulfate and pressed into a slug. The slug is heated at 450\u00a0\u00b0C and the residue is dissolved in deionized water. As a result, most of the REEs are leached out, while most of the NiO remains in the residue. The REEs in the solution can be precipitated in the form of oxalates and calcined to REEs oxides. Table 9\n provides a summary of other methods in the literature for the recovery of REEs from NiMHBs.\nFig. 9\n illustrates the process flow of the key hydrometallurgical techniques for recovering REEs from NiMHBs. NiMHBs black mass or REEs-rich slag is utilized as the raw material, which can be leached out through conventional leaching using inorganic acids or supercritical fluid extraction technique. The solution obtained from the leaching process, comprising REEs and other ions, is then processed through hydrometallurgical techniques such as solvent extraction, ion exchange, and precipitation to selectively recover REEs from other metallic ions.Molten slag extraction is the primary pyrometallurgical method employed for recycling NiMHBs. The selection of the slag system is a critical step in this process, as the majority of the REEs present in the battery will end up in the slag. The main approaches for battery smelting are direct smelting and oxidized smelting, with the former being more suitable for industrial use as it requires fewer pre-treatment steps. While pyrometallurgy has been demonstrated to be an effective option for the concentration of REEs from various sources, the separation and purification of individual REEs are heavily dependent on hydrometallurgical techniques. Precipitation is a commonly employed technique for the group separation of REEs from different leachates due to its cost-effectiveness, high recovery yield, and the high purity of its product. However, the method exhibits poor selectivity toward individual LREEs. The precipitate formed is typically an organic or inorganic salt, depending on the precipitant and leaching medium utilized. Solvent extraction is the most widely reported method for industrial separation and purification of REEs. Nevertheless, the application of solvent extraction for the recovery of REEs from NiMHBs leaching solutions is limited, possibly due to the significant co-extraction of other metals alongside REEs. In recent times, concerns have been raised over the disposal of hazardous residues generated by the organic solvents used in this process. To address this issue, ionic liquids have been introduced as novel, environmentally friendly compounds that can be used as extractants, diluents, or both in solvent extraction systems. Ionic liquids have demonstrated significant potential for REEs separation on a laboratory scale. However, further research is necessary to evaluate their industrial feasibility [100]. The application of adsorbents is generally limited to leachates with low metal concentrations due to the limited uptake capacity of the conventional adsorbents. Supercritical fluid extraction of REEs from various sources is a novel technique that combines autoclave leaching and solvent extraction processes. The method has several advantages over conventional solvent extraction, including high mass transfer and extraction rate, and the simultaneous leaching and recovery of metals. However, it is more appropriate for the intragroup separation of REEs, and the process parameters must be carefully controlled [125]. As a novel method, the ABS process has demonstrated promising results for the recovery of REEs as an alternative to solvent extraction, enabling the separation of REEs from complex matrices. However, the stripping yield and regeneration of the utilized extractants remains an area of improvement. Additionally, there are other methods that involve combined hydro-pyrometallurgical approaches [79,121,124], some of which have been industrialized.The industrial recovery of REEs from NiMHBs has gained significant attention in recent years due to the increasing demand for REEs in various industries. The market for REEs is highly dependent on their end use, which can be affected by political and economic factors. The price and demand for REEs can fluctuate greatly, making it difficult to predict the profitability of recovering them from NiMHBs. The limitations and challenges of the industrial recovery of REEs from NiMHBs can be classified into several key categories, including.\n\n\u2022\nComplex composition: Ni-MH batteries contain a complex mixture of metals and other materials, making it difficult to separate and recover the REEs.\n\n\n\u2022\nEconomic feasibility: REEs are present in NiMHBs in relatively low concentrations, which makes it challenging to economically recover them. Traditionally, Ni and Co have been the main target elements in an industrial recovery process. However, due to the increased attention to REEs as strategic metals, different industries have modified their processes to efficiently recover these elements from the batteries.\n\n\n\u2022\nEnvironmental concerns: The processes used to recover REEs from batteries and the generated wastes can be environmentally damaging, and there are concerns about the potential release of toxic substances during the recovery process.\n\n\n\u2022\nTechnological limitations: There are currently limited technologies available for the efficient recovery of REEs from NiMHBs, which makes it difficult to scale up the process.\n\n\n\u2022\nLack of standardization: There are currently no widely accepted or consistent methods for recycling these batteries, which makes it difficult to track and recover rare earth elements. This lack of standardization can also make it difficult for recyclers to identify and separate batteries that contain rare earth elements, which further complicates the recovery process. Additionally, the lack of standardization can also make it difficult for recyclers to ensure that the batteries are being recycled in an environmentally responsible manner.\n\n\nComplex composition: Ni-MH batteries contain a complex mixture of metals and other materials, making it difficult to separate and recover the REEs.Economic feasibility: REEs are present in NiMHBs in relatively low concentrations, which makes it challenging to economically recover them. Traditionally, Ni and Co have been the main target elements in an industrial recovery process. However, due to the increased attention to REEs as strategic metals, different industries have modified their processes to efficiently recover these elements from the batteries.Environmental concerns: The processes used to recover REEs from batteries and the generated wastes can be environmentally damaging, and there are concerns about the potential release of toxic substances during the recovery process.Technological limitations: There are currently limited technologies available for the efficient recovery of REEs from NiMHBs, which makes it difficult to scale up the process.Lack of standardization: There are currently no widely accepted or consistent methods for recycling these batteries, which makes it difficult to track and recover rare earth elements. This lack of standardization can also make it difficult for recyclers to identify and separate batteries that contain rare earth elements, which further complicates the recovery process. Additionally, the lack of standardization can also make it difficult for recyclers to ensure that the batteries are being recycled in an environmentally responsible manner.\nTable 10\n offers a comprehensive comparison of the principles, features, and technical advantages and disadvantages of various methods for recovering REEs from NiMHBs.A techno-economic analysis of the recovery of REEs from NiMHBs involves evaluating the technical feasibility of the process, the involved methods and equipment, as well as the purity and quantity of the recovered REEs. The economic analysis assesses the costs of the process, including the cost of the equipment, materials, and waste management, as well as the revenue generated from selling the recovered REEs, considering the market demand and REEs price. The number of studies on the techno-economic analysis of the recovery of valuable metals from NiMHBs is limited. Furthermore, the cost of the recovery process can vary depending on the techniques employed, making it challenging to conduct a comprehensive analysis that encompasses all aspects of the process.Lin and colleagues conducted a preliminary economic analysis to compare the recovery of valuable metals from NiMHBs through thermal and mechanical processes [126]. According to their work published in 2016, roughly 619 and 821 USD can be obtained in profits by recovering valuable metals from each ton of spent NiMH batteries through thermal melting and mechanical processes, respectively [126]. The diagram in Fig. 10\n shows the flow of materials in the recycling process and the potential economic value of the recovered products as per market rates in 2022. Based on revenue potential per unit mass, didymium (Nd\u00a0+\u00a0Pr) metal and high-grade nickel metal are the two most valuable co-products which are recovered via recycling of the batteries. Despite comprising less than 1% of the total recovered materials by mass, didymium generates over 14% of the total potential revenue from all products recovered. Negative revenue represents the cost of disposing of waste products. The value of REE products and Ni metal are based on an expected purity of 99% or higher, as determined by the hydrometallurgical separation process used in the study [127].The individual separation of REEs is challenging due to their similar chemical properties, and as such, group recovery of REEs is the most commonly reported practice. The lanthanide contraction, as explained in Section 1.1, makes it more feasible to separate REEs with a large difference in atomic number (e.i., LREEs from HREEs) than adjacent REEs (except for Ce and Eu). Due to the limitations of pyrometallurgical processes, individual separation of REEs is typically achieved through hydrometallurgical techniques. Some methods, such as solvent extraction, require multiple repetitive stages to attain the desired purity of an element, while others necessitate precise attention to the solution's controlling parameters (e.g., for fractional precipitation). Additionally, factors such as process efficiency, environmental impact, investment and operational costs, and industrialization flexibility should be considered when selecting a recovery method.Given the presence of La, Ce, Nd, and Pr in NiMHBs, they can be isolated and purified collectively using methods such as precipitation. The solid products (e.g. REEs mixed oxides) can then be dissolved in acidic solutions for further processing. The current section outlines the latest studies on the individual separation of these elements to complete the recycling cycle of REEs from NiMHBs.Due to the versatility of the solvent extraction process and its scalability, as well as the diversity of options for the extraction media and system, solvent extraction is currently the leading technique for individual separation of REEs. The availability of various organic extractants and the ability to modify them through methods such as saponification, as well as the option to add complexing and auxiliary agents to the aqueous solution, have made solvent extraction adaptable to different target elements and aqueous media. The focus of many recent studies in this field has centered on the difference in REEs\u2019 affinities for complexation with various aqueous species, extractants, or both. These affinities can be derived from differences in thermodynamic stability, hydrophobicity, or the extraction electrochemistry of distinct REEs complexes.Organophosphorous extractants are commonly utilized for the extraction and separation of lanthanides due to their selectivity. However, the use of these extractants alone for individual separation of lanthanides has shown limited success. To improve the outcomes, various auxiliary approaches such as saponification, selective scrubbing, the use of complexing or buffering agents, or taking advantage of the synergistic effect between extractants are often employed. PC 88\u00a0A [128\u2013136], D2EHPA [130,132,133,137\u2013141], Cyanex 272 [130,133,135,140,142\u2013144], Cyanex 572 [132,145], TOPO [132,140,144], and TBP [139,140,143\u2013145] are the most commonly used extractants in REEs separation. However, some studies suggested that synthesized extractants may results in better yields and higher separation factors between elements than conventional organophosphorus compounds [54,146]. The majority of published research in this area has focused on REEs separation from synthetic solutions, and the results may differ when using leaching solutions of NiMHBs due to the presence of other elements and associated interferences. Table 11\n provides the key physical and chemical properties of conventional organic extractants used in REEs recovery and separation.The extraction of REEs via cation exchangers results in the liberation of H+ which leads to an increase in the acidity of the aqueous solution (as per Eq. (25)). To mitigate this effect, a pre-treatment technique known as saponification may be employed. The technique entails the treatment of the organic phase with an alkaline solution, such as sodium hydroxide or ammonium hydroxide solutions, to reduce the amount of H+ released during the extraction reaction by exchanging the hydrogen in the extractant molecule with the cation of the alkaline agent (e.g. Na\u00a0+\u00a0or NH4\n+). Saponification can stabilize the process, enhance extraction efficiency, and improve the selectivity of REEs separation [92,133]. However, due to the high volume of wastewater generated, this method is being increasingly replaced by alternative techniques such as making use of synergistic effects between extractants [101] or adding buffering agents.Scrubbing is a technique utilized for the selective removal of targeted elements or impurities from a loaded organic phase (or any other type of adsorbent) using a scrubbing solution. One commonly reported approach in the literature for the separation of LREEs is to scrub impurities from the loaded organic phase using a pure solution of the target element. The target element refers to the element that is being purified. Considering X as the target element and Y as an impurity (where X and Y are REEs that are already loaded into the organic phase), Y can be removed from the loaded organic phases by scrubbing it with a pure solution of X. This process results in the substitution of X for Y in the organic phase and the transfer of Y into the aqueous phase, as outlined in Eq. (31) [129].\n\n(31)\n\n\n\nX\n\n+\n3\n\n\n+\n\n\nY\n\n\n(\n\nH\n\nA\n2\n\n\n)\n\n3\n\n\n\u203e\n\n\u21cc\n\n\nX\n\n\n(\n\nH\n\nA\n2\n\n\n)\n\n3\n\n\n\u203e\n\n+\n\nY\n\n+\n3\n\n\n\n\n\n\nThis approach has been demonstrated to be effective in countercurrent processes, where impurities (such as Y) are scrubbed from the organic phase after multiple sequential steps. Reports in the literature have documented the application of this method for the scrubbing of Pr from Nd, and La from didymium [128,129,140].Carboxylic acids (R\u2013COOH) are among the most commonly used buffering agents in the solvent extraction separation of LREEs. Synthesized complexing agents have also been shown to have great potential in achieving high separation factors between LREEs [138,147]. However, the number of reports on these agents is limited. Carboxylic acids are cheaper and less hazardous to the environment compared to the addition of complexing agents such as EDTA and DTPA. They create a buffer system in the aqueous phase, similar to the effect of saponification, which prevents drastic drops in acidity resulting from the cation exchange mechanism (Eq. (25)), as outlined in Eq. (32) [148].\n\n(32)\n\n\n\nH\n+\n\n+\nR\nC\nO\n\nO\n\u2212\n\n\u21cc\nR\nC\nO\nO\nH\n\n\n\n\nThe use of lactic acid, acetic acid, and citric acid as buffering agents is prevalent in the separation of adjacent lanthanides, among which lactic acid has shown superior performance [133,142,148,149]. The relative effectiveness of saponification versus the use of buffering agents may vary depending on the specific extraction system employed. For example, the application of lactic acid was found to be a more advantageous technique for La/didymium separation using D2EHPA, as compared to saponifying the extractant. However, saponification was determined to be more efficient in terms of improving selectivity for Cyanex 272. In the case of PC 88\u00a0A, there was no significant difference between the two approaches [133].In solvent extraction systems, the phenomenon of synergism refers to an increase in extraction efficiency when a combination of extractants is used, resulting in a performance greater than the sum of their discrete efficiencies [5], while in antagonism the opposite occurs. Synergism is a widely-used principle in the solvent extraction of REEs, as it can lead to increased extraction capacity and enhanced separation of the elements through increased hydrophobicity of the metal-extractant complex [5]. Studies have shown that synergism between Cyanex 272 and Alamine 336 is effective in the separation of didymium and La [150]. Similar results have been reported for the combination of D2EHPA and Cyanex 272 [140]. Likewise, the synergistic effect of 8-Hydorquinoline and PC 88\u00a0A was found to enhance the selective separation of Nd and Pr, with acetic acid used as a buffering agent [151].Recent advances in the synthesis of novel extractants have garnered significant attention in the individual separation of REEs due to their superior performance. Among the newly developed extractants, TiBDGA showed exceptional separation factor values of about 135 and 60 for the separation of Ce and La from Pr and Nd, respectively [146]. Similarly, DEHAPO has been identified as a promising extractant for the selective separation of Ce from La with a separation factor of up to 167 [54]. Despite these promising results, the number of studies on the application of novel extractants in the separation of adjacent lanthanides is still limited, and further research is required for their industrial application. Fig. 11\n illustrates the distribution ratio of lanthanides extracted by TiBDGA as a selective extractant for La and Ce, and DEHAPO as an extractant for selective separation of Ce4+.\nTable 12\n summarizes various solvent extraction approaches for the individual separation of Nd, Pr, La, and Ce. It is important to note that the majority of these studies have focused on the extraction of REEs from chloride media, with relatively fewer studies investigating extraction from sulfate media. This is likely due to the fact that the formation of less-extractable anionic sulfate metallic species, such as Nd(SO4)2\n\u2212 and Pr(SO4)2\n\u2212 can result in inferior REEs extraction efficiency in sulfate media [132].Ionic liquids have recently received attention as a potential green alternative for conventional organic materials in solvent extraction, particularly in the separation of adjacent REEs. Along this line, Gras and colleagues [159] investigated the use of [P66614][Tf2N] and [C1C4Pyrr][Tf2N] ionic liquids for the individual separation of Ce from La, Pr, and Nd. The study involved the oxidation of Ce(III) to Ce(IV) under alkaline conditions, while the oxidation state of the other lanthanide ions remained unchanged. The lanthanide mixed hydroxides formed in the oxidation step were then dissolved in a nitric acid solution, followed by selective separation of Ce(IV) from the other elements via ILs.The extraction mechanism of non-functional ILs is based on ion exchange, in which the cationic or anionic ligand of the molecule is released into the aqueous medium. However, this mechanism can result in environmental concerns and additional costs associated with the regeneration and reuse of the ILs [160,161]. In contrast, functional ILs, which contain task-specific coordination ligands, employ a solvation-based extraction mechanism rather than an ion-exchange mechanism, which improves the extraction of metal ions and prolongs the lifetime of the ILs [162]. Khodakarami and Alagha [162] studied the separation of adjacent LREEs from a nitrate solution using two functional ILs, [A336][DHDGA] and [OcGBOEt][DHDGA] with the molecular structures illustrated in Fig. 12\n. It was observed that the extractability of [OcGBOEt][DHDGA] was superior to that of [A336][DHDGA], despite the latter exhibiting superior selectivity for the individual separation of LREEs. The extraction equilibrium between REEs (M3+), nitrate ion, and the employed functional ILs can be described by Eq. (33), where Cn and A denote the cation and anion of the IL, respectively.\n\n(33)\n\n\n\nM\n\na\nq\n\n\n3\n+\n\n\n+\nx\n\n[\n\nC\nn\n\n]\n\n\n\n[\n\nA\nn\n\n]\n\n\no\nr\ng\n\n\n+\ny\n\n\n(\n\nN\n\nO\n3\n\u2212\n\n\n)\n\n\na\nq\n\n\n\u21cc\n\n\n[\n\nC\nn\n\n]\n\nx\n\nM\n\n\n[\n\nA\nn\n\n]\n\nx\n\n\n\n(\n\nN\n\nO\n3\n\n\n)\n\ny\n\n\n\n\n\nIonic liquids have been widely utilized as solvents to enhance the extraction conditions of conventional organic extractants. Dehaudt and colleagues [163] studied the utilization of TODGA as the extractant in both organic solvent (DIPB) and ionic liquid ([Cnmim][Tf2N] and [Cnmim][BETI], n\u00a0=\u00a02,4,6,8,10) media to individually separate La, Ce, Pr, and Nd from a synthetic nitrate solution. DTPA was used as the holdback reagent (buffered with citric acid) in the aqueous phase to partition the elements between the aqueous and IL phases. The holdback reagent improves the selectivity of the extractant by retaining certain elements in the aqueous phase. The reagent forms less-hydrophobic complexes with the elements as a function of their acidity and ionic radii. As demonstrated by Dehaudt and colleagues, using TODGA diluted in [C4mim][Tf2N] and DIPB, high selectivity for the separation of La over other REEs could be achieved. The method is also suitable for the individual separation of Ce, Nd, and Pr from their mixed solution [163]. Different approaches for using ILs in the individual separation of Ce, La, Nd, and Pr are reviewed in Table 13\n.After solvent extraction, adsorption/ion exchange and fractional precipitation are commonly utilized methods for the separation of REEs. Due to similarities in chemical behavior among REEs within a group (i.e. LREEs and HREEs), fractional precipitation has shown potential for group separation. However, taking advantage of different oxidation states, Eu and Ce can be individually separated from adjacent elements. Various methods for selective oxidation of Ce(III) from acidic media have been investigated, including photooxidation [172] and alkalinization [159], as well as oxidation using wet air [74,173], hydrogen peroxide [53,174\u2013176], hypochlorite [53,176\u2013178], and permanganate [53,176,179,180]. McNeice and colleagues [176] investigated the oxidation of Ce(III) to Ce(IV) in the mixed solution of Ce, La, Pr, Nd, and Te hydroxides using various oxidizing agents, including sodium hypochlorite, hydrogen peroxide, potassium permanganate, and Caro's acid (peroxymonosulfuric acid). Of these, KMnO4 was found to be the most effective oxidizing agent, resulting in a precipitation yield of 98.4% for Ce(IV) hydroxide, with minimal co-precipitation of other elements [176]. The application of adsorption techniques in REEs separation is primarily limited to the use of membranes and resins. Makowka and Pospiech [181] examined the separation of Ce from La (and other metals) using Cyphos 104 IL (as the extractant and ion carrier) from a nitrate solution, through the use of polymer inclusion membranes (PIMs). This study demonstrates the potential of PIMs for the competitive transport of Ce, La, Cu, Co, and Ni from the loaded ionic liquid, with the ionic liquid extracting more than 99% of Ce and 97% of La. However, the transport selectivity of the PIM for Ce and La was 67% and 15.7%, respectively, highlighting the potential of these membranes for individual separation of REEs.Reports on the application of impregnated adsorbents indicate that these materials generally exhibit low selectivity in the separation of individual LREEs. However, they possess substantial potential for the selective desorption of the elements by altering different elution parameters. Lee and colleagues [182] investigated the chromatographic separation of Ce, Pr, Nd, Sm, Zn, Al, Ca, and Fe from La using an Amberlite XAD-7 HP resin impregnated with D2EHPA. The study demonstrated that La could selectively be eluted from the loaded resin with a recovery rate of 90% [182]. In a similar study, a column packed with microcapsules containing PC 88\u00a0A extractant was employed to extract La, Ce, and Pr from a chloride solution. Within eight adsorption cycles, more than 95% of Pr was extracted while the concentration of La and Ce in the feed solution remained largely unchanged. Subsequently, La and Ce were co-extracted and then separated through controlled desorption [183]. Ashour and colleagues [184] also reported the application of this approach in using silica nanoparticles and porous microparticles functionalized with a monolayer of DTPA-derived ligands.Metal\u2212organic frameworks (MOFs) are an emerging class of porous crystalline materials which are size-selective and are well suited for individual separation of REEs. Wu and colleagues [185] synthesized a zinc-trimesic acid (Zn-BTC) MOF covered by nanoporous graphene (NG), which resulted in high separation factors between adjacent REEs (e.g., \u03b2Nd/Pr\u00a0=\u00a09.8 and \u03b2Ce/La\u00a0=\u00a020.05) during extraction. This is due to the fact that the pore size of the MOF is very similar to the hydrated REE ion diameter, which allows their bare ions enter the MOF channels to coordinate with existing oxygen and form stable structures similar to hydrated REEs ions. Moreover, the presence of a 2D nanopore structure with a controlled size and the surrounding oxygen-containing groups, provides NG with higher potential for separation and purification of the elements [185]. Fig. 13\n illustrates the separation factor between lanthanides for MOF/NG, ZnO/NG, and MOF alone.Despite their similar chemical properties, REEs can be separated individually through hydrometallurgical approaches. The separation process for REEs is more complex than that of other metals and often involves multiple consecutive steps. Among the REEs present in NiMHBs, only Ce possesses different oxidation states of +3 and\u00a0+\u00a04, making it more amenable to separation from other REEs. Solvent extraction is the most widely used technique in industry for the individual separation of REEs, as it offers various options for adjusting the process towards specific target elements. Techniques such as saponification of the extractant, selective scrubbing, the addition of complexing or buffering agents, and taking advantage of the synergistic effects between different extractants are commonly employed to adjust solvent extraction for REEs separation. Ionic liquids have recently gained attention as a more environmentally friendly alternative to organic compounds in the separation of adjacent lanthanides by solvent extraction techniques. ILs have been utilized as the primary extractant, solvent, or both in solvent extraction, and have demonstrated exceptional results. Additionally, the same approaches used to adjust organic extractants can also be applied to ionic liquids. The application of adsorbents and membranes in REEs separation is also an area of ongoing research, with various novel synthesized adsorbents showing superior selectivity towards REEs being introduced.The limitations and shortcomings of individual separation of La, Nd, Pr, and Ce are mainly caused by their similar chemical and physical properties, as well as the lack of specificity of the methods that can be used for their separation. Table 14\n provides limitations and shortcomings of the individual separation of lanthanides.In addition to above-mentioned points, recycling of the reagents can be difficult, which can potentially increase the cost of the separation process. Furthermore, some of the separation methods are not amenable to scale-up, although they may show high separation efficiency for individual REEs in laboratory scale. In some cases, the separation process may not be fully selective and may result in the presence of impurities in the final product. The challenges of separating La, Nd, Pr, and Ce from one another are significant, and the development of new, more selective and efficient separation methods is an active area of research.Rare earth elements (REEs) are of strategic importance for the world's technological development and play a crucial role in the ongoing efforts towards a more sustainable and environmentally friendly future. However, their limited availability of economic resources and the concentration of production and supply in a few countries pose significant challenges in meeting the increasing demand for REEs. Nickel-metal hydride batteries (NiMHBs) are primarily composed of steel casing and electrode materials containing large amounts of light rare earth elements (LREEs), Ni, and Co. Due to their widespread use in rechargeable devices, recycling end-of-life NiMHBs can make a substantial contribution to addressing the global demand for REEs. Molten slag extraction is the primary pyrometallurgical approach reported for recycling NiMHBs. The efficiency of this method is highly dependent on the properties of the slag system, as almost all of the REEs present in the battery are collected by a slag phase floating on the surface of a molten Ni-based alloy. Among hydrometallurgical methods, precipitation is the most frequently reported technique for group separation of REEs from leaching solutions of NiMHBs, MHA, or REEs-rich slags, owing to its economic, versatile, and scalable nature. Solvent extraction has remained the primary technique for individual separation of REEs, although in some cases, adsorption and fractional precipitation have also shown outstanding results. Studies on the application of ionic liquids (ILs) in solvent extraction have demonstrated the significant potential of these compounds for recovering REEs from various aqueous media. Not only do ILs exhibit exceptional performances on lab scale, but they also present more environmentally friendly alternatives to conventional organic extractants and solvents. Nevertheless, additional research is required to assess their industrial viability. Individual separation of LREEs has been an arduous task for decades due to the highly similar chemical properties of these elements. In solvent extraction, conventional organic extractants are often unable to achieve high separation factors between adjacent lanthanides, and their generated wastes can be harmful to the environment. Despite these limitations, the versatility of solvent extraction for industrial applications, as well as its potential for manipulation through mixing, complexation, and modification with other agents, have made it the preferred technique for individual separation of REEs. Most available adsorption and precipitation methods have shown poor selectivity for individual REEs. However, recent advancements in synthetic materials have led to their applications in REEs recovery and separation. This includes their use as extractants in solvent extraction and adsorption techniques and agents added to the solutions to improve complexation or selective precipitation. In light of the ongoing competition to achieve high degree of separation between the elements through green processes, it is anticipated that future REEs recovery efforts will rely on the development of novel extractants and agents, as well as implementing innovative and sustainable approaches.\nHossein Salehi: Conceptualization, methodology, investigation, data curation, validation, writing - original draft, visualization, software. Samane Maroufi: Conceptualization, writing - review & editing, supervision, project administration, funding acquisition. Sajjad S. Mofarah: Methodology, validation, writing - review & editing. Rasoul Khayyam Nekouei: Methodology, validation, writing - review & editing. Veena Sahajwalla: Writing-reviewing and 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 research was supported by the Australian Research Council's Industrial Transformation Research Hub funding scheme (project IH190100009).", "descript": "\n                  The recycling of nickel-metal hydride batteries (NiMHBs) has garnered significant attention in recent years due to the growing demand for critical metals and the implementation of national and international legislation aimed at achieving zero carbon emissions and reducing environmental impact. Typically, NiMHBs contain 10\u00a0wt% of rare earth elements (REEs) including La, Ce, Nd, and Pr. However, the majority of these REEs (>90%) are being discarded in landfills each year. The scarcity of these metals and the concentrated distribution of their ore deposits in only a few countries have prompted significant concern globally. One of the existing strategies to address this issue is extraction of REEs through urban mining. This study provides an in-depth fundamental and systematic review on the existing strategies and technologies for the recovery of REEs from spent NiMHBs. Further, the state-of-the-art approaches for the individual separation of La, Ce, Nd, and Pr from aqueous media are discussed, along with their corresponding challenges and shortcomings as well as the potential future directions. The research aims to provide a transformative understanding of various methods for the recovery of REEs from NiMHBs, the available techniques for the individual separation of REEs from different secondary resources, and potential improvements in the recycling process of spent NiMHBs. The outcome of this work will contribute to the development of more efficient and effective REEs recovery methods and help address the growing concern of REEs scarcity and extraction environmental impact.\n               "}
{"full_text": "This paper outlines a viable method to grow Pt/Ni nanoparticles on TiO2 nanotube surfaces, directly from solutions that resemble those available in industrial nickel processing. Global nickel resources originate from two different types of ores, sulfides and laterites, which in hydrometallurgical Ni recovery are dissolved to either chloride, sulfate or ammonia media, with or without pyrometallurgical pre-treatment steps. Ni recovery from these solutions is typically achieved by solvent extraction-electrowinning (i.e. a pure electrodeposition) route, although hydrogen reduction and different chemical precipitation methods may also be used. As a result, all Ni processing produces hydrometallurgical solutions that not only contain nickel but also a variety of other metals originally present in the ore - like cobalt, copper and precious metals - and the processes are usually optimised also to recover these other metals (Crundwell et\u00a0al., 2011). In spite of this, such solutions can still contain trace amounts of precious metals, which are not recovered from solutions due to a high concentration ratio between the base metal (such as Ni) and trace metals (such as Pt).This is of particular importance in view of the global scarcity of metal resources that has become an imminent concern (Peck et\u00a0al., 2015) and thus, improved recycling and recovering of metals from impure process solutions, acidic industrial side-streams and other waste streams is becoming a necessity. At the same time, emerging technologies like fuel cells may increase the demand for Pt as a catalyst material even further (Peck et\u00a0al., 2015) and therefore, identification of alternative (secondary) raw material sources for platinum group metal (PGM) based catalysts is essential.Ni-Pt combinations have been proved to be extremely effective catalyst materials (Debe, 2012; He et\u00a0al., 2011, 2013; K\u00fchl and Strasser, 2016; Shao et\u00a0al., 2016) and thus, process and side-streams associated with Ni metal (McDonald and Whittington, 2008a,b) could be potential raw material sources when trace amounts of Pt is present, especially as annual global nickel production is over 2000\u202fGg (i.e. 2 million tonnes) and expected to increase up to 140\u2013175% by 2025 (Elshkaki et\u00a0al., 2017). However, due to the high concentration ratio between Ni and precious metals, the hydrometallurgical solutions in nickel production have not been considered suitable for recovery of precious metals, and even less, for production of a high-added value Pt catalyst.On the other hand, the toxic nature of nickel solutions \u2013 which are inevitable in hydrometallurgy in order to fulfil the ever increasing global nickel demand \u2013would require new methodologies of how to fully exploit these solutions and also, the minor components present in them. The results presented here show that utilising redox replacement reactions the formation of Pt/Ni catalysts from such solutions is indeed possible, without any additional chemicals.Usually the redox replacement reaction is used in conjunction with electrochemical deposition and this is the case in our studies (Fig.\u00a01\n). Firstly, Ni is electrodeposited and then, the redox replacement reaction takes place spontaneously between electrodeposited Ni and Pt2+ ions due to the difference in the reduction potentials: the deposited Ni is oxidised by Pt2+ and dissolved back to solution as Ni2+, whilst Pt2+ is reduced to Pt and deposited on the electrode. The half-reaction for the oxidation of Ni is shown in Reaction 1a and the reduction of Pt2+ in Reaction 1b, together with their respective standard electrode potentials (E\u00b0 vs. SHE).\n\n(1a)\n\n\nN\n\ni\ns\n\n\u2192\nN\n\ni\n\na\nq\n\n\n2\n+\n\n\n+\n2\n\ne\n\u2212\n\n\nE\u00b0\n\n(\nox\n)\n\n\n\n=\n\n+\n\n0\n.\n25\n\nV\n\nvs\n.\n\nSHE\n\n\n\n\n\n\n(1b)\n\n\nP\n\nt\n\na\nq\n\n\n2\n+\n\n\n+\n2\n\ne\n\u2212\n\n\u2192\nP\n\nt\ns\n\n\nE\u00b0\n\n(\nred\n)\n\n\n=\n\n+\n\n1\n.\n20\n\nV\n\nvs\n.\n\nSHE\n\n\n\n\nThe overall redox reaction is displayed in Reaction 2, together with the standard reaction potential, which is the driving force for the spontaneous redox replacement reaction:\n\n(2)\n\n\nN\n\ni\ns\n\n+\nP\n\nt\n\na\nq\n\n\n2\n+\n\n\n\u2192\nN\n\ni\n\na\nq\n\n\n2\n+\n\n\n+\nP\n\nt\ns\n\n\nE\u00b0\n\n(\nRR\n)\n\n\n=\n\n+\n\n1\n.\n45\n\nV\n\nvs\n.\n\nSHE\n\n\n\n\nIn contrast to other electrochemical-redox replacement methods like electrochemical-atomic layer deposition (e-ALD) (Gregory and Stickney, 1991; Vaidyanathan et\u00a0al., 2006) or surface-limited redox replacement (SLRR) (Brankovic et\u00a0al., 2001), the goal here is not to deposit smooth monolayers but utilise electrodeposition-redox replacement method (EDRR) to create functional Pt/Ni catalysts directly from solutions resembling hydrometallurgical Ni process solutions.Moreover, this one step approach differs from those presented in literature about Pt/Ni surfaces prepared by EDRR (Papadimitriou et\u00a0al., 2008, 2010; Rettew et\u00a0al., 2009; Tegou et\u00a0al., 2010; Wang et\u00a0al., 2011; Zhang et\u00a0al., 2012) or electroless deposition-redox replacement (Tama\u0161auskait\u0117-Tama\u0161i\u016bnait\u0117 et\u00a0al., 2013, 2014). In these previous studies, all the solutions have been optimised for the application, i.e. Pt and Ni concentrations of synthetic solutions are tailored for the most effective catalyst formation and they do not represent industrial solutions. Conversely, this paper demonstrates \u2013 for the very first time \u2013 that non-optimised solutions and side-streams could be a potential raw material sources for functional Pt/Ni catalyst surfaces. These solutions are indeed challenging as in hydrometallurgical Ni process solutions the Ni content is high and Pt content extremely low: for a 10\u202fppm\u202fPt solution, Ni/Pt concentration ratio is typically \u2248 20,000.Earlier, the authors have demonstrated that Ag can be recovered from Zn based hydrometallurgical solutions by EDRR (Halli et\u00a0al., 2017) and Au from copper based solutions (Korolev et\u00a0al., 2018) whereas this current paper goes beyond that approach: instead of pure recovery of a precious metal, functional Pt/Ni catalytic surfaces are produced and used without any further modifications for photocatalytic H2 generation.From an industrial perspective, the possibility to utilise under sourced side-streams and process solutions for catalyst production makes EDRR already very attractive method but it has also further advantages: EDRR does not demand any additional chemicals, unlike cementation (precipitation) or solvent extraction traditionally used in hydrometallurgy for metal recovery, no neutralization chemicals are needed either, and when compared to electrowinning (pure electrodeposition), EDRR is more effective with solutions of trace amount of precious metals. This all makes the EDRR method more sustainable than the competing methods.Overall, this research demonstrates a new route for the exploitation underutilised industrial side-stream solutions, which not only leads to the formation of catalytic surfaces for clean energy production but also has the added benefit of reducing/eliminating the presence of potentially toxic material (Pt) from industrial Ni processing. Furthermore, this method provides a platform for the cost and material competitive large-scale catalyst production based on the principles of circular economy.Flat TiO2 surfaces were prepared by anodising Ti foil (99.9%) at 20\u202fV for 15\u202fmin in 0.5\u202fM H2SO4. In contrast, TiO2 nanotubes were prepared by the immersion of Ti foil in a tri-ethylene glycol electrolyte consisting of 0.3\u202fM NH4F and 3\u202fM H2O, at 60\u202fV\u202fat 60\u202f\u00b0C for 15\u202fmin. After anodising, all the samples were annealed in air at 450\u202f\u00b0C for 1\u202fh.Ni nanoparticles were prepared on TiO2 surfaces by the electrodeposition \u2013 redox replacement (EDRR) method (Ivium CompactStat) from a solution containing 60\u202fg/l nickel (from NiSO4\u22196 H2O, ACS grade, Sigma-Aldrich) and 10\u202fg/l H2SO4 (95\u201398%, p.a., Carl Roth), while Pt/Ni nanoparticles were deposited from the same base solution (60\u00a0g/l nickel\u00a0+\u00a010\u00a0g/l H2SO4) having 10\u202fppm or 100\u202fppm\u202fPt (from 1000\u202fmg/l AAS standard, Sigma-Aldrich). Either flat TiO2 or TiO2 nanotube surfaces \u2013 with a geometric area 0.5\u202fcm2 - were used as the working electrode, with a Pt sheet (6\u202fcm2) as the counter electrode and Ag/AgCl in 3\u202fM KCl as the reference electrode. The distance between working and counter electrode was 2\u202fcm and the volume of the solution was 20\u201325\u202fml.In EDRR method, the electrodeposition step (ED step) was performed galvanostatically and consisted of a total 74 short cathodic and anodic current pulses. After 37 cathodic-anodic pulse pairs, a redox replacement (RR) step was performed. During this step, no external current or voltage was applied but open circuit potential (OCP) was recorded until a pre-determined time had elapsed. After this, the EDRR cycle was repeated - first with 37 cathodic-anodic pulse pairs followed by a RR step.In the case of the flat TiO2 surfaces, the cathodic and anodic current pulses had a duration of 10\u202fms each and the current density was\u00a0\u2212100\u202fmA/cm2 and\u00a0+20\u00a0mA/cm2, respectively. The RR step time was either 10, 30 or 60\u202fs and the number of full EDRR cycles was varied (10, 20 or 30 cycles). For TiO2 nanotube surfaces the conditions were modified, such that the cathodic pulse durations were 4\u202fs\u202fat\u00a0\u221230\u202fmA/cm2 and anodic pulses 10\u00a0ms\u00a0at\u00a0+30\u00a0mA/cm2, to reflect the higher surface area and lower conductivity of the tubes. The associated RR time was set to be between 60 and 240\u202fs and the number of cycles was either 10 or 20 cycles.Field-emission scanning electron microscope (FE-SEM Hitachi S4800) was used to characterize the morphology of the samples, whereas X-ray photoelectron spectroscopy (XPS, PHI 5600) provided the chemical composition of the samples. In XPS, the signal intensity was divided by a relative sensitivity factor (RSF) and normalized over all of the elements detected. All data processing was performed using MultiPack v.9.6.0 software.Photocatalytic H2 generation measurements were conducted by irradiating the TiO2 samples with an AM 1.5 solar simulator (100\u202fmW/cm2) in a quartz tube containing a 20\u202fvol% ethanol-water solution for 5\u202fh. The amount of produced H2 was measured by using a gas chromatograph (GCMS-QO2010SE, Shimadzu) equipped with a thermal conductivity detector and a Restek micropacked Shin Carbon ST column (2\u202fm\u202f\u00d7\u202f0.53\u202fmm). The quartz reactor was purged with N2 gas for 10\u202fmin to remove O2 prior to the initiation of the photocatalytic experiments.Pt/Ni nanoparticles are formed on TiO2 surface during electrodeposition-redox replacement (EDRR) cycling and a typical EDRR measurement is shown in Fig.\u00a02\n: short cathodic\u00a0+\u00a0anodic current pulses (during the ED step) are followed by redox replacement (RR step) and the EDRR procedure is cycled a number of times. The EDRR profiles for all samples are shown in Supporting Information (Fig.\u00a0S1).The ED step (electrodeposition) consists of galvanostatic pulsing between cathodic and anodic currents. Firstly, Ni and possibly some Pt is deposited during the short cathodic current pulse, though simultaneous H2 evolution - that disturbs the deposition - may also take place. This is overcome by applying a short anodic current pulse that not only results in hydrogen desorption, but also makes more surface sites available for deposition in the following cathodic pulse (Kollia et\u00a0al., 1990; Spanou and Pavlatou, 2010). During the RR step (redox replacement) there is a spontaneous replacement of deposited Ni with Pt, due to the difference in electrochemical oxidation/reduction potentials - Pt2+ oxidises the electrodeposited Ni to soluble Ni2+ while it itself is simultaneously reduced to Pt and deposited to the surface (see Reactions 1\u20132).\nFig.\u00a03\n highlights the need of redox replacement (RR) step in the nanoparticle formation. The galvanostatic pulsing (i.e. ED step) alone is not an effective method for the formation of Pt/Ni nanoparticles as is clearly observed when comparing SEM images of a Pt/Ni nanoparticles prepared with galvanostatic pulsing to those prepared by EDRR method. The used parameters were the same in both of these cases, the only difference being that EDRR has an additional RR step/cycle. For comparison, Fig.\u00a03 also shows the deposition of pure Ni nanoparticles by EDRR method and it is evident that although Pt is not necessary for the nucleation of particles on the surface, it improves it. In addition, the presence of Pt results in a characteristically jagged appearance of the nanoparticles (see Supporting Information, S2). This observation is probably a result of Pt growth on the nanoparticles, both during the redox replacement step and possible co-deposition on previously replaced Pt in the subsequent ED steps.The positive effect of redox replacement step on nanoparticle formation is partly due to Ostwald ripening, leading to the presence of larger surface features, and partly due to the replenishing of solution nearby the electrode, reducing the possibility of mass-transfer limitation in ED step. Natter and Hempelmann (2003) have found a similar observation with pulse electrodeposition when varying t\noff (i.e. short current-off time between deposition pulses) for Au nanoparticles, i.e. the size of nanoparticles grew with longer t\noff time. It is important to note that - in addition to different materials and solutions (Au in literature (Natter and Hempelmann, 2003) cf. Pt/Ni presented here) - t\noff time has a different purpose than RR time. In pulse electrodeposition, t\noff is applied only for milliseconds between short deposition pulses and pulsing is performed in a single metal electrolyte in order to replenish the surface from adsorbed hydrogen, while RR time is clearly longer and performed after electrodeposition step in multi-metal electrolyte in order to redox replacement reaction to take place. As a result, enrichment of the more noble metal on the surface takes place.The effect of the initial EDRR cycles on Pt/Ni nanoparticle nucleation is presented in Fig.\u00a04\n, which comprises of Pt/Ni nanoparticles deposited to flat TiO2 surface using a single cycle or 5 cycles (the redox replacement time: 30\u202fs). As can be seen, already after a single EDRR cycle particles have nucleated on the surface, although the particle size is relatively small. When the number of cycles is increased, both the particle density and size of the particles increase substantially and start to show the characteristic jagged appearance. Data from XPS shows that the at-% of Pt is 0.29 after a single cycle and it increases to 5.77\u202fat-% after 5 cycles, whereas of Ni at-% remains low (0.77% after 1 cycle cf. 0.87\u202fat-% after 5 cycles), suggesting that even if Pt may co-deposit with Ni during electrodeposition step, it is deposited primarily during the redox replacement step.The formation of Pt/Ni nanoparticles is further studied as a function of number of cycles and RR time (Fig.\u00a05\n \u2013 SEM and Fig.\u00a06\n - XPS). The associated potential profiles of EDRR (Fig.\u00a0S1) are shown in Supporting Information.From Fig.\u00a05 it is seen that the size of nanoparticles increases both as a function of number of cycles and RR time. As previously discussed (Fig.\u00a03), the positive effect of RR time on the size and nanoparticle density can be associated with Ostwald ripening and replenishing the solution nearby the electrode: the dissolution of Ni from the surface during RR step leads to a higher local concentration in the vicinity of the electrode, further diminishing the possible mass-transport limitation in the following ED step.\nFig.\u00a06(a) shows examples of Ni2p and Pt4f spectra for a sample prepared from 100\u202fppm\u202fPt solution on a TiO2 surface using the EDRR method (the redox replacement time was 30\u202fs and number of cycles 30). As can be seen, the Pt4f region has well separated spin-orbit components (\u0394metal\u202f=\u202f3.35eV).The atomic-% (and weight-%) of Pt was determined by considering the doublet peak of Pt4f region, which can be de-convoluted into four peaks. The presence of two main peaks (69.75\u202feV and 73.12\u202feV) is ascribed to the Pt0 4f7/2 and Pt0 4f5/2, while the other small peaks at 71.0\u202feV and 75.26\u202feV correspond to Pt2+ 4f7/2 and Pt2+ 4f5/2. Ni2p peak, on the other hand, has split spin-orbit components (\u0394metal\u202f=\u202f17.3eV) that comprise of core level and satellite features, which can be resolved into eight peaks. Two peaks are located at 851.25\u202feV (Ni0 2p3/2) and 868.5\u202feV (Ni0 2p1/2), indicating the presence of Ni metal. Another two peaks at 852.40\u202feV (Ni2+ 2p3/2) and 869.76\u202feV (Ni2+ 2p1/2) are ascribed to the NiO. The other two peaks at 855.14\u202feV (Ni2+ 2p3/2) and 872.44\u202feV (Ni2+ 2p1/2) are due to the formation of Ni(OH)2. The peaks at 860.44\u202feV and 879.06\u202feV are the satellite peaks.As can be observed from Fig.\u00a06(b\u2013c), also the atom-% of Pt (and the respective weight-% of Pt, both determined by XPS) increases with longer RR times. This is due to the mass-transfer limitations related to the low Pt concentration: the mass-transfer quickly limits the redox replacement reaction when Pt content is present only in ppm levels and thus, increasing the replacement time provides longer time for Pt to reach the electrode surface, resulting in higher Pt at-% on the surface. In order to demonstrate more clearly the purity of the end-product, the Pt/Ni ratio is calculated from at-% - Fig.\u00a06(b) \u2013 and demonstrates that higher RR time results in higher end-product purity.The effectiveness of the EDRR, on the other hand, is best discussed in terms of enrichment, which is calculated by comparing the weight-% of Pt on the particles (determined by XPS) to weight-% of Pt in the solution. It is remarkable how effective EDRR is when compared to pure galvanostatic pulsing (Figs.\u00a03 and 6c). For example for a 100\u202fppm (1 ppm = 0.0001\u202fwt-%) Pt solution, utilising 60\u202fs RR time results in 42 weight-% of Pt on the surface after only 20 EDRR cycles, and this translates to over 4,200-fold enrichment. In comparison, the reference sample shown in Fig.\u00a03 (30 cycles of galvanostatic pulsing and no RR steps) has a significantly lower Pt content (4.6\u202fwt-%), resulting in a decade lower (460) enrichment, even if the current input in the reference sample is higher (20 cycles in EDRR cf. 30 cycles in galvanostatic pulsing).To further demonstrate the industrial feasibility of the EDRR method, the formation of nanoparticles was performed from a solution containing only 10\u202fppm of Pt but with the same Ni concentration (60\u202fg/l of Ni), resulting in concentration ratio Ni/Pt\u202f=\u202f20,000. As can be seen in Fig.\u00a07\n, the formation of Pt/Ni nanoparticles is successful even with such a low concentration of Pt in solution. Moreover, XPS analysis showed that with 60\u202fs replacement time 15\u202fwt-% of Pt was deposited on the surface, a level that is over three time higher than obtained by the pure galvanostatic pulsing (reference sample) from the 100\u202fppm\u202fPt solution. In the terms of the enrichment, EDRR results in over 10,000-time enrichment of Pt from 10\u202fppm solutions, suggesting that the method is extremely feasible with the solutions with a low level of precious metals.In order to investigate further the abilities of EDRR in the formation of the high-value added products, experiments were performed using TiO2 nanotube substrates (Fig.\u00a08\n \u2013 SEM and Fig.\u00a09\n - XPS). TiO2 nanotube surfaces are promising candidates for the photo-catalytic applications as the tubular configuration provides a high light absorption pathway and aids the prevention of the recombination of photo-generated electron/hole pairs (Tong et\u00a0al., 2012). Moreover, TiO2 nanotubes have demonstrated drastically enhanced photocatalytic activity in numerous studies when the nanotubes are decorated with \u201cco-catalyst\u201d metal nanoparticles (Christoforidis and Fornasiero, 2017; Liang et\u00a0al., 2013; Ni et\u00a0al., 2007; Papadimitriou et\u00a0al., 2008; Park et\u00a0al., 2013).\nFigs.\u00a08\u20139 demonstrate that the size and the amount of Pt/Ni nanoparticles on TiO2 nanotube surface can indeed be controlled by RR time and cycling when using the EDRR method (see also Fig.\u00a0S2b). The results also show that particles not only nucleate on the top of the nanotubes but also on the outer walls, allowing the exploitation of the 1D nature of the tubes for photocatalysis: the main advantages of 1D materials are the enhanced light absorption combined with short reaction paths of photogenerated carriers (Xiao et\u00a0al., 2015; Altomare et\u00a0al., 2016; Nguyen et\u00a0al., 2015, 2016). Fig.\u00a09(a) shows again an exemplar of the XPS data fitted for the Ni2p and Pt4f regions and the detected species were Pt0, Pt2+ and Ni species. Furthermore, the EDRR method allows the control over the Pt/Ni ratio (Fig.\u00a09(b), at-% determined by XPS) which has been shown to be a critically important factor in the electrocatalysis of oxygen reduction reaction in numerous studies (Jiang et\u00a0al., 2017; Toda et\u00a0al., 1999; Yang et\u00a0al., 2004).In order to demonstrate that the EDRR method could be used to produce photocatalytic surfaces from hydrometallurgical base metal streams, proof-of-concept measurements of photocatalytic H2 generation with the prepared Pt/Ni nanoparticle - TiO2 nanotube surfaces were performed. Fig.\u00a010\n shows that these surfaces indeed possess significant activity for H2 evolution, the highest being an over 30-fold enhancement (the redox replacement step\u202f=\u202f240\u202fs and number of cycles\u202f=\u202f20) when compared to a pure TiO2 nanotube surface. When the photocatalytic activity is compared to TiO2 nanotube surfaces covered with pure Ni nanoparticles, the H2 evolution levels are similar for fresh samples (see Supporting Information, S3). However, pure Ni nanoparticles suffers from aging whereas Pt or Pt/Ni are highly stable against oxidation (S3). Moreover, the catalytic activity shown here is comparable with literature, e.g. our results show a similar H2 evolution rate under 1.5 AM solar illumination as that obtained for atomic layer deposited Pt as co-catalysts on TiO2 nanotubes (Yoo et\u00a0al., 2018). The results also indicate that the Pt/Ni ratio is critical for H2 evolution: the sample with highest H2 production has also the clearly highest Pt/Ni ratio \u2248 8, while for all the other surfaces the Pt/Ni ratio \u2248 2 (Fig.\u00a09(b)). All these samples with a similar Pt/Ni ratio have also similar hydrogen evolution rates (Fig.\u00a010), thus indicating the critical role of Pt/Ni ratio in the particles.As the presented EDRR method is particularly powerful in tuning the Pt/Ni composition, these results are very promising in the view of preparing photocatalytic surfaces directly from sulfate based process streams or side streams of hydrometallurgical Ni metal plants. EDRR is truly attractive approach for the industrial solutions that contain only a small amount of platinum group metals (PGMs), especially as Pt/Ni nanoparticle formation consumes electricity only during the Ni deposition steps while Pt is \u201cenriched\u201d on the surface via an electroless redox replacement reaction, thus enhancing the process economics. It is also worth noting that as EDRR was successful from solutions with Ni/Pt ratio as high as 20,000, the industrial Ni process solutions and side-streams containing trace amounts of PGMs could potentially be \u201ca platinum mine\u201d for clean energy technologies if a future industrial process was developed. For example, in an average size base metal plant 10\u202fm3/h or even 100\u202fm3/h of such solutions are flowing in the processes, and with such amounts the large-scale production of catalysts \u2013 which has been identified as one of the main task of catalyst development (Debe, 2012) - can truly become possible.Real hydrometallurgical solutions also contain other metals than Ni and Pt and these will influence both the EDRR process and resultant surface. Previous results of Ag recovery from Zn/Ag solutions [Yliniemi et\u00a0al., 2018] have shown that although the presence of Fe3+ as in impurity may slightly reduce enrichment efficiency, it may improve product purity. This is most likely due to the selective dissolution of Zn by Fe3+ as the reduction potential of Fe3+/Fe2+ (E\u00b0\u202f=\u202f0.77\u202fV vs. SHE) is higher than that of Zn2+/Zn. Thus, both Fe3+ and Ag+ can oxidise Zn to Zn2+ but only Ag is enriched on the surface as Fe3+ is reduced to soluble Fe2+. Similar behaviour is expected in Ni/Pt solution and when considering performance, this may actually result in increased catalytic activity. Therefore, EDRR performed in hydrometallurgical solutions has huge potential for the recovery of precious metals like Ag (Halli et\u00a0al., 2017; Yliniemi et\u00a0al., 2018) and Au (Korolev et\u00a0al., 2018). Furthermore, this paper shows that EDRR can also directly produce functionalised surfaces with Pt or other trace metals present in hydrometallurgical solutions. Moreover, EDRR method allows control over not only particle size and density, but more importantly, over the precious metal/base metal ratio (here Pt/Ni) in the particles, i.e. EDRR provides also a control over the catalytic activity of these surfaces.The EDRR process outlined here is an effective method for the production of catalytic surfaces and simultaneously, exploiting the hydrometallurgical solutions fully by utilising also the minor components (such as Pt) present in them. Remarkably, the results presented here show a 10,000-time enrichment level of Pt onto the surface when Pt/Ni nanocatalysts are formed from solution simulating hydrometallurgical process streams on TiO2 surfaces by EDRR. It is also noteworthy, that such an enrichment is possible without any additional use of chemical or further modifications.By adjusting the different EDRR parameters (number of cycles or redox replacement time), the surface characteristics of the resultant catalytic nanoparticles can be tuned to control the desirable properties like nanoparticle size and distribution. Moreover, preliminary results of the produced Pt/Ni co-catalytic surfaces for photocatalytic H2 evolution demonstrated the level of performance that is comparable to the standard procedures for co-catalytic Pt/TiO2 surfaces found in literature.Overall, the findings offer a more sustainable circular economy platform, where minor concentrations of valuable metals present in base metal production solutions are used for the preparation of high-value products to be used as photocatalysts in clean energy sector.\nAcademy of Finland (NoWASTE - project no: 297962), Finland; Technology Industries of Finland Centennial/Jane and Aatos Erkko Foundation (Future Makers: Biorefinery Side Stream Materials for Advanced Biopolymer Materials - BioPolyMet), Finland; ERC, European Union; DFG (the Erlangen DFG cluster of excellence EAM, project EXC 315 (Bridge) and funCOS), Germany.The following is the supplementary data related to this article:Electrochemical \u2013 Redox replacement (EDRR) profiles during Pt/Ni nanoparticle formation (Fig.\u00a0S1), appearance of Pt/Ni nanoparticles (Fig.\u00a0S2) and photocatalytic activity for H2 generation by pure Ni nanoparticles on TiO2 nanotubes and Pt/Ni nanoparticles on TiO2 nanotubes (fresh and aged samples, Fig.\u00a0S3).\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data related to this article can be found at https://doi.org/10.1016/j.jclepro.2018.08.022.", "descript": "\n                  Solutions that simulate hydrometallurgical base metal process streams with high nickel (Ni) and minor platinum (Pt) concentrations were used to create Pt/Ni nanoparticles on TiO2 nanotube surfaces. For this, electrochemical deposition \u2013 redox replacement (EDRR) was used that also allowed to control the nanoparticle size, density and Pt/Ni content of the deposited nanoparticles. The Pt/Ni nanoparticle decorated titanium dioxide nanotubes (TiO2 nanotubes) become strongly activated for photocatalytic hydrogen (H2) evolution. Moreover, EDRR facilitates nanoparticle formation without the need for any additional chemicals and is more effective than electrodeposition alone. Actually, a 10,000-time enrichment level of Pt took place on the TiO2 surface when compared to Pt content in the solution with the EDRR method. The results show that hydrometallurgical streams offer great potential as an alternative raw material source for industrial catalyst production when coupled with redox replacement electrochemistry.\n               "}
{"full_text": "Data will be made available on request.Carbon dioxide (CO2) emissions to the atmosphere exhibit an economic burden and an environmental threat due to their significant contribution to climate change and global warming [1, 2]. In 2021, according to the International Energy Agency (IEA) the global CO2 emissions were projected to climb by 5% with a total of 1.5 billion tons. This would be the second-highest increase in history and the largest yearly increase in emissions since 2010 [3]. Consequently, different strategies including deploying carbon capture and storage (CCS) and carbon capture and utilization (CCU) were deployed to control the emissions of massive amounts of CO2 to the atmosphere and to replace depleted fossil fuels in the future. Reusing and recycling CO2 can contribute to decreasing the effects of global warming and the production of renewable fuels like methanol.The use of CCS has been a trending topic in industry and literature over the last three decades given the role of this technology in minimizing industrial CO2 emissions to the atmosphere [4\u20136]. The CCS is an environmental solution supporting the project\u2019s environmental sustainability rather than providing economic value. Consequently, carbon utilization technologies (CCU) have been studied for deployment within oil and gas infrastructures to sustain projects' environmental and economic importance. In CCU, captured and treated CO2 can be utilized for enhanced oil recovery to increase production or as a feedstock in different industries such as food, chemicals, and fuels. Hence, captured and treated CO2 can be either liquefied or compressed for direct selling to different sectors or utilized within the same plant by deploying CO2 monetization technologies to value-added products.Conversion of CO2 to methanol has been considered among the most favorable CO2 utilization processes in the industry in the last ten to fifteen years due to the maturity and stability of the investigated catalytic systems. [7\u201310]. Methanol is a liquid chemical that can be used as (i) a solvent; (ii) feedstock for producing chemicals such as acetic acid, methyl tert-butyl ether (MTBE), formaldehyde, and dimethyl ether (DME), or (iii) as a cleaner fuel in the transportation sector. In the transportation sector, pure methanol can be used directly as a marine or vehicle fuel, or blended with gasoline for vehicle utilization. Currently, Asia Pacific holds the largest market share of methanol due to the rapid increase in methanol consumption in the automotive, construction, and electronics industries. The global methanol market is projected to reach $26 billion by 2025, with a compound annual growth rate of 6.6% from 2019 to 2025 [11]. Traditionally, methanol has been produced from fossil fuels such as natural gas and coal. However, methanol production from captured CO2 is an emerging route aiming to assure a sustainable production of methanol after the depletion of fossil fuels and to support the efforts to control and mitigate CO2 emissions.The feasibility and efficiency of different chemical, electrochemical, and thermochemical reactors for methanol production have been studied in the literature in the past few years. For instance, Kim et al. [12] assessed the energy efficiency and economic feasibility of a solar-based process for the production of methanol from CO2 and water based on two-step routes. In the proposed process, the first step consists of a thermochemical reactor utilizing solar energy for converting CO2 to CO using a water gas shift reaction. Synthesis gas consisting of CO and H2 is then fed to a methanol catalytic reactor for methanol synthesis. The study concluded that the two-step solar-thermochemical pathway is a promising approach for CO2 utilization to fuels. However, the solar reactor sub-system is capital intensive, and much work must be done to improve the process from an economic perspective. On the other hand, Al-Kalbani et al. [13] modeled and compared the energy assessment of methanol production from CO2-based chemical vs electrochemical production processes. The authors reported that methanol production based on high-temperature CO2 electrolysis has double energy efficiency as CO2 hydrogenation utilizing H2 produced via water electrolysis. Nonetheless, from an economic perspective, CO2 hydrogenation to a methanol-based chemical route is found to be the most feasible solution.Methanol can be produced via catalytic CO2 hydrogenation using homogenous and heterogeneous catalyst systems wherein the reaction pathway mainly depends on the catalyst. To date, different research studies have investigated or developed the conversion of CO2 to methanol on a pilot scale using heterogeneous catalysts [14\u201317]. In 1996, the first commercial low-pressure methanol synthesis process was patented with process conditions below 150\u00a0bar and 300\u00a0\u00b0C using a Cu/Zn-based catalyst [18]. Since then, Cu/ZnO/Al2O3 catalysts have been widely used in industry and studied in the literature for methanol synthesis due to the superior advantages, where the promotor ZnO provides a high dispersion and stabilization level of Cu active sites and the metal oxide (Al2O3) provides support for the catalyst [19,20]; hence, contributing to enhancing methanol production reaction. Different industrially mature catalyst has been studied for CO2 hydrogenation to methanol from synthesis gas in the temperature and pressure ranges of 210\u2013250\u00a0\u00b0C and 50\u2013100 bars. In this regard, Van-Dal and Bouallou [21] designed and simulated a CO2 hydrogenation to methanol plant combined with a CO2 capture unit and hydrogen production unit using Aspen Plus software. In the CO2 hydrogenation section of the plant, Cu/ZnO/Al2O3 catalyst was used in the adiabatic reactor. The steam formed in the methanol synthesis unit was utilized as a CO2 capture unit. Atsonios et al. [22] investigated a techno-economic analysis of methanol production from CO2 hydrogenation using a membrane reactor. The study focused mainly on exploring the most economical operation conditions for captured CO2 utilization to methanol and revealed that hydrogen production costs largely influence the economic feasibility of the process. A thermo-economic approach for methanol production from different renewable sources was proposed by Rivarolo et al. [23]. The authors reported two plant configurations, for which CO2 is obtained from biogas or purchased from an external plant. Furthermore, the study mainly focused on electricity generation from renewable hydroelectric, wind, or photovoltaic plants and investigated the option of purchasing electricity if renewable resources are not available.Similarly, Bellotti et al. [8] reported a thermo-economic feasibility study of a power-to-fuel plant for methanol production from CO2. The studied system consists of a methanol production plant, a hydrogen production plant from water electrolysis, and an amine-based CO2 capture unit. The authors concluded that selling the by-product O2 is essential for economic feasibility results. However, neither this study nor the above-mentioned studies have considered the proposed plant's design and simulation. Other studies in the literature either focused on the reaction pathway and kinetics of CO2 hydrogenation to methanol over Cu/ZnO/Al2O3\n[24\u201328], or elaborated on catalyst preparation, catalyst formulation, and reaction mechanisms for CO2 hydrogenation to methanol over different catalysts [29\u201336]. To the best of the authors\u2019 knowledge, limited studies in the literature provided a comprehensive analysis of the CO2 hydrogenation process to methanol, wherein the majority of these studies focused on thermo-economic aspects of plant configuration or the feasibility of employing different technologies to support the feasibility of methanol production. Moreover, operating conditions such as temperature, pressure and H2/O2 feed ratio, and reactor types have not been intensively examined in previous studies. Consequently, The purpose of this work is to assess the technoeconomic-environmental feasibility of CO2 hydrogenation to the methanol process using the commercial catalyst, Cu/ZnO/Al2O3, with updated operating conditions for improved CO2 conversion. The methanol synthesis process is modeled and simulated using the commercial software Aspen Plus V11. Both isothermal and adiabatic reactors are studied for methanol synthesis under fixed feed conditions and catalyst specifications. Additionally, a sensitivity analysis is considered to investigate the influence of temperature, pressure, and variable hydrogen feed on the methanol yield. The optimized process is finally evaluated under environmental and economic aspects. In comparison with other studies that consider captured CO2 utilization for methanol production, this study emphasizes on the practicality and profitability of deploying CO2 to methanol monetization infrastructure within the biomass value chain for direct utilization of liquid pure CO2 by-product produced from a cryogenic biogas upgrade process. The proposed process could also be employed for the CO2 utilization from petrochemical processes where the raw materials (CO2 and H2) are available.The proposed process utilizes 76.46 kmol/hr of CO2 by-product produced from a cryogenic biogas upgrading process at 12.3\u00a0\u00b0C and 47.63\u00a0bar within the same plant, and 535.22 kmol/hr of hydrogen supplied at 25\u00a0\u00b0C and 30\u00a0bar. Depending on the plant capacity and purchase price of hydrogen, hydrogen can be supplied from a renewable source such as water electrolysis, or a fossil-fuel resource such as natural gas or coal. The deployment of a CO2 monetization process to value-added methanol within the biogas value chain achieves two main targets: (1) minimizing CO2 emissions, and (2) enhancing the economic performance of the biogas value chain. The proposed configuration of the full-scale biogas upgrading process is illustrated in Fig. 1\n. Although the proposed CO2 hydrogenation to methanol is connected to the biogas process, it could also be employed for the CO2 utilization from petrochemical processes where the required raw materials (CO2 and H2) are available.\nFig. 2\n presents the process used for CO2 hydrogenation to methanol. The process consists of a feed preparation section to meet the required conversion conditions; a reactor section where catalytic CO2 conversion takes place and a purification section to produce methanol with purity\u00a0\u2265\u00a098%. The methodology for modeling the methanol synthesis process was mainly inspired by Van-Dal and Bouallou [21]. For designing and simulating the CO2 hydrogenation process in Aspen Plus, Redlich-Kwong (RKS) equation of state can be optimally used to simulate the process kinetics as reported previously in the literature [37,38]. In contrast, other studies reported using different equations of states, such as Non-Random Two Liquid (NRTL) [7], or employed more than one equation of state, such as RKSMHV2 and NRTL-RK based on the stream pressure [21].The following is a thorough description of each of the sections in this process.In the first section of the conversion process, the feeds (CO2 and H2) are compressed to 78\u00a0bar, mixed, and heated to 210\u00a0\u00b0C to meet the reactor inlet specifications. An advantage of utilizing CO2 produced from the cryogenic biogas upgrade process is that the produced CO2 is at high pressure and does not require high power for compressing compared to other proposed processes in the literature where multi-stage compressors were used [21,37,39,40]. Additionally, the CO2 is supplied with high purity and does not require any treatment before feeding into the conversion process. If CO2 is supplied from other industries the treatment and compression work should be considered.In this section, CO2 is mixed with /H2 then compress to 75.7\u00a0bar and heated to 210\u00a0\u00b0C, and fed to the fixed bed plug flow reactor to produce methanol. Adiabatic and isothermal operating conditions were tested in this study. The reactor system contains 44,500\u00a0kg of Cu/ZnO/Al2O3 catalyst. The properties of the Cu/ZnO/Al2O3 are summarized in Table 1\n.Two parallel exothermic reactions Eq. (1) and (2) take place inside the reactor to produce methanol alone with an endothermic reverse-water gas shift (RWGS) reaction Eq. (3):\n\n(1)\n\n\nC\n\nO\n2\n\n\n\ng\n\n\n+\n3\n\nH\n2\n\n\n\ng\n\n\n\u21cc\nC\n\nH\n3\n\nO\nH\n\n\n\nl\n\n\n+\n\n\nH\n2\n\n\nO\ng\n\n\u0394\nH\n=\n-\n87\n\nK\nJ\n/\nm\no\nl\n\n(\n25\n\n\n\u00b0\n\nC\n)\n\n\n\n\n\n\n\n(2)\n\n\nC\nO\n\n\ng\n\n\n+\n2\n\nH\n2\n\n\n\ng\n\n\n\u21cc\nC\n\nH\n3\n\nO\nH\n\n\n\nl\n\n\n\u0394\nH\n=\n-\n128\n\nK\nJ\n/\nm\no\nl\n\n(\n25\n\n\n\u00b0\n\nC\n)\n\n\n\n\n\n\n\n(3)\n\n\n\n\nC\nO\n\n2\n\n\n\ng\n\n\n+\n\nH\n2\n\n\n\ng\n\n\n\u21cc\n\n\nC\nO\n\ng\n\n+\n\nH\n2\n\n\nO\ng\n\n\u0394\nH\n=\n41\n\nK\nJ\n/\nm\no\nl\n\n(\n25\n\n\n\u00b0\n\nC\n)\n\n\n\n\n\nIn the presence of the catalyst, the used kinetic model is based on Langmuir-Hinshelwood- Hougen- Waston (LHHW) mechanism and assumes CO2 as the primary source for methanol production in the presence of the RWGS reaction [41]. The kinetic model parameters were further modified by Mignard and Pritchard [42] to application ranges up to 75\u00a0bar as shown in Eq. (4) and (5), where the pressure is in bar and temperature in K. The kinetic constants, Eq. (6), follow the Arrhenius law, while the thermodynamic equilibrium constants, Eq. (7) and (8), are given by Graaf et al. [43]:\n\n(4)\n\n\n\nr\n\nC\n\nH\n3\n\nO\nH\n\n\n=\n\n\n\nk\n1\n\n\nP\n\nC\n\nO\n2\n\n\n\n\n\nP\n\nH\n2\n\n\n\n(\n1\n-\n\n1\n\nk\n\ne\nq\n2\n\n\n\n\n\n\nP\n\nH\n\n2\nO\n\n\n\n\nP\n\nC\n\nH\n\n3\nO\nH\n\n\n\n\n\n\n\nP\n\n\nH\n2\n\n\n3\n\n\nP\n\nC\n\nO\n2\n\n\n\n\n\n)\n\n\n\n\n\n\n1\n+\n\nk\n2\n\n\n\nP\n\nH\n2\nO\n\n\n\nP\n\nH\n2\n\n\n\n+\n\nk\n3\n\n\nP\n\n\nH\n2\n\n\n\n0.5\n\n\n+\n\nk\n4\n\n\nP\n\nH\n\n2\nO\n\n\n\n\n\n\n\n3\n\n\n\n\n\n\nm\no\nl\n\n\nk\n\ng\n\nc\na\nt\n\n\ns\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n(5)\n\n\n\nr\n\nR\nW\nG\nS\n\n\n=\n\n\n\nk\n5\n\n\nP\n\nC\n\nO\n2\n\n\n\n\n\n(\n1\n-\n\n1\n\nk\n\ne\nq\n1\n\n\n\n\n\n\nP\n\nH\n\n2\nO\n\n\n\n\nP\n\nC\nO\n\n\n\n\n\nP\n\nC\n\nO\n2\n\n\n\n\nP\n\nH\n2\n\n\n\n\n)\n\n\n\n(\n1\n+\n\nk\n2\n\n\n\nP\n\nH\n2\nO\n\n\n\nP\n\nH\n2\n\n\n\n+\n\nk\n3\n\n\nP\n\n\nH\n2\n\n\n\n0.5\n\n\n+\n\nk\n4\n\n\nP\n\nH\n\n2\nO\n\n\n\n\n)\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\nm\no\nl\n\n\nk\n\ng\n\nc\na\nt\n\n\ns\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n(6)\n\n\n\nk\ni\n\n=\n\nA\ni\n\nexp\n\n\n\n\nB\ni\n\n\nRT\n\n\n\n\n\n\n\n\n\n\n(7)\n\n\n\nlog\n10\n\n\n1\n\nK\n\ne\nq\n1\n\n\n\n=\n\n2073\nT\n\n+\n2.029\n\n\n\n\n\n\n(8)\n\n\n\nlog\n10\n\n\nK\n\ne\nq\n2\n\n\n=\n\n3066\nT\n\n-\n10.592\n\n\n\n\nThe equations(1 to 8) were rearranged in alignment with the type of accepted kinetic equations in Aspen Plus software and represented in equations (9 to 11). Table 2\n summarizes the model parameters used in the Aspen Plus software [21].\n\n(9)\n\n\n\nr\n\nC\n\nH\n3\n\nO\nH\n\n\n=\n\n\n\nk\n5\n\n\nP\n\nC\n\nO\n2\n\n\n\n-\n\nk\n6\n\n\nP\n\nH\n\n2\nO\n\n\n\n\nP\n\nC\n\nH\n3\n\nO\nH\n\n\n\nP\n\n\nH\n2\n\n\n\n-\n2\n\n\n\n\n\n\n\n1\n+\n\nk\n2\n\n\nP\n\n\nH\n2\n\nO\n\n\n\nP\n\n\nH\n2\n\n\n\n-\n1\n\n\n+\n\nk\n3\n\n\nP\n\n\nH\n2\n\n\n\n0.5\n\n\n+\n\nk\n4\n\n\nP\n\nH\n\n2\nO\n\n\n\n\n\n\n\n3\n\n\n\n\n\n\n\n\n\n\n\n\nm\no\nl\n\n\nk\n\ng\n\nc\na\nt\n\n\ns\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n(10)\n\n\n\nr\n\nR\nW\nG\nS\n\n\n=\n\n\n\nk\n5\n\n\nP\n\nC\n\nO\n2\n\n\n\n-\n\nk\n7\n\n\nP\n\nH\n\n2\nO\n\n\n\n\nP\n\nCO\n\n\n\nP\n\n\nH\n2\n\n\n\n-\n1\n\n\n\n\n1\n+\n\nk\n2\n\n\nP\n\nH\n\n2\nO\n\n\n\n\nP\n\n\nH\n2\n\n\n\n-\n1\n\n\n+\n\nk\n3\n\n\nP\n\n\nH\n2\n\n\n\n0.5\n\n\n+\n\nk\n4\n\n\nP\n\nH\n\n2\nO\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\nm\no\nl\n\n\nk\n\ng\n\nc\na\nt\n\n\ns\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n(11)\n\n\nln\n\nk\ni\n\n=\n\nA\ni\n\n+\n\n\nB\ni\n\nT\n\n\n\n\n\nAdditionally, a multi-tube reactor (# of tubes\u00a0=\u00a01000 tubes, length\u00a0=\u00a05\u00a0m, and diameter\u00a0=\u00a01\u00a0m) was considered. A pressure drop of 0.6\u00a0bar is allowed through the reactor, with an outlet stream leaving the reactor at 75\u00a0bar. Exact specifications are applied for the isothermal reactor, with a constant reactor operating temperature of 210\u00a0\u00b0C.Gases leaving the reaction were collected in the knock-out drum (KO101) to separate the products from unreacted reactants. Unreacted gases are recycled back to the reactor to enhance conversion and part of it is purged to the atmosphere at a split fraction of 0.1 to avoid by-product accumulation. The produced liquid methanol leaves the knock-out drum at 73.4\u00a0bar. Two parallel valves are used to reduce its pressure down to 1.2\u00a0bar before entering the flash drum (FLT101) for further purifications and removal of unreacted gases. The outlet liquid methanol from the flash drum at 1.2\u00a0bar and 14.95\u00a0\u00b0C is heated up to 80\u00a0\u00b0C before sending it to the distillation column (D101) for methanol/water separation. A distillation column with 15 stages and a reflux ratio of 2.12 is utilized to produce a high-grade methanol product at 64.92\u00a0\u00b0C and 1\u00a0bar, and water by-product at 101.91\u00a0\u00b0C and 1\u00a0bar. No pressure drop is assumed across the distillation column. The produced methanol is then compressed to 80.29\u00a0bar, heated to 80.29\u00a0\u00b0C, and fed to a final knock-drum (KO102) to increase the purity of the methanol to 99.41\u00a0mol%.In this process, all compressors are isentropic and operate at 72% efficiency. Moreover, a stream pressure drop between 0.1 and 2.3\u00a0bar is allowed in the heat exchangers. Table 3\n presents the main specifications of the designed methanol synthesis process.\nFig. 3\n illustrates the simulated CO2 to methanol process using Aspen Plus. The proposed simulated process utilizes high-pressure pure CO2 obtained from a cryogenic biogas separation unit within the same plant. As justified by Rivarolo et al. [23], we are utilizing CO2 produced from biogas, which offers more excellent economic performance and a more straightforward plant layout. The use of pure and high-pressure CO2 for methanol production reduces the project's overall costs due to: (1) the employment of a single CO2 compressor, and (2) limited need for CO2 collection and purification devices. Hydrogen was assumed to be purchased from a local market to supply the process.The hydrogenation of CO2 to methanol takes place in the catalytic reactor (R101). The reactor uses a commercial Cu/ZnO/Al2O3 as a catalyst. Consequently, designing and modeling the feed preparation section and the separation section mainly depend on the reactor\u2019s feed specifications and the composition of the outlet stream. The results revealed that feeding the reactor with CO2/H2 mixture at 210\u00a0\u00b0C and 75\u00a0bar achieved CO2 conversion of 99% and a methanol yield of 98%. The productivity of methanol and the conversion of CO2 are enhanced by recycling part of the unreacted gas mixture. This is in good agreement with the results reported by Leonzio et al., [44]. Although the recycled stream of gases contains CO, at lower feed gas temperatures, methanol synthesis from CO2/H2 is faster than CO/H2\n[45]. Based on Skrzypek et al. [24] who studied methanol synthesis kinetics over Cu/ZnO/Al2O3 catalyst in a high-pressure fixed bed plug flow reactor, the authors concluded that the surface reaction between CO2 and H2 is the rate-controlling step. The authors further reported that the selectivity is higher for a feed that consists of only CO2 and H2 without any CO. This reveals that CO2 is the primary source for methanol synthesis in the process [24,46]. It was observed that increasing the feed pressure up to 75.7\u00a0bar significantly improved methanol production and achieved overall CO2 conversion\u00a0\u2265\u00a099%. This was aimed at favorable operating conditions of the forward reaction following Chatelier's principle. Kiss et al. [7] simulated the process at 50\u00a0bar, which resulted in 100% process conversion using Cu/Zn/Al/Zr catalyst, while Atsonios et al. [22] simulated the process at 65\u00a0bar, using a membrane reactor and Cu/ZnO/Al2O3 catalyst, which resulted in 30.5% CO2 conversion. Table 4\n summarizes the main specifications of the inlet and outlet streams of the proposed methanol simulation process.Additionally, Table 5\n compares the CO2 conversions and methanol yields and/or selectivity of the processes reported by different authors in the literature using adiabatic/isothermal fixed bed flow reactors packed with Cu/Zn or Cu/ZnO2 based catalysts. As indicated in Table 5, the current proposed process's reported CO2 conversion and methanol yield are higher than other studies and relatively close to the results reported by Kiss et al. [7] carried out in an isothermal plug flow reactor operated at 50\u00a0bar and 250\u00a0\u00b0C. Nevertheless, other authors used the fibrous Cu/Zn/Al/Zr catalyst rather than the industrially mature Cu/ZnO/Al2O3 catalyst employed in this study. The obtained results highlight the importance of additional research to demonstrate the impact of reactor type on process conversion and to determine the optimal reactor configuration and operating conditions.Different studies studied the conversion of CO2 to methanol in adiabatic and isothermal reaction system [7,21,47]. Consequently, the productivity and the conversion of CO2 in both reactor types were explored and simulated in this study. Tests were performed at 210\u00a0\u00b0C and 75.8\u00a0bar using same previous flow rate and catalyst loading. The obtained results are illustrated in Table 6\n. The adiabatic reactor produced slightly less CO2 but had a higher methanol yield and selectivity. The required heat duty and residence time differed significantly between the two reactors. It was observed that the residence of the adiabatic reactor is 50% less than the isothermal reactor. Confirming that the adiabatic reactor system has more favorable operating conditions. Therefore, the adiabatic reactor was subjected to further analysis to understand the effect of operating conditions (Temperature and pressure) and the molar flow rate of H2 on methanol production and CO2 conversion. It was proved that methanol production is independent of the reactor temperature at various reactor temperatures due to the low activation energy, which was almost zero. Hence, the changing temperature in the Arrhenius equation does not influence the reaction kinetics.The influence of changing adiabatic reactor pressure and H2 molar flow rate on the methanol production is presented in Fig. 4\na and b. Results indicated that increasing the adiabatic reactor pressure is directly proportional to methanol conversion. Maximum pressure of 75\u00a0bar can be set as the operating pressure due to kinetic limitations. Moreover, increasing the H2 feed flow rate resulted in higher methanol yield, where a CO2/H2 ratio of 1:7 was optimally selected for the study. As can be seen in Fig. 4 (b), increasing the H2 flow rate beyond 535.22 kmol/hr does not significantly influence the yield, wherein expanding the molar flow rate by 34.6% would only enhance methanol yield by 4.5%.On the other hand, changing the temperature and/or the pressure of the feed disturbs the flash specifications. Hence optimal feed specifications of 210\u00a0\u00b0C and 75.8\u00a0bar were selected to achieve a CO2 conversion of 95.66% in an adiabatic reactor, which is relatively lower than the reported CO2 conversion in the literature. Further optimization of the overall process has been studied to investigate the optimal configuration.The presented process involves different endothermic and exothermic units where heat integration is crucial for improved process efficiency. The pinch analysis method proposed by Linnhoff and Hindmarsh [48] was considered for designing an optimal heat exchanger network (HEN) to (1) improve the overall process energy efficiency; (2) minimize the operational costs and utility consumption, and (3) minimize indirect CO2 emissions due to reduction of fuel consumption for steam generation. The commercial software Aspen Energy Analyzer V11 was used to conduct the pinch analysis where a minimum temperature difference (\n\u0394\n Tmin) of 5\u00a0\u00b0C was selected. The main trade-off when considering a low \n\u0394\n Tmin in pinch analysis is between energy/external utility reduction and increased capital costs due to extra heat exchangers. A low \n\u0394\n Tmin decreases utility costs but increases the capital costs for installing additional units.Consequently, the payback time on capital investment was also considered for evaluating the optimal heat integration scenario [49]. The optimal results from the Aspen Energy analyzer were then transferred to the primary Aspen Plus simulation for an updated process flow diagram after heat integration, illustrated in Fig. 5\n. The implementation of heat integration resulted in 63.19% energy savings after introducing three additional units, RE101, RE102, and RE103. A comparison of the required external utility requirement under optimized HEN vs total utility requirement in the absence of optimized HEN is illustrated in Fig. 6\n. Results after heat integration imply that external hot utilities were reduced from 3.13 to 0.014 GW achieving more than 99.55% of saving. In addition, the external cooling utilities requirements were reduced from 3.62 to 2.11 GW achieving around 41.7% saving. Consequently, deploying an optimized methanol production will significantly reduce the costs associated with utilities purchasing and/or generation.The utilization of high-pressure pure CO2 in the proposed process contributes to reducing both capital costs (Capex) and operating costs (Opex) compared to other models reported in the literature. Under optimized conditions, it was noticed that the proposed process required pure H2 at a pressure of 30\u00a0bar and a total specific power of 214 kWh/tMeOH to produce methanol with a purity of 99.41\u00a0mol%. This is economically feasible if compared with an operating pressure of 65\u00a0bar and specific power consumption of 113 kWh/tMeOH to produce methanol with a purity of 99.3\u00a0mol% methanol as reported by Atsonios et al. [50]. The reported value did not consider the power requirement for CO2 feed preparation as it was considered as part of the CO2 capture and treatment unit in the plant.Optimizing the CO2 conversion reduced total utility requirements by 63%, indicating that this process has the potential to generate additional revenue. The plant's economic viability depends on different factors, including Capex, utility costs, electricity costs, the project's lifetime, CO2 taxes in some countries, and methanol price in international markets. The net present value (NPV), equation (12), was used to assist the economy of the methanol production process and determine its profitability:\n\n(12)\n\n\nNPV\n=\n\n\u2211\n\nt\n=\n1\n\nn\n\n\n\nC\n\nF\nt\n\n\n\n\n\n\n1\n+\ni\n\n\n\nt\n\n\n\n\n\nwhere CF represents annual cash flow at any time (t); n is the service life of the project and i is the rate of return on the investment. The profitability of the methanol production process was based on a plant service life of 20\u00a0years and a rate of return on investment of 8%. The Capex of the process was determined using the step counting method following the procedure established by Timm\u2019s correlation for similar gas processes [51]:\n\n(13)\n\n\nCapex\n=\n13000\nN\n\n\nQ\n\n\n0.615\n\n\n\n\n\nwhere Capex is in US Dollars for 1998, N is the number of significant processing units, and Q is the annual plant capacity in metric tons (mt). When counting the significant processing units, only reactors, distillation columns, and compressors are considered to have substantial costs [52]. Moreover, since Timm\u2019s correlation results in Capex were conducted in 1998, cost indices were used to adjust the Capex value to the year 2021 [53].Both fixed and variable Opex must be addressed when estimating the Opex. In this analysis, fixed operating and maintenance costs were taken as 1.04% of Capex assumed previously by Bellotti et al., [8]. On the other hand, variable Opex relays on the production capacity, utility requirement, and fuel and electricity costs for running equipment and generating utilities. All compressors are electrically driven and purchased from an external local supplier in Qatar at a $0.036/kWh [54]. The steam generation total cost ($/lbsteam) was calculated using Eq. (14)\n[52]:\n\n(14)\n\n\nS\nt\ne\na\nm\n\nc\no\ns\nt\n=\nF\nu\ne\nl\n\nP\nr\ni\nc\ne\n\n\u00d7\n\n\n\nH\ne\na\nt\ni\nn\ng\n\nr\na\nt\ne\n\n\n\nB\no\ni\nl\ne\nr\n\ne\nf\nf\ni\nc\ni\ne\nn\nc\ny\n\n\n\n\n\nwhere fuel price was taken as a fixed average monthly Henry Hub natural gas price in 2021 of $3.62/MMBTU; the heating rate is the amount of energy needed to heat feed water to saturated low or high-pressure steam in (Btu/ lbsteam) [55], and boilers efficiency of a fixed value of 85.7% was considered [52,56].For cold utilities, sea water and chilled water were considered for cooling process streams down to 35 and 15\u00a0\u00b0C, respectively. However, the costs of raw water, makeup water, condensate return, water treatment, and power for pumping cooling water were not analyzed. Further detailed calculations can be considered when assessing the plant on a tactical level of project planning. For example, seawater can be used to cool down process streams down to 35\u00a0\u00b0C, and chilled water can be used to cool down the process stream (S116) entering FT101 to 15\u00a0\u00b0C.\nThe revenues of the proposed CO2 conversion plant are based on selling the produced liquid methanol at an average price of $692/mt [57]. The process profitability was supported by the availability of CO2 from nearby cryogenic biogas or petrochemical processes and the presence of gray H2. This latter is produced from steam methane reforming in the Middle East and supplied for $0.9/kg [58]. The NPV and payback period of the methanol plant with a production capacity of 23.4 kt/yr was determined to be $6.5 million and nine years, respectively based on 20\u00a0years of service life.It is difficult to identify the most competitive methanol production scheme. The economic performance depends on the electricity and/or fuel prices, hydrogen costs, and methanol selling price in international markets. Under fixed Opex and hydrogen supply costs, the profitability of the investment in CO2 hydrogenation to methanol process was investigated based on different methanol production capacities: 23.4 kt/yr, 33.6 kt/yr, and 44.8kt/yr for the project\u2019s lifetime of 20 and 25\u00a0years. As shown in Fig. 7\n, increasing the production capacity up to 3.36 kt/yr and 4.48 kt/yr for a project\u2019s lifetime of 20\u00a0years results in enhancing the NPV of the project by 147% and 411%, respectively. The NPV is further enhanced for all production scenarios when extending the project\u2019s lifetime by five additional years. This reflects the economic attractiveness of deploying a bio-methanol process due to the low Opex and the availability of affordable gray H2 supplied from local steam gas reforming processes. It is worth observing that due to the high requirement of H2 to satisfy the CO2/H2 ratio of 1:7 in the process, a maximum H2 supply price of $0.97/kg is required to break even the NPV for a 20-year project with annual methanol production of 2.34 kt/yr. Consequently, supplying renewable hydrogen from PEM electrolysis at a price between $4.2/kg and $5.2/kg will be economically infeasible for the proposed CO2 hydrogenation to methanol process [58].As shown in the proposed bio-methanol production process, CO2 emissions can be released into the atmosphere directly from the main process equipment or indirectly due to burning fuel for generating thermal energy and/or electricity (in case generated locally). The process releases three streams to the atmosphere containing CO2 for direct emissions: PUR-S111, S-117, and S-125. After heat integration, only burning fuel for generating low-pressure steam contributes to indirect CO2 emissions to the atmosphere. Consequently, the indirect emissions are mainly influenced by the fuel needed to generate steam utilized in the process. According to the US Energy Information Administration [59], burning 1 MMBtu of natural gas emits around 117\u00a0lb of CO2. A comparison between direct and indirect CO2 emissions before and after heat integration is presented in Fig. 8\n. The figure shows that the indirect emissions were reduced by around 98% after conducting heat integration since the process streams provided sufficient duty for the heaters.Additionally, when considering the optimized process, the estimated direct and indirect emissions are 98.6% and 96%, respectively, less than the values reported by [37], who reported direct emissions of 0.090 tCO2/tMeOH and indirect emissions of 0.136 tCO2/tMeOH for a European bio-methanol plant. The reduction in emissions is mainly attributed to the requirement of a smaller number of compressors and heat exchangers in this process for preparing CO2 feed to feed specifications. This reflects the added value of incorporating a CO2 to methanol process within the biomass supply chain for sustainable bio-methanol production.In the presented process, high-pressure liquid and pure CO2 were considered for methanol utilization. CO2 captured from flue gases is a potential feed that will not impact the thermodynamic properties of the primary catalytic conversion process. However, additional CO2 capture, treatment, and compression units will be needed to meet feed specifications. Hence, overall plant design and economic feasibility should be investigated. On the other hand, it is worth mentioning that different production routes can be utilized for CO2 to methanol production, including CO2 electrochemical reduction to methanol and two-step CO2 catalytic conversion to methanol. In the latter route, CO2 is first converted to CO, and CO is then hydrogenated to methanol in the second step. Both processes are still industrially immature and require further development. Moreover, despite the maturity of the CO2 hydrogenation to methanol technology, catalyst development is still a dynamic area of research where other studies investigated the utilization of Ni/Ga [33,60], ZnO/ZrO2\n[61], and InOx/ZrO2\n[62] catalysts. The studied catalysts are still under development and have not reached the stage of industrial commercialization yet.A state of art of catalytic conversion of CO2 to methanol is presented in this study. The economic and environmental feasibility of the proposed process under optimized operating conditions was explored. In comparison with previous studies, the assessed process involves less equipment due to the utilization of high-pressure and pure liquid CO2, produced or generated from a former cryogenic biogas separation process or petrochemical industries. Optimized CO2/H2 feed ratio of 1:7 to achieve an overall CO2 process conversion of 99% and methanol yield\u00a0\u2265\u00a099%. Simulation results indicated better performance for the adiabatic reactor than the isothermal reactor, with a reduced residence time of 48.46% and operating conditions of 210\u00a0\u00b0C. Overall energy efficiency was further improved by lowering external utilities by 63% after using the heat integration approach. Similar to the economic evaluation, which concluded that the process profitability is highly dependent on H2 supply price, in this analysis, the financial assessment demonstrated the requirement of a maximum H2 supply price of $0.97/kg to break even the NPV for a 20-year project lifetime in the Middle East with annual methanol production of 2.34 kt/yr. From an environmental perspective, the optimized process successfully contributes to reducing total CO2 emissions by 97.8% compared to the baseline process configuration. The catalyst Cu/ZnO/Al2O3 showed excellent efficiency for the industrial commercialization of CO2 hydrogenation to methanol. Future research on process configuration and simulation could involve testing the efficiency and stability of different novel Cu, Pd, or Zn-based catalysts for CO2 hydrogenation to methanol under varied operating conditions and CO2/H2 feed ratios.\nNoor Yusuf: Methodology, Software, Data curation, Writing \u2013 review & editing, Writing \u2013 original draft. Fares Almomani: Resources, 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.We would like to acknowledge the fund provided by QAFCO R&D grant # QUEX-CENG-QAFCO-20/21-1. The statements made herein are solely the responsibility of the authors. Open Access funding provided by the Qatar National Library.", "descript": "\n                  The hydrogenation of CO2 to methanol is one of the promising CO2 utilization routes in the industry that can contribute to emissions mitigation. In this work, improved operating conditions were reported for the sustainable catalytic hydrogenation of CO2 to methanol using Cu/ZnO/Al2O3 catalyst operated at 70\u00a0bar and 210\u00a0\u00b0C. The CO2 feedstock used for this process is pure CO2 produced from the cryogenic upgrading process of biogas or hydrocarbon industries and ready-to-use hydrogen purchased at 30\u00a0bar and 25\u00a0\u00b0C. The process was modeled and simulated using the commercial Aspen Plus software to produce methanol with a purity greater than 99% at 1\u00a0bar and 25\u00a0\u00b0C. The simulation results revealed that an adiabatic reactor operated with a CO2/H2 ratio of 1:7 produces methanol with a yield\u00a0\u226599.84% and a CO2 conversion of 95.66%. Optimizing the heat exchanger network (HEN) achieved energy savings of 63% and reduced total direct and indirect CO2 emissions by 97.8%. The proposed methanol process with an annual production rate of 2.34 kt/yr is economically sound with a payback period of nine years if the maximum H2 price remains below $0.97/kg. Hence, producing or purchasing gray H2 from a steam reforming plant is the most viable economic source for the process.\n               "}
{"full_text": "Since the 1970s societal focus on air quality and pollution control has introduced legislation requiring a continuous lowering of the sulfur content in transportation fuels implying that almost all crude oil (ca. 2500 million tons annually) is to be hydrotreated. In most of the world the maximum sulfur content in gasoline and diesel for road use is now 10\u00a0ppm, corresponding to a removal of 99.99% of the sulfur originally present. Hydrodesulfurization is the industrial catalytic process by which sulfur is removed from organosulfur molecules in mineral oil as H2S at high hydrogen pressure and temperature (250\u2013400\u00a0\u00b0C). The hydrodesulfurization process is sometimes more generally denoted hydrotreating as also organonitrogen compounds, aromatics and unsaturated carbon molecules in the oil undergo simultaneous conversion and/or are hydrogenated [1\u20133]. Over the last 40\u00a0years increasing severity of legislation has called for intense development of industrial hydrodesulfurization processes and catalysts. The main catalytic challenge for production of today\u2019s ultra-low sulfur diesel (<10\u00a0ppm sulfur) is the conversion of refractory alkyl-substituted dibenzothiophenes. These are molecules such as 4,6-dimethyldibenzothiophene (4,6-DMDBT) [4] that have alkyl substituents located in positions next to the sulfur atom and where the alkyl groups thus limit access to the sulfur atom.Dr. Henrik Tops\u00f8e (1944\u20132019) [5] was a key figure in the research that established a detailed fundamental understanding of the nature of the industrially employed hydrodesulfurization catalyst, its active site(s), the molecular reaction routes and inhibition phenomena under ultra-deep hydrodesulfurization. Many of his groundbreaking and seminal contributions will be mentioned below including the discovery of the so-called Co-Mo-S structure that is now a commonly accepted model for the working hydrotreating catalyst. Characteristic to his work was a continuous employment of new and more sophisticated experimental methods and the combined use of results from multiple experimental and theoretical approaches, all with the aim to steadily uncover the secrets of hydrotreating catalysis. At Haldor Topsoe A/S, Henrik pursued the fundamental research in close interplay with industrial developments and fostered a synergy for implementation of the findings in industrial practice as visualized in Fig. 1\n.In this paper, we will honor Henrik and his enormous impact on research and development in hydrotreating catalysis and acknowledge his unique generosity, mildness, humor, commitment and intellect, which made him a fantastic colleague and inspirational supervisor/mentor. The paper will present new findings for advancing hydrodesulfurization and thus lie at the heart of Henrik\u2019s interests. Although Henrik was not part of the present work, it testifies that the tradition of research and methodologies that he established in the laboratories of Haldor Topsoe A/S are deeply embedded and form the basis for continuous developments of even rather mature catalytic systems.Sweetening of gasoline was in the first part of the 1900s undertaken by non-catalytic processes involving absorption or conversion of the odor-repulsive mercaptans by means of agents such as metallic copper or lead sulfide. Subsequently conversion of organosulfur compounds was claimed using molybdenum-sulfur or tungsten-sulfur materials as catalysts, but actual catalysis was not documented before 1943 when it was discovered that the bimetallic combination cobalt-molybdenum had an extraordinary ability for catalytic hydrodesulfurization [6]. In the context of hydrotreating, cobalt is traditionally termed a promoter (to molybdenum) but as the boost in activity is a factor of 10\u201320 this terminology is somewhat misleading. The 1943 paper also examined the combinations iron-molybdenum, copper-molybdenum, zinc-molybdenum and aluminum-molybdenum to find that they had much inferior catalytic performance. Soon it was found that the combination nickel-molybdenum also had high catalytic activity and this combination is together with cobalt-molybdenum universally employed in industrial hydrotreating today. The chemical state of the catalytically active metals at process conditions was known to be at least partially sulfidic and in some early models sulfidic Mo was bonded via oxygen to the surface of the high-surface area alumina support employed. It was eventually realized that Mo was in a chemical state like MoS2, i.e., with no or very limited amounts of bonds to alumina surface. The precise role of the promoter element cobalt (also itself in a sulfidic state) was yet unresolved. Two models were considered [1]. The first, a contact synergy model in which separate Co9S8 and MoS2 nanostructures together were responsible for the hydrotreating catalysis with a spill-over of activated hydrogen. The second, an intercalation model in which the promoting Co atoms were located in the van der Waals gap between the layers of the layered MoS2 structure, similarly to how layered MoS2 structures may intercalate e.g., lithium ions. In 1976 a team led by Henrik Tops\u00f8e found by in situ M\u00f6ssbauer Emission Spectroscopy (MES) that cobalt in sulfidic cobalt-molybdenum catalysts occurred in 3 different forms [7]: (i) Co9S8, (ii) CoAl2O4 and (iii) a new form distinctly different from the two-former mentioned. Both Co9S8 and CoAl2O4 may sometimes be present in catalysts but neither of the compounds contribute in any significant extent to the catalytic activity [8\u201310]. On the other hand, a clear correlation between the size of the MES cobalt signal and the catalytic activity within a series of cobalt-molybdenum catalysts made it possible to assign the catalytically active center to what was named the Co-Mo-S structure [8,9]. It was in 1981 found by Extended X-ray Absorption Fine Structure (EXAFS) [11] that Mo was present as MoS2-like nanodomains and the so-called Co-Mo-S model was proposed which has Co atoms located on the edges of the layered MoS2 structure. A high affinity of Co for the edges of MoS2 has since been confirmed by Scanning Tunneling Microscopy (STM) [12] of Co-Mo-S structures synthesized on Au(111) and by Scanning Transmission Electron Microscopy (STEM) [13] of carbon supported Co-Mo-S synthesized by metals impregnation and subsequent gas phase sulfidation. In parallel with the technical progress in atomic resolution imaging, Density Functional Theory (DFT) methods evolved and have now been able to elucidate the catalytic mechanisms taking place at active sites located on the edges of Co-Mo-S nanocrystals [14\u201318].Generally, two hydrodesulfurization pathways are in play for HDS of dibenzothiophenes, the direct desulfurization (DDS) and the pre-hydrogenation (HYD) pathways [19]. By the DDS pathway, direct cleavage of the C\u2014S bond(s) in the dibenzothiophene molecule takes place and with a minimum amount of H2 consumed. By the HYD pathway, the dibenzothiophene molecule undergoes an initial hydrogenation in one of its six-rings followed by cleavage of C\u2014S bond(s). In this way the HYD pathway consumes more H2 than the DDS pathway. Whereas molecules such as thiophene and dibenzothiophene are primarily hydrodesulfurized by a DDS pathway then HDS of sterically hindered dibenzothiophenes, such as 4,6-DMDBT, proceeds mainly via a HYD pathway in the model feed studies required to detect and quantify the individual desulfurized product molecules [20\u201322]. The detailed reaction mechanisms are still debated. Following the DDS pathway, the current view of the catalytic cycle is that sulfur vacancies are created on cobalt atoms (associated with \n\n\n1\n\u00af\n\n00\n\n S-terminated edges of MoS2) by formation of H2S by means of atomic hydrogen formed by activation of H2 at the metallic so-called brim sites at the edge of the MoS2 structure [23\u201325]. The sulfur atom of an organosulfur molecule attaches to a cobalt atom at a coordinative undersaturated site (CUS) such as a sulfur vacancy and after transferal of more activated hydrogen atoms generated at the nearby brim sites the carbon part of the organosulfur molecule dissociates into a main hydrocarbon part and a single sulfur atom. Molecular hydrodesulfurization has now taken place and subsequently sulfur atom removal as H2S by means of yet more activated hydrogen from the brim sites takes place to close the cycle. STM imaging have revealed organosulfur molecules associated with edge sites for sufficiently long time to capture an STM image, thus providing unprecedented insights and direct visualization of the important first adsorption step in the catalytic process [26\u201328].Experimental microscopy techniques and DFT calculations give, for Co and Ni promoted MoS2, a mutually consistent image of the nature of the active sites on the MoS2 edges. No other first transition period metals have experimentally to the same substantial degree been found to promote the hydrodesulfurization reaction. For instance, an STM-based study [29] has shown that all four metals Co, Ni, Cu and Zn associate to MoS2 edges in identical ways, i.e., association to the edge is a necessary but not sufficient prerequisite for promoting the catalysis. As Cu and Zn do not have any promoting effect [29], the electronic structure of the potentially promoting element clearly plays a role and only for Co and Ni do the electronic structure seem suited for hydrodesulfurization catalysis. DFT calculations have shown that when Fe is placed on an MoS2 edge in the very same way as Co, then the Fe-Mo-S system is characterized by sulfur binding parameters that suggests low catalytic activity [30]. Going to the second and third transitions periods, Rh-Mo and Ir-Mo catalysts have experimentally [31\u201337] shown substantially higher activities than a Mo-only catalyst, i.e., the two elements Rh and Ir promote to a comparable extend to their first transition period counterpart Co. Oppositely, the element combinations Ru-Mo, Pd-Mo and Pt-Mo did not provide catalysts with a substantially enhanced HDS activity compared to a Mo-only reference [37\u201340]. All in all, only Co, Ni, Rh and Ir have been found to substantially promote MoS2.In industrial practice, a particular hydrotreating reactor is designed with a certain size and maximum pressure. To get the best performance, the refiner will choose a catalyst by considering both the specific oil feed (nature of crude, boiling point range) and the achievable hydrogen amount and pressure. In this context Co-Mo is typically the preferred catalyst for relatively low hydrogen partial pressures (<40\u00a0bar) whereas Ni-Mo is preferred for high pressure (>60\u00a0bar) applications. This has, over the years, led to many catalyst producing companies launching all-in-one catalysts that contain both Co and Ni as promoting elements. Although such element combinations may indeed at certain operating conditions give moderate advantages in terms of catalyst performance, it is certainly not the case that the combination provides the refiner with the very best of both Co and Ni. Rather, these Co-Ni-Mo catalysts have performance characteristics that are probably closer to a kind of average of the Co and Ni catalyst performances, i.e., the performance you would get from loading a physical mixture of Co-Mo and Ni-Mo catalysts. Bimetallic promotion of MoS2 with Ni-Cu has also been examined but with no great industrial breakthrough [41].The complexity of the hydrodesulfurization reaction mechanisms as well as the structural complexity of the bimetallic transition metal sulfide nanostructures have hampered a detailed understanding of all empirical findings of promotional effects. However, with the toolbox at hand today and with the multidisciplinary approach that Henrik was so dedicated to, we will in this paper explore a metal combination Co-Pt that has had very limited attention [42]. Specifically, we will demonstrate how, in our hands, highly activity-enhanced Pt-Co-Mo-S nanocrystals can be prepared by slight modifications of the otherwise standard Co-Mo-S catalyst and we will present an atomistic picture for the synergetic role of this tertiary transition metal sulfide catalyst system.Pt-containing Co-Mo catalysts were prepared in two different ways: (i) by post impregnation of Pt onto a commercial (unsulfided) Co-Mo/Al2O3 catalyst (16\u00a0wt% Mo and 3.5\u00a0wt% Co) and (ii) to illustrate the feasibility of a more industrially acceptable preparation route, by incipient wetness impregnation of alumina carrier extrudates with a Co-Mo liquor into which Pt had been incorporated. For the former preparation method [Pt(acac)2] (acac\u00a0=\u00a0acetylacetonate), corresponding to the desired wt% of Pt on the final catalyst, was dissolved in a volume of dichloromethane (DCM) corresponding to the pore volume, as determined by mercury porosimetry, of the oxidic Co-Mo catalyst to be modified. After 30\u00a0min impregnation time, the sample was air dried at room temperature for 30\u00a0min followed by drying at 250\u00a0\u00b0C in air for 1\u00a0h. For the latter preparation method, [Pt(NH3)4](HCO3)2 was dissolved into a commercial Co-Mo liquor, followed by pore volume filling impregnation of an alumina carrier. This also yielded catalysts with 16\u00a0wt% Mo and 3.5\u00a0wt% Co. After preparation the catalysts obtained by one or the other method differed neither in chemical properties nor activity within the uncertainty of the activity measurements. Samples with a specific nominal loading of Pt are in the following referred to as Pt-Co-Mo(wt%).The catalyst samples were tested in a 5-in-1 pilot unit in which 5 reactors are placed closely together thus experiencing the same thermal conditions. The reactors were loaded with a mixture of 15.0\u00a0mL neat catalyst volume (whole extrudates) and 40% fine inert SiC particles. In one 5-in-1 test 4 platinum-modified experimental catalysts were tested together with the commercial reference sample that was used as basis for the platinum impregnations. After loading of the reactors, the catalysts were sulfided for 24\u00a0h at test condition using a sulfur doped oil. After the sulfidation procedure a switch to the test feed was made and the pilot unit was then run for 110\u00a0h at steady conditions. Samples were collected simultaneously from all 5 reactors at run hours 80, 90, 100 and 110. The activities reported are based on the average sulfur level of the 4 samples collected. Sulfur-content variation between the 4 samples obtained at different run hours was minimal. The activities have been calculated using our internal kinetic model to obtain relative rate constants with the industrial reference catalyst used for the impregnation experiments defining activity of 100%. Test conditions: 355\u00a0\u00b0C, 30 barg H2, 1.5 LHSV, 490 H2/oil, 75/25 w/w blend of straight run diesel (LG) and cracked feed (LC). Feed properties: 1.22\u00a0wt% S, 356 wtppm N, SG 60/60 0.8735, calc. D86: 237\u00a0\u00b0C (10\u00a0vol%), 297\u00a0\u00b0C (50\u00a0vol%), 358\u00a0\u00b0C (90\u00a0vol%). The tested Pt-Co-Mo catalysts were collected from the reactor after cooling to RT. Then the samples were sieved and rinsed several times with an excess of pentane to wash away oil residues inside and outside the porous extrudates. All volatiles were removed by drying the sample at 40\u00a0\u00b0C in vacuum and the samples were subsequently stored in closed containers until characterization by electron microscopy or X-ray absorption was undertaken.Electron microscopy examinations were performed using an FEI Talos F200X (scanning) transmission electron microscope equipped with ultra-bright field emission gun (X-FEG) and Super-X EDX detectors. The microscope was operated at 200\u00a0keV in both scanning-beam and broad-beam modes. In the scanning-mode, a high-angle annular dark field (HAADF) detector was employed for imaging concurrently with EDX spectrum acquisition. In the broad-beam mode, a charge-coupled device camera was used for imaging. The combined HAADF and EDX data cube was acquired for 30\u00a0min over an area of 1024 pixels\u00a0\u00d7\u00a01024 pixels (0.26\u00a0nm pxl\u22121) with a probe current of 0.7 nA and EDX energy range of 0\u201320\u00a0keV (0.01\u00a0keV/channel). Samples for electron microscopy were prepared by crushing the catalyst pellets in a mortar and dispersing the fine powder onto a Cu-TEM grid with continuous carbon (SPI Supplies) in ambient conditions. Thus, the samples have been exposed to ambient conditions for a few days during transportation and storage before examination in the electron microscope.To increase the counting statistics of a single EDX pixel spectrum the data were binned by a factor of x8 to yield an effective pixel size of 2.1\u00a0nm and further processed in Br\u00fcker software (Esprit 1.9) by a Bremsstrahlung background subtraction, series deconvolution and Cliff-Lorimer quantification to display the net counts. The Pt-L\u03b1 (9.435\u00a0keV) signal, however, showed too few counts for a proper background subtraction and therefore are displayed as raw counts with an estimated signal-to-noise of 50% as determined from the peak-to-background in the summed EDX spectrum. The Pt-L region was emphasized because it separates from other elemental peaks, in contrast with e.g. the Pt-M\u03b1 signal (2.050\u00a0keV) that overlaps with Zr L\u03b1 (2.044\u00a0keV) signals as reminiscence from the microscope. Likewise, the present analysis focuses on Mo-K\u03b1 (17.480\u00a0keV), Co-K\u03b1 (6.931\u00a0keV) and S-K\u03b2 (2.465\u00a0keV) that all separates peaks stemming from the sample and microscope.High-resolution scanning transmission electron microscopy (HRSTEM) was performed on a JEOL ARM-200F equipped with cold field emission gun (CFEG) and CEOS probe (STEM)-Cs-corrector. The microscope was operated at 200\u00a0keV and probe aberrations up to 3rd order was corrected. The illumination system was set with a probe size of\u00a0~\u00a01\u00a0\u00c5, with a current of approximately 0.1 nA, and with a pixel dwell-time of 32\u00a0\u00b5s. The focusing of the sample was done prior to acquisition in an adjacent area as to record an image of a pristine area not previously exposed to the electron beam. Images were generated using a high-annular dark-field detector. Although residual oil is removed by pentane as described in section 2.2, HRSTEM revealed carbon deposition to a degree that varied from area to area. Occasionally areas allowed sufficient cleanliness to acquire an HRSTEM image of a clearly atom-resolved image and the atomic details blurred after a few scans, so it was necessary to move to a new area. It was such images that have been considered in the present study. Even though further optimization of beam energy and current could be pursued, Fig. S8 (Supplementary Material) indicates that some Pt atoms are stabilized in successive images suggesting they reveal their pristine locations. Complementary STEM image simulations were carried out in the QSTEM software suite with experimental details shown in Supplementary Material.The activity tested Pt-Co-Mo(0.5) catalyst was, in its sulfided state as retrieved from the reactor, characterized ex situ by X-ray absorption near edge structure (XANES) at the XAS beamline at the ANKA synchrotron source (Karlsruhe, Germany) using the Pt L3-edge (11.564\u00a0keV). The catalyst was crushed and pressed into a 13\u00a0mm pellet using polyethylene and measured in transmission mode. The XANES spectra were energy calibrated using a metal reference, background subtracted and normalized. References of PtS (ICSD_31131) and PtS2 (ICSD_41375) were also measured.To obtain total energies we employed the GPAW [43,44] density functional theory code in the finite difference mode with a spacing of 0.18\u00a0\u00c5. The exchange and correlation were treated using the BEEF-vdW functional [45]. As in ref. [25], the S-edge and Mo-edge of the Co-Mo-S particle were modelled with 4x4 stripes periodic in the x-direction and separated with vacuum in the y and z directions. The corner of the Co-Mo-S particle was modeled with a step stripped continuous in the x-direction separated by vacuum in the y and z directions, exposing 3xCo and 2xMo on the S- and Mo-edge respectively. A 2x1x1 Brillouin zone sampling was used in the x, y and z direction respectively. All structures were optimized until the maximum force was lower than 0.03\u00a0eV/\u00c5. The crystal structures of PtS2, MoS2 and Co8S9 were optimized using the stress tensor method available in ASE [46]. In the following, the free energy of gas phase H2 and H2S has been obtained using the ideal gas approximation [47]\n\n\n\n\n\n\nG\n\n\nx\n\n\n=\n\n\nE\n\n\nx\n\n\n+\n\n\nZPE\n\n\nx\n\n\n+\n\u0394\n\n\nH\n\n\nx\n\n\n0\n,\nT\n\n\n-\nT\n\n\nS\n\n\nx\n\n\nT\n\n\n+\n\n\nk\n\n\nB\n\n\nT\nln\n\n\n\n\n\n\np\n\n\nx\n\n\n\n\np\n\n\n\n\n\n\n\nwhere \n\n\nE\nx\n\n\n is the electronic energy, \n\n\n\nZPE\n\nx\n\n\n the zero-point energy, \n\n\u0394\n\nH\n\nx\n\n\n0\n,\nT\n\n\n\n the change in enthalpy from 0\u00a0K to T, \n\n\nS\n\nx\n\nT\n\n\n the entropy at T, \n\n\np\nx\n\n\n the pressure of the molecule in the gas, and \n\np\n\n the standard pressure, and the free energy of surfaces and bulk sulfides are assumed to be described by the 0\u00a0K electronic energy. The uncertainty of the calculation has been estimated using the BEEF-vdW ensemble [45], using an ensemble of 3000 energies. This uncertainty represents how much, e.g., an adsorption energy (at 0\u00a0K) can vary within the GGA functionals.We obtain the sulfur equilibrium termination at HDS conditions, 673\u00a0K and p(H2)/p(H2S)\u00a0=\u00a020, for the S-edge, Mo-edge and at the corner by gradually increasing the sulfur coverage and for each step calculating free energy change of the sulfidation according to:\n\n(1)\nCoxMoySz-1\u00a0+\u00a0H2S(g)\u00a0\u2194\u00a0CoxMoySz\u00a0+\u00a0H2(g)\n\nwhere x, y and z represent the number of Co, Mo and S atoms in the supercell, respectively.The adsorption energy of a molecule on a specific site, \n\n\u0394\n\nE\nx\n\n\n, has been obtained as follows:\n\n\n\n\u0394\n\nE\nx\n\n=\n\nE\n\nx\n\u2217\n\n\n-\n\nE\nx\n\n-\n\nE\n\n\u2217\n\n\n\n\n\nwhere \n\n\nE\n\nx\n\u2217\n\n\n\n is the energy of the adsorbed molecule on a specific site, \n\n\nE\nx\n\n\n the energy of the molecule in vacuum, and \n\n\nE\n\n\u2217\n\n\n\n the energy of the site.The stability of single atom Pt incorporation at the Mo-edge is described by the free energy of the following reaction:\n\n(2)\nCoxMoySz\u00a0+\u00a0PtS2(b)\u00a0+\u00a0nH2(g)\u00a0\u2194\u00a0PtCoxMoy-1Sz-n\u00a0+\u00a0MoS2(b)\u00a0+\u00a0nH2S(g)\n\nwhere CoxMoySz here represents the equilibrium structure of the Mo-edge, PtCoxMoy-1Sz is the equilibrium structure of the Pt doped Mo-edge, where 1 Mo at the edge has been substituted with 1Pt in the unit cell, and PtS2 and MoS2 are the reference metal sulfides of Pt and Mo, respectively. Similarly, at the Co-promoted S-edge and corner site, the stability of Pt is described by calculating the free energy change of the reaction:\n\n(3)\nCoxMoySz\u00a0+\u00a0PtS2(b)\u00a0+\u00a0(10/9+n) H2(g)\u00a0\u2194\u00a0PtCox-1MoySz-n\u00a0+\u00a01/9 Co9S8(b)\u00a0+\u00a0(10/9+n) H2S(g)\n\nwhere CoxMoySz here represents the equilibrium structure of the S-edge/corner of a Co-Mo-S particle, PtCox-1MoySz the equilibrium structure of the Pt doped S-edge/corner site, and PtS2 and Co9S8 the stable metal sulfides of Pt and Co respectively.The hydrodesulfurization (HDS) activity of the Pt-Co-Mo catalysts was measured relative to a commercial Co-Mo reference catalyst under industrially relevant conditions in a pilot unit. The activities are reported as relative volume activities. All (Pt)-Co-Mo catalysts measured have, within the experimental uncertainty of preparation using the same alumina carrier and the experimental uncertainty of subsequent chemical analysis, identical molar loads of molybdenum per volume (and weight) of catalyst. Fig. 2\na shows the influence of increased amounts of platinum in the catalysts. The activity increases approximately linearly up to about 1\u00a0wt% Pt (10000\u00a0ppm) and here reaches an unparalleled catalytic performance of 146%. Platinum in amounts higher than 1\u00a0wt% did not increase activity further and eventually a slight decrease in activity was found. This may suggest a saturation and a corresponding Pt waste of the promotional active sites at high platinum loadings. The activity of catalysts without any cobalt at all, Mo/Al2O3 and Pt-Mo/Al2O3, was found to be at least an order of magnitude lower than that of the Co-Mo reference catalyst of Fig. 2a. Thus, it must be the intimate contact of Co and Pt that is responsible for the boosted performance.\nFig. 2a shows a Pt promoted activity for a fixed cobalt amount. Samples with variations in Co amounts were prepared as well. Fig. 2b shows the effect of variations in the amount of both promoters (Pt and Co) while keeping the molar Mo amount constant. The contents of Pt and Co are shown as estimated volumetric concentrations (mmol/L) inside the reactor during catalytic testing. In Fig. 2b, the activity increases with an increased amount of Co (following the x-axis, y\u00a0=\u00a00). An increase in activity of 10% was obtained by an increase in the Co concentration from 479\u00a0mmol/L to 603\u00a0mmol/L in the reactor, corresponding to 124\u00a0mmol/L or 26% of the total Co in the reference sample. The Pt effect is much more pronounced: a similar activity increase of 10% is obtained by replacing only 10\u00a0mmol/L of Co (~2% of total Co) with Pt (following the y-axis). Thereby, in the concentration domains examined, the intrinsic promotional effect of platinum (per atom) is more than 12 times higher than cobalt in a rather large window of promotor concentration variations of the industrial Co-Mo catalyst. Because of the very consistent data, navigating in the activity contour plot by simple extrapolation now allows for tailoring the catalyst activity with the usage of cobalt and platinum in combination. For instance, at a fixed low Pt amount of 10\u00a0mmol/L, an activity lift of 20% RVA can readily be obtained, and likewise for 30\u00a0mmol/L of Pt an activity boost of more than 35% is expected as indicated by dashed lines in Fig. 2b.Selected Pt-Co-Mo catalysts were after catalytic HDS activity tests characterized by electron microscopy and X-ray absorption spectroscopy in order to unravel the nature of the Pt-promotion. Fig. 3\na shows a STEM image of the activity tested Pt-Co-Mo(0.5) catalyst with the corresponding EDX element maps displaying the distribution of cobalt, molybdenum and platinum. The metals are all uniformly distributed on the agglomerate together with EDX signals of sulfur, aluminum and oxygen in accordance with an alumina-supported (sulfided) Co-Mo catalyst (Fig. S3, Supplementary Material). The elemental composition of various sample agglomerates observed in the STEM imaging appeared surprisingly homogeneous (based on at least 20 agglomerates for the Pt-Co-Mo(0.5) sample) and despite a very little platinum detection signal in a single pixel spectrum (see Fig. S4, Supplementary Material) a Pt peak signal was verified throughout the catalyst from larger EDX sum-spectra areas. In fact, a quantification of the Pt amount from the full area EDX sum spectrum, as well as a selected smaller area, revealed a similar Pt content of 0.5\u00a0wt% demonstrating a high homogeneity within the sampled area, and indicates a uniformity on a larger scale as it matches the nominal platinum load in the prepared catalyst (Fig. S5, Supplementary Material). Such a highly dispersed platinum phase matching the nominal Pt weighting was also found in the highly active catalyst Pt-Co-Mo(1.0) of twice the platinum amount. However, in the catalyst with the highest Pt load Pt-Co-Mo(1.9) significant amounts of distinct Pt nanoparticles a few nm in size were observed in addition to the highly dispersed Pt phase (Figs. S5 and S6, Supplementary Material). No other elements from the element maps (e.g. sulfur) could be clearly associated with the Pt nanoparticles indicating likely metallic platinum (Fig. S6, Supplementary Material). Interestingly, a full-frame EDX quantification including both the dispersed Pt phase and Pt nanoparticles still matched the overall nominal Pt load (1.9\u00a0wt%), indicating a two-phase Pt distribution with an estimated fraction of the highly dispersed Pt phase corresponding to 1.2\u00a0wt% as measured in a selected area free of Pt nanoparticles (Fig. S5, Supplementary Material). The concentration of platinum in the highly dispersed Pt phases, as determined from the STEM-EDX analyses, thus scales with the activity gain of the catalysts at all Pt loadings (see Fig. 2a). On the other hand, the presence of Pt nanoparticles (observed occasionally in all Pt-samples samples but only to a substantial extent in the highest Pt loading samples) appear with limited or no correlation with the HDS activity. Therefore, we exclusively associate the highly dispersed platinum phase with the Pt promotional effect of the Co-Mo catalysts and attribute the activity saturation above 10000\u00a0ppm Pt in Fig. 2a to the onset of the separate Pt formation.To shed further light on the local structure of the dispersed platinum, the activity tested Pt-Co-Mo(0.5) catalyst was qualitatively addressed by ex-situ X-ray absorption near edge spectroscopy (XANES). The normalized XANES spectrum recorded at the Pt L3-edge (Fig. 3b) reveals edge features of the Pt-Co-Mo catalyst that in intensity and shape are close to those of PtS2, indicating a similar electronic configuration to platinum(IV) sulfide. Thereby the platinum in the Pt-Co-Mo catalyst is, in average, significantly different from both a metallic (Pt) state or an oxide (PtO2) state (Supplementary Material). This also indicates limited oxidation during the sample storage prior to the measurements.High-resolution TEM imaging was used to visualize the nano-scale structures of the tested catalysts. TEM images of the reference Co-Mo sample without Pt (rel. vol. activity\u00a0=\u00a0100%) and the Pt-Co-Mo(0.5) sample (rel. vol. activity\u00a0=\u00a0124%) are shown in Fig. 3c,d and reveal very similar appearance of elongated dark contrasted features with occasional two such features in pair with a separation of 0.62\u00a0nm, corresponding to the lattice distance of MoS2 (002). Thus, the elongated dark features are attributed to MoS2 slab structures viewed along the (001) basal plane, and as a cobalt promoted HDS catalyst the Co-Mo-S structure is assumed consistent with ref. [1,13]. The Co-Mo-S structures viewed along the (001) contour the terminating edges of the support and was not observed as free-standing, unsupported slabs. This indicates that the Co-Mo-S slabs are supported on their basal plane (001) by the alumina substrate. The length and stacking height of the Co-Mo-S slab structures were measured from the TEM images (16 different sample areas each) with very similar size distributions of predominantly single layer structures (~89%), double layers (~10%) or triple layers (<1%). The stacking degree calculated as the average number of layers in the MoS2 (001) direction was 1.12 and 1.13 for the Pt-Co-Mo and Co-Mo catalyst, respectively, revealing only a marginal difference of\u00a0<\u00a01%. The correspondingslab lengthswerefoundin the rangefrom1.2\u00a0nm to7.5\u00a0nmwith ameansizeof 2.38\u00a0nm and 2.48\u00a0nm for the Pt-Co-Mocatalystand Co-Mocatalyst, respectively, asdetermined from alognormalfit to the entire size distribution(Fig. 3e). Thus, the Co-Mo-S and Pt-Co-Mo-S structures are similarly distributed in stacking height and slab length, and this similarity is expected to extend below the detection limit for the slab length of 1.2\u00a0nm. Based on the minutechangeinthe average slab length theCo-Mo-S structuresin the Pt-Co-Mo catalystexposeabout4%moreedgesites compared to the Co-Mosample. This minor change in edge dispersion is insufficient to fully account for the activity boost of 24% caused by addition of Pt, indicating that Pt has additional functional effects.Recent work on enhancing the intrinsic catalytic properties of two-dimensional MoS2 for tuning the electronic properties in improved hydrogen evolution reaction (HER) catalysts points to single atom Pt dopants of the (inert) MoS2 basal plane [48]. The location of such Pt single atom has been reported to replace an Mo atom in the MoS2 structure (in which Pt\u2014S bonds are formed), or localized in S vacancies, and various unspecific positions in case of surface (carbon) contamination [49\u201351]. Therefore, we address possible interactions of single Pt atoms with the Co-Mo-S structure either by adsorption or substitution under HDS sulfiding conditions using density functional theory (DFT).We have with DFT investigated the possible stable sites for Pt substitution into and adsorption on the Co-Mo-S particle. In order to do so we have chosen the 3 model structures from ref. [25] to represent a Co-promoted MoS2 particle, namely the Mo-edge, a Co-promoted S-edge and a corner site. We start out by addressing the equilibrium structures of the 3 model sites by calculating the free energy of sulfidation, \n\n\n\nH\n\n\n2\n\n\nS\n\n\ng\n\n\n+\n*\n\n\u2194\n\n\nH\n\n\n2\n\n\n\n\ng\n\n\n+\nS\n*\n\n, at HDS conditions (see Supplementary Material). We find at equilibrium that the Mo- and S-edge is terminated by monomeric S, whereas the corner site has a S vacancy (Fig. 4\na), in accordance with [25].We can now address the most stable sites for Pt substitution into the Co-Mo-S particle. By substitution of one Mo atom with one Pt atom in the Mo-edge model, and one Co atom with one Pt atom in the S-edge and corner models, we first obtain equilibrium structures for the Pt-Co-Mo-S sites by calculating the free energy of sulfidation and desulfidation around the Pt-promoted site at HDS conditions (Supplementary Material). The substitution free energies have then been obtained for reaction (2) and (3) and are presented in the bottom of Fig. 4a together with the Pt-Co-Mo-S edge and corner equilibrium structures. As a reference for our DFT calculations we choose PtS2 (rather than PtS) since this particular platinum sulfide structure is what our XANES results indicate (Fig. 3b). We find it is energetically favorable to incorporate Pt single-atoms into both Mo- and S-edges and corner sites. The calculations indicate a slight preference of the Mo-edge over the corner site, and to a lesser extend the S-edge, however, within the uncertainty of calculations; Mo-edge: \u22120.50 (+/\u22120.44) eV, S-edge: \u22120.16 (+/\u22120.30) eV, and corner: \u22120.38 (+/\u22120.26) eV, the corner and the Mo-edge are probably equally preferred substitution sites. We have also calculated the stability of Pt and PtS adsorbed on several different adsorption sites on the Mo-edge, S-edge and the corner equilibrium structure of Co-Mo-S (see Supplementary Material). We find that Pt substitution into Co-Mo-S is significantly more stable than adsorption on the Co-Mo-S particle.Next, we address the structure of the Co-Mo-S and Pt-Co-Mo-S sites at HDS conditions (as shown in Fig. 4a) in more detail. In Fig. 4b we show how the fraction of sulfur vacancy sites on edges and corners of Co-Mo-S and Pt-Co-Mo-S structures relates to the free energy of sulfidation. We find that the free energy of sulfidation of a vacancy site at the Co-Mo-S Mo-edge is \u22120.11 (+/\u22120.19) eV, and for the corner vacancy 0.08 (+/\u22120.16) eV. Thus, as seen in Fig. 4b, vacancy and monomeric S sites are likely to co-exist at the Mo-edge and corner of the Co-Mo-S particle, however, to which degree is shadowed by the uncertainty of the calculation. On the other hand, platinum single-atoms incorporated into the edges and the corner of the Co-Mo-S particle leads to a weakening of the sulfur binding energy around the Pt atom compared to the non-Pt counterpart (Fig. 4b), such that the Pt sites at the Mo-edge and corner are indeed characterized by inherently stable vacancy sites. In fact, at the Mo-edge, Pt incorporation leads to a double CUS-vacancy around the Pt atom, which is a significant restructuring compared to the non-Pt counterpart. At the S-edge we see a slight shift in the free energy of sulfidation when Pt is incorporated (Fig. 4b), however this does not lead to vacancy sites.Double vacancy sites at the corners of Co-Mo-S have previously been suggested to be attractive for HDS [28], however, such sites are prohibited due to a high formation energy at HDS conditions. Here, we have also calculated the formation energy of such a double vacancy site from the vacancy sites at the corners of Co-Mo-S and Pt-Co-Mo-S (geometry given in Supplementary Material). At HDS conditions, we find a free energy of formation of 1.07\u00a0eV and 0.7\u00a0eV for Co-Mo-S and Pt-Co-Mo-S corners, respectively. Although Pt indeed lowers the formation energy of a double vacancy by\u00a0~\u00a00.4\u00a0eV relative to the corresponding Co corner, also the Pt-corner double vacancy at these sites are unstable at HDS reaction conditions.Visualization of the Pt-Co-Mo(0.5) catalyst at the atom-by-atom scale was approached using high-resolution STEM imaging [52\u201354]. Previously atomically resolved imaging of Co-Mo-S structures [13,55,56] has required use of a high-surface area carbon as carrier but we have here achieved such resolution for catalysts based on an industrially much more relevant high-surface area \u03b3-Al2O3 carrier. The high-angle annular dark-field (HAADF) image contrast scales approximately as Z1.7 where Z is the atomic number and the Pt atoms (Z\u00a0=\u00a078) therefore appear much brighter than single Mo (Z\u00a0=\u00a042) or Co atoms (Z\u00a0=\u00a027), providing the support is sufficiently light and thin. The industrial Co-Mo-S structures are supported on a few nm thin alumina crystallites, with dimensions and orientations occasionally thin enough for imaging single heavy metal atoms [57]. However, alumina is an insulator with a very poor electrical conductivity and charging of the sample in the electron beam complicates high-resolution imaging. Therefore, we dispersed the alumina-supported catalyst onto a TEM grid with a conducting amorphous carbon layer to compensate for the electrical resistivity of alumina at the expense of image contrast.\nFig. 5\na shows a high-resolution STEM image of the Pt-Co-Mo(0.5) sample and reveals a truncated hexagonal shaped nanocrystal about 3\u00a0nm in diameter with a periodic array of bright dots indicating heavy atom columns. An intensity profile across the nanocrystal shows four intensity peaks separated by about 0.32\u00a0nm consistent with the Mo\u2014Mo distance of MoS2 (Fig. 5b,c). The nanocrystal is therefore attributed a MoS2 structure viewed in (001) projection in agreement with previous studies [55]. We note that the size of the crystal is in accordance with the MoS2 slab lengths viewed edge-on in TEM images (Fig. 3) and we show below by comparison to STEM image simulations that the contrast is consistent with metal atoms in a single-layer slab structure.A wider image field-of-view (Fig. S5, Supporting Material) shows the presence of both hexagonally shaped MoS2 nanocrystals as well as more irregular shaped MoS2 nanocrystalline domains viewed in (001) projection with multiple corner sites and kinks of a concave geometry, and even isolated (metal) single-atoms or few-atom clusters. Such a co-existence of both regular and irregular shaped MoS2 structures in alumina-supported catalysts have previously been reported [53,54]. Furthermore, restructuring of the MoS2 morphologies was observed in successive recorded images as an effect of the electron beam (in line with [58]) under the present imaging conditions. However, such image series also revealed distinctively bright dots associated with the edges of the MoS2 crystals (as indicated by circles in Fig. 5b) in the first acquired images that either remained at the same position or has moved to new positions in the subsequent image (Fig. S6, Supporting Material). To address the atomic arrangement in such nanocrystals, we carry out a detailed image contrast analysis of the MoS2 nanocrystal in Fig. 5, which represents a pristine area not previously exposed to the electron beam.The intensity profile along the edge reveals three distinct contrast levels (Fig. 5c). To ease the interpretation of this contrast pattern, STEM image simulations of a supported MoS2 structure was performed. A single-layer MoS2 slab structure consisting of 16 metal atoms (Mo, Co, Pt) and 36 sulfur atoms was used as model for the STEM image simulations, with an edge length of 4 metal atoms (using the S-edge with 50% sulfur coverage). To overcome the effect of contrast reduction from any (alumina, carbon) support materials, the (Mo,Co,Pt)-MoS2 model structure was placed on top of an amorphous carbon model structure with a varying thickness of 0\u201315\u00a0nm. Not surprisingly, in the simulated HAADF-STEM images of an unsupported or thin (3\u00a0nm) carbon supported MoS2 the atomic (Mo) metal lattice as well as the sulfur sub-lattice could be resolved, whereas at thicker supports of 9\u201315\u00a0nm the sulfur columns showed significantly reduced contrast with intensities visually mixed up with the intensity fluctuations of the amorphous support (Fig. S7, Supplemental Material). This is in accordance with a thin, flat (crystalline) support like graphene or graphite being the preferred support materials for obtaining single atom sensitivity imaging of MoS2\n[55,58,59]. In contrast, however, the atomic imprints of Mo, Co or Pt single atoms were distinctly resolved in the simulated STEM images up to 15\u00a0nm of amorphous carbon support as a result of the high Z-number. In addition, image contrast of single atoms is insensitive towards small sample tilts away from a crystal zone-axis as opposed to e.g., diatomic (sulfur) columns. Thus, the following discussion will only focus on the metal atom positions.The image contrast of the individual supported metal atoms in the simulated STEM images was quantitatively addressed by measuring the intensity maxima of the corresponding atom peak positions to evaluate the confidence level for atom identification. We find that even for the unsupported MoS2 crystal, the peak maxima of the various Mo atoms show a range of intensities rather than a single value as a result of the comprehensive imaging model [60] used to simulate the sampling of the electron probe over the electrostatic potential of the specimen. Hence, a distribution of metal atom intensities measured from the HAADF-STEM images are expected. Importantly, the contrast of a single Pt atom was significantly brighter than any Mo atom or Co atom on a support thickness up to 15\u00a0nm (Fig. S9, Supplementary Information). On the other hand, the Co and Mo atoms were convincingly separated by contrast only at very thin supports (3\u00a0nm) as for thicker supports (9\u201315\u00a0nm) the contrast levels of Co and Mo started to overlap. The supported Co atoms, however, still appeared with lower contrast than the average Mo atom. Based on the maximum overlap of the peak intensity distributions the Co atoms could be discriminated from Mo with a confidence level of 84% on a 9\u201315\u00a0nm thick amorphous carbon support in the simulations.A comparison of the experimental data and the normalized image simulations is shown in Fig. 5c. The overall contrast levels of the (experimental) MoS2 structure and the adjacent support materials can be well described by the 9\u201315\u00a0nm amorphous (carbon) supported single-layer MoS2 model structure although the (experimental) support might not be completely flat and includes alumina as well. However, the detailed contrast levels could not be fitted with single Mo and Pt atoms only (shown in grey) without exceeding the uncertainty in the image contrast given by the statistical noise in the raw image (error bars). Instead, the simulations show that two Co atoms incorporated on that edge gives a better agreement (Fig. 5c,d). That is the relative contrast level analysis indicates the formation of a single-layer Pt-Co-Mo nanostructure. Moreover, the tertiary Pt-Co-Mo sulfide structure provided by a Pt corner atom in the Co-Mo-S structure is an energetically stable configuration according to our DFT results (Fig. 4). Thereby, we substantiate the argument that the nanocrystal in Fig. 5 is indeed a cobalt promoted single layer MoS2 crystal (Co-Mo-S phase) with a corner Pt atom. An unambiguous atom identification may possibly only be obtained using complementary atom-scale spectroscopy with careful optimized illumination and imaging schemes [13,52]. However, with the atomic assignments in place we point to the Co promoted edge as the S-edge of the MoS2 structure (Fig. 5d) and by symmetry, the opposite edge must be an Mo-edge.More interestingly, from an intensity analysis of the peak maxima over the entire nanocrystal (Fig. S10, Supplementary Material), six atom positions stand out by having a significantly larger intensity more than 3x sigma above the Gaussian distribution fit to all peak maxima and includes the corner Pt atom just identified. These six atom positions are all marked by circles in Fig. 5b and are assigned Pt single atoms with four corner site positions (C) and two Pt atoms associated with the low-indexed edges (E, E\u2019), i.e. the Mo-edge and S-edge, respectively. Based on the location of these six Pt atoms in the nanocrystal in Fig. 5 we thereby justify all the three plausible scenarios of single-atom Pt-Co-Mo-S interactions discovered by DFT calculations (Fig. 4) with indications of the corner site position being the more pronounced.Thus, the Pt promotional effect of the industrial Co-Mo catalysts with Pt added prior to a sulfidation step may be associated with incorporation of Pt in the Co-Mo-S edge structures (referred to as the Pt-Co-Mo-S phase) rather than adsorption or a separate Pt (-sulfide) phase, e.g. PtS which elsewhere have been observed as a stable phase at HDS reaction condition at higher temperatures (400\u00a0\u00b0C) [61], and also suggested in the case of platinum doping of a pre-sulfided NiW catalyst [62]. The Pt-Co-Mo-S phase, however, is consistent with a Pt(IV)-sulfide resembling structure with Pt\u2014S bonds as revealed by XANES. According to our DFT investigations, the platinum in the Pt-Co-Mo-S structure has a sulfur coordination of 3\u20134 under HDS conditions (Fig. 4). The linear increase in HDS activity with increased amounts of Pt (Fig. 2a) is consistent with single atoms gradually being incorporated into edge sites of a nanocrystal. In particular, the promotional effect of Pt and Co in combination at various contribution ratios (Fig. 2b) is rationalized from the incorporation of Pt in corner sites or edge sites of a partly cobalt decorated MoS2 in which Mo and Co atoms co-exist at the S-edge as visualized by electron microscopy (Fig. 5).In addition, the amount of Co and Pt relative to Mo per Co-Mo-S slab can be evaluated based on the nominal metal load on the catalysts using simple geometric considerations. Assuming single-layer Co-Mo-S slab structures of length\u00a0~\u00a02.5\u00a0nm, this corresponds to a cluster size of N\u00a0=\u00a061 metal atom sites in a regular hexagonal shape with a side length of 4 Mo\u2014Mo distances (5 atoms) and 9 metal atoms across the longest diameter. We find that 3.5\u00a0wt% Co corresponds to Co:Mo\u00a0=\u00a00.35 (molar ratio) which is equivalent to about 16 atoms out of the assumed average cluster size of 61 metal atoms, in case of full Co incorporation into Co-Mo-S. This would correspond to 66% of the edge metal atoms and agrees with a Co-Mo-S structure with Co decorating the S-edges only. Using a Co-Mo-S slab version with 16 Co atoms and 45 Mo atoms, the corresponding addition of Pt single atoms is Pt:Mo\u00a0=\u00a00.022, 0.044 or 0.067 (molar ratio) for 1, 2, or 3Pt atoms per Co-Mo-S slab, respectively. Hence the activity maximum at 146% of Fig. 2a occurs at a degree of Pt-promotion corresponding to Pt:Mo\u00a0=\u00a00.03\u20130.04 or about 2Pt atoms per Co-Mo-S slab on average. At saturation, Pt separates out into the observed competing second phase of Pt clusters at a high precursor density [50,63,64], which are probably poorly catalytically active. As minor local variations in Pt precursor or metal concentrations at the nano-scale may be expected during synthesis, it can be speculated that the origin of saturation by a few Pt atoms per Co-Mo-S slab on average can be geometrically constrained as the corner sites of the (hexagonal) Co-Mo-S structures are gradually filled and occupied by Pt at increasing load.With the Pt-Co-Mo-S catalyst model in mind we investigate the hydrodesulfurization of the sterically hindered 4,6-DMDBT molecules based on DFT calculations. Such 4,6-disubstituted species are present (see Supplementary Material) in the heavy end of the industrial feed used (a mix of straight run gas oil and light cycle oil) and are, due to sterically hindrance in the vicinity of the sulfur atom, the most difficult sulfur species to convert by hydrotreating. Analytics such as e.g. 2-dimensional gas chromatography show that 4,6-disubstituted DBTs constitute essentially all those organosulfur compounds that remain to be converted when a sulfur level of a few hundred ppm S has been achieved. To reach the desired ULSD target of only 10\u00a0ppm S, most of the catalyst volume in the reactor serves the purpose of desulfurizing the relatively very small amount of a few hundred ppm 4,6-disubstitued DBTs (Supplementary Material). At the test conditions employed in this study sulfur concentrations below 20\u00a0ppm were obtained in the oil exiting the reactors, i.e. concentrations well within the domain where essentially only 4,6-substituted DBTs are being converted. As the H2 partial pressure was 30\u00a0bar, our conditions are suited for evaluating the performance of catalysts aimed at the industrial so-called low-pressure segment, i.e. Co-Mo catalysts. Industrial refiners that have decided on process equipment to produce ULSD in the high-pressure segment would use Ni-Mo rather than Co-Mo catalysts.The exact mechanism for desulfurization of 4,6-DMDBT on Pt-Co-Mo-S is considered to be quite intricated giving the size of the molecule(s) and the complexity of the edge sites. To provide a first preliminary assessment we here address with DFT a possible effect of Pt by investigating the stability of reaction intermediates in two hypothetical DDS and HYD reaction pathways of 4,6-DMDBT desulfurization at Pt-Co-Mo-S and Co-Mo-S sites. We especially focus on the thiolate formation, since thiolate formation has been suggested to be a starting model for which sites participate in the C\u2014S cleavage of a sulfur containing molecule [25].\nFig. 6\n shows the hypothetical DDS and HYD pathways we investigate. Species denoted with an asterisk are adsorbed species. For both the DDS and HYD pathway the first step is the adsorption of 4,6-DMDBT on the (Pt)-Co-Mo-S particle. In the DDS pathway, a thiolate intermediate (DDS-thiolate) is formed after C\u2014S cleavage via hydrogenation. This species can then undergo a second round of C\u2014S bond breakage to form 3,3\u2032-DM-BP, which desorbs from the site. In the HYD pathway, the adsorbed 4,6-DMDBT is hydrogenated six times to 4,6-HH-DMDBT, such that one benzene ring has been fully hydrogenated. This species can now undergo C\u2014S bond breakage via hydrogenation of either i) the C\u2014S bond between the S atom and the aromatic ring or ii) the C\u2014S bond between the S atom to the cyclohexyl ring to form a HYD-thiolate. As for the DDS pathway the thiolate undergoes a second C\u2014S bond breakage via hydrogenation, and the desulfurized product, 3,3\u2032-DM-CHB, leaves the site.In Fig. 7\n we show the calculated energy diagram for both the DDS and HYD pathway of 4,6-DMDBT on the Co-Mo-S and Pt-Co-Mo-S equilibrium structures of the S-egde, Mo-edge and corner sites. We have chosen also to include the Mo-edge with a single vacancy, since both the 1S terminated Mo-edge and the single vacancies are likely (see Fig. 4b).We start out with the energetics of the DDS pathway in Fig. 7a where adsorption of 4,6-DMDBT is the first reaction step. Beginning with the Co-Mo-S structure, we find the 4,6-DMDBT molecule to adsorb strongest through physisorption at the brim site of the Mo- and S-edges, and the calculations indicate a slightly weaker physisorption to the brim of the defect (vacancy) Mo-edge site or corner site (see molecular configurations in Supplementary Material). At the Co-Mo-S corner, 4,6-DMDBT adsorbs equally strong physisorbed at the brim and in a chemisorbed state aligned in-plane with the particle in agreement with previous work [25] (only configurations free of the support were considered). Introducing the Pt sites in the Co-Mo-S structure slighty weakens the 4,6-DMDBT adsorption strength on both the Mo-edge and the corner site whereas essentially no effect is obtained at the S-edge.Next we address the DDS-thiolate from 4,6-DMDBT on both Co-Mo-S and Pt-Co-Mo-S sites. According to the calculations, the DDS-thiolate is most strongly adsorbed on corner and edge vacancy sites to which it can chemisorb, consistent with that thiolates in generel are found to adsorb strongest on defect sites [25]. However, at all investigated sites except the non-Pt promoted Co-Mo-S corner site we find, interestingly, that the DDS-thiolate formation reaction step is uphill in energy. Thus, since both the adsorption of 4,6-DMDBT and the thiolate formation reaction step is energetically preferred on Co-Mo-S (corner sites) over Pt-Co-Mo-S, these calculations suggest that Pt sites has limited or no promotional effect for the DDS pathway.We now turn to the HYD pathway for which the energetics is shown in Fig. 7b. Here the adsorbed 4,6-DMDBT is hydrogenated to 4,6-HH-DMDBT before the C\u2014S bond is cleaved and the corresponding thiolate is formed. For the Co-Mo-S and Pt-Co-Mo-S structure, as for 4,6-DMDBT we find that 4,6-HH-DMDBT preferentially physisorbs at the brim of the 1S-terminated Mo- and S-edges and with a weaker chemisorption in-plane with the particle for the corner sites (see Supplementary Material for molecular structures). The HYD-thiolate, resulting from 4,6-HH-DMDBT, adsorbs preferentially to the vacancy sites through chemisorption by the S-part of the molecule to the vacancy. This is valid for both the HYD-thiolate a and b (see Supplementary Material). However, on all sites, except on the vacancy site of the Mo-edge, the HYD-thiolate a, formed from breaking the C\u2014S bond between the aromatic ring and the S-atom, is more stable than HYD-thiolate b resulting from breaking the C\u2014S bond between the S-atom and the cyclohexyl ring. Importantly, we find the formation of the HYD-thiolate to be energetically favourable on vacancy sites and unfavourable on 1S-terminated sites, suggesting that vacancy sites could be key sites for the final steps of desulfurization of 4,6-DMDBT. Here, we have not considered energy barriers for thiolate formation, however, it has been found that barriers for thiolate formation specifically from thiophene and dibenzothiophene are significantly lower on defect sites at the Mo-edge and the corner, than on the 1S-terminated S-edge [25]. We also note that the HYD-thiolate formation is significantly more favourable on all vacancy sites of both the Co-Mo-S and Pt-Co-Mo-S structure than the strongest DDS-thiolate formation on the Co-Mo-S corner, indicating desulfurization of 4,6-DMDBT proceeds primarily via the HYD pathway, consistent with experimental findings on a Co-Mo catalyst [21,22]. Overall, this could suggest that different sites may participate in the desulfurization such that 4,6-DMDBT preferentially is hydrogenated at the 1S-terminated brim of the particle, where we find the strongest adsorption of 4,6-DMDBT and the hydrogenated intermediate, and the hydrogenated species then diffuse to a vacancy site, where the final desulfurization of 4,6-DMDBT takes place.The reactivity of corner sites of Co-Mo-S has recently attracted attention [25,65] and specifically adsorption of 4,6-DMDBT molecules has been observed by STM to occur particularly at corner (vacancy) sites [28]. Thus, a corner platinum site in the Co-Mo-S structure, as visualized in the present study, is obviously highly intriguing. Even though intermediates in the HYD pathway adsorb weaker to the Pt-corner in Pt-Co-Mo-S than the corresponding Co-Mo-S corner, the inherent vacancy site of the Pt-Co-Mo-S corner still favors HYD-thiolate formation (Fig. 7b and Supplementary Material). Likewise, at the Mo-edge the single-atom Pt incorporation promotes a double CUS site for which the calculations indicate HYD-thiolate formation is favored. Double CUS sites at the Mo-edge generated in the presence of thiophene was recently observed by STM [66] and suggested to activate the (Mo)-edges of MoS2 towards sterically hindered molecules. Thus, the Mo-edge of the Pt-Co-Mo-S structure provides such double CUS sites available intrinsically, which may also explain enhanced HDS activities [38] of Pt-MoS2 systems without cobalt. Therefore, we propose that the high reactivity of Pt-Co-Mo catalysts is obtained by lowering the sulfur binding energy around the Pt single-atoms at corners and (Mo)-edges in a Pt-Co-Mo-S phase making such vacancy sites more abundant (Fig. 4b). The Pt sites in the catalyst still has ability to bind 4,6-DMDBT and especially the HYD-thiolate sufficiently to favor its formation. This could suggest that despite a weaker S-binding, these sites maintain reactivity towards C\u2014S scission of hydrogenated reaction intermediates from 4,6-DMDBT, although more elaborate studies are needed to address that in detail.Finally, the presence of highly irregular shaped MoS2 crystal morphologies observed in the tested HDS catalysts indicates the availability of other site geometries than the low-index edge structures and corner sites considered in the DFT models in the present analysis, e.g., kink sites of a concave geometry, which can be speculated to mediate hydrogenation and desulfurization favorable sites in combination. Thus, the concentration of catalytically important edges under industrial operations, i.e. the evolution in the number of a particular atomic site geometry, are key to HDS catalysis which in turn depends on the activation procedure of the catalyst (e.g. gas phase sulfidation vs. liquid sulfidation [67,68]), operating conditions, and promoters as well.The present work demonstrates a tertiary transition metal sulfide-based catalyst for hydrotreating processes. Addition of ppm-levels of Pt to an industrial Co-Mo-S catalyst leads to a remarkable increase of up to 46%, as compared to the Pt-unpromoted catalyst, in the activity for low-pressure hydrotreating aimed at producing ultra-low sulfur diesel oil. This boosted performance is attributed to the Pt atoms incorporated into terminating edge and corner sites of the Co-Mo-S nanostructure as resolved by combining X-ray absorption spectroscopy and atomic-resolution electron microscopy. Interplay with density functional theory calculations suggests that Pt acts by reducing the sulfur binding energy under HDS conditions compared to the Co-Mo-S structure only. This mechanistic effect is most pronounced at the Pt-promoted corner site and at the Mo-edge. Since the Pt favors CUS formation and the simple probe HYD-thiolate adsorption is similar on the Pt promoted and unpromoted vacancy sites, the role of Pt is speculated to create sites for favorable adsorption of sterically hindered molecules such as 4,6-DMDBT to undergo desulfurizationutilizing a functionality of adjacent Mo- and S-edges, or corner sites synergistically.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 Christoffer Tyrsted for help with XANES measurements. We acknowledge use of facilities at NMI Natural and Medical Sciences Institute at the University of T\u00fcbingen (Germany), Center for Nanoanalysis, for part of this work, and Clementine Warres for kind support. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. SH acknowledges support from Danish National Research Foundation Center for Visualizing Catalytic Processes (VISION) (DNRF146).Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2021.03.008.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  We introduce a tertiary transition metal sulfide nanostructure, Pt-Co-Mo-S, for catalytic hydrodesulfurization (HDS) of sulfur-containing molecules in crude oil distillates with the aim to produce ultra-low sulfur transport fuels. The addition of ppm-levels of Pt to a standard industrial Co-Mo-S catalyst boosts the HDS activity by up to 46%. The promotional effect is examined by combining atomic-resolution Scanning Transmission Electron Microscopy (STEM), Energy Dispersive X-ray Spectroscopy (EDX), X-ray Absorption Near Edge Spectroscopy (XANES) and Density Functional Theory (DFT). It is shown that the Pt-Co-Mo-S catalyst contains predominantly single-layer MoS2 nanocrystals with Co atoms fully covering the S-edge terminations and Pt atoms uniquely attached to corner and edge sites in a platinum(IV) sulfide-like structural motif. Platinum is suggested to reduce the sulfur binding energy and increase the abundance of coordinately undersaturated sites (CUS) and not necessarily changing the reactivity towards 4,6-DMDBT molecules, although more elaborate studies are needed to address this in detail.\n               "}
{"full_text": "", "descript": "\n                  Activated carbon fiber, also known as the third generation of activated carbon material, is a new type of activated carbon material after powder and granules, made of the organic fiber. Activated carbon fiber has a strong adsorption capacity due to its large specific surface area, and can adsorb and remove organic matter in water, including some carcinogenic or toxic macro-aromatic substances. Physical adsorption is caused by electrostatic interaction between the surface molecules of the adsorbent and the adsorbate molecules, which is characterized by adsorption without selectivity. Chemical adsorption is due to the chemical bonding between the two molecules, forming a strong chemical bond and also the surface complex. Chemical adsorption has the certain selectivity, generally monolayer adsorption. With the large increase in vehicle usage and the exhaust emissions from equipment such as oil-fired boilers, the pollution of nitrogen oxides and carbon monoxide gas has become an important issue in environmental management. Adsorption or catalytic conversion of nitrogen oxides and carbon monoxide is an important means of treatment. This paper integrates the smart city scenario to construct the novel scenario for the analysis and research. The simulation results prove the effectiveness.\n               "}
{"full_text": "Over the last decades, the use of Bronsted acid catalysts has been increased in both laboratories and industries processes, such as biodiesel production, esterification and hydrolysis because of liquid phase limitations, hard separation, non-recovery acidic waste generation and the corrosion of reactors [1, 2, 3, 4]. Nano-substrates with a larger surface area and high surface-to-volume ratio have been applied in the solid acid catalysis field to increase available catalytic centers and enhance catalytic activity [5, 6]. Among solid nano catalysts, magnetic core-shell nano catalysts have been widely used because of their easy separation and recovery by applying a magnet as well as functionalization possibilities of the inorganic surface. Magnetic field separation is more effective than conventional separation methods such as purification and centrifuge because it can reduce catalyst waste, optimize operational cost and enhance the purity of products [7, 8, 9, 10]. By choosing the appropriate shell, the functionality of the catalyst in the shell is improved. Besides, this shell prevents the accumulation of magnetic nanostructure and enhances its chemical stability [11].Iron oxide magnetic core-shell nanoparticles have been prepared in various ways and have been modified by functional groups with catalytic properties [12]. Silica is one of the best coatings used for the synthesis of iron oxide core-shell nanoparticles because of stability, biocompatibility, functionality and resistance to acid or high temperature [13, 14]. The acid catalytic properties of iron oxide@SiO2 nanoparticles can be exceeded by the modification of the surface of nanoparticles with acid functional groups, such as HBF4 [15], sulfuric acid [16], R-NHMe2][H2PO4] [17] and sulfonic acid [18]. Solid acid catalyst functionalized with sulfonic acid could be used in different organic reaction [19, 20, 21]. The iron oxide coated with silica and functionalized with sulfonic acid (Fe3O4@SiO2\u2013SO3H) nanoparticles has been used as an effective catalyst in the synthesis of pyrimidinones [22], pyrazole [23], quinoline [24], pyrazolopyridines [25] and dihydropyrimidinones/thiones [26] as well as in the reduction of oximes to amines [27] and esterification reactions [28]. However, all the above-mentioned catalysts are nanoparticles as a zero-dimensional nanostructure. It seems that the use of other nanostructures with different morphology (such as one-dimensional nanostructure) as a catalyst in organic reaction is very limited and needs further study.Nanofibers are a new generation of one-dimensional nanostructures frequently used for their different applications like filtration [29], membrane [30], sensor [31], food and packaging [32], medical applications [33] and catalysis [34]. Electrospinning followed by calcination provides a tunable, simple and versatile way for generating ceramic nanofibers with unique properties including high surface area, large porosity, mechanical roughness, superb thermal stability, higher electron transfer and enhancement of the thermo electric merit and magnetic moment [35]. Such new properties lead to the potential application of ceramic nanofibers in lithium batteries, data storage devices, magnetic resonance imaging, sensing devices, energy storage, targeted drug delivery and catalysis [36, 37, 38]. Recently, some researchers have focused on the fabrication of porous ceramic nanofibers and their applications in heterogeneous reactions [39]. Pd\u2013TiO2 nanofibers have been applied in Suzuki coupling reaction with high conversion efficiency due to their high surface area and larger number of active sites [40]. Cu25O/Ni75O nanofibers can be used as anode catalysts for hydrazine oxidation in alkaline [41]. In one study, Pt/TiO2 nanofibers were used during electrochemical reaction for methanol oxidation and were found to effectively facilitate electron transport [42]. In another study, Pt supports composed of graphene sheets decorated by Fe2O3 nanorods and denoted as Pt/Fe2O3/N-RGO displayed higher catalytic activity than free Pt in the 4-nitrophenol hydrogenation reaction [43]. Previously, our research group prepared iron oxide nanofibers and applied them as catalysts in alcohol oxidation [44] and methyl orange degradation as a Fenton catalyst [45]. Also, in other studies, our group examined the fabrication and catalytic application of core-shell Fe2O3@SiO2 nanofibers as novel magnetic nanofibers for the one-pot reductive amination of carbonyl compound [46, 47]. It seems that by functionalization of the surface of Fe2O3@SiO2 nanofibers, this magnetic solid nano catalyst can be used in various organic reactions.Formamides are important crossroad intermediates in organic component synthesis such as formamidines [48], isocyanides [49], Vilsmeier reagents [50], fluoroquinolones [51] and imidazoles [52]. Hence, researchers have focused on the synthesis of formamides via the reaction of amines with various N-formylating agents such as carbon dioxide [53, 54], phenyl chloroformate [55], methyl formate [56], ammonium formate [57] and acetic formic anhydride [58]. Of these reagents, formic acid as a cheap, non-toxic, stable formylating compound is a good candidate for N-formylating amines. Unfortunately, their moderate reactivity demands increased reaction temperatures or catalyst presence, particularly for aromatic and sterically-hindered amines formylation. So, formic acid has been used in the presences of catalysts such as poly ethylene glycol [59], zeolite [60], ZnO [61], ZnCl2 [62], thiamine hydrochloride [63] and cobalt nanoparticles [64]. Nishikawa et\u00a0al. studied the synthesis of N-formamides with formic acid under mild conditions using tetraethylorthosilicate (TEOS) [65]. However, a long reaction time requires a Dean-Stark trap in reflux conditions and the difficult N-formylation of electron-withdrawing derivatives of anilines can limit the usage of these catalysts. Besides, the above-mentioned catalyst recycles need to be centrifuged or a long-time filtration leading to the unavoidable loss of solid catalyst in the process of separation. Thus, it seems that magnetic nanostructures modified with silica and reinforced with an acid functional group can be a good choice as a catalyst for N-formylation.Formamidines are widely used as effective drug agents and useful intermediate in the synthesis of purine [66] imidazole and quinazoline [67] compounds. Formamidines and related anions can bind transition metals. Recently, formamidines derivatives with Ni (II) [68] Au (I), Ag (I) [69] Co (II) [70] Cu (II) [71] Pt [72] and Pd [73] ions have been synthesized. Amidine functional group has various biological properties, such as anti-virus [74], anti-AIDS [75], anti-degenerative [76], anticancer [77], anti-platelet [78] and antimicrobial [79] activities. Finding simple and moderate conditions for the synthesis of formamidine can be useful in the production of many organic compounds.Continuing our previous research [44, 45, 46, 47], this study sought to prepare Fe2O3@SiO2\u2013SO3H nanofibers and study their use as a heterogeneous acid catalyst in organic reactions aiming at introducing a new generation of one-dimensional nano catalysts in organic reactions. For this purpose, iron oxide nanofibers, prepared by a combination of electrospinning and calcinations, were coated with silica using tetraethyl orthosilicate. Next, the silica surface was reacted with chlorosulfonic acid to obtain Fe2O3@SiO2\u2013SO3H nanofibers. Due to the significance of formamide group, we decided to examine the catalytic performance of Fe2O3@SiO2\u2013SO3H nanofibers as a novel magnetic nano catalyst in the N-formylation of amines using formic acid. We also decided to study the catalytic ability of Fe2O3@SiO2\u2013SO3H nanofibers in the synthesis of formamidine via the reaction of different amines with triethyl orthoformate as a second catalytic reaction.Poly (vinyl alcohol) (PVA) of 88,000 g/mol molecular weight and 88% degree of hydrolysis were obtained from Sigma-Aldrich (USA). The chemicals used in the study were all purchased from Merck Company. No further purification was done to the reagents with deionized water used as a solvent.Fe2O3@SiO2 nanofibers were prepared based on the method employed in our previous study [46]. In summary, 10 ml of 8% W/W PVA solutions were prepared in deionized water as a solvent. Then, 2 g ferric nitrate (Fe(NO3)3.9H2O) was added and stirred for 1 h at room temperature. The electrospinning process was conducted by Electroris (FNM Ltd., Iran, http://www.fnm.ir) that as an electrospinning device can control electrospinning parameters such as high voltage (1\u201335 kV), injection rate with syringe pump (0.1\u2013100 mL/h), drum rotating speed (0\u20131000 rpm), drum-to-nozzle distance (0\u2013300 mm), needle scanning rate (0\u2013100 mm/min) and the electrospinning media temperature. The electrospinning condition of the prepared solutions was as follows: 5ml syringe with an 18-gauge needle as a nozzle, a rotating drum with 300 rpm speed covered with an aluminum foil as a collector, the applied voltage of 20 kV, 2 ml/h for the rate of injection and 12 cm for the distance between the collector and nozzle. Next, the electrospun PVA/Fe(NO3)3.9H2O nanofibers were peeled off from foil and then calcinated at 600 \u00b0C for 6 h to entirely eliminate the organics with the heating rate of 20 \u00b0C/min, which produced iron oxide (Fe2O3) nanofibers.In the next step for coating with SiO2, a mixture of ethanol (15ml), deionized water (5ml) and aqueous ammonia solution (1 ml, 25 wt%) were obtained and stirred completely; then 25 mg of obtained Fe2O3 nanofibers were added to this mixture and stirred at room temperature. Then, the solution of 30 ml ethanol and 1 ml tetraethyl orthosilicate (TEOS) were mixed with dispersed Fe2O3 nanofibers using a syringe pump. Finally, after relaxation for 6 h, the core-shell nanofibers were separated using a super magnet and washed with ethanol and deionized water.200 mg of Fe2O3@SiO2 nanofibers were added into the mixture of diethyl ether (1 ml) and chlorosulfonic acid (0.6 ml) and stirred for 3h at room temperature until the release of hydrochloric acid gas was stopped. The residue was washed with diethyl ether and deionized water and dried for 6h at 100 \u00b0C in an oven to obtain the desired nanofibers.The amount of acid on nanofibers was determined using titration with base. For each titration operation, 50 mg of nanofibers was weighed and transferred to a beaker. Then, 2 ml from potassium hydroxide 0.1 M was added. The prepared suspension was stirred by a magnetic stirrer for 15 min. Then, the nanofibers were separated by a magnet, and the clear solution was titrated with 0.1 M hydrochloric acid in the presence of phenolphthalein.The characterization of obtained products was carried out using scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDS), transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and, vibrating sample magnetometer (VSM). SEM images were observed using SEM (PhilipsXL30 and S-4160) with gold coating equipped with EDS. TEM measurements were done at 120 kV (Philips, model CM120). Powder XRD spectrum was recorded at room temperature by a Philips X'pert 1710 diffractometer using Cu K\u03b1 (\u03b1\u00bc 1.54056 \u00c5) in Bragg-Brentanogeometry (\u03b8-2\u03b8). FT-IR spectra were obtained over the region 400\u20134000 cm\u22121 with NICOLETIR 100 FT-IR and spectroscopic grade KBr. The magnetic properties of catalyst were attained by Vibrating Sample Magnetometer/Alternating Gradient Force Magnetometer (VSM/AGFM, MDK Co., Iran, http://www.mdk-magnetics.com).For each reaction, 1 mmol of triethyl orthoformate was transferred to a small glass flask. Then, 1 mol% of catalyst (0.01 mmol H+ in the catalyst, 25 mg Fe2O3@SiO2\u2013SO3H nanofibers) was added. The obtained mixture was stirred by a magnetic stirrer for 15min. Then, 1 mmol of amine was added slowly to be stirred at room temperature (24\u201327 \u00b0C). The reaction progress was followed by thin-layer chromatography with hexane-ethyl acetate at a ratio of 1:4. After the completion of the reaction, 5 ml of dichloromethane was added, and the catalyst was separated by a magnet. The solvent was evaporated using vacuum distillation. The obtained precipitate in dichloromethane was dissolved and re-crystallized. Then, the isolated product was weighed on an electronic balance and used to compute percent yield based on isolated product weight in terms of the expected weight of the product. The synthesized compounds were identified by FT-IR, Mass and NMR spectroscopy and compared with those reported in the literature. The spectral data for selected products are presented below:\nN,N\u2032-Bis-(3,4-dicholoro-phenyl)-formamidine (Table\u00a03, entry 7): White solid, mp 134\u2013135 \u00b0C; 1HNMR (CDCl3, 400MHz): \u03b47.25 (m, 2H), 7.28 (d, J = 6.40 Hz, 1H), 7.41 (br s, 1H), 8.39 (m, 2H), 8.5 (s, 1H), 8.85 (s, 1H).FTIR: 697 cm_1, 749 cm_1, 820 cm_1, 866 cm_1, 1048 cm_1, 1100 cm_1, 1147 cm_1, 1296 cm_1, 1396 cm_1, 1468 cm_1, 1524 cm_1, 1587 cm_1, 1665 cm_1, 2895 cm_1, 3077 cm_1, 3242 cm_1, 3395 cm_1. MS m/z: 334(M+).\n1H-Benzimidazole (\nTable\u00a03, entry 9): Pale yellow solid, mp 169\u2013170 \u00b0C, 1H NMR (CDCl3, 400 MHz): \u03b47.10\u20137.23 (m, 2H),7.61\u20137.64 (m, 2H), 8.17 (s, 1H), 10.31\u201311.1 (br s, NH).FTIR: 619 cm_1, 743 cm_1, 876 cm_1, 950 cm_1, 1001 cm_1, 1125 cm_1, 1202 cm_1, 1243 cm_1, 1303 cm_1, 1405 cm_1, 1456 cm_1, 1591 cm_1, 1703 cm_1, 2736 cm_1, 2806 cm_1, 2854 cm_1, 2933 cm_1, 3061 cm_1, 3425 cm_1. MS m/z: 118 (M+).10 mg of Fe2O3@SiO2\u2013SO3H was added to 1.2 mmol aqueous formic acid. After five minutes of stirring, 1.0 mmol of amine was added and the reaction mixture was stirred at room temperature (24\u201327 \u00b0C). The reaction progress was followed by thin-layer chromatography with hexane-ethyl acetate at a ratio of 1:4. After the completion of the reaction, an external magnet was used to separate the catalysts. The reaction mixture was extracted with CH2Cl2 and H2O. The organic layer was then dried over anhydrous Na2SO4 and identified bythe1H NMR spectroscopy. The percent yield was calculated by the same procedure as mentioned in section 2.4. The spectral data for selected products are presented below:\n4- morpholin carbaldehyde (\nTable\u00a04\n, entry 8): colorless oil, 1H NMR (CDCl3, 400 MHz): \u03b43.37 (t, 2H, J = 5.1 Hz), 3.55 (t, 2H, J = 5.1 Hz), 3.62 (t, 2H, J = 5.1 Hz), 3.65 (t, 2H, J = 5.1 Hz), 8.04 (s, 1H).\nN-(tert-Butylamine) formamide (\nTable\u00a04, entry 6): colorless oil,1 HNMR (400 MHz, CDCl3): 50:50 (cis/trans) \u03b4 1.32 (s, 9H), 7.28 (d, 1H, J = 2.3, cis), 7.43 (br, 1H, cis), 7.51 (br, 1H, trans) 7.67 (d, 1H, J = 12.2, trans).The generation of well-controlled ceramic nanofibers is typically conducted as follows: (i) the preparation of an electrospinning solution containing a polymer and sol-gel precursor of the ceramic material, (ii) electrospinning the polymeric solution under appropriate conditions and (iii) the calcination of the polymer/precursor composite nanofibers at high temperature to remove polymers and obtain the ceramic phase [35, 36]. Coaxial electrospinning with two immiscible components or polymer in a core-shell nozzle followed by calcination is a conventional electrospinning method used for fabricating core-shell ceramic nanofibers [39]. However, similar to our previous study [46], in this study one-dimensional Fe2O3@SiO2 nanofibers were prepared by a different and new method with the idea of preparing of Fe2O3@SiO2 nanoparticles using a coating of iron oxide (Figure\u00a01\n). To do so, first, polymer/precursor composite nanofibers were fabricated by the routine uniaxial electrospinning of PVA polymer containing Fe(NO3)3 as an iron precursor. Iron oxide nanofibers were then obtained by calcinating polymeric nanofibers. Finally, these nanofibers were coated with silica by the sol-gel method in the vicinity of TEOS. The products of the three steps were characterized using SEM to approve one-dimensional nanostructure formation.The SEM images of PVA/Fe(NO3)3 nanofiber, Fe2O3 nanofiber and Fe2O3@SiO2 nanofibers are shown in Figure\u00a02\n. As can be seen in Figure\u00a02 a, b, the successful electrospinning of PVA/Fe(NO3)3 solution led to smooth and fine one-dimensional polymeric nanofibers without bead. However, when composite nanofibers were calcinated at high temperature controlled the rising speed, the surface of Fe2O3 nanofibers appeared with rough surface, bended shape and a few ruptures in the axial direction (Figure\u00a02 c, d). Finally, as shown in Figure\u00a02 e, f, successful and homogenous coating of iron oxide nanofibers with TEOS produced fine Fe2O3@SiO2 nanofibers. The average diameters of nanofibers were calculated by the measurement software based on 15 fibers at random in SEM image. The average diameters of PVA/Fe(NO3)3 nanofibers, Fe2O3 nanofibers and Fe2O3@SiO2 nanofibers were 198 \u00b1 20, 93 \u00b1 16 and 142 \u00b1 35 nm respectively, confirming that the diameter of composite nanofibers diminished during calcinations and the removal of organic phase whereas the diameters increased with the coating of Fe2O3 nanofibers.Continuing the coating, the functionalization of Fe2O3@SiO2 nanofibers was followed in this study in order to increase the acidity of surface to be used as novel heterogeneous acid catalyst. For this purpose, the silica surface was reacted with chlorosulfonic acid to obtain Fe2O3@SiO2\u2013SO3H nanofibers. Figure\u00a03\n a, b indicates the SEM analysis of Fe2O3@SiO2\u2013SO3H nanofibers with different magnification. It is observed that nanofibril structures were produced, but some rupture in fibers can be seen due to the increased acidity in the presence of strong acid functional that led to some corrosion in the direction of nanofibers, although the fiber structure with bigger direction than diameter is observable. The mean diameter of Fe2O3@SiO2\u2013SO3H nanofibers was determined using measurement software and the fiber diameter distribution histogram was drawn by Origin software. As can be seen in Figure\u00a03 c, diameter distribution is between 50 and 300 nm with a mean diameter up to 137 \u00b1 12 nm for Fe2O3@SiO2\u2013SO3H nanofibers. The TEM image of Fe2O3@SiO2\u2013SO3H nanofibers is shown in Figure\u00a03 d. As the TEM image shows, Fe2O3 nanofibers were coated with the uniform layer of silica that was functionalized with SO3H. It is observed that the dense layer of SiO2\u2013SO3H as thick as 7 nm exists on the surface of Fe2O3 nanofibers. In contrast to Fe2O3@SiO2 nanofibers (15 nm for SiO2 layer [46]), it seems that the thinner layer of shell surrounded the magnetic core probably as a result of the corrosion of silica surface during the functionalization with chlorosulfonic acid.In order to determine the elemental composition and confirm the functionalization of nanofibers, Fe2O3, Fe2O3@SiO2 and Fe2O3@SiO2\u2013SO3H nanofibers were evaluated by EDS analysis (Figure\u00a04\n).As can be seen, the Fe and O patterns exist as the main elements in the quantitative analysis of the three nanostructures. Furthermore, Si element can be seen in Fe2O3@SiO2 and Fe2O3@SiO2\u2013SO3H nanofibers patterns. The presence of S in the EDS analysis of Fe2O3@SiO2\u2013SO3H nanofibers confirmed the successful functionalization of Fe2O3@SiO2 nanofibers surface with sulfonic acid.\nFigure\u00a05\n shows the FT-IR spectra of PVA/Fe(NO3)3 nanofibers, Fe2O3 nanofibers, Fe2O3@SiO2 nanofibers and Fe2O3@SiO2\u2013SO3H nanofibers. The FT-IR of PVA/Fe(NO3)3 nanofibers in Figure\u00a05 a exhibited various transmittance peaks at 1300-1800 cm\u22121 which were attributed to the functional group of PVA. The main peak in 1721 cm\u22121 corresponding to the (C =O) residual from primary vinyl acetate can be seen in Figure\u00a05 a, but this peak decreased significantly in Figure\u00a05 b, c and d due to the removal of PVA during calcinations at high temperature. In Figure\u00a05 b two peaks appeared at 463 and 548 cm\u22121 which could be assigned to the Fe\u2013O vibration of the Fe2O3 nanofibers [46] shifting to 461 cm\u22121 and 581 cm\u22121 in Fe2O3@SiO2\u2013SO3H nanofibers. In all of Fe2O3@SiO2\u2013HA Bronsted acids, the band at 400\u2013650 cm\u22121 is assigned to the stretching vibrations of (Fe\u2013O) bond [15, 80].The strong peak at 1031 cm\u22121 in Fe2O3@SiO2 nanofibers spectra (Figure\u00a05 c) corresponds to the vibrations of the Si\u2013O bond that appeared as shoulder of broad peak in 1000\u20131100 cm\u22121 at Fe2O3@SiO2\u2013SO3H nanofibers spectra (Figure\u00a05 d) as well as the peaks around 800 cm\u22121 correspond to Si\u2013O\u2013Si [81]. The broad peak that appeared at about 3409 could be shown the stretching of the OH group in the SO3H moiety [16]. Based on the results of previous studies on Fe2O3@SiO2\u2013SO3H nanoparticle synthesis, it can be understood that functionalization with SO3H lead to a wider peak than Fe2O3@SiO2 nanoparticles at about 3400 cm\u22121 [16,23-28]. The same result could be found in Figure\u00a05 by comparing the spectra. The peak at 1631 cm\u22121 represents the physical absorption of the lower amount of water [28]. The peaks of 1089 cm\u22121 and 1384 cm\u22121 are likely hidden under the wide peak at 1000-1400 cm\u22121 that are related to O=S=O stretching vibration in \u2013SO3H groups [82,83].Diffraction peaks at around 23.97\u00b0, 28.29\u00b0, 29.97\u00b0, 33.09\u00b0, 35.49\u00b0, 40.29\u00b0, 49.41\u00b0, 53.97\u00b0, 62.37\u00b0 and 63.81\u00b0 which related to the (012), (031), (411), (104), (110), (512), (024), (116), (214) and (300) are readily recognizable from the XRD pattern (Figure\u00a06\n). The observed diffraction peaks agree well with the structure of Fe2O3 (1999 JCPDS file No 24-0072 and 16-0653). It seems that the silica sheath of core-shell nanofibers was in amorph structure without the distinct sharp peak in the XRD pattern. However, the broad peaks at position 10-20\u00b0 could be assigned to the small amount of silica.The magnetic properties of Fe2O3@SiO2\u2013SO3H nanofibers were characterized by a vibrating sample magnetometer (VSM) (Figure\u00a07\n). The magnetic saturation was about 13.4 emu g\u22121 that is a suitable amount for nanostructure as a magnetic catalyst. The amount of the magnetic saturation of Fe2O3@SiO2\u2013SO3H nanofibers was very close to the value of Fe2O3@SiO2 nanofibers (14 emu) [46], which can indicate that functionalization with SO3H had no effects on it. Also, the hysteresis loop was not observed confirming that the catalyst is a super paramagnetic material.The number of acids functionalized on the surface of core-shell nanofibers was detected using the reverse titration method. It was observed that 0.4 mmol\u22121 g of acid existed in the Fe2O3@SiO2\u2013SO3H nanofibers that could be enough for catalytic application. The data can be used to specify the meaning of mol% H+ in catalysts. Mole percent H+ is the percentage of the moles of H+ in proportion to the total number of moles in a mixture. If we use 1mmol of reagent, it is about 0.01 mmol H+ that equals 25 mg Fe2O3@SiO2\u2013SO3H nanofibers according to the calculation of the number of acids in the surface of Fe2O3@SiO2\u2013SO3H nanofibers.Although sulfuric acid acts industrially as an effective catalyst, its aqueous environment can interfere with many reactions because many reactants are sensitive to water that can accelerate the rearrangement and production of by-products, reducing the efficiency of the desired reaction. In addition, it can lead to high corrosion in industrial reaction devices. Using catalysts situated on the surface of a mineral solid, such as silica, can be very effective in avoiding the presence of water in the catalyst as well as raising its specific surface area. Core-shell magnetic silica with sulfuric acid as an insoluble, non-corrosive catalyst with high acidity brings ease of working and separation at the end of the reaction by the magnet; it also makes it possible to use it again in a similar reaction without reducing the catalytic power.Formamidines can be prepared in several ways using different starting materials [15] in most of which toxic solvents are used. High temperature, long reaction time, severe acidity conditions, low yield, hard separation and using additional amounts of reactant and catalyst are the problems of these reactions. Triethyl orthoformate is one of the good starting materials that reacts with amines to produce formamidine. In this study, the reaction was done for the first time in acetic acid under reflux at 140\u2013150 \u00b0C for 1.5\u201394 h [23]. The use of liquid acid, high temperature and long reaction time are the three disadvantages of this reaction. Due to the importance of formamidines, the catalytic ability of Fe2O3@SiO2\u2013SO3H nanofibers in the preparation of formamidines was studied in mild condition and without solvent. For this purpose, the reaction between triethyl orthoformate and aniline was selected as the model reaction (Figure\u00a08\n).First, the reaction progress of triethyl orthoformate and aniline at room temperature was followed by thin-layer chromatography in the presence of Fe2O3@SiO2\u2013SO3H nanofibers. After three hours, the product achieved a yield of 86%. Proper efficiency at low temperature indicates the catalytic activity of Fe2O3@SiO2\u2013SO3H nanofibers in the direct reaction of formamidine preparation. Next, the effect of an increase in the temperature on the model reaction was investigated (Table\u00a01\n). The increase in temperature could increase the speed and efficiency of the reaction. The addition of 1 mol% of catalyst at 70 \u00b0C synthesized 92 % formamidine during 1 h (Table\u00a01, entry 3). The reaction was remarkably fast. Next, the effect of catalyst amount on the reaction efficiency was investigated. By increasing the amount of catalyst to 3 mol%, the reaction efficiency increased to 96% (Table\u00a01, entry 4). Without a catalyst, no product was prepared at room temperature. At 70 \u00b0C, a crystalline amine was formed, but formamidine was not obtained meaning that the reaction needed the catalyst (Table\u00a01, entry 5).Moreover, the catalytic effect of iron oxide nanofiber with silica coating (Fe2O3@SiO2) was investigated in this reaction (Table\u00a01, entry 6). By adding the same values of this nanofiber to the reaction and performing it in optimal conditions, it was observed that after four hours, the product was obtained with a yield of 27%, indicating the effectivity of functional group on reaction efficiency, which increased by increasing the acidity of the catalyst surface.The comparison of the catalytic activity of some catalyst in formamidine synthesis was shown in Table\u00a02\n. Sheykhan et\u00a0al. used Fe2O3@SiO2-HA as a magnetic solid acid catalyst to synthesize formamidine (Table\u00a02, entry 3,4) [15]. As observed in Table\u00a02, the yield of reaction was low in the presence of Fe2O3@SiO2-HCLO4 nanoparticles (entry 3); however, better results were achieved with Fe2O3@SiO2\u2013SO3H nanofibers (entry 1). When Fe2O3@SiO2\u2013HBF4 nanoparticles (entry 4) were used as catalysts, the yield of reaction exceeded, but the reaction time was still more than Fe2O3@ SiO2\u2013SO3H nanofibers. It seems that catalysts that are functionalized with SO3H, such as sulfonated rice husk (RH-SO3H), had very good results (Table\u00a02, entry 5) but with a difficult catalyst separation process in spite of magnetic catalyst [84]. Archibald et\u00a0al. synthesized formamidine complex in reflux conditions and low yield of product with acetic acid as a liquid acid catalyst (Table\u00a02, entry 6) [85]. The reaction with cyclodextrin (CD) as a catalyst (Table\u00a02, entry 7) [86] was done at more time (14 h). It was revealed that Fe2O3@SiO2\u2013SO3H nanofibers as a novel magnetic catalyst performed this reaction in a shorter time and with higher efficiency. Also, all the above-mentioned studies used more amount of reactant (2 mmol amine) and catalyst (0.05 mmol Fe2O3@SiO2\u2013HBF4 nanoparticles), but the new procedure of this study included using 1mmol of amine and 0.03 mmol of Fe2O3@SiO2\u2013SO3H nanofibers.Finally, the optimum conditions were extended to all types of aniline derivatives. The results of the study revealed that in all cases, high-yield products were obtained and the procedure was applicable in the synthesis of a wide range of formamidine derivatives (Figure\u00a09\n, Table\u00a03\n). The reaction in the presence of aniline derivatives was carried out conveniently with the electron donation, withdrawing group and more than one substitution. This method proves to be effective with p-bromoaniline, p-fluoroaniline, 2,4-dichloroaniline and p-nitroaniline as derivatives of aniline with the electron-withdrawing group on the aromatic ring (Table\u00a03, entry 5, 6, 7, 8). As shown in Table\u00a03, the withdrawing electrons groups led to a decrease in efficiency. The yield of reaction with p-bromoaniline was 80% in 3h while the same product achieved in70% in 4h in the presence of Fe2O3@SiO2\u2013HBF4 nanoparticles [15]. In contrast to Fe2O3@SiO2\u2013HBF4 [15] nanoparticles and RH-SO3H [84], the novel magnetic nanofibers were successful at the synthesis of electron deficient rings such as 2,4-dichloroaniline and p-nitroaniline (Table\u00a03, entry 7,8).To test the possibility of inter-molecular reaction, the reaction of orthophenylenediamine with triethyl orthoformate was done and led to produce benzimidazole in a very high yield (Table\u00a03, entry 9). Among different derivatives of formamidine, benzimidazole had the highest yield due to the high speed of intermolecular reaction. Aiming to prepare unsymmetrical diaryl orthoformamidines, we decided to do a reaction using a mixture of 50:50 mol/mol of aniline and p-toluidine with triethyl orthoformate under the same reaction conditions. The result was a product with 84\u201386 \u00b0C meting point and three new stain in TLC (Table\u00a03, entry 10). It seems that the product is a mixture of unsymmetrical formamidine and two corresponding symmetrical formamidines which is consistent with Robert's finding in the same reaction in the presence of acid [87]. We also tried to synthesize aliphatic formamidine using this procedure. For this purpose, the reaction of t-butyl amine and triethyl orthoformate under the same reaction conditions was examined (Table\u00a03, entry 11). It has been observed in the literature that the formamidines with an aliphatic residue can quickly decompose to the corresponding amines when exposed to silica, and they are less stable than aryl formamidines [88].The magnetic recycling of catalysts was also investigated. Thus, the reaction between triethyl orthoformate and aniline was studied in the presence of 3 mol% of Fe2O3@SiO2\u2013SO3H nanofibers at 70 \u00b0C during 1h reaction time. After the completion of the reaction, an external magnet was employed to separate the mixture, and recovered Fe2O3@SiO2\u2013SO3H nanofibers were reused in a subsequent reaction without any significant decrease in activity even after five runs. The isolated product yield decreased slowly from 96% to 92% after five cycles (Figure\u00a010\n). In conclusion, we can introduce Fe2O3@SiO2\u2013SO3H nanofibers as a new and powerful recyclable magnetic catalyst for the conversion of amines to formamidine derivatives under solvent-free conditions. The notable advantages of this catalyst including industrial-scale production, reusability, low-cost separation process of the catalyst by using a magnet and the operational simplicity of the method and its solvent-free condition can present this catalyst as an important alternative to previously reported methods.The methods previously used for amine N-formylation [53, 54, 55, 56, 57, 58] have some disadvantages such as high temperature, long reaction time, additional amounts of reactants, the use of reactors, toxic and harmful solvents for the environment, low efficiency and use of expensive and harmful compounds. Because of the importance of the N-formylation reaction and the problems mentioned, the N-formylation reaction of amines was performed in this study using Fe2O3@SiO2\u2013SO3H nanofiber catalyst and N-formyl products were obtained under room temperature conditions and in short time with high yields.A variety of amines were used to investigate the magnetic Fe2O3@SiO2\u2013SO3H nanofibers catalysis in the formylation reaction (Figure\u00a011\n) and the results are summarized in Table\u00a04\n. The formylation of different types of amines, including aliphatic, acyclic, aromatic and heterocyclic compounds, were considered in this study. As shown in Table\u00a04, aromatic amines such as aniline and p-toluidine reacted in excellent yields to produce the corresponding N-formyl compounds (entries, 1, 2). The most effective result was gained with the amine/formic acid molar ratio of 1\u20131.2 with 0.01 g of the catalyst at room temperature in solvent-free conditions (Table\u00a04, entry 1,3). The same result can be found in the synthesis of formamide from the N-formylation of amines in the presence of Natrolite zeolite as catalyst [60]. However, more catalyst demand (0.02 g) and hard separation process are the disadvantages of the Natrolite zeolite catalyst. The sterically-hindered amines (entry 6) and poorly reactive ones such as p-nitroaniline (entry 4) were easily formylated providing corresponding formamides in good yields. The conversion of p-nitroaniline to the corresponding amides was carried out in 94% with Fe2O3@SiO2\u2013SO3H nanofibers as a catalyst while the yield of reaction was 83% and 90% in the presence of the Natrolite zeolite [60] and RH-SO3H [84] catalysts, respectively. In addition, Nishikawa et\u00a0al. were not able to do the N-formylation of p-nitroaniline probably due to the low acidity of TEOS asa catalyst [65]. It is worth mentioning that the main problem of TEOS is the hard recovery of the liquid catalyst. The method is effective for the formylation of aliphatic amines (entries 6, 8) with high yield. This method can be applied to convert orthophyllen-diamine to benzimidazole in high yields (entry 9) as an inter-molecular reaction. Imidazole as a heterocyclic amine reacted with formic acid with a 94% yield (entry 10). The chemo selectivity of reaction can be seen in the N-formylation of amine functional group in phenyl hydroxyl amine without O-formylation (Entry 11).The reaction of aniline and formic acid in the presence of Fe2O3@SiO2\u2013SO3H nanofibers was used to study the possibility of the magnetic recycling of catalysts. The catalyst was easily separated from the product using an external magnet due to the magnetic properties of the nanofibers. The separated catalysts were applied in the sample reaction over four cycles in high yields.In conclusion, Fe2O3@SiO2\u2013OSO3H nanofibers were prepared in the following steps: electrospinning, calcinations, coating with silica layer and functionalization with chlorosulfonic acid. The modified, novel, one-dimensional nanostructure was characterized by SEM, TEM, EDS, VSM and FT-IR approving the formation of a core-shell nanofibrous structure. The results revealed that magnetic core-shell nanofibers modified with acid can act as a novel catalyst for the synthesis of formamide and formamidine derivatives with high efficiency. This reaction normally requires high temperature, high acid amount, long reaction conditions and often a difficult catalyst separation process whereas in the method used in this study, most of the above-mentioned disadvantages were removed. The catalyst can be separated by magnet and reused several times in organic reactions without the significant reduction of the yield.Hakimeh Ziyadi: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper.Mitra Baghali: Performed the experiments; Wrote the paper.Akbar Heydari: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.This work was supported by Islamic Azad University, Medical Sciences Tehran, Iran (IAUMST).Data included in article/supplementary material/referenced in article.The authors declare no conflict of interest.No additional information is available for this paper.The authors would like to thank the Active Pharmaceutical Ingredients Research Center (APIRC) and Chemistry Department of Tarbiat Modares University for equipment and laboratory services.", "descript": "\n                  Over the past several decades, the fabrication of novel ceramic nanofibers applicable in different areas has been a frequent focus of scientists around the world. Aiming to introduce novel ceramic core-shell nanofibers as a magnetic solid acid catalyst, Fe2O3@SiO2\u2013SO3H magnetic nanofibers were prepared in this study using a modification of Fe2O3@SiO2 core-shell nanofibers with chlorosulfonic acid to increase the acidic properties of these ceramic nanofibers. The products were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray spectroscope (EDS), vibrating sample magnetometer (VSM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). The prepared nanofibers were used as catalysts in formamide and formamidine synthesis. The treatment of aqueous formic acid using diverse amines with a catalytic amount of Fe2O3@SiO2\u2013SO3H nanofibers as a reusable, magnetic and heterogeneous catalyst produced high yields of corresponding formamides at room temperature. Likewise, the reaction of diverse amines with triethyl orthoformate led to the synthesis of formamidine derivatives in excellent yields using this novel catalyst. The catalytic system was able to be recovered and reused at least five times without any catalytic activity loss. Thus, novel core-shell nanofibers can act as efficient solid acid catalysts in different organic reactions capable of being reused several times due to their easy separation by applying magnet.\n               "}
{"full_text": "Data will be made available on request.Lignocellulose, the natural composite of cellulose, hemicellulose and lignin, is the most abundant form of biomass [1]. Different from cellulose and hemicellulose, lignin is composed of aromatic units, such as sinapyl (S), coniferyl (G) and p-coumaryl (H) alcohols as well as ferulic (FA) and p-coumaric (pCA) acids [1]. Nowadays, cellulose and hemicellulose are fully utilized in pulping and the second-generation (2G) bioethanol industries. The 2G biofuel industry produces enzymatic hydrolysis lignin (EHL) as a low-value and large volume byproduct. As a renewable resource of aromatic molecules, EHL is an ideal feedstock for the sustainable production of commercial aromatic chemicals and fuels.Catalytic lignin solvolysis (CLS) is a promising route to directly produce aromatic chemicals at relatively mild reaction conditions [1]. In CLS reaction, lignin is firstly dissolved and depolymerized in a solvent before contact with a catalyst [2]. However, due to its complex and highly cross-linked structure, lignin cannot be efficiently dissolved in most of the solvents at ambient temperature, and a high reaction temperature (\u223c300\u00a0\u00b0C) is often needed for CLS to achieve complete lignin conversion. Nevertheless, high reaction temperature also promotes the repolymerization/condensation of active monomers and self-conversion of solvent, resulting in the formation of char and complex products [3\u20137]. Yan and his co-workers have examined CLS at relatively low temperature (100\u00a0\u223c\u00a0200\u00a0\u00b0C) in water with noble metal catalysts, but, due to the low lignin solubility in water, the monomer yields are only around 7\u00a0\u223c\u00a08\u00a0wt% [8,9].Ethylene glycol is a green solvent that can be produced from the conversion of cellulose and has been widely used for the production of chemicals and fuels [10]. Recently, ethylene glycol was used as a solvent for lignin depolymerization and fractionation, due to its high lignin solubility. For example, Song et al. [11] depolymerized lignosulfonate with a Ni/C catalyst in ethylene glycol at 200\u00a0\u00b0C under 5\u00a0MPa H2, and achieved 68\u00a0wt% conversion of lignosulfonate, with 4-propyl guaiacol and 4-ethyl guaiacol as the main monomer products. Ren et al. [12] used ethylene glycol as a solvent for fractionation of lignin in poplar sawdust with Ru/C and H2SO4 as co-catalysts, and obtained 24.1\u00a0wt% phenolic monomers at 185\u00a0\u00b0C for 6\u00a0h. Nevertheless, the mechanism of lignin dissolution in ethylene glycol is still not clear and the steps of lignin depolymerization at low temperatures also need to be further elucidated.Herein, EHL dissolution at room temperature and solvolysis at 200\u00a0\u00b0C in different solvents are examined. Ethylene glycol achieves complete EHL dissolution and gives the highest monomer yield among the solvents examined. The interaction between ethylene glycol and lignin molecules is investigated with 1H and 13C NMR and Gaussian simulation. The roles of Ni and NaOH catalysts in EHL solvolysis are discussed based on the GPC and HSQC-NMR results as well as molecular dynamics simulation. Based on these results, the mechanisms of EHL dissolution and solvolysis in ethylene glycol are proposed.The EHL was provided by Shandong Long Live biological technology Co., Ltd. which was obtained from the microbial enzymatic hydrolysis of corncob to produce ethanol. The composition of EHL, 91.2\u00a0wt% lignin, 0.12\u00a0wt% residual carbohydrate and 1.42\u00a0wt% ash, has been reported in our previous work [13]. The solvents (AR), including cyclohexane, ethyl acetate, isopropanol, ethanol, methanol and ethylene glycol, were purchased from VWR Chemicals. NiCl2\u00b76H2O (>99.9%), NaOH (>99.9%) and NaBH4 (>98%) MgO (>99.9%), ZrO2 (>99.9%) and Al2O3 (>99.9%) were purchased from Sigma Aldrich. Ferulic acid (>99.9%) and coniferyl alcohol (>98%) were also purchased from Sigma Aldrich. Anisole (99%) was supplied by Acros Organics.The Ni catalyst was prepared via the reduction of NiCl2\u00b76H2O with NaBH4. NaOH (0.5\u00a0g) and NaBH4 (1\u00a0g) were dissolved in 30\u00a0mL deionized water and the solution formed was dropped into the solution of NiCl2 (4.05\u00a0g NiCl2\u00b76H2O in 50\u00a0mL deionized water) with magnetic stirring at room temperature. The black precipitate, i.e., the Ni catalyst, was washed with 100\u00a0mL deionized water for 4 times and preserved in deionized water before use.Solid bases, including NaOH/MgO, NaOH/ZrO2, NaOH/Al2O3, were prepared through incipient wetness impregnation technique with prescribed 20\u00a0wt% NaOH loading. After drying at 100\u00a0\u00b0C overnight, the simples were calcined at 450\u00a0\u00b0C for 4\u00a0h.The mixture of EHL (1\u00a0g) and solvent (20\u00a0mL) was treated with ultrasound for 30\u00a0min at room temperature and then left to stand at room temperature for 72\u00a0h. The EHL solution and insoluble EHL were separated with filtration, and the insoluble EHL was dried at 80\u00a0\u00b0C for 24\u00a0h. The amount of dissolved EHL was calculated with Eq. (1).\n\n(1)\n\n\nT\nh\ne\n\na\nm\no\nu\nn\nt\n\no\nf\n\nd\ni\ns\ns\no\nl\nv\ne\nd\n\nE\nH\nL\n\n\n\n%\n\n\n=\n\n\nT\nh\ne\n\nw\ne\ni\ng\nh\nt\n\no\nf\n\na\nd\nd\ne\nd\n\nE\nH\nL\n-\nT\nh\ne\n\nw\ne\ni\ng\nh\nt\n\no\nf\n\ni\nn\ns\no\nl\nu\nb\nl\ne\n\nE\nH\nL\n\n\nT\nh\ne\n\nw\ne\ni\ng\nh\nt\n\no\nf\n\na\nd\nd\ne\nd\n\nE\nH\nL\n\n\n\u00d7\n100\n\n\n\n\n\n31P NMR spectra of methanol-soluble (M-soluble) and methanol-insoluble (M-insoluble) EHL were measured according to a method in literature [14]. The sample (40\u00a0mg) was dissolved in the mixture of pyridine and deuterated chloroform (1.6:1 v/v, 0.4\u00a0mL). Cholesterol (19\u00a0mg/ml, 0.2\u00a0mL) was added as an internal standard, while chromium-III-acetylacetonate (19\u00a0mg/ml, 0.05\u00a0mL) was applied as a relaxation reagent. 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (0.1\u00a0mL) was employed as a phosphitylation reagent. After phosphitylation for 2\u00a0h, the sample was moved into an NMR tube and measured with a Bruker AVANCE III HD 400\u00a0MHz instrument. The heteronuclear single quantum coherence nuclear magnetic resonance (HSQC-NMR) spectra of M-soluble and M-insoluble EHL were recorded with the same instrument. For HSQC-NMR, the sample (50\u00a0mg) was dissolved in DMSO\u2011d\n6 (0.6\u00a0mL) as the deuterated NMR solvent.The 1H NMR and 13C NMR spectra of EHL, the mixture of EHL and methanol (EHL-MET) and the mixture of EHL and ethylene glycol (EHL-EG) were acquired with the same instrument. 50 and 100\u00a0mg EHL were dissolved in 0.6\u00a0mL DMSO\u2011d\n6 for 1H NMR and 13C NMR spectra, respectively. For EHL-MET and EHL-EG, methanol (0.1\u00a0mL) and ethylene glycol (0.1\u00a0mL) were added into EHL and the DMSO\u2011d\n6 mixtures, respectively.EHL solvolysis was carried out in a 50\u00a0mL batch reactor (Parr 4597, Hastelloy C-276) equipped with a temperature controller and a pressure sensor. In a typical test, EHL (1\u00a0g), Ni (1\u00a0g), NaOH (0.5\u00a0g) and ethylene glycol (25\u00a0mL) were added into the reactor. The reactor was sealed and purged with nitrogen for six times, and then purged with hydrogen for three times, and finally pressurized to 3\u00a0MPa H2 at room temperature. The reactor was then heated to 200\u00a0\u00b0C and remained for 6\u00a0h with a fixed stirring rate of 600\u00a0rpm.After reaction, the mixture was filtrated to separate the solid catalyst and liquid product. NaOH in the liquid product is neutralized with HCl, and then the liquid product was extracted with deionized water (60\u00a0mL) and dichloromethane (30\u00a0mL). Floccule formed at the interface of two phases during extraction and was separated with a filtration technique. The monomer products were extracted into the dichloromethane phase and were analyzed qualitatively with Shimadzu GC\u2013MS (QP2010SE with Optic 4) and quantitatively with an Agilent 7890 GC equipped with an FID. For both GCs, the working conditions were the same. The oven temperature program was set from 45 to 250\u00a0\u00b0C at 10\u00a0\u00b0C/min and then held at 250\u00a0\u00b0C for 7\u00a0min. The solvent delay was set as 2\u00a0min for the MS detector. HP-5 MS capillary column (30\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a00.25\u00a0\u00b5m) and a split ratio of 50 were used. The mass detector was set to scan the m/z range from 10 to 500. Anisole was used as an internal standard to quantify the products. The total monomer yield was calculated with Eq. (2):\n\n(2)\n\n\n\nT\nh\ne\n\nt\no\nt\na\nl\n\nm\no\nn\no\nm\ne\nr\n\ny\ni\ne\nl\nd\n\n=\n\n\nT\nh\ne\n\nw\ne\ni\ng\nh\nt\n\no\nf\n\nt\no\nt\na\nl\n\nm\no\nn\no\nm\ne\nr\ns\n\n\n\nt\nh\ne\n\nw\ne\ni\ng\nh\nt\n\no\nf\n\nE\nH\nL\n\na\nd\nd\ne\nd\n\ni\nn\nt\no\n\nt\nh\ne\n\nr\ne\na\nc\nt\ni\no\nn\n\n\n\n\n\n\n\nThe average molecular weight of the EHL and floccule were determined with an Agilent HPLC system with Phenogel (5\u00a0\u03bcm\u20135\u00a0nm and 100\u00a0nm) columns and a UV detector at 280\u00a0nm\u00b7THF was used as an eluent at a rate of 1.0\u00a0mL\u00a0min\u22121 and the analysis was carried out at room temperature. Calibration was performed using polystyrene standards ranging from 30300\u00a0g\u00a0mol\u22121 to 208\u00a0g\u00a0mol\u22121. The EHL and floccule samples were acetylated before analysis to make them soluble in THF [15]. The HSQC-NMR spectra of EHL and floccule were recorded with the same Bruker instrument. The sample (50\u00a0mg) was dissolved in DMSO\u2011d\n6 (0.6\u00a0mL) as the deuterated NMR solvent.The infrared spectra of liquid ethylene glycol as well as adsorbed ethylene glycol and ethylene glycol-lignin monomers mixture were obtained with attenuated total reflectance-Fourier transform infrared spectrometer (ATR-FTIR, PerkinElmer Co.) The scan number was 200 and the spectral resolution was set as 4\u00a0cm\u22121. Adsorbed samples were prepared through heating Ni catalyst (0.5\u00a0g) in pure ethylene glycol (10\u00a0mL) or ethylene glycol-lignin monomer (0.1\u00a0g ferulic acid or coniferyl alcohol in 10\u00a0mL ethylene glycol) at 200\u00a0\u00b0C for 1\u00a0h. After cooling, these samples were washed with acetone and dried at 60\u00a0\u00b0C.The Gaussian simulation was carried out with the Gaussian16 package using M062X simulation method in conjunction with the 6\u201331\u00a0g(d) basis set [16]. The interaction energies between solvent and phenol or benzene are calculated according to Eq. (3), where H is the enthalpy and BSSE is the Basis Set Superposition Error acronym.\n\n(3)\n\n\nH\nint\n\n=\n\nH\n\nA\n+\nB\n\n\n-\n\n\n\nH\nA\n\n+\n\nH\nB\n\n\n\n+\n\nE\nBSSE\n\n\n\n\nThe non-covalent interaction (NCI) analysis is carried out with software Multiwfn and VMD [17]. Before NCI analysis, structures are firstly optimized with Gaussian simulation.The Forcite module in Material Studio was used for molecular dynamics simulation of the competitive adsorption of lignin dimer (1-(4-hydroxyphenyl)-2-phenoxypropane-1,3-diol (Fig. 4 (b)) and ethylene glycol over Ni surface. The Ni (111) facet with a (10\u00a0\u00d7\u00a014\u00a0\u00d7\u00a04) supercell was chosen as the Ni surface model, above which are 400 ethylene glycol and 2 lignin dimer molecules built with amorphous cell module in Material Studio. Before the simulation, the unit cell and atomic position of the model are firstly optimized. In order to quickly obtained the optimized model, it was quenched for 5 cycles from 26.85 to 226.85\u00a0\u00b0C, and then underwent an isobaric (NPT) molecular dynamics simulation for one picosecond (ps). After that, this model underwent NPT simulation at 200\u00a0\u00b0C for 500\u00a0ps, and then underwent isothermal molecular dynamics (NVT) simulation at 200\u00a0\u00b0C for 1000\u00a0ps. In all of these simulations, Nose and Berendsen were used as temperature and pressure control algorithm, respectively.The amount of EHL dissolved in 20\u00a0mL solvent was examined at room temperature, and the results are shown in Fig. 1\n(a). In cyclohexane, ethyl acetate, isopropanol, ethanol and methanol, the amount of dissolved EHL shows a positive linear relationship with the solvent polarity (\u03b4H). Nevertheless, this relationship does not fit EHL dissolution in ethylene glycol. The solvent polarity of ethylene glycol is close to that of methanol, but ethylene glycol achieves the complete dissolution of EHL, while methanol only dissolves 47.9% EHL. When the EHL-ethylene glycol solution is diluted with 60\u00a0mL ethanol, no EHL is precipitated. Increasing the dosage of EHL to 3\u00a0g with keeping the volume of ethylene glycol unchanged (20\u00a0mL), the residue remaining on filter paper is not the original EHL solid particles, but a viscous liquid (Fig. S1).The methanol insoluble EHL cannot be dissolved with fresh methanol, indicating that the soluble and insoluble parts have different structures. Therefore, the contents of hydroxyls in M-soluble and M-insoluble EHL were determined with 31P NMR, and the results are depicted in Fig. 1 (b). The contents of aliphatic-OH, aromatic-OH and carboxylic-OH in M-soluble EHL are 1.00, 1.37 and 0.68, respectively, much higher than that of M-insoluble EHL, which are 0.65, 0.74 and 0.35, respectively. The linkages in M-soluble and M-insoluble EHL were determined with HSQC-NMR, and the spectra are illustrated in Fig. 1 (c). In the spectrum of M-insoluble EHL, the intensity of the signal of \u03b2-O-4 linked structures (A\u03b3) obviously decreases, and more intense signals of C-C linkages are detected, compared to those in the spectrum of M-soluble EHL, indicating that M-insoluble EHL has more C-C linkages and less \u03b2-O-4 linkages than M-soluble EHL.Non-catalytic EHL solvolysis reactions in different solvents were examined at 200\u00a0\u00b0C under 3\u00a0MPa H2 for 6\u00a0h. As depicted in Fig. 2\n (a), the monomer yields obtained show a positive correlation with the amount of dissolved EHL in these solvents, and ethylene glycol gives the highest total monomer yield, i.e., 5.5\u00a0wt%. After that, catalytic EHL solvolysis reactions in ethylene glycol were examined with Ni catalyst, with NaOH, and with both Ni and NaOH as co-catalysts under the same reaction conditions. The Ni catalyst was prepared via the reduction of NiCl2 with NaBH4, which is a classic catalyst that has been widely employed in many hydrogenation reactions (its characterizations are shown in Fig. S2) [18,19]. Fig. 2 (b) and (c) show the total monomer yields and monomer structures obtained, and Scheme S1 gives the yields of individual monomers obtained. With Ni catalyst, the total monomer yield is 8.2\u00a0wt%, lower than that obtained with NaOH, which is 14.6\u00a0wt%. When Ni and NaOH catalysts co-existed, the total monomer yield is up to 18.8\u00a0wt%. Without a catalyst, most of the monomers obtained have carbon\u2013carbon double bonds in their side chains, and para-alkyl phenols (para-ethyl phenol, para-ethyl guaiacol and para-propyl guaiacol) and phenols without para side chains (phenol, guaiacol and syringol) are also detected. With Ni catalyst, carbon\u2013carbon double bonds are hydrogenated, and para-propanol syringol appears. With NaOH as a catalyst, C2-ketone and C3-ketone substituted syringol appear, which are typical products formed in soluble-base catalyzed lignin conversion reactions [20]. When Ni and NaOH catalysts co-existed, the product distribution is similar to that obtained with only NaOH.The effects of NaOH, with keeping 1\u00a0g Ni catalyst unchanged, and Ni, at 0.5\u00a0g dosage of NaOH, amounts on the total monomer yield are plotted in Fig. 2 (d). The total monomer yield is only 12.2\u00a0wt% when 0.25\u00a0g NaOH is added and increases to 18.8\u00a0wt% with 0.5\u00a0g NaOH. Further increasing the amount of NaOH results in the decrease of the total monomer yield. With the increase of the amount of Ni catalyst from 0.25 to 0.75\u00a0g, the total monomer yield increases from 16.4 to 18.9\u00a0wt%, and is not obviously changed when the Ni catalyst amount further increases to 1\u00a0g.The recyclability of the Ni catalyst is shown in Fig. S3 (a). The Ni catalyst (1\u00a0g) was separated from the liquid products with filtration and then directly reused with fresh NaOH (0.5\u00a0g). During 4 runs of the Ni catalyst, the total monomer yield slightly decreases from 18.8 to 17.0\u00a0wt%. Nevertheless, the XRD pattern of the used Ni catalyst indicates that the Ni catalyst has transformed from an amorphous phase (Fig. S2(a)) to a crystalline phase (Fig. S3 (b)) after the first time run. Hence, the phase transition of the Ni catalyst does not obviously affect its activity on EHL solvolysis.The activities of solid bases, including MgO and NaOH supported on different metal oxides (NaOH/MgO, NaOH/ZrO2, and NaOH/Al2O3), are also examined (Fig. 2 (e)). When 0.5\u00a0g solid bases are added with 1\u00a0g Ni as co-catalysts, the total monomer yields obtained are around 10\u00a0wt%, much lower than that obtained with 1\u00a0g Ni and 0.5\u00a0g NaOH as co-catalysts, indicating that all the solid bases examined show much lower activities than NaOH. Increasing the amount of MgO and NaOH/MgO from 0.5\u00a0g to 1\u00a0g with keeping 1\u00a0g Ni catalyst unchanged, total monomer yields are not obviously improved, slightly increasing from 9.1 to 10.7\u00a0wt% and from 11.2 to 12.4\u00a0wt%, respectively. Hence, these solid bases cannot efficiently catalyze EHL depolymerization at a low reaction temperature (200\u00a0\u00b0C).Although EHL was completely dissolved in ethylene glycol before and after the reaction with or without a catalyst, a floccule appeared between the water and CH2Cl2 phases during product extraction. The floccule is composed of lignin fragments that cannot be dissolved in water and CH2Cl2. The weight average molecular weights (Mw) and \u03b2-O-4 linkage contents of EHL and the floccule samples were analyzed with GPC (Fig. S4) and HSQC-NMR (Fig. S5), respectively, and the results are summarized in Fig. 2 (f). The Mw of EHL is 4333\u00a0g/mol. The Mw of the floccule obtained without a catalyst is much lower than that of EHL, which is 2216\u00a0g/mol. When a catalyst is added, the Mw decreases in an order: Ni (1768\u00a0g/mol)\u00a0>\u00a0NaOH (642\u00a0g/mol)\u00a0>\u00a0both Ni and NaOH (464\u00a0g/mol). The intensities of the peaks of \u03b2-O-4 linkages in HSQC-NMR spectra were normalized with the peak of DMSO. For EHL, the relative intensity of \u03b2-O-4 linkage signal is 0.22. These values of floccule obtained without catalyst and with Ni are similar, which are 0.15 and 0.13 respectively. Nevertheless, this value of floccule obtained with NaOH is only 0.04. When Ni catalyst and NaOH co-exist, the signal of \u03b2-O-4 linkage disappears and this value turns to 0.EHL, EHL-MET and EHL-EG samples were analyzed with 1H NMR and 13C NMR to reveal the interactions between EHL and solvent, and signal assignment is based on the published works [21\u201327]. Fig. 3\n (a) plots the 1H NMR spectra obtained. In the spectra of EHL-MET and EHL-EG, the peaks of H in phenolic hydroxyls (10\u20138\u00a0ppm) shift to higher field (lower \u03b4 value), compared to those in the spectrum of EHL. This is due to the cleavage of original intramolecular hydrogen bonds in lignin and the formation of new hydrogen bonds between solvent and phenolic hydroxyls [28]. Nevertheless, in the spectra of EHL-MET and EHL-EG, the positions of the peaks of H in phenolic hydroxyls are similar, indicating that the phenolic O-H\u22efO hydrogen bonds in EHL-MET and EHL-EG have similar strengths. In addition, the strong peaks of H in aromatic ring (7.5\u20136.3\u00a0ppm) and aliphatic chain \u2013CH3/\u2013CH2 (1.4\u20130.6\u00a0ppm) also shift to higher field in the spectrum of EHL-MET, and further shift to the same direction in the spectrum of EHL-EG, compared to those in the spectrum of EHL. In the 13C NMR spectra (Fig. 3(b)), the peaks ascribed to C4 in G unit (G4, 145.7\u00a0ppm), C2/C6 in pCA and H units (pCA2/6 and H2/6, 130.5\u00a0ppm), C3/C5 in pCA and H units (pCA3/5 and H3/5, 116.0\u00a0ppm) as well as C in CH2 in the aliphatic side chain (29.6\u00a0ppm) shift to lower field (higher \u03b4 value) in the spectrum of EHL-MET, and further shift to the same direction in the spectrum of EHL-EG, compared to those in the spectrum of EHL. The shifting of these 1H and 13C NMR peaks in the spectra of EHL-MET and EHL-EG suggests the existence of interaction between aromatic and aliphatic C-H in EHL and O in the solvent.Gaussian simulation was employed to verify the existence and strength of phenolic O-H\u22efO and aromatic C-H\u22efO interactions in EHL-MET and EHL-EG. Phenol and benzene were used to represent the phenolic OH and benzene ring in EHL, respectively, to exclude the influence of other functional groups. Fig. 4 (a) illustrates the stable structures and interaction energies of phenol-methanol (P-MET), phenol-ethylene glycol (P-EG), benzene-methanol (B-MET) and benzene-ethylene glycol (B-EG) complexes. For P-MET and P-EG, the interaction energies are similar, i.e., \u22120.44\u00a0eV and \u22120.59\u00a0eV, respectively. Hoverer, the interaction energy of B-EG is \u22120.36\u00a0eV, much lower than that of B-MET, which is only \u22120.06\u00a0eV. Therefore, the strengths of phenolic O-H\u22efO interaction formed in P-MET and P-EG are similar, but the aromatic C-H\u22efO interaction in B-EG is much stronger than that in B-MET.The strength of O-H\u22efO hydrogen bonds between aliphatic OH in a lignin dimer (1-(4-hydroxyphenyl)-2-phenoxypropane-1,3-diol) and OH in ethylene glycol and methanol are also calculated. As shown in Fig. 4\n (b), the interaction energies of C\u03b3-OH\u22efO and C\u03b1-OH\u22efO in lignin dimer-ethylene glycol (D-EG) are \u22120.55 and \u22120.69\u00a0eV, respectively, slightly lower than these value in lignin dimer-methanol (D-MET), which are \u22120.37 and \u22120.41\u00a0eV, respectively. This indicates that the aliphatic O-H\u22efO hydrogen bonds in D-EG are slightly stronger than those in D-MET.The NCI of this lignin dimer, lignin dimer-ethylene glycol (D-EG) and lignin dimer-methanol (D-MET) are further analyzed, and the results are shown in Fig. 4 (c). Blue and green clouds indicate the hydrogen bond and van der Waals interactions, respectively. In the lignin dimer, the van der Waals interaction between the two benzene rings in lignin dimer is ascribed to \u03c0-\u03c0 stacking interaction, which results in the overlapping of two benzene rings [29]. In D-EG, overlapping benzene rings are opened due to the van der Waals interaction between the lignin dimer and ethylene glycol, which are ascribed to the lone pair\u22ef\u03c0 interaction between the lone pair of O in ethylene glycol and \u03c0 electrons in benzene rings [30]. Nevertheless, in D-MET, methanol prefers to form a hydrogen bond with phenolic hydroxyl in lignin dimer, and the benzene rings are still stacked.The interaction between ethylene glycol and a lignin model molecule (Fig. 5\n (a)) consisting of five benzene rings and C-O and C-C linkages, i.e., \u03b1-O-4, \u03b2-O-4, 5-O-4, and \u03b2-1, was further examined. As shown in Fig. 5 (b) and (c), without ethylene glycol molecule, the lignin model molecule is aggregated due to the intramolecular hydrogen bond and \u03c0-\u03c0 stacking interactions. When seven ethylene glycol molecules are added, the aggregated lignin model molecule is stretched (Fig. 5 (d)), and both hydrogen bond interaction and van der Waals interactions, including C-H\u22efO and lone pair\u22ef\u03c0, form between ethylene glycol and the lignin model molecule (Fig. 5 (e)).The adsorption of ethylene glycol and lignin monomers, i.e., ferulic acid (FA) and coniferyl alcohol (CA), over Ni catalyst at 200\u00a0\u00b0C was examined with ATR-FTIR, and the spectra are shown in Fig. 6\n. In the spectrum of liquid ethylene glycol, the broad band at 3290\u00a0cm\u22121 is ascribed to the stretching vibration of \u2013OH, and the bands at 2936 and 2869\u00a0cm\u22121 are ascribed to the stretching vibration of \u2013CH2\u2013, whose bending vibrational bands appear in the range of 1455\u20131200\u00a0cm\u22121, and the bands at 1029 and 866\u00a0cm\u22121 are ascribed to the stretching and bending vibration of \u2013C\u2013O\u2013, respectively. In the spectrum of ethylene glycol adsorbed over Ni catalyst, the bond of \u2013OH shifts to 3664\u00a0cm\u22121 and its intensity is significantly weakened compared to that of liquid ethylene glycol, and the stretching vibrational bonds of \u2013CH2\u2013 and \u2013C\u2013O\u2013 also shift to higher wavenumber, appeared at 2985, 2894 and 1054\u00a0cm\u22121, respectively. The shift of these bonds results from the transformation of electrons from ethylene glycol to Ni atoms. When the mixtures of 10\u00a0mL ethylene glycol and 0.1\u00a0g FA or CA are adsorbed over Ni catalyst, the spectra obtained are the same as that of pure ethylene glycol adsorbed over Ni catalyst, indicating that these lignin monomers in ethylene glycol can not be adsorbed over Ni catalyst.The competitive adsorption of the mentioned lignin dimer (1-(4-hydroxyphenyl)-2-phenoxypropane-1,3-diol) and ethylene glycol molecules over the Ni surface was investigated with molecular dynamics simulation. Fig. 7\n (a) is the initial state of the model, in which one lignin dimer (Dimer A) is surrounded with ethylene glycol molecules and another one (Dimer B) is over the Ni surface. After 1000\u00a0ps simulation (Fig. 7 (b)), Dimer A is still in the liquid phase, while Dimer B remains adsorbed over the Ni surface. The distribution of ethylene glycol molecules along the z-axis (Fig. 7 (c)) indicates that ethylene glycol molecules are enriched over the Ni surface. The trajectory of the mass center of Dimer A and B during 1000\u00a0ps (Fig. 7 (d)) shows that Dimer A cannot go through the ethylene glycol molecular layer to adsorb over the Ni surface, while Dimer B steadily adsorbs over the Ni surface.EHL dissolution is the first step of EHL solvolysis. We found that ethylene glycol achieves complete EHL dissolution at room temperature, while other solvents, such as methanol, only dissolve part of EHL with high content of hydroxyls and \u03b2-O-4 linkages. Previous articles thought that lignin dissolution is mainly attributed to the O-H\u2026O hydrogen bond between solvent and lignin [28,31\u201333]. Nevertheless, for lignin molecules with low content of hydroxyls, the main obstacle to its dissolution is the \u03c0-\u03c0 stacking interaction [34\u201337]. Gaussian simulation indicates that ethylene glycol forms stronger C-H\u22efO and lone pair\u22ef\u03c0 interactions with benzene rings of EHL than methanol dose, because one ethylene glycol molecule contains two O atoms. These strong van Der Waals forces break original \u03c0-\u03c0 stacking in EHL and achieve complete EHL dissolution.The monomer yields obtained from non-catalytic EHL solvolysis show a positive correlation with the EHL solubility of solvents, and the highest monomer yield is obtained in ethylene glycol. As revealed with the GPC and HSQC-NMR results, linkages in EHL are already partly broken in ethylene glycol even at 200\u00a0\u00b0C without a catalyst, forming monomers and lignin fragments. We speculate that the strong van Der Waals forces between ethylene glycol and EHL may result in the shift of electrons in the benzene ring of EHL, reducing the bond energy of \u03b2-O-4 linkages in EHL, as shown in Scheme 1\n(a).It has been generally accepted that a large lignin molecule cannot be directly adsorbed on the surface of the solid catalyst due to its large three-dimensional structure [1,2,4]. Our results of molecular dynamics simulation further reveal that the adsorption of ethylene glycol hinders the adsorption of lignin dimer from the liquid phase to the surface of the Ni catalyst. Hence, catalytic EHL hydrogenolysis is not the main reaction pathway, and lignin depolymerization mainly occurs through solvolysis reaction.The comparison of the results of blank reaction without catalyst and catalytic reaction with Ni catalyst reveals that the Ni catalyst does play a role in the hydrogenation of carbon\u2013carbon double bonds in the side chains of lignin monomers. Nevertheless, the adsorption of lignin monomers is also hindered by the adsorption of ethylene glycol. Hence, the hydrogenation reaction may occur in the liquid phase. we guess that O atoms in ethylene glycol may attract adsorbed H atoms over the Ni surface due to their strong electronegativity, forming a hydrogen-ethylene glycol complex, which may desorb from the Ni surface and involve in the hydrogenation reaction in the liquid phase (Shame 1(b)). As reported, even at around 200\u00a0\u00b0C, lignin depolymerization and product repolymerization occur simultaneously in non-catalytic lignin alcoholysis, and intermediates with carbon\u2013carbon double bonds in their side chains more readily undergo repolymerization reactions [38\u201342]. The Ni catalyst stabilizes active monomers through hydrogenation reactions, suppressing repolymerization reactions. Hence, the addition of the Ni catalyst improves monomer yield and reduces Mw of floccule.As discussed above, Ni catalyst just plays a role in the hydrogenation reaction of carbon\u2013carbon double bonds in EHL solvolysis reaction. The hydrogenation of carbon\u2013carbon double bonds is relatively easy [43,44], and does not require a Ni catalyst with high hydrogenation activity. Hence, the activity of the Ni catalyst is insensitive to the phase transition of Ni from the amorphous phase to the crystalline phase.NaOH is soluble in ethylene glycol and serves as a homogeneous catalyst that directly promotes the breakage of C-O linkages in lignin [45]. Hence, NaOH is more efficient than solid base catalysts for catalyzing lignin depolymerization. When NaOH was used as the catalyst, the content of \u03b2-O-4 linkage and Mw of floccule were significantly reduced. When Ni and NaOH were used as co-catalysts, the product distribution obtained is similar to that obtained with only NaOH as a catalyst, indicating that the reaction mainly follows the soluble-base catalyzed route. Nevertheless, NaOH also promotes repolymerization/condensation of active monomers and intermediates, and hence too high NaOH amount results in the decrease of total monomer yield [45,46]. As mentioned above, the Ni catalyst stabilizes active monomers and intermediates, suppressing repolymerization reaction, and hence the combination of Ni and NaOH obtains a higher monomer yield than a single catalyst [47\u201349].Based on the presented results, the pathways of EHL solvolysis in ethylene glycol at 200\u00a0\u00b0C with Ni and NaOH as co-catalysts are proposed and presented in Scheme 2\n. Agglomerated lignin molecule is firstly dissolved and partly depolymerized in ethylene glycol, exposing more functional groups, e.g., \u2013OCH3 and \u2013OH. NaOH depolymerizes dissolved lignin fragments through attacking these functional groups, forming active monomers and intermediates with carbon\u2013carbon double bonds [45,50]. Active hydrogens are transformed from the Ni surface to the liquid phase with ethylene glycol as a porter, and involve into the hydrogenation of carbon\u2013carbon double bonds. After several cycling of base-catalyzed dehydroxylation and hydrogenation reactions, stable monomers are produced.Ethylene glycol shows high EHL solubility and achieves complete EHL dissolution at room temperature, while methanol only dissolves part of EHL with high content of hydroxyls and \u03b2-O-4 linkages. The Gaussian simulation results indicate that ethylene glycol forms strong Van Der Waals interactions with EHL, including C-H\u22efO and lone pair\u22ef\u03c0 interactions, and these interactions break original \u03c0-\u03c0 stacking in EHL, achieving complete EHL dissolution.The total monomer yields obtained from non-catalytic EHL solvolysis at 200\u00a0\u00b0C under 3 H2 for 6\u00a0h show a positive correlation with EHL solubility, and ethylene glycol gives the highest total monomer yield, i.e., 5.5\u00a0wt%, among the solvents examined. With Ni and NaOH as co-catalysts, the total monomer yield in ethylene glycol reaches 18.8\u00a0wt% under the same reaction condition.EHL is partly depolymerized at 200\u00a0\u00b0C without a catalyst due to the strong interactions between EHL and ethylene glycol. NaOH as the homogeneous catalyst directly attracts C-O bonds in EHL and depolymerizes EHL into active monomers and intermediates. The adsorption of lignin fragments over Ni catalyst via C-O bond is hindered with ethylene glycol, and Ni catalyst mainly plays a role in supplying active hydrogen atom to stabilize the active intermediates, suppressing repolymerization side 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 work has received funding from the European Union\u2019s Horizon 2020 research and innovation program, (BUILDING A LOW-CARBON, CLIMATE RESILIENT FUTURE: SECURE, CLEAN AND EFFICIENT EN-ERGY) under Grant Agreement No 101006744. The content presented in this document represents the views of the authors, and the European Commission has no liability in respect of the content. Y.S. Sang and G. Li would like to express their gratitude to both the China Scholarship Council (202006250156, 202208320030) and the EU-101006744 project.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2023.142256.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The dissolution and solvolysis processes of enzymatic hydrolysis lignin (EHL) in ethylene glycol are investigated. Ethylene glycol exhibits high EHL solubility and achieves complete EHL dissolution at room temperature. Gaussian simulation reveals that van de Waals interactions between ethylene glycol and EHL, including C-H\u22efO and lone pair\u22ef\u03c0 interactions, break the \u03c0-\u03c0 stacking in EHL, achieving complete EHL dissolution. EHL is partly depolymerized in ethylene glycol at 200\u00a0\u00b0C even without a catalyst due to the strong van de Waals interactions. When NaOH and Ni are used as co-catalysts, EHL is efficiently depolymerized at 200\u00a0\u00b0C, and the overall monomer yield reaches 18.8\u00a0wt%. Fourier transform infrared spectroscopy (FT-IR) and molecular dynamics simulation results indicate that the adsorption of ethylene glycol over Ni surface hinders the adsorption of lignin fragments and monomers. Hence, EHL catalytic solvolysis in ethylene glycol occurs in the liquid phase, where OH\u2212 of NaOH promotes the EHL linkage breakage and active hydrogen atoms formed on Ni surface stabilize the active monomers.\n               "}
{"full_text": "Energy and the environment are among the most important concerns of the current era. However, most of the energy we are using still comes from nonrenewable fossil fuels obtained from reserves that are ultimately unsustainable and that result in environmental pollution. The conversion of energy from renewable energy sources could reduce the dependence on fossil fuels significantly [1\u20133]. Among these renewable energy methods, electrochemical catalytic water-splitting is considered to be one of the most effective.Efficient electrolysis of water usually requires developing high-performance electrocatalysts with high stability, fast kinetics and low overpotential [4]. Noble metal-based materials such as Pt and Ru-based catalysts are the most widely used catalysts in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively. However, optimum operating conditions and reaction mechanisms are different for HER and OER electrocatalysts. Catalysts that are good for HER do not perform well in OER, and vice versa [5]. Pt and Ru-based catalysts, although they are effective bifunctional electrocatalysts, perform poorly in overall water splitting. Moreover, the expense and scarcity of these precious metals severely limits their large-scale application, and it is not economical to produce single-function electrocatalysts for each HER and OER, as this would increase manufacturing costs [6\u201310]. The development of efficient bifunctional electrocatalysts for water splitting will greatly reduce the preparation cost and simplify the electrolytic system as well. However, designing and preparing a catalyst that promotes both HER and OER in the same electrolyte remains a major challenge.In recent years, transition metal phosphides (TMPs) [9,11], carbides [12], nitrides [13,14] and sulfides [15,16] have been widely reported as potential electrocatalysts for OER or HER [17]. In particular, TMPs have attracted great interest from researchers due to their outstanding catalytic performance [18]. The negatively charged P atoms in the catalyst are capable of attracting protons and acting as active sites for H2 evolution. In the oxygen evolution reaction, transition metal phosphates are transformed into transition-metal oxyhydroxides on the surface of the catalysts which can act as active catalytical sites for O2 evolution [19]. Additionally, TMPs have been considered as advanced catalysts because of their superior electrical conductivity [20]. These properties make TMPs promising electrochemical catalysts for water splitting [21]. However, many non-noble metal phosphates do not exhibit satisfactory bifunctional properties. For instance, NiCoP catalysts have been reported as promising electrocatalysts of non-noble metals for overall water splitting in recent years [22,23], but they show inferior HER catalytic activity relative to the noble metal-based catalysts in the alkaline condition.For this reason, different strategies have been implemented to improve their bifunctional performance, such as developing single atom catalysts [24], doping other transition-metals and heteroatoms [25] and modulating the structure and composition [26]. It has been proved that the electrocatalytic performance of catalysts is tremendously affected by their morphology, active surface sites and electronic structure [27], which can be tailored by designing their heterostructures. Generally, heterostructured materials demonstrate better electrocatalytic performance than their individual building units because of the benefits from the strong coupling between different building components. It is an efficient approach to introduce a \u201ccollaborator\u201d that can form heterostructures to further improve HER and OER activity.CoMoO4 is considered an ideal choice as a \u201ccollaborator\u201d to provide active hydrolytic dissociation sites because Co and Mo have hydrogen adsorption energy close to Pt, and their binary metal oxides have higher constitutive activity than monomeric metal oxides, which significantly facilitates the proton supply. Nevertheless, due to the weak intrinsic conductivity of CoMoO4, its application in alkaline HER is limited. The synthesis of oxide catalysts with the incorporation of a P atom to tailor the electronic structure is an efficient strategy to address the problem of low conductivity [28]. Yaqiong Gong et\u00a0al. [28] utilized phosphorus-doping modulation to fabricate monoclinic P-CoMoO4 with an optimized electron structure supported on nickel foam (P-CoMoO4/NF) for alkaline HER via a facile hydrothermal method, followed by low-temperature phosphidation. Ivan P. Parkin et\u00a0al. [29] synthesized a series of P-doped CoMoO4 nanostructures on Ni foam by facile hydrothermal annealing and phosphidation modification, which enhanced their electrochemical performance significantly.Herein, we report a novel P-CoMoO4@NiCoP/NF heterostructured nanoarray catalyst as an efficient bifunctional electrocatalyst to promote the overall water splitting. P-CoMoO4@NiCoP/NF nanoarrays have been successfully prepared through the phosphatization of the CoMoO4@NiCo2O4 nanoarray precursor with NaH2PO2\u00b7H2O as the P source under heat treatment in the N2 atmosphere. Heterostructured P-CoMoO4@NiCoP rooted on Ni foam possesses a novel tree-like 3D structure, in which P-CoMoO4 nanosheets (leaves) are assembled on the surface of NiCoP nanowires (trunk). Due to the unique heterostructure, more exposed active sites and coordinated electronic structure, the P-CoMoO4@NiCoP/NF presents excellent HER and OER catalytic activity and shows outstanding overall water splitting performance.Analytical grade Ni(CH2COOH)2\u00b76H2O (\u226598%), Co(CH2COOH)2\u00b76H2O (\u226599%), Na2MoO4\u00b72H2O (\u226599%), NH4F (\u226596%), NaH2PO2\u00b7H2O (98%\u2013103%), KOH (\u226585%), urea (\u226599%) and ethanol were ordered from the Sinopharm Chemical Reagent Co. Commercial ruthenium dioxide (RuO2, 99.9%) and platinum on activated carbon (Pt/C, 20\u00a0\u200bwt%) were ordered from Aladdin Chemical Reagents Co. Ni foam (denoted as NF) was ordered from Shenzhen Meisen Electromechanical Equipment Co., Ltd. All chemical reagents were used without further purification. The experimental water was deionized water.A piece of Ni foam (2\u00a0\u200bcm\u00a0\u200b\u00d7\u00a0\u200b4\u00a0\u200bcm) was degreased in an acetone solution, ultrasonicated in 3.0\u00a0\u200bM HCl solution for 3-5\u00a0\u200bmin, then thoroughly washed with deionized water and ethanol alternately to clean the surface. Urea (10\u00a0\u200bmmol), NH4F (8\u00a0\u200bmmol), Ni(CH2COOH)2\u00b76H2O (0.333\u00a0\u200bmmol) and Co(CH2COOH)2\u00b76H2O (0.667\u00a0\u200bmmol) was dissolved in 36\u00a0\u200bmL of deionized water and stirred continuously to form a clear solution. The pre-treated NF was then transferred to a Teflon lined stainless steel autoclave (50\u00a0\u200bmL) containing the solution and maintained at 120\u00a0\u200b\u00b0C for 3\u00a0\u200bh. After natural cooling, the NiCo-based precursor of NF (denoted as NiCo-OH/NF) was taken out, rinsed with deionized water until there was no residue, washed with ethanol three times, and finally dried at 60\u00a0\u200b\u00b0C for 12\u00a0\u200bh NiCo2O4 nanowire arrays grown on Ni foam (denoted as NiCo2O4/NF) were obtained after annealing the NiCo-OH/NF sample at 450\u00a0\u200b\u00b0C for 2\u00a0\u200bh in air.The prepared NiCo2O4/NF nanoarrays were immersed in a solution containing 36\u00a0\u200bmL of deionized water, 1\u00a0\u200bmmol Na2MoO4\u00b72H2O and 1\u00a0\u200bmmol Co(CH2COOH)2\u00b76H2O. The reaction was carried out at 100\u00a0\u200b\u00b0C in a Teflon-lined stainless steel autoclave for 12\u00a0\u200bh. After natural cooling, the sample was taken out, washed with deionized water until there was no residue, washed with ethanol three times, and finally dried at 60\u00a0\u200b\u00b0C for 12\u00a0\u200bh to obtain CoMoO4@NiCo2O4/NF.P-CoMoO4@NiCoP/NF composite nanoarrays were obtained through phosphatization with NaH2PO2\u00b7H2O as the P source. The prepared CoMoO4@NiCo2O4/NF composite nanoarrays and NaH2PO2\u00b7H2O were put separately in a porcelain boat with the NaH2PO2\u00b7H2O powder at the upstream side, and then heated in a tube furnace at 300\u00a0\u200b\u00b0C (ramp rate of 5\u00a0\u200b\u00b0C min\u22121) for 180\u00a0\u200bmin under a N2 atmosphere.As comparison samples, NiCoP/NF and P-CoMoO4/NF nanoarrays based on Ni foam were prepared separately by the same method, as detailed in Supporting Information 1.\nFig.\u00a01\n illustrates the process of synthesizing P-CoMoO4@NiCoP/NF nanoarrays. Briefly, NiCo-OH/NF (Fig.\u00a0S1) was synthesized through hydrothermal synthesis, followed by annealing to form NiCo2O4/NF nanowire arrays (Fig.\u00a0S2). The CoMoO4 nanosheets were then synthesized on the NiCo2O4 nanowires to form CoMoO4@NiCo2O4/NF (Fig.\u00a0S3). Finally, heterostructured P-CoMoO4@NiCoP/NF nanoarrays were synthesized through phosphatizing the prepared CoMoO4@NiCo2O4/NF sample.\nFig.\u00a02\na shows the X-ray diffraction (XRD) pattern of the P-CoMoO4@NiCoP powder sample which was peeled off the Ni foam substrate. The reflections in the XRD pattern could be indexed to NiCoP (PDF No. 71-2336) and CoMoO4 (PDF No. 21-0868), indicating the partial phosphatization of the CoMoO4@NiCo2O4/NF sample. Under the experimental conditions, NiCo2O4 was completely phosphatized into NiCoP, while CoMoO4 did not show an obvious conversion to a phosphatized product. The 2\u03b8 values of 41.14\u00b0 and 45.06\u00b0 corresponded to the (111) and (201) crystal planes of NiCoP, and it was found that the 2\u03b8 values were shifted positively about 0.15\u00b0 compared with the standard card of NiCoP (PDF No. 71-2336). This was ascribed to the higher amounts of Co atoms and the mixed valence states of the Co ions [6,30]. Meanwhile, the 2\u03b8 values corresponding to the (002) and (021) crystal planes of CoMoO4 were 26.66\u00b0 and 23.54\u00b0, exhibiting 0.15\u00b0 and 0.21\u00b0 deviations from the standard values of CoMoO4 (PDF No. 21-0868), probably due to the incorporation of P in it. Further element mapping characterization (line scanning of P-CoMoO4@NiCoP in Fig.\u00a0S4) revealed that the P elements were highly concentrated in the core nanowires, but also with a uniform distribution of relative low content in the CoMoO4 nanosheets, indicating that the P element was partially incorporated into the CoMoO4 matrix [11]. Combining it with the XRD analysis, the phosphatized sample was denoted as P-CoMoO4@NiCoP/NF.The morphological and structural characteristics of the synthesized samples were investigated through field emission scanning electron microscopy (FESEM). First, the FESEM images of NiCoP/NF (Fig.\u00a0S5) with different magnifications showed that the NiCoP nanowire arrays were uniformly grown on the Ni foam. The length of the NiCoP nanowires was about 1.3\u00a0\u200b\u03bcm, and the diameter of the nanowires was ~100\u00a0\u200bnm. Each nanowire was directly in contact with the Ni foam, which can ensure efficient electron transport between the electrocatalyst and the Ni foam substrate, thereby contributing to the water splitting [31]. After a second hydrothermal reaction and phosphatization, the P-CoMoO4 nanosheets were closely and homogeneously covered on the Ni foam (Fig.\u00a0S6). Fig.\u00a02b shows the FESEM image of the P-CoMoO4@NiCoP/NF sample. It can be found that the P-CoMoO4 nanosheets with a thickness of about 40\u00a0\u200bnm (Fig.\u00a0S7) were uniformly assembled on the NiCoP nanowire arrays, constructing heterostructured P-CoMoO4@NiCoP/NF nanoarrays. The heterostructured P-CoMoO4@NiCoP anchored on the Ni foam exhibited a novel tree-like 3D structure in which the P-CoMoO4 nanosheets were assembled like leaves on the NiCoP nanowire trunk. The transmission electron microscopy (TEM) image of the P-CoMoO4@NiCoP (Fig.\u00a02c) also attests to the heterostructure, with the P-CoMoO4 nanosheets tightly aggregating around the NiCoP nanowires.The high-resolution TEM (HRTEM) image (Fig.\u00a02d) on the nanosheet (marked in red circle) reveals two clear lattice distances of 0.224\u00a0\u200bnm and 0.187\u00a0\u200bnm, corresponding to the (003) and (\n\n\n1\n\u00af\n\n\n33) crystal plane of CoMoO4, respectively. The corresponding selected area electron diffraction (SAED) image (Fig.\u00a02e) shows several bright rings with discrete spots, which match well with the (\n\n\n1\n\u00af\n\n\n31), (003), (\n\n\n1\n\u00af\n\n\n33) and (\n\n\n3\n\u00af\n\n\n52) planes of the CoMoO4. The corresponding elemental mapping image of P-CoMoO4@NiCoP (Fig.\u00a02f) illustrates that the Ni element is only distributed on the nanowires and Mo only on the nanosheets, while the P and Co elements are uniformly distributed on the NiCoP nanowires and P-CoMoO4 nanosheets. This confirms that the P-CoMoO4 nanosheets are uniformly grown on the NiCoP nanowires. These results demonstrate the successful preparation of the heterostructured P-CoMoO4@NiCoP/NF composite.The chemical composition and valence states of the elements were studied by XPS for the heterostructured P-CoMoO4@NiCoP/NF, along with those of bare NiCoP/NF and P-CoMoO4/NF. XPS survey spectra of P-CoMoO4@NiCoP/NF (Fig.\u00a0S8a) shows that P-CoMoO4@NiCoP/NF is mainly composed of Ni, Co, Mo, P and O elements. The peaks of Ni in P-CoMoO4@NiCoP/NF are weaker than those in NiCoP/NF because there is a dense vegetation of P-CoMoO4 nanosheets wrapped on the surface of the NiCoP nanowires, thus weakening the Ni peak intensity of the P-CoMoO4@NiCoP sample. The XPS spectrum of the Ni 2p in P-CoMoO4/NiCoP displays two peaks at 875.08 and 856.58\u00a0\u200beV, attributed to Ni2+ 2p1/2 and Ni2+ 2p3/2 (Fig.\u00a03\na) [32]. The peaks at 870.98\u00a0\u200beV and 852.98\u00a0\u200beV are assigned to the peaks of P-CoMoO4@NiCoP/NF, related to Ni0 2p1/2 and Ni0 2p3/2, respectively. Compared to those of NiCoP/NF, the Ni2+ peaks of P-CoMoO4@NiCoP/NF are negatively shifted about 0.1\u00a0\u200beV. Likewise, Fig.\u00a03b demonstrates the Mo 3d spectrum. The appearance of Mo4+ and Mo6+ can be attributed to the CoMoO4 and Mo-P species. In Fig.\u00a03b, the Mo 3d signal in the P-CoMoO4@NiCoP/NF exhibits a positive shift of about 0.3\u00a0\u200beV compared with that in the P-CoMoO4/NF. Generally, the increase of the valence electron charge will lead to the decrease of binding energy, and vice versa [33]. Therefore, the negative shift of binding energy for Ni 2p and the positive shift of binding energy for Mo 3d strongly demonstrate that there are strong electronic interactions between the NiCoP nanowire and P-CoMoO4 nanosheet, which can significantly accelerate the charge transfer. The Co 2p core-level spectrum (Fig.\u00a03c) exhibits Co2+ species at 797.98\u00a0\u200beV and 781.78\u00a0\u200beV. As can be seen, there are no peaks of Co\u03b4+ species in CoMoO4@NiCo2O4/NF, indicating that Co\u03b4+ (793.88\u00a0\u200beV and 778.88\u00a0\u200beV) is due to the formation of Co-P bonds (Fig.\u00a0S8b) [4]. Moreover, the peak intensity of the Co\u03b4+ species in NiCoP/NF and P-CoMoO4/NF is weaker than that in P-CoMoO4@NiCoP/NF, indicating that the formation of the heterogeneous interface facilitates the formation of Co\u03b4+ species, thereby improving the OER or HER performance [19]. In Fig.\u00a03d, the high-resolution P 2p spectra of P-CoMoO4@NiCoP/NF, with the peaks of P-M 2p3/2 and P-M 2p1/2, locate at a binding energy of 129.58 and 130.43\u00a0\u200beV, respectively. Compared to those of bare NiCoP/NF and P-CoMoO4/NF, the peak intensities of P-Metal (P-M 2p3/2 and P-M 2p1/2) demonstrate that the redistribution of the charge is caused by the strong interaction at the NiCoP and P-CoMoO4 interface in P-CoMoO4@NiCoP/NF, which could significantly enhance the electrocatalytic activity [34]. The peak at 133.9\u00a0\u200beV represents P-O species oxidized (PO4\n3\u2212, etc.) due to air exposure [22].Generally, a large number of air bubbles will be generated on the surface of the electrode during the electrocatalytic reaction under high current density. The gas bubbles produced in-situ, if not released immediately, will seriously hinder the electrolyte diffusion and produce dead areas that cannot participate in the catalytic reaction, thus hindering the mass transfer process and causing the catalytic performance to deteriorate [35].In order to study the hydrophilic and aerophobic properties of the samples, the contact angle of the water droplet and the underwater contact angle of the air bubble were tested. Dynamic testing showed that the water droplet spread immediately on the surface of the P-CoMoO4@NiCoP/NF, as well as the NiCoP/NF and P-CoMoO4/NF, once in contact with them, while the water droplet remained on the surface of the blank Ni foam (see the testing video in Supporting Information 2). As shown in Fig.\u00a0S9, the P-CoMoO4@NiCoP/NF, as well as NiCoP/NF and P-CoMoO4/NF, displayed a liquid contact angle of 0\u00b0, showing its superhydrophilic property, while the blank Ni foam showed its hydrophobic property, with a large static liquid contact angle of 120\u00b0. Fig.\u00a0S10 displays the measurement images of the underwater air bubble contact angles for those samples. The P-CoMoO4@NiCoP/NF demonstrates superaerophobic properties, and the air bubble shows complete estrangement from the surface of the contact film, which is beneficial to the release of the gas bubbles produced during the electrocatalytic water splitting. Similarly, the components comprising NiCoP/NF and P-CoMoO4/NF also exhibit superaerophobic properties, and the air bubbles become estranged from the surface of the film upon cessation of contact, while the air bubble can retain a contact angle of 134\u00b0 on the surface of the Ni foam, implying the underwater aerophilic property of the bare Ni foam surface. The videos about the underwater air bubble contact angle testing are provided in Supporting Information 3. The superhydrophilic and superaerophobic properties of P-CoMoO4@NiCoP/NF make it conducive to water dissociation and bubble separation in the electrocatalytic process, thus improving the catalytic performance.Supplementary video related to this article can be found at https://doi.org/10.1016/j.nanoms.2021.05.004\nThe following are the supplementary data related to this article:\n\nVideo 1\nVideo 1\n\n\n\n\n\nVideo 2\nVideo 2\n\n\n\nTo determine the HER performance, a standard three-electrode cell was used to perform linear sweep voltammetry in a 1.0\u00a0\u200bM KOH electrolyte. The iR-compensated polarization curves of P-CoMoO4@NiCoP/NF, NiCoP/NF, P-CoMoO4/NF, Pt/C/NF and Ni foam at 5\u00a0\u200bmV\u00a0\u200bs\u22121 are displayed in Fig.\u00a04\na, and their corresponding overpotentials at current densities of 10, 50, and 100\u00a0\u200bmA\u00a0\u200bcm\u22122 are presented in Fig.\u00a04b. The P-CoMoO4@NiCoP/NF offers outstanding HER performance with a low overpotential of 66\u00a0\u200bmV at 10\u00a0\u200bmA\u00a0\u200bcm\u22122, which is very close to that of the commercial Pt/C/NF catalyst (45\u00a0\u200bmV). In contrast, the NiCoP/NF and P-CoMoO4/NF exhibit inferior HER performance with high overpotentials of 139\u00a0\u200bmV and 125\u00a0\u200bmV at 10\u00a0\u200bmA\u00a0\u200bcm\u22122, respectively, suggesting significantly enhanced effects of the heterostructure catalyst on the HER performance. Furthermore, the P-CoMoO4@NiCoP/NF delivers a high current density (j\u00a0\u200b=\u00a0\u200b209\u00a0\u200bmA\u00a0\u200bcm\u22122) at an overpotential of 200\u00a0\u200bmV (Fig.\u00a0S11), which is about 4 times higher than those of NiCoP/NF (j\u00a0\u200b=\u00a0\u200b55\u00a0\u200bmA\u00a0\u200bcm\u22122) and P-CoMoO4/NF (j\u00a0\u200b=\u00a0\u200b56\u00a0\u200bmA\u00a0\u200bcm\u22122), indicating strong synergic effects derived from the P-CoMoO4@NiCoP/NF heterostructured interface [36]. In addition, Tafel plots were investigated to determine the HER rates of these catalysts (Fig.\u00a04c). The P-CoMoO4@NiCoP/NF presents the smallest Tafel slope of 75\u00a0\u200bmV dec\u22121 among the three catalysts, including NiCoP/NF (135\u00a0\u200bmV dec\u22121) and P-CoMoO4/NF (103\u00a0\u200bmV dec\u22121), demonstrating the fastest HER kinetics of the P-CoMoO4@NiCoP/NF catalyst [37]. Compared with some reported catalysts in recent literature, the P-CoMoO4@NiCoP/NF catalyst shows superiority in HER performance (Table\u00a0S1).Double-layer capacitance (C\ndl\n) reflects the electrochemical surface area of the catalyst. The capacitive current density difference between the anode and the cathode is proportional to the scan rate [15]. Cyclic voltammetry (CV) was used to study the electrochemical surface area (ECSA) at different scan rates (Fig.\u00a0S12). The C\ndl\n of the three catalysts was calculated based on their current density under different scan rates, as shown in Fig.\u00a04d, with 34.79\u00a0\u200bmF\u00a0\u200bcm\u22122, 27.15\u00a0\u200bmF\u00a0\u200bcm\u22122 and 16.26\u00a0\u200bmF\u00a0\u200bcm\u22122 for P-CoMoO4@NiCoP/NF, P-CoMoO4/NF and NiCoP/NF, respectively. The heterostructured P-CoMoO4@NiCoP/NF possesses the largest electrochemically active surface area, which endows it with the best electrocatalysis activity. The excellent performance of the hydrogen evolution is also related to the superhydrophilic and superaerophobic properties of the P-CoMoO4@NiCoP/NF. The superhydrophilicity of the P-CoMoO4@NiCoP/NF can facilitate electrolyte diffusion and make full contact with the electrolyte in the reaction process. In addition, the superaerophobic property is conducive to the release of gas bubbles for exposing more active sites and clearing the pathways for electrolyte diffusion, thus accelerating the hydrogen evolution reaction.Electrochemical impedance spectroscopy (EIS) was investigated in an alkaline medium to further reveal the relevant properties. The fitting circuit consisted of Rct (charge transfer resistance between electrolyte and catalyst interface) in parallel with CPE and then in series with Rs (the intrinsic resistance of the electrode and electrolyte). According to the equivalent circuit diagram shown in Fig.\u00a04e, the Rs of P-CoMoO4@NiCoP/NF (1.286\u00a0\u200b\u03a9) is smaller than that of NiCoP/NF (1.350\u00a0\u200b\u03a9) and P-CoMoO4/NF (1.409\u00a0\u200b\u03a9). Similarly, the Rct of P-CoMoO4@NiCoP/NF (0.353\u00a0\u200b\u03a9) is also smaller than that of NiCoP/NF (0.406\u00a0\u200b\u03a9) and P-CoMoO4/NF (0.435\u00a0\u200b\u03a9). The smaller Rs and Rct of the P-CoMoO4@NiCoP/NF electrode resulted from the tree-like 3D architecture with a heterostructured interface, which reduced the internal resistance and promoted electron transfer, thus improving the electrocatalytic performance [38]. As shown in Fig.\u00a0S13, the Rs (1.723\u00a0\u200b\u03a9) and Rct (0.918\u00a0\u200b\u03a9) of the CoMoO4@NiCo2O4/NF are significantly larger than those of P-CoMoO4@NiCoP/NF, indicating that phosphide and the incorporation of P can effectively increase the charge transfer property of the electrocatalyst.In addition, the P-CoMoO4@NiCoP/NF shows high durability during the 60\u00a0\u200bh chronopotentiometry test at 10\u00a0\u200bmA\u00a0\u200bcm\u22122, and the overpotential displays a negligible increase (Fig.\u00a04f). It basically maintains the original tree-like 3D heterostructure (Fig.\u00a0S14). This is attributed to the superaerophobic property of the P-CoMoO4@NiCoP/NF. The superaerophobic property of the P-CoMoO4@NiCoP/NF accelerates the release of bubbles, and will not produce a dead zone in the catalytic reaction, maintaining good performance in the stability test [35]. Fig.\u00a0S15 shows the XPS spectra of the P-CoMoO4@NiCoP/NF before and after the 60\u00a0\u200bh chronopotentiometry test for HER at 10\u00a0\u200bmA\u00a0\u200bcm\u22122 in 1.0\u00a0\u200bM KOH solution. Of note, the XPS spectra of the Ni 2p region does not change, but the Co\u03b4+ peaks disappear. The XRD of the P-CoMoO4@NiCoP/NF after the chronopotentiometry test for HER in 1.0\u00a0\u200bM KOH solution (Fig.\u00a0S16a) shows the presence of Co(OH)2, confirming the partial change from P-CoMoO4@NiCoP/NF to Co(OH)2 [9]. The high-resolution TEM (HRTEM) image (Fig.\u00a0S16c) on the nanosheet (marked in red circle in Fig.\u00a0S16b) reveals a clear lattice distance of 0.237\u00a0\u200bnm, corresponding to the (101) crystal plane of Co(OH)2. The corresponding selected area electron diffraction (SAED) image (Fig.\u00a0S16d) shows several bright rings with discrete spots, which match well with the (101), (102) and (111) planes of Co(OH)2. Phase transformation may cause the disappearance of Co\u03b4+. And the overall decrease in the peak intensity of P 2p and Mo 3d may be caused by the partial leaching out of P and Mo from the P-CoMoO4@NiCoP sample during the reaction process [39]. The existence of Co\u03b4+ related to the formation of the Co-P bond has been proved in Fig.\u00a0S8b and it has been reported in some literature [4,40]. The decrease of P-M bonds may be one of the reasons for the disappearance of Co\u03b4+.The OER catalytic activities of these samples were also investigated in 1.0\u00a0\u200bM KOH solution. The LSV curves are shown in Fig.\u00a05\na. To reach a current density of 100\u00a0\u200bmA\u00a0\u200bcm\u22122, the heterostructured P-CoMoO4@NiCoP/NF requires an overpotential of only 252\u00a0\u200bmV, which is much lower than that of NiCoP/NF (287\u00a0\u200bmV) and P-CoMoO4/NF (262\u00a0\u200bmV). To reach even higher current densities of 200 and 300\u00a0\u200bmA\u00a0\u200bcm\u22122, only 292 and 313\u00a0\u200bmV overpotentials are required for the P-CoMoO4@NiCoP/NF catalyst. The capacitance behavior of the non-Faradaic capacitance current range for P-CoMoO4@NiCoP/NF, NiCoP/NF, P-CoMoO4/NF and Ni foam was measured (Fig.\u00a0S17). The results show that the P-CoMoO4@NiCoP/NF, NiCoP/NF and P-CoMoO4/NF samples exhibit very strong capacitive behavior in comparison with the Ni foam sample. This may be one of the reasons that the polarization curve is raised in the non-Faradaic capacitance current range. In addition, the formation of Ni and Co oxidation peaks also raises the current, to some extent, which causes an obvious peak in the non-Faradaic region of the polarization curve [41,42]. Moreover, as shown in Fig.\u00a05b, it has the smallest Tafel slope of the three samples, P-CoMoO4@NiCoP/NF (126\u00a0\u200bmV dec\u22121), NiCoP/NF (150\u00a0\u200bmV dec\u22121) and P-CoMoO4/NF (148\u00a0\u200bmV dec\u22121), demonstrating the fastest OER kinetics of the P-CoMoO4@NiCoP/NF catalyst. Similarly, the P-CoMoO4@NiCoP/NF achieves a higher current density (j\u00a0\u200b=\u00a0\u200b231\u00a0\u200bmA\u00a0\u200bcm\u22122) at an overpotential of 300\u00a0\u200bmV than those of NiCoP/NF (j\u00a0\u200b=\u00a0\u200b124\u00a0\u200bmA\u00a0\u200bcm\u22122) and the P-CoMoO4/NF (j\u00a0\u200b=\u00a0\u200b183\u00a0\u200bmA\u00a0\u200bcm\u22122) (Fig.\u00a0S18), indicating the synergistic effect of NiCoP and P-CoMoO4 for improving the OER performance [36]. P-CoMoO4@NiCoP/NF's superhydrophilicity enables it to adsorb water molecules well and promotes the wettability of the electrolyte, thus promoting the surface activity of the catalyst, showing excellent oxygen evolution performance. In the EIS spectra (Fig.\u00a05c), the heterostructured P-CoMoO4@NiCoP/NF exhibits the lowest semicircle. The Rs of P-CoMoO4@NiCoP/NF (1.041\u00a0\u200b\u03a9) is lower than that of NiCoP/NF (1.181\u00a0\u200b\u03a9) and P-CoMoO4/NF (1.200\u00a0\u200b\u03a9), indicating that the P-CoMoO4@NiCoP/NF presents the lowest intrinsic resistance of the electrode and electrolyte among the three catalysts. Similarly, the Rct of P-CoMoO4@NiCoP/NF (0.646\u00a0\u200b\u03a9) is significantly smaller than that of NiCoP/NF (0.993\u00a0\u200b\u03a9) and P-CoMoO4/NF (0.858\u00a0\u200b\u03a9), indicating a faster charge transfer between the P-CoMoO4@NiCoP/NF electrode and the electrolyte [19]. Compared with the Rs (1.615\u00a0\u200b\u03a9) and Rct (1.717\u00a0\u200b\u03a9) of CoMoO4@NiCo2O4/NF (Fig.\u00a0S19), the charge transfer can be increased through phosphating, and then improve the electrical conductivity of the electrocatalyst [20]. In addition, a minimal increase in overpotential is observed even after continuous chronopotentiometry testing over 50\u00a0\u200bh at 100\u00a0\u200bmA\u00a0\u200bcm\u22122 (Fig.\u00a05d), confirming the high stability and durability of the P-CoMoO4@NiCoP/NF. Its superaerophobic property enables P-CoMoO4@NiCoP/NF to release bubbles rapidly to maintain stability. The morphology observation in Fig.\u00a0S20 shows that the original heterostructure is basically retained.Generally, the proposed mechanism of OER under alkaline conditions are considered as follows (M, surface metal sites)[43].\n\n(3-1)\nM\u00a0\u200b+\u00a0\u200bOH\u2013 \u2192 M\u00a0\u200b\u2212\u00a0\u200bOH\u00a0\u200b+\u00a0\u200be\u2013\n\n\n\n\n\n(3-2)\nM\u00a0\u200b\u2212\u00a0\u200bOH\u00a0\u200b+\u00a0\u200bOH\u2013 \u2192 M\u00a0\u200b\u2212\u00a0\u200bO\u00a0\u200b+\u00a0\u200bH2O\u00a0\u200b+\u00a0\u200be\u2013\n\n\n\n\n\n(3-3)\n2M\u00a0\u200b\u2212\u00a0\u200bO \u2192 O2\u00a0\u200b+\u00a0\u200b2\u00a0\u200bM\n\n\n\n\n(3-4)\nM\u00a0\u200b\u2212\u00a0\u200bO\u00a0\u200b+\u00a0\u200bOH\u2013 \u2192 M-OOH\u00a0\u200b+\u00a0\u200be\u2013\n\n\n\n\n\n(3-5)\nM-OOH\u00a0\u200b+\u00a0\u200bOH\u2013 \u2192 M\u00a0\u200b+\u00a0\u200bO2\u00a0\u200b+\u00a0\u200bH2O\u00a0\u200b+\u00a0\u200be\u2013\n\n\n\nThis indicates that oxides and hydroxides are all intermediates in the oxygen evolution reaction. It was also reported that the OER electrocatalytic activity of NiCo phosphides are attributable to the Ni-Co oxo/hydroxo species, which are key OER intermediates during oxygen evolution and are partially derived from the oxidization of Ni and Co atoms on the surface of the catalyst [4,9,44]. Fig.\u00a0S21 shows the XPS spectra of P-CoMoO4@NiCoP/NF before and after the 50\u00a0\u200bh chronopotentiometry test for OER at 100\u00a0\u200bmA\u00a0\u200bcm\u22122 in 1.0\u00a0\u200bM KOH solution. Of note, the peaks of Ni0 and Co\u03b4+ disappeared after 50\u00a0\u200bh OER testing, indicating Ni and Co have been oxidized. Meanwhile, the sole presence of the P-O bond and the disappearance of the P-M bonds are in close correlation with the oxidation of Ni and Co during the catalytic processes [45]. In order to further determine the surface oxidation, the samples were characterized by XRD after the OER stability test (Fig.\u00a0S22a), and the presence of NiO was found, indicating that surface oxidation did exist on the P-CoMoO4@NiCoP/NF sample. The high-resolution TEM (HRTEM) image (Fig.\u00a0S22c) on the nanosheet (marked in red circle in Fig.\u00a0S22b) reveals a clear lattice distance of 0.205\u00a0\u200bnm, may correspond to the crystal plane of Co oxo/hydroxo species. The selected area electron diffraction (SAED) image (Fig.\u00a0S22d) on the nanowire shows several bright rings with discrete spots, which match well with the (111), (200) and (220) planes of NiO. These results elucidate that an additional oxide catalyst layer is gradually formed on the surface of the P-CoMoO4@NiCoP.The above experimental results showed that the heterostructured P-CoMoO4@NiCoP/NF electrocatalyst presented a superior bifunctional electrocatalytic performance on HER and OER. Therefore, a two-electrode overall water splitting electrolyzer was constructed using P-CoMoO4@NiCoP/NF as both the anode and the cathode. As shown in Fig.\u00a06\na, P-CoMoO4@NiCoP/NF electrocatalysts are used as anode for OER and cathode for HER. In view of the superior bifunctional characteristics of P-CoMoO4@NiCoP/NF, its possible mechanism for overall water splitting can be illustrated by Fig.\u00a06b. During OER, electrons are transferred from the P-CoMoO4 nanosheets to the NiCoP nanowires via interface action. Then they are transferred from NiCoP to the Ni foam substrate, while the electron transmission path of HER is the opposite [15]. The superhydrophilic and superaerophobic properties of P-CoMoO4@NiCoP/NF make it conducive to water dissociation and bubble separation in the electrocatalytic process, thus improving the catalytic performance and stability. In addition, the heterostructured nanostructures ensure strong electronic interactions of the P-CoMoO4@NiCoP/NF [27]. Because of these advantages, this electrolyzer only needs a low potential of 1.62\u00a0\u200bV to achieve the current density of 20\u00a0\u200bmA\u00a0\u200bcm\u22122 (Fig.\u00a06c), which could be maintained well with almost no degradation after the 50\u00a0\u200bh chronopotentiometry test (Fig.\u00a06d), suggesting an impressive durability of overall water splitting. Such outstanding activity and durability enable the heterostructured P-CoMoO4@NiCoP/NF to be a potential alternative to noble metal electrocatalysts for energy-efficient and cost-effective water splitting [46]. Table\u00a0S1 compares the overall water splitting performance of P-CoMoO4@NiCoP/NF with some representative catalysts reported recently. It can be seen that it is comparable to or even outperforms its counterparts.In summary, we have developed an efficient strategy for improving the electrocatalytic activity of water splitting by engineering a 3D tree-like heterostructure of P-CoMoO4@NiCoP/NF, which could promote electron and charge transfer and provide abundant active sites. In addition, the superhydrophilic and superaerophobic properties of the P-CoMoO4@NiCoP/NF can facilitate good contact between the catalysts and electrolyte, which is very conducive to water electrolysis. In the half-cell evaluation, P-CoMoO4@NiCoP/NF exhibits excellent HER and OER performance with low overpotentials of 66\u00a0\u200bmV at 10\u00a0\u200bmA\u00a0\u200bcm\u22122 and 252\u00a0\u200bmV at 100\u00a0\u200bmA\u00a0\u200bcm\u22122. Furthermore, it also displays small Tafel slopes of 75 and 126\u00a0\u200bmV dec\u22121 in alkaline media, as well as high stability, even in the chronopotentiometric testing of 50\u201360\u00a0\u200bh. Moreover, as both the cathode and the anode, P-CoMoO4@NiCoP/NF exhibits good overall water splitting performance. To reach a current density of 20\u00a0\u200bmA\u00a0\u200bcm\u22122, P-CoMoO4@NiCoP/NF only needs a low potential of 1.62\u00a0\u200bV with 50\u00a0\u200bh durability. This excellent performance indicates that P-CoMoO4@NiCoP/NF is a promising bifunctional electrocatalyst for overall water splitting, which may make it possible to realize large scale, high efficiency catalytic electrolysis of water under high current density.The authors declare no competing financial interests.The authors acknowledge the National Natural Science Foundation of China (NSFC 91834301, 21808046 and 21908037) and Anhui Provincial Science and Technology Department Foundation (201903a05020021 and 202003a05020046) for funding 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.nanoms.2021.05.004.", "descript": "\n                  Improving catalytic activity and durabilty through the structural and compositional development of bifunctional electrocatalysts with low cost, high activity and stability is a challenging issue in electrochemical water splitting. Herein, we report the fabrication of heterostructured P-CoMoO4@NiCoP on a Ni foam substrate through interface engineering, by adjusting its composition and architecture. Benefitting from the tailored electronic structure and exposed active sites, the heterostructured P-CoMoO4@NiCoP/NF arrays can be coordinated to boost the overall water splitting. In addition, the superhydrophilic and superaerophobic properties of P-CoMoO4@NiCoP/NF make it conducive to water dissociation and bubble separation in the electrocatalytic process. The heterostructured P-CoMoO4@NiCoP/NF exhibits excellent bifunctional electrocatalysis activity with a low overpotential of 66\u00a0\u200bmV at 10\u00a0\u200bmA\u00a0\u200bcm\u22122 for HER and 252\u00a0\u200bmV at 100\u00a0\u200bmA\u00a0\u200bcm\u22122 for OER. Only 1.62\u00a0\u200bV potential is required to deliver 20\u00a0\u200bmA\u00a0\u200bcm\u22122 in a two-electrode electrolysis system, providing a decent overall water splitting performance. The rational construction of the heterostructure makes it possible to regulate the electronic structures and active sites of the electrocatalysts to promote their catalytic activity.\n               "}
{"full_text": "Direct methanol fuel cells (DMFC) are a type of efficient, \u2018green\u2019 power source in which chemical energy can be converted to electric energy via the oxygen reduction reaction (ORR) on the cathode and the methanol oxidation reaction (MOR) on the anode [1,2]. It is known that the performance of a DMFC is mainly determined by the MOR kinetics. The oxidization of methanol to CO2 is highly favoured since this oxidation mechanism is accompanied by the simultaneous release of 6 electrons [3]. Unfortunately, this reaction is sluggish. More seriously, CO intermediates are likely to be generated during the MOR. This so-called CO path can easily inactivate noble-metal catalysts with serious consequences [4]. Since the oxidation of CO to CO2 requires the presence of OH*, the adsorbed OH species is indispensable to facilitate CO desorption or convert CO to CO2\n[5\u20137]. Therefore, the design of catalysts with improved ability to adsorb OH species is of great importance in boosting the MOR and eventually enabling the assembly of high-performance DMFCs.One alternative to a Pt catalyst for the MOR is a Pd catalyst, which exhibits better CO resistance. This is due to its better oxophilic nature [8]. To facilitate the increased adsorption of OH species on the Pd catalyst for the MOR, other metals (e.g., Mn, Rh, Ni, Ag) with a high ability to adsorb OH species have been combined with the Pd catalyst. For example, a number of PdM alloys (M\u00a0=\u00a0Mn, Rh, Ni, Ag) have been synthesized [9\u201313]. The drawback of these alloys is the reduced utilization of the Pd catalyst, since the exposed atomic Pd sites in these alloys tend to be occupied by the introduced metals [14].A core\u2013shell structured Pd catalyst is expected to be better for MOR than an alloy, as a core\u2013shell catalyst would effectively have an increased number of atomic Pd sites in the shell. More importantly, it is possible to optimize the electronic structure of the Pd atoms via the strain effect [14\u201317]. This is because the d-band centre of Pd atoms in the shell can be modified according to the strain state formed [18\u201322]. Furthermore, the strong interaction between the metal core and the Pd shell facilitates electron transfer between them, which dramatically affects the electronic structure of the Pd shell [23,24]. Consequently, improved OH adsorption on the Pd shell is expected, eventually leading to increased activity for the MOR.In this work, a series of Ag-core/Pd-shell (Ag@Pd\nx\n) catalysts are synthesized. The Ag metal was selected as the core because, with its large lattice parameter, an Ag core can provide a tension strain for a Pd shell. Meanwhile, the thickness of the Pd shell is varied. In particular, the strain and electronic state of a Pd shell can be adjusted by changing the thickness of the Pd shell. The detailed MOR performance of the as-designed Ag@Pd\nx\n (x\u00a0=\u00a01,3,5) and related density functional theory (DFT) calculations are reported in this contribution.Silver nitrate (AgNO3), methanol (CH3OH), triphenylphosphorous (TPP), palladium acetylacetonate (Pd(acac)2), tri-n-octyl oxyphosphorous (TOPO), and borane tert-butylamine complex (BTB) were purchased from Aladdin Chemical Reagent Co. Ltd. Ketjen Carbon was supplied by Shanghai Cuike Chemical Technology. All reagents were of analytical grade and used as received without further purification.Ag@Pd\nx\n (x\u00a0=\u00a01,3,5) catalysts were synthesized by a seed-growth method as described previously [14]. A Pd/C catalyst and AgPd alloy were also synthesized for comparative purposes. Prior to employing these catalysts for the MOR, 10\u00a0mg of the catalyst and 90\u00a0mg of Ketjen Carbon were dispersed in hexane individually with the aid of sonication for 2\u00a0h. Subsequently, the black mixture was centrifuged and dried in a vacuumoven at 60\u00a0\u00b0C for 12\u00a0h. An ink was then prepared by dispersing 5\u00a0mg of this black product in 1\u00a0mL of ethanol and 20\u00a0\u00b5L of Nafion solution (Sigma Aldrich, 5\u00a0wt%) under sonication for 30\u00a0min. To form a working electrode, 5\u00a0\u03bcL of the resulting ink was coated onto a glassy carbon disk electrode (GCE, 5.0\u00a0mm in diameter). After that, the electrode was dried at room temperature for about 10\u00a0min.The phase characterization of the fabricated Ag@Pd\nx\n catalysts was carried out on a Bruker D8 advance X-ray diffraction (XRD) system (Karlsruhe, Germany) with Cu K\u03b1 radiation in the 2\u03b8 range from 25\u00b0 to 90\u00b0 at a scanning rate of 5\u00b0 min\u22121. Transmission electron microscopy images of these catalysts were recorded on a JEOL JEM-2100F (Tokyo, Japan), operated at 200\u00a0kV. High-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were also recorded. The EDX analysis was performed in the STEM mode using an aberration-corrected JEOL 2200FS electron microscope (Tokyo, Japan) equipped with a Bruker-AXS SDD detector (Karlsruhe, Germany). The X-ray photoelectron spectroscopy (XPS) was carried out on a Kratos Axis Ultra DLD 5 spectrometer (Manchester, UK) with an Al K\u03b1 (hv\u00a0=\u00a01486.6\u00a0eV) X-ray excitation source. All XPS spectra were calibrated using the C1s peak located at 284.6\u00a0eV.The MOR performance was evaluated in a three-electrode system using a CHI850 electrochemical workstation (Shanghai, China). A Pt sheet (1.5\u00a0\u00d7\u00a01.5\u00a0cm2) and a saturatedcalomelelectrode (SCE) were selected as the counter electrode and the reference electrode, respectively. The working electrode was the ink-coated GCE. The electrochemical active surface area (ECSA) was calculated by the equation: ECSA\u00a0=\u00a0Q / (m\u2219C), where Q is the calculated charge required to reduce PdO to Pd on the catalyst surface, m is the Pd atomic mass of the catalyst dropped on the GCE surface, and C is the theoretical charge (420 \u03bcC cm\u22122) needed to reduce a layer of PdO to Pd [25].All calculations were performed using the Perdew\u00a0\u2212\u00a0Burke\u00a0\u2212\u00a0Ernzerhof method implemented in the Vienna ab initio Simulation Package [26\u201328]. The interactions between ion cores and valence electrons were calculated with a projector augmented wave method [28]. The valence electronic states were expanded in the plane wave basis sets within a cut-off energy of 400\u00a0eV. The Pd(111) surface was modelled as a periodic slab with a p(3\u00a0\u00d7\u00a03) unit cell with four Pd layers. The two layers at the bottom were fixed in the slab while the two layers at the top, together with the adsorbates, were relaxed throughout the geometry optimization. For these surface slabs, the Brillouin zone of the surface calculations was sampled with 4\u00a0\u00d7\u00a04\u00a0\u00d7\u00a01 Monkhorst-Pack mesh. The geometric optimizations converged when the energy difference was smaller than 10-5 eV and the forces were less than 0.01\u00a0eV\u00a0\u00c5\u22121.The solvent reduction method used is illustrated schematically in Fig. 1\na. The Ag core is 3.2\u00a0nm in size (Fig. S1a, b). The thickness of the Pd shell grown on the surface of the Ag core is varied by altering the amount of the Pd(acac)2 precursor used. In this way, three Ag@Pd\nx\n nanoparticles were synthesized with Pd shells of different thicknesses. From the TEM images, one can see that they are uniformly distributed on the carbon support (Fig. 1b\u2013d). Their sizes (Fig. S2a\u2013c) are 3.7, 4.6, and 5.4\u00a0nm, respectively. In other words, their size increases when the shell is thicker or the number of Pd layers in the shell is increased. These particles are named Ag@Pd1, Ag@Pd3, and Ag@Pd5 catalysts, respectively, throughout this paper. As control experiments, well-dispersed Pd/C (2.6\u00a0nm) and AgPd alloy (7.6\u00a0nm) catalysts were also synthesized (Fig. S3). As an example, the HAADF-STEM image of a Ag@Pd3 nanoparticle (Fig. 1e) reveals an interplanar spacing of 0.234\u00a0nm in the inner layer and an interplanar spacing of 0.228\u00a0nm in the outer layer. These are indexed to Ag (111) and Pd (111), respectively. Note that the interplanar spacing of the Pd shell is larger than that of the standard Pd (111) (0.224\u00a0nm) [29]. In other words, the Pd shell is stretched by the Ag core. EDX element mapping of two Ag@Pd3 nanoparticles was then conducted (Fig. 1f), illustrating their core\u2013shell structure, in that the Pd element is distributed around the Ag element. In addition, the estimated thickness of the Pd shell is about 0.8\u00a0nm. According to the EDX line profiles (Fig. 1g), a Ag@Pd3 nanoparticle has nearly 3 Pd layers in the shell.XRD analysis of these nanoparticles was then performed (Fig. S4). The Ag core exhibits a typical FCC structure with diffraction peaks located at 38.2\u00b0, 44.4\u00b0, 64.3\u00b0, 77.5\u00b0, 81.4\u00b0, corresponding to (111), (200), (220), (311) and (222) of Ag, respectively [14]. As a control experiment, the visible diffraction peaks of the Pd/C catalyst are located at 39.6\u00b0, 46.9\u00b0, 67.5\u00b0, and 80.1\u00b0, associated with (111), (200), (220), (311) of Pd, respectively. After the growth of a Pd shell on this Ag core, no obvious changes are found in the XRD peaks of the Ag core. Meanwhile, no Pd diffraction peaks are seen in the XRD patterns of the Ag@Pd\nx\n catalysts, indicating the ultrathin nature of the Pd shell.In order to investigate the electronic interactions between the Ag core and the Pd shell in the Ag@Pd\nx\n nanoparticles, an XPS analysis was carried out. In the Ag 3d XPS spectra of metallic Ag and the Ag core, both Ag (0) and Ag (+1) are observed and most of the Ag is shown to be in the zero-valence state (Fig. 2\na). A very tiny shift in the binding energy of Ag is observed. For example, the binding energies of Ag (0) 3d5/2 for the Ag@Pd1 (367.43\u00a0eV), Ag@Pd3 (367.46\u00a0eV) and Ag@Pd5 (367.44\u00a0eV) nanoparticles are slightly more negative than that of the Ag core (367.73\u00a0eV). Moreover, the binding energies of Ag 3d5/2 in these Ag@Pd\nx\n particles are very similar. With respect to the Pd 3d XPS spectra of these nanoparticles (Fig. 2b), most of the Pd present is shown to be in the zero-valence state. Only a small amount of Pd (+2) is seen, probably due to surface oxidation. Compared with the Pd/C catalyst, the binding energy of Pd 3d5/2 in the Ag@Pd\nx\n nanoparticles shifts negatively by about 0.6\u00a0eV. The shift in this binding energy demonstrates electron transfer from the Ag core to the Pd shell. When this Pd shell becomes thicker, the negative shift in the related binding energy turns out to be weaker. This phenomenon confirms the reduced electronic effect in these Ag@Pd\nx\n nanoparticles.The electrochemical MOR performance of these Ag@Pd\nx\n nanoparticle-based catalysts was then tested in 1\u00a0M KOH solution (Fig. 3\na). The reduction peak of AgO to Ag is seen at ~0.05\u00a0V in the cyclic voltammogram (CV) of the Ag nanoparticles (or the Ag core). It is weakened and eventually disappears when the Ag core is coated with a Pd shell. Simultaneously, a new reduction peak appears at about \u22120.4\u00a0V when a Pd shell is coated on the Ag core. Obviously, this peak is due to the reduction of PdO to Pd. Using the area of this cathodic peak, the ECSAs of the catalysts were calculated and further utilized as an index of active sites inside these catalysts [25]. The ECSA of the Ag@Pd1 nanoparticle is 113\u00a0m2 g\u22121, larger than that of Ag@Pd3 (98\u00a0m2 g\u22121), Ag@Pd5 (81\u00a0m2 g\u22121), and Pd/C (61\u00a0m2 g\u22121) nanoparticles (Table S1). In other words, a thinner Pd shell exposes more active Pd sites. The MOR activity of these catalysts was then tested in 1\u00a0M KOH\u00a0+\u00a01\u00a0M CH3OH solution. Note that for all these catalysts it is necessary to activate them after several voltammetric cycles (Fig. S5). An example of such activated CVs is shown in Fig. 3b. Surprisingly, the MOR catalytic activity of the Ag@Pd3 (2369\u00a0mA\u00a0mg\u22121\nPd) catalyst is higher than that of Ag@Pd1 (1322\u00a0mA\u00a0mg\u22121\nPd), Ag@Pd5 (1703\u00a0mA\u00a0mg\u22121\nPd), and Pd/C (571\u00a0mA\u00a0mg\u22121\nPd) catalysts (Fig. 3b). This is probably due to the fact that the Ag@Pd3 catalyst has the best intrinsic activity among these catalysts. The specific current was further normalized by its ECSA. The Ag@Pd3 catalyst exhibits the highest intrinsic activity in that the peak current on the Ag@Pd3 catalyst is about 2.4\u00a0mA\u00a0cm\u22121 larger than that of the Ag@Pd (1.5\u00a0mA\u00a0cm\u22121), Ag@Pd5 (2.1\u00a0mA\u00a0cm\u22121) and Pd/C (0.94\u00a0mA\u00a0cm\u22121) catalysts (Fig. 3c, Table S1). Moreover, the Ag@Pd3 catalyst shows better MOR performance than the AgPd alloy with respect to the mass activity (985\u00a0mA\u00a0mg\u22121\nPd) and the specific activity (1.4\u00a0mA\u00a0cm\u22121) (Fig. S6, Table S1). In fact, the AgPd alloy exhibits enhanced MOR activity compared to the Pd/C catalyst, probably originating from a bifunctional mechanism [30]. Specifically, the OH radical can be adsorbed on the Ag sites of the AgPd alloy, leading to improved MOR activity. When compared with recently reported Pd-based catalysts, the mass activity and specific activity of the Ag@Pd3 catalyst are found to be greatly superior (Table S2).The cycling stability of the catalysts was also evaluated. After 1000 cycles of cyclic voltammetric tests, the mass activity of the Ag@Pd3 catalyst still retained 43% of its initial value (Table S1). This is much higher than the corresponding values for Ag@Pd1 (23%), Ag@Pd5 (18%), AgPd alloy (21%), and Pd/C (4%) catalysts. Moreover, the Ag@Pd3 catalyst still retains a pair of typical MOR peaks even after 1000 cyclic voltammetric cycles. This performance is much better than that of the Pd/C catalyst (Fig. S7). Actually, aggregation of the Ag@Pd3 catalyst occurring during the course of such a stability test is obviously reduced when compared with the Pd/C catalyst (Fig. S8). In order to further investigate the change in the active sites of the Ag@Pd3 catalyst during the stability test, the Pd 3d XPS spectrum of the Ag@Pd3 catalyst was recorded after such a stability test (Fig. S9). A much more pronounced Pd (+2) peak is visible. Consequently, the Pd (0) is oxidized to Pd (+2) during the MOR. This is the reason why its MOR activity decreases as a function of running time. Moreover, the atomic ratio of Ag:Pd in the Ag@Pd3 catalyst changes from 1:2 to 1:1.5, which suggests Pd dissolution during the MOR. This is another source of the decreased MOR activity.The MOR mechanism on the Ag@Pd\nx\n catalysts was further examined by DFT calculations using a VASP code. To evaluate the electronic and strain effects in the Ag@Pd\nx\n catalysts, simple Ag@Pd (111) and tensional Pd (111) models were constructed (Fig. S10). Compared with Pd (111)-0%, both d-band electron energy ranges of Pd (111)-3% and Pd (111)-5% are decreased. On the other hand, the d-band centre of Pd (111)-5% is \u22121.44\u00a0eV (Fig. 4\na). This is closer to the Fermi level than that of Pd (111)-3% (-1.47\u00a0eV) and Pd (111)-0% (-1.55\u00a0eV). As for Ag@Pd(111), its d-band electron state is more negative and narrower, although the strain state of Pd in Ag@Pd(111) is close to that in Pd(111)-5%. All these results confirm that there is an electronic effect between Ag and Pd. The projected partial density of states (PDOS) of Ag after being coated with a Pd layer exhibits several new weak peaks in the range from \u22120.3 to \u22122 eV. They are in line with those in the PDOS energy range from 0 to \u22123 eV for Pd in Ag@Pd (111) (Fig. 4b). Consequently, the tensional strain effect makes the d-band centre shift closer to the Fermi level, while the electronic effect causes the d-band centre to move far away from the Fermi level in the Ag@Pd system. Since the Pd atom plays an important role in the adsorption action, the adsorption energy of CH3OH on Pd (111) is increased when the Pd (111) is stretched, but the adsorb energy of CH3OH on Ag@Pd (111) is still smaller than the Pd (111)-0% (Table S3).The difference in the adsorption energies was further elucidated using these catalyst models. According to the PDOS of free and adsorbed CH3OH (Fig. 4c), the electronic energy of CH3OH is decreased after the adsorption of CH3OH on Pd (111)-0%. This indicates that the system of CH3OH-Pd (111)-0% is stable. Moreover, a higher reduction in energy is seen when CH3OH is adsorbed on Pd (111)-3% and Pd (111)-5%. In other words, CH3OH is more easily adsorbed on the tensional Pd (111). However, when CH3OH is adsorbed on Ag@Pd (111), the corresponding energy is higher than that on Pd (111)-0%. This result reveals that CH3OH has a weaker adsorption energy on Ag@Pd (111). On the other hand, the adsorption energy and PDOS of OH are similar to the action of CH3OH on the Pd-based models (Table S3, Fig. 4d). These results show that the strain effect in the Ag@Pd\nx\n catalysts is beneficial for the adsorption of CH3OH and OH, while the electronic effect in the Ag@Pd\nx\n catalysts is harmful for the adsorption of CH3OH and OH. In short, the Ag@Pd3 catalyst exhibits superior activity to its counterparts, which is mainly ascribed to the optimal combination of strain and electronic effects.In summary, a number of core\u2013shell Ag@Pd\nx\n catalysts have been synthesized with a Pd shell of different thicknesses. Optimizing the thickness of the Pd shell leads to enhanced MOR activity. As confirmed by DFT simulations, this high MOR activity is attributed to an optimal combination of strain and electronic effects in the Ag@Pd3 catalyst. This work offers new insight into strain and electronic effects in core\u2013shell structured electrocatalysts. It thus provides a novel approach to the design of various high-performance MOR electrocatalysts. Such catalysts are extremely promising for future use in the mass production of direct methanol fuel cells.\nXiaobo Yang: Investigation, Writing - original draft. Xili Tong: Project administration, Writing - review & editing, Supervision. Xingchen Liu: Investigation. Kaixi Li: Supervision. Nianjun Yang: Writing - 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 work is financially supported by the Joint Funds of the National Natural Science Foundation of China (U1710112) and the State Key Laboratory of Coal Conversion (2020BWL001).Supplementary data to this article can be found online at https://doi.org/10.1016/j.elecom.2021.106917.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The performance of a direct methanol fuel cell (DMFC) is strongly dependent on the catalytic anode. A high-performance anode is expected to offer enhanced intrinsic activity and/or a large electrochemical surface area. Herein, a series of Ag-core/Pd-shell (Ag@Pd\n                        x\n                     , x\u00a0=\u00a01,3,5) catalysts are synthesized in which the thickness of the Pd shell is varied. Both tensional strain and electron transfer between the Ag core and the Pd shell are found to affect the intrinsic activity of these Ag@Pd\n                        x\n                      catalysts. Of these, the Ag@Pd3 catalyst exhibits the best performance for the methanol oxidation reaction (MOR), showing 4.1 times higher mass activity and 2.6 times higher specific activity than a Pd/C catalyst. Furthermore, density functional theory calculations show that this high MOR performance stems from a stronger adsorption of CH3OH and OH on the Pd active sites. This catalyst is thus a promising candidate for inclusion in a high-performance DMFC.\n               "}
{"full_text": "Increasing concerns about energy consumption, exhaustion of fossil fuel resources and global warming have enhanced attention in making of bio-based chemicals/fuels through biorefineries [1\u20133]. Lignocellulosic biomass is the prompting abundant carbon source in making value added chemicals and fuels. Cellulose and hemicellulose are the key composites in lignocellulosic biomass [4\u20137]. Hexoses such as glucose and fructose are the major products obtained from cellulosic biomass. HMF, a product derived from hexoses by dehydration, considered as a platform chemical from bio-renewables [8,9]. The high functionality of HMF allows to convert into diverse biofuel molecules like ethyl levulinate (EL), 5-ethoxymethylfurfural (EMF), 2,5-dimethylfuran (DMF), 2,5-dimethyltetrahydrofuran (DMTHF) and value-added chemicals such as levulinic acid (LA), 2,5- diformylfuran (DFF), 2,5-furandicarboxylic acid, terephthalic acid, and caprolactone, etc. [10\u201315]. Among various chemicals, DMF is more attractive in particular among other HMF derivatives because of its elevated boiling point, better energy density and high octane number which are greater than the current gasoline alternatives like ethanol and butanol. DMF is insoluble in water and is easy to store. These properties make DMF as an effective gasoline alternative and can be considered as an alternative transportation biofuel. DMF can also be used as a diene in Diels\u2010Alder reactions [16\u201318].Noble metal catalysts such as Pt and Pd are widely used in HMF hydrodeoxygenation (HDO) due to their high reactivity in hydrogenation reactions. Different kinds of carbon supported Ru based catalysts are used for HMF hydrodeoxygenation to DMF with good yields [19\u201322]. Ru based bimetallic catalysts are also showed high yield of DMF with excellent reusability of the catalysts [23\u201325]. Due to the high reactivity of Ru, these catalysts lead to furan ring over hydrogenation and formation of ring opened products. Pt supported on mesoporous carbon with nitrogen rich and Pt on reduced graphin oxide catalysts are tested for HDO of HMF. These catalytic systems showed moderate DMF yields [26,27]. Further few research groups worked on Ru and Co based bimetallic catalysts to improve DMF yield [28,29]. Even though these catalysts showed good DMF yield, they found the Co metal leaching due to the weak interaction with the supports. Yadav et al., reported Pd on K-10 supported Cs-modified heteropoly acid catalyst with comparable activity [30]. Pd-Au and Pd-Zn based bimetallic catalysts also used for HMF hydrogenation with enhanced activity [31,32]. These noble metal catalysts showed good activity; however, their high cost, non-selective nature due to high reactivity and limited stability limited their usage for the conversion of HMF to DMF. In this context, non noble metals like Co, Ni and Cu catalysts have attracted great attention because of their low cost, less hazardous nature and high selectivity compared to noble metals. Co and Cu bimetallic catalyst are also active for this reaction only at prolonged reaction times [33\u201335]. Also, there are many bifunctional Ni based catalysts reported for the HMF hydrogenation with considerable activity. Many of these catalysts showed high selectivity towards DMTHF over DMF and also observed side products like 2-methyl furan, THF and ring opened products [36\u201338]. Comparative to Ni based catalysts Cu catalysts showed better selectivity towards DMF as Cu shows high reactivity to C=O bond over C=C bond. Dumesic et al., reported copper chromite catalyst for HMF hydrodeoxygenation. In that study they converted sugars and obtained DMF with 67% selectivity. However, the high copper loading and toxicity of chromium had a negative impact [39]. Barta et al., reported the activity of different commercially available copper catalysts such as, Cu, CuO, CuO-Fe2O3, CuZnFe2O4 and CuZn. Among the catalysts, CuZnO showed better activity at 220\u202f\u00b0C with 30\u202fbar H2 pressure in 6\u202fh of reaction time. This catalyst showed the activity drop up to 17% in its reusability [40]. Zhu et al., demonstrated CuZn catalyst with good activity towards DMF at 220\u202f\u00b0C with 15\u202fbar H2 in 5\u202fh. Most of the studies focused on the Cu based mixed oxide catalysts with high content of Cu up to 54%, which causes the unstable nature of the catalyst during recyclability [41]. Rupert and group also reported CuZnO catalyst with self-tuned properties towards BHMF and DMF. Moreover, most of these bulk Cu based catalysts are not stable during recycling experiments [42]. Esteves et al. demonstrated the use different metal oxide (Al2O3, Nb2O5, Al2O3-Nb2O3) supported Cu catalysts for HDO of HMF [43]. There is a need to develop non-noble metal like Cu based active catalysts which can efficiently catalyse 5-HMF hydrodeoxygenation at moderate reaction conditions within reasonable reaction time. The activity of Cu based catalysts mainly depends on Cu dispersion which can be improved if porous metal oxides with high surface area are used as support [44]. It is thought to prepare Cu based catalyst on mesoporous materials with high surface area to achieve high activity with selectivity during HMF conversion to DMF.Here in this paper, copper supported SBA-15 catalysts are prepared and studied for the hydrodeoxygenation of HMF to selectively yield DMF. The catalysts are characterized with various techniques to derive their surface-structure characteristics. All these characteristics are utilized in understanding the catalysts for their selective HDO activity of 5-HMF.A set of Cu supported on SBA-15 catalysts were designed and prepared by impregnation method. Initially, mesoporous silica SBA-15 was prepared based on the reported literature using a silica source tetraethyl orthosilicate (TEOS) [45,46]. In the preparation, P123 is used as structure directing material. An aqueous solution with the composition of TEOS:P123:2M HCl:H2O\u202f=\u202f4.25:2:60:15 (weight ratio) was stirred using magnetic bead at 40\u202f\u00b0C for 24\u202fh. Then this mixture was transferred to Teflon bottle for hydrothermal treatment for 24\u202fh at 100\u202f\u00b0C. After that the resultant gel solution was washed with distilled water for several times up to pH of the solution becomes neutral during filtration. The filtrate was oven dried for overnight at 80\u202f\u00b0C. The calcination of dried solid was carried out at 550\u202f\u00b0C in presence of air flow for 8\u202fh. The resultant mesoporous silica (SBA-15) was used as support. Cu supported on SBA-15 catalysts with various loadings (5, 10, 15, 20\u202fwt%) of Cu were prepared by wet impregnation method by using copper nitrate as precursor. The required quantity of the precursor dissolved in water was added to the SBA-15 and the mixture was dried with infrequent stirring on a hot plate. The solid mass was dried at 100\u202f\u00b0C in an air oven for 12\u202fh. Later calcined in air at 450\u202f\u00b0C for 5\u202fh. These samples were denoted as X%Cu/SBA-15, where\u202fX\u202frepresents the Cu weight percentage.HMF hydrogenation was carried out in a 100\u202fmL Parr autoclave reactor. Initially, the catalyst (0.15\u202fg) was reduced at 400\u202f\u00b0C in presence of H2 flow (35\u202fmL/min) for 2\u202fh. After reduction the sample was cooled to room temperature in presence N2 flow. The autoclave reactor was charged with the catalyst (0.15\u202fg), HMF (2\u202fmmol, 0.252\u202fg) in THF (20\u202fmL) and the reactor was flushed with H2 for three times. The reaction was performed at 180\u202f\u00b0C under 2.0\u202fMPa H2 pressure with a stirring rate of 300\u202frpm/min for stipulated reaction time. The reactor was cooled after completion of the reaction to room temperature and it was filtered for product analysis.Products were confirmed by GCMS (Shimadzu, GCMS-QP2010S) and conversion and yields were determined by using Shimadzu 2010 gas chromatography equipped with capillary column. Flame ionization detector was used to analyze the products by separating them on Inno wax capillary column (diameter: 0.25\u202fmm, length: 30\u202fm). Conversion of HMF and yields were estimated based on the equations shown below.\n\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\nMi\n-\nM\nf\n\n\nMi\n\n\n\u00d7\n100\n\n\n\n\n\n\n\n\nY\ni\ne\nl\nd\n\n\n(\n%\n)\n\n=\n\n\nPi\n\n\nMi\n\n\n\u00d7\n100\n\n\n\n\n\n\n\nMi\n\n refers the initial moles of HMF, \n\nMf\n\n refers the final moles of HMF and Pi stands for the moles of product i formed.The catalysts were characterized by different techniques like powder X-ray diffraction (XRD), BET surface area, temperature-programmed reduction (TPR), temperature-programmed desorption of NH3 (TPD-NH3) and transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The details of these experimental procedures are reported in our earlier publications [47\u201349].The dissociative N2O chemisorption method was carried on the instrument (BELCAT-II, BEL Japan) used for TPR studies to determine copper metal surface area, particle size and dispersion. The sample (100\u202fmg) was placed in the sample tube and pre-treated in Ar gas (50\u202fmL/min) flow for 30\u202fmin at 100\u202f\u00b0C. Firstly catalyst was pre-reduced by increasing the temperature to 400\u202f\u00b0C with a ramp of 10\u202f\u00b0C/min under 5% H2/Ar flow. This step is termed as TPR1. Then the sample was cooled in Ar flow to 50\u202f\u00b0C and exposed sequentially to 10% N2O/He gas for 1\u202fh, to re-oxidize metallic Cu to Cu2O by N2O dissociative chemisorption. The sample the purged in Ar flow at 50\u202f\u00b0C for 15\u202fmin, again temperature programmed reduction (TPR2) was carried to reduce the oxidized Cu2O species to copper metal. The copper metallic area, particle size and dispersion were estimated by the following equation\n\n\n\nDispersion\n\n\n\n\nD\n%\n\n\n\n=\n\n\n2\n\n\u00d7\n\n\nH\n2\n\nc\no\nn\ns\nu\nm\np\nt\ni\no\nn\n\ni\nn\n\nT\nP\n\nR\n2\n\n\n\n\nH\n2\n\nc\no\nn\ns\nu\nm\np\nt\ni\no\nn\n\ni\nn\n\nT\nP\n\nR\n1\n\n\n\n=\n\u00d7\n100\n\n\n\n\n\n\n\n\nSpecific\n\nC\nu\n\ns\nu\nr\nf\na\nc\ne\n\na\nr\ne\na\n\n\n\nS\n\n\n\n\n2\n\n\u00d7\n\n\nH\n2\n\nc\no\nn\ns\nu\nm\np\nt\ni\no\nn\n\ni\nn\n\nT\nP\n\nR\n2\n\n\u00d7\nN\n\n\n\nH\n2\n\nc\no\nn\ns\nu\nm\np\nt\ni\no\nn\n\ni\nn\n\nT\nP\n\nR\n1\n\n\u00d7\n\n\nM\n\nCu\n\n\n\u00d7\n\n1.4\n\n\u00d7\n\n\n10\n9\n\n\n\n\n=\n\n\nm\n2\n\n\ng\n\n-\n1\n\n\n\n\n\nWhere N\u202f=\u202fAvogadro constant, M Cu\u202f=\u202fatomic mass (3.45\u202fg\u202fmol\u22121) and the number of superficial Cu atoms per unit surface area as 1.47\u202f\u00d7\u202f1019 atoms/m2, and the density of copper as 8.92\u202fg/cm3.The surface properties of the catalysts were drawn from N2-adsorption and desorption isotherms and the profiles are shown in Fig. 1\nA. The isotherms of the catalysts and support representing type-IV with H2 like hysteresis loop related to the mesoporous nature of the catalysts [50]. These profiles show that the mesoporous structure of SBA-15 was retained in the catalysts even after the dispersion of CuO. The increase in N2 uptake was observed at high P/P0 could be initiated by capillary condensation of multi-layer adsorption. The textural properties of the catalysts are shown in Table 1\n. The immobilization of Cu species significantly altered the surface properties of SBA-15. The surface area of bulk SBA-15 is 676\u202fm2/g and the surface area values are decreased with increase in copper content on SBA-15. The inner pore blockage of SBA-15 with deposition of CuO causes decrease in the surface areas which is in good agreement with pore size distribution measurements. Pore size distribution curves were employed by BJH (Barrett-Joyner-Halenda) method considering by adsorption branches of isotherms, all the curves confirmed the pores are in mesoporous region as showed in Fig. 2(B). The presence of Cu particles with in the channels of SBA-15 block the pores of the support. Due to this pore blockage, there is a partial strain in the pores that leads to a marginal increase in pore diameter increase in Cu loading [51].Low-angle XRD patterns of Cu/SBA-15 samples along with support SBA-15 are shown in Fig. 2(A)\n\n. All the catalysts exhibited an intense peak at 2\u03b8\u202f=\u202f0.9\u00b0 designated to (100) plane and two less intense reflections at 1.6, 1.8\u00b0 corresponding to (110) and (200) planes in the 2D hexagonal pore arrangement [52]. A marginal shift in (100) plane 2\u03b8 value with the existence of CuO on SBA-15. This indicates the presence of intact ordered mesoporous structure with minor distortion after deposition of CuO.\nFig. 2(B) shows the wide angle XRD patterns of pure SBA-15 and copper loaded SBA-15 catalysts. All the composite samples displayed a broad intense peak around 23.5\u00b0, assigned to the amorphous nature of SiO2. The XRD patterns of the samples showed the presence of intense reflections at 2\u03b8\u202f=\u202f32.4\u00b0, 35.7\u00b0, 38.9\u00b0, 48.6\u00b0, 53.7\u00b0, 58.1\u00b0,61.5\u00b0, 66.1\u00b0, 67.9, 72.3\u00b0, which are attributed to the monoclinic CuO (JCPDS No. 65-2309). These results indicating the presence of crystalline CuO over SBA-15 support. Powder XRD patterns of reduced catalysts are inserted in Fig. 2(B). The most intense peaks at 2\u03b8\u202f=\u202f44.3\u00b0, 50.4\u00b0, and 74.1\u00b0 corresponding to (111), (200), and (220) planes of metallic copper are observed. No other peaks were noticed in the patterns, indicating that Cu existed as Cu0 species after the reduction.H2-TPR analysis was performed to know the reducibility of CuO particles that are in contact with support SBA-15 and the resulted profiles are shown in Fig. 3\n. The TPR profiles of Cu loaded SBA-15 catalysts showed two reduction peaks, one is in the range of 280\u2013300\u202f\u00b0C and another one is at 390\u202f\u00b0C. According to the literature the finely dispersed copper oxide species are easily reducible at lower temperatures comparatively the bulk species reduce at higher temperatures [53,54]. If the interaction between support and metal is weak then it would reduce at low temperature region and the strong interactions of metal species with support will be reduced at higher temperatures. When the Cu content enhanced from 5 to 20\u202fwt%, a shift in the main reduction peak towards high temperature was noticed and it became relatively broad. This indicates that, with increasing the Cu loading on SBA-15 resulted to form strong interaction with support SBA-15. The catalysts with high Cu content (15 and 20\u202fwt%) exhibited a small reduction peak at low temperature (220\u202f\u00b0C) might be related to the reduction of bulk CuO species present in the samples. The N2O chemisorption technique used to estimate copper dispersion, metal surface area and particle size and the results are presented in Table 1. The N2O chemisorption results suggesting that the dispersion of Cu particles on SBA-15 was decreased with the increase in Cu loading. This could happen due to the formation of Cu metal aggregated species, which resulted in the decrease of metal area and increase of particle size. N2O chemisorption results also supporting the TPR results.Temperature programmed desorption of NH3 performed to calculate the total acidity and acidic strength of the catalysts. The acidity profiles of pure SBA-15 and Cu supported on SBA-15 are shown in Fig. 4\n. In general, the desorbed peak of NH3 was classified into three temperatures corresponding to the acidic sites as weak (<200\u202f\u00b0C), medium (200\u2013400\u202f\u00b0C) and strong (>400\u202f\u00b0C). The Cu/SBA-15 catalysts showed all these three types of acidic sites. The increase in copper loading tend to increase area under NH3 desorbed peak. This indicates that the moderate acidic sites associated with Cu are increased with increase in its content in the catalysts. The acidity of the catalysts is mainly associated with the presence of ionic copper species [55]. The total acidity values calculated by considering the area under the desorbed peaks and the results are shown in Table 1. The results indicate that with the increase in copper loading led to the improvement in the total acidity.To visually investigate the distribution of the particles of copper on SBA-15 support, TEM was carried out. TEM micrographs of bare SBA-15 and 15%Cu/SBA-15 are shown in Fig. 5\n. The images show the hexagonal array of uniform channels, which justify the highly ordered mesostructured materials. Copper particles are uniformly distributed on the surface of the support. The average particle size of 15%Cu/SBA-15 was 6.3\u202fnm. TEM images further confirms that structural ordering is maintained even after the Cu-incorporation in SBA-15 matrices.SEM images of bulk SBA-15 support and prepared Cu/SBA-15 samples are shown in Fig. 5(D and E). The bare SBA-15 support showed a fine rod-like mesoporous structures. Hence this result confirms that SBA-15 has 2D hexagonal arrays of mesoporous structure. The morphology shows that the mesoporosity is retained even after impregnation with copper and catalysts exhibited homogeneously dispersed CuO particles. The SEM images of Cu/SBA-15 clearly indicate that the copper oxide is present in a highly dispersed state.Selective HDO of HMF was performed with copper loaded SBA-15 catalysts. The activity results are shown in Fig. 6\n. The 5% Cu/SBA-15 catalyst showed 100% of HMF conversion with 65% of DMF yield and 33% yield of methyl furfural (MFA). With increasing the copper loading, HMF is totally converted and DMF yield increased for catalyst with 15% Cu loading and at the same time MFA yield was decreased. Further increase in copper loading to 20%, the yield of DMF was decreased to 74% from 90% and dimethyl tetrahydrofuran (DMTHF) yield was increased to 13% and also the ring opening products like 2, 5-hexenediol and alcohol are formed. Among the catalysts 15% Cu/SBA-15 showed promising catalytic activity towards DMF.The aforementioned results suggesting that the 15% Cu/SBA-15 catalyst was resulted in maximum of 90% DMF yield in 8\u202fh of reaction time. The activity of the catalysts can be explained in the light of the characteristics of these catalysts derived from different analysis methods. The N2O chemisorption results suggesting that the dispersion and metal surface area of Cu particles on SBA-15 were decreased and particle size was increased with the increase in Cu loading. Metal particle size is increased with the Cu content on SBA-15, might be the reason for the marginal decrease in metal dispersion and surface area. The dispersion of copper species on support increases the number of active metal sites which are responsible for high hydrogenation activity of catalysts. Although the lower loading (5 and 10%) catalysts showed high metal surface area and high metal dispersion, these catalysts displayed low yields. This is due to the insufficient number of metal sites with low Cu content. The 15% Cu/SBA-15 catalyst with sufficient number of active metal sites and enough total acidity, showed high yield of DMF. Further increase in loading of Cu on SBA-15 resulted in the decline of DMF yield. The high amount of Cu and high acidity led to the formation of over hydrogenated product DMTHF and other ring opened products as by products. Similar kind of results were observed over Cu/SiO2 catalysts for HMF hydrogenation to DHMF [56]. They used mild reaction conditions of 100\u202f\u00b0C, 4\u202fh reaction time and 15\u202fbar H2 Pressure for the preparation of DHMF at high loading of Cu (50%) on SiO2. MFA and DMF yields were increased because of high copper loading and acidity, which promoted the further hydrogenation of DHMF. Zhu et. al., developed the Cu/Zn catalysts derived from minerals. The Cu-ZnO catalyst with molar ratio 2 showed a high yield of 92% DMF. This is attributed to well dispersion of Cu metal sites over support ZnO surface, which allows the high Cu metal concentration and suitable acidity [41]. Ruppert et al., reported Cu/ZnO catalysts for DHMF and DMF synthesis from HMF over 10% Cu/ZnO catalyst. The selectivity was related to the presence of acid sites at the catalyst surface [42]. In the present work, comparatively lower loading of Cu (15%) on SBA-15 and targeted for the DMF synthesis from HMF. Availability of well dispersed copper metal with sufficient number of acidic sites and total acidity might be the reason for the selective formation of DMF over DHMF. The acidity of the catalysts is responsible for the carbonyl group activation [57]. Hydrogenolysis is more preferable over hydrogenation in case of the high acidic catalysts. TPD of NH3 results suggesting that the 15%Cu/SBA-15 have enough acidity to activate the carbonyl group of 5-HMF. Additionally, the literature results suggesting that the low temperatures reduction peaks in TPR indicate the CuO with small particles size or oligomeric clusters with a relatively weak interaction with support. The high temperature reduction peak is usually indicating a relatively stronger interaction of CuO with the support [58]. From the TPR observations, it is supporting that CuO species in a strong interaction with silica facilitates the hydrogenolysis reaction more. Moderate metal surface area with small particle size and enough acidity makes 15%Cu/SBA-15 catalyst more active than other catalysts for selective formation of DMF by the HDO of HMF. The 15%Cu/SBA-15 catalyst was more active among the all catalysts and further used for optimization of reaction conditions.In order to verify the effect of catalyst loading on HDO of HMF to DMF, the experiments were conducted with different amount of catalyst. Catalyst loading was calculated based on the ratio of catalyst weight to total reaction mixture weight (only liquid mass). Catalyst weight percentage was varied in between 0.27 and 1.09\u202fwt% and the results are shown in Fig. 7\n. When the catalyst amount is 0.27\u202fwt%, the conversion of HMF was 70% with 37% yield of DMF with 30% of intermediate MFA. Upon increasing the catalyst amount conversion of HMF and DMF yields were increased up to the catalyst loading of 0.82\u202fwt%. Further increase in catalyst loading to 1.09\u202fwt%, DMF yield was decreased to 78% and DMTHF a ring hydrogenated product formation was increased to 10%. The maximum DMF yield was observed with catalyst weight of 0.82\u202fwt%. At low catalyst loading the availability of number of active metal sites are low, which were not sufficient to convert maximum HMF to DMF and as a result, the maximum amount of intermediate MFA was remained in the reaction mixture. When high amount of catalyst was used, the number of active metallic sites and acidity were high, resulted in the formation of ring hydrogenated product DMTHF and also ring opened products.The effect of reaction time was carried to understand the reaction path way. The output of reaction time effect is shown in Fig. 8\n. The reaction was performed in the time intervals of 2\u201310\u202fh. At the initial time of 2\u202fh, the conversion of HMF was reached to 69% with 33% of MFA as major product. With increasing the reaction time to 4\u202fh, the conversion of HMF increased to 76% with 46% yield of DMF along with 28% of MFA. The maximum HMF conversion (100%) and DMF yield (90%) was observed at 8\u202fh of reaction time. Further increase in time led to decrease in DMF yield (74%) and formation of DMTHF with a yield of 18% was observed. Some of the ring open products like 2, 5-hexanedione and 2, 5-hexanediols were observed at prolonged reaction time (10\u202fh). These results suggest that the HDO of HMF over Cu/SBA-15 catalyst going through the formation of MFA as an intermediate before the formation of desired DMF. Further increase in time led to the conversion of DMF to the over hydrogenated product DMTHF i.e, furan ring hydrogenated product and ring opened products like 2, 5-hexanedione and diols.The influence of hydrogen pressure on HMF conversion and DMF yield over the catalyst was also investigated and the results are shown in Fig. 9\n. When the pressure was 1\u202fMPa, the conversion and selectivity are 88% and 55% respectively. Upon increasing pressure to 1.5\u202fMPa the conversion of HMF is 100%, the selectivity of DMF (81%) is not maximum and the presence of intermediates were noticed. Further increasing pressure to 2\u202fMPa, the DMF selectivity reached maximum of 100%. Further enhancement in pressure, HMF conversion remained constant and DMF selectivity decreased to 71% due to the formation of over hydrogenated product DMTHF with 22% selectivity.Effect of reaction temperature on hydrodeoxygenation of HMF is one of the crucial parameters in optimizing reaction parameters. The temperature effect was studied in the range of 140\u2013200\u202f\u00b0C at a fixed H2 pressure of 2.0\u202fMPa and the results are presented in Fig. 10\n. The HMF conversion (65%) and DMF yield (34%) were low at 140\u202f\u00b0C. Further increase in temperature to 160\u202f\u00b0C led to the maximum conversion of HMF (100%) with 70% of DMF yield and 23% yield of MFA. The reaction at 180\u202f\u00b0C was given the maximum yield of 90% DMF with 5% of DMTHF and no MFA was observed at this temperature. Further elevation in the temperature to 200\u202f\u00b0C, a drop in DMF yield (79%) was observed with increased DMTHF yield to 16%. From the above results it is observed that lower reaction temperatures favor the hydrogenolysis of O\u2013H bond in HMF over Cu/SBA-15 catalyst which results in the formation of MFA as a major product. However, elevated temperatures encourage the C=O bond reduction along with the hydrogenolysis of O\u2013H bond, which favor the conversion of intermediate MFA to DMF and also ring hydrogenated and ring opened products. The optimum reaction temperature was set as 180\u202f\u00b0C based on these experiments.Based on the experiments carried at optimized reaction conditions at different time intervals, a plausible reaction mechanism was proposed for hydrodeoxygenation of HMF to DMF over Cu/SBA-15 catalyst (Scheme 1\n). During the hydrodeoxygenation reaction, hydrogen molecules dissociate on the Cu metallic sites. The alcohol group is removed as H2O on the acidic sites and forms methyl furfural. The formation of this intermediate is observed during the reaction. The carbonyl group of furfural moiety further hydrogenated to alcohol with the dissociated hydrogen present on Cu metal sites. Then the alcohol group is converted to methyl by the elimination water as mentioned above to form DMF.The present catalyst 15Cu/SBA-15 activity was compared with previously reported catalysts and the results are displayed in Table 2\n. Srivastava et al. reported the hydrogenation of HMF over Cu\u2013Co supported on Al2O3 and achieved 68.4% selectivity of DMF with 99.9% conversion of HMF in 8\u202fh reaction time at a temperature of 200\u202f\u00b0C with 3.0\u202fMPa H2 pressure [59]. J. Wang and co-workers prepared cobalt-based N-doped carbon catalyst and obtained 83.1% conversion of HMF with 83.1% selectivity of DMF [12]. Cobalt-based catalysts exhibited 83.3% selectivity of DMF with 100% conversion of HMF at 170\u202f\u00b0C in 12\u202fh in 1,4-dioxane solvent [60]. Laura M. Esteves group reported copper supported on niobium-alumina mixed oxide and achieved 84.5% yield of DMF but in second run the yield of DMF was drastically decreased to 49.5% [43]. Jiang Li et al. reported iron-based catalyst which are tested at a high temperature of 240\u202f\u00b0C for 12\u202fh long duration and achieved only 75.3% DMF yield [61]. The present Cu/SBA-15 catalyst showed 100% HMF conversion with 90% of DMF yield within 8\u202fh of reaction time with 2.0\u202fMPa H2 pressure at 180\u202f\u00b0C. This catalyst showed better yields at comparatively less H2 pressure and temperature than reported catalysts.The repeated use of catalyst was one of the key advantages of solid catalysts. Fig. 11\n indicates the reusability of Cu/SBA-15 catalyst. After completion of the reaction, the catalyst was extracted from reaction mixture using centrifugation. Thus, recovered catalyst was washed with THF and dried at 80\u202f\u00b0C before to use for next run. The Cu/SBA-15 catalyst was reused directly for the next run. The reused catalyst exhibited almost same activity after each cycle. The catalyst after being used for 5 cycles was characterized by XRD to know the structural stability and the results are shown in Fig. 12\n. There was no change in its diffractogram of the used catalyst.Hydrodeoxygenation of HMF towards DMF was achieved over Cu supported on mesoporous SBA-15 catalysts. The 15%Cu/SBA-15 catalyst was active among the prepared catalysts and showed complete conversion of HMF with a maximum of 90% DMF yield. The presence of well dispersed Cu species over the support having strong interaction with SBA-15 and its acidity commutatively makes 15% Cu/SBA-15 as superior catalyst. The catalyst was selective for O\u2013H bond cleavage and C=O bond hydrogenation, compared to the hydrogenation of the C=C bond of the furan ring. The reusability of the catalyst was examined and the catalyst was showed constant activity. The catalyst activity was also depended on the reaction conditions.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.Author D. Dhana Lakshmi acknowledge to UGC-New Delhi, India for financial support in the form of Senior Research Fellowship. We thank Director, CSIR-IICT for permitting to publish our results with a IICT communication number: IICT/Pubs./2021/183.", "descript": "\n                  Selective catalytic hydrodeoxygenation (HDO) of 5-hydroxymethylfurfural (HMF) to prepare 2, 5-dimethylfuran (DMF) was studied as this product is a good biofuel. A sequence of copper dispersed on SBA-15 catalysts are designed and tested their activity for HMF hydrodeoxygenation reaction. The physico-chemical characteristics of the catalysts are gained from powder XRD, TEM, N2-adsorption desorption, NH3-TPD, H2-TPR and N2O-chemisorptions studies. Characterization results indicate the fine dispersion of Cu metal on SBA-15 with high surface area and appropriate acidic sites. The catalyst with 15%Cu on SBA-15 showed high activity towards DMF with 90% yield. The optimized reaction conditions were 180\u202f\u00b0C of reaction temperature, 20\u202fbar H2 pressure, and a reaction time of 8\u202fh to achieve maximum yield. The catalyst is recyclable and exhibits consistent activity.\n               "}
{"full_text": "Hydrogen peroxide (H2O2) is one of the most important chemical compounds [1]. This environmentally friendly compound is used as an oxidizer in green chemistry due to its only by-product (H2O) harmless nature. Moreover, hydrogen peroxide has many uses in the pulp/paper industry, water purification, rocket fuel, and chemical synthesis owing to its high amount of active oxygen (47.1%) [2,3]. At present, most hydrogen peroxide is produced by the autoxidation (AO) process, involving continuous hydrogenation and oxidation of anthraquinone [4]. This procedure continuously produces hydrogen peroxide with a high concentration (45\u201370\u00a0wt%) and\u00a0no contact between H2 and O2 under mild reaction conditions [3]. However, it is plagued with some problems such as high cost, use of toxic solvents (i.e., octanol, naphthalene), and the difficulties attributed to the separation and transportation of the product [4]. Regarding these problems and the increasing demand for H2O2 in the international market, direct synthesis of H2O2 from hydrogen and oxygen has attracted a great deal of attention as a promising substitute for the AO method, thanks to its remarkable advantages such as the smaller amount of environmental pollution and lower cost [5].The main reaction and side reactions in the direct synthesis of hydrogen peroxide are displayed in Scheme 1\n. Since these reactions are characterized by negative Gibbs free energies, low selectivity of H2O2 is one of the main challenges of this process [2,6]. Therefore, many studies have been dedicated to find an efficient catalyst with high selectivity for the generation of H2O2, and many supported metallic catalysts have been tested, including Pt [7,8], Au [9\u201312], and Pd [13\u201321]. These studies have shown that the production of H2O2 was effectively performed in the presence of supported Pd catalysts. However, Pd is active not only for the production of H2O2 but also accelerates H2O generation [22,23].Consequently, various methods have been investigated to increase the selectivity of these catalysts toward H2O2. For instance, the addition of mineral acids and halides such as bromide can improve the selectivity owing to their adsorption on high energetic sites that generate H2O [24]. However, the presence of halides and acids in the reaction can lead to metal leaching, corrosion of the instruments, and deactivation of the catalyst at higher concentrations. One of the other effective methods to improve the selectivity towards H2O2 is adding of a second metal to the supported Pd catalysts. In recent years, this approach has been applied by many researchers and the effects of the other metals on the activity of the supported Pd catalysts have been extensively studied [25]. For instance, Menegazzo et\u00a0al. produced H2O2 with 61% selectivity in the presence of PdAu@ZrO2 under mild conditions [26]. Freakley et\u00a0al. used PdSn/TiO2 and PdSn/SiO2 catalysts and achieved a high H2O2 selectivity of up to 95% at 2\u00a0\u00b0C [27]. Gu et\u00a0al. synthesized H2O2 with 70.9% selectivity using PdAg/C catalyst at 2\u00a0\u00b0C, which was higher than the obtained value over Pd/C catalyst [28]. Wang et\u00a0al. prepared alumina-supported PdZn catalysts and studied the impact of Zn on the catalytic performance of these catalysts [29]. According to their work, H2O2 was produced with productivity's 25431\u00a0mol kgPd\n\u22121 h\u22121 and 8533\u00a0mol kgPd\n\u22121 h\u22121 in the presence of PdZn and Pd catalysts, respectively. Maity et\u00a0al. obtained H2O2 with 95% selectivity using PdNi catalysts and attributed the improved catalytic activity to the presence of Ni [30].Besides the beneficial effects of the presence of the other metals in the structure of supported Pd catalysts on their catalytic activity to produce H2O2 with high selectivity, the nature of the support is also very effective due to its influence on the electronic structure of the metals. Accordingly, various supports have been used for this process, such as SiO2, zeolites, TiO2, and active carbon [31\u201335].We herein used mesoporous silica (KIT-6) as an effectual support to prepare the novel bimetallic catalysts, owing to its three-dimensional structure and large interconnected pores, facilitating the transport and diffusion of reactants/products. Then, a series of CoPd/KIT-6 catalysts with different Co:Pd molar ratios were synthesized by an impregnation method at different calcination temperatures. These catalysts were used for the direct production of H2O2, and the effect of calcination temperature as an efficient factor on their catalytic activity was investigated. The best calcination temperature was selected based on the highest H2O2 selectivity, and a series of CoPd/KIT-6 catalysts with various Co:Pd molar ratios were calcined at the desired temperature. Afterward, the optimum reaction conditions for direct synthesis of H2O2 were perused in the presence of the catalyst with the best Co:Pd molar ratio. Catalyst reusability was investigated as well. During these investigations, we give much of our attention to improve H2O2 selectivity, focusing largely on the electronic interactions of Pd in the structure of CoPd/KIT-6 catalysts.The synthesis of the CoPd/KIT-6 catalysts was performed in two steps. Firstly, mesoporous KIT-6 was prepared using the hydrothermal procedure presented by Kishor et\u00a0al. [36]. Briefly, a mixture of 4.0\u00a0g P123 (Sigma\u2013Aldrich), 144\u00a0mL distilled water, and 7.9\u00a0g HCl solution (Merck, 35%) was stirred at 35\u00a0\u00b0C. Then, 4.0\u00a0g 1-butanol (Merck, 99.9%) was added to the former homogeneous solution. After stirring for 1\u00a0h, 8.6\u00a0g of TEOS (Dae-Jung) was added to the solution and stirred for 24\u00a0h at 35\u00a0\u00b0C. The mixture was then transferred to an oven for 24\u00a0h at 100\u00a0\u00b0C, and the obtained solid was filtered and washed by ethanol (Merck, 99.9%). Afterward, the synthesized KIT-6 was dried at 100\u00a0\u00b0C for 24\u00a0h and then calcined at 550\u00a0\u00b0C for 6\u00a0h. Secondly, the incorporation of Pd and Co into the KIT-6 structure was obtained by a wet-impregnation method. CoPd/KIT-6 catalysts with different Co:Pd molar ratios (0.5:1, 1:1, and 2:1) were synthesized using PdCl2 (Merck) and CoCl2\u00b76H2O (Merck) as the Pd and Co sources, respectively. To prepare the solutions of Pd and Co salts, 0.033\u00a0g of PdCl2 (dissolved in HCl) and a defined amount of CoCl2 (0.01, 0.02, and 0.04\u00a0g for 0.5CoPd, CoPd and 2CoPd, respectively) were separately added to 15\u00a0mL distilled water and stirred for 30\u00a0min at 80\u00a0\u00b0C. Subsequently, 0.5\u00a0g of the synthesized KIT-6 was added to 20\u00a0mL distilled water and stirred for 30\u00a0min. Then, this solution was added to the aqueous solutions of Pd and Co, followed by intense stirring for 12\u00a0h at room temperature. Then, the mixture was placed in an oven at 100\u00a0\u00b0C for 24. After synthesizing the catalysts using the above procedure, the CoPd/KIT-6 catalysts were calcined at different temperatures, namely 550, 450, and 350\u00a0\u00b0C; the corresponding obtained catalysts were named CoPd/KIT-550, CoPd/KIT-450, and CoPd/KIT-350, respectively. Also, the 0.5CoPd/KIT-350, and 2CoPd/KIT-350 catalysts were obtained via calcination of the 0.5CoPd/KIT and 2CoPd/KIT catalysts at 350\u00a0\u00b0C.All the experiments were performed using a Teflon-coated stainless-steel autoclave (volume: 60\u00a0mL, maximum working pressure: 15 MPa) equipped with a magnetic stirrer and a pressure gauge. Generally, 15\u00a0mL of 0.03\u00a0mol L\u22121\u00a0H2SO4/methanol solution and 2\u00a0mg catalyst were transferred to the autoclave. After that, the autoclave was purged with 10% H2\u00a0(1\u00a0MPa) for three times, filled with 10% H2 (1 MPa), and subjected to Ar to dilute H2 at the total pressure of 1.8 MPa. After stirring (1300\u00a0r min\u22121) for 5\u00a0min, O2 was added, and the total pressure of the reactor was increased to 2 MPa. Then, the reaction was carried out for 30\u00a0min at 25\u00a0\u00b0C. At the end of the reaction, the catalyst particles were collected, and the amount of produced H2O2 was evaluated by titration with KMnO4 (standardized with oxalic acid), while the amount of H2 was determined using gas chromatography (Teifgostar Faraz, GC-2552) equipped with a thermal conductivity detector (TCD) and Molecular-Sieve Packed Columns (6 m \u00d7 2 mm \u00d7 2 mm). Using the results of these investigations, H2O2 selectivity, H2 conversion, and H2O2 production rate were calculated as follows:\n\n(1)\n\n\n\nH\n2\n\n\u00a0conversion\n\n(\n%\n)\n\n=\n\n\nmmoles\u00a0of\u00a0reacted\u00a0\n\nH\n2\n\n\n\nmmoles\u00a0of\u00a0initial\u00a0\n\nH\n2\n\n\n\n\n\n\n\n\n\n(2)\n\n\n\nH\n2\n\n\nO\n2\n\n\u00a0selectivity\u00a0\n\n(\n%\n)\n\n=\n\n\nmmoles\u00a0of\u00a0produced\u00a0\n\nH\n2\n\n\nO\n2\n\n\n\nmmoles\u00a0of\u00a0reacted\u00a0\n\nH\n2\n\n\n\n\n\n\n\n\n\n(3)\n\n\n\nH\n2\n\n\nO\n2\n\n\nproduction\u00a0rate\n\n(\n\nmmol\u00a0g\n\u2212\n\ncat\n\n\u2212\n1\n\n\n\n\nh\n\n\u2212\n1\n\n\n\n)\n\n=\n\n\nmmoles\u00a0of\u00a0produced\u00a0\n\nH\n2\n\n\nO\n2\n\n\u00a0\n\n\ncatalyst\u00a0weight\u00a0\n\n(\ng\n)\n\n\u00d7\nreaction\u00a0time\n\n(\nh\n)\n\n\n\n\n\n\n\nFinally, the collected particles of CoPd/KIT-350 catalyst, showing the highest yield of H2O2, were recycled in two ways: 1) the catalyst was washed with water, dried at 120\u00a0\u00b0C and reused (this catalyst was called CoPd/KIT-350\n\nD\n). 2) The catalyst was washed with water, calcined at 350\u00a0\u00b0C and reused (this catalyst was called CoPd/KIT-350\n\nC\n).To investigate the hydrogenation and decomposition reactions of H2O2, a series of control experiments were performed. For this purpose, the initial concentration of H2O2 (Merck, 35%) was 1\u00a0wt% and the reaction was carried out as\u00a0described in section 2.2.1. However, decomposition tests\u00a0were fed with Ar, while the hydrogenation tests were performed in an atmosphere of Ar/H2 without the presence of O2.Brunauer\u2013Emmett\u2013Teller (BET) surface area was determined using nitrogen adsorption/desorption isotherms at \u2212196\u00a0\u00b0C on a PHS-1020 (PHS CHINA) apparatus. Before measurement, the samples were outgassed at 120\u00a0\u00b0C for 4\u00a0h. X-ray powder diffraction (XRD) patterns were evaluated from 2\u03b8\u00a0=\u00a00.66\u201380\u00b0 using an Asenware XDM-300 diffractometer by a Ni-filtered Cu K\u03b1 radiation (\u03bb\u00a0=\u00a01.5406\u00a0\u00c5). Transmission Electron Microscopy (TEM) analysis was performed using a Philips EM 208S microscope. Field emission scanning electron microscopy (FESEM) analysis was conducted with a QNANTA FEG 450 instrument. Also, Energy Dispersive X-ray Analysis (EDX) and elemental mapping were carried out in connection with SEM analysis. Inductively coupled plasma (ICP) spectrometry was performed using an Optima 7300\u00a0TV spectrometer. X-ray photoelectron spectroscopy (XPS) was carried out on a Kratos Axis UltraDLD spectrometer using a monochromatic Al K\u03b1 source (20\u00a0mA, 15\u00a0kV). The Kratos charge neutralizer system was used on all specimens. Survey scan analyses were carried out with an analysis area of 300\u00a0\u00d7\u00a0700 microns and at a pass energy of 160\u00a0eV. High-resolution analyses were carried out with the same analysis area and at a pass energy of 20\u00a0eV. The binding energy scale has been calibrated by setting the main line of the carbon 1s spectrum (adventitious carbon) to 284.8\u00a0eV. Spectra were analyzed using CasaXPS software (version 2.3.17). Fitting of the Pd 3d energy region was achieved using a pair of peaks for each Pd chemical state, fixing the area ratio between the Pd 3d5/2 and the Pd 3d3/2 components to 3:2 and the energy separation to 5.26\u00a0eV. In the case of Pd2+ species, symmetric peaks were used (using the GL (50) line shape in CasaXPS), while asymmetric line shape was chosen for Pd0 species (using the LA (2.5,7,10) line shape in CasaXPS).N2 adsorption\u2013desorption isotherms of KIT and CoPd/KIT catalysts are depicted in Fig.\u00a01\n. The curve of pure KIT displays type IV isotherm with H1 hysteresis loop, indicating its high-quality mesoporous structure with uniform pore size [37]. For the other samples, the shape of the isotherms and hysteresis loops are very similar to those observed for KIT. This observation demonstrates that the addition of the metals has not affected the mesoporous structure of original support. Also, the pore size distribution illustrated in Fig.\u00a01h clearly displays that all the samples possess narrow mesopores (between 1 and 5\u00a0nm) with a size distribution centered at\u00a0\u223c\u00a03.5\u00a0nm. The textural properties of these samples are given in Table 1\n. These results show that the BET surface area and pore volume of KIT decrease with increasing the metals loading, which indicates that the modification step has been done successfully, and the metals have been inserted inside the pores of KIT. Also, the results corresponding to CoPd/KIT-350, CoPd/KIT-450 and CoPd/KIT-550 demonstrate that the increase in calcination temperature leads to the increase in BET surface area probably due to sintering of metal nanoparticles.The low-angle XRD patterns of the samples are presented in Fig.\u00a02\na. The XRD patterns of all the cases show an intense peak at around 2\u03b8\u00a0=\u00a01\u00b0 corresponding to the (211) reflection of an ordered 3-D cubic symmetry with Ia3d space group, and another peak at 2\u03b8\u00a0=\u00a01.1\u00b0 corresponding to (220) reflections, indicating the well-ordered pore arrangement of the samples [38]. No change in the position of these peaks in the\u00a0XRD patterns of CoPd/KIT-350, CoPd/KIT-450, and CoPd-KIT-550 samples confirms that the change in calcination temperature, as well as addition of the metals, has not affected the mesoporous structure of KIT. The insignificant shift corresponding to (211) reflection in the XRD pattern of 2Co/Pd-KIT-350 is probably due to the incorporation of a large amount of Co into the KIT structure.The wide-angle XRD patterns of all the samples illustrated in Fig.\u00a02 show the broad peak corresponding to amorphous silica. For CoPd/KIT-550 sample (Fig.\u00a02f), the diffraction peaks at 2\u03b8\u00a0=\u00a033\u00b0 (101), 41\u00b0 (110), 54\u00b0 (112), 60\u00b0 (103) and 71\u00b0 (202) are related to the presence of tetragonal PdO (JCPDS 00-041-1107) and the peaks at 2\u03b8\u00a0=\u00a036\u00b0 (311) and 65\u00b0 (440) are attributed to the cubic phase of Co (JCPDS 01-078-1969). Also, the absence of any peaks corresponding to the crystalline forms of the metals in the XRD patterns of 0.5CoPd/KIT-350, CoPd/KIT-350, 2CoPd/KIT-350, and CoPd/KIT-450 samples confirms the good dispersion of the metal clusters on the mesoporous KIT.The FESEM images of KIT, Pd/KIT-350, CoPd/KIT-350, CoPd/KIT-450 and CoPd/KIT-550 samples are shown in Fig.\u00a03\n. The FESEM image of KIT (Fig.\u00a03a) shows that this material is composed of rock-like aggregated irregular particles. However, as shown in Fig.\u00a03b\u2013e, this surface morphology has changed after the addition of the metals. Also, the comparison of the SEM image of CoPd/KIT-350 and those of CoPd/KIT-450 and CoPd/KIT-550 indicates that the surface morphology has been strongly influenced by the calcination temperature. Besides, the results of EDX and elemental mapping analysis of the Pd/KIT-350 and CoPd/KIT-350 samples shown in Fig.\u00a04\n confirm the existence of Pd and Co elements and their uniform distribution on the KIT surface.The topology and structural details of KIT and CoPd/KIT samples acquired by TEM analysis are shown in Fig.\u00a05\n. Fig.\u00a05a and b reveals an ordered mesoporous structure with a bi-continuous network of channels for KIT sample, confirming the successful synthesis of this material [38]. The size distributions of the CoPd particles, obtained by TEM analysis for each of the samples, are reported in histograms in Fig.\u00a06\n. The data show that the particle size has decreased by an increase in Co amount, while this value has increased by an increase in calcination temperature.Surface compositions, electronic interactions between Pd and Co, and the chemical state of Pd species on the surface of the prepared catalysts were investigated using XPS analysis. The specimens were prepared by pressing few milligrams of the samples onto high purity indium foils (Sigma\u2013Aldrich). Low-resolution wide scans are reported in Fig.\u00a0S1 in the Supporting Information (SI) file, together with the high-resolution spectra obtained on O 1s, C 1s, and Si 2p regions (Fig.\u00a0S2). The high-resolution spectra collected on the energy windows typical for Pd 3d and Co 2p peaks are depicted in Fig.\u00a07\n (for samples Pd/KIT-350, Co/KIT-350, CoPd/KIT-350, 2CoPd/KIT-350, and 0.5CoPd/KIT-350) and Fig.\u00a08\n (for samples CoPd/KIT-450, CoPd/KIT-550, CoPd/KIT-350\n\nC\n and CoPd/KIT-350\n\nD\n); the results are summarized in and Table 2\n. As shown in Fig.\u00a0S1, the intensity of Pd 3d and Co 2p peaks is intensity extremely low, and long acquisition times were needed to obtain the spectra reported in Figs. 7 and 8. Here, the Pd 3d XPS spectra of the different bimetallic catalysts show two signals in the 334\u2013344\u00a0eV range, related to the Pd 3d5/2 and Pd 3d3/2 components. After deconvolution, two contributions are observed for Pd. The peaks at (336.7\u00a0\u00b1\u00a00.2) eV and (342.0\u00a0\u00b1\u00a00.2) eV can be assigned to Pd2+ species, while the asymmetric peaks centered at (334.9\u00a0\u00b1\u00a00.2) eV and (340.2\u00a0\u00b1\u00a00.2) eV are due to the presence of Pd0 species [39]. The Co 2p XPS spectra of all the catalysts suggest the presence of Co+2 species, as each spectrum is characterized by peaks at 780, 782, 785 and 790\u00a0eV [40].As stated above, the presence of two palladium species was revealed in all catalysts. It has to be noted that X-ray induced reduction of high valence Pd compounds has been reported, and cannot be entirely excluded in the present case, also in consideration of the long acquisition time needed [41]. However, Pd2+/Pd0 ratios presented in Table 2 suggest that the oxidation state of Pd atoms might have been affected by the amount of Co in the samples. Indeed, when the Co loading increases from 0.6 to 2.0\u00a0wt%, the concentration of Pd2+ increases as well, probably due to the electron transfer from Pd to Co [42,43].The results corresponding to CoPd/KIT-350, CoPd/KIT-450, and CoPd/KIT-550 samples show that the different calcination temperatures affect the relative concentration of Pd2+ and Pd0. As shown in Table 2, when the calcination temperature rises from 350 to 450\u00a0\u00b0C, the Pd2+/Pd0 ratio decreases from 2.65 to 2.59. However, a further increase in the calcination temperature to 550\u00a0\u00b0C leads to the highest Pd2+/Pd0 ratio value (17.69). Table 2 also shows the results of XPS analysis for CoPd/KIT-350\n\nC\n and CoPd/KIT-350\n\nD\n samples. These results demonstrate that the concentration of Pd2+ in both samples is higher than that of Pd0, with a higher Pd2+/Pd0 ratio for CoPd/KIT-350\n\nC\n. This observation suggests that calcination of the catalyst after the reaction can accelerate the oxidation of Pd.The results of the catalytic activity of the catalysts in the direct synthesis of H2O2 from H2 and O2 are listed in Table 3\n. The results corresponding to Co/KIT-350 and Pd/KIT-350 catalysts show that Co is inactive in the direct synthesis of H2O2, while the reaction is performed in the presence of Pd and results in the production of H2O2 with 26% selectivity. Moreover, the results of the catalytic activity of CoPd/KIT catalysts reveal that the addition of Co enhances the catalytic activity of the Pd catalyst. As observed, with an increase in the molar ratio of Co:Pd from 0.5:1 to 1:1, the catalytic activity increases and reaches the highest level of H2O2 selectivity of 50%, H2 conversion of 51% and H2O2 productivity of 520\u00a0mmol catalyst g\u22121 h\u22121. On the other hand, the results of the catalytic activity of CoPd/KIT-350, CoPd/KIT-450, and CoPd/KIT-550 catalysts show a trend of decreasing the H2O2 productivity along with the increase in calcination temperature. This is probably due to the decrease in the number of catalytic active sites or to the decrease in the concentration of the metals on the surface of the catalyst.\nTable 3 also shows the results corresponding to the catalytic activity of CoPd/KIT-350\n\nC\n and CoPd/KIT-350\n\nD\n. It is obvious that the catalytic activity of these catalysts has reduced compared to that of CoPd/KIT-350, most likely due to slight leaching of the metals from the support during the reaction. However, a comparison between the obtained results for CoPd/KIT-350\n\nD\n and CoPd/KIT-350\n\nC\n catalysts shows that calcination of the catalyst after the reaction results in an increase in the stability of the catalyst.Also, to investigate the effect of calcination temperature on the catalytic activity of the catalysts upon re-use, the catalysts were dried at 120\u00a0\u00b0C after the reaction and reused in the reaction of H2 with O2. The results of this investigation are summarized in Table 4\n. The catalytic activity of all the catalysts decreases after the reaction due to the leaching of the metals as discussed above. However, this leaching decreases with an increase in the calcination temperature. Also, the results corresponding to the CoPd/KIT-350 catalyst which was calcined at 350\u00a0\u00b0C after the reaction reveals that calcinating the catalyst can positively affect the catalytic activity of the catalyst upon re-use.Decomposition and hydrogenation of H2O2 are the main reason for the loss of selectivity in the direct synthesis of H2O2 [44]. Therefore, to investigate the catalytic activity of the catalysts in these reactions, the decomposition and hydrogenation experiments were performed in H2O2/methanol solutions under Ar and H2/Ar atmospheres, respectively. The results of these experiments are illustrated in Fig.\u00a09\n. These data show that the addition of Co to Pd/KIT-350 decreases the rate of the decomposition and hydrogenation reactions in the direct synthesis of H2O2. On the other hand, as shown in Fig.\u00a09, different calcination temperatures have significantly affected the activity of the catalysts for decomposition and hydrogenation of H2O2 and the activity of the catalysts has decreased at higher calcination temperatures. According to the results presented in Table 3 and Fig.\u00a09, it can be concluded that the addition of Co to the Pd catalyst and different calcination temperatures change the catalytic activity of the catalysts not only for H2O2 production but also for H2O2 decomposition and hydrogenation.For the direct synthesis of H2O2, it is well known that the interaction of molecular O2 and atomic H leads to the formation of H2O2, while O2 dissociation results in the production of H2O [45]. Therefore, inhibiting the O\u2013O bond scission both in the initial mixture and in the product is a key factor in the selective formation of H2O2. Previous studies, including theoretical calculations and experimental works, have evidenced that the addition of a second metal to the Pd catalyst lowers the catalytic activity toward the breaking of the O\u2013O bond by a change in the electronic interactions of Pd, creation of inactive sites for product decomposition, blocking Pd's active sites and increasing the Pd monomer sites [15,46,47].However, our results show that the introduction of Co into the Pd catalyst has not affected the electronic interactions of Pd significantly (Table 2, entries 1, 3, 4, and 5), while the H2O2 selectivity has increased from 26 to 49% (Table 3, entries 3, 4, 5, and 6). This observation confirms that the selectivity increment is more associated with the other factors mentioned above.Alternatively, a closer look at Table 3 (entries 5, 7, and 8) shows that calcination temperature can change the H2 conversion and H2O2 selectivity, as well. With regard to the effect of calcination temperature on the catalyst features, this change can be attributed to some factors such as a change in the electronic interactions of Pd (Table 2, entries 4, 6, and 7), an increase in the specific surface area (Table 1, entries 3, 5, and 6) and the particles size (Fig.\u00a06) of the catalyst and phase formation in the structure of the catalyst (Fig.\u00a02e and f). Therefore, to investigate the contribution of electronic interactions of Pd, the CoPd/KIT-550 catalyst, characterized by the highest Pd2+ content (Pd2+/Pd0 ratio\u00a0=\u00a017.69), was exposed to H2 for 18\u00a0h at 250\u00a0\u00b0C and used in the reaction of H2 with O2 under optimized reaction conditions. At the end of the reaction, the conversion increased from 35% to 47% due to the decrease in the amount of Pd2+. This observation implies that the change in the electronic interactions of Pd caused by an increase in calcination temperature has a considerable effect on this reaction.Although our results confirmed the effects of the Co addition and calcination temperature on the electronic interactions of Pd in the selective formation of H2O2, it should be noted that the presence of H2 and O2 in the reaction can also change the electron density of Pd. The comparison of XPS analysis for CoPd/KIT-350 and CoPd/KIT-350\n\nD\n (Table 2) clearly shows that the presence of H2 and O2 in the reaction has led to a change in the concentrations of Pd0 and Pd2+ and as a result, the H2O2 selectivity has been altered. To prove that the decrease in H2O2 selectivity results from the change in the electron density of Pd, CoPd/KIT-350\n\nC\n was used in the reaction of H2 with O2 under optimized reaction conditions. The results of this experiment (Table 2, entry 8 and Table 3, entry 9) show that the H2O2 selectivity increases again when the electron density of Pd approximately returns to the initial state (Table 2, entry 4 and Table 3, entry 5).From the above results, we could conclude that the electronic interactions of Pd can be one of the important factors affecting the H2O2 selectivity. However, the active oxidation state of Pd, which is responsible for the selective generation of H2O2, is still unknown. Some groups have claimed that Pd0 plays a more critical role than Pd2+ toward the selective formation of H2O2, and several other groups have confirmed that Pd2+ is more active in this reaction [15,22,28,29,48\u201353]. However, during our investigation, we found a relation between selectivity and Pd2+ and between conversion and Pd0. The results of these investigations are as follows:The results presented in Tables 3 and 4 are summarized in Fig.\u00a010\n to visualize better the relations between the catalytic activity of the catalysts and the active oxidation state of Pd. As shown in Fig.\u00a010c, the curves corresponding to H2O2 selectivity and Pd2+ content show a similar trend. This observation indicates that Pd2+ possesses the higher activity for H2O2 formation due to its particular electronic and geometrical structures which would lead to the weak interaction of Pd atoms with O2, OOH, H2, and H2O2 adsorbents. As a result, the dissociation of these adsorbents decreases, and the selectivity of H2O2 as the main product increases [22]. However, according to the mechanism of this reaction, decreasing the activity of the catalyst for breaking the H\u2013H bond is undesirable due to the decrease in H2 conversion (Fig.\u00a010b). Therefore, H2 conversion on Pd2+ is less active than that on Pd0. The relation between Pd0 content and H2 conversion is also shown in Fig.\u00a010a. As observed, H2 conversion increases by increasing the content of Pd0.The above results were confirmed by the final results of this reaction in the presence of CoPd/KIT-350\n\nD\n and CoPd/KIT-350\n\nC\n catalysts. For CoPd/KIT-350\n\nD\n catalyst, the content of Pd2+ decreased after the reaction, probably due to the presence of O2 and H2 at the high pressure in the reaction and as a result, the H2O2 selectivity decreased (Fig.\u00a010c) and H2 conversion increased (Fig.\u00a010b). However, for CoPd/KIT-350\n\nC\n catalyst, the content of Pd2+ increased due to the calcinating the catalyst after the reaction. Consequently, H2O2 selectivity increased, and H2 conversion decreased in the presence of this catalyst (Fig.\u00a010c and b).Since calcinating the catalyst after the reaction showed to be advantageous for the direct synthesis of H2O2 (Section 3.2.1), the CoPd/KIT-350\n\nC\n was selected and used for the reusability test. For this purpose, the catalyst was calcined at 350\u00a0\u00b0C after each run and re-used following the procedure stated in Section 2.2.1. The results are depicted in Fig.\u00a011\n. As can be seen, at the end of the second use, a significant loss of activity is observed, most likely due to the leaching of Pd atoms from the catalyst. However, no significant change in catalyst activity is seen in the next three successive runs. These results suggest that CoPd/KIT-350 catalyst has good stability for the direct synthesis of H2O2.In this work, the catalytic activity of the novel CoPd bimetallic catalysts supported on mesoporous KIT-6 was studied for the direct synthesis of H2O2 from H2 and O2. These catalysts were prepared using various Co:Pd ratios at different calcination temperatures and characterized with various analyses. By combining the results of the characterizations and the obtained catalytic activity, we determined that Co addition and calcination temperature have affected the electronic interactions of Pd as well as morphology, particle size, and BET surface area of the catalysts. Considering these changes in the structure of the CoPd/KIT catalysts, the electronic behavior of Pd during the reaction was investigated, and the results demonstrated that Pd2+ was the active phase for selective formation of H2O2 and could provide the higher selectivity for H2O2. In contrast, Pd0 resulted in low activity for H2O2 selectivity and increased the H2 conversion. As a result, the existence of Pd2+ alone in the structure of the catalyst is not enough to improve the process of direct synthesis of hydrogen peroxide because the conversion of H2 is dependent on the existence of Pd0. Also, the reusability experiments showed that calcinating the catalyst after the reaction increases the stability of the catalyst. Therefore, the results of this work not only provide novel insights into the catalyst design but also show that CoPd/KIT-350 is an efficient bimetallic catalyst for the selective generation of H2O2.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 for the financial support (Research Council Grant) provided by Isfahan University of Technology (Iran).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.2021.03.014.", "descript": "\n                  A series of CoPd/KIT-6 bimetallic catalysts with various Co:Pd molar ratios at different calcination temperatures were prepared and used for the direct synthesis of H2O2 from H2 and O2. These catalysts were characterized by nitrogen adsorption\u2013desorption, low and wide-angle X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), elemental mapping and energy-dispersive X-ray (EDX) methods. It was found that the particle size, electronic interactions, morphology, and textural properties of these catalysts as well as their catalytic activity in the reaction of H2 with O2 were affected by Co addition and different calcination temperatures. Also, the results showed that while the H2O2 selectivity depends on Pd2+ species, the H2 conversion is related to Pd0 active sites. Among these catalysts, CoPd/KIT-6 calcined at 350\u00a0\u00b0C (CoPd/KIT-350 catalyst) showed the best catalytic activity with 50% of H2O2 selectivity and 51% conversion of H2.\n               "}
{"full_text": "Now-a-days fuel cells are contributed as highly efficient and eco-friendly energy sources [1,2]. It converts the chemical energy into electrical energy by anodic fuel (ethanol/methanol) oxidation and cathodic oxygen reduction with very little environmental pollution [3]. Still, now it is challenging to use the same catalyst for both anode and cathode, which enhance the electro catalytic activity and durability in fuel cell [4, 5]. Generally platinum (Pt) and platinum related catalyst are mostly used for both anodic oxidation and cathodic reduction reaction [6\u20138] but due to limitation of Pt supply, high price, sluggish kinetics in cathode reaction (ORR), it is very important to use cost-effective some other non-Pt catalyst for large scale commercial application in fuel cell [9, 10]. In recent years, most of the researcher has been tried to decrease the use of expensive Pt based catalyst and increase the inexpensive metal or metal free catalyst for fuel cell [11\u201314]. Palladium nanoparticles (PdNPs) can be used instead of platinum because they are same group metals in the periodic table, cheaper than platinum and more abundant in the earth [15\u201318]. The catalytic properties of PdNPs can be improved by dispersing the PdNPs on carbon based support materials [19, 20]. It has been reported that the metal nanoparticles supported on carbon nanomaterial shows an improved catalytic activity [1, 21].Carbon nanotubes (CNTs) are one of the most promising carbon nano-materials for contributing in modern science [22, 23]. It possesses many fascinating physical properties such as, excellent mechanical strength, porous structure, electrical and thermal conductivities [22,23]. Due to its unique properties, CNT-based nanocomposites have been applied for a wide range of applications, such as energy conversion devices, batteries, supercapacitors, sensors and fuel cells [12, 24\u201327]. Moreover, Functionalization of CNT with some other hetero atoms (N,B, S and P) is an effective way to tune their intrinsic properties.Among them, nitrogen is the most effective dopants because of their similar small atomic size to that of the carbon atom and can be easily inserted in carbon nanotube frame [28\u201330]. Doping of CNT with nitrogen atoms can improves the physico-chemical properties due to conjugation between the lone-pair electrons of nitrogen and the \n\u03c0\n system of CNT [12]. It may improve the electro catalytic activity by providing the nucleation sites of metal nanoparticle and changing the electronic structure of the catalyst. Moreover, the incorporation of nitrogen atom on to CNT support may enhance the chemical binding energy between CNT and metal nanoparticles which is also favorable to increase the durability of the catalyst [30\u201332].Herein, we have synthesized nitrogen-functionalized carbon nanotube supported palladium nanoparticles (NCNT-Pd) via simple chemical reduction method at room temperature without using any surfactant (scheme\u00a01\n). Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Energy dispersion spectroscopy (EDS) analyses are performed to investigate the structural and morphological characteristics of the as synthesized catalyst, such as size and dispersion of the particles, crystallinity, and percentages of element. Then the NCNT-Pd was applied for oxygen reduction reaction in alkaline media by using cyclic voltammetry and hydrodynamic voltammetry. Furthermore, the catalytic performances of NCNT-Pd also investigated for electro-oxidation of ethanol in alkaline media by using cyclic voltammetry (CV). From the result, it is found that the NCNT-Pd catalyst not only showed higher catalytic activity for ORR but also showed excellent EOR in alkaline media. For comparison, we also synthesize CNT-Pd and NCNT by the same procedure.The pristine CNT was prepared by Carbon Nano Tech Co., Ltd. (Pohang, South Korea). Potassium tetrachloropalladate (99%) and hydrazine mono hydrate (65%) were purchased from sigma Aldrich. Methanol and ethanol (99.9%) were obtained from OCI Co., Ltd .All other reagents were analytical grade and used without further purification. Deionized (DI) water was used for preparing the electrolyte solution.The pristine carbon nanotubes (CNTs) were treated with a mixture of concentrated H2SO4/HNO3 (3:1, v/v) at 100\u00a0\u00b0C for 2\u00a0h to oxidize the surface of CNT [25,33]. The obtained product (CNT\u2013COOH) was filtered and washed with deionized (DI) water and dried in vacuum oven at 50\u00a0\u00b0C for 6\u00a0h. Then 10\u00a0mg of CNT-COOH\u00a0+\u00a030\u00a0mL DI water was taken in a 100\u00a0mL round bottle flask and ultrasonicated for 1\u00a0hour. Then 100 \u03bcL hydrazine monohydrate (N2H4\u2022H2O) followed by 30\u00a0mL ice cold sodium borohydride (0.074\u00a0mg/mL) was added under vigorous stirring. After that 15\u00a0mL (0.33\u00a0mg/mL) of potassium tetrachloropalladate (II) (K2PdCl4) was added drop wise and stir it for 20\u00a0h at room temperature. Finally, the black product was filtered and washed for several times with DI water. As prepared nitrogen-functionalized carbon nanotube supported palladium nanoparticles (NCNT-Pd) was then dried at 60\u00a0\u00b0C under vacuum condition for 24\u00a0h. For comparison, NCNT and CNT-Pd also prepared by the same method.The transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) were takenon TECNAI model FI-20 microscope at 200\u00a0kV by using a copper (Cu) grid. X-ray diffraction (XRD) patterns of the catalyst were carried out with Rigaku micro-area high-resolution X-ray diffractometer using Cu, K\u03b1 radiation \u03bb=1.5405Ao. The size and \nd\n-Spacing of Pd NPs were estimated from TEM images and XRD patterns by using image J and origin 9.1 software. X-ray photoelectron spectroscopy (XPS) were investigated by using a Multi-Lab 2000 with a monochromatic 14.9\u00a0keV Al K\u03b1 X-ray source. All electrochemical applications were investigated via cyclic voltammetry and hydrodynamic voltammetry with a CHI 700C (USA) potentiostat. The three electrode half-cell system assembled with a glassy carbon working electrode (GCE area: 0.0707 cm2), a Pt wire auxiliary and an Ag/AgCl reference electrode (Bioanalytical Systems Inc. 3\u00a0M NaCl) respectively. For hydrodynamic voltammetry measurements, a rotating ring disk electrode (disk area: 0.196 cm2; platinum ring area: 0.041 cm2) was used as an RRDE electrode. Electrochemical impedance spectroscopy (EIS) was investigated by using Versa STAT3 (Princeton Applied Research, USA) in three electrode cell system. All electrochemical experiments were conducted at 25\u00a0\u00b0C in 0.1\u00a0M KOH solution which is saturated by Ar and O2 for 30\u00a0min.XPS analysis was carried out to investigate the elemental composition and electronic structure of the as prepared NCNT-Pd catalyst. The survey spectrum of NCNT-Pd, NCNT and CNT (Fig.\u00a01\n(a)) reveals the presence of carbon (C1s, 284.08\u00a0eV), palladium (Pd 3d, 335.08\u00a0eV), nitrogen (N1s, 401.07\u00a0eV) and oxygen (O1s, 534.07\u00a0eV) which have similarities with the previous literatures [16,25] Fig.\u00a01.(b) shows the XPS spectrum of C1s for CNT and NCNT-Pd. It is clearly seen that, four different peaks are present in C1s spectrum which is corresponding to the sp2 carbon (284.78) eV and oxygenated carbon (CO; 285.07\u00a0eV C\u00a0=\u00a0O; 286.09\u00a0eV; OC\u00a0=\u00a0O eV; 287.28\u00a0eV) [16, 25, 28, 34]. The relative intensity of sp2 carbon is higher than the oxygenated carbon that means percentages of sp2 carbon is higher than the oxygenated carbon which plays an important role for ORR [35]. Moreover the peak shifting of C1s of NCNT-Pd to the higher binding energy compared to CNT indicates that strong interaction of metals in graphitic system of CNT [36]. The XPS spectra of N1s is deconvoluted in one main peak which is located at 400.08\u00a0eV [28, 37]. The N1s peak of NCNT-Pd (Fig.\u00a01(c)) suggested that carbon nanotube successfully functionalized by nitrogen and the percentages of N content is 2.15 wt%. The high resolution of Pd 3d spectrums are shown in Fig.\u00a01(d). There are two well-defined Pd peaks are corresponding to the Pd 3d5/2 (335.1\u00a0eV) and Pd 3d3/2 (340.5\u00a0eV) which has similarities with the previous literature values [16, 38]. The Pd3d5/2 signals give three peaks at 335.1, 336.2, and 337.2\u00a0eV, which can be ascribed to Pd0, PdO, and PdO2 states,respectively [38, 39]. The content of Pd in NCNT-Pd catalyst is 7.24 wt%.The TEM images of NCNT-Pd are shown in Fig.\u00a02\n(a-c). From the TEM images, it can be clearly seen that the Pd nanoparticles are uniformly decorated on the most of the outer surface of NCNTs. The average diameter of the decorated Pd NPs on NCNT surface is about 7.22\u00a0nm which increase the surface area of the catalyst. The interplanar \nd\n-spacings for the lattice fringes of Pd were 0.230\u00a0nm and 0.195\u00a0nm, which is corresponding to the (111) and (200) lattice planes of the face-centered cubic (fcc) Pd structure (Fig.\u00a02(c)) [15, 16]. The selected-area electron diffraction (SAED) pattern (Fig.\u00a02(d)) was taken from single Pd NPs which is given five different rings corresponding to the five different crystal plane on the Pd NPs. The observed five different ring represents the (111), (200), (220), (311) and (222) planes of the fcc Pd, indicating the polycrystalline structure [40, 41]. EDS mapping is conducted to analyses the elemental distribution of NCNT-Pd. As shown in Fig.\u00a02(e), only C, O, N and Pd are present in NCNT-Pd without any other species.XRD patterns of the NCNT-Pd are shown in Fig.\u00a02(f). The XRD peak at 2\u03b8=25.4\u00b0 corresponds to the C(002) peak of NCNT [12, 42]. In addition there are five diffraction peaks at 2\u03b8 of 40\u00b0, 46\u00b0, 68\u00b0, 82\u00b0 and 87\u00b0 which can be ascribed to (111), (200), (220), (311) and (222) crystal plane of face centered cubic (fcc) Pd, respectively [16, 43] and these results have similarities with the SAED pattern. The average particle size of the prepared NCNT-Pd nanoparticles (d) was calculated by using the Scherrer Eq.\u00a0(1)\n[43]. The diffraction peaks of Pd (111) at 2\u03b8 of 40\u00b0 were used to calculate the size of particles (d).\n\n(1)\n\n\nd\n=\n\n\nk\n\n\n\u03bb\n\n\n\nb\n\nC\no\ns\n\n\u03b8\n\n\n\n\n\n\nWhere, k is a constant (0.9),\n\n\n\u03bb\n\n is the wavelength of X-ray (0.15405\u00a0nm), b is the full width at half-maximum (FWHM) of the (111) diffraction peak (in radian) and \n\u03b8\n is the angle of the maximum peak position. The average size of the Pd nanoparticles was estimated from the XRD results was 7.46\u00a0nm for NCNT-Pd which is similar to the TEM analysis.The electrochemical impedance spectroscopic (EIS) experiment are conducted to evaluate the electrical conductivity and charge transfer behavior of the electrodes. The EIS experiments were carried out by inserting the electrode in a solution of 0.1\u00a0M KCl containing 1.0\u00a0mM K3Fe(CN)6 and K4Fe(CN)6 (1:1) at a frequency ranges from 105 to 10\u22122\u00a0Hz. The charge-transfer kinetics can be estimated from the intercept region and shape of the impedance spectrum. The Nyquist plots obtained from EIS of bare GCE, NCNT, CNT-Pd and NCNT-Pd catalyst modified electrodes are shown in Fig.\u00a03\n(a). In the Nyquist plots, the intercept on the real axis at high-frequency region represents the electrolyte resistance (Rs) and the diameter of the semicircle portion at low-frequency region represents the electron-transfer impedance between electrolyte and electrode interface (Rct) [44, 45] . The Rs value of NCNT-Pd (SBH), CNT-Pd, NCNT and bare GCE are 110, 115, 130 and 137 \u038f respectively and the Rct values for them are 115\u03a9, 124\u03a9, 1.25 k\u03a9 and 7.5 k\u03a9 for NCNT-Pd, CNT-Pd, NCNT and bare GCE respectively. From the EIS it is found that NCNT-Pd shows lowest Rs and Rct value which is an indication of higher conductivity low charge transfer resistance of the NCNT-Pd electrode [44, 45]. The results indicate that the NCNT-Pd modified electrode was highly conductive with excellent catalytic activity at the interface than the other electrodesThe electro-catalytic activities of NCNT-Pd and other three catalysts (CNT-Pd, NCNT and 20% commercial Pt/C) toward ORR were first investigated by cyclic voltammograms (CV) in 0.1\u00a0M KOH electrolyte (saturated by O2 gas) at a scan rate 50 mVs\u22121 (Fig.\u00a03(b)). For the NCNT-Pd, CNT-Pd and 20% Pt/C samples, peak I corresponds to hydrogen desorption and/or oxidation in this region [46]. A well-defined oxygen reduction peak started at \u22120.087\u00a0V (inset figure) is observed for O2-saturated electrolyte indicating that NCNT-Pd has an efficient catalytic activity for ORR in alkaline media. From the forward scan, peak II, which emerges at a potential of \u22120.4\u00a0V, correspond to the formation of Pd oxides and partially overlaps with the oxygen reduction peak. Peak III, centered at \u22120.33\u00a0V, can be attributed to the reduction of Pd oxide during the reduction sweep [16, 47]. Following the reduction sweep, peak IV (\u22120.8\u00a0V to \u22121.0\u00a0V) corresponds to the adsorption of hydrogen [46].There are some possible reasons for excellent ORR activity of NCNT-Pd. First, the functionalized N atoms are electron rich which breaks the electro-neutrality of sp2 carbon of CNT and produces charge sites which enhance the ORR activity by increasing oxygen adsorption [35]. Second, the hetero N atoms enhance the nucleation and uniformly dispersion of PdNPs on NCNT surface [16]. Third, the homogeneously disperse Pd NPs are increased the surface area of NCNT-Pd catalyst which is also favorable for oxygen reduction [1, 16].To further investigate the ORR kinetics of NCNT-Pd, CNT-Pd and NCNT modified electrode, we have conducted rotating disk electrode (RDE) measurement in 0.1\u2009M KOH electrolyte Fig.\u00a04.(a-c) represents the RDE voltammograms of NCNT-Pd, CNT-Pd and NCNT modified electrode at various rotation rate. It can be seen that catalytic current density increases with increasing rotation rate because of oxygen diffuse to catalyst modified electrode and reduced to directly OH by maximum current transfer [48].\nFig.\u00a04(a\u2032-c\u2032) shows the Koutecky\u2013Levich plots (J\u00a0\n\u2212\n\u00a0\n1\n\u00a0vs. \u03c9\u22121/2\n) at various electrode potential (data are taken from Fig.\u00a04(a-c). From the slope of Koutecky\u2013Levich plots (K-L plots), we can calculate the electron transfer number by using Koutecky-Levich Eq. (2) & (3)\n[48]]\n\n(2)\n\n\n\n1\nJ\n\n=\n\n1\n\nJ\nk\n\n\n+\n\n1\n\nj\nL\n\n\n=\n\n1\n\nJ\nk\n\n\n+\n\n1\n\nB\n\n\u03c9\n\n1\n2\n\n\n\n\n\n\n\n\n\n\n(3)\n\n\nB\n=\n0.62\nnFA\n\nD\n\no\n\n\n2\n3\n\n\n\n\nV\n\n\n\u2212\n\n1\n6\n\n\n\n\n\nC\n\n0\n\n\n\n\n\nWhere, J is the measured current density, Jk\n and JL\nare the kinetic and diffusion limiting current density, \u03c9 is the angular rotation rate of the electrode, n is the electron transferred number, F is the Faraday constant(F\u00a0=\u00a096,485 C mol\u22121), A is the geometric electrode area (cm2). K is the rate constant of the reaction, Co is the bulk concentration of O2 (1.2\u00a0\u00d7\u00a010\u22126\u00a0mol cm\u22123), DO is the diffusion coefficient of O2 in the 0.1\u00a0M KOH solution (1.73\u00a0\u00d7\u00a010\u22125 cm2 s\u00a0\u2212\u00a01) and V is the kinetic viscosity of the electrolyte (1\u00a0\u00d7\u00a010\u22122 cm2\ns\u00a0\u2212\u00a01) [12, 16, 48].The linearity and parallelism of the K-L plots indicate first-order reaction kinetics and similar electron transfer numbers for ORR at various potentials [49, 50]. The average electron transfer number which is calculated from the slope of Koutecky\u2013Levich plots is \u223c3.94 at \u22120.6 to \u22121.2\u00a0V, indicating NCNT-Pd catalyst proceed ORR process via 4e- transfer pathway as like as 20% commercial Pt/C catalyst.To quantify the production of peroxide species (HO2\n\u2212) during the ORR process, rotating ring disk electrode (RRDE) measurement are performed in O2-saturated 0.1\u00a0M KOH electrolyte [51]\nFig.\u00a05.(a) represents the comparison of RRDE measurements (at 2500\u00a0rpm) of NCNT-Pd, CNT-Pd, NCNT and 20% commercial Pt/C. It is found that NCNT-Pd shows higher disk current density (Fig.\u00a05(a-b)) and lower ring current density than the others two catalysts (CNT-Pd and NCNT) and also 20% commercial Pt/C. In RRDE, O2 defuse to disk electrode and reduced as well as generated peroxide species which is detected by ring electrode. It is well known that the generation of HO2\n\u2212 species during ORR process is not desirable because it decreases the efficiency of the catalyst and also can lead to the deterioration of it [52]. Our NCNT-Pd catalyst shows efficient catalytic activity with higher disk current and lower ring current that means the negligible amount of peroxide species produces during the ORR process. The peroxide species formation (% HO2\n\u2212) and transfer electron number (n) are calculated by the following equations (4 &5) [52, 53]:\n\n(4)\n\n\n%\n\nof\n\n\nHO\n2\n\u2212\n\n=\n200\n\u00d7\n\n\n\ni\nr\n\n/\nN\n\n\n\ni\nd\n\n+\n\ni\nr\n\n/\nN\n\n\n\n\n\n\n\n\n(5)\n\n\nn\n=\n4\n\u00d7\n\n\n\ni\nd\n\n\n\ni\nd\n\n+\n\n(\n\ni\nr\n\n/\nN\n)\n\n\n\n\n\n\n\nWhere, Id\n is disk current, Ir\n is ring current and N is current collection efficiency which was determined to be 0.37 from the reduction of K3Fe[CN]6.The calculated peroxide species (HO2\n\u2212) are below \u223c10% for NCNT-Pd and over the potential range of \u22120.4 to \u22121.2\u00a0V (Fig.\u00a05(d)). Also the calculated electron transfer number is \u223c 3.90 over the same potential range (Fig.\u00a05(c)). This result has similarities with 20% commercial Pt/C electrode (Fig.\u00a05(c-d)), that means NCNT-Pd is a promising cathode catalyst for the alkaline fuel cell. The summary of the ORR performance on NCNT-Pd, CNT-Pd, NCNT and 20% commercial Pt/C electrodes (2500\u00a0rpm) were enlisted in table\u00a01\n. The values NCNT-Pd are not only similar to that 20% commercial Pt/C, but also that of most reported Pt-based catalysts as presented in table\u00a02\n.Methanol crossover effect is one of the major concern to justify the fuel cell catalyst. chronoamperometry is used to show the methanol tolerance effect of NCNT-Pd and 20% commercial Pt/C in O2-saturated 0.1\u00a0M KOH at a constant voltage of \u22120.3\u00a0V (Fig.\u00a06\n(a)). As shown in Fig.\u00a06(a), after injection of 0.1\u00a0M methanol a sharp current response observed for Pt/C electrode which is an indication of methanol oxidation reaction and CO poisoning effect. In contrast, NCNT-Pd shows a tiny current response on methanol injection, demonstrating a better methanol tolerance than the 20% commercial Pt/C.Furthermore, we also conducted stability test for NCNT-Pd and 20% commercial Pt/C by using chronoamperometric measurements in O2-saturated 0.1\u00a0M KOH at a constant potential of \u22120.3\u00a0V. As shown in Fig.\u00a06(b), NCNT-Pd exhibits 84% retention current after 10 000\u00a0s. In contrast, 20% commercial Pt/C exhibits only 40% retention current after the same time. From the above result, it is clear that NCNT-Pd shows excellent methanol tolerance and better durability towards ORR.The cyclic stability of NCNT-Pd was also investigated in O2-saturated 0.1\u00a0M KOH electrolyte by cycling the catalyst between \u22120.2\u00a0V and 1.2\u00a0V (Fig.\u00a06(c)). After 500 continuous cycles, the NCNT-Pd modified electrode showed a very little decrease in half wave potential (\u223c15\u00a0mV) along with very small decline in limiting current density indicating that the NCNT-Pd catalyst also has great advantages in terms of cyclic stabilityNCNT-Pd catalyst is also tested for ethanol oxidation reaction (EOR). The electrocatalytic performances for EOR are investigated by CV measurements Fig.\u00a07.(a) represents the CV curves of NCNT-Pd, CNT-Pd and commercial Pd/C in Ar-saturated1M KOH electrolyte in the absence of ethanol at scan rate 50mVs\u22121.A s shown in Fig.\u00a07(a), the cathodic peak at around \u22120.4\u00a0V is ascribed to the reduction of PdO [16, 47]. The electrochemically active surface areas (ECSA) can be calculated from the reduction peak of PdO by using the equation ECSA= Q/(0.405\u00a0\u00d7\u00a0mPd), where Q is the integral columbic charge (mC) of the reduction area of PdO, 0.405 is the charge required for the reduction of PdO on catalyst and mPd is the Pd loading (g) on the electrode [54]. From the Fig.\u00a07(a), it is clear that NCNT-Pd shows higher PdO peak than other two catalyst that means NCNT-Pd has more active surface area. The calculated ECSA values of NCNT-Pd, CNT-Pd and Pd/C are 74.7 m2gpd\n\u22121, 52.1 m2gpd\n\u22121 and 38.2 m2gpd\n\u22121 respectively. The NCNT-Pd shows the largest surface area which is 1.43 and 1.85 times higher than the CNT-Pd, and commercial Pd/C suggesting doping N in CNT plays an important role for more nucleation of Pd NPs on NCNT.The electrocatalytic performances for ethanol oxidation are investigated by CV in Ar-saturated 1\u00a0M KOH +0.1\u00a0M ethanol solution at 50 mVs\u22121 scan rate (Fig.\u00a07(b)) Fig.\u00a07.(b) represents the comparison of ethanol oxidation on NCNT-Pd, CNT-Pd, and commercial Pd/C electrode, respectively. As seen from Fig.\u00a07(b), there are two well defined anodic oxidation peaks are observed for the forward and backward scan towards ethanol oxidation reaction. The oxidation peak in the forward scan is attributed to the oxidation of ethanol and the backward scan peak is produced by oxidation of remaining carbonaceous species which are not fully oxidized in the forward scan [55]\n[56]. The anodic oxidation peak current density of EOR on NCNT-Pd is 10.3\u00a0mA cm\u22122 which is 2.54 and 5.2 times higher current density than the CNT-Pd and commercial Pd/C. Moreover, the onset potential (in the forward scan) of NCNT-Pd for EOR is around \u22120.72\u00a0V which is shifted to more negative side than the CNT-Pd (\u221264\u00a0mV) and commercial Pd/C (\u221260\u00a0mV) indicating faster reaction kinetics for ethanol oxidation on the NCNT-Pd with lower over-potential. These results suggest that NCNT-Pd catalyst shows higher catalytic activity towards EOR than CNT-Pd and commercial Pd/C.With regard to the catalytic mechanism of EOR by Pd-based catalysts on ethanol in alkaline media can be represented in Eqs.\u00a0(6)-(10) [47, 57]\n\n(6)\n\n\nM\n+\nO\n\n\nH\n\n\u2212\n\n\u2194\nM\n\u2212\nO\n\nH\nads\n\n+\n\n\ne\n\n\u2212\n\n\n\n\n\n\n\n(7)\nM+ CH3CH2OH \u2194 M-(CH3CH2OH)ad\n\n\n\n\n\n(8)\nM-(CH3CH2OH)ads\u00a0+\u00a03OH\u2212\u2192 M-(CH3CO)ads\u00a0+\u00a03H2O\u00a0+\u00a03e\u2212\n\n\n\n\n\n(9)\nM-(CH3CO)ads\u00a0+\u00a0M-OHads\u00a0\u2192\u00a0M-CH3COOH\u00a0+\u00a0M\n\n\n\n\n(10)\nM-CH3COOH\u00a0+\u00a0OH\u2212\u00a0\u2192\u00a0M\u00a0+\u00a0CH3COO\u2212+ H2O\n\n\nWhere, M is the Pd based catalyst. The main intermediate species are CH3CO and OH produces during the reaction and OH facilitates the desorption of CH3CO releasing acetate as the main product [47]\nFurthermore, the stability of the catalysts is studied by the chronoamperometric method in 0.1\u00a0M CH3CH2OH\u00a0+\u00a01\u00a0M KOH solution at a constant potential \u22120.4\u00a0V Fig.\u00a08.(a) represents the stability of different catalysts for ethanol oxidation reaction and it is clear that the current density rapidly decreases with the time and after 100\u00a0s they decay slowly and attain steady state current density. The initial and steady state current density of NCNT-Pd is higher than the CNT-Pd and commercial Pd/C, indicating the excellent tolerance of intermediate carbonaceous species and higher catalytic activity of NCNT-Pd towards EOR.To evaluate the cyclic stability of NCNT-Pd for EOR, cyclic voltammograms have been cvonducted for ethanol electro-oxidation reaction in 1\u00a0M KOH containing 0.1\u00a0M C2H5OH electrolyte by cycling between \u22120.4\u00a0V and 1.0\u00a0V at scan rate 50 mVs\u22121 (Fig.\u00a08(b)). After 500 continuous cycles, the NCNT-Pd modified electrode showed around similer onset potential and a very small decrease in current density indicating that the NCNT-Pd catalyst also shows superior cyclic stability in EOR.The electrocatalytic parameters measured from the above voltammograms for EOR have been tabulated (table\u00a03\n.). From the table\u00a03, it can be seen that electroactive surface area of NCNT-Pd higher than that of CNT-Pd and Pd/C which is favorable for the high portion of ethanol absorption and enhances EOR. Also, NCNT-Pd shows more negative onset potential and higher current density than CNT-Pd and commercial Pd/C that means NCNT-Pd can be also used as an efficient catalyst for ethanol oxidation. A comparison of EOR at different material modified electrodes can be observed in table\u00a04\n. It is clear that the NCNT-Pd also shows higher catalytic activity toward EOR than other materials in terms of onset potential, ECSA and current density.The above ORR and EOR results have demonstrated that NCNT-Pd shows higher catalytic performance toward ORR and EOR. Some key factors are responsible for this performance, first, hetero atom electron rich which plays an important role for Pd NPs nucleation and results in more surface area and enhance ORR and EOR [16, 32]. Second, the lone pair of N atom breaks the elctroneutrality of sp2 carbon and keeping enough pi electron on CNT network for conduction [16]. Third, due to strong interaction NCNT and Pd, NCNT-Pd shows excellent electrocatalytic activity for both ORR and EOR.Nitrogen functionalized NCNT supported Pd NPs were synthesized by simple chemical reduction method at room temperature. The elemental and morphological properties have analyzed by XPS, XRD, TEM, and EDX. From the TEM images, it is revealed that small size Pd nanoparticles are successfully synthesized on NCNT. The as prepared NCNT-Pd is applied for ORR and EOR in alkaline media. The electrochemical studies for ORR reveal that, NCNT-Pd shows more positive onset potential and higher current density than the CNT-Pd and NCNT. The RRDE analysis demonstrated that ORR proceeds by \u223c4-electron transfer pathway with the negligible amount of peroxide species as like as commercial 20% Pt/C. Moreover, NCNT-Pd shows good catalytic performance toward EOR with higher peak current density and remarkable negative shift of onset potential. From the above discussion, it is clear that NCNT-Pd shows higher catalytic activity both ORR and EOR in alkaline media. So, the as synthesized NCNT-Pd can be used as an advance catalyst for both anode and cathode part in alkaline fuel cell.Graphical abstract.docxThe authors declare no competing interests.This research has supported by the National Research Foundation of Korea (NRF) and funded by the Ministry of Education, Science and Technology (NRF-2021R1F1A1047229).", "descript": "\n                  In this report, we have synthesized nitrogen-functionalized carbon nanotube (NCNT) supported palladium nanoparticles (NCNT-Pd) by using facile chemical reduction method at room temperature. This material has been used as electrocatalyst for electrochemical oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR). In the case of ORR, NCNT-Pd shows more positive onset potential, higher current density, as well as superior methanol tolerance and long term durability compared with the 20% commercial Pt/C catalyst. Moreover, NCNT-Pd exhibits enhanced catalytic activity for EOR with more negative onset potential and higher current density. The current density of NCNT-Pd for EOR is 2.54 and 5.2 times higher than that of CNT-Pd and 10% Pd/C, respectively. Due to nitrogen functionalization and synergetic effect between NCNT and Pd, NCNT-Pd exhibits superior catalytic performance in ethanol oxidation and oxygen reduction reactions.\n               "}
{"full_text": "Deactivation factorUnit conversion factor (\n\n\n\nmol\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\nmin\n\n\n\ng\n\n\nN\ni\n\n\n\u2215\n\n\nmmol\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\u2215\ns\n\u2215\n\n\nm\n\n\nc\na\nt\n\n\n2\n\n\n\n)Specific heat capacity of the macroparticle (J/kg\n\n\n\np\na\nr\nt\ni\nc\nl\ne\n\n\n/K)Specific heat capacity of the grain (J/kg\n\n\n\ns\nu\np\np\no\nr\nt\n\n\n/K)WeiszPrater criterion (Equation 37)Effective diffusivity in the macroparticle (\n\n\n\nm\n\n\n2\n\n\n\u2215\ns\n\n)Effective diffusivity in the product layer around the grains (\n\n\n\nm\n\n\n\u2215\n\n\ns\n\n)External heat transfer coefficient (\n\nW\n\u2215\n\n\nm\n\n\ni\nn\nt\ne\nr\nf\na\nc\ne\n\n\n2\n\n\n\u2215\nK\n\n)Heat conductivity of the macroparticle (W/m/K)Heat conductivity of the grain (W/m/K)External mass transfer coefficient (\n\n\n\nm\n\n\ng\na\ns\n\n\n3\n\n\n\u2215\n\n\nm\n\n\ni\nn\nt\ne\nr\nf\na\nc\ne\n\n\n2\n\n\n\u2215\ns\n\n)Kinetic rate constants (unit dependent on equation)Kinetic rate constants (unit dependent on equation)Equilibrium constant (atm)Molar concentration (mol/\n\n\nm\n\n\ng\na\ns\n\n\n3\n\n\n)Molar concentration outside the macroparticle (mol/\n\n\nm\n\n\ng\na\ns\n\n\n3\n\n\n)Initial molar concentration inside the macroparticle (mol/\n\n\nm\n\n\ng\na\ns\n\n\n3\n\n\n)Dimensionless concentration (Equation 32)Molar mass of methane (\n\n\n\nkg\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\u2215\n\n\nmol\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n)Thiele modulus (Equation 35)Number of grains in radial position rNumber of the grain layers in the macroparticleTotal number of the grainsPartial pressure of methane (atm)Partial pressure of hydrogen (atm)Initial reaction rate (\n\n\n\nmmol\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\u2215\n\n\ng\n\n\nN\ni\n\n\n\u2215\nmin\n\n)Actual reaction rate (\n\n\n\nmmol\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\u2215\n\n\ng\n\n\nN\ni\n\n\n\u2215\nmin\n\n)radial position in the macroparticle (m)Radial position in the grainPosition of the grain in the macroparticle (m)Dimensionless radial positionRadius of the macroparticle (m)Radius of the grain (m)Radius of the core of the grain (m)Average rate of reaction in given time and radial position (\n\n\n\nmol\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\u2215\n\n\nm\n\n\np\na\nr\nt\ni\nc\nl\ne\n\n\n3\n\n\n\u2215\ns\n\n)time (s)Dimensionless timeTemperature (K)Temperature outside the macroparticle (K)Initial temperature inside the macroparticle (K)Heat of reaction (kJ/\n\n\nmol\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n)Porosity of macroparticle (\n\n\n\nm\n\n\np\no\nr\ne\n\n\n3\n\n\n\u2215\n\n\nm\n\n\nm\na\nc\nr\no\np\na\nr\nt\ni\nc\nl\ne\n\n\n3\n\n\n\n)Density of the macroparticle (\n\nkg\n\u2215\n\n\nm\n\n\np\na\nr\nt\ni\nc\nl\ne\n\n\n3\n\n\n\n)Density of the grain (\n\nkg\n\u2215\n\n\nm\n\n\ns\nu\np\np\no\nr\nt\n\n\n3\n\n\n\n)Density of produced carbon layer, including porosity (\n\nkg\n\u2215\n\n\nm\n\n\nc\na\nr\nb\no\nn\n\n\n3\n\n\n\n)Hydrogen can be produced through different processes from different feedstocks, such as steam methane reforming, water splitting, and thermocatalytic decomposition of methane. Steam methane reforming coupled with CO2\u00a0capture and storage (CCS) technologies are the most known and investigated methods in the field of low carbon footprint hydrogen production. However, the separation of produced CO2\u00a0and handling and storage leads to costs for gas separation and storage management. Water splitting is an energy and capital intensive process and increases the final price of hydrogen. By contrast, methane decomposition to functional carbon materials and hydrogen has advantages to the alternative processes such as the elimination of additional purification/separation units and production of valuable carbon nanomaterials (tubes or fibers) instead of CO2. The potential applications of carbon nanomaterials in semiconductors, additives to building materials, energy storage, and catalytic materials due to their unique physicochemical properties such as high conductivity, high tenacity and mechanical strength, high specific surface area and semiconductor properties make TCD more economically and environmentally appealing\u00a0[1\u20136].Methane, in absence of oxidizing agents or a catalyst (including inert heterogeneities), decomposes naturally to hydrogen and amorphous carbon at high temperatures, >1300\u00a0\u00b0C (reaction (1))\u00a0[7]. The addition of a catalyst facilitates the decomposition of methane in two ways. First, the activation energy and therefore the required temperature for conversion decreases (between 500\u00a0\u00b0C to 950\u00a0\u00b0C dependent on the active material of the catalyst); Second, solid carbon can be produced in specific nano-structured shapes, depending on the support and active materials of catalyst and operating conditions. Nickel, iron, copper, and carbon are the most common materials used as the active sites of the catalysts. Many studies have been performed on the catalyst preparation, different support and active materials, thermodynamics and kinetics of the thermocatalytic decomposition of methane\u00a0[1,2,8\u201310]. In general, nickel has the highest activity and rate of methane decomposition. Nickel, compared to the others, is active in a lower temperature range and is also more likely to deactivate. However, the addition of a second (or even a third) metal such as iron or copper to the nickel, has shown improved stability at higher temperatures and less deactivation\u00a0[11\u201314]. \n\n(1)\n\n\nCH4(g)\n\u27f6\nC(s)\n+\n\n\nH\n\n\n2\n\n\n(g)\n\n\n\n\n\n\n\u0394\n\n\nH\n\n\n\n(\n298\n\nK\n)\n\n\n\n=\n+\n74\n.\n52\n\nkJ\u2215mol\n\n\n\n\nDespite the high potential of TCD for producing carbon nanomaterials and CO2-free hydrogen, it is greatly restricted for industrial applications due to inadequate productivity, uncertainties of process performance and operational challenges coming from carbon formation\u00a0[2]. Therefore, in addition to studies on catalysts, the TCD reactor and process has to be developed, designed and controlled thoroughly to become feasible at industrial scale. For a rational reactor and process design, modeling and experimental studies can provide the required understanding and basic data for this. This understanding facilitates identification of optimal process conditions for maximum carbon nanomaterial production. Modeling of the catalyst performance as function of equivalent process time is critical for understanding and predicting the product and catalyst evolution in the reactor. This performance can be expressed with the ratio of the mass of produced carbon to the mass of fresh catalyst used, called carbon yield (Eq.\u00a0(2)) and the change in catalyst particle size and density. \n\n(2)\n\n\nc\na\nr\nb\no\nn\n\ny\ni\ne\nl\nd\n=\n\n\nm\na\ns\ns\n\no\nf\n\np\nr\no\nd\nu\nc\ne\nd\n\nc\na\nr\nb\no\nn\n\n(g)\n\n\nm\na\ns\ns\n\no\nf\n\no\nr\ni\ng\ni\nn\na\nl\n\nc\na\nt\na\nl\ny\ns\nt\n\n(g)\n\n\n\n\n\n\nIn the literature, including the work of Ashik some studies have been reported on modeling at the molecular scale\u00a0[2,15,16], which helps scientists in catalyst evaluation and to develop a microscopic level of understanding of the complete chemical transformation. Although these models are helpful in the understanding of reaction mechanisms, a model that properly describes the behavior of a catalyst at the macro level has not been reported to the best of our knowledge. In particular, the formation of a functional carbon layer onto the catalyst phase leads to particle growth\u00a0[2]. The particle size and thus growth is a crucial parameter in designing a reactor. In the present work, for the first time, the multi-grain model (MGM) based on the analogy with the growth of polyolefin particles is developed to describe the macroscopic behavior of growing particles in TCD of methane. The model couples different phenomena involved inside the catalyst particle (which is called macroparticle in this study) such as heat and mass transfer and chemical reaction. A short review of kinetic studies in the literature is provided in Section\u00a02. In Section\u00a03, the model description is presented. The model validation and its reactor predictions assessed in Sections\u00a04 and 5 respectively.In TCD, the activity of the catalyst and kinetic rate of reaction decrease over time due to deactivation. The actual rate of reaction at time \n\nt\n>\n0\n\n is described using two parameters: the initial reaction rate and a time-dependent deactivation factor.The initial reaction rate and the reaction mechanism have been studied extensively\u00a0[2]. Douven et\u00a0al. and Yadav et\u00a0al. proposed reaction rate equations for carbon nano-tubes (CNT) production by TCD of the methane which is only dependent on methane concentration\u00a0[3,17], (see Eq.\u00a0(3) below). Yadav found out that multi-walled CNT is produced with a different kinetic rate than single-walled CNT, both only depend on the concentration of methane\u00a0[18], (see Eq.\u00a0(4)). Ashik et\u00a0al. 2017 proposed Eq.\u00a0(5) for the initial reaction rate which also does not depend on the hydrogen concentration\u00a0[19]. \n\n\n(3)\n\n\n\n\nr\n\n\n0\n\n\n\n[\nk\nm\no\n\n\nl\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\u2215\nk\n\n\ng\n\n\nc\na\nt\n\n\n\u2215\ns\n]\n\n=\n\n\n\n\nK\n\n\n1\n\n\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n[\na\nt\nm\n]\n\n\n\n\n\n\n\n1\n+\n\n\n\n\nK\n\n\n1\n\n\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n[\na\nt\nm\n]\n\n\n\n\n\nK\n\n\n2\n\n\n\n\n\n\n\n\n2\n\n\n\n\n\n\n\n\n(4)\n\n\n\n\nr\n\n\n0\n\n\n\n[\nk\nm\no\n\n\nl\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\u2215\nk\n\n\ng\n\n\nc\na\nt\n\n\n\u2215\ns\n]\n\n=\n\n\n\n\nK\n\n\n1\n\n\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n[\na\nt\nm\n]\n\n\n\n\n\n1\n+\n\n\n\n\nK\n\n\n1\n\n\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n[\na\nt\nm\n]\n\n\n\n\n\nK\n\n\n2\n\n\n\n\n\n\n\n\n\n\n\n\n(5)\n\n\n\n\nr\n\n\n0\n\n\n\n[\nm\nm\no\n\n\nl\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\u2215\n\n\ng\n\n\nc\na\nt\n\n\n\u2215\nm\ni\nn\n]\n\n=\n\n\nk\n\n\np\n\n\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n1\n.\n4\n\n\n\n[\na\nt\nm\n]\n\n\n\n\n\n\nThe majority of studies have revealed that the hydrogen concentration has a negative effect on the initial reaction rate due to thermodynamic factors equilibrium and occurrence of the reverse reaction\u00a0[5,7,20\u201323]. The kinetic models presented in Eqs.\u00a0(3)\u2013(5) must therefore be regarded as a simplified form of the actual kinetics. The equations Eqs.\u00a0(6)\u2013(8) that involve the effect of the hydrogen concentration have a very similar form, but differ mostly in the expression in the denominator. Amin et\u00a0al.\u00a0[7] and Snoeck et\u00a0al.\u00a0[21] derived Eq.\u00a0(6) while Borghei et\u00a0al.\u00a0[22] suggested different powers for H2\u00a0and CH4,\u00a0Eq.\u00a0(7), which may be due to the use of a different catalyst and the specific operating conditions used in their studies\u00a0[7,21,22]. Saraswat et\u00a0al.\u00a0[5] have reported a more extended form of the reaction rate, Eq.\u00a0(8), which includes effects of hydrogen and methane partial pressures in Langmuir\u2013Hinshelwood type of expressions\u00a0[5]. \n\n\n(6)\n\n\n\n\nr\n\n\n0\n\n\n\n[\nm\nm\no\n\n\nl\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\u2215\n\n\ng\n\n\nc\na\nt\n\n\n\u2215\nm\ni\nn\n]\n\n=\n\n\nk\n\n(\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n[\na\nt\nm\n]\n\n\u2212\n\n\n\n\nP\n\n\nH\n2\n\n\n\n\n2\n\n\n\n[\na\nt\nm\n]\n\n\u2215\n\n\nK\n\n\np\n\n\n)\n\n\n\n\n\n\n\n1\n+\n\n\nK\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n\n\nP\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n1\n.\n5\n\n\n\n[\na\nt\nm\n]\n\n+\n\n\nK\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n[\na\nt\nm\n]\n\n\n\n\n\n2\n\n\n\n\n\n\n\n\n(7)\n\n\n\n\nr\n\n\n0\n\n\n\n[\nm\no\n\n\nl\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\u2215\n\n\ng\n\n\nc\na\nt\n\n\n\u2215\nh\nr\n]\n\n=\n\n\nk\n\n(\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n[\na\nt\nm\n]\n\n\u2212\n\n\n\n\nP\n\n\nH\n2\n\n\n\n\n2\n\n\n\n[\na\nt\nm\n]\n\n\u2215\n\n\nK\n\n\np\n\n\n)\n\n\n\n\n\n\n\n1\n+\n\n\nK\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n\n\nP\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n0\n.\n5\n\n\n\n[\na\nt\nm\n]\n\n+\n\n\nK\n\n\n\n\nH\n\n\n2\n\n\n\n\n\u2217\n\n\n\n\nP\n\n\n\n\nH\n\n\n2\n\n\n\n\n1\n.\n5\n\n\n\n[\na\nt\nm\n]\n\n\n\n\n\n2\n\n\n\n\n\n\n\n\n(8)\n\n\n\n\nr\n\n\n0\n\n\n\n[\nm\no\n\n\nl\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\u2215\n\n\ng\n\n\nc\na\nt\n\n\n\u2215\ns\n]\n\n\n=\n\n\n\n\nk\n\n\n1\n\n\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n[\na\nt\nm\n]\n\n\u2212\n\n\nk\n\n\n2\n\n\n\n\n\n\nP\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n2\n\n\n\n[\na\nt\nm\n]\n\n\n\n\n\n\n\n1\n+\n\n\nk\n\n\n3\n\n\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n[\na\nt\nm\n]\n\n+\n\n\nk\n\n\n4\n\n\n\n\n\n\nP\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n0\n.\n5\n\n\n\n[\na\nt\nm\n]\n\n+\n\n\nk\n\n\n5\n\n\n\n\nP\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n[\na\nt\nm\n]\n\n+\n\n\nk\n\n\n6\n\n\n\n\n\n\nP\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n1\n.\n5\n\n\n\n[\na\nt\nm\n]\n\n\n\n\n\n2\n\n\n\n\n\n\n\n\n According to the literature\u00a0[5,7,23] The most likely mechanism of the reaction is based on molecular adsorption of methane as the first step, followed by a series of dehydrogenation reactions that are taking place one by one until it ends with separate adsorbed carbon and hydrogen atoms. Detachment of the first \nH\n from \n\nC\n\n\nH\n\n\n4\n\n\n\n is known to be the slowest and the rate determining step. Every two adsorbed hydrogen atoms form a single \n\n\nH\n\n\n2\n\n\n molecule, which is released from the catalytic surface. The carbon atom, however, can diffuse into the nickel catalyst and either forms nanomaterials or encapsulates the active site.The ratio of reaction rate at time \nt\n to the initial reaction rate (Eq.\u00a0(9)) is called the deactivation factor. The deactivation factor expresses the stability of the catalyst over time, which is a crucial factor in order to obtain a high carbon yield. \n\n(9)\n\n\na\n=\n\n\nr\n\n(\nt\n)\n\n\n\n\n\nr\n\n\n0\n\n\n\n\n\n\n\n\nSeveral different empirical or semi-empirical equations for the deactivation factor a have been defined. Borghei\u00a0[22] proposed Eq.\u00a0(10) for the deactivation factor, where b, c and d are constants and \n\n\nk\n\n\nd\n\n\n is defined by an Arrhenius type of temperature dependency. Douven\u00a0[3] reported that deactivation is reversible and probably due to the formation of amorphous carbon and encapsulation of active sites. Douven used a sigmoid Eq.\u00a0(11) as the deactivation factor, where parameter b is assumed to have only temperature dependency however parameters \n\n\nt\n\n\n0\n\n\n and c decrease slightly as the methane partial pressure increases. \n\n\n(10)\n\n\na\n=\n\n\n1\n\n\n\n\n\n\n1\n+\n\n(\nd\n\u2212\n1\n)\n\n\n\nk\n\n\nd\n\n\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\nc\n\n\n\n[\na\nt\nm\n]\n\n\n\nP\n\n\n\n\nH\n\n\n2\n\n\n\n\nb\n\n\n\n[\na\nt\nm\n]\n\nt\n\n[\nm\ni\nn\n]\n\n\n\n\n\n1\n\u2215\n\n(\nd\n\u2212\n1\n)\n\n\n\n\n\n\n\n\n\n(11)\n\n\na\n=\nd\n\u2212\nb\ntanh\n\n\n\n\nt\n\n[\ns\n]\n\n\u2212\n\n\nt\n\n\n0\n\n\n\n\nc\n\n\n\n\n\n\n\n\n\nEq.\u00a0(12) is proposed by Amin et\u00a0al.\u00a0[7] and is based on the proposed mechanism in 2.1, mass balance of species on the surface of the catalyst and the assumption that all the reaction steps except one are in equilibrium. So, the concentrations of intermediate species are negligible. All parameters (\n\n\nK\n\n\nd\n\n\n, \n\n\nK\n\n\nd\n,\nC\n\n\n, \n\n\nk\n\n\nd\n,\nC\n\n\nH\n\n\n4\n\n\n\n\n and \n\n\nk\n\n\nd\n,\n\n\nH\n\n\n2\n\n\n\n\n) are temperature dependent following the Arrhenius equation and are determined by fitting the expression to experimental data. \n\n(12)\n\n\na\n=\n\n\n\n\n\n\n1\n\n\n1\n\u2212\n0\n.\n5\n\n\nk\n\n\nd\n\n\n\n\n\n\nk\n\n\nd\n,\nC\n\n\n+\n\n\nk\n\n\nd\n,\nC\n\n\nH\n\n\n4\n\n\n\n\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n+\n\n\nk\n\n\nd\n,\n\n\nH\n\n\n2\n\n\n\n\n\n\nP\n\n\n\n\nH\n\n\n2\n\n\n\n\n0\n.\n83\n\n\n\n\nt\n\n\n\n\n\n\n\u2212\n0\n.\n8\n\n\n\n\n\n\nFor our model of the TCD reactor and the particle growth over time, the following physical phenomena have to be taken into account:\n\n\n1.\nThe transport of species into and out of the particle, being diffusion of methane into the catalyst pores and diffusion of hydrogen out of the catalyst pores.\n\n\n2.\nThe heat transfer into the macroparticle to provide the heat for the strongly endothermic reaction. This includes the transfer from the bulk gas to the external surface of the macroparticle, conduction within the macro macroparticle as well as conduction within the grains.\n\n\n3.\nThe decomposition of methane on the active sites of the macroparticles into hydrogen and solid carbon.\n\n\n4.\nThe accumulation of solid carbon onto the catalyst, with consequential growth of the catalyst and deactivation of the catalyst.\n\n\nThe transport of species into and out of the particle, being diffusion of methane into the catalyst pores and diffusion of hydrogen out of the catalyst pores.The heat transfer into the macroparticle to provide the heat for the strongly endothermic reaction. This includes the transfer from the bulk gas to the external surface of the macroparticle, conduction within the macro macroparticle as well as conduction within the grains.The decomposition of methane on the active sites of the macroparticles into hydrogen and solid carbon.The accumulation of solid carbon onto the catalyst, with consequential growth of the catalyst and deactivation of the catalyst.These phenomena are very similar to the olefin polymerization, which also experiences particle growth of the macroparticle due to solid product formation. For the solids formation and particle growth different modeling approaches have been developed\u00a0[24,25]. For our TCD process the most appropriate model, capable of modeling particle growth and convenient for further development to include fragmentation\u00a0[26], is the Multi-grain model (MGM).MGM is based on two assumptions. Firstly, the macroparticle is spherical with only profiles in the radial direction. So, it is a 1D model in the radial direction. Secondly, the macroparticle is composed of layers of identical non-porous grains (microparticles) with active sites on their surface\u00a0[25\u201329]. The growth and evolution of the macroparticle are due to the accumulation of produced carbon on the surface of grains. Fig.\u00a01 shows the schematic of the both concepts of macroparticle and internal grain layers before and after the reaction takes place. Fig.\u00a01(a) shows the schematic of a fresh porous macroparticle which consists of layers of non-porous grains that are illustrated as black spheres. Fig.\u00a01(b) shows a circular sector of the same macroparticle after entering the reactor. The gray shell around each grain is the produced carbon on the grain.\n\nMethane and hydrogen diffusion through the pores of the macroparticle and through the layer of accumulated carbon surrounding the catalyst fragments is modeled at two different scales. At the scale of the macroparticle the process is modeled by the diffusion\u2013reaction equation in spherical co-ordinates: \n\n(13)\n\n\n\n\n\u2202\nM\n\n(\nr\n,\nt\n)\n\n\n\n\u2202\nt\n\n\n=\n\n\n1\n\n\n\n\nr\n\n\n2\n\n\n\n\n\n\n\u2202\n\n\n\u2202\nr\n\n\n\n\n\n\nD\n\n\ne\n\n\n\n\nr\n\n\n2\n\n\n\n\n\u2202\nM\n\n(\nr\n,\nt\n)\n\n\n\n\u2202\nr\n\n\n\n\n\u2212\nR\n\n(\nr\n,\nt\n)\n\n\n\n\nWhere \nM\n is the molar concentration of a component, \nr\n is the radial position in macroparticle and \n\n\nD\n\n\ne\n\n\n is the effective diffusivity of the considered component. \n\nR\n\n(\nr\n,\nt\n)\n\n\n is the average rate of reaction at a given radial position: \n\n(14)\n\n\nR\n\n(\nr\n,\nt\n)\n\n=\n\n\n\n(\n1\n\u2212\n\u03b5\n)\n\n\n\n\u2211\n\n\ng\n=\n1\n\n\n\n\nN\n\n\ng\n,\nr\n\n\n\n\n\n\n4\n\u03c0\n\n\nR\n\n\ng\n0\n\n\n2\n\n\n\n.\nr\n\n(\nt\n)\n\n\n\n\n\n\n\n4\n\n\n3\n\n\n\u03c0\n\n\n\n\nr\n\n\n3\n\n\n\u2212\n\n\n\n(\nr\n\u2212\nd\nr\n)\n\n\n\n3\n\n\n\n\n\n\n\n\n\nWhere \n\u03b5\n is the porosity of catalyst, \n\n\nN\n\n\ng\n,\nr\n\n\n is the number of grains at radial position \nr\n and \n\n\nR\n\n\ng\n0\n\n\n is the radius of the core of the grains. Eq.\u00a0(13) can be solved with the following boundary and initial conditions: \n\n\n(15)\n\n\n\n\n\u2202\nM\n\n(\n0\n,\nt\n)\n\n\n\n\u2202\nr\n\n\n=\n0\n\n\n\n\n(16)\n\n\n\n\nD\n\n\ne\n\n\n\n\n\u2202\nM\n\n(\nR\n,\nt\n)\n\n\n\n\u2202\nr\n\n\n=\n\n\nk\n\n\no\nu\nt\n\n\n\n(\nM\n\u2212\n\n\nM\n\n\nb\n\n\n)\n\n\n\n\n\n(17)\n\n\nM\n\n(\nr\n,\n0\n)\n\n=\n\n\nM\n\n\n0\n\n\n\n\n\n\n\n\n\n\n\nk\n\n\no\nu\nt\n\n\n is the external mass transfer coefficient of the macroparticle, \n\n\nM\n\n\nb\n\n\n is the external concentration and \n\n\nM\n\n\n0\n\n\n is the initial concentration in the particle.The concentration at the grain scale is also modeled by the diffusion equation in spherical coordinates, Eq.\u00a0(18). Considering the assumption that the core of the grains are non-porous, so there is no hydrogen or methane inside the core of the grains. Therefore, the boundary conditions, Eqs.\u00a0(19) and (20) are defined at the surface of the core and outer surface of the grain particle. \n\n\n(18)\n\n\n\n\n\u2202\nM\n\n(\n\n\nr\n\n\ng\n\n\n,\nt\n)\n\n\n\n\u2202\nt\n\n\n=\n\n\n1\n\n\n\n\nr\n\n\ng\n\n\n2\n\n\n\n\n\n\n\u2202\n\n\n\u2202\n\n\nr\n\n\ng\n\n\n\n\n\n\n\n\nD\n\n\np\n\n\n\n\nr\n\n\ng\n\n\n2\n\n\n\n\n\u2202\nM\n\n(\n\n\nr\n\n\ng\n\n\n,\nt\n)\n\n\n\n\u2202\n\n\nr\n\n\ng\n\n\n\n\n\n\n\n\n\n\n(19)\n\n\n\n\nD\n\n\np\n\n\n\n\n\u2202\nM\n\n(\n\n\nR\n\n\ng\n0\n\n\n,\nt\n)\n\n\n\n\u2202\n\n\nr\n\n\ng\n\n\n\n\n=\nr\n\n(\nt\n)\n\n.\nC\nF\n\n\n\n\n(20)\n\n\nM\n\n(\n\n\nR\n\n\ng\n\n\n,\nt\n)\n\n=\nM\n\n(\n\n\nr\n\n\ng\n,\nr\n\n\n,\nt\n)\n\n\n\n\n\n\nWhere \n\n\nr\n\n\ng\n\n\n is the radial position in the grain, \n\n\nD\n\n\np\n\n\n is the diffusivity in the product layer around the grains, \n\n\nR\n\n\ng\n\n\n is the radius of whole-grain and \n\n\nr\n\n\ng\n,\nr\n\n\n is the position of grain in the macroparticle (Fig.\u00a01) and \n\nC\nF\n\n is the unit conversion factor. The initial condition is the same for the macroparticle (Eq.\u00a0(17)). Eqs.\u00a0(13)\u2013(17) are solved only once during each time step for the macro macroparticle, while Eq.\u00a0(18) and its initial and boundary conditions are solved for all the internal grain layers at each time step.The heat transfer mechanism at both scales is based on conduction. The temperature profile in the macro-particle is calculated by\u00a0Eq.\u00a0(21): \n\n(21)\n\n\n\n\n\u2202\nT\n\n(\nr\n,\nt\n)\n\n\n\n\u2202\nt\n\n\n=\n\n\n1\n\n\n\n\nr\n\n\n2\n\n\n\n\n\n\n\u2202\n\n\n\u2202\nr\n\n\n\n\n\n\n\n\nk\n\n\nh\n\n\n\n\n\u03c1\n\n\nC\n\n\nP\n\n\n\n\n\n\nr\n\n\n2\n\n\n\n\n\u2202\nT\n\n(\nr\n,\nt\n)\n\n\n\n\u2202\nr\n\n\n\n\n\u2212\n\n\n\u0394\nH\n\n\n\u03c1\n\n\nC\n\n\nP\n\n\n\n\nR\n\n(\nr\n,\nt\n)\n\n\n\n\nWhere \n\n\nk\n\n\nh\n\n\n, \n\u03c1\n and \n\n\nC\n\n\nP\n\n\n are the heat conductivity, the density and the specific heat capacity of the macroparticle respectively, \n\n\u0394\nH\n\n is the heat of reaction and \n\nR\n\n(\nr\n,\nt\n)\n\n\n is obtained from\u00a0Eq.\u00a0(14). Eq.\u00a0(21) can be solved with the following initial and boundary conditions (Eq.\u00a0(22) to (24)): \n\n\n(22)\n\n\n\n\n\u2202\nT\n\n(\n0\n,\nt\n)\n\n\n\n\u2202\nr\n\n\n=\n0\n\n\n\n\n(23)\n\n\n\n\nk\n\n\nh\n\n\n\n\n\u2202\nT\n\n(\nR\n,\nt\n)\n\n\n\n\u2202\nr\n\n\n=\nh\n\n(\nT\n\u2212\n\n\nT\n\n\nb\n\n\n)\n\n\n\n\n\n(24)\n\n\nT\n\n(\nr\n,\n0\n)\n\n=\n\n\nT\n\n\n0\n\n\n\n\n\n\n\n\nh\n is the external convective heat transfer coefficient outside the macroparticle which can be estimated from the Gunn correlation, \n\n\nT\n\n\n0\n\n\n and \n\n\nT\n\n\nb\n\n\n are respectively the initial temperature and the temperature outside the macroparticle.The radial temperature profile in the grains can be obtained from the heat conductivity equation in spherical co-ordinates for the whole domain of the core of the grain and the layer of carbon product. However, the heat conductivity, density and specific heat capacity (\n\n\nk\n\n\ng\n\n\n, \n\n\n\u03c1\n\n\ng\nr\n\n\n and \n\n\nC\n\n\n\n\nP\n\n\ng\n\n\n\n\n) have different values in the core of the grains and in the product layer. \n\n\n(25)\n\n\n\n\n\u2202\nT\n\n(\n\n\nr\n\n\ng\n\n\n,\nt\n)\n\n\n\n\u2202\nt\n\n\n=\n\n\n1\n\n\n\n\nr\n\n\ng\n\n\n2\n\n\n\n\n\n\n\u2202\n\n\n\u2202\n\n\nr\n\n\ng\n\n\n\n\n\n\n\n\n\n\nk\n\n\ng\n\n\n\n\n\n\n\u03c1\n\n\ng\nr\n\n\n\n\nC\n\n\n\n\nP\n\n\ng\n\n\n\n\n\n\n\n\nr\n\n\ng\n\n\n2\n\n\n\n\n\u2202\nT\n\n(\n\n\nr\n\n\ng\n\n\n,\nt\n)\n\n\n\n\u2202\n\n\nr\n\n\ng\n\n\n\n\n\n\n\n\n\n\n(26)\n\n\n\n\nk\n\n\ng\n\n\n\n\n\n\u2202\nT\n\n(\n\n\nR\n\n\ng\n0\n\n\n,\nt\n)\n\n\n\n\u2202\n\n\nr\n\n\ng\n\n\n\n\n=\nr\n\n(\nt\n)\n\n.\nC\nF\n.\n\u0394\nH\n\n\n\n\n(27)\n\n\nT\n\n(\n\n\nR\n\n\ng\n\n\n,\nt\n)\n\n=\nT\n\n(\n\n\nr\n\n\ng\n,\nr\n\n\n,\nt\n)\n\n\n\n\n\n\nOne can notice the analogy between mass and heat transfer at both scales of the macroparticle and the grains.In this study, Eq.\u00a0(6) and (12) and corresponding parameter values are provided in Table\u00a01 and are used in the model as the initial reaction rate and deactivation factor of reaction\u00a0[7]. Carbon formation increases the radius of the grains and consequently the size of the macroparticles. The growth rate of the radius of the grains is calculated by\u00a0Eq.\u00a0(28) and the growth rate of the macroparticle equals the summation of the growth rate of all grain layers, Eq.\u00a0(29). \n\n\n(28)\n\n\n\n\n\u2202\n\n\nR\n\n\ng\n\n\n\n\n\u2202\nt\n\n\n=\nM\n\n\nW\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n\n\n\nR\n\n\ng\n0\n\n\n2\n\n\n\nr\n\n(\nt\n)\n\n\n\n\n\n\u03c1\n\n\nc\na\nr\nb\no\nn\n\n\n\n\nR\n\n\ng\n\n\n2\n\n\n\n\n\n\n\n\n(29)\n\n\n\n\n\u2202\nR\n\n\n\u2202\nt\n\n\n=\n2\n\n\n\n\n\u2211\n\n\n1\n\n\n\n\nN\n\n\ng\n\n\n\n\n\n\n\u2202\n\n\nR\n\n\ng\n\n\n\n\n\u2202\nt\n\n\n\n\n\n\n\n\n\n\n\nThe performance and the results of the model are validated by comparing its results with analytical solutions and results obtained from an independent PDE solver. Two limiting cases are used to verify the implementation of the model. In the simplified case 1, it is assumed that there is only one layer of micro grains, the mass transfer limitation is low, and the reaction is first order without deactivation. In this case, the mass of carbon produced is calculated by\u00a0Eq.\u00a0(30): \n\n(30)\n\n\nc\na\nr\nb\no\nn\n\np\nr\no\nd\nu\nc\ne\nd\n\n(kg)\n=\n\n(\n4\n\u03c0\n\n\nR\n\n\ng\n0\n\n\n2\n\n\n.\nN\n.\nt\n.\nk\n.\n\n\nM\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n.\nM\n\n\nM\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n)\n\n\n\n\nWhere \n\n\nR\n\n\ng\n0\n\n\n is the radius of the core of micro grains, \nN\n is the number of micro grains in the only available layer in the macroparticle, \nt\n is the time passed since the start of the reaction, \nk\n is the kinetic coefficient of the first-order reaction per surface area of grain core \n\n(\nmol\n\u2215\n\n\nm\n\n\n2\n\n\n)\n\n and \n\nM\n\n\nM\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n is the molar mass of methane. Fig.\u00a02 illustrates the high accuracy of MGM in case 1, by showing that the MGM results matches\u00a0Eq.\u00a0(30).\nIn case 2, again it is assumed that the reaction is first order in methane and independent of the hydrogen concentration without any deactivation (\n\nr\n=\nk\n.\n\n\nP\n\n\nC\n\n\nH\n\n\n4\n\n\n\n\n\n). The second assumption is that the reaction takes place uniformly in the macroparticle. Finally, it is assumed that the physical properties of the macroparticle do not change with time as the reaction proceeds. In these conditions the methane concentration profile inside the particle can be calculated at any time by solving Eq.\u00a0(31) and its associated initial and boundary conditions. \n\n\n(31)\n\n\n\n\n\u2202\nM\n\n(\nr\n,\nt\n)\n\n\n\n\u2202\nt\n\n\n=\n\n\n1\n\n\n\n\nr\n\n\n2\n\n\n\n\n\n\n\u2202\n\n\n\u2202\nr\n\n\n\n\n\n\nD\n\n\ne\n\n\n\n\nr\n\n\n2\n\n\n\n\n\u2202\nM\n\n(\nr\n,\nt\n)\n\n\n\n\u2202\nr\n\n\n\n\n\u2212\nk\n.\nM\n\n(\nr\n,\nt\n)\n\n\n\n\n\n\n\nM\n\n(\nr\n,\nt\n=\n0\n)\n\n=\n\n\nM\n\n\n0\n\n\n\n\n\n\n\n\n\n\n\u2202\nM\n\n(\nr\n=\n0\n,\nt\n)\n\n\n\n\u2202\nr\n\n\n=\n0\n\n\n\n\n\n\nM\n\n(\nr\n=\nR\n,\nt\n)\n\n=\n\n\nM\n\n\nb\n\n\n\n\n\n\n The set of equations can be rewritten in dimensionless from via definition of the following dimensionless quantities: \n\n\n(32)\n\n\n\n\nM\n\n\n\u0302\n\n\n=\n\n\nM\n\n\n\n\nM\n\n\nb\n\n\n\n\n\n\n\n\n(33)\n\n\n\n\nr\n\n\n\u0302\n\n\n=\n\n\nr\n\n\nR\n\n\n\n\n\n\n(34)\n\n\n\n\nt\n\n\n\u0302\n\n\n=\n\n\nD\n\n\n\n\nR\n\n\n2\n\n\n\n\nt\n\n\n\n\n(35)\n\n\n\n\nM\n\n\nT\n\n\n=\nR\n\n\n\n\nk\n\n\n\n\nD\n\n\ne\n\n\n\n\n\n\n\n\n\n\n\n\n\n\nM\n\n\nT\n\n\n is a Thiele modulus and represents the ratio of reaction rate to diffusion rate. By using these dimensionless quantities in Eqs.\u00a0(31), they change to: \n\n\n(36)\n\n\n\n\n\u2202\n\n\nM\n\n\n\u0302\n\n\n\n(\n\n\nr\n\n\n\u0302\n\n\n,\n\n\nt\n\n\n\u0302\n\n\n)\n\n\n\n\u2202\n\n\nt\n\n\n\u0302\n\n\n\n\n=\n\n\n1\n\n\n\n\n\n\nr\n\n\n\u0302\n\n\n\n\n2\n\n\n\n\n\n\n\u2202\n\n\n\u2202\n\n\nr\n\n\n\u0302\n\n\n\n\n\n\n\n\n\n\nr\n\n\n\u0302\n\n\n\n\n2\n\n\n\n\n\u2202\n\n\nM\n\n\n\u0302\n\n\n\n(\nr\n,\nt\n)\n\n\n\n\u2202\n\n\nr\n\n\n\u0302\n\n\n\n\n\n\n\u2212\n\n\nM\n\n\nT\n\n\n2\n\n\n.\n\n\nM\n\n\n\u0302\n\n\n\n(\n\n\nr\n\n\n\u0302\n\n\n,\n\n\nt\n\n\n\u0302\n\n\n)\n\n\n\n\n\n\n\n\n\nM\n\n\n\u0302\n\n\n\n(\n\n\nr\n\n\n\u0302\n\n\n,\n\n\nt\n\n\n\u0302\n\n\n=\n0\n)\n\n=\n\n\n\n\nM\n\n\n\u0302\n\n\n\n\n0\n\n\n\n\n\n\n\n\n\n\n\u2202\n\n\nM\n\n\n\u0302\n\n\n\n(\n\n\nr\n\n\n\u0302\n\n\n=\n0\n,\n\n\nt\n\n\n\u0302\n\n\n)\n\n\n\n\u2202\n\n\nr\n\n\n\u0302\n\n\n\n\n=\n0\n\n\n\n\n\n\n\n\nM\n\n\n\u0302\n\n\n\n(\n\n\nr\n\n\n\u0302\n\n\n=\n1\n,\n\n\nt\n\n\n\u0302\n\n\n)\n\n=\n1\n\n\n\n\n\n Eq.\u00a0(36) has been solved with an independent PDE-solver (Matlab pdepe solver). Fig.\u00a03 compares the results of MGM with the Matlab solver. As is clearly illustrated, the results of the MGM are in almost perfect agreement with the Matlab solver over the time, from start of the reaction till the steady state condition has been reached (which is about one minute in this case). This confirms that the model works correctly and there are no errors in the calculation methods.\n\n\nThe results of MGM simulations are evaluated by means of a sensitivity analysis and the assessing importance of different parameters and operating conditions in TCD. The temperature range used in our models is limited to the operating conditions that are used to derive the kinetic coefficients of Eqs.\u00a0(6) and (12) (temperature: 500\u2013650\n\n\n\u00b0\nC\n\n and maximum hydrogen fraction: 10%) to ensure of the validity of results\u00a0[7]. Table\u00a02 provides the most important parameters used in the model for the following cases, unless otherwise stated or the importance of the parameter is evaluated.\n\nTo assess the impact of internal mass transfer limitations, the diffusivity was altered to one thousand times higher and lower values than the (base case) values stated in Table\u00a02. The results are summarized in Fig.\u00a04 and reveal that lowering the internal mass transfer rate does not affect the carbon yield. Hence, for the conditions of Table\u00a02 the effect of diffusion limitation is negligible. On the other hand, if the mass transfer limitation increases to one thousand times higher, the carbon yield decreases by about 35%.The importance of internal diffusional resistance is also confirmed by the Weisz\u2013Prater criterion (Eq.\u00a0(37)) that estimates the importance of the diffusion on the reaction rates in heterogeneous catalytic reactions\u00a0[30]. In the normal case, \n\n\n\nC\n\n\nW\nP\n\n\n=\n9.02\n\u00d7\n10\n\n\n\n\u22124\n\n\n\u226a\n1\n\n which means that internal mass transfer does not influence the production rate of carbon. However, in the case with one thousand times lower effective diffusion coefficient, \n\n\n\nC\n\n\nW\nP\n\n\n=\n0.902\n\n which implies that for such low diffusion coefficients, internal mass transfer limitation is not negligible anymore compared to the reaction rate. \n\n(37)\n\n\n\n\nC\n\n\nW\nP\n\n\n=\n\n\n\n\nr\n\n\n0\n\n\n\u03c1\n\n\nR\n\n\n2\n\n\n\n\n\n\nD\n\n\ne\n\n\n\n\nM\n\n\nb\n\n\n\n\n\n\n\n\nThe same procedure was applied to assess the role of the internal heat transfer limitation, by changing the thermal conductivity of the solid material composing the macroparticle. The results are presented in Fig.\u00a05. Since the carbon yield is not affected by lowering the heat transfer limitation, for the conditions of Table\u00a02 the heat transport resistance is negligible in comparison with other factors. However, in the case with 1000 times higher heat transfer limitation, the carbon yield is decreased dramatically. In this case, the temperature in the macroparticle increases relatively slowly, and as a result the reaction rate and therefore the slope of the curve increases gradually.In addition, there can prevail external heat and mass transfer limitations in the thin film around the macroparticle. However, it was observed that even with the highest external heat and mass transfer limitation, meaning a macroparticle in a stagnant gas phase (\n\nN\nu\n=\n2\n\n and \n\nS\nh\n=\n2\n\n) and with larger particle sizes (\n\n1000\n\n\n\n\u03bc\nm), the production rate of carbon is not reduced.\nThese observations regarding mass transfer importance and their effect on the TCD process are in agreement with literature findings derived from both experiments and the Weisz\u2013Prater criterion\u00a0[5,22].\nFig.\u00a06 illustrates in logarithmic scale how much the carbon yield changes if the initial reaction rate changes by a factor 1000. Reduction of the reaction rate leads to decrease by a factor 1000 in carbon yield. This finding is another confirmation of the fact that the reaction is the rate-determining step compared to the mass and heat transfer limitations (Section\u00a05.1). On the other hand, if the reaction is one thousand times faster, the carbon yield increases around 450 times. This means that in this case mass and heat transfer limitations become also important which again is in agreement with Section\u00a05.1.\n\nAs can be seen in Fig.\u00a07 adding inert gas (which means lowering the methane fraction) decreases the carbon yield. On the other hand, for a given fraction of methane, increasing the fraction of hydrogen leads to lower initial reaction rate and higher durability of the catalyst against deactivation. These effects are presented in Fig.\u00a08. These two effects together lead to higher carbon yield, however, in comparison a pure methane feed yields a higher amount of carbon in a shorter amount of time.\n\n\nTemperature has two opposing effects in the TCD process. On one hand, higher temperature leads to a higher initial reaction rate and therefore a higher carbon production rate. On the other hand, increasing the temperature results in faster deactivation of the catalyst and lowers the final carbon yield. These two phenomena are illustrated in Fig.\u00a09. At high temperatures deactivation proceeds more suddenly instead of gradual deactivation at lower temperatures. Thus, curves of 600\n\n\n\u00b0\nC\n\n and 650\n\n\n\u00b0\nC\n\n have a very short flat part, rather than longer flat tail.Increasing the temperature between 500\n\n\n\u00b0\nC\n\n to 650\n\n\n\u00b0\nC\n\n leads to a lower carbon yield due to the increased deactivation rate. However, it should be noted that this slightly lower amount of carbon is produced in a significantly shorter period of time. Therefore, in the examined conditions and with the used kinetic model, the optimum operating conditions will depend on economic considerations. It should be noted that using a different catalyst (and as a result, different kinetic models) may change this optimum condition.The number of micro grain layers in the macroparticle is a model parameter that is not straightforward to measure or estimate, as previous parameters were. As Fig.\u00a010 shows, this number has a significant impact on the carbon yield. The effect of the number of grain layers is not linear and becomes stronger with an increase in the number of layers. Although physically the number of micro grain layers can be translated to the specific surface area of the macroparticle, the internal structure of an actual catalyst particle is more complex than the structure defined by many layers of identical spheres. Therefore, the number of grain layers will be used as the tuning parameter of the model against validated data.\n\n\nA Multi-Grain Model has been developed to model the heat and mass transfer inside macroparticles coupled with the decomposition reaction of methane. The reaction rate model and deactivation factor from Amin\u00a0[7] are used, however, the model is suitable for the use of other kinetic models which can be easily accommodated.The effect of operating conditions and model parameters has been investigated by sensitivity analyses and it was found that the heat and mass transfer rates do not limit the carbon production rate and consequently the reaction is the rate-limiting step of the process. This fact is in agreement with experimental findings respected in the literature. However, if a catalyst is made with one thousand times higher ratio of kinetics rate to the mass and heat transfer rates (either by increasing the reaction rate or decreasing the mass and heat transfer rates), the heat and mass transfer limitations will affect the final carbon yield.It was found that, the presence of hydrogen causes a decrease in the reaction rate, however a higher carbon yield is achieved due to delayed deactivation of the catalyst. Moreover, increasing the operating temperature leads to a faster initial reaction rate and faster catalyst deactivation and hence an optimal, process dependent, temperature exists.In the future, it would be interesting to conduct experimental tests to tune, validate and further develop the model. The findings of the current article can be used in CFD models and enable researchers and industry to model, design and predict the behavior of multiple particles in the fixed or fluidized bed reactors employing TCD.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 Netherlands Organisation for Scientific Research (NWO) and the Netherlands Ministry of Economic Affairs\n .", "descript": "\n                  ThermoCatalytic Decomposition of methane (TCD) is studied as a method to convert natural gas into hydrogen and functional carbon. In these processes the carbon typically formed on top of a catalyst phase leading to particle growth. Therefore, the development of a particle growth model is necessary to understand the limitations of thermocatalytic decomposition of methane and to assess optimal parameters and process conditions. In this paper, a particle growth model is presented to describe the growth of functional carbon on the catalyst particle. This coupled model requires kinetic equations and information on deactivation rates which have been studied from literature. The morphology of the particle changes due to carbon formation, which leads to eventual deactivation. Therefore, these kinetic expressions are coupled to a particle growth model based on the analogy with the growth of particles in polyolefin production. To combine the effects of particle growth, kinetics, and internal heat and mass transfer, the Multi-Grain Model (MGM) was used. Results confirm that with the currently available catalysts the carbon yield is not affected by heat and mass transfer limitations, however, with the availability of more active catalysts these limitations will become important. Temperature, however, has a significant role in that it regulates the kinetic rate and thus growth rate, which in turn influences the catalyst deactivation. The optimum temperature for the production of nano-carbon, within a reasonable process time, therefore sensitively depends on the choice of catalyst.\n               "}
{"full_text": "In the past century, petroleum has made important contributions to the rapid development of economy and society. As a result of the gradual depletion and non-renewability of petroleum, the use of non-petroleum resources instead of petroleum is of great significance. This alternative route can be generally achieved through syngas (the mixture of H2 and CO) chemistry because syngas cannot only be derived from non-petroleum carbon resources such as natural gas, coal, biomass, and CO2 by gasification, water-gas-shift reactions, or reverse-water-gas-shift reactions (Scheme 1\n) but also directly or indirectly synthesize a wide variety of fuels and basic chemicals.\n1\u20133\n At present, the industrial technology for syngas production is relatively mature, but there are still many challenges in highly selectively and stably converting syngas into target products.\n4\u20136\n\nThe primary transportation fuel gasoline, which contains hydrocarbons with 5\u201311 carbons (C5\u201311) and is almost entirely derived from petroleum now, can also be produced from non-petroleum syngas.\n7\n As illustrated in Scheme 1, syngas can be initially converted into methanol via methanol synthesis (MS) reaction and then transformed into gasoline via a methanol-to-gasoline (MTG) reaction. Over 30 years ago, Mobil MTG technology was industrialized in New Zealand.\n8\n To reduce investment and save energy consumption, directly synthesizing gasoline from syngas (STG) without separating intermediate products has been receiving extensive attention.\n4\n It is well known that gasoline can be obtained from Fischer-Tropsch (F-T) synthesis (mode I). However, because of the limitation of the Anderson-Schulz-Flory distribution, the selectivity of C5\u201311 is less than 50%.\n9\n Moreover, the carbon chain-growth mechanism determines that n-paraffins with low-octane value are predominant.\n10\n Zeolites (or molecular sieves) have uniform pore structures and acidity, which can not only limit the distribution of hydrocarbons according to the size of the micropores but also catalyze the isomerization of n-paraffins.\n1\n\n,\n\n2\n Therefore, they are often used to mix with F-T catalysts (mode II) to upgrade F-T hydrocarbons. However, it is not easy for zeolite to effectively convert the inert short-chain hydrocarbons, such as methane and ethane, or the heavy hydrocarbons larger than its pores, resulting in a low C5\u201311 selectivity.\n6\n Enhancing diffusion of heavy hydrocarbons by increasing the mesoporosity of zeolites or improving the proximity of acid sites and metal by well designing core-shell structures can help alleviate this problem. For example, Tsubaki and colleagues obtained approximately 74% C5\u201311 at 34% CO conversion on mesoporous Y zeolite combined with Co,\n11\n while Khodakov and colleagues acquired approximately 61% C5\u201312 at 37% CO conversion over core-shell ZSM-5/Ru/ZSM-5.\n10\n Unlike the mode II catalyst, where hydrocarbons are initially produced on the F-T catalyst, the mode III catalyst, which is made by a mixture of MS and zeolite catalysts, yields hydrocarbons at acid sites of zeolite micropores. As a result, heavy hydrocarbons are difficult to generate due to the limitation of micropores. Conventional CuZnAl (CZA) MS catalyst is not suitable for preparing the mode III catalyst because its optimal reaction temperature (473\u2013543 K) cannot effectively start the reaction catalyzed by zeolite.\n12\n\n,\n\n13\n Therefore, high-temperature MS catalysts have usually been selected. For example, Bao and colleagues recently reported a 76.7% gasoline selectivity with 20.3% CO conversion at 633 K over ZnMnO\nx\n-SAPO-11 composite catalyst.\n14\n Compared with the mode II catalyst, the mode III catalyst tends to achieve high C5\u201311 selectivity, but because of the low activity of the MS catalyst at high temperature, the conversion efficiency of the latter is significantly lower than that of the former. Taking into account the inconsistency of the reaction temperature and lifetime of the metal (oxides) and zeolite catalysts, placing them in two independent reactors in tandem (dual-bed) is expected to achieve better performance.\n15\u201318\n Just like the mode II catalyst, the mode IV catalyst, which contains an F-T catalyst and zeolite separately in a dual-bed reactor, can also adjust the distribution of F-T products. Ding and colleagues recently reported that CO conversion and C5\u201311 selectivity could both achieve 67% over the dual-bed catalyst Fe3O4@MnO2/H-ZSM-5.\n19\n Besides the four types of catalysts mentioned above, the dual-bed mode V catalyst, which consists of a syngas-to-DME (STD) catalyst and DME-to-gasoline (DTG) zeolite catalysts in separate beds, can also transform syngas to gasoline. The STD process is very good at converting syngas, which helps to improve the efficiency.\n20\n In 1987, the Haldor Tops\u00f8e TIGAS process, based on the mode V catalyst, was successfully demonstrated in Houston, Texas.\n21\n\n,\n\n22\n In addition, the Karlsruhe Institute of Technology has developed a similar technology called the bioliq process.\n23\n Although some technologies for this dual-bed mode V process have existed, still few studies obtain high C5\u201311 selectivity at high syngas conversion or the mechanism of the zeolite characteristics on selectivity and stability.Here, we report an 80.6% C5\u201311 selectivity without CO2 at 86.3% CO conversion over a dual-bed catalyst (CZA\u00a0+ Al2O3)/N-ZSM-5(97) that includes a STD catalyst CZA\u00a0+ Al2O3 in the upper bed and a DTG catalyst N-ZSM-5(97) in the lower bed (Figure\u00a0S1). A low amount of acid and the nano-sized structure of the ZSM-5 zeolite are beneficial to C5\u201311 selectivity and stability, respectively. The deactivation mechanism is also explored and discussed.The performance of syngas conversion on various catalysts was compared at 573 K and 3.0 MPa with identical syngas feed. As shown in Figure\u00a01\nA, CZA\u00a0+ Al2O3 is a typical STD catalyst that gives a 90.4% DME selectivity at 64.5% CO conversion. When this STD catalyst is mixed with N-ZSM-5(97) (nano-sized ZSM-5 with Si/Al\u00a0= 97), the CO conversion and CO2 selectivity are significantly improved, and a considerable amount of the DME is converted to hydrocarbons. The selectivity of C1\u20132 light hydrocarbons is as high as 20.0%, whereas the selectivity of C5\u201311 liquid hydrocarbons without CO2 is only 26.1%. Increasing the proximity by three-component grinding and pressing leads to further decrease the selectivity of C5\u201311 hydrocarbons (Figure\u00a0S2). It is interesting to find that the reaction result of the dual-bed catalyst (CZA\u00a0+ Al2O3)/N-ZSM-5(97), which is configured by CZA\u00a0+ Al2O3 on the upper bed and N-ZSM-5(97) on the lower bed, is quite different from the mixed catalyst (CZA\u00a0+ Al2O3+N-ZSM-5(97)). The DME and MeOH are completely converted, the C5\u201311 selectivity reaches up to 75.9%, and the C1\u20132 selectivity is as low as 4.6% over (CZA\u00a0+ Al2O3)/N-ZSM-5(97). The effect of the structure and Si/Al ratio of ZSM-5 catalysts in the lower bed on the performance of the STG reaction was explored. The conversion of CO over the four dual-bed catalysts in Figure\u00a01B is close because the upper-bed STD catalysts and the reaction conditions are the same. However, their C5\u201311 selectivity and stability are quite different. As shown in Figures 1B and S3A\u2013S3D, the dual-bed catalyst with the nano-sized N-ZSM-5(97) or N-ZSM-5(21) has much better stability than the micro-sized M-ZSM-5(116) or M-ZSM-5(18), respectively. Furthermore, the dual-bed catalyst with N-ZSM-5(97) or M-ZSM-5(116) with a high Si/Al ratio exhibits considerably higher C5\u201311 selectivity than that with N-ZSM-5(21) or M-ZSM-5(18) with a lower Si/Al ratio, respectively. This suggests that the nano-sized structure, which generally means good diffusion ability,\n24\n is beneficial to extend the lifetime; meanwhile, the high Si/Al ratio, which usually represents a low amount of acid, is conducive to inhibit the formation of light hydrocarbons. It should be mentioned that the above results were obtained by a high-throughput reactor, and the reaction temperatures (573 K) of the upper and lower beds were the same. In fact, such a high temperature is not suitable for the STD reaction (Figure\u00a0S4). To gain better results, we studied in detail the STG reaction over the dual-bed (CZA\u00a0+ Al2O3)/N-ZSM-5(97) catalyst at different reaction temperatures for the two beds.The STG reaction on (CZA\u00a0+ Al2O3)/N-ZSM-5(97) was investigated at T (upper bed)\u00a0= 533 K, T (lower bed)\u00a0= 593 K, P\u00a0= 3.0 MPa, H2/CO\u00a0= 2, and gas hourly space velocity (GHSV)\u00a0= 1,500\u00a0mL g\u22121 h\u22121. As shown in Figure\u00a02\nA, the selectivity of C5\u201311, C3\u201311, aromatics, or CO2 was kept at approximately 79%, 34%, 98%, or 32%, respectively, with 87% CO conversion. The light C1\u20132 selectivity was less than 1.7%. The activity of this dual-bed catalyst did not decrease at all within 110\u00a0h on stream. The detailed distribution of gasoline-range C5\u201311 can be observed in Figure\u00a02A. The selectivity of iso-paraffins reached approximately 40%, whereas the selectivity of olefins was less than 2%. Moreover, the iso/n-paraffin ratio reached 18, which is much higher than that in F\u2013T products.\n6\n\n,\n\n25\u201328\n\nFigures S5 and S6 indicate that increasing GHSV or H2/CO ratio had little effect on C5\u201311 selectivity. Figure\u00a02C shows that increasing the pressure significantly increased the CO conversion without affecting the C5\u201311 selectivity, which is extremely valuable for improving the efficiency of the STG process. Notably, at P\u00a0= 4.0 MPa, H2/CO\u00a0= 2, and GHSV\u00a0= 3,000\u00a0mL g\u22121 h\u22121, the CO conversion was as high as 86.3%, while the selectivity of C5\u201311, C3\u201311, and CO2 reached 80.6%, 98.2%, and 30.6%, respectively. By calculation, the space time yield was up to 0.28\u00a0g C5\u201311 per hour per gram of dual-bed catalyst. The main research progress concerning the conversion of syngas to C5\u201311 liquid hydrocarbons (including aromatics) in the past 3 years is listed in Table S1.\n10\n\n,\n\n11\n\n,\n\n14\n\n,\n\n19\n\n,\n\n29\u201336\n It is apparent that, compared with F-T-based catalysts (modes I, II, or IV), this dual-bed catalyst (mode V) has significant advantages in suppressing the low-value light C1\u20132 and achieving a high C5\u201311 and iso/n-paraffin ratio. Besides, compared with MS-catalyst-based physically mixed catalysts (mode III), this mode V catalyst has remarkable advantages in achieving high CO conversion and regenerating the deactivated catalyst after long-term operation. The effect of the reaction temperature on the lower bed was studied. It can be seen from Figure\u00a02D that increasing the temperature is harmful to the generation of C5\u201311.Note that the dual-bed catalyst (CZA\u00a0+ Al2O3)/N-ZSM-5(97) for all of them was configured by 1.5\u00a0g CZA\u00a0+ Al2O3 (upper bed) and 1.0\u00a0g N-ZSM-5(97) (lower bed).The X-ray diffraction (XRD) patterns in Figure\u00a03\nA indicate that the four ZSM-5 zeolites in this study possess a typical pure MFI structure. The XRD patterns in Figure\u00a0S7 suggest that CZA and Al2O3 exhibit the structures of a conventional industrial CuZnAl MS catalyst and a \u03b3-Al2O3 catalyst, respectively.\n37\n The NH3-TPD results in Figure\u00a03B show that the acid amount follows the order M-ZSM-5(116)\u00a0< N-ZSM-5(97)\u00a0< N-ZSM-5(21)\u00a0< M-ZSM-5(18), which is exactly the reverse order of the Si/Al ratio (Figures 3C\u20133F). The NH3-TPD result in Figure\u00a0S8 proves that Al2O3 has an acid property, which should be derived from Lewis acid sites.\n20\n The XRF results listed in Table S2 show that the Cu/Zn/Al molar ratio is 4.8:1.8:1 for CZA. It can be observed from Figures 3C\u20133F that N-ZSM-5(97) and N-ZSM-5(21) are composed of approximately 200 and 50\u00a0nm particles, respectively, while M-ZSM-5(116) and M-ZSM-5(18) are both made up of 2\u20135\u00a0\u03bcm hexagonal crystals. Table S3 shows that N-ZSM-5(97) has the highest Brunauer-Emmett-Teller (BET) area and external area. Combined with the results in Figures 1B, S3, and 3C\u20133F, there is no doubt that reducing the crystal size of the lower-bed ZSM-5 zeolite will considerably prolong the lifetime of the overall dual-bed catalyst. In addition, it can be inferred from Figures 2B and 3B that, whether using nano- or micro-structured ZSM-5 zeolite, low acid content can facilitate the formation of C5\u201311 and depress the generation of light hydrocarbons. Generally, the growth of the carbon chains largely depends on the oligomerization of the initial light olefins.\n38\n However, the hydrogenation of light olefins, which can be catalyzed by the acid sites (especially Br\u00f8nsted acids) of H-form ZSM-5 zeolite,\n39\n\n,\n\n40\n is disadvantageous. Compared with oligomerization, a higher acid content for ZSM-5 zeolite can be more beneficial to the hydrogenation, which results in a lower C5\u201311.The four ZSM-5 zeolite catalysts in the lower bed after reaction shown in Figure\u00a01B were analyzed by the thermogravimetric method. As presented in Figure\u00a04\nA, their weight losses follow the order N-ZSM-5(97)\u00a0< N-ZSM-5(21)\u00a0< M-ZSM-5(116)\u00a0< M-ZSM-5(18). This demonstrates that for ZSM-5 zeolites with approximate Si/Al ratios, nano-sized structures are more resistant to coke, while for ZSM-5 zeolites with similar structures, a low Si/Al ratio (or high acid content) is ready to cause carbon deposits. Also, the catalytic results of a lower-bed ZSM-5 catalyst with a particle size of approximately 500\u00a0nm also prove that decreasing the zeolite particle size is beneficial to reduce coke and prolong lifetime (Figures S9A\u2013S9D). During syngas conversion reactions, after the zeolite catalyst is mixed with the metal catalyst, the coke formation rate will be generally significantly reduced, and the catalyst stability will be greatly improved.\n41\n\n,\n\n42\n As shown in Figure\u00a04A, the weight loss of the spent N-ZSM-5(97) in the dual-bed catalyst is close to that in the mixed catalyst, which implies that the dual-bed catalyst (CZA\u00a0+ Al2O3)/N-ZSM-5(97) has the potential for long-term use. After reaction, the four ZSM-5 zeolite catalysts in the lower bed were dissolved by hydrofluoric acid, and then the retained organic species were extracted by dichloromethane and analyzed by gas chromatography-mass spectrometry. As presented in Figure\u00a04C, the content of aromatics (species 1, 2, 5, 6, and 8) with no more than ten carbons (or no larger than tetramethyl-benzene) in the spent N-ZSM-5(97) is much less than that in the others. However, the selectivity of aromatics for (CZA\u00a0+ Al2O3)/N-ZSM-5(97) is substantially higher than that for the other three dual-bed catalysts in Figures S3A\u2013S3D. This indicates that the products no larger than the size of micropores (0.53\u00a0\u00d7 0.56\u00a0nm) for MFI topology are easily diffused out of N-ZSM-5(97). It also can be seen that the naphthalene derivatives (species 14\u201316) are hardly generated for N-ZSM-5(97). These polycyclic aromatics are too large to block micropores and cause catalyst deactivation.\n43\n Besides methylbenzenes and polycyclic aromatics, a considerable amount of oxygenates, which include the derivatives of 2-cyclopenten-1-one (species 3\u20136) and phenol (species 13), can be observed in Figure\u00a04B. The mass spectra of the typical oxygenates are shown in Figures 4C and S10. Our previous researches have proved that the 2-cyclopenten-1-one species, which can be produced via a series of C\u2013C bond formation reactions, such as carbonylation, aldol, prins, and hydroacylation, are important intermediates for the formation of single-ring aromatics.\n44\u201346\n Compared with the spent N-ZSM-5(97), the spent N-ZSM-5(21) obviously contains more 2-cyclopenten-1-one species. However, the selectivity of aromatics over (CZA\u00a0+ Al2O3)/N-ZSM-5(97) is apparently higher than that over (CZA\u00a0+ Al2O3)/N-ZSM-5(21) (Figures S3A and S3B). This means that there are other ways to generate aromatics. Recently, Wei and colleagues found that phenol species formed by the aldol cycle in the syngas atmosphere can be transformed to aromatics.\n47\n The amount of phenol species in spent N-ZSM-5(97) is more than that in spent N-ZSM-5(21), which is positively related to the selectivity of the aforementioned aromatics. This suggests that phenol species are likely to act as intermediates for aromatics. In the spent N-ZSM-5(97) of the mixed catalyst (CZA\u00a0+ Al2O3\u00a0+ N-ZSM-5(97)), 2-cyclopenten-1-one species can be hardly found, whereas phenol species can be detected. This mixed catalyst can actually produce few aromatics (6%), which also suggests that the speculation above is reasonable. Although the mechanism of synthesizing single-ring aromatics from these oxygenates, such as 2-cyclopenten-1-one and phenol species, has been well explored by the previous works of ourselves and others,\n15\n\n,\n\n44\n\n,\n\n45\n\n,\n\n47\n the pathway of generating polycyclic aromatics, which are the main factors leading to catalyst deactivation, has still not been revealed. The weight loss for the spent M-ZSM-5(18) is more than twice that for the spent N-ZSM-5(97) (Figure\u00a04A); however, the amount of their soluble organics is relatively close (Figure\u00a04B). This demonstrates that there is a large amount of insoluble heavy carbon deposits, which are generally polycyclic aromatics,\n43\n in the spent M-ZSM-5(18). Interestingly, some 2,3-dihydro-1H-inden-1-one oxygenates (species 11 and 12) can be definitely detected and identified in the spent M-ZSM-5(18) (Figures 4B, 4D, and S3D). We consider that they can be generated through carbonylation and condensation of phenol species and transformed to polycyclic aromatics by reactions such as isomerization and dehydration because this is similar to the mechanism for the formation and conversion of 2-cyclopenten-1-one species.\n44\u201346\n\nIn summary, high and selective conversion of syngas to gasoline-range C5\u201311 liquid hydrocarbons can be simultaneously achieved over a dual-bed catalyst (CZA\u00a0+ Al2O3)/N-ZSM-5(97) that consists of a STD catalyst CZA\u00a0+ Al2O3 (a mixture of CuZnAl MS catalyst and acidic \u03b3-Al2O3 catalyst) in the upper bed and a DTG catalyst N-ZSM-5(97) (nano-sized H-ZSM-5 zeolite with Si/Al ratio\u00a0= 97) in the lower bed. The selectivity of C5\u201311 and C3\u201311 can reach 80.6% and 98.2%, respectively, along with 86.3% CO conversion at T (upper bed)\u00a0= 533 K, T (lower bed)\u00a0= 593 K, P\u00a0= 4.0 MPa, H2/CO\u00a0= 2, and GHSV\u00a0= 3,000\u00a0mL g\u22121 h\u22121. This dual-bed catalyst exhibits an excellent stability during a 110-h test. The iso/n-paraffin ratio in the C5\u201311 is up to 18. By comparing four lower-bed ZSM-5 zeolite catalysts with various particle sizes and acid content, we found that the nano-sized structure is beneficial to reduce coke and prolong lifetime; meanwhile, the low acid content is advantageous to increase C5\u201311 selectivity. The 2,3-dihydro-1H-inden-1-one species can be definitely detected and identified in the spent lower-bed micro-sized M-ZSM-5(18). They are regarded as intermediates to generate polycyclic aromatics, which generally lead to catalyst deactivation. The dual-bed catalyst (CZA\u00a0+ Al2O3)/N-ZSM-5(97) suggests a promising application in producing gasoline from syngas.Full experimental procedures are provided in the supplemental information.Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Professor Zhongmin Liu (liuzm@dicp.ac.cn).This study did not generate new unique reagents.The published article includes all datasets generated or analyzed during this study.We acknowledge financial support from the National Natural Science Foundation of China (grant nos. 21978285, 21991093, and 21991090) and the \u201cTransformational Technologies for Clean Energy and Demonstration\u201d Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDA21030100). We thank Weichen Zhang for assistance in the experiments.Y.N. designed and performed the experiments, analyzed the data, and wrote the manuscript. K.W. discussed the results. W.Z. and Z.L. supervised the study, discussed the results, designed the experiments, and revised the manuscript.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.checat.2021.02.003.\n\n\nDocument S1. Supplemental experimental procedures, Figures S1\u2013S10, and Tables S1\u2013S3\n\n\n\n\n\nDocument S2. Article plus supplemental information\n\n\n\n", "descript": "\n                  Achieving high conversion of syngas to fuels and basic chemicals with excellent selectivity and stability remains a challenge. Here, we report an 80.6% selectivity of gasoline-range C5\u201311 hydrocarbons at 86.3% CO conversion over a dual-bed catalyst (CZA\u00a0+ Al2O3)/N-ZSM-5(97) that consists of an upper-bed syngas-to-dimethyl ether (DME) catalyst (CZA\u00a0+ Al2O3) and a lower-bed DME-to-gasoline catalyst (nano-sized N-ZSM-5(97) zeolite). This dual-bed catalyst exhibits an excellent stability in a 110-h reaction test. The iso/n-paraffin ratio in the C5\u201311 is up to 18. For the lower-bed zeolite catalyst, the nano-sized structure is beneficial to reduce coke and prolong lifetime; meanwhile, the low acid content is advantageous to increase C5\u201311 selectivity. The 2,3-dihydro-1H-inden-1-one species can be definitely detected and identified in the spent lower-bed micro-sized M-ZSM-5(18) catalyst. They are regarded as intermediates to generate polycyclic aromatics, which generally lead to catalyst deactivation. The dual-bed catalyst (CZA\u00a0+ Al2O3)/N-ZSM-5(97) suggests a promising application in producing gasoline from syngas.\n               "}
{"full_text": "With the current energy crisis and increasingly serious environmental issues, it is very inevitable to search for renewable and sustainable energies as alternatives to fossil fuels [1]. In the portfolio of renewable energies development, H2 energy from formic acid (FA) is generally regarded as one of the most promising paradigms. It has recently attracted tremendous research passion because FA with high H2 capacity of 4.3\u00a0wt% has high stability, low-toxicity, low-flammability, and biodegradability [2]. It is universally acknowledged that H2 evolution from FA involves a dehydrogenation pathway \n\n\n(\n\nH\nC\nO\nO\nH\n\u2192\nC\n\nO\n2\n\n\n+\n\n\nH\n2\n\n\n)\n\n\n [3] and a dehydration reaction \n\n\n(\n\nH\nC\nO\nO\nH\n\u2192\nC\nO\n\n+\n\n\nH\n2\n\nO\n\n)\n\n\n [4]. The produced CO in the later undesired reaction is highly poisonous to the catalysts. Thus, the main challenge in the FA decomposition for H2 production is to develop a highly efficient and selective catalyst to avoid the undesired CO generation. To date, supported Pd-based heterogeneous catalysts have been proven to be the most efficient for H2 evolution from FA system [3,5\u20138]. However, the sluggish kinetics of H2 evolution over Pd-based catalysts cannot satisfy the industrial applications [8]. From this point of view, the development of Pd-based catalysts with sufficient active sites and selectivity for H2 evolution from FA is of great importance to enter the hydrogen economy era.Supported Pd-based catalysts, including mono-, bi- and tri-metals, have been intensively investigated for FA decomposition in the literature [9\u201311]. The fact is widely regarded that the addition of exotic metals and the nature of the support matrix are critical to dictate the performance of catalyst [12,13]. On one hand, for the Pd-based catalysis FA decomposition reactions, the most common additives are Au (Pd\u2013Au) and Ag (Pd\u2013Ag) [13]. On the other hand, various supports have been demonstrated to be effective for the Pd-based catalysts, including carbon materials (activated carbon (AC), mesoporous carbon (MSC), reduced graphene oxidization (rGO), etc.), metal-organic frameworks (MOFs), metal oxides, mesoporous silica, and macroreticular basic resin [14\u201317]. However, the preparation procedure of above-mentioned Pd-based support materials is time-consuming and costly. Thus, it is of great interest to develop the supported Pb-based catalysts on cheap biomass resource matrix and to explore its catalytic performance for FA dehydrogenation.Herein, the present work proposes a facile and cost-effective approach to load PdAg bimetal nanoparticles on cellulose modified with polyetherimide (PEI) as an efficient catalyst for H2 generation from FA decomposition in a sodium formate-free aqueous system. The cellulose was obtained from Eucalyptus biomass through 70% FA fractionation. Interestingly, the resultant cellulose-derived PdAg bimetallic catalyst (PdAg-PEI-FAC) is a core-shell-like structure and shows a high catalytic performance with the turnover frequency (TOF) value of 2875 h\u22121. The as-obtained catalyst has excellent stability toward FA decomposition with no loss of catalytic activity after five recycles. The remarkable catalytic activities of this catalyst result from high dispersion Pd and synergistic effects between the PdAg bimetallic system. In order to give more insight into FA fractionation, the properties of isolated lignin and hemicelluose fractions from Eucalyptus biomass through 70% FA fractionation were also characterized in this present work as Supporting information. In addition, the probable catalytic mechanism and recyclability of PdAg-PEI-FAC for H2 evolution from FA solutions were also evaluated. .\nEucalyptus wood was kindly gifted from Yingqiang Wei, a staff of Gaofeng Forest Farm, Guangxi province, China. The contents of cellulose, hemicellulose and lignin of Eucalyptu biomass were calculated to be approx. 42.2%, 12.6% and 31.8%, respectively, according to the methods detailed in our previous work [18]. Formic acid (FA, 88%) and polyetherimide (PEI, Mw\u00a0=\u00a070,000\u00a0g mol\u22121) were purchased from Aladdin Biochemical Technology Co., Ltd., Shanghai, China. Glucose standard (99.5%) was bought from Guangfu Fine Chemical Co. Ltd., Tianjin, China. 5-Hydroxymethyl furfural (5-HMF, 99%) and furfural (99%) standards were of chromatographic-grade and purchased from J & K Scientific GmbH, Pforzheim, Germany. All other reagents including PdCl3 (AR, Pd content\u00a0\u2265\u00a059%), AgNO3 (AR, \u2265 99.8%), CoNO3\u00b76H2O (AR, \u2265 98.5%), AuCl3\u00b7HCl\u00b74H2O (AR, Au conent\u00a0\u2265\u00a048.7%), NiNO3(AR, \u2265 99%), NaBH4 (AR, \u2265 99%) and AlCl3 (AR, \u2265 99.8%) were of analytic grade and bought from Sinopharm Chemical Reagent Co., Ltd, Beijing, China.The fractionation flow of Eucalyptus biomass through 70% FA is depicted in Scheme 1\n. For this FA fractionation processing, the resultant cellulose was modified with PEI for preparation of the cellulose-derived PdAg bimetallic catalyst for H2 evolution, while the properties of lignin fraction and hemicellulose upgrade were also investigated.In brief, 44\u00a0g of the milled Eucalyptus biomass powder along with 445\u00a0mL 70% FA were loaded into a 2\u00a0L round-bottom glass flask, which was placed into an aluminum heating module with a magnetic digital stirring hotplate (Gongyi, Henan, China). The weight/volume ratio of biomass solid weight (g) to FA liquid volume (mL) was set at 1 : 10 (w/v). The solid/liquid slurry was stirred at 1300\u00a0rpm and 130\u00a0\u00b0C. After 3\u00a0h reaction, the reaction mixture was cooled to room temperature and filtered using 0.45\u00a0\u03bcm filter paper to separate the solid residue cellulose and filtrate. Approx. 18\u00a0g of the solid residue cellulose fraction was obtained and assigned as FAC (formic acid cellulose). 400\u00a0mL of the black liquor filtrate containing FA, soluble lignin and carbohydrate hydrolysates was collected in a glass flask. After evaporation in vacuum conditions of \u22120.1\u00a0MPa and 50\u00a0\u00b0C, about 340\u00a0mL of FA could be recovered from the filtrate for re-usage. Once the FA was recoved, lignin was precipitated at the bottom of flask. Subsequently, 300\u00a0mL distilled water was used to wash the precipitated lignin in duplicate. Through filtration, approx. 9 g of brown lignin pellets were achieved, and approx. 450\u00a0mL of\u00a0the aqueous liquid consisting of hemicellulose and cellulose\u00a0hydrolysates (including C5 and C6 monomeric and oligomers) were collected. These carbohydrate hydrolysates could be\u00a0further converted into furans using AlCl3 as a Lewis catalyst.The synthesis procedure of polyetherimide (PEI) modified cellulose-derived Pd/Ag bimetallic catalyst (PdAg-PEI-FAC) is shown in Fig.\u00a01\na in Section 3.2. First of all, FAC (200\u00a0mg) was modified with 40\u00a0mL of 1.0\u00a0wt% PEI with Mw\u00a0=\u00a070,000\u00a0g\u00a0mol\u22121 to obtain PEI-FAC support. Then, the as-prepared PEI-FAC was homogeneously dispersed in 80\u00a0mL of deionic water, followed by the addition of 3.6\u00a0mL of Na2PdCl4 (20\u00a0mmol L\u22121) and 2\u00a0mL of AgNO3 (35\u00a0mmol L\u22121) solution with the Pd/Ag mass/weight ratio of 3.75/3.75. The mixture solution was stirred at room temperature and 500\u00a0rpm for 30\u00a0min. Subsequently, the sample was reduced with 0.213\u00a0mol NaBH4 for 1\u00a0h\u00a0at the stirring rate of 500\u00a0rpm. After filtration, the black solid residue was washed with distilled water several times to remove the unreacted metal ions and redundant NaBH4. After lyophilization, PdAg-PEI-FAC catalyst was achieved and labeled as Pd3.75Ag3.75-PEI-FAC. When metal Ag in Pd3.75Ag3.75-PEI-FAC was substituted with other metal elements (M\u00a0=\u00a0Co, Ni, and Au) with the Pd/M mass/weight ratio of 3.75/3.75, three other bimetallic catalysts were obtained and labeled as Pd3.75Co3.75-PEI-FAC, Pd3.75Ni3.75-PEI-FAC and Pd3.75Au3.75-PEI-FAC. In addition, by changing the PdAg compositions (Pd/Ag mass/weight ratio\u00a0=\u00a01.5/1.5, 2.5/2.5, 5/5 and 3.75/0), four relative counterpart bimetallic catalysts of Pd1.5Ag1.5-PEI-FAC, Pd2.5Ag2.5-PEI-FAC, Pd5Ag5-PEI-FAC and Pd3.75-PEI-FAC were also prepared.In order to demonstrate that PEI-FAC is a suitable support material, several counterpart support matrices, such as reduced oxidative graphite (rGO), nitrogen-doped carbon (N@C), and carbon blank (C) were used to load PdAg bimetals as catalysts and labeled as Pd3.75Ag3.75-rGO, Pd3.75Ag3.75-N@C and Pd3.75Ag3.75-C, respectively. All those catalysts were compared to evaluate their catalytic performance of H2 generation from FA solution.The crystalline structure of PdAg-PEI-FAC was characterized by Rigaku Ultima IV X-ray diffractometer (XRD, Rigaku, Japan) with Ni-filtered CuK radiation operated at 40\u00a0kV and 40\u00a0mA in the air, with an environmental humidity of around 60%. Samples were tested under a diffraction angle, 2, in the range of 5\u00b0 \u223c30\u00b0 with a step interval of 0.5\u00b0 min\u22121.The elemental distribution and chemical state of catalyst was analyzed by XPS (ESCALAB 250, Thermo Fisher Scientific, USA). The data was acquired using Monochromated Al Kalph (150\u00a0W), a pass energy of 200\u00a0eV for survey, 30\u00a0eV for high resolution scans. The analyzed area was 500\u00a0\u00d7\u00a0500\u00a0\u03bcm.The metal Pd and Ag contents in PdAg-PEI-FAC samples were detected by PS-4 inductively coupled plasma atomic emission analysis (ICP-AES) spectrometer (Baird Co., USA). The data was analyzed by software PLASMA\u2163. The data accuracy was up to 10\u00a0\u00b1\u00a00.5%.The pore properties and surface area of the PEI-FAC and Pd3.75Ag3.75-PEI-FAC were tested by Brunauer Emmett-Teller (BET) analyses, which were performed on Autosorb IQ Quantachrome (USA) using N2. The samples were degassed at 250\u00a0\u00b0C for 12\u00a0h before measurement.The morphology and particle size of Pd3.75Ag3.75-PEI-FAC and Pd3.75Ag3.75-FAC were detected by high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images on JEOL JEM-ARM200F (Japan) at 200\u00a0kV.To examine the catalytic performance of PdAg-PEI-FAC catalysts for H2 production from FA decomposition, the effects of four critical factors on H2 generation were investigated, including FA concentration (0.5\u00a0mol L\u22121, 1\u00a0mol L\u22121, 1.5\u00a0mol L\u22121, 2.0\u00a0mol L\u22121, 5.0\u00a0mol L\u22121 and 10\u00a0mol L\u22121), reaction temperature (30\u00a0\u00b0C, 40\u00a0\u00b0C, 50\u00a0\u00b0C and 60\u00a0\u00b0C), Pd/Ag composition (1.25/1.25, 2.75/2.75, 3.75/3.75 and 5.0/5.0\u00a0wt%) and support matrix (rGO, N@C, carbon, FAC-PEI). The total reaction system was fixed at 5\u00a0mL, and the loading of the catalyst was set at 100\u00a0mg. The quantity of total gas volume (Vgas, or \n\n\n\nV\n\n\n\nH\n2\n\n\n+\n\nC\n\nO\n2\n\n\n\n\n) produced from the reaction was collected using a 500\u00a0mL gas burette by a drainage method.Apart from the total gas volume (Vgas, or \n\n\n\nV\n\n\n\nH\n2\n\n\n+\n\nC\n\nO\n2\n\n\n\n\n), the catalytic activities of PdAg-PEI-FAC were also evaluated from the following two aspects, turnover frequency (TOF, h\u22121) and activated energy (E\n\na\n, kJ mol\u22121).TOF was calculated through Pd content detected by ICP-AES according to Eq. (1):\n\n(1)\n\n\nT\nO\nF\n=\n\n\nT\no\nt\na\nl\n\n\nv\no\nl\nu\nm\ne\n\no\nf\n\n\nH\n2\n\n\ng\ne\nn\ne\nr\na\nt\ni\no\nn\n\n\n(\n\nm\nL\n\n)\n\n\n\nR\ne\na\nc\nt\ni\no\nn\n\nt\ni\nm\ne\n\n\n(\nh\n)\n\n\u00d7\n\nP\nd\n\nc\no\nn\nt\ne\nn\nt\n\n\ni\nn\n\nt\nh\ne\n\nc\na\nt\na\nl\ny\ns\nt\n\n\n\n(\ng\n)\n\n\n\n\n\n\n\n\nEa was calculated according to Arrhenius equation (Eq. (2)):\n\n(2)\n\n\nln\n\nT\nO\nF\n=\nln\n\nA\n\u2212\n\n\n\nE\n\u03b1\n\n\n\nR\nT\n\n\n\n\n\nwhere, A means frequency factor, R is gas constant, R\u00a0=\u00a08.314\u00a0J K\u22121 mol\u22121, T is reaction temperature, K. The value of Ea is obtained from the slope of the fitting straight line.Composition analyses of CO2, H2 and CO were conducted on a SP-2000 (Beijing Beifen Ruili Analytical Instrument Co., Ltd.). The compositions of H2 and CO2 were determined by a GC spectrum using a thermal conductivity detector (TCD), while the CO content was detected by a GC spectrum using a flame ionization detector (FID)-Methanator.The statistical analyses were determined using origin 10.0 (OriginLab Co., USA), and the final data were expressed by average\u00a0\u00b1\u00a0SD.In order to obtain the cellulose from the biomass to synthesize the cellulose-derived bimetallic catalyst, 70% aqueous FA was used for the efficient fractionation of Eucalyptus biomass (Fig.\u00a0S1). The reasons to choose 70% FA as fractionation solvent were: On one hand, FA is a biomass-derived and sustainable renewable solvent [19] and has been intensively employed for nondestructive fractionation of biomass in the literature [20]. On the other hand, other articles reported having employed concentrated FA (\u2265 80\u00a0wt%) [20], much lower FA concentration is rarely used for biomass fractionation. The lower FA concentration is, the less FA erodes the equipment. During FA fractionation processing, FA plays three major roles: solubilization effect dissolving lignin, catalyst function hydrolyzing hemicellulose, and reaction substrate formylation with hydroxyl groups on the surface of cellulose and lignin. Although increasing FA concentration and temperature could lead to high lignin dissolution and cellulose yield, it was prone to cellulose degradation and lignin repolymerization [20]. In this present work, approx. 73.18% of dissolved lignin is isolated using 70% FA as the fractionation solvent at 130\u00a0\u00b0C for 3\u00a0h, this value is similar to those reported by Zhou et\u00a0al. [21] and Li et\u00a0al. [22].The structural properties of the isolated cellulose were characterized by SEM and HPLC, the results are shown in Fig.\u00a01. As seen from Fig.\u00a01a\u2013f, the FAC morphology of SEM image is much smoother (Fig.\u00a01f), while the surface of the native cellulose from Eucalyptus is rough (Fig.\u00a01d and e). Subsequently, the compositions of native cellulose and FAC were analyzed by HPLC (Fig.\u00a01g), and results are shown in Fig.\u00a01h. After FAC was acid hydrolyzed before HPLC determination, 2.49% of formic acid was detected (Fig.\u00a01g and h), indicating that formylation has occurred on the surface of FAC during FA fractionation. This phenomenon was intensively demonstrated in the earlier reported papers [20\u201322]. The subtle change of formylation on the FAC surface is directly detrimental to cellulose enzymatic hydrolysis with only \u223c62% of saccharification efficiency. Our results agree with the report in the literature [21]. As an alternative promising option, FAC can be employed to synthesize cellulose-derived PdAg bimetallic catalysts after decoration with polyetherimide (PEI). The as-prepared PdAg bimetallic catalyst is used to produce hydrogen from FA decomposition at room temperature.Apart from cellulose, the other two fractions of lignin and hemicellulose were also obtained from FA fractionation of Eucalyptus biomass (Fig.\u00a0S1). As observed from the mass balance of the isolated principle fractions of cellulose, hemicellulose and lignin in Fig.\u00a0S2, it is worthily noticeable that 90.31% lignin and nearly 95% hemicellulose are dissolved in 70% FA aqueous solution, leaving behind a high purity of 80.14% cellulose as solid residue, which has a yield as high as \u223c82%. According to the data in the lab-scale level, 100\u00a0g of dry biomass can achieve 33\u00a0g of cellulose solid residue with the purity of 80.14%, 27\u00a0g of lignin with high purity of 90.31%, and 9.07\u00a0g of dried hemicellulose hydrolysate powder with xylose concentration of 65.62%.Importantly, more than 90% of FA can be recovered for recycleability. It has been demonstrated that the recycled FA shows the similar excellent fractionation capacity as the fresh FA in terms of isolated cellulose and lignin yields (Fig.\u00a0S3).In order to fully understand the FA fractionation, the properties of isolated lignin from Eucalyptus biomass through 70% FA fractionation were also characterized FT-IR, GPC, and TGA. As seen from FT-IR profile in Fig.\u00a0S4, albeit with formylation, lignin possesses hydroxyl groups very similar to the native lignin from sugarcane bagasse [23]. The typical strong absorbance peaks corresponding to aromatic skeleton vibrations at 1603\u00a0cm\u22121, 1509\u00a0cm\u22121, 1458\u00a0cm\u22121 and 826\u00a0cm\u22121 are observed in FT-IR for the resultant lignin. Table S1 shows FT-IR characteristic peaks of lignin isolated from FA fractionation of Eucalyptus biomass. To our delight, compared with the native lignin (Mp\u00a0=\u00a06217\u00a0g mol\u22121), GPC determination shows a decrease of mass weight of 2582\u00a0g mol\u22121 for the resultant lignin from FA fractionation (Fig.\u00a0S5), indicating approx. 17% of lignin are depolymerized into mono- or oligo-aromatic compounds, which are not detected in this work. Interestingly, depolymerization and condensation phenomena of the resultant lignin is not observed using low FA concentration (70%) as fractionation solvent, which is quite different from the previous reports in the case of high FA (\u2265 88%) fractionation of biomass [20,21]. TGA analysis shows that the depolymerization temperature related to inter-unit linkage of lignin backbone is 515.7\u00a0\u00b0C, and only approx 5.74% of mass residue was retained at 800\u00a0\u00b0C (Fig.\u00a0S6). It indicates that the resultant lignin isolated from FA fractionation has high purity (\u2265 90%), and is quite suitable for downstream upgrading towards carbon functional materials, such as lignin-derived catalysts [24].For FA fractionation of Eucalyptus, hemicellulose and part of cellulose are hydrolyzed to C5 and C6 sugars (30.2\u00a0g\u00a0L\u22121), including 19.8\u00a0g\u00a0L\u22121 xylose, 7.7\u00a0g\u00a0L\u22121 glucose, 1.2\u00a0g\u00a0L\u22121 xylose oligomers, and 1.8\u00a0g\u00a0L\u22121 glucose oligomers (Fig.\u00a0S7). These sugar mixtures can be converted into high value-added commercial building blocks, such as furural and 5-hydroxylmethylfurual (5-HMF) using AlCl3 as a Lewis catalyst [25]. Results in Fig.\u00a09d show that 80% furural derived from C5 sugars and 60% 5-HMF from C6 sugars are simultaneously obtained at 110\u00a0\u00b0C and 2\u00a0h. The results are similar with those using pure xylose and glucose conversion into furan compounds in the literature [26,27].As aforementioned, the subtle change of formylation on the FAC surface is directly detrimental to cellulose enzymatic hydrolysis with approx. 62% of saccharification efficiency. In the present work, FAC is decorated with PEI and then covalent with bimetallic ions Pd2+ and Ag+ to synthesize cellulose-derived PdAg bimetallic catalyst for hydrogen production from aqueous FA in a sodium formate (SF)-free solution. Fig.\u00a02\na shows the synthesis schematic flow of cellulose-derived PdAg bimetallic catalyst.X-ray photoelectron spectroscopy (XPS) is employed to examine the chemical valence states of Pd and Ag in the as-prepared catalysts. From the XPS profile of Pd in Pd3.75Ag3.75-PEI-FAC(Fig.\u00a02b), Pd0 (Pd 3d5/2 335.4\u00a0eV, Pd 3d3/2 340.7\u00a0eV) and Pd2+ (Pd 3d5/2, 337.7\u00a0eV; Pd 3d3/2, 342.7\u00a0eV) [28]. The existence of oxidized Pd species can be ascribed to the oxidation of metallic Pd in the air during preparation. While for Pd3.7-PEI-FAC without Ag composition, Pd0 (Pd 3d5/2 335.7\u00a0eV, Pd 3d3/2 341.2\u00a0eV) is shifted to higher bind energy and Pd2+ (Pd 3d5/2, 337.7\u00a0eV; Pd 3d3/2, 342.7\u00a0eV) remains stable. However, XPS profile of Pd3.75Ag3.75-FAC without PEI decoration shows that only Pd0 (Pd 3d5/2 335.7\u00a0eV, Pd 3d3/2 341.2\u00a0eV) is observed and oxidized Pd species (Pd2+) are not detected. The probable reason is the fact that Pd and Ag ions are reduced into nanoparticles by NaBH4 before deposition on the suface of the FAC. These phenomena demonstrate the transfer of electrons and the interaction among Pd\u2013Ag bimetals and Pd metal-support [29]. The function of PEI decoration can enhance the binding energy of metal-support and stabilize Ag nanoparticles with small size, which can improve the catalytic performance and durability of Pd3.75Ag3.75-PEI-FAC catalyst.\u00a0The XPS signal of Ag 3d orbit in Pd3.75Ag3.75-PEI-FAC (Fig.\u00a02c) gives the binding energies of the Ag 3d5/2 peak\u00a0at 368.0\u00a0eV and Ag 3d3/2 peak at 374.0\u00a0eV. The 6.0\u00a0eV gap between the two states is a typical characteristic of\u00a0metallic Ag\u00a0[29]. It reveals that the Ag species in Pd3.75Ag3.75-PEI-FAC exist predominantly in the metallic form. This data is good consistent with the previous report in the literature [30].As seen from X-ray diffraction (XRD) patterns in Fig.\u00a02d, no metal diffraction peaks are observed for PEI-FAC support, which preserves typical cellulose XRD feature of 2\u03b8\u00a0=\u00a021.6\u00b0 [31]. Interestingly, no diffraction peak of Pd (111) at 2\u03b8\u00a0=\u00a040.2\u00b0 (PDF#46-1043) is found in Pd3.75-PEI-FAC and Pd3.75Ag3.75-PEI-FAC, which may be attributed to atomic dispersed Pd in PEI-FAC support. However, the typical diffraction peak of Ag (111) at 2\u03b8\u00a0=\u00a038.1\u00b0 (PDF#04-0783) is observed in Pd3.75Ag3.75-PEI-FAC, confirming the presence of Ag metallic nanoparticle, which is also demonstrated by XPS measurement in Fig.\u00a02c. Based on the above observations, it is reasonably speculated that the Pd3.75Ag3.75-PEI-FAC catalyst has core\u2013shell structure with an Ag nanoparticle as the core in the in-layer and homo-dispersed Pd as the shell in the out-layer. Because the particle size of the catalyst has a significant effect on the catalytic performance, HAADF-STEM was employed to detect the nanoparticle diameter. From the upper TEM images of Pd3.75Ag3.75-PEI-FAC (Fig.\u00a03\na), it can be clearly seen that larger nanoparticles of Ag with the average diameter approx. 8.7\u00a0nm (yellow circle), and smaller nanoparticles of Pd with the average diameter approx. 2.3\u00a0nm (red circle). The Pd nanoparticle is encased in the surface of Ag nanoparticle. However, from the below TEM images of PdAg-FAC (Fig.\u00a03b), it can be seen that the AgPd nanoparticles were dispersed on the surface of the FAC support with the average diameters of 6.5\u00a0nm. Some Ag\u2013Pd alloy was also observed, because the lattice space is 0.231\u00a0nm, which is between the (111) lattice spacing of\u00a0face centered cubic Pd (0.22\u00a0nm) and Ag (0.24\u00a0nm) [32]. Large Pd nanoparticle in PdAg-FAC shows lower catalytic activity than small Pd nanoparticle in PdAg-PEI-FAC [32].BET surface area and pore diameter of the supported catalyst have a vital role in the catalytic dehydrogenation reaction. BET surface area and pore diameter of the support (PEI-FAC) and catalyst (Pd3.75Ag3.75-PEI-FAC) was tested by nitrogen adsorption desorption isotherms. BET profiles of the support and catalyst in Fig.\u00a04\n appear in their hysteresis loops, demonstrating the generation of mesopores. The content of Pd and Ag metal in Pd3.75Ag3.75-PEI-FAC was measured by ICP-OES, and the values are 0.8\u00a0wt% and 1.5\u00a0wt%, respectively. Pd and Ag contents in other PdAg catalysts with different compositions were shown in Table\u00a0S2.After illustrating the structural properties of the Pd3.75Ag3.75-PEI-FAC catalyst, we are further evaluating its catalytic activities of hydrogen generation from aqueous FA solution in view of three aspects, the total gas volume (Vgas (H2 + CO2)), turnover frequency (TOF, h\u22121) and activated energy (Ea, kJ mol\u22121).The support material has been demonstrated to be one of the most significant factors for the FA dehydrogenation because the interaction between the support and metal would subtly modify the physical and chemical properties of the catalyst and then enhance their activities [8]. It can be seen in Fig.\u00a05\na that Pd3.75Ag3.75-PEI-FAC (using PEI-FAC as support, 112.5\u00a0mL Vgas min\u22121) exhibits almost similar hydrogen generation rate as Pd3.75Ag3.75-rGO (using reduced oxidation graphene as support, 110\u00a0mL Vgas min\u22121), higher than Pd3.75Ag3.75-N@C (using N-doped carbon as support, 90\u00a0mL Vgas min\u22121). However, when black carbon is used as support matrix to load the Pd3.75Ag3.75, it shows no H2 generation activity, indicating N species plays a critical role in H2 evolution from FA. TOF values in Fig.\u00a05b further confirm that Pd3.75Ag3.75-PEI-FAC (2185 h\u22121) shows the similar TOF value as Pd3.75Ag3.75-rGO (2206 h\u22121) and Pd3.75Ag3.75-N@C (2179 h\u22121). Different from rGO and N@C support matrix, PEI-FAC is cost-effective, sustainable and prepared easily from biomass fractionation. From the viewpoint of Vgas data, it has been demonstrated that the amide group (-NH) in PEI-FAC support matrix plays a positive function for FA dehydrogenation activity over Pd3.75Ag3.75-PEI-FAC. The phenomenon was also confirmed by Li and coworker [22].As observed in Fig.\u00a05c and d, the effect of Pd/M compositions of bimetallic catalysts using PEI-FAC as support matrix on catalytic activities of catalyst is significant. After comparative examination of catalytic activities for different bimetallic compositions (Pd/Au, Pd/Ni, Pd/Co, and Pd/Ag) catalysts, Pd2.5Ag2.5-PEI-FAC is found to display the highest catalytic activity with TOF value of 1817 h\u22121, following with Pd2.5Au2.5-PEI-FAC\u00a0>\u00a0Pd2.5Ni2.5-PEI-FAC\u00a0>\u00a0Pd2.5Co2.5-PEI-FAC\u00a0>\u00a0Pd2.5-PEI-FAC, which may be attributed to electron transfer variation between Pd and adscititious metal elements [25]. The work function of Pd, Au, Ni, Co, and Ag are 5.12\u00a0eV, 5.1\u00a0eV, 4.6\u00a0eV, 5.0\u00a0eV and 4.26\u00a0eV, respectively [33]. Electrons tend to transfer from elements with lower work functions to those with higher ones. The larger the gap of the work function between Pd and the adscititious metal element is, the better the electron transfer will be. Therefore, the synergistic function between Pd and Ag plays a critical role resulting in the improvement of catalytic performance for Pd3.75Ag3.75-PEI-FAC, this phenomenon is consistent with the reportings of previous literature [23].Three major operational parameters affecting the H2 evolution from FA over Pd3.75Ag3.75-PEI-FAC were optimized, including FA concentration (0.5, 1, 1.5, 2 and 5\u00a0mol L\u22121), reaction temperature (30, 40, 50 and 60\u00a0\u00b0C) and PdAg mass/weigh ratio (1.25/1.25, 2.5/2.5, 3.75/3.75 and 5/5). The results are shown in Fig.\u00a06\n.As shown in Fig.\u00a06a, FA concentration shows negative effects on dehydrogenation activity of catalysts. The higher the FA concentration is, the lower the Vgas generation will be. For instance, approx. 350\u00a0mL of Vgas is obtained within 30\u00a0min\u00a0at 1.5\u00a0mol L\u22121 FA, while less than 125\u00a0mL of Vgas is released within 30\u00a0min\u00a0at 10\u00a0mol L\u22121 FA. After 20\u00a0min of reaction, the average conversion rate of FA to H2 is nearly up to 7.8\u00a0mL\u00a0min\u22121 mol\u22121, and the final concentration of FA is too low to detect by HPLC. As seen in Fig.\u00a06b, the reaction rate of hydrogen generation greatly depends on reaction temperature, and 238\u00a0mL of Vgas can be readily released within 5\u00a0min\u00a0at 60\u00a0\u00b0C, corresponding to almost full conversion of FA into H2 and CO2 without the detection of toxic CO by GC measurement. However, the reaction rate of hydrogen generation will be greatly reduced and only \u223c80\u00a0mL of total Vgas is released in 5\u00a0min\u00a0at 30\u00a0\u00b0C. Appropriate loading of Pd and Ag compositions in the catalyst is also important for hydrogen generation from FA system. The data in Fig.\u00a06c reveal that Pd3.75Ag3.75-PEI-FAC delivers the highest catalytic activity among all tested PdAg-PEI-FAC catalysts with different mass/weight ratio. Further increases of the PdAg mass/weight ratio up to 5/5 did not result in increased H2 evolution from FA over Pd5Ag5-PEI-FAC is not increasing in comparison with Pd3.75Ag3.75-PEI-FAC.To further explore the catalytic active site and the roles of Ag and the \u2013NH group in Pd3.75Ag3.75-PEI-FAC, the H2 generation from FA by several catalyst counterparts was investigated. The results are shown in Fig.\u00a07\n.As seen from Fig.\u00a07a, Ag3.75-PEI-FAC shows no hydrogen generation activity at 60\u00a0\u00b0C when Ag is solely loaded on the PEI-FAC, indicating Ag is not the active site. On the other hand, when Pd is solely loaded on the PEI-FAC, Pd3.75-PEI-FAC exhibits hydrogen generation activity with total Vgas of 125\u00a0mL within 10\u00a0min. The two comparative experiments indicate that Pd acts as the active site instead of Ag for the hydrogen generation from the FA system. Apple-to-apple comparison of Pd3.75-PEI-FAC (Vgas of 125\u00a0mL in 10\u00a0min) and Pd3.75Ag3.75-PEI-FAC (Vgas of 250\u00a0mL in 10\u00a0min) figures out that adding Ag with appropriate content can improve the catalytic activity of the PdAg bimetallic catalyst due to the synergistic effect between Pd and Ag [13]. Furthermore, significant differences of catalytic activities between Pd3.75Ag3.75-PEI-FAC (Vgas of 250\u00a0mL in 10\u00a0min) and Pd3.75Ag3.75-FAC (Vgas of \u2264 50\u00a0mL in 40\u00a0min) demonstrate that the amino group (-NH) of PEI fosters both the C\u2013H cleavage and H2 desorption step in the dehydrogenation of FA [24]. It is universally accepted that the major function of the \u2013NH group is not only to act as a proton scavenger during the catalytic FA dehydrogenation to expedite the C\u2013H cleavage, but also acts as a capping agent to avoid Ag nanoparticles enhancement during PdAg-PEI-FAC syntheses. To our delight, as seen from Fig.\u00a07b, the addition of sodium formate (SF) has a neglective effect on the catalytic activity of Pd3.75Ag3.75-PEI-FAC. It indicates that Pd3.75Ag3.75-PEI-FAC shows excellent hydrogen generation from FA system in SF-free conditions, which is very cost-effective and environmental friendly for H2 production in practical applications.On the basis of the above XRD and HAADF-STEM data, Pd3.75Ag3.75-PEI-FAC might have a core\u2013shell structure with Ag nanoparticles as the core in the in-layer and homo-dispersed Pd as the shell in the out-layer. Highly homo-dispersed Pd and synergistic effects between Pd and Ag of catalysts can improve hydrogen evolution from FA (Figs. 5 and 7). In addition, an amide (\u2013NH) group coated on cellulose surface to act as a proton scavenger can also efficiently enhance the catalytic performance of Pd3.75Ag3.75-PEI-FAC (Figs. 5 and 7). Therefore, a plausible catalytic mechanism pathway over Pd3.75Ag3.75-PEI-FAC was proposed in Fig.\u00a08\n.As seen from Fig.\u00a08, three reaction steps will take place for H2 evolution from FA over Pd3.75Ag3.75-PEI-FAC. In step I, O\u2013H bond cleavage provides a proton (H+), and then AgPd-formate (AgPd-[HCOO]-) intermediate is formed; For step II, C\u2013H bond dissociation affords an AgPd hydride (AgPd-[H]-) via isomerization, and one mole of CO2 is released; In step III, H2 evolution occurs through the recombination of AgPd-[H]- with a H+, and the catalyst is regenerated for next\u00a0reaction. Navlani-Garc\u00eda and co-workers pointed out that\u00a0the positive role of the bimetallic catalyst was to boost C\u2013H bond dissociation (Step II), in which a reduction of 48%\u00a0of the energy barrier calculated by density functional theory (DFT) for Pd catalyst was achieved by the bimetallic catalyst [5].The TOF value of Pd3.75Ag3.75-PEI-FAC is as high as 2875 h\u22121, which is superior to most of the previous PdAg bimetallic catalysts supported on other matrices, such as carbon (854 h\u22121) [34], N-rGO (171 h\u22121) [6], N-GCNT (413 h\u22121) [35], g-C3N4 (420 h\u22121) [36], and graphene (572 h\u22121) [37], for H2 generation from FA aqueous solution in the literature (Table S3). It indicates that cellulose isolated from biomass after modification with PEI is suitable for PdAg support as a bimetallic catalyst. However, some excellent PdAg heterogeneous catalysts showed much higher TOF values than Pd3.75Ag3.75-PEI-FAC in our work for H2 evolution from FA (Table S3), such as. Ag1Pd9@NPC with TOF of 3000 h\u22121 [32], Ag1Pd9-MnOx/carbonsphere with TOF of 3558 h\u22121 [35], PdAg@ZrO2/C with TOF of 9206 h\u22121 [32], PdAg@ZrO2/rGO with TOF of 4500 h\u22121 [15], PdAg/amine-MSC with TOF of 5638 h\u22121 [38], and Pd0.50Ag0.50/PDA-rGO with TOF of 6980 h\u22121 [35]. Different from Pd3.75Ag3.75-PEI-FAC, all those reported PdAg heterogeneous catalysts needed sodium formate as additive for H2 evolution. From the Arrhenius curve of ln(TOF) versus 1000/T, the apparent activation energy (Ea) of Pd3.75Ag3.75-PEI-FAC catalyst is calculated to be as low as 53.97\u00a0\u00b1\u00a02.31\u00a0kJ\u00a0mol\u22121 (Fig.\u00a09\na). To our delight, Pd3.75Ag3.75-PEI-FAC shows unique robustness and satisfactory re-usability, which is confirmed by the data in Fig.\u00a09b. This catalyst retains excellent stability with no loss of catalytic activity after five recycles (Fig.\u00a09b).In summary, FA fractionation of lignocellulose towards cellulose-derived catalysts has been successfully developed in this work. Owing to formylation, the resulting cellulose is not suitable for directly enzymatic hydrolysis for downstream production of C6 sugars and ethanol fermentation. However, it is a promising alternative option for the resulting cellulose to synthesize cellulose-derived PdAg bimetallic catalyst for hydrogen production from FA aqueous solution at room temperature. Among the as-prepared bimetallic catalysts in hand, Pd3.75Ag3.75-PEI-FAC shows the highest activity with approx. 350\u00a0mL of total hydrogen within 30\u00a0min from 1.5\u00a0mol L\u22121 FA aquous solution at 60\u00a0\u00b0C. The TOF value of Pd3.75Ag3.75-PEI-FAC reaches a high value of 2875 h\u22121, which greatly outperforms most of the previously reported Pd-based heterogenous catalysts for H2 generation from FA aqueous solution in the literature. Furthermore, the apparent activation energy (Ea) of Pd3.75Ag3.75-PEI-FAC is calculated to be 53.97\u00a0\u00b1\u00a02.31\u00a0kJ\u00a0mol\u22121. As expected, Pd3.75Ag3.75-PEI-FAC possesses high selectivity, durability and stability over 5 cycles with no loss of catalytic activity. The findings in this work open a new window for formosolv fractionation of biomass towards cellulose-derived PdAg bimetallic catalyst for hydrogen evolution from FA decomposition at room temperature, taking considerable account into biomass valorization simultaneously.YL (Yun Liu) completed conceptualization and supervision of the project, wrote and revised the manuscript. YY (Yanyan Yu) prepared the draft manuscript. HX (Huanghui Xu) completed methodology and investigation. HY (Hongfei Yu) finished XPS, HAADF-STEM and BET experiments. LH (Lihong Hu) characterized lignin's structure via GPC, FT-IR, and TGA.The authors declare no conflict of interests.This study was financially funded by the National Natural Science Foundation of China (NSFC, 21476016; 21776009), and Fundamental Research Funds for the Central Universities. The authors also acknowledge the special project for the construction of innovative province in Hunan Province of China (2019NK2031-3) The authors acknowledge professor Wensheng Qin, from Lakehead University of Canada, for checking English writing of revised manuscript.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.2020.08.006.", "descript": "\n                  The present work, in which cellulose isolated from formic acid fractionation (FAC) is decorated with polyetherimide (PEI) to attain highly efficient cellulose-derived PdAgbimetallic catalyst (PdAg-PEI-FAC), has been investigated, and the catalyst properties are characterized by XRD, XPS, BET, ICP-AES and HAADF-STEM. The as-obtained Pd3.75Ag3.75-PEI-FAC exhibits excellent catalytic performance for H2 evolution from a sodium formate-free formic acid (FA) aqueous medium at ambient temperature and the turnover frequency (TOF) reaches a high value of 2875 h\u22121, which is superior to most of the previously reported Pd-based heterogeneous catalysts supported on a carbon matrix in the literature. The remarkable catalytic activities of PdAg-PEI-FAC result from high dispersion Pd and synergistic effects between the PdAg bimetallic system. Furthermore, the amide (-NH) group in PEI coated on cellulose acting as a proton scavenger efficiently improves the catalytic property of catalyst. In addition, the critical factors affecting H2 release, such as FA concentration, reaction temperature, PdAg compositions and support matrix type, are also evaluated. Based on the experimental results, the probable three-step mechanism of H2 evolution from FA over Pd3.75Ag3.75-PEI-FAC is proposed. In the end, the activation energy (Ea) of Pd3.75Ag3.75-PEI-FAC catalyst is calculated to 53.97 kJ mol-1, and this catalyst shows unique robustness and satisfactory re-usability with no loss of catalytic activity after five recycles. The findings in this work provide a novel routine from lignocellulose fractionation towards cellulose-derived catalyst for H2 evolution.\n               "}
{"full_text": "As one of the most important inorganic chemicals, ammonia is responsible for supporting approximately 27 % of global population [1,2]. The Haber-Bosch process for ammonia synthesis is regarded as one of the greatest inventions of the last century [1]. Since then, a great deal of effort has been made to develop new ammonia synthesis catalysts to allow the operation of the Haber-Bosch process at reduced temperature and pressure. Intensive investigations have been carried on Ru-based catalysts promoted through a variety of different support materials Ru/HT-C12A7:e\u2212 [3], Ru/ Ba-Ca(NH2)2 [4], Ru/BaTiO3-xHx [5], Ru/TiH2, Ru/BaTiO2.5H0.5 [6] and Ru/BaCeO3-xNyHz [7]. Atomically dispersed Co supported on N-doped hollow carbon spheres also exhibit excellent catalytic activity at 350 \u00b0C [8]. Novel ammonia synthesis methods including electrochemical [9,10] and photocatalytic synthesis [11] allow ammonia synthesis at ambient conditions. The promotional effects of applied electric fields to ammonia synthesis have been reported [12]. The use of single atom catalysts for electrochemical synthesis of ammonia has been investigated through density functional theory calculations [13]. Due to the high cost of Ru, large scale application is limited with around ten ammonia synthesis plants using Ru-based catalysts globally (in which some plants are combined with Fe-catalysts too), all remaining plants use cheap fused Fe catalysts. A large Haber-Bosch ammonia synthesis plant may need 300 tons of catalyst thus cost is extremely important. Metal hydrides, oxyhydride and oxynitride hydride have been investigated as efficient promoters/supports for Ru, Ni, Fe and Co-based catalysts [5,7,14,15]. In general, metal hydrides are sensitive to air and moisture which may limit their practical large scale applications [15,16]. In laboratory conditions, most of the electride or hydride-based catalysts are handled in a glove-box to avoid their reaction with H2O [4\u20137]. In conclusion, no matter whether the catalyst is Fe/Co/Ni or Ru-based, they still fall short in terms of cost, moisture and oxygenate tolerance, therefore further improvements in this key area is still required.In ammonia industry, whether using Fe or Ru-based catalysts, a heavy gas purification process is applied to purify the feed gases, H2 and N2, to avoid catalyst poisoning. Trace amounts of oxygenates (as low as 10 atomic oxygen) such as O2, H2O, CO, and CO2 will deactivate the Fe-based catalyst [16\u201322]. In a recent report, it has been demonstrated that even impurities below 1 ppm of oxygen lead to a significant loss in activity for a state-of-the-art multi-promoted iron-based industrial catalyst [21]. A high purity gas feed of over 99.99995 % (0.5 ppm impurity), is normally used in reported papers for Haber-Bosch processes [4\u20137]. Inevitable intensive gas purification of both H2 and N2 will lead to a relevant increase in capital investment on the facility as well as additional energy inputs, lowering overall efficiency. One of the strategies to improve the oxygenate tolerance of the Fe or Ru catalysts is to prevent the particle growth, under the ammonia synthesis conditions through strong metal support interaction (SMSI) [22,23]. It has been reported that the strong interaction between Ru and defects in CNTs can significantly improve the catalytic activity of Ru-based catalyst for ammonia synthesis [24].In conventional fused Fe-based industrial catalysts, there are strong interactions between iron and the oxygen vacancies in the oxide promoters, although this intrinsic oxygen vacancy concentration is limited [18,22]. We previously reported that Ni promoted by BaZr0.1Ce0.7Y0.2O3\u2212\n\n\u03b4\n and Fe promoted by Ce0.8Sm0.2O2-\n\n\u03b4\n catalysts, display good ammonia synthesis activities due to the key role of extrinsic oxygen vacancies [22,25]. Anion vacancies, in particular nitrogen vacancy containing materials, provide the next step in this concept [26]. For the synthesis of ammonia through the Haber-Bosch process, the important role of nitrogen vacancies was also observed by Hosono and his co-workers in Fe, Co, Ru catalysts supported on Ba-CeO3-xNyHz and Ni supported on LaN, with it found that nitrogen vacancies on LaN can efficiently bind and activate N2 [7,27]. Using CeO2 as an example, on heating up to high temperatures, intrinsic oxygen vacancies will be generated through the reduction of Ce4+ ions and the loss of lattice oxygen atoms.\n\n(1)\n\n2\n\n\nC\ne\n\n\nC\ne\n\n\u00d7\n\n+\n\nO\nO\n\u00d7\n\n=\n2\n\n\nC\ne\n\n\nC\ne\n\n'\n\n+\n\nV\nO\n\n\u22c5\n\u22c5\n\n\n+\n\n1\n2\n\n\nO\n2\n\n(\ng\n)\n\n\nHere Kroger-Vink notations for defect chemistry is used in this article [28].Under the ammonia synthesis conditions, due to the presence of H2 at high temperature, some CeO2 will be reduced to oxygen deficient CeO2-\u03b4 thus forming more oxygen vacancies.\n\n(2)\n\n2\n\n\nC\ne\n\n\nC\ne\n\n\u00d7\n\n+\n\nO\nO\n\u00d7\n\n+\n\nH\n2\n\n(\ng\n)\n=\n2\n\n\nC\ne\n\n\nC\ne\n\n'\n\n+\n\nV\nO\n\n\u22c5\n\u22c5\n\n\n+\n\nH\n2\n\nO\n(\ng\n)\n\n\n\nThese oxygen vacancies will be a kind of nest, able to form strong interactions with the Fe or other transition metal atoms via SMSI, similar to the case for the Ag - CeO2 system [22,23,29]. Similarly, Goula et al. reported excellent catalytic activity and stability for Ni supported on Sm2O3, Pr2O3 and MgO promoted (doped) CeO2, attributed to the high concentration of oxygen vacancies [30].Oxygen vacancies introduced through thermal treatment or reduction are referred to as intrinsic oxygen vacancies. Their concentration is greatly related to temperature and oxygen partial pressure. If the promotion effects were solely related to intrinsic oxygen vacancies, it would be very limited. However, oxygen vacancies can be deliberately introduced into oxides through doping with another metal oxide with lower metal valence than the parent oxides theoretically existing in any temperature range, provided a stable solid solution is formed. These oxygen vacancies are referred to as extrinsic oxygen vacancies. The concentration of oxygen vacancies can be controlled through adjusting the doping level within the solid solution limit while more oxygen vacancies will be generated at higher doping levels. This technology is used for solid oxide fuel cells (SOFCs) and other electrochemical devices. For example, Sm2O3 can be used to dope CeO2, generating extrinsic oxygen vacancies,\n\n(3)\n\n\n\n1\n-\nz\n\n\nC\ne\n\nO\n2\n\n+\nz\nS\nm\n\nO\n1.5\n\n=\n\n\n1\n-\nz\n\n\n\n\nC\ne\n\n\nC\ne\n\n\u00d7\n\n+\n\n\n2\n-\n0.5\nz\n\n\n\nO\nO\n\u00d7\n\n+\nz\n\n\nS\nm\n\n\nC\ne\n\n'\n\n+\n0.5\nz\n\nV\nO\n\n\u2219\n\u2219\n\n\n\n\n\nIn order to maximize the concentration of oxygen vacancies, doping the oxygen with another more negatively charged anion, such as N3\u2212 ions is another option. It has been reported that when firing a porous CeO2 membrane in NH3 at 550 \u00b0C, N-doped CeO2 i.e. CeO2-xNy was formed, with x = 0.1 [31]. During the activation of the ammonia synthesis catalysts in mixed N2 and H2, it is inevitable that ammonia will be generated, further reacting with CeO2 or doped CeO2 to form N-doped CeO2, as shown in the reaction below.\n\n(4)\n\n\n\nC\ne\nO\n\n2\n\n+\n\n\n2\nx\n\n3\n\n\n\nN\nH\n\n3\n\n=\n\n\nC\ne\n\n\nC\ne\n\n\u00d7\n\n+\n\n\n2\n-\nx\n\n\n\nO\nO\n\u00d7\n\n+\n\n\n2\nx\n\n3\n\n\nN\nO\n'\n\n+\n\nx\n3\n\n\nV\nO\n\n\u2219\n\u2219\n\n\n+\nx\n\nH\n2\n\nO\n\n\n\nWhen Sm-doped CeO2, Ce1-zSmzO2-\u03b4, is exposed to NH3 at high temperature, cation Sm3+ and anion N3\u2212 co-doped CeO2, Ce1-zSmzO2-xNy, oxynitrides will be formed which have a higher concentration of oxygen/anion vacancies. Due to the second type of anions, N3\u2212 ions, the vacancies are more precisely described as anion vacancies, which could be either oxygen vacancies, \n\nV\nO\n\n\u2219\n\u2219\n\n\n or/and nitrogen vacancies, \n\nV\nN\n\n\u2219\n\u2219\n\u2219\n\n\n. Fig. 1\n shows the diagram to maximize the anion vacancies in CeO2 through both cation and anion co-doping. From the charge balance, more anion vacancies will be generated when some O2- ions are further replaced by lower valence ions such as N3\u2212 ions. Co-doping of Sm3+ and N3\u2212 ions in CeO2 will maximise the generation of extrinsic anion vacancies. It is anticipated that a significantly greater promotion effects can be achieved by using materials with more tailorable extrinsic anion vacancies such as Sm-doped cerium oxynitrides, Ce1-zSmzO2-xNy. At high anion vacancy concentrations the interaction between the positively charged anion vacancies and the electron-rich metal particles will be stronger, adding to an already strong SMSI, the nested or anchored metal particles on the anion vacancies will inhibit sintering and growth, improving metal catalyst stability [22,32\u201334].Another poisoning mechanism of oxygenates is the competitive adsorption between oxygen and nitrogen on the catalyst surface, with oxygen occupying active sites limiting catalytic activity [17]. The presence of large amounts of extrinsic anion vacancies on the surface of Sm-doped cerium oxynitrides will provide more active sites (anion vacancies) available for the adsorption of oxygenates such as O2, and H2O, reducing poisoning severity on the metal catalyst while retaining high activity even at high concentrations of oxygenate impurities. CeO2-based oxides have been reported as excellent oxygen storage materials for efficient catalytical conversion of H2, CO, CO2 and hydrocarbons [[35\u201338].Nitrogen is larger than oxygen, no matter in atomic or ionic format. At a coordination number of 4 (CN = 4), the environment of anions in fluorite structure, the ionic size of O2\u2212 and N3- ions is 1.38 and 1.46 \u00c5 respectively [39]. Thus, the size of the nitrogen atom in N2 may match better with nitrogen vacancies than oxygen vacancies, introducing extra nitrogen vacancies may further improve the promotion effect for efficient synthesis of ammonia through the Haber-Bosch process. Therefore, in this study, Sm3+ and N3- co-doped CeO2, Sm-doped cerium oxynitrides (Ce1-zSmzO2-xNy) with z \u2264 0.5 were synthesized and their promotion effects on ammonia synthesis were investigated in detail. The Ce1-zSmzO2-xNy promoter was synthesized in a simple one-step combustion synthesis method carried out in air. It is found that both the stability and catalytic activity of iron based catalysts have been improved by deliberately introducing extrinsic anion (oxygen and nitrogen) vacancies into the Ce1-zSmzO2-xNy promoter/co-catalyst.CeO2-xNy was prepared from a mixture of Ce(NO3)3\u00b76H2O (99.5 % Alfa Aesar) and urea with a mole ratio of 1 to 10 respectively [40]. 50 mL of water was then added to the mixture in a ceramic evaporating dish. The mixture was then treated at 120 \u00b0C for 24 h to form a gel like product which was combusted at 400 \u00b0C to for the desired CeO2-xNy powder. Part of this sample was then further calcined at 550 \u00b0C for 3 h in air to form the calcined CeO2-xNy powder. The resulting CeO2-xNy powder was then mechanically mixed with Fe2O3 and reduced using the same method previously reported [22].The solubility limit of Sm2O3 in Ce1-zSmzO2-\u03b4 is roughly at z = 0.5 [41]. The CeO2-xNy and Ce1-zSmzO2-xNy with z \u2264 0.5 were synthesised from cerium nitrate, samarium nitrate and urea through a simple combustion method [40]. Ce1-zSmzO2-xNy was prepared from a mixture of Ce(NO3)3\u00b76H2O (99.5 % Alfa Aesar), Sm(NO3)3\u22c56H2O (99.9 % Alfa Aesar), and urea with a ratio of 1 mole of total metal ions to 10 mole of urea. The rest of the synthesis process was the same as described above for CeO2-xNy and was repeated for z values of 0.1, 0.2, 0.3, 0.4, and 0.5. The rest is the same as for preparation of CeO2-xNy. The combustion method was used to prepare pure CeO2 as previously reported [22].Before materials characterisation, all samples were washed multiple times by water and ethanol to remove any residual urea or other hydrocarbons. The catalyst was characterised using X-ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Carbon, Hydrogen and Nitrogen (CHN), Raman spectroscopy, Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). XRD analyses were carried out on a Panalytical X\u2019Pert Pro Multi-Purpose Diffractometer (MPD), with Cu K \u03b11 radiation, working at 45 kV and 40 mA. The nitrogen content was measured by a CHN analysis, performed on a FlashEA\u00ae 1112 Element Analyzer at MEDAC Ltd adhering to UKAS ISO17025 accreditation, with a standard deviation of \u00b10.3 wt%. The same CHN facility has previously been used to determine the nitrogen content in ammonia synthesis catalyst (Ni,M)2Mo3N (M = Cu or Fe) [42]. The SEM images were obtained with ZEISS SUPRA 55-VP operating at 10 kV. Elemental compositions were analysed with an energy-dispersive X-ray spectrometer (EDX) attached to the SEM. X-ray Photoelectron Spectroscopy (XPS) data were collected at the Warwick Photoemission Facility, University of Warwick. Samples were attached to electrically conductive carbon tape and mounted on a sample bar loaded in to a Kratos Axis Ultra DLD spectrometer (base pressure < 2 \u00d7 10\u221210 mbar). Samples were illuminated using a monochromated Al K\u03b1 X-ray source (hv = 1486.7 eV). The core level spectra were recorded using a pass energy of 20 eV (resolution approx. 0.4 eV), from an analysis area of 300 \u03bcm x 700 \u03bcm. The data were analysed in the CasaXPS package, using Shirley backgrounds and mixed Gaussian-Lorentzian (Voigt) lineshapes. High resolution transmission electron microscopy (HRTEM) observations of the samples were carried out on a JEOL2100 microscope, operated at 200 kV, equipped with an Oxford Instruments 80 mm2 SDD EDX detector. EDX spectra were collected by focusing the electron beam onto a certain area of the samples.To measure the catalytic activity, a fixed bed stainless steel reactor was used with the catalyst held in place in the centre by quartz granules and glass fibre. The total weight after activation / reduction was approximately 0.3 g for each catalyst tested. Full details on the testing parameters used along with the gas purification process can be found in our previous report [22]. The data points in Fig. 9 were obtained through the purification and known impurity injection process detailed in our previous work [22].Produced ammonia was collected in dilute sulfuric acid (0.01 M) with concentration measured using an ISE Thermo Scientific Orion Star A214 ammonia meter [25]. The rate of ammonia production (in mol g\u22121 h\u22121) was calculated according to the following equation.\n\n(5)\n\n\nr\n\nN\n\nH\n3\n\n\n\n=\n\n\n\n\nN\n\nH\n4\n+\n\n\n\n\u00d7\nV\n\n\nt\n\u00d7\nm\n\n\n\n\nWhere [NH+\n4] is ammonia concentration in mol L\u22121, V is volume of 0.01 M H2SO4 in L, t is time in hours and m is catalyst mass in g.Carbon, Hydrogen and Nitrogen (CHN) analysis was carried out to precisely measure the overall nitrogen content in the synthesised oxynitrides. In our synthesis process of cerium oxynitride, urea was used as the nitrogen source. It is believed that, during the combustion process, ammonia is generated from urea to further react with CeO2 to form cerium oxynitride, CeO2-xNy. Assuming the charge for cerium is +4, from the charge balance, the formula of the cerium oxynitride can be written as \nC\ne\n\nO\n\n2\n-\nx\n\n\n\nN\n\n\n\n2\nx\n\n3\n\n\n\n\nV\n\n\nx\n3\n\n\n\n, where \u2018V\u2019 represents anion vacancies, oxygen or/and nitrogen vacancies. For Sm-doped cerium oxynitrides, the general formula can be written as \n\n\nC\ne\n\n\n1\n-\nz\n\n\n\n\nS\nm\n\nz\n\n\nO\n\n2\n-\n\n\ny\n+\nz\n\n2\n\n\n\n\nN\ny\n\n\nV\n\n\n\n3\ny\n+\nz\n\n2\n\n\n\n. As z is known from the starting materials, and since the weight percentage of nitrogen in the synthesised oxynitrides is measured, y can be then deduced. Therefore, the general formulae of these new oxynitrides can be determined based on the nitrogen content, which are listed in Table 1\n. For samples with z = 0 and 0.1, the nitrogen content is y = 0.07 for anion sites for both samples (Table 1). However, the anion vacancy concentration for the Sm-doped sample, Ce0.9Sm0.1O1.84N0.07, is 4.5 %, which is slightly higher than that of the Sm-free sample, CeO1.89N0.07 (3.5 %). This is due to the doping of low valent Sm3+ cations in the Ce0.9Sm0.1O1.84N0.07 sample, thus more anion vacancies are generated (Fig. 1, eq. 3). According to the data in Table 1, in general, the nitrogen content and anion vacancies increase with increased Sm-doping level. The only exceptional situation is that, the nitrogen content in sample Ce0.5Sm0.5O1.51N0.16 is slightly lower than that for sample Ce0.6Sm0.4O1.52N0.19. The possible reason is, at z = 0.5, the cation doping level is already very high leading to a high concentration of anion vacancies. If more oxygen is replaced by nitrogen, more anion vacancies will be generated. However, there is a limit on the anion vacancy concentration in the oxynitride in order to maintain the crystal lattice. Under this circumstance the nitrogen content is slightly reduced for the sample Ce0.5Sm0.5O1.51N0.16. For representative sample Ce0.5Sm0.5O2-xNy, the compositions were also estimated using X-ray Photoelectron Spectroscopy (XPS).\nFig. 2\n shows the selected XPS data for pure Ce0.5Sm0.5O2-xNy promoter plus the Ce0.5Sm0.5O2-xNy promoter with Fe2O3 and Fe before and after the activity test. For the pure oxynitride, XPS revealed a composition of Ce0.5Sm0.5O1.45N0.12, a slightly lower nitrogen content than that derived from the CHN results. The ratio of Ce(IV) to Ce(III) was found to be 3.8, with a corresponding ratio of Sm(III) to Sm(II) of 2.6, the XPS spectrum of Ce 3d and Sm 3d are shown in Figure S1. Analysis of the N 1s spectrum acquired from the Ce0.5Sm0.5O2-xNy sample is shown in Fig. 2b and corresponds to a nitrogen content of 4.67 at% (Table 1). The N 1s spectrum is further shown in Figure S1c, showing large deviation in the exp count values. This indicates that the nature of the nitrogen bonds obtained from the XPS spectra is unreliable with the main conclusion being the presence of nitrogen. The C 1s spectrum is shown in Figure S1d showing no CN bonds confirming no residual urea present in the sample. The discrepancy in nitrogen content derived from XPS and CHN is due to the surface specificity of XPS. While CHN measures the overall nitrogen content, the sampling depth in XPS is limited to the outermost few nm of the material and thus a little deviation is common. The deduced overall unoccupied anion sites, i.e., the total anionic vacancies is the same (16.5 %), due to the detection of Ce(III) and Sm(II) in the XPS results, allowing us to remove the assumption that all cerium has a charge of +4 and all Sm has a charge of +3 (Table S1).The compositions of the Fe2O3 and Fe promoted catalysts are less reliable with Ce to Sm ratio varying drastically from the expected value. This is due to the overlap between Ce 3d and the Fe Auger emission as well as Ce 4d and Sm 4d, both of which make the determination of the amount of Ce less accurate. To resolve this, a sample with low Ce content was examined in order to obtain a reliable line shape for the Fe LM23M23 Auger emission. For the two promoted catalysts, Fe2O3 and Fe before and after the activity test, examination of the Sm 3d region showed no Sm(II) present, with all samarium being Sm(III). Although no Sm(II) was detected on the surface in these samples it should be noted that the weight percent of oxynitride in these samples is only 20 wt%, much lower than in the pure Ce0.5Sm0.5O2-xNy sample and therefore signal for Sm(II) could be below the detection limit. However, for the calculation of composition it was assumed that only Sm(III) was present. The ratio of Ce(IV) to Ce(III) was 0.87 and 3.7 in 85 wt% Fe2O3 \u2013 15 wt % Ce0.5Sm0.5O2-xNy and Fe \u2013 20 wt % Ce0.5Sm0.5O2-xNy respectively. This deviation from the pure oxynitride sample was expected to be due to the overlap of emission spectra as described above. For the mixed 85 wt% Fe2O3 \u2013 15 wt % Ce0.5Sm0.5O2-xNy, the Fe spectra is shown in Fig. 2c. All Fe is in the form of Fe2O3. After the catalytic activity test, the Fe spectra for the 80 wt%Fe \u2013 20 wt % Ce0.5Sm0.5O2-xNy catalyst is shown in Fig. 2e. 76 % of this region is made up of Fe2O3 which is expected due to the reoxidation of the small Fe catalyst particles upon removal of the catalyst from the reactor. 21 % of the region is Fe (II) with the remaining 3 % being zero valence Fe(0). However, after reduction and the catalytic activity test, no nitrogen is detected in the 80 wt%Fe \u2013 20 wt % Ce0.5Sm0.5O2-xNy sample. One possibility is the nitrogen content is too low, beyond the measuring limit of XPS. Another possible reason is, the oxynitride sample is partially oxidized by the oxygenates present in the gas stream. However, the Sm-doped CeO2, Ce1-zSmzO2-\u03b4, still exhibit good stability and activity for ammonia synthesis, which is related to the extrinsic oxygen vacancies, although the overall activity is slightly lower [22].X-ray diffraction (XRD) was used to determine the phase and structure of the synthesised oxynitrides. As shown in Fig. 3\na, the XRD patterns of pure and Sm-doped CeO2-xNy are similar to CeO2, indicating they have the same or a similar crystal structure to CeO2. Rietveld refinement of these oxynitrides were carried out by GSAS + EXPGUI using the fluorite structure for pure CeO2 as the parent phase (Figure S2) [43,44]. During the refinement, cubic CeO2 with a space group \nF\nm\n\n3\n\u00af\n\nm\n\u2009\n(\n225\n)\n was used as the parent phase. It was assumed that Ce and Sm share the 4a (0,0,0) sites, O and N share the 8c (1/4,1/4,1/4) sites [45]. The oxygen and nitrogen occupancies were taken from the chemical composition of these oxynitrides measured by CHN analysis because CHN can provide the overall nitrogen content while XPS can only provide the information on the surface (Table 1). The real and calculated XRD patterns provide a good fit, indicating all these new materials are single phase. The lattice parameters, and cell volume thermal factors are listed in Table 2\n. The lattice parameters and cell volumes of Ce1-zSmzO2-xNy with z = 0 to 0.5 are also shown in Fig. 3b. It is believed that oxygen and nitrogen share the same 8c sites, ordering of nitrogen and oxygen, as has been observed in some oxynitrides, but does not happen in Ce1-zSmzO2-xNy. It is then reasonably deduced that nitrogen is homogeneously distributed in the bulk, although the defect concentration including anion vacancies are normally higher on the surface of a particle. From XPS analysis of sample Ce0.5Sm0.5O2-xNy, un-occupied anion sites is 16.5 %, which is the same as deduced from CHN analysis (Table 1). The crystal structure of Ce0.5Sm0.5O2-xNy is also consistent with the observed d-spacing from high resolution transmission electron microscopy (HRTEM) (Fig. 4\na), indicating it is correct. To the best of our knowledge, Sm-doped CeO2-xNy is the first cation doped fluorite oxynitride.It is noticed that the lattice parameter change in Ce1-zSmzO2-xNy with z = 0 to 0.5 does not follow the Vegard's law, i.e., the lattice parameter change should be proportional to the change of z in Ce1-zSmzO2-xNy (Fig. 3b). As Ce1-zSmzO2-xNy is both a cation (Sm3+) and anion (N3\u2212) co-doped solid state solution, which is more complicated. It may not necessarily follow Vegard's law, which normally applies to only cation doped materials. According to CHN analysis, based on the contents of nitrogen, it can be deduced that the formula for Ce1-zSmzO2-xNy with z = 0 and 0.1 is CeO1.89N0.07 and Ce0.9Sm0.1O1.84N0.07 respectively (Table 1). The ionic size for Ce4+ and Sm3+ ions at coordination number of 8 (CN = 8) is 0.97 and 1.079 \u00c5 respectively [39]. Doping of CeO2 by larger Sm3+ ions should lead to the lattice expansion. This has been previously observed in Sm-doped CeO2 [46]. However, the introduction of nitrogen in the lattice makes things more complicated. The lattice parameter of the synthesized CeO2-xNy is a = 5.4273(1) \u00c5. This is slightly lower than the reported a = 5.5133(1) \u00c5 for pure CeO2 [47]. The ionic size of N3- ions is larger than O2- ions [39]. From this point of view, partially replacing O2- ions with larger N3- ions in CeO2 should lead to an increased lattice parameter. On the other hand, this anion doping also generates anion vacancies (charged voids), as shown in Eq. (4). This may result in lattice shrinking. The final lattice parameters of CeO2-xNy is the combined effects of lattice expansion due to larger N3- ions and lattice shrinking due to the formation of anion vacancies. Lattice shrinking was also observed in some perovskite oxynitrides where some lattice O2- ions are replaced by large N3- ions, which is attributed to the formation of higher valent cation ions [48]. XPS analyses indicate both Ce4+ and Ce3+ for element Ce, and Sm3+ and Sm2+ for element Sm, exist on the surface of pure Ce0.5Sm0.5O2-xNy (Table S1), thus no higher valent cations are formed in Ce0.5Sm0.5O2-xNy. A similar situation may happen on sample Ce0.9Sm0.1O2-xNy, which means that the lattice shrinking for sample Ce0.9Sm0.1O2-xNy is likely due to the extra anion vacancies due to the doping of more negative N3\u2212 ions in the lattice (Fig. 3b). However, from z = 0.1 to 0.5, the lattice parameter gradually increases indicating that the effect of larger Sm3+ ions on the lattice parameters becomes more significant than that from anion vacancies (Fig. 3b). These results indicate that the relationship between lattice parameters and doping level in both the cation and anion-co-doped CeO2 is very complicated, and may not necessarily follow the Vegard's law.To study the oxygen vacancies in the as-prepared oxynitrides, Raman spectra of these samples were collected (Fig. 3c). Pure CeO2, and raw CeO2-xNy show a sharp F2g peak at 465 cm\u22121, corresponding to the typical fluorite structure of CeO2 [40,49,50]. The peak at 570 cm\u22121 is attributed to oxygen vacancies [40,50]. No peak was observed at 570 cm\u22121 for pure CeO2 indicating that for pure CeO2, intrinsic oxygen vacancy concentration is not high enough to be detected by the Raman spectroscope. The peak at 570 cm\u22121 for raw CeO2-xNy and Ce0.9Sm0.1O2-xNy is very weak, indicating a low concentration of oxygen vacancies. This peak becomes stronger with increased Sm doping level, indicating a higher concentration of oxygen vacancies. Sample Ce0.5Sm0.5O2-xNy with the highest doping level gives the strongest peak at 570 cm\u22121 indicating the highest concentration of oxygen/nitrogen vacancies. These results are consistent with the deduced chemical formula from CHN analyses and the corresponding anion vacancy concertation (Table 1).SEM imaging was employed to further examine the surface of the catalyst promoter as well as the promoted iron catalyst before and after reduction, EDS layering allowed for a clear distinction between promoter and iron catalyst to be seen, giving a further picture of how the promoter is distributed within the catalyst. The SEM/EDS images of sample CeO2-xNy are shown in Figure S3. Figure S3a shows the image for CeO2-xNy with EDS layering, it is observed from this images that there is a secondary particle size distribution of approximately 1\u20136 \u03bcm. In Figures S3b, 3c and 3d the unreduced catalyst, catalyst after carrying out the activity test, and catalyst after carrying out the stability test are shown. The size distribution of the CeO2-xNy promoter does not change showing good stability throughout the reduction and reaction processes. Figure S4 shows the SEM/EDS images of sample Ce0.5Sm0.5O2-xNy along with the unreduced \u03b1-Fe2O3 - Ce0.5Sm0.5O2-xNy promoted catalyst and the reduced catalyst after both the activity and stability test. It can be seen from Figures S4a and 4b that Ce0.5Sm0.5O2-xNy has a similar structure to that of CeO2-xNy with a secondary particle size distribution of approximately 1\u201310 \u03bcm. A similar particle size distribution can be seen for Ce0.5Sm0.5O2-xNy before reduction as well as for both reduced catalysts after activity and stability tests showing good stability throughout this process, again similar to CeO2-xNy.HRTEM images of the mixed 85 wt% Fe2O3-15 wt% Ce0.5Sm0.5O2-xNy catalyst before and after catalytic activity measurements are shown in Figs. 4a & 4b, respectively. In Fig. 4a, the closest match to 0.294 nm is the (111) spacing of Ce0.5Sm0.5O2-xNy (0.3134 nm) and, for 0.482 nm it is the (003) spacing of \u03b1-Fe2O3 (0.4582 nm). This indicates the crystal structure determined by Rietvild refinement for sample Ce0.5Sm0.5O2-xNy is correct. In Fig. 4b, the closest match to 0.318 nm is Ce0.5Sm0.5O2-xNy (111) (0.3134 nm), for 0.271 nm it is Ce0.5Sm0.5O2-xNy (200) (0.271 nm), and for 0.44 nm it is \u03b1-Fe2O3 (100) (0.436 nm). This is consistent with the XRD pattern of the Fe- Ce0.5Sm0.5O2-xNy catalyst after the catalytic activity measurement (Figure S5b). From XPS analyses, only 2.6at% of iron is in metallic Fe form in the sample after the catalytic measurement with the rest reoxidized by air when removed from the reactor at room temperature. Therefore it is difficult to find metallic Fe particles during TEM observations (Table S1). In Fig. 4b it can be seen that small \u03b1-Fe2O3 is present on the surface of Ce0.5Sm0.5O2-xNy particles providing indirect evidence of anchoring of Fe in Ce0.5Sm0.5O2-xNy (or Ce0.5Sm0.5O2-\u03b4 after oxynitride is oxidised to oxide by oxygenates) through SMSI. Iron is expected to be in the form of metallic Fe under ammonia synthesis conditions. However, when removed from the reactor the small particle size of metallic iron will cause re-oxidation, which has been confirmed by XPS analyses (Fig. 2 and Table S1). Figure S6 shows the TEM images with EDX for the 85 wt% Fe2O3-15 wt% Ce0.5Sm0.5O2-xNy catalyst before and after catalytic measurements. Before the catalytic measurement, it is a mixture of Fe2O3 and Ce0.5Sm0.5O2-xNy resulting from the mechanical mixture (Figure S5b & S6a). Element nitrogen was not detected by EDX because the content is too low, beyond the measuring limit of EDX. Figure S6b shows the presence of small particle sized Fe2O3 on the Ce0.5Sm0.5O2-xNy surface while a large portion of Fe or FeOx is not in direct contact with Ce0.5Sm0.5O2-xNy because it contains only 20 wt% in the composite.From the analyses above, single phase doped oxynitrides, Ce1-zSmzO2-xNy with a large amount of extrinsic anion vacancies have been successfully synthesised and confirmed. They are expected to be excellent promoters/co-catalysts for ammonia synthesis catalysts, which are investigated in detail below.These oxynitrides were investigated as promoters with a Fe catalyst for the synthesis of ammonia. The experimental details are described in the experimental section. In our experiments it was found that, under 3 MPa, pure Fe catalyst using \u03b1- Fe2O3 as the precursor, pure CeO2, pure Sm2O3, CeO2-xNy and Ce0.5Sm0.5O2-xNy all show no activity towards ammonia synthesis on their own at temperatures up to 600 \u00b0C with feed gas purity of 99.996 %. When \u03b1-Fe2O3 was mixed with CeO2-xNy, ammonia was successfully generated (Fig. 5\na&5b). This indicates the catalytic activity is a synergetic process between the \u03b1-Fe catalyst and the oxynitride promoter/co-catalyst. The oxynitride may not be a simple promoter as pure Fe from reduction of \u03b1-Fe2O3 itself does not show any activity. The oxynitride in the Fe-oxynitride composite is more likely a co-catalyst, which needs further investigation.For a synergetic process, normally there is an optimised ratio between Fe and the oxynitride, which exhibits the best catalytic activity. To find an optimal ratio between the Fe catalyst and the oxynitride promoter, the weight of oxynitride promoter was varied from 14 wt% to 26 wt% in the total Fe-oxynitride composite catalysts. The catalytic activities of these Fe-based catalysts were initially measured in a fixed bed reactor using BOC Zero grade N2 and H2 as the feed gases without further purification. The impurity level of these gases has been listed in a previous report [22]. Unless specified, all the activities were obtained from Zero grade feed gases.\nFig. 5a & b show the ammonia synthesis activity of CeO2-xNy promoted Fe catalysts at different weight percentages over a temperature range of 600 \u00b0C - 250 \u00b0C, at 1 MPa and 3 MPa respectively. Apart from 77 % Fe - 23 % CeO2-xNy and 74 % Fe - 26 % CeO2-xNy, both achieving their maximum activity at 500 \u00b0C and 1 MPa, all the other experiments suggested an optimum operation temperature of 450 \u00b0C. At 3 MPa the highest activity was 17.2 mmol g\u22121 h\u22121 with the optimum weight ratio of 80 % Fe \u2013 20 % CeO2-xNy. At 1 MPa, the highest activity was again achieved for 80 % Fe \u2013 20 % CeO2-xNy, 8.86 mmol g\u22121 h\u22121. The optimum mass ratio of Fe to CeO2-xNy is found to be 80:20, as could be reasonably expected according to the synergetic process between Fe and oxynitride promoter. It is expected that the greater the oxynitride content then the stronger SMSI effect will be, as described above [29]. More oxynitride means more anion vacancies thus higher activity. However, the content of Fe is also important as Fe is the actual catalyst or a co-catalyst. Therefore, if Fe is diluted too much then activity will be lower. A balance between these two effects can be achieved at an oxynitride weight percent of 20 %. The SMSI between Fe and the anion vacancies and the possible in situ Ce-H species formation on the CeO2-xNy surface possibly donating electrons to the nested/anchored \u03b1-Fe particle, facilitating the dissociation of N N bonds, thus improving the ammonia synthesis reaction [7]. The anion vacancies in CeO2-xNy may adsorb N2, which takes part in the ammonia synthesis process as proposed in nitride (Co3Mo3N, LaN), perovskite oxynitride hydride (BaCeO3\u2212xNyHz) catalysts [7,27,51].The apparent activation energy (Ea) of the Fe-CeO2-xNy composite catalysts at a temperature below 450 \u00b0C is obtained from the slope when plotting the logarithm of the ammonia synthesis rate vs. 1000/T (Fig. 6\na & b) [5,6]. At a temperature above 450 \u00b0C, limited by the thermodynamic equilibrium and the greater thermal decomposition of ammonia, the activity of Fe-based catalyst normally starts to decrease at a temperature between 450 \u2013 500 \u00b0C [4]. Due to limited data-points at low temperature, some of the obtained apparent activation energies may have a relatively large deviation. Considering the Ea for different compositions at both 1 MPa and 3 MPa, the 80 wt% Fe-20 wt% CeO2-xNy composite catalyst tends to have low apparent activation energy and high activity (Figs. 6a &b). The Ea for 80 wt% Fe-20 wt% CeO2-xNy composite catalyst is 66 \u00b1 4.78 and 50 \u00b1 8.22 kJ/mol at 1 MPa and 3 MPa respectively. This is comparable to the Ea of 70 kJ/mol for fused industrial Fe-catalyst (Haldor Topsoe KM1) at low pressure [5,52]. For ammonia synthesis catalysts using excellent promoters, a low apparent activity energy around 50 kJ/mol is normally observed [5,7,14,27,53]. As the 80 wt% Fe catalyst in the Fe-CeO2-xNy composites exhibits the optimum highest activity, the mass ratio between Fe and oxynitride is fixed to 80:20 in the composite catalysts in the following study.From previous reports and the analyses above, anion vacancies play a crucial role in the stability and catalytic activity of ammonia synthesis catalysts. To further increase anion vacancies, new Sm-doped cerium oxynitrides of Ce1-zSmzO2-xNy with z = 0.1 to 0.5 were synthesised. Catalytic activity of the Fe- Ce1-zSmzO2-xNy composite catalysts with mass ratio of Fe-catalyst to Ce1-zSmzO2-xNy of 80:20 at different temperatures and pressures, 1 MPa, 3 MPa were investigated respectively (Fig. 5c & d). At both 1 MPa and 3 MPa, the sample 80 wt%Fe-20 wt%Ce0.5Sm0.5O2-xNy where z = 0.5 achieved the highest activity. The introduction of samarium into the CeO2-xNy promoter/co-catalyst will create extrinsic anion vacancies, confirmed by Raman spectra (Fig. 3c). From CHN analyses, nitrogen content in Ce1-zSmzO2-xNy where x > 0.2 is significantly higher than in the other samples, indicating the introduction of appropriate amounts of Sm3+ ions in CeO2 also facilitate the incorporation of nitrogen into the lattice (Table 1). According to Eq. (4), the concentration of negatively charged nitrogen defects \n\nN\nO\n'\n\n will also be higher in samples with high nitrogen content. These negatively charged nitrogen defects, similar to negatively charged H\u2212 ions, theoretically may donate electrons to nearby Fe particles, helping in the dissociation of strong N N bonds, thereby leading to higher activities. At 3 MPa, the optimum temperature at which the highest activity was achieved was 500 \u00b0C for Fe-Ce0.8Sm0.2O2-xNy, 400 \u00b0C for Fe-Ce0.5Sm0.5O2-xNy, and 450 \u00b0C for the other Ce1-zSmzO2-xNy promoted Fe-catalysts (Fig. 5c & d). The optimum temperature of Fe-Ce0.5Sm0.5O2-xNy is similar to the expensive Ru-based catalysts [16]. It is noted that sample Ce0.5Sm0.5O2-xNy has the highest concentration of anion vacancies, which could be related to the lower optimum operating temperature. At 1 MPa the difference in activities is much less, although the highest activity was still obtained for the Fe-Ce0.5Sm0.5O2-xNy catalyst. At 3 MPa the highest activity of 18.8 mmol g-1 h-1 at 400 \u00b0C was obtained from the 80 % Fe \u2013 20 % Ce0.5Sm0.5O2-xNy catalyst. The weight hourly space velocity (WHSV) is 16000 mL g-1 h-1, which is less than half of those in most of the reported papers (Table S2). This low WHSV is a result of the high catalyst loading (300 mg vs 100 mg) and larger reactor (external diameter of \u00bd inch instead of 3/8 inch), compared to other research groups [5\u20137]. Considering the WHSV, at 400 \u00b0C, 1 MPa, the 80 wt%Fe-20 wt% Ce0.5Sm0.5O2-xNy composite catalyst exhibits an activity (5.6 mmol g-1 h-1 at 16000 mL g-1 h-1) comparable to the best industrial benchmark W\u00fcstite fused Fe catalyst (13.9 mmol g-1 h-1 at 36000 mL g-1 h-1, 400 \u00b0C, 0.9 MPa). However, purity of feed gas in this study is only 99.996 %, much lower than the 99.9999 % and 99.99995 % used in previous reports (Table S2).In the Haber-Bosch process, conversion and ammonia yield are limited by thermodynamic equilibrium at high temperatures as the reaction is exothermic [54]. Therefore, synthesis of ammonia at reduced temperature will have higher conversion and reduced energy consumption. At 1 MPa and 350 \u00b0C, the activity of 80 % Fe \u2013 20 % Ce0.5Sm0.5O2-xNy is 2.86 mmol g\u22121 h\u22121 at WHSV of 16000 ml g\u22121 h\u22121, comparable to Fe-LiH (11 mmol g\u22121 h\u22121 at WHSV of 60000 ml g\u22121 h\u22121 [14], Ni-LaN (5.2 mmol g\u22121 h\u22121 at WHSV of 36000 ml g-1 h\u22121, 0.9 MPa) [29] (Table S2). At 350 \u00b0C, the activity of our Fe-Ce0.5Sm0.5O2-xNy is among the highest for all reported non-Ru catalysts for the Haber-Bosch reaction despite lower feed gas purity (99.996 %) (Table S2). The loading of cheap Fe (80 wt%) in our composite catalysts is much higher than the 0.4 wt% and 1.2 wt% Fe in the BaTiO2.4H0.6 and BaCeO3-xNyHz supported catalysts making direct comparisons less meaningful. To some extent, they are different catalyst types as our oxynitride promoted Fe catalyst is closer to the industrial fused Fe catalysts which normally contain over 90 wt% Fe. It is estimated that the cost of our 80 wt%Fe-20 wt% Ce0.5Sm0.5O2-xNy catalyst would be much lower than the LiH, BaTiO2.4H0.6, BaCeO3-xNyHz or LaN supported catalysts as cheap iron makes up the majority of the composition in our catalysts. Those with only a few weight percentage of transition metal such as Ru, Fe, Co, Ni are typically supported catalysts. The Fe-oxynitrides with 80 wt% Fe should be classified as composite catalysts.The apparent activation energy for the 80 wt%Fe \u2013 20 wt% Ce1-zSmzO2-xNy catalysts with z = 0.1 to 0.5 at 1 MPa and 3 MPa are also obtained through the Arrhenius plots using the activity data at a temperature below 450 \u00b0C (Fig. 6c & d). With increased z in the Sm-doped cerium oxynitrides, the Ea tends to decrease. This tendency is quite clear when the Ea is plotted against z in Ce1-zSmzO2-xNy (Fig. 6e). For z = 0.1, at 1 MPa, the Ea is 76 kJ/mol, which is comparable to the 70 kJ/mol at 1 MPa representative industrial KM1 catalysts [5,52]. However, when the pressure is increased to 3 MPa, Ea is also increased from 76 to 98 kJ/mol. This is common as Ea for industrial fused Fe catalyst increased to 180 kJ/mol at 10 MPa [5,55]. When z \u2265 0.2, either at 1 MPa or 3 MPa, the Ea for all the Fe-Ce1-zSmzO2-xNy composite catalysts is lower than the Ea of 70 kJ/mol for the industrial Fe catalyst. When z \u2265 0.3, the Ea is in the range of 45 kJ/mol and is less sensitive to pressure change from 1 to 3 MPa. The Ea for sample Fe- Ce1-zSmzO2-xNy is 43 \u00b1 3.22 kJ mol\u22121 at 1 MPa, 44 \u00b1 6.27 kJ mol\u22121 at 3 MPa, around 45 kJ mol\u22121 (Fig. 6e). This is comparable with the lowest Ea in reported papers for promoters/co-catalysts such as Ru/C12A7:e (49 kJ/mol at 0.1 MPa) [5,53], Fe/LiH (46.5 kJ/mol at 1 MPa) [5,14], Fe/BaCeO3-xNyHz (46 kJ/mol at 0.9 MPa) [7], Fe/BaTiO2.4H0.6 (63 kJ/mol at 4 MPa) [5] and Ni-LaN (57.5 kJ/mol at 0.9 MPa) [27]. The low apparent activation energy of our Fe-Ce1-zSmzO2-xNy composite catalyst with z \u2265 0.3 is clearly related to the high concentration of anion vacancies. The doping of large Sm3+ ions and introduction of nitrogen vacancies through partial replacement of lattice oxygen by the larger nitrogen ions, leads to a lattice expansion (Fig. 3b). Please note the lattice parameters for Ce1-zSmzO2-xNy with z \u2265 0.3 (a \u2265 5.4281(1) \u00c5) are much larger than that for sample CeO2-xNy (a = 5.4273(1)\u00c5) (Table 2, Fig. 3b). Larger lattice parameters means the void (free volume) for mobile anions vacancies is also bigger, making the possible adsorption of large N2 molecular easier if the nitrogen vacancies take part in the ammonia synthesis reaction, as is the case for perovskite oxynitride hydride BaCeO3-xNyHz and LaN [7,27]. A large lattice parameter will lead to high mobility of anions, which also benefits the reaction. This will be discussed later.In order to figure out the relationship between lattice volume and the ammonia synthesis activity, the activity of the 80 wt%Fe-20 wt% Ce1-zSmzO2-xNy composite catalysts at different temperatures are plotted against the z values in the Ce1-zSmzO2-xNy (Fig. 7\n). For both 1 MPa and 3 MPa, at different temperatures, the lowest ammonia synthesis rate was observed for the Fe - Ce0.9Sm0.1O2-xNy catalyst. This is consistent with the relatively small lattice parameters (cell volume) for Ce1-zSmzO2-xNy samples (Fig. 3b). Comparing the nitrogen content and anion vacancy concentration for samples CeO2-xNy and Ce0.9Sm0.1O2-xNy, they have the same level of nitrogen content (y = 0.07) (Table 1), while the anion concentration in sample Ce0.9Sm0.1O2-xNy is higher than that for CeO2-xNy due to the doping of low-valent element Sm. However, the activity of Fe-Ce0.9Sm0.1O2-xNy is lower, which seems correlated to the smaller lattice parameters (cell volume). From this point of view, the catalytic activity is correlated to both the concentration of anion vacancies and the size/volume of the crystal lattice. In solid state ionics, large lattice parameters will lead bigger cell volume to more \u2018free volume\u2019 (void not occupied by ions), making the migration of anions much easier, leading high ionic conductivity. It has been reported that partially replacing Sm3+ ions in Ce0.8Sm0.2O2-\u03b4 with larger Ca2+ ions, leads to increased O2\u2212 ionic conductivity because of the increased \u2018free volume\u2019, making the migration of O2\u2212 much easier [56]. It is expected that the same situation may happen on Sm3+ and N3- co-doped CeO2, which shares the same crystal structure as Ce0.8Sm0.2O2-\u03b4 (Table 1). This indicates that high mobility of the anions, particularly N3- ions in the oxynitrides, which may participate in the reaction for ammonia synthesis, is another important parameter to achieve high catalytic activity [7]. Theoretically high anion conductivity, particularly N3- ion conductivity, can extend the reaction zone of ammonia synthesis reaction as N3- ions can be formed at anywhere on the surface, then quickly diffuse through the oxynitride particles to an active site. These active sites are the contact points between \u03b1-Fe and Ce1-zSmzO2-xNy, and is where the reaction is completed (Fig. 10), leading to higher catalytic activity. Introduction of O2\u2212 ionic conduction in electrodes to facilitate the electrode reaction has been widely used in SOFCs. This key strategy may also be employed to the catalytic reaction for ammonia synthesis to improve the activity of composite catalysts if anions such as N3- ions also take part in the reaction.The anion vacancy promotion effect due to the doping of nitrogen in CeO2, CeO2 mixed with the Fe-catalyst at the same mass ratio (20 : 80) was also investigated, as shown in Fig. 8\n\n\na&b. In the measured temperature range, Fe-CeO2-xNy shows much higher activity than Fe-CeO2, indicating the vacancy-rich CeO2-xNy is a better promoter. At 3 MPa and 450 \u00b0C, the ammonia formation rate increases from 9.7 mmol g\u22121 h\u22121 to 17.2 mmol g\u22121 h\u22121 when Fe\u2013CeO2 is replaced with Fe\u2013CeO2-xNy, almost doubling the activity. This provides further evidence that the anion vacancies, particularly nitrogen vacancies, may take part in the reaction, leading to increased activity [7,22,26,27]. The slightly increased activity of the Fe-CeO2 catalyst at 600 \u00b0C and 3 MPa (Fig. 3b) could be related to the formation of intrinsic oxygen vacancies at high temperature with the presence of high pressure H2 (Eq. (2)). The maximum rate allowed at our reactor conditions according to thermodynamic equilibrium is shown in Fig. 8a & b [54]. It can be seen that for both 1 MPa and 3 MPa, the reactions approach the equilibrium conversion as temperature increases. For both pressures, the catalyst reaches their peak activity at values lower than the thermodynamic equilibrium rate.The apparent activation energies of the Fe catalysts promoted by all three promoters are lower than 70 kJ/mol for the representative industrial Fe catalyst (Fig. 8c & d) [5,52]. The activation energies for Fe-CeO2 and Fe-CeO2-xNy are around 60 kJ mol\u22121, slightly higher than that for Fe-Ce0.5Sm0.5O2-xNy (Fig. 6e). The presence of a large amount of anion vacancies in Ce0.5Sm0.5O2-xNy benefits the ammonia synthesis reaction, reducing the apparent activation energy (Table 1).The ammonia synthesis rate is a key parameter when people talk about the activity of a catalyst. The ammonia synthesis rate is related to the activity of the catalyst, the loading of the catalyst and gas flow rate (space velocity). The effect of space velocity on the ammonia synthesis rate and conversion of 80 wt%Fe-20 wt% Ce0.5Sm0.5O2-xNy at 400 \u00b0C, 3 MPa is shown in Figure S7. The rate of formed ammonia increased as flow rate of the feed gases and WHSV was increased. However, total conversion increases at reduced feed gas flow rates due to the longer residence time of the reactants in the catalyst bed. A nearly linear relationship between total flow rate and outlet conversion was observed indicating good mass transfer properties between that catalyst and reactants. This experiment indicates that the ammonia synthesis rate is linear to the space velocity in the region tested, the ammonia synthesis rate will be doubled if the space velocity is doubled and vice versa. Therefore, WHSV should be taken into account when comparing the ammonia synthesis rate from different sources [22]. In Table S2, the ammonia synthesis rates from different Fe-based catalysts plus the representative Cs-Ru/MgO are listed together alongside their respective WHSV.Stability is an important parameter for industrial ammonia synthesis catalysts. Among all the investigated promoters (CeO2, CeO2-xNy and Ce1-zSmzO2-xNy), Ce0.5Sm0.5O2-xNy exhibits the highest promotion effect (or co-catalyst) to the Fe catalyst for ammonia synthesis. Industrial ammonia reactors usually run continuously for long periods of up to years at a time. Long term stability tests of the new catalysts are carried out as it is vital for commercial applications. The stability of the 80 wt% Fe \u2013 20 wt% CeO2-xNy catalyst was measured for nearly 200 h in Zero grade feed gas, at the optimised conditions, 450 \u00b0C and 3 MPa (Fig. 9a). There is a slight drop in activity over the first 50 h before the catalyst stabilises then remains stable over the rest of the entire test. This slight initial drop in activity is expected to be caused by the change in nitrogen content in the promoter material which needs to be stabilised in mixed N2 and H2 at high temperature and high pressure. At high temperature, it is possible that the Ce0.5Sm0.5O2-xNy is partially oxidised by the oxygenate impurities as nitrogen was not detected by XPS for the 80 wt% Fe \u2013 20 wt% Ce0.5Sm0.5O2-xNy sample after the catalytic activity at both 1 and 3 MPa to a temperature up to 600 \u00b0C (Fig. 2) although the nitrogen content in Fe - Ce0.5Sm0.5O2-xNy could be too low, beyond the measuring limit for XPS and CHN.The stability of the 80 % Fe \u2013 20 % Ce0.5Sm0.5O2-xNy catalyst at 400 \u00b0C and 3 MPa was also measured for over 200 h (Fig. 9b). Similar to the CeO2-xNy promoted Fe-catalyst, an initial slight drop in activity is also observed, remaining stable for the rest of the tested hours. Although the initial drop is at a similar extent to the CeO2-xNy promoted Fe-catalyst, the drop in activity for 80 % Fe \u2013 20 % Ce0.5Sm0.5O2-xNy catalyst continues over a longer period of time, about 100 h. Since the operating temperature is 50 \u00b0C (400 \u00b0C instead of 450 \u00b0C) lower, it could therefore take a longer time to stabilise the nitrogen content in the Ce0.5Sm0.5O2-xNy promoter.In conventional ammonia synthesis catalysts, low operating temperatures amplify the poisoning effect of impurities causing a more significant problem than at high temperatures [17,18]. The stability of 80 % Fe \u2013 20 % Ce0.5Sm0.5O2-xNy catalyst at 300 \u00b0C, 3 MPa was therefore also investigated. This catalyst is stable at 300 \u00b0C during the measured 290 h in which there is a \u223c 90 h break (Fig. 9c). During the break, the gas flow was stopped, the reactor was cooled down to room temperature with 3 MPa mixed N2 and H2 together with the generated NH3. This experiment indicates that, the oxynitride promoted Fe catalyst exhibits excellent stability in Zero grade feed gas, even at room temperature. This is very different from the conventional industrial fused Fe catalyst as the promoters we used are vastly different. In fused Fe catalysts, the stability relied on the homogeneously distributed Al2O3 additive [17,18]. In our catalyst, there is no added Al2O3 at all. The stability of our Fe-oxynitride composite catalysts relied on the SMSI between iron and oxynitride, prohibiting the growth or sintering of the iron particles. Therefore, our Fe-oxynitride catalyst is even more stable at temperatures as low as room temperature [22].Intermittent operation of the ammonia synthesis reactor is therefore possible as the catalyst will not be damaged during the heating/cooling process. This is particular useful for green ammonia production at a small scale using the surplus intermittent renewable electricity as the energy sources [2]. The dominant technology for ammonia production will be the Haber-Bosch reaction for the foreseeable future, including green ammonia production [2,18,57]. It has been reported that improving the efficiency of water electrolyser and/or developing new catalysts enabling the agile operation of the Haber-Bosch process are the keys to achieving green ammonia industry [58]. From this point of view, if our cation doped cerium oxynitride promoted catalysts are used in the localised green ammonia synthesis plants, then there is potential for this technology to be used for renewable electricity storage providing a better match with the intermittent nature of the renewable resources. During the stability test at 300 \u00b0C, no initial activity drop was observed indicating the nitrogen content is fairly stable at this temperature, which implies Ce0.5Sm0.5O2-xNy is not (partially) oxidised by the oxygenate impurity at 300 \u00b0C.It has been reported that high pressures can help to prevent the decomposition of oxynitrides to oxides and nitrogen, and high pressures facilitate the oxynitride synthesis process [59]. Therefore at 3 MPa both Ce0.5Sm0.5O2-xNy and CeO2-xNy promoters are expected to have a higher tolerance to decomposition caused by oxygenate impurities in the feeding gas. We tried to test the nitrogen content in the 80 wt%Fe\u201020 wt%Ce0.5Sm0.5O2-xNy catalyst after the stability test, however, due to the large Fe content of 80 wt%, the nitrogen content is too low, beyond the measurement limit of the XPS and CHN facility as the total nitrogen in the total composite catalyst is too small. The stability of oxynitrides is related to both the oxygenate concentration and reaction temperature. When the reaction temperature is reduced to 300 \u00b0C, it was found that the catalytic activity of Fe-Ce0.5Sm0.5O2-xNy catalyst is stable, indicating that the Ce0.5Sm0.5O2-xNy is stable in the less pure feed gas at 300 \u00b0C. It has been reported that the Ni-LaN catalyst is stable for ammonia synthesis when ultrapure feed gas (purity > 99.99995 %) was applied [27]. Ce0.5Sm0.5O2-xNy could also be chemically stable at higher temperatures when ultrapure feed gas is used for ammonia synthesis.It has been confirmed that the 80 % Fe \u2013 20 % Ce0.5Sm0.5O2-xNy catalyst exhibits the highest activity without further purification of the Zero grade H2 and N2 feed gases. In the past, catalyst tolerance towards impurity is normally tested at fixed concentration of O2 such as 5 ppm of an oxygenic compound [20], 1 ppm impurity [19,21]. In order to test the limit of our Fluorite oxynitride promoted Fe catalyst, a much higher concentration of impurities was used. The activities of our 80 wt% Fe \u2013 20 wt% Ce0.5Sm0.5O2-xNy catalyst at 475 \u00b0C and 3 MPa with different impurity levels up to 200 ppm were investigated (Fig. 9d). Here the temperature is the measured temperature of the tube furnace hot zone. This investigation into oxygenate tolerance to 200 ppm impurity was conducted in a newly designed reactor capable of achieving higher pressures with a large wall thickness (0.125 inch instead of 0.083 inch). Due to the less effective heat transfer between the thick wall reactor and hot zone of the furnace, the real temperature of the catalyst in new thick wall reactor is slightly lower [22]. An impurity of 10 ppm in the gas is expected to remain as the O2 and H2O traps cannot remove impurities such as CO, CO2, and hydrocarbons [22]. To achieve higher impurity levels, zero grade nitrogen was mixed with nitrogen with 1000 ppm O2 to reach higher quantifiable oxygenate concentrations desired. The activity of the 80 wt% Fe \u2010 20 wt% Ce0.5Sm0.5O2-xNy composite catalyst in Zero grade gas is about 86 % of that after the purification process indicating that the oxygenate impurities still exhibit an effect on activity. Higher activity can be obtained if very pure feed gas is applied in our Fe-oxynitride catalysts. Further increases in the total impurity level to 104 ppm, with known injected 61 ppm oxygen, provides a rate retention of 81 %. Please note 61 ppm oxygen equals to 122 ppm atomic oxygen. This is over 10 times of the maximum allowed oxygenate level (10 atomic oxygen) for industrial fused Fe catalysts [17]. The ammonia formation rate is still more than half (53 %) of the original rate in purified gas when total impurity level was 200 ppm with known injected 158 ppm O2. This experiment indicates the 80 wt% Fe \u2013 20 wt% Ce0.5Sm0.5O2-xNy composite catalyst exhibits excellent oxygenate tolerance properties. When a feed gas with 200 ppm impurities is used, by doubling the amount of catalyst / the size of the reactor, the same amount of ammonia can be produced compared to standard feed gas with 10 ppm atomic oxygenate. If an oxygenate tolerant catalyst, such as Fe \u2013 Ce1-zSmzO2-xNy composite is used in the reactor, purification requirements will be lower thus saving on initial facility cost and continued energy inputs significantly improving the overall efficiency for ammonia synthesis. However, there is a risk that the oxynitride may be partially oxidised by oxygenate impurities if their concentration is too high at high reaction temperature. The formed Sm-doped CeO2 will still exhibit high activity and stability but the activity will be slightly lower due to the loss of nitrogen vacancies (Fig. 9b) [22]. Therefore, if we want to take advantage of nitrogen vacancies in Ce1-zSmzO2-xNy to achieve the high activity it is required to minimise the oxygenate concentration. Developing more stable promoters with anion vacancies present in a high concentration, particularly nitrogen vacancies is therefore desired in order to achieve a high stability in oxygenates.The results presented above clearly show that the introduction of a large amount of anion vacancies in CeO2 through cation (Sm3+ ions) and anion (N3\u2212 ions) co-doping result in various changes in catalytic properties when used as support for low-cost Fe catalysts for ammonia synthesis. Both stability and activity of the Fe-Ce1-zSmzO2-xNy composite have been significantly improved, which is attributed to the anion vacancies, particularly nitrogen vacancies. The schematic diagram on the interaction between anion vacancies and \u03b1-Fe particles and the reactants, H2 and N2 for ammonia synthesis is shown in Fig. 10.As for the reaction mechanism of oxynitride or oxynitride hydride promoted transition metal (TM) catalysts, Kobayashi et al. reported that oxyhydride BaTiO3-xHx improves the activity of the transition metal catalysts through the oxynitride-hydride intermediate, where both lattice N3\u2212 ions and H- ions play important roles for the increased catalytic activity [5]. Kitano et al. proposed two possible reaction mechanisms for ammonia synthesis over TM / BaCeO3-xNyHz catalysts. Both are related to Mars \u2212 van Krevelen mechanism through anion vacancies with the participation of lattice N3\u2212 and H- ions [7]. The key evidence for the Mars \u2212 van Krevelen mechanism is the low apparent activation energy, 46\u221262 kJ/mol for TM/ BaCeO3-xNyHz catalysts [7]. The introduction of N3\u2212 ions to the BaTiO3-xHx lattice through an in situ formed oxynitride-hydride intermediate or, to the perovskite oxynitride hydride BaCeO3-xNyHz lattice at the very beginning, will generate anion vacancies from the charge balance. The more N3\u2212 ions are introduced into the lattice, the more anion vacancies will be generated. In our Ce1-zSmzO2-xNy, there are N3\u2212 ions in the lattice already, similar to BaCeO3-xNyHz. As for the H- ions, it has been widely reported that Ce-H species may be formed when CeO2 is exposed in H2 at high temperature while more Ce-H species can be formed when more oxygen vacancies are presented in the CeO2 lattice [60,61]. It is reasonably deduced that, under the ammonia synthesis conditions, in the presence of high concentration (\u223c 75 %) H2 at high temperature, some hydride intermediates may also be formed in our cerium oxynitrides. Following this our oxynitride would be a kind of oxynitride hydride, similar to BaCeO3-xNyHz, although exhibiting a fluorite structure instead of a perovskite structure. The apparent activation energy of the 80 wt% Fe- 20 wt% Ce1-zSmzO2xNy catalysts with z \u2265 0.3 is around 45 kJ/mol, which is fairly close to that for the TM/ BaCeO3-xNyHz catalysts [7]. The low activation energy plus the presence of N3- ions and possible indirectly formed H- ions through the intermediate in Ce1-zSmzO2-xNy, are very similar to the case for BaCeO3-xNyHz, thus they may share the same or similar reaction mechanism.Another important finding in this study is, the catalytic activity seems related to the lattice parameter of the Ce1-zSmzO2-xNy support. The larger the lattice volume with larger \u2018free volume\u2019, the higher the catalytic activity (Figs. 3b & 7). This can be considered in two aspects: (a) The size of anion vacancies. As shown in Fig. 10, both N2 and H2 may be adsorbed on the anion vacancies on the surface. It is believed that the bigger the vacancy (charged void), the easier for the adsorption of N2/H2 gases, thus the higher the catalytic activity. (b) The mobility of N3\u2212 and H- ions. When N2 and H2 are adsorbed on anion vacancies on the surface of Ce1-zSmzO2-xNy, it may dissociate to form N3- and H- ions, according to the proposed reaction mechanism of oxynitride hydride [7]. High mobility or ionic conductivity of N3- and H- ions will allow the reaction between N3- ions and adsorbed H- species on Fe or, between H- ions and N-species on Fe to happen all over the surface of the composite catalyst, rather than limited to the triple phase (Fe- Ce1-zSmzO2-xNy-gaseous reactants) boundary, similar to the case for the reaction on the anode of a solid oxide fuel cell. This will increase the probability for the formation of ammonia thus result in a higher activity. Large lattice volume means large \u2018free volume\u2019 thus the ionic conductivity or mobility of the N3-/ H- ions will be higher, leading to higher catalytic activity [62]. It should be noted that Sm-doped CeO2 is a well-known O2- ionic conductor with high ionic conductivity and has been used as an electrolyte for SOFCs. It is expected that the ionic conductivity for other anions such as N3- and H- ions in the Ce1-zSmzO2-xNy will also increase at increased lattice parameters thus larger \u2018free volume\u2019 for easy diffusion of ions, facilitating the ammonia synthesis reaction [56].The high concentration of anion vacancies in Ce1-zSmzO2-xNy will facilitate the nesting/anchoring of Fe particles, resulting in SMSI, which has been described above (Fig. 10). This SMSI can prohibit the sintering of Fe particles even under a strong oxidization environment [22,29]. Therefore, a large amount of anion vacancies in cation doped cerium oxynitride improved both the stability and catalytic activity for ammonia synthesis.It has been reported that, for cubic Fe, (111) plane is the most active for ammonia synthesis reaction. The second most active plane is (211) when exposed to the reactant gases [63]. If a Fe atom is nested onto a surface anion vacancy via SMSI, where the array of anions (O2\u2212 and N3- ions) are, ideally the (111) plane is in parallel or close to parallel to the plane of anion arrays. Therefore, the probability for (111) planes to be exposed to the reactants (H2 and N2) is very high, thus can maximize the ammonia production [22]. Theoretically N2 located away from the \u03b1-Fe may also be dissociated by nitrogen vacancies not in contact with Fe, then diffuse through the whole Ce1-zSmzO2-xNy particle, then reach the contact point between \u03b1-Fe and Ce1-zSmzO2-xNyto catalyze the ammonia synthesis reaction. From all aspects of the ammonia synthesis reaction, oxynitrides with a large amount of anion vacancies, particularly nitrogen vacancies, will benefit the reaction to improve both activity and stability.Cation doped fluorite oxynitrides, new single phase Sm-doped cerium oxynitrides have been synthesized for the first time. For the high Sm-doped cerium oxynitrides, approximately 16.5 % of the anion positions are not occupied. The introduction of nitrogen to form nitrogen vacancies will have better match to the adsorbed N2 in terms of size. The optimised composition is 80 wt% Fe \u2013 20 wt% Ce0.5Sm0.5O2-xNy which showed an activity of 18.8 mmol g\u22121 h\u22121 at 400 \u00b0C, 3 MPa (WHSV = 16000 mL g\u22121 h\u22121) using 99.996 % H2 and N2 as the feed gases, comparable to the industrial fused Fe catalyst at a much higher purity. At 350 \u00b0C and 1 MPa, the activity is among the highest in all reported non-Ru based catalysts. The apparent activity energy of our Fe-Ce1-zSmzO2-xNy catalysts with z \u2265 0.3 is in the range of 45 kJ/mol, among the lowest Ea for all reported ammonia synthesis catalysts [5,7,27]. It is believed that the reaction proceeds through the Mars \u2212 van Krevelen mechanism mediated by the anion vacancies, similar to perovskite oxynitride hydride BaCeO3-xNyHz. At 3 MPa and 475 \u00b0C, the activity retention of the Fe \u2013 20 wt% Ce0.5Sm0.5O2-xNy catalyst is 70 % with known injected 107.5 ppm O2 (150 ppm impurity level). The Fe-Ce0.5Sm0.5O2-xNy catalyst exhibits excellent stability at 300 \u00b0C, even after cooling to room temperature implying stability at room temperature. This is suitable for agile operation of localised green ammonia synthesis plants using intermittent energy from renewable electricity. Both catalysts starting from \u03b1-Fe2O3- oxynitride are stable in air at room temperature and are thus easy to handle. This article provides a new development strategy for the synthesis of novel oxygenate-tolerant ammonia synthesis catalysts that can be used for both existing large scale Haber-Bosch processes as well as small scale green ammonia synthesis from renewable energy sources. Agile operation is key for small scale ammonia plants utilising intermittent renewable energy, therefore, future work thoroughly investigating the catalyst tolerance to the thermal shock of reactor start-up/shut-down should be investigated based on the promising results highlighted in the stability test. The development of a large weight percent iron based catalyst will have vast cost advantages over expensive Ru and Co based catalysts. However, the replacement of relatively expensive rare earth elements in the oxynitride provides room for further improvements in this regard, which is under investigation.None.The authors thank EPSRC (Grant No. EP/G01244X/1) for funding.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2020.119843.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  For the first time, new Sm doped cerium oxynitrides with the formula Ce1-zSmzO2-xNy (z \u2264 0.5) are synthesized in order to maximize the concentration of anion vacancies. Single phase Sm-doped CeO2-xNy were confirmed by XRD, HRTEM and Rietveld refinement. These oxynitrides show a great promotion effect for the low-cost Fe catalyst for the ammonia synthesis. At 350 \u00b0C and 1 MPa, the activity of 80 wt% Fe- 20 wt% Ce1-zSmzO2xNy is one of the highest reported for non-Ru catalysts for the Haber-Bosch reaction. The apparent activation energy of the 80 wt% Fe- 20 wt% Ce1-zSmzO2xNy catalysts with z \u2265 0.3 is around 45 kJ/mol, which is in the lowest range among all reported ammonia synthesis catalysts. Introduction of nitrogen vacancies through doping may facilitate the mobility of nitrogen vacancies. This study demonstrates doped oxynitrides with a large concentration of anion vacancies, particularly nitrogen vacancies are excellent promoters/co-catalysts for ammonia synthesis.\n               "}
{"full_text": "The production of fuels, such as bio-kerosene for aviation transportation, directly from lignocellulosic biomass is of increasing interest for energy sustainability [1]. Unfortunately, bio-oil produced via thermal pyrolysis of biomass does not meet the requirements desired for a transportation fuel as it is very viscous, dense, corrosive and poorly energetic [2]. Therefore, the bio-oil formed is required to undergo a set of upgrading steps, in which the energetic value is increased by oxygen depletion and by chain elongation. Catalytic fast pyrolysis (CFP) takes place at high temperatures (\u223c500\u00a0\u00b0C), in an inert atmosphere (i.e., in the absence of oxygen) and with short residence times (i.e., few seconds) to lower the bio-oil oxygen content [3]. When CFP is performed in the so-called ex-situ mode [4], the catalyst is only contacted with the pyrolysis vapors after conducting the thermal pyrolysis step in a separate reactor. This way of operation favors the formation of aliphatics and olefins over aromatics, which are more interesting for bio-oil production. The bio-oil generated by ex-situ CFP can be further upgraded to advanced bio-fuels by subsequent deoxygenation reactions.Traditionally, zeolite ZSM-5 has been mostly used for the CFP step. This can be done solely with zeolite ZSM-5 as the active phase [5,6] or in combination with promoters, such as Ni or Ga [7,8], which boost its efficiency in obtaining a high-quality deoxygenated bio-oil. The good activity of zeolite ZSM-5 is related to its shape selectivity, acidity and thermal stability [5,6]. However, a common limitation of zeolite ZSM-5 in the CFP process is its deactivation due to coke formation, which results in clogging its micropores. Creating mesoporosity within zeolite ZSM-5, e.g. via desilication [9], can considerably lower the detrimental effects of pore blockage by coke formation [10]. It is important to note that the formation of coke deposits is induced by olefin polymerization [11], as well as other C-C coupling reactions, all of which can abundantly occur during biomass conversion processes. C-C coupling reactions are also required, however, as part of the necessary bio-oil upgrading, with ketonization [12,13] or aldol condensation [14,15] being typical examples, given the large amounts of carboxylic acids, ketones and aldehydes present in bio-oil [16]. Indeed, these reactions are often the topic of (model) bio-oil upgrading and deoxygenation studies and are commonly catalyzed by base catalysts, such as metal oxides (e.g., ZrO2, TiO2 or hydrotalcites) or alkali metal-exchanged zeolites [17,18].For bench-scale and pilot-scale testing, the use of shaped catalyst bodies is required to ensure mechanical strength and to avoid pressure drop issues in the chemical reactors used [19]. To make suitably shaped catalyst bodies, binder materials, such as clay minerals and alumina, are typically added to the powdered catalysts to obtain a mixture that upon extrusion or granulation generates the shaped catalyst bodies. However, binders can modify the catalyst properties and hence activity, e.g. by offering a source of metal cations that can exchange with and alter the catalyst\u2019s active sites, altering for example catalyst acidity within a zeolite [20,21]. Yet, binder materials can also affect the catalyst\u2019s stability. Deactivation of catalyst bodies has been studied, for instance, for fluid catalytic cracking (FCC) particles. These FCC particles are formed via e.g., a spray drying procedure of the different catalyst components and consist of a zeolite material, e.g. zeolite H-Y, promoted or not with active metals (e.g., La), binder and filler materials, such as SiO2, Al2O3 and clay minerals [22,23]. The binder and filler materials improve the stability of the active zeolite component, which would suffer from more structural damage under the hydrothermal conditions applied in the industrial reactor and regenerator systems if the binder and filler would not be present [24]. Common reasons for FCC catalyst deactivation are poisoning by coke [25,26] and metals deposition which leads to catalyst particle agglutination, impeded accessibility of the zeolite\u2019s pore network, and alterations in size and acidity, as illustrated by Meirer et al. [27\u201329].The nature of the binder material in a catalyst body has a significant impact on its performance. On extruded zeolite-based catalysts, Verkleij et al. reported on the higher resistance against deactivation for zeolite ZSM-5 when blended with Al2O3 rather than SiO2, in the 1-hexene oligomerization reaction. Indeed, the Al2O3-based system better favored elongated oligomers over branched ones, protecting the zeolite pores better against clogging [30]. The same catalyst systems, when applied in the transalkylation of aromatics [31,32], also showed differences in stability. In this case, the Al2O3-based catalyst extrudates showed more and more condensed coke deposits than the SiO2-based catalyst. These deposits were located mainly on the rim of the zeolite crystals, however, while in the case of the SiO2-based catalyst extrudates coke deposits could be found throughout the zeolite crystals, leading to faster deactivation.Indeed, the choice of the binder and its integration with the other components of the technical catalyst \u2013 i.e., the manner in which e.g. porosity, crystal size or acidity of the zeolite component is affected - will impact the mode, rate and extent of catalyst deactivation, as previously reported [33]. To reverse (totally or partially) certain causes of catalyst deactivation, oxidative regeneration cycles at high reaction temperatures can be carried out [34,35]. Indeed, by burning-off the coke deposits from spent catalysts, pore or active site blockage can in principle be reversed and the physicochemical properties of the catalyst material, such as surface area, pore volume and acidity, restored. As an example, Michels et al. demonstrated that upon regeneration in flowing air at 550\u00a0\u00b0C for 3\u00a0h, a zeolite ZSM-5-based catalyst extruded with attapulgite as binder material showed a significant recovery of the textural properties and catalytic activity in the methanol-to-hydrocarbons (MTH) reaction [36]. However, other causes of catalyst deactivation, such as morphological and textural changes, could not be repaired by such a regeneration process. Indeed, because of the high temperatures and steam generated upon coke combustion, changes in the structural and textural properties of the catalyst usually worsen [25]. In line with this observation, previous reports [37] emphasized the importance of applying intermediate regeneration temperatures (yet equal or higher than the reaction temperature), to ensure total combustion of the coke deposits formed within the zeolite catalyst but avoid irreversible structural damages.In this work, we report on the different modes of deactivation of two tailored catalysts used in cascade for a more efficient bio-oil deoxygenation in lignocellulose catalytic pyrolysis. This cascade reaction [38] combines the synergistic effect of a solid acid ZrO2/desilicated zeolite ZSM-5/attapulgite clay mineral (further denoted as ZrO2/ds-ZSM-5-ATP) -employed for the ex-situ CFP step- and a solid base K-grafted zeolite USY/attapulgite (further denoted as K-(USY-ATP)) -used in the subsequent bio-oil upgrading step-. While the acidity of the ZSM-5-based catalyst is key to promote cracking and alkylation reactions involved within the catalytic pyrolysis stage, the basicity of the alkaline-grafted USY catalyst is so for the subsequent upgrading of bio-oil via deoxygenating routes, such as aldol condensations [15].Understanding the origin and cause of the deactivation of these catalysts, as well as the extent to which deactivation can be reversed by a suitable regeneration treatment will allow optimizing the catalyst lifetime. To this extent, the catalyst materials were extensively characterized using bulk and spatially-resolved characterization techniques on fresh, spent and regenerated samples in their shaped form. The extent, nature and location of coke formation, the structural integrity and acid/base properties of the catalyst materials, and the regeneration effects on coke removal and recovery of the original properties have been assessed.The experimental details related to catalysts syntheses and characterization are presented in the Supporting Information.The bench scale set-up in which ZrO2/ desilicated ZSM-5-attapulgite catalyst (ZrO2/ds-ZSM-5-ATP) as solid acid and K-exchanged zeolite USY/attapulgite catalyst (K-(USY-ATP)) as solid base were tested is schematically depicted in Fig. 1\nc [39]. Biomass feedstock consisted of previously de-ashed wheat straw (4\u00a0g) heated to 550\u00a0\u00b0C and fluidized in 100 NmL/min N2. In a first stage, a catalytic bed of ZrO2/ds-ZSM-5-ATP is exposed to the pyrolytic vapors coming from the thermal pyrolysis stage. The subsequent stage consists of a fixed bed of K-(USY-ATP) catalyst for the for the treatment of the catalytic vapors, which are further upgraded. Both catalytic pyrolysis and upgrading processes operate at 450\u00a0\u00b0C, and on both processes the catalyst to biomass ratio (C/B) was of 0.6 (excluding the binder into the catalyst weight). The vapors leaving the reactor were condensed to collect the liquid bio-oil for approx. 10\u00a0min; non-condensable gases were collected in sampling bags at the end of the line. The energy yield associated with bio-oil product was calculated as the proportion of chemical energy (HHV) retained regarding that of raw biomass.Regeneration of both catalysts was carried out under static air by heating the catalyst bodies in an open crucible at a temperature ramp of 1.8\u00a0\u00b0C/min up to 550\u00a0\u00b0C and holding this temperature for 6\u00a0h.\nFig. 1 shows the activity of the cascade process, consisting of an ex-situ catalytic fast pyrolysis step (i.e., thermal and catalytic pyrolysis) with a ZrO2/ desilicated ZSM-5-attapulgite catalyst (ZrO2/ds-ZSM-5-ATP) as solid acid and a catalytic upgrading step with a K-exchanged zeolite USY/attapulgite catalyst (K-(USY-ATP)) as solid base. The activity is expressed as the catalytic bio-oil deoxygenation as a function of the mass yield (a) and the energy yield (b) for the cascade process (see reactor scheme in Fig. 1c) compared to the single-step CFP process run with ZrO2/ds-ZSM-5-ATP only.After applying this three-step cascade process for a catalyst/biomass (C/B) ratio of 0.6, and for a bio-oil mass yield of 40\u00a0wt%, the deoxygenation degree (compared to the non-catalytic thermal bio-oil) was \u223c60\u00a0wt% (see green triangle data in Fig. 1a). For the ex-situ CFP only experiment, the run with a C/B ratio adjusted at 1.2 to run at equal total amount of catalyst material used. The deoxygenation degree was ca. 15\u00a0wt%, clearly inferior to the cascade catalytic process (grey triangle data series). Also, it turned out that at the same weight conditions the energy yield (Fig. 1b) is higher when two catalytic processes are coupled (60%) with respect to only one catalytic step (57%). Accordingly, the cascade process leads to both enhanced bio-oil deoxygenation and energy yield.After ex-situ CFP, 6.8\u00a0wt% of coke was formed in the ZrO2/ds-ZSM-5-ATP catalyst, as determined by thermogravimetric analysis-mass spectrometry (TGA-MS) (Fig. S1). The main part of it (6.0\u00a0wt%) was highly deficient in hydrogen, indicated by the high temperature of combustion (\u223c480\u00a0\u00b0C, Fig. S1a-b) [40]. Given the likely insoluble character of the coke formed which precludes the analysis by chromatography, its polyaromatic nature was confirmed by FT-IR spectroscopy, showing the characteristic stretching bands of condensed ring aromatic structures (\u03bdC=C) in the 1560\u20131600\u00a0cm\u22121 spectral region and the C-H stretching vibrations (\u03bdCH) corresponding also to aromatic coke at 3067\u00a0cm\u22121 with contributions of methylene (2930 and 2860\u00a0cm\u22121) and methyl (2970\u00a0cm\u22121) groups [41] (Fig. S2a). Condensations, alkylations and isomerization reactions produced upon CFP may be responsible for the origin of these polyaromatic species. The FT-IR spectrum showed also signatures of aldehydes (\u03bdCH at 2745\u00a0cm\u22121, \u03bdC=O at 1714 and 1691\u00a0cm\u22121), olefins (\u03bdC=C 1636\u00a0cm\u22121) and organic acids (\u03bdC=O/\u03bdO-H 1616\u00a0cm\u22121), among other compounds, indicating the presence of oxygenates in the carbon deposits including phenols and furans. The corresponding UV\u2013Vis diffuse reflectance spectrum (Fig. S2b) further corroborated the aromatic nature of coke with the presence of species such as pyrenes (with absorption bands at \u223c250\u2013350\u00a0nm) [42], naphthalenes and anthracenes (with absorption bands at \u223c280\u2013400\u00a0nm) and more conjugated poly-aromatic carbonaceous species and/or graphite-like coke, likely insoluble, characterized by absorption bands with maxima at >400\u00a0nm [43].The size of the coke deposits formed on the spent ZrO2/ds-ZSM-5-ATP catalyst was estimated by Raman spectroscopy, based on the expression introduced by Ferrari and Robertson for disordered graphitic carbons and graphene (see Eq. (1) in the Supporting Information) [44,45]. Based on the D and G integrals (Fig. S3a), the average coke size is estimated to be between 5 and 10\u00a0\u00c5. Substituted pyrenes (Fig.S3b) are of this size and have been proposed by Guisnet and Magnoux as average component of the (soluble) coke formed within zeolite H-ZSM-5 material for a coke content close to 9\u00a0wt%, with an approximate boiling point close to 400\u00a0\u00b0C [46].The spent K-(USY-ATP) catalyst extrudate, employed for the catalytic deoxygenation of the bio-oils formed in the first CFP step, showed a lower amount of coke deposits than the ZrO2/ds-ZSM-5-ATP CFP catalyst. This is consistent with the process setup, with the zeolite ZSM-5-based catalyst being directly exposed to the raw pyrolytic vapors, while the K-loaded zeolite USY-based catalyst further upgrades the already treated vapors. TGA-MS analysis of the spent USY catalyst (Fig. S1c,d) showed a coke content of 5.6\u00a0wt%, being for the largest part again poly-aromatic [40] (4.9\u00a0wt%, combusting at \u223c415\u00a0\u00b0C), while a smaller fraction is attributed to hydrogen-richer coke (Table 1\n). The nature of the coke deposits was further characterized by FT-IR spectroscopy (Fig. S2c) and UV\u2013Vis DRS (Fig. S2d). FT-IR spectroscopy on the spent K-(USY-ATP) catalyst material (Fig. S2c) showed the presence of aromatics (\u03bdCH at 3067\u00a0cm\u22121) and coke (\u03bdC=C at 1574, 1600\u00a0cm\u22121). As expected, the lower content of aromatics and coke for the K-(USY-ATP) catalyst was indicated by the lower relative intensities of these bands (Fig. S2c) compared to the ZSM-5-based catalyst employed in the first catalytic stage (a). It should be noted that the FT-IR bands at 2745\u00a0cm\u22121 (\u03bdCH) and 1714\u00a0cm\u22121 (\u03bdC=O) are indicative of aldehydes, showing also oxygenates presence which originate from the deoxygenation activity carried out by the K-(USY-ATP) catalyst. The UV\u2013Vis DRS spectrum of the spent K-(USY-ATP) catalyst material (Fig. S2d) indicates the presence of hydrogen-rich coke types, such as alkylated benzenes, absorbing light in the range of 250\u2013270\u00a0nm; hydrogen-deficient coke types, such as naphthalenes and anthracenes, which absorb light in the range of 280\u2013400\u00a0nm [43]; and poly-aromatic carbonaceous species above 400\u00a0nm.As catalyst extrudates might be subject to diffusion limitations, any coke deposits formed could be non-homogeneously distributed over the shaped catalyst body. To study any spatial distribution of the coke deposits, the spent technical catalysts were studied by Confocal Fluorescence Microscopy (CFM) [47], as schematically depicted in Fig. 2\n.Visual inspection after cross-sectioning of the spent ZrO2/ds-ZSM-5-ATP catalyst showed a clear egg-shell distribution of the coke deposits formed. When the surface/shell (Fig. 2a) was irradiated with the excitation lasers of the CFM set-up, no fluorescence could be detected because of the high amount of poly-aromatic or (even) coke, which renders the surface opaque. When moving from the edge to the center of the catalyst body (Fig. 2b), fluorescence was seen, where there were less coke deposits and of softer nature -i.e. H-richer- than on the catalyst surface. Distinguished regions can be observed: brighter ones (highlighted in red, labelled as 2) with an extensive presence of naphthalenes and anthracenes (emitting light at wavelengths below 550\u00a0nm), and darker (green, 3) with higher presence of poly-aromatics (with more than 3 aromatic rings) which emit light above 550\u00a0nm [47,48]. The ratio between poly-aromatic and anthracene-like carbonaceous species was more pronounced in the regions located closer to the edge of the cross-section, (3, green), confirming the higher presence of more conjugated coke on more external locations of the catalyst body.Zooming in at a central position of the cross-section (1, blue, Fig. 2b and Fig. 2c) revealed various bright spots (A) assigned to the presence of zeolite crystals (0.5\u20132\u00a0\u00b5m) where coke preferentially forms in mesopore walls [49,50] given the high concentration of Br\u00f8nsted acid sites for the zeolite ds-ZSM-5 [51]. By contrast, the darker spots (B) might correspond to the presence of attapulgite crystals (0.5\u00a0\u00b5m\u00a0\u00d7\u00a030\u00a0nm), which given their absence of Br\u00f8nsted acid sites [52] would produce less coke deposits [53,54].The spent K-(USY-ATP) catalyst extrudate showed an egg-shell distribution of coke deposits too, visually confirmed by the gradient in color over the extrudate cross-section (Fig. S4). Indeed a larger proportion of poly-aromatics [47,48,55] was found on the external surface, while inner spots of the catalyst extrudate contained more coke rich in hydrogen (mainly naphthalenes/anthracenes emitting light between 400 and 500\u00a0nm). The shell, with an approximate thickness of \u00bd mm, was better distinguished when recording spectra on selected spots/regions due to the lower fluorescence intensity compared to the core of the section (Fig. 2d). The catalyst extrudate\u2019s surface (Fig. 2a) also contained regions of higher fluorescence intensity presenting a similar concentration profile as seen in the core of the cross-section, which is possibly associated with local attapulgite agglomeration. Note that for this catalyst material the unique source of (very weak) Br\u00f8nsted acid sites is the clay mineral given that the zeolite USY is highly dealuminated (i.e., a Si/Al ratio of \u223c400) [56].The influence of coke deposits and their subsequent regeneration on the textural properties of spent and regenerated catalyst bodies were determined by physisorption of Ar gas at \u2212196\u00a0\u00b0C. The results are shown in Fig. S5 and summarized in Table 1.The ZrO2/ds-ZSM-5-ATP catalyst, which showed a type I to type IIb isotherm with a steep H3 hysteresis loop [57,58] when fresh, (Fig. S5a, associated with a microporous material with additional mesoporosity) changed to a type I isotherm with a flatter H4 hysteresis loop after reaction. This was caused by the big loss in micropore and, in particular mesopore volume, partially blocked by the formation of coke deposits. The surface area was also significantly reduced after reaction (\u223c38% in drop, Table 1). The pore-size distributions plot (Fig. S5b) reveals that the micropores (filled at low relative pressures) and mesopores (filled at higher relative pressures) were partially shuttered after reaction. Notably, with the regeneration procedure applied the original textural properties were recovered to a large extent (up to \u223c92%) with micro- and mesoporosity, as indicated in Table 1.The textural properties of the K-(USY-ATP) catalyst extrudate were severely affected upon cascaded bio-oil deoxygenation. The isotherm for the spent catalyst sample revealed a substantial loss in micro- and mesopore volume (black series Fig. S5c). BJH analysis derived from pore-size distribution plot (Fig. S5d) confirmed that the mesopores were partially shuttered after reaction. Besides, an important loss in surface area after reaction was noted too. The affected textural properties were partially restored upon regeneration, as the improved values of micro- and mesoporosity and BET indicate, but to a much lesser extent than with the zeolite ZSM-5-based catalyst material.The acidity of fresh, spent and regenerated catalysts was assessed by FT-IR spectroscopy with pyridine as probe molecule.The Br\u00f8nsted acid sites (BAS) of the ZrO2/ds-ZSM-5-ATP catalyst originate from the zeolite component [51] and are indicated by the 1545 and 1636\u00a0cm\u22121 bands in Fig. 3\na [59,60]); the Lewis acid sites (LAS) come from the acidic Al3+ ions (indicated by the bands at 1455 and 1620\u00a0cm\u22121\n[59\u201362]), the Zr4+ from the ZrO2 component, and from different cations within the attapulgite (e.g., Fe2+/3+, Mg2+, Ca2+\n[62,63] indicated by the multiple bands in the range 1443\u20131455\u00a0cm\u22121. Quantification of the BAS was based on the integration of the 1545\u00a0cm\u22121 band -for fresh, spent and regenerated catalysts- while the ca. 1448\u00a0cm\u22121 band was used for determining the LAS [61]. After ex-situ CFP reaction the overall acidity (BAS\u00a0+\u00a0LAS) of the ZrO2/ds-ZSM-5-ATP catalyst dropped significantly, being specially affected the strong sites (\u223c75% drop) (Fig. 3b, Fig. S6a). This may indicate that the strong Lewis and Br\u00f8nsted acid sites are the main contributors to the catalyst activity and that deactivation occurs due to site poisoning [53,54]. Indeed, the induced mesoporosity of the desilicated ZrO2/ds-ZSM-5-ATP catalyst prevents from deactivation by pore occlusion [35,50]. Upon regeneration most but not all acidity could be recovered, however, being the strong acid sites those with better recoverability once combusted the bulky coke.The LAS recovered less than the BAS (Fig. 3b) which might point at likely changes in the attapulgite cations and Zr after reaction and regeneration. Indeed, it should be noted that the band located at 1612\u00a0cm\u22121, attributed to pyridine interacting with cus (coordinatively unsaturated sites) and Zr species [64,65] (Lewis acidity, PyL), was of considerably higher intensity for the fresh than for the regenerated catalyst material (Fig. S6b). However, when compared to the intensity of other cations interacting with pyridine \u2013 i.e., band at 1620\u00a0cm\u22121- the relative intensity of PyL-Zr was larger for the regenerated than for the fresh catalyst material. This stronger PyL-Zr interaction is in line with the predicted ZrO2 re-dispersion after reaction and regeneration.Pyridine FT-IR studies showed the very limited acidity of the K-(USY-ATP) catalyst in line with its high Si/Al ratio (\u223c400). Yet the bands at 1443\u00a0cm\u22121 (attributed to pyridine adsorbed onto Lewis acidic K+ cations [61,66]) and 1447\u00a0cm\u22121 (attributed to pyridine adsorbed onto a smaller cation present in the attapulgite, such as Al3+, Fe3+, Mg2+ and Ca2+) [56] showed the presence of weak LAS, which disappeared after outgassing at 150 \u2070C [61] (Fig. 3c). Upon cascaded bio-oil upgrading and regeneration was observed a good recovery of the sites within the attapulgite clay (indicated by the 1447\u00a0cm\u22121 band, see Fig. 3d). Quantification of the weak LAS, by integration of the 1447 and 1443\u00a0cm\u22121 bands, revealed that the total concentration was very low for the fresh and regenerated samples (Table 1). However, the bands attributed to K+ (1443\u00a0cm\u22121) lost their intensity, indicating some relocation or partial loss after reaction and regeneration. Note that K+-sites, located in the sodalite cages of the FAU structure [15,56], would be significantly hindered/blocked in case structural damage occurs.While acidity is the key to activity in catalytic pyrolysis, basicity is so for carrying out bio-oil deoxygenation. Bulk basicity of fresh, spent and regenerated K-(USY-ATP) catalyst extrudates was determined by CO2-TPD and summarized in Table 1. After performing the catalytic reaction, a decrease of 70% in the number of basic sites was quantified. This dramatic basicity drop might be likely due to the consumption of basic OH groups during reaction (note that the FT-IR spectra of the spent catalyst in Fig. S2c did not show the characteristic stretching band O-H) and to structural damaging [18]. In line with the latter, oxygen vacancies might get clogged and K-OH sites (located in the sodalite cages of the USY zeolite [15,56]) inaccessible.Rather than being recovered after regeneration, basicity dropped further due to likely zeolite structural damage suffered from the gases/steam generated by coke burning. Besides, the attapulgite phase may have been altered upon regeneration, getting affected oxygen anchoring to its basic sites in the form of alkaline cations (such as Mg2+, Ca2+ and K+\n[56]).X-ray diffraction (XRD) was employed to measure the structural integrity of the fresh, spent and regenerated catalyst samples.The orthorhombic phase of the zeolite ZSM-5 framework (Pnma space group, PDF 00-044-0003 [67]) remained well preserved after reaction (Fig. 4\na-b). Nevertheless, the shape of some X-ray diffraction peaks changed a bit, including the transformation of the initially split peak at 2\u03b8 26.8\u00b0 and 27.1\u00b0 into a single peak, and an intensity increase compared to the peak at 27.6\u00b0 (Fig. 4b). These changes are typically attributed to the incorporation of organics within the zeolite framework [68,69]. A carbon phase (PDF 00-026-1077) assigned to graphitic coke can be tentatively identified in the XRD pattern of the spent catalyst sample at 2\u03b8 31.5\u00b0 [70\u201372], which is absent in the fresh sample. By contrast, the XRD peak seen at 31.0\u00b0 (2\u03b8) for the fresh sample and assigned to quartz, present as impurity in the binder [73], disappeared after catalysis. This is likely due to either a phase change of attapulgite, or agglomeration of phases due to sintering upon high temperatures.The coke deposited during catalysis led to an increase in the \nb\n and \nc\n lattice parameters (Table 2\n) [69]. This was counterbalanced by a contraction of the lattice parameter \na\n, likely related to slight dealumination [74], caused by contact with moisture coming from biomass vapors [75]. As response to these mild structural changes, it is observed a slight reduction of the crystal domain size, indicated by the decrease of the LVOL-IB value in Table 2.Upon regeneration, the original crystallite size was almost totally restored, as evidenced by a LVOL-IB value close to the shown by the fresh sample (Table 2). The XRD pattern also showed the re-appearance of the doublet peaks at (2\u03b8) 26.8 and 27.1\u00b0, the original shape of the peak at 27.6\u00b0, together with the disappearance of the carbon phase at 31.5\u00b0 (Fig. 4b). Regeneration resulted in lattice parameter changes opposite to those observed after reaction: \nb\n and \nc\n shortened, an indication of coke depletion, and \na\n expanded back to the initial value of the fresh sample. The regeneration conditions, which expose the sample to high temperature and gas formation during the removal of coke deposits, could have possibly led to phase transformations of the attapulgite component. The absence of a quartz peak at 31.0\u00b0 (2\u03b8) in the regenerated sample indicates that the re-dispersion or phase change suffered upon reaction was irreversible. In addition, the unit cell size again increased in the regenerated sample, although the value of the fresh sample was not fully recovered.Aluminum speciation in the fresh, spent and regenerated ZrO2/ds-ZSM-5-ATP catalytic material has been assessed by 27Al MAS (Fig. 4c) and 27Al MQ MAS NMR (d) analyses. The main resonances of the fresh catalyst sample were located at 53\u00a0ppm (red series, Fig. 4c), assigned to framework tetrahedral coordinated Al species (A, AlIV) [76,77], and at ca. 3\u00a0ppm, assigned to extra-framework octahedrally coordinated aluminum species (E, AlVI) [78]. Interestingly, less well-defined extra-framework penta-coordinated Al (C, AlV) species were also present for the fresh sample at ca. 30\u00a0ppm. After reaction the signal intensity of framework AlIV species (so-called as A) decreased, in line with a loss in crystallinity, yet with concomitant signal increases in intensity of C and E at 30\u00a0ppm and 3\u00a0ppm, respectively (black series, c). Besides, the resonance of octahedral AlVI species, E, shifted to a lower chemical shift. This change was better observed in the 27Al MQ MAS NMR spectra (d) and believed to be related to distortions caused by pore coverage by coke [79]. In addition to the shift towards higher field, a shoulder emerged at 10\u00a0ppm (D, AlVI\u2019) (Fig. 4c), also attributed to broadening/structural distortions suffered upon coke build-up at high reaction temperature [74\u201376] (see expansion of D in Fig. 4d).The acquired NMR spectra were fitted according to the different resonances identified (A\u00a0\u2192\u00a0F) (Fig. S7) and the areas of the main deconvoluted spectra of the fresh, spent and regenerated catalyst samples were integrated to estimate the ratio between framework and extra-framework Al species (Table S1). The apparent drop of quantified framework Al sites after reaction is in line with the drop in Br\u00f8nsted acidity noted above [76,80]. Yet, after catalyst regeneration, most of these Al sites seemed to be restored -as indicated by the increased intensity of A (green spectrum in Fig. 4c)-, recovering the framework tetrahedral coordination lost upon reaction. The signal associated with extra-framework AlVI species, E, returned to its initial frequency position (c), while a large downfield distortion of signal D was noted. The increase in distortion of the AlVI species after regeneration compared to the spent sample is likely associated with re-arrangement processes [81,82] of the attapulgite binder material during the thermal treatment, as a result of the gases/steam created upon coke burning. However, the overall ratio between framework and extra-framework Al was notably recovered, as shown in Table S1.The XRD pattern of the K-(USY-ATP) catalyst material (red series, Fig. 4e-f) show the typical cubic pattern of the FAU phase of zeolite USY (PDF 00-045-0112) (see Table S2). As noted above, the diffraction peak observed at ca. 2\u03b8\u00a0=\u00a031.0\u00b0 (b) corresponds to the hexagonal phase of quartz (PDF 01-089-1961) [67], present as impurity in the attapulgite clay.No meaningful structural changes are observed for the spent K-(USY-ATP) catalyst (black series in Fig. 4e). Contrarily to what observed for ZrO2/ds-ZSM-5-ATP, no diffraction peak assigned to carbon was seen at 2\u03b8\u00a0=\u00a031.5\u00b0 for spent K-(USY-ATP) (black series), pointing at a more amorphous nature of coke.The XRD pattern of the regenerated sample (green series) did show considerable changes regarding the fresh and spent catalysts patterns, such as the lower intensity and the overall broader peak widths. These may be an indication of a drop in crystallinity, compared to the fresh and spent samples. Indeed, significant unit cell shrinkage and reduced size of the crystalline domains (nearly by half) were confirmed upon estimation of the lattice parameters and crystallite sizes (Vcell and LVOL-IB values, respectively, in Table 2). This decrease in crystallinity may have been provoked by the high temperature and the steam formed during coke burning upon catalyst regeneration, likely provoking hydrolysis of the grafted K species (SiOK+\u00a0+\u00a0H2O\u00a0\u2192\u00a0SiOH\u00a0+\u00a0KOH), and ultimately loss of the catalyst\u2019s basicity. Although smaller these ca. 30\u00a0nm crystals do keep their structural properties to a great extent; otherwise, a dramatic decrease of the textural properties would have been observed.When examining the morphology of the ZrO2/ds-ZSM-5-ATP catalyst no significant changes were seen after reaction and regeneration compared to fresh samples. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, which are shown in Fig. S8, reveal well-preserved zeolite ZSM-5 grains (of ca. 0.5\u20132\u00a0\u00b5m) agglomerated within 4 to 10\u00a0\u00b5m particles, and embraced by needle-like agglomerates of attapulgite (0.5\u00a0\u00b5m\u00a0\u00d7\u00a030\u00a0nm), accumulating more on edges and surface zeolite defects. Note that quartz impurities [73,83] were detected in the attapulgite in fresh (f) and regenerated samples (h). The TEM images show segregated domains of binder and zeolite, of varied sizes and shapes, unaltered after reaction and regeneration (Fig. S8c-h). Yet, a likely presence of coke-derived carbon flakes [75] were observed in TEM images of the spent catalyst, deposited on the edges of the zeolite crystal (left side Fig. S8g). Also, a high magnification TEM image of the fresh ZrO2/ds-ZSM-5-ATP catalyst (Fig. S8f) revealed the presence of nanosized ZrO2 particles distributed on the zeolite crystals where this component was originally deposited via impregnation, but also on the attapulgite clay binder material. This observation could indicate that ZrO2 was re-dispersed during granulation and calcination. Furthermore, ZrO2 might have re-dispersed after regeneration, as illustrated by the aggregated clusters formed onto the regenerated ZrO2/ds-ZSM-5-ATP catalyst (Fig. S8h).Re-dispersion effects were further studied by \u00b5-XRF (Fig. 5\na-b). On the fresh catalyst body (a) can be seen that Zr species preferably remained close to zeolite domains. It should be noted that zeolite domains are easily identified by the higher presence of Si -predominant in the zeolite- and in particular by the absence of a Mg signal which is unique for the attapulgite clay. On the contrary, Zr species seem to re-disperse upon reaction and regeneration, as indicated by the Zr signal of the regenerated catalyst body (Fig. 5b) which is also detected at the attapulgite domains.SEM (Fig. S9) and TEM images (Fig. 5c,f) of the fresh K-(USY-ATP) catalyst material showed particle sizes of 300\u2013500\u00a0nm. Attapulgite aggregates, identified by its characteristic needle-like morphology, were found on the zeolite crystal edges. Compared to the fresh sample, the zeolite particles seemed irreversibly clustered and of smaller size after reaction (d,g) and regeneration (e,h), in support of the structural modification noted above.In the high magnification image of the fresh catalyst (Fig. 5f) mesopores -generated via dealumination- with dimensions of ca. 20\u00a0nm can be distinguished. These were not observed with any clarity in the spent catalyst (g), however, presumably due to coverage by coke deposits. In the case of the regenerated sample (h), cavities can be seen, likely created by interconnection of mesopores [84].Upon ex-situ CFP, the ZrO2/ds-ZSM-5-ATP catalyst developed coke consisting of highly polyaromatic deposits, as indicated by TGA-MS, FT-IR and UV\u2013Vis DRS. Confocal fluorescence microscopy mapping showed that the coke deposits are distributed heterogeneously, i.e., with an egg-shell pattern, over the technical catalyst: being more poly-aromatic and abundant on the surface, and its concentration diminishing progressively towards the inside of the body. By applying Raman spectroscopy, the soluble fraction of coke was estimated to range between 5 and 10\u00a0\u00c5 in size. These large coke species locate in the zeolite\u2019s mesopores, partially blocking the strong acid sites, but also form externally, affecting the catalyst\u2019s textural properties too. This was demonstrated with Ar physisorption and py-FT-IR spectroscopy, which showed drops in both (meso)pore volume, surface area and acidity, thereby especially affecting the strong (and Lewis and Br\u00f8nsted) acid sites. Coke deposition also led to structural changes, as observed by XRD with the expansion of the lattice parameters.Another structural change pointed out by XRD is the re-distribution/phase change of attapulgite, evidenced by the observed contraction of the unit cell of the ZrO2/ds-ZSM-5-ATP catalyst after reaction. Such a change was also observed in the 27Al MAS NMR spectra, which showed an enhancement of the EFAl species in the spent catalyst at the expense of the tetracoordinated framework Al species. Penta- and, in particular octahedral Al species, increased in amount and extension, i.e., broadened, suggesting relocations upon catalytic reaction. No significant morphological changes were seen in the TEM images of the spent sample.Regeneration, carried out in static air for 6\u00a0h at 550\u00a0\u00b0C to burn-off the coke deposits, efficiently restored the textural properties and acidity previously affected by the formed coke, ruling-out any significant structural collapse. TNH3-TPD and FT-IR spectroscopy with pyridine as probe molecule did, however, confirm some irreversible loss in the acidity for the regenerated catalyst material. FT-IR also showed a relative larger intensity of the PyL-Zr band for the regenerated catalyst relative to the fresh catalyst, which suggests ZrO2 re-dispersion, as confirmed by \u00b5-XRF.The 27Al MAS NMR measurements indicate that the distortions created after regeneration are more pronounced than after reaction, likely due to re-arrangement processes of the attapulgite as a result of the gas/steam created under the regeneration conditions. Structural changes after regeneration were also observed by XRD, as indicated by the disappearance of the quartz impurity in the attapulgite binder. Despite some distortion of the EFAl species, the framework Al species were mostly recuperated. The attapulgite phase is believed to play an important role in this regeneration given its high content in SiO2 and Al2O3, which serve as reservoir upon Al distortion effects suffered during reaction.After bio-oil upgrading the K-(USY-ATP) catalyst extrudates suffered structural damage and pore blockage by coke. It was determined by TGA-MS, FT-IR, UV\u2013Vis DRS and confocal fluorescence microscopy that the coke deposits consisted of a large proportion of naphthalenes/anthracenes and poly-aromatics, distributed again in an egg-shell manner over the spent catalyst extrudate. However, these coke deposits were of a softer nature (i.e. H-richer) than the coke deposits formed on the ZrO2/ds-ZSM-5-ATP catalyst used for the catalytic fast pyrolysis stage. This is in line with the latter being upstream and directly exposed to the crude pyrolysis vapors, while the K-USY-based catalyst further upgrades the already treated vapors. Another factor for which less poly-aromatic coke formed onto the K-USY-Attapulgite catalyst is the lack of strong acid sites where it develops more easily. The textural properties of the spent catalyst nevertheless were severely affected by coke formation and morphological damages, as determined by physisorption and TEM measurements. The observed clustering had detrimental consequences for the required basicity of the catalyst material, as determined by CO2-TPD, by means of clogged oxygen vacancies or hindering accessibility to the K+ and OH\u2013 sites.Loss of structural integrity was enhanced upon catalyst regeneration due to the steam formed during coke burning, with the complete loss of basic sites. XRD measurements revealed the shrinkage of the zeolite unit-cell as well as a loss of crystallite size in the regenerated catalyst. The original textural properties, affected by coke deposition and regeneration, could only be partially recovered, less so than with the CFP catalyst. The Py-FT-IR spectroscopy studies indeed suggested some loss in K-loading after reaction and/or regeneration. The changes in physicochemical properties correlated with the morphological changes, as observed by TEM, for the zeolite material regenerated.A cascade process, consisting of a thermal pyrolysis followed by a two-step catalytic ex-situ catalytic fast pyrolysis (CFP) -catalyzed by a ZrO2/desilicated zeolite ZSM-5-/attapulgite material as solid acid- and the subsequent catalytic upgrading/deoxygenation of the formed oil -catalyzed by a K-(zeolite USY-attapulgite) material as solid base- was studied for the production of bio-oil from lignocellulosic biomass. A high bio-oil mass yield was achieved (40\u00a0wt%) with a remarkable deoxygenation degree (60\u00a0wt%), compared to a non-catalytic thermal bio-oil.Upon ex-situ CFP the solid acid ZrO2/desilicated zeolite ZSM-5-attapulgite catalyst suffers acid site coverage by the build-up of coke deposits and (reversible) changes in the Al coordination. In the case of the base K-(zeolite USY-attapulgite) catalyst, employed to further upgrade the formed bio-oil via CFP, mild pore blockage by coke formation and a partial loss of structural integrity was observed. Yet, the main deactivation cause is ascribed to clustering of the crystallites which hinders the catalyst\u2019s basicity.Regeneration of the deactivated catalysts by coke burn-off to a large extent reverted the negative effects of the coke deposition on the ZrO2/desilicated zeolite ZSM-5-attapulgite catalyst. Although the structural distortion suffered by the catalyst upon pyrolysis can be considered mild and reversible, the small losses in framework Al and acidity upon a complete reaction plus regeneration cycle will progressively attenuate its activity. This, together with the observed ZrO2 migration after reaction and regeneration will eventually require catalyst replacement by fresh material after few reaction cycles.In contrast, the regeneration procedure caused irreversible structural change in the K-(zeolite USY-attapulgite) catalyst. It is believed that the steam produced upon coke burning provokes hydrolysis of the grafted K species, with the KOH produced attacking the zeolite\u2019s structure, which ultimately experiences a total loss of basicity. This implies that after only one reaction cycle the K-(zeolite USY-attapulgite) catalyst would need to be replaced, with its associated economic implications for the process.Alternative regeneration procedures which efficiently restore the initial properties of the alkaline-exchanged USY catalyst material or the revision of the grafting procedure which, as seen, negatively impacts on the structure of the catalyst in the downstream process, might offer an alternative here. Among them, performing the regeneration at milder temperature conditions and in a flow reactor rather in a muffle furnace at static conditions would shorten the long exposure of the catalyst to the produced water vapors upon coke combustion, likely preventing structural damage and basicity loss. Also, to correct for hindered basicity a newly alkaline grafting process could be accomplished after regeneration, replenishing the extinguished K-OH sites. Testing of these newly proposed recoverability methods and the determination of the catalysts\u2019 lifespans, i.e., the number of regeneration cycles that the catalysts can survive before getting replaced, could be topics for future 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.Authors gratefully acknowledge the financial support from the European Union Seventh Framework Programme (FP7/ 2007-2013) under grant agreement n\u00b0604307 (CASCATBEL project). Dr. T. Hartman (Utrecht University, UU) is thanked for recording the Raman spectra, while Dr. A.-E. Nieuwelink (UU) is acknowledged for performing the \u00b5-XRF analysis. The NMR experiments were supported by the Netherlands Organization for Scientific Research (NWO) within the Middelgroot program (no. 700.58.102 to M.B.), and uNMR-NL, an NWO-funded National Roadmap Large-Scale Facility for The Netherlands (no. 184.032.207).A.M.H.G. contributed to the idea of this study, conducted experimental work, processed the results and wrote the manuscript. R.M.D. and E.T.C.V. calculated the XRD lattice parameters, interpreting the results. H.H. and D.P.S. run the catalytic tests and participated in the interpretation of the results. K.H. and M. B. performed the 27Al (MQ) MAS NMR measurements. P.C.A.B. contributed to the idea of this work and aided on the discussion of results and manuscript writing. B.M.W. contributed to the idea of this study, including manuscript design and writing.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2021.09.029.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The modes of deactivation -and the extent to which their properties can be restored- of two catalyst bodies used in cascade for bio-oil synthesis have been studied. These catalysts include a solid acid granulate (namely ZrO2/desilicated zeolite ZSM-5/attapulgite clay) employed in ex-situ catalytic fast pyrolysis of biomass, and a base extrudate (K-exchanged zeolite USY/attapulgite clay) for the subsequent bio-oil upgrading. Post-mortem analyses of both catalyst bodies with Raman spectroscopy and confocal fluorescence microscopy revealed the presence of highly poly-aromatic coke distributed in an egg-shell manner. Deactivation due to coke adsorption onto acid sites affected the zeolite ZSM-5-based catalyst, while for the base catalyst it is structural integrity loss, resulting from KOH-mediated zeolite framework collapse, the main deactivating factor. A hydrothermal regeneration process reversed the detrimental effects of coke in the acid catalyst, largely recovering catalyst acidity (\u223c80%) and textural properties (\u223c90%), but worsened the structural damage suffered by the base catalyst.\n               "}
{"full_text": "The current energy landscape based on fossil fuels created enormous economic and environmental problems such as greenhouse emission, climate change, solid waste pollution et\u00a0al. Hydrogen economy is a clean alternative to fossil fuels [1\u20137]. Looking for an efficient, low-cost and safe hydrogen storage method is the prerequisite to the large-scale applications of hydrogen economy [8]. Mg stand as a candidate for hydrogen storage due to its large hydrogen storage density(7.6\u00a0wt%, theoretical value), fair accessibility and low-cost [9\u201311]. However, because of its sluggish reaction kinetics and high thermodynamic stability, Mg/MgH2 is not able to meet the requirements for practical application [12,13].Researches of past decades attempted many hopeful efforts to improve its hydrogen absorption/desorption properties, including nano-structuring [14,15], catalytic doping using transition metals [16\u201320], metal oxides [21\u201323], and intermetallic compounds [24\u201327]. Tremendous physical and chemical methods such as ball-milling [28,29], thin-film deposition [30,31] and combustion synthesis [32], have been attempted to synthesize Mg-based composites. Since the last couple of years, high-energy planetary ball milling equipment is applied to produce structural defects in many kinds of materials [33,34]. Nevertheless, the particle size of magnesium will increase after ball milling because of its agglomerating and welding property. Thus, carbon materials, which are anti-welded, are frequently used as the additive during ball milling to prevent magnesium from agglomerating and welding.Many carbon materials such as carbon nanotubes (CNTs), carbon nanofibers and graphite themselves are hydrogen storage materials [35]. In order to improve the hydrogen storage performance of carbon nanotubes, Yoo et\u00a0al. [36] introduced defects and doped Pd on CNTs, the defective CNTs with Pd particles at 1\u00a0atm and 573\u00a0K stored 1.5\u00a0wt% hydrogen. Similarly, Hirai et\u00a0al. [37] produced Pd-doped graphite as the hydrogen storage material with PdCl2-graphite as the precursor. However, the limitations of carbon material for hydrogen storage are obviously. In order to store hydrogen in molecular state by physisorption, most studies have been carried out at high pressures (1\u201316\u00a0MPa) and low temperatures (80\u2013133\u00a0K). Although chemisorbed hydrogen concentration can reached up to a higher level, the chemically absorbed hydrogen cannot be desorbed reversibly at room temperature because of their strong CH covalent bonds [38]. Therefore, carbon material seems unfavorable to use as hydrogen storage material solely. Developing Mg-carbon materials by ball milling for hydrogen storage have drawn considerable research interest. Huang et\u00a0al. [39] investigated the effects of different carbon additives such as carbon black, graphite and multi-walled carbon on the hydrogen storage properties of magnesium and found that the composite containing graphite displayed a remarkable decrease in the desorption temperature. Liu et\u00a0al. [40] prepared MgH2 cluster with size below 5\u00a0nm. Multi walled carbon nanotubes and graphene nanoparticles were used to limiting the dimensions of clusters. The small clusters significantly reduced the hydrogen desorption temperature comparing with the bulk MgH2. Zhou et\u00a0al. [41,42] prepared crystallitic carbon from anthracite and used as additive during ball milling. The magnesium particles were milled to 20\u201360\u00a0nm and hydrogenated to \u03b2-MgH2. With the increase of milling time, \u03b3-MgH2 of orthorhombic crystal is formed. The endothermic peak of \u03b3-MgH2 is 53\u00a0\u00b0C lower than that of \u03b2-MgH2. Recently, Han et\u00a0al. [43] successfully prepared oxygen-rich activated carbon by the mulch-assisted ambient-air synthesis for hydrogen storage. In Pd-supported carbon-Mg hydrogen storage composites, carbon materials are considered as spillover agents [44]. First, H2 dissociates on Pd surface, then, H atoms spill onto the carbonaceous materials towards Mg bulk.Among the transition metals, Ni exhibit excellent catalysis for the hydrogen absorption/desorption of MgH2. Shi et\u00a0al. [45] attempted to elaborate the synergistic mechanism between Ni and carbon aerogel for Mg-based hydrogen storage composite. The results show that the synergistic catalytic effect was attributed to the charge transfer between Ni, carbon aerogel and MgH2. After the introduction of Ni/carbon aerogel, the dehydrogenation activation energy of the Mg-Ni composite was reduced to 86.3\u00a0kJ\u00a0mol\u22121. Yao et\u00a0al. [46] anchored uniform-dispersed Ni nanoparticles on grapheme oxide(GO) to prepare Ni@rGO as the catalyst for MgH2. The activation energy for the rehydrogenation of MgH2\nNi@rGO reduced to 47.6\u00a0\u00b1\u00a03.4\u00a0kJ\u00a0mol\u22121. Coincidentally, Liu et\u00a0al. [47] also used rGO as the supporter and anchored Ni3Fe on it to prepare the catalyst Ni3Fe/rGO. Ni3Fe/rGO shows an excellent synergistic effect on the hydrogen storage performance of MgH2. Ouyang et\u00a0al. [48\u201350] did many works about the synergistic catalysis on Mg-based alloys. They found out that adding metals and their hydrides, such as In and Ce, was an effective way to improve the hydrogen storage properties of Mg-based hydrogen storage materials. The sluggish kinetics of Mg/MgH2 has been significantly improved.The previous literatures show that Ni and carbon materials exhibit remarkable catalysis for the hydriding reaction of Mg. Moreover, hybrid catalysts usually show enhanced catalytic performance comparing with the single-phase catalysts. Herein, practical experiments show that C atom and Ni atom can be incorporated into Mg crystal. In the present study, the microphysical processes of H2 absorption, H2 dissociation and H diffusion on Ni/C synergistic incorporated Mg(0001) were studied by first-principles calculations. The mechanism of Ni/C atoms for catalyzing hydrogen storage of Mg crystal were comparatively discussed.Mg powder with a particle size of <\u00a00.074\u00a0mm was purchased from Tianjin Ruijinte Chemical Company, China, and used as received. Anthracite used as milling aid was purchased from Rujigou Mine, China. The anthracite has low volatile matter content (6.60\u00a0wt%, air dry basis), low ash content (8.55\u00a0wt%) and high fixed carbon content (83.00\u00a0wt%). Nano-nickel with a particle size of 20 to 100\u00a0nm was purchased from Aladdin Industrial Corporation. The hydrogen with a purity of >\u00a099.999\u00a0vol.% was purchased from Jinghui Gas Company, Beijing, China.The anthracite coal which purchased from Rujigou Mine was used as the precursor to prepare the crystallitic carbon by demineralization and carbonization (supporting information). The Ni/C co-incorporated Mg was prepared by ball milling on a Fritsch Pulverisette-6 planetary ball-mill. The number of stainless steel balls with diameter of 3, 5, 10 and 20\u00a0mm was 500, 20, 2 and 2, respectively. The work revolution of the mill used in this study were speed of 300\u00a0r\u00a0min\u22121, ball to sample weight ratio of 30:1, milling time of 3.0\u00a0h, weight ratio of Mg powder to nano-nickel to carbon was 7:1:2. As a contrast, C-incorporated Mg was prepared with the Mg powder to carbon weight ratio of 8:2 and Ni-incorporated MgH2 was prepared with the MgH2 to nano-nickel weight ratio of 9:1. The atmosphere in the vial during milling was Ar.X-ray diffraction (XRD) measurement was carried out on a D8 Discover X-ray diffractometer with Cu K\u03b1 radiation using a step of 0.02\u00b0. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were carried out on a JEOL JEM-2100F electron microscope operating at 200\u00a0kV.All the first-principles calculation based on density functional theory (DFT) was carried out on the DMol3 program package of Materials Studio 7.0. The generalized gradient approximation (GGA) parameterized by Perdew-Burke-Ernzerhof(PBE) was adopted for the exchange-correction calculation [51,52]. A (6\u00a0\u00d7\u00a06\u00a0\u00d7\u00a06) Monkhorst-Pack k-points was used in the optimization of Mg cell, while a (4\u00a0\u00d7\u00a04\u00a0\u00d7\u00a01) k-points was used in supercells optimization, and the vacuum space of different Mg(0001) supercells was set at 20\u00a0\u00c5 as a periodic boundary condition [53].The Mg(0001) supercell was constructed by 5-layers of (3\u00a0\u00d7\u00a03) Mg(0001) slabs containing 45 Mg atoms, as shown in Fig. S1(supporting information). There are four nonequivalent interstitial sites for the absorption of C and Ni atoms, namely top, bridge, hcp and fcc (Fig. S1, supporting information) [54,55]. The supercells of the Ni/C incorporated Mg(0001) were constructed by absorbing Ni/C atoms on the topmost surface of clean Mg(0001). Finally, fcc was preferred as the absorbing site to construct the incorporated supercells (supporting information).The convergence criteria were set as energy tolerance of 1.0\u00a0\u00d7\u00a010\u22125\u00a0Ha\u00a0atom\u22121, self-consistent field (SCF) tolerance of 1.0\u00a0\u00d7\u00a010\u22126\u00a0Ha\u00a0atom\u22121, maximum force gradient of 0.002\u00a0Ha\u00a0\u00c5\u22121, and maximum atomic displacement of 0.005\u00a0\u00c5. The Ni and C atoms were treated with spin polarization and performed using different orbitals for different spins. For clean and C-incorporated Mg(0001), the core treatment of All Electron and the smearing broaden of 0.005\u00a0Ha were chosen. While, for the Mg(0001) with Ni atom, the core treatment of DFT Semi-core Pseudopots and the smearing broaden of 0.01\u00a0Ha were chosen to make sure that the calculation processes are convergent and the results are reasonable. The Double-Numeric plus d-function was chosen as the global basis set.To clarify the structure of incorporated Mg models in calculation, the morphology and crystal structure of the crystallitic carbon and incorporated Mg samples are investigated. After demineralization and carbonization, the particles of crystallitic carbon become structured. Crystalline structure of the carbon shows that diffraction peak for graphite(0002) is obvious, suggesting that the carbon possess similar atomic arrangement as graphite. Moreover, the diffraction peak for graphite(10\n\n\n1\n\n\u00af\n\n1) which is the characteristic peak of the amorphous carbon is appeared. After structural simulation according to the diffraction, the detailed lattice parameters are listed in Table S2(supporting information). Compared with anthracite coal, a/b of the crystallitic carbon is increased and c is decreased. Correspondingly, the length of the pi bond increases from 1.35 to 1.40\u00a0\u00c5 and the interlayer spacing decreases from 3.28 to 3.26\u00a0\u00c5. Demineralization eliminates the mineral salt in the coal so that the interlayer spacing tends to shrink. Carbonization made the arrangement of C atoms regular and stretch pi bond. The crystallitic carbon can prevent Mg particles from cold welding and agglomerating and break the particles by its hard edges and protrusions to reduce the particle size of Mg [43]. Therefore, ball milling can crush the particle of Mg effectively. There are amounts of hexagon particles in the TEM image (Fig.\u00a01\nB) which are Mg particles. This structure is often seen in the Mg-based materials [56]. It indicates that monocrystal Mg flakes are peeled off during ball milling. The EDS mappings of Mg and C (Fig.\u00a01C,D) shows that Mg and carbon is mixed well and carbon is distributed on Mg evenly.In the Ni/C co-incorporated Mg sample, Mg and Ni are the main phases and Mg2Ni cannot be detected, as shown in the XRD pattern in Fig.\u00a02\nA1. After hydrogenation, Mg is hydrogenated and formed MgH2. The diffraction peaks of Mg2NiH4 is appeared. This means atomic Ni which incorporates into the Mg surface diffused into the crystal grain of MgH2. MgH2, Ni and Mg2NiH4 became the main phases as shown in Fig.\u00a02A2. Then, after dehydrogenation, the component of the material became complex. The diffraction peaks of Mg, Ni and incomplete dehydrogenated MgH2 are apparent. Mg2NiH4 releases H2 and turns into Mg2Ni indicating the incorporation of Ni is stable. The TEM image of Ni/C co-incorporated Mg after hydrogenation shows that MgH2 nano-crystals with tetragonal (P42/mnm) space group evenly distribute in the material (Fig.\u00a02B). Based on the HRTEM observation (Fig.\u00a02C), crystal domain with lattice fringes of 0.166 and 0.125\u00a0nm are apparent and can be indexed to (211) and (202) planes of MgH2, respectively. Diffraction spots of MgH2, Mg, Mg2NiH4 and Ni are presented in Fig.\u00a02D. The results agree with the previous literatures [46,57]. With hyriding and dehyriding reaction, Mg2Ni/Mg2NiH4 can in-situ form in the ball-milled Ni-Mg/MgH2 samples. The addition of Mg2Ni alloys can improve the de/hydrogenation performance of Mg/MgH2 system [58].The TEM image of Ni/C co-incorporated Mg shows the irregular shape of Mg crystallites (Fig.\u00a03\nA) and EDS mapping analysis shows the homogeneous distribution of Ni and C over Mg (Fig.\u00a03A1\u2013A3). Herein, Ni/C co-incorporated Mg(0001) is constructed to investigate the behavior of H2 on Mg surface. Fig.\u00a03B1\u2013B4 shows the clean Mg(0001), C-incorporated Mg(0001), Ni-incorporated Mg(0001) and Ni/C co-incorporated Mg(0001) after geometry optimization. Obviously, C atom stably adsorbs on the second layer of Mg(0001) and Ni atom adsorbs on the first layer of Mg(0001).The adsorption energy of hydrogen is calculated by Eq.\u00a0(1) as follows:\n\n(1)\n\n\n\nE\nads\n\n=\n\u2212\n\n(\n\n\nE\n\nH\n2\n\n\n+\n\nE\nMg\n\n\u2212\n\nE\ntotal\n\n\n)\n\n\n\n\n where Eads is the absorption energy of hydrogen, EH2 is the energy of hydrogen molecule, EMg is the energy of different Mg(0001) models and Etotal is the total energy of Mg(0001) model with H2. The results in detail are listed in Table S3(supporting information). It is undisputed that the incorporation of C and Ni atoms has an obvious effect on the H2 adsorption. With the incorporation of C and Ni atoms, the energy which emitted from the H2 adsorption decreases. The total charge density maps of different Mg(0001) with H2 adsorption are shown in Fig.\u00a04\nA\u2013D. The electron clouds of H2 and clean Mg(0001) has no obvious interaction. However, with the incorporation of C and Ni, the electron clouds of H2 and different Mg(0001) become overlapped. After C atom enters into Mg crystal lattice, the electronic structure of Mg(0001) surface is changed. The interaction between the electron clouds of H and Mg gradually become clear. While Ni atom incorporates on Mg(0001) surface, it has a direct impact on the adsorption of H2. As an anchor, the electron cloud of Ni has obvious overlap with that of Mg and H2. To clarify the role of C and Ni atoms in the Ni/C co-incorporated Mg(0001), the deformation charge density maps and density of states (DOS) are studied. Obviously, the electron enrichment regions are distributed in the surrounding of C and Ni atoms and the electron depletion regions are around Mg atoms. It shows that C and Ni atoms obtain electrons from Mg. The unoccupied orbits of Ni and H2 has obvious overlap. Meanwhile, the DOS of Ni/C co-incorporated Mg(0001) with H2 adsorption indicates that C s has obvious hybridization with Mg s and p at an energy of \u221210.0\u00a0eV, and C p has evident hybridization with Mg s and p at the range of \u22126.2 to 1.3\u00a0eV. While the Ni d has apparent hybridization with Mg p at an energy of \u22120.8\u00a0eV. The orbital hybridization verifies that Ni and C atoms influence the electronic structure of Mg(0001), while the \u03c3-bond of H2 still firm.The incorporation of C and Ni can reduce the barrier energy of H2 dissociation in different levels, as shown in Fig.\u00a05\n. The corresponding deformation charge density maps are shown in Fig. S2(supporting information). After dissociation, H atoms accept electrons from Mg. With the incorporation of C, the Mulliken charge of H reduces from \u22120.248 to \u22120.271. While when it comes to Ni, the Mulliken charge of H increases from \u22120.248 to \u22120.057. It indicates the influence of C and Ni incorporation for the electronic structure of H is opposite. Thus, the Ni/C co-incorporation may has an eclectic effect for the hydriding reaction of Mg. Actually, the incorporation of C can modulate the electronic structures of Ni and H, as shown in Fig. S2D (supporting information).With the incorporation of C, the barrier energy (Eb) of H2 dissociation on Mg(0001) reduced from 104.8 to 95.3\u00a0kJ\u00a0mol\u22121. But when it comes to Ni, the barrier energy significantly reduces to 0.9\u00a0kJ\u00a0mol\u22121, indicating that Ni catalyzes the dissociation of H2 effectively. However, the catalysis of C incorporation seems to be finite. While the Ni/C co-incorporation also made the barrier energy reduced dramatically due to the catalysis of Ni.The catalytic mechanism of Ni atom on H2 dissociation is clarified by the DOS and deformation charge density calculation as shown in Fig.\u00a06\n. The Ni d orbit and H s orbit both emerge at \u22125.6, \u22122.8 and \u22120.85\u00a0eV in DOS (Fig.\u00a06A), suggesting that the two orbits have obvious overlapping effect. To visualize the fix action of Ni d orbit on hydrogen absorption, deformation charge density distributions of the Ni/C co-incorporated Mg(0001) after H2 dissociation were calculated. It is found that Ni and H atoms combine together after H2 dissociation. The Ni dz\n\n2 orbit and H s orbit accept the electrons (red color regions), while the electron in Ni dxy\n orbit is concentrated around the regions closing to Mg atoms due to the delocalization of the free electrons of Mg (red color regions) and the dxy\n orbit which far from Mg is electron deficiency (blue color regions). Mg atoms which surrounding Ni atom, meanwhile, contribute the electrons (blue color regions). Ni d orbit plays an important role during H2 dissociation, as other transition metals such as Fe [59]. In contrast to the Ni/C co-incorporated Mg(0001), the Ni atom in Ni-incorporated Mg(0001) shows a closer combination with H atoms, as shown in Fig.\u00a06C,D. The electron deletion region of Ni dxy\n orbit is contracted around x axis due to the strong electron obtain ability of Ni. Without C atom, the Milliken charge of Ni become more negative (from \u22120.324 to \u22120.357), indicating that C can weaken the electron obtain ability of Ni which is beneficial for the H diffusion.After H2 dissociation, H atoms need to diffusion into the crystal lattice of Mg to form MgH2, as shown in Fig.\u00a07\n. The energy barrier (Eb) of H diffusion on clean Mg(0001), C-incorporated Mg(0001), Ni-incorporated Mg(0001) and Ni/C co-incorporated Mg(0001) are 43.0, 47.8, 69.0 and 35.8\u00a0kJ\u00a0mol\u22121, respectively. The diffusion of H in the crystal lattice of Mg is an endothermic process. Thus, the energy of reaction (Er) is positive. With the incorporation of C or Ni, the energy barrier of H diffusion increases while it compared with that of H diffusion on clean Mg(0001). Despite the incorporations of C and Ni can reduce the barrier energy of H2 dissociation, the incorporations hinder the diffusion of H at the same time. Though the incorporation of C hinders the diffusion of H, the energy barrier of H2 dissociation still higher than that of H diffusion which means the dissociation of H2 is the limiting step during Mg is hydrogenated. Therefore, the hindrance of C incorporation for H diffusion has no obvious effect on the hydrogenation of Mg. However, Ni incorporation can significantly reduce the energy barrier of H2 dissociation and makes the H diffusion become limiting step. During the diffusion of H, Ni atom plays a role of anchor and fixes H atom around it, as shown in Fig.\u00a06. So that the incorporation of Ni hinders the H diffusion on Mg(0001). The increase of the energy barrier of H diffusion against the hydrogenation of Mg. The Ni/C co-incorporation, which is different from the single incorporation of Ni and C, can reduce the energy barrier of H diffusion. During the dissociation of H2 on Mg, the Ni/C co-incorporation, similar to the Ni incorporation, can significantly reduce the energy barrier. Then, H diffusion becomes the limiting step. However, the Ni/C co-incorporation can reduce the energy barrier of H diffusion at the same time. Thus, the Ni/C co-incorporation shows the best catalysis for the improvement of the H2 absorption performance of Mg compared with the single incorporation of Ni and C.The analysis of H2 dissociation and H diffusion on different Mg(0001) show that H2 dissociation is the limiting step of the hydriding reactions of clean and C-incorporated Mg(0001) and H diffusion is the limiting step of the hydriding reactions of Ni- and Ni/C co-incorporated Mg(0001). Thus, the energy barriers of the critical step of clean, C-incorporated, Ni-incorporated and Ni/C co-incorporated Mg(0001) hydriding reaction are 104.8, 95.3, 69.0 and 35.8\u00a0kJ\u00a0mol\u22121, respectively. Obviously, the Ni/C co-incorproated Mg(0001) shows the best hydriding performance. Moreover, the value of its energy barrier is very close to the hydriding activation energy of Mg with Ni4@rGO6 in Refs. [46,39].In addition, the incorporation of C also can influence the incorporation of Ni on the co-incorporated Mg(0001) surface. In the experiment, Ni enters the Mg crystal and in-situ forms Mg2Ni/Mg2NiH4. Herein, the effect of C incorporation for Ni diffusion in Mg is investigated. The energy paths of Ni diffusion on clean Mg(0001) and C-incorporated Mg(0001) are shown in Fig.\u00a08\n. Obviously, the incorporation of C can significantly reduce the barrier energy of Ni diffusion on Mg(0001) surface. The barrier energy is reduced from 37.1 to 7.2\u00a0kJ\u00a0mol\u22121. This means C-incorporated structure is beneficial for the incorporation of Ni on Mg surface. It is conducive to the formation of Mg2Ni/Mg2NiH4. In addition, the deformation charge maps and Mulliken charge in Fig. S3(supporting information) show that during the diffusion of Ni, the Mulliken charge of C increased stepwise and Ni became more negative at the same time. This means electrons is transferred from C to Ni.In summary, the first-principles calculations are performed to investigate the effect of Ni/C incorporation for the hydriding reaction of Mg(0001). The morphology and crystal structure of the Ni/C co-incorporated Mg sample show that the pi bond of carbon is stretched and monocrystal Mg flakes are peeled off by ball milling. Carbon is distributed on Mg evenly to create conditions for the formation of C-incorporated structure. With Ni incorporation, Mg2NiH4/Mg2Ni is in-situ formed in the Ni/C co-incorporated Mg sample after hydrogenation and dehydrogenation. On the incorporated Mg(0001), the H2 molecule exhibits a better dissociation performance than that on clean Mg(0001). The catalytic effect of Ni on H2 dissociation can be ascribed to the bridging effect of Ni dxy\n orbit.However, the single incorporation of Ni or C is unfavorable for the H diffusion. The strong interaction between Ni and H hinders the activity of H on Mg(0001). This has a considerable effect on the hyriding reaction of Ni-incorporated Mg(0001) due to the H diffusion become limiting step of the hyriding reaction. But the incorporation of C can weaken the constraint of Ni for the H diffusion when it comes to Ni/C co-incorporated Mg(0001). The Ni/C co-incorporated structure in Mg(0001) shows an eclectic performance in H2 dissociation and H diffusion. The Ni/C co-incorporated Mg(0001) shows the best performance during hyriding reaction. The catalysis of Ni can effectively reduce the barrier energy of H2 dissociation and the incorporation of C can improve the H diffusion performance. Moreover, the incorporation of C is beneficial for the formation of Ni-incorporated structure. The present paper is helpful to clarify the catalytic roles of Ni and C in co-incorporated system on hydriding of Mg crystal.The authors would like to acknowledge the National Supercomputing Center in Shenzhen for their technical support of Materials Studio.This work is supported by the National Key R&D Program of China (Grant No. 2017YFB0103002), National Natural Science Foundation of China (Grant Nos. 51771056, 51371056, 51701043 and 52071141), Equipment Pre-research Field Foundation (Grant No. 6140721040101), Equipment Pre-research Sharing Technology (No. 41421060201), Changzhou Leading Talents Project (Grant No. CQ20183020), 333 Project in Jiangsu Province and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, Fundamental Research Funds for the Central Universities (Grant No. 2021MS051), Interdisciplinary Innovation Program of North China Electric Power University (grant number XM2112355).", "descript": "\n                  Ni and carbon materials exhibit remarkable catalysis for the hydriding reaction of Mg. But the underlying mechanism of Ni/C hybrid catalysis is still unclear. In this work, density functional theory (DFT) calculation is applied to investigate the effect of Ni/C co-incorporation on the hydriding reaction of Mg crystal. The morphology and crystal structure of the Ni/C co-incorporated Mg sample show that the co-incorporated structure is credible. The transition state searching calculation suggests that both the incorporations of Ni and C are beneficial for the H2 dissociation. But Ni atom has a dramatic improvement for H2 dissociation and makes the H diffusion become limiting step of the hyriding reaction. The Ni dz\n                     \n                     2 orbit and H s orbit accept the electrons and combine together compactly, while the Ni dxy\n                      orbit is half-occupied. The catalytic effect of Ni on H2 dissociation can be ascribed to the bridging effect of Ni dxy\n                      orbit. The incorporation of C can weaken the over-strong interaction between Ni and H which hindered the H diffusion on Mg(0001). The Ni/C co-incorporated Mg(0001) shows the best performance during hyriding reaction compared with the clean and single incorporated Mg(0001).\n               "}
{"full_text": "Currently, the production of sustainable and low-cost energy resources has become an urgent necessity to overcome the most serious issues that face the contemporary world about the future availability of fossil fuels\u00a0(Sayed et al., 2020; Basyouny et al., 2021). Biodiesel or the extracted diesel from the bio-resources was assessed extensively to be introduced as an alternative fuel for the fossil fuel that can be produced by simple, low cost, and sustainable techniques\u00a0(Lee et al., 2020; Abukhadra et al., 2020). Most of the addressed biodiesel products were extracted thermo-chemically from green resources (algae, biomass, vegetable oil, and extracts of plants) in addition to the animal fats\u00a0(Abukhadra et al., 2021). Technically, the determining viscosity, lubricity, cetane number, and flashpoint of the biodiesel products are of acceptable values to be used directly as fuels in the engines or to be mixed with other types of fossil fuels in blends\u00a0(Lawan et al., 2020; Zhong et al., 2020).Chemically, the biodiesel terms refer to series of fatty acid methyl esters that can be extracted from the transesterification process of triglycerides mainly in the vegetable and plants oils, or by esterification of free fatty acids\u00a0(Al\u00a0Hatrooshi et al., 2020). The spent or the waste products of cooking oil like corn oil and sunflower oil were evaluated as low cost, recyclable, and commercial raw oil instead of the fresh oils that are of expensive costs\u00a0(Ibrahim et al., 2020). The transesterification process involved the chemical conversion of the studied types of oil either edible or non-edible using alcohol and in the presence of a potential catalyst\u00a0(Sayed et al., 2020; Bin\u00a0Jumah et al., 2021). The heterogeneous catalysts were recommended to be used considering the several technical, economic, and environmental drawbacks of the commonly used homogeneous catalysts\u00a0(Rabie et al., 2019; Betiha et al., 2020). The solid heterogeneous catalysts that are of active basic and acidic functional groups were assessed as promising catalysts for their low cost, high reactivity, high thermal stability, significant recyclability, strong mixing, and dispersion properties, low hazardous byproducts, and facile separation properties\u00a0(Sayed et al., 2020; Mohadesi et al., 2020).The acidic forms of the heterogeneous catalysts were assessed widely as promising environmental catalytic materials that can be used in both esterification and transesterification processes with significant activity\u00a0(Nata et al., 2017; Wang et al., 2019). The applications of the acidic catalyst were recommended than the basic forms as it is of no saponification effects especially with the transesterification systems that contain low-grade oils as feeds of significant free fatty acid content\u00a0(Wang et al., 2019). The fabrication of the acidic catalysts involved acidification and acidic functionalization of different types of inorganic materials as montmorillonite, mesoporous silica, mica, zeolite, clay minerals, and some metal oxides\u00a0(Betiha et al., 2020; Farabi et al., 2019a; Cheng et al., 2019; Silva et al., 2018; Negm et al., 2019). The acidic inorganic catalysts exhibited some technical drawbacks during the transesterification processes related to their cost, low acidic densities, poor operational stability, and small pore sizes\u00a0(Zailan et al., 2021; Mendaros et al., 2020).The catalysts that were prepared by acidification or sulfonation of carbon and carbonaceous materials showed significant catalytic activities in addition to their promising mechanical and thermal stability during the transesterification processes\u00a0(Wang et al., 2019; Mateo et al., 2021; Shi et al., 2018). Most of the investigated carbon-based acidic catalysts were prepared by acidification processes for carbonized biomasses, biochar, hydrochar, activated carbon, polymers, cellulose, graphene oxide\u00a0(Betiha et al., 2020; Flores et al., 2019; Fonseca et al., 2020). Therefore, there are several limitations about the possible use of such catalysts considering the availability and the price of the precursors in addition to the cost of the carbonization processes\u00a0(Mendaros et al., 2020; Flores et al., 2019).Natural coal is of very high availability and has huge reserves in several countries with different ranks and qualities\u00a0(Shaban et al., 2017; Yu et al., 2018). Structurally, it is composed of polycyclic sheets that are of aromatic hydrocarbons in the polymeric form\u00a0(Tang et al., 2019). These aromatic rings are bonded to alkyl side chains and oxygen functional groups as carbonyl and hydroxyl\u00a0(Yu et al., 2017). The structure of coal contains several organic compounds related to its content cellulose, lignite, resign, and the other macerals. These are associated with several organic chemical groups as carbonyl, carboxyl, and hydroxyl groups\u00a0(Tang et al., 2019). Additionally, the present bridge bonds of -CH- or -O- induce the structural flexibility of coal which enhances the active groups loading process and reduces the steric hindrance impact\u00a0(Yu et al., 2018; Tang et al., 2019). Therefore, it is an ideal structure for the synthesis of highly active acidic catalysts for the transesterification and esterification processes. However, little has been introduced to evaluate the possible acidification of coal-based activated carbon in the esterification reactions; no previous studies have been developed to evaluate the acidification of the raw coal itself as an acidic catalyst for the transesterification reactions\u00a0(Tang et al., 2019). Egypt was pleased with about 50 million tons of coal as a geological reserve in the El-Mghara area, Sinai that is of no economic value until now.Therefore, the introduced study involved a novel investigation for the Egyptian raw coal as a potential acid catalyst without carbonization in the transesterification of waste sunflower oil either at low-temperature conditions or at high-temperature conditions. The acidification processes involved systematic sulfonation of the coal structure using different concentrations of \n\n\nH\n\n\n2\n\n\nSO4 with detailed inspection for the effect of the modification processes. The sulfonated coal catalyst (S.Coal) was applied in the transesterification processes considering different experimental factors. Additionally, the catalytic mechanism and the kinetic behaviors were addressed in the study.\n\nThe raw coal samples were obtained from El-Maghara coal deposits, Sinai, Egypt. The full ultimate and proximate chemical composition of the used sample was presented in Table\u00a01. Commercial sunflower oil and ultrapure methanol (99.8% purity; cornel Lab Company) were used in the transesterification studies. Analytical grade sulfuric acid (95%\u201398% purity; cornel Lab Company) was used as an acidification reagent for the coal sample. Commercial waste products for sunflower cooking oil were collected from the local restaurants and applied in the transesterification process. The fatty acid content as well as the physical properties of the studied oil sample was emphasized in Table S1.The raw coal fractions were ground gently using a normal home blender to size range from \n\n20\n\n\u03bc\nm\n\n to \n\n70\n\n\u03bc\nm\n\n. After the grinding, 10\u00a0g of the coal powder were immersed within 100 mL of sulfuric acid at 150\u00a0\u00b0C for 60\u00a0min at the inert condition of the nitrogen atmosphere. Then, the resulted suspension was cooled down to low-temperature conditions (40\u00a0\u00b0C) and washed extensively with distilled water until attending the neutral pH. Finally, the modified sample was dried for 180\u00a0min at 70\u00a0\u00b0C, kept in specified tubes, and labeled as SO3H/coal to be used in the further steps.The catalyst structure was investigated based on X-ray diffraction patterns utilizing a PANalytical X-ray diffractometer (Empyrean) with Cu-K\n\u03b1\n radiation considering the 2 Theta angle from 5\u00b0 to 70\u00b0. The morphological changes under the sulfonation effect were followed using SEM images of the Scanning electron microscope (SEM, (Gemini, Zeiss-Ultra 55)). The petrographic properties of raw coal, as well as the S.Coal sample, were addressed by inspecting their thin sections under a Nikon polarizing transmitted microscope. The chemical structure before and after the modification reactions were investigated based on their FT-IR spectra using Fourier Transform Infrared spectrometer (FTIR-8400S) within measuring range from within determination range 400\u00a0cm\u22121 to 4000\u00a0cm\u22121. The species of the bonded ions to the surface of S.Coal as catalyst were determined utilizing X-ray photoelectron spectroscopy on a Thermo scalable Thermofihsre instrument (250 Xi, USA) occupied with Al K radiation source of monochromatic properties (1468.7 eV) with binding energies measuring range from 0 up to 500 eV. The densities of the incorporated acid groups in the structure of the synthetic S.Coal were determined based on the Boehm titration method. 0.5\u00a0g of the prepared S.Coal catalyst was added to solutions of NaHCO3 (0.05 M; 17 mL), Na2CO3 (0.05 M; 17 mL), NaOH (0.05 M; 17 mL), and Na2SO4 (1.0 M; 20 mL). After that, the mixtures were shaken for 24\u00a0h and the solid particles were separated by the filtration process. Then, 5 mL of the four aliquot solutions were acidified using diluted HCl (0.05 M) and the present acidic groups were determined by simple titration using NaOH (0.05 M) in the existence of indicator (phenolphthalein). The surface area of S.Coal as catalyst was inspected considering its \n\n\nN\n\n\n2\n\n\n adsorption/desorption curve using Beckman Coulter surface area analyzer (SA3100 type).The transesterification experiments were conducted inside an airtight autoclave (150 mL) connected to a digital heater of magnetic stirrer (600 rpm). The affecting factors on the activity of the S.Coal catalyst were followed from 5\u00a0min to 120\u00a0min as the reaction time, 5:1 to 25:1 as the methanol\u2013oil\u200b ratio, 1\u00a0g to 5\u00a0g of S.Coal as the weight of the catalyst, and 40\u00a0\u00b0C to 120\u00a0\u00b0C as temperature. The experimental procedures were accomplished considering the essential step of careful filtration of the oil sample (38 mL) followed by gentle heating at 75\u00a0\u00b0C to remove the solid suspension and to avoid the side effect of the humidity. After that, a selected quantity of S.Coal was added to the oil sample and stirred for about 15\u00a0min to achieve homogeneous dispersion for the S. Coal within the sample. Then, the studied methanol volumes were added to the reactants at adjusted transesterification temperature for a certain time interval.By completing the selected time interval, the liquid phases were separated from the solid S. Coal fractions by filtrations using Whitman filter paper (\n\n40\n\n\u03bc\nm\n\n). Then, the sample was poured carefully within a separating funnel to confirm the separation of the glycerol byproducts at the bottom of the funnel. After, the complete removal of the glycerol layers, the oil sample was heated at about 75\u00a0\u00b0C for 4\u00a0h to remove the residual alcohol content. The formed fatty acid methyl esters were inspected using gas-chromatography (Agilent 7890\u00a0A). The determination was performed after the controlled dilution of the biodiesel sample with n-hexane in the presence of methyl heptadecanoate as the reference standard. Based on the determined values of fatty acid methyl ester (FAME), the biodiesel yields were calculated directly from Eq.\u00a0(1). \n\n(1)\n\n\nBiodiesel\u00a0yield\n\n\n(\n%\n)\n\n=\n\n\n\n(\nW\ne\ni\ng\nh\nt\n\no\nf\n\nb\ni\no\nd\ni\ne\ns\ne\nl\n\u00d7\n%\n\nF\nA\nM\nE\n)\n\n\n\nW\ne\ni\ng\nh\nt\n\no\nf\n\no\ni\nl\n\n\n\u00d7\n100\n\n\n\n\nThe gas chromatography technique of the Agilent 7890\u00a0A type was applied for the qualitative detection of the formed fatty acid methyl ester (FAME). The essential determination procedures included firstly controlled dilution of the collected oil samples after the tests using n-hexane. After that, the formed FAME components were determined using an Agilent-7890\u00a0A Series gas-chromatograph system which was connected with a split/splitless injector, flame ionization detector, and DB WAX capillary column (30\u00a0m\u00a0\u00d7\u00a00.25\u00a0m\u00a0\u00d7\u00a00.\n\n25\n\n\u03bc\nm\n\n) containing inert gas (\n\n\nH\n\n\n2\n\n\n) as a carrier. The temperature of both the detector and injector was adjusting during the analysis at 280\u00a0\u00b0C. The oven temperature was adjusted to be or regular increment from 120\u00a0\u00b0C up to 260\u00a0\u00b0C considering the increasing rate at a fixed value of 10 \u00b0C/min. The quantity of the present FAME was estimated considering the internal standard of methyl heptadecanoate injection.The physicochemical properties of the produced biodiesel in the studied system were followed based on the values of flash point, viscosity, cloud point, cetane number, pour point, density, and acid value. The actual value of viscosity was detected by Chongqing viscometer based on the recommended ASTM D445 test method. The flashpoint value was measured by the Penksy-martins flash tester based on ASTM D93 test method. The value of cloud point as well as pour point were measured by Lawler cloud point and pour point analyzer, respectively based on ASTM D2500 (cloud point) and ASTM D97 (pour point) tests. The value of the cetane number was determined by the Ignition quality tester based on the ASTM D613 test. The value of density was obtained by density hygrometer based on the ASTM D941 method. The acid value of the sample was determined by an automated titration system based on the ASTM D664 method.Regarding the structural properties, the XRD pattern of raw sub-bituminous coal declared the amorphous structure of the sample (Fig.\u00a01). This was confirmed by detecting the broad peaks of amorphous carbon at 2Theta angles of 8\u00b0\u201330\u00b0 and 40\u00b0\u201350\u00b0 which was assigned to the amorphous structure of the aromatic carbon sheets of lattice plane (002) and (101) (Fig.\u00a01A)\u00a0(Akinfalabi et al., 2017; Wong et al., 2020; Farabi et al., 2019b). Such aromatic sheets of carbon are of random orientation in the carbonaceous components of coal\u00a0(Akinfalabi et al., 2017; Farabi et al., 2019b).\nThe SO3H functionalized coal (S.Coal) showed noticeable changes in the pattern as the first broad peak was shifted to a high position (10\u00b0\u201332\u00b0) and the second peak declined strongly (Fig.\u00a01B). This demonstrates a change in the structure of the sample and a possible increase in the amorphization degree\u00a0(Farabi et al., 2019b; Niu et al., 2018b). This behavior is related to the effect of the sulfonation process in breaking the C-O-C cleavage bonding in the carbonaceous components of coal and the significant increase in the basal spacing\u00a0(Araujo et al., 2019). As a result, the sulfonated carbon altered significantly to be of more rigid and amorphous properties under the continuous disordering in the carbon units of the carbonaceous components of the coal\u00a0(Farabi et al., 2019b). Additionally, the expected dehydration effect of the carbonaceous components to form polyaromatic/carbon on the surface of the coal sample is of significant impact on such changes in the XRD pattern\u00a0(Akinfalabi et al., 2017). The obtained pattern for the spent is of no observable changes in the previously detected peaks of the amorphous carbon, only reduction in the intensity which declared the effect of the adsorbed oil or the glycerol on the surface of the S. coal catalyst (Fig.\u00a01C).Based on the FT-IR spectra of raw coal and S. Ccoal samples prepared at different concentrations of \n\n\nH\n\n\n2\n\n\nSO4 were presented in Fig.\u00a02. The raw sample demonstrated the common identification chemical groups of commercial coal (Fig.\u00a02A). The essential chemical groups were detected are O-H groups (3000\u00a0cm \u22121 to 3600\u00a0cm\u22121), aliphatic \u2013CH2 (2858\u00a0cm \u22121 and 2940\u00a0cm\u22121), ketone groups (2325\u00a0cm \u22121), C=C stretching (1716\u00a0cm\u22121), CO stretching (1616\u00a0cm\u22121), bending vibration of C-H of a methylene group (1450\u00a0cm\u22121), bending vibration of C-H in the methyl group (1372\u00a0cm\u22121), C-O stretching (1000\u00a0cm\u22121 to 1200\u00a0cm\u22121), and deformed C-H in the aromatic planes (500\u00a0cm\u22121 to 900\u00a0cm\u22121) (Fig.\u00a02A)\u00a0(Shaban et al., 2017; Yu et al., 2018).\nAfter the functionalization process, the recognized spectrum of the S. Coal declared a strong increment in the intensities and the broadness of the O-H related band around 3398\u00a0cm \u22121 (Fig.\u00a02B to F). This is related to the stretching vibration of the COOH groups that were formed as a result of the enrichment in the acidic chemical groups within the structure of the coal sample during the strong oxidation of the carbonaceous components by the sulfonation reactions\u00a0(Mateo et al., 2021; Wong et al., 2020). The intensities of the \u2013OH-related bands increased significantly with the different values of \n\n\nH\n\n\n2\n\n\nSO4 concentrations reflecting increment in the incorporated acidic groups (COOH) with testing higher concentrations of the acid (Fig.\u00a02B to F)\u00a0(Farabi et al., 2019b; Araujo et al., 2019). This was observed also for the absorption band that related to the CO vibrational modes of COOH groups around 1695\u00a0cm\u22121\u00a0(Mateo et al., 2021; Yu et al., 2018). It appeared at higher intensities for the samples with were treated with the highest \n\n\nH\n\n\n2\n\n\nSO4 concentrations. The incorporated sulfur-bearing groups were identified by SO3 stretching (1174\u00a0cm\u22121) of the \u2013SO3H groups, symmetric \n\n\n (1071\u00a0cm\u22121), and asymmetric \n\n\n (1001\u00a0cm\u22121) in addition to C-S groups (578\u00a0cm \u22121) (Fig.\u00a02B to F)\u00a0(Mateo et al., 2021; Fonseca et al., 2020). The essential bands of raw coal as the C=C stretching (2610\u00a0cm\u22121) and the C-H plane bending (1296\u00a0cm\u22121) were detected as reduced bands especially with using high concentrations of the acid (Fig.\u00a02B to F)\u00a0(Farabi et al., 2019b; Araujo et al., 2019). This might be related to the predicted destruction of the cyclic structure of cellulose and the oxidation of the present methyl groups in the structure during the oxidation process as they were converted into COOH groups\u00a0(Mateo et al., 2021). Also, the intensity of the CO stretching-related band show (1629\u00a0cm\u22121) an obvious increment as a result of the oxidation process and the formation of new carboxylic groups (Fig.\u00a02B to F)\u00a0(Fonseca et al., 2020).XPS analysis was also performed to investigate the functional groups which were formed during the formation of S.Coal as an acidic catalyst in the conversion of sunflower oil (Fig.\u00a03). The survey scan was estimated within the range from 0 to 1400 eV to confirm the change in the structure of the coal skeleton after the incorporation of \u2013SO3H functional groups. The high resolutions scans clarify the existence of the identification peaks of C 1s, O 1s, and S 2p (Fig.\u00a03A). The first spectrum of O1s spectrum reflected the existence of two chemical states for the oxygen ions at 531.7 (C=O bond) and 533.2 eV (S-O, S-OH, C\u2013OH bonds) (Fig.\u00a03B)\u00a0(Yu et al., 2018; Tang et al., 2019). The first spectrum of C 1s demonstrates the presence of three chemical states at 288.6 eV (to C=O/O\u2013C=O bond), 286.6 eV (C\u2013O bond), and 284.5 eV (C-C/C-H bond) (Fig.\u00a03C). Additionally, the S 2p spectrum reflects the chemical state of -SO3H at 168.7 eV confirming the incorporation of the sulfonic groups within the coal structure\u00a0(Yu et al., 2018) (Fig.\u00a03D).\n\n\n\nThe optical properties of the investigated coal sample before and after the modification reactions were evaluated based on the obtained images from the transmitted polarized microscope (Fig.\u00a04A, B, and C). The raw sample appeared with a characteristic macerals structure of coal that is composed mainly of the carbonized wood tissue (vitrinite) as the essential component. The present vitrinite contains several varieties of other species of macerals (Liptinite) as pollen, spores, resin, and cuticle of leaves (Fig.\u00a04A and B). Additionally, other inorganic components were identified as cryptocrystalline silica, pyrite, and clay impurities. The SO3H-functionalization and the related oxidation process resulted in strong changes in optical features of the coal sample (Fig.\u00a04C). The sample showed a strong reduction in the present inorganic impurities and the maceral structure appeared in a dark tone reflecting the influence of the oxidation processes (Fig.\u00a04C).Regarding the morphology, the SEM image of raw bituminous coal reflected the presence of the sample as compacted layers (Fig.\u00a04D). These layers are composed of irregular forms and stacked randomly above each other which are related mainly to the compression of the macerals and the wood tissue components (Fig.\u00a04D). The modification reaction resulted in strong changes in the morphologies and the surficial features of the coal sample (Fig.\u00a04E and F). The surface appeared to be of rugged and irregular topography with numerous nano nudes (Fig.\u00a03G and H). Moreover, the structure appeared to be of observable microporosity which might be related to the dissolving of the inorganic components (Fig.\u00a04H). This gives the sulfonated products a higher surface area and more exposed sites than the raw coal which will be of strong contribution in inducing the catalytic performance of the sample. Regarding the SEM image of the S.Coal after the transesterification process, the inspected featured reflected partial coating of the S.Coal particles with irregular materials that might be related to the adsorbed glycerol or the components of the studied S.SFO (Fig.\u00a04I).The weight of the used catalyst is of a significant influence on the biodiesel yield which can be obtained by the transesterification of the spent sunflower oil over S.Coal catalyst (95% \n\n\nH\n\n\n2\n\n\nSO4). The effect of the catalyst dosage was followed from 1\u00a0g up to 5\u00a0g and the other controlling factors were fixed at 60\u00a0min as reaction time, 15:1 as a methanol-to-oil ratio, 600 rpm as stirring speed, and 40\u00a0\u00b0C as low-temperature conditions (Fig.\u00a05A).There is a noticeable enhancement in the determined yields from 93.2% up to 98.4% with increasing the catalyst content from 1\u00a0g up to 3\u00a0g (Fig.\u00a05A). This behavior can be explained based on the predicted increase in the catalytic active sites and the interacted surface area which induce the catalytic performance of the conversion process\u00a0(Basyouny et al., 2021; Bhatia et al., 2020). Beyond 3\u00a0g, the used dosages of S.Coal (4\u00a0g and 5\u00a0g) are of negative effects and the produced yield decreased to 95.5% (4\u00a0g) and 89.2% (5\u00a0g) (Fig.\u00a05A). The further increases in the catalyst weight above the limit cause a reduction in the homogeneity properties of the reaction system causing a reduction in the resulted yields\u00a0(Sayed et al., 2020; Bin\u00a0Jumah et al., 2021). Additionally, the unreacted particles of S.Coal increase the mass transfer resistance between the different reaction components which diminish the conversion yield of the reaction\u00a0(Sayed et al., 2020).According to the stoichiometric equation of the transesterification reaction, each molecule of triglycerides reacts with three methanol molecules to create its FAME so the adjustment of the ratio between triglyceride and methanol is of a significant effect on the resulted yields\u00a0(Negm et al., 2017). The influence of the methanol-to-oil ratio as a controlling factor on the transesterification process of sunflower oil by S.Coal (95% \n\n\nH\n\n\n2\n\n\nSO4) was investigated within the range from 5:1 up to 25:1. The other affecting parameters were fixed at 3\u00a0g as catalyst dosage, 60\u00a0min the reaction time, 600 rpm as stirring speed, and at low-temperature conditions (40\u00a0\u00b0C) (Fig.\u00a05B).In the studied transesterification process for S.SFO over S.Coal as an acidic catalyst, the increment in the methanol ratio from 5:1 up to 20:1 resulted in an increase in the yields up to 98.8 % (Fig.\u00a05B). The accurate amount of methanol is of vital effect in accelerating the reaction in the forward direction and in reducing the mass transfer resistance between the different phases of the reaction\u00a0(Bin\u00a0Jumah et al., 2021; Rabie et al., 2019). Moreover, the suitable methanol content enhances the solubility of the reactants and decreases the viscosity inside the reaction system (Singh et\u00a0al. 2020). All the previous effects are of valuable impact in enhancing the catalytic activity of S.Coal and the achieved biodiesel yields for the studied S.SFO. At the studied methanol ratio of 25:1, the observed yield was declined slightly down to about 97.4% i.e.\u00a0the best methanol-to-oil ratio is 20:1 (Fig.\u00a05B). The reversible effect for the excessive content of methanol on the produced yields is related to the impact of the unreacted alcohol molecules in the deactivation of the catalytic sites and the dilution of the mixture which direct the reaction to the backward direction\u00a0(Abukhadra et al., 2019).The transesterification time interval is a vital factor in controlling the miscibility and the reaction balance\u00a0(Sayed et al., 2020). The influence of the time interval was inspected within the experimental range from 10\u00a0min up to 80\u00a0min for different temperature values from low-temperature conditions (40\u00a0\u00b0C) up to 120\u00a0\u00b0C as high-temperature conditions. The transesterification conditions were studied at 20:1 for the methanol/oil ratio, 3\u00a0g for the used S.Coal dosage (95% \n\n\nH\n\n\n2\n\n\nSO4), and 600 rpm for the stirring speed (Fig.\u00a05C).At the low-temperature conditions (40\u00a0\u00b0C), the yields increased gradually with increasing the transesterification interval up to 60\u00a0min and the obtained yield attend 98.8%. The further expansion of the reaction time resulted in diminishing the catalytic activity and the produced biodiesel yield declined to be 98% after 80\u00a0min (Fig.\u00a05C). This behavior is ascribed to the immiscible nature between the different reaction components which would hinder the catalytic activity of the S.Coal at the starting periods\u00a0(Toledo\u00a0Arana et al., 2019). Therefore, the reaction requires a considerable period to reach the equilibrium point (60\u00a0min) at which the mixing periods achieved the best homogeneity and low miscibility between the components. The elevation in the time to a higher interval than the equilibrium time (60\u00a0min) showed a desirable impact on the produced yield as the reaction was driven to the reversible direction\u00a0(Basyouny et al., 2021).With increasing the temperature, the reactions show similar behavior as observed at the low-temperature conditions regarding the enhancement of the yield with time (Fig.\u00a05C). There is a noticeable enhancement in the achieved yield at all the studied intervals as compared to the normal conditions of low-temperature conditions (40\u00a0\u00b0C). Moreover, the equilibration time and the required intervals to achieve the best yields declined strongly after the regular increase in the studied temperature. At 120\u00a0\u00b0C, biodiesel yields of 97.3% and 99.3% were determined after 10\u00a0min and 20\u00a0min, respectively (Fig.\u00a05C). Additionally, the reaction equilibration was attained after 30 min achieving the highest yield (99.6%). Based on these results, it can be concluded that the performance of the transesterification reaction inside an airtight autoclave under high-temperature conditions is of strong impact in enhancing the catalytic activity of S. Coal and decreasing the required time for effective conversion processes. This behavior is attributed to the endothermic nature of the transesterification reaction and the increase in the reaction temperature cause increase in the kinetic energies of the system. As a result, the mass transfer between the different phases of the reaction will increase strongly and the rate of reaction will be accelerated significantly\u00a0(Toledo\u00a0Arana et al., 2019; Seela et al., 2020).The sulfonation conditions are of great influence on the stability of \u2013SO3H and the number of active sites within the produced catalyst\u00a0(Fonseca et al., 2020). Therefore, the effect of the sulfonation conditions was inspected as function of the sulfuric acid concentration (70%, 75%, 80%, 85%, 90%, and 95 %). The transesterification conditions were studied at 20:1 for the methanol/oil ratio, 3\u00a0g for the used S.Coal dosage, 600 rpm for the stirring speed, 60\u00a0min as the interval at low-temperature conditions (40\u00a0\u00b0C), and 30\u00a0min as the interval at high-temperature conditions (120\u00a0\u00b0C) (Fig.\u00a05D).Based on the determined biodiesel yields, the highest percentages were obtained for the coal samples which were treated with the highest \n\n\nH\n\n\n2\n\n\nSO4 concentrations (85%, 90%, and 95%) either at the low-temperature conditions (40\u00a0\u00b0C) or at high-temperature conditions (120\u00a0\u00b0C) (Fig.\u00a05D). It can be detected that using the sulfuric acid at a concentration above 90% is of slight effect on the catalytic performance of S.Coal. Such enhancement in the catalytic activity of S.Coal with using high concentrations of \n\n\nH\n\n\n2\n\n\nSO4 is related to the vital role of the acid in inducing the stability and the acidity properties of the catalyst\u00a0(Tang et al., 2019). This can be clarified according to Luciatalia,s principle of equilibrium as the sulfonation process is a reversible exothermic reaction. Thus, decreasing the \n\n\nH\n\n\n2\n\n\nSO4 concentration would direct the reaction to the opposite direction and diminish the quantities of the incorporated sulfonated groups within the coal sample\u00a0(Wong et al., 2020). This was supported by the previously investigated FT-IR analysis that reflected a significant increment in the intensities of the identification bands of sulfur-bearing chemical groups as O=S=O, SO3, -SO3H, and C-S groups.Such results also are of high agreement with the measured acid densities of the coal samples after the sulfonation processes with the different concentrations of \n\n\nH\n\n\n2\n\n\nSO4 (Tabel.2). The measured densities increased from 2.95 mmol/g up to 7.53 mmol/g with increasing the \n\n\nH\n\n\n2\n\n\nSO4 concentration from 70% up to 95%. Additionally, the increase in the sulfur content with treating the coal samples with high \n\n\nH\n\n\n2\n\n\nSO4 concentration confirms the effect of the concentration in inducing the stability of the incorporated sulfur-bearing groups (Table\u00a02). Moreover, the slight enhancements in the surface area and the porosity with regular increase in the concentration of the used acid are of remarkable effect in inducing the activity of the S.Coal catalyst which was synthesized at high concentrations of \n\n\nH\n\n\n2\n\n\nSO4 (85%, 90%, and 95%) (Table\u00a02).\n\nThe recyclability properties of the produced S.Coal as acidic catalyst were assessed for five reusing cycles. The used S.Coal particles after the transesterification tests were collected washed carefully using distilled water three times and each time consumed 10\u00a0min. After that, the fractions were dried gently in an electric drier for 8\u00a0h at 60\u00a0\u00b0C to be reused in the next transesterification cycle. The transesterification conditions were selected at 600 rpm as the stirring speed, 20:1 as the adjusted methanol to oil ratio, 3\u00a0g as the incorporated S.Coal quantity, 60\u00a0min as the interval at low-temperature conditions, and 30\u00a0min as the interval at high-temperature conditions (120\u00a0\u00b0C) (Fig.\u00a05E).The achieved yields reflect the considerable stability of the S.Coal as an acidic catalyst for the transesterification of S.SFO considering the addressed five cycles either at low-temperature conditions or at high-temperature conditions (Fig.\u00a05E). The obtained yields for reusing cycles of S.Coal at the low-temperature conditions (40\u00a0\u00b0C) are 98.8% (Cycle 1), 98% (Cycle 2), 97.3% (Cycle 3), 95.4% (Cycle 4), and 93.5% (Cycle 5) (Fig.\u00a05E). The obtained values at high-temperature conditions (120\u00a0\u00b0C) are 99.6% (Cycle 1), 99.3% (Cycle 2), 97.4% (Cycle 3), 94.53% (Cycle 4), and 89.6% (Cycle 5) (Fig.\u00a05E). The linear decrease in the S.Coal with repeating the assessed cycles reflects the possible deactivation of its catalytic sites by the adsorbed glycerol and oil during the transesterification processes. Additionally, the expected leaching of the -\n\n\nS\n\n\n3\n\n\nOH groups during the tests affected negatively the activity of the S.Coal catalyst during the recyclability tests\u00a0(Araujo et al., 2019). The observed high stability of the S.Coal sable at the low-temperature conditions (40\u00a0\u00b0C) as compared to high-temperature conditions (120\u00a0\u00b0C) demonstrate the effect of high-temperature and pressure conditions in accelerating the leaching of the \u2013SO3H groups from the S-Coal sample.The effective transesterification processes that involve the promising conversion of S.SFO into biodiesel occur according to three steps as declared in Eq.\u00a0(2), Eq.\u00a0(3), and Eq.\u00a0(4). The three equations represent successive chemical interactions between the present triglyceride (TG) as well as diglyceride (DG) and monoglyceride (MG) with about 1 mole of methanol which resulted in the formation of FAME (1 mole). Finally, the reactions resulted in about 1 mole of glycerol and 3 moles of FTAM as declared in Eq.\u00a0(5)\u00a0(Naeem et al., 2021). Considering the assumption of Eq.\u00a0(5), the transesterification rate is controlled essentially by the concentrations of the liquid reactants either the TG or methanol content. \n\n\n(2)\n\n\nT\nG\n+\nMethanol\n\u2194\nD\nG\n+\nM\nE\n\n\n\n\n(3)\n\n\nD\nG\n+\nMethanol\n\u2194\nM\nG\n+\nM\nE\n\n\n\n\n(4)\n\n\nM\nG\n+\nMethanol\n\u2194\nG\nR\n+\nM\nE\n\n\n\n\n(5)\n\n\nT\nG\n+\n3\nMethanol\n\u2194\nG\nR\n+\n3\nM\nE\n\n\n\n\n\nThe transesterification as a chemical reaction is of reversible properties which make the high alcohol content is of valuable impact in directing the reaction in the forward side until the equilibrium\u00a0(Abukhadra et al., 2020). As the higher concentrations are of no significant impact without the existence of suitable concentrations of TG, the transesterification reactions appear to depend only on the availability of TG molecules. Therefore, the occurred reactions during the conversion of S.SFO over S.Coal are of pseudo-first-order kinetic properties\u00a0(Roy et al., 2020). The Pseudo-First order equation in its linear form can be expressed by Eq.\u00a0(6). \n\n(6)\n\n\n\u2212\nL\nn\n\n\n1\n\u2212\n\n\nY\n\n\nM\nE\n\n\n\n\n=\nk\nt\n\n\n\nThe kinetic investigation was conducted considering the time from 10\u00a0min up to 60\u00a0min, the temperature from 40\u00a0\u00b0C up to 120\u00a0\u00b0C, the methanol/oil ratio at 20:1, the S.Coal dosage at 3\u00a0g, and the stirring speed at 600 rpm (Fig.\u00a05F). The obtaining fitting degrees (R2 > 0.9) reflected strong agreement between the kinetic behaviors for the transesterification of S.SFO over S.Coal and the assumption of the Pseudo-first order kinetic model (Fig.\u00a05F; Table.S2). Such fitting results suggested the operation of the adsorption-surface reaction\u2013desorption mechanism during the process and the pseudo-First\u200b order behavior is reasonable to represent the transesterification systems of S.Coal\u00a0(Li et al., 2019).The value of the activation energy (41.05 kJ/mol) was obtained as a theoretical parameter based on the Arrhenius equation (Eq.\u00a0(7)) and the conducted linear regression plots of ln k vs 1/T (Eq.\u00a0(8)). This declares the suitability of the S.Coal transesterification system for S.SFO to operate effectively at mild conditions and low energy\u00a0(Naeem et al., 2021) (Fig. S1). \n\n\n(7)\n\n\nk\n=\ne\nx\np\n\n\n\u2212\n\n\n\n\nE\n\n\na\n\n\n\n\nR\nT\n\n\n\n\n\n\n\n\n(8)\n\n\nL\nn\nk\n=\nA\n.\n\n\n\u2212\n\n\nE\n\n\na\n\n\n\n\nR\nT\n\n\n+\nL\nn\nA\n\n\n\n\n\nThe sulfonation mechanism depends strongly on the presence of electrophilic species to functionalize the aromatic components within the coal structure\u00a0(Xiao and Hill, 2020). During the sulfonation process, the molecules of the used sulfuric acid will be affected by the protonation process as a result of the contentious reaction between them. This is related to the affinities of the -OH groups to grab hydrogen ions which causes a break for the present oxygen\u2013hydrogen bonds forming good leaving groups of \n\n\nH\n\n\n2\n\n\nO\u00a0(Tang et al., 2019). On the other hand, the protonated oxygen during this step will interact with one of its lone pair electrons creating bonds with the sulfur ions which leads to the formation of protonated trioxide\u00a0(Tang et al., 2019). Such positively charged sulfur ions in their trioxide forms are of high electronegativity and act as strong effective electrophiles that are of significant role during the sulfonation of the coal structure\u00a0(Yu et al., 2018) (Fig.\u00a06A and B). The attack of the sulfur ions on the benzene rings resulted in destruction for the double bonds in the aromatic ring and this induced the incorporation of sulfur-bearing groups as \u2013SO3H (Fig.\u00a06A and B) which was confirmed by the FT-IR analysis (Fig.\u00a02). On the other hand, two reacted molecules of sulfuric acid produced HSO\n\n\n\n4\n\n\n\u2212\n\n\n radicals which act as a base to remove the hydrogen protons\u00a0(Xiao and Hill, 2020). At the same time, there is strong oxidation process occurred for the OH groups converting them into carboxyl groups (COOH) (Fig.\u00a06A and B) as observed in the FT-IR spectrum. The existence of COOH and \u2013SO3H in addition to OH as highly active catalytic groups after the sulfonation process induced the catalytic efficiency of S.Coal as an acidic catalyst in the transesterification processes (Fig.\u00a07).\n\nThe observed declination in the catalytic efficiency of S.Coal samples with decreasing the concentrations of \n\n\nH\n\n\n2\n\n\nSO4 can be illustrated based on Luciatalia \n\n\n\n,\n\n\ns principle. According to the principal, such sulfonation reactions are of reversible properties and the decrease in the \n\n\nH\n\n\n2\n\n\nSO4 concentration directs the reversible direction which reduces the entrapment efficacy of the sulfuric groups in the structure of coal\u00a0(Mateo et al., 2021; Fonseca et al., 2020). Moreover, the oxidation processes at the diluted concentrations of \n\n\nH\n\n\n2\n\n\nSO4 are of faint effect which resulted in the formation of aldehyde groups instead of the active carboxyl groups which affect negatively the efficacy of the catalyst.For the transesterification conversion of S.SFO over S.Coal as an acidic heterogeneous catalyst, it can be illustrated based on the chemistry of the oil. Chemically the vegetable oils are unsaturated and saturated monocarboxylic acids are known as triglycerides (Fig.\u00a07)\u00a0(Abukhadra et al., 2020). The triglycerides can be reacted with alcohol molecules during the transesterification process forming monoesters in the presence of the catalyst. This reaction involves essentially hydrolysis reaction for the ester groups between the present fatty acids and the glycerol (Fig.\u00a07)\u00a0(Basyouny et al., 2021). This causing generation of new types of ester bonds between the fatty acids and the alcohol molecules and then replacement for the glycerol by three monohydric alcohols. The predicted mechanism for the transesterification of S.SFO over the synthetic sulfonated coals as acidic catalyst (S.Coal) and the expected nucleophilic attacks of the reacted ester groups was emphasized schematically in Fig.\u00a07\u00a0(Negm et al., 2017).The physical properties of the resulted biodiesel product at the best conditions over S.Coal as acidic heterogeneous catalyst were evaluated based on the international requirements for the suitable biodiesel as biofuels standards (ASTM D-6751 and EN 14214) (Table\u00a03). The viscosity and the density of the tested biodiesel sample are of high agreement with the suggested values for the recommended biodiesel products. Additionally, the cetane index was measured at a promising value (more than 45) which declares the safety properties of the product if it was applied directly in the engines\u00a0(Basyouny et al., 2021). Moreover, the measured value of flashpoint is suitable for the handling and the transport of the biodiesel product as fuel (Table\u00a03).\nThe results of the GC\u2013MS analysis reflect the formation of palmitoleic acid methyl ester, oleic acid methyl ester, and linoleic acid methyl ester as the essential fatty acid methyl esters (FAME) (Table\u00a03). Additionally, other species were detected at minor content including myristic acid methyl ester, palmitic acid methyl ester, eicosanoic acid methyl ester, stearic acid methyl ester, caprylic acid methyl ester, and behenic acid methyl ester (Table\u00a03).The activity of S.Coal as a heterogeneous catalyst for the transesterification of waste cooking oil was compared with other studied catalysts in literature either the basic catalysts or the acidic catalysts (Table\u00a04). The synthetic S.Coal as a heterogeneous catalyst displayed higher activity than several studied basic catalysts including zeolite-X, NiO, CaO, CaO/SiO2, KOH/Clinoptilolite, and, synthetic apatite. Additionally, it appears as a high active acidic catalyst as compared to the other acidic catalysts including some inorganic acidic catalysts (SO4/Fe-Al-TiO2, Fe2O3-MnO-SO4/ZrO2, and Ti(SO4)O) and sulfonated carbonaceous catalysts (Sulfonated graphene and Sulfonated AC from bamboo) (Table\u00a04).\nThe resulted high yield in addition to the determined acid densities in comparison with the other acid catalyst which were prepared by sulfonation of carbonaceous materials demonstrates the flexibility of the raw component to affect strongly by the sulfonation reactions in their natural form without carbonization. It was reported that the carbonization process is of negative effects on the carbon skeleton. The carbonization temperature causes accumulation for the structural carbon layers in a highly random orientation as a result of the destruction and collapse of carbon frameworks\u00a0(Yu et al., 2017). This reduces the quantities of the present hydroxyl and carboxyl active groups during the formation of the catalyst\u00a0(Flores et al., 2019). Therefore, raw coal without carbonization is a favorable precursor for the synthesis of acidic heterogeneous catalysts with significant acidic density and promising catalytic activity.Raw sub-bituminous coal was treated with sulfuric acid in a sulfonation process to produce potential acidic catalysts for the transesterification of S.SFO. The synthetic S.Coal catalyst using \n\n\nH\n\n\n2\n\n\nSO4 (95%) exhibits the best catalytic activity, acid density (8.4 mmo/g), and surface area (26.4\u00a0m2/g. At low-temperature conditions (40\u00a0\u00b0C), the S.Coal achieved a yield of 98.8 after 60\u00a0min using 3\u00a0g of the catalyst and a methanol/oil ratio of 20:1. However, at high-temperature conditions (120\u00a0\u00b0C), the yield was enhanced to 99.5%, and the time interval was reduced to 30\u00a0min only. The synthetic S.Coal catalyst is of considerable reusability and higher catalytic performances some addressed basic and acidic catalysts. Moreover, the extracted biodiesel at the best conversion conditions is of acceptable technical qualifications according to the international requirements. The results declare higher efficacy of the sulfonation process on the raw coal than the carbonized products.\nSherouk M. Ibrahim: Visualization, Formal analysis, Writing \u2013 original draft, Writing \u2013 review & editing. Ahmed M. El-Sherbeeny: Conceptualization, Project administration, Visualization, Writing \u2013 original draft, Writing \u2013 review & editing. Jae-Jin Shim: Writing \u2013 original draft, Writing \u2013 review & editing. Ali A. AlHammadi: Formal analysis, Writing \u2013 original draft, Writing \u2013 review & editing. Mostafa R. Abukhadra: Conceptualization, Project administration, Visualization, Formal analysis, 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 authors extend their appreciation to King Saud University, Saudi Arabia for funding this work through researchers supporting project number (RSP-2021/133), King Saud University, Riyadh, Saudi Arabia\nSupplementary material related to this article can be found online at https://doi.org/10.1016/j.egyr.2021.11.139.The following is the Supplementary material related to this article. \n\nMMC S1\n\nThe Supplementry materials containing the fatty acid composition of the used oil, the kinetic parameters, and fitting of the kinetic rate constnat with Arrhenius equation.\n\n\n\n\n", "descript": "\n                  Raw coal without carbonization was treated with sulfuric acid at different concentrations (70% to 95%) as a controlled sulfonation process to produce a simple and effective acidic catalyst. The synthetic catalyst (S.Coal) at the best \n                        \n                           \n                              H\n                           \n                           \n                              2\n                           \n                        \n                     SO4 concentration (95%) showed the best acid density (8.4 mmol/g), the best surface area (26.4\u00a0m2/g), and the best catalytic activity during the transesterification of the waste sunflower oil. The best yield (98.8%) at low-temperature conditions (40\u00a0\u00b0C) was achieved after 60 min using 3\u00a0g of the catalyst and at a methanol/oil ratio of 20:1. At high-temperature conditions (120\u00a0\u00b0C), the yield was enhanced to 99.5% after 30 min only considering the catalyst quantity and the methanol content as the same values. The S.Coal as a catalyst is of remarkable reusability for five transesterification runs either at low-temperature conditions (40\u00a0\u00b0C) or at high-temperature conditions (120\u00a0\u00b0C). Additionally, the catalyst is of higher catalytic performances than several basic and acidic catalysts demonstrate the efficiency of the sulfonation process on the raw coal without carbonization. The kinetic properties of the occurred transesterification reactions over S.coal followed the Pseudo-First order behavior and of low activation energy.\n               "}
{"full_text": "Due to the depletion of fossil fuel reserves, converting renewable and abundant biomass into fuels and chemicals becomes important [1\u20134]. Lignin is one of the main components of lignocellulose, mainly consisting of three aromatic units, i.e., p-coumaryl alcohol (H), coniferyl alcohol (G) and sinapyl alcohol (S), which are connected with C-O and C-C linkages. As the only renewable natural resource that contains aromatic rings, lignin is regarded as a suitable feedstock to replace fossil resources to produce aromatic chemicals [5\u20138] and fuels [9\u201311].Enzymatic hydrolysis lignin (EHL) is the by-product of bioethanol production industry, and has not been utilized effctively [12\u201314]. Currently, EHL as a low-grade fuel is burned to produce heat and power for the biorefining industry [15]. Compared to the kraft lignin, EHL has a high purity and low content of sulfur, and its structure is similar to the native lignin [16]. Therefore, EHL is a suitable feedstock for depolymerization reaction to produce aromatic chemicals [17].Catalytic solvolysis of lignin into aromatic chemicals has been widely investigated, some milestone works were reported. Barta et al. [18] found that supercritical methanol served both as the solvent and the hydrogen-source in the conversion of organosolv lignin into substituted cyclohexyl derivatives over a Cu-based porous metal oxide catalyst. Song et al. [19] depolymerized lignin in birch wood with Ni/C as a catalyst in methanol at 200\u00a0\u00b0C under Ar atmosphere, and obtained 54% phenolic monomers with propylguaiacol and propylsyringol as the main products. Our previous work verified that Mo-based catalysts were active for catalytic solovolysis of Kraft lignin. Ma et al. [20,21] achieved the complete conversion of Kraft lignin into small-molecule products, including C6\u2013C10 esters, alcohols, arenes, phenols, and benzyl alcohols, with MoC1\u2212x/AC as a catalyst ethanol at 280\u00a0\u00b0C. After that they reported that MoN2, Mo/Al2O3 and MoC1\u2212x/CuMgAlOx showed a similar activity for the Kraft lignin depolymerization and similar products. However, the complex products are usually obtained from lignin depolymerization, posing a challenge to the subsequent separation and purification process [22]. Recently, Wu et al. [23] reported that MoS2 showed a high selectivity for conversion of guaiacol to 2-(tert-butyl)-3-methylphenol (TBC), which has been used as an antioxidant in the polymer industry. Herein, MoS2 is employed in the depolymerization of EHL. The influences of reaction parameter are examined, and the mechanism of EHL depolymerization are discussed based on the results of FT-IR and monomers and dimers conversion. Furthermore, the active species and the deactivation of catalyst are also studied.EHL was purchased from Shandong Longlive Biotechnology Co., Ltd. The raw materials are dried at 60\u00a0\u00b0C for 12\u00a0h before use. Analytical-grade chemicals and solvents, including ethanol, methanol, and isopropanol, were purchased from Tianjin Guangfu Technology Development Co., Ltd. Ammonium molybdate, sulfur, hydrazine hydrate and the model compounds were purchased from Aladdin Co., Ltd.MoS2 sample was prepared with a hydrothermal method. Ammonium molybdate (1.53\u00a0g) and elemental sulfur (0.5\u00a0g) were dissolved in 60\u00a0mL distilled water and 8\u00a0mL of hydrazine hydrate was added drop by drop to the solution. This solution was then transferred into a 200\u00a0mL Teflon\u2013lined stainless autoclave and heated at 150\u00a0\u00b0C for 24\u00a0h. The resulting black precipitate was separated and washed with water and absolute ethanol and dried under vacuum at 60\u00a0\u00b0C for 12\u00a0h.The X-ray diffraction (XRD) patterns of fresh catalysts were recorded at room temperature using a Rigaku D/max 2500\u00a0v/pc instrument with Cu K\u03b1 radiation, operated at 40\u00a0kV and 40\u00a0mA at a scanning rate of 10\u00a0\u00b0/min in the 2\u03b8 range of 10 \u2013 90\u00b0. The morphology and structure of samples were observed with a scanning electron microscope (SEM, S-4800, Hitachi) and a transmission electron microscope (TEM, JEM-2100, JEOL). The X-ray photoelectron spectra (XPS) for both fresh and used catalysts were recorded with a PHI 1600 ESCA system spectrometer. The X-ray source was Mg K\u03b1 (1253.6\u00a0eV), and the binding energy was calibrated using C1s at 284.6\u00a0eV as the standard. The Raman spectra were obtained on a Renishaw instrument (532\u00a0nm).EHL depolymerization reactions were carried out in a 300\u00a0mL autoclave reactor (Kemi Co. Ltd, 250\u00a0mL, made of Hastelloy). In a typical run, 1.0\u00a0g EHL or a model compound, such as guaiacol, 70\u00a0mL methanol and 0.5\u00a0g MoS2 were charged into the autoclave reactor. The reactor was sealed and purged with pure nitrogen for 5 times, then heated to the desired temperature and pressurized with hydrogen and stirred at 600\u00a0rpm. After completion of the reaction, the reactor was rapidly cooled to room temperature, and the liquid products and the catalyst were subsequently separated by filtration.In the reusability tests, the used MoS2 catalyst was washed with 10\u00a0mL methanol and then was dried at 60\u00a0\u00b0C in vacuum for 1\u00a0h before the next run.All liquid products were analyzed and quantified with an Agilent 6890/5973\u00a0N GC\u2013MS and a 6890\u00a0N gas chromatography equipped with a flame ionization detector (FID) and a 30\u00a0m HP\u20135MS capillary column. The injection temperature of GC and GC-MS was maintained at 280\u00a0\u00b0C. The oven temperature increased from an initial temperature of 45\u00a0\u00b0C to a final temperature of 250\u00a0\u00b0C at a rate of 10\u00a0\u00b0C/min and kept at 250\u00a0\u00b0C for 7\u00a0min. The selectivity of TBC was calculated as follows:\n\n\n\nSelectivity\n\nof\n\nTBC\n\n\n\n\n%\n\n\n\n=\n\n\n\nweight\n\nof\n\nthe\n\nTBC\n\n\nweiht\n\nof\n\nthe\n\noverall\n\nproduct\n\n\n\u00d7\n100\n%\n\n\n\n\nFT-IR spectra of EHL and liquid products were collected in the transmission mode on a Nexus spectrometer (Thermo Nicolet Co.). The spectrum was obtained after 32 scans and recorded in the region 4000\u2013400\u2009cm\u22121 with a resolution of 4\u2009cm\u22121.The XRD patterns of the fresh and used MoS2 are depicted in \nFig. 1. In the pattern of fresh and used MoS2, the peaks of (100) and (105) planes of MoS2 are weak, suggesting that the synthesized MoS2 has amorphous structure. \nFig. 2(a\u2013c) displays the SEM images of the fresh and used MoS2. Fresh MoS2 has a fluffy flower-like structure, while the used MoS2 became agglomerated. The TEM image (Fig. 2(d)) confirms the MoS2 phase in the fresh MoS2 catalyst. The lattice spacing marked on the micrograph for the highlighted domain is 0.27\u2009nm and corresponds to the (103) plane of MoS2.The XPS spectra were also measured for the fresh and used MoS2. As shown in \nFig. 3(a), Three oxidation states including Mo4+, Mo5+and Mo6+ were detected in the fresh MoS2 samples. Two peaks located at 228 and 232.2\u2009eV were attributed to Mo3d5/2 and Mo3d3/2 of Mo4+ species. The two peaks at 229.6 and 233\u2009eV were attributed to Mo5+, and the other two peaks located at 232.8 and 235.9\u2009eV were assigned to Mo6+\n[24,25]. Besides, the signal of S 2\u2009s was observed at 227\u2009eV. Fig. 5(b) depicts the XPS spectra of S 2p energy region of the MoS2 catalysts. Four peaks located at 161.9, 161.2, 163.3 and 164.3\u2009eV were attributed to S2\u2212 2p3/2, S2\u2212 2p1/2, \n\n\nS\n\n\n2\n\n\n2\n\u2212\n\n\n2p3/2 and \n\n\nS\n\n\n2\n\n\n2\n\u2212\n\n\n 2p1/2, respectively [26]. the Mo5+ (229.6 and 233\u2009eV) and \n\n\nS\n\n\n2\n\n\n2\n\u2212\n\n\n (~164\u2013165\u2009eV) species was assigned to the MoOxSy phase [27,28]. After reaction, the Mo5+ was gradually transformed to either Mo6+ or Mo4+ partially, and its surface ratio decreased sharply from 41.6% to 2.2%. Meanwhile, the proportion of \n\n\nS\n\n\n2\n\n\n2\n\u2212\n\n\n decreased from 27% to 23.9% and the ratio of S2\u2212 increased from 73% to 76.1%, respectively.\n\nFig. 4 shows the Raman spectra of the fresh and the used MoS2 catalysts. Typically, the peaks at 308.2 and 403.5\u2009cm\u22121 of the fresh MoS2 are ascribed to the in-plane \n\n\nE\n\n\n2\ng\n\n\n1\n\n\n mode and out-of-plane A1\u2009g mode of the MoS2, respectively. In Raman spectra of the used MoS2, two peaks located at 1383.7 and 1581.6\u2009cm\u22121 are observed, which are attributed to the D (defected) and the G (graphitized) bands of carbon, [29] confirming the formation of char.\n\nFig. 5(a) shows the total ion chromatogram (TIC) of the liquid products obtained from EHL depolymerization over a MoS2 sample at 280\u2009\u00b0C for 6\u2009h in methanol. 23-Types of aromatic monomers are identified, with the total yield of 124.1\u2009mg/g, and TBC is the main products with the selectivity of 40.3%. Without MoS2 (Fig. 5(b)), only 62.6\u2009mg/g of monomers are formed, and the main products are eugenol, guaiacol, 4-ethylguaiacol, phenol and 4-ethylphenol.As shown in \nTable 1, The effect of different solvents, including methanol, ethanol, and isopropanol, were examined in EHL depolymerization on MoS2 sample. Among the solvents examined, ethanol gives the highest monomer yield, but monomers obtained in ethanol are quite complex. 2,4,6-tri-tert-butylphenol is main monomers formed in ethanol, and its selectivity is only 15.4%. In isopropanol, total yield is only 107.5\u2009mg/g, the maximum selectivity of the monomers (2,5-diisopropylphenol) is only 10%.The effect of reaction time is shown in \nFig. 6(a). The total monomer yield obtained at 2\u2009h is only 75.1\u2009mg/g. It rapidly increases from 2 to 3\u2009h, and generally increases from 2 to 8\u2009h, reaching 129.8\u2009mg/g at 8\u2009h. The selectively of TBC obviously increases from 2 to 3\u2009h, and generally increases during 3\u20136\u2009h, reaching 40.3%, but it is not obviously changed during 6\u20138\u2009h. Moreover, when the reaction time is 1\u2009h, a few G-type products (guaiacol, 2-methoxy-4-methylphenol, 4-ethyl-2-methoxyphenol etc.) appeared. When the reaction is prolonged to 3\u2009h, these G-type products disappeared and large amounts of alkylphenolsare formed.The effect of reaction temperature is presented in Fig. 6(b). As reaction temperatures increases from 260\u2009\u00b0C to 290\u2009\u00b0C, the total monomer yield increases from 89.3 to 154.1\u2009mg/g. However, the selectivity of TBC increases from 16.6% at 260\u2009\u00b0C to 40.3% at 280\u2009\u00b0C and then decreases to 36% at 290\u2009\u00b0C. Furthermore, the total yields at 260\u2009\u00b0C and 270\u2009\u00b0C are similar, which are 89.3 and 92.7\u2009mg/g EHL, respectively, but the selectivity of TBC at 270\u2009\u00b0C is 35.2%, much higher than that at 260\u2009\u00b0C (16.6%).\nFig. 6(c) shows the effect of initial hydrogen pressure at 280\u2009\u00b0C. The total yield has a positive correlation with the hydrogen pressure, while the selectivity of TBC shows a volcanic relationship with hydrogen pressure. Char is formed when initial hydrogen pressure is 0 and 1\u2009MPa\u2009H2, and no char is formed at 2 and 3\u2009MPa\u2009H2. From 0\u20133\u2009MPa\u2009H2, the total monomer yield dramatically increases from 69.4 to 139.8\u2009mg/g. The selectivity of TBC reaches a maximum of 42.4% at 1\u2009MPa of hydrogen and decreases to 38.2% at 0\u2009MPa\u2009H2 and 29.1% at 3\u2009MPa\u2009H2, respectively.The FT-IR spectra of EHL and the liquid products obtained from without catalyst and over MoS2 are presented in \nFig. 7, and the corresponding band assignments are summarized in \nTable 2\n[30\u201332]. Band 1 (3417\u2009cm \u22121) is ascribed to the O-H stretching vibration. Bands 2 (2848\u20132960\u2009cm\u22121) corresponds to C-H stretching of CH3 and CH2. Band 3 (1456\u2009cm\u22121) is ascribed to C-H deformations asymmetric in CH3 and CH2 and band 4 (1380\u2009cm\u22121) is ascribed to aliphatic C-H stretch in CH3. Band 5 (1334\u2009cm\u22121)\uff0cband 6 (1265\u2009cm\u22121), band 8 (1120\u2009cm\u22121) and band 9 (839\u2009cm\u22121) are related to syringyl ring and guaiacyl ring. Band 7 (1166\u2009cm\u22121) is ascribed to C-O stretch in ester group.Compared to the spectrum of EHL. the band of O-H (band 1) is obviously strengthened after reaction without MoS2, but this band is significantly weakened when MoS2 is added. Nevertheless, the bands of C-H in CH3 and CH2 (bands 2, 3 and 4) become stronger after the reaction without MoS2 and are further strengthened after the reaction with MoS2. The bands related to syringyl ring and guaiacyl ring (band 5, 6, 8 and 9) are also weakened after reaction without MoS2 and nearly disappear when MoS2 is added. In addition, the band of C-O stretch in ester group (band 7) shows the same trend as the band of syringyl ring and guaiacyl ring.The MoS2 catalyzed conversion of several lignin monomers were examined under the same conditions in the lignin depolymerization (\nTable 3). Guaiacol, 4-methylguaiacol and catechol are completely converted, and TBC is also the main products. Moreover, product formed in conversion of guaiacol, 4-methylguaiacol and catechol is nearly the same as that formed in EHL depolymerization (\nFig. 8). Nevertheless, only 64.0% of phthalic ether is converted with fresh MoS2, (entry 6 in Table 3), while its product distribution is nearly the same as with EHL. When 4-Ethylphenol is the feedstock, no TBC was observed. Compared to fresh catalyst, the used catalyst shows similar activity for conversion of guaiacol into TBC.A series of model dimers were used to examine the activity of MoS2 for cleavage of different C-C and C-O bonds. The results are shown in \nFig. 9. 43.3% of 4,4\u2032-oxydiphenol is converted to alkylphenols with the cleavage of 4-O-5 linkage, but 56.7% of 4,4\u2032-oxydiphenol is only alkylated without the cleavage of 4-O-5 linkage. In the conversion of 4-(benzyloxy)phenol, \u03b1-O-4 linkage is completely cleaved, yielding 50.5% toluene and 49.5% alkylated terephthalate. The conversion of 4,4\u2032-methylenediphenol yields 65.8% of alkylphenols, verifying that MoS2 has a activity for cleaving C-C bonds.Without a catalyst, 62.6\u2009mg/g monomers are produced, and the band of O-H stretching vibration is strengthened, indicating that EHL is partly depolymerizated even without a catalyst. It has been proved that alcohols such as methanol, ethanol, and isopropanol acted as the nucleophilic reagent and cleave ether linkages, depolymerizing lignin into fragments and monomers [6]. MoS2 has a high activity for cleavage of \u03b1-O-4 bond, as \u03b1-O-4 bonds in 4-(benzyloxy) phenol are completely cleaved with MoS2. Among all C\u2013O linkages in lignin, the 4-O-5 bond between two phenyl groups has the highest dissociation energy [33]. MoS2 catalyst partly cleaves 4-O-5 bond in 4,4\u2032-oxydiphenol without aromatic ring hydrogenation. EHL contains a high amount of inter-unit C-C bonds, including both native C-C linkages and new C-C linkages formed via condensation of reactive intermediates during delignification [34]. The formation of alkylphenols (65.8%) from 4,4\u2032-methylenediphenol verifies that MoS2 enables the cleavage of C-C bond.After reaction without catalyst, the bands of C-H stretching of methyl and methylene groups are strengthened, compared to those in EHL, indicating that alkylation occurs even without a catalyst. After reaction with MoS2, The bands related to CH3 and CH2 are all significantly strengthened, indicating that the addition of MoS2 significantly promotes alkylation reaction. MoS2 also shows a high activity for demethoxylation/demethylation reaction, as the bands related to syringyl and guaiacyl ring also disappear. Song et al. [35,36] also demonstrated that Mo has the ability to dehydroxylate. In addition, the weakness of the band of O-H stretching vibration in EHL depolymeriation with MoS2 and the formation of alkylphenol in conversion of monomers indicate that MoS2 has the activity for dehydroxylation.In EHL depolymerization, monomers with methoxy are detected at 1\u2009h, but disappear at 3\u2009h, and alkylphenols appear at 3\u2009h. Moreover, the products obtained from EHL depolymerization is nearly the same as with those obtained from guaiacol and methyl-guaiacol conversions, but is different with those obtained from 4-ethylphenol conversion. Meanwhile phenol is not observed in guaiacol conversion. Therefore, guaiacol and its derivant are the intermediates for alkylphenols, instead of phenols. The products obtained in catechol conversion is nearly the same as those obtained from guaiacol conversion and EHL depolymerization, indicating that guaiacols and its derivant first undergo demethylation and then undergo alkylation. Previous work [23] indicated that ortho-methylcatechol is the main intermediates for TBC. Phenolic hydroxyl group next to the methyl group on ortho-methylcatechol may be dehydroxylated firstly, and then alkylation reaction. The electron donating effect of OH and CH3 group in ortho-methylcatechol are main reason for the alkylation activation in the ortho-hydroxyl structure. Cui et al. [37] reported that higher alkylphenols (like tert-butylphenol) more likely to form via consecutive substitution of lower alkylphenols (like o-cresol) with methyl, ethyl or isopropyl groups supplied by solvent medium. The possible pathway of TBC formed from fragmentated lignin are proposed as in \nScheme 1.The reaction pathways are proposed as in \nScheme 2. EHL is firstly depolymerizated with methanol, forming lignin fragments and monomers, such as G-type monomers (guaiacol and 4-ethyl-2-methoxyphenol), S-type monomers (Syringol) and H-type monomers (phenol and 4-ethylphenol). Without catalyst, active monomers and intermediates are prone to undergo repolymerization reaction, forming a large amount of coke. MoS2 depolymerizated lignin fragments through cleavage of C-O and C-C linkages, and also stabilize active phenolic monomers through dehydroxylation, demethylation and alkylation reaction.MoOxSy species is identified to exist in the MoS2 according to the XPS analysis. In our previous works with MoS2 as catalyst [23], we confirmed MoOxSy as the active phase for the alkylation of guaiacol. After EHL depolymerization, the proportion of Mo5+ and \n\n\nS\n\n\n2\n\n\n2\n\u2212\n\n\n decreased from 41.6% and 27\u20132.2% and 23.9%, indicating that MoOxSy is converted to MoS2. In addition, the catalyst surface aggregates, and the wrinkles on the surface disappear after the reaction (Fig. 2(a,b)). Moreover, graphitic carbon was formed on the used MoS2 catalyst according to the Raman results. In summary, the loss of active phase (MoOxSy), the aggregation of catalyst surface and the formation of graphitic carbon are probably the reasons for the deactivation of MoS2.In summary, A simple one-pot method for preparing 2-(tert-butyl)-3-methylphenol (TBC) with high selectivity from EHL is reported. The highest TBC selectivity of 40.3\u2009wt% and the total monomer yield of 124.1\u2009mg/g lignin are achieved in methanol over MoS2 at 280\u2009\u00b0C for 6\u2009h under 2\u2009MPa hydrogen pressure. The selectivity of TBC shows a volcanic relationship with hydrogen pressure and reaction temperature. The result of FT-IR and the conversion of monomers indicate that MoS2 has the high activity for demethylation, dehydroxylation and alkylation. According to the results of dimer conversions, the C-C and C-O linkages in EHL are cleaved on a MoS2 sample. The effect of time on product distribution and monomer conversion proves the pathways for alkylphenol production from EHL. The main active species for selective conversion of EHL to TBC is likely MoOxSy composed of Mo5+ and \n\n\nS\n\n\n2\n\n\n2\n\u2212\n\n\n.\nYiming Ma: Methodology, Investigation and Writing \u2013 original draft. Hong Chen: Supervision, Conceptualization, Writing \u2013 review & editing. Yongdan Li: Supervision, Conceptualization, Writing \u2013 review & editing. Kai Wu: Software, Visualization. Qingfeng Liu: Software, Visualization. Yushuai Sang: Formal analysis, Software and 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.This work has received funding from the European Union\u2019s Horizon 2020 Research and Innovation program, (BUILDING A LOW-CARBON, CLIMATE RESILIENT FUTURE: SECURE, CLEAN AND EFFICIENT ENERGY) under Grant Agreement No 101006744. The content presented in this document represents the views of the authors, and the European Commission has no liability in respect of the content.", "descript": "\n                  Low selectivity and complex product distribution are the main challenges for the utilization of lignin. Herein, the selective production of 2-(tert-butyl)-3-methylphenol (TBC), an antioxidant in the polymer industry, from depolymerization of enzymatic hydrolysis lignin (EHL) on a hydrothermally synthesized MoS2 catalyst is studied. The total aromatic monomer yield is 124.1\u00a0mg/g EHL and the selectivity of TBC is up to 40.3\u00a0wt% in methanol at 280\u00a0\u00b0C under 2\u00a0MPa\u00a0H2 for 6\u00a0h. The FT-IR analysis of products reveals that MoS2 has a high activity for demethylation, dehydroxylation and alkylation, and the dimer conversions reveal that C-O and C-C bonds in EHL are broken with MoS2. The guaiacol and its derivants are identified as the intermediate for formation of TBC in EHL depolymerization according to the effect of time on product distribution and monomer conversion.\n               "}
{"full_text": "No data was used for the research described in the article.Montmorillonite with chemical formula (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2n(H2O) is a very soft phyllosilicate group of minerals was first described in 1847 for an occurrence in Montmorillon in the department of Vienne, France, more than 50\u00a0years before the discovery of bentonite in the US [1]. Its structure is well known nearly 7 decades [2]. Several classification systems were proposed at the same time but analysis of smectites is still difficult. MMT as a member of the smectite group, is a 2:1 clay, with a central octahedral sheet of alumina sandwiched by two tetrahedral sheets of silica [3]. Dehydroxylation of clay disregarded in classifications until now, so the structure of the octahedral sheet in smectites remained unconsidered. Although, the cis- and trans-vacant character of the dioctahedral smectites had been known for a long time but a manageable proof was lacking. Determination of the octahedral structure of the sheet for illites is possible by X-ray diffraction but not for smectites because of their turbostratic disorder. Octahedral sheet structure affected on dehydroxylation temperature of all dioctahedral 2:1 clay minerals. Dehydroxylation temperature is 550 and 700\u00a0\u00b0C for trans-vacant minerals and cis-vacant varieties, respectively. Mixed types with two dehydroxylation peaks also exist. Thus, simultaneous thermal analysis (STA) can be used for determination of the cis- or trans-vacant character of MMTs [4] (Table 1\n).In 1961, Grim and Kulbicki were classified MMTs based on phase transformations and recrystallization products of the H+-exchanged at high temperatures. Based on thermal behavior, they defined two Wyoming and Cheto-type differing primarily in the distribution of the calculated layer charge. Grim and Kulbicki neglected the dehydroxylation although considered layer charge and octahedral cation population and distribution. A low layer charge and low contents of Mg2+ substituting for Al3+, characterize the Wyoming-type in contrast to the Cheto-type, which has high a content of Mg2+ and a higher layer charge. There are also mixtures of Cheto- and Wyoming-type. These two types differ in phase transformations above 1000\u00a0\u00b0C. The Wyoming-type transforms into cristobalite and mullite whereas the Cheto-type tranformed to \u03b2-quartz, \u03b2-cristobalite and cordierite at high temperatures [5].In 1969, a classification developed by Schultz based on the amount and location of charge and the proportion of tetrahedral charge [6]. Dehydroxylation temperature and the amount of hydroxyl groups also measured but only one temperature peak was recorded, even if there were two peaks. Schultz defined seven types of MMTs and beidellites: 1) ideal MMTs Wyoming-type, 2) Chambers-type (which corresponds to the mixture of Cheto-and Wyoming-type of Grim and Kulbicki), 3) Tatilla-type, 4) Otaytype (which corresponds to the Cheto-type of Grim and Kulbicki), 5) ideal beidellite, 6) non-ideal beidellite and 7) non-ideal MMT. Ideal and non-ideal types dehydroxylate at about 700 and 550\u00a0\u00b0C, respectively. Wyoming-types display a low layer charge and only beidellite has a dominant tetrahedral charge. Differentiate between MMT and beidellite have been determined via using Greene-Kelly test [7].The ranges of composition for the different types gave by Brigatti [8] and Poppi and Brigatti [9] based on the Schultz' system [6]. Types of the dioctahedral MMT series were characterized based on crystallochemical data especially octahedral and tetrahedral distributions. Their classification turned special interest to the content of iron in the octahedral layer and 8 solid solution ranges were classified for smectites: 1) Wyoming, 2) Tatilla, 3) Otay, and 4) Chambers-type, 5) non-ideal MMT, 6) nontronite, 7) beidellite and 8) Fe-rich beidellite. Fe-rich MMT and beidellite correspond to non-ideal MMT and beidellite. The iron content in the octahedral sheet of MMT and beidellite is less than 15\u00a0% of the cations in the octahedral sheet and for non-ideal or Fe-rich MMTs and beidellites 15\u201330\u00a0%.Classification system was modified by new methods. In 1971, Lagaly and Weiss gave a new insight into the cation density and charge distribution of layer silicates through intercalation with alkylammonium [10]. Structural formula calculations should be performed according to K\u00f6ster which means the measured layer charge has to be involved in the calculation of the composition [11]. Tsipursky [12], Drits and Muller [4] and Drits et al. [13] explained that the thermal behavior of dioctahedral 2:1 clay minerals is depended to the structure of the octahedral sheet, directly. These two aspects are incorporated in the new classification system.There are samples in common classification cannot be classified as any type proposed in the literature. Samples originating from other places than Wyoming, Otay, Tatilla, etc. are difficult to characterize. Although demand is increasing for industrial applications but the names MMT/beidellite or even their trivial names Wyoming-type, etc. don\u2019t bear information of the minerals characteristics. Even in the smectite group, MMTs show distinct differences in chemistry, octahedral sheet structure, Fe-content, layer charge and location of charge. To describe these differences well defined adjectives are used. The adjective, that gives information on the chemistry of the mineral and is not considered to be part of the name [14,15]. It may precise the name and is not connected to it which makes variations possible [16]. It should be avoided to use the adjectives as hyphenated chemical prefix.In oil drilling industry, MMT used as a component of drilling mud, making the mud slurry viscous. It is kept the drill cool and removed the drilled solids [17\u201321]. As a component of foundry sand and as a desiccant, it is also removed moisture from air and gases [18]. In drought-prone soils, the clay also used as a soil additive to hold soil water. It is used in the construction of earthen dams and levees to prevent the leakage of fluids [22\u201326]. Swelling property of this clay makes MMT-bentonite be useful also as a protective liner for landfills and as an annular seal or plug for water wells [27]. Due to its adsorbent and clumping properties, Na-MMT is also used as the base of some cat litter products [28,29]. MMT has also been used in cosmetics [27,30,31]. In a fine powder form, MMT can also be used as a coagulant in ponds [32]. As it added into water, making the water \u201cclouded\u201d, attracts minute particles and then settles to the bottom. MMT is an effective absorbance for heavy metals but to date, its effect on human health is not known [33]. It's assumed that heavy metal adsorption is only applicable when the clay has direct contact to it. Hence, it will not help when ingested because almost doesn't pass through the intestinal mucous membranes, certainly. MMT has been used to treat contact dermatitis for external use [34]. Because the clay may provide some resistance to environmental toxins, it is added as an anti-caking agent to some animal's foods [35]. MMT clays have been extensively used in catalytic processes [36]. For over 60\u00a0years, MMT clays have been used as cracking catalysts [37\u201342]. Other acid-based catalysts use acid-treated MMT clays [43]. Other uses include use in papermaking to minimize deposit formation [44,45] and as a retention and drainage aid component [46].As mentioned before, MMT is a phyllosilicate mineral with nanolayered structure consists of stacked layers [47]. Thickness of layers is about 1\u00a0nm. Each layer is composed of one O-Al(Mg)-O octahedral sheet (about 100\u00a0nm\u00a0\u00d7\u00a0100\u00a0nm, in width and length) sandwiched by two O-Si-O tetrahedral sheets [48]. The layer is positively charged due to the isomorphous substitution, so cations are existed in the interlayered space of MMT. Van der Waals and electrostatic forces held neighboring layers together to form the primary particles of clay [49]. Secondary micrometer-scale to millimeter-scale particles are formed through aggregation of primary particles (Fig. 1\n) [47].IR spectra of MMT recorded by Dankov\u00e1 et al. presented in Fig. 2\n\n[51]. As can be seen in this figure, an absorption band exist about 3626\u00a0cm\u22121 attributed to the stretching vibrations of structural OH groups in MMT [52]. The bands observed at 916 and 840\u00a0cm\u22121 related to the Al-Al-OH and Al-Mg-OH bending vibrations, respectively [53,54]. A complex band at 1040\u00a0cm\u22121 corresponds to the stretching vibrations of Si\u2013O groups [53,54], whereas the Al-O-Si and Si-O-Si bending vibrations recorded at 523 and 470\u00a0cm\u22121\n[55]. The band at 625\u00a0cm\u22121 are assigned to the out of plane vibrations of Al-O and Si-O [56]. A broad band at the range of 3420\u20133450\u00a0cm\u22121 correspond to the H2O-stretching vibrations. The shoulder at about 3330\u00a0cm\u22121 is an overtone of the bending vibration of water at 1635\u00a0cm\u22121\n[57].SEM micrograph of MMT is shown a dense aggregate formed through condensation of the sheet structure-leaf-like crystals (Fig. 3\n) [51]. The layered structure of MMT is clear in this micrograph. The surface of clay hasn't homogenous dispersion. In addition, there are pores with different sizes distributed, randomly [58].In TEM image of natural sample of MMT from the Tagansoye deposit, reported by Krupskaya et al. in 2017, there is a significant amount of small and thin nano-sized particles among the laminar MMT particles with a size of 1\u20132\u00a0\u00b5m, covered the specimen and produced grey background in micrographs (Fig. 4\na) [59]. As can be seen in Fig. 4b which recorded by Alamri et al. in 2021, the clay has a porous-like surface and a nest-like form [60].Nitrogen adsorption\u2013desorption isotherms and Barrett-Joyner-Halender (BJH) pore size distribution of MMT, obtained by Alamri et al. in 2021, are shown in Fig. 5\n\n[60]. Surface area, pore volume, and particle size of MMT are 258.108\u00a0m2/g, 0.423\u00a0cm3/g, 8.092\u00a0nm, respectively.Alamri et al. were also prepared XPS spectrum of MMT, are shown in Fig. 6\n\n[60]. The spectrum indicates that Mg, O, C, Ca, Si, and Al existed on the surface of MMT.In 2018, M. Ahmadzadeh et al. prepared the EDX spectrum of MMT clay (Fig. 7\n) [61]. In this spectrum, three sharp peaks are observed which are related to Al, Si and O elements. Several weak peaks are also observed which belong to Mg Fe, Na and K.XRD pattern of the MMT is shown in Fig. 8\n and the crystallographic parameters are evaluated by measuring the (001) and (080) peaks. This pattern reported by Fil et al. in 2014 [50]. The peaks marked as MMT are indicate 2:1 swelling clay and confirm the characteristics of the MMT type clay and other peaks have been attributed to impurities corresponding to quartz. A diffraction peak of the (001) plane at 2\u03b8\u00a0=\u00a019.733 corresponds to its basal spacing of 4.99\u00a0\u00c5. The (080) reflection at 2\u03b8\u00a0=\u00a068.823 also indicates that MMT has a dioctahedral structure [62,63].Fil et al. are also used from X-ray fluorescence (XRF) method to identify the major minerals and chemical compounds present in the MMT. Their results summarized in Table 2\n\n[50].They also gave the pH profiles of clay as a function of time in a 1.5\u00a0wt% suspension at natural, acidic and basic conditions (Fig. 9\n) [50]. They were shown that when distilled water (pH 5.45) is added to MMT, pH raised to 8.15 in 45\u00a0min and to 7.7 after 75\u00a0min and then remained almost constant upon reaching to the equilibrium pH of 7.7. Increasing of pH in the first 45 minunes can be ascribed to the rapid adsorption of H+ ions in water onto the negatively charged MMT surface and as potential determining ions (pdi) in the electrical double layer (EDL) to provide electroneutrality. In addition, the H+ ions exchanged with some of the cations in the MMT lattice leading to the consumption of H+ ions.All of the above data confirm the structure of MMT clay.Hydrophilicity of MMT often causes the agglomeration of the nanoclay in the polymer matrix so it is not compatible to most of polymer matrixes [64\u201367]. Modification of the MMT surface, is the most important method to achieve homogenous dispersion of clay platelets in polymers. Nanoplatelets incorporated and distributed in polymeric matrixes, homogeneously. The organic cations as a modifier decreased the surface free energy of silicate layers and improve their compatibility with hydrophobic polymers [68\u201378]. Ion exchange is a method can be applied for MMT modification using cationic surfactants based on its cationic exchange capacity (CEC) [79\u201381]. For example, cationic smectites such as nontronite, laponite and MMT are the most common clays are modified through replacement of their exchangeable ions (Na+, Ca2+) with positively charged organic or biological molecules [82]. Organomodification involves amino groups which results in organic/inorganic hybrids with specific selectivity and reactivity [82,83]. Functionalization generate selectivity in catalysts via spatial constraints induction [84]. Generally, tremendous improvements in the wide range of physical and engineering properties of nanoclays have been observed, in recent years, [85\u201388].MMT was first discovered in 1847, in France [1]. Many researchers turned their attention to this clay due to its special properties, results small size of the particles and their unique crystal structures. These properties include cation catalytic abilities, exchange capabilities, swelling behavior, plastic behavior when wet and low permeability caused the clay be more applicable in many industries and processes [89]. Several studies have been also reported on the antibacterial properties of MMTs [90\u2013100].To date, many reports published in the field of MMTs and many researchers prepared review papers about different aspects of these materials. We have referred to several of this reports in the following.\u201cPolypropylene/MMT nanocomposites, synthetic routes and materials properties\u201d have been evaluated in a review published by Manias et al. in 2001. According to this review, the nanocomposite is formed either by using functionalized polypropylenes and common organo-montmorillonites, or by using neat/ unmodified polypropylene and a semi-fluorinated organic modification for the silicates. Hybrids can be formed by solventless melt-intercalation or extrusion. The resulting polymer/inorganic structures are characterized by a coexistence of intercalated and exfoliated MMT layers. Tensile characteristics, higher heat deflection temperature, retained optical clarity, high barrier properties, better scratch resistance, and increased flame retardancy improved by small additions of these nanoscale inorganic fillers [101].In 2007, Leszczynska et al. were also reviewed polymer/MMT nanocomposites with improved thermal properties and thermal stability of MMT nanocomposites based on different polymeric matrixes, with the aim to describe the basic changes in thermal behavior of different polymeric matrixes upon addition of MMT. They also gave a brief description of analysis of volatile and condensed products of degradation and the kinetics of the process decomposition in inert and oxidative environment [102].Leszczynska et al. prepared a review about \u201cfactors influencing thermal stability and mechanisms of thermal stability improvement on polymer/MMT nanocomposites\u201d in 2007. This work presents a detailed examination of factors influencing thermal stability, the role of chemical constitution of organic modifier, composition and structure of nanocomposites and mechanisms of improvement of thermal stability in polymer/MMT nanocomposites [103].In 2008, a review article published by Bhattacharyya and Gupta. The title was: \u201cAdsorption of a few heavy metals on natural and modified kaolinite and montmorillonite: A review\u201d. This article is a unique collection of vital information about the feasibility of using two important and common clay minerals, kaolinite and montmorillonite, as scavengers for removal of toxic heavy metals such as As, Cd, Cr, Co, Cu, Fe, Pb, Mn, Ni, Zn in their ionic forms from aqueous medium. The authors tried to incorporate how and why clays can be effectively used as a liner in water treatment plants [33].In 2009, Pagacz and Pielichowski provided a review article by this title: \u201cpreparation and characterization of PVC/montmorillonite nanocomposites\u2014a review\u201d. In this review, preparation and characterization of poly (vinyl chloride)/montmorillonite (PVC/MMT) nanocomposites are being presented. Flammability of PVC/MMT nanomaterials are also described. Finally, a future outlook is given [104].\u201cMontmorillonite supported metal nanoparticles: an update on syntheses and applications\u201d is the title of a review article of Varadwaj and Parida published in 2013 to cover numerous aspects of material syntheses and various fields of applications in which these materials show their significant efficacies. They concluded this review with a positive view for the future expansion of this field by the joint efforts of researchers from various scientific and industrial areas [105].An overview of the catalytic utility of MMT-K10 as solid acid, support for complex or metal nanoparticles in unimolecular and bimolecular reactions have been presented by Kumar et al. in 2014. The main part of this review is organized according to the role of clay in various organic reactions and an emphasis is given in highlighting the greenness of the processes. A fair comparison is also provided between clay catalysts with respect to other homogeneous or heterogeneous catalysts. Finally, the authors summarized their views in future trends and developments [106].Adsorbents based on MMT for contaminant removal from water are reviewed in a paper, published in 2016 by Zhu et al. The aim of this review article is to help the readers in choosing proper and developing novel clay mineral based adsorbents for target contaminants, and on the other hand can give a proper example to systematically show the various mechanisms for the uptake of contaminants on adsorbents. The mechanisms for uptake contaminants on adsorbents, various adsorbents based on MMT and uptake of contaminants by them, comparison of the adsorbents and disposal and reutilization of the spent adsorbents have been reviewed in this article [107].In 2018, a review paper published in the field of polymer nanocomposites based on silylated-montmorillonite by Bee et al. This paper focus on silylation, to coat silane coupling agents on the clay which is a type of covalent organic functionalization approach. Emphasis is placed on possible factors such as the silane configuration, reaction conditions and the nature of the solvent system, affected the degree of surface modification during silylation. In this review, the effect of impregnation of silylated-fillers in various macromolecular matrices is summarized and compiled based on recent collected literatures and their processing, morphologies, properties and future prospects are specifically detailed and discussed. The authors believe that this review provides a comprehensive overview on the effect of silylated MMT on the structure and properties of certain selected polymeric matrixes including polyamide, vinyl polymer, biodegradable based polymer, elastomeric rubber matrixes and epoxy resin [108].\u201cExfoliation of montmorillonite and related properties of clay/polymer nanocomposites\u201d is the title of a review paper published in 2019 by Zhu et al. The literature survey suggests that future work should place emphases on developing green and effective exfoliation methods, deepening understanding of exfoliation mechanisms and the interfacial interactions between the inorganic MMT nanolayers and organic monomers/polymers. It is suggested that future research assembling exfoliated MMT nanolayers with functional polymeric molecules or other nano-scale building blocks to produce functional hierarchical nanomaterials with practical applications [49].In 2019, Dlaminia et al. were published a review is titled \u201cCritical review of montmorillonite/polymer mixed-matrix filtration membranes: possibilities and challenges\u201d. They were reviewed the articles focused on clay/polymer (CP) based mixed-matrix membranes (MMM) for water treatment. Fabrication and the structure of clay/polymer nanoparticles (CPNs), CPN membranes for water filtration, and inconvenience of layered platelets to mass transport, are the subjects explained in this paper. They conclude that it is possible to achieve significant improvements in water flux without compromising solute rejection [109].In order to increase the activity of MMT in different applications, the surface of MMT has been modified and functionalized by different methods [110\u2013120]. In recent years, a great attention has been attracted to MMTs as the heterogeneous catalyst due to their unique properties. These solid catalysts have advantages such as high yield and simple isolation of product, simple and clean work up and reusability of the catalyst [121,122]. In this review article, we have provided an overview of using the MMTs as catalysts. We have been also evaluated the structure of some modified and functionalized MMTs and explained their catalytic application in the organic syntheses. In the following of our review research, several reports in the field of organic synthesis by non-modified MMT have been presented.Dintzner et al. generated 2,2-dimethylbenzopyran derivatives (3) via a one-pot condensation of substituted phenols (1) with phenyl bromide (2), catalyzed by MMT-K10 [123]. They reported that phenol could be directly condensed with phenyl bromide while in previously work which was originally observed by Dauben et al. in 1990, the major product was o-phenyl phenol (4) with minor amounts of p-phenyl phenol (5) and benzopyran (3) also generated under optimal conditions (Scheme 1\n) [124]. A detailed study of the intramolecular clay-catalyzed [1,3] shift reaction of 3-methyl-2- butenyl phenyl ether (6) also presented by Dintzner et al. in 2004 [125].In 1997, Li et al. used the MMT-K10 clay as a remarkable acetylation catalyst for acetylation of primary and secondary alcohols, thiols, amines and phenols with acetic anhydride in excellent yield. Mild conditions, high yield, easy separation and inexpensive and environmentally friendly catalyst are some advantages reported for these reactions [126].Bhaskar and Loganathan have developed an efficient, convenient and environment-friendly method for the acetylation of sugars (7) using the inexpensive MMT-K10 as the heterogeneous catalyst, in 1998 (Scheme 2\n) [127]. The authors believe that MMT-K10 as an inexpensive solid acid is shown to be an efficient catalyst for the per-O-acetylation of several mono-, di- and trisaccharides. The pyranose forms (8) accounted for 75\u2013100\u00a0% of the acetylatedproducts.In 2002, Yadav et al. reported that aryl amines (9) react smoothly with cyclic enol ethers (10) on the surface of MMT-KSF under mild reaction conditions to afford the corresponding pyrano- and furano[3,2-c]- quinolines (11 and 12) in high yields with high diastereoselectivity (Scheme 3\n) [128]. The authors described the notable features of this procedure which are greater selectivity, mild reaction conditions, cleaner reaction profiles, high yields of products and ready availability of the reagents at low cost.In 2006, different bismaleimides (16) and bisphthalimides (17) were synthesized by Habibi and Marvi through the condensation reaction of maleic (13) and phthalic (14) anhydrides with different diamines (15) on MMT-K10 clay as catalyst under microwave irradiations and solvent-free conditions (Scheme 4\n) [129]. Solvent-free reaction conditions, simple experimental and product isolation procedures, easy recovery and reuse of the natural clays, cleaner reaction profiles and availability of the reagents at low cost, high yields of products and enhanced rates are the notable features of this procedure.An efficient green protocol for the preparation of amidoalkyl naphthols (22), employing a three-component one-pot condensation reaction of 2-naphthol (18), aromatic aldehydes (19), amides (20) or urea (21) in the presence of MMT-K10 clay under solvent free conditions reported by Kantevari et al. in 2007 (Scheme 5\n) [130]. Recovery and reusability of catalyst, short reaction time, excellent yields and simple workup are the advantages of this method.In 2014, Rocchi et al. developed a solvent-free, inexpensive and fast microwave-assisted method for cross aldol condensations of aromatic aldehydes (19) and ketones (23) for synthesis of aryl and heteroaryl trans-chalcones (24) under microwave irradiation and solvent-free conditions in the presence of MMT-KSF (Scheme 6\n) [131]. They explained that in comparison to their previously reported methods, this protocol constitutes a user- and environment-friendly alternative that proceeds normally in good to excellent yields [132].In 2020, Iriti et al. developed a fast, cheap, simple and environmentally sustainable method for the synthesis of 1,2-bisubstituted benzimidazoles (26) and 2-substituted benzimidazoles (27) from ortophenylnediamine (25) and aldehyde derivatives (19) catalyzed by MMT-K10 under microwave assistance (Scheme 7\n) [133]. The reactions were carried out in a short reaction time under solvent-free conditions by using an inexpensive and environmentally friendly heterogeneous catalyst. The authors shown that the reaction process is applicable in the industrial fields. They also compared this procedure to their previous work [134] and found that the proposed method does not require a previous treatment for the preparation of deep eutectic solvents (DESs) as eco-friendly and sustainable solvent and catalytic systems.In addition to the above reports, there are many other reports related to the use of modified MMTs in the organic synthesis, which shows the high potential of this clay as the catalyst. In the continuation of this review article, some of these reports are mentioned.In 2000, Pai et al. were designed the reaction of benzylation of arenes (28 and 29) in the presence of different types of Fe-K10/MMT as catalyst (Scheme 8\n). Fe3+-K10, K10-FeOO, K10-FeOA and K10-FeAA were synthesized. Each catalyst was activated at 120, 280 and 550\u00a0\u00b0C for a period of 5\u00a0h. For example, K10-FeOO was activated at 120, 280 and 550\u00a0\u00b0C to obtain K10-FeOO120, K10-FeOO280 and K10-FeOO550 catalysts, respectively. K10-FeOO120 was the best. It was found that in the reaction monobenzylated product (30) was formed as the main product (<93\u00a0%) and the three isomers of dibenzylated product (31) were produced as byproducts [135].They prepared Fe3+-K10 catalyst by the reported procedure [136]. For K10-FeOO preparation, they dissolved FeCl3 in dry acetonitrile and added MMT-K10. They stirred resulting slurry at room temperature for 5\u00a0h and then filtered and washed the clay with acetonitrile and then with benzene. If the clay washed with deionised water instead of acetonitrile and benzene, K10-FeOA and K10-FeAA are prepared depending on the reaction conditions [135].Sulfonic acid functionalized ordered nanoporous Na-MMT (SANM) was easily prepared by the reaction of Na-MMT with chlorosulfonic acid (Scheme 9\n) by Shirini et al. in 2012 [137]. The modified catalyst used for methoxymethylation reaction of alcohols (32) with formaldehyde dimethyl acetal (33, FDMA) in chloroform under reflux conditions in good to excellent yields (Scheme 10\n). Short reaction times, heterogeneous nature of reaction conditions, use of relatively small amounts of FDMA, ease of preparation, stability of the reagent, recyclability, and easy workup procedure are important features of the reported method.In 2014, a multi-functionalized catalyst has been synthesized by Varadwaj et al. through supporting 3-aminopropyltriethoxysilane (35) and mercaptopropyl trimethoxysilane (36) on the surface of K10-MMT which possesses the ability to act as either base or acid. At first, they prepared SO3H@K10-MMT by adding MPTMS into K10-MMT. Then, APTES was added to the SO3H@K10-MMT and the prepared SO3H-APTES@K10-MMT was obtained (Scheme 11\n) [138].The catalytic activity of the prepared SO3H-APTES@K10-MMT was evaluated as a heterogeneous catalyst for one-pot deacetalization\u2013nitroaldol (Henry reaction) giving a 99.2\u00a0% product (38) from benzaldehydedimethylacetal (37), yield in just 2\u00a0h (Scheme 12\n). The authors reported that this material has also been shown outstanding capacity for the heavy metal cations adsorption and can be utilized as a potential candidate for the remediation of contaminated water. This material is also potent enough to carry out the Henry reaction without any significant loss of its activity [138].In 2017, Safari and Ahmadzadeh reported that an equimolar amounts of carbonyl compound (39 and 40), hydrazine hydrate (41), \u03b2-keto ester (42) and malononitrile (43), in the presence of zwitterionic sulfamic acid functionalized MMT nanoclay MMT-ZSA nanoclay at 90\u00a0\u00b0C under solvent free conditions formed pyrano[2,3-c]pyrazoles (44 and 45) (Scheme 13\n) in 84\u201395\u00a0% yields. Short reaction times, heterogeneous reaction conditions, a much mild procedure, a wide range of functional group tolerance, high reaction rates, absence of any tedious workup or purification, avoid of hazardous reagents/solvents and reusability of the catalyst are some advantages of this work. [139].For preparation of MMT-ZSA, at first, the MMT-NH2 nanoclay was prepared by silane condensation [140]. Then, for the synthesis of sulfonated MMT, chlorosulfunic acid added to MMT-NH2 (Scheme 14\n) [139].In 2017, Zarnegar et al. used NH2-MMT nanoclayas as an eco-friendly, nontoxic, inexpensive, and chemically highly stable nanocatalyst for the synthesis of azine (47) and 2-aminothiazole (50) derivatives in excellent yields at room temperature. (Scheme 15 and 16\n\n) [141]. Simplicity of performance, easier work-up procedure, short reaction times and high yields of the products are some advantages features that authors have been mentioned for this report.In order to preparation of NH2-MMT, the grafting of MMT with organic moieties containing amine was performed with APTES via silanization procedure (Scheme 17\n) [140,141]. NH2-MMT have been also used as nanocatalyst in a variety of chemical reactions and as a good support for heterogeneous catalytic processes, such as Ullmann coupling reaction [142], synthesis of heterocyclic compounds [143], Henry reaction [138], CS coupling reaction [140], Knoevenagel reaction [144], carbonylative sonogashira reaction [145] and synthesis of isoxazoles [146].In 2017, Pham et al. introduced an efficient and green synthesis of 4H-pyran derivatives (53) via one-pot, three-component condensation of aromatic aldehyde (19), ethyl acetoacetate (51) (or 5,5-dimethylcyclohexane-1,3-dione (52)) and malononitrile (43) under ultrasound irradiation in the presence of K2CO3 supported on acidic MMT at 50\u00a0\u00b0C in 50\u00a0% EtOH:H2O as solvent (Scheme 18 and 19\n\n) [147]. K2CO3 supported on the surface of acid treated MMT was changed to KHCO3 due to ions H+ presenting in the inter lamellar space react with K2CO3 during this procedure.In 2018, Ahmadzadeh et al. were synthesized isoxazole derivatives (55) in 87\u201396\u00a0% yields via one-pot multicomponent cyclocondensation of hydroxylamine hydrochloride (54), ethylacetoacetate (51) and benzaldehyde derivatives (19) in water under ultrasound irradiations in the presence of sn-MMT-K10 as catalyst (Scheme 20\n). This reaction is significant due to low-cost and eco-friendliness catalyst, rapid completion of the reactions, avoidance of using organic solvents, excellent yield and mild conditions [61].They prepared sn-MMT-K10 by ion exchange between SnCl2 according to the reported procedure in the literature [148].In 2019, Kancherla et al. used boric acid supported on MMT (H3BO3/MMT-K10) as an efficient reusable and eco-friendly heterogeneous nanocatalyst for the synthesis of 2,3-dihydroquinazolin-4(1H)-one derivatives (57) via condensation of anthranilamide (56) and benzaldehyde (19) (Scheme 21\n) [149]. This method is simple, requires cheaper reagents, the products are easy to separate from reaction mixture and the catalyst is reusable at least three times with negligible loss of activity. The authors believe that These catalysts can be used for many acid-catalyzed organic transformations wherein mild acidity is required.In 2021, another heterogeneous catalyst was also synthesized by Ahmadzadeh et al. named copper(II) anchored on amine-functionalized MMT (MMT-[(CH2)3-NHCHPy]-Cu(II)) [150]. In order to synthesize this catalyst, they first added MMT K10 to 3-aminopropyltrimethoxysilane (35) according to their previously reported method [146] to prepare MMT-(CH2)3-NH2. Then, MMT-[(CH2)3-NHCHPy] synthesized by adding 2-pyridine carboxaldehyde (58) and finally, MMT-[(CH2)3-NHCHPy]-Cu(II) prepared by adding copper acetate (Scheme 22\n).This modified MMT is behaved as a highly efficient catalytic system for the four-component condensation reaction of hydrazine hydrate (41), \u03b2-ketoester (42), malononitrile (43) and terephthalaldehyde (59) toward the synthesis of multisubstituted bispyrano[2,3-c]pyrazole derivatives (60). In aqueous media (H2O-EtOH) under reflux conditions (Scheme 23\n) [150]. The authors reported that the catalyst is environmentally friendly, has a simple synthesis and workup procedure and high synthesis yield, and has a short reaction time which has been recycled and reused five times for the reaction without losing its activity.We conclude this review with a brief explanation of acid treated and cation exchanged MMTs. These are high potential catalysts for organic reactions [151]. Oligomerization of alkenes [152\u2013154], dehydration of alcohols [155\u2013157], addition reactions [158\u2013164], alkane isomerization and rearrangements [165,166], Friedel crafts reaction [167\u2013172], miscellaneous (Diels alder, Suzuki and Heck) reactions [173\u2013175], aromatic alkylation [176,177], Michael reaction [178\u2013183] and Sakurai\u2013Hosomi reaction [184\u2013186] are examples of these reactions.Before the modification or functionalization of the clay structure, it is acted through the sites which have Lewis and Bronsted activity. Depending on the function, the modified clay act as an acid (Scheme 24\n) or base (Scheme 25\n) via the functions.Clays such as kaolinite [187\u2013190], smectite [191], illite [192], chlorite [193], vermiculite [194], talc [195] and pyrophyllite [196] have been used as a catalyst in organic reactions. These clays act similar to each other. The reactions are used of the acidic nature of acid-treated or cation-exchanged clay minerals. Both Lewis and Bronsted activity are common. Free acid (in acid activated clay minerals) or dissociation of interlayer water molecules coordinated to polarizing interlayer cations caused the Bronsted activity [197].\n\n\n\n\n\n\n\nM\n\n\n\n\nOH\n\n\n\nn\n\n\n\n\n\nm\n+\n\n\n+\n\nB\n\u2192\n\n\n\n\nM\n\n\n\n\nO\n\nH\n2\n\n\n\n\n\nn\n-\n1\n\n\nO\nH\n\n\n\n\n\n\n\nm\n-\n1\n\n\n\n+\n\n\n+\n\nB\n\nH\n+\n\n\n\n\nWhere B is water or an organic species in interlayer space [198].For many years, MMTs have been attracted a great attention in various fields specially as the heterogeneous catalyst. Remarkable advantages such as simple and clean work up, high efficiency, simple isolation of product and reusability of the MMTs, have been made them unique for different applications. In this paper, the structural features and various methods for modification of MMT reviewed. This review proves that is helpful for further research work on the catalytic activity of MMTs for synthesis of organic materials in different industries.\nNahid Yaghmaeiyan: Data curation, Writing \u2013 original draft, Visualization, Investigation, Software, Writing \u2013 review & editing. Mahdi Mirzaei: . Reza Delghavi: .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 to University of Kashan for supporting this work by Grant No. 159189/40.", "descript": "\n                  Microscopic crystals with soft phyllosilicate group of minerals which formed through precipitation of water solution, are called montmorillonite (MMT). It is concentrated and transformed by natural weathering in environment caves and left aluminosilicates which were contained in the bedrock. By the adding water, montmorillonite swells and expanded considerably more than other clays. The amount of expansion is depended on the type of exchangeable cation contained in the sample. The presence of sodium as the predominant exchangeable cation, is increased the swelling several times rather original volume. Hence, Na-MMT used as the major constituent in nonexplosive agents for splitting rock in natural stone quarries. Advantageous properties of montmorillonite made it appropriate for many applications such as use in oil drilling industry as a component of drilling mud, soil additive, component of foundry sand, desiccant to remove moisture from air and gases, catalyst and various medicinal and pharmacological applications. This review article consists the various synthetic methods for preparation of catalysts based on MMT for organic syntheses and assessing their catalytic activities.\n               "}
{"full_text": "The growing scarcity of water in the world today forces researchers to investigate more deeply various water conservation schemes and explore new water purification technologies. There is also a need to find solutions for the increasing presence of microbial and chemical pollutants in water (Brillas et al., 2009; Shannon et al., 2008). The scientific community has demonstrated the suitability of advanced oxidation processes (AOPs) for degrading contaminants present in wastewater. In AOPs, generally, the pollutants undergo mineralisation into different inorganic compounds such as salts, CO2, and water or they will be converted to readily degradable small organic molecules if treatment time is optimized (Oller et al., 2011; Comninellis et al., 2008; P\u00e9rez et al., 2006). The typical chemical feature that connects the AOPs is the formation of the hydroxyl radicals (\u2022OH) (Malato et al., 2009). A broader definition of AOPs also includes the techniques that involve oxidants such as SO4\n\u2022- and Cl\u2022 (Sun et al., 2012; Tan et al., 2011). The major areas in which research is undertaken on the photocatalytic degradation of contaminants in water are the Fenton process (Clarizia et al., 2017; Rahim Pouran et al., 2015; Herney-Ramirez et al., 2010; Pera-Titus et al., 2004; Babuponnusami and Muthukumar, 2014) and TiO2 photocatalysis (Pelaez et al., 2012; Kumar and Devi, 2011; Sin et al., 2012; Zaleska, 2008; Mccullagh et al., 2007; Fagan et al., 2016; Ola and Maroto-Valer, 2015).The history of Fenton reaction began in 1894 when H.J.H. Fenton performed a reaction with iron ions and oxidizing agents. He observed a higher oxidative capacity of the mixture in comparison to its components (Koppenol, 1993). Even though the Fenton reaction was initially formulated for Fe(II) and H2O2 many redox-active metals such as Cu, Mn, and Ni also display Fenton-like reactions (Masarwa et al., 1988; Goldstein et al., 1993).The general mechanism of the Fenton process can be represented as follows.\n\n(1)\nFe(II)+H2O2\u2192Fe(III)+\u2022OH+OH\u2212 k =40\u201380\u2009M-1 s-1\n\n\n\n\n\n(2)\nFe(III)+H2O2\u2192Fe(II)+HO2\n\u2022+H+ k =0\u00b7001\u20130\u00b701\u2009M-1 s-1\n\n\n\n\n\n(3)\nOrganic matter+\u2022OH\u2192degradation products\n\n\nHydrogen peroxide reacts with Fe(II) to generate \u2022OH. However, it is a challenging task to recover the Fe(II) (Eq. 2) because of the inherently slow Fe(III) to Fe(II) reduction kinetics (Wang, 2008). Studies report that a Fenton reaction proceeds through a non-radical iron(IV)-oxo (FeIVO)2+ species, but its mechanism awaits a proper experimental validation (Gonzalez-Olmos et al., 2011; Shen et al., 1992; Chen, 2019). Also, various studies illustrate the use of electrical energy (Nidheesh and Gandhimathi, 2012; Li et al., 2018; Plakas et al., 2016) and light energy (Feng et al., 2006) in speeding up the regeneration of Fe(II) in the Fenton reaction (Zhang et al., 2007; Rubio et al., 2013; Lin et al., 2014).Since sunlight can accelerate the Fenton process, it is explored as a cheap alternative. Here, [Fe(OH)]2+ is the crucial photoactive Fe(III) complex under solar light and is predominantly present at low pH of 2.8. In the presence of light irradiation, [Fe(OH)]2+ species get reduced to Fe(II), and that in turn generates further \u2022OH and enhances the contaminant degradation.\n\n(4)\n[Fe(OH)]2++hv\u2192Fe(II)+\u2022OH\n\n\nThe most desirable pH for the homogeneous photo-Fenton reaction is reported to be 2.8 (Clarizia et al., 2017). Even though photo-Fenton reactions are highly efficient in oxidising different contaminants, a pre and post-treatment of water is required to perform the reaction at pH 2.8. Also, when the pH of the solution is increased to neutral, that leads to iron sludge precipitation as iron hydroxides (Giannakis et al., 2016). The major pitfalls associated with the homogeneous photo-Fenton reaction is its narrow pH range and the need to remove iron sludge after the reaction; both these add up to the cost of water treatment. However, the heterogeneous photo-Fenton reaction can perform the water treatment at around neutral pH (Soon and Hameed, 2011). So advanced methods, or materials are warranted for performing the reaction at circum-neutral pH (O\u2019Dowd and Pillai, 2020). Therefore, the development of low cost, efficient, visible-light responsive materials for performing the Fenton reaction at around neutral pH is an active area of research in AOPs (Lv et al., 2010; Hou et al., 2013). There are different techniques employed for achieving this target. In the current review, various advanced materials developed for Fenton and photo-Fenton processes are discussed. The readers are redirected to other reviews for a detailed understanding of the reactor design, usage of chelates etc (O\u2019Dowd and Pillai, 2020; Ganiyu et al., 2018; Wang et al., 2016; Babuponnusami and Muthukumar, 2014; Clarizia et al., 2017).\nIllumination of semiconductors such as iron oxides, TiO2, ZnO etc. with light having energy equal or higher than the bandgap of the material leads to the formation of electrons and holes (Lee and Park, 2013). Those photo-induced electrons (excited from the valence band to the conduction band) transfer to an acceptor molecule and the molecule undergoes reduction. At the valence band, the generated hole (electron vacancy) receives an electron from a molecule which is adsorbed to the system, and that molecule gets oxidised. In the O2 atmosphere, generally O2 acts as an acceptor molecule and generates the superoxide anion (O2\n\u2022\n-). Also, the adsorbed hydroxyl groups (OH\u2212) capture the holes to produce hydroxyl radicals (\u2022OH). Similarly, many organic moieties will get oxidised to other smaller compounds. Various reactive oxygen species (ROS) have different capacity for oxidation and selectivity. The \u2022OH and O2\n\u2022\n- are the two dominant reactive oxygen species involved in the Fenton reaction (Cai et al., 2016; He et al., 2016; Wang et al., 2020c). The \u2022OH with a half-life of 10-9 s and high reduction potential (+2.80\u00a0V vs SCE, \u2022OH/H2O; under acidic conditions) is the most reactive oxygen species involved (Feng et al., 2018; He et al., 2016). Since \u2022OH are short-lived, it is generally produced in-situ by the illumination of UV light on H2O2 or O3 (Cho et al., 2010; Hodges et al., 2018). It is also possible to generate the H2O2 through the photo electrocatalytic mechanism, and that in-situ generated H2O2 can take part in the Fenton reaction and produce \u2022OH. The \u2022OH, which is the most active ROS, also has the capacity to disrupt the cell wall of microorganisms and can perform disinfection of water. One of the hindrances associated with \u2022OH based disinfection is the scavenging of \u2022OH by natural organic matter (NOM) present in the wastewater, which may diminish the efficiency of wastewater disinfection (Brame et al., 2014).At the initial stages of the development of photo-Fenton reaction, it was thought to be impractical to acidify the wastewater to perform the disinfection and removal of contaminants (Giannakis et al., 2016). As time progressed, researchers came up with the photo-Fenton methods of performing the disinfection of Escherichia coli (E. coli) at around neutral pH (Ruales-Lonfat et al., 2014; Rincon and Pulgarin, 2006; Spuhler et al., 2010; Rodriguez-Chueca et al., 2014). The mechanism of disinfection of microorganisms through a heterogeneous catalyst can occur through two pathways. At first, the normal semiconductor action resulting in the formation of electron-hole pairs and then to the creation of \u2022OH. The \u2022OH formation can also occur via the photo-Fenton action of H2O2 and Fe(II). A simplified summary of the mechanism of disinfection is given in \nFig. 1. The complexity of the cell wall of microorganisms can be related to their capacity to get inactivated by photocatalytic action. As the complexity of cell wall increases, it needs either harsh conditions or longer exposure time for the complete disinfection. The resistance level of different classes of bacteria and viruses towards various disinfectants are compared in \nFig. 2. Due to the unique barrier properties of the outer membrane of gram-negative bacteria, it is observed to be more tolerant to disinfectants compared to the gram-positive bacteria. In case of viruses, enveloped viruses (e.g. SARS-CoV-2) seem to be more susceptible to disinfectants than the non-enveloped viruses (Chu et al., 2019). Enveloped viruses consist of three building blocks; genetic material (DNA, RNA), protein capsid, and lipid bilayer. Non-enveloped viruses lack the outer lipid bilayer membrane (Holland Cheng et al., 1995). In general, disinfectants act on the lipid bilayer membrane of the enveloped viruses and deactivate the viruses (Chu et al., 2019). \u2022OH generated by solar photo-Fenton processes are capable of inactivating viruses by photo-oxidation of capsid protein (Giannakis et al., 2016).Iron-based materials are usually treated as superior heterogeneous Fenton catalysts because of their low cost, negligible toxicity levels, high catalytic activity and easy methods for recovery (Pereira et al., 2012; Nidheesh, 2015; Fu et al., 2014; Garrido-Ram\u00edrez et al., 2010; Rahim Pouran et al., 2014). A heterogeneous Fenton system can generate the \u2022OH by two methods. Either it could be the true heterogeneous catalytic mechanism or the homogeneous Fenton reaction occurring because of the leached iron from the solid catalyst (He et al., 2016). In 1998 Lin and Gurol (1998) proposed the widely accepted mechanism of heterogeneous catalytic decomposition of H2O2 by studying the reactions of H2O2 on the solid iron oxide catalyst (goethite).\n\n(5)\n\n\u2261 FeIII-OH+H2O2\u2194(H2O2)s\n\n\n\n\n\n(6)\n(H2O2)s\u2194(\u2261FeII\u2022O2H)+H2O\n\n\n\n\n(7)\n\n(\u2261 FeII\u2022O2H)\u2192\u2261 FeII+HO2\n\u2022\n\n\n\n\n\n(8)\n\n\u2261FeII+H2O2\u2192\u2261 FeIII-OH+\u2022OH\n\n\nIn the mechanism, the symbol \u2261 FeIII represents the iron present on the surface. Here the interaction of H2O2 at the goethite surface (\u2261FeIII-OH) forms the complex (H2O2)s (Eq. 5). Then a ligand to metal charge transfer leads to the formation of a transition state complex (\u2261 FeII\n\u2022O2H) (Eq. 6). Subsequently, the complex dissociates and forms hydroperoxyl radical (Eq. 7), and later \u2022OH is generated in the presence of \u2261FeII and H2O2. (Eq. 8). The mechanism depicts the recycling of Fe (III) and Fe (II) on the surface, so here goethite is treated as a heterogeneous catalyst.Apart from the pure heterogeneous Fenton process, the iron leached out from the solid catalyst enhances the reaction rate by homogeneous Fenton pathway (Ramirez et al., 2007; Wang et al., 2010; Hartmann et al., 2010). Zeng and Lemley (2009) reported the leaching of iron from amberlyst-15 ion-exchange resin while studying the kinetic modelling of degradation of the herbicide 4,6-dinitro-o-cresol (DNOC). Also, a faster rate of degradation of DNOC was observed during the addition of hydrochloric acid owing to the higher amounts of the leached ferrous ion at lower pH values. In another study, FeOx supported on CuFe2O4 and TiO2 was used as model systems for understanding the role of leached iron species in the heterogeneous Fenton reaction. This study pointed out that the methods such as gravimetry, X-ray fluorescence and energy dispersive X-ray analysis are not sensitive enough to account for the low metal ion leaching from the heterogeneous Fenton catalyst (Kuan et al., 2015). So, they have monitored the 4-chlorophenol (4-CP) degradation using inductively coupled plasma optical emission spectroscopy (ICP-OES) and UV\u2013vis spectroscopy under continuous pH monitoring. Time-dependent leaching of metal ions was observed with the pH variations, and even \u00b5M/sub-ppm concentrations of dissolved metal ions were responsible for the increase in degradation rate of 4-CP in the heterogeneous Fenton system.Iron oxides are generally considered to be biodegradable, non-toxic and environmentally friendly (Nidheesh, 2015; Pouran et al., 2014; Ruales-lonfat et al., 2015a; Xu et al., 2012). Usually, the physical properties of synthesised materials are dependent on their specific surface area, particle size, morphology etc. and these properties vary greatly based on their synthesis strategies. Some of the popular methods adopted for the synthesis of iron-based materials include solvothermal procedure, hydrothermal procedure, thermal decomposition, microemulsion process and co-precipitation method (Nidheesh, 2015). Until now, sixteen pure faces of oxides, hydroxides and oxy-hydroxides are reported in the literature (Giannakis et al., 2016; Usman et al., 2018).Iron oxides have the potential to act as photo-catalysts because of their semiconducting properties. The possible semiconducting mechanism of iron oxides can be detailed as follows (Cai et al., 2016; Ruales-Lonfat et al., 2015a).\n\n(9)\nIron Oxide+h\u03bd\u2192Iron Oxide (h++e\u2212)\n\n\n\n\n(10)\ne-(cb)+O2\u2192O2\n\u2022\n\u2212\n\n\n\n\n\n(11)\nh+(vb)+H2O\u2192H++\u2022OH\n\n\n\n\n(12)\nh+(vb)+OH\u2212\u2192\u2022OH\n\n\n\n\n(13)\nH2O2+ e\u2212\u2192\u2022OH + OH\u2212\n\n\n\n\n\n(14)\nFe(III)+ e\u2212\u2192 Fe(II)\n\n\nUpon light irradiation, the heterogeneous Fenton reaction gets enhanced by the production of Fe(II) from the reduction of Fe(III) to Fe(II).\n\n(15)\nFe(III)+h\u03bd+OH\u2212\u2192Fe(II)+\u2022OH\n\n\nAlso on the particle surface, \u2022OH is generated by the heterogeneous Fenton reaction between Fe(II) and H2O2 (Eq. 1). Later the \u2022OH reacts with organic matter leading to their degradation (Eq. 3).In the iron oxide systems, the ferrous ion is part of the crystal system of oxides. This feature enhances the stability of the catalyst towards the splitting of H2O2, and thus the leaching of ferrous ions from the catalyst is reduced. Magnetite (Munoz et al., 2015; Zubir et al., 2015; Du et al., 2017; Nguyen et al., 2017; Nidheesh et al., 2014; Costa et al., 2006), ferrihydrite (Zhu et al., 2018; Xu et al., 2017; Xu et al., 2016b; Zhang et al., 2014; Zhu et al., 2018), hematite (Pradhan et al., 2013; Patra et al., 2016; Jaramillo-Paez et al., 2017; Chen et al., 2016; Huang et al., 2016), goethite (Xu et al., 2016c; Wang et al., 2017; Hou et al., 2017; Krumina et al., 2017; Jin et al., 2017; Qian et al., 2018), schwertmannite (Duan et al., 2016; Yang et al., 2016; Wang et al., 2013), lepidocrocite (He et al., 2017; Sheydaei et al., 2014), and maghemite (Wang et al., 2008; Ma et al., 2018) are some of the classes of iron minerals utilised as Fenton-catalysts. The recent developments regarding these heterogeneous Fenton catalysts are discussed in the upcoming sections.Ferrihydrite (Fh) is a naturally occurring iron oxyhydroxide mineral used as a Fenton catalyst because of its large specific surface area (Liu et al., 2010; Zhang et al., 2014). In some of the recent studies Ag/AgBr/ferrihydrite (Ag/AgBr/Fh) and Ag/AgCl/ferrihydrite (Ag/AgCl/Fh) was established as heterogeneous Fenton catalysts (Zhu et al., 2018a, 2018b). AgBr is a semiconductor which absorbs light in the visible region (bandgap of 2.6\u2009eV). Ag nanoparticles absorb visible light because of their surface plasmon resonance (SPR) effect. Here, by introducing Ag/AgBr/Fh hybrid system, the study demonstrates the direct injection of electrons from the Ag/AgBr to the ferrihydrite (Zhu et al., 2018b). On the ferrihydrite surface, electrons help in the regeneration of Fe(II) which leads to an enhancement in catalytic degradation of bisphenol A (BPA, is an endocrine disruptor and one of the emerging contaminants of concern present in drinking water (Rubin, 2011). ). Since the electrons from catalyst are directly performing the recycling of Fe(II), the reaction is also efficient in terms of the amount of H2O2 consumed. XPS analysis was employed to determine the chemical state of different elements present in the catalyst such as Fe, O, Ag and Br (Zhu et al., 2018b). The peak at 711.72\u2009eV in the Fe 2p spectrum was attributed to the Fe(III) coordinated to the oxygen on Fh. In Ag 3d spectra two peaks at 368.5 and 374.5\u2009eV values represent the silver in the zero-oxidation state, and the peaks at 367.9 and 373.9\u2009eV were associated with the Ag+ in AgBr (Zhu et al., 2018b). In a similar study, Ag/AgCl/Fh hybrid catalyst also demonstrated BPA degradation (Zhu et al., 2018a). Ag/AgCl/Fh was synthesised by an impregnation-precipitation strategy followed by a photo-reduction under UV light. The rate of degradation of BPA by six percentage Ag/AgCl/Fh (6% weight ratio of Ag added to Fh) was measured to be 0.0506\u2009min-1 which is about five times the rate of pure Fh (k\u2009=\u20090.0099\u2009min-1).Another strategy to enhance the rate of regeneration of Fe (II) is to introduce electrons from semiconductors to the heterogeneous Fenton catalyst. In another study, a BiVO4/ferrihydrite (BiVO4/Fh) system was synthesised to understand the decolourisation efficiency of acid red-18 at near-neutral pH (Xu et al., 2017). EPR spectrum showed that the introduction of BiVO4 to the ferrihydrite enhanced the generation of \u2022OH. XPS studies and 1,10-phenanthroline spectrophotometric method concluded the increase in the concentration of Fe(II) on the surface of the BiVO4/Fh. Furthermore, enhanced H2O2 consumption was observed for BiVO4/Fh system compared to the pure ferrihydrite. This was rationalised by the Fe(II) regeneration on the surface, by the photogenerated electrons from BiVO4. Similarly, a 15% doped composite of cerium oxide (CeO2) and Fh showed 98.7% degradation of tetracycline antibiotic (Huang et al., 2020). Here, the mechanism details the critical role of Ce4+/ Ce3+ cycle in helping the regeneration of Fe(II). Furthermore, 7%TiO2/Fh nanohybrid depicted the efficient removal of cefotaxime antibiotic under UV light (Jiang et al., 2019).In a recent report, a composite material of oxidised multi walled carbon nanotubes (CNTs) and ferrihydrite (CNTs/Fh) was prepared and evaluated in the degradation of BPA (Zhu et al., 2020). A 3% CNTs/Fh system depicted seven times higher efficiency compared to simple Fh in degrading the pollutant. Cyclic voltammetry (CV) studies revealed a 14\u2009mV lowering of half-wave potential (E1/2) of CNTs/Fh (0.827\u2009V vs. RHE) compared to Fh (0.841\u2009V vs. RHE) revealing the fast reduction of Fe (III) (thermodynamic aspect). Also, the effective transfer of electrons from H2O2 to Fh was considered as the dynamic aspect of the increase in the rate of Fenton reaction. The possible mechanism deduced from DFT calculations, and CV characterisation is summarised in \nFig. 3.Transition metal-doped iron oxides with a spinel structure are normally named as ferrites. They have a general formula of MxFe3\u2212xO4 (M is a bivalent transition metal ion) with a face-centred cubic lattice formed by oxide ions. Among the different iron-based materials tested as heterogeneous photo-Fenton catalysts, the ferrites are of particular interest because of their narrow bandgap (1.9\u2009eV, for ZnFe2O4) and high stability (Sharma et al., 2015; Hou et al., 2013; Vald\u00e9s-Sol\u00eds et al., 2007; Wang et al., 2014). Ferrites are preferred as heterogeneous Fenton catalysts because of their easiness in recovery and reuse owing to their magnetic properties (Laurent et al., 2008; Polshettiwar et al., 2011; Sharma and Singhal, 2015). Ferrites are chemically stable (Yang et al., 2013), and because of their narrow bandgap, they are also active catalysts under visible light (Wang et al., 2011). Sharma and Singhal (2015) demonstrated the synthesis of magnetic nano-spinel having formula MFe2O4 (M=Cu, Zn, Ni and Co) using a sol-gel method. Among all the four ferrites, CuFe2O4 was found to be best (k\u2009=\u20090.228\u2009min-1) for the degradation of azo dye RB5, which was attributed to the coupling between Fe3+/Fe2+ and Cu2+/Cu+ redox pairs leading to the efficient production of more \u2022OH radicals. Similar studies also reported the use of copper ferrites for gallic acid removal (Fontecha-C\u00e1mara et al., 2016), degradation of sulfonamide antibiotic (Gao et al., 2018), and antibacterial therapy (Liu et al., 2019). Fontecha-C\u00e1mara et al. (2016) studied three commercially available iron oxides; copper ferrite, magnetite and ilmenite (FeTiO3) for the removal of gallic acid and the highest catalytic activity was displayed by copper ferrites. Cu2+ occupied the octahedral site of the copper ferrite spinel, and the collective effect of iron and copper ions significantly improved the rate of Fenton reaction by generating more \u2022OH radicals. In another study, ZnFe2O4 was synthesised from precursors such as Fe(NO3)3 and Zn(NO3)3 through a hydrothermal treatment procedure and depicted the visible light degradation of orange II (Cai et al., 2016). Experiments performed with various radical scavengers such as tert-butanol, sodium oxalate and iso-propanol showed that \u2022OH generated on the surface was the key species responsible for the degradation. A generalised scheme of the mechanism of generation of \u2022OH and the regeneration of Fe(II) on the surface of ferrites is summarised in \nFig. 4. The stability of the catalyst was understood by performing multiple runs with the recycled catalyst. After the first cycle, the reaction rate constant was observed to be 0.0468\u2009min-1. Even after five cycles, the catalyst showed a similar reaction rate constant (k\u2009=\u20090.0483\u2009min-1). Also, the amount of H2O2 decomposed by the catalyst during the five cycles was equivalent. Invariably all these indicate the reusability of the catalyst. Xiang et al. (2020) prepared ZnFe2O4 nanoparticles having yolk-shell structure and evaluated in the degradation of tetracycline under visible light. The yolk-shell structured ZnFe2O4 nanoparticles had a higher specific surface area and presented better visible light absorption capacity in comparison to the spherical ZnFe2O4 nanoparticles. The higher visible light absorption was correlated to the possibility of multi-scatterings of light in the inner yolk-shell structure of ZnFe2O4.\nLater, Hermosilla et al. (2020) reported an environmentally friendly synthesis of manganese ferrites (Mn\u2012Fe2O4) via routes such as sol-gel, combustion and reverse microemulsion. Bio-recalcitrant compounds such as ciprofloxacin (a fluoroquinolone antibiotic) (Davis et al., 1996) and carbamazepine (anti-depressive drug) (Ballenger and Post, 1980) were successfully degraded by the photo-Fenton action of visible light active Mn\u2012Fe2O4. The magnetisation is one of the crucial characteristics of a material for its separation and reusability. \u03b1-Fe2O3 (hematite) is reported as a weak ferromagnet, and its content has a trivial contribution to the magnetisation of the material (Raming et al., 2002). Here, the magnetisation value of the sol-gel synthesised Mn\u2012Fe2O4 came to be 41.0\u2009emu\u2009g-1 and the lowest value was reported for Mn\u2012Fe2O4 synthesised by reverse microemulsion route (3.7\u2009emu\u2009g-1). The relative content of \u03b1-Fe2O3 was lowest in the sol-gel Mn\u2012Fe2O4, and it was associated with its higher values of magnetisation.Some of the recent studies report the use of magnetite (Fe3O4) or its composites for disinfection of E. coli in water (Tong et al., 2020; Feng et al., 2019; Arshad et al., 2019). A composite material of Fe3O4 and flower-like MoS2 (Fe3O4/MoS2) effectively inactivated E. coli up to six log scale within 30\u2009min (Tong et al., 2020). The Fe3O4/MoS2 was active at a broad pH from 3.5 to 9.5, and the catalyst could be separated magnetically owing to its saturation magnetization value of 40.6\u2009emu\u2009g-1. Similarly, a graphene composite of Fe3O4 was successful in inhibiting the growth of Pseudomonas aeruginosa and S. aureus (Tong et al., 2020). Wang and co-workers developed a therapeutic approach by combining the copper ferrite antibacterial therapy with photothermal therapy (PTT) (Liu et al., 2019). The hydrothermally prepared haemoglobin functionalized copper ferrite nanoparticle (Hb-CFNPs), effectively generated the \u2022OH and initiated the cell membrane disruption. Further shining 808\u2009nm laser light (near-Infrared, NIR) increased the cell membrane permeability by hyperthermia and resulted in leakage of bacterial contents. In-vitro experiments revealed the broad-spectrum antibacterial activity over the E. coli (100% removal), and S. aureus (96.4% removal) bacteria and the therapeutic method showed significant results in the S. aureus infected abscess treatment. The coupling between Fe3+/Fe2+ and Cu2+/Cu+ redox pairs catalysed the production of \u2022OH and promoted the oxidative damage of bacterial cells. A brief illustration of the synthetic strategy and the therapeutic application of Hb-CFNPs is outlined in \nFig. 5.There are various reports of the application of iron oxide family of materials for wastewater treatment and microbial inactivation (Nieto-Juarez and Kohn, 2013; Pecson et al., 2012; Xu et al., 2012). In 2015, Ruales-Lonfat et al. (2015b) studied the microbial inactivation efficiency of four commercially available iron oxides. Hematite, goethite and wustite used O2 as electron acceptor and performed the photocatalytic activity even in the absence of H2O2. However, the magnetite was only active in the presence of H2O2. It is important to note that no bacterial growth was observed after the photo-Fenton treatment. These results are very significant because excluding H2O2 from the reaction decreases the cost of the water treatment to a great extent. The concentration of the catalyst used was 0.6\u2009mg/L Fe3+, and iron concentration similar to this scale is usually observed in natural water sources. This seems to be a useful strategy in a large-scale application for bacterial inactivation.Size and morphology of the nanomaterials have a significant correlation with the physical and chemical characteristics exhibited by them (Mai et al., 2005; Kundu and Jayachandran, 2013; Xie et al., 2013). Hematite (\u03b1-Fe2O3) having morphology such as microtubes, nanorods, nanorings are reported in the literature (Xiong et al., 2011; Vayssieres et al., 2005; Hu et al., 2007). Xiao et al. (2018) studied the morphological evolution of hematite by adjusting the hydrothermal reaction time. After 6\u2009h of hydrothermal treatment, the nanoparticles attained a spherical morphology, and upon further heating, elliptical, olive-like and burger-like morphologies were observed at 12, 18, 24\u2009h, respectively. Burger-like \u03b1-Fe2O3 was found to be better in the removal of acid red G in comparison to other morphologies of \u03b1-Fe2O3, and a 98% degradation efficiency was observed under visible light within 90\u2009min.Recently, a composite material of schwertmannite/graphene oxide (SCH/GO) was synthesised through an oxidation-co-precipitation route and demonstrated removal of tetracycline antibiotic under visible light. The photo-Fenton catalytic tests were performed in real wastewater matrices such as raw food wastewater and biogas fluid of anaerobically digested food. The SCH/GO nanocomposite was efficient in selectively degrading the tetracycline in the presence of a comparable concentration of moieties such as chlorides, sulfates, phosphates and nitrates. The SCH/GO system showed fifteen times higher rate constant of tetracycline degradation compared to the SCH, because of its improved optical absorption property and separation of electron-hole pairs (Ma et al., 2020).Similar to the Fenton reaction in the presence of H2O2, Fe(II/III)-oxalate system also reported having the superior capacity to degrade organic pollutants (Wei et al., 2013; Liu et al., 2012; Lan et al., 2008). A FeWO4 nanosheet material synthesised by a hydrothermal method showed facet dependent surface Fenton chemistry in the presence of oxalic acid (Li et al., 2019). Density functional theory (DFT) studies concluded that the {001} facets were efficient in producing reactive oxygen species in comparison to {010} facets. Also, DFT analysis confirmed that \u2022OH generated on the {001} facets diffused faster to the solution and kept the {001} facets vacant for the continuous activation of oxalic acid molecules into radicals.Apart from directly employing various iron minerals as heterogeneous Fenton catalysts, iron minerals can be incorporated into numerous supporting materials like zeolites (Soon and Hameed, 2011; Nidheesh, 2015; Hartmann et al., 2010), metal-organic frameworks (MOFs) (Liu et al., 2017; Cheng et al., 2018), clays (Garrido-ram\u00edrez et al., 2010; Navalon et al., 2010), graphene oxide (GO) (Nidheesh, 2017; Wang et al., 2019), silica (Gan and Li, 2013; Zhong et al., 2011) etc. Some of the desirable properties needed for the supporting materials for holding the iron-based catalysts for photo-Fenton reaction can be their ability to perform the reaction for multiple cycles and the lesser leaching of the Fe ions. Moreover, they need to be durable against highly reactive radicals. In this section, recent reports on various supporting materials for heterogeneous Fenton processes and their desirable properties are discussed.The incorporation of iron into clays can be performed by pillaring (Guimar\u00e3es et al., 2019; Tabet et al., 2006), impregnation (Herney-Ramirez et al., 2008; Hassan and Hameed, 2011) etc. Generally, inorganic supporting materials provide thermal stability, resistance to organic solvents and high mechanical strength (Cheng et al., 2006).Clay materials are abundantly present in the earth crust, but they may not be used as excellent Fenton catalysts because of their low iron content in them. Even though the layered clay materials have a large surface area, their interlamellar space is mostly inaccessible because of higher electrostatic interaction present between the layers (Garrido-ram\u00edrez et al., 2010). These shortcomings are circumvented by pillaring of clays. Among the different clay materials, pillared clays are of special interest because of their catalytic and adsorption properties. Through the pillaring process, (stacking and then connecting the 2D layers) various large-sized poly oxo-hydroxy metal cations are incorporated into the structure of clays by replacing the smaller ions (Aznarez et al., 2015; Nogueira et al., 2011). This process makes the interlamellar space accessible for the reactants and leads to a significant increase in the porosity and surface area of the material. Pillaring process also exposes some of the catalytic sites, and additional catalytic sites are added in case iron compounds make the pillars (Navalon et al., 2010). Since the crystal structures and other characteristic properties of the pure clay materials are well-defined, the difference in catalytic activity mainly arises from the pillars incorporated (Baloyi et al., 2018). Further, the Fe(III) species can be considered as immobilised in the interlayer spacing of pillared clays. So, the ion species is stable against the differences in solution pH, and that results in limited leaching of iron (Herney-Ramirez et al., 2010). There are various reports of the use of pillared clays for the degradation of dyes (Li et al., 2015; Ayari et al., 2019), pharmacologically important compounds (Hurtado et al., 2019; Khankhasaeva et al., 2017), and phenolic compounds (Hadjltaief et al., 2015; Catrinescu et al., 2012). However, the applications of pillared clay-based systems in more complex matrices, especially those with heavy organic load, are rare in literature. Recently, the photo-Fenton activity of Al-Fe smectite pillared clay has been demonstrated for the treatment of winery wastewater with high amounts of recalcitrant polyphenolic compounds (Guimaraes et al., 2019). The catalyst was prepared by intercalating poly-hydroxy aluminium (Al3(OH)4\n+5) and (Fe3(OH)4\n+5) species between the layers of natural smectite. The photo-Fenton studies performed under UV-C light radiation resulted in a 75.2% percentage total organic carbon (TOC) removal of the winery wastewater. In another study, Xu et al. (2016b) used hydroxy-iron montmorillonite (Fe/Mt) as a host material, and BiVO4 semiconductor was loaded into the interlayers of Fe/Mt. An 8%BiVO4/Fe/Mt composite demonstrated an 85.2% TOC removal of acid red-18 under visible light irradiation. The remarkable \u2022OH generation capacity of the system was associated with the synergistic effect between BiVO4 and Fe/Mt and the photo-induced injection of electrons from BiVO4 to Fe(III) ions.Similar to the examples of the addition of plasmonic systems with the ferrites, Ag/AgCl was impregnated onto sepiolite clay which was modified with hydroxy iron (Ag/AgCl/Fe\u2010S) (Liu et al., 2017). This catalyst exhibited excellent activity in degrading BPA. Electrochemical impedance spectroscopy (EIS) was performed to understand the charge transfer resistance (CTR) and ease of separation of electron-hole pairs in the catalyst. In a three-electrode electrochemical system, 0.1\u2009mol/L KCl was used as an electrolytic solution, and a glassy carbon electrode modified with prepared catalysts was employed as a working electrode. Several reports suggest that a lower charge transfer resistance can be correlated with facile separation of electrons and holes (Ganiyu et al., 2018). Here among the three catalytic systems studied, Ag/AgCl/Fe\u2010S was reported with the lover CTR, and that corroborate the enhanced degradation of the BPA.Layered double hydroxides (LDHs) are a class of clay-based materials having brucite-like sheet structures made of metal hydroxides (Gursky et al., 2006; Shao et al., 2013). Their intercalated anions/cations can be easily exchanged by cation exchange to alter their properties (Zhang et al., 2014; Zhang et al., 2012). The strong electrostatic interaction observed between the layers and interlayer anions provides a well-oriented structure for the LDHs (Zhang et al., 2020). This ordered layered structure endows the LDHs with plenty of sites for the interaction of pollutants and H2O2 (Jack et al., 2015). The general hydrophilic nature provided by the hydroxyl groups promotes distinct interaction of hydrophilic contaminants and H2O2 with the active catalytic sites on the surface (Yang et al., 2020). Bai et al. (2017) synthesised a Co/Fe LDH through a co-precipitation strategy and demonstrated the Fenton-like removal of nitrobenzene. The mechanism of the process was studied by an \u2022OH scavenger, and \u2022OH was identified to be the key radical involved. In another study, a Fe\u2012Ni LDH was reused for the synthesis of a magnetic catalyst (Ni3Fe/Fe3O4). Here, Fe\u2012Ni LDH was treated with the orange II dye and heated under nitrogen atmosphere to obtain the novel catalyst (Ni3Fe/Fe3O4). The thermo-magnetic curves depicted the superior magnetic properties of the synthesised catalyst. In the past decade, there have been various reports of using copper-containing LDHs for the mineralisation of phenol, a major waste generated in the petrochemical industry (Zhang et al., 2010a, 2010b). But in those scenarios, the degree of mineralization of phenol was comparatively low, and the system utilized a higher dosage of H2O2. Even though many of the studies discuss the synergistic effect of copper with other metals present, yet studies concerning the in-depth understanding of phenol degradation mechanism remains unaddressed (Zhou et al., 2011). In a similar perspective, Wang et al. (2018) prepared a series of CuNiFe LDHs by varying the Cu/Ni ratios. The specific band observed in the Raman spectra at 460\u2009cm-1, and 533\u2009cm-1 corresponds to the lattice vibration in LDHs. For the samples where the Cu/Ni ratio is higher than 0.5, a particular band is observed at 294\u2009cm-1, attributed to the Cu(OH)2. The intensity of this band increases with the increase in Cu/Ni ratio. Experiments revealed that the catalytic activity (phenol mineralisation) increased upon decreasing the Cu/Ni ratio. It is remarkable to mention that, when the concentration of H2O2 was kept near the theoretical value (M Hydrogen peroxide /M phenol =14) they observed mineralisation of 90% phenol. At lower Cu/Ni ratios, electron transferred from Ni2+ to Cu2+ and facilitated the generation of Cu+ species. Here Cu+ reacted with H2O2 in a Fenton like mechanism and produced the \u2022OH. The mechanism is summarised in \nFig. 6.The large-scale applications of these materials vary depending upon the local availability of the particular clay materials. Currently, many of the Fenton related studies using clay materials concentrate on the degradation and mineralisation of dyes. Extensive research is needed to develop new catalysts with disinfection properties and the capacity for removal of antibiotics.Perovskites are a class of ABX3 compounds in which the X anion is mainly O2- (Ferri and Forni, 1998; Zhu and Thomas, 2009). Perovskite compounds have a cubic geometry with A cation surrounded by 12 X anions, and B cation surrounded by 6 X anions (Smith et al., 2019; Quan et al., 2019). These compounds received their generic nomenclature from the mineral perovskite (CaTiO3). In the last decade, ABO3 perovskite family of oxides such as EuFeO3 (Ju et al., 2011) , LaFeO3 (Nie et al., 2015), BiFeO3 (Rusevova et al., 2014; Luo et al., 2010) garnered a great deal of attention as heterogeneous photo-Fenton catalysts for the degradation of various organic pollutants. A nano-BiFeO3 perovskite catalytic-system was demonstrated to have the degradation capacity of BPA. They have studied the capping action of various organic ligands such as oxalic acid (OA), formic acid (FA), glycine (Gly), nitriloacetic acid (NTA), and ethylenediaminetetraacetic acid (EDTA) on the nano-BiFeO3 catalyst. Studies show that the EDTA_BiFeO3 system was accelerating the BPA degradation and the efficiency of OA_BiFeO3 system was lower than the bare BiFeO3 catalyst (Wang et al., 2011). To further understand the system, density functional theory (DFT) studies were carried out on the OA_BiFeO3 and EDTA_BiFeO3 models. DFT studies gave the insight that unique hydrogen bonding interaction observed in the EDTA_BiFeO3 catalyst was responsible for the weakening of the O-O bond and the generation of \u2022OH (\nFig. 7).In a recent effort, Cu-substituted LaFeO3 perovskite was used for the degradation of BPA (Pan et al., 2020). Using a citric acid complexation method (Zhao et al., 2016), by altering the copper doping ratio they demonstrated the synthesis of novel Cu-substituted LaFeO3 catalysts. The generation of oxygen vacancy in the Cu-substituted LaFeO3 played a critical role in redistributing charge on the surface of the catalyst, and that helped the efficient decomposition of H2O2. The XRD phase evolution studies depicted that at calcination temperature of 700 \u00baC, the XRD spectrum became narrower and sharper. Also, the LaFeO3 perovskite structure of LaCuxFe1\u2212xO3-\u03b4 solid solution was retained when the x values changed from 0.1 to 0.5. The peaks corresponding to Miller indices (121) and (240) got widened and the observed peak-offset was attributed to the lattice contraction of the crystal. Theoretical calculations showed an approximate 0.05-angstrom decrease in the Fe\u2012O, and La\u2012O bond lengths after copper substitution and that also portrays the volume contraction of the LaCuxFe1\u2212xO3-\u03b4 unit cell. These parameters undoubtedly suggested that copper got incorporated into the LaFeO3 perovskite replacing Fe in the structure (Pan et al., 2020).In a similar study, Cu-doped LaFeO3 was used as a visible-light active catalyst for the photo Fenton degradation of methyl orange (To et al., 2018). The performance of 15\u2009mol% Cu-doped catalyst was better than that of pure LaFeO3 catalyst. In another study, Cu-doped BiFeO3 was synthesised by a sol-gel method, and it depicted the degradation of 2-chlorophenol under visible light (Soltani and Lee, 2017). In the above cases of copper doping, along with Fe(II), Cu(I) was also acting as an active species in a Fenton like manner in splitting the H2O2 to \u2022OH. In a different study, Chu et al. (2018) used the Ag-doped LaCaFeO3-\u03b4 (Ag-LaCaFeO3-\u03b4) perovskite as a peroxymonosulfate (PMS) activating agent for the effective removal of bacterial pathogens. EIS studies have suggested an increase in lattice oxygen vacancies in Ag-LaCaFeO3-\u03b4 than LaCaFeO3-\u03b4. A synergistic effect of free radicals (SO4\n\u2022- and \u2022OH) and silver ions towards the bacterial inactivation was observed. The studies demonstrated the antimicrobial effect of the catalyst on the E. coli and S. aureus (a methicillin antibiotic-resistant bacteria). The study also displays that the silver leaching observed after 48\u2009h of the reaction was 0.09\u2009mg/L, which comes below the guidelines of the World Health Organization (WHO) for safe drinking water.Carbon-based materials namely carbon nanotubes (CNTs) (Yao et al., 2016; Yang et al., 2018), activated carbon (AC) (Yao et al., 2013; Navalon et al., 2011), biochar (Fang et al., 2015; Yan et al., 2017), graphene oxide (GO) (Nidheesh, 2017; Divyapriya and Nidheesh, 2020), g-C3N4 (Sudhaik et al., 2018; Hasija et al., 2019) etc. have been exploited in the heterogeneous Fenton reactions. The recent developments in using two-dimensional carbon-based materials for Fenton related applications are discussed here.Many researchers have shown that incorporating carbon materials with heterogeneous Fenton catalysts helps in quick reduction of Fe(III) to Fe(II), because of its fast single electron transfer ability. Graphene is a two-dimensional monolayer of carbon atoms with superior electron mobility, mechanical stability and electrical conductivity (Dai, 2013; Bekyarova et al., 2013). It is reported that the presence of graphene provides support to the Fenton catalyst, and it enhances the performance of the Fenton reaction (Nidheesh, 2017; Divyapriya and Nidheesh, 2020; Han et al., 2014). In a GO\u2012Fe3O4 Fenton catalyst, GO is considered as a sacrificial electron donor (Zubir et al., 2015). The unpaired \u03c0 electrons present in the sp2 carbon domains (CC) of GO transfer electron to the iron centres of Fe3O4 and accelerate the reduction of Fe(III) to Fe(II) (Zubir et al., 2014). XPS analysis on the GO\u2012Fe3O4 system evidenced a continuous reduction of Fe(III) to Fe(II). Hence the strong electron transfer ability depicted by the graphene-related materials is a crucial factor that contributes to the enhanced catalytic efficiency of graphene-based material supported heterogeneous Fenton catalysts. The GO\u2012Fe3O4 catalyst showed a 97% removal of Acid Orange 7 (AO7) whereas Fe3O4 was only effective in removing 65% of AO7 under photo Fenton conditions. Similar recyclability of Fe(II) species is reported for CNTs supported FeS systems (Ma et al., 2015). The several chemical moieties present on the GO (carboxyl, hydroxyl, hydrophobic groups etc.) and the higher specific surface area promotes the adsorption of organic pollutants to the surface of GO and contributes to the effective removal of pollutants (Bagri et al., 2010; Su\u00e1rez-Iglesias et al., 2017). The hydrogen bonding, \u03c0-\u03c0 interaction, hydrophobic interaction and electrostatic interaction are the four possible interactions that cause the better adsorption of pollutants to the GO surface (Wang et al., 2019).\nBoruah et al. (2017) prepared a magnetically recoverable Fenton catalyst by decorating Fe3O4 nanoparticles on an amide-functionalized graphene sheet. The specific \u03c0-\u03c0 and electrostatic interaction between the sp2 carbon system of graphene and organic pollutants assisted the mineralisation of various phenolic compounds under sunlight irradiation. The catalyst was stable up to ten cycles, and lower electron-hole recombination was inferred from the photoluminescence studies. In a similar study, Wan and Wang (2017) used polyol process and an impregnation method to prepare Fe3O4/Mn3O4/reduced graphene oxide hybrid material. Under the optimum conditions (H2O2 =6\u2009mM, catalyst\u2009=0.5\u2009g/L, pH\u2009=\u20093) catalyst showed a 98% degradation of sulfamethazine, one of the pharmaceutically active compound. Zheng et al. (2018) loaded Fe3O4 nanoparticles on the functionalized graphene oxide (GO) nanosheets through urushiol molecules as a linker (Fe3O4\u2012U-rGO). Urushiol is known for its strong coordinating ability to the metal oxides, and it bonds with various materials through its phenolic hydroxyl groups (Zheng et al., 2011, 2014). The composite catalytic material prevented the iron sludge formation and unfavourable decomposition of H2O2 to H2O and O2. Hence the Fenton catalytic performance was significantly improved and the process exhibited complete degradation of rhodamine B and methylene blue. The catalyst also exhibited excellent reuse stability up to seven cycles with minimal sludge formation. The synthesis method of the composite catalyst and its reaction pathway are summarised in \nFig. 8. Graphene oxide membranes showed tremendous potential in water filtration technologies, but their low permeation flux was hindering their large-scale applications (Wang et al., 2016; Yin et al., 2016; Gao et al., 2013). Recently, a composite material of GO and metal-organic framework (MOF) exhibited six times higher permeation flux (26.3\u201330.6\u2009L\u2009m-2 h-1 bar-1) with an enhanced separation efficiency, compared to GO nano-sheets. Also, a forty-minute visible light photo-Fenton action on the material removed 97.27% of BPA compound (Xie et al., 2020).The need to remove iron sludge after the wastewater treatment makes the homogeneous Fenton process non-viable (Zhu et al., 2019). Guo et al. (2017) used a low amount of graphene (0\u20132\u2009wt%) to modify the ion-sludge obtained from homogeneous Fenton process to prepare a heterogeneous Fenton catalyst named as iron sludge-graphene (Fe\u2012G). The Fe\u2012G catalyst was characterised as FeOOH particles entrapped inside graphene sheet. Owing to the mesoporous structure and the increased adsorption ability of Fe\u2012G catalyst, it showed an improved degradation rate of metronidazole (an antibiotic) compared to the bare iron sludge.The development of graphene oxide (GO) composites for innovative disinfection technologies is an emerging research topic (Arshad et al., 2019; Moreira et al., 2018; Singh et al., 2020). Hu et al. (2018) prepared a hybrid material of reduced graphene oxide (rGO), silver nanoparticles (AgNP), and Bi2Fe4O9 (rGO-Ag/BFO) through an evaporation process. The hybrid material exhibited 100% bactericidal performance (> 6 logs) of E. coli. Generally, 3 logs of photocatalytic bacterial disinfection is achieved in 1\u20134\u2009h; (Laxma Reddy et al., 2017) however in the process using the hybrid catalyst, approximately 6 logs of disinfection was achieved in 20\u2009min. High performance of the material was associated with the synergistic effect of various mechanisms such as rGO/AgNP co-assisted photocatalysis, photo-Fenton reaction, and rGO assisted silver ion release. The rGO-Ag/BFO was also excellent in the disinfection of gram-negative P. aeruginosa and gram-positive S. aureus.\nGraphitic carbon nitride is a polymeric medium band-gap material (2.7\u2009eV) with efficient photocatalytic property (Zhao et al., 2015; Cao et al., 2015; Wang et al., 2012). Its remarkable thermal and chemical stability have set the stage for preparation of various state-of-the-art nanocomposites for solving the energy storage and environmental pollution issues (Sudhaik et al., 2018; Hasija et al., 2019; Wu et al., 2013; Liu et al., 2016). The Fe doped g-C3N4 modified with mesoporous carbon was effective in removing Acid Red 73 dye for a wide pH ranging from 4 to 10 (Ma et al., 2017). The XPS analysis identified the Fe\u2012N species formed on the N-rich C3N4 as the active sites for the Fenton reaction. Cyclic voltammetry (CV) experiments verified that the mesoporous carbon could accelerate the Fe (III) to Fe (II) cycle. Similarly, Hu et al. (2019) doped the g-C3N4 with different ratios of Fe(III) and the obtained Fe\u2012g\u2012C3N4 catalyst was successful in removing phenol, BPA and 2,4-dicholorophenol. The study reported that 5% doping of Fe in g-C3N4 was the optimum iron concentration for phenol removal, and the catalyst was efficient in degrading the components of a complex wastewater system such as cooking wastewater. Fe(III) forms strong \u03c3 and \u03c0 bonds with the triazine ring skeleton of the g-C3N4, and that helps the material to act as an efficient heterogeneous Fenton catalyst upon light irradiation.Many of the studies that deal with Fenton chemistry and disinfection only take care of inactivating bacteria in the system; but genetic material could stay active in the medium (Tong et al., 2020; Thakur et al., 2020). Through the horizontal gene transfer process, it is possible to transfer the antibiotic resistance gene (ARG) from one bacterium to another bacterium which does not possess the antibiotic resistance (Koonin et al., 2001; Thomas and Nielsen, 2005). Therefore, methodologies need to be developed to eliminate ARGs from the wastewater to prevent the rapid growth of antibiotic-resistant bacteria (ARB). A ternary nanocomposite system prepared by Saha et al. (2020) was successful in inactivating the commercially available plasmids pUC18 and pBR322 containing the ampicillin resistance gene (ampR). The synthesis of ternary nanocomposite of reduced graphene oxide (rGO), iron oxide and g-C3N4 was carried out by in-situ mixing of all the precursor chemicals. The fragmentation route of the plasmids was confirmed by agarose gel electrophoresis studies performed after treating the system with ternary catalyst, H2O2 and visible light. Initially, the plasmids were at a supercoiled fashion, and upon light irradiation over the system, plasmids transferred to a relaxed and single-stranded form. After an exposure time of around 30\u2009min, the plasmids disintegrated into smaller fragments. The various phenomena that lead to the inactivation of plasmids involve photocatalytic activity by iron oxide and g-C3N4, photo-Fenton activity, relaxation of charge carriers by rGO etc. and are summarised in \nFig. 9 (Adapted from reference (Saha et al., 2020)).Recent advancements show that incorporating goethite or hematite with the g-C3N4 can result in a heterogeneous photo-Fenton catalyst, capable of degrading tetracycline antibiotic under visible light (Wang et al., 2020b; Zhao et al., 2020). The combination of hematite (\u03b1-Fe2O3) and g-C3N4 resulted in a solid-state Z-scheme type catalyst (Wang et al., 2020b). In a similar perspective, a co-calcination approach of melamine (Hughes, 1941) (a cyclic compound of formula C3H6N6) with Fe-based metal-organic framework (MIL-53(Fe)), resulted in the formation of Z-scheme heterostructure catalyst \u03b1-Fe2O3 @g-C3N4 (Guo et al., 2019). The photoluminescence emission spectra suggested the enhanced separation ability of photo-generated electron-hole pairs. The higher number of electrons participated in the regeneration of Fe(II) boosted the production of \u2022OH and resulted in higher degradation of tetracycline antibiotic. Many-a-times, the large quantity of commercial H2O2 needed in the Fenton reaction considerably increases the operating cost of the reaction and hinders its industrial-scale applications (Comninellis et al., 2008). The g-C3N4 having a negative conduction band (CB) potential compared to the reduction potential of O2/H2O2, can transfer two electrons to the oxygen and result in an in-situ production of H2O2 (Kofuji et al., 2016; Moon et al., 2017). It is significant to note that, in the \u03b1-Fe2O3/g-C3N4 system, H2O2 was produced on the g-C3N4 and the in-situ produced H2O2 was decomposed to \u2022OH on the \u03b1-Fe2O3 surface. Also, the hole in the VB of hematite was capable of oxidising OH- to \u2022OH. These characteristics make the \u03b1-Fe2O3/g-C3N4 a promising candidate for wastewater purification applications (Wang et al., 2020b).Zeolites are framework aluminosilicate structures composed of linked MO4 tetrahedra (M= Si4+, Al3+) (Suib, 1993; Armbruster and Gunter, 2001; Weckhuysen and Yu, 2015). An array of zeolites are available with specific pore sizes. So they find distinct applications in separating mixtures of molecules based on size and they are also called as molecular sieves (Kita et al., 1997; Jia et al., 1994; Primo and Garcia, 2014). One of the striking features associated with zeolites is their selectivity towards the guest molecules compared to other high surface area materials such as activated carbon and silica gel (Lesthaeghe et al., 2007; Martinez-Macias et al., 2015). Fe-containing zeolites are widely studied because of their structural uniformity and high catalytic activity in removing different contaminants (Aleksic et al., 2010; Gonzalez-Olmos et al., 2009; Hartmann et al., 2010; Navalon et al., 2010). Different studies show the enhancement in the catalytic degradation rate by shining UV light on Fe-zeolite systems (Kasiri et al., 2008; Kusic et al., 2006; Noorjahan et al., 2005). Gonzalez-Olmos et al. (2012) reported the mineralisation of phenol and imidacloprid (an insecticide) using iron-containing zeolites, Fe-ZSM5 and Fe-beta. Studies performed at near-neutral pH demonstrated that Fe-ZSM5 catalyst was efficient in producing \u2022OH compared to Fe-Beta. They also performed these experiments in a pilot-scale under the solar light using a compound parabolic collector (CPC) (Gonzalez-Olmos et al., 2012). In a similar perspective, Fe-ZSM5 catalyst was prepared by an ion-exchange method and used for removal of diclofenac, an anti-inflammatory drug. Characterisation by scanning electron microscopy (SEM) and inductively coupled plasma mass spectrometry (ICP-MS) gave insights about the morphology and composition of the catalyst. After two hours of treating diclofenac under optimal conditions ([H2O2] = 50\u2009mM, [Fe]FeZSM5 = 2.0\u2009mM, UV-A), they reported low toxicity and biodegradability. Catalyst also gave a similar performance in consecutive runs (Perisic et al., 2016). In another study, Fe3O4 nanoparticles were deposited on zeolite-Y through a wet impregnation method (Yang et al., 2019). A 9% iron-loaded zeolite degraded 90% phenol at neutral pH within two hours. The possible adsorption of phenol on the catalyst and the splitting of H2O2 to generate \u2022OH is schematically represented in \nFig. 10.The ideas of improving the efficiency of Fenton catalyst are well explored in literature, but the strategies for improving the H2O2 utilization are relatively ignored. Since the Fenton reaction is applied for treating wastewater with massive loads of contaminants, it needs the external addition of large amounts of H2O2. Optimization of H2O2 amount could make the Fenton reaction considerably cheaper. Here the idea is to reduce the self-decomposition of H2O2 to H2O and O2 and to prevent the reaction of excess H2O2 with \u2022OH. In a recent study, Wang et al. (2020a) prepared a Fe3O4-zeolite-cyclodextrin catalyst (F-Z-C), which acted as a nano-reactor capable of controlling the local reactant concentration. Cyclodextrins are known for forming inclusion complexes with the guest molecules through van der Waals forces, chemical bonds etc (Saenger, 1980\n). In this process, the dispersed contaminants (e.g. methylene blue) adsorb on the F-Z-C catalyst which result in increased contaminant concentration over a local region. Then the \u2022OH generated on the catalyst-water interface reacts with the adsorbed contaminants and degrades them. Also, the F-Z-C catalyst can store excess H2O2 and release it once necessary. This \u2018storage- release\u2019 effect prevents the self-decomposition of H2O2 and improves the H2O2 utilization efficiency.Metal-organic frameworks (MOFs) are a class of supramolecular assemblies formed from the interaction of various metal ions and organic ligands (Deria et al., 2014; O\u2019Keeffe and Yaghi, 2012; Howarth et al., 2016). The high porosity, excellent surface area and capacity to act as nano-reactors attracted wide attention on MOFs for various applications such as gas storage, liquid-phase separations, catalysis, drug delivery etc (Li et al., 2011; Wang et al., 2015; Kuppler et al., 2009; Denny et al., 2016; Wang and Astruc, 2020; Horcajada et al., 2012; Li et al., 2009). In the past decade, a surge has been observed in the utilization of iron-contained MOFs for heterogeneous Fenton reactions owing to their efficient \u2022OH generation capacity (Liu et al., 2017; Cheng et al., 2018; Sharma and Feng, 2019). Fe-MOFs have a higher tendency for the recombination of generated electron-hole pairs resulting in lower photocatalytic activity (Liang et al., 2015; Liu et al., 2018). Nowadays, preparation of composite materials with Fe-MOFs and semiconductor is devised as a successful strategy to promote the charge transfer efficiency (Chandra et al., 2016; Shen et al., 2015). MIL (Materials Institute Lavoisier) group of metal-organic frameworks are one of the most explored classes of MOFs in the field of environmental remediation (Farha and Hupp, 2010; Janiak and Vieth, 2010). Li et al. (2018) formulated a one-pot solvothermal synthesis method for the preparation of TiO2 @MOF (TiO2 @NH2-MIL88B-Fe) heterostructures (abbreviated as SU-3, where three stands for the optimal molar ratio of Ti: Fe). The TiO2 @MOF displayed enhanced photodegradation of methylene blue (MB) under visible LED light. To identify the key reactive species in the Fenton system, experiments were performed with EDTA (h+ scavenger), p-benzoquinone (O2\n\u2022- scavenger) and tert-butyl alcohol (\u2022OH scavenger). The excess addition of tert-butyl alcohol slowed down the degradation of MB, and it indicated the vital role of \u2022OH in the Fenton reaction. Electron spin resonance (ESR) technique employed using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trapper resulted in a characteristic four peak signal (ratio 1:2:2:1) attributed do the DMPO-\u2022OH, indicating the presence of \u2022OH (Du et al., 2017). A plausible mechanism of the photocatalytic decomposition of MB by SU-3 is represented in \nFig. 11.In another study, Fe3O4 and carbon aerogel (CA) were combined with the MIL-100(Fe) and the composite material was evaluated for the removal of tetracycline hydrochloride (TC), a contaminant of emerging concern, under UV light. The CA incorporated nanocomposite with a higher surface area (389\u2009m2 g-1) enhanced the performance by 1.6 times compared to MIL-100(Fe)@\u2009Fe3O4. Also combining the system with CA resulted in improvement of the water stability of the MOF (Rasheed et al., 2018). In a similar study, Wu et al. (2020) prepared a series of Fe-MOFs and studied the role of Fe-oxo clusters in the framework for the degradation of TC. Fe-oxo clusters were regarded as the absorption antennae, and Fe-MOFs showed substantial absorption in the visible region. Among the MIL-101, MIL-53 and MIL-88B, photo-Fenton activity was highest in the MIL-101, because of its higher number of coordinatively unsaturated iron sites. Generally, the metal centres in a MOF are fully occupied by the organic ligands, and this inhibits the metal sites from activating H2O2. So, introducing coordinatively unsaturated metal sites becomes a successful strategy in activating H2O2. Tang and Wang (2018) prepared the CUS-MIL-100(Fe) having open iron centres and reported 100% degradation of sulfamethazine (a commonly used sulfonamide antibiotic). Also the Fenton experiments performed for multiple cycles, revealed excellent structural stability of the catalyst and minimal leaching of iron which is lower than the environmental standards demanded by European Union (<2\u2009mg/L).In a recent report, distinct nano-architectures of road-like, spindle-like, and diamond-like MIL-88A-Fe were prepared and studied to examine the correlation of different exposed crystal facets towards catalytic performance (Liao et al., 2019). The shape-selective synthesis of the catalyst was achieved by simply varying the water/DMF ratio during the solvothermal synthesis, and the contribution of (100) facet decreased upon increasing the quantity of DMF. The peak area analysis of XRD pattern showed that (100) facet has a ratio of 60%, 30% and 15% in the road-MIL-88A-Fe, spindle-MIL-88A-Fe and diamond-MIL-88A-Fe, respectively. Also, DFT studies revealed the easier activation of H2O2 on the (100) crystal facet compared to (101), and the rod-MIL-88A-Fe was chosen as the best catalyst for Fenton reaction.Zero-valent iron (ZVI) with a standard reduction potential of EH\n0 (Fe2+/Fe0) = \u2212440\u2009mV is considered as an effective reducing agent (Fu et al., 2014). ZVI can give out two electrons in the presence of H2O2 or O2 and form the Fe(II) species responsible for Fenton reaction (Eq. 16 and 17). ZVI is known for reacting differently in aerobic and anaerobic conditions. In aerobic conditions, it acts by oxidizing contaminants and in anaerobic condition by reducing contaminants (He et al., 2016).\n\n(16)\nFe0+O2+2\u2009H+\u2192Fe(II)+H2O2\n\n\n\n\n\n(17)\nFe0+H2O2+2\u2009H+\u2192Fe(II)+2\u2009H2O\n\n\n\n\n(18)\nFe(II)+H2O2\u2192Fe(III)+\u00b7OH+OH-\n\n\n\nOver the past decade, ZVI was demonstrated for treating different varieties of organic and inorganic contaminants such as dyes (Wang et al., 2017), phenolic compounds (Ambika et al., 2020; Minella et al., 2019), antibiotics (Zhou et al., 2019), nitroaromatic compounds (NACs) (Zarei et al., 2019), arsenic (Tucek et al., 2017), heavy metals (Li et al., 2017), chlorinated organic compounds (Ezzatahmadi et al., 2017), nitrates etc (Ezzatahmadi et al., 2017\n). The heavy metal iron content in wastewater poses significant challenges during its treatment (Demirbas, 2008; Fu and Wang, 2011). In the last decade, ZVI emerged as a catalyst for treating wastewater, having a large load of heavy metal content (Vilardi et al., 2018; O\u2019Carroll et al., 2013). Li et al. (2017) tested conventional adsorbents and precipitants such as activated alumina, Ca(OH)2, Fe3O4, nanoTiO2 etc. for removing heavy metal ions and compared their performance with the ZVI. However, only the ZVI demonstrated simultaneous removal of multiple heavy metals present (Ni(II), Zn(II), As(V) and Cu(II)). Since the standard reduction potential of copper is more positive compared to that of ZVI, it can easily receive electrons from ZVI. Interestingly, the chemical reduction method of Cu(II) and As(V) using ZVI is less susceptible to changes by the difference in pH or introduction of chelates. For metal ions such as Zn (II) and Ni(II) having standard reduction potential more negative than that of ZVI, precipitation, adsorption and electrostatic attraction are the major methods for the metal ion removal (Li et al., 2017). The oxyanions of arsenic were primarily removed by co-precipitation with Fe(II).Zero-valent iron microspheres have shown to be efficient catalyst for UV light photo-Fenton processes with a TOC removal of 99% for phenol and 83% for oxalic acid (Blanco et al., 2016). The studies also demonstrated the degradation of 90% 1,4-dioxane by ZVI microspheres under solar irradiation for 180-minutes. The results of this study showed similar values of degradation at various pH values and indicate that iron solubilisation is not an essential step in this process and the photo-Fenton reaction is taking place on the surface of the catalyst (Barndok et al., 2016). In a similar study, nano-ZVI was evaluated for the removal of ciprofloxacin antibiotic. With a ratio of 5:1 for nZVI: H2O2, a 99.3% removal of ciprofloxacin was achieved in 120\u2009min at neutral pH. The experiment performed under UV light demonstrated a 100% degradation of ciprofloxacin within 25\u2009min (Mondal et al., 2018). Kakavandi et al. (2019) reported the use of nZVI supported on kaolinite (a layered silicate mineral) for the removal of acid black 1 (AB1) dye. The reaction was performed at pH 2.0 with 0.3\u2009g/L of catalyst, and even after four cycles the catalyst was efficient in removing 72% of the dye, indicating catalyst durability and potential for reuse. In a recent study, Jiang et al. (2020) investigated the role of formic acid in the pathway for degradation of prechlorinated organic contaminants. One of the major concerns about chlorinated contaminants is their resistance to degradation via an oxidative pathway. In the study, formic acid acted as a scavenger of \u2022OH and generated carbon dioxide radical (CO2\n\u2022-). Carbon dioxide radical is known for transferring one of its electrons to chlorinated contaminants and performing the degradation by a reductive pathway. They studied the role of formic acid in the generation of CO2\n\u2022- and confirmed the presence of carbon dioxide radical by electron paramagnetic resonance (EPR) analysis. The generalised mechanism for oxidative and reductive routes of degradation followed by ZVI is summarised in \nFig. 12.The first report of inactivation of two waterborne viruses \u03a6X174 and MS-2 using ZVI came in 2005 (You et al., 2005; Hossain et al., 2014). Later Lee et al. (2018) demonstrated that nano zero-valent iron (nZVI) could act as a potent bactericide under anaerobic conditions. Under aerobic conditions, a more significant amount of nZVI was required for inactivation, possibly due to the surface corrosion and oxidation of nZVI by oxygen. Another study in 2009 by Diao and Yao (2009) reported the inactivation of Pseudomonas fluorescens (gram-negative bacteria), Bacillus subtilis var. niger (gram-positive bacteria) and Aspergillus versicolor (fungus) using nZVI. A sulfidated micro zero-valent iron (S-mZVI) was studied for the removal of antibiotic-resistant E. coli bacteria and antibiotic-resistant gene (ARG) TetB. The S-mZVI was prepared in a planetary ball mill by mixing sulfur and mZVI in a molar ratio of 20 (Fe/S). SO4\n\u2022\u2013 and \u2022OH radicals generated was attributed to the effective removal of ARG (Zhang et al., 2020). Similarly, a Fe/Ni nanoparticle system was synthesised by a liquid-phase reduction method using NaBH4. Here the bimetallic system displayed superior activity compared to the ZVI in the removal of f2 bacteriophage. An optimum ratio of 5:1 (Fe/Ni) showed the highest catalytic performance and both metals existed in the nanoparticle as Fe0 and Ni0 (Cheng et al., 2019).Chlorination is one of the popular methods employed for disinfection of drinking water, but it produces carcinogenic disinfection byproducts (DBPs) like chloroform, chloroacetic acid etc (Hrudey, 2009; Hua and Reckhow, 2007). Therefore, ZVI is of interest since it will not result in the production of any DBPs. Two of the limiting factors that prevent the practical application of ZVI include poor dispersibility and its low disinfection efficiency. Because of the lower disinfection efficiency, a higher dosage of ZVI is required, and that adds to the cost of the process. The electrostatic and magnetic interactions between ZVI particles lead to its aggregation and poor dispersibility. Also, under aerobic conditions, the iron oxide layers formed over the ZVI decelerates the electron transfer from the ZVI core to the exterior (Fu et al., 2014; Sun et al., 2019). Recently Sun et al. (2019) prepared amorphous zerovalent iron microspheres (A-mZVI) and crystalline nanoscale zerovalent iron (C-nZVI) for studying the efficiency of removal of E. coli under aerobic conditions. C-nZVI produced \u2022OH on the surface of a catalyst but A-mZVI generated \u2022OH by the iron dissolution and oxygen activation in the solution. SEM and TEM images of the E. coli treated with C-nZVI depicted the spherical nanoparticles on the surface of bacteria. But the sample treated with A-mZVI showed a thick covering of interconnected flakes on the surface of E. coli bacteria. In the A-mZVI, a fast dissolution of Fe2+ was observed, and later it deposited on the E. coli as iron oxy-hydroxy species. A thicker adsorption of oxide layers over the bacteria resulted in better inactivation efficiency by A-mZVI. Also, the magnetization studies revealed that the corrosion products of A-mZVI are essentially non-magnetic. Therefore, the sedimentation of the reaction mixture after treatment with A-mZVI was not influenced by a magnet. The difference in the mode of action of C-nZVI and A-mZVI and their physical mode of separation are schematically represented in \nFig. 13. Since this technique employs direct gravitational precipitation of the inactivated bacteria covered by iron oxides, it can also prevent the release of ARG into the water (Sun et al., 2019).ZVI is an abundantly available, non-toxic, and comparably low-cost material that has also shown applications as heterogeneous Fenton catalyst (Fu et al., 2014). It has successfully demonstrated the removal of microorganisms, heavy metals, and contaminants of emerging concern from drinking water, and it also functions without formation of toxic disinfection by-products (DBP) (Giannakis et al., 2016; Sun et al., 2019; Du et al., 2020). Because of its versatility, it has great potential for future applications in large-scale water treatment plants. However, it is a challenging task to understand the complex mechanism of action of ZVI because its mechanisms of action may involve oxidation, reduction, co-precipitation, surface adsorption etc. Its mechanism also varies according to the contaminants which it reacts with. Also, ZVI treatment may result in the formation of smaller quantities of corrosion products such as Fe(OH)2, Fe(OH)3, Fe2O2 etc. and they could be detrimental to the pipelines in water distribution channels (Fu et al., 2014). \nTable 1. summarises operational parameters used for performing the Fenton reaction and the catalytic activity observed for different classes of heterogeneous Fenton catalysts.The implementation of Fenton reaction for real-water applications becomes complicated due to the presence of a complex matrix of organic substances present in real water called as Natural Organic Matter (NOM) (Giannakis et al., 2016). Various studies have shown the positive impact of NOM in enhancing the efficiency of Fenton reaction (Spuhler et al., 2010; Huling et al., 2001; Georgi et al., 2007; Vione et al., 2006; Moncayo-Lasso et al., 2009). Fe(III) can form complexes with the NOM (Fe3+-NOM), and this complex is less prone to precipitation and depict higher absorption in the UV\u2013visible range (Voelker et al., 1997; Walte and Morel, 1984). Georgi et al. (2007) reported that the presence of humic acid (a type of NOM) in the Fenton system (50\u2013100\u2009mg/L) had shifted the optimum pH of the reaction towards the neutral condition. Ruales-Lonfat et al. (2015a) compared the E. coli inactivation of hematite with the Milli-Q water and water collected from Geneva lake. The natural water was not interfering with the photocatalytic semiconducting action of hematite (hematite/light/water) and with both water samples complete E. coli inactivation was observed within 120\u2009min. When H2O2 was introduced to the system (hematite/light/water/H2O2), natural water system showed slightly higher inactivation rate in comparison to Milli-Q water. This observation could be related to the photo Fenton action showed by iron species which got complexed and solubilised by the NOM. The formed complex enhances the reaction rate by participating in the homogeneous Fenton reaction.On the contrary, some other studies report the inhibition of Fenton process in the presence of NOM (Bogan and Trbovic, 2003; Lindsey and Tarr, 2000; Lindsey and Tarr, 2000). Fenton experiments performed to degrade polycyclic aromatic hydrocarbons in the presence of humic acid, and fulvic acid (classes of NOM) exhibited the inhibition of \u2022OH formation (Lindsey and Tarr, 2000). In a similar study Lindsey and Tarr (2020) observed four times lower radical formation in natural water compared to pure water. Since the classes of NOM present in real water varies depending on the source of water, detailed localised studies are needed to understand the effect of NOM and its interaction with the heterogeneous Fenton catalyst in either enhancing or inhibiting the Fenton reaction process.Various parameters such as light source, dosage of catalyst etc. need to be optimised for the efficient performance of Fenton reaction. Choice of the perfect light source for the photo Fenton reaction is a critical aspect considering its economic viability. The optimum light radiation required for the photo Fenton process is in the UV region and near-visible spectrum up to 560\u2009nm wavelength (Carra et al., 2015). Solar light is a sustainable source of energy, and in areas where the availability of sunshine is limited, artificial light sources are required. Mercury UV-lamps were the common source of light for photo Fenton reaction (Guimar\u00e3es et al., 2019). But considering its low energy efficiency and possible mercury contamination, Xenon lamp-based sun simulators are widely used for photo Fenton reactions (Hu et al., 2019; Cai et al., 2016). Recent studies report the use of light emitting diodes (LEDs) as an energy-efficient light source in the heterogeneous photo Fenton reaction (Zhu et al., 2018a, 2018b). LEDs have a longer lifespan compared to Xenon lamps, and they convert less amount of energy in the form of heat (Carra et al., 2015; Matafonova and Batoev, 2018).Optimising the dosage of catalyst, H2O2 etc. is an important procedure in minimising the operating cost and achieving the highest catalytic efficiency of Fenton reaction. The amount of natural NOM, carbohydrates, proteins and inorganic species (carbonate, nitrate, sulphate, etc.) present in water are the specific parameters that define a water matrix (Lado Ribeiro et al., 2019). Photo Fenton inactivation experiments performed on E. coli. bacteria in the urban wastewater sample resulted in a 2.43 log disinfection (Rodr\u00edguez-Chueca et al., 2012). When the water matrix was changed to distilled water, a log 5.81 inactivation was observed. In a recent report, Ling et al. (2018) studied the effect of chloride and phosphate on the ZVI in a Fenton reaction. Chloride ions were shown to have accelerated the decomposition of H2O2 and enhanced the reaction rate, but the phosphate ions were observed to inhibit the H2O2 decomposition. It was assumed that the insoluble iron phosphate layer formed on the ZVI surface could have blocked the catalytic sites on ZVI and thus resulted in a decreased reaction rate. A proper investigation of the critical parameters of the water matrix can provide a rational understanding of the possible interactions of catalyst and H2O2 with organic/inorganic compounds of the water matrix and obtain insights on the potential effects on Fenton reaction rate.Reusability/recyclability of heterogeneous Fenton catalysts is an essential parameter in view of its economic implications. Multiple cycles of Photo Fenton reaction performed with ammonia modified graphene/Fe3O4 catalyst resulted in the lowering of pollutant degradation efficiency of the catalyst (Boruah et al., 2017). The first cycle showed 92.43% degradation of phenol whereas, at the 10th cycle phenol degradation reduced to 75.50%. A similar decrease in the catalytic efficiency over multiple runs is observed for almost all types of heterogeneous Fenton catalysts (Tang and Wang 2018). An environmentally friendly sustainable model of reusing/recycling the heterogeneous Fenton catalyst is a highly sought-after area of research. Recently Serr\u00e0 et al. (2020) reported an interesting zero-carbon-emission circular process resulting in water remediation and energy production. Spirulina microalgae was cultivated in wastewater which was rich in iron and heavy metals. During the photosynthetic process, spirulina utilised carbon dioxide and released oxygen to the atmosphere. This microalgal biomass was fermented and produced bioethanol. The resultant biomass was dried and burnt for thermal energy generation. The iron-rich ash obtained after combustion was utilised as heterogeneous photo Fenton catalyst under sunlight for wastewater treatment. Finally, the mineralised water was reused for cultivating the next batch of microalgae and completed the cycle. The research area of zero-carbon-emission circular processes using heterogeneous Fenton catalysts is expecting significant advances in recent years. A large scale implementation of similar techniques will effectively contribute to tackling the current global issues like excessive CO2 emission and atmospheric pollution (Keijer et al., 2019).Even though most reports on Fenton catalysts are successful in showing better results of degradation of organic contaminants and transformation of inorganic contaminants, more work and insights are needed to understand the mechanistic aspects of the reactions involved. Theoretical understanding of the underlying mechanisms of the Fenton and Fenton-like reactions by density functional theory (DFT) studies can provide new insights and information in this area of research (Buda et al., 2001). DFT studies help in understanding the role of surface defects in enhancing the catalytic activity of heterogeneous Fenton reactions. Only by the in-depth understanding of the mechanism, further improvements in the catalyst performance can be achieved. Recently, static and dynamic DFT calculations performed by Hsing-Yin Chen and co-workers on the intermediates of Fenton reaction concluded that \u2022OH is the predominant species below pH 2.2 (English for Writing Research Papers Useful Phrases, 2016). This study also reported that as the pH increases from 2.2 to 4.6, iron(IV)-oxo complex [(H2O)5FeIVO]2+ was the major complex and at pH >7.9 a deprotonated dihydroxy species, [(H2O)3FeIVO(OH)2] was the active intermediate. Nonetheless, the high-level ab initio calculations question the presence of dihydroxy species in aqueous Fenton reactions and yet there are no reports of the successful synthesis and characterisation of [(H2O)3FeIVO(OH)2] species in the literature. Likewise, it is essential to explore the electron transfer mechanism in the Fenton catalyst by quantum chemical calculations (Qin et al., 2017; Vorontsov, 2019). Later the theoretical studies should be validated with necessary experimental evidence.The lack of standardised procedures for reporting the catalytic activity is a major concern in the heterogeneous photo-Fenton process. Different groups of researchers use various ratios of concentrations of catalyst: H2O2 for performing the reactions (See the catalytic activity summarised in Table1). Therefore, a standardised procedure for comparing the efficiency of the catalyst is inevitable. In many cases, even though the authors argue about carrying out the reaction at a neutral pH, the reaction pH changes during the experiment. Usually, the pH of the reaction mixture decreases due to the formation of smaller degradation products such as oxalic acid, formic acid etc (Kuan et al., 2015). Hence, use of buffers and/or continuous monitoring of pH is necessary while performing the reaction. Another challenge that needs to be addressed is the leaching of iron species from the catalyst and deactivation of catalytic sites by the adsorption of impurities from wastewater. So, more work is needed to understand catalyst stability and longevity as well as technologies that can use the catalyst in a sustainable way.2-dimensional (2D) nanomaterials with high surface area have played a significant role in various energy and environmental applications. 2D materials such as MXene composites are less explored in the Fenton chemistry, and a great deal of attention should be paid on them to unveil their potential. Also, many of the reports on heterogeneous Fenton catalysis show higher activity in the ultraviolet region of the electromagnetic spectrum. Since the sun can be a low-cost source of energy (i.e., depending on the technology for capturing and utilizing the radiation) and because of its inexhaustible nature, researchers need to target more on the synthesis of catalysts which are visible light active and have high activity under visible light spectrum. When it comes to the broad-scale application of photo-Fenton reaction for water treatment, materials that utilize a broader spectrum of sunlight and doing so at high quantum yields will have more potential for practical applications. Nowadays, the research community is witnessing computer-aided design of various catalysts. So, researchers should come forward for applying in silico tools in designing novel catalytic materials having excellent activity for photo Fenton reaction (Poree and Schoenebeck, 2017).The toxicity of nanomaterials is itself a widely debated topic. The nanoparticles embedded in different matrices get released into the environment, and they interact with the living cells in a dynamic fashion (Hamers, 2017). Nanomaterials which are synthesised with distinct properties, may undergo chemical changes once they are released into the environment. Hence a comprehensive understanding of the interaction of nanoparticles with the living matter is necessary for the large-scale application of these catalysts and for avoiding any future health hazards. Different reports present great achievements in degrading various contaminants of emerging concern via Fenton and Fenton-like processes, but excellent outcomes in complex wastewater matrices are still rare in the literature. So, opportunities for combining heterogeneous photo Fenton process with other established wastewater treatment technologies should be explored for commercial-scale applications.Photo-Fenton reaction is proved to be a promising method for the removal of bacterial and fungal pathogens from wastewater (O\u2019Dowd and Pillai, 2020; Garc\u00eda-Fern\u00e1ndez et al., 2012). Also, there are successful reports on the removal of MS2 coliphage (bacteriophage having similar properties to that of human enteric viruses) from wastewater through photo-Fenton treatment (Nieto-Juarez et al., 2010; Ortega-G\u00f3mez et al., 2015). Nieto-Juarez and Kohn (2013) investigated the fate of MS2 coliphage upon photo-Fenton treatment with four commercially available iron oxide species (hematite, goethite, magnetite and amorphous Fe(OH)3. The study reported 99.9% virus removal by all the iron species studied via a photo-Fenton process combined with physical removal such as adsorption. A recent study reports the effectiveness of homogeneous solar photo-Fenton for inactivation of hepatitis A virus (an extremely resistant non-enveloped virus) in water (Polo et al., 2018). Since the SARS-CoV-2 is an enveloped virus, which remains more susceptible to disinfectants compared to a non-enveloped virus (Chu et al., 2019), photo-Fenton reaction offers the possibility of effectively deactivating SARS-CoV-2 virus present in wastewater. Therefore, future studies should aim at identifying suitable heterogeneous Fenton catalysts for virus removal with respect to their inactivation efficiency and absorption capacity. The research area of heterogeneous photo-Fenton has a lot of room for development, and it has remarkable potential in addressing pressing challenges in industrial-scale water treatment.Photo-Fenton treatment of wastewater is an evolving technology for removing organic, inorganic, and microbial contaminants from water. In comparison to the homogeneous Fenton reaction, heterogeneous catalysts have displayed great potential for commercial applications in view of their wide pH range of application, low sludge formation, and reusability. Along with the recent literature reports on the advances in materials for heterogeneous Fenton reaction, schematic illustrations in this review provide a basic understanding of the electron transfer mechanisms and the formation of reactive oxygen species in Fenton reactions. The excellent magnetic properties of materials such as ferrites, magnetite, and their composites have enabled significant developments, due to easy separation and reusability of such materials after wastewater treatment. The incorporation of plasmonic materials with iron minerals to broaden their visible light absorption capacity is highlighted as a growing area of research. Various supporting materials used in the heterogeneous Fenton catalytic systems have a specific role in altering the catalytic activity of the system. Incorporation of semiconductor nanoparticles and carbon-based two-dimensional materials with iron minerals to speed-up the electron transfer and Fe(II) regeneration has shown promise and is expected to receive more attention in the coming years. The higher specific surface area provided by graphene-related supporting materials leads to enhanced adsorption of pollutants on the Fenton catalytic surface. Pillaring process of clay materials exposes the active site of the catalyst and the specific hydrophilic nature of LDHs assists the interaction of hydrophilic contaminants with LDHs. Literature review on studies on the application of Fenton, Fenton-like, and photo-Fenton technologies show these processes play important role in the inactivation of pathogenic microorganisms and destruction of contaminants of emerging concern in various types of wastewaters. Along these lines, there is potential for further enhancement of process performance when introducing catalysts of good selectivity, synergistic action, and versatility for the treatment of various types of source waters/wastewaters polluted with a variety of pathogenic microorganisms, organic and inorganic contaminants of concern. Fe-contained perovskites are seeing growing interest in Fenton-like processes, especially on aspects related to innovative strategies to dope or modify perovskites materials to achieve higher catalytic activity and better process performance. New developments on the synthesis and applications of MOFs and zeolite materials, especially for water treatment, demonstrate there have been important advances in recent years and point towards further progress, especially on topics focused on tailor-designed framework structures with improved functionality and higher activity for more efficient Fenton-like applications. Zero-valent iron-based technologies have seen huge interest in both mechanistic studies and field applications for the treatment of numerous pollutants due to the environmental compatibility of iron and versatility of the process towards oxidation or reduction reactions. It is expected that more applications will be seen in the future, especially if challenges on n-ZVI dispersibility, longevity, and reactivity control are addressed. The materials which can in-situ produce the H2O2 by a two-electron transfer to oxygen and the techniques that improve the H2O2 utilisation by reducing its self-decomposition can considerably decrease the operating cost of the reaction and pave the way for commercial-scale applications of Fenton reaction. Therefore, a sea of opportunities is wide open in this area for the cost optimisation of the existing technology and to develop brand new materials with extraordinary catalytic efficiency.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.\n\n\nImage 1\n This project has received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement number 820718, and is jointly funded by the European Commission and the Department of Science Technology of India (DST). Dionysiou also acknowledges support from the University of Cincinnati through the Herman Schneider Professorship in the College of Engineering and Applied Sciences.", "descript": "\n                  Heterogeneous Fenton catalysts are emerging as excellent materials for applications related to water purification. In this review, recent trends in the synthesis and application of heterogeneous Fenton catalysts for the abatement of organic pollutants and disinfection of microorganisms are discussed. It is noted that as the complexity of cell wall increases, the resistance level towards various disinfectants increases and it requires either harsh conditions or longer exposure time for the complete disinfection. In case of viruses, enveloped viruses (e.g. SARS-CoV-2) are found to be more susceptible to disinfectants than the non-enveloped viruses. The introduction of plasmonic materials with the Fenton catalysts broadens the visible light absorption efficiency of the hybrid material, and incorporation of semiconductor material improves the rate of regeneration of Fe(II) from Fe(III). A special emphasis is given to the use of Fenton catalysts for antibacterial applications. Composite materials of magnetite and ferrites remain a champion in this area because of their easy separation and reuse, owing to their magnetic properties. Iron minerals supported on clay materials, perovskites, carbon materials, zeolites and metal-organic frameworks (MOFs) dramatically increase the catalytic degradation rate of contaminants by providing high surface area, good mechanical stability, and improved electron transfer. Moreover, insights to the zero-valent iron and its capacity to remove a wide range of organic pollutants, heavy metals and bacterial contamination are also discussed. Real world applications and the role of natural organic matter are summarised. Parameter optimisation (e.g. light source, dosage of catalyst, concentration of H2O2 etc.), sustainable models for the reusability or recyclability of the catalyst and the theoretical understanding and mechanistic aspects of the photo-Fenton process are also explained. Additionally, this review summarises the opportunities and future directions of research in the heterogeneous Fenton catalysis.\n               "}
{"full_text": "The water electrolysis process occurs through two simultaneous half-cell reactions: the oxygen evolution reaction (OER) on the anode and the hydrogen evolution reaction (HER) on the cathode. The Alkaline OER is a 4-electron\u2013proton transfer process that makes the reaction sluggish with high overpotential and complex reaction mechanisms [1,2]. Nickel (Ni)-based compounds including Ni-based oxides and (oxy)hydroxides are among the most efficient precious-metal-free catalysts for alkaline OER due to their desirable advantages such as enhanced reaction kinetics and structure/performance stability [3]. Relationships between metallic Ni and various O-containing surface compounds formed during anodic oxidation of polycrystalline Ni in aqueous alkaline media can be described by the Bode diagram (Fig. 1\n) [4]. Mild anodic polarization of metallic Ni results in the reversible formation of \u03b1-Ni(OH)2; moderate anodic polarization results in the irreversible conversion of \u03b1-Ni(OH)2 into \u03b2-Ni(OH)2 as well as in the direct oxidation of Ni to \u03b2-Ni(OH)2; and, this process is accompanied by the development of NiO that is sandwiched between Ni and \u03b2-Ni(OH)2 (marked as a NiO sandwich in Fig. 2\n). The purple lines and the formation of \u03b3-NiOOH were suggested by Bode [5]. The \u03b3-NiOOH phase is believed to be the highest-achievable Ni oxidation state [6]. It is most commonly assumed that the \u03b2-NiOOH oxidation phase is most active towards the OER [7].So far, many research efforts have focussed on improving the OER performance of Ni by the design and optimization of the catalyst structure [6,8,9].Sonoelectrochemistry is the combination of ultrasound with electrochemistry. The use of ultrasound in electrochemistry offers many advantages including [10]: a) gas bubble removal at the electrode surface; b) solution degassing; c) disruption of the Nernst diffusion layer; d) enhancement of mass transport of electroactive specious through the double layer; and, e) activation and cleaning of the electrode surface. Recently, it was reported that ultrasonication greatly enhances the electrocatalytic properties of metallic surfaces [11\u201316]. Our group also investigated the effect of ultrasound on Ni(poly) in alkaline media and found that the rate of the HER was greatly enhanced.In this work, we investigated the effects of ultrasound (24\u00a0kHz) on the OER on polycrystalline Ni immersed in 1.0\u00a0M aqueous KOH solution at room temperature. We applied ultrasound (i) during linear sweep voltammetry (LSV) experiments and (ii) for surface treatment of the Ni(poly) electrode for 30\u00a0min and then we conducted the LSV experiments under silent conditions (in the absence of ultrasound).All electrochemical experiments were carried out using a potentiostat/galvanostat (BioLogic-SP 150) in a three-electrode configuration. The voltammetry experiments were performed using a double-jacketed sonoelectrochemical cell. Ultrasonication was applied by a f\u00a0=\u00a024\u00a0kHz ultrasonic probe (Hielscher UP200S, 200\u00a0W @ 60% fixed amplitude, the tip \u00d8 = 14\u00a0mm, and the tip area\u00a0=\u00a0153.9\u00a0mm2 (1.5386\u00a0cm2). The ultrasonic or acoustic power (P\nacous) was found to be 44\u00a0\u00b1\u00a01.40\u00a0W by calorimetrically using the methods of Margulis et al.\n[17] and Contamine et al. [18]. In order to keep the temperature at T\u00a0=\u00a0298\u00a0\u00b1\u00a01\u00a0K a refrigerated circulator (JULABO, Germany) was connected to the sonoelectrochemical cell.A polycrystalline nickel Ni(poly) disc (\u00d8 = 5\u00a0mm) of geometric surface area (A\ngeom) of 0.196\u00a0cm2 was used as a working electrode (WE). The WE was mechanically polished using alumina suspension (down to 0.05\u00a0\u03bcm, Buehler Micro polish) to obtain a mirror-like surface rinsed with UHP water, ultrasonicated in UHP water for \u223c30\u00a0s and finally rinsed in UHP water under ultrasonic conditions. The reference electrode (RE) was a homemade reversible hydrogen electrode (RHE) [19]. All potential values in this work are reported with respect to the RHE. The counter electrode (CE) was a Ni mesh (40 mesh woven from 0.13\u00a0mm diameter wire, 99.99% metal basis, Alfa Aesar, Germany) in a rectangle shape (20.67\u00a0\u00d7\u00a010.76\u00a0mm2). Its surface area was at least 10 times larger than that of the WE. The distance between the ultrasonic probe and the working electrode was ca. 3\u00a0cm. The experiments were carried out in N2 (g) (99.999%) saturated 1.00\u00a0M (pH\u00a0=\u00a013.7) aqueous KOH (Sigma-Aldrich, 99.99% in purity) solution prepared using ultra-high-purity water (Millipore, 18.2 M\u03a9 cm in resistivity).The performance of Ni(poly) towards the OER in aqueous alkaline electrolytes was investigated by a series of linear sweep voltammetry (LSV) in the potential region of\u00a0+\u00a01.10\u00a0\u2264\u00a0E\napp \u2264 +1.70\u00a0V vs. RHE at the potential scan rate of \u03bd\u00a0=\u00a00.30\u00a0mV\u00a0s\u22121 in 1.0\u00a0M KOH aqueous solutions in the absence of ultrasound (silent conditions), during (with) ultrasound and after 30\u00a0min ultrasound.The potential values from linear sweep voltammetry (LSV) experiments were IR corrected using the following equation (1):\n\n(1)\n\n\n\nE\n\nIR\n-\nc\no\nr\nr\ne\nc\nt\ne\nd\n\n\n=\n\nE\n\napp\n\n\n-\n-\nI\nR\n\n\n\nwhere I is the measured current and R is the electrolyte resistance, measured for each electrolyte employed. The R value was determined by electrochemical impedance spectroscopy (EIS) in the high-frequency region from the value of the real impedance (Z\u2019\n) where the imaginary impedance (Z\n\u2019\u2019) is zero in the Nyquist plot. The EIS experiments were carried out in the 100\u00a0kHz to 0.1\u00a0Hz frequency (f) range with a voltage perturbation of\u00a0+\u00a010\u00a0mV at an applied potential of +1.60\u00a0V vs. RHE at T\u00a0=\u00a0298\u00a0K.The surface structure and morphology of the Ni(poly) electrodes before and after ultrasound treatment were studied using a scanning electron microscope (SEM) Zeiss-Ultra 55-FEG-SEM operating at 10\u00a0kV accelerating voltage.In order to study the effects of power ultrasound on the electrochemical surface area of Ni(poly), the \u201ccapacitance\u201d and \u201c\u03b2-NiOOH\u201d methods were used. The \u201ccapacitance\u201d method consists of cycling the Ni electrodes at different scan rates in a non-faradic charging process to determine the electrochemical surface area (A\necsa) [20]. A series of cyclic voltammograms (CVs) on Ni(poly) in 1.0\u00a0M KOH were generated at different scan rates (5, 10, 20, 50, 100, 200, 300, 400\u00a0mV\u00a0s\u22121) in the potential region of\u00a0+\u00a00.80\u00a0V vs. RHE to\u00a0+\u00a00.90\u00a0V vs. RHE. The double-layer capacitance value (C\ndl) was obtained by plotting the charging current (I\nc, A) vs. scan rate (\u03bd, V s\u22121) and by using equation (2):\n\n(2)\n\n\nSlope\n=\n\nC\n\ndl\n\n\n=\n\n\n\u0394\n\nI\nc\n\n\n\n\u0394\nv\n\n\n\n\n\n\nThe electrochemical surface area was calculated by using the specific capacitance density (c) of 40 \u03bcF cm\u22122 and equation (3)\n[20,21].\n\n(3)\n\n\n\nA\n\necsa\n\n\n=\n\n\nC\n\ndl\n\n\nC\n\n\n\n\n\n\nFig. 2a and 2b show the CVs of the Ni(poly) electrode before and after 30\u00a0min of ultrasonication at different scan rates (5, 10, 20, 50, 100, 200, 300, and 400\u00a0mV\u00a0s\u22121) in the potential range of\u00a0+\u00a00.80\u00a0\u2264\u00a0E\napp \u2264 +0.90\u00a0V vs. RHE where non-faradic currents occur. Fig. 2c shows plots of current vs. scan rate at a potential of\u00a0+\u00a00.85\u00a0V vs. RHE before and after 30\u00a0mins of ultrasonic exposure.The \u201c\u03b2-NiOOH\u201d method consisted of integrating the \u03b2-NiOOH reduction peak once steady-state polarization was reached at a high scan rate. The \u03b2-NiOOH method was carried out by running 10 CV cycles from\u00a0+\u00a00.50\u00a0\u2264\u00a0E\napp \u2264 +1.60\u00a0V vs. RHE at a scan rate of \u03bd\u00a0=\u00a0100\u00a0mV\u00a0s\u22121 before and after 30\u00a0min US (Fig. 2d). The A\necsa values for this method were calculated using the \u03b2-NiOOH reduction peak of the 10th cycle (from 1.2 to 1.4\u00a0V vs. RHE) divided by the specific charge density of 420 \u03bcC cm\u22122 (equation (4)) [20].\n\n(4)\n\n\n\n\nA\n\n\ne\nc\ns\na\n\n\n=\n\n\nQ\n\n\n420\n\n\n\n\n\nwhere Q is the charge associated with the \u03b2-NiOOH reduction peak. The A\necsa values before and after 30\u00a0min of ultrasonication treatment for both capacitance and beta methods are summarised in Table 1\n. It needs to be mentioned that the difference between the A\necsa values from the \u201ccapacitance\u201d and the \u201c\u03b2-NiOOH\u201d methods is related to the basis of measurements of both methods. The capacitance method is related to conductivity and homogeneity of surface for double layer charging while the beta method is related to the faradaic reaction of nickel hydroxide to nickel oxyhydroxide transformation [20]. It can be observed from Table 1 that ultrasound does not seem to affect the electrochemical surface area of the Ni(poly) electrode, indicating that the electrochemical surface area was not significantly modified due to erosion caused by the implosion of acoustic cavitation bubbles on the electrode surface [22]. Fig. 2e and 2f show the SEM images of Ni(poly) before and after 30\u00a0min US. Before US a smooth surface is seen except the scratches due to mechanical polishing. After 30\u00a0min US some irregular pits could be observed, however, it is unclear whether these arose from the actions of inter-facial ultrasound. Such features are sometimes found widely scattered across non-sonicated surfaces (see, for example, some pits in non-sonicated electrode Fig. 2e). The pit areas in both non-sonicated and sonicated electrodes have been marked red in Fig. 2e and 2f. These pits have little influence on electrochemical measurements because there are very few of them and their contribution to total A\necsa is relatively small. Aqueous ultrasonication did not significantly roughen the electrode and the surface roughness remained almost unchanged [23].The effect of ultrasound on the oxygen evolution reaction (OER) at Ni(poly) in 1.0\u00a0M aqueous KOH solution was investigated by linear sweep voltammetry (LSV). Fig. 3\na shows the LSVs for the OER on Ni(poly) in N2 saturated 1.0\u00a0M KOH aqueous solutions at a low scan rate of \u03bd\u00a0=\u00a00.3\u00a0mV\u00a0s\u22121 before, during (with) and after 30\u00a0min ultrasonic treatment. It can be observed that the ultrasonic (US) treatment increases the OER activity. Fig. 3b demonstrates the Tafel plots obtained from the LSV curves in the OER region. Tafel slopes (b) at low and high overpotentials and the potential at\u00a0+\u00a010\u00a0mA\u00a0cm\u22122 (E\n+10 mA cm\n-2) are tabulated in Table 2\n. Results from Table 2 indicate that lower potential requires to reach\u00a0+\u00a010\u00a0mA\u00a0cm\u22122 in presence of ultrasound and after ultrasonic treatment. However, even when ultrasound is \u201con\u201d during the OER experiments, the lower overpotential at\u00a0+\u00a010\u00a0mA\u00a0cm\u22122 is required when compared to after ultrasonic treatment.Ni-based materials show the Tafel slope values between 40\u00a0mV dec-1 to 130\u00a0mV dec-1. Also, it is well known that there are generally two Tafel regions for the OER, separated at\u00a0\u223c\u00a01.5\u00a0V vs. RHE in 1.0\u00a0M KOH [6,7]. According to Table 2, the Tafel slopes of 52, 55, 50\u00a0mV dec-1 at low overpotentials and 141, 90 and 130\u00a0mV dec-1 at high overpotentials were obtained for the OER on Ni(poly) before ultrasonication (US), with US and after 30\u00a0min US, respectively. The Tafel slopes are in good agreement with the literature [7,24,25]. By comparing the Tafel slopes under different US conditions reported in Table 2, it can be concluded that ultrasound does not change the Tafel slopes significantly for the OER and does not affect the mechanism of OER. It is worth mentioning that the experiments have been repeated several times and almost the same values have been obtained showing the reproducibility of the work.\nFig. 3c illustrates the plot of E at\u00a0+\u00a010\u00a0mA\u00a0cm\u22122 (E+\n\n10 mA cm\n-2) vs. different ultrasonic conditions. It can be seen in Fig. 3c that the overpotential to reach\u00a0+\u00a010\u00a0mA\u00a0cm\u22122 decreases when US is \u201con\u201d during the OER experiment.\nFig. 3d shows the Nyquist representation of the impedance data of Ni(poly) before US, with US and after 30\u00a0min US at T\u00a0=\u00a0298\u00a0K and E = +1.60\u00a0V vs. RHE. For all US conditions, a depressed semi-circle can be seen. Accordingly, the data were approximated by the modified Randles circuit shown in Fig. 3d, whereas the capacitance is replaced by a constant phase element. Note, for \u03b1\u00a0=\u00a01 the CPE reflects an ideal capacitance. R\ns correlates with the cell ohmic resistance (electrodes). R\nct represents the charge transfer resistance and may also include other contributions such as the adsorption of intermediates. CPE is a constant phase element that is often associated with the capacitive charging of a rough electrode. The parameters obtained from the EIS measurement are shown in Table 3\n. According to Table 3, the Ni(poly) electrode after 30\u00a0min US treatment has the lowest charge transfer resistance compared to the two other conditions. While the R\ns are almost constant in all US conditions. Since no significant increase in the electrochemical surface has been observed on Ni(poly) by applying US, the enhancement of OER activity of Ni(poly) after ultrasonication treatment could be due to the reaction of radicals at the electrode/electrolyte interface such as (OH\u2022, H\u2022, H2O2, etc) caused by collapsing cavitation bubbles. It was reported before that such radicals could react with the electrolyte species and produce a secondary sonochemical reaction [15,16,26,27].We have developed a simple in-situ method to activate Ni(poly) electrodes in 1.0\u00a0M aqueous KOH solution towards the OER by ultrasonic treatment (24\u00a0kHz, 60% amplitude, 44\u00a0W) for 30\u00a0min. It was shown that ultrasound improves Ni(poly) OER activity by reducing the overpotential needed to achieve\u00a0+\u00a010\u00a0mA\u00a0cm\u22122 by \u201323\u00a0mV and charge transfer resistance from 98.5\u00a0\u03a9 before US to 11.1\u00a0\u03a9 after 30\u00a0min US treatment. However, the US treatment does not affect the electrochemical surface area of Ni(poly) or Tafel slope. The enhancement of OER activity of Ni(poly) could be attributed to the formation of free radicals by collapsing cavitation bubbles and the secondary sonochemical reactions at the electrode/electrolyte interface. However, understanding the exact reason and the mechanism will still need a wide range of experiments and spectroscopy measurements.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 NTNU and ENERSENSE for the 3-year financial support for FF\u2019s doctoral studies. FF would like to thank Professors Gregory Jerkiewicz and Christophe Coutanceau for their useful advice.", "descript": "\n                  The development of cost-effective and active water-splitting electrocatalysts is an essential step toward the realization of sustainable energy. Its success requires an intensive improvement in the kinetics of the anodic half-reaction of the oxygen evolution reaction (OER), which determines the overall system efficiency to a large extent. In this work, we designed a facile and one-route strategy to activate the surface of metallic nickel (Ni) for the OER in alkaline media by ultrasound (24\u00a0kHz, 44\u00a0W, 60% acoustic amplitude, ultrasonic horn). Sonoactivated Ni showed enhanced OER activity with a much lower potential at\u00a0+\u00a010\u00a0mA\u00a0cm\u22122 of\u00a0+\u00a01.594\u00a0V vs. RHE after 30\u00a0min ultrasonic treatment compared to\u00a0+\u00a01.617\u00a0V vs. RHE before ultrasonication. In addition, lower charge transfer resistance of 11.1\u00a0\u03a9 was observed for sonoactivated Ni as compared to 98.5\u00a0\u03a9 for non-sonoactivated Ni. In our conditions, ultrasound did not greatly affect the electrochemical surface area (A\n                     ecsa) and Tafel slopes however, the enhancement of OER activity can be due to the formation of free OH\u2022 radicals resulting from cavitation bubbles collapsing at the electrode/electrolyte interface.\n               "}
{"full_text": "The fossil-based energy system and the dependence on fossil fuels have exacerbated climate change, resulting in an environmental crisis [1]. A change in the fuel production and consumption strategies become necessary in order to reduce the greenhouse gases and other emissions responsible for the global warming. Any alternative to reduce the dependence on petroleum should address the production of energy and chemicals from renewable feedstocks, such as biomass [2]. Biomass has the potential to decrease net emissions of carbon since the used raw materials grow removing carbon dioxide from the atmosphere by photosynthesis [3].Hydrogen production from biomass, in addition to reducing greenhouse gas emissions, would contribute to the expansion and economic viability of the biorefinery [3]. Hydrogen has been globally accepted as an environmentally friendly fuel, since huge energy is contained in the H-H bond and its combustion only releases water to the environment [4]. Moreover, hydrogen can be used in several technologies such as fuel cells and internal combustion engines or turbines [5,6]. Hydrogen can be produced effectively from biomass through a sort of processes [7\u201310], among which is the aqueous-phase reforming [11].Aqueous-phase reforming (APR), which can be driven at relatively mild conditions, is able to manage diluted aqueous wastes of different oxygenated hydrocarbons to obtain valuable products (either hydrogen or other value-added chemicals) [12]. APR process was first introduced in 2002 by Dumesic and co-workers [13], and since then, had attracted a considerable R&D activities. In APR, reactants remain in liquid phase, unlike steam reforming (SR), what avoids an energetically demanding vaporization-step. Furthermore, low reaction temperatures shift the Water-Gas Shift (WGS) equilibrium towards further formation of hydrogen with the consequent reduction of carbon monoxide content [14].Glycerol, a major by-product of the biodiesel production process, is one of the 12 platform molecules for biorefineries proposed by the US Department of Energy [15]. A huge surplus of glycerol has been generated in the last years, thus, its valorisation represents a challenge for the biodiesel plants profitability [16]. Aqueous-phase reforming of glycerol comprises the decomposition (Eq. 1) and the Water-Gas Shift (WGS) (Eq. 2) steps:\n\n(1)\n\n\n\n\nC\n\n\n3\n\n\n\n\nH\n\n\n8\n\n\n\n\nO\n\n\n3\n\n\n\u2192\n4\n\n\nH\n\n\n2\n\n\n+\n3\nC\nO\n\n\n\n\n\n\n(2)\n\n\nC\nO\n+\n\n\nH\n\n\n2\n\n\nO\n\u2194\n\n\nH\n\n\n2\n\n\n+\nC\n\n\nO\n\n\n2\n\n\n\n\n\n\nThe overall reaction stoichiometry for the ideal APR of glycerol is given by reaction 3:\n\n(3)\n\n\n\n\nC\n\n\n3\n\n\n\n\nH\n\n\n8\n\n\n\n\nO\n\n\n3\n\n\n+\n3\n\n\nH\n\n\n2\n\n\nO\n\u2192\n7\n\n\nH\n\n\n2\n\n\n+\n3\nC\n\n\nO\n\n\n2\n\n\n\n\n\n\nAs an immature technology, this process requires constant investigation for active and stable catalytic materials and optimization of operating conditions to improve current results to the point of being profitable for the industry [17].Numerous works have focused on noble metal based catalysts for APR, especially Pt and Re, due to their high efficiency for C\u2013C, O\u2013H and C\u2013H bonds cleavage and WGS reaction [17\u201324]. Thanks to its higher availability and economy, Ni-based systems have also been widely studied as an alternative to those upscale metals [25\u201329]. Cobalt is another transition metal that has attracted attention for this type of process [30\u201332]. Nevertheless, leaching is a large drawback for transition metals. Both catalytic systems, based on precious and transition metals, present a certain deactivation, mainly due to hydrothermal instabilities [33]. Among the strategies considered, bimetallic catalysts upgrade glycerol conversion and gaseous products and improve stability [30,34\u201337]. For instance, bimetallic Pt-Co supported on multi-walled carbon nanotubes increases the glycerol reforming activity of the monometallic catalyst by 4, and the WGS activity by 32 [30].APR process is clearly impacted by operating variables as a substrate concentration, temperature and system pressure, and contact time, among others [38,39]. Several authors have optimized temperature and pressure conditions to enhance gaseous products [19,40\u201343], a few others have conducted research that address other parameters such as feed concentration, mass of catalyst/ reagent mass flow rate ratio, reaction time and feed flow rate [38,39,44]. The reported results, however, become contradictory since they depend on the interrelation with other variables and the reaction system [39]. Moreover, most of the literature is focused on catalyst performance with respect to either gas o liquid phase product distribution.The present work aims to investigate the effect that operating conditions exert on the product distribution during the APR of glycerol over a 0.3PtCoAl catalyst, in order to maximize the hydrogen production by APR. This catalyst, synthesized by impregnating Pt on cobalt aluminate support, has been previously tested in long-term reactions (100\u2009h TOS), proving to be efficient for H2 production and stable [45]. Due to its promising performance, in this work this optimized catalyst formulation was used as a benchmark to search the suitable reaction conditions for hydrogen production. For this purpose, the most handled process variables, such as glycerol concentration in the feedstream, coupled temperature/pressure and contact time, were investigated. The catalytic performance was evaluated based on the most commonly applied reaction indices, and a comprehensive analysis of both gaseous and liquid products is presented. In addition, exhausted catalyst was also characterized to gain knowledge in the main deactivation causes.Bimetallic 0.3Pt/CoAl catalyst was synthetized in two steps. First, cobalt aluminate with a nominal Co/Al mole ratio of 0.625, was synthesized by coprecipitation. An aqueous solution containing appropriate amounts of Co and Al precursors (10.3\u2009g of Co(NO3)2\u00b76H2O and 21.2\u2009g of Al(NO3)3\u00b79H2O) was added dropwise to a vigorously stirred solution containing sodium carbonate while pH was adjusted to 10 with NaOH solution (2\u2009M). The resulting slurry was aged for 24\u2009h at room temperature, filtered, washed several times with de-ionized water and dried in an oven at 110 \u00b0C overnight. The cobalt aluminate spinel was formed by calcination at 500 \u00b0C (heating rate 5 \u00b0C/min) for 5\u2009h in a static air atmosphere. Thereafter, Pt was impregnated (nominal loading 0.3\u2009wt%) using aqueous solution of tetraammineplatinum(II) nitrate as precursor, in the solution/support proportion of 1.5/1 (vol./vol.). After impregnation, the sodden solid was dried in an oven at 110 \u00b0C for 17\u2009h and finally, calcined at 350 \u00b0C (heating rate 5 \u00b0C/min) for 5\u2009h.The bulk composition of the catalyst was evaluated by ICP-AES. The specific surface area and the main pore size were estimated by the BET and BJH methods, respectively. The measurement was performed using nitrogen at 77\u2009K as an adsorbent gas (Tristar II 3020). Prior to the physisorption measurement, the sample was outgassed at 300 \u00b0C for 10\u2009h in order to clean the solid surface.XRD diffraction patterns of the calcined, reduced and spent catalyst were obtained on a PANalytical X\u00b4pert PRO diffractometer (CuK\u03b1 radiation, \u03bb\u2009=\u20091.5406\u2009\u00c5, graphite monochromator)), with a step size of 0.026\u00b0 (2\u03b8) and a counting time of 2\u2009s. The crystallite average size was calculated by Scherrer equation from the peak broadening and the identification of the crystal phases was carried out on the basis of ICDD database.\n27Al Solid State NMR measurements at 104.26\u2009MHz for 27Al were performed (9.4\u2009T Bruker AVANCE III 400 spectrometer). Chemical shifts were referenced externally to the AlCl3 aqueous solution at 0\u2009ppm. The spectra were acquired at a spinning frequency of 60\u2009kHz employing a PH MASDVT400W BL 1.3\u2009mm ultrafast probe head.The XPS analyses were performed on a SPECS spectrometer with Phoibos 150 1DDLD analyzer and a monochromatic X-ray beam Al K target (1486.7\u2009eV). The electron energy analyzer was operated at pass energy of 30\u2009eV and step size of 0.08\u2009eV. The C 1\u2009s photoelectron line (BE\u00a0=\u00a0284.8\u2009eV) was used to calibrate the binding energies of the photoelectron. The catalyst was analyzed either in calcined and reduced form. The reduction of the catalyst was carried out in-situ at 600\u2009\u00b0C with 20% H2/Ar flow, for 1\u2009h.Temperature programmed reduction of the fresh calcined (H2-TPR) catalysts was carried out in a Micromeritics AutoChem 2920 apparatus, equipped with a thermal conductivity detector (TCD). About 50\u2009mg of sample was initially heated in He stream at 550\u2009\u00b0C for 1\u2009h (heating rate 10\u2009\u00b0C/min). Then, sample was cooled down to room temperature into Ar flow, and switched to 5% H2/Ar flow while temperature was ramped to 950 \u00b0C at 10 \u00b0C/min, and hold for 1\u2009h.Temperature programmed hydrogenation (TPH) was conducted on the spent catalyst is order to analyse carbonaceous deposits. Sample was first heated at 550\u2009\u00b0C for 1\u2009h, under a He flow, and cooled down to ambient temperature. Then, a flow of 5% H2/Ar was passed through the sample heated at 10\u2009\u00b0C/min up to 950 \u00b0C and m/z\u2009=\u200915 (CH4) signal was recorded with mass spectrometer (Pffeifer Vacuum OmniStar).The amount of surface Pt and Co sites were evaluated by H2 pulse chemisorptions (5% H2/Ar, loop volume 0.5312\u2009mL) at 40 \u00b0C (Micromeritics AutoChem 2920 equipment). Initially, catalyst surface was cleaned by passing a He flow at 500 \u00b0C. First, H2 pulse was applied on sample reduced at 250 \u00b0C (to titrate the metallic Pt). Thereafter, sample was further reduced at 600 \u00b0C, and subsequent pulse chemisorption was completed (to titrate the total metallic sites). H/Me (Me=Pt, Co) stoichiometry of 1/1 was assumed. The exposed metallic area of Pt and Co (SPt\no and SCo\no) was calculated assuming 0.084\u2009nm2 and 0.0662\u2009nm2 per Pt and Co sites, respectively. The average Pt size was calculated by formula dPt\no (nm) =\u20096000/(\u03c1\u00b7SPt\no) [46].The surface acid and base properties of the reduced solid were evaluated by temperature programmed desorption (TPD) of NH3 and CO2, respectively, conducted in a Micromeritics AutoChem 2920 equipment coupled to Mass Spectrometer (MKS Cirrus). Previously, sample was cleaned by passing a He flow at 550\u2009\u00b0C for 1\u2009h and cooled down to room temperature. Then, the solid was reduced at 600 \u00b0C in 5% H2/Ar flow (heating rate 10 \u00b0C/min), hold for 2\u2009h and cooled down in He flow to 90 \u00b0C. Then, a series of 10% NH3/He or 5% CO2/He pulses were introduced at 90 \u00b0C. Subsequently, the reversibly adsorbed NH3 or CO2 was evacuated by flowing He for 60\u2009min. Finally, the temperature was ramped to 950\u2009\u00b0C at a heating rate of 5\u2009\u00b0C/min, and the signals m/z\u2009=\u200917 (NH3) and 44 (CO2) were monitorized (MS Pffeifer Vacuum OmniStar). The total amount of acid and basic sites was calculated from the integration of pulse areas, whereas the strength was evaluated from the corresponding TPD curve. The model reaction of skeletal isomerization of 33DM1B (3,3-dimethyl-1-butene) was used to characterize the Br\u00f8nsted acid sites. The catalyst (ca. 100\u2009mg) was in-situ reduced, and cooled down to the reaction temperature (300\u2009\u00b0C) under inert gas-flow. The 33DM1B partial pressure and flow rate were set at 20 kPa and 15.2\u2009mmol/h, respectively. The obtained products were online analysed by GC (column RTx-1, Restek) coupled to a flame ionization detector. The percentage of leached metal was measured by means of ICP-MS analysis of the resulting liquid aliquot.The APR activity tests were carried out in a \ufb01xed-bed up-flow reactor (Microactivity Effi, PID Eng&Tech). The catalyst (particle size between 40 and 160\u2009\u00b5m) was placed on a stainless steel frit, covered with a quartz wool plug, and in-situ reduced under 10% H2/He flow at 600 \u00b0C for 2\u2009h (heating rate 5 \u00b0C/min) at atmospheric pressure. The reactor pressure was regulated by He flow. When the desired pressure was reached, the He flow was switched to bypass and the liquid feedstream pumped into the reactor (Eldex optos 5985-1LMP pump) while the temperature was raised at 5 \u00b0C/min up to the reaction temperature. From the Weisz-Prater and Mears criteria it was confirmed that both external and intraparticle mass transfer effects were negligible in our experiments (Table S1, Supporting Information).The product stream was cooled down to 5 \u00b0C in a Peltier device around gas-liquid separator. The gas stream was on-line analysed by GC (\u00b5GC Agilent, 4 parallel columns MS5A, PPQ, Al2O3-KCl). The gaseous products were quantified by external calibration. The liquid phase product stream was periodically sampled and analysed by either off-line GC-FID (Agilent 6890\u2009N, HP-Wax bonded PEG column) or HPLC-RI (Waters 616, Hi-Plex H column). The liquid products identified were acetaldehyde (MeCHO), acetic acid (AcOH), acetone (ACTN), ethanol (EtOH), methanol (MeOH), ethylene glycol (EG), 1,2-propylene glycol (PG), hydroxyacetone (HA), propanal (EtCHO), propanoic acid (PA), 1-propanol (1-PrOH) and 2-propanol (2-PrOH). Pure reference compounds were used for quantification. The total organic carbon (TOC) was measured off-line on a Shimadzu TOC-L apparatus. The carbon balance was above 90% for all the experiments.The catalytic performance was evaluated based on parameters summarized in \nTable 1.The main physico-chemical properties of the 0.3Pt/CoAl catalyst are given in \nTable 2. Both the actual platinum loading and the Co/Al atom ratio were close to the nominal values. Regarding textural characteristics, both the calcined and reduced forms of the solid showed mesoporous nature (Fig. S1, Supporting Information) with isotherms of type IV and H1 hysteresis, both having unimodal pore size distribution. The textural properties of the catalyst barely varied upon reduction (i.e. SBET: 10.3% decrease; dpore: 13.5% increase). The former feature was due to the inherently lower surface area of the metallic Co and Pt, while the latter feature suggested that Pt was mostly deposited into the smallest pores.The 27Al NMR analysis (Fig. S2, Supporting Information) of the support Co/Al exhibited only two peaks at 6.9 and 71.8\u2009ppm, corresponding to aluminium ions in octahedral and tetrahedral symmetry, respectively [47]. In the bare support, strongly prevails the octahedral symmetry (Aloctahedral/Altetrahedral=96/4). After Pt impregnation, a resonance peak around 33\u2009ppm emerged, indicating the presence of penta-coordinated aluminium. This peaks represented about 8% of the total area. Concomitantly, the relative amount of octahedral aluminium decreased to 84%. These findings suggested that Pt ensembles anchored on octahedral sites.The oxidation state and concentration of surface elements of the sample were surveyed by XPS. The Co 2p spectrum of the calcined catalyst (\nFig. 1a) presented the characteristic pattern of cobalt oxide, with the Co 2p3/2 peak at 781.1\u2009eV and a strong shake up feature at 785.1\u2009eV. The 2p3/2-2p1/2 line separation is 15.6\u2009eV. These remarks virtually exclude the presence of Co3+ ions. It is worth pointing out that during XPS analysis the beam emitted may partially reduce the cobalt oxide species. Reduction of the sample by hydrogen at 600 \u00b0C gives rise to additional Co 2p3/2 feature at 778.2\u2009eV (Fig. 1b) that could be unambiguously assigned to metallic cobalt [48]. Detailed XPS spectra from Pt 4d and Al 2p levels for calcined and reduced samples are shown in Fig. 1c\u2013d. The Pt 4d5/2 spectra exhibited binding energy (BE) values of 316.8\u2009\u00b1\u20090.3\u2009eV for the calcined solid and shifted to 314.0\u2009\u00b1\u20090.3\u2009eV for the reduced solid. According to literature, the latter denotes the presence of fully reduced metallic Pt at the catalyst surface [49]. The Al 2p peak was measured at 74.2\u2009eV for both forms of the solid, either calcined and reduced, indicating that octahedral sites of Al3+ cations were dominant [50].XRD patterns of fresh and reduced solids are displayed in \nFig. 2a. The calcined form of the solid showed diffraction peaks consistent with both the standard cobalt oxide (PDF 00-042-1467) and cobalt aluminate (PDF 00-044-0160) spinel structure, in agreement with the support composition. In the reduced form of the solid, additional peaks, characteristic of Co0 in both hcp and fcc phases could be observed. The measured mean crystallite size of the spinel and metallic cobalt were 6.3 and 6.9\u2009nm, respectively (Table 2). The absence of reflections attributable to platinum phases (neither in the calcined nor the reduced forms) suggested that the size domains were below conventional XRD detection limit, and could be ascribed to the low loading and high dispersion of Pt.The H2-TPR profile of the fresh calcined solid (TPRa) is shown in Fig. 2b, and exhibits four reduction peaks. The peak assignation was done according to [45]. The low temperature peak (at 192 \u00b0C) was ascribed to the concomitant reduction of PtOX species and free surface Co3+ to Co2+ species promoted by hydrogen spillover over Pt0. The peak at 331 \u00b0C was ascribed to the reduction of Co3+ species in close interaction with the support. The intense reduction peak centered at 563 \u00b0C was assigned to Co2+ reduction to Co0. Finally, the peak at 763 \u00b0C was assigned to the reduction of cobalt ions in the cobalt aluminate (CoAl2O4) phase. In order to further investigate the temperature required for full reduction of both Pt ensembles and Co3+ species not in the aluminate spinel phase, two additional TPR experiments were done consecutively. First, TPRb, where the calcined solid was reduced up to 600 \u00b0C and hold for 1\u2009h; subsequently, after cooling down to room temperature, sample was again reduced up to 950 \u00b0C (TPRc). The TPRb reduction profile from room temperature to 600 \u00b0C was identical to TPRa, and represented around 70% of its hydrogen uptake. The TPRc profile showed a single, broad reduction peak at 780 \u00b0C, ascribed to the reduction of the cobalt aluminate spinel. No peaks at below 625 \u00b0C were detected. Therefore, it was confirmed that both platinum species and the cobalt as segregated Co3O4 were completely reduced at 600 \u00b0C. Based on the H2-TPR results, the catalyst was reduced at 600 \u00b0C for 2\u2009h prior to the catalytic runs.H2 pulse chemisorption (Fig. S3, Supporting Information) was carried out to titrate the metallic surfaces. As expected, the catalyst reduced at 600 \u00b0C for 1\u2009h showed about 5 times more metallic Co surface than metallic Pt surface (2.01\u2009m2/g vs 0.44\u2009m2/g). These values indicated that only 1.84% of the total surface was due to metals. For platinum, the calculated dispersion was 58% with an average diameter of 2.4\u2009nm, in agreement with the absence of XRD peaks.Ammonia and carbon dioxide TPD experiments revealed the amphoteric character of our spinel based catalyst (Table 2). Its surface was predominantly basic, as basic sites density was two-fold larger than acid sites density. In addition, the basic sites were primarily weak (88% contribution) while the acid sites were medium strength sites (86% contribution) (Fig. S4, Supporting Information). The very low activity in the 33DMB1 isomerization (Table 2) in comparison with other Lewis solids [51] indicated they are predominantly of Lewis-type.Based on the obtained liquid and gaseous products, Reynoso et al. [45] suggested a plausible reaction pathway for the glycerol APR on cobalt aluminate derived catalysts. Reaction pathway consisted on two main routes, which needed both acid and metallic sites (\nScheme 1). In outline, dehydrogenation to glyceraldehyde, preferably on metal sites, which undergoes decarbonylation to produce ethylene glycol, methanol and finally hydrogen. On the other hand, dehydration route, mainly on acid sites, first produces hydroxyacetone and, by subsequent dehydration/hydrogenation, yields C3 liquid products. Further transformation of the liquid products due to C\u2013O bond cleavage leads to the formation of alkanes, which decreases the evolution of hydrogen. In addition, CO can be converted by WGS, increasing H2 yield, or can be hydrogenated (together with CO2) to produce methane and alkanes by Fischer-Tropsch reaction, constituting a hydrogen selectivity challenge.The influence of feedstock concentration on the catalytic reaction was explored at 260 \u00b0C and 50\u2009bar, at WHSV of 6.8\u2009h-1 (flowrate: 0.1\u2009mL/min, catalyst mass: 0.9\u2009g) and varying the glycerol concentration (5, 10 and 20\u2009wt%). \nFig. 3 shows the effect of glycerol concentration in the feedstock on APR global results after 3\u2009h TOS. The global glycerol conversion was very high (>99%) for all the glycerol concentrations, pointing to very active catalyst for glycerol reforming. Others also reported about the promotional effect of Pt-Co catalysts in the APR reactions and attributed to the PtCo alloying [30,52].Larger differences were obtained in the carbon conversion to gas, which slightly decreased as the concentration of glycerol fed increased (e.g. 41% for lower concentration and 33% for the most concentrate feedstream). This decrease was more pronounced by increasing the glycerol content from 5% to 10%, since by increasing up to 20% the decrease was practically negligible (1.2%). This trend indicated that increasing glycerol concentration, increased the carbon content in the liquid products. Similar results were reported by others [41,53]. For more diluted feedstocks, the availability of the active sites (either metallic and acid/base) increases, thus reactions involved in the APR proceed more extensively to obtain more volatile (gas phase) compounds. Consequently, it can be deduced that feedstocks with low glycerol concentration were more advantageous for gas production (deeper degree of reforming), while more concentrated ones would be preferred for liquid production (i.e. for hydrogenolysis of glycerol by in-situ produced H2) [54,55]. It could also be observed that for the WHSV values used in this study, at glycerol concentrations of 10\u2009wt% or above, there were not enough available active sites to further decompose intermediate molecules, thus reaching almost constant Xgas. This behavior implies that reaction order with respect to glycerol concentration decreased with glycerol concentration.The most important effect of glycerol concentration was on the hydrogen yield, which showed a significant drop from 50.6% to 26.7% when glycerol concentration increased from 5% to 20%. This tendency is in line with the results reported by others [38,41] and was consistent with the above idea, that is, the surface coverage increased with glycerol concentration (i.e. less free sites being available). Moreover, the increase of liquid products yield with glycerol concentration was at the expense of hydrogen consumption, since the yield of products of hydrogenation increased (e.g. 1,2-propylene glycol).Both selectivity to hydrogen and to alkanes showed slight decreasing trend with glycerol concentration, which was also reflected in the almost constant H2/CH4 ratio (\nTable 3). High values of hydrogen selectivity (above 85%) were obtained for the three feedstream compositions. Selectivity to alkanes, above 10%, was considerably high in comparison to values reported in the literature for Pt supported on alumina (around 8%) [38], and could be due to cobalt, which is active for CO/CO2 hydrogenation reactions [56].Concerning the gas product distribution (Table 3), an increase in glycerol concentration affected both CO2 and H2 concentration in the opposite way, increasing the former and decreasing the latter. For example, passing from 5% to 20% glycerol concentration, H2 concentration decreased by 11% while CO2 concentration increased by 33%. A decreasing trend for H2 was reported by others [41], and was attributed to the slight increase in the yield of liquid products, being most of them formed though hydrogen consuming reactions. CO content increased with the glycerol concentration, especially at the highest glycerol concentration, due to the lower availability of free metallic centers for WGS. At the reactor outlet, H2/CO2 ratio decreased with glycerol concentration, from 3.7 to 2.5. In all cases, this ratio was above the theoretical (7/3). These results agreed with the decreasing trend of SCH4. The lowest hydrogen concentration in the gas product was 67%, when feeding 20\u2009wt% glycerol/water mixture. CH4 content decreased slightly with the increase of glycerol content. Indeed, H2/CH4 ratio was not almost varied with glycerol concentration, which agree with the almost constant SH2.The H2 production rate (FH2) did not increase in proportion to the increase in glycerol concentration (see Table 3). For instance, when glycerol concentration varied from 5% to 20%, two-fold increase on FH2 was obtained (218 vs 460\u2009\u00b5mol/(gcat\u00b7min)) when, by stoichiometry, a 4-fold increase would be expected. Opposite trend was shown by the molar flow of hydrogen per mole of converted glycerol (i.e. hydrogen selectivity ratio SRH2, which was limited to 7) which decreased from 3.95 to 1.44 passing from 5% to 20% glycerol feed. These features suggested that H2 lost in hydrogenation/hydrogenolysis reactions increased in higher proportion by increasing the glycerol concentration.Higher concentration of liquid products (\nTable 4) was obtained by increasing the glycerol content in the feed stream, in agreement with the decreasing trend of Xgas. An increase in glycerol concentration produced an increment in both hydroxyacetone (HA, primary product from glycerol dehydration) and 1,2-propylene glycol (PG, product from hydroxyacetone hydrogenation) yields. As previously reported for cobalt aluminates catalysts [57], hydrogenation reaction seems to occur more rapidly than dehydration. PG dehydration on acid sites can lead to the formation of acetone or propanal, depending of the primary or secondary hydroxyl elimination by dehydration [58]. Further hydrogenation of both intermediates produce 2-propanol and 1-propanol, respectively. This same route can also form propanoic acid, the main liquid product. Other authors also reported this product in the liquid stream of glycerol APR [39,59]. Among the liquid products, those whose yield was most affected corresponded to propanoic acid, which decreased at expense of the increase of hydroxyacetone and 1,2-propylene glycol, production of the later consuming hydrogen. The ratio of products from dehydrogenation route to products from dehydration route (\nFig. 4) presented a maximum at 10\u2009wt% glycerol. Therefore, it could be concluded that this glycerol concentration provides a balance between this two reaction routes. However, the yield of dehydration-route products had a more pronounced rise with glycerol concentration than those obtained via dehydrogenation-route.Experiments to determine the effect of coupled temperature and pressure variables on the catalyst APR performance were performed varying temperature in the 220\u2013260 \u00b0C range, while pressure was established to ensure a liquid-phase reaction mixture (1.8\u20134.2\u2009bar above the bubble point of the feedstock). This means that isolated effect of pressure was not analysed, but that of the coupled temperature and pressure. The following coupled temperature and pressure pairs were used (\u00b0C/bar): 220/25; 235/35; 245/40 and 260/50. The reaction conditions were 10\u2009wt% glycerol concentration and WHSV=\u20096.8\u2009h-1 (0.1\u2009mL/min of glycerol, 0.9\u2009g of catalyst). The obtained results are shown in \nFig. 5.Conversion of glycerol reached almost 100% except for the mildest conditions (Xgly=89%), the later indicating the endothermic characteristics of the reforming reaction [60]. The lowest carbon conversion to gas (21%) was achieved at the mildest operation condition. More severe conditions enhanced carbon conversion to gas and, consequently, decreased the yield of liquid products. Despite same Xgly trend, carbon conversion to gas exhibited a continuous increase with temperature-pressure, reaching a maximum of 43.2% at the most severe conditions (260\u2009\u00b0C/50\u2009bar). High temperatures promoted the reforming of glycerol and the intermediate liquids, by promotion of C-C and C-O bonds cleavage to obtain C-containing more volatile compounds [38].The effect of couple temperature and pressure variables strongly affected hydrogen yield, which increased with temperature, in parallel with Xgas. For instance, from 220\u2009\u00b0C/25\u2009bar to 260\u2009\u00b0C/50\u2009bar an overall increment of 126% was attained by YH2, as due to the endothermic nature of glycerol reforming [60], which favored the glycerol decomposition. The gas products include hydrogen and C-containing compounds (see \nTable 5). Produced hydrogen could be further reacted giving alkanes in the gas phase and intermediate liquid compounds. Hydrogen lost in alkane formation (SH2) was computed for each run, and the obtained trend is depicted in Fig. 5. For the mildest operation conditions, where 11% of glycerol was unreacted, selectivity to hydrogen was 88%. For the rest of conditions, where glycerol conversion was almost complete, SH2 increased with temperature reaching a maximum of 90% at the most severe conditions. At full glycerol conversion (higher T/P conditions), APR proceeded more extensively, through the reforming of intermediate liquid products, what allowed to obtain more hydrogen. The observed decreasing yield of methane (limited by thermodynamics) indicated less hydrogen consumption, what explained the increasing hydrogen selectivity trend. Similar features for SH2 were obtained by others [19]. Regarding alkane selectivity, it moderately increased with the operation temperature, being methane the most representative of them.Regarding the C-atom amount of the produced alkanes, the majority corresponded to methane, which accounted for 63% at the lowest temperature and 71% at the highest, once again suggesting that C-C scission reaction were promoted by temperature [61].\nTable 5 summarizes the gas product composition. Hydrogen was, by far, the most abundant product, with around 70% mole percentage, independent of the reaction conditions. Thanks to the increasing trend in conversion to gas, H2 production rate increased with temperature/pressure. CO2 was the main carbon-containing product, followed by methane (3.5\u20135.2% range), which was the most abundant alkane. Traces of ethane, ethylene, propane, and butane were also detected (compiled as C2 +). Alkanes were formed by either the subsequent reaction hydrogenation of CO/CO2 and Fischer-Tropsch reactions [62]. The formation of C4 +\u2009compounds suggested that Pt-Co catalysts had some activity in C-C coupling reaction, in addition to their recognized great activity in WGS reaction. The latter could be confirmed from the very low CO content in the gaseous product (< 0.2%) for all the temperatures studied.The H2/CO2 ratio was 2.8 and 2.9 in the mildest and the most severe conditions, respectively. These values slightly exceed the theoretical value of 7/3 for glycerol APR, which indicated that glycerol was partially reformed to intermediate species that can readily undergo dehydrogenation reactions while keeping carbon atoms. The large yield of propanoic acid agreed these results.Depending on the applied T/P conditions, around 57\u201379% of the carbon contained in glycerol came out in the liquid product. As the glycerol conversion reached almost 100% (except at 220 \u00b0C/25\u2009bar, with 89%), the production of intermediate oxygenated liquids was considerable. Indeed, the spatial velocity (6.8\u2009h-1) was insufficient for a deep reforming of glycerol molecules. The identified liquid products (\nFig. 6) comprised acids (acetic acid, propanoic acid), ketones (acetone, hydroxyacetone), aldehydes (acetaldehyde, propanal), C3 alcohols (1,2-propylene glycol, 1-propanol, 2-propanol) and C1-C2 alcohols (ethylene glycol, ethanol, methanol). Other peaks detected by chromatography, which accounted less than 5% of all area) could not be identified. The wide variety in the liquid fraction pointed out the complexity of the glycerol APR reaction network and the strong influence of coupled temperature/pressure variable. It must be said that 1,3-propanediol was not obtained in the liquid. Formation of 1,2-propylene glycol and 1,3-propylene glycol is competitive, their selectivity depends on which intermediate, hydroxyacetone or 3-hydroxypropanal, is preferentially produced. The former intermediate is produced by Lewis acid sites [63] while the later requires Br\u00f8nsted acid sites [64]. The dominant Lewis characteristics of the catalyst (Table 2) explained the absence of 1,3-propylene glycol.Most of the liquid products contained a three-carbon chain. On the other hand, the (C1 + C2)/C3 compounds yields ratio in the liquid stream indicated monotonous increase with the operation temperature (insert in Fig. 6) confirming that temperature promoted C-C cleavage. Most of the liquid products incremented its yield with reaction temperature, being 1,2-propylene glycol, hydroxyacetone and ethylene glycol the exceptions. The former two products resulted from the direct dehydration of glycerol (hydroxyacetone) and its subsequent hydrogenation (1,2-propylene glycol). These results indicated that an increase in reaction temperature favored the dehydrogenation pathway and explained the improvement in hydrogen yield at higher temperatures.The effect of contact time was studied in terms of WHSV (higher WHSV, shorter contact time), varying the flowrate of the feedstream from 0.02 to 0.5\u2009mL/min over 1.8\u2009g of 0.3Pt/CoAl catalyst. The experiments were performed at 260\u2009\u00b0C/50\u2009bar with a 10\u2009wt% glycerol in the feedstream. \nFig. 7 shows glycerol conversion, carbon conversion to gas, hydrogen yield and selectivity to hydrogen and methane.The effect of WHSV was very noticeable in all the parameters represented, except in Xgly, which remained close to 100% for a wide range of WHSV, only declining to 97% for the highest WHSV studied (17\u2009h-1). These results indicated again the pronounced activity of this catalyst to glycerol decomposition, even working at very high WHSV. Nevertheless, carbon conversion to gas was highly sensitive to contact time: as WHSV increased, carbon conversion to gas decreased. Augmenting feed flowrate from 0.02 to 0.1\u2009mL/min (WHSV = 0.68 and 3.4\u2009h-1, respectively) resulted in a 50% drop in the carbon conversion to gas. Further increase in WHSV resulted in a less severe decay in Xgas. Operation at high WHSV values (i.e. short contact time) hindered consecutive reforming reactions of the intermediate liquid products, thus resulting in less gaseous compounds. Similar trend was reported in the literature [19,41,59]. Interestingly, selectivity to hydrogen increased with WHSV, i.e., the shorter the contact time, less hydrogen was lost in gas phase products. Similar trend was reported by others [30] and was ascribed to a lowered rate of alkanes production. Though the space velocity employed was calculated on the liquid flowrate basis, the gases (H2, CO, CO2) flowed with the liquid stream. Therefore, the SH2 trend suggested that CO/CO2 hydrogenation (producing hydrogen loss in the gas phase) were lessened by the short contact times [65]. Analogous to Xgas, a decrease in hydrogen yield and selectivity to methane could be observed with WHSV, with a concomitant increase on CO and CO2. This was consistent with the increase on hydrogen selectivity. At lower WHSV, the contact time between the intermediate liquids, gases and catalyst was higher, thus enhancing the hydrogen consumption reactions (such as hydrogenation CO/CO2 and hydrogenolysis of the substrate and liquid intermediates) which decreased the hydrogen yield. Regarding SRH2, it decreased with WHSV, passing from 3.89 to 1.27\u2009molH2/molGlyc-converted when WHSV increased from 0.68 to 17\u2009h-1.The CO/H2 molar ratio in the gas stream increased with WHSV (\nFig. 8), i.e. long contact time favored WGS reaction, thus this reaction occurs to a lesser extent at high WHSV. The H2/CO2 ratio indicates the competition between C\u2013C and C\u2013O scission [66]. This value was slightly higher than the theoretical one (7/3) and practically remained above 2.7 regardless of the WHSV used, which indicated that 0.3Pt/CoAl catalyst had a high capacity for C-C bonds breakage prior to C-O bond breakage.The liquid products yields at different WHSV are provided in \nFig. 9. As seen before, low WHSV presented an exceptionally high carbon conversion to gas, and therefore, the yield to liquid products was insignificant. Largely, the yield of the liquid products increased with WHSV, in accordance with Xgas decrease. At higher WHSV, a wide variety of liquid products could be distinguished. Among the products obtained, a noticeably high yield of hydroxyacetone and 1,2-propylene glycol could be noted at the shortest contact time (yields of 37.6% and 41.5%, respectively). The yields of both compounds increased in similar way with WHSV, which suggests that at such short contact times, dehydration and hydrogenation reactions predominate at the same rate to the detriment of C-C cleavage. This lower activity for C-C bond scission caused a drop in the yield of C2 products from 13.4% at 6.8\u2009h-1 to 5% at 17\u2009h-1. Yields of propanoic acid, ethanol, acetone and both 1-propanol and 2-propanol reached their maxima at 3.4 and 6.8\u2009h-1, respectively, then decreasing dramatically as WHSV achieved 17.0\u2009h-1. It is evident that at short contact times less fragmentation of the initial molecule takes place. Contrariwise, for the longest contact time (0.68\u2009h-1), it is only possible to detect compounds resulting from several reaction stages such as propanols and acetone.After each reaction at different T/P conditions, the spent catalysts were analysed by analysed by N2 adsorption-desorption isotherms, XRD, ICP-MS and TPH analyses. The nitrogen isotherms and BHJ pore size distribution are shown in Fig. S5, Supporting Information. They still showed type IV isotherms with H1 hysteresis. The most notable differences with respect to the reduced form is that hysteresis loops were narrower in the spent catalysts, though they retained mesoporosity. In the spent catalysts, new generated small pores of around 4\u2009nm contributed to the total pore volume, suggesting the presence of new phases not observed in the fresh reduced form (e.g. gibbsite). As can be seen in \nTable 6, the specific surface area of the spent catalyst increased when the reaction was performed at the mildest condition (18% increase at 220\u2009\u00b0C/25\u2009bar) while decreased for the other conditions tested (14% decrease at the most severe conditions). At lower temperatures, a greater decrease in the average pore diameter was also observed (decreased by 36% at the mildest conditions), which had a tendency to increase with the reaction temperature without reaching the value of its reduced state. XRD diffractograms of the spent catalysts (Fig. S6, Supporting Information) showed peaks from cobalt spinel (either cobalt oxide and cobalt aluminate) and metallic Co, which remained in the spent catalysts. The later suggested that bulk cobalt remained in metallic form. As in the fresh catalysts, no peaks from Pt were visible, indicating that Pt remained highly dispersed. New, sharp peak emerged for the spent catalysts at about 18\u00ba (2\u03b8), which could be ascribed to the gibbsite phase (PDF 033-0018). The intensity of the gibbsite peak decreased with the operation temperature/pressure. The same trend observed in the textural properties indicated that gibbsite is surely related to this textural trend.Metal leaching in the liquid product was also investigated by ICP-MS. The results showed that the reaction conditions influenced cobalt leaching. Although the overall results confirmed low metal leaching, cobalt was found to be more leachable at low reaction temperatures (cobalt leaching was 2.30% after reaction at 220\u2009\u00b0C vs 0.74% at 235\u2009\u00b0C). These apparently inconsistent results agreed with results obtained with catalyst 0.625CoAl after 30\u2009h TOS at 235\u2009\u00b0C and 260\u2009\u00b0C [57]. A comprehensible explanation could be found in the re-deposition of hydroxylated alumina. At higher temperatures, pH of the reaction medium increased, thus greater leaching was expected. However, in hot water the solubility of the inorganic oxide materials is low, which facilitates the re-deposition of leached alumina, decorating the cobalt and therefore protecting it from leaching [67]. This was in agreement with the gibbsite phase detected by XRD. As for the other metals, both platinum and aluminium had a low percentage of leaching (below 4\u00b710-3% and 0.02%, respectively), understanding that the platinum conferred the stability of the catalyst during reaction. The production of stable catalysts against leaching is a challenge in biomass transformation processes. The most researched strategies to stabilize the supported metal nanoparticles focus on overcoat using techniques such as Atomic Layer Deposition (ALD) [68] or the embedment into support structure via strong metal\u2013support interaction (SMSI) [69\u201371].The quantification of the carbonaceous deposits was measured by TPH (Temperature-Programmed Hydrogenation). The results obtained by TPH did not show any relation to the reaction condition. The samples with the highest amount of carbonaceous deposits correspond to those for the reactions at 235 and 245\u2009\u00b0C. Surprisingly, the catalysts after the reaction at 260\u2009\u00b0C had a content of carbonaceous material very similar to that used in the reaction at 220\u2009\u00b0C (about 3 times less than at the other conditions). It is worth highlighting the much lower (three to four orders of magnitude) coke deposits in glycerol APR as compared to glycerol steam reforming [72,73]. This was due to the ability of hot compressed water to dissolve carbonaceous deposits [67].0.3Pt/CoAl catalyst, synthesized by impregnation of Pt over cobalt aluminate (nominal Co/Al = 0.625) support, was characterized and tested for glycerol aqueous-phase reforming under various reaction conditions. Specifically, the glycerol concentration in the feedstream, the coupled temperature/pressure variable and the space velocity (in terms of WHSV) process variables were studied. At the conditions studied (260\u2009\u00b0C/50\u2009bar), the glycerol conversion did not show significant variation when glycerol concentration was increased from 5 to 20\u2009wt% nor when the space velocity was increased from 0.68 to 6.8\u2009h-1. Only at 220\u2009\u00b0C/25\u2009bar (WHSV = 6.8\u2009h-1, 10\u2009wt% glycerol/water) did the glycerol conversion drop below 90%. The highest carbon conversion to gas was achieved at a lower glycerol concentration, at highest temperature/pressure, and at lengthy contact time. As expected, the conditions where the highest conversion to gas was achieved were the ideal ones to obtain higher hydrogen yield.Increasing the glycerol concentration in the feedstream from 5% to 20% (260\u2009\u00b0C/50\u2009bar, 10\u2009wt% glycerol/water, WHSV = 6.8\u2009h-1) also showed an increase in the yield of the liquid products formed through the dehydration/hydrogenation of glycerol such as hydroxyacetone and propylene glycol. Conversely, by increasing temperature/pressure from 220\u2009\u00b0C/25\u2009bar to 260\u2009\u00b0C/50\u2009bar (10\u2009wt% glycerol/water, WHSV = 6.8\u2009h-1) 1,2-propylene glycol yield decreased while ethanol yield increased. As well, higher hydrogen yield was achieved at a higher reaction temperature. On the topic of post-reaction characterization, temperature/pressure conditions undoubtedly affected cobalt leaching. However, further investigation is needed to clearly establish leaching mechanism, which will help to overcome this challenge.Increasing the feed flowrate, and consequently the WHSV, did not change the composition of the outflow gases. Nonetheless, due to the shorter contact time, the production of liquids increased, especially the liquids obtained by the direct reaction of glycerol (hydroxyacetone and 1,2-propylene glycol).\nA. J. Reynoso: Investigation and Writing \u2013 original draft. J.L. Ayastuy: Funding acquisition, Conceptualization, Writing \u2013 review & editing. U. Iriarte-Velasco: Formal analysis, Writing \u2013 review & editing. M.A. Guti\u00e9rrez-Ortiz: Resources, Funding acquisition and 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 grant PID2019-106692EB-I00 funded by MCIN/ AEI/10.13039/501100011033. The authors thank for technical support provided by SGIker of UPV/EHU and European funding (ERDF and ESF).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2022.107402.\n\n\nTable S1\n\nSupplementary material\n\n\n\n.", "descript": "\n                  This study examined the influence of process variables (glycerol concentration in feed, coupled temperature/pressure and space velocity) in the catalytic performance in the APR of glycerol over 0.3Pt/CoAl catalyst in a continuous fixed-bed reactor in order to maximize the production of H2. The effect of glycerol concentration in the feed was studied from 5 to 20\u00a0wt%, the coupled temperature/pressure varied from 225 \u00b0C/25\u00a0bar to 260 \u00b0C/50\u00a0bar and the spatial velocity was changed from 0.68 to 17\u00a0h-1. Our results reflected that H2 production was favored at higher reaction temperature/pressure (3.62 vs. 2.49\u00a0molH2/molGly-converted, at the most severe and mild conditions, respectively), lower WHSV (3.89 vs. 1.27\u00a0molH2/molGly-converted, at the lowest and highest space velocity, respectively) and more diluted feedstocks (3.95 vs. 1.44\u00a0molH2/molGly-converted, at the most diluted and concentrated freestreams, respectively). A threshold value at 10\u00a0wt% glycerol was found for the ratio of dehydrogenation to dehydration liquid products. The post-reaction catalyst was also characterized by several techniques, showing that Co leaching was the major drawback, especially at the mildest operation conditions, while carbonaceous deposits are negligible.\n               "}
{"full_text": "Because of the growing demand for energy, hydrogen production has evolved as a fossil fuel substitute due to its high heat and lack of environmentally concerning exhaust gases [1\u20135]. At present, as the application of HER, electrochemical catalysis and photoinitiated water splitting have opened up new avenues to obtain clean hydrogen [6\u20138]. Traditional electrochemical catalytic methods require noble metal catalysts with high catalytic performance, which are precious and rare. In the development of electrochemical catalysts for the HER, low-cost and earth-abundant catalysts with high catalytic efficiency have emerged to promote hydrogen evolution under water splitting conditions. Transition metal nitrides [9], phosphides [10], carbides [11] and dichalcogenides [12] have been considered promising replacements for traditional platinum-like catalysts, which exhibited a low free energy of H\u2217 (\u0394GH\u2217) and excellent conductivity.The HER mechanism in an acidic solution can be summarized in two steps: the Volmer reaction \n\n\n(\n\n\nH\n3\n+\n\nO\n+\n\n\ne\n\u2212\n\n\n+\n\nC\na\nt\n.\n=\n\nH\n\na\nd\ns\n\n\n\n+\n\n\nH\n2\n\nO\n\n)\n\n\n and the Heyrovsky reaction \n\n\n(\n\n\nH\nads\n\n\n\n\n+\n\nH\n\n3\n+\n\n\n\nO\n\n+\n\ne\n\n\u2212\n\n\n\n=\nH\n\n2\n\n\n\n\n+\n\nH\n\n2\n\nO\n\n)\n\n\n [13]. The adsorption/desorption of H\u2217 on the catalytic surface determines the process of hydrogen evolution. Therefore, the lower the \u0394GH\u2217 is, the higher the catalytic efficiency. Traditional noble catalysts for electrochemical water splitting with a low \u0394GH\u2217 have been widely reported [14\u201316]. The Pt-like d-band states can facilitate the adsorption/desorption of H\u2217 to reduce the \u0394GH\u2217. To date, heteroatoms (such as P, S and N) also exhibit a similar activity for the adsorption/desorption of H\u2217 [17\u201319]. For example, heteroatoms can induce a change in charge at transition metal atoms on the catalyst surface to form Pt-like d-band states. Xia Long et\u00a0al. reported a novel catalyst of iron-nickel sulfide with outstanding activity and stability in an acidic solution [20]. The overpotential (OP) was 105\u00a0mV\u00a0at 10\u00a0mA\u00a0cm\u22122 with a Tafel slope (Ts) of 40\u00a0mV dec\u22121. They investigated the catalytic mechanism with DFT calculations, which indicated that the S atom induced a change in charge of the Fe and Ni atoms on the catalyst surface to reduce the \u0394GH\u2217. The Pt-like d-band states contributed to the lower energy barrier for H+ adsorption and facilitated the HER.Heretofore, P has been widely studied due to its lone pair electrons [21\u201323]. These paired electrons in the 3p and hollow 3d orbitals can affect H\u2217 adsorption, thus benefiting the HER [24\u201326]. Recently, an increasing number of transition metal phosphides have been reported, such as Fe2P, CuxP, CoxP and NixP [22\u201326]. The traditional and widespread preparation method for phosphides is through high-temperature phosphorization with NaH2PO2 or NH4H2PO2 in a N2 atmosphere [27]. However, phosphides easily aggregate in the formation process, which decreases the activity of catalysts for the HER. To avoid this problem, many studies about different morphologies have been reported: Ni2P microspheres, Ni2P nanoparticles and porous carbon matrix loaded with Ni2P. Dan Ma et\u00a0al. investigated Ni2P using carbon-based substrates for HER activity [28]. N-doped reduced graphene oxide (N-RGO) was adopted as the substrate for Ni2P nanoparticles and the above construction showed enhanced HER performance. The electron density of Ni was modulated by P and the doped N, and this modulation was beneficial for H\u2217 adsorption and a low \u0394GH\u2217. The N-RGO substrates alleviated the aggregation of Ni2P and provided a large surface area for active site exposure.Currently, MoS2 has been the most promising candidate for HER catalysts with a similar \u0394GH\u2217 of H\u2217 adsorption to Pt [29,30]. The d orbitals of Mo are easily induced by the s and p orbitals of adjacent heteroatoms to expand and form Pt-like d orbitals, thus improving HER activity. However, because of poor conductivity and few active sites, pure MoS2 exhibits undesirable catalytic performance. To solve this issue, many efforts have been made to perfect a nanostructure or load other heteroatoms, such as MoS2 with a 2D-layered structure, amorphous MoS2 and heteroatom-doped MoS2 [31\u201333], which shows excellent HER activity in acidic solutions but demonstrates poor activity in alkaline or neutral solutions. Therefore, developing catalyst with outstanding catalytic performance in alkaline or neutral solutions is a research hotspot. The doping of single Ni atoms can enhance the catalytic performance for the HER in an alkaline or neutral solution. Qi Wang et\u00a0al. reported a novel catalyst of MoS2 decorated with single atoms of Ni, and this catalyst exhibited a low overpotential and Ts in solutions with a wide pH range [34]. The single Ni atoms were introduced into the MoS2 S-edge and H-sites of the basal plane, which reduced the \u0394GH\u2217 of H\u2217 adsorption and extended the pH range that can catalyze HER. Because of its excellent conductivity and plentiful defects, RGO has been widely reported as a substrate to enhance the catalytic performance of MoS2. Lin et\u00a0al. have prepared RGO loaded with Ni-doped MoS2 composite [35]. They found that the doped Ni atoms could facilitate the formation of H\u2217 and accelerate the adsorption and desorption of H\u2217 on the catalyst surface. In principle, the doping of Ni atoms into MoS2 could increase the HER activity at different pH values.Recently, heterogeneous interfaces between multicomponent catalysts have been proposed [35,36]. Because of the synergistic effect between the different components and heterogeneous interfaces, the charge distribution is changed, and active sites are formed, which improves catalytic performance. In this work, a novel Ni2P/MoS2 cocatalyst was prepared by using porous N-doped carbon as the substrate (Ni2P/MoS2-CC). The synergistic effect between Ni2P and MoS2 was investigated through theoretical calculations and experimental verification. Raman and XPS spectra proved the presence of synergistic effect between Ni2P and MoS2. The results showed that the cocatalysts had an outstanding catalytic performance for the HER not only in acidic solutions but also in alkaline solutions. The onset potential (OP) values were 280, 350 and 40\u00a0mV in acidic, phosphate-buffered saline and alkaline solutions, respectively.Details about the reagents used in this work are shown in the Supporting Information (SI).The detailed preparation route is presented in Scheme 1\n. Typically, pectin (1.5\u00a0g), nickel nitrate (0.87\u00a0g, 3\u00a0mmol), ammonium hypophosphite (1.0\u00a0g, 12\u00a0mmol) and melamine (2.5\u00a0g, 20\u00a0mmol) were placed into 70\u00a0mL of deionized water. Then, the solution was placed into a 150\u00a0mL hydrothermal reactor. The temperature was set to 150\u00a0\u00b0C and kept for 12\u00a0h. The resulting material was dried at 105\u00a0\u00b0C, and then it was carbonized under a N2 atmosphere at 900\u00a0\u00b0C to obtain Ni2P-loaded N-doped carbon substrates. The aforementioned Ni2P-loaded N-doped carbon substrates (0.1\u00a0g) and ammonium tetrathiomolybdate (0.01\u00a0g) were added into a 25\u00a0mL hydrothermal reactor with 15\u00a0mL deionized water, which was then kept at 150\u00a0\u00b0C for 20\u00a0h. The obtained materials were named Ni2P/MoS2-CC catalysts. The CC catalysts, Ni2P-CC catalysts and MoS2-CC catalysts were prepared using the same method without the addition of nickel nitrate, ammonium hypophosphite and ammonium tetrathiomolybdate, respectively.The DFT calculation details, structural details and computational methods are all described in the SI.The details about the preparation of the working electrode are presented in the SI. The used electrolytes were 0.5\u00a0mol\u00a0L\u22121\u00a0H2SO4, 1.0\u00a0mol\u00a0L\u22121 PBS and 1.0\u00a0mol\u00a0L\u22121 KOH. The voltages were referenced to a reversible hydrogen electrode (RHE), E (vs. RHE)\u00a0=\u00a0E (vs. SCE) + 0.224\u00a0V. The cyclic voltammetry (CV) curves were obtained from a potential range of 0.0\u20130.1\u00a0V (vs. RHE) at scan rates of 40, 60, 80, 100, and 140\u00a0mV\u00a0s\u22121. The electrochemical impedance spectroscopy (EIS) performance was tested from a frequency of 0.01\u2013105\u00a0Hz with potentials of 100, 125, 135, 140 and 150\u00a0mV and amplitude of 5\u00a0mV. A modified Randles equivalent circuit was adopted to fit the Nyquist plots and obtain the electrolyte resistance (Rs) and charge transfer resistance (Rct). The detailed methods for the above tests are presented in the SI.The morphologies and microstructures of the as-prepared materials were observed by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and a surface-area analyzer. The crystal structures of the prepared composites were investigated by X-ray diffractometry (XRD) and Raman spectroscopy. The surface components were obtained from X-ray photoelectron spectroscopy (XPS). The detailed methods for the above characterization procedures are presented in the SI.In order to investigate the morphology of Ni2P/MoS2-CC, scanning electron microscopy (SEM) images were obtained, and the results are presented in Fig.\u00a01\n. It can be clearly seen that the Ni2P/MoS2 nanoparticles were uniformly dispersed on the N-doped carbon substrate surface. Compared with Ni2P-CC (Fig.\u00a01a) and MoS2-CC (Fig.\u00a01b), Ni2P/MoS2-CC (Fig.\u00a01c and 1d) exhibited more scalloped edges to provide more loading sited. As shown in Fig.\u00a01c and 1d, a 3D-interconnected structure could be observed, which provided plentiful defects for active sites. The flower-like structure could reduce the agglomeration of MoS2, which improved catalytic performance of the prepared composite. The EDX images (Fig.\u00a01e\u20131k) corresponded to SEM results indicated the presence of C, N, O, Ni, Mo, P and S, which were uniformly distributed on the carbon substrate surface.As shown in Fig.\u00a02\na and b, uniformly distributed Ni2P/MoS2 nanoparticles were observed on the substrate surface, and the porous structure could be clearly seen. The d spacing 0.22\u00a0nm corresponded to the Ni2P (111) crystal lattice, respectively [37]. In addition, the interface between Ni2P and MoS2 could be clearly seen from Fig.\u00a02b, and Ni2P particles were coated with MoS2 sheets. This specific structure provided a synergistic effect that improved HER performance. Moreover, the lattice distance of MoS2 cannot be observed due to its amorphous structure [38,39].The crystal structure of the as-prepared catalysts were investigated by X-ray diffractometry (XRD), and the results are shown in Fig.\u00a03\na. Obviously, the peak at 26\u00b0 can be attributed to the graphite carbon diffraction (002), indicating that the carbon substrates possessed excellent conductivity for electron transport [37]. The peaks of Ni2P could be easily observed at 2\u03b8 values of 41\u00b0, 45\u00b0, 47\u00b0, 54\u00b0, 55\u00b0, 66\u00b0 and 75\u00b0, which corresponded to (111), (201), (210), (300), (211), (310) and (400) of Ni2P (PDF 01-074-1385). The corresponding peaks of Ni2P and carbon could be seen in CC, Ni2P-CC, MoS2-CC, Ni2P/MoS2 and Ni2P/MoS2-CC composites. In addition, a neglectable shift was observed, indicating that the each other have no effect on the crystal phase. This result was in accordance with the aforementioned analysis. However, the basic peak patterns of MoS2 could not be found, which illustrated that the as-prepared MoS2 had an amorphous structure.Raman spectra were obtained to determine the degree of graphitization and the presence of MoS2, and the results were presented in Fig.\u00a03b and 3c. From Fig.\u00a03b, it can be clearly seen that the characteristic peaks of graphite carbon (G line, 1572\u00a0cm\u22121) and amorphous carbon (D line, 1344\u00a0cm\u22121), and the value of ID/IG indicates the degree of graphitization. The G band in the Raman spectrum of carbon materials is assigned to the stretching bond of sp2 -hybridized carbon. Meanwhile, the D band is attributed to the disorder induced by structural defects and impurities. It could be observed that Ni2P/MoS2-CC exhibited the lowest degree of graphitization (ID/IG\u00a0=\u00a01.24), which indicated the presence of rich structural defects for the electron transport. The peaks of MoS2 could be clearly observed at 373 and 400\u00a0cm\u22121, which were assigned to E1\n2g and A1g of the Mo\u2013S phonon mode, respectively [35]. This result further suggested that Ni2P/MoS2-CC composite was successfully prepared. A shift in the Raman spectra of Ni2P/MoS2-CC could be clearly observed compared with that of MoS2-CC, and this shift was attributed to the presence of the interface between Ni2P and MoS2. The Ni2P and N-doped carbon structure affected the number of layers along the z orientation and improved the uniform distribution of MoS2 [35]. All of the above results enhanced catalytic performance for the HER. Fig.\u00a03d presented the FTIR spectroscopy of Ni2P/MoS2-CC, Ni2P and MoS2. The characteristic peaks of Ni2P and MoS2 could be observed in the Ni2P/MoS2-CC curve, which indicated that Ni2P and MoS2 successfully loaded on the N-doped carbon matrix surface. The presence of C\u2013N (1520\u00a0cm\u22121) and CO (2400\u00a0cm\u22121) were apparent in Ni2P/MoS2-CC, implying the presence of nitrogen and oxygen sources.X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic states of the elements on the surface of Ni2P/MoS2-CC. As shown in Fig.\u00a04\n, the peaks appeared at 286, 532, 401, 130, 855, 172 and 233\u00a0eV can be attributed to C, O, N, P, Ni, S and Mo elements, respectively [27,35,40]. The peaks at 284.7 and 286.0\u00a0eV were assigned to C\u2013C or CC and C\u2013O in Fig.\u00a04b, respectively. As shown in Fig.\u00a04c, the O 1s spectra were divided into two peaks of 531.0 and 532.4\u00a0eV, which were attributed to adsorbed oxygen and hydroxyl oxygen, respectively. In addition, the peaks in Fig.\u00a04d were attributed to pyridinic-N (398.7\u00a0eV), pyrrolic-N (400.9\u00a0eV), oxidized-N (402.8\u00a0eV), Ni\u2013N (396.2\u00a0eV) and Mo 3p (395.0\u00a0eV) [41]. The N dopants that were introduced into the carbon substrates and Ni2P/MoS2 could serve as electron acceptors to improve the catalytic performance for the HER.\u00a0Three peaks at 129.5, 130.2 and 133.2\u00a0eV corresponded to 2p3/2, 2p1/2 and P\u2013O bonding were observed in the P\u00a02p\u00a0spectra, respectively. The Ni 2p spectra were divided into six peaks: 853.2\u00a0eV (2p3/2), 868.4\u00a0eV (2p1/2), 872.3\u00a0eV (Ni-Ox), 856.7\u00a0eV (Ni-Ox), 860.8\u00a0eV (satellite) and 879.9\u00a0eV (satellite). The presence of Ni-Ox bonding was attributed to the oxidation of surface Ni atoms. A clear negative shift of 0.8\u00a0eV could be observed and it was caused by the strong interaction from the MoS2 charge density effect. The peaks located at 162.7, 161.3 and 169.0\u00a0eV corresponded to S 2p1/2, S 2p3/2 and SO4\n2\u2212, respectively. The Mo 3d spectra were fitted to Mo4+ 3d3/2 (228.9\u00a0eV), Mo4+ 3d5/2 (232.6\u00a0eV), Mo6+ (232.6\u00a0eV) and S 2s (226.1\u00a0eV) in Fig.\u00a04h. As shown in Fig.\u00a04g and 4h, a positive shift to a high binding energy could be observed. It is worth noting that the electronic interaction from the adjacent Ni2P could affect the charge distribution of Mo and S. This result demonstrated that the electronic interaction between Ni2P and MoS2 could facilitate the electron transport from MoS2 to Ni2P, which enhanced the activity of catalysts for the HER.The catalytic performance for the HER was studied by a three-electrode system in 0.5\u00a0mol\u00a0L\u22121\u00a0H2SO4 solution, 1.0\u00a0mol\u00a0L\u22121 PBS solution and 1.0\u00a0mol\u00a0L\u22121 KOH solution, respectively. The corresponding result curves of the as-prepared catalysts are presented in Fig.\u00a05\n. The onset potential (Eonset), Cdl and OP values at a current of 10\u00a0mA\u00a0cm\u22122 are listed in Table 1\n. Additionally, commercial Pt/C was also tested as the reference in this work. The commercial Pt/C exhibited an outstanding performance with a negligible Eonset, and the OP at a current of 10\u00a0mA\u00a0cm\u22122 was 10\u00a0mV in the 1.0\u00a0mol\u00a0L\u22121 KOH electrolyte (Fig.\u00a05a). As shown in Fig.\u00a05a, the pure N-doped carbon substrates showed negligible activity for the HER. Compared with Ni2P-CC, MoS2-CC composite and Ni2P/MoS2, the prepared Ni2P/MoS2-CC composite exhibited a better performance with a lower OP of 170\u00a0mV (vs. RHE) at a current of 10\u00a0mA\u00a0cm\u22122 (Table 1), which can be attributed to the synergistic effect between Ni2P and MoS2 at their interface and the N-doped carbon matrix. As previously reported, the use of only MoS2 revealed a good catalytic performance for the HER [42,43]. Clearly, the addition of Ni2P enhanced the activity of catalysts in alkaline and neutral solutions. It is worth noting that the rate-determining step in 1.0\u00a0mol\u00a0L\u22121 KOH was the Volmer reaction \n\n\n(\n\n\nH\n2\n\n\n\nO\n+\ne\n\n\u2212\n\n\u2192\n\nH\nads\n\n\n\n\n+\n\nOH\n\n\u2212\n\n\n)\n\n\n. For Ni2P, the Ni atom could play a hydroxide-acceptor role, and the P atom could play a proton-acceptor role, which was attributed to the enhancement of the unfilled d-orbital in Ni\u03b4+ caused by MoS2. The stronger the unfilled d-orbital in Ni\u03b4+ is, the higher the OH\u2212 adsorption ability. As presented in Fig.\u00a05b, the as-obtained catalyst also exhibited outstanding catalytic activity for the HER in acidic and neutral solutions with Eonset values of 280 and 310\u00a0mV, respectively. The aforementioned results indicated that the as-prepared catalyst had a wide pH range for the catalysis of the HER. Compared with previous reports (Table 2\n), the obtained Ni2P/MoS2-CC composite exhibited a better catalytic performance, indicating that this method opened a novel avenue to prepare catalysts for the HER [44\u201351].To investigate the reaction kinetics for the HER process, the Ts was obtained from the equation \n\n\u03b7\n=\nb\nl\no\ng\n\n(\nj\n)\n\n+\na\n\n (where b represents the Ts), and the plots are presented in Fig.\u00a05c and 5d. The rate-determining step of the Volmer reaction in 1.0\u00a0mol\u00a0L\u22121 KOH and 0.5\u00a0mol\u00a0L\u22121\u00a0H2SO4 was the OH\u2212 production \n\n\n(\n\n\nH\n2\n\n\n\nO\n\n+\n\ne\n\n\u2212\n\n\n+\n\nCat.\n\u2192\n\nH\nads\n\n\n\n\n+\n\nOH\n\n\u2212\n\n\n)\n\n\n and the H+ adsorption \n\n\n(\n\n\nH\n3\n+\n\n\n\nO\n\n+\n\ne\n\n\u2212\n\n+\nCat.\n\u2192\n\nH\nads\n\n\n\n\n+\n\nH\n\n2\n\nO\n\n)\n\n\n, respectively. As shown in Fig.\u00a05c and 5d and Table 1, Ni2P/MoS2-CC possessed lower Ts than the others, which indicated that Ni2P/MoS2-CC had favorable kinetics for H2 evolution. The synergistic effect between Ni2P and MoS2 at their interface introduced the charge density distribution of the d-orbital, which facilitated OH\u2212 adsorption. It should be noted that Ts values of 95 and 75\u00a0mV dec\u22121 in alkaline and acidic solutions were lower than 120\u00a0mV dec\u22121, which suggested that the reaction mechanism was the Volmer-Heyrovsky mechanism [17,22].In order to investigate the intrinsic catalytic performance of Ni2P/MoS2-CC composite for HER, the kinetic current density for hydrogen evolution/oxidation reactions (HER/HOR) was fitted with simplified Butler\u2013Volmer equation:\n\n\n\n\nj\nk\n\n=\n\nj\n0\n\n\n(\n\n\ne\n\n\n\n\u03b1\nF\n\n\nR\nT\n\n\n\u03b7\n\n\n\u2212\n\ne\n\n\n\n1\n\u2212\n\u03b1\n\n\nR\nT\n\n\n\u03b7\n\n\n\n)\n\n\n\n\nwhere j\n0 represents the exchange current density of intrinsic activity, \u03b1 represents the transfer coefficient regarding to the symmetry of the HER/HOR. F, R, and T are Faraday's constant (96,485\u00a0C\u00a0mol\u22121), the universal gas constant (8.314\u00a0J\u00a0mol\u22121\u00a0K\u22121) and the temperature (around 293\u00a0K), respectively. \u03b7 represents the applied overpotential (V). It could be clearly seen from Fig.\u00a05f that the fitted curves of Ni2P/MoS2-CC (0.76\u00a0mA\u00a0cm\u22122) showed a larger exchange current density than Pt/C (0.34\u00a0mA\u00a0cm\u22122), indicating that Ni2P/MoS2-CC had a faster HER kinetics. These results can be attributed to the H\u2217 adsorption/desorption energy on the active sites [52].To estimate the intrinsic catalytic activity, the electrochemical active surface areas (ECSA) of the catalysts were obtained through the calculation of the double-layer capacitance (Cdl) according to the cyclic voltammetry (CV) curves (shown in SI) at different scan rates, and the results were shown in Fig.\u00a05e. As presented in Fig.\u00a05e, the prepared Ni2P/MoS2-CC composite had a superior Cdl, up to 19\u00a0mF\u00a0cm\u22122, which was caused by the synergistic effect between Ni2P and MoS2 at their interface and the substrate nanostructure. Additionally, Ni2P/MoS2-CC composite possessed an outstanding specific surface area of 390\u00a0m2\u00a0g\u22121 with a pore size distribution of 4\u00a0nm (shown in the SI), which was beneficial for the transport of electrons and Hads. As shown in Fig.\u00a05f, 5g and 5h, Ni2P/MoS2-CC exhibited a higher TOF values and exchange current density, which also indicates that Ni2P/MoS2-CC composite has higher intrinsic activity [53].Electrochemical impedance spectroscopy (EIS) was conducted to study the kinetics of the HER, and the corresponding results are presented in Fig.\u00a06\n. The values of electrochemical impedance are listed in Table 3\n. As shown in Fig.\u00a06a and 6b and Table 3, the catalysts had small solution impedance. It can be observed that Ni2P/MoS2-CC composite exhibited a smaller semicircle in Fig.\u00a06a, which suggested a lower Rct value (87.2\u00a0\u03a9) and indicated that this catalyst possessed a higher activity for the HER than the others. The Rct value of Ni2P/MoS2-CC composite (87.2\u00a0\u03a9) was smaller than CC (591.6\u00a0\u03a9), Ni2P-CC (192.1\u00a0\u03a9) and MoS2-CC (490.2\u00a0\u03a9), indicating that the prepared Ni2P/MoS2-CC composite had high interfacial charge transfer efficiency and dynamic velocity. This result was attributed to the porous structure and the synergistic effect between Ni2P and MoS2 at their interface, which accelerated the transport of electrons and Hads [54,55]. The conductivity of the catalyst was improved by the addition of the N-doped carbon substrate and Ni2P. Compared with the other potentials, the Rct exhibited a clear change. As shown in Fig.\u00a06b, the semicircle gradually decreased as the overpotential decreased, indicating that a high overpotential was beneficial for the HER [56,57]. The experimental data were well fitted with the Randles equivalent circuit (Fig.\u00a06c). The Tafel slope calculated from the electrochemical impedance was 93\u00a0mV dec\u22121, which was similar to the value obtained from the LSV curves. This result reflected outstanding electrode kinetics of the as-prepared catalyst.To describe the durability of Ni2P/MoS2-CC, the LSV curves before and after 10,000 cycles and the voltage-time response were obtained, and the results are shown in Fig.\u00a07\n. Meanwhile, the structure of Ni2P/MoS2-CC composite after the stability test was also investigated. It can be observed that a negligible change occurred in Fig.\u00a07a and 7b, which indicated that the catalyst maintained good stability after a long-term experimental test. The characteristic peaks of Ni2P were observed in the XRD patterns, as shown in Fig.\u00a07c, which corresponded to the standard Ni2P. It should be noted that the peaks of MoS2 located at 373 and 400\u00a0cm\u22121 appeared in the Raman spectra, indicating that MoS2 was definitely present on the substrate. There was no change in the structure after analyzing the SEM images, and the porous structure was effectively retained. The aforementioned analysis proved that the catalyst had outstanding durability, which was attributed to the specific substrate structure.To shed more light on the synergistic effect between the Ni2P and MoS2 at their interface, DFT was adopted to calculate the Gibbs free energy (\u0394GH\u2217), and the crystal structures of different catalysts with different H\u2217 adsorption sites are shown in Fig.\u00a08\n and Fig.\u00a02S. The perfect activity for the HER is a zero value for \u0394GH\u2217, which is caused by the \u0394G offset from a proton reduction and \u0394G from the removal of adsorbed hydrogen [56,57]. As shown in Fig.\u00a08d, the H\u2217 adsorption sites of Mo and S have high absolute (\u0394GH\u2217) values. Clearly, the introduction of Ni2P into the MoS2 structure improved HER activity. The absolute \u0394GH\u2217 value of Ni2P or MoS2 was greatly reduced. The minimum value for S at the adsorption sites of Ni2P/MoS2 was 0.10\u00a0eV, which was much closer to zero, indicating that Ni2P/MoS2 has good HER performance. These results proved that the synergistic effect between Ni2P and MoS2 at their interface could improve the catalytic performance for the HER, which was also in accordance with the experimental results, demonstrating wide applications in composite materials [58\u201365].In summary, a Ni2P/MoS2-CC synergistic catalyst was prepared through a novel method. The synergistic interface between Ni2P and MoS2 greatly reduced the free energy barrier and improved the catalytic performance for the HER. The addition of N-doped carbon substrates and Ni2P provided excellent conductivity for electron transport, and the porous structure prevented the aggregation of MoS2 while also provided more passageways for the desorption of the adsorbed H\u2217 intermediate. Compared with Ni2P and MoS2 catalysts, the prepared Ni2P/MoS2-CC cocatalyst exhibited obvious improvements with a wide pH range for the HER. In addition, Ni2P/MoS2-CC cocatalyst showed low OP values of 280, 350 and 40\u00a0mV in acidic, phosphate-buffered saline and alkaline solutions, respectively, and the corresponding Ts values were 75, 121 and 95\u00a0mV dec\u22121, respectively. The DFT results revealed that the interface between Ni2P and MoS2 decreased the absolute (\u0394GH\u2217) value to accelerate proton/electron transfer. This work has opened a new avenue to prepare a cocatalyst with a specific interfacial structure that facilitates the HER process.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 that could be construed.The work described has not been submitted elsewhere for publication, in whole or in part, and all the authors listed have approved the manuscript that is enclosed.We greatly appreciate the financial support of the National Natural Science Foundation of China (No. 21872119, 22072127), the Natural Science Foundation of Hebei Province (No. B2021203016), the Science and Technology Project of Hebei Education Department (No. ZD2022147), and the Special Project for Local Science and Technology Development Guided by the Central Government of China (No. 216Z1301G).The following is the Supplementary data to this article:Materials, characterization, DFT calculation, preparation of the working electrode. Fig.\u00a01S(a\u2013d) CV curves of different catalysts; (e) N2 adsorption\u2013desorption curve; (f) Pore size distribution curve. Fig.\u00a02S. (a) Crystal structures of MoS2 using S as the adsorption sites; (b) crystal structures of MoS2 using Mo as the adsorption sites; (c) crystal structures of Ni2P using Ni as the adsorption sites; (d) crystal structures of Ni2P using P as the adsorption sites; (e) crystal structures of Ni2P/MoS2 using P as the adsorption sites.\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.2020.12.008.", "descript": "\n                  Electrochemical catalysts for the hydrogen evolution reaction (HER) have attracted increasing attentions. Noble metal-free cocatalysts play a vital role in HER applications. Herein, a novel strategy to prepare a Ni2P/MoS2 cocatalyst through a simple hydrothermal-phosphorization method was reported, and the prepared cocatalyst was then loaded on an N-doped carbon substrate with excellent conductive performance. The large surface area of the carbon substrate provided many active sites, and the interface between Ni2P and MoS2 improved the catalytic performance for the HER. Compared with pure Ni2P catalyst and MoS2 catalyst, the prepared Ni2P/MoS2 cocatalyst exhibited enhanced catalytic performance. In addition, the results indicate that the prepared cocatalyst has a wide pH range and low onset potential values of 280, 350 and 40\u00a0mV in acidic, phosphate-buffered saline and alkaline solutions, respectively, and the corresponding Tafel slopes are 75, 121 and 95\u00a0mV dec\u22121, respectively. Density functional theory (DFT) was adopted to calculate the hydrogen adsorption free energy (\u0394GH\u2217). The results showed that the interface between Ni2P and MoS2 reduced \u0394GH\u2217, which was beneficial to the adsorption of hydrogen. Present preparation of cocatalysts with unique interfaces provides a new strategy for improving the catalytic performance of HER.\n               "}
{"full_text": "Data will be made available on request.Hydrogen is the green energy with the greatest potential in the 21st century. The hydrogen produced industrially in the water-to-gas conversion processes or steam reforming processes contains other impurities, including CO2, N2 and CO, etc. [1]. Therefore, it is required to purify or separate hydrogen. For this purpose, membrane separation technology with low energy consumption and a high degree of automation can be utilized [2]. The most often used inorganic membranes for gas separation are palladium metal membranes, microporous ceramic membranes, and carbon molecular sieve membranes [3\u20135]. Previous researches have demonstrated that microporous silica membranes offer remarkable gas separation performances. The most common preparation processes are chemical vapor deposition (CVD) and sol-gel techniques [6,7]. The sol-gel method is a promising candidate approach with favorable properties, such as large surface areas and superior gas separation performances.However, the primary issues with pure silica membrane include undesirable gas permeance and selectivity, as well as insufficient hydrothermal stability when exposed to water vapor environments [8]. An increase in selectivity is commonly at the expense of a decrease in membrane permeance. In general, it is quite difficult for one single material to overcome the contradiction between permeance and selectivity, so membranes need to be modified. In silica structures, the hydrolysis reactions of siloxane (SiOSi) bonds and water molecules result in the fracturing of SiOSi bonds and the formation of new moveable silanol (Si-OH) groups. Microporous structures become dense or even collapse as a result of the rearrangement of Si-OH groups and the continuation of condensation reactions. This circumstance frequently causes confusion in industrial gas treatment. Therefore, silica membranes used in industry must have greater hydrothermal stabilities in order to maintain relatively stable gas permeances and desirable separation performances throughout their operational lifetime. There are two methods for enhancing the hydrothermal stabilities of pure silica membranes. One strategy is to introduce hydrophobic groups to reduce the Si-OH concentrations on the membrane surfaces, hence decreasing the physical adsorption of water molecules, including alkylamine, methyl and phenyl, etc. [9\u201314]. The other is to dope transition metals/metal oxides into the silica networks, such as alumina, nickel, cobalt, tantalum, magnesium, niobium, titanium and zirconium, etc. [15\u201324]. This approach effectively prevents the densification of membranes following high-temperature treatment, thereby enhancing the hydrothermal stability and reproducibility of membranes. This is because oxygen atoms and transition metal atoms generate more stable covalent connections than SiO bonds, so structures of metal-doped silica membranes are more stable than those of the SiO2 membrane [8]. Numerous studies have demonstrated that this method has a vast array of positive effects.Nickel and cobalt elements have been proven to be promising metal nanoparticles in the field of gas separation. Nickel is a relatively inexpensive transition metal with high hydrogen affinity. Kanezashi et al. [16] investigated nickel-doped silica membranes with various nickel contents (Si/Ni\u00a0=\u00a04/1\u20131/1). The steady permeances of He and H2 for the nickel-doped silica membrane (Si/Ni\u00a0=\u00a02/1) at 500\u00a0\u00b0C and 90 kPa were 1.6\u00a0\u00d7\u00a010\u22125 and 4.6\u00a0\u00d7\u00a010\u22126 mol\u00a0m\u22122 Pa\u22121 s\u22121, respectively, and the He/N2 and H2/N2 permselectivities were 1450 and 400, respectively. In addition, the high hydrothermal stability of cobalt-doped silica membranes in mixed air streams has the curiosity of academic researchers. According to Uhlmann et al. [17], the He permeance was 9.5\u00a0\u00d7\u00a010\u22128 mol\u00a0m\u22122 Pa\u22121 s\u22121 with an optimum activation energy of 15\u00a0kJ\u00a0mol\u22121 of cobalt-doped silica membrane. The He/N2 permselectivity rose from 350 to 1100 under dry gas conditions after exposure to water vapor. Liu et al. [18] synthesized cobalt-doped silica membranes at 500\u00a0\u00b0C with a He/CO2 permselectivity of 479 and a He permeance of 3.3\u00a0\u00d7\u00a010\u22127 mol\u00a0m\u22122 Pa\u22121 s\u22121. After hydrothermal treatment, the He and H2 permeances decreased by 28\u00a0% and 22\u00a0%, respectively, and the He/CO2 permselectivity dropped to 190.The problems with pure SiO2 membrane applications are their undesirable H2/CO2 separation performance and poor hydrothermal stability in hydrothermal environment. In this investigation, hydrophobic group modification and metal doping are intended to solve the problems. MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u00a0=\u00a00, 0.024, 0.04, 0.056 and 0.08) materials were synthesized using the sol-gel method, and membranes were fabricated using the coating process under N2 atmosphere. The physicochemical properties and microscopic morphologies of the materials and membranes were systematically characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscope (TEM), N2 adsorption-desorption measurements and scanning electron microscope (SEM). Subsequently, the H2 permeances, H2/CO2 permselectivities and hydrothermal stabilities of the membranes were discussed in detail. To our knowledge, this is the first study to investigate the effects of methyl groups and nickel-cobalt doping on gas separation performances and hydrothermal stabilities of silica membranes, with the hope of inspiring future researches on binary metal-doped silica membranes.The samples were prepared by the sol-gel method using tetraethylorthosilicate (TEOS, purchased from Xi\u2019an chemical reagent Co., Ltd., Xi\u2019an, China) and methyltriethoxysilane (MTES, purchased from Hangzhou Guibao Chemical Co., Ltd., Hangzhou, China) as silica sources, nickel nitrate hexahydrogen (Ni(NO3)2\u00b76H2O, purchased from Tianjin Fuchen Chemical Reagent Co., Ltd., Tianjin, China) as nickel sources, cobaltous nitrate hexahydrate (Co(NO3)2\u00b76H2O, purchased from Tianjin Fuchen Chemical Reagent Co., Ltd., Tianjin, China) as cobalt sources, absolute ethanol (EtOH, purchased from Tianjin Branch Micro-Europe Chemical Reagent Co., Ltd., Tianjin, China) as organic solvents, nitric acid (HNO3, purchased from Sichuan Xilong Reagent Co., Ltd., Chengdu, China) as catalysts and deionized water prepared in the lab. In our earlier research, Co/MSiO2 membranes with n\nCo/n\nTEOS =\u00a00.08 exhibited optimal gas permselectivity and hydrothermal stability [25]. In this study, Ni-Co/MSiO2 materials and membranes were prepared with (n\nNi +\u00a0n\nCo)/n\nTEOS =\u00a00.08. The molar ratio of TEOS/MTES/Ni(NO3)2\u00b76H2O/Co(NO3)2\u00b76H2O/EtOH/HNO3/H2O was 1/0.8/x/0.08-x/8.5/0.085/6.8 (n\nNi and n\nCo were denoted by x and 0.08-x, respectively). The contents of n\nNi (x) and n\nCo (0.08-x) in the samples for this investigation are summarized in \nTable 1.The TEOS, MTES and Co(NO3)2\u00b76H2O solutions were completely dissolved in an ethanol solution, then placed in an ice-water bath, and strongly stirred for 0.5\u00a0h. Subsequently, a mixture of Ni(NO3)2\u00b76H2O solution, deionized water and HNO3 solution was added to the aforementioned solution. The reaction mixtures were then heated and refluxed in a water bath at 60\u00a0\u00b0C for 3\u00a0h, then cooled to 25\u00a0\u00b0C (heating and cooling rate 0.5\u00a0\u00b0C\u00a0min\u22121) to obtain the MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u00a0=\u00a00, 0.024, 0.04, 0.056 and 0.08) sols [20].Sols of MSiO2 and NixCo0.08\u2212x/MSiO2 were vacuum-dried at 30\u00a0\u00b0C to produce gels. The gels were crushed into powders and calcined at 400\u00a0\u00b0C under N2 atmosphere for 2\u00a0h in a tube furnace (heating rate of 0.5\u00a0\u00b0C\u00a0min\u22121) to get the final unsupported MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u00a0=\u00a00.024, 0.04, 0.056, and 0.08) materials [26].The microporous \u03b1-Al2O3 composite disks supports (dimensions of \u03a620 mm \u00d7 1.5 mm, purchased from Yixing Damai Ceramic Technology Research Institute, China) were lightly sanded with 600 and 1200 grit sandpaper, washed with water and ethanol to remove inorganic and organic contaminants, and dried overnight at 70\u00a0\u00b0C. The \u03b1-Al2O3 supports were submerged in sols for 1\u00a0min to ensure that the sols were adequately absorbed into the pores and were withdrawn at a pace of 10\u00a0cm\u00a0min\u22121. The samples were dried at 30\u00a0\u00b0C for 3\u00a0h and calcined in a tubular furnace at 400\u00a0\u00b0C under N2 atmosphere for 2\u00a0h (heating rate 0.5\u00a0\u00b0C\u00a0min\u22121). To minimize any defects caused by dust in the air, the deep coating process was repeated 4 times to obtain the final MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u00a0=\u00a00, 0.024, 0.04, 0.056 and 0.08) membranes [27]. \nFig. 1 depicts the schematic diagram of preparation process for MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u00a0=\u00a00, 0.024, 0.04, 0.056 and 0.08) sols/materials/membranes.The as-prepared MSiO2 and NixCo0.08\u2212x/MSiO2 membranes were exposed to saturated steam in an incubator at 200\u2009\u00b0C and 75\u2009% RH for 288\u2009h to obtain the steam-treated MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes. Next, MSiO2 and NixCo0.08\u2212x/MSiO2 membranes were calcined at 400\u2009\u00b0C in a tubular furnace under N2 atmosphere for 2\u2009h (heating rate 0.5\u2009\u00b0C\u2009min\u22121) to obtain regenerated MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes [8]. The gas permeances of steam-treated and regenerated MSiO2 and NixCo0.08\u2212x/MSiO2 membranes were investigated, respectively.Fourier transform infrared spectroscopy (FTIR) was performed on a Nicolet 5700 instrument to measure the functional groups of materials in the wavelength measurement range of 400\u20134000\u2009cm\u22121 using the KBr compression method. X-ray diffraction (XRD) was performed on a Rigaku D/max 2200 X-ray diffractometer to determine the chemical compositions of materials via CuK\u03b1 radiation (40\u2009kV, 40\u2009mA), and the scanning range was 4\u201390\u00b0 with the speed of 8\u00b0/min. The crystallizations of materials were analyzed by transmission electron microscopy (TEM) using a JEOL-JEM 2100F instrument. The BET surface areas and pore volumes of materials were determined by the N2 sorption-desorption isotherms using an ASAP 2020 instrument. The morphologies of membrane surfaces and cross-sections were analyzed by scanning electron microscope (SEM) using a Hitachi JEOL-JSM-6300 instrument with a 5\u2009kV acceleration voltage.The schematic diagram of the experimental setups of the gas dead-end permeance system is performed in \nFig. 2. The membranes were sealed in a stainless steel module using O-rings. Testing could begin after ventilation achieved a stable level (at least 3\u2009h). Single gas permeance of MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes were examined at various pressures and temperatures. The internal and external pressure difference (0.1\u20130.4\u2009MPa) and temperature (25\u2013200\u2009\u00b0C) were adjusted to meet the test parameters. H2 (0.289\u2009nm) and CO2 (0.33\u2009nm) with high-purity (99.99\u2009%) were used as permeance gases. Each value for each membrane was based on three membranes. Each membrane was measured three times under the same condition. According to the nine values, the average value and standard deviation of each membrane were calculated. The values displayed in the data analysis plot were the average values, and the standard deviations were represented as the error bars. The gas permeance tests followed the standard dead-end volume procedure, in which the data were obtained from the end bubble flow meter and recorded when the equilibrium state was reached (2\u2009h after gas stabilization). The ratio of H2 permeance to CO2 permeance determined the permselectivity of H2/CO2, known as the ideal selectivity. The gas permeance is represented by Eq. (1) and the permselectivity is expressed by Eq. (2):\n\n(1)\n\n\nF\n\n=\n\n\n\nQ\n\u00d7\nP\n\n\nA\n\u00d7\n\u2206\nP\n\u00d7\nT\n\u00d7\nR\n\n\n\n\n\n\n\n\n(2)\n\n\n\u03b1 \n=\n\n\n\n\nF\n\n\na\n\n\n\n\n\n\nF\n\n\nb\n\n\n\n\n\n\n\nwhere F is the gas permeance (mol\u2009m\u22122 Pa\u22121 s\u22121), Q is the gas flow through the flow meter (m3 s\u22121), P is the standard atmospheric pressure in Xi\u2019an, China (Pa, 1.05\u2009\u00d7\u2009105), A is the effective area of the membrane (m2), \n\u2206\nP is the pressure difference across the membrane (Pa), T is the absolute temperature (K) and R is the gas constant (J\u2009mol\u22121 K\u22121), and \u03b1 is the permselectivity.For steam-treated and regenerated MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes, the H2 and CO2 permeances were determined by repeating the aforementioned procedures, respectively, and the H2/CO2 permselectivity values were calculated using the same approach, respectively.The infrared spectra curves of unsupported MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) materials are shown in \nFig. 3. The bending vibration peak of water molecules is displayed at the absorption peak of 1645\u2009cm\u22121\n[28]. The absorption peak at 3450\u2009cm\u22121 is attributed to the stretching vibration of -OH groups in structurally coordinated water and physically adsorbed water. The stretching vibration peak of -CH3 groups and the characteristic absorption peak of Si-CH3 groups for TEOS and MTES appear at 2978\u2009cm\u22121 and 1276\u2009cm\u22121, respectively, demonstrating that the hydrophobic -CH3 groups successfully branch onto the Si atoms. The absorption peaks at 790 and 443\u2009cm\u22121 are considered as symmetrical stretching vibration of the OSiO bonds and the bending vibration of the SiO\nSi bonds, respectively. In the MSiO2 material, the absorption peak at 1050\u2009cm\u22121 accompanied by a shoulder is ascribed to the asymmetric stretching vibration of the SiOSi bonds, confirming that the sol-gel process has been carried out satisfactorily [29]. The absorption peaks at approximately 1031, 1012 and 1023\u2009cm\u22121 are assigned to the vibration of the SiOSi bonds for Ni/MSiO2, Co/MSiO2 and Ni0.024Co0.056/MSiO2 materials, respectively. This is because the introduction of nickel or/and cobalt elements can destroy the symmetry of SiO2 and lead to the shift of the SiOSi bond position. In addition, there is an absorption peak at approximately 960\u2009cm\u22121 in nickel or/and cobalt-doped silica materials, but not in the MSiO2 materials. Metals can be covalently attached to siloxanes to form SiOM bonds, thereby forming a denser network structure [30,31]. Therefore, it can be speculated that the absorption peak is the SiOM (M\u2009=\u2009Ni, Co) bonds. The nickel and cobalt elements have been successfully integrated into the MSiO2 network to form Si-O-Ni or/and SiOCo bonds, which is conducive to overcoming the problem of the hydrolysis reaction of SiOSi bonds with water molecules under water vapor conditions. This minimizes the possibility of structural collapse after regeneration and sets the groundwork for enhancing the hydrothermal stability and repeatability of silica membranes.The XRD patterns of unsupported MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) materials are illustrated in \nFig. 4. The distinct diffraction peak at around 2\u03b8\u2009=\u200923\u00b0 can be assigned to the typical amorphous SiO2 phase of all samples [32,33], suggesting that methyl modification, nickel or/and cobalt doping does not change the phase structure of silica particles. The doping of nickel or/and cobalt elements reduces the SiO2 peak intensity. This is because some silica atoms are replaced by the introduced nickel or/and cobalt elements to form Si-O-Ni or/and SiOCo bonds, resulting in the decrease of SiOSi bonds. In addition to the amorphous SiO2 phase, another absorption peak in the Ni/MSiO2 material is detected at 2\u03b8\u2009=\u200943.31\u00b0, corresponding to the (200) crystalline plane of NiO (JCPDS no. 47-1049). The other diffraction peaks in the Co/MSiO2 and Ni0.24Co0.056/MSiO2 materials appear at 2\u03b8\u2009=\u200942.49\u00b0 and 61.64\u00b0, corresponding to the (200) and (220) crystalline planes of CoO (JCPDS no. 70-2855). The peaks of NiO and CoO are not pronounced due to the low concentrations of nickel and cobalt elements. The full-width at half maxima of the characteristic reflection with the highest intensity (200) is used to calculate the mean crystallite size with the aid of making use of the Scherrer equation [34], as indicated in Eq. (3):\n\n(3)\n\n\nD \n=\n\n\n\nk\n\u03bb\n\n\n\u03b2\ncos\n\u03b8\n\n\n\n\n\nwhere D is the size of NiO/CoO crystallites, k is the constant value of Scherrer (0.89), \u03bb is the wavelength of X-ray source (0.154\u2009nm), \u03b2 is the full width at half maximum intensity, and \u03b8 is the Bragg angle.The mean size of NiO crystal in the Ni/MSiO2 material is calculated as 4.5\u2009nm, and the mean size of CoO crystals in the Co/MSiO2 and Ni0.24Co0.056/MSiO2 materials is calculated as 4.8\u2009nm. The synthesis of NiO and CoO is advantageous for facilitating the surface diffusion of H2 molecules, and has positive effects on improving the H2 permeances and separation performances.\n\nFig. 5 depicts photos of unsupported MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) materials. In nickel or/and cobalt-doped silica materials, the color gradually becomes lighter as the cobalt content decreases and the nickel content increases. They are blue-violet, blue-gray, cyan-gray, gray-green and pale-yellow-green, respectively. It can be inferred that Co(NO3)2\u00b76H2O and Ni(NO3)2\u00b76H2O solutions are successfully thermally decomposed into CoO and NiO during the calcination processes (black-gray for CoO and yellow-green for NiO), which is consistent with the XRD analysis (Fig. 4).The TEM images of unsupported MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) materials are presented in \nFig. 6. The darker-contrast particles are attributed to the metal oxides due to the differences in electronic density. The presence of different-sized particles in the image indicates that the oxides of nickel and cobalt have been effectively doped into the amorphous silica matrix and are irregularly distributed as single-dispersed particles. The lighter-contrast structures are attributed to the silica carriers, which exhibit an amorphous three-dimensional mesh structure. The metal-doped silica materials have more dispersed three-dimensional structures, which is more advantageous to reducing the densification of the MSiO2 material.The N2 adsorption-desorption isotherms and pore size distributions of unsupported MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) materials are demonstrated in \nFig. 7. The N2 adsorption capacity rises dramatically at lower relative pressures (P/P0 <\u20090.1), which is caused by the strong adsorption potential energies in the pores. Therefore, it is speculated that there are a large number of micropores in the materials. Subsequently, as the relative pressure continues to increase, the amounts of N2 adsorption capacity is still slowly increasing. Except for single-layer adsorption, the pore structures contain multi-layer adsorption and even capillary condensation. This adsorption type corresponds to the Type I adsorption isotherm of the IUPAC classification. These samples possess the same characteristics as microporous materials. Notably, the adsorption isotherm of the MSiO2 material exhibits hysteresis and the loop is not closed. This may be because the MSiO2 material collapsing in liquid nitrogen at low temperatures and N2 molecules becoming trapped in micropores, resulting in non-overlapping adsorption-desorption curves. This phenomenon does not occur in other materials, because the doping of nickel or/and cobalt elements activates the pore size, resulting in larger microporous volumes, thereby forming more stable silica frameworks [35]. The pore size distribution curves in Fig. 7b are obtained by Horvath-Kawazoe (HK) and Barrett-Joyner-Halenda (BJH) methods. The pore size distributions tend to widen due to the doping of nickel and cobalt elements. The pore size distribution of the MSiO2 material ranges from 1.5\u2009\u00c5 to 10\u2009\u00c5, and that of nickel or/and cobalt-doped silica materials ranges from 1.5\u2009\u00c5 to 15\u2009\u00c5.The pore structure parameters of unsupported MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) materials are listed in \nTable 2. The doping of nickel or/and cobalt enlarges the BET surface area, total pore volume and average pore size of the MSiO2 material, which is because the bond lengths of NiO or/and CoO bonds is longer than those of the SiO bonds (the bond lengths of NiO, CoO and SiO are approximately 2.1, 2.3 and 1.64\u2009\u00c5, respectively). For membranes with gas-selective permeance, a larger total pore volume and a smaller average pore size can guarantee that the membrane works as an effective sieve, allowing for greater gas permeances while preventing the passage of larger gas molecules. The Ni0.24Co0.056/MSiO2 membrane is more beneficial as an effective sieve for gas separation. The optimized molecular structure models based on molecular dynamics simulations of MSiO2, Ni/MSiO2, Co/MSiO2 and Ni0.24Co0.056/MSiO2 materials are shown in \nFig. 8. The molecular structure distribution of the Ni0.024Co0.056/MSiO2 material is more uniform and tighter, which lays a foundation for the membrane to achieve superior gas separation and hydrothermal stability.\n\nFig. 9 illustrates SEM images of surfaces and cross-sections for MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes. The surface coverages are relatively complete without visible pinholes and cracks, and the surface particles are relatively homogeneous. Compared with the MSiO2 membrane, a small number of large particles are observed on the surfaces of the nickel or/and cobalt-doped silica membranes due to the formation of NiO or/and CoO. The membrane cross-section shows an asymmetric configuration and can be divided into two layers. The upper part is the selective layer, which is the medium of gas permselectivity. The thickness of selective layers for MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes are approximately 2.3\u20132.8\u2009\u00b5m. The \u03b1-Al2O3 support layer provides mechanical strength as the membrane system\u2019s foundation. The membrane cross-sections reveal irregular structures due to the shapes of the \u03b1-Al2O3 support bodies. Overall, the porous \u03b1-Al2O3 supports are successfully loaded with a selective layer for gas separation.\n\nFig. 10 displays influence of pressure difference on H2 permeances and H2/CO2 permselectivities for MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes at 25\u2009\u00b0C. The H2 permeances of MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes at 0.3\u2009MPa and 25\u2009\u00b0C are 2.4\u2009\u00d7\u200910\u22127\u20136.4\u2009\u00d7\u200910\u22127 mol\u2009m\u22122 Pa\u22121 s\u22121 and the H2/CO2 permselectivities are 19.2\u201356.8. With the increase of differential pressure, the H2 permeance and H2/CO2 permselectivity of the MSiO2 membrane is basically insignificant, but those of metal-doped silica membranes marginally increase. The H2 permeances of MSiO2, Ni/MSiO2, Co/MSiO2 and Ni0.24Co0.056/MSiO2 membranes at 0.3\u2009MPa and 25\u2009\u2103 rise by 0.3\u2009%, 1.0\u2009%, 1.4\u2009% and 2.0\u2009%, and the H2/CO2 permselectivities increase by 2.0\u2009%, 3.4\u2009%, 4.3\u2009% and 2.2\u2009%, respectively, compared with those at 0.1\u2009MPa and 25\u2009\u00b0C. The rise in pressure difference facilitates the adsorption and transport of H2 molecules, so the H2 permeances and H2/CO2 permselectivities of silica membranes are improved at a relatively high differential pressure (0.3\u2009MPa).The influence of temperature on H2 permeances and H2/CO2 permselectivities for MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes at a pressure difference of 0.3\u2009MPa are investigated in \nFig. 11. The H2 permeances and H2/CO2 permselectivities of all tested membranes increase practically linearly as the temperature within the measuring range rises. The H2 permeance and H2/CO2 permselectivity of the Ni0.24Co0.056/MSiO2 membrane at 0.3\u2009MPa and 200\u2009\u00b0C are 1.2 and 2.0 times higher than those at 0.3\u2009MPa and 25\u2009\u00b0C, respectively. The transport of H2 molecules is dominated by the activation-diffusion process, so the relatively high temperature is more conducive to the movement of H2 molecules, thereby increasing the H2 permeance. The CO2 permeances are not optimal at relatively higher temperatures due to the violent movement of CO2 molecules and the increase in the average free path. These factors are conducive to promoting the increase of H2/CO2 permselectivity. The results show the relatively high temperature (200\u2009\u00b0C) is favorable to promoting the H2 permeances and H2/CO2 permselectivities.The apparent permeance activation energy (E\na) quantifies the energy required for gas molecules to permeate through membrane pores. According to the Arrhenius Law [36], the permeance F is the temperature-dependent parameter represented by Eq. (4):\n\n(4)\n\n\nF\n\n=\n\n\n\nF\n\n\n0\n\n\nexp\n\n(\n\u2013 \n\n\n\n\nE\n\n\na\n\n\n\n\nRT\n\n\n)\n\n\n\nwhere F is the gas permeance (mol\u2009m\u22122 Pa\u22121 s\u22121), F\n0 is the maximum permeance at infinitely high temperatures (mol\u2009m\u22122 Pa\u22121 s\u22121), E\na is the apparent permeance activation energy (kJ\u2009mol\u22121), R is the gas constant (J\u2009mol\u22121 K\u22121) and T is the absolute temperature (K). Eq. (4) can be described in Eq. (5):\n\n(5)\n\n\nln\nF\n\n=\n\nln\n\n\nF\n\n\n0\n\n\n\u2013\n\n\n\n\nE\n\n\na\n\n\n\n\nRT\n\n\n\n\n\n\n\n\nFig. 12 performs the Arrhenius relationships of H2 and CO2 permeances in MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes. \nTable 3 lists the apparent permeance activation energy (E\na) of H2 and CO2 for MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes. There are percolative paths in the membranes that allow H2 and CO2 molecules to diffuse. The E\na values of H2 molecules are positive, while E\na values of CO2 molecules are negative, consistent with prior results [37]. The positive or negative E\na values are correlated with the gas transport mechanisms. The negative values of E\na are frequently interpreted as the strong adsorption of molecules on the pore surface. The magnitude of E\na value depends on the pore diameter, porosity and interaction between pore walls and gas molecules. The higher E\na value indicates the larger energy barrier that the gas must overcome during transport in the membrane pores, hence raising the difficulty coefficient of gas diffusion. The E\na value of H2 in the Ni0.024Co0.56/MSiO2 membrane is lower than that in other membranes, so H2 molecules can permeate into the membrane with less repulsive force. This is because the doping of nickel and cobalt elements enlarges the total pore volume of the MSiO2 membrane, which successfully reduces the densification of the SiO2 network. In addition, the NiO and CoO have the good affinities for H2 molecules, which facilitates the surface diffusion of H2 molecules. However, the E\na value of CO2 in the Ni0.024Co0.56/MSiO2 membrane is higher than that in other membranes. The NiO and CoO are alkaline metal oxides with significant adsorption effects on acidic CO2 molecules [38]. Therefore, CO2 molecules are adsorbed on the membrane pore wall, resulting in the contraction of pore walls and an increase in transport resistance of CO2 molecules [20]. The Ni0.024Co0.56/MSiO2 membrane exhibits the lowest diffusion resistance of H2 molecules and the highest diffusion resistance of CO2 molecules among all tested membranes, which effectively promotes H2 permeance and H2/CO2 permselectivity.\n\nFig. 13 demonstrates the H2 permeances and H2/CO2 permselectivities for MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes at a pressure difference of 0.3\u2009MPa and 200\u2009\u00b0C. The H2 permeances of MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) membranes are 3.6\u2009\u00d7\u200910\u22127\u20137.6\u2009\u00d7\u200910\u22127 mol\u2009m\u22122 Pa\u22121 s\u22121 and the H2/CO2 permselectivities are 41.2\u2013113.5. The binary nickel-cobalt doping is more favorable to achieving higher H2 permeances and H2/CO2 permselectivities than doping with the single metal. The total amount of nickel and cobalt elements in binary nickel-cobalt-doped silica membranes remain unchanged, but the increase in cobalt content is more conducive to promoting H2 permeances and H2/CO2 permselectivities. So it becomes clear that this specific Ni/Co substitution ratio of Ni0.024Co0.056/MSiO2 membrane is responsible for the exhibited maximum H2 permeance and H2/CO2 permselectivity. Therefore, this specific Ni/Co substitution ratio seems to create an average pore size that is the most effective for H2/CO2 separation.The Ni0.024Co0.056/MSiO2 membrane exhibits superior H2 permeance and H2/CO2 permselectivity than the MSiO2 membrane. Compared with the MSiO2 membrane, the H2 permeance and H2/CO2 permselectivity of the Ni0.024Co0.056/MSiO2 membrane increase by 2.1 and 2.8 times, respectively. The possible mechanisms in MSiO2 and Ni0.24Co0.056/MSiO2 membranes for H2/CO2 separation are presented in \nFig. 14. The separation of gas molecules is based on differences in pore sizes and differences in affinity for gas molecules on the surface of membranes. The introduction of nickel and cobalt elements enlarges the total pore volume and microporosity of the MSiO2 material (Table 2). The diffusion mechanism of H2 in the metal-doped membrane differs from that of the MSiO2 membrane. The NiO and CoO have the significant affinities for H2 molecules, which can enhance the surface diffusion of H2 molecules, hence facilitating adsorption and transport of H2 molecules, which is conducive to the growth of H2 permeance and H2/CO2 permselectivity [39].\n\nFig. 15 plots the H2 permeances and H2/CO2 permselectivities of steam-treated and regenerated MSiO2 and NixCo0.08\u2212x/MSiO2 membranes at a pressure difference of 0.3\u2009MPa and 200\u2009\u00b0C. The H2 permeance and H2/CO2 permselectivity of the steam-treated Ni0.024Co0.056/MSiO2 membrane increase, while those of the steam-treated MSiO2 membrane decrease. Compared with the fresh samples, the H2 permeance and H2/CO2 permselectivity of the steam-treated MSiO2 membrane drop by 9.3\u2009% and 8.7\u2009%, respectively, and those of the steam-treated Ni0.024Co0.056/MSiO2 membrane rise by 9.8\u2009% and 5.4\u2009%, respectively. The hydrophilic silanol group (Si-OH) on the MSiO2 membrane surface is the active physical adsorption center, so the Si-OH group easily absorbs water vapor when the MSiO2 membrane is in a humid heat state for a long time. This causes a hydrolysis reaction between the SiOSi bonds in the silica structure and the water molecules, which causes the SiOSi bonds to break and form new Si-OH groups. The Si-OH groups occupy part of pore spaces of the MSiO2 membrane, which increases the diffusion resistance of H2 molecules, thereby reducing the H2 permeance and H2/CO2 permselectivity. The doping of nickel and cobalt elements broadens the total pore volume and average pore size of the MSiO2 membrane, consequently decreasing the H2 resistance through the steam-treated Ni0.024Co0.056/MSiO2 membrane and enhancing the H2 permeance and H2/CO2 permselectivity. In addition, the nickel and cobalt elements formed SiONi and SiOCo bonds can prevent the network structure of the steam-treated Ni0.024Co0.056/MSiO2 membrane from breaking, therefore enhancing the hydrothermal stability.The H2 permeance and H2/CO2 permselectivity of the regenerated MSiO2 membrane exhibit the recovery trend, which is because the physically adsorbed water in membrane pores evaporate at high temperature, so the diffusion resistances of H2 through the MSiO2 membrane decrease, resulting in the inevitable increase of H2 permeance and H2/CO2 permselectivity. The H2 permeance and H2/CO2 permselectivity of the regenerated MSiO2 membrane are still lower than those of fresh samples, but those of the regenerated Ni0.024Co0.056/MSiO2 membrane continue to increase. Compared with the fresh samples, the H2 permeance and H2/CO2 permselectivity of the regenerated MSiO2 membrane decrease by 7.5\u2009% and 7.4\u2009%, respectively, and those of the regenerated Ni0.024Co0.056/MSiO2 membrane increase by 12.8\u2009% and 8.3\u2009%, respectively. The Si-OH groups on the MSiO2 membrane surface underwent further rearrangement and condensation reactions during the regeneration process, resulting in the compact and even collapse of the micropore structure, showing lower H2 permeance and H2/CO2 permselectivity than the fresh samples. This effect may be mitigated by the presence of Ni-O-Si and Co-O-Si bonds, so the Ni0.024Co0.056/MSiO2 membrane demonstrates more repeatability. The findings demonstrate that the introduction of nickel and cobalt significantly improves the hydrothermal stability and repeatability of the MSiO2 membrane.\n\nTable 4 demonstrates the pore sizes, E\na values, gas permeances and permselectivities, hydrothermal stability of various silica membranes. There are two methods to characterize the pore sizes. One is to measure the pore sizes by gas permeation measurements using the supported membranes, and the other is to calculate pore sizes by the N2 adsorption-desorption isotherms using the unsupported membrane materials. Since the drying stresses of unsupported silica membrane materials and supported silica membranes during heat treatment are not exactly the same, the two materials cannot be expected to have exactly the same structure. The particle size and pore structure data of unsupported membrane materials cannot be quantitatively converted to the situation of supported membranes, but can qualitatively indicate the changing trend of material structure in the treatment process. The silica membranes can be prepared by multi-step coating process which can aid in reduction of the number of defect sites and decrease the pore size that gas molecules go through. Generally speaking, a larger pore size may lead to a lower apparent permeance activation energy, a higher gas permeance and a lower gas permselectivity. The gas permeance and permselectivity are the two most important parameters of silica membranes, although there is always a trade-off between both. The increase of gas permeance is always at the expense of gas permselectivity and vice versa, so it is difficult to simultaneously enhance gas permeance and permselectivity. The introduction of nickel and cobalt elements can boost the surface diffusion of H2 molecules, hence promoting the adsorption and transport of H2 molecules, which is advantageous for achieving higher H2 permeances and H2/CO2 permselectivities. Furthermore, the NiOSi and CoOSi bonds formed by nickel and cobalt doping can improve the hydrothermal stability and repeatability of silica membranes. Binary nickel-cobalt metal doping provides a novel avenue to develop high-performance membranes with enhanced gas permeance and hydrothermal stability.In summary, MSiO2 and NixCo0.08\u2212x/MSiO2 (x\u2009=\u20090, 0.024, 0.04, 0.056 and 0.08) materials and membranes were successfully prepared using TEOS, MTES, Ni(NO3)2\u00b76H2O and Co(NO3)2\u00b76\u2009H2O solutions. The physicochemical properties and microscopic morphologies were systematically characterized by FTIR, XRD, TEM, N2 adsorption-desorption and SEM. The H2/CO2 permselectivities of membranes were evaluated by differential pressure, temperature, Ni/Co content and hydrothermal stability and other factors. The phase structure analysis proved that nickel and cobalt elements were successfully incorporated into SiO2 network in the form of SiONi/SiOCo bonds and NiO/CoO. The apparent activation energies of H2 permeances in MSiO2 and Ni0.024Co0.056/MSiO2 membranes were 2.67\u2009\u00b1\u20090.04 and 1.13\u2009\u00b1\u20090.06\u2009kJ\u2009mol\u22121, respectively. When operating at a pressure difference of 0.3\u2009MPa and 200\u2009\u00b0C, the H2 permeances of MSiO2 and NixCo0.08\u2212x/MSiO2 membranes were 3.6\u2009\u00d7\u200910\u22127 \u00d7\u200910\u22127 mol\u2009m\u22122 Pa\u22121 s\u22121and 7.6\u2009\u00d7\u200910\u22127 mol\u2009m\u22122 Pa\u22121 s\u22121, respectively, and the H2/CO2 permselectivities were 41.2 and 113.5, respectively. Compared with the MSiO2 membrane, the H2 permeance and H2/CO2 permselectivity of the Ni0.024Co0.056/MSiO2 membrane rose by 2.1 and 2.8 times, respectively. After steam treatment and regeneration, compared with the fresh samples, the H2 permeance and H2/CO2 permselectivity of the MSiO2 membrane fell by 7.5\u2009% and 7.4\u2009%, respectively, and those of the Ni0.024Co0.056/MSiO2 membrane increased by 12.8\u2009% and 8.3\u2009%, respectively. The doping of nickel and cobalt elements enhanced H2 permeances and H2/CO2 permselectivities, and counteracted numerous effects of water on silica matrixes, thereby improving the hydrothermal stability of the MSiO2 membrane. We will conduct additional comprehensive tests of binary nickel-cobalt-doped silica membranes for separating H2 from various gases (N2, CH4 and O2) in order to determine their practical efficiency in industrial applications.\nMengyu Yan: Conceptualization, Writing \u2013 original draft. Jing Yang: Conceptualization, Writing \u2013 review & editing, Funding acquisition. Ruihua Mu: Methodology, Project administration. Yingming Guo: Software, Project administration. Xinshui Cui: Supervision, Data curation. Jinghua Song: Formal analysis.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 Key Research and Development Projects of Shaanxi Province, China [2022SF-287 and 2021GY-147]; and the Scientific Research Plan Projects of Shaanxi Education Department, China [19JC017 and 21JK0650].", "descript": "\n                  Silica membranes possess gas separation characteristics, but their undesirable performances and poor hydrothermal stabilities hinder the application in industrial gas treatments. To solve the problems, we have devised a new membrane preparation method involving methyl group modification and nickel-cobalt doping. In this paper, methyl-modified silica (MSiO2) and nickel-cobalt-doped methyl-modified silica (NixCo0.08\u2212x/MSiO2, x\u00a0=\u00a00, 0.024, 0.04, 0.056 and 0.08) materials and membranes were synthesized using the sol-gel technique. The physical-chemical properties of materials were characterized by FTIR, XRD, TEM, N2 adsorption-desorption and SEM. The H2 permeances and permselectivities of membranes were evaluated with pressure difference, temperature, Ni/Co content and hydrothermal stability as the inferred factors. In Ni-Co/MSiO2 materials, nickel and cobalt elements were found in the form of Si-O-Ni/Si-O-Co bonds and NiO/CoO. The H2 permeance and H2/CO2 permselectivity of the Ni0.024Co0.056/MSiO2 membrane were 7.6\u00a0\u00d7\u00a010\u22127 mol\u00a0m\u22122 Pa\u22121 s\u22121 and 113.5, respectively, which were 2.1 and 2.8 times higher than the MSiO2 membrane at 0.3\u00a0MPa and 200\u00a0\u00b0C. After steam treatment and regeneration, the Ni0.024Co0.056/MSiO2 membrane increased from the original values in the H2 permeance and H2/CO2 permselectivity, while the MSiO2 membrane decreased. The final results revealed that Ni-Co/MSiO2 membranes possessed excellent H2/CO2 permselectivity and hydrothermal stability.\n               "}
{"full_text": "Although the lockdowns and other measures implemented to cope with the Covid-19 pandemic substantially reduced the global energy demand, it is expected that the trend will be on the rise again in the near future. By and large, fossil fuel is still an important source of energy for industries, households, especially for the various means of transportation. Due to the environmental impact such as greenhouse emissions and the climate change, many forms of alternative energy have been proposed to substitute the use of fossil energy. Considered as carbon neutral, energy from plants in the form of biofuel has gained interests worldwide since there is a wide variety of raw materials that can be used such as linseed oil, jatropha oil, coconut oil, soybean oil, sunflower oil, palm oil, etc. [1\u20137]. However, food security as well as economic viability are still the main issues to be considered when developing the processing plants.Crude palm kernel oil (CPKO) is vegetable oil obtained from the inner part of palm fruit without bleaching or refining process. Using CPKO as raw material for the production of biofuel certainly helps improve the economic performance of the process. CPKO consists of both saturated and unsaturated fatty acid chains esterified to the glycerol backbone, which can potentially be converted into other products such as alkanes, alkenes, aromatics, isomer compounds, or cyclic compounds depending on the type of catalyst and the operating conditions. Hence, CPKO can be used as raw material to produce biojet, in which can be used for the aviation industry. The conventional production of biojet relies heavily on the fossil resources. Four alternative methods have been proposed for producing jet fuel from non-fossil raw materials. The first method is alcohol-to-jet fuel using methanol, ethanol, butanol, and long-chain fatty alcohol as raw material. Another method is called gas-to-jet fuel, converting biogas, natural gas, syngas into jet fuel via fermentation and Fisher Tropsch synthesis. Various types of sugar such as sugarcane, corn, fruit residue can be used to produced jet fuel by the process called sugar-to-jet. The fourth method is oil-to-jet, which is the chemical conversion of vegetable oil into bio-jet fuel via hydro-processing, hydrotreating, deoxygenation, isomerization/hydrocracking [8\u201310]. This work is related to the application of oil-to-jet method, which involves the use of hydrogen gas to transform unsaturated hydrocarbon, aromatics, and heteroatoms into biojet fuel.Hydrocracking is a chemical reaction that usually requires high temperatures exceeding 400\u00a0\u00b0C in order to produce smaller molecules. Deoxygenation reaction (DO) of triglyceride with hydrogen can occur with the presence of metal catalyst at moderate temperatures (250\u2013450\u00a0\u00b0C) and the pressure of 10\u2013300\u00a0bar [8,11\u201315]. There are three different chemical reactions in this category including hydrodeoxygenation (HDO), decarbonylation (DCN), and decarboxylation (DCX). The first reaction eliminates oxygen in the form of water molecule. On the other hand, DCN and DCX release oxygen in the form of CO and CO2, respectively, causing a reduced number of carbon in the hydrocarbon product [16]. Platinum-group metals (Ru, Rh, Os, Ir, Pd, and Pt) have been used as catalyst for the conversion of vegetable oil to biofuel. Snare et al., [17] studied the effect of different metal catalysts (Pd, Pt, Ru, Mo, Ni, Rh, Ir, and Os) supported on carbon and metal oxide on the deoxygenation of stearic acid, which is a saturated fatty acid with 18 carbon atoms. The main product was n-heptadecane (n-C17). Both 5%Pd/C and 5%Pt/C provided high conversion and high selectivity toward C17 exceeding 90%. Pt/C was also reported as an outstanding catalyst for hydrogenation and deoxygenation reactions especially DCN and DCX [18,19]. This helps reduce the amount of water as by-product (from HDO) and enhances the overall biofuel yield. Fu et al. [20] performed hydrothermal conversion of five different fatty acids including stearic, palmitic, lauraic, oleic, and linoleic acid at high temperatures over Pt/C catalyst. Saturated fatty acids (stearic acid, palmitic acid, and lauric acid) were converted to n-alkanes via decarboxylation reaction with high selectivity exceeding 90%. For unsaturated fatty acid (oleic acid and linoleic acid), the major pathway was the hydrogenation reaction producing saturated fatty acid followed by decarboxylation to form n-alkanes. This research showed that Pt/C can be used as an efficient catalyst for both deoxygenation and hydrogenation reactions. Scaldaferri et al. [21] applied a group of catalysts including Pd/C, NbOPO4, ZSM5, and beta-zeolite for deoxygenation of soybean oil under 10\u00a0bar of nitrogen pressure in a batch reactor. Among these catalysts, Pd/C provided the highest yield of bio-jet fuel (62%) followed by niobium phosphate (50%). The fraction of oxygenated compound was only 1% for the case of Pd/C catalyst. The stable and oxygen-free hydrocarbon product tends to have low viscosity, improving flow characteristics at low temperatures [22]. These findings in the literature confirm that Pt/C catalyst is one of the promising catalysts for biojet application.Freezing point is the key property for jet fuel that should be kept below \u221247\u00a0\u00b0C in order to maintain flow properties at high altitudes (low temperature). The content of iso-alkanes, cycloalkanes and small molecules of hydrocarbons can greatly affect the freezing point of jet fuel [23]. The heat of combustion must be of at least 42.8\u00a0MJ/kg while the content of aromatics must not exceed 25\u00a0vol% according to standard specification for aviation turbine fuel (ASTM D1655-04a, ASTM D7566, JP-8 MIL-DTL-83133E) [24,25].The choice of reactor type is one of the important elements for the production of biojet. Many newly developed catalysts have been tested in a batch reactor where the reacting mixture is thoroughly mixed. In this system, the overall rate of reaction is impeded due to the dilution with reaction products and chemical equilibrium. Another issue is the separation and recycling of catalyst after the operation completed. To reduce the effects of dilution and chemical equilibrium while enhancing the productivity, this work applied a continuous fixed bed reactor, where the solid catalyst is placed inside. Moreover, the small footprint of a continuous process also improves the safety of the operation especially when dealing with high pressures.Therefore, this study applied 5%Pt/C as catalyst for deoxygenation of CPKO. The catalyst was packed in a continuous fixed bed reactor. The effect of operating parameters including reaction temperature, pressure, amount of catalyst, hydrogen flow rate, and CPKO flow rate on the reaction performance were investigated. The fuel properties of the product at the optimal conditions were compared with various standards and the reaction performance was compared with other systems as reported in the literature.CPKO was obtained from Univanich Palm Oil Public Company Limited, Thailand. It was kept in an amber glass bottle at \u221220\u00a0\u00b0C. Standard n-alkane (nC8-nC20, 40\u00a0mg/L each in hexane) solution was supplied by Sigma-Aldrich. Cyclohexane (GC grade, 99.8%) and methyl alcohol (HPLC grade, 99.9%) for esterification and transesterification reactions were purchased from Fisher Chemical. Sulfuric acid (ACS reagent grade, 98%) was purchased from Merck. The 5% Pt/C catalyst and H-ZSM-5 zeolite (Si/Al\u00a0=\u00a015) were supplied by Riogen Inc. Isopropanol (AR grade, 99.7%) was purchased from QReC. Sodium sulphate anhydrous (99.0%) and potassium hydroxide pellet (85%) were obtained from Carlo-Erba. Ultra-high purity helium (99.999%), compressed air (air zero), and high purity hydrogen (99.99%) were supplied by Linde (Thailand).The catalyst was characterized by Scanning Electron Microscope and Energy Dispersive X-ray Spectrometer (S-4800, Hitachi) to study the surface morphology and dispersion of metal on the support. Nitrogen physisorption (3Flex surface characterization analyzer, Micromeritics) was used to determine the specific surface area according to Brunauer-Emmett-Teller theory (BET). The total pore volume and the average pore size were calculated by Barrett-Joyner-Halenda (BJH) method. Ammonia (NH3) temperature programed desorption (BELCAT II, Thermo Finnigan) was used to determine the acidity of the catalyst.Pulse H2 chemisorption (AutoChem II 2920 chemisorption analyzer, Micromeritics) was applied to determine the metal dispersion of Pt on the activated carbon. The molecular coordination of hydrogen to metal was 1:2 [26]. The amount of 0.1\u00a0g of catalyst was placed in a U-glass tube. At 330\u00a0\u00b0C under inert atmosphere, the stream of argon flowing at a rate of 30\u00a0mL/min was used to flush the impurity off the catalyst for 90\u00a0min. Then the temperature was changed to 45\u00a0\u00b0C. Pulse chemisorption was performed until saturation was achieved.To obtain the profile of free fatty acids of CPKO (acid value was 33.95\u00a0mg KOH/g oil), two reaction steps were performed. The first step was the esterification of CPKO. After removing the moisture in CPKO, excess methanol and 4\u00a0wt% sulfuric acid were added. The reaction was carried out at 65\u00a0\u00b0C in a round-bottom flask with reflux. The product was washed with deionized water. After esterification reaction, the acid value was 0.83\u00a0mg KOH/g oil. Then the oil was transesterified with methanol at 65\u00a0\u00b0C for 1.5\u00a0h using 1.5\u00a0wt% KOH as catalyst. The obtained fatty acid methyl ester (FAME) was washed with deionized water, followed by removing the moisture. The sample was then analyzed by GC\u2013MS.Thermal gravimetric analysis (TGA/DSC 3+, Mettler Toledo) was used to study the decomposition of CPKO in the temperature range of 35\u2013650\u00a0\u00b0C with the heating rate of 10\u00a0\u00b0C/min and the nitrogen flow rate of 60\u00a0mL/min.The catalyst particles were packed in a tubular reactor made of stainless steel (od. \u00bc inch\u00a0\u00d7\u00a00.049 in WT). Two layers of glass wool (0.1\u00a0g for each layer) were used to sandwich the catalyst bed in the middle. The reactor was placed inside a tubular furnace, where the reaction temperature was adjusted by means of a temperature controller. For a typical experiment, the catalyst bed was treated with hydrogen flowing at a rate of 70\u00a0mL/min at 330\u00a0\u00b0C for 90\u00a0min. A certain amount of CPKO was ultrasonicated to eliminate any dissolved gas prior to be fed as reactant. Then the flow rate of hydrogen was adjusted to the desired value via a mass flow controller while the flow rate of CPKO was adjusted via HPLC pump. Both streams were mixed in a J-mixer (od. 1/16 in, 0.020 in thru hole) with liquid stream at right angle. Then the reacting mixture entered the reactor previously packed with catalyst particles. The exit-end of the reactor was equipped with a back-pressure regulator and a separator that was used to separate gas and liquid products as shown in Fig. 1\n. Liquid samples were collected every hour for the total of 10\u00a0h at the sampling port. Anhydrous NaSO4 was used to remove water from the samples prior to the analysis. The operating conditions applied are summarized in Table 1\n.For the analysis by GC-FID and GC\u2013MS, the oil samples were diluted with cyclohexane (CAS number 110-82-7). The volume of 1 \u03bcL of diluted oil sample was injected with a split ratio of 40:1 to the GC-FID (HP6890, Agilent) equipped with DB-5HT GC column (30\u00a0m, 0.25\u00a0mm in diameter and 0.25\u00a0\u00b5m film thickness). The injector temperature and detector temperature were set at 350\u00a0\u00b0C. The oven was heated from the initial temperature of 50\u00a0\u00b0C and held for 2\u00a0min followed by a temperature ramp to 130\u00a0\u00b0C at a heating rate of 10\u00a0\u00b0C/min and a final ramp to 365\u00a0\u00b0C at a rate of 15\u00a0\u00b0C/min. The system was held at this temperature for 15\u00a0min. Helium was used as a carrier gas with a constant flow rate of 2.0\u00a0mL/min. Standard normal-alkanes (linear alkane) were used to characterize the hydrocarbon product based on the retention times of GC-FID chromatogram. Eq. (1) was used to calculate the fraction of certain compound(s) in the sample. This can be used to determine the yield of that product fraction via Eq. (2). In this work, the desired product was categorized in two groups, which were nC8-nC16 and non nC8-non nC16. Note that the latter was distinguished by the peaks associated with retention times in the range of nC8-nC16 but not the same as that of standard n-alkanes [27]. Both groups were considered as biojet fuel range and GC\u2013MS was used to analyze the potential biojet products. The productivity was calculated via Eq (3) in order to compare the performance with other systems as reported in the literature. Eqs. (4)\u2013(6) were used to determine the yield of liquid, water, and gas product. The water yield was calculated based on the difference of the weight of oil before and after the use of NaSO4 as shown in Eq. (5). After that, the mass balance was used to determine the mass of gas product.\n\n(1)\n\n\n%\nA\nr\ne\na\n\nA\n=\n\n\nA\nr\ne\na\n\no\nf\n\nA\n\n\nT\no\nt\na\nl\n\na\nr\ne\na\n\no\nf\n\no\ni\nl\n\np\nr\no\nd\nu\nc\nt\n\n\n\u00d7\n100\n\n\n\n\n\n\n(2)\n\n\n%\nY\ni\ne\nl\nd\n\nA\n=\n\n\n%\n\nA\nr\ne\na\n\no\nf\n\nA\n\u00d7\nw\ne\ni\ng\nh\nt\no\nf\n\no\ni\nl\n\np\nr\no\nd\nu\nc\nt\n\n\nw\ne\ni\ng\nh\nt\n\no\nf\n\nC\nP\nK\nO\n\n\n\n\n\n\n\n\n(3)\n\n\nP\nr\no\nd\nu\nc\nt\ni\nv\ni\nt\ny\n=\n\n\n%\n\nA\nr\ne\na\n\u00d7\nw\ne\ni\ng\nh\nt\n\no\nf\n\no\ni\nl\n\np\nr\no\nd\nu\nc\nt\n\n\na\nm\no\nu\nn\nt\n\no\nf\n\nc\na\nt\na\nl\ny\ns\nt\n\u00d7\nt\ni\nm\ne\n\n\n\n\n\nwhere A is either nC8-nC16 or non nC8-non nC16 compounds.\n\n(4)\n\n\nL\ni\nq\nu\ni\nd\n\ny\ni\ne\nl\nd\n\n\n(\n%\n)\n\n=\n\n\nM\na\ns\ns\n\no\nf\n\nl\ni\nq\nu\ni\nd\n\np\nr\no\nd\nu\nc\nt\n\n\nM\na\ns\ns\n\no\nf\n\ns\nt\na\nr\nt\ni\nn\ng\n\nC\nP\nK\nO\n\n\n\u00d7\n100\n\n\n\n\n\n\n(5)\n\n\nW\na\nt\ne\nr\n\ny\ni\ne\nl\nd\n\n\n(\n%\n)\n\n=\n\n\nM\na\ns\ns\n\no\nf\n\nl\ni\nq\nu\ni\nd\n\np\nr\no\nd\nu\nc\nt\n\n\nM\na\ns\ns\n\no\nf\n\ns\nt\na\nr\nt\ni\nn\ng\n\nC\nP\nK\nO\n\n\n\u00d7\n100\n\n\n\n\n\n\n(6)\n\n\nG\na\ns\n\ny\ni\ne\nl\nd\n\n\n(\n%\n)\n\n=\n\n\nM\na\ns\ns\n\no\nf\n\ng\na\ns\n\np\nr\no\nd\nu\nc\nt\n\n\nM\na\ns\ns\n\no\nf\n\ns\nt\na\nr\nt\ni\nn\ng\n\nC\nP\nK\nO\n\n\n\u00d7\n100\n\n\n\n\nFatty acid profile of CPKO were identified via GC\u2013MS (5975C, Agilent) equipped with DB-FastFAME column, Agilent G3903-63011 (30\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a00.25\u00a0\u00b5m). The volume of 1 \u03bcL of diluted oil sample was injected. Initial temperature of 50\u00a0\u00b0C was ramped to 194\u00a0\u00b0C with heating rate 30\u00a0\u00b0C/min and held for 3.5\u00a0min followed by another temperature ramp to 250\u00a0\u00b0C at a heating rate of 5\u00a0\u00b0C/min. The system was held at this temperature for 1\u00a0min.GC\u2013MS (5975C, Agilent) equipped with DB-5MS column, Agilent 122\u20135532 (30\u00a0m, 0.25\u00a0mm in diameter and 0.25\u00a0\u00b5m film thickness) was used for the quantification of iso-alkanes, cycloalkanes, alkenes, aromatic, oxygenated compounds and other components in the oil product for P9 and P9HZSM0.06. The injector was set at 300\u00a0\u00b0C. The oven was heated from the initial temperature of 40\u00a0\u00b0C and held for 5\u00a0min followed by a temperature ramp to 300\u00a0\u00b0C at a heating rate of 2\u00a0\u00b0C/min. The system was held at this temperature for 10\u00a0min. The ion source temperature was at 230\u00a0\u00b0C. Helium was used as a carrier gas with a constant flow rate of 1.5\u00a0mL/min. The compounds in CPKO and oil product were identified based on the NIST mass spectral database.Mettler Toledo DSC 3+ was employed to identify the freezing point and melting point of CPKO and product samples. A small amount of sample (10.50\u00a0\u00b1\u00a00.5\u00a0mg) was subjected to two thermal treatments. The first temperature profile was a cooling process from the initial temperature of 40\u00a0\u00b0C to \u221275\u00a0\u00b0C at a rate of \u22125\u00b0C/min under the nitrogen flow at a rate of 50\u00a0mL/min. Then another temperature profile was imposed by ramping up from the final temperature of the previous step to 40\u00a0\u00b0C at a rate of 5\u00a0\u00b0C/min.The boiling range of oil product (ASTM D2887) was determined by another GC-FID (Varian CP3800) equipped with Zebron ZB-1XT Simdist capillary column (15\u00a0m\u00a0\u00d7\u00a00.53\u00a0mm\u00a0\u00d7\u00a00.25\u00a0\u00b5m). A temperature program started from the initial temperature of 40\u00a0\u00b0C where it was held for 1\u00a0min before ramping up to 370\u00a0\u00b0C at a heating rate 10\u00a0\u00b0C/min. The oven temperature was held at 370\u00a0\u00b0C for 5\u00a0min to complete the profile. The injector temperature and detector temperature were set at 370\u00a0\u00b0C. The volume of sample injected into the column was 1\u00a0\u00b5L using a spitless mode.To assess the change of oxygen content in the oil. Both CPKO and the product were analyzed by the Elemental analyzer (CHNS/O Analyzer, 628 series, Leco Coporation, USA) at 1300\u00a0\u00b0C. The acid value of CPKO was determine via titration (EN 14104) with 0.1\u00a0M alcoholic potassium hydroxide. Isopropanol was used as solvent and phenolphthalein was used as indicator. The acid value can be calculated by Eq. (7).\n\n(7)\n\n\nA\nc\ni\nd\n\nv\na\nl\nu\ne\n=\n\n\n\nC\n\nK\nO\nH\n\n\n\u00d7\n\nV\n\nK\nO\nH\n\n\n\u00d7\n56.1\n\n\nW\n\no\ni\nl\n\n\n\n\n\n\nwhere\n\n\n\n\nC\n\nK\nO\nH\n\n\n\u00a0=\u00a0concentration of potassium hydroxide solution (mol/L)\n\n\n\n\n\nV\n\nK\nO\nH\n\n\n\u00a0=\u00a0volume of potassium hydroxide solution used for titration (mL)\n\n\n\n\n\nW\n\nO\ni\nl\n\n\n\u00a0=\u00a0weight of oil sample (g)\n\n\n\n\n\nC\n\nK\nO\nH\n\n\n\u00a0=\u00a0concentration of potassium hydroxide solution (mol/L)\n\n\nV\n\nK\nO\nH\n\n\n\u00a0=\u00a0volume of potassium hydroxide solution used for titration (mL)\n\n\nW\n\nO\ni\nl\n\n\n\u00a0=\u00a0weight of oil sample (g)The heating value of oil product was measured using Parr 6400 calorimeter. Note that the minimum heating value of biojet fuel is 42.8\u00a0MJ/kg (ASTM D1655-04a, ASTM D7566).From nitrogen physisorption experiment, the adsorption isotherm of our fresh catalyst as presented in Fig. 2\na showed characteristics of microporous material (pore width\u00a0<\u00a02\u00a0nm). The pore size distribution is shown in Fig. 2b with the average pore diameter of 2.21\u00a0nm. Note that this average pore diameter was close to the boundary between micropores and mesopores. The adsorption isotherm revealed the S-shape with a narrow hysteresis loop. This was probably resulted from the partially fused micropores. The analysis revealed the pore volume of 0.57\u00a0cm3/g and high BET specific surface area of 1,038.66\u00a0m2/g, which was in the range of surface area of activated carbon [28], indicating that the surface area of catalyst was not significantly affected by the platinum loading.The morphology of catalyst surface is shown in Fig. 3\n. The SEM images indicate the porous structure with deposits of small particles, commonly observed for loaded activated carbon. The elemental mapping via SEM-EDX analysis suggested that the metal was uniformly distributed on the activated carbon support. The metal dispersion determined via H2-chemisorption was 6.49%, close to the amount of Pt loading in the manufacturing process (5\u00a0wt% Pt). Obtained from the NH3-TPD experiment, the acidity of catalyst is represented in Fig. 4\n, with two major peaks at 225\u00a0\u00b0C and 900.8\u00a0\u00b0C corresponding to the weak acid sites (0.05\u00a0mmol/g) and strong acid sites (1.773\u00a0mmol/g), respectively. It was possible that the shoulder peak around 600\u00a0\u00b0C was caused by the partial decomposition of carbon [29\u201331]. The peak between 500 and 900\u00a0\u00b0C from our result represented the strong acid sites of Pt/C catalyst, according to NH3-TPD technique. Prior to the temperature ramp, ammonia molecules adsorped on acid sites of catalyst. Upon increasing of temperature, ammonia molecules desorped from catalytic surfaces. The amount of ammonia leaving the catalyst was interpreted as the amount of acid sites despite the partial decomposition of carbon occurring at high temperatures. This was in line with the work of Lawal [32], reporting the NH3-TPD profiles of 5% Pt/C and 5% Pt/graphite. However, the cause of strong acid sites was not specified. Ob-eye et al. [33] reported the strong acidity of activated carbon. There is also a possibility that the strong acid sites resulted from the interaction of activated carbon and Pt. A further study is required to identify whether the strong acid sites were related to Pt or activated carbon, or the synergistic interaction. Note that high acidity of catalyst can be associated with the catalytic performance of the deoxygenation. The detailed characterization of HZSM-5 (used for improving the fuel properties in this research) was reported by Bangjang et al. [34].The decomposition of CPKO was obtained from thermal gravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG), as shown in Fig. 5\na. It was observed that the thermal degradation of CPKO consisted of 2 steps for decomposition of polyunsaturated and monounsaturated fatty acid, respectively [35,36]. Approximately 15.76% loss occurred between 204.49\u00a0\u00b0C and 313.37\u00a0\u00b0C, due to the decomposition of polyunsaturated fatty acids. The second step involved the decomposition of monounsaturated fatty acids as observed between 368.47\u00a0\u00b0C and 441.54\u00a0\u00b0C, accounting for the weight loss of 83.16%. According to the TGA/DTG analysis, the reaction temperature for our hydroprocessing of CPKO should not exceed 450\u00a0\u00b0C. Further increasing the temperature could cause the excess thermal cracking of liquid hydrocarbon to undesired product. Fig. 5b shows the temperature profiles for melting and freezing of CPKO obtained from differential scanning calorimetric analysis (DSC). On the cooling segment, the solidification of CPKO was observed from 15.10 to 11.54\u00a0\u00b0C, while the melting was observed from 12.63 to 28.28\u00a0\u00b0C with trace amount melted at \u221219.75\u00a0\u00b0C. These results will be used to compare with that of the hydroprocessing product.The acidity of CPKO, determined via titration method (EN14104), was 33.95\u00a0mg KOH/g oil. The fatty acid profile of CPKO was identified by GCMS. The total saturated fatty acid content, total unsaturated fatty acid, fatty acid profile, and the average molecular weight of CPKO are summarized in Table 2\n. The content of fatty acids in the jet fuel range (C8-C16) was 71.74%. Hence, the deoxygenation of CPKO over Pt/C can potentially lead to the high yield of jet fuel product. Besides, the unsaturated chain can undergo hydrogenation, isomerization, and cracking at the double bond chains. The saturated chains may also be dehydrogenated on metal catalyst followed by isomerization and cracking to produce jet fuel [37,38].One of the most important parameters for catalytic reaction strongly affecting the reaction performance is the reaction temperature. In this work, the temperature range of 350\u2013420\u00a0\u00b0C was used to study the effect of reaction temperature on the yield and properties of product. Considering experimental conditions P1-P4, where the pressure, amount of catalyst, feed rate of oil, and feed rate of hydrogen were kept constant at 500 psi, 0.05\u00a0g, 17.5\u00a0mL/min, and 0.04\u00a0mL/min, respectively. It was found that the color of liquid product changed with increasing reaction temperature. Fig. 6\n\n displays four sets of 10 samples collected hourly from the phase separator at each reaction temperature. Note that the collection time was based on the time when CPKO was introduced to the reactor. The color did not significantly change when the reaction temperature was below 400\u00a0\u00b0C; however, the samples appeared distinctively darker at 420\u00a0\u00b0C possibly due to the contribution of thermal cracking (see Fig. 8\n). All samples were stored at \u221220\u00a0\u00b0C. The product did not change the physical appearance as well as chemical composition after 4\u00a0months of storage at \u221220\u00a0\u00b0C.At high temperatures, the cracking reactions of triglycerides in CPKO led to small molecules of product, while the deoxygenation and hydrogenation reactions yielded saturated compounds in the form of normal chain length hydrocarbons (n-compounds) [17]. As shown in Fig. 7, the fraction of non n-alkane in the biojet range (peaks between nC8-nC16) including iso-alkane, cycloalkanes, oxygenated compounds increased with increasing temperature. Table 3\n shows the yield of liquid and gas products obtained from different operating conditions. The liquid yield was separated into two fractions, one was aqueous (mostly water) and another was organic (liquid hydrocarbon). For P1-P4, the yield of gas product increased with increasing reaction temperature. The presence of CPKO was observed in the liquid product obtained from P1-P4. The gas product constituted of CO (from DCN), CO2 (from DCX), light hydrocarbons such as CH4, and remaining H2. Note that the content of gas and aqueous products can reflect on the degree of conversion of CPKO via deoxygenation reactions. According to the aqueous content, indicating the presence of water produced as by-product from DCN and HDO, the reactions performed at 420\u00a0\u00b0C yielded a slightly higher content of water. However, this slight proportion suggested that DCX was the major reaction pathway. This was in line with the previous study by Liu et al. [39], who applied deoxygenation of palmitic acid over 5%Pt/C. In this case, the major product was pentadecane (C15), produced via decarboxylation (DCX).This was related to the increased conversion of CPKO, suggesting that the hydrogenation and hydrodeoxygenation reactions were significantly promoted. The fraction of nC8-nC16 was rather constant throughout the time-on-stream of 10\u00a0h. For the reaction temperature of 420\u00a0\u00b0C (P4), the average yield of biojet range fuel was 59%. Although the reaction performance was somewhat subdued, the biojet yield of approximately 40% was obtained at 400\u00a0\u00b0C. Also, the product had a slightly yellowish appearance (not dark). Both reaction temperatures (400\u00a0\u00b0C and 420\u00a0\u00b0C) were used for subsequent experiments in order to optimize other operating parameters.It is conceivable that the amount of catalyst should be balanced with the flow rate of CPKO to allow for efficient catalytic reactions, while suppressing the undesired reactions. Therefore, the effects of both catalyst amount and flow rate of CPKO on the reaction performance were investigated. In this set of experiments, the pressure and hydrogen flow rate were maintained at 500 psi and 17.5\u00a0mL/min, respectively. Fig. 8 shows samples collected for the reaction temperature of 400\u00a0\u00b0C and 420\u00a0\u00b0C. Based on the physical appearance of the samples, the color of product was affected by the operating conditions applied for our hydro-processing. For the reaction temperature at 400\u00a0\u00b0C and catalyst amount of 0.05\u00a0g (P3 and P5), the color of product appeared lighter when the flow rate of CPKO was decreased from 0.04\u00a0mL/min (P3) to 0.02\u00a0mL/min (P5). Then we increased the catalyst amount from 0.05\u00a0g (P5) to 0.07\u00a0g (P6) while other parameters were kept constant. At this condition, the color of product was transparent. This was owing to the efficient catalytic reaction of CPKO and hydrogen. In other words, the amount of catalyst of 0.07\u00a0g was sufficient to handle the CPKO flow rate of 0.02\u00a0mL/min. On the contrary, the product was slightly yellowish similar to that of P5 when the flow rate of CPKO was increased from 0.02\u00a0mL/min (P6) to 0.04\u00a0mL/min (P7). It was probable that a larger portion of intermediates was present. This trend was more apparent for the comparison between P3 and P4. Upon decreasing the flow rate of CPKO (P8) and increasing the amount of catalyst (P9), the color of product became lighter and almost transparent. The trend reversed when the flow rate of CPKO was increased from 0.02\u00a0mL/min (P9) to 0.04\u00a0mL/min (P10).Oil samples obtained from different operating conditions (P5-P10) were analyzed by GC-FID and the chromatograms are shown in Fig. 9\n. The main components in CPKO are represented by the peaks at retention times greater than 25\u00a0min. It can be observed that the product of P5 and P6 contained a significant portion of high-molecular weight hydrocarbons despite the complete disappearance of raw material. The presence of raw material was evident when the flow rate of CPKO was increased from 0.02\u00a0mL/min (P6) to 0.04\u00a0mL/min (P7), indicating that CPKO was not completely converted. A similar trend was also observed when changing the operating conditions from P8 to P9 and then to P10. However, the chromatograms of P8-P10 show that the distribution of product shifted to the left compared to that of P5-P7, suggesting a greater portion of low-molecular weight hydrocarbons. Consequently, we analyzed the yield of biojet range product of samples obtained from P8-P10 in comparison with that of P4, as presented in Fig. 10\n. The difference in the total yield of biojet was not significant. However, P9 provided the highest yield of nC8-nC16 and the lowest yield of non-nC8-non nC16. Note that normal-alkane (linear alkane) can burn very cleanly, enhancing the combustion characteristics. P10 was undesirable since the product still contained some unconverted CPKO. Fig. 10d shows the boiling point profiles of the product obtained from different reaction conditions. The shifting of boiling point profile is noticeable. For reaction at 400\u00a0\u00b0C, the fraction of low-boiling point was smaller than that of 420\u00a0\u00b0C as shown in Table 4\n. Moreover, the boiling point profile of P9 showed a significantly smaller content of high-boiling point hydrocarbons, providing 44.69% of jet fuel (boiling point in the range of 150\u2013280\u00a0\u00b0C) as shown in Table 4. This efficient conversion of CPKO was a combination of high reaction temperature, sufficient amount of catalyst, and appropriate feed rate of CPKO. The DSC analysis of the product obtained from P8-P10 revealed that P9 yielded the lowest freezing point (-0.34 to \u221246.95\u00a0\u00b0C). Hence, this operating condition of P9 (420\u00a0\u00b0C, 500 psi, 0.07\u00a0g of Pt/C, CPKO flow rate of 0.02\u00a0mL/min, hydrogen flow rate of 17.5\u00a0mL/min) was used to study the effect of hydrogen flow rate.The flow rate of hydrogen is considered as an important parameter affecting the performance of deoxygenation and hydrogenation of vegetable oil. High ratios of H2-to-oil during the hydroprocessing could lead to the product that contains the majority of saturated hydrocarbons with high ratio of hydrogen-to-carbon. This type of product can be associated with high combustion efficiency [40]. A set of experiments were performed by varying the hydrogen flow rate as 17.50, 35.0, and 70.0\u00a0mL/min designated by P9, P11, and P12. This corresponded to the H2-to-CPKO molar ratio of 28.02, 55.66, 110.92, respectively. Other operating parameters were kept constant at 420\u00a0\u00b0C, 500 psi, 0.07\u00a0g of Pt/C, and CPKO flow rate of 0.02\u00a0mL/min. Fig. 11\na\u2013c shows the effect of hydrogen flow rate on the %yield of biojet fuel. Results indicated that similar yields of biojet were obtained for all experiments (500 psi). Although the H2-to-CPKO ratio increased with increasing hydrogen flow rate, the residence time was negatively affected. In other words, the higher hydrogen flow rate, the shorter the residence time. The analysis via simulated distillation also revealed that the boiling range of the product was not significantly different, as indicated in Fig. 11d. The samples were analyzed by DSC to obtain the freezing profile, which was \u22120.34 to \u221246.95\u00a0\u00b0C, \u22120.67 to \u221248.30\u00a0\u00b0C, and 5 to \u221244.58\u00a0\u00b0C for P9, P11, and P12, respectively. Therefore, the operating condition P12 was not considered further due to the broad freezing profile, especially on the higher temperature range. Since, the fuel properties of product obtained from P9 and P11 were similar, P9 (lower hydrogen flow rate) was chosen for further investigation on the effect of reaction pressure. Decreasing the pressure from 500 psi to 250 psi led to the decreased content of n-alkane, owing to the promoted hydrogenation and deoxygenation. At this condition, the content of hydrocarbons in the jet fuel boiling range decreased to 18.60%. Hence, the suitable operating conditions were at 420\u00a0\u00b0C, 500 psi, 0.07\u00a0g of Pt/C, CPKO flow rate of 0.02\u00a0mL/min, hydrogen flow rate of 17.5\u00a0mL/min.To address the importance of catalyst for this process, a blank test was performed at the optimal conditions (P9). This result was compared with that obtained using 5%Pt/C, as shown in Fig. 12\n. Apparently, the color of fuel product was markedly different. Without catalyst, thermal reactions resulted in a dark brown liquid. The aqueous phase was not observed. It was possible that a small degree of deoxygenation occurred for the blank test. Therefore, for the liquid product collected, 7.14% of n-alkane in the range of biojet fuel (nC8-nC16) was possibly produced via random cracking of CPKO followed by hydrogenation. According to the fatty acid profile of CPKO (Table 2), the content of fatty acids in the jet fuel range (C8-C16) was 71.74%. Hence, hydrocarbons in the biojet fuel range were likely produced in the system. The content of n-alkanes of approximately 7.14% was much lower than 27.68% of the product obtained from P9. The content of non-n-alkanes from the blank experiment was 52.42%, suggesting that the deoxygenation was not significantly involved. It was probable that the n-alkanes were produced via cracking reactions of saturated chain fatty acids. The freezing point of 11.47\u00a0\u00b0C (obtained from DSC analysis) was much higher than those obtained with the use of catalyst. Therefore, in this case, it was necessary to apply the catalyst in order to achieve the desired properties of biojet fuel through the deoxygenation, isomerization, and hydrocracking.\nTable 5\n shows the comparison of reaction performance of hydroprocessing of vegetable oil using different systems such as batch, semi-batch, and continuous reactors. Despite the more extreme conditions in terms of reaction temperature and pressure, our system offered an extraordinarily high productivity (>9 gproduct/gcatalyst\u2219h) compared to other systems (<1.6 gproduct/gcatalyst\u2219h), due to the small amount of catalyst and the relatively high weight hourly space velocity (WHSV). The yield of biojet product was comparable to the literature data.One of the important fuel properties for jet fuel is the freezing point. The product obtained from P9 was analyzed by DSC. The result showed that the freezing point was in the range of \u22120.34 to \u221246.95\u00a0\u00b0C, which was higher than \u221247\u00a0\u00b0C according to the specification of aviation fuel (ASTM D1655-04a, ASTM D7566, and JP-8 MIL-DTL-83133E) [24,25]. Hence, the fuel product should be upgraded to improve the freezing point. For this, we modified the catalyst bed slightly to incorporate a section of HZSM-5 located behind the bed of Pt/C, with a layer of glass wool sandwiched in between. The operating conditions were kept the same as P9 except the catalyst bed. The amount of HZSM-5 was varied as 0, 0.02\u00a0g, 0.04\u00a0g, and 0.06\u00a0g. The product samples collected at 10\u00a0h of reaction time were analyzed by the elemental analyzer, DSC, and simulated distillation. The samples were designated as P9HZSM0.02, P9HZSM0.04, and P9HZSM0.06 for the amount of HZSM-5 of 0.02\u00a0g, 0.04\u00a0g, and 0.06\u00a0g, respectively.According to the elemental analysis, the oxygen content of CPKO, P9, P9HZSM0.02, P9HZSM0.04, and P9HZSM0.06 was 18.83, 14.96, 15.02, 12.46, and 7.64%w/w, respectively. These results confirmed that the oxygen content was reduced after our hydroprocessing. This was in line with the aqueous fraction of the product that increased with increasing amount of HZSM-5, produced via hydrodeoxygenation (HDO) [46]. The product tended to be non-polar compounds, suitable for blending with jet fuel. HZSM-5 also promoted the aromatization, cracking/isomerization [47\u201349]. This led to the increase of gas product when compared to the product obtained from P9, especially for the first 7\u00a0h of time-on-stream (see Fig. 13\na). The chromatograms of these products are presented in Fig. 9h\u2013j. Evidently, the use of HZSM-5 caused a significant shift of product peaks to the left side, indicating lower boiling point of product. This was also supported by the simulated distillation analysis as shown in Fig. 13b. The maximum content of jet fuel of 83.34% was obtained from P9HZSM0.06.\nTable 6\n shows the composition of product obtained from P9 and P9HZSM0.06. The content of aromatics (BTXs) significantly increased with the use of Pt/C and HZSM-5. According to the work of Qian et al. [50], who investigated the properties of diesel blends, increasing the content of aromatics lowered the boiling point and viscosity while increasing the density of the blend. Note that the increased fuel density can help minimize the fuel storage [23]. The catalytic performance of HZSM-5 is generally associated with the Si-to-Al ratio [51]. The lower the ratio, the higher acidity of catalyst, leading to the higher degree of aromatization [52]. In our system, the Si-to-Al was 15. There are two major reaction pathways involving hydrotreating over HZSM-5 catalyst [53]. The first one starts with cracking reactions, producing carbonium ions. Then the isomerization and hydrogenation occurred, resulting in iso-alkanes. The second pathway involves unsaturated hydrocarbons produced via cracking reactions, followed by aromatization via Diels-Alder reaction. Hence, the major product of this pathway is aromatic compounds. In our case, results suggested that the reaction mechanism was dominated by the second pathway. The use of zeolite with a larger Si-to-Al ratio may lead to the larger content of iso-alkanes.As mentioned previously, the content of cycloalkane in the biojet fuel can help decrease the freezing point and n-alkanes exhibit high freezing point [23]. According to Table 6, despite the fact that the content of cycloalkane (6.8%) in the product obtained from P9 was larger than that of P9HZSM0.06, this effect was overwhelmed by the content of aromatic compounds. It was conceivable that the reactions proceeded via hydrogenation and deoxygenation for the case of P9, resulting a larger content of n-alkane, alkyl-cycloalkane, and cycloalkane as compared to that of P9HZSM0.06.The samples were analyzed for the freezing point via DSC. Results are shown in Fig. 14\n. Increasing the amount of HZSM-5 led to the decrease of freezing point since high degree of cracking reaction caused high content of small molecules [23,47,49]. The content of BTXs significantly improved the cold flow property of our product [54]. The use of 0.06\u00a0g of HZSM-5 together with Pt/C could lower the freezing point by 30\u00a0\u00b0C compared to that of P9. It was also observed that using 0.02\u00a0g of HZSM-5 in the case of P9HZSM0.02 was not sufficient to cause a major difference in terms of gas yield, freezing point, and boiling range. P9 provided the content of 44.69% of jet fuel as compared to 50.53% obtained from P9HZSM0.02 (see Table 4). Therefore, the amount or ratio of HZSM-5 and Pt/C is one of the important parameters for the optimization of this process. The heating value of 43.43\u00a0MJ/kg of the upgraded biojet product was not significantly altered as compared to that of the non-upgraded product (43.15%). The production of biojet fuel (C8-C16) via deoxygenation reaction using Pt/C as a catalyst resulted in the high yield of biojet fuel. However, one of the important properties of jet fuel is the freezing point. Apparently, our product (P9) did not meet the criteria. Using HZSM-5 in combination with Pt/C offered the improved freezing point property of biojet product. Due to the possible over-cracking in our system, the acid strength of HZSM-5 should be varied to explore the effect on the chemical composition of the biojet product.The production of biojet fuel from CPKO via hydroprocessing was demonstrated in a continuous process (fixed bed reactor) using 5%Pt/C as catalyst for hydrogenation, cracking, and deoxygenation. The effects of operating parameters including reaction temperature, pressure, CPKO flow rate, hydrogen flow rate, and the amount of catalyst were investigated. The content of n-alkane C8-C16 was significantly improved at 500 psi as compared to that obtained at 250 psi. The hydrogen flow rate in the range investigated did not significantly affect the product quality. The optimal operating conditions were at 420\u00a0\u00b0C, 500 psi, hydrogen flow 17.50\u00a0mL/min, and CPKO flow 0.02\u00a0mL/min (H2-to-oil molar ratio of 28.02). The yield of biojet was 58.29%, with the major content of nC8-nC16 of 27.68%. The content of hydrocarbons in the biojet boiling range (ASTM D2887) was 44.69%. The oxygen content was 14.96% as compared to that of the feedstock of 18.83%. The biojet product was upgraded by the use of HZSM-5 in combination with Pt/C as catalyst to promote cracking, aromatization and isomerization. The freezing point was shifted down by 30\u00a0\u00b0C and the oxygen content was reduced to 7.64% when using 0.06\u00a0g of HZSM-5 and 0.07\u00a0g of Pt/C. The product with low content of oxygenated compound was classified as non-polar hydrocarbons, suitable for blending with commercial jet fuel. The content of hydrocarbons in the biojet boiling range was 83.34% for the case of combined catalyst. In our system, the high acid strength of HZSM-5 led to the high content of aromatics in the product, contributing to the shifting of boiling range of biofuel. The resulting small hydrocarbons also improved the cold flow property of product.\nMontakan Makcharoen: Visualization, Methodology, Investigation, Writing \u2013 original draft. Amaraporn Kaewchada: Supervision, Writing \u2013 review & editing, Conceptualization, Methodology, Writing \u2013 original draft. Nattee Akkarawatkhoosith: Visualization, Writing \u2013 review & editing, Conceptualization, Investigation. Attasak Jaree: Supervision, Writing \u2013 review & editing, Conceptualization, Methodology, Investigation, Resources, 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 acknowledge the financial support from the National Research Council of Thailand (NRCT).", "descript": "\n                  This work focused on the conversion of crude palm kernel oil (CPKO) to biojet fuel via deoxygenation reaction in a fixed bed reactor. The catalyst was Pt supported on activated carbon (Pt/C). The investigation involved the effects of various operating parameters such as reaction temperature (350\u2013420\u00a0\u00b0C), pressure (250 and 500 psi), flow rate of CPKO (0.02 and 0.04\u00a0mL/min), flow rate of hydrogen (17.5, 35.0, and 70.0\u00a0mL/min), and the amount of catalyst (0.05 and 0.07\u00a0g) on the reaction performance and the fuel properties of product. The biojet yield of 58.29%, with the major content of linear alkane in the range of jet fuel (nC8-nC16) of 27.68% and the productivity of 9.32\u00a0g product/g catalyst\u00b7h were achieved at the optimal operating conditions (420\u00a0\u00b0C, 500 psi, CPKO flow rate at 0.02\u00a0mL/min, hydrogen flow rate at 17.50\u00a0mL/min (H2-to-CPKO molar ratio of 28.02), and 0.07\u00a0g of Pt/C catalyst). The oxygen content was reduced from 18.83% (in CPKO) to 14.96% (in the product). To improve the freezing point of biojet fuel via cracking and aromatization, the catalyst bed was modified by adding HZSM-5 as a catalyst bed adjacent to the bed of Pt/C. The freezing point of product was significantly lowered by 30\u00a0\u00b0C.\n               "}
{"full_text": "Data will be made available on request.The number of daily activities that possess a serious threat to humanity is increasing, which worsens the situation globally. According to the World Economic Forum and the World Health Organization, antibiotic resistance is arguably the biggest threat of the 21st century and might be a \u201cpotential tragedy\u201d for human welfare and the global economy [1]. Alarmingly, antibiotic resistance is on the rise, necessitating the development of new classes of antibacterial candidates. Certain antibiotic drugs like Daptomycin, GAR936 Linezolid, and Oitavancin, had created the marketplace for antimicrobial chemotherapy [2,3]. Utilizing metallo-drugs is one of the therapeutic approaches under consideration. Metal complexes are explored as potential candidate as these complexes inhibit enzymatic action, interact with intracellular biomolecules, increase lipophilicity, modify the function of plasma membrane, break cell cycle, and many more [4]. In a similar way, chelation drastically alters the organic functionality of metal\u2013ligand complexes. A variety of metal quinoline based anti-infection drug like Norfloxacin and Ciprofloxacin, were investigated to exert better action than antibiotics alone [5\u20137]. The cytotoxic study of cisplatin has given a tremendous push to explore novel metal complexes. The success of cisplatin had accelerated metal-derived medicines and sparked a widespread research inspiring scientists to develop alternative methods with better pharmacological properties [8]. SB metallo-drugs were reported against a variety of bacterial and parasite species in countless researches. In 1864, Hugo Schiff firstly explained about the condensation reaction pathway between an amine and an aldehyde resulting SB as a product (Fig. 1\n(a)). SB ligands have ability to coordinate with diverse metal ions via imine nitrogen or other donor atoms i.e., oxygen and sulphur (Fig. 1(b)). Numerous researches were focused on the synthesis, adaptability, and reactivity of the central metal atom as well as the existence of the azomethine group, which played a vital role in understanding the biological transformation process and racemization reaction [9,10]. Particularly, the medicinal science was dominated by heterocyclic Schiff base metal chelates due to the diverse features [11,12]. A variety of SB complexes with heterocyclic moieties such as semicarbazone, thiosemicarbazone, 1,2,4-triazoles, pyrazoles, 4-aminoantipyrene, benzoxazoles, coumarins, and triazines have attracted a lot of attention [13\u201317]. Owing to the prospective applications in analytical chemistry, dying industry, food industry, pharmaceutical industries, and agrochemical endeavours, SB ligands and their metal chelates have been accounted repeatedly [18]. This review aimed to describe the synthesis of SB and their metal complexes along with the metal\u2013ligand stoichiometry. The review also covers the interaction of the Co(II), Ni(II) and Cu(II) complexes affecting bio-activities.Table 1.\n\nThe synthesis of SB under solvent free or without solvent can proceed in the presence/absence of catalyst [19]. These are as followed:Microwave-assisted synthesis is a green chemical method. It is a beneficial technique in the organic synthesis due to its simplicity, responsive and reducing hazard capacity. It can often minimize reaction time under solvent-free or less-solvent conditions, resulting in higher yields and easier work-up in comparison to traditional methods [20]\n. The synthesis of SB under non-solvent environment [21] has been described below (\nScheme 1\n\n).As the reaction mixture is mashed in a mortar-pestle, the formation of SBs can be achieved effectively at room temperature using a catalyst such as SnCl2 and CH3COOH.In this process, mixture of primary amine and aldehyde/ketone is well mixed with the help of mortal-pastel. The reaction takes two to three minutes to complete.Methanol and ethanol are suitable solvents which are generally used in the synthesis of SB ligands followed by the refluxing in acidic, basic or neural medium. The purification of the product is done by either recrystallization or chromatography (TLC/column) techniques. Schemes 2 and 3\n\n provide the reaction mechanisms in acidic and basic medium, respectively [22].In 2019, Vinusha and co-workers described the synthesis of SB ligand 3 by taking 5-amino-4H-1,2,4-triazole-3-thiol 1 and 3-hydroxy-4-methoxy benzaldehyde 2. The metal (Co(II), Ni(II) and Cu(II)) chloride salts were taken in 2L:1M stoichiometric ratio for the synthesis of metal complexes 4\u20136 (\nScheme 4\n\n). The structural analysis of 3 and its metal chelates was carried out by using proton/carbon NMR, mass, FT-IR and TGA spectroscopic techniques. The analysis results confirmed tridentate nature of 3 and octahedral geometry of 4\u20136 complexes. All compounds were examined for in vitro antibacterial activity by employing agar well diffusion method with nine food pathogens which included three gram\u00a0+\u00a0ve bacteria (B1, B12, B14) and six gram \u2013ve bacteria (B2, B4, B9, B15, B16, B17). DMSO and Amoxicillin were utilized to control negative and positive action, respectively. All the gram\u00a0+ve and gram \u2013ve bacteria showed inhibition zone with compounds 4\u20136. Whereas 3 showed better inhibition against B1, B2, B4, B12, B14 and B15\n[23].In 2020, SB 9 was synthesized by taking pyrazine-2-carbohydrazone 7 and 2-hydroxy-5-methylacetophenone 8. Co(II) 10, Ni(II) 11 and Cu(II) 12 complexes with the general empirical formula [M(L)(Cl)(H2O)2] were also reported (\nScheme 5\n\n). Microanalytical, magnetic susceptibility and various spectroscopic techniques such as proton/carbon NMR, IR, P-XRD, SEM, ESR and TGA were employed for the characterization of the compounds. The spectroscopic analysis validated the tridentate donor behavior (ONO) of 9. The molar conductance and physico-chemical suggested the non-electrolytic behavior and monomeric octahedral geometry of 10\u201312, respectively. All the compounds were screened for their antimicrobial activity using disc-agar diffusion method. Fungal strains F1 and F2 were used for antifungal screening and clotrimazole was used as standard drug. The results confirmed good antifungal activity of 9\u201312. In vitro antibacterial screening was carried out by taking gram\u00a0+ve (B1, B3), gram \u2013ve (B2, B18) strains. Ciprofloxacin, an antibacterial standard drug was used for the comparison purpose. The antibacterial actions of 10\u201312 complexes were more prominent against gram\u00a0+ve as compare to gram \u2013ve strains. The order of the antibacterial activity was 11\u00a0>\u00a0Ciprofloxacin\u00a0>\u00a012\u00a0>\u00a010. The chelation was found to be responsible for the enhanced antimicrobial activity of the 10\u201312 than 9\n[24].In the same year, Dhanaraj and Raj [25] described the synthesis of SB ligand 17 in two steps. First step included the refluxing of ethanolic solution of 4-aminoantipyrine 13 and acetamide 14 to yield 4-aminopyridine derivative 15. In the second step, ethanolic solutions of p-phenylenediamine 16 and 15 were refluxed in order to get the SB ligand 17. The acetate salts of Co(II), Ni(II) and Cu(II) were used for the synthesis of complexes 18, 19 and 20, respectively (\nScheme 6\n\n). Compounds 17\u201320 were analyzed by elemental and several spectroscopic techniques such as mass, IR, UV\u2013Visible and XRD. The nano-crystalline structure of 17\u201320 was confirmed by results. The complexes 18\u201320 were analyzed for DNA binding and cleavage activities. Antimicrobial studies were carried out against some bacterial (B1, B2, B3 and B4) and fungal (F1 and F2) species using agar well diffusion method. Cytotoxicity and anticancer action of all the compounds were also tested against L929 fibroblast cell line and SK-MEL-28 cell line, respectively. The compound 20 possessed the maximum potential for DNA binding, DNA cleavage and antimicrobial assay in comparison to 17, 18 and 19.In 2020, the synthesis of quinoxaline-based ligand 27 was carried out in multiple steps. Initially, aqueous solution of o-pheneylenediamine 21 and oxalic acid 22 was heated at 100\u00b0C to obtained 1,4-dihydro-quinoxalin-2,3-dione 23. The next step involved the refluxing of 23 and ethylenediamine 24 for 2 hrs, resulting 3-[2-(aminoethyl)amino]quinoxalin-2(1H)-one 25 which after reaction with salicylaldehyde 26 precipitated 27. The Co(II) 28, Ni(II) 29 and Cu(II) 30 metal chelates were also synthesized (Scheme 7\n) and characterized using FTIR, ESR, NMR, EDAX, magnetic moment, conductance, analytical and electronic spectral data. The ligand 27 acted as OO bidentate donor and coordinated through carbonyl oxygen and phenolic oxygen of quinoxaline ring. Complex 30 was analyzed theoretically. The compounds were tested for in vitro antimicrobial efficiency by well diffusion technique against gram\u00a0+ve (B1 and B3), gram \u2013ve (B4 and B5) bacteria and fungul (F1 and F2) strains. The compounds were also analyzed for their anticancer, in vitro antioxidant and DNA binding studies. The complexes 28 and 29 were found potent against human breast cells lines MCF7. The antioxidant activities of 28\u201330 were found moderated [26].A novel SB ligand 33 was synthesized [27] via condensation reaction of methyl-6-acetamide-2-amino-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate 31 and 2-hydroxy-3-methoxybenzaldehyde 32. Metal chelates were also prepared by refluxing 33 with CoCl2\u00b76H2O and NiCl2\u00b76H2O (\nScheme 8\n\n). The spectroscopic analysis confirmed octahedral geometry for Co(II) 34 and Ni(II) 35 complexes. The compounds 33\u201335 were screened for their antioxidant activity by four different methods i.e.,(a) FTC, (b) FRAP method, (c) DPPH free radical scavenging activity using Blosis method and (d) reduction force determination using CUPRAC method. The results confirmed better antioxidant activity of 34 and 35 in comparison to 33.Alothmanet al.,[28] described the synthesis of 38 through condensation reaction of 1-aminoquinolin-2(1H)-one 36 and 2-hydroxybenzaldehyde 37 along with its nano-sized Co(II) 39, Ni(II) 40 and Cu(II) 41 complexes (\nScheme 9\n\n). The structure elucidation was carried out via various elemental and spectroscopic techniques, confirming the neutral (NOO) tridentate nature of 38 and octahedral geometry of 39\u201341. The anticancer potential of 38\u201341 was carried out against calf thymus CT-DNA through UV\u2013Visible absorption process. To describe the DNA cleavage activity, solid-state DC electrical conductivity was measured at 576-696\u2103, confirming the semiconducting nature of 39\u201341. The potent cytotoxicity against Artemiasalina was possessed by 39 and 40 complexes with the value of LD50\u00a0=\u00a02.68 \n\n\u00d7\n\n 10-6 and 2.74 \n\n\u00d7\n\n 10-6, respectively.In 2020, a new antipyrine based tridentate ligand 48 was synthesized into three steps [29]. Firstly, 2-hydroxy-3-methoxybenzaldehyde 42 and 4-aminoantipyrin 43 were refluxed to obtain compound 44. Another compound 47 was synthesized through the reaction of 3-nitrobenzaldehyde 45 and hydrazine hydrate 46. Both the compounds 44 and 47 were refluxed to yield 48. Different complexes of Co(II) 49, Ni(II) 50 and Cu(II) 51 were also reported via condensation pathway (\nScheme 10\n\n). Various techniques such as magnetic susceptibility, molar conductance, elemental analysis, UV\u2013Visible, FT-IR, mass spectroscopy, proton/carbon NMR and TGA were employed for the characterization of 48\u201351. The analysis confirmed the octahedral geometry of 49 and 50 and distorted octahedral geometry of 51. In vitro antimicrobial actions against gram\u00a0+ve (B1, B7 and B8), gram \u2013ve (B2 and B4) bacterial and fungi (F1, F2, F3, F4 and F5) strains using Broth micro dilution method were evaluated confirming the considerable activity of 48\u201351. The complexes were also tested for the screening of anticancer efficiency against liver bilobular cancerous cells (LBir2754), SOD efficacy and DNA cleavage.In the same year, a reaction was carried out by refluxing glycylglycine 52, 4-nitrobenzaldehyde 53 and metal salts. The Co(II) 54, Ni(II) 55 and Cu(II) 56 transition complexes (\nScheme 11\n\n) were characterized using molar conductance, electronic spectra, magnetic moment and various spectroscopic methods such as TGA, ESR, P-XRD, IR and NMR, confirming octahedral geometry for 54\u201356. In vitro antimicrobial screening was performed against B1, B2, B3, B9, F1, F2 and F3 employing disc diffusion process. DNA cleavage against E.coli DNA and anticancer efficiency against colon cancer cells and human cervical cell lines were evaluated. The complex 55 was found effectively potent in comparison to 54 and 56\n[30].A novel SB ligand 59 was synthesized by the condensation of o-phenylenediamine 58 and 5-acetyl-4-hydroxy-2H-1,3-thiazine-2,6(3H)-dione 57. The Co(II) 60, Ni(II) 61 and Cu(II) 62 complexes were also prepared by using condensation pathway (\nScheme 12\n\n). The elemental analysis, magnetic moment, molar conductance and spectral analysis (IR, proton/carbon NMR, ESR, TGA, mass) were done. The monobasic tridentate nature of 59 was suggested due to the presence of three donor sites (phenolic oxygen, azomethine nitrogen and nitrogen of amino \u2013NH2 group). 61 and 62 complexes possessed octahedral geometry whereas complex 60 acquired square planner geometry. Compounds 59\u201362 were used for in vitro antimicrobial screening against B1, B10 strains and fungus F2 using disc agar diffusion method. The antitumor action of 59 and 60 was also tested against human hepatocelluar carcinoma cell [31]. Complex 62 showed highest antitumor activity than 59 with the IC50 value 120\u00a0\u00b5g/mL.A naphthalene\u2013functionalized SB 65 was reported and synthesized using 2-hydroxy-1-naphthaldehyde 63 and o-phenylenediamine 64 via condensation along with Co(II) 66, Ni(II) 67 and Cu(II) 68 chelates (Scheme 13\n\n). The stoichiometry and structure elucidation were done on the basis of elemental analysis and spectroscopic methods (UV\u2013Visible, FTIR, NMR, and mass). The analysis confirmed the tridentate NNO donor nature of 64 and six-coordinated geometry of 66\u201368. Microanalytical study verified 1:1 stoichiometry ratio of metal and ligand. All the compounds 65\u201368 were tested for antimicrobial, anticancer and cytotoxic activities. The antimicrobial screening was carried out against B1, B6, B2, B11, F1 and F6 strains employing disc diffusion method and anticancer activity was performed against colon carcinoma HCT-116 cell lines. The compounds 66\u201368 were found to be more active than 65\n[32].Alothman et. al. [33] in the same year, refluxed the ethanolic solutions of 1,8-diamino-3,6-dioxaoctane 69 and 3,5-dichloro-salicylaldehyde 70 to synthesize a novel SB ligand 71. Co(II) 72, Ni(II) 73 and Cu(II) 74 complexes of 71 were prepared via facile synthesis strategy (Scheme 14\n\n). The analytical and spectroscopic analysis confirmed the hexadentate donor nature of 71. Absorption (UV\u2013Visible) studies confirmed the octahedral geometry of 72-74 complexes. Agar well diffusion was used to check in vitro antimicrobial activities against three bacteria B1, B2, B9 and three fungi F1, F2 and F3strains. In vitro cytotoxic analysis was also carried out against MCF7 cancer cells. The experimental analysis confirmed complex 74 to be more active as compared to 71\u201373. Hence, complex 74 possessed enough potential to use as anticancer agent.In 2020, ethene-1,2-diamine based ligand 78 was prepared bycondensation of furan-2-carbaldehyde 75, 1,2-ethenediamine 76 and 2-hydroxybenzaldehyde 77. Co(II) 79 and Ni(II) 80 complexes were also prepared. The structural analysis was performed using UV\u2013Visible and magnetic moment analysis, suggesting octahedral geometry of 79 and 80 (Scheme 15\n\n). The compounds 78\u201380 were tested against B1, B2, F2 and F4 strains for antimicrobial potential. The poor inhibition zone (disc-agar diffusion method) confirmed the inactiveness of 78\u201380 against F4 strain, whereas 79 was moderately efficient and 80 was inefficient against F2. The metal coordination inability and low lipophilicity of 79 and 80 were responsible for the low antimicrobial efficiency of the complexes. Cytotoxicity analysis of 78\u201380 was carried out against breast carcinoma (MCF7) and contra liver carcinoma (HEPG2).Complex 80 demonstrated the admirable anticancer activity as compared to 78 and 79 with higher LD50 values. The DNA binding ability of complexes was also performed with calf thymus DNA (CT-DNA) and analyzed using viscosity and absorption methods. The complex 80 intensively bounded with CT-DNA as per the results [34].In the same year, a new class of SB ligand 83 was synthesized by refluxing 2-aminothiophenol 81 and 2-(p-tolyoxy)-quinoline-3-carbaldehyde 82. The Co(II) 84, Ni(II) 85 and Cu(II) 86 metal complexes were also synthesized (Scheme 16\n\n) and characterized via various spectroscopic (IR, UV\u2013Visible, mass, TGA, XRD, SEM and EDX) techniques [35]. Elemental and magnetic moment data confirmed 1:1 ligand and metal stochiometric ratio. In vitro antimicrobial activity and DNA cleavage study were carried out to check the potential of 83\u201386. Bacterial (B1, B2, B4 and B11) and fungal (F1 and F7) strains were used for microbial activity using cup plate agar diffusion method. The order of antibacterial activity was and 85\u00a0>\u00a086\u00a0>\u00a084 against B2 and B4,\n86\u00a0>\u00a085\u00a0>\u00a084 against B11 and 86\u00a0>\u00a084\u00a0>\u00a085 against B1. The antifungal potential order was found to be 86\u00a0>\u00a085\u00a0>\u00a084 against F1 and F7. The DNA cleavage study was performedusing agarose gel electrophoresis method and pUC18 DNA was used in cleavage process. Complexes 86 and 84 were abled for DNA cleavage and no cleavage was noticed with 83 and 85.Daravath et. al. described the synthesis of a thiazol-based ligand 89 by refluxing methanolic solution of 6-aminobenzothiozole 87 and 5-hydroxysalicylaldehyde 88. A series of Co(II) 90, Ni(II) 91 and Cu(II) 92 complexes were also synthesized via condensation reaction (Scheme 17\n\n). Elemental analysis and various spectroscopic techniques reported the square planer geometry of 90\u201392 in 1:2 stoichiometry (metal:ligand) ratio. Metal complexes were examined for CT-DNA binding and cleavage of Pbr322-DNA by using UV\u2013Visible absorption, fluorescence titrations and agarose gel study, respectively. The antimicrobial activity of 89\u201392 were carried out against two gram\u00a0+ve (B1 and B14) and gram \u2013ve (B2 and B4) bacterial strains. The antifungal screening was carried out against F8 and F9 by using paper disc method. The results showed that among all the complexes, 92 showed the higher degree of DNA cleavage, binding and antimicrobial activities [36].In 2020, a series of Co(II) 96, Ni(II) 97 and Cu(II) 98 complexes of new ligand 95 was synthesized by performing template reaction [37], in which methanolic solution of metal salts were added dropwise into the methanolic solution of o-vanillin 93 and glycine 94 and refluxed for two hrs (Scheme 18\n\n). The structure elucidation was carried out by using FT-IR, NMR, UV\u2013Visible techniques and X-ray crystallography. Tridentate dianionic donor nature of 95 and octahedral geometry of 96\u201398 was confirmed by the data. All the compounds 95\u201398 were assessed for their molecular docking and DNA cleavage activities. DNA cleavage activity was carried out against E.coli genome through agarose gel electrophoresis process. Tyrosine Kinase (1\u00a0T46) and EGFR (1\u00a0M17) were the two targets that were used in molecular docking study. It was concluded that 98 showed the highest activity for DNA cleavage as compared to 95\u201397.In the same year, the Cu(II) complexes 104 and 105 of two SB ligands were synthesized by Medani and co-team. Ligand 101 and ligand 103 were synthesized by refluxing 2-phenylacetohydrazide 99 with 2-hydroxyacetophenone 100 and 1-hydroxy-2-napthaldehyde 102, respectively (Scheme 19\n\n). The structure elucidation was carried out using various elemental and spectroscopic methods. The X-ray study suggested the formation of binuclear complexes with 101 and 103. Compounds 101, 103\u2013105 were screened for in vitro antimicrobial activity against bacteria (B1 and B2) and fungi (F2 and F3) stains. Ampicillin and Amphotericin B were used as standard antibacterial and antifungal drug, respectively. The antioxidant survey was also done through DPPH free radical scavenging method. On the basis of the antimicrobial screening (agar well diffusion method), antioxidant analysis and molecular docking studies, confirmed the DNA binding ability of 101, 103\u2013105. Complexes 104 and 105 also exhibited better potential for DNA interaction as compared to 101 and 103\n[38].Three novel hydrazone SBs named as (HL1=(E)-N'-(pyridin-2-ylmethylene)benzohydrazide 106, H2L2=(E)-2-(2-hydroxybenzylidene)hydrazine-1-carboxamide 107 and HL3=(E)-2-(pyridin-2-ylmethylene)hydrazine-1-carboxamide 108 (Scheme 20\n\n) and their Co(II) 109, Ni(II) 110 and Cu(II) 111 complexes were synthesized by Fekri et al. [39]. The compounds were analyzed using numerous techniques and tested for in vitro antibacterial as well as anticancer activities. Antibacterial activities were performed against B1, B2, B3 and B4 employing minimum bactericidal concentration and minimum inhibitory concentration methods. The anticancer activity was done against human colon cancer (SW742) and human gastric cancer (AGS) cell lines using MTT assay. The results illustrated the higher efficiency of 109\n\u2013\n111 for anticancer and antibacterial activities.In the same year, Salicylaldehyde based three novel SB ligands (123\u2013125) with aromatic systems and aliphatic spacers were synthesized successfully via diamine precursors taking dinitro compounds 112 and diamine compounds (113\u2013115) to yield intermediates (116\u2013118) and diamine precursors (119\u2013121)\n[40]. These percussors and salicylaldehyde (1 2 2) were refluxed in ethanolic medium to give the respective SBs and further used (Scheme 21\n\n) in the synthesis of Cu(II) complexes (126\u2013128). Firstly, The FT-IR data showed the appropriate chelation sites of 123\u2013125 for the metal ions. The p-p* transition in phenyl ring and n-p* transition of azomethine group in the Cu(II) complexes were confirmed by the electronic absorbance spectroscopy. The biological investigation (antibacterial, antitumor, DPPH free radical scavenging, brine shrimp and DNA cleavage activities) of 123\u2013125, 119\u2013121 and 126\u2013128 were also studied. The antibacterial activity of 116\u2013126 and 128 was done against two gram\u00a0+ve (B1 and B12) and four gram -ve (B2, B15, B19 and B20) strains using well diffusion method. None of the tested compounds demonstrated appreciable antibacterial activity against these six bacterial strains. The ineffectiveness of the compounds was attributed to the microbial cell\u2019s impermeability, which prevented the tested compounds from interacting with all the six bacterial strains. The antifungal activity of 116\u2013126 and 128 was carried out against F1, F3, F10, F4 and F11. Furthermore, the growth of fungal strains was inhibited to various degrees by the compounds. The Potato disc antitumor analysis was used to test the efficiency of 116\u2013126 and 128. Compounds 116-126 and 128 illustrated the significant tumor suppression in a concentration-dependent fashion. The cytotoxicity of the compounds was checked against Artemiasalina and confirmed cytotoxic nature of 119\u2013121, 126 and 128 and non-cytotoxic nature of 116\u2013118 and 123\u2013125. DPPH free radical examination showed that complexes 123\u2013125 were extremely antioxidant. The nature of the compounds 116-122 were concentration-dependent DNA protective whereas 126 and 128 caused a considerable harm to the plasmid-DNA at all concentrations.A SB ligand 4-bromo-2-(((3-(methylamino)propyl)imino)methyl)phenol 131 was synthesized by refluxing N-methyl-1,3-diaminopropane 129 and 5-bromosalicylaldehyde 130 in ethanolic environment along with its Cu(II) complex 132 (Scheme 22\n\n). The structures of 131 and 132 were analyzed by various techniques such as FT-IR, EPR, electronic, solvatochromic studies, single crystal crystallography and Hirshfeld surface analysis. 131 and 132 were tested for their in vitro cytotoxic and antibacterial activities. Cytotoxic activity was performed against Dalton\u2019s lymphoma as cites cell, reporting 36\u00a0mg/m LIC50 value for 132. The antibacterial activity was carried out against B1, B2, B4 and B14 bacterial strains using well diffusion method. The complex 132 showed highest activity against B1 and B14 than B2 and B4\n[41].In 2020, Co(II) 136 and Cu(II) 137 complexes of novel SB [(L) 2-((1H-Benzo[d]imidazole-4ylimmino)methylphenol] ligand 135 were synthesized through condensation of 2-aminobenzimidazole 133 and hydroxybenzaldehyde 134 in ethanolic medium (Scheme 23\n\n). The structure elucidation of 135\u2013137 was carried out by magnetic moment, molar conductance and various spectral techniques (FT-IR, UV\u2013Visible, AAS, proton NMR). The spectral analysis and DFT study confirmed the bidentate donor nature of 135 and octahedral geometry of 136 and 137. The compound 137 possessed maximum antimicrobial efficiency against B1, B2, B9 and F2 using agar well diffusion method. The CT-DNA binding using UV\u2013Visible absorption studies demonstrated better DNA binding capacity of 137 than 135 and 136\n[42].Kareem et. al., carried out condensation reaction between 1,7-bis-(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione (curcumin) 138 and amino ethylene piperazine 139 to prepare a novel SB ligand 140. The Co(II) 141, Ni(II) 142 and Cu(II) 143 complexes were also synthesized (Scheme 24\n\n) and structural analysis was carried out using several spectroscopic methods. The molar conductance signified the non-electrolytic behavior of 141\u2013143. Micro analytical analysis confirmed the 1:1 stoichiometric ratio of 140 and its metal complexes. EPR and UV\u2013Visible spectroscopic analysis ascribed the octahedral geometry of 141 and 142 while 143 has the square planer geometry. The ability to scavenge free radicals was tested using revised Brand-Williams methods which proved 143 to be more potential antioxidant agent than 141 and 142. Compounds 140\u2013143 were also examined for in vitro cytotoxic activity against HeLa, KCL22 and MDA-MB231 cancer cell lines as well as normal PBMCs cells using MTT method. Complex 143 was found to be more effective on KCL22 and MDA-MB231 cell lines than 140\u2013142\n[43].A reaction of 2-aminobenzylalcohol 144 and ortho-vanillin 145 was carried out to produce a novel SB ligand ABOVL 146. Co(II) 147 and Cu(II) 148 complexes of ABOVL were also prepared (Scheme 25\n\n). The analytical and spectral techniques have been employed to analysis the structure of 146\u2013148. The spectral data confirmed the bidentate (O, N) donor nature of 146, octahedral and square planer geometry of 147 and 148, respectively. The CT-DNA binding analysis was achieved using absorption, viscosity and fluorescence data. The absorption values of 147 and 148 were 6.24\u00a0\u00b1\u00a00.04\u00a0\u00d7\u00a0104 M\u22121 and 5.76\u00a0\u00b1\u00a00.03\u00a0\u00d7\u00a0104 M\u22121, respectively. The viscosity experiments also disclosed the CT-DNA binding with 147 and 148 via intercalation. Furthermore, the data of Ksv (Stern-Volmer quenching constant) for 148 and 147, obtained from fluorescence experiment were 3.99\u00a0\u00d7\u00a0103 M\u22121 and 3.21\u00a0\u00d7\u00a0103 M\u22121, respectively. All the above analysis confirmed potential binding of CT-DNA with 148. Antimicrobial activity against B2, B21, B22, B23 bacterial and fungal F8 and F9 strains using agar diffusion method confirmed better microbial inhibition of 147 and 148 than 143. In vitro cytotoxic analysis was carried out against murine melanoma cancer cells (B16F10), human pancreatic carcinoma (MiaPacac2) and human cervical adenocarcinoma (HeLa) tumor cells line. cis-platin was used as control system using MTT assay. 148 demonstrated the maximum inhibitory efficiency with the higher IC50 value 49.13\u00a0mg/mL than 147\n[44].A novel SB ligand (4-nitrophenylimino)methyl)benzylideneamino)phenol 152 originated from the condensation reaction of 2-aminophenol 149, 1,3-isopthaldehyde 150 and 4-nitroaniline 151. The Co(II) 153, Ni(II) 154 and Cu(II) 155 complexes of 152 were also synthesized (Scheme 26\n\n). The structure analysis was carried out using molar conductance, elemental and spectroscopic techniques. The tridentate nature and octahedral geometry were confirmed for 152 and 153\u2013155, respectively. Utilizing the cyclic voltammetric and absorption studies, the interaction of 153\u2013155 with CT-DNA (calf thymus) was investigated. The findings of the experiments showed that complexes can bind with CT-DNA in the intercalation manner. DNA cleavage analysis was also carried out against Pbr322 plasmid DNA by utilizing the agarose gel electrophoresis analysis in H2O2. The DNA cleavage data confirmed the inactive nature of 152 and 154 whereas 153 and 155 showed higher DNA cleavage tendency into linear and open circular manner. In vitro antimicrobial screening utilizing paper disc assay against bacterial (B1, B2, B3 and B18) and fungal (F1, F2, F12 and F13) strains, illustrated better antimicrobial activity of 153\u2013155 than 152\n[45].Two SBs(Z)-1-(1H-benzo[d]imidazol-2-yl)-N-benzylidenemethanamine 159 and 1-(1H-benzo[d]imidazol-2-yl)-N-(4-nitrobenzylidene) methanamine 160 were obtained from the reaction of 2-(aminomethyl)benzimidazoledihydrochloride 156 with benzaldehyde 157 and 4-nitrobenzaldehyde 158 to yield respective ligands (159 and 160). Co(II) 161, Ni(II) 162 and Cu(II) 163 complexes were also reported (Scheme 27\n\n) and analyzed via different physicochemical and spectroscopic techniques. The square planer geometry of 161\u2013163 were confirmed on the basis of magnetic susceptibility, UV\u2013Visible and molar conductivity studies. Compounds 161\u2013163 were tested for CT-DNA binding using fluorescence spectroscopy, absorption, viscosity, circular dichroism and cyclic voltammetry. Moreover, the metallo nucleases properties of 161\u2013163 were reported using agarose gel electrophoresis assay. In vitro antimicrobial activity using disc diffusion assay against bacterial (B1, B2, B3, B4 and B18) and fungal (F1, F2, F11, F12 and F13) strains confirmed better inhibition of 161\u2013163 than 159 and 160. Complex 163 exhibited highest efficiency against all microbial strains as compare to 161 and 162\n[46].Two novel SB ligands, HPSL 167 and HPSA 168 were prepared by refluxing sodium-5-sulfonate-2-hydroxybenzaldehyde 164, two forms of amino acids i.e., D,L-leucine 165 and phenylalanine 166 in aqueous medium. Water soluble Cu(II) complexes, Cu-HPS 169 and CU-PSA 170 were also prepared (Scheme 28\n). The structural analyses established tridentate dibasic chelating nature of 167 and 168. Magnetic susceptibility analysis confirmed the paramagnetic behavior and square planar geometry of 169 and 170. Compounds 167\u2013170 were screened for their antimicrobial, CT-DNA binding and anticancer activities. The antimicrobial screening using agar well dilution method against bacterial (B1, B2 and B24) and fungal (F2, F3 and F14) pathogens strains against the standard drug Gentamycin and Fluconazole, respectively. The results demonstrated the better antimicrobial activity of 169 and 170 than 167 and 168. CT-DNA binding of 167\u2013170 was done using viscosity and gel-electrophoresis. Compounds 167\u2013170 possessed low lipophilicity owing to the presence of salting (Na-sulfonato) groups, resulting in significant electrostatic interaction with CT-DNA. The order of the interaction was HPSL\u00a0<\u00a0HPSA\u00a0<\u00a0Cu-PSL\u00a0<\u00a0Cu-PSA. The anticancer activity was performed against breast carcinoma (MCF7), colon carcinoma (HCT-116) and hepatocellular carcinoma (HepG2) cells. The findings confirmed the viability of 169 and 170 as anticancer therapeutic candidates [47].A new SB ligand (2-((E)-(4-trifluoromethoxy)phenylimino)methyl)-6-tert-butylphenol 173 was synthesized in methanolic medium condensation of 3-(tert-butyl)-2-hydroxybenzaldehyde 171 and 4-(trifluoromethoxy)benzenamine 172. [Co(L)2(H2O)2] 174, [Ni(L)2] 175 and [Cu(L)2] 176 complexes of 173 were also synthesized (Scheme 29\n\n) and characterized via different elemental and spectroscopic techniques. Compounds 175 and 176 possessed square planer geometry whereas 174 owned octahedral geometry with 2:1 (ligand:metal) stoichiometry ratio, which was also supported by single crystal XRD. Compounds 173\u2013176 were subjected for their CT-DNA binding, Pbr322 DNA cleavage and antimicrobial (paper disc technique) activities. The effective CT-DNA binding with 174\u2013176 was analyzed by viscosity measurements, fluorescence quenching and absorption spectroscopy. The order of binding compatibility was 176\u00a0>\u00a0175\u00a0>\u00a0174\u00a0>\u00a0173. The Pbr322 DNA cleavage activity was performed via gel electrophoresis method through oxidative and photolytic mechanism. Complex 176 reported efficient cleavage of Pbr322 supercoiled DNA into the liner or circular forms. The compounds 174\u2013176 exhibited better antimicrobial inhibition against bacterial (B2 and B12) and fungal (F8 and F9) strains as compared to 173\n[48].Co(II) 180, Ni(II) 181 and Cu(II) 182 complexes of neoteric salen-based quadridentate SB ligand 179 were obtained from the reaction of 4-fluoro-1,2-phenylenediamine 177 and 2-hydroxy naphthaldehyde 178 (Scheme 30\n\n). The characterization was carried out using thermal, elemental analysis and different spectroscopic methods. The electronic transition spectral analysis suggested the square planer geometry and 1:1 metal:ligand stoichiometry ratio of 180\u2013182 whereas magnetic moment data suggested the paramagnetic and diamagnetic nature of 180 and 182 and 181, respectively. Compounds 179\u2013182 were analyzed for their antimicrobial, DNA binding (fluorescence emission, UV\u2013Visible absorption and viscosity techniques), DNA cleavage and antioxidant activities. Mancozeb and Streptomycin were utilized as standard drugs for antifungal and antibacterial activities, respectively by employing the disc diffusion method. Complexes 180\u2013182 exhibited better antimicrobial potential against F8, F9, B1, B2, B9 and B12 strains than 179. Complex 182 showed the maximum potential for DNA binding than 180 and 181. The DNA cleavage action was performed against the Pbr322 DNA using photolytic and oxidative pathway by utilizing the agarose gel electrophoresis method. 180\u2013182 were able to cleavage the DNA into its linear and nicked form and 179 was found to be unaffected into the DNA cleavage process. On the other hand, the antioxidant activity of 180\u2013182 was performed using free radical DPPH scavenging method with the standard ascorbic acid. The IC50 value was maximum for 182 but less than the standard ascorbic acid [49].Rao and co-team [50] described the synthesis (\nScheme 31\n\n) and characterization of newly prepared Co(II) 186, Ni(II) 187 and Cu(II) 188 complexes of SB ligand 2-((E)-(6-ethoxybenzo[d]thiazol-2-ylimino)methyl)-4-nitrophenol 185, resulted from the reaction of 5-nitrosalicylaldehyde 183 and 2-amino-6-ethoxybenzothiazole 184. The ligand 185 exhibited the bi-dentate monobasic behavior and the complexes 186\u2013188 possessed square planer geometry. All the compounds 185\u2013188 were analyzed for their antimicrobial, DNA binding and DNA cleavage studies. CT-DNA binding was carried out using fluorescence emission, UV\u2013Visible absorption and viscosity measurement techniques, confirming the interaction through intercalative manner whereas CT-DNA cleavage efficiency was checked using agarose gel electrophoresis process by utilizing H2O2 as oxidant reagent. The antibacterial (disc diffusion method) activity was also performed against the B4 ATCC-15380, B2 ATCC-25922, B1 ATCC-25923, B9 ATCC-12454 and B8 ATCC-35552 bacterial strains. The cytotoxic activity was analyzed using MTT-assay against MCF7 and HeLa cell lines, which confirmed the better efficiency of 186\u2013188 towards both cell lines as compare to 185. The order of IC50 value was 188\u00a0>\u00a0186\u00a0>\u00a0187\u00a0>\u00a0l85. Compound 188 showed the highest potential for all the biological (DNA binding and cleavage, antioxidant and antibacterial activities.A series of Co(II) 192 and Ni(II) 193 complexes was synthesized by the condensation reaction of respective metal salts and SB ligand 191 derived from the reaction of 1,2-diaminopropane 189 and 2-hydroxy-6-isopropyl-3-methyl-bezaldehyde 190 in 1:2\u00a0M ratio (\nScheme 32\n\n).The synthesized compounds were analyzed through elemental analysis, magnetic and electrochemical measurements, molar conductivity and various spectroscopic techniques such as SEM-EDX, cyclic voltammetry. Furthermore, single crystal-XRD technique confirmed a dimeric form with the empirical formula [NiL]2 and the distorted square planar geometry around the Nickel center for 193. Compounds 191\u2013193 were analyzed for their antimicrobial, DNA cleavage and antioxidant activities. All compounds showed negligible antifungal activity against F1, F2, F3 and F15 than the standard drugs i.e., Fluconazole and Miconazole. Whereas, complex 192 exhibited maximum antibacterial activity against B1, B2, B3 and B9 strains using Ampicillin and Ciprofloxacin as standard drugs. Complexes 192 and 193 exhibited higher MIC values than Ampicillin but lower than Ciprofloxacin. 192 and 193 showed good antioxidant activity than191on comparing with ascorbic acid. The DNA cleavage activity was performed against Pbr322 DNA in the presence of H2O2 using agarose gel electrophoresis method. Complex 193 possessed maximum potential to convert the supercoiled form (I) into naked DNA form (II) [51].A novel SB ligand 196 was derived from the reaction of 4-methoxy salicylaldehyde 194 and 6-aminobenzothiazole 195. Co(II) 197, Ni(II) 198 and Cu(II) 199 complexes of 196 were also prepared using condensation pathway (\nScheme 33\n\n). All the compounds were characterized through various analytical and spectral techniques such as IR, UV\u2013Visible, TGA and ESR. According to the data, all the complexes 197\u2013199 possessed square planar geometry. The anticancer efficiency was checked against different cell lines such as cervical cancer cell (HeLa), breast cancer cell (MCF7) and adenocarcinomic human alveolar basal epithelial cells (A549) with cis-platin as standard drug. Complex 199 showed the maximum IC50 value. 97\u2013199 were more active against HeLa cells than A549 and MCF7 cancer cells. The toxicity order was \ncis\n\n-platin\u00a0>\u00a0199\u00a0>\u00a0198\u00a0>\u00a0197\u00a0>\u00a0196, due to the reduction of charge on metal ions which allowed the complexes to pass through the lipid layer of the cell membrane easily. The DNA binding study through intercalative mode via electronic absorption, viscosity and fluorescence quenching confirmed efficient binding of 199 with CT-DNA. On the other side, DNA cleavage action of 197\u2013199 was performed against super coiled Pbr322 DNA using agarose gel electrophoresis method. 199 reported endorsed translations of super coiled Pbr322 plasmid DNA into linear form more efficiently. Complex 199 also possessed highest antibacterial efficiency against gram\u00a0+ve B3 and gram \u2013ve B2, B4 and B26 strains amongst all the synthesized compounds, using disc diffusion method. This was due to chelation leading the complexes to behave as effective bactericidal agents [52].In recent decades, almost every field of science and technology has experienced enormous advancements. Despite the advancements, there is still a long way to go until therapeutic interventions against microorganisms and cancer treatment have been explored. Due to the symptoms and medication obstruction, the direct access of antibacterial and anticancer drugs is restricted. Although there has been a notable advancement in our understanding of the subatomic causes of microbial disease and tumor growth, optimal therapeutic approaches are still lacking. In light of these facts, it is imperative to promote the development of novel antibacterial and anticancer agents. It is possible to fully address the usefulness of the various types of atoms (both ligands and metal complexes) for designing of new and effective antibacterial and anticancer agents. Investigating SB metal chelates with various subatomic features and topologies as antibacterial and anticancer agents is therefore essential. Additionally, emphasizing and launching techniques may be beneficial for human development to overcome the drawbacks of available operators. Therefore, we concentrated on the pharmacologically potent SB and their chelates. Notably, metallo-compounds shown more potential pharmacological efficacy than the conventional SB, necessitating more research. Tweedy's chelation theory and Overtone's idea were used by the authors to explain the enhanced biological efficiency. We have made an effort to emphasize the synergistic work and opportunities of SB derived metal complexes in accordance with the scope of the study. The designing and synthesis of novel SBs are incredibly significant for inventing unique medication libraries and eliminating the challenge of various drug resistances. Many SB metal complexes have shown excellent effectiveness against microorganisms and in the treatment of cancer, however none of these complexes have gone in-depth research or been released onto the market. This might be because people neither think SB complexes can be utilized as antibacterial/anticancer medications nor they do not have a professional interest in them. Researchers have been inspired to create synthetic strategies for the synthesis of innovative metal-frameworks based on SB science by the advantageous properties. An effective technique to obtain novel analogues with improved properties is to coordinate the carbonyl group by the SB reaction to amine compounds that represent various pharmacological groups. Many materials, including metal\u2013organic frameworks, gels, porous organic cage, and nanocomposites, can be created through molecular self-assembly using a small subset of these substances. These assemblies currently have a broad variety of applications, from physics and materials science to pharmacology and health, and can be further tailored to simulate biological settings. This study covered various Co(II), Ni(II) and Cu(II) complexes with excellent DNA binding/cleavage, antimicrobial, anticancer and antioxidant inhibitory properties that may be useful for developing innovative therapeutic strategies for treating a variety of diseases. On the basis of this analysis, it is possible to draw the conclusion that creating SB metal complexes with crucial features is necessary in order to deliver medication to the product.\nAlka: Writing \u2013 original draft. Seema Gautam: Methodology. Rajesh Kumar: Formal analysis, Investigation. Prashant Singh: Visualization, Writing \u2013 review & editing. Namita Gandhi: Data curation. Pallavi Jain: Methodology, Validation, 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.The authors would like to thank SRM Institute of Science and Technology, NCR Campus, Modinagar, Ghaziabad, India for the guidance and support.", "descript": "\n                  Schiff bases are versatile chemical compounds that are frequently utilized and manufactured by reacting various amines with carbonyl compounds (aldehydes/ketones), resulting in the formation of the azomethine/imine (\u2013CN\u2013) group by condensation reaction. Medical science is driven to generate novel drugs with revolutionary bioactivities and functionalities to cure diseases that are rapidly evolving. Schiff base (SB) is a dynamic pharmacophore that, through chelation, can create complexes with metals of various oxidation states. SB metal complexes have already been recognized as an effective branch of investigation in coordination science. In the recent years, scientists have paid close attention to SBs and the metal complexes owing to versatile potential in the pharmaceutics sector, such as antifungal, antibacterial, antiviral and antimalarial, anti-HIV, anti-cancer, anti-tuberculosis, and many others. These compounds have also been identified as potent oxidants, with applications in sensing and nanotechnology. The ligand environment, metal ion complexation, and lipophilic nature have an impact on the biological activity of transition metal complexes. SB metal complexes are the attractive targets for the development of broad-spectrum medicines due to their combination of pharmacological properties. The review focuses on the synthesis, spectroscopic characterization and in-vitro biomedical applications (antimicrobial, anticancer, antitumor, DNA binding and cleavage, antioxidant) of SBs as well as their Co(II), Ni(II) and Cu(II) transition metal complexes.\n               "}
{"full_text": "Due to dwindling reserves of easily accessible fossil resources and the increasing demand for fuels and chemicals, there is a growing need to develop efficient catalytic routes from renewable lignocellulosic biomass to fuels and chemicals. Cellulose, which is a renewable feedstock derivable, can be converted to platform chemicals such as 5-hydroxymethylfurfural (HMF) [1], levulinic acid (LvA) [2], and \u03b3-valerolactone (GVL) [3]. In particular, GVL has attracted much attention in recent years, because it can be used in the chemical industry either directly or as an intermediate to food additives, nylon and green solvents [4]. GVL is also increasingly considered as a platform for the production of liquid biofuels [5].GVL can be obtained by gas- or liquid-phase hydrogenation of LvA using a suitable metal-based catalyst and a hydrogen source. In the past few years, many different catalyst systems including noble [6\u20138] and non-noble metals [9\u201313] have been evaluated for the hydrogenation of LvA to GVL. In addition to the use of renewable feedstock, it is also of increasing importance to develop catalysts that are based on cheap and abundantly available metals. Many catalysts used in the chemical industry are currently based on expensive precious group metals. Accordingly, there is a strong incentive to replace them with non-noble metals.The catalytic hydrogenation of LvA in the presence of molecular hydrogen using batch [14,15] and flow reactors [15\u201319] has been extensively studied in the last decade. The first example of catalytic hydrogenation of LvA was however already reported more than 50 years ago by Broadbent et al. [20] These researchers employed an unsupported Re black catalyst and were able to reach a 71 % yield of GVL after 18\u202fh reaction at 106\u202f\u00b0C at a H2 pressure of 150\u202fbar. The remaining products were mainly polymeric esters. Afterwards, a wide range of supported noble metal catalysts featuring mainly Ru [7,18,21], Ir [22], Rh [23], and Pt [17] as the key hydrogenation components were evaluated for their activity in LvA hydrogenation. Ruthenium-based catalysts have emerged as promising candidates, because they typically combine high activity and selectivity. Non-noble copper-based catalysts such as Cu/ZrO2 [24], Cu/SiO2 [25], Cu-Cr [26], and Cu-Fe [27] have also been reported to be effective for producing GVL from LvA, although they typically require a higher reaction temperature and/or long reaction time for achieving high LvA conversion. Homogeneous catalysts have also been studied providing good results under relatively mild conditions. For instance, Yi and collaborators reported the hydrogenation of levulinic acid to GVL using a homogeneous Fe complex in aqueous solution, obtaining GVL yields as high as 97 % in 2\u202fh at 100\u202f\u00b0C and 50\u202fbar [28].There is growing evidence of the positive effect of bimetallic catalyst formulations for the hydrogenation of oxygenated substrates [29,30]. For instance, the group of Weckhuysen reported on the beneficial effect of Ru-Au nano-alloying for the catalytic conversion of LvA to GVL [31]. Bimetallic catalysts containing noble and non-noble metals (i.e., Ni-Ru, Ni-Pt, Ni-Au, and Ni-Pd) supported on supports such as zeolite, ZrO2, \u03b3-Al2O3, and SiO2 have also been employed for the upgrading of biobased intermediates derived from lignin [32]. Supported Ni-Re [33] and Pt-Re [34] catalysts have been shown to be highly active for the selective hydrogenation of carboxylic acids. Higher conversion and selectivity of carboxylic acid hydrogenation were achieved with a Ni-Re catalyst compared to its single-metal and Pt-based counterparts [33]. Recently, Ni/Al2O3, Ni-Cu/Al2O3, Ni-Nb/TiO2 and Ni/HZSM-5 were employed for the hydrogenation of LvA [15,19,35]. A high reaction temperature (220\u2212275\u202f\u00b0C) was however required to achieve reasonable performance. Shimizu and co-workers first reported a noble-metal-free Ni-MoOx/C catalyst with a TON (turnover number) of 4950 [36], which is comparable to a state-of-the-art Ru catalyst for the hydrogenation of LvA to GVL at 250\u202f\u00b0C [37]. Grunwaldt et al. reported a solvent-free method to obtain a 92 % GVL yield for LvA hydrogenation using Ni/Al2O3 [14]. However, reuse of the Ni catalyst resulted in a significantly lower activity. Shimizu\u2019s group reported that Re/TiO2 is a promising catalyst for the selective hydrogenation of aromatic and aliphatic carboxylic acids. In their study, 3-phenylpropanol was produced in 97 % yield from 3-phenylpropionic acid under mild conditions (50\u202fbar H2 at 140\u202f\u00b0C) [38].Here we report a novel TiO2-supported Fe-Re bimetallic catalyst system, which is highly active in the hydrogenation of LvA to GVL under mild conditions. The catalysts were extensively characterized using H2-TPR, XPS, XANES, EXAFS, M\u00f6ssbauer spectroscopy, CO-IR spectroscopy and TEM. Strong interaction between Fe and Re was observed in terms of the formation of a Fe-Re-oxide phase, which upon reduction is partially converted into a metallic Fe-Re alloy covered by FeOx species. The interactions of Re and Fe with the titania support also play an important role in the formation of catalytically active nanoparticles. The strong synergy in levulinic acid hydrogenation is attributed to the interface between metallic Fe-Re particles and FeOx.All the supported catalysts were prepared by an incipient wetness impregnation method. Titania (P25 TiO2, Evonik-Degussa) was dried at 110\u202f\u00b0C overnight, prior to impregnation of the metal precursor. For the preparation of Fe-Re bimetallic catalysts, appropriate amounts of Fe(NO3)3\u00b79 H2O (\u226598.0 %, Sigma Aldrich) and perrhenic acid (HReO4) (99.99 %, 75\u221280\u202fwt% in H2O, Sigma Aldrich) precursors were dissolved in deionized water. Then, the required amount of titania was added very slowly under continuous stirring at room temperature. The sample was dried at 110\u202f\u00b0C overnight, and ground thoroughly and reduced in a furnace at 500\u202f\u00b0C for 2\u202fh (ramp rate 2\u202f\u00b0C/min) in a flow of 10 % H2/He (total 100\u202fmL/min). Two monometallic reference catalysts 2.0\u202fwt% Fe/TiO2 and 13\u202fwt% Re/TiO2 were prepared using the same method.Temperature-programmed reduction (TPR) experiments were performed in a Micromeritics AutoChem II 2920 instrument equipped with a fixed-bed reactor, a computer-controlled oven, and a thermal conductivity detector. Typically, samples (50\u202fmg) were loaded in a tubular quartz reactor. Prior to reduction, samples were pretreated at 150\u202f\u00b0C for 2\u202fh. The sample was reduced in 4\u202fvol% H2 in N2 at a flow rate of 8\u202fmL/min, whilst heating from room temperature up to 900\u202f\u00b0C at a heating rate of 10\u202f\u00b0C/min. The H2 consumption was monitored by a gas chromatography equipped with a thermal conductivity detector (TCD) and calibrated using a CuO/SiO2 reference catalyst.XPS measurements were performed using a Kratos AXIS Ultra spectrometer, equipped with a monochromatic X-ray source, and a delay-line detector (DLD). Spectra were obtained using an aluminum anode (Al K\u03b1\u202f=\u202f1486.6\u202feV) operating at 150\u202fW. Survey scans were measured at a constant pass energy of 160\u202feV and region scans at 40\u202feV. The background pressure was kept at 2 \u00d7 10\u22129 mbar. Quasi-in situ XPS measurements for all of the catalysts were performed after reducing them in a tubular quartz reactor with 10\u202f\u00b0C/min heating rate from room temperature to 500\u202f\u00b0C in a flow of 10\u202fvol% H2 in He (total flow 100\u202fmL/min). After cooling to room temperature, the lids at the inlet and outlet of the reactor were closed to prevent air exposure. The samples were prepared for XPS measurements in an Ar-flushed glove box and transferred in an air-tight transfer holder to the XPS apparatus. Data analysis was performed using CasaXPS software. The binding energy was corrected for surface charging by taking the C 1s peak of adventitious carbon as a reference at 284.6 eV.X-ray absorption fine structure (QEXAFS) measurements were done at the Fe K-edge (\u223c7112\u202feV) and Re L-3 edge (10535\u202feV) in transmission mode on beamline BM26 at ESRF (DUBBLE, Grenoble). The photon flux of the incoming and outgoing X-ray beam was detected with two ionization chambers I0 and It, respectively. The obtained absorption data were background-subtracted, normalized and fitted as difference spectra using Athena software. EXAFS analysis was performed using VIPER on k3-weighted data. The amplitude reduction factor S02\n was determined by fitting the first Re-Re coordination to 12, of Re foil. In a typical experiment, ca. 15\u202fmg catalyst sample (in tablet form) was placed in a stainless-steel XAS reactor equipped with two fire-rods and glassy carbon windows as described in ref [39]. Catalysts were reduced in this cell by heating at a rate of 3\u202f\u00b0C/min from 40\u202f\u00b0C to 500\u202f\u00b0C followed by an isothermal dwell of 0.5\u202fh in a flow of 20\u202fvol% H2 in He at a total flow rate of 50\u202fmL/min. During reduction, the state of the samples was followed by XANES, while EXAFS spectra were recorded at 50\u202f\u00b0C after the reduction.Transmission 57Fe M\u00f6ssbauer spectra were collected at -153.2\u202f\u00b0C with a sinusoidal velocity spectrometer using a 57Co(Rh) source. Velocity calibration was carried out using an \u03b1-Fe foil at room temperature. The source and the absorbing samples were kept at the same temperature during the measurements. The M\u00f6ssbauer spectra were fitted using the Mosswinn 4.0 program.Low-temperature infrared spectra of CO adsorbed on the catalysts was recorded using a Bruker Vertex V70v FT-IR spectrometer. The IR spectra were acquired at a resolution of 2\u202fcm\u22121 and 32 scans were averaged for each spectrum. Typically, an amount of ca. 20\u202fmg catalyst was pressed into a thin self-supporting wafer with a diameter of 13\u202fmm, which was then placed inside a controlled-environment IR transmission cell capable of heating and cooling, gas dosing, and evacuation. Prior to CO adsorption, the catalyst wafer was reduced at 500\u202f\u00b0C for 1\u202fh in flowing 10\u202fvol% H2 in He, followed by cooling to 100\u202f\u00b0C. The cell was then evacuated to \u223c10-6 mbar and further cooled to liquid nitrogen temperature. The sample was then subjected to pulses of CO via a sample loop (10\u202f\u03bcL) connected to a six-port sampling valve. CO was pulsed until saturation was reached as observed by saturation of the CO IR adsorption bands.Transmission electron micrographs were acquired on a FEI cubed Cs corrected Titan at 300\u202fkV. Typically, a small amount of the sample was ground and suspended in pure ethanol, sonicated and dispersed over a Cu grid with a holey carbon film. Samples were firstly reduced in 10\u202fvol% H2 in He (total flow 100\u202fmL/min) at 500\u202f\u00b0C for 2\u202fh, followed by passivation in 1\u202fvol% O2 in He for 10\u202fh. Bright field images (BF) were taken using a rather large objective aperture to enhance the contrast, specifically for lattice imaging. HAADF-STEM imaging was done to analyze the particle size. Elemental analysis was done with an Oxford Instruments EDX detector X-MaxN 100TLE.Aqueous-phase catalytic hydrogenation of LvA to GVL was performed in a 10\u202fmL autoclave (HOKE Swagelok) at various temperatures (130\u2013200\u202f\u00b0C) and a (cold) H2 pressure of 40\u202fbar. In a typical reaction, 2\u202fmmol LvA and 23\u202fmg reduced catalyst were loaded into the autoclave in a nitrogen-flushed glove-box. The autoclave was sealed using a rubber plug before removing it from the glove-box. An amount of 4\u202fmL degassed water was injected into the autoclave via the rubber plug using a syringe. The autoclave was then sealed and purged four times with H2 before the pressure was increased to 40\u202fbar. The reaction was started by heating the autoclave to the desired reaction temperature under continuous stirring (1000\u202frpm). At the end of the reaction, the autoclave was cooled rapidly to room temperature in an ice bath, after which the remaining H2 was released. The catalyst was separated from the solution by filtration (0.45 \u03bcm filters). The reaction products were subjected to NMR analysis.Quantitative analysis of the liquid products (LvA and GVL) was carried by 1H-NMR using 1,4-dioxane as an internal standard. An amount of 100\u202f\u03bcL 1,4-dioxane was added to the reaction mixture after the catalytic reaction. An aliquot of 300\u202f\u03bcL of the reaction mixture was transferred to a 5\u202fmm NMR tube together with 300\u202f\u03bcL deuterated dimethylsulfoxide-d6 (DMSO-d6) solvent. For quantitative 1H NMR analysis, 32 scans were averaged using a relaxation delay of 5\u202fs. All spectra were integrated using MestReNova software.The conversion of LvA (\nX\n) was calculated as follows:\n\n\n\nX\n\u2009\n\n%\n\n=\n\n\n\nC\n\nL\nv\nA\n,\n0\n\n\n\u2212\n\nC\n\nL\nv\nA\n\n\n\n\n\nC\n\nL\nv\nA\n,\n0\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\nThe yield of the liquid component \ni\n (\n\n\nY\ni\n\n\n) was calculated as follows:\n\n\n\n\nY\ni\n\n\u2009\n\n%\n\n=\n\n\n\nC\n\np\nr\no\nd\nu\nc\nt\n\u2009\ni\n\n\n\n\n\nC\n\nL\nv\nA\n,\n0\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\nA set of Fe-Re catalysts supported on TiO2 with different atomic Fe/Re ratios were prepared by wetness impregnation. The Fe loading for all of the Fe and Fe-Re catalysts was kept at 2.0\u202fwt%. The atomic Fe-to-Re ratio was varied between 5:1 and 1:2. The bimetallic catalysts are denoted as Fe-Re(x:y)/TiO2 in which x:y stands for the atomic Fe/Re ratio. Monometallic Fe-2.0\u202fwt%/TiO2 and Re-13\u202fwt%/TiO2 (denoted as Fe(2.0)/TiO2 and Re(13)/TiO2, respectively) were prepared in the same way and served as reference catalysts.We firstly screened these catalysts for their performance in the hydrogenation of LvA to GVL. For this purpose, the catalysts were reduced at 500\u202f\u00b0C for 2\u202fh and then tested in a batch reactor in water at 140\u202f\u00b0C and 40\u202fbar H2 for 4\u202fh. 0 Fig. 1\n compares the performance of the reduced catalysts. Fe(2.0)/TiO2 showed a very low GVL yield of less than 1%. The yield for Re(13)/TiO2 was 3%. A much higher catalytic performance was achieved using Fe-Re bimetallic catalysts. With Fe-Re(1:1)/TiO2 catalyst, a yield of 12 % GVL was obtained at the conversion of 14 %. The catalytic performance increased with increasing Re-to-Fe ratio. The best catalytic performance was obtained for Fe-Re(1:2)/TiO2, which gave 17 % yield of GVL at the conversion of 18 %. These results evidence a significant synergy between Fe and Re. The addition of Re to Fe strongly improved LvA conversion.TPR traces of the reduction of all catalysts are presented in Fig. 2\n. The Fe(2.0)/TiO2 sample shows a very small reduction feature around 285\u202f\u00b0C, which is due to the (partial) reduction of Fe3+ to Fe2+. A broad feature around 650\u202f\u00b0C can be attributed to the reduction of Fe2+ to metallic Fe [40]. The active phase in Re(13)/TiO2 sample is reduced at 350\u202f\u00b0C. This suggests that it is easier to reduce Re than Fe on titania. For the Fe-Re samples, the main reduction peak becomes sharper and its position shifts to slightly higher temperatures compared with the Re-only and Fe-only samples. It is also noted that the first (partial) reduction peak at 285\u202f\u00b0C, which is due to reduction of Fe3O4 or Fe2O3 to FeO, is not present for the bimetallic Fe-Re catalysts and the second reduction peak at 650\u202f\u00b0C shifts to higher temperature (cf. the dashed line in Fig. 2). A previous study showed that higher reduction temperature for bimetallic Fe-Re/SiO2 catalysts compared to the monometallic ones can be attributed to a strong interaction between Fe and Re in the mixed oxide [40]. For the Fe-Re/TiO2 samples, besides the small reduction feature of Fe2+ \u2192 Fe\u00b0, a single main reduction feature suggests that Fe and Re are present in a mixed-oxide phase.Reduced Fe, Re, and Fe-Re samples were further characterized by XPS, which is a surface-sensitive technique. Fig. 3\n shows the fitted Re 4f XPS spectra. Quantitative XPS data are collected in Table 1\n. A wide range of oxidation states of Re between +2 and +7 is observed for the reduced catalysts. At low Re content in Fe-Re(5:1)/TiO2, the intensity of the Re signal was too low for reliable fitting. For Fe-Re(2:1)/TiO2, the Re2+ : Re4+ : Re5+ : Re6+ : Re7+ ratio was 16 : 16 : 10 : 21 : 37. No metallic Re was observed for this sample. Samples with a higher Re content, Fe-Re(1:1)/TiO2 and Fe-Re(1:2)/TiO2, contained both metallic and oxidic Re. For reduced Fe-Re(1:1)/TiO2, the metallic Re fraction was 53 % and the remainder was present as Re-oxide species with a large contribution of Re2+. The Re\u00b0 content increased to 70 % for Fe-Re(1:2)/TiO2. It is important to mention that the metallic Re\u00b0 content in the Fe-Re(1:1)/TiO2 and Fe-Re(1:2)/TiO2 samples are higher in the Re(13)/TiO2 sample (29 %). These results confirm that the presence of Fe resulted in a higher reducibility of the Re component in the reduced materials.We also analyzed the oxidation state of Fe in these samples by XPS. The XPS spectra in the Fe 2p region and the results of their deconvolution are shown in Fig. 4\n and Table 1, respectively. The reduced Fe(2.0)/TiO2 sample contains a large amount of Fe2+ (78 %) with the remainder being Fe3+ (22 %). No metallic Fe was observed in this catalyst. This confirms that the first reduction peak in TPR is due to the partial reduction of Fe3+ to Fe2+. It is seen that the amount of Fe2+ is lower for the reduced Fe-Re(5:1)/TiO2, Fe-Re(2:1)/TiO2, and Fe-Re(1:1)/TiO2 samples in comparison to Fe(2.0)/TiO2. The finding that the Fe3+/Fe2+ ratios of these three catalysts are similar indicates that Re does not promote the reduction of Fe at a low Re content. Instead, its reducibility is decreased, which is likely due to the strong interaction of Fe in a mixed Fe-Re-oxide phase. On the other hand, at higher Re loading (Fe-Re(1:2)/TiO2), the fraction of Fe2+ in the reduced materials is already \u223c85 %, suggesting that the presence of Re promotes the reduction of Fe. The well-known mechanism for this kind of reduction promotion is the spillover of H atoms from the first reduced metal phase to the other components [41,42]. The difference is obviously due to the formation of metallic Re at higher Re content. Since the Fe content is constant in our samples, this may imply that a certain fraction of free Re-oxide is needed to obtain metallic Re.We also investigated the reduction of Fe and Re in more detail by in situ XANES, which is bulk sensitive in contrast to XPS. The XANES spectra of the selected Fe-, Re-, and Fe-Re samples were collected at reduction temperatures in the 50\u2212500\u202f\u00b0C range under a 50\u202fmL/min flow of a 10/40 v/v mixture of H2 and He. Reference materials including the corresponding metals and metal oxides of varying oxidation states were also measured. The energy of the half-edge-step was used to compare the oxidation state of Fe and Re during the reduction process. Fig. 5\na and c show the XANES spectra of Fe and Re reference samples, respectively. Fig. 5b and d show the energy corresponding to the half-edge-step for Fe and Re, respectively, during reduction, which is employed here as a qualitative indicator of the oxidation state. Although the pre-edge feature of Fe XANES spectra can be analyzed to analyze the oxidation and coordination state of Fe, [43] the data quality of the measured spectra was too low to extract useful information. Analysis of the energy of the half-edge-step shows a gradual reduction of Fe3+ to Fe2+ and Fe(0). The addition of Re increases the rate of Fe reduction. This is in line with the earlier finding that the addition of Re promotes the reduction of Fe3+ to lower oxidation state. On the other hand, while the Re(13)/TiO2 shows the highest reducibility, it is seen that the addition of Fe delays the reduction of Re to higher temperature. However, by comparing the energy at the half-edge-step of the Re-containing samples with the Re foil (Re\u00b0) reference, it can be concluded that all of the Re species can be reduced to their metallic state above 350\u202f\u00b0C. This seems to be inconsistent with the XPS results, where a larger amount of oxidized Re was observed. This difference is likely due to the in situ character of the XANES measurements, while XPS was carried out in a quasi in situ mode, including cooling to room temperature and sample transfer at ambient conditions via a glove-box. Although air exposure was avoided, traces of oxygen and water will likely oxidize reduced surface Re species. Obviously, the difference is substantial also because XPS is a surface-sensitive technique, while XANES probes the bulk as well. Notably, the Fe-Re bimetallic catalysts gave lower energies at the half-edge-step than the Re(13)/TiO2 catalyst and the Re foil (Fig. 5d). This points to the formation of a Fe-Re alloy with a different electronic structure than reduced Re nanoparticles. The presence of this Fe-Re alloy can explain the higher reducibility of the Re component in reduced samples, revealed by XPS analysis.In order to obtain more detailed structural information of the Fe-Re samples, extended X-Ray absorption fine structure (EXAFS) data were collected both at the Fe K-edge and the Re L3-edge after reduction at 500\u202f\u00b0C followed by cooling to 50\u202f\u00b0C. Due to the low Fe content in combination with the strong X-ray absorption by the titania support, the Fe Ke-edge EXAFS data quality was too low for reliable fitting. Fig. 6\n depicts the experimental and fitted k\n3-weighted R-space spectra for the Re foil and the Re(13)/TiO2 and Fe-Re(1:2)/TiO2 samples. The EXAFS fitted parameters are summarized in Table 2\n. The amplitude reduction factor was chosen such that the Re foil with its typical cubic closest packed structure has a Re-Re coordination number of 12. For the Re(13)/TiO2 sample, the first Re-Re shell coordination number due to a metallic Re-Re bond is significantly lower (CN\u202f=\u202f7.2), implying the formation of nanoparticles. This sample also contains a first shell contribution from Re-O scattering at 2.072\u202f\u00c5 with a coordination number of 1.1, indicating that the Re phase was not fully reduced. This may be attributed to the strong interaction with the titania support. The difference in Debye-Waller factors between this sample and the reference also hints at such interactions [44]. The fitting results for Fe-Re(1:2)/TiO2 are very different. First of all, the Re-Re bond distance is reduced compared with the other two Re-only samples and a second shell is seen, which can be fitted with a Re-Fe scatterer. The theoretically determined Re-Fe and Re-Re bond length values in a stoichiometric Re-Fe alloy are 2.587\u202f\u00c5 and 2.689\u202f\u00c5, respectively. Both are shorter than the Re-Re bond length (2.75\u202f\u00c5) in Re(13)/TiO2 and the Re foil. The shorter Fe-Re bond length is due to the smaller size of the Fe atom with respect to the Re atom. The Re-Re bond length is 2.653\u202f\u00c5 with a coordination number of 7.6, implying a similar coordination number as for the Re-only sample. The occurrence of a Fe-Re coordination of 1.2 at 2.453\u202f\u00c5 indicates that a Re-Fe alloy is formed with a high Re/Fe ratio. The inclusion of a Re-O contribution did not significantly improve the fitting results for the Fe-Re(1:2)/TiO2 catalysts.Given the poor quality of the Fe K-edge XAS data, we used M\u00f6ssbauer spectroscopy to characterize the Fe phase in our samples. The M\u00f6ssbauer absorption spectra were recorded at -153.2\u202f\u00b0C after reduction at a temperature of 500\u202f\u00b0C. The obtained spectra are presented in Fig. 7\n, while the M\u00f6ssbauer fit parameters are summarized in Table 3\n. The Fe(2.0)/TiO2 sample can only be reduced to metallic Fe to a small extent (6%). The remaining Fe is present as Fe3+ (bulky hematite, Fe2O3, 15 %), super-paramagnetic Fe3+ (small hematite particles, Fe2O3, 52 %), and Fe2+ (w\u00fcstite-like structures, FeO, 27 %). The hyperfine sextet with a hyperfine field of 51.9\u202fT is characteristic for large bulky hematite particles, indicating that a part of the Fe phase sintered during the reduction. The sample containing Re had a higher Fe reduction degree, evidenced by the increase of the relative intensity of the Fe\u00b0 singlet. The hyperfine sextet is no longer visible in the Re-containing samples, indicating that the sintering of Fe-oxide did not occur. This is very likely due to the interaction between Fe and Re. For the FeRe(1:2)/TiO2, the intermediate Fe2+ phase is not visible, which we take as an indication that only part of the Fe atoms can be promoted by Re and reduced at the applied temperature. The (remaining) non-reducible Fe3+ species are likely experiencing a strong interaction with the TiO2 support. The Fe reduction degree in this sample is close to 40 %. Given that the reduction degree probed by XPS was much lower, we speculate that the reduced Fe species might be part of a Fe-Re alloy. We cannot exclude however that part of the difference in Fe reduction degree can be due to the sensitivity of reduced Fe species to oxygen during the transfer from the XPS pre-chamber to the high-vacuum chamber.The reduced and passivated samples were analyzed on a Cs-corrected TEM. In all samples, nanoparticles were found on micron-sized agglomerates of titania particles. Although the bright-field images showed the presence of 1\u22122\u202fnm nanoparticles, the active phase could not be clearly observed in this way. Accordingly, HAADF-STEM images were recorded as well from which the particle size distribution was determined. EDX maps and particle size distributions are shown in Figs. 8\u201311\n\n\n\n. The average particle size for Fe-Re(2:1)/TiO2, Fe-Re(1:1)/TiO2, and Fe-Re(1:2)/TiO2 are roughly similar at 1.0\u202f\u00b1\u202f0.4\u202fnm. Re(13)/TiO2 contains slightly larger nanoparticles with an average diameter of 1.3\u202f\u00b1\u202f0.8\u202fnm. All samples contain few nanoparticles larger than 2\u202fnm. Notably, the Re-only sample contains a fraction of significantly larger nanoparticles (cf. Fig. 11). By comparing Fe-Re(1:2)/TiO2 and Re(13)/TiO2 with the same Re content, it can be stated that the presence of Fe in the bimetallic catalyst results in a better Re dispersion.EDX mapping in STEM mode showed the presence of Fe and Re over the surface of the titania support. The reported quantitative Fe signals (determined by wide electron probe EDX) were corrected for signals due to secondary electrons using the Co signal. In general, the Fe/Re ratio in areas where no clear nanoparticles are visible is higher than in areas where Re nanoparticles can be observed (Table 4\n). For instance, the Fe/Re ratio on the titania surface for Fe-Re(2:1)/TiO2 is higher (\u223c2.0) than the Fe/Re ratio on the nanoparticle displayed in Fig. 8. The EDX maps of the nanoparticle shown in Fig. 8d\u2013f also suggest a core-shell structure in which the shell contains more Fe than the core. The presence of Fe and Re across the titania surface is in agreement with recent aberration-corrected TEM images of a Ni-Re catalyst, which showed that Re is dispersed on the surface in the form of atoms, clusters, and nanoparticles [33]. In the present study, STEM can only image the nanoparticles. Accordingly, we can conclude that the titania-supported Fe-Re samples contain Re nanoparticles in close contact with Fe and very highly dispersed Fe and Re species homogeneously distributed over the titania surface.IR spectroscopy of adsorbed CO was used to investigate the active phase of the reduced catalysts. As XPS showed that the samples contain oxidic Re and Fe, we recorded the CO IR spectra at liquid N2 temperature. We also included the bare TiO2 support for comparison. Fig. 12\n shows the IR spectra of CO adsorbed on the samples as a function of the CO partial pressure in the cell (0.02\u22121\u202fmbar CO range). The IR spectra in the CO stretching region contain bands at 2178 cm\u22121, 2157\u202fcm\u22121, and 2042\u202fcm\u22121. According to the literature [45,46] CO stretching bands between 2200\u22122100\u202fcm\u22121 can be assigned to CO adsorption on Lewis-acidic metal cations and OH groups, while metals in a lower oxidation state usually give rise to lower CO stretch frequencies. In particular, bands between 1900 and 2100\u202fcm-1 can be assigned to linearly adsorbed CO on the surface of metals. For the bare TiO2 support, a sharp band at 2181\u202fcm\u22121 increased with CO pressure concomitant with a red shift to 2178\u202fcm\u22121. This feature is due to CO adsorbed on Lewis acidic Ti4+ sites. Another weaker band at 2156\u202fcm\u22121 can be assigned to weaker Ti\u2212OH\u00b7\u00b7\u00b7CO complexes [47]. The presence of surface OH groups is also evident from the OH stretching region. Similar IR spectra were obtained for Fe(2.0)/TiO2 (Fig. 12b). The presence of Fe on TiO2 (Fe(2.0)/TiO2) resulted in a lower intensity of the OH stretching bands in the 3500\u22123800\u202fcm\u22121 region, suggesting that during the preparation Fe3+ has reacted with titania OH groups forming Fe-O-Ti species. It is seen that the more acidic OH groups are preferentially consumed. There are no indications of the presence of metallic Fe, consistent with the other characterization data (Fig. 4, Table 2). The spectra did not contain other bands than those observed for bare TiO2, which suggests that Fe is present as a dispersed Fe-oxide phase, which does not adsorb CO. Based on the reported OH density of this type of titania (4.5 OH/nm2), we can estimate that a Fe content of 2\u202fwt% corresponds to about 80 % of the monolayer capacity.For the Re-containing samples, the intensity of the broad band at 2157\u202fcm\u22121 is much higher than for TiO2 and Fe(2.0)/TiO2. Moreover, these bands already appear at a much lower CO coverage and, importantly, before the band at 2178\u202fcm-1 appears. This completely different behavior suggests that a different and stronger CO adsorption complex gives rise to the 2157\u202fcm-1 band in these samples. This is further underpinned by the strong erosion of the band due to OH groups in the 3500\u22123800\u202fcm-1 range when Re is present. It is interesting to note that the OH stretch intensity becomes weaker with increasing Re content. On the other hand, the reduced Re(13)/TiO2 only contains a very weak band at 2158\u202fcm-1 band, which must be due to the consumption of most of the weakly acidic Ti\u2212OH groups. This is consistent with the 13\u202fwt% Re loading corresponding to 1.8 monolayer coverage of the titania surface hydroxyl groups. All of the IR spectra of the Re-containing samples contain a broad band in the 2030-2048 cm-1 regime, which can be associated with Re\u00b0. The intensity of this band for the Fe-Re(1:2)/TiO2 sample is much weaker than for the Re(13)/TiO2 one. We also notice a red shift of the Re\u00b0 feature with increasing Re content, which could be due to the close proximity of CO adsorbed on Re to FeOx and/or the formation of a bimetallic Fe-Re alloy.The CO IR spectra show that the Fe-Re(1:1)/TiO2 and Fe-Re(1:2)/TiO2 samples contain a lower amount of reduced Re\u00b0 surface sites than Re(13)/TiO2. On the other hand, while TEM shows that the nanoparticles in these catalysts are approximate \u223c1\u202fnm, XPS and TPR point to a substantially higher reduction degree of Re in the bimetallic catalysts. Together with the Fe/Re ratios on the nanoparticles derived by STEM-EDX maps, we can conclude that the metallic Re particles are covered by small Fe-oxide clusters that partially block the reduced Re sites. A similar conclusion has been drawn in studies of related Fe-Re/SiO2 [40] and Pd-FeOx/SiO2 [48] catalysts.The CO IR spectra of the bimetallic Fe-Re catalysts contain a feature at 2157\u202fcm\u22121, which shows a maximum at intermediate Fe/Re ratio. It is not likely that this feature is related to the highly dispersed Fe-oxide and Re-oxide species on the titania support, because the signal is absent for the monometallic catalysts. Therefore, we speculate that the 2157\u202fcm\u22121 is due to Lewis acid cations, likely Fe cations, at the interface between metallic Re nanoparticles and a partially reduced Fe-oxide (Scheme 1\n).In attempting to explain the Fe-Re synergy, we compare the highly active Fe-Re(1:2)/TiO2 catalyst with the nearly inactive Re(13)/TiO2 one. The Re reduction degree is much higher for the bimetallic catalyst, demonstrating that reduced Re is the active phase for LvA hydrogenation in line with previous literature [20]. Our characterization data show that the oxides of Fe and Re strongly bind to the titania surface via reaction with the OH groups. When the total Fe\u202f+\u202fRe content is higher than the monolayer capacity of TiO2 as for Fe-Re(1:2)/TiO2, hardly any OH groups are observed in the IR spectrum. Therefore, the higher Re reduction degree in the bimetallic catalyst can be related to a fraction of Re-oxide species that are not bound to the titania surface. When the Re loading is too low, a significant fraction of Re remains strongly bound to the titania surface and cannot be reduced, leading to a low hydrogenation activity. Thus, we can conclude that a role of Fe is to compete for surface OH groups of the titania support and decrease the interaction of Re with the support, thereby resulting in a higher reducibility of Re. This is supported by the M\u00f6ssbauer data, which shows that only 41 % of Fe can be reduced to the metallic state with the remainder being Fe3+ (Fig. 7, Table 2). Despite the higher Re reduction degree in bimetallic catalysts, the Re surface area probed by CO IR spectroscopy is small, which is due to the coverage of part of the Re nanoparticles with Fe-oxides (Scheme 1). It is less likely that a very small amount of reduced Fe forming an alloy can cause this, since the combined XPS and M\u00f6ssbauer data suggest that most of the reduced Fe species are in the core of a bimetallic Re-Fe alloy phase. We speculate that a second role of partially reduced Fe-oxides (FeO) is to dissociate water and provide slightly acidic OH groups, which can catalyze the dehydration step of 4-hydroxypentanoicacid intermediate to \u03b3-valerolactone in the mechanism of LvA hydrogenation (Scheme 1) [49,50]. This kind of promoting effect has been discussed for earlier dehydration relevant to aqueous phase reforming by bimetallic Ir-Re, Pt-Re, and Rh-Re catalysts [51\u201353]. A similar mechanism involving acid-catalyzed dehydration followed by Pt-catalyzed hydrogenation for selective glycerol hydrogenolysis was also proposed by Davis and co-workers [54]. The promotion of metal catalysts with partially oxidized oxophilic MOx species, such as ReOx-promoted Rh has also been suggested by a DFT study [55]. In those cases, the oxophilic nature of Re facilitated the activation of water, while we speculate that for the Fe-Re bimetallic catalysts partially reduced Fe-oxide species play this role. The close proximity of metallic Re, Fe-Re alloys and their oxides including titania can also improve the hydrogenation activity via fast heterolytic activation of H2 at their interface. Such metal-support interfaces are known to facilitate heterolytic H2 activation [56].Encouraged by these findings, we further optimized the reaction conditions by varying the reaction temperature. Fe-Re(1:2)/TiO2 was selected for this optimization study as it was the most active catalyst in the screening stage. Fig. 13\n shows that this catalyst is active for LvA hydrogenation at a temperature as low as 130\u202f\u00b0C. Increasing the reaction temperature resulted in a remarkable increase of the catalytic performance. Nearly full conversion was achieved after reaction at 180\u202f\u00b0C for 4\u202fh, yielding 95 % GVL. The results at 200\u202f\u00b0C were similar and demonstrate that the catalyst is very active and selective for LvA hydrogenation to GVL in water. The sigmoidal activation with respect to temperature might be due to the higher water coverage on the reduced Re surface at too low temperature, which may also lead to partial re-oxidation. Further operando spectroscopy would be required to investigate the catalytic surface during aqueous phase LvA hydrogenation.We performed additional LvA hydrogenation reactions by varying the reaction time from 30\u202fmin to 6\u202fh at a temperature of 180\u202f\u00b0C. The performance of Fe/TiO2, Re/TiO2, and Fe-Re(1:2)/TiO2 are compared in Fig. 14\n. It is clear that the bimetallic catalyst is much more active in LvA hydrogenation. About 50 % yield of GVL at 50 % LvA conversion was achieved after 2\u202fh reaction for Fe-Re(1:2)/TiO2, whereas Re(13)/TiO2 and Fe(2.0)/TiO2 were nearly inactive. Notably, the two monometallic catalysts became slightly active after prolonged reaction but afforded only GVL yields of 40 % and 5%, for Re(13)/TiO2 and Fe(2.0)/TiO2 after 6\u202fh. For Fe-Re(1:2)/TiO2, the maximum yield of GVL \u223c95 % was already reached after 4\u202fh.Hydrogenation of levulinic acid towards \u03b3-valerolactone is one of the most promising reactions in the fields of biomass valorization to fine chemicals and liquid transportation fuels. A series of Fe-Re supported on TiO2 (P25) catalysts were tested for hydrogenation of levulinic acid in water. Remarkable improvements in catalytic performance were observed for the Fe-Re bimetallic catalysts, in comparison with their monometallic counterparts, suggesting a synergistic effect. H2-TPR results show that the reduction peak of Fe-Re samples shifts to higher temperature regime due to the close interaction between Fe and Re species. XANES shows that the presence of Re promotes the reduction of Fe and there is an interaction between Fe and Re. EXAFS analysis further reveals the presence of Fe-Re alloy. XPS and low-temperature CO-FTIR results evidenced large fractions of FeOx and ReOx are present and part of the metallic Re is covered by FeOx. The M\u00f6ssbauer study shows that only part of Fe can be reduced to metallic Fe, due to the strong interaction between Fe and TiO2 support. The coverage of TiO2 surface hydroxy by Fe species was believed to be the reason for the improved reducibility of Re. The Fe-Re alloy, improved Re reducibility and FeReOx species are likely present in the bimetallic samples and believed to be the main reason for the enhanced catalytic activity. The presence of FeOx and ReOx are highly oxophilic and might introduce Re\u2212OH acidic groups via hydration during the reaction, facilitating the dehydration, a key intermediate step for levulinic acid hydrogenation. Under optimized conditions, nearly full conversion of levulinic acid could be achieved after reaction at 180\u202f\u00b0C for 4\u202fh, obtaining 95 % yield of GVL.\nXiaoming Huang: Methodology, Investigation, Data curation, Formal analysis, Validation, Writing - original draft, Visualization. Kaituo Liu: Methodology, Investigation, Data curation, Formal analysis, Validation, Visualization. Wilbert L. Vrijburg: Formal analysis. Xianhong Ouyang: Investigation. A. Iulian Dugulan: Investigation, Formal analysis. Yingxin Liu: Investigation. M.W.G.M. Tiny Verhoeven: Investigation, Formal analysis. Nikolay A. Kosinov: Methodology, Investigation, Formal analysis. Evgeny A. Pidko: Conceptualization, Supervision. Emiel J.M. Hensen: Conceptualization, Writing - review & editing, Supervision, Project administration.There are no conflicts to declare.This work was performed in the framework of the European Union FP7 NMP project NOVACAM (\u201cNovel Cheap and Abundant Materials for Catalytic Biomass Conversion\u201d, FP7\u2010NMP\u20102013\u2010EU\u2010Japan\u2010604319). E.A.P. thanks the Government of the Russian Federation (Grant 074\u2010U01) and the Ministry of Education and Science of the Russian Federation (Project 11.1706.2017/4.6) for supporting his research in the framework of his personal ITMO professorship. The authors would like to thank Rim van de Poll and Alexander Parastaev for helping with the XAS measurements, Bart Zijlstra for the FEFF calculations and Miao Yu for the useful discussions about EXAFS fitting.", "descript": "\n                  Hydrogenation of levulinic acid to \u03b3-valerolactone is a key reaction in the valorization of carbohydrates to renewable fuels and chemicals. State-of-the-art catalysts are based on supported noble metal nanoparticle catalysts. We report the utility of a bimetallic Fe-Re supported on TiO2 for this reaction. A strong synergy was observed between Fe and Re for the hydrogenation of levulinic acid in water under mild conditions. Fe-Re/TiO2 shows superior catalytic performance compared to monometallic Fe and Re catalysts at similar metal content. The hydrogenation activity of the bimetallic catalysts increased with Re content. H2-TPR, XPS, XANES, EXAFS, M\u00f6ssbauer spectroscopy, TEM, and low-temperature CO IR spectroscopy show that the bimetallic catalysts contain metallic Re nanoparticles covered by FeOx species and small amounts of a Fe-Re alloy. Under reaction conditions, the partially reduced surface FeOx species adsorb water and form Br\u00f8nsted acidic OH groups, which are involved in dehydration of reaction intermediates. Under optimized conditions, nearly full conversion of levulinic acid with a 95 % yield of \u03b3-valerolactone could be achieved at a temperature as low as 180\u202f\u00b0C in water at a H2 pressure of 40\u202fbar.\n               "}
{"full_text": "Adsorption energyBond lengthMayer bond orderReaction EnergyActivation EnergyWith the adjusted energy structure and change in the supply-demand relationship, the production capacity of fossil fuels in the petrochemical industry is decreasing gradually, while the production capacity of chemical raw materials must be urgently strengthened. Hydrocracking technology is an important process for the petrochemical industry to convert distillates into chemical raw materials (Bezergianni et\u00a0al., 2009; Choudhary and Saraf, 1975; K\u00f6seo\u1e21;lu and Phillips, 1987; Scherzer Jg, 1996). In general, hydrocracking catalysts contain acidic zeolites as the cracking center (Ali et\u00a0al., 2002; Martens et\u00a0al., 2001; Speight, 2020; Zhang et\u00a0al., 2007), and the nitrogen contents, particularly the basic nitrogen compounds in the cracking feedstock, are strictly limited. To remove the nitrogen compounds in the feedstock, a hydrocracking pretreatment catalyst is required in the hydrocracking process (Badoga et\u00a0al., 2020; Kohli et\u00a0al., 2019; Oh et\u00a0al., 2019; Prada Silvy et\u00a0al., 2019).The prevailing commercial pretreatment catalysts are highly active Mo\u2013Ni bimetal \u03b3-alumina-supported hydrotreating catalysts. Strong acidic supports and electronegative elements can significantly improve the removal of nitrogen compounds (Hu et\u00a0al., 2019; Tung et\u00a0al., 2017; Valles et\u00a0al., 2019; Yao et\u00a0al., 2017; Tang et\u00a0al., 2017). These prompters could cause electron deficiency or bring extra protons to the Ni\u2013Mo\u2013S active nanoclusters via inductive effects or charge transfer (Prins et\u00a0al., 1997; Tominaga and Nagai, 2010). With the rapid development of computer technology and the progress of quantum chemical calculations, theoretical calculations of complex catalytic processes, such as charge distribution effects on hydrodenitrogenation, can be implemented.In this study, quinoline, which is a typical basic two-ring nitrogen compound for hydrodenitrogenation (HDN) research (Li et\u00a0al., 2012; Lu et\u00a0al., 2007), is used as the probe, and a series of model Ni\u2013Mo\u2013S with different charge distributions are used as the active sites. The key processes of quinoline HDN, including adsorption, hydrogenation saturation, and C\u2013N bond cleavage on the Ni\u2013Mo\u2013S, are calculated by quantum chemistry calculations.The neutral Ni\u2013Mo\u2013S model in this study was a hexagonal single-layer nanocluster. The stable state of the Ni\u2013Mo\u2013S active sites under the hydrogenation reaction is shown in Fig.\u00a01\n (Ding et\u00a0al., 2018a). Previous studies have shown that on the Ni(Co)\u2013Mo\u2013S or MoS2, the hydrogenolysis active centers are mainly located on the (10-10) plane, denoted as the Ni(Co)\u2013Mo edge (Ding et\u00a0al., 2017a, 2017b, 2018a,b; Sun et\u00a0al., 2004; Sylvain et\u00a0al., 2004). In this study, the issues of quinoline HDN are focused on the Ni\u2013Mo edge of the Ni\u2013Mo\u2013S active sites. Considering the symmetry of the calculation model, in the electron deficiency case, three pairs of electrons were subtracted from the Ni\u2013Mo\u2013S, and the model is denoted as E-Ni-Mo-S. For additional protons, one proton was added to each Ni\u2013Mo edge, and the model was denoted as P\u2013Ni\u2013Mo\u2013S.Calculations were performed using the DMol3 code. The calculation function is the general gradient approximation-Perdew-Burke-Ernzerhof function, and the basis set is a double numerical plus polarization basis (Chigo Anota and Cocoletzi, 2014; Delley and B. 1982). To analyze the transition state, the open shell mode was used to treat the electron spin. The symmetry in the calculation was also canceled to meet the anisotropy in the HDN process. The orbital cut off is unified to 5.0\u00a0\u00c5 for every atom. To balance the calculation speed and accuracy, the effective core potential (ECP) method was used to simplify the core electron treatment, and thermal smearing was set to 5\u00a0\u00d7\u00a010\u22124 Hartree. The self-consistent field density convergence (SCF) was set to 2\u00a0\u00d7\u00a010\u22125, and the energy tolerance for the geometry optimization and transition state was 2\u00a0\u00d7\u00a010\u22125 Hartree. The force tolerance was 4\u00a0\u00d7\u00a010\u22123 Ha/\u00c5 geometry optimization and 3\u00a0\u00d7\u00a010\u22123 Ha/\u00c5 for the transition search. The Grimme 06 correction method was used to calculate the atomic dispersion. The exchange-correlation dependent factor s\n6 was set to 1.0, and the damping coefficient was set to 20.0. The dispersion parameters for the atoms involved in this calculation can be found in Table\u00a01\n (Grimme, 2010, 2011).During the process of HDN, the adsorption of reactants on the active sites relies on the interactions between the lone or conjugated electron pairs of the reactants and the unoccupied molecular orbitals of the active sites. According to acid-base theory, E-Ni-Mo-S can protonate basic nitrogen compounds. The changes in molecular orbitals before and after the protonation of quinoline (Q), tetrahydroquinoline (THQ), and decahydroquinoline (DHQ) (THQ and DHQ are important intermediates in the hydrodenitrogenation process of quinoline (Luan et\u00a0al., 2009) and are shown in Table\u00a02\n. The highest occupied molecular orbital (HOMO) of nonprotonated basic nitrogen compounds is mainly contributed by the lone pair electrons on the nitrogen atoms. When the nitrides are protonated by E-Ni-Mo-S, the lone pair electrons of atoms combine with H+. The newly generated HOMO has barely related to the nitrogen atoms, and the orbital eigenvalue is significantly reduced. This change will weaken the binding ability between the active center and the nitrogen compounds.The effects of charge distributions on the lowest unoccupied molecular orbital (LUMO) are shown in Table\u00a03\n. On the neutral Ni\u2013Mo edge, the LUMO is attributed to the d orbital of the tetracoordinated Ni atom and the pentacoordinated Mo atom with S atoms. The LUMO eigenvalue is\u00a0\u22124.53\u00a0eV. On the E-Ni-Mo-S, the composition and morphology of the LUMO orbitals do not change much, and they still consist of unoccupied d orbitals from aligned metal atoms. However, the LUMO eigenvalue significantly decreases to\u00a0\u22128.20\u00a0eV. On the P\u2013Ni\u2013Mo edge, the H+ bonds are stably coordinated with the pentacoordinated Mo atom, which is near the exposed Ni atom. This combination will satisfy the stable hexacoordination of the Mo atom. The Ni atom close to H+ will be more electron deficient, leading to a reduction in the LUMO eigenvalue. It could be concluded that both the lack of electrons and the extra protons will lower the LUMO eigenvalue and enhance the ability of receiving electrons from the reactants.During the HDN process, the reactant, some important intermediates and the ammonia have strong adsorption ability on the active centers. The calculation results of the adsorption of Q, THQ, DHQ and NH3 on Ni\u2013Mo-edge affected by different charge distributions are shown in Table\u00a04\n. On the neutral Ni\u2013Mo\u2013S, the formation of the nitrogen compounds adsorption is point to point. Specifically, the nitrogen atom of Q, THQ and DHQ bonded with nickel atom, forming an N\u2013Ni bond with 2.2\u20132.3\u00a0\u00c5 and 0.3\u20130.4 Mayer bond order. The nitrogen atom of NH3 prefers to bond with Mo atoms. The bond direction is in accord with the orientation of the LUMO morphology listed in Table\u00a02. Because of the similarity of LUMO morphology, the adsorption morphology of the nitrogen compounds on the E-Ni-Mo-S and Ni\u2013Mo\u2013S active sites are similar as well, whereas the significant difference is the adsorption energy. The adsorption energies of nitrogen compounds on E-Ni-Mo-S are approximately 20\u201330 larger than those on the neutral Ni\u2013Mo\u2013S. When the nitrogen compounds adsorb on the P\u2013Ni\u2013Mo\u2013S active sites, the H+ will transfer to the nitrogen, combing with the long pair electrons. The adsorption of nitrogen compounds will turn to flat model without forming the N\u2013Ni bond. Despite the lacking of the single strong chemisorption bonds, the weak interaction between the conjugate \u03c0-electrons and the unoccupied orbitals the extra dispersion force from the increasing contact area both enlarge the adsorption energy. According to the calculation results, both the electron deficiency and the extra proton will enhance the adsorption of nitrogen compounds on the Ni\u2013Mo-edge, whereas the ammonia desorption is inhibited which is negative to the recovery of the active center during the HDN process.On the Ni\u2013Mo edge, hydrogen activation is carried out by H2 molecule dissociation on the metal or sulfur atom. Hydrogen dissociation with adsorption of a quinoline molecule was calculated, and the results are shown in Table\u00a05\n. On the Ni\u2013Mo edge of neutral Ni\u2013Mo\u2013S, hydrogen dissociation is a strong endothermic step with a high energy barrier. At the corresponding position of E-Ni-Mo-S, this dissociation is an obvious exothermic process, and the activation energy significantly decreases to 108.51\u00a0kJ/mol. On the P\u2013Ni\u2013Mo\u2013S, the thermal effects and activation energy charge were less significant than those on E-Ni-Mo-S. It could be predicted that electron deficiency will promote hydrogen dissociation.The newly generated active hydrogen must transfer to the nitrogen compounds quickly in the case of self-combination. Among the several hydrogen transfers of quinoline HDN, the conversion from THQ to penta-hydroquinoline (PHQ) is a key speed control step (Ding et\u00a0al., 2017; Jian and Prins, 1998). This elementary reaction on the Ni\u2013Mo edge with different charge distributions is shown in Table\u00a06\n. The active hydrogen breaks the conjugated aromatic rings. The reaction energy is up to 40\u201370\u00a0kJ/mol, and the activation energy exceeds 100\u00a0kJ/mol. In comparison, hydrogen transfer on neutral Ni\u2013Mo\u2013S is relatively easier and most difficult on E-Ni-Mo-S. The difficulty of hydrogen transfer is adverse to hydrogen dissociation, indicating that the stronger the interaction between the hydrogen and active sites, the easier the hydrogen dissociation and the harder the hydrogen transfer.For quinoline, the main pathway of C\u2013N bond cleavage is the E2 elimination of DHQ. This process contains two elementary steps: the first step is hydrogen elimination of \u03b2-C, forming nona-hydroquinoline, and the second step is cleavage of the C\u2013N bond, forming a CC bond and amino group (Li et\u00a0al., 2012). Table\u00a07\n shows the elimination of the \u03b2-H of DHQ on Ni\u2013Mo-edges with different acid types. According to the calculated results, the transfer of \u03b2-H to the active sites is an endothermic process with high activation energy. During this step, the S accepts the hydrogen atom, and the \u03b2-C atom bonds with the Mo atom. The influence of the charge distribution is limited, whereas the H+ provided by B\u2013Ni\u2013Mo\u2013S returns to the active sites, and the reaction energy and activation energy both decrease. The C\u2013N bond cleavage of NHQ is shown in Table\u00a08\n. The results show that the C\u2013N break on the neutral Ni\u2013Mo\u2013S is a strong endothermic step with very high energy barrier. Meanwhile, the C\u2013N bond cleaves the newly generated CC bonds attached with the Mo atom. The electron deficiency on Ni\u2013Mo\u2013S does not change the pathway of C\u2013N bond cleavage, and the influence is quite limited. Attributable to the stronger adsorption ability of the LUMO, the energy barrier decreased by approximately 10\u00a0kJ/mol on the E-Mo-Ni-S. Notably, on the P\u2013Ni\u2013Mo\u2013S, the proton transferred to the Ni\u2013Mo-edge in the elimination step returns back to nitrogen compounds during C\u2013N bond cleavage. The proton not only lowers the electron density but also increases the coordination of the N atom, leading to a more stable transition state of C\u2013N bond cleavage. The activation energy decreased by approximately 40\u00a0kJ/mol, indicating that flexible H+ transfer between the nitrogen compounds and the active center significantly lowered the C\u2013N bond cleavage in the HDN of quinoline.In this study, the HDN catalytic activities of Ni\u2013Mo\u2013S with different charge distributions are calculated. The conclusions are as follows:\n\n1.\nElectron deficiency and extra protons could both lower the LUMO eigenvalue of Ni\u2013Mo\u2013S. The effects of electron deficiency on the morphology are limited, whereas extra protons could change the local morphology of LUMO.\n\n\n2.\nElectron deficiency and extra protons could both enhance the adsorption ability of Ni\u2013Mo\u2013S active sties to nitrogen compounds. On neutral Ni\u2013Mo\u2013S and E-Ni-Mo-S, the nitrogen compounds adsorb via the chemisorption N\u2013Ni bond, whereas on P\u2013Ni\u2013Mo\u2013S, the nitrogen compounds take flat adsorption. However, ammonia desorption is inhibited by electron deficiency and extra protons during the HDN process.\n\n\n3.\nElectron deficiency on N\u2013Mo\u2013S promotes the generation of active hydrogen but restricts hydrogen transfer to nitrogen compounds.\n\n\n4.\nDuring C\u2013N bond cleavage, the proton of P\u2013Ni\u2013Mo\u2013S can flexibly transfer between the nitrogen compounds and the active sites. In this way, the cleavage of C\u2013N is significantly promoted.\n\n\nElectron deficiency and extra protons could both lower the LUMO eigenvalue of Ni\u2013Mo\u2013S. The effects of electron deficiency on the morphology are limited, whereas extra protons could change the local morphology of LUMO.Electron deficiency and extra protons could both enhance the adsorption ability of Ni\u2013Mo\u2013S active sties to nitrogen compounds. On neutral Ni\u2013Mo\u2013S and E-Ni-Mo-S, the nitrogen compounds adsorb via the chemisorption N\u2013Ni bond, whereas on P\u2013Ni\u2013Mo\u2013S, the nitrogen compounds take flat adsorption. However, ammonia desorption is inhibited by electron deficiency and extra protons during the HDN process.Electron deficiency on N\u2013Mo\u2013S promotes the generation of active hydrogen but restricts hydrogen transfer to nitrogen compounds.During C\u2013N bond cleavage, the proton of P\u2013Ni\u2013Mo\u2013S can flexibly transfer between the nitrogen compounds and the active sites. In this way, the cleavage of C\u2013N is significantly promoted.The authors acknowledge the financial support from the Sinopec Science and Technology Department (Grant No. 121014-1).", "descript": "\n                  The charge distribution on Ni\u2013Mo\u2013S active sites can affect hydrodenitrogenation (HDN) activity. In this study, a series of model Ni\u2013Mo\u2013S were developed with various charge distributions. For comparison, the charge distribution effects on quinoline HDN were studied. The results show that a lack of electrons and extra protons can both lower the orbital eigenvalue of the Ni\u2013Mo\u2013S, leading to stronger adsorption of nitrogen-containing compounds and inhibition of ammonia desorption. Electron deficiency will improve the generation of active hydrogen on the active sites but inhibit hydrogen transfer to the nitrogen compounds; extra protons can provide H+ to the nitrogen compounds, which will flexibly transfer between the nitrogen compound and active sites, thus improving the cleavage of the C\u2013N bond.\n               "}
{"full_text": "Lignin, the most abundant renewable aromatic material on Earth, can be potentially exploited for the sustainable supply of fuels and chemicals which are currently derived from rapidly depleting and greenhouse gas emitting fossil resources [1\u20135]. Lignin, constituting 15\u201330 % of the biomass weight and up to 40 % of the biomass energy, is an amorphous and highly cross-linked macromolecule composed of the three primary phenylpropane monomers of p-coumaryl, coniferyl and sinapyl alcohols [6\u201310]. It can be processed via various depolymerization techniques for the production of high-value platform chemicals such as phenolics, aromatics and alkanes [11]. Today, industries such as pulp and paper manufacturing and lignocellulosics-to-ethanol processes produce large amounts of lignin as a by-product which is mostly burnt for use as an internal energy input [12\u201316]. Therefore, development of efficient processes for feasible utilization of lignin is highly important both in terms of environmental and economic aspects. In recent years, different thermochemical approaches (e.g., liquefaction and pyrolysis) have been applied for processing of lignin materials under reductive, neutral and oxidative atmospheres to produce value-added products (e.g., aromatic and cycloalkane hydrocarbons and phenolic compounds) [17,18]. However, there is a major problem in the processing of lignin, which is the high formation of char solid residues remaining from the conversion of lignin, causing a low yield of desired target products [19\u201324]. This happens since lignin fragments from degradation of lignin polymer are highly reactive and undergo rapid repolymerization to form large amounts of char [25]. This necessitates the development of the catalytic systems which can effectively suppress char-forming condensation reactions.In the reductive approaches, aiming to develop lignin-to-hydrocarbon processes, the most commonly tested catalysts can be divided into three groups: (i) transition (e.g., Ni, Cu)/noble (e.g., Pd, Pt, Ru) metal-based catalysts used in metallic form; (ii) metal oxide catalysts (e.g., MoOx, ReOx); (iii) conventional sulfide catalysts (e.g., NiMo/Al2O3, CoMo/Al2O3) currently being applied in refineries for hydrotreating purposes [19,26\u201336]. Meanwhile, different metal phosphide (e.g., MoP, Ni2P), nitride (e.g., Mo2N) and carbide (e.g., Mo2C) catalysts have also been widely tested for the hydrodeoxygenation (HDO) of lignin model compounds [3]. The problem with the first two groups is that, although they mostly have high hydrogenation efficiency, their application is limited to sulfur-free lignin materials since they can be readily poisoned and deactivated in the presence of sulfur. The conventional sulfide catalysts have also been reported to give a high char yield from the conversion of lignin. Agarwal et al. [31] reported a char yield of 23.3 wt% produced in the liquefaction of kraft pine lignin at 450 \u00b0C and 100 bar H2 using CoMo/Al2O3 as a conventional hydrotreatment sulfide catalyst. In another work performed by the same group [32] for hydrotreatment of kraft lignin at 350 \u00b0C and 100 bar H2, the solid residue yields of 20.5 and 35.4 wt% were obtained over conventional sulfide catalysts of NiMo/Al2O3 and CoMo/Al2O3, respectively. Considering that the majority of commercially available lignins have sulfur content (sulfite and kraft lignins with 3.5\u20138.0 % and 1.0\u20133.0 % sulfur, respectively), development of novel sulfur-resistant catalysts with high HDO efficiency can be an important strategy for the future supply of fuels and chemicals from lignin feedstocks [2,37]. Sulfur-resistant catalysts could also be applied for co-processing of lignin materials with other sulfur-containing feedstocks. This is particularly important for the feasibility of the integration of lignin processing with the existing petroleum refinery units with sulfur-containing input streams to improve the cost-effectiveness of lignin valorization.To meet the above-mentioned challenges, and as a step towards an applicable and efficient lignin-to-hydrocarbon process, this work aimed to develop a catalyst with three major properties: (i) sulfur resistance; (ii) high char-suppressing potential; (iii) high HDO efficiency. In this study, rhenium sulfide was tested as a catalyst for HDO of m-cresol (as a model lignin-derived phenolic compound) and reductive liquefaction of kraft lignin, and its performance was compared with that of nickel-molybdenum sulfide which is a well-established conventional sulfide catalyst. To the best of our knowledge, this is the first use of rhenium sulfide for the conversion of a lignin feedstock. Rhenium sulfide has been reported in literature to be an active catalyst for hydrodesulfurization and hydrodenitrogenation reactions [38], and metallic rhenium is known as a catalyst with high hydrogenation efficiency [39]. Recently, hydrodeoxygenation of some phenolic compounds have also been performed using different rhenium phases (metal, oxide and sulfide) [40\u201342]. Moreover, rhenium has a considerably lower price compared to noble metals like Pd, Pt, Ru, Rh and Ir, and it also may have a lower price in the future with the enhanced demand for rhenium compounds and its increased commercial exploitation. Therefore, rhenium sulfide was selected to study its HDO activity and catalytic performance in the conversion of lignin. \u03b3-Alumina, zirconia and desilicated HY zeolite were used as support materials for the rhenium sulfide catalysts in this work. An alkali-assisted depolymerization was also carried out to achieve an enhanced lignin depolymerization, and to study the correlation between depolymerization rate and stabilization rate as a key factor for suppressing char formation in a lignin liquefaction process.NiMo/Al2O3, Re/Al2O3, Re/ZrO2 and Re/HY were examined as catalysts in this work. HY, used as catalyst support, was a mesoporous zeolite obtained by desilication of a commercial Y zeolite (Zeolyst, CBV 780, SiO2/Al2O3 molar ratio: 80) through alkaline treatment in a 0.3 M NaOH solution with mild stirring at 80 \u00b0C for 60 min. Then, the sample was filtered, washed with distilled water, and dried at 110 \u00b0C overnight. Subsequently, the desilicated zeolite was converted to the protonic form by three successive ion exchanges with a 1 M aqueous NH4Cl solution at 80 \u00b0C for 4 h, followed by drying at 110 \u00b0C overnight and calcination at 550 \u00b0C for 12 h with a heating ramp of 2 \u00b0C min\u22121. As a result of desilication, the SiO2/Al2O3 molar ratio of HY zeolite was decreased from 80 to 36. Supported rhenium catalysts were obtained by incipient wetness impregnation of \u03b3-Al2O3 (Puralox SCCa 150/200, Sasol), ZrO2 (with monoclinic crystalline structure, SZ 31164, NORPRO) and HY with an aqueous solution of NH4ReO4 (Sigma-Aldrich). The NiMo/Al2O3 catalyst was prepared through incipient wetness co-impregnation of \u03b3-Al2O3 with an aqueous solution containing both (NH4)6Mo7O24\u00b74H2O and Ni(NO3)2\u00b76H2O (Sigma-Aldrich). The amount of rhenium loaded on all the supports was approximately 3 wt% (2.8, 2.8 and 2.7 wt% on Al2O3, ZrO2 and HY supports, respectively), and the loading amounts of nickel and molybdenum metals on alumina support were 4.8 and 14.6 wt%, respectively (determined by quantitative XRF). At these metal loading amounts, both rhenium and nickel-molybdenum catalysts gave similar conversions for the HDO of m-cresol (based on initial experiments). Therefore, these metal loading amounts were selected to study the performance of the catalysts in the liquefaction of lignin. After impregnation, the catalysts were dried first at 60 \u00b0C (12 h) and then at 110 \u00b0C (12 h), with subsequent calcination at 550 \u00b0C for 12 h. Prior to reaction, the prepared catalyst was sulfided with dimethyl disulfide (DMDS, \u2265 99 %, Sigma-Aldrich) in the presence of 20 bar hydrogen (99.9 %, AGA) at 340 \u00b0C for 4 h in a Parr autoclave reactor.The crystalline structure of the catalysts was determined using X-ray diffraction (XRD) on a Bruker AXSD8 Advance X-ray powder diffractometer with Cu K\u03b1 radiation (\u03bb = 1.542 \u00c5). The chemical analysis of catalysts was carried out using an X-ray fluorescence (XRF) instrument (PANalytical Epsilon 3XL). The textural properties of the samples were determined by nitrogen isothermal (\u2212196 \u00b0C) adsorption-desorption using a TriStar 3000 instrument. Transmission electron microscopy (TEM) images were acquired with a high angle annular dark field (HAADF) detector using a FEI Titan 80\u2013300 operating at the accelerating voltage of 300 kV. The electronic states of the supported metals were determined by X-ray photoelectron spectroscopy (XPS) measurement using a PerkinElmer PHI 5000 VersaProbe III Scanning XPS Microprobe.The acidity of the catalyst samples was measured by temperature programmed desorption of ammonia (NH3-TPD) and ethylamine (ethylamine-TPD) using an experimental setup consisting of mass flow controllers (MFC, Bronkhorst) for gas mixing, a quartz tube containing the sample in a temperature-controlled furnace and a mass spectrometer (MS, Hiden HPR-20 QUI) for measuring the amount of ammonia or ethylene in the outlet stream. Before ammonia or ethylamine adsorption, the presulfided catalyst was pretreated in Argon at 100 \u00b0C for 30 min. Then, it was exposed to 1555 ppm of NH3 or 543 ppm of ethylamine at 100 \u00b0C for 2 h. Afterwards, the sample was flushed with argon for 30 min to eliminate physisorbed ammonia/ethylamine. The desorption measurement was performed by heating the sample to 800 \u00b0C with a ramp of 10 \u00b0C min\u22121 under an argon flow (20 ml min\u22121). Thermogravimetric analysis (TGA) of the samples showed that no thermal decomposition occurs up to 800 \u00b0C (shown in Fig. S1, Supplementary Information). The samples were heated from 35 to 800 \u00b0C with a heating ramp of 10 \u00b0C min\u22121 in a stream of nitrogen gas (30 ml min\u22121).The hydrodeoxygenation (HDO) of m-cresol (\u2265 99 %, Sigma-Aldrich) and reductive liquefaction of kraft lignin (product number: 370959, Sigma-Aldrich) (with 2.1 wt% sulfur content measured by ICP-AES and ICP-SMS) were conducted in a 300 ml Parr autoclave reactor. The elemental and proximate compositions of the kraft lignin sample are presented in Table S1, Supplementary Information. In each experiment, 3 g reactant, 90 ml hexadecane solvent (\u2265 99 %, Sigma-Aldrich) and a certain amount of presulfided catalyst were added to the reactor. The lowest catalyst-to-feed ratio applied in this work was 1:3 (at least 1 g solid catalyst) in order to minimize mass transfer limitations for a better comparison of the catalytic performance of the different catalysts. The loaded reactor was sealed, and the air inside it was evacuated by pressurizing/depressurizing the reactor three times with first nitrogen and then hydrogen gas. Afterward, the reactor was pressurized with 30 bar of hydrogen gas and then heated up to the reaction temperature (340 or 400 \u00b0C). The reactor pressure was 56\u201357 and 65\u201368 bar at the reaction temperatures of 340 and 400 \u00b0C, respectively. The reactions were carried out with a stirring rate of 1000 rpm for a duration of 3 h for HDO of m-cresol and 6 h for lignin conversion. In the experiments for HDO of m-cresol, 88 mg DMDS was added to maintain the sulfidation of catalysts during the reaction, and in some experiments for lignin conversion, NaOH (\u2265 98 %, Sigma-Aldrich) was added for enhanced depolymerization of lignin via alkali-catalyzed degradation. The liquid composition in HDO of m-cresol was monitored by collecting samples at intervals of 30 min. When the reaction was complete, the reactor was immediately quenched to room temperature and the solid phase was separated from the liquid product by vacuum filtration. The solid residue remaining from lignin conversion reactions was washed with acetone to remove the organics and solvent absorbed on the solid particles. After acetone extraction, the solid fraction (catalyst, char residues and unconverted lignin) was dried at 110 \u00b0C and weighed. Subsequently, the solid fraction was washed with dimethyl sulfoxide (DMSO, \u2265 99.9 %, Sigma-Aldrich) to dissolve and remove unconverted lignin. Then, the solids were washed with acetone to remove DMSO and dried at 110 \u00b0C overnight. The difference in the weight of solids before and after DMSO extraction was assigned to the amount of unconverted lignin which was almost negligible (below 2 wt% on feed) in all the experiments. The liquid phase products were analyzed by a two-dimensional gas chromatography system (GC \u00d7 GC, Agilent 7890\u22125977A). The products were separated by two columns with different polarity (a DB-5 ms column (30 m \u00d7 0.25 mm \u00d7 0.25 mm) for the first dimension and a BPX-50 column (2.5 m \u00d7 0.10 mm \u00d7 0.10 mm) for the second dimension), and detected by mass spectrometer (MSD) and flame ionization (FID) detectors for qualitative and quantitative analysis, respectively. The product yields were measured by an external standard calibration method, with calibration curves using several known concentrations (mass) that were related to the FID peak areas (with R2 > 0.99). This calibration was conducted for a number of individual compounds such as toluene, ethylbenzene, propylbenzene, methylcyclohexane, cyclohexane, guaiacol, m-cresol, phenol, propylphenol, naphthalene, methylnaphthalene, tetralin, dimethyltetralin, methylbiphenyl, benzyl phenyl ether, biphenol and phenanthrene. Experiments were repeated 2\u20133 times to ensure the reproducibility of the data.\nFig. 1\n presents m-cresol conversion levels and product selectivities with time over sulfided Re/Al2O3 and NiMo/Al2O3 catalysts at 340 \u00b0C. Both catalysts exhibited a similar trend for the conversion of m-cresol, giving a complete conversion after 2.5 h reaction. Considering that the number of rhenium atoms loaded on alumina support is almost ten times less than that of molybdenum atoms (the loading amounts of Re and Mo were approximately 3 and 15 wt%, respectively), it could be inferred that rhenium sulfide is more active than molybdenum sulfide as a hydrogenation promoter. After 3 h reaction, the mass yields of methylcyclohexane, methylcyclohexene, ethylcyclopentane and toluene were 63.9, 0.0, 6.2 and 10.9 wt% over NiMo/Al2O3, and 75.1, 0.6, 8.2 and 5.2 wt% over Re/Al2O3, respectively (shown in Fig. 2\n).As can be seen from the products obtained by the conversion of m-cresol, the HDO reaction proceeds through both ring hydrogenation (HYD) and direct deoxygenation (DDO) pathways over both NiMo/Al2O3 and Re/Al2O3 catalysts. In the DDO mechanism, m-cresol is adsorbed through its oxygen atom on the catalyst active site which is a sulfur vacancy, and the double bond on the phenolic ring close to the Caromatic-OH bond is hydrogenated to a single bond. This results in a temporary removal of the electron delocalization effect of the out-of-plane lone pair electron orbital of oxygen onto the phenolic ring \u03c0 bond orbital and, in turn, a weaker CO bond which can be easily cleaved by dehydration over an adjacent acid site, giving toluene as the final aromatic hydrocarbon product [43\u201345]. In the ring hydrogenation pathway, the co-planar adsorption of m-cresol on the catalyst surface leads to ring saturation, producing methylcyclohexanol as an intermediate. This is then followed by dehydration to form methylcyclohexene which undergoes subsequent hydrogenation to be converted into methylcyclohexane as the saturated cyclic hydrocarbon product [43,46]. Ring hydrogenation was the dominant HDO pathway over both catalysts, giving high methylcyclohexane-to-toluene molar ratios of 5.5 and 13.6 over NiMo/Al2O3 and Re/Al2O3, respectively. The higher methylcyclohexane-to-toluene ratio over Re/Al2O3 indicates higher hydrogenation activity of this catalyst. In a study for hydrodeoxygenation of 2-ethylphenol over sulfided Mo-based catalysts, it was suggested that ring hydrogenation via co-planar adsorption requires two neighboring sulfur vacancies as active site, while DDO mechanism occurs on a single sulfur vacancy [47]. It is also inferred from the product selectivities over time that toluene, produced via DDO mechanism, does not undergo ring hydrogenation and remains unchanged by the end of the reaction. Another difference between the two examined catalysts is the rate of the hydrogenation of methylcyclohexene which is lower over Re/Al2O3. Methylcyclohexene was almost undetectable during the reaction using NiMo/Al2O3, indicating that this intermediate only exists for a short time before it is rapidly hydrogenated to methylcyclohexane. In contrast, methylcyclohexene was observed in relatively high quantities in the first 2 h of the reaction over Re/Al2O3. This might be ascribed to the slower adsorption of this intermediate on hydrogenation active sites on the surface of Re/Al2O3, more likely due to the higher number of phenolic rings hydrogenated over this catalyst. Ethylcyclopentane was also produced in low yield over both catalysts via acid-catalyzed ring contraction of methylcyclohexane [48].The yields of monocyclic products and char residues obtained by the conversion of kraft lignin over different catalysts are shown in Table 1\n. Lignin was almost fully converted (> 98 %) in all the experiments, and other products (not shown in Table 1) are mainly heavy oligomers (non-detectable by GC), tetralins, indenes, naphthalenes, water and gas products. A comparison of the monocyclic product yields of Re/Al2O3 and NiMo/Al2O3 at the reaction temperature of 340 \u00b0C (entries 2 and 3, Table 1) reveals a remarkable superiority of rhenium sulfide catalyst; the total monocyclic product yields achieved over Re/Al2O3 and NiMo/Al2O3 were 21.5 and 4.6 wt%, respectively. This is mainly due to the different amounts of char remaining from the conversion of lignin over these two catalysts, with the yields of 40.6 wt% over NiMo/Al2O3 and 11.2 wt% over Re/Al2O3 (the images of char residues remaining from lignin conversion are shown in Fig. S2, Supplementary Information). This significant difference clearly illustrates the high catalytic efficiency of Re/Al2O3 for suppressing char formation which is a major problem in thermochemical processes for conversion of lignin. The typical high char yields from lignin conversion is a result of the low stability and high reactivity of lignin-derived intermediates which undergo condensation reactions to form heavy compounds as solid char residues [49]. Radical coupling, quinone methide and vinyl condensation are some significant condensation mechanisms which lead to high amounts of char remaining from lignin conversion [25]. Low char yield obtained over alumina-supported rhenium sulfide reveals that this catalyst is highly effective for stabilizing the lignin-derived reactive compounds and, in turn, suppressing condensation reactions. This could be due to higher activity of the rhenium sulfide catalyst for hydrogenating free radicals and preventing radical coupling in a reducing atmosphere. The char-suppressing effect of hydrogenation could also be observed by a comparison of the char yield of the alumina-supported hydrogenation promoters (rhenium and nickel-molybdenum sulfides) with that of the pure alumina support. As shown in Table 1, entries 1\u20133, the highest char yield (48.2 wt%) was obtained using the pure alumina support, indicating a higher condensation rate in the absence of hydrogenation active sites. Meanwhile, almost no monocyclic hydrocarbons were produced over Al2O3 due to the absence of hydrogenation activity.The high efficiency of the rhenium sulfide catalyst should be recognized in light of the fact that the low char yield of 11.2 wt% was obtained in this case using hexadecane as solvent, which is not a good solubilizer of lignin-derived components. Therefore, rhenium sulfide could be effectively used in the absence of the oxygen-containing polar solvents (e.g., alcohols) which are typically used for lignin liquefaction due to their higher solubility for lignin fragments. This makes rhenium sulfide a potential catalyst to be used for co-processing of lignin with hydrocarbon feedstocks in conventional petroleum refinery units. Moreover, considering the resistance of this metal sulfide catalyst to sulfur poisoning, it can be effectively used in hydrotreating of sulfur-containing lignin feedstocks. In addition, although rhenium is more expensive than molybdenum, but it has a lower price compared to the other noble metals with high hydrogenation activity (e.g., Pt, Ru, Ir and Rh). It could also be noticed that the low loading of rhenium on the catalyst support (like 3 wt% in this work compared to the typically high loading amounts of molybdenum (12\u201315 wt%) in commercial molybdenum-based catalysts) can increase the cost-effectiveness of rhenium-based catalysts. They could also be more economically attractive in the future with the increased commercial exploitation of rhenium due to enhanced demand for rhenium compounds.One characteristic of rhenium which makes it a highly active metal for catalyzing deoxygenation reactions is its high oxophilicity [39]. The oxophilic rhenium species are well known to be efficient for activation of oxy-compounds by strong adsorption of oxygen-containing functional groups to the surface of catalyst. Hence, this facilitated adsorption and strengthened interaction could be a reason for the effective performance of rhenium sulfide for lignin degradation (through cleavage of ether linkages) and HDO of lignin-derived phenolic compounds.The high catalytic activity of rhenium sulfide can also be correlated to the low binding energy shift between the rhenium sulfide phase and rhenium metal. As depicted by XPS analysis, presented in Fig. 3\n and Table 2\n, the binding energies of the Re 4f7/2 component of the 4f doublet for the ReOx/Al2O3 catalyst are 44.03 and 46.15 eV which are assigned to Re6+ (ReO3) and Re7+ (Re2O7), respectively [50,51]. After sulfidation, this catalyst displayed two Re 4f7/2 contributions with binding energies of 41.36 and 42.57 eV which are attributed to ReS2 species and some oxysulfide species (S-Re-O), respectively [52,53]. As the relative proportion values show, rhenium sulfide on alumina support exists mainly (87 %) as ReS2 with the binding energy close to that of rhenium metal (40.4\u201340.7 eV) [54,55]. The low binding energy difference between the rhenium sulfide phase and rhenium metal indicates that a high degree of the characteristic of metal is preserved during sulfidation, giving a metal-like nature to the metal-sulfur valence molecular orbitals. As metallic rhenium is believed to be highly effective for activation of H2 molecules [39], this metal-like character of rhenium sulfide species leads to a facilitated uptake of hydrogen and a high rate of the dissociation of molecular H2, giving an enhanced hydrogenation efficiency. The low char yields obtained from the liquefaction of lignin in the presence of rhenium-based catalyst could be associated to the high hydrogenation efficiency of rhenium sulfide; the highly reactive lignin derivatives can be stabilized via rapid hydrogenation, and thus, undesired coupling reactions and repolymerization to char residues can be effectively inhibited. The XPS analysis of the spent alumina-supported rhenium sulfide catalyst used for the HDO of m-cresol shows that the sulfide state of rhenium was maintained during the HDO reaction (presented in Fig. S3). The binding energy of ReS2 species on the spent catalyst was similar to that of the fresh catalyst, while the Re 4f7/2 contribution attributed to oxysulfide species disappeared, indicating the complete sulfidation of these species during the HDO reaction.Rhenium-induced acidity can also play an important role in catalytic performance of rhenium species by improving acid-catalyzed reactions. As revealed by NH3-TPD data presented in Fig. 4\n and Table 2, the catalyst acidity was increased by the addition of rhenium species; the total acidity of Al2O3 and ReS2/Al2O3 are 0.364 and 0.451 mmol g\u22121, respectively. The amount of acidity induced by rhenium sulfide species should be higher than the difference in acid amounts of Al2O3 and ReS2/Al2O3, since the impregnated metal species cover a portion of the acid sites of the support, and the amount of ammonia desorption (in TPD analysis) from support in ReS2/Al2O3 is less than that from alumina support alone. It is also noticeable that, based on the acid strength distribution of these two catalysts, rhenium-induced acidity is mostly of medium strength; the densities of weak, medium and strong acid sites are 0.124, 0.145 and 0.095 mmol g\u22121 in Al2O3, and 0.129, 0.204 and 0.118 mmol g\u22121 in ReS2/Al2O3, respectively. This increased acidity can particularly improve the cleavage of ether linkages of lignin, causing an enhanced rate of depolymerization [56]. As a result, lignin fragments can be converted into monomeric compounds before they undergo repolymerization to form heavy solid residues. Moreover, the acidity provided by rhenium species can also improve the dehydration step of HDO reaction, leading to an enhanced deoxygenation and increased hydrocarbon yield [45,57]. According to ethylamine-TPD analysis, the rhenium-induced acidity is mainly Lewis acidity. In ethylamine-TPD, ethylamine is adsorbed on Br\u00f8nsted acid sites, and the ethylammonium ions (formed via proton transfer) undergo the Hofmann elimination reaction to produce ethylene and ammonia at higher temperatures [58,59]. Therefore, the ethylene detected during ethylamine-TPD is quantified to measure Br\u00f8nsted acidity. Based on the ethylene desorption profile (shown in Fig. S4), the density of Br\u00f8nsted acid sites of ReS2/Al2O3 is 0.022 mmol g\u22121 which constitutes 5% of the total acidity (0.451 mmol g\u22121, measured by NH3-TPD) of this catalyst, indicating that the catalyst acidity is mainly Lewis type.The reaction pathway and product distribution in a lignin liquefaction process is a strong function of reaction temperature mainly due to the temperature dependence resulting from varying activation energies for the different series and parallel reactions taking place during lignin conversion. The significance of reaction temperature is more realized when it is considered that it greatly affects the rate of repolymerization reactions of lignin derivatives which lead to undesired formation of solid char residues. At low reaction temperatures (usually below 300 \u00b0C), low lignin depolymerization occurs due to low thermal cracking and inefficient catalytic degradation (e.g., hydrogenolysis), and instead, the repolymerization of highly reactive lignin-derived compounds yields a high char formation since these compounds cannot be catalytically stabilized at low temperatures [22]. Similarly, at high reaction temperatures (usually above 400 \u00b0C), significant char-forming reactions happen as a result of severe carbonization [21,22]. Therefore, the applied reaction temperature should be high enough to provide the activation energies required for both depolymerization of lignin and stabilization of reactive lignin derivatives (via e.g. hydrogenation and alkylation) on one hand, and not too high in order to cause carbonization reactions on the other hand. As mentioned before, Al2O3-supported rhenium sulfide could efficiently suppress char-forming reactions at 340 \u00b0C. In order to examine the stabilizing efficiency of this catalyst at an elevated temperature, the reaction temperature was increased to 400 \u00b0C while keeping other parameters constant. This caused a reduction in char yield from 11.2 to 8.5 wt% (entries 3 and 4, Table 1), indicating that Re/Al2O3 catalyst could more effectively stabilize lignin fragments at the higher temperature of 400 \u00b0C through an enhanced hydrogenation of free radicals. Importantly, this temperature increase caused a remarkably improved deoxygenation efficiency more likely due to the enhanced dehydration activity of the Re/Al2O3 catalyst at elevated temperature; the monocyclic hydrocarbon yield was increased from 7.3 to 16.8 wt%, and the monocyclic phenolic yield was decreased from 14.2 to 0.7 wt% by an increase of temperature from 340 to 400 \u00b0C (entries 3 and 4, Table 1). Consequently, the enhanced oxygen removal at 400 \u00b0C resulted in a lower monocyclic product yield of 17.5 wt% which was slightly higher (21.5 wt%) at 340 \u00b0C. Moreover, HDO reaction selectivity was also affected by the increase of temperature, and the higher temperature of 400 \u00b0C favored direct deoxygenation over ring hydrogenation; the monocyclic aromatic hydrocarbon selectivity was increased from 38.7 to 48.3 mol% by increasing temperature from 340 to 400 \u00b0C (shown in Fig. 5\n). This could be caused by the decreased availability of hydrogen on the surface of the catalyst at elevated temperature as a result of the reduced hydrogen adsorption due to its exothermic nature [60]. Lower hydrogen availability is favorable for the DDO mechanism which requires less hydrogen consumption compared to the HYD reaction route. This is consistent with several previous studies reporting that HYD and DDO are the dominant reaction pathways taking place at low and high temperatures, respectively [61,62].As can be seen from the yields and selectivities of monocyclic products obtained over Re/Al2O3, Re/ZrO2 and Re/HY, presented in Table 1 (entries 4\u20136), catalyst support has a significant effect on catalytic performance and reaction pathway. For a better comparison of the catalytic activities of the supports, HY zeolite was desilicated to generate a mesoporous zeolitic structure with lower diffusion limitations of lignin fragments. The textural properties of the supports are shown in Table S2. HDO activity was remarkably affected by the choice of catalyst support, and monocyclic hydrocarbon yield was reduced in the order: Re/Al2O3 > Re/ZrO2 > Re/HY. The monocyclic hydrocarbon yields were 16.8, 11.2 and 8.5 wt%, and monocyclic phenolic yields were 0.7, 10.6 and 14.9 wt% over Re/Al2O3, Re/ZrO2 and Re/HY catalysts, respectively, indicating that the use of alumina as catalyst support led to the highest deoxygenation efficiency. As a result of the enhanced oxygen removal over Re/Al2O3, this catalyst gave a lower mass yield of total monocyclic compounds compared to Re/ZrO2 and Re/HY catalysts. It can be seen from the data shown in Fig. 5 that HDO reaction selectivity was not largely influenced by catalyst support, and the monocyclic aromatic hydrocarbon selectivity was similar (44.7\u201348.3 mol%) over Al2O3-, ZrO2- and HY-supported rhenium catalysts. Similar to the HDO activity trend of the catalysts, the stabilizing efficiency was also decreased in the order: Re/Al2O3 > Re/ZrO2 > Re/HY, giving the char yields of 8.5, 10.7 and 11.8 wt%, respectively.To study the effect of the addition of a zeolitic catalyst to Re/Al2O3, a combination of Re/Al2O3 (1 g) and HY (1 g) was used as the catalytic system for the conversion of kraft lignin at 400 \u00b0C. The addition of zeolite had a negative effect and resulted in a reduction of monocyclic product yield from 17.5 to 13.8 wt% (entries 4 and 7, Table 1). This could be due to the condensation of lignin-derived fragments catalyzed by zeolite acid sites in the absence of a hydrogenation promoter, converting both hydrocarbons and oxygenates into catalytic coke deposited inside the zeolite channels [63].TEM images, shown in Fig. 6\n, illustrate that rhenium species are well dispersed on all the supports as spherical particles. This is in agreement with previous studies reporting a high dispersion of rhenium particles on different catalyst supports [38,45,64]. The spherical structure of rhenium sulfide species on a \u03b3-alumina support was also observed in a study by Quartararo et al. [65]. However, Escalona et al. [38] reported the presence of rhenium sulfide as both layered crystallites and spherical particles (with different ratios) on a \u03b3-alumina support, suggesting that the ratio between the two structures depends on the sulfiding condition. The same group later showed that ReS2 species supported on different materials had layered structure which was transformed to spherical structure (with lower S/Re ratio) when exposed to the electron beam for 15 min, as a result of the desulfurization to metallic Re under the beam [52]. However, this structural change under electron beam was not observed in our work, and the rhenium sulfide species appeared as spherical particles from the beginning of the exposure to the beam. The histograms of size distribution, presented in Fig. 6, indicate that the mean particle diameter of rhenium sulfide species is 1.1, 1.4 and 0.9 nm on Al2O3, HY and ZrO2 supports, respectively. Besides, the two main lattice fringes in the TEM image of ReS2/Al2O3 have interplanar spacings of about 0.20 and 0.14 nm, which correspond to the (400) and (440) reflections of the \u03b3-alumina phase, respectively (shown in Figs. S5 and S6a). The difference in the catalytic performance of the supports could be correlated with their different acidic properties measured by NH3-TPD analysis. The data presented in Table 2 and Fig. 4 illustrate that the acid site density of the catalysts decreased in the order: Re/Al2O3 (0.451 mmol g\u22121) > Re/ZrO2 (0.236 mmol g\u22121) > Re/ HY (0.214 mmol g\u22121). The higher acidity of Re/Al2O3 promotes the dehydration step of the HDO reaction to remove the oxygen atom of lignin-derived phenolics as water. Hence, it is supposed that the promoted dehydration, which is an acid-catalyzed reaction, leads to the high hydrocarbon yield achieved over the Re/Al2O3 catalyst. Moreover, as revealed by XPS data, shown in Fig. 3 and Table 2, catalyst support also affects the chemical state of the supported rhenium sulfide species, which this, in turn, can influence the reaction rate and pathway. The binding energy of ReS2 species loaded on different supports decreased in the order: ReS2/ZrO2 (41.94 eV) > ReS2/HY (41.62 eV) > ReS2/Al2O3 (41.36 eV). Therefore, rhenium sulfide loaded on alumina support has the minimum binding energy difference with that of metallic rhenium, and in turn, exhibits the most metal-like behavior. Besides, based on the relative proportion values of Re 4f7/2 components, the alumina support leads to the highest degree of sulfidation of rhenium oxide species (87 %) followed by zirconia (84 %) and HY zeolite (75 %) supports. Furthermore, the atomic ratios of S/Re on the different supports reduced in the order: ReS2/ZrO2 (3.1) > ReS2/HY (2.4) > ReS2/Al2O3 (2.2), indicating that rhenium loaded on alumina support has the minimum attached sulfur ligands, resulting in the most sulfur-deficient sulfide phase and, in turn, the highest number of sulfur vacancies which can contribute to hydrogenation/hydrogenolysis reactions.Base-catalyzed cleavage of lignin linkages, particularly aryl-alkyl ether bonds, is a well-studied approach for lignin depolymerization. A wide variety of low-cost and commercially available catalytic reagents such as NaOH, LiOH and KOH have been applied for alkaline degradation of lignin [3]. As an example for the mechanism of base-catalyzed degradation of lignin, the cleavage of \u03b2-O-4 linkages, as the most dominant aryl-alkyl ether bonds in lignin structure, occurs by the polarization promoted by a base catalyst [66,67]. Using NaOH as base catalyst, it is proposed that the sodium cation polarizes the \u03b2-O-4 ether linkage via formation of a cation adduct with lignin. As a result, the oxygen atom of the ether bond gains an increased negative partial charge, and in turn, less energy is required for heterolytic cleavage of this bond. This leads to the formation of a sodium phenolate along with a carbenium ion transition state which is subsequently neutralized by the hydroxide ion.In this study, NaOH was added to the reaction system for an increased depolymerization degree and a higher yield of monomeric products. However, the ratio of the amount of Re/Al2O3 to the amount of added NaOH was critical for the fate of the converted lignin as either the desired target monomeric products or undesired solid char residues. The effect of this ratio is shown in Table 1 (entries 8\u201310). The addition of 1 g NaOH resulted in a large amount of char formation from lignin conversion (44.3 wt%), giving a monocyclic product yield of 16.3 wt% and monocyclic hydrocarbon yield of 11.9 wt% which were less than those obtained in the absence of NaOH (17.5 and 16.8 wt%, respectively). This is due to the high amount of additional NaOH which resulted in a high rate of base-catalyzed degradation and, in turn, a large number of lignin fragments produced in a short period of time at the beginning of the reaction. The amount of Re/Al2O3 catalyst (1 g) in the reaction medium was insufficient in order to stabilize the rapidly derived intermediates, so that they underwent thermal condensation to repolymerize into char residues. The high rate of condensation at the beginning of the reaction was confirmed by carrying out another experiment with similar amounts of Re/Al2O3 (1 g) and NaOH (1 g) for a total reaction time of 20 min; the yield of char generated in the first 20 min was 40.9 wt% which is 92 % of the char produced in the 6 h reaction experiment. As expected, an increase of the Re/Al2O3-to-NaOH ratio, by using 2 g Re/Al2O3 and 1 g NaOH led to a lower char yield of 23.1 wt% and, in turn, an increased monocyclic product yield of 20.9 wt%. This was further improved by a higher Re/Al2O3-to-NaOH ratio, and the combination of 2 g Re/Al2O3 and 0.5 g NaOH exhibited an even better catalytic performance, giving a high monocyclic product yield of 24.6 wt% (monocyclic saturated and aromatic hydrocarbon yields of 13.5 and 11.1 wt%, respectively) and a low char yield of 11.3 wt%. To examine whether this improvement was caused by a well-optimized Re/Al2O3-to-NaOH ratio or the use of an increased amount of Re/Al2O3, another experiment was performed using 2 g Re/Al2O3 with no addition of NaOH. This resulted in a monocyclic product yield of 18.6 wt% which was slightly higher than that obtained over 1 g Re/Al2O3 (17.5 wt%) and considerably lower than that of a catalytic system including 2 g Re/Al2O3 and 0.5 g NaOH (24.6 wt%), indicating that the optimum Re/Al2O3-to-NaOH ratio was the main reason to achieve a high monomer production. However, the char formation increased from 5.4 to 11.3 wt% when adding 0.5 g NaOH. The significance of the stabilizing effect of Re/Al2O3 could also be realized by the high char yield of 47.0 wt% obtained in a NaOH-catalyzed depolymerization of lignin in the absence of Re/Al2O3 (entry 11, Table 1).A comparison of the yields of monocyclic products and char residues obtained using NaOH, Re/Al2O3 and the combination of NaOH and Re/Al2O3 illustrates that the highest lignin conversion efficiency could be achieved in the presence of both NaOH and Re/Al2O3 (at optimum amounts). NaOH results in a high depolymerization of lignin to the intermediates which undergo condensation in the absence of Re/Al2O3, leading to a low yield of monocyclic products (4.2 wt%) (entry 11, Table 1). A higher monocyclic product yield of 18.6 wt% was obtained over Re/Al2O3 since this catalyst could effectively stabilize lignin fragments and suppress char-forming reactions (entry 12, Table 1). However, the highest monocyclic product yield of 24.6 wt% was achieved using a combination of NaOH and Re/Al2O3 due to the high depolymerization rate of lignin via base-catalyzed degradation on one hand, and the efficient stabilization of lignin-depolymerized fragments (suppression of condensation reactions) over Re/Al2O3 on the other hand (entry 10, Table 1). It could also be inferred from the results obtained at different Re/Al2O3-to-NaOH ratios that the correlation between the rate of lignin degradation and the rate of stabilization of lignin-depolymerized fragments is a key factor for suppressing char formation in a lignin liquefaction process. The rates of lignin depolymerization and subsequent stabilization of lignin derivatives should be well balanced to ensure that lignin derivatives can be stabilized by the applied catalytic system before they undergo repolymerization to form a large amount of char.It is also noteworthy that no phenolics were detected in the liquid product using the combination of 2 g Re/Al2O3 and 0.5 g NaOH, indicating the complete HDO of phenolic compounds to hydrocarbons. Using this catalytic system, complete conversion of lignin was observed, and monocyclic saturated hydrocarbons, monocyclic aromatic hydrocarbons, tetralins, indenes and naphthalenes were the main GC-detectable organic products with the yields of 13.5, 11.1, 2.1, 1.7 and 1.2 wt%, respectively (shown in Table 3\n). Meanwhile, the monocyclic hydrocarbon distribution presented in Fig. 7\n show that methylcyclohexane (16.5 wt%), (1-methylethyl)cyclohexane (7.9 wt%) and 1-ethyl-2-methylcyclohexane (7.1 wt%) were the most dominant monocyclic saturated hydrocarbons, and 1-ethyl-4-methylbenzene (6.9 wt%), ethylbenzene (5.7 wt%) and toluene (4.4 wt%) were the major monocyclic aromatic hydrocarbons. The monocyclic hydrocarbon products were composed of 23.8 wt% C7-C9 arenes, 21.4 wt% C10-C14 arenes, 46.5 wt% C6-C9 cycloalkanes and 8.3 wt% C10-C13 cycloalkanes. All GC-detectable liquid products are listed in Table S3. The molecular weight distribution of the liquid products is also shown in Fig. S7.\nScheme 1\n presents a proposed network of the major series and parallel reaction pathways taking place in the catalytic liquefaction and hydrodeoxygenation of kraft lignin. First, lignin polymer is fragmented into lower weight oligomers by cleavage of CC and COC linkages through thermal cracking and NaOH-catalyzed degradation. These smaller lignin fragments can diffuse into the channels of the solid hydrotreating catalyst (e.g., ReS2/Al2O3) and be converted into monocyclic phenolics via hydrogenolysis. Monocyclic phenolics can undergo either direct deoxygenation or ring hydrogenation to produce monocyclic aromatic or saturated hydrocarbons, respectively. The HDO reaction selectivity could be controlled by several parameters including solvent type (polar or non-polar; solvent polarity affects solvent-reactant interactions and, in turn, the orientation of reactant adsorption on the catalyst surface), catalyst properties (oxophilicity, catalytic performance of hydrogenation active sites, hydrogen dissociation efficiency and catalyst channel size), reaction temperature (affecting hydrogenation activity and hydrogen availability on the surface of catalyst) and reaction time. In the case where free radicals of oligomers and phenolic derivatives are not well stabilized by hydrogenation, irreversible radical coupling reactions lead to rapid condensation which forms solid char residues. Therefore, the high-yield production of monocyclic hydrocarbons from the conversion of kraft lignin, using the catalytic combination of NaOH and ReS2/Al2O3, occurs through four major steps: (i) depolymerization of lignin to oligomers via thermal cracking and alkali-catalyzed degradation; (ii) hydrogenolysis of oligomers to monocyclic phenolics over ReS2/Al2O3; (iii) stabilization of the free radicals intermediates by ReS2-catalyzed hydrogenation; (iv) hydrodeoxygenation of monocyclic phenolics to monocyclic hydrocarbons.ReS2/Al2O3 was a highly efficient catalyst for high-yield production of monocyclic hydrocarbons in the reductive liquefaction of a sulfur-containing lignin due to: (i) high stabilizing efficiency via hydrogenation of the free radicals of lignin-depolymerized fragments, resulting in significant suppression of char-forming condensation reactions; (ii) high hydrodeoxygenation activity leading to an efficient oxygen removal from lignin-derived phenolic compounds; (iii) resistance to sulfur poisoning. Compared to NiMo/Al2O3 as a conventional sulfide catalyst, ReS2/Al2O3 led to a significantly lower char yield and higher monocyclic product yield; in the reductive liquefaction of kraft lignin at 340 \u00b0C, the alumina-supported nickel-molybdenum and rhenium sulfide catalysts resulted in char yields of 40.6 and 11.2 wt%, total monocyclic product yields of 4.6 and 21.5 wt%, and monocyclic hydrocarbon yields of 4.6 and 7.3 wt%, respectively. Char suppression over ReS2/Al2O3 was also observed at an elevated temperature of 400 \u00b0C, giving a low char yield of 8.5 wt%. Moreover, it was shown that HDO activity is greatly affected by the choice of catalyst support; Al2O3-, ZrO2- and HY-supported ReS2 catalysts resulted in considerably different monocyclic hydrocarbon yields of 16.8, 11.2 and 8.5 wt%, respectively. This could be ascribed to different rates of dehydration and hydrogenation reactions which are affected by the acid property of the support and the sulfidation degree of the supported phase, respectively. Oxophilicity, sufficient acidity, metal-like behavior of rhenium sulfide and high dispersion of the supported phase are the significant characteristics of ReS2/Al2O3, making this catalyst highly effective for reductive conversion of lignin into monocyclic hydrocarbons. The alkali-assisted depolymerization of lignin by an addition of NaOH clearly illustrated the significance of ReS2/Al2O3-to-NaOH ratio which needs to be well optimized for a positive effect of NaOH addition. Otherwise, the addition of a depolymerization promoter (e.g., NaOH) higher than its optimum amount leads to an insufficient stabilization of the lignin-depolymerized fragments, and in turn, shifts the reaction pathway towards a rapid condensation and high char formation. The ReS2/Al2O3-to-NaOH ratio of 4 g/g led to a high monocyclic hydrocarbon yield of 24.6 wt% and a low char yield of 11.3 wt%.\nPouya Sirous-Rezaei: Conceptualization, Methodology, Investigation, Validation, Writing - original draft. Derek Creaser: Writing - review & editing, Supervision. Louise Olsson: Writing - review & editing, Supervision, Resources, Funding acquisition.There are no conflicts of interest to declare.The Swedish Energy Agency (P47511-1), Formas (2017-01392) and Area of Advance Energy at Chalmers are greatly acknowledged for their financial support. We would also like to acknowledge the Chalmers Material Characterization (CMAL) facilities for STEM and XPS measurements.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2021.120449.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  Thermochemical processing of lignin ends up with a major problem which is the high yield of char remained from lignin conversion, causing low yields of desired products. The ReS2/Al2O3 catalyst, used in this work, exhibited a high char-suppressing potential and high hydrodeoxygenation efficiency in the reductive liquefaction of kraft lignin. Compared to NiMo/Al2O3, as a conventional sulfide catalyst, ReS2/Al2O3 showed significantly better catalytic performance with 72.4 % lower char yield, due to its high efficiency in stabilizing the lignin-depolymerized fragments. The remarkable catalytic performance of ReS2/Al2O3 is attributed to its high oxophilicity, the metal-like behavior of rhenium sulfide and sufficient acidity. The effects of reaction temperature and different catalyst supports (Al2O3, ZrO2 and desilicated HY zeolite) were also studied. In an alkali (NaOH)-assisted depolymerization of lignin, it was revealed that ReS2/Al2O3-to-NaOH (stabilization-to-depolymerization) ratio plays a crucial role in determining the reaction pathway toward either solid char residues or liquid monomeric products.\n               "}
{"full_text": "The increasing environmental pollution and the depletion of nonrenewable energy have raised awareness towards the development of hydrogen storage systems as a viable source of clean energy. However, the suitability for on-board application has been a challenge to realizing the much-anticipated hydrogen economy [1]. MgH2 is one of the most promising candidates for hydrogen storage and has been extensively investigated over the last few decades. This is because of its large gravimetric and volumetric hydrogen capacities of about 7.6 wt% and 110\u00a0g/L, respectively. MgH2 also has the benefits of natural abundance, low cost, and non-toxicity [2,3]. However, the sluggish sorption rates and high operating temperature of MgH2 ranging between 300 and 400\u00a0\u00b0C which result from high kinetic barriers and stable thermodynamics (dehydrogenation enthalpy \u0394Hf\n \u2265 75\u00a0kJ/mol), prevent its mobile application [4,5]. To overcome these barriers, tremendous efforts have been given to nano-structuring [6\u20138], alloying [9\u201314], and catalysis [15\u201319] in the past few years with significant progress made on the sorption properties. However, hydrogen capacity loss by the formation of a multicomponent alloy and the introduction of confined porous materials that result from nano-structuring and alloying [6,12] has drawn attention towards catalytic doping as one of the most effective ways of improving the hydrogen storage properties of MgH2. The use of catalysts such as transition metals [20,21], transition metal oxides [19,22,23], carbides [18,24], hydrides [25,26], etc., have been shown to improve the kinetics of MgH2.Among the various catalysts, TiO2 has recently attracted great interest due to its particle size and tunable Ti valence state (via oxygen vacancy creation) which can enhance the hydrogen storage properties of MgH2 [27,28]. For instance, MgH2 doped with 5 mol% of rutile, anatase, and P25 TiO2 synthesized by high-energy ball milling were investigated to enhance the hydrogenation properties of Mg [27]. From the results, the rutile TiO2 doped composite showed the fastest absorption kinetics and highest capacity; this was attributed to the formation of an ultrafine nanocomposite MgH2-TiO2. Pandey et\u00a0al. [29] also observed that MgH2 catalyzed by 7 and 50\u00a0nm TiO2 exhibited the optimum catalytic effect for hydrogen desorption and absorption respectively among the particle sizes of 7, 25, 50, 100, and 250\u00a0nm. It was stressed that the TiO2 could partially be reduced at temperatures below 340\u00a0\u00b0C to form defective TiO2-x with oxygen vacancies during the sorption process of MgH2. Furthermore, nanocrystalline TiO2 supported on carbon (TiO2@C) has also shown good catalytic performance in the hydrogen storage reaction of MgH2\n[30]. The addition of 10 wt% of the catalyst into MgH2 reduced the onset dehydrogenation temperature to 205\u00a0\u00b0C with the release of 6.5 wt% H2 within 7\u00a0min at 300\u00a0\u00b0C and reabsorption of 6.6 wt% H2 within 10\u00a0min at 140\u00a0\u00b0C. The mechanism behind the improvement was based on the weakening/breaking of the Mg-H bond by TiO2 as obtained from DFT calculations which agreed with the experimental results. Some recent investigations have revealed that hydrothermally synthesized TiO2-based catalysts such as the 2D graphene-like TiO2 nanosheet, and graphene-supported TiO2 nanoparticles (TiO2@rGO), could improve the sorption properties of MgH2 [31,32]. The perovskite oxides of titanium also demonstrated some level of efficiency towards improving the sorption properties of MgH2. For example, Zhang et.al reported the catalytic performance of Na2Ti3O7 nanotubes, with a diameter of 10\u00a0nm, which could facilitate the hydrogen de/absorption kinetics of MgH2 by providing a lot of hydrogen diffusion channels [33]. With 5 wt% of the catalyst, MgH2 could desorb 6.5 wt% H2 at 300\u00a0\u00b0C in 6\u00a0min and absorb 4.1 wt% H2 at 150 \u00b0C in 10\u00a0min. Some other examples include MgH2 catalyzed with Li2TiO3\n[17], SrTiO3\n[34], BaTiO3\n[35], and NiTiO3\n[36]. On a general note, it was resolved that fairly stable metal oxides with a large number of possible structural defects and high valence states possess high catalytic efficiency on the sorption reaction of MgH2 [22,37,38].Given the above investigations, it is reasonable to examine the catalytic effect of light metal hydride (NaH) pre-activated TiO2 in the improvement of MgH2 hydrogen storage performance. The choice of NaH as a reducing agent for TiO2 is based on its strong reducing ability which can help boost vacancy concentration, and defect sites in TiO2 under a mild preparatory condition; this was confirmed in our previous works [39,40] by using Na/NaCl induced-oxygen vacancy in TiO2 for hydrogenation and water-gas shift reactions. High-energy milling of TiO2 with NaH under room temperature yields black TiO2 powder (TiO2-x) which is reportedly characterized by surface disorders, surface defects, and oxygen vacancies [41,42]. Introducing 5 wt% of this powder reduces the operating temperature of MgH2 to \u223c185\u00a0\u00b0C, with room temperature absorption of 4.5 wt% H2 in 45\u00a0min. 2.5 wt% of the catalyst also enables MgH2 to undergo 100 cycles of de/absorption at 300\u00a0\u00b0C. To the best of our knowledge, no investigation has been conducted before now on this branch of knowledge.Pure MgH2 (98%), pure NaH (95%), and anatase TiO2 (99.8% metal basis 25\u00a0nm particle size) powders were commercially purchased from Alfa-Aesar, Sigma Aldrich, and Macklin chemical respectively. The powders were used without further treatment. All samples were prepared under an argon atmosphere in the glovebox with a circulative purification system (O2 < 10\u00a0ppm, H2O < 0.1\u00a0ppm) to avoid the influence of oxygen and moisture. NaH doped TiO2 catalyst was prepared by ball milling NaH with TiO2 separately in mole ratios of 0, 0.5, 1, and 2 to 1 for 3 hr. After that, a preselected amount of each catalyst obtained was added into fresh MgH2 powder and ball-milled. Pristine MgH2 was also ball-milled separately with and without TiO2. Ball milling of all the MgH2 composites lasted for 16 hr with batch weight kept at 2 g. All of the milling processes were performed using Retsch PM 400 at room temperature with a rotation speed of 200 RPM. The mixtures were sealed in 150\u00a0ml stainless steel vials in a glovebox with a ball-to-powder weight ratio (BPR) of about 80:1. The milling process was interrupted for 2.5\u00a0min after every 10\u00a0min of rotation to dissipate accumulated heat.Powder X-ray diffraction (XRD) measurements were conducted using an X'Pert3 Materials Research Diffractometer (Malvern Panalytical) with Cu K\u03b1 radiation (\u03bb\u00a0=\u00a00.154\u00a0nm) at 40\u00a0kV and 40\u00a0mA. Samples were measured into the steel sample holders and covered with Kapton to avoid contamination during the measurement. Each measurement was done at a scan speed of 2\u00b0/min over diffraction angles of 10\u00b0 to 90\u00b0. The microstructure and morphology of the samples were investigated using Hitachi S-4800 scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) analysis unit and an FEI Tecnai G2 F20 S-TWIN transmission electron microscopy (TEM). For TEM analysis, the samples were dispersed in hexane, sonicated, and drop cast on a copper grid. Image processing was performed using Digital Micrograph (Gatan) software. X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific, ESCALAB 250Xi, Al-K\u03b1\u00a0=\u00a01486.6\u00a0eV) technique was used to analyze the surface state of the catalyst. The binding energy was calibrated using C-C binding energy at 284.4\u00a0eV as a reference.The thermal decomposition properties of the samples were first investigated by using a homemade temperature-programmed desorption system equipped with a mass spectrometer (HPR20, Hiden) (TPD-MS). About 15\u201320\u00a0mg of the samples were loaded into an air-tight sample holder and sealed to the reactor in the glovebox. The analysis was carried out between room temperature and 400\u00a0\u00b0C at a heating rate of 2\u00a0\u00b0C/min under 20\u00a0mL/min argon flow.The volumetric desorption of the samples was carried out using Gas Reaction Controller (Advanced Material Corporation, USA). 120\u2013150\u00a0mg of each sample was tested. Samples were heated up from room temperature to 400\u00a0\u00b0C with a heating rate of 2\u00a0\u00b0C/min under 0.001\u00a0bar of H2. However, room temperature absorption was conducted at 10, 30, and 50 bars of H2 backpressure.Thermodynamic and kinetic de/re-hydrogenation behaviors of samples were evaluated by using a conventional Hy-Energy PCT pro-2000 pressure-composition-isotherm (PCI) analyzer. 200\u2013250\u00a0mg of each sample was loaded into a standard autoclave steel reactor. Isothermal desorption was investigated at 260, and 290\u00a0\u00b0C under 0.001\u00a0bar of H2pressure, while isothermal absorption was investigated at 50, 100, 200, 230, and 260\u00a0\u00b0C under 30 bars of H2 pressure. The composite's reversibility was evaluated at 300\u00a0\u00b0C under 0.001 and 30\u201350 bars of H2 pressure for de/absorption. PCI desorption measurement was performed at 300, 320, and 340\u00a0\u00b0C. The thermodynamic property was determined by using the Van't Hoff equation [43], which is expressed as a function of the equilibrium pressures recorded during PCI measurements.\n\n(1)\n\n\nIn\n\n(\n\nP\n\nH\n2\n\n/\n\nP\n\u03b8\n\n\n)\n\n=\n\u2212\n\n\u0394\n\nH\n/\nRT\n+\n\n\u0394\n\nS\n/\nR\n\n\n\nWhere PH2, \u0394H, and \u0394S are the hydrogen equilibrium pressure, enthalpy, and entropy change, respectively.The apparent activation energy of each sample under investigation was determined using the Kissinger method [44] through the mass spectra data obtained from TPD-MS. Samples were heated from room temperature to 400\u00a0\u00b0C with heating rates of 2, 6, 8, and 10\u00a0\u00b0C/min under 20\u00a0mL/min of argon flow. The method is as described in equation 2.\n\n(2)\n\n\nIn\n\n(\n\u03b2\n/\n\nT\n\np\n\n2\n\n)\n\n=\n\u2212\n\nE\na\n\n/\nR\n\nT\np\n\n+\nA\n\n\n\n\nWhere \u03b2 is the heating rate, Tp\n2 is the peak temperature of desorption given by the result of TPD-MS, R is the gas constant, A is a linear constant and Ea\n is the activation energy calculated from the slope value of the Kissinger plot.The optimum ratio of NaH to TiO2 with the best catalytic performance was determined by TPD-MS (Fig. S1), which indicates that NaH doped TiO2 in a 1:1\u00a0mole ratio (designated as NaTiOxH) has the best catalytic effects. Following that, the dehydrogenation properties of as-milled MgH2, MgH2\u20135 wt% TiO2, and MgH2-ywt% NaTiOxH (y\u00a0=\u00a02.5, 5, and 10) composites were measured by TPD-MS and the results are summarized in Fig.\u00a01\na. The results show that 10 wt% NaTiOxH catalyzed MgH2 starts to desorb hydrogen from \u223c174\u00a0\u00b0C and reaches its peak at 237\u00a0\u00b0C; which is \u223c100\u00a0\u00b0C lower than the as-milled MgH2. However, reducing the amount of NaTiOxH from 10 to 5 and 2.5 wt% only influences the dehydrogenation peak slightly.To clearly show the catalytic effects of the different catalysts and doping amounts, temperature-programmed volumetric desorption of the prepared samples was measured and plotted in Fig.\u00a01(b). It shows that 2.5, 5, and 10 wt% NaTiOxH catalyzed MgH2 begin to release H2 from temperatures below 200\u00a0\u00b0C, while they desorb \u223c6.9, 7.2, and 6.2 wt% H2 at \u223c260\u00a0\u00b0C; finally, the composites release off \u223c7.5, 7.4, and 6.5 wt% H2 at \u223c320\u00a0\u00b0C. However, as-milled MgH2 starts to dehydrogenate at \u223c 295\u00a0\u00b0C which is \u223c100\u00a0\u00b0C higher than these catalyzed samples, and it liberates \u223c7.5 wt% H2 at \u223c375\u00a0\u00b0C. Taking the kinetics and H2 capacity into consideration, the 5 wt% NaTiOxH catalyzed MgH2 was selected for further investigations.The hydrogenation properties of MgH2\u20135 wt% NaTiOxH were measured under varying conditions as shown in Fig.\u00a02\n. At room temperature (Fig.\u00a02a), the dehydrogenated MgH2\u20135wt% NaTiOxH absorbs \u223c4.5 and more than 5.0 wt% H2 under 50 bars of H2 pressure within the first 45 and 120\u00a0min, respectively. Meanwhile, under 10 and 30 bars of H2 pressure, the composite respectively absorbs \u223c4.1 and 5.5 wt% H2 within the first 180\u00a0min; \u223c5.5 and 6.0 wt% H2 in 6 hr; \u223c6.0 and 6.4 wt% H2 (\u223c87.7% of the total) in 12 hr (shown in Fig. S2). The observed fluctuation of absorption curves between 30 and 50 bars at the apex could be ascribed to an uncontrollable variation in environmental temperature. Furthermore, a moderate temperature absorption measurement of the composite at 30 bars of H2 pressure (Fig.\u00a02b) shows an absorption capacity of about \u223c4.4 and 4.5 wt% H2 within the first 30\u00a0min at 50\u00a0\u00b0C and 100\u00a0\u00b0C, respectively; while as-milled MgH2 could barely absorb at these temperatures. This enhanced absorption kinetics further confirms the catalytic efficiency of NaTiOxH catalyst on MgH2.Subsequently, a comparative measurement of isothermal de/re-hydrogenation at high temperatures of both the non-catalyzed and catalyzed MgH2 was conducted at four (4) different constant temperatures of 200, 230, 260, and 290\u00a0\u00b0C under 0.001\u00a0bar and 30 bars of H2, respectively (Fig.\u00a03\n). Figure\u00a03(a) shows that the composite releases \u223c7.2 wt% H2 within the first 15\u00a0min at 290\u00a0\u00b0C, and \u223c6.9 wt% in 60\u00a0min at 260\u00a0\u00b0C. However, as-milled MgH2 releases only \u223c3.1 wt% H2 after 120\u00a0min at 290\u00a0\u00b0C, with no significant release at 260\u00a0\u00b0C. Furthermore, as shown in Fig.\u00a03(b), the composite absorbs \u223c6.6 wt% H2 within the first 120\u00a0s at 260\u00a0\u00b0C and \u223c6.9 wt% after 20\u00a0min at 230\u00a0\u00b0C, while as-milled MgH2 only absorbs \u223c6.0 and 6.1 wt% H2 within the same time range at 230\u00a0\u00b0C and 260\u00a0\u00b0C, respectively. Lastly, the composite charges \u223c6.0 wt% H2 after 30\u00a0min at 200\u00a0\u00b0C while the as-milled sample absorbs less.A reversibility test was carried out to investigate the stability of the de/re-hydrogenation kinetics. In pursuit of a high hydrogen capacity with reasonable de/re-hydrogenation kinetics, MgH2\u20132.5 wt% NaTiOxH was chosen for cycling measurement across 100 cycles at 300\u00a0\u00b0C with a total of 475 hr (Fig.\u00a04\n). Dehydrogenations were measured under 0.001\u00a0bar of H2 while hydrogenations were measured under 30, 40, and 50 bars of H2, respectively. The data profiles of the 1st, 50th, and 100th cycles of de/absorption (Fig. S3 and S4) show that the composite remains fairly intact with only a slight variation in the hydrogen capacity. Aside from the drop in kinetics, after 100 cycles, the hydrogen desorption capacity remained at \u223c6.1 wt%, equivalent to \u223c84% capacity retention and 0.012 wt% hydrogen loss per cycle. The kinetic relaxation observed could be attributed to the agglomeration of Mg/MgH2 particles and their separation from the catalyst during cycling. The zero-point slight variation noticeable between the 50th and 100th cycle absorptions could be attributed to the H2 backpressure increase from 40 to 50 bars.The dehydrogenation kinetic improvement of MgH2\u20135 wt% NaTiOxH was investigated by applying the Kissinger model to calculate the apparent activation energy (Ea). First, the mass spectra data of as-milled and catalyzed MgH2 were collected at heating rates of 2, 6, 8, and 10\u00a0\u00b0C/min as shown in Fig.\u00a05\n (a and b). The as-milled MgH2 shows clearly two-step desorption behavior similar to reported studies [36,45,46]. This was attributed to either the formation of metastable high-pressure \u03b3-MgH2 or the non-uniformity of its grain/particle sizes. As shown in Fig.\u00a05(c), the Kissinger plots of as-milled and 5 wt% NaTiOxH catalyzed MgH2 indicate that ball milling pristine MgH2 could reduce its Ea\n from the reported value of \u223c180\u00a0kJ/mol [34,45,47,48] to \u223c101 (\u00b14) kJ/mol, and further reduction to \u223c57 (\u00b11) kJ/mol by adding NaTiOxH catalyst. The PCI desorption measurement of the composite at 300, 320, and 340\u00a0\u00b0C exhibits a distinct plateau region at each isotherm as shown in Fig.\u00a05(d). The Van't Hoff plot of equilibrium pressure against temperature (inset) provides the dehydrogenation enthalpy change of \u223c77 (\u00b11.5) kJ/mol-H2. This indicates that NaTiOxH does not have any thermodynamic improving effect on MgH2.A comparative tabulation of the room temperature absorption capacity, activation energy, and reversibility of MgH2 catalyzed by a few representative TiO2-based catalysts is shown in Table\u00a01\n. This confirms the improved H2 storage performance of MgH2 by adding NaTiOxH catalyst.X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) analysis unit were used to evaluate the phase composition, morphology, and crystallography of the synthesized NaTiOxH catalyst. X-ray photoelectron spectroscopic (XPS) investigation was also conducted to examine the chemical status of the component species consisted in the catalyst as shown in Fig.\u00a06\n. The XRD profile of NaTiOxH (Fig.\u00a06a) displays the diffraction peaks corresponding to TiO2. The reduced intensity and broadness of some of the peaks have previously been attributed to an amorphous layer of defective TiO2-x nanoparticles formed near the surface [52]. In addition, a careful resolution of the peaks (Table S1) shows the emergence of a few crystalline phases of Ti3O5 and Na2Ti3O7 near the TiO2. The presence of these crystalline phases and the formation of black TiO2-x powder suggest that a reaction occurred between NaH and TiO2 during ball milling [42,53,54]. The SAED pattern of the catalyst (Fig.\u00a06b) exhibits a typical point-ring diffraction characteristic of polycrystalline materials. The agreement in the calculated d-spacing values (Table S1) of indexed Ti3O5, and Na2Ti3O7 between the SAED and XRD patterns of the catalyst (shown in Fig.\u00a06a and b), confirms their in-situ formation. In addition, the analysis of selected areas on the HRTEM image of NaTiOxH (Fig.\u00a06c) shows the lattice fringes corresponding to Ti3O5 (110), and Na2Ti3O7 (101 and 103). The Fast Fourier Transformed (FFT) image (inset) reveals a rod-like diffraction pattern of the crystalline Na2Ti3O7 on the (101) plane. More TEM images of the catalyst that show the distribution and lattice fringes of Na/TiOx species on different sides could be seen in Fig. S5 and S6. For the morphology, the SEM micrograph of pristine TiO2 anatase (Fig.\u00a06d) shows spherical nanoparticles (\u223c23\u00a0nm) with reduced size and increased surface area that is noticeable after milling with NaH (Fig.\u00a06e). Bright patches on the particles\u2019 surface could be attributed to contaminations on exposure to air before/during measurement. Furthermore, sodium 1s XPS spectrum of the catalyst (Fig.\u00a06f) reveals its existence as Na-O-Ti, and Na-O/OH positioned at 1070.5 and 1072.1\u00a0eV, respectively [55]. The energy peak at 1069.2\u00a0eV is attributed to the Ti-Auger effect (Ti LMM) [56,57]. Titanium 2p XPS of the catalyst (Fig.\u00a06g) shows that it exists in four (4) different valence states of +4, +3, +2, and 0; as distinct from the spectra of pure TiO2 (Fig. S7). This is due to the formation of defects and oxygen vacancies after milling with NaH; a phenomenon that could generate trapped-in electron densities around vacant 3d orbitals of the corresponding adjacent Ti atoms [49,52,58,59]. The appearance of the peak corresponding to Ti3+ at 457.5\u00a0eV could be attributed to the Ti3O5 and/or its related species with other possible oxyhydrides [60,61]. Ti2+ peak at 455.1\u00a0eV could be due to TiO species probably covered up in the amorphous layer of the defective TiO2-x [28,61,62], while Ti0 peak at \u223c452.5\u00a0eV is considered to be the characteristic peak of Ti metal [28,60,63]. The 1\u00a0s XPS spectrum of oxygen (Fig.\u00a06h) identifies its existence as O-Ti/Na (530.0\u00a0eV), H-O-Ti/Na, and/or peroxides (531.9\u00a0eV) with the binding energy slightly shifting to the higher region as compared to oxygen 1s spectra of pure TiO2 sample (Fig. S8). An adsorbed contaminant on exposure of the sample to air before measurement accounts for the peak at 532.4\u00a0eV, while the peak at 536.8\u00a0eV is attributed to the Na-Auger effect (Na KLL) [64\u201366]. Summarily, ball milling NaH with TiO2 liberates defective TiO2-x species which is characterized by reduced valences of titanium, and oxygen vacancies. A few crystalline phases of Ti3O5 and Na2Ti3O7 also emerge in situ during the reaction.As revealed in Sections\u00a03.1 and 3.2, NaH doped TiO2 in a 1:1\u00a0mole ratio (NaTiOxH) exhibits excellent catalytic performance on the hydrogen storage properties of MgH2. To unravel the mechanism behind this performance, XRD patterns of the as-milled and de/re-hydrogenated samples before and after cycling were collected as shown in Fig.\u00a07\n(a). The patterns show characteristic peaks of MgH2, Mg, and MgO. The MgO peak could arise due to oxygen contamination before/during the XRD measurement [67] or the presence of MgO characteristic of the oxide additives loaded MgH2\n[28]. Aside from these regular phases of MgH2/Mg/MgO, the phase stability of Ti3O5, Na2Ti3O7, and defective TiO2-x after 100 cycles show a probable reason behind the improved de/absorption behaviors of MgH2. Meanwhile, the SAED image of the as-milled composite (Fig.\u00a07b) shows the plane corresponding to Ti3O5 and Na2Ti3O7 species. The TEM image of the as-milled composite reveals the contacting catalyst nanoparticles around the MgH2 particles (Fig.\u00a07c), while the HRTEM images of the as-milled and hydrogenated composites reveal the corresponding lattice fringes of MgH2, MgO, Na2Ti3O7, and TiO2-x species (Fig.\u00a07d and e). The calculated d-spacing values of the resolved lattice fringes as detected by SAED and TEM agree with both the XRD pattern and the standard values of d-spacing as shown in Table S2. More TEM images of the composites with some lattice fringes on different sides are shown in Fig. S9 and S10. Moreover, from the morphological perspective, the SEM micrograph of pristine MgH2 (Fig.\u00a07f) shows micro-particles with a size distribution >30\u00a0\u00b5m. High energy milling reduces the size below 10\u00a0\u00b5m (Fig.\u00a07g) with the size distribution profile shown in Fig. S11. The addition of NaTiOxH catalyst (Fig.\u00a07h) further reduces the size of MgH2 coupled with an increased contact which could help promote its kinetics of de/re-hydrogenation [34]. The SEM micrograph of the dehydrogenated sample (Fig. S12) shows fine sponge-like Mg particles with bright patches on the surface attributed to MgO. A slight expansion of the particles could be observed on hydrogenation (Fig. S13). This expansion is believed to promote contact between the particles of Mg/MgH2 and NaTiOxH which could facilitate the reversibility process as observed in Fig.\u00a04. Likewise, EDS mapping of the composite (Fig.\u00a07i) also reveals the bright spots of Na, Ti, and O species well dispersed around the network of MgH2; a phenomenon that could positively influence the electron dynamics around MgH2 for an improved hydrogen storage performance. Furthermore, titanium 2p XPS spectral of the as-milled sample (Fig.\u00a07j) shows its existence in four (4) different states similar to Fig.\u00a06(g) above. However, the addition of MgH2 causes a significant electronic effect around Ti3+ at 457.0\u00a0eV, and Ti2+ at 454.8\u00a0eV as compared to the Ti spectra of NaTiOxH catalyst in Fig.\u00a06(g). This kind of shift in binding energy (\u223c1\u00a0eV difference) was previously attributed to the reduction in ionic contribution in the respective titanium chemical bond formation; the unstable ionic character could arise due to factors such as lattice distortion, hybridization, and crystal field stabilization effects [68\u201371]. On dehydrogenation, the compositional changes around the valences due to H2 release result in a slight shift to the higher binding energy region. Interestingly, the Ti0 state at 452.5\u00a0eV in the as-milled sample which corresponds to titanium metal disappears. Considering the high chemical reactivity between titanium and oxygen, the reason for this disappearance could be attributed to the oxidation of the titanium metal into higher state sub-oxides even as more oxygen atoms are likely to be present after dehydrogenation [41,54,72\u201374]. After re-hydrogenation, a similar pattern to the as-milled sample in Fig.\u00a07(j) above re-emerges (Fig. S14). This indicates probably the occurrence of a redox reaction via H2 insertion and removal; a process that causes some sort of vacancy creation and elimination between the valence and conduction bands of Ti-O bonds as previously reported by the following studies [17,75]. The Oxygen 1s data plot of the as-milled sample (Fig. S15) shows the bond states corresponding to phases of Mg-O, Na/Ti-O, hydroxides, and/or peroxides [65]. The binding energy shift of the corresponding O-based species (while Mg-O binding energy peak remains) is also consistent with the redox process as shown in Fig. S16. It should be noted at this point that the Na2Ti3O7 species formed in-situ remains after 100 cycles of de/re-hydrogenation from XRD analysis (Fig.\u00a07a); hence, a logical conclusion could be reached that a \u2018topotactic\u2019 reaction probably occurs between Mg-H and this catalytic species similar to the reported investigations on TiO2 surface topotactic reactions [73,76]. This reaction probably generates nonstoichiometric layers around the Na-doped TiO6 octahedrons which provide multiple diffusion channels that enhance the Mg-H bond de/association [17,33]. In addition, the disappearance and re-appearance of Ti3O5 species in the respective XRD patterns of the dehydrogenated and re-hydrogenated composites, and the unstable valences of Ti3+/Ti2+/Ti0 species in the composites (consistent with H2 insertion and removal), evidently confirm that the redox process around these defects facilitates hydrogen diffusion in Mg/MgH2 [17,32,75]. This is schematically illustrated in Fig.\u00a08\n. However, apart from the multivalent titanium states, and the Ti3O5/Na2Ti3O7 species formed in situ, some other components of the defective TiO2-x could have played their respective roles towards improving the electronic conductivity around the MgH2 bond; this creates the gap for further investigations.This work presents NaH doped nanocrystalline TiO2 NaTiOxH as an effective catalyst for the hydrogen storage properties of MgH2. The introduction of 2.5 wt% of the catalyst stabilized the reversibility of Mg/MgH2 up to 100 cycles with \u223c6.1 wt% H2 retained afterward. Interestingly, 5 wt% of the catalyst could influence a remarkable absorption of \u223c4.5 wt% H2 in 45\u00a0min at room temperature under 50 bars of H2 pressure. This composite also absorbed \u223c5.0 wt% H2 in 60\u00a0min at 50\u00a0\u00b0C under 30 bars of H2 pressure. In addition to these improvements, the desorption analysis revealed that 5 wt% NaTiOxH catalyzed MgH2 could start to release H2 from \u223c185\u00a0\u00b0C and desorb \u223c7.2 wt% H2 within 15\u00a0min at \u223c290\u00a0\u00b0C; thanks to the apparent activation energy of desorption calculated to be \u223c57\u00a0kJ/mol which is 44\u00a0kJ/mol below as-milled MgH2, and 123\u00a0kJ/mol below pristine MgH2. However, the observed dehydrogenation enthalpy of \u223c77 (\u00b11.5) kJ/mol-H2 indicates that NaH only acted as a reducing agent for TiO2 without any positive influence on the thermodynamic property of MgH2. The results obtained from XPS measurement revealed that NaTiOxH is an effective catalyst for MgH2 possibly due to the existence of reduced titanium valences (Ti<4+) which showed partial/full reversibility via hydrogen insertion and removal, due to vacancy creation and elimination. Additional information from XRD, TEM, SEM, and EDS complementarily revealed an intimate contact between the homogeneously dispersed NaTiOxH and MgH2 particles. It was also observed that the presence of catalytically active Ti3O5 and \u201crod-like\u201d Na2Ti3O7 formed in-situ with some other possible multivalent titanium sub-oxides in the defective black TiO2-x powder could enhance the hydrogen storage performance of MgH2 by providing multiple diffusion channels during the de/absorption.The authors declare no competing interest.\nFigure S1. Temperature programmed desorption curves of MgH2 catalyzed with 10wt% NaH @ TiO2 at ratio 0.5, 1, and 2:1.\nFigure S2. Room-temperature absorption curves of MgH2\u20135 wt% NaTiOxH in 12 hr at 10, and 30 bars of hydrogen pressure.\nFigure S3. 1st, 50th, and 100th desorption cycles of MgH2\u20132.5 wt% NaTiOxH at 300\u00a0\u00b0C and 0.001\u00a0bar of H2 pressure.\nFigure S4. 1st, 50th, and 100th absorption cycles of MgH2\u20132.5 wt% NaTiOxH at 300\u00a0\u00b0C and 30\u201350 bars of H2 pressure.\nTable S1. Comparisons between calculated d-spacing of phases in the catalyst and standards.\nFigure S5. TEM image of NaTiOxH catalyst.\nFigure S6. HRTEM images of NaTiOxH catalyst.\nFigure S7. XPS profile of Titanium 2p in pure TiO2 sample.\nFigure S8. XPS profile of Oxygen 1s in pure TiO2 sample.\nTable S2. Comparisons between calculated d-spacing of phases in the composite and standards.\nFigure S9. HRTEM images of as-milled MgH2\u20135 w% NaTiOxH.\nFigure S10. HRTEM images of hydrogenated MgH2\u20135 w% NaTiOxH.\nFigure S11. Size distribution profile of as-milled MgH2.\nFigure S12. SEM image of dehydrogenated MgH2\u20135 wt% NaTiOxH.\nFigure S13. SEM image of hydrogenated MgH2\u20135 wt% NaTiOxH.\nFigure S14. XPS profile of Ti 2p in MgH2\u20135wt% NaTiOxH at different phases.\nFigure S15. XPS profile of Oxygen 1s spectra in MgH2\u20135wt% NaTiOxH.\nFigure S16. XPS profile of O 1s in MgH2\u20135wt% NaTiOxH at different phases.The authors acknowledge the Project supported by the National Key R&D Program of China (2019YFE0103600, 2018YFB1502101), the Key R&D Program of Shandong Province, China (2020CXGC010402), the National Natural Science Foundation of China (51801197), the Liaoning Revitalization Talents Program (XLYC2002076), the Dalian High-level Talents Program (2019RD09), the Youth Innovation Promotion Association CAS (2019189) and K.C. Wong Education Foundation (GJTD-2018\u201306).", "descript": "\n                  This paper presents the catalytic effect of NaH doped nanocrystalline TiO2 (designated as NaTiOxH) in the improvement of MgH2 hydrogen storage properties. The catalyst preparation involves ball milling NaH with TiO2 for 3 hr. The addition of 5 wt% NaTiOxH powder into MgH2 reduces its operating temperature to \u223c185\u00a0\u00b0C, which is \u223c110\u00a0\u00b0C lower than the additive-free as-milled MgH2. The composite remarkably desorbs \u223c7.2 wt% H2 within 15\u00a0min at \u223c290\u00a0\u00b0C and reabsorbs \u223c4.5 wt% H2 in 45\u00a0min at room temperature under 50\u00a0bar H2. MgH2 dehydrogenation is activated at 57\u00a0kJ/mol by the catalyst. More importantly, the addition of 2.5 wt% NaTiOxH catalyst aids MgH2 to reversibly produce \u223c6.1 wt% H2 upon 100 cycles within 475 hr at 300\u00a0\u00b0C. Microstructural investigation into the catalyzed MgH2 composite reveals a firm contact existing between NaTiOxH and MgH2 particles. Meanwhile, the NaTiOxH catalyst consists of catalytically active Ti3O5, and \u201crod-like\u201d Na2Ti3O7 species liberated in-situ during preparation; these active species could provide multiple hydrogen diffusion pathways for an improved MgH2 sorption process. Furthermore, the elemental characterization identifies the reduced valence states of titanium (Ti<4+) which show some sort of reversibility consistent with H2 insertion and removal. This phenomenon is believed to enhance the mobility of Mg/MgH2 electrons by the creation and elimination of oxygen vacancies in the defective (TiO2-x) catalyst. Our findings have therefore moved MgH2 closer to practical applications.\n               "}
{"full_text": "For hydrogen to become competitive compared to fossil fuels, cost effective and sustainable catalytic materials are needed for low-temperature water-electrolysis. The oxygen evolution reaction (OER) is the main limiting step of the overall water-splitting process, as it presents a large overpotential in comparison to the thermodynamic limit of 1.229\u00a0V vs. RHE. This is the main driving force for the current research on electrolyzers for the OER. Well-studied RuO2 and IrO2 present good efficiency, however, sustainability requires the replacement of these critical elements by materials containing Earth-abundant elements [1,2]. In the last decades, an increased interest appeared for mixed hydroxides of Ni and Fe for OER application, as described in the recent review of Gao and Yan [3]. These include layered double hydroxides, which structurally consist of positively-charged, brucite-like layers with edge-sharing M(OH)6 octahedrons (M\u00a0=\u00a0Ni, Fe), charge-balanced by interlayer anions plus water molecules. These Ni-Fe layered double hydroxides (LDH) were reported to present low overpotential and low Tafel slope for OER in alkaline media [4,5], which are very important catalytic indicators.Although the particular platelet like structure of LDH is often mentioned as an advantage leading to a large surface area of the catalyst [6,7], it is not yet clear why this specific material is efficient towards OER and whether it is due to the elemental composition or to its particular structure. A better understanding of the OER reaction mechanisms is necessary to highlight the characteristics allowing faster kinetics in order to obtain an efficient design of this type of catalysts. The impact of several factors on the OER catalytic activity of Ni-Fe LDH has been evaluated, such as the Ni/Fe ratio of the cationic layer [8,9], the nature of the intercalated anion [10], or the effect of delamination of the LDH [6,11]. However, the wide range of experimental conditions and material analysis procedures reported, the lack of information to fully replicate the published works and, consequently, the scattering of results obtained in different works leads to difficult comparison of the published results. As a result, newcomers in the field can easily doubt on the main factor(s) behind the good efficiency claimed for these materials.It is worth to mention that current alkaline electrolyzers industrially used are often Ni-based electrodes, at which surface the reaction takes place [2]. On these electrodes, a strong layer of hydroxides with different structures and level of hydration is formed upon cycling. Some groups studied the effect of different amounts of Fe in Ni hydroxides on the efficiency towards the OER, but without ever mentioning the name of LDH structure [12]. Others reported the interesting properties of combining Ni and Fe elements in electrodeposited [12], sputtered [12], or chemically bulk-synthesized hydroxides[1].In the case of Ni-Fe in LDH structure, the observation we made is that the literature rarely considers and discuss the importance of both the ratio and the crystallinity of the catalyst in a single work, or at least not explicitly, which is the objective of this work.As regards to the ratio of metallic cations, Oliver-Tolentino et al. reported the bulk preparation of Ni-Fe LDH of MII/MIII ratios 1.5 and 2, obtaining better efficiency towards OER with ratio 1.5, containing more Fe, which is considered to allow the activation of Ni centers by partial-charge transfer mechanism [8]. Zhou et al. prepared Ni-Fe samples with different ratios, from pure Ni hydroxide to Fe oxide [9]. Of all these compositions, we consider that only the sample with MII/MIII ratio of 2 can be considered to have a LDH structure and it is the one that presented the highest efficiency on their work. Later, G\u00f6rlin et al. studied nanosized Ni-Fe oxyhydroxides catalysts of different metallic ratios and obtained an optimal activity towards OER for a MII/MIII ratio of 0.8 [13]. They suggested that the good OER efficiency of the catalyst was related to the distortion of the matrix around the metallic active sites [13].These two last works open the discussion on the role of the structure and the atomic ordering of the material. It is well known that, for any catalyst, the arrangement of the atoms and structure of the material play an important role in its electrochemical behavior. In particular, it is commonly accepted that a low degree of crystallinity can lead to high amount of defects which are considered to be good active sites [14]. Hall et al., when reviewing Ni(OH)2 materials [15], verified that the reasons for this increased efficiency are still uncertain, being not clear whether the electron vacancies are induced by impurities or by high level of disorder. Electrodeposited Ni-Fe hydroxides [16] and sprayed Ni-Fe oxides [17] with amorphous structures are claimed to be very good catalysts thanks to the rough surfaces, high structural disorder and improved charges transfer rate resulting from the low order range nature of the material. Regarding the Ni-Fe LDH, the crystallinity of these materials is still poorly studied. Only three papers are reported in the review of Diogini and Strasser in 2016 under this topic [18]. Trotochaud et al. prepared amorphous electrodeposited Ni-Fe oxyhydroxide subjected to electrochemical aging, leading to an increase of long range order perpendicular to the metal cation sheets [19]. In the electrochemical tests, they obtained no clear difference of OER efficiency with or without aging, which led them to the conclusion that the long range order had no impact on the activity of the material and that structural defects did not enhance the activity [19]. This conclusion is in line with the work of Song et al., who prepared highly crystalline LDH to then exfoliate them to obtain single sheets (no stacking of the metal hydroxides sheets) [11]. Although the ordering within the 2D layer was not mentioned, they got a noticeable increase in efficiency, which was not entirely due to the increase in electrochemical surface area of the material and which may be due to a higher amount of edges giving a different electronic configuration of the active sites, similarly to defects. Later, the work of Xu et al. focused on the effect of the intercalated anion and the crystallinity of the LDH on the efficiency towards OER [20]. They used a Ni/Fe ratio of 3 and observed that the higher crystallinity led to a decrease of efficiency. Nevertheless, the smaller size of the lengthly hydrothermally treated particles also induced a higher surface area which may be the cause of the improvement. In the work of Xiong et al., highly crystalline LDH were prepared and then reduced to create defects through oxygen vacancies allowing better performance [21]. Thus, based on this literature review, it is possible to wonder to which extent is the LDH ordered structure, with its characteristic platelet morphology, relevant towards OER, when compared to a disordered Ni/Fe oxyhydroxide.To help answer this question, the present work gives insights on the importance of the ratio and crystallinity of the Ni-Fe LDH catalyst for its efficiency towards the OER. The approach followed to extract the electrochemical performance parameters can be divided into three main parts. First, the experimental setup and measurements carried out were validated using RuO2 as benchmark. This intends to prove that the conditions in which the measurements were done are line with the literature. Second, NiFe LDHs prepared with different degrees of crystallinity and Ni/Fe ratios were characterized using a method proposed by McCrory et al [22,23]. This part aims to generate data that can be directly compared with other works performed under the same conditions, thereby reducing differences in performance resulting from the breath of experimental conditions used for the preparation of the electrodes and not of the electrocatalyst properties per se. Third, the data obtained, namely overpotential and Tafel slopes, were not just reported but critically analyzed: in the case of overpotential values, statistical analysis was performed in order to identify trends beyond random fluctuations, while in the case of Tafel slopes, O2 measurements using microelectrodes were performed in order to unambiguously identify the region of O2 evolution and evaluate whether Tafel slopes could be extracted or not. Altogether, this is an original approach and it is expected to shed light on the properties that can actually affect the electrocatalyic properties of NiFe LDH family towards OER, with implications on the design of electrolyzers at industrial level.Herein, Ni-Fe LDH intercalated with carbonates were synthesized with different degrees of crystallinity through a simple hydrothermal method, for different Ni/Fe ratios. The carbonate intercalated hydroxide was chosen as it is the most stable form of Ni-Fe LDH and, hence, would present less degradation by anion-exchange in electrolytes containing potential impurities, making it an \u201capplication-friendly\u201d catalyst. Their efficiency as catalysts for the OER was evaluated and compared with standard RuO2 as a benchmark. The Tafel slopes and overpotentials were correlated with the structural features of the synthesized nanostructured Ni-Fe. Finally, the difficulty in comparing the electrochemical results with published data is discussed.Ni-Fe/CO3 LDH were synthesized using co-precipitation method in aqueous solution to obtain materials with a Ni/Fe ratio of 2, 3 and 4 [8,24]. The LDH were prepared by dropwise addition of an aqueous solution (25\u00a0mL; pH\u00a0=\u00a012.5) of NaOH (1.17\u043c) and Na2CO3 (0.34 \u043c) to an aqueous solution (25\u00a0mL) of nitrate metallic salts (0.5\u043c), under vigorous stirring at room temperature. The metallic salt solution was prepared with the desired Ni/Fe ratio. After complete addition, the resulting brownish slurry was stirred for two hours (final pH\u00a0=\u00a09). Half of the slurry volume is kept aside and called Ni/Fex-AsPrep (x\u00a0=\u00a0Ni/Fe ratio). The rest of the slurry was thermally treated in an autoclave at 120\u00a0\u00b0C for 24\u00a0h and named Ni/Fex-HT. All six samples were rinsed with DI water and centrifuged 3 times to eliminate any salt residues.The LDH slurries were dried overnight in oven at 80\u00a0\u00b0C and ground to a fine powder. The powder X-Ray diffraction was recorded using a PANalytical Xpert Pro instrument with Cu K\u03b1 radiation (\u03bb\u00a0=\u00a01.5418\u00a0\u00c5) and graphite monochromator. Phase analysis was performed using the PDF-4\u00a0+\u00a02019 database from the International Center for Diffraction Data. The profile matching of the obtained pattern was performed with the FullProf software, using a cell of space group R-3\u00a0m (166).The morphology of the materials was observed by Scanning Electron Microscopy with a Hitachi S-4100 system using an electron beam energy of 25\u00a0keV. The sampling was made by drop-casting a suspension of the LDH powder on a Si wafer. LDH samples were also analyzed by HR-TEM, using a transmission electron microscope energy-filtered TEM EF 200\u00a0kV, JEOL brand, model 2200FS, high-resolution electron gun Schottky emission (SE), omega type energy filter column spectrometry with electron energy loss EELS.The particles sizes and Zeta potentials were measured with a Malvern ZetaSizer Nano ZS apparatus and LDH dispersions prepared from dried powders and sonicated at least 5\u00a0min in water to ensure good particles dispersion.Attenuated total reflectance infrared spectra of dry LDH powders were collected with a Bruker Optics tensor 27 Fourier Transform-IR spectrometer, equipped with a Golden Gate ATR accessory plate. The spectra were collected at room temperature in ambient air, and 128 scans were averaged for each sample.Atomic Absorption Spectroscopy was performed on the samples dissolved in HCl (37%) using an Avanta apparatus from GBC Scientific equipment with an air-acetylene flame to measure the amount of Fe (Lamp: 248.3\u00a0nm at 6\u00a0mA, with slit of 0.2\u00a0nm) and Ni (Lamp: 232.0\u00a0nm at 5\u00a0mA, with slit of 0.2\u00a0nm).A glassy carbon rotating disk electrode (GC RDE) with 3\u00a0mm diameter was used as supporting electrode after being polished successively with SiC papers (P2500 and P4000) and suspensions of alumina powder (1\u00a0\u00b5m and 0.3\u00a0\u00b5m particle size), rinsed and sonicated 2\u00a0min in DI water after each polishing and dried in air. Aqueous suspensions of catalysts (0.5\u00a0mg\u00a0mL\u22121) were prepared from the LDH powder and sonicated 30\u00a0min. The glassy carbon RDE were modified by drop-casting the suspension (11\u00a0\u00b5L) and let dry 30\u00a0min in air, giving a final catalyst loading of 28\u00a0\u00b5g\u00a0cm\u22122. The RDE was then covered with a thin film of ion conductive polymer by drop-casting a Nafion solution (11\u00a0\u00b5L of 5\u00a0wt% Nafion\u00ae perfluorinated resin solution from Sigma-Aldrich diluted 100 times in ethanol) and let to dry 5\u00a0min at room temperature to avoid the detachment of the catalyst. The electrode was then installed as prepared in the electrochemical cell. A powder of ruthenium (IV) oxide (Alfa Aesar), deposited with the same procedure on the electrode, was used as a benchmarked electrocatalyst to evaluate the performance of the synthesized Ni-Fe LDH.The electrochemical experiments were performed using an Autolab PGSTAT 302\u00a0N potentiostat with the GPES software. The measurements were carried out at room temperature, inside a Faraday cage, in a cell with three-electrode configuration, with the GC RDE as working electrode, a saturated calomel electrode (SCE) as reference and a Pt wire as counter electrode. The testing electrolyte was 0.1\u00a0M KOH electrolyte. Before each experiment oxygen was bubbled in the cell for at least 20\u00a0min and the RDE rotated at 1600\u00a0rpm driven by an Autolab rotator and motor controller. The electrochemical characterization followed McCrory et al. [22,23]. A conditioning stage was performed consisting of 20 cyclic voltammetry sweeps in the 0\u20130.8\u00a0V vs. SCE potential range at a scan rate of 10\u00a0mV\u00a0s\u22121. This step allowed the stabilization of the catalyst and Nafion layers, verified by at least 5 identical last scans. Then, three cycles were measured at 5\u00a0mV\u00a0s\u22121 between 0 and 0.8\u00a0V vs. SCE to extract the figures of merit: overpotential and Tafel slope. The stability of the modified electrode was tested by chrono-potentiometry for 2\u00a0h at 10\u00a0mA\u00a0cm\u22122. All polarization potentials reported are relative to the reversible hydrogen electrode (RHE) and current densities per geometric area (0.196\u00a0cm2). The small catalyst loading and the fact of being impregnated in Nafion, prevented the use of XRD or XPS to verify compositional and structural changes after the experiments, as discussed in other works [25].Electrochemical data (overpotential values; n\u00a0=\u00a05 per each material) was submitted to statistical analysis. Data normality and homoscedasticity were previously tested using Shapiro-Wilk and the Spearman tests (p\u00a0<\u00a00.05), respectively. A two-way analysis of variance was then used to compare the statistical OER differences among synthetized materials, considering the factors \u201ctreatment type\u201d and \u201cNi/Fe ratio\u201d, followed by the Tukey\u2019s multiple comparison test, whenever significant differences were observed (p\u00a0<\u00a00.05). The statistical analysis was performed with the software Prism version 8.Ni-Fe/CO3 LDH were synthesized by co-precipitation with a Ni/Fe ratio of 2, 3 and 4. Part of the powders were further subjected to a hydrothermal treatment (HT). As such, six samples were produced, three resulting directly from the synthesis, and named Ni/FeX-AsPrep, and three coming from the HT and named Ni/FeX-HT (where X\u00a0=\u00a0Ni/Fe ratio)The aspect of the prepared materials is observed by scanning electron microscope (SEM) (Fig. 1\n). The as-prepared samples present aggregates of poorly defined shapes, while the heat-treated samples display individual platelets with approximate hexagonal shape, as reported for several LDH-type materials [26\u201328]. More particularly, the sample Ni/Fe4-HT has the aspect of a desert rose, probably due to the higher content of Ni, as this morphology is characteristic of some Ni(OH)2 materials [15].The average particle size (Z-Ave) of the studied LDH dispersed in deionized water, which is presented in Table 1\n, varied between 242\u00a0nm and 758\u00a0nm. These values should be considered more as an order of magnitude than as the exact particle sizes, because the particles have platelet-like shape and not the spherical form required by the dynamic light scattering theory [29]. Moreover, the measured entities seem to have been mainly aggregates composed of smaller particles, appearing on SEM images with widths between 50\u00a0nm and 250\u00a0nm. Table1 also presents the zeta potential (ZP) of the particles. The values ranged between\u00a0+\u00a040 and\u00a0+\u00a050\u00a0mV, allowing for the stabilization of a dilute suspension of particles in deionized water due to electrostatic repulsions (concentration\u00a0<\u00a00.1\u00a0mg\u00a0mL\u22121), which is important for preparing well dispersed suspensions of LDH for the electrochemical measurements.The attenuated total reflectance infrared (ATR-FTIR) measurements (Fig. 2\n) confirm the presence of CO3\n2\u2013 in the LDH samples, with a strong absorption band at 1350\u00a0cm\u22121. The large band around 3500\u00a0cm\u22121 corresponds to O-H stretching and the one at 1630\u00a0cm\u22121 to the H-O-H deformation, showing qualitatively the hydration of the LDH samples. Bands under 1000\u00a0cm\u22121 correspond to bonds between oxygen and metallic atoms forming the hydroxide layers [30].The results of the chemical analysis of the dissolved materials through atomic absorption spectroscopy (AAS) are displayed in Table2\n. The molar ratios of the synthesized materials correspond to the expected ones, except for the sample Ni/Fe2-HT. The solution of Ni/Fe2-HT contained some undissolved particles remaining, corresponding to an oxide phase, which was hence not analyzed with the rest of the solution, leading to a lower amount of Fe measured than for Ni/Fe2-AsPrep. This observation proves that the LDH phase obtained in the case of the Ni/Fe2-HT has a higher Ni/Fe ratio than expected.\nFig. 3\n\na displays the X-ray diffraction patterns for the three compositions investigated in this work, both as-prepared and after hydrothermal treatment. The pattern is matched with the peaks corresponding to the rhombohedral structure of Iron-Nickel Carbonate Hydroxide Hydrate of formula Ni6Fe2(OH)16(CO3)\u00b74H2O (computed pattern JCPDS 01\u2013082\u20138040). The peaks (003), (006) and (009) are characteristic of the layered structure of the 3R LDH polytype, from which can be extracted the basal spacing of 7.7\u00a0\u00b1\u00a00.15\u00a0\u00c5, typical of a carbonate-intercalated LDH [28]. The sample Ni/Fe2-HT has a pattern with more defined peaks in the region 2\u03b8\u00a0=\u00a035\u00b0 to 60\u00b0. This region corresponds to the reflections of the interlayer, which let us suppose that the molecules forming the interlayer are more ordered in this sample.For each sample, the average crystallite size in the a direction is estimated by the Scherrer equation using the full width at high maximum of the (110) peak, determined by fitting together (110) and (113), often overlapped. The obtained crystallite sizes are ranging between 8 and 33\u00a0nm (Fig. 3b). It is worth to note that these values are a low-end estimate of the actual crystallite sizes, as the broadening of the diffraction peak can be due to local imperfections in the lattice such as strain and chemical heterogeneities. As expected, the crystallite size increased after hydrothermal treatment, for all the Ni-Fe LDH compositions considered in this work. Moreover, bigger crystallite sizes were obtained for samples with lower Ni/Fe ratio, which suggest a link between the amount of MIII cations in the structure and the propensity of the hydroxide layers to stack more regularly. In the case of well-defined hexagonal platelets for Ni/Fe2-HT and Ni/Fe3-HT visible on SEM pictures, the actual crystal sizes correspond to the width of the platelets, which is ranging between 80 and 120\u00a0nm, meaning 3 to 15 times the estimated crystallite sizes. Hence, the values estimated through the Scherrer equation are not to be considered as the actual size of the LDH crystal but as a parameter to assess the quality of the long-range order of the structure.To explore further the crystallinity of the samples, the profile matching of the diffraction patterns was performed using the FullProf software using a R\u20133\u00a0m symmetry to extract the cell parameters. Both parameters a and c were found to increase with the Ni/Fe ratio (Fig. 3c). This is consistent with the size of the cations, as the bivalent nickel is 0.08\u00a0\u00c5 bigger than the trivalent iron, according to the Shannon tables [31]. The c parameter increases with the Ni/Fe ratio, which is also consistent with the fact that the layers are less positively charged and hence, the attraction between layer and interlayer is smaller, leading to a less \u201ccompact\u201d stacking.The XRD pattern of the sample Ni/Fe2-HT presents additional peaks which do not correspond to the Ni-Fe carbonate hydroxide structure (Fig. 3a). Such pattern has also been observed in other works synthesizing Ni-Fe LDH by hydrothermal technique [24,32] and were assigned to the spinel structure of the NiFe2O4 phase (JCPDS: 00\u2013054-0964). The hydrothermal treatment in autoclave, carried out with the aim to enhance the growth of the LDH particles, promoted the segregation of an iron-rich phase. A similar work, where the material is thermally treated under lower conditions of temperature (50\u00a0\u00b0C) and pressure does not present the NiFe2O4 phase [8], which shows that this segregation is due to the higher temperature applied to the sample. It is worth of mention that the spinel structure is consistent with the presence of insoluble particles found when dissolving the Ni/Fe2-HT sample for AAS measurements was attempted (cf. Table 2).Transmission electron microscopy (TEM) was used to more clearly identify some features evidenced in SEM and XRD analyses. TEM images depicted in Fig. 4\n reveal that the obtained Ni-Fe LDHs present a plate-like morphology, with some hexagonal shapes being identified, especially for samples subjected to hydrothermal treatment (Ni/Fe2-HT, Ni/Fe3-HT, Ni/Fe4-HT). Moreover, particle size of individual particles increases after hydrothermal treatment.The sample Ni/Fe2-HT, which revealed a secondary phase by XRD (recall Fig. 3), actually presents a few particles which have different shape and density (highlighted with an orange circle in Fig. 4). This could be associated with the NiFe2O4 formed after hydrothermal treatment. However, it must be mentioned that due to the overlapping of these particles with LDH particles, diffraction analysis could not be performed to unambiguously identify this secondary phase.Cyclic voltammograms (CV) obtained for the neat glassy carbon electrode, the RuO2 benchmark and the LDH samples are displayed in Fig. 5\n after ohmic drop correction of the data. The glassy carbon substrate presents no current in the studied potential range. The electrochemical procedure was first validated by experiments using commercial RuO2 powder as a benchmark. Working electrodes were prepared following the procedure reported by Jung et al. with commercial RuO2\n[22], with a catalyst loading of 800\u00a0\u00b5g\u00a0cm\u22122. We obtained a similar Tafel slope (67\u00a0mVdec-1) and an overpotential of 290\u00a0\u00b1\u00a011\u00a0mV, lower than the reference value (the overpotential at 10\u00a0mA\u00a0cm\u22122 was 380\u00a0\u00b1\u00a020\u00a0mV and the Tafel slope was 65\u00a0mVdec-1) [22]. Following these tests, the LDH loading was reduced to 28\u00a0\u00b5g\u00a0cm\u22122 in order to obtain a stable film, not possible with the 800\u00a0\u00b5g\u00a0cm\u22122 loading.The CVs of the different prepared electrodes (Fig. 5) exhibit an increasing catalytic current for potentials more positive than 1.5\u00a0V vs. RHE, corresponding to the onset of the OER. In comparison, the electrode prepared with RuO2 showed a catalytic current lower than the LDH samples, leading to an overpotential of 500\u00a0mV at 10\u00a0mA\u00a0cm\u22122. The curves of the LDH are very similar to one another, however, the current slope corresponding to the OER was higher for AsPrep samples, leading to lower overpotentials at 10\u00a0mA\u00a0cm\u22122 compared to the hydrothermally treated samples. In the case of LDH with a Ni/Fe ratio of 4, an additional wave appeared at 1.49\u00a0V (Ni/Fe4\u2013AsPrep) and 1.52\u00a0V (Ni/Fe4-HT), which was attributed to the NiII/NiIII oxidation [8]. In samples with lower Ni/Fe ratio, this feature is less visible due to the lower Ni content and to its overlapping with the OER peak. Confirming the change of the Ni state at this potential, a change of color from light to dark brown was detected, more perceptible in the materials with higher Ni. In the reverse scan, the NiIII/NiII reduction peak appeared in each Ni/Fe LDH sample, from 1.43\u00a0V to 1.40\u00a0V, in the order Ni/Fe2-HT\u00a0>\u00a0Ni/Fe2-AsPrep\u00a0>\u00a0Ni/Fe3-HT\u00a0>\u00a0Ni/Fe3-AsPrep\u00a0>\u00a0Ni/Fe4-HT\u00a0>\u00a0Ni/Fe4\u2013AsPrep. This order may illustrate the availability of Ni atom in each LDH for changing its oxidation state upon polarization.In the E vs. log(i) plots of the cyclic voltammetry data obtained for each of our samples (Fig. 6\n\na), different regions are visible. The main linear region appears at low currents (0.01 to 1\u00a0mA\u00a0cm\u22122) and is identified with red lines in the graph. It is attributed to the oxidation of NiII to NiIII, with a low Tafel slope, between 20 and 30\u00a0mVdec-1, highlighting the fast reaction kinetics. To confirm that OER was taking place at higher potentials, the voltammograms were repeated with an O2 micro-sensor close to the electrode surface for detecting the potential at which O2 was formed. For these experiments the glassy carbon electrode did not rotate and was facing up. The micro-sensor was a 10\u00a0\u03bcm platinum disk polarized at a potential were O2 is reduced with limiting control (in this experiment \u22120.8 to \u22121V\nvs\nSCE). The idea is that the current measured by the microelectrode will be constant and proportional to the concentration of dissolved O2 in the bulk solution but should immediately increase as soon as O2 starts to be generated in the glassy carbon electrode with catalyst. The result is presented in Fig. 6 b) and shows the superposition of the current measured in the GC electrode with LDH catalyst and the current measured by the O2 sensing microelectrode. The\u00a0\u00d7\u00a0axis depicts the potential at the GC. The O2 reduction current measured at the microelectrode is negligible until the GC potential reaches 0.5\u00a0V\nvs\nSCE. Then, for more positive potentials, the current increases rapidly until the saturation of the measuring device (1nA) is reached. This experiment was performed with all LDH samples and confirmed that O2 was produced at potentials higher than the linear region of the Tafel plot. In these plots, the region corresponding to the OER does not present a Tafelian behavior, making difficult its determination with confidence. This supports the suggestion of McCrory et al. about the possible change of reaction mechanisms with change of potential [23]. Based on these findings it was decided to not use in this work the Tafel slope as a figure of merit to assess the efficiency of the catalyst.\nFig. 6\nThe overpotential values measured on the ohmic drop-corrected plots at 10\u00a0mA\u00a0cm\u22122 are reported in Fig. 7\n\na. Each electrode was also submitted to a stability test to observe the evolution of the overpotential while applying a current of 10\u00a0mA\u00a0cm\u22122 for two hours (Fig. 7b). No important degradation of the electrodes has been observed as the increase of the overpotential was kept under 50\u00a0mV for all electrodes. The mean overpotentials varied between 330\u00a0mV and 410\u00a0mV. These values are higher than the reported for the best materials [4,20,21], but it is important to note that in this study a low catalyst loading was used and no conductive carbon material was added to the ink. Furthermore, the aim of this paper is to compare samples in the same conditions, as the comparison with materials from other publications using the overpotential is not accurate, owing to the number of experimental parameters that can influence the results. Qualitatively, two main findings can be highlighted: (i) the overpotential decreased in the AsPrep samples with increasing Ni/Fe ratio and (ii) the overpotential of the Ni/FeX\u2013HT samples was larger for any of three Ni/Fe ratios surveyed, which seems to indicate that the higher crystallinity due to the hydrothermal treatment leads to lower efficiency.\nFig. 7\nThe observations performed in the previous paragraph are based on the comparison of the mean values of the figure of merit chosen to describe the efficiency of the electrocatalysts (cf. Fig. 7a). An analysis of variance (ANOVA) of the results of the overpotential was performed using the 2-way ANOVA test to evaluate the effect of the hydrothermal treatment and of the Ni/Fe ratio. To the best of our knowledge, no study on NiFe LDH for OER catalysis takes into consideration the importance of samples replication and the data variability. In the present study, five replicates of electrode for each LDH were tested, from which we extract the overpotential value. The assumptions of data normality and homoscedasticity were successfully verified through Shapiro-Wilk and Spearman\u2019s test, respectively, before performing the 2-way ANOVA tests. The ANOVA statistical analysis highlighted the effective impact on the thermal treatment on the catalyst efficiency: a hydrothermally treated LDH sample is more crystalline and present a significantly higher overpotential than its non-treated counterpart (F(1,24)\u00a0=\u00a022.48, p\u00a0<\u00a00.0001), while the Ni/Fe ratio caused no statistically relevant differences in measured OER values (F(2,24)\u00a0=\u00a02.29, p\u00a0=\u00a00.1225) (pls. cf. Table S1).The Tukey\u2019s multiple comparison test showed that there is difference in the overpotential values measured between samples of Ni/Fe4 LDH AsPrep vs. HT (pls. cf. Table S1). No significant overpotential differences were found in the comparison AsPrep vs. HT for the other two ratios, neither between samples with similar hydrothermal treatment and different ratios (pls. cf. Table S1).This study raises awareness about results reported in the literature taking conclusion on the impact of metal ratios or crystallinity of the sample without mentioning repeatability of the electrochemical measurements. The preparation of the working electrode can have as much impact as the difference of Ni/Fe ratio in the ranges studied in this work.Firstly, the electrochemical characterization presented in section 3 highlights that the layered double hydroxides prepared in this work present a lower overpotential than the RuO2 reference material, justifying once again the efforts in better understanding and developing NiFe electrocatalyst LDHs.Then, when comparing samples within AsPrep or HT group separately, we observe that the increase of Ni/Fe ratio implies an increase in the cell parameters a and c linked with a decrease of the positive charge of the LDH layer due to less divalent Fe atoms. Many references [8,9,13], report a better efficiency for lower Ni/Fe ratios (1.5, 2 and 0.8 respectively), as if a higher concentration of Ni would be unfavorable for the catalyst efficiency. However, from the results of this study, the changes in composition and structure do not introduce significant electrocatalytic differences.On the other hand, the statistical analysis of the electrochemical data shows that using a more crystalline material on the electrode results in a lower electrocatalytic performance. In our work, it was seen for NiFe4-AsPrep vs. NiFe4-HT but less clearly for lower Ni/Fe ratios. These results are consistent with a work reported by Xu et al. in 2015, which shows, through the comparison of overpotentials, that a LDH material with a Ni/Fe ratio of 3 presents lower efficiency when it is hydrothermally treated, mentioning that lower crystallinity provides less confined active sites [20]. However, the discussion does not make the distinction between the atoms ordering and the size of the LDH particles inducing a higher surface area, which could be the cause for the improvement. Another work, from Gao et al. also concludes that the amorphous nature of the material makes it more flexible and hence more stable over time to electrochemical processes in comparison to crystalline materials [16], although their material is preconditioned for a long time to achieve the higher catalytic efficiency, which eventually may lead to a rearrangement of the material in more stable phases. The interest of defects for electrocatalysis is also discussed in the work of Trotochaud et al., in 2014, which mentions that the increase of efficiency of a \u03b2-(NiOH)2 is due to the inclusion of Fe impurities, and not to the more ordered structure than in the \u03b1-(NiOH)2\n[19]. They conclude that the long-range order in the material seems unimportant. G\u00f6rlin et al. discussed this point in a work on nanosized Ni-Fe oxyhydroxides catalysts of different metallic ratios and highlighted that the higher OER efficiency of the catalyst is related to the distortion of matrix around the metallic active sites [13]. Hence, from the results obtained and what is seen in the literature, it seems that more disordered material is more efficient, in terms of overpotential.Mostly, the present study reinforces that the importance of the LDH phase in the NiFe mixed hydroxide lies in the fact that this structure allows a \u201cmeta-stable\u201d phase of the hydroxide, preventing phase segregation, but the long-range order or the platelet-like morphology does not seem necessary and would even be detrimental for the electrochemical efficiency of the material. In parallel, the effect of the metallic cation ratio is not clear. More fundamental studies would help determine if the discussion of the influence of the Ni/Fe ratio is overrated as a single factor or if it is a combination of the presence of Fe with a distorted \u03b2-Ni2(OH) structure that can be the right direction for the development of better NiFe-based electrocatalysts.From an application-based point of view, the statement that the hydrothermal treatment is detrimental for the catalytic efficiency is interesting as it allows to remove a time and energy-intensive step from the catalyst production process.On a more general note, there are several recent works in the literature reporting promising eletrocatalysts for different reactions [33\u201335]. The strategy followed in this work could be extended in a general way to the design of similar materials.This work exposes views on the currently highly studied NiFe layered double hydroxides for efficient catalysis of the OER. Both the Ni/Fe ratio and the crystallinity of the synthesized material are investigated to highlight their influence on the electrocatalytic activity of the Ni/Fe LDH. No evidence was found for any impact of the Ni/Fe ratio in the efficiency of the OER but the hydrothermal treatment performed to obtain a higher crystallinity of the catalysts leads to a decrease of their efficiency. In conclusion, this work renders insights on the structure and Ni/Fe ratio, which are relevant for the design of NiFe LDHs. Future works will involve the use of other catalyst supports to increase the loading of catalyst and test the material closer to the application conditions, namely using larger current densities.\nData availability\nThe raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.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.Thanks are due to FCT/MCTES for the financial support to CICECO-Aveiro Institute of Materials (UIDB/50011/2020; UIDP/50011/2020) and CESAM (UIDP/50017/2020\u00a0+\u00a0UIDB/50017/2020), through national funds. We thank also the European Commission funding the project NANOBARRIER (Reference N\u00b0280759) through the programme FP7-NMP. This study was also carried out in the framework of the NANOGREEN R&D project (CIRCNA/BRB/0291/2019) funded by national funds (OE), through FCT. Roberto Martins and Alexandre Bastos funded by national funds (OE), through FCT \u2013 Funda\u00e7\u00e3o para a Ci\u00eancia e a Tecnologia, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of the article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of July 19 (CEECIND/01329/2017), and in frame of SMARTAQUA project, which is funded by the Foundation for Science and Technology in Portugal (FCT), the Research Council of Norway (RCN), Malta Council for Science and Technology (MCST), and co-funded by European Union\u2019s Horizon 2020 research and innovation program under the framework of ERA-NET Cofund MarTERA (Maritime and Marine Technologies for a new Era).Supplementary data to this article can be found online at https://doi.org/10.1016/j.matdes.2021.110188.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The Oxygen Evolution Reaction (OER), half-reaction of the water-splitting process for hydrogen production, suffers from sluggish kinetics. NiFe materials appeared as interesting catalytic materials for this reaction in alkaline electrolyzers and have been studied, particularly in the form of NiFe Layered Double Hydroxides (LDH). However, the impact of the specificity of the atomic arrangement in the LDH and of its composition on the catalytic efficiency of the material are still unknown. Herein, LDH are synthesized with Ni/Fe ratios from 2 to 4 and different levels of crystallinity to assess their electrocatalytic behavior in 0.1\u00a0M KOH. Statistical analysis of the electrochemical results allows to highlight that, while no effect from the atomic ratio is observed, an increase in the crystallinity of the LDH seem detrimental to the catalytic efficiency.\n               "}
{"full_text": "Benzophenone reduction is a common process in chemical industry. Its main product, diphenylmethanol (also known as benzhydrol), has an important role in perfume and pharmaceutical manufacturing. Many different benzophenone reduction methods have already been reported such as photoelectrochemical method [1], electrocatalysis [2,3], simple reduction using reductive compounds [4\u20136], or catalytic hydrogenation [7\u201315]. From these methods, catalysis seems to be the most promising because of its lower chemical need, high conversion, and good selectivity. Furthermore, according to the paper of Cirtiu et\u00a0al. [2], catalytic hydrogenation is 10 times faster than electrocatalysis; thus, its investigation seems to be the most beneficial. Depending on the reaction conditions, over-reduction of benzophenone to diphenylmethane is possible (Fig.\u00a01\n).Carbon-based catalysts are often used in heterogeneous catalysis due to their high specific surface area, good adsorption, and chemical stability [16\u201318]. Activated carbon might be the most used carbon material (e.g. in granulated form under the trademark name of Norit) in catalysis, but carbon nanotubes also seem to be promising and available because of their constantly decreasing price. Carbon-supported catalysts have already been tested in many different hydrogenation reactions with high conversion and selectivity [19\u201325]. However, many publications evidence that carbon nanotubes in other hydrogenation reactions can be even better support than the commonly used activated carbons [26\u201328]. According to other researchers, the presence of covalently bonded nitrogen renders catalysts to be selectively inhibited to produce an intermediate or a product with high selectivity [29,30]. Based on this observation, one can expect N-doped carbon nanotubes to be selective in hydrogenation processes. In this paper we compare the efficiency and selectivity of palladium-decorated activated carbon (Norit) and carbon nanotubes in benzophenone hydrogenation.For catalyst synthesis anhydrous palladium(II) acetate (Fluorochem Ltd.)\u00a0were used. The hydrogenation tests were carried out using benzophenone (99%, Alfa Aesar), hydrogen (4.5, Messer), and tetrahydrofuran (\u226599%, VWR Chemicals). As catalyst support, activated charcoal Norit (Norit RBAA-3, rod, Sigma-Aldrich) was used in grinded, powder form. For GC (gas chromatography) calibration the following compounds were used: benzophenone (99%, Alfa Aesar), benzhydrol (99%, Acros Organics), dicyclohexyl ketone (98%, Sigma-Aldrich), dicyclohexylmethanol (98%, Alfa Aesar), diphenylmethane (\u226599%, Alfa Aesar), cyclohexyl phenyl ketone (98%, Sigma-Aldrich), and cyclohexyl(phenyl)methanol (99%, Sigma-Aldrich). Reaction samples for gas chromatography coupled with mass spectrometry (GC-MS) analysis were diluted using methanol (99.8%, GC grade, Merck). Acetophenone (\u226599%, Sigma-Aldrich) was used as an internal standard for GC-MS analysis.Nitrogen-doped BCNTs, which was used for the Pd containing catalyst preparation, were produced by the catalytic chemical vapor deposition (CCVD) method. MgO with 5\u00a0wt% nickel was used as catalyst in CCVD synthesis at 973\u00a0K. The carbon source (butylamine) was fed into the reactor by a syringe pump (6\u00a0mL\u00a0h\u22121), where it vaporized and was carried into the catalyst bed with nitrogen gas (100\u00a0mL\u00a0min\u22121). Because the carbon source is a nitrogen containing substance, this method results in BCNTs. The nickel was removed from the nanotubes by hydrochloric acid. The purity of BCNTs was checked by using of thermogravimetric analysis, the carbon content was 95.9\u00a0wt%.The carbon-based material (Norit or BCNT, 1.9\u00a0g) was dispersed in distilled water by a Hielscher Ultrasound tip homogenizer. Into the dispersion, aqueous solution of Pd(OAc)2 (211\u00a0mg in 10\u00a0mL water) was added, and the mixture was sonicated for 10\u00a0min. After evaporating the water using a vacuum rotary evaporator, the residue was dried at 378\u00a0K overnight. The catalysts were activated in a hydrogen flow at 673\u00a0K, 30\u00a0min. The efficiency of the reduction was checked by high resolution transmission electron microscopy (HRTEM) and X-ray diffractometry (XRD). The theoretical metal content was 5\u00a0wt%, which was checked and supported by inductively coupled plasma - optical emission spectrometry (ICP-OES).The morphology and size of the noble metal particles on the support surface were examined by HRTEM (FEI Tecnai G2) operating at an accelerating voltage of 20\u00a0kV. The sample preparation was carried out by dropping from the aqueous suspension onto 300 Mesh Cu grid (Ted Pella Inc.) The size of the nanoparticles was measured by using the ImageJ software. Based on the size of the scalebars of the HRTEM images, 100 nanoparticles were measured randomly. The size distribution histograms were created by OriginPro 8.The confirmation of metallic phases of the noble metal was checked by a Bruker Advance D8 X-ray diffractometer (Cu-K\u03b1 source, 40\u00a0kV and 40\u00a0mA generator settings), in parallel beam geometry (G\u00f6bel mirror) and with Vantec1 detector.To measure the surface area of the catalyst, Brunauer\u2013Emmett\u2013Teller (BET) method was used. The nitrogen adsorption measurements were carried out using a TriStar 3000 type instrument on 77\u00a0K.The nitrogen content of the carbon supports was measured by Vario Macro CHNS element analyzer. The certificated standard material was sulphanilamide (N: 16.25%, C: 41.81%, S18.62%, H: 4.65%, Elementar Analysensysteme GmbH). The carrier gas was helium (99.9990%), while oxygen (99.995%) was used for oxidation.The incorporated nitrogen forms were characterized by the X-ray photoelectron spectroscopy method with SPECS XPS equipped with a PHOIBOS 150 MCD analyzer (MgK\u03b1 and AlK\u03b1).The turn-over number (TON) was calculated using eq. (1), for both catalysts to compare their efficiency.\n\n(1)\n\n\nT\nO\nN\n=\n\n\n\nn\n\nb\ne\nn\nz\nh\ny\nd\nr\no\nl\n\n\n\nn\n\nP\nd\n\n\n\n\n\n\nwhere nbenzhydrol is the amount of the product (mol), while nPd stands for the amount of Pd (mol). The TON value is suitable to compare catalysts with different metal content because it correlates the amount of the product to a unit of catalytically active metal [31].The catalytic hydrogenation was carried out in a stainless steel reactor (200\u00a0mL total volume) equipped with a heating jacket (B\u00fcchi Uster Picoclave system). The load volume was 150\u00a0mL. The concentration of the benzophenone solution was 10\u00a0mM. Tetrahydrofuran was used as the solvent. The pressure of hydrogen was constant 20\u00a0bar during the tests. Catalyst loading was constant, 0.1\u00a0g in all tests. After starting the hydrogenation, samples were withdrawn from the reactor at 5, 10, 15, 20, 30, 40, 60, 80, 120, 180 and 240\u00a0min and analyzed using GC-MS technique. Conversion of benzophenone (X %) and selectivity for benzhydrol (S %) were calculated according to eqs. (2) and (3), respectively, where n is the amount of substance in moles. (The stoichiometric ratio is 1:1 in all the possible reduction steps.)\n\n(2)\n\n\nX\n\n%\n=\n\n\nn\n\nb\ne\nn\nz\no\np\nh\ne\nn\no\nn\ne\n,\n\nc\no\nn\ns\nu\nm\ne\nd\n\n\n\nn\n\nb\ne\nn\nz\no\np\nh\ne\nn\no\nn\ne\n,\n\ni\nn\ni\nt\ni\na\nl\n\n\n\n\n\u00d7\n\n100\n%\n\n\n\n\n\n\n(3)\n\n\nS\n\n%\n=\n\n\n\nn\n\nb\ne\nn\nz\nh\ny\nd\nr\no\nl\n\n\n\nn\n\np\nr\no\nd\nu\nc\nt\ns\n\n\n\n\n\n\u00d7\n\n100\n%\n\n\n\n\nThe GC-MS\u00a0is a suitable method for both qualitative and quantitative determination of the possible reaction products. The progress of the hydrogenation was followed by a Shimadzu GCMS-QP2020 mass spectrometer-coupled gas chromatograph. Chromatographic separation was performed using a Stabilwax-MS capillary column (30\u00a0m length\u00a0\u00d7\u00a00.25\u00a0mm i.d., 0.25\u00a0\u03bcm film thickness) from Restek Corp. The column temperature program was as follows: 130\u00a0\u00b0C (1\u00a0min), 130\u2013250\u00a0\u00b0C (10\u00a0\u00b0C\u00a0min\u22121), 250\u00a0\u00b0C (2\u00a0min). The injector and the detector were set at 250\u00a0\u00b0C and 230\u00a0\u00b0C, respectively. Helium was used as the carrier gas at 0.86\u00a0mL\u00a0min\u22121 column flow rate. The electron ionization mode was performed with 70\u00a0eV electron energy. MS cut time was 3\u00a0min. 1\u00a0\u03bcL of the sample was injected by a Shimadzu AOC-6000 autosampler. The split was set at 1:10. Before injection, 40\u00a0\u03bcL of the reaction mixture sample plus 100\u00a0\u03bcL of the internal standard solution (acetophenone in methanol) was diluted to 1000\u00a0\u03bcL with methanol.The nitrogen content of the two carbon structures was measured by the CHNS (carbon, hydrogen, nitrogen, sulfur) method. According to the results, the BCNTs contained 6.19% of nitrogen, whereas the Norit had only 0.54% of nitrogen. This large difference had a significant effect on the outcome of the hydrogenation reactions [32].The metal content of the catalysts was checked by the ICP (inductively coupled plasma) method. In case of the Norit-based catalyst, 2.94 w/w%, meanwhile in case of the BCNT-based 3.53 w/w% Pd content were determined.According to the specific surface area measurements, the BET surface areas of 5\u00a0wt% Pd/Norit and 5\u00a0wt% Pd/BCNT were 487.47\u00a0m2/g and 146.62\u00a0m2/g, respectively.The confirmation of metallic phases of the noble metal was checked by the XRD method. The diffractogram of the Pd/Norit catalyst shows reflexions of (111), (200), and (220) phases at 39.9\u00b0, 46.5\u00b0, and 68.0\u00b0 two theta degrees, respectively, confirming the presence of the elemental Pd phase (Fig.\u00a02\n A). The C(002) and C(010) phases are also present in the XRD pattern. On the XRD pattern of the Pd/BCNT catalyst, the reflexion peaks of the elemental Pd are visible (Fig.\u00a02 D). Characteristic reflections of carbon (002, 010) were also found. Moreover, the Ni(011) peak is present at 44.8\u00b0 two theta degree. This can be explained by the synthesis method of the carbon nanotube, during which nickel as a catalyst metal was used to grow the BCNTs. These nickel nanoparticles are closed inside of the carbon nanotubes; thus they are not removable by chemical purification (SI Fig.\u00a01). In this sense, the nickel particles are not available for catalysis.The morphology of the catalysts was examined by HRTEM. In the HRTEM picture of the Pd-Norit catalyst, Pd nanoparticles with small size and homogenous dispersibility on the surface of the activated carbon layer can be seen (Fig.\u00a02 B). According to the size measurements, most of the nanoparticles are in the range of 3\u201310\u00a0nm. The average size is 5.3\u00a0\u00b1\u00a03.2\u00a0nm (Fig.\u00a02 C). In case of the Pd-BCNT the average size of Pd particles was 8.1\u00a0\u00b1\u00a01.8\u00a0nm (Fig.\u00a02 D).The particle size is influenced by the force of interaction between the adsorbent and the adsorbed metal and their ions. More adsorption interaction mechanisms play role at the same time during the anchoring of catalytically active metals and their ions. Namely ion-exchange, electrostatic interaction, complexation, and physical adsorption provide the adsorption of metal ions on CNT\u00a0surfaces. N-BCNTs have more defect sites than their single-walled or multiwalled counterparts, and because ofthe incorporated nitrogen atoms\u00a0they also have special adsorption points which are excellent spots for catalytically active metal particles. The N-doped carbon nanotubes are easier to oxidize than their non-doped counterparts owing their structure; thus on the surface of N-CNTs many oxygen contained surface groups are located, which are formed during the synthesis. The mentioned functional groups mainly hydroxyl\u00a0and carboxyl groups, which play role in the metal ions adsorption. Moreover, the adsorption mechanism is also affected by the complex formation between palladium and the incorporated nitrogen atoms in case of N-BCNT. The nitrogen incorporation weakens the \u03c0\u2013\u03c0 interaction between the neighboring carbon atoms; thus the metal ions (palladium) can establish donor\u2013acceptor interaction with the \u03c0-system of N-BCNTs. These stable adsorption interactions help to avoid the excessive crystal growth; thereby small nanoparticles are formed.The carbon nanotube\u2013based catalyst was also examined by the XPS method (Supplementary Information I).Compositions of the reaction mixtures after a given reaction time at different temperatures are shown in Fig.\u00a03\n. Besides benzophenone, benzhydrol, and diphenylmethane, other possible reaction products and intermediates, namely dicyclohexyl ketone, dicyclohexylmethanol, cycohexyl(phenyl)methanol, and cyclohexyl phenyl ketone, were produced usually either in trace amount, or their sum of yields was below 0.5%. At 283 and 293\u00a0K, conversion of benzophenone was faster using the BCNT than using the Norit-supported catalyst. This trend changed at 323\u00a0K, while at 313\u00a0K the speed of conversion was roughly equal with the two catalysts. As it is concluded later on from the Arrhenius plots, the conversion rate for the Norit-supported catalyst is more temperature dependent than that of the BCNT-supported catalyst. With Norit, depending on the temperature, 14.3\u201398.4% of the benzophenone was converted in 240\u00a0min. Using BCNTs, the conversion was in the range of 20.8\u201396.3% (Fig.\u00a04\n). Regarding the product distribution, there is a marked difference in the selectivity of the two types of support. While BCNT is highly selective (98.5\u201399.3%) toward\u00a0the formation of benzhydrol at all the studied temperatures, only 38.0\u201376.6% selectivity for benzhydrol could be achieved using the Norit-based catalyst (Fig.\u00a04). With BCNTs, the concentration of benzhydrol is continuously increasing throughout the studied time period (240\u00a0min) independent of the reaction temperature. In case of Norit, the same trend can be seen at lower temperatures (283\u00a0K and 293\u00a0K), while at 313\u00a0K and 323\u00a0K it reaches a maximum at about 180\u00a0min and 80\u00a0min, respectively. Diphenylmethane concentration remained low for the BCNT runs (0.9\u20131.5% selectivity). Using Norit, it was much higher (23.4\u201362.0% selectivity).Kinetics of the reactions was also studied. Plots of benzophenone concentration over time showed first-order kinetics (Fig.\u00a05\n). The reaction rate constants (k) were determined by applying non-linear regression using eq. (4).\n\n(4)\n\n\n\nc\n\nb\ne\nn\nz\no\np\nh\ne\nn\no\nn\ne\n\n\n=\n\nc\n\nb\ne\nn\nz\no\np\nh\ne\nn\no\nn\ne\n,\n\ni\nn\ni\nt\ni\na\nl\n\n\n\u00d7\n\ne\n\n\u2212\nk\n\u00d7\nt\n\n\n\n\n\n\nThe results are summarized in Table\u00a01\n.The lower rate constants observed at 283 and 293\u00a0K for Norit becomes higher as reaction temperature is increased to 313 and 323\u00a0K. Clearly, the rate constant for Norit exhibits steeper temperature dependence, than BCNT. Activation energies for both catalysts were determined based on the Arrhenius plots (Fig.\u00a06\n). Using linear regression, activation energies of 64.0\u00a0\u00b1\u00a03.0\u00a0kJ/mol for Pd/Norit and 45.2\u00a0\u00b1\u00a03.6\u00a0kJ/mol for Pd/BCNT catalysts were calculated. R\n\n2\n values were 0.995 and 0.987, respectively.The TON\u00a0was calculated using eq. (1), for both catalysts to compare their efficiency (Table\u00a02\n). As it can be seen the TON values of the BCNT catalyst is continuously increasing by the temperature. However, in case of the Norit catalyst the TON values start decreasing at around 313\u00a0K. This phenomenon is caused by the over-hydrogenation effect of the Norit catalyst, as it started to produce increased amount of diphenylmethane (only 38% benzhydrol selectivity at 323\u00a0K). Furthermore, despite the lower reaction rate of the BCNT-based catalyst than the Norit, it still produced more benzhydrol.In this research, two Pd-contained carbon-based catalysts were characterized and used in benzophenone hydrogenation. Both the Norit (activated carbon) and the nitrogen-doped carbon nanotube based catalysts were tested on four different temperatures for kinetic calculations. At lower temperatures (283\u00a0K and 293\u00a0K) the BCNT and at higher temperatures (313\u00a0K and 323\u00a0K) the Norit-based catalyst had higher reaction rate. At the highest temperature applied (323\u00a0K) both catalysts provided high benzophenone conversion (BCNT: 96.3%, Norit: 98.4%). However, in case of the Norit the selectivity was relatively low (76.6%), while using BCNT the result achieved was excellent (99.3%). The high benzhydrol selectivity might be explained by the presence of covalently bonded nitrogen atoms in the catalyst (BCNT: 6.19 w/w%, Norit 0.54 w/w%) that can inhibit the over-hydrogenation process; thereby BCNTs are better catalyst supports for benzhydrol production than the commonly used activated carbon supported catalysts. Furthermore, in spite of the approximately three times lower BET surface are of the BCNT-based catalyst than the Norit, the BCNT-based catalyst seemed to be more efficient than the commonly used activated carbon\u2013based one.This research was supported by the European Union and the Hungarian State, co-financed by the European Regional Development Fund in the framework of the GINOP-2.3.4-15-2016-00004 project, aimed to promote the cooperation between the higher education and the industry.\n\u00c1. Prekob: Conceptualization, Investigation, Writing - original draft. L. Vanyorek: Conceptualization, Supervision, Writing - review & editing. Z. Fejes: Conceptualization, Validation, 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.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.2020.100409.", "descript": "\n                  Catalytic activity of palladium catalysts with two different types of carbon support, Norit (an activated carbon), and bamboo-shaped carbon nanotubes (BCNT) have been tested for benzophenone hydrogenation. The selectivity toward\u00a0the two possible reaction products (benzhydrol and diphenylmethane) can be directed by the catalyst support. It has been found that the Norit support preferred the over-hydrogenation of benzhydrol to diphenylmethane. The BCNT support proved to be much more selective and resulted as much as 99.3% benzhydrol selectivity at 96.3% benzophenone conversion. The high benzhydrol selectivity might be explained by the presence of covalently bonded nitrogen atoms in the catalyst (BCNT: 6.19 w/w%, Norit 0.54 w/w%) that can inhibit the over-hydrogenation process, thereby BCNTs are better catalyst supports for benzhydrol production than the commonly used activated carbon\u2013supported catalysts.\n               "}
{"full_text": "Hydrogen generation from photoelectrochemical (PEC) water-splitting provides a promising route to store the intermittent solar energy, and serves as a sustainable and environmentally-benign alternative to the existing hydrogen production technologies. The essential component of these PEC water-splitting systems is the electrocatalyst that efficiently expedites the kinetics of electrode reactions [1], including oxygen evolution reaction (OER) at anode and hydrogen evolution reaction (HER) at cathode. The state-of-art electrocatalysts are based on noble metals, such as Pt for HER [2], and IrOx for OER [3], but their scarcity and high cost limit their practical application on a global scale. Consequently, great efforts have been dedicated to developing highly efficient and robust electrocatalysts, which are low-cost and can be prepared and integrated into the PEC water-splitting systems by simple and scalable methodologies. For example, Earth-abundant MoS2 [4,5], nickel-iron hydroxides [6,7], metal phosphides [8,9], cobalt phosphate [10], calcium iron oxides [11] can be prepared and integrated onto the photoelectrodes for PEC water-splitting simply by solution based approaches, such as (photo-)electrodeposition or spin-coating.Hydrogen generation can also be assisted from the reforming of renewable organics and chemical waste [12\u201320]. In particular, hydrogen generation from photocatalytic reforming of plastics, such as polyethylene terephthalate (PET), synthesized by condensation polymerization of ethylene glycol (EG) with terephthalic acid (TA), provides several distinct advantages over water-splitting, including less energy required for hydrogen generation, and mitigation of the environmental threat posed by plastics waste. To date, several electrocatalytic and photo-(electro-)catalytic systems have been developed for the depolymerisation or reforming of plastics into hydrogen. For example, Hori et al. reported an electrocatalytic system, consisting of Pt/C anode and Pt/C cathode, for conversion of plastic waste into hydrogen at 200 \u00b0C [16]. Jiang et al. developed solar thermo-coupled electrochemical process using nickel as the electrocatalyst for the depolymerization of polypropylene into methane and hydrogen [17]. Ag2+ ions was found to be capable of mediating the electrochemical oxidation of aliphatic polymers [21]. On the other hand, efficient hydrogen production from reforming of plastics can also be achieved photocatalytically by using Pt modified TiO2 nanoparticles [18], toxic CdS/CdOx quantum dots [19], and nickel phosphide modified C3N4 [20] as photocatalysts. Nonetheless, these developed systems suffered from wide distribution of products from the oxidation of plastics [16\u201321]. The wide product distribution due to the low selectivity requires complicated separation steps and therefore imposes an additional cost for whole hydrogen generation process. Consequently, to establish an efficient and economically attractive PEC or photocatlytic platforms for hydrogen generation from reforming of plastics, the development of the electrocatalysts that can efficiently catalyze plastics oxidation with high selectivity towards specific chemicals, especially for valuable platform chemicals (e.g., formic acid), is, therefore, of great importance.Nickel phosphides (NiPx) and their alloys have been discovered as efficient electrocatalysts for the electrochemical reactions of importance [22\u201331]. For example, their unique surface structure, consisting of both proton-accepting sites and hydride-accepting sites, promotes the catalysis of HER [22,28,29]. Besides, under appropriate anodic conditions, their surface can be in-situ converted into active nickel oxyhydroxide species responsible for OER [23,29] and electrocatalytic oxidation of organics [27,31]. To date, several synthetic strategies have been established for the preparation of NiPx, but most of them required costly, environmentally harmful, and energy-intensive conditions [22\u201327,29\u201331], which not only prevents the large-scale production of NiPx, but also impedes direct integration of Ni-P into the PEC devices for hydrogen generation.In the present contribution, we report on the facile electrosynthesis of nickel-phosphorous alloy nanoparticulate thin film (nanoNi-P) and its applications towards overall water-splitting and reforming of EG and PET plastics. Through the detailed investigation on the effects of electrosynthetic conditions, the factors influencing the chemical composition, surface morphology, and the resultant HER activity of nanoNi-P are elucidated. nanoNi-P and its composite with carbon nanotubes with high HER activity can be directly deposited onto the semi-conducting and conducting substrates with high surface roughness less than half minute at room temperature by our developed electrosynthetic approach. In addition, we also show that the prepared nanoNi-P is an excellent pre-catalyst not only for OER, but also for the selective oxidative conversion of EG and PET plastics into formic acid. It is the first time that high selectivity (\u223c100 %) towards of the oxidation of EG and PET have been realized using electrocatalyst solely made of Earth-abundant materials. Finally, efficient and selective generation of hydrogen and formic acid from PEC reforming of PET plastics was demonstrated using an Earth-abundant PEC platform based on nanoNi-P modified TiO2 nanorods photoanode and nanoNi-P based cathode. The production of formic acid and hydrogen from photoelectrocatalytic reforming of EG and PET at ambient conditions sets a sharp contrast to the conventional production of formic acid and hydrogen via an energy-intensive and high-pressure processes involving the use of fossil fuel-based reactants. This work also paves a path for developing artificial leaf for simultaneous environmental mitigation and photosynthesis of renewable fuels and valued chemicals.Nickel-phosphorous alloy nanoparticulate thin film modified electrodes (nanoNi-P) were prepared by electrochemical deposition in a deaerated plating solution containing NiCl2 (0.2 M), NaH2PO2, and NH4Cl (0.25 M) at a specific applied current density (Idep) for 27 s. The electrodeposition was carried out using a CHI 760E potentiostat (CH Instruments, Inc., USA) connected with a conventional three-electrode electrochemical cell with screen-printed carbon electrode (diameter: 3 mm; Zensor R&D, Taiwan) or carbon paper (TPG-H-60, Alfa Aesar) working electrode, Ag/AgCl (sat\u2019d KCl) reference electrode, and Pt foil (1 cm \u00d7 4 cm) counter electrode. Prior to the electrodeposition, carbon paper was cleaned sequentially in nitric acid (65 %, Honeywell Fluka\u2122), ethanol (99.5 %, ECHO), and de-ionized water (DIW) under ultrasonication for 10 min. The synthetic parameters, including Idep and the concentration of NaH2PO2 (Chypophosphite), were adjusted to tune the chemical composition, surface morphology, and the overall electrocatalytic acitivity of the resultant nanoNi-P.The nanocomposite of carbon nanotubes with Ni-P alloy nanospheres (CNT/nanoNi-P) was electrodeposited onto the SPCE and carbon paper (exposed surface area: \u223c1.0 cm2) in a deaerated plating solution containing NiCl2 (0.2 M), NaH2PO2 (0.04 M), NH4Cl (0.25 M), 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (0.5 mg mL\u22121), and multiwall carbon nanotubes (CNTs, 0.25 mg mL\u22121) at Idep of -20,000 \u03bcA cm-2 for 27 s. Note that the plating solution was subjected to ultrasonication at least for 30 min to disperse CNTs prior to the electrodeposition.TiO2 nanorod photoanode, designated as nanoTiO2, was prepared by the direct growing TiO2 nanorods onto the fluorine-doped tin oxide coated glass substrate (FTO) using hydrothermal method. Briefly, FTO substrates were cleaned at 70 \u00b0C for 30 min in an aqueous ammonia-hydrogen peroxide solution. Thereafter, the cleaned FTO substrate was placed inclined against the wall of a Teflon liner (volume: 23 mL) with its conducting side facing down. Thereafter, the Teflon liner was filled with a precursor solution containing 15.73 mL HCl (6.0 M) and 0.27 mL titanium (IV) isopropoxide, and was placed into a stainless steel autoclave. Finally, the sealed autoclave was heated in an oven at 150 \u00b0C for 3 h. After being rinsed with DIW and dried under N2 purge, the samples were annealed in air at 500 \u00b0C for 1 h.nanoNi-P modified nanoTiO2 photoanode, designated as nanoTiO2|nanoNi-P, was prepared by subjecting the prepared nanoTiO2 photoanode (exposed surface area: 6.0 cm2) to the electrodeposition process at Idep of -20,000 \u03bcA cm\u22122 for 1 or 27 s in a deaerated plating solution containing NiCl2 (0.2 M), NaH2PO2 (0.04 M), and NH4Cl (0.25 M).Characterization on surface morphology and film composition of the prepared modified electrodes were performed using characterized using scanning electron microscope (SEM, Hitachi SU-8010) equipped with energy-dispersive X-ray spectroscopy (EDS). High-resolution transmission electron microscopy (HRTEM) and EDS elemental mapping were performed using a JEM-2100F Transmission Electron Microscope (JEOL Ltd., Japan) to analyze the chemical composition, elemental distribution, and structure of CV-activated nanoNi-P particle and nanoTiO2|nanoNi-P, which were detached from electrode substrates under ultrasonication. A Horiba Jobin Yvon JY 2000\u22122 ICP optical emission spectrometer was used to quantify the amount of nickel species in the nanoNi-P, CNT/nanoNi-P, and nanoTiO2|nanoNi-P electrodes. X-ray photoelectron spectra (XPS) of the nanoNi-P modified electrodes were measured using a PHI 5000 VersaProbe X-ray spectrometer with Al X-ray beam as excitation source. The binding energies (BE) shown in the XPS were corrected by referencing the C 1s peak to 284.6 eV. Ni K-edge X-ray-absorption near-edge structure (XANES) and K-edge extended X-ray-absorption fine-structure (EXAFS) spectra of the nanoNi-P samples were measured at Taiwan Light Source (TLS) beamline 17C1 in National Synchrotron Radiation Research Center (NSRRC). Soft X-ray absorption spectra (sXAS) were recorded at TLS beamline 20A1 in NSRRC. Ni L-edge and O K-edge absorption spectra of the samples were measured in both total-electron-yield (TEY) and total-fluorescence-yield (TFY) modes. Raman spectra of nanoNi-P samples were measured using a Raman spectrometer (Protrustech Corporation Ltd.) with 532 nm laser with a light power of 300 mW.The activity of the nanoNi-P and CNT/nanoNi-P modified electrodes towards HER were analyzed utilizing a potentiostat (CHI 760, CH Instruments, Inc., USA) connected with a nitrogen-purged two-compartment, separated with an anion exchange membrane (AEM-025, Johnson Matthey, UK), three-electrode electrochemical cell with Hg/HgO (1 M NaOH) reference electrode and Pt foil (1 cm \u00d7 4 cm) counter electrode. IR drop was compensated for all the electrochemical experiments, and all potentials are referenced to the reversible hydrogen electrode (RHE) with Eq. (1):\n\n(1)\n\n\n\u2009\nE\n\n\nV\nv\ns\n.\nRHE\n\n\n=\n E\n\n\nV\nv\ns\n.\n Hg\n/\nHgO\n\n\n+\n0.140\n+\n0.059\n\u00d7\npH\n\n\n\nThe electrochemically active surface area (ECSA) of the prepared nanoNi-P modified electrodes was calculated by firstly determining the double-layer capacitance (Cdl) utilizing cyclic voltammetry, followed by dividing the determined Cdl value with Cdl of pure nickel thin film (\u223c16 \u03bcF cm\u22122) [32].Two kinds of turnover frequencies (TOF) were used to evaluate the activity of nanoNi-P modified electrodes, including one based on the loading amount of nickel species (TOFNi) and the other one based on surface sites (TOFECSA). TOFNi, determined by using Eq. (2), was used to evaluate the overall activity, whereas TOFECSA, determined by using Eq. (3), was used to access the intrinsic activity [22,33].\n\n(2)\n\n\nTO\n\nF\nNi\n\n=\n\nnumber of hydrogen turnover per geometric surface area\nnumber of loaded nickel atom per geometric surface\n\n\n\n\n\n\n\n(3)\n\n\nTO\n\nF\nECSA\n\n=\n\nnumber of hydrogen turnover per geometric surface area\nnumber of surface site per real surface area\n\n\n\n\n\nThe number of loaded nickel atom per geometric surface area (NNi) was determined by ICP-OES, whereas the number of hydrogen turnover per geometric surface area (#H2), corresponding to specific current density, and the number of the surface site per real surface area (#surface site) were calculated by using Eqs. (4) and (5), respectively:\n\n(4)\n\n\n\n#\n\n\nH\n2\n\n\n\n=\n(\nj\n\nmA\n\nc\n\nm\n2\n\n\n\n)\n\u00d7\n(\n\n\n1 C\n\ns\n-1\n\n\n1000 mA\n\n)\n\u00d7\n(\n\n\n1 mol\n\ne\n\u2212\n\n\n\n96485\n C\n\n\n)\n\u00d7\n(\n\n\n1 mol\n\nH\n2\n\n\n\n2 mol\n\ne\n\u2212\n\n\n\n)\n\u00d7\n(\n\n\n6\n.022\n\u00d7\n1\n\n0\n23\n\n\nH\n2\n\n molecules\n\n\n1 mol\n\nH\n2\n\n\n\n)\n=\n3.12\n\u00d7\n\n10\n15\n\n\n\n\nH\n2\n\n/s\n\n\nc\n\nm\n2\n\n\n\nper\n\nmA\n\nc\n\nm\n2\n\n\n\n\n\n\n\n\n\n(5)\n\n\n\n#\nsurface site\n\n=\n\n\n\n\n\n\n4\n atoms/unit cell\n\n\n43.76\n\u00d7\n\n10\n\n\u2212\n24\n\n\nc\n\nm\n3\n\n/unit cell\n\n\n\n\n\n\n\n2\n3\n\n\n\n=\n2.029\n\u00d7\n\n10\n15\n\natoms c\n\nm\nreal\n-2\n\n(\nmetallic nickel\n)\n\n\n\n\nSubsequently, a plot of current density (j) vs. overpotential (\u03b7) can be converted into that of TOF vs. \u03b7 by Eqs. (6) and (7):\n\n(6)\n\n\nTO\n\nF\nNi\n\n=\n\n\n(\n3.12\n\u00d7\n\n10\n15\n\n\n\n\nH\n2\n\n/s\n\n\nc\n\nm\n2\n\n\n\nper\n\nmA\n\nc\n\nm\n2\n\n\n\n)\n\u00d7\n|\nj\n|\n\n\n\nN\nNi\n\n\n\n\n\n\n\n\n\n(7)\n\n\nTO\n\nF\nECSA\n\n=\n\n\n(\n3.12\n\u00d7\n\n10\n15\n\n\n\n\nH\n2\n\n/s\n\n\nc\n\nm\n2\n\n\n\nper\n\nmA\n\nc\n\nm\n2\n\n\n\n)\n\u00d7\n|\nj\n|\n\n\n#\nsurface site\n\u00d7\n\nA\nECSA\n\n\n\n\n\n\n\nNote that as the exact hydrogen binding site is unknown, the number of the surface atom was used as #surface site in the TOFECSA calculation instead. Besides, as the number of the surface atom for Ni (\n\n2.029\n\u00d7\n\n10\n15\n\natoms c\n\nm\nreal\n-2\n\n\n), Ni2P (\n\n2.001\n\u00d7\n\n10\n15\n\natoms c\n\nm\nreal\n-2\n\n\n), and Ni3P (\n\n2.023\n\u00d7\n\n10\n15\n\natoms c\n\nm\nreal\n-2\n\n\n) are similar, and considering the low P content of the nanoNi-P prepared in this work, we used the number of the surface atom for Ni, i.e., \n\n2.029\n\u00d7\n\n10\n15\n\natoms c\n\nm\nreal\n-2\n\n\n, in the TOFECSA calculation.As the nanoNi-P modified electrode, prepared with Idep= -20000 \u03bcA cm\u22122 and Chypophosphite = 0.02 M, exhibited the best overall HER activity (vide infra), it was selected as the pre-catalyst for OER and electrochemical reforming of EG and PET. In addition, prior to its application for the reactions as above-mentioned, it was activated by cyclic voltammetry (CV) between 1.2\u20131.65 V vs. RHE at a scan rate of 50 mV s-1 for 225 cycles. Catalytic properties of the CV-activated nanoNi-P modified electrodes towards OER and electrochemical reforming of EG and PET were performed utilizing a CHI 760 potentiostat (CH Instruments, Inc., USA) connected with a nitrogen-purged three-electrode electrochemical cell consisting of Hg/HgO (1 M NaOH) reference electrode and Pt foil (1 cm \u00d7 4 cm) counter electrode. PET lysate for PET reforming was prepared by soaking 4.0 g of PET flakes, obtained from cutting commercially available PET bottles, in 100 mL KOH solution (2.0 M) in a sealed vial at 80 \u00b0C for 24 h, and subsequent diluting the resultant solution with an equal amount of DIW. The prepared PET lysate was used for the reforming process directly without further purification or filtration. The amount of PET dissolved in the PET lysate, determined by subtracting the solid content in the PET lysate from the amount of PET flakes added, was found to be 0.55 \u00b1 0.03 g, corresponding to the solubility of 2.73 \u00b1 0.16 g L-1. As the exact molecular weight of PET in commercially available PET bottle is unknown, the concentration of PET repeating unit (C10H8O4) in PET lysate (1.42 \u00b1 0.08 mM), determining by dividing the solubility of PET in PET lysate with the molecular weight of PET repeating unit (C10H8O4), was used as the basis for the calculation of conversion rate in the (photo-)electrochemical PET reforming.Characterization on the PEC properties of nanoTiO2 and nanoTiO2|nanoNi-P were carried out utilizing a MultiPalmSens4 potentiostat (PalmSens BV, Netherlands) in a home-made electrochemical cell containing deaerated KOH solution (1.0 M, pH 14.0) under light irradiation (AM 1.5 G 100 mW cm\u22122) using an XES-40S2-CE solar light simulator (SAN-EI Electric). Prior to the photoelectrochemical measurements, nanoTiO2|nanoNi-P was activated by potential cycling at 50 mV s\u22121 between 1.2\u20131.65 V vs. RHE for 225 cycles.Electrochemical impedance spectroscopy (EIS) analyses of nanoTiO2 and nanoTiO2|nanoNi-P were carried out in the PET lysate under light illumination using MultiPalmSens4 potentiostat equipped with EIS spectrum analyser. The applied potential, AC amplitude, and frequency range were open-circuit potential, 10 mV, and 1 MHz to 0.1 Hz, respectively. The obtained EIS data were fitted with the equivalent circuit [34] using built-in ZView\u00ae software to retrieve the parameters associated with the kinetics of interfacial charge transfer.Quantification of formate generated from the (photo-)electrochemical reforming of EG and PET was performed using a 883 Basic IC plus ion chromatography (Metrohm) equipped with Metrosep Organic Acids Guard (4.6 \u00d7 50 mm) and Metrosep Organic Acids column (7.8 \u00d7 250 mm). Note that as a small amount of formate (0.67 \u00b1 0.04 \u03bcmole) was found in the PET lysate, presumably resulted from the decomposition of impurity in the commercially available PET bottle, prior to the reforming experiments, the difference in the amount of formate in PET lysate before and after reforming experiments was reported as the actual amount of formate generated from (photo-)electrochemical PET reforming.Quantification of hydrogen generated from PEC reforming of PET plastics was carried out by headspace gas analysis using an Agilent 7890A Series gas chromatography equipped with a thermal conductivity detector. The GC oven holding the 5 \u00c5 molecular sieve column was set at 40 \u00b0C.The nanoNi-P modified electrodes were prepared by electrochemical deposition under various applied current densities (Idep) for 27 s (see Experimental Section for the details). The composition and the amount of nickel species in the prepared nanoNi-P modified electrode were summarized in Table 1\n. Fig. 1\n shows the XPS spectra of the nanoNi-P modified electrodes prepared with different Idep. Features in XPS spectra confirm the formation of Ni-P alloy regardless of Idep, including (i) peaks in Ni 2p3/2 region at binding energy (BE) of 851.8 and 853.2 eV corresponding to metallic nickel and Ni\u03b4+, respectively, in nickel phosphide [35,36], and (ii) peaks in P 2p region at BEs of 129.1 and 130.0 eV corresponding to P\u03b4\u2212 in nickel phosphide [35,36]. The existence of small peaks at BEs of 855.8, 132.8, and 530.8 eV is indicative of the presence of NiOx and POx species, presumably formed by the oxidation of surface NiPx upon exposure to air [37], on the electrode surface. The analyses on the composition depth profile of all the prepared samples (Figure S1 and Figure S2) indicate that P atoms were well-dispersed in metallic nickel matrix, and the P content of the prepared nanoNi-P modified electrodes decreased as Idep was increased. Figure S3 shows the chronopotentiograms recorded during the electrochemical preparation of the nanoNi-P modified electrodes. As revealed, the required potential shifted to more negative potential as Idep was increased. The negative shift in the deposition potential promoted the hydrogen evolution reaction (HER), which consequently decreased the Faraday efficiency for the deposition of nanoNi-P (Table 1). In addition, in contrast to the electrochemical deposition of nickel (Eq. 8), the electrochemical reduction of hypophosphite ions into P\u00b0 (Eq. 9) requires the transfer of protons. As a result, the depletion of proton in the vicinity of electrode surface due to HER discouraged the electrochemical reduction of P, resulting in lower P content of the nanoNi-P modified electrode prepared at higher Idep (Figures S1-S2 and Table 1). It is interesting to note that the amount of nickel in the nanoNi-P modified electrode prepared with lower Idep exceeded the theoretical value based on Faraday\u2019s law. This additional amount of metallic nickel was deposited most likely via electroless deposition as the reduction of Ni2+ ions to metallic nickel by hypophosphite (Eqs. (10) and (11)) is thermodynamically favourable [38]. Although the deposition rate of electroless deposition is low at room temperature [39], the relative long electrodeposition duration (Figure S3) required for the preparation of the nanoNi-P electrode at Idep = -12.5 \u03bcA cm-2 allowed the noticeable amount of deposits.\n\n(8)\n\n\n\n\n\n\nN\n\ni\n2+\n\n+\n2\n\ne\n\u2212\n\n\u2192\nNi\n\n\n\n\n\nE\n0\n\n=\n\u2212\n0.25\n V\nv\ns\n.\nN\nH\nE\n\n\n\n\n\n\n\n\n\n\n(9)\n\n\n\n\n\n\n\nH\n2\n\nP\n\nO\n2\n\u2212\n\n+\n2\n\nH\n+\n\n+\n\ne\n\u2212\n\n\u2192\nP\n+\n2\n\nH\n2\n\nO\n\n\n\n\n\nE\n0\n\n=\n\u2212\n0.248\n V\nv\ns\n.\nN\nH\nE\n\n\n\n\n\n\n\n\n\n\n(10)\n\n\n\n\n\n\nN\n\ni\n2+\n\n+\n\nH\n2\n\nP\n\nO\n2\n\u2212\n\n+\n\nH\n2\n\nO\n\u2192\nNi\n+\n\nH\n2\n\nP\n\nO\n3\n\u2212\n\n+\n2\n\nH\n+\n\n\n\n\n\n\nE\n\nr\ne\na\nc\nt\ni\no\nn\n\n0\n\n=\n 0\n.254 V\n\n\n\n\n\n\n\n\n\n\n(11)\n\n\n\n\n\n\n2N\n\ni\n2+\n\n+\n\nH\n2\n\nP\n\nO\n2\n\u2212\n\n+\n2\n\nH\n2\n\nO\n\u2192\n2\nNi\n+\n\nH\n2\n\nP\n\nO\n4\n\u2212\n\n+\n4\n\nH\n+\n\n\n\n\n\n\nE\n\nr\ne\na\nc\nt\ni\no\nn\n\n0\n\n=\n 0\n.514 V\n\n\n\n\n\n\n\n\n\nFigure S4 shows the SEM images of the nanoNi-P modified electrodes prepared at different Idep. As revealed, all the nanoNi-P modified electrodes consisted of the aggregates of particles with the decreasing size with increasing Idep. For example, nanoNi-P, prepared with Idep of -12.5 \u03bcA cm\u22122, comprises the aggregates of particles with a size of 0.5\u223c 1.0 \u03bcm, but that, prepared with Idep of -200,000 \u03bcA cm\u22122, consists of the aggregates of nanoparticles with a size of \u223c200 nm. The decrease in particle size at higher Idep could be attributed to the competing HER, as the hydrogen bubbling on the as-deposited Ni-P nuclei would limit the access of the plating ions and discourage the growth of Ni-P particles. The analyses on the electrochemically available surface area (ECSA), shown in Table 1 and Figure S5, reveal that the nanoNi-P modified electrode, prepared with higher Idep, exhibited higher ECSA, which is in agreement with SEM results (Figure S4) that the decrease in nanoNi-P particle size increased the surface area.The prepared nanoNi-P modified electrodes were subjected to the electrochemical characterizations on their HER activity, and the results are shown in Fig. 2\n. The HER activity indexes, including exchange current density (i0), Tafel slope, the required overpotential (\u03b7) to achieve a geometric current density (Jgeo) of -10 mA cm\u22122, and TOFNi and TOFECSA at \u03b7= \u2212200 mV, were included in Table 1. As revealed, the nanoNi-P modified electrode, prepared with Idep of -20,000 \u03bcA cm\u22122, exhibited the best HER activity. In terms of overall activity based on TOFNi, (Fig. 2\nb and Table 1), it exhibited a TOFNi of 187.2 \u00b1 7.2 h-1 at \u03b7= \u2212200 mV, which is about 1.3, 3.7, 36.1, and 80.0 times higher than those prepared with Idep of -200,000, -2,000, -200, and -12.5 \u03bcA cm\u22122, respectively. In terms of intrinsic activity based on the electrochemically available surface sites, i.e., TOFECSA, (Fig. 2\nc and Table 1), it exhibited a TOFECSA of 7.40 \u00b1 0.29 s-1 at \u03b7= \u2212200 mV, which is about 2.0, 3.0, 26.6, and 23.7 times higher than those prepared with Idep of -200,000, -2,000, -200, and -12.5 \u03bcA cm\u22122, respectively. The above results suggest that the superior electrocatalytic activity of nanoNi-P, prepared with Idep of -20,000 \u03bcA cm\u22122, was not simply resulted from the differences in ECSA or loading amount of nickel species. Tafel analyses (Fig. 2d) show that nanoNi-P, prepared with higher Idep, exhibited lower Tafel slope, which indicates that Idep had significant influences on the HER mechanism.\nFigure S6 shows the X-ray-absorption near-edge spectra (XANES) of the Ni K edge. As indicated, the Ni K edge position of nanoNi-P is slightly more positive than that of nickel foil, which is in line with the XPS results that Ni-P alloy formation induced an electron transfer from Ni to P to form Ni\u03b4+ species. The presence of negatively-charged P atoms (P\u03b4\u2212) is beneficial as they can act as proton-acceptor sites to facilitate HER [28]. The bonding environment of nickel atoms was also studied by Fourier-transformed extended X-ray-absorption fine-structure (EXAFS) at Ni edge, and results along with the fitted parameters are shown in Fig. 3\n and Table S1, respectively. The nanoNi-P modified electrodes, prepared with smaller Idep, was found to exhibit a higher coordination number in Ni-P bonding (CNNi-P) but a lower coordination number in Ni-Ni bonding (CNNi-Ni). The increasing CNNi-P/CNNi-Ni ratio by decreasing Idep indicates that the incorporation of more P atoms in the nickel crystal lattice decreased the concentration of isolated nickel sites (hydride-acceptor sites) and discouraged the first electron transfer to water molecules (Volmer step) during HER [40], resulting in higher Tafel slope and decreased activity (Fig. 2). In contrast, the nanoNi-P modified electrodes, prepared with significantly high Idep (i.e., -200,000 \u03bcA cm\u22122), exhibited zero CNNi-P, which suggests insufficient proton-acceptor sites available for HER, resulting in lower intrinsic activity than that prepared with Idep = -20,000 \u03bcA cm-2. These results are in agreement with the previous report that the highest HER activity of Ni-P catalyst relied on the coexistence and collaboration of hydride-acceptor sites (isolated nickel atoms) and proton-acceptor sites (negatively-charged P atoms) [28].The above findings indicate that Idep is crucial in determining the P content and bonding environment of the nanoNi-P modified electrodes. To further investigate the effect of P content on the HER activity, we prepared nanoNi-P modified electrodes with optimized Idep (i.e., -20,000 \u03bcA cm\u22122) in a plating solution containing hypophosphite of various concentrations, and the film composition along with the amount of nickel species in the prepared nanoNi-P modified electrodes are summarized in Table 2\n. P content of the prepared modified electrodeswas found to increase with increasing hypophosphite concentration (Chypophosphite) in the plating solution, suggesting that the P content of the nanoNi-P modified electrode can be controlled simply by tuning Chypophosphite. The SEM characterizations (Figure S7) reveal that the nanoNi-P modified electrode, prepared without hypophosphite, consists of aggregates of particles with irregular shape and smooth surface. In contrast, those prepared with hypophosphite exhibited aggregates of particles with spherical shape and rough surface. These difference in surface morphology resulted in the variation of ECSA (Figure S8).\nFig. 4\n shows the electrochemical characterization on the HER activity of the nanoNi-P modified electrodes prepared with various Chypophosphite. The HER activity indexes, including i0, Tafel slope, \u03b7 required to reach Jgeo of -10 mA cm\u22122, and TOFNi and TOFECSA at \u03b7= \u2212200 mV, were included in Table 2. As revealed from Fig. 4a-c and Table 2, there was an optimal P content to achieve the highest HER activity. In terms of the overall activity, the nanoNi-P modified electrode, prepared with Chypophosphite = 0.04 M, exhibited the highest TOFNi of 216.0 \u00b1 3.6 h-1 at \u03b7= \u2212200 mV. Nonetheless, in terms of the intrinsic activity, the nanoNi-P modified electrode, prepared with Chypophosphite = 0.2 M, exhibited the highest TOFECSA of 7.40 \u00b1 0.29 s-1 at \u03b7= \u2212200 mV. The analyses of XANES (Figure S9) and EXAFS (Figures S10 and Table S2) spectra reveal that the nanoNi-P modified electrode prepared with Chypophosphite = 0.04 M exhibited no Ni-P bonding. The lack of sufficient proton-acceptor sites for HER would therefore be the main cause resulting in lower intrinsic activity than that prepared with Chypophosphite = 0.2 M. Tafel analyses (Fig. 4d) show that the nanoNi-P modified electrode, prepared in the presence of hypophosphite, exhibited lower Tafel slope than that prepared in the absence of hypophosphite, which suggests P played a role in determining the mechanism of HER, and the incorporation of suitable amount of P atoms as the proton-acceptor sites can facilitate the HER kinetics. Since the nanoNi-P modified electrode, prepared with Idep= -20,000 \u03bcA cm\u22122 and Chypophosphite = 0.04 M, exhibited the highest overall HER activity, it was designated as nanoNi-Pop and selected for further investigation.To further improve the overall HER activity, we prepared carbon nanotube-supported Ni-P alloy nanospheres (CNT/nanoNi-Pop) modified electrodes by the electrodeposition of nanoNi-P in the presence of CNTs under optimal electrodeposition condition (i.e., Idep= -20,000 \u03bcA cm\u22122 and Chypophosphite = 0.04 M). As revealed in Fig. 5\na, the CNT/nanoNi-Pop modified electrode consisted of Ni-P nanospheres with a size of \u223c150 nm embedded in between entangled CNTs. Characterization of electrocatalytic activity (Fig. 5b-c) indicates that the CNT/nanoNi-P modified electrode exhibited significantly higher HER activity than nanoNi-Pop, though the loading amount of nickel species are similar (2.62 \u00b1 0.02 vs. 2.64 \u00b1 0.02 \u03bcmole cm-2). Notably, the CNT/nanoNi-Pop modified electrode required \u03b7 of -149.7 \u00b1 6.1 and -206.3 \u00b1 9.2 mV to reach jgeo of -10 and -100 mA cm-2, respectively, which were about 20 mV smaller than nanoNi-Pop. Furthermore, at \u03b7= \u2212200 mV, the CNT/nanoNi-Pop modified electrode exhibited a TOFNi value of 431.2 \u00b1 59.6 h-1 that is about two times higher than nanoNi-Pop. Finally, as revealed in Fig. 5\nd and Figure S11, the CNT/nanoNi-Pop modified electrode also exhibited high stability at both Jgeo of -10 and -100 mA cm-2. The excellent activity and stability of the CNT/nanoNi-P modified electrode place itself among the most active earth-abundant HER catalysts in alkaline aqueous media (Table S3).Encouraged by its excellent HER activity, the applications of the nanoNi-Pop modified electrode for OER and electrochemical oxidation of EG were further investigated. In addition, the nanoNi-P modified electrode prepared without hypophosphite, designated as nanoNi for simplicity, was also included for comparison. Moreover, prior to the application for the OER and electrochemical reforming of EG, both modified electrodes were subjected to a CV-activation process (see Experimental Section for the details), and the CVs recorded during the activation process are shown in Figure S12. As revealed, the charge under the redox peaks remained constant during the last 10 cycles of the CV-activation, suggesting that the growth of oxy-hydroxide layer on the surfaces of nanoNi-Pop and nanoNi completed after the CV-activation process.\nFig. 6\na-b show TEY sXAS of nanoNi-Pop and nanoNi before and after CV-activation. Additional signals at 854.2 eV in the Ni L3-edge region (Fig. 6a), reflecting the formation of Ni3+ [41,42], and at 528.3 eV in O K-edge region (Fig. 6b), resulted from the hybridization of O(2p)-Ni(3d)eg [41,42], were observed after CV-activation, which confirms that NiOOH species formed on both nanoNi-Pop and nanoNi after CV-activation process. Besides, the intensity ratio of the double-peak features at 854.2 and 852.1 eV (I854.2/I852.1) in Ni L3-edge region, an index of the oxidation state of the nickel atoms [41], was found to be influenced by P incorporation. nanoNi after CV-activation, designated as nanoNi(CV) for simplicity, exhibited a higher I854.2/I852.1 value (0.37 vs. 0.31) than nanoNi-Pop after CV-activation, designated as nanoNi-Pop(CV), which implies that nickel species in nanoNi(CV) had higher average oxidation state than those in nanoNi-Pop(CV). Nonetheless, analysis of TFY sXAS spectra of NiL-edge (Fig. 6c) reveals that no change in peak position nor the appearance of any new peak after CV-activation for both samples. As TFY is a bulk-sensitive probe and has higher probing depth than TEY mode [43,44], the observed different features in TFY and TEY modes suggest that CV-activation only induced a very thin oxyhydroxide layer on the surface of both nanoNi-Pop(CV) and nanoNi(CV). For example, as revealed from the TEM analyses (Figure S13), the thickness of oxyhydroxide layer formed on nanoNi-Pop(CV) was <10 nm. Fig. 6d shows the CVs of the nanoNi-Pop(CV) and nanoNi(CV) electrodes in KOH (1.0 M) solution. As revealed, both nanoNi-Pop(CV) and nanoNi(CV) electrodes exhibited CV features characteristic to the redox reaction of Ni(OH)2/NiOOH (Eq. (12)):\n\n(12)\n\n\nNi\n\n\n\nOH\n\n\n2\n\n+\nO\n\nH\n\u2212\n\n\u21cc\nNiOOH\n+\n\ne\n\u2212\n\n+\n\nH\n2\n\nO\n\n\n\n\n\n\n(13)\n\n\nNiOOH\n+\nC\n\nH\n2\n\nOHC\n\nH\n2\n\nOH\n\u21cc\nNi\n\n\n\nOH\n\n\n2\n\n+\nproduct\n\n\n\n\nIn addition, both nanoNi-Pop(CV) and nanoNi(CV) exhibited multiple cathodic peaks during the reverse scan, which could be ascribed to the various routes of reductive transformation of NiOOH to Ni(OH)2 [45,46]. Due to the difference in the energetics of the reductive transformation of \u03b2-NiOOH and \u03b3-NiOOH, these reductive transformations occurred at different potentials, and cathodic peaks r1, r2, and r3 are assigned to the transformations of \u03b2-NiOOH to \u03b2-Ni(OH)2, \u03b3-NiOOH to \u03b2-Ni(OH)2, and \u03b3-NiOOH to \u03b1-Ni(OH)2, respectively [45,46]. In other words, the multiple cathodic peaks observed in Fig. 6d indicate that \u03b2-NiOOH and \u03b3-NiOOH coexisted in both nanoNi-Pop(CV) and nanoNi(CV). The molar ratio of \u03b2-NiOOH to \u03b3-NiOOH, determined by dividing the charge under the cathodic wave r1 to that under cathodic waves r2, and r3 (Figure S14), was found to be 2.74 and 1.27 for nanoNi-Pop(CV) and nanoNi(CV), respectively. This finding suggests that the \u03b2-NiOOH content of nanoNi-Pop(CV) was higher than that of nanoNi(CV), and P played a role in suppressing further transformation of \u03b2-NiOOH into \u03b3-NiOOH. Additional analyses on the formation of oxyhydroxide on the nanoNi-Pop(CV) and nanoNi(CV) electrodes using Raman spectroscopy were attempted, but the results (Figure S15) didn\u2019t show any features characteristics to these oxyhydroxides, which can be attributed to the fact that the oxyhydroxide layer is too thin (Figure S13) to be observed by Raman spectroscopy.\nFigures S16 shows electrochemical characterizations on the OER activity of both nanoNi-Pop(CV) and nanoNi(CV) electrodes in KOH solution (1.0 M). It can be found that nanoNi-Pop(CV) exhibited significant higher activity, in terms of \u03b7, than nanoNi(CV). Notably, the nanoNi-Pop(CV) modified electrode required \u03b7 of \u223c400 mV to achieve jgeo of -100 mA cm\u22122, which is \u223c 60 mV smaller than nanoNi(CV) (Figures S16a-b). The Tafel analyses (Figure S16c) reveal that nanoNi-Pop(CV) exhibited a significantly lower Tafel slope than nanoNi(CV) (44 vs. 61 mV dec-1), which implies that the mechanism of OER was different at these two catalysts, and OER proceeded in a more feasible fashion at nanoNi-Pop(CV).\nFig. 7\n shows the electrochemical characterizations on the activity of both nanoNi-Pop(CV) and nanoNi(CV) electrodes towards electrochemical oxidation of EG in KOH solution (1.0 M). In the absence of EG, both nanoNi-Pop(CV) and nanoNi(CV) electrodes exhibited CV features characteristic to Ni(OH)2/NiOOH redox reactions. Besides, upon addition of EG, both nanoNi-Pop(CV) and nanoNi(CV) electrodes exhibited additional features, including a notable enhancement in the peak current of o1 wave and the decrement in the peak current of r1 wave. These changes in CV features indicate that the oxidation of EG at both nanoNi-Pop(CV) and nanoNi(CV) electrodes involved electrochemical formation of active NiOOH species (Eq. (12)) and a follow-up reaction between EG and NiOOH (Eq. (13)). Moreover, as compared with the extent of decrement in the cathodic current of r2 and r3 peaks in the presence of EG (Fig. 7a and b), the significantly pronounced decrement in the cathodic current of r1 peak in the presence of EG was noticed. As discussed in the previous section, the cathodic peak r1 is resulted from the reductive transformation of \u03b2-NiOOH to \u03b2-Ni(OH)2, and the pronounced decrease in the current of peak r1 in the presence of EG suggests that \u03b2-NiOOH would be the active species responsible for the electrocatalysis of EG. Furthermore, nanoNi-Pop(CV) exhibited higher catalytic current responses to EG than nanoNi(CV). Notably, nanoNi-Pop(CV) exhibited a Jgeo of 38.3 mA cm\u22122 at 1.5 V vs. RHE in EG solution (0.1 M), which is about 1.6 times higher than nanoNi(CV) (\u223c24.6 mA cm\u22122). The higher electrocatalytic activity of nanoNi-Pop(CV) than nanoNi(CV) would be attributed to the higher amount of active \u03b2-NiOOH sites available for EG oxidation. This result suggests that P played a beneficial role in reserving active \u03b2-NiOOH species for electrochemical EG oxidation. Product analyses after 1-h electrolyses at 1.5 V vs. RHE (Fig. 7\nc and d) reveal that the electrocatalytic EG oxidation at both nanoNi-Pop(CV) and nanoNi(CV) electrodes mainly generated formate (Faradic efficiency: \u223c100 %), which suggests that EG oxidation at both electrodes not only involves an electrocatalytic scheme consisting of the electrochemical generation of active \u03b2-NiOOH species (Eq. (12)) and its ensuing redox reactions with ethylene glycol and intermediates, but also involve the CC bond cleavage to generate formate as the sole product (see Scheme 1\n) [47,48]. Besides, the rate of formate generation (Rformate) at nanoNi-Pop(CV) was about 1.3 times higher than nanoNi(CV) (244.6 \u00b1 15.0 \u03bcmole cm\u22122 h\u22121 vs. 182.9 \u00b1 26.0 \u03bcmole cm\u22122 h\u22121). These results suggest that \u03b2-NiOOH content affected the kinetics of the electrochemical EG oxidation, but didn\u2019t exhibit any effect on the reaction pathway of EG oxidation. To the best of our knowledge, it is the first time that high conversion of EG into formate with high selectivity has been achieved using precious metal-free electrocatalysts. Note that as Rformate at nanoNi-Pop(CV) was much higher than that at nanoNi(CV) electrode, the pH drop, due to the dissociation of formic acid, nearby the surface of nanoNi-Pop(CV) electrode was therefore more pronounced. The pronounced pH drop would result in the positive shift in the redox potential of Ni2+/Ni3+ redox couple, and decrease in the overpotential available for the electrochemical EG oxidation during the controlled-potential electrolysis [15,49], which in turn gradually reduces the catalytic current and thus causes curving of charge transient of nanoNi-Pop(CV) electrode (line ii in Fig. 7c).Further application of nanoNi-Pop(CV) towards photoelectrochemical (PEC) EG oxidation was explored by integrating nanoNi-Pop(CV) onto the TiO2 nanorods (nanoTiO2) photoanode. Initial attempt was performed by electrodepositing nanoNi-Pop onto nanoTiO2 photoanode with charge passage (C) of 0.54 C cm\u22122, and subsequently CV-activating the resultant electrode (see Experimental Section for the details). The results of SEM (Figure S17) and XRD (Figure S18) analyses confirm the prepared photoanode, designated as nanoTiO2|nanoNi-Pop(CV) (C = 0.54 C cm\u22122), consisted of Ni-P submicron-sized spheres decorated rultile TiO2 nanorods. However, as revealed in Figure S19 and Fig. 8\ng, the nanoTiO2|nanoNi-Pop(CV) (C = 0.54 C cm\u22122) photoanode exhibited inferior PEC performance than the bare nanoTiO2 photoanode. The poor performance can be attributed to the large size of nanoNi-Pop(CV), which short-circuits photo-induced charges between the neighbouring TiO2 nanorods and disables the hole transfer to EG. To improve the PEC performance of the nanoNi-Pop(CV) modified nanoTiO2 photoanode by reducing the size of nanoNi-Pop(CV), we reduced the duration of electrodeposition process from 27 s to 1 s, which means that only 0.02 C cm\u22122 charge passed for the deposition of nanoNi-Pop. The SEM and EDS analyses (Figures S20) reveal that nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) consisted of nanoNi-Pop(CV) nanoparticles with size of 10\u223c20 nm uniformly decorated onto nanoTiO2. Fig. 8a-f show the analyses of HR-TEM and EDS elemental mapping for nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122). The clear lattice fringes with a lattice spacing of 0.206 nm (Fig. 8\nb) corresponds to the (011) lattice plane of metallic nickel (PDF no. 45-1027). The SAED pattern, shown in the inset of Fig. 8b, consists of the bright spots and weak diffuse ring. The bright spots indicate that the prepared rutile TiO2 nanorod is single crystal, whereas the diffuse ring is indexed to the (011) plane of low-crystalline nickel. The results of EDS elemental mapping (Fig. 8c-f) suggest that the surface of nanoTiO2 is uniformly covered with nanoNi-Pop(CV). Fig. 8g-h show the PEC characterizations of the bare nanoTiO2 and nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) photoanodes. As revealed, nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) exhibited superior PEC performance than bare nanoTiO2. To begin with, nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) exhibited a lower photocurrent onset than bare nanoTiO2 (0.24 vs. 0.26 V vs. RHE). In addition, the photocurrent response of nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) was found to be about 5 times higher than bare nanoTiO2 (\u223c0.23 mA cm\u22122 vs. \u223c46.0 \u03bcA cm\u22122) at 0.4 V vs. RHE. Finally, nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) exhibited Rformate of 2.1 \u00b1 0.2 \u03bcmole cm\u22122 h\u22121 with Faradaic efficiency (FEformate) of 57.2 \u00b1 3.1 % at 0.5 V vs. RHE, whereas bare nanoTiO2 only showed Rformate of 0.3 \u00b1 0.1 \u03bcmole cm\u22122 h\u22121 and FEformate of 27.0 \u00b1 4.8 % at the same applied potential. This remarkable enhancement in PEC performance by surface modification of nanoNi-Pop(CV) further confirms the crucial role of nanoNi-Pop(CV) in catalyzing EG oxidation. Note that PEC EG oxidation by nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) resulted in lower FEformatethan electrochemical EG reforming by nanoNi-Pop(CV), which could be attributed to the fact that the surface of nanoTiO2 is not fully covered with nanoNi-Pop(CV) due to the small loading amount of nanoNi-Pop(CV) (95.0 \u00b1 3.0 nmole cm\u22122), and the direct hole transfer from nanoNi-Pop(CV)-free TiO2 surface to EG is possible, resulting in the lower FEformate.As the EG is one of the monomers constituting PET, the direct application of nanoNi-Pop(CV) to the reforming of PET was also attempted and investigated. Figures S21 shows electrochemical characterizations on the activity of the nanoNi-Pop(CV) electrode towards PET reforming in KOH solution (1.0 M). It can be found from Figure S21a that nanoNi-Pop(CV) exhibited higher catalytic current in PET lysate than in KOH solution, indicating nanoNi-Pop(CV) had promising activity towards electrochemical PET reforming. Besides, product analyses after 2-h controlled-potential electrolyses indicate that nanoNi-Pop(CV) can also selectively reform PET into formate with Rformate of 50.7 \u00b1 6.7 \u03bcmole cm\u22122 h\u22121 and FEformate of 103.9 \u00b1 4.7 %. The PET conversion rate, based on the amount of dissolved PET repeating unit (C10H8O4), was found to be 16.8 \u00b1 2.2 %. Note that nanoNi-Pop(CV) exhibited negligible activity in electrocatalysing the oxidation of terephthalic acid (TA) and exhibited high selectivity (FEformate: 95.7 \u00b1 2.7 %) towards the generation of formate from the electrochemical EG oxidation in the presence of TA (Figure S22a). These findings not only indicate the oxidation of terephthalate unit of PET was unlikely involved in the PET reforming, but also suggest the PET reforming at nanoNi-Pop(CV) would start with the electrocatalytic oxidation of EG unit at \u03b2-NiOOH active sites and follow-up CC bond cleavage to release formate. The high activity of nanoNi-Pop(CV) towards PET reforming would be therefore attributed to the beneficial role of P in the reserving high amount of active \u03b2-NiOOH species. It is also important to note that the presence of TA slightly reduced the activity of nanoNi-Pop(CV) towards the electrocatalytic EG oxidation. For example, Rformate from the electrocatalytic oxidation of EG (0.1 M) in the presence of TA (0.1 M) was 203.3 \u00b1 26.0 \u03bcmole cm\u22122 h\u22121, about 1.2 times lower than that obtained in the absence of TA. The apparent decrease in the activity can be attributed to the fact that the presence of TA (0.1 M) induced a drop in bulk pH (from 14.01 to 13.90), which in turn shifts in the redox potential of Ni2+/Ni3+ redox couple to the positive side (Figure S22b), and decreases the overpotential available for the electrocatalytic EG oxidation during the controlled-potential electrolysis. This result also suggests that the release of TA during the PET reforming would be one of the main reasons causing the gradual decrease in the catalytic current during the controlled-potential electrolysis of PET (Figure S21b). On the other hand, due to the limited solubility of PET (2.73 \u00b1 0.16 g L\u22121), PET concentration available for reforming is very low (i.e., in the order of \u03bcM). During PET reforming, the thickness of the diffusion layer nearby electrode surface would increase significantly with this low PET concentration, resulting in the fast decrease in the concentration gradient and thus contributing to the loss in catalytic current.Encouraged by the promising activity of nanoNi-Pop(CV) and CNT/nanoNi-Pop towards PET reforming and HER, respectively, we subsequently examined their applicability towards PEC PET reforming. For this purpose, a nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122)//CNT-nanoNi-Pop two-electrode PEC device, consisting of nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) photoanode and CNT/nanoNi-Pop cathode, was established, and its PEC performance was characterized and shown in Fig. 9\n\nb. For comparison, PEC characterization of another two-electrode PEC device based on nanoTiO2 photoanode and Pt foil, designated as nanoTiO2//Pt, was also included in Fig. 9\nb. Note that to prepare cathode with the same geometry surface area to that of nanoTiO2|nanoNi-Pop(CV) photoanode, CNT/nanoNi-Pop cathode was prepared by electrodepositing CNT/nanoNi-Pop onto the carbon paper instead of SPCE (see Experimental Section for the details). The SEM results (Figure S23a) reveal that the surface morphology of CNT/nanoNi-Pop deposited on the carbon paper was similar to that deposited on the SPCE substrate (Fig. 5a), suggesting that the proposed deposition methodology is suitable for both flat substrate and substrate with 3D porous structure. Electrochemical analysis, shown in Figures S23b-c and Fig. 9a, reveals that CNT/nanoNi-Pop modified carbon paper required an additional \u03b7 = 40\u223c60 mV to achieve the same Jgeo with Pt foil in the current density range between -0.1 to -70 mA cm\u22122, but showed comparable activity at Jgeo \u2265 -90 mA cm\u22122. Particularly, CNT/nanoNi-Pop modified carbon paper required \u03b7 of \u223c \u2212180 mV to achieve a Jgeo= -100 mA cm\u22122, which is about 30 mV lower than Pt foil. In addition, as compared with bare nanoTiO2, nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) exhibited enhanced photoelectrocatalytic activity towards PET reforming. For example, nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) showed a photocurrent density of \u223c310 \u03bcA cm\u22122, which is about 1.6 times higher than bare nanoTiO2. The EIS analyses, shown in Table S4 and Figure S24, indicate that nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) exhibited smaller interfacial charge transfer resistance (709.3 vs. 1104.0 \u03a9) than nanoTiO2. Consequently, the enhancement in photocurrent can be mainly attributed to the improved kinetics of interfacial charge transfer by nanoNi-Pop(CV). In typical 4-h PEC PET reforming at an external bias of 0.5 V, nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122)//CNT-nanoNi-Pop generated 6.4 \u00b1 1.4 \u03bcmol formate (FEformate: 57.1 \u00b1 1.7 %) and 12.5 \u00b1 1.1 \u03bcmol H2(FEhydrogen: 76.8 \u00b1 7.8 %), whereas nanoTiO2//Pt produced 1.0 \u00b1 0.1 \u03bcmol formate (FEformate: 14.7 \u00b1 2.5 %) and 7.1 \u00b1 2.4 \u03bcmol H2(FEhydrogen: 65.7 \u00b1 11.7 %). The PET conversion rate, based on the amount of dissolved PET repeating unit (C10H8O4), for nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122)//CNT-nanoNi-Pop and nanoTiO2//Pt were found to be 14.9 \u00b1 3.3 and 2.4 \u00b1 0.2 %, respectively. The significantly enhanced PET conversion and product generation by nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122)//CNT-nanoNi-Pop indicate that the overall PEC performance was limited by the oxidative conversion of PET at nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122) photoanode, which further confirms the crucial role of nanoNi-Pop(CV). Note that a gradual decrease in photocurrent for both devices (Fig. 9b) was observed during the photoelectrochemical PET reforming. The apparent loss in photocurrent could be attributed to the (i) the pH drop induced by the release and dissociation of formic acid and TA during the reforming of PET, and (ii) decrease in the PET concentration gradient by the increase in the thickness of diffusion layer. To stabilize the photocurrent by minimizing the accumulation of acids and stabilizing the thickness of diffusion layer, the development of flow-type photoelectrochemical system for PET reforming is currently under investigation in our lab.nanoNi-P with various P content and morphologies have been successfully synthesized, using a facile and simple electrodeposition method, and their applications in catalyzing HER, OER, and reforming of EG and PET were explored and investigated. P content of nanoNi-P was found to play an crucial role in regulating the relative amount of hydride-acceptor sites and proton-acceptor sites, and thus determining the intrinsic activity of nanoNi-P towards HER. An ultrathin layer of nickel-oxyhydroxide formed onto nanoNi-P after the CV-activation process, and both \u03b2-NiOOH and \u03b3-NiOOH was found to coexist in this thin oxyhydroxide layer. Nonetheless, the presence of P in nanoNi-Pop(CV) played an important role in suppressing further transformation of \u03b2-NiOOH into \u03b3-NiOOH, and nanoNi-Pop(CV) showed higher \u03b2-NiOOH cotent than nanoNi(CV). \u03b2-NiOOH was the main active species responsible for the (photo-)electroforming of EG and PET, rendering higher activity of nanoNi-Pop(CV) over nanoNi(CV). Efficient and selective generation of hydrogen and formate from PEC PET reforming was successfully realized using an Earth-abundant nanoTiO2|nanoNi-Pop(CV) (C = 0.02 C cm\u22122)//CNT-nanoNi-Pop PEC device. Our work opens a sustainable avenue for simultaneous mitigation of plastic pollution and photosynthesis of renewable fuel and valued chemicals.\nChia-Yu Lin: Conceptualization, Writing - original draft, Funding acquisition, Supervision. Shih-Ching Huang: Investigation, Validation, Writing - original draft. Yan-Gu Lin: Funding acquisition, Writing - review & editing. Liang-Ching Hsu: Investigation, Validation. Chih-Ting Yi: Investigation, Validation.The authors report no declarations of interest.We gratefully acknowledge the Ministry of Science and Technology, Taiwan for the financial support (Grant number 110-2218-E-006-016-, 109-2218-E-006-023-, 108-2112-M-213-002-MY3). The research was supported in part by Higher Education Sprout Project, Ministry of Education to the Headquarters of University Advancement at National Cheng Kung University (NCKU). We would also like to thank Dr. Shu-Chih Haw and Prof. Jih-Jen Wu for their technical support. The assistance in HR-TEM analyses from Center for Micro/Nano Science and Technology of National Cheng Kung University was also acknowledged.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2021.120351.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  Photoelectrochemical reforming of plastic waste offers an environmentally-benign and sustainable route for hydrogen generation. Nonetheless, little attention was paid to develop electrocatalysts that can efficiently and selectively catalyze oxidative transformation of valueless plastic wastes into valued chemicals. Herein, we report on facile electrosynthesis of nickel-phosphorus nanospheres (nanoNi-P), and their versatility in catalyzing hydrogen generation, water oxidation, and reforming of polyethylene terephthalate (PET). Notably, composite of nanoNi-P with carbon nanotubes (CNT/nanoNi-P) requires \u2212180 mV overpotential to drive hydrogen generation at -100 mA cm\u22122. Besides, CV-activated nanoNi-P (nanoNi-P(CV)) was shown to be capable of reforming PET into formate with high selectivity (Faradic efficiency= \u223c100 %). Efficient and selective generation of hydrogen and formate from PET reforming is realized utilizing an Earth-abundant photoelectrochemical platform based on nanoNi-P(CV)-modified TiO2 nanorods photoanode and CNT/nanoNi-P cathode. This work paves a path for developing artificial leaf for simultaneous environmental mitigation and photosynthesis of renewable fuels and valued chemicals.\n               "}
{"full_text": "Porous carbons have been widely used as catalyst supports for the preparation of heterogeneous catalysts because of their high surface area, high chemical and thermal stabilities, diverse surface properties, and tunable macroscopic shape [1\u20136]. However, the main limitation of carbon materials as support for metal catalysts is their poor stability under oxidation conditions. Nowadays, owing to their attractive performance, porous carbons have been increasingly used as catalyst supports in mild oxidative catalytic reactions that do not deteriorate their structure, including fuel cell [7,8], oxidation of volatile organic compounds (VOC) [9,10], decomposition of NOx [11,12], oxidative dehydrogenation [13,14] and so on. Nevertheless, oxidation-resistant carbon supports are in high demand for the increasing variety of catalytic reactions. A series of methods have been proposed for improving the oxidation resistance of carbon materials, such as graphitization [15], purification [16], surface coating [17] or modification [18], composite fabrication [19] and others (Fig. S1). Purification can slightly improve the oxidation resistance of porous carbon. However, other methods result in serious porosity degradation.Since its experimental discovery in 2004 [20], graphene has quickly attracted tremendous attention and has become one of the most explored nanomaterials in the scientific world. It is possible to prepare graphene-based porous carbon with exceptional properties and a vast application potential from graphene oxide (GO) colloids by chemical reduction [21], intercalation with pillar molecules [22] or nanoparticles [23], thermal exfoliation [24], and KOH activation [25], among them KOH activation method gives rise to porous graphene with the highest porosity. The structure of porous graphene can be further tuned by high-temperature heating under Ar atmosphere. Heat treatment of porous graphene at the temperature region of 1873 ~ 2273 K can partly recover its crystallinity and electrical conductivity by sacrificing a certain amount of porosity according to our previous study [26]. To the best of our knowledge, heat-treated porous graphene has the highest graphitization degree among the high surface area porous carbons except single wall carbon nanotubes (SWCNTs). High quality SWCNTs are still too expensive for a wide variety of applications. The gasification of carbon atoms at the edge of graphene unit through oxidation is 102 to 103 times faster than that on the basal planes [27]. The heat-treated porous graphene with partially graphitized structure should have higher oxidation resistance than conventional porous carbons due to its less prismatic edges and more basal planes, being a promising catalyst support for reactions under oxidative conditions. It is well known that metal catalysts can promote the gasification of carbons under an oxidative atmosphere. Therefore, the development of porous carbon with sufficient oxidation resistance, especially in the presence of metal is necessary. This paper reports the excellent oxidation resistance of heat-treated porous graphene as a metal catalyst support.The GO colloid was prepared using natural graphite (Madagascar graphite from Madagascar) as described elsewhere [28]. The prepared GO suspension was mixed with KOH at a weight ratio of KOH/C\u00a0=\u00a010, followed by a unidirectional freeze-drying method [29,30]. The obtained GO monolith mixed with KOH was heated up to 573 K at a heating rate of 1.5 K min\u22121 to prepare the porous graphene monolith-KOH mixture, which was heated up to 1073 K at a heating rate of 30 K min\u22121 and maintained at this temperature for 1 h. The entire process was conducted under a 400 mL min\u22121 Ar flow. The mixture was washed with distilled water up to pH\u00a0=\u00a07 of the supernatant, and then soaked in 1 mol L\u22121 HCl solution at 353 K for 24 h to completely remove the residual KOH. The obtained monolith was rewashed with distilled water and dried in a vacuum oven at 393 K for 24 h. Finally, the monolith was ground into a powder for further use. The as-prepared sample is denoted as porous graphene (PG). A pitch-based activated carbon fiber ACF-A20 (abbreviated as ACF, AD'ALL Co., Ltd) was studied for comparison. ACF-A20 used here is a metal free porous carbon which is extremely suitable for the research that needs to eliminate the interference from metal impurities.The samples were placed in a graphite-resistance furnace and heated to the target temperature at a heating rate of 20 K min\u22121 and then maintained at the target temperature for 30 min before cooling down. The entire process was conducted under a pure argon flow (1 L min\u22121). Thermal treating within the temperature region of 1873 ~ 2473 K enables partial graphitization of PG without degrading all the porosity according to our previous study [26]. We need porous graphene with both high surface area and high graphitization degree as catalyst support, then the heating temperature of 1873 and 2273 K are selected in present work. The yields of the PG and ACF after heat treatment at the two selected temperatures are about 80-85%. The porous carbon (C) heat-treated at T K is denoted as C@T in this study.Metal-loaded carbons were prepared via incipient wetness impregnation method [31,32] with Pd(O2CCH3)2 (toluene solution), Ni(NO3)2\u20226H2O (ethanol solution), and Fe(NO3)3\u20229H2O (ethanol solution). All reagents were supplied by Wako Pure Chemical Industries, Ltd (Japan). The metal loading amounts of samples were adjusted to 1.0wt% or 3.0wt%. The impregnated samples were dried at room temperature, then heated up to 623 K at a heating rate of 1.5 K min\u22121 and maintained at the target temperature for 10 min before cooling down. The entire process was conducted under a pure argon flow (200 mL min\u22121). The carbon sample (C) loaded with metal M is denoted as M-C in this study.The microscopic morphology of the carbon sample was observed using a field emission scanning electron microscope (SEM, JEOL JSM-6330F, Japan) and a transmission electron microscope (TEM, JEOL 2100F, Japan). The crystallinity change of carbon was examined by synchrotron X-ray diffraction (XRD, Aichi Synchrotron Radiation Center, Japan, \u03bb\u00a0=\u00a00.07997 nm) and Raman spectroscopy (Renishaw, inVia reflex 785S, UK, \u03bb\u00a0=\u00a0532 nm). Porosity analysis of porous carbons was performed using a surface analyzer (Micromeritics, ASAP 2020, US) by N2 adsorption at 77 K. Before N2 adsorption, the samples were pre-evacuated at 523 K for 5 h. The surface area was determined by the subtracting pore effect (SPE) method [33,34] with carbon black (Mitsubishi 32#) as a reference. The Brunauer-Emmett-Teller (BET) method was also employed to analyze the N2 adsorption isotherms in P/P0 range of 0.05-0.30 for comparison. The micro- and mesopore volumes were determined using the quenched solid density functional theory (QSDFT) with a slit-pore model [35]. The total pore volume was determined using the Gurvitch rule with the N2 adsorption amount at P/P0\u00a0=\u00a00.98 [36]. The thermal gravimetric (TG) analysis was performed using a thermogravimetric analyzer (Rigaku, Thermo Plus TG 8120, Japan) from 303 K to 1123 K under a dry air flow (200 mL min\u22121) at a heating rate of 2 K min\u22121.The high-resolution TEM images of the pristine and heat-treated carbons (Fig.\u00a01\n) clearly indicate that the graphene units of different porous carbons have different stacking structures depending on the heat treatment. PG mainly consists of crumpled graphene sheets of considerably large size (Fig.\u00a01(a1)), which is very different from conventional highly porous carbon (such as activated carbon or ACF) that consists of small graphene units [37,38]. The micropores and mesopores constructed by entangled and crumpled graphene sheets will be further discussed in Section\u00a03.3. Heat treatment at 1873 K produces partially stacked graphene sheets of larger size (Fig.\u00a01(a2)), and heat treatment at 2273 K leads to the formation of a well-ordered graphitic layer together with a few disordered parts (Fig.\u00a01(a3)). The stacking behavior of graphene sheets of PG under heat treatment has been discussed in detail in our previous study [26]. ACF mainly consists of randomly oriented graphitic units of approximately 2 nm (Fig.\u00a01(b1)), which is similar to the structure of the traditional microporous carbons [37,38]. Heat treatment at 1873 K does not cause significant structural changes of ACF (Fig.\u00a01(b2)), but further heating at 2273 K produces entangled and few-layer-stacked graphene layers (Fig.\u00a01(b3)). SEM images of pristine and heat-treated carbons indicate that the heat treatment at 1873 and 2273 K does not significantly change their exterior morphology (Fig. S2). The graphitization of each carbon under heat treatment will be further discussed with their Raman spectra and XRD patterns in the following section.The graphitic states of different carbons were evaluated by Raman spectroscopy. Pristine PG has two overlapping broad bands located at ~ 1350 and ~ 1590 cm\u22121 (Fig.\u00a02\n(a1)), which are associated with the defects or edges in the graphene unit (D band) and in-plane motion of carbon atoms in the aromatic planes (G band), respectively. Heat treatment at 1873 K leads to a decrease in the D band and an increase in the sharpening of the G band. The decrease in full width at half maximum (FWHM) can be attributed to the growth of the graphitic structure [39]. Further heating at 2273 K gives a very sharp G band and a negligible D band. The intensity ratios of the D band to the G band (ID/IG value) of PG, PG@1873, and PG@2273 are 1.61, 0.39, and 0.06, respectively, indicating the significant graphitization of PG through high-temperature treatment. It is worth mentioning that, PGs prepared from different graphite precursors show different graphitization behavior under heat treatment. PG heat-treated at 1873 K in our previous study [26] doesn't possess high graphitization degree as PG@1873 prepared in the present work does. The reason could be that PG prepared from Bay carbon graphite has smaller and more defective graphene sheets compare to those prepared from Madagascar graphite in current study.The shapes of the Raman spectra and ID/IG values of pristine ACF are similar to those of PG (Fig.\u00a02(b1)). Heat treatment of ACF at 1873 K results in a sharpened D band, and the ID/IG values decrease from 1.63 to 1.42. Further heat treatment at 2273 K remarkably decreases the D band intensity, and the ID/IG value becomes 0.51, which is much larger than that of PG@2273. PG has a more graphitizable structure than ACF. The in-plane crystallite sizes La\n of graphitic structures obtained using the Tuinstra-Koenig equation [40,41] are shown in Table S1, together with the values of ID/IG and FWHM. The peaks corresponding to the overtone mode longitudinal optical phonons (~2450 cm\u22121) and the 2D band (~2700 cm\u22121) become distinct and sharp, while the peak of the (D\u00a0+\u00a0G) combination mode (~2930 cm\u22121) becomes weaker with the increase in heat treatment temperature, suggesting the growth of ordered graphitic structure during the thermal treatment [26].PG has broad (002), (10), and (11) XRD peaks around 13\u00b0, 22\u00b0, and 39\u00b0 (Fig.\u00a02(a2)) with three-dimensional X-ray reflection peaks indicating that the stacking structure of graphene units is turbostratic. After heat treatment at 1873 K, all the above-mentioned peaks become sharper, and the (004), (105)\u00a0+\u00a0(006), (201) peaks start to appear at 28\u00b0, 41\u00b0, and 45\u00b0, indicating the growth of three-dimensional ordering. Intensive heat treatment at 2273 K results in sharp and distinctive XRD peaks, suggesting the development of a three-dimensional crystalline structure in PG@2273 (Table S2). On the other hand, heat treatment does not significantly change the XRD patterns of ACF (Fig.\u00a02(b2)), although the stacking of graphene layers can be observed in the TEM images (Fig.\u00a01(b3)). The classical work on the graphitization of carbon by R.E. Franklin [42, 43] indicates that graphitizable carbons have weakly linked, compact, and nearly parallel-oriented carbon crystallites, while non-graphitizable carbons have rigidly linked carbon crystallites with random orientations. ACF is a typical non-graphitizable carbon and does not form a highly crystalline structure even upon heat treatment at 2273 K. On the contrary, PG has a representative graphitizable nature even if the as-prepared PG has a disordered graphene unit structure. The observed graphitizable behavior of PG is ascribed to the microscopic assembly structure and property of disoriented graphene units, which means a large planar size and high flexibility.The N2 adsorption isotherms of pristine and heat-treated carbons at 77 K can provide their surface and porosity information. The adsorption isotherm of PG is a combination of type I and IV (Fig.\u00a03\na) according to the IUPAC classification [44]. A significant N2 uptake occurs at low P/P0 region and a distinct hysteresis above P/P0\u00a0=\u00a00.4, indicating the presence of both micropores and mesopores. ACF has an N2 adsorption isotherm of type \u2160(b) (Fig.\u00a03b), indicating the presence of wider micropores and/or narrow mesopores [44]. Heat treatment at 1873 K induces a slight decrease in N2 uptake for both PG and ACF, while heating at 2273 K results in a significant decrease in N2 uptake due to reduction in porosity. The porosity evolution of PG (Bay carbon graphite as precursor) with the increase of heat-treatment temperature has been systematically investigated in our previous study [26]. The porosity degradation mostly occurs within the temperature region of 1373 ~ 2273 K at an accelerating elevation and finishes at 2473 K. This is because the mutual-stacking of graphene layer in PG needs to overcome a certain energy barrier. The porosity of PG degrades very fast once this energy barrier is broken through. The PG and ACF heated at 1873 K must be applicable to metallic catalyst supports. The porosity parameters of the pristine and heat-treated carbons are listed in Table S3.Heat treatment can induce the graphitization of porous carbons by decreasing their defects and edges, thereby increasing their oxidation resistance. Herein, TG measurements of pristine and heat-treated carbons under dry air were performed. Pristine PG shows a slow weight loss up to 600 K (Fig.\u00a04\na), which is related to the decomposition of oxygen functional groups and desorption of adsorbed gas. Then, it starts burning rapidly and finishes at approximately 800 K. The temperatures at 5% and 50% burn-off ratios are 668 and 802 K, respectively. These two temperatures are defined as \u201cburning threshold temperature\u201d (BTT) and \u201chalf-burned temperature\u201d (HBT) in this study to evaluate the oxidation resistance. The pristine PG residual after TG measurement is less than 0.1wt%. PG@1873 has a BTT and HBT of 830 K and 906 K, respectively, and is more thermally stable than pristine PG. The PG@1873 residual after TG measurement is undetectable. PG@2273 shows almost no weight loss at temperatures below 900 K, and starts burning rapidly at higher temperatures. The BTT and HBT of PG@2273 are 945 K and 1016 K, respectively.Heat treatment induces a similar change to ACF compared with PG, i.e., the BTT and HBT of ACF increase with the heating temperature (Fig.\u00a04b, Table S4). However, the increase in BTT and HBT of PG is more significant than those of ACF (Fig.\u00a04c, 4d), which results into higher thermal stability of heat-treated PGs. Pristine PG, on the other hand, shows lower thermal stability than pristine ACF. The increase in the thermal stability of carbon through heat treatment can be ascribed to the following two factors. The heat treatment can remove trace amounts of metal impurities [45, 46], which act as catalysts during the gasification of carbon. Another reason is that graphitization reduces the edges and defects of carbon, on which the oxidation is 102 to 103 times faster than that on the basal planes [27]. Pristine ACF is metal-free carbon, which has a higher oxidation resistance than that of pristine PG, which contains trace amounts of metal impurities from the Hummer process and KOH activation even after soaking in 1 mol L\u22121 HCl solution. Heat treatment can effectively remove trace amounts of metal impurities in PG, thus diminishing the catalytic effect of oxidation. Moreover, PG has a more graphitizable structure than ACF; its defects and edge carbons decrease faster than those of ACF, ensuring a more pronounced increase in thermal stability under heat treatment.Porous carbons are often used as supports in many catalytic reactions. Consequently, the high oxidation resistance of porous carbon in the presence of metal is of significant importance. PG@1873 prepared in the present study has both a large surface area and high crystallinity, and is used as the model for further investigation. Herein, a TG study was performed on PG@1873 loaded with three kinds of metals (Pd, Ni, and Fe; 1.0wt%). Metal-loaded ACF@1873 was studied in the same way for comparison. The synchrotron XRD patterns of all metal-loaded carbons show no extra diffraction peaks except for the carbon supports (Fig. S3), suggesting a highly dispersed state of the loaded metals. The above three metals are usually dispersed in single-atom states if they are loaded at low weight ratios (around 1.0wt%) on carbons [47\u201349]. TG measurements indicate that the BTT and HBTs of all carbons decrease after the metal loading (Fig.\u00a05\na, 5b and Table S5). By comparing the TG curves of PG@1873 and ACF@1873 loaded with the same metal, it was found that M-PG@1873 showed a smaller decrease in both BTT and HBT than M-ACF@1873 (Fig.\u00a05c, 5d and Fig. S4), which means that PG@1873 is more oxidation-resistant than ACF@1873 in the presence of a metal catalyst. In particular, the oxidation resistance of PG@1873 for Fe catalysts is evident; the BTT of Fe-PG@1873 is 130 K higher than that of Fe-ACF@1873. Molecular dynamic studies indicate that Ni atoms at the edge of graphene can fracture the C-C bond and promote the diffusion of carbon atoms, while those at the graphene plane are much less active [50]. PG@1873 prepared in this work has a highly crystalline graphene plane with fewer edges and defects, on which the metal catalyst cannot break the C-C bond effectively, and therefore is more thermally stable than less ordered metal-loaded ACF@1873.It is important to understand the oxidation resistance as the metal loading increases. Here we also studied the oxidation resistance of PG@1873 and ACF@1873 with Fe loading of 3.0wt%. The synchrotron XRD pattern of thus prepared Fe-PG@1873 show a few diffraction peaks from Fe-contained compound (Fig. S5), while that of Fe-ACF@1873 show no extra diffraction peaks. Nanoparticles with higher crystallinity may be preferentially produced on graphene walls with higher graphitization degree. Nevertheless, the BTT and HBT of both PG@1873 and ACF@1873 decrease with the heating temperature, and PG@1873 shows much smaller decrease in both BTT and HBT than ACF@1873 with Fe loading of 1.0wt% and 3.0wt% (Fig. S6). In particular, PG@1873 loaded with 3.0wt% of Fe still have considerably high oxidation resistance.The oxidation resistance of metal-loaded PG@1873 is also compared with those of other reported carbon supports (Fig.\u00a06\na, 6b, 6c). Because of the difficulty in many studies to determine the BTT caused by the decomposition of metal compounds for catalysts at lower heating temperatures, we only compared the HBT of different carbons. The nomenclature, abbreviation, and detailed information of these metal-loaded carbons are given in Tables S6, S7, and S8. It is evident that the oxidation resistance of PG@1873 is remarkable compared with most of the carbon supports in the presence of metal, while that of ACF@1873 is average. The oxidation resistance of metal-containing carbon is highly related to its graphitization degree, as discussed in this section. The high graphitization degree of PG@1873 ensures superior oxidation resistance than in case of other carbons. It is worth mentioning that MWCNT-2, MWCNT-4 and MWCNT-5 show higher oxidation resistance than the PG@1873 reported in this study. It is explained by the almost perfect crystal structure of the well-prepared multiwall carbon nanotubes (MWCNT). The TEM image and XRD pattern of MWCNT-2 clearly indicate its highly ordered graphene layers and sharp (002) diffraction peak [51], suggesting that it has a higher ordering than that of PG@1873. Nevertheless, MWCNT has the limitation of small specific surface area and nonporous features as catalyst supports. A high surface area is essential for high-quality catalytic support. We plotted the HBT versus specific surface areas for the carbon supports whose surface areas are available in their references. The present PG@1873 is situated near the right-upper corner of Fig.\u00a06d, which is far from other catalyst supports. The partially graphitized porous graphene PG@1873 prepared in the present work has both the merits of high oxidation resistance and large surface area, showing great potential as catalyst support for reactions under mild oxidative conditions.This study reveals that heat treatment can improve the oxidation resistance of porous graphene more efficiently than conventional porous carbon, such as activated carbon fiber. Porous graphene has a more graphitizable structure than ACF because of its unique constitutional graphene units with large planar size and high flexibility. The partially graphitized porous graphene PG@1873 prepared in this work has both the merits of large surface area and high oxidation resistance. After metal loading, the oxidation resistance of PG@1873 is well maintained, while that of ACF@1873 deceases. Such unique properties make PG@1873 a promising catalyst support for catalytic reactions conducted under oxidative conditions.\nShuwen Wang: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing-original draft. Yasunori Yoshikawa: Investigation. Zhipeng Wang: Investigation. Hideki Tanaka: Formal analysis, Investigation. Katsumi Kaneko: Conceptualization, Methodology, Writing- 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 work was supported by a Grant-in-Aid for Scientific Research (B) [grant number 17H03039] and the OPERA Japan Science Technology Agency project, Japan.Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cartre.2021.100029.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Porous graphene (PG) prepared from reduction and KOH activation of graphene oxide was heat-treated under argon atmosphere to obtain a partially graphitized porous carbon with high oxidation resistance. Transmission electron microscopy, Raman spectroscopy, synchrotron X-ray diffraction, and N2 adsorption isotherms (77 K) clearly illustrate the structural ordering and porosity change of PG under heat treatment. Pitch-based activated carbon fiber (ACF) was studied for comparison. PG is more graphitizable than ACF under heat treatment because it consists of highly flexible graphene units of larger size than those in ACF. Thermogravimetric studies indicate that heat treatment enhances more the thermal stability of PG than ACF, and metal-loading has a less detrimental effect on the thermal stability of heat-treated PG than heat-treated ACF and other reported carbon supports. Heat-treated PG shows great superiority to other carbon supports due to its both splendid oxidation resistance and high surface area. This study provides a promising route for the preparation of carbon-based catalyst supports for mild oxidative environments.\n               "}
{"full_text": "The steam reforming of hydrocarbons is a widely used process for obtaining hydrogen. Nowadays, almost 50 % of its worldwide production is acquired via steam reforming reaction [1,2]. The process analyzed in this research occurs between methane and steam, specifically. It produces a mixture of H2 and CO, commonly known as syn-gas. Until the two last decades, H2 was mainly obtained using large scale reforming reactors, for the needs of the industrial ammonia synthesis on the way of the Haber-Bosch process [3]. Currently, the demand for scaled-down reactors is rising, due to the rapid development of fuel cells [4] and the necessity of producing hydrogen on-the-spot [5]. Small-scale reactors currently surpass the large-scale reactors, due to the lack of proper pipelines for the hydrogen transport. For now, gas distribution networks are not suitable for transporting hydrogen [6]. Therefore, it is more cost-effective to set up a small-scale reactor, directly in the location of hydrogen demand [6]. The small-scale reactors are also advantageous when it comes to the processing of distributed or stranded resources for hydrogen production [7]. These facts force a growing interest in the improvement of the small-scale reactor's performance. However, scaling down the steam reforming reaction is not a simple task. To keep the cost of a small-scale unit relatively low, they often have to come in a modular and standardized design. The operating conditions of the small-scale applications are also required to be less severe than in the industrial-scale reactors, to reduce the material cost of a single reactor [6]. Following, the temperature of the process has to be reduced. That results in a reduction of the methane conversion [8], being just one of the issues connected with scaling down the process [6].By far, researches were mostly focused on the parametric study and optimization of the reaction conditions [9,10]. However, this approach has limited effectiveness, as the reaction conditions can be improved just to a certain level. Therefore, further development of the process is pursued by the introduction of new materials and design concepts, like new catalyst structures [11,12], introducing new kinds of catalyst supports [13], or by rethinking the design of the reactor itself [14\u201316]. Other works focus on an optimization of the process' cost effectiveness by the introduction of renewable sources for hydrogen production [17\u201319]. The cited researchers have focused on different aspects of the steam reforming and the possibilities of its enhancement and cost reduction.Another captivating opportunity was pointed out by Palma et\u00a0al. [20], who introduced a structured catalyst for the intensification of the reforming reaction. Their work concentrated on the importance of an effective heat transfer and its beneficial influence on the reaction rate. They confirmed that improving axial and radial temperature distribution results in better performance of the reactor, due to rising catalytic activity. Yun et\u00a0al. [21] have also focused on the enhancement of heat transfer during the process. They proposed a reactor design with a maximized heat transfer area, which had also led to better performance of the reforming unit. Dubinin et\u00a0al. have stated that proper handling of heat in the reforming process can also lead to an increase of the process' cost-effectiveness, as excess heat could be reused by applying a heat exchanger and incorporating the reactor into a mini-CHP unit [22].The strong endothermic character of the reaction causes the development of a non-uniform temperature field inside the reformer [23]. Rapid temperature decay at the upstream region has a consequence in the presence of thermal stresses inside the reactor, leading to its uneven degradation and reduction of the unit's lifetime [24]. Thus, the unification of the temperature distribution may not only improve the operational conditions but also achieve easier control of the process. Therefore, our team decided to develop an original strategy for the improvement of the temperature distribution across a steam reforming reactor. Motivated with promising results obtained by Settar et al. [25], we decided to improve their idea of the macro-patterned active surfaces [25] with the introduction of metallic foam matrices [26]. Their first research, which was reported in Ref. [25] introduced the division of the catalytic region of a wall-coated reformer and separating the created zones with inter-catalytic spacing, which led to an intensification of the hydrogen production. The second one [26] focuses on providing advantageous thermal conditions for the reaction, as metallic foams were previously reported to deliver a considerable heat transfer area [27], leading to the improvement of the temperature distribution inside the reactor. First of all, instead of plating a catalyst only on the wall surface, we decided to fill a whole reactor's volume with a catalytic composite of nickel and yttria-stabilized zirconia (Ni/YSZ), to maximize its active surface contained in the reactor. Then, the reforming unit was divided into separate segments, and part of them got substituted with non-catalytic metallic foam, to adjust the intensity of the reaction proceeding, leading to the unification of the thermal field inside the reactor. The acquired results showed that the macro-patterning concept could be successfully used to alter the temperature field in the reforming reactor and enhance the reaction rate [16]. This was also confirmed by further investigations by Settar et al. [28,29].Basing on the previously cited works, we decided that the proper optimization of the segments' alignment is necessary, to achieve a uniform temperature distribution, simultaneously maintaining as high a methane conversion rate as possible. The presented research regards the optimization of the particular segments distribution and adjustment of Ni/YSZ densities used in the catalytic ones, for the proper altering of the reforming reaction rate (Section Mathematical model). Optimization of the given issue is meant to allow the unification of the temperature distribution in the macro-patterned reactor [16]. However, it has to be conducted with respect to maintaining high hydrogen production effectiveness, determining that the problem we are dealing with is a multiobjective optimization. The objectives behave contrarily, as the temperature gradients can be only reduced by limiting the amount of the catalyst used in a particular reactor, which has a consequence in the decreasing of the methane conversion rate. The less a catalyst is used, the more a reaction rate is diminished. The described problem becomes even more complex if we consider the amount of possible segments combinations (Section Genetic algorithm). Many configurations can be falsely recognized as a global optimum when being only an optimal solution in the currently investigated part of the search space. Moreover, the steam reforming numerical simulation has a considerable computational time, as it combines heat and mass transfer phenomena, as well as chemical reactions occurring during the process [30]. Therefore, its implementation in a proper optimization algorithm is a demanding task.The genetic algorithm (GA) was chosen to be the mean of this optimization. Due to its non-deterministic nature and a vast search region, it is expected to handle with even adversely conditioned problems successfully. Moreover, it has a high plausibility of finding the global optimum in a limited time [31]. Previous researches have confirmed the applicability of GAs to the steam reforming optimization. Taji et\u00a0al. have successfully conducted a multiobjective optimization of the methane conversion rate and hydrogen yield in an industrial hydrogen plant [32]. The optimization have taken into account the catalyst deactivation over time. On the other hand, Zheng et\u00a0al. have reported the results of a single objective GA optimization of the methane conversion, by the adjustment of the reaction parameters and the catalyst load in a micro-reactor [33]. The presented works validate GA to be an effective technique for the steam reforming optimization, as the method allows for definition of many objectives, which may be pursued effectively, regardless of the number of considered parameters [33,34]. Although, implementations of GA in the reforming optimization have been reported previously, no papers regarding a multiobjective optimization of the reactor design has been found. The multiobjective optimization of the catalyst distribution in a small-scale steam reforming reactor constitutes the original contribution of this paper. The presented work introduces new aspects in the development of the steam reforming process, as follows:\n\n\u2022\nincorporation of the physical properties of the metallic foams into the mathematical model, both for the catalytic and non-catalytic segments,\n\n\n\u2022\nthe multiobjective optimization of the segments alignment and their morphological features using a genetic algorithm,\n\n\n\u2022\nthe improvement of the temperature distribution, by altering the amount of catalyst loaded into a single reactor, maintaining a relatively significant methane conversion rate.\n\n\nincorporation of the physical properties of the metallic foams into the mathematical model, both for the catalytic and non-catalytic segments,the multiobjective optimization of the segments alignment and their morphological features using a genetic algorithm,the improvement of the temperature distribution, by altering the amount of catalyst loaded into a single reactor, maintaining a relatively significant methane conversion rate.The geometry of the reformer considered in this analysis is presented in Fig.\u00a01\n. It was assumed to be an axisymmetric tubular reactor. The analyzed model is steady. The reforming unit consist of a cylindrical pipe, divided into thirty separate segments, indicated with the red dashed line. The segments vary in values of porosity, pore diameter and density of the used catalytic material. The flow of gases is assumed to be laminar, steady and in one direction. The fluids taking part in the process are assumed to be Newtonian. The properties of the substances included in the reactions were taken from the literature [35].The reactor is supplied with a mixture of hydrogen (H2) and steam (H2O). The relation between their amounts is described with steam-to-carbon ratio (SC). This parameter is very vital for the steam reforming, as too low a SC may have a consequence in carbon deposition, limiting the active surface of the catalyst [36]. The two main reactions occurring during the considered process are as follows:\n\n-\nmethane/steam reforming (MSR) reaction:\n\n\n\n\n(1)\n\n\nC\n\nH\n4\n\n+\n\nH\n2\n\nO\n\u2192\n3\n\nH\n2\n\n+\nC\nO\n\n,\n\n\u0394\n\nH\n\nM\nS\nR\n\n\n=\n206\n.\n1\n\n\n\nk\nJ\n\n\nm\no\nl\n\n\n,\n\n\n\n\n\n-\nwater-gas shift reaction (WGS):\n\n\n\n\n(2)\n\nCO\n+\n\nH\n2\n\nO\n\u21cc\n\nH\n2\n\n+\n\nCO\n2\n\n,\n\n\u0394\n\nH\nWGS\n\n=\n\u2212\n41.15\n\n\nkJ\nmol\n\n.\n\n\n\nmethane/steam reforming (MSR) reaction:water-gas shift reaction (WGS):Equations (1) and (2) were incorporated into the model by the formulation of a proper expression describing the rates of the reactions. The MSR reaction proceeds rather slowly and its rate can be described with:\n\n(3)\n\n\nR\nMSR\n\n=\n\n\nw\n\u02d9\n\ncat\n\n\nA\nMSR\n\nexp\n\n\n\u2212\n\n\nE\na\n\n\n\nR\n\u00af\n\nT\n\n\n\n\n\np\n\nCH\n4\n\n\u03b1\n\n\np\n\n\nH\n2\n\nO\n\n\u03b2\n\n.\n\n\n\nThe exact values of parameters contained in (3) were acquired during experimental research conducted earlier by our team [30]. The WGS reaction, expressed with (2) has a contrary nature, as it proceeds fast. It has been assumed to remain in equilibrium under the conditions present during the considered process, as explained in Ref.\u00a0[37]. This was also confirmed by analyzes conducted previously [38\u201341]. According to the given literature review, this assumption can be validated. Therefore, CO, CO2, H2 and H2O have to satisfy the equilibrium equation, given below:\n\n(4)\n\n\nK\nWGS\n\n=\n\n\nk\nWGS\n+\n\n\nk\nWGS\n\u2212\n\n\n=\n\n\n\np\n\nCO\n2\n\n\n\np\n\nH\n2\n\n\n\n\n\np\nCO\n\n\np\n\n\nH\n2\n\nO\n\n\n\n\n=\nexp\n\n\n\u2212\n\n\n\u0394\n\nG\nWGS\n0\n\n\n\n\nR\n\u00af\n\nT\n\n\n\n\n,\n\n\nallowing to formulate the WGS reaction rate equation, as follows:\n\n(5)\n\n\nR\nWGS\n\n=\n\nk\nWGS\n+\n\n\np\nCO\n\n\np\n\n\nH\n2\n\nO\n\n\n+\n\nk\nWGS\n\u2212\n\n\np\n\nH\n2\n\n\n\np\n\nCO\n2\n\n\n.\n\n\n\nThe value of Eq. (5) can be acquired through the analysis of the reforming process stoichiometry and balancing the chemical species. Following that, we are able to specify the methane conversion rate \n\nx\nc\nr\n\n and carbon monoxide conversion rate \n\ny\nc\nr\n\n:\n\n(6)\n\nx\nc\nr\n=\n1\n\u2212\n\n\n\nn\n\nCH\n4\n\ninlet\n\n\u2212\n\nR\nMSR\n\nV\n\n\nn\n\nCH\n4\n\ninlet\n\n\n,\n\n\n\n\n\n(7)\n\ny\nc\nr\n=\n\n\n\nK\nWGS\n\n+\n3\nx\nc\nr\n\u2212\n\n\u03c7\n\u2212\n\u03c9\n\n\n\n2\n\n\n\nK\nWGS\n\n\u2212\n1\n\n\n\n\n,\n\n\nwhere:\n\n(8)\n\n\u03c7\n=\n\n\n\n\nK\nWGS\n\nS\nC\n+\n3\nx\nc\nr\n\n\n2\n\n,\n\n\n\n\n\n(9)\n\n\u03c9\n=\n4\n\nK\nWGS\n\nx\nc\nr\n\n\n\nK\nWGS\n\n\u2212\n1\n\n\n\n\nS\nC\n\u2212\nx\nc\nr\n\n\n,\n\n\nand consequently calculate the partial pressures included in Eq. (4), basing on the reactions' stoichiometry as well, leading to the following relation [30]:\n\n(10)\n\n\nR\nWGS\n\n=\n\n\nn\n\nCH\n4\n\noutlet\n\nV\n\n=\n\n\n\nn\n\nCH\n4\n\ninlet\n\nx\nc\nr\n\nV\n\ny\nc\nr\n.\n\n\n\nAfter connecting Eq. (6) with Eq. (10) and applying simple mathematical transformations, a final expression for the WGS reaction's rate is formed:\n\n(11)\n\n\nR\nWGS\n\n=\n\nR\nMSR\n\ny\nc\nr\n.\n\n\n\nThe mass consumption and production rates of the mentioned reactions (Eqs. (1) and (2)) are summarized in Table 1\n. These values are further applied into the heat transfer model (Section Heat and mass transfer model). Now, the chemical reactions model is almost complete, as only thermodynamic heat generation rates of the reactions are left for formulation. They can be acquired by multiplying the reaction rates (Eqs. (3) and (11)) by their enthalpies:\n\n(12)\n\n\nQ\nMSR\n\n=\n\u2212\n\u0394\n\nH\nMSR\n\n\nR\nMSR\n\n,\n\n\n\n\n\n(13)\n\n\nQ\nWGS\n\n=\n\u2212\n\u0394\n\nH\nWGS\n\n\nR\nWGS\n\n.\n\n\n\nThe mathematical model used in this analysis is based on the fundamental transport equations (Eqs. (14)\u2013(18)). The governing equations were derived using volume-averaging method, due to application of porous structures [42]:\n\n(14)\n\n\n\n\u2202\n\n\n\n\u03c1\n0\n\n\nU\nx\n\n\n\n\n\n\u2202\nx\n\n\n+\n\n1\nr\n\n\n\n\u2202\n\n\nr\n\n\u03c1\n0\n\n\nU\nr\n\n\n\n\n\n\u2202\nr\n\n\n=\n0\n,\n\n\n\n\n\n(15)\n\n\n\n\u03c1\n0\n\n\n\u03b5\n0\n2\n\n\n\n\n\nU\nx\n\n\n\n\u2202\n\nU\nx\n\n\n\n\u2202\nx\n\n\n+\n\nU\nr\n\n\n\n\u2202\n\nU\nx\n\n\n\n\u2202\nr\n\n\n\n\n=\n\u2212\n\n\n\u2202\nP\n\n\n\u2202\nx\n\n\n+\n\n\u03bc\n\n\u03b5\n0\n\n\n[\n\n\n\n\u2202\n2\n\n\nU\nx\n\n\n\n\u2202\n\nx\n2\n\n\n\n+\n\n1\nr\n\n\n\u2202\n\n\u2202\nr\n\n\n\n\nr\n\n\n\u2202\n\nU\nx\n\n\n\n\u2202\nr\n\n\n\n\n\u00a0\n\u2212\n\n\u03bc\n\nK\np\n\n\n\nU\nx\n\n\u2212\n\n\n\n\u03c1\n0\n\n\nc\nine\n\n\n\n\nK\np\n\n\n\n\nU\nx\n\n\n\nU\nx\n2\n\n+\n\nU\nr\n2\n\n\n,\n\n\n\n\n\n(16)\n\n\n\n\u03c1\n0\n\n\n\u03b5\n0\n2\n\n\n\n\n\nU\nx\n\n\n\n\u2202\n\nU\nr\n\n\n\n\u2202\nx\n\n\n+\n\nU\nr\n\n\n\n\u2202\n\nU\nr\n\n\n\n\u2202\nr\n\n\n\n\n=\n\u2212\n\n\n\u2202\nP\n\n\n\u2202\nr\n\n\n+\n\n\u03bc\n\n\u03b5\n0\n\n\n\n\n\n\n\n\u2202\n2\n\n\nU\nr\n\n\n\n\u2202\n\nx\n2\n\n\n\n+\n\n1\nr\n\n\n\u2202\n\n\u2202\nr\n\n\n\n\nr\n\n\n\u2202\n\nU\nr\n\n\n\n\u2202\nr\n\n\n\n\n\u2212\n\n\nU\nr\n\n\nr\n2\n\n\n\n\n\u2212\n\n\u03bc\n\nK\np\n\n\n\nU\nr\n\n\u2212\n\n\n\n\u03c1\n0\n\n\nc\n\ni\nn\ne\n\n\n\n\n\nK\np\n\n\n\n\nU\nr\n\n\n\nU\nx\n2\n\n+\n\nU\nr\n2\n\n\n,\n\n\n\n\n\n(17)\n\n\n\u03c1\n0\n\n\nC\np\n\n\n\n\nU\nx\n\n\n\n\u2202\n\nT\nloc\n\n\n\n\u2202\nx\n\n\n+\n\nU\nr\n\n\n\n\u2202\n\nT\nloc\n\n\n\n\u2202\nr\n\n\n\n\n=\n\n\u2202\n\n\u2202\nx\n\n\n\n\n\n\u03bb\neff\n\n\n\n\u2202\n\nT\nloc\n\n\n\n\u2202\nx\n\n\n\n\n+\n\n1\nr\n\n\n\u2202\n\n\u2202\nr\n\n\n\n\nr\n\n\u03bb\neff\n\n\n\n\u2202\n\nT\nloc\n\n\n\n\u2202\nr\n\n\n\n\n+\n\nQ\ns\n\n,\n\n\n\n\n\n(18)\n\n\n\u03c1\n0\n\n\n\n\nU\nx\n\n\n\n\u2202\n\nY\nj\n\n\n\n\u2202\nx\n\n\n+\n\nU\nr\n\n\n\n\u2202\n\nY\nj\n\n\n\n\u2202\nr\n\n\n\n\n=\n\n\u2202\n\n\u2202\nx\n\n\n\n\n\n\u03c1\n0\n\n\nD\n\nj\n,\neff\n\n\n\n\n\u2202\n\nY\nj\n\n\n\n\u2202\nx\n\n\n\n\n+\n\n1\nr\n\n\n\u2202\n\n\u2202\nr\n\n\n\n\nr\n\n\u03c1\n0\n\n\nD\n\nj\n,\neff\n\n\n\n\n\u2202\n\nY\nj\n\n\n\n\u2202\nr\n\n\n\n\n+\n\nS\nj\n\n.\n\n\n\nThe effective mass diffusivity of species \n\n\nD\n\nj\n,\neff\n\n\n\n is calculated using Eq. (19), which is explained below [43]:\n\n(19)\n\n\nD\n\nj\n,\neff\n\n\n=\n\n\n1\n\u2212\n\n1\n\u2212\n\n\u03b5\n0\n\n\n\n\n\nD\nj\n\n.\n\n\n\nThe permeability \n\n\nK\np\n\n\n of the specific segment is calculated using Eq. (20), basing on the information about its porosity \n\n\n\u03b5\n0\n\n\n [44]:\n\n(20)\n\n\nK\np\n\n=\n\n\n\n\u03b5\n0\n\n\n\n1\n\u2212\n\n\n\n1\n\u2212\n\n\u03b5\n0\n\n\n\n\n1\n/\n3\n\n\n\n\n\n\n36\n\n\n\n\n\n1\n\u2212\n\n\u03b5\n0\n\n\n\n\n1\n/\n3\n\n\n\u2212\n\n\n1\n\u2212\n\n\u03b5\n0\n\n\n\n\n\n\n\n\nd\np\n2\n\n,\n\n\nwhere d\np stands for an average pore diameter. The inertial coefficient c\nine was calculated using [45]:\n\n(21)\n\n\nc\nine\n\n=\n0.0095\n\ng\ns\n\n\u2212\n0.8\n\n\n\n\n\n\u03b5\n0\n\n\n3\n\n\n\u03c4\n\u2212\n1\n\n\n\n\n\n\n\n\n1.18\n\n\n\n\n1\n\u2212\n\n\u03b5\n0\n\n\n\n\n3\n\u03c0\n\n\n\n\n1\n\ng\ns\n\n\n\n\n\n\u2212\n1\n\n\n,\n\n\nwhere tortousity \u03c4 and shape function \n\n\ng\ns\n\n\n are expressed with following equations [44,45]:\n\n(22)\n\n\u03c4\n=\n\n\n\u03b5\n0\n\n\n1\n\u2212\n\n\n\n1\n\u2212\n\n\u03b5\n0\n\n\n\n\n1\n/\n3\n\n\n\n\n,\n\n\n\n\n\n(23)\n\n\n\ng\ns\n\n=\n1\n\u2212\nexp\n\n(\n\n\u2212\n\n\n1\n\u2212\n\n\u03b5\n0\n\n\n0.04\n\n\n)\n\n.\n\n\n\n\nThe properties of gases taking part in the process were taken from the literature [46]. The heat transfer model applied in this analysis, allows calculation of the effective thermal conductivity \n\n\n\u03bb\neff\n\n\n [47]. It describes heat propagation in a structure of a metallic foam, which can be calculated using:\n\n(24)\n\n\n\u03bb\neff\n\n=\n\n\n\n2\n\nl\n\n\n2\n\n\n\nR\nA\n\n+\n\nR\nB\n\n+\n\nR\nC\n\n+\n\nR\nD\n\n\n\n\n\n,\n\n\nwhere R\nA\u2212R\nD stand for the thermal resistances of the porous media cell subsections and can be calculated as follows [47]:\n\n(25)\n\n\nR\nA\n\n=\n\n\n4\nd\nl\n\n\n\n\n2\n\ne\n2\n\n+\nd\n\u03c0\n\n\n1\n\u2212\ne\n\n\n\n\n\n\u03bb\nsolid\n\n\n\n+\n\n\n4\nd\nl\n\n\n\n\n4\n\u2212\n\n\n2\n\ne\n2\n\n+\nd\n\u03c0\n\n\n1\n\u2212\ne\n\n\n\n\n\n\n\n\u03bb\nmix\n\n\n\n,\n\n\n\n\n\n(26)\n\n\nR\nB\n\n=\n\n\n\n\ne\n\u2212\n2\nd\n\n\nl\n\n\n\ne\n2\n\n\n\u03bb\nsolid\n\n+\n\n\n2\n\u2212\n\ne\n2\n\n\n\n\n\u03bb\nmix\n\n\n\n,\n\n\n\n\n\n(27)\n\n\nR\nC\n\n=\n\n\n\n\n\n2\n\n\u2212\n2\ne\n\n\nl\n\n\n\u03c0\n\nd\n2\n\n\n\u03bb\nsolid\n\n\n2\n\n+\n\n\n2\n\u2212\n\u03c0\n\nd\n2\n\n\n2\n\n\n\n\n\u03bb\nmix\n\n\n\n,\n\n\n\n\n\n(28)\n\n\nR\nD\n\n=\n\n\n2\ne\nl\n\n\n\ne\n2\n\n\n\u03bb\nsolid\n\n+\n\n\n4\n\u2212\n\ne\n2\n\n\n\n\n\u03bb\nmix\n\n\n\n.\n\n\n\nFormulas (25)\u2013(27) require knowledge of the foam ligament radius d, acquired by solving [47]:\n\n(29)\n\nd\n=\n\n\n\n\n2\n\n\n\n2\n\u2212\n2\n\n\u03b5\n0\n\n\u2212\n\n\n3\n\n2\n\n\n4\n\n\ne\n3\n\n\n\n\n\n\u03c0\n\n\n3\n\u2212\ne\n\u2212\n4\ne\n\n2\n\n\n\n\n\n\n.\n\n\n\nFor the needs of the numerical analysis, the dimensionless cubic node length e was set to be equal to 0.0339, as explained in Ref.\u00a0[48]. The values of the thermal conductivity \n\n\n\u03bb\nsolid\n\n\n for the catalytic material and metallic foam were taken from the literature [49,50]. The heat conductivity of the gases mixture \n\n\n\u03bb\nmix\n\n\n inside the reformer was calculated using the mixing laws [46].After having defined the mathematical model for the methane/steam reforming (Section Mathematical model) preparation of an adequate numerical model is needed. The Finite Volume Method was chosen for the discretization of the governing equations used in the mathematical model [51,52]. Each of the partial differential equations, described in Section Heat and mass transfer model, can be written in a generalized form, as follows Eq. (30):\n\n(30)\n\n\n\u03a8\nx\n\n\n\n\u2202\n\u03c6\n\n\n\u2202\nx\n\n\n+\n\n\u03a8\nr\n\n\n\n\u2202\n\u03c6\n\n\n\u2202\nr\n\n\n=\n\n\u2202\n\n\u2202\nx\n\n\n\n\n\u0393\n\n\n\u2202\n\u03c6\n\n\n\u2202\nx\n\n\n\n\n+\n\n1\nr\n\n\n\u2202\n\n\u2202\nr\n\n\n\n\nr\n\u0393\n\n\n\u2202\n\u03c6\n\n\n\u2202\nr\n\n\n\n\n+\n\n\nS\n.\n\n\u00af\n\n\n\n\nThe coefficients given in Eq. (30) originate from the transport Eqs. (17 and 18), and their values are gathered in Tables 1 and 2\n Although, if we consider a non-catalytic segment, the sources \n\n\nS\n\u00af\n\n\n for Eqs. (17) and (18) are equal to 0, as the chemical reactions are assumed to be suppressed on these segments [16].The discretized transport Eq. (31) was obtained after the integration of Eq. (30) over the created control volumes. Following simple mathematical transformations, it can be presented in a form, given by Ref.\u00a0[51]:\n\n(31)\n\n\n\n\n\n\n\n\u03a8\nx\n\n\u03c6\n\n\ne\n\n\u2212\n\n\n\n\n\u03a8\nx\n\n\u03c6\n\n\nw\n\n\n\n\nr\nm\n\n\u0394\nr\n+\n\n\n\n\n\nr\n\n\u03a8\nr\n\n\u03c6\n\n\nn\n\n\u2212\n\n\n\nr\n\n\u03a8\nr\n\n\u03c6\n\n\ns\n\n\n\n\u0394\nx\n=\n\n\n\n\n\n\u0393\n\n\n\u2202\n\u03c6\n\n\n\u2202\nx\n\n\n\n\ne\n\n\u2212\n\n\n\n\u0393\n\n\n\u2202\n\u03c6\n\n\n\u2202\nx\n\n\n\n\nw\n\n\n\n\nr\nm\n\n\u0394\nr\n+\n\n\n\n\n\nr\n\u0393\n\n\n\u2202\n\u03c6\n\n\n\u2202\nr\n\n\n\n\nn\n\n\u2212\n\n\n\nr\n\u0393\n\n\n\u2202\n\u03c6\n\n\n\u2202\nr\n\n\n\n\ns\n\n\n\n\u0394\nx\n+\n\nS\n\u00af\n\n\nr\nm\n\n\u0394\nr\n\u0394\nx\n,\n\n\n\n\n\n(32)\n\n\na\nP\n\n\n\u03c6\nP\n\n=\n\na\nE\n\n\n\u03c6\nE\n\n+\n\na\nW\n\n\n\u03c6\nW\n\n+\n\na\nN\n\n\n\u03c6\nN\n\n+\n\na\nS\n\n\n\u03c6\nS\n\n+\nb\n,\n\n\nwhere:\n\n(33)\n\n\na\nP\n\n=\n\na\nE\n\n+\n\na\nW\n\n+\n\na\nN\n\n+\n\na\nS\n\n,\n\n\n\n\n\n(34)\n\nb\n=\n\nS\n\u00af\n\n\nr\nm\n\n\u0394\nr\n\u0394\nx\n,\n\n\n\nThe Power Law scheme was applied to calculate the fluxes crossing the control volume faces. Each of these fluxes is represented by a specific coefficient \n\n\na\nj\n\n\n [51]. The subscripts E, W, N, S dicate the location of a specific face, relating to the analyzed node. They correspond to the geographic directions, therefore \n\n\na\nE\n\n\n represents a face on the left side of the node, \n\n\na\nW\n\n\n on the right, \n\n\na\nN\n\n\n a face above and \n\n\na\nS\n\n\n underneath the node. The momentum equations were solved using the SIMPLE algorithm, which allowed to calculate the velocity values in the specific grid points [51]. The created systems of partial differential equations were solved using a prepared Gauss-Seidel linear solver, due to its high robustness [51]. Afterwards, the numerical model was implemented in the C++ programming language. The complete numerical analysis (Section Numerical Results) was conducted using the designed in-house numerical code. The convergence criterion set for this analysis was defined as acquiring the difference between the values of the mass and heat sources, in two subsequent iterations, lower than 10\u22125. The grid's resolution was established at 150 elements in the longitudinal and 25 elements in the radial directions, resulting in a square-shaped element. These dimensions were assumed to be sufficient for the conducted analysis, as our previous research [16] already confirmed it.For this analysis, the \n\nS\nC\n\n was set to be equal to 2.0, as this ratio was previously reported to be high enough to effectively prevent carbon deposition [53]. Each segment of a specific reactor can be filled with non-catalytic steel foam or catalytic Ni/YSZ of various densities, calculated basing on the segment's porosity (Section Genetic algorithm). The segments' alignment is the most crucial aspect for altering the temperature distribution inside the reformer [16]. The endothermic character of the reaction results in a temperature drop within the catalytic segments and the higher density of the catalyt in a specific segment. The drop is expected to be greater, as the reaction becomes more intense [8]. The metallic foam segments were introduced for the limitation of the temperature decrease. They tend to have good thermal conducting properties, what combined with their considerable surface of heat exchange, is expected to allow the gases mixture to reheat effectively [27]. The value of \n\n\n\u03bb\nsolid\n\n\n was set at 22\u00a0W\u00a0m\u22121\u00a0K\u22121 for the catalytic material [49] and at 30\u00a0W\u00a0m\u22121\u00a0K\u22121 for the steel foam [50]. The temperature of the fuel flowing inside the reforming unit is considered to reach the temperature of the reformer instantly. The symmetry boundary conditions were applied at the symmetry axis, whereas the no-slip boundary conditions were set at the wall of the reactor. All of the boundary conditions were summarized in Fig.\u00a02\n. The thermal conditions in this analysis were set as follows:\n\n\u2022\ninlet temperature \n\nT\n=\n\nT\nin\n\n=\n900\n\n K at \n\nx\n=\n0\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,\n\n\n\u2022\noutlet temperature \n\n\u2202\nT\n/\n\u2202\nx\n=\n0\n\n at \n\nx\n=\nL\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,\n\n\n\u2022\nsymmetry boundary condition \n\n\u2202\nT\n/\n\u2202\nr\n=\n0\n\n at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\n0\n\n,\n\n\n\u2022\nwall temperature \n\nT\n=\n\nT\nwall\n\n=\n900\n\n K at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\nR\n\n.\n\n\ninlet temperature \n\nT\n=\n\nT\nin\n\n=\n900\n\n K at \n\nx\n=\n0\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,outlet temperature \n\n\u2202\nT\n/\n\u2202\nx\n=\n0\n\n at \n\nx\n=\nL\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,symmetry boundary condition \n\n\u2202\nT\n/\n\u2202\nr\n=\n0\n\n at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\n0\n\n,wall temperature \n\nT\n=\n\nT\nwall\n\n=\n900\n\n K at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\nR\n\n.The thermal boundary condition was chosen to be of the first-type, as setting a constant temperature value has the most negative influence on the unification of the temperature distribution inside the reformer. Due to this fact, the results of the conducted analysis are more reliable. The boundary conditions are also essential for calculating the mass transport equations:\n\n\u2022\ninlet mole fractions \n\n\nY\nj\n\n=\n\nY\n\nj\n,\nin\n\n\n\n at \n\nx\n=\n0\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,\n\n\n\u2022\noutlet mole fractions \n\n\u2202\n\nY\nj\n\n/\n\u2202\nx\n=\n0\n\n at \n\nx\n=\nL\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,\n\n\n\u2022\nsymmetry boundary condition \n\n\u2202\n\nY\nj\n\n/\n\u2202\nr\n=\n0\n\n at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\n0\n\n,\n\n\n\u2022\nno-slip boundary condition \n\n\nY\nj\n\n=\n0\n\n at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\nR\n\n.\n\n\ninlet mole fractions \n\n\nY\nj\n\n=\n\nY\n\nj\n,\nin\n\n\n\n at \n\nx\n=\n0\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,outlet mole fractions \n\n\u2202\n\nY\nj\n\n/\n\u2202\nx\n=\n0\n\n at \n\nx\n=\nL\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,symmetry boundary condition \n\n\u2202\n\nY\nj\n\n/\n\u2202\nr\n=\n0\n\n at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\n0\n\n,no-slip boundary condition \n\n\nY\nj\n\n=\n0\n\n at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\nR\n\n.The boundary conditions for the Navier-Stokes equations were provided as follows:\n\n\u2022\ninlet velocity U=U\nin=0.15 m s\u22121 at \n\nx\n=\n0\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,\n\n\n\u2022\noutlet velocity \n\n\u2202\nU\n/\n\u2202\nx\n=\n0\n\n at \n\nx\n=\nL\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,\n\n\n\u2022\nsymmetry boundary condition \n\n\u2202\nU\n/\n\u2202\nr\n=\n0\n\n at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\n0\n\n,\n\n\n\u2022\nno-slip boundary condition \n\nU\n=\n0\n\n at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\nR\n\n.\n\n\ninlet velocity U=U\nin=0.15 m s\u22121 at \n\nx\n=\n0\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,outlet velocity \n\n\u2202\nU\n/\n\u2202\nx\n=\n0\n\n at \n\nx\n=\nL\n\n and \n\n0\n\u2264\nr\n<\nR\n\n,symmetry boundary condition \n\n\u2202\nU\n/\n\u2202\nr\n=\n0\n\n at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\n0\n\n,no-slip boundary condition \n\nU\n=\n0\n\n at \n\n0\n\u2264\nx\n<\nL\n\n and \n\nr\n=\nR\n\n.The genetic algorithm (GA) was used to find the most optimal catalyst distribution in the methane/steam reforming process. This kind of optimization technique was developed by John Holland and his associates [31]. It is an example of a stochastic method, having its origins in the evolution process. Thus, vocabulary originating from biology and genetics is used for their description [31]. Although GAs are qualified as search algorithms, they can be distinguished from the conventional ones.First of all, they operate not with the parameters themselves, but with their representations in the form of finite-length strings, binary in the case of this analysis. GAs start their operation with a whole population of solutions, instead of a single solution. This approach is intended to limit the risk of the algorithm engage in a local extremum when a multimodal search space is considered. After acquiring results for the given set of solutions, they are evaluated with fitness functions, assigning a specific fitness value for each of the specimens. The calculated fitness is further used for determining which specimens perform best and should be preferred for the composition of a consecutive population of solutions. A gene is the smallest element of the GA, and it represents a single character of a code string. A chromosome stands for a complete code string, and in this analysis, it represents the parameters of a specific segment of the macro-patterned reactor, which are the segment's porosity and pore size. A set of chromosomes composes a genotype, which should be understood as a single specimen in a specific population.Having introduced the essential vocabulary allows to describe the principles of the GA used in this analysis. Its operation starts with randomizing the parameters of an initial population, composed of thirty reactors, each divided into thirty independent segments. The algorithm starts with defining segments\u2019 average pore size and porosity, which was constrained to be selected from values between 0.5 and 0.8. As for the lower porosities the pressure drop was reported to be overly significant [54]. The upper boundary was set at 0.8, due to the lower accessibility of the manufacturing methods for metallic foams of higher porosities [55]. Having the porosity for a specific segment, the algorithm proceeds to the selection of its average pore diameter. Its values were constrained as explained in Ref. [56]. What is more, digital material representations (DMR) of metallic foams were used to acquire exact ranges of pore size for foams of particular porosity (Section Digital material representation). After this part is done, it randomizes if a specific segment is meant to be a non-catalytic or catalytic one. If the later is chosen, the catalyst density \n\n\n\n\nw\n\u02d9\n\n\ncat\n\n\n is calculated with:\n\n(35)\n\n\n\n\n\nw\n\u02d9\n\n\ncat\n\n=\n\n\u03c1\ncat\n\n\u22c5\n\n(\n\n1\n\u2212\n\n\u03b5\n0\n\n\n)\n\n,\n\n\n\nwhere \n\n\n\u03c1\ncat\n\n\n was equal to 5.3448 106 g m\u22123, as the ratio of Ni to YSZ used for derivation of the reaction kinetics (Section Mathematical model) was 60:40 and the solid Ni density is equal to 8.908 106 g m\u22122. Afterwards, the GA calls the reforming simulation code over each of the specimens and when calculations are complete, it gets to the evaluation process, basing on two separate fitness functions, determining that we deal with a multiobjective optimization [57]. First of the fitness functions Eq. (36) calculates the methane conversion rate, determining the amount of methane which has undergone conversion during the reaction. The second one, expressed with Eq. (37), is based on the difference between the maximal and the minimal temperatures present inside reactors, \n\n\nT\nmax\n\n\n and \n\n\nT\nmin\n\n\n respectively. What is more, the described functions are in conflict, as maximization of Eq. (36) is connected with rising of the catalyst density used in the specific reactor and minimization of Eq. (37) requires an opposite approach, making the whole optimization process even more arduous. The equations for fitness calculation were formulated as follows:\n\n(36)\n\n\nf\n\nCH\n4\n\n\n=\n\n\nf\nr\na\n\nc\n\nCH\n\n4\nin\n\n\n\n\u2212\nf\nr\na\n\nc\n\nCH\n\n4\nout\n\n\n\n\n\nf\nr\na\n\nc\n\nCH\n\n4\nin\n\n\n\n\n\n,\n\n\n\n\n\n(37)\n\n\nf\nT\n\n=\n1\n\u2212\n\n\n\nT\nmax\n\n\u2212\n\nT\nmin\n\n\n\n\u0394\n\nT\nmax\n\n\n\n,\n\n\nwhere \n\n\u0394\n\nT\nmax\n\n\n stands for the temperature difference acquired for a reference case (Section Numerical results). After calculating values of Eqs. (36) and (37) a single fitness value f is needed. This can be acquired by application of the weighted-sum method [58]:\n\n(38)\n\nf\n=\n\u2211\n\nw\nj\n\n\nf\nj\n\n.\n\n\n\nThe weights \n\n\nw\nj\n\n\n were chosen arbitrarily and their values are equal to 0.6 for \n\n\nf\n\n\nCH\n4\n\n\n\n\n and 0.4 for \n\n\nf\nT\n\n\n, as it was decided that methane conversion is a more important factor in this process. Afterwards, the algorithm chooses the two best performing reactors, which advance to the next population intact and the crossover procedure begins. The crossover starts with a partially-random selection of potential parent specimens. Basing on the acquired fitness values, the probability of selection for each specimen is calculated. The probability value is acquired by dividing the fitness of each specimen by the sum of the fitness values acquired for the whole population. Then, specimens are paired using a roulette wheel selection and the recombination of their chromosomes starts [31]. The point of crossover is randomized, both chromosomes split and their bit strings are interchanged. In the case of this analysis, the algorithm performs two crossovers, first explicit and second implicit. The explicit one occurs first and is used for a crossover of the pore size and the porosity of a considered segment. Following that, the implicit one determines if that segment is supposed to be a catalytic one or not. It happens via a crossover of the catalyst density value. If its results are contained in the permissible range, which boundaries are calculated using Eq. (35), the segment is set to be catalytic and the catalyst's density is calculated basing on the porosity value acquired during the explicit crossover procedure. Next, the mutation is performed. It is a random process, converting a single gene in the bit string representing the segment's parameters, to have an opposite value. This mechanism helps avoiding the local extremum trap, by the introduction of genes unable to be acquired during the crossover process, as they are limited by the parameters generated for the initial population [31]. The mutation is described with its rate, corresponding to the probability of its occurrence. Depending on the literature source, many different rates are advised. Typical GAs with numerous populations are suggested to have mutation rate equal to 2 % [31] or \n\n1\n/\nX\n\n %, where X stands for the bit string's length [59]. However, in the case of this analysis, the population is strictly limited to thirty individuals and according to Ref. [60], a mutation rate equal to 10 % was applied. The mutation is individually performed for each segment, meaning that it is randomized thirty times for a specific reactor. This part of the GA's operation is repeated until a new population is complete. Then, the whole process starts over again, until the convergence criteria are met, which were set to be the methane conversion rate over 60 % and a temperature difference lower than 25 degrees. The algorithm operation is summarized in Fig. 3\n.For the improvement of pore size range accuracy, a set of over three hundred digital representations of metallic foam was generated. The process was conducted using an in-house code, developed basing on the random geometric graphs. The DMR generation approach is very similar to the algorithm presented in Neumann et al. [61].Exemplary foams are provided in Fig. 4\n. The composed algorithm starts its operation with the generation of a random graph in a cube of specified dimensions. Every node and connection is a source of a field similar to the gravity field. Afterwards, it creates a grid of voxels, composing a cuboid. Every voxel has its phase assigned basing on the phase force coefficients defined by the user. The force coefficients decide how highly a node or a connection can influence the given voxel.The generated set of DMRs was further uploaded to the Avizo software, which allowed for a quantitative analysis of the structures\u2019 morphology. Basing on the acquired results, the upper border of the average pore size for the considered range of porosity, is set at 0.002 m. The lower border of the average pore size range is described by the following equation:\n\n(39)\n\n\nd\np\n\n=\n3.2894\n\n\u03b5\n0\n\n\u2212\n0.6315\n.\n\n\n\nDefining the borders of an admissible average pore size completes the necessary assumptions. Now, the GA is provided with every information essential for calculations commencement.Having developed the described mathematical model and optimization algorithm allows to perform a set of numerical calculations. The strongest emphasis is put on analyzing the thermal conditions inside the reactor and the reaction's products. The pursued reactor has to combine improvement in the thermal conditions, maintaining as high a methane conversion rate as possible. Otherwise, the application of the identified solution would be unprofitable.Preceding the optimization process, a definition of a reference case is needed. The reference reactor has a homogeneous catalyst distribution, and its density in the particular segments is set to have the maximal possible value. Following, a reactor with maximal CH4 conversion and thermal conditions typical for a conventional reactor is composed. The maximal Ni/YSZ density is acquired for segments of the lowest admissible porosity, equal to 0.5, resulting in a density value equal to 2.67 \n\u22c5\n 106 g m\u22123 (Eq. (35)).The fitness value calculated for the reference case is equal to 0.53 and its temperature distribution is presented in Fig. 5\n. After analyzing the temperature field, the most significant decrease in its value can be noticed at the inlet of the reactor. The observed drop in the tempaerature value occurs due to the activation of the reforming reaction [8]. A conclusion can be drawn, that the MSR reaction dominates the temperature field formation. Closing to the reactor's outlet, less CH4 remains left for conversion. Thus, less heat is consumed by the MSR reaction and the temperature gradients start to diminish.For the reference case, the methane conversion rate reached 0.82. The distribution of the mole fractions is presented in Fig. 6\n. The mole fractions change in the longitudinal direction, confirm the previous conclusion about change of temperature gradient, being correlated to the amount of CH4 left for conversion. Therefore, it can be concluded that the thermal conditions can be moderated by changing the rate of the reforming reaction.The developed genetic algorithm starts its operation with the generation of the initial population. It is composed of thirty specimens, with fully randomized parameters of the segments. Afterwards, the methane/steam reforming simulation is called over each of them. The initial set's distribution of thermal fitness \n\n\nf\nT\n\n\n and methane conversion rate \n\n\nf\n\n\nCH\n4\n\n\n\n\n, is presented in Fig. 7\n. The calculated fitness value for the best solution included in the initial population is equal to 0.47. Its temperature distribution (Fig. 8\n) and mole fractions are presented in Figs. 8 and 9\n.The results acquired after computing ten subsequent generations brought a substantial improvement. Both, in the unification of the temperature field and methane conversion rate, when compared to the results for the initial generation. The fitness of the best specimen improved to 0.68.The overall distribution of fitness values in the 10 th generation is presented in Fig. 10\n. The specimens begin to be focused in the pursued region of the search space, what can be noticed in Fig. 10. The temperature distribution acquired for the most fit specimen in this generation is provided in Fig. 11\nb). Its methane conversion rate reached 0.56 and the exact changes of mole fractions in the longitudinal direction are presented in Fig. 12\n.The 30th generation brought only a slight improvement, considering the optimal solution. The fitness values acquired for the specimens became to be focused in a very narrow region of the search space (Fig. 13\n). Therefore, the algorithm has ended global exploration and started to search for the optimum locally.The fitness value acquired for the best solution contained in the 30th generation is equal to 0.74. However, no considerable improvement has been noted, considering the thermal conditions (Fig. 14\n). Therefore, the reason of improvement of the fitness value should be sought in the methane conversion rate. As expected, it rose to 0.61. The exact change of the mole fractions in the longitudinal direction of the fittest reactor is presented in Fig. 15\n.The final set of solutions characterizes itself with an improvement of the methane conversion rate. The acquired thermal conditions are similar to the ones in the 30th generation. The overall distribution of fitness values for the 50th generation is presented in Fig. 16\n. The best solution reached a fitness of 0.78. Its temperature distribution is provided in Fig. 17\nb). Like for the 30th generation, only \n\n\nf\n\n\nCH\n4\n\n\n\n\n has improved and the value of f\n\n\nT\n\n was maintained (Fig. 16). The mole fractions distribution is presented in Fig. 18\n. The CH4 conversion rate is equal to 0.64, setting around 80 % of the value acquired for the reference case.The observed decrease of f\nCH4 is directly connected with the reduction of the Ni/YSZ amount used. Apparently, the algorithm detected the region of the highest temperature decrease at the reactor's inlet. Thus, the amount of the catalytic material in that region was reduced (Fig. 19\nb), to improve the thermal conditions. As it can be observed, the Ni/YSZ amount is increasing, closing to the reactor outlet. The algorithm did it on purpose, to maximize the CH4 conversion rate for the presented solution. The overall amount of catalytic material was strictly limited, resulting in ten out of thirty catalytic segments only (Fig. 19b).To summarize the algorithm's operation, the differences between subsequent specimens should be confronted. Radius averaged temperature distribution in the reactor is an adequate method of comparison between fittest solutions in particular generations. Following the results presented in Fig. 20\n, a significant change in the temperature distribution can be noted. The temperature difference value was reduced to 23.7\u00a0K, from 44.8 K acquired for the reference case. However, the presence of peaks in the temperature values may induce catalyst degradation, just like for the reference case. The observed peaks may be a consequence of a single segment's dimensions. Possibly, narrower segments would result in smoothing the noticed peaks, although their future manufacturing would be more challenging.The presented analysis aimed to improve the methane/steam reforming process, through the optimization of the thermal conditions inside a reforming reactor. The macro-patterning concept was introduced, as it appears to be a valid strategy for the improvement of the reforming process. The concept's principle is to divide the reformer's tube into separate segments, which are further filled with non-catalytic metallic foam or Ni/YSZ. The optimization was conducted using a genetic algorithm. The GA altered the segments composition and their porosity, to maximize the CH4 conversion rate and minimize the temperature gradients.Implementation of the morphological properties of the metallic foams allowed to check the influence of their introduction on the heat and mass transfer in the reforming reactor. The generated DMRs brought information about flow characteristics and made it possible to define how much the process has improved precisely.After the calculation of fifty generations, a solution with improved thermal conditions has been acquired. The algorithm decided to use only 33 % of the reference amount of the catalytic material. Despite a significant reduction of the Ni/YSZ present, the CH4 conversion fell only by 22 %. Simultaneously confirming that the effectiveness of the reforming process can be elevated by improvement of the thermal conditions inside the reactor.Considering the catalytic segments' parameters, the acquired solution might be suspected to be a local optimum only. The Ni/YSZ segments have similar parameters, which propagated since the GA's early operation. Therefore, the algorithm requires further development, to ensure that the global optimum was found. The mechanism of computing multiobjective fitness has to be revised, as the weighted sum approach appears to be inferior. Other possible improvements have to be sought in the introduction of an adaptive mutation rate, the enhancement of a crossover procedure and a fitness calculation. Moreover, changing the thermal fitness function to optimize the temperature gradients might alleviate the elimination of peaks in the average temperature profile.The following is the Supplementary data to this article:\n\nMultimedia component\nMultimedia component\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.02.228.", "descript": "\n                  The presented research focuses on an optimization design of a catalyst distribution inside a small-scale methane/steam reforming reactor. A genetic algorithm was used for the multiobjective optimization, which included the search for an optimum of methane conversion rate and a minimum of the difference between highest and lowest temperatures in the reactor. For the sake of computational time, the maximal number of the segment with different catalyst densities was set to be thirty in this study. During the entire optimization process, every part of the reactor could be filled, either with a catalyst material or non-catalytic metallic foam. In both cases, the porosity and pore size was also specified. The impact of the porosity and pore size on the active reaction surface and permeability was incorporated using graph theory and three-dimensional digital material representation. Calculations start with the generation of a random set of possible reactors, each with a different catalyst distribution. The algorithm calls reforming simulation over each of the reactors, and after obtaining concentration and temperature fields, the algorithms calculated fitness function. The properties of the best reactors are combined to generate a new population of solutions. The procedure is repeated, and after meeting the coverage criteria, the optimal catalyst distribution was proposed. The paper is summarized with the optimal catalyst distribution for the given size and working conditions of the system.\n               "}
{"full_text": "This study did not generate any datasets.Catalysis is a key process in science that allows to control all kinds of chemical transformations. In the presence of a suitable catalytic material the reaction rate can be dramatically increased, which enables the optimal use of resources, increasing the yield of desired products and at the same time avoiding waste formation as well as reduce specific energy requirements.\n1\u201310\n Nowadays, 90% of all modern processes in the chemical industry apply catalytic technologies.\n7\u201310\n In addition to this crucial role in chemical sciences, catalysts also provide the basis of innovation for many other industries based on life and material sciences as well as energy technologies. Thus, new catalytic materials including molecularly defined and nanostructured systems are continuously prepared by scientists all over the world and tested for all kinds of transformations.\n1\u201310\n Regarding the potential new catalysts, in particular, 3D-metal-based systems are gaining increasing importance and provide the basis for an advanced and sustainable chemical synthesis.\n11\u201317\n Due to the inherent beneficial aspects such as stability, recycling, and reusability, heterogeneous nanostructured materials, especially, are of prime importance.\n17\u201320\n\nTypically, for a specific benchmark reaction or more importantly for a given industrial process the \u201cbest\u201d catalyst is desired. Especially, for the bulk chemical industry it is important to apply state-of-the-art catalysts with optimal activity (TOF, turnover frequency), productivity (TON, turnover number), and selectivity to be cost competitive on a global scale.\n21\n However, apart from such highly optimized systems, there is also significant interest in catalysts, which can be applied in a general way for various processes. This is especially true for applications in organic synthesis, for drug discovery, and for basic sciences. Here, it is a common practice to develop new catalysts only for one specific synthetic methodology and the generality of a given catalyst is measured by its robustness toward different reaction conditions, but especially by its functional group tolerance and a wide substrate scope. Considering that elementary steps of many chemical processes are similar, we believe that \u201cgeneral\u201d catalysts can be developed more efficiently by not only focusing on one specific transformation. As an example, in the oxidation of alcohols diverse compounds B\u2013F can be formed. In general, alcohol (A) is oxidized to the corresponding aldehyde (B) first, which then can react with different nucleophiles such as H2O, alcohol, and ammonia to generate either geminal diol (X), hemiacetal (X), hemiaminal (X), or primary imine (Y), respectively, as intermediates (X and Y). All these intermediates might be further oxidized to produce the corresponding acid (C), ester (D), primary amide (F), and/or nitrile (E), respectively (Figure\u00a01\n).\n22\u201328\n\nLooking at the individual steps of Figure\u00a01, clearly the conversion of A to B and X to C, D, and F are mechanistically related and indeed can be performed with similar type of catalysts. However, traditionally each of these methodologies is studied separately using different catalyst systems, which is time and resource consuming.Among the many kinds of chemicals, functionalized aromatic and heterocyclic compounds are most valuable, which provide the basis for countless products of our daily life. In fact, synthetic organic chemistry and drug discovery majorly rely on the valorization of such compounds.\n29\u201331\n Among these, (hetero)aromatic carbonyl compounds (B), carboxylic acids (C), esters (D), nitriles (E), and amides (F) represent valuable fine and bulk chemicals widely used in research laboratories and industries.\n32\u201338\n Notably, these compounds can be easily functionalized/upgraded. Hence, they serve as precursors and intermediates for the synthesis of advanced chemicals, pharmaceuticals, agrochemicals, biomolecules, and materials. Moreover, many life science molecules, natural products, fragrances, and cosmetics as well as other daily life products contain, \u2013CHO, \u2013C=O, \u2013COOH, \u2013COOR, \u2013CN, and \u2013CONH2 functionalities, which play vital roles in their physical properties and functions.In general, products B\u2013F can be conveniently accessed by oxidation of benzylic alcohols and related heteroaromatic compounds,\n39\u2013102\n which are broadly commercially available. As an example, more than >200 benzylic alcohols are available from Sigma-Aldrich.\n103\n Regarding potential oxidants, air is ideal because it is abundant, inexpensive, and green, and it produces only water as by-product.\n104\n Favorably, air is much safer and more convenient to use than dioxygen. To perform the oxidation of alcohols using molecular oxygen or air to produce B\u2013F, both homogeneous and heterogeneous catalysts based on precious and non-precious metals were developed in the past (Figure\u00a02\n).\n39\u2013102\n\nDespite these achievements, until now, there is no single general catalyst developed or applied for the oxidative conversion of alcohols to synthesize carbonyl compounds (aldehydes and ketones; B), carboxylic acids (C), esters (D), nitriles (E), and amides (F).In this regard, here, we show that a general catalyst development can be achieved efficiently by directly including different related benchmark reactions and parallel testing of the catalyst materials under investigation. Following the presented strategy, we demonstrate that it is possible to develop graphitic-shell-encapsulated cobalt nanoparticles as a \u201cmost general\u201d oxidation catalyst, which can not only be applied in one of the above-mentioned aerobic oxidation reactions but many related transformations. The highly stable and reusable catalyst allows for the synthesis of functionalized and structurally diverse aromatic and heterocyclic aldehydes, ketones, carboxylic acids, esters, nitriles, and primary amides in good to excellent yields.In the past decade, we prepared a variety of nanostructured 3D-metal (Fe, Co, Ni, and Cu)-based materials by immobilization of either organometallic complexes or metal organic frameworks on inorganic supports and subsequent pyrolysis under inert atmosphere.\n105\u2013108\n Some of these materials proved to be highly active and selective\u00a0for catalytic hydrogenations, oxidations, and reductive amination reactions.105\u2013108 A typical feature of these active catalysts is the core-shell structure\u00a0of the metal nanoparticles, which are embedded in graphene or graphitic layers.\n105\u2013108\n To obtain this specific structure, ligated metal complexes have been used as precursors.\n105\u2013108\n To be cost-efficient, the respective ligands should be as simple, abundant, and inexpensive as possible. In this respect, amines and carboxylic acids are interesting as a plethora of them is easily accessible.In continuation of our previous work,\n105\u2013108\n we started to prepare a library of supported 3D-metal nanoparticles using Co, Mn, Fe, and Cu salts with piperazine (PZ) and DL-tartaric acid (TA) as ligands, which will form metal coordination polymers or metal organic frameworks. As an example, Co(NO3)2\u00b76H2O was dissolved in DMF, and then this mixture was heated to 150\u00b0C. At this temperature, PZ and TA were added and stirring was continued for 30\u00a0min. After addition of the support (carbon; Vulcan XC72R) and 4\u00a0h of additional stirring, the solvent was removed, and the resulting dark solid material was grinded and pyrolyzed at different temperatures (400\u00b0C\u20131,000\u00b0C) under argon atmosphere for 2\u00a0h to provide the desired cobalt-based nanoparticles supported on carbon (Figure\u00a03\n). Similarly, other 3D-metal nitrates (Fe(NO3)3\u00b79H2O, Mn(NO3)2\u00b76H2O, and Cu(NO3)2\u00b73H2O) were applied following the same procedure. For comparison, metal salts without ligands were pyrolyzed on carbon, and Ru- as well as Pd-containing materials were made using PZ and TA ligands, too.Following our concept to develop a universal oxidation catalyst, we evaluated the generality and applicability of the prepared materials not only for one type of reaction, but five different aerobic oxidation reactions were chosen. More specifically, all potential catalysts as well as selected commercial ones were tested for their activities in the conversion of benzyl alcohol (A1) to benzaldehyde (B1), benzoic acid (C1), methyl benzoate (D1), benzonitrile (E1), and benzamide (F1) (Figure\u00a04\n). In general, all these benchmark reactions were performed in the presence of air (1\u00a0bar or 10 bar) at 55\u00b0C\u2013120\u00b0C using either alcohols, water, or heptane as solvent. Interestingly, aldehyde and ester formation are observed at ambient pressure and low temperature, while the formation of acid, amide, and nitrile proceeded at temperatures >100\u00b0C and 10\u00a0bar of air vide infra.First, we tested in a parallel manner, the materials prepared by the pyrolysis of Fe-, Mn-, Co-, and Cu-nitrates on carbon (Fe(NO3)3@C-800, Mn(NO3)2@C-800, Co(NO3)2@C-800, and Cu(NO3)2@C-800) (Figure\u00a04). All these materials exhibited no or poor activities for all the benchmark reactions (<16% yields of the corresponding products B1\u2013F1). Next, we tested catalysts prepared by the impregnation and pyrolysis of PZ- and TA-ligated metal complexes (Fe-PZ-TA@C-800, Mn-PZ-TA@C-800, Co-PZ-TA@C-800, and Cu-PZ-TA@C-800) (Figure\u00a04). Among these materials Fe-PZ-TA@C-800 was completely inactive for the formation of benzoic acid and methyl benzoate, whereas it showed low to moderate activity for the synthesis of benzaldehyde, benzonitrile, and benzamide in 16%, 20%, and 60%, respectively. Mn-PZ-TA@C-800 was even more specific producing only 20% of B1, while no or very little activity is observed in the other model reactions. Interestingly, Co-PZ-TA@C-800 exhibited remarkable activity and selectivity in all the benchmark reactions and produced almost quantitative of yields (>98%) of benzaldehyde, benzoic acid, methyl benzoate, benzonitrile, and benzamide. Finally, Cu-PZ-TA@C-800 was tested and showed no activity for the formation of C1 and D1 as well as very low activity for B1 formation (30%). However, this material was found to be efficient for the preparation of benzonitrile (98%) and benzamide (97%). Because of the unique behavior of the cobalt-based material, variation of the pyrolysis temperature of the templated Co-PZ-TA@C was performed. However, materials prepared by pyrolysis at 400\u00b0C, 600\u00b0C, and 1,000\u00b0C showed lower activity. Similarly, pyrolysis of cobalt-complexes with single ligands either PZ or TA (Co-PZ@C-800 or Co-TA@C-800) gave less active materials and provided the desired products B1\u2013F1 in 50%\u201368% yields. Using the Fe, Mn, or Co salts in the absence and presence of PZ and TA under homogeneous conditions exhibited no or minor activity in all five benchmark tests (<5%) (Table S2). Likewise, the non-pyrolyzed supported pre-catalysts (metal-PZ-TA@C) behave. However, in the presence of the homogeneous Cu-PZ-TA system and its supported derivative some activity for the formation of benzaldehyde (10%\u201315%) is observed (Table S2).To compare the activities and selectivities of the optimal system (Co-PZ-TA@C-800) with commercially available precious-metal-based catalysts, Ru/C and Pd/C were also applied in the benchmark reactions (Figure\u00a04). Under similar conditions, Ru/C showed no activity for alcohol to ester oxidation, and in all other cases, product yields were lower compared with Co-PZ-TA@C-800, while Pd/C exhibited only high activity for the preparation of benzaldehyde. Likewise, Ru-PZ-TA@C-800 and Pd-PZ-TA@C-800, exhibited moderate to low activity for most reactions. Thus, among all the tested materials Co-PZ-TA@C-800 was found to be the most general oxidation catalyst, which allows for diverse aerobic oxidations of benzyl alcohols to produce a variety of product classes in a selective manner.To demonstrate the stability, recycling, and reusability of this Co-material (Co-PZ-TA@C-800), the synthesis of benzonitrile from benzyl alcohol in presence of aqueous ammonia and air was performed for seven times under standard conditions. Notably, in the presence of ammonia supported nanoparticles easily encounter stability and reusability problems. Nevertheless, as shown in Figure\u00a05\n,\u00a0Co-PZ-TA@C-800 was stable and is conveniently recycled and reused up to 7th run.To know the structural features and to understand the catalytic activities, we carried out detailed characterizations of the most active (Co-PZ-TA@C-800), moderately active (Co-PZ@C-800), (Co-TA@C-800), and less active (Co(NO3)2@C-800) materials using X-ray powder diffraction (XRD), scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). The XRD patterns of the most active catalyst, Co-PZ-TA@C-800, showed the presence of mainly metallic cobalt particles (Figure\u00a0S1), while the moderately active catalysts Co-PZ@C-800 and Co-TA@C-800 contained a mixture of metallic cobalt and oxidic cobalt (Co3O4) particles (Figure\u00a0S1). STEM analysis of Co-PZ-TA@C-800 proved the formation of metallic cobalt particles with different sizes ranging from 1 to 7\u00a0nm and from 25 to 40\u00a0nm (Figure\u00a06\nA). However, some bigger particles with sizes up to 80\u00a0nm were also observed. The smaller particles are usually found in groups, while other areas of the material contained no cobalt. Interestingly, most of the particles in this material are surrounded by few layers of graphitic carbon (Figure\u00a06A, right image). In addition to metallic cobalt, the presence of a very small amount of cobalt oxide is observed (Figures 7\nA, 7B, and S2). Co-PZ@C-800 contained also metallic and oxidic cobalt; however, the presence of the oxide seems to be more than in Co-PZ-TA@C-800, and it can be found either on the surface or as partially oxidized particles (see e.g., the biggest particle in the left image in Figures 6B and S3).The sizes of these particles are in the range between 25 and 60\u00a0nm with few particles being bigger up to 100\u00a0nm and fewer below 25\u00a0nm compared with Co-PZ-TA@C-800. In the case of metallic cobalt, these particles are covered by graphitic layers (Figure\u00a06B, right image). Likewise, Co-TA@C-800 showed the presence of both metallic and oxidic cobalt particles with sizes of 15\u201350\u00a0nm and only very few below this size. Similar to Co-PZ-TA@C-800, the nanoparticles of metallic cobalt are surrounded by graphitic layers in both Co-PZ@C-800 and Co-TA@C-800 (Figure\u00a06C). The least active material, cobalt nitrate@C-800, contained completely Co3O4 particles, which are not surrounded by graphitic layers (Figure\u00a0S4). The material obtained after three reaction cycles using the active catalyst Co-PZ-TA@C-800 showed that there is not much difference in the structure compared with the fresh catalyst (Figure\u00a06D). In this reused material, metallic nanoparticles with sizes of 3\u201310 and 25\u201340\u00a0nm are observed, which are in few cases partially oxidized at the surface. However, analysis of the material after 7 reaction cycles revealed that cobalt is oxidized in more proportion (Figure\u00a0S5). This implies that the cobalt is successively oxidized during the reaction cycles. EELS was applied to analyze the elemental composition of a selected area in the most active material, Co-PZ-TA@C-800 (Figures 7A and 7B). Analysis of the edge features of the elements enables the visualization of the spatial distribution of the corresponding elements (C, N, O, and Co) in a single-color elemental map as shown in Figure\u00a07 (right). As can be seen there the support mainly consists of carbon (Figure\u00a07, red map, C-K edge) and some content of nitrogen, which is originated from the ligands (Figure\u00a07, green map, N-K edge). Inspection of the distribution of the Co-L edge signal and the O-K edge signal (Figure\u00a07, yellow and blue map, respectively) reveal that the two bigger particles consist of a metallic cobalt core and a shell of cobalt oxide. Two selected spectra that show different features of selected areas in the material are shown in Figure\u00a07B.To obtain further insights into the surface chemistry of these materials, we performed XPS analysis. The sample surfaces of all the four catalysts (Co-PZ-TA@C-800, Co-PZ@C-800, Co-TA@C-800, and Co-PZ-TA@C-800 recycled) consists mainly of C with small concentrations of Co, O, N, S, and Si (Table S1) with the last two probably originating from Vulcan XC-72R and N from the starting chemicals such as ligands and cobalt nitrate. As found by STEM the Co particles are surrounded by carbon layers leading to the very low surface concentrations of Co between 0.2 and 0.4 atom% for the fresh catalysts and 0.9 atom% for the recycled catalysts. The high-resolution Co 2p spectra of all four samples (see Figure\u00a08\nA) confirmed the presence of metallic Co as sharp peaks at 778.7 (Co 2p3/2) and 793.8 eV (Co 2p1/2) as well as oxidic structures as broad peaks at higher binding energies.\n109\n Considering the satellite features at around 786 and 803 eV, a mixture of CoO and Co3O4 seems to be present. Looking at the recycled catalyst (three reaction cycles) Co-PZ-TA@C-800R (see Figure\u00a08A) an oxidation of the surface can be observed so that only a minor part Co is still in the metallic state. Note that the Co concentration at the surface increases to 0.9 atom% (Table S1) in the recycled catalyst (three reaction cycles), which indicates a partial breakup of the protective carbon shell probably also leading to the observed oxidation during the use of the catalyst. In case of the recycled catalyst after 7th run, only oxidized Co is observed on the surface (Figure\u00a0S6).The N 1s spectra in Figure\u00a08B are fitted with four peaks which can be assigned to pyridinic-N at binding energies around 398.8 eV, pyrollic-N, and/or N bonded to a metal in a Me\u2013Nx center (\u223c400.1 eV), graphitic N (\u223c401.3 eV) as well as oxidized pyridinic-N (\u223c404 eV).\n110\n Interestingly the concentration of N is higher in Co-PZ-TA@C-800 (1.7 atom %) compared with the other fresh catalysts (0.6 and 0.7 atom%; Table S1). After recycling the N concentration becomes even higher (4.7 atom%) and is dominated by pyridinic and pyrollic-N/Me\u2013Nx. This is explained by ammonia side-reactions on the catalyst surface.All these characterization data revealed that the immobilization and pyrolysis of cobalt-complexes containing PZ and/or TA ligands produced dissimilar kinds of cobalt nanoparticles supported on carbon, which in turn revealed varying catalytic activities. The material (Co-PZ-TA@C-800) containing predominately metallic cobalt nanoparticles exhibited highest activity. Apparently, fully oxidized cobalt has a negative impact on the overall catalytic performance as such particles have not been observed in the most active catalyst, and even not in the recycled one. Catalytic performance likely depends on the particle nature, sizes, and their distribution. The combination of PZ and TA ligands seems to favor the formation of a higher share of smaller cobalt containing particles and thus induce an increased number of accessible active sites in the catalyst, Co-PZ-TA@C-800. Hereafter, we represent the most active catalyst Co-PZ-TA@C-800 as Co/GS@C, where GS denote graphitic shell.After having a general catalyst system Co/GS@C (Co-PZ-TA@C-800) in hand, we performed additional tests with >90 different alcohols. As shown in Figures 9, 10, 11, and 12\n\n\n\n, simple substituted as well as functionalized and structurally diverse aromatic and heterocyclic aldehydes, ketones, acids, esters, nitriles, and amides can be prepared in good to excellent yields. For example, alkyl- and phenyl-substituted alcohols produced the corresponding products B\u2013F in up to 98% yield (Figure\u00a09; products B2, B6, B7, B11, C2, C3, C8\u2013C10, D2\u2013D5, E2\u2013E4, F2\u2013F4, and F8).Similarly, fluoro- and thio-trifluoromethyl-substituted products were obtained yields in up to 96% (Figure\u00a09; products B3\u2013B5, B12, B13, C4\u2013C7, D6, D7, E5\u2013E7, and F5). Such compounds are interesting building blocks for the discovery of new pharmaceuticals and agrochemicals.\n111\n In addition to benzyl alcohols, related condensed arenes gave corresponding products in up to 89% yields (Figure\u00a09; products B8, B9, D8, D9, E8\u2013E10, F6, and F7). Likewise, benzophenone, and 1-phenylbutan-1-one were obtained in 83%\u201385% yields (Figure\u00a09; products B14\u2013B15). Notably, in the oxidative cross-esterification reaction apart from methanol, other aliphatic alcohols can be used to provide ethyl, propyl, iso-propyl, butyl, and hexyl benzoates in up to 90% yields (Figure\u00a09; products D10\u2013D14). Interestingly, in case of ammoxidation to give nitriles, Co-PZ-TA@C-800 showed good activity for aliphatic alcohols at elevated temperature (140\u00b0C). As a result, 4-phenylbutanenitrile, and several alkyl nitriles were obtained in up to 78% yield (Figure\u00a09; products E12\u2013E15).Next, the ability and selectivity of Co-PZ-TA@C-800 for the refinement of more complex molecules as well as the tolerance of functional and sensitive groups was studied. Thus, functionalized as well as multi-substituted benzylic alcohols were subjected to aerobic oxidation under the optimized conditions (Figure\u00a010). Chloro-, bromo-, and iodo-substituted benzylic alcohols smoothly reacted to the corresponding halogenated benzaldehydes, acetophenones, benzoic acids, methyl benzoates, benzonitriles, and primary benzamides in good to excellent yields (Figure\u00a010; products B16\u2013B18, B27, B36\u2013B38, C11\u2013C14, C19, D15\u2013D17, D25, E16\u2013E19, F9\u2013F13, and F18). These further functionalized halogenated molecules are indispensable for many applications and serve as valuable starting materials and intermediates.\n112\n As an example, 2,6-dichloro-benzyl alcohol was reacted in presence of ammonia in water and produced the corresponding benzamide in 85% yield (F18). Substrates containing ether, hydroxyl, amine, nitro, ester, boronic ester, or nitrile substituents, were selectively converted to desired products B19, B20, B23, B26\u2013B33, B39, B40, C15, C16, C20, D18, D19, D21\u2013D25, E21, E23, E27, F14, F15, and F17. Interestingly, sulfur-containing alcohols were also selectively converted without oxidation of S-moiety (Figure\u00a010; products B24, B25, C17, D20, E25, E26, and F19). In case of 1,3- and 1,4-benzenedimethanol, both CH2\u2013OH groups were selectively oxidized and produced terephthalaldehyde B30 and terephthalonitrile E24 in 95%\u201396% yields. In addition, di- and multi-substituted substrates, which possess additional challenges, were efficiently oxidized, and gave products B21, B22, B26\u2013B29, C18, C19, D23, D25, E20, E22, and F16\u2013F18 in high yields (Figure\u00a010). Even the dinitro-substituted benzyl alcohol produced the corresponding benzaldehyde B28 in 85% yield. Sterically hindered tri-methyl benzyl alcohol also reacted to provide the corresponding benzonitrile in 86% yield (Figure\u00a010; product E22). In addition to benzylic alcohols, allylic alcohols such as cinnamyl and perillyl alcohols can be efficiently transformed to cinnamaldehyde, perillyl aldehyde, and cinnamyl nitrile (Figure\u00a010; products B34, B35, E28). Furthermore, aliphatic cyclic secondary alcohols were oxidized to produce cyclic ketones (Figure\u00a010; products B43 and B44).Subsequently, the synthesis of heterocyclic carbonyl compounds, carboxylic acids, esters, nitriles, and amides from corresponding alcohols was explored. In general, heterocyclic compounds find wide range of applications, especially in life sciences. Indeed, such scaffolds are ubiquitous in pharmaceuticals, natural products, agrochemical, and other biomolecules. Thus, they play a pivotal role in modern small molecule drug discovery processes.\n113\n\n,\n\n114\n As shown in Figure\u00a011, different kinds of heterocyclic alcohols were oxidized to give the desired compounds. Interestingly, nicotinic derivatives such as nicotinaldehyde, methyl nicotinate, nicotinonitrile, nicotinic acid (niacin), and nicotinamide\u2014the latter two are used as food supplement and nutrition medications\u2014as well as 3-acetylpyridine are smoothly prepared from 3-pyridinemethanol in up to 97% yield (Figure\u00a011; products B45, B61, C21, D26, E29, F20). Similarly, bromo-, di-methoxy-, and di-chloro-substituted 2- and 3-pyridinemethanol are selectively oxidized to produce B47\u2013B49 and D28. Other N-heterocycles such 2-pyrazine and quinolinemethanol are well accepted and provided the respective products in 88%\u201394% (products E30, E31, and F21). Interestingly, 2-thiophenmethanol also allowed for selective oxidation in up to 95% yield (Figure\u00a011; products B50, B62, D29, E41, and F22). At this point, it should be noted that sulfur-containing compounds constitute common poisons for most heterogeneous catalysts. However, Co-PZ-TA@C-800 tolerated the presence of many sulfur-containing molecules and a variety of sulfur-containing products, e.g., B24, B25, B50, B52, B53, B62, C17, C22, D20, D29, E25, E26, E33, E34, E41, F19, and F22 were obtained in good to excellent yields. Apart from the oxidation of hydroxymethyl-substituted N-, O-, and S-heterocycles a variety of benzylic alcohol containing heterocyclic motifs such as thiazole, morpholine, pyrazine, tetrahydropyran, diazepane, N-methyl diazepane, and triazole underwent aerobic oxidation under the previously optimized conditions and furnished the desired products (Figure\u00a011; B51\u2013B53, B55\u2013B60, and E33\u2013E40). As an example, 2,1,3-benzothiadiazol-5-yl-methanol gave corresponding aldehyde and nitrile (Figure\u00a011; products B52, E34. Other notable examples include the oxidation of 3,4-(methylendioxy)-benzylalcohol, an important motif present in drugs and natural products (Figure\u00a011, products B51, C23, D30, E32, F23) and 3,5-dimethyl-1-phenyl-1H-pyrazol-4-yl-methanol as well as 2-(2-morpholinoethoxy)phenyl-methanol (Figure\u00a011, products B54, B60, E40, E42).In recent years, the valorization of hydroxymethylfurfural (HMF, A2) and furfuryl alcohol (A3) attracted significant interest for the preparation of sustainable polymers and fuels (Figure\u00a012).\n115\n\n,\n\n116\n Among these, the synthesis of 2,5-furandicarboxylic acid (FDCA) and dimethyl furan-2,5-dicarboxylate (FDCM) from HMF is of actual interest to produce poly(ethylenefuranoate) (PEF) polymer.\n115\n Applying our Co/GS@C catalyst FDCM, (D31) is prepared in up to 85% yield. More sensitive furan-2,5-dicarbaldehyde (B66) can be also obtained in up to 87% yield. Furthermore, furan-2,5-dicarbonitrile (E43) is available from this latter intermediate. Similarly, furfuryl alcohol (A3) was selectively transformed to corresponding aldehyde (B67), carboxylic acid (C24), methyl ester (D32), nitrile (E44), and amide (F24) in good to excellent yields (Figure\u00a012). Again, these products have various interesting applications, for example, 2-furoic acid is a known preservative, flavoring ingredient, food, and color additive in food,\n117\n while 2-furonitrile has been suggested as a potential sweetening agent, which has about thirty times the sweetening power of sucrose.\n118\n\nIn general, catalytic oxidations were performed in 50\u2013150\u00a0mg scale with respect to substrate. To demonstrate the utility of this catalyst system, reactions of five alcohols were also performed on 1\u201310\u00a0g (Figure\u00a013\n). The yields of the desired products from these upscaling experiments were similar to those obtained from the smaller scale.Further, we calculated TONs and TOFs of our Co-catalyst for the oxidation of benzyl alcohol to benzaldehyde (Table S3). Under standard conditions (0.5\u00a0mmol alcohol, 35\u00a0mg catalyst, 80\u00b0C, 24 h) these values are found to be 15.6 and 0.65 h\u22121, while at 100\u00b0C and increased amount of substrate (2.5\u00a0mmol of benzyl alcohol, 35\u00a0mg catalyst, 24 h) both values increased (TOF and TON are 46.8 and 1.95 h\u22121). These numbers are at least comparable to reported non-noble metal-based catalysts for the individual transformations (Table S3).We performed kinetic investigations on the Co/GS@C-catalyzed oxidation of benzyl alcohol to benzaldehyde and examined the effect of (1) reaction time, (2) reaction temperature, (3) catalyst amount, and (4) substrate (benzyl alcohol) concentration (Figure\u00a0S7). By increasing the time, temperature, or catalyst loading the yield of benzaldehyde increased, and quantitative yield was obtained for 24 h, at 80\u00b0C with 35\u00a0mg of catalyst. On the other hand, increasing the substrate (benzyl alcohol) concentration, the yield of benzaldehyde is decreased. Next, we calculated the reaction order with respect to substrate (benzyl alcohol), which is found to be \u22120.85 (Figure\u00a0S7E). This also confirmed that the substrate has a negative effect on the rate of the reaction.Next, we conducted experiments to identify the formation of possible reactive oxygen species (ROS) during the Co/GS@C-catalyzed aerobic oxidation reactions. For this purpose, under standard conditions, the oxidation of benzyl alcohol to benzaldehyde was tested in the presence of different radical quenchers/trapping agents such as NaN3, i-PrOH, and p-benzoquinone (PBQ) (Table S4). All these reagents have been used to trap singlet oxygen (1O2), hydroxyl (\u22c5OH) or super oxide (O2\n\u22c5\u2212) radicals, which are considered as the ROS in aerobic oxidations. These experiments showed that there is no effect after adding i-PrOH or NaN3 on the reactions. However,\u00a0the reaction is inhibited after the addition of 80\u00a0mg PBQ. This makes the formation of super oxide (O2\n\u22c5\u2212) species likely. In addition, we performed an experiment for trapping super oxide (O2\n\u22c5\u2212) species using butylated hydroxytoluene (BHT) (Figure\u00a0S8). Under similar experimental conditions, without the substrate (35\u00a0mg Co/GS@C, 0.5\u00a0mmol BHT, 1\u00a0bar air, 10\u00a0mol % K2CO3, 2\u00a0mL n-heptane, 80\u00b0C, 24\u00a0h), we performed the reaction with BHT and observed the formation of BHT-OOH, which is detected by GC-MS (Figure\u00a0S8). These experiments indicate that super oxide (O2\n\u22c5\u2212) is formed during the reaction.Further to prove the formation of a superoxide radical intermediate, EPR spin-trapping studies using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as spin-trap reagent were performed. The EPR spectrum of the reaction mixture containing a suspension of Co/GS@C catalyst, Cs2CO3 and benzyl alcohol in heptane after heating at 80\u00b0C for 3\u00a0min under bubbling of O2 followed by addition of DMPO exhibited a signal at g\u00a0= 2.006 characteristic of the DMPO-OOH spin adduct indicating again the formation of a superoxide radical intermediate during the catalytic reaction (Figure\u00a014\n). It should be noted that no EPR signal is detected in the absence of benzyl alcohol suggesting that its adsorption on the surface of the Co/GS@C-800 catalyst induce the activation of molecular oxygen and superoxide formation.Regarding the general mechanism, in all these oxidations, the first step is the Co/GS@C-catalyzed oxidative conversion of benzyl alcohol (A) to benzaldehyde (B). Thus, for the formation of carboxylic acid (C), ester (D), nitrile (E), and amide (F), (B) serves as the key intermediate (Figure\u00a0S9). Indeed, for all transformations the formation of benzaldehyde was detected by GC-MS. In case of benzoic acid, the aldehyde reacts with water and generates benzaldehyde hydrate (X) as another intermediate, which is then oxidized in the presence of Co/GS@C and gives the corresponding acid. Similarly, in the formation of benzoic acid esters, aldehyde reacts with another alcohol and provides hemiacetal (X\n/ ) as another intermediate, which finally converts to the corresponding ester in presence of Co/GS@C and air. For the formation of nitrile, benzaldehyde couples with ammonia and generates primary imine (Y), which finally yields the corresponding nitrile. In case of amide synthesis, two pathways are possible: (1) the aldehyde can react with ammonia to form hemiaminal as the intermediate, which could be then oxidized to give the corresponding primary amide or (2) formation of benzonitrile takes place, which can undergo hydrolysis to form the primary amide. To prove these two pathways, we performed the reaction of benzonitrile in water in presence of ammonia and air using Co/GS@C catalyst at 120\u00b0C for 24 h. From this experiment, we obtained 98% of benzamide (Figure\u00a0S10). Thus, we conclude the formation of amide occurred mainly by the hydrolysis of benzonitrile. It should be noted that the intermediates, aldehyde hydrate (X), hemiacetal (X\n/), and primary imine (Y) are unstable, and we were not able to detect or isolate them.Based on the identified active oxygen species and the proposed reaction pathways and intermediates, we suggest the following general mechanism for the different aerobic oxidations of primary alcohol in the presence of Co/GS@C (Figure\u00a015\n). In the first step, (1) adsorption and activation of alcohol and oxygen takes place on the catalyst surface. During this process, the generation of the observed superoxide species occurs. In the next step (2), oxidation of the activated alcohol takes place. In the last step (3), the desorption of the product, aldehyde takes place by the regeneration of catalyst. Similar catalytic cycles for the formation of esters, carboxylic acid, and nitrile are proposed. The hydrolysis of benzonitrile to benzamide occurs best in presence of catalyst, water, ammonia, and air.In conclusion, we demonstrate that a new catalyst can be efficiently developed not only for one specific synthetic transformation but also for related methodologies with similar elementary reaction steps. In particular, we show that the here presented cobalt catalyst is able to perform the selective aerobic oxidation of alcohols to a variety of functionalized aromatic products. This catalyst is based on carbon-supported graphitic-shell-encapsulated specific cobalt nanoparticles, which are prepared by immobilization of in-situ-generated cobalt-PZ-TA template on carbon and subsequent pyrolysis under argon at 800\u00b0C. Applying the optimal material, functionalized and structurally diverse (hetero)aromatic aldehydes, ketones, carboxylic acids, esters nitriles, and primary amides were prepared in selective manner from alcohols in the presence of air. The resulting compounds represent valuable fine and bulk chemicals, which serve as key starting materials and intermediates for the synthesis of advanced chemicals, pharmaceuticals, agrochemicals, and materials. We believe that the presented concept is not only valid for the here-described case of alcohol oxidations but offers manifold opportunities for other chemical transformations, too.Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Matthias Beller (matthias.beller@catalysis.de).All materials generated in this study are available from the lead contact without restriction.We gratefully acknowledge the European Research Council (EU project 670986-NoNaCat) and the State of Mecklenburg-Vorpommern for financial and general support. We thank the analytical team of the Leibniz-Institut f\u00fcr Katalyse e.V. for their excellent service.R.V.J. and M.B. supervised the project. T.S., R.V.J., and M.B. planned and developed the project. T.S. prepared catalysts and performed catalytic experiments. V.G.C. performed catalytic experiments and reproduced the results. N.R. performed TEM measurements and analysis. J.R. conducted EPR measurements. S.B. performed XPS measurements and analysis. R.V.J., T.S., M.B., and V.G.C. wrote the paper.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.chempr.2021.12.001.\n\n\nDocument S1. Figures S1\u2013S224, Tables S1\u2013S4, supplemental experimental procedures, and supplemental references\n\n\n\n\n\nDocument S2. Article plus supplemental information\n\n\n\n", "descript": "\n                  Functionalized (hetero)aromatic compounds are indispensable chemicals widely used in basic and applied sciences. Among these, especially aromatic aldehydes, ketones, carboxylic acids, esters, nitriles, and amides represent valuable fine and bulk chemicals, which are used in chemical, pharmaceutical, agrochemical, and material industries. For their synthesis, catalytic aerobic oxidation of alcohols constitutes a green, sustainable, and cost-effective process, which should ideally make use of active and selective 3D metals. Here, we report the preparation of graphitic layers encapsulated in Co-nanoparticles by pyrolysis of cobalt-piperazine-tartaric acid complex on carbon as a most general oxidation catalyst. This unique material allows for the synthesis of simple, functionalized, and structurally diverse (hetero)aromatic aldehydes, ketones, carboxylic acids, esters, nitriles, and amides from alcohols in excellent yields in the presence of air.\n               "}
{"full_text": "Hydrogen is hailed as an environmentally benign alternative to traditional fossil fuels because it has a high energy density and produces zero pollution [1,2]. However, to date, hydrogen is primarily produced by steam reforming of fossil resources. From an environmental and sustainability perspective, hydrogen production from electrochemical water splitting, which produces no carbon emissions, is a desirable method. In general, water splitting is divided into two half reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) [3\u20136]. To date, Pt-based and Ir-based catalysts are still the most popular systems for the HER and OER, due to their low overpotentials and small Tafel slopes. Nevertheless, the limited supplies and high cost of precious metals make them impractical for large-scale application in electrocatalytic water splitting reactions [7].Recently, extensive studies have focused on exploiting cost-effective catalysts based on abundant transition metals (TMs), such as chalcogenides [8,9], phosphides [10,11], nitrides [12,13], and carbides [14,15]. However, these require a high operating voltage for water splitting due to their inferior anodic OER kinetics. To address this problem, highly efficient, energy-saving hydrogen production has been achieved by combining the HER with oxidation reactions that have low theoretical voltages and use small organic molecules, including methanol [16,17], hydrazine [18,19], urea [20,21], and 5-hydroxymethylfurfural [22,23]. The urea oxidation reaction (UOR) has an extremely low theoretical voltage of 0.37 V, would generate a 70% energy saving, and offers the potential to purify industrial and sanitary wastewater [24]. The UOR is a six-electron transfer reaction and is also limited by intrinsically sluggish kinetics (CO(NH2)2\u00a0+\u00a06OH\u2013\u00a0\u2192\u00a0CO2\u00a0+\u00a0N2\u00a0+\u00a05H2O\u00a0+\u00a06e\u2013). Therefore, exploring highly efficient bifunctional catalysts with high HER and UOR activity to achieve urea-assisted energy-saving hydrogen production remains a formidable task.Transition metal phosphides (TMPs) have been explored as intriguing electrocatalysts for water electrolysis by virtue of their low electrical resistance and similarity to hydrogenase [25]. TMPs (Ni2P, CoP, MoP, etc.) with various morphologies and structures have been fabricated and have yielded good performance. However, most of them exhibit only monofunctional catalytic activity. Among the various strategies for preparing excellent bifunctional catalysts for both oxidation and reduction reactions, constructing a synergistic interface with two different electrocatalysts is an effective method. Interfacial engineering can tailor the electronic environment, expose abundant active sites, promote electron transfer, and optimize the adsorption of reaction intermediates [26\u201329]. Experiments as well as theoretical calculations have shown that bimetal catalysts with abundant heterogeneous interfaces usually display better electrocatalytic performance than the corresponding single compound [30]. Wang et\u00a0al. [31] reported a heterogeneous bimetallic Mo-NiPx/NiSx catalyst for robust overall water splitting that exhibited a low voltage of 1.42 V at 10 mA cm\u20132. Yu et\u00a0al. [32] designed a heterogeneous bimetallic material, Ni2P-FeP, that proved to be a startling effective bifunctional catalyst for overall water splitting, revealing rich active sites and an improved transfer coefficient. Hence, the rational design of a hierarchical structure to engineer coupling interfaces is important for optimizing catalytic activity and creating high-performance bifunctional catalysts for the HER and UOR.Herein, we design and prepare a Ni2P/NiMoP nanosheet catalyst with a hierarchical architecture on a nickel foam (NF) substrate, using simple hydrothermal and phosphorization methods. The Ni2P/NiMoP electrode demonstrates excellent performance for both the HER and the UOR, requiring an overpotential of only 22 mV for the HER and a small working potential of 1.33 V for the UOR at a current density of 10 mA cm\u20132. Moreover, a two-electrode system using Ni2P/NiMoP as a bifunctional catalyst shows an ultralow cell working voltage of 1.35 V at 10 mA cm\u20132 and long-term durability for 80 h. Its intriguing activity can be ascribed to the engineered heterostructure interface, which leads to charge redistribution, regulates the electronic structure, and promotes electron transfer during the reactions.The procedure for synthesizing Ni2P/NiMoP nanosheets grown on NF is schematically illustrated in Fig.\u00a01\na. First, the NiMoO4 precursor nanosheets were grown directly on NF through a simple hydrothermal method previously described [33]; these nanosheets were characterized using SEM and XRD (Figures S1 and S2). Next, uniform Ni2P/NiMoP nanosheets were prepared via a phosphorization process that left the original morphology undamaged (Fig.\u00a01b\u2013d). The resulting uniformly distributed nanoflake arrays on the NF substrate were beneficial for exposing active sites and for allowing electrolyte permeation and gas release [34]. The TEM images in Fig.\u00a01e show the typical nanosheet structure. HRTEM images show 2.21 \u00c5 and 2.29 \u00c5 spacing between the lattice fringes, which correspond to the (111) planes of Ni2P and NiMoP (Fig.\u00a01f and g), confirming the existence of a heterointerface structure between Ni2P and NiMoP. The heterogeneous interfaces may have modulated electron distribution and enhanced electron transfer, thereby exposing abundant active sites and optimizing the material's chemical adsorption capacity. For comparison, Ni2P was also prepared without the Mo precursor (Figures S3 and S4). The HRTEM image and energy-dispersive X-ray (EDX) elemental maps in Fig.\u00a01h show that elemental Ni, Mo, and P were evenly distributed in the as-prepared samples, implying the successful fabrication of metal phosphides.\nFig.\u00a02\na presents the XRD pattern of the as-prepared Ni2P/NiMoP scraped from the NF substrate. The diffraction peaks are easily indexed to the Ni2P phase (PDF#03-0953) and the NiMoP phase (PDF#31-0873), indicating the successful synthesis of a Ni2P/NiMoP heterostructure. XPS was conducted to investigate the chemical composition and electronic states of the Ni2P/NiMoP and Ni2P samples. Comparison of the XPS survey spectra (Figure S5) confirmed the presence of elemental C, P, O, Mo, and Ni in the Ni2P/NiMoP. In the high-resolution Ni 2p spectrum (Fig.\u00a02b), the two peaks centered at 852.9 and 870.0 eV are consistent with Ni-P bonds [31]. The peaks located at 856.3 and 874.2 eV are assigned to Ni-O bonds, arising from surface oxidation, while the remaining two signals are satellite peaks. In the P 2p spectrum (Fig.\u00a02c), the two peaks at binding energies of 129.2 and 130.3 eV are compatible with P 2p3/2 and P 2p1/2, respectively, and the P-O species (133.8 eV) is due to surface oxidation [35]. In the Mo 3d spectrum (Fig.\u00a02d), the peaks at 230.3 and 233.3 eV are ascribed to Mo 3d5/2 and Mo 3d3/2, respectively. It should be noted that the peaks of P 2p and Ni 2p for Ni2P/NiMoP are positively shifted compared with those for Ni2P, indicating charge redistribution due to the strong coupling interfaces between Ni2P and NiMoP, which could have promoted electrocatalytic activity. In addition, the higher valence state of Ni in Ni2P/NiMoP would have been conducive to optimizing the binding interaction between the catalyst and reaction intermediates [27,36,37].The HER performance of each as-prepared sample was recorded using a typical three-electrode setup in 1 M KOH electrolyte. We first investigated the effect of phosphorization temperature on the HER activity (Figure S6), finding the Ni2P/NiMoP catalyst prepared at 350 \u00b0C has the best catalytic activity. The linear sweep voltammograms (LSV) in Fig.\u00a03\na indicate the Ni2P/NiMoP has notable electrocatalytic activity for the HER compared with bare NF, Ni2P, NiMoO4, and commercial Pt/C catalysts. As shown in Figs.\u00a03b and S7, the Ni2P/NiMoP displayed an ultralow overpotential of 22 and 91 mV to yield current densities of 10 and 100 mA cm\u20132, which were much lower than those for NF (169 and 374 mV), Ni2P (92 and 189 mV), and NiMoO4 (144 and 318 mV). It also outperformed most TM-based HER materials (Table S1) and was comparable to or even higher than the benchmark catalyst, Pt/C (16 and 121 mV). Tafel slopes were calculated to investigate the reaction kinetics (Fig.\u00a03c). Ni2P/NiMoP exhibited a low value of 34.5 mV dec\u20131, which was close to that of Pt/C and smaller than those of NF, Ni2P, and NiMoO4, indicating the fast HER kinetics of Ni2P/NiMoP.To determine the origins of this incredibly high HER catalytic activity, we conducted electrochemical impedance spectroscopy (EIS) analyses. Ni2P/NiMoP had a lower charge transfer resistance (R\nct) than the other materials, suggesting fast kinetics in the electrocatalytic HER process (Fig.\u00a03d and Table S2). The electrochemically active surface area (ECSA) was estimated by measuring the double-layer capacitance (Cdl), which is correlated with ECSA. Figure S8 shows that Ni2P/NiMoP exhibited a Cdl value of 135.5 mF cm\u20132, about twice that of Ni2P (61.5 mF cm\u20132), demonstrating that the engineered heterostructure interface of Ni2P/NiMoP made more active sites accessible. When the LSV curves were normalized by the ECSA, the Ni2P/NiMoP still showed excellent catalytic performance (Figure S9).To further assess the intrinsic specific activity of Ni2P/NiMoP, we also investigated the turnover frequency (TOF) (Fig.\u00a03e). At an overpotential of 200 mV, the Ni2P/NiMoP exhibited the largest TOF value, at 0.92 s\u20131, about twice that of Ni2P and NiMoO4, confirming the Ni2P/NiMoP heterostructure strongly enhanced the intrinsic catalytic activity. Stability is also a critical parameter in assessing catalyst activity. Notably, the LSV curves of the Ni2P/NiMoP electrode remained almost identical after 5,000 and 10,000 cycles (Fig.\u00a03f), implying it had excellent cycling stability. We also assessed long-term electrochemical durability using chronoamperometry at 100 mA cm\u20132, and the overpotentials remained reasonably stable for 60 h of continuous operation (Figure S10). These results demonstrate the robustness of the Ni2P/NiMoP electrode for the HER.We also investigated the morphology and structure of Ni2P/NiMoP after the HER, finding the nanosheet morphology to be intact after long-term durability testing (Figure S11). The XRD pattern also showed no significant differences from the original (Figure S12), and the heterointerface structure was preserved (Figure S13). These results indicate that the engineered heterostructure interface of Ni2P/NiMoP enhanced the charge transfer rate and the number of accessible active sites, resulting in admirable electrocatalytic activity.Given the sluggish kinetics of the OER, we replaced it with the UOR, in light of the latter's distinctly low theoretical voltage of 0.37 V. Testing was performed in 1 M KOH with 0.33 M urea. First, the polarization curves of Ni2P/NiMoP catalyst for the OER and UOR were compared (Fig.\u00a04\na). The UOR current density was 10 mA cm\u20132 at a voltage of 1.33 V, much lower than for the OER (10 mA cm\u20132 at 1.49 V), confirming the crucial role of urea in lowering the anodic potential. We also assessed the effect of urea on the HER performance of Ni2P/NiMoP catalyst (Figure S14) and found negligible difference in the HER polarization curves in the presence of urea, indicating Ni2P/NiMoP resisted urea interference during the HER. The UOR performance of NF, Ni2P, NiMoO4, and commercial Pt/C were probed for comparison (Figs.\u00a04b and S15), with Ni2P/NiMoP emerging as better than all of these (Fig.\u00a04c and Table S3), requiring an ultralow potential of 1.37 V to reach 400 mA cm\u20132. The Tafel slope of Ni2P/NiMoP was only 23.3 mV dec\u20131, far lower than that of NF (159.1 mV dec\u20131), Ni2P (31.8 mV dec\u20131), NiMoO4 (57.2 mV dec\u20131), and Pt/C (102.0 mV dec\u20131), implying faster UOR kinetics (Fig.\u00a04d). The low R\nct meant Ni2P/NiMoP had a faster charge-transfer process for the UOR (Fig.\u00a04e).Next, to assess the prospects of Ni2P/NiMoP for industrial applications, we tested its electrochemical stability at a density of 500 mA cm\u20132 for the HER and UOR. As shown in Figs.\u00a04f and S16, the voltage underwent negligible change after 10 h of testing, indicating Ni2P/NiMoP is suitable for industrial application. We also investigated the morphology, phases, and chemical states of Ni2P/NiMoP after UOR stability testing. An SEM image (Figure S17) shows the nanosheet morphology was well preserved, indicating its excellent structural stability. The XRD pattern showed that in the post-UOR sample, a new phase of Ni(OH)2 had been formed and the phosphide phase had deteriorated (Figure S18), which is attributable to surface oxidation on the electrode. After UOR stability testing (Figure S19), no lattice fringe was evident, indicating the crystalline Ni2P/NiMoP had been transformed into amorphous (oxy)hydroxide species. XPS was conducted to investigate variations in the chemical valence states of the post-UOR sample (Figure S20). The disappearance of the Ni-P peak and the notably decreased peak intensities of P and Mo after UOR experimentation were ascribed to surface corrosion and the formation of hydroxide species [38]. Based on the above experimental evidence, the (oxy)hydroxide species produced on the surface may have served as the actual catalytic active sites for the UOR, and the remaining phosphides as the core to support the efficient transport of electrons.In view of the Ni2P/NiMoP catalyst's extraordinary performance for the HER and UOR, a two-electrode setup employing a Ni2P/NiMoP electrode as a bifunctional catalyst in 1 M KOH with 0.33 M urea was used instead of the traditional water splitting method (Fig.\u00a05\na). The \u0394E values of urea-assisted water electrolysis were 1.34 and 1.53 V at 10 and 100 mA cm\u20132, much lower than for traditional overall water splitting (Fig.\u00a05b), suggesting the prospects for application are good. As depicted in Fig.\u00a05c, the LSV curve of the Ni2P/NiMoP electrode exhibited a voltage of 1.50 V to reach 10 mA cm\u20132 for traditional water splitting. In contrast, the full cell voltage dropped noticeably to 1.35 V at 10 mA cm\u20132 after the introduction of 0.33 M urea (Fig.\u00a05d), showing that energy-saving hydrogen generation can be achieved by substituting the anodic OER with UOR. The urea electrocatalysis performance of Ni2P/NiMoP also exceeded those of the most-reported catalysts (Table S4). This two-electrode system was able to steadily generate hydrogen for 80 h of operation at 10 mA cm\u20132, and the operating voltage showed no significant decay, demonstrating the system's excellent durability (Fig.\u00a05e).Based on the above experimental results and previous reports, the compelling activity of Ni2P/NiMoP may stem from the following features. First, the engineered interface heterostructure could have led to charge redistribution, thereby regulating the electronic structure and optimizing the adsorption capacity of active species during the catalytic process. Second, the hierarchical architecture of the Ni2P/NiMoP nanosheets on the NF substrate not only might have ensured an efficient charge transfer rate but also could have improved the number of accessible active sites and the release of gas. Third, the hydroxide species produced on the phosphide surface may have served as the actual catalytic active sites, favoring the electrocatalytic UOR reaction.In conclusion, we developed a Ni2P/NiMoP nanosheet catalyst with a hierarchical architecture on a NF substrate. The structure and the strong coupling interfaces between Ni2P and NiMoP were proved by XRD and XPS. The interface engineering may have led to charge redistribution, regulating the electronic structure and optimizing the adsorption capacity of active species during the catalytic process. Moreover, the uniform nanosheets on the NF substrate could have promoted charge transfer and improved the number of accessible active sites. The Ni2P/NiMoP catalyst exhibited notable HER and UOR properties. Importantly, a two-electrode electrolyzer assembled with Ni2P/NiMoP as a bifunctional catalyst for both the anode and the cathode required an ultralow cell voltage of 1.35 V to achieve a current density of 10 mA cm\u20132 and exhibited excellent long-term durability during 80 h of operation. This work offers great potential for developing TMP catalysts to use in energy-saving high-purity hydrogen production via the engineering of heterojunctions.L.F. Jiao proposed the concept. T.Z. Wang and X.J. Cao performed the experiments. T.Z. Wang wrote the manuscript. All authors participated in data analysis and manuscript discussion.The authors declare no competing financial interests.This work was financially supported by the National Natural Science Foundation of China (52025013, 51622102), Ministry of Science and Technology of China MOST (2018YFB1502101), the 111 Project (B12015), and the Fundamental Research Funds for the Central Universities (63191523, 63191746).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.esci.2021.09.002.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Electrochemical water splitting is a sustainable and feasible strategy for hydrogen production but is hampered by the sluggish anodic oxygen evolution reaction (OER). Herein, an effective approach is introduced to significantly decrease the cell voltage by replacing the anodic OER with a urea oxidation reaction (UOR). A Ni2P/NiMoP nanosheet catalyst with a hierarchical architecture is uniformly grown on a nickel foam (NF) substrate through a simple hydrothermal and phosphorization method. The Ni2P/NiMoP achieves impressive HER activity, with a low overpotential of only 22 mV at 10 mA cm\u20132 and a low Tafel slope of 34.5 mV dec\u20131. In addition, the oxidation voltage is significantly reduced from 1.49 V to 1.33 V after the introduction of 0.33 M urea. Notably, a two-electrode electrolyzer employing Ni2P/NiMoP as a bifunctional catalyst exhibits a current density of 10 mA cm\u20132 at a cell voltage of 1.35 V and excellent long-term durability after 80 h.\n               "}
{"full_text": "The accumulation of waste plastics in landfills and oceans has caused a global environmental crisis.\n1\u20133\n In particular, microplastics have been entering the food chain and become a potential threat to human health (B. Liebmann et al., 2018, Microplastics 2018, conference). Although there are thousands of plastic materials in use, only six of them\u2014polyethylene (PE, high and low density), polypropylene, poly(vinyl chloride), polystyrene (including expanded polystyrene), polyurethane, and poly(ethylene terephthalate)\u2014are widely used. Collectively, ~6.3 billion metric tons of plastic waste were produced by 2015, of which 79% was landfilled, 12% was incinerated, and only 9% was recycled.\n4\n PE is the polymer with the most massive volume produced globally, and the production could reach over 100 million metric tons per year.\n4\n\n,\n\n5\n Therefore, the efficient upcycling of waste plastics, especially PE, is critical to mitigating the severe environmental problem.Technologies for recycling waste plastics mainly include three types: mechanical recycling, incineration, and chemical recycling. Mechanical recycling is the only technology used commercially for the large-scale plastic recycling process, but it still suffers from decreasing product quality after the consecutive melting and remolding cycles.\n6\n Although incineration converts mixed waste plastics to heat and electricity, the energy recovery efficiency cannot be as much as that from chemical recycling because of the massive loss of energy.\n7\n Therefore, chemical recycling is considered a promising process for valorizing waste plastics, whereby plastics are the low-cost feedstock for producing value-added chemicals or fuels.Recently, pyrolysis has been extensively investigated as a chemical recycling technology. The world's largest resin producers, including Chevron Phillips Chemical (CPC), Saudi Basic Industries Corporation, and BASF, have been using this technology to produce circular polymers from plastic waste.\n8\u201310\n Indeed, CPC has already accomplished the first commercial-scale production of circular PE in the United States. In addition to the commercial application, catalytic pyrolysis has also drawn much interest from research communities. The production of syngas or liquid hydrocarbon fuels from PE waste is technically feasible.\n11\n However, elevated temperatures (>300\u00b0C) are needed in catalytic pyrolysis processes,\n12\u201314\n which might not be economically sound given the high energy consumption. Moreover, it is challenging to control product distribution at high temperatures. In addition to linear alkanes, branched, cyclic, and aromatic hydrocarbons are produced during pyrolysis.\n15\u201317\n Aromatics are of value, but they can readily be transformed into coke that might cause catalyst deactivation.\n18\u201321\n Even though the catalyst could be regenerated after the coke is burned, the operation cost would increase substantially.Therefore, developing effective catalytic processes that could selectively convert PE to high-value chemicals under mild reaction conditions is of utmost importance for chemical upcycling of PE waste plastics.\n22\n For instance, Sadow and coworkers\n23\n designed a mesoporous catalyst with a Pt core@SiO2 shell structure to selectively convert high-density PE (HDPE) into a narrow distribution of diesel- and lubricant-range alkanes in a solvent-free system (300\u00b0C, 24 h, 1.38 MPa H2). The polymer molecules thread and bind into the silica pores, and the small-molecule products desorb and exit the pores after the cleavage from the polymer end at the active sites on the Pt metal catalyst surface. Likewise, Scott and coworkers\n24\n developed a tandem solvent-free hydrogenolysis-aromatization process to produce valuable alkyl aromatics from PE with a Pt/Al2O3 catalyst at 280\u00b0C. Although these solvent-free methods provided a strategy for manufacturing higher-value products from PE waste, the kinetic performance is still an issue because it requires an extended processing time (24 h).In general, compared with solvent-free pyrolysis, PE depolymerization can be promoted dramatically with the use of solvents, where mass transfer and heat transfer rates can be improved.\n25\u201327\n Adams et\u00a0al.\n28\n used ionic liquids to convert PE at 120\u00b0C, and the yield of low-molecular-weight hydrocarbons reached 95% in 72 h. Although the reaction temperature was much lower, the reaction time had to be prolonged to achieve satisfactory outcomes. Meanwhile, the separation might be an issue given that another solvent was needed for extracting the products from the ionic liquid solvent. Jia et\u00a0al.\n29\n reported that PE was degraded into transportation fuels and waxes through cross-alkane metathesis with hexane, 98% of which were converted into liquid hydrocarbon oils at 150\u00b0C in 3\u00a0days. Ideally, a well-designed solvent system with appropriate heterogeneous catalysts could promote highly selective PE depolymerization under mild conditions. However, for the current solvolysis process, catalytic deconstruction rates still need to be enhanced. Practically, the recovery, reuse, and lifetime of solvents and catalysts could also be limiting factors for large-scale applications.In our previous study, we found that ruthenium on a carbon (Ru/C) catalyst was able to convert n-heptadecane into short-chain hydrocarbons under mild conditions. The Ru catalyst is known to be capable of cleaving the C\u2013C bond.\n30\n\n,\n\n31\n The dehydrogenative chemisorption of the hydrocarbons is considered the first step in the mechanism of hydrogenolysis on active metal, and then the formed hydrogen-deficient surface species go through C\u2013C bond scission.\n32\n After the cleavage of C\u2013C, the reaction is finally completed by hydrogenation and desorption. PE, consisting of long hydrocarbon chains, has the simplest structure of any of the polymers. While our manuscript was under review, the remarkably high activity of the Ru catalyst in the hydrogenolysis of PE was also reported by Rorrer et\u00a0al. in the absence of solvent.\n33\n We hypothesize that Ru catalysts can break the C\u2013C bonds in PE polymers by using a suitable solvent. Hence, in the current study, we investigated the conversion of PE to liquid fuels with a Ru/C catalyst in the liquid-phase reaction, which has not been reported previously to the best of our knowledge.\nTable 1\n shows the structural parameters of fresh and spent Ru/C catalysts. The specific surface area, the metallic surface area, and the active-metal dispersion decreased after the first run but remained the same after the second run. The result showed that the catalyst structure became stable after the first cycle. The decrease in Ru dispersion could be partly due to metal leaching during the reaction. The Ru particle size increased from 2.9 to 4.1\u00a0nm, indicating that sintering occurred after the first run. These structural changes could explain the decrease in the catalytic activity after the first run.Transmission electron microscopy (TEM) images of the fresh and spent Ru/C catalysts are displayed in Figure\u00a01\n, showing that the Ru nanoparticles were well dispersed on the C support. The mean particle size on the fresh catalyst was approximately 3.1\u00a0nm. A slight shift in the particle-size distribution was observed on the used catalysts, although the particle size was in the range of 2\u20135\u00a0nm. According to the TEM images, the mean particle size of the spent Ru/C catalysts after the first and second cycles was 4.2 and 4.0\u00a0nm, respectively, which is consistent with the CO pulse chemisorption result. Both characterization results demonstrated that the aggregation occurred on the Ru/C catalyst after the first cycle, whereas the Ru particle size was nearly unchanged in the subsequent cycles.We employed X-ray photoelectron spectroscopy (XPS) to investigate the change in valence state in the Ru particles before and after the reaction. Because the Ru 3d doublet overlaps C 1s, Ru 3p is commonly used for characterizing the change in the Ru element valence state. Figure\u00a02\n shows that the Ru 3p1/2 and 3p3/2 binding energies of the fresh Ru/C catalyst were 462.9 and 485.0 eV, respectively, whereas those of the spent catalyst shifted to low values, 462.4 and 484.8 eV, respectively, after the reaction, indicating that Ru oxide on the catalyst was reduced by H2 during the reaction. Meanwhile, the Ru atomic percentage decreased from 1.6% to 1.05% after the first cycle, whereas it remained the same after the second cycle, which is consistent with the trend of the decrease in the metallic surface area in Table 1.The crystalline structures of the fresh and used catalysts before and after the HDPE depolymerization, respectively, were characterized through X-ray diffraction (XRD) (Figure\u00a03\n). Two XRD peaks at about 2\u03b8\u00a0= 25\u00b0 and 43\u00b0 are associated with the (002) and (100) phases of the C support, respectively. No Ru or Ru oxide peaks were observed, indicating that the Ru particles were very small and dispersed on the C support very well.\n34\n No significant change in the XRD patterns was observed before or after the reaction, implying that the catalyst's crystal structure might be unchanged.The HDPE depolymerization reaction was investigated with a variety of C-supported metal catalysts under the same reaction conditions. The experimental results in Table 2\n show that the copper, iron, palladium, platinum, and nickel catalysts displayed no effect on the HDPE depolymerization at 220\u00b0C. Although other groups have reported that iron, palladium, and nickel can promote PE deconstruction, high temperatures (e.g., 430\u00b0C) are still necessary for such processes.\n35\n\n,\n\n36\n Recently, Pt@SiO2 catalysts were reported to carry out the hydrogenolysis of HDPE in a solvent-free system for an extended reaction time, 24 h, at a relatively low temperature (250\u00b0C).\n23\n In contrast, in our study, only <0.5 wt % of the HDPE depolymerization products (C8\u2013C38) was detected on gas chromatography-mass spectrometry (GC-MS) with the Pt/C catalyst in n-hexane even when it was reacted for 6\u00a0h at 250\u00b0C. The solvent system's poor performance could be ascribed to HDPE's low solubility in supercritical n-hexane (critical temperature: 234.5\u00b0C). Rhodium (Rh) was reported to have catalytic ability in C\u2013C cracking, which is similar to Ru.\n37\n However, with the Rh/C catalyst, no detectable liquid hydrocarbon products by GC-MS were observed at 220\u00b0C, although there was no residue after the reaction. Long-chain hydrocarbons (>C45) with high molecular weights, which are beyond the detection limit of our mass spectrometer, could be the main products. As the temperature increased to 280\u00b0C, an ~75.3 wt % yield of alkanes in the range of C8\u2013C38 was obtained (Figure\u00a0S1A), demonstrating that Rh is also active for C\u2013C hydrogenolysis at elevated temperatures. In contrast, the full conversion of HDPE to hydrocarbon fuels by pyrolysis with the Ru/Y-zeolite catalyst was accomplished at 600\u00b0C. However, the severe coke deposition on the catalyst in pyrolysis raised concerns about the catalyst\u2019s stability.\n38\n Here, we found that the Ru/C catalyst was superior among all the screened catalysts in this study. The HDPE strips were converted to 60.8 wt % jet-fuel-range and 14.1 wt % diesel-range alkanes at 220\u00b0C in just 1\u00a0h with the Ru/C catalyst in n-hexane, and no long-chain products could be detected (Figure\u00a0S1B). Compared with other metals, Ru metal was reported to have the lowest activation energy in ethane hydrogenolysis, favoring the C\u2013C bond cleavage.\n32\n In the comparison of ethane hydrogenolysis on transition-metal catalysts, \u2217CHCH\u2217 was found to be the primary intermediate in the C\u2013C bond scission for Ru, Rh, and Pt because it has the lowest free-energy barrier in C\u2013C bond cleavage.\n39\n Meanwhile, both \u2217CHCH\u2217 and \u2217CH3CH\u2217 were considered dominant intermediates for Pd. Among these transition metals, the turnover rate in \u2217CHCH\u2217 cleavage decreases in the order Ru\u00a0> Rh > Pt > Pd, which is consistent with our result that Ru could cleave the C\u2013C efficiently and that Pd has the lowest cleavage turnover rate.The temperature effect on the HPDE depolymerization is shown in Figure\u00a04A. We detected no cracking product at 150\u00b0C. When the depolymerization was carried out at 200\u00b0C, a complete HDPE conversion to liquid-phase alkanes was obtained. With increasing temperature, the yield of high-molecular-weight alkane products decreased. The yield of the jet-fuel-range alkanes (C8\u2013C16) reached a maximum of ~60 wt %, whereas that of the diesel fuels (C17\u2013C22) was ~15 wt % at 220\u00b0C, and almost all long-chain hydrocarbons (C number > 23) were converted to short-chain alkanes in 1 h. As the temperature increased to 230\u00b0C, the yields of jet- and diesel-fuel-range alkanes decreased to ~55 and ~5 wt %, respectively, as a result of excess cracking. The HDPE polymer is not easily solvated in a supercritical solvent. At 240\u00b0C, which is higher than n-hexane's critical temperature (234.5\u00b0C), we observed an abrupt change in the product distribution compared with that at 230\u00b0C. The yield of the long-chain hydrocarbon products (C17\u2013C38) increased dramatically from <5 to ~50 wt % as the temperature increased just 10\u00b0C (from 230\u00b0C to 240\u00b0C), implying that the low solubility of HDPE in the supercritical n-hexane solvent could lead to much slower C\u2013C bond cracking rates.The reaction time is another crucial parameter for determining the product distribution. Here, the effect of reaction time on the HDPE depolymerization was also investigated, and the results are shown in Figure\u00a04B. Surprisingly, HDPE was rapidly degraded to liquid hydrocarbons (C number\u00a0< 38) in only 0.5\u00a0h at 220\u00b0C. With increasing reaction time, the yield of jet-fuel-range alkanes increased first and then decreased as a result of excess cracking. The maximum yield (~60 wt %) of jet-fuel-range alkanes was achieved in 1 h. Almost no high-molecular-weight products were observed after 1 h.Further, we also investigated the catalyst loading effect on the depolymerization by varying the amount of catalyst. As shown in Figure\u00a04C, the depolymerization reaction did not occur in the absence of a catalyst. The depolymerization reaction rate increased with increasing catalyst loading. With a low loading of the catalyst ([Ru]/[HDPE] ratio was 2.1%), the yield of lubricant-range hydrocarbons (C24\u2013C35) reached 31.6%. While the [Ru]/[HDPE] ratio increased to 8.3%, the yield of jet-fuel-range alkanes achieved the maximum value (~60 wt %). As the catalyst amount continued to increase, the corresponding jet-fuel yield decreased. Meanwhile, more short-chain hydrocarbons (C number\u00a0< 8) were observed after the [Ru]/[HDPE] ratio surpassed 1.2%, indicating that an increasing amount of catalyst would promote the cracking reaction.\nFigure\u00a05\n shows that hydrogen pressure played a significant role in the HDPE depolymerization. In the absence of H2, no product was detected. With increasing H2 pressure from 0 to 60 bar, the depolymerization reaction rate increased first and then decreased after the H2 pressure passed 30 bar, indicating that higher hydrogen pressure could inhibit the depolymerization reaction. Iglesia and coworkers also observed that hydrogenolysis of the linear and branched alkanes (C2\u2013C8) was reduced as the H2 pressure increased.\n40\n They found that H2 pressure could also influence the C\u2013C bond cleavage position in long-chain alkanes, probably as a result of the dehydrogenated intermediates formed by quasi-equilibrated adsorption and dehydrogenation.\n41\n\n,\n\n42\n At low hydrogen pressures, the hydrogenolysis rates were proportional to the concentration of the reactive unsaturated intermediate [\u2217CnH2n+2\u2212y\u2217], and the rates increased with hydrogen pressure.\n43\n At high hydrogen pressures, the surface was mainly occupied by chemisorbed hydrogen atoms (H\u2217), hindering the adsorption of intermediates and decreasing the hydrogenolysis rates.\u00a0Note that Iglesia and coworkers studied the alkane hydrogenolysis in the gas phase, which could significantly differ from PE's hydrogenolysis in solvents. HDPE's structure resembles those of long-C-chain linear alkanes (varying in C chain length), consisting of only Csecondary\u2013Cprimary and Csecondary\u2013Csecondary bonds. Hence, the Ru-catalyzed HDPE hydrogenolysis includes primarily two independent reactions: regioselective hydrogenolysis of the easily accessible C\u2013C bonds (e.g., Csecondary\u2013Csecondary) and hydrogenolysis of Csecondary\u2013Cprimary bonds (i.e., chain-end scission).\n44\n Thus, the scission of Csecondary\u2013Csecondary is preferred for acquiring more valuable long-chain hydrocarbons.Also, the hydrogenolysis mechanism of linear liquid-phase alkanes would be analogous to the dissociation mechanism for the C\u2013C bonds in HDPE and its degradation intermediates. Herein, the hydrogen pressure effect was further explored with eicosane, a C20 linear alkane, as the probe reactant (Figure\u00a06\n). We found that at low H2 pressure (10 bar), the C19 alkane, n-nonadecane, was the dominant product, indicating that terminal dissociation was the main pathway. As the H2 pressure increased to 60 bar, the main products were octadecane and heptadecane (C18H38 and C17H36), demonstrating that the primary pathway was changed to internal dissociation. Nakagawa et\u00a0al. reported that with a Ru/CeO2 catalyst and the absence of solvents, the reaction order to the H2 partial pressure for cracking n-hexadecane (C16H34) was 0.4. Non-stoichiometric methane formation from n-hexadecane ([methane] \u2212 [C15]\u00a0= \u22120.8) was observed, indicating that high hydrogen pressure suppressed excess methane formation, i.e., the cleavage of Csecondary\u2013Cprimary.\n44\n The same group also observed that under higher hydrogen pressures, the yield of C15 from terminal dissociation was lower than the average of the internal dissociation product yields, which is similar to our result that only a low yield of C19 was obtained at 60\u00a0bar of H2. Notably, Nakagawa et\u00a0al. found no significant difference between the yields of C2\u2013C14 hydrocarbons, whereas we observed that the main products, C18 and C17, were acquired with the presence of a solvent.Likewise, HDPE is a linear alkane polymer containing predominantly secondary C atoms and a few primary C atoms; the influence of hydrogen pressure on the hydrogenolysis of HDPE seems similar to that of eicosane. At low H2 pressures, the liquid alkane products might mainly be generated from the terminal dissociation, which was suppressed with increasing H2 pressure. After the H2 pressure passed a threshold value, the internal dissociation became dominant. At 60\u00a0bar of H2, ~90% of HDPE was converted to C8+ liquid hydrocarbon products, implying that internal dissociation is the primary depolymerization pathway at high H2 pressures. However, both terminal and internal dissociation can coexist in a wide range of H2 pressures during HDPE depolymerization.Solute solubility and thermodynamic equilibrium coefficients are critical parameters that affect the reaction kinetics in solutions.\n45\n Here, the role of different organic solvents in HDPE depolymerization was investigated. In a polar solvent, e.g., water, the HDPE degradation rate was found to be very slow at 220\u00b0C, as shown in Figure\u00a07\n. Typically, PE can be degraded in supercritical water whose dielectric constant is comparable to those of the polar organic solvents.\n46\n\n,\n\n47\n Although the supercritical hydrolysis process requires a very high energy input, the low polarity of supercritical water facilitates PE's dissolubility and thus promotes the reaction rate. However, at 220\u00b0C, subcritical water is much denser and more polar than supercritical water, leading to a low PE solubility and thus a slow depolymerization reaction rate. Meanwhile, we observed that the HDPE strips were transformed into spherical solid particles after the reaction, which was different from that in the organic solvents (Figure\u00a0S2). These plastic strips usually melted at over 150\u00b0C.\n48\n The formation of spherical solids indicated that the plastic strips were melted but were not solvated in the water at 220\u00b0C as a result of the low-solubility HDPE in subcritical water. Therefore, non-polar solvents were preferred for PE dissolution and depolymerization. Figure\u00a07 shows that n-hexane was the optimal organic solvent for HDPE degradation with the Ru/C catalyst, whereas other non-polar solvents exhibited much different performance in the depolymerization reaction. Notably, no cracking products were detected in n-pentane solvent, although the polarity of n-pentane is very similar to that of n-hexane. Here, the reaction temperature (220\u00b0C) was higher than n-pentane's critical temperature (196.45\u00b0C) but lower than n-hexane's critical temperature (234.5\u00b0C). Therefore, the supercritical pentane solvent behaved very differently from those at lower temperatures. HDPE polymers might not be solvated in the supercritical n-pentane, causing high resistance to mass and heat transfer. We also observed that the HDPE strips were transformed into spherical particles in the supercritical n-pentane after the reaction, implying that HDPE was melted rather than dissolved.We evaluated the solvation effect by using the Hansen solubility parameters, which are based on the theory of \u201clike dissolves like.\u201d\n49\n As shown in Tables S1 and S2, the relative energy difference (RED) of water and PE is much larger than 1, indicating that water is not a suitable solvent for PE. The RED values are less than 1 for other organic solvents that show a high affinity, consistent with the experimental results that HDPE polymer could be dissolved in these solvents. It is reasonable that PE solvation in the solvents is the first step in the degradation reaction (Scheme 1\n). We observed that the solvent molecular structure profoundly affects the depolymerization, as shown in Figure\u00a07. For instance, methylcyclohexane was not as efficient as n-hexane for depolymerization because of its obstructive cyclic molecular structure. Under identical reaction conditions, the dominant products with the n-hexane solvent are the medium-chain n-alkanes (C8\u2013C16), whereas the longer-chain n-alkanes (C17\u2013C38) are the main products in methylcyclohexane. Nevertheless, the appropriate inhibition effect on the PE depolymerization in methylcyclohexane was desired for controlling the product distribution given that the long-chain hydrocarbons (C17\u2013C38) are the target products, such as lubricants, with a higher profit margin than the medium-chain n-alkanes (C8\u2013C16), which are jet-fuel components. A similar steric hindrance effect was also observed with decalin as the solvent, whereby no cracking liquid hydrocarbon products were detected after the reaction. The solvated polymer molecules in decalin might be obstructed from being in contact with the heterogeneous Ru/C catalyst surface. Note that the molecular size of n-hexane is 1.03\u00a0nm (length) \u00d7 0.49\u00a0nm (width) \u00d7 0.4\u00a0nm (height), which is much larger than methylcyclohexane (0.79\u00a0\u00d7 0.73\u00a0\u00d7 0.5\u00a0nm) and slightly longer than decalin (0.91\u00a0\u00d7 0.72\u00a0\u00d7 0.5\u00a0nm).\n50\u201352\n Nevertheless, the linear molecules, e.g., n-hexane, were more flexible, compensating for their bulky molecular size.\n53\n\n,\n\n54\n The similarity in shape between n-hexane and HDPE could facilitate the diffusion of large PE oligomer molecules in the solvent, which allows the access of bulky reactant substrates to the Ru/C catalyst surface. In addition, methylcyclohexane and decalin are known as the hydrogen-donor solvents,\n55\n which can transfer hydrogen even in the H2 atmosphere. The solvent-donated H\u2217 could quickly react with the polymer radicals, terminating the consecutive cracking reactions.\n56\u201358\n\nAccording to the results of the molecular dynamics (MD) simulations, PE adopts a compact conformation in pentane and hexane, with the lowest radius of gyration value (Rg), followed by water and methylcyclohexane, and finally it adopts an extended conformation in trans-decalin (Table S3). The extended conformation of PE in decalin can be attributed to the high degree of hydrophobicity of decalin solvent. A PE molecule is also hydrophobic in nature and thus prefers to be in hydrophobic solvents, resulting in the fully extended conformation of the PE molecule in hydrophobic solvents such as decalin. To understand the influence that the structure might have on dynamic properties, we computed the end-to-end polymer chain autocorrelation function in different solvents, as shown in Figure\u00a08\n, which estimates how readily the polymer relaxes in a particular solvent.\nFigure\u00a08 shows that PE polymer decorrelates fastest in n-pentane and n-hexane, followed by methylcyclohexane, water, and decalin. The decorrelation order of the PE polymer is in accordance with the amounts of short-chain hydrocarbon molecules produced in the experiment, except for n-pentane. In our simulations, PE decorrelated fastest in n-pentane; however, PE did not depolymerize in n-pentane to produce short-chain hydrocarbon molecules in the experiment. In this case, the difference observed between experiment and simulation is due to the limitation of simulations to capture supercritical behaviors of n-pentane. In our simulations at 493 K, n-pentane still behaved like a normal fluid rather than a supercritical one. As a result, the behavior of PE in n-pentane is similar to that in n-hexane.The PE end-to-end length decorrelation rate correlates to the affinity of PE polymer toward the solvent it is immersed in, as described by the radius of gyration results. As the simulation progresses, the interaction between PE polymer and solvent molecules causes the conformation of the PE polymer to change. The cases in which the PE end-to-end length decorrelates fast (for example, in n-hexane) indicate that the PE polymer does not have a high affinity toward solvent molecules, thus causing the PE polymer to coil. We propose that the coiled polymer adsorbs in this state on the catalyst surface and undergoes cracking reactions. The coiled structure has a high tendency to pass through the solvent molecules to reach the catalyst surface. In contrast, the slow decorrelation rate of PE in decalin shows PE affinity toward decalin, where PE polymer can sustain extended conformations for a longer time. The comparison between PE conformation in decalin and in hexane after 500\u00a0ns of NVT (substance, volume, and temperature) simulations is shown in Figure\u00a09\n. The straight PE chain in decalin has a high affinity toward solvent molecules, preventing the straight PE chain from reaching the catalyst surface for depolymerization reactions and leading to poor kinetic performance. The extended configurations of PE in decalin suggest higher relative thermodynamic stability in the bulk solvent as a result of increased entropy arising from the chain flexibility in the solvent. This result suggests that considering both the solvent quality and the adsorption affinity of collapsed and extended PE chains could determine an additional important screening characteristic for solvents used in depolymerization processes.The catalyst stability is a big hurdle in plastic depolymerization via catalytic pyrolysis.\n59\n\n,\n\n60\n In our study, the catalyst did not show severe deactivation in the n-hexane solvent after being used for five cycles (Figure\u00a010\n). The yield of jet-fuel-range alkanes (C8\u2013C16) decreased only slightly after first use and then became stable in the subsequent runs, indicating that the catalyst stability would be reliable for depolymerization. We observed that more short-chain hydrocarbons were generated after the first cycle, which could be ascribed to the increase in Ru particle size. Nakagawa et\u00a0al. found that the terminal dissociation was more prevalent if the Ru particle size increased from <1.5 to >2\u00a0nm.\n44\n Therefore, smaller particle size might favor the yield of jet-fuel-range products. Furthermore, the thermal gravimetric analysis (TGA) curves showed that the Ru loading decreased by 0.62% after the first cycle and remained almost the same after the second cycle (Figure\u00a0S3), which is consistent with the trend of decrease in the metallic surface area in Table\u00a01. Both results demonstrated that Ru would not continuously leach after the first use.Because of the high catalytic activity of the Ru catalyst in cleavage of the C\u2013C bond, the solvent stability is important for the PE hydrogenolysis process. A blank experiment was conducted without the addition of HDPE (0.05\u00a0g Ru/C, 25\u00a0mL n-hexane, 220\u00b0C, p(H2) 20 bar, 1 h, 700\u00a0rpm). Approximately 5.6 wt % of the solvent (including 5.1 wt % loss by evaporation) was lost after the reaction, which was much lower than in the cross-alkane metathesis process for PE depolymerization (15.1 wt % loss) with light alkanes as both the solvent and the feedstock and (t-Bu2PO-t-BuPOCOP)Ir(C2H4)/\u03b3-Al2O3 and Re2O7/\u03b3-Al2O3 as catalysts at 175\u00b0C for 4\u00a0days.\n29\n Moreover, for process optimization, the short-chain hydrocarbon products from HDPE depolymerization could be reused as the makeup solvent in the process.In summary, we have demonstrated an efficient liquid-phase hydrogenolysis process with the heterogeneous Ru/C catalyst for selective depolymerization of waste HDPE plastic under mild conditions. Approximately 90 wt % HDPE was converted to C8+ liquid hydrocarbon products in an n-hexane solvent within 1\u00a0h under 30\u00a0bar H2 at 220\u00b0C. We were able to tune the product distribution by adjusting the process conditions, including catalyst loading, reaction temperature, hydrogen pressure, and reaction time. With high catalyst loading, high reaction temperature, or prolonged reaction time, excess cracking occurred during the reaction and led to the production of less valuable short-chain hydrocarbons. Hydrogen pressure played a significant role in the polymer dissociation pathway. Under low H2 pressures, terminal dissociation was dominant, whereas internal dissociation was prevalent when the H2 pressure increased.Furthermore, solvents also profoundly affected the depolymerization reaction kinetics and product selectivity. The solvation ability of PE in solvents was a key factor for depolymerization. The degradation of HDPE in subcritical water was slow because of its low solubility in polar solvents. Among the non-polar hydrocarbon solvents, n-hexane (a linear alkane) was better for HDPE depolymerization than the cyclic alkanes (methylcyclohexane and decalin). The highest yield of jet-fuel-range hydrocarbons (C8\u2013C16) reached 60.8 wt % in the n-hexane solvent at 220\u00b0C. The MD simulations suggest that the interaction between PE polymers and solvent molecules causes the conformation of the PE polymer to change. The PE polymer with a low affinity toward solvent molecules tends to coil and then sieve through solvent molecules and get to the catalyst surface, where it will get cracked. PE adopts a compact coil conformation in pentane and hexane, followed by water, methylcyclohexane, and decalin. Although the steric hindrance from the solvents' cyclic molecular structure inhibited PE depolymerization, it promoted the production of long-chain hydrocarbons, such as lubricants.Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Hongfei Lin (hongfei.lin@wsu.edu).This study did not generate any new unique reagent or material.This study did not generate codes, software, or algorithms.The feedstocks, HDPE plastic water jugs, were collected from the local recycling center in Pullman, Washington. Before the experiment, the jugs were cleaned with deionized water, dried at 100\u00b0C, and then cut into strips (5\u00a0\u00d7 5\u00a0mm). All chemicals were used as received without further treatment. The catalysts (Ru/C [5% Ru basis], Pd/C [5% Pd basis], Pt/C [5% Pt basis], and Rh/C [5% Rh basis]), the catalyst precursors (copper(II) nitrate trihydrate [99%] and iron(III) nitrate nonahydrate [98%]), and the self-synthesized catalyst support (activated charcoal Norit) were supplied from Sigma-Aldrich. Nickel(II) nitrate hexahydrate (99%) was purchased from Millipore Sigma. p-xylene (99%) was purchased from Alfa Aesar. Ultrapure water (specific resistance of 18.2 M\u03a9 cm\u22121), n-pentane (Alfa Aesar, 98%), n-hexane (J.T. Baker, 95%), methylcyclohexane (Alfa Aesar, 99%), and decalin (Tokyo Chemical Industry, 99%) were used as the solvents.5% Cu/C, 5% Fe/C, and 5% Ni/C were synthesized through impregnation with copper nitrate trihydrate, iron nitrate nonahydrate, and nickel nitrate hexahydrate, respectively, as the metal precursors and activated charcoal Norit as the support. After being dried, the as-prepared 5% Ni/C, 5% Fe/C, and 5% Ni/C samples were calcined at 350\u00b0C (Ni/C) or 500\u00b0C (Fe/C and Ni/C) for 3\u00a0h in an atmosphere of nitrogen. Finally, the catalysts were reduced in H2 flow at 400\u00b0C (Ni/C) or 500\u00b0C (Fe/C and Ni/C) for 5\u00a0h prior to use.The specific surface area of the catalysts was determined through single-point adsorption of N2 at 77 K with a Micromeritics Autochem II 2920. The samples were prepared in helium at 200\u00b0C for 1\u00a0h before nitrogen adsorption (30% N2/He).The CO pulse chemisorption was used for determining the metal dispersion, active-metal particle size, and metallic surface area. The test was carried out on a Micromeritics Autochem II 2920. The sample was reduced for 2\u00a0h at 300\u00b0C with 10% H2/Ar at a 50\u00a0mL/min flow rate and then purged with helium for 1\u00a0h at a flow rate of 50\u00a0mL/min. After the sample was cooled to ambient temperature, 10% CO/He was added at each pulse, and the CO uptake profile was measured with a thermal conductivity detector (TCD) until no CO was adsorbed. The Ru dispersion was calculated under the assumption of a CO/Ru stoichiometry of 1:1.\n61\n\nThe fresh and spent Ru/C catalysts were characterized by TEM on a JEOL 2010\u00a0J microscope at an accelerating voltage of 200 kV. The Gatan Digital Micrograph software was used for conducting data processing and analysis. The catalyst powder samples were dispersed on Formvar film nickel grids (200 mesh).The XPS analyses were carried out on a Kratos AXIS-165 with a monochromatized Al-K\u03b1 X-ray anode (1,486.6 eV) with the C 1s peak at 284.6 eV as the internal reference. The deconvolutions of Ru 3p were analyzed with the software XPSPEAK version 4.1.The crystalline catalyst structure was evaluated by X-ray powder diffraction (Rigaku Miniflex 600), with a Co-K\u03b1 radiation source (\u03bb\u00a0= \u00c5) at a 2\u03b8 step of 10\u00b0\u201390\u00b0 with a step size of 0.02\u00b0.TGA was performed with a TA Instruments Q50. The samples were loaded in aluminum crucibles and heated in airflow (60\u00a0mL/min) from 25\u00b0C to 600\u00b0C at a heating rate of 10\u00b0C/min.The depolymerization experiments were carried out in a 45\u00a0mL elevated pressure and temperature Parr Series 5000 multiple reactor system with a 4871 temperature controller. In a typical experiment, a certain amount of HDPE strips and catalyst were loaded in 25\u00a0mL solvent. The vessels were sealed and purged five times with 400 psi N2 and three times with 400 psi H2 and then pressurized with H2 to the set pressure at ambient temperature. Then the reactor was heated up to the set reaction temperature with magnetic stirring at 700\u00a0rpm. After the reaction, the vessel was quenched in a cold bath for fast cooling.After the reaction, the reactor was connected to a gas chromatograph Shimadzu GC-2014 with a TCD for analysis of the gas-phase product samples. The columns included a right 12.5\u00a0m (l) \u00d7 0.32\u00a0mm (i.d.) packed column, which comprised 3\u00a0m Hayesep D, 4\u00a0m HS, and 2.5\u00a0m HN, and a left 2\u00a0m (l) \u00d7 0.32\u00a0mm (i.d.) 10% Carbowax 20\u00a0m Ch packed column. After the reactor was disassembled, the solid catalyst and non-dissolvable residues were filtered out of the liquid phase. Then the liquid product samples were collected, and the internal standard, p-xylene, was added. The liquid samples were analyzed by a QP-2020 (Shimadzu) gas chromatograph-mass spectrometer for identifying and quantifying the unknown products. The QP-2020 was equipped with a Shimadzu SH-Rxi-5SIL MS column (30\u00a0m \u00d7 0.25\u00a0mm i.d., 0.25\u00a0\u03bcm film thickness), a flame ionization detector, and a high-performance ion source. The following definitions were used for quantitating the weight yield (y):\n\n\n\ny\n=\n\n\n\u2211\n\nm\nx\n\n\n\n\nm\n0\n\n\n\u00d7\n100\n\n%\n,\n\n\n\nwhere m0 is the weight of the HDPE feedstock before reaction and mx is the weight of the alkane hydrocarbons after the reaction, where x means the C number.A PE molecule C100H202 in length was packed into five different simulation boxes of 10\u00a0\u00d7 10\u00a0\u00d7 10\u00a0nm3. Each box was filled with one of the five different solvents: methylcyclohexane, n-pentane, n-hexane, water, or decalin. Water was modeled with the SPC/E water force field,\n62\n while the force fields for the organic solvents were obtained from the Automated Topology Builder repository.\n63\n For decalin, the isomer used was trans-decalin because trans-decalin is more stable than its cis counterpart as a result of its diequatorial chair conformation. Each system was simulated with the GROMACS 2018.3 simulation package.\n64\n The steepest descent algorithms were used for removing unfavorable contacts in the initial configuration. Electrostatic interactions were calculated with the particle mesh Ewald summation method\n65\n with an electrostatic cutoff value of 1.0\u00a0nm and van der Waals cutoff value of 1.0\u00a0nm. The system was evolved in the NPT ensemble (temperature 493 K, pressure 1 atm) for 2\u00a0ns with the Donadio-Bussi-Parrinello thermostat\n66\n (time constant \u03c4\u00a0= 0.1 ps) and the Berendsen barostat\n67\n (time constant \u03c4\u00a0= 1 ps). A temperature of 493 K was chosen to be consistent with the experiment. All the dimensions of the box were allowed to change during the NPT simulation. The production runs were carried out in the NVT ensemble (temperature 493 K), where the temperature was maintained by the Donadio-Bussi-Parrinello thermostat (time constant \u03c4\u00a0= 0.1 ps) for\u00a0500\u00a0ns.The polymer structure in the solvent was captured through the average radius of gyration calculated over the entire simulation time of 500\u00a0ns. To assess the dynamic behavior of the polymer in different solvents, we calculated the end-to-end autocorrelation function according to the following equation:\n\n\n\n\ne\n2\ne\n\n(\nt\n)\n\n\u2261\n\n\n\u27e8\n\nA\n\n(\nt\n)\n\n\u22c5\nA\n\n(\n0\n)\n\n\n\u27e9\n\n\n\u27e8\n\nA\n\n(\n0\n)\n\n\u22c5\nA\n\n(\n0\n)\n\n\n\u27e9\n\n\n,\n\n\n\nwhere \n\nA\n\n is the vector from the first C atom to the last C atom along the polymer chain.This project was partially funded by the Washington State University internal fund and Washington Research Foundation. C.J. is thankful for the Chambroad Fellowship from the Gene and Linda Voiland School of Chemical Engineering and Bioengineering at Washington State University. Computational simulation in this research was supported by NSF-CBET award 1703638 and was facilitated through the use of advanced computational, storage, and networking infrastructure provided by the Hyak supercomputer system at the University of Washington. The authors would like to acknowledge Zengran Sun and Prof. Steven R. Saunders for their support in the thermal gravimetric analysis. The authors also thank Dr. Baoming Zhao and Prof. Jinwen Zhang for valuable discussions.H.L. proposed, designed, and guided the project and revised the manuscript. C.J. performed most of the experiments and drafted the manuscript. S.X. and W.Z. also took part in the experiments and revised the manuscript. N.I., J.S., and J.P. performed the molecular dynamics simulations. All authors checked the manuscript.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.checat.2021.04.002.\n\n\nDocument S1. Supplemental experimental procedures, Figures S1\u2013S3, and Tables S1\u2013S3\n\n\n\n\n\nDocument S2. Article plus supplemental information\n\n\n\n", "descript": "\n                  Polyethylene (PE) is the most popular plastic globally, and the widespread use of plastics has created severe environmental issues. High energy consumption in the current process makes its recycling a challenging problem. In our report, the depolymerization of high-density PE was conducted in various liquid-phase solvents with the Ru/C catalyst under relatively mild conditions. The maximum yields of the jet-fuel- and lubricant-range hydrocarbons were 60.8 and 31.6 wt %, respectively. After optimization of the reaction conditions (220\u00b0C and 60\u00a0bar of H2), the total yield of liquid hydrocarbon products reached approximately 90 wt % within only 1 h. The product distribution could be tuned by the H2 partial pressure, the active-metal particle size, and the solvents. The solvation of PE in the different solvents determined the depolymerization reaction kinetics, which was confirmed by the molecular dynamics simulation results.\n               "}
{"full_text": "With the rapid industrial development, an increasing number of organic pollutants are discharged into the aquatic environment, producing negative impacts on humans, plants, and animals, as well as the entire ecosystem (Trejo-Castillo et\u00a0al., 2021). Benzotriazole (BTA), a widely used chemical, has become a common additive in mineral flotation agents (Yao et\u00a0al., 2021), circulating cooling water treatment agents (Yang et\u00a0al., 2021), anti-icing fluids, dishwasher detergents, and metal corrosion inhibitors (Castaldo et\u00a0al., 2020). This causes significant amounts of BTA to be discharged into the aquatic environment through industrial excess discharges (Cheng et\u00a0al., 2021) or mixed surface runoff collected in sewage systems (Yang et\u00a0al., 2021). Because of its persistence, bioaccumulation, and toxicity, BTA inhibits the growth and reproduction of aquatic organisms, and its estrogenic potential may have deleterious effects on the sex differentiation system of many organisms (Feng et\u00a0al., 2020). In addition, BTA interferes with aquatic species and soil microbial communities, and it is carcinogenic and mutagenic in mammals (Li et\u00a0al., 2020a). As a result, BTA is classified as an emerging polar pollutant. Given that almost no enzymes in organisms can degrade BTA, conventional wastewater treatment technologies can only remove about 30% of BTA in effluent (Yin et\u00a0al., 2021). Thus, the existing BTA treatment technology needs to be improved effectively.Advanced oxidation technology (Liu et\u00a0al., 2009) is a widely used and effective method to degrade organic pollutants. Photoelectrocatalysis (PEC) (Brillas, 2020) has gained attention because it is an efficient and simple method that does not produce secondary contamination and improves the cavity time, and it has shown excellent performance in the degradation of BTA (Hu et\u00a0al., 2016). PEC has effectively degraded BTA using TiO2-coated electrodes, with a removal rate of 82.1% after 180\u00a0min (Ding et\u00a0al., 2009). Wu et\u00a0al. (2013) used ZnFe2O4 as a catalyst and performed a photoelectric-Fenton like reaction for 180\u00a0min to effectively remove 91.2% of BTA.The key to practical application of PEC is to develop a cheap and efficient catalyst. The doping of transition metal ions has become popular in environmental studies. Metals, such as Fe, Ni, Cu, and Zn, and their oxides in various valence states are dominant (Zhang et\u00a0al., 2020a). Their self-doping can lead to the generation of oxygen vacancies that act as trap sites for holes, thereby facilitating the separation of photogenerated carriers (Zhang et\u00a0al., 2021) and improving the PEC activity. Fe2O3 is one outstanding semiconductor with an ideal band gap (1.9\u20132.2\u00a0eV) structure (Leao-Neto et\u00a0al., 2020). However, pure Fe2O3 has not been commonly used as an electrical conductor due to its excessively short vacancy diffusion length (2\u20134\u00a0nm) (Asif et\u00a0al., 2021) and excited state lifetime (shorter than 10 ps) (Hannan et\u00a0al., 2021). Cu2O is a p-type metal oxide semiconductor material, and it is stable and nontoxic (Wang et\u00a0al., 2020a). Its 3d and 4s orbitals do not overlap, resulting in a semiconductor energy band structure with an empty conduction band, a full valence band (Li et\u00a0al., 2020c), and a stereocrystalline configuration (Tan et\u00a0al., 2019). Thus, Cu2O can precisely compensate for the easy compounding and low efficiency of Fe2O3 carriers in degrading pollutants (Polat, 2020). Cheng et\u00a0al. (2021) prepared a CuO\u2013Cu2O/WO3 film for the anode using a two-step deposition method and degraded oxygenated phenol in photocatalytic degradation with an efficiency of 87.6% after 180\u00a0min. Machreki et\u00a0al. (2021) successfully developed porous Fe2O3 films for photocatalytic degradation of B41 dye wastewater, achieving a degradation efficiency of about 68% after 70\u00a0min. Given that experiments combining Cu2O and Fe2O3 are complex and expensive, studies have rarely been conducted on composites of Cu2O and Fe2O3. As a result, it is critical to develop materials that are simple to prepare with a high degradation efficiency.In this study, Fe2O3/Cu2O (FC) composites were produced by a simple one-pot hydrothermal process, and they were used as catalysts for efficient PEC degradation of BTA. Afterwards, the PEC degradation efficiency of BTA was determined, and the optimal operating conditions for the system and the degradation mechanism of BTA were investigated.Analytical reagents of CuCl2\u00b72H2O, FeCl3\u00b73H2O, polyethylene glycol-4000 (PEG-4000), CH3COONa, Na2SO4, H2SO4, NaOH, CH3CH2OH, and BTA were obtained from Aladdin Biochemical Technology Ltd (Shanghai, China). Deionized water was used for all experiments.In this study, FC composites were prepared using the one-pot hydrothermal method (Wang et\u00a0al., 2021). Furthermore, appropriate amounts (molar ratios of 1:2, 1:1, 2:1, 3:1, and 4:1) of FeCl3\u00b73H2O and CuCl2\u00b72H2O were weighed, mixed with 40\u00a0mL of anhydrous CH3CH2OH, and stirred for 30\u00a0min. Next, 1\u00a0g of PEG-4000 and 3.6\u00a0g of CH3COONa were added and stirred vigorously for 4\u00a0h before being transferred to an autoclave at 200\u00b0C for 12\u00a0h of heating. The samples were filtered and cooled before being rinsed three times with distilled water and anhydrous ethanol to remove residual ions. They were then dried for 12\u00a0h at 60\u00b0C. The materials made were recorded as FC1/2, FC1, FC2, FC3, and FC4. Cu2O (or Fe2O3) was prepared without adding FeCl3\u00b73H2O (or CuCl2\u00b72H2O).Scanning electron microscopy (SEM) observation and energy spectrum analysis were performed using a field emission scanning electron microscope. X-ray diffraction (XRD) maps were obtained using a Bruker D8 advance X-ray diffractometer. X-ray fluorescence (XRF) analysis was performed using a Panalytical Axios FAST simultaneous wavelength dispersive XRF spectrometer (the Netherlands). X-ray photoelectron spectroscopy (XPS) analysis was conducted using a Thermo Fisher EscaLab 250Xi XPS analyzer. Ultraviolet\u2013visible (UV\u2013Vis) spectra were measured with a UV-2550 spectrophotometer (Shimadzu, Japan). Electrochemical measurements were performed with a CHI 660A electrochemical workstation (Shanghai Brilliance Instruments Co. Ltd., China) in a conventional three-electrode system. The modified electrode, platinum wire, and saturated glycerol electrodes were used as the working electrode, counter electrode, and reference electrode, respectively. The intermediates were analyzed using high performance liquid chromatography (HPLC) (Dinonex Ultimate 3000 UHPLC Column, Hypersil GOLD, 100\u00a0mm\u00a0\u00d7\u00a02.1\u00a0mm, 1.9\u00a0\u03bcm)\u2013mass spectrometry (MS) (Thermo Scientific Q Exactive). HPLC l (C18 column, 5\u00a0\u03bcm, 4.6\u00a0mm\u00a0\u00d7\u00a0150\u00a0mm, Agela ODS) was conducted for identification of the intermediates.\nFig.\u00a0A.1 shows the experimental setup for PEC degradation of BTA. The device used a quartz cylinder as the photoelectric reaction tank and used a titanium plate and a ruthenium-iridium loaded titanium plate (5\u00a0cm\u00a0\u00d7\u00a05\u00a0cm) as the cathode and anode, respectively (2\u00a0cm apart from each other). A 100-W high-pressure mercury lamp was used to simulate UV lamp irradiation with a primary emission wavelength of 365\u00a0nm. Circulating condensate was used to reduce the slight thermal effect of the UV lamp, and the reaction temperature was always maintained at room temperature. The simulated wastewater was mixed with 125\u00a0mL of BTA solution (20\u00a0mg/L) and 25\u00a0mL of Na2SO4 solution (0.2\u00a0mol/L). During the degradation process, BTA water samples were collected at different time intervals, and their concentrations were measured after filtration with 0.45-\u03bcm filters.The concentration of BTA was measured and analyzed using liquid chromatography (Wukong Instruments K2025) with a C18 column (with a wavelength of 268\u00a0nm). The removal efficiency (R) of BTA was calculated as follows:\n\n(1)\n\n\nR\n=\n\n\n\nC\n0\n\n\u2212\n\nC\nt\n\n\n\nC\n0\n\n\n\u00d7\n100\n%\n\n\n\nwhere C\n0 is the initial concentration of BTA; and C\n\nt\n is the concentration of BTA at the treatment time t. The chemical oxygen demand (COD) of BTA was determined according to the standard dichromate potassium method (HJ828-2017) proposed by the State Environmental Protection Administration of China.The optical properties of FC were studied using UV\u2013Vis spectroscopy, and the band gap energy was calculated using the following equation:\n\n(2)\n\n\n\u03b1\nh\nv\n=\nA\n\n\n(\n\nh\nv\n\u2212\n\nE\ng\n\n\n)\n\n\u03b7\n\n\n\n\nwhere \u03b1 is the absorption coefficient; h is Planck constant; v is the frequency of light; A is the absorbance; E\ng is the band gap energy; and \u03b7 is a variable depending on the nature of the optical leap, with \u03b7\u00a0=\u00a00.5 for the direct energy band leap and \u03b7\u00a0=\u00a02 for the indirect energy band leap (Wang et\u00a0al., 2020b).The main degradation products were analyzed and identified using HPLC\u2013MS. For HPLC, the mobile phase was 50% acetonitrile and 0.1% formic acid aqueous solution (0\u201310\u00a0min) at a flow rate of 0.2\u00a0mL/min, and its detection wavelength was 280\u00a0nm. For MS, the ion source electron spray ionization was used, the trans-gas rate was 40\u00a0mL/min, the auxiliary gas rate was 10\u00a0mL/min, the spray voltage was 3.0\u00a0kV, the capillary temperature was set to 300\u00b0C, the scan mode of S-lens50 was positive ion fullms-ddms2 top5, the resolution reached 70 000, and the scan range was 50\u2013750 (mass-to-charge ratio).The SEM image of FC (Fig.\u00a01\n(a)) showed that many small disks of different sizes were attached to the cubic crystals. The rounded Fe2O3 increased the contact area for pollutants and easily trapped the electrons transferred to Cu2O in the cubic crystals in the reaction, thereby improving the degradation efficiency. The corresponding energy dispersive spectrometer (EDS) spectrum (Fig.\u00a01(c)) confirmed the presence of Cu, Fe, and O. Additionally, the characterization analysis using XRF (Table A.1) showed that the contents of Fe2O3 and Cu2O agreed with those demonstrated by EDS, indicating the successful preparation of the composites.\nFig.\u00a02\n shows the XRD characteristics of different materials. The diffraction peaks (Srivastava and Ingole, 2020) of \u03b1-Fe2O3 appeared at diffraction angles (2\u03b8) of 24.20\u00b0, 33.14\u00b0, 35.67\u00b0, 40.82\u00b0 49.43\u00b0, 54.11\u00b0, 62.45\u00b0, and 64.02\u00b0, corresponding to Fe2O3 crystals of (012), (104), (110), (113), (024), (116), (214), and (300), and the main diffraction peak of Fe2O3 crystals appeared at 2\u03b8\u00a0=\u00a035.67\u00b0 (Sheikholeslami et\u00a0al., 2020). Cu2O diffraction peaks (Sarto et\u00a0al., 2019) appeared at 2\u03b8\u00a0=\u00a029.54\u00b0, 36.50\u00b0, 42.24\u00b0, 51.98\u00b0, 61.34\u00b0, 73.61\u00b0, and 77.29\u00b0, corresponding to Cu2O cubic crystals of (110), (111), (200), (211), (220), (311), and (222) (Shi et\u00a0al., 2019), and the main diffraction peak of Cu2O cubic crystals was at 2\u03b8\u00a0=\u00a036.50\u00b0 (Gaim et\u00a0al., 2019). Clearly, the characteristic diffraction peaks of the composites covered all characteristic peaks of \u03b1-Fe2O3 and Cu2O, further confirming that Fe2O3 and Cu2O coexisted.The chemical states of the elements in the FC composites were analyzed using XPS. The XPS spectrum (Fig.\u00a03\n(a)) demonstrated the presence of O, Fe, and Cu elements in FC. These results were consistent with the EDS image (Fig.\u00a01(c)). In addition, the fitted O 1s spectrum (Fig.\u00a03(b)) contained four peaks at 531.9\u00a0eV, 530.6\u00a0eV, 530.1\u00a0eV, and 529.7\u00a0eV. 530.6\u00a0eV and 530.1\u00a0eV were mainly produced by the oxides of Fe (Bullen et\u00a0al., 2020) and Cu (Liu et\u00a0al., 2020), while the peaks at 529.7\u00a0eV and 531.9\u00a0eV were primarily attributed to lattice oxygen and surface hydroxyl groups. As shown in Fig.\u00a03(c), the peaks of Fe 2p3/2 and Fe 2p1/2 were at 710.6\u00a0eV and 724.2\u00a0eV, respectively (Bagus et\u00a0al., 2020), and the appearance of their satellite peaks at 718.4\u00a0eV indicated the presence of Fe3+ in the prepared samples (Imrich et\u00a0al., 2021). Additionally, the detected peaks of Cu 2p3/2 and Cu 2p1/2 of Cu(I) were located at 932.3\u00a0eV and 952.3\u00a0eV, respectively (Fig.\u00a03(d)) (Joseph and Sugunan, 2021), which were consistent with the peaks of Cu2O in Zhang et\u00a0al. (2020b).The optoelectronic properties of the catalysts were further investigated in order to judge the suitability of FC for photocatalysis. The optical properties of FC were investigated using UV\u2013Vis spectroscopy (Fig.\u00a04\n(a)). Pure Fe2O3 and Cu2O presented absorption fringes at 587\u00a0nm and 603\u00a0nm, corresponding to the band gap energies of 2.09\u00a0eV and 1.86\u00a0eV, respectively (Fig.\u00a0A2). Fe2O3 had the same bad gap energy (2.09\u00a0eV) as that reported by Khasawneh et\u00a0al. (2019), while Cu2O had a lower band gap energy than that (1.90\u00a0eV) reported by Li et\u00a0al. (2020b). The decrease in Cu2O band gap energy was due to the appearance of d-orbit bands in the band gap. FC with a band gap of 1.96\u00a0eV not only improved PEC but also optimized the disadvantage of a narrow band gap prone to electron-hole complexation.\nFig.\u00a04(b) shows the photoluminescence (PL) spectral analysis of Fe2O3, Cu2O, and FC, which was used to determine formation, transfer, and complexation of photoexcited electrons and holes. At 472\u00a0nm, a strong emission peak was observed when excited by a 550-nm laser. The peak intensity of pure Fe2O3 was nearly twice that of FC, showing that electron-hole complexation occurred much more slowly in pure Fe2O3 than in pure Cu2O. The reduction in PL intensity of FC was due to the successful loading of Fe2O3 onto Cu2O where the built-in internal electric field at both loads provided an efficient path for charge carriers.The electrical properties were characterized by cyclic voltammetry (CV) curves. Prior to the test, the CV curves of the materials were determined using a Na2SO4 (0.2\u00a0mol/L) solution as the electrolyte. The CV curves of different materials (Fig.\u00a04(c)) showed that the oxidation and reduction peaks of the pure materials were significantly lower than those of FC at \u22120.187\u00a0V and 0.253\u00a0V. This result shows that FC had a stronger redox activity due to the superposition of the valence changes of Fe3+/Fe2+ as well as Cu2+/Cu+ (He et\u00a0al., 2017), and the two electron pairs interacted with each other to make the degradation of BTA more efficient.As shown in Fig.\u00a04(d), there was no significant change in the photocurrent generated by all composites in the absence of light, and all photocurrents converged to zero. This indicated that photogenerated holes and electrons were not generated by the composites in the absence of light. The photocurrent generated by FC was much higher than that generated by the pure material. This confirmed that the separation of photogenerated electrons and holes was promoted by light exposure, which agreed with the PL analysis.The degradation of BTA in different processes was investigated. As shown in Fig.\u00a05\n(a), only 2% of FC was adsorbed after 90\u00a0min, meaning that FC had no adsorption effect on BTA. Under electrocatalysis with a current density of 20\u00a0mA/cm2, the removal efficiency of BTA was low, and 18.08% of BTA was removed in 90\u00a0min. This indicated that the degradation of BTA with electrocatalysis was not effective, a result that agreed with the findings of Li et\u00a0al. (2021). The UV-catalyzed removal rate could only reach 26.25% for the same time period. This was mainly attributed to the fact that longer light exposure darkened the solution and reduced its light transmission, thereby preventing the degradation of BTA. The removal rate of BTA with PEC was significantly higher than under photocatalytic and electrocatalytic reactions. This was mainly because PEC generated superoxide radicals (\n\u00b7\n\nO\n2\n\u2212\n\n) or oxidized radicals (\u00b7OH) and reduced the compound rate of photogenerated electron-hole pairs. This allowed these electron-hole pairs to release heat or migrate to the electrode plate, thereby reacting with nearby BTA. Therefore, PEC was suitable for the degradation reaction of BTA.\nFig.\u00a05(b) shows the effect of different materials on the degradation of BTA. The removal rate of BTA using FC under PEC treatment was 90.78%, significantly higher than with Fe2O3 and Cu2O (77.17% and 55.87%, respectively). This was due to the successful loading of Fe2O3 on Cu2O to reduce the electron-hole complexation rate.\nFig.\u00a05(c) shows the effect of the FC compounding ratio on the degradation of BTA. The degradation rate of BTA decreased significantly as the percentage of Cu2O increased. This was because the cubic crystal structure of Cu2O had a small contact area with BTA. The small circular shape of Fe2O3 compensated for this shortcoming of Cu2O with a large contact area with BTA, thereby significantly improving its catalytic efficiency through its internal electron transfer path. With an Fe:Cu molar ratio of 2:1, the highest BTA degradation rate reached 90.87%. However, when the percentage of Fe2O3 continued to increase, FC entered the solution and reduced the transparency of the solution. This hindered the photocatalysis and thus reduced the removal rate. Therefore, an Fe:Cu ratio of 2:1 (FC2) was considered the most suitable for subsequent experiments.\nFig.\u00a05(d) displays the effect of FC2 dosage on the degradation of BTA. When the dosage of FC2 was increased from 0 to 0.05\u00a0g/L, the 90-min removal rate of BTA increased from 54.94% to 94.87%. This was because the increase in the dosage enhanced the active site of FC, thereby augmenting the degradation rate of BTA. However, the removal efficiency decreased significantly with the further increase in FC2 dosage owing to the decreased solution transparency under high FC2 dosages. Therefore, 0.05\u00a0g/L was selected as the optimal dosage for subsequent experiments. The kinetic fitting curves (Fig.\u00a06\n(a)) showed that the reaction process was in accordance with the primary reaction kinetics, and the slope of the kinetic curve with a dosage of 0.05\u00a0g/L was higher than those with other dosages, demonstrating that FC2 showed an efficient PEC activity with a dosage of 0.05\u00a0g/L.\nFig.\u00a05(e) shows the effect of solution pH on the PEC degradation of BTA. The solution pH was adjusted using 0.1\u00a0mol/L of H2SO4 and NaOH solution. At a pH value of 3.06, BTA was completely degraded in 60\u00a0min. With the increase in pH, the removal efficiency decreased. This was due to the fact that BTA under acidic conditions is in a free state and can be easily degraded. In contrast, BTA under alkaline conditions has a molecular structure. As a result, the initial solution pH was adjusted to be 3.06 for further experiments. The kinetic fitting curves (Fig.\u00a06(b)) showed that the reaction process accorded with the primary reaction kinetics, and the slope of the curve with a pH value of 3.06 was higher than those with other pH values. Therefore, FC2 demonstrated an efficient PEC activity at a pH value of 3.06.\nFig.\u00a05(f) shows the effect of current density on the degradation of BTA. When the current density was increased from 5\u00a0mA/cm2 to 25\u00a0mA/cm2, the removal efficiency of BTA increased because more photoelectrons were generated for the oxidative degradation of BTA, and strong oxidative radicals were generated to catalyze the degradation of BTA. When the current density was further increased, the degradation rate of BTA tended to be stable and did not increase. The kinetic fitting curves (Fig.\u00a06(c)) showed that the reaction process fitted the primary reaction kinetics, and that the slopes of the curves with current densities greater than 20\u00a0mA/cm2 were higher than those with other current densities. This demonstrated that an efficient PEC activity was achieved using FC2 with a current density greater than 20\u00a0mA/cm2. Based on green energy-saving principles, 20\u00a0mA/cm2 was chosen as the optimal current density for subsequent experiments.In practical applications, the most important properties of PEC materials are their activity levels and their stability for long-term use. Stability experiments were performed by extracting, washing, and drying the recovered material. As shown in Fig.\u00a07\n, the removal rate of BTA using FC was maintained above 80.0% even after five repeated uses. This indicated that the FC catalyst was stable and retained a high level of PEC activity after repeated use. FC has significant potential for application in removal of organic wastewater with PEC.The COD removal rate can indirectly show the mineralization of organic pollutants, some of which are degraded to produce organic intermediates (Akintayo et\u00a0al., 2021). Therefore, analysis of COD aids in understanding the degradation of organic pollutants (Can-G\u00fcven, 2021). Fig.\u00a08\n shows the removal efficiency of COD. The removal efficiency of COD was lower than that of BTA. This implied that intermediates formed during the BTA degradation. After 90\u00a0min of treatment, the removal rate of COD reached 96.61%. In contrast, the removal rate of BTA was already 100% at 60\u00a0min. This indicated that almost all degraded BTA and its intermediates were mineralized. Compared with photocatalysis, electrocatalysis, and PEC alone, PEC with FC as a catalyst was more efficient in degrading BTA and removing COD.\n\n\u00b7\n\nO\n2\n\u2212\n\n, h+, and \u00b7OH are the major active substances in the degradation of organic pollutants using PEC (Jiang et\u00a0al., 2020). To understand the PEC mechanism of FC composite, the \n\u00b7\n\nO\n2\n\u2212\n\n trapping agent p-benzoquinone, the photogenerated h+ trapping agent ammonium oxalate, and the \u00b7OH trapping agent isopropanol were used to investigate the major active substances in the degradation of BTA using the PEC system (He et\u00a0al., 2020). As shown in Fig.\u00a09\n(a), each trapping agent reduced the PEC removal efficiency of BTA to some degree. The addition of ammonium oxalate had a significant inhibitory effect on the removal of BTA, and the addition of isopropanol also had a certain inhibitory effect on the performance of PEC. Their 60-min removal rates were 47.2% and 55.1%, respectively. The experiment with p-benzoquinone had the least effect on PEC, with a removal rate of 71.8% after 60\u00a0min. As a result, h+, \n\u00b7\n\nO\n2\n\u2212\n\n, and \u00b7OH were all involved in the process of BTA removal using PEC, and the degree of involvement of the three free radicals in the oxidation of BTA was as follows: h+\u00a0>\u00a0\u00b7OH > \n\u00b7\n\nO\n2\n\u2212\n\n. As a result, h+ in the composite PEC was mostly concentrated on the VB end of Fe2O3 rather than on the VB end of Cu2O (Liang et\u00a0al., 2020) because the VB potential of Fe2O3 was higher than the VB potential of Cu2O (Fig.\u00a010\n) (He et\u00a0al., 2014).N element in BTA is presumably oxidized to NH3 and released into the atmosphere. Fig.\u00a09(b) shows the generation of NH3-N during the degradation process. Clearly, the concentration of NH3-N increased as the concentration of BTA decreased. At 60\u00a0min, the concentration of NH3-N reached a maximum of 3.05\u00a0mg/L. At the same time, BTA was completely degraded. During the degradation of BTA, N element was finally released into the air in the form of NH3.To understand the photocatalytic degradation mechanism, UV full spectrum scanning and HPLC analysis were performed on the BTA solution. As shown in Fig.\u00a09(c), the peak of the intermediate products appeared around 400\u00a0nm, which was determined to be the peak of N=N. This indicated that the intermediate product contained diazotrophic substances, which also contributed to the darkening with the degradation solution.As shown in Fig.\u00a09(d), in addition to the major BTA peak, other small peaks appeared at 2.7\u00a0min and 3.8\u00a0min on the curves for 10\u00a0min, 20\u00a0min, 30\u00a0min, and 60\u00a0min but not on the curves for 0\u00a0min and 90\u00a0min. The peaks at 2.7\u00a0min were much larger than those at 3.8\u00a0min. This indicated that both peaks represented the intermediate products of BTA degradation. These intermediate products might be the substances that were not dehydrogenated after the opening of the ring. In addition, after 60\u00a0min of BTA degradation, the intermediate products were completely degraded during the remaining 30\u00a0min, with support from the change in the COD degradation rate.In order to investigate the possible degradation pathway of BTA, liquid mass spectra were measured in PEC for 40\u00a0min to observe the intermediates. As shown in Table A.2, diazo intermediates were produced during the degradation process, which was consistent with the UV full spectrum scan results showing intermediates after de-aminoamination. From this observation, the possible degradation pathway of BTA can be conceptualized as shown in Fig.\u00a010. BTA exists in ionic form under acidic conditions, providing suitable conditions for PEC. PEC generates strong oxidation radicals for the oxidative decomposition of BTA. The radicals break the N\u2013N bond, and an open loop forms for easy degradation. The oxidation of BTA by anode and materials occurs simultaneously during the degradation process. Thus, the reaction catalytic time is greatly reduced, resulting in cost savings and improving the degradation efficiency. Finally, BTA is oxidized to nontoxic H2O, NH3, and CO2. All these substances are discharged into the environment without producing secondary pollutants. Therefore, this FC-based PEC system provides a harmless and effective method for the degradation of BTA.In this study, composite FC was successfully prepared and used as a photocatalyst for the photocatalytic degradation of BTA. The results showed that, under optimal conditions, this photocatalytic treatment of 20-mg/L BTA for 60\u00a0min increased the BTA removal efficiency to 100%, and reduced the COD concentration of BTA by 96.61% after 90\u00a0min of treatment. After FC was added to the PEC system, it suppressed the compounding rate of photogenerated electrons and holes and improved the PEC degradation rate and mineralization rate of BTA, reflecting the strong photoelectric properties of FC. As a result, FC can be used as an effective catalyst for PEC. FC-based PEC provides an effective and promising method for degrading new organic pollutants.The authors declare no conflicts of interest.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.wse.2022.06.003.", "descript": "\n                  Given the difficulties of degrading benzotriazole (BTA), this study used a one-pot hydrothermal method to prepare \u03b1-Fe2O3/Cu2O (FC) composites for photoelectrocatalytic (PEC) degradation of BTA. The characterization of FC structure showed that Cu2O in cubic crystals was loaded with circular sheets of Fe2O3. Owing to this structure, FC showed efficient PEC degradation of BTA when exposed to ultraviolet light. The experimental results demonstrated that FC efficiently degraded BTA. When the PEC degradation continued for 60\u00a0min, 100% degradation of BTA was achieved because FC enhanced the photoelectron-hole separation and the separation and transfer of articulated carriers. High performance liquid chromatography\u2013mass spectrometry showed that intermediates formed during the PEC degradation of BTA. Finally, various pathways for degradation of BTA were postulated. This FC-based PEC system provides a harmless and effective method for degradation of BTA.\n               "}
{"full_text": "Data will be made available on request.Solid carbon is imperative for numerous applications, and its diverse properties stem from the many allotropes, and various compounds and composites that may be formed from carbon precursors, e.g. diamond, graphite, carbon fibers, cokes, carbon black, activated carbon, and carbon aerogels [1]. Some of these may in turn be surface-modified with functional groups to provide new properties [2,3]. The natural abundance of carbon presents a vast array of precursors for production of both mundane and advanced materials, in which heat treatment is often a key step. Pyrolysis, a term typically used when organic materials are involved, is a thermal treatment in an inert atmosphere, to prevent combustion and favor decomposition of the carbon precursor [4]. For advanced materials, the interest lies in the formation of solid carbon products, termed char. The pyrolysis conditions, such as temperature, heating rate, and dwell duration, are key variables for controlling the amount of char relative to gases and liquids, i.e., degree of carbonization. Char formation and degree of graphitization are also highly dependent on the carbon precursor and how it is treated, e.g., molecular structure, porosity, and the presence of impurities like metals [5].Porous carbons are utilized in several material applications, such as catalysis and energy storage, where transition metal nanoparticles are often confined within the carbon structure or pores [6]. Such materials have commonly been produced by impregnation or deposition-precipitation of pre-synthesized porous carbon structures, e.g. activated carbons, carbon nanofibers, and carbon nanotubes. Both methods are simple to execute, and impregnation grants control over metal loading but may result in poor metal dispersion at high metal loadings and pore blocking, which are often undesirable features [7]. A different approach is pyrolysis of carbon precursors with metal ions bound to their structure. A straightforward tactic is to use a polymer with appropriate functional groups that can bind metal ions, or advanced precursors such as metal-organic frameworks (MOFs), followed by pyrolysis [8,9].A polymer with great potential for this application is alginate \u2013 a biopolymer extracted from brown seaweed. Alginate is comprised of two monomers: (1\u20134)-linked \u03b2-D-mannuronate (M), and its C-5 epimer, \u03b1-L-guluronate (G) (\nFig. 1a). The spatial orientation and relative content of the two monomers will affect the interbonding and intrabonding properties in the presence of multivalent cations. The GG-diads have historically been of importance for cross-linking alginate chains, proposed to form so-called \u201cegg-box structures\u201d (Fig. 1b) [10]. This enables alginate to serve as a scaffold to obtain atomic dispersion of metal cations between the macromolecular chains.Studies of heating of metal alginates have focused on their flame-retardant properties [11\u201315] and the decomposition of alginate [16\u201318], typically only investigating the alginate or carbon by in situ characterization techniques such as thermogravimetric analysis (TGA) and Fourier-Transformed infrared spectroscopy (FTIR). The resulting materials after pyrolysis of metal alginates have been utilized for sorption of heavy metal ions and dyes from aqueous solutions [19\u201321], as anode materials [22,23], supercapacitors [24], electrocatalysts for oxygen reduction reaction (ORR) [25], and as catalysts for CO2 hydrogenation [26] and NOx abatement [27]. Specifications related to the M/G-content and monomer distribution in the alginate are rarely reported, and only alginate solutions with low concentrations have been utilized, less than 5\u2009wt%. The pyrolysis temperature was typically in the range of 800\u2013900\u2009\u00b0C, resulting in well-carbonized carbon materials, but with extensive sintering of the metal particles, as there were no reports of particles below 10\u2009nm. The metal species were usually only characterized after pyrolysis with ex situ techniques such as X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM). The transition from metal-alginate to the final product remains unresolved but understanding the evolution of both carbon structure and metal species during pyrolysis is vital for tailoring material properties.In our previous work, the green and Na-alginate was ion-exchanged with different transition metal ions, followed by pyrolysis at 500\u2009\u00b0C to produce [28]. In particular, pyrolysis of Fe-alginate yielded desirable material properties for heterogeneous catalysis. Due to the promising nature of this material, herein, the evolution of Fe and C species during pyrolysis has been corroborated by both ex situ characterization and advanced in situ techniques such as XAS and M\u00f6ssbauer spectroscopy. This was mainly performed to understand how the pyrolysis conditions affect the material properties, but catalysts were also prepared by performing pyrolysis in a range of suitable temperatures to see how these material properties affect their performance in high-temperature Fischer-Tropsch synthesis (FTS). In FTS, synthesis gas (syngas, CO and H2) from biomass may react through a polymerization reaction to form green fuels and chemicals (hydrocarbons of varying lengths), and H2O as a by-product. Additionally, Fe catalysts exhibit water-gas shift activity, a reaction where H2O and CO form H2 and CO2. This enables Fe catalysts to use a syngas feed with low H2/CO ratios, which is typically the result when syngas is produced from carbon-rich feedstocks such as biomass.Sodium alginate (Protanal LFR 5/60: G monad frequency (FG)\u2009=\u20090.65\u20130.70, G diad frequency (FGG)\u2009=\u20090.5\u20130.6, average length of G-blocks (NG>1)\u2009=\u200911\u201320) was supplied by Dupont Nutrition Norge AS). Iron(III) nitrate nonahydrate (>\u200998\u2009%) was supplied by Sigma-Aldrich, with impurities such as Cl- (<\u20095\u2009ppm), SO4\n2- (<\u20090.01\u2009%), Ca2+ (<\u20090.01\u2009%), Mg2+ (<\u20090.005\u2009%), K+(<\u20090.005\u2009%), Na+ (<\u20090.05\u2009%). Ethanol (96\u2009%) was supplied by VWR. Deionized water was produced by using a Milli-Q water purification system.Na-alginate was dissolved in deionized water by a magnetic stirrer to form a 20 w/w% alginate/water solution. A solution of 0.1\u2009M Fe(NO3)3-solution was prepared, with five times greater volume than that of the alginate. The Na-alginate solution was dripped into the Fe(NO3)3-solution, which formed alginate beads on contact, and was kept in solution for 24\u2009h. The alginate beads were then washed by placing them in 200\u2009mL deionized water for 5\u2009min, discarding and replacing the water, repeated three times before they were immersed in ethanol-water solutions of increasing concentration over time to gradually transform the beads from hydrogels to alcogels. The initial ethanol concentration was 10\u2009%, which was discarded and increased by 20\u2009% every 10th\u2009min up to 90\u2009%, before finally leaving the beads in a 96\u2009% ethanol solution for 24\u2009h. The beads were collected from the ethanol solution and dried at 80\u2009\u00b0C overnight, followed by mortaring. Approximately 1\u2009g of the dried powder was placed in a calcination reactor that allows gas to pass through the sample. Pyrolysis was performed by using a heating rate of 2\u00b0\u2009min\u22121 in 100\u2009mL\u2009min\u22121 N2 to the desired temperature, and dwelled at this temperature for a given time, as listed in \nTable 1. The samples were passivated in 1\u2009% O2 in Ar for 2\u2009h at room temperature after pyrolysis, which is important due to the samples\u2019 pyrophoric nature.Powder XRD was recorded at ambient temperature with a Bruker D8 A25 DaVinci X-ray Diffractometer using a Cu K\u03b1-radiation (\u03bb\u2009=\u20090.15432\u2009nm) X-ray tube and LynxEye\u2122 SuperSpeed detector. The samples were scanned in the range 2\u03b8\u2009=\u200910\u201380\u00b0 for 60\u2009min, using a 0.2\u00b0 divergence slit. The powder diffraction files (PDF) used as standards were \u03b1-Fe (7-9753), FeO (PDF 6-615), \u03b3-Fe3O4 (9-2285), \u03b3-Fe2O3 (21-3968), \u03c7-Fe5C2 (36-1248), \u03b8-Fe3C (35-0772).N2 adsorption-desorption experiments were performed with a Micromeritics Tristar II 3000. Approximately 100\u2009mg sample was degassed and evacuated for 24\u2009h, at 353\u2009K for the dried samples, and 473\u2009K for the pyrolyzed samples. To determine the surface area, pore-volume, and pore diameters of the samples, the Brunauer-Emmett-Teller (BET) isotherm and Barrett-Joyner-Halenda (BJH) method (desorption) were used.Thermogravimetric analysis (TGA) with differential scanning calorimetry (DSC) was performed with a Netzsch Jupiter 449 unit. The analysis was performed in pure argon (100\u2009mL\u2009min\u22121) or air (75\u2009mL\u2009min\u22121), with a ramp rate of 10\u2009\u00b0C\u2009min\u22121\n, heating from RT to 900\u2009\u00b0C. A QMS 403\u2009C A\u00ebolos quadrupole mass spectrometer (MS) was used to analyze the effluent gases.The elemental compositions of the iron alginate samples were measured by inductively coupled plasma mass spectrometry (ICP-MS). Between 10 and 30\u2009mg of dried samples were mixed with 2\u2009mL concentrated nitric acid (HNO3) in perfluoroalkoxyalkane (PFA) vials. Further, the samples were digested in an UltraClave, heated to 245\u2009\u00b0C, and pressurized to 50\u2009bar. The resulting solutions were diluted to a total volume of 216.6\u2009mL, then 16\u2009mL of this solution was sent for analysis along. Three blank samples were used to correct the results. The elemental analysis was performed with a High Resolution Inductively Coupled Plasma ELEMENT 2 connected to a mass spectrometer.The pyrolyzed samples were imaged by high-resolution (HR)-TEM. Experiments were performed with a JEOL JEM-2100 (LaB6-filament, side-mounted Gatan 2k Orius CCD) and a JEOL JEM-2100F (200k Schottky field emission gun (0.7\u2009eV energy spread) and bottom-mounted Gatan 2k Ultrascan CCD) both with Oxford X-Max 80 SDD energy-dispersive X-ray (EDX) (solid angle 0.24 sr) and scanning option with bright-field (BF) and high-angle annular dark-field (HAADF) detector. For sample preparation, the samples were suspended in isopropanol, then deposited on a Cu-grid with lacey carbon.The D- and G-bands of the carbon in the pyrolyzed samples were analyzed with Raman spectroscopy. Experiments were performed with a Horiba ASD with a laser wavelength of 633\u2009nm, employing a 600\u2009g\u2009mm\u22121 grating, \u00d7\u200950 LWD objective, 15 acquisition, 3 accumulations, 25\u2009% filter, and hole of 200. The powdered samples were placed on glass slides for analysis.Transmission 57Fe M\u00f6ssbauer spectra were collected at different temperatures with conventional constant-acceleration or sinusoidal velocity spectrometers using a 57Co(Rh) source. Velocity calibration was carried out using an \u03b1-Fe foil at room temperature. The source and the absorbing samples were kept at the same temperature during the measurements. The M\u00f6ssbauer spectra were fitted using the Mosswinn 4.0 program [29]. The experiments were performed in a state-of-the-art high-pressure M\u00f6ssbauer in-situ cell \u2013 recently developed at Reactor Institute Delft [30]. The high-pressure beryllium windows used in this cell contain 0.08\u2009% Fe impurity whose spectral contribution was fitted and removed from the final spectra. The experiment at 700\u2009\u00b0C was performed in a standard tubular reactor and the pyrolyzed sample was measured quasi in-situ (via glovebox transfer).In situ XRD and X-ray absorption spectroscopy (XAS) was performed at the Swiss-Norwegian beamlines (BM31, European Synchrotron Radiation Facility (ESRF), France). The Fe-alginate sample was placed between two quartz wool plugs in a quartz capillary reactor (o.d. 1\u2009mm), resulting in a bed length of 10\u2009mm. The capillary was placed in an in situ cell, described elsewhere [31]. During the pyrolysis experiments, the capillary was purged with 10\u2009N\u2009mL\u2009min\u22121 He, heating from RT (20\u2009\u00b0C) to 700\u2009\u00b0C at a rate of 2\u2009\u00b0C\u2009min\u22121 at atmospheric pressure. The effluent gases that evolved were analyzed with an online MS. X-ray diffraction data were collected with a 2D plate detector (Mar-345) using monochromatic X-rays with a wavelength of 0.4975\u2009\u00c5. A lanthanum hexaboride (LaB6) standard was used as a calibration reference. X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) were measured at the Fe K-edge, ranging from 7.05 to 8.2\u2009keV, in transmission mode. The XAS data were analyzed using DLV Excurve and Feffit. A linear pre-edge was subtracted, and the data were normalized by the edge-jump. The background was subtracted to yield the data in \u03c7(k), which was k2-weighted before applying the Fourier-transform. EXAFS data were generally fitted in a k-range of 3.5\u20139.5\u2009\u00c5\u22121, due to the deterioration of the signal at high temperatures, and an R-range of 1.0\u20133.8\u2009\u00c5\u22121. For the amplitude reduction factor (S0), metallic Fe was extracted from the metallic Fe foil measurement, and the Fe-alginate (RT) was extracted from Fe(NO3)3. Coordination number (N), energy shift (\u0394E0), scattering distance (R) and Debye-Waller factor (2\u03c32) were refined for each scattering path.The absorbances from the XANES measurements were placed in a matrix A with m rows (energy) and n columns (sample). The variance in the dataset was ranked by using singular-value decomposition (SVD), by the following equation:\n\n\n\nA\n=\nU\n\u00d7\nS\n+\nV\n\n\n\nwhere U (m x m) is the left singular matrix, S is the singular-value matrix and V\n\nT\n is the right singular matrix. The S matrix gives us the variance of each point in the dataset, for the initial evaluation of the number of pure components. Simple-to-use interactive self-modeling analysis (SIMPLISMA) was used to provide initial estimates for the pure components in the data set [32]. These estimates were then analyzed by multivariate curve resolution alternating least-square regression (MCR-ALS), to approximate the composition of components in a dataset with mixtures of components, using the equation [33]:\n\n\n\nA\n=\nC\n\u00d7\n\n\nS\n\nT\n\n+\n\u03b5\n\n\n\nwhere C is the concentration of the relative concentration of the initial components contained in S, and \u03b5 is the error. The analysis assumes that the pure components have the lowest noise relative to the other spectra in the data. Constraints of non-negativity and normalization were applied, to obtain concentrations summing up to 100\u2009%.Diffuse reflectance infrared Fourier Transform spectroscopy (DRIFTS) measurements of all samples were conducted in a Thermo Scientific Nicolet iS50 FT-IR Spectrometer with a Harrick Praying Mantis\u2122 high temperature in situ cell, flushed with 30\u2009mL\u2009min\u22121 Ar. A spectral range of 4000\u2013600\u2009cm\u22121 was used, with a resolution of 4\u2009cm\u22121 and 32 scans being averaged for each spectrum. The samples were heated from RT to 500\u2009\u00b0C at a rate of 5\u2009\u00b0C\u2009min\u22121, halting the heating at every 50\u2009\u00b0C to perform measurements.The FTS experiments were performed in a 10\u2009mm i.d. tubular stainless-steel fixed bed reactor at 340\u2009\u00b0C, 20\u2009bar, and H2/CO\u2009=\u20091.0. The catalysts (0.10\u2009g, 90\u2013250\u2009\u00b5m sieve fraction) were diluted and mixed with inert SiC (10\u2009g, 90\u2013250\u2009\u00b5m sieve fraction) to minimize temperature gradients. To keep the catalyst bed fixed, the mixture was loaded into the reactor between two plugs of quartz wool. The reactor was mounted between two aluminum blocks in an electrical furnace to further improve heat distribution. The catalysts were reduced in H2 (100\u2009N\u2009mL\u2009min\u22121) at 3 bars with a heating rate of 2\u00b0\u2009min\u22121 to 400\u2009\u00b0C, with a dwell time of 3\u2009h. Then the reactor was cooled to 330\u2009\u00b0C and pressurized to 20\u2009bar with 56,100\u2009N\u2009mL\u2009g\u22121 h\u22121 H2. Syngas (48.5\u2009% CO, 48.5 H2, 3\u2009% N2) were then introduced in steps, replacing 25\u2009% of the H2 flow every 5th minute, keeping the gas-velocity constant. The product stream was passed through a hot trap (90\u2009\u00b0C) and a cold trap (25\u2009\u00b0C) to collect condensable FT products, i.e., wax, light hydrocarbons, water, and oxygenates. The gas-phase products were analyzed with an Agilent Technologies 6890N gas chromatograph (GC) equipped with a stainless steel Carbosieve S-II and an HP-plot Al2O3 column with a thermal conductivity detector (TCD) and flame-ionization detector (FID). The 3\u2009vol% N2 in the syngas mixture was used as an internal standard for the GC.The molar flows of H2O and H2 were estimated by using oxygen and hydrogen mass balances, where oxygenate formation was disregarded. The WGS equilibrium constant (Keq) was calculated by Keq =\u200910((\u22122.4198)+(0.0003855*(T)+(2180.6/T))), and was compared to the WGS quotient (QWGS) based on the fraction of products and reactants.The results are divided into two sections. First, ex situ characterization after pyrolysis of Fe-alginate at 400, 500, 600, and 700\u2009\u00b0C will be regarded. These materials were also tested for FTS, to understand how the resulting material characteristics affect the catalytic performance. Second, in situ pyrolysis experiments were investigated to elucidate the evolution of both iron and carbon species.One batch of Fe-alginate was synthesized and split into four portions that were subjected to pyrolysis at different temperatures. The samples pyrolyzed at 400, 500, and 600\u2009\u00b0C used a dwell duration of 8\u2009h, whereas 1\u2009h was selected for the 700\u2009\u00b0C treatment to limit the extent of sintering.The porosity inherent in Fe-alginate can be attributed to the interconnection of alginate macromolecules, which is facilitated by ion-exchanged Fe3+. The measured BET (Brauner-Emmett-Teller) specific surface area of Fe-alginate was 178\u2009m2 g\u22121, with pores primarily in the mesopore size range. After pyrolysis, a significant increase in BET surface area was observed for every step increase in temperature (Table 1, Fig. S1). In all cases, the pyrolysis treatment yielded pore sizes larger than for the Fe-alginate, and the total pore volume was also observed to increase with increasing temperature (Fig. S2). The observed changes in porosity can be attributed to both advancement of polymer decomposition and the development of by-products, which can lead to both expansion and formation of new pores.The dried Fe-alginate sample contained 10.3\u2009wt% Fe, dictated by the amount of Fe3+ that can be crosslinked within the alginate gel. The decomposition of alginate constituents innately increased the Fe loading with increasing pyrolysis temperature. Relatively to Fe-alginate, the Fe loading doubled for P400, while for the other samples, roughly a threefold increase was achieved (Table 1). This indicates that a significant decomposition of alginate occurred above 400\u2009\u00b0C.The powder X-ray diffractograms of the pyrolyzed samples, cooled to ambient temperature and exposed to air, are presented in \nFig. 2. Broad and indistinct diffraction peaks were discerned for P400, which may originate from magnetite (\u03b3-Fe3O4) or maghemite (\u03b3-Fe2O3). These iron oxides are isostructural, the only difference is that magnetite has 2/3 Fe3+ and 1/3 Fe2+, whereas maghemite contains Fe3+ ions exclusively. This grants two highly similar diffraction patterns that are difficult to differentiate with the low-range ordering present in the present samples. Similar diffraction patterns were also present in P500 and P600, more defined yet still broad, indicative of increasing ordering due to particle growth. With increasing pyrolysis temperature, diffractions corresponding to ferrite (\u03b1-Fe) emerged, first for P500, then with an increase in intensity for P600, while the oxide diffraction diminished. If present, oxide phases were not discernible for P700, but the Fe species in the sample had reduced drastically and partially carburized into cementite. The most intense \u03b1-Fe diffraction peak, Fe(110), at 2\u03b8\u2009=\u200945\u00b0 overlaps with \u03b8-Fe3C(211) and (103), but the Fe(200) diffraction peak at 2\u03b8\u2009=\u200965\u00b0 confirmed the presence of \u03b1-Fe.Images captured with TEM (\nFig. 3) showed amorphous carbon structures containing densely packed spherical Fe particles. As the average Fe particle size for P400 and P500 was relatively similar (Table 1), the increase in pyrolysis temperature from 400 to 500\u2009\u00b0C did not lead to significant particle growth. The analysis of crystallinity and lattice fringes was not straightforward due to the size of the Fe particles. Larger particles were observed for P600, where some iron particles were completely reduced (Fe/FexCy), and some had a reduced core and a surrounding oxide shell, supported by an EDS line scan (Fig. S3). The EDS scan revealed a higher S density on the Fe particles, while Na was distributed more evenly in the sample, which is sensible, as S originated from the Fe precursor and the Na from the alginate. As the density difference of Fe atoms in iron oxides and metallic structure change the extent of electron transmission, darker areas indicate reduced Fe. Even larger particles and a broader particle size distribution were observed for P700 (Fig. S4). Similar Fe particle structures to P600 were seen for P700, but darker particles were also observed, indicative of more reduced Fe.The D-band (1350\u2009cm\u22121) and G-band (1580\u2009cm\u22121) region were measured with Raman spectroscopy, to investigate the bonding modes present in the carbon support. The relative intensity of the bands (ID/IG) is commonly used to quantify defects in graphitic and diamond-like structures, but the pyrolysis temperatures employed here are not likely sufficient to form graphite. Thus, the assignment of D- and G-band could be misleading. A study on pyrolysis of saccharose showed that the ID/IG ratio increased up to 2000\u2009\u00b0C [34]. The ID/IG ratio of the pyrolyzed samples (\nFig. 4) increased with increasing temperature, similar to the aforementioned study, and also a blue-shift of the D-band and a red-shift of the G-band. An increasing ID/IG ratio in this temperature regime has been associated with increasing size of the structural carbon units [35]. The carbon material appears to be amorphous, but the increased ordering of the carbon structure was observed between 400\u2013500\u2009\u00b0C, and 600\u2013700\u2009\u00b0C.The samples were reduced in H2 at 400\u2009\u00b0C for 3\u2009h before being tested as catalysts at high-temperature FTS conditions, where a high initial CO conversion level was observed for all samples (\nFig. 5a). Because identical sample amounts were used, the samples with higher Fe content achieved a higher conversion level. However, the activities in terms of iron-time yield (molCO gFe\n\u22121 h\u22121) showed an inversely proportional relation to the pyrolysis temperature (Fig. 5b), where P400, P500, and P600 obtained similar activity profiles and the deactivation mainly occurred during the first 40\u2009h on stream. Before reaction, P700 was the only sample containing \u03b8-Fe3C, and the induction period experienced by this catalyst may be related to a slower interconversion of \u03b8-Fe3C to \u03c7-Fe5C2. As both FTS and WGS are highly exothermic, the high activity of P400 resulted in higher temperature (Fig. S12), which in turn increases the activity further. The oven temperature was unchanged during the first 100\u2009h of stream due to the high heat of inertia in our system and to observe the deactivation profile. To obtain comparable data, the temperature was adjusted to 340\u2009\u00b0C after 100\u2009h, as reported in \nTable 2.Shorter hydrocarbons were produced by P400 and P500, compared to P600 and P700, but the C2-C4 olefins and paraffins selectivities were comparable for all samples. The changes in selectivity during reaction (Fig. S8), show similar results as discussed above, but P700 differs from the others by not stabilizing after 40\u2009h on stream. All samples obtained the same CO2 selectivity, which also implies that the WGS activity is comparable, as the amount of CO2 is directly related to the former.At no point did the catalysts reach WGS equilibrium (Fig. S10).For comparison, Table 2 contains FTS performance data for relevant carbon-supported catalysts from literature. The samples in the current work have fair activity compared to the referenced works, but they do not have particularly specific selectivity. Compared to the previous work on this type of catalyst (Fe/15C [28]), a higher activity and olefin selectivity was reported due to higher Na and S content from the catalyst synthesis. Sodium and sulfur were not added to any of these catalysts by intention, but a much higher olefin selectivity could be achieved by optimizing the promoter amount, akin to the iron catalyst support on graphene oxide and promoted with potassium (Fe/K1-rGO [36]). Optimization of the catalysts for FTS is beyond the scope of this work.The diffractograms of the spent P400, P500 and P600 were similar, containing broad diffractions of \u03c7-Fe5C2 and \u03b3-Fe2O3, while P700 obtained more defined \u03c7-Fe5C2 diffractions and a less prominent oxide phase (Fig. S7). At high-temperature FT conditions, \u03c7-Fe5C2 is considered the active phase and is also the thermodynamically favored iron carbide.The measured particle size from TEM images of the spent samples (Fig. S6) revealed a relatively similar number-average particle size (\u00b1\u200995\u2009% confidence interval), with sizes of 12.6\u2009\u00b1\u20090.3\u2009nm (P400), 15.2\u2009\u00b1\u20090.5\u2009nm (P500), 15.1\u2009\u00b1\u20090.3\u2009nm (P600) and 15.6\u2009\u00b1\u20090.4\u2009nm (P700). The exception was P400, which experienced less sintering than the other sample, likely due to lower metal loading. As the core is much darker than the surrounding shell, there is an indication that the spent particles had an \u03b1-Fe or \u03c7-Fe5C2 core and an outer layer comprised of iron oxide.M\u00f6ssbauer spectroscopy on spent samples that had been exposed to air revealed that the amount of \u03c7-Fe5C2 increased from 12\u2009% to 15\u2009% for P400, P500, and P600, up to 38\u2009% in P700, while all samples also contained 6\u20138\u2009% \u03f5\u2019-Fe2.2C (Table S3). Deducing the extent of carburization during reaction from the contents of samples exposed to air is questionable, however, it seems reasonable that P700 achieved a greater extent of carburization. Before reduction in H2 and introduction of syngas in FTS, P700contained partially reduced and carburized particles, as seen from XRD (Fig. 2) and M\u00f6ssbauer (Table S2), which when activated yielded more \u03c7-Fe5C2 than the other treatments after over 100\u2009h on stream. No apparent catalytic activity benefit was observed from the increased content of \u03c7-Fe5C2. The oxide contributions originated from \u03b3-Fe2O3, but with various particle sizes. For P700, only very small and superparamagnetic (SPM) \u03b3-Fe2O3 was observed, while for the other samples, intermediately sized \u03b3-Fe2O3 was observed in addition.An in situ M\u00f6ssbauer experiment of the P500 sample after reduction and 20\u2009h of FTS, showed that the catalyst consisted of 18\u2009% \u03c7-Fe5C2 and 82\u2009% Fe1\u2212xO. The fitting parameters show that a relatively crystalline \u03c7-Fe5C2 phase was formed, while the oxide phase was relatively disordered. After the experiments, the sample was exposed to air and measured again, where the sample contained only 8% \u03c7-Fe5C2 and the remainder Fe3+, due to oxidation (Table S2). The linewidth of \u03c7-Fe5C2 increased significantly upon oxidation after reaction, indicating that the crystalline ordering decreased.The mass-loss and the accompanying effluent gases from the pyrolysis of the Fe-alginate were investigated using TGA-DSC coupled with MS. The sample was heated from RT to 900\u2009\u00b0C in Ar at a rate of 10\u2009\u00b0C\u2009min\u22121 (\nFig. 6). The decomposition was divided into four different segments, where the first segment (I) involved dehydration (H2O; m/z\u2009=\u200918) only, with an endothermic DSC signal. This process peaked between 100 and 120\u2009\u00b0C, and the total dehydration yielded a mass-loss of 10\u2009%. The second segment (II) took place between 160 and 350\u2009\u00b0C, with the largest mass loss (50\u2009%) throughout the entire experiment, the main effluent gases being H2O and CO2 (m/z\u2009=\u200940). This was followed by the third segment (III) between 350 and 550\u2009\u00b0C, where a minor decomposition of 11\u2009% mass loss occurred, with more CO2 than H2O evolving, compared to the previous segment. In the final segment, another mass loss of 11\u2009% was observed, which was assigned to the reduction of the iron oxide, an endothermic process. The mass loss can be attributed to the carbon acting as a reductant, resulting in the release of both CO (m/z\u2009=\u200928) and CO2. A study of the pyrolysis of Fe2+-biopolymers (gelatin, chitosan, and alginate) demonstrated that a reduction of FeO to \u03b1-Fe or \u03b8-Fe3C was observed around 650\u2009\u00b0C [39].M\u00f6ssbauer measurements were conducted after Fe-alginate was subjected to different heat treatments in Ar (\nFig. 7, Table S1, Fig. S13) and subsequently cooled to 120\u2009K (300\u2009K for samples subjected to air after treatment). The untreated Fe-Alginate contained 90\u2009% Fe3+ and 10\u2009% Fe2+, which was almost unchanged after heating the sample to 100\u2009\u00b0C. At 200\u2009\u00b0C, Fe2+ was almost exclusively observed, which was also the case at 300\u2009\u00b0C, but small amounts of superparamagnetic (SPM) \u03b1-Fe could be fitted. For very small particle sizes \u03b1-Fe becomes SPM, when about half of their atoms are located at the surface, resulting in a nanoparticle that acts as a single magnetic domain. The quadrupole splitting (QS) of Fe2+ reduced as the temperature increased from 100\u2009\u00b0C to 200\u2009\u00b0C, implying higher charge symmetry at a higher temperature due to the removal of H2O ligands during drying.After treatment at 400\u2009\u00b0C and a dwell time of 8\u2009h (P400), the fraction of \u03b1-Fe (SPM) increased to 25\u2009%, while the remainder consisted of Fe2+. Two different Fe2+ species were observed, where one was fitted as Fe1\u2212xO (6\u2009%) as it has a hyperfine field and a QS close to zero. The remainder (69\u2009%) has a very different QS (1.42\u2009mm\u2009s\u22121) than the measurement at 300\u2009\u00b0C (2.54\u2009mm\u2009s\u22121), and due to having no hyperfine field, it was assigned to very small particles of Fe1\u2212xO that are superparamagnetic (Table S1), adding up to a total of 75\u2009% Fe1\u2212xO. Concerning this assignment of Fe1\u2212xO, this entails that the Fe2+ species with high QS observed at 200\u2009\u00b0C (2.72\u2009mm\u2009s\u22121) and 300\u2009\u00b0C are not Fe1\u2212xO, and the low charge symmetry implies that these are Fe2+ ions still bound to alginate.The heat treatment at 500\u2009\u00b0C with no dwell time yielded the same composition as for 400\u2009\u00b0C for 8\u2009h, but with species having smaller line widths, which could be due to higher crystallinity as an effect of higher temperature. However, dwelling at 500\u2009\u00b0C for 8\u2009h (P500) halved the number of Fe2+ (35%), and in addition to the SPM \u03b1-Fe (16\u2009%), there was also SPM FexC (25\u2009%) and \u03b1-Fe (24\u2009%). The formation of the latter indicates that an increase in temperature has led to some particle growth, as some \u03b1-Fe has lost its SPM properties. When this sample was cooled down and exposed to air, it was almost completely oxidized (96\u2009% Fe3+), containing small amounts of \u03b1-Fe (4\u2009%) (Table S2). A sample was also measured after treatment at 700\u2009\u00b0C for 1\u2009h (P700), which yielded large quantities of \u03b8-Fe3C (68\u2009%), some \u03b1-Fe (24\u2009%), and small amounts of SPM FexC (4\u2009%) \u2013 the sample is completely reduced. Upon exposure to air, the sample was partially oxidized to Fe3+ (43\u2009%), but the \u03b8-Fe3C (51\u2009%) appeared to be difficult to re-oxidize.The synchrotron experiment was performed by heating Fe-alginate from RT to 700\u2009\u00b0C, while alternately recording XRD and XAS (\nFig. 8, Fig. S16), with an experimental objective to elucidate the fate of the Fe species during pyrolysis. Initially, the dried Fe-alginate sample was XRD-amorphous \u2013 the broad diffraction peak around 21\u00b0 originating from the quartz capillary, but also overlaps with alginate (002) [40]. At 380\u2009\u00b0C, very low-intensity diffraction peaks corresponding to FeO appeared, which increased in intensity with increasing temperature, indicating growth of the FeO particles. At 634\u2009\u00b0C, FeO diffractions were promptly transformed into \u03b1-Fe and were accompanied by the release of CO \u2013 a stable signal until this point \u2013 and some CO2 (Fig S14). At the next measurement (660\u2009\u00b0C), sharp \u03b1-Fe diffractions were observed, indicating a rapid reduction and particle growth.The initial XANES measurements exhibited a high absorption edge threshold energy (E0, maximum of the derivative signal), indicative of Fe3+ \u2013 the same species introduced into the alginate matrix in the material synthesis. At 150\u2009\u00b0C, E0 was lowered, implying a reduction of Fe3+ to Fe2+, accompanied by a shift of the pre-edge to lower energy. Approaching 350\u2009\u00b0C, E0 did not change significantly, but the spectra developed more characteristic features. The pre-edge also gained a wide feature towards the main edge, which is characteristic of FeO but may also have a contribution from broad features of the \u03b1-Fe edge. The next significant change was observed at 630\u2009\u00b0C, where the spectra rapidly transformed to features that are distinct for \u03b1-Fe.A total of 189 XANES spectra were measured over the temperature range, and MCR in conjunction with SIMPLISMA and SVD was utilized to estimate compounds that are not among the measured standards, and their concentration. Many pure components may be obtained, but only those that can account for a significant degree of variance, have meaningful spectra, and are significantly different from the other components, were used. This evaluation resulted in four calculated components, which are shown in Fig. 8(b) along with the concentration of these components over the temperature range (Fig. 8(d)) The pure components were matched with measured standards, where component 1 had an edge position similar to \u03b1-Fe2O3 and Fe(NO3)3, however, the spectrum was relatively featureless compared to both of these, and was assigned as Fe3+ bound to alginate. Components 2 and 4 are somewhat similar in terms of edge position, but component 4 has features that are very close to FeO and were assigned accordingly. Component 2 contained less prominent features, similar to component 1, and had a more defined pre-edge that was shifted towards lower energies, and is therefore likely a Fe2+ species, as it does not resemble any spectrum among the measured standards. Component 3 has features matching with the Fe-foil and was assigned to \u03b1-Fe. Including more components than the four chosen here, yielded several variations of component 3 (\u03b1-Fe), where some may be linked to iron carbides. The concentration plot serves to give an estimate of the contributions of the selected components during the pyrolysis experiments. Some contributions, like \u03b1-Fe at the beginning and Fe3+ species between 400\u2009\u00b0C and 600\u2009\u00b0C, are not realistic but they provide the best fit given the limited set of components.An in situ DRIFTS experiment was performed to investigate the pyrolysis temperature\u2019s effect on the alginate structure. The measurement obtained at RT (\nFig. 9(a)), shows the bands present in Fe-alginate, which were; the broad band ranging from ~3000\u20133600\u2009cm\u22121 (O-H stretching mode, \u03bd(O-H)s); a weak signal located at 2938\u2009cm\u22121 (aliphatic C-H stretching mode, \u03bd(C-H)s); the intense peaks at 1629\u2009cm\u22121 (antisymmetric COO stretching mode, \u03bd(COO)asym) and 1429\u2009cm\u22121 (symmetric COO stretching mode \u03bd(COO)sym); the weak band at 1236\u2009cm\u22121 (C-C-H and O-C-H deformation, \u03b4(CCH) and \u03b4(OCH)); the intense bands between 1133 and 1109 (C-O stretching vibrations of the pyranose rings \u03bd(C-O)s); and the band at 1054 (C-O or C-C stretching, \u03bd(C-O)s and \u03bd(C-C)s). The band at 1749\u2009cm\u22121 was not observed for Na-alginate nor when gelated with divalent cations and does not appear to match with a carboxylic acid, and we have previously proposed that it might be related to an ester [41].The Fe-alginate was heated from RT to 500\u2009\u00b0C in He, measuring DRIFTS spectra every 50\u2009\u00b0C. The absorbance was calculated by using the single-beam data of the sample measured at RT \u00b0C as I0, to visualize the change towards 500\u2009\u00b0C (Fig. 9(b)), while the raw single-beam data can be found in Supporting information (Fig. S18). The reduction of the wide \u03bd(O-H)s band shows that water was gradually removed with increasing temperature, but the shoulder at 3480\u2009cm\u22121 persisted until 200\u2009\u00b0C. The position of this shoulder indicates intermolecularly bonded hydroxyl groups, perhaps linked to Fe-species, but it ultimately diminished when transitioning to 300\u2009\u00b0C. The absorbance in the O-H region dropped severely from 250 to 350\u2009\u00b0C. The absorbance of \u03bd(C-H)s increased initially, but was diminished at 350\u2009\u00b0C and appeared to be removed as the temperature reached 450\u2009\u00b0C, at which point the signal deteriorated rapidly.In the region of 1800\u20131000\u2009cm\u22121, the overall absorbance increased initially but reduced with increasing temperature after 150\u2009\u00b0C. The most dramatic loss of overall absorbance was observed between 300\u2009\u00b0C and 350\u2009\u00b0C, affecting the entire region. A band appeared at 1767\u2009cm\u22121 at 50\u2009\u00b0C, accompanied by a gradually increasing band at 1801\u2009cm\u22121. We assign the bands at 1767\u2009cm\u22121 and 1801\u2009cm\u22121 to the formation of an acid anhydride, where the peaks correspond to the symmetric and asymmetric stretching vibrations of the carbonyl groups, respectively. The higher intensity observed for the asymmetric peak indicates that the acid anhydride forms intermolecular bonds between the alginate chains, via the association of two carboxylate groups [42].The \u03bd(CO)s at 1749\u2009cm\u22121 and \u03bd(COO)sym were the first bands in this region to decrease in absorbance, between 200 and 250\u2009\u00b0C. Simultaneously, new bands appeared at 1706\u2009cm\u22121 and 1596\u2009cm\u22121, and 1510\u2009cm\u22121. The band at 1596\u2009cm\u22121, which we assign to \u03bd(CC)s, was removed by 350\u2009\u00b0C and appears to be an intermediate structure. The change for \u03bd(COO)asym is difficult to assess precisely due to overlapping bands. The bands at 1510\u2009cm\u22121 and 1706\u2009cm\u22121 have frequencies that indicate \u03bd(CC)s,skeletal and \u03bd(CO)s, respectively [43,\n44], and they persisted throughout the investigated temperature range. There were also bands emerging at 1390 and 1200\u2009cm\u22121, in the region where O-H bending and C-O stretching is typically located.The Fe3+ introduced during the synthesis step was also observed when characterizing the dried Fe-alginate. Distinguishing the interaction between alginate and Fe3+ requires a powerful tool that may investigate the local alginate structure, such as nuclear-magnetic resonance (NMR). Unfortunately, the paramagnetism of the iron species makes high-resolution NMR unfeasible. The interaction of divalent transition metals with alginate is well-studied, but these results do not translate for trivalent cations. Not only is the binding strength between alginate and trivalent cations much higher than for the divalent cations, but the ionic radii are also of importance - a smaller ionic radius allows for tighter intermolecular interactions between the alginate macromolecules. Investigations of Al3+-alginate by NMR showed two different octahedral six-fold coordination sites [45]. The Fe3+ ion is slightly larger than Al3+, but similarities in binding mode can be expected. In our previous work, Fe3+-alginate was compared to alginates cross-linked with divalent cations, where the latter had more distinct \u03bd(COO)asym and \u03bd(COO)sym than Fe3+-alginate [41]. An EXAFS measurement was performed of Fe-alginate at RT (Table S4, Fig. S15), which indicated that the Fe atoms on average were coordinated with 6 oxygens. However, the oxygen can stem from both carboxyl and hydroxyl groups, and it is also difficult to differentiate light scatterers such as oxygen and carbon.During pyrolysis, the fate of the iron and alginate was linked, due to their initial interaction in Fe-alginate. At first, water was removed by drying as seen from the first mass-loss segment in TGA, but the iron species appeared unchanged. The reduction of Fe3+ to Fe2+ took place between 150 and 200\u2009\u00b0C without involving the common reduction route through \u03b3-Fe3O4. Instead, a direct reduction to Fe2+ was observed, seemingly while still bound to alginate. The reduction of Fe3+ liberated some carboxyl groups and might have resulted in the formation of intermolecular bonded acid anhydride. This was the beginning of the largest mass-loss (50\u2009%) segment in TGA that started at 160\u2009\u00b0C, and lasted up to 350\u2009\u00b0C, encompassing H2O formation due to loss of hydroxyl groups and possibly also due to dehydration during the development of acid anhydrides and freeing up of carboxylates. The formation of FeO occurred in the range of 200\u2013400\u2009\u00b0C, which further destabilized the carboxyl groups, and possibly also hydroxyl groups, but crystallites with sufficient ordering were observed at 380\u2009\u00b0C. This mass-loss was also observed for divalent metals in the same temperature range [41]. New CC and CO double bonds were also formed, while the bands related to the pyranose rings were severely reduced by 400\u2009\u00b0C, implying that the ring structures of the monomers are fractured or transformed, which is consistent with the third mass-loss (11\u2009%) segment.The carbon structure was further investigated by observing the D-band and G-band, which indicated that the carbon support carbonized by removing heteroatoms and formation of small carbon subunits up to 700\u2009\u00b0C. Above 630\u2009\u00b0C, the extent of carbonization appears to be sufficient to let carbon act as a reductant, as FeO was rapidly reduced to Fe, releasing CO and CO2 that resulted in a mass-loss of 11\u2009%. Our previous investigations of the pyrolysis of metal-alginates with Fe, Co, Ni, and Cu, showed that Fe-Alginate was the only sample to have a mass loss in TGA after 550\u2009\u00b0C, accompanied by CO and CO2 loss. The in situ XRD showed only clear diffractions of \u03b1-Fe, while both M\u00f6ssbauer and XRD of the passivated P700 sample indicated that \u03b8-Fe3C was also formed. Differentiating \u03b1-Fe to \u03b8-Fe3C with XANES is not straightforward, but its absence during in situ XRD could be due to the low ordering of the FexC/\u03b8-Fe3C and might require dwelling at 700\u2009\u00b0C to develop.Analysis of the spent samples indicated that the Fe-alginate sample pyrolyzed at 700\u2009\u00b0C was carburized to a greater extent and was able to activate more iron particles than those treated at lower temperatures. Yet, the samples pyrolyzed at low temperatures yielded the highest catalytic activity. A correlation was observed between catalytic activity at the end of FTS experiment and the average particle size in the spent sample (Fig. S10) \u2013 the sample pyrolyzed at 400\u2009\u00b0C achieved noticeably lower Fe loading (20\u2009%) than the others (27\u201333\u2009%), due to less decomposition of the support. This was beneficial, as low metal loading reduced the proximity of the metal particles, effectively lowering the extent of sintering.All catalysts exhibited the same iron phases during reaction, although with different compositions. The active phase during reaction is \u03c7-Fe5C2, but having a higher extent of carburization did not seem to be beneficial \u2013 the greater extent of carburization for P700 did not yield higher activity. The catalytic stability of P700 seemed to be the best due to the sample having relatively large particles before reduction and reaction, and therefore the loss of catalytic activity due to sintering was limited. The in situ FTS measurement of the sample pyrolyzed at 500\u2009\u00b0C indicated that the sample consisted of 18\u2009% \u03c7-Fe5C2 after 20\u2009h of FTS. Thus, the relatively small amount of the total iron that is carburized and responsible for the catalytic activity of P500 must be highly dispersed. Sintering should be limited to maintain the activity, as it appears to be the main cause of deactivation.While there were no apparent differences in olefin selectivity, more short-chain hydrocarbons were produced by the catalysts pyrolyzed at lower temperatures. All samples had the same amount of Na and S relative to Fe, but higher pyrolysis temperatures could potentially increase the segregation of sulfur to the surface of the Fe particles, but due to the low amount of S, it could not be quantified by XPS. Relative to the previous study (0.13\u2009wt% Na and 0.19\u2009wt% S) on this type of catalyst, a more efficient washing procedure was employed, which lowered the Na content to 0.05\u2009wt% and reduced the C2-C4 olefin/paraffin (O/P) ratio from 2.0 to 1.0 [28]. The selectivity of C2-C4 olefins could therefore be improved by the addition of Na by incipient wetness impregnation. For this type of study, it is important to take into consideration that the Na and S originating from the precursor in the synthesis step are distributed differently in the material than if it is added to the pores of the support by impregnation.All the treatments performed on Fe-alginate resulted in materials with desirable catalytic properties, as the events that led to the decomposition of the alginate structure and initiated the carbonization took place around 400\u2009\u00b0C. P400 obtained the most desirable catalytic properties for FTS, with high activity and without significant changes in hydrocarbon selectivity.The pyrolysis process of Fe-Alginate to form carbon-supported iron catalysts was investigated. The in situ characterization revealed that the Fe3+ species in alginate reduce to Fe2+ around 180\u2009\u00b0C, at the same time as the alginate starts to restructure and decompose. The formation of FeO crystallites led to the loss of carboxyl and hydroxyl groups, and new CC and CO bonds in the alginate residues. At temperatures exceeding 630\u2009\u00b0C, a complete reduction to \u03b1-Fe took place, where the carbon in the support acts as a reductant, with the observed release of CO and CO2. The resulting carbon support is formed by the deterioration of the alginate, and the most critical events that aid the formation of a catalyst with desirable properties took place at temperatures up to 400\u2009\u00b0C. The results provide valuable knowledge to the rational design of metal-alginate-based materials with tailored structures and properties for various applications.Catalysts were synthesized by performing pyrolysis at temperatures between 400 and 700\u2009\u00b0C, all of which resulted in appreciable material characteristics for heterogeneous catalysts. With increasing pyrolysis temperature, the particle size increased, as well as the reductive nature of the treatment, and a larger surface area but smaller pores were formed. All the catalysts exhibited great performance in high-temperature FTS, but higher catalytic activity was observed for catalysts synthesized at milder temperatures due to restricted alginate mass-loss. This resulted in lower Fe loading, which limited particle growth and had a beneficial effect on the catalytic surface area and activity. A greater extent of carburization during FTS was observed for the sample pyrolyzed at 700\u2009\u00b0C, but this did not enhance the catalytic activity nor selectivity to a significant extent.\nJoakim Tafjord: Conceptualization, Methodology, Validation, Software, Formal analysis, Investigation, Resources, Data curation, Writing \u2013 original draft, Writing \u2013 review & editing, Visualization. Samuel K. Regli: Methodology, Software, Formal analysis, Investigation, Writing \u2013 review & editing. Achim Iulian Dugulan: Methodology, Formal analysis, Investigation, Data curation, Writing \u2013 review & editing. Magnus R\u00f8nning: Investigation, Resources, Supervision, Writing \u2013 review & editing. Erling Rytter: Conceptualization, Writing \u2013 review & editing. Anders Holmen: Conceptualization, Writing \u2013 review & editing. Rune Myrstad: Resources, Writing \u2013 review & editing. Jia Yang: Conceptualization, Investigation, Resources, Writing \u2013 original draft, Writing \u2013 review & editing, Supervision, Project administration, Funding acquisition.The manuscript was written through the contributions of all authors. All authors have approved 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.The authors would like to acknowledge support from the Research Council of Norway through the Norwegian Center for Transmission Electron Microscopy, NORTEM (197405/F50); the iCSI (industrial Catalysis Science and Innovation) Centre for Research-based Innovation (Contract no. 237922); and the Swiss-Norwegian Beamlines at ESRF (Grant no. 296087). Funding from the Norwegian University of Science and Technology (NTNU) is also acknowledged. The assistance of Ljubisa Gavrilovic and the staff at the Swiss-Norwegian Beamlines at ESRF during the synchrotron beam-time is also acknowledged.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcata.2022.118834.\n\n\n\nSupplementary material\n\n\n\n\n", "descript": "\n                  Transition metals supported on carbons play an important role in catalysis and energy storage. By pyrolysis of metal alginate, highly active catalysts for the Fischer-Tropsch synthesis (FTS) can be produced. However, the evolution of the carbon (alginate) and transition metal (Fe3+) during pyrolysis remains largely unknown and was herein corroborated with several advanced in situ techniques. Initially, Fe3+ was reduced to Fe2+, while bound to alginate. FeO nucleated above 300\u00a0\u00b0C, destabilizing the alginate functional groups. Increasing temperatures improved carbonization of the carbon support, which facilitated reduction of FeO to \u03b1-Fe at 630\u00a0\u00b0C. Catalysts were produced by pyrolysis between 400 and 700\u00a0\u00b0C, where the highest FTS activity (612\u00a0\u00b5molCO gFe\n                     \u22121 s\u22121) was achieved for the sample pyrolyzed at low temperature. Lower metal loading, due to less decomposition of alginate, moderated sintering and yielded larger catalytic surface areas. The results provide valuable knowledge for rational design of metal-alginate-based materials.\n               "}
{"full_text": "The typical strategies for NOx removal in automotive exhaust include Three-way Catalytic Conversion (TWC), NOx Storage Reduction (NSR) and Selective Catalytic Reduction (SCR) technologies [1,2]. A three-way catalyst requires to be operated under near stoichiometric conditions, while NSR and SCR catalysts are functioning for NOx removal during lean-burn operation. In the SCR technique, the reducing agent must be injected to react with NOx over the SCR catalyst, which demands enough space and complicated control system [3]. As for lean-burn gasoline and light-duty diesel vehicles, use of SCR system is limited due to the confined space. However, an NSR catalyst is very suitable to effectively remove NOx in the exhaust of vehicles equipped with lean-burn gasoline or light-duty diesel engines [3,4]. Zhang [5] et al. reported that addition of Mn into the traditional Pt/Ba/Al2O3 catalyst can significantly improve NOx removal efficiency, owing to the increased activity for NO oxidation and the superior NOx storage efficiency.From the perspective of environmental catalysis, a high N2-yield is the expected result for NOx removal. The PtRh bimetallic NSR catalysts have been widely employed to ensure the effective NOx conversion towards N2. The benefit of the bimetallic Pt/Rh/BaO/\u03b3-Al2O3 catalyst with a Rh/Pt weight ratio of 0.5 is to largely accelerate the release of stored NOx and significantly promote the NOx reduction activity, but the simultaneous presence of Pt and Rh lightly suppresses the NO oxidation and NOx storage under lean-burn conditions [6]. The bimetallic separated Pt/Al2O3\u00a0+\u00a0Rh/BaCO3 catalyst with a Rh/Pt weight ratio of 1 shows a better NSR performance, in comparison with that of the bimetallic Rh/Pt/BaCO3/Al2O3 catalyst [7]. Castoldi group [8] found that the Pt/Rh-Ba/Al2O3 catalyst with a Rh/Pt weight ratio of 0.5 shows higher selectivity of N2O and NH3 by-products than Pt-Ba/Al2O3 in the rich-phase during isothermal lean-rich cycles, resulting in a poorer N2-selectivity. Addition of ceria (CeO2) can improve the low-temperature NOx storage and reduction performance [9].However, it is still unclear how the Rh/Pt weight ratio impacts on the overall NOx removal efficiency and nitrogen yield, which limits the rapid and rational design of a highly efficient NSR catalysts. In the present work, a series of Pt/Rh bimetallic NSR catalysts with different Rh/Pt weight ratio was prepared by mechanical mixing of Pt/Ba/Mn/Al2O3 and Rh/CeO2 powders to investigate the impacts of the Rh/Pt weight ratio on the overall NOx removal efficiency and nitrogen yield.The Mn/Al2O3 powder with the target MnO2 loading of 10\u00a0wt% was prepared by the incipient wetness impregnation of \u03b3-Al2O3 with an aqueous solution of Mn(CH3COO)2, followed by drying at 120\u00a0\u00b0C overnight and calcination at 550\u00a0\u00b0C in static air for 3\u00a0h. The Ba/Mn/Al2O3 powder was prepared by the same procedure as the Mn/Al2O3 powder, using Ba(CH3COO)2 aqueous solution and the prepared Mn/Al2O3. The target BaO loading was 15\u00a0wt% in the Ba/Mn/Al2O3 support. The prepared Ba/Mn/Al2O3 powder was further calcined at 850\u00a0\u00b0C in static air for 4\u00a0h to yield the final Ba/Mn/Al2O3 support.The Pt/Ba/Mn/Al2O3-x catalysts with different Pt loadings were prepared by the incipient wetness impregnation of the Ba/Mn/Al2O3 support with an aqueous solution of hydroxylamine Platinum(II), followed by drying at 120\u00a0\u00b0C overnight and calcination at 590\u00a0\u00b0C in static air for 2\u00a0h. The target Pt loadings of the Pt/Ba/Mn/Al2O3\u20131, Pt/Ba/Mn/Al2O3\u20132, Pt/Ba/Mn/Al2O3\u20133, Pt/Ba/Mn/Al2O3\u20134 and Pt/Ba/Mn/Al2O3\u20135 were 1.07, 1.20, 1.27, 1.33 and 1.20\u00a0wt%, respectively. The Pt/Al2O3 and Pt/Ba/Al2O3 catalysts with a target Pt loading of 1.2\u00a0wt% were also prepared by the same method to investigate the impacts of Mn and Ba on Pt dispersion. The Rh/CeO2-y catalysts were prepared by the same procedure as the Pt/Ba/Mn/Al2O3-x catalysts, using Rhodium(III) nitrate aqueous solution and CeO2 powder. The target Rh loadings of the Rh/CeO2\u20131, Rh/CeO2\u20132, and Rh/CeO2\u20133 were 2.34, 1.19 and 0.60\u00a0wt%, respectively.To achieve the model NSR catalyst with a target metal loading of 1.2\u00a0wt%, 5\u00a0g of the Rh/CeO2-y catalyst was mechanically mixed with 45\u00a0g of the Pt/Ba/Mn/Al2O3-x (y\u00a0=\u00a0x) catalyst to obtain Rh0.2Ce-Pt0.8BMA, Rh0.1Ce-Pt0.9BMA and Rh0.05Ce-Pt0.95BMA samples with Rh/Pt ratios of 0.25, 0.11 and 0.05, respectively. Similarly, 5\u00a0g of pure CeO2 powder was mechanically mixed with 45\u00a0g of the Pt/Ba/Mn/Al2O3\u20134 catalyst to obtain CePt1.00BMA sample. The Pt/Ba/Mn/Al2O3\u20135 catalyst was marked as Pt1.00BMA.Powder X-ray diffraction (XRD) patterns were recorded on a Philips X'pert Pro diffractometer in the 2\u03b8 range of 5\u201390\u00b0 with an increment step of 0.02\u00b0, using a Ni filtered Cu K\u03b1 radiation (\u03bb\u00a0=\u00a00.15418\u00a0nm) source. The X-ray tube was operated at 40\u00a0kV and 30\u00a0mA.The actual Pt loading was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES), and the content of metal oxides was measured by an X-ray fluorescence (XRF) spectrometry. Pt dispersion of the Pt/Al2O3, Pt/Ba/Al2O3 and Pt1.00BMA catalysts was measured by the CO-pulse method following the procedure reported in our previous work [10]. The specific surface area and total pore volume of the catalysts were determined by nitrogen adsorption at \u2212196\u00a0\u00b0C using a Quantachrome NOVA2000e analyzer. The specific surface area was estimated using the Brunauer-Emmett-Teller (BET) equation and the total pore volume was estimated from the single nitrogen adsorption amount at the P/P\n0 of ~0.98. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images and EDS maps were collected on a Tecnai G2 TF30 microscope operating at 300\u00a0kV.Temperature-programmed reduction (H2-TPR) experiments were carried out on a Quantachrome CHEMBET 3000 automated chemisorption instrument equipped with a thermal conductivity detector (TCD) to monitor H2 uptake. After pretreatment at 350\u00a0\u00b0C for 30\u00a0min in Ar gas stream, H2-TPR experiments were performed in 10\u00a0vol% H2/Ar gas mixture with a total flow rate of 80\u00a0mL/min from 50 to 360\u00a0\u00b0C at a heating rate of 10\u00a0\u00b0C/min.The catalytic activity of the various NSR solids (Section 2.1) was evaluated using a continuous flow fixed-bed microreactor. The catalyst was pre-treated at 500\u00a0\u00b0C in 5% O2/N2 gas stream for 40\u00a0min. Then, the catalyst was cooled or heated to the reaction temperature of interest. The activity experiment was performed in lean/rich (45\u00a0s/15\u00a0s) cycling gas streams with a gas hourly space velocity (GHSV) of 120,000\u00a0mL\u00a0h\u22121\u00a0g\u22121. Twenty cycles of lean-to-rich switching gas stream were performed at every temperature, and the last five cycles were used for estimation of catalyst activity. The inlet composition of the lean gas stream was 400\u00a0ppm NO, 5% O2, 5% H2O, 5% CO2 and\u00a0~\u00a085% N2. The inlet composition of the rich gas stream was 3500\u00a0ppm CO, 1000\u00a0ppm C3H6, 5% H2O, 5% CO2 and\u00a0~\u00a089.5% N2. The outlet gas concentrations were analyzed using an online MultiGas FT-IR Analyzer (2030DBG2EZKS13T). The NOx removal efficiency, X\n\nNOx\n (%), was estimated by the following Eq. (1) [11]:\n\n(1)\n\n\nX\n\nNO\nx\n\n\n\n%\n\n=\n100\n\u00d7\n\n\n\n\u222b\n0\n300\n\n\n\n\nNO\nx\n\n\nin\n\n\u2212\n\n\n\nNO\nx\n\n\nout\n\nd\nt\n\n\n\n\n\u222b\n0\n300\n\n\n\n\nNO\nx\n\n\nin\n\nd\nt\n\n\n\n\n\n\nThe various product yields were calculated according to the following Eqs. (2)\u2013(5):\n\n(2)\n\n\nY\n\nNH\n3\n\n\n\n%\n\n=\n\n\n\n\u222b\n0\n300\n\n\n\n\nNH\n3\n\n\nout\n\nd\nt\n\n\n\n\n\u222b\n0\n300\n\n\n\nNO\n\nin\n\nd\nt\n\n\n\n\n\n\n\n\n(3)\n\n\nY\n\n\nN\n2\n\nO\n\n\n\n%\n\n=\n\n\n2\n\n\u222b\n0\n300\n\n\n\n\n\nN\n2\n\nO\n\n\nout\n\nd\nt\n\n\n\n\n\u222b\n0\n300\n\n\n\nNO\n\nin\n\nd\nt\n\n\n\n\n\n\n\n\n(4)\n\n\nY\n\nNO\n2\n\n\n\n%\n\n=\n\n\n\n\u222b\n0\n300\n\n\n\n\nNO\n2\n\n\nout\n\nd\nt\n\n\n\n\n\u222b\n0\n300\n\n\n\nNO\n\nin\n\nd\nt\n\n\n\n\n\n\n\n\n(5)\n\n\nY\n\nN\n2\n\n\n\n%\n\n=\n\n\n\n\u222b\n0\n300\n\n\n\n\n\nNO\n\nin\n\n\u2212\n\n\nNO\n\nout\n\n\u2212\n\n\n\nNO\n2\n\n\nout\n\n\u2212\n2\n\n\n\n\nN\n2\n\nO\n\n\nout\n\n\u2212\n\n\n\nNH\n3\n\n\nout\n\n\n\nd\nt\n\n\n\n\n\u222b\n0\n300\n\n\n\nNO\n\nin\n\nd\nt\n\n\n\n\n\nwhere [NOx]\nin\n, [NH3]\nout\n, [N2O]\nout\n and [NO2]\nout\n present the transient concentrations of the inlet NOx and outlet NH3, N2O and NO2, respectively, during the lean-to-rich cycling. [NOx]\nout\n is the outlet NOx concentration. The N2 yield is estimated indirectly through a material balance based on the other measured nitrogen-containing compounds, as presented in Eq. (5). The lean NOx storage efficiency was calculated according to the following Eq. (6) [12]:\n\n(6)\n\n\n\u014b\n\nNO\nx\n\n\n\n%\n\n=\n\n\n\n\u222b\n0\n45\n\n\n\n\n\nNO\n\nin\n\n\u2212\n\n\nNO\n\nout\n\n\u2212\n\n\n\nNO\n2\n\n\nout\n\n\n\nd\nt\n\n\n\n\n\u222b\n0\n45\n\n\n\nNO\n\nin\n\nd\nt\n\n\n\n\n\n\nIn Eqs. (1)\u2013(5) and (6), the time duration of each cycle was 300\u00a0s and 45\u00a0s, respectively.Before the activity test, blank calibration under lean/rich (45\u00a0s/15\u00a0s) cycling was conducted at every reaction temperature in order to achieve the transient concentrations of the inlet NO for the estimation of \u222b0\n300[NO]\nin\ndt .The specific surface area, total pore volume, and chemical composition of each catalyst are summarized in Table 1\n. The values of specific surface area of Pt1.00MBA, CePt1.00MBA, Rh0.05Ce-Pt0.95MBA, Rh0.1Ce-Pt0.9MBA and Rh0.2Ce-Pt0.8MBA samples are 93.5, 97.5, 109.9, 110.7 and 110.3\u00a0m2/g, respectively. The corresponding values of total pore volume are 0.27, 0.39, 0.32, 0.32 and 0.28\u00a0cm3\u00b7g\u22121, respectively. The actual.Rh/Pt weight ratios of Rh0.05Ce-Pt0.95MBA, Rh0.1Ce-Pt0.9MBA and Rh0.2Ce-Pt0.8MBA samples are 0.06, 0.12 and 0.22, respectively, according to the results of actual Pt and Rh loadings listed in Table 1. The actual contents of BaO, MnO2 and Al2O3 in the Ce-containing samples are lower compared to that of Pt1.00MBA sample, which can be attributed to the dilution induced by CeO2 or Rh/CeO2 addition.Powder X-ray diffractograms of the catalysts are shown in Fig. 1\n. All XRD patterns exhibit the characteristic reflections of alumina components at 2\u03b8\u00a0\u2248\u00a037.2, 45.9 and 66.7\u00b0 [13]. The characteristic peaks of Rh species were not detected in the Rh-containing samples because the concentration of Rh was lower than the detection limit of powder XRD technique [14]. The clear Pt reflections were also not observed in the X-ray diffractograms, although the calcination temperature was higher than that of the Pt/Al2O3 catalyst reported in our previous work [15]. The estimated Pt dispersion of the Pt/Al2O3, Pt/Ba/Al2O3 and Pt1.00BMA catalysts was 12.4, 25.3 and 35.4%, respectively. These results indicate that addition of Mn and Ba is advantageous for improving Pt dispersion.The XRD peaks centered at 2\u03b8\u00a0\u2248\u00a023.9, 24.3, 34.6, 42.0, 44.9 and 47.0\u00b0 can be assigned to the characteristic reflections of the orthorhombic BaCO3 phase, thereby confirming the decomposition of Ba(O2CCH3)2 into crystalline BaCO3 during catalyst calcination [16]. The estimated primary crystallite sizes of BaCO3 particles as summarized in Table 1, suggesting the commensurate dispersion of the crystalline BaCO3 phase in all catalysts. The characteristic XRD peaks appeared at 2\u03b8\u00a0\u2248\u00a025.8, 31.4 and 41.2\u00b0 for the Pt1.00MBA sample illustrates the formation of BaMnO3 phase due to the reaction between MnO2 and BaO during calcination [13]. By contrast, the Ce-containing samples show slightly weaker intensities of the peaks assigned to BaMnO3, BaCO3 and Al2O3 components, which can be ascribed to the reduced relative concentrations. From XRD patterns of the Ce-containing samples, additional diffraction peaks at 2\u03b8\u00a0\u2248\u00a028.6, 33.1, 47.5, 56.3, 76.7, 79.2 and 88.6\u00b0 can be clearly observed, which are closely related to the characteristic reflections of CeO2 crystallites [10]. The estimated primary crystallite sizes of CeO2 particles in the Ce-containing samples are summarized in Table 1, showing that by increasing the Rh loading conduces to the growth suppression of CeO2 crystallite size.The chemical elements distribution was clearly confirmed by HAADF-STEM images, and EDS maps are shown in Fig. 2\n. In the Pt1.00MBA catalyst, the entire overlapping of the EDS map of Ba element with that of Mn i indicates, (indirectly) the formation of BaMnO3. Mn-rich phase was also observed, which illustrates the presence of MnO\nx\n species, although it is not detected by the powder XRD technique. From the EDS map of Pt element in Pt1.00MBA, it was confirmed that Pt location on Mn and Ba species occurs to a greater extent than Pt location on Al2O3. Rh distribution on the surface of CeO2 is observed from the EDS maps of Rh and Ce elements in the Rh0.2Ce-Pt0.8MBA catalyst.\nFig. 3\n shows H2-TPR profiles of the catalysts investigated. The H2-TPR profile of Pt1.00MBA presents a slight negative peak at 175\u00a0\u00b0C and three H2 consumption peaks at 292, 315 and 333\u00a0\u00b0C. Observations of the negative peak can be ascribed to the temperature-driven desorption of the very weakly chemisorbed hydrogen. The H2 consumption peak at 292\u00a0\u00b0C corresponds to the reduction of MnO2 to Mn2O3, whereas the peaks at 315 and 333\u00a0\u00b0C should be respectively attributed to the reduction of Mn4+ to Mn2+ in the BaMnO3 and the stepwise reduction of MnO\nx\n species [5]. Addition of CeO2 into the Pt1.00MBA sample leads to additional H2 consumption at 147 and 208\u00a0\u00b0C, corresponding to surface oxygen species in the CeO2 crystallites [10,15]. Compared with the CePt1.00MBA catalyst, the additional H2 consumption peaks appeared at 130, 116 and 104\u00a0\u00b0C in the H2-TPR profiles of the Rh0.05Ce-Pt0.95MBA, Rh0.1Ce-Pt0.9MBA and Rh0.2Ce-Pt0.8MBA samples, respectively, can be assigned to the reduction of active surface oxygen species in the vicinity of Rh sites [17].As shown in Fig. 4\n, all the catalysts show a low NOx removal efficiency at 200\u00a0\u00b0C, which is much lower than the NOx storage efficiency, because the stored NOx is released and reduced under the rich phase [18,19]. By increasing the temperature to 350\u00a0\u00b0C leads to a significantly enhanced NOx storage and removal, which agrees well with the literature [9]. The Rh-containing samples showed the smaller gaps between NOx storage and overall removal efficiency compared to the Rh-free samples, below 350\u00a0\u00b0C, implying that the introduction of Rh sites promotes the reduction of the stored NOx. The decrease of NOx storage and overall removal efficiency above 400\u00a0\u00b0C is a result of the shift to an equilibrium-limited regime [20]. The results displayed in Fig. 4 indicate that by increasing the Rh/Pt weight ratio enhances NOx reduction under the rich phase in the kinetic regime but shows a negative effect in the equilibrium-limited regime.N2 yield directly reflects the ability to eliminate the hazardous N-containing gaseous pollutants. In Fig. 5\n, no observation of the clear advantage of Rh-containing catalysts in the N2 yield below 250\u00a0\u00b0C is seen, and this is the result of the larger NH3 yield than in the Rh-free catalyst, due to the superior low-temperature activity of the water-gas shift reaction over the Rh/CeO2 catalyst [9,21]. In the region of 250\u2013400\u00a0\u00b0C, by increasing the Rh/Pt weight ratio a clear advantage in N2 yield can be ibserved, corresponding to less N2O and NH3 production. Ammonia adsorbed species start to decompose into N2 and H2 at ~250\u00a0\u00b0C over the Rh-containing catalyst, which results in more N2 production [8]. N2 yield is improved with increasing Rh/Pt weight ratio due to the higher reactivity of Rh than Pt in the NH3 decomposition reaction [8]. With further increasing the reaction temperature, N2 yield declined although only tiny amounts of by-products were formed. Such observation is mainly ascribed to the thermodynamic limitation of NO conversion at 400\u00a0\u00b0C [22].Mn species existed in the form of MnO\nx\n and BaMnO3 phases in the model NSR catalysts investigated, whereas the identified Ba species included BaCO3 and BaMnO3. By increasing the Rh/Pt weight ratio enhanced NOx reduction under the rich phase and improved overall N2 yield in the kinetic regime of NO conversion but shows a negative effect in the equilibrium-limited regime above 400\u00a0\u00b0C. An NSR catalyst should be operated below 500\u00a0\u00b0C, and the Rh/Pt weight ratio should be selected based on the actual operating temperature that is dependent on the installation position. However, the NSR catalysts must keep the Rh/Pt weight ratio as low as possible, unless the overall operating temperature is below 350\u00a0\u00b0C, because rhodium is much more expensive than platinum.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 that could be construed as influencing the position presented in the manuscript entitled \u201cRh/CeO2+Pt/Ba/Mn/Al2O3 model NSR catalysts: Effect of Rh/Pt weight ratio\u201d.This research was supported by the National Natural Science Foundation of China (21862010), the Provincial Applied Fundamental Research Program of Yunnan (202101AT070237), the Major Science and Technology Programs of Yunnan (2019ZE001-2, 202002AB080001-1), and the National Engineering Laboratory for Mobile Source Emission Control Technology (NELMS2019C01).", "descript": "\n                  A series of model NOx storage-reduction (NSR) catalysts with different Rh/Pt weight ratios were prepared independently by wet impregnation and then mechanically mixed to investigate the effect of the Rh/Pt weight ratio on the overall NOx removal efficiency and nitrogen yield. XRD, EDS and H2-TPR studies indicated the coexistence of BaCO3, MnO\n                        x\n                      and BaMnO3 phases in the catalysts. Increasing Rh/Pt weight ratio enhanced NOx reduction under the rich phase, and improved overall N2-yield in the kinetic regime of NO conversion, but shows a negative effect in the equilibrium-limited regime above 400\u00a0\u00b0C.\n               "}
{"full_text": "In the chemical industry, the catalytic dehydrogenation reactions are commonly used, and in the preparation of pharmaceuticals and fine chemicals, such reactions involving the oxygenated compounds are particularly important (Singh and Vannice, 2001). The main procedure for cyclohexanone production is the catalytic dehydrogenation of the cyclohexanol, which in the synthesis of pharmaceuticals and fine chemicals is an important intermediate. For instance, producing of the caprolactam and adipic acid, the main raw materials in manufacture of the polyamide fiber in nylon textiles such as nylon-6 and nylon-6.6, respectively. Most of the uses of the caprolactam depend on the type and amount of impurities which it contains. In this case, higher purity requirements must be increasingly met by the raw materials (Sim\u00f3n et al., 2012a,b).With catalysts based on the copper, cyclohexanol dehydrogenation processes are carried out as when they are carefully reduced, they present a highly dispersed copper phase. In this manner, they usually operate under mild conditions. Due to the copper sintering, these copper catalysts are not used at high temperature (Sim\u00f3n et al., 2012a,b). Recently, the field of alloy catalysts owing to their enhanced catalytic performance has been attracting a lot of interest compared to the individual components.Because of the unique biological, electronic, optical, magnetic, and specifically catalytic properties, the metallic nanoparticles (NPs) constructed from more than one metal have aroused great interest (Singh and Xu, 2013). In this case, owing to the interplay between electronic and lattice effects of the neighboring metals, multimetallic NP-based catalysts often show superior catalytic activities to their monometallic counterparts (Ataee-Esfahani et al., 2010). However, so far the controlled synthesis of the NPs consisting of multiple (n\u202f\u2265\u202f3) metal components has remained relatively unexplored and scientists have focused mainly on bimetallic systems (Wang and Li, 2011).In order to enhance their properties, metals such as Rh (Mendes and Schmal, 1997), Co, Zn, Fe, Cr, Pd, Ni (Nagaraja et al., 2011) added to the copper catalysts. For catalytic activity of tri metallic catalysts, the shape and size distribution, surface segregation and crystalline structure, bulk and surface compositions are crucial factors (Ranga Rao et al., 2012).On the other hand, as a low cost support for heterogeneous catalysts one can use a suitable support and type of interaction in industry with support material graphene to replace current metal oxide based catalyst supports, in order to maximize the catalytic activity of a catalyst (He et al., 2013). In this case, due to their high external surfaces, it is expected to be efficient, excellent high electrical conductivity and thermal/chemical stability which are leading to increase the selectivity and rate of the reactions, preventing poisoning and inhibition of sintering of the active metal surface by coke deposition (Julkapli and Bagheri, 2014).This research focused on the formulation of the catalyst supported by nitrogen doped graphene (N-rGO) formulation of two different copper based supported catalysts (Cu, and tri metallic alloy CuNiRu), characterization by BET, XPS, TPR-H2, TPD-NH3 and XRD. For dehydrogenation of the cyclohexanol, testing the catalytic activity had been carried out for the two different catalysts to evaluate the role of the promoters (Ni and Ru), the effect of these two different catalysts on the activity and selectivity at different operating conditions (T\u202f=\u202f200, 225, 250, 260 and 270\u202f\u00b0C, P\u202f=\u202f1\u202fatm, liquid flow rate of reactant\u202f=\u202f(0.1\u202fml/min), gas flow rate of the carrier(N2 gas)\u202f=\u202f25\u202fml/min, and time of the reaction \u223c8h.For unpromoted and promoted catalysts designated as CuNiRu/N-rGO and Cu/N-rGO, the dehydrogenation of cyclohexanol carried out. By Modified Staudenmaier\u2019s method, the Graphite oxide is prepared as reported by (Ambrosi et al., 2012); however, N-rGO was prepared as described in (Ning et al., 2013). Using incipient wetness impregnation of the Cu-precursor into the N-rGO support, the Cu/N-rGO catalyst was synthesised. The mass of the impregnated precursor was estimated to be equivalent to 1\u202fwt% of the Cu. The N-rGO support was dried overnight at 110\u202f\u00b0C, and the wet slurry containing the Cu-precursor (Cu(NO3)2\u00b73H2O, R&M chemicals) calcined and subsequently reduced for 3\u202fh to obtain as-synthesised 1\u202fwt%Cu/99\u202fwt%N-rGO catalyst in a flow of N2/H2 (10% (v/v)) at 275\u202f\u00b0C. In order to synthesise CuNiRu/N-rGO catalyst, the same preparation method of Cu/N-rGO catalyst was followed. The Cu-Ni-Ru-precursors (Ni(NO3)2\u00b76H2O, M.\u202f=\u202f290.79\u202fg/mole), (Cu(NO3)2\u00b73H2O, M.\u202f=\u202f241.60\u202fg/mole), and (Ru(NO)(NO3)3, M.\u202f=\u202f317.09\u202fg/mole) (R&M chemicals). The mass of the impregnated precursor was estimated to be equivalent to 0.5\u202fwt%Cu, 0.25\u202fwt%Ni and 0.25\u202fwt%Ru.Temperature programmed desorption using NH3 (TPD-NH3) was utilized to measure the acidity with temperature programmed reduction (TPR) studies of the catalysts were performed on a Thermos Finnigan TPDRO1100 series with 5% H2-Ar as reducing and carrier gas. For specific surface area, by adsorption-desorption isotherm using Brunauer-Emmett-Teller (BET method), all the catalysts were characterized using a Micromeritics Pulse Chemisorb 2700 instrument. Before measurements, the samples were oven dried at 393\u202fK for 12\u202fh and flushed in-situ with He gas for 2\u202fh. As X-ray source operated at 25.6\u202fW (beam diameter of 100\u202f\u00b5m), the surface analysis of selected catalyst is carried out using the XPS (Ulvac-PHI, ULVAC-PHI Quantera II, INC.), with monochromatic Al-K\u03b1 (hv\u202f=\u202f1486.6\u202feV). In this manner, the wide scan analysis was performed by using a pass energy of 280\u202feV with 1\u202feV per step. While, the narrow scan was performed using a pass energy of 112\u202feV with 0.1\u202feV per step. On a RIGAKU miniflex II X-ray diffractometer capable of measuring powdered diffraction pattern from 3 to 145\u00b0 in 2\u202f\u03b8\u202fscanning range, the XRD patterns of the catalysts in reduced forms recorded. The X-ray source is Cu K\u03b1 with wavelength (\u03bb) of 0.154\u202fnm radiation. In this case, the XRD has been set up with the latest version of PDXL, RIGAKU full function powder-diffraction analysis software.In a stainless steel fixed bed reactor Cyclohexanol dehydrogenation was carried out using a nitrogen gas cylinder which serves as the carrier gas and a liquid micro pump for feeding of the cyclohexanol into the reactor. Fig. 1\n shows the schematic of the experimental set up. In this case, by external electric heater, the reactor was heated and insulated with glass wool. Using a K-type thermocouple, the temperature of the catalytic bed was monitored. Reaction was normally conducted under the following standard conditions: 200, 225, 250, 260 and 270\u202f\u00b0C temperatures, 0.1\u202fg catalyst weight, atmospheric pressure, 0.1\u202fml/min pure cyclohexanol feed flowrate, \u223c8\u202fh reaction time.Here, the reaction sequence was as follows: with the appropriate amount of catalyst, the reactor has been loaded. In order to run for 8\u202fh under the above mentioned conditions, the reaction was allowed. In this case, by GC (Shimadzu 17A, FID, CP-Sil 24 CB 30\u202fm 0.25\u202fmm 0.25 um), the Liquid samples were analyzed. The detected liquid products were as phenol and cyclohexanone. In case, the Gas analysis was performed in (Shimadzu 17A, TCD, HP-MoLesieve, 19095P-M50 30\u202fm\u202f*\u202f0.530\u202fmm\u202f*\u202f50\u202fMm) and the gaseous product was mainly hydrogen.1: Liquid feed tank, 2: Liquid pump, 3: Valves, 4: Preheater, 5: Fixed bed reactor, 6: Thermocouple, 7: External electric heater, 8. Catalyst bed, 9: Rotameters, 10: Cooler, Gas-liquid separator, 11: Liquid sample and 12: Gas sample (H2 gas).The heterogeneous catalysts are generally porous in nature. This characteristic plays an important role, in their catalysis application. The catalytic activity is closely linked to the available surface area for adsorption. The BET specific surface area, pore size and pore volume distributions of the CuNiRu/N-rGO and Cu/N-rGO samples analysed by the N2 adsorption/desorption isotherms. For all of the classified samples, the N2 adsorption-desorption isotherm (Balbuenat and Gubbins, 1993), in which two branches are almost parallel over a wide range of the P/Po due to the IUPAC classification. For Cu/N-rGO and CuNiRu/N-rGO samples, the N2 adsorption-desorption isotherm displaying the type-III isotherm (see Fig. 2\n(a and b)). Indeed, the samples have macroporous (pore size\u202f>\u202f50\u202fnm) structures. The Cu/N-rGO and CuNiRu/N-rGO samples have a hysteresis of the type H3 according to the IUPAC classification. Attributed to the formation of the slit-shaped pores, these samples have plate-like structures. Compare to that of the Cu/N-rGO catalyst, the CuNiRu/N-rGO catalyst recorded a decreasing about 90% in pores diameters and an increasing \u223c75% in the BET surface area (Table 1\n). Benefiting from the unique structure uniform distribution of the optimal size, the results imply that the Ni and Ru promoters provide a large surface area of the catalyst. In this way, with adding the different promoters such as Zn, Zr and Al, the improvement of the copper catalyst supported reduced graphene oxide (Fan and Wu, 2016). In this research, they found that by adding the promoters to the copper catalyst and the pore diameters deceased, the surface area of the catalyst would be enhanced.The NH3-TPD analysis is used to measure the samples acidity. The quantitative estimation of the acidic site of the samples is summarized due to the desorbed amount of the ammonia (Table 2\n). Based on the peak temperature, in three different regions as 200\u2013400\u202f\u00b0C, 400\u2013600\u202f\u00b0C and above 600\u202f\u00b0C, the results illustrate that the acidic sites are distributed. In this manner, the bimetallic Cu-Ni catalysts supported on the \u03b3-Al2O3 was prepared by Pudi et al., (2015) and they stated that to the ammonia desorption from the weak acidic sites, the first region is attributed while the second region refers to the medium strength acidic sites. In case, from the strong acidic sites, the third region represents the ammonia desorption.The desorption peaks at a range of medium acid sites illustrated by the CuNiRu/N-rGO and Cu/N-rGO catalysts. In this manner, the temperature at which the peaks desorption occurs related to the acidic strength. Table 2 shows that both of the CuNiRu/N-rGO and Cu/N-rGO and catalysts correspond to the medium acidic sites on their surfaces respectively with the NH3 total amount desorbed of 4.64\u202f\u00d7\u202f103 and 2.57\u202f\u00d7\u202f104\u202f\u00b5mol/g. In this way, suggested by the NH3-TPD results, the addition of the promoters can change the total acidity and acidic strength of the catalyst surface. Ji et al. (2007) has been found to provide the suitable monovalent copper active sites based on the study of the dehydrogenation of the cyclohexanol over the Cu-ZnO/SiO2 catalysts, the ZnO help as a promoter. The total acidity of the trimetallic catalyst CuNiRu/N-rGO by 2.11\u202f\u00d7\u202f104\u202f\u00b5mol/g was higher than the Cu/N-rGO. However, the acidic strength is weaker than the Cu/N-rGO due to the rule of the promoter\u2019s Ni and Ru as encouraging increasing the activity of the catalyst and providing the suitable active sites as well.The XPS is used more widely than the others to analyse the surface composition and oxidation states of the industrial catalysts. The photoelectron lines of the wide and narrow scan spectrum evidently present the Cu according to the XPS results as illustrated in Fig. 3\n. The metals loading in terms of the weight percent of the Cu to Ni and Ru and the loading of the CuNiRu alloy nanoparticles on the N-rGO support are close to the ratio of 50%:25%:25%. In case, it was found that due to the low content, there is no absorption peak of the Ni and Ru. Here, one can find the atomic percent (at%) of each of the elements for all the tested samples (see Table 3\n). From the atomic percentage results for the catalysts, the presence of the C, Cu, N and O in the sheets of the Cu/N-rGO and CuNiRu/N-rGO are proved.\nFig. 3 shows the high resolution spectrum of the Cu element. The peak at 933.2\u202feV is assigned to the Cu2p3/2 as per previous studies, which is attributed to either of the Cu and/or Cu2O (Jia et al., 2015; Durando et al., 2008). Indeed, it is difficult to distinguish between these two species based on the Cu2p3/2 binding energies, as they are much close. Attributing to the binding energies of the Cu and/or Cu2O, the CuNiRu/N-rGO catalyst illustrate the peak at 933.1\u202feV as well as the Cu/N-rGO. However, the peak shifted from the observed Cu2p3/2 of the CuNiRu/N-rGO catalyst. In this case, by the shift of the Cu2p3/2 binding energy, a strong electronic interaction between the Cu and Ni with the Ru elements in the metallic alloy are indicated.The H2-TPR usually is used to determine the reduction behavior of the catalysts. One can categorize the reduction behaviors of the CuNiRu/N-rGO and Cu/N-rGO catalysts into two stages of the reduction behavior (Fig. 4\n(a and b)). Due to the reduction behavior of the monovalent copper active sites (Cu2O) to the metallic copper active sites (Cuo), based on the results reported by Bridier et al. (2010), the first stage of the H2-TPR profile recorded at lower temperature. In the case of the Cu/N-rGO and CuNiRu/N-rGO catalysts, as described in Schlapbach et al. (2001), the second peak possibly refer to the adsorbed H2 on the C surface of the N-rGO. The CuNiRu/N-rGO catalyst comparing the Cu/N-rGO catalyst indicates decreasing about 18\u202f\u00b0C in the reduction temperature. Due to the strong interaction between the Cu2O species and the promoters\u2019 Ni and Ru causes highly dispersion of the copper active sites on the N-rGO support, the reduction temperature of CuNiRu/N-rGO catalyst could be shifted. In this case, the higher dispersion of the Cu2O species responsible for their ease of reduction indicated by the shift clearly. In this way, in the study of the promoted copper catalysts, the results are in good agreement with the obtained results by Lin et al. (2010).The crystallinity and phase determination of the samples were analyzed using the X-ray diffraction. Fig. 5\n(a and b) shows the XRD diffraction patterns of the Cu/N-rGO and CuNiRu/N-rGO where the XRD pattern of the Cu/N-rGO catalyst illustrates there are five diffraction peaks centered at 2\u03b8\u202f=\u202f26.6\u00b0, 37.7, 43.28\u00b0, 50.68\u00b0 and 61.7\u00b0 (Fig. 5(a)). In this case, the 2\u03b8\u202f=\u202f37.7\u00b0, 43.28\u00b0 and 61.7\u00b0 might be indexed to the (111), (200) and (220) facets of the cubic phased Cu2O (ICDD card number 00\u2013001-1241) due to the surface oxidation of the Cu NPs, the first short and weak peak at 2\u03b8\u202f=\u202f26.6\u00b0 can be attributed to the (002) planes of the N-rGO. However, the strongest peak can be attributed in the Cu/N-rGO pattern to the (111) facet of the Cu2O. These finding are in good agreement with the results reported by Zhang et al. (2016) within providing the Cu2O/rGO in one step of the reduced GO and loading the Cu2O. The presence of the other phases of the copper interestingly indexed to the (200) confirming of the formation of the metal Cu and associated to the existence of the Cu crystallites detected at 2\u03b8\u202f=\u202f50.6\u00b0. In this case, the crystallite size of the Cu2O at 2\u03b8\u202f=\u202f37.7\u00b0 of the (111) planes from Scherrer equation was estimated as 3.8\u202fnm. In case, the presence of the Cu2O and the metal Cu phases on the N-rGO support revealed by the XRD results.\nFig. 5(b) shows the XRD patterns of the CuNiRu/N-rGO catalyst. In this case, due to the presence of the amorphous carbon in N-rGO, the weak and broad peak at 2\u03b8\u202f=\u202f26.7\u00b0 relates to the (002) planes. Attributed to the copper oxide (Cu2O) for the (111), (200) and (220) planes, the diffraction peaks centered at 2\u03b8\u202f=\u202f37.80\u00b0, 43.71\u00b0 and 61.9\u00b0 respectively. By the existence of the broad and weak diffraction peak at 2\u03b8\u202f=\u202f50.6\u00b0, the presence of the metal Cu could be confirmed which can be indexed to the (200) planes. The peak at 2\u03b8\u202f=\u202f37.80\u00b0 attributed to Cu2O, NiO and RuO2 respectively for (111), (111) and (101) planes. Compare to the Cu/N-rGO, this peak illustrates a slightly shift in angles to the higher value. The shift in the diffraction angle is due to the formation of the CuNiRu alloy as indicated by Bai et al. (2015) in their pioneering research on preparation of the hollow PdCu alloy supported on the N-rGO. Moreover, the crystal size of the CuNiRu/N-rGO catalyst estimated to be 1.13\u202fnm which is smaller than those of the Cu/N-rGO about 2.67\u202fnm. As an evidence for the existence of the CuNiRu/N-rGO catalyst in a strong interaction alloy one can consider the mentioned results.The obtained results from the XRD diffraction support the H2-TPR. As it is observed, the reduction behavior of the catalysts samples encourages one step reduction process due to the reduction of the Cu2O (monovalent Cu+) to the Cuo (metallic Cu). For the CuNiRu/N-rGO and Cu/N-rGO catalysts, from the XRD diffraction results the CuNiRu/N-rGO recorded the highly presence of the Cu2O active sites and the smallest crystal sizes as well.\nFigs. 6\u20138\n\n\n illustrates the effect of the reaction temperature on conversion of the cyclohexanol over the CuNiRu/N-rGO and Cu/N-rGO catalysts and the cyclohexanone yields and selectivity. Here, the process temperature varies between 200 and 270\u202f\u00b0C. In general, it is obvious that the cyclohexanol conversions the cyclohexanone yields (Figs. 6 and 7) and increases when the reaction temperature increases, while the selectivity of the cyclohexanone decreases at the same operating conditions (see Fig. 8).This is consistent with the activation energy trends for temperature dependent reactions, whereby the catalytic performance increases with respect to the temperature increasing in terms of the conversion of the reactants and the yield of the products (Smith, 2008). Compare to the catalyst Cu/N-rGO, the catalyst CuNiRu/N-rGO has an increase of 3.7% of the cyclohexanol conversion, 8.1% of the cyclohexanone yield, 8.8% of the cyclohexanone selectivity, at 270\u202f\u00b0C and 60\u202fmin (Figs. 6\u20138).As shown in Fig. 8, with rise of the temperature (200\u2013270\u202f\u00b0C), the cyclohexanone selectivity on both catalysts decreases. This can be due to the formation of the other by products e.g. phenol and cyclohexene via the side reactions occurring at the elevated temperatures such as the aromatization of the cyclohexanol to phenol and dehydration of the cyclohexanol to cyclohexene (Ji et al. (2007).Not only for the reaction of the dehydrogenation of alcohol to the ketone the sites of the metallic copper are active sites but also for the reaction of aromatization of the cyclohexanol to the phenol. Therefore, the metallic copper active sites are not selective. As indicated from the NH3-TPD results, for this particular reaction, the CuNiRu/N-rGO is more suitable as within the actual dehydrogenation of the cyclohexanol to the cyclohexanone prefers the lower acidity active sites (Chang and Abu Saleque, 1993).The stability of the Cu/N-rGO and CuNiRu/N-rGO catalysts was evaluated based on the times on stream (TOS) \u223c8\u202fh (530\u202fmin). In this manner, the three different reaction stages can be identified as per Figs. 6\u20138, for the Cu/N-rGO as follows:\n\n\u2022\nFor TOS\u202f=\u202f0\u201360\u202fmin and reaction temperature 200\u2013270\u202f\u00b0C, the activation stage (Stage 1) shows increasing about 23.3% of the conversion of the cyclohexanol, 7% of selectivity of the cyclohexanone and 14% of yield of the cyclohexanone. The results\u2019 increasing is due to the activity of the fresh Cu/N-rGO catalyst.\n\n\n\u2022\nFor TOS\u202f=\u202f60\u2013110\u202fmin Steady state stage (Stage 2) shows that the results are almost constant.\n\n\n\u2022\nThe deactivation stage (Stage 3) shows a decline in the results for TOS\u202f=\u202f110\u2013530\u202fmin and reaction temperature 200\u2013270\u202f\u00b0C. The reduction in the results in terms of the reductive values between temperatures 200\u2013270\u202f\u00b0C as: the cyclohexanol conversion was about 10%, the selectivity of the cyclohexanone was 7%, yield of the cyclohexanone was 14%. The results suggest that the deactivation affects the reactions.\n\n\nFor TOS\u202f=\u202f0\u201360\u202fmin and reaction temperature 200\u2013270\u202f\u00b0C, the activation stage (Stage 1) shows increasing about 23.3% of the conversion of the cyclohexanol, 7% of selectivity of the cyclohexanone and 14% of yield of the cyclohexanone. The results\u2019 increasing is due to the activity of the fresh Cu/N-rGO catalyst.For TOS\u202f=\u202f60\u2013110\u202fmin Steady state stage (Stage 2) shows that the results are almost constant.The deactivation stage (Stage 3) shows a decline in the results for TOS\u202f=\u202f110\u2013530\u202fmin and reaction temperature 200\u2013270\u202f\u00b0C. The reduction in the results in terms of the reductive values between temperatures 200\u2013270\u202f\u00b0C as: the cyclohexanol conversion was about 10%, the selectivity of the cyclohexanone was 7%, yield of the cyclohexanone was 14%. The results suggest that the deactivation affects the reactions.In this manner, one can identify the three different reaction stages for the CuNiRu/N-rGO as (see Figs. 6\u20138):\n\n\u2022\nThe activation stage (Stage 1) shows increasing in conversion of the cyclohexanol for TOS\u202f=\u202f0\u201360\u202fmin and reaction temperature 200\u2013270\u202f\u00b0C and selectivity and yield of the cyclohexanone. In this case, by the activity of the fresh CuNiRu/N-rGO catalyst the increasing in the results is caused.\n\n\n\u2022\nThe steady state stage (Stage 2) shows that the results are almost constant for TOS\u202f=\u202f60\u2013380\u202fmin.\n\n\n\u2022\nThe deactivation stage (Stage 3) shows a decline in the results for TOS\u202f=\u202f380\u2013530\u202fmin and reaction temperature 200\u2013270\u202f\u00b0C. In terms of reductive values of the temperatures 200\u2013270\u202f\u00b0C, in the results the reduction in the cyclohexanol conversion, yield and selectivity of cyclohexanone. Thus, by the deactivation based on the obtained results, the reactions slightly affected.\n\n\nThe activation stage (Stage 1) shows increasing in conversion of the cyclohexanol for TOS\u202f=\u202f0\u201360\u202fmin and reaction temperature 200\u2013270\u202f\u00b0C and selectivity and yield of the cyclohexanone. In this case, by the activity of the fresh CuNiRu/N-rGO catalyst the increasing in the results is caused.The steady state stage (Stage 2) shows that the results are almost constant for TOS\u202f=\u202f60\u2013380\u202fmin.The deactivation stage (Stage 3) shows a decline in the results for TOS\u202f=\u202f380\u2013530\u202fmin and reaction temperature 200\u2013270\u202f\u00b0C. In terms of reductive values of the temperatures 200\u2013270\u202f\u00b0C, in the results the reduction in the cyclohexanol conversion, yield and selectivity of cyclohexanone. Thus, by the deactivation based on the obtained results, the reactions slightly affected.The Cu/N-rGO catalyst observed the decreasing of 67.1% of the cyclohexanol conversion, 46.4% of the cyclohexanone yield and 51.4% of the cyclohexanone selectivity. All the results support the idea of the fast deactivation of the Cu/N-rGO vigorously occurred after 110\u202fmin from the reaction started.While, the CuNiRu/N-rGO catalyst has illustrated better stability at high TOS up to 380\u202fmin, and then a slightly reduced in the results was observed up to the end of the reaction. Compared to that of the Cu/N-rGO in terms of increasing the reaction conversion the excellent CuNiRu/N-rGO performance and its stability at different TOS of the reactions could be attributed to the promotional effect of Ru and Ni in the formation of CuNiRu/N-rGO catalyst.To investigate the catalytic performance of the supported catalysts in the dehydrogenation of cyclohexanol to cyclohexanone and to analyse the properties of the synthesised catalysts using TPD-NH3, BET, XPS, TPR-H2, TGA and XRD techniques, this research was revolved to formulate two types of the supported catalysts namely supported copper (Cu/N-rGO) and supported tri metals alloy (CuNiRu/N-rGO) in paper forms. The briefly conclusions from the major findings of this study are:\n\n\u2022\nThe promoters (Ni and Ru) were added to the Cu/N-rGO catalyst and raised supervisor behaviors in terms of the provided suitable and selective active sites for catalytic dehydrogenation of the cyclohexanol to the cyclohexanone reaction with the smallest crystals size, higher thermal stability and larger surface area.\n\n\n\u2022\nThe reaction results in terms of the highest cyclohexanol conversion, cyclohexanone yield and selectivity and hydrogen productivity in case of CuNiRu/N-rGO catalyst detected much improvement with in a reduction in phenol yield and selectivity as well.\n\n\n\u2022\nDue to the copper sintering, coke deposition and reduced active sites, the fastly deactivated in the Cu/N-rGO which might be. Thus, the CuNiRu/N-rGO has a much longer operational life. This is based on the performance evaluation of the catalysts.\n\n\nThe promoters (Ni and Ru) were added to the Cu/N-rGO catalyst and raised supervisor behaviors in terms of the provided suitable and selective active sites for catalytic dehydrogenation of the cyclohexanol to the cyclohexanone reaction with the smallest crystals size, higher thermal stability and larger surface area.The reaction results in terms of the highest cyclohexanol conversion, cyclohexanone yield and selectivity and hydrogen productivity in case of CuNiRu/N-rGO catalyst detected much improvement with in a reduction in phenol yield and selectivity as well.Due to the copper sintering, coke deposition and reduced active sites, the fastly deactivated in the Cu/N-rGO which might be. Thus, the CuNiRu/N-rGO has a much longer operational life. This is based on the performance evaluation of the catalysts.This work was fully sponsored by the Ministry of Higher Education, Malaysia under the Fundamental Research Grant Scheme - (03-02-1522FR). All analyses, otherwise specified, were conducted at the Material Characterization Laboratory, Universiti Putra Malaysia.", "descript": "\n                  In different hydrocarbons reactions, copper based catalysts have industrial importance especially in the synthesis of the cyclohexanone from dehydrogenation of the cyclohexanol. At operating conditions, one of the significant problems in the industrial process is fast deactivation of the copper based catalysts. The present work focuses on the formulation of two types of the supported catalysts namely supported tri metals alloy (CuNiRu/N-rGO) in paper forms and supported copper (Cu/N-rGO), analysing the properties of the synthesised catalyst support (N-rGO) by Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), Temperature-programmed desorption (TPD-NH3), Temperature Programmed Reduction (TPR-H2) and X-ray diffraction (XRD) as well as to investigate the catalytic performance of the two supported catalysts in the dehydrogenation of cyclohexanol to the cyclohexanone. All experiments on the catalytic performance were conducted at moderate temperatures (200\u2013270\u202f\u00b0C), 1\u202fatm, 0.1\u202fml/min cyclohexanol flow rate and \u223c8\u202fh time on stream (TOS). The performances of the catalysts were evaluated in the gas phase dehydrogenation of cyclohexanol to the cyclohexanone. The conversion of the cyclohexanol using CuNiRu/N-rGO is 4% higher compare to use of the Cu/N-rGO. The selectivity for cyclohexanone in case of the Cu/N-rGO catalyst is about 83.88%, whilst, the CuNiRu/N-rGO illustrated approximately 6% better performance. The yield of the cyclohexanone using the Cu/N-rGO is about 78%, while by adding the Ni and Ru as promoters with the improvement of the Cu/N-rGO the yield of cyclohexanone was improved by 8%. The duration of the steady state period significantly improved by using CuNiRu/N-rGO which was proposed up to 7\u202ftimes. This research shows that the CuNiRu/N-rGO catalyst provides the suitable and selective active sites for the dehydrogenation of cyclohexanol to the cyclohexanone reaction.\n               "}
{"full_text": "Data will be made available on request.An increasing demand for efficient electrochemical energy storage and conversion system in modern society has stimulated the development of novel rechargeable batteries that can realize the portable electronic revolution, in which highly sophisticated portable devices such as drones are widely and intensively utilized [1\u20135]. Rather than using conventional lithium-ion batteries that catch fire and explode, the emerging metal\u2013air batteries in particular have been gaining increasing attention as they have the potential to contribute to the development of sustainable electrochemical energy and storage systems due to their high energy density and enhanced environmental friendliness [6,7]. For example, rechargeable zinc\u2013air batteries (ZABs) have the potential to provide four times more energy (1086\u00a0Wh\u00a0kg\u22121) in a sustainable and environmentally friendly manner than the most advanced lithium-ion batteries [8\u201314]. Additionally, the ZABs utilize a two-electrode system consisting of a catalytic air cathode and a nonprecious zinc anode, which significantly reduces the anthropological cost of these batteries [7,15\u201317]. However, practical challenges in zinc\u2013air batteries, such as low cycle life, poor reversibility, and low energy conversion efficiency [18\u201320], persist because of higher energy barriers, including the slow kinetics in the electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and poor electrochemical durability in alkaline electrolytes [21,22].For the improved bifunctional activity of electrocatalysts in ZABs, precious metals (e.g., Pt-based for ORR and Ir\u2013Ru-based for OER) [23\u201325] and their corresponding oxides (IrO2 and RuO2) have typically been studied as the catalytic air cathode that uses molecular oxygen as fuel for the electrocatalytic reaction [26,27]. Alternatively, nonprecious metals (i.e., Fe, Co, and Ni), transition-metal composites (oxides, nitrides, and phosphides), and metal-supported carbon\u2013based hybridization materials have also been investigated [28\u201332]. However, the long-term operation of these single metals and metal oxide\u2013based composite catalysts is characterized by an inherent low conductivity and rapid deactivation of metal sites, resulting in an unbalanced symmetry of OER and ORR [33\u201335].To address the aforementioned issues, intensive research has been conducted on the development of heterogeneous electrocatalysts containing both OER and ORR elements [15,36]. Accordingly, multielement random alloy (MRA) catalysts, in which more than one metal elements are randomly mixed together to produce high-tech metal materials with modified crystal and electronic structure, have been considered for the desirable bifunctionally in active catalysts owing to their high conductivity, surface area, and selective active sites [37\u201345]. In particular, the electrocatalytic features of MRA can be further extended when it change the chemical composition (i.e., mixing ratio of each component), which give a rise to the resultant crystal structure to determine a stability of active metal particles and a charge distribution with internal resistance. Unfortunately, however, predicting the desirable crystal structure and relevant electrocatalytic properties of MRA depending on the metal composition have not been investigated yet due to the thermodynamic complex of each component. In parallel, sophisticated control of metal composition and crystal structure of MRA during the fabrication are also required to ensure their electrochemical catalytic properties for the efficient rechargeable zinc-air batteries. In addition, the electrocatalytic properties of MRA can be enhanced further by coupling with a metal oxide-based electrocatalyst. Because, the integrating MRA into metal oxide system and building a new multi-component dual-phase (MRA/metal oxide) electrocatalyst with different heterointerface can induce a synergistic effect between various metals as well as expose more active sites and efficient charge transfer/redistribution, resulting in a high symmetry of OER and ORR, providing superior electrocatalytic performance [46,47]. Unfortunately, the electrical properties and durability of dual-phase electrocatalysts continue to lag behind commercialization due to a lack of knowledge of their composition\u2013structure\u2013transport relationship.Herein, we report a novel dual-phase electrocatalyst comprised of AgNi random alloy and CoNb2O6 nanocube as the bifunctional multicomponent system derived using a sequential hydrothermal synthesis. By employing virtual crystal approximation (VCA), the optimal composition design and crystal structure of AgNi can be determined to have a specific atomic ratio of 6:4 (Ag:Ni) and a hexagonal closed-packed (hcp) structure, resulting in the highest electrical conductivity (\u03c3 \u223c2\u00a0\u00d7 107 Scm\u22121) and ionized potential (\u223c\u22125.4\u00a0eV). Based on this information, we have successfully fabricated Ag0.6Ni0.4 electrocatalysts that are distributed on top of CoNb2O6 nanocubic electrocatalysts via a sequential hydrothermal process, allowing sophisticated control of chemical composition. The resultant dual-phase CoNb2O6 @Ag0.6Ni0.4 offers a high surface-to-volume ratio, exposed active sites and defect-enriched surface, thereby enhancing the OER/ORR bifunctional activity and charge transports. The CoNb2O6 @Ag0.6Ni0.4 catalyst exhibits outstanding electrochemical activity (Ej = 10 (OER) \u2212\u00a0E1/2 (ORR) =\u00a00.49\u00a0V) and excellent ORR and OER cycle durability. Furthermore, the dual-phase CoNb2O6 @Ag0.6Ni0.4 catalysts are directly applied as an air cathode for zinc\u2013air batteries, providing a stable discharge/charge voltage gap of 0.81\u00a0V over 587\u00a0h at a current density of 10 mAcm\u22122, also delivers excellent peak power density (178.9\u00a0mW\u00a0cm\u22122 at 213\u00a0mA\u00a0cm\u22122) and specific capacity (806.8\u00a0mA\u00a0h\u00a0g\u22121). From a practical perspective, we also designed pouch-type zinc\u2013air batteries using CoNb2O6 @Ag0.6Ni0.4 catalysts as the air cathode, which exhibit an excellent rate capability, peak power density (135.6\u00a0mW\u00a0cm\u22122 at 150\u00a0mA\u00a0cm\u22122) and long-term stability for more than 158.6\u00a0h at a current density of 10\u00a0mA\u00a0cm\u22122.Through density functional theory (DFT), we initially examined the electronic structural and electric transport properties of Ag1\u2212xNix random alloy as a function of Ag content for hcp and fcc structures. Ag1\u2212xNix random alloy (0\u00a0\u2264x\u00a0\u2264\u00a01) was thoroughly optimized using the VCA (R1) provided in the Vienna ab initio simulation package (VASP) (R2) for the hcp and fcc structures. Our DFT calculations employed the frozen-core projector augmented wave method (R3) to describe the core-valence interaction using the generalized gradient of Perdew, Burke, and Ernzerhof (R4) for the exchange-correlation functional with a cut-off energy of 450\u00a0eV for plane waves, a set of 500 k-points for the irreducible Brillouin zone, self-consistent-field convergence threshold of 10\u00a0\u2212\u00a05\u00a0eV, and atomic force of 0.1\u00a0meV/\u00c5. The electric transport properties of Ag1\u2212xNix random alloy (0\u00a0\u2264x\u00a0\u2264\u00a01) for the hcp and fcc structures were simulated at 400\u00a0K using the BoltzTrap code (R5) with dense k-mesh, specifically a set of 10,000 k-points for the irreducible Brillouin zone.The multielement random alloy based CoNb2O6 @Ag0.6Ni0.4 heterogeneous electrocatalyst was prepared using a sequential two-step process.Initially, highly crystalline CoNb2O6 nanocubes were synthesized using hydrothermal methods. In a typical synthesis of CoNb2O6 nanocubes on an fluorine-doped tin oxide FTO substrate, cobalt nitrate hexahydrate [Co(NO3)2.6H2O, 0.01\u00a0mol] and niobium ethoxide [Nb (OCH2CH3)5, 0.01\u00a0mol] were mixed in 50\u00a0mL of aqueous citric acid (0.02\u00a0mol). The reaction solution was homogeneously mixed for 40\u00a0min at 30\u00a0\u00b0C and magnetically stirred. The solution was agitated for 2\u00a0h after the addition of ethylene glycol (EG, 18\u00a0mL), polyvinyl alcohol (2.6\u00a0mmol), and hexamethylenetetramines (HMT, 0.002\u00a0mol). The resultant mixed solution was transferred to a Teflon-lined autoclave (100\u00a0mL) containing a CoNb seed layer-coated FTO plate (see the supporting Information for details on FTO cleaning and CoNb seed layer coating) and heated at 150\u00a0\u00b0C for 15\u00a0h. The obtained sample was rinsed with deionized water and ethanol before being dried at 30\u00a0\u00b0C for 1\u00a0h. The obtained sample was calcined in a muffle furnace at 500\u00a0\u00b0C for 2\u00a0h to produce a cube-shaped CoNb2O6 nanostructure.In accordance with standard protocol, nickel nitrate hexahydrate [Ni(NO3)2\u00b76\u00a0H2O, 8\u00a0mmol], polyvinylpyrrolidone (PVP, 20\u00a0mg), ethylene glycol (EG, 12\u00a0mmol), and hexadecyltrimethylammonium bromide (CTAB, 2\u00a0mmol) were dissolved in 50\u00a0mL citric acid (8.4\u00a0g) to form a clear solution. Then, sodium borohydride (NaBH4, 5\u00a0mL) and silver nitrate (Ag(NO3)\u00b7H2O 18\u00a0mmol) were added to the solution, which was stirred in a round-bottom flask equipped with a reflux condenser at 50\u00a0\u00b0C for 30\u00a0min. The reaction mixture was transferred to a Teflon-lined autoclave and heated at 120\u00a0\u00b0C for 6\u00a0h. The as-obtained precipitate was collected and washed with ethanol and water, and the dry product was calcined at 600\u00a0\u00b0C for 2\u00a0h in a 10% H2/N2 flow at a heating rate of 2\u00a0\u00b0C/min to produce an Ag0.6Ni0.4 random alloy. Ag0.2Ni0.8, Ag0.4Ni0.6, and Ag0.8Ni0.2 random alloy nanoparticles were also prepared in the similar manner. For comparison, Ag and Ni metal particles were also prepared in the same way, with exclusion of Ni(NO3)\u00b7H2O or Ag(NO3)\u00b7H2O, respectively. After heat treatment, Ni, Ag and Ag0.6Ni0.4 samples were used for the textural characterization and electrochemical measurement (see the details in Figs. S1 and S2 of the Supplementary Material). Elemental stoichiometric ratio of the prepared AgNi random alloy samples were examined with ICP-OES (ICP-OES, Perkin Elmer Optima 8300, see the Table S1).The prepared Ag0.6Ni0.4 random alloy atoms were embedded in the CoNb2O6 nanocubes using a simple hydrothermal method. To embed the CoNb2O6 nanocubes, 30\u00a0mg of Ag0.6Ni0.4 random alloy powder was dispersed into a solution comprising a mixture of 20\u00a0mL acetyl-acetone (nanoparticle-capping agent) and 5\u00a0mL \u03b1-terpinol (binding agent). The resulting mixture was sonicated for 30\u00a0min at room temperature. After diluting the colloidal mixture with a 1:1 ratio of ethanol and water, it was transferred to a 100\u00a0mL Teflon-lined stainless steel autoclave containing a CoNb2O6 nanocubes-coated FTO plate and heated at 100\u00a0\u00b0C for 1\u00a0h. The collected sample was dried and then kept at 400\u00a0\u00b0C for 1\u00a0h to obtain a final CoNb2O6 @Ag0.6Ni0.4 catalyst. Bulk composition of CoNb2O6 and CoNb2O6 @Ag0.6Ni0.4 samples were examined with ICP-OES analysis (see the Table S2) CoNb2O6 @Ag0.2Ni0.8, CoNb2O6 @Ag0.4Ni0.6, and CoNb2O6 @Ag0.8Ni0.2 samples were also prepared in the similar manner. For comparison, CoNb2O6 @Ag and CoNb2O6 @Ni catalysts were also prepared in the same way, with exclusion of Ni(NO3)\u00b7H2O or Ag(NO3)\u00b7H2O, respectively.The physicochemical properties of the prepared catalyst were studied via X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HR-TEM), N2 adsorption/desorption, X-ray photoelectron spectra (XPS), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), electron paramagnetic resonance (EPR), and grazing incident wide-angle X-ray scattering (GIWAX) (see Supplementary Material for detailed instrumentation and experimental protocols).Linear-sweep voltammetry (LSV) and cyclic voltammetry (CV) data were collected using potentiostat/galvanostat/ZRA based on a conventional three-electrode set-up (Gamry reference 300). Electrochemical impedance spectra were conducted using an AUTOLAB/PGSTAT 128\u00a0N analyzer at a frequency range of 100\u00a0kHz\u20130.10 mHz in a 1\u00a0M KOH solution. The oxygen (O2) evolution performance was estimated in a closed cell through in situ gas chromatography (YL instrument 6500 GC system) to analyze the headspace. ORR activity of catalysts was evaluated using a rotating disk electrode (RDE, Biologic Science Instruments) connected to a Dy 2300 potentiostat in an O2-saturated 0.1\u00a0M KOH solution (see the Supplementary Material for detailed experimental protocols).A two-electrode cell with a CoNb2O6 @AgNi catalyst\u2013dispersed gas diffusion layer (GDL) as the air cathode and a polished Zn plate as the anode was used to test the performance of the rechargeable Zn\u2013air battery. Catalyst ink was spray-coated onto a GDL surface to prepare the air cathode for the Zn\u2013air battery. In accordance with the standard technique, 10\u00a0mg catalysts, 8\u00a0mL deionized water, 2\u00a0mL isopropanol, and 0.05\u00a0mL Nafion were mixed and sonicated in an ultrasonic bath to produce a homogenous ink. Subsequently, 10\u00a0\u00b5L of this ink was coated onto the GDL surface and dried to obtain a mass loading of \u223c25\u00a0\u00b5g/cm2. The Zn\u2013air battery performance of catalysts was examined in a 6\u00a0M KOH solution containing 0.2\u00a0M zinc acetate (ZnC4H6O4) electrolyte.The rechargeable Zn\u2013air battery performance of the catalysts was evaluated using a two-electrode cell in a 6\u00a0M KOH solution containing a 0.2\u00a0M zinc acetate (ZnC4H6O4) electrolyte. The discharge polarization curve and peak power density of catalysts were measured using a Biologic potentiostat (Biologic, VMP-3) device. The discharge/charge cycling performance of catalysts was measured using the Wonatech cycler system (Wonatech, WBCS3000). Additional details of the Zn\u2013air batteries measurement and the module of Zn\u2013air battery pouch cell fabrication have been provided in the Supplementary Material.A typical crystal structure of neat Ag and Ni is hcp and fcc, respectively (\n\nFig. 1\na). To understand the impact of metal composition on their crystal structure, electrical properties, and stability, we consider Ag1\u2212xNix random alloy (0\u00a0\u2264x\u00a0\u2264\u00a01) at the first stage using VCA (see more details in Supplementary Material and experimental section) [48,49]. Interestingly, we found that the calculated relative energy of Ag1\u2212xNix random alloys are in between hcp and fcc structure and prefer to have the hcp structure with the lowest energy minima when approaching the portion of Ni at x \u223c 0.4 (\nTable S3\n). The calculated work function of Ag1\u2212xNix random alloy gradually increased after adding the Ag component and approached the maximum value of \u223c\u2212\u00a05.4\u00a0eV at x \u223c 0.4 (\nFig. 1\nb). This indicated that the increasing portion of Ni upon Ag can promote a structural phase of transition from fcc to hcp structure with the lowest energy minima, which is in a good agreement with the earlier publications [50,51]. We anticipated that the change crystal structure of Ag1\u2212xNix random alloy influences their electrical properties. In fact, the electrical conductivity \u03c3 of Ag1\u2212xNix random alloy (0\u00a0\u2264x\u00a0\u2264\u00a01) increases gradually and then increases drastically when x \u223c 0.4 with increasing Ag content, especially electrical conductivity \u03c3 of pure Ag is significantly greater than that of pure Ni (\nFig. 1\nc).The significant change in the electrical conductivity of Ag1\u2212xNix random alloy as a function of composition ratio prompted the hypothesis that the magnetic moment of Ag1\u2212xNix random alloy can be modulated. The calculated magnetic moment of Ag1\u2212xNix random alloy (0\u2009\u2264x\u2009\u2264\u20091) for the hcp and fcc structures are provided in Fig. 1\nd. The magnetic moment disappeared near x \u223c 0.4, indicating that the electronic structure of Ni at the Fermi level should be derived from the s/p-state rather than the d-state. Hence, we evaluated the electronic structures of pure Ag, pure Ni, and Ag0.6Ni0.4, which revealed that the Fermi region of the pure Ni system is dominated by an incompletely filled d-state; however, the d-orbitals of pure Ag and Ag0.6Ni0.4 are completely filled (\nFig. S3\n). This implied that the Fermi region of pure Ag and Ag0.6Ni0.4 are contributed from s/p states, where the effective mass m*\u2009is substantially decreased. Therefore, it is plausible to conclude that the rapid increase in electrical conductivity (\u03c3) of Ag1\u2212xNix random alloy (0\u2009\u2264x\u2009\u2264\u20091) at x \u223c 0.4 is due to the removal of magnetism associated with the electronic structure near the Fermi level, which is mostly contributed by the s/p-state rather than the d-state.The facile design and formation route for multielement random alloy-based CoNb2O6 @Ag0.6Ni0.4 heterogeneous electrocatalyst is illustrated in Fig. 2a. The corresponding distinct grazing incident wide angle X-ray scattering (GIWAXS) patterns and summary of lattice parameters of the Ag, Ni, Ag0.6Ni0.4, and CoNb2O6 @Ag0.6Ni0.4 samples are shown in Figs. S4a-4d and Table S4. The observed diffraction peaks of Ag metal particles for q (\u00c5\u22121) =\u20091.5019, 2.3787, 2.6351, and 3.0474 are indexed to the (100), (001), (120), and (200) crystal planes of the hcp structure, respectively. Additionally, the diffraction peaks of Ni metal for q (\u00c5\u22121) =\u20093.0082, 3.5939 and 4.9896 are associated with the reflection planes (111), (200), and (220), respectively, corresponding to the fcc strucutre. Interestingly, we found a minor displacement of the diffraction angles upon Ag0.6Ni0.4 nanoparticles as compared to those of Ag and Ni metal particles. This indicated the formation of Ag0.6Ni0.4 random alloy, conforming to hexagonal crystal structure with lattice parameters a =\u20093.3357\u2009\u00c5, b =\u20093.3357\u2009\u00c5, c =\u20093.4151\u2009\u00c5, \u03b1\u2009=\u200990\u00b0, \u03b2\u2009=\u200990\u00b0 and \u03b3\u2009=\u200960\u00b0, which is in good agreement with the results of VCA above. Moreover, a structural analysis based on the diffraction peaks of CoNb2O6 revealed that their crystal structure correspond to an orthorhombic unit cell with lattice parameters a =\u20095.7219\u2009\u00c5, b =\u200914.149\u2009\u00c5, c =\u20095.0489\u2009\u00c5, \u03b1\u2009=\u200990\u00b0, \u03b2\u2009=\u200990\u00b0 and \u03b3\u2009=\u200990\u00b0. After Ag0.6Ni0.4 random alloy was deposited on top of CoNb2O6 nanostructures, the GIWAXS patterns for CoNb2O6 @Ag0.6Ni0.4 nanostructures indicated that they remained their own crystal structure. The X-ray diffraction patterns of CoNb2O6, Ag0.6Ni0.4, and CoNb2O6 @\u2009Ag0.6Ni0.4 are displayed in Fig. S5, which corresponds to the standard PDF cards (ICDD \u2013PDF-032\u20130304, JCPDS-04\u20130783 and JCPDS-04\u20130850). Interestingly, the diffraction peak of AgNi random alloy was found on the CoNb2O6 @AgNi XRD result, proving that a dual-phase CoNb2O6 @AgNi was successfully synthesized.The morphological characteristics of the prepared samples were analyzed through SEM and TEM. The low- and high-magnification SEM images of CoNb2O6 exhibit the typical hierarchical nanocube morphology (\n\nFig. 2\nb, c and\nFig. S6a\n, b). Subsequently, the atomic percentage and purity of Co, Nb, and O (14.40/27.26/58.34) were confirmed through corresponding energy dispersive X-ray spectra and elemental mapping (\nFig. S7\n). Fig. 2\nd shows the spherical-like structure of Ag0.6Ni0.4 random alloy particles with porous nature. The EDX spectra with elemental mapping showed that the atomic percentages of Ag and Ni were 58.43% and 41.57%, respectively, which was very close to the stoichiometric ratio of Ag0.6Ni0.4\n(\nFig. S8\n).After the incorporation of Ag0.6Ni0.4 random alloy nanoparticles on CoNb2O6 nanocubes, the morphology of the CoNb2O6 nanocubes was found to be highly preserved, as depicted by the SEM image shown in Fig. 2\ne. Additionally, the SEM-EDX elemental mapping of CoNb2O6 @Ag0.6Ni0.4 demonstrates that Ag0.6Ni0.4 random alloy particles are uniformly distributed over the CoNb2O6 nanocubes (\nFig. S9\n). SEM images and EDX elemental mapping of CoNb2O6 @Ni and CoNb2O6 @Ag exhibited a similar trend to that of CoNb2O6 @Ag0.6Ni0.4, as shown in Figs. S10, S12 and Figs. S11, S13\n, respectively.To investigate the crystal structure and lattice spacing of CoNb2O6 @Ag0.6Ni0.4, HR-TEM measurement was performed on the Ag0.6Ni0.4 sample. The Ag0.6Ni0.4 nanoparticles exhibited typical spherical shapes, and their lattice spacing of 0.204 and 0.236\u2009nm was ascribed to the (111) and (111) plans (ICDD-00\u2013004\u20130850, 00\u2013004\u20130783) of metallic Ni and Ag of Ag0.6Ni0.4 random alloy, respectively (\nFig. 2\nf, g and h). The corresponding TEM-EDS mapping validated the formation of the Ag0.6Ni0.4 random alloy shown in Fig. 2\ni, j and k. The low and high-magnification images of CoNb2O6 showed a nanocube structure, which is consistent with SEM images (\n\nFig. 3\na and Figs. S14a, b\n). The well-resolved lattice fringe distances of 0.172\u2009nm observed in the HR-TEM image of Fig. 3\nb correspond to the (062) crystal plane of CoNb2O6 (ICDD-00\u2013032\u20130304). Furthermore, the elemental mapping image of CoNb2O6 shows that the nanocubes consist of only Co, Nb, and O (\nFig. 3\nc). The TEM image of CoNb2O6 @Ag0.6Ni0.4 shows a homogeneous dispersion of Ag0.6Ni0.4 random alloy on the surface of the CoNb2O6 nanocubes, maintaining the original nanocube shape without structural degradation (Fig. 3\nd, e and Figs. S14c-S14e\n). The high-resolution HR-TEM image of CoNb2O6 @Ag0.6Ni0.4\n(\nFig. 3\nf, g, h, i and\nFigs. S15\n) reveals a distinct interface between CoNb2O6 and Ag0.6Ni0.4, confirming the coexistence of CoNb2O6 and Ag0.6Ni0.4 phase. The lattice spacing of 0.219\u2009nm corresponds to the (231) crystal plane of CoNb2O6 (ICDD-00\u2013032\u20130304) while the observed 0.236 and 0.204\u2009nm lattice distances relate to the metallic Ag (111) and Ni (111) crystal planes of Ag0.6Ni0.4 random alloy, respectively.Selected area electron diffraction (SAED) patterns of CoNb2O6 @Ag0.6Ni0.4\n(\nFig. 3\ng insets) display polycrystalline rings corresponding to the (130), (131) and (220), (311) crystal plans of CoNb2O6 (ICDD-00\u2013032\u20130304) and Ag0.6Ni0.4 (ICDD-00\u2013004\u20130850, 00\u2013004\u20130783) random alloys, respectively. Additionally, EDX elemental mapping of CoNb2O6 @Ag0.6Ni0.4\n(\nFig. 3\nj) reveals the homogeneous dispersion of Ag0.6Ni0.4 random alloy particles on the surface of CoNb2O6 nanocubes, demonstrating the successful synthesis of CoNb2O6 @Ag0.6Ni0.4 via the proposed sequential hydrothermal method.Through XPS, the elemental composition and surface valence state of the Ag0.6Ni0.4 random alloy incorporation effect were determined. The survey XPS spectrum of the CoNb2O6 @Ag0.6Ni0.4 catalyst implies the existence of Co, Nb, Ag, Ni, and O elements (\nFig. S16\n). Negative shifts were observed in the core level Co 2p and Nb 3d peaks of CoNb2O6 @Ag0.6Ni0.4 relative to CoNb2O6 (see the details in supplementary material Figs. S17a and S17b). In contrast, the Ag 3d peaks of CoNb2O6 @Ag0.6Ni0.4 have higher binding energies than those of Ag0.6Ni0.4 random alloy (see the details in Fig. S17c and Fig. S17d).The peaks at 853.3 and 870.9\u2009eV and 855.2 and 872.8\u2009eV [52\u201354] in the high-resolution Ni 2p region of Ag0.6Ni0.4 random alloy were attributed to spin-orbit doublets Ni0 and Ni2+ (Ni(OH)2), respectively (\nFig. S17e\n). Ni0 peak at 853.7 and 871.4\u2009eV and Ni (OH)2 peak at 855.5 and 873.1\u2009eV were shifted to higher binding energies (\u223c0.4\u20130.5\u2009eV) following the integration of the Ag0.6Ni0.4 random alloy on top of CoNb2O6\n(\n\nFig. 4\na). The intensity of Ni0 peak decreased more compared to those of pristine Ag0.6Ni0.4, indicating changes in Ni oxidation states. Notably, the new peak at 854.1\u2009eV [55,56] corresponds to NiO while the peak at 857.3\u2009eV indicates the abundance of the Ni3+ state on the surface of CoNb2O6 @Ag0.6Ni0.4. Hence, it was plausible to conclude that the formation of Ni3+ species can play crucial roles in oxygen electrolysis [57,58].Three significant peaks belong to lattice oxygen (529.5\u2009eV for M\u2013O), hydroxyl species (531.1\u2009eV for OH), and surface-adsorbed H2O (532.4\u2009eV) in the O 1\u2009s spectra of CoNb2O6\n(\nFig. 4\nb). However, the O 1\u2009s spectra of CoNb2O6/Ag0.6Ni0.4 shows a new peak at 530.4\u2009eV [59,60], which should be attributed to highly oxidative oxygen species (O2\n2\u2212/O\u2212) and is associated with surface oxygen vacancies [61]. The relative ratio of peak area for four oxygen species is summarized in Table S5. A relative concentration of 33.23% O2\n2\u2212/O\u2212 species on the CoNb2O6 @Ag0.6Ni0.4 surface was reported to contribute to the superior ORR and OER activities [62].To evaluate the OER electrocatalytic activity, the synthesized catalyst and commercial RuO2 were used as working electrodes in a three-electrode test system containing a 1\u2009M KOH electrolyte. As displayed in \nFig. 5\na, the I\u2013R corrected LSV polarization curve of the CoNb2O6 @Ag0.6Ni0.4 catalyst exhibits an overpotential of 110\u2009mV for 10 mAcm\u22122, which is much less than those of CoNb2O6 (330\u2009mV), CoNb2O6 @Ag (250\u2009mV), CoNb2O6 @Ni (220\u2009mV), and RuO2 (300\u2009mV), and outperforms the recently reported metal catalysts at comparable conditions (Table S6). The OER performance of CoNb2O6 with different AgNi random alloy compositions is also depicted in Supplementary Material (Fig. S18\n,\nTable S7\n). In Particular,CoNb2O6 @Ag0.6Ni0.4 required a relatively low overpotential (\u03b7) of 400\u2009mV to achieve a current density of 100\u2009mA\u2009cm\u22122, compared to CoNb2O6 @Ni (520\u2009mV), CoNb2O6 @Ag (620\u2009mV), and CoNb2O6 (700\u2009mV). Accordingly, it was hypothesized that the outstanding activity of CoNb2O6 @Ag0.6Ni0.4 resulted from a strong electronic interaction between CoNb2O6 and Ag0.6Ni0.4 that modifies the local electronic structure of CoNb2O6 @Ag0.6Ni0.4, and it can be confirmed that Ag0.6Ni0.4 activates more reactive sites of OER. Furthermore, CoNb2O6/Ag0.6Ni0.4 exhibited a small Tafel slope value of 40\u2009mV dec\u22121, which is lower than CoNb2O6 @Ni (50 mVdec\u22121), CoNb2O6 @Ag (54 mVdec\u22121), and CoNb2O6 (100\u2009mV dec\u22121) counterparts and RuO2 (89\u2009mV dec\u22121), indicating its fast OER reaction kinetics [63\u201365]\n(\nFig. 5\nb). Reasonably, the Tafel slope of CoNb2O6 @Ag0.6Ni0.4 decreases due to the introduction of the Ag0.6Ni0.4 random alloy to provide a good connectivity of the CoNb2O6 interface, which enhances charge and mass transfer during OER.The excellent intrinsic activity of the CoNb2O6 @Ag0.6Ni0.4 catalyst is also evaluated by their larger turnover frequency (TOF) (0.2300\u2009s\u22121 at an overpotential of 320\u2009mV), which is significantly higher than those of CoNb2O6 @Ag (0.0118\u2009s\u22121), CoNb2O6 @Ni (0.0166\u2009s\u22121), and CoNb2O6 (0.0062\u2009s\u22121) under the same overpotential condition, indicating more favorable reactive sites on the CoNb2O6 @Ag0.6Ni0.4 for OER reaction (details of the calculations are provided in the Supporting Information). Moreover, to gain a better understanding of the synergistic OER activity of CoNb2O6 and Ag0.6Ni0.4 random alloy, the electrochemical active surface area (ECSA) of the catalysts was determined using the electrochemical double layer capacitance (Cdl) derived from the CV curves in a non-Faradic potential region (\nFigs. S19a-S19d\n). As shown in Fig. S19e in the Supplementary Material, CoNb2O6 @Ag0.6Ni0.4 exhibited a larger Cdl of 4.89 mF cm\u22122 than CoNb2O6 @Ag (2.10 mF cm\u22122), CoNb2O6 @Ni (2.60 mF cm\u22122), and CoNb2O6 (1.80 mF cm\u22122), indicating the highest ECSA for CoNb2O6 @Ag0.6Ni0.4 after incorporation of Ag0.6Ni0.4 (see calculation details in the Supplementary Material). Moreover, the calculated roughness factor (RF) of the catalysts has a trend similar to that of the ECSA following the order of CoNb2O6 @Ag0.6Ni0.4 >\u2009CoNb2O6 @Ag >\u2009CoNb2O6 @Ni >\u2009CoNb2O6\n(\nTable S8\n). CoNb2O6 @Ag0.6Ni0.4 with a higher RF value has been shown to have a more favorable exposed active surface for oxygen electrolysis [66].The stability test of CoNb2O6 @Ag0.6Ni0.4 was performed using continuous chronoamperometric responses, which revealed that the initial OER current density was almost maintained for 264.8\u2009h at 220\u2009mV, which is significantly better than the CoNb2O6 @Ni (200.8\u2009h at 280\u2009mV), CoNb2O6 @Ag (165.5\u2009h at 330\u2009mV) and CoNb2O6 catalyst (a loss of 0.80% was observed after the continuous chronoamperometric operation of the CoNb2O6 catalyst for 149\u2009h at 380\u2009mV (\nFig. 5c,\nFig. S20\n)). Furthermore, there is low deterioration (0.01\u2009V) and a positive shift of the OER polarization for CoNb2O6 @Ag0.6Ni0.4 after 264.8\u2009h of continuous stability testing (\nFig. 4\na, red dotted line), indicating the outstanding structural stability of CoNb2O6 @Ag0.6Ni0.4. After performing the continuous OER activity, the structure of CoNb2O6 @Ag0.6Ni0.4 electrode was reexamined by the XPS, SEM and TEM under the same conditions as the initial measurement. The high resolution XPS spectra of the Co 2p, Nb 3d, and Ni 2p level spectrum in CoNb2O6 @Ag0.6Ni0.4 are shown in Fig. S21. The energy level XPS spectra of Co 2p and Nb 3d show slight negative peak shifts and the spectra of Ni exhibits a positive shift after the OER electrolysis. This suggests that constructing dual phase CoNb2O6 @Ag0.6Ni0.4 can effectively maintained the electronic structure of the Co, Nb core and optimize the adsorption of reaction intermediates, hence promoting electrocatalytic stability. SEM and TEM images of the CoNb2O6 @Ag0.6Ni0.4 electrode during and after a stability test are shown in Figs. S22, S23. The overall shape and structure of the catalyst was maintained and still retained original nano cube structure of CoNb2O6 @Ag0.6Ni0.4, indicating the structural stability of the catalyst surface.The corresponding ORR activity of the catalysts was evaluated using a RDE in 0.1\u2009M KOH. LSV and cyclic voltammetry (CV) results obtained in N2-saturated and O2-saturated 0.1\u2009M KOH for various catalysts are shown in Supplementary Material Fig. S24, S25. Polarization curves reveal that CoNb2O6 @Ag0.6Ni0.4 exhibits superior ORR activity compared to all other prepared catalysts (CoNb2O6, CoNb2O6 @Ni, CoNb2O6 @Ag, and commercial Pt/C) in terms of a more positive onset potential of 1.12\u2009V, higher half-wave potential (E1/2) of 0.85\u2009V, and a higher limiting current density of -5.60 mAcm-2\n(\nFig. 5\nd and Table S9\n). Notably, the ORR activity of the CoNb2O6 @Ag0.6Ni0.4 catalyst outperforms the nonprecious metal catalyst reported in the literature (\nTable S6\n). Additional ORR polarization curves of CoNb2O6 with various AgNi random alloy compositions are also shown in the Supplementary Material (Fig. S26\n,\nTable S7\n). The superior ORR performance of CoNb2O6 @Ag0.6Ni0.4 led us to believe that the random alloy composition of integrated CoNb2O6 and Ag0.6Ni0.4 can provide synergistic effects for the ORR electrolysis.Additionally, we found that the measured Tafel slope of CoNb2O6 @Ag0.6Ni0.4 is 50 mVdec\u22121, which is less than that of CoNb2O6 @Ag (69 mVdec\u22121), CoNb2O6 @Ni (73 mVdec\u22121), CoNb2O6 (75 mVdec\u22121), and Pt/C (64 mVdec\u22121), indicating the fast ORR kinetics on the CoNb2O6 @Ag0.6Ni0.4\n(\nFig. 5\ne). The electron transfer number, n, of the CoNb2O6 and CoNb2O6 @Ag0.6Ni0.4 catalysts was also determined from the Koutecky\u2013Levich (K\u2013L) plots under various potentials (0.3\u20130.7\u2009V), which were obtained from ORR polarization curves at various rotation speeds (400\u20131600\u2009rpm, Figs. S27a and S27b). The K\u2013L plots of CoNb2O6 and CoNb2O6 @Ag0.6Ni0.4 at different potentials exhibit excellent linearity and near parallel fitting, revealing typical first-order reaction kinetics (Figs. S27c and S27d\n). Furthermore, the calculated electron transfer number (n per O2) for the CoNb2O6 and CoNb2O6 @Ag0.6Ni0.4 catalysts was 3.9 and 4, respectively, indicating that the ORR process followed the four-electron transfer pathway closely [67]. The ORR stability of the CoNb2O6, CoNb2O6 @Ag, CoNb2O6 @Ni and CoNb2O6 @Ag0.6Ni0.4 catalysts was further evaluated through chronoamperometry testing in an O2saturated 0.1\u2009M KOH solution at 0.7\u2009V (\nFig. 5\nf,\nFig. S28\n). The CoNb2O6 @Ag0.6Ni0.4 catalyst retains 98% of the initial ORR current density after 24.5\u2009h of continuous chronoamperometric performance, indicating that the CoNb2O6 @Ag0.6Ni0.4 catalyst has a well ORR-stability. In comparison to the CoNb2O6 @Ag0.6Ni0.4 electrocatalyst, CoNb2O6 @Ag (96% after 20.1\u2009h), CoNb2O6 @Ni (96% after 18.6\u2009h) and pristine CoNb2O6 (95% after 13\u2009h) exhibit lower stability, which further confirming that the constructed dual-phase sample of CoNb2O6 @Ag0.6Ni0.4 has the best ORR electrocatalytic performance. Additionally the nearly identical ORR curves before and after durability test provide further evidence of the excellent stability of CoNb2O6 @Ag0.6Ni0.4 (depicted using the red dotted line in Fig. 5\nd).The plotted histogram compared the OER overpotential at 10\u2009mA\u2009cm\u22122 and the ORR half-wave potential of prepared catalysts, revealing the significantly enhanced ORR and OER performance of CoNb2O6 @Ag0.6Ni0.4, which is superior to CoNb2O6 and RuO2 and surpasses that of Pt/C (\nFig. 5\ng). Due to the excellent oxygen electrolysis, the overall OER and ORR activities of catalysts were further analyzed (\nFig. 5\nh). The potential difference \u0394E (\u0394E = Ej = 10\u2009\u2212 E1/2) between the OER potential at 10 mAcm\u22122 and the ORR half-wave potential (E1/2) can be used to estimate the bifunctional activity of a catalyst. In general lower the \u0394E value of an electrode, the greater its bifunctionality [68]. As shown in Fig. 5\nh, CoNb2O6 @Ag0.6Ni0.4 exhibits a small \u0394E value of 0.49\u2009V, which is smaller than those of CoNb2O6 (0.99\u2009V) and the metal-based bifunctional catalysts as reported in Tables S6.Additionally, in situ gas chromatographic analysis was also conducted to confirm the amount of oxygen evolution. The Faradaic efficiency of CoNb2O6 and CoNb2O6 @Ag0.6Ni0.4 catalysts at 10\u2009mA\u2009cm\u22122 with oxygen evolution time was measured. As shown in Fig. 5\ni and Fig. S29, the amount of O2 volume detected with increasing operation duration is comparable to the volume calculated at constant current density, demonstrating a nearly 100% Faradaic efficiency during the OER. Furthermore, CoNb2O6 @Ag0.6Ni0.4 exhibited excellent methanol tolerance without any change in the ORR current when 0.5\u2009mL (3\u2009M) methanol was added to the O2-saturated electrolyte at 520\u2009s (\nFig. S30\n).Overall, the electrocatalytic efficiency of CoNb2O6 @Ag0.6Ni0.4 for OER and ORR has improved significantly, likely due to the efficient charge transport and redistribution at the interface between CoNb2O6 and Ag0.6Ni0.4. To gain a deeper understanding of the catalytic interaction at the CoNb2O6 @Ag0.6Ni0.4 interface, we conducted DFT calculations to calculate the electrical conductivity of CoNb2O6 based on its charge carrier density (ne) and oxidation numbers (\nFig. 6\na). Interestingly, we found that the conductivity of CoNb2O6 could be further enhanced through oxidation or reduction compared to neutral CoNb2O6, with a conductivity of approximately 3\u2009\u00d7\u2009103 S/m. We also observed that the conductivity of CoNb2O6 gradually increased as its oxidation number increased up to +\u20093, but decreased when its oxidation number decreased down to +\u20093. These results can be explained by changes in the density of states (DOS). When electrons are removed from CoNb2O6 (i.e., cation), the Fermi level moves closer to the broad valence band maximum (VBM), referred to as 1, which leads to a decrease in effective mass (m*) and an increase in conductivity (Fig. 6\nb). Conversely, when electrons are added to CoNb2O6 (i.e., anion), the Fermi level moves closer to the sharp conduction band maximum (CBM), referred to as 2, which leads to an increase in m*\u2009and a decrease in conductivity.Then, we compared the relative energy of CoNb2O6 and Ag0.6Ni0.4 using DFT calculations, considering their oxidation levels (Fig. 6\nc). The relative energy of CoNb2O6 was found to be slightly lower than that of Ag0.6Ni0.4 when an electron was removed from CoNb2O6. Conversely, the relative energy of Ag0.6Ni0.4 was lower than that of CoNb2O6 after an electron was added to CoNb2O6. These results imply that it is possible to secure a favorable oxidation state for efficient charge transport.The local electronic properties, a coordination environment and bond distance of Ag0.6Ni0.4, CoNb2O6, and CoNb2O6 @Ag0.6Ni0.4 catalysts were analyzed through XANES. As shown in \nFig. 7\na, the normalized Co L\n3pre-edge XANES for CoNb2O6 @Ag0.6Ni0.4 showed a slightly lower energy compared to pristine CoNb2O6, indicating a shift in local coordination due to the presence of Ag0.6Ni0.4 random alloy. The Ni L-edge XANES spectra for CoNb2O6 @Ag0.6Ni0.4 showed apparent changes from those of Ag0.6Ni0.4 random alloy (\nFig. 7\nb). The fine multiple splitting of the Ni L3 and Ni L2 peaks was attributed to the crystal field effects from the electronic structure. These characteristics, which result from the dipole transition from the 2p to the empty 3d state, were sensitive to changes in the oxidation state and local geometry of Ni [69].Ni and Co coordination conditions in CoNb2O6 @Ag0.6Ni0.4 were further investigated using K-edge XANES spectra for Ni and Co. As shown in Fig. 7\nc, the shape of the pre-edge (1\u2009s to 3d transition) and absorption edge (multiple scattering) of the normalized Ni K-edge spectra obtained from CoNb2O6 @Ag0.6Ni0.4 differs from that of the Ag0.6Ni0.4 random alloy, indicating that there are probably local lattice strain changes around the targeted Ni atom. The Ni K-edge spectra of CoNb2O6 @Ag0.6Ni0.4 exhibited an increase in the white line and a shift of the absorption edge toward high photon energy compared to those of Ag0.6Ni0.4 random alloy, indicating that the Ni has more empty d\u2010orbital states and less electron density. Thus, the results matched the oxidation valence of Ni (Ni2+ is oxidized to Ni3+/Ni4+) [70]. Comparing the Co K-edge XANES spectra (\nFig. 6\nd) of CoNb2O6 @Ag0.6Ni0.4 to those of CoNb2O6, the white line is significantly reduced and the absorption edge shifts to slightly negative values, confirming the higher d\u2010orbital occupancy due to the surface charge polarization caused by the electron transfer [71].\nFig. 6\ne shows the Fourier transform (FT) of EXAFS R-space Ni K-edge spectra (k2-weighted) for CoNb2O6 @Ag0.6Ni0.4. The dominant peak in the first coordination shell at 1.6\u2009\u00c5 corresponds to the scattering path of Ni\u2013O, whereas the second coordination shell peak at 3.5\u2009\u00c5 originated from the scattering path of Ni\u2013Ni [72,73]. Comparatively, the decreased peak intensities of the Ni\u2013O and Ni\u2013Ni coordination shells are associated with lower coordination numbers and defects in the structure, reflecting changes to the electronic structure [74,75]. Besides, the FT curve of Co K-edge R-space spectrum for CoNb2O6 @Ag0.6Ni0.4 exhibits the lowest EXAFS when compared to CoNb2O6, demonstrating its more disordered octahedral coordination around the Co atom [76] due to theAg0.6Ni0.4 random alloy incorporation (\nFig. S31\n). Additionally, the Co\u2013O and Co\u2013Co bond lengths of CoNb2O6 @\u2009Ag0.6Ni0.4 are 1.4 and 2.6, [77] which is shorter than those of CoNb2O6 at 1.49 and 2.66. The contraction of Co\u2013O and Co\u2013Co bond length reflects the enhanced interaction between Ag0.6Ni0.4 random alloy and CoNb2O6, which shifted the CoNb2O6 @Ag0.6Ni0.4 peak toward the lower energy side, as shown in Fig. S31.As shown in Fig. 7\nf, the CoNb2O6, CoNb2O6 @Ag0.6Ni0.4, and Ag0.6Ni0.4 random alloy samples exhibited significant changes near the O K-edge, indicating alterations in the local chemical and electronic structure around O atoms [78]. Notably, the main absorption edge peak below 535\u2009eV corresponds to metal 3d band electronic transitions while the peaks at 536\u2013543\u2009eV relate to metal 4sp band electronic transitions [79]. Additionally, the normalized O K-edge intensity of CoNb2O6 @\u2009Ag0.6Ni0.4 is lower than that of CoNb2O6, and the adsorption pre-edge shifts to higher energy coupled with a new peak at 533.4\u2009eV, indicating characteristics of oxygen vacancies [80]. From the XANES- and EXAFS data, we concluded that the incorporation of Ag0.6Ni0.4 random alloy into CoNb2O6 surface had been significantly affecting the Ni atoms and oxidized to high-valence states (Ni3+), accompanying surface generate oxygen vacancies, which synergistically contribute to high ORR and OER catalytic activity of CoNb2O6 @Ag0.6Ni0.4.In addition, the K-edges of the Co and Ni of CoNb2O6 @Ag0.6Ni0.4 were found to shift compared to the bare CoNb2O6 and Ag0.6Ni0.4. These shifts can be attributed to the formation of the interface between CoNb2O6 and Ag0.6Ni0.4 and promising interfacial charge transfer from Ag0.6Ni0.4 to CoNb2O6. This drastically affected the local configuration of Ag0.6Ni0.4 and CoNb2O6 structural sites. These combined electronic and chemical properties of CoNb2O6 @Ag0.6Ni0.4 alter the ORR/OER properties of the catalyst surface, generating an architecture with high conductivity and potential for active site exposure. Additionally, they offer an intuitive means of revealing bifunctional activity by monitoring the strong electronic bonding between Ag0.6Ni0.4 random alloy and CoNb2O6. This allows them to maintain structural stability even when electrochemical conditions are extremely severe.To further validate the existence of oxygen vacancies, EPR spectra of the CoNb2O6 @Ag0.6Ni0.4 catalyst were acquired. As shown in Fig. 7\ng, the CoNb2O6 @Ag0.6Ni0.4 catalyst exhibits a strong EPR signal at g =\u20092.001, which arises from the unpaired electrons trapped in the CoNb2O6 @Ag0.6Ni0.4 catalyst, thus proving the existence of oxygen vacancies induced by Ag0.6Ni0.4 random alloy incorporation. Thus, the CoNb2O6 @Ag0.6Ni0.4 catalyst is expected to minimize the charge transfer resistance during the electrolysis. Furthermore, Fig. 7\nh displays the Nyquist plots derived from electrochemical impedance spectroscopy fitting for the CoNb2O6, CoNb2O6 @Ag, CoNb2O6 @Ni, and CoNb2O6 @Ag0.6Ni0.4 electrodes. The fitting parameters are estimated and listed in Table S10 of the supporting Information. The results revealed a substantially lower charge transfer resistance (Rct) compared to CoNb2O6 from the fitting result, indicating that the significantly improved interfacial charge transfers can promote the reaction kinetics of the oxygen electrolysis.Additionally, the N2 sorption isotherms (type IV isotherm with H3 hysterics loop) of the CoNb2O6 @Ag0.6Ni0.4 catalyst indicate the existence of mesopores (\nFig. 5\ni). Moreover, the catalyst exhibits a high Brunauer\u2013Emmett\u2013Teller (BET) surface area of 198\u2009m2 g\u22121 compared with that of CoNb2O6 electrode (93\u2009m2 g\u22121), which indicates that the Ag0.6Ni0.4 random alloy incorporation positively enlarges the intrinsic surface area of CoNb2O6 @Ag0.6Ni0.4. Fig. 7\ni inset depicts the Barrett\u2013Joyner\u2013Halenda pore size distribution of CoNb2O6 @Ag0.6Ni0.4 catalyst, with the majority of pores falling into a mesopores size range of 5\u201350\u2009nm; these mesoporous and high specific surface area of CoNb2O6 @Ag0.6Ni0.4 significantly allow more active site, facilitating accessible mass transfer and collection efficiencies during electrolysis [81,82]. The estimated pore size, BET surface area, and pore volume of the catalysts are summarized in Table S11.In addition to their excellent ORR and OER bifunctional electrocatalytic activity, rechargeable Zn\u2013air batteries were evaluated to demonstrate the charge\u2013discharge performance of CoNb2O6 and CoNb2O6 @Ag0.6Ni0.4 catalysts. \nFig. 8\na illustrates a schematic diagram of a customized two-electrode Zn\u2013air battery system. The Zn\u2013air cell driven by optimized CoNb2O6 @Ag0.6Ni0.4 displays an open circuit potential (OCV) of 1.425\u2009V (\nFig. S32\n), which is higher than the OCV of the Zn\u2013air cell driven by pristine CoNb2O6 (1.41\u2009V). This finding demonstrates the lower internal resistance of CoNb2O6 @Ag0.6Ni0.4. The assembled zinc\u2013air batteries polarization (I\u2013V) curves and corresponding power density (P\u2013V) plots are shown in Fig. 8\nb. Based on the discharge curve, the peak power density of the CoNb2O6 @Ag0.6Ni0.4-air cathode battery (178.9\u2009mW\u2009cm\u22122 at a current density of 213\u2009mA\u2009cm\u22122) is higher than that of the Pt+C/RuO2 (131.8\u2009mW\u2009cm\u22122 at a current density of 173\u2009mA\u2009cm\u22122) and CoNb2O6-based Zn\u2013air battery (107\u2009mW\u2009cm\u22122 at a current density of 155\u2009mA\u2009cm\u22122), indicating the high catalytic activity of CoNb2O6 @Ag0.6Ni0.4 even in practical Zn\u2013air battery conditions. The specific capacity of air cathode at various current densities is shown in Fig. 8\nc. At a discharge current density of 10\u2009mA\u2009cm\u22122, the CoNb2O6 @Ag0.6Ni0.4 air-cathode\u2013based battery demonstrates an excellent specific capacity of 806.8\u2009mA\u2009h\u2009g\u22121, compared to 606.3\u2009mA\u2009h\u2009g\u22121, 576.6\u2009mA\u2009h\u2009g\u22121 of Pt+C/RuO2 and CoNb2O6. Furthermore, when the current density reached 20 and 50\u2009mA\u2009cm\u22122, the CoNb2O6 @Ag0.6Ni0.4 cathode was still capable of discharging capacities of 788.2 and 726.4\u2009mA\u2009h\u2009g\u22121, respectively, confirming the practical capability of the as-designed catalysts. Fig. 8\nd displayed the Galvanostatic discharge\u2013charge polarization cycles for Zn\u2013air batteries based on CoNb2O6 @Ag0.6Ni0.4, RuO/PtO and CoNb2O6 air cathodes. The cycle tests were measured at room temperature at a current density of 10 mAcm\u22122. The Zn\u2013air batteries with CoNb2O6 @Ag0.6Ni0.4 cathodes were able to operate a long life cycle at a voltage gap of 0.81\u2009V without voltage loss for 587\u2009h; however, the discharge voltage of Zn\u2013air batteries with Pt+C/RuO2 and CoNb2O6 air cathodes decreased within 156\u2009h and 286\u2009h, respectively.Owing to the excellent power density, specific capacity, and cycling stability of CoNb2O6 @Ag0.6Ni0.4, the portable Zn\u2013air pouch cell was constructed with CoNb2O6 @Ag0.6Ni0.4 as the air cathode (see the details in Supplementary Material). Images of the CoNb2O6 @Ag0.6Ni0.4 air cathode\u2013based rechargeable Zn\u2013air battery pouch cell are shown in Fig. 8\ne and Fig. S33. The fabricated Zn\u2013air pouch cell using CoNb2O6 @Ag0.6Ni0.4 catalyst as the air cathode (\nFig. 8\nf) demonstrates an OCV of 1.41\u2009V and a specific capacity of 716.6\u2009mA\u2009h\u2009g\u22121 (at 30\u2009mA\u2009cm\u22122 (Fig. S34\n)) based on the mass of consumed Zn (93.4% of the theoretical capacity). The galvanostatic discharge measurements (\nFig. 8\ng) of the CoNb2O6 @Ag0.6Ni0.4-catalyzed Zn\u2013air pouch cell show a small voltage drop between 5 and 50\u2009mA\u2009cm\u22122. Additionally, when the current density is lowered to 10\u2009mA\u2009cm\u22122, the discharge can be reversed, demonstrating that the discharge voltage rate performance and their reversibility are excellent. The peak power density of the pouch cell was measured to be 135.6\u2009mW\u2009cm\u22122 at 150\u2009mA\u2009cm\u22122\n(\nFig. 8\nh). As shown in Fig. 8\ni, the cycling stability of a CoNb2O6 @Ag0.6Ni0.4 air-cathode\u2013based battery was studied at a current density of 10\u2009mA\u2009cm\u22122 and a charging and discharging duration of 10\u2009min each. The Zn\u2013air pouch cell with CoNb2O6 @Ag0.6Ni0.4 air cathode has a small charge\u2013discharge voltage gap of 0.86\u2009V. After 158.6\u2009h of cycling, CoNb2O6 @Ag0.6Ni0.4-based batteries exhibited a minor voltage drop of 0.03\u2009V, demonstrating their excellent stability. Following cycling performance, three fabricated pouch cells integrated in series with ultrasonic welding (OCV is 4.23\u2009V, a single battery is 1.41\u2009V) lit an LED house for several hours (\nFig. 8\nj), demonstrating the application potential of the prepared CoNb2O6 @Ag0.6Ni0.4 air cathode.In conclusion, we provided an efficient sequential hydrothermal method for the fabrication of highly dispersive mesoporous Ag0.6Ni0.4 random alloy nanoparticles on CoNb2O6 nanocubes. The random alloy based CoNb2O6 @Ag0.6Ni0.4 heterogeneous catalyst demonstrated outstanding ORR and OER activities and remarkable stability in alkaline environments. Detailed simulation and characterization data, including VCA, XPS, XNEAS, XRFA, and advanced electrochemical experiments, revealed that the improved electrochemical performance of the CoNb2O6 @Ag0.6Ni0.4 catalyst is likely attributable to the incorporation of Ag0.6Ni0.4 random alloy. This modified the electrical and chemical properties of CoNb2O6 and created CoNb2O6 @Ag0.6Ni0.4 with high conductivity and the possibility for active site exposure. The formation of defect-enriched surface, Ni3+ active intermediates, an abundance of highly oxidative oxygen species, and a mesoporous structure resulted in additional catalytic support, which improved overall electrochemical performance. The strong electronic bonding and structural advantages of CoNb2O6 @Ag0.6Ni0.4 facilitated charge transfer and ensured structural stability even under extreme electrochemical conditions. The CoNb2O6 @Ag0.6Ni0.4 air cathode delivered excellent specific capacity (806.8\u2009mA\u2009h\u2009g\u22121 at 10 mAcm\u22122), power densities (178.9\u2009mW\u2009cm\u22122 at 213\u2009mA\u2009cm\u22122), and stable cycling life in Zn\u2013air battery applications (587\u2009h at 10\u2009mA\u2009cm\u22122). Exceptionally, the designed pouch-type zinc\u2013air batteries possessed a peak power density of 135.6\u2009mW\u2009cm\u22122 at 150\u2009mA\u2009cm\u22122, excellent rate capability, and a stable discharge/charge cycle life of over 158.6\u2009h at 10\u2009mA\u2009cm\u22122. The proposed fabrication of random alloy dispersed CoNb2O6 @Ag0.6Ni0.4 catalysts facilitated the construction of various metal alloy catalyst systems for numerous sustainable energy conversion technologies.\nChandran Balamurugan: Project conception and organization, Investigation, Sample and device fabrication, Formal analysis, Interpretation of data, Visualization, Writing - original draft, Writing-review.\u00a0Changhoon Lee:\u00a0DFT calculation & Interpretation, Visualization.\u00a0Kyusang Cho:\u00a0Helped in device fabrication & characterization, Validation.\u00a0Jehan Kim:\u00a0GIWAXS measurements & Validation.\u00a0Byoungwook Park:\u00a0Graphical support. Woochul Kim:\u00a0Helped in materials characterization.\u00a0Namsoo Lim:\u00a0Helped in materials characterization.\u00a0Hyeonghun Kim:\u00a0Helped in materials characterization.\u00a0Yusin Pak:\u00a0Equipment provision and discussions.\u00a0Keun Hwa Chae:\u00a0Materials characterization, Validation.\u00a0Ji Hoon Shim:\u00a0Equipment provision and discussions.\u00a0Sooncheol Kwon: Project conception and organization, supervision, Interpretation of data, Writing & 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 supported by the Young Researchers Program of the NRF funded by the Ministry of Science, ICT & Future Planning (NRF-2021R1A2C4001904). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2022R1I1A1A01072238). This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2022M3H4A1A04074153; NRF-2020M3H4A2084417; NRF-2022M3C1A3091988). This research at MPK/POSTECH was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2021R1F1A1063478).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2023.122631.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  The integration of bifunctionally active sites of multielement random alloy catalysts with other metal oxide electrocatalysts is a promising strategy for efficient electrochemical reactions. In this study, a novel combination of virtual crystal approximation and hydrothermal synthesis was used to investigate the composition-dependent structure and electrical property in a Ag1\u2212xNix catalyst. The combination showed that a hexagonal closed-packed structure of Ag1\u2212xNix with a compositional ratio of 6:4 (Ag:Ni) had electrical conductivity of \u223c2\u00a0\u00d7\u00a0107 S\u2219cm\u22121 and an ionization potential of \u2212\u00a05.4\u00a0eV. Furthermore, the bifunctional oxygen electrocatalytic efficiencies of Ag0.6Ni0.4 were improved by forming a heterointerface with the CoNb2O6 electrocatalyst, resulting in a discharge-charge voltage gap of 0.81\u00a0V over 587\u00a0h, peak power density of 178.9\u00a0mW\u2219cm\u22122, and specific capacity of 806.8\u00a0mA\u2219h\u2219g\u22121 in a zinc\u2013air battery. This approach was applied to pouch-type zinc\u2013air batteries, resulting in long-term stability of over 158.6\u00a0h at 10\u00a0mA\u2219cm\u22122.\n               "}
{"full_text": "Hydrogen (H2) economy is an envisioned future in which the main source of energy of a nation is derived from hydrogen storage and application in the short, medium, and long term [1]. H2 produced from green chemistry based on water electrolysis \u2013 mostly derived from the use of wind and solar energy sources [1] \u2013 is required to be stored and distributed in a proper fashion [2] until usage.Electrolyzers normally use platinum-group metals (PGM) - which are noble metals, as eletrocatalysts for the electrolysis of water [3] and for maintaining the green concept involving the application of H2 as a low-carbon energy source. H2 is used in fuel cell systems [4] which also normally employ noble metals as electrocatalysts for the generation of clean energy [3]. The reactions involved in water electrolysis and fuel cell systems are generally hydrogen and oxygen evolutions (HER and OER), hydrogen oxidation (HOR), and oxygen reduction (ORR) reactions [3].Generally, Pt-based electrocatalysts are the benchmark electrocatalysts for ORR and HER processes, while RuO2 and IrO2 are regarded the ideal electrocatalysts for OER processes [5]. The major disadvantages regarding the use of these noble metals/oxides lie in their high costs and scarcity. In view of that, there has been an increasingly growing interest in the search for cheap Earth-abundant elements which are equally efficient for application as electrocatalysts in ORR, HER and OER processes in place of the aforementioned noble metals/oxides [5,6].In the search for cheap Earth-abundant elements, Bezerra and Maia [5] and Martini and Maia [6] found crystalline NiCo2O4 and CoMoSe/GNR (Co(OH)2\u2012CoMoO4\u2012MoSe2/GNR) as extremely stable and highly efficient when applied as electrocatalysts for OER.Other catalysts reported to have improved OER electrocatalytic responses include the following: MoOx formed on the surface of N-doped MoS2 (MoOx@N-doped MoS2\u2212\n\nx\n) [7]; one-dimensional CoS2\u2212MoS2 nano-flakes decorated MoO2 sub-micro-wires (CoMoOS) [8]; MoS2 quantum dots (MSQDs) [9]; metallic octahedral type molybdenum disulfide (1T MoS2) [10]; Fe-doped MoS2 nanosheets (Fe-MoS2 NSs) [11]; phosphorus incorporated cobalt molybdenum sulfide on carbon cloth (P-CoMoS/CC) [12]; MoSe2 nanosheet/MoO2 nanobelt/carbon nanotube membrane (MoSe2 NS/MoO2 NB/CNT-M) [13]; one-dimensional MoO2\u2013Co2Mo3O8@C nanorods [14]; porous NiMoO4\u2212\n\nx\n/MoO2 hybrids [15]; CeO2 shells on the surfaces of ZIF-67-derived porous N-doped Co3O4@Z67-NT (Co3O4@Z67-NT@CeO2, T\u00a0=\u00a0temperature) [16]; ultrathin Co3O4 nanomeshes (Co-UNMs) [17]; hierarchically structured Co3O4/NiCo2O4/Ni foam (CO/NCO/NF) composite [18]; defect-activated Co3O4 (DA-Co3O4) [19]; porous Co3O4/CoMoO4 nanocages [20]; Co3O4/MoS2 heterostructure [21]; crystal lattice distorted ultrathin cobalt hydroxide (CLD-u-Co(OH)2) nanosheets [22]; mixed NiO/NiCo2O4 nanocrystals [23]; and porous NiO/NiCo2O4 heterostructure [24].The central idea behind the development of the present work (the use of Co/Mo-based catalysts) was derived from our recently published work which reported the combination of graphene nanoribbons (GNRs), cobalt salt, and commercial MoSe2 for the synthesis of OER electrocatalysts; the combined application of these materials resulted in the construction of a suitable electrocatalyst containing Co and Mo oxides, some MoSe2, and GNR (CoMoSe/GNR electrocatalyst), which was highly efficient for OER [6]. The application of non-commercial MoS2 nanosheets and MoSe2 nanoribbons, in place of GNR, as supporting materials for the Co/Mo-based electrocatalysts (synthesized and studied in the present work) was found to be very useful. Clearly, the efficient performance of the Co/Mo-based electrocatalysts in OER helped confirm the suitability of the non-commercial MoS2 nanosheets and MoSe2 nanoribbons as supporting materials for the OER electrocatalysts proposed in this study.Taking these considerations into account, in the present work, the main idea was to use Ni and/or Co salts and urea in combination with MoSe2 and MoS2 to produce nanocomposites which are effectively capable of electrocatalyzing OER in alkaline solution through the application of hydrothermal and calcination methods. The physical characterizations of the nanocomposites revealed the following: i) CoMoO4 nanoparticle oxides are supported on MoSe2 nanoribbons for the NiCoMoSe nanocomposite; ii) NiCo2O4 (and CoMoO4) nanoparticle oxides are supported on MoSe2 nanoribbons for the NiCoMo nanocomposite; iii) Co2Mo3O8 (and CoMoO4) nanoparticle oxides are supported on MoSe2 nanoribbons for the CoMo nanocomposite; iv) CoMoO4, Co2Mo3O8, and Co3O4 nanoparticle oxides are supported on MoS2 nanosheets and MoSe2 nanoribbons for the CoMoSe nanocomposite. The factors that mainly contributed to the improvement of OER electrocatalysis for the four catalysts mentioned above can be summarized as follows: 1) N atoms, from the urea used in the synthesis, bonded to relatively high amount of Ni and/or Co (and Mo); 2) the electrons released (3 to 2 electrons) from the oxidation of Co from the 2+ to 3+ state (one electron released), Ni from the 2+ to 3+ state (one electron released), and Mo from the 4+ to 6+ state (two electron released) identified by the presence of Ni2+ and Ni3+, and/or Co2+ and Co3+, and Mo4+ and Mo6+ in the nanocomposites; and 3) the presence of MoS2 nanosheets and MoSe2 nanoribbons as supporting material for the metal oxides in the nanocomposites contributed to a reduction in both the ECSA values and the charge transfer resistance.The key advantages of the chosen method lie in the fact that it is a much simpler and straightforward technique which involves the production of efficient and relatively cheaper non-commercial supporting materials for OER catalysts compared to the tedious technique involving the production of GNRs. The proposed technique was used for the production of non-commercial MoS2 nanosheets and MoSe2 nanoribbons with markedly low charge transfer resistance (Rct), which were used as supporting materials in a relatively simple synthesis for the development of highly efficient and innovative low-cost catalysts (nanocomposites) for OER. Carbon paper was employed as electrode substrate due to its low cost, good conductivity, and relative stability in OER.All reagents used in this work were of analytical purity and were not subjected to any previous treatment before use. CoCl2\u20226H2O, MoSe2 (this compound was derived from two different sources; as such, it will be referred to as MoSe2\u20131), IrO2, and RuO2 were purchased from Sigma Aldrich. NiCl2\u20226H2O, KOH, and HNO3 were acquired from Vetec, and urea and H2SO4 were obtained from Neon. MoSe2 (MoSe2\u20132) and MoS2 were synthesized from stoichiometric mixtures of the elements, in evacuated and sealed ampoules (see below).All electrochemical measurements were performed in a three\u2012electrode system. A graphite plate was used as counter electrode, a reversible hydrogen electrode was used as reference electrode, and a carbon paper (CP) HCP030N (sheet of 1.0 cm2) or Au-disk/Pt-ring RRDE (0.196 and 0.11 cm2 geometric areas, respectively) with a collection efficiency of N\u00a0=\u00a00.26, according to the manufacturer's information, was employed as working electrode.Synthesis of MoS2: Molybdenum metal was annealed at 1000\u00a0\u00b0C in H2 stream prior to the synthesis of the materials. Molybdenum powder (3.258\u00a0g, 0.03395\u00a0mol) and crystalline sulfur (2.178\u00a0g, 0.06791\u00a0mol) were placed in a quartz ampoule, evacuated under dynamic vacuum, and sealed. The ampoule was heated in a muffle furnace up to 650\u00a0\u00b0C for 7\u00a0h, kept at this temperature for 72\u00a0h, and cooled thereafter in the furnace. The MoS2 product obtained was a black homogeneous powder.Synthesis of MoSe2\u20132: Molybdenum powder (1.936\u00a0g, 0.02018\u00a0mol) and selenium powder (3.183\u00a0g, 0.04031\u00a0mol) were placed in a quartz ampoule, evacuated under dynamic vacuum, and sealed. The ampoule was heated in a muffle furnace up to 850\u00a0\u00b0C for 9\u00a0h, kept at this temperature for 100\u00a0h, and cooled thereafter in the furnace. The MoSe2 product obtained consisted of a black homogeneous powder with metallic luster.To prepare CoMoSeS, 16\u00a0mg of MoSe2\u20131, 16\u00a0mg of MoS2, 100\u00a0mg of CoCl2\u20226H2O and 500\u00a0mg of urea were weighed and mixed together, and 30\u00a0mL of ultrapure water was added to the mixture. The mixture was subjected to ultrasonication for 20\u00a0min. After that, the mixture was transferred into a Teflon\u2012lined stainless\u2012steel autoclave and subjected to heating in a muffle furnace at a temperature of 180\u00a0\u00b0C for 24\u00a0h (Scheme\u00a01\n). After it was cooled to room temperature, the mixture was washed with ultrapure water by centrifugation several times and dried at 40\u00a0\u00b0C in an oven for 12 hTo prepare CoMo, an amount of CoMoSeS was placed inside a quartz boat, taken to a muffle furnace and heated at a temperature of 600\u00a0\u00b0C for 3\u00a0h (Scheme\u00a01). CoMo/AL was produced by adding 30\u00a0mL of a solution of 0.5\u00a0M HNO3:0.5\u00a0M H2SO4 to CoMo and keeping the mixture under magnetic stirring and heating (50\u00a0\u00b0C) for 8\u00a0h (Scheme\u00a01). Thereafter, the mixture was washed with ultrapure water by centrifugation until neutral pH was obtained. The mixture was then dried at 40\u00a0\u00b0C for 12\u00a0h in an oven.NiCoMoSe was prepared by hydrothermal method. Specifically, 16\u00a0mg of MoSe2\u20132, 100\u00a0mg of NiCl2\u20226H2O, 100\u00a0mg of CoCl2\u20226H2O, 500\u00a0mg of urea and 30\u00a0mL of deionized water were mixed and kept under ultrasonic bath for 20\u00a0min. The dispersion was transferred into a Teflon\u2012lined stainless\u2012steel autoclave and heated to 180\u00a0\u00b0C for 24\u00a0h (Scheme\u00a01). After cooling to room temperature naturally, the product was washed with ultrapure water several times by centrifugation, and finally dried in an oven at 40\u00a0\u00b0C for 12 hTo produce NiCoMo, a fraction of the prepared NiCoMoSe was calcined in a muffle furnace at 600 \u00baC for 3\u00a0h (Scheme\u00a01).The sequence of syntheses described above (Scheme\u00a01) is similar to the syntheses performed by Bezerra, Martini, and Maia (2020,2021) [5,6].The CP sheet was cleaned by sonication in deionized water several times. The Au disk electrode was polished with alumina (1 and 0.05 \u03bcm meshes) until a mirror-like surface was obtained. After that, the electrode was sonicated in ultrapure water, acetone, and then again in ultrapure water for 5\u00a0min in each solvent. The electrode was then subjected to 10 voltammetry cycles using a scan rate of 50\u00a0mV s\u00a0\u2212\u00a01 and potential range of 0.05 to 1.70\u00a0V in N2 saturated 0.5\u00a0M H2SO4. During this analysis, the solution was changed when needed [6].The modified working electrodes were prepared by dripping an aqueous solution of the catalysts (1\u00a0mg mL\u22121 concentration) on the surface of the electrodes; this produced a uniform thin film with a 150\u00a0\u00b5g cm\u20122 loading. After drying at room temperature, the modified electrodes were immersed in deionized water prior to being immersed in the electrolyte (1\u00a0M KOH aqueous solution), which was saturated with N2 (5.0 purity) or O2 (4.0 purity) - both gasses were acquired from White Martins.The procedure employed by Trotochaud et\u00a0al. [25] was used for the purification of Fe in order to test the interference of Fe from the KOH electrolyte (see details in the Supporting Information) in the OER responses.The morphology and distribution of the nanocomposites and nanoparticles were analyzed by TEM, STEM, and EDX using JEOL JEM 2200F, FEI TECNAI G\u00b2 F20 HRTEM, and JEOL JEM 2100 plus microscopes with electron beam at 200\u00a0kV. Non-electrochemically (i.e., as synthesized material) and electrochemically stabilized (denoted by es; i.e., material obtained after long-term electrochemical stability test) CoMoSeS, CoMo, NiCoMoSe, and NiCoMo nanocomposites were diluted in ultrapure water and applied by dripping on a 400-mesh copper grid from Ted Pella with ultrathin carbon films supported on lacey carbon film. The microstructure of the nanocomposites and nanoparticles was visually characterized by SEM using a JEOL JSM-6380LV scanning electron microscope operating at 20\u00a0kV [5]. Field-emission scanning electron microscopy (FESEM) imaging of the synthesized nanomaterials was performed using Gemini scanning electron microscope (Germany) operating at an accelerating voltage of 9-7\u00a0kV.X-ray diffraction (XRD) analysis was carried out in order to determine the crystalline structure of the samples using a Rigaku X-ray diffractometer (model: ULTIMA IV, Rigaku, Japan). The XRD equipment employed operated at a scanning rate of 3\u00b0 min\u22121 in 2\u03b8 ranging from 5 to 100\u00b0, with CuK\u03b1 X-ray radiation (\u03bb\u00a0=\u00a01.54056\u00a0\u00c5). The crystallite size, Dhkl, was calculated using the Scherrer equation Dhkl\u00a0=\u00a0K \u03bb / (Bhkl cos \u03b8), where K is the crystallite-shape factor (0.94), \u03bb is the wavelength of the X-rays, Bhkl is the width of the diffraction peak in radians, and \u03b8 is the Bragg angle [26,27].The composition of the samples and the chemical states of the elements were also investigated by X-ray photoelectron spectroscopy (XPS) using Omicron spectrometer assembled with hemispherical analyzer (SPHERA), a 400 Al K\u03b1 (1486.7\u00a0eV) X-ray source (DAR), a Thermo-Scientific ESCALAB Xi+ spectrometer with a monochromatic Al K\u03b1 X-ray source (1486.6\u00a0eV), and a spherical energy analyzer operating in the CAE (constant analyzer energy) mode using the electromagnetic lens mode. The CAEs employed for the survey spectra and high-resolution spectra were 100\u00a0eV and 50\u00a0eV, respectively. In the course of the analysis, the chamber was evacuated at 1\u00a0\u00d7\u00a010\u22128 mbar and the spectra were deconvoluted using a Voigt-type function with Gaussian (70%) and Lorentzian (30%) combinations [5].The thermogravimetric analyses (TG) of MoS2, MoSe2\u20131, MoSe2\u20132, CoMo, CoMoSeS, NiCoMoSe, and NiCoMo nanocomposites were performed using a Shimadzu TGA-50 thermogravimetric analyzer under a synthetic air gas (5.0 FID) with flow rate of 50\u00a0mL min\u22121, heating rate of 10\u00a0\u00b0C min\u22121, and in alumina crucibles.An AFP2 WaveDriver 20 bipotentiostat\u2012galvanostat (Pine Research Instrumentation) was used to perform the cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry (CA) experiments.A PGSTAT128N potentiostat-galvanostat (Autolab) equipped with FRA2.X module was used to perform the electrochemical impedance spectroscopy (EIS) experiments, running at open-circuit potential (OCP) in the frequency range of 10 mHz-100\u00a0kHz, with potential perturbation of 10\u00a0mV (rms).The LSV and CA curves normalized by geometric area or mass of catalysts or electrochemical active surface area (ECSA)[5,6,28\u201331] were corrected by manual 100% iR drop compensation \u201cRu (= uncompensated resistance) - imparted iR drop\u201d [5,6,32], verified through EIS high frequency intercept (the average Ru values in 1\u00a0M KOH was 4.5 \u03a9), based on the works of Maia et\u00a0al. (2020, 2021) and Anantharaj et\u00a0al. (2018) [5,6,32], using the following equation:\n\n(1)\n\n\n\niR\n\ndrop\n\nfree\n\n\n\nE\nOER\n\n=\n\nE\nRHE\n\n\u2212\n\nE\niR\n\n=\n\nE\nRHE\n\n\n\u2212\n\u2212\n\n\n(\n\n\nI\nmea\n\n\u00d7\n\nR\nu\n\n\n)\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\nwhere ERHE and Imea are measured potentials and currents, respectively.The ECSA values were calculated using the double-layer capacitance (Cdl) values, which were obtained through CV analysis. The CVs were obtained in a 0.1\u00a0V potential window without faradaic current response and centered on OCP. The CV analyses performed in different potential scan rates were initiated from more positive potential to negative potential and the potential was held steady for 10\u00a0s at each potential extremity. Thus, the Cdl was determined by the following relation [5,6,32]:\n\n(2)\n\n\n\nC\ndl\n\n=\n\n(\n\n\n(\n\n\n\n\u0394\n\nI\n\n2\n\n)\n\n=\n\n(\n\n\n\nI\na\n\n\u2212\n\nI\nc\n\n\n2\n\n)\n\n\n)\n\n/\n\u03bd\n\n\n\nwhere Ia and Ic are the anodic and cathodic currents at OCP, respectively, and \u03bd is the potential scan rate. The ECSA value was obtained by dividing the Cdl by the specific capacitance (Cs) values, considering the Cs value as 0.040 mF cm\u22122 in KOH 1\u00a0M [33].\nFigs.\u00a01\n and S1 present the scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), electron diffraction pattern, annular bright-field scanning transmission electron microscopy (ABF-STEM), and energy-dispersive X-ray (EDX) mapping images of MoSe2\u20131, MoSe2\u20132, MoS2, NiCoMoSe, NiCoMo, CoMoSeS, and CoMo nanocomposites and nanoparticles.As can be observed in Figure S1, the MoSe2\u20131, MoSe2\u20132 and MoS2 nanocomposites exhibited sheet-shaped particles. However, Fig.\u00a01A and the inset of the figure show that the NiCoMoSe nanocomposite was constituted mainly by nanoribbons with extension of up to approximately 5\u00a0\u00b5m. The TEM images confirmed the presence of nanoribbons with width of 23\u00a0nm on average and length ranging from 0.1 to 1\u00a0\u00b5m (Figs.\u00a01B-C). The HR-TEM images (Figs.\u00a01D-E) showed the presence of dark spots on the nanoribbons, with lattice fringe of 0.22\u00a0nm which was assigned to (003) plane of CoMoO4\n[34] (Fig.\u00a01E). The inset of Fig.\u00a01E exhibited a crystalline electron diffraction pattern for the NiCoMoSe nanocomposite. The EDX elemental mapping analysis of the NiCoMoSe nanocomposites (Fig.\u00a01F) showed the presence of all the starting hydrothermal synthesis metals as well as Se and O under a seemingly uniform distribution with minor presence of Mo and smaller amount of Se (see Table S1).Although the surface morphology of NiCoMo (Figs.\u00a01G and inset) was found to be similar to that of NiCoMoSe in terms of the presence of ribbons, the nanoribbons in the NiCoMo nanocomposite were found to be covered with small flake-like nanoparticles, which may be linked to the fact that the sample has been subjected to high temperature thermal treatment. The TEM images in Figs.\u00a01H-I showed a noticeable alteration in the structure of the NiCoMo nanocomposite after calcination at 600 \u00baC compared to the NiCoMoSe nanocomposite (Figs.\u00a01B-C). The nanoribbons, which previously exhibited defined edges, showed a wrinkled, rounded shape with width of 24\u00a0nm on average and length ranging from 0.1 to 1\u00a0\u00b5m (Figs.\u00a01H-I); the diameter of the rounded shape was about 20\u00a0nm on average. The nanoparticles can be found to have been well-distributed on the smooth surface of the nanoribbons in the NiCoMo nanocomposite. The HR-TEM images in Figs.\u00a01J-K enabled us to identify the dark rounded nanoparticles distributed over the nanoribbon-shaped structures of the NiCoMo nanocomposite, with lattice fringe of 0.29\u00a0nm which was assigned to (220) plane of NiCo2O4 [35,36] (Fig.\u00a01K). The inset of Fig.\u00a01K exhibited a mixed crystalline electron diffraction pattern for the NiCoMo nanocomposite. The elements in the NiCoMo nanocomposite (Fig.\u00a01L) can be found to be uniformly distributed with minor presence of Ni and smaller amount of Se (see Table S1).As can be seen in Fig.\u00a01M, the SEM images of CoMoSeS showed the presence of structures composed of nanoribbons and seemingly rounded nanoparticles with size of approximately 0.1\u00a0\u2212\u00a00.2\u00a0\u00b5m. The images in Figs.\u00a01NO showed the presence of nanoribbons with width of 49\u00a0nm and length of around 1\u00a0\u00b5m as well as rounded nanoparticles with a diameter of 53\u00a0nm, both supported on nanosheets with length of 600\u00a0nm and width of 200\u00a0nm on average. The HR-TEM image displayed lattice fringe of 1.07\u00a0nm (Figure S2) for the nanosheets (Figs.\u00a01P and S2) which was assigned to (002) plane of MoS2\n[37] and the rounded nanoparticles exhibited lattice fringe of 0.49\u00a0nm which was associated with (002) plane of Co2Mo3O8 [14,38] (Fig.\u00a01Q). The inset of Fig.\u00a01Q exhibited a mixed crystalline electron diffraction pattern for the CoMoSeS nanocomposite. The elements on the CoMoSeS nanocomposite can be found to be uniformly distributed with smaller amount of Se, though one can find some agglomeration of Mo and S in some regions of the mapping image (highlighted in Fig.\u00a01R) (see Table S1).As can be observed in Fig.\u00a01S, the SEM images of CoMo showed the presence of structures composed of nanoribbons and seemingly rounded nanoparticles with size ranging from approximately 0.2\u00a0\u2212\u00a00.5\u00a0\u00b5m. The TEM images in Figs.\u00a01T-U showed the presence of nanoribbons with width of 90\u00a0nm and length of approximately 557\u00a0nm, as well as rounded nanoparticles with diameter of 56\u00a0nm on average. The increase in width of the nanoribbons in the CoMo nanocomposites can be attributed to the wrinkling of the nanosheets present in the CoMoSeS nanocomposites prior to calcination at 600 \u00baC; similarly, the increase in length of the nanoribbons in the CoMo nanocomposites can be attributed to the wrinkling of the nanoribbons present in the CoMoSeS nanocomposites prior to calcination at 600 \u00baC. The HR-TEM images exhibited lattice fringes of 0.26\u00a0nm, 0.15\u00a0nm, 0.49\u00a0nm, and 0.18\u00a0nm which were related to the nanoparticles and corresponded to (\u2212222) and (\u2212424) planes of CoMoO4\n[39], (002) plane of Co2Mo3O8 [14,38], and (331) plane of Co3O4 [40,41], respectively (Figs.\u00a01V-W). The junction of different crystal planes revealed different angles; these included the following: crystal planes of CoMoO4 (\u2212222) and Co2Mo3O8 (002) with junction angle of 147.8\u00b0; crystal planes of Co3O4 (331) and CoMoO4 (\u2212222) with junction angle of 140.7\u00b0; crystal planes of CoMoO4 (\u2212424) and Co3O4 (331) with junction angle of 145.5\u00b0. The following angle was recorded for the junction of the same crystal planes: crystal planes of Co3O4 (331) and Co3O4 (331) with junction angle of 103.6\u00b0 (Figs.\u00a01V-W). The inset of Fig.\u00a01V exhibited a mixed crystalline electron diffraction pattern for the CoMo nanocomposite. The elements in the CoMo nanocomposite can be found to be uniformly distributed with minor presence of Se (Fig.\u00a01X) (see Table S1).\nFig.\u00a02\n shows the TEM, HR-TEM, electron diffraction pattern, ABF-STEM, and EDX mapping images of NiCoMo-es, CoMoSeS-es, and CoMo-es nanocomposites and nanoparticles.The TEM images of the NiCoMo-es (Fig.\u00a02A and inset) and NiCoMo nanocomposites (Figs.\u00a01H-I) exhibited similar structures (Figs.\u00a01H-I). The nanoribbons of the NiCoMo-es nanocomposite were characterized by a wrinkled, close rounded shape with width of 24\u00a0nm on average and length of 0.2\u00a0\u00b5m (Fig.\u00a02A); the diameter of the rounded shape of the nanoribbons was 22\u00a0nm on average. The nanoparticles were found to be well distributed on the smooth surface of the nanoribbons of the NiCoMo-es nanocomposite. The HR-TEM images (Figs.\u00a02B-C) enabled us to identify the dark rounded nanoparticles distributed over the nanoribbon-shaped structures of the NiCoMo-es nanocomposite, with lattice fringes of 0.19, 0.22, 0.26, and 0.39\u00a0nm which were assigned to (421), (003), (\u2212222), and (021) planes of CoMoO4 [34,39] (Figs.\u00a02B-C). The junction of the different crystal planes showed the existence of a 115\u00b0 angle between the CoMoO4 (003) and CoMoO4 (021) planes (Fig.\u00a02B). In addition, a great defect was observed between at least two different crystal planes of CoMoO4 (see the red dashed line in Fig.\u00a02C). The inset of Fig.\u00a02C exhibited a mixed crystalline electron diffraction pattern for the NiCoMo-es nanocomposite. The elements in the NiCoMo-es nanocomposite (Fig.\u00a02D) were also found to be uniformly distributed with the presence of smaller amount of Mo. The identification of Ni (Fig.\u00a02D) in the NiCoMo-es nanocomposite indicated the presence of Ni nanoparticles in the nanocomposite even though we were unable to identify lattice fringes for NiCoO4 nanoparticles as observed for the NiCoMo nanocomposite (Fig.\u00a01K). Also, the fact that Se was not identified in the NiCoMo\u2013es (Fig.\u00a02D) points to the corrosion of this element after the OER long term stability test.The TEM images of CoMoSeS-es (Figs.\u00a02E and inset) showed nanoribbons with width of 7\u00a0nm and length of around 59\u00a0nm as well as rounded nanoparticles with a diameter of 100\u00a0nm on average; both the nanoribbons and nanoparticles were supported on nanosheets (Figs.\u00a02E and inset). The TEM images of the CoMoSeS-es nanocomposite were found to be similar to the TEM images of the CoMoSeS nanocomposite (Figs.\u00a01NO) even under different dimensions; this may be attributed to the fact that different regions of the nanocomposite samples were selected for analysis. The HRTEM images exhibited lattice fringes of 0.23\u00a0nm which were related to the nanosheets and corresponded to (103) plane of MoS2 [42,43] (Figs.\u00a02F-G), while the nanoparticles exhibited lattice fringes of 0.26 and 0.49\u00a0nm which were associated with (\u2212222) plane of CoMoO4\n[39] and (002) plane of Co2Mo3O8 [14,38], respectively (Figs.\u00a02F-G). The junction of the different crystal planes revealed the presence of a 90\u00b0 angle between the MoS2 (103) and CoMoO4 (\u2212222) planes (Fig.\u00a02F), a 124\u00b0 angle between the CoMoO4 (\u2212222) and CoMoO4 (\u2212222) planes (Fig.\u00a02F), and a 93\u00b0 angle between the MoS2 (103) and MoS2 (103) planes (Fig.\u00a02G). In addition, the lattice fringes observed were assigned to MoS2 (nanosheets) and Co2Mo3O8 (002) planes in the CoMoSeS nanocomposite (Figs.\u00a01P-Q). The inset of Fig.\u00a02G exhibited a mixed crystalline electron diffraction pattern for the CoMoSeS-es nanocomposite. There was a uniform distribution of elements in the CoMoSeS-es nanocomposite with minor presence of Se and smaller amount of S (Fig.\u00a02H); the distribution of elements in the CoMoSeS-es nanocomposite was found to be very similar to that observed in the CoMoSeS nanocomposite (Fig.\u00a01R).The TEM images of CoMo-es (Figs.\u00a02I and inset) showed the presence of nanoribbons with width of 49\u00a0nm and length of approximately 239\u00a0nm, as well as rounded nanoparticles with diameter of 80\u00a0nm on average. The TEM images recorded for the CoMo-es nanocomposite were found to be similar to those of the CoMo nanocomposite (Figs.\u00a01T-U) even under different dimensions; this may be linked to the fact that different regions of the nanocomposite samples were selected for analysis. The HRTEM images exhibited lattice fringes of 0.26, 0.49, and 0.18 and 0.23\u00a0nm for the nanoparticles which were assigned to (\u2212222) plane of CoMoO4\n[39], (002) plane of Co2Mo3O8 [14,38], and (331) and (220) planes of Co3O4 [40,41,44], respectively (Figs.\u00a02J-K). The junction of the different crystal planes revealed the presence of different angles between the nanocomposites: 130\u00b0 angle between Co2Mo3O8 (002) and Co3O4 (220); 83.4\u00b0 angle between Co3O4 (220) and CoMoO4 (\u2212222); 53.2\u00b0 angle between CoMoO4 (\u2212222) and Co3O4 (331); and an angle of 88\u00b0 between the same crystal planes of Co3O4 (331) and Co3O4 (331) (Figs.\u00a02J-K). Although similar lattice fringe distances were identified for the different crystals, these lattice fringes resulted in different angles (this was probably related to the fact that a different region of the nanocomposite sample was selected for analysis) between the junction of different crystal planes for the CoMo nanocomposite in comparison with the CoMo-es nanocomposite; furthermore, different angles were also observed for the junction of similar crystal planes for the CoMo nanocomposite in comparison with the CoMo-es nanocomposite (compare Figs.\u00a01V-W with 2J-K). The inset of Fig.\u00a02K exhibited a mixed crystalline electron diffraction pattern for the CoMo-es nanocomposite. The elements in the CoMo-es nanocomposite were found to be uniformly distributed with minor presence of Se and smaller amount of S (Fig.\u00a02L), as observed in the CoMo nanocomposite (Fig.\u00a01X).The nanocomposites crystalline structures were investigated by XRD analysis (Figure S3), and the results obtained showed that all the samples were characterized by a mixed composition. The XRD diffraction pattern for the NiCoMoSe nanocomposite (Figure S3) exhibited peaks at 13.6\u00ba, 37.8\u00ba, and 47.3\u00ba which corresponded to the (002), (103), and (105) planes of MoSe2 (JCPDS card 029\u20130914), respectively. The peak at 17.3\u00ba was assigned to the (002) plane of Co2Mo3O8 (JCPDS card 034\u20130511). The peaks at 26.7\u00ba; 33.8\u00ba, 39.7\u00ba, and 59.9\u00ba were associated with the (002), (\u2212222), (003) and (\u2212352) planes of CoMoO4 (JCPDS card 021\u20130868) and the peak at 62.6\u00ba corresponded to the (220) plane of NiO (JCPDS card 047\u20131049). Based on the diffraction peaks of the nanocomposite, the crystallite size of CoMoO4 was 25.1\u00a0nm on average and the lattice fringes assigned to the (003) plane of CoMoO4 was identified in Fig.\u00a01E.Compared to the NiCoMoSe nanocomposite, the NiCoMo nanocomposite exhibited a different diffraction pattern (Figure S3), which resulted from the modification of the crystallinity of the material after calcination at 600 \u00baC (Scheme\u00a01). The peaks at 18.8\u00ba, 31.1\u00ba, 36.9\u00ba, 44.8\u00ba, 59.2\u00ba, and 65.0\u00ba corresponded to the (111), (220), (311), (400), (551), and (440) planes of NiCo2O4 (JCPDS card 020\u20130781). The diffraction peaks at 26.4\u00ba, 40.2\u00ba, 43.3\u00ba, and 62.6\u00ba were linked to the (002) and (003) planes of CoMoO4 (JCPDS card 021\u20130868), and the (200) and (220) planes of NiO, respectively (JCPDS card 047\u20131049). Based on the diffraction peaks of the nanocomposite, the crystallite size of NiCo2O4 was 17.4\u00a0nm on average and the lattice fringes assigned to the (220) plane of NiCo2O4 was identified in Fig.\u00a01K.The XRD pattern of the CoMoSeS nanocomposite (Figure S3) indicated the presence of (002), (103), (006) and (105) planes of MoS2 at the following peaks: 14.4\u00b0, 39.5\u00b0, and 44.3\u00b0 and 49.9\u00b0, respectively (JCPDS card 006\u20130097). The peak at 17.2\u00b0 was correlated to the (002) plane of Co2Mo3O8 (JCPDS card 034\u20130511). The peaks at 26.5\u00b0, 33.7\u00b0, 47.1\u00b0, and 60.4\u00b0 were linked to the (002), (\u2212222), (241), and (\u2212424) planes of CoMoO4 (JCPDS card 021\u20130868). Furthermore, the peak at 32.6\u00b0 was correlated to (100) plane of Co(OH)2 (JCPDF card 074\u20131057). Based on the diffraction peaks of the nanocomposite, the crystallite size of MoS2 was 54.5\u00a0nm on average and the lattice fringes assigned to (002) planes of MoS2 and Co2Mo3O8 were identified in Figures S2 and 1Q, respectively.The change in crystalline structure between the NiCoMoSe and NiCoMo nanocomposites was evidently clear, with the disappearance of the peaks attributed to MoSe2, Co2Mo3O4, and CoSe2 and the presence of the peaks related to NiCo2O4 phase in the NiCoMo nanocomposite. This change was also confirmed by the results obtained from TG analysis (Figure S4) once the NiCoMo nanocomposite was found to present negligible mass loss while the NiCoMoSe nanocomposite exhibited a mass loss starting effectively at 230\u00a0\u00b0C, reaching a mass loss close to 26% at 600\u00a0\u00b0C, and finally recording 31% of mass loss at 900\u00a0\u00b0C (Figure S4).The second mass loss for the NiCoMoSe nanocomposite recorded between 531 and 629\u00a0\u00b0C coincided approximately with the first mass loss for MoSe2\u20132 (551\u2013632\u00a0\u00b0C). MoSe2\u20132 presented additional (second) mass loss effectively after 776\u00a0\u00b0C and finally recorded close to 100% of mass loss at 900\u00a0\u00b0C (Figure S4).The MoSe2\u20131 (commercial sample) sample presented its first mass loss between 381\u2013463\u00a0\u00b0C; this was around 170\u00a0\u00b0C lower compared to the first mass loss observed for MoSe2\u20132. The difference in temperature was associated with the different structures of the two MoSe2 materials identified through the SEM images (Figure S1). However, the additional (second) mass loss observed for MoSe2\u20131 was exactly equal to that of MoSe2\u20132, and the former recorded a mass loss of approximately 100% at 900\u00a0\u00b0C (Figure S4).The MoS2 sample exhibited its first mass loss between 518\u2013629\u00a0\u00b0C, followed by an additional (second) mass loss effectively after 767\u00a0\u00b0C, and finally recording a mass loss of approximately 80% at 900\u00a0\u00b0C (Figure S4).The first mass losses observed for MoSe2\u20131 and MoSe2\u20132 were related to the burning of Se, which was gasified into SeO2 [6,45,46] (Figure S4 and Table S1). The pyrolysis of MoSe2 followed the reaction 2 MoSe2\u00a0+\u00a07 O2\u2192 2 MoO3\u00a0+\u00a04 SeO2; as the temperature reached the sublimation temperature of SeO2, SeO2 underwent volatilization [6,46] (see Figure S4). MoO3 sublimed [6,47] effectively after 776\u00a0\u00b0C in both samples (see Figure S4).The first mass loss observed in MoS2 was related to the burning of S, which was gasified into SO2\n[48] (Figure S4 and Table S1). The pyrolysis of MoS2 followed the reaction 2 MoS2\u00a0+\u00a07 O2\u2192 2 MoO3\u00a0+\u00a04 SO2; as the temperature reached the sublimation temperature of SO2, SO2 underwent volatilization [48] (see Figure S4). MoO3 sublimed [6,47] effectively in the same temperature in both the MoSe2\u20131 and MoSe2\u20132 samples (see Figure S4).For the NiCoMoSe nanocomposite, the first mass loss was related to the partial burning of Se, which was gasified into SeO2 [6,45,46] (Figure S4 and Table S1) and the second mass loss was related to the partial sublimation of MoO3 [6,47]. The occurrence of gasification at much lower temperatures can be attributed to the presence of additional metals (Ni and Co) in the structure of the NiCoMoSe nanocomposite, which facilitated the burning of the nanocomposite in comparison with the pure sample of MoSe2\u20131 and MoSe2\u20132 which did not contain these metals.For the NiCoMoSe nanocomposite, the wt.% for Co, Ni, Se, and Mo was 29.8%, 27.4%, 5.7%, and 1.8%, respectively (Table S1). For the NiCoMo nanocomposite, the wt.% recorded for Co, Ni, Se, and Mo was 33.5%, 32.0%, 0.1%, and 6.0%, respectively (Table S1). The burning (Scheme\u00a01) of the NiCoMoSe nanocomposite enriched the Co, Ni, and Mo compositions in the NiCoMo nanocomposite (Table S1). In addition, these composition responses (or changes in composition) (Table S1) were found to be in relative consonance with the EDX mapping images of the NiCoMoSe and NiCoMo nanocomposites (Figs.\u00a01F and 1L).For the CoMoSeS nanocomposite, the first mass loss was related to the partial burning of Se, which was gasified into SeO2 [6,45,46] (Figure S4 and Table S1). The gain in mass observed between 534\u2013624\u00a0\u00b0C can be attributed to the production of MoO3 and SO2 which remain in the structure of the CoMoSeS nanocomposite as a result of the pyrolysis of MoS2 - based on the reaction 2 MoS2\u00a0+\u00a07 O2\u2192 2 MoO3\u00a0+\u00a04 SO2\n[48]; MoO3 and SO2 are released/gasified at temperatures greater than 624 up to 748\u00a0\u00b0C, reaching close to 27% of mass loss at 900\u00a0\u00b0C (see Figure S4). The CoMo nanocomposite presented negligible mass loss (see Figures S4). For the CoMoSeS nanocomposite, the wt.% recorded for S, Co, Se, and Mo was 4.1%, 49.3%, 3.8%, and 6.0%, respectively (Table S1). For the CoMo nanocomposite, the wt.% recorded for S, Co, Se, and Mo was 0.4%, 45.5%, 0.4%, and 13.8%, respectively (Table S1). The burning (Scheme\u00a01) of the CoMoSeS nanocomposite enriched the Mo composition in the CoMo nanocomposite (Table S1). In addition, the composition responses (Table S1) were found to be in line with the EDX mapping images of the CoMoSeS and CoMo nanocomposites (Figs.\u00a01R and 1X).\nFigs.\u00a03\n and S5\u20136 present the XPS survey spectra as well as the high-resolution XPS (HR-XPS) spectra for CoMoSeS, CoMo, NiCoMoSe, and NiCoMo, and for the CoMoSeS-es, CoMo-es, and NiCoMo-es nanocomposites.Looking at the survey spectra in Figure S5, one can observe the presence of the peaks (on average) at 54, 64, 106, 231, 284, 397, 531, 643, 714, 780, 855, and 927\u00a0eV related to Se 3d, Co 3p, Co 3\u00a0s, Mo 3d, C 1\u00a0s, N 1\u00a0s, O 1\u00a0s, Ni LMN, Co LMN, Co 2p, Ni 2p, and Co 2\u00a0s respectively, for the NiCoMoSe and NiCoMo, and NiCoMo-es nanocomposites. For the NiCoMo-es nanocomposite, the Se 3d peak was unable to be identified. For the CoMoSeS nanocomposite survey spectrum (Figure S5), the peaks corresponding to Se 3d, Ni LMN, and Ni 2p were unable to be identified, and apart from the existing peaks in the survey spectrum, there is an additional peak around 163\u00a0eV, which corresponds to S 2p. For the CoMoSeS-es nanocomposite, the peaks related to S 2p, Mo 3d, and N 1\u00a0s were unable to be identified. The XPS survey spectra of the CoMo nanocomposites exhibited peaks related to Co 3p, Co 3\u00a0s, Mo 3d, C 1\u00a0s, N 1\u00a0s, O 1\u00a0s, Co LMM, Co 2p, and Co 2\u00a0s at approximately 63, 104, 232, 284, 400, 532, 716, 782, and 930\u00a0eV, respectively. The C 1\u00a0s peak at 284\u00a0eV present in the samples was associated with the CP used as supporting material for the samples.For the NiCoMoSe and NiCoMo nanocomposites, the atomic contents recorded for Co and Ni were approximately 10% each (atomic content ratio close to 1:1; see Table S2). The atomic content of N was about 12% (derived from the urea used in the synthesis, Scheme\u00a01), while the atomic content of Se was about 0.9% (see Table S2). A comparison between the NiCoMo nanocomposite and the NiCoMoSe nanocomposite showed that the atomic content of Mo in the former was 8.4 times higher than that observed in the latter and the atomic content of O in the former was 7% lower than that observed in the latter (see Table S2); the lower O atomic content observed in the NiCoMo nanocomposite can be attributed to the fact that this nanocomposite was obtained from the burning of the NiCoMoSe nanocomposite at 600\u00a0\u00b0C (Scheme\u00a01 and Figure S4).With regard to the NiCoMo-es nanocomposite, the atomic contents recorded for Co and Ni were significantly lower - around half and a quarter, respectively, compared to the Co and Ni atomic contents recorded in the NiCoMo nanocomposite (see Table S2); this implies strong oxidation and loss (corrosion) of Co and Ni from the surface of the NiCoMo-es nanocomposite during OER. The occurrence of strong oxidation is corroborated by the increase in atomic content of O observed in the NiCoMo-es nanocomposite compared to the NiCoMo nanocomposite (see Table S2). In addition, it is clear that the following factors point to the oxidation/corrosion of the NiCoMo nanocomposite when subjected to electrochemical stabilization: i) the non-detection of Se, ii) the decrease of 18% in Mo atomic content; and iii) the decrease of approximately 30% in atomic content of N (see Table S2).With regard to the CoMoSeS nanocomposite, the atomic content recorded for Co was 20%. The atomic content of N was 5.7% (derived from the urea used in the synthesis, Scheme\u00a01), while the atomic contents recorded for S, Mo, and O were 3.5%, 1.3%, and 69.3%, respectively (see Table S2). The atomic contents of O and Co in the CoMoSeS nanocomposite were close to the combined atomic contents of O, Co and Ni in the NiCoMoSe nanocomposite (see Table S2). A comparison of the CoMoSeS and NiCoMoSe nanocomposites showed that the atomic content of Mo in the former was 2.3 times higher than that of the latter, and the atomic content of S in the former (CoMoSeS) was 4.2 times higher than the atomic content of Se in the latter (NiCoMoSe) (see Table S2).For the CoMoSeS-es nanocomposite, the atomic content recorded for Co was 18%; this value was very close to that observed in the CoMoSeS nanocomposite (see Table S2). However, considering that there was a significant increase in the atomic content of O (\u223c82%) and S, Mo, and N were unable to be detected, this shows that the CoMoSeS nanocomposite underwent corrosion when it was electrochemically stabilized (see Table S2).With regard to the CoMo nanocomposite, the atomic content of Co was half the value recorded for the content of the element in the CoMo-es nanocomposite (the atomic content of Co was 25.6%, which was slightly higher than the Co atomic content recorded in the CoMoSeS nanocomposite; see Table S2). This can be attributed to the following: i) significant reduction of the atomic content of N (to around a quarter of the value recorded before electrochemical stabilization) in the CoMo nanocomposite after electrochemical stabilization (the atomic content of Mo is also reduced to around one third of the value), and ii) a 10% increase in the atomic content of O after electrochemical stabilization (see Table S2); in essence, this suggests the occurrence of oxidation/corrosion of N and Mo in the CoMo nanocomposite after electrochemical stabilization.The Ni 2p high resolution HR-XPS spectra for the NiCoMoSe and NiCoMo, and NiCoMo-es nanocomposites exhibited two pairs of peaks which were linked to Ni 2p3/2 and Ni 2p1/2 levels, with a content ratio of about 2.2:1 [5,49], and two other peaks which were related to satellites peaks [5,49] (Fig.\u00a03). The deconvoluted peaks were attributed to the following: Ni2+ 2p3/2 and Ni2+ 2p1/2 at approximately 854.5 and 872.1\u00a0eV; Ni3+ 2p3/2 and Ni3+ 2p1/2 at approximately 856.2 and 874.2\u00a0eV; and their respective satellites peaks at 860.3, 878.0, 862.9, and 881\u00a0eV [5,49] (Table S3). The presence of Ni3+ and Ni2+ species on the nanocomposite surfaces was confirmed by the spin-orbital splitting observed at around 18\u00a0eV between the peaks, in addition to the presence of satellites peaks [5,49] (Fig.\u00a03 and Table S3). With regard to the NiCoMoSe nanocomposite, the content percentages of Ni2+ and Ni3+ were approximately 25.1 and 24.4%, respectively (Table S3). For the NiCoMo nanocomposite, the content percentages of Ni2+ and Ni3+ were 19.9 and 31.3%, respectively, while the content percentages of Ni2+ and Ni3+ in the NiCoMo-es nanocomposite were 32.6 and 21.2%, respectively (Table S3). Interestingly, the content percentages of Ni2+ and Ni3+ appeared to have been inverted in terms of quantity after the NiCoMo nanocomposite was electrochemically stabilized (Table S3). The CoMoSeS and CoMo nanocomposites did not present HR-XPS signal for Ni.The Co 2p HR-XPS spectra exhibited two peaks corresponding to Co 2p3/2 and Co 2p1/2 levels with a ratio of about 2.0:1, in addition to the respective satellite peaks for the CoMoSeS, CoMo, NiCoMoSe, and NiCoMo, as well as for the CoMoSeS-es, CoMo-es, and NiCoMo-es nanocomposites (Fig.\u00a03). The deconvoluted peaks were attributed to the following: Co3+2p3/2 and Co3+2p1/2 at approximately 780 and 794.1\u00a0eV; Co2+2p3/2 and Co2+2p1/2 at approximately 782 and 798\u00a0eV; and their respective satellites peaks at 782.9, 802, 787, and 804.6\u00a0eV [5,6,50] (Table S3). The presence of Co3+ and Co2+ species on the nanocomposites surfaces was further confirmed by the spin-orbital splitting occurring at around 18.8\u00a0eV between the peaks, in addition to the presence of the satellites peaks [5,6,50] (Fig.\u00a03 and Table S3). The content percentages recorded for Co3+ and Co2+ in the CoMoSeS nanocomposite were 22.7 and 52.4% respectively. The content percentages of Co3+ and Co2+ recorded in the CoMo nanocomposite were 41.8 and 37.5% respectively. With regard to the NiCoMoSe nanocomposite, the content percentages recorded for Co3+ and Co2+ were 31.3 and 22% respectively. The content percentages recorded in the NiCoMo nanocomposite for Co3+ and Co2+ were 29.5 and 38.1%, respectively (Table S3). The content percentages recorded for Co3+ and Co2+ in the CoMoSeS-es nanocomposite were 22.8 and 51.5%, respectively. With regard to the CoMo-es nanocomposite, the content percentages of Co3+ and Co2+ recorded in the sample were 48 and 33.4%, respectively. The content percentages of Co3+ and Co2+recorded in the NiCoMo-es nanocomposite were 46.2 and 36.6%, respectively (Table S3).The Mo 3d HR-XPS spectra exhibited nearly 2\u20134 peaks mostly in the 3d5/2 and 3d3/2 regions for the CoMoSeS, CoMo, NiCoMoSe, and NiCoMo nanocomposites, as well as for the CoMo-es and NiCoMo-es nanocomposites (Fig.\u00a03). The deconvolution of Mo 3d HR-XPS spectra resulted in four peaks which were attributed to Mo4+3d5/2 and 3d3/2 species at approximately 231 and 233\u00a0eV, respectively, and Mo6+ 3d5/2 and 3d3/2 species at approximately 234 and 235\u00a0eV [6,38,51], respectively (Fig.\u00a03 and Table S3). The CoMoSeS nanocomposite exhibited an additional shoulder at 226\u00a0eV (Fig.\u00a03) which was attributed to S 2\u00a0s [52,53]. The deconvoluted peaks corresponding to Mo4+ species at lower binding energies which were attributed to the NiCoMoSe nanocomposite were no long observed in the Mo 3d HR-XPS spectrum of the NiCoMo nanocomposites (Fig.\u00a03); this shows that the molybdenum species on the surface of these nanocomposites were oxidized to the 6+ state \u2013 with content percentage of 100% (see Table S3) \u2013 after calcination at 600\u00a0\u00b0C (Scheme\u00a01). The content percentages of Mo4+ and Mo6+ species recorded in the CoMoSeS nanocomposite were approximately 52 and 48%, respectively (Table S3). The content percentages of Mo4+ and Mo6+ species recorded in the CoMo nanocomposite were 60.5 and 39.5%, respectively (Table S3). The content percentages of Mo4+ and Mo6+ species recorded in the NiCoMoSe nanocomposite were 57.1 and 42.9%, respectively (Table S3). The content percentages of Mo4+ and Mo6+ species recorded in the CoMo-es nanocomposite were 51.2 and 48.8%, respectively (Table S3).The S 2p HR-XPS spectrum related to the CoMoSeS nanocomposite exhibited two peaks in the 2p3/2 and 2p1/2 regions [53,54] and a third peak in the sulfate region [52,54] (Fig.\u00a03); this was the only nanocomposite that presented XPS signal for S. The S 2p HR-XPS spectrum was deconvoluted into the following peaks: S2\u2212 2p3/2 and 2p1/2; S2\n2\u2212 2p3/2 and 2p1/2; and sulfate peaks at 162.8 and 164.2; 165.7 and 166.9; and 171.5\u00a0eV, respectively [10,52,54\u201356] (see Table S3). The content percentages recorded for S2\u2212 and S2\n2\u2212 were found to be equal to 42.4% (Table S3).The Se 3d HR-XPS spectrum related to the NiCoMoSe and NiCoMo nanocomposites exhibited one peak and a shoulder (Fig.\u00a03), which were deconvoluted into Se 3d5/2 and Se 3d3/2 levels [13,57] at 53.9 and 54.8\u00a0eV, respectively (see Table S3). The content percentages of Se 3d5/2 level recorded in the NiCoMoSe and NiCoMo nanocomposites were 78.4 and 65.1%, respectively (Table S3).The O 1\u00a0s HR-XPS spectra exhibited a broad peak and a shoulder for the CoMoSeS, CoMoSeS-es, CoMo, CoMo-es, NiCoMoSe, and NiCoMo nanocomposites, and two peaks for the NiCoMo-es nanocomposite (Figure S6); these were deconvoluted into four peaks centered around 530, 531, 532.5, and 534\u00a0eV (Table S3), which were assigned to metal oxides, metal hydroxides, oxygen atoms located at defect sites, and adsorbed water molecules, respectively [13,15,58]. The average content percentages recorded were as follows: M-O: 34%, OH: 43.6%, and O defect sites: 16.7% (Tables S3). These results were in total agreement with the results obtained from the XRD analysis (Figure S3).The N 1\u00a0s HR-XPS spectra exhibited a larger broad peak and a shoulder for the CoMoSeS nanocomposite and a broad peak for the CoMo, NiCoMoSe, and NiCoMo nanocomposites (Figure S6). The CoMo-es and NiCoMo-es nanocomposites (Figure S6) exhibited two broad peaks, which were deconvoluted into three or four peaks for different N species coexisting on the surfaces of the nanocomposites; these peaks were centered around the following: i) at 394.7, 399.3, and 402.7\u00a0eV which corresponded to N-Mo, NCo, and NH for the CoMoSeS nanocomposite; ii) at 396.5, 399.1, and 404.5\u00a0eV which corresponded to N-Mo, NCo, and NH for the CoMo nanocomposite; and iii) at 396, 397.7, 398.7, and 400.5\u00a0eV which corresponded to N-Mo, NNi, NCo, and NH, respectively, for the NiCoMoSe, NiCoMo and NiCoMo-es nanocomposites surfaces (Figure S6) [59\u201361]. The content percentages of NCo recorded in the CoMoSeS and CoMo, and CoMo-es nanocomposites were 57.6% on average, while the content percentages of NNi and NCo combined recorded in the NiCoMoSe and NiCoMo and NiCoMo-es nanocomposites were 84% on average (Tables S3); these results show that N derived from urea is bonded to the metals in the nanocomposites structures.The CV profiles obtained for the bare CP and modified CP electrodes are shown in Figs.\u00a04\nA and S7A.The CV profiles obtained for the bare and modified electrodes (Figs.\u00a04A and S7A) exhibited mostly small capacitive current densities, as observed by Bezerra and Maia [5]. However, for the IrO2/CP electrode (inset of Fig.\u00a04A), the responses of the current densities were found to be high and were similar to the result obtained by Souza et\u00a0al. [62]. Similarly, the responses of the current densities recorded for the RuO2/CP electrode (Figure S7A) were found to be similar to the result obtained by Martini and Maia [6]. For the CoMoSeS/CP electrode (Fig.\u00a04A), the values of the current densities obtained were found to be similar to those observed for the CoMoSe/GNR/CP electrode [6], though with different CV shapes. The increased current densities observed for the modified CP electrodes shown in Figs.\u00a04A and S7A were attributed to the different active sites present in these electrodes.The LSV curves obtained for the bare CP and modified CP electrodes are shown in Figs.\u00a04B and S7B. Considering the overpotential required to achieve a current density of 10\u00a0mA cm\u22122, which corresponds to a solar-to-fuel device operating at 10% efficiency illuminated under 1 sun [63,64], one can say that the best OER electrocatalysts obtained in the present study were IrO2, NiCoMo, CoMoSeS, CoMo, and NiCoMoSe nanocomposites; these catalysts exhibited overpotentials (\u03b7\nj at 10\u00a0mA cm\u20122) of 280, 356, 375, 375, and 390, respectively, which are superior to the overpotential of the commercial RuO2 (Table S4). The low \u03b7 value recorded for the IrO2 catalyst was expected since it is a benchmark catalyst for OER [5] and its low \u03b7 is attributed to its high ECSA value (378.8 cm2, see Table S4); however, the stability of the catalyst was found to be very poor, as will be proved below. The low \u03b7 value recorded for the NiCoMo nanocomposite was found to be close to the values reported in the literature (Table S5).The mass-specific current density [5,6,28\u201332,64] obtained at \u03b7\nj of 10\u00a0mA cm\u20122 was 68.5 A g\u22121 (Fig.\u00a04B); this value was close to the value reported by Bezerra and Maia [5]. At 10.0 A g\u22121, the NiCoMo/CP modified electrode recorded the lowest overpotential of 260\u00a0mV (Fig.\u00a04B); this value was close to the values reported by Bezerra, Martini, and Maia [5,6], and was relatively close to the overpotential of the IrO2/CP modified electrode - which was 220\u00a0mV (Fig.\u00a04B).The specific current densities of the NiCoMo/CP, CoMoSeS/CP, and CoMo/CP modified electrodes (current per ECSA [5,6,28\u201332,64]; see the values below for ECSA and in Table S4) at 10.0\u00a0mA cm\u22122\nby ECSA iR free yielded \u03b7 of approximately 320\u00a0mV (Fig.\u00a04C). As the IrO2/CP modified electrode exhibited considerably high ECSA value (Table S4), its specific current densities disappeared in Fig.\u00a04C.It is worth noting that the addition of Co and Ni to MoS2 and MoSe2 and the formation of the oxides (Figs.\u00a01 and S3) contributed toward the improvement of the OER catalysis (compare Fig.\u00a04B with Figure S7B). This outcome can also be linked to the structures formed in the materials (compare Fig.\u00a01 with Figure S1), since the four samples with similar morphology \u2013 NiCoMo, NiCoMoSe, CoMoSeS, and CoMo \u2013 presented similar OER catalytic performance.An increase is observed in the ECSA value of the NiCoMo nanocomposite (Table S4) due to the following: i) the NiCoMo nanocomposite is constituted mainly by nanoribbons and nanoparticles, with the nanoparticles supported on the nanoribbons; ii) the presence of NiCo2O4 and NiO oxides in the NiCoMo nanocomposite; and iii) the calcination of the nanoribbons (MoSe2\u20132) at 600 \u00baC, which leads to the wrinkling of the nanoribbons, where they present junction (and defect) of different crystal planes for the CoMoO4 oxides. It is worth noting that the increase in the ECSA value of the NiCoMo nanocomposite enhances its catalytic performance in OER.The CoMoSeS and CoMo nanocomposites are constituted mainly by nanosheets, nanoribbons and nanoparticles, and the last two are supported on nanosheets. The hydrothermal heating of MoSe2\u20131 and MoS2 gives rise to nanoribbons and nanosheets, respectively. When they are calcined at 600 \u00baC, the nanoribbons (MoSe2\u20131) become wrinkled and the nanosheets (MoS2) are generally shrunken into nanoribbons. In addition, the CoMoSeS and CoMo nanocomposites exhibit junctions of different crystal planes for MoS2 and Co2Mo3O8, CoMoO4, and Co3O4 oxides (in addition to the presence of hydroxide Co(OH)2). All these factors lead to the enhancement of the OER activity of CoMoSeS and CoMo nanocomposites, thus enabling the catalysts to present the second best OER catalytic performance. In addition to the presence of MoSe2 and CoSe2, Co2Mo3O8, CoMoO4 and NiO oxides, the presence of dark spots (nanoparticles) on the surface of the nanoribbons in the NiCoMoSe nanocomposite did not lead to a significant improvement in the OER catalytic performance of the NiCoMoSe nanocomposite.The improved OER catalytic performance which is linked to the atomic contents recorded in the nanocomposite surfaces (Table S2) \u2013 caused by the different components used and the different steps of synthesis employed based on the application of the same element \u2013 leads us to the following analytical observations: i) the NiCoMo nanocomposite presents a slightly higher atomic content of Co in comparison with Ni (11.8 and 10.9%, respectively) \u2013 considered an ideal atomic content of Co (and Ni) in the nanocomposite surface \u2013 and this leads to a considerable improvement in the OER catalytic performance of the nanocomposite; ii) the CoMo nanocomposite presents Co atomic content of 12.7%, which yields a good OER activity for the nanocomposite; iii) the increase in the atomic content of Co to 20.3%, as was the case of the CoMoSeS nanocomposite, does not lead to additional improvements in the OER catalytic performance of the nanocomposite. Finally, the NiCoMoSe nanocomposite presents Co and Ni atomic contents of 10.1 and 11.2%, respectively, and this contributes toward the deterioration of the OER catalytic performance.The Tafel plots are shown in Figs.\u00a04E-F, S7C-D, and S9 (S9 obtained from S8). The NiCoMoSe, CoMoSeS, CoMo, and NiCoMo nanocomposites presented the following Tafel slope values: 59 (closer to the value recorded for IrO2), 60, 63, and 83\u00a0mV dec\u22121, respectively; these values were found to be lower than the values recorded for CoMo/AL, RuO2, MoS2, and MoSe2\u20132 (120, 138, 175, and 178\u00a0mV dec\u22121, respectively) (Table S4). The Tafel slope values recorded for the samples in this study are close to the values reported in the literature (Table S5). The Tafel slope values obtained from the chronoamperometry data were quite close to the values obtained from the SLV experiments (Figure S9). It is generally accepted that the determining step of the reaction rate is closer to the final step of a series of reactions when a decrease is observed in the Tafel slope; essentially, this is a sign of a good OER electrocatalyst [64]. Thus, the combination of Co and Ni in MoS2 and MoSe2 was found to be convenient as it helped produce efficient OER electrocatalysts.The Tafel slope values obtained suggest the involvement of 3 to 2 electrons (60 and 90\u00a0mV dec\u22121, respectively) [5,6,32] in the OER mechanism involving the application of the following catalysts: NiCoMoSe, CoMoSeS, CoMo, and NiCoMo.The number of electrons released (3 to 2 electrons), which was based on the Tafel slope values, was derived from the following oxidation processes: Co from the 2+to 3+ state (one electron released), Ni from the 2+ to 3+ state (one electron released), and Mo from the 4+ to 6+ state (two electrons released), respectively, for the catalysts containing the respective metals in their compositions [5,6,29,31,32,65,66]. Also, the oxidation states recorded for the aforementioned metals were confirmed by the results obtained from the HR-XPS spectra analyses (Fig.\u00a03).In addition, the mass surface percentage values (Table S2) of Co and/or Ni and N were found to be extremely high (Mo presented relatively lower mass surface percentage values), as confirmed by the XPS survey results (Figure S5); these results were also in agreement with the EDX mass percentage values (the only exception here was N; see Table S1).The electrocatalytic OER mechanism involving these electron numbers can be summarized as follows [5,6,29,31,32]:Initially, the oxides/hydroxides [5,6,32] are involved in the following way:\n\n(4)\n\n\n\n\n\nM\n\n\n\n2\n+\n\n\n\u2212\nOH\n+\n\nO\n\n\n\n\nH\n\n\n\u2212\n\n\n(\naq\n)\n\n\u2192\n\n\n\nM\n\n\n\n3\n+\n\n\n\n(\n\nO\n\n)\n\n\u2212\nOH\n+\n\n\n\nH\n\n\n+\n\n\n(\naq\n)\n\n+\n\n\n\ne\n\n\n\u2212\n\n\n\n\n\n\n\n(5)\n\n\n\n\n\nM\n\n\n\n3\n+\n\n\n\n(\n\nO\n\n)\n\n\u2212\nOH\n+\n\nO\n\n\n\n\nH\n\n\n\u2212\n\n\n(\naq\n)\n\n\u2192\n\n\n\nM\n\n\n\n2\n+\n\n\n\u2212\nOH\n+\n\n\n\nH\n\n\n+\n\n\n(\naq\n)\n\n+\n\n\nO\n\n2\n\n\n(\n\ng\n\n)\n\n+\n\n\n\ne\n\n\n\u2212\n\n\n\n\nwhere M is Co or Ni. M stands for Mo; note that the mechanism begins with M4+.One needs to consider the occurrence of OER mechanisms via i) electrochemical oxide (reactions 6\u20138) and ii) oxide (reactions 9\u201311) [5,6,29,32]:\n\n(6)\n\n\n\nS\n\n+\n\nO\n\n\n\n\nH\n\n\n\u2212\n\n\n(\naq\n)\n\n\u2192\n\nS\n\n\u2212\nOH\n+\n\n\n\ne\n\n\n\u2212\n\n\n\n\n\n\n\n(7)\n\n\n\nS\n\n\u2212\nOH\n+\n\nO\n\n\n\n\nH\n\n\n\u2212\n\n\n(\naq\n)\n\n\u2192\n\nS\n\n\u2212\n\nO\n\n+\n\n\nH\n\n2\n\n\nO\n\n+\n\n\n\ne\n\n\n\u2212\n\n\n\n\n\n\n\n(8)\n\n\n2\n\nS\n\n\u2212\n\nO\n\n\u2192\n2\n\nS\n\n+\n\n\nO\n\n2\n\n\n(\n\ng\n\n)\n\n\n\n\n\n\n\n(9)\n\n\n\nS\n\n+\n\nO\n\n\n\n\nH\n\n\n\u2212\n\n\n(\naq\n)\n\n\u2192\n\nS\n\n\u2212\nOH\n+\n\n\n\ne\n\n\n\u2212\n\n\n\n\n\n\n\n(10)\n\n\n2\n\nS\n\n\u2212\nOH\n\u2192\n\nS\n\n\u2212\n\nO\n\n+\n\nS\n\n+\n\n\nH\n\n2\n\n\nO\n\n\n\n\n\n\n\n(11)\n\n\n2\n\nS\n\n\u2212\n\nO\n\n\u2192\n2\n\nS\n\n+\n\n\nO\n\n2\n\n\n(\n\ng\n\n)\n\n\n\n\nwhere S stands for surface active sites [5,29], which consist of Co, Ni, and Mo.To study the stability of the catalysts, chronoamperometry analysis was performed for 24\u00a0h using a potential that could produce a current density of 10\u00a0mA cm\u22122 (Figure S10). The current vrs. time responses for CP-modified CoMoSeS, CoMo and NiCoMo electrodes are shown in Figure S10. After every 4\u00a0hrs, the cell system was shaken to remove eventual O2 covering the catalyst surfaces; this was linked to the presence of pulse current densities (Figure S10).After the long-term stability test, the LSV responses were compared with the LSV initially obtained prior to the test (Fig.\u00a04D). The CoMoSeS-es nanocomposite exhibited a decrease of 43% in the maximum current density (45\u00a0mA cm\u22122) and a negligible shift in overpotential at 10\u00a0mA cm\u22122. The CoMo-es nanocomposite exhibited a decrease of 26% in the maximum current density (60\u00a0mA cm\u22122) and an increase of only 10\u00a0mV in overpotential at 10\u00a0mA cm\u22122. The NiCoMo-es nanocomposite exhibited a decrease of only 5% in the maximum current density (33\u00a0mA cm\u22122) and a decrease of 12\u00a0mV in overpotential at 10\u00a0mA cm\u22122. These results point to the stability and reliability of the materials in terms of catalytic activity in OER, once they did not show any signals of deactivation, even after operating for a long period of time (after long-term stability test, Fig.\u00a04D). The IrO2-es nanocomposite exhibited a significant shift in overpotential at 10\u00a0mA cm\u22122 \u2013 from 280 to 420\u00a0mV (Fig.\u00a04D); this clearly points to the instability of IrO2 as a good catalyst for OER long-term stability test.While it is evident that there is a corrosion of nanoparticles, nanosheets and nanoribbons (compare the element atomic contents in Table S2 with the CoMoSeS, CoMo, and NiCoMo nanocomposites) in the CoMoSeS-es, CoMo-es, and NiCoMo-es nanocomposites, the difference in electrochemical stability between these nanocomposites can be associated with the atomic contents of the element recorded on the nanocomposites surfaces (Table S2), and this leads us to the following observations: i) the NiCoMo-es nanocomposite presents a high atomic content of Co in comparison with Ni (5.4 and 2.5%, respectively) \u2013 a good amount of Co (and Ni) atomic content on the nanocomposite surface helps ensure a better OER catalytic performance for this nanocomposite; ii) the CoMo-es nanocomposite exhibits Co atomic content of 25.6%, and this negatively affects the OER catalytic performance of the nanocomposite; iii) the Co atomic content of 18.0% in the CoMoSeS-es nanocomposite also negatively affects the OER catalytic performance of the nanocomposite (Table S2). With regard to the CoMoSeS-es nanocomposite, there are no traces of Mo, S, and N (Table S2) on the nanocomposite surface, and this contributes toward worsening the OER catalytic performance of the nanocomposite.The results obtained from the OER activity for the NiCoMo/Au electrode before and after the long-term stability test in O2-saturated purified 1\u00a0M KOH solution (Fe free) (Figure S11) presented responses quite similar to those shown in Fig.\u00a04D obtained from the application of 1\u00a0M KOH solution containing Fe impurities. The conclusion that can be drawn here is that the Fe impurities present in the 1\u00a0M KOH solution are clearly not responsible [62] for the high electrocatalytic performance and stability of the NiCoMo catalyst.Figure S12 presents the ring current responses obtained for the bare Pt ring along with the HLS curves for the CoMoSeS/Au and NiCoMo/Au disk electrodes; the figure shows the occurrence of residual currents related to the Pt oxidation without any currents related to the oxidation of HO2\n\u2212 which could be derived from OH-oxidation during OER. This implies that the OER effectively resulted in O2 production instead of the production of HO2\n\u2212[5,6].The integration (Figure S13B) of CV responses (Figure S13A) was used to quantify the surface concentration of active sites per cm2 (\u0393) [5,6,31,32,66]. Taking into account the values related to the current densities and redox pairs, the CVs recorded for the CoMoSeS/Au and NiCoMo/Au disk electrodes were considerably different (Figure S13A). The CoMoSeS/Au disk electrode exhibited a redox pair at 1.1\u00a0V with high current densities; this redox pair was attributed to Co2+/3+ oxidation/reduction [6,66] and Co2+/3+ was the main active site present in the CoMoSeS electrocatalyst. The NiCoMo/Au disk electrode exhibited only shoulders with maximum small current densities at 0.94 and 0.58\u00a0V, respectively, for Ni2+/3+ oxidation and Ni3+/2+ reduction [5,65] (Figure S13A-B), where Ni2+/3+ was the main active site present in the NiCoMo electrocatalyst.The CV integration (Figure S13B) values divided initially by 0.05\u00a0V s\u00a0\u2212\u00a01 and additionally divided by the electron charge (1.602\u00a0\u00d7\u00a010\u221219C) [5,6,32] yielded \u0393 values of 1.915\u00a0\u00d7\u00a01016 and 7.52\u00a0\u00d7\u00a01013 atoms cm\u22122 for the CoMoSeS/Au and NiCoMo/Au disk electrodes, respectively. The \u0393 values obtained here were close to the \u0393 values reported by Bezerra, Martini, and Maia [5,6]. The \u0393 values and hydrodynamic SLV responses (Figure S13C) were used to calculate the relationship between the turnover frequency (TOF, see equation S1) and potential (Fig.\u00a05\n).The TOF values obtained for the CoMoSeS/Au disk electrode were 0.42 and 13.37 s\u00a0\u2212\u00a01 at 1.56 and 1.68\u00a0V, respectively, and the TOF values obtained for the NiCoMo/Au disk electrode were 1.06 and 331.26 s\u00a0\u2212\u00a01 at 1.50 and 1.66\u00a0V, respectively (Fig.\u00a05). These TOF values were found to be close to the values reported by Bezerra, Martini, and Maia [5,6].The faradaic efficiency (FE, see equation S2) relative to the potential (Figure S13F) was calculated using the current densities of the hydrodynamic linear potential scan starting from 1.0\u00a0V for the NiCoMo/Au modified disk electrode in N2\u2012saturated 1.0\u00a0M KOH simultaneously with the current densities of the bare Pt ring set at 0.4\u00a0V (Figure S13E) [5,6]. The O2 produced from the OER in the NiCoMo/Au modified disk electrode under purely diffusion control [5,6,29] was detected in the bare Pt ring electrode.However, as the O2 produced from the OER in the disk electrode increased at higher current densities, the O2 was found to accumulate in the disk-ring interspace [67] (Teflon interspace, Figure S13D), and despite using a Ti or Au thin wire close to the Teflon interspace surface to dislodge the O2 bubbles formed specifically on the interface between the disk and ring spacer [67] (Teflon interspace), we were not able to improve the ring current densities in the potential range of the high current densities in the disk (Figure S13E); this resulted in a narrow potential window where FE could be determined with some degree of reliability (Figure S13F).The FE values obtained for the NiCoMo/Au disk electrode were close to 100% at 1.55 (without Ti or Au wire), 1.57 (with Au wire), and 1.59\u00a0V (with Ti wire) (Figure S13F). The decrease observed in the FE at more positive potentials (Figure S13F) was attributed to both the huge production of O2 (including O2 bubbles [68]) and the bubbles nucleation in the Teflon spacer between the disk and the ring [69], which was caused by a sudden increase in gas concentration in the solution that flowed past the spacer [70], decreasing the current densities on the surface of the bare Pt ring [5,6] (Figures S13E).The double layer capacitance (CDL) was obtained from CV in a non-faradaic potential region at different scan rates [5,6,68] (Figure S14 and Eq.\u00a0(2)). CDL divided by specific capacitance [33] yields the electrochemically active surface area (ECSA) [5,6,68] of the catalytic surface (Figure S15), as shown in Table S4. The calculated ECSA values obtained for the bare electrode and the modified MoS2, MoSe2\u20132, RuO2, and IrO2 electrodes were approximately 0.04, 0.04, 0.29, 10.8, and 378.8 cm2, respectively. Considering the geometric current densities of the electrode, the extremely high ECSA value obtained for the CP-modified IrO2 electrode is found to be responsible for the improvement in OER activity presented by the catalyst. When the specific current densities of the electrode are taken into account, one notices that the extremely high ECSA value obtained for the CP-modified IrO2 electrode is responsible for the worsening of the OER activity observed for the catalyst (Figs.\u00a04B-C). The CoMoSeS, CoMo and CoMo/AL modified electrodes exhibited ECSA values of approximately 0.05 cm2; this result shows that there was no variation in the number of active sites after the calcination and acid leaching process during the synthesis. The NiCoMoSe and NiCoMo recorded ECSA values of 0.05 and 0.7 cm2, respectively; these relatively low ECSA values were responsible for the good OER activity obtained in these catalysts considering the geometric and specific current densities of the nanocomposites (Figs.\u00a04B-C).With the exception of RuO2 and IrO2, all the samples recorded an increase in ECSA values after the OER and long-term stability (when applied) experiments; this justifies the inefficient performance of IrO2 after OER long-term stability test (Fig.\u00a04D). The NiCoMo nanocomposite recorded a 1.74-fold increase in the ECSA value after OER in comparison to its initial ECSA value, and its ECSA value after the stability experiment was equal to that observed after the OER experiment. The CoMoSeS and CoMo nanocomposites exhibited very similar behavior, with both recording an 8.6-fold increase in the ECSA values after OER in comparison to their initial ECSA values; furthermore, both nanocomposites recorded about 1.25-fold increase in the ECSA values after the stability test compared to the ECSA values they recorded after OER. These increases in ECSA value justify the efficient performance of NiCoMo, CoMoSeS, and CoMo after the OER long-term stability test (Fig.\u00a04D).Electrochemical impedance spectroscopy analyses were performed for the bare CP and the CP-modified electrodes at OCP in N2-saturated 1\u00a0M KOH before and after OER, and after long-term stability experiments; the results obtained are shown by Nyquist plots in Figure S16. There were very small variations in Ru values for the experiments conducted before and after OER experiments; the mean Ru values recorded for the CP-modified electrodes and the bare CP electrode were 4.5 \u03a9 and 6.4 \u03a9, respectively (Figure S16). The average Rct values obtained for the experiments conducted before and after OER, and after long-term stability were as follows: 2 \u03a9 for CoMoSeS/CP, CoMo/CP, NiCoMoSe/CP, MoS2/CP, and RuO2/CP modified electrodes; 8 \u03a9 for the MoSe2\u20132/CP modified electrode; 40\u03a9 for the NiCoMo/CP modified electrode; 230 \u03a9 for the CoMo/AL/CP modified electrode (Figure S16). The IrO2/CP modified electrode presented Rct value of 2 \u03a9 before OER; there was a significant increase in the Rct value after the OER and the long-term stability test; this points to the instability of the IrO2/CP modified electrode after the long-term stability test (Fig.\u00a04E).The NiCoMo/CP modified electrode recorded a decrease in the Rct value before the OER experiment compared to the value it recorded after the OER experiment; this explains why it was chosen as the best OER electrocatalyst in the present study. As the CoMoSeS/CP and CoMo/SP modified electrodes presented very low Rct values, these electrodes were chosen as the second best OER electrocatalysts (Figure S16).MoSe2\u20132 presents a slightly higher Rct value in relation to MoS2; thus, when the oxide nanoparticles that compose the NiCoMo nanocomposite are supported on MoSe2\u20132 nanoribbons, the Rct value presented by this nanocomposite continues to be slightly higher. The NiCoMoSe nanocomposite presents a relatively smaller Rct value in comparison with the NiCoMo nanocomposite; this can be mainly attributed to the different oxides (less resistant to charge transfer; for example, NiCo2O4 was not identified in the NiCoMoSe nanocomposite) that constitute the oxide nanoparticles supported on MoSe2\u20132 nanoribbons.As MoS2 presents relatively lower resistance to charge transfer in comparison with MoSe2, the oxides nanoparticles supported on the nanosheets and/or nanoribbons (MoS2) tend to keep the Rct values rather low in the CoMoSeS and CoMo nanocomposites.The present study showed that the factors responsible for the best OER catalytic response for the NiCoMoSe, NiCoMo, CoMo and CoMoSeS nanocomposites included the following:\n\n(1)\nFirstly, the presence of different oxide nanoparticles supported on MoSe2 nanoribbons helped enhance the OER catalytic response. For CoMoSeS in particular, the presence of different oxide nanoparticles supported on MoS2 nanosheets and MoSe2 nanoribbons contributed toward an improvement in the OER catalytic activity.\n\n\n(2)\nThe second factor that contributed toward the improvement in catalytic activity was related to the main oxides identified in the synthesis process; these oxides included CoMoO4, NiCo2O4 (and CoMoO4), Co2Mo3O8 (and CoMoO4), and CoMoO4, Co2Mo3O8, and Co3O4, for the NiCoMoSe, NiCoMo, CoMoSeS, and CoMo nanocomposites, respectively. Apart from that, the junction of the different oxide crystal planes generated additional suitable sites that contributed toward the improvement of OER electrocatalysis.\n\n\n(3)\nThirdly, the N atoms, from the urea used during the synthesis, bonded to relatively high amount of Ni and/or Co (and Mo) on the surface of the nanocomposites and this helped enhance OER electrocatalysis. Similarly, the presence of Ni2+ and Ni3+, and/or Co2+ and Co3+, and Mo4+ and Mo6+ on the nanocomposite surfaces also contributed toward enhancing the OER electrocatalytic activity of the nanocomposites.\n\n\n(4)\nFinally, the electrons released (3 to 2 electrons) from the oxidation of the following: i) Co from the 2+ to 3+ state (one electron released); ii) Ni from the 2+ to 3+ state (one electron released); and iii) Mo from the 4+ to 6+ state (two electron released), contributed toward enhancing the OER electrocatalytic activity.\n\n\nFirstly, the presence of different oxide nanoparticles supported on MoSe2 nanoribbons helped enhance the OER catalytic response. For CoMoSeS in particular, the presence of different oxide nanoparticles supported on MoS2 nanosheets and MoSe2 nanoribbons contributed toward an improvement in the OER catalytic activity.The second factor that contributed toward the improvement in catalytic activity was related to the main oxides identified in the synthesis process; these oxides included CoMoO4, NiCo2O4 (and CoMoO4), Co2Mo3O8 (and CoMoO4), and CoMoO4, Co2Mo3O8, and Co3O4, for the NiCoMoSe, NiCoMo, CoMoSeS, and CoMo nanocomposites, respectively. Apart from that, the junction of the different oxide crystal planes generated additional suitable sites that contributed toward the improvement of OER electrocatalysis.Thirdly, the N atoms, from the urea used during the synthesis, bonded to relatively high amount of Ni and/or Co (and Mo) on the surface of the nanocomposites and this helped enhance OER electrocatalysis. Similarly, the presence of Ni2+ and Ni3+, and/or Co2+ and Co3+, and Mo4+ and Mo6+ on the nanocomposite surfaces also contributed toward enhancing the OER electrocatalytic activity of the nanocomposites.Finally, the electrons released (3 to 2 electrons) from the oxidation of the following: i) Co from the 2+ to 3+ state (one electron released); ii) Ni from the 2+ to 3+ state (one electron released); and iii) Mo from the 4+ to 6+ state (two electron released), contributed toward enhancing the OER electrocatalytic activity.The presence of MoS2 nanosheets and MoSe2 nanoribbons as supporting material for the metal oxides led to a decrease in both the charge transfer resistance and the ECSA values; this resulted in the improvement of the OER electrocatalytic activity of the electrocatalysts.The TOF and FE values obtained for the NiCoMo and CoMoSeS nanocomposites were high under low potentials. In addition, after the OER long-term stability test, the NiCoMo, CoMoSeS, and CoMo nanocomposites were found to be stable despite the fact that some elements of corrosion were detected; this shows that the electrocatalysts have high OER electrocatalytic activity.The Fe impurities present in the 1\u00a0M KOH solution were not found to be responsible for the high OER electrocatalytic performance and stability of the electrocatalysts.The authors are grateful to the LCE/DEMa/UFSCar for the support with TEM analyses and to LAMAS \u2013 Laborat\u00f3rio Multiusu\u00e1rio de An\u00e1lise de Superf\u00edcies from UFRGS, for providing us with the XPS facilities. The authors also acknowledge the financial support provided by the following Brazilian funding agencies: CNPq (grants 303759/2014-3, 303351/2018-7, and 405742/2018-5), Fundect-MS (grant 59/300.184/2016), CAPES-PRINT (grant 88881.311799/2018-01), and CAPES \u2013 Finance Code 001. This study was partly funded by the Federal University of Mato Grosso do Sul \u2013 UFMS/MEC \u2013 Brazil. L.S.B. and B.K.M. are grateful to CAPES for the fellowship granted them during the course of this research.", "descript": "\n                  It is well known that the benchmark electrocatalysts for OER in alkaline solution are RuO2 and IrO2; however, the high cost, scarcity, and instability of these metal oxides impede their ample use in OER processes, and this has fueled the search for cheap Earth-abundant elements which are equally efficient for application as electrocatalysts in OER. The present work reports the use of hydrothermal and calcination methods for the synthesis of nanocomposites made up of Ni and/or Co salts and urea in combination with MoSe2 and MoS2 and their application as efficient and stable electrocatalysts for OER in alkaline solution. The NiCoMoSe, NiCoMo, CoMoSeS, and CoMo nanocomposites constructed in the present study presented high OER electrocatalytic activity and stability mainly as a result of the following: the combination of N atoms bonded to Ni and/or Co (and Mo); the electrons released from the oxidation of Co from the 2+ to 3+state, Ni oxidation from the 2+ to 3+state, and Mo oxidation from the 4+ to 6+state; and the metal oxides (CoMoO4, NiCo2O4, Co2Mo3O8, and Co3O4) supported on MoS2 nanosheets and MoSe2 nanoribbons which contributed to a decrease in charge transfer resistance, apart from keeping the ECSA values relatively low.\n               "}
{"full_text": "The Heck coupling reactions has become one of the most useful C(sp2)-C(sp2) bond-forming reactions in organic synthesis [1\u20135]. The reaction has been applied to many areas, including bioactive compounds, natural products, drug intermediates, fine chemical syntheses, UV absorbers, antioxidants and industrial applications [6\u20139]. However, existed protocol gained increasing importance both in large-scale industrial processes and the development of new materials and biologically active compounds [10\u201312]. But in the other hand, existed protocol suffers from the inherent drawback of the required pre-synthesizing the organic halides and accompanied formation of a stoichiometric amount of hazardous halide salt, which can cause significant environmental concerns.Furthermore, the examination of ionic liquid functionalized novel catalytic systems involving other transition metals such as Co [13\u201315], Ni [16,17] and Cu [18,19] have also been receiving much attention other than palladium (Heck reaction) catalysts [20\u201323]. In general, copper based catalytic systems are easily available because of their cheaper price, functional group tolerance and large-scale procedures. Nevertheless, only limited reports are available in the copper catalysed Heck type reaction and are suffering the problems concerning extraction from the reaction mixture, waste production, high toxicity & price, air-sensitivity and leaching [24]. Henceforth, we felt it would be of keen interest to eliminate these negative aspects of the copper complex. In this regard, we have developed a stable, selective, suitable ligand that leads to efficient heterogeneous copper complex with high turnover and reprocessability.In the last two years, Nemanja Vucetic et\u00a0al., have described bis-layer supported ionic liquid catalyst with an unprecedented activity in the Heck reaction [25] and Daniel Rauber et\u00a0al., have been reported fluorinated phosphonium ionic liquid in Heck reactions [26]. Furthermore, Saithalavi Anas et\u00a0al., have proposed polymer supported copper catalyst for the Heck reaction [27]. However, Issa Yavari et\u00a0al., has illustrated copper-catalysed Mizoroki-Heck coupling reaction using an efficient and magnetically reusable Fe3O4@SiO2@PrNCu catalyst [28]. All the above techniques provide excellent yield, but some have disadvantages such as lengthy work-up procedure, harsh reaction conditions (organic co-solvents) and requires absolutely dry and inert media. To the best of our knowledge, no one has reported copper functionalized 1-glycyl-3-methyl imidazolium chloride (Fig.\u00a01\n), which proved to be highly efficient at ambient temperature for C\u2013C bond formation under solvent free condition.Though, recovery and leaching can occur in the extractive work up leading to a loss of the catalyst in the reaction mix on the one hand and requests supplementary effort to purify the extracted product. To overcome such problems, novel complex was developed [29] by covalent linking of organo catalytic unit with an ionic liquid moiety (often chloroglycine). This imparts a low solubility of catalyst in the solvents used for extraction of the product on the one hand and high solubility in the reaction medium on the other hand [29]. This strategy was applied to Heck reaction providing high yield and good recyclability of the organo catalyst.The objectives of the present study are to: (i) develop an efficient synthetic process for the facile conversion of Heck reaction. The present method developed for the Heck reaction offer many advantages including high conversion, solvent free and the involvement of non-toxic reagents.At the initial of this project, the influence of various reaction parameters has been investigated for the 1-iodo-4-methoxybenzene and styrene as model reaction like best catalytic system, temperature, solvent and base. To begin, the impact of different bases to the model reaction was studied and these results are summarized in Table\u00a01\n. Initially, no product was found when the absence of base (Table\u00a01, entry 1). However, reaction was carried using several bases with 14%\u201355% yields (Table\u00a01, entries 2\u20139). Furthermore, Et3N is superior in comparison to various bases (Table\u00a01, entry 10). After inventing the best catalyst systems, we further optimized the reaction conditions in presence of Et3N. The experiments showed the time decreasing from 24\u00a0\u200bh to 10\u00a0\u200bh gave the same results (Table\u00a01, entries 11 and 12). But the yields were dropped appreciably the time decreasing from 10 to 8\u00a0\u200bh (Table\u00a01, entry 13). Further, the amount of the base was increased from 2 mmol to 3\u00a0\u200bmmol, the yield increased 96%\u201397% (Table\u00a01, entry 14), in the other hand dissimilar yield was achieved when decreasing the base amount from 2 mmol to 1\u00a0\u200bmmol (Table\u00a01, entry 15). By all these investigation results, it was concluded that 2\u00a0\u200bmmol Et3N is sufficient to brought out the complete Heck reaction in 10\u00a0\u200bh at 25\u00a0\u200b\u00b0C.Next, in order to find an appropriate solvent for the model reactions, the coupling of 1-iodo-4-methoxybenzene and styrene was carried out with different solvents and Et3N. Among the reports, in the absence of solvent was the most productive, as compared with the polar and non-polar solvents (Table\u00a02\n, entries 1\u201311). This may be due to the easy coordination of complex with organic co-solvents. It has also been reported that H2O molecule sometimes is required to activate the Cu(II) catalyst. In our case, carrying out the reaction in water gave a negative effect on the product yield in comparison with solvent free condition (Table\u00a02, entry 14). This lower yield could be due to complex delocalization under aqueous condition. However, temperature also plays an important role in the model reaction. When we conducted the Heck reaction of 1-iodo-4-methoxybenzene and styrene as the model substrate at 30\u00a0\u200b\u00b0C, there was no change in the yield (Table\u00a02, entry 12).Subsequently, in order to optimize the reaction conditions for a particular catalyst, the coupling reaction of 1-iodo-4-methoxybenzene and styrene was performed in presence of Et3N by using different catalysts and the results are given in Table\u00a03\n. Unfortunately, no product was detected in the absence of catalyst (Table\u00a03, entry 1). To our delight, when using a various copper salts, the coupling reaction gave trace to 14% (Table\u00a03, entries 2\u20136). Following, we studied the activity of synthesized 1-carboxy ethyl-3-methyl imidazolium bromide [Cemim]Br, 1-(2-aminoethyl)-3-methylimidazolium bromide [Aemim]Br, 1-glycyl-3-methyl imidazolium chloride [Gmim]Cl catalyst and it was initiate that [Gmim]Cl catalysed furnished less yield of desired product (Table\u00a03, entries 7\u20139). An additionally we tested the combination of CuCl2 and the different ionic liquids led to significant activity to the model reaction (Table\u00a03, entries 10\u201312). However, Among them heterogeneous [Gmim]Cl\u2013Cu(II) catalyst was found to be the best catalyst providing 96% yield in 10\u00a0\u200bh (Table\u00a03, entry 13). After, similar yield was reached when escalating the catalyst amount from 0.1 to 0.2\u00a0\u200bmol% (Table\u00a03, entry 14) and it was found that conversion decreases with decrease in catalyst loading (Table\u00a03, entry 15). Later, the reaction conditions for the active catalyst system were further optimized the reaction duration. The testing showed that on increasing the time from 10 to 11\u00a0\u200bh gave the same (Table\u00a03, entry 16). Though, the yields were dropped significantly on decreasing the time from 10 to 9\u00a0\u200bh (Table\u00a03, entry 17). In contrast to other protocols, this coupling reaction can be performed even at 30\u00a0\u200b\u00b0C, while at 25\u00a0\u200b\u00b0C only 10\u00a0\u200bh reaction time is necessary to complete the Heck coupling reaction without any loss (Table\u00a03, entry 18).With suitable conditions in hand, we examined the generality and limitation of the catalyst system. As shown in Table\u00a04\n, more than 22 different stilbenes derivatives were synthesized in moderate to excellent yields. More specifically, 9 different aryl chlorides were tested: para- and ortho-methyl substituted chlorobenzenes gave the corresponding products (Table\u00a04, entries 2 and 3). Similarly, 4-nitro and 4-methoxy substituted aryl chlorides were successfully transformed in 82%\u201384% yields (Table\u00a04, entries 4 and 5). In addition, aryl bromides with electron-withdrawing substituent (CN) also work well under our standard conditions with 730 TON (Table\u00a04, entry 6). Next, we used methyl acrylate olefins. When 4-methyl and 4-nitro substituted aryl chloride are treated with methyl acrylate under the optimized reaction condition, substituted stilbene product was isolated only in 78% and 83% yields (Table\u00a04, entries 8 and 9). However, 7 different aryl bromides were tested: aryl bromide showed good reactivity with other activated substituted groups such as para-(methyl/methoxy/chloro) and afforded the corresponding coupling products in moderate to good yields (Table\u00a04, entries 10\u201313). Moreover, 4-methoxy and 4-methyl substituted aryl bromide are treated with methyl acrylate gave substituted products in reasonable yields (Table\u00a04, entries 15 and 16). Likewise, functionalized aryl iodides containing both electron withdrawing and electron donating groups felt efficient coupling reaction with styrene. Nevertheless, para-methoxy and methyl substituted aryl iodides reacted well with styrene gave the corresponding products in excellent yields (Table\u00a04, entries 18 and 19). Additionally, excellent reactivity was observed, when para-CN-aryl iodine were subjected to coupling with styrene (Table\u00a04, entry 20). While, reaction of 4-methoxy iodo benzene with methyl acrylate resulted in 87% yield (Table\u00a04, entry 22). Notably, in all the cases, complete trans form was detected and the formations of cis or homo coupling products were not observed.Isolation of the heterogeneous catalyst was easily performed by extraction or centrifugation. The isolated catalyst was washed with diethyl ether and dried in air. The regenerated catalyst was used for the reaction of 1-iodo-4-methoxybenzene with styrene for nine runs to afford trans-stilbene with 96%\u201391% isolated yields (Table\u00a05\n). The precise mechanism of the catalytic reaction needs to be elucidated, but it is noticeable that the mechanism is strongly modified depending of the halobenzene employed, obtaining trans-stilbene as the main product (Scheme 1\n).In conclusion we have introduced [Gmim] Cl\u2013Cu(II) as a catalyst for Heck reaction with Et3N in the absence of an organic co-solvent. Aryl iodides were reacted efficiently with styrene and methyl acrylate at 25\u00a0\u200b\u00b0C in the presence of the catalyst. Noteworthy features of this catalyst system are (1) its catalytic activity was tested in Heck reaction; (2) 0.1\u00a0\u200bmol% of catalyst was sufficient to furnish the trans-stilbenes with excellent yields (up to 96%). (3) The catalyst can be readily recovered and reused without significant loss of its activity.All solvents and chemicals were commercially available and used without further purification unless otherwise stated. The 1H NMR spectra were recorded on a Bruker 500\u00a0\u200bMHz using CDCl3/DMSO\u2011d\n6 as the solvent and mass spectra were recorded on JEOL GC MATE II HRMS (EI) spectrometer. FT-IR was recorded on AVATRA 330 spectrometer with DTGS detector. Column chromatography was performed on silica gel (200\u2013300 mesh). Analytical thin-layer chromatography (TLC) was carried out on precoated silica gel GF-254 plates.[Gmim]Cl\u2013Cu (II) complex was synthesized following the literature [30].In a conical flask (100\u00a0\u200bmL), a mixture of 1-iodo-4-methoxybenzene (1\u00a0\u200bmmol), styrene (1.2\u00a0\u200bmmol), triethylamine (2\u00a0\u200bmmol) and [Gmim]Cl\u2013Cu (II) (0.1\u00a0\u200bmol%) was added and stirred at 25\u00a0\u200b\u00b0C for a period as indicated in Table\u00a04 (The reaction was monitored by HPLC and TLC). The heterogeneous mixture was extracted with ethyl acetate or diethyl ether (3\u00a0\u200b\u00d7\u00a0\u200b5\u00a0\u200bmL). The organic phase was separated and dried over anhydrous Na2SO4 and evaporated. The resulting crude was purified by flash chromatography to give the desired pure product with excellent yield.1-Iodo-4-methoxybenzene (1\u00a0\u200bmmol) was reacted with styrene (1.2\u00a0\u200bmmol) in the presence of [Gmim]Cl\u2013Cu(II) (0.1\u00a0\u200bmol%) and triethylamine (2\u00a0\u200bmmol) at 25\u00a0\u200b\u00b0C. After completion of the reaction (TLC/HPLC), the product was extracted as stated in the preceding general method. The white solid [Gmim]Cl\u2013Cu(II) was isolated by centrifugation. Furthermore, the recovered complex was washed with diethyl ether and dried in air. The resulting catalyst was charged to another batch of the similar reaction. This was repeated for 9 runs to complete the reaction in 10\u00a0\u200bh to give the desired product with 96%\u201391% yield (Table\u00a05).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 the Management of PC-Campus for providing required facilities and SIF (IITM) for providing the spectral data.", "descript": "\n                  A novel 1-glycyl-3-methyl imidazolium chloride-copper(II) complex [[Gmim]Cl\u2013Cu(II)] was found to be a heterogeneous catalyst for an efficient Heck reaction with good to excellent yield under solvent free condition. This protocol provides a simple strategy for the generation of a variety of new C\u2013C bonds under environmentally benign condition. The catalyst was reused up to nine consecutive cycles without any significance loss in its activity.\n               "}
{"full_text": "Today, lower quality fuels (high viscosity) containing high amounts of sulfur are extracted due to decreasing oil reserves [1]. As it is known, when fuels are burned, organic sulfur compounds in them are oxidized and emit SO2 gas which is harmful to the atmosphere and the environment, and these gases cause acid rain and corrosion [2]. Therefore, it is of crucial importance to desulfurize these low quality fuels.Hydrodesulfurization (HDS) as conventional desulfurization is widely used in the world. In HDS, organic sulfur compounds react with H2 gas and H2S is released as a result of the carbon\u2013sulfur bond cleavage in organic compound [3]. However, HDS has some disadvantages [4\u20136]: the use of high temperature, high pressure, expensive H2 gas and expensive catalysts with high chemical stability and high thermal resistance that must not be affected by severe operating conditions. Also, aliphatic sulfur compounds are easy to remove in HDS, while refractory aromatic sulfur compounds are difficult to remove [7].To eliminate these disadvantages, alternative desulfurization processes such as adsorptive desulfurization [8], extractive desulfurization [9], oxidative desulfurization [10], biodesulfurization [11] are used. Among them, the most advantageous and promising method is oxidative desulfurization (ODS). In ODS, at relatively low temperatures such as 20\u201360\u202f\u00b0C, at atmospheric or near atmospheric pressures, organic sulfur compounds are oxidized by using H2O2 and a catalyst to convert first to their sulfoxides and then to their sulfones, which are more polar compounds, and finally these oxidized sulfur compounds are removed from the fuel by extraction with a polar extractant such as methanol, acetonitrile, dimethyl formamide etc. or by adsorption [12].Desulfurization is also carried out with simultaneous oxidation and extraction [13]. In HDS, it is difficult to remove aromatic sulfur compounds, especially alkyl-substituted aromatic sulfur compounds which are prevented from accessing into the catalyst pores due to steric hindrance [14]. On the contrary, in ODS, using a liquid homogeneous catalytic system such as formic acid or acetic acid- H2O2 (HP) oxidant [15], alkyl-substituted aromatic sulfur derivatives are easier to remove due to an increase in electron density [1,16\u201319] on the sulfur atom as shown in Fig. 1\n. In particular, bonding the naphthenic ring to the thiophenic ring significantly increases the ODS yield of the compounds such as THBNT, THDBT and OHDNT [20]. When the phosphotungstic acid-HP system is used, the molecular size of the catalyst becomes important. Since phosphotungstic acid is a bulky molecule [21], the ODS reactivity of aromatic sulfur compounds having alkyl groups adjacent to the sulfur atom decreases due to spatial obstacle [22]. In a study [23] in which ODS of model sulfur compounds was performed by phosphotungstic acid-HP, it was reported that sulfur removal decreased in the order DBT\u202f>\u202f4-MDBT\u202f>\u202f4,6-DMDBT. When the solid heterogeneous catalyst is used, the sulfur atom is prevented from entering the catalyst pore and its interaction with the sulfur atom due to the steric hindrance of alkyl groups adjacent to sulfur becomes weak, consequently causing a decline in the ODS reactivity [24]. Desulfurization using t-butylhydroperoxide in the presence of Mo/Al2O3 catalyst is in the order DBT\u202f>\u202f4-MDBT\u202f>\u202f4,6-DMDBT\u226b BT [25]. With the use of TiO2 anatase-supported V2O5 catalyst and HP, the ODS yield is in the order DBT\u202f>\u202fBT\u202f>\u202f4-MDBT\u202f>\u202f2-MT\u202f>\u202f2,5-DMT\u202f>\u202f4,6-DMDBT [26]. In the H3PW12O40/TiO2-HP system, the desulfurization at 30\u202f\u00b0C increases in the order 4,6-DMDBT\u202f<\u202fBT\u202f<\u202fDBT [27].In ODS reactions, the mixture consists of two immiscible liquid phases as organic phase (real fuel or model fuel solution containing sulfur compounds such as DBT, 4,6-DMDBT dissolved in a non-polar solvent such as n-hexane, n-heptane or iso-octane) and aqueous phase (H2O2 solution). Therefore, quaternary ammonium salts as phase transfer catalysts (PTCs), one end of which is hydrophilic and the other end hydrophobic, are generally used, reducing the liquid\u2013liquid interface tension [28] and enabling the transfer of oxidizing species to organic phase, so that the ODS increases significantly [29]. Sometimes using ionic liquid (IL) instead of aqueous phase, ODS is further increased such that the IL acts as extractant during oxidation [30]. For the last 20\u201330\u202fyears, ultrasound wave has been used to accelerate oxidation reactions and increase ODS more. Sonication has two simultaneous effects in accelerating ODS reactions. The ultrasound wave creates cavitation bubbles in liquid and the implosion of these bubbles produces very high temperatures and pressures locally in the liquid. At the extremely high temperatures, chemical bonds of organic compounds are broken and reactive radicals are generated (Sonochemical effect). Microjet, microturbulence and shock waves created by imploding cavitation bubbles significantly accelerate the mass transfer by increasing the emulsification of the organic and aqueous phase (Sonophysical effect). Thus higher desulfurization efficiencies are achieved in a shorter time [31].ODS reactions are generally heterogeneous reactions, i.e., there are two or more phases in the mixture that are immiscible with each other. The solution of the organic phase, which is formed by dissolving model sulfur compounds in a non-polar solvent such as hexane, heptane or toluene, has been referred as to denotations such as model fuel, model diesel, model liquid fuel, model sulfur solution. The aqueous phase consists of an oxidant and a catalyst. In many studies, the reactivity of the model sulfur compounds has been determined and the optimum conditions (temperature, oxidant volume, catalyst amount, organic phase/aqueous phase volume ratio, time etc.) for maximum desulfurization have been found. These conditions have then been applied to real fuels to achieve desulfurization.Many solid, liquid and gas oxidizers have been evaluated. Inorganic chemicals such as oxone [32], sodium persulfate [33], potassium superoxide [34], potassium dichromate [35], sodium percarbonate [36], sodium perchlorate [37], hydrogen peroxide [38], sodium hypochlorite [39], solid oxidizers such as cyclohexanone peroxide [40] and organic chemicals such as t-butylhydroperoxide [41] and cumene hydroperoxide [42] as liquid oxidizers are used. The most distinctive feature of cyclohexanone peroxide as solid organic oxidizers and cumene hydroperoxide and t-butylhydroperoxide as liquid organic oxidizers is that they can all dissolve in the organic phase or fuel, thereby directly oxidizing sulfur compounds [43,44]. The structural formulas of oxidizing substances are shown in Fig. 2\n. Gaseous oxidants are generally oxygen [45], nitrogen dioxide [46] and ozone [47], and the solubility [48\u201351] of these gases in non-polar solvents is generally higher than that in water.Catalysts used in ODS are divided into two types; homogeneous catalysts soluble in liquid phase and heterogeneous catalysts insoluble in liquid phase.Catalysts used in heterogeneous catalysis are solid and insoluble in liquid mixture [52]. Nanoparticles improve the adsorption of sulfur compounds due to their large surface area [53]. Photocatalyst under UV [54] or visible light [55], nano-sized silica particles including mesoporous silica [56], aluminum oxide particles [57], transition metal oxides [58], activated carbons [59], modified metal\u2013organic frameworks [60], Ni catalyst also called sponge metal [61], nanocomposite [62], graphene oxide [63], activated carbon (AC)-supported phosphotungstic acid [64] and fly ash-modified fenton catalysts [65] are used. In the case of using heterogeneous catalysts, the catalytic ODS mechanism [66\u201370] is illustrated in Scheme 1\n. DBT, which is transferred from the organic phase to the aqueous bulk phase by ultrasound, diffuses to the outer surface of the solid catalyst by passing across the liquid film (boundary layer) around the supported catalyst particle. DBT is adsorbed on active sites on the external surface of the catalyst or on active sites on the internal surface of the inner pores by diffusing through the pore. HP interacts with active sites on the inner and outer surface and forms oxidizing active complexes. After DBT adsorbed on these active centers is converted into its sulfones by undergoing an oxidation reaction, DBT sulfone is desorbed and transferred successively to the boundary layer, aqueous phase and organic phase. In addition to enhancement of adsorption and desorption, ultrasound significantly increases not only the external and internal diffusion but also the collision frequency of reactants with active sites, thus causing increased UAODS performance.Matsuzawa et al. [71] carried out the photocatalytic oxidation of DBT using a Hg-Xe lamp of 200\u202fW at wavelength\u202f>\u202f290\u202fnm in the presence of anatase-type TiO2 (P25) as a heterogeneous photocatalyst and air (in which oxygen acts as an electron scavenger [72], thus causing oxidation only by electron vacancy (h+) [73] of TiO2\n[74\u201376]) in a polar acetonitrile solution. They found the photooxidation rate in combination with H2O2, photocatalyst and indirect ultrasound (45\u202fkHz and 50\u202fW) was higher than the oxidation rate with H2O2 and photocatalyst, and this effect was due to the reactivation of the TiO2 surface and increased mass transfer. However, they stated that the direct oxidation rate of 4,6-DMDBT using only H2O2 under photoirradiation was higher than the photooxidation rates in the cases of HP-photocatalyst-US and HP-photocatalyst. In addition, it is reported that the oxidation reaction rate of the methyl group in 4,6-DMDBT increased by using aliphatic and cyclic alkanes as a non-polar solvent instead of the polar solvent acetonitrile, since oxygen was more soluble in non-polar solvents [77].In another study [78], using photocatalytic anatase TiO2, 30\u202fwt% HP and ultrasound with duty cycle, the catalytic oxidative desulfurization of gum turpentine, which is similar to crude sulphated turpentine and a by-product of Kraft process [79] to obtain wood pulp, spiked with dimethyl disulfide was investigated and a desulfurization efficiency of 100% was found at 28\u202f\u00b0C, 120\u202fW power dissipation and 20\u202fkHz US frequency, 70% duty cycle, 15\u202fg L\u20131 HP concentration, 4\u202fg L\u20131 TiO2 loading for 50 ppmw DMDS initial concentration. Also, it was reported that total treatment cost (0.31 $ L\u20131) with (US\u202f+\u202fH2O2\u202f+\u202fTiO2) system is less as compared to US, only 30\u202fwt% H2O2, only Fenton, only TiO2, US\u202f+\u202f30\u202fwt% H2O2, US\u202f+\u202fFenton and US\u202f+\u202fTiO2. In addition, the authors investigated the effects of individually US/Fenton and US/TiO2 processes on desulfurization, but it was found that the desulfurization efficiencies of those processes were lower than the desulfurization efficiency of the US/HP/TiO2 process. It has been explained that the reason for the very high desulfurization of the US/HP/TiO2 process is the production of more hydroxyl radicals from HP along with the support of the TiO2 catalyst and the generation of additional hydroxyl radicals as a result of the increase of active sites by deformation of the catalyst under US. It was also stated that homogeneous distribution of the catalyst particles and better mixing due to the high turbulence caused by the collapsed bubbles enhance the sulfur removal. Cavitational yields (4.65\u202f\u00d7\u202f10\u20139, 4.71\u202f\u00d7\u202f10\u20139 and 6.61\u202f\u00d7\u202f10\u20139 g J\u22121 for US/Fenton reagent, US/TiO2 and US/HP/TiO2, respectively) were calculated by the authors to confirm the differences in desulfurization in the three processes. In this study, it was determined that the total cost of the other treatment methods was 2.22, 43.12, 14.69, 17.50, 1.255, 0.70 and 0.595 $ L\u22121 for US, only HP(30%(v/v), only Fenton, only TiO2, US\u202f+\u202fHP(30%(v/v), US\u202f+\u202fFenton and US\u202f+\u202fTiO2, respectively. Although a high sulfur removal is obtained from gum turpentine in the presence HP and TiO2 under US, oxidative desulfurization of DMDS as an aliphatic sulfur compound is quite easy, the initial sulfur quantity (50\u202fppm DMDS) is very low, and the reaction time is 120\u202fmin. Therefore, it is not a favorable method.In the studies performed by Yu et al. [80] and Zhao et al. [81], sonophotocatalytic oxidative desulfurization of hydrotreated diesel oil and model diesel oil using CdO as semiconductor and H2O2 as oxidant was investigated and desulfurization efficiencies were found to be 72.7 and 99.47%, respectively. The high desulfurization in the latter under 20\u202fkHz and 150\u202fW US can be attributed to primarily the use of the model sulfur solution prepared by dissolving the organosulfur compound in a solvent instead of hydrogenated diesel fuel, which consists of a complex mixture of aliphatic hydrocarbons and aromatic hydrocarbons [82], and acetic acid to increase the oxidizing power of H2O2 and secondarily to the catalyst with a smaller grain size (i.e. larger surface area) and more homogenized structure, which is synthesized under ultrasound [83], hence causing a higher catalytic activity.Behin and Farhadian [84] performed the ODS (followed by extraction with a binary solvent of methanol and water in ratio of 1:1 in volume) of nonhydrotreated kerosene with a total S content of 1553 ppmw at 0.05\u202fcm\u202fs\u22121 superficial gas velocity for 15\u202fmin. by passing ozone as a homogeneous photocatalyst through an airlift reactor and using H2O2 under both US of 20\u202fkHz frequency (60\u202fW power) and UV in a wavelength range of 280\u2013400\u202fnm. Despite a 48% loss of aromaticity due to ozone, and to a lesser extent polar solvent, a desulfurization efficiency of 91.7% was reached. It is revealed that the high desulfurization yield at optimum conditions was due to HO\u00b7 (oxidation potential [85] of hydroxyl radical, 2.80\u202fV) and HO2\u00b7 (oxidation potential [85] of hydroperoxyl radical, 1.7\u202fV) radicals formed in the mixture during the reaction rather than the increased mass transfer and the physical properties of raw kerosene are almost unchanged.In addition, sonolysis of sulfur compounds in water was carried out at high ultrasonic frequencies without using catalysts and oxidants. The dilute solution containing 21.46\u202fppm S BT in water was subjected to sonodegradation at 21\u202f\u00b0C under 352\u202fkHz and 80\u202fW US, and it was explained that the dominant mechanism was the oxidation of BT as a result of the formation of hydroxyl radicals from water [86]. However, in the sonolysis of a dilute T solution containing 32\u202fppm S in water at 22\u202f\u00b0C under 850\u202fkHz and 40\u202fW US power, it was revealed that the dominant mechanism was pyrolysis as a result of high temperature caused by collapsed cavitation bubbles rather than hydroxyl radical formed in the medium since T can diffuse readily into the cavitation bubble due to T's lower boiling point (i.e., more volatile) than BT [87].AOPs were utilized in combination with sonolysis. Despite high desulfurization under both US and UV or visible light in AOPs [71,80,81], where photocatalysts are used, these high desulfurization yields were reached in 6,5 and 3\u202fh, respectively, for the respective studies. In photocatalysis, a light energy such as UV or visible light is absorbed by photocatalyst (e.g., TiO2), and the electron is excited by passing from the valence band to the conduction band, and thus an electron-hole pair is formed on photocatalyst. The positive electron holes (h+) react with the water adsorbed on the catalyst to produce hydroxyl radicals. In addition, oxygen on catalyst surface reacts in series with the excited electron (e\u2013) to produce hydroxyl radicals and also US generates hydroxyl radical from HP. Consequently, enhanced hydroxyl radical production renders sonophotocatalytic ODS yield high. The reactions are as follows [88,89]:\n\n\n\nPhotocatalyst\n\n\n\u2192\n\n+\n\nh\n\u03bd\n\n\n\n\n\n\ne\n-\n\n\n+\n\n\nh\n+\n\n\n\n\n\n\n\n\nH\n2\nO\u202f+\u202fh\n+\u202f\u2192\u202fH\n+\u202f+\u202fHO\u2022\n\n\n\n\n\n\n2\n\nH\n2\n\nO\n\n+\n\n2\n\nh\n+\n\n\n\u2192\n\nH\n2\n\n\nO\n2\n\n\n+\n\n2\n\nH\n+\n\n\n\n\n\n\n\n\n\n\nH\n2\n\n\nO\n2\n\n\n\u2192\n\n)\n)\n)\n\n\n\n2\nH\nO\n\u00b7\n\n\n\n\n\n\n\n\n\nO\n2\n\n\n+\n\n\ne\n-\n\n\u2192\n\nO\n2\n\n\u00b7\n-\n\n\n\n\n\u2192\n\n\n2\n\nH\n+\n\n+\n\n\ne\n-\n\n\n\n\nH\n2\n\n\nO\n2\n\n\n\u2192\n\n\n\nH\n+\n\n+\n\n\ne\n-\n\n\n\n\nH\nO\n\u00b7\n\n+\n\nH\n2\n\nO\n\n\n\n\nIn the Sono-Fenton process, FeSO4 is used along with HP under US irradiation. In the Fenton reaction, Fe2+ is first oxidized by HP to produce the HO\u00b7 radical and then the reaction of Fe3+ with HP produces the complex intermediate Fe-OOH2+ which decomposes rapidly to form HO2\u00b7 radical and Fe2+ under US [90]. Fenton reaction is substantially accelerated by US [91]. As a result, sulfur removal further increases due to enhancement of hydroxyl radicals in organic-aqueous phase interfacial area. The medium must be acidic to maximize production of free radicals [92]. The reactions in the Sonofenton process are as follows [93]:\n\n\n\nF\n\ne\n\n2\n+\n\n\n\n+\n\n\nH\n2\n\n\nO\n2\n\n\n\u2192\nF\n\ne\n\n3\n+\n\n\n+\n\nH\nO\n\u00b7\n+\n\nO\n\nH\n-\n\n\n\n\n\n\n\n\n\nF\n\ne\n\n3\n+\n\n\n\n+\n\n\nH\n2\n\n\nO\n2\n\n\n\u2192\nF\ne\n\n\n-\n\n\nO\nO\n\nH\n\n2\n+\n\n\n\n+\n\n\nH\n+\n\n\n\n\n\n\n\n\n\nFe\n\n\n-\n\n\nO\nO\n\nH\n\n2\n+\n\n\n\n\u2192\n\n\n)\n)\n)\n\n\n\n\nF\n\ne\n\n2\n+\n\n\n\n+\n\nH\n\nO\n2\n\n\u00b7\n\n\n\n\nAs noted above, reaction times are very high in studies [71,80,81], where photocatalyst was used. Therefore, this will lead to higher electrical energy consumption for US, UV and heating, if any, increasing the operating cost in AOP.In the study [84], in which ozone and HP were used as oxidant under US-UV, it was explained that the reason for high sulfur removal in a short time was indirect hydroxyl radical production from O3 and direct hydroxyl radical from HP by UV and US. In addition, it is stated that ultrasound greatly accelerates the gas\u2013liquid mass transfer through micro-streaming produced by the violent collapse of bubbles and allows ozone to react with sulfur compounds by increasing the gas\u2013liquid interfacial area. Moreover, dissolved ozone gas acts as nucleation sites to form cavitation bubbles, causing the formation of more cavitation bubbles [94]. Thus, this synergistic effect accelerates significantly the ultrasound-assisted photo oxidative desulfurization reaction rate.In a sonophoto-fenton process [95] in which oxalic acid was used, a sulfur removal of>93% was achieved from 100\u202fppm DBT in toluene at 0.05\u202fmol L\u22121 Fe2+ concentration, 0.15\u202fmol L\u22121 oxalate concentration, pH\u202f=\u202f2, a volume ratio (organic phase/HP) of 10:1, 25\u202f\u00b0C and 15\u202fmin under both 37\u202fkHz, 95\u202fW indirect US and UV in the presence of air. It was revealed that FeII(C2O4), which is formed by the reaction of Fe2+ with oxalate anion (C2O4\n2\u2212) in the reaction medium, as well as \n\n\n\nFe\n\n\nII\n\n\n\n\n(\n\nC\n2\n\n\nO\n4\n\n)\n\n\n2\n\n\n2\n-\n\n\n\n complex which is formed by the reaction of FeII(C2O4) with C2O4\n2\u2212, is responsible for this high desulfurization. The authors reported that FeII(C2O4) and \n\n\n\nFe\n\n\nII\n\n\n\n\n(\n\nC\n2\n\n\nO\n4\n\n)\n\n\n2\n\n\n2\n-\n\n\n\n caused the formation of HO\u00b7, HO2\u00b7 and O\u00b7 radicals in the aqueous phase to oxidize DBT under US and UV irradiation. It was stated that Fe(II)-oxalate complex as catalyst can be reused three times (a decrease of 1.33 and 1.56% for the first and second run, respectively) without significantly losing its activity by regenerating it after each reaction.The effect of solid catalysts to increase ODS has also been studied [96], and it was found that the use of US for total desulfurization of 2,3-DMBT and 2,3,7-TMBT, which are the two most abundant components in JP-8 fuel, in the presence of H2O2, formic acid and phosphoric acid-activated carbon increases the total desulfurization in the absence of US (mechanical stirring) by around 2.4-fold. It is also reported that desulfurization by chemically activated carbon (MW-99) with phosphoric acid is superior to desulfurization by thermally activated carbon (Norit SX-1) due to the larger surface area of MW-99 and the greater number of its surface acid centers. Sulfur removal of 98 and 94% (followed by adsorption with activated alumina) from JP-8 and diesel, respectively, was performed with MW-99 under optimum conditions (65\u202f\u00b0C, 2\u202fh, 60% amplitude, 20\u202fkHz sonication, pH\u202f=\u202f1.4).Khlaif and Bded [97] carried out the ODS (followed by extraction) of crude oil containing 1.95% total S by weight in the presence of US and AC using different volumes of acetic acid and 50\u202fwt% H2O2. As a result of the increase of the amount of AC used from 3 to 9\u202fg, the number of active sites in AC increased, thus improving ODS and an optimum desulfurization of 81.325% was obtained by using 9\u202fg AC, 40\u202fmL H2O2, 30\u202fmL acetic acid at 50\u202f\u00b0C.Using phosphotungstic acid (H3PW12O40@ TMU-17-NH2) incorporated in robust zinc-based MOF with enhanced efficiency as a solid catalyst, simultaneous extraction and oxidation of model oil containing BT, DBT and 4,6-DMDBT, each of which has concentration of 500\u202fmg L\u20131, were performed in the presence of acetonitrile under indirect sonication of 37\u202fkHz [98]. Although the pore volume and surface area (137\u202fcm3 g\u22121 and 814\u202fm2 g\u22121) of the composite MOF catalyst formed by encapsulating H3PW12O40 in TMU-17-NH2 were lower as compared to those of the neat MOF (239\u202fcm3 g\u22121 and 1050\u202fm2 g\u22121), a sulfur removal of 98, 87 and 71% was reached with 20\u202fmg of the MOF composite containing 20\u202fwt% phosphotungstic acid at model oil/MeCN 1:1\u202fvol ratio, O/S ratio of 2:1 and room temperature for DBT, 4,6-DMDBT and BT, respectively, at the end of 15\u202fmin. The reason for the lower reactivity of 4,6-DMDBT compared to DBT is that the alkyl substituted aromatic compound is sterically prevented from entering the 3D framework. Also lower desulfurization was achieved with DMF solvent instead of MeCN depending on the fact that adsorption of solvent on the heterogeneous catalyst increases with increasing boiling point [99,100] and polarity [101\u2013103]. The low desulfurization with DMF can be attributed to the fact that not only the boiling point of DMF (153\u202f\u00b0C) is significantly higher than that of MeCN (82\u202f\u00b0C) [104] but also higher polarity [105] of the former compared to the latter causes stronger interaction with Zn2+ in the modified MOF composite [106], thus reducing adsorption of DBT. The former is bound to Zn2+ cations in the MOF composite [106]. The three possible adsorption mechanisms [107,108] are \u03c0-\u03c0 interaction between sulfur compounds and aromatic rings of modified MOF, hydrogen bonding between NH2 groups and S, and strong Zn2+-S interaction between phosphotungstic acid-TMU-17-NH2 and aromatic sulfur compounds. TMU-17-NH2 is probably structurally similar to TMU-16-NH2 with positive zeta potential [109]. H2O2 and aromatic sulfur compounds are adsorbed on the catalyst, the phosphotungstic acid anion is oxidized with hydrogen peroxide and as a consequence, the polyoxoperoxo complex anion formed oxidizes aromatic sulfur compounds [110]. In addition, water in the reaction medium can result in the radical decomposition of H2O2 by forming an aqueous complex with Zn2+ in Zn(II)-based MOF, hence generating a strong oxidant radical HO\u00b7[111] and electrophilic activation of hydrogen peroxide to convert sulfur compounds to their sulfoxides as oxidized sulfur compounds is caused by Zn-based MOF [112].Metalloporphyrin [113] and metallophthalocyanine [114] catalysts, which are metal complexes, are also used in ODS reactions. Metal removal from the latter is not easy compared to the former [115]. The degree of ODS can be changed by adding different electron-withdrawing or electron-donating substituents to these complexes [113,116]. In addition, the stability of these complexes can be increased by forming nanocomposite catalysts, thus ensuring that they can be reused in oxidation reactions [117].Wang et al. fulfilled two separate studies [118,119] concerning sonocatalytic ODS (followed by extraction with methanol) of benzothiophene in the presence of H2O2 at 60\u202f\u00b0C using core\u2013shell nanosphere modified with metallophthalocyanine (tetra-substituted carboxyl iron phthalocyanine, FeC4Pc) encapsulated into magnetic mesopore silica nanoparticles and silica nanotube catalyst with magnetite nanoparticles-coated interior surface and FeC4Pc-modified inner and outer surface. Higher desulfurization of the former (at the same conditions, desulfurization near 94.5%) compared to the latter (76% desulfurization yield at 30\u202fmin and molar ratio of H2O2/S\u202f=\u202f15) can be considerably clarified by the fact that the particle size (60\u202fnm) and the average pore size (2.6\u202fnm) of the nanosphere composite catalyst are smaller than the outer diameter of the nanotube catalyst (200\u202fnm), hence providing larger surface area for adsorption, though the catalyst loading is not specified in the latter. In these two studies, it was reported that high desulfurization is due to the radical decomposition of H2O2 to HO\u00b7 on metallophthalocyanines. HO\u00b7 radical from H2O2 by ultrasound wave can also be formed [120]. It is also stated that both catalysts can be easily isolated from the mixture by applying an external magnetic field after the reactions due to their superparamagnetic properties and reused in the next reactions.Uniform Ni skeletal catalyst was synthesized at a size of 2.5\u201310\u202f\u00b5m under 90\u202fkHz ultrasound and crude oil containing 2.645% S by weight is subjected to oxidation with two treatment cycles using a mixture of ozone-air and 0.2% by weight catalyst based on the oil volume for 5\u202fmin in a US bath with frequency of 22\u202fkHz [61]. Sulfur removals from gasoline and diesel fractions in crude oil were found to be 52 and 27.4%, respectively, as well as improvement of gasoline and diesel fractions.By using 0.5\u202fg of the modified GO/COOH solid catalyst with increased surface acidity formed by the addition of \u2013CH2COOH group to the epoxy or hydroxyl groups of GO as a result of the reaction of graphene oxide (GO) with chloroacetic acid, a desulfurization of 95%, which is higher than desulfurization in the case of using non-acidified GO, was performed from the DBT solution containing 1000\u202fppm S with 30\u202fwt% H2O2 within 300\u202fmin on sonication [121]. It was put forward that the adsorption-oxidation mechanism is the conversion of DBT to DBT sulfone by the peroxyacid group formed on the GO/COOH surface via activation of H2O2 by the carboxyl group in GO, and then \u03c0-\u03c0 interaction of DBT sulfone with GO/COOH and adsorption of DBT sulfone through hydrogen bonding. In addition, it was stated that ultrasound contributes to high desulfurization due to the increase in the surface area caused by the exfoliation of GO/COOH as well as the increased collision frequency of the reactants due to the significantly increased mass transfer.As phosphotungstic acid hydrate as oxidizing agent is dissolved in the aqueous phase, thus making it difficult to be reused by recovery [122], activated carbon-supported phosphotungstic acid (PTA) catalysts were synthesized and two separate studies [123,124] were carried out on UAODS of 2000 ppmw DBT. In the first study [123], a DBT conversion of 93.4% was reached using 40\u202fmL of model oil, at PTA/AC-10 catalyst/model oil 1.25: 100 mass ratio and H2O2/model oil 0.1\u202fvol ratio under 70\u202fW US power at 60\u202f\u00b0C and 10\u202fmin, while in the second study [124] under the same conditions except the use of US at 100\u202fW power, DBT conversion well below the conversion reached in the first study was obtained. The reason for the low conversion can be attributed to the weakening of the ultrasound wave (bubble shielding effect) as a consequence of absorption and scattering of US waves by these bubbles by resulting in the formation of dense cavitation bubble cloud around the transducer under high power [125]. Therefore, an optimum power intensity is needed as an important factor for high conversion in liquid phase reactions. In both studies, it was reported that desulfurization improved due to the increase in the number of surface acid sites by the increase in the amount of phosphotungstic acid in AC, and beyond a certain phosphotungstic acid amount, the sulfur removal is unchanged due to the reduction in surface area as a result of the destruction of microchannels in AC and the occupation of pores in AC by phosphotungstic acid.In a similar study [126] where the same catalyst (HPW/AC-10) was synthesized, the optimum conditions were determined using RSM for reasonable desulfurization of the model oil containing 2800\u202fppm S consisting of a mixture of DBT, BT and T in the presence of individually, 30, 20 and 10\u202fwt% H2O2 at different catalyst quantities, different AP/OP volume ratios and different times under 37\u202fkHz US. By applying these optimum parameters to kerosene with 1370 ppmw S, a 99% desulfurization was successfully achieved, followed by four-cycle extraction.In a study [127] where O2 in air was used as oxidant instead of thermally unstable H2O2, modified heteropolyacid catalysts (H5PV2Mo10O40/SiO2 and H5PV2W10O40/SiO2) supported on silica were synthesized. At optimum conditions (catalyst weight/model oil volume 11.09\u202fg L\u20131, POM weight /SiO2 (wt. %) 39.879, sonication time 199.209\u202fmin.) found using the response surface method at 65\u202f\u00b0C and 1.3 L min\u22121 air flow rate, a higher desulfurization (90 vs. 70%) of DBT was achieved in a shorter time (199 vs. 360\u202fmin.) under 20\u202fkHz and 360\u202fW direct US compared to the desulfurization in the case in which ultrasound is not used. It was demonstrated that the reason for low desulfurization is the polymerization of DBT due to the low concentration of oxygen dissolved in the organic phase (limited aerobic medium) under magnetic stirring, thus causing the polymer formed to accumulate on the modified heteropolyacids. While this polymerization is thought to be probably initiated by the DBT cation radical formed as a result of electron transfer from DBT to vanadium incorporated heteropolyacid [128], it was found that US increases the dissolved oxygen concentration and prevents polymer deposition on the catalyst surface. DBT conversion 10% more with H5PV2W10O40/SiO2 than the conversion percentage with H5PV2Mo10O40/SiO2 was obtained since the standard reduction potential of V5+ and W6+ (1 and \u22120.090\u202feV, respectively) is higher than that of Mo6+ (-0.913\u202feV), thus having stronger oxidizing power [129,130]. The oxidation mechanism [131,132] in the UAODS system can be elucidated by the electron transfer-oxygen transfer (ET-OT) reaction, in which oxygen is involved, between the modified heteropolyacid and DBT as follows:\n\n\n\n\nC\n12\n\n\nH\n8\n\nS +\n\n\n\n\n\nP\n\nV\n\n2\n\n5+\n\n\nW\n10\n\n\nO\n40\n\n\n\n\n\n5\n-\n\n\n\n\n\u2192\n\n\nET\n\n\n\n\nC\n12\n\n\nH\n8\n\n\n\nS\n\n\n+\n\u2219\n\n\n\n+\n\n\n\n\n\nP\n\n\nV\n\n5+\n\n\n\nV\n\n4+\n\n\nW\n10\n\n\nO\n40\n\n\n\n\n\n6\n-\n\n\n\n\n\n\n\n\n\n\n\nC\n12\n\n\nH\n8\n\n\n\nS\n\n\n+\n\u2219\n\n\n\n+\n\n\n\n\n\nP\n\n\nV\n\n5+\n\n\n\nV\n\n4+\n\n\nW\n10\n\n\nO\n40\n\n\n\n\n\n6\n-\n\n\n\n\n\u2192\n\n\nOT\n\n\n\n\n\n\n\nP\n\n\nV\n\n5+\n\n\n\nV\n\n4+\n\n\nW\n10\n\n\nO\n39\n\n\n\n\n\n6\n\u2013\n\n\n\u2013\nO\n\u2013\n\nC\n12\n\n\nH\n8\n\nS\n\n\n\n\n\n\n\n\n\n\n\n\nP\n\n\nV\n\n5+\n\n\n\nV\n\n4+\n\n\nW\n10\n\n\nO\n39\n\n\n\n\n\n6\n\u2013\n\n\n\u2013\nO\n\u2013\n\nC\n12\n\n\nH\n8\n\nS\n\n\n\u2192\n\n\nOT\n\n\n\n\n\nC\n12\n\n\nH\n8\n\nSO+\n\n\n\n\n\nP\n\nV\n\n2\n\n4+\n\n\nW\n10\n\n\nO\n39\n\n\n\n\n\n5\n\u2013\n\n\n\n\n\n\n\n\n\n\n\n\n\n\nP\n\nV\n\n2\n\n4+\n\n\nW\n10\n\n\nO\n39\n\n\n\n\n\n5\n\u2212\n\n\n\n\n\n\u2192\n\nOxidation by oxygen\n\n\n\n\n+\n\n\n\nO\n\n2\n\n\n+ 2\n\n\nH\n\n+\n\n\n\n\n\n\n\nP\n\nV\n\n2\n\n5+\n\n\nW\n10\n\n\nO\n40\n\n\n\n\n\n5\n-\n\n\n\n+\n\n\nH\n2\n\nO\n\n\n\n\nModel oil with 1000\u202fppm total S content containing BT, DBT and 4,6-DMDBT was sonicated at 300\u202fW, 45% amplitude and 20\u202fkHz fixed frequency using 30\u202fwt% H2O2 in the presence of MoO3 supported on \u03b3-Al2O3 catalyst for 30\u202fmin [133] and at the optimum conditions (H2O2/S\u202f=\u202f3\u202fmolar ratio, 45\u202f\u00b0C, 30\u202fg L\u20131 catalyst/model oil ratio) found by RSM with central composite design, a DBT\u202f\u2192\u202fDBT sulphone conversion above 98% was found. Moreover, a desulfurization improvement of over 95% was achieved for DBT even after 6 cycles without losing the catalyst effect, due to US, which prevents the agglomeration of catalyst particles and H2O2 and causes desorption of adsorbed polar sulfones and water impurities from the catalyst surface. For BT, DBT and 4,6-DMDBT, the highest desulfurization was achieved when the MoO3 content on the catalyst was 10\u202fwt% and at this loading, it was proved by XRD analysis that MoO3 is homogeneously dispersed on the support and MoO3 crystals are not seen. It was suggested that the sulfur compounds are oxidized by highly reactive molybdenum peroxide and molybdenum diperoxides formed in situ.In a similar study [134] where the same reagents and the same ultrasonic parameters were used, complete oxidation of DBT in the model oil containing 600 ppmw total S was achieved in the presence of MoO3 loading of 10 wt.%/Al2O3 at H2O2/S\u202f=\u202f3.8\u202fmolar ratio, 30\u202fg L\u20131 catalyst/model oil ratio, 45\u202f\u00b0C and 30\u202fmin. Besides, the addition of aromatic compounds (tetralin, naphthalene and 2-methyl naphthalene) individually to the model oil formed by dissolving DBT in hexane to mimic diesel fuel appreciably reduced the UAODS yield although the resulting DBT selectivity is high due to the competitive adsorption of the aromatic compounds on the catalyst surface. Further, in both studies [133,134] it was shown that the active sites responsible for the adsorption of sulfur compounds are tetrahedrally coordinated Mo6+ oxides, above a Mo-saturated monolayer coverage (which is at 10\u202fwt% Mo loading), agglomeration of amorphous MoO species results in the formation of MoO3 crystals and cause a reduction in the number of active sites, as well as the reduction of surface area, by blocking micropores of the catalyst [135], thus reducing the UAODS.Using persulfate agent in toluene and hexane as solvent, 98\u202fwt% H2O2 and 1% Si-Al/Al2O3 as solid catalyst, 99.72% of sulfur (followed first by extraction with acetone, then by adsorption with activated charcoal and ultimately by sonication under 30\u202fkHz US of the diesel sample treated with acetic acid) in hydrotreated diesel fuel containing 766.73 ppmw total S was removed at around 65\u202f\u00b0C and atmospheric pressure [136]. It can be thought that the oxidation mechanism [137] is based on sulfate ion radical caused by thermal activation of persulfate, hydroxyl radical formed as a result of the reaction of sulphate ion radical with H2O2 and activation of S2O8\n2\u2212 by hydrogen peroxide, which causes the formation of hydroxyl radical. Moreover, US can cause homolytic cleavage of the persulfate agent [138] and hydrogen peroxide [90]. The surface hydroxyl groups [139] on Al2O3 (Fig. 3\n) in the solid catalyst in the reaction medium can induce the formation[137] of \n\nS\n\nO\n\n4\n\n\n\u2219\n-\n\n\n\n radical from persulfate by interacting with H+ formed by the reaction (4) and, hence accelerating the UAODS reaction.\n\n(1)\n\n\n\nS\n2\n\n\nO\n8\n\n2\n-\n\n\n\u2192\n\n2\nS\n\nO\n4\n\n\u00b7\n-\n\n\n\n\n\n\n\n\n(2)\n\n\n\nS\n2\n\n\nO\n8\n\n2\n-\n\n\n\n+\n\n\nH\n2\n\n\nO\n2\n\n\n\u2192\n2\nS\n\nO\n4\n\n\u00b7\n-\n\n\n\n+\n\n2\nO\nH\n\u00b7\n\n\n\n\n\n\n(3)\n\n\n2\nS\n\nO\n4\n\n\u00b7\n-\n\n\n\n+\n\n\nH\n2\n\n\nO\n2\n\n\n\u2192\n2\nS\n\nO\n4\n\n2\n-\n\n\n\n+\n\n2\nO\nH\n\u00b7\n\n\n\n\n\n\n(4)\n\n\n\nS\n2\n\n\nO\n8\n\n2\n-\n\n\n+\nO\nH\n\u00b7\n\u2192\n\nS\n\nO\n4\n\n2\n-\n\n\n+\n\nS\n\nO\n4\n\n\u00b7\n-\n\n\n+\n\n1\n/\n2\n\nO\n2\n\n+\n\n\nH\n+\n\n\n\n\n\nSince homogeneous Fenton catalysts (FeSO4) dissolve in the aqueous phase and consequently, making their recovery difficult [140] after ODS reactions, water-insoluble Fenton-like catalysts supported on coal fly ash (which is a very cheap waste from coal-fired power plants) were synthesized [141]. Approximately 30% desulfurization was carried out as a result of simultaneous oxidation and extraction of sulfur compounds from commercial diesel fuel containing 595\u202fppm S using 10\u202fwt% H2O2 and ethanol solvent in the presence of the Fenton-like catalyst in an ultrasonic bath at 47\u202fkHz frequency and 147\u202fW power [65]. It has been suggested that the oxidation stems from the hydroxyl radicals formed from the reaction between Fe2+ and H2O2. Hydroxyl radicals [90] formed from the decomposition of H2O2 by US may also contribute to this desulfurization. Furthermore, since coal fly ash contains metal oxides [142], H2O2 helps desulfurization by being adsorbed on the supported catalyst as well as forming surface-bound hydroxyl radicals on the support [143].US has also been applied to oil sands [144] as an oil deposit consisted of a mixture of clay, sand, bitumen and water. A total sulfur removal efficiency of 82% has been reported by simultaneous oxidative and extractive desulfurization of semi-solid Alberta bitumen containing 5.2\u202fwt% S using 3\u202fwt% H2O2, saturated NaOH and tetrahydrofuran under a 28\u202fkHz frequency and 200\u202fW powerful indirect ultrasound at 20\u202f\u00b0C and 20\u202fmin [145]. Then, an 88% bitumen recovery from oil sand and a 42% sulfur reduction from bitumen was fulfilled using the same reagents, the same reaction conditions and ultrasonic parameters simultaneously. In addition, possible metalloporphyrins [146\u2013148] in bitumen can accelerate the UAODS reaction of bitumen. Moreover, it was stated that since ionic NaOH cannot dissolve oil sand sufficiently and effectively, mid-polar THF is used owing to its high dissolving power.The UAODS process was not limited to liquid fuels, but also applied to mesophase materials [149]. It was demonstrated a sulfur removal (followed by extraction with equal volumes of methanol and sodium hydroxide (0.5\u202fwt%)) of 91.1% from coal tar pitch with 0.9\u202fwt% S containing predominantly polycyclic aromatic hydrocarbons (also called polynuclear aromatic hydrocarbons) was carried out using xylene as dispersant and solvent, trichloroacetic acid as catalyst, 30\u202fwt% H2O2 in the absence of surfactant under 20\u202fkHz and 300\u202fW direct US at 60\u202fmin. and 70\u202f\u00b0C [150]. On the other hand, the use of surfactant did not increase UAODS.Apart from hydrogen peroxides, organic peroxide has also been used as oxidant. In this type of study [151], approximately 35% desulfurization (followed by extraction three times with acetonitrile) was performed from a high-viscosity bunker-C oil MFO 380 (max kinematic viscosity 380 cSt) with 3.17\u202fwt% S using viscosity-reducing heptane and 3\u202fmL of t-butyl hydroperoxide as oxidant in the presence of 0.2\u202fg MoO3 as solid catalyst under direct US at a frequency of 20\u202fkHz and 70% amplitude at atmospheric pressure, 90\u202fmin and 80\u202f\u00b0C. Unlike HP, TBHP has the advantage of being soluble in both aqueous and organic phases, therefore, in desulfurization reactions where the aqueous phase is not used, it is in direct contact with sulfur compounds without the need for mass transfer. It was reported that the much higher-reactivity peroxo molybdenum complex formed as a result of the reaction of t-BHP with MoO3 is responsible for the oxidation of sulfur compounds to their sulfones. When ultrasonic cavitation bubbles in sonochemistry implode, very high temperatures and pressures occur locally in the liquid (hot spot theory) [152]. Therefore, it can be deduced that reactive oxygen species, which are generated by thermal decomposition of t-BHP in this reaction, such as t-butoxyl (H3C)3 - O\u00b7, hydroxyl HO\u00b7 and t-butyl peroxyl (H3C)3 - O - O\u00b7 radicals [153], further contributes to the oxidation of bunker-C oil.In the presence of heterogeneous catalysts with which sulfur compounds interact electronically on the solid surface, adsorption, where mass transfer is an important factor, takes place through catalyst pores [154], whereas homogeneous catalysts dissolve in liquid (ie, aqueous phase). After UAODS reactions, isolation, recovery and reuse of homogeneous liquid catalysts, as well as the homogeneous solid catalysts dissolved in the aqueous phase, from the reaction mixture are quite problematic since they are in the same phase as reactants, which increases the process cost [155].Reactions, in which homogeneous catalysts are involved, can be divided into two classes; 1) Reactions in the absence of PTC 2) Reactions in the presence of PTC. Among the homogeneous solid catalysts, catalysts such as phosphotungstic acid [156] as polyoxometallate class, Fe(II)SO4\n[157] and CuSO4\n[158] were employed, while organic acids such as acetic acid [159] and formic acid [160] were utilized as homogeneous liquid catalysts.In the absence of PTC, the ODS mechanism [161] is shown in Scheme 2\n. Peroxyformic acid formed in situ by the reaction of HP and formic acid in aqueous phase is transferred to the organic phase where DBT is oxidized, by the effect of ultrasound.In a study [162] where the sonoreactor was optimized to increase the UAODS yield, a sulfur removal of 98.25% was achieved from model fuel containing 1000 ppmw DBT in n-decane using 16\u202fmL of 34.5\u202fwt% H2O2 and 40\u202fmm-diameter sonotrode with an immersion depth of 3\u202fcm at acetic acid/H2O2 64: 300\u202fmolar ratio in 7.4\u202fcm-diameter glass reactor under 20\u202fkHz, 500\u202fW and 80% amplitude direct US at 48\u202f\u00b0C within 30\u202fmin.UAODS of a model fuel containing 100 ppmw DBT (10.8\u202fmM/l) in toluene was performed using FeSO4, acetic acid and 30\u202fvol% hydrogen peroxide (HP) [163]. It was stated that the hydroperoxyl radicals formed were responsible for the oxidation of the sulfur compound rather than the hydroxyl radicals formed, hence by explaining that lower scavenging of HO2\u00b7 radicals is important. An DBT removal of approximately 33.34\u202fwt% from model oil has been reached at acetic acid/HP\u202f=\u202f2\u202fvol ratio, toluene/HP\u202f=\u202f10\u202fvol ratio, at 1.5\u202fM Fe2+ concentration, 90\u202fmin and atmospheric pressure under 70\u202fW and 35\u202fkHz indirect US at 25\u202f\u00b0C.In a similar study [164] in which desulfurization of benzothiophene (BT), 3-methyl thiophene (3-MT) and thiophene (T) was performed using 25\u202fmL of 30\u202fvol% HP\u202f+\u202fCH3COOH and Fe2+, sulfur removals of 79.4, 77.9, 77% \u2212 76.3, 76.9, 77.6% and 77.5, 76.5, 76.1% were obtained from concentrations of 100, 300 and 500\u202fppm for BT, 3-MT and T, respectively, under 2.5\u202fbar, 35\u202fW and 35\u202fkHz indirect US at 90\u202fmin and 25\u202f\u00b0C, such that these conversions were higher than those obtained at atmospheric pressure due to the elimination of transient cavitations at high pressure. In addition, according to the cavitation bubble dynamics model, it was revealed that the high desulfurization is caused by the sonophysical effect (microconvection) of US.In a study [165] in which a sample of raw coal containing 2.16\u202fwt% total S as solid fuel was treated with peroxyacetic acid, oxidative desulfurization of raw coal improved due to the increased reactivity of the coal depending on the increased specific surface area, the total pore volume and the mean pore size of \u200b\u200bthe treated coal compared to those of the untreated coal since abrasion of coal particles upon sonication occurs; 17.59% of the total sulfur present in the coal was removed using 10\u202fmL 98\u202fwt% acetic acid and 50\u202fmL 30\u202fwt% HP under 20\u202fkHz and 720\u202fW direct US at 30\u202f\u00b0C within 5\u202fmin. It was shown that the greatest contribution to desulfurization is that US increases the production of hydroxyl radical in the presence of HP and acetic acid in the mixture, whereas the hydroxyl radical production rate is significantly low when there is only HP.In a similar study [166] in which the same reactants were used, the raw coal was subjected to ultrasonic treatment followed by microwave. The US applied reduced the particle size of the coal, increased its total porosity (i.e., specific surface area, total pore volume and average pore diameter of the raw coal are 0.88\u202fm2.g\u22121, 0.00213\u202fcm3.g\u22121 and 9.68\u202fnm, respectively, whereas specific surface area, total pore volume and average pore diameter of the coal sample after US treatment are 1.66\u202fm2.g\u22121, 0.00771\u202fcm3.g\u22121 and 18.56\u202fnm, respectively) and increased hydroxyl radicals. But at the same time, microwave increased the reaction rate dramatically as the reactants in the mixture absorbed the electromagnetic radiation generated [167]. At the end of the ultrasonic treatment at acetic acid (98\u202fwt%)/HP (30\u202fwt%) 1:5\u202fvol ratio under 20\u202fkHz and 720\u202fW direct US for 50\u202fmin at 40\u202f\u00b0C, followed by microwave treatment under 600\u202fW power at a frequency of 2.45\u202fGHz at 100\u202f\u00b0C for 6\u202fmin, a desulfurization of nearly 22% was obtained from raw coal containing 1.93\u202fwt% organic S, which results from the resonance nature of the thiophenic compound according to mercaptan and sulfoether, whereas the percentage of pyritic sulfur (in the form of FeS2) removed as inorganic sulfur was reported to be about 85%.In another study [168] using the same reactants, two coal samples (XS with 0.85\u202fwt% organic S and YN with 2.69\u202fwt% organic S) completely free of inorganic sulfur as a result of pretreatment with dilute nitric acid were subjected simultaneously to ultrasonic and microwave treatment with a power of 560\u202fW each for 50\u202fmin. Sulfur removals of 23.53 and 76.58% were achieved for XS and YN, respectively. Consequently, it turns out that from these three studies concerning coal, simultaneous operation (US-MW) is more efficient.In desulfurization of model fuels prepared by dissolving model sulfur compounds in a non-polar solvent such as octane, heptane or hexane, an extraction step is not required since the sulfones as oxidized sulfur compounds are easily determined by instrumental devices such as GC-FID, HPLC, hence easily finding the conversion to sulfones. However, as there are also aliphatic and aromatic hydrocarbons in addition to sulfur compounds in real fuels, it is not possible to determine the sulfur compounds with these devices. After separating sulfones by an extractant, the total sulfur percentage in the fuel can be determined by using devices such as microcoulometric analyzer, sulfur analyzer with UV fluorescence, XRF and GC-SCD.Alkaline solutions have also been used in UAODS. In simultaneous oxidative and extractive desulfurization [169] of ultra low-sulfur diesel spiked with 500 ppmw 4,6-DMDBT, it has been shown that desulfurization in single step can be improved without an extraction step mainly due to the hydroxyl radicals formed as well as secondarily the formation of carbonate radical \n\nC\n\nO\n\n3\n\n\n-\n\u2219\n\n\n\n by resulting in radical decomposition of HP under US in the range of pH 6\u202f\u223c\u202f8 with basic sodium carbonate. Approximately 94% desulfurization was reported at diesel/acetonitrile 1: 2\u202fvol ratio, 0.8\u202fM HP 30\u202fwt%, 30\u202fmM Na2CO3 under 23\u202fkHz frequency direct ultrasonic pulse at 60\u202f\u00b0C in 2\u202fh.As shown in Table 1\n, the other studies [170\u2013175] using acetic acid as organic acid in addition to HP are common in the literature. In addition, acetic acid is relatively low-cost [176]. In studies [97,165,177] in which desulfurization of crude oil, coal and model diesel fuel with the help of US by using acetic acid-HP oxidant system was performed, it was indicated that high desulfurization efficiency is reached in a short time at relatively low temperatures. The oxidation of sulfur compounds is caused by peroxyacetic acid and hydroxyl radicals formed in situ in the aqueous phase. It has also been shown that nitrogen compounds have an inhibitory effect on oxidative desulfurization as the oxidation reactivity of the nitrogen compounds present in the fuel (e.g. quinoline) is higher than that of the sulfur compounds [172]. Moreover, the effects of different US loop reactor types on UAODS were also examined [173]. It is stated that the aqueous phase separated after the UAODS reaction and the extractant separated after the extraction step can be reused for the fresh feedstocks containing 208 ppmw S DBT and the same feedstocks subjected to oxidation treatment, respectively, though the desulfurization efficiencies in reuses are lower than those in their first uses [171]. UAODS efficiencies of diesel fuel feeds containing different sulfur amounts in the presence of acetic acid under the relevant reaction conditions are shown in Table 1.One of the most important reasons why HDS is still widely used today is that fuel loss after HDS process is very low [178]. In laboratory-scale studies, after the ODS process, the properties of the fuel are almost unchanged [172,179\u2013186], but the loss of fuel in the extraction step (i.e., the reduction of fuel recovery) after the ODS process on large scales can pose a major problem. Moreover, whether the properties such as density, viscosity, cetane number, boiling range distribution of the desulfurized fuel produced in large quantities (factory scale) have changed is a matter of investigation separately and must be checked one by one. In most research papers [171,187\u2013189], when H2O2/S mole ratio initially increases, desulfurization generally increases, then reaches a certain value and decreases slowly after this optimum value. It was reported that this decline is due to dilution of the aqueous phase.In a study [172] in which nitrogen was removed by US from a synthetic fuel solution with 252 ppmw N prepared by dissolving quinoline in a hydrotreated petroleum product feed containing 3.6\u202fppm S, 92% nitrogen removal (followed by extraction with methanol) was achieved in the case where only acetic acid is used in the absence of HP as oxidant. It is stated that this value is higher than the value (79% nitrogen removal) obtained without oxidation treatment by only liquid\u2013liquid extraction with methanol, hence underlining that acetic acid has the capacity to extract nitrogen compounds.The effects of different sonoreactor types on desulfurization and denitrogenation (followed by silica gel adsorption) of hydrotreated diesel fuel containing 241 ppmw S and 161 ppmw N were also evaluated [173]. It was shown that the most effective reactor in terms of cost and performance optimization was sonitube.In an oxidation study [190] accomplished under 20\u202fkHz and 70\u202fW direct US followed by extraction with DMF, it was stated that while the initial sulfur content in the model fuel containing DBT increased from 1220.80 ppmw to 3976.86 ppmw, desulfurization also increased to 98.35%. In the UAODS [175] followed by extraction, as acetic acid/oil ratio increased to 1.50\u202fwt%, the desulfurization of diesel containing 849 ppmw S improved. This was attributed to the strong oxidant peracetic acid formed in situ.Heterogeneous reactions with solid\u2013liquid systems using solid oxidants were also carried out. HP-acetic acid at S/oxidant 1:10\u202fmolar ratio, KO2-Acetic acid, Na2S2O8 alone, Na2S2O8-acetic acid and oxone alone at S/oxidant 1:10 and 1:30\u202fmolar ratios at different times at 80\u202f\u00b0C were used [186] for UAODS of model oils and diesel fuel. Sulfur and nitrogen removal were individually performed by ultrasonic horn device under 21.1\u202fkHz and 80\u202fW direct US and ultrasonic cup horn device under 19.9\u202fkHz and 80\u202fW US from mild hydrotreated diesel feedstock containing 226\u202f\u00b1\u202f2.17 ppmw total S and 158\u202f\u00b1\u202f2.81 ppmw total N as well as three model solutions containing 1.2\u202fmg\u202fmL\u22121 DBT or DMDBT and 1.2\u202fmg\u202fmL\u22121 quinoline individually. In UAODS reactions of model solutions in both reactor types, when oxone alone is used at a molar ratio of S/oxidant\u202f=\u202f1:30 without acetic acid, very high desulfurization efficiencies compared to other oxidant systems (100% sulfur removal for DBT and DMDBT in 90\u202fmin, a nitrogen removal of 40% for quinoline in the same time) were achieved. For scale-up purposes, the US cup horn was chosen as it closely resembles the geometry of continuous flow reactors and sulfur was removed (followed by SiO2 adsorption) from hydrotreated diesel fuel at molar ratios of (S\u202f+\u202fN)/oxidant 1:10, 1:20 and 1:30 by oxone at different times. In addition to obtaining a diesel fuel containing 0.91\u202f\u00b1\u202f0.48 ppmw N (a nitrogen removal of 99.4%) at a molar ratio of 1:30 in 90\u202fmin, a sulfur removal of 99% was achieved. In the case of extraction with MeOH instead of adsorption, significantly low desulfurization (65%) was obtained for the same molar ratio and the same time, but diesel fuel recovery with SiO2 adsorption was lower than that with methanol extraction by 11%. It was stated that excess oxone can be reused for the same diesel fuel without losing its activity in four treatment cycles followed by adsorption with SiO2 each (from 84% sulfur removal at the end of the 1st cycle up to 95% at the end of the 4th cycle). Although oxone is a relatively inexpensive oxidant and provides high desulfurization, a 15% diesel loss after adsorption with SiO2 makes it very difficult to use in large scales, on the contrary, low desulfurization efficiencies were obtained by extraction with methanol due to low extractive performance of the extractant selected for oxidized sulfur compounds. This major difference between extraction and adsorption performance could possibly be due to SiO2 adsorbing not only oxidized sulfur compounds but also sulfur compounds [191].After biphasic UAODS reactions in the presence of HP and acetic acid, how to valorize the aqueous phase or eliminate the sulfur compounds and their oxidized counterparts in the aqueous phase is a crucial environmental issue.A 96.45% sulfur removal [192] (followed by extraction with acetonitrile at 1000\u202frpm mechanical stirring speed for 25\u202fmin at room temperature) was achieved from model diesel fuel containing 3976.861\u202fmg S L\u20131, which is prepared by dissolving DBT in homogeneous solution (n-dodecane\u202f+\u202fn-heptane\u202f+\u202fn-hexadecane), using 10\u202fmL HP and 10\u202fmL acetic acid under 20\u202fkHz frequency, 70\u202fW power and 80% amplitude direct US at 70\u202f\u00b0C in 30\u202fmin. The aqueous phase (total organic carbon TOC content 1200\u202fmg L\u20131) containing DBT, DBTO2 and acetic acid, that is separated after the heterogeneous UAODS reaction and called diesel wastewater, was diluted individually 10- and 20-fold with distilled water and subsequently subjected to homogeneous ODS reaction at C(Fe2+)\u202f=\u202f2\u202fmmol L\u20131 and C(HP)\u202f=\u202f20\u202fmmol L\u20131 Fenton's reagent concentration (with acetic acid by adjusting pH to 3.1) under 200\u202fW and 20\u202fkHz direct US for 120\u202fmin. At the end of the homogeneous ODS reactions of the two aqueous phase samples diluted 10- and 20-fold with pure water, a removal of 75 and 76% for DBTO2, 28 and 66% for TOC, respectively, were obtained. HPLC analysis of the treated diesel wastewater confirmed the formation of benzoic acid followed by aliphatic carboxylic acids (e.g., oxalic acid) after 30\u202fmin as a result of oxidative degradation of small amounts of remaining DBT. It was stated that this sono-Fenton process has the potential to remove organic pollutants from diesel waste water and the treated water can be reused.In order to further remove the sulfur in the fuel (i.e., to obtain ultra-low or low-sulfur fuel), advanced oxidation processes, which are used in the removal of organic pollutants from wastewater, have also been utilized in UAODS reactions. For this purpose, FeSO4 was added to the aqueous phase containing HP-acetic acid and a 98.32% desulfurization degree [193] (followed by extraction two times at DMF/oil 1:1\u202fvol ratio for 2\u202fmin each at room temperature) of hydrotreated Middle Eastern diesel fuel containing 568.75 ppmw total S was obtained at optimum conditions (40\u202f\u00b0C, Fe2+/HP 0.05\u202fmol/mol, pH\u202f=\u202f2.10 and reaction time of 15\u202fmin) under 200\u202fW and 28\u202fkHz direct US. Explaining that the high desulfurization is due to the Fe2+ ion which generates more hydroxyl radicals from HP, it has been determined that the US-Fenton\u2019s reagent system follows the second order reaction kinetics.In a similar study [184] where Fenton\u2019s reagent as oxidizer and acetic acid were used, 97.5% sulfur removal from original diesel fuel containing 1936.48 ppmw total S (followed by extraction at DMF/oil 1:1\u202fvol ratio under vigorous mixing at room temperature) has been achieved at optimum operating conditions (70\u202f\u00b0C, 10\u202fmin, 8\u202fW\u202fcm\u22122 ultrasonic intensity, O/S molar ratio 6: 1, FeSO4/HP mass ratio 2:10 and acetic acid/HP volume ratio 1:2) under direct US at 28\u202fkHz frequency. It was reported that the diesel loss after oxidation-extraction is less than 8\u202fwt% and although the density and cetane index decreased a little, the other properties of diesel fuel did not change much.By virtue of very severe process conditions (Hydrotreated diesel fuel with 421.45 ppmw total S obtained as the feeding material by hydrotreatment of diesel fuel containing 9997 ppmw total S for two-stage HDS, 7\u202fMPa, 628\u202fK, LHSV 1.8\u202fh\u22121) necessary to reduce very high-sulfur diesel fuels by HDS to less than 10 ppmw S (9.5 ppmw S), diesel fuel containing 9997 ppmw total S was first processed by HDS in milder conditions (with 99.8% diesel fuel recovery) to obtain a fuel containing 421.45 ppmw S and then subjected to oxidation reaction (followed by extractions two times at DMF/oil 1: 1\u202fvol ratio for 2\u202fmin each at room temperature) at 70\u202f\u00b0C, HP/Diesel Oil 3/100\u202fvol ratio, pH\u202f=\u202f2.1 and Fe2+/HP 0.05\u202fmol\u202fg\u22121 in the presence of Fenton\u2019s reagent and acetic acid under 28\u202fkHz and 200\u202fW direct US in 15\u202fmin [178]. Along with the 92.2% diesel fuel recovery, diesel fuel containing 9 ppmw total S (97.86% sulfur removal) was obtained. Therefore, it was stated that integrating the ODS unit as a complement to the HDS unit is potentially advantageous in terms of overall process cost and efficiency.It was reported that by using individually Fenton\u2019s Reagent and Fenton-type reagent (Cu2+-HP), which is used to enrich hydroxyl radicals, in the presence of acetic acid (pH\u202f=\u202f1.9\u202f\u223c\u202f2.1), a desulfurization degree (followed by extraction twice at DMF/fuel 1:1\u202fvol ratio at room temperature for 10\u202fmin each) of 95.2 and 89.2%, respectively, was achieved for FCC diesel fuel [185] with 1936.48 ppmw total S at 60\u202f\u00b0C, HP/S 6:1\u202fmolar ratio and M2+ (Fe2+ or Cu2+)/HP 0.05\u202fmol\u202fmol\u22121 under 28\u202fkHz and 200\u202fW direct US in 15\u202fmin, which is an indication that metal ions catalyze the UAODS reaction creating a synergistic effect.In a study [179] conducted to remove sulfur from a straight run diesel oil sample containing 960\u202fppm S (followed by extraction one time with DMF at extractant/oil volume ratio of 1:2), a desulfurization yield of 94.7% was obtained at the optimum conditions (HP/formic acid (FA) 1: 1\u202fvol ratio, (HP\u202f+\u202fFA)/oil 1:10\u202fvol ratio, 50\u202f\u00b0C and 10\u202fmin) under 28\u202fkHz\u201340\u202fkHz and max 200\u202fW direct US. It was observed that the degree of desulfurization almost does not increase due to the decomposition of HP after the optimum reaction time, the sulfur removal is slightly reduced due to side reactions after the optimum oxidant/oil volume ratio, and the desulfurization removal does not change beyond the optimum temperature. Moreover, it was stated that beyond optimum conditions, oil recovery decreases and also production costs will increase.In a similar study [180] under the same optimum conditions as the previous study [179] (except that extractant DMF/oil volume ratio is 1: 1 and extraction time is twice), the effect of HP/FA volume ratio under direct US was investigated and a sulfur removal of 92.8% has been obtained from FCC diesel oil containing 1948 ppmw total S at the end of the UAODS process. Beyond the optimum oxidant/catalyst volume ratio (1:1), it was reported that desulfurization decreases due to nonproductive decomposition of excess HP to oxygen and water as there is not enough formic acid in the medium to form high-concentration peroxyformic acid in-situ by reaction of HP with FA.The effect of extraction on desulfurization after the oxidation reaction of sulfur compounds in FCC diesel containing 1985\u202fppm total S with HP-FA oxidant system under indirect US was investigated [194]. Taking into account oil recovery and the consumption of extraction solvent, a desulfurization of 94.2% was achieved as a result of extraction two times at DMF/oil volume ratio of 1:1 at 30\u202f\u00b0C for 20\u202fmin each.Recently, RSM-Box-Behnken Design has been used to find the optimum desulfurization, to examine the effect of reaction parameters and interactions between the parameters on UAODS yield and also to find which parameter or parametric relationships are more important on desulfurization such that fewer experiments are performed with this program, thus resulting in less time-consuming study.Using RSM [181], a sulfur removal of 95.46% from kerosene containing 2490 ppmw total S was achieved at the ratio of nO/nS\u202f=\u202f15.02, nacid/nS\u202f=\u202f107.8 and US power/fuel volume\u202f=\u202f7.6\u202fW\u202fmL\u22121 (followed by extraction with acetonitrile, extractant/kerosene volume ratio\u202f=\u202f1, extraction stage\u202f=\u202f1, ambient temperature, 700\u202frpm, 30\u202fmin.) at 20\u202fkHz frequency and 400\u202fW direct US at 50\u202f\u00b0C within 10.5\u202fmin. It was observed that above the optimum nacid/nS and nO/nS ratios, the desulfurization was almost unchanged as performic acid formation and decomposition reactions occur together in an acidic medium and the equilibrium concentration of peroxyformic acid was reached due to the decomposition of HP. When the two ratios in the relation of power/volume and nO/nS to sulfur removal are above a certain value, no increase in desulfurization was observed due to dilution in the aqueous phase and the weakening of the ultrasonic wave emitted to the mixture by enlargement of the bubble cloud at the probe tip at high power. The fact that there is no significant increase in desulfurization above a certain value of the two ratios in the relation between power/volume and nacid/nS is due to the reason mentioned above. While a sulfur removal of 29.92% from kerosene is achieved by extraction alone employing acetonitrile without oxidation reaction, the desulfurization is 74.9% by oxidation and water washing without extraction process, which shows that formic acid extracts oxidized sulfur compounds sulfoxides and sulfones during the oxidation reaction.In a similar study [195] with the same oxidant system by applying RSM, a sulfur removal higher than 98% was achieved at HP/S molar ratio of 10.82, FA/S molar ratio of 379.75 and 52\u202f\u00b0C (which are the three independent reaction parameters selected) under 70\u202fW and 20\u202fkHz direct US and at 15\u202fmin for model fuel containing 500\u202fppm total S prepared by dissolving BT in toluene. With the same values of these 3 optimum parameters found, a sulfur reduction of approximately 95.6% (followed by extraction at acetonitrile/kerosene volume ratio of 1 for 30\u202fmin at room temperature) was achieved from kerosene containing 2720 ppmw total S under 250\u202fW direct US in 20\u202fmin. The results revealed that the decrease in desulfurization at low acid/S and high O/S values is due to the dilution of the formic acid by increased surplus HP, thus lowering peroxyformic acid concentration and also the formation of vapor-filled bubbles rather than gas-filled bubbles with increasing HP. It was found that the importance degree of the independent reaction parameters was in descending order: Acid/S molar ratio\u202f>\u202fHP/S molar ratio> (Acid/S molar ratio)2 according to the ANOVA results of the quadratic correlation equation (where the smaller than 0.05 the P value and the larger the F value, the more important the parameter).In a study [196] with the same oxidant system, using the RSM-Box-Behnken Design (BBD), where temperature and US power/gas fuel volume (W mL\u22121) were selected as constant parameters and O/S, Acid/O molar ratios and sonication time as process variables, 87% sulfur removal from gas oil containing 2210 ppmw total S (followed by one-time extraction at acetonitrile/gas oil volume ratio of 1: 1 under vigorous stirring for 30\u202fmin at room temperature) was achieved at O/S 46.36\u202fmolar ratio, acid/O 3.22\u202fmolar ratio in 19.81\u202fmin for 50\u202f\u00b0C and 7.78\u202fW\u202fmL\u22121 under a direct US of 20\u202fkHz. However, in the case of 4-step extraction, 96.2% of the sulfur present in the gas oil was removed, but it was reported that the recovery of gas oil decreased to 81.25%. After the oxidation reaction under the same conditions, the extraction performances under mechanical mixing and under direct US were compared. It was observed that the desulfurization yields were approximately the same, thus showing that US does not have a positive effect on extraction. In addition to these, as a result of the preliminary cost analysis of this batch process, it was determined that a total operating cost of $ 0.43 was incurred for the treatment of 1 L gas fuel and also 31.7 and 56.3% of this total cost were liquid\u2013liquid extraction and US Power/gas oil volume, respectively. It was stated that this calculated cost will be less in continuous-flow UAODS systems as there are stagnant zones in the mixture in batch UAODS systems, thus leading to a higher consumption of US power density per unit volume of fuel in the batch systems. According to ANOVA analysis, it was determined that the importance of variables is in the order: sonication time\u202f>\u202facid/O molar ratio\u202f>\u202fO/S molar ratio> (acid/O\u202f\u00d7\u202fsonication time)> (sonication time)2> (acid/O molar ratio)2> (O/S molar ratio)\u202f\u00d7\u202f(acid/O molar ratio)> (O/S molar ratio)2. It was explained that sulfur removal decreased due to the scavenge of hydroxyl radicals at high acid/O molar ratio and enhancement of side reactions in case there is excess HP in the medium towards high O/S molar ratio. In high acid/O and high O/S molar ratios, it was explained that peroxyformic acid stabilizes at low pH of the aqueous phase as a result of very high concentration of formic acid after a certain value, thus resulting in a lower desulfurization by limiting the production of active oxidizing radicals, which are generated by the decomposition of performic acid.The RSM-BBD was applied to a batch reactor in a continuous study [188] in which the aqueous phase consisting of HP and FA is injected by nozzles of different diameter to just below the bottom end of the probe (which is the active site where radicals are produced). O/S molar ratio, acid/S molar ratio and sonication time were selected as independent variables at 50\u202f\u00b0C under 20\u202fkHz and 360\u202fW direct US and the optimum parameters (nO/nS\u202f=\u202f38.88, nacid/nS\u202f=\u202f116.47 and sonication time 29.2\u202fmin.) were determined under batch conditions. According to ANOVA, it is stated that the most important terms are in the order: acid/S molar ratio> (O/S molar ratio\u202f\u00d7\u202facid/S molar ratio)\u202f>\u202fsonication time. These optimum parameters have been applied to two continuous reactors in series (where in the first reactor, the aqueous phase was injected to the lower end of the probe) at different feed rates (thus causing different retention times) and different fuel phase/aqueous phase volume ratios (herein (Vacid/VO)\u202f=\u202f1.117). For non-hydrogenated diesel fuel containing 1550 ppmw total S, a desulfurization of 83.39% (followed by a single extraction with acetonitrile/organic phase volume ratio of 1:1 at 1000\u202frpm mixing speed for 30\u202fmin at room temperature) was reached at Vf (volume of the fuel phase)/Vaq (volume of the aqueous phase) 5: 1\u202fvol ratio, 40\u202fmL\u202fmin\u22121 total outlet flow rate (33.33\u202fmL\u202fmin\u22121 diesel fuel\u202f+\u202f6.67\u202fmL\u202fmin\u22121 aqueous phase), a residence time of 3\u202fmin in the first reactor and 2.5\u202fmin in the second reactor using 1.5-mm-diameter nozzle from the point of the lowest retention time and lowest aqueous phase volume to minimize the process cost. It was explained that when the nozzle diameter decreases from 1.5\u202fmm to 0.43\u202fmm, the desulfurization decreased to 68.74% due to a decline in the ratio of the hydrodynamic momentum flow rate generated by the US probe to the hydrodynamic momentum flow rate of the dispersed aqueous phase (in which case, aqueous phase will stay in the active zone for much less time as the increasing flow rate by use of the smaller nozzle diameter leads to the increased momentum). In addition, it was shown that the increase of the aqueous phase flow rate from 10 to 40\u202fmL\u202fmin\u22121 for all the nozzle diameters leads to a decrease in desulfurization due to the reason mentioned above. Batch sonoreactor and sonoreactors in series operating at different times at a constant volume ratio of Vf/Vaq\u202f=\u202f2.96\u202fmL\u202fmL\u22121 and at different Vf/Vaq ratios at constant sonication times of 5.5\u202fmin were compared and it was reported that in all cases, the sulfur removal per power density consumed in continuous sonoreactors in series is higher than that in the batch sonoreactor.The effect of pressure on UAODS in a sonoreactor was investigated [197] and the optimum conditions (390\u202fW US power at 20\u202fkHz frequency, gauge pressure 0.03 barg and 22\u202fmin) were found by applying RSM-BBD in which pressure, US Power and sonication time were selected as independent variables at T\u202f=\u202f50\u202f\u00b0C, nO/nS\u202f=\u202f15.02 and nacid/nS\u202f=\u202f107.8. A sulfur removal of 96.7% (followed by one-time extraction at acetonitrile/kerosene 1:1\u202fvol ratio under 500\u202frpm stirring speed for 30\u202fmin at room temperature) was obtained from kerosene with 2490 ppmw total S. Also, it was disclosed that according to computational fluid dynamics (CFD), desulfurization decreased at pressures above atmospheric pressure (1 barg and 2 barg) due to the progressively decreasing vapor volume fraction, the decreasing bubble collapse pressure, the low dispersion of the aqueous phase into the organic phase and a significant increase in the aqueous phase volume fraction. The authors suggested that the marked rise in the aqueous phase volume fraction did not result in finer emulsion droplets, thus causing the interfacial area between the aqueous and organic phase to diminish. In addition, it was stated that when the US Power increased from 100 to 400\u202fW, the max acoustic pressure and micro-streaming speed increased according to the calorimetric analysis, thus desulfurization was improved due to the increase in mass transfer rate. It was determined that the most important terms affecting desulfurization are in the order: time\u202f>\u202fPressure\u202f>\u202fPressure\u202f\u00d7\u202fPower\u202f>\u202fPower according to ANOVA.In a continuous cylindrical sonoreactor with multiple probes (3 probes) and two nozzles [182], through which the aqueous phase is injected just below the first and the second probe tips from the left side of the inside of the reactor, the optimum conditions (Vacid/VO (mL mL\u22121) 1.12, Vaq = (Vacid\u202f+\u202fVO) 733.33\u202fmL, Vf\u202f=\u202f3666.67\u202fmL, Vf/Vaq (mL mL\u22121) 5 and temperature 50\u202f\u00b0C) were determined under direct ultrasound, each of which has a power of 400\u202fW and a frequency of 20\u202fkHz (all ultrasonic processors ON). >97% of sulfur (followed by extraction with DMF) from diesel fuel containing 1550 ppmw total S was removed using two 1.5-mm-diameter nozzles at 15\u202fmin residence time, 277.2\u202fW electrical power, 48.90\u202fmL\u202fmin\u22121 total aqueous phase volumetric flow rate (flow rate of each nozzle 24.45\u202fmL\u202fmin\u22121) and fuel phase volumetric flow rate of 244.44\u202fmL\u202fmin\u22121. According to the CFD simulation results, it was explained that this high desulfurization is due to the higher hydrodynamic momentum ratio (momentum of ultrasonic jet-like streaming/momentum of the aqueous phase injected by the nozzle) as well as secondarily, further oxidation reactions of DBT derivatives with oxidizing radicals (HO2\u00b7, O\u00b7 and HO\u00b7) in the active zone just below the probe tips not only when larger-diameter (1.5\u202fmm) nozzles are used instead of 0.4- and 0.9-mm-diameter nozzles but also when each of the aqueous phase flow rates is lower (using two nozzles with an aqueous phase flow rate of 24.45\u202fmL\u202fmin\u22121 each instead of using a single nozzle with the aqueous phase flow rate of 48.89\u202fmL\u202fmin\u22121). In this case, it was suggested that the aqueous phase is dispersed more homogeneously into fuel when compared to smaller diameter nozzles at higher flow rates.The operating cost of the UAOD system was investigated [183] in a continuous flow jacketed glass reactor where the glass nozzle through which the aqueous phase (85\u202fwt% FA\u202f+\u202f35\u202fwt% HP) flows is placed 3\u202fcm below the US probe tip. Residence time (min), FA/S molar ratio and oxidant/S molar ratio were selected as independent variables at a reaction temperature of 50\u202f\u00b0C as constant value and RSM based on BBD was applied. A sulfur removal of 86.90% (followed by one-time extraction at DMF:oil 1:1\u202fvol ratio at room temperature and 875\u202frpm stirring speed for 30\u202fmin) was obtained from the partially hydrotreated diesel fuel containing 2760 ppmw total S at optimum conditions (retention time of 16\u202fmin, molar ratio of na/nS 54.47 and molar ratio of nO/nS 8.24) under 360\u202fW and 20\u202fkHz direct US. Under these optimum conditions, it was reported that the organic phase/aqueous phase volume ratio is 4.34 and the operating cost (chemical consumption\u202f+\u202felectricity due to ultrasound irradiation) is 7.73 cents per liter of oxidized diesel fuel. As the largest part of the operating cost was HP consumption, the organic phase/aqueous phase volume ratio was increased to 10 in order to significantly reduce the aqueous phase consumption at residence time 16\u202fmin and FA/HP volume ratio 3.16. Eventually, a sulfur removal of 84.38% was achieved with an operating cost of 4.66 cents per liter of oxidized diesel fuel at na/nS 23.64\u202fmolar ratio, nO/nS 3.58\u202fmolar ratio, 7.07\u202fmL\u202fmin\u22121 diesel flow rate and 0.71\u202fmL\u202fmin\u22121 aqueous phase flow rate (0.54\u202fmL\u202fmin\u22121 85\u202fwt% FA\u202f+\u202f0.17\u202fmL\u202fmin\u22121 35\u202fwt% HP). According to ANOVA results, it was determined that the most important terms affecting desulfurization in this process are in the order: residence time\u202f\u2248\u202fna/nS\u202f>\u202f(residence time)2\u202f>\u202f(na/nS)2\u202f>\u202f(na/nS\u202f\u00d7\u202fnO/nS)\u202f>\u202f(nO/nS)2.Sono-desulfurization of gasoline and crude oil was performed at optimum conditions found by applying RSM-BBD in which ultrasonic power, irradiation time and oxidant amount are selected as independent variables [198]. A desulfurization of 80.87% (followed by extraction three times at DMSO/gasoline 1:1\u202fvol ratio and water washing four times) was obtained for gasoline containing 1207 ppmw S at optimum conditions (464.7\u202fW direct ultrasonic power (pulsed ultrasound 2\u202fs on, 2\u202fs off), 5.5\u202fmin irradiation time and 8.1\u202fmL HP (HP: FA volume ratio 1:1)), whereas a sulfur removal of 73.37% (followed by first magnetic stirring of oil sample for one h and then extraction with 60\u202fmL of a mixture at acetonitrile:methanol:water 1:1:1\u202fvol ratio) was achieved from the crude oil containing 28,620 ppmw S at optimum conditions (785.1\u202fW direct ultrasonic power, 6.2\u202fmin irradiation time, 11.4\u202fmL HP (HP: FA, the same volume ratio) with the same pulsed ultrasound. It was stated that after the oxidation of the gasoline sample, adding distilled water up to 1% of the DMSO volume to DMSO for the extraction of oxidized sulfur compounds decreases desulfurization by 20% compared to extraction alone with DMSO. It was explained that this low desulfurization is due to the fact that water reduces the extraction ability of DMSO as the DMSO and water dipole moments [199] are 3.96 and 1.85 D, respectively, (hence DMSO has greater polarity). The differences between mechanical stirring-heating and desulfurization under US were compared and these differences were reported to be approximately 10 and 30% for gasoline and heavy crude oil, respectively, which demonstrates that UAODS is more effective for high-sulfur fuels. This threefold higher difference can be attributed to the emergence of the higher cavitation intensity [200] as heavy crude oil has higher density, higher viscosity and higher surface tension than gasoline. In addition, the high vapor pressure of extremely volatile gasoline compared to heavy crude oil can limit violent implosion of cavitation bubbles in the liquid mixture [125].RSM-Box-Behnken Design (BBD) was used to evaluate the effects of nformic acid/nS, nO/nS, ultrasound power (UP)/simulated oil volume and temperature on UAODS and to optimize these reaction parameters on the purpose of max attainable desulfurization efficiency [187]. A sulfur removal of approximately 97% from DBT containing 500 ppmw S in toluene is reported at nO/nS\u202f=\u202f26.7, nformic acid/nS\u202f=\u202f74.6, UP/model oil volume\u202f=\u202f7\u202fW\u202fcm\u22123 and at 50\u202f\u00b0C under 20\u202fkHz and 400\u202fW direct US in 630\u202fs. Besides, it was stated that the FA (formic acid)/HP molar ratio should be at a certain value (1.4\u20132.8) in order to maximize the concentration of peroxyformic acid (HCOOOH), which is formed in the equilibrium reaction between HP and HCOOH in the aqueous phase in desulfurization reactions and oxidizes the sulfur compounds.In a study [161] where a computational fluid dynamic (CFD) model was used to examine the hydrodynamic and mass transfer characteristics of model fuel in the ultrasonic horn reactor, it was explained that high desulfurization is caused by physical effects such as jet stream, high turbulence intensity rather than the chemical effect of ultrasound, and the reaction is controlled by chemical kinetic due to the very high mass transfer rate. In the mentioned study, a sulfur removal of 96.35% from the model fuel containing 500 ppmw DBT in toluene was achieved at nO/nS\u202f=\u202f26.7, nformic acid/nS\u202f=\u202f74.6, UP/Model Oil Volume\u202f=\u202f26.7\u202fW\u202fmL\u22121 under 20\u202fkHz direct US at 50\u202f\u00b0C in 210\u202fs.It was observed in the studies [178,181\u2013185] that the properties of diesel fuels (density at 15\u202f\u00b0C, kinematic viscosity at 40\u202f\u00b0C, flash point, water content, cetane index) almost did not change after UAODS process followed by extraction.Three organic acid catalysts (FA, acetic acid and trifluoroacetic acid) were compared and a 76.5% sulfur reduction [201] (followed by extraction at a DMF/oil volume ratio of 1:1) was achieved for the catalytic cracking diesel containing 1452 ppmw total S by using trifluoroacetic acid at oxidant/oil 1:10\u202fvol ratio, 70\u202f\u00b0C and 60\u202fmin as the optimum operating conditions under indirect 20\u202fkHz US, which is higher than the sulfur removals obtained in the case of using acetic acid and FA catalysts as the acidity [202] of trifluoroacetic acid (pKa\u202f=\u202f0.18) is higher than that of formic acid (pKa\u202f=\u202f3.75) and acetic acid (pKa\u202f=\u202f4.75), thus causing the oxidizing power of the peroxycarboxylic acid formed to increase further.In a study [189] where 1-butyl-3-methyl imidazolium hydrogen sulfate [Bmim][HSO4] and 1-octyl-3-methyl imidazolium hydrogen sulphate [Omim][HSO4] with two different alkyl lengths were synthesized and used instead of aqueous phase, approximately 100% desulfurization yield of the model fuel containing 500 ppmw DBT in n-decane was obtained using [Omim][HSO4] at O/S\u202f=\u202f5\u202fmolar ratio and mass ratio IL/model fuel\u202f=\u202f2 under 30\u202fW power and 25\u202fkHz direct US at 25\u202f\u00b0C in 3\u202fmin. In the experiments in the absence of ultrasound, it was explained that the desulfurization with [Omim][HSO4] is higher than the desulfurization with [Bmim][HSO4] by applying the same optimum operating conditions as the case of using ultrasound under stirring at 900\u202frpm. It was noted this high desulfurization is due to the longer alkyl chain of the cation of [Omim][HSO4]. In addition, the reactivity of different sulfur compounds under the same operating conditions was compared and it was reported that the UAODS was in descending order DBT\u202f>\u202fBT\u202f>\u202fT\u202f>\u202f4,6-DMDBT. It was stated that the lowest desulfurization for 4,6-DMDBT is due to the steric hindrance of two alkyl groups adjacent to the sulfur atom, hence weakening the \u03c0-\u03c0 interaction between the aromatic sulfur compound and the ionic liquid. Under the same optimum conditions, a UAODS efficiency of 76.3% was obtained for the real diesel fuel containing 746 ppmw total S. Moreover, it was reported that [Omim][HSO4] can be used six times without losing its activity in UAODS reactions of the model fuel by regenerating it after each reaction and the solubility of the model fuel in this ionic liquid is very low (1.45\u202fwt%), thus suggesting that the synthesized ionic liquid has the potential to be used both as an extractant and as a catalyst.However, the high viscosity of ionic liquids, their costly synthesis [203], and the change in the solubility [204] of the fuel in the ionic liquid according to the anions and cations formed depending on the starting raw materials, and more importantly, the presence of aromatic groups [177,189] such as imidazolium in IL significantly that reduces the desulfurization reactivity of thiophenes, especially abundant in petroleum products, due to steric hindrance make the UAODS process very difficult to be feasible using ionic liquid.One of the two identical hydrotreated diesel feeds containing 231 ppmw S and 115.5 ppmw N to use expensive oxidants in lower quantities was subjected to pre-extractive desulfurization and the other to pre-adsorptive desulfurization (diesel/methanol volume ratio 1:1 for EDS/N and diesel/fuller's earth (V/W)\u202f=\u202f1:0.2 for ADS/N) and then, the UAODS/UAODN reaction (followed by EDS/N and ADS/N individually at the same ratios as those in the pre-treatments) of the two partially desulfurized and denitrogenized fuel samples (S\u202f=\u202f196 ppmw and N\u202f=\u202f85 ppmw after pre-EDS/N and S\u202f=\u202f184 ppmw and N\u202f=\u202f52 ppmw after pre-ADS/N) was performed using oxone or HP in US Cup Horn at 80\u202f\u00b0C under 80\u202fW and 19.9\u202fkHz direct US for 90\u202fmin [205]. As a result of all these processes, diesel fuel with 11 ppmw S and 6 ppmw N is obtained by the pre- and post-ADS/N process, while diesel fuel with 78 ppmw S and 25 ppmw N is obtained by the pre- and post-EDS/N process, thus suggesting that it would be economically feasible to use cheap and efficient adsorbent fuller\u2019s earth instead of expensive extractant methanol. It was stated that this process can be proposed to be complementary to HDS.According to the ODS mechanism [28,29,206] (Scheme 3\n) using phosphotungstic acid in the presence of PTC, the phosphotungstate anion in aqueous phase is oxidized to the peroxophosphotungstate anion (1) by HP, then this active oxidizing complex anion is transferred (3) to organic phase by forming an ion pair (2) with the lipophilic cation of PTC. This complex anion is reduced to phosphotungstate anion by oxidizing the sulfur compounds in organic phase (4). The phosphotungstate anion is transferred to the aqueous phase by the lipophilic cation (5) and the cycle is completed.A DBT removal of 100% from model fuel [206] containing 4000 ppmw S DBT in toluene was performed using HP 30\u202fvol% (phosphotungstic acid concentration of 0.6\u202fmM in aqueous phase and tetraoctylammonium bromide (TOAB) concentration of 7.32\u202fmM in organic phase) under 600\u202fW and 20\u202fkHz direct US at 75\u202f\u00b0C in 7\u202fmin. The same conditions were applied to diesel fuels with different sulfur content at certain times (18\u202fmin for diesel A with 7744 ppmw S, 10\u202fmin for diesel B with 3011 ppmw S and 10\u202fmin for diesel C with 1867 ppmw S) at 75\u202f\u00b0C and a desulfurization yield (followed by extraction with acetonitrile three times at solvent/oil mass ratio of 1:2 at room temperature for 2\u202fmin each) of 98.2, 98.7 and 99.4%, respectively, was achieved along with a fuel recovery of 82.8, 87.2 and 85.5\u202fwt%. It was reported n-paraffins, n-alkyl cyclohexanes, n-alkyl benzenes and alkyl naphthalenes as component classes in the diesel C sample selected as representative were not adversely affected during oxidation, but alkyl naphthalenes among the four main components have relatively high polarity and thus they were extracted by acetonitrile.In a similar study [207] (where the temperature, tetraoctyl ammonium fluoride (TOAF) concentration, sonication time, phosphotungstic acid concentration and HP purity were 70\u202f\u00b0C, 7.5\u202fmM, 10\u202fmin, 0.7\u202fmM and 30\u202fvol%, respectively) with the research [206], under the same direct US power and frequency in a continuous flow sonoreactor, marine fuel with less than 23 ppmw S and jet fuel with 1 ppmw S (each followed by adsorption with activated, acidic Al2O3), respectively, were obtained from marine gas oil containing 1710 ppmw S and Jet Fuel (JP-8) containing 863 ppmw S. 33-fold lower consumption of Al2O3 compared to acetonitrile, loss of alkyl naphthalene less than 1\u202fwt%, regeneration with 94% alumina recovery by washing with DMF solvent and maintaining 99% of its adsorption capacity by calcination at 550\u202f\u00b0C have revealed that alumina has the potential of being used in large-scale continuous systems.In another study [208] where 30\u202fwt% HP and phosphotungstic acid were used, the UAODS performances of DBT in the presence of different phase transfer catalyst types at 70\u202f\u00b0C under 20\u202fkHz and 600\u202fW direct US were evaluated. It was stated that desulfurization reactions of DBT took place in the presence of TOAB (49.57% conversion), tetrabutylammonium bromide (TBAB) (38.34% conversion), methyltributylammonium chloride (MBAC) (11.4%), methyltributylammonium hydroxide (MBAH) (11.10%) and tetramethylammonium fluoride (8.20%) as cationic-type PTCs, whereas desulfurization reactions did not occur in the presence of 1-octanesulfonic acid as anionic-type PTC, Tween 80 as non-ionic PTC and in the absence of PTC. In addition, in the presence of TOAF and tetraoctadecylammonium bromide (TODAB), 90.30% (97.53% in 20\u202fmin for TOAF) and 56.89% conversions were performed in 10\u202fmin, respectively. From these results, it was emphasized that the biggest positive effect on UAODS is the long alkyl chain (hence more lipophilic cation) bound to the quaternary cation, and the less positive effect is the hydrophilic anion of quaternary salt. It was stated that the smaller (i.e., the more hydrophilic) the size of the monoatomic anion of quaternary salt for the same alkyl chain length, the more effective the PTC. It was determined by GC-PFPD analysis that 3-bromobenzothiophene and 2-bromobenzothiophene sulfone were formed as intermediates when TOAB was used in UAODS reactions of BT, while in the case of TOAF, intermediate products were not formed. The formation of the byproducts can be shown representatively in Scheme 4\n: either by the radical mechanism [209,210] where aromatic sulfur compounds react with bromine radical which is formed by homolytic cleavage [211] of molecular bromine on sonication or by direct reaction [212] with Br2 formed. Bromine radical can also be formed by the reaction of hydrogen peroxide with bromide anion [213].The reason for the absence of intermediates can be explained as follows: the standard reduction potential [214] of fluorine and HP is E\u00b0 (V) = +2.87 and E \u00b0(V) = +1.77, respectively. In case of quaternary ammonium salt containing fluoride anion, H2O2 cannot oxidize the fluoride anion to fluorine as the standard reduction potential of F2/F\u2212 is +2.87\u202fV. Therefore, fluoride-containing organosulfur compounds are not found in the reaction products. But as a result of the dissociation of the quaternary ammonium salts containing the other halide anions except fluoride in aqueous acidic media, the halide ions reduce hydrogen peroxide to water, causing the decomposition of hydrogen peroxide [215]. For example, when TOAB is used, HP in the aqueous acidic phase is reduced by oxidizing the bromide anion released by dissociation [216] of the quaternary ammonium salt in water according to the following reaction as E\u00b0 (V) of Br2 is\u202f+\u202f1.07 [214].\n\n\n\n\nCatalytic decomposition [217,218] of HP in acidic medium in the presence of bromide ion is as follows:\n\n(1)\n\n\n\nH\n2\n\n\nO\n\n2\n(\na\nq\n)\n\n\n\n+\n\n2\nB\n\nr\n\naq\n\n-\n\n+\n\n2\n\nH\n\naq\n\n+\n\n\n\u2192\nB\n\nr\n2\n\n\n+\n\n2\n\nH\n2\n\nO\n\n\n\n\n\n\n(2)\n\n\n\nH\n2\n\n\nO\n\n2\n(\na\nq\n)\n\n\n\n+\n\nB\n\nr\n\n2\n(\na\nq\n)\n\n\n\n\u2192\n\n\nO\n2\n\n\n+\n\n2\nB\n\nr\n-\n\n+\n\n2\n\nH\n+\n\n\n\n\n\nBr2 formed in reaction 1 reacts with H2O2 in reaction 2 forming bromide ion again. The sum of reaction 1 and 2 is written as\n\n\n\n2\n\nH\n2\n\n\nO\n\n2\n(\na\nq\n)\n\n\n\u2192\n2\n\nH\n2\n\n\nO\n\n(\nl\n)\n\n\n+\n\n\nO\n\n2\n(\ng\n)\n\n\n\n\n\n\nBr2, which is formed according to reaction (1), participates in bromination reaction with sulfur compounds in organic phase and forms bromo intermediates. Besides, as mentioned before, bromination reaction can be carried out by bromine radical Br\u00b7 formed by homolytic decomposition of Br2 molecule under US. The reason of the decreased desulfurization in this case can be explained as follows: as HP is decomposed in an acidic environment, the amount of peroxo-phosphotungstate formed in situ may decrease significantly. Additionally, a small amount of peroxo-phosphotungstate, which has a higher ability to oxidize organic compounds than hydrogen peroxide [219], reacts very quickly with Br2 in the medium, causing the amount of peroxo-phosphotungstate to decrease much more. Therefore, the desulfurization under US can be significantly lower. In the case of the quaternary ammonium salt containing fluoride for the same alkyl chain length, HP is not reduced by fluoride, thus high desulfurization efficiencies can be achieved by the high amount of peroxotungstate formed and no intermediates are formed. A similar phenomenon can occur when carboxylic acids such as formic acid are used instead of phosphotungstic acid. The reaction mechanism under ultrasound irradiation in the presence of a quaternary ammonium salt with bromide anion can be explained as follows:\n\n(3)\n\n\nBr\n-\nB\nr\n\u2192\n\nB\nr\n\u00b7\n\n+\n\nB\nr\n\u00b7\n\n\n\n\n\n\n(4)\n\n\n\nH\n2\n\n\nO\n2\n\n\u2192\n2\nO\nH\n\u00b7\n\n\n\n\n\n\n(5)\n\n\nHCOOOH\n\n\u21cc\n\nH\nC\nO\nO\n\u00b7\n+\n\nH\nO\n\u00b7\n\n\n\n\n\n\n(6)\n\n\n\nH\n2\n\n\nO\n2\n\n+\n\nB\nr\n\u00b7\n\u2192\nH\n\nO\n2\n\n\u00b7\n+\n\nH\nB\nr\n\n\n\n\n\n\n(7)\n\n\nH\n\nO\n2\n\n\u00b7\n\n+\n\n\n\nH\n2\n\n\nO\n2\n\n\u2192\nH\nO\n\u00b7\n+\n\n\nO\n2\n\n+\n\n\nH\n2\n\nO\n\n\n\n\n\n\n(8)\n\n\nHO\n\u00b7\n\n+\n\n\nH\n2\n\n\nO\n2\n\n\u2192\nH\n\nO\n2\n\n\u00b7\n\n+\n\n\nH\n2\n\nO\n\n\n\n\n\n\n(9)\n\n\nBr\n\u00b7\n\n+\n\nB\nr\n\u00b7\n\u2192\n\nB\n\nr\n2\n\n\n\n\n\n\n\n(10)\n\n\nHO\n\u00b7\n\n+\n\nH\nO\n\u00b7\n\u2192\n\n\nH\n2\n\n\nO\n2\n\n\n\n\n\n\n\n(11)\n\n\nHO\n\u00b7\n\n+\n\nB\nr\n\u00b7\n\u2192\nH\nO\nB\nr\n\n\n\n\n\n\n(12)\n\n\nH\n\nO\n2\n\n\u00b7\n\n+\n\nB\nr\n\u00b7\n\u2192\n\nH\nO\nO\nB\nr\n\n\n\n\nAccordingly, Br2 formed through the reaction 1, the PFA formed in-situ and HP in the reaction solution bring about a series of reaction 3\u201312 generating hydroxyl [158,161,220] and bromine radicals [211] by the decomposition of PFA and HP and the homolytic bond cleavage of Br2. Consequently, the hydroxyl and hydroperoxyl radicals play a dominant role in oxidation of the organosulfur compounds. In a study [221] in which HP reacts with FA at 30\u202f\u00b0C in the presence of TBAB, it was confirmed by titrimetric analysis that the HP concentration decreased significantly by the decomposition of HP and the peroxyformic acid concentration was too low. The change of the transparent color of the aqueous solution containing HP, FA and PFA in the absence of TBAB to the yellow color of the bromine water formed by the dissolution of Br2 in water in the presence of TBAB is an additional indicative of the decomposition. The resulting performic acid (or peracetic acid formed in the case of using acetic acid) can also react as follows:\n\n(13)\n\n\nRCOOOH\n\n+\n\n2\nB\n\nr\n-\n\n\n+\n\n2\n\nH\n+\n\n\n\u2192\nR\nC\nO\nO\nH\n\n+\n\n\nH\n2\n\nO\n\n+\n\nB\n\nr\n2\n\n\n\n\nwhere R is H or CH3 and its concentration may decrease depending on the concentration of Br ion in the medium.Moreover, formic acid can react with the resulting Br2 according to the following reaction [222,223] (14), thus causing formic acid concentration to decrease.\n\n(14)\n\n\nHCOOH\n\n+\n\nB\n\nr\n2\n\n\n\u2192\nC\n\nO\n2\n\n\n+\n\n2\n\nH\n+\n\n\n+\n\n2\nB\n\nr\n-\n\n\n\n\n\nIn the reaction mechanism in the case of using HP, FA and TOAF, peroxyformic acid generates formyloxyl radical and hydroxyl radical by homolytic cleavage under US [220]. As a result of the reaction of peroxyformic acid with hydroxyl radicals, formyl radical and peroxyformyl radical are formed, which is similar to the reactions [224] of peracetic acid with the hydroxyl radicals. Therefore, in addition to the hydroxyl radicals formed and the high concentration of performic acid, highly reactive formyloxyl and peroxyformyl radicals may also be responsible for the high desulfurization.Diesel fuel [208] containing 0.1\u202fg TOAF was undergone ODS reaction (followed by extraction four times at acetonitrile/oil 1:1 mass ratio at room temperature for 1.5\u202fmin each) with an equal volume of 30\u202fvol% HP solution containing 0.2\u202fg of phosphotungstic acid under the same US frequency and power at the same temperature as the previous study [207]. After UAODS reactions of 10\u202fmin followed by extraction four times, a sulfur removal of 95, 98.8, 87.5, 99.9 and 96.1% was achieved from F-76 containing 4222 ppmw S, MGO containing 1710 ppmw S, JP-5 containing 113.7 ppmw S, JP-8 containing 863 ppmw S and transportation fuel containing 259 ppmw S, respectively. In addition, after a 98.8% UAODS yield from MGO containing 1710\u202fppm S in the presence of TOAF, the aqueous phase was reused for two fresh MGO samples with 1710 ppmw S each in the presence and absence of TOAF and a UAODS of 98.15 and 96.01%, respectively, was obtained. Again, under the same conditions, this time using dilute HP (3\u202fvol%), a UAODS of 97.90 and 94.8% was obtained for MGO and F-76, respectively. It was stated that after the UAODS reaction of organic sulfur compounds, 99.49% of the tungsten remained in the aqueous phase according to ICP analysis, hence it could be completely recovered.In a study [225] investigating the effect of quaternary ammonium salts with four different alkyl lengths as PTC on UAODS, using PTC (optimum concentration 0.0116\u202fmol L\u20131) in the range of 0.03\u20130.25\u202fg, 12\u202fmL 30% HP and 12\u202fmL formic acid, 28.37, 42.37, 70.02, 86.57 and 94.67% sulfur removal, respectively, were obtained without PTC and in the presence of TMAB, TEAB, TPAB and TBAB at 50\u202f\u00b0C in 1.5\u202fh under direct US for 0.028\u202fmL of thiophene dissolved in 24\u202fmL of n-heptane. The highest desulfurization with TBAB was attributed to the bigger radius (thus more stable complex formation [HCOOO\u2013--Q---Br] by higher electron delocalization) of the phase transfer cation TBA+ compared to the radii of the other phase transfer cations for the transfer of [HCOOO\u2013] to the organic phase in the presence of the same anion (Br\u2013) and the higher extraction constant of TBAB. It was revealed that the reaction follows pseudo first order kinetics.In a study [226] in which the effect of two different types of continuous flow reactors on UAODS (followed by extraction at acetonitrile/oil 1: 1 mass ratio at room temperature for 1.5\u202fmin with vigorous shaking) of MGO was investigated, a 92.74% sulfur removal was performed using 25\u202fg 30\u202fvol% HP, 0.1\u202fg TOAF, 0.2\u202fg phosphotungstic acid under 600\u202fW US in power at 70\u202f\u00b0C in 20\u202fmin for treating 20\u202fg MGO containing 1710 ppmw total S in a probe-type reactor operating at 20\u202fkHz, while using 625\u202fg 30\u202fvol% HP, 2.5\u202fg TOAF, 5\u202fg of phosphotungstic acid to treat 500\u202fg of MGO per h in a portable tubular sonoreactor operating at 40\u202fkHz, a desulfurization degree of 92.36 and 89.78% was achieved at 25\u202f\u00b0C for 100\u202fW US power-60\u202fmin and 200\u202fW US power-30\u202fmin, respectively. Then, this tubular sonoreactor was scaled up to a treatment rate of 12.5\u202flb MGO h\u22121 and a 92.42% desulfurization performance was accomplished using 7.09\u202fkg 30\u202fvol% HP, 56.75\u202fg TOAF and 28.13\u202fg phosphotungstic acid under 100\u202fW US power at 25\u202f\u00b0C in 60\u202fmin. It has also been stated that sonoreactors can be connected in parallel to treat more fuel (25\u202flb\u202fh\u22121) with the same removal percentage. In addition, by using diluted HP (3\u202fvol%), a sulfur removal of 91% was reached in this sonoreactor in 120\u202fmin. Moreover, it is predicted that chemical costs can be reduced by recycling the processed phosphotungstic acid, TOAF and HP by connecting sonoreactors in parallel to treat larger quantities of fuel (four times the recycle rate) and electricity consumption can be reduced by using low power US. Thus, in terms of total cost, it was reported that this parallel sonoreactor type has the potential to be applied in large-scale processes and has a greater advantage over batch-operated probe-type reactors for industrial and commercial applications.In a study [227] where ionic liquid was used instead of the aqueous phase, 97.6, 99.4 and 98.9% sulfur removal (followed by stirring for 170\u202fmin), respectively, was obtained from 511 ppmw thiophene, 524 ppmw benzothiophene and 530 ppmw dibenzothiophene using 5\u202fg 30\u202fvol% HP, 1.5\u202fg 20% trifluoroacetic acid and 0.3\u202fg TOAF at 50\u202f\u00b0C in 10\u202fmin in the presence of 5\u202fg of 1-n-butyl-3-methyl imidazolium methylsulfate ionic liquid under 600\u202fW and 20\u202fkHz direct US. A 100% desulfurization was achieved by applying the same conditions for Navy diesel (F-76) containing 4220 ppmw total S instead of model compounds. It is reported that the limitation of this method is that the ionic liquid used can extract sulfur-free aromatic compounds present in the fuel.Nowadays, due to the increase in oil consumption, urban and industrial wastes have been used as an energy source. In the presence of 0.1\u202fg TOAB, 30\u202fvol% HP and 0.2\u202fg phosphotungstic acid, a sulfur removal [228] of 27.5 and 61.8% (followed by extraction three times at acetonitrile/oil 1: 1 mass ratio with vigorous agitation at room temperature for two min each) under 20\u202fkHz direct US at 88\u202f\u00b0C in 20\u202fmin, respectively, was achieved from pyrolysis oil containing 8800 ppmw total S obtained by pyrolysis of the waste tire at 650\u202f\u00b0C for use as clean fuel and also diesel fuel containing 960 ppmw total S. As high carbon black and different hydrocarbon compounds in pyrolysis oil led to low desulfurization efficiency, after UAODS reaction, oxidized compounds were adsorbed in a 6-cm-length column filled with 30\u202fg Al2O3 and a sulfur removal of 68.2 and 99.7% was performed for pyrolysis oil and diesel, respectively. Nevertheless, this sulfur removal value was not considered sufficient as pyrolysis oil contains more benzothiophene and thiophene groups with the lowest ODS reactivity compared to diesel fuel according to GC-SCD analysis. Therefore, two continuous UAODS reactors were connected in series and a desulfurization efficiency of 89% was obtained for the pyrolysis oil containing 0.88\u202fwt% total S under the same UAODS reaction conditions, followed by adsorption in a 6-cm-length column filled with 30\u202fg Al2O3.A series of UAODS experiments [229] were conducted by selecting sonication time, thiophene solution/phosphotungstic acid mass ratio, thiophene solution/HP mass ratio and thiophene solution/TOAB mass ratio as independent variables. At the optimum conditions found by principal component analysis (T:HP:TOAB:Phosphotungstic acid\u202f=\u202f1:1.5:0.005:0.01 mass ratio), an approximately 73.5% conversion of thiophene at 500 ppmw concentration to its sulfones has been carried out in the range from 75 to 85\u202f\u00b0C in 20\u202fmin under 20\u202fkHz direct US. The same conditions were applied to solutions of other model sulfur compounds and the ODS reactivity following the pseudo first-order reaction kinetics was in the order: 4,6-DMDBT\u202f>\u202f4-MDBT\u202f>\u202fDBT\u202f>\u202f2-MBT\u202f>\u202fBT\u202f>\u202fT. It was explained that the low reactivity of thiophene is due to the low electron density on S atom and the relatively high reaction temperature near the boiling point (84\u202f\u00b0C) of thiophene.The conversion of 99% (55.5% at 0.02\u202fM HP) and 99.9% (99.1% at 0.02\u202fM HP), respectively, was achieved from model fuel 1 (500\u202f\u00b5g BT mL\u22121) and model fuel 2 (500\u202f\u00b5g DBT mL\u22121) using 0.2\u202fg phosphotungstic acid, 0.1\u202fg TOAB, 0.65\u202fM HP at 80\u202f\u00b0C in 15\u202fmin under 20\u202fkHz direct US [230]. The activation energies for the oxidation reactions of DBT and BT following the pseudo first-order reaction kinetics were found to be 45.01 and 60.52 kJ mol\u22121, respectively.An economic analysis of the study [228] was also evaluated. A sulfur removal [231] (each followed by adsorption in a 6-cm-length column filled with Al2O3) of 68 and 90.91%, respectively, was obtained from pyrolysis oil with high-sulfur content (8800 ppmw total S) obtained by pyrolysis of waste tires in one continuous sonoreactor and two continuous sonoreactors connected in series at pyrolysis fuel/phosphotungstic acid 100:1 mass ratio, 30\u202fvol% HP sol./TOAB 250:1 mass ratio and the convenient feed rates of aqueous and organic phase in such a way that fuel/water volume ratio is 1:1 in the reactors at room temperature and atmospheric pressure in 20\u202fmin under 20\u202fkHz direct US. As a result of the benefit-cost analysis, it is explained that a single UAODS unit can be feasible at industrial scales as the benefit/cost ratio is 1.16 and 0.86 for a single reactor and reactors in series, respectively. A recycle rate of 95, 92, 99 (which is obtained by regeneration at 500\u2013600\u202f\u00b0C) and 95% was reached for phosphotungstic acid, HP, Al2O3 and PTC, respectively, in a single sonoreactor.A 47% yield (which is higher than the desulfurization efficiency at atmospheric pressure under the same conditions) of UAODS was obtained for the model fuel [232] with 100 ppmw DBT concentration prepared by dissolving DBT in toluene using 0.05\u202fg of TOAB, 2\u202fmL of 30\u202fvol% HP and 4\u202fmL of formic acid at 25\u202f\u00b0C in 90\u202fmin under high pressure of 1.8\u202fbar and 35\u202fkHz and 70\u202fW indirect US. It was explained that this relatively high desulfurization is caused by the stable complex formation of TOAB with HP and the elimination of transient cavitation by high pressure, thus preventing the production of reducing species such as H2 and CO, which consume oxidizing species formed by the collapse of transient cavitation bubbles in the organic phase. It was stated that as US emulsifies the aqueous and organic phase highly (hence creating a higher interface area) and the mass transfer resistance is relatively large in the absence of PTC under mechanical mixing, the effect of PTC under US on sulfur removal is lower than that under stirring.In a similar study [233] where the effects of PTC on UAODS were elucidated by cavitation bubble dynamics and thermodynamic analysis, at HP/HCOOH 0.6\u202fmolar ratio, HP/TBAB 16.11\u202fmolar ratio (0.5\u202fg of TBAB) and solvent/oxidant 3.33\u202fvol ratio, a sulfur reduction of approximately 96.65 and 77.63% was achieved from 20\u202fmL of model fuel containing 100 ppmw DBT in toluene with 35\u202fkHz and 70\u202fW indirect US at 40\u202f\u00b0C in 90\u202fmin under atmospheric pressure and nitrogen atmosphere of 1.8\u202fbar, respectively. On the contrary to the study [232], it was reported that this low desulfurization at high pressure occurs due to lower emulsification and lower interfacial area compared to the situation at atmospheric pressure although transient cavitation is eliminated. It was declared that DBT undergoes almost complete oxidation due to the intensive microconvection with the help of US and the enhanced UAODS by transferring fast the oxidant anion of PTC to the organic phase by a large amount of PTC and oxidant in the medium compared to DBT although UAODS in the presence of PTC is based on an ionic mechanism (with higher activation energy than the activation energy of the UAODS reaction in the absence of PTC) rather than radical mechanism. In addition, it was reported that the effect of PTC under mechanical mixing is less pronounced than the effect under US due to the higher activation energy, the higher \u0394G and the lower \n-\n \u0394S value of the stirring system compared to the ultrasonic system.UAODS reactions [234] of two model fuels containing 500 ppmw model sulfur compound each prepared by dissolving BT and DBT in toluene were carried out using 50\u202fwt% HP and TOAB with different polyoxomethalate catalysts at 30, 50 and 70\u202f\u00b0C in the range of 2 to 30\u202fmin and it was found that the highest reactivity was obtained with a DBT conversion of 94.8% after 30\u202fmin of reaction by using NaPW under 500\u202fW power 20\u202fkHz and 40% amplitude (200\u202fW power output) direct US at 70\u202f\u00b0C. According to the BT and DBT conversion results, it was found that the UAODS catalytic activity was in the order Na3PW12O40\u202f>\u202fH3PW12O40\u202f>\u202fH3PMo12O40\u202f>\u202fH4SiW12O40 as well as an increase in sulfur removal with increasing temperature for each catalyst. It was stated that the reason for the activity order H3PW12O40\u202f>\u202fH3PMo12O40 is that the peroxotungsten complex formed is more catalytically active than the peroxomolybdenum complex even though the standard reduction potential of Mo(VI) is higher than W(VI). However, it was noted that the acidity of the aqueous phase in the case of phosphotungstic acid does not affect UAODS yield much when compared to the desulfurization results obtained in the case of the most active catalyst, sodium phosphotungstate.At optimum conditions (21.96\u202fmL oxidant volume, 1\u202fg catalyst, 0.1\u202fg PTC and 100% amplitude) found by RSM, in which volume of oxidant (40\u202fvol% HP), catalyst (phosphotungstic acid) mass, TOAB mass and ultrasonic wave amplitude are selected as independent variables, using Minitab 15 software, a desulfurization (followed by extraction at acetonitrile/oil 1:1 mass ratio) of 94.5% for gas oil [235] containing 250 ppmw total S was performed at 65\u202f\u00b0C in 20\u202fmin under 20\u202fkHz and 750\u202fW direct US. In this study, it was reported that the importance of process independent variables and their interactions according to UAODS results was in the order oxidant volume\u202f>\u202fultrasonic wave amplitude\u202f>\u202foxidant volume\u202f\u00d7\u202fultrasonic wave amplitude\u202f>\u202fcatalyst mass\u202f>\u202fPTC mass\u202f>\u202foxidant volume\u202f\u00d7\u202fPTC mass\u202f>\u202fcatalyst mass\u202f\u00d7\u202fPTC mass\u202f>\u202fPTC mass\u202f\u00d7\u202fultrasonic wave amplitude and after a certain HP volume, excess HP causes a reduction in sulfur removal by creating a radical scavenging effect.In a study [236] aimed at reducing the kinematic viscosity and sulfur of diesel oil, using the Box-Behnken design as RSM by Design Expert v.7.0.0 software, HP volume (X1), acetic acid volume (X2), PTC (TOAB) mass (X3), the amount of transition metal catalyst (phosphotungstic acid) (X4) and time (X5) were chosen as independent variables. As a result of the screening of the variables, time was found to be insignificant with respect to the desulfurization performances. After applying RSM by screening out the time variable, the importance of the relevant four variables and their interactions with each other for UAODS according to the results of ANOVA was in the order X1\n2\u202f>\u202fX4\n2\u202f>\u202fX3\n2\u202f>\u202fX2\n2\u202f>\u202fX1X2\u202f>\u202fX2\u202f>\u202fX1. Under the optimum conditions found (13.17\u202fmL HP, 17.26\u202fmL acetic acid, 0.15\u202fg TOAB and 1.5\u202fg phosphotungstic acid), an S removal (followed by extraction one time at 166.7\u202fg L\u20131 NaOH (caustic soda solution)/oil 1:1\u202fvol ratio for 2\u202fmin) of 68.85% was achieved from diesel oil containing 5044 ppmw total S at 50\u202f\u00b0C in 5\u202fmin under 20\u202fkHz frequency, 700\u202fW power and 40% amplitude direct US. After a certain amount of PTC, the mass transfer was slowed down due to the formation of a thick turbid layer in the mixture, thus leading to a reduction in UAODS. A similar trend of sulfur removal to the trend with PTC has been also observed for the transition metal catalyst, but due to the large volume of phosphotungstic acid and the small surface area of the particles. As a result of the screening analysis, it was stated that as the viscosity of diesel fuel, which has a kinematic viscosity of 3.96 cSt at 40\u202f\u00b0C, decreases by max 20% after UAODS process, the relevant independent variables have no effect on the viscosity, and therefore the kinematic viscosity as a dependent variable was not taken into account.In a study [237] investigating the mechanism of the UAODS system in the presence of different catalysts (phosphotungstic acid, acetic acid and formic acid), it has been underlined that the desulfurization reaction is based on the ionic mechanism (caused by the transport of the peroxo-metallate anion and the anion of peracids from the aqueous phase into the interface by the lipophilic cation of PTC) in the presence of phase transfer catalyst, whereas in the absence of PTC, the desulfurization reaction is based on the radical-based mechanism (caused by the formation of active oxygen radicals such as acetyl radical CH3CO\u00b7 and hydroperoxy radical HO2\u00b7 by resulting in decomposition of peracids and HP by the collapse of cavitation bubbles formed). It was found that the sulfur removal efficiencies achieved at 1.8\u202fbar for all three catalysts were lower than the desulfurization performances at atmospheric pressure, mainly due to the reduction in microconvection intensity within the mixture under high pressure, resulting in lower mass transport. In this study, in contrast to the other two studies [164,233] in which n-hexane and toluene were used as solvents, it was reported that as n-decane has a high boiling point and therefore has a very low vapor pressure, no reducing species such as H2 and CO, which reduces oxidizing species, were formed as a result of ultrasonic cavitation at atmospheric pressure. At n-decane (organic phase)/HP (aqueous phase) volume ratio of 10, a maximum desulfurization of about 74% with a rate constant of 0.0155\u202fmin\u22121 was performed using 60\u202fmg L\u20131 TBAB, 4\u202fmL FA and 2\u202fmL HP at 50\u202f\u00b0C in 90\u202fmin under 35\u202fkHz and 70\u202fW indirect US for the model fuel containing 100 ppmw DBT in n-decane. Excessive use of PTC prevented mass transfer, decreasing UAODS relatively. The excess of the transition metal catalyst acts as an emulsion in the mixture by covering the emulsion droplets with a thin film and creating a barrier in the mass transport of the oxidant into the interface, thus causing the UAODS yield to be levelled off.The optimum conditions, which led to a sulfur removal of 60.75% without extraction, found for the batch reactor in the study [236], were applied to the continuous tube-type flow-through sonoreactor [238] by scaling up 2.5 times and under direct US with two transducers operating at a frequency of 20\u202fkHz and a sonication power of 48\u202fW each, a sulfur removal efficiency of 80.79% was achieved from final gas oil containing 5044 ppmw total S using 30\u202fmL HP, 45\u202fmL acetic acid, 0.375\u202fg TOAB and 3.75\u202fg phosphotungstic acid at equal feed and outlet flow rates in 5\u202fmin. It was explained that this higher conversion compared to that in the batch reactor is due to the lack of temperature control (hence leading to an increase in the temperature of the mixture as a result of cavitation under US) in the continuously operating sonoreactor and the fact that every fluid element does not reside for exactly 5\u202fmin as in the batch reactor (i.e., resided for 5\u202fmin on average). The kinematic viscosity of the relevant gas oil decreased by 9.40% within 5\u202fmin under the UAODS conditions, while a 13.5% reduction in kinematic viscosity was achieved by using US alone in the same minute. It has been noted that US gives off some of its energy to split HP and peracetic acid into their radicals under oxidation conditions, while under US alone, it converts the gas oil into lighter fractions by giving off its energy to cleave the C - C and C - S bonds. However, for the cases of US alone and UAODS, no significant change was observed in kinematic viscosity at treatment times of 15\u202fmin, compared to the kinematic viscosity before the treating of gas oil. In the absence of acetic acid, besides final gas oil containing 5044 ppmw total S, other feedstocks (atmospheric gas oil with 10,700 ppmw total S, atmospheric kerosene with 4980 ppmw S, Isomax gas oil with 181 ppmw total S) were subjected to oxidation reaction under direct US with 48\u202fW max power and it was stated that the UAODS efficiency is in the order atmospheric kerosene\u202f>\u202fatmospheric gas oil\u202f>\u202ffinal gas oil\u202f>\u202fIsomax gas oil and the sulfur removal from high-sulfur gas oils is higher. As for kerosene, since lighter fractions as well as the small number of condensed aromatic sulfur compounds (thus lower specific gravity, lower kinematic viscosity, and lower boiling range of kerosene, compared to gas oils) were present, the best desulfurization improvement has been achieved.In a study [239] where crude oil containing 2133 ppmw total S was desulfurized and upgraded (simultaneous extraction and oxidation process) under 40\u202fkHz indirect US, 65.28% S removal was achieved with 200\u202fppm oxidant, 60\u202fppm demulsifier dosage and distilled water at 65\u202f\u00b0C in 10\u202fmin and it was determined that the physical properties of the treated crude oil have improved (ie, decrease in density, decrease in kinematic viscosity at 20\u202f\u00b0C, increase in cetane number, decrease in 10% carbon residue on residuum/%).At optimum conditions (17\u202fmin, 180.3\u202fmmol HP and 25\u202fppm FeSO4) found by applying RSM based on central composite design (CCD) in which HP amount, catalyst (FeSO4) amount and time are selected as independent variables, a 90% desulfurization of gas oil [240] containing 9500 ppmw total S was performed by three-stage UAODS process (followed by extraction three times at a volume ratio of methanol/oil 4:5 at room temperature for 2\u202fmin each after every UAODS reaction) using isobutanol as PTC in the presence of acetic acid (ie, in acidic medium where the catalyst is active at pH less than 3) at 62\u202f\u00b0C under 24\u202fkHz and 400\u202fW direct US. In the presence of TOAB as PTC instead of isobutanol, 21.99% sulfur removal from gas oil was performed by a one-step UAODS process under the same conditions, while in the presence of isobutanol, a 67.70% reduction in total sulfur was achieved by one-step UAODS (followed by extraction). Moreover, it was stated that isobutanol is very cheap, can be mixed into the fuel and burned, and it has economic viability as it does not require separation after UAODS reactions. After the oxidation reactions, the extractions with methanol were carried out under US and the sulfur removal was the same as that obtained by the extraction under stirring, thus demonstrating that ultrasound has no effect on extraction in this study. According to the F-test of the regression model, it was revealed that the effect of time variable and time\u202f\u00d7\u202fHP interaction on UAODS is not of importance.At the optimum conditions (16.4\u202fmin sonication time, 122.1\u202fmg TOAB, organic phase/aqueous phase 29.7\u202fmL/10.3\u202fmL volume ratio and 204.8\u202fppm Fe(VI) for BT, 29.5\u202fmin sonication time , 111.6\u202fmg TOAB, organic phase/aqueous phase 16.2\u202fmL/23.8\u202fmL volume ratio and 245.3\u202fppm Fe(VI) for DBT) found by applying RSM based on BBD for which the ultrasonication time, TOAB amount, organic phase/aqueous phase volume ratio and Ferrate concentration in ppm unit are selected as independent variables, a sulfur removal of 88.3 and 91.8%, respectively, was obtained using 0.1\u202fN acetic acid (pH\u202f=\u202f4) from two model fuels (500 ppmw BT in toluene and 500 ppmw DBT in toluene) at 70\u202f\u00b0C [241]. The optimum conditions found for BT and DBT were individually applied to diesel fuel containing 1428.6 ppmw total S, resulting in 85.7% BT and 91% DBT reduction in diesel oil. It was explained that these lower desulfurization yields compared to model fuels is due to the presence of many different sulfur compounds in diesel fuel that make oxidation difficult. The effect of different amounts of Ferrate and TOAB on UAODS was also investigated under 20\u202fkHz frequency, 500\u202fW and 40% amplitude direct US. When the ferrate concentration increased to a certain value, sulfur removal gave a maximum and after a certain value, sulfur removal decreased. This was attributed to the fact that as the ferrate concentration increased, the pH of the aqueous phase slightly increased (i.e., more basic medium), thus leading to a decrease in the oxidation capacity of the ferrate in basic medium (lower reduction potential (+0.72\u202fV) in basic medium [242]). However, the standard reduction potential [243] of ferrate in acidic medium is\u202f+\u202f2.20\u202fV. With the excessive use of TOAB, the sulfur removal decreased, which has been attributed to the slowing of mass transfer due to turbidity of the mixture and to sterically prevention of electrophilic oxidation of sulfur compounds by the high concentration of alkyl groups. According to ANOVA results, it was reported that OP/AP volume ratio, PTC\u202f\u00d7\u202f(OP:AP) volume ratio interaction and PTC2 have the greatest effect on UAODS for BT, whereas OP/AP, US time\u202f\u00d7\u202fPTC interaction, US time\u202f\u00d7\u202fFerrate concentration interaction, PTC\u202f\u00d7\u202fferrate concentration, PTC2, (OP:AP)2 and (Ferrate conc.)2 have the greatest effect on the sulfur removal for DBT. It was determined that the amount of PTC for both model sulfur compounds is not important to UAODS. It has been pronounced that potassium ferrate has higher oxidation capacity and higher stability than HP and HP decomposes thermally at high temperature despite its lower cost, which is another important advantage of this process. Moreover, thermal decomposition [244] of potassium ferrate occurs above 198\u202f\u00b0C. The oxidation mechanism is based on the formation of protonated Fe(VI) as a reactive complex [245\u2013247] (which is much stronger oxidant than FeO4\n2 \u2212) by reaction of ferrate with acetic acid and, subsequently the transfer of the complex into the organic phase (where organic sulfur compounds are oxidized) by binding to the lipophilic cation of the phase transfer agent.By applying the Pareto-optimal analysis-based fuzzy logic model [248] in which US time, TOAB amount, organic phase/aqueous phase volume ratio and ferrate concentration are selected as four independent variables to maximize the sulfur reduction and, also US energy consumption, TOAB amount and the Ferrate amount are selected as three independent variables to minimize the operating cost, in the presence of acetic acid (pH\u202f=\u202f4) at 70\u202f\u00b0C under 20\u202fkHz direct US with 200\u202fW power output (500\u202fW, 40% amplitude), it was reported that a conversion of 93.79% was achieved per operating cost of $ 0.830 at the optimum conditions (15.86\u202fmin US time, 107.7\u202fmg TOAB, 30\u202fmL:10\u202fmL organic phase/aqueous phase volume ratio and 100\u202fppm ferrate concentration) for 500 ppmw BT, while a conversion of 88.36% was achieved per $ 0.769 operating cost at the optimum operating conditions (10\u202fmin US time, 100.1\u202fmg TOAB, organic phase/aqueous phase volume ratio 16.96\u202fmL/23.04\u202fmL and 300\u202fppm ferrate concentration) for 500 ppmw DBT. It was shown that the desulfurization efficiencies obtained in this study are comparable with two sonoreactors in series in the previous studies [228,231], whereas the operating cost in this study is lower than that in the continuous sonoreactors connected in series, hence having the potential to be applicable for scaling up purposes.UAODS is performed at relatively much lower temperatures (i.e., in the range of room temperature to 90\u202f\u00b0C), atmospheric or near atmospheric pressures, and generally shorter times than HDS. Process efficiency in UAODS is very important in terms of commercial applicability. In addition, US power intensity [125], defined as the power transferred to the liquid per surface area of the ultrasonic probe, and amplitude are important. It is beneficial to use low-amplitude ultrasound from the point of lower power and lower electricity consumption.As mentioned before, reaction and ultrasonic parameters have a very important effect on desulfurization. Increasing the amount of PTC up to a certain value improves UAODS by allowing more PTC-oxidant complexes to transfer into the organic phase and then ODS decreases slightly as a result of the slowing down of mass transfer between the aqueous-organic phase in the liquid mixture due to the formation of a thick turbid layer above an optimum amount of PTC [236,237,241]. As known, the reaction rate constant increases exponentially with increasing temperature according to Arrhenius equation, consequently increasing the reaction rate as well [249]. Nevertheless, above an optimum temperature, the collapse intensity of cavitation decreases as more solvent vapors will accumulate in cavitation bubble [120,250,251] in addition to decomposition of HP into water and oxygen, thus decreasing UAODS yield. Temperature can be increased unless the collapse intensity of the cavitation bubble reduces the total reaction rate [200]. Above an optimum reaction volume, sulfur removal decreases due to the lower ultrasonic power density [78,196]. With increasing HP concentration (i.e., a more concentrated HP solution) up to a certain value in aqueous phase, UAODS usually increases due the formation of more HO\u00b7 radicals than HP [65,126]. Above an optimum concentration, HP can have a scavenging effect on hydroxyl radicals [157]. The sulfur removal increases up to a certain ultrasonic intensity, whereas dense bubble clouds, which show the cavitation shielding effect, will accumulate near the probe above a certain intensity [184]. Therefore, UAODS yield can decrease at high intensities and consequently, an optimum US intensity is required. Although generally, dissolved gases such as helium and oxygen in liquid mixture act as nucleation sites, facilitating the formation of the cavitation bubble, reaction rates change depending on the solubility, the thermal conductivity and the specific heat of the gases used [91,200]. However, dissolved gas above a certain concentration in cavitation bubble can cushion the collapse of the cavitation bubble, consequently causing a lower collapse intensity [252,253]. Therefore, it is necessary to find the optimum dissolved gas concentration in liquid mixture to increase the UAODS reaction rates unless the dissolved gas quantity decreases the cavitation effect. Pressure can have two opposite effects. As pressure increases, the intensity of the cavitation bubble implosion increases [254]. However, above an optimum pressure, much less bubbles, which can have almost no impact on overall reaction rate, can be produced due to increasing cavitation threshold of the liquid mixture [200]. The effect of pressure on sulfur removal varies as shown in Table S1 in the Supplementary Information and the boiling point of the solvent in the organic phase or the boiling range of fuel becomes crucial. For low boiling point solvents such as hexane, toluene, it is observed that sulfur removal increases with increasing pressure at relatively low operating temperature [164,232], while sulfur removal decreases with increasing pressure at high operating temperature [233]. For high boiling point solvents, it was reported that sulfur removal decreases with increasing pressure at relatively high temperature [197,237]. These differences observed in sulfur removal at high pressures can be attributed to a decrease or increase in the collapse intensity of cavitation bubbles. Nonetheless, much more effort is needed to establish a clear relationship between pressure and temperature in terms of cavitation intensity. In summary, in order to maximize total UAODS reaction rate, it is necessary to consider in combination the effects of reaction and ultrasonic parameters on UAODS yield.Desulfurization process efficiency (DPE\u202f=\u202fUAODS yield/MR(H2O2/S)) can be defined as the UAODS yield per molar ratio of reactants used (i.e., the molar ratio of hydrogen peroxide to sulfur). The less the amount of HP, the larger the quantity of fuel used to remove sulfur and the higher the UAODS yield, the higher the process efficiency. Figs. 4, 5 and 6\n\n\n show DPEs calculated using heterogeneous catalysts, homogeneous catalysts in the absence of PTC and homogeneous catalysts in the presence of PTC, respectively. The operating conditions of UAODS reactions with heterogeneous catalyst, homogeneous catalyst in the absence of PTC and homogeneous catalyst in the presence of PTC are given in Tables S2, S3 and S4, respectively. From the three figures, it can be seen that the DPEs under indirect US (ultrasonic bath) are mostly lower than the DPEs under direct US. This low process efficiency can be attributed to the fact that the intensity of the indirect US (in this case, the ultrasonic wave generated by the transducer passes first through the walls of the sample container and then through the liquid) is much lower compared to the intensity of ultrasound in direct contact with liquid using the ultrasonic probe [255]. Also, in an ultrasonic bath, ultrasonic wave cannot propagate equally in all directions into each fluid element in a liquid, thus resulting in heterogeneous dissipation [256\u2013258].It can be seen that in the case of using heterogeneous catalysts, the DPEs are generally higher than DPEs with and without PTC using homogeneous catalysts. These high DPEs can be due to both the adsorption of sulfur compounds on the catalyst surface and the oxidation of sulfur compounds by forming an active oxidizing complex caused by HP on the surface, as well as the adsorption of oxidized sulfur compounds. There are many advantages of using solid catalyst in liquid under US irradiation: solid particles function as nucleation sites to form cavitation bubbles, thus causing free radicals to increase further. Sonication results in an increase in surface area by reducing the particle size of solid catalysts and inactive catalyst becomes reactive as a result of desorption of adsorbed sulfones (passivating surface coating) due to the surface cleaning caused by liquid jet streams which are formed by implosion of cavitation bubbles [259]. In addition, more collision occurs between reactants and catalysts due to microstreaming [250] and agglomeration of catalysts is prevented [260]. Moreover, the high heat generated by the collapse of cavitation bubbles near solid catalysts can propagate inside catalyst, consequently leading the reaction rate to be higher and it is emphasized that the largest sonochemical effect occurs in macropores >50\u202fnm in diameter [261]. On the other hand, too many catalyst particles can attenuate US waves propagating through liquid [125]. Therefore, an optimum catalyst loading is necessary in UAODS reactions.There is an exception in the case of using potassium ferrate in Fig. 6. As potassium ferrate is a stronger oxidant in acidic environment than HP and the active complex consisting of ferrate and acetic acid has higher oxidation power than ferrate alone, DPEs are very high.DPEs for acetic acid-HP and formic acid-HP in Fig. 5 are generally higher than those for the phosphotungstic acid-HP system in Fig. 6, which is due to the small molecular size of acetic acid [262] (ca. 0.4\u202fnm) and formic acid [263] (ca. 0.3\u202fnm), thus alkyl substituted aromatic sulfur compounds do not cause steric hindrance. The reason that DPEs are lower in the case of using phosphotungstic acid-HP system in the presence of PTC in Fig. 6 compared to DPEs in the case of using homogeneous catalysts without PTC is that the alkyl groups adjacent to the sulfur atom of compounds such as 2,5-DMT, 4-MDBT and 4,6-DMDBT in fuel lead to the steric hindrance due to bulky size of the oxidizing polyoxoperoxo complex composed of phosphotungstic acid and HP. However, when organic acids such as formic acid and acetic acid are used in combination with phosphotungstic acid, DPE increases considerably by creating a synergistic effect due to the polyoxoperoxo complexes and peracids formed [236,238]. The reason for using PTCs in the case of phosphotungstic acid is the transfer of the formed polyoxoperoxo complex anion to the organic phase, otherwise DPE without PTC may be low. Also, phosphotungstic acid decomposes as pH increases from 1 to 8.3 [264] and thus an acidic medium is favorable to the UAODS reactions. Since phosphotungstic acid is thermally stable [265] up to 400\u202f\u00b0C, it can form stable polyoxoperoxo complexes with HP and hence ODS can be performed at relatively higher temperatures, which are below 100\u202f\u00b0C, compared to the temperatures in the case of acetic acid and formic acid. Performic acid [266] and peracetic acid [267] undergo dramatically thermal decomposition, especially at temperatures of 45\u202f\u00b0C and above.Formic acid and acetic acid have the capacity to extract sulfur compounds and peracids formed as a result of emulsification by US effect can easily be transferred into the organic phase or the organic-aqueous phase interface. Therefore, it can be deduced that PTC has no significant effect on DPE. In the studies in Fig. 6, it is seen that PTC is used in addition to phosphotungstic acid. The reason for using PTC may be due to the low desulfurization obtained by using phosphotungstic acid in the absence of PTC.In Fig. 4, modified Metal-organic Framework (MOF) was used in the study where DPE of 49, 43.5 and 35.5% was obtained. The reason for the high DPE can be both the entrapment of phosphotungstic acid into amino-functionalized MOF with large surface area and pore volume (hence aromatic sulfur compounds are effectively adsorbed and oxidized on phosphotungstic acid@TMU-17-NH2), and the simultaneous extraction of oxidized sulfur compounds using acetonitrile. In addition, ultrasonic synthesis, which is more environmentally friendly and performed at lower reaction time at room temperature than solvothermal process carried out at high temperature, may have contributed to high desulfurization as MOFs synthesized under ultrasound have generally higher surface area, lower particle size, higher crystallinity, more uniform morphology and size distribution compared to those obtained by conventional preparation methods.Reactor configurations also affect DPE. In Fig. 5, the high DPE of 23.57 is due to the nozzle, through which the aqueous phase consisting of FA and HP flows in a very low amount (0.71\u202fmL\u202fmin\u22121), placed just below the tip of ultrasonic probe, thus causing an increase in sulfur removal by generating active radicals in this efficient region and dispersing the aqueous phase more homogeneously into the organic phase.In ODS, ionic liquids have also been tried instead of the aqueous phase. However, their synthesis is generally high cost and it is difficult to transport them due to their high viscosity. In addition, as more US power is needed to fully emulsify the high viscosity ionic liquid phase and organic phase, the operating cost will increase due to electrical energy consumption. Moreover, since the ionic liquid loses its activity after a certain recycle, its regeneration will also lead to an additional cost. Therefore, the use of ionic liquids in continuous processes is not practical.In the studies, one of the biggest problems of UAODS is fuel loss during extraction and/or adsorption process to remove oxidized sulfur compounds after oxidative treatment. During the separation processes, other polar hydrocarbons in fuel pass into the extractant phase or are adsorbed on the adsorbent. Although it has been shown in laboratory and pilot studies that the physicochemical properties of the fuel after the UAODS process change in acceptable ranges according to the fuel specifications for petroleum fractions, how these properties will change in large-scale industrial production is a separate research topic. In addition, the ultrasonic probe must be replaced with the new one as the tip surface erodes by pitting in long service life [125,268], otherwise it becomes inoperable.One of the biggest reasons for the widespread use of HDS is that it has a high fuel recovery as well as a very little negative effect on fuel properties. In addition, hydrotreatment of diesel consisting of paraffinic, aromatic and naphthenic components saturates the aromatic compounds in the diesel, resulting in an increase in the cetane number [269].After UAODS, how to eliminate the waste sulfones generated and accumulated is an environmental issue. Elemental sulfur, which is mainly used for sulfuric acid production [270], can be produced by the reaction of SO2 with H2S generated in HDS units after the waste sulfones are converted to SO2 as a result of thermal decomposition [271] by burning them in high temperature furnace operating at 1093\u20131427\u202f\u00b0C in the Claus process [272,273] or by pyrolysis [274].In a study [275] evaluating the desulfurization process economics by using Aspen Plus simulation, it has been shown that the UAODS process is not cost-effective for fuels containing high sulfur (i.e., in the range of several thousand ppmw) due to high chemical consumption to drastically reduce the sulfur content of fuel and very high amounts of extraction solvent required to separate the huge amounts of sulfones formed, therefore it is not competitive with HDS. Therefore, detailed research taking fuel loss into account is still needed to achieve cost savings and high sulfur removal in the UAODS process by using low amounts of reagents, performing reactions at the lowest possible temperature in the shortest possible time and using the most efficient extraction solvent in the lowest possible amounts.Concluding remarks and future directions can be presented as follows:\n\n-\nIn order to increase the sulfur removal per power density consumed as well as to reduce the process cost, one continuous-flow sonoreactor or two continuous-flow sonoreactors in series can be used at low flow rate of the aqueous phase feed and, short retention times. At high conversions, continuous sonoreactors can be connected in parallel to treat more fuel.\n\n\n-\nDesulfurization can be increased by the addition of heterogeneous catalysts to continuous sonoreactors connected in series.\n\n\n-\nPotassium ferrate with a much higher reduction potential than HP under acidic conditions can be activated by HCl, HNO3, H2SO4, HClO4 and HCOOH instead of CH3COOH. To reduce the process cost, UAODS reactions can be carried out using potassium ferrate in acidic medium in the absence of relatively expensive PTCs.\n\n\n-\nLow temperatures in the range of 20\u201340\u202f\u00b0C favor UAODS reactions since the decomposition of performic and peracetic acid increases drastically above 40\u202f\u00b0C in the case of homogeneous catalysts. To observe the change of concentration of peroxycarboxylic acid over time, the reactions of HP and carboxylic acids (i.e., HCOOH or CH3COOH) can be carried out at different temperatures, different times and various molar ratios in the absence of both PTC and organic phase under US irradiation and consequently, peroxyformic acid or peroxyacetic acid (HCOOOH or CH3COOOH) concentration at any time t during the reaction can be readily determined by titrimetric analysis. Eventually, the time, at which peroxyformic acid or peroxyacetic acid concentration is maximum, is found for each temperature. Therefore, UAODS reactions can be performed at those times, thus reducing the process cost due to short reaction times and increasing the sulfur removal efficiency. Alternatively, UAODS reactions can be performed at different temperatures and by taking an aliquot of the aqueous phase at certain times during the UAODS reaction for each temperature, the change of the concentration of the peroxycarboxylic formed can be followed by titrimetric analysis. Consequently, a relationship between the sulfur removal and peroxycarboxylic acid concentration can be established and sono-oxidative desulfurization reaction conditions can be optimized.\n\n\n-\nIndirect ultrasonic application in UAODS reactions is not as effective as direct US application from the point of view of DPE.\n\n\nIn order to increase the sulfur removal per power density consumed as well as to reduce the process cost, one continuous-flow sonoreactor or two continuous-flow sonoreactors in series can be used at low flow rate of the aqueous phase feed and, short retention times. At high conversions, continuous sonoreactors can be connected in parallel to treat more fuel.Desulfurization can be increased by the addition of heterogeneous catalysts to continuous sonoreactors connected in series.Potassium ferrate with a much higher reduction potential than HP under acidic conditions can be activated by HCl, HNO3, H2SO4, HClO4 and HCOOH instead of CH3COOH. To reduce the process cost, UAODS reactions can be carried out using potassium ferrate in acidic medium in the absence of relatively expensive PTCs.Low temperatures in the range of 20\u201340\u202f\u00b0C favor UAODS reactions since the decomposition of performic and peracetic acid increases drastically above 40\u202f\u00b0C in the case of homogeneous catalysts. To observe the change of concentration of peroxycarboxylic acid over time, the reactions of HP and carboxylic acids (i.e., HCOOH or CH3COOH) can be carried out at different temperatures, different times and various molar ratios in the absence of both PTC and organic phase under US irradiation and consequently, peroxyformic acid or peroxyacetic acid (HCOOOH or CH3COOOH) concentration at any time t during the reaction can be readily determined by titrimetric analysis. Eventually, the time, at which peroxyformic acid or peroxyacetic acid concentration is maximum, is found for each temperature. Therefore, UAODS reactions can be performed at those times, thus reducing the process cost due to short reaction times and increasing the sulfur removal efficiency. Alternatively, UAODS reactions can be performed at different temperatures and by taking an aliquot of the aqueous phase at certain times during the UAODS reaction for each temperature, the change of the concentration of the peroxycarboxylic formed can be followed by titrimetric analysis. Consequently, a relationship between the sulfur removal and peroxycarboxylic acid concentration can be established and sono-oxidative desulfurization reaction conditions can be optimized.Indirect ultrasonic application in UAODS reactions is not as effective as direct US application from the point of view of DPE.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.Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2021.105845.The following are the Supplementary data to this article:\n\nSupplementary Data 1\n\n\n\n", "descript": "\n                  Recently, environmental pollution has increased significantly due to petroleum-based fuels widely used in vehicles. This environmental pollution is mainly due to the acidic SO2 gas generated by the combustion of fuels and emitted into the atmosphere. SO2 gas causes not only acid rain but also corrosion of metal parts of engines in vehicles. In addition, it functions as a catalyst poison in catalytic converters in exhaust system. Due to these damages, strict regulations have been introduced to reduce the amount of sulfur in fuels. As of 2005, the permissible amount of sulfur in diesel fuels in Europe and America has been limited to 10 and 15\u202fppm by weight, respectively.\n                  Due to the decreasing oil reserves in the world, high viscosity petroleums containing high sulfur and heavier fractions (i.e., low-quality oils) are increasing, thus making desulfurization difficult and leading to high costly process. Since time and economic loss are very important today, these two terms have to be reduced to a minimum. Recently, ultrasound wave in ODS shown as an alternative to HDS is utilized to further increase desulfurization in shorter times. Ultrasound wave locally creates high temperatures and high pressures (hot-spot theory) in liquid, causing the desulfurization reaction to accelerate further.\n                  In this review, the advantages and difficulties of oxidative desulfurization, the economics of ultrasound-assisted oxidative desulfurization are summarized and recommendations for improving the process are presented.\n               "}
{"full_text": "The extensive combustion of fossil carbon is most probably responsible for the growing concentration of greenhouse gas CO2 in the atmosphere. The concerns about global warming turned attention towards the production of biofuels by upgrading non-edible and waste vegetable oils and animal fats [1\u20138]. The most widely used production method of diesel range bio-oil, generally referred to as biodiesel, is catalytic transesterification of latter renewable triglycerides by lower alcohols [1,6,7]. However, the biodiesel cannot fully replace conventional diesel oil, because of its lower energy density, higher viscosity, moderate oxidation stability, and limited compatibility with fossil fuel [4,6]. A better alternative of triglyceride upgrading is deoxygenation via hydroprocessing that is providing a mixture of hydrocarbons. The hydrocarbon mixture is second generation biofuel, often referred to as biogasoil or green diesel. Biogasoil has comparable or even better fuel properties than conventional diesel fuel [6,7,9].Catalytic triglyceride HDO can be carried out at moderate temperature (200 \u2013 400\u00a0\u00b0C) and hydrogen pressure (<50\u00a0bar). The applied catalysts are those, routinely used in the petroleum industry for hydroprocessing, such as, sulfided cobalt and nickel molybdate catalysts, supported noble (Pt, Pd, Ru) and non-noble metals (Ni, Cu, Co), metal phosphides, metal oxides, etc. [3,4,6,10]. Application of monometallic transition metal catalysts are very common for the deoxygenation of fatty acids and triglycerides [3,6,10]. Palladium based catalysts are especially preferred because of the peculiar ability of the palladium metal to activate hydrogen for reaction [10]. Nevertheless, there is a general agreement that the catalyst support does not only provide high surface area to stabilize high metal dispersion, but it also has significant contribution to the HDO activity and selectivity by its acid-base property [4,6,10]. Supports, having strong Br\u00f8nsted acidity, such as H-zeolites, are less favored because they initiate cracking of long chain paraffins and condensation reactions, producing coke precursors and coke that deactivates the catalyst [6,10]. Therefore, supports of mild-to-moderate acidity, such as activated carbon, TiO2, ZrO2, SiO2, and Al2O3 were found suitable for HDO catalysts [4,6,10]. The most often used support is \u03b3\u2013Al2O3\n[4,8].The hydroconversion of triglycerides to paraffins proceeds in the consecutive steps of ester bond hydrogenolysis, giving carboxylic acid, and deoxygenation of the acid to paraffin. The deoxygenation reaction is the rate determining step of paraffin formation [3,4,6,11]. Former studies showed that the deoxygenation reaction of the carboxylic acid intermediate over Pd catalysts follows pathways resulting in the formation of CO (hydrodecarbonylation, HDCO), CO2 (hydrodecarboxylation, HDCO2), and H2O (H2-reduction of oxygen, HDH2O). Over supported metal catalysts the HDCO was found to be the major reaction route, whereas the HDCO2 and HDH2O were reaction routes, which had only minor contribution to the HDO reaction [7,8]. The mechanisms of these HDO pathways are not fully understood. It was suggested that the HDCO reaction proceeded through direct hydrogenolysis of the carboxylic acid to paraffin and formic acid (C\u2013C bond scission), which reaction step was followed by quick decomposition of HCOOH to CO and H2O [6,8,11]. Deoxygenation on the HDH2O route was proposed to proceed by deoxygenation of carboxylic acid to aldehyde intermediate [2,3,5,8]. Accordingly, formation of aldehyde from carboxylic acid involves hydrogenation/dehydration reactions (hydrogenation of C\u00a0=\u00a0O bond to CH\u2013OH followed by H2O formation involving C\u2013O bond scission). The surface-bound aldehyde intermediate is then further hydrogenated to paraffins by releasing either H2O or CO, corresponding to routes HDH2O and HDCO, respectively [3,5].In the present study, alumina-supported Pd catalysts were prepared and studied to learn more about the mechanism of triglyceride HDO reaction. The effect of support phosphatization on the catalyst structure, acid-base properties, and activity was investigated. The HDO activity was tested using tricaprylin and valeric acid model compounds. Quasi\u2013operando DRIFTS investigation provided insight in the chemistry of surface intermediate formation during the catalytic reaction and permitted to come to important conclusions, concerning some mechanistic details.Alumina-supported palladium catalyst was perepared by impregnating 10 gramms of \u03b3\u2013Al2O3 (Ketjen CK-300, Alfa Aesar) by 10\u00a0cm3 of aqueous Pd(NH3)4(NO3)2 (product of Strem) solution. The concentration of the solution was adjusted to get catalyst of 0.5\u00a0wt% Pd content. The sample was dried at 110\u00a0\u00b0C for 16\u00a0h. To decompose the metal precursor salt the sample was calcined. It was heated first at a heating rate of 2\u00a0\u00b0C\u00a0min\u22121 to 150\u00a0\u00b0C, kept at this temperature for 1\u00a0h, and then the temparature was raised at a heating rate of 4\u00a0\u00b0C\u00a0min\u22121 to 350\u00a0\u00b0C. The catalyst was kept at this tempreture for additional 4\u00a0h. The obtained catalyst sample was designated as Pd/Al2O3.The phosphatized-alumina-supported Pd catalyst samples were prepared following the same procedure as above, except that the alumina support was phosphatized first to different extents. Supports with 1.0, 2.5, and 5.0\u00a0wt% phosphorous content were prepared by impregnating 10\u00a0g of \u03b3\u2013Al2O3 with 10\u00a0cm3 of a solution, containing calculated amount of phosphoric acid. The impregnated samples were dried at 110\u00a0\u00b0C for 16\u00a0h then calcined in air at 550\u00a0\u00b0C for 4\u00a0h to generate the phosphatized alumina supports. The thus obtained phosphatized \u03b3\u2013Al2O3 catalyst supports are designated as Al2O3\u20131P, Al2O3\u20132.5P and Al2O3\u20135P, respectively. These supports were used to prepare catalysts of 0.5\u00a0wt% Pd content. The corresponding supported Pd catalysts were designated as Pd/Al2O3\u20131P, Pd/Al2O3\u20132.5P, and Pd/Al2O3\u20135P.The P and Pd content of the catalyst samples were determined by using Inductively Coupled Plasma Optical Emission Spectroscopic (ICP-OES) method (Spectro Genesis ICP-OES apparatus).Nitrogen adsorption isotherms of the catalyst samples were determined at \u2212196\u00a0\u00b0C using an automatic, volumetric adsorption analyzer (The \u201cSurfer\u201d, product of Thermo-Fisher Scientific). The sample was dehydrated before the measurement at 250\u00a0\u00b0C under high vacuum (10-6\nmbar) for 4\u00a0h. The SSA of the catalyst samples was determined by the BET method, whereas the pore-size distribution was calculated using the Barett-Joyner-Halenda (BJH) method.The XRPD measurements were carried out using a Philips PW 1810/3710 X-ray powder diffractometer in a Bragg-Brentano parafocusing arrangement applying monochromated Cu K\u03b1 (\u03bb\u00a0=\u00a01.5418 A) radiation.The dispersion of Pd in the catalysts was determined by CO pulse chemisorption method. About 100\u00a0mg of the sample was placed into a quartz microreactor (I.D.: 4\u00a0mm) and reduced in situ in hydrogen flow at 450\u00a0\u00b0C for 1\u00a0h. It was flushed then by He flow at 450\u00a0\u00b0C for 30\u00a0min and cooled to room temperature in the He flow. In 3\u00a0min intervals carbon monoxide pulses of 10\u00a0\u00b5L volume were injected sequentially into the He flow, passing through the catalyst bed. The CO concentration of the reactor effluent was monitored using thermal conductivity detector (TCD). The TCD signal was processed by computer. The introduction of CO pulses was continued until the chemisorption sites were saturated. After calibration the molar amount of chemisorbed CO was calculated from the areas of the TCD signals.The CO2-TPD measurement was used to characterize the basicity of the supports. About 150\u00a0mg of the sample was placed into a quartz microreactor (I.D.: 4\u00a0mm) and activated in O2-flow at 550\u00a0\u00b0C for 1\u00a0h. The sample was then flushed with N2 for 15\u00a0min at 550\u00a0\u00b0C, evacuated at the same temperature for 30\u00a0min and cooled to room temperature. Adsorption of CO2 was carried out by contacting the sample with CO2 gas at 13.3\u00a0kPa for 15\u00a0min. The system was flushed by He and the temperature of the reactor was ramped up at a rate of 10\u00a0\u00b0C\u00a0min\u22121 to 700\u00a0\u00b0C. The CO2 concentration in the gas flow was monitored by TCD.A Nicolet 6700 FT-IR (Thermo Scientific) instrument was used in transmission mode to record the spectra of the surface species present on the neat supports and catalysts and obtained from adsorption of compounds. Self-supporting wafer of the examined sample having a \u201cthickness\u201d of 5\u201310\u00a0mg\u00a0cm\u22122 was placed into the sample holder of a stainless steel spectroscopic cell equipped with CaF2 windows and a furnace section for in situ activation of the sample either in atmospheric gas flow or under high vacuum. Spectra were taken by averaging 512 scans at a nominal resolution of 2\u00a0cm\u22121.The acidity and basicity of the supports were studied by determining the spectra of adsorbed pyridine (Py) or CO2, respectively. Prior to Py adsorption the sample was pretreated at 450\u00a0\u00b0C under high vacuum (10-6\nmbar) for 1\u00a0h then the temperature was lowered to 200\u00a0\u00b0C and the sample was contacted with Py vapor at 5\u00a0mbar for 30\u00a0min. The sample was cooled then to 100\u00a0\u00b0C. The Py vapor was removed from the cell by successive evacuation at temperatures 100, 200, 300, 400, and 450\u00a0\u00b0C for 30\u00a0min at each temperature. After each evacuation a spectrum was recorded at room temperature. The spectrum of the wafer, recorded before Py adsorption, was subtracted from each spectrum to obtain the spectrum of the adsorbed species only.A procedure, similar to that described above, was followed to determine the spectra of the species obtained from adsorption of CO2. Wafer of the activated sample was contacted with CO2 at 15\u00a0mbar at room temperature for 30\u00a0min. The spectrum of the adsorbed species from CO2 was measured after successive evacuation at room temperature, 100, 200, 300, and 400\u00a0\u00b0C under high vacuum for 30\u00a0min at each temperature.The electronic state of Pd in the catalysts was characterized by analyzing the FTIR spectrum of adsorbed species formed from CO. Prior to CO adsorption, the Pd-containing catalyst wafer was reduced at 450\u00a0\u00b0C in H2 stream for 1\u00a0h. The catalyst was contacted with CO gas at 5\u00a0mbar at room temperature for 20\u00a0min then the spectrum of the carbonyl species formed was recorded after 20\u00a0min evacuation under high vacuum at room temperature. Each absorbance spectra were scaled to 5\u00a0mg\u00a0cm\u22122\nwafer thickness to allow quantitative comparisons.Spectra were recorded using a Varian NMR System spectrometer operating at 600\u00a0MHz 1H frequency (242.74\u00a0MHz for 31P and 156.26\u00a0MHz for 27Al) with a Chemagnetics 3.2\u00a0mm narrow-bore triple resonance T3 probe in double resonance mode. The 31P direct polarization spectra were recorded (160 transients) at 20\u00a0\u00b0C with 12\u00a0kHz of spinning rate and 300\u00a0s repetition delay. The 27Al \u2013 1H cross polarization spectra were recorded (12000 transients) with 0.5\u00a0ms contact time, 5\u00a0s of repetition delay at 20\u00a0\u00b0C with 12\u00a0kHz of spinning rate. For both experiments SPINAL 1H decoupling was used. As chemical shift reference ammonium dihydrogen phosphate (\u03b4iso\u00a0=\u00a00.81\u00a0ppm with respect to 85\u00a0wt% H3PO4 solution) for the 31P and sodium aluminate (\u03b4iso\u00a0=\u00a079.3\u00a0ppm) for the 27Al measurements was used.Hydroconversion of tricaprylin was investigated using a high pressure fixed-bed flow-through microreactor system. The catalytic reactor (I.D.: 8\u00a0mm) was filled with 2.0\u00a0g of catalyst using its 0.315\u20130.630\u00a0mm sieve fraction. Prior to the catalytic run, the catalyst was reduced in situ in 50\u00a0cm3\nmin\u22121 flow of H2 at 450\u00a0\u00b0C for 2\u00a0h at atmospheric pressure, then the temperature was lowered to the desired reaction temperature (300 or 350\u00a0\u00b0C) and the pressure was increased to 21\u00a0bar total pressure. The tricaprylin reactant was fed into the reactor using a high-pressure syringe pump (ISCO) at a weight hourly space velocity (WHSV) of \n\n4\n\ng\n\nt\nr\ni\nc\na\np\nr\ny\nl\ni\nn\n\n\n\ng\n\nc\na\nt\na\nl\ny\ns\nt\n\n\n-\n1\n\n\n\n\nh\n\n\n-\n1\n\n\n\n, whereas the H2/tricaprylin molar ratio was 20. The product mixture was cooled to room temperature and the liquid products were separated from the gas products in a reflux condenser downstream of the reactor. The effluent gas leaving the condenser through a back pressure regulator valve contained mainly H2, and CO (as main product), CO2, and minor amount of CH4. The effluent was analysed on-line using a gas chromatograph (GC) equipped with a TCD detector and a 60/80 Carbonex-1000 (L 15.0ft\u00a0\u00d7\u00a0OD 1/8\u2033) stainless steel column. The liquid product from the presurized collection vessel was first transferred into a closed atmospheric vessel. During this transfer, the pressure of the vessel reached the system pressure, which was then released via a transfer line connected to a syringe, where the gas, expanded to atmospheric pressure, was collected. Note that this latter expanded gas contained all of those products, which were in liquid phase under system pressure but appeared as gases at atmospheric pressure (mainly propene, and some ethane in H2). The liquid sample, drained now from atmospheric pressure, was analyzed by GC equipped with flame ionization detector (FID) using CP-FFAP CB (L 25.0\u00a0m\u00a0\u00d7\u00a0ID 0.32\u00a0mm\u00a0\u00d7\u00a0df 0.3\u00a0\u03bcm) capillary column, whereas the expanded gas products were analysed by GC-TCD-FID using ShinCarbon ST (L 2.0\u00a0m\u00a0\u00d7\u00a0ID1/8in.\u00a0\u00d7\u00a0OD 2.0\u00a0mm) column. Samples were taken in every hour after the steady state was reached (after one hour time of stream). At high tricaprylin conversions, when the reactant was converted almost completely into paraffins, the liquid product split into two clear, colorless hydrocarbon and water phases. Since the lower aqueous phase contained only a negligible amount of organic components, in such cases only the composition of the upper organic phase was analyzed. In some cases, when the tricaprylin conversion was not complete, and/or was not completely converted into paraffins, a single phase liquid product was obtained, which was analysed for the organic components. In some cases, when unknown components were also formed, GC\u2013MS equiped with Rxi-5Sil MS (L30.0\u00a0m\u00a0\u00d7\u00a0ID0.25\u00a0mm\u00a0\u00d7\u00a0df0.25\u00a0\u03bcm) column was used for peak assignments.Quasi-operando DRIFT spectroscopic investigation of the adsorbed species formed from carboxylic acid intermediate under catalytic conditions is crucial in order to reveal structure\u2013activity relationships of the catalyst in the HDO reaction of triglycerides or fatty acids. The experiments were carried out using an FT-IR spectrometer (Thermo Nicolet iS10) equipped with a Collector II diffuse reflectance mirror system and a flow-through DRIFT spectroscopic reactor cell (I.D.: 5\u00a0mm, height of catalyst bed\u00a0\u223c\u00a04\u00a0mm) filled with about 20\u00a0mg of powdered catalyst sample. The design of the cell allows carrier gas or gas phase reactant mixture to flow through the catalyst bed in the sample cup. The reactant is introduced into the cell by switching the carrier gas flow to a gas saturator containing the reactant at room temperature. Due to the experimental difficulties related to the very low vapor pressure of caprylic acid, valeric acid (VA) was used as model compound in these experiments. First, the catalyst was pre-treated in situ in a 50\u00a0cm3\nmin\u22121 H2 flow at 450\u00a0\u00b0C for 2\u00a0h and then the spectrum of the activated catalyst powder was collected (512 scans at a nominal resolution of 2\u00a0cm\u22121) at the desired reaction temperatures (300 or 350\u00a0\u00b0C). The reaction of VA was initiated by switching the H2 or He flow (50\u00a0cm3\nmin\u22121) to the gas saturator. The thus obtained H2/VA or He/VA mixture contained 258\u00a0ppm VA. The spectrum taken in the presence of the reacting gas mixture at 300 or 350\u00a0\u00b0C was corrected with the spectrum of the pure catalyst at the same temperature. Since the contribution of the gas phase spectrum was found negligible under the applied conditions, the thus obtained difference spectrum practically reflects the bands of surface species formed (positive bands) or consumed (negative bands) in the VA adsorption/reaction process.The effluent gas leaving the reactor cell was continuously monitored by online mass spectrometer (MS; VG ProLab, Thermo Scientific) following the characteristic masses of the major reaction products: butane (m/z\u00a0=\u00a058, C4H10\n+), pentane (m/z\u00a0=\u00a072, C5H12\n+), CO (m/z\u00a0=\u00a028, CO+), and CO2 (m/z\u00a0=\u00a044, CO2\n+). All signals were corrected for the contribution of other reaction products giving a fragment at the same m/z value.The measured Pd and P contents of the catalysts, listed in Table1\n, are in good agreement with the values that follow from the applied conditions of catalyst preparation.The nitrogen adsorption isotherms, shown in Fig.\u00a0S1, are characteristic for mesoporous oxides. They are classified as type IV isoterms, having H2 type hysteresis loop. The SSA of the catalysts decreased as their phosphorous content was increased (Table1). The SSA of the Pd/Al2O3\u20135P catalyst is about 40% lower than that of the Pd/Al2O3 catalyst. These results suggest that phosphate groups can block some pores of the alumina support and thereby decrease the SSA.XRPD patterns of the phosphatized catalysts and that of the parent \u03b3\u2013Al2O3 support are shown in Fig. 1\n. The diffractograms of the support and all the catalysts were similar, i.e., no new crystalline phase could be detected (Fig. 1). The results suggest that the size of the Pd or PdO particles on the support is well below the detection limit of the XRPD method (the diameter was less than about 5\u00a0nm).The Pd dispersion (D\nPd) was obtained as the ratio of the number of surface Pd atoms and the total number of Pd atoms in the catalyst. The molar amount of chemisorbed CO was taken to be equivalent with half of the molar amount of surface Pd atoms [12].Assuming spherical particle shape and that the three low-index planes are in equal proportions on the polycrystalline surface of the face-centered cubic crystals of the metal, the mean Pd particle size (d\nPd) was calculated from the dispersion by the equation\n\n(1)\n\n\n\nd\n\nP\nd\n\n\n=\n6\n\nv\n\na\n\n\nD\n\n\nP\nd\n\n\n\n\n\n\n\n\nwhere v is the volume occupied by a single Pd atom in the bulk of metal (1.47\u00b710\u20132\nnm3), and \n\na\n\n is the average surface area occupied by one Pd atom (7.93\u00b710\u20132 nm2) [13]. The Pd dispersions and the mean particle sizes, listed in Table1, barely changed with the phosphorous content of the catalysts.The FT-IR spectra measured for the phosphate-free and phosphatized \u03b3\u2013Al2O3 supports in the range of stretching vibration of surface OH groups are shown in Fig. 2\n. The assignment of the alumina OH bands, first given by Kn\u00f6zinger and Ratnasamy [14], was later refined by Busca et al. [15\u201317]. The \u03bdOH bands above about 3700\u00a0cm\u22121 were assigned to terminal OH groups. The band at 3792\u00a0cm\u22121, appearing as a weak shoulder in the spectrum of pure \u03b3\u2013Al2O3 (Fig. 2a) was attributed to \u03bdOH vibration of groups linked to tetrahedral aluminum ions. The band at 3773\u00a0cm\u22121 belongs to OH groups next to a coordinately unsaturated tetrahedral aluminum atom, i.e. next to a Lewis acid site. In a very recent paper [18] the assignment of the band at about 3780\u20133770\u00a0cm\u22121 was made more accurate. The band was shown to stem from hydroxyl groups bound to octahedrally coordinated surface Al3+ ions (O5AlVI-OH sites) that transforms to pentacoordinated Al3+ surface sites (\u201cbare\u201d O5AlV sites) upon thermal dehydroxylation.The relatively broad band, centered at 3730\u00a0cm\u22121 (Fig. 2a) was assigned to OH groups bound to octahedrally coordinated Al3+ ions [15\u201317]. The presence of a coordinately unsaturated octahedral aluminum atom adjacent to an OH group, results in OH band, shifted to the 3740\u20133700\u00a0cm\u22121 frequency region [15\u201317]. However, we could not distinguish these bands because of the band broadening and strong overlapping (Fig. 2a). The OH bands below about 3700\u00a0cm\u22121 are attributed to different bridging OH species [15\u201317]. Thus, the broad band centered approximately at 3670\u00a0cm\u22121 can be assigned to bridging OH species, whereas the very broad feature around 3590\u00a0cm\u22121 can be attributed to triply bridging OH species or OH groups in hydrogen bonding interactions [15\u201317].The intensity drop of the OH bands (Fig. 2, b-d) clearly indicates OH consumption in a reaction with phosphoric acid. The reaction has been described as sort of surface acid-base neutralization reaction resulting in the formation of surface phosphate species and water [19\u201321]. The formation of phosphate species is clearly indicated by the appearance of the band at 3677\u00a0cm\u22121, assigned to the \u03bdOH vibration of P\u2013OH species (Fig. 2, b-d) and a strong \u03bdP=O vibrational band of the phosphate groups in the range of 1150\u20131250\u00a0cm\u22121 frequency (not shown). Interestingly, the characteristic \u03bdOH band of the terminal OH groups on tetrahedral aluminum ions at 3792\u00a0cm\u22121 gained in intensity as the concentration of the surface phosphate species increased (Fig. 2, a-d), suggesting that a surface reaction between phosphoric acid and alumina surface is not a simple acid-base neutralization reaction.The surface basicity of the phosphate-free and phosphatized \u03b3\u2013Al2O3 supports was characterized by the carbonate-like surface species obtained from CO2 adsorption. The IR spectrum of these species was recorded and the spectral features were assigned on the basis of the available literature [17,21,22] (Fig. 3\n). The CO2 uptake of the supports resulted mainly in the formation of two types of bicarbonate species as indicated by the appearance of the asymmetric \u03bdO-C=O vibrations at 1645\u00a0cm\u22121 for both B1 and B2 type bicarbonate species and the symmetric vibrations at 1454\u00a0cm\u22121 of the B1 and 1482\u00a0cm\u22121 of the B2 type bicarbonate species (Fig. 3A). The corresponding \u03bdOH bands of the bicarbonate species appear as positive bands at 3610 (B2 type) and 3595\u00a0cm\u22121 (B1 type, as a shoulder) in the range of the O\u2013H vibration frequencies (Fig. 3B). The most intense negative \u03bdOH bands at 3773 and around 3700\u00a0cm\u22121 (Fig. 3B) suggest that mainly those OH groups were involved in the formation of bicarbonates, which have a coordinately unsaturated tetrahedral or octahedral aluminum atom in their neighborhood. Note that the involvement of these hydroxyl groups in the CO2 adsorption could be just due to their location near to Lewis acid sites [16]. The lack of negative band at 3677\u00a0cm\u22121, where the \u03bdOH band of the phosphorus\u2013bound hydroxyls appears, implies that the P\u2013OH groups do not participate in bicarbonate formation (Fig. 3B). In harmony with the conclusion of other authors [21,23], this result permitted for us to conclude that phosphatization does not generate basic centers for CO2 uptake, i.e., the P\u2013OH group is stronger acid than the CO2.The CO2 uptake on the supports results also in the formation of bidentate chelating carbonate that gives weak IR band at 1670\u00a0cm\u22121 (shoulder) and monodentate carbonate bands at 1542and 1404\u00a0cm\u22121 (shoulders) frequencies (Fig. 3A) [17,22]. Formation of these carbonate species probably takes place with the involvement of coordinately unsaturated aluminum cation and oxide anion pairs, which oxide ions are Lewis base sites on the alumina surface [16,17,21]. It is important to note that the strength of the characteristic absorption bands of the carbonate species is inversely proportional to the phosphate loading (Fig. 3A, a-d). These results suggest that formation of phosphate species is accompanied not only by consumption of basic surface OH groups, but also by elimination of Lewis base sites.The concentration and distribution of basic sites on the parent and phosphatized \u03b3\u2013Al2O3 supports were determined by CO2-TPD measurements. The TPD curves in Fig. 4\n indicate the presence of at least three overlapping component bands. According to Wang et al. [22], a peak appearing around 80\u00a0\u00b0C is due to the decomposition of bicarbonate species formed on weak basic sites, whereas the component peaks observed around 160 and 250\u00a0\u00b0C can be attributed to decomposition of chelating bidentate carbonate species formed on medium strength basic sites and monodentate carbonate species formed on strong basic sites, respectively. These assignements are clearly supported by the observed thermal stability of different carbonate species (Fig. S2). An additional high temperature peak, attributed to bridging bidentate carbonate species formed on strong basic sites, may also appear at around 325\u00a0\u00b0C temperature. Therefore, the CO2-TPD curves shown in Fig. 4 were resolved by peak fitting using four component peaks. The peak fitting process resulted essentially in three major component peaks with a maximum around 85, 135, and 220\u00a0\u00b0C, representing weak, medium strength and strong basic sites (the fourth component peak had a negligible intensity on each sample, therefore it was ignored). The total amount of different basic sites and their distribution was calculated from the area under the corresponding curves using the result of a previous calibration measurement (Table\u00a0S1). The concentrations of the weak-to-moderate strength basic sites and the strong basic sites are listed in Table2\n. Results show that introduction of phosphate groups significantly decreased the concentration of all types of basic centers on the surface of the alumina support, which dropped by more than 90% for the support, having the highest phosphorous content.Adsorption of Py on the Lewis acid sites of alumina results in the formation of coordinately bonded Py giving absorption bands in the range of 1630\u20131590 (8a band) and 1460\u20131430\u00a0cm\u22121 (19b band) [16,17,25]. The pair of bands observed at 1623 and 1456\u00a0cm\u22121 in Fig. 5\na can be assigned to Py adsorbed on tri-coordinated Al3+ cations (tetrahedral Al cations with coordinative unsaturation), which represent the strongest acid Lewis sites of alumina. The second pair of bands at 1615 and 1451\u00a0cm\u22121 can be also attributed to Py adsorbed on Al ions with coordinative unsaturation, which are most probably in octahedral coordination [16,17,25] and represent weaker acid Lewis sites than those of the coordinately unsaturated tetrahedral aluminum cations.The spectra of adsorbed CO2 support above identification of Py sorption sites. Bands at 2360 and 2345\u00a0cm\u22121, which belong to the so called \n\n\n\u03a3\n\nu\n\n+\n\n\n mode of linearly coordinated (end-on adsorbed) CO2\n[17,21,26], were found to develop in the presence of CO2 gas in the IR cell (Fig.\u00a0S3). These bands are most probably bands of CO2, bound to strong and weak, coordinately unsaturated Lewis acid Al centers in tetrahedral and octahedral coordination, respectively.The intensity of Py bands is lower for the supports having higher phosphate concentration (Fig. 5, a-d), suggesting that formation of surface phosphate affected the concentration of both types of Lewis acid sites.The bands of linearly adsorbed CO2 are also weaker for the phosphatized supports, indicating that surface phosphate eliminates Lewis acid sites of alumina (Fig.\u00a0S3, a-d).Evacuation at 400\u00a0\u00b0C resulted in the total desorption of Py from the weaker Lewis acid sites (see the intensity drop of the bands at 1615 and 1451\u00a0cm\u22121), whereas the strongest Lewis acid sites still withheld Py (see the bands at 1623 and 1456\u00a0cm\u22121) (Fig. 5, dashed lines). Assuming that the total number of Lewis sites and the number of strong Lewis sites are proportional to the band intensities observed after evacuation at 100\u00a0\u00b0C and 400\u00a0\u00b0C, respectively, the corresponding concentrations of Lewis acid sites were determined by using the extinction coefficient given in ref. [24]. The concentrations of the weak-to-moderate strength Lewis acid sites and the strong Lewis acid sites are listed in Table2.The type of surface phosphate species was detected by 31P MAS NMR. The spectrum of the Al2O3\u20131P sample, having the lowest P content, shows peaks at \u221210 and \u201322\u00a0ppm (Fig.6A, b), which peaks can be assigned to phosphorous in monomeric and polymeric phosphate species, respectively [22,23,27,28]. At higher phosphorous contents, both peaks were stronger, while the peak of the polymeric species gained even more intensity (Fig. 6\nA, b-d). In line with expectations [23], the higher surface phosphate concentration favors the formation of polymeric species via condensation of P\u2013OH groups.The changing local environment of aluminum atoms on the surface of alumina upon phosphate modification was characterized by 1H and 27Al CP/MAS NMR (Fig.6B). The 1H spectra reflect the local environment of those Al atoms which are near to protons at the surface or near to surface-attached OH groups [23,29]. On the parent alumina support 27Al resonance peaks can be observed at 14, 38, and 75\u00a0ppm (Fig.6B, a), which can be attributed to octahedral (AlVI), pentagonal (AlV), and tetrahedral (AlIV) surface aluminum atoms [27\u201330]. Note that pentagonal aluminum atoms (AlV) are often observed in high surface area transition aluminas in minor concentrations and their presence is associated with oxygen defects adjacent to aluminum nucleus or substitution of lattice oxygen in octahedral symmetry by hydroxyl groups [30,31]. When the alumina surface was modified with increasing amount of phosphate, it was clearly visible that relatively broad component peaks developed at lower chemical shifts near to these peaks at about 54, 26, and 5\u00a0ppm (Fig. 6B, b-d) indicating the changes in the local environment of the corresponding surface Al atoms due to the formation of Al-O-P bonds [27,32]. Results show that all types of surface Al atoms were involved in the formation of Al-O-P bonds, suggesting the non-selective binding of the phosphate to the alumina surface. This observation is in line with the spectral changes found in the \u03bdOH region (Fig. 2) indicating the consumption of all the different types of OH groups upon phosphating the alumina surface.It is generally accepted that the carbonyl bands of CO adsorbed on highly dispersed Pd catalysts appear in the spectral range below and above about 2000\u00a0cm\u22121, attributed to bridging and linearly-bound CO, respectively [12,33,34]. The FTIR spectrum of the species formed from CO adsorption on alumina-supported Pd catalysts are shown in Fig. 7\n. Carbonyl bands are clearly discernible at about 1860, 1945, 2050, and 2085\u00a0cm\u22121. Following the band assignments of Lear et al. [34,35], the broad band at about 1860\u00a0cm\u22121 can be assigned to \u00b53 hollow-bonded CO on Pd [111] planes or \u00b52 bridge-bonded CO on Pd [100] planes, whereas the band near to 1945\u00a0cm\u22121 can be attributed to the \u00b52 bridge-bonded CO on Pd [100] facets and CO, bridge bonded to particle edges. The linear CO peaks at around 2050 and 2085\u00a0cm\u22121 can be ascribed to CO bound to Pd [111]/[111] and Pd [111]/[100] particle edges, and CO bound to particle corners, respectively [34,35]. These bands are all present both on the non-phosphatized and phosphatized-alumina-supported Pd catalysts, although some deviation from the published relative intensities, are apparent (Fig. 7). In particular, the linear features relative to the bridge-bonded features became more pronounced at increasing phosphorous content of the support, which suggest somewhat greater contribution of the Pd particle edges and corners to the adsorption. Note that phosphating the alumina support hardly affected the metal dispersion and the Pd particle size in the catalysts (Table1).Results of catalytic hydroconversion of tricaprylin (TC) are shown in Fig. 8\n. The organic liquid product contained the unreacted TC (if any) and caprylic acid, propyl caprylate, 1-octanol, octyl caprylate, heptane, octane, and some other minor products, mainly octanal, 8-pentadecanon, 9-nonanone, and dicaprylates formed by the hydrogenolysis of only one ester bond of TC. Caprylic acid and propyl caprylate is formed by the hydrogenolysis (HYS) reaction of three or two ester bonds, respectively, whereas octyl caprylate could have been formed by esterification of caprylic acid by octanol. Heptane and octane were produced via deoxygenation of caprylic acid. Octanol is a possible intermediate of paraffin formation [11]. The effluent gas contained mainly propane (from HYS reaction) and CO. Minor amounts of CO2, ethane, and methane could be also detected. The dominance of heptane over octane and CO over CO2 in the liquid and gas phase product mixture, respectively, suggests that hydrodecarbonylation (HDCO) is the main deoxygenation route, whereas hydrodecarboxylation (HDCO2) and oxygen hydrogenation (HDH2O) represent minor reaction routes [6,7].At the reaction temperature of 300\u00a0\u00b0C the conversion of TC was low (18.5%) on the Pd/Al2O3 catalyst, but reached virtually 100% on the Pd/Al2O3\u20132.5P and Pd/Al2O3\u20135P catalysts (Fig. 8, left side). When the reaction temperature was raised to 350\u00a0\u00b0C, all the phosphatized-alumina-supported Pd catalysts showed high activity in the HYS reaction resulting in 100% TC conversion (Fig. 8, right side). In line with earlier findings [11], these results suggest that caprylic acid intermediate was formed by facile HYS reaction from TC, which was followed by consecutive, rate-limiting deoxygenation (mainly HDCO) reaction of the intermediate. Interestingly, the yield of the paraffin products (heptane and octane) dramatically increases with the phosphorous content of the alumina support reaching nearly 100% on the Pd/Al2O3\u20135P catalysts (Fig. 8, right side). These results clearly suggest that phosphatization of alumina surface resulted in the change of catalyst structure so that the rate of the hydrodeoxygenation (mainly HDCO) reaction was significantly enhanced.The carboxylate species formed from adsorption of valeric acid, and their reactivity was investigated under catalytic conditions by quasi-operando DRIFT spectroscopy. The results obtained for the Pd/Al2O3 and Pd/Al2O3\u20135P catalysts are presented in Figs. 9 and 10\n\n, respectively.Molecularly adsorbed carboxylic acid could not be observed under the conditions of experiments, i.e., the characteristic \u03bdC=O band of valeric acid expected to appear at\u00a0\u223c\u00a01780\u00a0cm\u22121 could not be detected. The negative bands in the \u03bdOH region (Figs. 9-10, Section A) and positive bands in the \u03bdO\u2013C\u2013O region (Figs. 9-10, Section B) clearly suggest that the adsorption of the carboxylic acid resulted in the consumption of surface OH groups and in the simultaneous formation of surface carboxylate species [36,37]. This surface reaction (dissociative adsorption) is described as the deprotonation of the carboxylic acid by the combination of acid hydrogen with a surface hydroxyl group to produce surface carboxylate species and H2O [36\u201338]. Results shown in Fig. 9A and 10A indicate that practically all types of OH groups on the support can be involved in the formation of carboxylate species, including the P\u2013OH groups of the phosphatized support (Fig. 2). Note that phosphatization resulted in consumption of surface Al\u2013OH groups and formation of new P\u2013OH groups (Fig. 2). Both the remaining Al\u2013OH and the new P\u2013OH groups are available for the adsorption of carboxylic acid and their total number determines the surface concentration of the carboxylate groups.The position of the asymmetric and symmetric \u03bdO-C-O stretching bands appearing over and below about 1500\u00a0cm\u22121, respectively, in addition to the difference between their peak positions (\u0394\u03bd\u00a0=\u00a0\u03bdas-\u03bds) are indicative of the bonding structure of carboxylate species [36,37,39,40]. The frequency of the \u03bdas vibration and the corresponding \u0394\u03bd value were shown to increase in the following order: chelating bidentate\u00a0<\u00a0bridging bidentate (\u2248 free ionic)\u00a0<\u00a0monodentate carboxyl species. The intense pair of bands observed at 1575 and 1470\u00a0cm\u22121 (\u0394\u03bd\u00a0=\u00a0105\u00a0cm\u22121) for the Pd/Al2O3 sample (Fig. 9B) and a similar pair of bands at 1585 and 1470\u00a0cm\u22121 (\u0394\u03bd\u00a0=\u00a0115\u00a0cm\u22121) for the Pd/Al2O3\u20135P sample (Fig. 10B) can be assigned to the asymmetric and symmetric vibrations of chelating bidentate carboxylate species [36,37,39,40]. A second type of carboxylate species gives an intense asymmetric \u03bdO-C-O stretching band at 1650\u00a0cm\u22121 (Fig. 9B). The identification of its symmetric pair is, however, difficult due to the appearance of overlapping C\u2013H deformation vibrations in the frequency range below 1500\u00a0cm\u22121, most probably due to the appearance of the \u03b4as(CH3), \u03b2s(CH2), and \u03b4s(CH3) vibrations of the \u2013CH3 and \u2013CH2- groups of the hydrocarbon chain (the \u03b4s(CH3) vibration is clearly discernible at around 1350\u00a0cm\u22121) [36,40,41]. However, we found a band at about 1390\u00a0cm\u22121 that showed parallel of intensity with that of the 1650\u00a0cm\u22121 band, if reaction conditions were varied. It was substantiated that a band at about 1390\u00a0cm\u22121 is the pair of the 1650\u00a0cm\u22121 band, stemming from symmetric \u03bdO-C-O stretching vibration (Fig. 9B, 10B, and S4, S5). The relatively high frequency of the asymmetric \u03bdO-C-O vibration band (1650\u00a0cm\u22121) and the large frequency separation from the corresponding symmetric vibration band (\u0394\u03bd\u00a0=\u00a01650 \u2013 1390\u00a0=\u00a0260\u00a0cm\u22121) clearly suggest that this second type of carboxylate group can be identified as monodentate carboxylate species bonding to surface aluminum atom [39,40]. Note that the concentration of the monodentate carboxylate species is much lower over the Pd/Al2O3\u20135P sample than over the Pd/Al2O3 sample (Fig.\u00a0S4, Sections C and D). Phosphatization of \u03b3\u2013alumina surface eliminated mainly those sites, where monodentate carboxylate species could have been formed.Upon contacting the catalysts with He/VA mixture at 300\u00a0\u00b0C gradually carboxylate bands developed (Fig.\u00a0S4) until their surface concentration reached steady state (Fig. 9B and 10B, spectra\na). Virtually the same steady state concentrations were reached at 350\u00a0\u00b0C (Fig. 9B and 10B, spectra\nb). As expected, no reaction products were formed in the absence of H2.When the reactant flow was changed to H2/VA, the intensity of the bands at 1575\u20131585\u00a0cm\u22121 and 1470\u00a0cm\u22121 decreased, suggesting that mainly the concentration of the corresponding bidentate carboxylate species decreased, whereas the surface concentration of the monodentate carboxylate species (bands at 1650 and 1390\u00a0cm\u22121) hardly changed. The consumption of the bidentate carboxyl species was accompanied by the appearance of the main deoxygenation reaction products, such as, butane, pentane, CO and CO2. Higher reaction temperature resulted in a higher reaction rate and, therefore, in higher product concentrations in the effluent gas (Fig. 9C and 10C). The dominance of butane and CO in the product mixture suggests that the main route of VA deoxygenation is the HDCO reaction. The results suggest that the surface of the Pd/Al2O3 catalyst is covered by more reactive bidentate carboxylate species and less reactive monodentate carboxylate species. In contrast, the surface coverage of the Pd/Al2O3\u20135P catalyst by the more active carboxylate is substantially higher, and it is much lower by the less active species than the corresponding coverages of the Pd/Al2O3 catalyst.Product formation was accompanied by recovery of surface OH groups as indicated by the intensity drop of some negative OH bands (Fig. 9A and 10A). Because the reaction of carboxylic acid and OH groups is accompanied by release of H2O, the recovery of OH groups had to involve C\u2013O bond hydrogenolysis. Mainly lower-frequency OH groups recovered, showing that the less basic OH groups were involved in the formation of reactive carboxylates [16,17].The effect of phosphatization on the alumina surface seems to be twofold: it consumes surface Al\u2013OH groups (Fig. 2) and also reduces the concentration of Lewis acid sites (Table 2) and consequently the concentration of the Lewis acid (Al\u2295) \u2013 Lewis base (O\u229d) pair sites. Note that latter oxygen atoms can behave as Br\u00f8nsted base (proton acceptor) or Lewis base (electron pair donor) sites depending on the nature of the adsorbate [42].The reaction of phosphoric acid with surface OH groups is well documented [20,21]. It is considered to be an acid-base reaction, as shown in Scheme1\nA. The reaction results in the formation of surface phosphate groups and water. Phosphate species formed on adjacent OH groups can condensate via P\u2013OH groups to form polymeric phosphate species [23], which appeared as dominating species at higher (\u22652.5\u00a0wt%) phosphate loadings (Fig. 6A). It is important to note that phosphatization results not only in the appearance of P\u2013OH groups but also in the formation of non-reactive terminal AlIV-OH groups (Fig. 2). A similar phenomenon was observed in a former study and was related to the reaction between a bridging OH group (Al\u2013O(H)\u2013Al) and phosphoric acid giving a terminal OH group and a [H2PO4]\u2013 ligand attached to a surface Al atom [20]; however, it was not clarified how the charge neutrality was preserved in this process.The present study evidenced that formation of new terminal AlIV\u2013OH groups was accompanied by the elimination of strong acid Lewis sites (i.e., coordinately unsaturated tetrahedral Al sites) and their charge balancing Lewis base (oxide ion) pair sites. We rationalize these observations by the reaction shown in Scheme1B. The strong Lewis acid sites were suggested to become reversibly reconstructed by establishing a weak bond with a nearby oxide ion and thus they could appear more as a distorted tetrahedral ion than as a tri-coordinated one (see left side of Scheme1B) [16]. However, the very weak coordination bond in the strained species can be easily broken in the presence of a base or an acid [16,43]. Our results substantiate that phosphoric acid reacts with these Lewis acid (Al\u2295) \u2013 Lewis base (O\u229d) pair sites resulting in the formation of terminal AlIV-OH species and [H2PO4]\u2013 groups completing the coordination sphere of coordinately unsaturated tetrahedral Al sites (right side of Scheme1B). Note that latter oxide ions behave as proton acceptor Br\u00f8nsted base sites in the process, whereas the [H2PO4]\u2013 groups cannot be distinguished from those formed with the involvement of surface OH groups (vide supra). The process proposed here clearly indicates how the charge neutrality can be maintained during the formation of the terminal OH groups, and also accounts for the elimination of Lewis acid (Al\u2295) \u2013Lewis base (O\u229d) pair sites (Scheme1B). This is in line with the suggestion of DeCanio et al. [43] for the bonding of fluoride ions to strong Lewis acid sites upon HF treatment of alumina support.The quasi-operando DRIFTS investigation revealed the formation of two types of surface carboxylate species under catalytic conditions (Figs. 9-10). Bidentate carboxylate species were formed via acid-base reaction between fatty acid and a surface hydroxyl groups to produce stable surface carboxylate and H2O as shown in Scheme2\nA [36\u201338]. It is most likely that the generated water desorbs from the surface at the reaction temperatures (300 and 350\u00a0\u00b0C) applied here during the DRIFTS investigations [37]. This process shows close resemblance to that observed for the surface reaction with phosphoric acid (see above and in Scheme1A). The second type of carboxylate could be identified as monodentate carboxylate group [39,40]. It is rational to think that these latter species were formed in a similar process over Lewis acid (Al\u2295) \u2013 Lewis base (O\u229d) pair sites than that we proposed for the surface reaction with phosphoric acid (cf. Scheme1B and 2B). The result is a monodentate carboxylate group, in which one of the oxygen forms an ester-like bond with a surface aluminum atom, while the other oxygen forms an H-bond with the neighboring OH group (right side of Scheme2B) [39,44]. The reactive adsorption of carboxylic acid on strong Lewis acid (Al\u2295) \u2013 Lewis base (O\u229d) pair sites is clearly supported by the good correlation between the concentration of strong Lewis acid sites (determined by Py adsorption) and the integrated absorbance values of the asymmetric \u03bdO-C-O band at 1650\u00a0cm\u22121, assigned to monodentate carboxylate species (Table\u00a0S2).In relation to the catalytic properties, the most important consequence of phosphatization is that it reduces the concentration of Al\u2295\u2013 O\u229d pair sites, where the less reactive monodentate carboxylate species are formed. The DRIFTS investigations showed that both types of carboxylates are strongly-bound surface intermediates, but the bidentate carboxylate species are more susceptible to deoxygenation reaction leading to paraffin product than the monodentate carboxylate intermediate (Figs. 9 to 10). The dominance of the bidentate carboxyl species on the phosphatized alumina support explains the substantially higher deoxygenation (mainly HDCO) activity of the phosphate catalyst as compared to the Pd/Al2O3 catalyst (Fig. 8). Note that phosphatization did not affect noticeably either the particle size, or the morphology of the Pd particles, therefore the enhanced deoxygenation activity and paraffin selectivity can be attributed to changes in the surface properties of the alumina support. Nevertheless, the reaction between the active surface intermediate and hydrogen most probably should take place at the metal/support interface where activated hydrogen is available for the reaction.We found that preferentially weak base OH groups were recovered during the HDO reaction (Fig. 9A and 10A). The recovery of surface OH groups must involve the scission of a carboxylate C\u2013O bond. The most likely product is aldehyde (Scheme2C), which is considered as an important intermediate of the HDO reaction of carboxylic acids [2,5,8]. Note that the process depicted in Scheme\u00a02C is the reverse process than that observed for surface carboxyl formation from aldehyde on metal oxides [36]. Our results suggest that the bidentate carboxylate species, formed in reaction with weak base hydroxyls, are more ready to react with hydrogen than the carboxylates formed in reaction with strong base surface sites. In this context, replacement of OH groups by P\u2013OH groups favorably affects the HDO activity.Paraffin formation proceeds mainly via hydrodecarbonylation (HDCO) of the aldehyde intermediate, whereas oxygen reduction reaction (HDH2O) of aldehyde represents a minor reaction route. The paraffin chain of fatty acids is shortened in HDCO but it is preserved in HDH2O reaction [2,5,7]. This latter reaction was suggested to proceed via primary alcohol intermediate and requires hydrogenation, dehydration, and/or hydrogenolysis steps [2,8]. One can speculate that latter reaction goes on a less complex reaction pathway in which the cleavage of both C\u2013O bonds in the surface carboxylate species occurs, however, the justification of such reaction route still requires further investigation.This study provides insights into the structure \u2013 activity relationships, determining the triglyceride HDO activity of alumina-supported Pd catalysts. The Pd/\u03b3-A2O3 catalyst showed relatively good activity in the ester bond hydrogenolysis of triglyceride, but poor activity in the consecutive deoxygenation of the obtained carboxylic acid intermediate to paraffin product.Surface modification of the \u03b3-alumina support with phosphate resulted in surprisingly high activity enhancement of the catalyst in the later rate-determining step of paraffin formation. The phosphatization of the alumina surface resulted in (i) partial elimination of basic Al\u2013OH groups and the concomitant formation of weak acid P\u2013OH groups on the alumina surface, (ii) a decrease in the number of Lewis acid (Al\u2295) \u2013 Lewis base (O\u229d) pair sites, whereas (iii) it did not affect noticeably either the particle size, or the morphology of the Pd particles on the support.Quasi-operando DRIFTS investigation under catalytic conditions revealed that both surface Al\u2013OH and P\u2013OH groups serve as sites for fatty acids to form bidentate carboxylate groups, which can further react with hydrogen to form paraffin products. If the OH groups, involved in the carboxylate formation are less basic, the formed carboxylate groups are more ready to react with hydrogen.Carboxylic acids can also react with strong Lewis acid (Al\u2295) \u2013 Lewis base (O\u229d) pair sites giving monodentate carboxylate species, having low reactivity. Since phosphatization significantly reduced the number of these pair sites, the concentration of the less reactive monodentate carboxylate groups decreased substantially. The dominance of reactive bidentate carboxylate groups on the phosphatized alumina surface explains the substantially higher HDO, mainly hydrodecarbonylation (HDCO) activity of the phosphatized-alumina-supported Pd catalyst than the HDO activity of the Pd/\u03b3\u2013Al2O3 catalyst.The product formation from the bidentate carboxylate surface intermediate is accompanied by OH group recovery, suggesting that the deoxygenation reaction must start with the hydrogenolysis of a carboxylate C\u2013O bond. This reaction leads to the formation of aldehyde that must be the intermediate of deoxygenation to paraffin.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 of the project by the 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). The Project is supported by the European Union. Thanks are also due to the Interreg V-A Slovakia - Hungary Cooperation Program, SKHU/1902, (Project No: SKHU/1902/4.1/001) and to the Project (Project No. VEKOP-2.3.2-16-2017-00013) supported by the European Union and the State of Hungary, co-financed by the European Regional Development Fund. This work was also supported by the TKP2020-IKA-07 project financed under the 2020-4.1.1-TKP2020 Thematic Excellence Programme by the National Research, Development and Innovation Fund of Hungary.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2021.08.052.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  The mechanism of catalytic hydrodeoxygenation (HDO) of fats, vegetable oils, and fatty acids was studied using alumina-supported Pd catalysts and tricaprylin and valeric acid as model reactants. The chemistry of fatty acid/catalyst interaction was studied by quasi-operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The Pd/\u03b3\u2013Al2O3 catalyst showed good activity in the hydrogenolysis reaction of the ester bonds to convert tricaprylin to caprylic acid, but they were of poor activity in the consecutive hydrodeoxygenation (HDO) of the acid to paraffin. The surface modification of the support alumina by phosphate groups significantly increased the HDO activity of the Pd catalyst and, consequently, the paraffin yield. The activity change was accounted partly for the partial replacement of the weak base Al\u2013OH groups by weak acid P\u2013OH groups but mainly for the partial elimination of Lewis acid (Al\u2295) \u2013 Lewis base (O\u229d) pair sites on the surface of the support. Both surface Al\u2013OH and P\u2013OH groups were shown to participate in the reaction with carboxylic acid and formed bidentate surface carboxylate species, which further reacted with hydrogen to give paraffin. Carboxylates of less basic surface sites were found to be more prone to HDO reaction than those of strong base sites. Monodentate carboxylates, formed on Al\u2295 O\u229d pair sites were of low reactivity. Phosphatizing eliminated most of the Lewis type acid-base pair sites, therefore, reactive bidentate carboxylates represented the most abundant surface intermediate (MASI) during the HDO reaction of triglyceride. The hydroxyl coverage of the carboxylated surface was shown to become somewhat higher under steady-state reaction conditions. The increased hydroxyl coverage implies that C\u2013O bond hydrogenolysis of the surface carboxylate proceeds, regenerating OH groups and forming aldehyde that could be intermediate of paraffin formation.\n               "}
{"full_text": "The emergence of antidepressants has effectively addressed symptoms for depression patients in recent years. Nefazodone (Nefa)\u2013a novel third generation antidepressant agent and phenyl piperazine derivative\u2013has been widely used in the treatment of depression. Several studies have reported the recent abuse of Nefa, leading to its release into natural water courses, which can cause a serious impact on the antipredator behavior or alter the reproductive behavior of aquatic species [1\u20133]. However, there are few reports on the degradation of Nefa. It is therefore essential to develop useful treatments to eliminate Nefa in aqueous media.Photocatalysis technology has recently received significant attention because it is environmentally friendly, low cost, and convenient, and has been shown to effectively remove antibiotics and many halogenated organic pollutants [4\u20137]. The selection of suitable photocatalysts for superefficient photocatalytic degradation of pollutants under visible light irradiation is therefore important. Graphitic carbon nitride (g-C3N4) has been shown to be highly effective for removing organic contaminants and offers benefits such as a tunable bandgap, low toxicity, and easy modification [8\u201211]. Doping modification is one of the most effective strategies for further enhancing the photocatalytic activity of g-C3N4 [12,13]. Among the metal-free dopants, phosphorus (P) plays an important role in modifying the structure of g-C3N4, which reduces the band gap energy, increases the separation efficiency of photogenerated charge carriers, and enhances the optical response [14]. For example, P can be directly doped into g-C3N4 frameworks to give a coral-like porous tube morphology [15]. In addition, transition metal phosphides can serve as cocatalysts to improve the activity and stability of g-C3N4 by constructing heterojunctions [16,17]. However, the roles of P-doping in g-C3N4\u2013based photocatalysts do not have been adequately studied.A previous study, in which a co-doped g-C3N4\u2013based photocatalyst for tetrachlorobisphenol A removal was prepared using P source calcination and transition metal layered double hydroxides (Co and Ni LDHs) as soft-templates, focusing on the effect of P atoms on the electron structure of Co and Ni transition metals [18]. However, the role of P doping in Co and Ni-loaded carbon nitrides and the effects of different P-doping strategies were not discussed in depth. In this study, four different photocatalysts are prepared to investigate the role of P doping in Co and Ni-loaded carbon nitride photocatalysts. The morphology and structure of the as-prepared samples are characterized. The optical performance is tested, including the optical absorption and photogenerated charge carrier transfer. The photocatalytic activity of the samples is evaluated in terms of Nefa degradation under visible light irradiation. Corresponding degradation mechanisms and routes are proposed based on analysis of the active species and intermediate products.Prior to synthesis of the composites, CN and CoNi LDH were prepared in accordance with previously reported methods [18]. Four different photocatalysts were then synthesized using a P annealing treatment with NaH2PO2 (1\u00a0\u200bg) as a P source (PH3) and denoted PA-1, PA-2, PA-3, and PA-4. For PA-1, the PH3-treated CN (0.2\u00a0\u200bg) and PH3-treated CoNi LDH (0.01\u00a0\u200bg) were mixed by ultrasonication and calcined at 350\u00a0\u200b\u00b0C for 2\u00a0\u200bh (5\u00a0\u200b\u00b0C/min) under a N2 atmosphere in a tube furnace. After cooling to room temperature, PA-1 was washed with ultrapure water for several times and dried in an oven at 60\u00a0\u200b\u00b0C. PA-2 and PA-3 were prepared using a similar process, but pristine CN and CoNi LDH were used as precursors, respectively. For PA-4, pristine CN (0.2\u00a0\u200bg) and CoNi LDH (0.01\u00a0\u200bg) were mixed and placed in a tube furnace. NaH2PO2 was then introduced at the upstream side and the calcination conditions were the same as that for PA-1. A schematic diagram of the synthetic methods is shown in Fig.\u00a01\n.The morphology, structure, and optical performance were characterized by scanning/transmission electron microscopy (SEM/TEM), aberration-corrected high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM), X-ray photoelectron spectroscopy (XPS), electron spin-resonance spectroscopy (ESR), UV-vis diffuse reflectance spectroscopy (DRS), room-temperature photoluminescence emission spectroscopy (PL), and electrochemistry. The concentration and degradation products of Nefa were analyzed by liquid chromatography and high-resolution mass spectrometry. The details of the characterization are shown in the Supplementary Information.A 100-mL solution containing Nefa (10\u00a0\u200bmg/L) and photocatalyst (20\u00a0\u200bmg) was added to a quartz reactor. Prior to the photocatalytic experiments, the quartz reactor was placed in a dark environment for 30\u00a0\u200bmin to reach saturation adsorption. The light source for photocatalytic degradation was a 300\u00a0\u200bW Xenon lamp (PLS-SXE300, Beijing Perfectlight) with a 420-nm cut-off filter. A given volume of solution was sampled at regular intervals, and the samples were measured following passage through a 0.22-\u03bcm syringe filter.Different calcination strategies were used to determine the role of P doping in Co and Ni-loaded carbon nitride photocatalysts. The SEM images (Fig.\u00a0S1) show that all samples had the inherent porous morphology of CN, which provided a large surface area. PA-1 and PA-3 exhibited clear agglomeration due to \u03c0-\u03c0 stacking, indicating that the introduced PH3 may destroy the inherent porous morphology of CN and result in the agglomeration. TEM images of pure CN and the samples are shown in Fig.\u00a0S2 and Fig.\u00a02\na\u2013d. For PA-1, PA-2, and PA-3, a lattice spacing of 0.21\u00a0\u200bnm related to the metal phosphides (CoNiP\nx\n) was observed [19,20], while PA-4 exhibited bright spots attributed to Co and Ni single atoms based on the findings of a previous study [18]. The observations indicate that different calcination strategies could change the local environment of single-atom catalysts. In addition, the EDS elemental mapping images in Fig.\u00a02e show that C, N, Co, Ni, and P were well distributed in PA-4.The structure of the samples was analyzed by XPS. The N/C of CN, PA-1, PA-2, PA-3, and PA-4 was determined to be 1.23, 1.20, 1.12, 1.13, and 1.16, respectively. The reduced ratios for the annealed samples compared with pure CN indicate that the direct contact between CN and the P source led to the formation of more nitrogen defects, resulting in improved optical performance. The formation of nitrogen defects in PA-4 also provided a favorable environment for anchoring single atoms [21]. Three conventional peaks located at 284.8\u00a0\u200beV (adventitious carbon), 286.4\u00a0\u200beV (C\u2013N), and 288.2\u00a0\u200beV (N\u2013CN) were observed in the C1s XPS spectra (Fig.\u00a03\na). The N1s XPS spectra show that all samples exhibited three peaks at 398.6, 399.6, and 401.6\u00a0\u200beV corresponding to bi-coordinated (N2C), tri-coordinated (N3C), and NH\nx\n groups in the heptazine framework, respectively (Fig.\u00a03b) [22,23]. The spectra of the PH3-treated CN (P\u2013CN) control are shown in Fig.\u00a0S3. The component ratios in P\u2013CN show similar values to that of CN, implying a slight influence of P treatment on the pure CN. Compared with the ratios of N\u2013CN (39.2%) and N2C (27.9%) in the CN, the ratios in all the P-doped samples were reduced, indicating that the nitrogen defects are due to the cracking of N\u2013CN structure. In the P2p XPS spectra (Fig.\u00a03c), all the P-doped samples showed two peaks at 129.0 and 133.8\u00a0\u200beV related to Metal\u2013P and P\u2013N/C, respectively [24\u201326]. The greater intensities of Metal\u2013P and P\u2013N/C for PA-4 imply that more P-doping structures were formed by the concurrent P annealing process in the presence of CN and CoNi LDH. The loading amounts of Co and Ni in the P-doped samples are listed in Table\u00a0S1. For PA-1 and PA-2, metal phosphides were first prepared using the CoNi LDH soft-templates by effective P-annealing reactions, which led to no available single atom precursors. For PA-3, the introduced P first reacted with carbon nitride to give P\u2013CN, which changed the coordination environment of the single atoms. In PA-4, the introduced P was able to concurrently react with CoNi LDH and carbon nitride, allowing the etching of the CoNi LDH soft-templates, while also doping into the coordination structure to form single atoms.The presence of nitrogen defects was also revealed by ESR spectroscopy. The g value at 2.004 is attributed to N vacancies (Fig.\u00a03d) [27]. The ESR intensity of all samples, PA-1, PA-2, PA-3, and PA-4, increased markedly compared with that of CN. PA-4 generated more N vacancies than the other samples owing to its synthesis process, which is consistent with the observed N/C ratios. For PA-4, the CN and CoNi LDH precursors were first mixed and then subjected to the PH3 annealing process, which retained the porous structure of CN, and provided more P sources to react with the precursors and generate more active sites.The optical absorption characteristics of the as-fabricated samples were measured by DRS. Fig.\u00a04\na shows that the absorption edge of CN was 542\u00a0\u200bnm. Compared with CN, the P-doped samples exhibit red-shifts (>550\u00a0\u200bnm), and the absorption intensity was boosted in both UV and visible light regions, indicating that the addition of P significantly promoted the optical absorption capacity [28,29]. PL spectra of the as-prepared samples were acquired to determine the separation of photogenerated carriers. Fig.\u00a04b shows that all P-doped samples exhibited a lower PL signal than that of the pure CN, suggesting that the recombination of electron-hole pairs was impeded by the introduction of P [30]. As anticipated, PA-4 had the highest charge separation efficiency, indicating that the formation of single atoms might make the electron cloud shift to the metal active sites and inhibit the recombination. The transient photocurrent response and electrochemical impedance spectra (EIS) were also determined. Fig.\u00a04c shows that PA-4 exhibited the best photocurrent response intensity\u2013approximately 4 times to that of CN. The arc radius of the P-doped samples was much smaller than that of CN (Fig.\u00a04d). Among the samples, PA-4 showed the smallest radius related to the lowest charge transfer resistance, which might be due to the formation of N vacancies and more P-doping induced single atom sites.The photocatalytic activity of the as-fabricated samples was determined by analyzing the removal of Nefa from water under visible light irradiation. Fig.\u00a05\na shows that Nefa was not eliminated without photocatalysts under visible light conditions. All P-doped samples exhibited an improved catalytic performance compared with CN. PA-4 showed the highest activity among all samples. Its degradation efficiency reached 99.9% within 40\u00a0\u200bmin, which is faster than that of the pure CN (50%). For PA-1, PA-2, and PA-3, the enhanced activity might be resulted from the construction of heterojunctions from P-doped CN and CoNiP\nx\n [31,32]. In PA-4, the reaction of P with CoNi LDH and CN generated single atoms, which altered the structure of the carbon nitride framework and accelerated the charge carrier transfer [33]. Additionally, the corresponding kinetics were well matched with the pseudo-first-order model, and the k values of CN, PA-1, PA-2, PA-3, and PA-4 were found to be 0.021, 0.032, 0.049, 0.046, and 0.065\u00a0\u200bmin\u22121, respectively (Fig.\u00a05b). As shown in Fig.\u00a05c, the degradation rate of Nefa decreased by 2.73% after five experiment cycles, indicating that PA-4 exhibited good catalytic stability.To determine the photocatalytic degradation mechanism of PA-4 for Nefa, the main active species were investigated: superoxide radicals (O2\n\u2022\u2212), electrons (e\u2212), hydroxyl radicals (\u2022OH), and holes (h+). \u03c1-Benzoquinone (BQ), AgNO3, tertbutyl alcohol (TBA), and sodium oxalate (SO) were used as scavengers for O2\n\u2022\u2212, e\u2212, \u2022OH, and h+, respectively [34]. As shown in Fig.\u00a05d, the degradation of Nefa was completely suppressed in the presence of BQ, indicating that O2\n\u2022\u2212 plays a major role in the photocatalytic process. Similarly, the degradation efficiency was negatively affected by the addition of AgNO3, suggesting that e\u2212 also plays an essential role. The addition of TBA and SO slightly limited the degradation efficiency, implying that \u2022OH and h+\u00a0\u200bplay minor roles. The ESR spectra also indicated the presence of the active species mentioned above (Fig.\u00a06\n).Analysis of the intermediate products (Fig.\u00a0S4) allowed possible Nefa degradation routes over PA-4 under visible light irradiation to be proposed (Fig.\u00a07\n). P1 and P2 are believed to be generated following hydroxylation reactions. These intermediate products are then thought to be broken into smaller units such as \u03c1-hydroxy-(m-chlorophenyl) piperazine (P3), 1-(m-chlorophenyl) piperazine (P4), m-chloroaniline (P5), and piperazine (P6) by N-dealkylation reactions [35,36]. Another route is related to the removal of chlorobenzene from the Nefa structure and hydroxylation reaction. The piperazine structure is then disrupted to generate P9 and P10. All the intermediate products are then further oxidized and finally generate CO2, H2O, and Cl\u2212 among other products.Four P-doped Co and Ni-loaded carbon nitride photocatalysts were prepared using different P-doping strategies. Characterization results showed that all the P-doped samples presented N defect structures, which enhanced their optical absorption performances and inhibited photogenerated charge carrier recombination compared with pure CN. Among the P-doped samples, PA-4\u2013prepared using a concurrent P annealing process in the presence of CN and CoNi LDH\u2013presented single atom structure, which effectively improved its photogenerated charge carrier transfer. Additional structure characterizations indicated that the introduced P in PA-4 was able to etch the soft-templates and get doped into the coordination environment to form Co and Ni single atom photocatalysts, which inhibited photogenerated charge carrier recombination and improved the photocatalytic activity. All the active species, O2\n\u2022\u2212, \u2022OH, and photogenerated electrons and holes, contributed to the degradation of Nefa. PA-4 exhibited a catalytic removal of 99.9% for Nefa degradation within 40\u00a0\u200bmin under visible light irradiation, which was faster than that of pure CN (50%).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 financially supported by the National Natural Science Foundation of China (No. 52100076). The authors thank Suqian Ningbiao Technology Testing Co., Ltd. for the characterization support.Information associated with this article can be found in the Supplementary material.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.efmat.2022.05.001.", "descript": "\n                  To understand the role of phosphorus doping in Co and Ni-loaded carbon nitride photocatalysts, four P-doped samples are prepared using different strategies. Morphological characterization shows that Co and Ni single atoms were prepared using a concurrent P annealing process in the presence of carbon nitride and CoNi layered double hydroxides (PA-4). In addition, structural characterization indicates that the introduced P can etch CoNi soft-templates and be doped into the coordination environment. The PA-4 structure is believed to enhance the photogenerated charge carrier transfer. The as-prepared PA-4 samples exhibit better photocatalytic activity for nefazodone (Nefa) degradation in water (99.9% within 40\u00a0\u200bmin) than other P-doped samples. Quenching experiments indicate that O2\n                     \u2022\u2212, \u2022OH, and photogenerated electrons and holes contribute to the degradation of Nefa. Analysis of the intermediate products suggests that the degradation routes primarily involve hydroxylation reactions, N-dealkylation reactions, and piperazine cracking. The findings provide an alternative strategy for the preparation of P-doped Co and Ni-loaded carbon nitride photocatalysts for contaminant degradation and elucidate the role of P doping.\n               "}
{"full_text": "Direct N-acylation of amines through the use of carboxylic acid, most particularly, acetic acid as an acylating agent to form amide linkage is a challenging tasks in synthetic chemistry [1]. This is interesting though not surprising because the amide functionality is an essential key part of many biologically as well as industrially important organics, such as peptides [2] polymers [3] natural products, and pharmaceuticals [4]. Moreover, N-acylation is also an important synthetic target in organic chemistry for the protection of amino groups during the multistep transformations [5].The most popular route of N-acylation or amide synthesis involves the use of activated carboxylic acid derivatives, such as acid chlorides, anhydrides, and esters [6]. For example, Goodreid et al. [7] reported direct amidation reaction between metal carboxylate salt of the corresponding acid and amine. Though this protocol gives a good quantitative yield (61%) within 2\u00a0h, it involves the tedious step of carboxylate salt formation by carboxylation of lithiated terminal alkyne. The cross-coupling reaction between aryl esters and aniline to form amide linkage is also reported [8]. This again is a tedious two-step protocol, where carboxylic acids require conversion into the corresponding esters. The direct amidation between aryl chloride or bromide and amines has also been reported [9]. In this reaction protocol, activation of halides is required to be carried out by expensive palladium-based organometallic catalyst. Thus, all these conventional substrates have serious limitations, such as corrosion effects, high cost, hygroscopicity, tedious workup, and low atom efficiency [10]. The one-step (direct) reaction between amines with carboxylic acids would therefore be both economically and environmentally a benign route to amide synthesis.In addition to homogeneous catalysts, a large number of heterogeneous catalysts are used for amide formation reaction. In the direct amidation of indoles by using ZnCl2 catalyst, expensive electrophilic reagent N-[(benzenesulfonyl)oxy] amides are used for the selective 3-amidation of indole [11]. Here, a series of primary amides were prepared by amidation reaction between twenty-five different carboxylic acids and urea by using ZrOCl2\u00b78H2O and cerium ammonium nitrate as heterogeneous catalysts under solvent-free condition [12]. The problems with the aforementioned two protocols are the need for microwave irradiation, and reproducibility. Another class of heterogeneous Lewis acid catalysts for direct amidation reaction are metal triflates, but they are difficult to synthesize, and therefore of high cost. The synthesis of N-(pyridine-2-yl) amides by a reaction between aldehydes and 2-aminopyridines was carried out under mild reaction conditions by Cu(OTf)3 catalyst [13]. A long chain peptide and protein linkages were degraded into amide fractions by Sc(OTf)3 as heterogeneous catalyst [14]. Tris (methoxyphenyl) bismuthanes was used to activate primary carboxylic acids, and later on coupled with a series of amines and alcohols to yield the corresponding amides and esters, respectively [15]. The protonated zeolite Y (Zeolite HY) catalyst was used by several workers for direct amidation reaction [5,16,17]. In this protocol, the structural modification of zeolite catalyst is an essential step to obtain amides in good yield. Direct amidation co-catalyzed by Ag/Al2O3 and Cs2CO3 between alcohols and amines yielded thirteen different amides, but this method operates only in the presence of harmful toluene solvent under refluxing condition [18]. Though the above-discussed materials are ingenious and effective catalysts for amide synthesis, all possess some practical constraints, which includes high cost, toxicity, corrosive nature, difficulty in separation, non-recyclability, harsh reaction conditions, polluting behavior, and low thermal stability [19].Bare metal oxides, such as Nb2O5, ZnO, Al2O3, TiO2, etc., have also been used as heterogeneous catalysts in direct amidation reaction. Metal oxide catalysts are the most suitable green, reasonable catalysts for direct amidation. The zinc oxide in the form of nanofluid was used as a pseudo-homogeneous catalyst for the direct amidation between aliphatic carboxylic acids and primary amines under solvent-free conditions. A tedious protocol is needed in this reaction to prepare a new reaction media, and the catalyst is recovered in the form of ZnO nanoparticles, which again needed to be converted into nanofluid ZnO for further reaction cycles [20]. Lewis acidic Nb2O5, as a basic group tolerant heterogeneous catalyst is used for the direct amide synthesis. But this reaction protocol followed continuous azeotropic refluxing of the reaction mixture with toluene for a longer time (30\u00a0h) [21]. Other metal oxides with high Lewis acidity, like Al2O3\n[22,23] and TiO2\n[24] were also found to be efficient for direct amidation between non-activated carboxylic acids and amines. However, most of them show low tolerance towards basic functionalities present either on acid or on the amine substrates and they also utilize hazardous solvents in the reaction protocol.For further enhancement of catalytic activity, the metal oxides were combined with other organic or inorganic compounds to form binary or ternary mixed metal oxide systems. For example, the thiol modified binary Pt/TiO2 was used as a heterogeneous catalyst for the selective hydrogenation of nitroarenes to the corresponding amines [25]. Vitamin B12-TiO2 was used as an efficient oxygen controlled catalyst for the preparation of esters and amides from trichlorinated organic compounds just by irradiating the reaction mixture with light [26].The introduction of Br\u00f8nsted acidity along with Lewis acidity in the metal oxide heterogeneous catalyst proved to be a promising practice for researchers in the intervening years. Such Br\u00f8nsted acidity was introduced by anchoring acidic groups, like sulfonic [27] and phosphoric [28] groups onto the surface of the catalyst. Synthesis of amides from fatty acids as well as benzoic acid derivatives was carried by using nanosulfated TiO2 as a solid acid catalyst [6]. The direct amidation between carboxylic acids and amines was carried out with sulfated tungstate as a green solid acid catalyst [10]. The acid-functionalized TiO2-based binary nanocomposites with improved Lewis as well as Br\u00f8nsted acidities having synergetic effect of constituent metal oxides have been effective in various organic transformations. For example, sulfated Fe2O3/TiO2 nanocomposite was used as an efficient visible active photocatalyst [29]. The ultrasensitive sulfated graphene/TiO2 nanocomposite was studied by some workers for the detection of global antioxidant capacity [30]. Ryoo et al. reported the catecholic chelation of the benzene disulfonate with titanium ion of the binary SiO2/TiO2 composite [19]. The anchoring of two \u2013SO3H groups on the robust network of the composite surface plays a crucial role in the acylation of ethanol by using acetic acid at ambient temperature of 80\u00a0\u00b0C. The esterification of bio-based organic acid, like levulinic acid, with ethanol in the presence of sulfated TiO2 and sulfated ZrO2/TiO2 composite was studied [31]. The report well established that the sulfated ZrO2/TiO2 composite with a large number of sulfonic groups anchored on its surface showed high catalytic activity towards the esterification of levulinic acid as compared to the bare sulfated TiO2 catalyst. Such promising catalytic performance of sulfated TiO2 based mixed metal oxide nanocomposites is encouraging research on such materials.Herein, we report sulfated binary TiO2/SnO2 nanocomposite as a green heterogeneous catalyst for the direct amide formation reaction between various amines and acetic acid. The overall reaction protocol involves no use of any coupling agent or solvent.All chemicals purchased were from S D Fine-Chem Limited, Mumbai, India and of AR grade.The bare TiO2 and bare SnO2 NPs were prepared as per our previously reported synthetic protocol [32].In our reported work [32] TiO2/SnO2 NC (with 4:1\u00a0wt% of TiO2 and SnO2 respectively) showed higher catalytic activity as compared to its single and binary counterparts. In the present investigation, this catalyst is further acid-functionalized to enhance its catalytic activity. A typical synthesis of TiO2/SnO2 NC (with 4:1\u00a0wt% of TiO2 and SnO2, respectively) was carried out by the wet impregnation method. During the synthesis of SnO2 NPs, the amount of TiO2 NPs to the desired stoichiometric ratio was added, and the resulting mixture was stirred vigorously at 60\u00a0\u00b0C for 3\u00a0h. Finally, all the samples were calcined at 450\u00a0\u00b0C for 3\u00a0h, to yield white colored NCs.For sulfation, 2\u00a0g of TiO2/SnO2 NCs was taken into a three-necked round bottom flask kept in an ice bath. Then, 2\u00a0mL of Chlorosulfonic acid was added dropwise into the flask by using a pressure-equalizing funnel fitted on the middle neck of the flask. The other two openings of the flask were packed by corks in order to avoid contact with atmospheric moisture. The resulting mixture was stirred for 2\u00a0h to assure proper anchoring of the sulfonic groups onto the surface of TiO2/SnO2 NCs. Then the mixture was removed from the flask and washed thoroughly with deionized water several times to remove unreacted chlorosulfonic acid. The white colored product was dried at 100\u00a0\u00b0C on hot plate and calcined at 300\u00a0\u00b0C in a furnace for 3\u00a0h.The crystallographic information of sulfated TiO2/SnO2 NCs was derived by X-ray diffractometry (XRD) with Ni-filtered Cu K\u03b1 radiation of 1.54056\u00a0\u00c5 (X\u2019 pert PRO, Philips, Eindhoven). High-resolution transmission electron microscopy (HRTEM) with selected area electron diffraction (SAED) imagery was obtained (JEOL-3010 and Tecnai G2 F20). The chemical composition of the samples was determined by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000, Thermo VG Scientific, UK) with a monochromatic Mg-K (1253.6\u00a0eV) radiation source. The UV\u2013visible DRS spectra of samples was recorded by UV\u2013visible spectrophotometry (LabIndia 3092). Ammonia-Temperature programmed desorption (NH3-TPD) analysis was used to elucidate the total acidity of sulfated sample (Micromeritics, AutoChem II 2920 chemisorption analyzer, USA, equipped with thermal conductivity detector (TCD)). BET analysis (N2 adsorption and desorption isotherms) was carried out (Quantachrome Nova Win instrument). 1H NMR and 13C NMR spectra of the compounds were recorded by Bruker AC-300 spectrometry using tetramethylsilane as an internal standard. The IR spectral analysis of the compounds was done by Perkin-Elmer FT-IR 783 spectrophotometer.The direct amidation reactions were performed in 25\u00a0mL round bottom flask under reflux at 115\u00a0\u00b0C and constant stirring (250\u00a0rpm). Equimolar volumes of glacial acetic acid (1.0\u00a0mL, 17\u00a0mM) and the corresponding amine (17\u00a0mM) were taken into the flask. Then, 50\u00a0mg of catalyst was added into the mixture. The progress of the reaction was evaluated by thin layer chromatography. The formation of the product was confirmed through FT-IR, 1H NMR, and 13C NMR measurements. After completion of the reaction, 10\u00a0mL of ethyl acetate was added into the reaction mixture. The mixture was centrifuged to remove the catalyst and the filtrate was washed with NaHCO3 (3\u00a0\u00d7\u00a010\u00a0mL), and finally with deionized water to obtain the product.\nFig. 1\n shows the powder X-ray diffractograms of pure TiO2/SnO2 NC and sulfated TiO2/SnO2 NC (with 4:1\u00a0wt% of TiO2 and SnO2, respectively). In both the diffractograms, the reflections of TiO2 (2\u03b8 = (25.22, 48.44, 54.27, 55.49) \u00b0, etc.) were well-matched with the anatase TiO2 reflections (JCPDS #21-1272). Meanwhile, the reflections of SnO2 (2\u03b8 = (26.56, 34.12, 52.03)\u00b0, etc.) were well-matched with the tetragonal cassiterite SnO2 reflections (JCPDS #41-1445). The overlapping of representative peaks of TiO2 and SnO2 reveals the formation of the well-intermixed composite. The XRD pattern of sulfated NC appearing like the non-sulfated NC confirms the uniform dispersion of sulfonic groups onto the surface of the sulfated NC [33]. A slight decrease in the intensities of characteristic peaks was observed in the sulfated NC due to the surface-adsorbed sulfonic (-SO3H) groups which is in line with earlier reports [34].The presence of surface-adsorbed \u2013SO3H groups onto the surface of NCs was ascertained by FT-IR analysis (Fig. 2\n). The FT-IR spectrum of non-sulfated NC (Fig. 2 a)) exhibited broadband and sharp band at (3,400 and 1,632) cm\u22121, which were attributed to the stretching and bending vibrations of surface adsorbed \u2013OH groups of water molecules, respectively [32]. These bands are also present in the FT-IR spectrum of sulfated NC (Fig. 2 b)). The characteristic Ti\u2013O bending vibration band at 1,400\u00a0cm\u22121 and Ti\u2013O stretching vibration band in the range (400 to 900) cm\u22121 are evidenced in both non-sulfated as well as sulfated NCs, which is in line with the previous reports [32]. The Sn\u2013O stretching bands in the range (750 to 455) cm\u22121 for SnO2 were mixed and embedded in the broad peak (M\u2212O stretching vibration band region, where, M\u00a0=\u00a0Ti or Sn) [32]. The \u2013SO3H groups anchored onto the surface of NC, confirmed by the existence of the four characteristic bands in the range (1232 to 956) cm\u22121 (Fig. 2 b)) [35].The morphology and particle size of the sulfated TiO2/SnO2 NC were determined by using TEM, HRTEM, and SEAD patterns. The bright-field TEM image (Fig. 3\n a)) of the sample shows a close aggregation of particles having an average size of between (10 and 25) nm, which is in good agreement with the crystallite size values obtained by XRD analysis. The HRTEM image (Fig. 3 b)) clearly shows intense lattice fringes corresponding to the anatase TiO2 with \u2018d\u2019 value 0.352\u00a0nm for (101) planes and cassiterite SnO2 with \u2018d\u2019 value 0.352\u00a0nm for (110) planes; which further supports the good polycrystallinity of the sulfated TiO2/SnO2 NC [32]. The SAED pattern of the sample (Fig. 3 c)) focused on some more structural details. In this pattern, the observed diffraction rings are indexed and are perfectly matched with anatase TiO2 and cassiterite SnO2\n[32]. The visibly sharp reflections of rings in the pattern clearly confirm the high polycrystallinity of the sulfated TiO2/SnO2 NCs.\nFig. 4\n shows the elemental composition and oxidation states of various elements in the sample that were investigated by using XPS technique.The survey spectrum represented in Fig. 4 a shows the presence of all expected elements with their characteristic peak positions. Fig. 4 b shows the XPS high-resolution spectrum of oxygen (O 1s). In the spectrum, the main peak at binding energy 530.8\u00a0eV is deconvoluted into two shoulder peaks, which indicates that the oxygen is present in several chemical states according to the measured binding energy. The lower binding energy peak at 530.8\u00a0eV is attributed to oxygen bonded to the metal \u2013M\u2013O (M\u00a0=\u00a0Ti, Sn), while the peak at 531.9\u00a0eV is attributed to adsorbed hydroxyl species (\u2013O\u2013H) [36]. The Ti 2p spectrum (Fig. 4 c)) reveals spin\u2013orbit splitting of Ti 2p1/2 and Ti 2p3/2 core level states at the positions (465.3 and 459.5) eV, respectively. The spin\u2013orbit splitting difference is 5.8\u00a0eV, which clearly confirms that the Ti element in the sample possesses\u00a0+\u00a04 oxidation state that is contributed by TiO2\n[36]. In the high-resolution XPS spectrum of tin (Sn 3d) (Fig. 4 d)), the two symmetric peaks appearing at binding energies (487.3 and 495.8) eV are assigned to the lattice SnO2. The spin\u2013orbit splitting difference between the Sn 3d5/2 and Sn 3d3/2 levels (8.5\u00a0eV) matches the standard spectrum of Sn (+4 oxidation state) bonded with O in SnO2 lattice [37]. Fig. 4 e shows the high resolution XPS spectrum for S 2p. The single broad peak situated at the position 169.5\u00a0eV corresponds to S element in the sample having\u00a0+\u00a06 oxidation state, which is contributed by the surface adsorbed sulfonic groups [29].The acidic strengths of non-sulfated TiO2/SnO2 and sulfated TiO2/SnO2 NCs were studied by NH3\u2013TPD (Fig. 5\n) technique in which 10% NH3\u2013He was used as a probe molecule to elucidate the total acidity of catalysts.NH3\u2013TPD analysis of the non-sulfated TiO2/SnO2 NCs shows the presence of two desorption peak maxima at 231.9 and 313.2\u00a0\u00b0C. Similarly, sulfated TiO2/SnO2 NCs shows two desorption peak maxima at slightly higher temperatures at (245.8 and 340.8) \u00b0C. In sulfated TiO2/SnO2 NCs, a broad desorption peak area in the medium temperature range (200 to 400) \u00b0C is assigned to NH3 adsorbed on acid sites having medium strength [38]. Table 1\n shows that for non-sulfated and sulfated TiO2/SnO2 NCs. The total acidity values obtained are (0.17405 and 0.21951) mmol/g, respectively (\u2020 Electronic supplementary information, ESI). Thus, it can be concluded that the sulfated NC is more acidic than non-sulfated NC.The surface area and pore structure of the non-sulfated and sulfated TiO2/SnO2 NCs were measured by using the nitrogen adsorption\u2013desorption isotherms and pore-size distribution obtained by BET analysis as shown in Fig. 6\n a and b, respectively. According to the Brunauer\u2013Deming\u2013Deming\u2013Teller (BDDT) classification, both the isotherms correspond to type IV isotherms, typically signifying the mesoporous nature of the prepared samples. The observed hysteresis loop situated in between medium relative pressure values (P/P0 = (0.4 to 0.8)), suggested the mesoporous nature of the samples [39]. For non-sulfated TiO2/SnO2 NC (Fig. 6 a)), the pore size distribution shows pores with the average size of 5.62\u00a0nm, and the specific surface area 71.04\u00a0m2.g\u22121. In the case of sulfated TiO2/SnO2 NC (Fig. 6 b)), the pore-size distribution shows pores with an average size of 5.52\u00a0nm, which is in good agreement with the nature of type IV isotherm. The BET analysis also revealed that the specific surface area of sulfated TiO2/SnO2 NC is 61.684\u00a0m2.g\u22121, which is higher than its bare counterparts, and comparable with that of the non-sulfated TiO2/SnO2 NC. A slight decrease in the surface area of sulfated NC in comparison to the non-sulfated NC was attributed to the surface adsorbed \u2013SO3H groups in the case of sulfated NC. A significantly high surface area of sulfated NC could result in high catalytic activity during amide formation reaction.As a primary objective, the sulfated TiO2/SnO2 NC was tested as a heterogeneous catalyst for the direct amide formation reaction. In this direction, various experimental conditions required for direct amidation were optimized (see Figs. S1 and S2 of the \u2020 Electronic supplementary information, ESI). In a typical procedure, glacial acetic acid was used as an acylating reagent as well as a solvent for the reaction. In a 25\u00a0mL RB flask, glacial acetic acid (1.0\u00a0mL, 17\u00a0mmol) and aniline (Table 2\n, entry 1) (1.6\u00a0mL, 17\u00a0mmol) were taken, and 50\u00a0mg of prepared catalyst was added with constant stirring at the temperature of 115\u00a0\u00b0C. The progress of the reaction was monitored by thin-layer chromatography. The reaction is completed within 2\u00a0h with a 95% yield. It is confirmed that the product is not formed in the absence of a catalyst. Scheme 1\n shows the representative amide formation reaction.The well-optimized reaction protocol used for the direct amidation of aniline was also used for the amidation of various derivatives of aniline with electron-donating and electron-withdrawing groups to establish the general applicability of the sulfated TiO2/SnO2 NC as a heterogeneous catalyst for the direct amidation. Table 2 shows that equimolar amounts of aniline derivative and acetic acid were treated with each other; excellent yields of (65 to 97) % of the corresponding amides were observed in (2 to 6) h under solvent-free condition. Moreover, to compare catalytic selectivity and activity, the turnover number (TON) and turnover frequency (TOF) of reactions were calculated and are given in Table 2. The TON and TOF values clearly show that the electron-donating groups (\u2013CH3, \u2013OCH3) favor amidation reaction, while in contrast, the electron-withdrawing groups (\u2013NO2, \u2013Cl) harm amidation reaction.After the usual reaction, all the products obtained were pure (confirmed by spectral data and melting points), and did not require additional efforts to purify them. The products were characterized by 1HNMR, 13CNMR, and IR spectroscopic techniques. All the spectroscopic data obtained were found to be identical with the literature data of known compounds (\u2020 ESI).\nN-phenylacetamide (1a). White lustrous solid; m. p.\u00a0=\u00a0113 \u2013 114\u00a0\u00b0C; 1H NMR (300\u00a0MHz, CDCl3) \u03b4 2.15 (3H, s, \u2013CH3), 7.10\u20137.12 (1H, d, Ar-H), 7.27\u20137.32 (2H, t, Ar-H), 7.52\u20137.55 (2H, d, Ar-H), 8.77 (1H, s, \u2013NH-); 13C NMR (75.47\u00a0MHz, CDCl3) \u03b4 24.33, 120.36, 124.29, 128.88, 138.16, 169.47; IR cm\u22121 (KBr)\nN-(-2-methylphenyl) acetamide (1b). Colorless solid; m. p.\u00a0=\u00a0110 \u2013 111\u00a0\u00b0C; 1H NMR (300\u00a0MHz, CDCl3) \u03b4 2.20 (3H, s, \u2013CH3), 2.26 (3H, s, Ar-CH3), 7.07\u20137.11 (1H, m, Ar-H), 7.18\u20137.23 (1H, m, Ar-H), 7.28 (1H, m, Ar-H), 7.71\u20137.74 (1H, m, Ar-H); 13C NMR (75.47\u00a0MHz, CDCl3) \u03b4 17.78, 24.22, 123.62, 125.39, 126.71, 129.54, 130.47, 135.64, 168.44; IR cm\u22121 (KBr)\nN-(-4-methylphenyl) acetamide (1c). White solid; m. p.\u00a0=\u00a0149 \u2013 150\u00a0\u00b0C; 1H NMR (300\u00a0MHz, CDCl3) \u03b4 2.15 (3H, s, \u2013CH3), 2.32 (3H, s, Ar-CH3), 7.10\u20137.28 (1H, d, Ar-H), 7.46\u20137.49 (2H, d, Ar-H), 9.79 (1H, s, \u2013NH); 13C NMR (75.47\u00a0MHz, CDCl3) \u03b4 20.85, 24.45, 120.12, 129.44, 133.92, 135.37, 168.46; IR cm\u22121 (KBr)\nN-(-4-methoxylphenyl) acetamide (1d). Colourless solid; m. p.\u00a0=\u00a0128 \u2013 130\u00a0\u00b0C; 1H NMR (300\u00a0MHz, DMSO) \u03b4 2.00 (3H, s, \u2013CH3), 3.70 (3H, s, -O-CH3), 6.84\u20136.87 (2H, d, Ar-H), 7.37\u20137.58 (1H, m, Ar-H); 13C NMR (75.47\u00a0MHz, DMSO) \u03b4 24.21, 55.56, 114.22, 121.02, 132.95, 155.48, 168.24; IR cm\u22121 (KBr)\nN-(-3-nitrophenyl) acetamide (1e). Yellow solid; m. p.\u00a0=\u00a0150 \u2013 152\u00a0\u00b0C; 1H NMR (300\u00a0MHz, DMSO) \u03b4 2.08 (3H, s, \u2013CH3), 7.53\u20137.58 (1H, m, Ar-H), 7.82\u20137.88 (2H, m, Ar-H), 8.59 (1H, m, Ar-H), 10.42\u201310.44 (1H, d, \u2013NH); 13C NMR (75.47\u00a0MHz, DMSO) \u03b4 24.42, 113.43, 117.96, 125.31, 130.49, 140.76, 148.35, 169.60, 169.98; IR cm\u22121 (KBr)\nN-(-4-nitrophenyl) acetamide (1f). Yellow solid; m. p.\u00a0=\u00a0215 \u2013 216\u00a0\u00b0C; 1H NMR (300\u00a0MHz, DMSO) \u03b4 2.10 (3H, s, \u2013CH3), 6.57\u20136.72 (1H, m, Ar-H), 7.78\u20137.94 (2H, m, Ar-H), 8.17\u20138.20 (2H, m, Ar-H), 10.56 (1H, s, \u2013NH); 13C NMR (75.47\u00a0MHz, DMSO) \u03b4 24.64, 112.83, 118.99, 125.37, 126.81, 136.10, 142.44, 145.86, 156.12, 169.84; IR cm\u22121 (KBr)\nN-(-4-chlorophenyl) acetamide (1g). Colourless solid; m. p.\u00a0=\u00a0177 \u2013 179\u00a0\u00b0C; 1H NMR (300\u00a0MHz, DMSO) \u03b4 2.03 (3H, s, \u2013CH3), 7.31\u20137.33 (2H, m, Ar-H), 7.58\u20137.61 (2H, m, Ar-H), 10.08 (1H, s, \u2013NH); 13C NMR (75.47\u00a0MHz, DMSO) \u03b4 24.40, 120.95, 126.99, 128.98, 138.69, 168.93; IR cm\u22121 (KBr)As part of the systematic study, we also explored the reusability of the catalyst as an important criterion of any heterogeneous catalyst. After each cycle of amidation reaction, the solid catalyst was removed by ultrafiltration and washed thoroughly with deionized water. Then the catalyst was dried in oven to remove adsorbed organic moieties (if any) as well as water at 110\u00a0\u00b0C. The equimolar amounts (17\u00a0mM) of aniline and acetic acid were further treated with each other in the presence of a dried catalyst for the next cycle of amidation reaction. Fig. 7\n shows that the catalyst retained its activity even after four successive cycles. Thus, it has been established that the catalyst has good stability, and therefore can be efficiently recycled and reused for repeated cycles of amidation reaction with an appreciable recovery of product yield.In summary, we have prepared sulfated TiO2/SnO2 NC by using chlorosulfonic acid as a sulfating agent, and investigated various physicochemical properties of the as-prepared NC using different instrumental techniques. This NC was then explored as a green heterogeneous catalyst for direct amidation reaction between diverse primary amines and acetic acid. The sulfated TiO2/SnO2 NC catalyzed amidation protocol offers a number of advantages, such as recyclability of the catalyst without loss in its activity, easy work-up, large-scale availability of catalyst, appreciable-to-high product yields of (65 to 97) % in short reaction time of (2 to 6) h, solvent-free condition, and easy separation of catalyst from the reaction mixture by filtration. All of these important outcomes of the study contribute to making this process more advanced, economical, and green, in terms of the environmental aspects.\nS.M. Patil: Data curation, supervision, writing - reveiw and editing. S.A. Vanalakar: Data curation, supervision, writing - reveiw and editing. S.A. Sankpal: Visualization, Investigation. S.P. Deshmukh: Writing - review & editing. S.D. Delekar: Supervision, Writing - 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.SMP is thankful to the University Grants Commission, New Delhi, India for financial assistance under the award of UGC-FIP [F. No. 36-40/14 (WRO)], which is gratefully acknowledged.Supplementary data to this article can be found online at https://doi.org/10.1016/j.rechem.2021.100102.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Heterogeneous binary TiO2/SnO2 nanocomposite (with 4:1\u00a0wt% of TiO2 and SnO2, respectively) catalyst was prepared by a sol\u2013gel method, and further sulfated by chlorosulfonic acid. X-ray diffraction technique revealed the structure of the nanocrystalline catalyst to be tetragonal. Fourier transformed infrared technique elucidated the presence of surface-anchored sulfonic (\u2013SO3H) groups on the catalyst. Morphological details of the catalyst were obtained by transmission electron microscopy. Elemental analysis of the catalyst was carried out by X-ray photoelectron spectroscopy. NH3-TPD technique was used to elucidate the surface acidity of the catalyst. The active surface area and mesoporosity of catalyst were studied by the BET method. Thereafter, this sulfated catalyst was utilized for the direct amidation reaction between a series of amine derivatives and acetic acid. The reaction gives excellent product yield within 120\u00a0min, and at a relatively moderate temperature of ~115\u00a0\u00b0C.\n               "}
{"full_text": "As an important class of chemicals, carbonate has many applications in chemical industry such as organic synthesis, printing and dyeing, electrochemistry and petrochemical sectors [1\u20134]. Diphenyl carbonate (DPC) is one of the versatile carbonates, and is often used as a key monomer for the green production of polycarbonates (PC) to replace the hazardous phosgene. It is also widely employed in the synthesis of various organic compounds and used as starting materials for the fine chemical and pharmaceutical industries like plastic plasticizer, solvent carrier, medicine and agriculture, and among others [5\u20138]. At present, there are several routes to produce DPC, mainly including phosgene method, oxidative carbonylation and transesterification methods [9\u201312]. Among these routes, using toxic phosgene as the raw materials in the process has a hazardous threat to the environment and human health, and such a route needs a large amount of NaOH to neutralize the produced inorganic acid, resulting in a great deal of waste salt and waste water. For oxidative carbonylation route, noble metal Pd catalysts have to be used in the synthetic process that suffers from the high cost of the production. Fortunately, the transesterification route is considered to be green and economic for the synthesis of DPC (Scheme 1\n), because the employed raw material is no toxicity, no pollution, inexpensive, and readily available. Besides, only CH3OH is the by-product in this process.Generally, this reaction goes through two-step process [13,14]: (i) methyl phenyl carbonate (MPC) of the intermediate is produced by the transesterification of DMC and phenol (Scheme 1-(1)), and (ii) the further disproportionation of MPC (Scheme 1-(2)). Nevertheless, there are several problems for this reaction. Firstly, DMC is the raw material and is also the product, which makes this reaction slower. Secondly, the equilibrium constants K1 (6.3\u00a0\u00d7\u00a010\u22125, 25\u00a0\u00b0C) and K2 (0.19, 25\u00a0\u00b0C) are very small in the absence of catalyst even at elevated temperature, leading to the very low yield of DPC. Therefore, the key to these problems is to develop efficient catalytic system. In the past years, various catalytic systems and reaction technologies have been reported for this process, but most of them have been focused on the first-step transesterification process of DMC and phenol since it is the rate-controlling step. Actually, to improve the total yield of DPC and the reaction efficiency, the second-step disproportionation of MPC is also very important. In recent years, we have devoted to the catalytic researches of MPC disproportionation by using titanium/tin organic compounds and metal/mixed metal oxides [15\u201319], but the activity and selectivity are not high enough, and high reaction temperature and long reaction time have to be required. In addition, these catalysts all contain transition metals as active centers which are highly toxic and environmentally harmful, and affect the purity of DPC as well. Metal-free catalysts would be an attractive alternative for replacing metal-containing catalysts, but no work has been reported in this area. Consequently, it is great of significance to design and develop metal-free catalysts toward clean DPC synthesis under facile operation condition.In recent years, ionic liquids have attracted growing attention in the application of solvents and/or catalysts for chemistry and chemical industry due to their green and tunable properties [20\u201322]. Thus, ionic liquids may be considered to be a novel metal-free catalyst for the synthesis of DPC. However, their utilization in homogenous catalysis brings disadvantages of using ionic liquid in large quantities and difficulty in their separation after reaction. Importantly, ionic liquids can be chemically grafted on solid materials to share the excellent catalytic activity of homogeneous catalyst and the easy-separation feature of heterogeneous catalyst [23\u201325]. Under these circumstances, the selection of solid supports is of critical importance. Mesoporous silica materials, especially SBA-15, have plenty of mesopores, high thermal stability and high specific surface area, which enable them to be good candidates for the grafting of ionic liquids.In this work, we designed a series of ionic liquids with different functional active groups as metal-free catalysts, which were immobilized on SBA-15 by chemical bonding grafting. These catalysts were then used for the synthesis of DPC (Fig.\u00a01\n), and the effect of reaction parameters as well as the recyclability of the catalysts was studied in detail. It was found that [SBA-15-IL-OH]Br displayed outstanding catalytic performance, and MPC conversion of 80.5% and DPC selectivity of >99% could be achieved with a low catalyst loading under mild reaction conditions. The TOF value was thrice higher than that by using previously reported traditional transition metal-based catalysts. Moreover, the surface properties of the catalysts were thoroughly analyzed by elemental analysis, FT-IR, XRD, TG, SEM-EDS, TEM and BET techniques to explore the relationship between structure and performance.Firstly, 1-(3-(triethoxysilyl)propyl)-1H-imidazole was synthesized according to the modified literature procedures [26,27], and the synthesis process was shown in Scheme 2\n. In a typical procedure, imidazole (13.6\u00a0g) (3-chloropropyl)triethoxysilane (48.2\u00a0g) and toluene (400\u00a0mL) were added into a round-bottomed flask under nitrogen atmosphere and then the mixture was stirred under refluxed condition for 24\u00a0h. The reaction was monitored through TLC. Then, triethylamine (20.2\u00a0g) was dropwise added to the solution and the resulting mixture was refluxed for another 2\u00a0h. After completion of the reaction, the mixture was filtrated off and washed three times with toluene (3\u00a0\u00d7\u00a020\u00a0mL). Then the solvent was evaporated at reduced pressure and the obtained product was purified through column chromatography with neutral alumina. Finally, the resulting solution was removed by rotary evaporation to give a colorless viscous liquid (Compound 1). The structure of Compound 1 was confirmed by 1H NMR, 13C NMR and FT-IR (Figs. S1\u2013S2, Supplementary Information).In the second step, SBA-15 material was synthesized by following a previously reported procedure with some modifications [28,29]. For this purpose, triblock copolymer P123 (5.0\u00a0g) was dissolved in 2\u00a0mol L\u22121 of aqueous HCl (180\u00a0mL) until transparent solution was formed. Then, TEOS (12.0\u00a0g) was slowly added into the solution under stirring. The mixture was kept at room temperature for 24\u00a0h under vigorous stirring, and then aged in a Teflon-lined autoclave at 100\u00a0\u00b0C for 24\u00a0h. The as-made sample was recovered by filtration and washed extensively with ethanol and deionized water, and dried overnight in an oven at 80\u00a0\u00b0C. Finally, the obtained white solid was calcined at 500\u00a0\u00b0C for 5\u00a0h in air under static conditions.For the synthesis of compound 2, 1.0\u00a0g of SBA-15 was dispersed in 50\u00a0mL of dry toluene by ultrasonic stirring for 30\u00a0min under nitrogen atmosphere, and then 2.0\u00a0g of Compound 1 (see Scheme 2) in 5\u00a0mL of dry toluene was added, and the reaction mixture was stirred and refluxed at 90\u00a0\u00b0C for 24\u00a0h through the condensation reaction between the surface silanol groups (Si\u2013OH) of SBA-15 and the ethoxyl groups (\u2013OCH2CH3) of organotriethoxysilanes [30,31]. After cooled down to room temperature, the reaction mixture was filtered and washed three times with absolute ethanol, and then the obtained white solid was Soxhlet extracted with dichloromethane. The hybrid SBA-15 was recovered and dried at 60\u00a0\u00b0C under vacuum for 24\u00a0h (Compound 2, see Scheme 2). The structure of Compound 2 was verified by FT-IR, SEM-EDS, 29Si MAS NMR and 13C MAS NMR measurements (Figs. S3\u2013S5 and Table S1).The synthesis of ionic liquid-SBA-15 hybrid catalysts with different functional groups including \u2013OH, \u2013COOH, \u2013CH3, \u2013SO3H and \u2013NH2 was illustrated in Scheme 3\n. The OH-functionalized ionic liquid hybrid SBA-15 was displayed in Scheme 3-(1). Briefly, compound 2 (1.0\u00a0g) was suspended in 50\u00a0mL of dry toluene to form a uniform dispersion by sonication in a round-bottomed flask. Then, 1.02\u00a0g of 3-bromo-1-propanol was added to the solution and the reaction mixture was stirred for 24\u00a0h under reflux conditions. After reaction, the mixture was cooled down to room temperature, the produced solid was filtered and washed with toluene repeatedly, followed by drying at 60\u00a0\u00b0C under vacuum overnight to afford OH-functionalized ionic liquid hybrid SBA-15. For convenience, the obtained catalyst was denoted as [SBA-15-IL-OH]Br. Besides, the ionic liquids with \u2013OH group and different alkyl chain lengths on the imidazolium cations were also prepared (Scheme S1), and the synthetic procedures were similar to that of [SBA-15-IL-OH]Br. The only difference was that the corresponding material was used to replace 3-bromo-1-propanol, which was designated as [SBA-15-IL-xC-OH]Br where xC is the carbon atom number in alkyl chain of the IL (x\u00a0=\u00a02, 4, 6 and 8).Based on the similar procedure except that the starting material was 4-bromobutyric acid, the COOH-functionalized ionic liquid hybrid SBA-15 shown in Scheme 3-(2) was prepared. The obtained catalyst was denoted as [SBA-15-IL-COOH]Br. For the preparation of CH3-functionalized ionic liquid hybrid SBA-15, SO3H-functionalized ionic liquid hybrid SBA-15 and NH2-functionalized ionic liquid hybrid SBA-15 shown in Scheme 3-3,3-4 and 3-(5), the procedures were presented in Supplementary Information (Section S.2-S.4). The obtained catalyst was denoted as [SBA-15-IL-CH3]Br [SBA-15-IL-SO3H]Br and [SBA-15-IL-NH2]Br, respectively.In addition, OH-functionalized ionic liquid hybrid SBA-15 with BF4\n\u2212, PF6\n\u2212, HSO4\n\u2212 and OH\u2212 anions were also prepared for comparison according to the procedures reported in literatures [32,33], and the procedures were shown in Scheme S2. The obtained catalysts were designated as [SBA-15-IL-OH]BF4 [SBA-15-IL-OH]PF6 [SBA-15-IL-OH]HSO4 and [SBA-15-IL-OH]OH, respectively.Powder X-ray diffraction patterns (XRD) were recorded on a DX-2700B diffractometer with the Ni-filtered CuK\u03b1 radiation (1.5418\u00a0\u00c5). FT-IR spectra were acquired by Thermo Nicolet 380 spectrometer using KBr pellet technique. Scanning electron microscopy (SEM) and Energy dispersive X-ray (EDS) analysis were performed with a JSM-7500F instrument. Transmission electron microscopy (TEM) measurements were carried out on JEM-1011 apparatus with a field-emission gun operating at 200\u00a0kV. N2 adsorption\u2013desorption profiles were determined on a 3H\u20132000PS2 adsorption instrument. Element analysis was performed on an Elementar Vario EL cube (EA) to determine the chemical composition of the samples. Thermal gravimetric (TG) analysis was performed using a Netzsch Thermoanalyzer STA 449C analyzer under N2 atmosphere. 1H NMR and 13C NMR spectra were recorded on a Bruker 600\u00a0MHz spectrometer. 13C MAS NMR and 29Si MAS NMR measurements were performed on a Bruker Advance III 400\u00a0MHz NMR spectrometer.MPC disproportionation was performed in a three-necked, round-bottomed flask under magnetic agitation. In a typical experiment, 150\u00a0mmol of MPC and catalyst (0.4\u20131.2\u00a0g) was added into the three-neck flask under N2 atmosphere. Afterwards, the reaction mixture was heated at the desired temperature. During the reaction progress, DMC product was distilled out by a liquid dividing head attached to a receiver flask to break the equilibrium limit thus shifting the reaction towards the production of target product DPC. Upon completion, the reactor was cooled down to room temperature and the catalyst was filtered off from the mixed solution, washed with acetone and then dried under vacuum. The filtrate was detected by GC\u2013MS HP-6890/5973 instrument with HP-5 capillary chromatography packed column. The products were quantitatively analyzed with 7890A gas chromatograph equipped with flame ionization detector (FID) and a DB-35 capillary chromatography packed column.The FT-IR spectra of the samples were showed in Fig.\u00a02\n to examine the structure of catalysts. As shown in Fig.\u00a02(a), for parent SBA-15, the intensive absorption peaks at 3442 and 1635\u00a0cm\u22121 were ascribed to the stretching and bending vibrations of surface O\u2013H groups, and the typical peaks at around 1099, 806, and 458\u00a0cm\u22121 were attributed to the asymmetric stretching, symmetric stretching and bending modes of Si\u2013O\u2013Si condensed silica network [34], respectively. Besides, the peak at 971\u00a0cm\u22121 was associated with the bending vibration of framework Si\u2013OH group [35]. Clearly, the FT-IR spectra of functional ionic liquids hybrid SBA-15 in Fig.\u00a02(b-f) exhibited the respective SBA-15 characteristic peaks, but the peak at 971\u00a0cm\u22121 weakened and even disappeared, which indicates that the bonding interactions of the surface Si\u2013OH of SBA-15 with \u2013OCH2CH3 of the functionalized ionic liquids took place during the preparation process, resulting in the removal of the Si\u2013OH. At the same time, some new peaks were observed obviously, for instance, two new characteristic peaks at 3013\u20132829\u00a0cm\u22121 could be seen corresponding to the CH2 asymmetric and symmetric stretching vibrations of the propyl chain of ionic liquids, and they all exhibited four typical peaks at around 1596\u20131395\u00a0cm\u22121 corresponding to the stretching vibrations of imidazolium ring [36\u201338]. These results suggest that functionalized ionic liquids with different functional groups were successfully grafted onto the surface of SBA-15 through chemical bonding.The XRD patterns of the samples were showed in Fig.\u00a03\n. It can be seen from Fig.\u00a03(A), the small-angle patterns for both of the parent SBA-15 and ionic liquids hybrid SBA-15 exhibited three remarkable diffraction peaks in low angle region. The intense diffraction peak at 0.99\u00b0 was assigned to the diffraction of the (100) plane of the mesoporous structure with a remarkable long-range ordering degree, and the other two weak peaks at 1.59 and 1.82\u00b0 corresponding to (110) and (200) reflections were attributed to the two-dimensional hexagonal planes of the mesoporous structure [32,39,40]. However, compared to the parent SBA-15, the diffraction peak (100) for ionic liquids hybrid SBA-15 only became broader and weaker slightly, which was probably resulted from the occupation of the porous channel and the partial blockage of small pores by ionic liquids. This phenomena was consistent with the unit cell parameter calculated by 2d100/(3)1/2 in Table 1\n, where the unit cell volume a0 increased from 10 to 10.2. Fig.\u00a03(B) displayed the wide-angle XRD patterns of the samples. For parent SBA-15, the strong and broad peak at 23\u00b0 was evidently observed, which was attributed to the amorphous nature of mesoporous silica, and the characteristic peaks of ionic liquids hybrid SBA-15 in the wide-angle ranges were similar to that of parent SBA-15. As a result, it could be inferred that the ordered mesoporous structure of SBA-15 remained perfect after grafting of the functionalized ionic liquids, which was further evidenced by TEM analysis below.Structural properties of the samples were evaluated by N2 adsorption\u2013desorption technique. As shown in Fig.\u00a04\n, both of the SBA-15 and the hybrid SBA-15 exhibited type IV isotherms with clear H1 hysteresis loops, which is a characteristic feature of the typical mesoporous silica material according to the IUPAC classification scheme [41,42]. The increase of steep step in adsorption volume for parent SBA-15 in the ranges of 0.65\u20130.92\u00a0P/Po corresponded to the characteristic of capillary condensation in the mesopore, however as for hybrid SBA-15, they appeared in the range of 0.57\u20130.72\u00a0P/Po. The pore size distributions in Fig.\u00a0S6 displayed that the parent SBA-15 exhibited homogeneous distribution with average pore diameter of 6.6\u00a0nm. However, average diameter of the hybrid SBA-15 by ionic liquids decreased to around 5.7\u00a0nm. At the same time, it was noted that BET surface area and pore volume of the samples were declined evidently after hybridization of ionic liquids (Table 1), suggesting that the grafting of ionic liquids was in the pores, and the pore wall thickness was increased as compared with that of the parent SBA-15. These results suggested that the functionalized ionic liquids were mainly grafted inside the pore channels of SBA-15.The surface morphology and microstructure of the samples were investigated by SEM-EDS and TEM techniques. As shown in Fig.\u00a05\n(a), the parent SBA-15 exhibited long rod-like morphology and smooth surface with uniform and homogeneous particle size of around 1\u20133\u00a0\u03bcm. After hybridization of the functionalized ionic liquids, the surface morphology of SBA-15 with regular appearance in Fig.\u00a05(b) and Fig.\u00a0S7 was not obviously changed, but the particles were better dispersed, suggesting that the hybrid SBA-15 had good structure integrity and morphology. The EDS spectra of the ionic liquid hybrid SBA-15 were shown in Fig.\u00a0S8, and the existence of the corresponding elements in their structure further confirmed that functionalized ionic liquids were well bonded to SBA-15, consistent with the FT-IR analysis.To investigate the pore structure of these catalysts, TEM was used to probe the mesopore architecture of the particles. As shown in Fig.\u00a05(c), the parent SBA-15 displayed a two-dimensional hexagonal network and long-range mesopore architecture with parallel and perpendicular directions, and the pore diameter and wall thickness were 6.8\u00a0nm and 3.5\u00a0nm, respectively. After hybridization, the pore diameter decreased and the wall thickness increased, but the pore channels were not blocked (Fig.\u00a05(d) and Fig.\u00a0S9). Moreover, the values of wall thickness and pore diameter observed by TEM were in good agreement with those from XRD and BET investigation (Table 1). It may also be noted that the hexagonal order channels of SBA-15 were still kept well after hybridization.TG analysis was employed to investigate the thermal stability of the samples, and the TG curves were displayed in Fig.\u00a0S10. For the parent SBA-15, the weight loss below 100\u00a0\u00b0C could be assigned to the desorption of adsorbed moisture, and the weight loss over 100\u00a0\u00b0C was negligible. As we can see, the ionic liquids hybrid SBA-15 could endure the temperature of about 250\u00a0\u00b0C, the weight loss in the range from 250 to 450\u00a0\u00b0C was related to the removal of ionic liquid components from the silica surface, suggesting that these catalysts could be used for the reactions investigated in the present work owing to their high thermal stability. From the TG analysis, the ionic liquid contents were calculated as 0.68, 0.67, 0.64, 0.62 and 0.59\u00a0mmol g\u22121 for SBA-15-IL-CH3, SBA-15-IL-OH, SBA-15-IL-COOH, SBA-15-IL-SO3H and SBA-15-IL-NH2, respectively, which agreed with the results of element analysis shown in Table S2.The ionic liquids-SBA-15 hybrid catalysts with different functional groups were used for the synthesis of DPC, and the results of catalytic reaction were given in Table 2\n. It was shown that SBA-15 support was almost inactive for the synthesis of DPC due to the absence of active sites (Table 2, entry 1). However, hybridization of the functionalized ionic liquids on SBA-15 displayed excellent catalytic activity in real catalytic conditions (Table 2, entries 2\u20136), and the major by-products were anisole and phenol. The order of the activity was as follows [SBA-15-IL-SO3H]Br\u00a0>\u00a0[SBA-15-IL-NH2]Br\u00a0>\u00a0[SBA-15-IL-COOH]Br\u00a0>\u00a0[SBA-15-IL-OH]Br\u00a0>\u00a0[SBA-15-IL-CH3]Br, which was presumedly resulted from the influence of different active groups [43,44].Among these catalysts [SBA-15-IL-COOH]Br and [SBA-15-IL-NH2]Br showed high catalytic activities with more than 81% MPC conversion (Table 2, entries 4 and 5), but gave the lower DPC selectivity (about 90%). In this case, high by-product yields were ascribed to the decarboxylation of MPC and the methylation of MPC by DMC into anisole and phenol catalyzed by strong acidic or basic sites on the functional groups, because ionic liquids with these groups were strong active sites in some catalytic reactions [39,45,46]. Therefore [SBA-15-IL-SO3H]Br with stronger acidic \u2013SO3H group exhibited much higher MPC conversion (Table 2, entry 6). On the contrary [SBA-15-IL-CH3]Br revealed the higher DPC selectivity but a relatively lower MPC conversion due to the fact that \u2013CH3 group was weak acid site (Table 2, entry 2). It is interesting to note that [SBA-15-IL-OH]Br with moderately acidic strength of \u2013OH group showed the best catalytic performance regarding both the conversion and selectivity, giving 80.5% MPC conversion and 99.6% DPC selectivity (Table 2, entry 3). Compared with the other catalysts investigated here, the high catalytic activity of [SBA-15-IL-OH]Br may be attributed to the large surface area, superior pore size and abundant active sites [26,47,48]. Furthermore, the effect of the alkyl chain length of the cation with the same functional group and the counterpart anion was studied (Table 2, entries 7\u201310), revealing the decrease activity with the increase of the chain length, which suggested that the alkyl chain played an important role in the catalytic activities as reported in literatures for other reactions [49\u201352]. Meanwhile [SBA-15-IL-2C\u2013OH]Br using 50\u00a0mg was tested in the reaction, and 4.6% DPC yield was obtained, indicating that the reaction was carried out in no stoichiometry regime. In addition, the effect of anionic\u00a0structures of the ionic liquids was also examined for [SBA-15-IL-OH]Br catalyst, and BF4\n\u2212, PF6\n\u2212, HSO4\n\u2212 and OH\u2212 were chosen for this purpose. It was found that the anionic structures had a strong impact on the catalytic activities (Table\u00a02, entry 3 and entries 11\u201314), and at the same time the\u00a0high yields of by-products were also obtained. The catalytic activity of these anions decreased in the order: HSO4\n\u2212\u00a0>\u00a0OH\u2212\u00a0>\u00a0Br\u2212\u00a0>\u00a0BF4\n\u2212\u00a0>\u00a0PF6\n\u2212. By contrast, SBA-15-IL-OH with Br\u2212 anion showed the best catalytic performances (Table 2, entry 3), and was thought to be more suitable for the target catalytic reaction.The grafted content of ionic liquids had great influence on the yield of DPC, and the appropriate content could ensure that the catalyst had more active sites. Therefore, the effect of the grafted content of ionic liquid on the catalytic activity of [SBA-15-IL-OH]Br was examined in no stoichiometry and the result was displayed in Fig.\u00a06\n(a). It could be seen that the conversion of MPC increased from 40.1% to 80.6% as the ionic liquid loading increased from 4% to 12%, while the selectivity was not dependent on the loading content. When the loading content was above 12%, the conversion reduced gradually. In this case, the specific surface area was greatly declined and the pore channels of SBA-15 was also partially blocked as confirmed by BET measurements (Fig.\u00a0S11 and Table S3). Thus, the appropriate loading content was 12%.The effects of different parameters such as reaction temperature, catalyst dosage and reaction time were also investigated by using [SBA-15-IL-OH]Br as an example. As shown in Fig.\u00a06(b), the catalytic activity was sensitive to the variation of the reaction temperature. The conversion of MPC increased from 40.3% to 81.1% as the reaction temperature increased from 150\u00a0\u00b0C to 170\u00a0\u00b0C and the selectivity of DPC was almost 99% because the reaction was an endothermic process. However, the selectivity dropped to 91.9% when the reaction temperature was further raised to 190\u00a0\u00b0C, some by-products such as anisole and phenol were produced at such a high reaction temperature due to the decomposition of MPC decarboxylation and the methylation of DMC, which was similar to the results reported previously in literatures [14,17]. Thus, the suitable reaction temperature was 170\u00a0\u00b0C for this reaction system.The effect of catalyst dosage on the reaction was illustrated in Fig.\u00a06(c), increasing the catalyst dosage accelerated the conversion and [SBA-15-IL-OH]Br exhibited increased catalytic activity. When the catalyst usage increased from 0.4 to 0.8\u00a0g, the MPC conversion increased from 34.3% to 80.6% with a DPC selectivity of 99.5%, which could be attributed to the increase of the catalytic active sites. When catalyst usage was further reached to 1.2\u00a0g, the conversion increased to 81.6% but the selectivity was significantly reduced. This was plausibly correlated with the mass transfer between the catalyst and the reactants. When the catalyst was excessive, the heat transfer resistance could lead to more by-products, thus the selectivity decreased. Therefore, the optimal catalyst amount was 0.8\u00a0g.The change of MPC conversion and DPC selectivity with reaction time was shown in Fig.\u00a06(d). The MPC conversion increased from 36.2% to 80.4% as the reaction time increased from 1\u00a0h to 2\u00a0h, and the selectivity was almost 99.5%. Continue to extend the reaction time to 3.5\u00a0h, the conversion only increased to 82.6%, but the selectivity decreased a little. Thus, this result illustrated that the reaction could be nearly completed and reached the equilibrium within 2\u00a0h.Based on the above results, the optimal condition could be summarized as follows: a loading of 12% ionic liquid, reaction temperature of 170\u00a0\u00b0C, catalyst amount of 0.8\u00a0g and reaction time of 2\u00a0h. Under these optimal conditions, conversion of 80.5% for MPC and yield of 80.2% for DPC were achieved. Furthermore, the catalytic performances of SBA-15-IL-OH and other heterogeneous catalysts reported in the literatures were compared in Table 3\n. It can be seen that [SBA-15-IL-OH]Br may work at relatively mild reaction condition, and exhibited a good catalytic performance. And the TOF value up to 174 h\u22121 was obtained, which is three times higher than the highest value (58 h\u22121) reported in literatures by using traditional transition metal-based catalysts. Thus, it is worth noting that [SBA-15-IL-OH]Br is an efficient catalyst to avoid the use of metal-based chemicals.The hot filtration test is conducted to examine the heterogeneity of the ionic liquid hybrid SBA-15. This test was carried out in the presence of 0.5\u00a0g [SBA-15-IL-OH]Br at 170\u00a0\u00b0C for 1\u00a0h. Subsequently, the catalyst was removed by filtration and the reaction was continued for another 4\u00a0h in the absence of the catalyst. It was found that the yield of the product DPC in the filtrate was not changed (Fig.\u00a0S12), indicating that [SBA-15-IL-OH]Br was a stable heterogeneous catalyst.It is well known that the recyclability of the catalyst plays an essential role in practical application. In order to examine the reusability of [SBA-15-IL-OH]Br, reusable experiments were performed under the same reaction conditions. After completion of each reaction, the catalyst may be readily separated by filtration and the recovered catalyst was washed with acetone, dried under vacuum at 60\u00a0\u00b0C for 12\u00a0h, and then used for the next run. As shown in Fig.\u00a07\n, the catalyst could be reused for at least six times without obvious decrease of activity, and the conversion and selectivity were remained 77.6% and 99.4%, respectively. Besides, after the sixth cycle, the recovered catalyst was subjected to XRD, FT-IR and TEM measurements. It is noted from Figs. S13 and S14 that [SBA-15-IL-OH]Br still exhibited the consistent characteristic peaks after six cycles. Moreover, the morphology and pore channel of the recovered catalyst had no significant change compared with the fresh one (Fig.\u00a0S15). These results further verify that the catalyst possesses remarkable stability and reusability in this reaction, indicating the potential in the future application.Based on the results obtained in this work and reported in the literatures [47,48,51], a possible mechanism is proposed for the process over [SBA-15-IL-OH]Br, as shown in Scheme 4\n. First, the carbonyl group of MPC is activated by the hydrogen bonding interactions between C2\u2013H of the imidazolium cation and carbonyl group of the MPC (step 1). At the same time, the H atom of OH of the imidazolium cation coordinates with the O atom of carbonyl group of the MPC through a hydrogen bond interaction, which cooperatively activates the reactant molecules. Subsequently, the O atom of methoxyl group of another MPC molecule attacks the activated carbonyl group by nucleophilic addition reaction, thereby forming an interior ring carbonate complex (step 2). Finally, the carbonate complex is opened up to yield the desired DPC via an intramolecular nucleophilic substitution and catalyst regeneration to complete the catalytic cycle (step 3). In the catalysis process, there are more than one active site in the catalyst, they can active multiple MPC molecules simultaneously by hydrogen bonding interactions, which make the catalysis more efficient, thereby improved the catalytic activity and yield of DPC.DFT calculations were also performed to understand the possible mechanism of the reaction. The geometries of reactants, intermediates and products were fully optimized without any restriction at the theoretical level of M06\u20132X/6-311g (d,p) [47,51]. Frequency calculations were further carried out to confirm that these stationary points are real minimums on potential energy surfaces. The result of calculation was shown in Fig.\u00a0S16. The hydrogen bond of O--H1 (between C2\u2013H of the imidazolium cation and carbonyl group of the MPC) was formed with a bond length of 3.446\u00a0\u00c5 (Intermediate I) after complexation of [SBA-15-IL-OH]Br and MPC, and simultaneously the hydrogen bond between O--H2 (between H atom of OH of the imidazolium cation and the O atom of carbonyl group of the MPC) was formed with a bond length of 2.097\u00a0\u00c5, which displayed a stronger interaction with MPC than O--H1. The Gibbs free energy of intermediate I to the reactants is \u221221.9\u00a0kcal\u00a0mol\u22121, which indicates a spontaneous process. This suggests that MPC can be activated cooperatively by imidazolium C2\u2013H and hydroxyl. Subsequently, the C--O bond was formed with a bond length of 3.090\u00a0\u00c5 by intra-molecular nucleophilic substitution (Intermediate II), and the bond length of the O--H1 was elongated to 5.064\u00a0\u00c5 in this process that made the cleavage of carbonate complex much easier, thus accelerating the reaction. The Gibbs free energy of intermediate II is slightly higher than intermediate I, which could be remedied by the energy released from the C\u2013O cleavage in the reactant. These results demonstrate that the hydrogen bond interactions between [SBA-15-IL-OH]Br and MPC indeed played an important role in the promotion of the reaction investigated here.In summary, ionic liquids hybrid SBA-15 with different functional groups were prepared and used as metal-free catalysts for the effective and solvent-free synthesis of DPC. It was found that [SBA-15-IL-OH]Br with \u2013OH group showed the best catalytic performance with 80.5% MPC conversion and 99.6% DPC selectivity. Notably, the TOF value up to 174 h\u22121 was obtained, which was three times higher than the best value reported in literatures by using traditional metal-containing catalysts. This catalytic system could be easily isolated and recovered from the reaction mixture by filtration, and reused for six recycling without remarkable loss of catalytic activity. The findings reported here provides a new platform based on ionic liquids-SBA-15 hybrid materials as green metal-free catalysts to accomplish the highly efficient synthesis of DPC in the absence of solvent, revealing a remarkable potential for application in the future.There are no conflicts to declare.We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21808048 and U1704251), Training Plan for University's Young Backbone Teachers of Henan Province (2021GGJS121), Program for Science & Technology Innovation Talents in Universities of Henan Province (23HASTIT014), Postgraduate Education Reform and Quality Improvement Project of Henan Province (YJS2022KC22), Project funded by China Postdoctoral Science Foundation (No. 2018M632782), Project funded by Postdoctoral Research Grant in Henan Province (No. 001802030), Key Project of Science and Technology Program of Henan Province (No. 222102230109, 212102310330 and 182102210050), and the Science Research Start-up Fund of Henan Institute of Science and Technology (No. 2015031).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.2021.02.010.", "descript": "\n                  Diphenyl carbonate (DPC) is one of the versatile carbonates, and is often used for the production of polycarbonates. In recent years, the catalytic synthesis of DPC has become an important topic but the development of a highly active metal-free catalyst is a great challenge. Herein, a series of ionic liquids-SBA-15 hybrid catalysts with different functional groups have been developed for the synthesis of DPC under solvent-free condition, which are effective and clean instead of the metal-containing catalysts. It is found that in the presence of [SBA-15-IL-OH]Br catalyst, methyl phenyl carbonate (MPC) conversion of 80.5% along with 99.6% DPC selectivity is achieved, the TOF value is thrice higher than the best value reported by using transition metal-based catalysts. Moreover, the catalyst displays remarkable stability and recyclability. This work provides a new idea to design and prepare eco-friendly catalysts in a broad range of applications for the green synthesis of carbonates.\n               "}
{"full_text": "The progressive climate change is a globally relevant issue, and accordingly to the Intergovernmental Panel on Climate Change (IPCC) reports, humanity has so far caused a rise in global temperature of about 1\u00b0C. To avoid drastic adverse effects on biodiversity, the melting of ice caps, and further rise of sea levels, a value of 1.5\u00b0C related to the pre-industrial level should not be exceeded (Abram et\u00a0al., 2019). Regarding the potent greenhouse gas CO2, the average global atmospheric concentration exceeded 400 ppm in 2016, which is the highest level ever recorded. In addition, the worldwide fossil fuel emissions of CO2 increased by more than 2% in 2018 (Abram et\u00a0al., 2019). Therefore, strategies are needed that prevent further increase of the CO2 concentration in our atmosphere. The options discussed in this context are the capturing of carbon dioxide as well as its direct conversion. Along this line, the electrochemical reduction of CO2 via appropriate catalysts into value-added products is a promising strategy. The obtainable products include C1 compounds, such as CO, CH4, HCOOH, or CH3OH, as well as multicarbon products, e.g. acetic acid and ethylene, and C2+ alcohols (ethanol and propanol). This review focusses on the formation of multicarbon alcohols to summarize recent developments, which moved the electrochemical CO2 reaction a few steps closer to an industrial realization. Multicarbon alcohols are important target products in electrochemical CO2RR, as they are valuable basic chemicals for the chemical industry, can be used for energy production or as fuel additives (Jouny et\u00a0al., 2018). An additional route for ethanol production thus further diversifies the feedstock for various products.With regard to the current state of the art, only CO and HCOOH are commercially viable. However, market analyses show that by further development of catalysts, electrodes, and cells and with it a consequent reduction in energy and product separation costs, higher alcohols are promising products for the future (Jouny et\u00a0al., 2018; Somoza-Tornos et\u00a0al., 2021). They have a larger market potential than CO and formic acid (Jouny et\u00a0al., 2018). According to Jiao and coworkers, a yield of at least 62% should be achieved for n-propanol, and 77% for ethanol at \u22120.7\u00a0V to become economically feasible. The current densities should be in the range of 200\u2013400 mA cm\u22122 (Jouny et\u00a0al., 2018). A more detailed techno-economic analysis of the CO2RR products can be found in Review Electrochemical CO2 reduction - The macroscopic world of electrode design, reactor concepts & economic aspects.Multicarbon alcohols are formed during carbon dioxide reduction reaction (CO2RR) according to the following reaction equations:\n\n\n\n\n\n2\nCO\n\n2\n\n+\n\n\n9\nH\n\n2\n\nO\n+\n\n\n12\ne\n\n\u2212\n\n\u2192\n\nCH\n3\n\n\nCH\n2\n\nOH\n+\n\n\n12\nOH\n\n\u2212\n\n\n\n\n\n\n\n\n\n\n\n3\nCO\n\n2\n\n+\n\n\n13\nH\n\n2\n\n\n\nO\n+\n18\ne\n\n\u2212\n\n\u2192\n\nCH\n3\n\n\nCH\n2\n\n\nCH\n2\n\nOH\n+\n\n\n18\nOH\n\n\u2212\n\n\n\n\n\nBoth the mechanism for the formation of C2+ alcohols and catalysts that enable the selective electrocatalytic CO2RR to C2+ alcohols are considered. Furthermore, the influence of process conditions and techno-economic considerations are also explained in more detail.In addition, it can be highly effective to couple electrochemical CO2 reduction with biocatalyzed methods. Schmid and coworkers achieved an FE of almost 100% for the conversion of CO2 to butanol and hexanol with a fermentation following the CO2RR using bacterium Clostridium autoethanogenum and C. kluyveri (Haas et\u00a0al., 2018).In terms of the electrocatalytic conversion, different types of electrolysers are described in literature. Basically, they can be divided into three main types: liquid-phase, gas-phase, and solid-oxide electrolyser cell (Kibria et\u00a0al., 2019). The oxygen evolution reaction (OER) usually takes place at the anode of the electrolyzers and the CO2RR at the cathode. One ubiquitous and dominating problem with CO2RR in general is the competing, parasitic reduction of water to H2 (hydrogen evolution reaction, HER) (Lv et\u00a0al., 2018b; Albo et\u00a0al., 2019; Gabardo et\u00a0al., 2019; Gao et\u00a0al., 2019; Marti\u0107 et\u00a0al., 2019, 2020; Xiang et\u00a0al., 2019; Chang et\u00a0al., 2020; Dutta et\u00a0al., 2020; Kim et\u00a0al., 2020b; Song et\u00a0al., 2020; Wei et\u00a0al., 2020; Zhang et\u00a0al., 2020c; Herzog et\u00a0al., 2021; Wang et\u00a0al., 2021), which occurs in the same potential range as the CO2 reduction. Thereby, the Faraday efficiency for the formation of hydrogen in CO2RR with the target product ethanol is typically reported to be above 30% (Kim et\u00a0al., 2020b). However, especially for the CO2RR to higher alcohols, the preferential formation of ethylene is a further problem and numerous studies focus on the selectivity inversion between ethylene and ethanol (Gu et\u00a0al., 2021; Kim et\u00a0al., 2021; Santatiwongchai et\u00a0al., 2021; Wang et\u00a0al., 2021).To indicate the selectivity of a catalyst or electrode, the so-called Faraday efficiency (FE, Equation\u00a01) is given by\n\n(Equation 1)\n\n\nF\nE\n=\n\n\n\nz\n\n\u00b7\nn\n\u00b7\nF\n\n\nI\n\n\u00b7\nt\n\n\n\n\n\u00b7\n100\n\n\n\n(z - number of electrons transferred; n - amount of substance of product; F - Faraday constant, I - current applied; t - reaction time).Particularly for studies that focus on catalyst design and synthesis, H-type cells are widespread, despite the severe limitations of those systems (Burdyny and Smith, 2019). Its name is derived from its H-like form with cathode and anode compartments filled with liquid electrolyte, separated via an ion exchange membrane. The catalyst is usually deposited on glassy carbon or carbon paper and the CO2 is dissolved in the electrolyte. While this setup allows for simple and rapid testing of catalysts, it suffers from mass-transport limitation due to the low solubility of CO2 and can due to carbonate formation not be operated with alkaline electrolytes like KOH, which have been shown to improve CO2RR activity and C2+ selectivity (Carroll et\u00a0al., 1991; Kibria et\u00a0al., 2019). The low CO2 solubility and therefore availability limits the maximum current densities in H-type cells to about 100 mA cm\u22122, rendering them not feasible to be used in industrial CO2RR processes (Weekes et\u00a0al., 2018). However, as the local conditions and, thus, selectivity are highly dependent on the current density and potential applied, the results obtained in an H-type cell make it difficult to draw significant conclusions about the catalyst performance under industrially relevant conditions. Those limitations demand the use of alternative setups for testing and optimizing of catalysts under realistic conditions at higher current densities (Weekes et\u00a0al., 2018; Burdyny and Smith, 2019). This means that catalyst testing should be carried out under reasonable conditions like current densities of at least 200 mA cm\u22122 and stability tests of the catalysts and electrodes used of at least 24\u00a0h (Burdyny and Smith, 2019; Marti\u0107 et\u00a0al., 2019; Siegmund et\u00a0al., 2021).Therefore, flow cells or gas-phase electrolysers (by using membrane electrode assemblies) should be used, in which gas and electrolyte streams are continuously supplied and cycled, respectively, to achieve the industrially relevant current density of >200 mA cm\u22122 (Weekes et\u00a0al., 2018; Li et\u00a0al., 2019b; Marti\u0107 et\u00a0al., 2019).The following brief overview describes the mechanistic background of the formation of multicarbon alcohols during the electrochemical CO2 reduction.In addition to general considerations on the mechanism of C-C coupling at the beginning of the chapter, various mechanisms found for diverse catalysts are further presented with only few catalysts being addressed here as examples. A more detailed discussion of the different catalysts and their operating principles is given in chapters \"Structural properties and crystal orientations\" ff. For CO2RR, copper plays a special role here, because it can form a variety of products and is the only metal capable of forming higher hydrocarbons and oxygenates. The diversity of possible products obtained by copper catalysts illustrates the complexity of the reduction reaction (Hori, 2008; Kuhl et\u00a0al., 2012; Nitopi et\u00a0al., 2019). For systematic optimization, a comprehensive understanding of the underlying reaction mechanism is fundamental.In the electrochemical CO2 reduction process, an initial electrochemical transfer of H+/e\u2212 to CO2 occurs. The resulting intermediate can bind to the electrode surface either via oxygen or via carbon. In the former case, formation of HCOOH can be expected, whereas in the latter case CO (Figure\u00a01\n) is obtained, making this step crucial for the formation of the products in CO2RR (Cheng et\u00a0al., 2016; Feaster et\u00a0al., 2017; Chernyshova et\u00a0al., 2018). Thereby, CO is widely considered as a key intermediate for further reduced C1 and C2 products, supported by investigations on the reduction of CO as well as in situ measurements (Hori et\u00a0al., 1994, 1997; Wuttig et\u00a0al., 2016; Gunathunge et\u00a0al., 2017; P\u00e9rez-Gallent et\u00a0al., 2017b; Bertheussen et\u00a0al., 2018; Birdja et\u00a0al., 2019; Nitopi et\u00a0al., 2019).The C-C bond formation is the crucial reaction step that separates the pathways for single and multicarbon products. The dimerization of two \u2217CO species is commonly considered a key step for the C-C bond formation, resulting in bidentate \u2217CO\u2217CO as intermediate species. Figure\u00a01 shows the proposed mechanistic pathway for the formation of ethanol and n-propanol (Cheng et\u00a0al., 2021). The subsequent reduction steps to \u2217CO\u2217CHOH or \u2217CO\u2217COH have been considered as possible follow up intermediates (Calle-Vallejo and Koper, 2013; Kortlever et\u00a0al., 2015; Montoya et\u00a0al., 2015; Goodpaster et\u00a0al., 2016; Cheng et\u00a0al., 2017, 2021; Garza et\u00a0al., 2018; Hanselman et\u00a0al., 2018; Jiang et\u00a0al., 2018; Todorova et\u00a0al., 2020). Herein, the \u2217COCOH intermediate could be observed via in situ IR spectroscopy (P\u00e9rez-Gallent et\u00a0al., 2017a). Furthermore, operando Raman spectroscopy results suggest that the dimerization of \u2217CO is competing with the hydrogenation to \u2217COH or \u2217CHO, which are further reduced to C1 products (Todorova et\u00a0al., 2020). Along this line, C-C coupling steps via reaction of \u2217CHO or \u2217COH with CO to \u2217COCHO or \u2217COCOH also have been postulated (Goodpaster et\u00a0al., 2016; Xiao et\u00a0al., 2016; Garza et\u00a0al., 2018; Jiang et\u00a0al., 2018). Methylcarbonyl represents the most likely intermediate where a distinction takes place as to whether hydrogenation to ethanol or acetaldehyde occurs or whether further coupling with \u2217CO and thus the formation of propanol takes place. In this case, the \u2217CO attacks the carbonyl carbon of the acetaldehyde (Chang et\u00a0al., 2020).Notably, the structure and properties of the (copper) electrodes have a significant influence on the C-C coupling step (Gao et\u00a0al., 2019; Fan et\u00a0al., 2020). It has been shown that the selectivity of CO2RR is dependent on the exposed copper facets. For example, Cu(110) and Cu(551) facets promote the formation of C2 products (Hori et\u00a0al., 2002; Schouten et\u00a0al., 2012, 2013; Kim et\u00a0al., 2016). Engineering of catalyst size and morphology has been proven successful in steering the selectivity toward C2 products due to the exposed facets and differences in surface features like defect density, grain boundaries, and overall surface. Furthermore, various studies showed that morphological changes of the catalysts under the chosen process conditions have a significant effect on the product selectivity (Gregorio et\u00a0al., 2020; Hou et\u00a0al., 2020). In particular, too large particles as well as high current densities were identified as crucial parameters leading to aggregation and consequently to an altered product selectivity (Huang et\u00a0al., 2018). The influence of structure on the pursued reaction mechanism was investigated for oxide-derived (OD) copper. It was shown that step square sites (s-sq) support the formation of C2+ alcohols, due to favorable thermodynamics for hydrogenation. In addition, the bond length between CO and the active site was correlated with the observed preferential product formation. For example, ethanol is preferentially formed at s-sq sites, which have the shortest determined bond length of 1.296\u00a0\u00c5 compared to planar-square and concave square, where ethylene formation preferentially occurs (Cheng et\u00a0al., 2021). Another way of tuning catalyst selectivity is by adjusting the copper oxidation state. While the increased selectivity and activity of oxide-derived materials has partially been assigned to morphologic effects resulting from the reduction, results indicate that Cu+ and subsurface oxygen species play a role, too (Mistry et\u00a0al., 2016; Favaro et\u00a0al., 2017; Xiao et\u00a0al., 2017b; Luna et\u00a0al., 2018; Pander et\u00a0al., 2018; Zhou et\u00a0al., 2018). Recent results show that for copper-oxide-containing electrodes, reduction of the oxide layer occurs first before product formation due to CO2RR and HER (L\u00f6ffler et\u00a0al., 2021). The difference with pure copper electrodes is that the reduction of the oxide leads to the increased occurrence of defects and grain boundaries, resulting in a highly active surface.After C-C coupling, subsequent reduction steps lead to the multicarbon reduction products ethylene and ethanol. The possible intermediates and conceivable branching in the mechanistic pathway are, however, still under debate (Todorova et\u00a0al., 2020). Bell and coworkers described \u2217COCHO as first dimer intermediate followed by reduction to either glyoxal or \u2217CO\u2217CHOH, and depending on the products formed, the reaction pathway proceeds either ethanol or ethylene, respectively. Glyoxal is subsequently reduced to acetaldehyde and ethanol (Garza et\u00a0al., 2018). Acetaldehyde has been confirmed as an important intermediate toward ethanol formation via in situ NMR spectroscopy as well as mass spectrometry (Bertheussen et\u00a0al., 2016; Clark and Bell, 2018). Other authors describe (as also can be seen in Figure\u00a01) \u2217CO\u2217COH as the key coupling product, whereby the mechanism then follows a different path via the reduction to \u2217CCO. According to Goddard and coworkers, the next intermediate \u2217CH\u2217COH is either dehydrated to form \u2217CH\u2217C, which yields ethylene after another hydrogenation step, or to \u2217CHCHOH, which is converted to ethanol via three further hydrogenation steps (Cheng et\u00a0al., 2017; Xiao et\u00a0al., 2017a). According to Calle-Vallejo and coworkers, acetaldehyde is the selectivity determining intermediate, which is converted to either ethylene or ethanol after further reaction steps (Calle-Vallejo and Koper, 2013; Hanselman et\u00a0al., 2018). Contrarily, Asthagiri and coworkers postulated acetaldehyde and the two further hydrogenated species \u2217CH2CH2O\u2217 and CH3CH2O\u2217 as three possible points where the pathways diverge (Luo et\u00a0al., 2016). Hirunsit and coworkers mention the dissociation of the C-O bonds as most important for following the pathway either toward ethanol or ethylene formation (Santatiwongchai et\u00a0al., 2021). Investigations on Cu(100) surfaces have shown that the protonation steps five to seven are decisive and if the C-O bond is about to break later, EtOH will be formed instead of ethylene. To conclude, this work shows that the following intermediates lead to ethanol: \u2217CH3CO, \u2217CH3CHO, \u2217CH3CHOH, and \u2217CH3CH2O whereas \u2217CH2CH, \u2217CCH2, and \u2217CHCH lead to ethylene. \u2217CHCHOH, \u2217CH2CHO, \u2217HOCH2CH2O, \u2217CH2CH2OH, \u2217CH2CHOH, and \u2217HOCH2CH2OH are the intermediates which can result in either ethanol or ethylene formation.To increase the selectivity toward multicarbon alcohols, multimetallic catalysts are frequently used. For example, the ethanol to ethylene ratio could be increased by a factor of 12.5 by introducing zinc as a co-catalyst to copper (Ren et\u00a0al., 2016). This is where the so-called spillover effect occurs. The effect was described not only for Cu-Zn (Ren et\u00a0al., 2016) but also for Cu-Ag (Dutta et\u00a0al., 2020; Marti\u0107 et\u00a0al., 2020; Ting et\u00a0al., 2020), Cu-Pd (Rahaman et\u00a0al., 2020), and for catalysts with Cu nanoparticles and pyridinic nitrogen in N-doped carbon (Han et\u00a0al., 2020a). One of the mechanisms proposed for bimetallic catalysts is shown in Figure\u00a02\n. In this process, CO2 is reduced to CO at Zn, Ag, Pd, or pyridinic N sites, where CO is only weakly adsorbed (Ren et\u00a0al., 2016; Han et\u00a0al., 2020a; Rahaman et\u00a0al., 2020) and CO migration to active copper sites can be achieved. There, CO is bound superiorly and will either be further reduced or undergo further reactions with adjacent \u2217C1 and \u2217C2 intermediates (Han et\u00a0al., 2020a). With respect to the Cu-Ag-containing catalysts, the ratio of Cu: Ag is expected to have a direct influence on the product distribution\u00a0due to an altered electronic structure (Marti\u0107 et\u00a0al., 2020). The interaction of copper and silver results in a shift of the Ed value, which represents the location of the center of the d-band, from that of copper at-3.30 eV by \u22120.56 eV toward that of silver (\u22125.36 eV). The electronic change results in less binding of CO2RR and HER intermediates, leading to preferential CO formation with FEs ranging from 55% to 68%. The main liquid product was ethanol with about 25% FE at 400 mA cm\u22122. Furthermore, the selectivity of 34.2% for ethanol in phase-blended Ag-Cu catalysts has been shown to be three times higher than with pure Cu2O (Lee et\u00a0al., 2017). The authors emphasized the importance of the biphasic boundary for improved ethanol to ethylene selectivity. Upon modification of the distance between CO-producing Ag and Cu sites, increased insertion of CO and consequently formation of EtOH (demonstrated by \u2217C2) can be achieved (Figure\u00a03\n).Ag-Cu foams could be activated for ethanol production via a 12\u00a0h thermal annealing in air at 200\u00b0C. The obtained oxide-derived bimetallic catalyst showed a maximum FE of 33.7% for ethanol at \u22121.0\u00a0V and 6.9% for propanol at \u22120.9\u00a0V vs. RHE, while the formation of those products was negligible without the mentioned thermal treatment of the catalyst (Dutta et\u00a0al., 2020).In addition to the spillover effect in bimetallic compounds, the combination of Cu nanorods (nr) and NGQ (nitrogen-doped graphene quantum dots) also enables an interesting mechanism. Oxygenated C2 intermediates were stabilized at the NGQ/Cu-nr, and by allowing both Cu-nr and NGQ to form C2 products, the formation of the multicarbon products is promoted by dual active sites. On both components, the existence of \u2217CO as intermediate could be detected, but there was no evidence for a spillover or tandem effects (Chen et\u00a0al., 2020a). Both effects describe the same process from a different point of view. However, while the term spillover effect describes the adsorption of the CO formed and its migration on the catalyst surface, the term tandem effect refers to the catalyst, i.e. that it has different domains on which different reaction steps take place. Therefore, a dual active-site mechanism was suggested, indicating the presence of active sites in NGQ as well as in Cu for the formation of C2+ products. In addition, the catalyst was found to stabilize the intermediate \u2217CH2\u2217CHO, which is crucial for the higher FEs (52.4%) of multicarbon alcohols.Heteroatom-doped nanostructured carbon materials have also been examined as catalysts for the reduction of CO2 to alcohols. Their performance can be tuned via the nature and amount of heteroatom sites as well as the carbon morphology (Wu et\u00a0al., 2019). A nitrogen- and boron-doped nano diamond catalyst reached a high ethanol selectivity of 93.2% at \u22121.0\u00a0V vs. RHE due to the synergistic effects of the heteroatom sites. The measurements were performed in H-type cells, with a CO2-saturated 0.1\u00a0M NaHCO3 electrolyte, and the total current densities were below 2 mA cm\u22122 (Liu et\u00a0al., 2017). Because boron has an electron-poor p-orbital, it acts similarly to transition metals with an empty d-orbital and thus represents an active site for adsorption and subsequent reduction of CO as well as for CO2 (Zhu et\u00a0al., 2021). For nitrogen-doped porous carbons, the high ethanol selectivity of 77% and 78% at \u22120.56\u00a0V vs. RHE has been attributed to synergistic effects between the carbon structure and active sites (Song et\u00a0al., 2017, 2020). In addition, P-doping of catalysts could be used to adjust the adsorption strength for the CO intermediate. Thus, with P-doping, 2.8 times as much ethanol (15%) could be obtained with Cu0.92P0.08 C2+ product yield (Kong et\u00a0al., 2021). Likewise, catalysts combining doped nanocarbons and copper catalysts have been described, reporting, e.g. tandem effects of heteroatom and metal sites with up to 64.8% FE for ethanol and 8.7% for propanol at \u22121.05\u00a0V vs. RHE (Song et\u00a0al., 2016; Karapinar et\u00a0al., 2019; Han et\u00a0al., 2020a). However, it must be emphasized that the FEs of over 60% for ethanol achieved herein by different groups obtained under conditions of extremely low current densities between 2 and 16 mA cm\u22122. Hence, further improvements in systems allowing for higher current densities above 200 mA cm\u22122 are required to establish an industrial relevant process.In general, many mechanistic insights are obtained using computational methods such as DFT. Here, DFT is often used to show potential pathways for a target-oriented catalyst design and can reveal mechanistic information, e.g. regarding detailed reaction pathways (Li et\u00a0al., 2020; Malkani et\u00a0al., 2020; Santatiwongchai et\u00a0al., 2021). The use of in situ techniques such as isotope labeling or the application of in situ spectroscopy such as XAS (X-ray absorption spectroscopy) or surface-enhanced vibrational spectroscopy methods can further help to complete the mechanistic understanding (P\u00e9rez-Gallent et\u00a0al., 2017a; Malkani et\u00a0al., 2020; Wang et\u00a0al., 2020b). Studies of surface reconstruction in copper electrodes during CO2RR were e.g. conducted in 2017 by Waegele and coworkers as well as Koper and coworkers using Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR) (Gunathunge et\u00a0al., 2017; P\u00e9rez-Gallent et\u00a0al., 2017a). Furthermore, XAS has already been used to study the electronic as well as the coordinative structure on Cu catalysts during ongoing CO2RR (Xu et\u00a0al., 2020; Herzog et\u00a0al., 2021). Xu and coworkers describe in detail the advantages that in situ techniques offer, such as identifying the metals that provide the adsorption sites in the electrocatalytic reaction and analyzing metal-adsorbate interactions (Malkani et\u00a0al., 2020). This contributes to a broader understanding of the mechanistic processes involved in CO2RR and for a more in-depth discussion on these techniques we refer to such papers.\nTable 1\n provides an overview of recent developments in CO2RR to multicarbon alcohols, including the catalysts and electrolytes used as well as the resulting Faraday efficiencies. Firstly, copper and copper oxide as well as copper-oxide-derived (OD) catalysts are listed, followed by copper-carbon catalysts as well as copper catalysts, which were doped e. g. with boron or modified with halides, catalysts made of copper and another metal, and lastly miscellaneous catalysts, which do not fit in one of the categories mentioned before. The dominant usage of copper can be explained by its ability of producing multicarbon products during the reduction of CO2 (Loiudice et\u00a0al., 2016; Garza et\u00a0al., 2018; Karapinar et\u00a0al., 2019; Malkhandi and Yeo, 2019; Jeong et\u00a0al., 2020; Lei et\u00a0al., 2020). The use of Cu electrodes in CO2 reduction experiments allows for the formation of a broad variety of products. Cyclic voltammetry (CV) measurements yielded CO, allyl alcohol, propionaldehyde, n-propanol, acetaldehyde, EtOH, ethylene, and methane in varying amounts and ratios (Clark and Bell, 2018). The table furthermore summarizes the FEs of the respective products. Thereby, it becomes visible that the selective formation of multicarbon alcohols still possesses a challenge. The products marked \u201cC2+\u201d usually contain high amounts of C2H4, which is often the main reason for the high overall FEs. This effect is a result of the fact that ethylene is generally preferred to ethanol formation in copper-based electrodes (Ren et\u00a0al., 2016).Nevertheless, catalysts of various compositions already achieved FEs above 50% for multicarbon products. Best results were obtained with up to 85% FEEtOH using Ag-graphene-NCF (Nano Carbon Fibers) as the catalyst, but the resulting current density was less than 1 mA cm\u22122, essentially not allowing any conclusive results on potential applications in larger scale (Lv et\u00a0al., 2018b). Catalysts made of Cu-N-C (Karapinar et\u00a0al., 2019), Cu-NPC (Han et\u00a0al., 2020a), or consisting of micropores in N-doped mesoporous carbon (Song et\u00a0al., 2020) also reached high Faraday efficiencies above 55% for ethanol. However, all of these catalysts/electrodes were operated at industrially irrelevant current densities of less than 20 mA cm\u22122. An intriguing question is what the performance or product distribution of these catalysts and electrodes will be at higher current densities. In contrast, higher current densities with simultaneously increased FEs for ethanol were obtained with Cu sputtered on PTFE and NC (FEEtOH 52% at partial current densities of 156 mA cm\u22122) (Wang et\u00a0al., 2020b) or N-doped graphene quantum dots on Cu-OD Cu nanorods with a FEC2+ alcohol of 52.4% at a total of 282 mA cm\u22122 (Chen et\u00a0al., 2020a).As can also be seen from Table 1, the most frequently used electrolytes are KHCO3, CsHCO3, and KOH. However, because the influences on the resulting selectivity of the catalysts is multifactorial and involves not only the electrolyte but also other aspects such as cell design, membrane, temperature, and other parameters, the influence of those is discussed in detail in the chapter \u201cProcess Conditions\u201d.The syntheses of solid electrocatalysts, which are capable of producing ethanol during the electrochemical reduction of CO2, are manifold. In the most common cases, precipitation methods or electrodeposition were used, as well as sputtering of thin films. To further optimize the performance of the catalysts, surface modifications or reconstructions were also frequently carried out, or the catalyst layer was created by means of evaporation (e.g. via chemical vapor deposition).During electrodeposition, the catalyst is plated directly onto a substrate from an electrolyte solution, whereby the substrate is used as a working electrode and the deposition can be galvanostatic or potentiostatic. Electrodeposition has so far been used to coat gas diffusion layers, like carbon paper (Aeshala et\u00a0al., 2012; Hoang et\u00a0al., 2017, 2018; Lee et\u00a0al., 2017; Kong et\u00a0al., 2021), but also other substrates like metal foams, polished Cu discs, or Cu foil (Dutta et\u00a0al., 2016, 2020; Ren et\u00a0al., 2016; Rahaman et\u00a0al., 2017, 2020; Kim et\u00a0al., 2020b), which were then often applied in H-cells. Often, these catalyst materials were deposited from sulfuric acid, CuSO4, and other metal-sulfate-containing electrolytes (Aeshala et\u00a0al., 2012; Dutta et\u00a0al., 2016, 2020; Ren et\u00a0al., 2016; Hoang et\u00a0al., 2017, 2018; Rahaman et\u00a0al., 2020). In addition, additives such as sodium citrate (Dutta et\u00a0al., 2020) or citric acid (Kong et\u00a0al., 2021), 3,5-diamino-1,2,4-triazole (DAT) (Hoang et\u00a0al., 2017, 2018), as well as lactic acid (Ren et\u00a0al., 2016; Lee et\u00a0al., 2017) were added to the electrolyte solution. Sodium citrate was used in the deposition of Ag15Cu85-foam on Cu foil (Dutta et\u00a0al., 2020). The deposition was realized from silver and copper(II)-sulfate-containing electrolyte at 3 A cm\u22122. In this process, the competing HER commonly results in the formation of gas bubbles as a geometric template for foam formation. Figure\u00a04\n schematically shows the process of deposition of porous copper using the resulting hydrogen as a template. The sodium citrate used should have an impact on the growth characteristics through chemisorption at the cathode surface. However, the electrodeposition of foams on Cu wafers was also successfully carried out without additives using sulfuric acid/CuSO4 solution at 3 mA cm\u22122 (Dutta et\u00a0al., 2016). Owing to the mesoporous structure of the resulting Cu foam, there is an increased formation of C2 products such as ethane and ethylene. In the case of the Cu-Ag foams, subsequent calcination at 200\u00b0C and the associated formation of Cu2O also led to increased Faraday efficiencies for EtOH and PrOH of up to 33.7% and 6.9%, respectively (Dutta et\u00a0al., 2020). Calcination was also carried out following the electrodeposition of Cu dendrites on electropolished meshs (Rahaman et\u00a0al., 2017) and a Cu-Pd foam on Cu foils (Rahaman et\u00a0al., 2020), to activate the catalyst as this thermal treatment may result in a higher FE for ethanol instead of CO due to segregation of the phases (Dutta et\u00a0al., 2020). Zeng et\u00a0al. electrodeposited Cu onto carbon paper and used thermal annealing to dope the Cu with phosphorus at 400\u00b0C and under N2 atmosphere using NaH2PO2\u2219H2O (Kong et\u00a0al., 2021). The yield of C2+ products was thus increased by 1.9 times, and the FE for EtOH was even 2.8 times higher (15%) than without any doping. Another used additive is (3,5-diamino-1,2,4-triazole) DAT, which acts as an inhibitor for Cu deposition before reaching \u22120.18\u00a0V (vs RHE) (Hoang et\u00a0al., 2017, 2018). As a result, it was possible to deposit Cu films with a high surface area and activity for CO2RR. It was possible to achieve 5\u20136 times higher current densities, when DAT was used as an additive in the deposition process than without (Hoang et\u00a0al., 2018). Lactic acid was also used as an additive as it stabilizes the Cu ions in the solution (Ren et\u00a0al., 2016; Lee et\u00a0al., 2017).Another commonly used method for catalyst synthesis is precipitation, and a broad variety of starting materials and products have been used or obtained. In some cases, the syntheses were carried out in the microwave, such as a precipitation reaction for Bi-MOFs (Albo et\u00a0al., 2019), or in autoclaves as in the synthesis of Cu(OH)F from Cu(II) nitrate in DMF, with the addition of NH4HF2 for 4\u00a0h at 160\u00b0C (Ma et\u00a0al., 2020b). In CO2RR, the resulting Cu(OH)F catalyst enabled the formation of C2+ products with FEs up to 65.2% at a maximum current density of 800 mA cm\u22122. Another example of the use of autoclaves is the preparation of paramelaconite (Cu4O3) from a Cu(II) nitrate-DMF-EtOH mixture after the addition of formic acid and dimethylamine at 130\u00b0C (Marti\u0107 et\u00a0al., 2019). The catalyst achieved an FE for C2+ products of over 61%. Cu nanoparticles can also be prepared by precipitation over the formation of copper oxides as shown by Jiao and coworkers. They precipitated Cu(OH)2 nanorods from a mixture of aqueous copper nitrate solution with ammonia and converted them to porous CuO by thermal annealing in the following (Lv et\u00a0al., 2018a). After applicating the nanorods onto a GDL, the reduction to copper nanoparticles was performed at 10 mA cm\u22122. Another example is the precipitation of Cu(OH)2 followed by thermal annealing under a H2/Ar atmosphere. The catalyst was then partially oxidized by storing it in air before applying it to CO2RR (Shang et\u00a0al., 2019). This procedure yielded core-shell Cu@Cu2O catalysts, which led to an FE of EtOH of 29% during CO2RR. Another example is the precipitation of Cu nanoparticles from a Cu(II)-containing solution with the addition of NaBH4 (Ma et\u00a0al., 2016; Wei et\u00a0al., 2020), which led to an FE for C2+ products of up to 80% (Wei et\u00a0al., 2020). If a NaBH4 solution is combined with a CuCl2 solution, boron-doped copper can be obtained as a precipitate, which achieves FEs for EtOH up to 27% (Zhou et\u00a0al., 2018). In addition to pure Cu precipitates, mixed oxides as well as other compounds with several metals have been successfully synthesized via precipitation and used for electrochemical CO2 reduction. For example, a catalyst of graphene oxide, ZnO, and Cu2O was prepared by precipitation and produced up to 30% propanol (Geioushy et\u00a0al., 2017). Precipitation of Ag2Cu2O3 with aqueous NaOH from a solution containing Cu and Ag nitrate was also successfully carried out under inert conditions, and the Faraday efficiency for this catalyst was about 25% for EtOH (Marti\u0107 et\u00a0al., 2020). Another catalyst that produced nearly 30% ethanol, when used in a GDE, is a CuPb-0.7/C (Pb shell thickness is 0.7\u00a0nm) catalyst, which was precipitated from a copper acetate, PbCl2, ascorbic acid, diphenyl ether, and oleylamine-containing solution (Wang et\u00a0al., 2020a). With 25% FE, slightly less ethanol was produced by V-Cu2S nanoparticles, which were prepared using Cu acetylacetone and dodecanethiol (Zhuang et\u00a0al., 2018).Another method for the preparation of catalysts or electrodes, which has already led to materials providing high Faraday efficiencies at industrially relevant current densities, is sputtering. In most cases, Cu was sputtered from a pure Cu target onto a PTFE membrane (Dinh et\u00a0al., 2018a; Gabardo et\u00a0al., 2019; Garc\u00eda de Arquer et\u00a0al., 2020; Li et\u00a0al., 2020) (pore size 0.45\u00a0\u03bcm). Subsequently, either carbon black (Dinh et\u00a0al., 2018a; Gabardo et\u00a0al., 2019), graphite (Gabardo et\u00a0al., 2019), Nafion, or a mixture of Nafion and Cu-NPs (Garc\u00eda de Arquer et\u00a0al., 2020) was spray coated onto the sputtered layer. Spray coating of porphyrin-based complexes (FeTTP) onto sputtered copper ultimately resulted in an FE for ethanol of 41% (Li et\u00a0al., 2020), as did co-sputtering of Cu and Ag onto a PTFE membrane (Li et\u00a0al., 2019b). The highest EtOH yield was obtained with an FE of 52% by first sputtering Cu and then a layer of N-C onto the membrane (Wang et\u00a0al., 2020b).Some catalysts were also synthesized via evaporation and vapor deposition onto a substrate. For example, compared to pure Cu, alcohol formation occurred at >265\u00a0mV more positive electrode potentials on a polycrystalline Cu foil coated with gold (Carlos G. Morales-Guio et\u00a0al., 2018). Furthermore, CVD of boron- and nitrogen-doped diamond on a Si substrate was performed and the resulting electrode led during CO2RR to an FE of 93.2% for ethanol, but with current densities below 2 mA cm\u22122 (Liu et\u00a0al., 2017).For modifying or reconstructing the surface of Cu foils/substrates, various ways including electrochemical and plasma activation were used. However, the resulting catalysts were always used in H-type cells, which lead to very low current densities. One possibility of surface modification for copper foil is to cyclize it. For example, the FE for ethanol could be increased from 2.2% to 7.7% by cyclizing the foil for three cycles between \u22121.1 and 0.9\u00a0V for 20\u00a0mV/s in a 0.1\u00a0M KHCO3 solution, containing 4\u00a0mM KCl (Schouten et\u00a0al., 2011). Cyclization in copper nitrate solution led to the formation of single crystal Cu2O nanocubes and an FE for C2+ products of 60% was obtained (Jiang et\u00a0al., 2018). Another possibility to modify the catalysts surfaces is plasma activation in O2 plasma (FEC2+ 69%) (Gao et\u00a0al., 2018) or heating a copper substrate in an oven to 1100\u00b0C followed by quenching in air, leading to the formation of sponge-like structures and an FE for C2+ products of 70% (Lei et\u00a0al., 2020). Wet chemical modification of the surface by oxidation with H2O2 and diluted HCl leads to the formation of CuCl on the surface, followed by the formation of Cu2O by immersion in KHCO3 (Kibria et\u00a0al., 2018). Subsequent electrochemical CO2 reduction then led to FEs for C2+ products above 80%. Also, modification with halides was obtained by immersing Cu foils in solutions containing CuBr2 (Wang et\u00a0al., 2021). Here, CuBr tetrahedrons formed on the surface which were subsequently immersed and thus uniformly coated in dodecanethiol. The application of the coated catalyst in CO2RR resulted in almost 36% FE for EtOH.Apart from the synthesis routes described so far, various catalyst syntheses can be found which were only used by a few groups including special synthesis routes\u2014e.g. a 4-step organometallic synthesis of co-corroles (48% EtOH, \u22120.56 V, total 2.5 mA cm\u22122) (Gonglach et\u00a0al., 2019), the synthesis of Cu-N-C by low-energy ball milling followed by pyrolysis in an argon stream (55% EtOH, \u22121.2 V, total 16 mA cm\u22122) (Karapinar et\u00a0al., 2019), the impregnation of melamine foam in a silver nitrate-graphene oxide solution followed by calcination (79%\u201385% EtOH, \u22120.5 to \u22120.7 V, total 0.3 mA cm\u22122) (Lv et\u00a0al., 2018b), or the preparation of carbon supported Cu catalysts by using an amalgamated Cu-Li method (91% EtOH. \u22120.7 V, total 1.2 mA cm\u22122) (Xu et\u00a0al., 2020).Regarding the synthesis and study of electrocatalysts versus industrial applicability, our group has recently published a perspective article (Siegmund et\u00a0al., 2021). There we defined the following evaluation criteria: (1) The issue of complexity and price required to synthesize the catalyst: Synthesis routes such as multiple steps synthesis are considered problematic in this regard, as they are accompanied by great complexity, as well as costly purification steps. Precipitation reactions, sputtering, or electrodeposition, on the other hand, are in simple principle and can be carried out in just a few steps. The processes described under \u201cSurface modification\u201d can also be described as predominantly less complex. (2) The issue of producing the catalyst in sufficiently large quantities (Siegmund et\u00a0al., 2021): E.g. it is possible to sputter large areas without any problems, which is already used for the production of thin-film solar cells (Edoff, 2012). Precipitation reactions are also common processes in industry and have the potential to be carried out on a large scale, as does electrodeposition of metals. However, individual considerations would need to be given to each catalyst synthesis in terms of its scalability. More problematic are synthesis routes which contain discontinuous processes, e. g. evaporation processes. (3) The issue of (long-term) stability of the catalyst materials at relevant current densities (Siegmund et\u00a0al., 2021): Some catalyst materials mentioned above have already been tested for their stability over longer time periods, e.g. Co-corroles showed stable electrolysis over 140 h, but at very low current densities of \u22122.5 mA cm\u22122 (Gonglach et\u00a0al., 2019). Also, sputtered electrodes were already stable over 150\u00a0h electrolysis (at up to 100 mA cm\u22122) (Dinh et\u00a0al., 2018a). For many of the catalysts, however, evidence of long-term stability under industrially relevant conditions is lacking, which is urgently needed to evaluate the applicability of the materials.In addition to the composition of the catalyst, its surface morphology and crystal face orientation were determined to be decisive factors in the selective reduction of CO2 to C2+ alcohols and therefore the factors that increase the FE for multicarbon product formation are discussed here.Several studies have already shown that Cu(100) surfaces are more selective for C2+ products, while Cu(111) is more likely to lead to the production of CH4 (Jiang et\u00a0al., 2018; Wang et\u00a0al., 2019; Gregorio et\u00a0al., 2020; Han et\u00a0al., 2020b; Ting et\u00a0al., 2020). However, an excess of CO at Cu(111) sites could also lead to EtOH formation. Cu(100), on the other hand, supports the dimerization of \u2217CO, which is formed as intermediate (Han et\u00a0al., 2020b). The selectivity via the surface orientation is also evident when using Cu nanocubes and Cu nanospheres. As more Cu(100) is present on the surface in the former, the ethylene formation under alkaline condition is more pronounced (Jiang et\u00a0al., 2018; Wang et\u00a0al., 2019). Another example for the advanced C-C coupling on Cu(100) can be observed on CuCl-derived Cu electrodes as they show an increased selectivity for C2 products (Kibria et\u00a0al., 2018). Compared to electropolished electrodes, those CuCl-derived ones show a change in preferential crystal orientation from Cu(111) to Cu(100). Upon transition from Cu(111) to Cu(100), FEs for C2+ products increased from 30% to 73%, that of propanol from 0% to 5%.When comparing Cu cubes and Cu octahedrons, the formation of C2H4 was also highest at the cubes, whereas CH4 formation was more pronounced at the octahedrons (Gregorio et\u00a0al., 2020). Furthermore, it was shown, using Cu-Zn catalysts as an example, that the roughness factor of the surface directly influences the product distribution. Higher roughness correlated with higher FEs for C2+ products (da Silva et\u00a0al., 2020). Figure\u00a05\n shows the influence of surface morphology on CO2RR in terms of C2+ product distribution and the influence of Cu-Zn ratio on catalytic activity. While the Faraday efficiency for the formation of C2+ products increases with increasing surface roughness, it simultaneously decreases for CH4 and H2 (Jeong et\u00a0al., 2020). The presence of corners and steps on the surface promotes the adsorption of C1 products and this, in turn, leads to an improvement in the dimerization to C2+ products (Hoang et\u00a0al., 2018). The improvement in C2+ production due to both more sharply defined structures and more curved surfaces is expected to occur as a result of improved bubble nucleation, a concentration of stabilizing cations as well as high local fields and thus increased current density (Luna et\u00a0al., 2018). Electro-redeposition is expected to lead to these electronic and morphological effects, which improves selectivity and activity of Cu in the production of C2+ during CO2RR. Furthermore, the yield of C2-C3 products could be significantly increased by in situ structural transformation of densely packed Cu-NPs by electrolysis to cube-shaped catalytically active structures (Kim et\u00a0al., 2017).Besides the surface roughness, porosity also plays an important role in the electrochemical performance of CO2RR (Han et\u00a0al., 2020a). For example, the transport of CO2 through the electrolyte-electrode interface at high current densities is facilitated when using GDEs with highly porous structures (Lv et\u00a0al., 2018a). In addition, the micropores are also expected to play an important role in the adsorption capacity of CO2 by the catalysts (Han et\u00a0al., 2020a).In addition to the catalysts themselves, the type of electrode and its manufacture also have a significant influence on the final performance in CO2RR (Tan et\u00a0al., 2020). Catalyst ink-based preparation techniques, for example, offer the possibility to influence catalyst surfaces via multiple parameters. In addition to using different techniques such as dropcasting, airbrushing, or hand painting, the drying temperature can also be adjusted. Overall, thinner porous catalyst layers, e.g. obtained by dropcasting or hand painting, should result in fewer C2+ products being formed. If, on the other hand, the catalyst layer is enlarged, there is better CO2 mass transfer within the porous layer. Simulations suggest that the layer thickness is more important than the porosity for controlling the local concentration of CO2 (Tan et\u00a0al., 2020). In addition to the layer thickness, the loading of catalyst also influences the results. For example, the study of Cu-NPs in combination with pyridinic N species in N-doped porous carbon showed that a copper loading of 10% was not sufficient, whereas 30% was too much and led to preferential ethylene formation instead of EtOH and PrOH. The highest yields for multicarbon alcohols were obtained at 20% Cu loading (Han et\u00a0al., 2020a).Whether to use copper, copper oxide, or OD-copper electrodes is a frequently discussed topic. In comparison to pure copper electrodes, oxide-derived copper electrodes contain remaining oxides, which should simplify the adsorption of \u2217CO and the C-C coupling (Ting et\u00a0al., 2020). Thus, OD-Cu should increase the selectivity for C2+ products (Iijima et\u00a0al., 2019). Furthermore, investigations have shown that a thin layer of metastable Cu2O on an electrode made of OD-Cu can result in an increase in selectivity in favor of C2 products due to an improved stabilization of intermediates of CO2RR (Shah et\u00a0al., 2020). It was also shown that current densities are higher on plasma-activated Cu foil (CuO2) than on electropolished Cu (Singh et\u00a0al., 2016; Gao et\u00a0al., 2018). This is not due to structural changes but can rather be understood as a chemical effect of Cu+ species. An increase in the product ratio for CO2RR of C2+/C1 with FEs of up to 61% for C2+ products was also shown by using a GDE, which is carbon-based and contains Cu derived from Cu4O3. Partial current densities of 185 mA cm\u22122 were obtained and due to the same reaction paths and intermediates of EtOH and ethylene, an increase in both C2H4 and EtOH yield was obtained by improvement with OD-Cu compared to normal Cu (Marti\u0107 et\u00a0al., 2019). An improvement in C2+ selectivity was also recently achieved by using Cu catalysts with nanocavities in which carbonaceous intermediates are trapped (Yang et\u00a0al., 2020). The intermediates would not only cover the surface of the catalyst but also stabilize the Cu+ present there, which is thus also retained during CO2RR and allows the selectivity to be increased (75.2% FE at 267 mA cm\u22122) (Yang et\u00a0al., 2020).However, other investigations show that only metallic copper is active, while oxides remaining in OD electrodes are unstable and inactive under CO2RR conditions during catalysis (Ting et\u00a0al., 2020). Spectroscopic investigations have shown that there is a low CO intermediate formation on Cu2O, resulting in a low activity toward CO2RR. According to Han and coworkers, CO2RR takes place on Cu0 and not Cu+ or Cu2+ and the oxides are not decisive for selectivity toward C2+ products. Instead, they examined the grain sizes and found a decrease of selectivity in the order Cu0 > Cu+ > Cu2+, and that the reduction of the oxides leads to fragmentation and thus to an increase in surface roughness (Lei et\u00a0al., 2020). A direct comparison of electropolished Cu electrodes with those containing Cu oxide or Cu hydroxide showed that electrodes containing Cu oxides or hydroxide showed better selectivity for C2+ products while suppressing the formation of CH4 (Lei et\u00a0al., 2020). The best results were obtained with Cu oxide electrodes with an FE of 68.2% and up to 64 times higher current densities than the pure Cu electrode. In the catalyst, three Cu species coexisted in different layers\u2014Cu0, Cu+, and Cu2+. Within 1\u00a0h of CO2RR, all species were reduced to Cu0, but fragmentation to irregular nanoparticles also took place. The resulting network shows an enrichment of highly active sites, which facilitates CO adsorption. Furthermore, more high-index facets were exposed. These effects resulted in the improved selectivity (Lei et\u00a0al., 2020). An investigation on Cu(100) surfaces using pulsed potential sequences (0.6\u00a0V and \u22121.0\u00a0V for 1\u00a0s each) also led to an increase in selectivity for C2+ products. While potentiometric measurements at \u22121\u00a0V on Cu single-crystal electrodes achieved FEs for EtOH of 8% and for ethylene of 45%, the overall value increased to 76% for the products, with ethanol FEs around 30%. The increased selectivity for ethanol is explained via a continuous in situ regeneration of Cu(I) and thus the co-existence of Cu(I) present as Cu2O and Cu(0) on the surface, the Cu(100) domain, and the defect sites (Ar\u00e1n-Ais et\u00a0al., 2020).Sargent and coworkers showed with the help of XAS measurements of GDEs that a direct reduction to metallic copper in the catalyst layer was achieved within 16 s, which implies that Cu0 is responsible for the selectivity toward EtOH and not the presence of oxides (Wang et\u00a0al., 2020b). In addition to the question to what extent oxides themselves have an influence on the selectivity of CO2RR at Cu electrodes, the influence of interparticle distances between CuOx nanoparticles was also investigated (Jeong et\u00a0al., 2020). It was shown that increasing the distance between those NPs improves the C2+ selectivity, as long as it is still\u00a0< 1nm. The C1 product formation was lowered and the obtained current densities were up to 12 times higher than with the unmodified catalyst. The reason for this was a higher surface roughness (increased ECSA) and a lowered energy barrier for CO2RR. Again, Cu+ was reduced to Cu0 during the reduction reaction (Jeong et\u00a0al., 2020).Another strategy is the combination of copper oxides with copper in the catalyst via the formation of Cu@Cu2O core-shell catalysts (Shang et\u00a0al., 2019). The synergy between Cu0 and Cu+ leads to an increase in selectivity and efficiency in the formation of C2+ products, whereby dimerization should be facilitated by promoting the formation of a positive- and a negative-charged carbon atom (Shang et\u00a0al., 2019).Besides the influence of surface activation or, for example, the use of OD-Cu electrodes, the influence of halides in Cu-based electrodes was also of interest for the CO2RR. Therefore, Wang and coworkers produced halide-containing copper catalysts via a precipitation process and found during electrochemical measurements in a flow cell that the adsorption capacity increases in the following order: Cu\u00a0< I-Cu\u00a0< Br-Cu\u00a0< Cl-Cu\u00a0< F-Cu (Ma et\u00a0al., 2020b). Overall, the C-C coupling works better the higher the coverage of the surface with \u2217CO is. The authors suggested that the presence of Cu+ sites may increase CO adsorption. In connection with C2H4 formation, they found that in the Cu-halides catalysts with increasing electronegativity of the halide, only a slight decrease of the onset potential could be observed. This indicates that the copper catalysts modification with halides promotes the first step after the \u2217CO intermediate formation. Furthermore, a dependence on the local pH value was observed. Thus, a significant increase in C2+ formation (with FEs of EtOH up to 15%) for F-Cu catalysts with increasing local pH was observed when using different 0.5\u00a0M electrolytes in the following order: K2HPO4\u00a0< K2CO3\u00a0< K2SO4 (Ma et\u00a0al., 2020b). Figure\u00a06\n shows both the influence of the halide on the formation of C2+ products and the influence of the KOH concentration for the Cu-F catalyst.Another recently published study shows the production of a halide-containing copper catalyst by oxidative-reductive recycling of polycrystalline copper in KHCO3 solution with addition of the corresponding potassium salt (Han et\u00a0al., 2020b). While Cl\u2212 and Br\u2212 stabilized Cu+ and thus tend to be promoters of Cu dissolution, I\u2212 inhibited it by forming an almost insoluble polyhedral CuI, along with the associated passivation of the surface. This cycling of copper in KHCO3 solution resulted in different structures on the surface of the Cu electrode depending on the halide. Although the reconstructed (re) Cu-I electrode had less Cu(100) on the surface compared to re-Cu-Br and re-Cu-Cl electrodes, the best selectivity for these electrodes was obtained for the copper electrodes modified with iodide with 80% FEC2. XAS measurements showed the same ratio of Cu0 to Cu+ for all three electrodes, rendering it not decisive for the selectivity. However, a correlation of the electrochemical performance during CO2RR was observed with the porous, in the case of re-Cu-I intertwined and spiderweb-like, hierarchical structure on the surface. The intermediately generated CO is supposed to be trapped inside the pores, providing an increased \u2217CO-coverage, which leads to an increased dimerization (Han et\u00a0al., 2020b). The question of how the addition of halides in the electrolytes affects the electrochemical performance of the catalysts and electrodes is discussed in the following chapter about process conditions.In addition to the previously discussed modifications of the catalysts with halides, there are also investigations on the influence of hydroxide. It was shown that the presence of OH groups near the catalyst surface improves the reaction kinetics and stabilizes the oxygen in CuxO catalysts during the reduction reaction (Xiang et\u00a0al., 2019). Also, with increasing number of OH\u2212 bound to Cu, the adsorption of CO and thus also dimerization should be supported (Iijima et\u00a0al., 2019). The adsorption energy of CO will be increased compared to pure Cu surfaces, because the OH layer will probably bring CO molecules closer together while a simultaneous reduction to C2+ products takes place.A further modification reported in the literature is the coating of a Cu foil with a 50\u00a0nm thick polyaniline film (PANI), whereby an improvement of the C2+ selectivity from an FE from 15% to 60%, for the coating of Cu nanoparticles even to 80%, was achieved (Wei et\u00a0al., 2020). The PANI layer is intended to increase the coverage of the surface with CO and improves the interaction of these molecules. At the same time, HER is significantly reduced, probably due to the increased hydrophobicity. Moreover, Mougel and coworkers created a superhydrophobic surface on their applied electrode by treating Cu dendrites with 1-octadecanthiol, resulting in an FE of 56% for C2H4 and 17% for EtOH at neutral pH (Wakerley et\u00a0al., 2019). The gas was captured at the electrode-electrolyte interface, which resulted in an increase in CO2RR and C2+ selectivity. Modification of surface hydrophobicity and adsorption energies is also possible by combining the use of halides and organic compounds (here dodecanethiol) (Wang et\u00a0al., 2021). Dodecanethiol lowers the selectivity for H2 and CH4 by decreasing the amount of adsorbed H\u2217. The bromide introduced into the copper catalyst, on the other hand, shifts the selectivity to ethanol by stabilizing positive Cu valence sites, which are expected to have a significant effect on the product distribution in CO2RR (Wang et\u00a0al., 2021).In addition to varying the oxidation states of copper or creating specific structures on the catalyst surface, bifunctional catalysts can be used to improve the selectivity for C2+ products. The potential for the formation of CO at the co-catalyst should correspond to the potential range for the formation of the target product at copper (Ren et\u00a0al., 2019). As discussed before in multimetallic and bifunctional catalysts, the combination with ZnO can increase the C-C coupling kinetics by increasing the local concentration of the intermediate CO (Zhang et\u00a0al., 2020b). In the case of Cu/ZnO tandem electrodes, additional CO was generated at the ZnO, and the resulting CO excess increased the C2+ selectivity by facilitating C-C coupling. The electrodes showed a stability of 10\u00a0h at 600 mA cm\u22122 (Zhang et\u00a0al., 2020b). The use of ZnO for increased selectivity of C2+ products in a Cu/ZnO tandem catalyst as a bifunctional catalyst with different domains was also shown by other groups. Gr\u00e4tzel and coworkers modified CuO nanowires via atomic layer deposition with ZnO, thus shifting the selectivity of CO and HCOO\u2212 (selectively formed on Cu nanowires) toward EtOH (Ren et\u00a0al., 2019). Herein, the additional active sites of zinc available for CO intermediate formation increase the amount of CO for C-C coupling and thus reduce HER at the same time. Figure\u00a07\n shows the proposed mechanism in more detail including the impact of varying the overpotential. Higher overpotentials lead to higher production of \u2217CH3, which can then be coupled with CO to form ethanol. Another example for bifunctional catalysts in CO2RR is the combination of copper with silver for obtaining enhanced yields for C2+ product (Hoang et\u00a0al., 2018). Sargent and coworkers made efforts in designing catalysts that favor the CO2RR pathway to ethanol. The diverse binding sites, existing in Ag-Cu bimetallic catalysts, led to a destabilization of the ethylene intermediates, probably due to a disruptive influence of Ag on ethylene-forming Cu sites. This resulted in an increased ethanol selectivity of 41% at \u22120.67\u00a0V vs. RHE, compared with an FE of 29% at best for the pure Cu catalyst (Li et\u00a0al., 2019b). Cu-Pd foams also revealed good catalytic activity toward CO2RR to C2+ products. This catalyst shows phase segregation in the nm range, with Cu- and Pd-rich domains present. These lead to a 2 times higher selectivity toward PrOH instead of EtOH. The methane pathway (C1) is suppressed and a concerted spillover effect of \u2217CO and \u2217H adsorbed on Pd domains results in the preferential formation of C3 products (Rahaman et\u00a0al., 2020). A catalyst for selective alcohol formation is an OD-Ag-Cu-foam of stoichiometry Ag15Cu85 (Dutta et\u00a0al., 2020). CO is selectively formed in the silver domains and is transferred by surface diffusion to copper, where it is converted to alcohols by C-C coupling. The excess of CO at the catalysts surface leads to good selectivity with up to 34% FE for ethanol. Furthermore, a selective activation of the copper by oxide deposition and the subsequent reduction under CO2RR conditions takes place and enhances the selectivity as well. Doping biphasic (BP) copper(I) oxide with silver also yields significant improvements in EtOH yield, including a shift in product selectivity from ethylene to divalent alcohols (Lee et\u00a0al., 2017). The FE for EtOH was raised from 11% to 35% for Ag-Cu2OBP compared to the undoped catalyst. Another option is the destabilization of the ethylene reaction path in favor of an increased EtOH production by using a Ag/Cu-alloy phase catalyst (Li et\u00a0al., 2019b). Ethylene is preferentially formed at highly coordinated surfaces and the introduction of an element with a weaker bonding capacity to carbon than copper reduces the probability of the formation of ethylene intermediates by increasing the variety of available bonding sites. On Cu (111), there are four bonding sites available, on Ag-doped Cu (111), there are 16.A concept that has been applied several times is the use of core-shell catalysts, where CO is enriched inside the nanocaves by reduction of the core, and is then converted by the shell into the target product (Zhuang et\u00a0al., 2018; Ren et\u00a0al., 2019; Shang et\u00a0al., 2019; Zhang et\u00a0al., 2020a). An example for the production of ethanol is a catalyst consisting of Cu2O nanocavities with embedded gold nanoparticles, which shifts the selectivity for CO2RR from C1 to C2 products (Zhang et\u00a0al., 2020a). The gold core reduces CO2 to CO in the nanocavities, resulting in a high local concentration of this intermediate. EtOH is then formed at the copper shell. Another core-shell catalyst developed by Sargent and coworkers consists of a Cu2S core and a Cu-V shell (Zhuang et\u00a0al., 2018). This catalyst achieved an FE of 32% for alcohols, with 25% for EtOH and 7% for PrOH at a partial current density of 120 mA cm\u22122, resulting in a 6-fold improvement of the EtOH: ethylene ratio from 0.18 to 1.2 compared to pure Cu nanoparticles.Another possibility for increasing selectivity toward multicarbon alcohols is the use of metal organic frameworks. The use of Cu(II)- and Bi(III)-based MOFs resulted in a FEEtOH of 28.3% (Albo et\u00a0al., 2019). However, these electrodes are only stable for 5 h. The increased EtOH formation can be explained by the reduction of CO2 at Bito HCOO\u2212, which is then transferred to Cu and reduced to alcohol. Owing to longer diffusion paths within the MOFs compared to other catalysts, a longer contact of the products is guaranteed and a reduction of MeOH to EtOH under C-C coupling can take place. A longer stability with up to 140\u00a0h was achieved for electrodes by using Co-corrole carbon paper electrodes (Gonglach et\u00a0al., 2019). The mechanism here is not based on CO as an intermediate but the formic acid pathway and Co-corroles stabilize various radical intermediates. EtOH could be obtained with an FE of 48%.Furthermore, metals were incorporated into various carbonaceous support materials, e.g. an N-doped porous carbon-supported copper catalyst was used for CO2RR to multicarbon alcohols (Han et\u00a0al., 2020a). Pyridinic N-species were probably the CO-producing sites and copper the catalytic sites for the production of EtOH and PrOH. An increase in pyridinic nitrogen atoms improved both selectivity and activity toward multicarbon alcohols. The carbon support influenced the copper concerning structure and size, resulting in improved CO2 adsorption and CO production. Pyridine nitrogen was also used in a catalyst consisting of Ag nanoparticles in a 3D-graphene-wrapped nitrogen-doped carbon foam, as it can bind \u2217CO intermediates better than other N-species (Lv et\u00a0al., 2018b). EtOH is then gradually formed at the Ag-NPs. The catalyst is also characterized by high conductivity. The direct comparison of Cu nanorods with nitrogen-doped graphene quantumdots (NGQ) and Cu nanorods clearly shows higher EtOH and PrOH yields (Chen et\u00a0al., 2020a). The reason for the increased formation of multicarbon alcohols is the better stabilization of the oxygen-containing intermediates. As mentioned and discussed before in multimetallic and bifunctional catalysts, there is also a synergistic effect, as C-C couplings occur at both the copper nanorods and the NGQ, and the formation of the desired C2+ products is greatly enhanced by these dual active sites.Besides the mentioned combination of metals and carbonaceous supports and the usage of bimetallic catalysts, a molecule-metal composite has been proposed. The porphyrin-based co-catalyst increased \u2217CO coverage on the metal surface, promoting C-C coupling and favoring the ethanol pathway. The FE for ethanol was 41% at \u22120.82\u00a0V vs. RHE, higher than 29% FE at \u20130.84\u00a0V observed for pure Cu (Li et\u00a0al., 2020). Molecular cobalt corrole complexes have been described, with the electron-donating ligands favoring a square-planar cobalt(I) complex as active species. It could reach an ethanol FE of 48% at \u22120.8\u00a0V vs. RHE (Gonglach et\u00a0al., 2019). Also, acetate as potential C2 product could be obtained using a manganese corrole complex with 63% FE at \u22120.67\u00a0V vs. RHE (Schoefberger et\u00a0al., 2020).Metal-free catalysts have also already been used for selective EtOH production, e.g. N-doped mesoporous carbon. High local electrical potentials within the mesoporous channel walls lead to an improved activation of CO2. In addition, this also facilitates C-C coupling through the pyridine and pyrollic nitrogen atoms. The micropores contained in the channel walls increase the selectivity of the catalyst for EtOH as well as the reactivity (Song et\u00a0al., 2020).In recent years, there has been a steady stream of new investigations of CO2RR with constantly new catalysts (Table 1) and a wide variety of production methods (Chapter 3.2). Overall, although the catalyst has a great impact on the selectivity and efficiency of the electrosynthesis, it is very difficult to compare catalysts due to large differences in electrode preparation, the test setup itself, and different electrolyte solutions, pressures, temperatures, etc. Here, standardized cells and reaction conditions could help to classify the potential of the catalysts in a reasonable way. In this area, there are already initiatives such as NFDI4Cat, which deals with the sharing of metadata in the entire field of catalysis and thus aims to create a research data infrastructure (Wulf et\u00a0al., 2021). Regarding a potential industrial application, an additional focus should be on simplicity, scalability, and the lowest possible cost of production, as well as on long-term stability as numerous catalysts have been tested only for their capability in reducing CO2 for few minutes. In addition, more emphasis should be given to the use of flow cells or MEAs for testing the catalysts to achieve higher current densities. Finally, as already discussed mechanistic studies, for example, by using in situ methods and carrying out of operando studies, should be given greater emphasis.Important for the successful electrolysis of CO2 to valuable products is not only the choice of the appropriate catalyst but also suitable process conditions.One key parameter with a strong influence on catalyst/electrode performance is the electrolyte. For example, compared to KHCO3, a higher selectivity to carbonaceous products using KOH was shown. High local pH values, which can be favored by an electrolyte with low buffer capacity, have been shown to improve the product distribution toward higher hydrocarbons (Hori et\u00a0al., 1989, 1997; Schouten et\u00a0al., 2014; Varela et\u00a0al., 2016; Xiao et\u00a0al., 2016; Wang et\u00a0al., 2018). Thus, alkaline electrolytes have been used in flow cells with promising results (Ma et\u00a0al., 2016; Dinh et\u00a0al., 2018a; Garc\u00eda de Arquer et\u00a0al., 2020).Owing to the competition between CO2 reduction and hydrogen evolution, alkaline conditions are required for an efficient performance of CO2 electrolysis (P\u0103tru et\u00a0al., 2019). In addition, the electrolyte used should be as conductive as possible in order to achieve higher energy efficiencies for the CO2RR (Dinh et\u00a0al., 2018a). How much this affects the overall cell performance is shown by a comparison between 10\u00a0M KOH and 0.1\u00a0M KHCO3, according to which the ohmic losses in the formation of C2H4 were reduced by a factor of 47 under the highly alkaline conditions (Dinh et\u00a0al., 2018a). Even when comparing 1\u00a0M KOH with 1\u00a0M or 0.1\u00a0M KHCO3, clear differences can already be seen. Although the same current densities can be achieved in principle with both electrolytes, the same current densities can be reached with 1\u00a0M KOH at considerably lower voltages, because the CO2RR activity is significantly higher there (Dinh et\u00a0al., 2018b) \u2014 in a catholyte with a higher basicity, less energy is therefore required for the CO2RR (Xiang et\u00a0al., 2019). In addition, the use of 1\u00a0M KOH also shifts the selectivity toward carbonaceous products (Dinh et\u00a0al., 2018b; Lv et\u00a0al., 2018a; Xiang et\u00a0al., 2019). Thus, by changing from 1\u00a0M KHCO3 to 1\u00a0M KOH at an Ag/PTFE-GDE, instead of 80%, an FE of 90% for CO could be achieved (Dinh et\u00a0al., 2018b). Furthermore, C2 products should be obtained mainly at KOH concentrations above 0.5\u00a0M (Xiang et\u00a0al., 2019). An increase in FE for these was observed with a) more negative potentials and b) higher KOH concentrations. The current density was also significantly increased by a higher KOH concentration. Furthermore, OH groups in the vicinity of the catalyst surface should improve the reaction kinetics and, in the case of CuxO catalysts, stabilize the oxygen of the catalyst during the reduction reaction (Xiang et\u00a0al., 2019). However, a recent study by Zhang and coworkers showed the opposite trend with a decrease in overall C2+ product formation (from 76.1%, 1\u00a0M KOH) and ethanol with increasing KOH concentration 7\u00a0M (60.4%, 7\u00a0M KOH) (Figure\u00a08\n (right)) (Duan et\u00a0al., 2021). The authors explained this deviation from previous publications with the high carbonate formation due to the high current densities of 400 mA cm\u22122 used. The described dependence on KOH concentration was performed on a poly(ionic liquid)-based Cu0-CuI tandem catalyst and also shows a significant increase of C2+ products for using 1\u00a0M KOH instead of 1\u00a0M KHCO3 or 1\u00a0M KCl. While the formation of hydrogen decreases from 22.7% (KHCO3) to 6.6% (KOH), the FE for ethanol increases significantly (Figure\u00a08 (left) (Duan et\u00a0al., 2021)). However, there are also studies which do not only deal with the basicity and thus the OH\u2212 concentration, but focus on the cation of the electrolyte solution. Thus, there are also results that indicate that OH\u2212 is not the promoter of CO2 reduction. In this study, the concentration of Na+ and OH\u2212 was varied while keeping the other ionic content constant and the result was that the main supporting effect in the formation of C2+ products is caused by the sodium cation (Li et\u00a0al., 2019a).However, there are also disadvantages of using basic electrolytes, such as the already mentioned instability of imidazolium-based ionomers in alkaline environments (Kutz et\u00a0al., 2017), but also the required stability of the catalysts and GDE. For example, C-based GDEs degrade after about 2\u00a0h when using a basic electrolyte (Dinh et\u00a0al., 2018b). Furthermore, the dilution of CO2 in basic electrolyte leads first to the creation of HCO3\n\u2212, followed by the conversion into CO3\n2\u2212 (Leonard et\u00a0al., 2020; Yang et\u00a0al., 2020). This results in an indirect slowing down of the kinetics by initiating a shift of the pH value toward more neutral values and to the formation of barriers within the gas diffusion electrodes due to salinization, which in turn hinders the CO2 flow, promotes hydrogen formation and decreases the current density continuously (Endr\u0151di et\u00a0al., 2019; Yang et\u00a0al., 2020). In addition, this storage of CO2 in the electrolyte can lead to an overestimation of the products FEs (Ma et\u00a0al., 2020a), to conductivity losses within the system, as well as to energy efficiency losses in the overall electrolytic cell (Gabardo et\u00a0al., 2019).Finally, it should be noted that the amount of electrolyte used also influences the CO2RR performance. If there are larger amounts of electrolyte between the membrane and cathode, kinetics of HER is suppressed and separation of the liquid products is simplified, but larger ohmic losses occur within the cell, leading to higher cell voltages at moderate current densities (Chen et\u00a0al., 2020b).The local pH value has a strong impact on the product distribution, as a more alkaline environment promotes CO and multicarbon product formation and suppresses HER and CH4 formation (Burdyny and Smith, 2019). Thus, the local pH has an influence on the energetics of the different products of the CO2RR. It has been observed that the pH in weak buffering solutions, such as KHCO3 or KCl, at the electrode can be shifted up to 6 units in the beginning of the electrolysis. The large pH difference can also cause difficulties in determining the equilibrium potential between the working and reference electrode correctly, which in turn influences the onset potentials. Within the catalyst layer, pH values above 12 may occur at current densities >200 mA cm\u22122. The use of acidic electrolytes in CO2RR is often regarded to be no alternative as hydrogen formation would become too dominant (Burdyny and Smith, 2019). However, recently Sargent and coworkers have shown CO2RR at 1.2 A cm\u22122 in 1\u00a0M H3PO4 yielding 50% FE of C2+ products, which is possible due to the drop of local pH during operation (Huang et\u00a0al., 2021). Regarding the F-Cu catalyst investigated by Wang and coworkers, a correlation between the local pH value and the catalyst could be found (Ma et\u00a0al., 2020b). It was shown that the local pH value at the electrode increases significantly in the order K2HPO4\u00a0< K2CO3\u00a0< K2SO4 due to the high concentration of OH\u2212 produced during CO2RR, which cannot be buffered by electrolytes like K2SO4; however, the buffering of the pH value is better with, e. g. K2HPO4 (Lv et\u00a0al., 2018a). At the same time, there is also an increase in C2+ formation in this order, which is more pronounced compared to the pure copper catalyst. In conclusion, it is however difficult to precisely estimate the extent of the pH influence on the catalyst (Ma et\u00a0al., 2020b). A recent study by Jung and coworkers on Cu/Cu2O aerogel catalysts also shows that the use of electrolytes with lower buffer effect leads to higher FEs of ethanol at simultaneously lower FEs for HER (Kim et\u00a0al., 2021). Thus, solvents with a higher buffering capacity should neutralize the OH\u2212 generated during CO2RR and thus oppose the local pH effects. Figure\u00a09\n shows the FEs of EtOH and H2 as a function of the selected electrolyte, with an increase observed for ethanol in the order K2HPO4\u00a0< KHCO3\u00a0< KClO4\u00a0< KCl. Studies on an electrode with electrodeposited copper showed that the local pH at the oxidized copper electrode decreases from 10.4 to 9.3 with increasing negative applied potential ranging between \u22120.4 and \u22121.2\u00a0V and using 1\u00a0M KOH (Henckel et\u00a0al., 2021). The decrease in pH is due to the formation of HCO3\n\u2212, while at the same time, malachite is formed at the electrode at the beginning of the reduction of the copper oxide. Malachite shows highest thermodynamic stability between pH 8.0 to 10.5 and precipitates at the Cu surface of the electrode due to the carbonate-rich environment. These processes could influence the CO2RR product distribution. Thus, it should lead to higher Faraday efficiencies for the formation of ethylene than pure Cu foil. The subsequent further reduction of copper oxide and malachite finally leads again to a pH value of >11 (Henckel et\u00a0al., 2021).Recently, the local pH for pulsed electrolysis at CO2RR in CsHCO3 and LiHCO3, respectively, was determined by simulations (Kim et\u00a0al., 2020a). These show deviating values depending on the applied potential. In the period of the pulse of \u22120.8 V, a high concentration of CO2 is present at the cathode surface; the local pH value of nine is low. When the potential is raised to \u22121.15 V, the pH also increases to 11, and the CO2 concentration decreases from the previous 31\u00a0mM to13\u00a0mM. It is likely that this results in additionally increased adsorption of \u2217CO compared to \u2217H, which is accompanied by increased C2+ selectivity with a concomitant decrease in HER (Kim et\u00a0al., 2020a).Studies on the most appropriate electrolyte also considered the optimal choice of cations. With respect to alkali metals, the following trend was found for the selective formation of CO and EtOH (Karapinar et\u00a0al., 2019) and for the current densities (Gao et\u00a0al., 2018): Li+\u00a0< Na+\u00a0< K+\u00a0< Cs+. The difference is also very clear when comparing the FEs for the formation of ethanol using different electrolyte solutions. The use of a Cu-N-C catalyst achieved an FE for EtOH of 2% in CO2RR at \u22121.2\u00a0V in LiHCO3 solution, but 42% in CsHCO3 (Karapinar et\u00a0al., 2019). If larger cations are used, these are less strongly hydrated, facilitating the adsorption on the surface of the catalyst (Karapinar et\u00a0al., 2019; Kibria et\u00a0al., 2019). The adsorption of those cations leads to a more positive potential of the outer Helmholtz layer (OHP), which in turn reduces the H+ concentration at the electrode, consequently lowering the extent of HER (Karapinar et\u00a0al., 2019; Kibria et\u00a0al., 2019; Lamaison et\u00a0al., 2020). In addition, hydrated Cs+ ions near the catalyst surface can buffer pH changes and increase the amount of locally dissolved CO2 (Kibria et\u00a0al., 2019; Jeong et\u00a0al., 2020). Furthermore, the hydrated Cs+ ions would impose an electric field on the external OHP and thus promote C-C bond formation by coupling adsorbed \u2217CO with \u2217HCO (Jeong et\u00a0al., 2020). With regard to the anions used, the best selectivity has so far been shown for OH\u2212 in both CO and C2+ formation (Lv et\u00a0al., 2018a; Kibria et\u00a0al., 2019). Jiao and coworkers investigated the influence of the anions using KOH, KHCO3, KCl, and K2SO4 while keeping the K+ concentration constant (Lv et\u00a0al., 2018a). The pH value in bulk was determined before and after electrolysis. It was found that KOH and KHCO3 showed hardly any changes in pH value in contrast to an increase of up to 4 pH units in the other two non-buffering electrolytes. The clearly best current densities for C2+ formation were obtained in KOH; in KCl and K2SO4, a high resistance was measured at higher overpotentials and a rapid overloading of the system occurred. The reason for the high resistance is probably the poor ionic conductivity of the membranes in these two electrolyte solutions (Lv et\u00a0al., 2018a).As already described with respect to the modification of catalysts with halides, the use of chloride, bromide, and iodide exerts a significant influence on the cell performance. Not only modifying catalysts but also adding halides to the electrolyte leads to considerable changes. For example, the addition of KX salts (X\u00a0= Cl\u2212, Br\u2212 and I\u2212) led to significantly increased current densities for the reduction of CO2 within an H-cell of plasma-activated copper catalysts in the order Cl\u2212\u00a0< Br\u2212\u00a0< I\u2212 (Ma et\u00a0al., 2020b). The FE of the C2+ products remained unchanged; current densities and formation rate for the products increased with increasing electronegativity of the halides. Investigations with KI addition showed that the increased activity for CO2RR takes place by accelerated hydrogenation of adsorbed CO intermediates (Dinh et\u00a0al., 2018a). A significant rise in current density was also observed when CsI was added to a CsHCO3 electrolyte solution (Gao et\u00a0al., 2018). It is assumed that the iodide adsorbs and thereby increases the roughness of the catalyst by quasi I\u2212-induced nanostructuring. Thus, Cu+ is also stabilized by the iodide. According to the previous discussion concerning the cations' choice on the CO2RR, it can be observed that CsI, due to its larger cation, supports the CO2RR more than the addition of KI (Gao et\u00a0al., 2018).Because most studies currently focus on the formation of CO or C2+ products in general, it is necessary that further research on the influence of electrolyte solutions on the CO2RR to higher alcohols should be performed. Herein, preferential formation of C2+ products was reported frequently when using basic electrolyte solutions such as 1\u00a0M KOH and high local pH values. A disadvantage here, however, is the formation of carbonates in the GDE, which block diffusion pathways and facilitate HER. Contrary, recent results show that acidic electrolytes also have great potential for the formation of multicarbon products. A greater focus should therefore also be given to these systems, as it might be feasible to reduce carbonization effects in the GDE and enable a long-time stable system. In the context of zero-gap reactors, investigations should also be carried out using different solid electrolytes. In particular, materials should be found, which contribute to a low cell resistance, but at the same time are stable against the alcohols produced.Another impact which was investigated on CO2RR is that of temperature. When using H-type cells, where the availability of CO2 at the cathode depends on the solubility of this gas in the used electrolyte, lower temperatures have been shown to facilitate a higher ratio of CO2RR to HER due to the better availability of CO2 (Ahn et\u00a0al., 2017). Palmore and coworkers investigated the influence of the temperature on the CO2 reduction on polycrystalline copper. They reported that the temperature affects various electrolyte parameters like CO2 solubility, pH, resistance of the solution, and diffusion rate of the reactants. While the FE for methane increased with lower temperatures and peaked at 2\u00b0C, ethylene FE increased with higher temperatures reaching its maximum at 22\u00b0C. Activity for HER rose with increasing temperatures (Ahn et\u00a0al., 2017). The effect of increased ethylene FEs with simultaneously lower methane FEs at elevated temperatures has also been described by other authors (Hori et\u00a0al., 1986; Cook, 1988; Kim et\u00a0al., 1988).Besides H-type cells, investigation of temperature effects have been performed in flow cells. Klemm and coworkers researched the impact on the Sn catalyst-based CO2RR to formate in a liquid-phase flow cell. The optimum performance was observed at 50\u00b0C with over 80% formate FE at 1 A cm\u22122, while higher and lower temperatures led to increased HER (Figure\u00a010\n). The increased HER at other temperatures than 50\u00b0C can be assigned due to the oppositional effects of reduced CO2 solubility and increased diffusion coefficients as well as faster reaction kinetics with increased temperature (L\u00f6we et\u00a0al., 2019). McIlwain and coworkers reported a reduction of the cell voltage by 1.57\u00a0V at 70 mA cm\u22122 during CO2RR to syngas, using a liquid-phase electrolyzer with an Ag-based GDE when the temperature is raised from room temperature to 70\u00b0C (Dufek et\u00a0al., 2011). According to Sargent and coworkers, increasing the temperature to 60\u00b0C and the associated faster reduction kinetics and extended mass transport through the ionomer layer resulted in obtaining comparable Faraday efficiencies for C2+ products even at lower overpotentials (Garc\u00eda de Arquer et\u00a0al., 2020).In gas-phase electrolyzers, higher temperature can increase the electrochemical performance. According to Park and coworkers, reduction to formate on Sn nanoparticles increased more than 2-fold, when rising the temperature from 30\u00b0C to 90\u00b0C (Lee et\u00a0al., 2018). Aric\u00f2 and coworkers reported a significantly increased methanol production rate on a PtRu catalyst with higher temperatures, in a MEA-type setup, however, with overall low FEs (Sebasti\u00e1n et\u00a0al., 2017). Sinton and coworkers also investigated the performance of their copper catalyst-based MEA electrolyzer at temperatures of 20\u00b0C, 40\u00b0C, and 60\u00b0C. An increase of temperature herein led to higher current densities for ethylene and hydrogen as well as higher FEs for the latter. Higher temperatures also increased the obtained ethanol output at the cathode side from 0.5 wt\u00a0% at 20\u00b0C, peaking at 40\u00b0C with 2.3 wt\u00a0%. This was attributed to an enhanced transport of water from anode to cathode side as well as increased vaporization of ethanol. The increased temperature was suggested to be a key factor in facilitating a highly concentrated output stream of liquid products (Gabardo et\u00a0al., 2019). The influence of temperature on the MEA in terms of current density at different overpotentials as well as the described influence on ethanol yield is shown in Figure\u00a011\n. The maximum temperature in gas-phase electrolyzers is limited due to a high rate of water crossover and the performance of the membrane (Kibria et\u00a0al., 2019).Initial studies have been carried out regarding temperature effects showing increased selectivity for ethanol at elevated temperatures. However, most studies addressing these reaction conditions deal with the formation of other products like formic acid or CO which means that there is still a lack on investigations showing the impact of various temperatures for a wider range of catalysts and electrodes for multicarbon alcohol production.Another important parameter is the pressure of CO2. For cell types using CO2 dissolved in the electrolyte, the solubility of CO2 is increased with rising pressure, which results in higher current densities. As early as 1995, Sakata and coworkers investigated various metals at an elevated pressure of 30.4\u00a0bar in an autoclave H-type cell. The use of Ag, Au, Zn, Pb, and In for the CO2RR results in the preferential formation of CO and formic acid at standard conditions, and an increase in pressure also provided higher resulting current densities due to reduced overpotentials. For metals in groups 8\u201310 such as Fe, Co, Rh, Ni, Pd, and Pt, applying a pressure of 30.4\u00a0bar resulted in a shift in selectivity from HER to the formation of CO and HCOOH (Hara et\u00a0al., 1995). A possible explanation for the change in selectivity is the facilitated desorption of CO under higher CO2 pressure (Hori and Murata, 1990; Kudo et\u00a0al., 1993; Kibria et\u00a0al., 2019). Using a dendritic Ag-Zn catalyst in an H-type cell containing 0.1\u00a0M CsHCO3, Vlugt and coworkers achieved a stable FE of over 90% for 40\u00a0h at 10 mA cm\u22122 and about \u22121.0\u00a0V vs. RHE. Pressurized measurements were performed in a single-chamber cell at 200 mA cm\u22122. Raising the pressure from 1 to 3\u00a0bar resulted in an increase in the partial current density for CO formation from about 30 mA cm\u22122 to 131 mA cm\u22122 at \u22122.0\u00a0V vs. RHE. A further increase to 6\u00a0bar even provided partial current densities of 188 mA cm\u22122 at \u22121.2\u00a0V vs. RHE (Ramdin et\u00a0al., 2019). Mul and coworkers used Cu-NPs and KHCO3 to evaluate the resulting FEs in CO2RR under variation of pressure between 1 and 9.1 bar. The pressure increase raised the FE of ethylene from 10.8% to 43.7% while lowering the FE(CH4) from 21.3% to 1.8% and decreasing the HER. Although a lower local pH was calculated for higher pressures, the increased ethylene selectivity is associated with increased surface coverage of CO, which also causes higher yields of CO under pressure (Kas et\u00a0al., 2015). Sakata and coworkers varied the pressure for the study of Cu electrodes in an H-cell between 1 and 60.8 bar, with an initial shift in selectivity from HER to hydrocarbons formation. The maximum was obtained at 40.5 bar, and as the pressure was increased further, the product spectrum shifted further toward CO and HCOOH (Hara, 1994).Variation of pressure was also investigated in flow cells. So far, studies have mainly been performed on Ag-GDEs. Using these GDEs, a significant reduction in cell voltages was achieved by increasing the pressure (Hara, 1997; Dufek et\u00a0al., 2012). McIlwain and coworkers combined the simultaneous increase of temperature and pressure (Dufek et\u00a0al., 2012). Raising the temperature from 60\u00b0C to 90\u00b0C and increasing the pressure to 18.7\u00a0bar reduced the cell voltage from 4.01\u00a0V to below 3\u00a0V in the CO2RR to CO, with an FE(CO) of 82% (Dufek et\u00a0al., 2012). Sinton and coworkers studied pressures from 1 to 7.1\u00a0bar using KOH electrolyte. The high pressure combined with 7\u00a0M KOH resulted in a low overpotential for reduction to CO of 300\u00a0mV at 300 mA cm\u22122 and an FE of almost 100%. Furthermore, the highest half-cell energy efficiency (EE) of 81.5% was achieved here compared with lower pressures (Gabardo et\u00a0al., 2018). Recent studies on sputtered Ag-GDEs also show the effect of improved overall energy efficiency in CO2RR to CO of up to 67% at 202 mA cm\u22122, where the pressure was 50\u00a0bar and 5\u00a0M KOH was used (Edwards et\u00a0al., 2020). In contrast, experiments by Schmid and coworkers on a silver-based GDE in a liquid flow cell showed no dependence of the FEs for CO when increasing the CO2 pressure from 0 to 25 bar. The CO2 pressure increase only caused a cell potential rise from 6 to 7 V, resulting in overall poorer energy efficiencies while no changes in FE were observable for CO formation (Krause et\u00a0al., 2020).In addition to CO2RR under pressure using aqueous electrolyte solutions, studies have also been conducted on the reduction of CO2 from supercritical CO2. CO2 behaves as a supercritical fluid meeting the critical pressure and temperature of 73.8\u00a0bar and 31.0\u00b0C (Span and Wagner, 1996). In this state, CO2 has the density of a liquid but the viscosity of a gas and is infinitely miscible with other gases (Abbott and Eardley, 2000; Melchaeva et\u00a0al., 2017). Battistel and coworkers studied CO2 reduction on Cu electrodes in supercritical CO2 using acetonitrile as cosolvent and tetrabutyl-ammonium hexafluorophosphate to increase conductivity. In addition, protic solvents of different pH values were added to allow the formation of hydrocarbons and to influence the selectivity. The use of water and 1\u00a0M CsHCO3 resulted in FEs of 11.1% for ethanol and 7.5% for methanol. However, the overall FE was limited to 34%, possibly due to reoxidation at the anode and other sources of loss. In addition, corrosion of the Cu electrodes was observed (Melchaeva et\u00a0al., 2017). Our group recently showed that under high pressure conditions in supercritical CO2, suppression of HER from an FE of 60% to below 8% is possible. The resulting shift in product distribution compared to measurements under ambient conditions led to current efficiencies of up to 66% for the formation of formic acid (Junge Puring et\u00a0al., 2020).Overall, the adjustment of process parameters could allow further optimization of electrochemical CO2 reduction, but the influence of temperature and pressure, especially for the reduction of CO2 to alcohols, has only been researched to a limited extent. In addition, aspects relevant for industrial implementation, such as long-term stability and integration into upstream and downstream processes, need to be evaluated.Finally, it can be stated that within the last few years, enormous efforts have been made regarding the design of catalysts for the electrochemical CO2RR to multicarbon products especially alcohols. The basis for these catalysts was almost exclusively copper-based systems. Overall, increasingly better Faraday efficiencies are being achieved for the formation of higher alcohols, with some exceeding 50% for C2+ alcohols (Karapinar et\u00a0al., 2019; Chen et\u00a0al., 2020a; Han et\u00a0al., 2020a; Song et\u00a0al., 2020; Wang et\u00a0al., 2020b; Zhang et\u00a0al., 2020a). Promising results were obtained by using alloys or bimetallic catalysts of Cu and, for example, Ag (Hoang et\u00a0al., 2018; Li et\u00a0al., 2019b; Dutta et\u00a0al., 2020; Kim et\u00a0al., 2020b; Marti\u0107 et\u00a0al., 2020; Zhang et\u00a0al., 2020a), Zn (Ren et\u00a0al., 2019; da Silva et\u00a0al., 2020), or Pd (Rahaman et\u00a0al., 2020). Likewise, catalysts with Cu and N-doped carbon showed encouraging results (Karapinar et\u00a0al., 2019; Chen et\u00a0al., 2020a; Han et\u00a0al., 2020a; Wang et\u00a0al., 2020b). These catalysts were prepared using different methods, of which sputtering (Li et\u00a0al., 2019b, 2020; Wang et\u00a0al., 2020b), electrodeposition (Dutta et\u00a0al., 2020; Kim et\u00a0al., 2020b; Rahaman et\u00a0al., 2020; Kong et\u00a0al., 2021), and precipitation conceivably followed by calcination, should be highlighted (Lv et\u00a0al., 2018a; Zhou et\u00a0al., 2018; Marti\u0107 et\u00a0al., 2019; Wei et\u00a0al., 2020).However, the selectivity does not solely depend on the catalyst, but also on the overall system. To achieve industrially relevant current densities, it is necessary to use flow cells or cells utilizing membrane electrode assemblies. H-cells, in which the CO2 transport to the catalyst is largely determined by its solubility in the electrolyte, should therefore be avoided in the future, especially because some studies show large differences in the product distribution for the same catalyst occur between H-cell and flow cell (Kibria et\u00a0al., 2018; Gregorio et\u00a0al., 2020; Wang et\u00a0al., 2020a). In addition to the cell design, the design of the electrodes themselves is also of great importance. For a more detailed consideration of these two points, reference is made to the second part of the review \u201cElectrochemical CO\n\n2\n\nreduction - The macroscopic world of electrode design, reactor concepts & economic aspects\u201d. There are few studies so far on the influence of temperature and pressure on the formation of multicarbon alcohols in CO2RR, but it was shown, for example by Sinton and coworkers, that a higher yield for EtOH could be obtained at a temperature of 40\u00b0C than at RT or at 60\u00b0C (Gabardo et\u00a0al., 2019). Initial results are also available on the influence of pressure, even for using supercritical CO2 (Melchaeva et\u00a0al., 2017).Owing to the many influences, for example, from the process conditions, but also from the design of the electrodes and cells, further development of catalysts in terms of their selectivity is indeed sensible, but the following points in particular should be given more attention:\n\n\u2022\nCatalyst synthesis routes that are as simple as possible and do not involve particularly cost-intensive process parameters (such as high pressure) for insignificantly better selectivities (Siegmund et\u00a0al., 2021) \u2014 better catalysts capable to reverse ethylene:ethanol selectivity are required\n\n\n\u2022\nA particular focus should be placed on the development of further tandem catalysts, as these materials have already shown promising results in terms of selectivity to multicarbon alcohols\n\n\n\u2022\nThe targeted investigation and use of confinement effects, as already used for thermal catalysis (Mouarrawis et\u00a0al., 2018)\n\n\n\u2022\nResearch on ethanol and propanol selectivity in context of temperature increase should be investigated in detail\n\n\n\u2022\nIncreasing the long-term testing and stability of the catalysts (for industrial implementation more than 1000\u00a0h tests are required (Masel et\u00a0al., 2021))\n\n\n\u2022\nReducing the cost of carbon capture by developing catalysts, electrodes, and cells that show good selectivities in terms of CO2RR even with lower CO2 concentrations\n\n\n\u2022\nDeveloping new electrode and cell designs (including membrane development) that allow for more selective and energy-efficient CO2RR (further discussion see \u201cElectrochemical CO\n\n2\n\nreduction - The macroscopic world of electrode design, reactor concepts & economic aspects\u201d)\n\n\n\u2022\nLess catalyst testing in H-cells, because achievable current densities below 100 mA cm\u22122 are not industrially relevant\n\n\n\u2022\nDespite better CO2RR product distribution with regard to multicarbon products, turning away from KOH, because of the formation of carbonates and the oxidation of copper without applied potential\u2014here more research regarding electrolyte influence on ethanol/propanol formation is essential\n\n\n\u2022\nMoving away from the use of liquid electrolytes through the application of MEAs to realize lower cell voltages and counteract flooding of electrodes, thus enabling higher long-term stability and continuous CO2RR\n\n\nCatalyst synthesis routes that are as simple as possible and do not involve particularly cost-intensive process parameters (such as high pressure) for insignificantly better selectivities (Siegmund et\u00a0al., 2021) \u2014 better catalysts capable to reverse ethylene:ethanol selectivity are requiredA particular focus should be placed on the development of further tandem catalysts, as these materials have already shown promising results in terms of selectivity to multicarbon alcoholsThe targeted investigation and use of confinement effects, as already used for thermal catalysis (Mouarrawis et\u00a0al., 2018)Research on ethanol and propanol selectivity in context of temperature increase should be investigated in detailIncreasing the long-term testing and stability of the catalysts (for industrial implementation more than 1000\u00a0h tests are required (Masel et\u00a0al., 2021))Reducing the cost of carbon capture by developing catalysts, electrodes, and cells that show good selectivities in terms of CO2RR even with lower CO2 concentrationsDeveloping new electrode and cell designs (including membrane development) that allow for more selective and energy-efficient CO2RR (further discussion see \u201cElectrochemical CO\n\n2\n\nreduction - The macroscopic world of electrode design, reactor concepts & economic aspects\u201d)Less catalyst testing in H-cells, because achievable current densities below 100 mA cm\u22122 are not industrially relevantDespite better CO2RR product distribution with regard to multicarbon products, turning away from KOH, because of the formation of carbonates and the oxidation of copper without applied potential\u2014here more research regarding electrolyte influence on ethanol/propanol formation is essentialMoving away from the use of liquid electrolytes through the application of MEAs to realize lower cell voltages and counteract flooding of electrodes, thus enabling higher long-term stability and continuous CO2RRInvestigation of mechanistic understanding, i. e. use of in situ technologies and operando methods like Raman or IR spectroscopy to realize better catalyst design resulting in higher selectivity toward multicarbon alcohols as products in CO2RROwing to the enormous number of studies in the research field of CO2 reduction and the steadily increasing number of reports, it is not possible to know and cite every publication. It is pointed out that no author was specifically excluded. In order to give a comprehensive overview despite the high number of publications on the topic, two review parts have been written. This part deals especially with the catalysts/mechanisms/influences of the formation of higher alcohols during CO2RR; for other products please check other review papers.The authors are grateful for financial support from the German Federal Ministry for Economic Affairs and Energy (projects \u201cElkaSyn \u2013 Steigerung der Energieeffizienz der elektrokatalytischen Alkoholsynthese\u201d, grant 03ET1642C, and \u201cE4MeWi \u2013 Energie-Effiziente Erneuerbare-Energien basierte Methanol-Wirtschaft\u201d, grant 03EI3035A-D). U.-P. A. is grateful for the financial support by the Deutsche Forschungsgemeinschaft (under Germany\u2019s Excellence Strategy \u2013 EXC-2033 \u2013 Project number 390677874) and the Fraunhofer Internal Programs under Grant No. Attract 097-602175.Conceptualization: T.J., A.G., and D.S..; Investigation: T.J., A..G., and J.H.; Writing (Original Draft), T.J., A.G., D.S., and H.L., Writing (Review & Editing): T.J., A.G., D.S., H.L., U-P.A., and E.K.; Supervision: U-P.A. and E.K.The authors declare no competing interests.", "descript": "\n                  Tackling climate change is one of the undoubtedly most important challenges at the present time. This review deals mainly with the chemical aspects of the current status for converting the greenhouse gas CO2 via electrochemical CO2 reduction reaction (CO2RR) to multicarbon alcohols as valuable products. Feasible reaction routes are presented, as well as catalyst synthesis methods such as electrodeposition, precipitation, or sputtering. In addition, a comprehensive overview of the currently achievable selectivities for multicarbon alcohols in CO2RR is given. It is also outlined to what extent, for example, modifications of the catalyst surfaces or the use of bifunctional compounds the product distribution is shifted. In addition, the influence of varying electrolyte, temperature, and pressure is described and discussed.\n               "}
{"full_text": "At present, the demand in olefin hydrocarbons steadily grows due to the rising production of polymers and chemical compounds on the basis thereof. Thermal and catalytic cracking of heavy hydrocarbons (mainly oil) and dehydrogenation of paraffins are the major industrial methods to manufacture the unsaturated hydrocarbons. The processes of catalytic dehydrogenation of hydrocarbons (STAR, Oleflex, BASF-Linde process, Catofin, FDB by Snamprogetti and Yarsintez) are among the priority processes in the petrochemical industry (Sattler et al., 2014), with the dehydrogenation of light paraffins with the oxidants (oxygen or carbon dioxide) over various catalysts being widely discussed in the literature (Fattahi et al., 2013, 2011; Darvishi et al., 2016; Bugrova et al., 2019).Various catalysts have been proposed for the dehydrogenation of light paraffins, including conventional catalysts based on deposited precious metals, primarily platinum (Long et al., 2014; Li et al., 2017; Zhou et al., 2017), chromium (S\u0142oczy\u0144ski et al., 2011; Li et al., 2016; Cheng et al., 2015), and vanadium (Rodemerck et al., 2016, 2017; Tian et al., 2016) oxides, and also the catalysts based on ordered mesoporous materials (Xu et al., 2013; Shee and Sayari, 2010), metal\u2013organic frameworks (Zhao et al., 2013), activated carbon (Xu et al., 2014; Li et al., 2015), zirconia (Otroshchenko et al., 2015, 2016) and even bare alumina (Rodemerck et al., 2016). The \u03b3-Al2O3-supported chromium oxide catalysts are widely used in industry (Ruettinger et al., 2010; Busygin et al., 2013) in the processes with fluidized bed of microspherical catalysts (Kataev et al., 2015; Gilmanov et al., 2015) and with fixed bed of palletized catalysts (Catofin). According to the patent (Fridman, 2012), the isobutane conversion of ~56% with the selectivity towards isobutylene of 92% may by achieved at ~540\u00a0\u00b0C when fresh CrOx/Al2O3 catalyst is used. The aged catalyst is characterized by the isobutane conversion of 48% and selectivity of 87.6% at ~565\u00a0\u00b0C.There are a number of drawbacks of fluidized-bed paraffin dehydrogenation process such as high toxicity of microspherical Cr-containing catalysts, equipment deterioration, high catalyst consumptionn, and relatively low alkene yields. Thus, the global tendency is connected with the dehydrogenation of paraffins in the fixed-bed reactor or reactors with the moving bed of catalyst as more effective and eco-friendly processes. The synthesis of such catalysts is complicated by the need to prepare the catalyst granules with high porosity, strength and homogeneous distribution of supported active components.Impregnation technique is mostly used to synthesize the supported catalysts due to simplicity and effectiveness. The \u03b3-Al2O3 support granules with diameter of ~3\u00a0mm (1/8\u2033) are impregnated with an aqueous solution containing the precursors of the active component and modifiers to prepare the CrOx/Al2O3 catalyst (Fridman, 2012; Ruettinger and Jacubinas, 2016; Salaeva et al., 2020). The CrO3 is used as a precursor of the active component in industry because of its high solubility (it allows introducing up to ~28 %wt. of Cr2O3 into the alumina support), the minimal amount of aggressive gases released during the catalyst calcination as well as the adsorption of chromate ions on the surface of \u03b3-Al2O3 leading to the uniform distribution of the active component and its stabilization in a highly dispersed state (Spanos et al., 1994). The main disadvantage of CrO3 application is low pH of the impregnation solution (pH\u00a0\u2248\u00a00) because of high concentration of H2CrO4 therein that requires the use of special acid-resistant equipment. Aluminum oxide is also known to be dissolved in acids that implies additional requirements to the impregnation conditions.One of the approaches that makes it possible to shorten the contact time of the impregnation solution with the alumina support is the vacuum impregnation-drying method used in particular to produce microspherical CrOx/Al2O3 catalysts (Bekmukhamedov et al., 2016). The reduced pressure inside the pores of alumina support provides penetration of the impregnating solution in small pores that leads to more homogeneous distribution of the active components in catalyst granules. The impregnation under moderate vacuum may be taken into account for catalyst preparation, since it can be implemented in industry.The aim of this work was to reveal the influence of impregnation conditions (pressure during impregnation) of a molded alumina support on the properties of the produced alumina catalysts and their activity in the dehydrogenation of paraffin hydrocarbons with a fixed bed catalyst. A series of catalysts were synthesized with different pressures during the impregnation, studied by a complex of physical\u2013chemical methods, and tested in the fixed-bed isobutane dehydrogenation.The alumina support was prepared using a thermochemically activated aluminum trihydroxide (TCA THA) that was extruded with a small amount of nitric acid and 2\u00a0wt% of wood flour as a porogen (Bugrova et al., 2016) into cylindrical granules with a diameter of ~3\u00a0mm and a length of 4\u20136\u00a0mm. The support granules were dried at 120\u00a0\u00b0C and calcined at 700\u00a0\u00b0C for 4\u00a0h. The temperatures of drying and calcination were previously optimized taking into account the phase transformation of alumina hydroxides and boehmite into \u03b3-Al2O3 without the transformation into \u03b4- or \u03b1-Al2O3 with lower specific surface area (Zykova et al., 2015). The conditions were close to those used to prepare the industrial CrOx/Al2O3 catalysts (Fridman, 2012).To study the stability of the alumina support towards impregnating solution, a model impregnating solution with pH\u22480 containing 2.29 %wt. of chromium and 0.35 %wt. of potassium were prepared by dissolving the corresponding amounts of CrO3 (chemically pure, Vekton, Russia) and KNO3 (chemically pure, Vekton, Russia). The support granules were placed into the model impregnation solution, then the probe of solution (~100\u00a0\u03bcl) was taken every 5\u20137\u00a0min to determine the content of Cr, K, and Al by atomic emission spectroscopy (\u201cAgilent 4100\u2033 spectrometer with microwave plasma).Chromia-containing catalysts were prepared by impregnation using an aqueous solution of precursors of the active component (CrO3, chemically pure) and alkaline modifier (KNO3, chemically pure). The excess of impregnating solution (15.2\u00a0ml) was prepared by dissolution of CrO3 (10.3\u00a0g) and KNO3 (1.7\u00a0g) in distilled water. The loadings of the components in the catalyst were 20 %wt. Cr2O3 and 2 %wt. K2O, which was close to those in industrial catalysts (Li et al., 2015; Otroshchenko et al., 2016; Fridman and Urbancic, 2015). Thealumina support granules were dried at 150\u00a0\u00b0C overnight to remove the moisture. Then the granules (10\u00a0g) were put into the three-neck flask equipped with the vacuum pump and dropping funnel with a pressure compensator. The support granules were degassed at 1.0, 0.85 and 0.70\u00a0atm for 20\u00a0min, then the excess of the impregnating solution was added from the dropping funnel at pressures of 1.0, 0.85, and 0.7\u00a0atm. After that, the flask was opened accurately, and the pressure was normalized to the atmospheric one. The excess of impregnating solution was drained. The impregnated granules were immediately dried at 95\u00a0\u00b0C and calcined at 400\u00a0\u00b0C for 2\u00a0h.Chemical analysis of the samples was carried out by dissolving them in a mixture of sulfuric and nitric acids and analyzing the resulting solution by atomic emission spectroscopy (AES) using the \u201cAgilent 4100\u2033 spectrometer with microwave plasma. The porous structure of the samples was carried out by low-temperature (77\u00a0K) nitrogen adsorption using the \u201dTristar 3020\u2033 analyzer (Micromeritics, USA). The determination of the specific surface area (SBET) was carried out using the multipoint BET method to straighten the nitrogen adsorption isotherm in the range of relative pressures p/po from 0.05 to 0.30. The pore size distribution was obtained using the BJH-desorption method analyzing the desorption branch of nitrogen adsorption\u2013desorption isotherm. The Hg intrusion porosimetry measurements were carried out using the Poremaster-33 (Quantachrome, USA).The phase composition of the synthesized catalysts was studied by powder X-ray diffraction (XRD) using the Shimadzu XRD 6000 diffractometer with CuK\u03b1 radiation and a Ni-filter. The diffraction peaks of the crystalline phases were processed using the POWDER CELL 2.4 software and compared with those peaks of standard compounds from the PCPDFWIN database. The sizes of the crystallites of the metal oxides were calculated using the Scherrer equation. The scanning electron microscopy (SEM) was used to characterize the broken granules. The SEM 515 (Philips) microscope with the accelerating voltage of 30\u00a0kV was used. The features of the sample reduction were studied by the temperature-programmed reduction (TPR-H2). The experiments were carried out on the chemisorption analyzer ChemiSorb 2750 (Micromeritics, USA) for as-prepared samples using a 10% H2/Ar gas mixture at a flow rate of 20\u00a0ml/min and a heating rate of 10\u00a0deg/min.Catalytic properties of the samples were studied through the isobutane dehydrogenation. The experiments were carried out on the \u201cKatakon\u201d flow catalytic unit (Katakon, Russia) in a tubular metal reactor with a stationary catalyst bed at temperatures of 570, 590, 610\u00a0\u00b0C. A reaction mixture of 15% i-C4H10 and the balance of Ar was fed through a catalyst bed (10\u00a0cm3 of catalyst pellets, 8.5\u20139.0\u00a0g) at a rate of 25.2\u00a0l/h. The experiment was carried out in a cyclic mode: reduction (H2/Ar mixture, 3\u00a0min), dehydrogenation (isobutene, 9\u00a0min), regeneration (atmosphere-air, 9\u00a0min). The Ar flow (3\u00a0min) was fed through the catalysts between each step. The gas probe for analysis was taken at 7th min of the dehydrogenation step. Analysis of the reaction mixture and reaction products was carried out using the gas chromatograph \u201cKhromos GKh-1000\u201d (Khromos, Russia) with a flame ionization detector and two microcatharometers. The products were separated at 50\u00a0\u00b0C using the quartz capillary column with poly(trimethylsilyl)propene (PTMSP), a packed column with Chromosorb 106 (60/80 mesh) and a packed column with NaX molecular sieves (45/60 mesh). The quantitative calculation of the volume fraction of the components of the gas mixture was determined using the Khromos software 2.16.43. Before the catalytic test, the experiment with quartz balls was carried out. The isobutane conversion was 3\u20135% at 570\u2013610\u00a0\u00b0C that indicates the negligible insignificant influence of the inert balls, reactor walls and homogeneous reaction on the catalytic results.To study the stability of the alumina support in contact with the impregnating solution, the support granules (particles with sizes of 0.5\u20131\u00a0mm) were placed in a model impregnating solution containing 2.35 %wt. of chromium (as CrO4\n2\u2212 ions) and 0.35 %wt. of potassium (as K+ ions) with pH\u00a0\u2248\u00a00. Fig. 1\na shows the concentration dependences for chromium, potassium and aluminum in the solution as a function of contact time of alumina granules with the solution. The Cr concentration in the solution drops sharply during the first five minutes of impregnation from 2.29 %wt. to ~1.7 %wt. indicating the sorption of negatively charged chromate ions (CrO4\n2\u2212) on the alumina support surface positively charged at pH\u00a0~\u00a00. During the next 40\u00a0min, the Cr concentration in the solution changes insignificantly indicating the achievement of the adsorption\u2013desorption equilibrium. The K concentration in the impregnating solution does not change indicating the absence of adsorption of positively charged potassium ions on the positively charged alumina surface.It is noteworthy that the Al concentration in the model impregnating solution grows rapidly enough indicating the dissolution of alumina support in acidic conditions under the action of a chemically aggressive impregnating solution. At 50\u00a0min of granule contact with the solution, the Al concentration in the solution increases up to 0.274 %wt., which corresponds to ~0.8 % support dissolution. The support dissolution in the impregnating solution is a negative effect leading to the formation of mixed chromium-aluminum oxides inactive in the dehydrogenation reaction (Nemykina et al., 2010), decreased specific surface area (Bugrova and Mamontov, 2016), etc. The aluminum presence in excess of the impregnating solution limits the reuse of the latter for impregnating the next support batch under industrial conditions (Mamontov et al., 2017).Thus, the contact of the impregnating solution containing chromic acid with the alumina support granules was shown to lead to rather rapid alumina dissolution. Reducing of impregnation time is essential to minimize this negative effect. On the other hand, time of alumina granule impregnation should be enough for the impregnating solution to penetrate inside the granules providing homogeneous distribution of active component and alkali modifier. Fig. 1b shows the intact and broken granules of alumina-chromia catalysts prepared by impregnation at atmospheric pressure during 5 and 15\u00a0min. The gradient of color from dark-green to yellow and white across the breaks of granules (\u201cegg-shell\u201d structure) is observed after 5\u00a0min of alumina support contact with the impregnating solution. This means that 5\u00a0min is not enough for homogeneous distribution of active component in the catalyst granules. Impregnation of alumina support during 15\u00a0min leads to more homogeneous distribution of active component across the granule diameter (absence of color gradient at the granule break, \u201cwhole-egg\u201d structure). Thus, the impregnation time should be optimized to achieve both introduction of active component and minimize the support dissolving. The granule impregnation at reduced pressure may be used to increase the solution penetration rate inside the support pores and minimize the contact time with the solution. This technique may be implemented in industry only in the case of insignificant pressure reduction to 0.5\u20130.7\u00a0atm (Ertl et al., 2008).Three alumina-chromia catalysts were prepared by varying the pressure during the impregnation (1, 0.85 and 0.7\u00a0atm). Table 1\n shows the chemical analysis data and textural characteristics of the prepared catalysts. The contents of potassium and chromium oxides in all catalysts are close to the given values \u200b\u200b(2 %wt. and 20 %wt., respectively). The observed overestimation of the chromium oxide content by about 1%wt. (the nominal loading of 20\u00a0wt%) may be a consequence of the Cr precursor sorption on the support during the impregnation from the excess of the impregnating solution. The Cr sorption is observed during the alumina support contact with the model impregnating solution (Fig. 1a). Thus, from the data obtained, it can be concluded that the pressure change during the impregnation does not significantly influence on the catalyst chemical composition.\nFig. 2\n shows the SEM images of the catalyst granules (broken immediately before the SEM studies). It can be seen from the SEM images that catalyst structure feature roughness that results from the packing of alumina powder particles during the extrusion. Wide pores with sizes of 10\u201350\u00a0\u03bcm and pores with size of from few hundreds nm to few micrometers are observed.The presence of these pores is important for both the penetration of the impregnating solution during the catalyst preparation and for effective reagent transport during the high temperature catalytic process. The size of primary alumina particles constituting the catalyst structure ranges from several micrometers to ~20\u00a0\u03bcm (Fig. 2b). The space between these primary particles provides high volume of macropores, and some of them cannot be detected by the SEM.\nFig. 3\n shows the N2 adsorption\u2013desorption isotherms and pore size distributions for alumina support and the catalysts on the basis thereof. The support is characterized by a mesoporous structure that is evidenced by the presence of a hysteresis loop in the region of relative pressures of 0.5\u20131.0 on the N2 adsorption\u2013desorption isotherms. The pore size distribution resides in the region from 2 to 20\u00a0nm with the distribution maximum at ~5\u00a0nm. The support specific surface area and the pore volume are 139\u00a0m2/g and 0.350\u00a0cm3/g, respectively (Table 1). The strength of alumina support granules is calculated as P\u00a0=\u00a0F/D*h, where F is a force of granule breaking, D is a granule diameter, and h is a granule length. The measurement for 30 granules show the strength of 8.7\u00a0\u00b1\u00a00.5\u00a0MPa that is sufficient for industrial catalysts. Thus, the presence of macropores shown by SEM does not decrease the granule strength.The catalysts are characterized by the decreased specific surface area (56\u201391\u00a0m2/g) and pore volume (0.166\u20130.225\u00a0cm3/g). It is clearly seen from Table 1 and Fig. 3a that the pressure decreasing during the impregnation leads to consistently reduced pore volume and specific surface area without significant changes of chemical composition (Table 1). A decreased pore volume throughout the pore size range for the catalysts (Fig. 3b) indicates the active component distribution in the support pores. The shifting of the pore size distribution maximum for Cr/Al2O3-0.7 catalyst towards smaller sizes can be caused by coarsening of the CrOx particles in mesopores with sizes of 5\u201320\u00a0nm. The pore size distribution in the range of 2\u20133.5\u00a0nm for all samples is practically the same, which may indicate the difficulties in the active component penetration into the fine support pores, even if vacuum impregnation is used.To study the porosity, the Hg intrusion porosimetry was applied for Cr/Al2O3-1.0 catalyst. The differential pore size distribution (Fig. 3c) features two types of pores, namely, mesopores with diameter from few nm to ~40\u00a0nm and wide macropores with sizes of from 50\u00a0nm to ~8\u00a0\u03bcm with a maximum at ~200\u00a0nm. The presence of macropores may be caused by using the wood flour as a porogen during the granule extrusion. These results are consistent with the SEM data. Thus, the catalyst granules are characterized by the hierarchical porous structure (meso- and macropores).The XRD patterns for the catalysts and the results of qualitative and quantitative analysis of XRD are shown in Fig. 4\n and Table 2\n, respectively. All catalysts feature high content of amorphous phase (68.1\u201371.5%). The support constitutes the \u03b3-Al2O3 and amorphous phases. The active component (CrOx) exists in the amorphous state, with a small part comprising \u03b1-Cr2O3 phase. The content of the \u03b1-Cr2O3 phase increases from 3.4 %wt. to 5.3 %wt. with a pressure decrease during the impregnation from 1.0 to 0.7\u00a0atm. The \u03b1-Cr2O3 particle size (coherent scattering region) increases from 7.0 to 15.3 and 20.4\u00a0nm for pressure of impregnation of 1.0, 0.85 and 0.7\u00a0atm, respectively. This may explain the decreased specific surface area of the catalysts obtained under reduced pressure due to the partial blocking of the alumina support pores by chromia particles.\nFig. 5\n shows the TPR results for the obtained samples. All catalysts are characterized by the hydrogen consumption in the temperature range from 200\u00a0\u00b0C to 450\u00a0\u00b0C associated with the Cr(VI) reduction to Cr(III) (Bugrova et al., 2019). Two peaks of hydrogen consumption are observed for Cr/Al2O3-1 catalyst indicating coexistence of two Cr(VI) states. The first peak with a maximum at 320\u00a0\u00b0C can be attributed to the reduction of monomeric and/or oligomeric forms of Cr (VI) (Fridman et al., 2016). In our previous work (Salaeva et al., 2020), using Raman spectroscopy, we showed the key roles of monomeric and dimeric chromia species in isobutane dehydrogenation. A high-temperature peak at 420\u00a0\u00b0C may be attributed to reduction of small particles of Cr (VI) oxide or potassium chromates (Neri et al., 2004; Rombi et al., 2003). The shifting of TPR peak from 320\u00a0\u00b0C to 360\u00a0\u00b0C may be caused by an increase in the particle size of the chromium (VI) compounds or increased interaction of chromia species with alumina, which both lead to the decreasing of the catalytic activity (Salaeva et al., 2019).Thus, from the TPR results it can be concluded that aside from \u03b1-Cr2O3 detected by XRD, the catalysts contain the Cr(VI) species that are reduced at 200\u2013450\u00a0\u00b0C. The reductive pretreatment of the catalysts before the catalytic experiments leads to Cr(VI) reduction into Cr(III) sites (Bugrova and Mamontov, 2018). The real state of the active catalyst surfaces during the dehydrogenation is presented by two types of Cr(III) species: the first one is Cr(III) found in the as-prepared catalysts (including \u03b1-Cr2O3 detected by XRD) and Cr(III) formed due to the Cr(VI) species reduction. The activity of these species depends on both their dispersion and distribution on the catalysts surface. According to N2 physisorption and TPR results, the chromia species distribution is more homogeneous for the catalyst prepared at atmospheric pressure. Thus, the use of vacuum impregnation allows obtaining the catalysts with high content of Cr(VI) species, but with the decreased dispersion of these species. Consequently, the decreased Cr(VI) dispersion may be the reason for reduced catalytic activity.The catalytic properties of the prepared catalysts were studied in isobutane dehydrogenation in a fixed-bed reactor (Fig. 6\n). The real catalyst granules were tested under conditions close to the industrial ones to show the real opportunity for catalyst application in the process. The isobutane conversion growth is observed for all catalyst as the temperature increases from 570\u00a0\u00b0C to 610\u00a0\u00b0C with the corresponding decreasing of isobutylene selectivity. The conversion growth in this temperature region indicates the minimal diffusion limitations that are attributed to the presence of macropores in the catalyst granules shown by the SEM. The catalyst synthesized by the impregnation at atmospheric pressure is characterized by higher catalytic activity. Thus, the isobutylene yield is ~70% (71.4% conversion and 95.6% selectivity) at 610\u00a0\u00b0C under the process conditions (15% i-C4H10 in Ar). These results are close to the performance of industrial CrOx/Al2O3 catalysts tested under similar conditions (Xing and Fridman, 2019).The conversion for the Cr/Al2O3\u20130.7 catalyst synthesized by vacuum impregnation at 0.7\u00a0atm. is lower by 11\u201321 %mol. compared to the sample impregnated at 1.0\u00a0atm. The isobutylene selectivity for Cr/Al2O3\u20130.7 is also slightly lower by 0.2\u20131.3 %. Thus, a regular decreasing in conversion and selectivity is observed for a number of catalysts prepared by impregnation at 1.0, 0.85 and 0.7\u00a0atm.To study the stability, the Cr/Al2O3-1.0 catalyst was tested during 25 catalytic cycles including oxidative regeneration and reductive activation between the dehydrogenation. It can be seen from Fig. 6b that relatively high stability is observed. The isobutane conversion is kept at 52\u201362% at 590\u00a0\u00b0C at a selectivity of 96\u201398%. This indicates the stability of active catalyst surface during the high-temperature oxidative-reductive treatments and relatively low amount of coke formed. The amount of coke was measured by TGA-DSC for Cr/Al2O3-1.0 catalysts after 3 catalytic cycles and cooling in inert atmosphere. The amount of coke was 1.26 %wt. after the cycle at 610\u00a0\u00b0C. This value is not high and the coke formation is an important process in the isobutane dehydrogenation in a fixed-bed reactor. The coke burning during the oxidative treatment leads to the catalyst bed overheating and this heat is used in the endothermic dehydrogenation process.Thus, the analysis of results of catalytic and physical\u2013chemical studies shows that for the catalysts prepared at 1.0, 0.85, and 0.7\u00a0atm, the following regularities are observed:\n\n\u2013\na decreasing of specific surface area and pore volume due to partial blocking of pores by chromia,\n\n\n\u2013\nan increased amount of \u03b1-Cr2O3 with the growth of the particle size for this phase that is undesirable because the \u03b1-Cr2O3 phase is characterized by very low dehydrogenation activity,\n\n\n\u2013\nan increased temperature of Cr(VI) reduction indicating the Cr(VI) species agglomeration or enhanced chromia-support interaction that is also undesirable and leads to the decreased activity of chromia species,\n\n\n\u2013\nthe decreased activity in a row Cr/Al2O3-1.0\u00a0>\u00a0Cr/Al2O3-0.85\u00a0>\u00a0Cr/Al2O3-0.7.\n\n\na decreasing of specific surface area and pore volume due to partial blocking of pores by chromia,an increased amount of \u03b1-Cr2O3 with the growth of the particle size for this phase that is undesirable because the \u03b1-Cr2O3 phase is characterized by very low dehydrogenation activity,an increased temperature of Cr(VI) reduction indicating the Cr(VI) species agglomeration or enhanced chromia-support interaction that is also undesirable and leads to the decreased activity of chromia species,the decreased activity in a row Cr/Al2O3-1.0\u00a0>\u00a0Cr/Al2O3-0.85\u00a0>\u00a0Cr/Al2O3-0.7.Thus, the preparation of CrOx/Al2O3 catalysts is accompanied by many challenges: dissolving of alumina support in acidic impregnating solution, limited penetration of the impregnating solution inside the alumina support granules, etc. The decrease in the pressure during the impregnation of the granules of alumina support is a promising way to decrease the time of contact of support granules with the impregnating solution and enhance the penetration of impregnating solution inside the granules because of pressure change. However, the impregnation under vacuum leads to blocking of the support pores by chromia that is confirmed by the decreased SBET and pore volume. A reduced catalytic activity may be a result of both reduction of active catalyst surface and increased amount of inactive \u03b1-Cr2O3 phase.It is known that the dehydrogenation of light paraffins at 570\u2013610\u00a0\u00b0C may be hindered by the internal diffusion of reagent to the active site and product elimination from the catalyst pores (Barghi et al., 2012,2014). A porous structure of the catalyst plays an important role in high-temperature processes (Lee and Kim, 2013). Wide mesopores and macropores provide isobutane diffusion towards active sites on the catalysts surface. Therewith the reaction products (isobutene and hydrogen) should be released from the active surface to prevent the hydrogenation reaction. Wide mesopores provide product transport from the catalyst granules. Blocking of the mesopores by chromia nanoparticles may be a reason for additional diffusion limitation and decreased activity of the catalysts prepared by vacuum impregnation. Besides, the Cr/Al2O3-1.0 catalyst prepared at atmospheric pressure is characterized by both relatively high surface area and the presence of macropores that was demonstrated by SEM and Hg intrusion porosimetry. Thus, the combination of macropores and relatively high surface area for this catalyst is favorable for relatively homogeneous distribution of the active component inside the alumina granules (\u201cwhole-egg\u201d structure, Fig. 1b), high activity and stability in the isobutane dehydrogenation.Thus, it was shown that the production of alumina-chromia catalysts is significantly limited by dissolution of the alumina support by impregnating solution containing chromic acid. Optimization of the impregnation conditions is required to synthesize highly active CrOx/Al2O3 catalysts for dehydrogenation of light paraffins in a fixed-bed reactor. On the one hand, the time of impregnation should be minimized to prevent the alumina support dissolution by the impregnating solution containing chromic acid. On the other hand, the time and pressure of impregnation should provide penetration of the impregnating solution inside the alumina granules with a diameter of ~3\u00a0mm for the homogeneous distribution of active component and modifiers on the catalysts surface. The vacuum impregnation allowed us to introduce the impregnating solution rapidly, but it led to the decreased catalyst porosity as well as decreased activity in isobutane dehydrogenation. The role of macropores in the active component distribution during the impregnation and the role of macropores in minimization of diffusion limitations was demonstrated.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 within the framework of the state assignment of the Ministry of Science and Higher Education of the Russian Federation (project No. 0721-2020-0037). Author thanks Mrs. Elena Blokhina (Tomsk State University) for AES studies.", "descript": "\n                  The development of catalysts for dehydrogenation of light paraffin hydrocarbons in a fixed bed reactor is of great importance for the world petrochemical industry. The preparation of granules (~3 mm in diameter) of CrOx/Al2O3 catalysts is hindered by such problems as homogeneous distribution of active component and modifiers, high strength of granules, etc. In this paper, the alumina support dissolution in the impregnating solution containing chromic acid and the opportunity to apply vacuum impregnation to minimize this effect in the preparation of CrOx/Al2O3 catalysts are discussed. A series of catalysts is synthesized at different impregnation pressures (1, 0.85, and 0.7\u00a0atm), characterized by a complex of physical\u2013chemical methods (low-temperature N2 adsorption, SEM, XRD, TPR-H2), and tested in isobutane dehydrogenation. The use of vacuum impregnation is shown to lead to the reduction of the specific surface area of the catalysts from to 91 to 56\u00a0m2/g and the growth of content of CrOx phases that decreases the catalytic activity in dehydrogenation. The isobutylene yield at 610\u00a0\u00b0C decreases from 68% to 54% for the catalyst prepared at P\u00a0=\u00a00.7\u00a0atm as compared with the one prepared at atmospheric pressure. The high activity and stability are connected with the hierarchical structure of the alumina support and homogeneous chromia distribution on its surface.\n               "}
{"full_text": "The progress of modern industrialization is accompanied by the massive consumption of fossil fuels, particularly oil and coal, leading to the shortage of these unrenewable resources and emission of large amounts of carbon dioxide (CO2) [1\u20133]. Recently, CO2 concentrations in the atmosphere have exceeded 415\u00a0ppm, which is about 50% higher than pre-industrial levels. Induced by massive CO2 emissions, global warming has seriously threatened the balance of natural ecosystems [4]. Similar to CO2, global emissions of methane (CH4), which is another major greenhouse gas [5], have increased by nearly 10% over the past two decades, and its atmospheric concentration has set a new record of 1.875\u00a0ppm [6,7]. Given the warming power of CH4 is 80 times as high as that of CO2, CH4 is believed as the second most prevalent greenhouse gas from human activities. Subsequently, how to deal with these greenhouse gasses is an urgent problem to be solved [6,8,9]. Conversion of two major greenhouse gasses into value-added syngas (\n\nC\n\nH\n4\n\n+\nC\n\nO\n2\n\n\u2192\n2\nC\nO\n+\n2\n\nH\n2\n\n\n\n\u0394\n\n\nH\n\n298\nK\n\n\n\n=\n247\n\nk\nJ\n/\nm\no\nl\n)\n\n\n, which is also called CO2 reduction by methane (CRM), is considered to be one of the most promising approaches to achieve sustainable development [10].Different metal catalysts have been employed as active components for CRM reaction, such as Fe, Co, Ni, Ru, Rh and so on. Noble metals have high catalytic activity, but their high costs and limited availability prevent their practical large-scale applications. Ni has become the most widely used catalyst for its comparably high activity and low cost, but it suffers from deactivation due to carbon deposition and sintering of Ni nanoparticles (NPs). Carbon deposition mainly comes from the two side reactions, i.e., of methane dissociation (\n\nC\n\nH\n4\n\n\u2192\nC\n+\n2\n\nH\n2\n\n\n\n\u0394\n\n\nH\n\n298\nK\n\n\n=\n75\nk\nJ\n/\nm\no\nl\n\n) and carbon monoxide disproportionation (\n\n2\nC\nO\n\u2192\nC\n+\nC\n\nO\n2\n\n\n\n\u0394\n\nH\n=\n\u2212\n172\nk\nJ\n/\nm\no\nl\n\n) [11,12]. Reducing the size of NPs has been demonstrated to limit carbon nucleation and growth, but nanoparticles tend to aggregate into large particles at high reaction temperature, leading to poor stability. Confining metal NPs inside mesoporous materials, such as porous shells and matrixes, has been reported to be effective in mitigating sintering [13,14]. Nevertheless, some active sites are inevitably covered and inhibited to interact with reaction gasses, leading to decreasing CRM activities. Therefore, enabling metal NPs to possess small sizes, high activity and high stability simultaneously is still a daunting challenge.Another concern of CRM is its highly endothermic nature, so that massive thermal energy is needed to drive reactions. Emerging solar-driven CRM not only supplies thermal energy required in a low carbon way, but also can store solar energy in the form of important feedstocks or fuels [15\u201318], thus serving as a promising candidate to tackle global energy and climate change problems simultaneously. The key parameter determining whether solar-driven CRM can be widely deployed is high solar-to-fuel efficiency. People have devoted extensive efforts to enhancing solar-to-fuel efficiency. For example, many different catalysts have been developed to improve the catalytic activity and efficiency [19\u201326], and the highest solar-to-fuel efficiency reported under mild conditions is only 33.8%. Further enhancing solar-to-fuel efficiency over 35% is still a desire.Here, we proposed interconnected Ni/MgAlO\nx\n nanoflakes grown on SiO2 particles to achieve highly efficient solar-driven CO2-to-fuel conversion (shown in Scheme\u00a01\n). An extremely large light-to-fuel efficiency of 35.7% and very high fuel production rates of H2 and CO (136.6 and 148.2\u00a0mmol min\u22121 g\u00a0\u2212\u00a01) are achieved under focused illumination. Excellent spatial confinement of active sites, strong metal-support interactions, improved CO2 absorption and activation, and decreased apparent activation energy of C* and CH* species under direct light illumination are considered the main mechanisms, as confirmed by both experimental measurements and DFT calculations. In addition, the lattice oxygen of MgAlO\nx\n in the nanocomposite takes part in the reaction which helps to decrease carbon species as will be discussed later.Mg(NO3)2\u00b76H2O (0.164\u00a0g, 0.64\u00a0mmol), Al(NO3)3\u00b79H2O (0.135\u00a0g, 0.36\u00a0mmol), SiO2 (0.12\u00a0g) and CO(NH2)2 (2.7\u00a0g) were dispersed in 8\u00a0ml deionized water. 9\u00a0ml C2H5OH and 8\u00a0ml Ni(NO3)2\u00b76H2O (0.1\u00a0M) were then added. After stirring for five hours, the solution was dropped in a Teflon-lined stainless steel autoclave heating for 36\u00a0h at 190 \u00b0C. The resulting suspension was centrifuged and washed three times with ethanol, and then the product was dried overnight. Finally, the powders were reduced under 10% H2/Ar at 700 \u00b0C for 3\u00a0h. The reduced sample was labeled as Ni/Mg1.78AlOx@SiO2. The samples with the Mg/Al molar ratio of 0.67, 1.22 and 2.03 were prepared after following similar procedures. Correspondingly, 0.26\u00a0g Mg(NO3)2\u00b76H2O (or 0.37\u00a0g Al(NO3)3\u00b79H2O), 0.12\u00a0g SiO2 and 2.7\u00a0g CO(NH2)2 were employed to obtain Ni/MgO@SiO2 or Ni/Al2O3@SiO2. The procedures were the same as Ni/Mg1.78AlOx@SiO2. Ni@SiO2 sample was synthesized via the same procedures except that Mg(NO3)2\u00b76H2O was not added.The CO2-to-fuel conversion was conducted in a homemade reactor with a quartz window. We put 0.019\u00a0g of catalysts in the reactor for every test. A stream of CH4/CO2/N2 (43.2%/43.2%/13.6%) was continuously fed to the reactor at 104.2\u00a0ml min\u22121. A 300\u00a0W Xe lamp was used as the light source without using any other heating devices. The irradiation power focused on samples was measured by a laser power meter, which was calibrated by AM 1.5 global solar light with a standard Si solar cell. The power of the focused UV\u2013Vis-IR\u00a0illumination is measured to be 12.0\u00a0W. Since the spot diameter is 6\u00a0mm, the irradiation density reaches 424.6\u00a0kW m\n\u2212\n2.The light-to-fuel efficiency(\u03b7) is defined as follows\n\n\n\n\u03b7\n=\n\n\n(\n\n\nr\n\nH\n2\n\n\n\u00d7\n\n\n\u0394\n\nc\n\n\nH\n\n\nH\n2\n\n\n0\n\n+\n\nr\n\nC\nO\n\n\n\u00d7\n\n\n\u0394\n\nc\n\n\nH\n\nC\nO\n\n0\n\n\u2212\n\nr\n\nC\n\nH\n4\n\n\n\n\u00d7\n\n\n\u0394\n\nc\n\n\nH\n\nC\n\nH\n4\n\n\n0\n\n\n)\n\n\nP\nirradiation\n\n\n\n\n\nwhere r\nH2 and r\nCO are the molar production rate of H2 and CO, respectively, and r\nCH4\n is the reaction rate of CH4.\n\n\n\n\u0394\n\nc\n\n\nH\n\n\nCO\n2\n\n\n0\n\n\n,\n\n\n\n\n\u0394\n\nc\n\n\nH\n\nCO\n\n0\n\n\n\nand\n\n\n\n\n\u0394\n\nc\n\n\nH\n\n\nCH\n4\n\n\n0\n\n\n are the standard heat of combustion (\n\n\n\n\u0394\n\nc\n\n\nH\n0\n\n\n, 298.15\u00a0K) of H2, CO and CH4 fuel, respectively (note: CO2 is not a fuel, so \n\n\n\n\u0394\n\nc\n\n\nH\n\n\nCO\n2\n\n\n0\n\n\n of CO2 is 0), and P\nirradiation is the irradiation power focused on the reactor.The structure and components of prepared catalysts were investigated by X-ray diffraction (XRD). MgO and Al2O3 are in the form of Mg-phyllosilicate Mg3Si4O10(OH)2 (PDF 29\u20131493) and Al-phyllosilicate Al2Si2O5(OH)4 (PDF 14\u20130164), respectively, both of which are at the same peak (2\u03b8 =42.6\u00b0) in Fig.\u00a01\na. When MgO is combined with Al2O3, it exists in the form of MgAl2Si2O6(OH)4 (PDF 35\u20130489). This fully shows that there is a strong interaction between magnesium oxide, aluminum oxide and silicon oxide [27]. All the samples show three diffraction peaks at 2\u03b8 of 44.5\u00b0, 51.8\u00b0and 76.3\u00b0, which correspond to the (111), (200) and (220) crystal plane of Ni [28,29], respectively. As shown in Fig.\u00a01b, there is one more peak at 26.619\u00b0 belonging to MgAl2Si2O6(OH)4 in Ni/Mg1.78AlO\nx\n@SiO2, suggesting that different Mg/Al molar ratios affect the combination of MgO, Al2O3 and SiO2. XRD results confirm that Mg2+ and Al3+ exist in the form of phyllosilicates instead of bulk MgO and Al2O3, illustrating strong interactions between Mg-Al phyllosilicates and the support SiO2.The surface morphology and element mapping analysis of the catalysts were investigated by a transmission electron microscope (TEM) and scanning electron microscopy (SEM). TEM images (Fig.\u00a01c) show that the structure of the Ni/Mg1.78AlO\nx\n@SiO2 catalyst is similar to a sphere. Internal pores are beneficial to enhance the surface area and limit the agglomeration of metal particles. According to N2 adsorption/desorption measurements (Fig. S1), the surface area of Ni/Mg1.78AlO\nx\n@SiO2 is 112.5 cm2 (Table\u00a01\n). Although this value is not the largest among different samples, the average particle size of Ni nanoparticles before the reaction is the smallest with a value of only 8.65\u00a0nm (Fig. S3). Small Ni nanoparticles can suppress coking since carbon nanofibers are more difficult to nucleate, not to mention subsequent growth [28,30]. Ni nanoparticles are shown as bright dots in Fig.\u00a01d, and their interplanar distance is 0.208\u00a0nm as shown in Fig.\u00a01e [31]. Due to the addition of excessive urea, the solution is alkaline, so SiO2 particles partially dissolve and form Mg-Al phyllosilicate with Mg2+ and Al3+ (Fig.\u00a01f). MgAl-phyllosilicate crystal lattice effectively suppressed the growth and migration of Ni nanoparticles. Through the corresponding element mapping of Ni, Mg, Al, Si, O (Fig.\u00a01g), all the elements are dispersed uniformly on the surface of the carrier. Good dispersion and small size of Ni nanoparticles help to achieve both good catalytic activity and durability.It is well known that the metal-support interaction is an important factor affecting CRM performances [32]. Here, H2-temperature programmed reduction (H2-TPR) experiments of the samples were conducted to check their metal-support interactions (Fig.\u00a02\na). There is a broad peak between 400\u00a0\u00b0C and 600\u00a0\u00b0C for each catalyst, which is corresponding to the medium interaction between Ni species and the support [29,33]. It is worth noting that catalysts containing Mg-Al supports have a narrow reduction peak over 600\u00a0\u00b0C, which indicates that a small part of NiO has strong interactions with the support. An additional peak centered on 732.4\u00a0\u00b0C is found for Ni/Mg1.78AlO\nx\n@SiO2, which suggests even stronger metal-support interaction [13]. A higher reduction temperature means the sintering resistance of metal nanoparticles is better. That also indicates the combination of magnesium and aluminum effectively increases metal-support interactions, which contributes to the small sizes of Ni nanoparticles (Fig. S3).The basic sites of catalyst surfaces have a great influence on the adsorption and dissociation of CO2 [34,35]. Strong alkalinity can effectively promote CO2 adsorption and dissociation [36]. As shown in Fig.\u00a02b, samples exhibited a low temperature desorption peak between 100 and 200 \u00b0C which is attributed to weak basic sites in catalysts. The desorption peak between 400 and 500 \u00b0C is attributed to strong basic sites. Interestingly, Ni/MgO@SiO2 has a strong desorption peak around 860\u00a0\u00b0C, suggesting it has strong alkalinity at high temperature. Therefore, Ni/Mg1.78AlO\nx\n@SiO2 displays the most basic sites that contributed to strong carbon dioxide adsorption capacity [37]. CO2 is the only oxygen-containing reactant in CRM, and generated active oxygen from CO2 dissociation can interact with carbon species to avoid continuous deposition of carbon on the catalyst surface (\n\nC\nO\n2\n+\nC\n\u2192\n2\nC\nO\n\n). The adsorbed CO2 reacts with newly formed carbon species, which plays a significant role in eliminating carbon deposition. Thus, it is expected to achieve low carbon deposition and good stability for those catalysts possessing more basic sites, such as Ni/Mg1.78AlO\nx\n@SiO2.The photothermocatalytic activity of CRM was conducted in a homemade reactor with a quartz window (Fig. S5). Upon the focused UV\u2013vis-IR irradiation, the surface temperature of the samples reached the equilibrium temperature quickly (Fig. S6). Gas chromatography was employed to detect both reactants and products. As shown in Fig.\u00a03\na, b, for Ni@SiO2, the production rates of H2 (r\nH2) and CO (r\nCO) are 45.1\u00a0mmol min\u22121 g\u22121 and 62.4\u00a0mmol min\u22121 g\u22121, respectively. The r\nH2 and r\nCO of Ni/Al2O3@SiO2 increase to 92.8\u00a0mmol min\u22121 g\u22121 and 115.4\u00a0mmol min\u22121 g\u22121, and the r\nH2 and r\nCO of Ni/MgO@SiO2 become 89.7\u00a0mmol min\u22121 g\u22121 and 109.6\u00a0mmol min\u22121 g\u22121 respectively. It is obvious that magnesium aluminum silicate as the support increases both activity and stability significantly. For Ni/Mg0.67AlO\nx\n@SiO2, r\nH2 and r\nCO are 120.8\u00a0mmol min\u22121 g\u22121 and 138.4\u00a0mmol min\u22121 g\u22121, respectively. When the Mg/Al molar ratio is 1.22, its r\nH2 and r\nCO increase further to 115.6\u00a0mmol min\u22121 g\u22121 and 135.5\u00a0mmol min\u22121 g\u22121, respectively. Especially, Ni/Mg1.78AlO\nx\n@SiO2 exhibits the best catalytic performance. Its r\nH2 and r\nCO are 136.6\u00a0mmol min\u22121 g\u22121 and 148.2\u00a0mmol min\u22121 g\u00a0\u2212\u00a01, respectively. Reaction rates of CH4(r\nCH4) and CO2 (r\nCO2) are 75.0\u00a0mmol min\u22121 g\u22121 and 81.2\u00a0mmol min\u22121 g\u22121, respectively. In addition, r\nH2 is slightly lower than r\nCO in all experiments, so that H2/CO ratio is less than 1 (Table\u00a02\n), which is attributed to the existence of the reverse water-gas shift reaction (CO2+H2\nCO+H2O, RWGS). Catalysts with magnesium aluminum silicate as the support have a higher molar ratio of H2/CO relatively, demonstrating their capabilities of inhibiting RWGS reaction.The change of light-to-fuel efficiency \u03b7 over time is shown in Fig.\u00a03c, the average light-to-fuel efficiency \u03b7 of Ni@SiO2 is 11.6%. The reason is that when metal particles are directly exposed to the outer surface of SiO2 without any restriction, metal particles tend to grow or aggregate, so that catalytic activities will be inhibited. The average light-to-fuel efficiency \u03b7 of Ni/Al2O3@SiO2 is 26.7%, and is lower than that of Ni/MgO@SiO2 (\u03b7 is 27.4%). The main problem lies in relatively poor stability of Ni/Al2O3@SiO2, whose performance has an obvious decline with time compared with Ni/MgO@SiO2. The average light-to-fuel efficiency \u03b7 of Ni/Mg0.67AlO\nx\n@SiO2 reaches 29.6%. As the molar ratio of Mg/Al increases, the light-to-fuel efficiency \u03b7 further rises. The average light-to-fuel efficiency of Ni/Mg1.22AlO\nx\n@SiO2 is 34.5%. Ni/Mg1.78AlO\nx\n@SiO2 has the highest average light-to-fuel efficiency \u03b7 of 35.7%. However, as the proportion of Mg2+ continues to increase, the performance begins to decline. The average light-to-fuel efficiency \u03b7 of Ni/Mg2.03AlO\nx\n@SiO2 is only 31.7%. In contrast to other strategies for solar thermochemical CO2 reduction by CH4 below 800 \u00b0C (Fig.\u00a03d), Ni/Mg1.78AlO\nx\n@SiO2 possesses a record-high light-to-fuel efficiency and its conversion of CH4 is as high as 70.9% (Table\u00a02), which is a significant advantage. Excellent photothermocatalytic durability is another advantage. After 24\u00a0h of reaction (Fig.\u00a03e), it's r\nH2 and r\nCO values slightly decrease, and the \u03b7 value remains as high as 34.6%. And the average size of Ni nanoparticles of used catalysts is 13.7\u00a0nm (Fig.\u00a03f), which maintains a small size. Furthermore, there are no obvious carbon species on the catalyst. Other catalysts with molar ratios of Mg/Al of 1.22, 0.67 and 2.03 also show relatively better stability than those catalysts containing single metal support during experiment tests (Fig. S7). Another important reason why Ni/MgAlO\nx\n@SiO2 catalysts have different catalytic performances lies in different Ni nanoparticle sizes (Fig. S3). Basically, the smaller the metal size, the better activity and stability the catalyst exhibits.It is well known that carbon deposition properties have a heavy influence on catalyst activity [40,41], thus thermal gravimetric (TG) was employed (Figs.\u00a04\na and S7) to quantify the amount of carbon deposited during reactions. All spent catalysts show a significant weight loss in the range between 500 and 700\u00a0\u00b0C, due to the oxidation of these carbon species. These carbon species largely covering active sites severely damage catalysis performance. The amount of carbon deposited on the Ni/MgO@SiO2 and Ni/Al2O3@SiO2 are 45.57% and 42.05% respectively. In contrast, the Ni/Mg1.78AlO\nx\n@SiO2 catalyst displays the lowest weight loss of 12.15%, suggesting that the quantity of active and graphitic carbon formed is minimal. Ni/Mg1.78AlO\nx\n@SiO2 also has the lowest carbon deposition rate of 0.005 gcg\u22121\ncat\nh\u00a0\u2212\u00a01 (Fig.\u00a04b). XRD analysis shows that the deposited carbon exists in the form of graphite 2H (PDF 75\u20131621) in spent samples of Ni/Mg1.78AlO\nx\n@SiO2 (Fig. S8). The carbon deposition rates of Ni/Al2O3@SiO2 (r\nc=0.056 gcg\u22121\ncat\nh\u00a0\u2212\u00a01) and Ni/MgO@SiO2 (r\nc=0.070 gcg\u22121\ncat\nh\u00a0\u2212\u00a01) are 11.2 times and 14 times as high as that of Ni/Mg1.78AlO\nx\n@SiO2, respectively. The results indicate Ni/MgAlO\nx\n@SiO2catalysts are good at inhibiting carbon deposition. According to Fig.\u00a04c, carbon species type, active or graphitic, can be determined with the help of the Raman spectrum [42]. One peak at 1342 cm\u22121 could be assigned to D band, coming from active carbon, and the other at 1578cm\u22121 could be assigned to G band, coming from graphitic carbon [43,44]. The D band is deemed to be the vibration of carbon atoms with dangling bonds in an amorphous carbon network while the G band is contributed by the CC stretching vibrations of graphite layers [29,45]. The relative intensity of IG/ID could reflect the ratio of active carbon to graphite carbon [13,46] (Fig.\u00a04d). The high IG/ID ratio of Ni @SiO2, Ni/MgO@SiO2 and Ni/Al2O3@SiO2 confirmed that main carbon species are active carbon [13]. Activated carbon can cover more catalytic active sites, which is more harmful to the stability of the reaction compared with graphitic carbon. The IG/ID of Ni/MgAlO\nx\n@SiO2 samples is lower, demonstrating the advantage of combining MgO and Al2O3 in enhancing stability. The IG/ID of Ni/Mg1.78AlO\nx\n@SiO2 is 0.92, which is the lowest among all samples, agreeing with its excellent stability shown in Fig.\u00a03c.In most cases, CH4 dissociation and CO disproportionation are generally sources of carbon deposition [21,38]. To investigate the main source, temperature-programmed CH4 decomposition (TPMD) and CO disproportionation (TPCD) were conducted. As shown in Fig. S12, Ni/MgAlO\nx\n@SiO2 catalysts display a relatively stronger TCD signal with CH4 decomposition rate rising, implying the formation of carbon deposition. CO disproportionation begins to occur above 300 \u00b0C (Fig.\u00a04e). Notably, Ni/MgO@SiO2 and Ni/Al2O3@SiO2 show the strongest and weakest CO consumption peaks, respectively. This suggests that Al2O3 helps to inhibit the side reaction of CO disproportionation. Although peaking around 395\u00a0\u00b0C, TPCD signals of Ni/MgAlO\nx\n@SiO2 catalysts also have two relatively weak peaks around 450 \u00b0C and 550 \u00b0C. Around practical operation temperature of 700 to 800 \u00b0C, TPCD signals become weak since CO disproportionation itself is an exothermic reaction and will be inhibited at high temperature conditions. The carbon deposition amounts of TPMD and TPCD are determined by TG. Carbon deposition rates of TPMD are higher than that of TPCD for all samples (Fig.\u00a04f), illustrating that methane cracking is the main source of carbon deposition. Note that the carbon deposition of Ni/Mg1.78AlO\nx\n@SiO2 during TPCD or TPMD is not the lowest, but these carbon species can be quickly oxidized by CO2. This is because Ni/Mg1.78AlO\nx\n@SiO2 has a strong adsorption of CO2 as confirmed by previous CO2-TPD measurements (Fig.\u00a02b). That explains why Ni/Mg1.78AlO\nx\n@SiO2 has the lowest carbon deposition rate during practical operation tests (Fig.\u00a04b).To identify whether the lattice oxygen of MgAlO\nx\n@SiO2 contributes to inhibiting carbon deposition, an isotope labeling experiment using 12C18O2 and 12CH4 was performed (Supplementary Information). The gas in the reactor cavity was injected into GC\u2013MS for detection before turning on the lamp. Only three peaks can be observed corresponding to carrier gas (Ar), and reaction gas 12CH4 and C18O2 (Fig.\u00a05\na). The retention time is located at 8.4\u20138.6\u00a0min corresponding to 12CH4, and m/z\u00a0=\u00a016.1,15.1 and 14.1 belong to 12CH4 and its fragments. The retention time of C18O2 is located at 11.4\u201312.0\u00a0min, m/z\u00a0=\u00a048.1, 46.1, and 44 belong to 12C18O2,12C18O16O and 12C16O2, respectively (Fig.\u00a05b), indicating that the air in the reactor had been cleaned in advance. Then we turned on the Xe lamp for 2\u00a0h to introduce concentrated light irradiation. After that, the reacted gas was injected into GC\u2013MS for measurement. The intensity of m/z\u00a0=\u00a016.1,15.1,14.1(CH4) and m/z\u00a0=\u00a048.0 (C18O2) is weakened (Fig.\u00a05b), and the additional crack peak at 7.6\u20137.8\u00a0min was attributed to CO (Fig.\u00a05a). Corresponding intensities of fragments of m/z\u00a0=\u00a028.1 (12C16O) and 30.1 (12C18O) significantly increased (Fig.\u00a05b), illustrating that CRM reaction occurred. The fragment strength corresponding to 12C18O2 decreased, while the fragment strength corresponding to 12C16O2 and 12C16O18O significantly increased. During the reaction, 12C18O2 is the only oxygen-containing reactant. As a result, the only source of 16O is the catalyst, which comes from the lattice oxygen in MgAlO\nx\n@SiO2. This is beneficial to reducing carbon deposition and promoting a highly active and stable photothermocatalytic reaction.In order to understand the improvement of Ni nanocluster catalysts with different substrates, we modeled pyramidal NPs loaded on slabs of MgAl2Si2O10H4, Mg3Si4O12H2 and Al2Si2O9H4. For comparison, Ni (111) surface models were also established. All substrates are constructed with 4\u00a0\u00d7\u00a02\u00a0\u00d7\u00a01 supercell with the (001) facet cleaved. Ni NPs are built as a pyramid of exposed (111) surfaces with 30 atoms. Calculations are carried out in the framework of DFT using the generalized gradient approximation (GGA) of Perdew\u2013Burke-Ernzerhof (PBE) [47]. The VASP (Vienna ab initio simulation package) package is employed with the projected augmented-wave method [48,49]. The kinetic cutoff energy for the plane-wave basis is set to 400\u00a0eV. The Brillouin zone integration was performed on a Gamma-centered 1\u00a0\u00d7\u00a01\u00a0\u00d7\u00a01\u00a0K mesh. All the atoms are fully relaxed until the force on each atom is less than 0.05\u00a0eV /\u00c5. To analyze the performance of catalysts, we used the periodic slab models with a vacuum layer of 15\u00a0\u00c5. Our unit cell contained four layers with two bottom layers fixed to relax the module of the slab. Transition state searches were conducted using the climbing image nudged elastic band (CI-NEB) [50,51].CH4 and CO2 are activated to form active species, which are the premise of CRM. The elementary steps of CRM reaction mainly include three parts: CH4 activation dehydrogenation, CO2 activation and oxidation of CH* and C* species. It is crucial for CRM reaction to oxidize CH* or C* species to remove the carbon deposition and suppress the deactivation of the catalyst. The reaction energy (\u0394E) and activation energy (Eact) of the elementary steps of the DRM reaction are shown in Table S1. For simplifying the calculation, the Eact calculates several key steps in the reaction. The reaction energy diagram for CRM on Ni (111) surfaces is depicted in Fig.\u00a05c. It can be seen that the Eact for CH4 to remove an H atom to become CH3* on the surface of Ni/Mg1.78AlO\nx\n@SiO2 is lower than that of other supports, which only needs 0.85\u00a0eV. This ensures a high reaction rate (entry 1 of Table S1) of CH4 activation and is also consistent with our experimental results. Besides, from the perspective of reducing carbon deposition, the Eact value of CH* oxidation to CHO* (1.02\u00a0eV) is less than the Eact value of CH* dissociation to C* (1.22\u00a0eV) for Ni/Mg1.78AlO\nx\n@SiO2 (Fig. S13). Subsequently, C* species formation is suppressed. On the contrary, Ni/Al2O3@SiO2 needs the highest energy to oxidize CH*(1.44\u00a0eV), while the Eact value of CH* dissociation is only 1.24\u00a0eV, which causes more carbon species deposition (Table S1).It has been reported that Ni nanoparticles can simultaneously act as active sites and plasmonic promoters under light illumination [52\u201354]. Hence, distinct catalysis performances may be observed between light illumination and dark conditions. To reveal how the light affects the photothermocatalytic CRM reaction, optical absorption properties of Ni@SiO2, Ni/MgO@SiO2, Ni/Al2O3@SiO2 and Ni/Mg1.78AlO\nx\n@SiO2 were measured firstly (Fig.\u00a06\na). Since light excites surface plasmon resonances of Ni nanoparticles [55], all samples show good solar absorption properties. Although Ni/Mg1.78AlO\nx\n@SiO2 has an intermediate absorptance compared with Ni/MgO@SiO2 and Ni/Al2O3@SiO2, its value is still over 80% across the entire solar spectra. To check whether high temperature plays a vital role in the solar-driven CRM, the experiment was conducted at near room temperature for Ni/Mg1.78AlO\nx\n@SiO2. No H2 and CO were detected (Fig. S14). This demonstrates that the high photothermocatalytic activity of Ni/MgAlO\nx\n@SiO2 is derived from light-driven thermocatalytic CRM.To directly compare differences between photothermocatalysis and thermocatalysis, CRM reactions over Ni/Mg1.78AlO\nx\n@SiO2 were performed under light irradiation and dark conditions. It can be seen from Fig.\u00a06b that whether driven by light or heat, the reaction rate of the reactants increases with temperature, indicating that high temperature is conducive to the catalytic reaction. At any temperature between 660\u00a0\u00b0C and 860\u00a0\u00b0C, the reaction activity of Ni/Mg1.78AlO\nx\n@SiO2 under light irradiation is better than that in dark conditions. The ratio of H2/CO under dark conditions is always lower than that of photothermocatalysis at the same temperature (Fig.\u00a06c), although it increases with temperature for both cases since RWGS is inhibited by high-temperature conditions.Kinetic studies are conducted to further explore how light irradiation affects solar CRM performances. Arrhenius plots using the conversion rate of CH4 under both UV\u2013vis-IR illumination and dark conditions are presented in Fig.\u00a06d. These curves demonstrate a good linear relationship, and have a good agreement with the Arrhenius equation [26,56] (\n\nk\n=\nA\n\ne\n\n\u2212\n\nE\na\n\n/\nR\nT\n\n\n\n). Accordingly, the apparent activation energy for CH4 of Ni/Mg1.78AlO\nx\n@SiO2 with focused UV\u2013vis-IR irradiation is 31.2\u00a0kJ mol\u22121, which is much less than that under dark conditions (70.3\u00a0kJ mol\u22121). The decrease in apparent activation energy can be ascribed to the excitation of hot electrons in metallic Ni. It has been demonstrated by several references that excited Ni can dramatically decrease the activation energy of CO2 dissociation and CH* oxidation compared with ground states (dark conditions) [16,19,20,52]. This explains the reduced apparent activation energy and promoted activity of Ni/Mg1.78AlO\nx\n@SiO2 under direct light illumination.In summary, highly efficient solar-driven CO2 conversion with CH4 is achieved via interconnected Ni/MgAlO\nx\n nanoflakes grown on SiO2 particles with an ultrahigh light-to-fuel efficiency of 35.7% below 800 \u00b0C. The excellent performance can be ascribed to the following three aspects. First of all, highly dispersed nickel nanoparticles with small sizes and strong metal-support interactions are realized on Ni/MgAlO\nx\n@SiO2. And the formation of Mg-Al phyllosilicate provides many basic sites, promoting the absorption and activation of CO2 molecules. Secondly, the active oxygen in the carrier participates in the solar-driven CRM reaction, which is beneficial to suppressing the formation of carbon species produced by CH4 dissociation and CO disproportionation. DFT calculations also demonstrate that the reaction on MgAlO\nx\n@SiO2 has a lower activation energy of CH* oxidation to CHO* and improves the dissociation of CH4 to CH3*. At last, full-spectrum solar energy can be efficiently captured and the light-driven CRM greatly reduces the apparent activation energy, thereby significantly improving catalytic activities under direct light illumination. Our work demonstrates that Ni/MgAlO\nx\n@SiO2 can realize solar-driven CO2 conversion with ultrahigh light-to-fuel efficiency and superior stability, thus is promising to provide new opportunities for tackling global climate change and energy shortage problems.The authors declare no conflicts of interest in this work.This work was financially supported by the Basic Science Center Program for Ordered Energy Conversion of the National Natural Science Foundation of China (Grant No. 51888103), the National Key R&D Program of China (Grant No. 2021YFF0500700) and the Basic Research Program of Frontier Leading Technologies in Jiangsu Province (Grant No. BK20202008).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.fmre.2022.04.011.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Solar-driven CO2-to-fuel conversion assisted by another major greenhouse gas CH4 is promising to concurrently tackle energy shortage and global warming problems. However, current techniques still suffer from drawbacks of low efficiency, poor stability, and low selectivity. Here, a novel nanocomposite composed of interconnected Ni/MgAlO\n                        x\n                      nanoflakes grown on SiO2 particles with excellent spatial confinement of active sites is proposed for direct solar-driven CO2-to-fuel conversion. An ultrahigh light-to-fuel efficiency up to 35.7 %, high production rates of H2 (136.6\u00a0mmol min\u22121 \n                     \n                        g\n                     \u00a0\u2212\n                     1) and CO (148.2\u00a0mmol min\u22121 g\u22121), excellent selectivity (H2/CO ratio of 0.92), and good stability are reported simultaneously. These outstanding performances are attributed to strong metal-support interactions, improved CO2 absorption and activation, and decreased apparent activation energy under direct light illumination. MgAlO\n                        x\n                     @SiO2 support helps to lower the activation energy of CH* oxidation to CHO* and improve the dissociation of CH4 to CH3* as confirmed by DFT calculations. Moreover, the lattice oxygen of MgAlO\n                        x\n                      participates in the reaction and contributes to the removal of carbon deposition. This work provides promising routes for the conversion of greenhouse gasses into industrially valuable syngas with high efficiency, high selectivity, and benign sustainability.\n               "}
{"full_text": "The high demand for energy in modern society has resulted in anthropogenic activities of humans leading to a sharp increase in the atmospheric CO2 concentration (>400\u00a0ppm) causing major concern for attempts to manage global warming and the greenhouse effect [1\u20133]. In this regard, CO2 valorization is attracting interest among researchers around the globe. Carbon dioxide is considered an alternative carbon source from which to prepare a variety of C1 feedstock chemicals and fuels (including such as formaldehyde, formic acid, methanol, and methane) through the hydrogenation reaction [4\u20136]. Among these hydrogenation products, formic acid is an attractive commodity chemical with numerous applications involving several industries such as food and agriculture, leather, pharmaceuticals, and textiles [7\u20139]. Moreover, it is a promising source for use in storing carbon\u2013neutral hydrogen [10,11]. However, the H2 gas required for CO2 hydrogenation is currently produced from fossil fuels through methane steam reforming, which produces a large amount of CO2 and signifies the need for alternative renewable and environmentally benign H2 sources [12,13].Numerous alcohols have been utilized as liquid hydrogen sources in several catalytic transfer hydrogenation (CTH) reactions [14\u201318]. However, most of the alcohols were obtained from non-renewable sources and their dehydrogenated products are of low value. From this view, glycerol could emerge as a viable H2 source for two basic reasons. First, glycerol is renewable and its overproduction in the biodiesel industry makes it cheap and abundant. Second, the dehydrogenation of glycerol offers value-added chemicals such as aldehydes, ketones, diols, or carboxylic acids, which improves the economics of the biodiesel industry [19]. Along with several other important uses of glycerol [20,21], it has also been successfully utilized as a hydrogen source for the reduction of CC and CO compounds with high product yields under strongly basic aqueous conditions [22,23]. Nevertheless, the reduction of thermodynamically stable C\u2013O bonds in a CO2 molecule is more challenging than the reduction of the C\u2013O bond in carbonyl groups of organic compounds [24]. To overcome the high stability of gas-phase CO2 (\u0394G\u00b0298 K\u00a0=\u00a033\u00a0kJ\u00b7mol\u22121), hydrogenation reactions were performed in aqueous media, where CO2 was in equilibrium with HCO3\n\u2212 (pKa1\u00a0=\u00a06.35) and the reaction became slightly exergonic (\u0394G\u00b0298 K\u00a0=\u00a0\u22124\u00a0kJ\u00b7mol\u22121) [25\u201327].Interestingly, the simultaneous conversion of one industrial waste and one side product (CO2 and glycerol) to create value-added commodity chemicals (formic acid and lactic acid) can be regarded as a \u201ctwo birds, one stone\u201d strategy. Several researchers studied the reduction of CO2 in the form of bicarbonate or carbonate with glycerol and alcohol in high-temperature water (HTW), with or without transition metals (Fe, Zn, Ni, etc.) as catalysts [28\u201331]. Jin et al. reported the hydrogen-transfer reduction of NaHCO3 with glycerol and isopropanol in alkaline HTW without catalyst; however, the formic acid yield was less than 10% relative to the initial NaHCO3 concentration, indicating the low bicarbonate reduction efficiency of this reaction system [28,29]. Furthermore, the same group studied the reduction of bicarbonate in the presence of transition metals in alkaline HTW. Transition metals acted as reducing agents to reduce CO2 to formate using water as a hydrogen source. The metals were oxidized simultaneously. In a second step, glycerol might reduce metal oxides back to metals by H2 donation [30,31]. Nevertheless, the use of an equimolar (or higher) amount of metal relative to bicarbonate, higher reaction temperature (300\u00a0\u00b0C), and higher leaching probability of the catalyst due to continuous phase-change, limit the process for large scale applications.Recently, homogeneous Ir-carbene catalysts were utilized for CO2 and carbonate transfer hydrogenation (TH) using glycerol [32\u201334]. The catalysts selectively afford FA and LA with high turnover numbers (TON) within a moderate range of reaction temperature (150\u2013180\u00a0\u00b0C). Although these catalysts are active at low reaction temperatures, the development of new and more efficient heterogeneous catalysts are indispensable considering the catalyst-separation issue in homogeneous catalysis. From this perspective, Lin et al. screened several commercial, supported, noble-metal heterogeneous catalysts in one-pot aqueous-phase TH involving glycerol and bicarbonates to increase the LA and FA yield at milder reaction temperatures [35]. Carbon supported Pd showed the best results among the catalysts tested: 55% LA and 30% FA yield at 240\u00a0\u00b0C. Very recently, our research group also demonstrated that meticulously prepared, ZIF-11 derived, graphitic nanoporous carbon (Ru/NCT) catalysts could efficiently produce LA and FA from simultaneous conversion of glycerol and carbonates [36]. Moreover, this was achieved with high TON and space\u2013time yield (STY). These studies point out the enormous scope for further development of new and efficient heterogeneous catalysts.Encouraged by these previous studies, we decided to develop laboratory synthesized, supported noble-metal catalysts for the one-pot conversion of glycerol and carbonates. Among the noble metals, Pt-based catalysts are most effective for alcohol and glycerol dehydrogenation reactions [37\u201341]. Additionally, supported Pt catalysts have also been successfully utilized in several transfer hydrogenation reactions [22,42\u201344]. In this context, we planned to investigate the performance of supported Pt catalysts for the simultaneous conversion of glycerol and carbonate involving both dehydrogenation and hydrogenation steps. The reaction parameters, including temperature, time, the concentration of glycerol and carbonate, catalyst loading, water amount, and effect of the CO2 substrate, were studied systematically. The catalyst was recycled in four consecutive cycles with little change in its activity and product selectivity. Several methods were adopted for the characterization of fresh and used catalysts to better understand the factors governing the catalytic activity and the changes that occurred in the recycled catalyst.Glycerol (99%), formic acid (99%), potassium carbonate anhydrous (99.55%), potassium bicarbonate (99%), sodium carbonate anhydrous (99%), and sodium bicarbonate (99%) were purchased from Samchun Chemicals, Korea. Lactic acid solution (50% in water), glyceraldehyde (90%), 1,2 propanediol (99.5%), potassium hydroxide (90%), chloroplatinic acid hexahydrate, ruthenium(III) chloride hydrate, potassium tetrachloropalladate(II) (98%), and zirconium oxide ceramic grade (99%), were procured from Sigma-Aldrich. Aluminum oxide activated acidic gamma (96%) and activated carbon were obtained from Alfa Aesar and Strem chemicals, respectively. All the chemicals were of commercial grade and were used without further purification.Oxide-supported metal catalysts were prepared by the wet-impregnation method as follows. A portion (2\u00a0g) of the solid support and an aqueous solution (100\u00a0mL) of the metal precursor was stirred for 12\u00a0h at ambient temperature. Water was evaporated by a rotary evaporator at 50\u00a0\u00b0C under reduced pressure. After drying in an oven, the solid was calcined at 400\u00a0\u00b0C for 4\u00a0h with a heating ramp of 2\u00a0\u00b0C/min in the muffle furnace under constant airflow (150\u00a0cc/min). 1\u00a0g of calcined powder pressed into the pellet, crushed and sieved (No. 20\u201340 mesh) to make granules, and then reduced at 300\u00a0\u00b0C for 2\u00a0h (2\u00a0\u00b0C/min heating ramp) with 10% H2 in N2 flow (50\u00a0mL/min) in a conventional stainless-steel fix-bed reactor (i.d. 5\u00a0mm, length 300\u00a0mm). For the platinum-on-carbon catalyst, the calcination temperature was set to 300\u00a0\u00b0C for 3\u00a0h with a heating ramp of 2\u00a0\u00b0C/min and reduction was done in a tube furnace at 300\u00a0\u00b0C for 2\u00a0h (2\u00a0\u00b0C/min heating ramp) with 5% H2 in Ar flow (300\u00a0mL\u00a0min\u22121).Powder X-ray diffraction patterns (PXRD) of all the catalysts were obtained using a Rigaku D/Max-2200\u00a0V X-ray diffractometer (Cu K\u03b1-radiation, \u03bb\u00a0=\u00a01.5406\u00a0\u00c5) at 40\u00a0kV and 40\u00a0mA. The N2 adsorption\u2013desorption isotherms were measured at 77\u00a0K using a Micromeritics Tristar 3000 system. The samples were dehydrated under vacuum at 423\u00a0K for 12\u00a0h before analysis. The specific surface areas were evaluated using the Brunauer-Emmett-Teller (BET) method. Scanning transmission electron microscopy (STEM) analysis of the reduced catalysts was performed to analyze the distribution of metal particles, and a minimum of two hundred metal particles were considered for calculation of average metal particle size. TEM-Talos (F200X system) operating at an accelerating voltage of 200\u00a0kV was used. X-ray photoelectron spectra (XPS) were measured on a Kratos AXIS SUPRA instrument (UK) with a monochromatic Al K\u03b1 X-ray source operated at 20\u00a0eV pass energy. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was conducted on an iCAP 6500 duo series (ICP-AES) analyzer (Thermo, USA). Temperature programmed desorption (TPD) profiles of the catalysts were measured on a Micromeritics AutoChem II 2920\u00a0V5.02 apparatus equipped with a thermal conductivity detector. About 100\u00a0mg of samples were pretreated at 400\u00a0\u00b0C for 1\u00a0h in a helium flow (50\u00a0mL/min), before the adsorption step. Subsequently, pretreated samples were exposed to NH3 or CO2 gas at 40\u00a0\u00b0C for 15\u00a0min with a flow rate of 50\u00a0mL/min. Physically adsorbed NH3 or CO2 gases were removed by purging with helium gas for 30\u00a0min at the same temperature and flow rate. TPD data were recorded between 40 and 400\u00a0\u00b0C at a heating rate of 10\u00a0\u00b0C/min. Temperature programmed reduction (TPR) experiments were carried out using an AutoChem II-2920\u00a0V4.06 (Micromeritics) analyzer. About 100\u00a0mg of sample was used for each experiment, preconditioned at 400\u00a0\u00b0C/h in He gas flow at 50\u00a0mL/min. This was followed by sample analysis from room temperature to 600\u00a0\u00b0C at a heating rate of 5\u00a0\u00b0C/min under 10% Ar-H2 gas at a constant flow rate of 50\u00a0mL/min. FTIR spectra were recorded on a Nicolet FTIR spectrometer (MAGNA-IR 560) using KBr pellets.All experiments were carried out in a locally made high-temperature, high-pressure, 100\u00a0mL capacity batch reactor fitted with a standard mechanical stirrer and thermocouple. The required amount of glycerol, carbonate/bicarbonate, water, and catalyst was placed in the reactor. The reactor was sealed, purged three times with nitrogen and pressurized to 400\u00a0psi. The reaction mixture was stirred at 500\u00a0rpm and the reaction time was counted once the desired reaction temperature was reached. After reaction, the reactor was cooled to room temperature, the pressure was released by the vent valve, and the off-gas was collected in a gasbag (only for a couple of reactions). The reaction mixture was centrifuged to separate the catalyst from the liquid solution. The recovered catalyst was washed with water and methanol and dried at 100\u00a0\u00b0C in an oven before being subjected to the next cycle. Glycerol conversion and product yield were calculated based on the following formulas.\n\n\n\nC\no\nn\nv\ne\nr\ns\ni\no\nn\n\n\n%\n\n\n=\n\n\nI\nn\ni\nt\ni\na\nl\n\nm\no\nl\ne\ns\n\no\nf\n\ng\nl\ny\nc\ne\nr\no\nl\n-\nF\ni\nn\na\nl\n\nm\no\nl\ne\ns\n\no\nf\n\ng\nl\ny\nc\ne\nr\no\nl\n\n\nI\nn\ni\nt\ni\na\nl\n\nm\no\nl\ne\ns\n\no\nf\n\ng\nl\ny\nc\ne\nr\no\nl\n\n\n\u00d7\n100\n\n\n\n\n\n\n\n\nY\ni\ne\nl\nd\n\n\n\n%\n\n\n\n=\n\n\nM\no\nl\ne\ns\n\no\nf\n\np\nr\no\nd\nu\nc\nt\n\u00d7\nC\n\na\nt\no\nm\ns\n\ni\nn\n\np\nr\no\nd\nu\nc\nt\n\n\nM\no\nl\ne\ns\n\no\nf\n\nr\ne\na\nc\nt\na\nn\nt\n\u00d7\nC\n\na\nt\no\nm\ns\n\ni\nn\n\nr\ne\na\nc\nt\na\nn\nt\n\n\n\u00d7\n100\n\n\n\n\nAqueous samples were collected, diluted with water, and passed through a 0.22\u00a0\u03bcm pore-size filter. The aqueous portion was then subjected to high-performance liquid chromatography (HPLC) analysis. HPLC analysis was performed using a YL9100 HPLC system equipped with a UV\u2013VIS detector (YL9120) at 218\u00a0nm and a refractive index detector (YL9170). The samples were separated in an Aminex 87-H column from Bio-Rad, using 5\u00a0mM H2SO4 as the mobile phase at 0.5\u00a0mL/min flow and a column temperature of 45\u00a0\u00b0C. The measure of LA, FA, glyceraldehyde (GLA), and 1,2 propanediol (1,2-PDO) was calibrated using multiple-point external standard methods to calculate their yield via HPLC. Typical HPLC chromatograms with UV and RI detectors were depicted in Fig. S1. With our HPLC operational conditions, glycerol, LA, and FA peaks overlapped in the RID detector, which makes quantification of glycerol difficult. For this reason, its conversion was calculated from proton nuclear magnetic resonance (\n1H NMR) spectroscopy using a Bruker 500\u00a0MHz NMR spectrometer. A typical 1H NMR spectrum of a liquid sample is shown in Fig. S2. The analysis sample was prepared by dissolving a known amount (~5\u00a0mg) of isonicotinic acid as an external standard in 2.5\u00a0mL of product solution, followed by dilution with D2O. Collected gas samples were analyzed by a gas chromatograph (GC) equipped with a Carboxen 1010 PLOT column and a thermal conductivity detector (TCD). A typical chromatogram is depicted in Fig. S3.In general, bicarbonates/carbonates are utilized as CO2 sources considering their high solubility in water, operational simplicity, and accuracy in the quantification of CO2\n[28,29,32\u201336,45]. Moreover, because it was decided to perform the reaction using a more environmentally friendly method, highly corrosive strong bases were avoided. Therefore, carbonate was chosen over bicarbonate to increase the initial pH of the reaction that required to promote glycerol dehydrogenation.As shown in Table 1\n, a catalyst screening study was performed for simultaneous conversion of potassium carbonate and glycerol at 180\u00a0\u00b0C. The four major reaction products (LA, FA, GLA, and 1,2-PDO) were observed for almost all the catalysts tested. GLA is the most common product of glycerol dehydrogenation (the first and key step) and subsequently undergoes several consecutive reactions that include dehydration, keto-enol tautomerization, and Cannizzaro reaction to produce lactic acid (Scheme 1\n) [46]. On the other hand, formation of 1, 2-PDO and formic acid was attributed to the hydrogenation of intermediate pyruvaldehyde and a CO2 source, respectively. Importantly, the hydrogen eliminated from glycerol in the dehydrogenation step was utilized for these hydrogenation reactions.The glycerol conversion and product yields were negligible in the absence of the catalyst (Table 1, entry 1); however, when Pt/\u03b3-Al2O3 was used for the reaction (Table 1, entry 5) 25% glycerol conversion and ~13% yields of LA and FA were achieved. This highlighted the role of the catalyst in the simultaneous conversion of glycerol and carbonate. Pt/\u03b3-Al2O3 catalyst lowers the reaction temperature required for this reaction, which in the absence of a catalyst, is reported in previous studies to require ~300\u00a0\u00b0C [28,29]. Moreover, PtOx/Al2O3 (before reduction) was found less effective than Pt/\u03b3-Al2O3 (after reduction) (Table 1, entry 2 and 5). This suggests that metallic Pt species on the surface of the catalyst are more active than PtOx species, confirmed through XPS analysis (Table S1 and Fig. S4). It is also possible that some of the PtOx gets reduced during the reaction by the hydrogen released from glycerol.In an aqueous-phase glycerol conversion reaction, formic acid formation through the decomposition of glycerol derived products such as lactic acid cannot be ignored [46,47]. Hence, to determine whether FA is generated through the hydrogenation of carbonate or from the degradation of lactic acid, we performed two sets of controlled experiments. In the first set (Table 1, entry 4), the reaction was performed in the absence of K2CO3; however, KOH was added to adjust the pH of the reaction mixture to 11.7 (initial pH with K2CO3). In the second set (Table 1, entry 3), the reaction was carried out without the addition of glycerol. In both cases, no FA was detected, which confirmed that FA was formed through the reduction of K2CO3 by hydrogen released from the glycerol.We screened a variety of supported noble-metal catalysts for the simultaneous conversion of glycerol and carbonate under the same reaction conditions. First, \u03b3-Al2O3-supported Pt, Pd, and Ru catalysts were tested and it was found that the Pt catalyst (Pt/\u03b3-Al2O3) showed the highest activity, followed by Pd and Ru (Table 1, entry 5\u20137). Pt/\u03b3-Al2O3 catalyst showed potential for both glycerol dehydrogenation and hydrogenation of carbonate, whereas Pd/\u03b3-Al2O3 was just a little behind the gamma-alumina supported Pt catalyst in terms of glycerol conversion and LA yield however, it produced an equal amount of FA, indicating its potential in the hydrogenation reaction. Glycerol conversion and acid yields were significantly lower in the case of Ru/\u03b3-Al2O3 catalyst as compared to Pt and Pd catalysts supported on gamma-alumina. In addition, when we compared these catalysts in terms of TON, once again Pt/\u03b3-Al2O3 was significantly ahead of other catalysts (the highest TON: 459 and 116 for LA and FA, respectively). To investigate the difference in catalytic activity, catalysts were subjected to various characterization techniques, and their textural and chemical properties are reported in Table S1 and Table 2\n. As shown in Table 2 the actual metal content of the catalysts measured by ICP-AES analysis is close to their theoretical content (3%). Despite having a higher metallic content (M0), alumina-supported Ru showed lower activity than Pt and Pd catalysts did. This was primarily attributed to its poor distribution on the alumina support, and significantly bigger particle size discussed thoroughly in the subsequent section.As shown in Fig. 1\n, Pt and Pd particles are finely distributed on the alumina support, with a narrow size distribution in the ranges 0.8\u20133.2\u00a0nm and 1.6\u20134.8\u00a0nm, and average particle size of 1.7 and 3.0\u00a0nm, respectively (Fig. 1a and b). In contrast, Ru particles are poorly distributed on the alumina support, with broader particle-size distribution in the range 10\u201350\u00a0nm and an average particle size of 20.6\u00a0nm (Fig. 1c). Despite having similar metal content, a significant difference between the dispersion of Ru particles and Pt and Pd particles on gamma-alumina support is most probably associated with the location of metal in the precursor. Pt/\u03b3-Al2O3 and Pd/\u03b3-Al2O3 prepared from H2PtCl6 and K2PdCl4 precursors, respectively, where metals are in anionic position, whereas Ru/\u03b3-Al2O3 prepared from RuCl3 precursor with Ru in cationic part. This may further suggest that precursor with metal in the anionic group is favorable for preparing catalyst with better dispersion and smaller particle size [48]. However, some other effects such as valance state of metal in precursor, the content of chloride ions in precursor, and pH of metal precursor solution in water on decomposition procedure of precursor and dispersion of metal particles on support cannot be ruled out completely. Therefore, a further detailed investigation is required in this matter. The results from the STEM analysis, which are in good agreement with the PXRD analysis results, reveal large, visible diffraction peaks of Ru0. The peaks for Pd0 are small, yet visible, and there is no observable diffraction peak for Pt0. This confirms its high dispersibility on the alumina support (Fig. 2\n).The reducibility of the gamma-alumina supported catalysts was examined by H2-TPR analysis, and the results are shown in Fig. 3\n. All three catalyst profiles feature essentially one main reduction peak related to the reduction of respective metal oxides to metal, and are consistent with previous reports [49\u201351]. However, such events are observed at different reduction temperatures, probably due to differences in strength of the metal-support interaction. A sharp, intense, reduction peak at 85\u00a0\u00b0C was observed for Ru/\u03b3-Al2O3, whereas less intense and broader peaks were detected for alumina-supported Pd and Pt catalysts in the temperature range 40\u2013250\u00a0\u00b0C. This indicates that Pt strongly interacts with the gamma-alumina support (followed by Pd and Ru, respectively). This strong metal-support interaction leads to fine dispersibility of Pt on gamma-alumina as we have already confirmed through PXRD and STEM analysis.To check the role of the support in simultaneous conversion of glycerol and carbonate, Pt was further impregnated on ZrO2 and carbon supports and then their catalytic activities were compared with Pt/\u03b3-Al2O3 (Table 1 entry 8 and 9). Using Pt/ZrO2 catalyst, yields of LA and FA are much lower than with Pt/\u03b3-Al2O3 catalyst, even after achieving a similar level of glycerol conversion. On the other hand, Pt/C showed much higher glycerol conversion (41.7%) and similar LA yield (11.7%). However, the FA yield (4.8%) was significantly lower than with Pt/\u03b3-Al2O3. Both Pt/ZrO2 and Pt/C catalyst seem less active in transferring hydrogen to carbonate and end-up with very low FA yields. Moreover, it is noteworthy that selectivity for LA from glycerol is higher on all the gamma-alumina-supported catalysts, in comparison with the carbon- and zirconium oxide-supported catalysts.To rationalize the differences in catalytic activities, catalysts were characterized using various techniques. Although the surface area of Pt/ZrO2 is ~20 times smaller than that of Pt/\u03b3-Al2O3, we found very little difference in the glycerol conversions. In contrast, Pt/C has around 10 times larger surface area than Pt/\u03b3-Al2O3 (Table 2, Fig. S5). However, that perspective enhancement in glycerol conversion was not significant, indicating that the surface area of the catalyst alone does not have a substantial effect on its catalytic activity. STEM analysis (Fig. 1a, d, and e) showed that the Pt metal particles are well dispersed on all three of the supports. The average metal particle size of Pt on the alumina or carbon support is the same (1.7\u00a0nm), while it is slightly higher on the zirconium oxide support (2.2\u00a0nm). This high dispersibility of Pt on different supports was further confirmed with the results from PXRD analysis (Fig. 2). There, no metal diffraction peak was observed for Pt/C or Pt/\u03b3-Al2O3, and a very small peak was observed for Pt/ZrO2. Because all three catalysts showed fine metal dispersion on their respective support and did not have a significant difference in metal particle size, we deepened our study by accessing the chemical properties of the catalyst using TPD analysis.The acidic properties of the catalysts were studied using NH3-TPD and the results presented in Fig. 4\n, Fig. S6, and Table 2. All three gamma-alumina-supported catalysts (Ru, Pd, and Pt) possess a high concentration of weak to moderate acid sites (0.6\u20130.74\u00a0mmol/g) in the temperature range of 40\u2013400\u00a0\u00b0C. However, when we compare acidity of Pt supported on three different supports (Fig. 4), it can be seen that Pt/\u03b3-Al2O3 show weak and moderate acid sites in the temperature ranges 40\u2013215\u00a0\u00b0C and 215\u2013400\u00a0\u00b0C, respectively. In contrast, the results for the other two catalysts indicate the presence of only weak acid sites in the temperature range 40\u2013215\u00a0\u00b0C. In Table 2, the total amount of acid sites from the results of the integration of the TPD profile was reported as moles of NH3 per unit weight. The concentration of acid sites is higher in the case of the Pt/\u03b3-Al2O3 catalyst than with the other two catalysts (Fig. 4). The Pt/C catalyst showed little acidity, probably due to the presence of surface functional groups such as OH and COOH, as shown in Fig. S7. When we correlated the acid properties of the catalysts with their catalytic reactivity, it was found that the Pt/\u03b3-Al2O3 catalyst, which has a large number of acid sites with weak to moderate strength, might be favorable for the adsorption of glycerol and carbonate on its surface. In this process, it may engage with carbonate/bicarbonate anions in the aqueous medium and decrease OH\u2212 ions availability. However, the concentration of carbonates/bicarbonates is still higher than the concentration of acid sites of the catalyst utilized in the reaction, which provides room to proceed. In compensation, Pt/\u03b3-Al2O3 would have higher carbonate molecules adsorbed on its surface than Pt/C and Pt/ZrO2 surfaces, resulting in nearly three times higher formate yield for earlier compared to later catalysts (Table 1, entry 5, 8, and 9). Furthermore, the high dispersibility and small particle size of Pt on its surface facilitates the reaction to produce LA and FA. Sievers et al. thoroughly studied the surface interaction of glycerol with various metal oxides having acid and base properties using FTIR spectroscopy. The study results reveal that even in the presence of water, the primary alcohol groups of glycerol strongly bonded to the Al sites of gamma-alumina to form bridging alkoxy bonds. The secondary alcohol groups of glycerol exhibited an additional hydrogen-bonding interaction with surface oxygen atoms of the gamma-alumina [52]. This implies that the \u03b3-Al2O3 support has a clear advantage over ZrO2 and carbon support for the adsorption of glycerol molecules to its surface.Based on preliminary catalyst screening tests and a characterization study, higher activity was observed with Pt/\u03b3-Al2O3. This is attributed to the high dispersion of Pt nanoparticles on the alumina support and its strong interactions with glycerol due to the presence of a large number of acidic sites with weak to moderate strength, compared to the other tested supports. The reaction temperature used for catalyst screening in this work was much lower (180\u00a0\u00b0C) than that used with the previously studied heterogeneous, carbon-supported noble-metal catalysts at 240\u00a0\u00b0C, for the same reaction [35]. This resulted in lower catalytic activity. Glycerol-to-lactic acid occurs via an endothermic reaction, it is favored at high temperatures. This is because, under mild conditions, it is difficult to remove OH groups from glycerol and then oxidize it to glyceraldehyde [53]. Considering this point, we examined the effect of the reaction temperature on catalyst activity and product yield, as shown in Fig. 5\na. Glycerol conversion was low at lower reaction temperatures (from 160 to 200\u00a0\u00b0C); however, thereafter, rapid improvement was noticed. Lactate and formate yields also increased with reaction temperatures up to 220\u00a0\u00b0C; however increasing the temperature further to 240\u00a0\u00b0C, resulted in decease in the formate yield. This was probably due to the decomposition of potassium formate in the presence of catalyst at a higher temperature. This kind of decomposition pattern for LA and FA salts was also reported over Pd/C catalysts, although at higher temperature 270\u00a0\u00b0C [35].\nFig. 5b and c show the effect of glycerol and the K2CO3 concentration on product yields, respectively. As the concentration of glycerol declined from 2\u00a0M to 0.5\u00a0M without changing the K2CO3 concentration, the glycerol conversion and lactate yield increased steadily from ~49 to 97% and from ~22 to ~46%, respectively. In contrast, the formate yield progressively decreased from ~24 to ~12%. In hydrothermal media, OH\u2212 ions generated from equilibrium reactions between carbonates and water [54] might also act as a catalyst to promote the glycerol-to-lactate reaction [46,55]. At low glycerol concentration, more CO3\n2\u2212 ions generate more OH\u2212 ions, which promote the dehydrogenation of glycerol and increase the lactate yield. On the other side, because the lower quantity of glycerol hydrogen produced from the dehydrogenation step is insufficient to reduce the carbonate, the result is a decrease in the formate yield.To achieve higher formate yield, we reduced the concentration of carbonate from 0.5\u00a0M to 0.125\u00a0M while the glycerol concentration was kept constant at 0.5\u00a0M, as shown in Fig. 5c. As we expected, the FA yield increased from ~12% at 0.5\u00a0M to 26% at 0.25\u00a0M. However, it dropped to ~17% with further decline in the carbonate concentration to 0.125\u00a0M. In contrast, a decrease in the carbonate concentration ultimately reduced the reaction pH and the concentration of OH\u2212 ions in the aqueous media. Because fewer OH\u2212 ions were available to promote the reaction, glycerol conversion dropped from 97 to ~67%, and the corresponding lactate yield from ~46 to 36%. This observation indicates that an optimal ratio of glycerol to carbonate is required to maximize the lactate and formate yields. The hydrogenation of bicarbonate/carbonate, and the decomposition of formate, are both reversible reactions. Thus, it is very likely that at a certain temperature and pressure, the formate yield is limited by the reaction equilibrium.Owing to the importance of subcritical water in an aqueous-phase glycerol transformation reaction, the effect of the amount of water on the product formation was examined (Fig. 5d). With increasing water amount from ~38 to ~58\u00a0mL, conversion dropped slightly from 84 to 78%. This could be related to a slight drop in the reaction pH. The formate yield remains stable at ~25 to 26% while a moderate improvement from ~41 to 50% in the lactate yield was observed. This is probably due to the higher distribution of glycerol molecules on the catalyst surface, or to lower decomposition of lactate under the higher-pressure conditions generated by increased volume of the reaction solution.The effect of catalyst amount is shown in Fig. 6\na. It can be seen that, by increasing the amount of the catalyst from 0.1 to 0.2\u00a0g, glycerol conversion increases (54\u201378%), along with the yield of lactate (22\u201350%) and formate (10\u201326%). Catalyst amounts higher than 0.2\u00a0g inversely affect the glycerol conversion and lactate yield (drop to 61 and 28%, respectively, using 0.4\u00a0g of the catalyst). This might be associated with the presence of an excess amount of acidity from the gamma-alumina support, which could engage with carbonate anions in the aqueous medium and thereby decrease OH\u2212 ions responsible for promoting lactate formation through the glycerol dehydrogenation step. Another reason for a drop in lactate yield evident at higher catalyst loading hinted at the possibility of lactate decomposition in the presence of excess acidity and metallic sites. On the other hand, an increase in formate yields at higher catalyst loading is probably associated with an increase in metallic sites and the decomposition of lactate. In brief, a catalyst amount beyond 0.2\u00a0g inhibits the formation of lactate and promotes the formation of formate. Therefore, 0.2\u00a0g of catalyst amount was chosen for further experiments.The influence of reaction time on the conversion and product yields was studied at 220\u00a0\u00b0C (Fig. 6b). The yield of lactate and formate increased rapidly with extension of the reaction time. Lactate reaches a plateau at 12\u00a0h with 50% yield, whereas formate yield starts declining after reaching the maximum yield of 29% at 8\u00a0h. The yield of 1, 2-PDO was initially stable (near 6% up to 12\u00a0h), but then climbed to 10% at 16\u00a0h. In similar results, Huo et al. reported that longer reaction resulted in the decomposition of formic acid and a drop in its yield during hydrogenation of bicarbonate over skeletal CuAlZn catalyst [45]. Therefore, 8\u00a0h of reaction time was chosen for further experiments.As shown in Fig. 6c, the various CO2 sources (K2CO3, Na2CO3, KHCO3, NaHCO3, and CO2) were tested as hydrogen acceptors against glycerol. It has already been reported that only bicarbonate is known to undergo hydrogenation to produce FA [25]. Among the CO2 sources used herein, carbonates were favored over bicarbonates in terms of LA and FA yield. Moreover, the best result with K2CO3 (compared to those with the other CO2 sources) is attributed to the initial high pH of the solution, which is essential for the dehydrogenation of glycerol to produce LA. Furthermore, alkali cations on carbonates can also affect the reaction. This is probably due to their different solubility and basicity, as well as to the formation of alkali salt products. The latter could be realized due to the dissimilar results when using K2CO3 and Na2CO3 as CO2 sources. Direct use of gaseous CO2 after adjusting the initial pH with KOH (same value as with K2CO3) produces 1,2-PDO as a predominant product with very little lactate and no formate. This indicates that the addition of CO2 makes the reaction solution acidic. This causes utilization of a different reaction path from pyruvaldehyde to 1,2-PDO, instead of to LA. It has already been reported that a basic condition is required to facilitate the production of LA from glycerol and that 1,2-PDO is favored by lowering the reaction pH [56]. These results are similar to those in previous studies using homogeneous and heterogeneous catalysts for the reduction of direct CO2 using glycerol as hydrogen source: very low yield of lactate and formate [32,35].The recycle tests of the Pt/\u03b3-Al2O3 catalysts were performed under optimized reaction conditions; however, they were done at lower conversions in order to clearly observe the changes in catalytic activity. After each cycle, the catalyst was simply washed with water and methanol to remove surface substances, and then reused in the next run after drying at 100\u00a0\u00b0C in an oven. As shown in Fig. 6d, the Pt/\u03b3-Al2O3 catalyst can be reused four times, albeit with a little change in catalytic activity and product distribution. Surprisingly, after the first cycle, both glycerol conversion and lactate yield increased marginally from 70-85% and 34\u201341%, respectively. The catalyst remained stable thereafter until the fourth cycle. On the opposite side, formate yield dropped moderately from 22% to 16%. The substance 1, 2-PDO, a hydrogenation product of glycerol and a competitor with formate for accepting the H2 released from glycerol was increased slightly. In addition, no Pt species was detected by ICP analysis of the solution after the reaction. However, 2.5% and 3.1% Al leaching from the support was noticed after the first and fourth cycles, respectively. Dissolution of the catalyst support in a high-temperature aqueous environment was reported earlier, where up to 2% of the available aluminum was dissolved from Pt/\u03b3-Al2O3 catalyst after 4\u00a0h in the water at 200\u00a0\u00b0C [57]. It was also reported that dissolved aluminum cations can have a catalytic role [57]; therefore, we performed a hot filtration leaching test (see Table S2) to check the effect of dissolved Al species. In this experiment, after 2\u00a0h the reaction was stopped, the catalyst was filtered in a hot condition, and then the reaction continued for the next 6\u00a0h without catalyst. The lactate yield was almost unchanged while the formate yield decreased from 20% to 13%. This indicates the decomposition of formate in the presence of leached Al species.To rationalize the changes in catalytic activity and product yields, the spent catalyst was characterized using PXRD, STEM, XPS, and NH3-TPD analyses, as shown in Figs. 7\u20139\n\n\n, Fig. S6, and Table S3. Fig. 7 shows the PXRD patterns of the fresh and used Pt/\u03b3- Al2O3 catalyst after the first and fourth cycles. It can be seen clearly that amorphous gamma-alumina converted to crystalline boehmite after the first use, and that the crystallinity further increased after consecutive reuses. Due to the high dispersion of Pt on \u03b3-Al2O3, the diffraction peak of Pt was absent; however, it was clearly evident on the boehmite phase at 40 degree 2\u03b8 angle. The crystallite size of the metallic Pt calculated from the Scherrer equation was 3.3 and 4.2\u00a0nm after the first and fourth cycles, respectively, as denoted in the inset of Fig. 7. This kind of phase change of the Pt/\u03b3-Al2O3 support from \u03b3-Al2O3 to boehmite was also reported previously for aqueous-phase reforming of glycerol, and liquid-phase reforming of lignin at 220 and 225\u00a0\u00b0C, respectively [58,59].STEM analysis of fresh and used Pt/\u03b3-Al2O3 catalysts suggested the aggregation of Pt nanoparticles after the first recycle test (Fig. 8). The size of the Pt nanoparticles increased from 1.7\u00a0nm to 5.2 and 5.6\u00a0nm after the first and fourth cycles, respectively.The surface chemical states of platinum in the fresh and used catalysts were further investigated by XPS (Fig. 9). Although the Pt 4f levels produce the most intense XPS line, this energy region became overshadowed by the presence of a very strong Al 2p peak from the support. Consequently, the energy region of the less intense Pt 4d peak was recorded. The binding energies are summarized in Table S3. All three samples show a broad and asymmetric Pt 4d5/2 peak that could be resolved (after curve fitting) into two components with binding energies of 314.2\u2013314.4 and 315.7\u00a0eV, corresponding to the Pt0 and Pt2+ species, respectively [60\u201362]. The fresh catalysts show high content of oxidized Pt species on the surface alumina, probably due to strong Pt-support interaction that hinders reduction [63]. The used catalysts contained a high content of Pt0 species, ascribed to the potential for in situ reduction of Pt2+ in the presence of hydrogen released from glycerol during the reaction. H2 formation during the reaction was confirmed through GC-TCD analysis of the vent gas (Fig. S3). The changes in acidic properties of fresh and spent catalysts were probed by NH3-TPD (Fig. S6). It can be seen that spent catalyst shows weak to moderate acid sites in the temperature region of 40\u2013400\u00a0\u00b0C as like fresh catalyst, however, in a lower concentration. The concentration of total acid sites decreased from 0.64 to 0.36\u00a0mmol/g after the first cycle.Therefore, the results obtained from the characterization of used catalysts demonstrated that change in the catalytic activity and product distribution mainly arose from phase-change on the support, which was responsible for three other consequences (i.e., leaching of Al species into the reaction solution, sintering of Pt nanoparticles and decrease in acidity of the catalyst). Glycerol conversion and lactate yield increases with the enlargement of the Pt particles. This could be correlated with the general fact that the larger the metal-particle size, the lower the undercoordinated sites and the higher the fraction of plane sites. A similar phenomenon was reported by Shimizu et al., where bigger Pt atoms showed higher activity than smaller undercoordinated Pt atoms for oxidant-free dehydrogenation of glycerol to form lactic acid [32]. Moreover, the metallic platinum content is higher in used catalyst than in fresh catalyst, enhancing the glycerol conversion and lactate yield. This is because Pt0 showed higher activity than PtOx did during the catalyst screening test (Table 1, entries 3 and 5). In addition, a decrease in acid sites in the spent catalyst might have also been helped to promote the glycerol conversion and LA yield (since the availability of OH\u2212 ions would be higher). On the other hand, the phase change of the support, sintering of the metallic Pt nanoparticles and decrease in acid sites (adsorption of carbonates would be lower) had adverse effects on the formation of formate from carbonate. Moreover, a higher percentage of dissolved Al species in the reaction solution after the first cycle could also be responsible for the reduction in the formate yield as per the results from the hot filtration leach test.In summary, we have extended the work on a very new and challenging topic comprising one-pot conversion of glycerol and a CO2 source into lactic and formic acid salts. Among the several supported noble-metal catalysts tested, Pt/\u03b3-Al2O3 showed higher catalytic activity and product selectivity. This was due to the high dispersibility of Pt nanoparticles and to the higher acidity of gamma-alumina support. This probably helps to form strong interactions between glycerol and the carbonate molecules on its surface. Several reaction parameters were optimized and up to 50% lactate and 26% formate yields were achieved, most importantly without using any additional strong base and external H2, which otherwise a mandatory requirement to obtain high lactic and formic acid salts from glycerol and CO2 source, respectively. The catalyst was reused for four consecutive cycles with little variation in catalytic activity and product distribution. Phase transfer of support from amorphous gamma-alumina into crystalline boehmite causes aggregation of Pt nanoparticles and leaching of \u2018Al\u2019 species in reaction media which subsequently responsible for the change in the catalytic activity of the used catalyst. Our future research work on this topic will be focusing on the development of a robust bimetallic catalyst system that can overcome the issue of catalyst stability and low product yields. By utilizing this novel approach, waste biomass products from bio-refineries can be integrated with CO2 (industrial waste) to produce value-added chemicals which will ultimately help in the improvement of overall bio-refinery economics with considerable beneficial impact on the environment.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 Next Generation Carbon Upcycling Project (2017M1A2A2043143) by the National Research Foundation of Korea, and was partially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry, and Energy (MOTIE) of the republic of Korea (No. 20202020800330). The authors would like to thank Dr. D. Y. Hong and Dr. D. W. Hwang for helpful discussions.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jiec.2021.06.023.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Glycerol and carbonates (used as CO2 sources) were simultaneously converted to carboxylic acid salts under mild hydrothermal media over supported Pt catalysts. The dehydrogenation of glycerol produced lactate (LA); at the same time, hydrogen molecules released from glycerol were effectively transferred to reduce carbonate or bicarbonate ions to formate (FA). Several reaction parameters, including temperature, time, glycerol and carbonate concentration, water amount, catalyst loading, and CO2 source were evaluated. Under the optimized reaction conditions, ~50% yield of LA from glycerol and ~26% yield of FA from potassium carbonate were achieved concomitantly over Pt/\u03b3-Al2O3 catalyst. Importantly, this was done without using external H2 or additional strong base. The textural, structural, and chemical properties of the catalysts were evaluated using N2 adsorption\u2013desorption, powder X-ray diffractometry (PXRD), inductively coupled plasma-atomic emission spectrometry, scanning transmission electron microscopy (STEM), temperature programmed reduction, and temperature programmed desorption analysis. The catalyst was reused for four consecutive cycles with little variation in catalytic activity and product distribution. Used catalysts were further characterized using, PXRD, STEM, and X-ray photoelectron spectral analysis to better understand the structural and chemical changes that occurred in the recycled catalysts, and the factors governing change in the catalytic activity. A plausible reaction pathway was proposed based on the catalytic results and the product distribution data obtained.\n               "}
{"full_text": "Data will be made available on request.Urban centers around the planet face problems generated by the high level of polluting gases and environmental deterioration which in conjunction with the depletion of fossil fuels and the growing energy demand, have led to the search for alternative energy sources through more efficient systems and sustainable processes [1\u20132]. In this context, the use of biomass has begun to spread since it turns out to be an available and sustainable alternative. Biomass comprises materials derived from plants or animals and is considered a renewable energy source, some examples of biomass are wood, crops, animal manure, among others. In particular, lignocellulosic biomass represents the most renewable and abundant carbon resources and is recognized as the most sustainable alternative to fossil resources [3], it can be divided into three categories, softwood, hardwood and grass, and is made up of cellulose (40\u201360\u00a0%), hemicellulose (10\u201340\u00a0%) and lignin (15\u201330\u00a0%) [4]. Especially, lignin is an important component of lignocellulosic biomass and due to its aromatic distinctiveness, this biopolymer becomes the perfect raw material to obtain low molecular weight aromatic substances. Lignins are phenylpropanoid polymers made up of three main units: syringyl, guaiacyl and hydroxyphenyl, which are recognized as the most abundant renewable aromatic carbon source on earth and their effective utilization is critical for the accelerated development of lignocellulosic biorefinery [5].Although biomass seems unlikely to replace oil, it has great potential as raw material for the generation of high-value chemical products in industry. The ideal depolymerization process involves the selective degradation of lignin into monomeric products [6].A great variety of techniques can be found for the depolymerization of lignin, such as pyrolysis, hydrocracking, hydrogenolysis, hydrolysis and oxidation. To know, the most studied strategies have been the oxidative approach, the solvolytic (basic or acid) and the pyrolytic process. Unfortunately, many of these methods remove too much oxygen and/or disrupt the aromatic ring to produce low-value chemicals.Hydrogenolysis is considered a promising method for the efficient depolymerization of lignin and lignin fragments since it requires less severe reaction conditions, and a moderate yield of monomers can be obtained. Heterogeneous hydrogenolysis catalysts based on transition metals (for example, Fe [7], Co [8] and Ni [9\u201310]) which are abundant on earth are actively explored with the aim to replace precious metal catalysts. Recently, Liu et al. reported the use of Ni@ZIF-8 catalyst in the reductive catalytic fractionation (RCF) of eucalyptus sawdust to produce phenolic compounds and \u03b2-O-4 structures using hydrogen (3\u00a0MPa) at 220\u2013260\u00a0\u00b0C [9]. On the other hand, Li et al. found that nickel single atom catalysts exhibited twice higher activity in lignin depolymerization compared to nickel cluster catalysts, Scheme 1\n\n[11]. However, the preparation of noble metal-free single-atom catalysts continues to be a challenge in metal anchorage, while for their part, avoiding the single-sites agglomeration that favors the nanoparticle formation encompasses another unmet need. Additionally, atomically dispersed supported metal catalysts maximize the efficiency of metal utilization and act as a robust support for metal nanoparticles [12]. MNC (Fe, Co, Ni) single-atom catalysts (SACs) with metals strongly linked to surrounding N atoms have shown promising performances in electrochemical [13] and thermochemical reactions [14].On the other hand, metal organic frameworks have emerged as a class of promising materials given their structure and adsorption capabilities [15] and their use as gas adsorbent materials, sensors [16] and catalysts [17\u201318], as previously described.In this study, we prepared and characterized catalysts based on NiNC material to evaluate their activity in transfer hydrogenolysis reactions using formic acid as hydrogen source finding excellent activity and selectivity in lignin-derived aryl ethers and kraft lignin. Our results in the catalytic hydrogenolysis reactions of guaiacylglycerol-\u03b2-guaiacyl and kraft lignin using nickel catalysts evidenced that nanometric and sub nanometric species are coexisting and having a synergic effect on the reaction. Additionally, zinc chloride and scandium triflate Lewis acids successfully catalyzed the hydrogenolysis of 1-(o-tolyloxy)propan-2-ol, a compound obtained from the nickel-catalyzed reaction of guaiacol and propylene carbonate. Overall, these results provide a synergic catalytic strategy to achieve selective hydrogenolysis of CO bonds in guaiacylglycerol-\u03b2-guaiacyl, 1-(o-tolyloxy)propan-2-ol, and kraft lignin.\nNi-1 catalyst was prepared from Ni(OAc)2\u00b74H2O and 1,10-phenanthroline ligand, followed by pyrolysis at 600\u00a0\u00b0C for 2\u00a0h and subsequent acid leaching as schematically detailed in Fig. 1\na, similar to the procedure reported by Wang et al., who described acid-resistant NiNC single-atom catalyst (SAC) to hydrogenation of some unsaturated cellulose-derived substrates [19].However, we found nickel nanoparticles formation of diameters larger than 5\u00a0nm which suggests the existence of sub nanometric species, Fig. 1b. As previously described, the presence of nanoparticles may not exclude the presence of single atoms, where they in fact coexist, it results difficult to obtain atomically dispersed species since they hold a high surface energy thus tending to agglomerate as clusters or nanoparticles in order to decrease such energy [20\u201321].In PXRD patterns, carbon-supported Ni materials have a broad shoulder diffraction peak around 25\u00b0 due to the graphitized carbon support.In PXRD patterns, carbon-supported Ni materials have a broad shoulder diffraction peak around 25\u00b0 due to the graphitized carbon support. In the case of Ni-1, other XRD signals are not detected due to the low metal loadings in single atoms or small clusters [22], whereas Ni/C material has a diffraction peak attributed to NiO (220) and metallic Ni [(111), (200), (220), (220); JCPDF No. 65\u20130380] [23\u201324], Fig. 2\na. On the other hand, EPR spectra of Ni-1 exhibited a signal at g\u00a0=\u00a02.003 due to carbon radicals coming from the material support at 77\u00a0K, while the Ni/C spectral profile is indicative of nanoparticles formation, Fig. 2b. To explore the chemical environment and bonding to Ni-1, which has the highest activity in the hydrogenolysis reaction, we carried out an X-ray photoelectron spectroscopy (XPS) analysis. The fitted Ni2p and N1s spectra are shown in Fig. 2c.Moreover, N1s spectrum can be fitted by four peaks indicating the presence of pyridinic N (398.89\u00a0eV), pyrrolic N (400.24\u00a0eV), graphitic N (402.14\u00a0eV) and, oxidized N (405.5\u00a0eV) species [25\u201326]. The wt.% of nickel in Ni-1 was found to be 4.14\u00a0wt%, calculated by XPS.We then proceeded to assess the evaluation of Ni-1 and other nickel sources toward guaiacylglycerol-\u03b2-guaiacyl ether (L1), an abundant linkage in lignin. Table 1\n shows the L1 transfer hydrogenolysis with formic acid as a hydrogen donor at 150\u00a0\u00b0C for 2\u00a0h.\nNi-1 showed the highest activity in CO cleavage compared to Ni/C and Ni/Zn-2-Meth.\nNi-1 catalyzed the CO bond cleavage selectively to guaiacol and isoeugenol (2-metoxy-4-[(E)-1-prophenyl] phenol) using an ethanol/H2O solvent system. When the reaction time surpassed 24\u00a0h, formation of 2-methoxy-4-propylphenol product was observed, entry 6.[a] All yields were determined by GC\u2013MS with dibenzothiophene as standard. [b] Ni:MgO ratio 1:160. [c] for 24\u00a0h.Based on these results and previously described works [27], we propose a reaction pathway for L1 hydrogenolysis to produce guaiacol and isoeugenol. First, a dehydration reaction of L1 occurs which is promoted by medium acidity. Then, compound A hydrogenation is reached by formic acid as hydrogen source, this reaction is catalyzed by Ni-1 to generate the alcohol function (compound D) and guaiacol, while subsequent dehydration and hydrogenation steps lead to the isoeugenol synthesis, Scheme 2\n\n.\nSubsequently, the reaction between guaiacol and the product of propylene carbonate decarboxylation catalyzed by Zn/Ni-2-Meth and ZIF-8 to form 1-(o-tolyloxy)propan-2-ol was performed, thus investigating the reaction scheme with different phenol-derived substrates. Fortunately, we obtained excellent conversion values, above 90\u00a0% as shown in Table 2\n. As mentioned, to compare with the guaiacol and propylene carbonate reaction scope, the phenolic substituents here explored were 2-metoxy-5-methylphenol, 2,6-dimethoxyphenol, 3-(dimethylamino)phenol, 2-metoxy-4-(2-propyl)phenol and 2-amino-4-tert-butylphenol, where the Ni/Zn-2-Meth catalyst successfully reproduced the reaction conversions above 90\u00a0% regardless of the substituent nature. Notably, the synthesis of these compounds was carried out under neat conditions since the reagents also acted as reaction solvents. As can be seen, those reaction products in the phenol reaction scope study were analogous to product C.It is proposed that in the first step of the reaction mechanism a phenol deprotonation occurs, whose species undergo a nucleophilic attack on propylene carbonate and promotes the ring-opening, Scheme 3\n.In addition, the hydrogenolysis of lignin-related molecules in the presence of Lewis acids (LAs) has been described, exerting a synergic effect with palladium compounds in the monomer formation from lignin [28\u201329] Motivated by these results and the ability of LAs to effectively polarize CO bonds making them more susceptible to hydrogenolysis, we then turned to explore the effect of different LAs in the CO bond cleavage of lignin and lignin model molecules, Scheme 4\n\n. Previously, Abu-Omar et al. described the synergic effect between Pd/C and Zn2+ in lignin hydrodeoxygenation [30]. Benzodioxanes can be obtained from 1-(o-tolyloxy)propan-2-ol with substoichiometric amounts of scandium triflate, the formation of guaiacol from the breaking of the CO bond requires the presence of formic acid, the disadvantage of using Lewis acids in the hydrogenolysis of L1 molecule relates to the inherent generation of multiple compounds without any selectivity. Tabanelli et al. prepared 2-hydroxymethyl-1,4-benzodioxane through the decarboxylation of cyclic carbonates and catechol as nucleophile in basic medium [31]. Additionally, Cui et al. described the use of NiAlOx catalyst in the presence of La(OTf)3 to produce aryl\u2013alkyl derivatives through the reductive hydrogenolysis and hydrogenation under hydrogen pressure (4\u00a0bar) [32].Results of CO bond cleavage for 1-(o-tolyloxy)propan-2-ol with ZnCl2 are shown in Table 3\n. However, only the esterification product, 7\u00a0% conversion of guaiacol and traces of benzodioxane were obtained, entry 4. On the other hand, when we changed the Lewis acid to Sc(OTf)3 it was possible to obtain the CO bond cleavage product above 50\u00a0% conversion in addition to increasing the yield of benzodioxanes formation.On the other hand, if comparing entries 1 and 3 (Table 4\n\n), it is observed that although they have the same reaction conditions, the solvent effect is notorious because PC (entry 1) tends to favor compound \nC2 contrasted to toluene (entry 3) which continues to favor the production of guaiacol. When examining entries 1, 4 and 5 we have the reaction times as the major difference resulting in different products. Further, as observed from entries 4 and 5, significant selectivity for guaiacol and compounds C1 and \nC2 (G:C1:C2) was obtained. In fact, at larger times (6\u00a0h) the guaiacol product was obtained in a higher conversion and no C1 esterification product was observed. To note from entry 1, having a time of 6\u00a0h translates into a higher product conversion, highlighting the formation of both, guaiacol in a greater extent, and compound \nC2. Here, in is worth mentioning the relevant role of formic acid as a hydrogen source, as concluded from entry 7.So far, unique results have been obtained for the hydrogenolysis of guaiacylglycerol-\u03b2-guaiacyl, particularly, hydrogenolysis of kraft lignin conducted with Ni-1 and formic acid in water/EtOH mainly produced guaiacol and various monomer products of lignin depolymerization (Fig. 3\n). Optimization for lignin and/or lignin kraft hydrogenolysis reactions has yet to be found, preliminary studies indicate that a change in lignin structure occurs, hydrogenolysis of different lignins are currently under way. Notably, the catalytic hydrogenolysis of pinus lignin was carried out with Ni-1 using formic acid as hydrogen source at 150\u00a0\u00b0C for 2\u00a0h. The gel permeation chromatography (GPC) analysis shows a Mw\u00a0=\u00a02930 corresponding to dioxasolv lignin while the Ni-1 catalyst utilized in the depolymerization reaction of lignin favored the formation of an oil product with a Mw\u00a0=\u00a01790, and significant amounts of lignin monomers, Fig. 4\n.In summary, we prepared and evaluated different nickel catalysts, where Ni-1 showcased the higher activity and selectivity for the \u03b2-O-4 bond cleavage in Lignin-Derived Aryl Ethers such as guaiacylglycerol-\u03b2-guaiacyl ether to obtain guaiacol and isoeugenol selectively. Interestingly, the CO bond cleavage of alkyl-aryl ether compounds was demonstrated with scandium triflate, which in combination with the Ni-1 catalyst, a higher degree of kraft lignin depolymerization was achieved. Overall, the CO bond cleavage represents a crucial step in the valorization of lignin and lignin-related molecules, here, the development of more robust and versatile catalyst will impact the chemical space exploration through sustainable methodologies. The utility of the Ni-1 catalyst here described certainly provides new opportunities in the transfer hydrogenolysis reactions of lignin derivatives. Finally, the role of the Lewis acid in native lignin was crucial in the CO bond cleavage, assisting the lignin depolymerization into monomer products.All manipulations were conducted under an argon atmosphere unless otherwise specified.All chemicals were commercially obtained and used without additional purification. 4\u0301hydroxy-3\u0301-metoxyacetophenone, 2,6-dimethoxyphenol, 99\u00a0%; guaiacol; 1,10-phenanthroline\u00a0\u2265\u00a099\u00a0%; 3,4-dimetoxyacetophenone, 97\u00a0%; nickel(II) acetate tetrahydrate, Ni(OAc)2\u00b74H2O 98\u00a0%, methanol, 98\u00a0%; anhydrous toluene, 99.8\u00a0%, anhydrous propylene carbonate, 99.9\u00a0%; formic acid\u00a0\u2265\u00a095\u00a0%, zinc chloride, \u226595\u00a0%, scandium (III) triflate, 99\u00a0%, nickel nitrate hexahydrate, puriss\u00a0\u2265\u00a098.5\u00a0%; (Ni(NO3)2\u00b76H2O), magnesium oxide nanopowder, 2-methylimidazole, 99\u00a0% and zinc nitrate hexahydrate, purum ((ZnNO3)2\u00b76H2O)), anhydrous ethanol, methylene chloride, anhydrous 1,4-dioxane were purchased from Sigma Aldrich. ZIF-8\n[33], Zn/Ni-2-Meth\n[9]and Ni/C\n[34] were prepared according to reported procedures. L1\n[35] were prepared according to previously reported procedures without further modification. Some experiments were carried out in an Anton Parr Monowave 50\u00a0+\u00a0P apparatus.\nNi-1 catalyst was prepared similarly to a procedure reported by Wang group [19] with some modifications. Briefly, for Ni-1 a 100\u00a0mL Schlenk flask was charged with Ni(OAc)2\u00b74H2O (0.25\u00a0mmol) and 1,10-phenanthroline (0.75\u00a0mmol), then 25\u00a0mL of anhydrous ethanol was added and sonicated for 10\u00a0min at room temperature. To this solution, 1.6\u00a0g of MgO were added and sonicated for 10\u00a0min. The resulting suspension was stirred at 60\u00a0\u00b0C for 12\u00a0h. The solvent was removed under reduced pressure and dried for 24\u00a0h in vacuum. The resulting solid was ground in an Agate mortar, and the fine powder was transferred to a glass ampule and sealed under vacuum, then the ampule was pyrolyzed at 600\u00a0\u00b0C for 2\u00a0h. The obtained black solid was stirred with 100\u00a0mL of 0.5\u00a0M H2SO4 at 80\u00a0\u00b0C overnight to remove MgO support. The black solid was washed with deionized water and dried under vacuum for 24\u00a0h.In a typical experiment, a 25\u00a0mL Schlenk flask equipped with a Rotaflo valve, and a magnetic stirring bar was loaded with 0.038\u00a0g of L1 (0.12\u00a0mmol), 20\u00a0mg of Ni-1 and 2\u00a0mL of deionized water and 2\u00a0mL of ethanol. Then 14.5\u00a0\u03bcL of FA (3 equiv. 0.36\u00a0mmol) and finally 82.4\u00a0\u03bcL (5 equiv.0.6\u00a0mmol) of NEt3 were added. The flask was heated at 150\u00a0\u00b0C for 24\u00a0h and the solvent was removed with vacuum for 6\u00a0h, then the mixture was dissolved with methylene chloride for GC\u2013MS quantification.A 25\u00a0mL Schlenk flask equipped with a Rotaflo valve and a stirring bar, was charged with guaiacol (0.58\u00a0mL), propylene carbonate solvent (0.5\u00a0mL) and Ni/Zn-2-Meth catalyst (12\u00a0mg, 1\u00a0mol%) under argon atmosphere and placed in an oil bath for 24\u00a0h at 150\u00a0\u00b0C. At the end of the heating time a TLC was taken to qualitatively observe the products, its composition was analyzed by NMR and CG-MS. Column chromatography using hexanes: EtOAc, 7.5:2.5 v/v was subsequently carried out to isolate the 1-(2-methoxyphenoxy) propan-2-ol oil (C). Finally, the solvent was evaporated under vacuum until compound C was obtained.\nCompound C. Colorless oil. 1-(o-tolyloxy)propan-2-ol. 1H NMR (400\u00a0MHz, chloroform\u2011d1\n) \u03b4 6.87 \u2013 6.73 (m,4H), 4.20 (d, J\u00a0=\u00a03.0\u00a0Hz, 1H), 4.12 (qd, J\u00a0=\u00a06.5, 3.3\u00a0Hz, 1H), 3.82 (dd, J\u00a0=\u00a09.5, 3.6\u00a0Hz, 1H), 3.75 (dd, J\u00a0=\u00a09.6, 7.5\u00a0Hz, 1H), 3.70 (s, 3H), 1.19 (d, J\u00a0=\u00a06.4\u00a0Hz, 3H).13C NMR (101\u00a0MHz, CDCl3) \u03b4 149.81, 148.19, 121.98, 121.48, 121.05, 115.01, 113.72, 112.11, 111.92, 77.48, 77.16, 76.84, 75.65, 65.93, 55.80, 18.53. IR (ATR, cm\u22121): 3476, 2970, 2930, 1795, 1593, 1503, 1454, 1250, 1222, 1178, 1122, 936, 740. Mass (EI)\nm/z:182.Reactions were carried out in 25\u00a0mL Schlenk flasks equipped with a magnetic stirring bar and a Teflon valve, which were charged with 1-(2-methoxyphenoxy) propan-2-ol (60\u00a0mg), propylene carbonate solvent (2\u00a0mL) and Ni/Zn-2-Meth catalyst (5.8\u00a0mg, 1\u00a0mol%), under argon atmosphere. The Schlenk was placed in an oil bath for 24\u00a0h and 150\u00a0\u00b0C, at the end of the heating time a TLC was taken to qualitatively observe the products, the solvent was evaporated under vacuum. The crude product composition was analyzed by GC\u2013MS.Dioxasolv lignin was isolated based on previously reported methods [35]. Briefly, 30\u00a0g of softwood sawdust was extracted with 1,4-dioxane/water (8:2, 200\u00a0mL) containing 0.1\u00a0M HCl at reflux for 1\u00a0h. The mixture was then cooled, filtered, and concentrated in vacuum. The resulting oil was dissolved in acetone/water (8:2) and precipitated in water (10 vols). The resulting powder was collected by filtration and air dried. The crude lignin was then dissolved in acetone/methanol (9:1) and precipitated in diethyl ether (10 vols). The purified lignin was collected by filtration and air dried to give the softwood dioxasolv lignin (0.403\u00a0g).In a typical experiment, a 10\u00a0mL borosilicate vial equipped with a magnetic stirring bar was loaded with 0.050\u00a0g of dioxasolv lignin or kraft lignin, 20\u00a0mg of Ni-1 and 2\u00a0mL of deionized water and 2\u00a0mL of ethanol. Then, 14.5\u00a0\u03bcL of FA (3 equiv. 0.36\u00a0mmol) and finally 82.4\u00a0\u03bcL (5 equiv.0.6\u00a0mmol) of NEt3 were added. The flask was heated at 150\u00a0\u00b0C for 2\u00a0h, and the solvent was removed with vacuum for 6\u00a0h, then the mixture was dissolved with DMSO\u2011d\n6 for NMR characterization.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 acknowledge the financial support from PAPIIT TA200121. We are grateful with Prof. Diego Solis-Ibarra and Prof. Edilso Reguera Ruiz for granting us access to several instruments. We are also grateful to Virginia G\u00f3mez-Vidales (EPR), M. Le\u00f3n, and E. Tapia from Laboratorio Nacional de Ciencias Patrimonio Cultural LANCIC-IQ-UNAM, CONACYT (LN 232619, LN 260779, LN 279740, LN 293904, LN 271614 y LN 293904), Josue Romero (LUME), Elizabeth Huerta Salazar (NMR), Elizabeth Hern\u00e1ndez \u00c1lvarez (ICP-MS, Instituto de Geof\u00edsica), Adriana Romo (ATR, Instituto de Qu\u00edmica) and Salvador L\u00f3pez (GPC analysis, IIM) for technical assistance.Supplementary data to this article can be found online at https://doi.org/10.1016/j.rechem.2022.100729.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Renewable aromatic carbon sources and their transformation into value-added chemicals represent one of the most promising approaches of sustainable development. Here, we prepared and characterized nickel catalysts and evaluated their activity in transfer hydrogenolysis reactions using formic acid as hydrogen donor, finding excellent activity and high selectivity to CO bond cleavage in lignin and lignin model compounds using water/ethanol or propylene carbonate as solvents. Particularly, a nickel catalyst named Ni-1, was used in the guaiacylglycerol-\u03b2-guaiacyl hydrogenolysis to selectively obtain guaiacol and isoeugenol. Thus, in the case of Ni-1 (NiIINx) we found that small clusters and nanoparticles of 3 \u20135 nm diameters coexist in the material. Further, the Sc(OTf)3 Lewis acid (LA) successfully promoted the CO bond cleavage of 1-(o-tolyloxy)propan-2-ol to obtain guaiacol. Additionally, we describe the synthesis of benzodioxanes catalyzed by scandium triflate from 1-(o-tolyloxy)propan-2-ol as product of the propylene carbonate decarboxylation catalyzed by ZIF-8.\n               "}
{"full_text": "Use of natural gas as an alternative vehicle fuel to gasoline and diesel is growing worldwide due to the fact that it produces significantly less emissions of greenhouse gases, while being cheaper and safer [1]. However, more technological efforts are required to increase the efficiency of catalytic converters to reduce the trace amounts of unburned methane in the exhaust gases due to its notable environmental impact. It is widely accepted that palladium catalysts are the most active candidates for methane oxidation [2]. Alternatively, the main cheaper noble metal-free substitutes are cobalt-based catalysts, namely those based on cobalt oxides such as the spinel-type Co3O4 due to the remarkable mobility of its oxygen species [3,4]. This material has already been extensively investigated for numerous applications such as CO oxidation [5,6], N2O abatement [7,8] or oxidation of VOCs [9] and soot [10].Most studies on the design of efficient catalysts are referred to powdered or pelleted systems although the real implementation in natural gas vehicles would need a more suitable catalyst geometry that minimises gas flow resistance and facilitates intensification of the catalytic process of lean methane oxidation. Monolith catalysts are usually the preferred option due to their good thermal and mechanical resistance [11,12]. However, an alternative solution has lately appeared in the form of open cell foams made of ceramic or metallic materials, which are characterised by a cellular structure with interconnected and often non-ordered pores with a large volume [13]. Typically, only 5\u201325% of the total volume of the foam is the base material. The alleged advantage of this type of structured substrates when compared with more conventional monoliths lies on their high surface/volume ratio and random disposition of the void volume, which can aid in the mass and heat transfer between the gas and the solid phase and allow rector operation at relatively high flow rates [14]. The use of foam-supported catalysts is currently focused on both pollution abatement processes (catalytic converters) [15\u201317] or conventional catalytic processes such as methane reforming or CO2 methanation [18\u201320].The incorporation of a powdered catalyst onto a structured support can be mainly carried out by two methodologies. The most commonly applied procedure on an industrial scale is to prepare a washcoating slurry with the powdered catalyst and a fluid phase such as water or a water/glycerine mixture. The structured support is then dipped into the slurry until it is thoroughly coated. Next, the samples is dried and calcined to stabilise the catalytic phase material [21]. A second approach involves applying impregnation-based routes to deposit the catalyst formulation directly onto the surface of the structured support. The most frequently used methods in this case are wet impregnation (often in the presence of some surfactants) and solution combustion synthesis (SCS) [22,23]. Essentially, SCS is understood as a self-sustained reaction of metal nitrates and an organic fuel with varying chemical nature, which induces a high-temperature reaction between fuel and oxygen-containing species derived from the decomposition of the nitrates. This methodology entails a series of advantages. For instance, in addition to avoiding the intermediate and time-consuming steps of washcoating, the SCS route usually leads to well crystallised nanosized clusters after thermal stabilisation [24,25]. In this sense, when evaluating cobalt catalysts supported on \u03b1-Al2O3 coated monoliths for N2O decomposition, W\u00f3jcik et al. [26] evidenced a better catalytic performance of deposited Co3O4 by SCS with respect to conventional impregnation. The key operational parameters of the SCS route are basically the selection of the fuel and the appropriate fuel-to-oxidiser (metallic nitrates) ratio, which is typically denoted as \u03a6. These two factors strongly influence the mechanism of the combustion process and, in turn, the morphological properties of the active phase.In this work attention was paid to analysing the use of two different fuels, namely urea and glycine, since these are cheap and readily available commercially, while the \u03a6 ratio was varied from 0.25 to 1.0, which corresponded to 25\u2013100% stoichiometric amount of fuel, respectively. An \u03b1-Al2O3 open cell foam was chosen due to its stability at relatively high temperatures and chemical inertness. Based on our previous study [27] dealing with the design of \u03b1-Al2O3 supported Co-Ce powdered catalysts for lean methane oxidation, the selected active phase was Co3O4 with a loading of 10%wt modified with controlled amounts of cerium as a promoter (Ce/Co molar ratio of 0.05). Both active phases were simultaneously incorporated in the same SCS step. The set of structured catalysts prepared by solid combustion synthesis was examined in the oxidation of lean methane under realistic conditions (relatively high space velocity and simultaneous presence of notable amounts of H2O and CO2 in the flue gas) for a prolonged reaction time interval (285\u00a0h at 550 \u00baC). Catalytic results were kinetically analysed in terms of the reaction rate normalised to the Co3O4 mass for a selected reaction temperature (400 \u00baC).An \u03b1-Al2O3-based open cell foam (Lanik, s.r.o., 45 ppi, length\u00a0=\u00a030\u00a0mm and diameter\u00a0=\u00a08\u00a0mm) was selected as the structured support. Table S1, Supplementary Material, summarises the main geometric properties of the foam substrate. The average strut thickness (0.42\u00a0mm) and pore size (1\u00a0mm) were estimated from various SEM micrographs similar to those shown in Fig. S1, Supplementary Material. The calculated porosity or voidage was 0.78. The followed procedure for estimating this physical parameter, which depends on the average strut thickness and the average pore diameter, is detailed in the Supplementary Material. On the other hand, it should be noted that, in addition to alumina, appreciable amounts of silica (18%wt.) and magnesia (1%wt.) were present as determined by WDXRF.The foam catalysts were synthesised by solid combustion synthesis using urea and glycine as fuels. Samples were prepared with varying fuel/oxidiser ratio (\u03a6), namely 0.25, 0.50, 0.75 and 1.00. The selected cobalt loading was 10%wt.Co3O4 with a Ce/Co molar ratio of 0.05 that was equivalent to a 1\u00a0wt.CeO2%. The SCS impregnation gel was an aqueous solution of cobalt nitrate hexahydrate (Co(NO3)2.6\u00a0H2O) 0.4\u00a0M and cerium nitrate hexahydrate (Ce(NO3)3.6\u00a0H2O) 0.02\u00a0M in which adjusted amounts of the used fuel for the various selected \u03a6 ratios were dissolved. In all cases, the open cell foams were dipped vertically into 25\u00a0ml of the corresponding impregnation solution for 5\u00a0min. Then, the excess was removed with compressed air. The impregnated foam was subsequently placed in an oven at 250\u00a0\u00b0C for 20\u00a0min with the aim of inducing the SCS reaction. This coating procedure was repeated several times to reach the desired cobalt and cerium concentration. After the last impregnation step, the coated foams were calcined at 600\u00a0\u00b0C for 4\u00a0h to produce the final catalysts. The samples were labelled as F(U) and F(G) when using urea and glycine, respectively.The chemical reactions that ideally occur between the metal nitrates and the selected fuels during the combustion step as a function of the \u03a6 ratio are the following:\n\n\n\nCo\n\n\n\n\n\n\nNO\n\n\n3\n\n\n\n\n\n\n2\n\n\n+\n\n\n14\n\n\n9\n\n\n\u03a6\nCO\n\n\n\n\nN\n\n\nH\n\n\n2\n\n\n\n\n\n\n2\n\n\n+\n\n\n7\n\n\n3\n\n\n\n\n\n\u03a6\n\u2212\n1\n\n\n\n\n\nO\n\n\n2\n\n\n\n\u2192\n\n\n1\n\n\n3\n\n\n\n\nCo\n\n\n3\n\n\n\n\nO\n\n\n4\n\n\n+\n\n\n14\n\n\n9\n\n\n\u03a6\n\n\n\nCO\n\n\n2\n\n\n+\n\n\n28\n\n\n9\n\n\n\u03a6\n\n\nH\n\n\n2\n\n\nO\n+\n\n\n\n\n\n14\n\n\n9\n\n\n\u03a6\n+\n1\n\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n\n\n\n\n\nCe\n\n\n\n\n\n\nNO\n\n\n3\n\n\n\n\n\n\n3\n\n\n+\n\n\n7\n\n\n3\n\n\n\u03a6\nCO\n\n\n\n\nN\n\n\nH\n\n\n2\n\n\n\n\n\n\n2\n\n\n+\n\n\n7\n\n\n2\n\n\n\n\n\n\u03a6\n\u2212\n1\n\n\n\n\n\nO\n\n\n2\n\n\n\n\u2192\nCe\n\n\nO\n\n\n2\n\n\n+\n\n\n7\n\n\n3\n\n\n\u03a6\n\n\n\nCO\n\n\n2\n\n\n+\n\n\n14\n\n\n3\n\n\n\u03a6\n\n\nH\n\n\n2\n\n\nO\n+\n\n\n\n\n\n14\n\n\n6\n\n\n\u03a6\n+\n\n\n3\n\n\n2\n\n\n\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n\n\n\n\n\nCo\n\n\n\n\n\n\nNO\n\n\n3\n\n\n\n\n\n\n2\n\n\n+\n\n\n28\n\n\n27\n\n\n\u03a6\n\n\nC\n\n\n2\n\n\n\n\nH\n\n\n5\n\n\n\n\nNO\n\n\n2\n\n\n+\n\n\n7\n\n\n3\n\n\n\n\n\n\u03a6\n\u2212\n1\n\n\n\n\n\nO\n\n\n2\n\n\n\n\u2192\n\n\n\n\n1\n\n\n3\n\n\nCo\n\n\n3\n\n\n\n\nO\n\n\n4\n\n\n+\n\n\n56\n\n\n27\n\n\n\u03a6\n\n\n\nCO\n\n\n2\n\n\n+\n\n\n70\n\n\n27\n\n\n\u03a6\n\n\nH\n\n\n2\n\n\nO\n+\n\n\n\n\n\n14\n\n\n27\n\n\n\u03a6\n+\n1\n\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n\n\n\n\n\nCe\n\n\n\n\n\n\nNO\n\n\n3\n\n\n\n\n\n\n3\n\n\n+\n\n\n14\n\n\n9\n\n\n\u03a6\n\n\nC\n\n\n2\n\n\n\n\nH\n\n\n5\n\n\n\n\nNO\n\n\n2\n\n\n+\n\n\n7\n\n\n2\n\n\n\n\n\n\u03a6\n\u2212\n1\n\n\n\n\n\nO\n\n\n2\n\n\n\n\u2192\nCe\n\n\nO\n\n\n2\n\n\n+\n\n\n28\n\n\n9\n\n\n\u03a6\n\n\n\nCO\n\n\n2\n\n\n+\n\n\n35\n\n\n9\n\n\n\u03a6\n\n\nH\n\n\n2\n\n\nO\n+\n\n\n\n\n\n7\n\n\n9\n\n\n\u03a6\n+\n\n\n3\n\n\n2\n\n\n\n\n\n\n\nN\n\n\n2\n\n\n\n\n\n\nThus, when \u03a6\u2009=\u20090 the reaction corresponds to the simple thermal decomposition of the metallic (cobalt or cerium) nitrate. If \u03a6\u2009=\u20091, the corresponding stoichiometric redox reaction is then made explicit. Taking into account that the required Co3O4 and CeO2 concentrations were 10%wt and 1%wt., respectively, the extent of the redox reactions involving cobalt nitrate was comparatively more noticeable since a larger amount of this salt was used in the synthesis.The foam catalysts were characterised 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, inductively coupled plasma atomic emission spectroscopy (ICP-AES), wavelength dispersive X-ray fluorescence (WDXRF), N2 physisorption, X-Ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-Ray photoelectron spectroscopy (XPS), temperature-programmed reduction with hydrogen (H2-TPR) and temperature programmed reaction with methane (CH4-TPRe). Except for SEM-EDX, the structured catalysts were crushed and milled to a fine powder before analysis. Although the experimental details are included elsewhere [28,29], some relevant details on the characterisation details are given below.Scanning Electron Microscopy images were obtained in a JEOL JSM-7000\u2009F Schottky-type field emission microscope operated at 10\u2009kV. The electronic microscope was equipped with a INCA X-sight Si(Li) series pentaFET EDX detector to allow for elemental analysis of the observed surfaces. On the other hand, High-Resolution Transmission Electron Microscopy and Scanning Transmission Electron Microscopy images were obtained with a in a Cs-image-corrected Titan (Thermofisher Scientific) at a working voltage of 300\u2009kV with a 2k x 2k Ultrascan CCD camera (Gatan) positioned before the filter for TEM imaging (energy resolution of 0.7\u2009eV). The microscope was equipped with a CCD camera (Gatan), a HAADF detector (Fischione) and an Ultim Max detector (Oxford Instruments) that allowed for EDX elemental mapping.The elemental composition of the synthesised catalysts was determined by ICP-AES, using a Thermo Elemental Iris Intrepid apparatus, and WDXRF with a PANalytical AXIOS sequential spectrometer. The textural properties, in terms of specific surface area (BET method) and pore volume (BJH method), were determined by nitrogen physisorption at \u2212\u2009196\u2009\u00b0C in a Micromeritics TriStar II apparatus. Before the analysis, outgassing of the samples was carried out on a Micromeritics SmartPrep apparatus at 300 \u00baC for 10\u2009h with a N2 flow.XRD analysis were carried out using Cu K\u03b1 radiation (\u03bb\u2009=\u20091.5406\u2009\u00c5) on a X\u2032PERT-PRO X-Ray diffractometer equipped with a Ni filter and operated at 40\u2009kV and 40\u2009mA. The samples were scanned from an initial value of 2\u03b8 =\u20095\u00b0 to a final value of 2\u03b8 =\u200980\u00b0, with a step size of 0.026\u00b0 and a counting time of 26.8\u2009s. From the obtained diffractograms, the cell parameter of the Co3O4 phase was obtained by full profile matching using FullProf.2k software. On the other hand, the Raman spectra of the samples were obtained with Renishaw InVia Raman spectrometer, coupled to a Leica DMLM microscope, with an ion-argon laser (Modu-Laser, 514\u2009nm). For each sample, five scans in the spectral window of 150\u2013900\u2009cm\u22121 and a spatial resolution of 2\u2009\u00b5m were accumulated. Finally, XPS measurements were performed in a Kratos AXIS Supra spectrometer using a 225\u2009W Al K\u03b1 radiation source with a pass energy of 20\u2009eV.The redox properties of the catalysts were investigated on a Micromeritics Autochem 2920 apparatus coupled to a TCD detector by means of Temperature-Programmed Reduction with hydrogen (H2-TPR) and Temperature-Programmed Reaction with methane (CH4-TPRe). In both cases an initial pre-treatment step with a 5%O2/He mixture at 300\u2009\u00b0C for 30\u2009min was performed with the aim of removing impurities form the surface of the samples while at the same time fully restoring the oxygen vacancies of the spinel lattice before the analysis of the reducibility. After cooling down to room temperature with flowing He, the experiments were conducted up to 600\u2009\u00b0C, with a 5%H2/Ar mixture and a 5%CH4/He mixture, respectively. In the CH4-TPRe the composition of the gaseous stream was monitored with a MKS Cirrus Quadrupole Mass Spectrometer.The efficiency of the foam catalysts for the complete oxidation of dilute methane was examined in a fixed bed quartz tubular (10\u2009mm ID) reactor in the 200\u2013600\u2009\u00b0C temperature range with a heating rate of 1 \u00baC min\u22121. The runs were carried out with a single piece of foam catalysts (with a mass of 650\u2013700\u2009mg) that were deposited on a glass frit located near the bottom of the reactor tube. The GHSV calculated on the basis of the total volume of the foam catalyst (1.5\u2009ml) was around 4000\u2009h\u22121. This corresponded to a WHSV of 85\u2009l\u2009gCo3O4\n\u22121 h\u22121. In order to avoid gas channelling each structured catalyst was wrapped with an aluminium foil. Light-off tests were repeated at least three times to assure reproducibility, with an average 12\u2009h of use for each structured catalyst. The composition of the feed stream was 1%CH4/10%O2/89%N2 with a total flow of 100\u2009ml\u2009min\u22121. Note that the typically encountered O2/CH4 molar ratio can vary between 2 and 6 for stoichiometric engines and between 10 and 70 for lean engines. The used ratio in this work falls within this last range. The composition of the reaction gases was continuously analysed by a SRS RGA200 quadrupole mass spectrometer following the m/z\u2009=\u200944 (CO2), 32 (O2), 28 (CO) and 16 (CH4) signals. The analysis of the product stream was carried out in steps of 25 \u00baC, typically after 15\u2009min on stream. Each analysis was performed in triplicate in order to check reproducibility. A margin of error of less than 1% was found. Methane conversion was determined by the difference between inlet and outlet CH4 molar flows. Additionally, the effect of the presence of water (10\u201330%vol.) and carbon dioxide (10%vol.) on the catalyst stability with time on stream was investigated at constant temperature (550 \u00baC) for a total reaction interval of 285\u2009h. The influence of GHSV in the 4000\u201360,000\u2009h\u22121 range (85\u2013850\u2009l\u2009gCo3O4\n\u22121 h\u22121) was also studied.The suitability of SCS as an attractive methodology for producing efficient cobalt catalysts for lean methane oxidation was initially addressed. Thus, a bulk Co3O4 oxide was obtained using cobalt nitrate as precursor and glycine as fuel (\u03a6\u2009=\u20091). As aforementioned, the reactive mixture was heated at 250\u2009\u00b0C for 30\u2009min in order to activate the SCS reaction. The resulting sample was then calcined at 600\u2009\u00b0C for 4\u2009h. For comparative purposes, a reference Co3O4 catalyst was synthesised by simple calcination of the same cobalt precursor under identical thermal conditions (600\u2009\u00b0C/4\u2009h). Both samples were prepared without cerium as promoter. Their performance in the oxidation of methane was examined at 30\u2009l\u2009gCo3O4\n\u22121 h\u22121 in the 200\u2013600\u2009\u00b0C temperature range. The composition of the feedstream was 1%CH4/10%O2/89\u2009N2%. Three consecutive light-off runs were recorded. While a slight decrease in activity with temperature was observed in the second run with respect to the first run, the third light-off curve was virtually identical to the second run. Hence, the light-off curves corresponding to the third cycle of each catalyst are shown in \nFig. 1. It was found that the sample synthesised with glycine showed a T50 (temperature at which 50% conversion was attained) of 455\u2009\u00b0C, while its counterpart required 480\u2009\u00b0C. 90% conversion was obtained at 525 and 575\u2009\u00b0C, respectively. A significantly higher reaction rate under differential conditions (375\u2009\u00b0C) was also noticed (1.2 vs 0.8\u2009mmol CH4 gCo3O4\n\u22121 h\u22121). It must be pointed out that despite the fact that the reaction rate, on a surface area basis, of the sample prepared by calcination is twice that of the one prepared by SCS, the calcination method cannot produce catalysts with high specific surface areas, and therefore with a large population of active sites.The superior oxidation ability of the sample prepared by SCS was connected with its appreciably better textural properties although no significant difference in the crystallite size were found (84\u201389\u2009nm). Hence, the increased volume of gases produced during the fuel-assisted combustion process provoked a higher porosity as revealed by the larger surface area (14 vs 5\u2009m2 g\u22121) and pore volume (0.04 vs 0.02\u2009cm3 g\u22121) and the smaller mean pore size (170 vs 355\u2009\u00c5). Likewise, a favoured reducibility at low temperatures was observed over the oxide synthesised with glycine as revealed by H2-TPR (Fig. S2, Supplementary Material). It was found that the onset reduction temperature was 280\u2009\u00b0C compared with 300\u2009\u00b0C. Besides, the H2 uptake at low temperatures (250\u2013325\u2009\u00b0C) was markedly larger 3.9 vs 2.6\u2009mmo g\u22121). In sum, this preliminary catalytic evaluation accompanied by the characterisation of the textural and redox properties evidenced the potential of the combustion route aided by a fuel (glycine) for preparing promising oxidation cobalt catalysts [30,31], and therefore justifies a deeper analysis for its optimisation in the removal of lean methane. As stated earlier, our interest will be now focused on the investigation of this methodology for intensifying the methane oxidation process with highly active Ce-promoted cobalt catalysts supported on open-cell \u03b1-Al2O3 foams. In this sense, it must highlighted that the use of low amounts of Ce as an additive has been observed to promote the performance of cobalt catalysts notably since it increases the mobility of active oxygen species [27,28].For defining the number of cycles required to achieve the desired amount of CeO2-modified Co3O4 (approximately 10%wt.Co3O4 and 1%wt.CeO2, which corresponded to a Ce/Co molar ratio of 0.05) loaded onto the foam substrate, the evolution of the Co3O4 oxide mass concentration as function of the number of cycles is shown in Fig. S3, Supplementary Material. This graph includes the mean oxide concentration for each cycle, which was estimated from gravimetric measurements by the difference of the coated and base foams prepared in duplicate with both fuels and the entire \u03a6 range (0.25\u20131.0). Thus, 16 measurements were averaged for each cycle. Hence, the consecutive cycles led to a gradual increase in Co3O4 concentration from about 1% (1st cycle), 2% (2nd cycle), 6% (3rd cycle) to 10% (4th cycle). It is worth pointing out that irrespective of the synthesis conditions (type of fuel and \u03a6 ratio) the amount of oxides coated in each cycle was quite reproducible. Hence, four cycles were tentatively required to attain the target concentration (10%wt.Co3O4). It must be pointed out that the metal concentration by chemical analysis was not determined after each coating step, since that would have destroyed the sample after the corresponding coating step. Therefore, the actual metallic loadings were determined by ICP-AES was only measured for the foam catalysts coated after four consecutive runs. As will be shown later on, a significantly lower metallic content was found with respect to that expected from gravimetric measurements although the Ce/Co molar ratio was always equal to nominal value (0.05).On the other hand, the adhesion of the catalytic coating onto the open cell foams was examined by ultrasonic treatment. Several samples were submerged in a 50% isopropanol/50% water solution and subjected to sonication at 40\u2009kHz and 200\u2009W for 1\u2009h in a Selecta ULTRASONS-H ultrasonic cleaner. Before and after the test, the samples were dried at 110\u2009\u00b0C for 1\u2009h and weighted to measure the mass loss owing to the sonication treatment. These tests revealed a minimal mass loss (0.2\u20130.4%wt.) after the sonication treatment, thus suggesting that the utilised combustion route was adequate to obtain structured samples with a relatively high mechanical stability of the deposited Ce-Co oxide.SEM analysis of the foam catalysts was carried out to ascertain differences in the morphology and homogeneity of the Ce-Co coating as function of the used fuel. Thus, \nFig. 2 and Fig. S4, Supplementary Material include representative SEM images of the two samples prepared with urea and glycine with \u03a6\u2009=\u20091, along with images of the pristine foam. Note that the surface morphology of the bare foam substrate was rough and consisted of an agglomeration of crystallites with various sizes and shapes, probably due to the foam being a mixture of several ceramic materials. Complementary EDX analysis was performed for semi-quantitatively estimating the elemental composition of the catalyst surface (a depth of 0.5\u20131\u2009\u00b5m). Thus, around 65 spot analyses on selected regions of both samples were carried out. Attention was paid to estimating the Co/Al molar ratio as a criterion for comparing the dispersion of cobalt on the foam, and more importantly, the Ce/Co molar ratio for characterising the contact between these two metals. Fig S5 (Supplementary material) shows the relative distribution of this ratio at the surface of the foam catalysts.After depositing the Ce-Co catalyst with urea, the formation of a distinct catalytic layer could not be distinguished. In fact, the observed structural morphology (x1300 magnification) was rather similar to that of the bare foam. The averaged Co/Al molar ratio derived from EDX was very low, around 0.06, which evidenced a relatively poor accessibility of cobalt species located on the surface of the foam. Although the mean Ce/Co molar ratio (0.06) was close to that determined by ICP-AES (0.05), a great variability in the relative abundance was detected on various regions of the catalyst, which suggested a non-homogenous distribution of these two metals. Hence, Ce-rich areas were identified on some regions (25% and 12% of the spot analysis evidenced a Ce/Co molar ratio higher than 0.06 and 0.1, respectively), while other zones were characterised by a low concentration of cerium species (43% of the spot analysis revealed a Ce/Co molar ratio lower than 0.03). On the other hand, high-magnification SEM images (x25,000\u2013150,000) shown in Fig. 1 revealed that the surface was covered by round patches with sizes ranging 450\u2013500\u2009nm, although some smaller clusters of around 50\u2013200\u2009nm were also visible. Likewise, uncovered areas of foam could be observed.Conversely, when the catalyst was prepared with glycine the surface of the foam substrate was not visible due to being fully covered with a clearly observable catalytic layer, which in addition presented a porous, foamy morphology with large voids in its microstructure. Judging from the images at medium magnification (x1300\u20131800), the oxides were homogeneously deposited and well anchored on the structured support although superficial debris were also found (Fig. 2). On average, the estimated Co/Al molar ratio derived from spot EDX measurements was around 1.3, substantially higher than that of the urea-based counterpart (0.06). This suggested a better distribution of cobalt on the surface of the foam. On the other hand, a transversal cut from a piece of this catalyst (Fig. S5, Supplementary Material) revealed that the thickness of the catalytic coating was around 7\u2009\u00b5m, with a part of the deposited cobalt being able to filter through the pores among the ceramic particles of the foam substrate. By zooming in on the foamy microstructure of the layer (x22,000\u2013150,000) it was observed that it was actually formed by the aggregation of crystallites around 25\u201330\u2009nm in size. The spongy structure and the relatively small Co3O4 crystallite were assigned to the easier and more violent combustion of the glycine nitrate gel and to the large amount of gases released during the combustion process that simultaneously inhibited sintering and favoured the creation of a porous network [32\u201334]. As for the relative abundance of cerium and cobalt species, the measured mean Ce/Co molar ratio was 0.06, close to the nominal value (0.05), thereby revealing an intimate mixing of both elements. Interestingly, almost 95% of the spot analysis evidenced a Ce/Co molar ratio between 0.05 and 0.06, which suggested a homogeneous relative distribution of both metals on the surface of the foam.Complementary HAADF-STEM coupled to EDX mapping was useful to determine the differences in the spatial distribution of cobalt and cerium on the surface of these two foam catalysts (\nFig. 3). This analysis required a previous crushing of the foams until obtaining a fine powder. As already noted, the surface of the urea-prepared sample was sparsely covered with bulky patches of catalytic material. Furthermore, both cobalt and cerium species were generally present as isolated entities, with very low mixing between the two metals. Therefore, it was clearly evidenced that the dispersion of both cobalt and cerium was certainly poor. In contrast, in the case of catalyst F(G), the surface of the foam was completely covered with both metals. Moreover, the cerium species exhibited good dispersion and mixing with cobalt, with almost no segregated clusters of ceria. These results evidenced the appreciably better structural properties of the supported catalyst prepared with glycine with respect to the urea-based counterpart.The crushed samples were also investigated by ICP-AES, N2 physisorption, XRD, HRTEM, Raman spectroscopy, XPS, H2-TPR and CH4-TPRe, with the aim of studying the effect of the type of fuel and \u03a6 ratio on the physico-chemical properties of the deposited metal oxides. Firstly, the composition of the samples after the fourth SCS cycle was determined by ICP-AES. The corresponding results are given in \nTable 1. A slightly lower oxide loading was detected (6.8\u20138.8%wt.Co3O4 and 0.80\u20131.01%wt.CeO2) when compared with the estimate given by thermogravimetric measurements. Nevertheless, the actual Ce/Co molar ratio of all foam catalysts determined by chemical analysis was virtually identical to that expected theoretically (0.05). On the other hand, by means of Raman spectroscopy, the possible presence of carbonaceous species derived from the thermal decomposition of organic fuels during the SCS process was examined. Hence, the absence of the signals at about 1340 and 1580\u2009cm\u22121 assigned to the so-called D and G bands [35] suggested that the combustion reaction of the fuel was complete. Accordingly, the observed mass loss of the samples by dynamic thermogravimetry (10\u2009\u00b0C\u2009min\u22121, Setaram Setsys Evolution) up to 900\u2009\u00b0C under oxidative conditions was negligible.The textural properties of the ceramic substrate and the two foam catalysts prepared with glycine and urea (\u03a6\u2009=\u20091) were compared. BET measurements of the bare foam were expected to reveal its macroporous character with a very low surface area (about 0.2\u2009m2 g\u22121). Interestingly, an appreciable increase in surface area up to 2\u2009m2 g\u22121 was found for the F(G) catalyst. This finding was consistent with the porosity of the deposited oxide catalyst as observed by SEM analysis. An estimate of the intrinsic surface area of the metallic phase resulted in around 26\u2009m2 g\u22121. By contrast, the surface area (0.6\u2009m2 g\u22121) of the F(U) catalyst was close to that of the blank substrate. The X-Ray diffractograms (with a step size of 0.026\u00b0 and a counting time of 2.0\u2009s) of all structured catalysts are shown in \nFig. 4. The diffraction pattern of the bare foam substrate was also included for the sake of comparison. Its pattern was characterised by the intense signals of the trigonal phase of the alpha-alumina support (2\u03b8 = 25.7, 37.8, 43.5, 52.6, 57.6, 61.4, 66.6, 68.4 and 77.0\u00b0) (ICDD 01\u2013081\u20131667). However, it must be pointed out that additional signals were noticed, which were assigned to impurities such as mullite (ICDD 01\u2013074\u20132419) at 2\u03b8 =\u200916.5, 23.7, 26.3, 31.0, 33.3, 35.3, 37.1, 39.3, 41.0, 42.7, 49.5, 54.2, 60.8, 64.7, 70.5 74.4 and 75.2\u00b0; cristobalite (ICDD 01\u2013076\u20130935) at 2\u03b8 =\u200921.9, 46.4, 48.5 and 36.1\u00b0 and cordierite (ICDD 01\u2013084\u20131221) at 2\u03b8 =\u200910.5, 28.4 and 29.6\u00b0, as can be seen in Fig. S6, Supplementary Material. The set of signals corresponding to the presence of the cobalt spinel oxide (Co3O4) at 2\u03b8 =\u200919.0, 31.3, 36.8, 38.5, 44.8, 59.4 and 65.2\u00b0 (ICDD 00\u2013042\u20131467) were identified for all foam catalysts. The presence of CoO or Co that could be formed by reduction of Co3O4 in the presence of the organic fuel was ruled out. Also, a weak signal at 2\u03b8 =\u200928.6\u00b0, attributable to the cubic phase of segregated CeO2 (ICDD 00\u2013004\u20130593), was visible. Accordingly, HRTEM images of the samples prepared with \u03a6\u2009=\u20091 (Fig. S7, Supplementary Material) allowed the resolution of lattice fringes of Co3O4 (0.29 and 0.24\u2009nm, which corresponded to the {220} and the {311} planes, respectively) and CeO2 crystallites (0.31 and 0.27\u2009nm, which corresponded to the {111} and {200} planes, respectively). The observation of the latter oxide phase suggested that a fraction of cerium was not incorporated into the framework of the cobalt spinel. The mean crystallite size of Co3O4, determined by the Scherrer equation, was around 50\u2009nm for the catalysts prepared with urea and between 19 and 28\u2009nm for the F(G) catalysts (Table 1). Apparently, these sizes were not greatly influenced by the used \u03a6 ratio for each fuel, and were in the same range as those reported by Toniolo et al. [36] for Co3O4 oxides synthesised with glycine (23\u201337\u2009nm) and urea (50\u201377\u2009nm) with varying \u03a6 (0.25\u20131). On the other hand, a comparison of this crystallite size with that estimated by SEM analysis evidenced that the Co3O4 particles observed in the sample prepared with urea, unlike those present in the glycine-based counterpart, were formed by the apparent agglomeration of smaller crystallites.Finally, as indicated above, the introduction of cerium into the lattice of the cobalt spinel was not complete since relatively small crystallites of cerium oxide were observed. The estimated size of CeO2 crystallites ranged between 10 and 12 and 7\u20138\u2009nm for the F(U) and F(G) catalysts, respectively. The extent of insertion of cerium atoms into the lattice of Co3O4 could be qualitatively evaluated by analysing its cell parameter, as shown in \nFig. 5. The cell parameter was calculated via a full profile fitting of the high-resolution diffractograms by using FullProf.2k software. In principle, a larger value could be associated with a greater abundance of cerium atoms given the larger ionic radii of Ce4+ (101\u2009pm) and Ce3+ (115\u2009pm) compared with Co3+ (69\u2009pm) and Co2+ (79\u2009pm). As a reference, the cell parameter of the bulk Co3O4 sample prepared by SCS (glycine) was estimated (8.0988\u2009\u00c5). Regarding the F(U) samples, it was significantly larger (8.1060\u20138.1070\u2009\u00c5) although no appreciable differences were noticed with varying \u03a6 ratio. However, in the case of the F(G) samples the insertion of cerium atoms was substantially promoted, and highly dependent on the used amount of glycine. Thus, the cell parameter was 8.1210\u2009\u00c5 for the sample with \u03a6\u2009=\u20091. The distortion of the spinel structure caused by cerium as a function of the synthesis conditions (type and amount of fuel) was also followed by Raman spectroscopy. In this way, a shift of the main Raman bands could be taken as an evidence of the extent of this structural change. The Raman spectra of the F(U) and F(G) catalysts are shown in Fig. S8, Supplementary Material. The spectra of the bulk Ce-free Co3O4 sample was also included for comparative purposes as it was taken as a reference to examine the eventual shift corresponding to the Ce-modified foam catalysts. This pure oxide exhibited the five expected vibrations of the Co3O4 lattice, namely three bands at 187, 506 and 602\u2009cm\u22121 from the F2\u2009g vibration modes, an Eg vibration mode at 462\u2009cm\u22121 and finally a signal at 667\u2009cm\u22121 attributed to the A1\u2009g vibration mode [37]. Fig. 4 includes the observed shift of the latter band (A1\u2009g) as a function of the \u03a6 ratio for both fuels. As for the F(U) samples, the shift was similar for all samples (11\u201312\u2009cm\u22121), thereby revealing that cerium insertion was not favoured with increasing amounts of urea. However, when using glycine the shift was more marked (13\u201321\u2009cm\u2212\n1). The results evidenced a greater distortion of the lattice, particularly for \u03a6\u2009=\u20091. To sum up, both XRD and Raman results suggested that the SCS route was suitable for partially doping the lattice of cobalt oxide, and that the extent of cerium insertion and the subsequent lattice distortion was favoured when large amounts (\u03a6\u2009=\u20090.75\u20131) of glycine were used. Although the possible insertion of cobalt into the ceria lattice could not be ruled out, this could not be evidenced by XRD and Raman spectroscopy, probably due to the low cerium content of the samples.The surface chemical state of the foam catalysts was investigated by XPS. Hence, the near-surface composition and distribution of cobalt, cerium, and oxygen species was determined by deconvolution and integration of the Co2p3/2 (777\u2013792\u2009eV), Ce3d (881\u2013917\u2009eV) and O1s (526\u2013536\u2009eV) spectra, respectively, as shown in Fig. S9, Supplementary Material. Additionally, the Al2p and Si2p spectra were integrated to include these elements in the surface elemental composition. The surface charging effect in the spectra was compensated against the C-H states in the C 1\u2009s spectra with the energy assumed to be 284.6\u2009eV. As shown in \nTable 2, it was found that cerium tended to be located preferentially on the surface of the samples with a concentration that was approximately ten times higher (7.8\u20139.1%wt.) than the corresponding bulk concentration (0.7\u20130.8%wt.) estimated by ICP-AES. This accumulation could be ascribed to low surface energy of cobalt species compared to ceria, which in turn results in the favoured presence of ceria on the outer surface. Therefore, Ce/Co molar ratios in the 0.22\u20130.38 range were observed in the foam catalysts.The Co2p3/2 spectra were deconvoluted into three main contributions and two satellites. The first two main components, centred at 779.5 and 780.6\u2009eV, were attributed to the existence of Co3+ and Co2+ ions, respectively, while the third one, centred at 782.3\u2009eV, was assigned to the presence of Co2+ as CoO [38]. Typically, the contribution of the latter component was less than 10% of the total surface Co concentration. This oxide was presumably formed due to in situ partial reduction of surface Co3O4 species under vacuum conditions in the XPS spectrometer. Therefore, it could be assumed that this phase was not present in the catalyst formulation. The signals located at 785.2 and 789.4\u2009eV were identified as the shake-up satellite peaks of the Co2+ and Co3+ ions, respectively [39]. It must be pointed out that the position of the main bands as well as their satellite bands did not vary markedly among all foam catalysts. Nevertheless, the deconvolution of the Co2p3/2 band in the components from Co2+ and Co3+ suggested notable differences in the oxidation state of cobalt on the surface. Hence, as a general behaviour it was observed that the Co3+/Co2+ molar ratio of the F(U) samples was slightly lower (0.80\u20130.93) compared with the F(G) counterparts (0.83\u20131.08). It was then possible to establish that smaller crystallites sizes were characterised by the presence of more oxidised cobalt species. On the other hand, it was also remarkable that for the samples prepared with the lowest amount of fuel (\u03a6\u2009=\u20090.25) were characterised by the lowest Co3+/Co2+ molar ratio. This was reasonably connected with the severe substoichiometric conditions of the combustion synthesis. In contrast, higher amounts of fuel resulted in a favoured presence of Co3+ species.On the other hand, the O1s spectra were deconvoluted into four signals (Fig. S9, Supplementary Material). The first two contributions, located at 529.7 and 531.0\u2009eV, were assigned to the lattice oxygen species from the cobalt oxide and the ceramic substrate, respectively. The third signal, centred at 532.1\u2009eV, was attributed to weakly adsorbed oxygen species on the surface of the samples. Note that these oxygen species could be located indistinctively on the surface of both the ceramic support and the Ce-Co active phase. Finally, the last signal, located at 533.0\u2009eV was attributed to the presence of carbonates, water and hydroxyl species [40]. Given the high ability of lattice oxygen species of Co-based catalysts for methane oxidation [41], its relative abundance was estimated as the Olatt/Otot molar ratio for all samples. The amount of Olatt species was assumed to be proportional to the area under the signal peaking at 529.7\u2009eV. It is noteworthy that the estimated amount of adsorbed oxygen species may be affected by air exposure. However, it is highly likely that this contamination did not result in a remarkable effect on the quantification of the amount of lattice oxygen species, since those are strongly bonded to Co or Ce atoms. Consequently, the Olatt/Ototal molar ratio of all samples would be overestimated. However, since all catalyst exhibited comparable specific surface areas, that overestimation could be assumed to be similar between all samples. Therefore the comparison among the various samples would be meaningful. Regarding the F(U) catalysts, this ratio barely varied with the \u03a6 ratio with values around 0.10\u20130.11. However, a significantly higher ratio was found for the F(G) samples, which notably depended on the \u03a6 ratio. Thus, it increased from 0.11 (\u03a6\u2009=\u20090.25) to 0.21 (\u03a6\u2009=\u20091). In sum, the use of increasing amounts of glycine as fuel favoured the presence of oxygen lattice species in the resulting Ce-Co oxide, which in turn was strongly related to the abundance of Co3+ ions (Fig. S10, Supplementary Material).To define the eventual relationship between the distribution of cerium species and the Co3+/Co2+ molar ratio, the Ce3d spectra of all samples were fitted with eight peaks corresponding to four pairs of spin-orbit doublets (Fig. S9, Supplementary Material). Following the convention adopted by Murugan et al. [42], letters U and V were used to refer to the 3d5/2 and 3d3/2 spin-orbit components, respectively. Of the four pairs of peaks, three of them (namely V, U; V\u2019\u2019, U\u2019\u2019 and V\u2019\u2019\u2019, U\u2019\u2019\u2019) were associated with electrons from Ce4+ while the remaining pair (V\u2032, U\u2032) was attributed to electrons from Ce3+ species. The Ce3+/Ce4+ molar ratios were obtained from the areas of the 3d5/2 and 3d3/2 components for each species. It must be noted that the estimation of this ratio could be affected by the possibility of cerium reduction under the conditions of spectra recording, thereby resulting in an overestimation of the proportion of Ce3+. Although it was difficult to quantify the extent of this eventual reduction, and since the samples were submitted to the same experimental analysis conditions and the ceria particle size of the foam catalysts was relatively similar (7\u201312\u2009nm), it was assumed that the samples of the same set of foam catalysts (F(G) or F(U)) would exhibit a similar tendency to form Ce3+. Therefore the estimated Ce3+/Ce4+ molar ratios could at least qualitatively compared. It was found that this ratio decreased as the Co3+/Co2+ molar ratio increased, which could be explained in terms of the equilibrium Ce3++Co3+\u2194Ce4++Co2+ established by the charge balance requirement within the cations of the spinel lattice [43]. Hence, an increase in Co3+ population at the expense of Co2+ resulted in a decrease of Ce3+ ions in favour of Ce4+.H2-TPR analysis was used to characterise the reducibility of the foam catalysts, since this is one of the main parameters governing the performance of Co3O4-based catalysts in redox reactions. A 5%H2/Ar mixture was used as the reducing gas and the experiments were carried out with a heating ramp of 10 \u00baC min\u22121 between 50 and 900 \u00baC. \nFig. 6 shows the corresponding reduction patterns (up to 600 \u00baC) of the samples prepared with the two fuels and varying \u03a6 ratio. It should be pointed out that the observed H2 consumption would correspond to the reduction of deposited cobalt and cerium species, which is expected to occur simultaneously. In order to decouple the reduction process of Co3+, Co2+ and Ce4+ cations, our attention will be first paid to analysing the reducibility of cobalt species. Regardless the synthesis conditions, the reduction process of all catalysts was dominated by a main reduction event at around 350 \u00baC and a more or less perceptible signal at lower temperatures (300 \u00baC). Thus, the onset temperature was approximately 250\u2009\u00b0C and 300\u2009\u00b0C for F(G) and F(U) samples, respectively. These findings were in accordance with the sequential reduction of Co3+ \u2192 Co2+ \u2192 Co0\n[44]. It must be pointed out that above 400\u2009\u00b0C no significant H2 uptake was observed, which ruled out the presence of highly stable cobalt species in the form of cobalt aluminate [45]. This was coherent with the high chemical stability of alpha alumina that prevented the formation of this undesired spinel.After integrating the profiles of the F(G) samples (\nTable 3) it was found that the total specific H2 uptakes were in all cases higher than those theoretically expected (16.6\u2009mmol\u2009gCo3O4\n\u22121), thereby suggesting the appreciable contribution to the overall reducibility of the cerium species present in the samples, mainly inserted in the spinel lattice as Ce4+ cations. The reduction of these species was expected to occur in the same temperature window (250\u2013400\u2009\u00b0C) as cobalt species and could be activated due to the transfer of hydrogen by metallic cobalt onto the ceria [46]. The H2 uptake ranged between 16.9 and 17.1\u2009mmol\u2009gCo3O4\n\u22121, and apparently depended on the amount of fuel used in the synthesis. These results would be in agreement with a favoured incorporation of cerium with high \u03a6 ratios, as suggested by XRD and Raman spectroscopy. The contribution of Ce4+ reduction to the overall reducibility of the Ce-Co catalysts was evaluated by analysing a post-run sample, particularly the one prepared with \u03a6\u2009=\u20091, by XPS. Thus, it was observed that its Ce3+/Ce4+ molar ratio substantially increased from 0.23 over the fresh sample to 0.44. On the other hand, the improvement in the redox properties of the samples prepared with increasing amounts of glycine was also reflected in the shift of the reduction temperatures to lower values. Furthermore, when taking a temperature of 300\u2009\u00b0C as a criterion, the low-temperature uptake increased with the \u03a6 ratio, from 2.1\u2009mmol\u2009gCo3O4\n\u22121 for \u03a6\u2009=\u20090.25\u20134.1\u2009mmol\u2009gCo3O4\n\u22121 for \u03a6\u2009=\u20091. As for the F(U) catalysts, their total H2 uptake was comparable (16.7\u201316.9\u2009mmol\u2009gCo3O4\n\u22121), and slightly larger than the theoretical consumption. This suggested that the amount of cerium species in the lattice was comparable irrespective of the \u03a6 ratio, in line with the results given by XRD and Raman spectroscopy. The low-temperature uptake was rather similar (1.5\u20131.7\u2009mmol\u2009gCo3O4\n\u22121) for all F(U) catalysts, and appreciably lower in comparison with their glycine-based counterparts. Analogously, a post-run sample (the one prepared with \u03a6\u2009=\u20091) was characterised by XPS. In this case, no marked differences in its Ce3+/Ce4+ molar ratio were found (0.35 for the fresh sample and 0.40 for the sample after the H2-TPR run). Finally, it must be pointed out only the diffraction signals of metallic cobalt were clearly distinguished (ICDD 00\u2013015\u20130806) for both used catalysts. No signals related to cerium species were visible. This was expected due to the low Ce content as CeO2 (lower than 1%wt.CeO2) of the samples.In addition to H2-TPR, the intrinsic reactivity of the oxygen species present in each set of foam catalysts was also characterised by studying the ability of a given sample for oxidising methane (5%CH4/He) in the absence of oxygen with increasing temperature (CH4-TPRe). These experiments were conducted up to 600 \u00baC with a heating rate of 10 \u00baC min\u22121 followed by an isothermal step for 30\u2009min. The amounts of evolved CO2 (m/z\u2009=\u200944) as the main oxidation product and CO (m/z\u2009=\u200928) and H2 (m/z\u2009=\u20092) as by-products derived from possible reforming processes during the run were measured. These results can be helpful in understanding the lean methane oxidation reaction in the light of the widely accepted Mars \u2013 van Krevelen mechanism. As shown in Fig. S11 (Supplementary Material), the process is dominated by the large formation of CO2 (and CO and H2, not shown) at 600\u2009\u00b0C that corresponded to the full reduction of cobalt species, which eventually catalysed the conversion of methane into syngas and CO2. However, more valuable data could be extracted from the detected production of CO2 at lower temperatures (400\u2013550\u2009\u00b0C), since this could be exclusively ascribed to the full oxidation of methane by the active oxygen species of the Co-Ce foam catalysts. Thus, an enlarged view of the CO2 generation profile in this temperature window is included in \nFig. 7. It was observed that for the F(U) catalysts the reactivity of oxygen species was relatively similar in view of their comparable peak oxidation temperature around 525\u2013530\u2009\u00b0C, except for the sample prepared with a \u03a6 ratio of 0.25 (550\u2009\u00b0C). In addition, a comparable oxygen consumption was observed from this set of samples ranging between 0.31 and 0.35\u2009mmol\u2009O2 gCo\n\u22121. The onset temperature for methane oxidation was in the 415\u2013455\u2009\u00b0C range. The onset temperature was defined as the temperature at which 5% of the total CH4 uptake in the low temperature range (below 550 \u00baC) was consumed. Interestingly, the catalysts prepared with glycine were considerably more active as revealed by the lower onset (380\u2013415\u2009\u00b0C) and peak oxidation temperatures (495\u2013535\u2009\u00b0C). Also, more appreciable was the amount of oxygen species involved in the oxidation process over these samples that ranged between 0.31 and 0.56\u2009mmol\u2009O2 gCo\n\u22121. All the results suggested that the use of glycine produced samples with improved properties for methane oxidation that could be associated with the favoured oxygen mobility induced by cerium insertion in the Co3O4 lattice. Thus, the most promising samples were the foam catalysts prepared with high \u03a6 ratios (0.75 and 1.00).\n\nFig. 8 shows the corresponding light-off curves of the oxidation of lean methane (85\u2009l\u2009gCo3O4\n\u22121 h\u22121) over the foam catalysts prepared with each fuel and varying \u03a6 ratio. The GHSV was around 4000\u2009h\u22121, calculated based on the total volume of structured foam catalyst (1.5\u2009ml). Carbon dioxide and water were the only detected reaction products. The absence of mass and heat transfer limitations within the reactor was checked in order to ensure that they did not affect the obtained kinetic results. Taking into account that the transfer regimes for a structured catalyst significantly varies with respect to their powdered counterparts, four different criteria were checked following the recommendations given by Ercolino et al. [16] and Italiano et al. [47], namely Carberry (external mass transfer), Weisz-Prater (internal mass transfer), Mears (external heat transfer) and Anderson (internal heat transfer) criteria. The mathematical equations related to each criterion are listed in Table S2, Supplementary Material. As an example, corresponding values derived from the estimated reaction data at several temperatures (300\u2013600\u2009\u00b0C) for the foam catalyst prepared with glycine and \u03a6\u2009=\u20091 are included in this table. Judging from the obtained results it was verified that inter- and intra-phase concentration and temperature gradients were negligible below 500\u2013550 \u00baC. Note that the contribution of heat/mass transfer limitations expectedly was significant at high temperatures (600 \u00baC) since the estimated values were only one order of magnitude lower with respect to the corresponding threshold.As for the foam catalysts synthesised with urea, an appreciable conversion (10%) was noticed at 400\u2013425\u2009\u00b0C. In view of the T50 values (\nTable 4) the foam samples with a \u03a6\u2009=\u20090.75 and \u03a6\u2009=\u20091.0 showed a similar efficiency with values in the 515\u2013520\u2009\u00b0C range, whereas the sample with \u03a6\u2009=\u20090.5 required 530\u2009\u00b0C for this conversion level (50%). Clearly, the poorest performance was shown by the catalyst prepared with the lowest amount of urea (\u03a6\u2009=\u20090.25). Accordingly, the conversion trend at 600\u2009\u00b0C followed the same order, namely, 85% conversion (\u03a6\u2009=\u20090.75\u20131.0), 80% conversion (\u03a6\u2009=\u20090.5) and 70% conversion (\u03a6\u2009=\u20090.25). By contrast, the use of glycine as a fuel comparatively resulted in markedly more efficient foam catalysts. Hence, at 400\u2013425\u2009\u00b0C a conversion as high as 20% was already noticed. No substantial differences were observed among the samples with a \u03a6 ratio of 1.0 (T50 = 450\u2009\u00b0C), 0.75 (T50 = 455\u2009\u00b0C) and 0.5 (T50 = 460\u2009\u00b0C), as shown in Table 4. Thus, these three samples achieved at least 95% conversion at 600\u2009\u00b0C. Again, the catalyst prepared with the lowest \u03a6 ratio exhibited a considerably poorer performance (T50 = 500\u2009\u00b0C).Despite the differences in activity found among the foam catalysts obtained with both fuels, the apparent activation energies of all examined samples were in the 70\u201376\u2009kJ\u2009mol\u22121 range. These values were comparable to those exhibited by bulk Co3O4 oxides [48,49], and suggested that the obtained kinetic results were not affected by diffusional limitations, in line with the results reported in Table S2, Supplementary Material. The apparent activation energy was estimated by assuming a first pseudo-order for methane and a zeroth pseudo-order for oxygen [50]. The integral method was applied to estimate the apparent activation energy when considering a first pseudo-order for methane and a zero pseudo-order for oxygen. Conversions between 10% and 90% were fit to the following linearized equation for the integral reactor (Eq. 1)\n\n(1)\n\n\nln\n\n[\n\n\u2212\n\nln\n\n(\n\n1\n\u2212\nX\n\n)\n\n\n]\n\n=\n\nln\n\n[\n\n\n\nk\n\n\n0\n\n\n\n\nC\n\n\nC\n\n\nH\n\n\n40\n\n\n\n\n\n(\n\n\nW\n\n\n\nF\n\n\nC\n\n\nH\n\n\n40\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\nR\nT\n\n\n\n\n\nwhere X is the fractional conversion of methane, k0 is the pre-exponential factor of the Arrhenius equation and W/FCH40 is the weight hourly space velocity. The goodness of the numerical fit is shown in Fig. S12 (Supplementary material).Having proven that glycine was a more suitable fuel for depositing the active phases on the open cell foams by solid combustion synthesis, the determination of the optimal \u03a6 ratio was attempted by comparing the specific reaction rate at a selected temperature of 400\u2009\u00b0C. This reaction rate was calculated under differential conditions (conversion<20%). Therefore, it was estimated as the ratio between the experimental conversion and the weight hourly space velocity (W/FCH40). Results included in Table 4 and Fig. 8 revealed a notable dependence of the intrinsic activity with the amount of fuel for the F(G) catalysts. Hence, the normalised reaction rate notably increased from 1.9 (\u03a6\u2009=\u20090.25) to 3.8\u2009mmol CH4 gCo3O4 h\u22121 (\u03a6\u2009=\u20090.5). This promotion, although less noticeable, was also evident with larger amounts of fuel. In this way, the foam catalyst prepared with the highest \u03a6 ratio (\u03a6\u2009=\u20091) exhibited a reaction rate of 5.3\u2009mmol CH4 gCo3O4 h\u22121. For comparative purposes, obtained results of the catalysts prepared with urea were included in \nFig. 9 as well. As dictated by the light-off curves, the intrinsic activity at 400\u2009\u00b0C was remarkably lower, in the 1.3\u20131.9\u2009mmol CH4 gCo3O4 h\u22121 range. The differences in performance among the various examined \u03a6 ratios were rather less obvious when using this fuel.A reasonable correlation was found between the low-temperature O2 consumption of the foam catalysts, as determined by CH4-TPRe analysis, and their specific reaction rate (\nFig. 10). This relationship would be the confirmation of the methane oxidation reaction followed a Mars \u2013 van Krevelen mechanism, since the catalysts that exhibited larger O2 uptakes due to their favoured mobility of oxygen species evidenced a higher catalytic activity. The reason for this behaviour seemed to lie on the larger abundance of Co3+ ions on the surface of the catalysts prepared with high \u03a6 ratios of glycine, which in turn resulted in a more abundant presence of lattice oxygen species with high mobility, as evidenced by the complementary correlations depicted in Fig. 9 among the normalised reaction rate and the Co3+/Co2+ and Olatt/Otot at the surface. The superior performance of the glycine-based catalysts prepared with glycine with respect to their urea-based counterparts was ultimately associated with a more efficient insertion of cerium into the lattice of the spinel, thus promoting the presence of Co3+ ions within it. This induced a more marked distortion that led to improved redox properties at low temperatures. Structurally the F(G) catalysts also exhibited a well anchored, homogeneous catalytic coating on the surface of the ceramic substrate characterised by a good dispersion of both cobalt and cerium, and a relatively high porosity.The performance of the most active catalyst, namely the sample synthesised with glycine and a \u03a6\u2009=\u20091, was studied at varying GHSV in the 4000\u201360,000\u2009h\u22121 range (equivalent to a WHSV in the 85\u20131275\u2009l\u2009gCo3O4\n\u22121 h\u22121 range) at 600 \u00baC. Results included in Fig. S13 (Supplementary material) correspond to the averaged conversion for 4\u2009h in steps of 4000\u2009h\u22121. Expectedly, a gradual decrease in conversion was found at lower residence times, from 95% at 4000\u2009h\u22121, to 82% at 16,000\u2009h\u22121 and 72% at 60,000\u2009h\u22121. Interestingly, upon returning to the baseline GHSV (4000\u2009h\u22121) and after a total accumulated time interval of 56\u2009h at 600\u2009\u00b0C, the conversion recovered to the same initial value (close to 95%), thereby suggesting a reasonably good thermal stability of the foam catalyst.Additionally, the effect of the presence of water (10%vol.) and carbon dioxide (10%vol.) on the catalyst stability with time on stream was investigated under isothermal conditions (550 \u00baC) for a total reaction interval of 285\u2009h (85\u2009l\u2009gCo3O4\n\u22121 h\u22121). Firstly, the following feed mixtures were alternated every 25\u2009h: 1%CH4/10%O2/N2 - 1%CH4/10%O2/10%CO2/N2 - 1%CH4/10%O2/N2 - 1%CH4/10%O2/10%H2O/N2 - 1%CH4/10%O2/N2 - 1%CH4/10%O2/10%H2O/10%CO2/N2. Finally, conversion was again recorded under a 1%CH4/10%O2/N2 atmosphere (5\u2009h). The evolution of methane conversion under these reaction conditions is included in \nFig. 11. During the first 25\u2009h of operation, the samples showed a marked thermal stability with no evidence of deactivation. Hence, a relatively constant conversion at around 86% was noticed. After the subsequent admission of carbon dioxide to the feedstream during additional 25\u2009h, conversion was hardly affected. Upon returning to base conditions, the same conversion was still maintained. However, the addition of water caused a significant decrease to a stable value of 58% due its adsorption on the catalyst surface. Interestingly, when water was subsequently cut off, the methane conversion was almost fully recovered, with a value similar (82%) to that observed under dry conditions. Thus, it was evidenced that this temporary inhibiting effect of water did not lead to a significant irreversible deactivation of the sample. Finally, attention was paid to examining the effect of the simultaneous presence of carbon dioxide and water for 25\u2009h in an attempt to mimic a real exhaust gas from a natural gas-fuelled engine. Interestingly, the decrease in conversion provoked by water was not accentuated to a greater extent when combined with carbon dioxide, since a mean conversion of 56% was noted. When returning to the base conditions (1%CH4/10%O2/N2) the mean conversion along 5\u2009h was 81%.After the first 155\u2009h with alternating conditions, the influence of the presence of larger amounts of water vapour in the feed stream was then analysed. For this reason, varying concentrations of water vapour, from 10% to 30%vol. were admitted into the reactor during consecutive periods of 25\u2009h. It was found that, despite the high used concentrations, the detrimental effect to the methane conversion was relatively limited. Hence, the average conversions for the various water vapour concentrations were 53% (15%H2O), 50% (20% H2O), 47% (25% H2O) and 45% (30% H2O), thus evidencing that the catalyst was relatively resistant to increased concentrations of water in the feed stream. Moreover, after returning to the base dry conditions, the achieved conversion was 78%, which pointed out that the irreversible deactivation phenomenon was also limited even after exposure of the catalyst to a feedstream containing 30%H2O. It must be pointed out that the eventual formation of CO and H2 derived from reforming processes of methane (steam and/or dry reforming) was not observed. Even in the presence of 10%CO2 and up to 30%H2O the selectivity to CO2 was 100%. In other words, it could be assumed that the reactivity of methane with oxygen (10%) was highly preferential, even when admixtured with water vapour (30%) and/or CO2(10%).The (fresh) catalyst was subjected to a similar stability test as well, but operating at under a higher space velocity, in order to assess the influence of water vapour in conditions closer to those found in real natural gas engines exhausts. During consecutive reaction time intervals of 25\u2009h at 600 \u00baC, the catalytic performance was evaluated under dry and humid conditions (10\u201330%H2O) at 4000\u2009h\u22121 (85\u2009l\u2009gCo3O4\n\u22121 h\u22121) and 40,000\u2009h\u22121 (850\u2009l\u2009gCo3O4\n\u22121 h\u22121). Results shown in Fig. S14 (Supplementary material) for the first 100\u2009h were in agreement with the previous results on stability (Fig. 10). Hence, the conversion under dry conditions at 4000\u2009h\u22121 was around 95%, decreasing to 77% when adding 10%H2O and recovering again to the initial value after cutting off the admission of water. When the water concentration was raised to 30%vol. the conversion decreased to 59%. As for the second 100\u2009h-time interval at higher space velocity, the negative effect of the addition of water was found to be less marked with respect to that observed at 4000\u2009h\u22121, probably due to the water having a shorter residence time to adsorb on the surface of the catalyst. Thus, under the dry conditions the average conversion was 73% and decreased to 58% with 10%H2O and to 48% in the presence of 30%H2O.Finally, the catalyst subjected to the 285\u2009h-stability test (at 550\u2009\u00b0C and 4000\u2009h\u22121 in the presence of H2O and CO2) was characterized in order detect any structural or chemical differences with its fresh counterpart, which could be responsible for the slight deactivation caused by the long term exposure to water vapour. Thus, XRD analysis found no abnormal crystalline phases in the aged catalyst, although the estimated average Co3O4 crystallite size was found to be appreciably larger (32\u2009nm vs. 18\u2009nm). On the other hand, when comparing SEM images (Fig. S15, Supplementary material), a notable deterioration of the superficial structure was detected in the used sample, with multiple cracks and rifts that spread from the numerous pores of the original foamy structure. Images taken at high magnification also confirmed the results given by XRD, with the Co3O4 crystallites exhibiting poorly defined borders and generally larger sizes (35\u201355\u2009nm). These findings evidenced that exposure to water vapour induced a slight sintering.The spent catalyst was also submitted to CH4-TPRe analysis in order to assess the effect of water vapour ageing on the redox properties and mobility of oxygen species. The profile of CO2 production (m/z\u2009=\u200944) of both fresh and used catalysts, shown in Fig. S16 (Supplementary material), revealed a marked worsening in the reducibility of the used sample, given the increase in both the onset reduction temperature, from 392\u00b0 to 412\u2009\u00b0C, and in the peak reduction temperature, from 495\u00b0 to 500\u2009\u00b0C. However, after integration of the profiles it was found that the low-temperature O2 consumption of both samples was identical (0.56\u2009mmol\u2009O2 gCo\n\u22121). Thus, the decrease in the reducibility of the used sample was merely a side effect of the aforementioned sintering of the Co3O4 crystallites, and not due to any detrimental effect on the intrinsic chemical properties of the catalyst. This was in line with the identical Ce/Co molar ratio (0.06) found by EDX on the surface of both fresh and used samples.In this work the intensified lean methane oxidation with novel Co3O4(10\u2009wt%)-CeO2(1\u2009wt%) catalysts supported over \u03b1-Al2O3 open cell foams was investigated. The structured catalysts were prepared by solution combustion synthesis using urea or glycine as fuel while varying the fuel/oxidiser ratio (\u03a6 ratio) between 0.25 and 1.0. The textural, structural, morphological and redox properties were examined by a wide number of analytical techniques including SEM-EDX, STEM-HAADF coupled to EDX mapping, ICP-AES, WDXRF, N2 physisorption, XRD, HRTEM, Raman spectroscopy, XPS, H2-TPR and CH4-TPRe. The catalytic performance was evaluated under realistic reaction conditions in terms of a relatively high gas hourly space velocity and the simultaneous presence of water and carbon dioxide in the exhaust gas of the natural gas fuelled vehicle.Glycine was found to produce catalysts with a considerably better performance than the urea-based counterparts, with specific reaction rates being around 3 times higher. From a structural and morphological point of view, the reason behind this behaviour was closely related to the intrinsic porosity of the Ce-Co catalytic layer deposited onto the foam substrate, relative dispersion of deposited cobalt and cerium species and the Co3O4 crystallite size resulting from the used type of fuel. In particular, the catalysts prepared with glycine resulted in the formation of a highly porous catalytic coating containing relatively small, well dispersed spherical oxide crystallites. In contrast, the samples synthesised with urea did not lead to the formation of a distinct, homogeneous Ce-Co layer. In fact, some areas of the foam were not fully covered while other areas presented large flat patches of cobalt oxide. Moreover, the intimate mixture of cobalt and cerium when using glycine as fuel allowed for a more efficient insertion of Ce ions into the lattice of the cobalt spinel, which translated into a more favoured presence of Co3+ ions within the Co3O4 structure. This, in turn, led to an increased presence and mobility of the lattice oxygen species that led to a better performance for lean methane oxidation. The optimal fuel/oxidiser ratio for glycine was found to be the stoichiometric one. This foam catalyst exhibited a notable activity even at low residence times. Moreover, this sample showed a marked thermal and hydrothermal stability under isothermal conditions (550\u2013600 \u00baC). While the catalytic performance was not affected by the presence of carbon dioxide, the observed inhibiting effect of water was found to be almost reversible, although exposure to humid conditions eventually caused an appreciable sintering of the Co3O4 crystallites.\nAndoni Choya: Investigation, Writing \u2013 original draft. Sylwia Gudyka: Investigation. Beatriz de Rivas: Methodology, Validation. Jose Ignacio Guti\u00e9rrez-Ortiz: Methodology, Formal analysis. Andrzej Kotarba: Methodology, Conceptualization. Ruben L\u00f3pez-Fonseca: 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 work was supported by the Spanish Ministry of Science and Innovation [PID2019\u2013107105RB-I00 AEI/FEDER, UE]; the Basque Government [IT1297\u201319]; and the University of the Basque Country UPV/EHU [PIF15/335 and DOCREC21/23]. The technical and human support provided by Advanced Research Facilities-SGIker (UPV/EHU), and Central Scientific and Technological Research Services/Atomic Spectroscopy Division (University of C\u00e1diz) is acknowledged. 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.apcata.2022.118511.\n\n\n\nSupplementary material.\n\n\n\n.", "descript": "\n                  A series of CeO2-modified Co3O4 catalysts supported over \u03b1-Al2O3 foams was prepared by solution combustion synthesis and examined for the lean methane oxidation. Two different fuels were used namely, urea and glycine, with varying fuel/oxidiser (\u03a6) ratio. The catalysts were characterised by SEM-EDX, STEM-HAADF coupled to EDX mapping, ICP-AES, WDXRF, N2 physisorption, XRD, HRTEM, Raman spectroscopy, XPS, H2-TPR and CH4-TPRe, and their activity in the abatement of methane was analysed under realistic conditions. The use of glycine produced catalysts with significantly better morphological and structural properties. Likewise, a favoured insertion of cerium cations into the Co3O4 spinel lattice was observed, which caused a significant distortion of the spinel structure, thereby leading to a higher amount of mobile oxygen species capable of oxidising methane. These beneficial structural alterations were more pronounced with higher \u03a6 ratios.\n               "}
{"full_text": "Transition metal and nitrogen co-doped carbonaceous catalysts (M/N/C, M\u00a0\u200b=\u00a0\u200btransition metal) are a category of non-precious metal catalysts that are recognized as the most promising substitutes for the expensive platinum-based catalysts currently used for proton exchange membrane fuel cells (PEMFCs) and metal-air/oxygen batteries (MABs). M/N/C catalysts have high activity towards the oxygen reduction/evolution reactions (ORR/OER), are much cheaper than precious metal catalysts, and use abundant rather than scarce resources. Unsurprisingly, they have become one of the most important subjects of research in this area during the past decade.In 1964, Jasinski [1] first confirmed that cobalt cooperated with nitrogen to yield catalytic activity toward the ORR in an alkaline medium. He then proved that a transition metal catalyst together with nitrogen and carbon could, through heat treatment, achieve better ORR performance [2]. To date, it has been confirmed that co-doping with one or two transition metals, such as Fe [3], Co [4], Ni [5], Cu [6], Mn [7], and Zn [8], can result in significant catalyst enhancement, and that additional co-doping with other non-metallic elements, such as S [9], P [10], and Se [11], further improves catalyst performance.M/N/C analogue catalysts are generally prepared by the following process. First, precursors containing carbon or carbon/nitrogen are pyrolyzed at high temperature in an inert or ammonia atmosphere, then treated with acid to remove inactive metal compounds, and annealed at high temperature for additional pyrolysis and graphitization to further enhance their activity and stability [12,13]. Nitrogen doping is achieved via pyrolysis under an ammonia flow instead of in an inert atmosphere, or by adding nitrogen-containing materials (e.g., melamine, ammonia chloride etc.) as the doping agents. Washing the pyrolysis product with acid, then annealing it at high temperature, can further enhance the catalytic performance.The catalyst\u2019s activity arises from positively charged carbon atoms caused by C\u2013N bonds, because C has a lower electro negativity than N. These are considered the main active sites, but another type of active site can be formed by transition metal atoms coordinated with nitrogen [14\u201317]. The structure and active sites of M/N/C catalysts are shown in Fig.\u00a01\n.The ORR and OER play important roles in next-generation energy conversion and storage technologies, including PEMFCs and MABs [18,19]. Due to the sluggish kinetics of these reactions at the cathode, the catalysts with high activity are required. Platinum-,ruthenium-, and iridium-based catalysts are considered efficient choices for these reactions, but their scarcity and high cost hinder their application on a large scale [20,21]. M/N/C analogue catalysts hold the promise of a solution to this issue, as the requisite metals are abundant and inexpensive and have been proven to exhibit good ORR performance in an alkaline medium \u2014 even higher than Pt-based catalysts \u2014while in an acidic medium, their performance is only slightly lower than that of Pt-based catalysts. Furthermore, their performance toward the OER can be enhanced by doping or the use of a support.The research in this area has made tremendous progress in the past decade, and in this paper, we briefly review what has been achieved.Their high activity and much lower cost has made M/N/C catalysts a focus of interest for many researchers working on PEMFCs. Significant progress has been made on their activity in an acidic medium, and the half-wave potential with a rotating disk electrode (RDE) is close to that of a commercial Pt/C catalyst, establishing a foundation for the application of M/N/C catalysts in PEMFCs [22,23]. Numerous studies have concentrated on synthesizing high-performance M/N/C catalysts for this use. In 2011, Wu et\u00a0al. [24] reported preparing a catalyst with Fe and Co incorporated into a carbon\u2013nitrogen skeleton via pyrolysis, and investigated its performance as the cathode in a H2\u2013O2 PEM fuel cell. With a loading of 4\u00a0\u200bmg\u00a0\u200bcm\u22122, the single cell achieved a maximum power density of 0.55\u00a0\u200bW\u00a0\u200bcm\u22122. Since then, the additional M/N/C catalysts have been developed for the same purpose, but their performance in a single PEM fuel cell has not managed to reach 0.6\u00a0\u200bW\u00a0\u200bcm\u22122 [25\u201330]. However, the new precursors and new preparation technologies continue to be explored, so the catalysts\u2019 performance in PEMFCs should continue to increase [31\u201340].Porous organic compounds/polymers or metal-organic frameworks are promising precursors for preparing M/N/C catalysts, not only for enhancing catalytic activity but also to achieve better conductivity and mass transfer to counteract the sluggish ORR and the complex mass transfer process in the membrane electrode assembly (MEA). The researchers have found that the porosity of organic compounds/polymers can improve the exposure of active sites and the mass transfer process in the MEA. Chung et\u00a0al. [41] used two nitrogen precursors (cyanamide and polyaniline) to prepare an Fe/N/C catalyst with atomic-level hierarchical porosity, in which the active FeN4 sites were inserted into the carbon. The hierarchical porosity boosted mass transport when the catalyst was applied in a H2\u2013O2/air PEMFC, and the maximum power density reached 0.87 and 0.94\u00a0\u200bW\u00a0\u200bcm\u22122 at P\nO2 of 1.0 and 2.0\u00a0\u200bbar, respectively (Fig.\u00a02\n). Fu et\u00a0al. [42] adopted an ammonium chloride salt-assisted approach to tailor Fe/N/C catalysts derived from polyaniline to increase the number of FeN4 active sites on the edge side, which boosted ORR performance. In H2\u2013O2 and H2-air PEMFCs, the peak power density reached 0.86 and 0.43\u00a0\u200bW\u00a0\u200bcm\u22122, respectively.In addition, because the three-dimensional structure of metal-organic frameworks (such as zeolitic imidazolate frameworks, ZIF-8) can accommodate MN4 moieties and promote mass transfer (Fig.\u00a03\n), they have become promising precursors for preparing M/N/C catalysts with greater densities of MN4 moieties and better mass transfer results [43]. To date, PEMFCs have achieved better performance using M/N/C catalysts derived from metal-organic frameworks precursors. Our group has worked in this field and made some progress. We prepared Fe/N/C catalyst with hollow carbon nano-polyhedrons derived from hollow ZIF-8 with ferric acetylacetonate and g-C3N4. The optimized catalyst yielded excellent ORR catalytic activity in an acidic medium and satisfactory performance in a H2\u2013O2 PEMFC, with a current density of 133 and 400\u00a0\u200bmA\u00a0\u200bcm\u22122 at 0.8 and 0.7\u00a0\u200bV, respectively [44]. Its high performance was due to the polyhedral structure and the high density of Fe\u2013N4 moieties achieved by adding g-C3N4 during the preparation process. Subsequently, we further improved this catalyst\u2019s performance in a PEMFC by adopting a novel preparation strategy, inserting iron into the ZIF-8 framework by evaporating ferrocene during the first heat treatment, so that atomic iron could disperse uniformly with nitrogen to form active sites (Fig.\u00a04\n) [45]. Surprisingly, our catalyst exhibited excellent performance in a H2\u2013O2 PEMFC, reaching a current density of 1100 and 637\u00a0\u200bmA\u00a0\u200bcm\u22122 at 0.6 and 0.7\u00a0\u200bV with a low catalyst loading of 1\u00a0\u200bmg\u00a0\u200bcm\u22122 in the cathode catalyst layer (conventional loading reported in the literature was 4\u00a0\u200bmg\u00a0\u200bcm\u22122).Shui\u2019s group [46] prepared a concave Fe/N/C single-atom catalyst (SAC) with a high density of Fe\u2013N4 moieties by pyrolyzing mesoporous SiO2-coated ZIF-8 and an iron source, which greatly improved both catalytic activity and mass transport (Fig.\u00a05\n). After assembly in the MEA cathode, the Fe/N/C SAC (TPI@Z8(SiO2)-650-C) achieved a peak power density of 1.18\u00a0\u200bW\u00a0\u200bcm\u22122with 2.5\u00a0\u200bbar\u00a0\u200bH2\u2013O2, and its current density was 0.047\u00a0\u200bA\u00a0\u200bcm\u22122 at 0.88 ViR-free and 1.0\u00a0\u200bbar\u00a0\u200bH2\u2013O2, reaching the US Department of Energy\u2019s 2018 precious metal-free catalyst activity target. Shui\u2019s group [47] also synthesized Co/N/C catalysts from aZIF-8 precursor with different densities of CoN4 active sites and examined the relationship between the CoN4 active sites and the PEMFC\u2019s power density. At optimal active site density, the catalyst reached a peak power density of 826\u00a0\u200bmW\u00a0\u200bcm\u22122 in a PEMFC cathode. Wu\u2019s group [48] prepared a Co/N/C catalyst with a core\u2013shell structure by pyrolyzing a precursor of Co-doped ZIF-8, assisted by a surfactant; the peak power density reached 0.87\u00a0\u200bW\u00a0\u200bcm\u22122 in a H2\u2013O2 PEMFC. This high performance was attributed to the atomic dispersion of the Co active sites, the presence of micropores, and the high N content obtained from the surfactant layer, which prevented the single atomic Co from aggregating.\nFig.\u00a06\n presents the major breakthroughs that have been made in the application of M/N/C catalysts in PEMFCs.Although a great deal strides have been made in the performance of M/N/C catalysts, their application in PEMFCs still faces significant challenges when it comes to stability. Currently, the catalyst performance quickly decays during PEMFC operation, especially in the initial working hours, as shown in Fig.\u00a07\n [49]. Multiple researchers have worked on determining the attenuation mechanisms and improving the stability of these catalysts. Now, it is generally believed that the attenuation factors are as follows (Fig.\u00a08\n): (1) demetallation of the active sites [50,51]; (2) protonation of the active sites [52,53]; (3) flooding of the micropores and catalyst layer [54,55]; (4) carbon corrosion [56,57]; and (5) attacks by H2O2 or free radicals [58,59].Understanding these attenuation mechanisms can enable the development of effective strategies to improve stability, and work in this area is underway. Wang et\u00a0al. [60] focused on preventing water flooding and carbon corrosion (see Fig.\u00a09\n), so they prepared Fe/N/C catalyst by surface fluorination. The electron-withdrawing and hydrophobic properties of the surface fluorination agent (trifluoromethylphenyl) prevented water flooding and inhibited the oxidative corrosion of the carbon matrix, yielding improvement in the catalyst\u2019s stability. Wei et\u00a0al. [61] looked at eliminating H2O2 by modifying the catalyst with CeO2 in order to clear the H2O2 produced during the ORR process. Open circuit voltage tests of a single PEMFC showed that the peak power density loss of Fe/N/C with CeO2 was much less. Similarly, Bai et\u00a0al. [62] enhanced Fe/N/C catalyst\u2019s stability by combining CeO2 nanoparticles into the catalyst structure; the nanoparticles acted as H2O2 scavengers and thereby protected the active sites, but the stability results didn\u2019t carry over into the MEA. Unfortunately, there are few reports, to date, on successful improvement of M/N/C catalyst stability in a PEMFC, so concerted efforts are still required in this area.As mentioned earlier, M/N/C catalysts have excellent ORR performance in both acidic and alkaline media [63]. And it has been proved by many research works that the MABs with M/N/C catalysts as air cathode exhibited excellent activity and stability [64\u201369]. The application of M/N/C catalysts in various kinds of MABs has been widely and intensively explored, including in Li-air/oxygen batteries, Zn-air batteries, Al-air batteries, and Mg-air batteries. M/N/C catalysts generally exhibit good performance in these types of batteries, comparable to that of precious-metal catalysts.Shui et\u00a0al. [70] first applied the Fe/N/C as cathode catalyst in Li\u2013O2 battery, which could reduce the overpotential, enhance battery efficiency and improve lifespan. But they did not discuss the capacity of discharge/charge of the material in detail. However, this work revealed the possibility that Fe/N/Chad potential to apply in MABs. Li et\u00a0al. [71] prepared the Fe/N/C catalyst with many exposed active sites through constructed special pore-in-pore structure, the material achieved a specific capacity of 7250\u00a0\u200bmA\u00a0\u200bh\u00a0\u200bg\u22121 at 70\u00a0\u200bmA\u00a0\u200bg\u22121 in Li\u2013O2 battery.More and more attentions have been attracted on designing M/N/C as the cathode catalysts for Zn-air battery. Zhang et\u00a0al. [72] constructed the Fe/N/C electrocatalyst with FeN4 sites and hierarchically ordered porous, gaining a high power density of 235\u00a0\u200bmW\u00a0\u200bcm\u22122 and a high capacity of 768.3\u00a0\u200bmA\u00a0\u200bh\u00a0\u200bg\u22121 when applying the catalyst as cathode of Zn-air battery. Chenet\u00a0al. [73] prepared the Fe/N/C catalyst with high density of FeNx active sites and achieved an unexpected ORR performance in alkaline medium. While applying in Zn-air battery, the peak power density could reach 266.4\u00a0\u200bmW\u00a0\u200bcm\u22122 and a specific capacity was 795.3\u00a0\u200bmA\u00a0\u200bh\u00a0\u200bg\u22121. Except Fe/N/C, Co/N/C is also an effective catalyst for the cathode of Zn-air battery. Luo et\u00a0al. [74] designed the Co/N/C bi-functional catalyst for the cathode of Zn-battery with 3D brush-like nanostructure, exhibiting a high peak power density (246\u00a0\u200bmW\u00a0\u200bcm\u22122) and better cycle performance.M/N/C analogue catalysts can also be applied in other MABs, such as Al-air battery, Mg-air battery. Li et\u00a0al. [75] enhanced the performance of Fe/N/C by using Cu replaced partial Fe, while it used as cathode of a Al-air battery, a higher discharge voltage and better stability achieved. Ye et\u00a0al. [76] chose Fe-doped ZIF-8 as precursors to fabricate Fe/N/C catalyst with high density of FeNx active sites and mesopores, and the good power density of 72\u00a0\u200bmW\u00a0\u200bcm\u22122 was gained at 0.72\u00a0\u200bV when applying in Mg-air battery.Capacity and cycling performance are two key indicators for MABs, so designing M/N/C catalysts with high capacity and good cycling stability is key for their application in these batteries [77\u201379]. However, M/N/C catalysts generally exhibit higher charging overpotential in rechargeable metal air/oxygen batteries due to their inferior OER performance.To realize the desired charge\u2013discharge performance of M/N/C catalysts in rechargeable MABs, it is essential to enhance their OER performance. Notably, decorating these catalysts with other compounds that yield unique structures can greatly improve their capacity and cycling performance [80,81]. He et\u00a0al. [82] developed an inexpensive, easy way to prepare an efficient catalyst for the Li\u2013O2 battery using N-doped graphene decorated with Fe/Fe3N/Fe4N nanoparticles, as shown in Fig.\u00a010\n. Excellent discharge\u2013charge capability was obtained when it was used in the cathode. Wang et\u00a0al. [83] prepared Fe/N/C catalyst embedded with Fe\u2013FeC3 formed from a compound MOFs system that contained MIL-100(Fe) and ZIF-8. The catalyst possessed a high initial discharge capacity of 8749\u00a0\u200bm Ah gCat+C\n\u22121 and a charge capacity of 8104\u00a0\u200bm Ah gCat+C\n\u22121 after use in a Li\u2013O2 battery. Chao et\u00a0al. [84] introduced a second transition metal (Co) into Fe/N/C catalyst. They used Fe, Co bimetallic MOF to prepare Fe, Co co-doped carbon\u2013nitrogen composite that yielded outstanding electrochemical activity and good cycling stability in Li\u2013O2 batteries due to its accessible active sites and unique catalyst structure, shown in Fig.\u00a011\n.Progress has also been made with Zn-air batteries. Qin et\u00a0al. [85] improved the capacity and cycling performance of a Zn-air battery by preparing a N-, P co-doped carbon catalyst inserted with FePx nanoparticles and a Fe/N/C moiety. Because of the high hydrogen evolution reaction performance of FePx and the structural properties achieved through co-doping, the resulting Zn-air battery showed a high specific capacity and outstanding stability. Co/N/C catalysts have proven even more suitable for Zn-air batteries. Amiinu et\u00a0al. [86] designed Co/N/C with abundant Co\u2013N coupling at the center as a high-performance bifunctional catalyst for the Zn-air battery, gaining a peak power density of 193.2\u00a0\u200bmW\u00a0\u200bcm\u22122 and an energy density of 853.12\u00a0\u200bWh kgZn\n\u22121. A Zn-air battery assembled with this catalyst had high cycling performance and operated for 80\u00a0\u200bh without attenuation. The battery\u2019s improved performance was due to the high density of active sites for the ORR/OER as well as the catalyst\u2019s porosity and high surface area. Liu et\u00a0al. [4] reduced the discharge\u2013charge overpotential and improved the cycling performance of a Zn-air battery by applying single-atom dispersed Co/N/C catalyst in the cathode. The results showed that abundant Co vacancies enhanced the ORR and OER performance, and the large amount of exposed pyridinic N inhibited the accession of zincate ions and the precipitation of ZnO (Fig.\u00a012\n). For these reasons, the Zn-battery achieved a power density of 155.3\u00a0\u200bmW\u00a0\u200bcm\u22122 and excellent cycling performance.From the reported work it can be seen that excellent breakthroughs have been achieved in improving the capacity and cycling stability of MABs by using M/N/C analogue catalysts in the cathode. Although some problems remain, these catalysts have great potential for application in MABs. The studies described above suggest that M/N/C catalysts yield better results, particularly in cycling performance, when decorated with other compounds, as shown in Table\u00a01\n.In summary, the application of M/N/C analogue catalysts in PEMFCs and MABs has been widely investigated, and great progress with some breakthroughs have been made. For PEMFCs, results comparable to those with Pt/C catalysts have been reported in recent years, especially with well-designed and well-prepared Fe/N/C and Co/N/C catalysts. Due to the high density of active sites dispersed in the skeleton and the active sites exhibit high catalytic activity towards the ORR process, making M/N/C analogue catalysts being the most promising catalysts to replace precious-metal catalysts that applying in cathode of PEMFCs. However, many problems and challenges still remain to be addressed before practical application of these catalysts can occur. One issue is the poor stability/durability of M/N/C catalysts in acidic PEM surroundings, caused by the dissolution of doped Fe/Co, carbon corrosion, and protonation of the active sites. Although some strategies have been suggested to address these challenges, it is still necessary to design and explore new carbon-based catalysts with better stability and durability. On the other hand, the application of M/N/C analogue catalysts in MABs has made great strides, and their practical application is almost feasible. While applying in cathode of MABs, these catalysts generally exhibit charge\u2013discharge performance far exceeding that of the traditional manganese oxide-based air electrode, and excellent stability/durability. Although M/N/C analogue catalysts exhibited high oxygen reaction performance in cathode of MABs, the problems and challenges are also existed. Capacity and cycling performance should be further enhanced with the real application of these catalysts in MABs, especially the cycling performance. The OER performance of M/N/C analogue catalysts will influence the cycling performance of MABs. To enhance the cycling performance, decorating the M/N/C catalysts with other compounds may be an effective strategy. In summary, to realize the practical application in PEMFCs and MABs, M/N/C analogue catalysts needs to be further improved to enhanced its performance, especially the stabilities.The manuscript has been reviewed and approved by each signed author and its publication is approved by all authors and explicitly by the responsible authorities where the work was carried out.The manuscript has not previously been published in any form, nor submitted for reviews to any other journal currently. And we will not submit it elsewhere until a decision has been made by your journal. If accepted, the manuscript will not be published elsewhere in the same form or in any other language without the written consent of your journal. Therefore, there will not be any conflict of interest for this manuscript.This work was supported by the National Key Research and Development Program of China (Project Nos. 2017YFB0102900and 2016YFB0101201), the National Natural Science Foundation of China (NSFC Project Nos. 21476088 and 21776105), the Guangdong Provincial Department of Science and Technology (Project No. 2015B010106012), and the Guangzhou Science, Technology and Innovation Committee (Project Nos. 201504281614372 and 2016GJ006).", "descript": "\n                  To meet the sharp increase in demand for clean and renewable energy, it is necessary to develop new energy-conversion and storage technologies, such as proton exchange membrane fuel cells (PEMFCs) and metal-air/oxygen batteries (MABs). Due to the sluggish reaction kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in the cathodes of PEMFCs and MABs, significant amounts of precious metal catalysts need to be used, driving up the cost of fuel cells and MABs and thereby hindering their commercialization on a large scale. Transition metal and nitrogen co-doped carbonaceous catalysts (M/N/C) have high catalytic activity towards the ORR and OER once the catalysts are modified with certain promoters/additives. In addition, M/N/C catalysts can be prepared from abundant, inexpensive materials, making their cost negligible compared with precious metal catalysts, a development that would efficiently decrease the cost of PEMFCs and MABs. In last decade, numerous researchers have attempted to realize these applications of M/N/C catalysts, and some exciting results have been achieved, making these promising replacements for precious metal catalysts. However, some serious problems and significant challenges remain. In this paper, we review the research on the application of M/N/C analogue catalysts in PEMFCs and MABs in the last 10 years, indicate the remaining challenges, and suggest the future research directions.\n               "}
{"full_text": "Methane, the main component of natural gas is available in large quantities and it is therefore a major energy source nowadays. Produced from bio derived sources, bio-methane has the potential to contribute substantially to future climate-neutral energy generation [1,2]. Both natural gas and biomethane are attractive fuels for decentralized power generation such as fuel cell-based combined heat and power (CHP) units. Such a CHP system has been engineered and built at Fraunhofer IMM with an electrical power equivalent of 50 kWel [3]. It relies on hydrogen generation by steam-reforming of natural gas, which is also combusted to supply heat to the reforming process. Micro plate heat exchanger technology allows the efficient coupling of both reactions in catalyst coated microchannels [4].For the combustion of various hydrocarbons including VOCs, Pt-based catalysts are known for their high activity [5]. However, in case of methane, which is the hydrocarbon most difficult to combust due to the symmetry of the molecular structure and resulting weak adsorption capacity on various supports, Pt-based catalysts have several drawbacks, namely, poor low-temperature activity and insufficient stability due to sintering and self-poisoning of metallic Pt sites by the dissociative adsorption of O2, especially under O2-rich (lean) conditions [6\u20138]. To overcome these limitations, Pd and mixed PtPd catalyst are suggested, which show significantly improved low-temperature activity [9\u201311]. On the other hand, Pd-based catalysts are quite unstable at higher temperatures which can be explained by the reaction mechanism. Generally, the Mars- van Krevelen mechanism is the widely accepted route for methane combustion over Pd [12\u201314].According to this mechanism, methane is adsorbed on PdO, the latter being reduced. In a second step, Pd is oxidized by O2 from the gas phase. However, PdO decomposes quite rapidly at temperatures exceeding 600\u00a0\u00b0C so that the adsorption of CH4 is inhibited, whereas metallic Pd is much less active compared to PdO [15,16]. In addition, Pd catalysts are also very sensitive to small amounts of sulphur species in the feed such as SO2 or SO3 [17]. Although adsorptive desulfurization will always be performed in practical processes upstream the combustion step, a practical catalyst should be capable of tolerating temporary breakthrough of low amounts of sulphur.Several other catalysts and support materials for low-temperature methane combustion are described, e.g. Co3O4/CeO2, core-shell systems, bimetallic Ru-Re/Al2O3, pervoskites and Ni-based catalysts but either nothing is reported about their stability at high reaction temperatures or their stability is insufficient [18\u201322]. For decentralized power generation units, a high-temperature stable combustion catalyst is required, because natural gas or biomethane steam-reforming is performed at T\u00a0>\u00a0700\u00a0\u00b0C [23\u201326]. To supply the steam reforming reaction with energy, a combustion catalyst formulation is lacking, which is stable at these temperatures. Rh catalysts are often reported to be stable at high reaction temperatures, but very few papers exist dealing with their performance as catalysts for the combustion of hydrocarbons [27]. Therefore, a systematic comparison of their activity with platinum and palladium catalysts is required, which is carried out in this study with special emphasis on the stability at 600\u00a0\u00b0C. Samples were prepared by keeping the total metal loading constant at 5 wt.-%, and replacing Pt with Rh up to 4 wt.-% Rh. Because Pd-containing catalysts are more common in methane combustion, a 5 wt.-% Pd catalyst is also tested for comparison.The catalysts were prepared by the incipient wetness impregnation method. The support \u03b3-Al2O3 (Puralox SBa-200, SASOL) was impregnated with a solution containing the calculated amounts of tetraamineplatinum(II) nitrate (Alfa Aesar) and a Rhodium(III) nitrate solution (abcr GmbH) to obtain 1\u20135 wt.-% Pt with up to 4\u00a0wt% Rh-containing alumina based catalysts, hereinafter denoted as xPt-(5-x)Rh. A nitric acid solution of Palladium(II) nitrate (Sigma-Aldrich) was used as the Pd precursor. The overall metal loading was kept constant for all catalysts.After calcination at 450\u00a0\u00b0C, the as-prepared powders were deposited as washcoats onto stainless steel microchannels by means of an aqueous suspension containing polyvinyl alcohol (PVA) as a binder. The plates containing the microchannels (14 channels with 500\u00a0\u03bcm width, 250\u00a0\u03bcm depth and 25\u00a0mm length) were dried at room temperature and calcined at 450\u00a0\u00b0C. From each two plates and input and output capillaries, a reactor was assembled by laser welding. Details of the microreactors applied for testing have been described elsewhere [28].The surface area was determined by nitrogen adsorption using a Sorptomatic 1990 (Thermo Fisher Scientific Inc.) automatic apparatus and calculated by the Brunauer-Emmett-Teller (BET) method. The actual metal loadings were detected by X-ray fluorescence on an ED-XRF Canberra-Packard 1510 spectrometer.For powder XRD measurements, samples were mounted on a polyacetate foil using a glue based on collodion and recorded on a Stoe Stadi P diffractometer using Cu-K\u03b11 radiation from a sealed tube X-ray source operating at 40\u00a0kV and 30\u00a0mA.XPS spectroscopy measurements were performed by using a multi-chamber UHV system (PREVAC, Poland). It was equipped with a monochromated Al source (XM650 X-ray monochromator) operated at 360\u00a0W and a hemispherical electron analyser (Scienta R4000). For survey scans, a pass energy of 200\u00a0eV was fixed. The background pressure of the ultra-high vacuum (UHV) chamber was 5\u00a0\u00d7\u00a010\u22128\u00a0mbar. All spectra were calibrated by setting the position of the C1s line to 284.7\u00a0eV. CasaXPS (ver. 2.3.16 PR 1.6) software was used to process the recorded spectra and for deconvolution of the signals. This procedure introduced a measurement uncertainty of about +/\u2212 0.2\u00a0eV.High resolution TEM images were recorded on a Titan G2 60\u2013300\u00a0kV (FEI Company) equipped with a monochromator operating on a Schottky field emission gun (FEG), three condenser lenses system, the objective lens system, Cs image correction, HAADF detector and EDS (Energy Dispersive X-Ray Spectroscopy). The samples were prepared by grinding in an agate mortar to fine powders, which were transferred to a slurry in 99.8% ethanol. After homogenisation by sonication, the material was supported on a 200-mesh copper grid with lacey formvar and stabilized with carbon and left on a filter paper for ethanol evaporation. Studies of the catalysts were carried out at an accelerating voltage of 300\u00a0kV.The microreactors were placed into a metal block powered by a heating cartridge regulated by a PID temperature controller with a K-type thermocouple inserted next to the catalyst coating. A temperature program from 450 to 750\u00a0\u00b0C was carried out for each catalyst. Also, the stability at 600\u00a0\u00b0C was investigated for 40\u00a0h. Experiments were carried out at a WHSV of 400\u00a0L gcat\n\u22121\u00a0h\u22121 of the feed flow. The O/C ratio was adjusted to 7.4, which corresponds to a methane concentration of 5.13%. Synthetic air and methane (purity >99.95%) were provided by Bronkhorst mass flow controllers. All tests were carried out at atmospheric pressure. Product gases were analysed by an online gas chromatograph (Agilent Technologies GC 7890A system) equipped with two thermal conductivity detectors and a flame ionization detector. The conversion of CH4 was calculated according to the following equation:\n\n(1)\n\nX\n\n\nC\n\nH\n4\n\n\n\n=\n\n\n\n\n\nC\n\nH\n4\n\n\n\ninlet\n\n\u2212\n\n\n\nC\n\nH\n4\n\n\n\noutlet\n\n\n\n\n\nC\n\nH\n4\n\n\n\ninlet\n\n\n\u00d7\n100\n%\n\n\n\n[CH4]inlet is the inlet concentration of CH4 and [CH4]outlet is the CH4 concentration at the outlet. The catalysts were heated in 20\u00a0mL\u00a0min\u22121 air to the reaction temperature prior to the catalytic tests.\nTable 1\n provides an overview of BET surface area and XRF measurements as determined for all catalysts under investigation. In Fig. S1a-e (Electronic Supporting Information, ESI), representative HR-TEM images of the fresh samples of the Pt and PtRh catalysts are shown. For all fresh catalyst samples, small metal particles of the active phase are very highly dispersed and evenly distributed on the \u03b3-Al2O3 carrier. In addition, some larger metal particles can also be observed, but in much smaller quantity. Some amorphous areas and characteristic rod-like \u03b3-Al2O3 crystals are observed for the support material. The small size of the noble metal particles gets obvious from Table 2\n, where the average dimensions of the metal particles of each catalyst are provided. Both 5Pt and mixed PtRh catalysts exhibit similar average metal particle sizes in the range of 1.4\u20132.3\u00a0nm as fresh catalyst. Phase identification adapted from HR-TEM images revealed the active phase of 5Pt/Al2O3 catalyst as Pt(111) and Pt(200) facets based on the interplanar distance of 1.96\u00a0\u00c5 and 2.26\u00a0\u00c5, respectively. For PtRh catalysts, mixed PtRh particles exposing Pt/Rh(222), Pt/Rh(200) and Pt/Rh(111) facets were identified based on the interplanar distance of 1.11, 1.93 and 2.22\u00a0\u00c5, respectively.In Fig. S2a-e (ESI) representative HR-TEM images of the spent catalyst samples after a stability test of 40\u00a0h duration are presented. Fig. S2 (ESI) provides evident that the particles on the spent samples increased in size in the course of reaction. This indicates aggregation and subsequent sintering. However, the average particle sizes of the spent samples also shown in Table 2 reveal smaller values with increasing Rh content. Even a low amount of Rh in the 4Pt1Rh catalyst leads already to a drastically lower average particle size compared to the 5Pt catalyst. This suggests less stable anchoring of the Pt crystallites in the absence of Rh on the alumina support. Thus, the introduction of Rh leads to a more stable catalyst. Similar to the fresh samples, mixed particles in the form of Pt/Rh(111) were identified for the PtRh catalysts with the exception of 4Pt1Rh catalyst, where only Pt(111) was detected likely originating from the low amount of Rh in the sample.\nFig. 1\n. shows the wide-angle XRD patterns of all catalyst under investigation. Peaks for cubic \u03b3-Al2O3 can be found at 2\u03b8\u00a0\u2248\u00a038.2\u00b0, 45.1\u00b0, 60.5\u00b0, 66.8\u00b0 and 84.7\u00b0, corresponding to the (311, 400, 333), (440, 444) facets (ICDD 01\u2013074-2206). Pt reflexes are identified at 2\u03b8\u00a0\u2248\u00a039.6\u00b0, 67.4\u00b0 and 81.1\u00b0 for catalysts with high Pt-content (5Pt, 4Pt1Rh, 3Pt2Rh) corresponding to the (111,220,311) facets, which are characteristic of face-centered cubic structure of metallic Pt (ICDD01-070-2431). Diffraction peaks at 2\u03b8\u00a0\u2248\u00a033.3\u00b0, 39.8\u00b0 and 84.5\u00b0 were assigned to the Rh2O3 (ICDD 01\u2013076-0148) component for catalysts with high Rh content (1Pt4Rh, 2Pt3Rh).X-ray photoelectron spectroscopy (XPS) was used to investigate the surface composition of the PtRh alloy present in the developed catalysts. The XPS spectra of Pt 4f for the 5Pt and PtRh catalysts are shown in Fig. 2A. Two different Pt 4f deconvolution peaks appeared at 70.3 and 75.1\u00a0eV for the 5Pt catalyst. The first binding energy is attributed to the presence of metallic Pt0 and the second binding energy corresponds to the presence of Pt4+. The 4Pt1Rh and 3Pt2Rh catalysts shows three deconvolution peaks at 70.8\u201371.0\u00a0eV, 73.3\u00a0eV and 75.0\u00a0eV. The first binding energy is mainly due to the presence of Pt0, whereas the latter two peaks correspond to Pt2+ and Pt4+, respectively. It must be noted that the higher Rh content in 2Pt3Rh and 1Pt4Rh catalysts provides only one deconvolution peak at 74.9\u00a0eV due to presence of Pt4+. This result indicates that the first three catalysts mainly consisted of metallic Pt species along with oxidized Pt species, while the latter two catalysts consisted only of oxidized Pt4+ species. It could be assumed that not enough Pt is present on the surface and the presence of larger amounts of Rh prevent reduction of the Pt species in the 2Pt3Rh and 1Pt4Rh catalysts. As shown in Fig. 2B, the XPS spectra of the PtRh catalysts show the Rh 3d peaks at 310.0\u00a0eV, suggesting the presence of Rh3+ in all the catalysts, which is in line with the powder XRD measurements.The following results shown in Fig. 3\n were obtained for the catalysts in the temperature program test. For all tests, no CO was detected, so that full selectivity towards CO2 was achieved.It is obvious that the low-temperature activity is improved by the partial replacement of Pt with Rh. A CH4 conversion of 99.4% is already achieved at 500\u00a0\u00b0C for the 1Pt4Rh catalyst and full conversion is observed for 3Pt2Rh, 2PtRh and 1Pt4Rh catalysts at 600\u00a0\u00b0C. On the other hand, for the 3Pt2Rh and 4Pt1Rh catalysts, a small decrease of conversion is observed at 750\u00a0\u00b0C, implying that these catalysts are not stable at this temperature. However, this is not the case for the 5Pt catalyst, which shows by far the lowest activity up to 600\u00a0\u00b0C among all catalysts tested. The Pd catalyst shows a different behavior with relatively high conversion obtained at 500\u00a0\u00b0C, which increases only slightly up to 750\u00a0\u00b0C, where 96.1% conversion is achieved.The results of the stability tests as determined for the fresh samples are presented in Fig. 4\n. The experiments reveal that Rh also improves the stability of the catalysts at 600\u00a0\u00b0C, whereas the Pd catalyst loses its initial activity quickly and stabilizes at 44% conversion. A high initial conversion is obtained for the catalysts containing \u22652 wt.-% of Rh, which is in line with the measurements performed at 600\u00a0\u00b0C shown in Fig. 3. Although the 5Pt catalyst is also quite stable during the test, it shows much lower conversion than the Rh-containing catalysts, as it was expected from the tests performed at different temperatures (Fig. 3). The 4Pt1Rh catalyst shows a much higher initial conversion but deactivates rather quickly. A moderate deactivation is observed for 3Pt2Rh and 2Pt3Rh catalysts, which initially show full conversion. The stability of the initially highly active 1Pt4Rh catalyst is the best of all PtRh catalysts. However, a small decline in conversion is observed after 40\u00a0h on reaction stream and from HR-TEM images (see Fig. S2e, ESI) indicating that sintering also occurs on this catalyst. A higher degree of sintering is observed with higher Pt-loading of the catalyst.The stability of the PtRh catalysts corresponds directly to the average crystallite size of the spent samples, and a higher stability is obtained with smaller particle size. Furthermore, XPS results could explain the improved activity and stability of the 2Pt3Rh and 1Pt4Rh catalysts as both consisted of only oxidized Pt4+ species compared to the other catalysts, which consisted mainly of metallic Pt species along with oxidized Pt species.The partial replacement of Pt with Rh leads to a significantly improved low-temperature activity at 450\u00a0\u00b0C reaction temperature compared to the sample containing only Pt. Mixed PtRh crystals were observed in the corresponding fresh samples and their crystal size was as low as 2\u00a0nm. Also, a stability test of 40\u00a0h duration revealed improved stability of the mixed PtRh catalysts compared to the samples containing only Pt or Pd. This improved stability during methane combustion is believed to originate from the improved stability of the crystals of the active metal species with increasing Rh-content. The synthesized PtRh catalysts are a more versatile formulation compared to Pd- or Pt- containing catalysts, because they show both higher low-temperature activity and improved thermal stability at elevated temperatures. This is not the case for Pd-containing catalyst systems, as reported in the literature [15,16] and observed in the current work. However, HR-TEM images revealed that sintering is still an issue even for the most stable 1Pt4Rh catalyst.Therefore, further measures have to be taken to avoid sintering in future work, and to make the catalyst particles more resistant against aggregation. According to the literature, two strategies could be followed. The addition of a second metal could prevent the direct contact between the catalytic active Rh (and Pt) centers [29,30]. Furthermore, anchoring of the catalytically active rhodium to a sintering stable metal oxide could help to improve the longevity of the catalyst, as indicated in the work of Fan et al. [31].None.\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.2020.106202.", "descript": "\n                  For the combustion of methane, a partial replacement of platinum by rhodium leads to a significantly improved light-off behavior of the corresponding mixed Pt-Rh/\u03b3-Al2O3 catalysts compared to a catalyst containing only Pt supported on \u03b3-Al2O3. For all Pt-Rh/\u03b3-Al2O3 catalysts, mixed PtRh crystals of low particle size were found. At 600\u00a0\u00b0C, the stability of catalysts, was found to increase with increasing rhodium content. The crystal size was determined by HR-TEM for spent catalyst samples after 40\u00a0h stability test. The samples containing higher amounts of rhodium showed smaller crystal size suggesting this as the origin of the increased catalyst stability.\n               "}
{"full_text": "The water fraction derived from the Fischer-Tropsch (FT) process contains organic, water-soluble compounds that are challenging for conventional wastewater treatment processes [1]. These compounds consist of oxygenated hydrocarbons such as alcohols that can be converted into valuable products including hydrogen by aqueous-phase reforming (APR) [2,3]. The conversion of the alcohols in the FT water fraction into hydrogen may enhance the economic efficiency of renewable fuel production through FT-synthesis and reduce the organic content in the water fraction directed to wastewater treatment.Aqueous-phase reforming takes place at low temperatures, 200\u2009\u00b0C to 250\u2009\u00b0C, and above the bubble point pressure of the feedstock [4], avoiding an energy demanding evaporation step. APR constitutes a suitable candidate to process wastewater with diluted organic compounds because it is energetically efficient compared to steam and autothermal reforming [5]. The energy efficiency becomes significant because evaporation of the highly diluted organic solution is avoided in APR. Consequently, a number of research groups have studied the APR of oxygenated hydrocarbons [6\u20138]. The mass fraction of oxygenated hydrocarbons in the water stream from the FT process is typically below 10%. A mixture of short-chain alcohols (C1-C3) is the largest group of organic constituents in the FT water fraction [9,10]. Although polyols such as ethylene glycol and glycerol have been the main model compounds applied in APR, monohydric alcohols have also been considered [3]. Methanol [11\u201314] and ethanol [15,16] were model compounds in APR for hydrogen production over platinum-based catalyst. Iridium supported on different metal oxides was also utilized as catalyst in the APR of methanol [17], and iridium, rhodium and rhenium supported on TiO2 in the APR of ethanol [18,19]. The APR of ethanol has been additionally conducted over nickel-based catalyst supported on hydrotalcite-like compounds [20], alumina [21], and ceria [22]. The APR of C3 alcohols has been investigated over Pt-based catalysts supported on alumina [23,24] and on polymer-derived carbon [25]. Moreover, real FT derived water fractions have been processed over Ru supported on active carbon and on metal oxides to produce alkanes via hydrodeoxygenation [26,27].In a previous work on the APR of methanol, doping of nickel on alumina with copper or cerium enhanced the hydrogen production compared to the monometallic catalyst [28]. Furthermore, nickel on ceria-zirconia catalysts exhibited high performance in terms of methanol conversion and hydrogen production [29]. Several authors have similarly described the positive effect of cerium on the catalyst activity in APR. This effect has been attributed to oxygen vacancies that may promote reforming to hydrogen and carbon monoxide, and the conversion of carbon monoxide through the water-gas shift (WGS) reaction. The addition of cerium may also enhance the stability of the catalyst [22,30\u201336]. Furthermore, copper has been applied as a catalyst dopant to improve the hydrogen selectivity in APR by limiting the formation of side products such as alkanes [37\u201339].This work focuses on comparing the APR of methanol, ethanol, propan-1-ol and propan-2-ol. Water solutions of these alcohols were selected as model feedstock because they are representative of the FT-derived water fraction. Self-prepared nickel catalyst on ceria-zirconia supports with different ceria contents, and nickel doped with cerium on \u03b3-alumina were selected due to their high activity and hydrogen selectivity, reported in previous studies [28,29]. Furthermore, nickel doped with copper or cerium supported on ceria-zirconia were considered as potential catalysts to improve the hydrogen production in APR. The results obtained from the APR of C1-C3 alcohols elucidate the effect of alcohol chain-length, the influence of the location of the hydroxyl group in alcohols and the type of catalyst applied on the reaction pathway and the product distribution.Ceria-zirconia supports with mass percentage of ceria in zirconia equal to 17% or 25% were supplied by MEL Chemicals in powder form. Engelhard supplied the \u03b3-Al2O3 support. The metal precursors used in impregnation were nickel (Ni(NO3)2\u22196H2O, \u226597.0%), copper (Cu(NO3)2\u22193H2O, 99\u2013104%) and cerium (Ce(NO3)3\u22196H2O, \u226599.0%) nitrates. These chemicals were supplied by Sigma-Aldrich. The feedstock were aqueous solutions with 5% mass fraction of either methanol (MeOH), ethanol (EtOH), propan-1-ol (1-PrOH) or propan-2-ol (2-PrOH). The chemicals were supplied by VWR Chemicals (assay on anhydrous substance is 100%), Altia Industrial (Etax Aa, assay of 99.5%), VWR Chemicals (assay of 100%) and Fluka (assay of >99.9%) respectively.The catalysts listed in Table 1\n were prepared through incipient wetness impregnation, similarly as in [29]. The ceria-zirconia supports were calcined at 450\u2009\u00b0C for 10\u2009h in flowing synthetic air, pelletized, crushed and sieved to 200\u2013300\u2009\u03bcm prior to metal impregnations. The bimetallic catalysts supported on 25% ceria-zirconia were prepared through co-impregnation of nickel and copper or cerium precursors in water solutions. After impregnating the metal precursors on 17% and 25% ceria-zirconia supports, the impregnated materials were kept for 24\u2009h at room temperature, followed by drying at 110\u2009\u00b0C and calcination in flowing air at 500\u2009\u00b0C for 4\u2009h. The bimetallic catalyst supported on \u03b3-Al2O3 was prepared through sequential impregnation of first cerium precursor followed by nickel precursor. The catalyst was dried at 80\u2009\u00b0C under vacuum and calcined at 500\u2009\u00b0C for 2\u2009h in flowing air after impregnation. Prior to the APR experiments, the catalysts were reduced in situ at 450\u2009\u00b0C and 2.5\u2009MPa for 2\u2009h with a H2:N2\u2009=\u20091 gas flow of 10 dm3 \u2009h\u22121. The values of target mass percentage included in Table 1 were calculated as the mass of metal in zero oxidation state per total mass of catalyst.The equipment and methods utilized for the characterization of catalysts were detailed in [29] and briefly described here. The supports, after calcination in the case of mixed-oxide materials, calcined catalysts and spent catalysts were characterized using atomic absorption spectroscopy (AAS) and inductively coupled plasma - optical emission spectroscopy (ICP-OES) to analyse metal loadings. For the AAS, 200\u2009mg of catalyst was dissolved in aqua regia at 120\u2009\u00b0C and subsequently diluted with Milli-Q water. Ni and Cu loadings were determined with a Varian AA240 AAS equipment applying air-acetylene flame, and Ce loading with a Perkin Elmer 7100 ICP-OES. Nitrogen physisorption was applied to determine BET surface areas and BJH method to determine pore volumes and pore sizes distribution. Nitrogen physisorption was conducted in an Thermo Fisher Ultra Surfer after degassing the calcined catalyst samples at 200\u2009\u00b0C for 3\u2009h in vacuum, and the samples of spent catalysts at 120\u2009\u00b0C for 5\u2009h. X-ray diffraction (XRD) was conducted to identify crystalline phases and to determine the crystallite sizes of nickel species. A PANalytical X-pert PRO MPD Alpha-1 diffractometer with Cu K\u03b11 radiation (45\u2009kV and 40\u2009mA) was utilized to obtain the XRD data. The scanning was continuous and ranged from 10\u00b0 to 90\u00b0 (2\u019f) with step size of 0.0131\u00b0. Based on peak broadening, Scherrer equation [40] was applied to estimate the particle size of nickel species. The X-Ray wavelength of Cu K-alpha was assumed to be 0.154\u2009nm, and a crystallite shape-factor of 0.94 was applied, considering sphere-like catalyst particles. Attempts to identify nickel species and determine their particle size with a scanning transmission electron microscope (STEM) were made with no success. In the results form STEM, Ni species were not detected, most likely because the atomic weight of nickel is considerably lower than the atomic weight of the metals in the support, cerium and zirconium [29].Aqueous solutions prepared with Milli-Q water and 5% mass fraction of either MeOH, EtOH, 1-PrOH or 2-PrOH were processed in APR over the catalysts listed in Table 1. The experiments were conducted over 1.5\u2009g of catalyst in a continuous fixed-bed reactor described in detail elsewhere [29]. The gaseous products were analysed with an online Agilent 490 Micro GC Biogas Analyzer with two thermal conductivity detectors (TCD), and the liquid products were analysed offline with an Agilent GC 6890 series with a flame ionization detector (FID) according to the detailed methods described in [29]. The operating conditions were set to be 230\u2009\u00b0C, 3.2\u2009MPa of inlet pressure, and 2.0 cm3\u2009min\u22121 of aqueous solution flow.Ideally, the reforming of MeOH, EtOH, 1-PrOH or 2-PrOH results in the formation of H2 and CO (Eqs. 1\u20134). At low temperatures, the WGS reaction (Eq. 5) is favoured to convert CO with H2O into CO2 and H2. The Gibbs free energy changes presented in this work were calculated at 503\u2009K with HSC Chemistry 8, software from Outotec. In addition to the APR operating conditions, potentially spontaneous reactions (Eqs. 2\u20134) due to slightly positive Gibbs free energy changes [41], and the type of catalyst and feedstock may facilitate different reaction pathways and the formation of side products, for instance through hydrogenation of carbon oxides (Eqs. 6 and 7). Accordingly, full reforming denotes in this work the reaction where the alcohol in the feedstock is converted into H2 and CO; and aqueous-phase reforming is considered as the process where different reactions, in addition to full reforming, such as WGS, and side reactions including methanation may take place.Methanol full reforming:\n\n(1)\n\n\n\n\nC\n\nH\n3\n\nO\nH\n\n(\nl\n)\n\u2009\n\n\u2194\n\n\nH\n2\n\nO\n\n\n\n\u2009\nC\nO\n\u2009\n\ng\n\n+\n2\n\nH\n2\n\n\u2009\n\ng\n\n\n\n\u0394\n\nG\n503\n\n=\n-\n\u2009\n24.8\n\u2009\nk\nJ\n\n\n\n\n\n\nEthanol full reforming:\n\n(2)\n\n\n\n\n\nC\n2\n\n\nH\n5\n\nO\nH\n\u2009\n\nl\n\n+\n\nH\n2\n\nO\n\nl\n\n\n\u2009\n\u2194\n\n\u2009\n2\nC\nO\n\u2009\n\ng\n\n+\n4\n\nH\n2\n\n\u2009\n\ng\n\n\n\n\u0394\n\nG\n503\n\n=\n+\n\u2009\n8.5\n\u2009\nk\nJ\n\n\n\n\n\n\nPropan-1-ol full reforming:\n\n(3)\n\n\n\n\n\nC\n3\n\n\nH\n7\n\nO\nH\n\u2009\n+\n\n\n2\nH\n\n2\n\nO\n\u2009\n\nl\n\n\n\u2009\n\u2194\n\n\u2009\n3\nC\nO\n\u2009\n\ng\n\n+\n6\n\nH\n2\n\n\u2009\n\ng\n\n\n\n\u0394\n\nG\n503\n\n=\n+\n\u2009\n19.9\n\u2009\nk\nJ\n\n\n\n\n\n\nPropan-2-ol full reforming:\n\n(4)\n\n\n\n\n\nC\n3\n\n\nH\n7\n\nO\nH\n\u2009\n+\n\n\n2\nH\n\n2\n\nO\n\nl\n\n\n\u2009\n\u2194\n\n\u2009\n3\nC\nO\n\u2009\n\ng\n\n+\n6\n\nH\n2\n\n\u2009\n\ng\n\n\n\n\u0394\n\nG\n503\n\n=\n+\n\u2009\n47.1\n\u2009\nk\nJ\n\n\n\n\n\n\nWGS reaction:\n\n(5)\n\n\n\n\nC\nO\n\u2009\n\ng\n\n+\n\nH\n2\n\nO\n\u2009\n\nl\n\n\u2194\n\u2009\nC\n\nO\n2\n\n\ng\n\n+\n\nH\n2\n\n\u2009\n\ng\n\n\n\n\u0394\n\nG\n503\n\n=\n-\n\u2009\n33.6\n\u2009\nk\nJ\n\n\n\n\n\n\nCOx hydrogenation\n\n(6)\n\n\n\n\nC\nO\n\u2009\n\ng\n\n+\n\n\n3\nH\n\n2\n\n\u2009\n\ng\n\n\u2194\n\u2009\nC\n\nH\n4\n\n\ng\n\n+\n\nH\n2\n\nO\n\u2009\n\nl\n\n\n\n\u0394\n\nG\n503\n\n=\n-\n\u2009\n82.1\n\u2009\nk\nJ\n\n\n\n\n\n\n\n\n(7)\n\n\n\n\nC\n\nO\n2\n\n\u2009\n\ng\n\n+\n4\n\nH\n2\n\n\u2009\n\ng\n\n\u2194\n\u2009\nC\n\nH\n4\n\n\ng\n\n+\n\n\n2\nH\n\n2\n\nO\n\u2009\n\nl\n\n\n\n\u0394\n\nG\n503\n\n=\n-\n\u2009\n67.1\n\u2009\nk\nJ\n\n\n\n\n\n\nThe results presented in this work are based on product analyses taken at approximately 6\u2009h on stream, when the amount of gases in the outlet stream had stabilized to nearly constant concentrations after a gradual increase. The parameters used to evaluate the experimental results are mass balance (MB, Eq. 8), conversion (X, Eq. 9), selectivity to liquid products (Sk\n, Eq. 10), hydrogen production rate (H2 PR, Eq. 11), hydrogen efficiency (H2 Eff, Eq. 12), hydrogen molar fraction among gaseous products (xH2\n, Eq. 13), yield of gaseous compounds (Yi\n, Eq. 14) and yield of liquid compounds (Yk\n, Eq. 15). Eq. 12 includes a H2/CO2 stoichiometric reforming ratio (RR) that has been traditionally used to evaluate the efficiency of H2 production [42]. This factor considers the stoichiometric production of H2 through full reforming (Eqs. 1\u20134) and WGS reactions (Eq. 5). Therefore, RR equals to 3 for MeOH, 6 for EtOH, and 9 for 1-PrOH and 2-PrOH. The H2 molar fraction in Eq. 13 evaluates the fraction of H2 produced among gases, and liquid products are disregarded in this formula. In addition, when referred to H2, Eq.14 considers the amount of H2 in the outlet stream per amount of alcohol fed into the system. This equation disregards water as a reactant, although water constitutes the hydrogen source when the WGS reaction takes place. Accordingly, the hydrogen yields reported in this work might be higher than 100%.\n\n(8)\n\nM\nB\n\u2009\n\n%\n\n=\n\n\n\nm\n\no\nu\nt\n\n\nl\ni\nq\n\n\n\u2219\n\u2009\n\nw\n\no\nu\nt\n\nj\n\n\n\n\nm\n\ni\nn\n\n\nl\ni\nq\n\n\n\u2219\n\u2009\n\nw\n\ni\nn\n\nj\n\n\n\n+\nX\n\n\n\n\n\n(9)\n\nX\n\n%\n\n=\n\n\n\nx\n\ni\nn\n\nj\n\n-\n\nx\n\no\nu\nt\n\nj\n\n\n\n\nx\n\ni\nn\n\nj\n\n\n\n\n\n\n\n\n(10)\n\n\nS\nk\n\n\u2009\n\n%\n\n=\n\n\n\nx\n\no\nu\nt\n\nk\n\n\n\n\nx\n\ni\nn\n\nj\n\n\n\n-\nx\n\n\no\nu\nt\n\nj\n\n\n\n\n\n\n\n\n(11)\n\n\nH\n2\n\n\u2009\nP\nR\n=\n\n\n\nn\n\u02d9\n\n(\n\nH\n2\n\n)\n\n\n\nm\n\nc\na\nt\na\nl\ny\ns\nt\n\n\n\n\n\n\n\n\n\n(12)\n\n\nH\n2\n\n\u2009\nE\nf\nf\n.\n=\n\n\n\nn\n\u02d9\n\n(\n\nH\n2\n\n)\n\n\nR\nR\n\u2219\n\nn\n\u02d9\n\n(\n\nj\n\ni\nn\n\n\n)\n\n\n\n\n\n\n\n(13)\n\n\u2009\n\nx\n\n\nH\n2\n\n\n\n\u2009\n(\n%\n)\n=\n\n\n\nn\n\u02d9\n\n(\n\n\n\nH\n2\n\n\n\no\nu\nt\n\n\n)\n\n\n\u2211\n\n\nn\n\u02d9\n\n(\n\n\ng\na\ns\n\n\no\nu\nt\n\n\n)\n\n\n\n\n\n\n\n\n(14)\n\n\nY\ni\n\n\u2009\n\n%\n\n=\n\n\n\nn\n\u02d9\n\n(\n\ni\n\no\nu\nt\n\n\n)\n\n\n\nn\n\u02d9\n\n(\n\nj\n\ni\nn\n\n\n)\n\n\n\n\n\n\n\n(15)\n\n\nY\nk\n\n\u2009\n\n%\n\n=\n\n\n\nx\n\no\nu\nt\n\nk\n\n\n\n\nx\n\ni\nn\n\nj\n\n\n\n\n\n\nIn Eqs. 8\u201315, j refers to the alcohol in the aqueous solution, i refers to a gaseous product and k refers to a liquid product. The m\nliq is the total mass of aqueous solution fed into (in) or collected from (out) the system, wj\n is the mass fraction of the alcohol in the aqueous solution fed into (in) or collected from (out) the system, x is molar fraction and \u1e45 is molar flow rate, and m\ncatalyst is the mass of catalyst loaded into the reactor. In Eqs. 9, 10 and 15, mol fractions of liquid compounds were applied instead of molar flow rates, because the molar flow rate of the outlet liquid was unavailable due to experimental limitations to measure it accurately. Accordingly, these equations do not take into account possible changes in the total number of moles.The ceria-zirconia-supported catalysts were impregnated with either nickel, or nickel and cerium or copper, and the metal content in the catalyst was determined with AAS analysis (Table 2\n). Compared to the targeted amounts (Table 1), about 90% of the Ni target was successfully impregnated on 17CeZr and 25CeZr supports, whereas when Cu or Ce were additionally impregnated, only about 70% of the target Ni was deposited. The mass percentage of Cu, 3.8%, was also lower than originally targeted, 5%. The amount of Ce detected in NiCe/25CeZr was affected by the cerium in the support and the impregnation efficiency cannot be evaluated in this case. The alumina-supported catalyst contained as much Ni as targeted (13%). The impregnation success on the alumina support can be attributed to its larger surface area, 159\u2009m2\u2009g\u22121, compared to 17CeZr and 25CeZr supports, with 112\u2009m2\u2009g\u22121 and 99\u2009m2\u2009g\u22121 respectively (Table 2). The slight difference of surface area between 17CeZr and 25CeZr had no obvious effect on the amount of Ni impregnated on the catalyst.\nTable 2 additionally includes the metal loadings in the spent catalysts. The APR of MeOH induced no significant change on the metal content of ceria-zirconia-supported catalysts. In the spent alumina-supported catalyst, about 30% less Ni and a 70% less Ce was observed compared to the calcined catalysts. In addition to potential leaching of Ni and Ce, the decrease of metal mass fractions can be attributed to the weight increase of the catalyst caused by the phase change to boehmite undergone by alumina (Fig. 1\n). The metal content of spent Cu-doped catalysts was similar to the amount in the calcined catalyst. Therefore, using Cu as a promoter improved the stability of the catalyst and prevented leaching. In contrast, during the APR of C2 and C3 alcohols, leaching of 20% of the Ni in Ni/17CeZr and Ni/25CeZr was observed. In a previous study [43], nickel leaching was attributed to the acidity of the reaction medium due to carbonic acid formed from CO2. However, leaching was not observed for the most acidic feedstock applied in the present work, MeOH, which also yielded the highest amount of CO2.The surface area, and pore volume and average pore diameter of supports, and calcined and spent catalyst are also included in Table 2. Metal impregnation on 17CeZr and calcination decreased its surface area by 35%, whereas impregnations on 25CeZr and calcination caused a decrease between 15%\u201325%. The surface area of spent catalysts was generally lower than the surface area of calcined catalysts, which is attributed to partial obstruction of pores, confirmed by lower pore volume. The alumina-supported catalyst showed a considerable decrease of surface area during the APR experiments from 129\u2009m2\u2009g\u22121 to 22\u2009m2\u2009g\u22121. This decrease was caused by a phase change from \u03b3-alumina to boehmite in the aqueous medium, observed in the XRD results. The structural change of the support (Fig. 1) and subsequent surface area decrease may have caused metal leaching, and collapse of pores (Table 2). The surface area of CeZr-supported catalysts decreased by 4%\u201313% during the APR experiments; nonetheless, the type of alcohol processed had no significant effect on the surface area and pore volume of the same catalyst. The average pore size of Ni/17CeZr and Ni/25CeZr remained close to 8\u2009nm and 11\u2009nm respectively, during APR. The average pore diameter increased from 10\u2009nm in calcined NiCu/25CeZr to 12\u2009nm in the spent catalyst. The surface area of NiCe/25CeZr was 5% lower in the spent catalyst and the pore volume and average pore diameter were unaffected by the reaction conditions.\nFig. 1 presents the X-ray diffractograms of supports, calcined catalysts and spent catalysts. Compared to the pure supports, NiO peaks were identified in the X-ray diffractogram of Ni-containing catalyst at 2\u03b8 positions 36\u00b0 and 43\u00b0. A CuO peak was identified for the NiCu-based catalyst at 38\u00b0 2\u03b8. In contrast, the addition of cerium was undetected in the diffractograms of NiCe/Al and NiCe/25CeZr. After reducing the catalysts in situ, nickel remained in the metal form also after the APR experiments were carried out, regardless of the type of catalyst or feedstock applied. The diffractograms of spent catalysts presented peaks of metallic Ni at 2\u03b8 positions 44.4\u00b0, 51.9\u00b0 and 77.1\u00b0. The diffractograms of spent NiCu/25CeZr additionally presented a peak at 2\u03b8 position 43.3\u00b0 that corresponds to metallic Cu. Although the peaks of metallic Ni and Cu at 2\u03b8 position 44.4\u00b0 and 43.3\u00b0 are not completely separated, the appearance of two different peaks indicates that the complete formation of an alloy can be discarded [44]. Regarding the alumina-supported catalyst, the previously mentioned phase change from \u03b3-alumina to boehmite (Section 3.1) is confirmed in the diffractogram of spent NiCe/Al. Boehmite can be identified in the peaks at 2\u03b8 positions 14.5\u00b0, 28.2\u00b0, 38.4\u00b0, 48.7\u00b0, 49.3\u00b0, 55.3\u00b0, 64.1\u00b0 and 72.0\u00b0.The crystallite size of Ni species in calcined catalysts and in spent catalysts determined by Scherrer equation are presented in Table 3\n. For the calcined catalysts, the most intense characteristic peak of NiO, 43.3\u00b0 2\u03b8, was considered to determine its crystallite size. The peak at 44.4\u00b0 2\u03b8, characteristic of metallic Ni, was considered for the spent catalysts. NiO crystallite size on the CeZr-supported calcined catalysts was approximately 20\u2009nm, except for NiCe/25CeZr where larger particles were determined (28\u2009nm). NiO crystallite size was 8\u2009nm in the calcined NiCe/Al catalyst. APR caused no obvious effect on the nickel crystallite size of NiCu/25CeZr regardless of the feedstock applied. Accordingly, Cu promoted the stability of the catalyst, which was also indicated by the results included in Table 2. APR over the other catalysts caused different changes in the crystallite size when different feeds were processed. The APR of ethanol induced a significant growth of nickel particles in Ni/17CeZr and Ni/25CeZr. A similar effect has been described in APR over a ruthenium-based catalyst, whose metal dispersion decreased from 25% to 19% attributed to metal sintering in the APR of ethanol [19]. Moreover, a significant increase in the size of Ni particles was observed in Ni/17CeZr after the APR of methanol was conducted. As indicated in the footnote of Table 3, the Ni particle size of spent Ni/17CeZr and Ni/25CeZr was determined after the catalysts had been 12\u2009h on stream and had been reduced twice. Accordingly, Ni/17CeZr has lower tolerance to the reduction and APR conditions that caused the increase of Ni particles compared to Ni/25CeZr. On the other hand, the alumina supported catalyst suffered obvious Ni agglomeration in the APR of MeOH due to the phase change to boehmite.To maximize the H2 production was one of the main targets of this work. Hydrogen constituted the main gaseous product in the APR of C1-C3 alcohols over different catalysts, with H2 molar fraction in the gas phase between 63% and 95% (Table 4\n). Hydrogen production and yield are useful parameters to evaluate the overall amount of hydrogen produced in the APR process independent of the reaction pathways. In the APR of MeOH, the highest values of H2 production rate were reached, between (1.9\u20132.4) mmol\u00b7min\u22121\u00b7g catalyst\n\u22121, and H2 yields, between 93% and 110% over Ni/17CeZr, Ni/25CeZr and NiCu/25CeZr. In contrast, NiCe/25CeZr and NiCe/Al exhibited poorer performance with 70% lower hydrogen production rate and yield. The APR of other alcohols produced different amounts of H2 depending on the catalyst. Over Ni/17CeZr, the hydrogen production and yield were higher in the APR of EtOH than in the APR of 1-PrOH. Similarly, the APR of 1-PrOH resulted in lower amounts of hydrogen over NiCu/25CeZr, in this case, compared to the amount obtained from 2-PrOH. The lowest amounts of hydrogen were obtained from the APR of 1-PrOH over Ni/17CeZr, 0.27\u2009mmol\u00b7min\u22121\u00b7 g catalyst\n-1 and 24% H2 yield, and over NiCu/25CeZr with 0.15\u2009mmol\u00b7min\u22121\u00b7g catalyst\n-1 of H2 and 13% H2 yield, and from the APR of EtOH over Ni/25CeZr 0.25\u2009mmol\u00b7min\u22121\u00b7g catalyst\n-1 of H2 and 17% H2 yield. Over Ni/25CeZr and NiCu/25CeZr, H2 was produced in similar amounts in the APR of 2-PrOH, (0.45 and 0.50) mmol\u00b7min\u22121\u00b7g catalyst\n-1, and 41% and 45% H2 yield respectively.H2 efficiency indicates the extent of full reforming to gases and WGS (Eq. 1\u20134 and 5, Section 2.3). Those alcohols whose reaction pathway in APR was mainly full reforming to gases and subsequent WGS will show higher H2 efficiency. The APR of MeOH produced only gases regardless of the catalyst applied. In the APR of MeOH, H2 efficiency values were around 35% over Ni/17CeZr, Ni/25CeZr and NiCu/25CeZr. It is worth noticing that NiCu/25CeZr was able to reach a H2 efficiency similar to Ni/17CeZ and Ni/25CeZr with 20% lower MeOH conversion. The APR of MeOH over NiCe/25CeZr and NiCe/Al resulted in low conversions, around 15%, with similarly low H2 efficiency, below 10%. In the APR of C2-C3 alcohols, different reaction pathways to full reforming and WGS to produce H2, and side reactions that consume H2 explain considerably lower H2 efficiency.Mainly gaseous products were obtained also in the APR of EtOH over Ni/17CeZr and Ni/25CeZr. However, 30% selectivity to liquid products indicates that full conversion to gases and WGS (Eqs. 2 and 5) were not the only reaction pathways, and side reactions to produce ethanal took additionally place. Ni/17CeZr and Ni/25CeZr reached also similar conversions close to 15%. However, H2 efficiency over Ni/17CeZr, 7%, was more than twice the value achieved over Ni/25CeZr, which suggest higher selectivity to the full reforming and WGS pathway (Eqs. 2 and 5) over Ni/17CeZr.The APR of 1-PrOH resulted in liquid product selectivities around 25%, over Ni/17CeZr, Ni/25CeZr and NiCu/25CeZr. Therefore, as in the APR of EtOH, full conversion to gases and WGS (Eqs. 3 and 5) was not the only reaction pathway and side reactions to produce liquid compounds took additionally place. The conversion of 1-PrOH over Ni/25CeZr, 44%, was twice as high as over NiCu/25CeZr, and three times as high as over Ni/17CeZr. H2 efficiency was the highest over Ni/25CeZr, 7%, which suggest relatively higher selectivity to the reaction pathway that involves full reforming to gases and WGS (Eqs. 3 and 5). Over Ni/17CeZr and NiCu/25CeZr, the APR of 1-PrOH resulted in lower H2 efficiency around 3% and 1% respectively.In the APR of 2-PrOH, similar results were obtained over Ni/25CeZr and NiCu/25CeZr differing from the APR of other alcohols in the liquid selectivity. The APR of 2-PrOH resulted in higher liquid product selectivity around 65% and its conversion was comparable to that achieved in the APR of MeOH, close to 60%. However, H2 efficiency was relatively low, 5%. High selectivity to liquids and low H2 efficiency with high conversion indicates that 2-PrOH was converted and H2 was produced through a reaction pathway different to full reforming to gases and WGS (Eqs. 4 and 5).The discussion included in the previous paragraphs suggests that the APR reaction pathway of C1-C3 is more complex than that explained by Eqs. 1\u20135. Therefore, the following subsections will be devoted to the evaluation of the product distribution obtained in the APR of C1-C3 over different catalysts to achieve a better understanding of the reaction pathways in the APR.The APR of MeOH over different Ni-based, and Cu- and Ce-containing catalysts was conducted to evaluate the effect of metal dopants on the catalyst performance. Methanol conversion and H2 yield decreased in the order Ni/25CeZr\u2009>\u2009NiCu/25CeZr\u2009>\u2009NiCe/Al\u2009>\u2009NiCe/25CeZr (Table 4 and Fig. 2\n). Both Ce-doped catalysts, NiCe/25CeZr and NiCe/Al, showed significantly poorer performance than the other catalysts. The H2 yield over the Ce-doped catalysts was less than 40% of the H2 yield over Ni/25CeZr (Table 4 and Fig. 2). The lower performance of NiCe/Al catalyst can be attributed to the phase change undergone by \u03b3-Al2O3 to boehmite and consequent decrease in the surface area, and metal agglomeration and leaching (Section 3.1). The results obtained over NiCe/25CeZr are surprisingly poor compared to those obtained over similar catalysts such as Ni/25CeZr or NiCu/25CeZr. The poorer results over NiCe/25CeZr reveal the negative effect of nickel particle growth (Table 3) on the performance of this catalyst, compared to the other 25CeZr-supported catalysts. Over NiCu/25CeZr, the MeOH conversion was lower than over Ni/25CeZr, and the H2 yields were similar over NiCu and Ni on 25CeZr, which explains the higher H2 molar fraction among gases over the Cu-doped catalyst (Table 4), as similarly reported in [29]. In addition, the product distribution was similar over Ni/25CeZr and NiCu/25CeZr (Fig. 2). The presence of CO2 among the gases confirms WGS reaction activity (Eq. 5) over both catalysts. The detected CH4 indicates that methanation of carbon oxides with hydrogen consumption took place, also observed over NiCe/25CeZr. Conversely, the only products observed over NiCe/Al were H2 and CO2, which indicates that CO conversion through the WGS reaction (Eq. 5) was highly promoted. No side products over NiCe/Al suggest that the selectivity was superior to that over the other Ce-doped catalyst, NiCe/25CeZr. Lower conversion over NiCe/Al hinders the comparison in terms of selectivity with the other 25CeZr-supported catalysts.The yields obtained over NiCu/25CeZr, compared to that of Ni/25CeZr, suggest that copper addition promoted the WGS reaction and methanation was less favourable. Additionally, MeOH conversion was lower over NiCu/25CeZr than over Ni/25CeZr. To evaluate the effect of Ni content on the APR of MeOH, the Ni loading was used to calculate the H2 production rate per mass of Ni using the values of H2 production rate in Table 4. Ni/25CeZr had a Ni loading of 9% mass fraction whereas NiCu/25CeZr had 7% mass fraction of Ni (Table 2). Accordingly, the H2 production rate was 27\u2009mmol\u00b7min\u22121\u00b7 gNi\n\u22121 over Ni/25CeZr and 30\u2009mmol\u00b7min\u22121\u00b7 gNi\n\u22121 over NiCu/25CeZr. These Ni-based H2 production rate indicates that lower H2 production rate over NiCu/25CeZr (Table 4) could be attributed to its lower amount of Ni compared to Ni/25CeZr; moreover, a possible negative effect of Cu on MeOH reforming could also explain it.Methanol is a simple molecule with no CC bonds, and a C/O stoichiometry of 1:1 that allows high selectivity towards hydrogen in APR [45]. Accordingly, the APR of methanol resulted in high conversions and hydrogen yields. However, longer chain alcohols present CC bonds and different C/O stoichiometry, which, along with the catalyst, has a noticeable effect on the alcohols conversion and H2 yield (Table 4 and Fig. 3\n). Fig. 3 summarizes the hydrogen yield versus alcohol conversion obtained in the APR of C1-C3 alcohols over different catalyst (data from Table 4).Conversion and H2 yield were the highest over Ni/25CeZr (Fig. 3, black) when MeOH and 1-PrOH were applied. In contrast, the APR of 2-PrOH over NiCu/25CeZr (Fig. 3, white) resulted in slightly higher conversion, and in the APR of EtOH, a noticeably higher yield was reached over Ni/17CeZr (Fig. 3, grey). Alcohol conversion and H2 yield in the APR of EtOH and 1-PrOH followed different trends over different catalysts.MeOH (Fig. 3, spheres) was converted more easily into H2 than longer-chain alcohols, although the number of hydrogen atoms contained in MeOH is lower. Conversion of 2-PrOH (Fig. 3, triangles) achieved the level of MeOH conversions; however, the H2 yields were considerably lower. The conversion of EtOH (Fig. 3, cubes) was significantly lower than the conversion of the other alcohols and similar over Ni/17CeZr and Ni/25CeZr. The APR of EtOH caused the agglomeration of Ni particles in both catalysts (Table 3), which decreased the number of active sites and likely resulted in lower conversions. H2 yield in the APR of EtOH was higher over Ni/17CeZr than over Ni/25CeZr. The conversion of specially 1-PrOH (Fig. 3, diamonds) varied considerably over different catalysts. The alcohol was converted to a larger extent and resulted in higher H2 yield over Ni/25CeZr than over Ni/17CeZr. Accordingly, higher amount of Ce in the support could have resulted in higher catalytic activity in this case.The main reaction pathways in the APR of MeOH can be deduced from the product distribution (Figs. 2 and 4\n). Methanol is accompanied by longer-chain alcohols in real water fractions derived from FT synthesis. Therefore, the product distribution from the APR of C2 and C3 alcohols was additionally studied to understand the effect of CC bonds, higher C/O ratio and higher number of hydrogen atoms in the alcohols on the product distribution. The effect of the hydroxyl group location in C3 alcohols on the bond scissions and consequent product distribution was also addressed. The product distribution derived from the APR of C2 and C3 alcohols was evaluated to enhance the understanding of the reaction pathways in APR of different alcohols over different catalysts. Fig. 4 shows the conversion of different alcohols and product yields over three different catalysts. The information presented in Fig. 4 will be discussed in this section to propose potential reaction pathways followed by different C1-C3 alcohols in APR over Ni/17CeZr, Ni/25CeZr and NiCu/25CeZr.The APR of MeOH produces H2 and CO (Eq. 1). Moreover, CO2 is produced through WGS reaction (Eq. 5), which decreases the amount of CO while favouring the H2 yield (Fig. 4). In addition, CH4 produced in methanation (Eqs. 6 and 7), was identified among the products from APR of MeOH over the three catalysts, Ni/17CeZr, Ni/25CeZr and NiCu/25CeZr. Ethane and propene were detected in negligible amounts over the monometallic catalysts. Undesired alkane formation was restricted over NiCu/25CeZr, which resulted in lower amounts of methane. Furthermore, no liquid products were detected in the liquid samples from the APR of MeOH over either of the catalysts.In the APR of EtOH, H2, CH4 and CO2 were the main gaseous products, in addition to negligible amounts of C2H6 and C3H6, and ethanal was the only liquid product (Fig. 4 a and b). Ni/17CeZr produced considerably larger amounts of hydrogen with similar conversion to that achieved over Ni/25CeZr. The difference in the H2 yields suggests that the APR of EtOH followed different pathways over Ni/17CeZr and Ni/25CeZr, which is also indicated by the different formation ratio of CO2 and CH4. Large amounts of H2 can be produced via full reforming of EtOH to gases and WGS (Eqs. 2 and 5). Additionally, H2 can be produced through EtOH dehydrogenation (Eq. 16) or decarbonylation (Eq. 18).Ethanol dehydrogenation\n\n(16)\n\n\nC\n2\n\n\nH\n5\n\nO\nH\n\u2009\n\nl\n\n\u2009\n\n\u2194\n\n\nH\n2\n\nO\n\n\n\u2009\n\nC\n2\n\n\nH\n4\n\nO\n\u2009\n(\nl\n)\n+\n\u2009\n\nH\n\n2\n\u2009\n\n\n(\ng\n)\n\n\n\n\n\n\n\n\n\n\n\u0394\n\nG\n503\n\n=\n+\n21.6\n\u2009\nk\nJ\n\n\n\nEthanal decarbonylation\n\n(17)\n\n\nC\n2\n\n\nH\n4\n\nO\n\u2009\n\nl\n\n\u2009\n\n\u2194\n\n\nH\n2\n\nO\n\n\n\u2009\nC\nO\n\u2009\n\ng\n\n+\nC\n\nH\n4\n\n\u2009\n(\ng\n)\n\u2009\n\n\n\n\n\n\n\n\n\n\u0394\n\nG\n503\n\n=\n-\n95.2\n\u2009\nk\nJ\n\n\n\nEthanol decarbonylation\n\n(18)\n\n\nC\n2\n\n\nH\n5\n\nO\nH\n\u2009\n\nl\n\n\u2009\n\n\u2194\n\n\nH\n2\n\nO\n\n\n\u2009\nC\n\nH\n\n4\n\u2009\n\n\n\ng\n\n+\n\u2009\nC\nO\n\u2009\n\ng\n\n+\n\u2009\n\nH\n2\n\n\u2009\n(\ng\n)\n\n\n\n\n\n\n\n\n\n\n\u0394\n\nG\n503\n\n=\n-\n73.6\n\u2009\nk\nJ\n\n\n\nIn the dehydrogenation of EtOH, ethanal is formed, as previously suggested in a different study [46], whereas the decarbonylation of EtOH involves the formation of CH4 and CO, as proposed in [47]. Ethanal yield was low and similar over Ni/17CeZr and Ni/25CeZr, which indicates that EtOH dehydrogenation was low regardless of the catalyst. The formation of ethanal is thermodynamically unfavourable (Eq. 16), and thus, ethanal yields were low. EtOH decarbonylation (Eq. 18) was more thermodynamically favourable, also compared to full EtOH reforming to gases (Eq. 2). However, considering the stoichiometry of Eqs. 2 and 18 and the product distribution in Fig. 4 a and b, it can be assumed that additionally, full reforming to gases took place over Ni/17CeZr and Ni/25CeZr because the H2 yield was higher than that of CO and CH4. Nevertheless, full reforming to gases was more favourable over Ni/17CeZr according to the significantly higher H2 yield and lower CH4 yield compared to Ni/25CeZr.The formation of CH4 and CO can take place via three different pathways in the APR of EtOH: (i) ethanal decarbonylation (Eq. 17); (ii) EtOH decarbonylation (Eq. 18), which also produces H2 as previously indicated; and (iii) EtOH full reforming to CO and H2 (Eq. 2) followed by hydrogenation of carbon oxides into CH4 (Eqs. 6 and 7). Considering the stoichiometry of Eqs. 2,17 and 18, the product distribution obtained in the APR of EtOH (Fig. 4) suggests that EtOH decarbonylation (Eq. 18) was more favourable over Ni/25CeZr than over Ni/17CeZr. This reaction pathway (Eq. 18) explains the lower H2 and CO2 yields and larger amount of CH4 over Ni/25CeZr compared to the product distribution obtained over Ni/17CeZr, where full reforming was more favoured. Furthermore, similarly negligible CO yields over Ni/17CeZr and Ni/25CeZr indicate that CO2 is formed via WGS reaction (Eq. 5), which allows the formation of additional H2.The APR of 1-PrOH was conducted over Ni/17CeZr, Ni/25CeZr and NiCu/25CeZr. Hydrogen was the main product over these three catalysts. Over NiCu/25CeZr, CH4 was also formed, whereas over Ni/17CeZr and Ni/25CeZr, CO, CO2, C2H6 and a small amount of C2H4 were additionally observed. The main liquid product detected over the three catalysts was propanal (Fig. 4). Hydrogen yield was higher over Ni/25CeZr than over Ni/17CeZr due to higher 1-PrOH conversion. On the other hand, although Ni/17CeZr and NiCu/25CeZr allowed similar 1-PrOH conversions, the H2 yield was higher over Ni/17CeZr. The differences in the H2 yield suggest that 1-PrOH followed different reforming pathways over different catalysts, which is also indicated by the different formation ratio of CO2 and CH4 (Fig. 4).Hydrogen can be produced via full reforming of 1-PrOH to gases and WGS (Eqs. 3 and 5). Additionally, H2 can be produced through 1-PrOH dehydrogenation (Eq. 19) or decarbonylation (Eq. 21) [48]. In the dehydrogenation of 1-PrOH, propanal is formed, whereas the decarbonylation of 1-PrOH involves the formation of C2H4 and CO, as proposed in Ref. [47]. Propanal yield was low and similar over Ni/17CeZr and NiCu/25CeZr, and slightly higher over Ni/25CeZr due to higher conversion. Thus, 1-PrOH dehydrogenation (Eq. 19) took place at a relatively low extent regardless of the catalyst. Nonetheless, the reaction stoichiometry of Eq. 19 matches the product distribution obtained over NiCu/25CeZr. In contrast, the product distribution obtained over Ni/17CeZr and Ni/25CeZr indicates that full reforming of 1-PrOH to gases and WGS (Eqs. 3 and 5) was the main reaction pathway to produce hydrogen, which was obtained in significantly larger amounts than the other products.Propan-1-ol dehydrogenation\n\n(19)\n\n\nC\n3\n\n\nH\n7\n\nO\nH\n\u2009\n\nl\n\n\u2009\n\n\u2194\n\n\nH\n2\n\nO\n\n\n\u2009\n\nC\n3\n\n\nH\n6\n\nO\n\u2009\n\nl\n\n+\n\nH\n2\n\n\u2009\n\ng\n\n\u2009\n\n\n\n\n\n\n\n\n\n\u0394\n\nG\n503\n\n=\n+\n9,5\n\u2009\nk\nJ\n\n\n\nPropanal decarbonylation\n\n(20)\n\n\nC\n3\n\n\nH\n6\n\nO\n\u2009\n\nl\n\n\u2009\n\n\u2194\n\n\nH\n2\n\nO\n\n\n\u2009\n\nC\n2\n\n\nH\n6\n\n\u2009\n(\ng\n)\n+\nC\nO\n\u2009\n(\ng\n)\n\u2009\n\n\n\n\n\n\n\n\n\n\u0394\n\nG\n503\n\n=\n-\n82.7\n\u2009\nk\nJ\n\n\n\nPropan-1-ol decarbonylation\n\n(21)\n\n\nC\n3\n\n\nH\n7\n\nO\nH\n\u2009\n\nl\n\n\u2009\n\n\u2194\n\n\nH\n2\n\nO\n\n\n\u2009\n\nC\n2\n\n\nH\n4\n\n\ng\n\n+\n\u2009\nC\nO\n\u2009\n\ng\n\n+\n\u2009\n2\n\nH\n2\n\n\u2009\n(\ng\n)\n\n\n\n\n\n\n\n\n\n\n\u0394\n\nG\n503\n\n=\n+\n\u2009\n1.3\n\u2009\nk\nJ\n\n\n\nOver Ni/17CeZr, C2H6 was produced in stoichiometric amounts with CO, in agreement with Eq. 20. An additional source of CO could have been the 1-PrOH decarbonylation (Eq. 21) accompanied by the production of C2H4 and H2. However, low C2H4 yield indicates that this reaction did not take place to a significant extent over Ni/17CeZr. CO2 and CH4 resulted from the WGS reaction (Eq. 5) and methanation of carbon oxides (Eqs. 6 and 7) respectively. However, CO2 and CH4 low yields suggest that WGS and methanation were less favoured. Over Ni/25CeZr, C2H6 was also produced accompanied by CO via propanal decarbonylation (Eq. 20). Carbon monoxide might have been also formed in the 1-PrOH decarbonylation (Eq. 21), which explains the production of C2H4 and additional H2. Higher CO2 yield than CO over Ni/25CeZr indicates that WGS (Eq. 5) was more favourable than over Ni/17CeZr. The presence of CH4 in the gases indicates that methanation of oxides (Eqs. 6 and 7) took also place over Ni/25CeZr.Carbon monoxide was not detected in the APR of 1-PrOH over NiCu/25CeZr and CO2, C2H6 and C2H4 were observed in negligible amounts. The presence of CH4 in the gas stream obtained over NiCu/25CeZr suggests that hydrogenation of carbon oxides took place (Eqs. 6 and 7). Accordingly, we conclude that NiCu/25CeZr mainly follows the reaction pathway in Eq. 19, which has been previously suggested by Lei et al. [24]. However, that suggestion differs from the observation by Wawrzetz et al. [23], who stated that decarboxylation of propionic acid to C2H6 and CO2 was the main reaction after formation of propanal from 1-PrOH.The conversion and product distribution in the APR of 2-PrOH was similar over Ni/25CeZr and NiCu/25CeZr (Fig. 4 b and c). Considering the main reaction products, acetone and H2, 2-PrOH dehydrogenation to the ketone (Eq. 22) was assumed to be the predominant reaction pathway, which has been previously proposed in [47]. Further reaction of acetone through decarbonylation (Eq. 23) results in CH4 and CO. As observed in Fig. 4 b and c, decarbonylation of acetone was limited over both catalysts. Nonetheless, higher acetone decarbonylation over Ni/25CeZr slightly lowered the H2 yield and increased the amount of CH4 among the products.Propan-2-ol dehydrogenation\n\n(22)\n\n\nC\n3\n\n\nH\n7\n\nO\nH\n\n(\nl\n)\n\u2009\n\n\u2194\n\n\nH\n2\n\nO\n\n\n\n\u2009\n\nC\n3\n\n\nH\n6\n\nO\n\u2009\n\nl\n\n+\n\nH\n2\n\n\u2009\n(\ng\n)\n\u2009\n\n\n\n\n\n\n\n\n\n\u0394\n\nG\n503\n\n=\n-\n12.4\n\u2009\nk\nJ\n\n\n\nAcetone decarbonylation\n\n(23)\n\n\nC\n3\n\n\nH\n6\n\nO\n\n\nl\n\n+\n\nH\n2\n\n\u2009\n(\ng\n)\n\n\u2009\n\u2009\n\n\u2194\n\n\nH\n2\n\nO\n\n\n\u2009\n2\nC\n\nH\n4\n\n\u2009\n\ng\n\n+\nC\nO\n\u2009\n(\ng\n)\n\u2009\n\n\n\n\n\n\n\n\n\n\u0394\n\nG\n503\n\n=\n-\n119.2\n\u2009\nk\nJ\n\n\n\nIn a different study [23] on the APR of 2-PrOH over Pt/Al2O3, acetone has been reported to be the only product and no H2 had been observed, in contract to the present work. No CO was detected among the gaseous products resulting from the APR of 2-PrOH (Fig. 4 b and c). This suggest CO conversion to CO2 through WGS reaction (Eq. 5), or hydrogenation of CO, and also CO2, to form CH4 (Eqs. 6 and 7) might have taken place under the reaction conditions. Negligible amounts of C2H6 and C3H6 were additionally detected.The product distribution in the APR of different alcohols originates from the CH and OH bond cleavage of those bonds adjacent to the CO functional group [46]. For MeOH, the cleavage of these bonds led to full reforming to gases according to the thermodynamically favourable Eq. 1 with negative Gibbs free energy. For the longer-chain alcohols, Gibbs free energies of full reforming to gases (Eqs. 2\u20134) have positive values. Therefore, full reforming of EtOH, 1-PrOH and 2-PrOH to gases was expected to happen to a lesser extent than from MeOH. Fig. 5\n shows the main reaction pathways proposed for the APR of MeOH, EtOH, 1-PrOH and 2-PrOH.The APR of MeOH proceeds through OH and CH bonds scission (Fig. 5 a). First the OH bond cleaves resulting in the formation of methoxy intermediates before decomposition to CO and H2 [46]. Every hydrogen atom in MeOH is activated to produce molecular hydrogen, which explains the high H2 yield reached in the APR of MeOH (Table 4 and Fig. 4). To maximize the H2 production, CO should be converted in the WGS reaction (Eq. 5) and limit CO bond cleavage that takes place in side reactions, such as methanation, where H2 is consumed to produce methane (Eqs. 6 and 7). NiCu/25CeZr successfully limited CO bonds scission in the APR of MeOH.The APR of EtOH proceeds through OH and CH, and CC bonds scission when full reforming to gases and decarbonylation reactions take place (Fig. 5b). The experimental results elucidated that the reaction pathways followed by EtOH in APR appear to be different over Ni/17CeZr and Ni/25CeZr. The product distribution obtained over Ni/17CeZr suggests that full reforming to gases (Eq. 2) was dominant in accordance with the larger H2 yield obtained. Lower H2 yield and relatively significant amounts of CH4 suggest that EtOH decarbonylation was more favourable over Ni/25CeZr. Accordingly, the cleavage of multiple CH bonds from the alkyl group was more favourable over Ni/17CeZr. When only OH and CH bonds cleave in EtOH, ethanal was formed. The ethanal formation pathway via alcohol dehydrogenation (Eq. 16) was less favourable than the gas formation that involved CC bond cleavage over both Ni/17CeZr and Ni/25CeZr (Eqs. 2 and 18), in agreement with the negative reaction Gibbs free energy changes and the obtained product distributions (Fig. 4).Full reforming of 1-PrOH to gases (Eq. 3) was less thermodynamically favourable than the dehydrogenation or decarbonylation of the alcohol (Eqs. 19 and 21). However, the experimental results indicates that the APR of 1-PrOH proceeds through OH and CH, and CC bonds scission when full reforming and decarbonylation reactions took place (Fig. 5 c). These pathways were the most favourable over Ni/17CeZr and Ni/25CeZr. However, conversely to the APR of EtOH, full reforming and the cleavage of multiple CH bonds in the alkyl group were more favourable over Ni/25CeZr. When only OH and CH bonds cleave in 1-PrOH, propanal was formed. This reaction pathway was less favourable over Ni/17CeZr and Ni/25CeZr. In contrast, propanal formation was the preferred reaction pathway over NiCu/25CeZr. These results over NiCu/25CeZr confirm that full reforming to gases was inhibited in the APR of 1-PrOH by the addition of Cu to the catalyst.The APR of 2-PrOH mainly proceeds through CH and OH bond cleavage to form acetone (Fig. 5 d) over Ni/25CeZr and NiCu/25CeZr. Therefore, 2-PrOH dehydrogenation was the main reaction pathway in agreement with the spontaneous Gibbs free energy of Eq. 22. The CC and CH bond cleavages involved in full reforming to gases (Eq. 4) were unfavourable. Further CC of acetone to CH4 and CO was neither a significant pathway. However, this reaction pathway took place to a larger extent over Ni/25CeZr than over NiCu/25CeZr.Catalytic APR of C1-C3 alcohols was conducted over different nickel-based catalysts. The results of these experiments allowed the evaluation of the product distribution to propose potential reaction pathways followed by different alcohols in APR over nickel-based catalyst. In addition, Cu and Ce were used as dopants to assess their effect on the performance and stability of ceria-zirconia and alumina supported catalysts. The addition of Cu to the Ni-based 25CeZr-supported catalyst promoted the catalyst stability and more selective production of H2. The addition of Ce to the Ni-based 25CeZr-supported catalyst adversely affected the catalyst stability and activity. The other Ce-doped catalyst, NiCe/Al, promoted CO-free hydrogen production, and the undesired formation of CH4 was prevented in the APR of MeOH.Focusing on Ni/17CeZr, Ni/25CeZr and NiCu/25CeZr, these catalysts performed differently in the APR of C1-C3 alcohols. The suggested reaction pathways in the APR of C2\u2013C3 alcohols comprises full reforming to gases, and alcohol dehydrogenation and decarbonylation. The extent in which these reactions took place depended on the type of feedstock and catalyst. In the APR of MeOH, H2 yield was high due to high MeOH conversion via full reforming to gases and the subsequent WGS reaction. Larger amounts of ceria in the support allowed a higher MeOH conversion, and Cu-doping limited CH4 formation. In the APR of longer-chain alcohols, Ni/17CeZr and Ni/25CeZr were active in the cleavage of OH, CH and CC bonds for full reforming to gases. However, side reactions such as alcohol dehydrogenation and decarbonylation were significant. Over NiCu/25CeZr, C2-C3 alcohols mainly followed the dehydrogenation pathway. Thus, Cu restricted the full reforming of alcohols to gases due to lower activity in the CC bond cleavage, which limited the H2 yield.Ni/17CeZr, Ni/25CeZr and NiCu/25CeZr are potential catalysts to process the oxygenated hydrocarbons in FT-derived water fractions. The monometallic Ni/17CeZr and Ni/25CeZr are preferred to maximize the hydrogen production. Nonetheless, NiCu/25CeZr could be additionally considered because of its improved stability during the experiments, compared to the monometallic catalyst, and when more selective production of hydrogen among gases is required.The authors thank Dr. Pekka Simell, Prof. Klaus Hellgardt and Prof. Leon Lefferts for their guidance and support. We are grateful to Aleksi Rinta-Paavola for his help with the preparation and characterization of catalysts, to Tyko Vierti\u00f6 and Eveliina M\u00e4kel\u00e4 for their help with the adsorption isotherm measurements, and to Laura Lonka for her help with the APR experiments. The Bioeconomy Infrastructure and the Raw materials research infrastructure (RAMI) that permitted conducting the experimental work for this study at both VTT and Aalto University. This work was funded by Academy of Finland (AQUACAT Project no. 285398).", "descript": "\n                  Catalytic aqueous-phase reforming (APR) can be applied to process the organic compounds in the water fractions derived from the Fischer-Tropsch (FT) synthesis. This work aimed at finding an active nickel-based catalyst to convert organic compounds typically found in FT-derived waters, such as alcohols, into hydrogen. In addition, this work aimed at proposing potential reaction pathways that explain the product distribution resulting from the APR of C1\u2013C3 alcohols. Solutions with 5% mass fraction of either methanol, ethanol, propan-1-ol or propan-2-ol in water were processed in APR at 230\u2009\u00b0C and 3.2\u2009MPa over different nickel-based catalysts in a continuous packed-bed reactor. Methanol was successfully reformed into hydrogen and carbon monoxide with conversions up to 60%. The conversion of C2\u2013C3 alcohols achieved values in the range of 12% to 55%. The results obtained in the APR of C2\u2013C3 alcohols suggest that in addition to reforming to hydrogen and carbon monoxide, the alcohols underwent dehydrogenation and decarbonylation. The most stable catalyst, nickel-copper supported on ceria-zirconia, reached feedstock conversions between 20% and 60% and high hydrogen selectivity. Monometallic nickel supported on ceria-zirconia catalysts reached higher H2 yields; however, the yield of side products, such as alkanes, was also higher over the monometallic catalysts. Accordingly, ceria-zirconia nickel-based supported catalysts constitute suitable candidates to process the alcohols in the water fractions derived from the FT synthesis.\n               "}
{"full_text": "Copper surface area, (\n\n\n\nm\n\n\nCu\n\n\n2\n\n\n\u00b7\n\n\ng\n\n\n-\n1\n\n\n\n)Avogadro\u2019 s number, (\n\n-\n\n)Copper molecular weight, (\n\ng\n\u00b7\n\n\nmol\n\n\n-\n1\n\n\n\n)Copper dispersion, (\n\n%\n\n)Average surface-volume copper diameter, (\n\nnm\n\n)Copper density, (\n\ng\n\u00b7\n\n\nm\n\n\nCu\n\n\n-\n3\n\n\n\n)H2 consumption from 1st and 2nd TPR, respectively, (\n\n\n\nmL\n\n\n\n\nH\n\n\n2\n\n\n\n\n\u00b7\n\n\ng\n\n\ncat\n\n\n-\n1\n\n\n\n)Copper crystallite dimension from XRD, (\n\nnm\n\n)Catalyst solid density, (\n\nkg\n\u00b7\n\n\nm\n\n\ncat\n,\ns\n\n\n-\n3\n\n\n\n)Catalyst apparent density, (\n\nkg\n\u00b7\n\n\nm\n\n\ncat\n\n\n-\n3\n\n\n\n)Catalyst porosity, (\n\n\n\nm\n\n\npores\n\n\n3\n\n\n\u00b7\n\n\nm\n\n\ncat\n\n\n-\n3\n\n\n\n)BET surface area, (\n\n\n\nm\n\n\n2\n\n\n\u00b7\n\n\ng\n\n\n-\n1\n\n\n\n)Pore volume, (\n\nc\n\n\nm\n\n\n3\n\n\n\u00b7\n\n\ng\n\n\n-\n1\n\n\n\n)Pore diameter, (\n\nnm\n\n)Gas hourly space velocity, (\n\nNL\n\u00b7\n\n\nkg\n\n\ncat\n\n\n-\n1\n\n\n\u00b7\n\n\nh\n\n\n-\n1\n\n\n\n)CO2 conversion, (\n\n%\n\n)Yield of product i, (\n\n%\n\n)Space time yield of product i, (\n\nmmol\n\u00b7\n\n\nh\n\n\n-\n1\n\n\n\u00b7\n\n\ng\n\n\ncat\n\n\n-\n1\n\n\n\n)Selectivity of product i, (\n\n%\n\n)Catalyst weight, (\n\nkg\n\n)Molar flow rate of component i, (\n\nmol\n\u00b7\n\n\ns\n\n\n-\n1\n\n\n\n)Molar fraction of component i, (\n\n-\n\n)Root mean square error of component i, (\n\n-\n\n)Objective function, (\n\n-\n\n)Number of experimental data, (\n\n-\n\n)Inlet volumetric flow rate, (\n\nNL\n\u00b7\n\n\nh\n\n\n-\n1\n\n\n\n)Temperature, (\n\nK\n\n)Total pressure, (\n\nbar\n\n)Stoichiometric number of component i in reaction j, (\n\n-\n\n)Rate of reaction j, (\n\nmol\n\u00b7\n\n\ns\n\n\n-\n1\n\n\n\u00b7\n\n\nkg\n\n\ncat\n\n\n-\n1\n\n\n\n)Total number of reaction, (\n\n-\n\n)Kinetic constant of reaction j, (\n\nmol\n\u00b7\n\n\ns\n\n\n-\n1\n\n\n\u00b7\n\n\nkg\n\n\ncat\n\n\n-\n1\n\n\n\n)Pre-exponential factor of the kinetic constant of reaction j, (depending on model)Adsorption constant of component i, (depending on model)Equilibrium constant of reaction j, (depending on reaction)Standard enthalpy of adsorption of component i, (\n\nJ\n\u00b7\n\n\nmol\n\n\n-\n1\n\n\n\n)Standard entropy of adsorption of component i, (\n\nJ\n\u00b7\n\n\nmol\n\n\n-\n1\n\n\n\u00b7\n\n\nK\n\n\n-\n1\n\n\n\n)Activation energy of reaction j, (\n\nJ\n\u00b7\n\n\nmol\n\n\n-\n1\n\n\n\n)Gas constant, (\n\nJ\n\u00b7\n\n\nmol\n\n\n-\n1\n\n\n\u00b7\n\n\nK\n\n\n-\n1\n\n\n\n)Carberry\u2019s number, (\n\n-\n\n)Second Damkohler number, (\n\n-\n\n)Effective diffusivity, (\n\n\n\nm\n\n\n2\n\n\n\u00b7\n\n\ns\n\n\n-\n1\n\n\n\n)Observed reaction rate per volume of catalyst, (\n\nmo\nl\n\u00b7\n\n\ns\n\n\n-\n1\n\n\n\u00b7\n\n\nm\n\n\ncat\n\n\n-\n3\n\n\n\n)Order of reaction with respect to component i, (\n\n-\n\n)Gas-solid mass transfer coefficient, (\n\nm\n\u00b7\n\n\ns\n\n\n-\n1\n\n\n\n)Forward rate of reaction of component i, (\n\nmol\n\u00b7\n\n\ns\n\n\n-\n1\n\n\n\u00b7\n\n\nkg\n\n\ncat\n\n\n-\n1\n\n\n\n)Concentration of species i in the bulk phase, (\n\nmol\n\u00b7\n\n\nm\n\n\n-\n3\n\n\n\n)Concentration of species i on the catalyst surface, (\n\nmol\n\u00b7\n\n\nm\n\n\n-\n3\n\n\n\n)Statistic indicator of the F-test, (\n\n-\n\n)Critical value of F-, from Fisher distribution tables, (\n\n-\n\n)Variance of the lack of fit, (\n\n-\n\n)Variance of the experimental error, (\n\n-\n\n)Number of variables (parameters of kinetic models), (\n\n-\n\n)Partial pressure of component i, (\n\nbar\n\n)Adsorption term of Bussche and Froment kinetic model, ( \n\n-\n\n)Adsorption terms of Graaf\u2019s kinetic model, (\n\n\n\nbar\n\n\n-\n0.5\n\n\n\n)Adsorption terms of Seidel\u2019s kinetic model, (\n\n-\n\n)Total amount of reduced centers, (\n\n-\n\n)Relative contact free energy of Cu and CeZr, (\n\n-\n\n)Inlet reactor conditionCO2 hydrogenation to methanol reactionReverse water gas shift reactionCO hydrogenation to methanol reactionExperimental valueCalculated valueThe combustion of hydrocarbons to produce energy entails a critical global challenge that needs to be tackled with urgency. The usage of fossil fuels correlates directly to the release of greenhouse gasses \u2013 especially CO2 \u2013 into the atmosphere, which is the main responsible of global warming [1,2]. Hence, in the last century research has been focusing on the development of carbon capture and storage technologies (CCS) first and, more recently, on the alternatives for CO2 utilization (CCU) [3\u20136]. An interesting approach for CCU is the CO2 reduction with renewable H2 to produce valuable chemicals and/or energy carriers [7]. In this context, the CO2 conversion to methanol is particularly appealing due to the high methanol demand worldwide (i.e. about 200 kton of methanol are used every day as chemical feedstock and transportation fuel) [8]. Indeed, methanol could be used directly as an alternative fuel or as intermediate for the production of dimethyl ether, olefins, gasoline and aromatics [9\u201311]. The CO2 hydrogenation to methanol is a catalytic gas phase process which follows three main reactions: the direct hydrogenation of CO2 to methanol (reaction 1), the production of CO through the r-WGS reaction (reaction 2) and the hydrogenation of CO to methanol (reaction 3).\n\n(1)\n\nCO2 hydrogenation: CO2+3H2 \u21c4 CH3OH\u00a0+\u00a0H2O \u0394H0=-49.5\u00a0kJ/mol\n\n\n\n\n(2)\n\nReverse water gas shift: CO2+H2 \u21c4 CO+H2O \u0394H0=+41.2\u00a0kJ/mol\n\n\n\n\n(3)\n\nCO hydrogenation: CO+2H2 \u21c4 CH3OH \u0394H0=-90.5\u00a0kJ/mol\n\n\nAmong these reactions, the CO2 hydrogenation to methanol is the most desired. Inevitably, the r-WGS takes place in parallel, accelerating the H2 depletion and, at the same time, contributing to the production of water. As a matter of fact, water is the main reaction by-product, which limits the system thermodynamically and causes catalyst deactivation [12]. Depending on the catalyst, the CO hydrogenation to methanol (reaction 3) could take place simultaneously, partially balancing the negative effect of the r-WGS. Nowadays, methanol is produced industrially from syngas feedstock (i.e., mixture of CO, H2 and c.a. 3% of CO2) at pressures of 50\u201380\u00a0bar and temperatures of 200\u2013300 \u2070C, over CuO/ZnO/Al2O3 catalytic beds [13,14]. Since the benchmark technology involves only traces of CO2 in the feedstock [15], the corresponding catalyst is not necessarily optimal when using pure CO2, i.e., a thermodynamically very stable molecule, as the sole carbon source. Usually CO2 adsorption is not strong enough [16] and efforts are required specifically on novel catalyst formulations [17]. Over the years, researchers have proposed a variety of different catalysts for the CO2 hydrogenation to methanol, with particular focus on Cu-based systems, in combination with different metal oxides as carrier and/or promoters [18\u201321]. First, important research efforts aimed at replacing the hydrophilic Al2O3 support, which could deactivate in presence of the large amounts of water produced in all the reactions [22]. In most of the catalyst formulations, the ZnO oxide still acts as main promoter, since it guarantees both a higher Cu dispersion and the formation of Cu\u03b4+ sites at the Cu-ZnO interface [23\u201325]. On the other hand, various carriers/promoters have been proposed in literature such as ZrO2\n[17,25\u201329], CeO2\n[22,30\u201334], Fe2O3\n[34\u201336], SiO2\n[37\u201339], and TiO2\n[30,40\u201343]. Most recently, the synergistic effect of CeO2-ZrO2 mixed oxides has received particular attention due to their high redox ability, improved thermal stability [44] and superior oxygen storage capacity (OCS) [45], properties that have proved highly beneficial for different reactive systems, such as the oxidation of aliphatic C2 [46], the conversion of NOx [47], the reduction of NO by propene [48] and, most recently, for the CO2 hydrogenation to methanol [49\u201351]. The introduction of smaller Zr4+ ions into the CeO2 tetrahedron creates a defective fluorite structure, which facilitates the adsorption of oxygen [52]. Shi et al., [49] proposed for the first time a ternary CuO/CeO2/ZrO2 catalyst for the CO2 hydrogenation to methanol. They found that a Ce:Zr mass ratio of 1 optimizes the basicity of the system in favour of the CO2 adsorption capacity. Their Cu30Ce35Zr35O catalyst showed excellent reducibility and Cu dispersion, as well as a balanced distribution of Cu0 and strong basic sites to enhance the H2 dissociative-adsorption and the formation of the H2CO intermediate, which preferentially hydrogenates to form methanol. Wang et al., [50] investigated the reaction pathway via in situ DRIFTS analysis. They showed that a calcination temperature of 450 \u2070C increases the CuO surface area and the formation of Cu-Ce-Zr sites, which favour the formation of H* and bi/m-HCOO*, responsible for the high selectivity to methanol.In any catalytic process, kinetic modelling is an essential tool to support efforts on catalyst development, to elucidate reaction mechanisms as well as to aid reactor design and process optimization. Numerous kinetic models have been proposed over the years to describe the methanol synthesis, mostly on commercial catalysts [13,53\u201357]. However, the majority of the kinetic models trace back to the works of Graaf et al., [58] and Bussche and Froment [59]. Both models propose a Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism with the dissociative adsorption of H2. Graaf et al., established a dual-sites mechanism (i.e., one for CO and CO2 and one for H2O and H2) where methanol is produced from CO2 and CO simultaneously. On the contrary, Bussche and Froment considered a mechanism where Cu is the sole active site and CO2 is the only carbon source for the methanol production. Even today, literature shows disagreements on the relative contribution of CO and CO2 to the methanol synthesis. For example, Liu et al., [60] propose at least four parallel reactions: CO-CO2 exchange, CO hydrogenation, CO2 hydrogenation and WGS, while Bowker et al., [61] proposed that CO2 is the only responsible of methanol synthesis, even when feeding CO/CO2/H2 mixtures. Interestingly, Yang et al., [62] proved that the CO2 and CO contributions to methanol synthesis varies with the operating conditions. More recently, Niels et al., [63] found that CO2 is the immediate source for methanol (i.e., CO2 pathway is one order of magnitude faster), whereas the presence of CO is inhibitory at low conversion due to competitive adsorption, and beneficial at higher conversion due to the removal of water via the WGS. Finally, L.C. Grabow and M. Mavrikakis [64] showed through DFT calculations that about 2/3 of the methanol comes from CO2 in the conventional process (i.e., syngas feed). However, the situation could be completely different with CO2-rich streams and other catalyst formulations.More recently, Park et al., [65] developed a model considering three-sites adsorption, where CO2 and CO adsorb on two distinct sites. In this study the authors carried out a rate determining step analysis (RDS) based on the mechanistic hypotheses earlier proposed by Graaf et al., to find the rate expressions that best fit the experimental data. Seidel et al., [56] reviewed the elementary steps involved in the three-sites adsorption mechanism, proposing an even more complex kinetic model. This was recently simplified by Slotboom et al., [66], who reduced the number of kinetic parameters considerably.Despite the extensive literature database of kinetic models and rate expressions for the Cu:ZnO system supported on either Al2O3 (i.e., benchmark formulation) or other metal oxides, kinetic modelling of the methanol synthesis remains an intriguing research topic, with at least two important open questions: 1) what are the type and number of the catalyst active sites involved in the methanol synthesis; and 2) which is the dominant C-source for methanol formation (i.e., CO/CO2) and the corresponding prevailing reaction pathway. In addition, and to the best of our knowledge, the kinetics of this reaction on novel catalysts such as Cu-Ce-Zr mixed oxides (i.e., better performant catalysts for the conversion of CO2) has not been investigated yet.Herein, the kinetic model of methanol synthesis from CO2 and H2 over a Cu-Ce-Zr mixed oxide catalyst is investigated by means of an RDS analysis for the single-site, dual-site and three-sites adsorption kinetic model, based on the most relevant mechanistic hypotheses retrieved from literature. A total of 6 kinetic models are compared with a complete set of 96 experimental data in the range of temperature, pressure, H2:CO2 molar ratio and GHSV of 200\u2013260 \u2070C and 10\u201340\u00a0bar, 3\u20137 and 7500\u201324000 \n\nNL\n\u00b7\n\n\nkg\n\n\ncat\n\n\n-\n1\n\n\n\u00b7\n\n\nh\n\n\n-\n1\n\n\n\n, respectively.The preparation of the ternary catalyst according to the works of Shi et al., [49] and Wang et al., [50] is followed by in depth catalyst characterization and extensive kinetic tests. Statistical analysis of the data combined with physicochemical constraints are used as tool for model discriminations. This work pays particular attention to the relative contribution of CO2 and CO to the formation of methanol (i.e., methanol synthesis from direct and indirect route, respectively) under various reaction conditions, by means of a theoretical differential analysis. The identification of the kinetic model, together with a detailed analysis of the reaction rates and the interplay between CO2 and CO hydrogenation will lead to a better understanding of this system. In this study, we will gain insights into the reaction mechanisms, identify the active sites and their role within the methanol formation, which is key for further improvement of this catalyst formulation, as well as an essential tool for reactor and process design.To elucidate on the reaction pathway involved in the CO2 hydrogenation to methanol over a copper-cerium-zirconium mixed oxides catalyst, the most relevant kinetic models reported in literature have been explored and re-parametrized. All the available kinetic models can be sorted in three groups, based on the number of active sites considered in the formulation of the mechanism. A detailed discussion is given below.The most relevant kinetic model considering a single-site adsorption mechanism is the one developed by Bussche and Froment in 1996 [59]. The most important assumption is that CO2 is the sole carbon source for methanol synthesis. As a result, only reaction (1) and (2) take place on the Cu surface of the catalyst, where both H2 and CO2 undergo dissociative adsorption. According to the authors, the rate determining steps are: 1) the CO2 dissociation on the active sites, which releases surface oxygen for the rWGS reaction and 2) the hydrogenation of the formate species for the CO2 hydrogenation to methanol. The rate equations are reported in Eq. 3\u20135.\n\n(3)\n\n\n\nr\n1\n\n=\n\nk\n1\n\n\np\n\n\nCO\n\n2\n\n\n\np\n\nH\n2\n\n\n\n\n\n1\n-\n\n1\n\nK\n\n1\n\n\neq\n\n\n\n\n\n\np\n\n\nH\n2\n\nO\n\n\n\np\n\n\n\nCH\n\n3\n\nO\nH\n\n\n\n\n\np\n\n\nH\n2\n\n\n3\n\n\np\n\n\nCO\n\n2\n\n\n\n\n\n\n\n\n\n\u03b2\n\n3\n\n\n\n\n\n\n\n(4)\n\n\n\nr\n2\n\n=\n\nk\n2\n\n\np\n\n\nCO\n\n2\n\n\n\n\n\n1\n-\n\n1\n\nK\n\n2\n\n\neq\n\n\n\n\n\n\np\n\n\nH\n2\n\nO\n\n\n\np\n\nCO\n\n\n\n\n\np\n\nH\n2\n\n\n\np\n\n\nCO\n\n2\n\n\n\n\n\n\n\n\u03b2\n\n\n\n\n\n\n(5)\n\n\n\u03b2\n=\n\n\n\n\n1\n+\n\nb\n\n\nH\n2\n\nO\n/\n\nH\n2\n\n/\n8\n/\n9\n\n\n\n\np\n\n\nH\n2\n\nO\n\n\n\np\n\nH\n2\n\n\n\n+\n\nb\n\nH\n2\n\n\n\np\n\n\nH\n2\n\n\n\n0.5\n\n\n+\n\nb\n\n\nH\n2\n\nO\n\n\n\np\n\n\nH\n2\n\nO\n\n\n\n\n\n\n-\n1\n\n\n\n\n\n\nThe Bussche and Froment model considers three adsorption constants (i.e., \n\n\nb\n\n\nH\n2\n\nO\n/\n\nH\n2\n\n/\n8\n/\n9\n\n\n\n, \n\n\nb\n\nH\n2\n\n\n\n and \n\n\nb\n\n\nH\n2\n\nO\n\n\n\n) and two kinetic constants (i.e., \n\n\nk\n1\n\n\n and \n\n\nk\n2\n\n\n), for a total of 10 parameters to be optimized.The most important and widely employed kinetic model describing the methanol synthesis is the model developed by Graaf et al., [58] in 1988. In their first publication, the authors had already recognized the lack of agreement in the literature on whether the carbon source for the methanol production is CO or CO2. As a result, they developed a model including both pathways (reaction 1, 2 and 3). All the reactions are assumed to be based on a dual-site LHHW mechanism, where CO and CO2 adsorb competitively on one site (s1) and H2 and H2O adsorb competitively on a second site (s2), with dissociation of H2. The adsorption of methanol is once again neglected. The rate equations are reported in Eq. 6\u20138, with the two adsorption terms (i.e., \n\n\n\u0398\n1\n\n\n and \n\n\n\u0398\n2\n\n\n) related to the site s1 and site s1 described in Eq. 9\u201310.\n\n(6)\n\n\n\nr\n1\n\n=\n\nk\n1\n\n\nb\n\n\nco\n\n2\n\n\nC\n\n\u0398\n1\n\n\n\u0398\n2\n\n\n\n\n\n\n\n(7)\n\n\n\nr\n2\n\n=\n\nk\n2\n\n\nb\n\nc\n\no\n2\n\n\n\nB\n\n\u0398\n1\n\n\n\u0398\n2\n\n\n\n\n\n\n\n(8)\n\n\n\nr\n3\n\n=\n\nk\n3\n\n\nb\n\nco\n\n\nA\n\n\u0398\n1\n\n\n\u0398\n2\n\n\n\n\n\n\n\n(9)\n\n\n\n\u0398\n1\n\n=\n\n\n\n\n1\n+\n\nb\n\nco\n\n\n\np\n\nco\n\n\n+\n\nb\n\n\nCO\n\n2\n\n\n\np\n\n\nCO\n\n2\n\n\n\n\n\n\n-\n1\n\n\n\n\n\n\n\n\n(10)\n\n\n\n\u0398\n2\n\n=\n\n\n\n\n\np\n\n\nH\n2\n\n\n\n0.5\n\n\n+\n\n\nb\n\n\nH\n2\n\nO\n\n\n\n\n\nb\n\nH\n2\n\n\n\n\n\n\np\n\n\nH\n2\n\nO\n\n\n\n\n\n\n-\n1\n\n\n\n\n\nwhere \n\nA\n\n, \n\nB\n\n, and \n\nC\n\n represents the driving force of the CO hydrogenation, r-WGS and CO2 hydrogenation to methanol, respectively. As a matter of fact, the authors provided also different expressions for the driving forces terms, which depends on the particular RDS for the specific reaction. All the (48) combinations are reported in Table 1\n and were tested in this study.The model from Graaf et al., includes 3 kinetic constants (i.e., \n\n\nk\n1\n\n\n, \n\n\nk\n2\n\n\n and \n\n\nk\n3\n\n\n) and 3 adsorption constants (i.e., \n\n\nb\n\nco\n\n\n\n, \n\n\nb\n\n\nCO\n\n2\n\n\n\n and \n\n\n\nb\n\n\nH\n2\n\nO\n\n\n\n\n\nb\n\nH\n2\n\n\n\n\n\n\n), for a total of 12 kinetic parameters.More recently, Henkel modified the model developed by Graaf et al., excluding the CO hydrogenation to methanol [54]. His reparameterization was based on two sets of experimental results, obtained from two distinct set-ups: 1) a Berty reactor and 2) a micro-fixed bed reactor, from which he obtained two different set of kinetic parameters [55]. The rate equations proposed for the CO2 hydrogenation and the rWGS are reported in Eq. 11\u201312, which lead to a total of 10 kinetic parameters.\n\n(11)\n\n\n\n\nr\n\n\n1\n\n\n=\n\n\n\n\nk\n\n\n1\n\n\n\n\nb\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\np\n\n\nc\n\n\no\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n1.5\n\n\n\n\n\n1\n-\n\n\n\n\np\n\n\n\n\nCH\n\n\n3\n\n\nO\nH\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n3\n\n\n\n\nK\n\n\n1\n\n\neq\n\n\n\n\n\n\n\n\n\n\n\n1\n+\n\n\nb\n\n\nco\n\n\n\n\np\n\n\nco\n\n\n+\n\n\nb\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n0.5\n\n\n+\n\n\nb\n\n\n\n\nH\n\n\n2\n\n\nO\n/\n\n\nH\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\n\n\n\n\n\n\n\n(12)\n\n\n\n\nr\n\n\n2\n\n\n=\n\n\n\n\nk\n\n\n2\n\n\n\n\nb\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\np\n\n\nc\n\n\no\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n\n1\n-\n\n\n\n\np\n\n\nCO\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\nK\n\n\n2\n\n\neq\n\n\n\n\n\n\n\n\n\n\n\n1\n+\n\n\nb\n\n\nco\n\n\n\n\np\n\n\nco\n\n\n+\n\n\nb\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n0.5\n\n\n+\n\n\nb\n\n\n\n\nH\n\n\n2\n\n\nO\n/\n\n\nH\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\n\n\n\n\n\nIn 2014, Park et al., [65] proposed a reaction pathway, based on the mechanism developed by Graaf et al., with the introduction of a third adsorption site exclusively for CO2. The authors considered the methanol dehydration to dimethyl ether in their reaction scheme, which was discarded in our analysis since no traces of DME were detected during the experimentation. The rate equations are summarized in Eq. 13\u201315. The model from Park et al., involves 14 kinetic parameters to be optimized.\n\n(13)\n\n\n\n\nr\n\n\n1\n\n\n=\n\n\n\n\nk\n\n\n1\n\n\n\n\nb\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\n\n\n\np\n\n\nc\n\n\no\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n1.5\n\n\n-\n\n\n\n\np\n\n\n\n\nCH\n\n\n3\n\n\nO\nH\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n1.5\n\n\n\n\nK\n\n\n1\n\n\neq\n\n\n\n\n\n\n\n\n\n\n\n1\n+\n\n\nb\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\n\n\n\n1\n+\n\n\nb\n\n\n\n\nH\n\n\n2\n\n\n\n\n0.5\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n0.5\n\n\n+\n\n\nb\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\n\n\n\n\n\n\n\n(14)\n\n\n\n\nr\n\n\n2\n\n\n=\n\n\n\n\nk\n\n\n2\n\n\n\n\nb\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\n\n\n\np\n\n\nc\n\n\no\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n-\n\n\n\n\np\n\n\nCO\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\nK\n\n\n2\n\n\neq\n\n\n\n\n\n\n\n\n\n\n1\n+\n\n\nb\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\n\n\n\n1\n+\n\n\nb\n\n\n\n\nH\n\n\n2\n\n\n\n\n0.5\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n0.5\n\n\n+\n\n\nb\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\n\n\n\n\n\n\n\n(15)\n\n\n\n\nr\n\n\n3\n\n\n=\n\n\n\n\nk\n\n\n3\n\n\n\n\nb\n\n\nCO\n\n\n\n\n\n\n\np\n\n\nco\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n1.5\n\n\n-\n\n\n\np\n\n\n\n\nCH\n\n\n3\n\n\nO\nH\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n0.5\n\n\n\n\nK\n\n\n3\n\n\neq\n\n\n\n\n\n\n\n\n\n\n\n1\n+\n\n\nb\n\n\nCO\n\n\n\n\np\n\n\nCO\n\n\n\n\n\n\n\n1\n+\n\n\nb\n\n\n\n\nH\n\n\n2\n\n\n\n\n0.5\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n0.5\n\n\n+\n\n\nb\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\n\n\n\n\n\nFew years later, in 2018, Seidel et al., [56] developed an even more detailed model based on three adsorption sites, reviewing also the elementary reactions involved and the rate determining step of each reaction. The active sites are distinguished as follows:\n\n\n\u2299\n\n for oxidized surface centers, assumed as active center for CO hydrogenation\n\n\n\u22c7\n\n for reduced surface centers, assumed as active center for CO2 hydrogenation\n\n\n\u2297\n\n as the active surface center for the decomposition of H2\nThe rate expressions are reported in Eq. 16\u201318 with the corresponding adsorption terms in Eq. 19\u201321.\n\n(16)\n\n\n\n\nr\n\n\n1\n\n\n=\n\n\n\u03d5\n\n\n2\n\n\n\n\nk\n\n\n1\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n2\n\n\n\n\n\n1\n-\n\n\n\n\np\n\n\n\n\nCH\n\n\n3\n\n\nO\nH\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n3\n\n\n\n\nK\n\n\n1\n\n\neq\n\n\n\n\n\n\n\n\n\n\n\n\u0398\n\n\n\u22c7\n\n\n\n\n2\n\n\n\n\n\n\n\u0398\n\n\n\u2297\n\n\n\n\n4\n\n\n\n\n\n\n\n\n(17)\n\n\n\n\nr\n\n\n2\n\n\n=\n\n\n\n\n\u03d5\n\n\n1\n-\n\u03d5\n\n\n\n\n\n\nk\n\n\n2\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\n\n1\n-\n\n\n\n\np\n\n\nCO\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n\n\n\n\np\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\nK\n\n\n2\n\n\neq\n\n\n\n\n\n\n\n\n\n\u0398\n\n\n\u22c7\n\n\n\n\n\u0398\n\n\n\u2299\n\n\n\n\n\n\n\n\n(18)\n\n\n\n\nr\n\n\n3\n\n\n=\n\n\n\n1\n-\n\u03d5\n\n\n\n\n\nk\n\n\n3\n\n\n\n\np\n\n\nCO\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n2\n\n\n\n\n\n1\n-\n\n\n\n\np\n\n\n\n\nCH\n\n\n3\n\n\nO\nH\n\n\n\n\n\n\np\n\n\nCO\n\n\n\n\np\n\n\n\n\nH\n\n\n2\n\n\n\n\n2\n\n\n\n\nK\n\n\n3\n\n\neq\n\n\n\n\n\n\n\n\n\n\u0398\n\n\n\u2299\n\n\n\n\n\n\n\u0398\n\n\n\u2297\n\n\n\n\n4\n\n\n\n\n\n\n\n\n(19)\n\n\n\n\n\u0398\n\n\u2299\n\n=\n\n\n\n\n1\n+\n\nb\n\nCO\n\n\n\np\n\nCO\n\n\n\n\n\n\n-\n1\n\n\n\n\n\n\n\n\n(20)\n\n\n\n\n\u0398\n\n\u22c7\n\n=\n\n\n\n\n1\n+\n\n\n\nb\n\n\nH\n2\n\nO\n\n\n\nb\nO\n\n\n\nb\n\nH\n2\n\n\n\n\n\np\n\n\nH\n2\n\nO\n\n\n\np\n\nH\n2\n\n\n\n+\n\nb\n\n\nCO\n\n2\n\n\n\np\n\n\nCO\n\n2\n\n\n+\n\nb\n\n\nH\n2\n\nO\n\n\n\np\n\n\nH\n2\n\nO\n\n\n\n\n\n\n-\n1\n\n\n\n\n\n\n\n\n(21)\n\n\n\n\n\u0398\n\n\u2297\n\n=\n\n\n\n\n1\n+\n\n\n\nb\n\nH\n2\n\n\n\n\n\np\n\n\nH\n2\n\n\n\n0.5\n\n\n\n\n\n\n-\n1\n\n\n\n\n\n\nThe parameter \n\n\u03d5\n\n represents the total amount of reduced center, while \n\n\n\n1\n-\n\u03d5\n\n\n\n represents the number of oxidized centers. Slootbom et al., [66] have recently corrected the definition of \n\n\u03d5\n\n, assuming a maximum coverage of the reduced center of 90% (Eq. 22).\n\n(22)\n\n\n\u03d5\n=\n\n\u03d5\nw\n\n-\n0.1\n\n\n\n\nThe authors used the relation of Ovesen et al., [67] for the calculation of \n\n\n\u03d5\nw\n\n\n, as follows:\n\n(23)\n\n\n\n\u03d5\nw\n\n=\n\n1\n2\n\n\n\n\n1\n-\n\n\n\u03b3\n\n\u2217\n\n\n\n\u03b3\n0\n\n\n\n\n\n\n\n\nwhere \n\n\n\n\u03b3\n\n\u2217\n\n\n\n\u03b3\n0\n\n\n\n is the relative contact free energy of Cu and Zn, for the benchmark formulation, and of Cu and the CeZr solution for our system. The \n\n\n\n\u03b3\n\n\u2217\n\n\n\n\u03b3\n0\n\n\n\n ratio is calculated according to Eq. 24\u201325, with the introduction of a new kinetic parameter (\n\n\n\n\u0394\nG\n\n3\n\n\n).\n\n(24)\n\n\n\n\n\u03b3\n\n\u2217\n\n\n\n\u03b3\n0\n\n\n=\n\n\n1\n-\n\n\n\nK\n3\n\n\n\n\np\n\nH\n2\n\n\n\np\n\nCO\n\n\n\n\n\np\n\n\nH\n2\n\nO\n\n\n\np\n\n\nCO\n\n2\n\n\n\n\n\n\n\n\n1\n+\n\n\n\nK\n3\n\n\n\n\np\n\nH\n2\n\n\n\np\n\nCO\n\n\n\n\n\np\n\n\nH\n2\n\nO\n\n\n\np\n\n\nCO\n\n2\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n(25)\n\n\n\nK\n3\n\n=\ne\nx\np\n\n\n\n\n\n\n\u0394\nG\n\n3\n\n\nRT\n\n\n\n\n\n\n\n\n\nIn this model, the adsorption constant dependency on temperature is neglected. This lead to a total of 12 parameters, if the \n\n\n\n\nb\n\n\nH\n2\n\nO\n\n\n\nb\nO\n\n\n\nb\n\nH\n2\n\n\n\n\n group is parametrized as a single constant.In 2020 Slotboom et al., [66] simplified the three-sites model, drastically reducing the amount of parameters (i.e., 6 in the simplified version). The authors revisited the elementary reaction steps of Bussche and Froment, thus, considering only CO2 as the carbon source for methanol production, with the updates from recent literature. As Graaf et al., proposed in their study for the dual-sites adsorption mechanism, Slotboom et al., provided a tool for identifying the rate determining step for both the CO2 hydrogenation and the rWGS (i.e., the CO hydrogenation is neglected). All the possible rate expressions are summarized in Table 2\n, with a total of 30 kinetic models, with 6 parameters each. The adsorption term, \n\n\n\n\u03b8\n\n\u22c7\n\n\n, is defined by Eq. 26.\n\n(26)\n\n\n\n\n\u03b8\n\n\u22c7\n\n=\n\n\n\n\n\nb\n\nH\n2\n\n\n\np\n\n\nH\n2\n\n\n\n0.5\n\n\n+\n\nb\n\n\nH\n2\n\nO\n/\n9\n\n\n\np\n\n\nH\n2\n\nO\n\n\n+\n\np\n\n\n\nCH\n\n3\n\nO\nH\n\n\n\n\n\n\n-\n1\n\n\n\n\n\n\nThe Cu-Ce-Zr mixed oxides catalyst was prepared with a CuO loading of 50\u00a0wt%, to be comparable to the benchmark formulation, and a CeO2:ZrO2 mass fraction of 1, as recently optimized by Shi et al., [49]. The catalyst was synthesized via the gel-oxalate coprecipitation method [27]. The required amount of metal nitrate precursors (i.e., Cu(NO3)2\u00b72.5H2O, Ce(NO3)3\u00b76H2O and ZrO(NO3)2\u00b76H2O) were solubilized in ethanol and coprecipitated by adding an oxalic acid solution (20\u00a0wt% excess) dropwise, at room temperature and under continuous stirring. The precipitate was stirred for 3\u00a0h, aged overnight, centrifuged and washed with deionized water, dried at 95 \u2070C for 16\u00a0h and calcined at 450 \u2070C for 4\u00a0h. The catalyst was pelletized, crushed and sieved to produce 50\u2013125\u00a0\u00b5m particle size, to be used for the characterization techniques and reaction tests. The chemical composition of the synthesized catalyst was measured via microwave plasma atomic emission spectroscopy, using an Agilent MP-AES 4200 elemental analyzer. Prior to the analysis, about 0.1 g of catalyst sample was digested in 70 v.% HNO3 solution at 80 \u2070C overnight and then diluted with 5 v.% HNO3 solution (i.e., blank solution), to reach ppm values of the metal content. The specific surface area (S.A.) and pore volume (P.V.) were determined via the BET and BJH elaboration of the N2 adsorption\u2013desorption isotherms at \u2212196 \u2070C, obtained using a Micromeritics ASAP 2020 gas adsorption device. Before the measurement, the sample was degassed at 250 \u2070C for 2\u00a0h. The catalyst reducibility was studied via temperature programmed reduction (TPR) analysis performed using a Micromeritics AutoChem 2920 equipment with a TCD detector. The analysis was carried out in the range 50\u2013400 \u2070C with a heating rate of 10 \u2070C\u00b7min\u22121, feeding 50\u00a0mL\u00b7min\u22121 of a 10% H2/Ar mixture. Prior to the TPR analysis, the sample was outgassed under inert conditions as for the N2 physisorption. The copper surface area (\n\n\nS\n\nCu\n\n\n\n), dispersion (\n\n\nD\n\nCu\n\n\n\n) and average surface-volume diameter (\n\n\nd\n\nCu\n\n\nSV\n\n\n\n) were determined via N2O oxidation followed by H2 titration method developed by Van der Grift [68]. The analysis was carried out in the same equipment used for the TPR and consists in performing a first TPR measurement, whose hydrogen consumption is indicated by \n\nX\n\n. Thereafter, the temperature was reduced to 90 \u2070C and the sample was outgassed under Ar flow for 2\u00a0h. The surface copper was oxidized feeding 50\u00a0mL\u00b7min\u22121 of a 2% N2O/Ar mixture for 1\u00a0h. A second TPR analysis was carried out, whose hydrogen consumption (\n\nY\n\n), is indicative of the number of Cu atoms dispersed on the surface of the catalyst. The copper surface area, dispersion and diameter were calculated with Eq. 26, Eq. 27 and Eq. 28, respectively, considering a Cu/N2O\u00a0=\u00a02 titration stoichiometry and a surface atomic density of 1.4\u00b71019 Cuat\u00b7m\u22122.\n\n(26)\n\n\n\n\nS\n\n\nCu\n\n\n=\n\n\n2\nY\n\u00b7\n\n\nN\n\n\nav\n\n\n\n\nX\n\u00b7\n\n\nM\n\n\nCu\n\n\n\u00b7\n\n\n1.410\n\n\n19\n\n\n\n\n\n\n\n\n\n\n(27)\n\n\n\n\nD\n\n\nCu\n\n\n=\n\n\n2\nY\n\n\nX\n\n\n100\n%\n\n\n\n\n\n\n(28)\n\n\n\n\nd\n\n\nCu\n\n\nSV\n\n\n=\n\n\n6\n\n\n\n\nS\n\n\nCu\n\n\n\u00b7\n\n\n\u03c1\n\n\nCu\n\n\n\n\n\n\n\nwhere \n\n\nN\n\nav\n\n\n\n, \n\n\nM\n\nCu\n\n\n\n and \n\n\n\u03c1\n\nCu\n\n\n\n are the Avogadro\u2019s number, the copper molecular weight and density, respectively. X-ray diffraction (XRD) analysis in the 2\u03b8 range 10-120\u00b0 was performed on the reduced catalyst with a MiniFlex600 machine (Rigaku) operating with a Ni \u03b2-filtered Cu-K\u03b1 radiant at 40\u00a0kV and 30\u00a0mA and a scan step of 0.05\u00b0/min. The diffraction peaks were identified according to the JCPDS database of reference compounds. The average diameter of the Cu-crystals was estimated via the Scherrer\u2019s equation (Eq. 29).\n\n(29)\n\n\n\nd\n\nCu\n\n\n=\n\n\nb\n\u03bb\n\n\nF\nW\nM\nH\nc\no\ns\n(\n\u03b8\n)\n\n\n\n\n\nwhere \n\n\nd\n\nCu\n\n\n\n is the dimension of the crystallites as if they were cubes, monodisperse in size, \n\n\u03bb\n\n is the wavelength, \n\nFWMH\n\n is the width of the peak, \n\n2\n\u03b8\n\n is the scattering angle and \n\nb\n\n is a constant usually varying between 0.89 and 0.94. XPS measurements were performed both on the calcined and reduced catalyst, using a Kratos AXIS Ultra spectrometer, equipped with a monochromatic X-ray source, and a delay-line detector (DLD). Spectra were obtained using an aluminum anode (Al K\u03b1\u00a0=\u00a01486.6\u00a0eV) operating at 150\u00a0W. The binding energies were internally calibrated setting the C1s peak position at 285\u00a0eV. The catalyst real density (\n\n\n\u03c1\n\ncat\n\n\n\n) was measured using an automatic gas pycnometer instrument (Ultrapyc 1200e). The apparent density of the catalyst (\n\n\n\u03c1\n\nb\n,\nc\na\nt\n\n\n\n) was calculated via the catalyst porosity (\n\n\n\u03b5\n\ncat\n\n\n\n), determined from the N2 physisorption analysis. The catalytic tests were carried out in a stainless-steel reactor (dint, 10\u00a0mm), loaded with 0.25\u00a0g of catalyst, diluted with 0.75\u00a0g of SiC, to ensure isothermal operation and prevent sintering phenomena. The catalyst and the SiC used for dilution were introduced in the reactor with the same particle size of 50\u2013125\u00a0\u00b5m. Larger SiC particles were used as pre-heating bed, separated from the catalytic bed with c.a. 1\u00a0cm3 of quartz-wool. The reactor was placed in an electric oven and heated more precisely via a heating mantle. The temperature was measured with two thermocouples, one at the beginning of the catalytic bed and one placed at the exit of the gases. Prior to the reaction tests, the catalyst was reduced in situ at 250 \u2070C, with 50\u00a0mL\u00b7min\u22121 of a 50% H2/N2 mixture for 4\u00a0h. The reaction mixture was analysed with a compact gas chromatograph (Global Analyzer Solution TM, G.A.S.) equipped with a TCD detector and two packed columns (HayeSep Q 60\u201380 mesh and 5A molecular sieve) for the analysis of permanent gases (i.e., H2, CO2, CO and N2) and an FID detector with capillary columns (Rtx-1, MTX-1 and MTX-QBond) for the analysis of the hydrocarbons. The experimental setup is sketched in Fig. 1\n. The reaction tests were performed in a range of temperature and pressure of 200\u2013260 \u2070C and 10\u201340\u00a0bar, respectively, feeding H2/CO2/N2 mixtures in different proportion, to have a H2:CO2 molar ratio from 3 to 7, and a GHSV ranging from 7500 to 24000 \n\nNL\n\u00b7\n\n\nkg\n\n\ncat\n\n\n-\n1\n\n\n\u00b7\n\n\nh\n\n\n-\n1\n\n\n\n. The carbon balance in the reaction was respected with a maximum error of 3%. The catalyst stability was observed within a long-term (100\u00a0h) test performed at 250 \u2070C, 30\u00a0bar, H2:CO2 molar ratio of 3 and a GHSV of 9600 \n\nNL\n\u00b7\n\n\nkg\n\n\ncat\n\n\n-\n1\n\n\n\u00b7\n\n\nh\n\n\n-\n1\n\n\n\n. The CO2 conversion (\n\n\nX\n\n\nCO\n\n2\n\n\n\n), product yield (\n\n\nY\ni\n\n\n), product space time yield (\n\n\n\nSTY\n\ni\n\n\n) and product selectivity (\n\n\nS\ni\n\n\n) were calculated according to Eq. 30\u201333, where \n\ni\n\n is either methanol or CO and \n\n\nw\n\ncat\n\n\n\n is the catalyst weight. Methanol and CO where detected as the sole carbon species in the product mixture.\n\n(30)\n\n\n\nX\n\n\nCO\n\n2\n\n\n=\n\n\n\nF\n\n\n\nCO\n\n2\n\n\n\nin\n\n\n-\n\nF\n\n\n\nCO\n\n2\n\n\n\nout\n\n\n\n\nF\n\n\n\nCO\n\n2\n\n\n\nin\n\n\n\n\n\n\n\n\n\n(31)\n\n\n\nY\ni\n\n=\n\n\nF\n\ni\n\n\nout\n\n\n\nF\n\n\n\nCO\n\n2\n\n\n\nin\n\n\n\n\n\n\n\n\n\n(32)\n\n\n\n\nSTY\n\ni\n\n=\n\n\nF\n\ni\n\n\nout\n\n\n\nw\n\ncat\n\n\n\n\n\n\n\n\n\n(33)\n\n\n\n\nS\n\n\ni\n\n\n=\n\n\nY\n\n\ni\n\n\n\u00b7\n\n\nX\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\n\n\nA commercial Cu/ZnO/Al2O3 catalyst from Johnson Matthey (i.e. Katalko-51) was tested in the exact same conditions, to compare the novel catalyst with the benchmark technology.The fitting procedure was carried out entirely in MATLAB R2019a. The kinetic parameters were determined via the fminsearch optimization procedure, based on the Nelder-Mead simplex algorithm [69], which minimizes an error objective function (OF) that we defined as the sum of the root mean square errors (RMSE) between the experimental and calculated molar fraction of the carbon containing species (i.e., CO2, CO and methanol) as follows:\n\n(34)\n\n\n\n\nRMSE\n\ni\n\n=\n\n\n\n\n\n\u2211\n\nk\n=\n1\n\n\nN\n\ndata\n\n\n\n\n\n\n\n\ny\n\ni\n,\nk\n\n\ncalc\n\n\n-\n\ny\n\ni\n,\nk\n\n\nexp\n\n\n\n\n\n2\n\n\n\nN\n\ndata\n\n\n\n\n\n\n\n\n\n\n\n(35)\n\n\nOF\n=\n\n\nRMSE\n\n\n\nCO\n\n2\n\n\n+\n\n\nRMSE\n\n\nCO\n\n\n+\n\n\nRMSE\n\n\nMeOH\n\n\n\n\n\n\nWhere \n\n\nN\n\ndata\n\n\n\n is the number of experimental data used for the kinetic fitting and \n\n\ny\n\ni\n\n\nexp\n\n\n\n and \n\n\ny\n\ni\n\n\ncalc\n\n\n\n are the molar fractions of the component \n\ni\n\n at the exit of the catalytic bed determined experimentally and via the model prediction, respectively. The experimental data were imported in terms of \n\n\ny\n\ni\n\n\nexp\n\n\n\n, together with the corresponding boundary conditions, such as inlet flow (\n\n\n\u03d5\no\n\n\n), inlet composition (\n\n\ny\n\ni\n\n0\n\n\n), temperature (\n\nT\n\n) and total pressure (\n\nP\n\n). On the other hand, the \n\n\ny\n\ni\n\n\ncalc\n\n\n\n were determined, within the algorithm iterations, via the integral analysis method, thus solving the ODEs describing the mole balance equations in a fixed bed reactor (Eq. 36).\n\n(36)\n\n\n\n\nd\n\nF\ni\n\n\n\nd\n\nw\n\ncat\n\n\n\n\n=\n\n\u2211\n\nj\n=\n1\n\n\nN\nr\n\n\n\n\n\n\nr\nj\n\n\n\u03bd\n\nji\n\n\n\n\n\n\n\n\nwhere \n\n\nF\ni\n\n\n is the molar flow rate of the component \n\ni\n\n, \n\n\nw\n\ncat\n\n\n\n is the catalyst weight, \n\n\nN\nr\n\n\n is the total number of the reactions involved, \n\n\n\u03bd\n\nji\n\n\n\n is the stoichiometric number of the component \n\ni\n\n in the reaction \n\nj\n\n, and \n\n\nr\nj\n\n\n is the corresponding reaction rate expression, which is unknown. The mole balance equations were solved under the hypothesis of steady state regime, isothermal operation, negligible pressure drop along the catalytic bed and absence of internal diffusion and external mass transfer limitation. The first three hypothesis were confirmed experimentally: 1) the reaction performance was evaluated at steady state (i.e., when no changes in the outlet composition were recorder over time); 2) the temperature difference between the gas inlet and outlet positions was less than 1 \u2070C and 3) the pressure difference between the gas inlet and outlet positions was less than 0.2\u00a0bar. The absence of mass transfer limitations was explored with preliminary experiments (details in S.I.) and was later confirmed with the Mear\u2019s [70] and Weisz-Prater [71] testing criteria.The reaction rates (\n\n\nr\nj\n\n\n) are function of the partial pressure of the components, and parameters such as kinetic (\n\n\nk\nj\n\n\n), adsorption (\n\n\nb\ni\n\n\n) and equilibrium constants (\n\n\nK\n\nj\n\n\neq\n\n\n\n). The equilibrium constants (Table 3\n) were retrieved from literature [72].The kinetic constant of each reaction (\n\n\nk\nj\n\n\n) and adsorption constants of the components (\n\n\nb\ni\n\n\n) are the parameter to be optimized throughout the algorithm. The kinetic constants were described as a function of a pre-exponential factor and an activation energy, following the Arrhenius\u2019 law (Eq. 37) [73]. The adsorption constants, instead, were expressed as a function of the standard entropy (\n\n\u0394\n\nS\n\nads\n,\ni\n\n0\n\n\n) and enthalpy of adsorption (\n\n\u0394\n\nH\n\nads\n,\ni\n\n0\n\n\n), according to the van \u2019t Hoff equation (Eq. 38). However, in some of the kinetic models considered in this study, the dependency on temperature of the adsorption constants is neglected (i.e., \n\n\u0394\n\nH\n\nads\n,\ni\n\n0\n\n\u2248\nR\nT\n\n) [5666]. Furthermore, to reduce the number of fitting parameters, some authors lumped the adsorption constants of some components together.\n\n(37)\n\n\n\nk\nj\n\n=\n\nk\n\nj\n,\n0\n\n\ne\nx\np\n\n\n\n-\n\n\nE\n\na\n,\nj\n\n\n\nRT\n\n\n\n\n\n\n\n\n\n\n\n(38)\n\n\n\n\nb\n\n\ni\n\n\n=\ne\nx\np\n\n\n\n\n\u0394\n\n\nS\n\n\nads\n,\ni\n\n\n0\n\n\n\n\nR\n\n\n\n\n\u00b7\ne\nx\np\n\n\n\n-\n\n\n\u0394\n\n\nH\n\n\nads\n,\ni\n\n\n0\n\n\n\n\nRT\n\n\n\n\n\n\n\n\n\nThe selected algorithm (fminsearch) requires an initial guess for the fitting parameters. Kinetic constant found in literature for the Cu-Zn-Al catalyst were implemented as initial guess, assuming these are likely of similar order of magnitude that the corresponding for our Cu-Ce-Zr catalyst [5513]. This minimizes the strong dependence that the algorithm has on the initial guess itself, and therefore increases the probability of obtaining meaningful results. To increase robustness of the model results, we setup a routine that evaluated the sensitivity of the model to the initial guess. This procedure consists of running the optimization algorithm in a loop, with newly obtained results as the initial guess. Thus, the convergence was reached when the difference between the algorithm output and the initial guess was less than 1%. Once the parameters of the best fit were obtained, the covariance matrix was computed with a second algorithm based on Levenberg-Marquardt method (lsqnonlin). From the covariance matrix, the standard deviation and the 95% confidence intervals, first indicators of the quality of the fit, were determined using the nlparci function in MATLAB. However, model discrimination techniques were necessary to find the set of rate expressions that best describe our system and, therefore, to gain insight into the reaction mechanism. A model was discarded at first when the physicochemical constraints (Table 4\n) were not respected. Thereafter, the significance of the model was assessed via the comparison of the variance of the lack of fit (\n\n\ns\n\n1\n\n2\n\n\n) and the experimental error (\n\n\ns\n\n2\n\n2\n\n\n), where \n\n\ns\n\n1\n\n2\n\n>\n\ns\n\n2\n\n2\n\n\n. The F-test (Eq.40) was carried out in combination with the analysis of the p-value (Eq. 41) (i.e., probability that the data belong to the non-critical area of the Fisher distribution), assuming 95% level of confidence (i.e., \n\n\u03b1\n=\n0.05\n\n). The \n\n\nF\n\nstatistic\n\n\n\n was first calculated according to Eq. 39, where \n\n\ns\n\n1\n\n2\n\n\n is the variance of the lack of fit and \n\n\ns\n\n2\n\n2\n\n\n is the variance of the experimental error. The \n\n\nF\n\ncritical\n\n\n\n was retrieved from the Fisher distribution tables, considering \n\n\nN\n\nvar\n\n\n\n and \n\n\n\n\n\n(\nN\n\n\ndata\n\n\n-\nN\n\n\nvar\n\n\n\n)\n\n\n as degree of freedom, where \n\n\nN\n\nvar\n\n\n\n is the number of variables (parameters). The \n\n\nF\n\nstatistic\n\n\n\n was then compared to the \n\n\nF\n\ncritical\n\n\n\n (F-test, Eq.40).\n\n(39)\n\n\n\nF\n\nstatistic\n\n\n=\n\n\ns\n\n1\n\n2\n\n\ns\n\n2\n\n2\n\n\n\n\n\n\n\n\n(40)\n\n\n\nF\n\nstatistic\n\n\n<\n\nF\n\ncritical\n\n\n=\n\nF\n\n\n\n1\n-\n\u03b1\n\n\n\n\n\n(\n\nN\n\nvar\n\n\n;\n\n\n\nN\n\ndata\n\n\n-\nN\n\n\nvar\n\n\n)\n\n\n\n\n\n\n\n(41)\n\n\np\n>\n\u03b1\n\n\n\n\nAs a result, the kinetic models fulfilling the physicochemical constraints were evaluated according to: 1) the value of the objective function, 2) the parity plots of the experimental and calculated flow rates of the carbon species, and 3) the outcome of the F-test and p-value.The absence of mass transfer (MT) limitations was evaluated according to the criteria reported in Table 5\n, where the Carberry (\n\nCa\n\n) and the second Damkohler number (\n\n\n\nDa\n\n\nII\n\n\n\n) are defined per component. The order of reaction with respect to the component i (\n\n\nn\ni\n\n\n) was estimated with Eq. 42, where \n\n\nr\n\ni\n\n+\n\n\n is its forward reaction rate [74]. Such derivation is specifically defined for complex reaction rates equation such as a LHHW kinetic. The correlations used for the mass transfer coefficient (\n\n\nk\n\ngs\n\n\n\n) and the effective diffusivity (\n\n\nD\ne\n\n\n) are reported in SI.\n\n(42)\n\n\n\nn\ni\n\n=\n\np\ni\n\n\n\u2202\n\n\u2202\n\np\ni\n\n\n\n\n\nl\nn\n(\nr\n\n\ni\n\n+\n\n\n)\n\n\n\n\n\n\nTable 6\n summarizes the main physical properties of the Cu/CeO2/ZrO2 catalyst. The textural properties of the catalyst are in line with the literature [49,50]. The N2 physisorption analysis revealed an isotherm of type IV with hysteresis (\nFigure S1\n), which is typical of a mesoporous material (i.e., pores in the range of 2\u201350\u00a0nm). The TPR profile (\nFigure S4\n\na) exhibits two peaks at 204 \u2070C and 231 \u2070C, after deconvolution. No further reduction of the support, due to H2 spillover, was measured. As a result, a reduction temperature of 250 \u2070C is believed to be sufficient to reduce all the CuO, prior to the reaction tests. The XRD spectra on the calcined and reduced catalyst (\nFigure S2\n) show the typical diffraction peaks of CuO at 2\u03b8 of 35.5\u2070 and 38.7\u2070 and of Cu at 2\u03b8 of 43.3\u2070 and 50.4\u2070, respectively. The disappearance of the CuO peak in the XRD spectrum of the reduced sample (\nFigure S2\n\nb) does not necessarily indicate the presence of sole metallic Cu, as CuO crystals smaller than 3\u20135\u00a0nm cannot be detected, as well as the Cu that is in contact with the Ce-Zr phases via O-bridges. The more complex Ce-Zr oxide phase was analyzed via XPS (\nFigure S3\n). We confirmed the presence of the Ce3+ valence, which is introduced by the zirconia phase, as reported elsewhere [49,50]. The CuCeZr catalyst of this study is characterized by a Ce4+/Ce3+ ratio of c.a. 3.53, which was calculated through the integration of the corresponding peaks of the XPS spectra. A detailed discussion on the XPS results is given in S.I. The catalyst composition according to the MP-AES method is 52\u00a0wt% of CuO, 22\u00a0wt% of CeO2 and 26\u00a0wt% of ZrO2. This composition is very close to the theorical value, indication of the reliability of the synthesis method. In Fig. 2\n\na the catalyst performance during CO2 hydrogenation is compared to that of the benchmark formulation (i.e., the CuZnAl from JM). The CuCeZr catalyst shows a much higher methanol production compared to CO, with a crossover temperature (i.e., \n\n\nT\n\ncross\n\n\n\n temperature beyond which \n\n\n\nS\nT\nY\n\n\nC\nO\n\n\n>\n\n\nS\nT\nY\n\n\nM\ne\nO\nH\n\n\n\n) of ca. 240 \u2070C. On the contrary, the benchmark catalyst shows a \n\n\n\nS\nT\nY\n\n\nC\nO\n\n\n\n larger than \n\n\n\nS\nT\nY\n\n\nM\ne\nO\nH\n\n\n\n over the entire temperature range. Since our catalyst formulation and preparation methods is a reproduction of previous works [49,50], we also compare the performance of this catalyst with that of the original reports by Shi et al., [49] (Fig. 2\nb). The catalyst synthesized in this work shows higher methanol yield with respect to the different formulations proposed by Shi et al., However, the physicochemical properties of our CuCeZr are further improved with the calcination temperature (i.e., 450 \u2070C), according to the optimization reported by Wang et al., [50]. Unfortunately, insufficient details on the results reported by Wang et al., made a direct comparison with our results unreliable. However, assuming a catalyst density of 2.56 \n\ng\n\u00b7\nc\n\n\nm\n\n\n-\n3\n\n\n\n (i.e., value we measured), the \n\n\n\nS\nT\nY\n\n\nM\ne\nO\nH\n\n\n\n they obtained at the same conditions is c.a. 6.6 \n\nm\nmol\n\u00b7\n\n\nh\n\n\n-\n1\n\n\n\u00b7\n\n\ng\n\n\ncat\n\n\n-\n1\n\n\n\n, which compares with the value reported in Fig. 2\nb. The agreement of our results with literature underline the credibility of the method. Furthermore, they emphasize the promising performance of the CuCeZr catalyst with respect to the benchmark technology, in view of the CO2 valorization to methanol.\nTable 7\n reports the information required for the model discrimination procedure, as discussed in Section 4.1. The rate determining step analysis (RDS) is carried out only when the author(s) reported the details behind the model derivations (i.e., Graaf and Slotboom). The discrimination between the different options proposed by Graaf and Slotboom is reported in S.I. and is based on the same criteria shown here. Both the models developed by Park and Seidel did not fulfil all the physicochemical constraints, thus, the statistics analysis was not carried out. It is worth noticing that all the models which do not consider the formation of methanol from CO (reaction 3), resulted in a low \n\np\n\n-\n\nMeOH\n\n (i.e., \n\np\n\n-value for methanol) which indicates the tendency of the model towards a scarce prediction of the methanol outlet molar fraction. This result anticipates the importance of considering the contribution of both CO and CO2 to the methanol synthesis, especially when CO2 is the sole carbon source.The model with the lowest RMSE (which corresponds to the final value of the objective function) and the largest \n\np\n\n-values for the carbon species is the one proposed by Graaf. As a result, we select this model (Graaf-\n\n\nA\n3\n\n\nB\n1\n\n\nC\n3\n\n\n, where \n\n\nA\n3\n\n\nB\n1\n\n\nC\n3\n\n\n refer to the specific combination of RDS) to be the most representative of our system. The kinetic parameters obtained from the fitting procedure are provided in Table 8\n. The accuracy of the parameter estimation is represented by the parity plots of CO2, CO, H2 and methanol (Fig. 3\n). The orders of magnitude of all the parameters are in line with the literature, especially when compared to the values retrieved from Graaf et al. Nevertheless, given the differences in the reaction rate expressions (i.e., \n\n\nA\n3\n\n\nB\n1\n\n\nC\n3\n\n\n for our CuCeZr catalyst versus the \n\n\nA\n3\n\n\nB\n2\n\n\nC\n3\n\n\n for the CuZnAl reported by Graaf), the comparison between the two kinetic models \u2013 and catalyst \u2013 is fair when observed in terms of reaction rates, rather than kinetic constants. Such analysis is addressed in Section 5.5. On the other hand, the adsorption term corresponding to the first and second active sites (i.e., \n\n\n\u0398\n1\n\n\n and \n\n\n\u0398\n2\n\n\n, respectively) do not differ from the original model. Therefore, in Table 9\n we compare the values of our adsorption constants to the same constants calculated by Graaf at 200 and 260 \u2070C. From the reaction rate expressions (Eq. 6\u201310), we see that the adsorption constants of CO2 and CO contribute also to the driving force (i.e., numerator of the reaction rate). As a result, the prediction of their effect on the reaction rate is not straightforward. On the contrary, the combined adsorption of H2O and H2 (i.e., \n\n\nb\n\n\nH\n2\n\nO\n\n\n/\n\n\n\nb\n\nH\n2\n\n\n\n\n\n) contributes only to the adsorption term in the denominator, hindering the reaction rate. Since our constant is order of magnitude higher than the one derived by Graaf, this leads to the conclusion that our catalyst is either more sensitive to water or to H2 adsorption.The model discrimination allows us not only to identify a model which better predicts the performance of our catalyst, but, most importantly, to gain some insights into the reaction mechanism itself. According to the assumptions behind the model developed by Graaf et al., we can distinguish between two active centers in the structure of the CuCeZr catalyst, which is in agreement with what was hypothesized in literature [49]: 1) the metallic copper (i.e., Cu0), where the dissociative adsorption of H2 occurs and 2) the oxygen defects within the Ce/Zr interface, where the CO2 molecule adsorbs and activates. The H species spillover towards the carbon atom of the activated CO2 to begin a series hydrogenation steps, knowns as \u201cformate\u201d route. The reaction pathway is sketched in Fig. 4\n. According to the formate path, methanol can be either synthesized directly from CO2 (direct route) or indirectly from the CO produced via the rWGS reaction. The two routes overlaps when the H2CO intermediate forms, sharing the last two steps which then lead to the formation of methanol. However, at the point where the COs1 (i.e., CO adsorbed) intermediates appears, an equilibrium between the adsorbed CO and the CO released to the gas phase explains a certain selectivity to CO. The relative contribution of CO and CO2 to the methanol synthesis depends on different factors such as temperature, H2 concentration and the distribution of the Cu0 active sites with respect to the oxygen vacancies. Nevertheless, for a fixed catalyst composition, only reaction conditions can affect the fraction of methanol produced via the direct and indirect paths. A detailed discussion on this aspect is given in Section 5.5.In Fig. 4, the limiting steps of the three reactions are also marked. In particular, the slowest steps are the formation of the H3COOs1, HCOOs1 and H3COs1 intermediates for the reaction 1, 2 and 3, respectively. This result is in agreement with the in situ DRIFT studies carried out by Wang et al., [50], where the formation of the formate (i.e., HCOOs1) through the first hydrogenation of the carbon atom is defined as \u201cthe slowest and key step\u201d.At this stage, being the reaction rate expressions determined, we could estimate the order of the reaction with respect to CO2 (\n\n\nn\n\nC\n\nO\n2\n\n\n\n\n) and evaluate the \n\nCa\n\n and \n\n\n\nDa\n\n\nII\n\n\n\n numbers. We observed that in our experimental conditions, \n\n\nn\n\nC\n\nO\n2\n\n\n\n\n ranges between 0.094 and 0.62. Furthermore, both the external mass transfer and internal diffusion limitation resulted to be negligible, being the maximum value of \n\nCa\n\n and \n\n\n\nDa\n\n\nII\n\n\n\n of 1.4\u00b710-3 and 7.1\u00b710-5, respectively. This result confirms our earlier conclusion that the experiments were carried out under kinetic regime.In this section, we discuss the catalyst performance as a function of the reaction conditions explored both experimentally and via model predictions. Experimental points and simulation results are combined in the same graphs, to show at the same time the quality of the fit. A detailed analysis of the thermodynamic equilibrium is reported in S.I.. Furthermore, to underline the compatibility of our results with the adopted equilibrium constant, the catalyst performance as a function of temperature and pressure \u2013 both experimental and modeling data \u2013 are reported together with the corresponding thermodynamic limit in \nFigure S10\n\n.\nFirst, from Fig. 5\n we observe that the kinetic model (solid lines) describes accurately the experimental reaction performance (points), in terms of \n\n\nX\n\n\nCO\n\n2\n\n\n\n (Fig. 5\na), \n\n\nY\n\nMeOH\n\n\n\n (Fig. 5\nb) and \n\n\nY\n\nCO\n\n\n\n (Fig. 5\nc) as a function of the space velocity (GHSV) at various temperatures. As expected from a kinetically controlled system, the conversion decreases with the space velocity, being the contact time of the gases with the catalytic bed shorter. Furthermore, \n\n\nY\n\nMeOH\n\n\n\n and \n\n\nY\n\nCO\n\n\n\n show the same trend as \n\n\nX\n\n\nCO\n\n2\n\n\n\n, leading to the conclusion that the contact time does not affect the product distribution in the range we explored. As a result, when employing a GHSV in the range 7500\u201324000 \n\nNL\n\u00b7\n\n\nkg\n\n\ncat\n\n\n-\n1\n\n\n\u00b7\n\n\nh\n\n\n-\n1\n\n\n\n, the CO contribution to the formation of methanol appears instantaneously, so that CO does not require additional contact time to react with the adsorbed hydrogen. Indeed, if that was the case, we would have observed an optimum in \n\n\nY\n\nCO\n\n\n\n as a function of GHSV. Additionally, \n\n\nX\n\n\nCO\n\n2\n\n\n\n, \n\n\nY\n\nMeOH\n\n\n\n and \n\n\nY\n\nCO\n\n\n\n all show a clear increase with temperature, resulting from the positive effect that temperature has on all the reaction rates. Finally, it is important to notice that at the lowest GHSV (\nFigure S10\n), the catalyst performance approach the thermodynamic equilibrium only at 260 \u2070C (i.e., highest reaction rate), where the thermodynamic value of \n\n\nX\n\n\nCO\n\n2\n\n\n\n, \n\n\nY\n\nMeOH\n\n\n\n and \n\n\nY\n\nCO\n\n\n\n at 30\u00a0bar is 21.1 %, 8.04 % and 13.1 %, respectively. As a result, in the temperature region 200\u2013260 \u2070C \n\n\nX\n\n\nCO\n\n2\n\n\n\n, \n\n\nY\n\nMeOH\n\n\n\n and \n\n\nY\n\nCO\n\n\n\n still displays an exponential increase with temperature (i.e., kinetic regime).\nFig. 6\n displays the effect of temperature and total pressure on the methanol (\n\n\nY\n\nMeOH\n\n\n\n) and CO yield (\n\n\nY\n\nCO\n\n\n\n). As anticipated from Fig. 5, temperature positively affects all the reactions, since the effect of kinetics (i.e., Arrhenius type) overcomes the thermodynamics. Besides, we observe that the effect of temperature on \n\n\nY\n\nMeOH\n\n\n\n (Fig. 6\na) is more significant as total pressure increases (i.e., the increase in \n\n\nY\n\nMeOH\n\n\n\n from 200 to 260 \u2070C is of 91% and 193% at 10 and 40\u00a0bar, respectively). On the contrary, \n\n\nY\n\nCO\n\n\n\n decreases with pressure and, at the same time, it keeps the same trend vs temperature, independently on the total pressure. As a result, the temperature of crossover shifts to higher values when pressure increases: at 10\u00a0bar the crossover occurs at c.a. 216 \u2070C, while at 40\u00a0bar \n\n\nY\n\nMeOH\n\n\n>\n\nY\n\nCO\n\n\n\n in the temperature region we explored (i.e., 200\u2013260 \u2070C).In Fig. 6\nb the effect of total pressure in the range of 10\u201340\u00a0bar is underlined: \n\n\nY\n\nCO\n\n\n\n and \n\n\nY\n\nMeOH\n\n\n\n exhibit two opposite trends, and the effect becomes more significant at higher temperatures (i.e., faster increase/decrease vs pressure). As discussed in section 4.2, methanol is formed via two parallel routes: 1) the direct one, which involves only reaction 1 and 2) the indirect one, which involves reaction 2 and 3, in series. As a result, when feeding only CO2 and H2 or, more generally, with CO2-rich streams, the direct route is faster than the indirect, since the latter needs the formation of CO first (i.e., \n\n\nr\n3\n\n\n is negligible for low values of \n\n\np\n\nCO\n\n\n\n). As soon as CO is formed, \n\n\nr\n3\n\n\n increases, causing an increase in methanol formation and, at the same time, a consumption of CO, which acts both as a product and a reactant. Such an effect is more noticeable at greater temperatures, because of faster reactions (i.e., the effect of pressure anticipates).Besides the effect of total pressure, a higher H2 concentration in the feed (i.e., higher molar feed ratio H2:CO2) causes an increase in both \n\n\nY\n\nMeOH\n\n\n\n and \n\n\nY\n\nCO\n\n\n\n (Fig. 7\n\na), independently of temperature. However, \n\n\nY\n\nMeOH\n\n\n\n increases more than \n\n\nY\n\nCO\n\n\n\n, shifting again the crossover point towards higher temperatures. As shown in Fig. 7\na, the model describes quite precisely the crossover point (\n\n\nT\n\ncross\n\n\n\n). Therefore, we used the model to predict the reaction performance in a wider range of H2:CO2 (1\u201310). We found that \n\n\nT\n\ncross\n\n\n\n monotonically increases up to an asymptotic value of 258 \u2070C at around H2:CO2 of c.a. 7 (Fig. 7\nb). As a matter of fact, a higher H2 concentration facilitates its adsorption on the active sites, increasing the surface concentration of Hs2. As a result, as soon as CO forms, its hydrogenation is faster than its desorption to the gas phase, which enhances the indirect pathway once again (i.e., higher \n\n\nY\n\nMeOH\n\n\n\n). However, when all the active sites for H2 adsorption (i.e., Cu0) are saturated with H2, a further increase in its partial pressure does not affect the reaction rates anymore.Once defined the reaction rates, we tested the predictive capability of the model by using the model to calculate both \n\n\nX\n\n\nCO\n\n2\n\n\n\n and \n\n\nY\n\nMeOH\n\n\n\n and comparing those values to an independent set of experiments (i.e., experimental data not used for the kinetic fitting). The kinetic model predicts quite accurately the experimental points obtained at lower GHSV (i.e., 2880 \n\nNL\n\u00b7\n\n\nkg\n\n\ncat\n\n\n-\n1\n\n\n\u00b7\n\n\nh\n\n\n-\n1\n\n\n\n, last 4 points) and at lower pressure (i.e., 28\u00a0bar, first two points), with a maximum deviation of 2.1% and 2.2% for \n\n\nX\n\n\nCO\n\n2\n\n\n\n and \n\n\nY\n\nMeOH\n\n\n\n, respectively (Fig. 8\n).In this section, we analyse in more details the reaction rates and the relative contribution of the CO2 and CO hydrogenation (i.e., direct and indirect pathway, respectively) to the methanol formation. First, we calculate the reaction rates at different temperatures, via a theoretical differential analysis (i.e., assuming conversion values lower than 5%) at 30\u00a0bar and H2:CO2 ratio of 3 (Fig. 9\n\na). We observe that \n\n\nr\n1\n\n\n is the highest reaction rate at temperatures below c.a. 240 \u2070C. Therefore, at low temperatures, the CO2 hydrogenation to methanol is the fastest reaction, being its activation energy the lowest (Table 8). However, \n\n\nr\n1\n\n\n is the only reaction rate showing an optimum in the temperature range we explored. It is clear indeed, that \n\n\nr\n1\n\n\n approaches the equilibrium as temperature increases, being its value very close to zero at 260 \u2070C. As a result, we observe here the two opposite effects of kinetics and thermodynamics of an exothermic reaction. On the contrary, \n\n\nr\n2\n\n\n and \n\n\nr\n3\n\n\n are quite far from the equilibrium and both display the typical exponential behaviour of kinetically controlled reactions. In addition, reaction 2 and 3 proceed with similar velocities, with \n\n\nr\n2\n\n\n being slightly faster as temperature increases (i.e., \n\n\nr\n2\n\n/\n\nr\n3\n\n=\n1.1\n\n at 260 \u2070C). We clearly see that the two pathways for methanol formation behave differently with temperature. As a result, the relative contribution of CO2 and CO to methanol synthesis changes as temperature increases (Fig. 9\nb). At 200 \u2070C, CO and CO2 contributes almost equally (i.e., 51.5% and 47.4% at 200 \u2070C, respectively). As temperature increases, CO-to-MeOH and CO2-to-MeOH exhibit opposite trends, with CO-to-MeOH reaching a value of c.a. 100% at 260 \u2070C. This result reveals why methanol selectivity does not decay with temperature as fast as it does on the CuZnAl catalyst (Fig. 2\na) and underlines the importance of designing a catalyst in such a way that CO adsorption is strong enough, to be able to proceed with the hydrogenation steps and form methanol, rather than desorb to the gas phase and contaminate the product stream.In Fig. 10\n\na, instead, we report the reaction rates as a function of the H2:CO2 ratio at 200 \u2070C and 30\u00a0bar. All the reaction rates remarkably increase with the H2 concentration. In particular, when H2:CO2 goes from 1 to 10, \n\n\nr\n1\n\n\n, \n\n\nr\n2\n\n\n and \n\n\nr\n3\n\n\n increase by ca. 30, 17 and 60%, respectively. As a matter of fact, all the direct reactions exhibit a positive order with respect to H2. However, expectedly, when the Cu0 active sites are saturated with H2, a further increase in the H2 concentration corresponds to a dilution of the carbon species, such as CO2 and CO, which also influence positively the reaction rates. This explains the slight decrease of the reaction rate (more noticeable for \n\n\nr\n2\n\n\n and \n\n\nr\n3\n\n\n) beyond H2:CO2 of c.a. 7, which is in agreement with the result reported in Fig. 7\nb. For completion, in Fig. 10\nb we also report the relative contribution of CO2-to-MeOH and CO-to-MeOH crosses at H2:CO2 of c.a. 1.5, with CO showing the predominant contribution beyond the crossing point. This is a clear consequence of the influence that the H2:CO2 ratio has on the reaction rate. For H2:CO2 larger than 1.5, \n\n\nr\n3\n\n>\n\nr\n1\n\n\n and the contribution of CO surpasses that of CO2, following a trend which corresponds to the reaction rates \n\n\nr\n3\n\n\n and \n\n\nr\n1\n\n\n, respectively.To underline the potential of the CuCeZr catalyst, we propose here a comparison with the benchmark formulation (i.e., CuZnAl) in terms of reaction rates. First, the model derived by Graaf et al., was implemented and validated with the experimental results obtained for the CuZnAl catalyst (details on the validation are given in S.I.). Therefore, the kinetic model we adopted for such comparison is representative of the CuZnAl system and can be used for predictive studies. As depicted in Fig. 11\n\na, the CO2 consumption rate (\n\n-\n\nr\n\nC\n\nO\n2\n\n\n\n\n) increases exponentially with temperature and it is quite similar for both catalysts, with the CuCeZr showing a slightly faster consumption. However, the CuCeZr catalyst converts CO2 more selectively to methanol \u2013 including both direct and indirect route \u2013 than the CuZnAl catalyst. As shown in Fig. 11\nb, methanol formation rate (\n\n\nr\n\nMeOH\n\n\n\n) its higher for the CuCeZr and crosses with CO (\n\n\nr\n\nCO\n\n\n\n) only at c.a. 256 \u2070C. On the contrary, the CuZnAl shows a much faster production of CO than methanol over the entire temperature range, which indicates that the CO hydrogenation does not contribute significantly to the synthesis of methanol, being CO the main reaction product. This demonstrates that the CuCeZr catalyst allows for a delay in the selectivity decay with increasing temperature when compared to the benchmark. It is clear that, in principle, lower temperatures favour the methanol production over CO. On the contrary, a higher temperature would correspond to much faster reactions, requiring less amount of catalyst to achieve equilibrium. In the end, when the desired product \u2013 in this case methanol \u2013 comes from an exothermic reversible reaction, the choice of the optimal temperature lies on a trade-off between reaction performance and economics. However, it is clear that the CuCeZr would facilitate the conflict between the demand of high performance and catalyst/reactor costs, since it allows to achieve higher methanol selectivity and faster CO2/CO conversion at higher temperature, when compared to the benchmark formulation.In this work, we investigate the kinetics of the CO2 conversion to methanol over a Cu/CeO2/ZrO2 catalyst, which remarkably outperforms the conventional Cu/ZnO/Al2O3 in terms of methanol yield/selectivity. The cross-over temperature (i.e., \n\n\nT\n\ncross\n\n\n\n, defined as the temperature above which the yield to CO exceeds that of methanol) increases up to 240 \u2070C for the CuCeZr, while CuZnAl shows a higher selectivity to CO in the entire temperature range.We analyse in detail the one-site, dual-site and three adsorption sites kinetic models, based on hypothesis retrieved from literature, and accordingly derived the kinetic parameters of all the models via an optimization algorithm based on the minimization of the RMSE (root mean square error). Physicochemical constraints and statistical indicators were used as tool for model discrimination. The best performing kinetic model (i.e., dual-site model of Graaf et al.,) suggests that the reaction mechanism proceeds via the adsorption of one of the oxygens of CO2 on the oxygen vacancies of the CeO2-ZrO2 phase (i.e., 1st active site), while H2 adsorbs and dissociate on the metallic copper (i.e., 2nd active site). The adsorbed hydrogen preferentially hydrogenates the carbon atom giving rise to the formate route. According to this mechanism, methanol can be formed either directly from CO2, or indirectly from the CO produced via the rWGS. The resulting kinetic model (i.e., rate expressions and fitted parameters) predicts the experimental data quite accurately, particularly the cross-over temperature (i.e., the model predicts that \n\n\nT\n\ncross\n\n\n\n stabilizes at 258 \u2070C at around H2:CO2 of c.a. 7.) Further, analysis of the individual reaction rates and the relative contributions of CO2 and CO to the methanol synthesis (i.e., COx-to-MeOH) reveal that CO2 and CO contribute evenly at 30\u00a0bar, H2:CO2 of 3 and 200 \u2070C (i.e., 51.5% and 47.4%, respectively), while the pathway CO-to-MeOH takes over at higher temperatures and/or higher H2 concentration. For H2:CO2 above 1.5, the CO contribution is predominant and exhibits an optimum at c.a. H2:CO2 of 7 (at 30\u00a0bar and 200 \u2070C) , which likely corresponds to the saturation of the Cu0 sites. This analysis underlines the importance of the indirect CO hydrogenation pathway in the reaction mechanism.In conclusion, these findings lead to a deeper understanding of the reaction mechanism of CO2 hydrogenation to methanol on novel CuCeZr systems, and serve as basis for future research into this catalyst formulation. For example, a more hydrophobic surface (i.e., weaker H2O adsorption and faster desorption from Cu0 sites) could lead to faster reaction rates and lower H2 requirement in the feed. Furthermore catalyst modification that lead to stronger CO binding would facilitate CO hydrogenation and, thus increase the selectivity to methanol even at higher temperatures.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\u2019s Horizon 2020 research and innovation programme under grant agreement No 838014 (C2Fuel project).Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2022.134946.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  This work addresses the kinetics of the CO2 hydrogenation to methanol over a Cu/CeO2/ZrO2 catalyst studied using single-site, dual-sites and three adsorption sites kinetic models. Physicochemical constraints and statistical indicators are used as tool for model discrimination. The best performing model is used to elucidate the reaction mechanism and the relative roles of the Cu-sites and oxygen vacancies. The results show that the dissociative adsorption of H2 occurs on the Cu0 sites, while CO2 is attracted to the oxygen vacancies created by the CeO2-ZrO2 solid solution. Then, the adsorbed H interacts preferentially with the carbon atom, favouring the so-called \u201cformate\u201d route. The CO formed via the r-WGS reaction could either desorb to the gas phase or react via hydrogenation to methanol. Analysis of the relative contributions of the CO2 and CO hydrogenation (i.e. direct and indirect pathways, respectively) to the methanol synthesis reveals that the latter is in fact preferential at high temperatures (i.e. about 100% of methanol is produced from CO at 260 \u2070C and 30\u00a0bar), and it shows an optimum vs the H2:CO2 ratio (c.a. 7 at 200 \u2070C and 30\u00a0bar), which corresponds to the saturation of the Cu0 sites with H2. Thus, this work provides an essential tool (i.e., kinetic model) for the design of reactors and processes based on novel catalysts, and importantly, it offers a deeper understanding of the reaction mechanism as basis for further catalyst development.\n               "}
{"full_text": "Magnesium alloys are constantly at the center of attraction in the selection and design of engineering materials due to its unique physical and mechanical properties for many years such as having low density and high strength-to-weight ratio [1,2]. In addition, magnesium metal is abundant on the earth's crust. Recent improvements in the mechanical properties of magnesium alloys have increased the usability of these alloys in aerospace and automotive industry applications too [3,4]. Moreover, they can be applied to 3C products (Computer, Communication and Consumer Electronics) where lightness, thinness and performance are a priority. However, one of the major factors that considerably restricts their application fields is the poor corrosion resistance of magnesium [5\u20137]. Thus, there are many studies focused on improving the corrosion resistance of magnesium alloys over the last few decades [8,9].There are many alternative surface modification methods to alter and enhance the corrosion resistance of magnesium alloys, such as physical vapor deposition (PVD) [10,11], chemical vapor deposition (CVD) [12,13], conversion coating [14,15], sol-gel method [16,17], electroless Ni-P plating [18\u201321] and micro-arc oxidation (MAO) [22\u201325]. The MAO process is quite similar to conventional anodizing with the notable exception of using high voltages within the range 200\u202fV to 600\u202fV. MAO coatings have high adhesion strength and exhibit exceptional wear resistance and distinguished corrosion resistance compared to coatings produced by other surface treatment techniques [26,27]. The MAO is also an environmentally friendly technology that has been widely used to produce relatively hard ceramic coatings on metals and alloys. During the reaction, a high potential is applied between the electrodes in the prepared electrolytes and the ratio of electrolytes affects the structure and corrosion resistance of the layer [28\u201331]. Therefore, many scholars have studied the effects of different electrolyte systems, such as silicate [32,33], phosphate [34\u201336], etc. on properties of micro-arc oxidation layers.Electroless nickel-phosphorus (Ni-P) plating has excellent properties, such as uniform deposition, high corrosion resistance, high wear resistance, good electrical as well as thermal conductivity, good lubricity and good ductility [37,38]. Electroless Ni-P plating on Mg alloy has attracted many scholars to study. However, a potential difference between the electroless Ni-P coating and the Mg alloy substrate which induces galvanic corrosion between the electroless Ni-P coating and the Mg alloy substrate seems to be a problem [39,40]. According to some studies, a MAO layer as an intermediate layer [41,42] with an outer electroless Ni-P coating on Mg alloy can provide good corrosion resistance.The aim of this study is to investigate how electrolyte composition influences the microstructure of the MAO coating and the corrosion resistance of the resulting MAO/Ni-P bilayer coating. The morphology, structure, adhesion and corrosion behavior of the bi-layered composite coating has been investigated by scanning electron microscopy (SEM), 3D white light interferometry. To the best of our knowledge, there are no scientific studies in the literature investigating the effect of phosphate concentration composite coatings on the AZ31B Mg alloy that include MAO and electroless Ni-P coating techniques. In this study, the MAO coatings were produced and the influence of phosphate addition on the corrosion resistance of MAO/Ni-P bi-layer coated AZ31B Mg alloy is then evaluated.The AZ31B Mg alloy (50\u202fmm\u202f\u00d7\u202f50\u202fmm\u00d70.2\u202fmm) was used as a substrate in this study. Before MAO treatment, the AZ31B plates were grounded by sandpaper from 400# to 1200#, then washed with deionized water, degreased with alcohol and dried at room temperature. The MAO operation uses a ui-polar power mode with duty cycle 30\uff05 and constant voltage of 400\u202fV in an alkaline solution. The electrolyte composition and processing conditions for MAO treatment are listed in Table 1\n. After MAO treatment, the sample was cleaned off with de-ionized water, wiped with alcohol, and dried at room temperature. And then, the electroless Ni-P coating was deposited on the MAO coated samples. The bath compositions and operation conditions of the electroless plating process were referring to our previous study [43]. The composition of the alloy is given in Table 2\n.Scanning electron microscope (SEM, JEOL JSM-IT100) equipped with energy dispersive spectroscopy (EDS) microanalysis hardware was used to observe the surface and cross-section morphologies of the MAO and MAO/Ni-P composite coated samples. The samples were coated with Pt in 60\u202fs to increase the conductivity. The average pore size and porosity were calculated using Image J software. 3D white light interferometry (Chroma 7503) was used to analyze surface roughness of the MAO. The XRD experiments were performed on a Bruker D2 PHASER X-ray diffractometer (\u03bb =1.54184\u202f\u00c5, 30\u202fkV and 10\u202fmA) with Cu K\u03b1 radiation. The scanning range of diffraction angle (2\u03b8) was 10\u00b0 and 90\u00b0 with a step width of 0.05\u00b0 and time step of 0.5\u202fs. The Posi-test AT-M pull-off adhesion tester (Defelsko) was used to measure the adhesion strength of the epoxy coating on the coated samples. The adhesion test was conducted according to ASTM D4541. The aluminum dollies with 20\u202fmm diameter were attached onto the surface of the coated samples using two-component adhesives and were held for 1\u202fh at 90\u2103 temperature to ensure that the adhesive was completely cured. Adhesion of Ni-P coatings on MAO coated samples were investigated by the pull-off test. Before applying force to pull the trolley from the surface of the sample, cut the area around the trolley all the way to the substrate, and then use the handle to repeat the up and down movement and increase the pull-up pressure for testing until the epoxy coating is removed from the substrate. All the tests were repeated five times to ensure the repeatability of the obtained results.Electrochemical tests were carried out using a VersaSTAT 4 potentiostat/frequency to analyze the corrosion behavior of Ni-P composite coating. A three-electrode cell, with a Pt flake counter electrode, a saturated Hg/Hg2Cl2/KCl as reference electrode and the sample as the working electrode (a circle with a diameter of 1\u202fcm is the measurement area). Potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) of samples were measured in 3.5\u202fwt.% NaCl solution. The samples were immersed in 3.5\u202fwt.% NaCl solution for 20\u202fmin to achieve stable open circuit potential before electrochemical measurement. The potentiodynamic polarization test was performed from \u2212300\u202fmV to 500\u202fmV in reference to the OCP with a scanning rate of 5 mVs\u22121. The EIS data were obtained at the open circuit potential and ambient temperature with a voltage amplitude of 10\u202fmV in the frequency range from 10-2 Hz to 105 Hz for 24\u202fh, 48\u202fh and 72\u202fh, respectively. The EIS data processing was carried by ZSimpWin 3.21 software. The salt spray test was conducted according to ASTM B-117. The sample was placed in a 5\u202fwt.% NaCl solution at a pH value range from 6.5 to 7.2. This solution was atomized into a mist and the heating chamber was kept at 35\u202f\u00b0C.Surface morphologies of the MAO coating treated in the different baths on AZ31B Mg alloy are shown in Figs. 1 and 2\n\n. Electrolyte composition has a crucial impact on the MAO coating structure and the final coating characteristics such as surface morphology, thickness, roughness and corrosion resistance [28,30,31]. A typical cratered structure with micro-pores (pancakes-like structure) can be observed on the surface of both samples. The micro-pore in the cratered region is a discharge channel through which molten material was ejected from the coating/substrate interface due to the high temperature and strong electric field. After ejection Mg alloy rapidly solidified upon contact with the electrolyte. The pancake-like structure is rapidly solidified around the discharge channels [44]. In Figs. 1(a) and 1(b), the surface morphology of the MAO coating treated in the phosphate-containing bath shows similar pancake-like features and micro-pores which irregularly arranged on the coating surface. Luo et al. [35] and Ma et al. [36] reported that phosphate ion concentration accelerated the reaction rate during coating formation leading to an increase in the pore size, thickness and roughness of MAO coatings. It is found in Fig. 1(a) that the size of most of the pores (98%) is about 1\u20135\u202f\u03bcm and the rest of the pores (2 %) has size of 5\u201310\u202f\u03bcm in diameter. On the other hand, Figs. 1(c) and 1(d) shows that the micro-pores size of the MAO coating treated in the phosphate-free bath is approximately 0.8\u20131.5\u202f\u03bcm, which is smaller than that of MAO coating treated in the phosphate-containing bath. Therefore, structurally independent morphologies in terms of pore size distribution and internal cracks are due to the different plasma thermochemical interactions between the substrate metal and different ions in the electrolyte solution [28,35,36]. Moreover, cracks can be obviously observed in the MAO coating prepared from the phosphate-containing bath. It implies that the roughness of MAO coating obtained from the phosphate-containing bath is higher than that from the phosphate-free bath.The cross-sectional morphologies of the MAO coatings on AZ31B Mg alloy were examined by SEM, as shown in Fig. 2. In Fig. 2(a), the cross-sectional morphology of the MAO coating treated in the phosphate-containing bath reveals the existence of an irregular metal-oxide interface. It exhibits some porosities or empty inclusions, where the crack appeared in the coating. It can be seen that this crack feature starts from the barrier layer between the substrate and the coating, and then propagates to the outer layer of the coating. Crack is generated due to thermal stress caused by the difference in the thermal expansion coefficient of Mg substrate and coating [44]. After MAO treatment in a phosphate-free bath, the thickness of the oxide layer becomes more homogenous. It can clearly be seen that the barrier layer of this coating has no porosities and more compact than the phosphate-containing sample in (Fig. 2(b)). Therefore, the thickness of the phosphate-containing sample (10.5\u201313.2\u202f\u03bcm) is larger than the phosphate-free sample (7.8\u201310.3\u202f\u03bcm).The XRD patterns of MAO coatings produced from different solutions are shown in Fig. 3\n. The major crystalline phase is Mg2SiO4 in the phosphate-containing solution. Depending on the short coating time and therefore the thin coating thickness obtained, the structure of Mg2SiO4 is not detected in the phosphate-free solution. This finding is in accordance with the literature [45]. For both MAO coatings, MgO and elemental Mg are observed in the XRD pattern regardless of the compounds in the solution. The peaks that are related to elemental Mg is coming from the substrate metal [46\u201348]. The other crystalline structures are formed due to the plasma thermochemical reactions in the discharge channels of the MAO process [49\u201352].The corrosion resistance of the MAO coated samples on AZ31B alloy was conducted by the potentiodynamic polarization tests. Polarization curves of the bare AZ31B and the MAO coated AZ31B plates in 3.5\u202fwt.% NaCl solution are displayed in Fig. 4\n. Corrosion current densities (icorr\n) and corrosion potentials (Ecorr\n) are evaluated from the intersection of the linear anodic and cathodic branches of the polarization curves, shown in Table 3\n. Ecorr\n means the corrosion potential where current density hugely increases according to the behavior of metal dissolution until it reaches a critical value. icorr\n means the corrosion rate. From Fig. 4 and Table 3, the phosphate-free sample has higher Ecorr\n (\u22121360\u202fmV) and lower icorr\n (1.61\u202f\u00d7\u202f10\u22123 \u03bcA/cm2) than those of the phosphate-containing sample (Ecorr\n: \u22121360\u202fmV; icorr\n: 6.87\u202f\u00d7\u202f10-3 \u03bcA/cm2). Due to the intense chemical interaction in the phosphate-containing electrolyte generates the formation of larger discharge pores and inner cracks at the MAO coatings. These porous structures and cracks play a key role in the corrosion process of MAO coatings [28,35,36,53]. On the other hand, the phosphate-free sample has a dense and continuous ceramic oxide layer. This protective layer prevents corrosive ions from reaching into the substrate metal compared to the phosphate-containing sample. Thus, the corrosion resistance of the phosphate-free sample is better than the phosphate-containing sample.The surface morphology of electroless Ni-P coating deposited on the different MAO coatings is shown in Fig. 5\n. All images show a homogeneous, uniform and nodular-like structure on the electroless Ni-P coating. No distinct particles are observed over the coating surface because no grain boundaries are seen. It indicates that the uniform surface and amorphous structure of the Ni-P deposit can be obtained under our setting coating conditions.\nFig. 6\n shows the polarization curves of the bare AZ31B and the various MAO/Ni-P composite coated AZ31B plates in 3.5\u202fwt.% NaCl solution. The potentiodynamic polarization data are also listed in Table 3. The average icorr\n of bare AZ31B, phosphate-containing-MAO/Ni-P coating and phosphate-free-MAO/Ni-P coating are about 65.4\u202f\u03bcA/cm2, 2.36\u202f\u03bcA/cm2 and 0.661\u202f\u03bcA/cm2, respectively. It is obviously seen that the icorr\n value of phosphate-free-MAO/Ni-P coating is lower than others, indicating that this coating would form a more compact and protective passive film. On the other hand, the Ecorr\n of bare AZ31B, phosphate-containing-MAO/Ni-P coating and phosphate-free-MAO/Ni-P coating are about \u22121530\u202fmV, \u2212360\u202fmV and \u2212360\u202fmV, respectively. In comparison with bare AZ31B, the Ecorr\n value of both composite coated AZ31B samples is obviously higher than that of bare AZ31B. Because of the Ecorr\n value is a parameter affected by the composition of the coating, which the MAO/Ni-P composite coating is completely covered and consistent with the results of Fig. 5. More positive Ecorr\n and smaller icorr\n imply that the phosphate-free-MAO/Ni-P composite coated AZ31B sample possesses better electrochemical resistance.The salt spray test was carried out for 120\u202fh in 5\u202fwt.% NaCl solution to investigate the corrosion protection of the MAO/Ni-P composite coated AZ31B plates. The data of the specimens were recorded each 24\u202fh during SST for the purpose of studying the corrosion process, as shown in Fig. 7\n. The phosphate-containing-MAO/Ni-P composite coated AZ31B plate has obvious pitting corrosion after 24\u202fh of SST. Moreover, at the end of the test (120\u202fh), this plate had more than 4.5 % corrosion area, which indicates severe corrosion. Contrary to the phosphate-containing-MAO/Ni-P composite coated AZ31B plate, the phosphate-free-MAO/Ni-P composite coated AZ31B plate still displayed the excellent corrosion resistance after 120\u202fh of SST.The Table 1 shows that the phosphate-free-MAO/Ni-P composite coated AZ31B plate can provide better corrosion protection capability than other counterparts. According to our previous study [43], the electroless Ni-P coating causes severe damage to MAO coating. It might be ascribed to that local failure occurs resulting from the localized poor adhesion. Then, the electrolyte easily penetrates into the defect (micro-pores or cracks) and the corrosion of the coatings will initiate. Therefore, it is necessary to identify the adhesion of MAO/electroless Ni-P coating interface.The pull-off test was carried out to investigate the effect of adhesion for MAO/electroless Ni-P coating interface, as shown in Fig. 8\n. The results of the adhesion test for the phosphate-containing-MAO/Ni-P composite coated AZ31B and the phosphate-free-MAO/Ni-P composite coated AZ31B are about 1.4\u202f\u00b1\u202f0.21\u202fMPa and 7.5\u202f\u00b1\u202f0.15\u202fMPa, respectively. The adhesion between the electroless Ni-P coating and the phosphate-containing-MAO coating is found to be weak and the electroless Ni-P layer easily strips off the MAO coating. In contrast, due to good adhesion to the phosphate-free-MAO coating, the electroless Ni-P coating directly is deposited and metallurgically integrated with the MAO coating.Appropriate surface roughness and microstructure are required in order to produce coatings with superior adhesion and corrosion resistance. Fig. 9\n shows the 3D white light diagram of the different MAO coated AZ31B plates. The results of the average surface roughness (Ra\n) for the phosphate-containing MAO coated AZ31B and the phosphate-free MAO coated AZ31B are about 0.935\u202f\u03bcm and 0.330\u202f\u03bcm, respectively. Although the phosphate-free sample has a lot of micro-pores and the microstructure morphologies are smooth (Figs. 9(c) and 9(d)), there is a good adhesion between the coating and the electroless Ni-P coating. The reason is that the surface of the phosphate-free sample forms a uniform oxide layer with good structure, so that it can improve the adhesion very well.\nFig. 10\n shows the cross-sectional SEM and BSE images of the MAO/Ni-P composite coatings. The electroless Ni-P coating is completely covered on both MAO coatings with the same thickness (approximately 10\u202f\u03bcm), which is consistent with the results of Fig. 5. From Fig. 1, it illustrates that the micro-pores distribution of the MAO coating treated in the phosphate-free bath is uniform and dense. Besides, the catalyst used for the electroless Ni-P coating penetrates inside the porous structure and interacts well with the pores in the interface area between MAO/Ni-P composite coatings [42]. It can clearly be seen that the micro-pores on the electroless Ni-P coating/MAO coating interface were fully covered (Fig. 10(d)). They are adhered by mechanical and physical interlocking force [54]. The electroless Ni-P coating is deposited on the MAO coating treated in the phosphate-free bath on AZ31B with increasing adhesion. This leads to the conclusion that the incorporation of the catalyst has a significant effect on the improvement of mechanical properties for both samples (shown in Section 3.3).\nFig. 11\n shows the EDS/elemental mapping of the cross-sectional image of the phosphate-free-MAO/Ni-P composite coated AZ31B plate. It is evident that the element of the triangular-shaped plating part is Ni and the surrounding elements are Mg, O, and Si, which indicate that the electroless Ni-P coating completely covers the micro-pores of the MAO coating.\nFigs. 12\n(a)-12(f) show the Nyquist and Bode plots for the experimental and fitting curves for all AZ31B MAO/Ni-P composite coatings after immersion in 3.5\u202fwt.% NaCl solution for different time periods up to 48\u202fh (recorded every 12\u202fh), respectively. Considering the microstructure characteristics and EIS behavior of the composite coated samples, the models chosen for the fitting of the different MAO/Ni-P composite coated samples are depicted in Fig. 13\n. These equivalent circuits are commonly used on Mg alloy [55\u201358], where Rs\n is the solution resistance. The constant phase element (CPE1) and resistance (R1) represent the properties of the electroless Ni-P plating; a parallel combination of a constant phase element (CPE2\n) and resistance (R2\n) represent the characteristic of the MAO coating. A good fit was shown between the experimental data and the simulated values (Figs. 12(a)-12(f)). The results are listed in Table 4\n. The phosphate-containing-MAO/Ni-P composite coated AZ31B exhibits the highest total impedance in the as-deposited state (0\u202fh), as shown in Figs. 12(a)-12(b). After immersion for 12\u202fh, the total impedance shows a decrease as immersion time increases (from 0\u202fh to 48\u202fh). The change in the phase angle is also apparent (Fig. 12(c)), shifted to a higher frequency as immersion time increases. On the contrary, the total impedance for the phosphate-free-MAO/Ni-P composite coated AZ31B shows an increase as immersion time increases, as shown in Figs. 12(d)-12(e). Since the passive layer is formed on the surface of the Ni-P coating during the immersion process, the corrosion resistance can be improved [41]. According to the results of the adhesion test (Fig. 8), the electroless Ni-P coating was found to be weak adhesion to the phosphate-containing-MAO coating and easily stripped the MAO coating. Furthermore, there were larger micro-pores and micro-cracks on the phosphate-containing-MAO coating (Fig. 1), so the chloride ions easily corrode and penetrate the coating.The absolute impedance of the composite coating at low frequency (0.01\u202fHz) is plotted as a function of time in Fig. 14\n. The figure shows that in all cases that the impedance (|Z|) of the phosphate-free-MAO/Ni-P composite coated AZ31B is greater than the other samples. The result indicates that the phosphate-free-MAO/Ni-P composite coated AZ31B provides good protection. When compared with the corrosion resistance of these two samples, it can be seen that the phosphate-free-MAO/Ni-P composite coated sample showed consistently higher corrosion resistance in 3.5\u202fwt.% NaCl solution at all electrochemical tests than the phosphate-containing-MAO/Ni-P composite coated sample. This is mainly due to the flat and dense microstructure of the MAO coating. Also, evenly distributed pores on the surface help catalyst to be absorbed easily at the surface of the MAO coating. Thus, the Ni-P composite coating has good adhesion and corrosion resistance. It is concluded that the firm oxide layer was essential for improving the quality of the Ni-P layer coating so that AZ31B is well protected.\nFig. 15\n shows the cross-section morphology of the MAO/Ni-P composite coatings after the EIS measurement with immersion in 3.5\u202fwt.% NaCl solution for 48\u202fh. It is evident that the phosphate-containing-MAO/Ni-P composite coated sample is peeling and the Mg matrix is dissolved, which demonstrates that the coating is damaged and the corrosive solution reacts with the Mg substrate, as shown in Fig. 15(a). In contrast, the phosphate-free-MAO/Ni-P composite coated sample remains whole and the corrosion product plugs the micro-cracks and micro-pores in the coating, which indicates that this sample has good adhesive bonding and a compact structure with few defects in MAO layer, as shown in Fig. 15(b). As a final point, it is vital to possess coating structures such as a phosphate-free-MAO coating layer before applying the electroless Ni-P coating process, which is consistent with the results of Fig. 14.In this study, the influence of phosphate addition on the morphology and the corrosion resistance of MAO/Ni-P bi-layer coated AZ31B Mg alloy have been investigated.The morphology of the MAO coating treated in the phosphate-free bath was smooth and had a dense coating layer. The pores were evenly distributed at the surface and almost no defects were observed in the cross-section. On the other hand, the phosphate-containing MAO layer had many defects and uneven pores distribution, resulting in low adhesion values to the Ni-P coating. This caused poor adhesion protection to the substrate. However, the Ni-P coating formed a strong mechanical and physical interlock with the phosphate-free MAO coating due to the uniform distribution of pore size and shape. Therefore, it had better adhesion to the Ni-P coating with a value of 7.5\u202f\u00b1\u202f0.15\u202fMPa.The potentiodynamic polarization test showed that the phosphate-free-MAO/Ni-P coating had 2.8 times lower current density than phosphate-containing-MAO/Ni-P coating. The phosphate-free-MAO/Ni-P composite coated sample was shown to elevate the impedance in the EIS analysis for long-term immersion, indicating that this intact coating structure provided better corrosion protection. Finally, the salt spray test showed that the phosphate-free-MAO/Ni-P coating still maintained better corrosion resistance for 120\u202fh. Therefore, it is concluded that the treated phosphate-free-MAO/Ni-P coating is superior to the phosphate-containing MAO coating in terms of mechanical and chemical properties. So, it is proposed that a new method of protecting the magnesium alloy has been developed.The authors declare no conflicts of interest.This study was financially supported by the Ministry of Science and Technology of Taiwan, Republic of China, under Grant No. MOST 106-2221-E-606-013-MY3.", "descript": "\n                  A bi-layer coating is deposited on the surface of the AZ31B Mg alloy for corrosion protection of the Mg alloy. The bi-layer coating is composed of a micro-arc oxidation coating (MAO), and an electroless plated Ni-P coating. The micro-arc oxidation (MAO) treatment in the electrolyte with or without the addition of phosphate ions is carried out under unipolar power mode. The microstructure and composition of the MAO coatings are analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The roughness of the MAO coatings is obtained by using 3D white light interferometry. The corrosion resistances are evaluated by potentiodynamic polarization test as well as electrochemical impedance spectroscopy (EIS) in a 3.5\u202fwt.% NaCl solution and salt spray test. The adhesion test results of the MAO/Ni-P composite coatings by drawing test machine. The results showed that the micro-pores size of the MAO coating treated in the phosphate-free bath is uniform and smaller than the phosphate-containing solution, which could decrease the roughness and enhance the corrosion resistance. The corrosion resistance of MAO coating with the phosphate-free solution for 2.8 times was better than that of phosphate-containing solution. Moreover, phosphate-free MAO/Ni-P composite coating showed the corrosion rate is more than 4 times lower than the phosphate-containing MAO/Ni-P composite coating. Furthermore, the MAO/Ni-P composite coatings prepared by the MAO coating treated in the phosphate-free bath could suppress the erosion of aggressive media during exposure after 120\u202fh salt spray test (corrosion area is approximately 5%) and present excellent adhesion.\n               "}
{"full_text": "Plastic pyrolysis, one of the routes to recycle low value polymeric wastes, is a well-known technology to produce useful liquid fuels and presents lower greenhouse gas net emissions than other current technologies such as incineration and gasification [1]. The liquid fraction derived from plastic pyrolysis, mainly in the range of gasoline and kerosene/diesel, can be used as feedstock for refineries or fuel for diesel engine generators [2,3]. However, many studies focused on application of pyrolysis oil are limited because they used selected neat polymers such as polyethylene (PE) and polypropylene (PP) for pyrolysis. In other words, the liquid product obtained from the pyrolysis of contaminated real municipal plastic wastes cannot be used in a direct industrial application, because of the presence of chlorine coming from poly(vinyl chloride) (PVC). There are many studies in the literature focused on reducing the chlorine content in the liquid products (in particular, the light liquid fraction) such as pyrolysis in the presence of a mixture of Ca(OH)2, Ni/SAPO-11, and Redmud [4]. ZnO/CoO adsorbent showed the highest dechlorination ability of a PVC pellet using superheated steam as the pyrolysis medium [5]. Stepwise pyrolysis of PVC containing plastic wastes also reduced the chlorine content in a liquid product [6]. However, in the case of high-boiling heavy wax, few studies on enhancement of properties and applications have been reported, likely due to the difficulty to find a suitable use thus far. The high-boiling heavy wax is a dark brown in a semi-solid phase and has a wide carbon-range distribution with high chlorine content. An adequate solution for application of the worthless heavy wax is thermal, catalytic, or hydro-cracking of heavy wax into lighter hydrocarbons in the range of gasoline and kerosene/diesel [7\u20139]. Catalytic cracking of heavy fractions obtained from the pyrolysis of automotive waste plastics was performed using a commercial FCC equilibrium catalyst in a fixed bed reactor with the flow of nitrogen at 525 \u2103 [10]. The heavy wax was diluted in atmospheric or hydrotreated vacuum gas oils and was converted into high yields of gas and gasoline fractions, which were dependant on the kinds of gas oils and polymers. Raw pyrolysis wax oil from municipal plastic wastes (refuse plastic fuel, RPF) was catalytically upgraded on the zeolites under atmospheric pressure at 450 \u2103 [8]. Catalytic cracking of heavy wax oil using HZSM-5 catalyst resulted in high yields of light hydrocarbons. However, chlorine issues were not discussed in relation to catalytic cracking of heavy fractions due to the use of pyrolysis oil obtained from selected polymers such as PE and PP. For the removal of chlorine compounds in the pyrolysis oils (chlorine content, 600 ppm), metal oxides including iron oxide, ZnO, MgO and Redmud were used as catalysts [11], Among them, iron oxide and an iron oxide carbon composite were active and stable to remove organic chlorine compounds from the oils (chlorine content, < 100 ppm). This result shows that iron oxide is suitable for the refinement of low-boiling pyrolysis oil as there is no considerable change in the carbon number distribution of the oils. Chlorinated pyrolysis oils (chlorine content, 0.8\u223c0.9 wt.%) were refined by using Redmud, which mainly contains Fe2O3, in a batch reactor at 325 \u2103 with auto-generated pressure [12]. About 45\u223c71 % of the light liquid fraction was produced with no olefins and very low chlorine content (< 0.1 wt.%). In comparison to thermal and catalytic cracking of oils, catalytic cracking using Redmud was slightly more effective to remove chlorine from liquid products. Most chlorine including HCl and chlorinated organic compounds was present in a gaseous phase in the case of thermal cracking, whereas chlorine is trapped physically and chemically in the form of HCl and FeCl3 in a solid phase (Redmud and heavy residue) during the catalytic cracking. Furthermore, it was reported that Redmud promoted the cracking reaction of chlorinated pyrolysis oil as well as a dechlorination reaction. However, in the aforementioned studies, upgrading and the cracking degree were not adequately evaluated because the chlorinated oils contained a large fraction of gasoline (above 80 %, C5\u223cC10), meaning that it is questionable whether Redmud can break up the long-chain carbon bonds in the heavy wax oil. As a result, it is necessary to achieve an integrated catalytic upgrading of chlorinated heavy wax including catalytic cracking of wax into a light oil fraction and dechlorination to obtain light oil with very low chlorine content to find suitable applications of worthless heavy wax.Here, we performed catalytic cracking of chlorinated heavy wax obtained from pyrolysis of contaminated real municipal plastic wastes using iron oxide impregnated zeolite catalysts and discussed the catalyst effects on properties of liquid products such as cracking degree of heavy wax and chlorine content in the products.Chlorinated heavy wax was obtained through the pyrolysis of RPF in a commercial rotary kiln-type pyrolyzer (COcom Co., South Korea). RPF was added to the externally heated rotary kiln reactor, which was operated at 400 \u223c 500 \u2103 and the residence time of 10 \u223c15 h. The chlorinated heavy wax used in this work was yellow-brown and in a solid state at room temperature (Fig. 1\n). Elementary analysis (EA), carbon-distribution analysis using a simulated distillation gas chromatography system (SIMDIS-GC), and paraffins-olefins-naphthenes-aromatics (PONA) analysis using gas chromatography (GC) were carried out to obtain information of the raw material. Each analysis method is described in the following section in detail. A thermogravimetric analysis (TGA) of the heavy wax was conducted using a TGA Q500 with air to provide the amount of non-volatile components in the wax.Pellet-type (1.5 mm diameter, clay binder) HY zeolite (SiO2/Al2O3 = 100) was purchased from TOSOH USA, Inc. and it was pretreated at 550 \u00b0C for 4 h in air prior to being used as a substrate. Iron-impregnated catalysts were prepared by the incipient wetness impregnation method using an aqueous solution of FeCl2\u00b74H2O (Sigma-Aldrich). The pretreated HY zeolite pellets were mixed with the iron- aqueous solution (8\u223c9 wt% solution) in a rotary bottle (1000 mL), followed by vacuum evaporation (B\u00dcCHI, R-100) at 70 \u00b0C and about 350 mbar until the water was gone. The Fe-impregnated catalysts were dried at 105 \u00b0C overnight, followed by calcination at 550 \u00b0C for 4 h in air. The prepared catalysts were designated as Fe[x]/HY, where x means the impregnated weight percentage of Fe based on the HY support.The physical properties of the prepared catalysts were determined using Brunauer-Emmett-Teller (BET) technique using a BELSORP-mini II (MicrotracBEL Co. Ltd.).The catalysts were pretreated at 105 \u00b0C. NH3-temperature programmed desorption (NH3-TPD) was conducted to determine the surface acidity of catalysts using an AutochemII 2920 analyzer (Micromeritics Co. Ltd). After pretreatment with He at 400 \u00b0C for 1 h, ammonia was adsorbed on the surface of the catalyst at 50 \u00b0C for 1 h and then was switched to He to remove NH3 physically adsorbed on the catalyst. The adsorbed NH3 was desorbed with an increase of temperature according to the NH3-TPD routine. An X-ray diffraction (XRD) analysis was conducted to verify the impregnated iron oxide states using a Rigaku Smartlab XRD system with Cu K\u03b1 radiation. TGA and a differential thermogravimetric analysis (DTG) of the used catalysts were conducted using a TGA Q500 with air to provide information on the volatility of coke and chlorine deposited on the used catalysts.A schematic drawing of the experimental set-up for catalytic cracking of heavy wax and the product-recovery system is presented in Fig. 2\n. The received heavy wax was heated using a heating mantle (80 \u00b0C) to make it flow well and the preheated wax was fed to a reactor using HPLC pump (Eledx Laboratories, 0.001\u201310 ml) at 0.29 g/min. The amount of loaded wax was measured using a data logger, connected with a balance (A&D Company) during the experiments. The line between the head of the HPLC pump and the inlet of the reactor was also heated using a heating band, at temperature of 170 \u00b0C. The prepared catalysts were placed in a quartz bulb-reactor (ID = 4 mm) situated in a heating furnace. Weight-hourly-space-velocity (WHSV) was set to 2 h\u22121, unless otherwise stated and N2 (50 m/min) was used as a carrier gas. A K-type thermocouple was located in the reactor to control the reaction temperature (450 \u2103) and the internal pressure of the reactor was displayed. The produced vapor was first condensed at 70 \u00b0C and then re-condensed at 4 \u00b0C to easily recovery the liquid products. HCl in non-condensable gas was trapped in a water trap (10 in Fig. 2) and then water was also removed in the next trap (11 in Fig. 2).The composition of the non-condensable gas was analyzed using an on-line gas chromatograph (GC, Agilent 7890A) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID) and the total flow rate of gas was determined using a gas-flow meter (Sensidyne Gilibrator2). The liquid product was recovered for 1 h after reaching a steady-state of the reaction system, unless otherwise stated. EA, SIMDIS-GC, and GC\u2013MS analyses were conducted for the liquid products. EA including C, H, N, O and Cl content was measured by an element analyzer (Thermo Scientific, Flash 2000). A SIMDIS-GC (Agilent 24001.070) was used to obtain the carbon distribution in the liquid product and compare the characteristics of liquid products obtained from the various reaction conditions. The GC was equipped with an AC (Analytical Controls) capillary column and a FID. The sample was diluted with carbon disulfide according to ASTM D2887 prior to the SIMDIS-GC analysis. A PONA analysis of the liquid product was performed using a GC/FID (Agilent 6890 N) and a GC\u2013MS (Shimadzu Q-2010 plus) equipped with a ZB-DHA-PONA column. The sample oil (0.1 g) was diluted 20 times with acetone and hexane, respectively. The diluted samples were centrifuged at 10,000 rpm for 2 min. (Smart 13, Hanil Science Inc.), followed by filtration with a 0.45 \u339b syringe filter (Sartorius). The compounds from the GC\u2013MS profile were identified by comparison of their retention times with those of the standard compounds from a P-I-A-N-O standard kit (Sigma-Aldrich), composed of 11 n-paraffins mix (0.1 mL), 37 isoparaffins mix (0.1 mL), 37 aromatics mix (0.1 mL), 30 naphthenes mix (0.1 mL), 25 olefins mix (0.1 mL), and P-I-A-N-O mix (138 n-paraffins, isoparaffins, aromatics, naphthenes, and olefins, 0.1 mL).Chlorine quantification of the liquid products and HCl-trapped water was carried out using an AC600 Semi-auto Calorimeter (LECO. Co., USA), a Metrohm Ion Chromatograph with Mitsubishi combustion module, and a Dionex Integrion HPIC System (Dionex Co., USA), respectively. For examination of chlorine and coke formation on the surface of the used catalysts, a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDX, S-4700, Hitachi) analysis was performed.The liquid fraction (wt.%) was calculated from the amount of the liquid product based on the amount of reactant fed into the reactor during the reaction time. The solid fraction (wt.%) including coke, char, and residues which could deposit on the surface of the catalyst and in the wall of the reactor, was determined by dividing the amount of solid by the amount of reactant fed. In here, the solid weight was calculated from the difference between the two weights of the catalyst and the quartz reactor before and after the reaction. In addition, the gas fraction (wt.%) was calculated from the difference between 100 and the liquid and solid fractions. Note that the small amounts of resides and tar that accumulated in the feeding line from the pump to the inlet of the reactor were ignored and trapped HCl gas was also ignored in the calculation.A photograph (a) and the carbon-number distribution (b) of heavy wax are shown in Fig. 1. The SIMDIS-GC data showed that the heavy wax has a much higher boiling point and a very wide carbon distribution: 1.54 wt.% of gasoline fraction (C5-C10), 37.91 wt.% of kerosene/diesel fraction (C11-C22) and 60.55 wt.% of heavy wax fraction (\u2265C23). Based on the results of the PONA analysis, n-paraffin (87.74 wt.%) was the main components of the wax in all carbon ranges (not shown here). Table 1\n shows the elemental composition of the heavy wax. The sample mainly consisted of carbon and hydrogen with a few heteroatoms and 0.14 wt.% of chlorine was detected, which should be removed for the utilization of such heavy waxes. The heavy wax had the low-level non-volatile components (0.51 wt%), based on the TGA data (up to 900 \u2103 in air). A strong weight loss begun at around 150 \u00b0C and most of the heavy wax was volatilized at around 530 \u00b0C.The textural properties of the prepared catalysts are summarized in Table 2\n. It was found that the BET surface area and total pore volume decreased by impregnation of Fe on the pellet-type HY zeolite. In the case of the Fe[20]/HY zeolite, the surface area and total pore volume decreased by 27 % and 31 %, respectively, compared to those of the HY zeolite.\nFig. 3\n displays the XRD patterns of the prepared catalysts (a to d). The diffraction peaks (2theta = 6.3, 10.3, 15.9 and 24.1\u00b0) observed in the XRD pattern of the HY zeolite could be assigned to the characteristics of a faujasite zeolite (FAU) structure [13]. For all Fe impregnated HY zeolite, iron oxide (\u03b1-Fe2O3) was observed (2theta = 33, 35, 49, 55, 62 and 64\u00b0) and its intensity increased with an increase of the Fe loading amount.The acidic properties of the catalysts evaluated by an NH3-TPD analysis are also summarized in Table 2. The amount of acid sites was measured from three distinct desorption peaks divided by the desorption temperature of 120\u2013300 \u2103 (weak acid sites), 300\u2013500 \u2103 (medium acid sites), and 500\u2013700 \u2103 (strong acid sites), respectively. All prepared catalysts had only low- and medium-strength acids. The amount of total acid of the parent HY zeolite was 0.593 mmol/g. When 3 and 5 wt.% of Fe were impregnated on the HY zeolite, the amount of total acid sites increased to 0.688 and 0.845 mmol/g, respectively. However, the total acidity of Fe[20]/HY was lower (0.595 mmol/g) than that of Fe[5]/HY. This means that weak- and medium-strength acid sites could be derived from the interaction between the surfaces of iron oxide and HY zeolites. However, excessive impregnation of Fe appeared to partially cover the acid sites on the surface of the HY catalyst. The volcano trend of the total acidity for Fe impregnated HY catalysts was in line with a previous study [14].Results of thermal cracking and catalytic cracking at 450 \u2103 are shown in Fig. 4\n:(a) liquid, solid and gas fraction, (b) carbon distribution in the liquid product, and (c) PONA distribution in the liquid product. As shown in Fig.4(a), thermal cracking without any catalysts showed 69.8 wt.% of liquid yield, 18.6 wt.% of gas yield and 11.7 wt% of solid. When using the HY zeolite in the cracking reaction, the liquid yield increased to 71.3 wt.% and the solid was significantly reduced to 0.5 wt.%. However, the highest gas yield (28.2 wt.%) was obtained due to the active performance of HY toward the cracking reaction. In the case of the Fe impregnated HY zeolite, the highest liquid yield (74.2 wt.%) was obtained, indicating that heavy wax oil was less cracked compared to the case of using HY zeolite, and 9.6 wt.% of solid was formed. The generated gases were methane, ethane, ethylene, propane, propylene, etc. in all cases and a trace amount of hydrogen was generated. With regard to the carbon distribution in the liquid products (Fig. 4(b)), thermally cracked liquid consisted of heavy hydrocarbons (63.5 wt.%, \u2265C23) and kerosene/diesel hydrocarbons (36.0 wt.%, C11\u223cC22), which is similar to the fraction of raw heavy wax. Considering gas yield (18.6 wt.%), the cracking reaction of hydrocarbons occurred, but high-boiling hydrocarbons and cokes were mainly formed during the thermal cracking reaction. For the HY catalyst, most of the heavy fraction was broken down into kerosene/diesel (45.1 wt.%) and gasoline (49.4 wt.%) fractions. When using Fe[3]/HY, the largest kerosene/diesel fraction (52.2 wt.%) was achieved, but 9.9 wt.% of heavy fraction remained. For the PONA distribution of the liquid products (Fig. 4(c)), the kerosene/diesel fraction in the thermally decomposed liquid showed mainly n-olefins. The heavy fraction was a mixture of n-paraffins and n-olefins. Although the carbon distribution of the thermally cracked liquid was similar to that of heavy wax, the main type of hydrocarbon were different; n-olefin for the thermally cracked liquid and n-paraffin for heavy wax. In the case of catalytic cracking of heavy wax, regardless of Fe impregnation on HY zeolite, n-paraffins in heavy wax disappeared and various types of hydrocarbons were detected in the liquid product: i-paraffins, naphthenes and aromatics as well as n-olefins.The effect of the Fe loading amount to the HY zeolite on the product yield (a), carbon distribution in the liquid product (b), and PONA distribution in the liquid product (c) is shown in Fig. 5\n. Unlike the solid yield (0.5 wt.%) of the parent HY catalyst, the solid yields of Fe impregnated HY catalysts increased although a trend was not found in the relationship between loading amount of Fe and solid yield (Fig. 5.(a)). However, a correlation between total acid sites and activity toward the cracking reaction was identified. The Fe[5]/HY catalyst containing the largest amount of total acid sites presented the highest gas yield (22.6 wt.%), whereas Fe[20]/HY with the lowest total acid sites among the Fe impregnated HY catalysts showed the lowest gas yield (12.4 wt.%). The influence of different loadings of Fe on the carbon distribution in the liquid products also demonstrates that Fe[20]/HY has the lowest cracking acitivty: 20.3 wt.% for gasoline fraction and 27.4 wt.% for heavy fraction in the liquid product (Fig. 5(b)). Herein, it should be mentioned that, although the parent HY and Fe[20]/HY catalysts showed similar total acidity (Table 2), the two catalysts showed clearly different yields and carbon distribution in the liquid product. The differences are likely due to the presence of different Br\u00f8nsted and Lewis acid sites: HY zeolite with more Br\u00f8nsted acid sties and Fe[20]/HY with more Lewis acids sites [14], where HY having strong Br\u00f8nsted acid sties is more favorable for catalytic cracking of wax [15]. The results obtained from the PONA analyses of the liquid products (Fig. 5(c)) demonstrate that all Fe impregnated catalysts consisted of i-/n-paraffins, n-olefins, naphthenes, and aromatics and the contents of n-olefins increased with Fe loading amount.\nFig. 6\n shows a comparison of chlorine contents in the liquid and gas products (a) and on the surface of used catalysts (b). The longer reaction time (time on stream (TOS) = 2 h) for catalytic cracking reaction than that of thermal cracking reaction (TOS =0.5 h) was applied to effectively compare the effect of chlorine removal of the prepared catalysts than thermal cracking reaction. For the thermal cracking of heavy wax, the liquid product contained a high concentration of chlorinated compounds (0.09 wt.%) and the concentration of chlorine captured from non-condensable gas product was 0.11 ppm. When using the HY catalyst, the chlorine content in the liquid product was much lower (320 ppm) than that in the case of thermal cracking, although the reaction time of catalytic cracking was longer than that of thermal cracking. As expected, the presence of iron oxide on the surface of HY resulted in lower chlorine content in the liquid and gas products, likely due to the chlorine adsorption ability of iron oxides. The impregnation of 20 wt.% Fe yielded liquid product with the lowest chlorine content (60 ppm) among the tested catalysts. An SEM-EDX analysis was performed to observe chlorine captured by catalysts and the results are presented in Fig. 6(b). Unlike the EDX result of HY zeolite, chlorine was detected on the Fe impregnated catalysts: 1.2 wt.% for Fe[3]/HY and 3.5 wt.% for Fe[20]/HY. These results point out that iron oxide adsorbs chlorine in the form of iron chloride [11], although the characteristic peaks (2theta = 33 and 64\u00b0) of FeCl2 was not observed in the XRD pattern (Fig. 3(e)), likely due to the small amount of Cl contents on the surface of the catalysts. Lingaiah et al. [11] suggested that FeCl2 also has activity to remove chlorine compounds like iron oxide, based on their results. Therefore, it is found that Fe impregnated HY catalysts are effective to produce cracked liquid products with very low chlorine content (< 100 ppm) from chlorinated heavy wax.\nFig. 7\n presents the changes of liquid yield and carbon distribution in the liquid products during catalytic cracking of the chlorinated heavy wax on Fe[5]/HY catalyst at 450 \u2103 and WHSV = 1.2 h\u22121. The liquid yield was very low (37.1 wt.%) and gasoline fraction was relatively high (33.3 wt.%) for the first hour after reaching a steady-state of the reaction system, due to relatively low WHSV and active acid sites of Fe[5]/HY catalysts. While the liquid yield reached to about 60 wt.% with TOS, gasoline fraction was gradually reduced to 22.1 wt.%, but heavy fraction increased from 6.8 wt.% to 16.5 wt.%, indicating a gradual loss of the acid sites. In case of chlorine contents, 0.01 wt.% was detected in liquid samples at TOS = 2 h and 4 h, but chlorine did not detected in other samples. This means that catalytic cracking reaction related to acid sites is more sensitive to operation time than dechlorination over iron oxides under this reaction conditions.SEM-EDX analyses of the spent catalysts revealed coke deposits as well as chlorine adsorption on the surface of the catalysts, as shown in Fig. 6(b). The regeneration of the spent catalyst is one of the important factors to promote catalyst usability. Fig. 8\n presents the TGA/DTG thermal scan of the HY zeolite and Fe[20]/HY zeolite. The TGA curves show total mass loss of about 5.34 wt.% for HY and 6.78 wt.% for Fe[20]HY with two mass-loss peaks at around 72 \u2103 (removal of water) and around 470 \u2103 (removal of soft coke) [16]. Based on the TGA/DTG, the regeneration condition of the spent catalysts was set as 700 \u2103 and 4 h in air.\nFig. 9\n shows SEM-EDX data of the regenerated Fe[20]/HY catalyst: chlorine was completely removed and coke was almost fully removed. This demonstrates that thermal treatment in air is effective to remove adsorbed chlorine and deposited coke on the catalyst. When using the regenerated Fe[20]/HY for the catalytic cracking of chlorinated heavy wax, a similar trend to the case of the fresh Fe[20]/HY was found: 86.1 wt.% for liquid yield, 9.6 wt% for gas yield and 4.3 wt% for solid yield, and 16.7 wt.% for gasoline, 53.0 wt.% for kerosene/diesel, and 30.3 wt.% for the heavy fraction in the liquid product.Catalytic cracking and dechlorination of chlorinated heavy wax obtained from pyrolysis of RPF were investigated using an iron oxide impregnated HY zeolite. The heavy wax as a raw material had a very wide carbon distribution (1.54 wt.% of gasoline fraction (C5-C10), 37.91 wt.% of kerosene/diesel fraction (C11-C22), and 60.55 wt.% of heavy wax fraction (\u2265C23)) and contained 0.14 wt% of chlorine. It was found that the largest worthy liquid fraction (gasoline and kerosene/diesel, 66.9 wt%) was achieved when using Fe[3]/HY among the impregnated HY catalysts, which is a similar result to the parent HY zeolite (67.4 wt.%). However, the presence of iron oxide on the surface of HY resulted in lower chlorine content in the liquid product than the parent HY catalyst, due to chlorine adsorption of iron oxides with a form of iron chloride. This indicated that the Fe impregnated HY catalyst had a dual function of catalytic cracking of the HY zeolite and dechlorination of iron oxide. Excessive impregnation of Fe, Fe[20]/HY, showed the lowest cracking activity of heavy wax owing to the catalyst having lowest total acid sites. Significant amount of soft coke and chlorine were deposited on the spent catalysts, and they were restored by thermal treatment in air (700 \u2103). The restored catalytic activity was confirmed by using the regenerated Fe/HY catalyst for the cracking of chlorinated heavy wax.\nKyung-Ran Hwang: Conceptualization, Methodology, Experiment, Analysis, Investigation, Visualization, Writing - original draft, and Writing - review & editing. Sun-A Choi: Experiment, Formal analysis. Il-Ho Choi Experiment, Formal Analysis. Kyong-Hwan Lee: Conceptualization, Supervision, Project Administration and Writing - review & editing.Data are openly available in a repository that issues datasets with DOIs.The authors declare no conflict of interest.This work was conducted under the framework of the research and development program of the Korea Institute of Energy Research (C0-2427-03).", "descript": "\n                  Catalytic conversion of useless chlorinated heavy wax (chlorine, 0.14 wt%) obtained from pyrolysis of refuse plastic fuel was studied using iron oxide impregnated HY zeolite to produce a useful liquid product. It was found that the largest liquid fraction (gasoline and kerosene/ diesel, 66.9 wt.%) with very low chlorine content was achieved when using Fe[3]/HY among impregnated HY catalysts. This demonstrated that the Fe impregnated HY catalyst had a dual function of catalytic cracking of HY zeolite and dechlorination of iron oxide. Excessive impregnation of Fe, i.e., Fe[20]/HY, showed the least cracking activity of heavy wax owing to the catalyst having lowest total acid sites, but yielded liquid product with the lowest chlorine content (60 ppm) among the tested catalysts. The spent catalysts were deposited by a significant amount of soft coke and chlorine, and they were totally restored by thermal treatment in air (700 \u2103).\n               "}
{"full_text": "In a world governed by a constant increase in energy demand, there is a need for new, sustainable, and ecologically acceptable power sources. Given the rate of fossil fuel consumption, researchers predict that they will be almost completely depleted in approximately fifty to a hundred years [1]. One of the proposed alternatives to the current fossil fuel-based economy is the so-called hydrogen economy [2,3]. Briefly, this approach takes energy from a non-carbon source, which can be either renewable sources like solar and wind energy or nuclear power plants, converts it, and stores it in the form of hydrogen gas. This gas can then be transported and converted back into electric energy when needed.In the heart of the hydrogen economy is the unitized regenerative fuel cell (URFC). This device is capable of running in two modes. The first one, when there is an inflow of electric energy, consists of running in electrolysis mode and converting water to hydrogen and oxygen, which can then be stored in pressurized containers. The second mode is the fuel cell mode, in which the device runs as a standard fuel cell, consuming hydrogen and oxygen and producing electricity and water. Theoretically, such a device can run indefinitely long, producing zero pollutants since the only products (depending on running mode) are hydrogen, oxygen, and water.The main problem with operating such a device, which is common to most fuel cells, is the sluggish kinetics of the oxygen reduction reaction (ORR) [4]. In the case of URFCs, the criteria for ORR catalyst selection are even more demanding because a catalyst must sustain transition from one operating mode to another and be stable for catalyzing both ORR and oxygen evolution reaction (OER) for an extended period. Although some catalysts are traditionally considered the best ones for either ORR or OER, there are some difficulties in their application in URFCs technology. For instance, Pt is regarded as the best catalyst for ORR. However, the thermodynamic potential of Pt versus standard hydrogen electrode is 1.23\u00a0V, and at such a high potential, the metal surface undergoes oxidation. This practically means that the surface of Pt catalysts consists of a Pt and PtO mix, which effectively lowers the open circuit potential, depending on Pt to PtO ratio. On OER potential large portion of Pt surface is covered with PtO, which inhibits its activity toward OER catalysis.On the other hand, ruthenium (Ru) and iridium (Ir) are considered the most efficient OER catalysts. However, these catalysts\u2019 low natural abundance and high cost rule them out of commercial applications. These and some other issues have led researchers to synthesize and characterize new catalysts that can catalyze both ORR and OER and endure harsh conditions in URFCs.Recently, a number of different catalysts based on carbon-supported transition metal nanocomposites and metal-oxide-supported nanocomposites have been reported as substitutes to noble-metal electrocatalysts [5,6]. Carbon-supported materials are still highly investigated catalysts due to the high porosity of carbon support. However, the durability of carbon under OER polarization conditions remains a challenge due to high potentials and a highly oxidative environment, resulting in a change in the material\u2019s morphology, composition, and structure and irreversibly affecting its activity toward OER [7]. Nam et al. [8] reported ternary Ni46Co40Fe14 dispersed in carbon (C@NCF-900) as a highly efficient bifunctional electrocatalyst with half-wave potential, E1/2, of 0.93\u00a0V vs. RHE and low potential to achieve a current density of 10\u00a0mA\u00a0cm-2, E10, in OER operation mode (1.66\u00a0V vs. RHE). Fu et al. [9] reported N-doped carbon nanofibers decorated with NiCo as a bifunctional electrocatalyst superior to commercial Pt/C in terms of ORR catalysis and RuO2 in terms of OER activity. Hollow-structured carbon-supported NiCo2O4/C was further reported by Wang et al. [10] as an electrocatalyst of superior durability under both ORR and OER conditions compared to the commercial Pt/C in alkaline solution with a potential of 1.67\u00a0V required to achieve 10\u00a0mA\u00a0cm-2 in OER operation mode and a potential difference between E1/2 and E10, \u0394E, of 0.96\u00a0V, lower than that of commercial Pt/C (1.06\u00a0V).In this work, four different catalysts to be used in URFC technology were prepared. Catalysts were synthesized via simultaneous supercritical carbon dioxide deposition method, and their electrocatalytic ability was assessed. Introducing another metal along with Pt can influence the electronic structure of Pt by changing interatomic distance in Pt-Pt bonds, resulting in a catalyst of superior activity and stability for the two investigated reactions. Moreover, lowering the Pt content in the catalyst also lowers its price. Graphene nanoplatelets (GNPs) were used as support to increase the surface area and improve catalyst/support durability. Grafting metal nanoparticles onto GNPs provides a structure with uniformly distributed, abundant active sites, decreasing the electrical contact resistance between neighboring metal nanoparticles, thus delivering good electrical contact and fast transport of reactants and their plentiful incidence with active catalytic sites. The unique structure of graphene may improve electrode durability by strengthening the interaction between the catalyst particles and the graphene support. Additionally, the degree of graphitization also plays a significant role in the oxidation resistance of carbon-based materials. The higher graphitic content has also been linked to a stronger interaction between metal and carbon support. An increase in the degree of graphitization results in stronger \u03c0-sites (sp2 hybridized carbon) on the support (which are the anchoring sites for the electrocatalyst), strengthening the metal-support interaction.Nickel (II) hexafluoroacetylacetonate hydrate (C10H2F12NiO4.xH2O, MW: 472.79, 98% purity), copper (II) hexafluoroacetylacetonate hydrate (C10H2CuF12O4.xH2O, MW: 477.65) and iron (III) tris (2,2,6,6-tetramethyl-3,5-heptanedionate) (C33H57FeO6, MW: 605.65) supplied by Sigma-Aldrich, were used as metal precursors. 1,5-dimethyl platinum cyclooctadiene (Me2PtCOD) supplied by Strem Chemicals, was used as Pt precursor. GNPs (759\u00a0m2 g-1 area) used as support material was purchased from XG Science (xGnP\u00ae Grade C). The CO2 and N2 gases were purchased from Haba\u015f.GNPs-supported Pt-M (M= Ni, Fe, Cu) catalysts were synthesized simultaneously through supercritical carbon dioxide (scCO2) deposition technique, \nFig. 1.Briefly, 0.1\u2009g of GNPs was placed in a pouch made of filter paper and placed in a custom-made high-pressure stainless-steel vessel having sapphire windows along with a stirring bar. For the desired metal loading onto GNPs support, the amounts of the corresponding precursors were determined by using the adsorption isotherm of the precursor onto the GNPs [11]. Next, a predetermined amount of organometallic Pt and other metal precursors were added, and the vessel was closed. To provide a supercritical environment, the vessel was heated to 333\u2009K and CO2 gas was introduced into the vessel up to 24\u2009MPa with a syringe pump. These conditions were maintained for a period of 24\u2009h to ensure that the system reached equilibrium. At the end of the 24th hour, the vessel was depressurized, and the pouch was removed. Subsequently, the resultant material was placed in a tube furnace to convert metal precursors to their metal forms. The conversion was carried out thermally at 400\u2009\u00b0C under N2 flow for 4\u2009h. Consequently, GNPs supported Pt-M catalysts were obtained [11,12].The metal loading over GNPs was analyzed by inductively coupled plasma mass spectrometry (ICP-MS) technique using an Agilent 7800 ICP mass spectrometer. The morphology and elemental distribution of the catalysts were characterized with transmission electron microscopy (TEM, Hitachi HighTech HT7700) and energy-dispersive X-ray spectroscopy (EDX). Catalyst compositions and surface oxidation states were characterized by X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific K\u03b1 X-ray photoelectron spectrometer.Electrochemical investigation of the synthesized catalysts was performed using Gamry Interface 1010 galvanostat/potentiostat in a standard three-electrode electrochemical cell connected to Gamry rotator (Gamry RDE710 rotating electrode) and with Pt mesh and saturated calomel electrode as counter and reference electrode, respectively. For easier comparison of results, all potentials are expressed versus the reversible hydrogen electrode (RHE). Catalytic inks for working electrodes preparation were made using Nafion as a binder [13]. High purity gases (O2 or N2, Messer, 99.9995\u2009vol%) were bubbled into the 0.1\u2009M KOH electrolyte/electrochemical cell to control the atmosphere.ORR and OER stability tests were performed in the chronoamperometry mode. OER tests were performed in Arbin Instruments equipment by applying a 30-second OER pulse (potential of 1.7\u2009V) followed by a 120-second pulse at the ORR potential. In this way, O2 formed during OER pulse was reduced during the ORR mode, thus preventing material peeling from the substrate due to the intense bubble formation and, at the same time, switching between the two modes of URFCs was simulated.\n\nFig. 2 shows the metal concentrations of the prepared catalysts determined by ICP-MS. It can be seen that the second metal loading is much lower compared to the Pt loading in all catalysts. This may be explained by the solubility of metallic precursors in the supercritical medium. The Pt loading in the plain Pt/GNPs catalyst was 15.8\u2009wt%. The addition of the second metal resulted in an increase in the total metal loading, as well as in the Pt loading, either in sequential or simultaneous supercritical carbon dioxide deposition of the metals [11,12]. Additionally, other factors such as the relative size of the metal ion and the ligand, the number of ligands attached to the metallic-ion, CO2-philic tendency, and the polar nature of the precursor in scCO2 can influence the relative compositions of the metallic phase yield in PtM/GNPs [14].To investigate the distribution of the different elements in the catalysts, TEM, EDX, and elemental mapping measurements were performed, and the results were presented in \nFig. 3. The TEM images indicate that the uniform and well-dispersed metal nanoparticles with a size range of 2\u20133\u2009nm are tightly anchored onto the GNPs support. The elemental mapping results reveal that the metal distribution on the GNPs is consistent with the loading amount of the catalysts. As expected, the corresponding EDX spectra show the C, Pt, Ni, Fe, and Cu signals from the respective catalyst. The observed dominant peak at around 8\u2009keV for PtNi/GNPs and PtFe/GNPs catalysts can be attributed to the Cu TEM grid.To examine the surface nature of the synthesized catalysts, XPS measurements were performed. \nFig. 4a shows the XPS survey spectra of the catalysts. The observed peaks were found to be compatible with the prepared catalyst structure and literature reports [15\u201317]. Additionally, the presence of these peaks in the structure suggests that the GNPs supported Pt-M catalysts were successfully prepared with scCO2 deposition technique. Fig. 4b shows the Pt 4\u2009f XPS spectra of the prepared catalysts. In Pt/GNPs catalyst, the spectra comprised two peaks allocated to the Pt 4\u2009f7/2 and Pt 4\u2009f5/2 of metallic Pt, respectively. This result illustrates that the organometallic precursor was efficiently reduced to its metallic form. In comparison with Pt/GNPs catalyst, the Pt 4\u2009f core level peaks of the PtNi/GNPs and PtCu/GNPs bimetallic catalysts slightly shifted to lower binding energies, remaining the same for the PtFe/GNPs catalyst, which has the lowest second metal content. The observed shifting could be attributed to the electron interaction between Pt and secondary metal atoms (Ni, Cu). Changes in the d-band center are believed to accompany similar variations in the surface of core-level shifts in the same direction [18]. Thus, the negative shift in the binding energy of the Pt 4\u2009f core level reflects the downshift of its d-band center relative to the Fermi level. The binding energy strongly influences the adsorption/desorption capability of reaction species on the catalyst surface. According to the Hammer and Norskov model [19], it can be said that the downshifted d-band center in the catalysts would weaken the chemical adsorption strength of oxygenated intermediates such as OHads in alkaline solution. This would cause Pt-OHads bonds to break easily, leading to more electrochemically active sites available for the oxygen reduction reaction (ORR). On the other hand, the downshift of the d-band center can also weaken the chemical adsorption strength of active oxygen, and block O-O band breaking, which deteriorates the ORR kinetics. Therefore, to maintain the balance between the two opposite effects, there should be an optimum Pt 4\u2009f binding energy corresponding to the best catalytic activity.In addition, high-resolution XPS scans of the catalysts were also carried out, as shown in \nFig. 5. In the high-resolution Ni 2p spectrum of PtNi/GNPs, displayed in Fig. 5a, the main peaks are observed at binding energies of 855.6\u2009eV for Ni 2p3/2 and 873.5\u2009eV for Ni 2p1/2; these are the characteristics of PtNi and two shake-up satellites [20]. The spectrum shows that Ni is in an oxidized state, corresponding to NiO and Ni(OH)2, and NiOOH, respectively. The major component is Ni(OH)2. These oxygen species (e.g., NiO, Ni(OH)2, NiOOH) present in the Pt-Ni catalysts represent surface and subsurface oxidative states and not crystalline oxidative states as no such oxide peaks are apparent in the XRD patterns of the catalysts [12]. Namely, XRD analysis of the samples synthesized following the same procedure has been previously performed [12]. XRD pattern of Pt/GNPs catalyst revealed distinct diffraction peaks corresponding to reflections from (111), (200), (220) and (311) planes of Pt in the face-centered cubic (fcc) crystal structure as well as a broad peak (at 2\u03b8 = 26\u00b0) corresponding to the (002) plane of the graphite-like crystalline structure of the GNPs. Diffraction peaks of the PtM/GNPs shifted to higher 2\u03b8 values relative to Pt/GNPs, evidencing the successful synthesis of PtM (M = Ni, Fe, Cu)/GNPs. Moreover, no other diffraction peaks were observed for any of the catalysts, confirming the formation of alloys of Pt and secondary metals.Furthermore, the obtained XPS results are comparable with the results of PtNi alloy-graphene catalyst, as discussed previously by Li et al. [21]. In the high-resolution Fe 2p spectrum of the PtFe/GNPs, the peaks centered at around 711 and 725\u2009eV can be assigned to the Fe 2p3/2 and Fe 2p1/2, respectively [22]. The Fe 2p region is deconvoluted to resolve each component. This confirmed the existence of zero-valent Fe0, oxidized Fe2+, and Fe3+ with a broad satellite peak. Similarly, in the case of PtCu/GNPs, two kinds of Cu species are observed in a close view of the Cu 2p spectrum, at ca. 932.1 and 952.2\u2009eV, attributed to Cu 2p3/2 and Cu 2p1/2, respectively [23]. After deconvolution, the spectrum shows that although most Cu is in the form of metallic Cu (Cu0, 932.1 eV), a signal from Cu2+ (934.3 and 954.3\u2009eV) also exists. The presence of Cu2+, further confirmed by a satellite peak at 943.7 and 940.1\u2009eV, can be ascribed to the easy oxidation of surface Cu atoms in the air [24].To investigate double-layer capacitance (Cdl), cyclic voltammograms (CVs) were recorded in the 200\u2009mV range around the open circuit potential (OCP), \nFig. 6. Capacitance was then determined as slope of \u0394j =\u2009f(\u03bd) plot, where \u0394j =\u2009ja - jc (mA cm-2) and \u03bd is electrode polarization rate (mV s-1), Fig. 6\ninset. Pt/GNPs sample showed the highest value of Cdl (8.2 mF cm-2) among the studied samples. PtFe/GNPs showed the highest Cdl value (7.2 mF cm-2) among the bimetallic Pt-M samples, comparable to that of Pt/GNPs. Samples containing PtNi and PtCu showed somewhat lower values of double-layer capacitance, i.e., 4.5 and 5.1 mF cm-2, respectively. Slight distortion of CVs with the increase in the polarization rate is observed for PtNi/GNPs and PtCu/GNPs, indicating the existence of ohmic resistance in parallel with Cdl\n[25]. It is worth mentioning that all catalysts tested in this work showed a higher Cdl value than commercial Pt/C (40\u2009wt% Pt) catalyst (3.1 mF cm-2) tested under the same condition [13], with these higher values of Cdl indicating larger active surface of the synthesized catalysts than that of commercial Pt/C catalyst.Activity toward ORR was investigated by performing a series of linear scan voltammetry (LSV) experiments in the ORR potential region using different rotation rates, \nFig. 7. It can be seen that all tested catalysts show relatively high diffusion-limited current densities (j\n\nd\n), which are comparable to that of commercial Pt/C (40\u2009wt% Pt) catalyst (\u22126.43\u2009mA\u2009cm-2 at 1800\u2009rpm) [13]. jd of \u2212\u20094.65\u2009mA\u2009cm-2 and \u2212\u20094.37\u2009mA\u2009cm-2 was reached during ORR at PtFe/GNPs and PtCu/GNPs, respectively, while a slightly lower value was observed for PtNi/GNPs sample (\u22123.65\u2009mA\u2009cm-2). jd value of \u2212\u20093.71\u2009mA\u2009cm-2 was reached during ORR at Pt/GNPs reference sample.Tafel slope, representing the reaction\u2019s sensitivity toward applied potential, was calculated from LSV plots at 1800\u2009rpm, \nFig. 8a and \nTable 1\n. Tafel analysis was performed in the current range from the onset potential, Eonset, to ca. 70\u201380% of the limiting current density. Pt/GNPs and PtFe/GNPs showed two values of Tafel slope each: 65\u2009mV dec-1 and 102\u2009mV dec-1 for Pt/GNPs vs. 81\u2009mV dec-1 and 66\u2009mV dec-1 for PtFe/GNPs, depending on the potential range. Dual values of the Tafel slope indicate a change in the ORR mechanism at these catalysts with a change in potential. The other two samples showed only one value of Tafel slope: 102\u2009mV dec-1 in the case of PtNi/GNPs and 67\u2009mV dec-1 in the case of PtCu/GNPs.Half-wave potential, E1/2, was also determined from the LSV studies. Combining it with the potential at which OER reaction achieves a current density of 10\u2009mA\u2009cm-2, we were able to calculate the difference between these two potentials, \u0394E, a key parameter in benchmarking catalysts for URFCs application (see below) [26]. As expected, the catalyst containing just Pt showed the highest value of E1/2 (0.98\u2009V), followed by PtNi and PtFe catalysts (0.94\u2009V and 0.91\u2009V, respectively). The lowest value of E1/2 was observed for PtCu/GNPs catalyst (0.90\u2009V). These values are comparable/somewhat higher than that reported in the literature (Table 1), indicating somewhat higher ORR activity of the herein studied catalysts.Koutecky-Levich analysis was used for assessing the number of exchanged electrons during ORR. Constructed Koutecky-Levich plots (Fig. 8b) represented straight lines that indicate a first-order reaction. The highest value of n was calculated for PtNi/GNPs sample (n\u2009=\u20093.93), indicating that four electrons are involved during the O2 reduction at this catalyst. Values calculated for the rest of the catalysts are between 3.61 and 3.77 electrons, Table 1. All calculated n values are comparable with that for ORR at commercial Pt/C (40\u2009wt% Pt) catalyst (3.97) [13] and with the n values found in the literature for ORR, Table 1.Although there is still a big debate about the precise mechanism of ORR, it is widely accepted that in alkaline media it can occur by either four- or two-electron mechanism. The first one concerns bidentate molecule adsorption (adsorption of two O atoms) or a direct four-electron pathway to generate OH- (Eq. 1) [7]:\n\n(1)\n\n\n\n\nO\n\n\n2\n\n\n+\n2\n\n\nH\n\n\n2\n\n\nO\n+\n4\n\n\ne\n\n\n\u2212\n\n\n\u2192\n4\nO\n\n\nH\n\n\n\u2212\n\n\n\n\n\n\n\n\n(2)\n\n\n\n\nO\n\n\n2\n\n\n+\n\n\nH\n\n\n2\n\n\nO\n+\n2\n\n\ne\n\n\n\u2212\n\n\n\u2192\nH\n\n\nO\n\n\n2\n\n\n\u2212\n\n\n+\nO\n\n\nH\n\n\n\u2212\n\n\n\n\n\n\n\n\n(3)\n\n\nH\n\n\nO\n\n\n2\n\n\n\u2212\n\n\n+\n\n\nH\n\n\n2\n\n\nO\n+\n2\n\n\ne\n\n\n\u2212\n\n\n\u2192\n3\nO\n\n\nH\n\n\n\u2212\n\n\n\n\n\n\n\n\n(4)\n\n\n2\nH\n\n\nO\n\n\n2\n\n\n\u2212\n\n\n\u2192\n2\nO\n\n\nH\n\n\n\u2212\n\n\n+\n\n\nO\n\n\n2\n\n\n\n\n\n\n\nEqs. (2\u20134) correspond to the so-called two-electron pathway in which O2 is first converted to HO2\n- and further generates OH- in alkaline electrolytes. However, the reactivity of each of these steps is strongly dependent on the O2 adsorption energy, dissociation energy of O-O bond, and on the binding of OH- to surface.Thus, the superior activity of PtFe/GNPs toward ORR may result from its lower Gibbs free energy of oxygen adsorption determined by the geometric and electronic effect [35]. Fe is believed to represent active adsorption sites that \u201ccapture\u201d oxygen and increase adsorption capacity towards oxygen, so the presence of copious Fe sites on the catalyst\u2019s surface can reduce the oxygen adsorption Gibbs energy in the first step of the ORR. The interaction between Fe and Pt can further impact ORR activity by affecting the catalyst\u2019s structure, including the Pt-Pt bond distance [35]. Namely, the addition of Fe causes the lattice contraction decreasing the interatomic distance that influences the adsorption and transfer of oxygen-containing species in ORR. Lattice distortion further affects the overlap of orbital, modifying the electronic properties on the active site and thus affecting the surface reactivity and the catalyst's performance.Stability tests for all catalysts were performed in chronoamperometry mode with constant O2 bubbling for 4\u2009h, \nFig. 9. Relatively good stability in the ORR operation mode was observed for all tested catalysts with no significant drop of current densities with time. Steady current densities indicate a stable performance of the synthesized catalysts in long-term exploitation in ORR operating mode.Similar to ORR, OER is also a multi-electron transfer process, which in alkaline media could be represented by Eqs. 6 \u2013 10,\n\n(6)\n\n\nM\n+\nO\n\n\nH\n\n\n\u2212\n\n\n\u2192\nMOH\n+\n\n\ne\n\n\n\u2212\n\n\n\n\n\n\n\n\n(7)\n\n\nMOH\n+\nO\n\n\nH\n\n\n\u2212\n\n\n\u2192\nMO\n+\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n(\nl\n)\n\n\n+\n\n\ne\n\n\n\u2212\n\n\n\n\n\n\n\n\n(8)\n\n\n2\nMO\n\u2192\n2\nM\n+\n\n\nO\n\n\n2\n(\ng\n)\n\n\n\n\n\n\n\n\n(9)\n\n\nMO\n+\nO\n\n\nH\n\n\n\u2212\n\n\n\u2192\nMOOH\n+\n\n\ne\n\n\n\u2212\n\n\n\n\n\n\n\n\n(10)\n\n\nMOOH\n+\nO\n\n\nH\n\n\n\u2212\n\n\n\u2192\nM\n+\n\n\nO\n\n\n2\n\n\n\ng\n\n\n\n\n\n+\n\n\nH\n\n\n2\n\n\n\n\nO\n\n\n(\nl\n)\n\n\n+\n\n\ne\n\n\n\u2212\n\n\n\n\n\nwhere M is the catalyst\u2019s active site, and MOH, MO, and MOOH are the reaction intermediates. The reactivity of each step is again highly dependent on the adsorption energy of oxygen species. It is often reported that the formation of MO from MOH or the evolution of MOOH from MO are the rate-determining steps (RDS) for OER [7].OER kinetics at four studied catalysts was investigated using LSV experiments at 1200\u2009rpm, \nFig. 10a. Key parameters as exchange current density, j\n\n0\n, overpotential for reaching benchmark current density of 10\u2009mA\u2009cm-2, \u03b7\n\n10\n, the current density at an overpotential of 400\u2009mV, j\n\n400\n, and Tafel slope were determined to further characterize the investigated catalysts.One can see that the best performing catalyst in terms of the reached current densities at a given potential is PtFe/GNPs, followed by PtNi/GNPs and PtCu/GNPs catalysts. The lowest current density was achieved with Pt/GNPs catalyst, concluding that Fe, Ni, and Cu are indeed active sites for OER. Oxide film forming under OER polarization conditions most likely occurs on Pt as indicated by Damjanovic et al. [36], leading to the degradation of Pt-based catalysts performance and a low value of reached current density. Namely, contrary to the ORR, the OER occurs on the oxidized surface being constrained either by the strong adsorption of OOHads or by the weak adsorption of Oads\n[37]. The optimal performance is achieved for metal oxides with a moderate binding energy of the OER intermediates. This corresponds to the top of the volcano plot constructed by plotting the overpotential to reach 1\u2009mA\u2009cm-2 as a function of the difference in the free binding energy between O*\u2009and OH. One can observe that, for example, NiO is located closer to the optimal value when compared to PtO2.Another important difference between ORR and OER is that during the ORR only the outer surface atoms are active once the reaction comes to be diffusion-controlled with increasing overpotential [37]. Opposite, reaction sites in the \u201cinner\u201d surface turn to be active with increasing OER overpotential, i.e., the reaction proceeds within the inner catalysts layers as well. Therefore, the porosity of a catalyst plays a crucial role in its performance during OER. N2-sorption analysis carried out in our previous study [12], revealed the highest electrochemical active surface area (ECSA) of 136\u2009m2 gPt\n-1, in case of PtFe/GNPs, followed by PtNi/GNPs (132\u2009m2 gPt\n-1) and PtCu/GNPs (122\u2009m2 gPt\n-1). Notably, lower ECSA ranging between 43.6 and \u221287.2\u2009m2 gPt\n-1 were determined for Pt/GNPs catalysts for different metal loading.The present study aimed to develop bifunctional oxygen catalysts that are vital for the operation of unitized regenerative fuel cells and rechargeable metal-air batteries. These are often obtained by combining highly active ORR catalyst (typically Pt) with highly active OER catalyst (typically RuO2 and IrO2). Within the present study, Pt was combined with low-cost transition metals instead of expensive RuO2 or IrO2.For further comparison of the catalysts\u2019 OER performance, Tafel slope values were evaluated and found to be relatively high, varying from 280\u2009mV dec-1 for PtFe/GNPs to 490\u2009mV dec-1 for PtCu/GNPs (Fig. 10b), suggesting impediment of electron transfer within the studied catalysts during OER.In terms of other calculated OER parameters (\nTable 2), results are in good agreement with the initial observations based on OER LSV experiments. Namely, the PtFe/GNPs catalyst, which achieved the highest current density in the investigated potential region, also showed the lowest overpotential to reach a current density of 10\u2009mA\u2009cm-2 (0.572\u2009V), followed by PtNi/GNPs (0.652\u2009V). Higher values of this parameter were calculated for Pt/GNPs and PtCu/GNPs with \u03b710 of 0.765 and 0.660\u2009V, respectively. The highest current at overpotential of 400\u2009mV was achieved with PtCu/GNPs (3.27\u2009mA\u2009cm-2), followed by PtFe/GNPs (2.89\u2009mA\u2009cm-2) and PtNi/GNPs (2.64\u2009mA\u2009cm-2) catalysts. As anticipated from the initial LSV results, and due to the mentioned oxide film forming, the lowest value of j400 was achieved with the control Pt/GNPs catalyst, which only has Pt as an active site for OER.After determining the potential at the current density of 10\u2009mA\u2009cm-2, we were able to calculate \u0394E for each catalyst. It was found that this parameter has the lowest value of 0.890\u2009V for PtFe/GNPs catalyst. Although all of the studied catalysts show mildly higher overpotential to reach benchmark current density of 10\u2009mA\u2009cm-2 (Table 2), \u0394E values are comparable or lower than some materials reported in the literature [10,27\u201330,33,34], indicating the promising performance of studied catalysts in URFC application.\nFig. 10\nc and Table 2 present the results of electrochemical impedance spectroscopy (EIS) experiments for the studied catalysts. A small difference in the electrolyte resistance, Rs was attributed to the small changes in cell geometry and distance between electrodes. Furthermore, PtFe/GNPs showed the lowest value of charge transfer resistance, Rct, of 53\u2009\u03a9 (Fig. 10c inset), followed by PtNi/GNPs catalyst with significantly higher Rct of approximately 450\u2009\u03a9. For Pt/GNPs and PtCu/GNPs catalysts, Rct was found to be even higher (ca. 480\u2009\u03a9 and 550\u2009\u03a9, respectively).OER stability test for all catalysts was performed for 10\u2009h by switching between ORR and OER polarization conditions. For clarity, only OER curves are shown in Fig. 10\nd. A significant initial drop in OER currents was observed for all studied catalysts within the first hour of the experiment. In the next nine hours, currents continued to drop, though slower. This suggests the relatively low stability of the studied catalysts for OER. It is worth mentioning that it is not necessary degradation of catalyst that led to the current drop in the stability test. The partial loss of catalytic ink from the electrode due to intense bubble formation in OER mode is one of the possible causes, suggesting further investigation to optimize ink preparation and composition to withstand the harsh conditions when switching between URFC\u2019s two operation modes.Pt and PtM (M = Ni, Fe, Cu) supported on GNPs were synthesized by simultaneous scCO2 deposition method. XPS confirmed the successful preparation of the catalysts via scCO2 deposition method by showing the presence of the basic elements (C 1\u2009s, O 1\u2009s, Pt 4\u2009f, and Pt 4d) in all samples with the signals of Ni 2p, Fe 2p, and Cu 2p in the PtNi/GNPs, PtFe/GNPs, and PtCu/GNPs catalysts, respectively. TEM analysis revealed the formation of metal nanoparticles of 2\u20133\u2009nm size uniformly distributed over GNPs. The catalysts were tested for ORR and OER as electrode reactions in URFCs. PtFe/GNPs showed promising performance for ORR regarding the high diffusion-limited current density, low Tafel slope, number of exchanged electrons close to 4, and high double-layer capacitance. The same material showed the best performance toward OER, evidenced by the highest current density and the lowest Tafel slope. Furthermore, this material performance was comparable to that of commercial Pt/C electrocatalyst containing double the amount of Pt.\nDu\u0161an Mladenovi\u0107: Investigation, Formal analysis, Visualisation, Writing \u2013 original draft. Elif Da\u015f: Investigation, Formal analysis, Writing \u2013 original draft. Diogo M.F. Santos: Visualisation, Writing \u2013 review & editing. Ay\u015fe Bayrak\u00e7eken Yurtcan: Conceptualisation, Investigation, Writing \u2013 original draft. \u0160\u0107epan Miljani\u0107: Conceptualisation, Supervision. Biljana \u0160ljuki\u0107: Conceptualisation, 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.The authors would like to thank the Ministry of Education, Science and Technological Development of Republic of Serbia (contract number: 451-03-68/2022-14/200146). The Portuguese Foundation for Science and Technology (FCT) is acknowledged for contract IST-ID/156/2018 (B. \u0160ljuki\u0107) and a research contract in the scope of programmatic funding UIDP/04540/2020 (D.M.F. Santos).", "descript": "\n                  Pt and Pt-M (M = Ni, Fe, Cu) nanoparticles supported on graphene nanoplatelets (GNPs) were synthesized by simultaneous supercritical carbon dioxide deposition method. Morphology analysis by TEM revealed the formation of metal nanoparticles of 2\u20133\u00a0nm size uniformly distributed over GNPs, while XPS was used to determine their oxidation states. Four materials were tested as electrocatalysts for ORR and OER in unitized regenerative fuel cells and rechargeable metal-air batteries. PtFe/GNPs exhibited favorable ORR kinetics in terms of the highest diffusion-limited current density, the lowest Tafel slope, and a high number of exchanged electrons (n\u00a0=\u00a03.66), which might be attributed to its high double-layer capacitance and, thus, high electrochemically active surface area. Furthermore, this material performance was comparable to that of commercial Pt/C electrocatalyst containing double the amount of Pt. The same material showed the best performance toward OER as evidenced by the highest current density, the lowest value of exchange current density, and overpotential to reach a current density of 10\u00a0mA\u00a0cm-2, as well as the lowest Tafel slope.\n               "}
{"full_text": "Developing new routes for the effective utilization of non-petroleum carbon resources (such as coal, biomass, natural gas, and waste) to produce clean fuels and value-added chemicals is of great interest due to the shortage of petroleum resources and the environmental problems [1]. Aromatics, acting as significant bulk chemicals, are widely applied for the production of solvents, medicines, dyes and polymers in the chemical industry [2]. Currently, aromatics are mainly obtained by the petroleum refining process, which is environmentally unfriendly and energy-intensive [3,4]. Catalytic conversion of syngas (a mixture of CO/H2) derived from aforementioned non-petroleum resources into aromatics has attracted extensive attention because of its prominent function in sustainable development [5]. By means of the well-known Fischer\u2013Tropsch synthesis (FTS), various hydrocarbons can be produced from syngas, but almost no aromatics can be obtained due to the limitation of Anderson-Schulz-Flory distribution (ASF) [6,7]. The composite catalysts which integrating Fe-based FTS catalyst with H-ZSM-5 zeolite (such as Fe\u2013Pd/H-ZSM-5 [8], Fe\u2013MnO/GaZSM-5 [9], Na\u2013Zn\u2013Fe5C2/H-ZSM-5 [10], and Fe3O4@MnO2/H-ZSM-5 [11]) is a useful means to optimize the aromatization performance. However, their aromatics selectivity is still unsatisfactory.Recently, an effective oxide-zeolite (OX\u2212ZEO) bifunctional composite catalysts design strategy has been put forward to directly synthesize aromatics from syngas (STA reaction), among which CO and H2 are activated into C1 oxygenated intermediates over metal oxides, while the zeolite is mainly responsible for C\u2013C coupling [12\u201319]. For example, Wang et\u00a0al. [12]. Reported a composite catalyst combining Zn\u2013ZrO2 oxide with H-ZSM-5 zeolite could obtain 80% aromatics selectivity at 20% CO conversion. Bao et\u00a0al. [13]. Presented that a composite catalyst integrating ZnCrOx oxide with H-ZSM-5 zeolite could achieve about 73.9% aromatics selectivity at 16% CO conversion. Nevertheless, the catalytic performance still needs to be further improved to meet the demands of industrial production, which will depend on the in-depth comprehending of the structure\u2013performance relationship.Previous studies have pointed out that the structure (such as crystal size) of the H-ZSM-5 zeolite in composite catalysts has a great influence on the catalytic performance for STA reaction [19,20]. Wei et\u00a0al. [19] reported that the H-ZSM-5 with short size along the b-axis presented low molecular-diffusion resistance, leading to high selectivity of tetramethylbenzene. Xie et\u00a0al. [20] observed that the light aromatics selectivity is closely related to the crystal size of H-ZSM-5, which could be enhanced by increasing the size of the b-axis. In comparison, there is little knowledge about the influence of the oxide structure in composite catalysts for STA reaction. Generally, the oxide with a smaller size can result in more defect sites (oxygen vacancies) [17,21], and it has been reported that oxygen vacancies over metal oxide surfaces are conducive to CO hydrogenation activation [1,16,22]. Therefore, we wondered whether oxide with a smaller size will have a beneficial effect on the catalytic performance. However, little in-depth research about the size effect of oxide component in composite catalysts for STA reaction has been reported.In this work, we investigate the size effect by selecting ZnCr2O4 spinel oxide, which is often used for syngas conversion, as a probe oxide, mixing with H-ZSM-5 zeolite as a composite catalyst for STA reaction. To achieve this goal, a series of ZnCr2O4 spinel oxides with distinct size were synthesized, and we compared the catalytic performance of this series of ZnCr2O4&H-ZSM-5 composite catalysts as a function of size of ZnCr2O4. We further reveal the intrinsic reason of the size effect by in situ DRIFTS characterizations. Moreover, by decreasing the crystal size of ZnCr2O4 oxide, the space-time yield (STY) of aromatics can reach as high as 4.79\u00a0mmol gcat\n\u22121\u00a0h\u22121, which outperforms the previously reported some typical composite catalysts (Fig.\u00a0S1 and Table S1).A series of ZnCr2O4 spinel oxides with diverse sizes were synthesized by employing a conventional co-precipitation method. Typically, 48\u00a0g of Cr(NO3)3\u00b79H2O and 17.85\u00a0g of Zn(NO3)2\u00b76H2O were dissolved in 100\u00a0mL of deionized water to make a salt solution, and 47.10\u00a0g of ammonium carbonate (NH4)2CO3 was added in 100\u00a0mL of deionized water to prepare a precipitant solution. After that, two peristaltic pumps were used to simultaneously inject the above two solutions into a beaker at 70\u00a0\u00b0C under persistent stirring to form a precipitate. Meanwhile, the pH value of the blended solution was maintained at around 7.0 by adjusting the flow rate of the two peristaltic pumps. After precipitation, the synthesized mixture was further aged for 5\u00a0h at the same temperature. Then the suspension was filtered and washed with deionized water several times to obtain a filter cake, which followed by drying overnight at 120\u00a0\u00b0C and finally calcined in air with a heating rate of 2\u00a0\u00b0C min\u22121 for 6\u00a0h. The resulting powder oxides with different sizes were denoted as ZnCr2O4-T, where T representing the calcination temperature, which varied from 400 to 700\u00a0\u00b0C. For example, ZnCr2O4-400, ZnCr2O4-500, ZnCr2O4-600 and ZnCr2O4-700 represent samples calcined at 400, 500, 600 and 700\u00a0\u00b0C, respectively.H-ZSM-5 zeolite with a Si/Al ratio of 96 used in this work was the same as our previous study [17,23]. The detailed physical properties of this H-ZSM-5 zeolite are exhibited in Fig.\u00a0S2 and Table S2, which embraces a typical nano-sized MFI structure. All the composite catalysts were acquired by physical mixing of the two components (oxides and zeolites) with a mass ratio of 3:1. For example, for the preparation of the ZnCr2O4-400&H-ZSM-5 composite catalyst, ZnCr2O4-400 oxide and H-ZSM-5 zeolite were firstly placed in an agate mortar and triturated together into powder, then followed by pressing under 30\u00a0MPa, crushed, and finally screened to granules between 40 and 60 mesh sizes (0.3\u20130.45\u00a0mm).The powder X-ray diffraction (XRD) data were obtained on a PANalytical X'Pert PRO X-ray diffractometer equipped with a Cu K\u03b1 radiation source radiation (\u03bb\u00a0=\u00a01.5406\u00a0\u00c5). XRD patterns were measured in the 2 theta range of 5-90\u00b0. The average crystal size of ZnCr2O4 spinel oxides was calculated according to the Scherrer equation by selecting the two representative diffraction peaks at 30.3\u00b0 and 35.8\u00b0, which correspond to the (022) and (113) lattice planes of spinel cubic ZnCr2O4, respectively.The chemical composition was determined on a Philips Magix-601 X-ray fluorescence (XRF) spectrometer. The micropore volume, BET specific surface areas and average pore width were recorded by N2 adsorption\u2013desorption at \u2212196\u00a0\u00b0C using a Micromeritics ASAP 2020 system, the samples were pre-degassed in vacuum at 120\u00a0\u00b0C for 24\u00a0h before measurement. The morphology of catalysts was observed by utilizing a Hitachi SU8020 field-emission scanning electron microscope (FE-SEM). A JEM-2100F microscope was used to acquire the transmission electron microscopy images (TEM) and high-resolution transmission electron microscopy images (HRTEM). The average particle size of each sample was estimated by counting at least 50 particles. The X-ray photoelectron spectroscopy (XPS) was conducted on a Thermofisher Excalab X+ spectrometer with monochromatized Al K\u03b1 as the exciting radiation, the binding energy of all the data was calibrated by utilizing the C 1s of 284.8\u00a0eV as the reference. The CO-temperature programmed desorption profiles (CO-TPD) were carried out by using a Micromeritics AutoChem II 2920 analyzer equipped with a TCD, helium and 10% CO\u2013He were used for reference and adsorption, respectively. The H2-temperature programmed reduction profiles (H2-TPR) were performed on a Micromeritics AutoChem II 2920 analyzer equipped with a TCD, argon and 10% H2\u2013Ar were used for reference and reduction, respectively. The NH3-temperature programmed desorption profiles (NH3-TPD) were tested by a Micromeritics AutoChem II 2920 equipped with a TCD detector.The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were performed on a Bruker Tensor 27 instrument equipped with a MCT detector to detect the change of intensity of surface intermediate species. Typically, a diffuse reflectance infrared cell with a ZnSe window was loaded with 50\u00a0mg of the sample, which was pretreated with 30\u00a0mL\u00a0min\u22121 of H2/N2 flow (H2/N2\u00a0=\u00a01/5) under 0.1\u00a0MPa at 310\u00a0\u00b0C and followed by sweeping with pure N2 at a flow rate of 30\u00a0mL\u00a0min\u22121. After that, the temperature was heated up to 390\u00a0\u00b0C and the background spectrum was recorded. Then 5\u00a0mL\u00a0min\u22121 of syngas (H2/CO/Ar\u00a0=\u00a047.5/47.5/5) was introduced into the infrared cell under 0.1\u00a0MPa at 390\u00a0\u00b0C and the in situ DRIFT spectra were acquired at 16 scans with a resolution of 4\u00a0cm\u22121.All the catalytic reactions were carried out in a high-pressure fixed-bed stainless steel tubular reactor (internal diameter of 8\u00a0mm). The reaction effluent products were heated to maintain in the gas phase and analyzed by an Agilent 7890B online gas chromatograph, which equipped with a thermal conductivity detector (TCD) connected to a TDX-1 packed column and a flame ionization detector (FID) connected to a PoraPLOT Q-HT capillary column. A concentration of 5% Argon contained in syngas was utilized as an internal standard gas. CO conversion and CO2 selectivity were calculated according to the following equation based on the carbon atoms number.\n\n(1)\n\n\n\nSel\n\n\nC\n\nO\n2\n\n\n\n\n=\n\n\nC\n\nO\n\n2\n\no\nu\nt\nl\ne\nt\n\n\n\n\n\nC\nO\n\n\n\ni\nn\nl\ne\nt\n\n\n\u2212\nC\nO\n\n\n\no\nu\nt\nl\ne\nt\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\nCO\n2outlet\n: moles of CO2 at the outlet\n\n(2)\n\n\n\nConv\u00a0\n\nC\nO\n\n\n=\n\n\n(\nC\nO\n\n\n\ni\nn\nl\ne\nt\n\n\n\u2212\nC\nO\n\n\n\no\nu\nt\nl\ne\nt\n\n\n)\n\n\nC\nO\n\n\n\ni\nn\nl\ne\nt\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\nCO\n\ninlet\n: moles of CO at the inlet CO outlet: moles of CO at the outlet.The selectivity of hydrocarbons (CnHm), MeOH and DME among the carbon products (excluding CO2) were calculated according to the total carbon atoms of the products detected by FID detector.\n\n\n\n\nSel\n\n\nC\nn\n\n\nH\nm\n\n\n\n\n=\n\n\nn\n\n\n\nC\nn\n\n\nH\nm\n\n\n\n\nt\no\nt\na\nl\n\nc\na\nr\nb\no\nn\n\na\nt\no\nm\ns\n\no\nf\n\np\nr\no\nd\nu\nc\nt\ns\n\nd\ne\nt\ne\nc\nt\ne\nd\n\nb\ny\n\nF\nI\nD\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n\n\nSel\n\nM\ne\nO\nH\n\n\n\n=\n\n\nn\n\nM\ne\nO\nH\n\n\n\nt\no\nt\na\nl\n\nc\na\nr\nb\no\nn\n\na\nt\no\nm\ns\n\no\nf\n\np\nr\no\nd\nu\nc\nt\ns\n\nd\ne\nt\ne\nc\nt\ne\nd\n\nb\ny\n\nF\nI\nD\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(3)\n\n\n\nSel\n\nD\nM\nE\n\n\n\n=\n\n\nn\n\nD\nM\nE\n\n\n\nt\no\nt\na\nl\n\nc\na\nr\nb\no\nn\n\na\nt\no\nm\ns\n\no\nf\n\np\nr\no\nd\nu\nc\nt\ns\n\nd\ne\nt\ne\nc\nt\ne\nd\n\nb\ny\n\nF\nI\nD\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\nn\nCnHm: carbon atoms number of CnHm\n\n\n\n\nn\nMeOH: carbon atoms number of MeOH\n\n\n\nn\nDME: carbon atoms number of DME\n\n\n\nn\nCnHm: carbon atoms number of CnHm\n\nn\nMeOH: carbon atoms number of MeOH\nn\nDME: carbon atoms number of DMEA series of ZnCr2O4 oxides were prepared by a co-precipitation method at different calcination temperature [24], which were named as ZnCr2O4-T, where T representing the calcination temperature (see Experimental section). The XRD patterns (Fig.\u00a01\na) show that all ZnCr2O4 oxides are of the typical cubic ZnCr2O4 spinel phase (PDF #98-009-5832), but as the calcination temperature decreases from 700\u00a0\u00b0C to 400\u00a0\u00b0C (from ZnCr2O4-700 to ZnCr2O4-400), the full width half maximum (FWHM) broadens from the XRD analysis (Fig.\u00a01a), suggesting that lowering the calcination temperature may lead to a decrease in the crystal size of ZnCr2O4 spinel oxides [25]. The calculation results from Scherrer equation further verify this speculation, the average crystal size of ZnCr2O4 spinel oxides changes dramatically from 32.1 to 7.9\u00a0nm (Table 1\n). The variation of the crystal size can attribute to the different crystallite growth rate in different calcination temperature [26,27].The N2 physical adsorption\u2013desorption experiments (Fig.\u00a01b) present that reducing the crystal size of ZnCr2O4 spinel oxides (from ZnCr2O4-700 to ZnCr2O4-400) can result in the formation of hysteresis loops, indicating the existence of mesopores, which may be caused by the stacking of oxides particles (Fig.\u00a0S3). In addition, it can be seen from Table 1 that ZnCr2O4-700 with a larger size exhibits a very low BET specific surface area (8.63\u00a0m2 g\u22121) and pore volumes (0.02\u00a0cm3\u00a0g\u22121). Surprisingly, reducing the crystal size of ZnCr2O4 spinel oxides (from ZnCr2O4-600 to ZnCr2O4-400) can greatly increase the BET specific surface areas (\u223c1.6\u201315 times) and pore volumes (\u223c1.6\u201320 times).The FE-SEM images (Fig.\u00a0S3) and TEM images (Fig.\u00a01c\u2013f and Fig.\u00a0S4) further provide convincing evidence that this series of ZnCr2O4 spinel oxides are consist of irregular nanoparticles with different particle sizes. The corresponding average particle sizes of ZnCr2O4 spinel oxides estimated by the TEM are 33.03\u00a0nm (ZnCr2O4-700), 16.63\u00a0nm (ZnCr2O4-600), 11.87\u00a0nm (ZnCr2O4-500) and 7.29\u00a0nm (ZnCr2O4-400), which is almost completely consistent with the average crystal size calculated by the Scherrer equation (Table 1). In addition, the lattice spacing of 0.208, 0.251, 0.295, and 0.480\u00a0nm in HRTEM images (Fig.\u00a0S5) can be ascribed to the (004)\u00a0(113)\u00a0(022), and (111) planes of ZnCr2O4 spinel phase (PDF #98-009-5832), respectively. This further indicates that this series of ZnCr2O4 oxides are of the typical cubic ZnCr2O4 spinel structure.Generally, the oxide with a smaller size and larger specific surface area can result in more surface defect sites (oxygen vacancies) [17,21], which are widely considered as active sites for CO hydrogenation activation [1,16,22,28,29]. To explore the concentration of oxygen vacancy over oxides, we performed X-ray photoelectron spectroscopy (XPS) measurements. As clearly depicted in Fig.\u00a02\na, two diverse signal peaks can be recognized from the O 1s XPS spectra of ZnCr2O4 oxides. One peak at a lower binding energy of 530.0\u00a0\u00b1\u00a00.3\u00a0eV can be regarded as the lattice oxygen atoms (Olattice) [30], while another peak situated at a higher binding energy of 531.0\u00a0\u00b1\u00a00.3\u00a0eV can usually be attributed to the oxygen atoms in the vicinity of the oxygen vacancy (Ovacancy) [29,31]. Obviously, the ratios of the Ovacancy peak for these oxides are distinct, indicating that their corresponding oxygen vacancy concentration could be quite disparate. On the basis of the calculated results from the deconvolution of the O1s XPS signal (Table 1 and Table S3) [32], the ZnCr2O4-400 exhibited the highest oxygen vacancy concentration, followed by ZnCr2O4-500, ZnCr2O4-600, and ZnCr2O4-700. It is not surprising that ZnCr2O4-400 owns more surface vacancies than the other three oxides due to its smaller size and larger specific surface area.Furthermore, we performed CO temperature-programmed desorption (CO-TPD) experiments to investigate the adsorption and desorption behaviors of CO on the ZnCr2O4 oxides. As shown in Fig.\u00a02b, the low-temperature peak (<300\u00a0\u00b0C) can be attributed to the weak adsorption of CO in the bulk phase, whereas the high-temperature peak (>300\u00a0\u00b0C) can be linked to the CO strongly absorbed at the oxygen vacancies [33\u201335]. The amount of CO desorption follows this order: ZnCr2O4-400\u00a0>\u00a0ZnCr2O4-500 > ZnCr2O4-600 > ZnCr2O4-700, indicating that the oxygen vacancy concentration also follows the same order, which is quite consistent with the XPS results (Fig.\u00a02a and Table 1). Moreover, it has been widely reported that stoichiometric ZnCr2O4 spinel (normal spinel) is hard to reduce [19,36]. However, the existence and increase of the oxygen vacancies could promote the reducibility of ZnCr2O4 spinel oxides [19,36]. Therefore, the sharp reduction peak in the temperature range of 200\u2013350\u00a0\u00b0C (Fig.\u00a02c) could be assigned to the ZnCr2O4 spinel oxides with oxygen vacancies [19,36]. Distinctly, the reduction peak intensity also follows this order: ZnCr2O4-400 > ZnCr2O4-500 > ZnCr2O4-600 > ZnCr2O4-700, which further demonstrates the difference of oxygen vacancy concentration (Table 1).The above characterization results demonstrate that we have successfully prepared a series of ZnCr2O4 spinel oxides with different size and surface defect sites. Then, we wondered whether these oxides with their unique structural properties would affect the activity and product selectivity for STA reaction. Therefore, ZnCr2O4 spinel oxides with distinct size were blended with the same H-ZSM-5 as composite catalysts for STA reaction at 390\u00a0\u00b0C, 3.0\u00a0MPa, oxides: H-ZSM-5\u00a0=\u00a03:1 (mass ratio) and GHSV\u00a0=\u00a01500\u00a0mL gcat\n\u22121 h\u22121. The results in Table 2\n and Fig.\u00a0S6 indicate that this series of ZnCr2O4&H-ZSM-5 composite catalysts exhibit obviously different catalytic performance. From ZnCr2O4-700&H-ZSM-5 to ZnCr2O4-400&H-ZSM-5, the CO conversion, aromatics selectivity and STY of aromatics are significantly promoted. Since the zeolite components for all the composite catalysts are completely consistent, it can be reasonably inferred that the difference in reaction results is mainly related to the difference in the structural properties of the ZnCr2O4 spinel oxides.Therefore, we compared the catalytic performance of this series of ZnCr2O4&H-ZSM-5 composite catalysts as a function of size of ZnCr2O4 spinel oxides. As presented in Fig.\u00a03\na, the CO conversion and aromatics selectivity are greatly affected by the crystal size of ZnCr2O4. For example, the CO conversion reaches as high as 32.6% for ZnCr2O4 with a crystal size of 7.9\u00a0nm, but it dramatically declines to 3.4% when it grows to 32.1\u00a0nm. Correspondingly, the aromatics selectivity exhibits a similar trend, i.e., it is 76% over the former ZnCr2O4 but decreases to 50.4% over the latter. Meanwhile, the crystal size of ZnCr2O4 also significantly affects the STY of aromatics (Fig.\u00a03b). It reaches as high as 4.40\u00a0mmol gcat\n\u22121 h\u22121 for 7.9\u00a0nm ZnCr2O4 crystals in contrast to only 0.31\u00a0mmol gcat\n\u22121 h\u22121 for those with a crystal size of 32.1\u00a0nm, enhancing by about 14.2 times. The same variation trend is also found for average particle size (Fig.\u00a0S7).Furthermore, we also noticed that the smaller the crystal size is, the higher oxygen vacancy concentration exists, which correlates monotonically with the CO conversion, aromatics selectivity and the STY of aromatics (Fig.\u00a03). Oxygen vacancies are widely considered as active sites for CO hydrogenation activation in syngas conversion [1,16,22,28,29], which can influence the formation of surface intermediate species over oxides. This could be the intrinsic reason for the size effect of oxides on the catalytic performance for STA reaction.Therefore, in order to obtain further insights into the size\u2013performance relationship, the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to monitor the evolution of surface intermediate species over different ZnCr2O4 oxides. As illustrated in Fig.\u00a04\na\u2013d, the adsorbed surface formate species (2957, 2870, 2743, 1578 and 1358\u00a0cm\u22121) [21,37,38] and carbonate/bicarbonate species (1492, 1380 and 1305\u00a0cm\u22121) [21,29,38] are clearly identified over all the ZnCr2O4 oxides after exposing to the syngas atmosphere under reaction condition, it has been widely reported that these active C1 oxygenated species are regarded as the crucial intermediate species. These active oxygenated intermediates can be further transformed to form MeOH, DME and olefins intermediates [21,23,37], or/and carbonyl compounds intermediates [38,39], which can be consumed by H-ZSM-5 (Fig.\u00a0S8) and continuously converted to generate aromatics in H-ZSM-5 finally [21,23,38,39]. Noticeably, when tracking the intensity variation of the IR signals for these surface intermediate species, it can be seen that the formation of these intermediate species over ZnCr2O4-400 is extremely faster and the corresponding intensity is also significantly stronger, followed by ZnCr2O4-500, ZnCr2O4-600 and ZnCr2O4-700 (Fig.\u00a04a\u2013d), which is well consistent with the above-mentioned catalytic performance (Table 2 and Fig.\u00a03).According to the results found above, we summarize the reasonable reason for the size effect of oxides on the catalytic performance for STA reaction: (1) The ZnCr2O4 oxides with smaller size and larger specific surface area can result in more surface defect sites (higher oxygen vacancy concentration), which are widely considered as active sites for CO hydrogenation activation in syngas conversion [1,16,22,28,29], thus resulting in higher CO conversion. (2) Syngas can be activated over oxygen vacancies of oxides to form C1 oxygenated intermediate species [21,27,37,38], which can be further transformed by H-ZSM-5 to produce aromatics [21,23,38]. ZnCr2O4 oxides with smaller size and higher oxygen vacancy concentration can lead to the rapid formation of more C1 oxygenated intermediate species, thus resulting in higher aromatics selectivity and higher STY of aromatics. The above results demonstrate that smaller ZnCr2O4 particles undoubtedly have a beneficial effect on the catalytic performance for STA reaction. Additionally, based on the understanding of the size\u2013performance relationship, ZnCr2O4-400 with a smaller size mixing with H-ZSM-5 can achieve as high as 4.79\u00a0mmol gcat\n\u22121\u00a0h\u22121 of aromatics STY, which outperforms the previously reported some typical catalysts (Fig.\u00a0S1 and Table S1) [12\u201319,40\u201344].In conclusion, we investigated the size effect of ZnCr2O4 spinel oxide in oxide-zeolite composite catalysts for syngas to aromatics (STA) reaction. The CO conversion, aromatics selectivity and space-time yield (STY) of aromatics are all significantly improved when the crystal size of ZnCr2O4 oxide decreases, which can mainly ascribe to the higher oxygen vacancy concentration and thus the rapid generation of more C1 oxygenated intermediate species. Based on the understanding of the size\u2013performance relationship, ZnCr2O4-400 with a smaller size mixing with H-ZSM-5 can achieve 32.6% CO conversion with 76% aromatics selectivity. The STY of aromatics reaches as high as 4.79\u00a0mmol gcat\n\u22121\u00a0h\u22121, which exceeds the previously reported some typical catalysts. These results demonstrate the importance of the oxide size in oxide-zeolite composite catalysts for STA reaction and may be helpful to design more efficient catalysts for conversion of syngas to aromatics.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 acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21978285, 21991093, 21991090), and the \u201cTransformational Technologies for Clean Energy and Demonstration\u201d, Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA21030100). We acknowledge Mr. Yijun Zheng and Mrs. Yanli He for their assistance in the structural characterization of catalysts.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.2021.07.003.", "descript": "\n                  Direct conversion of syngas to aromatics (STA) over oxide-zeolite composite catalysts is promising as an alternative method for aromatics production. However, the structural effect of the oxide component in composite catalysts is still ambiguous. Herein, we investigate the size effect by selecting ZnCr2O4 spinel, as a probe oxide, mixing with H-ZSM-5 zeolite as a composite catalyst for STA reaction. The CO conversion, aromatics selectivity and space-time yield (STY) of aromatics are all significantly improved with the crystal size of ZnCr2O4 oxide decreases, which can mainly attribute to the higher oxygen vacancy concentration and thus the rapid generation of more C1 oxygenated intermediate species. Based on the understanding of the size\u2013performance relationship, ZnCr2O4-400 with a smaller size mixing with H-ZSM-5 can achieve 32.6% CO conversion with 76% aromatics selectivity. The STY of aromatics reaches as high as 4.79\u00a0mmol gcat\n                     \u22121\u00a0h\u22121, which outperforms the previously reported some typical catalysts. This study elucidates the importance of regulating the size of oxide to design more efficient oxide-zeolite composite catalysts for conversion of syngas to value-added chemicals.\n               "}
{"full_text": "Substituted thiocarbamides and their transition metal complexes are one such group of compounds that have promise anticancer activity (Pandey et al., 2019; Pandey et al., 2018). These compounds are more effective anticancer agents, most likely due to the presence of intramolecular hydrogen bonding in the structural framework, which increases lipophilicity and improves hydrogen bonding interactions with DNA (Mahendiran et al., 2018). The coexistence of hard nitrogen, oxygen, and soft sulphur donor atoms in the structural motif of substituted thiocarbamides (Pandey et al., 2019; Almalki et al., 2021; Al-Qahtani et al., 2021; Alkhamis et al., 2021; Abu-Dief et al., 2021) gives rise to structural variety in transition metal complexes. Moreover, ligands contains triazole moiety and their derivatives show an essential function as an antimicrobials (Emam et al., 2020; Gaber et al., 2020), anticancer (Gaber et al., 2020) and antitumoral (Matesanz et al., 2020). The significance of thiourea chemistry was improved by association with metal ions which strengthen antimicrobial activity (Maalik et al., 2019), antioxidant activity (Maalik et al., 2019; Rahman et al., 2020) and anticancer activity (Maalik et al., 2019; Abbas et al., 2020). Also, thiourea derivatives offers excessive promise as a metal transition sensor ionophore (Yahyazadeh and Ghasemi, 2013; Fakhar et al., 2016; Razak et al., 2020). Because of dyes' resistance to aerobic digestion and their stability toward heat, light, and oxidizing agents (Robinson et al., 2002; Han and Yun, 2007), many challenges arise when attempting to handle wastewater-containing dyes. Such dyes are toxic, carcinogenic, and mutagenic (Anliker, 1979; Chung and Stevens, 1993). Catalytic oxidation has recently been reported as a viable technique for curing colored water (Santos et al., 2009). Metal complexes are important cellular components that participate in a variety of biochemical processes in living organisms. Minerals have a variety of properties, such as reactivity to organic substrates, altered coordination patterns, and redox activity, to name a few. As a result, developing special coordination complexes, whether drugs, is regarded as a primary goal in the development of effective diagnostic tools (Hambley, 2009). Several bioactive minerals are being studied for their potential use in the development of new pharmaceuticals. Transition metals such as Co(II), Cr(III) and Zn(II) are required for many biological processes such as electron transfer and catalysis, and they are commonly found in enzyme or protein active sites (Thompson and Orvig, 2003).As a result of the extensive range of biological properties (Pandey et al., 2019; Pandey et al., 2018; Emam et al., 2020; Gaber et al., 2020; Matesanz et al., 2020) of thiocarbamides and triazole and derivatives, the preparation of the thiocarbamide derivative and its Cr(III), Ni(II) and Zn(II) chelate is recorded in the current research. According to the spectral and theoretical studies, various types of chelation have been suggested for metal complexes. Furthermore, the isolated Zn(II) complex's catalytic activity was reduced during the decomposition of organic Erichrome Black T (EBT) dye. Moreover, zeta potential estimation, molecular docking calculations, antimicrobial and antioxidant activities of the investigated compounds were evaluated.Solvents, metal(II) chlorides, benzoyl isothiocyanate and 1H-1,2,4-triazol-3-amine have been purchased from Sigma-Aldrich.PerkinElmer-2400 series-II analyzer have been used for partial elemental analysis. Ordinary methods (Jeffery et al., 1989) were being used to estimate the content of chloride and metal in investigated complexes. The Infra-red spectra were measured by KBr pellets via FT-IR spectrophotometer \u201cMattson 5000\u2033. The electronic spectra are analyzed by via UV / Vis Spectrophotometer (Unicam). The NMR spectra of 1H and 13C with Brucker 400\u00a0MHz were handled on ligand H2L that is identified in the solvent (DMSO). The photoluminescence spectra of investigated compounds were made in DMSO solution on excitation by using a LS50B PerkinElmer Fluorimeter. Jenway 4010- conductivity meter was used to assess the molar conductivity of prepared DMSO solution of metal complexes (0.001\u00a0mol/dm\u22123). The spectrum of X-ray diffraction was described in detail utilizing Cu, Wavelength 1.5406\u00a0\u00c5 source on diffractometer \u201dthe Bruker AXS Advance\u201c. Mass spectra were obtained with ionization mode (EI) in the range of m/z\u00a0=\u00a040\u20131000 with Varian Mat 311. The Gouy procedure for scientific magnetic susceptibility to Sherwood has been used to determine the effective magnetic moment \u00b5eff, at room temperature per metal atom. The zeta potential measurements for the H2L and its metal complexes in water were performed via Malvern Zeta-size Nano at 25\u00a0\u00b0C. The thermal analyzer TGA-50H was used for thermal evaluation (TGA / DTG) under measured conditions such as the temperature rises by rate equal to 10\u00a0\u00b0C/min.The H2L ligand derived from thiocarbamide moiety and metal chelates have been synthesized in accordance with the procedures described in Scheme 1\n. The products obtained were crystallised a number of times with absolute EtOH then with Et2O and then desiccated over anhydrous calcium chloride. TLC was performed to verify the purity of the H2L ligand. The physical and analytical results are summarized in Table 1\n.Unfortunately, we could not get single crystals from the investigated compounds, thus structure optimization data have been measured with DMOL3 application in the Materials Studio software (Delley, 2002). Optimized complex frameworks were estimated using the DFT (Modeling and Simulation Solutions for Chemicals and Materials Research and Studio, 2011) process. Computations of DFT semi-core pseudopods (dspp) have been generated via double number basis sets and functional polarisation (DNP) (Warren, 1986) that was much additional successful than the duplicate gaussian basis groups (Kessi and Delley, 1998). Furthermore, The optimistic interchange-correlation feature was based on the functional (GGA) and (RPBE) (Hammer et al., 1999).The organic dye was oxidatively degraded at a pH 7.0 (buffered aqueous solution) in an air atmosphere in the presence of known doses of green oxidizing agent H2O2. 0.2\u00a0mg of the Zn2+ complex as a catalyst was added to 10\u00a0ml of the dye solution (30\u00a0mg/l), followed by an appropriate dose of H2O2 (30%) and stirring. At the end of each experiment, the flask contents were filtered, and the concentration of the dye in each filtrate was measured at \u03bbmax of 530\u00a0nm (Hassani et al., 2015). At a constant dose of H2O2 (0.2\u00a0ml) and constant dye concentration (30\u00a0ppm), the effect of time was investigated by running the reaction for 5-, 10-, 15-, 20-, and 30-minute intervals. Initially, the effect of temperature was investigated by performing the reaction at 30, 45, and 60\u00a0\u00b0C. Finally, the effect of H2O2 dose was investigated using 0.2, 0.3, 0.4, 0.5, and 0.6\u00a0ml of H2O2 and constant dye concentration (30\u00a0ppm and 30\u00a0\u00b0C, respectively).Hana Instrument 8519 digital pH meter was used to perform pH-metric measurements. Titrations were carried out at 298, 308, and 318\u00a0K. The Van Uitert and Hass relation (Uitert and Haas, 1953) is used to correct pH-meter readings in a 50 percent (v/v) dioxane-water mixture:\n\n\n\n-\nlog\n\n\n\n\n\nH\n\n+\n\n\n\n\n=\nB\n+\nlog\n\nU\n\nH\n\no\n\n+\nlog\n\n\u03b3\n\u00b1\n\n\n\n\n\nWhere \n\nlog\n\nU\n\nH\n\no\n\n\n and \n\nlog\n\n\u03b3\n\u00b1\n\n\n are the correction factors for the solvent composition and ionic strength, respectively and B is the reading.The titrations of solution mixtures towards standardized free carbonate NaOH solution (9.65x10-3 M) in 50 percent (v/v) water - dioxane at constant ionic strength are used in the experiment (1\u00a0M KCl solution). Figure S1, Supplementary Materials, depicts this. The solution mixtures were made in the following manner:\n\na.\n1.25\u00a0ml HCl (1.04\u00a0\u00d7\u00a010\u22122\u00a0M)\u00a0+\u00a01.25\u00a0ml KCl (1\u00a0M)\u00a0+\u00a010\u00a0ml bidistilled H2O\u00a0+\u00a012.5\u00a0ml dioxane.\n\n\nb.\n1.25\u00a0ml HCl (1.04\u00a0\u00d7\u00a010\u22122\u00a0M)\u00a0+\u00a01.25\u00a0ml KCl (1\u00a0M)\u00a0+\u00a02.5\u00a0ml (5\u00a0\u00d7\u00a010\u22123\u00a0M) H2L\u00a0+\u00a010\u00a0ml bidistilled H2O\u00a0+\u00a010\u00a0ml dioxane.\n\n\nc.\n1.25\u00a0ml HCl (1.04\u00a0\u00d7\u00a010\u22122\u00a0M)\u00a0+\u00a01.25\u00a0ml KCl (1\u00a0M)\u00a0+\u00a02.5\u00a0ml (5\u00a0\u00d7\u00a010\u22123\u00a0M) H2L\u00a0+\u00a09.5\u00a0ml bidistilled H2O\u00a0+\u00a010\u00a0ml dioxane\u00a0+\u00a00.5\u00a0ml metal ion (Ni2+) (5\u00a0\u00d7\u00a010\u22123\u00a0M).\n\n\n1.25\u00a0ml HCl (1.04\u00a0\u00d7\u00a010\u22122\u00a0M)\u00a0+\u00a01.25\u00a0ml KCl (1\u00a0M)\u00a0+\u00a010\u00a0ml bidistilled H2O\u00a0+\u00a012.5\u00a0ml dioxane.1.25\u00a0ml HCl (1.04\u00a0\u00d7\u00a010\u22122\u00a0M)\u00a0+\u00a01.25\u00a0ml KCl (1\u00a0M)\u00a0+\u00a02.5\u00a0ml (5\u00a0\u00d7\u00a010\u22123\u00a0M) H2L\u00a0+\u00a010\u00a0ml bidistilled H2O\u00a0+\u00a010\u00a0ml dioxane.1.25\u00a0ml HCl (1.04\u00a0\u00d7\u00a010\u22122\u00a0M)\u00a0+\u00a01.25\u00a0ml KCl (1\u00a0M)\u00a0+\u00a02.5\u00a0ml (5\u00a0\u00d7\u00a010\u22123\u00a0M) H2L\u00a0+\u00a09.5\u00a0ml bidistilled H2O\u00a0+\u00a010\u00a0ml dioxane\u00a0+\u00a00.5\u00a0ml metal ion (Ni2+) (5\u00a0\u00d7\u00a010\u22123\u00a0M).The antimicrobial activity of H2L and its respective complexes was examined against Aspergillus flavus fungus and Candida albicans fungus (ATCC 7102) as well as the bacteria G-bacteria: Escherichia Coli (ATCC 11775) and G+: Staphylococcus Aureus (ATCC 12600) by a modified disc dispersion methodology (Scheme S1, additional materials); Kirby-Bauer testing (Pfaller et al., 1988). The solution from individual substance, of investigated compounds and standard drug (Amphotericin B Antifungal Agent and Ampicillin Antibacterial Agent) in DMSO solution, were arranged for testing against spore germination. The inhibition regions diameter was expressed in millimetres (Abu-Dief et al., 2021; Aljohani et al., 2021; Abdel-Rahman et al., 2016; Abu-Dief et al., 2020).Mammary gland (MCF-7) breast cancer, and hepatocellular carcinoma HepG2 liver cells have been established using a technique stated by Mauceri, H.J.et\nal (Mauceri et al., 1998). Percentage of relative cell viability was determined by:\n\n\n\nTherelativecellviability\n%\n=\n\n\n\nA\n570\n\n\no\nf\n\nT\nr\ne\na\nt\ne\nd\n\nS\na\nm\np\nl\ne\ns\n\n\n\nA\n570\n\n\no\nf\n\nU\nn\nt\nr\ne\na\nt\ne\nd\n\nS\na\nm\np\nl\ne\ns\n\n\n\u00d7\n100\n\n\n\n\n2\u00a0ml of 2,2\u2032-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) solution (60\u00a0mM) and 3\u00a0ml of MnO2 solution (25\u00a0mg / ml) was added to each of the studied compounds as well as all solutions have been prepared in 5\u00a0ml of buffer solution with pH 7, using 0.1\u00a0M of aqueous phosphate. The mixture was stirred, centrifugated, filtered, and the absorption at \u03bb734 nm of the produced green\u2013blue solution (ABTS radical solution) was attuned to about 0.5. The spectroscopical grade of phosphate buffer / MeOH was then applied to the 50\u00a0ml (2\u00a0mM) solution of the examined species (1:1). Absorption was measured and the decrease in colour was stated as a percentage of inhibition. L-ascorbic acid is a regular antioxidant as a positive control. while sample without ABTS was a negative control (Lissi et al., 1999; El-Gazzar et al., 2009; Aeschlach et al., 1994).The inhibition% of free radical ABTS was calculated by the equation:\n\n\n\nI\n%\n\n=\n\n(\nA\nb\nl\na\nn\nk\n\n-\n\nA\ns\na\nm\np\nl\ne\n)\n\n/\n\n(\nA\nb\nl\na\nn\nk\n)\n\n\u00d7\n100\n\n\n\n\nA large number of studies have shown that epidermal growth factor receptor (EGFR) is a potential therapeutic target for the treatment of various tumors (such as colorectal and breast tumors) (Avdovi\u0107 et al., 2020). The inactivation of this receptor can affect the spread of cell cancer and promote cancer cell apoptosis. thus, we analyzed the potential inhibitory effects of ligand, H2L and its metal complexes.Molecule Operating Environment (MOE) software is used to evaluate the binding affinity between EGFR protein and investigated molecules (Integrated Computer-Aided Molecular Design Platform, Molecular operating environment, Chemical Computing Group, 2019). Use the specified minimization algorithm to minimize the energy of all 3D molecule structures. Then the charge of the atom is processed and the minimized potential energy is corrected. The inspected target substances are saved as a new database in MDB format (Shah et al., 2020). The crystal structure of the EGFR receptor (PDB: 3W2S) is taken from the protein database (Sogabe et al., 2013). MOE has been used to adjust the structure of molecularly bound proteins by removing ligands, adding hydrogen, and minimizing the energy of 3W2S. The minimized energy structure is used more as a binding receptor. The largest active center of 3W2S (PHE 856, LYS 745, ASP 855, CYS 797, ARG 841, ASN 842, MET 793, LEU 718, GLY 719, SER 720, VAL 726, ASP 800, and PHE 997) is the MOE position obtained by the site finder algorithm. The docking uses various features (initial re-scoring methodology: London dG with poses 10, final re-scoring methodology: GBVI/WSA dG with poses 5, placement: triangle matcher, and refining: rigid receptor) to identify and evaluate the compounds' connections with 3W2S. The S value is a rating value that measures the affinity of a compound to the receptor and is calculated by the standard MOE rating function. The RMSD is also used to compare the binding conformation with the binding reference configuration.Quantitative and spectrometric results of the ligand, H2L and its coordinated metals suggest that the metal\u2013ligand stoichiometry is 1:1 for all complexes except the Ni(II) complex, that has 2:1 (M: L) stoichiometric ratio. The synthetic procedure of the planned complex structure is displayed in Scheme 1. The designed complexes were stable, and insoluble in the most popular organic solvents, excepting DMSO and DMF. The molar conductivity of the synthetic compounds is of a non-electrolytic behavior.The H2L 1H NMR spectrum (Figure S2, Supplementary Materials) in DMSO solution showed two signals related to SH and NH protons at 11.855 and 8.445\u00a0ppm, respectively. the presence of SH signal indicated that H2L solution was in the form of thiol. All previous signals have vanished by addition D2O (Figure S3, Supplementary Materials). The aromatic ring protons have been found between 7.334 and 7.880\u00a0ppm.The 13C NMR spectrum (Figure S4, Supplementary Materials) revealed three signals related to the C-S and C\u00a0=\u00a0O carbons at 168.30 and 166.00\u00a0ppm, respectively (Hosny et al., 2021; Hosny et al., 2018). Signals occurring at 151.86, 153.62 and 158.78\u00a0ppm attributed to the C\u00a0=\u00a0N carbon atoms of the hetero ring and the open chain, separately.The 1H NMR spectrum of Zn(II) complex (Figure S5, Supplementary Materials) in DMSO solution showed the signal related to NH proton at 8.449\u00a0ppm as well as the disappearance of signal attributed to SH proton which confirmed the proposed structure of Zn(II) complex.A brief evaluation has been made between the Infra-Red spectral data of the H2L ligand and its complexes in order to investigate the coordination action of H2L towards the metal ions. The most significant IR bands of absorption were displayed in Table 2\n and Figure S6, Supplementary Materials. The IR spectrum of H2L shows distinct vibrations at 1612, 1638, 1697 and 2053\u00a0cm\u22121, related to \u03bd(C\u00a0=\u00a0N)ring, \u03bd(C\u00a0=\u00a0N)*, \u03bd(C\u00a0=\u00a0O) and \u03bd(SH), respectively (Rakha, 2000; Abdel-Monem et al., 2018; Abdel-Monem and Abouel-Enein, 2017; Hosny et al., 2018; Abdel-Rhman et al., 2019). The intense vibration at 2053\u00a0cm\u22121, due to the v(SH) group, indicates that, H2L is existing as thiol form in the solid phase. The vibrations referring to \u03bd(NH) groups cannot be determined since a large destruction of 3160\u20133400\u00a0cm\u22121 overshadows their asymmetrical and symmetrical vibrations. The bending system \u03b4 of the (C\u00a0=\u00a0N)ring appeared at 623\u00a0cm\u22121, which has changed to a larger wavenumber when it is participate in complexing.In the Cr(III) complex, the H2L coordinates the Cr(III) ion as neutral bidentate via the recently formed groups of (C\u00a0=\u00a0N)* and (C\u00a0=\u00a0N) hetero ring. This type of complexation is indicated by the presence of \u03bd(SH) and \u03bd(C\u00a0=\u00a0O) at the higher and same wavenumbers, separately. This shows that these locations are not participate in coordination. Furthermore, the change of \u03bd(C\u00a0=\u00a0N)* to the lower wavenumber and the change of \u03bd/\u03b4(C\u00a0=\u00a0N)triazole ring to lower and higher wavenumbers (Rastogi and Sharma, 1974), respectively, indicated mutual coordination between these locations. In addition, in Zn(II) chloride complex, the H2L works as mono-negative tridentate via deprotonated-SH, (C\u00a0=\u00a0O) and (C\u00a0=\u00a0N) ring groups. This statement was concluded by the absence of \u03bd(SH) (Pandey et al., 1993), the change of \u03bd(C\u00a0=\u00a0O) to less wavenumber (Abdel-Monem and Abouel-Enein, 2017) and the change of \u03b4 and \u03bd of (C\u00a0=\u00a0N) ring to higher and lower wavenumbers, respectively (Abdel-Monem et al., 2018). Finally, the H2L operates as a binegative tetradentate throughout the binuclear Ni(II) complex. This proposal is verified by the disappearance of both \u03bd(C\u00a0=\u00a0O) and \u03bd(SH) (El-Sawaf et al., 2020) with the parallel presence of \u03bd(C\u00a0=\u00a0N)** and \u03bd(C-O) (Liu et al., 2012) as well as the changing of \u03bd/\u03b4(C\u00a0=\u00a0N) vibrations (Rastogi and Sharma, 1974; Pandey et al., 1993).Recent vibrations in regions 410\u2013470 and 511\u2013585\u00a0cm\u22121 were related to \u03bd(M\u2212N) (Ferraro and Walker, 1965) and \u03bd(M\u2212O), respectively. As well, the wide vibrations at\u00a0\u2248\u00a03432\u20133380\u00a0cm\u22121 confirm the existence of H2O in the complex (Chubar et al., 2003). The values of weight loss from the TGA data were used to distinguish between coordinated and crystallised H2O.The electronic spectra of ligand (H2L) as well as metal complexes were recorded in DMSO solution have been revealed in (Table S1, supplementary materials and Figure S7, supplementary materials). The ligand H2L presented bands in the ranges 242 and 216\u00a0nm, that could have been allocated to the (\u03c0\u00a0\u2192\u00a0\u03c0*)Ar and (\u03c0\u00a0\u2192\u00a0\u03c0*)ring transitions, respectively (Tossidis et al., 1987). The band at a value of 300\u00a0nm is due to the carbonyl moiety n\u00a0\u2192\u00a0\u03c0* transition (Tossidis et al., 1987). The electronic spectra of Cr(III) complex shows three absorption bands at 610, 450 and 336\u00a0nm attributable to 4A2g(F)\u21924T2g(F)(\u03bd1), 4A2g(F)\u21924T1g(F)(\u03bd2) and 4A2g(F)\u21924T1g(P)(\u03bd3) transitions, respectively characteristic for octahedral Cr(III) complexes (Parmar et al., 2010). Furthermore, the magnetic moment value, (\u00b5eff.\u00a0=\u00a03.36B.M.) can be taken as an extra indication for the octahedral geometry of Cr(III) complex. The complex, [Ni2(HL1)Cl2(H2O)2].4H2O complex have a magnetic moment values 2.63B.M., which is lesser than the measured value for a single nickel atom of d8-octaedral and/or tetrahedral complexes and larger than the diamagnetic square-planar complexes. This value may indicate the presence of Ni(II) complex in mixed stereochemistry (El-Asmy et al., 1990). This interpretation is also verified by the two bands at 346 and 484\u00a0nm assignable to 3T1(F)\u21923T1(P) and 3T1(F)\u21923T2(F) transition, respectively compatible with the tetrahedral configuration as well as one band at 396\u00a0nm is appearing because of forbidden d\u2013d transition, reliable with the square planar geometry of Ni(II)-complex (Saha et al., 2016).\nTauc\u2019s equation, \u03b1h\u03c5\u00a0=\u00a0A(h\u03c5 - Eg)r was used to calculate optical band gaps for Cr(III) and Ni(II) metal complexes, where (r = \u00bd or 2 for indirect, and direct transitions, respectively), (A): energy independent constant, and (Eg): optical band gap (Hosny et al., 2020). Eg Values are estimated from the plot of (\u03b1h\u03c5) with (h\u03c5) (\nFig. 1\n\n). According to the curves, the direct band gaps for Cr3+, and Ni2+ complexes are 4.64, and 4.66\u00a0eV, respectively. while, the indirect band gaps are 4.22, and 4.26\u00a0eV, respectively This information reveals that these complexes are magnetic insulators with insulating properties, high-spin frameworks, and antiferromagnetic ordering on a regular basis, i.e., metal cations in close proximity have opposite spin. Furthermore, the electronic structure of these complexes is distinguished by a predominance of metal d-orbitals in both the valence and conduction bands. (Cipriano et al., 2020).The mass spectrum of H2L (Figure S8, Supplementary Materials) revealed that the molecular ion peak [M]+ appeared at the value of m / z equal to 247, that was identical to the M.wt. of ligand. The fragmentation path of the H2L ligand was given in Scheme 2\n.The photoluminescence spectrum of H2L show emission broad band at 349\u00a0nm. Moreover, its Cr(III), Ni(II) and Zn(II) complexes, show emission broad bands at 356, 349 and 359\u00a0nm, respectively, Fig. 2\n. These bands could be assigned to as L-M charge transfer (Singh et al., 1999; Etaiw et al., 2018). The emission bands of Cr(III) and Zn(II) complexes indicate that both are traditional blue complexes. Furthermore, the intensity of fluorescence in all complexes is substantially lower than that of free ligand. This could be because the transition metal (M)\u2013fluorophore (F) interaction is too strong, resulting in fluorescence quenching (Zhao et al., 2010).The XRD patterns of separated [Ni2(HL)Cl2(H2O)2].4H2O complex is depicted in Figure S9, Supplementary Materials and its (2\u03b8)\u00b0 value for peaks, the peak indexing, and inter-planar spacing (d-values) were showed in Table S2, supplementary materials. The lattice parameters of Ni(II) complex has been evaluated by using match software (\nhttps://www.crystalimpact.com/match/\n). The [Ni2(HL)Cl2(H2O)2].4H2O complex has a triclinic space group with P \u22121 and lattice parameters a\u00a0=\u00a012.34\u00a0\u00c5, b\u00a0=\u00a012.50\u00a0\u00c5, c\u00a0=\u00a024.45\u00a0\u00c5, \u03b1\u00a0=\u00a0100.09\u00b0 \u03b2\u00a0=\u00a090.31\u00b0, \u03b3\u00a0=\u00a095.43\u00b0 whose unit cell volume is 3772.94\u00a0\u00c53. The lattice parameters of [Ni2(HL)Cl2(H2O)2].4H2O complex exhibits a good harmony with the Crystallography Open Database (COD) No. 4,332,969 (Hern\u00e1ndez-Molina et al., 2006).Such lattice parameters were determined by using the next relationship:\n\n\n\ntriclinic\n\n1\n\nd\n\nhkl\n\n2\n\n\n=\n\n\n\n\nh\na\n\n\n\n\n\n\n\n\n\nh\na\n\n\n\n\ncos\n\u03b3\n\n\n\n\ncos\n\u03b2\n\n\n\n\n\n\n\nk\nb\n\n\n\n\n1\n\n\n\ncos\n\u03b1\n\n\n\n\n\n\n\nl\nc\n\n\n\n\n\ncos\n\u03b1\n\n\n\n1\n\n\n\n\n\n\n\n+\n\nk\nb\n\n\n\n\n\n\n\n\n1\n\n\n\nh\na\n\n\n\n\ncos\n\u03b1\n\n\n\n\n\n\ncos\n\u03b3\n\n\n\n\nk\nb\n\n\n\n\ncos\n\u03b1\n\n\n\n\n\n\ncos\n\u03b2\n\n\n\n\nl\nc\n\n\n\n1\n\n\n\n\n\n\n\n+\n\nl\nc\n\n\n\n\n\n\n\n\n1\n\n\n\ncos\n\u03b3\n\n\n\n\nh\na\n\n\n\n\n\n\ncos\n\u03b3\n\n\n\n1\n\n\n\nk\nb\n\n\n\n\n\n\ncos\n\u03b2\n\n\n\n\ncos\n\u03b1\n\n\n\n\nl\nc\n\n\n\n\n\n\n\n\n\n\n\n.\n\n\n\n\n\n\n\n\n1\n\n\n\ncos\n\u03b3\n\n\n\n\ncos\n\u03b2\n\n\n\n\n\n\ncos\n\u03b3\n\n\n\n1\n\n\n\ncos\n\u03b1\n\n\n\n\n\n\ncos\n\u03b2\n\n\n\n\ncos\n\u03b1\n\n\n\n1\n\n\n\n\n\n\n\n\n-\n1\n\n\n\n\n\n\nThe crystalline-particle parameters were calculated using standard equations in the FWHM method (Velumani et al., 2003). the parameters were particulate sizes\u00a0=\u00a00.2897\u00a0\u00c5 lying in the nanometer scale, (2\u03b8)\u00b0=23.99, d spacing\u00a0=\u00a03.7102\u00a0\u00c5, FWHM\u00a0=\u00a05.1099, the crystal strain (\u03b5)\u00a0=\u00a01.5346 and the dislocation density (\u03b4)\u00a0=\u00a011.9127\u00a0\u00c5\u22122. The regular-crystal lattice of the Ni(II) complex could be calculated from the minimised quantities of dislocation\u2010density (\u03b4) and the crystal strain (\u03b5) (El-Metwaly et al., 2020).The Zeta-potential measurements provide information regarding the stability of the colloidal suspension; the colloidal suspension is stable when the forces generating particle mutual repulsion whichplaysa prominent role. The higher absolute value of the zeta potential exhibitsthe greater the repulsion between the particles forming the suspension and thus the higher stabilityof suspension, whereas low values of ZP (\u00b15 mV) indicate more flocculation between the particles and thus a higher tendency for instability (Bhagat et al., 2019). In this research, [Ni2(HL)Cl2(H2O)2].4H2O complex had the largest positive-positive repulsion with a potential of 20.3\u00a0mV suggesting the stability of this colloidal suspension. But, the ligand (H2L) and Cr(III) complex displayed potential of \u22129.34 and \u22126.91\u00a0mV, respectively which are in the negative range, this results shows the lower stability of the suspension of the ligand (H2L) and its Cr(III) complex. Furthermore, Zn2+ complex demonstrated a potential of 2.69\u00a0mV, indicating the instability of Zn(II) complex colloidal solution as the particles of this suspensions prefer to flocculate.Prediction of coordinated or crystallised H2O molecules may be performed utilizing TGA data (Rakha et al., 1989; Zaky et al., 2014) (Figure S10, Supplementary Materials). It can be assumed that there is an alignment between the TGA data and the proposed molecular formula. For instance, the Cr(III) complex has 5-degradation stages. The first one seems to have a weight loss of 8.05 percent between 35 and 124\u00a0\u00b0C, suggesting the elimination of two hydrated water molecules. The step two has a weight loss of 19.67 percent in the 124\u2013352\u00a0\u00b0C temperature range, demonstrating the elimination of the H2O co-ordinating molecule and two HCl molecules. The 3rd step (from 352 to 448\u00a0\u00b0C) has a weight loss of 41.36 percent, referring to the elimination of the molecule C4H3ClN5S. The fourth stage in the 448\u2013547\u00a0\u00b0C range has a weight loss of 6.27 percent, relating to the elimination of C2H4. Finally, unoxidized carbon and metal oxide existed as a residue. Table S3, Supplementary Materials, demonstrates the steps of decomposition of complexes.Coats\u2013Redfern (Coats and Redfern, 1964) and Horowitz\u2013Metzger (Horowitz and Metzger, 1963) techniques were accustomed to predict the kinetic and thermodynamic variables of the designed complexes (Figures S11-S16, supplementary materials). Tables 3 and 4\n\n demonstrate the various kinetic parameters (A, Ea, \u0394S*, \u0394H* and \u0394G*) of the separated complexes. we may notice that:\n\n(i)\nA resemblance was found between the data collected from both approaches.\n\n\n(ii)\nThe good stability of the complex was proved by the high value of the activation energy.\n\n\n(iii)\nThe positive value of \u0394G* shows that the degradation stage is non-spontaneous process, also, the\u00a0+\u00a0ve value of \u0394H* suggested endothermic operations (Abu-Dief et al., 2019; Abu-Dief et al., 2020).\n\n\n(iv)\nThe negative \u0394S* of certain degradation steps indicate that the activated fragments have a more orderly composition than the un-decomposed fragment and the degradation reactions become slow (Moore and Pearson, 1961). Although\u00a0+\u00a0ve values can indicate that the disorder of the decomposed fragments rises much faster than the un-decomposed fragment (Kenawy et al., 2001).\n\n\nA resemblance was found between the data collected from both approaches.The good stability of the complex was proved by the high value of the activation energy.The positive value of \u0394G* shows that the degradation stage is non-spontaneous process, also, the\u00a0+\u00a0ve value of \u0394H* suggested endothermic operations (Abu-Dief et al., 2019; Abu-Dief et al., 2020).The negative \u0394S* of certain degradation steps indicate that the activated fragments have a more orderly composition than the un-decomposed fragment and the degradation reactions become slow (Moore and Pearson, 1961). Although\u00a0+\u00a0ve values can indicate that the disorder of the decomposed fragments rises much faster than the un-decomposed fragment (Kenawy et al., 2001).The optimized ligand, H2L and its metal complexes structures that labeled with the atom symbol and its number were shown in figure S17, supplementary materials and are listed in tables S4-S11, supplementary materials. Coordination induces a minor difference in bond angles and lengths existing in the thiocarbamide structure of H2L; the major initiatives in the angles of H2L were N(14)-C(13)-N(11), N(15)-N(14)-C(13), S(12)-C(10)-N(8), O(9)-C(7)-N(8), O(9)-C(7)-C(7)(2), N(11)-C(10)-N(8), N(17)-C(13)-N(11), S(12)-C(10)-N(11), C(13)-N(11)-C(10), N(8)-C(7)-C(7)-C(2), and N(17)-C(13)-N(14). Analyzing the result of H2L and the separated complexes, the following observations can be stated:\n\n(i)\nAs predicted, the Cr(III) structure has angles close with those predicted for octahedral complexes with sp3d2 hybridization (El-Gammal, 2010; El-Morshedy et al., 2019). In addition, the optimized structure of the Zn(II) complex tends to be tetrahedral (Moore and Pearson, 1961). moreover, the Ni(II) complex produced mixed geometry (tetrahedral and square planar) including sp3 and dsp2 hybridization.\n\n\n(ii)\n(C\u00a0=\u00a0O), (C-O), (C-S), (C\u00a0=\u00a0N)azomethine, and (C\u00a0=\u00a0N)ring moities have larger bond lengths than those found in the ligand (H2L) attributed to the formation of metal\u2013oxygen and metal-nitrogen bonds (Moore and Pearson, 1961).\n\n\ni.\nThe bond angles of coordination atoms of ligand moiety will be changed in all complexes due to the formation of chelate rings (Fukui et al., 1954).\n\n\n(iii)\nIn the Zn(II) complex, metal ion is tri-coordinated to the H2L ligand in a tetrahedral geometry with bond angles; Cl19-Zn18-S12\u00a0=\u00a0132.55\u00b0, Cl19-Zn18-O9\u00a0=\u00a0117.666\u00b0,\n\n\n(iv)\nN(14)-Zn18-O9\u00a0=\u00a0106.487\u00b0, S12-Zn18-O9\u00a0=\u00a091.074\u00b0, N14-Zn18-S12\u00a0=\u00a092.107\u00b0 and Cl19-Zn18-N14\u00a0=\u00a0112.157\u00b0 which give a small deviation from tetrahedral geometry.\n\n\nAs predicted, the Cr(III) structure has angles close with those predicted for octahedral complexes with sp3d2 hybridization (El-Gammal, 2010; El-Morshedy et al., 2019). In addition, the optimized structure of the Zn(II) complex tends to be tetrahedral (Moore and Pearson, 1961). moreover, the Ni(II) complex produced mixed geometry (tetrahedral and square planar) including sp3 and dsp2 hybridization.(C\u00a0=\u00a0O), (C-O), (C-S), (C\u00a0=\u00a0N)azomethine, and (C\u00a0=\u00a0N)ring moities have larger bond lengths than those found in the ligand (H2L) attributed to the formation of metal\u2013oxygen and metal-nitrogen bonds (Moore and Pearson, 1961).The bond angles of coordination atoms of ligand moiety will be changed in all complexes due to the formation of chelate rings (Fukui et al., 1954).In the Zn(II) complex, metal ion is tri-coordinated to the H2L ligand in a tetrahedral geometry with bond angles; Cl19-Zn18-S12\u00a0=\u00a0132.55\u00b0, Cl19-Zn18-O9\u00a0=\u00a0117.666\u00b0,N(14)-Zn18-O9\u00a0=\u00a0106.487\u00b0, S12-Zn18-O9\u00a0=\u00a091.074\u00b0, N14-Zn18-S12\u00a0=\u00a092.107\u00b0 and Cl19-Zn18-N14\u00a0=\u00a0112.157\u00b0 which give a small deviation from tetrahedral geometry.\nFig. 3\n displays the computed IR spectrum of the ligand, H2L in the vacuum and its observed spectrum. A slight variation between the observed and the computed can be found since the observed spectrum was evaluated for the solid material. Figure S18, Supplementary Materials is the relationship chart between the computed and the observed wavenumbers demonstrate the linear relationship according to the given equation \u03bdcal\u00a0=\u00a00.878 \u03bdExp\u00a0+\u00a0119.321 whereas R2\u00a0=\u00a00.9866.By the aid of Density functional theory (DFT), we have been able to determine different quantum variables as ELUMO, EHOMO, dipole moment, binding energy, and compounds' total energy (Table 5\n) (Liu et al., 2012; Yousef et al., 2012; Govindarajan et al., 2012; Abu El-Reash et al., 2013; Pearson, 1989; Padmanabhan et al., 2007; Gaber et al., 2018). Figure S19, supplementary materials includes the energy of frontier molecular orbitals (FMOs, that includes both orbitals of HOMO and LUMO). The data designated that:\n\n(i)\nthe stability of studied metal complexes was demonstrated by the high Ea value and it was verified by the negative EHOMO and ELUMO value (Gaber et al., 2018; Abu El-Reash et al., 2013).\n\n\n(ii)\nGenerally, the HOMO orbital was dispersed on O(9), S(12), N(8), N(11), N(15), N(14), and N(17) atoms, which are the expected position for nucleophilic attacks in the metal ion.\n\n\n(iii)\nThe stability of metal complexes than the ligand, H2L has been explained from the total energy measurements (Aljahdali and El-Sherif, 2013).\n\n\nthe stability of studied metal complexes was demonstrated by the high Ea value and it was verified by the negative EHOMO and ELUMO value (Gaber et al., 2018; Abu El-Reash et al., 2013).Generally, the HOMO orbital was dispersed on O(9), S(12), N(8), N(11), N(15), N(14), and N(17) atoms, which are the expected position for nucleophilic attacks in the metal ion.The stability of metal complexes than the ligand, H2L has been explained from the total energy measurements (Aljahdali and El-Sherif, 2013).Physical and electrostatic potential behavior is estimated by the theoretical or diffraction strategies. MEP was expressed based on electronic density (Zalaoglu et al., 2010) since has been used as a parameter in the description of nucleophilic and electrophilic attack locations and also the interaction of hydrogen bonds. Figure S20, Supplementary Materials shows the MEP which illustrated for the compounds in the study that shows that the greenish color region pointed to the neutral electrostatic potential field, whereas the blue region is the favored position for the nucleophilic attack that had the lowest zone of e's (Tanak et al., 2011). However, the reddish color section related to the region rich in e's as well as the position needed for electrophilic attack.Mulliken atomic charge has a major part to play in the mathematical interpretation of the molecular construction. Figure S21, Supplementary Materials and Tables S12-S15, Supplementary Materials show the distribution of charges of H2L, while oxygen and nitrogen atoms provide a negative value, however most carbons and hydrogens atoms provide positive values. This may be due to the e-donating potential of oxygen and nitrogen atoms.The catalytic activity of [Zn(HL)Cl]0.0.5H2O complex was investigated in the oxidative degradation of an organic dye, such as EBT dye, using H2O2 as the oxidizing agent \u201cdue to its green character\u201d. The oxidation did not occur in the absence of the H2O2 or Zn(II) catalyst, but when both oxidant and catalyst were used, the dye degraded. As a result, the impact of H2O2 dose was studied in combination with the impact of time and temperature to determine the ideal conditions for the reaction. The effect of time was investigated at a constant dose of H2O2 (0.2\u00a0ml) and constant dye concentration (30\u00a0ppm) by running the reaction for 5-, 10-, 15-, 20-, and 30-minute intervals. Early, the effect of temperature was investigated by performing the reaction at room temperature (30 \u02daC), 45, and 60\u02daC. Finally, the effect of H2O2 dose was investigated by using 0.2, 0.3, 0.4, 0.5, and 0.6\u00a0ml of H2O2 and constant dye concentration and temperature (30\u00a0ppm, and 30 \u02daC, respectively). The results of the experiment are shown in (Fig. 4\n), which shows that the degradation of the dye increases with the time of the reaction, with about 45 percent of the dye removed after 30\u00a0min. By increasing the H2O2 dose, the degradation first improved and then began to be constant or decrease, as shown by self-quenching of OH radicals according to the succeeding OH equation:\n\nH2O2\u00a0+\u00a0OH \u0307\u2192 H2O\u00a0+\u00a0HO2\n\n\n\n\n\nHO2 \u0307 + OH \u0307 \u2192 H2O2\u00a0+\u00a0O2\n\n\n\nAfter 10\u00a0min, at a constant dose of H2O2 (0.2\u00a0ml) and constant dye concentration, the effect of temperature on the reaction was investigated (30\u00a0ppm). The study indicate that the capacity of dye removal increases with temperature, with only 57 percent of the dye remaining after 10\u00a0min at 60\u00a0\u00b0C compared to 85 percent at 30\u00a0\u00b0C.\nTable 6\n shows the stoichiometric protonation constants of the investigated Ligand (L). The ligand compound investigated here has two protonation constants, corresponding to the protonated N-triazol ring and C-SH groups. As shown in Scheme 3\n, the N-triazol ring has the highest pKa value (pKa1\u00a0=\u00a08.60, at 25\u00a0\u00b0C) and the Sulphur group has the lowest (pKa2\u00a0=\u00a04.74, at 25\u00a0\u00b0C). Fig. 5\n depicts the species distribution of the ligand (L). The ligand (L2+) from Sulphur group to form HL+ tends to lose it protons after the pH is raised to the pH range of 4.71\u20134.74. As the conditions become more alkaline, the second proton aims to be deprotonated to a free ligand (L).The stability constants of binary chelated ligand (L) with Ni(II) metal ion as an example of divalent transition metal ions. The comparison of the titration curves of free ligand with the complexed ligand shows that adding Ni(II) ion to the free ligand solution lowers the pH. As a result, the curves associated with complexes are found at lower pHs than that of the free ligand because they require more alkali to raise the pH to the level of the free ligand. The release of protons from the coordinated ligand is an implication of complex formation. The stoichiometric stability constants associated with the inspected ligand's Ni(II) complex were estimated in 50 %(V/V) Dioxane-Water at various temperatures and are shown in Table 6.\nTable 6 shows the logarithms of the stability constants for all complex systems evaluated by potentiometric equilibrium titration process (1) and (2) (simplicity charges are omitted):\n\n(1)\n\n\nM\n+\nL\n\u21cc\nM\nL\n\u03b2\n=\n\n\n\n\nM\nL\n\n\n\n\n\n\nM\n\n\n\n\n\nL\n\n\n\n\n\n\n\n\n\n\n\n(2)\n\n\nM\n+\n2\nL\n\u21cc\nM\n\nL\n2\n\n\u03b2\n=\n\n\n\n\nM\n\nL\n2\n\n\n\n\n\n\n\nM\n\n\n\n\n\n\nL\n\n\n\n2\n\n\n\n\n\n\n\nThe Ni-L process distribution diagram (Fig. 6\n) is investigated with the goal of investigating the changes in concentration of the Ni(II) complex with pH. The Ni-L complex is generated at pH 5.5 with a maximum of 95 %, while at pH 8, the complex Ni(L)2 is formed.The protonation of the ligand and its Ni2+ complex is attributed to the data of thermodynamic parameters that are relative to the temperature data shown in Tables 7 and 8\n\n. \u0394S and \u0394H values were determined by establishing a correlation between equilibrium constant values (ln K) and temperature reciprocal values (1/T) (ln K = - \u0394H/RT\u00a0+\u00a0\u0394S/R) resulting in an intercept \u0394S/R, and a slope \u2013\u0394H/R (Figs. 7 and 8\n\n). There are several conclusions, which are summarized below:a. The reaction involving ligand protonation is exothermic and has a net negative \u0394G (Table 7).b. The data in Table 8 show that the values of log10 K1 - log10 K2 for binary complexes are positive, implying that the first ligand molecule coordinates to the metal ion is preferable to its bonding to the second (El-Sherif et al., 2012). This may demonstrate the significance of the steric effects caused by the addition of the second molecule of the ligand i.e., the NiL2 (1:2) species cannot be formed until the NiL(1:1) species is formed. This could be due to: (i) an improvement in free metal ion Lewis acidity (M+n) when compared to a 1:1 chelated ion (ML+ n\u20131) and (ii) steric weakness resulting from the addition of a 2nd bulky ligand to the chelated ion ML+ n\u20131.c. All of the negative values correlated with the complex formation of the Gibb free energy reflect the random existence of the Ni-L complex formation reactions.d. The negative heat content (\u0394H) values indicate that the complex formation operation is exothermic, implying that the chelation operation works better at low temperatures.e. The ligand complex (\u0394S) values are positive, suggesting that the metal complex formation is entropically favorable (El-Sherif and Eldebss, 2011), and the complexation mechanism is related tothe production of hydrogen ion (H+) and H2O molecules (Jeragh et al., 2007). During the production of metal chelates, the ligand displaces water molecules from the metal ion's main hydration sphere. The number of particles in the process thus increases, implying that the system's randomness increases with the next equation.\n\n\n\n\n\n\n\nM\n\n\n\n\n\nH\n2\n\nO\n\n\n\nn\n\n\n\n\n\n\n\na\nq\n\n\n\n\n2\n+\n\n\n+\n\nL\n\n\n\na\nq\n\n\n\n-\n\n\u21cc\nM\n\nL\n\n\n\na\nq\n\n\n\n+\n\n+\nn\n\nH\n2\n\nO\n\n\n\n\nf. indicate that the complexation mechanism is spontaneous and exothermic, implying that the process of complex formation is entropically favorable.It can be concluded that log K1\u00a0>\u00a0log K2 indicates that Ni(II) ion empty locations are more readily available for binding the 1st ligand than the 2nd one. The chelation mechanism-induced reaction is exothermic, spontaneous, and entropically favorable.The antimicrobial effects of the Schiff base ligand and metal complexes were undertaken toward S. aureus, C. Albicans, E. coli and A. flavus\n(\nTable 9\n), while Amphotericin B and Ampicillin were being used as guideline for anti-fungal and antibacterial behavior, respectively. The Cr(III) complex displayed the highest inhibitory effect towards all microorganisms under investigation based on the estimated area diameter (mm/mg Sample). In addition, all compounds were evaluated no action against A. flavus fungus stain. Also, the Zn(II) complex did not show any activity against those fungus stain. Various antibacterial potency can be due to variations in the composition of the cell wall of the microorganisms (Koch, 2003).All prepared compounds have been evaluated for ABTS- antioxidant behavior Table 10\n. Ni(II) complex displayed significant antioxidant potency with percent inhibition\u00a0=\u00a070.50% relative to ascorbic acid. Whereas ligand, H2L and other M2+-complexes exhibited moderate potency.2-various cell lines, HePG2 (liver carcinoma), and MCF-7 (breast carcinoma), have been used to determine the cytotoxicity of the prepared compounds under investigation (in vitro). The represented Fig. 9\n demonstrates the relationship between concentration and cell viability. By such plots, IC50 values (IC50 is the concentration that inhibits 50 percent) can be determined as shown in Table 11\n. The results may be summarized in the following points:\n\na.\nThe Ni(II)-complex demonstrates effective strong activity towards MCF-7 and HePG2 cells with IC50 values of 10.96\u00a0\u00b1\u00a01.0 and 8.31\u00a0\u00b1\u00a00.9\u00a0\u00b5M, respectively, identifying that Ni(II) complex acts as chemo-therapeutically substantial (Yousef et al., 2014).\n\n\nb.\nCr(III)-complex exhibited moderate activity toward MCF-7 and HePG2 cell lines with IC50 values 41.03\u00a0\u00b1\u00a02.9 and 27.71\u00a0\u00b1\u00a02.1\u00a0\u00b5M, respectively.\n\n\nc.\nThe weak action of Zn(II) complex towards both cancer cells with IC50\u00a0=\u00a054.30\u00a0\u00b1\u00a03.5 and 61.97\u00a0\u00b1\u00a03.7\u00a0\u00b5M.\n\n\nd.\nFinally, H2L exhibited weak action towards HePG2 cancer cells with IC50 values equal to 54.14\u00a0\u00b1\u00a03.3\u00a0\u00b5M and also moderate potency towards MCF-7 cancer cells with IC50 values 33.82\u00a0\u00b1\u00a02.5\u00a0\u00b5M.\n\n\nThe Ni(II)-complex demonstrates effective strong activity towards MCF-7 and HePG2 cells with IC50 values of 10.96\u00a0\u00b1\u00a01.0 and 8.31\u00a0\u00b1\u00a00.9\u00a0\u00b5M, respectively, identifying that Ni(II) complex acts as chemo-therapeutically substantial (Yousef et al., 2014).Cr(III)-complex exhibited moderate activity toward MCF-7 and HePG2 cell lines with IC50 values 41.03\u00a0\u00b1\u00a02.9 and 27.71\u00a0\u00b1\u00a02.1\u00a0\u00b5M, respectively.The weak action of Zn(II) complex towards both cancer cells with IC50\u00a0=\u00a054.30\u00a0\u00b1\u00a03.5 and 61.97\u00a0\u00b1\u00a03.7\u00a0\u00b5M.Finally, H2L exhibited weak action towards HePG2 cancer cells with IC50 values equal to 54.14\u00a0\u00b1\u00a03.3\u00a0\u00b5M and also moderate potency towards MCF-7 cancer cells with IC50 values 33.82\u00a0\u00b1\u00a02.5\u00a0\u00b5M.It is believed that molecular binding is very important in drug discovery. The investigated compounds related to the most suitable active site of 3W2S of EGFR (PHE 856, LYS 745, ASP 855, CYS 797, ARG 841, ASN 842, MET 793, LEU 718, GLY 719, SER 720, VAL 726, ASP 800, and PHE 997) that predicted by the site-finder algorithm in MOE (Fig. 10\n and Figures S22-S24). The largest binding pocket was assigned and all hits were docked against the most active site using the MOE docking software, Table 12\n. As a glance in this table, the S values of investigated compounds are close to each other. Thus, the inhibitory activity may be compared according to the type and number of interaction bonds of the tested compounds with EGFR protein. According to the interaction with EGFR, the inhibitory activity order is [Ni2(HL)Cl2(H2O)2].4H2O\u00a0>\u00a0H2L\u00a0>\u00a0[Cr(H2L)Cl3(H2O)].2H2O\u00a0>\u00a0[Zn(HL)Cl]0.0.5H2O. This order is harmonious with experimental data. It has been observed that the contact between H-donor and H-acceptor is the most common type of interaction with EGFR receptor while Zn(II)-complex doesn\u2019t show any interactions with EGFR receptor. Based on the results tabulated, it can be deduced that the Ni(II)-complex (Figure S23, Supplementary Materials) has the highest inhibitory activity of the EGFR protein which is similar to experimental data. In this complex, two nitrogen atoms of ligand build two H-acceptor interactions with LYS 745 and LEU 858 of EGFR (with distances 2.70 and 3.49\u00a0\u00c5) also oxygen atoms of water molecules build two H-donor interactions with ASP 837 and ASN 842 of EGFR (with distances 2.52 and 2.98\u00a0\u00c5). While in EGFR- H2L interaction (Fig. 10), there is one H-donor and one H- acceptor interactions with nitrogen and oxygen atoms with PHE 856, and LYS 745, respectively (with distances 3.21 and 2.96\u00a0\u00c5). Finally, Cr(III)- complex shows only \u03c0-cation interaction of five-membered ring of ligand with LYS 745 of EGFR (distance\u00a0=\u00a04.01\u00a0\u00c5).In the present manuscript, a new thiocarbamide derivative (H2L), was produced by the reaction of benzoyl isothiocyanate with 3-amino triazole. Its Cr(III), Ni(II) and Zn(II) complexes were synthesized and characterized using various spectroscopic techniques. The ligand operates as a neutral bidentate, mono-negative tridentate and binegative tetradentate in the Cr(III), Zn(II) and Ni(II) complexes, respectively. The photoluminescence spectra of ligand and its metal complexes exhibits that fluorescence quenching of complexes than free ligand. The suggested frameworks of these complexes have been optimized using the DFT analysis. Coats-Redfern and Horowitz-Metzger methods have been used to calculate the kinetic parameters (Ea, A, \u0394H*, \u0394S* and \u0394G*) for titled complexes of all thermal degradation stages. The catalytic activity The Zn(II) complex demonstrated promising activity in the degradation of organic dyes, indicating that it can be used as a starting point for developing catalysts in such features. The greater cytotoxicity and ABTS-antioxidant activity were observed in the Ni(II) complex relative to the other studied compounds. Whereas Cr(III) complex exhibits the highest antimicrobial activity towards E. coli, S. aureus and C. albicans. According to molecular docking interaction, Ni(II) complex exhibits the highest inhibitory activity to the EGFR protein that agree with the experimental anticancer data.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.I wish to thank Dr. Mohammed M. El-Gamil, expert of Toxic and Narcotic Drug, Forensic Medicine, Ministry of Justice, Egypt for his help in preparing metal complexes and for his instructions for theoretical studies.Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2022.104104.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Throughout this research, the thiocarbamide derivative (H2L), and its Cr(III), Ni(II) and Zn(II) complexes have been reported. The thiocarbamide moiety was established with a reaction of benzoyl isothiocyanate and 1H-1,2,4-triazol-3-amine. Structural elucidation of such compounds was achieved using elementary examination, spectral and magnetic experiments. The octahedral construction of the Cr(III) complex, the tetrahedral geometry of the Zn(III) complex and the mixed geometry (tetrahedral and square planar) of the Ni(II) complex have been verified by the optimization of structure using DFT. The action of Zn2+ complex in the oxidative degradation of an organic azo-dye was investigated, and it showed promising results. The thermal degradation behavior of thiocarbamide metal complexes were studied as well as the calculation of the kinetic data for title compounds (Ea, A, \u0394H*, \u0394S* and \u0394G*) of thermal degradation steps has been tested utilizing two different techniques. Liver carcinoma (HePG2) and breast carcinoma (MCF-7) cytotoxicity as well as ABTS-antioxidant activities demonstrated the effective inhibitory of the Ni(II)-complex relative to other tested compounds. The antimicrobial activity of the compounds suggests that Cr(III) has the highest activity. Furthermore, the Molecular Operating Environment (MOE) program was used to calculate the binding affinity between the EGFR protein and the compounds under investigation.\n               "}
{"full_text": "Biomass is an effective energy carrier, contributing to the growing demand for clean and everlasting energy sources for the sustainable development of society. Biomass can be converted to biofuel through biochemical technologies (e.g., fermentation and anaerobic digestion) and thermochemical technologies (e.g., pyrolysis, liquefaction, gasification and torrefaction) [1]. One of the leading technical barriers to industrialising biomass-derived energy is the high energy input of conversion processes, especially for biomass with high moisture content. Wet bio-feedstocks require energy-costly drying operation, reducing the efficiency of the energy conversion process. However, supercritical water gasification (SCWG) technology may overcome this problem as the wet biomass can be directly gasified without an energy-intensive drying step. For high moisture biomass, the energy required for the drying process is higher than that of heating the water to a supercritical point. It was reported that the total efficiency of heat utilisation of biomass SCWG is higher than that of thermal gasification when the moisture content exceeds 27% [2]. SCWG of wet biomass can produce H2-rich syngas, which has a high heating value and can be used as a cleaner alternative of fossil fuels.Biomass gasification is a process that converts feedstock materials into gaseous products such as syngas at high temperature conditions (above 700\u00a0\u00b0C), with a controlled amount of oxygen and/or steam but without combustion. One of the primary benefits of SCWG technology is associated with the high-pressure/high-temperature water that is used as the reaction medium. The physical properties of water drastically change when the pressure/temperature conditions are above its critical point. As the reaction medium, supercritical water (SCW) offers several advantages, such as low viscosity, high diffusion coefficient, and complete miscibility with varying organics and gases, thereby enhancing the mass transfer and reaction rate in the reactor [1,3].Cellulose is one of the main structural components of lignocellulose biomass, constituting 40\u201350\u00a0wt% of lignocellulosic biomass on a dry weight basis [4]. Moreover, it is reported that the contribution of cellulose to H2 production during the gasification process is more than that of hemicellulose and lignin [5]. Therefore, it is essential to investigate the conversion mechanism of cellulose during the SCWG process. Extensive experimental studies on SCWG of cellulose have been carried out [1,3,6]. Cellulose comprises glucose monomers linked together by \u03b2-1,4 d-glucopyranose bonds, forming strong intramolecular and intermolecular hydrogen bonds [3]. Cellulose undergoes rapid hydrolysis and decomposes to its monomer (e.g., glucose) at very short residence times under elevated pressure/temperature. Then glucose undergoes hydrolysis to liquid-phase organic intermediates, followed by the slower formation of small quantities of stable light gases [7]. Therefore, cellulose is one of the most refractory substances that are difficult to dissolve in hot water [8], requiring harsh operating conditions to convert it into biofuel. It is widely accepted that a higher operating temperature favours the formation of H2 [1,5]. However, heating the feedstock and water to supercritical conditions is an energy-consuming process. Lowering the reaction temperature of SCWG and improving the conversion efficiency are essential for promoting the commercial utilisation of SCWG.The use of catalysts in SCWG is one of the most promising approaches to improve the gas yield while minimising the heat requirements, which can reduce the operating costs of the process [9]. Nickel-based catalyst is one of the most effective transition metal catalysts in biomass gasification for improving the gas yield and preventing the formation of tar (heavy hydrocarbons produced during SCWG, which can contaminate equipment and lead to increased maintenance costs) [10]. Extensive studies have been carried out to investigate the catalytic effect of Ni on the gaseous product yield in the biomass SCWG process [9,10]. It is widely accepted that Ni could effectively promote water gas shift reaction and steam reforming reaction [11,12], which are the two main reactions occurred in SCWG to produce hydrogen. Nickel (Ni) is known for its tendency to catalyse the cleavage of C\u2013C, C\u2013O, and O\u2013H bonds [13,14], which promotes the formation of various carbonaceous products. The cracking products can be effectively dehydrogenated to produce more hydrogen [15]. Kumar and Reddy [16] investigated the impacts of Ni, Ru, and Fe on the gas yield during SCWG of banana pseudo-stem. The results showed that Ni has the highest activity in H2 generation. Ruppert et\u00a0al. [14] studied the thermochemical conversion of cellulose for hydrogen production with Ni/ZrO2. They considered that the organic intermediates probably undergo dehydrogenation on the metal surface, hence increasing H2 yield. The cleavage of C\u2013C and C\u2013O bonds can occur to form various carbonaceous products. However, the proposed mechanisms were primarily based on the analysis of products detected during the SCWG process. Detailed structural changes at the molecular level, such as radicals and intermediates in cellulose dissociation and steam reaction, can hardly be captured through experimental methods. The exact mechanism of Ni catalytic thermal decomposition of cellulose has not been fully understood and further investigation is needed.Molecular dynamics (MD) simulation provides an opportunity to investigate the underlying mechanisms of catalytic SCWG of biomass at an atomic level. MD with ReaxFF can simulate the cleavage and forming of chemical bonds to identify elementary pathways. ReaxFF MD simulations were adopted to study the SCWG of lignin [17\u201320]. The structural evolution of lignin and the chemical reactions of forming CO, CO2, CH4, and H2 were obtained. Zhang et\u00a0al. [21] conducted a molecular study on SCWG of glucose under microwave heating. They found that the external electric fields promote glucose decomposition to produce formaldehyde and hydrogen-free radicals, increasing H2 yield. The ReaxFF approach has been successfully employed to study the SCWG of biomass catalysed by the metal catalyst. The work of Monti et\u00a0al. [22] showed that the ReaxFF approach was\u00a0able to obtain an atomic-level characterisation of the crucial steps of the adsorption of the lignin molecules on the\u00a0Palladium catalyst, including their fragmentation and desorption. The SCWG of lignin with Pt and Ni nanoparticles was studied by using ReaxFF simulation [23]. It was found that the Pt and Ni reduce the degradation temperature, accelerating the aromatic ring-opening process. The ReaxFF simulation study of Fe-catalysed SCWG of lignin revealed that Fe iron with a low oxidation state contributes to the formation of CO, while iron with a high oxidation state was beneficial to increasing CO2 yield [24]. The evolution of the lignin decomposition catalysed by Ni was investigated with ReaxFF simulation [25]. The results indicated that Ni could potentially accelerate the scission of C\u2013O bonds and destroy the conjugated \u03c0 bond of the aromatic ring during the ring-opening process. The generation process of H2 molecules occurring on the Ni surface was presented. The thermal stability of carboxymethyl cellulose on the Fe2O3 surface was studied by Saha et\u00a0al. [26]. It was found that cellulose can be adsorbed on the metal surface via the formation of bonds between Fe and oxygen atoms. The chemisorption would bulge the Fe slightly out of the Fe2O3 surface.Although significant experimental work has been performed on Ni catalytic SCWG of biomass, there is a lack of detailed understanding on the chemical processes involved at the molecular level. Further fundamental modelling studies are required to deepen the understanding of the catalytic and micro reaction degradation mechanisms during the SCWG process. This could provide a basis for optimising operating conditions and developing high-efficiency catalysts, thereby promoting the utilisation of SCWG technology. To the best of our knowledge, the ReaxFF simulation of Ni catalysed SCWG of cellulose has not been carried out. This study investigated the effect of Ni on cellulose depolymerisation and ring-opening process. The effects of temperature and cellulose-to-water mass ratio (C/W) on gaseous products were investigated. The detailed gaseous product generation pathways were analysed. Besides, the influence of temperature and C/W on carbon deposition behaviour on Ni nanoparticle (NiNP) was investigated.All MD simulations in this work were conducted using the ReaxFF force field [27]. The description of connectivity-dependent interactions in the ReaxFF force field is based on bond order formalism. Bond order is determined by interatomic distance using an empirical formula, including contributions from \u03c3, \u03c0 and \u03c0\u03c0 bonds. The chemical reactions during the time intervals can be analysed based on the interatomic potential and the bond order. Nonbonded interactions, such as Coulomb and van der Waals interaction, are calculated independently. The charge equilibration (QEq) method adjusts the partial charge on individual atoms. The following equation calculates the energy of each particle:\n\n(1)\n\n\nE\ns\ny\ns\nt\ne\nm\n=\nE\nb\no\nn\nd\n+\nE\no\nv\ne\nr\n+\nE\nu\nn\nd\ne\nr\n+\nE\nl\np\n+\nE\nv\na\nl\n+\nE\nt\no\nr\n+\nE\nv\nd\nW\na\na\nl\ns\n+\nE\nc\no\nu\nl\no\nm\nb\n\n\n\nwhere E\n\nbond\n\n, E\n\nover\n\n, E\n\nunder\n\n, E\n\nlp\n\n, E\n\nval\n\n, E\n\ntor\n\n, E\n\nvdWaals\n\n, and E\n\ncoulomb\n stand for bond energy, overcoordination energy penalty, undercoordination stability, lone pair energy, three-body valence angle energy, four-body torsional angle energy, van der Waals energy, and Coulomb energy, respectively. In this study, the C/H/O/Ni parameter set [28,29] developed for modelling hydrocarbon chemistry catalysed by Ni was adopted to study the cellulose SCWG catalysed by Ni nanocatalysts. The verification of the adopted force field was carried out in our previous work [30].Cellulose (C6H10O5)n is a polysaccharide consisting of a linear chain of several hundred to many thousands of \u03b2-1,4 linked d-glucopyranose units. Fig.\u00a01\n shows the model used in the MD simulations. The model construction starts with a monomer, and the unimolecular d-glucopyranose was built and optimised using the Materials Studio [31] Forcite module. Ten d-glucopyranose monomers were connected to form a polymer, as shown in Fig.\u00a01(c). Face-centred cubic lattice of NiNP was created on a web-based crystallographic tool [32]. The minimum surface energy of corresponding Miller indices of (111), (100) and (110) was adopted from the work of Chen et\u00a0al. [30]. The melting temperature of NiNP depends on its size, and the melting temperature decreases with decreasing radius of NiNP. The melting temperature of 3\u00a0nm NiNP simulated with the ReaxFF force field is around 1700\u00a0K [30], which is lower than the simulation temperature (1800\u00a0K\u223c2200\u00a0K) in this study. The simulated melting temperature of 4.0\u00a0nm NiNP is around 2000 K by using the same ReaxFF force field [30]. Although the 4.0\u00a0nm NiNP would melt to some degree when the temperature is above 2000K, the NiNP still keeps a spherical shape. Therefore, NiNP with a diameter of 4.0\u00a0nm was adopted to maintain the integrity of NiNP during the simulation in this study.Eight reaction systems S1\u2013S8 were built to investigate the Ni catalytic SCWG of cellulose, as listed in Table 1\n. Different cellulose molecule numbers have been tested to eliminate the effect of atom number on the simulation results. Ten cellulose molecules were adopted to ensure the validity of simulation results. To observe the reactions that occur on the Ni surface, the catalyst to biomass ratio considered is relatively high. Cases S1\u2013S6 are used to study the effects of temperature and catalyst on the SCWG of cellulose. Cases S5, S7, and S8 are used to study the effect of cellulose-to-water mass ratio (C/W) on the catalytic SCWG of cellulose. The system pressure would affect the yield of gaseous products. An increase in pressure will shift the methanation reactions (\n\nCO\n+\n3\n\nH\n2\n\n\u2194\n\nCH\n4\n\n+\n\nH\n2\n\nO\n\n, \n\n\nCO\n2\n\n+\n4\n\nH\n2\n\n\u2194\n\nCH\n4\n\n+\n2\n\nH\n2\n\nO\n\n) to the right, thereby enhancing the formation of CH4 [1]. Therefore, the simulation box dimensions were adjusted to keep the same pressure in systems S7 and S8 with varying water molecules.Sorensen and Voter [33] pointed out that an elevated temperature could accelerate the reaction process and thus significantly extend the simulation time scale, which has become a familiar and effective strategy in ReaxFF MD simulation [24,34]. For reaction rates described by the Arrhenius equation, increasing the temperature would increase the reaction rates but not the activation energy barrier. Salmon et\u00a0al. [35] studied coal pyrolysis using ReaxFF simulation at an elevated temperature. They compared the simulated product distribution with experimental results and concluded that elevated temperature did not influence the reaction pathways during coal pyrolysis. Accordingly, elevated reaction temperatures were chosen in this work to study the catalytic mechanism of Ni during the SCWG of cellulose.Initial configurations of all models were built by using Packmol [36]. Cellulose and water molecules are distributed randomly into the cubic box, and the box dimensions are listed in Table 1. In catalytic SCWG (e.g., cases S4\u2013S8), NiNP was fixed in the centre of the unit cell and water and cellulose molecules are distributed around, as shown in Fig.\u00a01(e). After system energy minimisation, the simulation cell was relaxed at 300\u00a0K for 20 ps. Subsequently, the equilibrated system was heated to the final reaction temperature with a heating rate of 15\u00a0K/ps. Then, the simulations would last at the target temperature for 2 ns. All simulations were performed using an isochoric-isothermal NVT (fixed atom numbers, volume, and temperature) ensemble. A time step of 0.25 fs was assigned. The trajectories and species information were outputted every 100 steps. The linear and angular momentum of NiNP was zeroed every 10 timesteps.The periodic boundary condition was applied in all directions. The initial velocities for all atoms were generated randomly following the Maxwell-Boltzmann distribution. Nos\u00e9-Hoover thermostat and barostat were adopted to control the system temperature and pressure with a temperature and pressure damping constant equal to 100 times and 1000 times of the time step, respectively. A bond order of 0.3 was employed to identify chemical bonds between pairs of atoms [28,30]. All simulations were repeated three times with different initial configurations and velocity distributions. The ReaxFF MD simulations were performed with the REAXC package [37] in the Large-scale Atomic/Molecular Massively Parallel Simulation (LAMMPS) [38].The first step in cellulose conversion involves its depolymerisation to oligomers or d-glucopyranose [4], which undergoes hydrolysis to form liquid-phase organic intermediates via scission of C\u2013C and C\u2013O bonds. Guo et\u00a0al. [9] established the mechanism of Ru catalytic gasification of d-glucopyranose. Hydroxyl groups are adsorbed to the catalytic Ru surface predominantly through oxygen atoms. The reactant undergoes dehydrogenation on the catalyst surface, followed by subsequent cleavage of C\u2013C or C\u2013O bonds, which results in syngas production. However, the detailed adsorption and degradation process on catalyst surfaces are not readily accessible by experiments.The depolymerisation and ring-opening percentage of cellulose during the heating period in cases S2 and S5 are shown in Fig.\u00a02\n(a) and (b). The depolymerisation and ring-opening percentage are computed by the following equations:\n\n(2)\n\n\nD\ne\np\no\nl\ny\nm\ne\nr\ni\ns\na\nt\ni\no\nn\n\np\ne\nr\nc\ne\nn\nt\na\ng\ne\n=\n\n\nC\nl\ne\na\nv\na\ng\ne\n\no\nf\n\n\u03b2\n-\n1\n,\n4\n\nl\ni\nn\nk\na\ng\ne\n\n\nI\nn\ni\nt\ni\na\nl\n\nn\nu\nm\nb\ne\nr\n\no\nf\n\n\u03b2\n-\n1\n,\n4\n\nl\ni\nn\nk\na\ng\ne\n\n\n\u00d7\n100\n(\n%\n)\n\n\n\n\n\n\n(3)\n\n\nRing\n-\nopening\u00a0percentage\u00a0\n=\n\u00a0\n\nNumber\u00a0of\u00a0opened\u00a0ring\u00a0\nInitial\u00a0number\u00a0of\u00a0ring\n\n\u00d7\n100\n\u00a0\n\n(\n%\n)\n\n\n\n\n\nIt can be seen that Ni could accelerate the depolymerisation and ring-opening process of cellulose. The depolymerisation and ring-opening occur at around 75 ps in the absence of Ni catalyst, and almost all \u03b2-1,4 linkages are cracked after 125 ps. While the start points of \u03b2-1,4 linkage cleavage and ring-opening are around at 50 ps in the presence of Ni catalyst, these processes were completed after 100 ps. The results show that decomposition of cellulose can occur at a lower temperature, which helps reduce the cost of biomass SCWG. The two cases show a similar onset time and evolution trend in depolymerisation and ring-opening processes, demonstrating that the ring-opening takes place immediately after cellulose is depolymerised into monomers. This is because the cleavage of \u03b2-1,4 linkage would lead to the structural instability of the corresponding monomer, resulting in the ring-opening of d-glucopyranose.The ring-opening of the d-glucopyranose monomer can be achieved by the cleavage of the C\u2013C or C\u2013O bond. Fig.\u00a02(c) shows the cleavage percentage of different types of bonds. Around 64% of rings were opened by the cleavage of the C\u2013O bond in the absence of Ni, while this figure increases to 70% when Ni was added. This result suggests that more rings of d-glucopyranose tend to be opened via the cleavage of the C\u2013O bond under the effect of Ni catalyst.Both high temperature and catalyst would promote bond breaking. The energy of atoms increases with increasing temperature, and the bonds between the atoms become more unstable and eventually break. Catalysts make this process more efficient by lowering the activation energy. If an atom forms a chemical bond with Ni atoms, the cleavage of other chemical bonds connected to this atom will take place. The cracking of such bonds is considered as catalytic cleavage and the others are considered as thermal cleavage. The thermal cleavage and catalytic cleavage of bonds during depolymerisation and ring-opening process in catalytic SCWG (case S5) are shown in Fig.\u00a03\n. It can be seen that 87% and 88% of bond breakings take place via thermal cleavage during depolymerisation and ring-opening process, respectively. Thermal cleavage of bond plays a dominant role in the bond-breaking process. Therefore, depolymerisation and ring-opening rate in noncatalytic and catalytic SCWG are similar after 100 ps, when the temperature of the system reaches a certain value, as shown in Fig.\u00a02.As the skeleton of organic matters, the dissociation kinetics of C\u2013C and C\u2013O bonds play a vital role in cellulose decomposition. The decomposition reactions of d-glucopyranose were considered to be first-order reactions [39]. Initial and equilibrium numbers of C\u2013C and C\u2013O bonds can be used to calculate the activation energy of the corresponding bond [40,41]. The reaction rate constant, K, is determined by the following equation [40]:\n\n(4)\n\n\nln\n\nN\n0\n\n\u2212\nln\n\nN\n\nt\ne\nq\n\n\n=\n\u00a0\nK\n\nt\n\ne\nq\n\n\n\n\n\nwhere N\n\n0\n and N\n\nteq\n are the numbers of C\u2013C or C\u2013O bonds at initial and equilibrium stages. The reaction rates are analysed by the Arrhenius equation:\n\n(5)\n\n\nK\n=\nA\nexp\n\n(\n\n\u2212\n\n\nE\na\n\n\nR\nT\n\n\n\n)\n\n\n\n\nwhere R is the universal gas constant. The activation energy (Ea) and the pre-exponential factor (A) in Eq. (5) are calculated by linear fitting. Fig.\u00a04\n shows the change in the activation energy of C\u2013C and C\u2013O bonds in the absence and presence of a catalyst. The activation energy of the C\u2013C bond is 25.33\u00a0kJ/mol without catalyst, and this figure decreases to 24.02\u00a0kJ/mol when Ni catalyst is added. Activation energies for C\u2013O bonds without and with Ni are calculated as 24.95 and 22.97\u00a0kJ/mol, respectively. It can be seen that the activation energy of C\u2013O bonds is lower than that of C\u2013C bonds. Thereby, the ring-opening of d-glucopyranose monomers is prone to take place via the cleavage of C\u2013O bonds. Moreover, the activation energy reduction of C\u2013O bonds (1.98\u00a0kJ/mol) under the effect of catalyst is more significant than that of C\u2013C bonds (1.31\u00a0kJ/mol), demonstrating Ni is more efficient in the cleavage of the C\u2013O bond than the C\u2013C bond. Consequently, the proportion of C\u2013O cleavage increases when Ni is added during the ring-opening process, as shown in Fig.\u00a02(c). It has been reported that cellulose is easier to gasify than lignin in the presence of Ni catalyst [42], which can be explained by the structural differences between cellulose and lignin because there are more C\u2013O bonds in cellulose molecules than in lignin.\nFig.\u00a05\n shows the time evolution of the total number of H2, CO, and CO2 molecules at different temperatures during SCWG of cellulose in the absence and presence of Ni catalyst. Temperature is one of the most dominant parameters that affect the gaseous product yield, especially when the reaction occurs without a catalyst [1]. Generally, the gaseous produce yield increases with an increase in reaction temperature as high temperatures favour the scission of C\u2013C and C\u2013O bonds. The simulation results show that H2 yield increases with increasing temperature, which is consistent with the experimental results [43]. The free radical reactions in water are believed to be temperature-dependent. When the conditions are above the critical point of water, free radical reactions dominate over ionic reactions [3]. Therefore, water splitting at higher temperature generates more H free radicals. In addition to water splitting reaction, biomass dehydrogenation reaction would also be enhanced at high temperatures [44]. The increase of H free radical number leads to the increase in H2 yield, which will be discussed subsequently.Ni catalyst will significantly increase the yield of H2, as shown in Fig.\u00a05. The H2 generation pathways were analysed to explore the effect of Ni on the H2 production mechanism. The generation of H2 is mainly through the following three pathways:\n\n\n\n\n\n\u2460\n\nR-H\n+\n\nH\n\u2022\n\n\u2192\n\nR\n\u2022\n\n+\n\nH\n2\n\n\n\n\n\n\u2461\n\n\nH\n2\n\nO\n+\n\nH\n\u2022\n\n\u2192\n\nH\n2\n\n+\n\nOH\n\n\n\u2022\n\n\n\n\n\n\u2462\n\n\nH\n\u2022\n\n\n+H\n\u2022\n\n\u2192\n\nH\n2\n\n\n\n\n\n\nwhere pathway \u2460: hydrogen transfer reactions, where H radical interacts with the H atoms in cellulose to produce H2 [17,45]; pathway \u2461: H radical interacts with water to produce H2 and \n\nOH\n\n at elevated temperature [17,46]; pathway \u2462: H radical termination reaction, where two H radicals interact with each to produce H2 [17,45]. R is the abbreviation for any other groups.The occurrence frequency and proportion of pathways \u2460-\u2462 during SCWG of cellulose in the absence and presence of Ni catalyst are presented in Fig.\u00a06\n(a) and Fig.\u00a06(b), respectively. The results suggest that pathway \u2460 plays a dominant role in the H2 generation in the absence of Ni catalyst, especially at relatively low temperatures. It can be seen that around 68% of H2 was produced via pathway \u2460 in 1800\u00a0K. Only a few H2 molecules were generated through pathway \u2462 without Ni catalyst. However, pathway \u2462 becomes the main H2 generation path with the addition of Ni catalyst. More than 70% of H2 molecules were generated via pathway \u2462, and this figure reached 82% in 1800\u00a0K. This is because a large number of H free radicals would be generated by the water splitting and cellulose dehydrogenation reactions on the Ni surface. Increasing the concentration of H free radical enhances the occurrence frequency of pathway \u2462. Meanwhile, the frequency of all three pathways increases with increasing temperature, and the increase of pathway \u2461 is the most significant. Therefore, the proportion of pathway \u2461 increases with increasing reaction temperature, as shown in Fig.\u00a06(b).The yield of H2 is closely related to the number of H free radicals. The generation paths of H free radicals were analysed to investigate the effect of Ni on H2 yield. The H radicals could be produced from the water splitting and cellulose dehydrogenation reactions, as listed in the following pathways:\n\n\n\n\n\n\u2463\n\n\nH\n2\n\nO\n\u2194\n\nOH\n\n\n\u2022\n\n+\n\nH\n\u2022\n\n\n\n\n\n\u2464\n\nR-O-H\n\u2194\n\nR-O\n\u2022\n\n+\n\nH\n\u2022\n\n\n\n\n\n\u2465\n\nR-C-H\n\u2194\n\nR-C\n\u2022\n\n+\n\nH\n\u2022\n\n\n\n\n\n\nwhere pathway \u2463: free radical reaction of water, where H2O splits into \n\n\nH\n\u2022\n\n\n and \n\n\nO\n\n\n\u2022\n\nH\n\n or the radicals recombine into H2O at elevated temperature [3]; pathway \u2464 and pathway \u2465: dehydrogenation reaction of cellulose, where the H atom connects to or disconnects from oxygen and carbon atoms respectively [41]. Due to the high activity of free radicals, the reaction pathways \u2463-\u2465 are reversible.\nFig.\u00a07\n(a) shows the frequency differences of forward and reverse reaction of pathway \u2463-\u2465 in cases S2 and S5. It should be noted that the frequency of reverse reaction could be higher than that of the forward reaction, which is because the reactants involved in the reverse reaction could be produced from other reactions. For example, \n\n\nO\n\n\n\u2022\n\nH\n\n could be generated from pathway \u2463, while it can also be produced from the cleavage of the C\u2013O bond in cellulose (e.g., \n\nR\n-\nOH\n\u2192\n\nR\n\u2022\n\n+\n\nO\n\n\n\u2022\n\nH\n\n). The cleavage of \u03b2-1,4 linkage and C\u2013O bond in d-glucopyranose would produce \n\n\n\nR\n-\nO\n\n\u2022\n\n\n that involved in the reverse reaction of pathway \u2464. The increase in these reactant concentrations would shift the reactions into the reverse direction. The results show that the H free radicals mainly come from the dehydrogenation reaction of H atoms connected to C atoms, e.g., pathway \u2465. The H radicals generated from pathway \u2465 increase with the addition of Ni, which can be attributed to the promotion effect of Ni on C\u2013H bond cleavage [47].Although the water splitting reaction would produce H radicals, the \n\n\nO\n\n\n\u2022\n\nH\n\n generated from water and cellulose would consume H radicals to form water simultaneously. The frequency of pathway \u2463 reverse reaction is higher than that of the forward reaction, leading to an increase in water molecule number. This is due to the structural features of cellulose, which has a large number of hydroxy groups. The increase in \n\n\nO\n\n\n\u2022\n\nH\n\n concentration shifts the reaction of pathway \u2463 to the reverse direction, especially with the addition of Ni catalyst, since Ni could promote the scission of C\u2013O bonds to produce more \n\n\nO\n\n\n\u2022\n\nH\n\n.Experimental results suggested that one of the roles of water in SCWG is being a source of hydrogen and free radicals [3]. However, quantitative information on such effect is difficult to obtain through experimental approaches. In ReaxFF MD simulation, the role of water in producing hydrogen and free radicals can be identified by tracing the evolution of the original water (the water that was added to the system in the initial stage). Fig.\u00a07(b) indicates the total water molecule number and the source of O atom in water in cases S2 and S5. Although it was observed that the frequencies of forward and reverse reaction of pathway \u2463 are high, especially on Ni surface, only a small part of the original water split into \n\n\nH\n\u2022\n\n\n and \n\n\nO\n\n\n\u2022\n\nH\n\n in the end, as indicated by the red line in Fig.\u00a07(b). The results indicate that only a limited number of H radicals produced from pathway \u2463 forward reaction could be the source of H2 generation. Moreover, Ni could promote the splitting reaction of water, and the reduction of original water in the presence of Ni is more significant. The increased water in two cases is ascribed to the combination of H radicals and hydroxy groups dissociated from cellulose since the oxygen atom in increased water mainly comes from cellulose, as indicated by the violet line in Fig.\u00a07(b). The increase of total water molecule number in Ni catalytic SCWG is more significant than in noncatalytic SCWG as Ni could promote the cleavage of C\u2013O bond to produce more hydroxy groups. It can be deduced that water plays a limited role in providing H free radicals to produce H2, while the hydrogen atoms in cellulose are the primary source of H2 generation.A schematic diagram of H2 generation pathways during the noncatalytic and Ni-catalytic SCWG of cellulose is shown in Fig.\u00a08\n. In the absence of a catalyst, H free radicals would be generated via dehydrogenation of cellulose, and a small amount of water would also be split into H free radicals and \n\n\nO\n\n\n\u2022\n\nH\n\n. H radicals generate H2 through pathways \u2460-\u2462, where pathway \u2460 plays a leading role, followed by pathway \u2461. In the presence of Ni catalyst, the decomposed molecular fragments and water would be adsorbed on the Ni surface, where the scission of the C\u2013H and O\u2013H bonds is enhanced to produce a large number of H radicals. H radicals undergo radical termination reactions to produce H2 (pathway \u2462), which dominates the H2 generation. Meanwhile, the generated \n\n\nO\n\n\n\u2022\n\nH\n\n would consume H radicals to form water. In the absence of catalyst, mainly comes from pathway \u2461. Nevertheless, cellulose also produces some \n\n\nO\n\n\n\u2022\n\nH\n\n on the Ni surface via the cleavage of C\u2013O bonds when Ni is added. The \n\n\nO\n\n\n\u2022\n\nH\n\n generated through serval pathways would consume a relatively large amount of H radicals, leading to an increase in H2O molecule number. It can be found that the generation of H2O is an H radical consumption process. The concentration of H radicals would increase if the number of \n\n\nO\n\n\n\u2022\n\nH\n\n in the reacting system can be suppressed, thereby increasing the yield of H2.It can be seen in Fig.\u00a05 that the yield of CO is enhanced at elevated temperature in the absence of a catalyst, which is consistent with the experimental results [48]. The elevated temperature would promote the cleavage of C\u2013C bonds, thereby enhancing the yield of gaseous products. CO yield decreased significantly when the catalyst was added. Yoshida et\u00a0al. [42] attributed the reduction of CO to the enhancement of water-gas shift reaction (\n\nCO\n+\n\nH\n2\n\nO\n\u2192\n\nCO\n2\n\n+\n\nH\n2\n\n\n), and the disproportionation of carbon monoxide adsorbed on the catalyst surface (\n\n2\nCO\n\u2192\n\nCO\n2\n\n+\nC\n\n). However, this study found that the molecular fragments produced by cellulose dissociation would be adsorbed on the catalyst surface [30]. The C\u2013O bonds would be cracked under the catalytic effect of Ni, as shown in Fig.\u00a09\n(a). Oxygen atoms that might be used to generate CO are prone to be detached from organic fragments to produce water by interacting with H radicals. It is demonstrated that the deoxygenation and dehydroxylation of organic fragments on Ni surface are the main reason for CO reduction.There was a slight change in the yield of CO2 when the temperature increased, as shown in Fig.\u00a05, which is ascribed to the structural features of cellulose. The carbon atom connected with two oxygen atoms is the primary source of CO2, as illustrated in Fig.\u00a09(b). There are a small number of \u03b2-1,4 linkage in cellulose; thus, the yield of CO2 is lower than H2 and CO [49]. Consequently, the effects of temperature and catalyst on CO2 production are weaker than those on H2 and CO.\nFig.\u00a010\n presents the yields of H2, CO, and CO2 under different C/W at 2000 K. The H2 yield increases slightly with the increasing number of water molecules, which is consistent with experimental results [23]. Increasing the water molecule number favours the forward reaction of pathway \u2463, leading to an increase in H radical number, as shown in Fig.\u00a011\n(a). In the presence of Ni catalyst, a large number of H radicals would be generated via water splitting and cellulose dehydrogenation reactions on the Ni surface. Then H radicals are continuously consumed to produce H2 and H2O. A high concentration of H radicals contributes to the formation of H2.Typically, water and small organic fragments would be adsorbed on metal catalyst [50], as shown in Fig.\u00a011(b). It was observed that the addition of water occupies a part of the active sites on the catalyst surface, which weakens the adsorption capacity of the catalyst to organic fragments [23]. When most of the active sites of the Ni catalyst are occupied by water or hydroxyl group, the small dissociative fragments outside Ni surface will generate CO via the cleavage of C\u2013C bonds. Therefore, the yield of CO increases with the addition of water, as shown in Fig.\u00a010. The influence of C/W on CO2 production is negligible since CO2 mainly comes from the carbon that is connected to the two oxygen atoms, and the production of CO2 is relatively low as stated before.Deactivation of catalysts is unavoidable in the catalytic SCWG reaction process. Carbon/coke deposition on catalyst surface is regarded as one main problem for the deactivation of Ni-based catalysts [51]. To investigate the carbon deposition and permeation behaviour in SCWG of cellulose, the carbon deposition and permeation on Ni surface under different temperatures and C/W were analysed. The block of NiNP was divided into three zones (inside the spherical shell) according to different radii, as illustrated in Fig.\u00a012\n(a). Carbon deposition rate is determined by the number of carbon atoms in different zones. Fig.\u00a012(b) shows carbon migration on the catalytic surface and in catalyst pores. There are a small number of carbon atoms in Zone 3\u00a0at 100 ps, and then the carbon atoms permeate into the inside of NiNP at 250 ps. The evolution of carbon number in different zones under varying temperatures is presented in Fig.\u00a012(d). The results show that carbon atoms infiltrate into Ni over time, and there is no noticeable difference in the total number of carbon when reaching an equilibrium state. However, the difference in the deposition rates under varying temperatures is appreciable. A small number of carbon atoms can be detected in Zone 2\u00a0at 250 ps when the temperature is 1800\u00a0K. With the increase in temperature, carbon would reach Zone 2\u00a0at an earlier time, at around 150 ps. The time instants for carbon reaching Zone 1\u00a0at 1800\u00a0K, 2000 K, and 2200\u00a0K are around 375 ps, 180 ps, and 130 ps, respectively. The results indicate that the permeation rate of carbon increases with increasing temperature. Nevertheless, the number of carbon molecules at the equilibrium state in different zones is roughly the same, which suggests that temperature has a negligible impact on the degree of carbon permeation.To uncover the carbon permeation mechanism on NiNP at different temperatures, the atomic order of Ni atoms was analysed. Steinhardt's bond orientational order parameters Q\nl\n (where l can take an integer value between 0 and infinity) [52] were used to explore the local atomic environment. These order parameters are mathematically defined based on certain rotationally invariant combinations of spherical harmonics calculated between atoms and their nearest neighbours, providing information about local atom environments. Q\nl\n has been used for various purposes, such as the structure identification of solid and liquid systems [53]. Commonly Q6 is used in the identification of cubic lattice structure. All the particles in a perfect ordered structure have the same value of Q6. As a results, Q6 values can be used to determine whether an ordered structure is beginning to turn into a disordered structure. Therefore, Q6 was adopted to characterise the atomic order of NiNP, which was built as a face-centred cubic lattice structure in this study. The magnitude of Q6 is large when the Ni atoms are ordered and small when the Ni atoms are disordered.\nFig.\u00a012(c) shows the sectional view of NiNP atomic Q6 values at different times. The internal atoms of NiNP are in an ordered state in the initial stage and then in transition to a disordered state over time. The evolution of averaged atomic Q6 of Ni atoms in different zones is presented in Fig.\u00a012(e). The equilibrium Q6 values decrease with increasing temperature. The position of atoms in the outermost shell (e.g., Zone 3) shifted first, and then the order degree of internal atoms decreased over time as heat was transferred to the interior region. The Q6 values in Zone 2 and Zone 1 decreased more slowly under lower temperatures. For example, the Q6 value in Zone 1\u00a0at 2200\u00a0K decreases significantly at around 125 ps, while the time instant for Q6 dramatical reduction at 1800\u00a0K is about 310 ps. The decrease in Q6 value represents that the crystal structure of NiNP is destroyed, and there is a relatively large displacement between Ni atoms. It is easier for carbon atoms to infiltrate into the NiNP when the displacement between Ni atoms becomes larger.\nFig.\u00a013\n shows the evolution of carbon numbers in different zones under varying C/W. The time that carbon reaches Zone 2 and Zone 1 is roughly the same under different C/W conditions, demonstrating that C/W has a negligible influence on carbon permeation on NiNP surface and in NiNP pores. Nevertheless, high C/W would inhibit the carbon deposition number on NiNP. The equilibrium carbon numbers in total and in different zones decrease with the increase in water molecule number. Wu and Liu [51] also reported that the increase of the steam to carbon ratio could favour carbon elimination during bio-oil gasification. The carbon elimination from the catalyst surface can be ascribed to the addition of water occupying the active sites on the catalyst surface, preventing the dissociative carbon atoms from attaching to NiNP.In this study, Nickel catalysed gasification of cellulose in supercritical water is investigated by using reactive MD simulation. The depolymerisation and ring-opening process of cellulose, effects of Ni and C/W on gaseous product yield, and carbon deposition behaviour on Ni catalyst were investigated. This study provides detailed information on Ni-catalysed cellulose SCWG at an atomic level.Calculated activation energies show that Ni can decrease the activation energy of C\u2013C and C\u2013O bond cleavage, promoting cellulose depolymerisation and ring-opening process. Cellulose could be gasified at a lower temperature with the addition of Ni. The activation energy reduction of C\u2013O is more significant than that of C\u2013C bonds under the effect of Ni.The H2, CO, and CO2 yields increase with increasing temperature. H2 yield increases significantly in the presence of Ni due to the large number of hydrogen free radicals generated by the cleavage of C\u2013H and O\u2013H bonds on the surface of NiNP. H radicals can not only interact with each other to produce H2 but also interact with H atoms on water and cellulose to generate H2. The \n\n\nO\n\n\n\u2022\n\nH\n\n generated would consume H radicals, leading to an increase in H2O number. The concentration of H radicals would increase if the number of \n\n\nO\n\n\n\u2022\n\nH\n\n in the reacting system can be suppressed, thereby increasing the yield of H2. Simulation results show that water plays a limited role in providing H free radicals to produce H2, the hydrogen atoms in cellulose are the primary source of H2 generation. The cellulose cracking fragments would be adsorbed on the NiNP surface, where these fragments undergo deoxygenation and dehydroxylation reactions, leading to a reduction of CO and CO2 yields. The addition of water will occupy the active sites on Ni surface, reducing the probability of molecular fragments attaching to the Ni surface. The small dissociative fragments outside Ni surface tend to generate more CO.The carbon deposition on the NiNP surface results in the deactivation of the catalyst. Due to the movement of Ni atoms at high temperature, the adsorbed carbon would infiltrate into the NiNP. Results suggest that carbon permeation rate increases with increasing temperature as the relative displacement of Ni atoms would be increased under higher temperatures. The increase in water mass fraction can favour carbon elimination from the catalyst surface because water would occupy the active sites on the NiNP surface, resulting in the failure of carbon adsorption. This study elucidated the detailed mechanism of Ni-catalysed cellulose SCWG from the molecular point of view, providing a basis for further biomass utilisation and cost reduction.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.Supercomputing time on ARCHER is provided by the \u201cUK Consortium on Mesoscale Engineering Sciences (UKCOMES)\u201d under the UK\nEngineering and Physical Sciences Research Council Grant No. EP/R029598/1. This work made use of computational support by CoSeC, the Computational Science Centre for Research Communities, through UKCOMES.", "descript": "\n                  Reactive force field (ReaxFF) molecular dynamic simulation was performed to elucidate the mechanism of Ni-catalysed supercritical water gasification of cellulose considering the effects of temperature and cellulose to water ratio. Simulations showed that Ni could decrease the activation energy of C\u2013C and C\u2013O bond cleavage, promoting the depolymerisation and ring-opening process of cellulose. The yields of gaseous products increase with the increasing temperature. H2 yield mainly depends on H free radical number, which can be generated from cellulose dehydrogenation and water splitting reactions. These two reactions were promoted on Ni surface, leading to an increase in H2 yield. In the presence of Ni catalyst, water plays a limited role in providing H free radicals to produce H2, while the hydrogen atoms in cellulose are the primary source of H2 generation. Meanwhile, reducing the concentration of \n                        \n                           \n                              O\n                              \n                              \n                              \u2022\n                           \n                           H\n                        \n                      could enhance H2 production as the combination of \n                        \n                           \n                              O\n                              \n                              \n                              \u2022\n                           \n                           H\n                        \n                      and \n                        \n                           \n                              H\n                              \u2022\n                           \n                        \n                      is a H radical consumption process. Small organic fragments would be absorbed on the Ni surface, where they undergo deoxygenation via the cleavage of C\u2013O bonds, resulting in a decrease in CO and CO2 yields. The increase in water mass fraction would promote the H2 yield as more H radical would be produced due to water splitting reaction. Moreover, the addition of water would occupy the Ni active sites and prevent the adsorption of organic fragments. These dissociative fragments are prone to produce more CO. The carbon deposition on the Ni surface results in the deactivation of the catalyst. Simulation results suggested that carbon deposition and permeation increase with increasing temperature. In contrast, the increase in water mass fraction can favour carbon elimination from the catalyst surface.\n               "}
{"full_text": "Cu is the only elemental metal electrocatalyst that can reduce CO2 to a wide range of hydrocarbons and/or oxygenates, but a specific product pathway usually shows low current density and Faradic efficiency [1\u20135]. In recent years, extensive research efforts have been devoted to addressing the above challenges. Different strategies focus on the modulation of properties of catalysts through both intrinsic routes including morphology design of crystal surfaces (crystal surface orientation, steps, edges, roughness, and particle size, etc.) and phase engineering, or extrinsic controls, such as oxidation state tuning, defect engineering, doping, composition variation, and so on [6\u201312]. For instance, the product distribution on Cu was shown both experimentally and theoretically to depend critically on the characteristics of Cu surfaces [7,13,14]. Montoya et\u00a0al. reported that Cu(100) exhibits a lower CO dimerization activation barrier than Cu(111) [15]. Bagger et\u00a0al. found that Cu(100)\u00a0\u00d7\u00a0(110) step is the specific ethanol-producing site by experimental and geometric analysis [7]. Regarding defect effects, the grain boundary density engineering in Cu could yield higher activity and selectivity for multi-carbon oxygenates [16]. In addition, a new Cu phase with 4H atomic arrangement exhibited enhanced overall activity and ethylene selectivity in CO2RR compared to the conventional FCC Cu [12]. Taking advantage of dopant-modulated oxidation state of Cu, Zhou et\u00a0al. demonstrated enhanced selectivity of C2 products by doping boron into Cu [17]. More generally, N\u00f8rskov et\u00a0al. suggested that alloying Cu-based catalysts by dopants with different oxygen affinity can be adopted to break the scaling relationship among different reaction intermediates to lower the overpotential [4]. Thereafter, plenty of Cu-based intermetallic compounds have been proposed, such as CuAl nanoalloy [11], AuCu nanoparticles [18], and polymer-supported CuPd nanoalloys [19].Recently, a new type of catalyst for CO2RR, single-atom alloys (SAAs), serving as model systems for understanding fundamental catalytic properties, has attracted a lot of attention [20]. SAAs are a class of single-atom catalysts, with small amounts of isolated metal atoms dispersed across the surface of a metal matrix. The isolated metal atoms typically have different catalytic activities and/or selectivities compared with host metals on CO2RR. The unique geometry of SAAs has the advantages in both increasing noble metal utilization ratio and breaking the limits of scaling relationships among different intermediates [20\u201323]. Up to date, SAAs has been successfully applied to various electrocatalytic reactions, including PtPd SAAs for the oxygen reduction reaction [24], AuRu and PtPd SAAs for hydrogen evolution reaction (HER) [24,25]. However, there have been only few studies on SAAs for CO2RR, let alone the extensive investigation on different combinations of metal species in SAAs and their effects on CO2RR.Although CO2RR catalysts have experienced rapid development with improved activity and selectivity in past decades, the large amount of data obtained in these studies combined with the complex reaction processes and the sensitivity to local environment of CO2RR make it challenging to develop a unified understanding of underlying mechanisms and design catalysts purposefully. First-principles simulations without empirical parameters are useful for facilitating the design of catalysts for CO2RR [26\u201332]. Based on theoretical calculations of free energies and microkinetic model, the thermodynamic descriptors of catalysts, i.e., intermediates adsorption energies, can be used to predict their properties, taking advantage of Sabatier-type trade-off between the adsorption of reactants and desorption of products [10,11,33\u201336]. Generally, the CO adsorption energy is a widely used descriptor to assess the activity of catalysts for CO2RR instead of extensive thermodynamic and kinetic simulations, which greatly accelerates the efficiency of catalyst design [10,33]. After identifying the optimal active sites, the surfaces of catalysts could be rationally engineered to maximize the presence of such sites, in particular when combined with advanced ML technique [10,11,37\u201341].In this work, combining high-throughput density functional theory calculations and ML techniques, we have screened optimal active sites of 43 Cu-based SAAs on different Cu surfaces with total 2669 configurations for promising CO2RR catalysis activity, and developed a general model to understand critical factors influencing the catalytic properties. With proper feature sets selected, an ML model based on gradient boosting regression (GBR) was trained to predict CO adsorption energies on Cu-based SAAs with low root mean square error (RMSE). What's more, a cross-group learning scheme was adopted to extend the capacity of ML model to make accurate predictions of CO adsorption energies on Cu-based SAAs with alloying elements unseen to the training set. Finally, by considering key constraints on CO adsorption energy and selectivity over HER, ZnCu, AgCu, GaCu, GeCu and\u00a0PCu SAAs are identified to exhibit highly active sites for\u00a0CO2RR.We consider Cu facets with different miller indexes which exhibit distinct catalytic activities and product selectivity on CO2RR. Structurally, different surfaces can be characterized by a set of first-nearest-neighbor coordination number (CN) of surface atoms. Based on the analysis of radial distribution function for all considered Cu-based SAAs, the first and second peaks were identified at 2.57 and 3.63\u00a0\u00c5, respectively, as shown in Fig.\u00a01\na. Hereby, 3\u00a0\u00c5 was adopted as the cutoff radius for first-nearest neighbors. Accordingly, ten distinct Cu surfaces were constructed, with their first-nearest-neighbor CNs listed in Table S1. It can be seen that the first-nearest-neighbor CNs for ten surfaces are limited to the range from 6 to 10. Therefore, Cu(100), (111), (110), (210), and (411), covering all first-nearest-neighbor CNs (6\u201310), were selected as SAA host surfaces, as shown in Fig.\u00a01b and f. By choosing 43 different elements as the alloying atoms, including 26 subgroup (d-block) elements and 17 main group \u2162A-\u2165A (p-block) elements (details in Fig.\u00a0S1). 731 different Cu-based SAA surfaces were constructed in total.Different surface sites with the specific combination of local CNs and alloying element are then searched for optimal CO adsorption energies, which closely relates to high-performance catalytic properties of Cu-based SAAs. To generate a sufficiently large database of CO adsorption energies on Cu-based SAAs for ML, various adsorption sites around single alloying atoms were considered. Based on the adsorption energies and structural characteristics of optimized configurations, possible duplicates in the dataset were removed to avoid potential overfitting. Finally, 2669 CO adsorption configurations on specific sites were obtained from 3150 initial structures.To characterize the different catalytic sites of Cu-based SAAs, 12 features, including intrinsic elemental characteristics, such as alloying atom mass (m), alloying atom Bohr radii (R), Pauling electronegativity (\u03c7), valence electron number (N\n\nv\n), the number of p electron of p-block element or d electron of d-block element (N\n\np/d\n), electron affinity of the alloying atom (E\n\nea\n), ionization potential (IP), and energy level center of valence electrons of specific elements (E\n\nc\n); structural characteristics, such as generalized CN of adsorption sites (GCN), the interplanar distance of Cu crystal surfaces (d\n\nhkl\n), the distance between adsorption site and alloying atom (D\n\nsa\n), and CO bond number on SAAs (CBN) were selected. It is worth noting that both (210) and (111) surfaces contain the first-nearest-neighbor CN of 9, while (411) surface has the same first-nearest-neighbor CNs of 8 and 7 with (100) and (110). To further distinguish the Cu-base SAA surfaces with the same first-nearest-neighbor CNs, GCN incorporating the information of every first-nearest-neighbor atom's CNs, which has been successfully applied in describing various electrocatalytic reactions on metal surfaces [42,43], is introduced as a more accurate description for local adsorption environment (Details in methods). Meanwhile, to characterize different CO adsorption configurations (on-top, bridge, hollow, etc.) on Cu-based SAAs, the bond number between CO and surface M atoms (M represents Cu or single alloying atom), i.e. CBNs, is adopted. The representation of active sites with different CBNs requires corresponding number of GCNs of surface atoms, constituting a vector. For instance, the adsorption on top of M (CBN\u00a0=\u00a01) corresponds to 1 GCN, while the bridge site adsorption of CO (CBN\u00a0=\u00a02) corresponds to 2 GCNs, and some hollow sites result in 3 or more GCNs. As a result, our GCN dataset exhibits variable dimensions. To address this problem, we have adopted a padding method, by which all GCN vectors are described with the same and maximum dimensions for all possible configurations, while those with smaller dimensions are appended by empty values set to zero. Similarly, D\n\nsa\n also has different dimensions, thus, the same padding procedure as in dealing with GCN is applied, but with the empty values set to \u22121. For GCN and D\n\nsa\n vectors, their components are arranged in ascending order, following by corresponding padding values. For instance, the smallest D\n\nsa\n, i.e. D\n\nsa1\n, is the first component, and D\n\nsa1\n\u00a0=\u00a00 corresponds to adsorption configurations with CO on alloying atom. By covering both surface structural information and chemical nature of catalysts, parts of these features have been successfully used to describe the binding energies of molecules and radicals on intermetallic and single-atom catalysts [44\u201346].Before ML model training, feature correlation was analyzed by Pearson correlation coefficient [47]. As shown in Fig.\u00a02\na, N\n\np/d\n, N\n\nV\n and E\n\nc\n show high linear correlation, implying that one of them could in principle replace another to reduce the feature set without losing key information. Thus, N\n\np/d\n and E\n\nc\n were neglected for feature importance analysis.GBR, support vector regression (SVR), and random forest regression (RFR) were used as the regression algorithms to train the models for CO adsorption energies on Cu-based SAAs. In order to supervise the bias-variance tradeoff and prevent overfitting, 64%: 16%: 20% of the DFT database were randomly divided as the training, validation, and testing sets, respectively. These three sets were separately used for model training, selection of hyper-parameters of model and test of model generalization on unseen data. R\n2 and RMSE were chosen to evaluate the accuracy of predicted energies. We used 10-fold cross-validation to select the best hyper-parameters. Afterwards, the model was tested on a test set which was split in advance to make sure that no severe overfitting was observed. With the same ten features of Cu-based SAAs, all three models exhibit high accuracies. The GBR model (testing set: R\n2\u00a0=\u00a00.956, RSME\u00a0=\u00a00.094\u00a0eV, Fig.\u00a0S2a) is slightly better than SVR (testing set: R\n2\u00a0=\u00a00.910, RSME\u00a0=\u00a00.133\u00a0eV, Fig.\u00a0S2b) and RFR (testing set: R\n2\u00a0=\u00a00.928, RSME\u00a0=\u00a00.120\u00a0eV, Fig.\u00a0S2c). It should be mentioned that configurations with significant reconstruction due to large difference in radius between alloying atoms and Cu were removed from training. These systems include ACu (A\u00a0=\u00a0C or O) and a small portion of YCu and NCu SAAs (Supplementary information for details, Fig.\u00a0S3). What's more, 63 non-adsorption configurations with positive adsorption energies were excluded (Supplementary information for details, Fig.\u00a0S4). For all three ML models, though test error is higher than training error, they remain in a relatively low level, which are comparable with those reported previously [10,37,48]. Moreover, the model complexity was supervised as shown in Fig.\u00a0S5. When the number of estimators was set to 50, an optimal bias-variance tradeoff of the model was reached. Based on GBR method, mean impact value (MIV) [49] analysis illustrates that the selected features have different importance levels in describing CO adsorption on Cu-based SAA surfaces, as shown in Fig.\u00a02b. The top two important features, i.e., N\n\nv\n and GCN\n\n1\n, contain both the electronic and structural properties of alloying atoms, which indicates our selected feature group is rational for Cu-based SAAs. Based on the importance analysis above, features were optimized by removing the few least important ones, including CBN and high-order components of GCN and D\n\nsa\n, i.e. GCN\n\n3\n, GCN\n\n4\n, GCN\n\n5\n, D\n\nsa3\n, D\n\nsa4\n, D\n\nsa5\n. The new model based on GBR with nine features maintains similar accuracy (R\n2\u00a0=\u00a00.953 and RMSE\u00a0=\u00a00.097\u00a0eV) as that learned from all features, as shown in Fig.\u00a02c. In addition, predicting energy based on the data set extracted from the initial structures is significant and challenging. By using GBR method, the models based on the data sets from relaxed configurations and unrelaxed counterparts have been trained, respectively. It shows the model based on the data set from relaxed configurations (testing set: R\n2\u00a0=\u00a00.910, RMSE\u00a0=\u00a00.097, Fig.\u00a0S6a) is more accurate in CO adsorption energy prediction than the latter (testing set: R\n2\u00a0=\u00a00.850, RMSE\u00a0=\u00a00.169\u00a0eV, Fig.\u00a0S6b), which is expected. Obtaining the high-precision ML model based on the original structure without DFT calculations of the database is a key issue, which deserves a further study.A well-performed ML should not only be able to determine an effective regression method within the scope of training set and finally generate a high-precision model, but also have the capacity to predict the CO adsorption energies on Cu-based SAAs with unknown alloying elements, which is later referred as target, using the model trained on the source dataset. Herein, in order to test the model generalization between Cu-based SAAs with different alloying elements, the whole dataset is grouped into 39 subsets according to alloying element, excluding N, Y, C and O alloyed SAAs. Starting from initial source dataset of CO adsorption energies on one specific SAA (corresponding to one type of alloying element), and gradually expanding the source dataset by increasing the number of different types of SAAs (x), energies in target dataset consisting of CO adsorption energies on one SAA were predicted. However, the model doesn't show a trend towards increased accuracy with the increase of the number of alloying elements in the source dataset. The RMSEs as a function of the number of different types of SAAs in training group were irregular, as shown in Fig.\u00a0S7. These results suggest that the model generalization is sensitive to data. This observation is further proved by predicting every single type of SAA using the model trained on all other 38 types of SAAs. As shown in Fig.\u00a0S8, the generalization of our model to different types of SAAs is obviously distinct. For example, our model can predict the energies with RMSE smaller than 0.1\u00a0eV for SAAs including As, Cd, Ga, Ge, Hf, In, P, Sb and Sn, while for elements like B, Al, Fe, Rh, Sc, W, it shows RMSE larger than\u00a00.25\u00a0eV.In order to understand the underlying cause, the nine-dimensional features were reduced to a two-dimensional space by Isomap [50], as displayed in Fig.\u00a0S9. It shows that the distribution of features for different SAAs exhibits a consistent pattern in the feature space. Accordingly, from a probabilistic point of view, the marginal probability distribution of both source and target, P(X) with X representing feature, could be regarded as the same. Although the ML model trained on source set couldn't be directly used to make accurate predictions on target SAAs with the alloying elements not included in the source, the information contained in the source is believed to be beneficial and could still be utilized. Now referring to the learning scheme as \u2018cross-group\u2019 learning: re-fit the former trained model by including 10 additional instances from target set. As is shown in Fig.\u00a02d, after re-fitting the model on the additional 10 instances, the RMSEs of predicted CO adsorption energies on target SAAs decrease notably to 0.2\u00a0eV or smaller. The results indicate excellent flexibility of our ML model for predicting CO adsorption energies on SAAs with new elements. Accurate predictions could be guaranteed with a supplement of at most 10 instances into the source training set, which implies that in this case over 90% of computation cost could be saved for new SAAs by cross-group learning scheme.In order to identify specific active sites for CO2RR, 2669 sites on Cu-based SAAs surfaces involving 43 alloying elements were systemically studied. Following the Sabatier principle, too weak binding of the intermediates on the surface leads to their quick desorption and thus weak charge transfer between intermediates and catalysts, while too strong binding results in catalyst poisoning. Accordingly, optimal adsorption energies are required to maximize catalytic activity. For CO2RR, CO is involved in many key reaction steps, and its adsorption energy on catalysts has been proved as an effective descriptor with the optimal CO adsorption energy of \u223c0.67\u00a0eV using revised Perdew\u2013Burke\u2013Ernzerhof (PBE) functional [10,33]. The distribution of calculated CO adsorption energies on Cu-based SAAs was analyzed for p- and d-block alloying elements, respectively. Fig.\u00a03\na shows a dominant single-peak distribution of adsorption energies on Cu-based p-block-element SAAs, and 35.5% of adsorption energies are within 0.1\u00a0eV of the optimal value of \u22120.67\u00a0eV. In contrast, the CO adsorption energies on Cu-based d-block-element SAAs are more diversely distributed as shown in Fig.\u00a03b. Besides 33.0% optimal CO adsorption sites, d-block-element SAAs generally show stronger binding for CO with broader adsorption energy distribution in the range of \u22122.30\u00a0eV\u00a0<\u00a0\u0394ECO\u00a0<\u00a0\u22120.77\u00a0eV. The wide distribution of adsorption energies in d-block-element SAAs suggests great tunability of alloying different d-block elements for different catalytic reactions. The enhanced binding strength results from stronger charge transfer and hybridization between Lewis acid CO and transition metals.Based on the feature importance analysis in Fig.\u00a02b, the five most important features of Cu-based SAAs were selected for further discussion, including N\n\nv\n, GCN, D\n\nsa\n, IP, and R. Compared to N\n\nv\n associated with atomic characteristics, the transferred charge (Q\n\nt\n) of alloying atom can better reflect the bonding properties and the key effects on surrounding Cu. Therefore, the charge transfer between various alloying atoms and Cu is explored to understand the fundamental mechanism. It can be found that Q\n\nt\n corresponding to sites within optimal CO adsorption energy range distributes widely in every segment, as shown in dark purple and dark brown in Fig.\u00a03c and d. Interestingly, both the negative and positive Q\n\nt\n could give rise to optimal adsorption energies. It suggests that charge transfer between alloying atoms and surrounding Cu atoms could activate surface sites, no matter whether it leads to positively or negatively charged active sites. Differently, previous reports showed that usually the negative Q\n\nt\n induced modulation of oxidation state in Cu2O, Cu2S or Cu through doping and alloying can improve CO2RR activity and selectivity [17,51\u201354]. Here, many alloying atoms of optimal sites exhibit positive Q\n\nt\n values, such as Si, Ga, Ge, In, Sn, Sb, Bi in p-block-element SAAs, and Cr, Mn, Fe, Co, Ni, Zn, Mo, Tc, Cd, W in d-block-element SAAs, indicating possible different mechanisms. The distribution of N\n\nv\n, R, and IP related to the sites with optimal energies were nearly uniform (Fig.\u00a0S10).Compared to elemental and electronic properties, surface structural features exhibit different distribution patterns. For GCN\n\n1\n, the range of [5,6] is the optimal coordination environment for both p-block and d-block elements as displayed in Fig.\u00a03e and f, indicating under-coordinated sites on Cu-based SAAs are highly active. Comparing the distribution of optimal adsorption energies on different surfaces of SAAs shown in Fig.\u00a0S11, the high-index surfaces, (210) and (411), have more active unsaturated sites than those of (100), (111) and (110), showing higher catalytic activity for CO2RR. Furthermore, Dsa reflects indirectly the effects of alloying atoms for CO adsorption on nearby Cu. Generally, the larger the D\n\nsa\n, the weaker the effects of alloying atoms on CO adsorption. Fig.\u00a03g and h show the partitions of D\n\nsa1\n according to adsorption sites on p- and d-block-element SAAs, respectively, where 0, \n\n\n(\n\n0\n,\n\n\n\n\n\n3.5\n\n]\n\n\n, and \n\n\n(\n\n3.5\n,\n\n\n\n\n\n4.3\n\n]\n\n\n \u00c5 correspond to cases with CO bonding to alloying atoms, first-nearest neighbor, and second-nearest neighbors of alloying atoms, respectively. For p-block-element alloying, CO mainly bonds to the first-nearest-neighbor Cu atoms (70.55%), which is attributed to the stronger electronegativity of p-block elements compared with Cu, leading to the modulation of Cu oxidation state. Differently, for d-block-element alloying, besides first-nearest-neighbor Cu atoms (54.79%), CO bonding to alloying atoms also takes an important part (34.67%). The strong tendency to top-site adsorption originates from the strengthened adsorption as illustrated in Fig.\u00a03b. Notably, active sites with optimal CO adsorption energies on both p-block- and d-block-element SAAs are mainly the first-nearest neighbors, as shown in Fig.\u00a03i and j.The above analysis indicates that the modulation of either oxidation or reduction states on Cu atoms could improve the catalytic activity for CO2RR. To understand these effects, we further investigate the electronic structures of representative CO adsorption configurations with three different charge states, including AsCu(411), ZnCu(411), and PdCu(111). Q\n\nt\n of As in AsCu(411) and Zn in ZnCu(411) are \u22120.32 and\u00a0+\u00a00.22 \n\n\n|\ne\n|\n\n\n, respectively. In these two cases, CO prefers to adsorb on the first-nearest-neighbor Cu sites. From the partial density of states (PDOS) plot in Fig.\u00a04\na and b, both the increased occupation of Cu 3d states in the range of [\u22127.8, \u22126.3] eV (indicated by red arrows) and their enhanced hybridization with C 2p states contribute to the improved adsorption strength compared with pure Cu cases. While for PdCu(111), Q\n\nt\n is \u22120.39 \n\n\n|\ne\n|\n\n\n. The strong CO adsorption strength results from effective overlap between Pd d states and C 2p orbitals, as shown in Fig.\u00a04c. This could be the main reason for more negative adsorption energies in d-block-element SAAs. CO adsorption energies on PdCu (111), AsCu (411) and ZnCu (411) SAAs are 0.26, 0.07 and 0.03\u00a0eV lower than that of Cu counterpart, respectively (Table S2). In addition, the charge density difference and Bader charge analysis show that CO on these three different surfaces keeps as an electron acceptor. The interaction between CO molecule and local charged atoms were further explored by analyzing the projected density of states of the catalysts before and after CO adsorption, shown in Fig.\u00a0S12. For all considered examples, the bonding d-\u03c3 and antibonding d-\u03c0\u2217 orbitals form after CO adsorption. With Fermi level crossing d-\u03c0\u2217 orbital, the excess electrons from surface atoms d orbitals back to antibonding \u03c0\u2217 orbital help stabilize the adsorption configurations. Therefore, the increased charges in CO can be used to roughly qualitatively estimate the contribution of the back-donating effect. Table S3 shows that |\u0394Q(CO)| for ZnCu and PdCu were larger than those of Cu counterparts, consistent with the enhanced CO adsorption. It is noted the excess electrons on CO was transferred from negative Cu around positive Zn or negative Pd, highlighting the importance of the negative charged metal atom center for enhancing CO2RR activity, which has been proposed for single-atom catalysts for oxygen evolution reaction [55]. In contrast, for AsCu, CO adsorbs on positive Cu site around As, leading to decreased |\u0394Q(CO)|. In this case, the slightly increased CO adsorption energy might result from the enhanced d-\u03c3 interaction. Furthermore, the charge transfer between Cu and alloying atoms could render CO molecules on alloying atoms and adjacent Cu oppositely charged, inducing enhanced Coulomb attraction interaction between them which promotes the activity and selectivity of C2 products [54].The active sites on Cu-based SAAs can then be screened against the competing hydrogen evolution reaction (HER). For HER, H adsorption energies \u0394EH, the key indicator for HER activity, were calculated for the screened configurations with optimal CO adsorption strength, with the screening criteria \u0394EH\n\n\n\u2209\n\n [\u22120.37,-0.17] eV [10]. Figs. S13a and S13b are two-dimensional distribution of CO adsorption energies versus adsorption configuration and element on p- and d-block-element SAA surfaces within (\u22120.77, \u22120.57) eV, respectively. Out of total 1102 and 1567 CO adsorption energies on p-block- and d-block-element SAAs, there are 391 and 517 sites showing optimal values, respectively. The proportion of optimal adsorption sites to total sites for a specific Cu-based SAAs could reflect its activity, which gives rise to the following sequences: P\u00a0>\u00a0Ga\u00a0>\u00a0As\u00a0>\u00a0Ge\u00a0>\u00a0Sb\u00a0>\u00a0Sn\u00a0>\u00a0In\u00a0>\u00a0Si for p-block-element SAAs, and Pd\u00a0>\u00a0Cd\u00a0>\u00a0Ag\u00a0>\u00a0Au\u00a0>\u00a0W\u00a0>\u00a0Zn\u00a0>\u00a0Tc\u00a0>\u00a0Re\u00a0>\u00a0Mo\u00a0>\u00a0Pt\u00a0>\u00a0Ru for d-block-element SAAs. The optimal CO adsorption sites were then screened by H adsorption energy out of [\u22120.37, \u22120.17] eV, as shown in Figs.\u00a0S14 and S15. Consequently, target SAAs with promising high-activity and selectivity over HER include PCu, AgCu, GaCu, SbCu, ZnCu, SnCu, GeCu, InCu, WCu, MoCu and\u00a0SiCu. Here, due to the high cost or toxicity, Pd, Au, Pt, Cd, Ru, Re, Tc and As were excluded, despite their promising CO2RR activity.In order to check the feasibility of Cu-based SAAs in the experiments, their thermodynamic stabilities were first estimated by formation energies shown in Fig.\u00a0S16. In addition, we also compared the energy differences between SAAs and dopant dimmer configurations with the same concentrate for one of the most promising surfaces, Cu (411), as listed in Table S4. \u0394E is calculated as \u0394E\u00a0=\u00a0ESAA - EDimer, where ESAA and EDimer are the energies of SAAs and the dopant dimmer counterparts with the same concentrate. It shows that the energies of MoCu and WCu with dopant dimmers are much lower than those of SAAs, indicating Mo or W atoms in Cu based SAAs tend to form clusters. For the candidates with a small energy difference (0\u00a0<\u00a0\u0394E\u00a0<\u00a00.2\u00a0eV), Ab initio molecular dynamics (AIMD) simulations were further performed at 300K for 10 ps. Our results show that Sb atoms tend to adsorb on the surface rather than doping, as shown in Fig.\u00a0S17. After excluding SAAs with too high energy (MoCu and WCu) and poor thermal stability (SbCu), eight promising catalyst materials were screened out, including PCu, AgCu, GaCu, ZnCu, SnCu, GeCu, InCu, and SiCu. Experimentally, dilute Zn, Ga, Ag, and Pt alloyed Cu have been synthesized [53,56,57]. Notably, some of the predicted results have been confirmed by recent CO2RR experiments, verifying the reliability of our work. For instance, CuZn nanoparticles with low Zn concentration showed enhanced CH4 Faraday efficiency compared to pure Cu [53]. CuAg with the surface radio 7:1 demonstrated a higher activity for yielding a single liquid product, acetaldehyde, in contrast to pure Cu which forms seven different products at the same potential [57]. Following these reports, the recommended synthesis routes include, but not limited to, surface replacement reaction in the solution and selective dealloying of bimetal materials [58].Based on the analysis of feature distributions, the low-coordination high-index crystal surfaces exhibit more active sites than those of low-index surfaces. Therefore, Cu (411) surface was selected to evaluate the limiting potential of SAAs for CO2 reduction to CO by considering reaction pathways R1 and R2 as following:\n\n(R1)\n\u2217\u00a0+\u00a0CO2\u00a0+\u00a0(H+\u00a0+\u00a0e\u2212) \u2192 \u2217COOH\n\n\n\n\n(R2)\n\u2217COOH\u00a0+\u00a0(H+\u00a0+\u00a0e\u2212) \u2192 \u2217CO\u00a0+\u00a0H2O\n\n\nBased on the implicit solvent models, the detailed Gibbs free energies diagrams for the reduction of CO2 to intermediate product \u2217CO on eight suggested SAAs are calculated in Fig.\u00a05\na. Compared with pure Cu(411), all SAA surfaces show enhanced \u2217COOH and \u2217CO binding, leading to improved catalytic performance. In particular, the limiting potential decreases from 1.53\u00a0V for pure Cu (411) surface to 1.21 and 1.18\u00a0V for Si and P SAAs. Moreover, their selectivity over HER was also explored by calculating the thermodynamic diagram shown in Fig.\u00a05b. Here, the \u2217H adsorption models were constructed according to most stable \u2217CO configurations. The obtained results in Fig.\u00a05b show that all |G(\u2217H)| on the selected SAAs were larger than that on pure Cu (411), indicating alloying further improves the selectivity of SAAs over HER.The active sites on Cu-based SAAs catalysts of CO2RR have been extensively explored based on 2669 CO adsorption configurations by DFT calculations. A simple ML model with high accuracy was generated for predicting CO adsorption energies. Our trained ML model is further applied to accurately predict CO adsorption energies on Cu-based SAAs with alloying element unseen to the training set by a cross-group learning scheme. Extensive ML analysis demonstrates valence electron number of alloy atoms and GCN are two key features to characterize the optimal active sites of Cu-based SAAs for CO2RR. By screening of optimal CO adsorption sites with high selectivity over HER, PCu, AgCu, GaCu, ZnCu, SnCu, GeCu, InCu, and SiCu SAAs, were recognized as promising catalysts for CO2RR. This work provides a feasible strategy to design Cu-based and other SAAs catalysts materials for improved CO2RR.Five Cu miller\u2013index facets with different coordination characteristics, including (100), (111), (110), (210), and (411), were constructed. Because catalysis mainly occurs near the surface, substitutional sites for alloying atoms were located in the surface and subsurface with specific coordination numbers. In this sense, our SAA models can be viewed as single atom surface alloy, a simplified model of the SAAs. The surface models were constructed with each cell vector of surfaces at least 7.2\u00a0\u00c5, and in c direction at least four layers of Cu were chosen. For the description of geometric structure, GCN of a specific site x is introduced as \n\nG\nC\nN\n\n(\nx\n)\n\n=\n\n\n\u2211\n\n\ny\n=\n1\n\n\nn\nx\n\n\nC\nN\n\n(\ny\n)\n\n\nn\ny\n\nC\n\n\nN\n\nm\na\nx\n\n\n\n\u2212\n1\n\n\n\n, where CN of first-nearest-neighbor atom y is weighted by \n\n\nn\ny\n\nC\n\n\nN\n\nm\na\nx\n\n\n\n\u2212\n1\n\n\n\n, with ny and CNmax representing the number of first-nearest-neighbor atoms y with the same CN(y) and the maximum CN in the bulk, respectively. For FCC crystals, CNmax is 12 [42,43].Radial distribution function, g(r), was used for atomic pairwise distance analysis, in order to shed light on the reasonable selection of cutoff distance for first nearest neighbors. By dividing the space volume into shells dr, it is possible to compute the number of atoms dn(r) at a distance between r and r\u00a0+\u00a0dr from a given atom:\n\n\n\nd\nn\n\n(\nr\n)\n\n=\n\nN\nV\n\n\u00b7\ng\n\n(\nr\n)\n\n\u00b7\n4\n\u03c0\n\nr\n2\n\n\u00b7\nd\nr\n\n\n\nwhere N and V represent total number of atoms and the whole volume of the system.First-principles calculations were performed within the framework of DFT, as implemented in the Vienna ab initio simulation package [59]. Projector-augmented wave potential was used with a plane-wave cutoff energy of 400\u00a0eV [60]. Exchange and correlation were described by the generalized gradient approximation in the scheme of revised PBE [61,62]. Spin-polarized calculations were also performed for magnetic systems. The Brillouin-zone integration was done using a Monkhorst\u2013Pack grid of k-point sampling [63]. K-spacing was set to 0.3 for all structures to allow the smallest spacing between k-points in units of 0.3\u00a0\u00c5\u22121. A vacuum distance larger than 20\u00a0\u00c5 was employed to avoid interactions between neighboring images, and the bottom two layers were fixed during relaxation. Adopting the conjugated gradient method, geometrical optimizations were carried out with the convergence threshold set at 1\u00a0\u00d7\u00a010\u22124\u00a0eV atom\u22121 in energy and 0.05\u00a0eV\u00a0\u00c5\u22121 in force. AIMD simulations were performed at 300\u00a0K, in an NVT ensemble using the Nos \n\n\ne\n\u2032\n\n\n-Hoover heat method for 10 ps with a time step of 1.0 fs. CO and H adsorption energies were calculated from \n\n\u0394\nE\n=\n\nE\n\na\nd\n\n\n\u2212\n\nE\n\nS\nA\nA\n\n\n\u2212\n\nE\n\nC\nO\n/\nH\n\n\n\n, where Ead and ESAA are the energies of SAA surfaces with and without adsorbates (CO/H), ECO and EH are the energies of H2 and CO, respectively. The formation energies of SAAs was calculated by \n\n\nE\nf\n\n=\n\nE\n\nS\nA\nA\n\n\n\u2212\n\nE\n\nC\nu\n\n\n\u2212\n\n\u03bc\nA\n\n+\n\n\u03bc\n\nC\nu\n\n\n\n, where ESAA and ECu are the energies of SAA surfaces and Cu counterpart, \n\n\n\u03bc\nA\n\n\n and \n\n\n\u03bc\n\nC\nu\n\n\n\n are the chemical potentials of alloying element and Cu.Gradient boosting regression (GBR), support vector regression (SVR), and random forest regression (RFR) were carried out using scikit-learn package [64]. The correlation of features was evaluated by Pearson correlation coefficient (p) [47], which is determined by\n\n\n\np\n=\n\n\n\n\n\u2211\n\ni\n\n\n(\n\nf\ni\n\n\u2212\n\nf\n\u00af\n\n)\n\n\n(\n\nF\ni\n\n\u2212\n\nF\n\u00af\n\n)\n\n\n\n\n\n\n\n\u2211\n\ni\n\n\n\n(\n\nf\ni\n\n\u2212\n\nf\n\u00af\n\n)\n\n2\n\n\n\n\n\n\n\n\u2211\n\ni\n\n\n\n(\n\nF\ni\n\n\u2212\n\nF\n\u00af\n\n)\n\n2\n\n\n\n\n\n\n\n\n\nHere, p is correlation coefficient between feature \n\n\nf\ni\n\n\n and \n\n\nF\ni\n\n\n within the range of [\u20131,1]. The feature importance was analyzed by MIV, determined as [49].\n\n\n\n\nm\n\n(\ni\n)\n\n\n=\n\n\n\n\ny\n\u02c6\n\n1.1\n\n(\ni\n)\n\n\n\u2212\n\n\ny\n\u02c6\n\n0.9\n\n(\ni\n)\n\n\n\n\n\n\n\u2211\n\n\nj\n=\n1\n\nn\n\n(\n\n\ny\n\u02c6\n\n1.1\n\n(\nj\n)\n\n\n\u2212\n\n\ny\n\u02c6\n\n0.9\n\n(\nj\n)\n\n\n)\n\n\n\n\n\n\nHere, \n\n\n\ny\n\u02c6\n\n1.1\n\n(\ni\n)\n\n\n\n and \n\n\n\ny\n\u02c6\n\n0.9\n\n(\ni\n)\n\n\n\n represent the prediction values of the model after the corresponding feature is multiplied by 1.1 and 0.9, respectively. The summation loops over all n features. Intuitively, the analysis depicts the sensitivity of the model prediction to a certain feature. If the prediction varies dramatically as the certain feature fluctuates, the corresponding feature is considered to be important.The accuracy of models was identified by RMSE and the coefficient of determination (R2), which are defined as\n\n\n\nR\nM\nS\nE\n=\n\n\n\n1\nN\n\n\n\n\u2211\n\ni\nn\n\n\n\n(\n\nY\ni\n\n\u2212\n\n\ny\n\u02c6\n\ni\n\n)\n\n2\n\n\n\n\n\n\n\n\n\n\n\n\nR\n2\n\n=\n1\n\u2212\n\n\n\n\n\u2211\n\ni\n\n\n\n(\n\nY\ni\n\n\u2212\n\n\ny\n\u02c6\n\ni\n\n)\n\n2\n\n\n\n\n\n\u2211\n\ni\n\n\n\n(\n\nY\ni\n\n\u2212\n\nY\n\u00af\n\n)\n\n2\n\n\n\n\n\n\n\n\n\n\nY\ni\n\n\n and \n\n\n\ny\n\u02c6\n\ni\n\n\n are the adsorption energies by DFT calculations and model prediction, respectively, and \n\n\nY\n\u00af\n\n\n is the average energy of DFT calculations.Isomap, one representative of isometric mapping methods, is used for nonlinear dimensionality reduction. It seeks a lower-dimensional embedding which maintains geodesic distances between all points [50].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 (Grant Nos. 62006219 and 62001266) Guangdong Innovative and Entrepre-neurial Research Team Program (grant No. 2017ZT07C341), the Bureau of Industry and Information Technology of Shenzhen for the 2017 Graphene Manufacturing Innovation Center Project (No. 201901171523), the China Postdoctoral Science Foundation (No. 2020M680506) and Guangdong Basic and Applied Basic Research Foundation (No. 2020A1515110338). D. Wang and R. Cao contributed equally.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.2021.10.003.", "descript": "\n                  Various strategies, including controls of morphology, oxidation state, defect, and doping, have been developed to improve the performance of Cu-based catalysts for CO2 reduction reaction (CO2RR), generating a large amount of data. However, a unified understanding of underlying mechanism for further optimization is still lacking. In this work, combining first-principles calculations and machine learning (ML) techniques, we elucidate critical factors influencing the catalytic properties, taking Cu-based single atom alloys (SAAs) as examples. Our method relies on high-throughput calculations of 2669 CO adsorption configurations on 43 types of Cu-based SAAs with various surfaces. Extensive ML analyses reveal that low generalized coordination numbers and valence electron number are key features to determine catalytic performance. Applying our ML model with cross-group learning scheme, we demonstrate the model generalizes well between Cu-based SAAs with different alloying elements. Further, electronic structure calculations suggest surface negative center could enhance CO adsorption by back donating electrons to antibonding orbitals of CO. Finally, several SAAs, including PCu, AgCu, GaCu, ZnCu, SnCu, GeCu, InCu, and SiCu, are identified as promising CO2RR catalysts. Our work provides a paradigm for the rational design and fast screening of SAAs for various electrocatalytic reactions.\n               "}
{"full_text": "Magnetic ferrite nanoparticles in the spinel phase have been considered as an influential class of materials, which are employed in various high-frequency device applications [1]. The cubic spinel structure has the chemical formula of AB2X4, where the anions\u00a0X\u00a0are occupied by O atom as metal oxides forming the cubic close-packed lattice, tetrahedral interstices fill the A site as the \u2018network formers\u2019 and octahedral interstices occupy the B site as the \u2018modifiers\u2019, called the Ferro-spinel and semiconductor in nature [2\u20135]. Most spinel ferrites belong to the space group of Fd3m (No. 227, Z\u00a0=\u00a08), which provide the highest symmetrical face-centered cubic (FCC) spinel structure. A spinel unit supercell's crystal is formed by 8 A-sites and 16 B-sites cations. Based on the distribution of divalent metal ions and trivalent ferric ions over A and B-sites, spinel ferrites are of three classes; normal spinel, inverse spinel, and mixed spinel [6].Magnetically soft spinel ferrites are used in a large spectrum of biomedical and industrial applications including medical treatments, magnetic resonance imaging, antenna fabrication, computer memories, energy storage in supercapacitors, high-density information storage, high-frequency transformers, hyperthermia treatment, multi-layered chip inductors, water purification methods, sensing of nucleic acid, separation of DNA and RNA, gene therapy and delivery, ferrofluids and so on [7\u201313]. The advancement of electronic devices is now moved to integrated circuits-based technologies, where the use of highly efficient transistors is increasing gradually in accordance with the Moore\u2019s law, thus requiring the nano-level engineering and fabrication. Therefore, in contrast to bulk materials, researchers are now focusing on nanocrystalline ferrites\u2019 for utilizing them in the latest nano-technological devices. The physical and chemical characteristics of ferrite nanomaterials mostly depend on their scale size, shape, or morphology. The structural parameters such as crystal size and lattice parameters are somehow linked to the electrical and magnetic properties of ferrite nanoparticles. Therefore, the controlling of several factors such as the particle size, surface-to-volume ratio, and magnetic anisotropy eventually improves the electronic properties of magnetic nanoparticles in the spinel phase, owing to their transition from bulk to nano-shape.Researchers are continuously paying efforts to employ an easy and efficient method for yielding the nanocrystalline ferrites to tailor their structural, dielectric, electric, and magnetic properties under favorable environmental conditions. Various techniques have been used to synthesize nanostructured ferrite materials such as the sol\u2013gel auto combustion, co-precipitation, high-energy milling, hydrothermal synthesis, precursor method, mechano-chemical route, and microwave hydrothermal [7\u20139,14\u201317]. Among these, the sol\u2013gel route appears to be a prominent method for preparing highly crystalline ferrite nanoparticles, as it is an eco-friendly, less expensive, and effective approach to maintaining a good stoichiometry during the synthesis process. The sol\u2013gel is a wet chemical method, which is widely used due to its potential advantages such as enhanced control over homogeneity, elemental composition, and powder morphology with a uniform narrow particle size distribution at relatively low temperature [7,16\u201318].Researchers previously attempted sporadically to study the effects of doping on the structural, electrical, morphological, photocatalytic, and magneto-dielectric properties of Ni-Cu ferrite NPs. Doping is an effective method to ameliorate the applications to a broad range by achieving excellent optoelectronic properties. Investigations are still continued with various dopants or substitutions in A and B-sites to tailor the physical, structural and electromagnetic properties of Ni-based ferrite nanoparticles [19\u201328]. Munir et al.\n[29] conducted an experiment with a noble nanocomposite CuFe2O4/Bi2O3 by introducing Bi2O3 nano-petals into the porous CuFe2O4 and observed a significant increase in the photocatalytic activity in the effect of photo-degradation activity. The investigated nanocomposite showed an excellent magnetic separation at room temperature for the reduced recombination and improved separation of electron-hole pairs. Carbon coated highly active magnetically recyclable hollow nanocatalysts were synthesized by Shokouhimehr et al. [30], where the authors projected that the prepared nanocomposite can be used as a general platform for loading other noble metal catalyst nanoparticles, resulting in the high yields (up to 99 percent) in selective nitroarenes reduction and Suzuki cross-coupling reactions. Furthermore, magnetic properties revealed that the catalysts could be easily separated using a suitable magnetic field and recycled five times in a row. Moreover, Rahman et al.\n[31] thoroughly investigated the photocatalytic efficiency and recycling stability of rGO-supported cerium substituted nickel ferrite nanoparticles under visible light illumination. According to their findings, NiCeyFe2-yO4/rGO (NCFOG) nanocomposite outperformed NiCeyFe2-yO4 nanoparticles by two times in photocatalytic efficiency and recycling stability, which is attributed to the formation of NCFOG heterojunction that enables the separation of photo-induced charge carriers with maintaining a strong redox ability. Recently, M. Arifuzzaman et al. [17] studied Cu substituted Ni-Cd ferrite NPs and reported the decrease of average crystallite size and saturation magnetization of Ni0.7-xCuxCd0.3Fe2O4 with Cu-substitution up to x\u00a0=\u00a00.2. Besides, V. A. Bharati et al. [32] reported the influence of doping of both Al3+ and Cr3+ on the structural, morphological, magnetic, and M\u00d6ssbauer properties of Ni ferrite NPs and justified their suitability in HF device applications.In [33], K. Bashir et al. revealed the electrical and dielectric properties of Ni-Cu ferrite NPs with doping of Cr3+, which made them the potential for HF applications and photocatalytic activity. Le-Zhong Li et al. [34] examined the Al3+ substituted Ni-Zn-Co ferrites and observed a decrease in saturation magnetization at x\u00a0>\u00a00.10. They reported the metal\u2013semiconductor transition behavior of Ni-Zn-Co ferrites as an effect of varying temperature and the increase of dc resistivity with Al content. The structural and magneto-optical properties of Ni ferrite NPs were reported in [35], where the authors calculated the electronic bandgap of 1.5\u00a0eV and observed a decrease in saturation magnetization and Tc with Al3+ content. In addition, a density functional theory (DFT) was used in estimating the electronic structure of CuO NPs with the optimized geometric crystal calculation, which showed the variation of energy band gap with Al content [36]. The effect of doping materials on the characteristics of different spinel ferrite nanoparticles is available in the literature, Zn ferrite [37,38], Ga ferrite [39], Co ferrite [40\u201342], Fe ferrite [43], Mg ferrite [25,44], and Ni-Zn ferrite [45,46].However, as per the literature survey, no study has been found yet on the structural, dielectric, and electrical properties of Al3+ substituted nanocrystalline Ni-Cu spinel ferrites. Therefore, it is important to perceive the role of Al3+ substitution on the structural, physical, and dielectric characteristics of Ni-Cu ferrite NPs. Henceforth, the present study aims to explore the influence of Al3+ substitution on the structural, dielectric, and electrical properties of the synthesized Ni0.70Cu0.30AlxFe2-xO4 (x\u00a0=\u00a00.00 to 0.10 with a step of 0.02) through the sol\u2013gel process.To synthesize the studied nanocrystalline Ni-Cu spinel ferrites, analytical-grade reagents of nickel (II) nitrate Ni(NO3)2\u00b76H2O (98%), copper (II) nitrate Cu(NO3)2\u00b73H2O (95\u2013103%), ferric (III) nitrate Fe(NO3)3\u00b79H2O (98%), and aluminum (III) nitrate Al(NO3)3\u00b79H2O (98%) were used in this experiment, which were purchased from the Research-Lab Fine Chem.Derivatives of Ni0.70Cu0.30AlxFe2-xO4 (0\u00a0\u2264\u00a0x\u00a0\u2264\u00a00.1) nanoparticles with a step of 0.02 were synthesized by the sol\u2013gel process. In this process, raw materials of Ni(NO3)2\u00b76H2O, Cu(NO3)2\u00b73H2O, Fe(NO3)3\u00b79H2O, and Al(NO3)3\u00b79H2O dissolved in ethanol and mixed properly with a magnetic stirrer to make the homogeneous solution. The pH of the mixture was kept at 7 using the liquid NH4OH solution and the sol was continued to heat up to a temperature of 70\u00a0\u00b0C and stopped when it was turned into a dry gel. In an electric oven, the dried gel was heated at 200\u00a0\u00b0C for 5\u00a0h, during which a self-ignition occurred and the compositions gradually became fluffy-loose powder. To obtain the resulting ingredients in a highly crystalline form, the derived powder was further annealed at 700\u00a0\u00b0C for another 5\u00a0h to eliminate any impurity in the samples. The powder was then grinded in a hand-milling process in a mortar to make it more homogeneous. A hydraulic press of 65\u00a0MPa was then applied to the samples for 2\u00a0min to condense and turned into disk shapes.The prepared samples were 12\u00a0mm in diameter and 2.3\u00a0mm in thickness. Powder samples were finally used for dielectric and electrical measurements.The structural parameters of the yielded nanocrystalline ferrites were determinedthrough the powder x-ray diffractometer (XRD) analysis using the model PW3040, with CuK\u03b1 radiation of \u03bb\u00a0=\u00a01.5418\u00a0\u00c5. The lattice parameter, crystal size (D), and displacement density were retrieved by using the XRD data. The theoretical density (\u03c1th), micro-strain (\u03b5ms), lattice strain (\u03b5ls), and stacking faults in the crystal structure were also determined. The lattice parameter (\n\na\n\n) and crystallite size (D) were measured by the following relations [20]:\n\n(1)\n\n\na\n=\n\nd\n\nhkl\n\n\n\n\n\n\n\n\nh\n\n2\n\n+\n\n\nk\n\n2\n\n+\n\n\nl\n\n2\n\n\n\n\n\n\n\n\n\n\n\n(2)\n\n\nD\n=\n\n\n0.9\n\u03bb\n\n\n\n\u03b2\n\nhkl\n\n\nc\no\ns\n\u03b8\n\n\n\n\n\nwhere, \u03bb, \u03b2hkl, \u03b8, and dhkl, respectively, indicate the wavelength of the X-ray, the full width at half maximum (FWHM) at the most prominent peak (311), the Bragg\u2019s angle, and the distance between adjacent planes. The cubic spinel phase of the prepared samples was reconfirmed through Fourier transform infrared spectroscopy (FTIR). The morphology of the studied materials has been interrogated by the Field Emission Scanning Electron Microscopy(FESEM) (JEOL-JSM 7600F model).The Wynne- Kerr Impedance Analyzer (model: 6500B) was used to determine the complex dielectric (\u03b5*), complex electric modulus (M*), complex impedance (Z*), and AC resistivity (\u03c1AC) of the investigated Ni-Cu-Al ferrite nanoparticles.XRD patterns of Al3+ substituted Ni-Cu ferrites annealed at 700\u00a0\u00b0C, are illustrated in.\nFig. 1\n, where the peaks are resulted due to diffractions from the planes of (111), (220), (311), (222), (400), (422), (511), and (440). The peaks are found well-defined with a homogeneous distribution of nanoparticles, which verify their high crystalline nature with no impurity. The existence of such peaks indicates the cubic single-phase formation of the spinel materials [32\u201334]. The peak diffracted from the plane (311) is found to have a high intensity, which was used to determine the average crystallite size (see Table 1\n) of the materials using Debye-Scherer\u2019s equation. The lattice constant (a0\n) values of the samples are calculated by the Nelson-Riley technique and unit cell volumes (V) of the compositions are listed in Table 1. A decreasing trend in the variation of lattice constant and cell volume with Al3+ content is observed as shown in Fig. 2\n , which is due to the replacement of larger ionic (0.67\u00a0\u00c5) cations by cations with smaller radius (0.51\u00a0\u00c5). As Fe3+ is replaced by Al3+ in the investigated ferrites, the unit cell becomes shrinkage, as a result, both a\n0 and V decrease linearly with Al3+ content, which is well satisfied by the Vegard's law [47,48]. As appeared in Table 1 and Fig. 2, the average crystallite size decreases with Al3+ content, which might be because of the ionic radius difference between Al3+ and Fe3+. The redistribution of cations in A and B sites ultimately causes the increase in stress and strain of the samples. Lattice spacing is determined by the following equation:\n\n(3)\n\n\nd\n=\n\n\nn\n\u03bb\n\n\n2\ns\ni\nn\n\u03b8\n\n\n\n\n\nwhere d is the inter-spacing distance between crystal planes and the value of n is taken as 1, which represents the order of diffraction.The sharp diffraction peaks from XRD confirm the higher crystallinity of the analyzed ferrite nanoparticles. The percentage crystallinity of the prepared nanoparticles is measured by the following equation [49,50]:\n\n(4)\n\n\n%\nC\nr\ny\ns\nt\na\nl\nl\ni\nn\ni\nt\ny\n=\n\n\nA\nr\ne\na\n\nu\nn\nd\ne\nr\n\nt\nh\ne\n\nc\nr\ny\ns\nt\na\nl\nl\ni\nn\ne\n\np\ne\na\nk\ns\n\n\nA\nr\ne\na\n\no\nf\n\nt\nh\ne\n\na\nl\nl\n\np\ne\na\nk\ns\n\n\n\u00d7\n100\n\n\n\n\nThe theoretical density (\u03c1th\n) is calculated by the following relation [51]:\n\n(5)\n\n\n\n\u03c1\n\nth\n\n\n=\n\n\n8\n\nM\nw\n\n\n\n\nN\na\n\n\na\n\n0\n\n3\n\n\n\n\n\n\nwhere Mw and Na indicate the molecular weight of the samples and Avogadro\u2019s number, respectively. The experimental density (\u03c1ex\n) is calculated by the following equation:\n\n(6)\n\n\n\n\u03c1\n\nex\n\n\n=\n\nM\n\n\u03c0\n\n\nr\n\n2\n\nl\n\n\n\n\n\nwhere M, r, and \n\nl\n\n represent the mass, radius, and height of the synthesized samples in tabloid shape, respectively.The experimental density and estimated density of the samples annealed at 700\u00a0\u00b0C are listed in Table 1. The porosity is found to increase as presented in Table 1, which is due to the discontinuity of the grain size, resulting in the decrease in density. The percentage porosity of the samples is calculated by the following relation:\n\n(7)\n\n\nP\n\n(\n%\n)\n\n=\n\n\n\n\u03c1\n\nth\n\n\n-\n\n\u03c1\n\nex\n\n\n\n\n\u03c1\n\nth\n\n\n\n\u00d7\n100\n%\n\n\n\n\nThe porosity is the contribution of inter-granular and intra-granular as shown by the following equation:\n\n(8)\n\n\nP\n\n\n\n%\n\n\n\n=\n\nP\n\ninter\n\n\n+\n\nP\n\nintra\n\n\n\n\n\n\nThe total displacement length per unit volume of the crystal structure is referred as the dislocation density (\u03b4) and the way to reduce it to annealing the samples at high temperatures, which in turn increases their grain size [36]. This annealing is also considered the regulator of the strength and flexibility of the crystal structure. The visible parallel lines and random lines in the crystal may indicate the displacements, which means these lines may result due to the displacement. Displacement density and particle size follow an inverse relationship with giving an error called the linearity error. The dislocation density is calculated by the following equation:\n\n(9)\n\n\n\u03b4\n=\n\n1\n\n\nD\n\n2\n\n\n\n\n\n\nThe length due to the deformation of an object is closely related to the pressure applied, known as the lattice strain (\u03b5ls). The defects caused by imperfections in the crystal structure compel atoms to deviate slightly from their normal position [52]. These structural flaws include interstitial and/or impurity atoms that cause lattice strain, which can be determined by the following relation:\n\n(10)\n\n\n\n\u03b5\n\nls\n\n\n=\n\n\u03b2\n\n4\nt\na\nn\n\u03b8\n\n\n\n\n\nwhere \u03b8 represents the angle of diffraction and \u03b2 indicates the full width at half maximum. The stacking faults are induced by the atomic planes of the crystal due to interruption of the layered arrangement in a normal lattice structure. The stacking fault [SF] is determined by the following equation:\n\n(11)\n\n\nS\nF\n=\n\n\n2\n\n\n\u03c0\n\n2\n\n\n\n45\n\u221a\n(\n3\nt\na\nn\n\u03b8\n)\n\n\n\n\n\n\nVarious defects in the crystal structure such as displacement, plastic deformation, point defects, and domain boundary defects are considered as key factors of deformation in the structure. The deformation is assumed to occur in one per million parts of the materials, which is defined as the micro strain (\u03b5ms). A notable feature of the micro strain is that it maximizes the peak and the following equations are introduced to comprehend it [53]:\n\n(12)\n\n\n\n\u03b5\n\nms\n\n\n=\n\n\n\u03b2\nc\no\ns\n\u03b8\n\n4\n\n\n\n\n\nThe ionic radii of A and B sublattices are calculated by the following relations [9,54]:\n\n(13)\n\n\n\nr\nA\n\n=\n\n\n3\n\n\n\na\n0\n\n\n(\nu\n-\n0.25\n)\n\n-\n\nr\no\n\n\n\n\n\n\n\n(14)\n\n\n\nr\nB\n\n=\n\na\n0\n\n\n(\n0.625\n-\nu\n)\n\n-\n\nr\no\n\n\n\n\nwhere ro and u represent the radius of oxygen (1.32\u00a0\u00c5) and oxygen parameter having the value of \n\n\n3\n8\n\n\n, respectively. The distance between the centers of adjacent ions is the hoping length, whose values for A-A sites, B-B sites, and A-B sites are calculated by using the following equations [54,55]:\n\n(15)\n\n\n\nL\n\nA\n-\nA\n\n\n=\n\n\n\na\no\n\n\n\n3\n\n\n\n4\n\n\n\n\n\n\n\n(16)\n\n\n\nL\n\nA\n-\nB\n\n\n=\n\n\n\na\no\n\n\n\n11\n\n\n\n8\n\n\n\n\n\n\n\n(17)\n\n\n\nL\n\nB\n-\nB\n\n\n=\n\n\na\no\n\n\n2\n\n\n2\n\n\n\n\n\n\n\nwhere a\u2080 represents the lattice constant.To confirm the structure of the spinel phase in all prepared samples, Fourier transform infrared (FTIR) spectroscopy technique was utilized. Fig. 3\n depicts the FTIR spectra of nanocrystalline Ni0.70Cu0.30AlxFe2-xO4, measured over the frequency region of 450\u20134000\u00a0cm\u22121. Two fundamental strong absorption bands v1 and v2 are observed in the effect of the metal\u2013oxygen (M\u2212O) bonds at the tetrahedral and octahedral sites. The entity of the high-frequency v1 band is found in the range of 585\u2013615\u00a0cm\u22121, which is formed by the internal stretching of the M\u2212O bond at tetrahedral sites whereas the low-frequency v2 band appears at around 400\u00a0cm\u22121, which corresponds to that of octahedral site [56]. The formation of spinel structures in the prepared Ni-Cu ferrite nanoparticles is ascertained by the observed bands. The bands observed in this investigation are consistent with previous findings. [57,58]. The absorption peaks, however, are induced by the tetrahedral site of the metal's intrinsic stretching vibration. Moreover, the stretching vibration of both sites is influenced due to changes in the lattice parameter. The tetrahedral stretching frequency band (v1) is found to shift towards the higher frequency region with the Al3+ substitution increases. The cause of the band shifting with Al3+ content as illustrated in Fig. 3, might include the fact of cations redistribution over the tetrahedral and octahedral sites [56,58,59].\nFig. 4\n shows the FESEM micrographs of nanocrystalline Ni0.70Cu0.30AlxFe2-xO4 annealed at 700\u00a0\u00b0C. As depicted in Fig. 4(A-F), the grains are formed of semi-spherical shapes with a uniform and even distribution in multi-domains separated by grain boundaries. The average grain size of the synthesized ferrite nanoparticles is measured by [54]:\n\n(18)\n\n\n\nG\na\n\n=\n\n\n1.5\nL\n\n\nXN\n\n\n\n\n\nwhere L, X, respectively, indicate the total length in cm and the magnification of the micrographs, and N is the number of intercepts. Fig. 5\n illustrates the EDX analysis of Ni0.70Cu0.30AlxFe2-xO4, which ensures the presence of all expected elements in each sample with appropriate proportions. The sum of elements in each composition provides 100%, which confirms the accuracy of the sol\u2013gel synthesize technique and manifests its novelty.\nFig. 6\n(A, B) demonstrates the variation in real (\u03b5\u2032) and imaginary (\u03b5\u2032\u2032) parts of the complex dielectric constant of nanocrystalline Ni0.70Cu0.30AlxFe2-xO4 annealed at 700\u00a0\u00b0C with varying frequency. The dielectric property of ferrites is contingent to different factors such as preparation method, chemical composition, grain size, electronic di-polarity, and so on. The \u03b5\u2032, \u03b5\u2032\u2032, and dielectric loss tangent (tan \u03b4E\n) are calculated by the following relations:\n\n(19)\n\n\n\n\n\u03b5\n\n\u2032\n\n=\n\n\nCt\n\n\n\n\u03b5\no\n\nA\n\n\n\n\n\n\n\n\n(20)\n\n\n\n\n\u03b5\n\n\u2033\n\n=\n\n\n\u03b5\n\n\u2032\n\nt\na\nn\n\n\u03b4\nE\n\n\n\n\nand\n\n(21)\n\n\ntan\n\n\u03b4\nE\n\n=\n\n1\n\n\u03c9\n\n\u03b5\no\n\n\n\n\u03b5\n\n\u2032\n\n\u03c1\n\n\n\n\n\nwhere C is the capacitance, \u03c9\u00a0=\u00a02\u03c0f with representing f as the applied field frequency, \u03b5o\n is the free-space permittivity, t is the thickness and A is the area of the contact surface of the tabloids.\nFig. 6(A) illustrates that \u03b5\u2032 decreases with frequency up to 105 Hz and thereafter remains almost constant with showing a very low value. On the contrary, the imaginary part (\u03b5\u2032\u2032) reveals higher values at low frequency regime and decreases vigorously with frequency as observed in Fig. 6(B). The observed dielectric dispersion of the investigated materials can be described by the Maxwell\u2013Wagner interfacial theory of polarization supported by Koop\u2019s phenomenological theory [54,55,60]. The grain boundaries are more active at low frequencies, whereas at high frequencies grains are more contributing. At the low frequency regime, the value of \u03b5\u2032 is higher because of the high resistive grains attributed to the space charge polarization [47,48]. The decreasing trend of the real dielectric constant (\u03b5\u2032) with frequency is found in Fig. 6A. This happens as the grains come into action at higher frequencies and the hopping electrons cannot follow the applied electric field after a certain frequency, which causes the polarization to be decreased. As a result, the value of \n\n\u03b5\n\n\u2032 appears to be very low, becoming almost constant [61].The sample with\u00a0x\u00a0\u00a0=\u00a00.1 shows the maximum value of \n\n\u03b5\n\n\u2032 because of the redistribution of Fe3+ at both A- and B-sites in Ni0.70Cu0.30AlxFe2-xO4. The substitution of Fe3+ by Al3+ in the compositions results in the transfer of Al3+ to A-sites and replaces some Fe3+ to B-sites, which causes the enhancement of Fe3+ ions in the grain and assembles them in the grain boundary [48,61]. Consequently, the space charge polarization is increased and caused a higher value of dielectric constant. Heat generated by the high flow of electricity in dielectric materials is dissipated and considered as the material\u2019s loss that is characterized as the imaginary part (\u03b5\u2032\u2032) of dielectric constant [54]. From Fig. 6(B), it is observed that the value of \u03b5\u2032\u2032 increases significantly with increasing Al3+ content in ferrites. The decrease of \u03b5\u2032\u2032 with frequency is occurred due to the high resistive effect of the grain boundaries. The electrons reverse their direction of motion frequently at higher frequencies and the hopping electrons can no longer follow the applied electric field after certain frequencies. Therefore, the probability of charge transport at the grain boundary decreases, resulting in the decrease of polarization, which gives the low value of \u03b5\u2032\u2032 [17,55,60,61].\nFig. 7\n shows the variation of dielectric loss tangent (tan \u03b4E\n) of the synthesized samples annealed at 700\u00a0\u00b0C with varying frequencies. Due to impurities and imperfections, the polarization lags behind the applied voltage, causing tan\u03b4E to form there [54,61]. The highest value of tan\u03b4E\n is found under the relaxation condition of \u03c9\u03c4\u00a0=\u00a01, where \u03c9\u00a0=\u00a02\u03c0fmax\n, and \u03c4\u00a0=\u00a01/2P represent the peak frequency and the relaxation time, respectively and both of which are closely related to the hopping or jumping probability. Electron sharing between Fe3+ and Fe2+ requires very little energy and the maximum peak is achieved when the hopping frequency between them is well-matched with the frequency of the applied electric field. Koop\u2019s theory explains in a very simple way how tan\u03b4E\n of the investigated materials decreases with frequency [62,63]. It is noted that at lower conductive grain boundaries, tan\u03b4E\n exhibits the maximum value as more electrons are available to be conductive at the low-frequency region. There is energy loss that occurred during the electrons sharing between Fe3+ and Fe2+, therefore high energy is required [47,64,65].The role of microstructure is important in determining the tan\u03b4E\n. H. Jia et al. showed that the grain boundaries and porosity between polycrystalline crystals affect the \u03b5\u2032 and \u03b5\u2032\u2032 [66]. The inter-relation among porosity, grain boundaries, and dielectric loss is defined by the following relation:\n\n(22)\n\n\ntan\n\n\u03b4\nE\n\n=\n\n\n\n1\n-\nP\n\n\n\nt\na\nn\n\n\u03b4\no\n\n+\n\nC\nm\n\n\n\nP\n\nn\n\n\n\n\nwhere Cm is the material-dependent constant, P represents the porosity and tan\u03b4o\n is the dielectric loss of material with full densification. Uniform density and lower porosity reduce the \u03b5\u2032 and \u03b5\u2032\u2032, respectively and the intrinsic and extrinsic fault are responsible for the dielectric loss.The variation in ac resistivity (\u03c1ac\n) of the investigated samples with frequencies (annealed at 700\u00a0\u00b0C) is depicted in Fig. 8\n, which is explained based on the hopping mechanism. The \u03c1ac\n is calculated by the following equation [54]:\n\n(23)\n\n\n\n\u03c1\n\nac\n\n\n=\n\n1\n\n\n\u03b5\n0\n\n\u03b5\n\u2032\n\u03c9\nt\na\nn\n\n\u03b4\nE\n\n\n\n\n\n\nwhere \u03c9 defines the angular frequency. According to the hopping mechanism, electrons jump from one state to another, which prefer to be distributed over the sites in the lattice. In Fig. 8, it is anticipated that at lower frequencies, the \u03c1ac of the investigated ferrites has higher values and depletes with increasing frequency. After a certain frequency, it gets almost saturation showing a very small value. This variation of \u03c1ac\n with frequency can be described by the frequency dependency of grains and grain boundaries. The conductivity mechanism illustrates the particle\u2019s ability to be highly conductive [67,68].The high-resistive grain boundaries are more active at lower frequencies, which impedes the movement of free charges and thus the hopping of electrons between Fe2+ and Fe3+ is less, resulting in the higher values of \u03c1ac [54]. To increase the hopping of electrons between Fe2+ and Fe3+, it must be operated at higher frequencies, which plays a critical role in reducing the \u03c1ac\n value. The main reason for the low values of \u03c1ac\n is that the hopping of electrons almost stops after a certain frequency. As shown in Fig. 8, the maximum value of \u03c1ac\n is found for the sample Ni0.70Cu0.30Fe2O4.With the increase of Al3+ concentration in Ni\u2013Cu ferrites, the AC conductivity increases. The sample with Ni0.70Cu0.30Al0.1Fe1.9O4 shows the minimum value at the low-frequency region. The conduction takes predominantly through the highly resistive grain boundaries at low frequencies, whereas it occurs through low resistive grains at high frequencies [65\u201367].The electric relaxation mechanism in the materials can be explained through the spectroscopy of electric modulus (M*), which is resolved into two components [63] as given in the following:\n\n(24)\n\n\nM\n\n\u2217\n\n=\n\n1\n\n\n\u03b5\n\n\n\u2217\n\n\n\n=\n\n1\n\n\u03b5\n\u2032\n-\ni\n\u03b5\n\u2033\n\n\n=\n\n\n\n\u03b5\n\n\u2032\n\n\n\n\n\u03b5\n\n\n\u2032\n2\n\n\n+\n\n\n\u03b5\n\n\n\u2033\n2\n\n\n\n\n-\ni\n\n\n\n\u03b5\n\n\u2033\n\n\n\n\n\u03b5\n\n\n\u2032\n2\n\n\n+\n\n\n\u03b5\n\n\n\u2033\n2\n\n\n\n\n=\nM\n\u2032\n+\ni\nM\n\u2033\n\n\n\nwhere \n\n\n\nM\n\n\u2032\n\n=\n\n\n\n\u03b5\n\n\u2032\n\n\n\n\n\u03b5\n\n\n\u2032\n2\n\n\n+\n\n\n\u03b5\n\n\n\u2033\n2\n\n\n\n\n\n is the real and \n\n\n\nM\n\n\u2033\n\n=\n\n\n\n\u03b5\n\n\u2033\n\n\n\n\n\u03b5\n\n\n\u2032\n2\n\n\n+\n\n\n\u03b5\n\n\n\u2033\n2\n\n\n\n\n\n is the imaginary part of the electric modulus. From the above equations, both the real (M\u2032) and imaginary (M\u2032\u2032) parts of the modulus are found to be frequency-dependent, which plays a key role in investigating the relaxation mechanism of the materials. From Fig. 9\n(A), it is perceived that M\u2032 responds very well to higher frequencies exhibiting the highest value for x\u00a0=\u00a00.00. It indicates the lower value of \u03b5\u2032 at high frequencies. The inadequacy of the restorative force and the release of space charge polarization near the grain boundary helps to attain its saturation. This phenomenon occurs at higher frequencies and at the same time ensures frequency independency in the electrical properties of the materials [47,63,64].To illustrate the peaking behavior, one has to look at the variation of M\u2032\u2032 as shown in Fig. 9(B). The hopping mechanism is used to illustrate the peaking behavior better, as it more accurately explains the transition of the charge carriers. In the figure above, it is clear and understand that charge carriers contributing to the hopping process cover long distances at low frequencies. On the other hand, charge carriers are able to cover short distances at higher frequencies, which indicates the relaxation in the polarization process [69,70].The relaxation of the material is distinguished by the cole\u2013cole plot (M\u2032\u2032 vs M\u2032) of the electric modulus as presented in Fig. 10\n. The grain and grain boundary is thought to be responsible for this separation [62,69]. A clear non-Debye type relaxation is found by looking closely at the non-overlapping semicircular pattern in Fig. 10. Nanoparticles annealed at 700\u00a0\u00b0C show two identical non-overlapping semicircular patterns [47].To study the electrical behavior of the material, the impedance spectroscopy was employed in this study for the synthesized nanoparticles. This is a long-established method to distinguish the impedance contributions of the materials\u2019 grains, grain boundaries and electrodes. The complex impedance (Z*) includes both the resistive and reactive components of the impedance as follow:\n\n(25)\n\n\n\n\n\n\nZ\n\n\n\u2217\n\n\n=\n\n\nZ\n\n\u2032\n\n-\nj\nZ\n\n\u2033\n\n\n\n\nwhere the resistive part is designated as the real part Z' which is the horizontal component of the complex impedance denoted by Z' = |Z*|cos\u03b8 and the imaginary part is designated as the reactive (capacitive) part expressed by Z\u201c = |Z*|sin\u03b8. However, these two components are combined impedance effect of resistance and capacitance due to grain and grain boundary, which are embroiled to dielectric and electric modulus parameters following the relation:\n\n(26)\n\n\nt\na\nn\n\u03b4\n=\n\n\n\n\u03b5\n\n\u2033\n\n\n\n\u03b5\n\n\u2032\n\n\n=\n\n\n\nZ\n\n\u2033\n\n\n\nZ\n\n\u2032\n\n\n=\n\n\n\nM\n\n\u2033\n\n\n\nM\n\n\u2032\n\n\n\n\n\n\nThe variation in the real part of complex impedance (Z\u2032) of the investigated ferrites annealed at 700\u00a0\u00b0C is illustrated in Fig. 11\n(A) with varying frequencies. The higher values of Z\u2032 of the synthesized ferrites are revealed at lower frequencies with dispersed behavior and drop sharply up to 1 KHz and thereafter it remains almost constant at high frequencies.Besides, the variation in imaginary part (Z\u2032\u2032) of the complex impedance for the synthesized Ni0.7Cu0.3AlxFe2-xO4 nano-ferrites is illustrated in Fig. 11(B). As observed in Fig. 11(B), the materials show higher Z\u2032\u2032 values at the lower frequencies likewise the real part (Z\u2032) and decrease rapidly with increasing frequency (up to 10 KHz), as the conductivity of the ferrites increases. However, at higher frequencies \n\n\u2265\n\n 100KHz, Z\u2032\u2032 shows frequency-independent behavior with small constant values in the effect of reduction in polarization [54,61,71,72]. Both Z\u2032 and Z\u2032\u2032 in Fig. 11 show the similar trend as the dielectric properties of the materials. For all compositions, the impedance curves are appeared to merge at higher frequencies; indicating the predominance contribution of low resistive grains. Moreover, the space-charge polarization is considered important only when the materials are resolved into grains and grain boundaries [61,68]. The curves tend to converge at higher frequencies owing to a decrease in space charge polarization and this behavior elucidates the increasing tendency of ac conductivity with frequency, confirming the semiconducting behavior of the prepared nanocrystalline spinel ferrites [47,71].The Nyquist impedance plot (known as the cole\u2013cole plot) of the prepared Ni0.7Cu0.3AlxFe2-xO4 annealed at 700\u00a0\u00b0C is shown in Fig. 12\n. This plot is the combined response of the RC circuit-connected resistor and capacitor in parallel, which reveals the contribution of grain and grain boundary resistance. The heterostructure nature of synthesized materials along with the characteristic nature of complex impedance spectra are revealed through the appearance of multiple electrical responses. Those responses are resulted due to grain resistance Rg, grain boundary resistance Rgb, and electrode effects), which can be easily determined by the semicircular arcs that appeared in the cole\u2013cole plots [47,54,72]. By looking at Fig. 12, the two semicircular arcs are clearly visible which are formed with their center placed below the real axis, which manifests the single-phase nature of Al3+ substituted nanocrystalline Ni-Cu materials. The diameter of the semicircle arcs is found to decrease with increasing Al3+ concentration, which is actually caused by the resistance of grain boundaries. However, at lower frequencies, the Rg dominates the appearance of the first semicircle, whereas at higher frequencies, the Rgb dominates the appearance of the second semicircle. The difference in relaxation time is considered the main catalyst behind the separation of semicircle arcs [61].The sol-gel method was used to synthesize a series of highly crystalline nanomaterials of Ni0.7Cu0.30AlxFe2-xO4. The single-phase cubic spinel structure of the investigated materials was confirmed through the XRD study with showing no impurity. The surface morphology was studied through the FESEM measurements, which illustrated the distribution of semi-spherical grains separated by the grain boundaries with a homogenous distribution of particles on the surface. The structural parameters were determined using the XRD and FESEM data. The electrical and dielectric properties were carried out by using the impedance analyzer supported by the modulus and impedance spectroscopy. Both the average crystallite size and the average grain size of the studied materials are found to be in the nano-scale range (55.63\u201370.74\u00a0nm) and (59.00\u2013 65.00\u00a0nm), respectively. The dielectric dispersion nature of the materials was revealed through the dielectric study of the materials. The electrical response of the materials was inspected by means of impedance and modulus spectroscopy, which resolved the contribution of grains and grain boundaries as the electrical response of the investigated nanoparticles. The relaxation phenomena in the materials was justified through the cole\u2013cole analysis of both impedance and modulus spectra. A little substitution of Al3+ is found to be influential in the structural, dielectric, and electrical properties of Ni-Cu spinel ferrites prepared by the cost-effective sol gel method.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 to the center of excellence of the Department of Mathematics and Physics at North South University (NSU), Dhaka 1229, Bangladesh. This research is funded by the NSU research grant CTRG-20/SEPS/13.", "descript": "\n                  In this study, a series of nanocrystalline Ni0.7Cu0.30AlxFe2-xO4 (x\u00a0=\u00a00.00 to 0.10 with a step of 0.02) has been synthesized through the sol\u2013gel auto combustion technique. The structural, morphological, dielectric, and electrical properties of the synthesized nanoparticles are analyzed due to the substitution effect of Al3+ content. The structural study has been performed through the XRD and FTIR analyses. The extracted XRD patterns assure the single-phase cubic spinel structure of all samples in high crystalline nature with maintaining their homogeneity. The average crystallite size of the nanoparticles is found in the range of 55.63\u201370.74\u00a0nm, and the average grain size varies from 59.00 to 65.00\u00a0nm. FTIR study also confirms the formation of spinel structures in the prepared Ni-Cu ferrite nanoparticles. The surface morphology of the materials has been studied through the FESEM study linked with EDX analysis. The dielectric dispersion of the materials is reflected at lower frequencies up to 10\u00a0kHz. The impedance spectroscopy confirms the non-Debye relaxation phenomena of the synthesized nanomaterials. The contribution of grains and grain boundaries is resolved through the modulus study of the materials. The trend in variation of AC resistivity with frequency has been explained by the hopping mechanism.\n               "}
{"full_text": "Calcium (Ca) is the fifth richest element in the Earth's crust. It is one of the cheapest and most biocompatible metals, with high content in the human body. The price of Ca is close to three millionths of the price of noble metal Pt of the same quality (Hill et\u00a0al., 2016). Like other alkaline earth metals, calcium has, in its outermost S orbital, two valence electrons which are easily given up in chemical reactions. Therefore, calcium is usually bivalent in its compounds and exists in ionic forms. The application of calcium in catalytic reactions could be sustainable, economical and green. However, due to the lack of a d-orbital to enable its oxidation state to change rapidly and reversibly, (a prerequisite for many catalytic cycles) (Harder, 2010; Zhu et\u00a0al., 2020a, 2020b), calcium metal is generally considered as a stoichiometric reagent with no catalytic performance in heterogeneous catalysts (Gerken et\u00a0al., 2014; Zhu et\u00a0al., 2015).Differing from the rare usage of calcium in heterogeneous catalysis, applications of calcium in homogeneous catalysis have made tremendous progress during the past decade (Hill et\u00a0al., 2016; Harder, 2010). For example, calcium alkoxide and calcium amide complexes are sufficiently reactive to promote many catalytic reactions. In some cases carbanions, such as benzyl calcium complexes or (Me3Si)2HC-stabilized alkyl calcium reagents, are highly effective as well. So far, calcium metal complexes have been reported to play a central role in the catalytic cycles of alkenes polymerization (Begouin and Niggemann, 2013), intramolecular hydroamination of aminoalkenes (Crimmin et\u00a0al., 2005) and hydrosilylation and alkene hydrogenation (Harder and Brettar, 2006). The rapid development of Ca compounds for homogeneous catalysis is mainly based on the viewpoints that the d0 valence configuration of a Ca2+ center in the calcium metal complexes will give it a certain level of \u2018lanthanide mimetic\u2019 characteristics so that a catalytic cycle can be constructed (Hill et\u00a0al., 2016).Recently, Zhou and coworkers found that alkaline earth metal elements Ca, Sr, and Ba can form stable octacarbonyl compound molecules which meet the 18-electron rule and exhibit typical transition metal bonding characteristics (Wu et\u00a0al., 2018). This indicates that the heavy alkaline earth metal elements may behave like transition metals in certain heterogeneous catalytic processes. However, there are few reports on the use of alkaline earth metals for heterogeneous catalysis. For example, Xia et\u00a0al identified through theoretical calculations that alkaline earth metals, placed in a covalent organic framework, can become effective electrocatalysts for oxygen reduction reaction (ORR), which is the major reaction for hydrogen fuel cells and metal-air batteries (Lin et\u00a0al., 2017). Chen et\u00a0al. proved experimentally that Mg, atomically dispersed in the graphene framework, has extremely high ORR activity under both alkaline and acidic conditions (Xu et\u00a0al., 2019). However, due to the lack of more experimental results, there is still insufficient evidence to show that alkaline earth metals have enough active catalytic sites in heterogeneous catalysis. In addition, the catalytic mechanism of alkaline earth metals in heterogeneous catalytic reactions can be an exciting field for renewable hydrogen production.Single atom catalysts (SACs) are an innovative type of heterogeneous catalysts in which each isolated active metal atom is fixed on supporting materials (Wang et\u00a0al., 2019; Kaiser et\u00a0al., 2020; Zhuo et\u00a0al., 2020). Although SACs are classified as the heterogeneous catalysts, the presence of single metal atoms in SACs is very similar to that in homogeneous catalysts (Yang et\u00a0al., 2017). The surface atoms of the supporting materials can be considered as ligand molecules in homogeneous catalysts, which not only stabilize the active metal atoms but also engage in the catalytic reactions (Wang et\u00a0al., 2019; Wu et\u00a0al., 2019). The similarity between SACs and homogeneous catalysts has driven us to explore the use of calcium metals for heterogeneous catalytic hydrogen evolution reaction (HER).In this research, we have found that atomically confined Ca in nitrogen-doped graphene (Ca1-NG) can be an effective heterogeneous catalyst to boost the electrocatalytic hydrogen evolution (EHE) and photocatalytic hydrogen evolution (PHE) reactions. To the best of our knowledge this is the first report that calcium single atoms have been used as catalysts for the HER. The performance of Ca1-NG loaded CdS is comparable to that of noble metal Pt loaded CdS for PHE under the same experimental conditions. Density functional theory (DFT) calculations have shown that the excellent performance of Ca1-NG can be attributed to the optimal adsorption capacity of hydrogen atoms on the Ca-doped active centers.Ca1-NG was prepared using a facile method previously described for the preparation of Co1-NG and Ni1-NG (Zhao et\u00a0al., 2017; Zhao et\u00a0al., 2018; Fei et\u00a0al., 2015). Briefly, a complete mixture of graphene oxide (GO) and CaCl2 was thermochemically treated in an NH3 atmosphere to form the Ca1-NG. During this process GO was reduced to NG (supplemental information\nFigure\u00a0S1), and the N dopants were incorporated into the graphene lattice to form a strong interaction with metal atoms (Wang et\u00a0al., 2019; Zhao et\u00a0al., 2017).No diffraction peaks of Ca oxides or carbides were detected in the X-ray diffraction (XRD) patterns of Ca1-NG samples (supplemental information, Figure\u00a0S1). Transmission electron microscopy (TEM) images show that there are no Ca-related nanoparticles in the prepared Ca1-NG samples (Figure\u00a01\nA). However, the energy-dispersive X-ray elemental mapping spectroscopy (EDS) indicated that Ca, N, and C elements are distributed evenly on the prepared Ca1-NG (Figure\u00a01D). The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Ca1-NG further demonstrated that Ca species were homogeneously dispersed in the substrates. As shown in Figures 1B and 1C, a small number of bright spots with diameters less than 0.2\u00a0nm are well dispersed on the substrates. The absence of Ca clusters has been confirmed with careful examination at several randomly picked locations during HAADF-STEM observations. These results indicate that all Ca species are atomically dispersed in the Ca1-NG. The loading content of Ca in Ca1-NG is 0.52 wt.% based on the analysis of inductively coupled plasma optical emission spectrometer (ICP-OES).X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate the chemical composition and valence state of Ca1-NG (Figure\u00a02\n and Table S1). The survey spectrum with major C peaks and some smaller peaks of N, O and Ca confirms the presence of C, N, O and Ca in Ca1-NG (Figure\u00a02A). The high-resolution N 1s spectrum shows that Ca1-NG catalyst contains mainly pyridinic N (398.0 eV) as well as a small amount of pyrrolic (399.5 eV), graphitic (400.8 eV), and oxidized N species (402.1 eV) (Figure\u00a02B). The presence of pyridinic N is not only favorable for hydrogen evolution activity of Ca1-NG but also serves as anchoring sites for single metal atoms. Figure\u00a02C shows the high-resolution Ca 2p spectrum of Ca1-NG. According to the National Institute of Standards and Technology XPS database (Naumkin et\u00a0al., 2012), the 347.2 eV and 350.8 eV binding energy peaks can be attributed to Ca 2p3/2 and Ca 2p1/2, respectively. The absence of metallic Ca 2p3/2 spectrum (344.9 eV) indicates that the scattered Ca species (shown in HAADF-STEM images) in Ca1-NG are Ca2+ cations. The Raman spectra of the resultant catalysts in Figure\u00a02D exhibit a D-band for defected graphite and a G-band for the doubly degenerate zone center E2g mode (Ferrari and Basko, 2013). The intensity ratio of D band to G band (ID/IG) for Ca1-NG (0.98) is close to that of NG (1.03). This result indicates that the dispersion of individual Ca atoms in the NG matrix has little effect on the degree of disorder and structural defects in the NG laminar structure (Zhao et\u00a0al., 2018).The atomic dispersion of Ca cations in Ca1-NG was further confirmed by the X-ray absorption near-edge structure (XANES) spectroscopy and the extended X-ray absorption fine structure (EXAFS) spectroscopy, which are sensitive to the local environment of metal atoms. Figure\u00a03\nA shows the Ca K-edge of XANES curves of Ca1-NG and CaO. Usually a metal foil is used for energy calibration. However, because calcium metal is very active in air, the Ca K-edge XANES spectrum of CaO was used as calibration reference material. As shown in Figure\u00a03A, the adsorption edge position of the Ca1-NG XANES curves is comparable to that of CaO, indicating that Ca metal atoms in Ca1-NG are in cationic states. This agrees well with the results of the XPS spectra (Figure\u00a02C). Further structural information was obtained from Ca K-edge EXAFS analyses (Table S2). Figures 3B and 3C show the Ca K-edge EXAFS K-space and R-space plots, respectively, for the Ca1-NG. It is noted that the EXAFS curve of Ca1-NG is obviously different from that of CaO. The R space plots of Ca1-NG show a sharp peak at approximately 2.1\u00a0\u00c5. However, CaO shows two strong bonding features at around 1.9\u00a0\u00c5 and 3.1\u00a0\u00c5, which are attributed to the Ca-O bond and Ca-O-Ca bonds, respectively. The major peak for Ca1-NG at approximately 2.1\u00a0\u00c5 can be corresponded to the formation of Ca-N bond, which is longer than that of Ca-O bond (1.9\u00a0\u00c5) in CaO. Atomic structure simulations indicate that the anchored Ca single atoms are located at the defective sites of NG derived from pyridine-N (Figure\u00a03D and supplemental information\nFigure\u00a0S2). The fitting results indicated a CN of 2.8 for Ca-N contribution in Ca1-NG. This result corresponds well to DFT calculations (Figure\u00a0S17), which indicate that single Ca atoms anchored in pyridinic N defects in graphene are stable (Detailed information can be found in the following DFT calculation section of this research).Experimental results have shown that the prepared Ca1-NG exhibits more enhanced activities for HER under both acidic and weak basic conditions than do other obtained catalysts (Figures 4A and 4B and supplemental information\nFigure\u00a0S3). The HER activities of Ca1-NG were evaluated, both in 0.5\u00a0M H2SO4 and 1.0\u00a0M (NH4)2SO3 solutions using a standard three-electrode electrochemical cell. The commercial 20 wt.% Pt/C and the prepared GO and NG were also evaluated as baseline catalysts. All potentials were referenced to the reversible hydrogen electrode (RHE) and with iR-corrected. As shown in Figure\u00a04A, Ca1-NG shows HER activity in acidic solution with an onset potential (Eonset) of 21\u00a0mV and an overpotential of 151\u00a0mV to deliver a current density of 10 mA cm\u22122. For comparison, NG and GO show poor activities toward HER, requiring much greater overpotentials of 297\u00a0mV and 338\u00a0mV, respectively, to generate the same 10 mA cm\u22122 current density.The enhanced HER activity of Ca1-NG is further confirmed by the smaller Tafel slope of 76\u00a0mV dec\u22121 for Ca1-NG as compared to 137\u00a0mV dec\u22121 for NG and 147\u00a0mV dec\u22121 for GO (Figure\u00a04B). The small Tafel slope indicates that the rate-determining step of Ca1-NG is either the electrochemical desorption of H or the discharge reaction, following the Volmer-Heyrovsky mechanism (Dong et\u00a0al., 2018). Although the Tafel slope of Ca1-NG is higher than that for the benchmarked 20 wt.% Pt/C catalyst (36\u2009mV dec\u22121), it is significantly lower than that of NG without Ca single atoms. This result suggests that the new Ca single atom can be effectively used as the catalytically active site of HER. In addition, Ca1-NG has also shown more favorable HER activity under neutral (or weak basic) conditions of 1.0\u00a0M (NH4)2SO3 with pH\u00a0= 8.0, as the obvious shift of the polarization curve for Ca1-NG catalyst to a lower overpotential (Figure\u00a0S3). These results indicate that the incorporation of calcium single atoms into N-doped graphene can lead to a profound enhancement of the HER activity for Ca1-NG under both acidic and weak basic conditions.The electrochemical active surface area (ECSA) of prepared catalysts was analyzed by means of Cdl in Figure\u00a0S4. The results showed that the capacitances of GO, NG, and Ca1-NG are 3.07, 3.68, and 5.22 mF cm\u22122 in a 0.5\u00a0M H2SO4 solution, corresponding to 76.8, 92.0, and 130.5\u00a0cm2 ECSAs, respectively. The ECSA of a 20 wt.% commercial Pt/C catalyst was measured using the underpotential deposition hydrogen (UPD-H) adsorption/desorption voltammetry method, which is usually used for the determination of ECSAs for noble-metal electrocatalysts. As shown in Figure\u00a0S5, the ECSA for Pt/C was determined to be 285.7\u00a0cm2. The turnover frequencies (TOFs) of the testing catalysts were calculated to evaluate the intrinsic activities of the catalysts. At overpotential of 100\u00a0mV, the TOF values of the GO, NG, and Ca1-NG were 0.125, 0.147, and 1.134 H2 s\u22121, respectively. These values revealed that Ca1-NG had intrinsic HER activity excelling other catalysts. An equivalent circuit simulation for electrochemical impedance spectroscopy (EIS) tests was carried out from 10\u22122 Hz\u2013106 Hz (Figure\u00a0S6). Ca1-NG shows a smaller arc radius compared to those of GO and NG, which means that the electrochemical impedance of Ca1-NG is smaller than those of GO and NG.Electrochemical stability is an important indicator used to evaluate the catalytic performance of catalysts. The result of the i-t curve (Figure\u00a0S7) shows that the catalytic current remains constant at about 17 mA cm\u22122 at 200\u00a0mV for over 30,000 s. This result indicates the high stability of Ca1-NG catalyst in a 0.5\u00a0M H2SO4 solution. The XPS and STEM analyses of Ca1-NG after HER are shown in Figures S8 and S9. The results show that the XPS and STEM characterizations of Ca1-NG do not change significantly after the reaction, proving that the structure of Ca1-NG is stable.On the other hand, the prepared Ca1-NG can significantly enhance the performance of CdS for PHE. The formation of the Ca1-NG loaded CdS composite photocatalysts (Ca1-NG/CdS) was confirmed via TEM images (Figure\u00a0S10). XRD patterns and UV-visible light absorption spectra show that a small amount of Ca1-NG loading does not affect the crystal structure of CdS but significantly improves the light absorption capacity of the photocatalyst (Figures S11 and S12). The PHE performance of Ca1-NG/CdS under visible light irradiation at 420\u00a0nm was evaluated using (NH4)2SO3 as an electron donor. As shown in Figures 4C and 4D, Ca1-NG/CdS exhibits much higher PHE activities compared to bare CdS and NG loaded CdS. The rate of hydrogen evolution for 0.5 wt.% Ca1-NG/CdS (92.0\u00a0\u03bcmol/h) is 8.1 times greater than that of bare CdS (11.3\u00a0\u03bcmol/h) and 1.6 times greater than that of 0.5 wt.% NG/CdS photocatalyst (57.7\u00a0\u03bcmol/h). Moreover, the catalytic performance of 0.5 wt% Ca1-NG/CdS was comparable to that of 0.5 wt.% Pt/CdS (Figure\u00a0S13), an active photocatalyst for PHE. In order to verify the role of CaO nanoclusters in hydrogen production we loaded CaO onto the surface of NG and successfully prepared CaO-NG/CdS. The hydrogen evolution performance of CaO-NG/CdS is shown in Figure\u00a0S14. It is noted that CaO nanoclusters have no catalytic effect on the HER. Therefore, we can conclude that it is the Ca single atoms in Ca1-NG/CdS that play a major catalytic role, rather than the CaO nanoclusters. It is noted that the loading content of Ca in Ca1-NG is only 0.52 wt.% (based on ICP-OES analysis). That means that very few Ca atoms, only 26 parts per million mass of CdS, are needed in order to facilitate the PHE reactions. This result also indicates that single calcium atoms in NG play an important role in the improvement of the hydrogen evolution activity, which agrees well with the enhanced EHE performance for Ca1-NG.The effect of Ca1-NG loading concentration was investigated and the results are shown in Figure\u00a04E. The rate of hydrogen evolution increases from 11.3\u00a0\u03bcmol/h to 92\u00a0\u03bcmol/h as the Ca1-NG loading on CdS photocatalysts increases from 0.0 to 0.5 wt.%. Further increasing Ca1-NG loading, however, results in a significant drop in the rate of hydrogen evolution. This decline is possibly due to the light blockage effect of Ca1-NG on the surface of CdS. The optimal loading of Ca1-NG on CdS is about 0.5 wt.% under the present reaction conditions. The apparent quantum efficiency of the optimal Ca1-NG/CdS photocatalysts for hydrogen production is 57.5% at 420\u00a0nm wavelength. As illustrated in Table S3, this efficiency (57.5%) is one of the greatest ever reported for non-noble-metal cocatalysts.The stability of Ca1-NG/CdS photocatalyst was verified by a three PHE reaction cycles test. As shown in Figure\u00a04F, no significant decrease in the rate of hydrogen evolution was observed during the cyclic test. During the three PHE cycles test a total of 1.264\u00a0mmol H2 was produced. The turnover numbers (TONs), which are defined as the total hydrogen atoms evolved per mole of CdS photocatalyst and per mole of Ca anchored in Ca1-NG/CdS, are 73 and 779363, respectively. These large TONs indicate that hydrogen is produced from the photocatalytic reduction of water rather than from the photo-corrosion of either CdS or Ca in the Ca1-NG/CdS photocatalysts. Additionally, XRD, TEM and ICP tests have shown that there are no significant differences for the Ca1-NG/CdS photocatalyst before or after the stability test (Figures S10 and S6 and Table S4), indicating that Ca1-NG/CdS is stable during PHE processes. The stability of Ca1-NG/CdS was further confirmed by a long-term photoelectrocatalytic test in 0.5\u00a0M H2SO4. As can be seen in Figure\u00a0S15, the linear sweep voltammetry (LSV) curves for Ca1-NG/CdS show no clear difference before or after a long-term photoelectrocatalytic hydrogen evolution test. This result also indicates that Ca1-NG/CdS is a stable catalyst for PHE.Photoluminescence (PL) and time-resolved photoluminescence (TRPL) decay spectra measurements were carried out to evaluate the charge carrier trapping and transfer mechanism in Ca1-NG/CdS photocatalyst during photocatalytic reactions (Figures 5A and 5B). The weak peak around 475\u00a0nm in PL spectra can be ascribed to the band edge emission of CdS, while the higher broad band at around 550\u00a0nm originates from the trap states (Veamatahau et\u00a0al., 2015; Mathew et\u00a0al., 2011). Clearly, the PL intensity of Ca1-NG/ CdS is much weaker than that of bare CdS, indicating that the photogenerated electron-hole pair recombination is effectively suppressed after Ca1-NG is loaded onto the surface of CdS. This may result from the effect of co-catalyst trapping photogenerated electrons (Chen et\u00a0al., 2010). Moreover, the PL intensity of Ca1-NG/CdS was weaker than those of NG/CdS and GO/CdS. This result is consistent with the better PHE performance for Ca1-NG/CdS.The transfer efficiency of photogenerated charge carriers was further confirmed by the TRPL decay spectra (Figure\u00a05B). The decay curves easily approximate a biexponential function. As shown in Table S5, the average lifetime of the PL decay in bare CdS was 2.37\u00a0ns. However, after NG and Ca1-NG loading, the PL lifetimes of the NG/CdS and Ca1-NG/CdS photocatalysts were reduced to 1.88 and 0.93\u00a0ns, respectively. These results suggest that the presence of Ca1-NG provides a new pathway for the electron transfer from CdS to Ca1-NG, leading to a significant decrease in the PL decay lifetime (Jiang et\u00a0al., 2017). In addition, the lower PL average decay lifetime of Ca1-NG/CdS compared to that of NG/CdS further confirms that Ca single atoms anchored in NG result in more effective separation of the photogenerated carriers, thereby leading to higher photocatalytic activity.To further understand the role of Ca1-NG cocatalyst in PHE, the transient photocurrent-time curves of Ca1-NG/CdS, NG/CdS, GO/CdS and bare CdS samples underwent several on-off cycles of intermittent irradiation at 420\u00a0nm. As shown in Figure\u00a05C, all the samples demonstrated a prompt photocurrent generation during the on and off illumination cycles. These on-off cycles also show high reproducibility. It is noteworthy that Ca1-NG/CdS exhibits greater photocurrent compared to NG/CdS and bare CdS. The photocurrent intensity of Ca1-NG/CdS was almost two times higher than that of NG/CdS, suggesting the positive roles of Ca doping in the acceleration of charge separation, which agrees with the results shown in Figures 5A and 5B.From a charge transfer viewpoint, EIS further shows the positive roles of Ca single atoms in Ca1-NG/CdS for PHE. In this research, EIS was carried out under visible light illumination and using a typical three-electrode setup. A smaller semicircle radius of an EIS curve generally means a lower charge transfer resistance and thus faster interface charge transmission of a photocatalyst (Zheng et\u00a0al., 2020; Shi et\u00a0al., 2020; Yao et\u00a0al., 2019). As shown in Figure\u00a05D, the Nyquist plots of Ca1-NG/CdS have much smaller semicircles than those of NG/CdS and bare CdS, suggesting a more efficient charge separation and transfer within Ca1-NG/CdS and, therefore, a better PHE performance.LSV tests under 420\u00a0nm visible light irradiation using 1.0\u00a0M (NH4)2SO4 aqueous solution as a photolyte show that Ca1-NG loading can effectively reduce the overpotential of CdS for PHE. As shown in Figure\u00a05E, the overpotential for Ca1-NG/CdS at \u221210 mA cm\u22122 is 0.62 V, much lower than those of NG/CdS (\u22120.76 V) and bare CdS (\u22120.90 V). (Note that a lower overpotential means a lower required activation energy for the HER (2H+(aq)\u00a0+ 2e\u2212 \u2192 H2(g)) (Kweon et\u00a0al., 2020) and is also favorable for photocatalytic H2 production (Shi et\u00a0al., 2020; Yao et\u00a0al., 2019; Luo et\u00a0al., 2015)). Additionally, the conduction band (CB) potentials of Ca1-NG and CdS were estimated to be \u22120.54 and \u22120.39\u00a0V (vs. NHE) using the Mott-Schottky method (Figure\u00a0S16). A more negative CB position indicates that photogenerated electrons in CdS under light irradiation can migrate from CdS to Ca1-NG (Figure\u00a05F), which agrees well with the results of PL and TRPL decay spectra.We can conclude, based on these characterization results, that Ca1-NG can serve not only as an electron storage medium to effectively inhibit the recombination of charge carriers, but also as active sites to accelerate the HERs. In addition, single Ca atoms doping in NG plays a key role in the improvement of catalytic performance of Ca1-NG and Ca1-NG/CdS for hydrogen evolution.DFT simulations were carried out to provide an in-depth theoretical understanding of the roles the Ca single-atoms play in the HER and PHE.Our calculations show that the Ca atom is located on the central axis after structural relaxation (Figures S17), which is consistent with experimental observations (Lin et\u00a0al., 2015). Both the SV+3N\u00a0+ Ca and DV+4N\u00a0+ Ca structures (two most common carbon vacancies: single vacancy (SV, refer Figure\u00a0S17A) and double vacancy (DV, refer Figure\u00a0S17B), exist in the NG. Creation of an SV (DV) leads to three (four) carbon atoms having dangling electrons (CN equals 2). Replacing one of these three/four carbon atoms by N results in a pyridinic-N that coexists with an SV/DV. In principle we can replace multiple carbon atoms to form SV\u00a0+ xN (x\u00a0= 1, 2, 3) and DV\u00a0+ yN (y\u00a0= 1, 2, 3, 4) structures, and the Ca atom is about 1.77\u00a0\u00c5 and 1.30\u00a0\u00c5 above the 2D plane, forming identical N-Ca bonds with lengths of 2.18\u00a0\u00c5 and 2.26\u00a0\u00c5, respectively (slightly longer N-Ca bonds in the DV+4N\u00a0+ Ca structure indicates weaker N-Ca bonding strength. This is because the two valence electrons of Ca split into only 3 Ca-N bonds in the SV+3N\u00a0+ Ca structure, whereas they have to split into 4 Ca-N bonds in the DV+4N\u00a0+ Ca structure, resulting in less electron density forming each Ca-N bond in the latter case). In a previous study, we found that a Ni single atom could also be supported above the SV+3N structure, but it would drop into the double vacancy surrounded by 4 N, making it no longer useful for HER. Here, a Ca single atom could be supported above the plane for both SV+3N and DV+4N structures, as the size of the Ca atom is larger than most transition metal atoms. The calculated adsorption energies for Ca single atoms adsorbed at the centers of SV+3N and DV+4N structures are \u0394ECa\u00a0= \u22124.50 eV and \u22125.96 eV, respectively. These adsorption energies are much more negative than the Ca crystal cohesive energy of about \u22121.84 eV (Lee et\u00a0al., 2009), indicating that the Ca adsorption at the SV+3N and DV+4N centers is extremely stable.Next, we studied H adsorption on these four structures and examined how the Ca single atom affects the H adsorption energy. We considered H adsorption at both Ca-sites and N-sites (we ignore H adsorption at the C-sites because they are not stable and are much less affected by the Ca atom). Because each structure involves several N atoms, we denote them as N1, N2, N3 (and N4), as illustrated in Figure\u00a06\n. We consider all possible situations, with several H adsorbed onto a combination of Ca and N atoms, and we name an H adsorption configuration by the H adsorption sites. For example, [N1, N2, Ca\u2217] denotes a configuration with three H atoms adsorbed onto N1, N2, and Ca, respectively. When discussing \u0394GH\u2217 values of a particular H within a configuration involving several H, we further denote the adsorption site of the discussed H using \u2217. For instance, in the former example [N1, N2, Ca\u2217], the discussed H is on Ca\u2217 site. Various H adsorption configurations for the SV+3N versus SV+3N\u00a0+ Ca and DV+4N versus DV+4N\u00a0+ Ca structures are shown in Figures 6 and 7\n. In particular, for the structures involving Ca, H+ coming from solution above graphene can adsorb onto the Ca-site, and H+ from underneath graphene can adsorb onto the N-site. With the same number m (m > 1) of adsorbed H, the catalytic system can have these H atoms (i) all adsorb onto the N-sites (Figure\u00a06B) or (ii) it can have one adsorbed onto Ca and the remaining m-1\u00a0H adsorbed onto N-sites (Figures 6C and 7). Although the energies (i) and (ii) might be slightly different, both structures could exist in solution with sufficient lifetime for catalyzing HER. It is difficult to have structural transition from one to the other since H on Ca-site and H on N-site are spatially separated on different sides of graphene. Therefore we considered both structures.In the case of a single Ni atom supported on SV+3N or DV+4N structures, we find that Ni-N bond could be broken if too many N and Ni sites are adsorbed with H. Here the Ca-N bonds are not broken, even if all the N and Ca sites are adsorbed with an H (Figure\u00a06). This is due to the unique property of Ca, that it can host a large CN (Yoon et\u00a0al., 2008). This unique property makes the Ca structure extremely stable/robust in the dynamic solution and makes sure all the H adsorption sites can contribute to HER.\nFigures 6 and 7 show clearly that with a Ca atom adsorption we get not only an extra Ca-site for H adsorption, but also more than three times as many possible processes for H adsorption. In addition, many of these H adsorption configurations are associated with small |\u0394GH\u2217| values, as highlighted in bold in Figures 6 and 7.All H adsorption processes in the four structures and corresponding \u0394GH\u2217 values are illustrated in Scheme 1\n, where \u0394GH\u2217 is reflected by the G difference between the initial configuration and final configuration. For the structures without Ca (black curves in Scheme 1), most |\u0394GH\u2217| values are very large. There is only one small |\u0394GH\u2217| (0.34eV), when an H is adsorbed to N2 of the SV+3N [N1] configuration, as highlighted in red. On the contrary, the two structures with Ca (blue curves in Scheme 1) both involve many |\u0394GH\u2217| values close to zero, as highlighted by red or orange. In particular, the red processes are especially useful for HER as they not only involve small |\u0394G\nH\u2217| values but also start from configurations that are highly likely to exist in the solution, because the starting configurations are either the initial configuration without any H or a configuration requiring a small or even negative \u0394G\nH\u2217 from the initial configuration. The SV+3N and DV+4N structures involve only one red H adsorption process, while the SV+3N\u00a0+ Ca and DV+4N\u00a0+ Ca structures involve 4 red H adsorption processes. In addition, all the red lines H adsorption are on the Ca-site, indicating that the Ca atom plays an essential role in providing many suitable H adsorption configurations to catalyze HER.Before explaining the detailed mechanism of Ca atoms in HER we first examine the effect of H coverage in the structures without Ca. For both SV+3N and DV+4N structures higher H coverage induces larger \u0394GH\u2217 values (Figure\u00a06A), consistent with the general trend that H binding becomes less stable when more H are adsorbed in the vicinity. When H coverage is low (Figure\u00a06A black boxes), the \u0394GH\u2217 values are very negative (\u22122.32 eV, \u22121.53 eV, and \u22121.34 eV). (The delta G_H value for the first H\u2217 in the SV+3N defect is about 0.8eV more negative than that in the DV+4N defect because the former configuration also involves more interaction between H\u2217 and N2, N3 (more details are given in SI), indicating a very strong H binding. Adding one more H to either structure dramatically increases \u0394GH\u2217 to quite positive values (0.34 eV and 0.78 eV), indicating a significant reduction in the binding strength of additional H. The abrupt reduction of H binding strength can be understood from their atomic structures. For the three configurations with low H coverages (Figure\u00a06A black boxes) the H atom(s) is located inside the vacancy hole and the whole structure is well within a 2D plane (see side view), where H and N form a sp2-like bond. In addition, the in-plane H interacts with other N atom(s) via a quasi H-N bond as they are spatially close enough for the charge densities to sufficiently overlap (Figure\u00a08\nA), which further enhances the binding strength of H (Especially in the [N1] configuration of SV+3N, as H forms quasi H-N bonds with two other N atoms). However, if more H atoms were added in, they would be too squeezed within the small vacancy hole. Hence they become out-of-plane due to Pauli repulsion (Figure\u00a06A, below black boxes). Some N atoms also move out-of-plane. This changes the H-N bonding from sp2-like toward more sp3-like and also reduces the charge density interaction between H and other N atoms. Hence the H binding strength is significantly reduced.When a Ca atom is deposited onto SV+3N or DV+4N, the Ca atom strongly binds to all the N atoms. This increases the CN of each H adsorbed N from three to four (Figure\u00a06B) and changes the N-H bonding nature to sp3-like. In addition, H is pushed out-of-plane substantially, losing quasi-bonding interaction with other N atoms (Figure\u00a08B). This significantly reduces the H binding strength on N-sites. As a result, the corresponding \u0394GH\u2217 values become quite positive, but with |\u0394GH\u2217| closer to 0 than in the structures without Ca (Figure\u00a06B boxes vs Figure\u00a06A boxes). This is one effect of Ca single atoms, namely reducing the H binding strength on N-sites to better values for HER by changing the H-N bond nature to more sp3-like and reducing the charge density interaction between H and other N atoms.In fact, the Ca atom too significantly reduces the H binding strength at the N-site, making \u0394GH\u2217 a bit too positive for HER (Figure\u00a06B boxes). The influence of Ca could be slightly weakened by further adsorbing an H on top of Ca, which adjusts \u0394GH\u2217 (of H adsorption on N) to less positive values, making the systems more suitable for HER (Figure\u00a06C blue boxes). For example, the [N\n\n1\n\n\u2217, Ca] configuration of SV+3N\u00a0+ Ca exhibits \u0394GH\u2217 values of 0.06 eV, superior for catalyzing HER. Here the influence of Ca on the graphene structure is reduced because the H-Ca bond weakens Ca-N interactions, as can be seen by the increase of the Ca to graphene-plane distance (Figure\u00a06C vs 6B). This makes the whole graphene structure deviate less from a 2D planar layer, hence allowing most of the N-H bonds to become less sp3-like and more sp2-like (Figure\u00a06C vs 6B). The returning of the H toward the 2D planar layer also enhances the charge density interactions between H and other N atom(s) (Figure\u00a08C vs 8B). Hence the overall H binding to N-sites is strengthened and becomes more suitable for HER.For the four structures outside the boxes in Figure\u00a06A, the H bindings are very weak because these H atoms are already repulsed out-of-plane by other H, even without a Ca deposition. Depositing a Ca atom makes slight differences to the H binding strength. Furthermore, adsorbing an H onto Ca also makes little change to the H binding strength on N-sites.Besides the two effects discussed above, the Ca atom itself also serves as an H adsorption site with exceptional \u0394GH\u2217 values. Our calculations predict that an H atom adsorbs on top of Ca, with \u0394GH\u2217 values of 0.46 eV and 1.50 eV for SV+3N\u00a0+ Ca and DV+4N\u00a0+ Ca, respectively (Figure\u00a07). The latter case has a weaker H-Ca bond because its Ca CN is higher. Although the two \u0394GH\u2217 values are too positive for HER, they can be reduced to very good values by adsorbing H atoms onto the N-sites (Figure\u00a07). For example, for the [\n\nN\n\n\n\n1\n\n\n\n, Ca\u2217\n\n] and [\n\nN\n\n\n\n1\n\n\n\n, N\n\n\n\n2\n\n\n\n, Ca\u2217\n\n] configurations of SV+3N\u00a0+ Ca, and the [\n\nN\n\n\n\n1\n\n\n\n, N\n\n\n\n3\n\n\n\n, Ca\n\n\u2217] configurations of DV+4N\u00a0+ Ca, \u0394G\n\nH\u2217\n\nare reduced to exceptional values for HER: 0.19, 0.22, and 0.16 eV, respectively. However, this excludes the [N\n\n1\n\n, N\n\n2\n\n, N\n\n3\n\n, Ca\u2217] configuration of SV+3N\u00a0+ Ca, where \u0394GH\u2217 is slightly increased compared to [Ca\u2217] configuration.Adsorbing H on the N-sites can reduce the \u0394GH\u2217 values of H adsorption on Ca sites, because the H-N bond weakens the N-Ca bonds, as can be seen by the N-Ca bond length increase (Figure\u00a07 rows 2-5 compared to row 1). The weakening of N-Ca bonds is also reflected in the charge density plots in Figure\u00a08. When there is no H adsorbed on any N we clearly see charge density overlap between Ca and the three/four N atoms. After adsorbing one or more H onto the N sites, most overlapping between Ca and N diminishes substantially. As a result, more valence electrons of Ca are involved to form a stronger Ca-H bond. Again, the strengthening of the Ca-H bond is reflected in both decreased Ca-H bond length and increased charge density overlap between Ca and H (Figure\u00a07 row 2-5 compared to row 1).In summary, we have explained three effects of Ca single atoms. First, the Ca atom makes H binding on N sites less stable by changing the H-N bonding nature more toward sp3-like and reducing the charge density interaction between H and other N atoms. Secondly, the H-N binding is over-weakened by a Ca single atom. With an extra H adsorbed on top of Ca the H-N binding can be strengthened. Thirdly, the Ca atom itself serves as an H adsorption site, with the adsorption strength adjustable by H adsorbed onto N. The latter two effects both result in many H adsorption processes with perfect \u0394GH\u2217 values. In particular, without Ca single atoms there are 7 unique processes of H adsorption and most of them have \u0394GH\u2217 values that are either too negative or too positive. Depositing a single Ca atom generates 23 unique processes of H adsorption and many of them are better than the 7 processes in former situations for HER. Therefore, we conclude that the Ca single atom significantly enhances the HER activity of N-doped graphene.Atomically confined calcium in NG (Ca1-NG) was successfully synthesized as an efficient catalyst for electrocatalytic and photocatalytic hydrogen evolution. HADDF-STEM images and X-ray absorption spectroscopy analyses confirm the uniformly dispersed single Ca atoms on the NG substrate. Ca K-edge EXAFS fitting curves and DFT calculations indicate the Ca single-atoms are anchored in the pyridinic-N defects in graphene to form a Ca-N3 structure. DFT calculations suggest that Ca atoms are trapped in SV+3N and DV+4N centers and Ca clustering is prevented. The high catalytic activity of Ca1-NG for HER and PHE results from the Ca single-atoms in NG, which leads to multiple H adsorption configurations with very favorable \u0394GH\u2217 values for HER. This research has pointed to a new approach for the development of high performance HER catalysts using non-transition metals.Here we have revealed that atomically confined Ca in NG (Ca1-NG) can effectively boost the electrocatalytic and photocatalytic HERs (EHE and PHE). Catalyst characterizations have shown that Ca single atoms anchored in NG can efficiently enhance the HER performance, improve the interfacial charge transfer, and suppress the photo-generated charge recombination. However, one limitation of this study is that the loading concentration of single-atom Ca prepared by the current method is low. We will further improve the single-atom preparation method to increase the loading in our future 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\ncadmium sulfide\nSinopharm Chemical Reagent (Shanghai, China)\nCAS:1306-23-6\n\n\nanhydrous calcium chloride\nSinopharm Chemical Reagent (Shanghai, China)\nCAS:10043-52-4\n\n\nconcentrated sulfuric acid\nSinopharm Chemical Reagent (Shanghai, China)\nCAS:7664-93-9\n\n\npotassium permanganate\nSinopharm Chemical Reagent (Shanghai, China)\nCAS:7722-64-7\n\n\ngraphite powder\nMacklin Reagent Co., Ltd\nCAS:7782-42-5\n\n\nsodium nitrate\nMacklin Reagent Co., Ltd\nCAS:7631-99-4\n\n\n30% hydrogen peroxide\nMacklin Reagent Co., Ltd\nCAS:7722-84-1\n\n\nammonium sulfite monohydrate\nAladdin Reagent Co., Ltd\nCAS:7783-11-1\n\n\n\nSoftware and algorithms\n\n\n\nVienna Ab-initio Simulation Package (VASP)\nTongji University\n\nhttp://software.tongji.edu.cn/Home/IndexPage\n\n\n\n\nOther\n\n\n\nJEOL JEM-2100F/HR transmission electron microscope\nJEOL (BEIJING) CO., LTD.\n\nhttp://www.jeol.com.cn/product/detail/617\n\n\n\nJEOL JEM-ARM200F microscope\nJEOL (Beijing) Co., Ltd.\n\nhttp://www.jeol.com.cn/product/detail/402\n\n\n\nBRUKER-D8 X-ray diffractometer\nBruker (Beijing) Scientific Technology Co. Ltd.\n\nhttps://www.bruker.com/zh/products-and-solutions/diffractometers-and-scattering-systems/x-ray-diffractometers/d8-advance-family/d8-advance-eco.html\n\n\n\nLab RAM high-resolution (HR) evolution Raman spectrometer\nHORIBA Jobin Yvon\n\nhttps://www.horiba.com/cn/scientific/markets-industries/display-technologies/\n\n\n\nESCALAB250 spectrometer\nThermofisher Scientific(China)Co.,Ltd.\n\nhttps://www.thermofisher.cn/order/catalog/product/SID-10148252?SID=srch-hj-ESCALAB250%20spectrometer#/SID-10148252?SID=srch-hj-ESCALAB250%20spectrometer\n\n\n\nInductively coupled plasma optical emission spectrometer (ICP-OES) Optima 8000\nPerkinElmer Management (Shanghai) Co., Ltd\n\nhttps://www.perkinelmer.com.cn/searchresult?searchName=Optima%25208000&_csrf=f3b614e8-0109-41be-a294-dfb37e7310da\n\n\n\nFluorescence Detector (RF-10A, Shimadzu, Japan)\nShimadzu (Japan) Co., Ltd.\n\nhttps://www.shimadzu.com.cn/an/gc/index.html\n\n\n\nEdinburgh FLS9800\nEdinburgh Instruments Ltd.\n\nhttps://www.selectscience.net/companies/edinburgh-instruments-ltd/?compID=7445\n\n\n\nNicolet iS10 (Thermo Fisher, USA) infrared spectrometer\nThermofisher Scientific(China)Co.,Ltd.\n\nhttps://www.thermofisher.cn/cn/zh/home.html\n\n\n\nbeamline XAFCA\nSingapore Synchrotron Light Source (SSLS)\n\nhttps://lightsources.org/cms/?pid=1000130\n\n\n\n\n\n\nFurther information and requests for resources should be directed to and will be fulfilled by the lead contact, Weifeng Yao (yaoweifeng@shiep.edu.cn).This study did not generate new unique reagents.This study did not generate any unique datasets or code.Graphene oxide (GO) was synthesized using the traditional Hummer method. In detail, 2.000\u00a0g of graphite powder, 1.000\u00a0g of NaNO3 and 46\u00a0ml of H2SO4 were added into a beaker soaked in an ice bath and well stirred. Under stirring, then 6.000\u00a0g of KMnO4 powder was slowly added to the above mixture for 10\u00a0minutes. The mixture was then heated to 35\u00b0C for 30\u00a0minutes. Next, after adding 92\u00a0mL of deionized water the mixture was heated to 98\u00b0C. At the same time 60\u00a0mL 30% H2O2 was slowly added to the mixture to prevent graphene oxidation. Finally, the mixture was centrifuged and washed repeatedly with deionized water. A golden yellow suspension was obtained by dispersing the obtained precipitate in water and then filtering. GO was obtained by freeze-drying the golden yellow suspension.Single calcium atom anchored nitrogen doped graphene (Ca1-NG) was synthesized via an impregnation method, followed by a calcination process under NH3 atmosphere. The synthesis details are as follows: 100.0\u00a0mg of GO and 1.0\u00a0mg of CaCl2 were dispersed into 50\u00a0mL deionized water. The mixture was sonicated for 4 hours to form a uniformly dispersed suspension. Then liquid nitrogen was added into the suspension to form a solid mixture, followed by freeze-drying for 24 hours. The resulting product was named Ca-GO. Finally, Ca-GO powder was calcinated under NH3 at 750\u00b0C for 1 hour to synthesize Ca1-NG. The method for the preparation of CaO-NG was adopted from a similar method reported, except that in this research it was calcined in air at 750\u00b0C for 1h before calcining under NH3. The prepared Ca1-NG (or CaO-NG) was then coupled with CdS using an impregnation method. Briefly, certain amounts of Ca1-NG (or CaO-NG) and CdS were added into an ethanol solution. Then the mixture was stirred at room temperature until the ethanol had completely evaporated. The obtained dark yellow powder was Ca1-NG/CdS (or CaO-NG/CdS).Photocatalytic hydrogen evolution activity was measured at 420\u00a0nm wavelength. 5.0\u00a0mg prepared catalysts were dispersed in 10\u00a0mL 1.0\u00a0M aqueous (NH4)2SO3 solution. The solution was degassed with N2 for 1\u00a0h to remove dissolved oxygen before being irradiated with a single-wavelength (420\u00a0nm) light-emitting diode (LED) monochromatic lamp (CEL-LED 100). The H2 evolution volume was analyzed via an online gas chromatograph (Techcomp Limited Co., GC7890II) equipped with a thermal conductivity detector. Ultra-pure nitrogen was used as a carrier gas.The apparent quantum efficiency (AQE) was measured and calculated according to the following equation:\n\n\n\nA\nQ\nE\n\n(\n%\n)\n\n=\n\n\nn\nu\nm\nb\ne\nr\n\no\nf\n\nr\ne\na\nc\nt\ne\nd\n\ne\nl\ne\nc\nt\nr\no\nn\ns\n\n\nn\nu\nm\nb\ne\nr\n\no\nf\n\ni\nn\nc\ni\nd\ne\nn\nt\n\np\nh\no\nt\no\nn\ns\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n\n\n=\n\n\n\nnumber\u00a0of\u00a0evolved\u00a0H\n2\n\nmolecules\u00a0x\u00a0\n2\n\nnumber\u00a0of\u00a0incident\u00a0photons\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n\n=\n\n\n\n2\n\u00d7\n\nn\n\nH\n2\n\n\n\n\n\nI\n0\n\n\u00d7\nt\n\n\n\u00d7\n100\n%\n\n\n\nwhere \n\n\nn\n\nH\n2\n\n\n\n is the mole numbers for hydrogen evolution from t\u00a0= 0 to time t. and I\n\n0\n is the Einstein of incident photons per second measured at \u03bb\u00a0= 420\u00a0nm.Electrochemical properties of catalysts were measured using a CHI 660E electrochemical workstation in a standard three-electrode cell. 5.0\u00a0mg catalysts were dispersed in a solution consisting of 500\u00a0\u03bcL water, 500\u00a0\u03bcL ethanol and 80\u00a0\u03bcL 5.0 wt.% Nafion solution. The above mixture was then sonicated for 1\u00a0h to form a homogeneous suspension. A working electrode was prepared by dropping 5\u00a0\u03bcL of the suspension onto the surface of a glassy carbon electrode (GCE), which was then dried in air. The electrode surface area is 0.07\u00a0cm2 with 0.265\u00a0mg cm-2 catalyst loading density. A saturated calomel electrode and a Pt foil were used as the reference electrode and the counter electrode, respectively. Linear-sweep voltammograms (LSV) were carried out at a scan rate of 2\u00a0mV S-1 in two electrolytes: one was a 0.5\u00a0M H2SO4 aqueous solution, and the other was a 1.0\u00a0M (NH4)2SO3 aqueous solution.The turnover frequency (TOF) values were calculated according to the Equation\u00a0below:\n\nTOF\u00a0= j x A (2F x n)\n\n\nWhere, j is the current density obtained at overpotential of 100\u00a0mV, A is the surface area of the electrode, F is the Faraday efficiency (96,485 mol-1), and n is the mole numbers of catalysts deposited onto electrodes.Cyclic voltammetry (CV) measurements were performed with scanning rates from 20 to 100\u00a0mV s-1 and potential ranges from 0.00 - 0.10\u00a0V (vs. RHE) in a 0.5\u00a0M H2SO4 solution. Double-layer capacitances (Cdl) were estimated based on current density variation as a linear function of scan rate. \u0394j\u00a0= (ja - jc)/2 was obtained at 50\u00a0mV vs. RHE. The electrochemically active surface area (ECSA) was determined by the double layer capacitance (Cdl). The following equation was used to calculate ECSA:\n\nECSA (cm2)\u00a0= Cdl/Cs\n\n\n\nThe specific capacitance (Cs) of a flat surface is usually in the range of 20 \u223c 60\u00a0\u03bcF cm-2. We assumed Cs was 40\u00a0\u03bcF cm-2 in the calculation of the ECSA.The ECSA of 20 wt.% Pt/C was calculated using the under-potential deposition hydrogen (UPD-H) adsorption/desorption voltammetry based on the following equation:\n\n\n\nE\nC\nS\nA\n\n(\n\ncm\n2\n\n)\n\n\n=\n\n\n0.5\n\u00d7\n\n\nS\nH\n\n/\nv\n\n\n0.21\n\n\n(\nm\nC\n\u00b7\n\ncm\n\n\u2212\n2\n\n\n)\n\n\n\n\n\n\nWhere SH was the integral area of the adsorption/desorption region for H atoms (0.05 V\u20130.40 V), which was marked red in Figure\u00a0S5, v is the scan rate.In this research, we also estimated the CB potentials of Ca1-NG and CdS using the Mott\u2212Schottky method. As shown in Figure\u00a0S16, the slopes of the Mott\u2212Schottky plots for CdS and Ca1-NG are greater than 0.00, suggesting that CdS and Ca1-NG are both n-type semiconductors. Their flat band potentials (Efb) are determined to be \u22120.58\u00a0V and \u22120.43\u00a0V (vs. SCE) for CdS and Ca1-NG, respectively. In general, the CB edge potential (ECB) is more negative by about \u22120.10 or \u22120.20\u00a0V than the Efb for the n-type semiconductors. Therefore, the ECB for CdS and for Ca1-NG are \u22120.78\u00a0V and \u22120.63\u00a0V (vs. SCE), that is \u22120.54\u00a0V and \u22120.39\u00a0V (vs. NHE) (normal hydrogen electrode). This result indicates that under light irradiation, photogenerated electrons in CdS can migrate from CdS to Ca1-NG at the heterojunction interfaces between Ca1-NG and CdS.Photoelectrochemical properties of catalysts were measured using a CHI 660E electrochemical workstation in a typical three-electrode system. The working electrode was prepared by dropping 50\u00a0\u03bcL of photocatalyst suspension onto the surface of a fluorine-doped tin oxide (FTO) conducting glass support with an area of 1.0\u00a0\u00d7 1.0\u00a0cm2 and then dried in air. An Ag/AgCl and a Pt foil were used as the reference electrode and the counter electrode, respectively. 0.1\u00a0M Na2SO4 aqueous solution was used as the electrolyte, which was purged with N2 to remove dissolved O2. The light source was a single-wavelength (420\u00a0nm) LED monochromatic lamp, which was identical to the light source for photocatalytic H2 evolution.XPS analyses were performed using an ESCALAB250 spectrometer equipped with a monochromatized Al K\u03b1 (1486.6 eV) source. The survey spectra were recorded in a 0.5 eV incremental with a pass energy of 140 eV. Detailed scans spectra were recorded in a 0.1 eV incremental with a pass energy of 140 eV. The elemental spectra were all corrected with respect to C1s peaks at 284.8 eV.To verify the above EXAFS results a least-squares curve fitting analysis was carried out for the first coordination shell spreading from 1.5 to 2.5\u00a0\u00c5. All backscattering paths were calculated based on the structures provided by ab initio simulations. The energy shift (\u0394E) was constrained for scatters at the same level. The path length R, coordination number (CN), and Debye\u2013Waller factors \u03c32 were left as free parameters. The fit was completed in R space with k range of 3.5\u201312.6\u00a0\u00c5\u22121 and k2 weight.All structures are calculated using density functional theory (DFT) implemented in the Vienna Ab-initio Simulation Package (VASP) (Kresse and Furthm\u00fcller, 1996). The exchange-correlation interaction is described by generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional (Perdew et\u00a0al., 1996). The Ca_sv pseudopotential is used. The vdW interaction is considered by using the DFT-D3 method (Grimme et\u00a0al., 2010) and spin-polarization effect is included. The electron wavefunctions are expanded using plane waves with an energy cutoff of 400 eV. Slab model is used for all calculations with a fixed cell thickness of 15\u00a0\u00c5 to ensure sufficient vacuum space. All structures are relaxed until all final residual forces on the atoms are smaller than 0.005 eV/\u00c5. They are built from a graphene unit cell with lattice constant of 2.467\u00a0\u00c5, as relaxed using the above parameters with a k-point mesh of 12\u00a0\u00d7 12\u00d71. A supercell of 4\u00a0\u00d7 4\u00d71 and k-mesh of 3\u00a0\u00d7 3\u00d71 are employed for all structures.\u0394G\nH\u2217 includes three parts: the difference in electronic energy \u0394E\nH, the difference in zero point energy \u0394E\nZPE, and the difference in entropy T\u0394S\nH\n\n\n(Equation\u00a01)\n\u0394G\nH\u2217\u00a0= \u0394E\nH\u2217\u00a0+ \u0394E\nZPE \u2013 T\u0394S\nH\u2217.\n\n\nAll the differences are between H in the adsorbed phase (H\u2217) and in the gas phase (H2). The vibrational frequency in H2 is much higher than in H\u2217 phase, so \u0394S\nH mainly results from the H2 molecule, namely, T\u0394S\nH \u223c 0.5\u00d7TS\nH2 \u223c 0.205 eV at the standard condition (300 K, 1 bar) (N\u00f8rskov et\u00a0al., 2005). The difference in zero point energy is usually very small. For example, \u0394E\nZPE is around 0.02 eV for H adsorbed onto the double-coordinated N of graphitic-C3N4 (Gao et\u00a0al., 2015) and around 0.035eV for H adsorbed onto Cu (111) surface (N\u00f8rskov et\u00a0al., 2005). Here, we use these two values for H adsorbed on pyridinic-N and Ca single atom, respectively. In particular, we use\n\n(Equation\u00a02)\n\u0394G\nH\u2217\u00a0= \u0394E\nH\u2217\u00a0+ 0.23 eV for H adsorbed on pyridinic-N\n\n\n\n\n(Equation\u00a03)\n\u0394G\nH\u2217\u00a0= \u0394E\nH\u2217\u00a0+ 0.24 eV for H adsorbed on Ca\n\n\nThe major contribution to \u0394G\nH\u2217 is the H adsorption energy, calculated as\n\n(Equation\u00a04)\n\u0394E\nH\u2217\u00a0= E(catalyst\u00a0+ mH) \u2013 E(catalyst+(m-1)H) \u2013 0.5\u00d7E(H2),\n\nwhere E(catalyst\u00a0+ mH) and E(catalyst+(m-1)H) refer to the total energies of the catalytic system with and without the adsorbed H that we are studying; E(H2) is the total energy of a gas phase H2 molecule. These three structures are all with the same supercell size and sufficiently relaxed. When more than one H is adsorbed onto the structure we consider the adsorption of H atoms one by one. In other words, when we consider the m\nth H atom, we use the structure with m-1\u00a0H atoms as the reference system.Similarly to defining the H adsorption energy in Equation\u00a0(4), we define the Ca adsorption energy as \n\n(Equation\u00a05)\n\u0394E\nCa\u00a0= E\nNG+Ca \u2013 E\nNG - E\nisolated_Ca_atom\n\n\nwhere E\nNG+Ca and E\nNG refer to the total energies of the N-doped graphene (NG) with or without Ca adsorption, and E\nisolated_Ca_atom is the total energy of an isolated single Ca atom.This work was financially supported by the Natural Science Foundation of Shanghai (19ZR1420200), Science and Technology Commission of Shanghai Municipality (19DZ2271100), and Shanghai Committee of Science and Technology (17DZ2282800). The authors thank Prof. Song Hong from Beijing University of Chemical Technology for his help on the electron microscopy characterization at the atomic level. S.L. acknowledges the postdoc fellowship provided by Agency for Science Technology and Research (A\u2217STAR) of Singapore. The computations in this paper were performed on the Odyssey cluster supported by the FAS Division of Science, Research Computing Group at Harvard University. S.L. also thanks Prof. Efthimios Kaxiras for helpful discussions.W.Y. designed the research. J.S., Q.Z., and Q.W. performed the syntheses, most of the structural characterizations, electrochemical and photocatalytic tests. S.L. and W.C. performed DFT simulations. The paper was co-written by W.Y., S.L., and C.H. The research was supervised by W.Y. and Q.X. All authors discussed the results and comments on the manuscript.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.isci.2021.102728.\n\n\nDocument S1. Figures S1\u2013S17 and Tables S1\u2013S5\n\n\n\nThe following reference appears in the Supplemental information: Chen et\u00a0al., 2019; Gopannagari et\u00a0al., 2017; Hu et\u00a0al., 2019; Irfan et\u00a0al., 2019; Li et\u00a0al., 2019a; Li et\u00a0al., 2019b; Liu et\u00a0al., 2020; Ran et\u00a0al., 2017; Rangappa et\u00a0al., 2020; Ruan et\u00a0al., 2020; Sun et\u00a0al., 2020; Wang et\u00a0al., 2020; Ye et\u00a0al., 2019; Zhang and Jin, 2019; Zhang et\u00a0al., 2019; Zhang et\u00a0al., 2020.", "descript": "\n                  Calcium is one of the most abundant and cheapest elements on earth. However, due to the lack of d-orbitals for chemical adsorption, it is generally considered as a stoichiometric reagent with no catalytic activities in heterogeneous catalysis. In this research, we have revealed that atomically confined Ca in nitrogen-doped graphene (Ca1-NG) can be an effective heterogeneous catalyst to boost both electrocatalytic and photocatalytic hydrogen evolution reactions (HER). Ca single atoms anchored in NG can efficiently enhance the HER performance due to the improvement of the interfacial charge transfer rate and suppression of the photo-generated charge recombination. Density functional theory calculations show that the high catalytic activity of Ca1-NG results from the Ca single atoms in NG, which leads to multiple H adsorption configurations with favorable \u0394GH\u2217 values for HER. This research can be valuable for the designing of environmentally friendly, economical and efficient catalysts for renewable hydrogen production.\n               "}
{"full_text": "Nowadays, the increasing environmental pollution and emissions of greenhouse gases stemming from the fast depletion of fossil resources imposed an increasing attention for developing efficient transformations of abundant and renewable lignin-based biomass resources. In this regard, lignin derivatives can be converted into high value-added chemicals and biofuels through the catalytic hydrodeoxygenation (HDO) process to partly replace at least non-renewable fossil resources [1\u20133]. For instance, anisole is often considered as a representative model substrate of lignin derivatives. For the catalytic HDO of anisole, due to their high catalytic activity, numerous metal oxides supported noble metal heterogeneous catalysts (e.g., Ru [4,5], Re [6], Pd [7], and Pt [8]) were widely explored. In addition, some transition metals and metal phosphides (e.g., Ni [9], Mo [10], CoMo [11], and Ni2P [12]) were investigated. Despite the high selectivities to deoxygenated products obtained, HDO processes are usually conducted under harsh reaction conditions, namely: high reaction temperatures (T\u00a0>\u00a0250\u00a0\u00b0C), high hydrogen pressure (3\u20136\u00a0MPa), and high metal loading amounts. Additionally, there exist some disadvantages of high catalyst cost and difficulty in catalyst reusability in some cases. Therefore, despite numerous research efforts, it is a huge challenge to develop highly efficient heterogeneous catalysts for the HDO of lignin derivatives.On the other hand, reducible Co3O4 is often used as a heterogeneous catalytic material [13,14]. Specifically, surface defects on reducible Co3O4 may benefit the activation of oxygen-containing functional groups, thereby promoting the catalytic performance of Co3O4-based catalysts. Recently, metal oxide-based macroporous materials have been applied in the fields of adsorption and catalysis (e.g., environmentally benign oxidation of volatile organic compounds [15,16]), because of their higher porosity and mass transfer rates, and larger concentration of surface defects, in comparison to bulk materials. However, there have been no reports on the development of macroporous Co3O4 supported noble metal catalysts for catalytic hydrogenation applications.In this communication, we fabricated a new three-dimensional (3D) ordered macroporous Co3O4-supported Ru catalyst (Ru/OM-Co3O4). Subsequently, he latter catalytic structure was investigated in the HDO of anisole under mild reaction conditions (i.e., 250\u00a0\u00b0C and 0.5\u00a0MPa hydrogen pressure). For comparison, the HDO reaction was also performed over Ru supported on other 3D ordered macroporous NiO and Al2O3 structures (OM-NiO and OM-Al2O3). The results showed that the present Ru/OM-Co3O4 catalyst could attain a much higher cyclohexane selectivity of 92.4% at a complete anisole conversion, compared with the commercial Co3O4, OM-NiO and OM-Al2O3 supported catalysts. The high catalytic efficiency of Ru/OM-Co3O4 was associated with both the beneficial activation of oxygen-containing groups in anisole at the surface oxygen vacancies of the OM-Co3O4 support, and the presence of highly dispersed Ru NPs, as well as the full exposure of active reaction/adsorption sites and favorable mass transfer related to the ordered macroporous structure of OM-Co3O4 support.Polymethyl methacrylate (PMMA) template beads were prepared by emulsion polymerization [17]. OM-Co3O4 support was fabricated by a sacrificial hard template method, and the resulting OM-Co3O4 supported Ru sample having a Ru loading of about 1.1\u00a0wt% was synthesized by the liquid-phase reduction process using sodium borohydride as reductant (see details in the Electronic Supporting Information, ESI). Other 3D ordered macroporous NiO and Al2O3 (denoted as OM-NiO or OM-Al2O3) and resulting supported Ru catalyst samples were synthesized according to identical procedures to those for OM-Co3O4 and Ru/OM-Co3O4 samples.Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), H2 temperature programmed desorption (H2-TPD), H2 temperature programmed reduction (H2-TPR), and Raman spectroscopy (see details in the Electronic Supporting Information, ESI).Details of catalytic HDO tests are included in the Electronic Supporting Information (ESI).As presented in Fig. S1 (ESI), XRD patterns for Ru/OM-Co3O4 sample exhibit several diffraction peaks, which match well those of the cubic Co3O4 spinel phase (JCPDS 42\u20131467). No diffractions corresponding to metallic Ru0 phase were observed, mainly owing to the small size (< 4\u00a0nm) of Ru0 particles and the low Ru loading used (~ 1.1\u00a0wt%) determined by ICP-AES analysis. The results reflect the good dispersion of Ru species on the surface of OM-Co3O4 support. As shown in Fig. S2 (ESI), Ru/OM-Co3O4 displays a 3D honeycomb-like ordered macroporous structure, in which hollow spheres are interconnected together through walls. TEM images of Ru/OM-Co3O4 (Fig. 1\n) depict that the large quantities of small Ru NPs with an average diameter of ~2.53\u00a0nm are uniformly attached on the surface of a nearly uniform OM-Co3O4 support, thereby forming the close interface between them. This is well consistent with the powder XRD results. Meanwhile, one can discern the lattice fringes of the (101) and (511) crystal facets of Ru0 and Co3O4 phases with facet spacing of 0.206 and 0.155\u00a0nm, respectively. In this case, both the ordered macroporous framework of OM-Co3O4 and the high dispersion of Ru NPs may favor full exposure of the adsorption and reaction active sites. In contrast, in addition to a few aggregates of particles, larger Ru particles with the size of ~15\u201320\u00a0nm are found to be distributed over the surface of the commercial Co3O4- supported Ru sample (Fig. S3, ESI). The above results illustrate that OM-Co3O4 support shows a promotional effect on the improvement of the dispersion of Ru and the formation of smaller Ru NPs.The structural defects can be easily generated through calcination or reduction treatments during the synthesis of supported catalysts, and further promote their catalytic performance of catalysts [18,19]. Therefore, XPS characterization was performed to identify surface electronic states of metal and oxygen species on Ru-based samples (Fig. 2\n). In the XPS of Ru 3d5/2 region for the Ru/OM-Co3O4 and Ru/Co3O4 samples, a peak with a binding energy at ~280.3\u00a0eV is observed, minoring the presence of metallic Ru0 species (Fig. S4, ESI). In the deconvoluted Co 2p region, Co 2p3/2 and Co 2p1/2 core levels appear at 777\u2013785 and 792.5\u2013801\u00a0eV, respectively, indicative of the presence of Co2+ and Co3+ species [20]. Notably, the surface fraction of Co2+ in the total Co species on the Ru/OM-Co3O4 (0.51) is larger than that on Ru/Co3O4 (0.42), reflecting the formation of more defective Co2+ sites. Meanwhile, XPS of the O 1\u00a0s region depicts the existence of three kinds of oxygen species at ~529.6, 531.3 and 532.8\u00a0eV, respectively, which correspond to lattice oxygen (OI), oxygen species adsorbed on defects (e.g., oxygen vacancies) or hydroxyl species (OII), and surface carbonate ions (OIII) [21]. Noticeably, the surface OII/(OI\u00a0+\u00a0OII\u00a0+\u00a0OIII) fraction on the Ru/OM-Co3O4 (0.48) is higher than that on the Ru/Co3O4 (0.41), likely suggestive of the generation of more oxygen vacancies.Raman spectra provide insight into the defective crystal structures. As illustrated in Fig. S5 (ESI), compared with those for Ru/Co3O4, five characteristic Raman peaks (F2g\n1, E2g, F2g\n2, F2g\n3, and A1g) of Co3O4 phase for the Ru/OM-Co3O4 solid all shift to low frequencies at the 532-nm laser wavelength, despite the reduced peak intensities. These results demonstrate the presence of lattice distortion/strain of Co3O4 spinel phase, and thus the formation of more Co2+-Ov-Co2+ like structural defects (Ov: oxygen vacancies) in the vicinity of Co2+ species on the Ru/OM-Co3O4 [22,23], mainly thanks to the multiple calcination processes conducted during the synthesis of OM-Co3O4.\nFig. 3\n shows the variation of anisole conversion and product distribution with reaction time after HDO reaction at 250\u00a0\u00b0C and 0.5\u00a0MPa over the Ru/OM-Co3O4 catalyst. The main deoxygenated products are benzene (BEN) and cyclohexane (CHA), along with the formation of small amounts of methoxycyclohexane (MCHA) and cyclohexanol (CHOL) by-products, and trace amounts of cyclohexanone (CNON) and phenol. With prolonged reaction time, the benzene selectivity gradually decreases, whereas the cyclohexane selectivity progressively increases. Besides a small amount of benzene with a yield of 4.2%, a large amount of cyclohexane with a high yield of 92.4% is obtained at complete conversion after 5\u00a0h of reaction. Over the pure OM-Co3O4 support, almost no conversion of anisole was obtained. As shown in Table 1\n (entry 1), Ru/Co3O4 exhibits a low catalytic activity for the HDO reaction with a much lower conversion, ca. 38.3% and a lower selectivity to deoxygenated products (ca. 66.3%) after 1\u00a0h. Notably, compared to Ru/Co3O4, the catalytic HDO performance of Ru/OM-Co3O4 is significantly improved, along with high conversion (90.2%) and selectivity to deoxygenated products (87.7%) (Table 1, entry 2). Compared with Ru/OM-Co3O4, the other two Ru/OM-NiO and Ru/OM-Al2O3 reference catalysts deliver lower conversions and selectivity to deoxygenated products (Table 1, entries 3 and 4).Since the activity and selectivity to deoxygenation products over the Ru/Co3O4 are much inferior to those over the Ru/OM-Co3O4 catalyst in the HDO process, one can confirm that surface Ru species and OM-Co3O4 support should play important roles in controlling the HDO process of anisole. In the present Ru/OM-Co3O4 catalyst, OM-Co3O4 support with a higher surface area of 24.3\u00a0m2/g can effectively serve as a support for achieving higher dispersion of small-sized Ru NPs, compared with the commercial Co3O4 with smaller specific surface area, ca. 4.5\u00a0m2/g. Further, TEM observations reveal the formation of highly dispersed and small-sized Ru NPs on the OM-Co3O4, which greatly facilitate the accessibility of active metallic Ru sites to substrates, and thus the dissociation of molecular hydrogen. Meanwhile, XPS and Raman results demonstrate the presence of more defects on Ru/OM-Co3O4, in comparison to Ru/Co3O4. Furthermore, the present Co3O4-supported Ru catalysts should be activated by H2 at high temperature before testing, and correspondingly the Co3O4 support could be reduced partly, as evidenced by the XPS analysis of the used catalysts (Fig. S6, ESI), thus leading to the increased surface Co2+/(Co2++Co3+) ratio (0.56 for Ru/OM-Co3O4 and 0.47 for Ru/Co3O4), and the OII fraction (0.65 for Ru/OM-Co3O4 and 0.5 for Ru/Co3O4) after the HDO reaction. Such abundant surface defective structures probably lead to the easier activation of methoxy group in anisole through the interaction between defective Co2+ species and oxygen atom of the methoxy group, and thus direct deoxygenation process to form benzene [5]. Subsequently, a further ring\u2011hydrogenation of benzene can produce cyclohexane.In this work, we further carried out H2-TPD experiments to determine the ability of H2 dissociation and the occurrence of hydrogen spillover on Ru-based catalyst samples. As presented in Fig. S7 (ESI), in the case of Ru/OM-Co3O4 sample, two desorption peaks located at ~92 and 300\u00a0\u00b0C correspond to desorption of hydrogen from Ru particles and highly dispersed Ru species strongly interacting with the support, respectively. The desorption peak at 535\u00a0\u00b0C is assigned to spillover hydrogen adsorbed on the support [24,25]. In contrast, no hydrogen spillover occurs on the Ru/Co3O4, besides a remarkably reduced desorption originating from the absence of highly dispersed Ru species. What's more, H2-TPR traces display that compared to pure OM-Co3O4 (386 and 501\u00a0\u00b0C, respectively), Ru/OM-Co3O4 exhibits lower reduction temperatures for Co3+ to Co2+ and Co2+ to Co0 species (360 and 489\u00a0\u00b0C, respectively, Fig. S8, ESI), confirming the occurrence of a much stronger hydrogen spillover from highly dispersed Ru0 species on the Co3O4 surface of the Ru/OM-Co3O4 catalyst.Therefore, thanks to the highly dispersive character of Ru species on the OM-Co3O4 support, and its abundant defective structure, both H2 dissociation and hydrogen spillover take place more easily on the Ru/OM-Co3O4 than on the Ru/Co3O4, thereby, significantly improving the catalytic HDO performance of Ru/OM-Co3O4. Also, the unique macroporous structure of OM-Co3O4 support provide a large number of open channels, which may likely favor the exposure of active reaction/adsorption sites and the facile diffusion of reactants and products, thereby promoting the catalytic HDO performance of Ru/OM-Co3O4 to some extent. It can be concluded that the higher catalytic performance of Ru/OM-Co3O4 should be closely associated with the surface cooperation between highly dispersed Ru NPs and defective sites present in the OM-Co3O4 support, as well as the unique macroporous framework of OM-Co3O4 support.The influence of reaction temperature and hydrogen pressure on the HDO of anisole was also investigated over the Ru/OM-Co3O4 catalytic system. As presented in Fig. S9 (ESI), with the elevated reaction temperature from 200 to 275\u00a0\u00b0C, the anisole conversion gradually increases from 42.5 to 98.6%, while the selectivity to deoxygenated products (benzene and cyclohexane) progressively increases from 44.7 to 94.7%. These results demonstrate that the high reaction temperature can promote the HDO of anisole. Fig. S10 (ESI) shows that benzene selectivity becomes quite low (<1.0%) above 1.0\u00a0MPa of hydrogen pressure. This is because the high hydrogen pressure favors the hydrogenation of benzene ring to form methoxycyclohexane and cyclohexane, thus inhibiting the direct cleavage of the methoxy group to form benzene. As summarized in Table S1 (ESI), compared to Ru-based catalysts previously reported, the present Ru/OM-Co3O4 catalytic system possesses better or at least comparative catalytic performance in the anisole HDO under mild reaction conditions.The stability of heterogeneous catalysts is one of the key indexes for their practical application. As displayed in Fig. 4\n, the selectivity to each product is almost unchanged, and the conversion is only decreased by ~1.3% after five successive HDO tests using the Ru/OM-Co3O4 catalytic system. Further, SEM and TEM images of the used catalyst (Fig. S11, ESI) reveal that the macroporous structure of OM-Co3O4 is kept unchanged, and no structural collapse occurs. It is indicated that Ru/OM-Co3O4 catalyst has good structural stability and reusability, mainly thanks to the strong interactions developed between Ru NPs and OM-Co3O4 support.In summary, we synthesized a new supported Ru catalyst on the OM-Co3O4 carrier, and utilized the OM-Co3O4 with appropriate surface defects to enhance the dispersion of small-sized Ru nanoparticles and create strong metal-support interactions. The Ru/OM-Co3O4 could afford a 92.4% yield of cyclohexane in the anisole HDO under mild reaction conditions (0.5\u00a0MPa hydrogen pressure and 250\u00a0\u00b0C), indicative of high activity and selectivity to deoxygenated products (benzene and cyclohexane). The formation of highly dispersed Ru species and more surface defects could favor the adsorption and activation of reactants on the catalyst surface. A significant diffusion behavior of reactants and products originating from the unique macroporous framework structure of OM-Co3O4 is very likely to account for the high catalytic efficiency of Ru/OM-Co3O4. It is expected that the present reported approach using OM-Co3O4 as catalyst support is novel and reproducible, and would be a promising approach for designing other high-performance supported catalysts applied in several other heterogeneous catalytic processes.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 thank the National Natural Science Foundation of China (21776017;21991102; U19B6002) for financial support.\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.2021.106302.", "descript": "\n                  A three-dimensional ordered macroporous Co3O4 (OM-Co3O4) supported Ru catalyst was developed for the efficient hydrodeoxygenation (HDO) of anisole. It is revealed that small-sized Ru nanoparticles evenly distributed over the surface of OM-Co3O4 with large quantities of oxygen vacancies could strongly capture Ru0 species, thereby resulting in strong Ru-Co3O4 interactions. Compared with commercial Co3O4 supported Ru catalyst, Ru/OM-Co3O4 displays a better catalytic HDO performance, with a high cyclohexane yield of 92.4% at 250\u00a0\u00b0C and 0.5\u00a0MPa hydrogen pressure after 5\u00a0h on stream. Such a significant efficiency of Ru/OM-Co3O4 is mainly attributed to both high dispersion of Ru0 species and an enhanced formation of surface defects, as well as the unique macroporous framework of OM-Co3O4 support.\n               "}
{"full_text": "The oligomerization of linear terminal alkenes is one of the significant problems in the linear \u03b1-olefins (LAOs) production in both industry and academia [1\u20136], together with hydrogenation of alkanes [7]. Linear alpha olefins, normally produced by ethylene oligomerization processes [8\u201311], and by Fischer\u2013Tropsch synthesis followed by purification [12], found applications as co-monomers in high density and linear low density polyethylene (HDPE and LLDPE) production, and as detergents, plasticizers, surfactants, and lubricants.[13,14]. Traditional technology for LAO synthesis is based on a full-range production process in which ethylene oligomerizes to achieve a broad range of the products. It is a non-selective approach and cannot match the constantly growing market demand. Accordingly, the change of a statistical ethylene oligomerization process into selective approach appears highly demanded [13\u201315]. The selective oligomerization of ethylene has recently attracted considerable attention [16,17]. In this regard, the catalytically selective trimerization of ethylene to 1-hexene has been extensively studied [18\u201323]. Amongst all the systems, the catalysts based on chromium metal has attracted more attention in the recent years [23,24]. This metal is the main center of Phillips\u2019 Cr-pyrrolide catalysts [25]. BP's (o-OMe)PNP catalysts [26], Sasol's PNP/SNS trimerization catalysts [27,28], and PNP tetramerization catalysts [29]. Catalysts based on other transition metals such as Zr, Ti, V, Ta, or Ni have been less studied [30,31].Hessen\u02bcs group in 2001 for the first time reported that the change of R from methyl to phenyl in [(\u03b7\n5\u2010C5H4C(Me)2RTiCl3]/MAO, switches the reaction from ethylene polymerization into ethylene trimerization and facilitates formation of 1-hexene as the major product. The hemilabile behavior of cyclopentadienyl ligand with the arene group is the main reason of this significant change in catalyst behavior [32]. Deckers et\u00a0al. in 2002 synthesized a new family of highly active catalysts for the trimerization of ethylene based on (arene-cyclopentadienyl) titanium complexes [(\u019e\n5-C5H3R-(bridge)-ArTiCl3] activated by MAO co\u2010catalyst. Selectivity to produce 1-hexene not only depends on the presence of the arene pendant group but also the bridge nature between cyclopentadienyl (Cp) and arene. In the absence of arene, polyethylene was the main product [33,34]. Huang's group synthesized a half\u2010sandwich titanium complex containing pendant thienyl group and used it in ethylene trimerization. In the reported results, they affirmed the important role of thiophene in ethylene trimerization [35]. Cp-based ligands have been widely studied as important ligands in organometallic chemistry and most studies on Cp modification can be focused on the type of the bearing pendant group on it. In this regard, in 2004, Huang et\u00a0al. used half-sandwich titanium complexes with the pendant ethereal group activated by MAO for ethylene trimerization [36]. In 2013, Zhang et\u00a0al. synthesized half-sandwich indenyl-based titanium complexes [Ind-(bridge)-Ar]TiCl3 bearing pendant arene group on the indenyl ring and examined the selective ethylene trimerization in the presence of MAO co-catalyst [37\u201339]. In the following of previous works, Zhang et\u00a0al. synthesized another series of half-sandwich indenyl-based titanium complexes with the thienyl group (Cp(Ind)-bridge-thienyl]TiCl3, which showed high selectivity in ethylene trimerization and its conversion to 1-hexene [23]. In 2015, Varga et\u00a0al. synthesized and characterized two titanium\u2010based heterogeneous catalysts using different methods including grafting through a covalent Ti-O-Si bond as well as through a pendant flexible tether from the Cp ligand [40]. The catalyst synthesized by the second method did not show any activity in ethylene trimerization because the active species were too close to the support surface [40]. In 2015, Duchateau et\u00a0al. synthesized different types of phenoxy-imine titanium catalyst according to the Fujita method and examined homogeneous and heterogeneous types [41]. Despite the high activity and selectivity of the synthesized titanium catalysts, very little polyethylene was produced as a by-product. In this work, an attempt was also made to stabilize the catalyst and prevent the formation of the polymer, while maintaining the desired catalyst activity and selectivity. In this line, MAO co-catalyst and phenoxy-imine titanium catalysts were stabilized using a two-step process on silica carrier [41]. In 2019, Mohamadnia et\u00a0al. successfully synthesized and identified three titanium-based catalysts {[\u019e\n5-C9H6-C(R)]-C4H3S}TiCl3 with different bridges (cyclohexane, cyclopentane and dimethyl) active in ethylene trimerization [29]. Factors affecting the catalyst activity in the production of 1-hexene including catalyst concentration, ethylene pressure, and reaction temperature were optimized [29].In the following of our research on the selective trimerization of ethylene, due to the high importance of \u03b1-olefins in the petrochemical industry, here a series of indenyl half\u2010sandwich titanium complexes, namely [Ind-C(R)-phenyl]TiCl3 was synthesized for possible application in ethylene trimerization process. The main aim is to fulfill the process at mild operating conditions such as low pressure and temperature to reduce operator-related risks, and to reach high economic efficiency by reducing catalyst consumption. In this regard, the effect of the arene bridge and indenyl ring, temperature, ethylene pressure, and MAO and catalyst concentrations on the catalytic efficiency was investigated. Furthermore, the effect of ligand type on the 1-hexene selectivity was also examined using DFT simulations by considering the energy path for each catalyst (C1-C4, see Scheme\u00a01\n), during ethylene oligomerization process [42\u201344].All manipulations of water- and/or air-sensitive compounds were performed using standard Schlenk and glove-box techniques under deoxygenated argon or nitrogen. The modified MAO (MMAO) co\u2010catalyst (7 wt% in toluene), titanium tetrachloride (TiCl4), n\u2010butyllithium (n\u2010BuLi; 2.5\u00a0M in n\u2010hexane), phenyllithium (1.5\u00a0M in dibutyl ether), and molecular sieve were obtained from Aldrich (Germany). Indene, pyrrolidine, 4-tert-butylcyclohexanone, cyclohexanone, cycloheptanone, acetone, ethanol, n\u2010hexane, diethyl ether, toluene, Na2CO3, MgSO4, NaCl, Na2SO4, sodium, and NaOH were purchased from Merck (Germany). Ethylene was provided by Bandar Imam Petrochemical Company (Iran) and purified by passing through NaOH, activated silica gel, and molecular sieve (3 \u00c5) columns, respectively. Methanol, n\u2010hexane, toluene and diethyl ether were dried and vacuum-distilled using calcium hydride (CaH2) and sodium metal consecutively before use.The 1H NMR and 13C NMR spectra have been recorded by the Bruker 400\u00a0MHz Ultra shield NMR instrument (Germany) at room temperature. The progress of the catalyst synthesis and trimerization reactions was followed by thin-layer chromatography (TLC) and gas chromatography GC system (Varian CP 3800), respectively. The inductively coupled plasma analysis (ICP), model 3410 ARL made in Switzerland, was used to determine the metal components of the catalyst. The UV-visible spectrophotometer (Pharmacia Biotech Ultrospec 4000) was used to further examine the spectral characteristics of synthetic complexes. Elemental analysis was performed using a Vario EL III CHNS elemental analyzer.The different fulvene precursors F1\u2013F4 were synthesized with a slight change according to the method proposed by Stone and Little (Scheme\u00a02\n) [45]. For this purpose, freshly distilled indene (5\u00a0mmol, 0.58\u00a0mL) and freshly distilled pyrrolidine (3\u00a0mmol, 0.25\u00a0mL) were dissolved in 2\u00a0mL of methanol under argon atmosphere at ambient temperature. Then, different ketones)2\u00a0mmol(, such as cyclohexanone, cycloheptanone, 4-t-butyl cyclohexanone, and acetone, were added dropwise to the stirred solution and the reaction mixture was stirred for 12\u00a0h. At the end of the reaction, acetic acid (3\u00a0mmol, 0.18\u00a0mL) was added to neutralize the residual base, and dilution was performed with diethyl ether (10\u00a0mL). To separate the remaining indene, and other unreacted materials, extraction was performed with deionized water (3\u00a0\u00d7\u00a010\u00a0mL) followed by brine (2\u00a0\u00d7\u00a010\u00a0mL). Finally, the water remaining in the organic phase was dried by anhydrous MgSO4. Synthesized fulvenes were purified using column chromatography by silica gel (petroleum ether as eluent). The pure fulvenes were characterized using 1H NMR, 13C NMR, and FT\u2010IR spectroscopies.Fulvene (C15H16, F1) was obtained as white crystals in 70% yield. 1H NMR (400 MHz, CDCl3, \u03b4, ppm): 1.70\u20131.79 (2H, m, C9H6-C(cyclo\u2010C5\nH\n10)), 1.79\u20131.9 (4H, m, C9H6-C(cyclo\u2010C5\nH\n10)), 2.75 (2H, t, C9H6-C(cyclo\u2010C5\nH\n10)), 3.05 (2H, t, C9H6-C(cyclo\u2010C5\nH\n10)), 6.79 (1H, d, C9\nH\n6-C(cyclo\u2010C5H10)), 6.94 (1H, d, C9\nH\n6-C(cyclo\u2010C5H10)), 7.17\u20137.27 (2H, m, C9\nH\n6-C(cyclo\u2010C5H10)), 7.37 (1H, d, C9\nH\n6-C(cyclo\u2010C5H10)), 7.9 (1H, d, C9\nH\n6-C(cyclo\u2010C5H10)). 13C NMR (100 MHz, CDCl3, \u03b4, ppm): 26.40, 28.10, 28.76, 32.29, 34.48 (CH2), 121.05, 123.73, 124.64, 126.01, 127.30, 128.25 (CH), 133.65, 135.87, 144.56, 152.46 (C\nq). FT\u2010IR (KBr, \u03c5max, cm\u22121): 3010, 3014 and 3064 (sp2 C-H), 2848 and 2921 (sp3 C-H), 1780\u20131930 (overtone of aromatic ring), 1619 (C=C), 1443 (CH2), 723 and 740 (=C-H) (Figs. S1\u2013S3).Fulvene (C12H12, F2) was obtained as a yellow oil in 70% yield. 1H NMR (400 MHz, CDCl3, \u03b4, ppm): 2.31 (3H, s, C9H6-C(CH\n3)2), 2.54 (3H, s, C9H6-C(CH\n3)2), 6.84 (1H, d, C9\nH\n6-C(CH3)2), 6.91 (1H, d, C9\nH\n6-C(CH3)2), 7.25\u20137.32 (2H, m, C9\nH\n6-C(CH3)2), 7.41 (1H, d, C9\nH\n6-C(CH3)2), 7.82 (1H, d, C9\nH\n6-C(CH3)2). 13C NMR (100 MHz, CDCl3, \u03b4, ppm): 22.84, 25.00 (CH3), 121.03, 123.52, 124.71, 126.02, 127.61, 128.35 (CH), 135.74, 136.70, 143.38, 143.97 (Cq). FT\u2010IR (KBr, \u03c5max, cm\u22121): 3600 (adsorbed water), 3030\u20133090 (sp2 C-H), 2913 and 2854 (sp3 C-H), 1930\u20131780 (overtone of aromatic ring), 1630 (C=C), 1450 (CH2), 727 and 750 (=C-H) (Figs. S4\u2013S6).Fulvene (C19H24, F3) was obtained as yellow oil in 75% yield. 1H NMR (400 MHz, CDCl3, \u03b4, ppm): 0.9 (9H, s, C9H6-C(4-tBu-cyclo-C5H9)), 1.4 (3H, m, C9H6-C(4-tBu-cyclo\u2010C5\nH\n9)), 2.2 (2H, t, C9H6-C(4-tBu-cyclo\u2010C5\nH\n9)), 2.4 (2H, m, C9H6-C(4-tBu-cyclo\u2010C5\nH\n9)), 3.2 (1H, d, C9H6-C(4-tBu-cyclo\u2010C5\nH\n9)), 3.8 (1H, d, C9H6-C(4-tBu-cyclo\u2010C5\nH\n9)), 6.86 (1H, d, C9\nH\n6-C(4-tBu-cyclo\u2010C5H9)), 6.96 (1H, d, C9\nH\n6-C(4-tBu-cyclo\u2010C5H9)), 7.24-7.30 (2H, m, C9\nH\n6-C(4-tBu-cyclo\u2010C5H9)), 7.40 (1H, d, C9\nH\n6-C(4-tBu-cyclo\u2010C5H9)), 7.95 (1H, d, C9\nH\n6-C(4-tBu-cyclo\u2010C5H9)). 13C NMR (100 MHz, CDCl3, \u03b4, ppm): 27.6 (CH3), 28.1, 29.3, 31.5, 33.8 (CH2), 47.5, 121.4, 124, 124.8, 126.1, 127.5, 128.3 (CH), 32.8, 133.6, 136, 144.8, 152.1 (Cq). FT\u2010IR (KBr, \u03c5max, cm\u22121): 3040\u20133090 (sp2 C-H), 2870 and 2956 (sp3 C-H), 1940\u20131780 (overtone of aromatic ring), 1627 (C=C), 1446 (CH2), 727 and 748 (=C-H) (Figs. S7\u2013S9).Fulvene (C16H18, F4) was obtained as yellow oil in 75% yield. 1H NMR (400 MHz, CDCl3, \u03b4, ppm): 1.68 (4H, m, C9H6-C(cyclo\u2010C6\nH\n12)), 1.86 (2H, m, C9H6-C(cyclo\u2010C6\nH\n12)), 1.97 (2H, m, C9H6-C(cyclo\u2010C6\nH\n12)), 2.93 (2H, t, C9H6-C(cyclo\u2010C6\nH\n12)), 3.17 (2H, t, C9H6-C(cyclo\u2010C6\nH\n12)), 6.87 (1H, d, C9\nH\n6-C(cyclo\u2010C6H12)), 6.95 (1H, d, C9\nH\n6-C(cyclo\u2010C6H12)), 7.25\u20137.32 (2H, m, C9\nH\n6-C(cyclo\u2010C6H12)), 7.41 (1H, d, C9\nH\n6-C(cyclo\u2010C6H12)), 7.81 (1H, d, C9\nH\n6-C(cyclo\u2010C6H12)). 13C NMR (100 MHz, CDCl3, \u03b4, ppm): 26.2, 28.6, 28.8, 29.5, 34.5, 34.9 (CH2), 121.2, 123.6, 124.7, 124.9, 126, 127.2 (CH), 128.3, 136.1, 144.1, 154 (C\nq). FT\u2010IR (KBr, \u03c5max, cm\u22121): 3010\u20133090 (sp2 C-H), 2852 and 2921 (sp3 C-H), 1790-1940 (overtone of aromatic ring), 1627 (C=C), 1448 (CH2), 721 and 750 (=C-H) (Figs. S10\u2013S12).Indenyl\u2010based ligands were prepared according to a slightly modified literature method (Scheme\u00a02) [26]. Solution of synthetic fulvenes derivatives (0.5\u00a0mmol) in diethyl ether (3\u00a0mL) was added dropwise to the phenyllithium solution in dibuthyl ether (2\u00a0mmol, 1.3\u00a0mL, 1.9\u00a0M) in 5\u00a0mL of dry diethyl ether under argon atmosphere at -40\u00a0\u00b0C. The mixture was stirred at room temperature for 12\u00a0h. After one day, the reaction mixture was hydrolyzed by 10\u00a0mL of cold water. The aqueous layer was extracted with light petroleum ether (three times), and the organic layer was dried with anhydrous MgSO4. The solvent was removed under vacuum. The obtained L1\u2013L4 ligands were purified using column chromatography via petroleum ether as eluent.Ligand (C21H22, L1) was obtained as a white solid in 91% yield. 1H NMR (400 MHz, CDCl3, \u03b4, ppm): 1.4\u20131.55 (1H, m, [C9H7-C(cyclo\u2010C5\nH\n10)]-C6H5), 1.51\u20131.66 (5H, m, [C9H7-C(cyclo\u2010C5\nH\n10)]-C6H5), 2.23\u20132.25 (2H, m, [C9H7-C(cyclo\u2010C5\nH\n10)]-C6H5), 2.39\u20132.42 (2H, m, [C9H7-C(cyclo\u2010C5\nH\n10)]-C6H5), 3.44 (2H, d, [C9\nH\n7-C(cyclo\u2010C5H10)]-C6H5), 6.56 (1H, t, [C9\nH\n7-C(cyclo\u2010C5H10)]-C6H5), 7.02\u20137.1 (3H, m, [C9\nH\n7-C(cyclo\u2010C5H10)]-C6\nH\n5), 7.15\u20137.21 (1H, m, [C9H7-C(cyclo\u2010C5H10)]-C6\nH\n5), 7.28\u20137.31 (2H, m, [C9H7-C(cyclo\u2010C5H10)]-C6\nH\n5), 7.4\u20137.49 (3H, m, [C9\nH\n7-C(cyclo-C5H10)]-C6H5). 13C NMR (100 MHz, CDCl3, \u03b4, ppm): 23.03, 26.61, 36.46, 37.48 (CH2), 44.55 [C9H7-\nC(cyclo-C5H10)]-C6H5, 122.35, 123.64, 123.86, 125.35, 125.67, 127.06, 128.13, 129.61 (CH), 143.93, 145.22, 147.38, 149.96 (Cq). FT\u2010IR (KBr, \u03c5max, cm\u22121): 2974 (sp2 C-H), 2933 and 2921 (sp3 C-H), 1610 (C=C), 1461 (CH2bending), 700\u2013800 (=C-H) (Figs. S13\u2013S15).Ligand (C18H18, L2) was obtained as a yellow oil in 90% yield. 1H NMR (400 MHz, CDCl3, \u03b4, ppm): 1.75 (6H, s, [C9H7-C(CH\n3)2]-C6H5), 3.47 (2H, d, [C9\nH\n7-C(CH3)2]-C6H5), 6.54 (1H, t, [C9\nH\n7-C(CH3)2]-C6H5), 6.73\u20136.78 (1H, d, [C9\nH\n7-C(CH3)2]-C6H5), 7.01\u20137.08 (1H, t, [C9\nH\n7-C(CH3)2]-C6\nH\n5), 7.10\u20137.17 (1H, t, [C9\nH\n7-C(CH3)2]-C6H5), 7.20\u20137.26 (1H, t, [C9H7-C(CH3)2]-C6\nH\n5), 7.27\u20137.34 (2H, t, [C9H7-C(CH3)2]-C6\nH\n5), 7.35\u20137.42 (2H, d, [C9H7-C(CH3)2]-C6\nH\n5), 7.48\u20137.52 (1H, d, [C9\nH\n7-C(CH3)2]-C6H5). 13C NMR (100 MHz, CDCl3, \u03b4, ppm): 29.65 (CH3)2, 37.8 (CH2), 40.5 [C9H7-C(CH3)2]-C6H5, 122.34, 123.70, 123.96, 125.47, 125.82, 126.21, 127.42, 128.29 (CH), 143.79, 145.28, 148.15, 152.35 (Cq). FT\u2010IR (KBr, \u03c5max, cm\u22121): 3072 (sp2 C-H), 2800\u20133000 (sp3 C-H), 1677 (C=C), 1465 (CH2bending), 780 (=C-H) (Figs. S16\u2013S18).Ligand (C25H30, L3) was obtained as a white solid in 92% yield. 1H NMR (400\u00a0MHz, CDCl3, \u03b4, ppm): 0.8 (9H, s, [C9H7-C(4-tBu-cyclo\u2010C5H9)]-C6H5), 1.1\u20131.2 (1H, m, [C9H7-C(4-tBu-cyclo\u2010C5\nH\n9)]-C6H5), 1.3\u20131.53 (2H, m, [C9H7-C(4-tBu-cyclo\u2010C5\nH\n9)]-C6H5), 1.6\u20131.80 (2H, t, [C9H7-C(4-tBu-cyclo\u2010C5\nH\n9)]-C6H5), 1.86\u20132.07 (2H, t, [C9H7-C(4-tBu-cyclo\u2010C5\nH\n9)]-C6H5), 2.58\u20132.85 (2H, m, [C9H7-C(4-tBu-cyclo\u2010C5\nH\n9)]-C6H5), 3.5 (2H, d, [C9\nH\n7-C(4-tBu-cyclo\u2010C6H9)]-C6H5)), 6.63\u20136.79 (1H, t, [C9\nH\n7-C(4-tBu-cyclo-C5H9)]-C6H5), 6.9\u20137.2 (4H, m, [C9\nH\n7-C(4-tBu-cyclo\u2010C6H9)]-C6\nH\n5), 7.2\u20137.34 (2H, m, [C9H7-C(4-tBu-cyclo\u2010C6H9)]-C6\nH\n5), 7.3-7.5 (3H, m, [C9\nH\n7-C(4-tBu-cyclo-C5H9)]-C6H5). 13C NMR (100\u00a0MHz, CDCl3, \u03b4, ppm): 24.1 (CH2), 27.5 [C9H7-C(4-tBu-cyclo-C5H9)]-C6H5, 32.5, 36.9 (CH2), 37.7 (CH2), 44.5, 48 [C9H7-C(4-tBu-cyclo-C5H9)]-C6H5, 122.5, 123.5, 123.9, 125.3, 125.7, 126.2, 128.2, 131.5 (CH), 144.1, 145.1, 147.5, 148.9 (Cq). FT\u2010IR (KBr, \u03c5max, cm\u22121): 3010\u20133085 (sp2 C-H), 2974 (sp3 C-H), 1606 (C=C),1458 (CH2bending), 700\u2013800 (=C-H) (Figs. S19\u2013S21).Ligand (C22H24, L4) was obtained as a white solid in 90% yield. 1H NMR (400 MHz, CDCl3, \u03b4, ppm): 1.57\u20131.92 (8H, m, [C9H7-C(cyclo\u2010C6\nH\n12)]-C6H5), 2.24\u20132.50 (4H, m, [C9H7-C(cyclo\u2010C6\nH\n12)]-C6H5), 3.48 (2H, d, [C9\nH\n7-C(cyclo\u2010C6H12)]-C6\nH\n5), 6.51\u20136.55 (1H, t, [C9\nH\n7-C(cyclo\u2010C6H12)]-C6H5), 6.75\u20136.79 (1H, d, [C9\nH\n7-C(cyclo\u2010C6H12)]-C6H5), 6.98\u20137.01 (1H, t, [C9\nH\n7-C(cyclo\u2010C6H12)]-C6H5), 7.07\u20137.10 (1H, t, [C9\nH\n7-C(cyclo\u2010C6H12)]-C6H5), 7.15\u20137.19 (1H, t, [C9H7-C(cyclo\u2010C6H12)]-C6\nH\n5), 7.25\u20137.31 (2H, t, [C9H7-C(cyclo\u2010C6H12)]-C6\nH\n5), 7.36\u20137.38 (2H, d, [C9H7-C(cyclo-C6H12)]-C6\nH\n5), 7.43\u20137.54 (1H, d, [C9\nH\n7-C(cyclo\u2010C6H12)]-C6H5). 13C NMR (100 MHz, CDCl3, \u03b4, ppm): 24.07, 31.63, 37.36, 39.11 (CH2), 47.80 [C9H7-C(cyclo\u2010C6H12)]-C6H5, 122.5, 123.58, 123.87, 125.37, 125.57, 126.69, 127.49, 128.22 (CH), 144.09, 145.27, 148.77, 151.83 (Cq). FT\u2010IR (KBr, \u03c5max, cm\u22121): 3025 (CHaromatic, sp2 C-H), 2971 and 2861 (CHaliphatic, sp3 C-H), 1610 (C=C), 1467 (CH2bending), 792 (=C-H) (Figs. S22\u2013S24).For the synthesis of the desired catalysts, ligands L1\u2013L4 (0.5\u00a0mmol) in dry petroleum ether (5\u00a0mL), was added dropwise to a solution of the 0.5\u00a0mmol n\u2010BuLi (0.2\u00a0mL, 2.5\u00a0M) solution in petroleum ether at -70\u00a0\u00b0C under an argon atmosphere. After mixing for four hours at the same temperature, white milky salt was obtained. Then, 0.5\u00a0mmol (0.05\u00a0mL) of TiCl4 was added to the reaction mixture at -78\u00a0\u00b0C. As soon as the TiCl4 was added, a dark red solution was obtained. The liver red solution was maintained at room temperature for 12 h. The unreacted solvent and TiCl4 were then removed by vacuum and the residue were dissolved in 10\u00a0mL of dry petroleum ether and then centrifuged. After cooling the solution containing the catalyst to -20\u00a0\u00b0C, the catalysts were obtained (Scheme\u00a02). Catalyst C1 was obtained as dark red crystals in 70% yield. Elemental analysis (%): calculated for C21H21TiCl3 (found): C 58.99 (59.30), H 4.95 (5.12), Ti 11.19 (10.53), Cl 24.87 (\u2014). Catalyst C2 was obtained as dark red solid in 65 % yield. Elemental analysis (%): calculated for C18H17TiCl3 (found): C 55.79 (54.90), H 4.42 (4.40), Ti 12.35 (11.95), Cl 27.44 (\u2014). Catalyst C3 was obtained as dark red solid in 68% yield. Elemental analysis (%): calculated for C25H29TiCl3 (found): C 62.08 (62.22), H 6.04 (6.11), Ti 9.90 (9.12), Cl 21.99 (\u2014). Catalyst C4 was obtained as dark red solid in 73% yield. Elemental analysis (%): calculated for C22H23TiCl3 (found): C 59.83 (59.92), H 5.25 (5.22), Ti 10.84 (9.93), Cl 24.08 (\u2014).The ethylene trimerization reactions were performed using synthetic titanium catalysts in a pressurized steel reactor equipped with a mechanical stirrer. The pressure and temperature inside the reactor and the mixer speed were controlled by the reactor's digital displays. In this regard, the reactor was first purged with dried pure argon at 120\u00a0\u00b0C for 2\u00a0h. It was reached the desired temperature and the solvent and MMAO were injected subsequently. After 10\u00a0min, the solution containing catalyst/solvent was injected and trimerization was started by charging the reactor with ethylene monomer. The reaction temperature and ethylene pressure were fixed constant throughout the process. After 30\u00a0min, the reactor was cooled to -10\u00a0\u00b0C. Then, the liquid phase including, 1-C6 and probable by\u2010products were collected and analyzed using GC instrument. The produced polyethylene by\u2010product was also washed with acidified ethanol (3% HCl), and dried under vacuum at 60\u00a0\u00b0C to a constant weight.DFT static calculations were performed at B3LYP level [46] with the Gaussian16 package [47]. The electronic configuration of the system was described with the standard split valence basis set with a polarization function for all the atoms (def2SVP keyword in Gaussian) of Ahlrichs and co-workers [48]. Geometry optimizations were performed without symmetry constrain, and analytical frequency calculations performed the characterization of the local stationary points. These frequencies were used to calculate unscaled zero-point energies as well as thermal corrections and entropy effects at 298\u00a0K and 1 atm. The transition states were located using the synchronous transit-guided quasi-Newton (QST3) approach and the extrema have been checked by analytical frequency calculations. All transition states have associated only one imaginary frequency. Solvent effects were estimated in single point energy calculations on the gas phase optimized structures based on the polarizable continuous solvation model (PCM) [49], as implemented in Gaussian16, using toluene (Tol) as a solvent. Energies were obtained using the B3LYP functional [46], in conjunction with the triple-\u03b6 basis set cc-pVTZ for all the atoms [50], together with the Grimme D3 correction term [51] to the electronic energy. The reported free energies in this work include energies obtained at the B3LYP/cc-pVTZ level of theory corrected with zero-point energies, thermal corrections and entropy effects evaluated at 298 K, achieved at the B3LYP/def2SVP level, without translational entropy corrections [52].In the structure of synthetic catalysts, [C9H6-C(R)-C6H5]TiCl3, due to the presence of halide groups and bulky indenyl moiety, the coordination number of Ti is IV [53,54]. In general, for this group of complexes, tetrahedral and square\u2010planar structures are observed. Due to the presence of bulky ligands in the structure and electrostatic repulsion between them, the complex tends to have a deviated tetrahedral structure. For the latter geometry, the steric hindrance has the minimum value [55], and in agreement the relative stability of the complex increases. According to the valence bond theory, orbitals of the valence layer of the central atom in the tetrahedral structure are hybridized either as sp3 or as d3s [29]. In our synthesized titanium-based catalysts as well as TiCl4, the electron arrangement of the central atom is d0. As shown in Fig.\u00a01, due to the d0 spin of Ti atom in the synthesized catalysts, d\u2192d transitions were not observed for the studied complexes. Actually, the peaks observed at 260\u2013280\u00a0nm were related to \u03c0\u2192\u03c0 (intra\u2010ligand) transitions. In addition to this type of transition, the ligand\u2010to\u2010metal transition at about 240\u00a0nm was also detected. However, due to the increase in the length of the resonance system, the intra\u2010ligand transition in all catalysts has shifted to higher wavelengths (Figs. S25\u2013S27).The ethylene trimerization reaction using the as-synthesized titanium\u2010based catalysts activated by MMAO afforded 1\u2010hexene and some by-products. The key role of co\u2010catalyst, between other functions such as removal of oxygen and water and eliminating environmental pollution, is to reduce the oxidation state of titanium metal and to facilitate the production of cationic species [56]. In general, co-catalyst facilitates alkyl abstraction from the catalyst pioneer to yield an anionic co\u2010catalyst species [RX\u2212] and a cationic metal species [LnM+], which together represent the active catalytic system as an ion pair with [LnM+][RX\u2212] formula (Scheme\u00a03\n) [54]. The MMAO\u2010activated system was highly active and selective in ethylene trimerization reaction [57]. In fact, an analysis of the liquid fraction by GC disclosed that ethylene trimerization via catalyst/MMAO under different position achieved 1\u2010hexene with high selectivity.The results of ethylene oligomerization using the C2 catalyst are shown in Table\u00a01\n. First, the effect of reaction temperature on catalyst activity was investigated (entries 3, 5-7). To do this, the reaction was performed at four temperatures of 20, 40, 60, and 80\u00a0\u00b0C with 1.5\u00a0\u03bcmol of C2 catalyst solution, and 8 bar ethylene pressure. From Fig.\u00a02, it is clear that for reaction temperature there is an optimal value of 60\u00a0\u00b0C, in which catalyst activity and 1-C6 selectivity have their maximum amount of 2593\u00a0kg 1-C6/mol-Ti h and 78.77%, respectively. It was reported that the pendant ring in the complex structure can coordinate easily to the electron deficient cationic Ti species due to the higher catalyst flexibility at elevated temperatures which subsequently causes a substantial decrease in the catalyst activity [33,58,59].Due to the negligible productivity difference in the productivity values at the reaction temperatures of 40\u00a0\u00b0C and 60\u00a0\u00b0C (2389 and 2593\u00a0kg 1-C6/mol-Ti h, respectively), the temperature of 40\u00a0\u00b0C was chosen for the next studies. In the next step, effect of ethylene pressure was taken into account. According to Fig.\u00a03, increasing the ethylene pressure from 3 to 8 bar, the activity of the C2 catalyst increased due to the enhanced solubility of ethylene gas at higher pressures. However, after P\u00a0=\u00a08 bar, the reaction switched to ethylene polymerization, so that at P\u00a0=\u00a012 bar, the activity decreased to 337, and 465\u00a0kg 1-C6/mol-Ti h, at the T\u00a0=\u00a020 and 40\u00a0\u00b0C (entries 4 and 10, Table\u00a01), respectively. Therefore, the next experiments were conducted at P\u00a0=\u00a08 bar as the optimum ethylene pressure value.Worth mentioning, effect of ethylene pressure on C2 catalyst activity and 1-C6 selectivity was considered at T\u00a0=\u00a020\u00a0\u00b0C, as well (entries 1\u20134, Table\u00a01 and Fig.\u00a04). Notably, in this condition the same trend as it at T=40\u00b0C was observed. Indeed, the highest catalyst activity and 1-C6 selectivity of 1179\u00a0kg 1-C6.mol Ti\u22121.h\u22121 and 74% were obtained at P\u00a0=\u00a08 bar (entry 3) after which the process was switched to the polymerization reaction with catalyst activity of 1294\u00a0kg PE/mol-Ti h.In the following, effect of Al/Ti molar ratio on the C2 catalyst activity and selectivity was investigated, Fig.\u00a05. Obviously, by increasing Al/Ti molar ratio up to 2000, catalyst activity increases due to the formation of more catalytic active sites. As the Al/Ti ratio raised above 2000, the activity decreased due to the poisoning of catalytic sites. Therefore, Al/Ti=2000 molar ratio was selected as the desired value, due to high activity of C2 catalyst toward 1-C6 formation.According to the results, C2, pressure of 8 bar, T\u00a0=\u00a040\u00a0\u00b0C and catalyst dosage of 1.5\u00a0\u03bcmol were selected as the optimum reaction conditions for achieving high catalytic activity and 1\u2010hexene selectivity. Finally, the effect of bridge type on the catalytic efficiency was elucidate. According to the previous studies conducted in these catalyst systems, there is no coordination between dangling arene (phenyl) and Ti metal at the primary precatalyst. However, after activation with the MAO co\u2010catalyst, it happens easily which facilitates the formation of 1-C6 product [26,60]. The bridge between the indenyl and phenyl moieties has a significant effect on the direction of phenyl ring relative to the titanium metal center and the size of C(Ind)C(bridge)C(phenyl) angle. When the catalyst is activated by the co\u2010catalyst, the dangling phenyl will go toward the cationic metal with a low oxidation number. Therefore, the type of bridge and its size have a direct effect on the coordination of arene and Ti. In the C1 with the bridge C(cyclo-C5H10), it has stronger coordination than the same complex with the CMe2 bridge. Catalyst C4 has the most space constraints since the pendant arene moiety has a very strong coordination with Ti. Consequently, this can prevent ethylene from approaching the metal active site. That is, the bridge has a dual effect on catalyst efficiency. While the cycloheptane bridge leads to the largest spatial hindrance and angular pressures, and thus low activity, the catalyst with a cyclohexane bridge with stable chair structure, high activity and selectivity. Table\u00a02\n shows the trimerization results for C1\u2013C4 catalysts (see Scheme\u00a01 for the detail of the catalysts).According to Table\u00a02, it is noteworthy that, regardless of the polymer produced in the trimerization, the selectivity for 1-hexene in all catalysts was higher than 90%. However, considering the unwanted polymer by-product, the selectivity of 1-hexene for all four catalysts decreased. 1-hexene selectivity for catalyst C4 was obtained due to the production of 1.7\u00a0g of polyethylene in the reaction of only 30% (Fig.\u00a06).\nIn the last part of our study, to compare quantitatively the effect of ligand type on the energy values of ethylene trimerization reaction path, and shed light on the structural parameters, DFT calculations were carried out. In this regard, the main responsible and effective steps in the reaction pathways were considered (Scheme\u00a04\n).The catalyst activation steps by the MAO co-catalyst (Scheme\u00a03) were not considered and M0 is our starting active catalyst (see Scheme\u00a04), in which the titanium has an oxidation state II. Thus, it has a severe electron deficiency and tends to coordinate rapidly with the two ethylene molecules leading to M1 (note that in the energy profile the insertion of only one ethylene molecule (M1\u2019) has also been considered) [61], from which a five-membered metallacycle is formed (M2), switching the corresponding oxidation of the metal center from Ti(II) to Ti(IV). It is possible that as a result of the ring-opening reaction 1-butene will be formed through two different reaction pathways. The first mechanism is the \u03b2-hydrogen transfer to Ti, forming a hydride, and the subsequent elimination, the second mechanism is the transfer of intramolecular \u03b2-hydrogen and the formation of M3, which means a reduction of the metal center again to Ti(II), and the olefin is bonded to titanium. Since 1-butene is not formed, instead the reaction will lead to the formation of M4 and the third ethylene molecule will be coordinated. After ethylene insertion (M5), the reaction likely continues with a ring-opening (M6) to finally produce 1-C6 (experimentally observed). However, a new ethylene molecule may bond to titanium and M7 may be formed, leading to the release of 1-octene or even higher olefins such as 1-decene. However, the latter species was not observed experimentally. After considering the general path shown in Scheme\u00a04, the structure of the catalysts was optimized. Since 1-hexene was observed experimentally as the main product and 1-octene as the by-product, only the energy of these steps was investigated for the four catalysts included in Fig.\u00a05. The energy diagram for ethylene oligomerization was then obtained for the four catalysts. In summary, the catalysts can evolve via four main stages of release of 1-butene, the formation of a seven-membered ring, the release of 1-hexene, and the formation of a nine-membered ring.The reaction mechanism proposed in Scheme\u00a04 has been studied for the four different catalysts C1\u2013C4. In general, results in Fig.\u00a07\n are similar for the different candidates. In all cases, the formation of the seven-membered metallacycle (M5) through an insertion of a third molecule of ethylene (T4) is more kinetically favorable than the release of a 1-butene molecule through the intramolecular \u03b2-hydride transfer (T2B). This difference is more significant in systems C2 (9.5\u00a0kcal\u2022mol\u22121) and C4 (8.3\u00a0kcal\u2022mol\u22121) than in systems C1 (2.7\u00a0kcal\u2022mol\u22121) and C3 (3.2\u00a0kcal\u2022mol\u22121). These observations match with the experimental evidence reported above, since it has not been observed any release of 1-butene molecules. After the formation of the seven-membered ring (M5) the mechanism proposes both the insertion of a fourth ethylene molecule subsequently forming a nine-membered metallacycle (M8) or, the release of a 1-hexane molecule through, again, an intramolecular \u03b2-hydride elimination, leading to M6, being the latter, the winner not thermodynamically, but kinetically speaking. The transition state involving the intramolecular migration of the \u03b2-hydrogen (T5) (see Fig.\u00a08a) is thus lower in energy than the transition state involving the formation of the nine-membered metallacycle (T7) (see Fig.\u00a08b). Matter of fact, in all cases the expected product is the 1-hexene while 1-octane becomes only a by-product. The catalysts showing better selectivity towards 1-hexene release is the system C1 as the difference in energy is around 2.9\u00a0kcal\u2022mol\u22121, being C4 the one showing less selectivity as this difference is only 0.8\u00a0kcal\u2022mol\u22121, with C2 and C3 in between, with values of 1.6 and 2.2\u00a0kcal\u2022mol\u22121, respectively. Again, these observations match with the experimental data reported before.According to the summarized data included in Table\u00a03\n (all the optimized geometries and more relevant distances are collected in Table S1 in the supporting information), the energy barrier of \u0394ETS for the formation of the seven-membered ring is lower than for the release of 1-butene. On the other hand, the energy barrier of \u0394ETS to release 1-hexene is lower than for the formation of a nine-membered ring. Because the amount of \u0394E2 is less than that of \u0394E1, the reaction tends to form 1-hexene. Calculations for other catalysts are shown in Table\u00a03, indicating that the most feasible path is the formation of 1-hexene. The C4 catalyst has a lower selectivity than other catalysts. Anyway, still the next kinetic barrier leading to M9 is higher, and thus the 1-octene formation is even more disfavored than the 1-hexene one, for the four catalysts C1-C4.To evaluate the sterics among the series C1\u2013C4 the %VBur of the rate determining intermediate M5 was evaluated [62,63]. The values are 26.9, 25.4, 27.0 and 26.3%, respectively. Even though the cyclohexane based systems C1 and C3 have a more hindered metal center [64], the difference is scarce and not enough to describe any trend (see Tables S28\u2013S31 for further details) [65\u201367]. Nor can emphasis be placed on weak interactions such as non-covalent ones [68], since for the four catalysts the H-bonds are almost identical.High selectivity and productivity of 1\u2010hexene production was obtained using the four titanium\u2010based catalysts C1\u2013C4, in good comparison with recent work [69]. Ethylene trimerization was performed using these synthetic catalysts with changes in MAO concentration, reaction temperature, and ethylene pressure. Using the C2 catalyst, the concentration of 1.5\u00a0\u03bcm for the C2 catalyst, the selectivity, and activity of the three catalysts C1, C3, and C4 were also examined in this optimal concentration. The activity of all four catalysts increased with increasing temperature to 60\u00a0\u00b0C, indicating the thermal resistance of synthetic catalysts at high temperatures. Increasing ethylene pressure always increases the solubility of ethylene and increases the activity, which was also true for the synthesized titanium-based catalysts but, the C2 catalyst moves from ethylene trimerization to ethylene polymerization at an ethylene pressure of 12 bar at 20 and 40\u00a0\u00b0C.The authors state that \u201cThere are no conflicts to declare\u201d.This work was supported by the Iran National Science Foundation (INSF) through the grant number of 98020308. A.P. is a Serra H\u00fanter Fellow and ICREA Academia Prize 2019, and thanks the Spanish MINECO for project PGC2018-097722-B-I00 and CTQ2017-85341-P. G.P. gratefully acknowledges the support of Institut de Qu\u00edmica Computacional i Cat\u00e0lisi (IQCC) and the computer resources and technical support provided by the Barcelona Supercomputing Center (BSC).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.mcat.2021.111636.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Different types of [Ind-C(R)-Phenyl]TiCl3 catalysts based on pendant arene containing indenyl (Ind) ligand bearing various types of bridges (R=cyclo\u2010C5H10 (C1), (CH3)2 (C2),\u00a04-tBu-cyclo\u2010C5H9 (C3),\u00a0and cyclo\u2010C6H12 (C4)) have been synthesized, and used in the ethylene trimerization to 1-hexene in the presence of methyl aluminoxane (MAO) as co-catalyst. The reaction conditions were first optimized in C2 catalyst case, where the highest 1-hexene product was achieved at the catalyst concentration, temperature and ethylene pressure of 1.5\n                        \n                           \n                           \u00d7\n                        \n                      10\u22123 M, 40\u00a0\u00b0C, and 8 bar, respectively. During this optimization and under specific reaction conditions, a switching behavior from ethylene trimerization to polymerization was also detected, as an undesired reaction. At the optimized conditions, synthesized catalysts showed the following trend toward both 1-hexene yield and selectivity: C1>C2>C3>C4. Then, to shed light on the possible reaction mechanisms and to confirm the activity trend obtained in experimental section, density functional theory (DFT) calculations were employed. In this line, obtained results for activity trend in the simulation studies fit well with the experiments. According to both experimental and DFT results, the highest catalytic activity was observed in the presence of the catalyst with a cyclohexane middle bridge (C1).\n               "}
{"full_text": "Ceria-based compounds are very attractive materials for electrochemical applications, such as solid oxide fuel cells (either electrolyte or anode [1,2]), oxygen storage materials [3], oxygen sensors or catalysts of partial oxidation of hydrocarbons [4,5]. They present high mobility of oxygen ions [3], high oxygen storage capacity (OSC) [3], attractive redox catalytic properties [1\u20135], chemical compatibility with water and carbon dioxide at high temperatures [4] and sufficient resistance to reduction under relatively low oxygen partial pressures [4]. The high oxygen mobility in ceria promotes the mechanism of carbon removal, which in turn, should contribute to the stability of the catalysts on hydrocarbon conversion reactions [3].Undoped ceria (CeO2) has a fluorite-type structure with 8-fold coordination of cations. In reductive atmospheres the oxygen vacancies are compensated by the reduction of Ce4+ to Ce3+. As a result, n-type electronic conduction through small polaron thermally activated hopping occurs [4,6]. To introduce the oxygen vacancies and to increase the ionic conductivity of these compounds, cerium atoms in the structure can be substituted with some aliovalent cations. Among them one can find e.g., Gd3+ or Sm3+ [7,8]. Another interesting type of dopant is praseodymium [9,10], which shows a significant redox activity under reductive conditions. At reduced pO\n\n2\n Pr acts in a similar manner to Gd or Sm. It is a fixed valent dopant non prone to changes in the oxygen vacancy concentration with temperature [9,10]. At high pO\n2 the concentration of oxygen vacancies becomes strongly dependent on temperature and oxygen partial pressure [9,10].In order to obtain a ceria-based compound with a considerable concentration of oxygen vacancies, the strategy of aliovalent co-doping of ceria with Gd and Pr has been suggested in the literature [9]. In this case oxygen nonstoichiometry is formed as a result of charge compensation. Another pair of dopants, Nd and Sm, led to improved electrical conductivity thanks to the lowering of an association enthalpy of the oxygen vacancy and the dopant ions [11\u201313]. Other factors, which may play a role in co-doping effect are: raise of a configurational entropy, modification of an elastic strain in the crystal lattice and changes in the grain boundary composition [11\u201313].In this paper, aliovalent co-doping procedure by Sm and Pr has been suggested. Then oxygen vacancies should be formed to compensate effectively negatively charged Sm3+, Pr3+ and Ce3+ that partially substitute Ce4+. Assuming ideal behaviour for the reduction of Pr and Ce in the lattice (invariant values of reaction entropies and enthalpies) the expected defect reactions leading to a formation of oxygen vacancies can be described by the following reactions [9]:\n\n(1)\n\n\nS\n\nm\n2\n\n\nO\n3\n\n\n\n\u2192\n\nC\ne\n\nO\n2\n\n\n\n\n2\nS\n\nm\n\nC\ne\n\n\u2032\n\n+\n\nV\nO\n\n\u22c5\n\u22c5\n\n\n+\n3\n\nO\nO\nx\n\n\n\n\n\n\n\n(2)\n\n\n2\nP\n\nr\n\nC\ne\n\nx\n\n+\n\nO\nO\nx\n\n\u2194\n2\nP\n\nr\n\nC\ne\n\n\u2032\n\n+\n\nV\nO\n\n\u22c5\n\u22c5\n\n\n+\n\n1\n2\n\n\nO\n2\n\n\n(\ng\n)\n\n\n\n\n\n\n\n(3)\n\n\n2\nC\n\ne\n\nC\ne\n\nx\n\n+\n\nO\nO\nx\n\n\u2192\n2\nC\n\ne\n\nC\ne\n\n\u2032\n\n+\n\nV\nO\n\n\u22c5\n\u22c5\n\n\n+\n\n1\n2\n\n\nO\n2\n\n\n(\ng\n)\n\n\n\n\nwhere \n\n\nO\nO\nx\n\n\n is the oxide ion in its lattice site, \n\n\nV\nO\n\n\u22c5\n\u22c5\n\n\n\n is an oxygen vacancy, \n\nS\n\nm\n\nC\ne\n\n\u2032\n\n\n is Sm3+ ion in Ce4+ ion site, \n\nP\n\nr\n\nC\ne\n\nx\n\n\n and \n\nP\n\nr\n\nC\ne\n\n'\n\n\n are Pr4+ and Pr3+ ions in Ce4+ site, respectively.It is clear from the above equations that both dopants lead to a formation of oxygen vacancies, which may have a positive effect on the partial oxidation of hydrocarbons catalysed with the use of these compounds. Therefore, in this work various nanocrystalline compounds of Pr and Sm co-doped ceria (with up to 20\u00a0mol.% of dopants) were fabricated by the reverse microemulsion synthesis method. Next, they were deposited in the form of layers on the surface of SOFC anode in aim to act as electrochemically active materials for the biogas reforming process. The aim of such a SOFC anode modification was to investigate the influence of these functional layers on a lifetime and efficiency of the commercially available solid oxide fuel cell operating under biogas without the need of an external reformer.The following compositions: CeO2-\u03b4, Ce0.9Sm0.1O2-\u03b4, Ce0.9Pr0.1O2-\u03b4, Ce0.8Pr0.15Sm0.05O2-\u03b4, Ce0.8Pr0.1Sm0.1O2-\u03b4 and Ce0.8Pr0.05Sm0.15O2-\u03b4 have been fabricated via a reverse microemulsion method. A detailed description of the applied procedure is reported elsewhere [14].The phase composition of the investigated materials was analysed using the X-ray diffraction method (XRD) by an X'Pert Pro MPD Philips diffractometer with Cu K\u03b1 (1.542\u00a0\u00c5) radiation at room temperature. The size of crystallites was estimated based on the Scherrer formula: C\u00a0=\u00a0k\u03bb/[(Be\u2212Bt)cos\u03b8], where C is an average diameter of the crystalline grain, k is a constant (assumed to be 0.9), \u03bb is the X-ray wavelength, \u03b8 is the diffraction angle, Be is the measured width of a peak profile and Bt is the instrumental width of a peak. The XRD patterns were also analysed by the Rietveld re\ufb01nement method using a HighScore Plus software with the pseudo-Voigt pro\ufb01le function applied. As a starting point of the analysis, crystal structure parameters of CeO2 (Fm-3m space group) were used [15]. The morphology of fabricated materials was examined using the FEI Quanta FEG 250 Scanning Electron Microscope (SEM). The thermal expansion coefficient of doped-ceria pellets (previously sintered at 1000\u00a0\u00b0C for 2\u00a0h) was determined using the Netzsch DIL 402\u00a0PC dilatometer operating in 100\u20131000\u00a0\u00b0C temperature range under nitrogen atmosphere with 3\u00a0\u00b0C/min heating/cooling rate.To form pastes, the obtained powders were ground in a mortar for about 1\u00a0h with ESL 403 organic binder (ElectroScience Laboratory, USA). The prepared pastes were deposited on the anode surface of a traditional 1-inch Solid Oxide Fuel Cell (Ni-YSZ anode, YSZ electrolyte and LSM-YSZ cathode). The catalytic layer was a circle of 16\u00a0mm diameter and 30\u00a0\u03bcm thickness. Finally, the modified fuel cells with deposited catalytic layers were fired at 1000\u00a0\u00b0C for 2\u00a0h.Such prepared fuel cells were mounted in a measurement rig [16]. They were heated up to 800\u00a0\u00b0C with argon delivered to the anode side and then, to reduce nickel oxide, humidified hydrogen (3% H2O) was supplied at 800\u00a0\u00b0C for 30\u00a0min and further at 750\u00a0\u00b0C for 20\u00a0h. After this time the hydrogen was replaced by wet synthetic biogas (3% H2O) consisting of methane and carbon dioxide mixed at a volume ratio of 60:40. The total flow rate of the inlet gas mixture was 21\u00a0cm3\u00a0min\u22121, ensuring constant gas supply to the FTIR system at the outlet of the fuel cell. This study was focused more on the comparison of the activity of additional catalytic layers and its influence on degradation rate, disregarding the optimal fuel utilisation factor discussed in other papers [17,18]. Two types of electrical measurements were collected during fuel cell operation in biogas: a current density versus voltage and a current density versus time at 0.65\u00a0V for the 90\u00a0h at 750\u00a0\u00b0C. A scheme representing a general procedure of the experiment applied in this work is shown in Fig.\u00a01\n.Simultaneously with electrical tests, an analysis of the composition of the outlet gases from SOFC was performed using a Fourier Transformed Infrared Spectroscopy (PerkinElmer Spectrum 100 with ZnSe optical windows). FTIR spectra were collected every 10\u00a0min within the wavenumber range of 4000\u2013500\u00a0cm\u22121 with a resolution of 4\u00a0cm\u22121. Concentrations of methane, carbon dioxide and carbon monoxide were then calculated. Although H2 gas is not visible in FTIR spectra, after a calibration process, we were able to determine its concentration as a difference from 100% of summed CH4, CO2 and CO concentrations. Such an approach is correct and in line with expectations. Moreover, the conversion rates of CH4 and CO2 as well as the yields of H2 and CO were calculated. A detailed description of the measuring system and analysis methods was reported elsewhere [16].The XRD patterns of Ce(Pr,Sm)O2-\u03b4 powders fabricated by a reverse microemulsion method are presented in Fig.\u00a02\n. All diffraction peaks can be attributed to CeO2-\u03b4, which indicates that all materials are single-phase. Therefore it can be stated that the amount of dopants applied in this work (20\u00a0mol%) is below the solubility limit of Pr and Sm in ceria, what is in agreement with literature reports [19]. The obtained XRD data also allowed to estimate the size of the crystallites in the fabricated compounds. The results of the analysis are shown in Table 1\n. It can be found that all of the materials are nanocrystalline and that addition of dopant reduces the average size of crystallites (from ca. 10 to 7\u00a0nm). The unit cell parameters of analysed compounds both with the goodness of fit (GOF) values for Rietveld refinement are also presented in Table 1. The lowest lattice parameter was found for pure CeO2-\u03b4 and Ce0.9Pr0.1O2-\u03b4, because Ce4+ and Pr4+ ions have the smallest and comparable ionic radii, as shown in Table 1. However, when both Pr and Sm dopants were introduced into ceria, then there is no linear dependence between lattice parameter and the amount of dopant, what suggests that praseodymium exists in mixed-valence state (3+/4+) in these compounds [4,9].The morphology of fabricated powders examined using Scanning Electron Microscopy is shown in Fig.\u00a03\n. All doped-ceria compounds have similar, uniform microstructure with round-shape grains of an average size of 20\u00a0nm. The only exception is pure CeO2-\u03b4 in which two types of grain shape are visible: round and flakes-like. The former has an average size of 20\u00a0nm and the latter up to 100\u00a0nm.In the next step the chemical compatibility of the fabricated compounds with the NiO-YSZ anode material was investigated. For this purpose the powders of the doped ceria materials were mixed with NiO-YSZ powder at 50\u00a0vol% ratio, ground in an agate mortar, uniaxially pressed into pellets and subsequently sintered at 1000, 1100 and 1200\u00a0\u00b0C for 2\u00a0h. Then the XRD analysis was performed. In Fig.\u00a04\n, a part of XRD pattern of Ce0.8Pr0.1Sm0.1O2-\u03b4 \u2013NiO/YSZ composite after sintering at different temperatures for 2\u00a0h is shown. It is representative also of other compositions. It clearly indicates that at temperature above 1000\u00a0\u00b0C a chemical reaction between ceria and YSZ phase takes place. As a result, the secondary phase of Ce0.33Zr0.67O2-\u03b4 is formed. This observation allows us to conclude that catalytic layers of ceria deposited on the surface of NiO-YSZ anode should be sintered at temperature not higher than 1000\u00a0\u00b0C to prevent the zirconium diffusion. Nevertheless, a potentially negative effect of the existence of this additional Ce\u2013Zr\u2013O phase depends on the part of the fuel cell in which it is formed [21\u201327]. For example, it was reported by Patel et\u00a0al. [24] that when this phase occurs at the Ni/YSZ/CeO2 anode operating with direct hydrocarbon feeds, it plays a key role in suppressing carbon formation and associated cell cracking. When Ciementi et\u00a0al. [25] used Zr0.35Ce0.65O2-\u03b4 to decorate the Ni-YSZ anode operating in methanol fuel, they also found that the addition of this compound not only enhanced the coking resistance due to its oxygen storage capability, but also increased the activity of the anode for fuel electro-oxidation due to the increased conductivity, as well as it affected the type of carbon that was formed [26]. On the other hand, if this phase forms also at the anode-electrolyte interface, then it will deteriorate the ionic conductivity of the thin YSZ electrolyte, lowering the performance of a whole fuel cell [27].In the next stage of the experiment the doped-ceria powders were mixed with an organic binder to the form of a paste and deposited on the surface of NiO-YSZ fresh anode. Then they were sintered at 1000\u00a0\u00b0C for 2\u00a0h in the air. The cross sections of these samples were analysed by SEM to examine the quality of the interface between the ceria catalytic layer and the anodic support. Exemplary SEM images of Ce0.9Sm0.1O2-\u03b4 and Ce0.8Pr0.05Sm0.15O2-\u03b4 interfaces are shown in Fig.\u00a05\n. They are representative of two groups of compounds: without and with praseodymium dopant. Catalytic layers without Pr have quite uniform microstructure, both in the case of grain size and porosity. In the interface with the anode support there are no visible cracks and no traces of delamination. When Pr is used as a dopant then the quality of the layer is quite poor. Large agglomerates, non-uniform porosity as well as a poor adhesion to the support are visible.To better understand the source of this phenomenon the dilatometry studies of fabricated doped-ceria materials were performed. For reference also the NiO-YSZ anode was examined. The results are presented in Fig.\u00a06\n. The calculated values of the expansion coefficient in defined temperature ranges are shown in Table 2\n. The thermal expansion coefficient of NiO-YSZ is equal to 12.4\u00a0\u00d7\u00a010\u22126\u00a0K\u22121 and is in agreement with literature reports [28]. Among presented compounds only Ce0.9Sm0.1O2-\u03b4 fits well to NiO-YSZ anode (TEC\u00a0=\u00a012.6\u00a0\u00d7\u00a010\u22126\u00a0K\u22121). This observation explains a very good quality of the interface between the Ce0.9Sm0.1O2-\u03b4 catalytic layer and the anode support. It can also be noticed that all Pr-doped ceria samples show linear expansion below 400\u00a0\u00b0C. The expansion coefficients measured in this temperature range are close to that of NiO-YSZ. However, the dilatometry curves display a non-linear behaviour with an inflection point at ~550\u00a0\u00b0C. This deviation from linearity increases with increasing Pr content. Such behaviour has been previously reported in the literature for Pr-doped ceria compounds [9,10,29,30]. It is explained by a chemical strain originating from the combination of slight contraction of the unit cell upon formation of oxygen vacancies and expansion of the unit cell upon partial reduction of praseodymium from Pr4+ to Pr3+ (according to Eqs. (1) and (2)) [9,10]. Therefore, the quality of the interface between the NiO-YSZ anode and the Pr-containing ceria catalytic layer may be poor. In consequence, it may be less adherent to the substrate and thus be more prone to mechanical damage. However, this weaker interface should not affect the catalytic properties of the layer itself. As long as effective gas diffusion can occur in the layer, biogas reforming chemical reactions should be performed, regardless of contact with the support. In turn, electrocatalytic reactions will require good electrical contact between the NiO-YSZ substrate and the layer (to allow transport of O2\u2212 ions), but even point contact enabling the formation of a percolation path should be sufficient and the layer will fulfil its role.After the analysis of structural properties of the fabricated compounds and their compatibility with the NiO-YSZ anode support they were investigated as an additional anode catalytic layers in SOFCs fueled by biogas. The results of electrical measurements are presented as current density plots versus time in Fig.\u00a07\n. The data were normalised to the standard value of 100% in the moment of fuel switching from hydrogen to biogas. Such a presentation allows us to exclude the effect of different gas diffusion through a layer due to a different microstructure, which may influence the absolute value of a power density. To make it possible to know the actual current density values at which the fuel cells operated, an I\u2013V plot for the reference SOFC without a layer was added to Supplementary Materials (Fig.\u00a0S4). In Fig.\u00a07 one can see that after fuel switching from hydrogen to biogas a rapid drop of current density takes place. This is due to fuel dilution with CH4 and CO2 and low fuel utilisation factor in this experiment. Among all investigated compounds the Ce0.8Pr0.05Sm0.15O2-\u03b4 layer seems to be the most resistant to fuel change whereas the Ce0.8Pr0.15Sm0.05O2-\u03b4 provides the biggest drop of current density after fuel switching. However, after the initial deterioration Ce0.8Pr0.05Sm0.15O2-\u03b4 ensures the best long-term stability, which was not observed for other investigated catalytic layers. For the rest of the presented layers further biogas feeding causes progressive degradation of the cell, which is responsible for a constant decrease in the current density (even up to 10% within 90\u00a0h of biogas feeding). Therefore one may state that none of these layers is a perfect catalyst for a direct internal reforming of biogas. However, comparing these results with the performance of a reference fuel cell (without a catalytic layer) in a similar experiment, then a beneficial effect of using the catalytic layer is clearly visible. All of the investigated layers give lower current density drop after hydrogen/biogas switching which may be explained by the ability of more effective direct internal reforming of biogas.To better understand the direct internal reforming of biogas and its influence on SOFC performance it is necessary to analyze also the composition of the outlet gases from the fuel cell. All possible reactions occurring at the anode side are presented in Table 3\n. Among them, one can find three undesired processes: CH4 pyrolysis (5), Boudouard reaction (6) and CO reduction (7) leading to a formation of a solid carbon, which can block the active area of the catalyst, impede the diffusion of fuel to the triple phase boundary (TPB) as well as it can even destroy the anode structure. The first reaction is an endothermic process, while the others are exothermic.The results of the in situ FTIR analysis for the selected representative compositions: Ce0.9Pr0.1O2-\u03b4 and Ce0.9Sm0.1O2-\u03b4 (monodoped) and Ce0.8Pr0.1Sm0.1O2-\u03b4 (co-doped) are presented in Fig.\u00a08\n as time dependencies of concentration of the particular outlet gases and corresponding catalytic parameters: the conversion rates of CH4 and CO2, the CO and H2 selectivities and the yields of CO and H2. The results obtained for all investigated compounds are given in the Supplementary Materials (Figs. S1 and S2). A detailed description of how these parameters were calculated can be found in our previous paper [16].The most stable in time composition of outlet gases were noticed for SOFC with the Ce0.9Sm0.1O2-\u03b4 layer. The initial increase of CH4 and CO2 concentration is related to a dilution of initial synthetic biogas mixture (60% of CH4 and 40% of CO2) by the hydrogen remaining in the measuring rig as well as by the initial very intensive internal reforming performed in the whole volume of the catalyst. After a few hours of operation under biogas a kind of equilibrium state is reached. This stabilisation is visible also in corresponding catalytic parameters (Fig.\u00a08 right): conversion rates of CH4 and CO2, the CO and H2 selectivities and the yields of CO and H2. Among two other presented catalysts the worst is Ce0.9Pr0.1O2-\u03b4, regarding the lowest stability in time as well as the least effective internal reforming (the highest amount of unreacted fuel with simultaneous the lowest amount of products). The co-doped Ce0.8Pr0.1Sm0.1O2-\u03b4 composition is clearly between the other two compounds. It should be noted that additional beneficial effect of co-doping is visible in more detailed non-equilibrium chemical analysis, which was performed based on FTIR measurements. It allowed to determine a contribution and direction of the particular chemical reactions to the direct internal reforming process and to recognise which of them are mostly responsible for carbon deposition. A detailed description of the procedure was shown in our previous paper [16]. The results for Ce0.8Pr0.1Sm0.1O2-\u03b4 are shown in Fig.\u00a09\n. Both time dependencies of reaction quotients (Qr) for reactions (1)\u2013(4) from Table 3 (Fig.\u00a09 left) and carbon activity coefficients (\u03b1C,r) for reactions (5)\u2013(7) from Table 3 (Fig.\u00a09 right) are presented.It is generally agreed that Qr~10\u22123 stands for the situation when mostly reactants are present in reaction area, while Qr~103 when nearly all substances are products and finally if 10\u22123<Qr\u00a0<\u00a0103 then significant amounts of both reactants and products are visible [16,34]. Therefore, related to our results, steam reforming of methane (1) is a dominant reaction, most probably due to the existence of an observable amount of water in reaction atmosphere. Both dry reforming (2) and RWGS (3) oscillate around an equilibrium, what is in agreement with other literature reports [35]. In turn, methanation reaction (4) is fully shifted towards reactants and takes part in decomposition of CH4 rather than its formation.The next parameter, carbon activity coefficient (\u03b1C,r) should be equal to 1\u00a0at the equilibrium point. When \u03b1C,r\u00a0>\u00a01, then a solid carbon formation is promoted [36,37]. Based on our results it can be concluded that both Boudouard reaction (6) and CO reduction (7) are rather shifted towards reactants (are lower than unity) and do not lead to carbon deposition on the anode side. This is probably due to an addition of water into fuel stream and a formation of additional water molecules by electrochemical hydrogen oxidation (8), which promote rather carbon gasification than deposition. Only \u03b1C,r for CH4 pyrolysis is significantly higher than 1 over whole measurement time, which clearly indicates that this reaction is responsible for carbon accumulation in investigated SOFC.This dominance of a steam reforming of methane (1) in a complex internal biogas reforming process as well as CH4 pyrolysis (5) as a main reaction responsible for carbon deposition was also observed for all other investigated SOFCs with catalytic layers. The only difference between them is the dynamics of time changes of the Qr and \u03b1C,r parameters. Therefore time dependencies of Qr for steam reforming and \u03b1C,r for CH4 pyrolysis were set in one graph for all samples and shown in Fig.\u00a010\n. To better understand these graphs it should be explained that the rate of time changes of quotients' values can illustrate how far from equilibrium point is each of reactions at a given time. Indirectly, a course of plotted function can deliver an overall view on how efficiently different reactions are trying to reach their equilibrium points. The higher is the change of calculated Qr in time, the bigger is the difference between the actual concentration of products and equilibrium composition [38]. On the other hand, the time changes of \u03b1C,r parameter, that is a reciprocal of Qr, should be understood in an opposite way. The more rapidly particular reactions responsible for coking move away from the equilibrium point, the lower might be the rates of these reactions, leading to slower carbon accumulation [38]. To prove it, additional plots of carbon balance in time can be drawn. This parameter is calculated as a difference between the numbers of moles of carbon in the inlet and the outlet stream of gases and gives us the information about an average rate of carbon deposition during dwell time. A comparison of time changes of carbon balance for all investigated SOFCs can be found in Fig.\u00a011\n. The lower is the decrease in carbon balance over time, the more stable is the operation of an analysed fuel cell. A rapid drop of this parameter is an undesired phenomenon, as it may indicate that the reforming process has slowed down (most probably due to the carbon accumulation and/or a change in the microstructure of the catalyst).Therefore, based on the data collected in Figs. 10 and 11 and explanation given upwards, it may be concluded that Ce0.9Sm0.1O2-\u03b4 and Ce0.8Pr0.05Sm0.15O2-\u03b4 are the most attractive compounds towards steam reforming of methane, whereas Ce0.8Pr0.15Sm0.05O2-\u03b4 is the most stable in time for this reaction, but not so effective. The activity of Ce0.9Sm0.1O2-\u03b4 and Ce0.8Pr0.05Sm0.15O2-\u03b4 leads to more efficient conversion of methane and production of CO/H2, what is in agreement with a composition of outlet gases and catalytic parameters shown in Fig.\u00a08. Moreover, in general, it also corresponds well with higher values of current density (Fig.\u00a07) obtained for these fuel cells. However, current density depicts us an efficiency of fuel utilisation and the ability to perform electrochemical reactions in particular compounds, what may be (but it doesn't have to be) in agreement with concentration of outlet gases.Regarding the carbon accumulation in particular catalytic layers, the Ce0.9Pr0.1O2-\u03b4 is the least stable (the biggest decrease of carbon balance over time, see Fig.\u00a011) while the carbon deposition due to CH4 pyrolysis is the slowest (the biggest increase of \u03b1C,r over time, see Fig.\u00a010 right). However, the latter one goes also in pair with the slowest carbon removal indicated by a reverse direction of Boudouard reaction (6) and CO reduction (7). Finally, the SOFC with Ce0.9Pr0.1O2-\u03b4 is very unstable in time, what corresponds well with the results of electrical measurements (Fig.\u00a07). The deterioration of its catalytic parameters is comparable with that observed for a reference fuel cell without any catalytic layer [16].All these observations also allow to conclude that high praseodymium content in ceria-based compounds is not desirable. It does result in a slight improvement in catalytic properties, but undoubtedly it leads to the problems with TEC mismatch between the catalytic layer and NiO-YSZ support. In consequence, many cracks in the catalytic layer/anode interface occur, limiting the electrical contact between these constituents. Further, it deteriorates electrical properties of the operating SOFC, as well as limits electrochemical reactions leading to the oxidation of deposited solid carbon (reaction 11).Finally, it is also worth to notice that in time dependence plots for all analysed parameters of Ce0.9Pr0.1O2-\u03b4 compound there is a characteristic extremum point appearing at ~65\u00a0h of biogas feeding. At this point there is a local maximum of current density (Fig.\u00a07), corresponding with a maximum of carbon balance (Fig.\u00a011) and a minimum of carbon activity coefficient for CH4 pyrolysis (Fig.\u00a010 right). It correlates with a significant change in a fuel composition (Fig.\u00a08), where unreacted methane starts to be a dominant component in an outlet stream of gases. Then, for a short time, a more intensive carbon accumulation takes place, giving a better electrical contact on the anode side and leading to higher current density. After 3\u20135\u00a0h a new equilibrium state is achieved and the internal biogas reforming further proceeds. The post-mortem SEM image of Ce0.9Pr0.1O2-\u03b4 layer is shown in Fig.\u00a012\n. It is uniform, with slight cracks randomly distributed. No visible traces of deposited carbon can be found, nor in the surface as well as in the cross section. This result is consistent and representative with other layers. It confirms, that although the investigated Pr and/or Sm doped ceria does not improve the DIR-SOFC electrical parameters, it protects from a significant deposition of a solid carbon, allowing SOFC to operate with biogas for a much longer time than without the additional layer. Even if a small amount of carbon is deposited at the end of the experiment, it seems to be very hard to determine its amount. Although the carbon balance (Fig.\u00a011) could be integrated over time, it should be remembered that this is a dynamic measurement and depositing carbon is oxidised and removed from the cell on an ongoing basis. Only a structural form of possible carbon deposits could be determined using Raman spectroscopy [39,40] or Temperature Programmed Oxidation (TPO) [40\u201343]. However, for our layers, these methods did not give any reasonable results, as the amount of deposited carbon was too low.The CeO2-\u03b4, Ce0.9Sm0.1O2-\u03b4, Ce0.9Pr0.1O2-\u03b4, Ce0.8Pr0.15Sm0.05O2-\u03b4, Ce0.8Pr0.1Sm0.1O2-\u03b4 and Ce0.8Pr0.05Sm0.15O2-\u03b4 oxides, deposited in the form of layers on the surface of SOFC anode directly fed by biogas, were studied in aim to determine the influence of Pr and Sm on the fuel cell performance.Regardless of the composition, the applied reverse microemulsion synthesis method allowed to obtain single-phase, nanocrystalline powders. It was found that in order to avoid the reaction between these oxides and the anode material (NiO-YSZ) sintering temperature should not exceed 1000\u00a0\u00b0C, since above that temperature a formation of the secondary Ce1-xZrxO2-\u03b4 phase was noticed.It was also shown that Pr-doped ceria catalytic layers suffered from poor adhesion to the NiO-YSZ anode support due to a large mismatch in total thermal expansion coefficients of these materials. These observations are in agreement with previous literature reports. However, co-doping with Sm decreased the TEC significantly, leading to a better adhesion of the layer to the anode.The studies of the influence of the catalytic layers on the direct internal reforming of biogas showed that Pr dopant deteriorates the properties of pure ceria. Single samarium doping or, if appropriate, an addition of a small amount of praseodymium is preferred. The Ce0.9Sm0.1O2-\u03b4 and Ce0.8Pr0.05Sm0.15O2-\u03b4 materials are the most attractive towards steam reforming of methane, which is a dominant reaction among all processes occurring simultaneously in direct internal reforming of biogas. Their high activity led to more efficient conversion of methane and production of CO/H2, what was in agreement with a composition of outlet gases and catalytic parameters. In turn, CH4 pyrolysis was found to be a dominant reaction responsible for carbon accumulation on the anode side, but both carbon activity coefficient and carbon balance parameters confirmed that the Ce0.9Sm0.1O2-\u03b4 and Ce0.8Pr0.05Sm0.15O2-\u03b4 show the highest stability over time and thus are the most attractive candidates for catalytic materials enhancing direct internal reforming of biogas in SOFC.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 Science Center under grant No. NCN 2017/26/D/ST8/00822.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.2020.07.146.", "descript": "\n                  The Pr and Sm co-doped ceria (with up to 20\u00a0mol.% of dopants) compounds were examined as catalytic layers on the surface of SOFC anode directly fed by biogas to increase a lifetime and the efficiency of commercially available DIR-SOFC without the usage of an external reformer.\n                  The XRD, SEM and EDX methods were used to investigate the structural properties and the composition of fabricated materials. Furthermore, the electrical properties of SOFCs with catalytic layers deposited on the Ni-YSZ anode were examined by a current density-time and current density-voltage dependence measurements in hydrogen (24\u00a0h) and biogas (90\u00a0h). Composition of the outlet gasses was in situ analysed by the FTIR-based unit.\n                  It has been found out that Ce0.9Sm0.1O2-\u03b4 and Ce0.8Pr0.05Sm0.15O2-\u03b4 catalytic layers show the highest stability over time and thus are the most attractive candidates as catalytic materials, in comparison with other investigated lanthanide-doped ceria, enhancing direct internal reforming of biogas in SOFCs.\n               "}
{"full_text": "Methane (CH4) is a commercially important fuel and alternative chemical resource replacing petroleum oil [1,2], and it is abundant in shale gas, natural gas, and organic-waste-digested biogas [3,4]. While clean liquid fuels and platform chemicals can be obtained through the gas-to-liquid process using methane-derived synthesis gas [5\u20138], oxidative or non-oxidative coupling of methane can also produce olefins and paraffins for further manufacturing of polymers and aromatic compounds.Oxidative coupling of methane (OCM) is a promising approach for the production of higher hydrocarbons and olefins compared to conventional synthesis gas-based processes as the exothermic reaction does not include equilibrium limitations and has lower energy requirements. During the OCM process, reactions occur on the catalyst surface and in the gas phase [9\u201312]. CH4 is adsorbed on the catalyst surface and its CH bond decomposes into methyl radicals and hydroxyl groups on the oxide surface. The methyl radicals diffuse into the gas phase, while the surface hydroxyls are removed to form water and leave oxygen vacancies on the catalyst surface. This surface reaction is called CH4 activation (surface reduction). The oxygen vacancies are filled by oxygen species in the gas phase through oxygen activation (surface oxidation). In the gas phase, coupling of the methyl radicals produces C2H6 [13]. C2H6 and paraffins can undergo oxidative dehydrogenation to produce olefins as the CH activation energy of C2H6 is similar to that of CH4 [14]. Furthermore, the deep oxidation of oxygen-containing intermediates or radicals can form COx (CO2 and CO) and hydrocarbons [10,11,15,16].Among OCM catalysts, Mn-Na2WO4/SiO2 is a highly selective and active catalyst [9,17\u201321]. Partial replacements of Mn and WO4\n2\u2212 with other metals (Ti, Co, Fe, Cu, Ni, Zn, Y, Zr, Mo, Pd, La, Ce, Nd, Eu, Tb, and Hf) [22\u201325] and anions (MoO4\n2\u2212, SO4\n2\u2212, PO4\n3-, CO3\n2\u2212, SiO3\n2\u2212, and P2O7\n4-) [26\u201328], respectively, have been attempted; however, the tri-component Na-W-Mn catalyst still exhibits the highest OCM activity [23]. CH4 is activated at the W sites, thereby leading to the reduction of W6+ to W5+, whereas the Mn compound supplies oxygen to re-oxidize W5+ to W6+; this was recently confirmed by means of a combined in-situ X-ray diffraction (XRD) and operando analysis [3,17,29,30]. Mn-Na2WO4/SiO2 is activated in the reaction temperature range 700\u2013900 \u00b0C [31,32]. However, at such high temperatures, deep oxidation to CO2, and the decomposition of hydrocarbons into coke can occur, which can cause the loss of valuable products and possibly, plugging, because of the coke formation. Perovskite catalysts, which exhibit a high stability and require low activation temperatures, have also been reported for the OCM [11,33\u201338]. Perovskite, with an ABO3 formula, contains A- and B-site cations forming cuboctahedral and octahedral oxygen anions, respectively. The A- and B-site cations can be doped with lanthanides, alkaline metals, alkaline-earth metals, and transition metals. The doping increases the mobility and activity of oxygen in the perovskite structure and improves the activation of the CH4 and O atoms at low temperatures during the OCM. For example, in a previous study performed in our lab, a SrO-deposited BaTiO3 perovskite catalyst achieved a 17.6 % C2+ (olefins and paraffins) yield at 725 \u00b0C; however, the low C2+ selectivity (45\u201350 %) and a lower degree of oxidative dehydrogenation led to a low olefin selectivity [33].In this study, a hybrid BaTiO3 perovskite and Mn-Na2WO4 catalyst was prepared for use in the OCM, in which high olefin and paraffin selectivities were achieved at a lower temperature (700 \u00b0C) than required for the conventional Mn\u2010Na2WO4/SiO2 catalyst (approximately 800 \u00b0C). The effects of the metal (Na, Mn, W)-matrix (BaTiO3) interactions on the catalytic OCM results were also investigated for the prepared catalysts. A detailed analysis of the W and Mn redox reactions that occur on BaTiO3 was performed to elucidate the mechanism of the low-temperature activated OCM catalysts.All chemicals were used as received without further purification. Titanium isopropoxide (Ti(OCH2CH2CH2CH3)4, 97 %) and barium nitrate (Ba(NO3)2, 99 %) were purchased from Sigma-Aldrich (Milwaukee, Wisconsin, USA). Sodium tungstate dihydrate (Na2WO4\u22192H2O, 99 %) was purchased from Yakuri Pure Chemicals (Kyoto, Japan). Manganese nitrate hexahydrate (Mn(NO3)2\u22196H2O, 98 %) was purchased from Kanto Chemicals (Tokyo, Japan). Silica gel 60 (0.060\u20130.2 mm, 70\u2013230 mesh) and citric acid (C6H8O7, 99.5 %) were purchased from Alfa Aesar (Ward Hill, Massachusetts, USA). Deionized (DI) water (18.2 M\u03a9\u2219cm) was prepared using an aquaMAX-Ultra 370 series water puri\ufb01cation system (YL Instruments, Anyang, Korea).Citric acid, barium nitrate, and sodium tungstate dihydrate were added to DI water in an alumina cup at room temperature under continuous stirring (360 rpm) for 10 min. Next, manganese nitrate hexahydrate and titanium isopropoxide were consecutively added dropwise to the mixture. The mixture was heated to 80 \u00b0C and aged until a transparent gel (metal-citrate complex) formed. The gel was dried in air at 140 \u00b0C for 3 h, leading to the formation of ivory powder. The prepared powder was further crushed and calcined in air at 900 \u00b0C for 6 h. The detailed compositions of the catalysts are listed in Table S1. The prepared catalysts were denoted as m-n-BTMW, where m = Mn/W (atom/atom), n = (Mn + W)/(Ba + Ti) (atom/atom), and Ba/Ti = 1.08 atom/atom [33]. The other catalysts prepared in this study consisted of Ba/Ti = 1 atom/atom for BaTiO3 (BTO), Ba/Ti = 1 atom/atom and Mn/(Ba + Ti) = 0.4 atom/atom for the Mn-doped BTO (Mn-BTO), Ba/Ti = 1 atom/atom and W/(Ba + Ti) = 0.4 atom/atom for Na2WO4-doped BTO (W-BTO), Mn/W = 3.5 atom/atom for the complex consisting of manganese oxide and Na2WO4, and Ba/Ti = 1 atom/atom, Mn/W = 3.5 atom/atom, and (Mn + W)/(Ba + Ti) = 0.4 atom/atom for the Mn and Na2WO4-doped BTO (Mn-W-BTO). Mn\u2010Na2WO4/SiO2, containing 5 wt% Na2WO4 and 2 wt% Mn, was prepared by the slurry method as described in a previous study [39]. Silica gel (3 g) was dispersed in DI water (70 mL), and sodium tungstate dihydrate (0.1811 g) was added to the slurry. This was followed by the addition of manganese nitrate hexahydrate (0.1178 mL) while stirring. The prepared powder was calcined in air at 800 \u00b0C for 5 h. The BET surface areas of catalysts were measured by N2 physisorption analysis using a Micromeritics ASAP 2020 apparatus (Norcross, Georgia, USA) (Table S2).O2 and CO2 temperature-programmed desorption (O2-TPD and CO2-TPD, respectively), and H2 temperature-programmed reduction (H2-TPR) were performed using a BELCAT-B (MicrotracBel, Osaka, Japan) with a thermal conductivity detector (TCD). A mass spectrometer (BELMASS, MicrotracBEL, Osaka, Japan) was also connected to the BELCAT-B to identify the products during the TPD and TPR measurements. For O2-TPD, the catalysts were pretreated at 900 \u00b0C for 1 h in a 5% (vol/vol) O2/He flow, followed by treatment in a 5% (vol/vol) O2/He flow at 50 \u00b0C for 1 h. For the measurement, the temperature was increased to 900 \u00b0C at a heating rate of 5 \u00b0C/min in a He flow. For the CO2-TPD analysis, the catalysts were pretreated at 550 and 850 \u00b0C for 1 h under a He flow, followed by treatment in a 5% (vol/vol) CO2/He flow at 50 \u00b0C for 1 h. For the analysis, the temperature was increased to 900 \u00b0C at a heating rate of 5 \u00b0C/min in a He flow. For H2-TPR, the catalysts were oxidized in a 5% (vol/vol) O2/He flow at 900 \u00b0C for 1 h, and then cooled to 50 \u00b0C, followed by flushing with Ar at 50 \u00b0C for 20 min. The measurement was performed in the temperature range 50\u2013900 \u00b0C at a heating rate of 5 \u00b0C/min.The X-ray diffraction (XRD) results were obtained at 25, 600, 700, and 800 \u00b0C using an X\u2019Pert3 PRO powder diffractometer (Malvern PANanalytical, Malvern, UK) with Cu K\u03b11\n radiation (\u03bb =1.54059 \u00c5), operated at 40 kV and 30 mA. At the given temperature, the catalysts were treated in a CH4 flow to obtain the XRD results of the CH4-treated catalysts. The catalysts were flushed with N2 and oxidized in an air flow to obtain the XRD results of the air-treated catalysts. The fractions of hexagonal BaTiO3 phase in the catalysts were roughly measured using the diffraction peak intensity ratio ((103)hexagonal/[(103)hexagonal + (111)tetragonal]) [40], where (103)hexagonal and (111)tetragonal are the diffraction peak intensities of hexagonal (103) and tetragonal (111), respectively.The X-ray photoelectron spectroscopy (XPS) results were obtained using an angle-resolved X-ray photoelectron spectrometer (Theta Probe AR-XPS system, Thermo Fisher Scienti\ufb01c, Waltham, Massachusetts, USA) with a monochromated Al K\u03b1 X-ray source (h\u03bd = 1486.6 eV) operated at 15 kV and 100 W at the Korea Basic Science Institute (Busan, Korea). The binding energies of all the XPS data were calibrated using the C 1s peak at 284.59 eV (Fig. S1). The Oads/(Oads+Olatt), Mn2+/(Mn2++Mn3++Mn4+), and Ti3+/(Ti3++Ti4+) ratios were calculated using the corresponding deconvoluted peak areas, where Oads and Olatt are the adsorbed and lattice oxygen atoms on the surface, respectively.The morphologies and elemental compositions of the catalysts were determined using a TEM (Talos F200X, FEI, Hillsboro, Oregon, USA) equipped with high-performance energy dispersive X-ray spectroscopy (EDS, Super-X EDS system, Bruker, Billerica, Massachusetts, USA) at the Korea Institute of Science and Technology Advanced Analysis Center (Seoul, Korea). A STEM (Titan TM 80-300, FEI, Hillsboro, Oregon, USA) connected to a high-angle annular dark-field (HAADF) detector was also used to observe the catalysts.The Raman spectra were obtained using a Raman microscope (In Via Raman Microscope, Renishaw, Wotton-under-Edge, UK) with a 532 nm laser excitation source. The Raman spectra were collected at 25, 600, 700, and 800 \u00b0C while the catalysts were heated from room temperature to 800 \u00b0C under a N2 flow.The OCM was performed in a fixed-bed reactor system under atmospheric pressure (Fig. S2). A straight quartz tube reactor with a height of 370 mm and an inner diameter of 6 mm was used. The catalyst (0.2 g), located close to the furnace thermocouple position, was placed on supporting quartz wool. The remaining reactor volume was filled with inactive zirconia silica beads. The catalyst was heated from room temperature to 650\u2013800 \u00b0C in a mixed O2 (20 mL/min) and N2 flow (30 mL/min) at a heating rate of 10 \u00b0C/min. A reactant mixture of CH4, O2, and N2 (19.4, 6.5, and 6.5 mL/min, respectively, achieving CH4/O2/N2 = 3/1/1 mol/mol/mol) was fed into the reactor. Weight hourly space velocity (WHSV) was fixed at 9720 mL h\u22121 g\u22121. Water vapor produced during the reaction was removed using a cold trap (\u22122 \u00b0C). The gas products (CO, CO2, CH4, C2, and C3 compounds) were analyzed after 30 min of reaction at each reaction temperature using a gas chromatography system (7890A, Agilent Technologies, Santa Clara, California, USA) equipped with a ShinCarbon ST micropacked column. The products were quantified using a TCD and a methanizer-connected flame-ionization detector (FID). The selectivity of molecule i (Si, %), methane conversion (XCH4, %), O2 conversion (XO2, %), yield of molecule i (Yi, %), and carbon-based olefin-to-paraffin ratio (olefin/paraffin, mol/mol) were calculated using the following equations:\n\n(1)\n\n\nS\ni\n\n\n%\n\n=\n\n\n\nN\ni\n\n\u00d7\n\u2009\n\nF\ni\n\n\n\n\n\u2211\n\n\nN\ni\n\n\u00d7\n\u2009\n\nF\ni\n\n\n\n\u2009\n\n\n\u00d7\n100\n\n\n\n\n\n(2)\n\n\nX\n\n\n\nC\nH\n\n4\n\n\n\n\u2009\n(\n%\n)\n=\n\n\n\nF\n\n\n\nC\nH\n\n4\n\n,\n\u2009\nf\ne\ne\nd\n\n\n\u2009\n-\n\nF\n\n\n\nC\nH\n\n4\n\n\n\n\u2009\n\n\n\nF\n\n\n\nC\nH\n\n4\n\n,\n\u2009\nf\ne\ne\nd\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n(3)\n\n\nX\n\n\nO\n2\n\n\n\n\u2009\n(\n%\n)\n=\n\n\n\nF\n\n\nO\n2\n\n,\n\u2009\nf\ne\ne\nd\n\n\n\u2009\n-\n\nF\n\n\nO\n2\n\n\n\n\u2009\n\n\n\nF\n\n\nO\n2\n\n,\n\u2009\nf\ne\ne\nd\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n(4)\n\n\nY\ni\n\n(\n%\n)\n=\n\u2009\n\n\n\nS\ni\n\n\u2009\n\u00d7\n\u2009\n\nX\n\n\n\nC\nH\n\n4\n\n\n\n\n100\n\n\n\n\n\n\n(5)\n\n\nS\n\n\nC\n\n2\n+\n\n\n\n\n\n%\n\n=\n\nS\n\n\nC\n\n2\n\u2009\n\n\n\nH\n2\n\n\n\n+\n\nS\n\n\nC\n\n2\n\u2009\n\n\n\nH\n4\n\n\n\n+\n\nS\n\n\nC\n\n2\n\u2009\n\n\n\nH\n6\n\n\n\n+\n\nS\n\n\nC\n\n3\n\u2009\n\n\n\nH\n6\n\n\n\n+\n\nS\n\n\nC\n\n3\n\u2009\n\n\n\nH\n8\n\n\n\n\n\n\n\n\n(6)\n\n\nS\n\n\n\nC\nO\n\nx\n\n\n\n\n%\n\n=\n\nS\n\nC\nO\n\n\n+\n\nS\n\n\n\nC\nO\n\n2\n\n\n\n\n\n\n\n\n(7)\n\n\n\nO\nl\ne\nf\ni\nn\n\n\nP\na\nr\na\nf\nf\ni\nn\n\n\n(\nm\no\nl\n/\nm\no\nl\n)\n=\n\n\n\u2009\n2\n\u00d7\n\u2009\n\nF\n\n\nC\n2\n\n\nH\n4\n\n\n\n+\n3\n\u00d7\n\u2009\n\nF\n\n\nC\n3\n\n\nH\n6\n\n\n\n\n\n2\n\u00d7\n\u2009\n\nF\n\n\nC\n2\n\n\nH\n6\n\n\n\n+\n3\n\u00d7\n\u2009\n\nF\n\n\nC\n3\n\n\nH\n8\n\n\n\n\n\n\n\nwhere Ni is the number of carbon atoms in molecule i, Fi is the molar flow rate of molecule i in the product mixture, FCH4, feed is the molar flow rate of methane in the feed, FO2, feed is the molar flow rate of O2 in the feed, C2+ is the mixture of olefins and paraffins, and COx is the mixture of CO and CO2. The carbon balance is defined as the ratio of carbon molar flow rate out of the reactor to the carbon molar flow rate of the feed. The carbon balance is a measure of the carbon loss that is caused by the formation of solid products (coke) or heavy gases (C4+) during the reaction. The oxygen balance roughly indicates the difference between the ideal and real oxygen consumptions, as reported in our previous works (see the supplementary data) [11,41].\n\n(8)\n\nC\na\nr\nb\no\nn\n\u2009\nb\na\nl\na\nn\nc\ne\n\u2009\n(\n%\n)\n=\n\n\n2\n\u00d7\n\u2009\n\nF\n\nC\n2\n\n\n+\n3\n\u00d7\n\u2009\n\nF\n\nC\n3\n\n\n+\n\nF\n\nC\n\nO\n2\n\n\n\n+\n\u2009\n\nF\n\nC\nO\n\n\n+\n\u2009\n\nF\n\nC\n\nH\n4\n\n\n\n\n\nF\n\nC\n\nH\n4\n\n,\nf\ne\ne\nd\n\n\n\n\u00d7\n100\n\n\n\n\n\n(9)\n\n\nOxygen balance (%)\n\n\n=\n\n\n\n(O\n2\n\n\nconsumed by the formation of hydrocarbons and CO\nx\n\n)\n+\n\nF\n\nO\n2\n\n\n\n\nF\n\n\nO\n2\n\n, feed\n\n\n\n\u00d7100\n\n\n\n\nThe prepared mixed oxides, 3.5-0.4-BTMW, containing Ba, Ti, Mn, Na, W, and O contain two main types of well-defined nanostructures (Fig. 1\nA): BaTiO3-based hexagonal particles, containing highly dispersed Mn, Na, and W (Fig. 1B), and MnO2-rich rods (Fig. 1C).The hexagonal particles contain highly dispersed Ba, Ti, Mn, Na, and W (Figs. 1B and S3), although the overall structure is hexagonal BaTiO3 (Fig. 2\nA and B) [42]. On the surface of the particles, Na2WO4 is present on top of Na-Mn-Ti-O (Figs. S4 and S5A), which is deposited on the BaTiO3 matrix (Figs. S4 and S5B). These observations indicate that the excess Mn present, after incorporating into the Mn-doped BaTiO3 structure, exists outside the BaTiO3 particles. The W atoms, which are larger than the Mn atoms, could not be incorporated into the BaTiO3 matrix and Na-Mn-Ti-O; thus, separate Na-W-O particles were formed (Fig. S4). Notably, the ratio of Na/W = 0.39 atom/atom (determined by EDS analysis, Fig. S5B), which is lower than that of Na2WO4 (Na/W = 2 atom/atom), indicates that the small Na+ cations easily migrate to Na-Mn-Ti-O through the BaTiO3 particles. The presence of Ti in the Na-Mn-Ti-O structure confirms that Ti was substituted into the MnO6 octahedra in Na-Mn-Ti-O, with the Na layers interposed between the MnO6 layers [43]. The observed distance between the layers is 4.9 \u00c5, which is lower than that of Na0.8MnO2 (5.1 \u00c5) reported in the literature [44]. This indicates that the Na content in Na-Mn-Ti-O is lower than in Na0.8MnO2 (i.e., x < 0.8).The nanorods contain 1 \u00d7 1 (T1) and 2 \u00d7 1 (T2) MnO2 tunnel structures in the bulk of the rods, as depicted in the STEM images (Fig. 2C and D) [45\u201347]. These can accommodate the Na+ ions of Na2WO4. The EDS results also indicate a high concentration of Ti, not Ba, (Fig. S6A), suggesting that Ti-dispersed MnO2 rods are formed. In addition to the MnO2-based rods, further observations of the Na2WO4 particles on the surfaces of the rods (Fig. S7A) suggest that a Na-Mn-Ti-O layer covered by a Na2WO4 layer formed through a reaction between Na2WO4 (as the Na source) and MnO2 [48]. This will be discussed in Section 3.2.The structures of the BTMW catalysts were elucidated by performing XRD analysis. The prepared BTO, Mn-BTO, and Mn-W-BTO were also observed. The XRD results indicate incorporation of Mn and a less amount of W into BaTiO3 (Fig. 3\n). Tetragonal BaTiO3 perovskite (t-BaTiO3, PDF#81-2201) is present in all the catalysts, indicating that BaTiO3 forms the base structure of all the mixed oxide catalysts. Furthermore, small diffraction peaks corresponding to BaCO3 (PDF#41-0373) are present in the XRD results of BTO and Mn-BTO. For Mn-BTO, peaks representing TiOx (Ti9O17, PDF#85-1061) and MnO2 (PDF#44-0142) are present, indicating the substitution of Ti with Mn and the addition of excess Mn, respectively. For W-BTO, strong sharp diffraction peaks representing Na2WO4 are present and this confirms a weak interaction between Na2WO4 (or W) and BaTiO3. For the Mn-W-BTO and 3.5-0.4-BTMW catalysts, the incorporation of Mn into the TiO6 octahedra distorted the tetragonal BaTiO3 structure as a consequence of the Jahn-Teller effect [49,50]. The high Mn content led to the formation of hexagonal BaTiO3, which coexists with tetragonal BaTiO3. The formation of Na-Mn-Ti-O (NaxMn(Ti)O2, PDF#21-1140 for Na0.44Mn(Ti)O2) [51,52] is attributed to a reaction between sodium manganese oxide and the Ti atoms, which were replaced by Mn from the Mn-doped BaTiO3. These observations correspond to the TEM, STEM, and TEM-EDS results (Figs. 1 and 2). Because 3.5-0.4-BTMW contains a higher Ba content (Ba/Ti = 1.08 atom/atom) than Mn-W-BTO (Ba/Ti = 1.0 atom/atom), Ba2TiO4 (PDF#72-0135) was used as a reference for 3.5-0.4-BTMW.The formation of highly dispersed Na-W-O layers on the BaTiO3-rich particles was further confirmed by CO2-TPD (Fig. 4\n) and TEM/STEM/TEM-EDS (Fig. S8). As basic catalysts are considered good OCM catalysts [12,53], the basicity of the m-n-BTMW catalysts (m = 3.5, n = 0\u20130.4) was determined by CO2-TPD (Fig. 4A and B). CO2 adsorption and desorption did not occur on the BTMW catalysts annealed at 850 \u00b0C under an inert He flow (Fig. 4A). Significant CO2 adsorption and desorption is observed at approximately 700 \u00b0C for the catalysts pre-annealed at 550 \u00b0C under an inert He flow (Fig. 4B), which is, however, attributed to the thermal decomposition of BaCO3, present on the BTMW catalysts, to BaO and CO2 because the curves of the catalysts pre-annealed under an inert He flow at 850 \u00b0C does not exhibit strong peaks at 700 \u00b0C. The presence of BaCO3 in the Ba-containing mixed oxides was also confirmed by the XRD results of BTO and Mn-BTO, which exhibit diffraction peaks for BaCO3 (Fig. 3a and b). Interestingly, the peak attributed to the decomposition of BaCO3 to BaO and CO2 at approximately 700 \u00b0C decreases with an increase in the (W + Mn) content and is not present in the result of 3.5-0.4-BTMW. This indicates that BaCO3 is not present in the BTMW catalysts that contain W and Mn (Fig. 4B). The strong interaction between Na-W-O and BaTiO3 may suppress the formation of BaCO3 in the structure. In the absence of BaCO3 decomposition peaks, the two broad peaks in the result of 3.5-n-BTMW (without Mn and W, n = 0) at 100\u2013400 \u00b0C and 400\u2013800 \u00b0C are assigned to weak and strong basic sites, respectively. Compared to 3.5-0-BTMW, the results of Mn-BTO and W-BTO did not exhibit clear CO2 desorption peaks, which can be attributed to the poor basicity of Mn-BTO and formation of the Na-W-O layer which fully occupies the surface of W-BTO. Notably, the TCD peaks observed in the results of 3.5-0.3-BTWM, 3.5-0.4-BTWM, and Mn-BTO at 400\u2013800 \u00b0C can be attributed to O2 formation, not to CO2 desorption, during the thermal decomposition of the manganese oxides, as indicated in the O2-TPD results (Fig. S9).The TEM and STEM images of W-BTO confirm the formation of a Na-W-O shell on the BaTiO3 surface (Figs. S8A\u2013C), which is also observed in the SAED image (Fig. S8D). EDS also revealed the formation of a Na-W-O layer on the BaTiO3-rich particle surface (Fig. S8E). Among the 3.5-n-BTWM catalysts containing varying amounts of n, BaTiO3 was fully covered by Na2WO4 in 3.5-0.2-BTWM or in the catalysts with n \u2265 0.2 (Fig. 4B).The formation of nanostructures, as depicted by TEM (Fig. 1), were confirmed by H2-TPR (Figs. 5\n and S10). Among the components, the reduction of Mn was focused with the assumption that Mn is an oxygen supplying component. The H2-TPR results of Mn2O3, Na2WO4, BaTiO3, and the other Na-Mn-W-Ba-Ti-O composites were obtained to elucidate the complex structure of the BTMW catalysts (Fig. S10 and Table S3).By adjusting m = Mn/W between W-BTO (m = 0) and Mn-BTO (m = \u221e) (Fig. 5A), the peaks at 500\u2013700 \u00b0C increases with an increase in the amount of Mn added; thus, these peaks are attributed to the reduction of Mn species, T\u03b1 (Mn4+ to Mn3+) and T\u03b2 (Mn3+ to Mn2+). With the addition of Mn, the T\u03b1 and T\u03b2 peaks appeared at 544\u2013578 \u00b0C and 600\u2013629 \u00b0C, respectively. W-BTO, not containing Mn, exhibits a weak reduction at 400\u2013550 \u00b0C, which can be attributed to the reduction of BaTiO3 (Fig. S10), and very small reduction peaks for the W species at 600 \u00b0C or higher (Figs. 5A and S10). The T\u03b1 and T\u03b2 reduction peaks do not significantly change for the compounds containing W (W = 2.3\u201328.6 %), however, the T\u03b1 peak shifts to a lower temperature by approximately 100 \u00b0C for Mn-BTO (no W species). This indicates that the W species suppress the T\u03b1 reduction at lower temperatures. In addition, the T\u03b2 reduction, or the oxygen supply from Mn3+, should be easier for m-0.3-BTMW with m = 1\u20133.5 exhibiting the lower reduction temperatures, compared to those with m = 0.11 and 4\u20139.A change in the reduction of Mn was also observed by adjusting n = (Mn + W)/(Ba + Ti). For Mn-BTO (Fig. 5A) in which n = 0.1 (Fig. 5B), Mn-BTO (containing MnO2 and BaTi(Mn)O3) is reduced at higher temperatures (T\u03b1 =483 \u00b0C, T\u03b2 =662 \u00b0C) compared to Mn-BTO with n = 0.1 (T\u03b1 =461 \u00b0C, T\u03b2 =609 \u00b0C). BaTi(Mn)O3 is the structure in which Ti of BaTiO3 was partially substituted with Mn. This indicates that MnO2 increases the reduction temperature of Mn3+ to Mn2+ (T\u03b2). As depicted in Fig. 5A, the surface-occupying W species increases the reduction temperatures of T\u03b1 and T\u03b2. For n = 0.1 depicted in Fig. 5B, lower reduction temperatures are observed for T\u03b1 and T\u03b2 compared to Mn-BTO with n \u2265 0.2\u20130.3. These observations can be attributed to the presence of surface W species which were not incorporated into the BTO structure partially occupying the BTO surface. Less BaTi(Mn)O3 and more Mn oxide can form on the BTO surface if n = (Mn + W)/(Ba + Ti) increases; this leads to an increase in the reduction temperatures of T\u03b1 and T\u03b2. Mn-W (n = \u221e) exhibited the highest reduction temperature of 748 \u00b0C. For n = 0.2\u20130.7, in addition to the changes in the reduction temperature, the T\u03b2 peak intensity does not significantly change, while the T\u03b1 peak intensity increases, confirming the formation of a larger amount of NaxMn(Ti)O2 (PDF#21-1140 in Fig. 3).The structures of the catalysts during the OCM process were determined through in-situ XRD. The analysis was performed through catalytic CH4 activation and subsequent air oxidation at 25, 600, 700, and 800 \u00b0C (Fig. 6\n and Table 1\n). Na2WO4 forms on the surface of the W-containing catalysts (W-BTO and Mn-W-BTO), which transform from a cubic structure (c-Na2WO4, PDF#74-2369) at room temperature to an orthorhombic structure (o-Na2WO4, PDF#20-1163) at 600 \u00b0C [54], and then, melt at 600\u2013700 \u00b0C, with the complete disappearance of the Na2WO4 peaks at 700 \u00b0C (Fig. 6B). The Na2WO4 melt can highly disperse WO4\n2\u2212 on the catalyst surface and decrease the reduction temperature of W during the OCM reaction at a temperature of 700 \u00b0C or greater, thereby improving the CH4 activation [55].For BTO that does not contain W, Mn, and Na (Fig. 6A), the room temperature tetragonal BaTiO3 (t-BaTiO3, PDF#81-2201) transforms to an elongated cubic structure at 600\u2013800 \u00b0C, followed by a small fraction of the cubic domains transforming into hexagonal structure (h-BaTiO3, PDF#82-1175) at 700 \u00b0C. The presence of CH4 or air does not significantly affect the phase behavior of BaTiO3. BaTiO3 forms a cubic perovskite phase (c-BaTiO3, PDF#79-2263) with a slightly elongated c-axis (a \u2248 c, c/a = 1.002\u20131.003) at 600\u2013800 \u00b0C (Table 1). The elongation along the c-axis of the cubic structure decreases the dipole movement of Ti4+ along this axis in octahedral TiO6, forming a tetragonal-like deformed structure, which is stabilized as tetragonal BaTiO3 (c/a = 1.007) at room temperature [56,57]. The hexagonal BaTiO3 phase (11.1 %) forms at 700 \u00b0C. Notably, the BaCO3 diffraction peaks observed at lower temperatures, disappear at 800 \u00b0C. The disappearance of the BaCO3 peaks corresponds with the decomposition of BaCO3 at temperatures lower than 850 \u00b0C, as indicated in the CO2-TPD results (Fig. 4A\u2013B).With the addition of W to BTO to prepare W-BTO, cubic BaTiO3, without its hexagonal polymorph, is observed at 600\u2013800 \u00b0C (Fig. 6B), unlike what is observed for pure BTO, which undergoes a transformation from elongated cubic to hexagonal phase (Fig. 6A). Although only cubic BaTiO3 is observed for W-BTO in a flow of air, the addition of CH4 at 800 \u00b0C leads to elongation of the cubic BaTiO3 structure by expanding the c-axis (c/a = 1.002 at 800 \u00b0C) and forming more Ti3+ and oxygen vacancies [58].Tetragonal and hexagonal BaTiO3 phases co-exist at room temperature in Mn-BTO. The tetragonal phasetransforms partially to a hexagonal phase when exposed to a flow of CH4, which reversibly transforms back to the tetragonal phase in a flow of air (Fig. 6C and Table 1). Under CH4 flow, the Mn in the bulk BaTi(Mn)O3 is reduced by the activation of CH4 molecules and the creation of oxygen vacancies, which dimerizes TiO6-TiO6 (corner-shared octahedra in the tetragonal phase) into Ti2O9 (face-shared octahedra in the hexagonal phase) (Scheme 1\n) [59]. The peaks corresponding to TiO2-x (PDF#85-1061 for Ti9O17, [60]) disappear under a flow of CH4 in the temperature range 700\u2013800 \u00b0C. This disappearance of peaks is not observed at a temperature of 600 \u00b0C or less, indicating the activation of CH4. These peaks are observed under a flow of air. However, the Mn species in MnO2 on the surface of BaTi(Mn)O3 are not reduced at 600 \u00b0C in a flow of CH4, unlike the reduction observed for bulk BaTi(Mn)O3. The reduction of MnO2 is suppressed by the oxygen atoms that are supplied from bulk BaTi(Mn)O3 to MnO2, as suggested by the H2-TPR results (Fig. 5). At higher temperatures (700\u2013800 \u00b0C), MnO2, under a flow of air, partially transforms to Mn3O4 (PDF#18-0803) [61,62], which is further reduced to MnO (PDF#72-1533) with a switch in the gas flow to CH4. This process is reversible, as a switch back to air re-oxidizes MnO to Mn3O4. Under the flow of CH4, Mn3+ (Mn3O4) on the surface is reduced to Mn2+ (MnO).For Mn-W-BTO (Fig. 6D), the BaTiO3 structure transformation depends on the temperature and the CH4/air environment, similar to that of Mn-BTO (Table 2\n). NaxMn(Ti)O2 was analyzed in a temperature range from room temperature to 800 \u00b0C in a flow of air. A shift to lower 2\u03b8 values is observed with an increase in the temperature (2\u03b8 = 15.98, 36.95, and 43.02\u00b0 at room temperature vs. 2\u03b8 = 15.56, 36.74, and 42.61\u00b0 at 700 \u00b0C and 2\u03b8 = 15.51, 36.74, and 42.52\u00b0 at 800 \u00b0C) and several new peaks (2\u03b8 = 19.09, 19.39, 33.93\u00b0) appear at 700 and 800 \u00b0C. Thus, NaxMn(Ti)O2 is reconstructed by the thermal conversion of Mn4+ to Mn3+, as observed in the CO2-TPD and O2-TPD results (Figs. 4 and S9) [43]. The mobile Na+ cations migrate into the bulk Mn-Ti-O structure to neutralize the electronically unsaturated state (Mn3+ compared to Mn4+), which forms NaxMn(Ti)O2 (x<1, Na0.7Mn(Ti)O2 in this case) in a flow of air at 700 and 800 \u00b0C [51]. Thus, a large fraction of Mn3+ species in NaxMn(Ti)O2, which is composed of MnO6, MnO5 anions, and Na+ cations [52], improves the formation of MnO5 polyhedron anions. When Mn-W-BTO is exposed to a flow of CH4 for 30 min, CH4 decomposes into methyl radicals and surface-adsorbed hydrogen species, and Mn in Na0.7Mn(Ti)O2 is reduced to Mn2+, which deconstructs the mixed oxide into Mn3O4 and MnO. Although MnWO4 is expected to be formed from Mn-Na2WO4/SiO2 through a reaction between the Mn2+ cations and excited WO4\n2\u2212 anions [17], in this study, MnO is formed instead of MnWO4. This indicates that the WO4\n2\u2212 anions can easily combine with other cations including Ba2+.The crystal structures of the m-n-BTMW catalysts, determined at temperatures up to 800 \u00b0C, are summarized in Scheme 2\n. At room temperature, the cubic Na2WO4 crystal occupies the surface of both NaxMn(Ti)O2 and BaTi(Mn)O3, topped by small BaTi(Mn)O3 and NaxMn(Ti)O2 particles, respectively (Figs. S4 and S6B). NaxMn(Ti)O2, which contains a lower quantity of Na, has a structure similar to MnO2, where MnO6 in MnO2 structure is partially substituted with TiO6 building tunnels constructed from TiO6 and MnO6. In BaTi(Mn)O3, the incorporation of Mn leads to the formation of hexagonal and tetragonal BaTiO3 structures. The hexagonal BaTiO3 contains a large fraction of Mn4+, in which MnO6 and TiO6 coexist in a face-shared octahedral arrangement. The tetragonal BaTiO3 contains a large fraction of Mn3+, which forms corner-shared MnO6 and TiO6 octahedra. At 700\u2013800 \u00b0C, however, the Na2WO4 crystals melt to a liquid phase, in which the Na+ cations and WO4\n2\u2212 anions are highly mobile. Mn4+ in NaxMn(Ti)O2 is thermally reduced to Mn3+, and the Na+ cations migrate to NaxMn(Ti)O2 to neutralize the electronically less stable state. The WO4\n2\u2212 anions more easily combine with Ba+ and Na+ cations in NaxMn(Ti)O2 and BaTi(Mn)O2, which highly disperses W on the catalyst surface.The activity of the catalysts in the OCM was determined at 700\u2013800 \u00b0C using a CH4/O2/N2 ratio of 3/1/1 mol/mol/mol (Table 2). The BTO, Mn-BTO, Mn-W-BTO, and 3.5-0.4-BTMW catalysts exhibit high CH4 (20.3\u201339.1 %) and almost 100 % O2 conversions over the entire temperature range, while the W-BTO and W-Mn catalysts exhibit lower CH4 (\u2264 8.6 %) and O2 (\u2264 21.9 %) conversions. Another factor that influences the CH4 activation ability is the dehydrogenation activity of the catalysts, indicated by the olefin/paraffin ratios. Mn-W-BTO and 3.5-0.4-BTMW exhibit higher dehydrogenation activities (olefin/paraffin = 1.3\u20132.2 mol/mol), while Mn-BTO, W-BTO, and W-Mn exhibit lower dehydrogenation activities (olefin/paraffin = 0.0\u20130.8 mol/mol).For BTO, a moderate C2+ selectivity (48.6 %) is observed with a lower dehydrogenation activity (olefin/paraffin =0.9 mol/mol) at 700 \u00b0C (Table 2). CH4 reacts with the lattice oxygen (Olatt) in perovskite BaTiO3 to create oxygen vacancies on the catalyst surface and form Ti3+ in BaTiO3. The larger fraction of Ti3+ present in the spent BTO catalyst indicates that the adsorption of oxygen (Oads) in the oxygen vacancies is slower than the loss of Olatt during the OCM reaction (Table 3\n and Fig. S12). The amount of Oads on the spent BTO and Mn-BTO catalysts is higher (Oads/(Oads + Olatt) = 0.14 and 0.16, respectively) than on the other catalysts (Oads/(Oads + Olatt) = 0.08\u20130.09), which may improve the deep oxidation of the methyl radicals to CO and CO2 (Table 3 and Fig. S13) [11].Mn-BTO and W-BTO fully oxidize the CH4 molecules, and high COx selectivities (90.0 % and 81.1 %, respectively) are achieved at 700 \u00b0C. A high oxidation activity and COx selectivity (1.0\u20133.6 % CO and 80.6\u201389.0 % CO2) is achieved by Mn-BTO, which consists of MnO2 but not W on the catalyst surface (Fig. 6C). An increase in Ti3+ and Mn2+ in the spent Mn-BTO catalyst is observed in the XPS results of the catalysts (Table 3, Figs. S12, and S14), which indicates that the oxygen atoms of MnO2 and BaTi(Mn)O3 activate the CH4 molecules to produce methyl radicals. For W-BTO, the high W coverage on the BTO surface, because of the incorporation of less W into BaTiO3 compared to Mn into BaTiO3, suppresses the reaction on the BaTiO3 surface, which significantly decreases the O2 conversion from 99.9 % for BTO to 1.3\u20135% for W-BTO. The higher amount of Ti3+ in the spent W-BTO catalyst indicates that the oxygen vacancies in BaTiO3 is not rapidly oxidized, leading to the formation of more oxidized tungsten species, which leads to further oxidation of the reactants to COx products (selectivity of 13.6\u201335 % for CO and 33\u201367.5 % for CO2).A significantly high C2+ selectivity (64.6\u201366.3 %) and high olefin/paraffin ratio (1.3\u20131.4 mol/mol) are observed at 700 \u00b0C for the Mn-W-BTO and 3.5-0.4-BTMW catalysts, which contain BaTiO3, Mn, and Na2WO4 as active components. The faster oxidation of the oxygen vacancies in BaTi(Mn)O3 and NaxMn(Ti)O2, which exhibits a change of W(+5)\u2014\u25a1 (\u25a1 as an oxygen vacancy) to W(+6)\u2014O, is confirmed by the decrease in Ti3+ and Mn2+ in the spent catalysts (Table 3, Figs. S12, and S14). The oxygen supply in the system suppresses further reduction of W(+5), which is formed through the in-situ reduction of W(+6), increasing the C+2 selectivity and O2 and CH4 conversions. 3.5-0.4-BTMW exhibits a slightly higher C2+ selectivity, because of the formation of Ba2TiO4 (Fig. 3) which improves its performance in the OCM reaction [33].3.5-0.4-BTMW was determined to be the optimum OCM catalyst based on its OCM activity and optimization of the BTMW catalyst composition performed at 700 \u00b0C. Through the optimization of m and n, it was confirmed that the Mn species activate gas-phase O2 to supply O atoms to the W species. First, m = Mn/W in the m-0.3-BTMW catalyst was manipulated (Fig. 7\nA and B). In the absence of Mn and at small m = Mn/W values, low O2 and CH4 conversions are observed, which reaches 96.8 % at m \u2265 2. The conversion of CH4, which is activated by the oxygen supply, slightly decreases with an increase in m = Mn/W. This can be attributed to difficulties associated with the supply of oxygen from the Mn3+ species, or the higher temperature required to reduce Mn3+ to Mn2+ at a higher m = Mn/W, as depicted in Fig. 5A.The value of n = (W + Mn)/(Ba + Ti) in the 3.5-n-BTMW catalysts was also adjusted (Fig. 7C and D). At n = 0\u20130.1, the 3.5-n-BTMW catalysts contain a limited amount of Mn and W species on the surface, and their catalytic activities are similar to that of BTO. High COx selectivities and O2 conversions are obtained on the exposed BaTiO3 surfaces incompletely covered by W. At n = 0.2, although the BTO surface is fully occupied by Na2WO4 (see H2-TPR results, Fig. 5B), a lower fraction of Mn species, including NaxMn(Ti)O2 and BaTi(Mn)O3, forms on the surface, suppressing O2 activation and decreasing the O2 conversion to 74 %. At n = 0.3\u20130.4, more Mn species form, leading to an O2 conversion of approximately 100 % and a CH4 conversion of 38.5\u201339.1 %; a C2+ selectivity of 66.3\u201366.5 % was achieved. At n \u2265 0.5, the catalyst contains less (Ba + Ti) and more (Mn + W). The W species that fully cover the BTO surface can limit the oxygen supply from BaTiO3, leading to a decrease in the O2 and CH4 conversions. The temperature required to reduce Mn3+ to Mn2+ also increases with an increase in n = (Mn + W)/(Ba + Ti), suppressing the oxygen supply and decreasing the O2 and CH4 conversions.To achieve the optimum yield of C2+, m should be between 2 and 4, and n between 0.3 and 0.4 at 700 \u00b0C, as indicated in Fig. 7B and D. The catalytic OCM activity was stable for 100 h, as observed on the representative optimal catalyst (3.5-0.3-BTMW) (Fig. S15). With an increase in the reaction temperature, the C2+ yield decreases, which indicates that deep oxidation to COx occurs in expense of the formation of the desired C2+ compounds.The catalytic activity of 3.5-0.4-BTMW in the OCM was further compared with that of the low-temperature active BaTiO3 perovskite and Mn-Na2WO4/SiO2 catalysts (Fig. 8\n). The CH4 and O2 conversions achieved by 3.5-0.4-BTMW at 700 \u00b0C are higher than those by Mn-Na2WO4/SiO2 in the temperature range 775\u2013800 \u00b0C (Table S4) [39,41,63\u201365]. BaTiO3 exhibits the highest CH4 and O2 conversions at 675 \u00b0C; however, the conversions decreased below those of the other catalysts at higher temperatures. These observations indicate that higher temperatures are required for the activation of CH4 and O2 on the Mn and W species. 3.5-0.4-BTMW achieved the highest C2+ selectivity at 700 \u00b0C, but a lower olefin/paraffin ratio compared to Mn-Na2WO4/SiO2 at 775 \u00b0C. Because the oxidative dehydrogenation of paraffins to olefins are accompanied by the production of COx, a lower olefin/paraffin ratio corresponds to a higher C2+ selectivity. BaTiO3, as the more active oxidation catalyst, achieves a lower C2+ selectivity and olefin/paraffin ratio, or higher COx selectivity, compared to the other catalysts, although it is activated at the lowest temperature (650 \u00b0C).The spent catalysts were characterized prior to proposing a reaction mechanism. The high-temperature XRD and Raman spectroscopy results of the spent 3.5-0.4-BTMW catalyst in a flow of inert N2 are depicted in Fig. 9\n. No MnO is observed at 25\u2013800 \u00b0C in the XRD result of 3.5-0.4-BTMW (Fig. 9A), while Mn2+ (MnWO4) is detected in the XRD result of Mn-Na2WO4/SiO2 [3,66]. NaxMn(Ti)O2 is thermally decomposed to Mn2O3(PDF#06-0540) and Mn3O4 at 700\u2013800 \u00b0C under a flow of N2; however, this is not observed under a flow of air (Fig. 6D). The high-temperature Raman results (Fig. 9B) indicate the presence of Mn2O3 and Mn3O4 at 700\u2013800 \u00b0C under a flow of N2, which is confirmed by the bands at 200, 321, and 348 cm\u22121 corresponding to the out-of-plane bending modes of Mn2O3 and Mn3O4 [67,68]. The Raman peaks at 535, 659, and 750 cm-1 indicate the formation of hexagonal BaTiO3 [69], while the peak at 628 cm-1 corresponds to the stretching mode of O-Mn-O, and the shifted peak at 685 cm-1 is attributed to Mn2O3 which formed through the decomposition of NaxMn(Ti)O2 [70]. The peaks at 827 and 922 cm-1 correspond to the asymmetric and symmetric stretching vibrations of Na2WO4, respectively [54]. Furthermore, the peak at 915 cm-1 corresponds to BaWO4 [71], indicating that the free WO4\n2- anions strongly interact with BaTi(Mn)O3 and NaxMn(Ti)O2 to form BaxWO4 and NaxWO4 on the catalyst surface, respectively. Through these interactions, highly dispersed single WO4\n2- anions, which are not oligomerized, can form on the surface of the catalyst.Based on the above discussion, a reaction mechanism for the BTMW catalysts is proposed, as indicated in Scheme 3\n. First, CH4 interacts with the oxygen atoms in W\u2014O (in NaxWO4 or BaxWO4) to form methyl radicals in the gas phase and oxygen vacancies in W\u2014\u25a1 on the catalyst surface. In the gas phase, coupling of the methyl radicals leads to the formation of C2H6. Second, the oxygen vacancies on the catalyst surface are occupied by oxygen present on the surface of NaxMn(Ti)O2, which reduces Mn3+ to a mixture of Mn2+ and Mn3+ (in Mn3O4). Mn2+ and Mn3+ (on the surface) are re-oxidized to Mn3+ by the oxygen supplied by bulk BaTi(Mn)O3. Reduction of Mn3+ to Mn2+ in NaxMn(Ti)O2 may not occur because of the oxygen supplied by BaTi(Mn)O3. The partial reduction of Mn3+ to a mixture of Mn2+ and Mn3+ can destroy the electronic balance in the crystal structure, improving the migration of the free surface Na cations into the bulk structure to form NaxMn(Ti)O2. When re-oxidized, the fraction of Mn3+ in NaxMn(Ti)O2 increases, and the Na cations are removed from the bulk to the surface. In bulk BaTi(Mn)O3, the reduction of Mn produces more oxygen vacancies to dimerize TiO6 into Ti2O9 in perovskite BaTiO3 and transform the tetragonal structure to a hexagonal BaTiO3 structure. In the final step, Mn2+ in BaTi(Mn)O3 is re-oxidized to Mn3+ by O2 gas to transform the hexagonal phase back to tetragonal phase.m-n-BTMW catalysts, hybrid catalysts consisting of perovskite BaTiO3 and Mn-Na2WO4, have been prepared and applied in the OCM. The strong interactions between BaTiO3, Na2WO4, and Mn created active sites composed of NaxMn(Ti)O2, BaTi(Mn)O3, and NaxWO4 under the OCM reaction conditions. The strong interaction between NaxMn(Ti)O2 and BaTi(Mn)O3 promoted the activation of oxygen, supplying oxygen to W on the surface of the catalysts at low temperatures, and increasing the C2+ selectivity through the activation of CH4 on NaxWO4 at temperatures close to the melting temperature of Na2WO4 (approximately 700 \u00b0C). In addition, fast recovery of the Mn oxidation state in these Mn compounds via the activation of CH4 increased the CH4 conversion achieved by the m-n-BTMW catalysts. The lower reducibility of Mn in NaxMn(Ti)O2/BaTi(Mn)O3 in the m-n-BTMW catalysts led to desirable activity for the OCM reaction compared to the conventional Mn\u2010Na2WO4/SiO2 catalyst; an optimal reaction temperature and increased C2+ yield was achieved. The optimized m-n-BTMW catalyst, exhibiting the best OCM activity at 700 \u00b0C, contained m and n values of 2\u20134 and 0.3\u20130.4, respectively.\nLien Thi Do: Conceptualization, Methodology, Investigation, Writing - original draft. Jae-Wook Choi: Validation, Methodology. Dong Jin Suh: Validation. Chun-Jae Yoo: Methodology, Supervision. Hyunjoo Lee: Resources. Jeong-Myeong Ha: Conceptualization, Methodology, Writing - review & editing.The authors report no declarations of interest.This research was supported by the C1 Gas Refinery Program (2015M3D3A1A01064900) and the Technology Development Program to Solve Climate Changes (2020M1A2A2079798) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT.Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apcatb.2021.120553.The following are Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  The oxidative coupling of methane (OCM) using hybrid catalysts containing BaTiO3 perovskite and Mn-Na2WO4 exhibited high activity and high selectivity at low temperature: 66.3 % C2+ (olefins and paraffins) selectivity and 25.9 % C2+ yield at 700 \u00b0C which is 100 \u00b0C lower than that used for Mn-Na2WO4 catalysts (800 \u00b0C). Upon the preparation of complex catalysts, the insertion of Mn into BaTiO3 (BaTi(Mn)O3) and the formation of NaxMn(Ti)O2 improved the oxygen-supplying ability of the catalysts. Additionally, strong interactions between the WO4\n                     2\u2212 anions and BaTi(Mn)O3 or NaxMn(Ti)O2 stabilized the WO4\n                     2\u2212 anions on the surface and improved the methane activation ability of the catalysts for the favorable production of hydrocarbons. The nanoscopic modification of catalysts was confirmed using H2\u2013temperature-programmed reduction, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy results.\n               "}
{"full_text": "Transformation of biomass, especially non-edible and waste materials, to high-value chemicals, fuels, and materials is a promising approach for reducing dependence on fossil resources. Recently, interest among researchers in the production of lactic acid has been increasing due to its growing market demand. Lactic acid is a naturally occurring organic acid that is one of the main ingredients in the food, cosmetic, and textile industry. It is currently considered as one of the most potential feedstock monomers [1,2], that can be converted to chemicals such as pyruvic acid, acrylic acid, 1,2-propanediol, 2,3-pentanedione, lactate esters and polylactic acid (PLA) [1,3\u20137]. Lactic acid can be produced by chemical synthesis or by the fermentation of carbohydrates that are present in the biomass. Today, lactic acid is commercially produced mainly through the fermentation of sugars, including glucose and sucrose derived from starchy feedstocks [1,2,8\u201311]. However, the current fermentative production of lactic acid has several drawbacks. These include environmental and scaling-up issues arising from the raw material choices, long reaction times, waste generation as well as from the separation problems in recovering the pure lactic acid [2,9,11]. Therefore research groups have focused on novel chemocatalytic methods, which are more desirable and cost-effective, for directly converting cellulosic biomass or sugars into lactic acid or its alkyl esters [1,5,12,13].The proposed reaction route (Scheme 1\n) in the conversion of cellulosic biomass-based hexoses such as glucose includes three key steps: (1) the isomerization of C6 monomers (glucose) into fructose, (2) the retro-aldol reaction of fructose to C3 triose intermediates, such as dihydroxyacetone, pyruvaldehyde, and glyceraldehyde, followed by (3) the conversion of trioses to lactic acid via several tandem reactions. In the reaction, two molecules of lactic acid are formed from one molecule of hexose. The main byproducts, 5-(hydroxymethyl)furfural (HMF), levulinic acid, and formic acid are also formed [14\u201316].The traditional chemocatalytic methods for the conversion of biomass to lactic acid or its esters from lignocellulose through C6 monosaccharides and C3 trioses have been investigated over the last decade. Catalytic research with homogeneous catalysts has been focusing on the use of metal salt-based catalysts using Sn, Al, and Pb, as well as transitional metal salts, such as Zn, Ni, Fe, Co, and Cr [16\u201319]. Alkaline conditions have been used in the synthesis to improve the conversion and yield of lactic acid [5,17,20]. Although some of these homogeneous water-soluble salts can selectively convert glucose into lactic acid, these salts are generally expensive, highly toxic, corrosive, and/or difficult to recover afterwards [21]. Besides traditional homogeneous catalysts, solid heterogeneous catalysts are more attractive from an industrial perspective. Heterogeneous catalysts, such as Zr, Cr, Sn, Mo, W, and Pb metal oxides, as well as supported metal catalysts on zeolites and aluminum and silica oxides have been tested for the conversion of trioses, sugars, and cellulose to lactic acid [10,14,22\u201327]. Compared to some homogenous catalysts, the lactic acid yields obtained with heterogeneous metal oxide catalysts were lower. Marianou et al. tested SnCl2, SnO and Sn/\u03b3-Al2O3 catalysts in glucose conversion to lactic acid with yields of ca. 33 %, 18 % and 20 %, respectively [26]. Takagi et al. used \u03b3-AlO(OH) in the conversion of glucose to lactic acid in aqueous phase resulting yield of ca. 30 % [27]. Sn-beta-zeolites provided slightly higher yields of lactic acid. Dong et al. studied conversion of glucose with SnO2, ZnO, Sn-Beta and Zn-Sn-Beta obtaining lactic acid yields of 6%, 12 %, 23 % and 48 %, respectively [15]. However, only a few studies on the use of activated carbon in the production of lactic acid can be found in the literature. Some studies on the oxidation of glycerol to lactic acid with carbon-supported Pt and Pd noble metal catalysts have been carried out [28,29]. Onda et al. tested the conversion of glucose to lactic acid and gluconic acid by noble metals supported on activated carbon in an alkaline, aqueous solution with the lactic acid yield of 43 % [30]. Zhang et al. used activated carbon together with metallic Zn and Ni to improve the hydrothermal conversion of glucose into lactic acid (Y = 55 %) [20]. However, high temperatures and/or alkaline conditions were used in these studies.In catalysis, the support can play an important role, increasing the surface area and the stability of the catalyst [31,32]. The support may also improve the catalytic activity by acting as a co-catalyst and its chemical properties can be changed by specific surface functional groups and physical properties tuned by controlling the pore structures. Carbon-based support materials have been used for catalytic applications because of their properties, such as high surface areas, high thermal and chemical stability, low corrosion capability, and easy recovery from the reaction mixture [31]. Moreover, when compared to alumina and silica supports, activated carbon supports are less expensive, and the active phase can be recovered after use by burning away the carbon support [31]. As an attractive point, waste and residue biomasses can be used as a raw material in the preparation of activated carbon.In this study, various activated carbon-based heterogeneous metal oxide catalysts were tested in the conversion of glucose to lactic acid in aqueous solution. Sn, Al, and Cr oxides were used as catalysts on activated carbon supports. The prepared activated carbon supports and catalysts were characterized by multiple techniques. As a raw material for activated carbon, hydrolysis lignin, a waste fraction from lignocellulosic biomass hydrolysis was used. The effect of the metal and the activated carbon support prepared from hydrolysis lignin by chemical or steam activation was tested in a pressurized batch system. Finally, the effect of changes in the reaction conditions (temperature, time, and pressure) on the conversion of glucose and yield of lactic acid were studied, and catalyst reusability experiments were conducted. To our knowledge, metal oxides supported on biomass-based activated carbon have now been used to convert glucose to lactic acid in aqueous solution for the first time.Hydrolysis lignin from the biomass hydrolysis process was obtained from Sekab Ab, Sweden. The following catalyst preparation materials were used: anhydrous AlCl3 (99 %) from Alfa Aesar, CrCl3\u22196H2O (98 %) from VWR, SnCl2\u22192H2O (99 %) from Merck, ZnCl2 (97 %) from VWR, and HNO3 (65 %) from Merck. Anhydrous glucose (99 %) from Alfa Aesar, fructose (99 %) from Acros Organics, formic acid (>98 %) from Merck, hydroxyacetone (95 %) from Alfa Aesar, 5-(hydroxymethyl)furfural HMF (98 %) from Acros Organics, anhydrous lactic acid (98 %) from Alfa Aesar, levulinic acid (98 %) from Acros Organics, NaOH from VWR, NaHCO3 (99.7 %\u2013100.3 %) from Alfa Aesar, Na2CO3 (99.5 %) from Alfa Aesar, and HCl (32 %) from Merck were used as reagents and/or standard materials.The activated carbon support was prepared from hydrolysis lignin by chemical activation with ZnCl2 or by physical activation using steam. Composition of used hydrolysis lignin is provided in Table S1 (Supplementary material). Hydrolysis lignin was dried in the oven at 105 \u00b0C and crushed to a particle size of < 0.42 mm for further use. Chemical activation was done by impregnation of the dried lignin with zinc chloride using a 2:1 mass ration of ZnCl2:biomass. ZnCl2 dissolved in H2O was mixed with the biomass for 3 h at 85 \u00b0C and then dried in the oven for about 3 days at 105 \u00b0C until a constant weight was reached. The carbonization and activation of the dried ZnCl2-impregnated lignin was done in a stainless steel tube in a tube furnace (Nabertherm RT200/13) at 600 \u00b0C for 2 h using a heating ramp of 10 \u00b0C min\u22121. During the thermal heating process, the reactor was flushed continuously with inert N2 gas (at a flow rate of 10 ml min\u22121). Carbonization, followed by physical activation of the biomass with steam, was done in a one-step process in a stainless-steel tube in a tube furnace using a heating ramp of 10 \u00b0C min\u22121 to a temperature of 800 \u00b0C; at the target temperature steam was added by feeding water at a flow rate of 0.5 ml min\u22121 into the reactor for 2 h. During the thermal heating process, the reactor was flushed continuously with inert N2 gas (at a flow rate of 10 ml min\u22121). The resulting activated carbon supports were washed with hot water, dried overnight at 105 \u00b0C, and crushed and sieved to a fraction size of 0.1\u20130.42 mm. The supports were named ACZ (AC zinc chloride-activated and water-washed) and ACS (AC steam-activated and water-washed). To modify the surface, the support materials were treated with 3 mol L\u22121 HNO3. The supports were named ACZN (AC zinc chloride-activated and HNO3-treated) and ACSN (AC steam-activated and HNO3-treated). The treatment was performed in a round-bottom flask, with a ratio of 10:1 mass ratio of acid:support and heated for 4 h at 85 \u00b0C. After the acid treatment, the supports were filtrated and washed with hot distilled water until a constant pH was obtained, and dried in the oven at 105 \u00b0C.Prior to the incipient wetness impregnation method, the pore volumes of the support materials were measured by the N2-physisorption method to calculate the volume of the impregnation solution. The amount of metal salts (SnCl2\u22192H2O, AlCl3, and CrCl3\u22196H2O) added by impregnation on the support were calculated by assuming the targeted concentration of metal (Sn, Al, Cr) in the catalyst was 2.5\u201310 wt.% of the total catalyst mass. Metal salts were added in distilled water, with a drop of concentrated HCl to dissolve the precursor salts. The impregnation solution was mixed with the support, matured for 4\u20135 h at room temperature, and finally dried in an oven at 105 \u00b0C for 16 h. The catalysts were thermally treated in a quartz tube in a tube furnace under a nitrogen atmosphere, using a constant flush of N2 (at a flow rate of 10 ml min\u22121). The thermal treatment was carried out at 350 \u00b0C, using a 5 \u00b0C min-1 ramp and a 3 -h holding time at the target temperature. All catalysts were tested in the reaction without further reduction treatment in a hydrogen atmosphere.Specific surface areas (SAs) and pore size distributions were determined from the physisorption adsorption isotherms using nitrogen as the adsorbate. Determinations were performed with a Micromeritics ASAP 2020 instrument (Micromeritics Instrument, Norcross, GA, USA). Portions of each sample (0.2 g) were degassed at a low pressure (0.27 kPa) and a temperature of 140 \u00b0C for 3 h to remove the adsorbed gas. Adsorption isotherms were obtained by immersing the sample tubes in liquid N2 (\u2212196 \u00b0C) to achieve constant temperature conditions. Gaseous nitrogen was added to the samples in small doses, and the resulting isotherms were obtained. SAs were calculated from the adsorption isotherms according to the Brunauer\u2013Emmett\u2013Teller (BET) method. The precentral distribution of pore volumes (vol.%) was calculated from the individual volumes of micropores (pore diameter < 2 nm), mesopores (pore diameter 2\u201350 nm), and macropores (pore diameter > 50 nm) using the density functional theory (DFT) model.The morphology of the catalyst particles was studied using a JEOL JEM-2200FS energy-filtered transmission electron microscope (EFTEM) equipped for scanning transmission electron microscopy (STEM) at the Centre for Material Analysis, University of Oulu. The STEM model is used for images, energy-dispersive X-ray spectroscopy (EDS) analysis, and quantitative mapping of the catalyst. The catalyst samples were dispersed in pure ethanol and pretreated in an ultrasonic bath for several minutes to create a microemulsion. A small drop of the microemulsion was deposited on a copper grid pre-coated with carbon (Lacey/Carbon 200 mesh copper) and evaporated in air at room temperature. The accelerating voltage in the measurements was 200 kV, while the resolution of the STEM image was 0.2 nm. The metal particle sizes were estimated visually from high-resolution STEM images of each sample.The metal contents of the supports and catalysts were measured by ICP-OES using a Perkin Elmer Optima 5300 DV instrument. Samples weighing 0.1\u20130.2 g were first digested with 9 ml of HNO3 at 200 \u00b0C for 10 min in a microwave oven (MARS, CEM Corporation). Then, 3 ml of HCl was added, and the mixture was digested at 200 \u00b0C for 10 min. Finally, 1 ml of HF was added, and the mixture was again digested at 200 \u00b0C for 10 min. Excess HF was neutralized with H3BO3 by heating at 170 \u00b0C for 10 min. Afterwards, the solution was diluted to 50 ml with water, and the elements were analyzed by the ICP-OES method.The total ash content was determined by using SFS-EN 14,775 standard method.X-ray diffractograms were recorded with the PANalytical X\u2032Pert Pro X-ray diffraction (XRD) equipment using monochromatic CuK\n\u03b11 radiation (\u03bb =1.5406 \u00c5) at 45 kV and 40 mA. Diffractograms were collected in the 2\u03b8 range of 5\u00b0\u201380\u00b0 at 0.017\u00b0 intervals, with a scan step time of 110 s. The crystalline phases and structures were analyzed with the HighScore Plus program.X-ray photoelectron spectroscopy analyses were performed using the Thermo Fisher Scientific ESCALAB 250Xi XPS System. The catalyst samples were placed on an indium film, with a pass energy of 20 eV and a spot size of 900 \u03bcm; the accuracy of the reported binding energies (BEs) was \u00b10.3 eV. Sn, Al, Zn, Cr, O, C, and N elemental data were collected for all samples. The measured data were analyzed with the Avantage V5 software. The monochromatic AlK\u03b1 radiation (1486.7 eV) was operated at 20 mA and 15 kV. Charge compensation was used to determine the presented spectra, and the calibration of the BEs was performed by applying the C1s line at 284.8 eV as a reference. The approximate detection depth of the analysis was < 10 nm.Elemental analysis of the prepared AC supports was performed by using a Flash 2000 CHN-O Organic elemental analyzer by Thermo Scientific. The ground and dried sample (about 1 mg) was placed in the analyzer and mixed with vanadium pentoxide (V2O5, 10 mg) to enhance the burning. The prepared sample was combusted at 960 \u00b0C for 600 s using methionine as a standard for the elements: C, H, and N, whereas the standard used for oxygen was 2,5-(bis(5-tert-butyl-2-benzoaxazol-2-yl)thiophene (BBOT).The surface acidity and basicity of the ACs and catalysts were characterized according to the Boehm titration method [33,34]. The samples (0.1\u20130.2 g) were weighed and separately mixed with 50 ml of 0.01 mol L\u22121 solutions of HCl, NaOH, NaHCO3, or 0.005 mol L\u22121 Na2CO3, and shaken for 72 h in sealed vials at room temperature. The solutions were filtered with ashless filter paper. Acidic groups were determined by the back-titration method\u2014taking 10 ml of each filtrate, mixing with 20 ml of 0.01 mol L\u22121 HCl and finally back-titrating with 0.01 mol L\u22121 NaOH using potentiometric titration. The acidic groups on the AC were calculated [35] based on the theory that NaOH neutralizes carboxylic, lactonic, and phenolic groups and Na2CO3 neutralizes carboxylic and lactonic groups, while NaHCO3 neutralizes only carboxylic groups. Basic sites were calculated by back-titration with 0.01 mol L\u22121HCl; the total number of basic groups were calculated with the assumption that HCl neutralizes the basic groups on the AC surface.Fourier Transform Infrared (FTIR) spectra were obtained using an ATR-FTIR spectrometer (Perkin Elmer Spectrum One) with diamond/ZnSe crystal. The scans were obtained in the spectral range of 4000\u2013650 cm\u20131 with a resolution of 4 cm\u20131, and 20 scans for each sample. Reference (blank) FTIR spectra were obtained from clean crystal.For catalyst testing, HEL\u2019s manual DigiCAT pressure reactor with the hotplate and stirrer system and three parallel Mini-Range stainless steel reactors (50 mL), each with individual manometers for pressure control, was used. In a typical reaction, the catalyst was added to the reactor with 0.100 g of glucose in 20 ml of ultrapure H2O. The reactor was purged with nitrogen and continuously mixed at 500 rpm. Heating was started after purging with nitrogen. When a reaction temperature of 180 \u00b0C was reached (in about 25 min), nitrogen was fed into the system until the desired final reaction pressure of 30 bar was attained (in about 5 min). The temperature was maintained constant through the heating of the external aluminum block in which the parallel reactors and external temperature probe were placed. After the reaction time (0\u2013300 min), samples were collected from the reactor through the outlet vent and filtrated with a 0.45 \u03bcm (polyethersulfone) membrane filter for further product analysis. Two tests were run in parallel and the error was presented as a percentage of the average standard deviation of the two parallel tests. Recycling of the catalyst was performed after filtering the catalyst out from the reaction solution and washing it with 10 ml ethanol three times. The catalyst was then used under the same reaction conditions as in the previous cycle. Due to collection loss, the weight of the catalyst was slightly lower than the initial weight after recycling; the corresponding glucose loading was reduced to keep the weight ratio of catalyst to glucose constant.The product analysis was performed with a Shimadzu High-Performance Liquid Chromatograph (HPLC) with a Shodex RI detector. The quantification was based on external calibration using standard solutions of glucose, fructose, lactic acid, levulinic acid, HMF, formic acid and hydroxyacetone. The liquid samples were analyzed with a Shodex SUGAR SH1821 column (8.0 mm ID \u00d7 300 mm) with pre-column SUGAR SH-G using 5 mmol L\u22121 H2SO4 as a mobile phase with flow rate 1.0 ml min\u22121 and a column temperature of 40 \u00b0C. Conversion and yield were calculated from the results of the quantification by HPLC by using the following equations:\n\n(1)\n\nC\no\nn\nv\ne\nr\ns\ni\no\nn\n\u2009\n(\n%\n)\n\u2009\n=\n\n\n\nC\n\ng\nl\nu\nc\no\ns\ne\n\u2009\ni\nn\ni\nt\ni\na\nl\n\n\n-\n\nC\n\ng\nl\nu\nc\no\ns\ne\n\u2009\na\nt\n\u2009\ne\nn\nd\n\u2009\no\nf\n\u2009\nr\ne\na\nc\nt\ni\no\nn\n\n\n\n\n\nC\n\ng\nl\nu\nc\no\ns\ne\n\u2009\ni\nn\ni\nt\ni\na\nl\n\n\n\n\n\n\n\n\n\n(2)\n\nY\ni\ne\nl\nd\n\u2009\n(\n%\n)\n\u2009\n=\n\n\n\nC\n\nm\ne\na\ns\nu\nr\ne\nd\n\u2009\nL\na\nc\nt\ni\nc\n\u2009\nA\nc\ni\nd\n\u2009\ni\nn\n\u2009\nt\nh\ne\n\u2009\ns\na\nm\np\nl\ne\n\n\n\u2009\n\n\n\nC\n\nt\nh\ne\no\nr\ne\nt\ni\nc\na\nl\n\u2009\nm\na\nx\n.\n\u2009\no\nf\n\u2009\nL\na\nc\nt\ni\nc\n\u2009\nA\nc\ni\nd\n\u2009\ni\nn\n\u2009\nt\nh\ne\n\u2009\ns\na\nm\np\nl\ne\n\n\n\n\n\n\n\nThe theoretical maximum of lactic acid in the sample (Eq. 2) is calculated by assuming that 2 mol of lactic acid is obtained from 1 mol of glucose.Compounds from product mixture were identified using Agilent 8890 GC System equipped with a mass detector (MS) and an Agilent HP5-MS Ultra Inert GC column (30 m x 0.25 mm x 0.25 mm). The oven temperature was programmed at 70 \u00b0C for 1 min, then increased to 280 \u00b0C at 10 \u00b0C min\u22121 and kept for 5 min. The injection volume was 1 ml and He flow 1 ml min\u22121. Mass spectra were collected with an electron impact ionization of 70 eV. The Full-scan acquisition was performed with the mass detection range set at m/z 35\u2013500. Data acquisition and analysis were executed by 5977B GC/MDS (Agilent Technologies).The BET SAs, average pore diameters, and DFT pore volumes and pore distributions of the AC supports and catalysts were measured by N2-physisorption analysis, and the results are listed in Table 1\n. According to the analysis, the SA and the pore volume values of chemically activated ACZ were higher (1595 m2 g\u22121 and 0.78 cm3 g\u22121) than those of steam-activated ACS (760 m2 g\u22121 and 0.47 cm3 g\u22121). After treatment with HNO3, the SAs and pore volumes of both ACS and ACZ decreased about 30 %, probably due to addition of functionalities (see XPS and Boehm titration analysis) or due to the collapse of the pore walls [36\u201338]. However, the mesoporous volumes of both catalyst supports (ACZN and ACSN) treated with HNO3 were similar, and the main difference was in the micropore volumes; the chemically activated support ACZN had more micropores than the ACSN support. The ACZN support was mainly microporous carbon, with 66 vol.% micropores and 34 vol.% mesopores. In contrast, ACS and ACSN were about 50:50 vol.% of micro:meso pores. For all supports, macropore volumes were zero. With metal impregnation, SAs and pore volumes decreased, indicating the addition of metal in the pores (Table 1). The addition seemed to occur mainly in the mesopores, since a greater decrease in their volumes was detected, though microporous volumes were also partly filled. For some catalysts, a greater decrease in porosity seemed to occur, in the following order: Sn/Al5/2.5@ACS > Sn/Al5/5@ACZN > Sn/Al5/2.5@ACZN > Sn/Cr5/2.5@ACZN > Sn10@ACZN > Sn/Al5/2.5@ACSN.Metal contents of the supports and catalysts were measured by total ash content analysis and by ICP-OES. Results of the ash analysis and ICP-OES analysis from activated carbon supports ACZ, ACZN, ACS, and ACSN are presented in Table S2 (Supplementary material). Without acid treatment, the chemically activated ACZ support contained about 5 wt.% of zinc (Table S2) and total metal content was 7.6 wt.% by ash analysis. After ACZ was treated with HNO3, zinc metal was removed from the support and ash content decreased to zero. The steam-activated support ACS contained 0.5 wt.% of Ca and Na; other metals, such as Fe, Mn, Mg, K, and Zn were present at less than 0.1 wt.% and total ash content was 2.3 wt.%. After acid treatment, total residual metal content was less than 0.3 wt.% for both ACZN and ACSN.The active metal contents of the catalysts Sn, Al, and Cr were measured by ICP-OES analysis. The results are presented in Table 1. From the ICP-OES analysis, determined Sn and Al contents were close to the targeted ones. Slightly lower amounts of Sn and Al were detected from the non-acid-treated catalyst Sn/Al5/2.5@ACS. In some cases, acid treatment and the presence of oxygen functionalities (see XPS and Boehm-titration) on the surface has been claimed to be important, by making the surface more accessible for metal precursors at the catalyst preparation and impregnation step\u2014i.e., acting as anchoring sites for the metal precursors on the surface\u2014or by making the surface more hydrophilic, so that the metal precursors can adsorb to the internal surface of the pores [31,39,40].The morphology of the catalyst particles was studied with an EFTEM/STEM microscope. The STEM mode was used for images and combined with EDS analysis and quantitative mapping to detect the elemental composition of the materials. The STEM-high angle annular dark field (STEM-HAADF) image of the Sn/Al5/2.5@ACSN catalyst and the quantitative mapping presented in Fig. 1\n show an equal distribution of aluminum and tin on the surface of the AC. The STEM-HAADF images from all the supports and catalysts are presented in Fig. S1 and Fig. S2 (Supplementary material). From the images of the supports, large particles of zinc oxide are seen on ACZ (Fig. S1 c), which disappear from ACZN after acid treatment (Fig. S1 d). From the catalyst images (Fig. S2), the particle sizes on the surfaces were estimated to be less than 10 nm, with a particle size of approximately 3\u20135 nm; some aggregations were also detected from the surfaces (\u223c 20 nm). The N2-physisorption analysis indicated that mainly mesopores and some of the micropores were filled by impregnation of the catalyst particles on the support. The STEM images verified that small nanoparticles were able to enter into the support pores. However, some of the smallest pores can be blocked by the particles at the pore entrance. Especially for the catalysts Sn/Al5/5@ACZN and Sn/Al5/2.5@ACZN (Figs. S2 b and c), there appeared to be some aggregations, which could be due to an accumulation of nanoparticles in the pores. These could be the reason for the larger decrease in the BET SA and pore volumes. Overall, the particles seemed to be distributed uniformly on surfaces with small particle sizes.XRD analysis was performed to verify the metal phases of the catalysts. The diffraction patterns are presented in Fig. S3 (Supplementary material). For all supports and catalysts, broad peaks at 23.8\u00b0 and 44.2\u00b0 were detected, representing amorphous carbon. For ACZ support, peaks at 31.7\u00b0, 34.3\u00b0, 36.1\u00b0, 47.4\u00b0, 56.5\u00b0, 62.7\u00b0, 67.8\u00b0, and 68.9\u00b0, representing ZnO (JCPDS file No. 04-003-2106) were detected, which disappeared after acid treatment (see Fig. S3 a, XRD pattern of ACZ and ACZN). For Sn-catalysts (Fig. S3 b), the oxidized metal phase of tin(IV)oxide (SnO2) (JCPDS file No. 00-001-0657) was detected, though the peaks representing SnO2, at around 26.3\u00b0, 33.5\u00b0, 51.7\u00b0, and 64.2\u00b0, were rather small for bi-metal catalysts. For Al and Cr, no clear peaks could be detected, probably because of the small particle size or a low concentration of the catalyst material. Overall, diffractograms presented no high crystallinity for Sn, Al, or Cr, indicating the presence of amorphous phases and/or very small particles on the surface of the support, which were also seen on the STEM images.The metal phases of the catalysts were confirmed with XPS (Fig. S4 and Table S3, Supplementary material). According to the analysis, metal oxides with peaks at 487.2 eV and 495.6 eV (Fig. S4 a) corresponded to Sn3d5/2 and Sn3d3/2 [25], respectively, representing tin(IV) oxide (SnO2), and with peaks at 74.5 eV (Fig. S4 b) corresponded to Al2p oxide, most likely representing AlO(OH) [41], were detected from the spectrums of catalysts containing Sn and Al on support ACS, ACSN, and ACZN. The XPS spectrum peaks of the Sn/Cr catalyst on ACZN (Figs. S4 a and c) at 487.2 eV and 495.6 eV corresponded to Sn3d5/2 and Sn3d3/2, respectively, representing SnO2, and peaks at 577.4 eV and 586.9 eV corresponded to Cr2p3/2 and Cr2p1/2 [42], respectively, representing Cr2O3. The support ACZ spectrum (Fig. S4 d) peaks at 1045.7 eV and 1022.6 eV corresponded to Zn2p1/2 and Zn2p3/2, respectively, representing ZnO [43].The surfaces of the AC supports were analyzed with XPS, FTIR and Boehm titration for information about their content and functionality. The elemental analysis (C,H,N,O) of the support material was compared to the total carbon, oxygen and nitrogen content obtained by XPS; however, elemental analysis analyses the total bulk material, while XPS analyses only the uppermost layer. Examples of the C1s and O1s spectra from the XPS analysis are presented in Fig. S5 (Supplementary material). The peaks from the C1s spectra indicated that most of the carbon was present as graphitic conjugated carbon (at 284.8 eV) and non-conjugated carbon (at \u223c285 eV). Also, carbon-oxygen type functionalities were present at 286.6 eV (from phenolic, alcoholic, or etheric functional groups) and at 288.8 eV (from carboxylic, anhydride, ester, or lactone groups) [44\u201347] (Table S3, Supplementary material). The highest total carbon content, detected by both XPS and elemental analysis, was in steam-activated ACS and it decreased after the chemical treatments due to the addition of heteroatoms (Table 2\n). The more drastic decrease in the total carbon content, detected by XPS rather than by elemental analysis, indicated that the functional groups were attached to the surface. The XPS O1s analysis of the chemically activated ACZ showed a higher total oxygen content than that of ACS, most likely due to oxygen atoms bound to Zn as ZnO [48], which were present from the preparation step. When activated carbon was treated with nitric acid, the total oxygen content of the ACSN was three times higher than on the surface of the plain steam-activated catalyst support (ACS) (Table 2). The nitric acid treated support ACZN showed a higher content of total oxygen than ACZ and four times higher than ACS. Similarly, confirmed by elemental analysis, the oxygen content of the HNO3 treated supports was about four times higher than of the untreated ones and was highest in the ACZN support. The increase of oxygen functionalities after HNO3 treatment was detected mainly as oxygen functionalities from the O1s scan at 532.3 eV (Table S3, Supplementary material), which can be identified as carbonyl oxygen from functionalities such as lactone, ester, carboxylic or anhydride, and oxygen atoms from phenol or ether groups. However, the identification is not clear since the same functionality can give a signal at different BEs, and it also depends on the fittings of the peaks [44,45,47,49\u201351]. It has been reported that oxidation by nitric acid treatment increases the oxygen content on the activated carbon, and especially the number of acidic functionalities such as carboxyl groups [36,38,51,52]. A small increase in the nitrogen content was also detected on ACs after nitric acid treatment. The nitric acid oxidation is also known to result in a number of nitro groups via the nitration mechanism of aromatic ring [53]. This was indicated by the FTIR spectrum of ACZN (Fig. S6 a, Supplementary material).The acidic and basic group concentration of the supports was studied with the Boehm titration method (Table 2). The titration indicated that only minor amounts of acidic functionalities and some basic functionalities (e.g., chromene or pyrone [31]) were present on ACS. The explanation in the literature is that the high temperatures (800 \u00b0C) used for steam activation can destroy most of the functional groups on the surface of AC [51], seen also from the FTIR spectrum (Fig. S6 a). A slightly higher total acidic group concentration was observed on chemically activated ACZ as compared to the steam-activated ACS. This could be due to the lower temperature (600 \u00b0C) used in the preparation of ACZ, leaving more functional groups on the surface, or from the presence of ZnO. According to the Boehm method (Table 2), after nitric acid treatment, a higher content of total acidic groups, at 1.2 mmol g\u22121 and 2.7 mmol g-1 were detected on ACSN and ACZN, respectively, than on untreated ACS and ACZ, due to an increase in carboxylic, lactonic, and phenolic groups (Fig. S7, Supplementary material). This was in good agreement with the results from the XPS analysis. No basic groups were detected on the ACZ, ACZN, or ACSN surfaces with Boehm titration.After the addition of metals to the supports, the total carbon content decreased, while the oxygen content increased, most likely due to the addition of metal oxides or hydroxides, which can be seen from the metal-oxygen bonds at 531.2 eV or 532.3 eV [54\u201356] (Table S3, Supplementary material) according to the XPS analysis. A decrease in nitrogen content was also noted after the addition of metal (Table S3), indicating that nitrate/nitro groups introduced to the surface during oxidation with nitric acid decreased after catalyst preparation, since the nitro groups begin to decompose at about 270 \u00b0C [53]. Boehm titration analysis was conducted for the catalysts by back-titrating with NaOH to determinate their total acidic group concentration. Besides the acidic groups (carboxylic, lactonic, and phenolic) on the aromatic carbon framework, inorganic components such as metal hydroxides may take up protons and/or precipitate during the acidification step in the Boehm titration procedure, thereby affecting the total acidity of the sample [57]. The total acidity increased after the addition of metal oxides to ACS and ACSN, and was 1.1 mmol g\u22121 and 1.7 mmol g-1 for Sn/Al5/2.5@ACS and Sn/Al5/2.5@ACSN, respectively. This is most likely due to oxidized metals or acidic species created during the metal impregnation on AC, seen also as an increase in O1s XPS analysis at 532.3 eV, indicating an increase in metal-oxygen bonds or carbon-oxygen bonds (Table S3, Supplementary material). On the other hand, for the catalysts impregnated on ACZN, the total acidity did not change after the addition of metals and was about 2.6 mmol g\u22121 for Sn10@ACZN, Sn/Al5/2.5@ACZN and Sn/Cr5/2.5@ACZN. Further investigations using Boehm titrations were performed for the heat-treated ACs. Heat treatment was performed in a similar manner as for the catalysts\u2014at 350 \u00b0C for 3 h under N2. As a result, the total acidity decreased by about 20 %, and mainly the carboxylic groups seemed to break down (Fig. S7, Supplementary material); these are groups that break easily at lower temperatures [44,51]. This could be why the total acidity did not increase with the addition of metal oxides on the ACZN support\u2014it contained more carboxylic groups, which could have decomposed in the heat treatment process during the catalyst preparation. Presence of carbonyl groups after catalyst preparation and heat treatment on carbon was verified from the FTIR spectrum (Fig. S6 b), however, peaks indicating the presence of nitro groups were absent. This was in agreement with the XPS and titration analyses.Preliminary studies with chlorides of Sn, Al, and Cr, and chlorides of Sn + Al, Al + Cr, and Sn + Cr metal combinations were done to study the conversion of glucose to lactic acid under the following conditions: 2 h at 180 \u00b0C, 30 bar, and 500 rpm, using 0.100 g of glucose in 20 ml of H2O. SnCl2*2H2O, AlCl3, or CrCl3*6H2O were used as single catalysts at a concentration of 0.1 mmol, and at concentrations of 0.05 mmol each metal in combination. The reaction parameters and catalyst amounts were adapted from Deng et al. [16]. Results from the test are shown in Fig. 2\n. For all the homogeneous metal salts tested, except SnCl2, 100 % conversion of glucose was obtained. The lactic acid yield of the single Sn catalyst was low (7%). Single Cr and Al catalysts had higher lactic acid yields (\u223c20 %); however, levulinic acid was produced in an almost 1:1 ratio with lactic acid. The lactic acid yield from the Al + Cr combination was about the same as that from single Al and Cr catalysts, though a lower amount of levulinic acid was produced. The highest lactic acid yields, 37 % and 36 %, were obtained from the Sn + Cr and Sn + Al combinations, respectively. Based on the preliminary study results, the best working metal combinations (tin combined with aluminum or chromium) were selected as heterogeneous catalysts on activated carbon for further analyses of lactic acid production.The conversion of glucose (0.100 g) by heterogeneous metal catalysts supported on activated carbon was studied at 180 \u00b0C, 500 rpm, 30 bar with 0.100 g of the catalyst in 20 ml of H2O (Fig. 3\n). The effect of the metal was studied using a carbon-supported tin oxide catalyst as well as tin oxide combined with oxides of aluminum or chromium on the carbon support. Furthermore, the ratio of the metals in the catalysts was modified. Chemically activated and nitric acid-treated activated carbon (ACZN) was used as support. ACZN treated in a manner similar to the catalysts, in a N2 atmosphere at 350 \u00b0C for 3 h, was used as a reference sample.After two hours, without a catalyst in the reaction system, the conversion of glucose was 48 %, and HMF was the main synthesis product with a yield of 23 % with a minor lactic acid yield of 5%. When ACZN was added in the reaction, a slightly higher fructose yield was noticed; however, the lactic acid and HMF yield was about the same as that without the catalyst. With the addition of 10 wt.% of tin to the support (corresponding to 0.084 mmol of Sn in 0.1 g of catalyst), the conversion was almost complete (94 %) and the formation of lactic acid increased to 19 % with Sn10@ACZN. Results indicated that the presence of SnO2 in the support promotes the conversion of glucose and the formation of lactic acid instead of HMF. With the addition of aluminum at 5 wt.% with 5 wt.% of tin (0.042 mmol of Sn and 0.19 mmol of Al), the lactic acid yield increased to 27 %, and complete conversion of glucose was achieved with the Sn/Al5/5@ACZN catalyst. However, when the aluminum/tin ratio was changed to 5/2.5 wt.% (0.042 mmol of Sn and 0.093 mmol of Al), the highest lactic acid yield (31 %) was reached with the Sn/Al5/2.5@ACZN catalyst. The trend indicated that the addition of a small amount of Al improved the formation of lactic acid, while the higher loading did not improve it further. Deng et al. noticed that for homogeneous catalysts, selectivity for lactic acid was highest when the molar ratio of Sn/(Sn + Al) was 0.5, and it decreased for lower or higher ratios [16]. In our studies, we see a similar trend, with lactic acid yield decreasing in the order Sn/Al5/2.5@ACZN > Sn/Al5/5@ACZN > Sn10@ACZN, where the Sn/(Sn + Al) molar ratio was 0.30, 0.19, and 1.00, respectively. Lewis acids, such as tin, are found to catalyze the isomerization of glucose into fructose, as well as the retro-aldol reaction of hexoses into trioses [10,26] (see Scheme 1). Other research groups have studied glucose conversion with supported Sn10/\u03b3-Al2O3 catalysts that yielded 20 % lactic acid and 25 % HMF [26]. Holm et al. tested Sn-zeolites and obtained a lactic acid yield of 26 % in a water solution [10]. Aluminum oxide was also used as a solid Lewis acid catalyst for converting trioses, with a lactic acid yield of 28 % [27]. Rasrendra et al. found that the aluminum and chromium metal salts were the most active catalysts converting trioses such as dihydroxyacetone and glyceraldehyde to lactic acid in water [19]. Takagaki et al. found that a chromium oxide catalyst could easily transform pyruvaldehyde to lactic acid, with a yield of 46 % [22]. Xia et al. found that the Cr/(Cr:Sn) molar ratio of 0.5 on a Cr-Sn-Beta zeolite gave the highest yield of lactic acid in the conversion of glucose [58]. However, in this study, the addition of chromium oxide with tin oxide to the Sn/Cr5/2.5@ACZN catalyst (0.042 mmol of Sn and 0.044 mmol of Cr, at a molar ratio of 0.5), did not improve the production of lactic acid, as compared to the Sn/Al catalyst. Instead, the addition of chromium oxide seemed to direct the reaction towards the production of HMF and levulinic acid, in contrast to the Sn/Al catalyst.To conclude, the highest yield of lactic acid produced was with the combination of Sn with Al, at a ratio of 5:2.5 (Sn/Al wt.%) on ACZN. This combination was selected for use in the following examinations for conversion of glucose to lactic acid.The effect of the catalyst support on the conversion of glucose to lactic acid was studied at 180 \u00b0C, 30 bar, 500 rpm using 0.100 g of the catalyst (0.042 mmol of Sn and 0.093 mmol of Al) and 0.100 g of the glucose in 20 ml of H2O. The activated carbon supports ACS, ACSN, ACZ, and ACZN were tested as reference samples (treated in an N2-atmosphere at 350 \u00b0C for 3 h similar to the catalysts). The supports were impregnated with the Sn/Al catalyst at a ratio of 5/2.5 wt.%. ACZ was used as a support only after treating it with nitric acid, since it contained 5 wt.% zinc (see Table S2). The results of the conversion and main product yields are presented in Fig. 4\n. Other byproduct yields were not quantified.After two hours, the conversion of glucose in the batch system for ACSN and ACZN reached about the same conversion rate as that without the catalyst (Fig. 3), and no notable differences in the yield of lactic acid were observed. With ACS, a higher conversion rate was noticed, but the lactic acid yield was about the same as before. Furthermore, slightly higher fructose and HMF yields were noticed with an increasing order of ACS < ACSN < ACZN, which was in the same order as the increase in total acidity of the ACs (see Table 2). This could be due to the Br\u00f8nsted acids (acidic groups) on the ACs, which can catalyze fructose to HMF and its derivatives [59]. In contrast, ACZ yielded 27 % lactic acid and a 97 % conversion rate, and the HMF yield shifted to levulinic acid. It was found that ZnO in the support (see ICP-OES, XRD, and XPS results) can act as a catalyst and convert glucose to lactic acid [15,34]. However, ACZ was not used as a support for Sn/Al without treating it first with nitric acid to remove residual ZnO as it already contained the active metal oxide.With the addition of Sn and Al oxides to the ACS support, the Sn/Al5/2.5@ACS catalyst provided glucose conversion of 98 % and a lactic acid yield of 24 %. With the acid-treated Sn/Al-catalysts, the glucose conversion rate reached 100 % and the highest lactic acid yields of 31 % and 34 % were reached with the Sn/Al5/2.5@ACZN and Sn/Al5/2.5@ACSN catalysts, respectively. Also, there was a shift from HMF to levulinic acid as the main byproduct. Results indicated that nitric acid treatment of the catalyst support had an impact on the conversion of glucose to lactic acid, seen in comparison to the untreated catalyst support (Sn/Al5/2.5@ACS). Further analysis of the reaction mixture was performed with GCMS. GCMS chromatograms and MS spectra (Fig. S8 and S9, Supplementary material) revealed that lactic acid was the main product with all catalysts. Also, other monocarboxylic acids such as formic acid and acetic acid were identified from the product mixture. With more acidic catalysts (Sn/Al5/2.5@ACSN and Sn/Al5/2.5@ACZN) byproducts formed besides the HMF and levulinic acid were other furfural derivatives such as 5-methyl furfural and 2,5-furandicarboxaldehyde and amount of byproducts varied during the reaction time (Fig. S10, Supplementary material). Compounds such as dihydroxyacetone, glyceraldehyde were also identified from the product mixture indicating that reaction towards lactic acid was going through the formation of dihydroxyacetone as intermediate (Fig. S10). The main byproduct with ACZ catalyst was identified as acetol (hydroxyacetone) (Fig. S8). It has been found that acetol and lactic acid are formed competitively from the same intermediate, i.e. pyruvaldehyde, via hydrogenation on metal surfaces [60\u201362]. Without the presence of external hydrogen in the reaction system, the hydrogenation can occur via transfer hydrogenation by hydrogen donor such as formic acid for example [63]. With ACZ, the acetol production was prominent compared to more acidic Sn/Al catalysts, which on the other hand, directed the reaction towards HMF and its derivative\u2019s production. Other byproducts formed such gaseous products or humines in the reaction mixture were not analyzed. It has been claimed that Lewis acid sites on heterogeneous catalysts are crucial for lactic acid production, while the Br\u00f8nsted acid sites play no role at all [64]. Rasrendra et al. found that Br\u00f8nsted acids, namely H+ ions, had a positive effect on the conversion of dihydroxyacetone to pyruvaldehyde while Lewis acid sites played a key role with conversion of trioses to lactic acid [65]. Clippel et al. studied dihydroxyacetone conversion to lactic acid and it\u2019s esters with bifunctional carbon-silica catalysts and demonstrated that the presence of weak Br\u00f8nsted acid sites originating from oxygen-containing functional groups in the carbon part were crucial in accelerating the dehydration reaction [66]. This indicated that acidic sites on the carbon surface could participate on the conversion of dihydroxyacetone to pyruvaldehyde and further to higher yields of lactic acid, explaining why more acidic Sn/Al5/2.5@ACSN was performing better than less acidic Sn/Al5/2.5@ACS (see chapter 3.1.1). Furthermore, when comparing Sn/Al5/2.5@ACSN and Sn/Al5/2.5@ACZN, the higher acidic group concentration on ACZN compared to ACSN did not seem to have notable effect on the conversion rate or the lactic acid yield or on changes in the byproduct yields after addition of Sn/Al. The higher SA of the Sn/Al5/2.5@ACZN catalyst also did not seem to contribute to the higher activity or lactic acid yield for the catalyst, as compared to the steam-activated Sn/Al5/2.5@ACSN with a lower SA. Both catalysts had almost similar mesoporous structures, indicating that having a more microporous structure is not beneficial for lactic acid production. It is possible that the molecules are too big to fit in the smallest micropores, or that the desorption of the products is difficult. This could be also the reason why higher acidic group concentration on the Sn/Al5/2.5@ACZN did not have an effect on the conversion rate or the lactic acid yield if the active acidic sites in the smallest micropores were not accessible for the reactants or were blocked. It has been shown that oxidized carbon catalysts, specially carboxylic groups on carbon, take place in dehydration reactions, however, not only the number of the acidic groups but also their location and accessibility are relevant for the reaction [67\u201369]. Results indicated that the environmentally hazardous ZnCl2 treatment step could be removed from the pretreatment process of the AC support, since the more microporous surface has no beneficial role to play in the conversion process or in the yield of lactic acid after the addition of Sn/Al to the support. Even though, it was noted that presence of ZnO and a lower amount of acidic groups in the catalyst seemed to produce lactic acid and inhibit the formation of HMF and its derivatives, the addition of Sn/Al oxides on acidified catalyst support resulted in the best lactic acid production levels. Moreover, the addition of zinc on the support cannot be controlled since it is left over from the activation process. Detailed studies regarding the preparation of carbon-supported zinc oxide catalysts and their utilization in lactic acid production are ongoing and will be reported at a later date.Based on the discussion above, we propose a reaction route presented in Scheme 2\n. as the reaction pathway for production of lactic acid and main byproducts with the AC based catalyst. The detailed reaction mechanism remains in question at the moment.Based on studies of the effect of metal and support interactions on glucose conversion to lactic acid, the Sn/Al5/2.5@ACSN catalyst was selected for optimization studies in the aqueous phase. The effects of catalyst loading, time, temperature, and pressure on the conversion of glucose to lactic acid were studied. Glucose at a concentration of 0.100 g was used and mixed at 500 rpm in 20 ml of H2O in all the experiments. The effect of catalyst loading at a temperature of 180 \u00b0C, with increasing catalyst amounts (0.050, 0.100, 0.150, and 0.200 g) was studied (Fig. 5\n). A catalyst load of 0.050 g was too low, as 100 % conversion of glucose was not achieved even after 3 h. With higher catalyst loading (\u2265 0.100 g), the glucose conversion rate reached 100 % within 60 min, and a catalyst concentration of 0.100 g yielded 34 % of lactic acid. Using 0.150 g of the Sn/Al5/2.5@ACSN catalyst, a yield of 37 % was achieved after 30 min. The highest yield of lactic acid (42 %) was achieved in 30 min using 0.200 g of the catalyst, and the yield was constant even at a reaction time of 180 min. An overview of some of the used heterogeneous catalysts in the conversion of biomass to lactic acid is presented in Table 3\n. Comparing our results to those, our catalyst provided reasonable yields of lactic acid in glucose conversion.The effect of the reaction temperature on the conversion of glucose to lactic acid was studied at 160, 180 and 200 \u00b0C using 0.200 g of the Sn/Al5/2.5@ACSN catalyst and 30 bar reaction pressure (Fig. 6\n). The complete conversion of glucose at 160 \u00b0C took 120 min to achieve and was slower than at higher temperatures (180 and 200 \u00b0C) where the reaction rate increased and conversion was almost complete (\u2265 98 %) at 20 min. Besides, the increase in the lactic acid yield from 35 % to 42 % was detected when the temperature was increased from 160 \u00b0C to 180 \u00b0C, but a further increase in the temperature did not increase the lactic acid yield. During the reaction temperature increase, byproducts (HMF and levulinic acid) were formed faster (Fig. S11, Supplementary material). At 200 \u00b0C, the color of the reaction solution was much darker (Fig. S12, Supplementary material), even though the lactic acid yield was not higher, indicating that other, unwanted soluble byproducts (including polymeric materials, such as humines) could have formed from the decomposition of reaction products at higher temperatures [22]. This suggested that a temperature of 180 \u00b0C was optimum for the reaction and a further increase in temperature was unfavorable for the selectivity of the reaction.Different types of atmospheres notably influence the yields of lactic acid, and an atmosphere of pure nitrogen was found to yield the highest amount of lactic acid compared to oxygen and air [70,71]. Sun et al. found that an increase in pressure gave higher lactic acid yields up to 40 bar (He), after which the yield started to decrease [72]. An atmosphere of nitrogen was used in our catalytic studies. The effect of increasing pressure on the conversion rate and the yield of lactic acid was tested at \u223c5, 20, 30, and 40 bar. The pressure increase was carried out by the addition of inert gas (N2) into the system. The pressure of the reaction itself was about five bar, when no excess pressure was added. As seen in Fig. 7\n, the pressure increase from 5 to 40 bar did not seem to have a major effect on either the conversion rate or the yield of lactic acid. A slightly lower conversion rate was noted after 20 min when no pressure was added, but the difference was only 4%, and after 60 min, the conversion rate was 100 % for all pressures. The lactic acid yield was about the same at every tested pressure. This was noted as a positive result since excess pressure does not need to be added to the system.Reusability tests of the Sn/Al5/2.5@ACSN catalyst were performed to obtain information about the stability of the catalyst. Tests were performed at 180 \u00b0C, at 5 bar (pressure of the reaction itself) and 30 bar with 0.2 g of the catalyst and 0.1 g of glucose in 20 ml of H2O. The catalyst was filtered after 120 min, washed with ethanol, dried in the oven at 105 \u00b0C, and tested in the same conditions as in the first run at four cycles. Ethanol wash was selected instead of water after reuse, as the lower-polarity solvent could access the catalyst pores easier. As a result of the reuse (Fig. 8\n), glucose conversion decreased from 100 % to 86 % after the first cycle, and finally to 80 % after four cycles. Yield of lactic acid decreased from about 40 % after the first cycle to 15 % after four cycles. More fructose was detected in the reaction solution, indicating the reaction was not completed, and yield of HMF increased from 5% to 15 %. No differences were detected in the conversion rates and yields at the different reaction pressures used (\u223c5 to 30 bar).After catalyst reuse, the reaction liquid was analyzed using the ICP-OES method, and the metal content was determined after each recycle of the catalyst. The hot, acidic medium can promote the solubility of some metal oxides and cause deactivation of the heterogeneous catalysts by leaching the metal or metal oxides [73]. In these experiments, the pH of the final product solution was about 2. After the first run, 0.2 % of tin was leached from the initial amount of Sn added to the catalyst; 15 % of aluminum was leached from the initial metal content added to the catalyst. After the second run, leaching was 0% for Sn and 1.0 % for aluminum. This indicated that some of the Al was leaching; however, it was relatively minor, and Sn was quite stable on the catalyst. The SA and the pore volumes of the catalyst were also determined after four recycles. The BET SA of the catalyst was 97 m2 g\u22121, with a pore volume of 0.06 cm3 g\u22121, resulting in an 82 % decrease of the BET SA and a 75 % decrease of the pore volume, leading to the conclusion that most of the SA and porosity was lost during the use of the catalyst. Blocking of the catalytic sites on the porous structure of catalyst by adsorbed reaction products or carbon deposits seems to be the cause of the catalyst deactivation. Moreover, the product distribution after fourth reuse seems to be similar as with the plain AC support, indicating blocking of the active metal sites on catalyst support. In the aqueous solution, carbonaceous byproducts, such as humines, can be formed during the reaction, which can block the pores, deposit on the active sites, and finally deactivate the catalyst [66,74,75]. In other research, calcination at mild conditions was used successfully to reactivate the catalyst after residue deposition [76]. After fourth reuse, the catalyst was calcined in air at 300 \u00b0C for two hours to remove the deposits on the catalyst surface. After calcination, The BET SA of the catalyst was 640 m2 g\u22121, indicating that the pore structure was opened. However, lactic acid yield was the same as after second reuse (Fig. 8), suggesting that leaching of aluminum was also the reason for decrease in lactic acid yield after reuse. This verified the fact that co-operation of the two metals; tin and aluminum was important for the catalyst selectivity to lactic acid.The aim of this work was to illustrate the possibility to utilize lignocellulosic side stream, hydrolysis lignin as a raw material for activated carbon, which in turn could be used as catalyst support for metal oxide catalysts. The modification of the AC support by chemical treatments with ZnCl2 and HNO3 was also studied. The prepared lignin-based activated carbon-supported tin-, aluminum- or chromium-containing catalysts were studied in the conversion reaction of glucose to lactic acid. All of the tested carbon-supported metal oxide catalysts showed a high rate of glucose conversion (> 94 %) and were able to convert glucose to lactic acid in 20\u2013120 min, depending on the reaction conditions. The addition of tin oxide along with aluminum oxide resulted in higher lactic acid production yields, in contrast to chromium oxide, which directed the reaction towards byproduct formation. It was noted that activated carbon support prepared by chemical activation with zinc chloride caused the deposition of zinc oxides on the support; this by itself was able to convert glucose to lactic acid and should be studied more in the future. Moreover, treatment of the AC surface with nitric acid had a positive effect on the lactic acid yield when tin and aluminum oxides were added on to the support, and was related to the higher acidity of the catalyst. The highest yield of lactic acid was produced with steam-activated and nitric acid-treated carbon support containing Sn/Al oxides at 5/2.5 wt.% (Sn/Al5/2.5@ACSN)\u2014within 20 min, the catalyst Sn/Al5/2.5@ACSN yielded 42 % lactic acid at 180 \u00b0C without addition of excess pressure. However, the reusability experiments indicated that the catalyst was not stable in an aqueous solution, resulting in a decrease in lactic acid yield within four cycles. This was probably caused by the deposition of carbonaceous byproducts, such as humines, on the catalyst active sites and leaching of aluminum oxides as active metal. In conclusion, lignin-based activated carbon supports provided interesting results and opened a window for later studies with lactic acid production. Furthermore, improved catalyst and reaction condition design is needed for better selectivity and to obtain the recyclable catalyst without deactivation.\nRiikka Kupila: Conceptualization, Investigation, Formal analysis, Writing - original draft. Katja Lappalainen: Conceptualization, Supervision, Writing - review & editing. Tao Hu: Formal analysis, Writing - review & editing. Henrik Romar: Writing - review & editing. Ulla Lassi: Supervision, Writing - review & editing.The authors declare no conflict of interest.Authors R.K. and K.L. would like to thank the Green Bioraff Solutions Project (EU/Interreg/Botnia-Atlantica, 20201508) for funding this research. Hanna Prokkola is thanked for performing the elemental analysis. Mia Pirttimaa and Sari Tuikkanen are thanked for methods developed with HPLC analytics.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2021.118011.The following are Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  In this study, heterogeneous biomass-based activated carbon-supported metal oxide catalysts were prepared and tested for lactic acid production from glucose in aqueous solution. Activated carbons were produced from hydrolysis lignin by chemical (ZnCl2) or steam activation and modified with a nitric acid treatment and Sn, Al, and Cr chlorides to obtain carbon-based metal oxide catalysts. The modification of the carbon support by nitric acid treatment together with Sn and Al oxides led to an increase in lactic acid yield. The highest lactic acid yield (42 %) was obtained after 20 min at 180 \u00b0C with the Sn/Al (5/2.5 wt.%) catalyst on steam-activated carbon treated by nitric acid. Reusability of the catalyst was also studied with the conclusion that the deposition of carbonaceous byproducts and leaching of Al oxides led to a decrease in catalyst selectivity to lactic acid.\n               "}
{"full_text": "While worldwide energy consumption continues to grow, unfortunately, nearly 88% of the current energy economy relies on fossil fuels [1]. It is only a matter of time before fossil fuels become either completely depleted or unprofitable to retrieve. Despite their outsized share of the energy portfolio, the era of fossil fuels is coming to an end [2]. Apart from the problem of diminishing availability, the negative externalities of fossil fuels have posed a prominent risk to the global ecosystem. At present, the world\u2019s main source of energy is the combustion of fossil fuels. The byproducts of this combustion (e.g., CO2, NO\nx\n, SO\nx\n, and fine particles) seriously pollute the air, soil, and water [3\u20135]. It is urgent that we adopt a fresh mindset in order to find solutions to these problems and to devise a future with a more secure and sustainable energy supply. Renewable energy will play a vital role in the world\u2019s energy future. However, there is a market barrier that stems from a major difference between renewable and conventional energy sources [6\u20139]. The amount of energy (mainly in the form of electricity) yielded by renewable energy sources can change unpredictably over a short period of time. For example, solar systems only produce energy when the sun is shining. Unfortunately, other renewable sources such as wind and tidal movements also possess adverse inconstancy [10]. This inconstancy makes the current generation of renewable energy less reliable than fossil-fuel-derived energy, because its output is highly dependent on weather conditions (i.e., clouds or wind) and time (i.e., day or night). For renewable energy to be practical on a very large scale, highly efficient electricity conversion and the high-density storage of electricity are required to enable energy-distribution technologies.Electrochemical hydrogen\u2013water conversion (H2\u00a0+\u00a0O2\u00a0\u2194\u00a0H2O) is a clean and efficient execute solution for a sustainable energy system [11,12]. To be specific, renewable energy can be converted into chemical energy stored in hydrogen (H2) through water electrolysis [13]. Inversely, hydrogen molecules can be electrochemically recombined into water (H2O) in order to output electricity through fuel cells. In this energy system, hydrogen acts as the energy carrier, and the energy conversion is independent of thermal cycles [1,14,15]. Because it is based on electrochemical reactions, hydrogen\u2013water conversion can drastically reduce the release of climate-changing gases and compounds that are harmful to the natural environment and to human health. However, for the purposes of practical application, hydrogen must first be obtained, then be stored, and finally be converted back to water to release the stored energy [16]. In order to achieve this target, efficient, low-cost water electrolysis and fuel cell technology must be efficiently integrated together. Electrochemical processes are the core of these energy conversion technologies, including the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in the water electrolyzer, and the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) in the hydrogen\u2013oxygen fuel cell [17,18]. The output performance of the aforementioned energy conversion technologies is significantly influenced by the efficiency of these four electrochemical reactions. Thus, the most critical problem in this sustainable energy system is how to effectively catalyze these reactions on the catalytic electrode surface in order to attain the lowest overpotential and highest current density [19\u201322]. Apart from the potential drop induced by electrochemical reactions, the electrical resistances and transport-related resistances affect the overall cell voltage of the water electrolysis and fuel cell. Accelerating the electron and proton transfer and the product emission by optimizing the electrode structure is another problem that requires extensive attention.This article provides a comprehensive review of recent advances toward the structural engineering of electrocatalytic catalysts for electrochemical hydrogen\u2013water conversion. Two major issues are addressed in this review: \u2460 the origin of energy dissipation in the electrochemical hydrogen\u2013water conversion system; and \u2461 the structural design of electrocatalysts for high energy-conversion efficiency, driven by the combination of fundamental science and practical technology. In the second section, after a brief introduction of the electrochemical process that occurs in hydrogen\u2013water conversion, we present a review of the energy dissipation in the two functioning technologies\u2014that is, water electrolysis and the fuel cell\u2014from a practical standpoint, and use classical kinetics to analyze the key barrier of the electrochemical reactions occurring on the catalyst surface. With the aid of scaling relations among reactive intermediates, we develop a framework to understand catalytic trends, which ultimately provides rational guidance toward the development of improved catalysts for a wide range of reactions. In the third section, we summarize general strategies on designing higher-performance electrocatalysts and discuss their advantages and drawbacks. Featured examples of state-of-the-art electrocatalysts that have been achieved for each reaction by structural design are presented in this part, demonstrating the successful combination of synthetic chemistry, electrocatalytic chemistry, and computational chemistry. The last section outlines the key scientific problems in the electrochemical hydrogen\u2013water conversion system and provides further development direction for catalyst design for a renewable and clean energy system with a high energy efficiency.As illustrated in Fig. 1\n, two different functioning technologies are involved in this renewable and clean energy system: water electrolysis and the fuel cell. The two electrolyzers are mainly composed of four parts: the electrolyte (e.g., H2O), the ion-exchange membrane (e.g., a Nafion membrane), the anode electrode, and the cathode electrode [23,24]. In order to accelerate water splitting, the two electrodes are always coated with a highly active and stable catalyst layer. In the water electrolyzer, electrical energy is consumed to split water into gaseous hydrogen (H2) and oxygen (O2). Taking acid water electrolysis as an example, water is oxidized to form oxygen molecules and protons at the anode, as the electrons pass through the external circuit and the protons pass through the membrane down to the cathode. Meanwhile, the protons and electrons combine at the cathode to form hydrogen molecules.The electrochemical reaction that occurs in the fuel cell is the exact opposite of the water electrolysis process [25]. A spontaneous \u201ccold\u201d combustion of hydrogen and oxygen occurs in the fuel cell device, in which hydrogen acts as the fuel and oxygen acts as the oxidizer. In general, hydrogen diffuses to the anode surface by penetrating through the electrode pores. Through the catalytic action of the catalyst layer, the adsorbed hydrogen is ionized and releases an electron at the electrode. Next, the hydrogen ions that pass through the electrolyte and the electrons that pass through the external circuit all reach the cathode, recombine with oxygen molecules to form water molecules, and release electricity. The heat generated by the internal reaction and electrical resistance can be removed by applying suitable water or air cooling systems. Table 1\n summarizes the half-cell reactions of water electrolysis and the fuel cell in different media. The four reactions can be grouped into two reversible reaction couples: hydrogen-involving HER and HOR with an equilibrium potential (U\n0) of 0\u00a0V versus reversible hydrogen electrode (vs RHE); and oxygen-involving ORR and OER with a U\n0 of 1.23 V vs RHE. From a chemical point of view, hydrogen\u2013water conversion is composed of two redox couples: The water/oxygen couple at high potential and the water/hydrogen couple at relatively low potential [26]. In acidic media, hydrated protons transfer charges from the anode to the cathode, while hydroxide ions act as charge carriers from the cathode to the anode in alkaline electrolytes.According to the thermodynamics of hydrogen\u2013water conversion, reversible water electrolysis and the H2\u2013O2 fuel cell have the same electrical onset potential of 1.23\u00a0V under standard conditions. However, the actual onset potential for the two reactions is far from the standard electrical potential. Under practical operating conditions, the cell voltage of the H2\u2013O2 fuel cell and of water electrolysis is always below 0.9\u00a0V and higher than 1.8\u00a0V, respectively, even utilizing state-of-the-art noble metals as the electrocatalysts [2,27\u201329]. In practice, in order to drive the electrochemical reaction process, there are a number of barriers that must be overcome, including the electrical resistance of the circuit, activation energies of the electrochemical reactions, blockage of the electrode surfaces by the product gas bubbles and water, and ionic transfer resistances across the electrolyte solution [24]. These barriers, which require a sufficient electrical energy supply, greatly reduce the energy conversion efficiency and cause the work potential to fall short of the thermodynamic potential, which is the so-called phenomenon of polarization (or overpotential, or overvoltage) [30]. Fig. 2\n shows the resistances (i.e., the barriers) in a typical cell with a liquid electrolyte [31,32]. The first resistance from both ends (R\na\n\u03a9 and R\nc\n\u03a9) is the external electrical circuit resistance, which includes the internal resistance of the wiring and connections at the anode and cathode, and the resistance of the electrons across the catalyst layers, which are not always good electronic conductors [33]. R\na\nct and R\nc\nct originate from the overpotential of the half-cell reactions on the surface of the anode [34]. R\na\nD and R\nc\nD are caused by the diffusion layers close to the electrode surface when mass transport phenomena are involved or gaseous species are formed [35]. As for water electrolysis, the partial coverage of the electrode surface by generated bubbles hinders the contact between the electrode and electrolyte. Similarly, the product water acts as a blockage between the electrode and the H2 and O2 input, inducing a resistance of mass transport in an alkaline fuel cell (AFC). R\nions originates from the transport of ions in the electrolyte and R\nsep stems from the resistance of the cell separator [36]. A similar situation is found in other types of cells, such as zero-gap and proton-exchange membrane (PEM) cells [37,38].The resistances in cell systems can be classified into three categories: activation resistances (losses due to electrochemical reactions), ohmic resistances (losses due to ionic and electronic conduction), and concentration resistances (losses due to mass transport). Each of the three major losses contributes to the characteristic shape of the current\u2013voltage (i\u2013V) curve of the electrochemical cell [39,40]. Fig. 3\n shows a typical i\u2013V curve for water electrolysis and for a fuel cell. In the case of water electrolysis (Fig. 3(a)), the current starts to flow across the cell above the thermodynamic electrolysis voltage of 1.23\u00a0V. Additional voltage is required to overcome the resistances discussed above. At low current densities, the voltage drops caused by ohmic resistances are small, and the reaction activation overvoltage accounts for the dominant part of the voltage drop. The logarithmic shape of the polarization curve (the Tafel area) is attributed to the charge transfer phenomena at the anode and cathode. As the overvoltage increases further, the reaction activation barrier decreases, and the shape of the polarization curve becomes linear. This linear shape indicates that the ohmic resistance is now the key kinetic parameter of the cell. In the case of the fuel cell (Fig. 3(b)), the activation resistances mostly affect the initial part of the curve, the ohmic resistances are mainly apparent in the middle section of the curve, and the concentration resistances are significant in the tail. Although the reactions that occur in the two functioning technologies are reversible, the shapes of the i\u2013V curves are not the same: the i\u2013V curve for water electrolysis generally obeys the Butler\u2013Volmer model even at very high overpotentials, while the i\u2013V curve for a fuel cell tends to show a constant value at high overpotentials due to the limitation in the mass transfer rate.To improve the energy efficiency of the two electrochemical cells and thus improve the performance of the energy system, an understanding of these resistances must be grasped in order to minimize them. Ohmic losses are caused by the electrode material\u2019s resistance to the electron flow and the electrolyte\u2019s resistance to the ion flow; these can be reduced by utilizing highly conductive materials as the wiring and electrode substrate, and by diminishing the distance between the two electrodes [41\u201343]. The concentration losses, which are attributed to mass transport, can be relieved by increasing the pressure of the gaseous reactants or the concentration of the liquid electrolyte [44,45]. The two kinds of voltage drops mainly depend on the cell design and operation conditions. In addition to the above two resistances, the majority (>\u202f60%) of the voltage drop in an electrochemical cell is induced by the Gibbs free-energy change for the endergonic transformation of the half-cell reactions [39]. Depending on the direction of the reactions, the activation polarizations greatly increase or decrease the anode voltage where the oxidation reaction takes place, and decrease or increase the cathode voltage where the reduction reaction occurs.In electrochemistry, the Butler\u2013Volmer relationship is used as the primary departure point to relate the overvoltage \n\n\u03b7\n\n across a metal\u2013electrolyte interface to the current density j (in A\u00b7cm\u22122) across this interface [46]:\n\n(1)\n\n\nj\n=\n\nj\n0\n\n\n\n\n\n\ne\n\n\n\u03b1\nn\nF\n\u03b7\n/\n(\nR\nT\n)\n\n\n-\n\n\ne\n\n\n-\n(\n1\n-\n\u03b1\n)\nn\nF\n\u03b7\n/\n(\nR\nT\n)\n\n\n\n\n\n\n\n\nwhere \n\n\u03b7\n\n is the overvoltage\u2014that is, the difference between the actual voltage across the interface and the equilibrium voltage; \n\n\nj\n0\n\n\n is the exchange current density in A\u00b7cm\u22122; \n\n\u03b1\n\n is the coefficient of the charge transfer; \n\nn\n\n is the number of electrons transferred in the electrochemical reaction; F\u00a0\u2248\u00a096\u00a0485\u00a0C\u00b7mol\u22121 is the Faraday constant; \n\nR\n\n is the constant of a perfect gas (0.082\u00a0J\u00b7(K\u00b7mol)\u20131); and \n\nT\n\n is the absolute temperature in K. The Butler\u2013Volmer equation basically reveals that the current produced by an electrochemical reaction increases exponentially with the activation overvoltage and exchange current density. In fact, improving the reaction energy efficiency focuses on increasing \n\n\nj\n0\n\n\n, which represents the \u201crate of exchange\u201d between the reactant and product at equilibrium. Taking the forward reaction for simplicity and including the concentration effects, \n\n\nj\n0\n\n\n is defined as follows:\n\n(2)\n\n\n\nj\n0\n\n=\nn\nF\nc\nf\n\n\ne\n\n\n-\n\u0394\n\nG\n\na\nc\nt\n\n\n/\n(\nR\nT\n)\n\n\n\n\n\nwhere \n\nc\n\n is the reactant surface concentration, f is the decay rate to products, and \u0394G\nact is the activation energy barrier for the forward reaction. Eq. (2) clearly shows that decreasing the size of the activation energy barrier (\u0394G\nact) will increase \n\n\nj\n0\n\n\n under a given environmental condition. In the actual reaction, only species in the activated state can undergo the transition from reactant to product. In fact, the activation energy of the reactions is strongly influenced by the electrode material [47]. A catalytic electrode is a site for species activation and transition. Using a highly catalytic electrode can significantly lower the activation barrier for the reaction, and therefore provides a way to dramatically increase \n\n\nj\n0\n\n\n. To reduce the activation energy of the electrode reactions, continuing research efforts are focusing on the design of efficient catalytic electrode materials based on an understanding of the relationship between the activation energies, electrode materials, and surface configurations.In terms of the reaction mechanism, the activation energy barrier (\u0394G\nact) can be quantified by the Gibbs free-energy change (\u0394G\nmax) for the rate-determining step (RDS) at the equilibrium potential, and its theoretical value on different catalytic materials can be calculated by means of density functional theory (DFT) calculation. In this way, the relationship between the activation energy and the electrode material is built, as a volcano-shaped plot is obtained by plotting \n\n\nj\n0\n\n\n versus \u0394G\nmax. The most common shape of the volcano plot is the HER rate description based on the Langmuir type of adsorption with the maximum located near the position where the hydrogen adsorption free energy (\u0394G\nH*) is zero [48]. In the HER, the reaction species is first adsorbed on the catalyst surface to form the reaction intermediate (M\u2013Hads). After the aforementioned Volmer step, hydrogen molecules can be formed by the coupling of an electron and a proton in the electrolyte through a Heyrovsky step, or their direct combination via a Tafel step [30,49,50]. As a result, the \u0394G\nH* is the overall decisive rate for HER [51,52]. In recent years, the DFT-calculated \u0394G\nH* has been widely used as the activity descriptor [53\u201355], for many traditional metals, metal composites/metal alloys, and nonmetallic materials. As shown in Fig. 4\n(a), different metals show significant differences in HER exchange current density, and the highly active metals (e.g., Pt) located near the top of the volcano plot possess optimal \u0394G\nH*\n[56]. If the catalytic material has a weak adsorption force on hydrogen, the hydrogen atom can barely be absorbed on the surface of the material, and the overall reaction rate is determined by the adsorption step of hydrogen (Volmer step). On the other hand, a too-strong adsorption of hydrogen atoms onto catalytic materials results in difficulty breaking the M\u2013Hads bond to form H2, and the RDS is the desorption step (Heyrovsky/Tafel). As the reverse process of HER, the RDS of HOR is the dissociative adsorption of H2 on the catalyst surface, which involves electron transfer from the surface to the \u03c3* antibonding orbital of the H2 molecule [57]. Consequently, the interaction of M\u2013Hads also plays a dominant role in the kinetics of HOR, and the activity of HOR follows the same trend as HER on noble metal surfaces due to the high reversibility of these two reactions (Fig. 4(b)) [58\u201361].In addition to the hydrogen-involved reactions, the relationship between \n\n\nj\n0\n\n\n and \u0394G\nmax can be applied to the oxygen-related reactions that occur in hydrogen\u2013water conversion. As shown in Figs. 4(c) and (d), the shapes of volcano plots for these reactions are quite similar, except for the reaction intermediates determining the reaction rate. In general, the ORR includes either a four-electron pathway to reduce oxygen to water, which is attractive for fuel cells, or a two-electron pathway, which is desirable for producing hydrogen peroxide [62]. In fact, a direct four-electron mechanism can either be a dissociative or associative process, depending on the oxygen dissociation barrier on the catalyst surface [63]. As a result, the oxygen adsorption strength (\u0394G\nO*) is associated with the ORR activity to construct a volcano plot (Fig. 4(c)) [63,64]. For metals that bind oxygen too strongly, the reaction rate is limited by the removal of O* or OH* species. For metals that bind oxygen too weakly, the reaction rate is limited by splitting of the O\u2013O bond in O2 (dissociative mechanism) or, more likely, by the transfer of electrons and protons to the adsorbed O2 (associative mechanism), depending on the applied potential [63]. As indicated by the volcano plot in Fig. 4(c), there seems to be some room for improvement, as even platinum (Pt) is not at the absolute peak. Metals with a somewhat lower oxygen-binding energy than Pt should have a higher ORR activity. Based on the above thermodynamic volcano plot, Viswanathan et al. [65] and Hansen et al. [66] developed a microkinetic model for ORR given that the OH binding energy is varying. They found a kinetic activity volcano that is in close agreement with the thermodynamic activity volcano, and identified an activity optimum at a 0.1\u00a0eV weaker O* binding than Pt(111) for the reduction of O2 to H2O through a four-electron pathway.The OER volcano plot has a long history starting in 1984, when Trasatti used the transition enthalpy from the lower to higher oxidation state of metal in metal oxides as a descriptor for the OER electrocatalytic activity of oxide electrodes [67]. That pioneering work viewed the OER process as a transition between two different configurations of the surface coordination complex. Accordingly, all metal oxides that are difficult or easy to oxidize are not very active for the OER. Difficult oxidization means that the intermediates are weakly adsorbed; therefore, water dissociation is the RDS. On the other hand, easy oxidization indicates that the intermediates are strongly adsorbed, and the removal of the O* or OH* species is the RDS. In this case, the OER reactivity has been related to the oxygen adsorption free energy \u0394G\nO*\n[68,69], as in the case of ORR. However, the single descriptor of \u0394G\nO* for OER activity is incomplete, as the four-electron OER involves multiple intermediates (OOH*, OH*, and O*), the binding energies of which are strongly correlated and can hardly be decoupled [63,70]. A linear scaling relation exists between the binding energies of the different surface intermediates [70]; that is, if the energy associated with one reaction step is changed, the energies of the others also change. Thus, Man et al. [70] took the difference between the energy states of two subsequent intermediates (\u0394G\nO*\u2212\u0394G\nOH*) as a descriptor for the catalytic activity of several compounds, including rutile, perovskite, spinel, rock salt, and bixbyite oxides (Fig. 4(d)), whose activity obeys the volcano plot quite well. In fact, the binding energies of OH* and OOH* (whether in OER or ORR) are related to each other by a constant energy value of approximately 3.2\u00a0eV in broad classes of metal oxide materials, regardless of the binding site [70,71]. As a result of this non-ideal scaling between OOH* and OH*, a real catalyst generally shows a minimum theoretical overpotential of 0.3\u20130.4\u00a0V [63,72,73], even for materials at the top of the OER and ORR volcano plots, including the extensively studied RuO2 for OER [70] and Pt-based catalysts for ORR [74].It is noticeable that the volcano plot appropriately demonstrates the Sabatier principle [75]; that is, an ideal catalyst should bind the reaction intermediates neither too weakly nor too strongly. In other words, optimal catalytic activity can be achieved using a catalyst surface with appropriate binding energies for reactive intermediates. To be specific, the best approximation to an ideal HER/HOR catalyst would be a material that is capable of minimizing the absolute value of \u0394G\nH*, and the ideal ORR and OER catalysts would be able to optimize the \u0394G\nO* and \u0394G\nO*\u2212\u0394G\nOH*, respectively. In fact, aside from decreasing the activation barrier, there is another significant way to increase \n\n\nj\n0\n\n\n, which is not apparent from Eq. (2): that is, to increase the number of possible reaction sites per unit area [76\u201379]. \n\n\nj\n0\n\n\n represents the current density, or the reaction current per unit area, and the area for current density is generally based on the projected geometric area of an electrode. The true electrode surface area of an electrode with an extremely rough surface can be orders of magnitude greater than the geometric electrode area, and can thus provide many more reaction sites. Therefore, the effective \n\n\nj\n0\n\n\n of a rough electrode surface will be greater than that of a smooth electrode surface. Another simple way to increase the density of active sites is to enlarge the amount of catalyst on a given electrode. However, an excessive amount of catalyst will hinder the charge and proton transfer on the electrode surface. As a result, the activity of the electrode does not increase linearly with the amount of catalyst.In conclusion, there are two general methods for increasing the activity (or rate of reaction) of an electrocatalyst system: \u2460 improving the intrinsic activity of each active site; and \u2461 increasing the density of active sites on a given electrode. Both methods have pros and cons. The difference in intrinsic activity between different catalysts may be more than ten orders of magnitude, while the difference in activity caused by catalyst loading will be only 1\u20133 orders of magnitude. Improving the intrinsic activity of each active site is the most fundamental and effective way to achieve high activity, and its realization must be based on a deep understanding of the reaction mechanism and material properties. Increasing the number of active sites is an easier strategy, but the activity growth is limited. At the same time, activity promotion by increasing catalyst loading is obtained through the sacrifice of increasing the electrode cost and the charge and proton transfer blockage. In practice, the two methods are not mutually exclusive and can ideally be implemented simultaneously, thus greatly enhancing the activity of catalyst.It is well known that the current density of a catalyst increases as the density of the active sites increases. Exposing more active sites is important in achieving high catalytic performance. Nanoarchitecture has been considered to be the most effective strategy, as it allows the density of active sites to be directly enriched and utilized, and thus efficiently optimizes the electrocatalytic activity [80\u201384]. The relation between the actual active surface area and the overall performance of an electrocatalyst was first recognized in transition-metal alloy systems. As early as 1982, Brown et al. [85] found that an alloy surface is generally rougher than that of a single metal, and can provide more active sites for a catalytic reaction. With the aid of nanostructuring and selective etching of molybdenum (Mo) in Ni\u2013Mo alloys [85\u201387], the surface area of Ni\u2013Mo alloys greatly increased, resulting in an obvious improvement in catalytic reactivity. With the rapid development of synthesis techniques, a series of electrocatalytic nanomaterials with different morphologies have been achieved in the past decade, including nanocages, nanofibers, nanoflowers, nano-foam, nano-nets, nano-needles, nano-rings, nano-shells, and nanowires [77,88\u201391]. Faber et al. [92] reported metallic cobalt disulfide (CoS2) as a highly active catalyst for HER, and demonstrated the crucial role of geometric structure in determining its overall catalytic performance. Compared with the common morphologies of nanoparticles and nanofilm, an increase in active surface area drastically improves the HER catalytic performance of microstructured and nanostructured electrodes (Fig. 5\n), endowing the CoS2 nanowire electrodes with overpotentials as low as 145\u00a0mV for driving a current density of \u221210\u00a0mA\u00b7cm\u22122. In addition, nanostructuring possesses a dual function in terms of both operational stability and reaction rate through the facilitation of mass transport and the removal of generated gas bubbles or water from the catalyst surface. Our group synthesized Mo2C/C with a two-dimensional (2D) lamellar structure via a controllable synthesis using a self-assembly and pre-shaping strategy [93]. The highly dispersed Mo2C nanoparticles and the 2D lamellar structure effectively boosted the mass and charge transfer across the Mo2C active sites, facilitating the electrochemical HER process. Moreover, our group further synthesized a series of three-dimensional (3D) nanostructured catalytic materials, including the NiCo2(SOH)\nx\n nanoflower [94], the coral-like FeNi(OH)\nx\n\n[95], the Ni\u2013VC nanoboscage [96], Ni\u2013Mo2C nanowire [97], and the Ni(OH)2@Ni2P nanopillar [98]. All these materials possessed highly active surfaces, fast electron transfer, and gas escape channels, which are beneficial for catalyzing water electrolysis.To be specific, a rapid loss resulting from water flooding may take place in ORR catalysts, apart from deactivation during a long operation time [99,100]. Water flooding will interrupt the O2 supply to active sites as the porous channels are obstructed by the accumulation of water, resulting in the termination of ORR in the flooded region [101,102]. In order to quantify the mass transfer and anti-flooding performance with the pore characteristics of electrocatalysts in a fuel cell, Wang et al. [103] designed a special \u201crattle-drum\u201d-like work electrode for ORR catalysts. Benefiting from a bigger pore volume and regular pore arrangement, the dual-porosity Pt/C catalyst exhibited four times the quantified mass transfer and anti-flooding efficiency of a commercial catalyst. In fact, different types of pores have special functions in the ORR process. Mesopores and macropores may be significant in mass transport during the ORR process [104\u2013106], while micropores are beneficial for hosting most catalytic sites [107,108]. With the purpose of constructing hierarchically porous structures, the sacrificial template method has been widely adopted, using silica colloid [105,109\u2013111], ordered mesoporous silica [106,112\u2013114], polystyrene microspheres [115], and some other oxides [116,117] as templates. For example, a colloidal silica template was used by Liang et al. [118] for the synthesis of N-doped carbon catalyst with a high specific surface area of 1280\u00a0m2\u00b7g\u22121, a hierarchically porous structure with meso/micropore distribution. However, the subsequent removal of the template can be time consuming and usually requires the usage of a strong acid or alkaline solution, which is dangerous to researchers and harmful to the environment. To avoid these disadvantages, our group developed a morphology-controlled approach using NaCl as the template, in which the template can be removed using hot water and then recycled [119\u2013122]. A nanostructured polyaniline (PANI) with a special structure was encapsulated in the NaCl crystal via salt recrystallization, and then accurately converted into a carbon nanomaterial under high temperature (Fig. 6\n). Moreover, a mass of pores was created in the carbon nanomaterial by gasification in a closed nanoreactor. Due to the multiple types of pores and high utilization of active sites, the 3D Fe/N\u2013C catalysts exhibited excellent catalytic performance toward the ORR.Facet engineering is another widely studied method to modulate the catalytic performance of materials for a given reaction. The reactivity of catalytic materials is highly related to their exposed facets because the adsorption strength of the intermediate species of the catalytic reaction varies greatly on different surface facets of the catalyst. Facets are always denoted by Miller indices. The exposed facet(s) of a nanomaterial strongly correlate to the shape of the nanoparticle [123,124]. In general, faceted nanomaterials can be categorized into low-index and high-index faceted types [125]. Low-index facets are those for which the sum of the three components of the Miller indices (hkl) is small, whereas a high-index facet contains at least one Miller index greater than unity.The structural sensitivity of the reactivity of low-index faceted nanomaterials has been demonstrated with regard to single-crystal Pt for HER. The surface morphology of Pt(hkl) with well-defined surfaces was confirmed by scanning tunneling microscopy (STM, Fig. 7\n), and the degree of activity was observed to follow the order (110) > (100) > (111) in alkaline solution [126]. Importantly, the activity in alkaline and acid electrolyte was found to differ substantially [127\u2013129]. This pH effect involved structure\u2013function relationships in the HER, and was further studied by Strmcnik et al. [130] with a focus on both Pt(111) and Pt(111) modified by Pt islands. Compared with the HER activity of the pristine Pt(111) surface, the HER on the Pt islands/Pt(111) electrode was found to be 5\u20136 times more active in alkaline solution, but only 1.5 times more active in acid electrolyte. The effect of pH on HER activity was shown to be due to the special ability of edge-step sites to dissociate water [131\u2013135]. In addition, the activity order of Pt(hkl) is in line with the density of low-coordinated Pt atoms due to the accelerated water dissociation step on their metal surfaces [136\u2013139].A similar phenomenon was observed in the case of ORR activity on single-crystal Pt surfaces following a descending order in nonadsorbing HClO4 electrolytes [140]. However, when the electrolyte was replaced by H2SO4, Pt(100) was found to be more active than Pt(111) [141\u2013143]. This activity difference was attributed to the special adsorption behaviors of the bisulfate anion on Pt(111). The bisulfate anions can be adsorbed on the Pt(111) surface more strongly than on Pt(100), resulting in an impeditive ORR process. Systematic investigations confirmed that the different properties of the respective facets have a significant effect on their catalytic performance. Following this work, many studies focused on developing a facet-control method to construct more active facets on a catalytic surface, from ideal single-crystal metals to more practical nanomaterials [144\u2013150]. Narayanan and El-Sayed [144] were the first to demonstrate facet-controlled synthesis in the case of Pt nanocrystals, including (100)-terminated nanocubes, (111)-bounded nanotetrahedra, and nanospheres with both (111) and (100) facets. A high-temperature organic phase method was reported by Wang et al. [145] for the synthesis of monodispersed (100)-terminated Pt nanocubes for ORR. The facet-controlled Pt nanocubes showed a specific ORR activity that was more than double the activity of a commercial Pt catalyst in acid electrolyte.Because of the higher density of low-coordinated atoms, steps, edges, and kinks, metals and compounds with high-index facets generally possess greatly improved reactivity in comparison with typical low-index materials [151]. However, such high-index facets are thermodynamically instable due to their higher surface energy [152,153]. Accordingly, the synthesis of high-index faceted nanomaterials has become a herculean task. In the last few years, a wide variety of synthetic protocols have been exploited for the synthesis of metallic nanomaterials with high-index facets for the purpose of improved catalytic reactivity. Yu et al. [154] developed a simple reduction route in aqueous solution to prepare Pt concave nanocubes (c-NCs) enclosed by high-index facets of (510), (720), and (830). Pt c-NCs have also been fabricated by using glycine to manipulate the reduction kinetics of H2PtCL6\n[155]. Employing electrochemical means, Tian et al. [156] synthesized tetrahexahedral (THH) Pt nanocrystals with facets including (730), (210), and (520). In addition to these single-metal materials, multi-metallic high-index faceted nanocrystals have been developed [157\u2013160]. As shown in Fig. 8\n, Luo et al. [159] reported a new class of Pt3Fe zigzag-like nanowires (z-NWs) with stable high-index facets and a nanosegregated Pt-skin structure. These unique structural features endowed the Pt-skin Pt3Fe z-NWs with a mass and specific ORR activity of 2.11\u00a0A\u00b7mg\u22121 and 4.34\u00a0mA\u00b7cm\u22122, respectively, at 0.9\u00a0V vs RHE.With the purpose of reducing catalyst cost, the design and preparation of low-cost metal nanomaterials with different exposed reactive facets have been a recent trend in facet engineering [161\u2013167]. Su et al. [168] studied the growth mechanism of the NiO crystal and found that the surface energy of the NiO facets followed the order of (100)\u00a0<\u00a0(113)\u00a0<\u00a0(101)\u00a0\u2248\u00a0(110). Han et al. [161] provided a template-free hydrothermal method for the controllable fabrication of a surface-tailored Co3O4 nanocube (NC), nanotruncated octahedron (NTO), and nanopolyhedron (NP), with facets of (001), (112), (001) and (111). The different crystal planes endowed the Co3O4 nanocrystal with different exposed surface atomic configurations of the Co2+ and Co3+ active sites. The unusual (112) plane-enclosed Co3O4 nanoparticle on reduced graphene oxide (rGO) with abundant Co3+ sites exhibited superior activity for both OER and ORR. In addition to these metal oxides, many other metal compounds with special facets have been reported. Feng et al. [163] further synthesized Ni3S2 nanosheet arrays with stable (2\n\n\n1\n\u00af\n\n\n0) facets, and demonstrated them to be efficient and ultra-stable electrocatalysts for HER and OER. Wang et al. [162] obtained flower-like nickel phosphide with different crystalline structures (Ni5P4 and Ni2P) and ascribed the excellent HER activity to the hierarchical structure with high-energy (001) facets.Apart from tuning the exposed facets of the nanocrystal, modulating the atomic scale arrangement (i.e., the transformation of the crystalline phase) can affect the intrinsic activity of a catalyst, due to the fundamental changes in its physical and chemical properties. Transition-metal disulfides with several unique polymorphs are typical cases that have been widely studied. Among these polymorphs, the metastable 1T phase has recently aroused great research interest owing to its metallic behavior, which is beneficial to electrocatalytic processes [169\u2013174]. Lukowski et al. [169] synthesized metallic 1T-MoS2 nanosheets from semiconducting 2H-MoS2 via a lithium intercalation method. The resulting 1T-MoS2 exhibited a dramatic improvement in electrocatalytic HER performance compared with the corresponding 2H polymorphs (Fig. 9\n). Similarly, metallic 1T tungsten disulfide (1T-WS2) was further synthesized by Lukowski et al. [173] via a simpler microwave-assisted intercalation. The polymorph engineering endowed the 1T-WS2 with faster electrical conductivity and more intensive active sites, boosting its HER activity.Not only can unique properties be induced by atomic arrangement, but the active sites of 1T-catalysts may also differ from those of the traditional 2H-structured phase. In this context, Voiry et al. [175] obtained highly conductive 1T-MoS2 nanosheets with excellent HER activity after removing excess negative charges from the surface of chemically exfoliated MoS2 nanosheets. Interestingly, after partial oxidation of the 1T- and 2H-MoS2, a sharp contrast in HER activity changes was observed. Although there was almost no shift in the HER activity of 1T-MoS2 after edge oxidation, the activity of 2H-MoS2 seriously decreased. It is well known that the edges of the usual 2H-MoS2 crystal are the main active sites for HER. The significant difference in HER activity between the partially oxidized 1T- and 2H-MoS2 nanosheets revealed that the main active sites of 1T-MoS2 for driving HER catalysis are not the edges of the nanosheets, but the basal planes of the nanosheets.The catalytic performance of metal oxides may also vary with their crystal phases. Our group found that reversing spinel crystalline structure has a great influence on the ORR catalytic activity of spinel (Fig. 10\n) [176,177]. By adjusting the iron (Fe) content, the spinel structure of a Co\u2013Fe-based crystal can be changed from its normal structure to the inverse structure and then back again [178]. The electrochemical results revealed that the inverse spinel {Co}[FeCo]O4/NG (nitrogen doped graphene) had the best ORR activity, outperforming commercial Pt/C. DFT results further disclosed that the higher ORR activity of the inverse-structured {Co}[FeCo]O4 could be ascribed to the modulated oxygen adsorption energy and elongated adsorbed oxygen bond induced by the dissimilarity effect of Fe and Co atoms at the octahedral site. The effect of crystal phase on the reaction pathway of ORR has also been studied. Karunagaran et al. [179] synthesized four kinds of iron oxide nanoparticles with different phases incorporated inside 3D rGO aerogels and determined their electrochemical, catalytic, and electron transfer properties for ORR. The results showed that ORR was catalyzed by all four catalysts via a two-electron pathway under higher potentials (0.70\u00a0V). On the other hand, when the potentials decrease to 0.20\u00a0V, rGO composites containing magnetite, maghemite, and goethite proceeded via four-electron transfer kinetics, whereas the hematite-containing composite went through two-electron transfer kinetics.Configuration distortion induced by the Jahn\u2013Teller effect for transition-metal compounds has also been studied in relation to the electrocatalytic performances of such compounds [180]. Recently, Liu et al. [181] observed an obvious structural distortion in Co3S4 atomically thin nanosheets (CSATNs) via the ultrasound exfoliation treatment of an intermediate Co3S4/TETA hybrid precursor. The structural distortion of CSATNs generates an electronic configuration change. Compared with bulk samples, the shift from spectral to lower magnetic fields (Figs. 11\n(a) and (b)) implies that the spin state of Co3+ in the octahedral sites (t2g\n4eg\n2) of CSATNs adjusts from low spin to high spin. High-angle annular dark field (HAADF) images showed that the octahedral coordinated cations were solely exposed in the planes, which further revealed the existence of Jahn\u2013Teller elongation (Figs. 11(c)\u2013(f)). Due to the synergistic adjustment in the atom and electron configuration, CSATNs possess significantly enhanced OER performance in comparison with bulk samples. In fact, the Jahn\u2013Teller effect is attributed to the uneven electron distribution of the central ions in degenerate d orbitals (t2g or eg). Thus, the filling state of electrons in the eg orbital may have a significant role in the catalytic properties of the transition-metal compounds. A volcano relationship between the intrinsic ORR activity and the filling states of the eg orbital in the B ions of perovskite-based oxides (ABO3) was discovered by Suntivich et al. (Fig. 11(g)) [182]. The perovskite-based oxides with only one electron filling in the eg orbital (defined as eg\u00a0\u2248\u00a01) were demonstrated to possess the highest ORR activity, as O2 can adsorb on the B sites end-on with an optimal binding energy. The eg occupancy theory can be further extended to spinel oxides, although the ORR active sites of spinel are not tetrahedral sites but octahedral sites (Fig. 11(h)) [183].Amorphization to modulate the atomic scale arrangement, and thus increase the catalytic performance, is another research hotspot [184\u2013189]. Short-range atomic arrangements of amorphous phases are beneficial for increasing the density of active sites [190\u2013194]. As early as 1995, Weber et al. [190] investigated the structural units of the amorphous compound MoS3 and found that all molybdenum is present in the Mo4+ oxidation state, while sulfur atoms occur in two different types of coordination: S2\u2013 and S2\n2\u2013. Merki et al. [191] and Benck et al. [192] then confirmed that the amorphous MoS2 is more active in catalyzing HER. Structural measurements demonstrated that an amorphous MoS\nx\n film is extremely rough in surface and sulfur-rich in composition, resulting in a large active area and intensive active sites for HER catalysis. Benck et al. [192] further revealed that the increase in the HER activity of amorphous molybdenum sulfide is contributed to by the large number of active sites caused by the amorphous structure and rough, nanostructured morphology, as the activity scales with the electrochemically active surface area. Meanwhile, Li et al. [195] and Li et al. [196] systematically studied the origin of the catalytic activity of the amorphous MoS2 in terms of the composition and crystallinity. Interestingly, the experimental results revealed that the crystallinity is crucial for determining the catalytic performance, whereas the composition is not particularly significant.In addition to the HER catalysis, Smith et al. [185] demonstrated that amorphous materials are more active than the comparable crystalline materials for OER catalysis, based on a study of the mixed-metal oxides of iron, nickel (Ni), and cobalt (Co). Due to the amorphous structure, the distribution of the metals in the amorphous films is homogeneous and their compositions can be accurately controlled. Modulated a-Fe100-\n\ny\n\n-\n\nz\nCo\ny\nNi\nz\nO\nx\n with an optimal element content exhibited excellent catalytic property that was even comparable to that of commercial noble metal oxide catalysts. Thanks to the controllable composition of amorphous materials, the effect of metal composition on electrocatalytic performance can be further studied along with the effect of amorphization. Smith et al. [184] prepared 21 complex metal oxide films for electrocatalytic water oxidation, and demonstrated the excellent stoichiometric concentrations of Fe, Co, and Ni in each sample. Structural characterization and electrochemical measurement confirmed that iron content is important for lowering the Tafel slope, and that cobalt or nickel are beneficial in reducing the overpotential (Fig. 12\n). For scale-up production, Kuai et al. [186] proposed an aerosol-spray-assisted method by which amorphous mixed-metal oxides can be sustainably obtained, which is very suitable for industrial applications. The obtained Fe6Ni10O\nx\n exhibited a low overpotential of 0.286\u00a0V for driving 10\u00a0mA\u00b7cm\u22122 and a small Tafel slope of 48\u00a0mV\u00b7decade\u22121 for the electrochemical OER, exceeding the best catalytic performance of all investigated Fe\u2013Ni\u2013O\nx\n series.Although the active sites of amorphous catalysts can be greatly enhanced by amorphous engineering, the electrical conductivity of the amorphous materials will be decreased due to short-range disorder in the crystal structure. Coupling these low-conductive materials with highly conductive materials is an effective route to guarantee the excellent electrocatalytic performance of amorphous catalysts. For example, Lee et al. [197] synthesized amorphous MnO\nx\n nanowires supported by Ketjenblack (KB) carbon as highly efficient ORR electrodes. The low-cost and highly conductive KB acts as a supporting matrix for the catalyst, greatly accelerating the electron transfer during the electrocatalytic processes. Many other amorphous/conductive composite materials, such as amorphous MoS\nx\n/carbon composite catalyst [198], amorphous MoS\nx\n/polypyrrole copolymer film (PPy/MoS\nx\n) [199], and amorphous MoS\nx\n/N-doped CNT (NCNT) forest hybrid catalyst [200], have also been reported. The highly conductive skeletons in these composite materials can overcome the barriers induced by the low electrical conductivity of amorphous catalysts, leading to a remarkable increase in catalytic activity (Fig. 13\n). Porous metal nanostructures, such as Ni foam [201] and nanoporous gold [202], are also used as conductive substrates to support an amorphous MoS\nx\n catalyst, of which the HER activity can be significantly enhanced.Defects exist widely in nanomaterials. It has been realized that the surface of catalysts with defects always exhibit higher reactivity than the defect-free sites [203\u2013206]. Accordingly, defect engineering has gradually developed as an effective method to tune the electronic and surface properties of nanomaterials [207\u2013209]. Cheng et al. [210] synthesized tetragonal or cubic M\nx\nMn3\u2013\n\nx\nO4 spinels by reducing the amorphous MnO2 in aqueous M2+ solution under ambient conditions. Due to its highly active area and abundant defects, nanocrystalline Co\nx\nMn3\u2013\n\nx\nO4 is endowed with considerable catalytic activity for both ORR and OER. Similarly, Ma et al. [211] synthesized oxygen vacancy (OV) defect-rich mesoporous MnCo2O4 materials, and found that their stability and methanol tolerance ability even exceeded those of a Pt/C catalyst. In order to obtain insight into the effect of defects in catalytic performance, our group performed a DFT\u00a0+\u00a0U calculation of OV concentration on the electronic structure of \u03b2-MnO2 catalysts and their catalytic performance for ORR [212]. As shown in Fig. 14\n, a moderate concentration of bulk OVs will greatly increase the electric conductivity of MnO2, while excessive OVs will hinder the ORR process. Such a curvilinear relationship between the electronic structure and OV concentration suggests that the conductivity and ORR catalytic activity of \u03b2-MnO2 can be modulated by the OV concentration. Defect engineering can also be applied to increase the density of active sites of nanomaterials for electrocatalysis [203,213,214]. Xie et al. [203] designed a reaction with a high concentration of precursors and different amounts of thiourea, thus realizing controllable defect modulation in as-formed MoS2 ultrathin nanosheets. Due to the defect-rich structure, many tiny cracks formed on the basal surfaces, resulting in 13 times more active sites for the defect-rich MoS2 ultrathin nanosheets than for the defect-free MoS2.Similar to element vacancy in metal compounds, intrinsic defects in carbon-based electrocatalysts are universal but have been ignored for a long time [209]. Defects easily form after heteroatom doping, and act as the active sites favoring electrocatalysis [215,216]. However, the electrocatalytic reactivity of carbon-based materials has mainly been ascribed to the induced changes of heteroatoms doping. As time goes on, some research has found that the catalytic activity of carbon electrocatalysts with intrinsic defects is even better than that of heteroatoms-doped carbon materials [217,218]. For example, Jiang et al. [219] found that defective carbon nanocages (CNC) possess a high ORR activity that exceeds that of B-doped carbon nanotubes. In this case, defect-rich CNC were successfully synthesized with many typical defect locations, but without any dopants (Fig. 15\n(a)). The electrochemical results indicated that the resultant CNC material with the highest defect density showed the best electrochemical ORR activity (Fig. 15(c)). The DFT results further indicated that the high ORR activities of these defect materials could be attributed to the pentagon and zigzag edge defects (Fig. 15(d)). Zhao et al. [220] used first principles calculations to predict that a type of 585 defect on graphene would be even more active than the N-doped sites for ORR, and obtained strong support for this theoretical prediction through experimental investigations. With the defect mechanism in mind, Zhao et al. [221] prepared a porous carbon (PC) material lacking any elemental doping by carbonizing Zn-MOF at 950\u00a0\u00b0C. With the benefit of the removal of zinc (Zn) atoms, defects could be formed on the PC catalyst, endowing the PC catalyst not only with excellent ORR activity, but also with a stability comparable to that of a commercial Pt/C catalyst. Furthermore, with the exception of the ORR process, the individual electrocatalytic activities for the other three electrochemical reactions in the energy conversion from water to water\u2014that is, HOR, OER, and HER\u2014were demonstrated to be particularly sensitive to the types of defects derived by the removal of heteroatoms from graphene [222].Atomic doping is the most widely used strategy for modulating the properties of catalytic materials. By reasonably introducing one or more metallic or nonmetallic elements into the lattice of the material, the electron structure of the original material can be adjusted, thus effectively improving the catalytic performance of the material [223\u2013232]. Taking MoS2 as an example, many metallic elements such as Ni, Co, Fe, vanadium (V), lithium (Li), and copper (Cu) have been reported to be successfully doped into its crystal structure, positively affecting the physical and chemical properties [171,233\u2013237]. Among these doped metallic elements, Ni and Co tend to locate around the S in MoS2, which will decrease the hydrogen adsorption energy at the S edge and increase the density of the active sites in MoS2\n[233\u2013235]. Unlike Ni- and Co-doped MoS2, V doping cannot increase the number of active sites, but will enhance the conductivity of MoS2\n[236]. Interestingly, our group explored the influence of the Ni-doping of molybdenum carbide on its surface electronic structure and its relationship with HER performance by combining experimental and theoretical evidence [97]. As shown in Figs. 16\n(a)\u2013(d), one-dimensional (1D) NiMo2C nanowire arrays were directly constructed onto conductive 3D Ni foam (NiMo2C/NF) via a facile and controllable strategy combined with hydrothermal and post-carburization treatment. The binder-free integrated NiMo2C/NF electrode showed superb HER catalytic activity in comparison with Mo2C and Ni catalysts (Fig. 16(e)). The DFT calculations clearly demonstrated that the incorporation of Ni into the Mo2C lattice brought about changes in the charge distribution on the catalyst, which resulted in a synergistic effect of Ni and Mo2C that decreased the hydrogen binding energy (Figs. 16(f) and (g)).Apart from metallic elements, research on doping with nonmetallic elements is also very active. Xie et al. [238] successfully synthesized oxygen-doped MoS2 ultrathin nanosheets, on which the synergistic modulations of both the active sites and the conductivity could be rationally realized. According to the DFT calculations, the smaller differential binding free energy of the oxygen-incorporated MoS2 revealed a lower energy barrier for driving the HER process. Our group further proposed a partial phosphorization of metal oxide precursors to construct oxygen-incorporated NiMoP2 with enhanced HER activity [239]. As illustrated in Figs. 16(h)\u2013(i), the H adsorption energy on the NiMoP2 surface was optimized by the oxygen incorporation, as the \u0394G\nH* of the O\u2013NiMoP2 is much closer to zero than its undoped equivalent. In addition, the Ni and Mo in O\u2013NiMoP2 possessed more positive charge, which was beneficial for adsorbing and activating water molecules, greatly accelerating the water dissociation in the alkaline HER.Aside from these metal compounds, carbon-based materials have been intensively used as doping objects, greatly enlarging the scope of catalyst research [240\u2013245]. Early in 2013, our group reported a phosphorus (P)-doped graphene with an ORR catalytic performance comparable to that of commercial Pt/C [246]. Furthermore, N, P dual-doped graphene/carbon materials were prepared as electrocatalysts for both ORR and OER, and their catalytic activities exceeded those of the benchmark Pt/C catalyst [247]. In order to reveal the underlying reasons for the high activity of the heteroatom-doped carbon, our group conducted a comprehensive DFT calculation on graphene doped by a series of different heteroatoms for ORR [248]. The DFT results indicated that there was a triple effect of the carbon sites\u2014namely, the charge, spin density, and ligand effect\u2014determining the intrinsic catalytic activity of the doped carbon catalysts and their ORR mechanism (Fig. 17\n). When the carbon materials are doped by a single heteroatom, the carbon sites around the doped atom can only be activated by the triple effect separately. This causes the ORR to proceed via the associative mechanism, and there is a limitation with an intrinsic overpotential of 0.44\u00a0V. However, when carbon materials are doped by metal or dual-heteroatoms, the ORR follows the dissociative mechanism, as double carbon sites can be activated by the triple effect. Thus, the activity limitation of the associative mechanism will no longer be in effect, leading to enhanced ORR activity. Our group also synthesized graphene co-doped with metallic and nonmetallic elements, and revealed the roles of nitrogen configuration in N-doped graphene as well as that of the trace atomic Ni in HER [249]. We found that quaternary nitrogen (N) is the most active site of the three N types in HER, whereas when doping with trace atomic cobalt, the planar (pyridine and pyrrolic) N becomes the most active. In contrast, when trace atomic Co was replaced by Ni, the planar (pyridine and pyrrolic) N exhibited depressed HER activity.Hybrid nanomaterials have an interface located at the boundary of two components [250]. It is extremely important for a heterogeneous catalyst to have a proper interfacial structure because the interface region always presents unique physical and chemical properties [251]. These unique properties can facilitate the capability of the resultant material to bind, convert, and transport the surface species (e.g., adsorbents, electrons, and intermediates), greatly promoting the catalytic reactions occurring on the surface [252\u2013255]. In recent years, abundant research studies have reported the design and synthesis of electrocatalysts for water\u2013hydrogen electroconversion with the assistance of interface engineering. In general, depending on the relative locations of the components, hybrid materials can be classified as supported structures, heterostructures, or core\u2013shell structures [256]. The special character of a supported structure is that the support component is much larger than the other components; in contrast, the components in a heterostructured material have similar sizes. In a core\u2013shell structure, one component is covered by another component, with interfaces existing at the boundary between the two components. These three types of hybrid materials with different interface structures have been reported as a result of research assembling metals, metal oxides, nonoxides, and so forth. For example, Zhang et al. [257] reported on a MoNi4 electrocatalyst immobilized on MoO2 cuboids (MoNi4/MoO2@Ni) with a supported structure that was made by controlling the outward diffusion of nickel atoms during calcination. By heating the NiMoO4 precursor under a reduction atmosphere, MoNi4 nanoparticles (20\u2013100\u00a0nm) supported by MoO2 cuboids (~1 \u03bcm) were synthesized, and the supported hybrid catalyst was found to exhibit excellent HER activity in alkaline solution (Figs. 18\n(a) and (b)). Heterostructured Co/CoP nanoparticles were prepared by Xue et al. [258] via the gradual phosphidation of Co metal into CoP components. By changing the weight ratios of the NaH2PO2 and Co species, the CoP contents in Janus Co/CoP nanoparticles could be controllably modulated, affecting the interface zone in the Co/CoP catalyst (Figs. 18(c)\u2013(f)). As illustrated in Figs. 18(g) and (h), Xu et al. [259] fabricated NiCo-based porous microrod arrays composed of carbon-confined NiCo@NiCoO2 core@shell nanoparticles (NiCo@NiCoO2/C PMRAs (porous microrod arrays)) by the reductive carbonization of bimetallic (Ni, Co) metal\u2013organic framework microrod arrays and subsequent controlled oxidative calcination. The obtained NiCo@NiCoO2/C PMRAs combined several desirable qualities for OER, including a large surface area, good conductivity, and multiple electrocatalytic active sites.For the purpose of rational catalyst design, the origin of the enhanced catalytic performance of these hybrid materials with an abundant interface has been deeply explored. The modulation of the electronic structure in the interface of hybrid materials has been well proven [260\u2013267]. An observed electron transfer was evidenced by Yu et al. [268] using X-ray absorption near-edge spectroscopy (XANES) for a Ni(OH)2/Pt catalyst. As pointed out by the authors, the pre-edge and main absorption edge of Ni for both the \u03b1- and \u03b2-Ni(OH)2/Pt electrode are shifted to lower energies, and the absorption intensities decrease in comparison with their counterparts, demonstrating that the electrons transfer from the Pt substrate to the hydroxide. Han et al. [269] revealed a strong metal-support interaction (SMSI) effect between CoS2 and Pt in the Pt/CoS2 hybrid system for highly active water splitting. A downward shift in the Pt d-band structure was proved by DFT calculation and was further supported by X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) analyses. Although the electron transfer has been well demonstrated in these metal/metal compound catalysts, the underlying mechanisms for the high catalytic activity are ambiguous. In order to uncover this mystery, our group studied the chemical properties of the metal/metal oxide interface, and found an interface-induced synergistic effect\u2014the \u201cchimney effect\u201d (Figs. 19\n(a) and (b)) [270]. As revealed by the DFT results (Figs. 19(c) and (d)), the neighboring sites around the interface are immune to H2O* and OH* species but selectively adsorb H*, which prevents the poison effect of H2O* and OH* on active sites. Meanwhile, the active sites on the interface have good capability for the adsorption and desorption of the H* reactant for the H2 product, as its \u0394G\nH* is close to 0. Due to these features, the hydrogen evolution on metal oxide/metal catalysts behaves similarly to a chimney for generating hydrogen productive continuously. The strong positive correlation between the HER activity of the metal oxide/metal composites and the interfacial metal atoms (Fig. 19(e)) further confirmed that the hydrogen generation can be greatly accelerated by increasing the amount of active site. With the exception of metal/metal compound catalysts, the origin of the performance enhancement of metal compound hybrid catalysts was further investigated by our group through the casting of monometallic NiO\u2013Ni3S2 heteronanosheets [271]. The electron transfer from Ni\u2013S to Ni\u2013O that occurs at the interface of NiO\u2013Ni3S2 results in a superior activity of overall water splitting that is even superior to the benchmark Pt/C and RuO2 assembly (Fig. 19(g)). DFT calculation results further demonstrated that the activation barrier of the hydrogen (or oxygen-containing) intermediates on the NiO\u2013Ni3S2 interfaces are significantly lowered, which is beneficial for the OER and HER processes (Figs. 19(h) and (i)).Vacancy generation is another important factor in developing interfaced materials with improved catalytic performances [272\u2013277]. A typical example was reported by Li et al. [278], who prepared FeS2/CoS2 hybrid nanosheets (NSs) for overall water electrolysis. In the synthesis process, CoFe2O4 nanoparticles were first synthesized by a coprecipitation method, and then transformed into the FeS2 and CoS2 phases. During the latter sulfuration process, interfaces with defect sites were created in the FeS2/CoS2 NSs. Electron paramagnetic resonance (EPR) spectra revealed a stronger EPR signal at g\u00a0=\u00a02.007 recorded with the FeS2/CoS2 NSs composite, indicating abundant S vacancies. Extended X-ray absorption fine structure (EXAFS) was used to further investigate the local structure of the obtained samples, and a distinct decrease in peak intensity in Fe K-edge EXAFS was detected with FeS2/CoS2 NSs, demonstrating the coordination deficiency from Fe. Similar results were reported by Gao et al. [279] in CeO2/NiO with an embedded configuration (Ce\u2013NiO\u2013E) or the surface-loaded configuration (Ce\u2013NiO\u2013L). An increasing trend of Ni3+ and oxygen defects for NiO (Ni3+ 62%, O defect 24%), Ce\u2013NiO\u2013L (Ni3+ 69%, O defect 26%), and Ce\u2013NiO\u2013E (Ni3+ 71%, O defect 32%) correlated well with the activity trend, indicating that these vacancy sites at the interface region contribute greatly to the enhanced activity. In fact, electron modulation and vacancy do not exist independently, and the catalytic performances of the interfaced materials may be influenced by the two factors simultaneously.Alloying is the diffusion penetration of the metal atoms of two or more metals, or the addition of nonmetallic elements into metals through melting, sintering, or vapor deposition processes. Metal alloying is an effective strategy for improving the performance of metal catalysts. It can not only refine the grain size to improve the mechanical strength and surface area of catalysts, but also selectively reduce the amount of a single component (e.g., Pt, Au, Ru, and other precious metals) in order to reduce the catalyst cost. Moreover, the catalytic activity and selectivity of metals can be changed by incorporating other metals to form alloys, due to the synergistic effect between metal components [280\u2013284]\nAccording to the Brewer\u2013Engel valence bond theory [285], alloying a metal with an unfilled d orbital and a metal with internally paired d-electrons can modulate the hydrogen adsorption energy on the alloy surface, thus improving the hydrogen evolution activity. Raj [286] prepared a series of Ni-based binary composites by means of electrodeposition techniques. The trend in their HER activities in alkaline water followed this order: Ni\u2013Mo\u00a0>\u00a0Ni\u2013Zn\u00a0>\u00a0Ni\u2013Co\u00a0>\u00a0Ni\u2013W\u00a0>\u00a0Ni\u2013Fe\u00a0>\u00a0Ni\u2013Cr. Due to its excellent activity, the Ni\u2013Mo alloy has been considered to be the most promising HER catalytic material. Zhang et al. [287] successfully constructed a layer of Ni\u2013Mo alloy nanorods with uniform size and uniform element distribution on the surface of a nickel substrate by means of magnetron sputtering technology, and found that the HER exchange current density was almost ten times higher than that of single-metal Ni or Mo catalysts. This research showed that the excellent catalytic activity of the Ni\u2013Mo alloy electrode mainly comes from two factors: \u2460 There is an increase in the specific surface, caused by the grain refinement of the two-component metal in the growth process; and \u2461 the electronegativity difference between the elements Ni and Mo causes electrons to concentrate around Mo, thus forming a synergy for electrocatalysis.In order to reduce the usage of precious metals, many non-noble metals (i.e., Co, Ni, Fe, Cu, V, Cr, Mn, Zn, etc.) have been incorporated with noble metals to form alloys as electrocatalysts [206,288]. Studies of ORR performances on PtM alloys revealed the following activity order [289\u2013291]: PtFe/C\u00a0>\u00a0PtCo/C\u00a0>\u00a0PtV/C\u00a0>\u00a0PtNi/C\u00a0>\u00a0Pt/C. The following stability order was also determined [292]: Pt3Ir(111)\u00a0>\u00a0Pt3Co(111)\u00a0>\u00a0Pt3Ni(111)\u00a0>\u00a0Pt3Fe(111). In addition, Greeley et al. [293] found a classic volcano-shaped dependency based on the ORR activity of Pt alloys versus the d-band center position of 3d metals (Fig. 20\n). According to that study, the ORR mechanism on Pt3M catalysts was either O2 dissociation or proton/electron transfer to molecular O2, and the optimal ORR catalyst should have a weaker oxygen-binding energy than Pt. Bampos et al. [294] further synthesized a series of carbon-supported Pd\u2013M (where M\u00a0=\u00a0Ag, Co, Cu, Fe, Ni, or Zn) bimetallic catalysts as ORR electrocatalysts in acidic solution. The ORR activity of these Pd-based alloys was found to descend as follows: PdZn/C\u00a0>\u00a0PdNi/C\u00a0>\u00a0Pt/C\u00a0>\u00a0PdAg/C\u00a0\u2265\u00a0PdCo/C\u00a0>\u00a0PdFe/C\u00a0>\u00a0PdCu/C\u00a0>\u00a0Pd/C. Notably, PdZn/C exhibited a specific activity three times higher than Pt/C at a potential between 0.35 and 0.5\u00a0V versus Ag/AgCl.In addition to metallic elements, the introduction of nonmetallic elements can improve the catalytic performance of alloys [295\u2013299]. For example, Kiran et al. [296] prepared few-layer MoS2(1\u2013\n\nx\n\n)Se2\n\nx\n alloys that possessed higher HER activity than pristine MoS2 and MoSe2. A systematic structure-activity relationship was revealed by tuning the Se/S incorporation in MoS2(1\u2013\n\nx\n\n)Se2\n\nx\n, with MoS1.0Se1.0 having an Se/S ratio of 1:1 showing the highest HER activity. Similar results regarding Mo\u2013S\u2013Se alloys were reported by the groups of Gong et al. [299] and Xu et al. [297]. Xu et al. [298] successfully controlled the composition of sulfur (S) and selenium (Se) in ternary WS2(1\u2013\n\nx\n\n)Se2\n\nx\n nanotubes on carbon fibers. The disordered atom arrangement introduced in the alloyed structure resulted in WS2(1\u2013\n\nx\n\n)Se2\n\nx\n having excellent electrocatalytic properties for HER. A ternary pyrite-type CoPS was further obtained by Cab\u00e1n-Acevedo et al. [300] for photo/electrochemical hydrogen evolution. Because of the higher electron-donating character of the P2\u2013 ligands in CoPS, this ternary pyrite CoPS was endowed with a more thermoneutral hydrogen adsorption, resulting in a higher HER activity than CoS2.The majority of the energy dissipation in electrochemical hydrogen\u2013water conversion is induced by the activation energy driving the electrochemical reactions involved in the energy system. Materials are at the core of a high-efficiency energy system, as the activation energy of the reactions involved in the system is strongly influenced by the catalytic materials. It is urgent to accelerate the development of active and robust electrocatalysts with acceptable cost in order to ensure that an efficient and sustainable energy system becomes a reality. In general, a catalyst\u2019s performance depends on two main factors: \u2460 the number of active sites in a given area; and \u2461 the intrinsic activity of each active site. Tuning the geometric structure (i.e., the nanoarchitecture and facet engineering) makes it possible to realize enhancement of the active sites, because when the catalyst size is reduced to the nanoscale, the catalyst possesses a high surface area with increased exposure of the catalytically active planes. Defect engineering and amorphization can also be applied to enrich active sites by exposing thermodynamically unstable active sites or active unsaturated atoms. Increasing the intrinsic activity of the active site is a more fundamental yet difficult strategy for activity improvement, which involves accurate modulation of the electronic structures of catalytic materials. Optimization of the intrinsic activity can be achieved based on a fundamental understanding of the particularities of the reactions and insight into the design of catalysts with targeted functionalities. The featured examples (their catalytic performances are summarized in Tables 2\u20135\n\n\n\n) discussed herein present successful modulation of the intrinsic activity via element doping, interface engineering, polymorph engineering, alloying, and so forth, with the aid of synergistic interaction between theoretical and experimental studies.At present, with the rapid development of chemical synthesis techniques, many catalytic materials have been reported with outstanding electrocatalytic performance. Meanwhile, developments in physical characterization methods and theoretical calculation are providing more evidence and guidance for researchers, allowing us to better understand the performance-improvement mechanism of catalysts. However, the new-generation electrode catalysts are mainly being produced through traditional trial-and-error processes rather than by means of rational design. This research model not only wastes social resources and the energy of scientific researchers, but also greatly delays scientific development. Thus, the development of a performance-oriented design strategy for electrocatalyst design from the most fundamental level to a practical application level is urgent. This target requires computational chemistry, electrocatalytic chemistry, and synthetic chemistry working in concert; unfortunately, however, all three fields require improvement at present. Current theoretical calculation models are still imperfect, as most do not take into account the influence of ionic strength, the double-layer effect, the solvation effect, and so forth, making it difficult to accurately reflect the actual reaction process of the catalytic material surface. Furthermore, although the reaction mechanisms of these hydrogen- and oxygen-involving reactions have been widely investigated, the actual mechanisms on different catalyst surfaces remain shrouded in mystery. In fact, it is also difficult to identify the actual active sites, as the surface structure of most catalysts varies during the electrocatalytic process. New progress will come from the merging of advanced theoretical calculations and experimental characterization (e.g., in situ, ex situ, and operando techniques), which will allow us to promote our understanding of the electrochemical reaction mechanism and the dynamic evolution of the catalysts involved at the molecular level. Finally, function-oriented catalyst preparation remains a major challenge. Existing material synthesis technologies are able to regulate the physical and chemical properties of specific catalysts to some extent at the nanoscale, but the use of controlled synthesis technology to modulate the electronic structure of catalysts at the atomic scale remains immature. The development of controllable synthesis methods with strong applicability and large-scale production is another focus of future research.We gratefully acknowledge financial support from the National Natural Science Foundation of China (21576032 and 51772037), the Key Program of the National Natural Science Foundation of China (21436003), the Major Research Plan of the National Natural Science Foundation of China (91534205), and the National Program on Key Basic Research Project of China (2016YFB0101202).Lishan Peng and Zidong Wei declare that they have no conflict of interest or financial conflicts to disclose.\n\n\nc\n\n\nreactant surface concentration, mol\u00b7m\u22123\n\n\n\nF\n\n\nFaraday constant, \u223c96\u00a0485\u00a0C\u00b7mol\u22121\n\n\n\nG\n\n\nGibbs free energy, J\u00b7mol\u22121\n\n\n\nf\n\n\ndecay rate to products and reactants, dimensionless\n\n\ni\n\n\ncurrent, A\n\n\nj\n\n\ncurrent density, A\u00b7cm\u22122\n\n\n\nj\n0\n\n\nexchange current density, A\u00b7cm\u22122\n\n\n\nn\n\n\nnumber of electrons transferred in the reaction, dimensionless\n\n\nR\n\n\nresistance, \u03a9\n\n\nT\n\n\ntemperature, K\n\n\nU\n0\n\n\nreaction equilibrium potential, V\n\n\nV\n\n\nvoltage, V\n\n\n\u03b1\n\n\ncharge transfer coefficient, dimensionless\n\n\n\u03bc\n\n\novervoltage, V\n\n\u0394G\nact\n\n\nactivation energy barrier, J\u00b7mol\u22121\n\n\n\u0394G\nH*\n\n\nhydrogen adsorption free energy\n\n\u0394G\nmax\n\n\ngibbs free-energy change, J\u00b7mol\u22121\n\n\n\u0394G\nO*\n\n\noxygen adsorption strength\n\n\nreactant surface concentration, mol\u00b7m\u22123\nFaraday constant, \u223c96\u00a0485\u00a0C\u00b7mol\u22121\nGibbs free energy, J\u00b7mol\u22121\ndecay rate to products and reactants, dimensionlesscurrent, Acurrent density, A\u00b7cm\u22122\nexchange current density, A\u00b7cm\u22122\nnumber of electrons transferred in the reaction, dimensionlessresistance, \u03a9temperature, Kreaction equilibrium potential, Vvoltage, Vcharge transfer coefficient, dimensionlessovervoltage, Vactivation energy barrier, J\u00b7mol\u22121\nhydrogen adsorption free energygibbs free-energy change, J\u00b7mol\u22121\noxygen adsorption strength", "descript": "\n                  In the context of the current serious problems related to energy demand and climate change, substantial progress has been made in developing a sustainable energy system. Electrochemical hydrogen\u2013water conversion is an ideal energy system that can produce fuels via sustainable, fossil-free pathways. However, the energy conversion efficiency of two functioning technologies in this energy system\u2014namely, water electrolysis and the fuel cell\u2014still has great scope for improvement. This review analyzes the energy dissipation of water electrolysis and the fuel cell in the hydrogen\u2013water energy system and discusses the key barriers in the hydrogen- and oxygen-involving reactions that occur on the catalyst surface. By means of the scaling relations between reactive intermediates and their apparent catalytic performance, this article summarizes the frameworks of the catalytic activity trends, providing insights into the design of highly active electrocatalysts for the involved reactions. A series of structural engineering methodologies (including nanoarchitecture, facet engineering, polymorph engineering, amorphization, defect engineering, element doping, interface engineering, and alloying) and their applications based on catalytic performance are then introduced, with an emphasis on the rational guidance from previous theoretical and experimental studies. The key scientific problems in the electrochemical hydrogen\u2013water conversion system are outlined, and future directions are proposed for developing advanced catalysts for technologies with high energy-conversion efficiency.\n               "}
{"full_text": "With increasing concerns of the fossil fuel\u2013related environmental crisis and global warming, there is an imperative demand for developing alternative green and sustainable energy conversion and storage technologies, such as batteries, fuel cells and water electrolysis [1\u20137]. As a crucial reaction of secondary metal-air batteries and electrochemical water splitting, oxygen evolution reaction (OER) plays an important role in the efficiency and operational stability of such systems [8\u201312]. High-performance electrochemical catalysts are thus urgently required to promote sluggish OER reaction kinetics, to provide low overpotentials (\u03b7) and excellent catalytic stability. Considering the rarity and high cost of noble metal OER catalysts (e.g. Ir- or Ru-based materials) [13,14], earth-abundant and cost-effective transition metal based electrocatalysts are clearly desired.Metal-organic frameworks (MOFs) are a class of porous materials composed of organic linkers and metal nodes with coordination bonding [15]. MOFs and derived materials have been used in a wide range of fields, e.g. gas storage and separation, batteries and catalysis etc., benefiting from their high specific surface area, tuneable porosity and abundant active metal sites [16\u201321]. Two-dimensional (2D) MOFs based materials have attracted growing attention for OER catalysts with unique physicochemical features. The 2D structure enables hydroxide units in the electrolyte to easily reach the active site and fast dissociation of generated O2, as well as shortening the electron transfer pathway through the thin film to the conductive support [22\u201324]. 2D MOFs can also be engineered to possess a large number of coordinatively unsaturated metal atoms exposed as the active sites [25,26]. The atomic surface structure and bonding arrangement can be elaborately controlled to facilitate the interaction between the active site and the reaction intermediates for superior OER electrocatalysts [27,28]. However, 2D MOFs have a high tendency to aggregate [29,30], leading to a decreased effective surface area during operation. Avoiding aggregation with high active surface area and improving the integral structural stability for superior OER catalysts are thus essential.To tackle these issues, there are increasing reports demonstrating that the introduction of functional nanoscale components (nanosheets or nanoparticles etc.) in a MOF composite could prevent the aggregation and enhance the integral structural stability during operation [30,31]. Meanwhile, the electronic structure of the metal units in the MOFs can be optimized by incorporation of heterogeneous metal-containing groups for superior OER performance [30\u201335]. For example, Qiao and coworkers have synthesized 2D Ni-BDC/Ni(OH)2 heterostructure, exhibiting a lower \u03b7 of 320\u00a0mV at 10\u00a0mA\u00a0cm\u22122 than that of Ni-BDC nanosheets, and a good catalytic durability of 20\u00a0h. The improved OER performance is attributed to the rational design of the composition and structure of the composite, and to the mitigation of aggregation of Ni-BDC by coupling with Ni(OH)2\n[30]. Similarly, Qin et al. have reported hybrids of Fe-Co polyphenolic network\u2013wrapped Fe3O4 nanocatalysts for enhanced OER with an \u03b7 of 260\u00a0mV at 10\u00a0mA\u00a0cm\u22122 and a durability of over 24\u00a0h, taking advantage of strong metal-polyphenolic ligand complexation that ensures robust metal-polyphenolic shells for prolonged operations [31]. Inspired by these reports, 2D Ni-based MOFs could be promising candidates for constructing hybrid electrocatalysts due to an excellent surface structure and physicochemical features. Meanwhile, considering Fe3O4 nanoparticles with good electrical conductivity (>100 S cm\u22121) and fast electron transfer between Fe2+ and Fe3+ in the crystals [31,36], the incorporation of Fe3O4 nanoparticles on the surface of 2D Ni-based MOFs could be promising candidate to be used for OER.Herein, we have successfully prepared ultrasmall Fe3O4 nanoparticles that are uniformly immobilized on 2D Ni-based MOFs (Fe3O4/Ni-BDC). The functionalized Fe3O4 nanoparticles (6\u00a0\u00b1\u00a02\u00a0nm) with abundant surface hydroxide groups are initially synthesized, and then either added directly during the synthesis of 2D Ni-BDC layers (4\u00a0\u00b1\u00a01\u00a0nm) or alternatively mixed with pre-synthesized 2D Ni-BDC. We have investigated the resulting morphology changes and electronic structure modulation to tackle the aggregation issue for OER via tuning the amount of Fe3O4 immobilized on the 2D Ni-BDC layers. The optimized Fe3O4/Ni-BDC-4 composite demonstrates significantly enhanced OER performance with an \u03b7 of 295\u00a0mV at 10\u00a0mA\u00a0cm\u22122, a Tafel slope of 47.8\u00a0mV dec-1 and an excellent catalytic durability over 40\u00a0h. DFT calculations are further conducted to identify the active site and help to understand how the valance states of the transition metals affect the OER performance.Iron (III) chloride hexahydrate (FeCl3\u00b76H2O, 97%), sodium bicarbonate (NaHCO3, \u226599.7%), L-ascorbic acid, terephthalic acid (1, 4-BDC, 98%), N,N-dimethylformamide (DMF, 99.8%), Nafion solution (10 wt%) were bought from Sigma-Aldrich. Triethylamine (TEA, 99%) was purchased from Merck (Germany). Nickel (II) chloride hexahydrate (NiCl2\u00b76H2O, 98%) was from BDH Chemicals Ltd Poole England. All chemicals were used as received without further purification. Ultrapure water (\u226518.25 M\u03a9\u00b7cm, Sartorius arium\u00ae pro, Germany) was used to prepare all the aqueous solutions.Water-dispersible Fe3O4 nanoparticles were synthesized according to a previous report [37]. Briefly, a 20\u00a0mL aqueous solution of 1\u00a0mM L-ascorbic acid was added into a 60\u00a0mL aqueous mixture of 6\u00a0mM FeCl3\u00b76H2O and 18\u00a0mM NaHCO3, under stirring for 20\u00a0min. The mixture was transferred to a 150\u00a0mL Teflon-lined stainless-steel autoclave, which was kept at 150\u00a0\u00b0C for 6\u00a0h. The product was separated using a magnet, washed with ultrapure water more than three times, leading to Fe3O4 nanoparticles that could be re-dispersed in water for further using.Ni-BDC was grown with or without the presence of water-dispersible Fe3O4 for Fe3O4/Ni-BDC composites. 63\u00a0mg of 1,4-BDC was first dissolved in a mixed solvent of DMF (15\u00a0mL), ethanol (1\u00a0mL), and ultrapure water (1\u00a0mL), into which 90\u00a0mg of NiCl2\u00b76H2O and a certain volume of water or Fe3O4 dispersion were added subsequently and under ultrasonication for 10\u00a0min, followed by a quick injection of 0.50\u00a0mL TEA. To optimize the ratio of Fe3O4 and Ni-BDC in the composite, different volumes of Fe3O4 dispersion (12\u00a0mg\u00a0mL\u22121; 1, 2, 3, 4 or 5\u00a0mL) were used. The above mixture was sealed and continuously ultra-sonicated for 6\u00a0h at room temperature. Finally, the precipitates were centrifuged and washed with ethanol three times, followed by drying in a vacuum oven at 60\u00a0\u00b0C for 12\u00a0h. The obtained composites were collected and labelled as Fe3O4/Ni-BDC-n (n\u00a0=\u00a01, 2, 3, 4 and 5), where n is the volume in mL of Fe3O4 dispersion used.The crystallinity of the synthesized materials was characterized by X-ray diffraction (XRD, D8 Advance X-Ray diffractometer (Huber)). X-ray photoelectron spectroscopy (XPS) was performed by drop-casting samples onto silicon substrates with a Thermo-Scientific system (Al-K\u03b1 radiation, 1484.6\u00a0eV). Fourier transform infrared spectroscopy (FTIR) measurements were performed on an Alpha-P FTIR spectrometer (Bruker) in the range of 4,000\u2013400\u00a0cm\u22121 with a resolution of 2\u00a0cm\u22121. The specific surface area was estimated by a surface area and pore size analyzer (ASAP 2020, Micromeritics). Elemental analysis was performed by inductively-coupled plasma optical emission spectrometry (ICP-OES). Micro-, nanostructure and composition characterization were conducted with scanning electron microscopy (SEM, Quanta FEG 200 ESEM, 15\u00a0kV), atomic-force microscopy (AFM, Agilent Technology 5500, tapping mode, a mica sheet as the substrate), and transmission electron microscopy (TEM, Tecnai G2 T20, 200\u00a0kV).In order to prepare catalyst inks, 4\u00a0mg of active materials (Fe3O4/Ni-BDC-n, Ni-BDC or Fe3O4), 4\u00a0mg of carbon black (Alfa Aesar\u2122) and 25\u00a0\u03bcL of Nafion solution (10 wt%) were mixed with 0.75\u00a0mL of 2-propanol and 0.25\u00a0mL of ultrapure water for a uniform ink after 1\u00a0h sonication. Prior to use, rotating disk electrodes (RDE) using glassy carbon electrodes (GCEs, d\u00a0=\u00a05.0\u00a0mm, A\u00a0=\u00a00.19625\u00a0cm2) and rotating ring disk electrodes (RRDEs, d\ndisk\u00a0=\u00a05.61\u00a0mm, A\ndisk\u00a0=\u00a00.2472\u00a0cm2, d\nring (inner)\u00a0=\u00a06.25\u00a0mm, d\nring (outer)\u00a0=\u00a07.92\u00a0mm, A\nring\u00a0=\u00a00.1859\u00a0cm2) with a GCE disk and a Pt ring were polished on a polishing pad with Al2O3 slurries with decreasing particle diameters (1.0, 0.3 and 0.05\u00a0\u00b5m). Afterwards, 10\u00a0\u00b5L of the catalyst ink was drop-cast onto the surface of the GCEs, leading to a mass loading of 0.398\u00a0mg\u00a0cm\u22122, and dried under room temperature. Electrochemical tests were carried out in a typical three-electrode setup with 1.0\u00a0M KOH solution as the electrolyte on an electrochemical workstation (Autolab PGSTAT12) with a graphite rod as the counter electrode and a Ag/AgCl (sat. KCl) as the reference electrode. Rotation of the RDE and RRDE were controlled on a Pine Instruments rotating system. The applied potentials were compensated for the solution resistance R\ns and current I via: E\nAg/AgCl-corr\u00a0=\u00a0E\nAg/AgCl - IR\ns\n[38], where the uncompensated Ohmic solution resistance (R\ns) in the high-frequency region was measured by electrochemical impedance spectroscopy (EIS) in a frequency range from 100\u00a0kHz to 0.1\u00a0Hz at 1.525\u00a0V vs. RHE. All measured potentials were calibrated to reversible hydrogen electrode (RHE) potential according to the following equation: E\nRHE\u00a0=\u00a0E\nAg/AgCl-corr\u00a0+\u00a00.197\u00a0+\u00a00.059\u00a0\u00d7\u00a0pH. To maintain the O2/H2O equilibrium at 1.23\u00a0V versus RHE, oxygen gas (O2\u00a0\u2265\u00a099.995%) flow was kept in the electrolyte during the test. The \u03b7 for OER was defined as: \u03b7\u00a0=\u00a0E\nRHE \u22121.23\u00a0V. For OER tests, working electrodes were initially scanned for 10 cycles using cyclic voltammetry (CV) to obtain stable signals. Then, linear sweep voltammetry (LSV) curves were obtained at a slow scan rate of 5\u00a0mV\u00a0s\u22121 at a rotational speed of 1,600\u00a0rpm to decrease capacitive currents and interference from generated gas bubbles. The electrode durability was evaluated by chronopotentiometry at a current density of 10\u00a0mA\u00a0cm\u22122. The Tafel slope (b) was calculated based on the Tafel equation [39,40]:\n\n(2.1)\n\n\n\u03b7\n=\na\n+\nb\n\u2219\nlog\nj\n\n\n\n\nand compared to b\u00a0=\u00a02.303RT/\u03b1nF, where j is current density of samples from the LSV test, \u03b1 is the charge transfer coefficient, n is\u00a0the number of transferred electrons during the redox reaction, F is\u00a0the Faraday constant (96485 C mol\u22121), R is\u00a0the gas constant (8.314\u00a0J\u00a0mol\u22121 K\u22121), and T is\u00a0the temperature (K).The electrochemical double-layer capacitance (C\ndl) was tested using CVs in a narrow potential range of 1.223\u20131.323\u00a0V vs. RHE, with scan rates of 40, 60, 80, 100, and 120\u00a0mV\u00a0s\u22121. The plot of \u0394j = (j\na\u00a0\u2212\u00a0j\nc), where j\na and j\nc are the anodic and cathodic current, respectively, at 1.24\u00a0V vs. RHE (no faradaic reaction occurring) against the scan rate had a linear relationship, whose slope was twice of C\ndl. The electrochemically active surface area (ECSA) relative to GCE and GCE-normalized current density were calculated according to the equations [41]:\n\n(2.2)\n\n\nECSA\n=\n\n\n\n\nC\n\n\nd\nl\n_\ns\na\nm\np\nl\ne\ns\n\n\n\n\n\n\nC\n\n\nd\nl\n_\nG\nC\nE\n\n\n\n\n\n\nA\n\n\ngeo\n\n\n\n\n\n\n\n\n(2.3)\n\n\n\n\nj\n\n\nE\nC\nS\nA\n_\nn\no\nr\nm\na\nl\ni\nz\ne\nd\n\n\n=\n\n\nj\n\n\nECSA\n\n\n\n\n\n\nHerein, C\ndl-GCE is the specific capacitance for a plane surface in the range of 20\u201360 \u03bcF cm\u22122, and C\ndl-GCE\u00a0=\u00a040 \u03bcF cm\u22122 was used [32]. A\ngeo is the geometric area of the GCE. To investigate the reaction mechanism for OER, RRDE voltammograms were recorded to determine the OER reaction pathway by measuring the HO2\n\u2212 formation, with the ring potential held at 1.50\u00a0V vs. RHE at 1,600\u00a0rpm, and linearly scanning the potential of the disk in O2-saturated 1.0\u00a0M KOH solution.Spin-polarized density functional theory (DFT) calculations were performed using the Vienna ab-initio simulation package (VASP) [42,43]. The Perdew\u00a0\u2212\u00a0Burke\u00a0\u2212\u00a0Ernzerhof (PBE) functional within the generalized gradient approximations (GGA) was used to account for exchange correlation effects. The projector-augmented wave method was used to represent the core-valence electron interaction. The valence electronic states were expanded in plane-wave basis sets with energy cutoff at 450\u00a0eV, and the force convergence criterion in the structure was set to be 0.05\u00a0eV/\u00c5. The transition states were searched using a constrained optimization scheme [44-48]. The Hubbard U approach was used to correct Ni and Fe 3d orbitals with U\neff\u00a0=\u00a03.8\u00a0eV [49-52]. The zero-point energies (ZPE) and entropy corrections to the free energies at room temperature (298.15\u00a0K) were applied. DFT-D3 method with Becke-Johnson (BJ) damping was employed to describe Van der Waals interaction [53,54]. The free energy of gas-phase O2 molecule is discussed in Supporting Information.On the basis of our XRD characterization (Fig. 1\na and Fig. S1) and previous report [52], the (200) surface of Ni-BDC was constructed and used to study the interactions between catalysts and adsorbates. A p(2\u00a0\u00d7\u00a01) Ni-BDC catalytic surface system was modelled with two metal oxide layers separated by one BDC layer (Fig. S22). A (5\u00a0\u00d7\u00a05\u00a0\u00d7\u00a01) k-point was used for geometry optimization with quasi-Newton algorithm. The (311) plane of Fe3O4, as an example of a curved surface of the ultrasmall nanoparticles, was used for constructing a model of the active surface, exposing five-coordinated Fe sites, two-coordinated and three-coordinated O sites (Fig. S23). Four stoichiometric layers of Fe3O4(311) on the Ni-BDC(200) surface was created to model a Fe3O4/Ni-BDC composited structure. A (1\u00a0\u00d7\u00a05\u00a0\u00d7\u00a02) k-mesh was used for geometry optimization. While there could be residual surface groups from the hydroxide groups and DHAA, the models used for DFT focus on the possible interaction in this Fe3O4/Ni-BDC composited structure.Fe3O4/Ni-BDC composites are synthesized through a two-step procedure as illustrated in Scheme 1\n. Water-dispersible Fe3O4 nanoparticles are initially prepared with a modified hydrothermal method [37]. Dehydroascorbic acid (DHAA) is oxidized from the ascorbic acid, serving as a stabilizer and capping ligand on surfaces of Fe3O4 nanoparticles interacted by carbonyl groups, ensuring a good dispersibility of Fe3O4 nanoparticles in aqueous solution [55,56]. During the subsequent sonication process, functionalized Fe3O4 nanoparticles are homogenously dispersed in an alkaline solution (pH\u00a0~\u00a010), immobilized on 2D Ni-BDC. Triethylamine (TEA) serves as the deprotonating agent and the modulator to promote the nucleation of pristine Ni-BDC for monodisperse film with the controllable orientation [57]. Introducing the Fe3O4 nanoparticles solution into the synthetic process of Ni-BDC leads to lower TEA concentration, restraining the continuous nucleation of Ni-BDC layers. As a consequence, smaller Ni-BDC layers with more defects and edges are obtained, and with Fe3O4 nanoparticles immobilized, leading to Fe3O4/Ni-BDC composites.The water-dispersible Fe3O4 nanoparticles (Fig. 1a) were characterized by XRD analysis, showing peaks matching well with the standard Fe3O4 phase (JCPDS no. 89\u20130688), with peaks at 18.3\u00b0, 30.0\u00b0, 35.4\u00b0, 43.0\u00b0, 56.9\u00b0 and 62.5\u00b0 fitting well with (111), (220), (311), (400), (511) and (440) planes of magnetite, respectively. The diffraction peaks of pure Ni-BDC (Fig. 1a) can be assigned based on Ni-BDC composites in literature [27,52]. The main diffraction peaks at 8.8\u00b0, 15.7\u00b0 and 17.9\u00b0 are ascribed to the (200), (201) and (-201) planes of Ni-BDC, respectively. Fe3O4/Ni-BDC-n composites with various amount of Fe3O4 are successfully synthesized, and XRD patterns of Fe3O4/Ni-BDC-4 (Fig. 1a) confirm the co-existence of the crystalline phases of Fe3O4 and Ni-BDC. Similar diffraction peaks are also observed on those of Fe3O4/Ni-BDC-1, 2, 3 or 5 (Fig. S1). It can be observed that the diffraction peaks of Ni-BDC become weaker with the increase of Fe3O4 content, a phenomenon due to the process that Fe3O4 nanoparticles limit the growth of Ni-BDC layers. In addition, FTIR spectra of Fe3O4, Ni-BDC and Fe3O4/Ni-BDC-4 are shown in Fig. 1b. The band at 553\u00a0cm\u22121 is assigned to the Fe-O stretching vibration of Fe3O4\n[58], with the bands in the range of 600\u20131300\u00a0cm\u22121 attributed to out-of-plane vibrations of the BDC linker [59]. The strong bands at 1373 and 1564\u00a0cm\u22121 are ascribed to the stretching modes of the coordinated carboxylate (\u2013COO-) of the terephthalate linker of Ni-BDC, indicative of the presence of both Fe3O4 and Ni-BDC [59]. The absorption band at 1647\u00a0cm\u22121 is regarded as the coordination between O atom of the carbonyl group (C\u00a0=\u00a0O) from DHAA and Fe units from the surfaces of Fe3O4 nanoparticles [37]. The broad band at 3315\u00a0cm\u22121 is attributed to the strong stretching mode of hydroxyl groups (\u2013OH) [37,59,60].The chemical bonding states of Fe3O4, Ni-BDC and Fe3O4/Ni-BDC composites are investigated by XPS. All signals originating from expected elements (Ni, Fe, O or C) are recorded in the survey spectra (Fig. S2). The high-resolution Ni 2p spectra (Fig. 1c) are deconvoluted into two satellite peaks and a couple of peaks for Ni2+ (854.8/872.5\u00a0eV for Fe3O4/Ni-BDC-4 and 860.2/878.5\u00a0eV for Ni-BDC) [27]. In addition, in high-resolution Fe 2p spectra (Fig. 1d), a spin\u2013orbit doublet at 709.2/722.6\u00a0eV and 709.4/723.0\u00a0eV is assigned to the Fe2+ in Fe3O4/Ni-BDC and Fe3O4, respectively [61]. Peaks at 712.2/725.9\u00a0eV and 712.2/725.9\u00a0eV are belong to the Fe3+ in Fe3O4/Ni-BDC and Fe3O4, respectively [34]. High-resolution Ni 2p (Fig. 1c) and Fe 2p (Fig. 1d) spectra indicate the Ni 2p3/2 (856.7\u00a0eV for Ni-BDC) and Fe 2p3/2 (710.4\u00a0eV for Fe3O4) shift to lower and higher binding energies (Fig. 1c, d and S3; Ni 2p3/2 and Fe 2p3/2 of Fe3O4/Ni-BDC-4), respectively [52,62]. The detailed binding energy data are summarized in Table S1. Binding energy level of Ni 2p3/2 decreases and while Fe 2p3/2 increases slightly with the amount of Fe3O4 nanoparticles among the five composites. This indicates that there is change in the bond strength of both Fe and Ni to varying degree in the different Fe3O4/Ni-BDC composites. The typical O 1\u00a0s spectrum of Fe3O4/Ni-BDC-4 (Fig. 1e) indicates peaks at 533.4, 532.4 and 531.6\u00a0eV, that are assigned to O\u2013H, O\u00a0=\u00a0C-O and Ni-O bonding, respectively, originating from the terephthalate linker and NiO6 octahedra in Ni-BDC [27]. Other peaks at 531.2, 530.7 and 529.8\u00a0eV are assigned to O\u2013H, O\u00a0=\u00a0C and Fe-O bonding, respectively, attributed to the surface hydroxyl and carbonyl groups from the DHAA and internal Fe-O units of the water-dispersible Fe3O4\n[61,63]. To quantify the specific surface area and pore sizes of Fe3O4/Ni-BDC-4, the typical adsorption\u00a0\u2212\u00a0desorption isotherm is recorded using nitrogen adsorption\u00a0\u2212\u00a0desorption measurements (Fig. 1f). The specific surface area is determined to be 136\u00a0m2 g\u22121, which is attributed to the slit-like structure formed by aggregation of Fe3O4/Ni-BDC-4 [52]. The pore size distributions of the composite are mainly centered at 40\u00a0nm. Besides, the pore size distribution indicates the presence of micropores (<2 nm), mesopores (2\u201350\u00a0nm) and macropores (>50\u00a0nm), confirming a hierarchical porous structure for Fe3O4/Ni-BDC-4, which is beneficial for mass transport for OER. Finally, ICP-OES is conducted to check the metallic components of various Fe3O4/Ni-BDC composites (Table S2). It verified that proportional Fe in the composite increases with the added amount of Fe3O4. Fe3O4/Ni-BDC-4 is composed of 24.7 at% Ni and 75.3 at% Fe.The morphologies of Ni-BDC, Fe3O4 and Fe3O4/Ni-BDC are characterized by SEM and TEM. SEM images of pure Ni-BDC (Fig. 2\na, Fig. S4a and b) show a hierarchical-layer structure comprised of aggregated 2D nanosheets. After immobilizing different amounts of Fe3O4 nanoparticles (Fig. 2b, Fig. S4c and d), the generated Fe3O4/Ni-BDC composites show changes in morphology and microstructures. With increasing the ratio of Fe3O4, the Fe3O4/Ni-BDC (Fig. 2c, Fig. S4e and f, Fig. S5) turns from a layered structure to a smoother structure with smaller Ni-BDC grains. Likely caused by Fe3O4 nanoparticles anchoring Ni-BDC on their surface hindering extended growth of Ni-BDC layers. As a control to assess if any leaching Fe from could cause MOF formation, samples following the same synthetic route of Fe3O4/Ni-BDC-4 without the addition of NiCl2\u00b76H2O are also fabricated, but were difficult to purify and separate from solution. SEM images and the corresponding EDS spectrum (Fig. S6) indicate that the control sample is the water-dispersible Fe3O4 nanoparticles with no formation of Fe based MOF. This further confirms the importance of Ni source (NiCl2\u00b76H2O) in the formation of Fe3O4/Ni-BDC composites. TEM image of pure Ni-BDC (Fig. 2d) clearly demonstrates a two-dimensional hierarchical-layer structure. The AFM image (Fig. S7) of partial Ni-BDC samples further indicates the thickness of Ni-BDC nanosheets is 5\u00a0\u00b1\u00a01\u00a0nm. TEM image of Fe3O4 (Fig. 2e) obviously shows ultrafine nanoparticles with a particle size of 6\u00a0\u00b1\u00a02\u00a0nm. The interplanar spacing of the lattice (Fig. 2f) is measured to be 0.485\u00a0nm, matching well with the (111) plane of magnetite Fe3O4 (JCPDS no. 89\u20130688) [64]. When the mass ratio of Fe3O4 is low, the high-resolution TEM image of Fe3O4/Ni-BDC-1 (Fig. S8) clearly shows the boundary between Fe3O4 and Ni-BDC. Fe3O4 nanoparticles are tightly anchored on the Ni-BDC layers, originating from the strong coupling effects between them. With a higher mass ratio of Fe3O4, the typical TEM images of Fe3O4/Ni-BDC-4 (Fig. 2g-h and S9) show that ultrafine Fe3O4 nanoparticles are homogenously immobilized on the Ni-BDC layers. The high-resolution TEM image of Fe3O4/Ni-BDC-4 (Fig. 2i) demonstrates a crystalline interplanar spacing of 0.297\u00a0nm, in accordance with the (220) plane of magnetite Fe3O4 (JCPDS no. 89\u20130688) [65]. The STEM-EDS elemental mapping images corresponding to a fragment of Fe3O4/Ni-BDC-4 (Fig. 2j-n) suggests the homogeneous distribution of nickel (cyan), iron (red), oxygen (green) and carbon (purple) elements, confirming that Fe3O4 nanoparticles are uniformly distributed on Ni-BDC sheets. Linear elemental distribution of Fe3O4/Ni-BDC-4 composite (Fig. S10) further verifies that ultrasmall Fe3O4 nanoparticles distributes on the surface of Ni-BDC.OER performance of the proposed electrocatalysts was investigated in a conventional three-electrode cell containing O2-saturated 1.0\u00a0M KOH solution by LSV at a scan rate of 5\u00a0mV\u00a0s\u22121. As controls, the OER activities of pristine Ni-BDC, Fe3O4 and commercial RuO2 with the same mass loading on GCE are examined. Catalytic performance of Fe3O4/Ni-BDC-n is tested for screening the optimal Fe ratio (Fig. 3\na and Fig. S11). The best OER performance is obtained with the Fe3O4/Ni-BDC-4 (Ni 24.7 %, Fe 75.3 %), exhibiting the lowest \u03b7 of 295\u00a0mV at a current density of 10\u00a0mA\u00a0cm\u22122. In comparison, large \u03b7 of 369, 465 and 339\u00a0mV (Fig. 3b) is registered for pristine Ni-BDC, Fe3O4 and commercial RuO2, respectively. It is noteworthy that introducing Fe3O4 nanoparticles, although themselves being poor OER catalysts, radically improves the overall water oxidation ability of Ni-BDC, decreasing \u03b7 with as much as 170\u00a0mV. The enhanced OER performance of Fe3O4/Ni-BDC-4 is attributed to the optimal electronic structure of transition metals and hierarchical-layer structure, which are confirmed by XPS and TEM results (Fig. 1c and d, Fig. 2g-i). To illustrate the role of electronic structure change upon OER performance, the \u03b7 of 337\u00a0mV at 10\u00a0mA\u00a0cm\u22122 of physical mixture of Fe3O4 and Ni-BDC (Fig. S12a) is significantly larger than that of Fe3O4/Ni-BDC-4. High-resolution Ni 2p and Fe 2p spectra (Fig. S12b and c) of physical mixed samples shows no shifts from the individual samples. In situ growth of Ni-BDC in the presence of Fe3O4 nanoparticles causes the binding energy changes of Ni 2p and Fe 2p in the composites (Table S1), optimizing the integral electronic structure of Fe3O4/Ni-BDC-4 composite for high OER performance. Tafel curves of Fe3O4/Ni-BDC-4, Ni-BDC, Fe3O4 and commercial RuO2 are shown in Fig. 3c. Tafel slope of Fe3O4/Ni-BDC-4 (47.8\u00a0mV dec-1) is considerably smaller than those of Ni-BDC (60.5\u00a0mV dec-1), Fe3O4 (148.1\u00a0mV dec-1) and commercial RuO2 (83.5\u00a0mV dec-1), revealing the significantly improved catalytic reaction kinetics on Fe3O4/Ni-BDC-4 [66]. The change of Tafel slope is related to the potential-determining step (PDS) of the electrochemical reaction, indicating the PDS of Fe3O4/Ni-BDC-4 is the second step for electron transfer (formation of O*) [40,67]. In addition, stability is also a critical parameter to evaluate the catalyst. The chronopotentiometric curve of Fe3O4/Ni-BDC-4 is shown in Fig. 3d. Compared with previously reported Fe3O4 or Ni-BDC based catalysts (Table S3), Fe3O4/Ni-BDC-4 demonstrates a superior durability over 40\u00a0h with a stable OER activity at a constant current density of 10\u00a0mA\u00a0cm\u22122. As a control, the OER catalytic stability of pure Fe3O4 (Fig. S13a) shows a sharp decay after 8\u00a0h, and the pristine Ni-BDC (Fig. S13b) exhibits a weak catalytic stability with a lifetime of less than 5\u00a0h. The good catalytic stability of Fe3O4/Ni-BDC-4 implies that the active sites continuously interact with the reaction intermediates for OER during operation. It\u2019s apparent that stability is achieved with the intermixing of Fe3O4 nanoparticles with Ni-BDC. The coupling effects between Fe3O4 nanoparticles and Ni-BDC layers could support the structural stability during OER process by efficiently preventing the aggregation of Ni-BDC. Furthermore, the hierarchical structure of the composite could offer abundant defects and edges.The key parameter involved the number of transferred electrons during OER is further investigated by a RRDE. A much lower current density (Fig. S14) on the ring electrode at 1.50\u00a0V compared with that on the disk electrode during OER process was recorded. It indicates a desirable four-electron reaction path (4OH-\u2192 O2\u00a0+\u00a02H2O\u00a0+\u00a04e-) occurs on Fe3O4/Ni-BDC-4 with negligible generation of hydrogen peroxide during OER [52,68]. This further confirms the observed disk current density results from water oxidation rather than other side reactions (Fig. S14). The above results validate that Fe3O4/Ni-BDC-4 is an efficient OER catalyst.Electrochemical behaviour of the samples was characterized by CV in O2-saturated 1.0\u00a0M KOH solution in a potential window of 1.123\u20131.573\u00a0V vs. RHE, a region without OER and pre-oxidation peaks are generally observed. Such pre-oxidation peaks are usually related to the oxidation of transition metals (from 2+ to 3+), which are involved in the OER process [25,52]. Ni-BDC (Fig. 4\na) and Fe3O4 (Fig. S15) show the anodic peak potential (E\npa) at 1.406 and 1.460\u00a0V vs. RHE, respectively [69,70]. The oxidation peak area ratio of Fe3O4/Ni-BDC-4 (Fig. 4a) normalized on the basis of Ni-BDC in CV curves is larger over others synthetic samples, and such an increased oxidation peak area is believed to be significant for enhanced OER catalytic ability [71,72], in good agreement with the above measured \u03b7 data. As summarized in Table S4 and Fig. 4b, E\npa of Fe3O4/Ni-BDC-n has a slight positive shift trend with increasing n (n\u00a0=\u00a01, 2, 3, 4), but the trend reverses when n\u00a0=\u00a05, revealing that the amount of Fe3O4 nanoparticles could affect E\npa. As a control, the main E\npa (1.411\u00a0V vs. RHE) in the CV of physical mixture of Fe3O4 and Ni-BDC (Fig. S16) shows a negligible change compared with the E\npa of pristine Ni-BDC, furtherly revealing that directly prepared Fe3O4/Ni-BDC composites modify the integral electronic structure for higher oxidation-state situations. With the increasing amount of Fe3O4 nanoparticles in the Fe3O4/Ni-BDC-n (n\u00a0=\u00a01, 2, 3, 4 and 5), more positive E\npa is observed than that of pristine Ni-BDC (Fig. 4a). The strong interaction between Fe3O4 and Ni-BDC suggests that the corresponded E\npa peaks from the synergistic effects of oxidation of Ni and Fe species, which shifts positively. The more positive E\npa peaks indicate the higher oxidation state of active sites accounting for good OER catalytic performance [25]. Most importantly, there is likely a link between E\npa and onset potential of OER (E\nonset). The optimal Fe3O4/Ni-BDC-4 composite demonstrates the highest E\npa (1.431\u00a0V vs. RHE) and the lowest E\nonset (i.e. smallest \u03b7) among Fe3O4/Ni-BDC-n. Previous reports [25,73] indicate this could be due to higher-oxidation-state metal originating from the coupling effects of Ni and Fe in the composite is responsible for an enhanced OER performance. Overall, we successfully demonstrate that the modulation of the oxidation state of elemental Ni and Fe of 2D Ni-BDC by incorporating with Fe3O4, which is revealed by XPS, leads to a high E\npa and a small \u03b7.ECSA is another crucial parameter, which is correlated to the number of active sites and has been determined via C\ndl measurement (Fig. S17) [32,41,74]. As displayed in Fig. 4c, the C\ndl of Fe3O4/Ni-BDC-4 is 478 \u03bcF cm\u22122, much higher than those of Fe3O4 (277 \u03bcF cm\u22122) and Ni-BDC (283 \u03bcF cm\u22122). Meanwhile, Fe3O4/Ni-BDC-n (n\u00a0=\u00a01, 2, 3, 5) exhibit C\ndl values of 249, 299, 360 and 407 \u03bcF cm\u22122, respectively (Fig. S18). Fe3O4/Ni-BDC-4 shows the highest C\ndl value, mainly attributed to assumption that the introduction of Fe3O4 nanoparticles on the surface of Ni-BDC layers can lead to the formation of hierarchical structure, offering abundant defects and edges. Besides, the coupling effects between Fe3O4 and Ni-BDC could effectively optimize the electronic structure modulation. These effects are favorable for the improvement of active sites, likely to be related to ECSA. Although ECSA can assess the number of active sites, it is hard to ensure all active sites measured by ECSA are catalytically active [41], we adopted ECSA value for normalizing the current density of LSV in Fig. 3a. Fe3O4/Ni-BDC-4 (Fig. S19) demonstrates the lower \u03b7 than those of pristine Ni-BDC and Fe3O4 after normalization. The normalized current density of Fe3O4/Ni-BDC-4 is considerably large, for example, reaching 2.6\u00a0mA\u00a0cm\u22122 at 1.55\u00a0V vs. RHE, in comparison to 0.19 and 0.05\u00a0mA\u00a0cm\u22122 for pristine Ni-BDC and Fe3O4, respectively. This result strongly indicates that the incorporation of Ni-BDC and Fe3O4 effectively promotes the catalytic activity. Further, EIS helps to understand charge transfer kinetics at the electrolyte/electrode interface. Nyquist plots of Fe3O4/Ni-BDC-4, Ni-BDC and Fe3O4 are shown in Fig. 4d. Diameter of the semicircles in high-middle frequency region corresponds to the charge-transfer resistance (Rct) [75]. Rct (10\u00a0\u03a9) of Fe3O4/Ni-BDC-4 during OER is significantly lower than those of pristine Ni-BDC (82\u00a0\u03a9) and Fe3O4 (746\u00a0\u03a9), implying a rapid charge transfer process on Fe3O4/Ni-BDC-4.The morphology and crystalline changes of Fe3O4/Ni-BDC-4 after duration test have been evaluated. XRD pattern of Fe3O4/Ni-BDC-4 (Fig. S20a) after LSV shows the disappearance of Fe3O4 peaks, indicating the possible amorphous transformation of Fe3O4. This observation may be attributed to the oxidation of ultra-small Fe3O4 nanoparticles (6\u00a0\u00b1\u00a02\u00a0nm) during OER, leading to the formation of oxy-hydroxides. Besides, TEM images (Fig. S20b and c) show iron oxides nanoparticles are still tightly anchored on the surface of Ni-BDC layers. While it is hard to obtain the clear crystalline interplanar of Fe3O4 nanoparticles in the high-resolution TEM image (Fig. S20d), further suggesting the amorphous transformation of Fe3O4 during the OER process. STEM-EDS mapping (Fig. S20e-i) demonstrates the uniform distribution of elemental Ni, Fe, O and C. Meanwhile, XPS results of Fe3O4/Ni-BDC-4 (Fig. S21) after long-term test indicate the partial transformation of metal units in Fe3O4/Ni-BDC-4 to high-oxidation state (Ni3+, Fe3+) due to the partial oxidation during OER process. In comparison to the pristine Fe3O4/Ni-BDC-4, larger satellite peaks in the XPS spectra (Fig. S21) after long-term test are observed, correlated with the oxidation of metal units in Fe3O4/Ni-BDC-4 during OER [27,76].DFT calculations have been performed to uncover the nature of Ni-BDC and Fe3O4/Ni-BDC catalyst and reveal their different performances on the oxygen evolution electrocatalytic process. The (200) surface of Ni-BDC was studied according to the XRD data (Fig. 1a), which is exposed with five-coordinated Ni atoms (Ni5c) and two-coordinated O atoms (O2c) (Fig. S26). The Bader charge analysis shows that the average charge of surface Ni in the Ni-BDC(200) system is\u00a0+\u00a01.345 |e| (Fig. 5\ne). After incorporation with Fe3O4, the XRD results (Fig. 1a and Fig. S1) show the gradual disappearance of the (200) main peak of Ni-BDC and emergence of (311) main peak of Fe3O4 in the Fe3O4/Ni-BDC composites. The Fe3O4(311) surface, therefore, has been considered as the possible representative active surface to investigate in the Fe3O4(311)/Ni-BDC(200) system (Fig. 5b and Fig. S26b). There are five-coordinated Fe atoms (Fe5c), two-coordinated O atoms (O2c) and three-coordinated O atoms (O3c) exposed on the Fe3O4(311) surface (Fig. S26b). Interestingly, the average charge of surface Ni in Fe3O4(311)/Ni-BDC(200) is slightly reduced to\u00a0+\u00a01.341 |e|. The average charge of surface Fe is\u00a0+\u00a01.867 |e| in Fe3O4(311)/Ni-BDC(200), higher than that in pristine Fe3O4(311) for\u00a0+\u00a01.522 |e|(Fig. 5e). The valence states of surface Ni sites are reduced while the surface Fe sites become oxidized on the Fe3O4(311)/Ni-BDC(200) surface. Those agree well with our XPS characterizations that the Ni 2p peak shifts to negative and Fe 2p peak shifts to positive (Fig. 1c and d, Fig. S3 and Table S1). It is safe to conclude that the valence states change of surface Ni and Fe sites are correlated to the improved OER performance. Furthermore, relative to the density of state (DOS) of the pristine Fe3O4 system [51], the density of states of Fe3O4(311)/Ni-BDC(200) system in Fig. 5d shows the Fermi level slightly left-shifting, indicating the electron donator role of the Fe3O4 in the composite system. The electrostatic potential analysis (Fig. S25) illustrates that the electrostatic potential of surface Ni layer is lower than that of Fe3O4 slab, indicating the partial electrons transferring from Fe3O4 to Ni-BDC in Fe3O4(311)/Ni-BDC(200) system, thereby leading a higher oxidation state of surface Fe sites and a reduced oxidation state of surface Ni sites. Therefore, the interaction mechanism of Fe3O4 and Ni-BDC from Fe3O4/Ni-BDC could be proposed. The local electronic environment of Ni nodes in the Ni-BDC is changed after coupling with Fe3O4, partial Ni nodes may be interacted or replaced by Fe3O4 nanoparticles. As BDC ligands are good electron acceptors [30], it suggests that Fe sites in the Fe3O4/Ni-BDC composites provide more electrons with a higher oxidation state in comparison to that of the pure Fe3O4, ensuring the successful formation of composites. Meanwhile, Ni sites in the Fe3O4/Ni-BDC composites will share the extra electrons from the Fe3O4, thus maintaining a reduced oxidation state in comparison to that of pristine Ni-BDC. The optimal electronic structureof Ni and Fe in the Fe3O4/Ni-BDC composites benefits the OER catalytic performance.To further understand the difference in OER electrocatalytic activity of Fe3O4/Ni-BDC and pristine Ni-BDC systems, we adopt the electrochemistry model developed by N\u00f8skov et al. and investigated the thermodynamics of four-electron reactive paths for OER from the free energy landscape (T\u00a0=\u00a0298.15\u00a0K) [77,78]. The elementary steps are shown as follows, with * denoted as the catalytic active sites or the adsorbed species:\n\n(3.1)\n\n\n\n\u2217\n\n+\n\nH\n2\n\nO\n\n(\nl\n)\n\n\u2192\n\n\nO\nH\n\n\n\u2217\n\n\n+\n\n\nH\n\n+\n\n+\n\n\ne\n\n-\n\n\n\n\n\n\n\n(3.2)\n\n\n\n\nO\nH\n\n\n\u2217\n\n\n\u2192\n\n\nO\n\n\n\u2217\n\n\n+\n\n\nH\n\n+\n\n+\n\n\ne\n\n-\n\n\n\n\n\n\n\n(3.3)\n\n\n\n\nO\n\n\n\u2217\n\n\n+\n\nH\n2\n\nO\n\n(\nl\n)\n\n\u2192\n\n\nO\nO\nH\n\n\n\u2217\n\n\n+\n\n\nH\n\n+\n\n+\n\n\ne\n\n-\n\n\n\n\n\n\n\n(3.4)\n\n\n\n\nO\nO\nH\n\n\n\u2217\n\n\n\u2192\n\nO\n2\n\n\n(\ng\n)\n\n+\n\n\nH\n\n+\n\n+\n\n\ne\n\n-\n\n\n\n\n\n\nFig. 5a and b show the free energy diagrams of OER on both Ni-BDC(200) and Fe3O4(311)/Ni-BDC(200) surfaces. The free energy diagrams show the step (3.2) referring to the formation of adsorbed O* species is the PDS for both two system (blue line at 0\u00a0V in Fig. 5a and b). Specifically, the free energy change of step (3.2) is 1.39\u00a0eV at the surface F5c site of the Ni-BDC(200) surface. The overpotential (\u03b7) is 0.16\u00a0V. In contrast, the free energy change for that step becomes 1.22\u00a0eV on the Fe3O4(311)/Ni-BDC(200) surface, 0.17\u00a0eV lower than that on the Ni-BDC(200) surface. This implies that the adsorbed O* species are more stabilized at the Fe3O4(311)/Ni-BDC(200) surface, thus enhancing the OER performance. The energy barriers for OH dissociation were also calculated in the two systems. The dissociation barrier of adsorbed OH to generate the adsorbed O is 0.13\u00a0eV on the Fe3O4(311)/Ni-BDC(200) surface, lower than that (0.21\u00a0eV) on the Ni-BDC(200) surface (see Fig. 5f). OH dissociation is more favourable on the Fe3O4(311)/Ni-BDC(200) surface, consistent with the free energy diagrams in this study. There are also four-coordinated Fe4c sites exposed on the Fe3O4(311)/Ni-BDC(200) surface and was also investigated to compare with Fe5c site. The step (3.1) was found to be the PDS (Fig. S27) with reaction energy of 1.67\u00a0eV. It is much higher than that at Fe5c site (0.96\u00a0eV) with an overpotential of 1.44\u00a0V. Thus the surface Fe5c site is more active towards OER on the Fe3O4(311)/Ni-BDC(200) surface. The Bader charge analysis shows that the adsorbed O* species on the surface Fe5c site possesses a charge of \u22120.60 |e|, while the charge of adsorbed oxygen at the surface Ni site on the pristine Ni-BDC(200) surface is \u22120.48 |e|. This indicates the stronger electronic interaction of Fe-O relative to that of Ni-O, stabilizing oxygen adsorption and lowering the free energy for step (3.2) in Fe3O4(311)/Ni-BDC(200) system. By applying a potential of 1.5\u00a0V (red lines of Fig. 5a and Fig. 5b), the free energy diagrams for both systems go down, showing the favourable thermodynamics for OER. The step 3.2 still has a lower free energy change in the Fe3O4(311)/Ni-BDC(200) system than the Ni-BDC(200) system (Fig. 5b), consistent with the better OER performance observed in the experiments. These theoretical findings propose the possible structure of Fe3O4/Ni-BDC system and reveal the Fe3O4/Ni-BDC catalyst favours the step of OH dissociation into adsorbed O species that boosts the OER performance. The introduction of balanced amount of Fe3O4 nanoparticles in the composite effectively modulates the electronic structure that lows the potential required for PDS, enhancing OER catalytic activity [25,73]. Besides, the 2D hierarchical-layer structure created by ultrafine Fe3O4 nanoparticles immobilized on 2D Ni-BDC layers provides a large surface area and promotes fast mass transport of the electrolyte to the reactive sites and the liberation of the generated oxygen gas. Meanwhile, the hierarchical composite structure could efficiently prevent the aggregation present in pure Ni-BDC layers that have poor stability (Fig. S13), maintaining the structural integrity during OER for a superior catalytic stability. The detailed XPS binding energy results (Table S1) differ from those in pristine Ni-BDC and Fe3O4, i.e. binding energy level of Ni 2p3/2 decreases, while the binding energy of Fe 2p3/2 increases slightly with the amount of Fe3O4 nanoparticles. This indicates that the Ni atoms in the composites possess higher electron densities than that of pristine Ni-BDC with the increasing amount of Fe3O4 nanoparticles. While Fe atoms in the composites have lower electron densities comparing with that of pure Fe3O4 and then trend to the stable electron densities. Initially, when the mass ratios of Fe3O4 in the composites are relatively low (Fe3O4/Ni-BDC-1, 2 and 3). The electronic densities of Fe and Ni atoms in the composites are tuned, offering improved OER performance. Further, Fe3O4/Ni-BDC-4 possesses the optimal electronic structure of Fe and Ni atoms, showing the best OER catalytic activity among five composites. When the Fe3O4 nanoparticles are further added to form Fe3O4/Ni-BDC-5, the electronic densities of Fe and Ni atoms in the composite is hardly changed in comparison to that of Fe3O4/Ni-BDC-4. Reversely, the further addition of Fe3O4 limits the OER performance with a reduced catalytic activity. All above results ensure that Fe3O4/Ni-BDC-4 could be a promising and stable OER electrocatalyst.Ultrafine Fe3O4 nanoparticles homogeneously immobilized on 2D Ni based MOFs (Fe3O4/Ni-BDC) were synthesized. The functionalized Fe3O4 nanoparticles (\u00d8 6\u00a0\u00b1\u00a02\u00a0nm) with abundant surface hydroxide groups are produced by a hydrothermal method, and then mixed into 2D Ni-BDC layers during synthesis (thickness: 4\u00a0\u00b1\u00a01\u00a0nm) creating strong interactions, which are not achieved by physically mixing the two components. Introduction of Fe3O4 modifies the integral electronic structure for reduced overpotential and prevents the aggregation of 2D Ni-BDC layers for enhanced OER catalytic stability. Different atom ratios of (Ni/Fe) in Fe3O4/Ni-BDC are tested for OER. Fe3O4/Ni-BDC-4 demonstrates the optimized OER performance with an \u03b7 of 295\u00a0mV at 10\u00a0mA\u00a0cm\u22122, a Tafel slope of 47.8\u00a0mV dec-1 and superior catalytic durability (40\u00a0h). DFT calculations further identify the active sites for Fe3O4/Ni-BDC as mainly contributed by Fe species with a higher oxidation state, and the PDS is the formation of the adsorbed O* species, which are facilitated in the Fe rich composite. The persistent stability during cycling (Fig. 3d) and the TEM images show that agglomeration is not occurring, indicating that this typically performance reducing effect can be handled. Such structure design methodologies for electronic structure and adsorbate modulation will inspire further development of promising catalysts for 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.Finance support from the Chinese Scholarship Council (201706220080) for W.H., the Natural Science Foundation of Hunan Province (2019JJ50526) for C. P., The Danish Council for Independent Research for the YDUN project (DFF 4093-00297) to J.Z., Villum Experiment (grant No. 35844) for X. X. is greatly acknowledged.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jechem.2021.05.030.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Two-dimensional (2D) metal organic frameworks (MOFs) are emerging as low-cost oxygen evolution reaction (OER) electrocatalysts, however, suffering aggregation and poor operation stability. Herein, ultrafine Fe3O4 nanoparticles (diameter: 6\u00a0\u00b1\u00a02\u00a0nm) are homogeneously immobilized on 2D Ni based MOFs (Ni-BDC, thickness: 5\u00a0\u00b1\u00a01\u00a0nm) to improve the OER stability. Electronic structure modulation for enhanced catalytic activity is studied via adjusting the amount of Fe3O4 nanoparticles on Ni-BDC. The optimal Fe3O4/Ni-BDC achieves the best OER performance with an overpotential of 295\u00a0mV at 10\u00a0mA\u00a0cm\u22122, a Tafel slope of 47.8\u00a0mV dec-1 and a considerable catalytic durability of more than 40\u00a0h (less than 5\u00a0h for Ni-BDC alone). DFT calculations confirm that the active sites for Fe3O4/Ni-BDC are mainly contributed by Fe species with a higher oxidation state, and the potential-determining step (PDS) is the formation of the adsorbed O* species, which are facilitated in the composite.\n               "}
{"full_text": "Data will be made available on request.Schiff base ligands, which can form stable coordination bonds with metal ions, have received considerable attention in recent years [1]. They have played an important role in the development of coordination chemistry due to their preparative accessibility and structural variety [2]. The chemistry of coordination compounds containing metal-nitrogen bonds has been of particular interest to researchers in recent years due to the extraordinary properties of many complexes [3]. Indeed, N4 and N2O2\n[4,5] M(II) nitrogen ligands have been used as catalysts in the reduction of organic substrates with nitro, olefinic, acetylenic and aldehyde groups under mild reaction conditions, as well as catalysts in the reduction of nitrogen. Other studies have shown that transition metal complexes based on Schiff NNNN base ligands function as excellent homogeneous and heterogeneous phase catalysts [6], precursors of electrocatalytic processes [7] and chemical sensors [8]. In particular, Schiff-based ligands are of considerable interest due to their structural properties of being potential models for several biological systems due to the chelation property of the ligands with most proteins [9]. These compounds also present biological, anticancer, antibacterial, antifungal activities. On the other hand, these ligands have an interesting application in the field of corrosion. Thus, in our laboratory, studies have shown that the L1 and L2 ligands are corrosion inhibitors of carbon steel in a HCl solution [10,11]. Indeed we have extended this study on the chelating behavior and the antimicrobial and antioxidant power of Cu(II),Ni(II),Co(II) and Zn(II) complexes based on the tetradentate ligand N4 [(N1Z, N2Z)-N1,N2-bis((1H-pyrrol-2-yl)methylene)ethane-1,2-diamine] (L) derived from 1H-pyrrole-2-carbaldehyde with ethylenediamine and to compare this potency with that of the free ligand. The structural study of the complexes was performed by chemical ionization mass spectrometry, UV\u2013visible spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR, 1H,13C). Schiff's base and its corresponding complexes were examined for their antibacterial and antioxidant activities against Gram (+) and Gram (-) bacteria, for their antioxidant activity against DPPH radical for their antimitotic and hemolytic biological activity.All chemicals used in this work were analytical reagent (AR) grade and of the highest purity. The reagents for the synthesis of the Schiff base ligand (L) were purchased from Alfa Aesar. Melting points were recorded in open capillaries by a Stuart Melting Point apparatus, SMP10. IR spectra of the compounds were recorded by FT-IR Fourier transform infrared FTIR TENSOR27 spectrometer using KBr pellets. The electronic absorption spectra were obtained by a Perkin-Elmer Lambda 35 UV\u2013vis spectrophotometer. Magnetic susceptibility of the metal complexes in the solid state was measured by the Gouy balance calibrated with mercuric tetrathiocyanatocobaltate (II). 1H NMR spectra of Schiff bases in DMSO\u2011d\n6 were recorded on a JEOL 500\u00a0MHz FT NMR System JNM-ECZ500R/S1 spectrometer. Thermal analyses (DTA and TG) were performed on a DTG-60H thermal analyzer from temperature 20\u00a0\u00b0C to 1000\u00a0\u00b0C at a heating rate of 20\u00a0\u00b0C/min. High resolution mass spectra (HRMS) were acquired by the electron boiling ionization (ESI) technique using a Bruker APEX-2.Ligand (L) was prepared by condensation of (1\u00a0g; 2 mmoL) 1H-pyrrole-2-carbaldehyde and ethylenediamine (0.31\u00a0g; 1 mmoL) in ethanol (30\u00a0mL) at reflux for 3\u00a0h. The progress of the reaction was monitored by TLC. The resulting pink crystalline precipitate was washed with ethanol and ether and dried under vacuum (Scheme 1\n).(L):R: 89\u00a0%; m.p. 182\u00a0\u00b0C; IR (KBr, cm\u22121) \u03bd: 3157(NH) 3088, 2940\u20132836C-Haleph, 1577 (CC), 1641 (NC), 1315(CN), 1315; 1H NMR (500\u00a0MHz,DMSO\u2011d\n6,ppm) \u03b4: 11,39 (1H, s, NH), 8.02 (1H,s,H-CN), 6.81\u20136.80 (1H,d,J\u00a0=\u00a05HZ), 6.38\u20136.37 (1H,m,Pyrrol) 6.05\u20136.04 (1H,m,Pyrrol) 2.46\u20132.45 (2H,t,J\u00a0=\u00a05HZ, ethylene); 13C NMR (125\u00a0MHz, DMSO) \u03b4:153,254 (CN), 130.48\u2013109.34 (Pyrrol-C), 61.97 (CH2\u2013 ethylene); MS [m/z]+=215.12;UV\u2013vis (DMSO): max (nm)\u00a0=\u00a0327,95; Solubility: DMSO, MeOH, EtOH, DMF, CHCl3.A solution of MCl2 (M\u00a0=\u00a0Zn, Cu, Ni and Co) dissolved in 15\u00a0mL of ethanol was added dropwise to an ethanolic solution (30\u00a0mL) of ligand L under stirring and reflux for 4\u00a0h. The precipitates obtained were filtered and purified by washing with ethanol and then dried under vacuum (Scheme 2\n).ZnL: solid black; R: 63\u00a0%; m.p \u00b0C:309.12; MS; conductivity\u039b(Scm2mole-1): 7; IR (KBr, cm\u22121) \u03bd: 2929\u20132864 (CH), 1602 (NC), 1559 (CC), 1286 (CN), 584 (M\u2212N); 1H NMR (500\u00a0MHz, DMSO\u2011d\n6, ppm) \u03b4:9.42 (1H,s,H-CN), 7.17\u20136.23 (3H,m,Pyrrol), 2,452 (2H,ethylene); \u03bceff (BM): dia; [m/z]+=277.04; UV\u2013vis (DMSO): max (nm)\u00a0=\u00a0330, 524; Solubility: DMSO, DMF, CHCl3.[CuL]Cl22H2O:solid black; Yield: 62\u00a0%; m.p\u00b0: 279,34; Molar conductivity\u039b (Scm2mole-1):57; Analysis Calculated for; IR (KBr, cm\u22121) \u03bd:3450 (H2O), 3302\u20133232(NH), 2937\u20132881(CH), 1635 (NC),1568 (CC), 1278 (CN), 528 (M\u2212N); 1H NMR (500\u00a0MHz, DMSO\u2011d\n6,ppm) \u03b4: 9.40 (1H,s,H-CN), 7.15\u20136.21 (3H,m,Pyrrol), 2.24 (2H,ethylene) 13C NMR (125\u00a0MHz, DMSO) \u03b4: (CN) 179.69, 140.29\u2013111,10 (Pyrrol-C) 59.92 (CH2-ethylene); \u03bceff (BM): dia;MS [m/z]+=278.07; UV\u2013vis (DMSO): max (nm)\u00a0=\u00a0330,400; Solubility: DMSO, DMF,CHCl3.[CoLCl2]H2O:Solid brown; Yield: 68\u00a0%; m.p:283,93; Molar conductivity\u039b (S cm2mole-1): 35.,9; Analysis Calculated for; IR (KBr, cm\u22121) \u03bd:3551 (H2O), 3216\u20133128 (NH),(CH)2959,1624 (NC), 1578 (CC), 1302 (CN), 525(M\u2212N); \u03bceff (BM):4.84 MS [m/z]+=344.01; UV\u2013vis (DMSO): max (nm)\u00a0=\u00a0365,640,708; Solubility: DMSO, DMF, THF.NiL: solid orange; Yield: 58\u00a0%; m.p: 345.12; Molar conductivity\u039b(S cm2mole-1): 4.2; Analysis Calculated for; IR (KBr, cm\u22121) \u03bd:2844\u20132915C-Haromatic, 1574 (NC), 1524(CC), 1303 (CN), 530 (M\u2212N); 1H NMR (500\u00a0MHz,DMSO\u2011d\n6,ppm) \u03b4: 7.57 (1H,s,H-CN), 6.68\u20135.89 (3H,m,Pyrrol), 2.45 (2H,ethylene) 13C NMR (125\u00a0MHz, DMSO) \u03b4: (CN) 160.63, 143.56\u2013111.30 (Pyrrol-C) 55.89 (CH2-ethylene); \u03bceff (BM): dia; MS [m/z]+=271,01; UV\u2013vis (DMSO): max (nm)\u00a0=\u00a0357,429,454,522; Solubility: DMSO, DMF, CHCl3.The 2,2-diphenyl-1-picrylhydrazyl (DPPH) is a stable free radical that accepts an electron or hydrogen radical to become a stable diamagnetic molecule [12] which is a stable scavenging activity in chemical analysis [13]. The antioxidant capacity of the tested products was estimated by comparison with a synthesized antioxidant (ascorbic acid). DPPH radical reduction capacity was determined by the decrease in its absorbance at 517\u00a0nm induced by the antioxidants. The absorption maximum of a stable DPPH radical in ethanol was at 517\u00a0nm. By using this method, it is possible to determine the free radical scavenging capacity of an antioxidant by measuring the decrease in absorbance of DPPH at 517\u00a0nm. Resulting from a color change from purple to yellow, the absorbance decreases when DPPH is trapped by an antioxidant, by hydrogen donation to form a stable DPPH molecule. The DPPH radical scavenging activity of the complexes was measured as indicated according to the protocol described by Lopes-Lutz et al [14]: each 1\u00a0mL of solution of the ligand and its complexes at different concentrations prepared in DMF, 2.5\u00a0mL of DPPH solution (2.4\u00a0mg/100\u00a0mL) was added. At the same time, a negative control was prepared by mixing 2.5\u00a0mL of DPPH with 1\u00a0mL of the methanol solution. After incubation in the dark for 30\u00a0min and at room temperature. Absorbance readings are taken at 517\u00a0nm using a spectrophotometer against a blank (1\u00a0mL of products at different concentrations\u00a0+\u00a02.5\u00a0mL of methanol). The ability to trap the DPPH radical was calculated using the following equation\n\n(1)\n\n\n%\nR\nS\nA\n=\n\n\nAc\n-\nA\ns\n\n\nAc\n\n\n\u00d7\n100\n\n\n\n\nAc: absorbance of the control (DPPH solution in the absence of the tested compound).As: absorbance in the presence of the tested compound.The microdilution test was used to determine the Minimum Inhibitory Concentration (MIC), in a96-well microplate. The ligand solution and its complexes were examined against two Gram-positive bacteria: Bacillus subtilis ILP1428B and Staphylococcus aureus CIP543154 (Pasteur Institute Collection), as well as two Gram-negative bacteria: Pseudomonas aeruginosaATCC27653 and Escherichia coli CIP5412 (American Type Culture Collection). Mueller HintonBroth was supplemented with the emulsifier (1\u00a0% (v/v) DMSO). Then, 50\u00a0\u03bcL of bacterial (106 CFU/mL) was deposed. Finally, bacterial growth was revealed by turning resazurin from purple to pink. The lowest inhibitory concentration of the ligand solution and its complexes corresponded to the lowest concentration that inhibited the reduction of blue resazurin dye into pink resorufin. MBC was determined by sub-culturing the contents of wells with greater concentrations than the MIC values on LB agar plates and incubating them at 37\u00a0\u00b0C for further 24\u00a0h. Experiments were repeated three times [15].The species used in this work is the Alenois Cress (Lepidium sativum). Seeds are washed with pure distilled water to eliminate any impurity and are tested for a germination rate higher than 95\u00a0% [16]. Seeds are sown in 50\u00a0mm diameter Petri dishes, lined with a layer of Whatman type filter paper, impregnated by 5\u00a0mL of aqueous solution of each ligand at a concentration of 1\u00a0mg/mL. The control box is impregnated by 5\u00a0mL of distilled water. Hydration is done once at the beginning of the test. All plates are placed in the oven at 25\u00a0\u00b0C and in the dark. The germination process and elongation of the seed radicles are observed directly in the Petri dishes every 24\u00a0h for one week. Three control replicates and three replicates of each of the products are used in Petri dishes containing 20 seeds each of average size i.e. (3\u00a0\u00d7\u00a020) seeds for each test. Data are expressed as the average radicle elongation and results are reported in mm.Calculation and expression of germination results.\n\n\u2022\nDetermination of the germination capacity of seeds of Lepidium sativum:\n\n\n\nSeed germination capacity is determined by calculating the seed germination rate expressed as a percentage:\n\n\n\n\n\n\n\n\nNumber\n\no\nf\n\ns\ne\ne\nd\ns\n\ng\ne\nr\nm\ni\nn\na\nt\ne\nd\n/\nT\no\nt\na\nl\n\nn\nu\nm\nb\ne\nr\n\no\nf\n\ns\ne\ne\nd\ns\n\n\n\n\n\u00d7\n\n100\n\n\n\n\n\n[Number of seeds germinated/Total number of seeds]\u00a0\u00d7\u00a0100\n\n\n\n\u2022\nDetermination of the germination inhibition index (GI):\n\n\nDetermination of the germination capacity of seeds of Lepidium sativum:Determination of the germination inhibition index (GI):This method is developed to determine the phytotoxicity of the ligand and its synthesized complexes. The monitoring of seed germination is determined every 24\u00a0h. The number of germinated seeds is noted and the percentage of germination inhibition is calculated as follows:\n\n\n\nIG\n%\n\n=\n\n\n\n\nPGte\n-\nP\nG\nt\nr\n/\nP\nG\nt\ne\n\n\n\n\n\u00d7\n\n100\n\n\n\n\nPGte: germination percentage of the control lot.PGtr: percentage of germination of the batch treated with the synthesized product.\n\n\u2022\nVigor of Lepidium sativum seedlings\n\n\nVigor of Lepidium sativum seedlingsAfter determining the germination rate for seven days for each of the replicas, the length of the radicle was measured. This quantity is expressed as the average of the radicle elongation and the results are reported in mm.\n\n\n\nSeedling\n\nv\ni\ng\no\nr\n\n=\n\nG\ne\nr\nm\ni\nn\na\nt\ni\no\nn\n\np\ne\nr\nc\ne\nn\nt\na\ng\ne\n\n\u00d7\n\ns\ne\ne\nd\nl\ni\nn\ng\n\nl\ne\nn\ng\nt\nh\n\n\n\n\nFresh human blood was collected in a sterile tube containing sodium citrate as an anticoagulant at a ratio of one volume to four volumes of blood. After decantation, the pellet was washed four times with sterile physiological water. In hemolysis tubes, 50\u00a0\u00b5L of the pellet is added to 1\u00a0mL of sterile physiological water mixed with 1\u00a0mg/ml of the component to be tested. The control tube is devoid of product. The preparations are then incubated in the dark for 1\u00a0h at room temperature and centrifuged at 1500\u00a0rpm for 3\u00a0min. Observation of hemolysis is performed directly with the naked eye. Each test is repeated three times.In test tubes, 1\u00a0mL of fresh human blood, collected on EDTA, is mixed with 1\u00a0mg of the test product. The control tube is devoid of product. The preparations are incubated in the dark for 1\u00a0h at room temperature. Thin smears are made, after homogenization of the blood, on clean slides. After drying, the smears are subjected to the May-Gr\u00fcnwald-Giemsa (MGG) staining. An observation is made under the optical microscope at a magnification of 1000x to illustrate the behavior of leukocytes towards the test products. Each experiment is repeated three times.All the synthesized metal complexes are solids and are in the form of colored powder. These products are stable in air and soluble in dimthylformamide (DMF) and dimethylsulfoxide (DMSO). The main physicochemical properties of the ligand (L) and the corresponding complexes are summarized in Table 1\n. Elemental analysis as well as FT-IR and UV\u2013vis measurements demonstrate that all the obtained complexes have a metal\u2013ligand stoichiometry (1:1) and present a composition in good agreement with the proposed formulas. The chelates were found to be stable under atmospheric conditions, whether stored in solution or as pure solids.The results of the conductimetric study of the 10-3 M concentration solutions in DMF of the ligand and its complexes are recorded in Table 1. The molar conductivities of the freshly prepared solutions of the complexes are between 4.2 and 57 Scm2mol\u22121.These values being low show that the complexes are not electrolytes [17]. The molar conductivity site of [CuL]Cl22H2O complex of 57 Scm2mol\u22121, respectively, referring to the electrolytic behavior of this complex. This result is proposed with the studies of Greenwood [18].The electronic spectra of the ligand (L) and its complexes (Fig. 1\n, Table 2\n) were recorded in DMF solutions (10-5 M) at room temperature. In the UV\u2013vis spectra of the parent ligand, an intense absorption band at about 327.95\u00a0nm has the n\u00a0\u2192\u00a0\u03c0* transition of the azomethine group. This band underwent a bathochromic shift for the complexes (Table 2), suggesting that Cu(II), Ni(II), Zn(II) and Co(II) are indeed coordinated [19]. Furthermore the electronic spectrum of the Co(II) complex shows the presence of two d-d bands at 640 and 708\u00a0nm attributed to the 4T1g\u21924A2g et 4T1g\u21924T1g(P) transitions respectively which suggests an octahedral geometry of the cobalt [20,21]. The value of magnetic moment for cobalt (II) complex was 4.84B.M. which was consistent with high spin octahedral geometry for Co(II) complex [22]. The electronic spectrum of complex Ni (II) shows two spin-allowed bands at. These absorption bands may be assigned to the 1A1g\u21921A2g and 1A1g\u21921Eg transitions, respectively, and reflect d8 ions in a square planar geometrical environment [23]. As for the copper complex the presence of an absorption band at 400\u00a0nm corresponds to the2Eg \u21902B1gtransition indicates the square planar geometry for Cu(II) complexes which is in perfect agreement with the literature data [21,24]. For the zinc complex, it does not show a d-d electronic transition due to the completely daughter d10 orbital. The appearance of the new transition absorption band at 524\u00a0nm is attributed to ligand\u2013metal charge transfer (LMCT) [20]. The complexes of Cu (II), Zn (II), and Ni (II) were also subjected to magnetic susceptibility test. They were diamagnetic, and supported by distinct signals in the 1H NMR spectra [21].IR spectra provide valuable information regarding the nature of the functional group. These studies are of great importance in the evaluation of important characteristic frequencies necessary for the comparative interpretation of the nature of the binding of Schiff base ligands and their metal complexes, molecular symmetry, electron distribution, and stability of the complexes formed. The main IR bands of the free ligand and their metal complexes are listed in Table 3\n.The IR spectra of the ligand and the Cu(II) and Co(II) complexes (Fig. 2\n) showed the presence of a characteristic band due to \u03bd(NH) around 3157\u20133088\u00a0cm\u22121, for the ligand and around [(3216\u20133128);(3302\u20133232) cm\u22121] for the Co(II)and Cu(II) complexes [25] the absence of this band for the zinc-nickel complex indicates deportation of the pyrolic nitrogen after coordination. An absorption band is systematically observed in the range 2862\u20132940\u00a0cm\u22121 for either the ligand or the complexes. This band is due to the deformation absorption of aliphatic CH The intense absorption band characteristic of the stretching vibrations of the azomethine group (-CN-) was observed at 1641\u00a0cm\u22121 in the spectrum of the Schiff base ligand. However, the band due to -CN- shifted to lower wavenumbers (1574\u20131636\u00a0cm\u22121) in the metal complex spectra [24]. This hypsochromic shift of the order of 5\u201367\u00a0cm\u22121 indicating the participation of the imine nitrogen in coordination. Also the vibrations of the double bond (CC) of the aromatic nuclei are characterized and maintained by a relatively intense and sharp band at 1577.97\u00a0cm\u22121 in the case of the ligand L and is shifted from about 1 to 53\u00a0cm\u22121 for the complexes of this same ligand [26,29]. In addition, new bands appear from 522 to 584\u00a0cm\u22121 in the spectra of the metal complexes which are attributed to \u03bd(M\u2212N) [27], confirming the coordination of copper(II), cobalt(II) zinc(II) and nickel(II) with the ligand [26]. After complexation, of the ligand with the copper and cobalt metal salts, a broad absorption band was observed around 3450\u20133551\u00a0cm\u22121 which is attributed to the presence of water of hydration molecules in the crystal lattice of Cu(II) and Co(II) complexes.\nTable 4\n shows the 1H NMR spectral data of the ligand (L) (Scheme3\n) and the corresponding Ni(II), Cu(II), Co(II) and Zn(II) complexes.The 1H NMR spectrum of the Schiff base (L) (Fig. S1), shows a singlet at 11.34\u00a0ppm which corresponds to the proton NH1\n[31].The signal at 8.018\u00a0ppm is attributed to the azomethine protons (-H5C\u00a0=\u00a0N) In the region 6.81 and 6.036\u00a0ppm, multiplets were observed, and can be attributed to the protons of the pyrrolic rings. The methylene protons (N-(CH2)6) appear at 2.46\u00a0ppm. Concerning the spectra of the Zn(II) and Ni(II) complexes (Figs. S2 and S3), also the absence of the NH proton of the pyrolic ring was observed thus suggesting the involvement of the ring nitrogen in the coordination with the metal ion. A shift of the signal of the azomethine protons (H5C\u00a0=\u00a0N) of the complexes with respect to those of the free ligand towards the strong fields [(9.42; 9.41; 7.57) ppm] for the Zn(II), Cu(II) and Ni(II) complexes which confirms the coordination of the azomethine group with the metal ion (data not shown).On the other hand, it is observed after coordination that the aromatic protons of the pyrrolic ring appear in the regions [(7.17\u00a0ppm-6.23\u00a0ppm); (7.15\u00a0ppm-6.21\u00a0ppm); (6.67\u20135.89\u00a0ppm)] for Zinc, Copper and Nickel. The aliphatic protons (CH2) of the complexes were detected at [2.45\u00a0ppm, 2.24\u00a0ppm and 2.45\u00a0ppm] for Zinc, Copper and Nickel (Table 4).The 13C NMR spectral data of the base ligand of Schiff L, shows that the peak appearing at 153.25\u00a0ppm is assignable to the imine carbon atoms. On the other hand, the signals observed in the region 130.48\u2013109.37\u00a0ppm are assigned to the signals of the pyrrolic ring carbons of the ligand. The aliphatic N-CH2 carbon peaks of the ligand was detected at 61.97\u00a0ppm (Fig. S4). The same signals are present in the 13C NMR spectra of the complexes (Figs. S5 and S6), but they are shifted to higher values invoking the coordination of the ligand to the metal ions by its azomethine group. The signals of the pyrrolic ring carbons appear in the regions [(140.29\u2013111.10); (143.56\u2013111.30) ppm] for the Cu(II) and Ni(II) complexes respectively. The aliphatic carbon (CH2) peaks are detected at 59.92 and 55.89\u00a0ppm for the Cu(II) and Ni(II) complexes (Table 5\n).The mass spectra of the ligand and its complexes were recorded on HPLC-MS equipment using the electrostatic spray ionization (ESI) technique. The mass spectrum of ligand (L) revealed a molecular ion peak at [m/z]\u00a0=\u00a0215 which confirms the proposed formula. The mass spectra of the Co(II), Ni(II), Cu(II) and Zn(II) complexes, were recorded and all spectra show peaks at molecular ions (M+). The proposed molecular formula of these complexes was confirmed by comparing their molecular formula weights with the m/z values. The molecular ion (M+) peaks obtained for the different complexes are (1)\nm/z\u00a0=\u00a0344 (Cobalt(II) complex), (2) m/z\u00a0=\u00a0271 (Nickel(II) complex) (3) m/z\u00a0=\u00a0278 (Copper(II) complex) (4) m/z\u00a0=\u00a0277 (Zinc(II) complex). These data are in good agreement with the proposed molecular formula for these complexes, i.e. [MLCl2] respectively [where M\u00a0=\u00a0Co(II), Ni(II)], and [ML] [where M\u00a0=\u00a0Cu(II) and Zn(II)], L\u00a0=\u00a0ligand. This confirms the formation of the Schiff base complex framework.The thermal behavior of transition metal complexes was studied to establish the decomposition process. To perform the measurements of this analysis, the temperature was increased from 20\u00a0\u00b0C to 1000\u00a0\u00b0C at a rate of 20C min\u22121 under air atmosphere. The thermograms obtained are shown in Figs. S7\u2013S10 and the thermal data of the complexes are shown in Table 6\n.The ATG curve indicates that the cobalt complex is decomposed in four main steps, the first is observed in the temperature range of 39.5\u2013200C\u00b0 with a weight loss of 4.67\u00a0%, associated with the loss of one molecule of water of hydration [28]. The DTA curve coincides exactly with the observed weight loss and presents a first endothermic reaction at 100.04\u00a0\u00b0C. The second decomposition step in the temperature range 201.09\u2013290.05\u00a0\u00b0C corresponds to the loss of a chlorine atom, with a mass loss of 9.28\u00a0% [29]. his is also highlighted by an exothermic peak at 288.03\u00a0\u00b0C. The third step c was attributed to the elimination of the organic part C12H12N4 with a weight loss of 51.77 accompanied by an exothermic peak observed at 478.5\u00a0\u00b0C. The fourth decomposition step in the temperature range 556.72\u2013616.92C corresponds to the loss of 9.33\u00a0% and was assigned to the elimination of a second chlorine atom in the presence of the ATD peak at 605.18\u00a0\u00b0C to finally give CoO as the residue. The thermogram of the Zn(II) complex showed a single decomposition step in the temperature range 296.21\u2013716.2\u00a0\u00b0C corresponds to the loss of one molecule of the ligand C12H12N4, with a mass loss of 79.14\u00a0%. The exothermic change between 306.73 and 569.71\u00a0\u00b0C in the presence of the ATD peak at 568.1\u00a0\u00b0C is related to the decomposition of the ligand. The TG plot of the Ni(II) complex showed a decomposition pattern with two well-defined decomposition steps The first decomposition step in the temperature range 237\u2013439\u00a0\u00b0C corresponds to the loss of C4H3N molecule with a mass loss of 18.83\u00a0% (Calc. 19.3\u00a0%) accompanied by an exothermic peak at 347.53\u00a0\u00b0C. The second decomposition step corresponds to the loss of one molecule C8H9N3 with a mass loss of 53.95\u00a0%, and this is confirmed by the presence of an exothermic peak at 496.86\u00a0\u00b0C on the DTA curve. The final product of the decomposition of the Ni(II) complex corresponds to nickel oxide. The first stage of decomposition of the Cu(II) complex is accompanied by a loss of mass of 11.93\u00a0% attributed to the departure of two molecules of water of hydration this is confirmed by the presence of an endothermic peak at 100.67\u00a0\u00b0C on the DTA curve. The second decomposition step in the temperature range 227.47\u2013525.47\u00a0\u00b0C gives a loss of mass of 67.26\u00a0% corresponding to the loss of C12H12N4 accompanied by an endothermic peak at 456.6\u00a0\u00b0C.The antioxidant activity of ligand (L) and its complexes Ni(II), Co(II), Zn(II) and Cu(II) was evaluated using the DPPH radical assay (Table 7\n) [30\u201332]. In the present study the analysis of the results obtained showed that the DPPH free radical scavenging activity of the complexes is higher than that of the free ligand. Zinc and copper complexes showed, for low concentrations, a very high activity compared to the ligand, as well as compared to nickel and cobalt complexes. This difference in activity can be attributed to the coordination environment and the redox properties which depend on several factors such as the size of the chelate ring, the axial ligation and the degree of unsaturation in the chelate ring [33]. Moreover, the low antioxidant activity of the Co(II) and Ni(II) complexes compared to the Cu(II) and Zn(II) complex could be due to the steric hindrance induced by the geometrical structure, preventing the approach of the DPPH radical towards the active centers of the complex [34]. The antioxidant activity of all the complexes studied is significantly higher than that of ascorbic acid. We can conclude that the complexation of this ligand promoted the antioxidant power which can be translated due to the electron withdrawing effect of M(II) ion (M\u00a0=\u00a0Zn(II),Cu(II),CoII) and Ni(II)) facilitates the release of hydrogen as a free radical in the presence of DPPH [35,36].The results of antibacterial effect are presented in Table 8\n. A careful observation of the results indicated a moderate antibacterial power against all tested strains. In addition, [CoLCl2]H2O and ZnL were more efficient against S. aureus, whichvalue of MIC 0.312\u00a0mg/mL, which in accordance with the results obtained by Kargar et al [31]. Furthermore, similar studies were performed to assess whether ligand and its complexes also inhibited \u03b1-amylase, as a carbohydrate-hydrolyzing enzyme. The results revealed that Zn possessed the highest inhibitory activity as compared to other complexes, whereas ligand had the lowest inhibitory effect [37]. On the other hand, B. subtilis and E. coli were more sensitive to [CoLCl2]H2Owith MIC value (0.019\u00a0mg/mL). These results showed that the chelation of the ligand increase the antibacterial effect, which was also confirmed by Polo-Cer\u00f3n [38]. It might be related to a modification in cation polarity caused by the hybridization of ligand filled orbitals with empty \u201cd\u201d orbitals of the metals. Generally, ligands and their respective metal complexes are moderately more potent inhibitors of Gram-positive rather than Gram-negative bacterial strains. This is due to the fact that Gram-positive bacteria have a thick peptidoglycan layer but no outside lipid membrane, whereas Gram-negative bacteria have a thin peptidoglycan layer but an outer lipid membrane [39].\n\n\n\u2022\n\nGermination capacity of Lepidium sativum seeds:\n\n\n\n\nGermination capacity of Lepidium sativum seeds:\nThe germination capacity of Lepidium sativum seeds after hydration with distilled water in the control batches was 95\u00a0% (Fig. 3\n).After treatment with the different products at a concentration of 1\u00a0mg/mL the germination percentages, after seven days of incubation, ranged from 0\u00a0% observed in the [CuL]Cl2H2O ligand batch to 60\u00a0% for that of [CoLCl2]H2O and ZnL (Fig. 4\n).\n\n\u2022\n\nGermination inhibition capacity of Lepidium sativum seeds\n\n\n\n\nGermination inhibition capacity of Lepidium sativum seeds\n\nFig. S11 shows the inhibition rate of the ligand and its complexes during the seven days of treatment and thus confirms the inhibitory effect of the five products from the first day of incubation. The results again show that [CuL]2H2O exerts a complete inhibition of germination on all seeds (100\u00a0% inhibition rate). The inhibition rate of the other components is 78\u00a0% for the L ligand, 57\u00a0% for the NiL products and 36\u00a0% for the [CoLCl2]H2O and [ZnL]Cl2.\n\n\n\u2022\n\nVigor of Lepidium sativum plantlet\n\n\n\n\nVigor of Lepidium sativum plantlet\nThe vigor of Lepidium sativum seedlings gives us information on the mitotic capacity of the radicles in the presence of the synthesized products compared to the control test, the greater their length the greater the speed of growth and multiplication of the cells and vice versa. Fig. 5\n shows that the length of Lepidium sativum radicles after 7\u00a0days of treatment is decreased by 100\u00a0% in the presence of [CuL]Cl22H2O, 91\u00a0% for ligand L, 66\u00a0% for ZnL and 86\u00a0% for [CoLCl2]H2O andNiL.These results of phytotoxicity tests indicate that the ligand and its corresponding metal complexes exhibit antimitotic activity with a higher capacity of this activity for [CuL]Cl22H2O and less effective for ZnL. These results are in agreement with those found by Abdou Saad El-Tabl et all whose copper-containing metal complex had antitumor activity on hepatic carcinoma cells (Hep-G2) [40], and also with the study of Tudor et all which showed that copper-based complexes showed an antimitotic effect on cervical cancer cell lines \u201cHeLa\u201d[35], as well as the study of Ceyhan et all who tested their copper complexes on the ovarian cancer cell line \u201cA2780\u2033, and proved that their synthetic substances induced a loss of deviability of the tested tumor cells [36].\n\n\u2022\n\nHemolysis test\n\n\n\n\nHemolysis test\nThis test is carried out in order to observe the in vitro effect of the ligand and its synthesized complexes on the behavior of human red blood cells in order to show the presence or absence of toxicity of these different tested components. Blood cells have a vital and important role on human health. They are very sensitive to changes in the composition and osmotic pressure of the surrounding environment. The results obtained show a hemolytic effect of [CoLCl2]H2O complexes following a lysis of the red blood cells releasing the hemoglobin, the tube presents a red pigmented supernatant (Figure S12). However, the other components have no hemolytic effect, the supernatant remains clear and therefore retains its hemoglobin content.\n\n\u2022\n\nEffect of the ligand and its corresponding complexes on leukocytes\n\n\n\n\nEffect of the ligand and its corresponding complexes on leukocytes\nThis test is carried out with the aim of concretizing the in vitro effect of the ligand and its synthesized complexes on the leukocytes which are the nucleated cells and whose role is important on the defense and maintenance of the immune system in man. Fig. 6\n illustrates the observation under the optical microscope of blood smears made from human blood treated with the different synthesized products. The control smear is made from fresh untreated blood. The results confirm the hemolytic effect of the [CoLCl2]H2O component, the red blood cells appear pale and emptied of their hemoglobin and the leukocytes are totally deformed, altered and have a defragmented nucleus. This complex acts not only on the integrity of the cell membranes but also on the genetic material of the cell, which confers to this product a genotoxic and lytic character for the blood cells. Microscopic observation of other smears taken from blood treated with the different products (L, [CuL]Cl22H2O, ZnL and NiL) showed no destructive effect on blood components. This tolerance test of eukaryotic and human cells towards the products synthesized for this study shows the destructive and thus toxic effect of complex [CoLCl2]H2O.The synthesis and structural study of the ligand [(N1Z,N2Z)-N1,N2-bis((1H-pyrrol-2-yl)methylene)ethane-1,2-diamine] (L) and its corresponding complexes of Cu(II), Co(II) and Zn(II) were carried out by IR, UV\u2013Visible, (1H,13C)NMR and Differential Thermal Analysis (DTA, TGA) spectrographic techniques and mass spectrometry. These last ones with the study of the conductivity showed that these complexes are not electrolytes on the one hand, and on the other hand that the ligand is coordinated to the metal by the four nitrogen atoms. The electronic spectral data of the complexes suggest an octahedral geometry for the cobalt and nickel complexes, tetrahedral for the zinc complex and square plane for the copper complex. The antioxidant activity of the ligand and its metal complexes was studied by DPPH free radical scavenging methods and showed that the ligand and its complexes have very good radical scavenging activity compared to standard ascorbic acid and that the zinc (II) complex has a higher antioxidant activity with an IC50\u00a0=\u00a00.0047\u00a0g/mL. Biological studies of these complexes show better activity compared to the ligand and the Zn(II) and Co(II) complexes have good activity against all bacterial strains. The toxicity study concludes that the ligand and its complexes have significant biological activity and inhibitory power on promising use in cancer treatment after further study on animal cells.\nIbtissam Elaaraj: Conceptualization, Methodology, Validation, Investigation, Data curation, Writing \u2013 original draft. Najia Moukrad: Methodology, Validation, Investigation, Data curation. Aziz Bouymajane: Investigation, Writing \u2013 review & editing. Safae Er Raouan: Investigation. Asmae Nakkabi: Investigation. Oumayma Oulidi: Investigation. Fouzia Rhazi Filai: Conceptualization, Resources, Supervision, Funding acquisition. Ibnsouda Koraichi Saad: Investigation. Francesco Cacciola: Resources, Writing \u2013 review & editing, Supervision, Project administration. Noureddine El Moualij: Investigation. Mohammed Fahim: Conceptualization, Methodology, Validation, Data curation, Writing \u2013 original draft, 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.Supplementary data to this article can be found online at https://doi.org/10.1016/j.rechem.2023.100787.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  In this contribution new Cobalt(II), Nickel(II), Copper(II) and Zinc(II) complexes based on the tetradentate ligand [(N1Z,N2Z)-N1,N2-bis((1H-pyrrol-2-yl)methylene)ethane-1,2-diamine] were synthesized and characterized by spectral techniques such as UV\u2013Visible, infrared, nuclear magnetic resonance (NMR, 1H,13C), thermal analysis (DTA and TGA) and mass spectrometry. Results showed that Co (II) and Ni (II) complexes have an octahedral geometry, Zn (II) complex has a tetrahedral structure, and Copper (II) complex has a square planar geometry. Furthermore, based on the molar conductivity values, these complexes were considered as non-electrolytes except the copper complex. The synthesized ligand and their complexes were assessed for their antioxidant and antibacterial activities. All complexes showed greater antioxidant and antibacterial activities with respect to the ligand. In addition, the ligand and its complexes were also tested biologically first on the germination of Lepidium sativum seeds, which is a phytotoxic and antimitotic test, and on the hemolytic and genotoxic behavior of human blood cells. A total inhibition on germination and radicle growth of Lepidium sativum seeds treated with [CuL]Cl22H2O solution was recorded. However, the [CoLCl2]H2O, caused a complete hemolysis of red blood cells and total alteration of nuclei and membranes of leukocytes.\n               "}
{"full_text": "Air pollution contamination resulting from volatile organic compounds (VOCs) is an internationally researched topic (Andelman, 1985; Harkov et\u00a0al., 1985; Abumaizar et\u00a0al., 1998; Clarke et\u00a0al., 2008; Cho et\u00a0al., 2018; Kim and Lee, 2018). VOCs are carbon-based volatile chemicals emitted into the atmosphere by industry and automotive exhaust causing different environmental and health problems (Bernstein et\u00a0al., 2008; Yu and Kim, 2012; Yang et\u00a0al., 2019). In recent years, there has been a dramatic increase in the rate of VOC emissions, directing research efforts to find an efficient and cost-effective method to reduce pollutants (Khan and Ghoshal, 2000; Li et\u00a0al., 2009; Jafari et\u00a0al., 2018; Piazzoli and Antonelli, 2018; Lyu et\u00a0al., 2020). Piumetti et\u00a0al. (2015) showed that the VOC emissions have a direct effect on the formation of ozone and smog in the troposphere as well as ozone depletion in the stratosphere. Different VOCs such as formaldehyde, naphthalene, chloroform paradichlorobenzene (1,4-dichlorobenzene), acetaldehyde, benzene and toluene are considered toxic and carcinogenic to humans (OEHHA, 2005; Batterman et\u00a0al., 2012; Hakim et\u00a0al., 2012; Chin et\u00a0al., 2013; Louie et\u00a0al., 2013; Chen et\u00a0al., 2017; Latif et\u00a0al., 2019; Nair et\u00a0al., 2019). Rezaee et\u00a0al. (2008) showed that the inhalation of toluene affects the nervous system by decreasing the ability to focus and think, memory loss, muscular deficiencies, and vision problems. Therefore, there is an urgent need for effective processes to remove and reduce VOCs from the environment, eliminating their effect on human health and improving environmental quality.Among VOCs, toluene (TOL) is considered a highly representative model as it contains aromatic hydrocarbons that resist biodegradation under normal conditions (Momani and Jarrah, 2009; Dole et\u00a0al., 2013). Different epidemiological studies have confirmed that exposure to TOL can cause severe health problems including effects on the nervous system, memory, and muscles (Djurendic-Brenesel et\u00a0al., 2016; Jafari et\u00a0al., 2019). Other studies have shown that the inhalation of TOL as a suspected factor of causing cancer (Dees et\u00a0al., 1996; Lee et\u00a0al., 2006, 2008). Toluene has been detected in different industrial processes such as printing, paint pressing and petrochemical industries (Jenck et\u00a0al., 2004; Kamal et\u00a0al., 2012; Dole et\u00a0al., 2013). Toluene (TOL) has a long half-life (Labeau et\u00a0al., 2003), and it also has important photochemical properties leading to the creation of tropospheric ozone (Derwent et\u00a0al., 1996; Zheng et\u00a0al., 2009; Nair et\u00a0al., 2019).The removal of VOCs via biotic (e.g., biodegradation) and abiotic processes (incineration, adsorption, micro-filtration) have been widely employed (Vandenbroucke et\u00a0al., 2011; Gil et\u00a0al., 2014; Li et\u00a0al., 2014). Biotic biodegradation using aerobic processes has been recognized as a reduction alternative for only biodegradable VOCs (Urase and Kikuta, 2005; Onesios et\u00a0al., 2009; Lahti and Oikari, 2011). Non-biodegradable and/or toxic contaminants are difficult to be reduced by these processes. Abiotic processes, on other hands, have many disadvantages including the production of secondary pollutants, high cost, and energy requirements, difficult operational conditions and low efficiencies under low VOC concentrations (Li et\u00a0al., 2014). Different studies have focused on developing economically feasible and effective treatment processes for the removal of VOCs (Everaert and Baeyens, 2004; Moulis and Kr\u00fdsa, 2013; Tejasvi et\u00a0al., 2015; Qian et\u00a0al., 2018).Amongst these processes, advanced oxidation technologies (AOTs) showed promising results in removing different VOCs from the environment (Gamal El-Din et\u00a0al., 2006; Al Momani, 2007; Al Momani and Jarrah, 2010; Hussain et\u00a0al., 2011; Almomani and Baranova, 2013; Liu et\u00a0al., 2019). Photo-catalytic oxidation processes (PHCOPs) are an important part of AOTs that can be performed through the exposure of the photo-catalyst to light photons and generate radicals that attack the pollutants leading to their degradation into eco-friendly products (Zhao et\u00a0al., 2016; Yang et\u00a0al., 2019). The PHCOPs have many advantages over other treatment processes including high efficiency, low utilization costs, long lifetime of the catalysts, mild operational conditions and the ability for breakdown wide varieties of complex pollutants into simpler compounds.In recent years, AOTs and mainly PHCOPs were operated with the solar irradiation to reduce the operational cost while maintaining high efficiency. In such combinations, titanium dioxide (TiO2) appears as the main photo-catalyst due to its low cost and high physio-chemical characteristics (chemical and thermal stability, non-toxic, and high photo-catalytic activity) (Fern\u00e1ndez-Garc\u00eda et\u00a0al., 2004; Khalifa, 2005; Chen and Zhang, 2008; Wu et\u00a0al., 2013; Tejasvi et\u00a0al., 2015; Nomura et\u00a0al., 2020; Zeng et\u00a0al., 2020). The use of TiO2 in AOTs showed high removals of TOL and ethylbenzene under UV irradiation (Chen and Zhang, 2008). However, due to high UV-driven activity triggered by TiO2\u2019s wide bandgap (\u22483.2\u00a0eV) and the rapid recombination of electron-hole pairs, its efficiency with the sunlight irradiation is very low. As such, the enhancement of the photo-catalytic ability of TiO2 under visible light is of great importance. Toward this aim, rigorous efforts have been given to the addition of dopant elements into the lattice of TiO2 to permit its use under visible light. Such enhancement will permit the use of sustainable energy resources (e.g. solar irradiation) rather than the need for the production of the expensive UV light (Labeau et\u00a0al., 2003; Almomani et\u00a0al., 2016, 2018a, 2018b).Different research works have reported that doping TiO2 with other metals could increase its service life and enhance its TiO2 activity under solar irradiation (Zhou et\u00a0al., 2006; Ni et\u00a0al., 2007; Devi et\u00a0al., 2009, 2010; Christoforidis et\u00a0al., 2012). The doped metal improves the photo-catalytic activity by shifting the adsorption spectrum to the visible light range, decreasing the bandgap energy, improving the production of electron-hole pairs, rising the speed of photon transfer to the surface of the catalyst and reducing the recombination of the electron-hole pairs. Moreover, the presence of metals with TiO2 can contribute to electron-hole separation and thus enhance the photo-catalytic activity of the process (Litter and Navio, 1996; Zhang et\u00a0al., 1998; Ad\u00e1n et\u00a0al., 2007; Li et\u00a0al., 2014). Kundu et\u00a0al. (2014) reported a 70% loxacin (25\u00a0ppm) photo-degradation using 1\u00a0g/L of Ni-doped on TiO2 prepared by hydrothermal method. Rahimi et\u00a0al. (2012) showed that the photocatalytic degradation performance of methylene blue using N- and S-co-doped TiO2 was 16% higher than naked TiO2. The higher photo-catalytic activities were related to the reduction in the band-gap of the co-doped TiO2. Similarly, Wen et\u00a0al. (2009) reported significant improvement in the photo-catalytic degradation of methylene blue using I\u2013F-co-doped TiO2. The improved photo-catalytic activity was related mostly to the significant increase in the surface area and stronger absorbance in the visible light range after doping with I and F (Almomani et\u00a0al., 2019). enhanced the solar photo-reduction of CO2 by adding Cu into the lattice of TiO2.Different procedures were proposed for the preparation of metal-doped TiO2 including precipitation (Dvoranova et\u00a0al., 2002), hydrothermal, solvothermal (Zhu et\u00a0al., 2006), chemical vapor deposition (Wu et\u00a0al., 2007) and electrospinning (Patil et\u00a0al., 2003). Among all methods, sol-gel method is advantageous for the synthesis of nano-powders due to the production of homogenous and high purity powders under controlled stoichiometry and ambient temperature (Akpan and Hameed, 2010; Bhosale et\u00a0al., 2016; Catauro et\u00a0al., 2017; Elsellami et\u00a0al., 2018; Xiao et\u00a0al., 2018; Ji et\u00a0al., 2019).Although different studies reported doping TiO2 with metal such as V, Cr, Fe, N, C, and S, up to our knowledge there is no study investigated the effect of doping of Cobalt (Co) into the TiO2 lattice. Moreover, a well-established rule concerning the optimum Co\u2013TiO2 composition to TOL removal efficiency has not been reported yet. There still a lack of knowledge of how doped metal affects the mechanism of solar PHCOPs. Discrepancies also exist regarding the optimal doping content in the TiO2 lattice for maximum VOC removal. Accordingly, the present work presents the preparation of a co-doped Co\u2013TiO2 photo-catalyst using a modified sol-gel method. A multi-characterization technique (UV\u2013Vis, N2 isotherms, XPS XRD, TEM) was used to examine the structural and electronic properties of the synthesized photo-catalysts. The as-synthesized catalyst was tested against sunlight toward the photo-oxidation of TOL. The catalyst selectivity and oxidation products were identified and the main mechanism of photo-catalytic oxidation of TOL under solar irradiation was presented.Analytical grades of Cobalt (II) acetate (CH\u2083COO)\u2082Co\u00b74H\u2082O, Merck, CAS#: 6147-53-1), mono-ethanolamine (MEA) (NH2CH2CH2OH, Merck, CAS#: 141-43-5), isopropanol alcohol (C3H8O, Merck, CAS #: 200-661-7), titanium butoxide (Ti(OCH2CH2CH2CH3)2, Merck, CAS #: 5593-70-4), ethyl alcohol (C2H5OH, CAS #: 64-17-5.) and toluene (C\u2086H\u2085CH\u2083, Merck, CAS #: 108-88-3) were used in this study.A sol-gel method was used to prepare the Co\u2013TiO2 photo-catalyst with a mass fraction of Co of 1, 2, 5 and 10\u00a0wt%. The preparation process is similar to the procedure proposed by (Bhatia et\u00a0al., 2016) with some modification. For each photo-catalyst, a specific amount of (CH\u2083COO)\u2082Co\u00b74H\u2082O, calculated based on the required mass fraction, was added stepwise to a mixture of 0.91\u00a0g NH2CH2CH2OH, 10\u00a0mL deionized water, and 15\u00a0mL C3H8O producing solution I. In a separate bottle, 10\u00a0mL of (Ti(OCH2CH2CH2CH3)2 was dispersed in 40\u00a0mL C2H5OH and sonicated for 20\u00a0min producing solution II. Solution II was added into the solution I in a stepwise fashion, mixed at 120\u00a0rpm and room temperature, and a stable sol was finally obtained after stirring for 2\u00a0h. The resulted sole was left reacting for another 6\u00a0h at room temperature, dried with dry air at 75\u00a0\u00b0C for 40\u00a0h, calcined at 500\u00a0\u00b0C for 2.5\u00a0h and used in tests.The composition and morphology of the photo-catalyst were examined using scanning electron microscopy (SEM\u2013EDS, Quanta 600 model). Tests were performed at an electron beam of 20\u00a0eV. The structure of the photo-catalyst was tested using XRD (Hiltonbrooks). A Brunauer\u2013Emmett\u2013Teller (BET) analyzer was utilized to estimate the surface area of the nano-catalyst. An XPS (Kratos Axis Ultra) was used to determine the chemical composition and electronic states of the photo-catalyst. A Porosimetry analyzer (Micromeritics Autopore IV 9500 V1.05) was used to determine the pore size and porosity of Co\u2013TiO2 under mercury (Hg) pressure in the range of 0.1\u201320,000 psia. The band-gap and visible light absorbance were measured by UV spectroscopy (UV\u2013Vis, Cary 300).Catalytic decomposition of TOL was carried out in a flow-type solar pilot plant (SPP) as shown in Fig.\u00a01\n. The SPP consists of synthetic air (20\u00a0vol % O2/N2), TOL tanks (Aldrich, purity 99.8%), a humidifier, a gas mixer, solar photo-reactor (SPHR) and a gas chromatograph. The SPHR contains four quartz tubes (L\u00a0=\u00a040\u00a0cm and ID\u00a0=\u00a01.6\u00a0cm) making a 45\u00b0 with the horizontal line. The photo-elements are hosted within a compound parabolic collector (CPC) to allow all the solar radiation (direct and reflected) arriving the solar platform to be available for TOL oxidation. The as-prepared photo-catalysis were attached to quartz tubes following the procedure presented in our previous work (Almomani et\u00a0al., 2016). The sun\u2019s movement was tracked automatically and a radiometer (Macam Q102 PAR) was used to determine the light intensity available for solar oxidation. The effluent line is connected to a humidity meter and Gas Chromatograph (GC) for off-gas analysis. The mass flow rate in the inlet line was controlled using a mass flow controller (MFC- SFC5300, USA). The temperatures of the TOL and humidifier were controlled at 25\u00a0\u00b1\u00a02\u00a0\u00b0C. The concentrations of inlet and outlet streams were determined by a GC-TCD (PerkinElmer Clarus 500).The photo-catalytic oxidation of TOL over TiO2 and CO\u2013TiO2 with different Co mass fractions was conducted at atmospheric pressure. The gas-phase inlet mixture (flow rate of 27.5\u201382.5 100\u00a0L/min) was prepared by mixing TOL vapor with a wet air stream producing a gas stream with the required inlet concentration of TOL. The mixture was circulated through the reactor for 80\u00a0min\u00a0h in dark. Then, the reactor cover was removed, and the photo-catalyst was left to react under natural solar irradiation. The inlet and outlet streams concentrations were analyzed by an online GC-TCD using several columns (Shin Carbon ST, Q PLOT, 2 OV101, and molecular sieve) to quantify TOL, O2, CO2, CO, Benzene and benzaldehyde. The detection limit for all these gases was determined to be in the range of 0.2\u00a0\u00b1\u00a00.02 ppmv. The rate of TOL photo-catalytic oxidation was followed under steady-state conditions, typically accomplished after 140\u00a0h of irradiation.Measured inlet and outlet concentrations of TOL and the concentration of effluent CO2 gases were used to calculate TOL conversion (%TNConv.) as in Eq. (1) and the degree of Mineralization (%Min) as in Eq. (2):\n\n(1)\n\n\n%\nT\n\nN\n\nC\no\nn\nv\n.\n\n\n=\n\n\n\n\n\n[\nT\nO\nL\n[\n\n\ni\nn\n\n\n\u2212\n\n\n[\nT\nO\nL\n]\n\n\no\nu\nt\n\n\n\n\n\n\n[\nT\nO\nL\n]\n\n\ni\nn\n\n\n\n\n\u2217\n100\n%\n\n\n\n\n\n\n(2)\n\n\n%\n\nM\ni\nn\n=\n\n\n\n\n\n[\nC\n\nO\n2\n\n]\n\n\no\nu\nt\nl\ne\nt\n\n\n\u2217\n100\n\n\n7\n\u2217\n\n\n[\nT\nO\nL\n]\n\n\ni\nn\ni\nt\ni\na\nl\n\n\n\u2217\nT\n\nN\n\nc\no\nn\nv\n.\n\n\n\n\n\n\n\nwhere \n\n\n\n[\nT\nO\nL\n]\n\n\ni\nn\n\n\n\na\nn\nd\n\n\n\n[\nT\nO\nL\n]\n\n\no\nu\nt\n\n\n\n are the initial and outlet concentrations of TOL, \n\n\n\n[\nC\n\nO\n2\n\n]\n\n\no\nu\nt\nl\ne\nt\n\n\n\nis the concentration of CO2 in the effluent gas, Q is the gas flow rate.The adsorption of TOL on TiO2 and Co\u2013 TiO2 was estimated following (3):\n\n(3)\n\n\n\nQ\ne\n\n=\n\n\n\n(\n\n\n[\nT\nO\nL\n]\n\n\ni\nn\n\n\n\n\u2212\n\n\n\n[\nT\nO\nL\n]\n\n\ne\n\n\n)\n\nV\n\n\nM\n\n\n,\n\n\n\nwhere \n\n\n\n[\nT\nO\nL\n]\n\ne\n\n\n is TOL equilibrium concentrations of (mg/L). V is the reactor volume of (L) and M is photo-catalyst the mass (mg). The experimental data were fitted to Langmuir isotherm (Eq. (4)) to estimate the adsorption constant, Ka:\n\n(4)\n\n\n\nQ\ne\n\n\n=\n\n\n\n\nQ\n\nm\na\nx\n\n\n\nK\na\n\n\n\n[\nT\nO\nL\n]\n\ne\n\n\n\n(\n1\n+\n\n\nK\ni\n\n\n[\n\n\n\nT\nO\nL\n]\n\ne\n\n\n)\n\n\n\n,\n\n\n\nwhere Qe (mg/gcat) is the amounts of TOL adsorbed on the photo-catalysis, Qmax (mg/gcat) is the maximum amount of TOL adsorbed and Ka (mg\u22121\u00b7L) is the adsorption constant. The kinetic of the solar photo-catalytic oxidation of TOL on TiO2 or Co\u2013TiO2 was tested against Langmuir\u2013Hinshelwood (L\u2013H) expression (Eq. (5), integrated form) (Momani and Jarrah, 2009).\n\n(5)\n\n\n\n\nln\n\n{\n\n\n\n\n\n[\nT\nO\nL\n]\n\n\ni\nn\n\n\n\n\n\n\n[\nT\nO\nL\n]\n\n\no\nu\nt\n\n\n\n\n\n}\n\n\n\n\n(\n\n\n[\nT\nO\nL\n]\n\n\ni\nn\n\n\n\n\u2212\n\n\n\n[\nT\nO\nL\n]\n\n\ne\n\n\n)\n\n\n\n=\n\n\n\n\nQ\n\nm\na\nx\n\n\nk\n\n(\n\n\nV\n\n\nV\n\u02d9\n\n\n\n\n)\n\n\n\n\n(\n\n\n[\nT\nO\nL\n]\n\n\ni\nn\n\n\n\n\u2212\n\n\n\n[\nT\nO\nL\n]\n\n\ne\n\n\n)\n\n\n\n\u2212\n\n\nK\ni\n\n\n\n\nwhere k is the constant of reaction, Ki is the adsorption equilibrium constant, and \n\n\nV\n\u02d9\n\n\n is the gas flow rate. The simultaneous adsorption and oxidation of TOL were proposed as in Eq. (6), linear form:\n\n(6)\n\n\n\u2212\n\n\nd\n\n\n[\nT\nO\nL\n]\n\nt\n\n\n\nd\nt\n\n\n\n=\n{\n\nK\n\nP\nH\n\n\n+\n\n\n\nQ\n\nm\na\nx\n\n\n\nK\na\n\n\n\n1\n+\na\n\n\n+\nK\n\n[\n\nO\nH\n\n]\n\n\n\n[\nT\nO\nL\n]\n\nt\n\n\n\n\nwhere KPH and K are the reaction rate constant to photon-energy and hydroxyl radical, respectively. \n\n\n\n[\nO\nH\n]\n\nt\n\n\n and \n\n\n\n[\nT\nO\nL\n]\n\nt\n\n\n are the concentration of the hydroxyl radical and TOL in the gas phase.\nFig.\u00a02\na shows the XRD pattern of TiO2 doped with Co at different weight percentages (0, 1.0, 2.0, 5 and 10.0\u00a0wt%). The diffraction peaks of the Co\u2013TiO2 structure can be indexed to the tetragonal anatase phase of TiO2 as confirmed by comparing the obtained diffraction power with standard card (JCPDS #21\u20131772). The peaks of Co in the mixture are mixed with the peaks of TiO2 as shown in the XRD of the Co\u2013TiO2 structure under all doping ratios. The obtained trends can be attributed to the high dispersion of Co within the TiO2 structure and the close ionic radius of Co (0.72 A) to with Ti4+(0.68A), conditions that make it difficult differentiate between the diffraction powers of TiO2 and Co in the lattice. The obtained trends agree with the findings of (Hamadanian et\u00a0al., 2010) and (Huang et\u00a0al., 2006), who showed no identified diffraction peaks of Co during the doping process of Co in TiO2. Barakat et\u00a0al. (2005) reported that the TiO2 anatase peaks occur for TiO2 structures calcined in the temperature range 723\u2013873\u00a0K. Co\u2013TiO2 structure calcined at higher temperature showed diffraction peaks for Co and TiO2. The crystallite size of Co\u2013TiO2 was calculated using Scherre equation to be in the range 10.03\u00a0\u00b1\u00a00.06 to 12.09\u00a0\u00b1\u00a00.06\u00a0nm with general trends showing higher crystallite size at a lower mass fraction of Co. Increasing the wt% of Co in the structure led to a noticeable decrease in the crystallite size. The specific surface area (Asp) of the Co\u2013TiO2 was determined to be in the range 75.31\u00a0\u00b1\u00a00.05 to 85.40\u00a0\u00b1\u00a00.05\u00a0m2/g. It was also observed that Asp increased by increasing the wt% of Co in the Co\u2013TiO2 structure. Besides, it was observed the structure has homogenous spherical characteristics with a diameter in the range 7\u201325\u00a0nm as estimated by TEM measurements (Fig.\u00a02b). The EDS measurements showed that Co is dispersed on the surface of TiO2 and the morphological structure was not changed during the doping process. Fig.\u00a02c\u2013e shows the chemical composition and the oxidation state of Co 2P, Ti 2P O 1s of Co\u2013TiO2 with 5\u00a0wt% of Co. The binding energy of Co 2p1/2 core at 796.4\u00a0eV and 2P3/2\u00a0at 780.4\u00a0eV belongs to Co(ll) oxide as confirmed by (Wagner, 2007). The satellite peak of Co 2P3/2 and Co 2P1/2 represent Co+2\u00a0at a binding energy of 786\u201310 and 802-5\u00a0eV, respectively. The obtained results show that Co exists within the TiO2 lattice in the form of Co+2. These observations were confirmed by (Huang et\u00a0al., 2006) and (Shifu et\u00a0al., 2008). The peaks of Co 2P before after the photo-catalytic reaction show no change. The binding energy of Ti 2P3/2 core level at 459\u00a0eV and Ti 2P1/2\u00a0at 465\u00a0eV represent Ti4+ in TiO2 structure. (Fig.\u00a02d). The XPS spectrum of O 1s region illustrates that O2 exists in two forms in the structure with a binding energy of 530 and 532\u00a0eV (Fig.\u00a02e). The peak at 530 is related to O in the bulk of TiO2, and the peak at 532\u00a0eV is related to O2 on the surface or oxygen within hydroxyl species (Peng et\u00a0al., 2012).\nFig.\u00a02f presents the absorption spectrum of TiO2 and Co\u2013TiO2 at different mass fractions of Co. Adding Co toTiO2 showed a redshift in the absorption spectrum toward the visible light region. The incorporated Co in TiO2 matrix is the main reason for this shift due to the charge transfer transition from the 3d orbitals of Co to the conduction band. The band-gap energies of the Co\u2013TiO2 catalyst were estimated to be in the range 2.51\u20133.04\u00a0eV with the largest shift observed at higher Co content. Shifu et\u00a0al. (2008) observed the same shift in the absorption spectrum upon adding Co to TiO2.The chemisorption measurements showed that the Co dispersion and surface area increased by decreasing the mass fraction of Co up to 5\u00a0wt%. Higher mass fraction decreased the Asp. Co\u2013TiO2 doped with l, 5 and 10\u00a0wt% of Co has Asp of 60.7, 85.6 and 63.4\u00a0m2/g, and a degree of dispersion of 90, 93 and 91%, respectively. The obtained results show that the higher the mass fraction in the structure, the more uniform and the higher dispersion on the TiO2 surface. However, the high load of Co might block the pore space in the photo-catalyst leading to lower Asp and decrease in the mean dispersion.\nFig.\u00a03\n presents the pore size distribution and N2 adsorption-desorption isotherms of Co\u2013TiO2 calculated by the Barrett, Joyner and Halenda (BJH) method. All Co\u2013TiO2 exhibits H1-type hysteresis attributed to the mesoporous structure of these photo-catalysts (Yu et\u00a0al., 2007). Increasing the CO mass fraction decreased the pore size diameter being 8.8, 8.0, 7.6 6.1 and 5.8\u00a0nm for the Co\u2013TiO2 with Co of 0, 1, 2, 5 and 10\u00a0wt%, respectively. The decrease in pore size diameter can be attributed to the increase in the amount of Co deposition on the TiO2, blocking the pores and reducing the pore diameters.Dark adsorption plays a key controlling step in determining solar oxidation efficiency. The results gathered from this step reflect the ability of the catalyst to adsorb the TOL and/or oxidize it. Results showed that the adsorption capacity of TOL decreased by increasing the Co content of the catalyst suggesting a beneficial enhancement in the treatment TOL, where most of the processed TOL undergoes solar oxidation. Fig.\u00a04\na shows the evolution of TOL concentration under dark, first 80min, and solar irradiation for an additional 60\u00a0min using TiO2 and 5\u00a0wt% Co\u2013TiO2. Tests were conducted with an inlet TOL concentration of 150 ppmv, the flow rate of 42 NL/min, a relative humidity of 50% and a solar light intensity of 0.1\u00a0W/cm2. The results revealed that the oxidation of TOL in dark is negligible. The concentration of TOL in the reactor effluent gradually increased to reach up to 95.5 and 90% of the inlet concentration for tests carried out under dark with TiO2 and 5\u00a0wt% Co\u2013TiO2, respectively. The observed 4.5 and 10% decrease in the effluent concentration of TOL is due to adsorption on the catalyst. The results also suggest that both photo-catalysts do not affect TOL oxidation in dark. No by-products were detected in the effluent confirming that no degradation nor mineralization occur for TOL. As the TiO2 and Co\u2013TiO2 catalysts were exposed to humidity, a reaction took place between H2O and the cation on the surface of the photo-catalyst leading to the formation of hydroxyl group (i.e. Ti\u2013OH or Co\u2013OH bonds) from the surface of TiO2 or Co\u2013TiO2 (Wen et\u00a0al., 2009; Almomani et\u00a0al., 2019). The hydroxyl groups after that bond to \u03c0-electron from the TOL aromatic ring and absorb TOL on the surface. In addition, direct electrostatic attraction between the aromatic ring in TOL and Co cations can increase the adsorption of TOL (Dvoranova et\u00a0al., 2002; Takeuchi et\u00a0al., 2012; Pham and Lee, 2015). Fig.\u00a04a also shows that the Co\u2013TiO2 achieved saturation when the concentration of TOL in the inlet stream was almost equal to the outlet stream after 71\u00a0min, while TiO2 took 76\u00a0min to reach the same.Under solar light irradiation, the concentration of TOL in the effluent gas stream showed an instantaneous increase, exceeding the inlet concentration due to the desorption of the TOL by the scrubbing effect of CO2 produced from the solar oxidation on TiO2 or Co\u2013TiO2 surfaces. The results achieved with Co\u2013TiO2 compared to naked TiO2 represent a significant enhancement in the photo-catalytic activity of Co\u2013TiO2, which is related to the presence of Co. As the addition of Co to TiO2 shifted the adsorption spectrum toward visible light and improved the solar light absorption, it is expected that the absorbed photon increases the photo-catalytic activity. It is expected that the bandgap energy of the photo-catalyst will decrease, the production of electron-hole pairs will enhance, and the movement of photons within the catalyst will be faster which will reduce the recombination of the electron-hole pairs. Consequently, the produced electrons and holes react with oxygen or water to produce different radicals including hydroxyl radicals (\u2022OH) and superoxide radicals (\u2022\n\n\nO\n2\n\u2212\n\n\n) (Rezaee et\u00a0al., 2008), attacking TOL and causing its decomposition into different byproducts including CO2, benzene, benzaldehyde and H2O. this result was confirmed by the increase in the concentration of these compounds in the effluent stream as shown in the second part of Fig.\u00a04b and c. The oxidation reaction of TOL over the Co\u2013TiO2 photo-catalyst can be represented by reactions 1-5\n\n\n\nC\no\n\u2212\nT\ni\n\nO\n2\n\n\n\n\n\u2192\n\ns\no\nl\na\nr\n\ni\nr\nr\na\nd\ni\na\nt\ni\no\nn\n\n\n\n\n\ne\n\nC\nB\n\n\u2212\n\n+\n\n\nh\n\nV\nB\n\n+\n\n\n\n(\nRe\na\nc\nt\ni\no\nn\n\u00a0\n1\n)\n\n\n\n\n\n\n\n\n\n2\n\nh\n\nV\nB\n\n+\n\n+\n2\n\n\nH\n2\n\nO\n\u2192\n\n2\n\n\nH\n+\n\n+\n\n\nH\n2\n\n\nO\n2\n\n\n\n(\nRe\na\nc\nt\ni\no\nn\n\u00a0\n2\n)\n\n\n\n\n\n\n\n\n\n\nH\n2\n\n\nO\n2\n\n\n\u2192\n\n\n2\n\nO\n\nH\n\u00b7\n\n\n\n(\nReaction\u00a0\n3\n)\n\n\n\n\n\n\n\n\n\n\n\n(\n\nO\n2\n\n)\n\n\na\nd\ns\n\n\n+\n\n\ne\n\nC\nB\n\n\u2212\n\n\n\u2192\n\nO\n2\n\n\u00b7\n\u2212\n\n\n\n\n\n\n(\nRe\na\nc\nt\ni\no\nn\n\u00a0\n4\n)\n\n\n\n\n\n\n\n\n\nO\n\nH\n\u00b7\n\n+\n\n\nC\n7\n\n\nH\n8\n\n\n\n\n\u2192\n+\n\u00a0\nC\n\nO\n2\n\n\n+\n\n\nH\n2\n\nO\n+\n\n\nb\ny\np\nr\no\nd\nu\nc\nt\ns\n\n\n(\n\nbenzene\n,\n\u00a0benzaldehyde\n\n)\n\n\n\n(\nReaction\u00a0\n5\n)\n\n\n\n\n\nThe low attachment of the generated CO2 in addition to its scrubbing effect enhanced the desorption of TOL adsorbed during the dark period, showing a sudden increase in the effluent TOL concentration. Then TOL underwent fast photo-catalytic oxidation over Co\u2013TiO2 resulting in a fast decrease in its effluent concentration and an increase in the production of CO2 (Fig.\u00a04b). Once the adsorbed TOL was detached from the surface of Co\u2013TiO2, a sharp decrease was observed. The concentration of TOL in the effluent stream reached a steady-state value of 4.5 and 60% of the initial TOL concentration after 120\u00a0min for tests carried out with 5\u00a0wt% CO\u2013TiO2 and TiO2, corresponding to %TNconv of 96.5% and 28.5%, respectively. The corresponding CO2 concentration in the effluent stream reached 962 and 140\u00a0ppm, respectively. Since the inlet concentration of TOL was kept constant at 150\u00a0ppm, TOL underwent continuous photo-catalytic oxidation on the surface of Co\u2013TiO2 and TiO2 producing a steady-state amount of CO2 of 962 and 124\u00a0ppm, corresponding to 91.3 and 33.2% of mineralization, respectively. The concentration of other byproducts (benzene and benzaldehyde) showed similar trends to CO2. The steady-state concentrations of benzene and benzaldehyde were 392 and 99\u00a0ppm, respectively. Fig.\u00a04b and c shows that the effluent concentrations of CO2, benzene, and benzaldehyde produced over Co\u2013TiO2 were 5, 9 and 12-fold higher than the same compounds produced over TiO2, confirming the superior photo-catalytic activity of the first photo-catalyst.Another set of experiments were carried out with solar irradiation started from the beginning of the experiment, eliminating the dark adsorption period, as shown in Fig.\u00a05\n. The effluent TOL showed a decrease in the concentration from the inlet concentration of 150\u00a0ppm to stable values of 3.1% and 39% of the initial concentration after 30\u00a0min of solar oxidation for the tests carried out with 5\u00a0wt% of Co\u2013TiO2 and TiO2, respectively. Initially, TOL was removed by both adsorption and photo-catalytic oxidation, leading to a decrease in the TOL effluent concentration. As the adsorption of TOL was disturbed by the produced CO2, the main byproduct of the photo-catalytic oxidation, the high % TNconv of TOL over Co\u2013TiO2 compared to TiO2 is due to the improvement in the photo-catalytic activity by doping with Co. The stable effluent of TOL after 50\u00a0min is related to continuous oxidation over the surface of Co\u2013TiO2. The effluent concentration of TOL was stabilized at 3.1 and 39% for tests carried out with 5\u00a0wt% of Co\u2013TiO2 and TiO2 corresponding to oxidation of 145.4 and 58.8\u00a0ppm of out the initial TOL (150\u00a0ppm). The CO2 production was increased from 0\u00a0at the beginning of the reaction to 1122\u00a0\u00b1\u00a020\u00a0ppm (%Min\u2248 93.4) after 50\u00a0min confirming the conversion of TOL to CO2 over the 5\u00a0wt% Co\u2013TiO2 catalyst. Other by-products such as benzaldehyde (99\u00a0ppm) and benzene (396\u00a0ppm) were detected in the effluent stream (Blount and Falconer, 2002). showed that the photo-catalytic oxidation of TOL over TiO2 and Pt/TiO2 was fast producing benzaldehyde and benzene as intermediates which react further to oxidized products.Considering the high inlet concentration of TOL (150\u00a0ppm) used in the present test, the effluent concentration 4.6\u00a0ppm is considered an excellent removal efficiency compared with other studied processes. European Union set the time-weighted average (TWA) of ambient air quality standard for toluene at 20\u00a0ppm, while the 8-h TWA in the workplace in Quebec, Canada was set at 100\u00a0ppm (Masih et\u00a0al., 2017; Golbabaei et\u00a0al., 2018).The photo-catalytic activity of Co\u2013TiO2 for the oxidation of TOL was tested under different initial concentrations of TOL in the range 20\u2013150\u00a0ppm and hydraulic retention time (HRT) in the range 50\u2013150\u00a0s at natural pH of 6.8. Fig.\u00a06\n shows that the inlet concentration of TOL had a minor impact on TOL degradation for tests conducted as high HRT \u2265100\u00a0s and mass fraction of Co in the lattice \u22655\u00a0wt%. Decreasing the HRT resulted in a decrease in TOL degradation under all the studied inlet concentrations. The percentage degradation of TOL at an initial concentration of 38\u00a0ppm was decreased from 62.2% at HRT of 150\u00a0s to 41.6% at 50\u00a0s for tests performed with 1\u00a0wt% Co\u2013TiO2. Tests carried out with higher concentrations (e.g. 100\u00a0ppm) showed a decrease in the percentage degradation of TOL by 15% as the HRT reduced from 100 to 50\u00a0s, over the same catalyst. The decrease in percentage degradation of TOL is attributed to low residence time which impacted the time required to achieve complete oxidation.\nFig.\u00a07\na shows that increasing the mass fraction of Co in the photo-catalyst from 1 to 5\u00a0wt% increased the percentage degradation of TOL. However, a further increase in the mass fraction of Co to 10\u00a0wt% resulted in a reduction in the degradation efficiency. Up to 95.6% of photo-catalytic degradation of TOL was achieved after 50\u00a0min of irradiation with photo-catalyst with a Co mass fraction of 5\u00a0wt%. Tests carried out with 1\u00a0wt % photo-catalyst showed 69.7% TOL degradation. The relationship between TOL oxidation and the Co mass fraction is due to a large number of active sites on Co\u2013TiO2 accessible for the photo-catalytic reaction. The Co\u2013TiO2 catalysts showed significant catalytic activity toward TOL oxidation compared with TiO2 alone. Tests performed in the dark revealed that the observed oxidation activities were fully attributable to solar photo-induced processes. The observed results also confirm that the change in surface morphology by adding Co to TiO2 lattice enhanced the solar photo-catalytic activities toward TOL oxidation. Moreover, the obtained results show solar oxidation of TOL is independent of the excitation wavelength. Nonetheless, the surface structure and electron generation ability are mainly responsible for the enhanced activity. The decrease in the photo-catalyst activity at a higher mass fraction of Co can be related to the shielding effect of Co, which decreases the solar light penetration to the catalyst.It is known that OH\u2022 radical play a key role in the photo-catalytic degradation of TOL (Fuerte et\u00a0al., 2002; Sleiman et\u00a0al., 2009). Adding Co to TiO2 shifted the absorption spectrum of the photo-catalyst toward visible light, improved electron-hole separation and thus enhanced the photo-activity. Moreover, the presence of Co in the structure increased the specific surface area and this improved oxidation efficiency.\nFig.\u00a07b illustrates that the naked TiO2 did not display significant activity for the solar photo-catalytic oxidation of TOL. It is known that TiO2 absorbs solar light with energy greater than or equal to its band-gap, transferring electrons from valence conduction bands and enhancing the production of electronic vacancies in the valence band (Reaction 6). The transfer electrons and the produced holes contribute to a series of reactions generating hydroxyl radicals that oxidize TOL (Reactions 4 and 7). The %TN\nconv\n reported for TiO2 suggest a limited ability of TiO2 to work under solar irradiation toward the solar oxidation of TOL.\n\n\n\nT\ni\n\nO\n2\n\n\n\n\n\u2192\n\nh\n\u03bd\n\u2264\n390\nn\nm\n\n\n\n\n\ne\n\nC\nB\n\n\u2212\n\n+\n\n\nh\n\nV\nB\n\n+\n\n\n\n(\nRe\na\nc\nt\ni\no\nn\n\u00a0\n6\n)\n\n\n\n\n\n\n\n\n\nO\n\nH\n\u2212\n\n+\n\nh\n\nV\nB\n\n+\n\n\n\u2192\n\nH\nO\n\n\n(\nRe\na\nc\nt\ni\no\nn\n\u00a0\n7\n)\n\n\n\n\n\nOn the other hand, the addition of Co to TiO2 structure enhanced visible light absorbance, enhanced the photo-catalytic activities and resulted in a significant tendency for the oxidation and mineralization of TOL to CO2 and other byproducts. As TiO2 has very low optical absorption properties under solar irradiation, the improvement in the solar oxidation activity is due to the presence of Co in the structure. Wang et\u00a0al. (2015) showed that TiO2 has a considerable absorption limit in the wavelength range 254\u2013370\u00a0nm. Adding Co to the TiO2 resulted in a significant enhancement of light absorption in the solar irradiation region (See Fig.\u00a02f). It was observed that the light absorption ability increased by increasing the mass fraction of Co in the Co\u2013TiO2 up to 5\u00a0wt%. Further increase in the mass fraction of Co to 10\u00a0wt% showed a slight increase in the light absorption. A small portion of the photons absorbed in the Co\u2013TiO2 is used as a source of heat, and the major part is used in exciting the electrons from the valence band to the conduction band in TiO2. In the absence of Co, the TiO2 has limited energy gaps between its valence band and conduction band (band-gap energy). Thus, high amounts of photon energy are required to surpass band-gap energy (\u223c3.2\u00a0eV) and excite the electrons to start the oxidation of TOL. Based on this, naked TiO2 absorbs only UV light to fulfill this energy requirement. Addition of the Co to TiO2 enhanced the excitation of the structure by the formation of Ti3+, Ti4+, and Co2+, providing the structure with the ability to absorb more energy photons and utilize them in transferring electrons from the valence band (O 2p) to the conduction band thus, enhancing the solar oxidation efficiency of TOL. As the mass fraction of Co2+ increases, the ability of Co\u2013TiO2 to absorb solar irradiation increased and the oxidation potential improved. However, a high mass fraction of Co might increase the surface coverage of TiO2 leading to a decrease in the light absorbed and inhibiting the photon energy from reaching the TiO2, resulting in a decrease in the ability to oxidize TOL. Therefore, a 5\u00a0wt% of Co was determined to be the optimum mass fraction of Co. The observed results suggest that Ti3+, Ti4+, and Co2+ play an important role in electron generation and transfer and the improvement in the electron-hole separation. The generated electrons increased the surface reaction with oxygen and water contributing more to TOL solar oxidation. The highest TOL removal and mineralization were 96.5 and 93.3% respectively, achieved by Co\u2013TiO2 with a mass fraction of 5\u00a0wt%. Although the band energies of Co\u2013TiO2 with a mass fraction of 1 and 2 %wt were 2.76 and 2.70\u00a0eV, respectively, were lower than that of 10\u00a0wt% (2.83\u00a0eV), their TOL photo-catalytic removal and mineralization degree were lower than those of the 10\u00a0wt %. This is because a higher mass fraction of Co blocked the lattice and decreased the incidental light absorption from reaching the TiO2 layer. Moreover, the excited electrons from the conduction band of Co would easily fall back to the valence band to recombine with holes before reacting with water or oxygen to produce oxy radicals, resulting in lower %TNconv.Another set of tests were conducted to study the effect of humidity on the solar oxidation of TOL. The summary of results is presented in Table\u00a01\n. The %TN\nconv\n increased by increasing the percentage relative humidity (%RH) from 10% up to 50%, after which the reported %TN\nconv\n slightly decreased. The observed trend was aimed at the contribution of water in the gas phase in the formation of hydroxyl radicals, which will increase the %TN\nconv\n as shown in reaction 8 combined with reaction 5. The decrease in %TN\nconv\n at high %RH is related to the competitive adsorption between the water and TOL on the photo-catalyst active site resulting in a decrease in the %TN\nconv\n (Momani and Jarrah, 2009).\n\n\n\n\nH\n2\n\nO\n\n\n\n\n\u2192\n\nh\n\u03bd\n\n\n\n\n\n\nH\n\u02d9\n\n\n+\n\n\n\nO\n\u02d9\n\n\nH\n\n\n\n\n(\n\nReaction\u00a0\n8\n\n)\n\n\n\n\n\nTo identify the main mechanism of photo-catalytic degradation of TOL, scavenger tests were performed to assess the ability of Co\u2013TiO2 to produce active hydroxyl radicals (OH\u2022 and \n\n\u2022\n\nO\n2\n\n\u2212\n.\n\n\n\n) and identify the role of each species in TOL oxidation. Tests showed that the concentration of OH\u2022 is significantly higher than.\n\n\u00a0\n\u2022\n\nO\n2\n\n\u2212\n.\n\n\n\n. It is known that OH\u2022 is the most powerful oxidizing agent, that can react with organic matter leading to its degradation (Shawaqfeh and Al Momani, 2010). The concentration of OH\u2022 was observed to increase by increasing the Co content in the Co\u2013TiO2 lattice up to 5\u00a0wt%, after that a decrease in the radical concentration was observed. The obtained results suggest that the prepared Co\u2013TiO2 photo-catalyst is capable of generating high concentrations of OH\u2022 under solar irradiation. The high production of hydroxyl radicals could be attributed to the decrease in the bandgap energy, enhancement in the production of electron-hole pairs and increase the rate of photons movement within the catalyst and decrease the recombination of the electron-hole pairs as results of solar light absorption enhancement (Reactions 1\u20135). The decrease in the OH\u2022 generation at higher Co mass fraction (Co\u00a0>\u00a010\u00a0wt %), can be related to the decrease in photons movement through the coupling interface between Co and TiO2. Electrons can be captured within the interface between Co and TiO2, leading to a sudden decrease in the concentrations of hydroxyl radicals.Based on the above analysis, the possible mechanism of the photo-oxidation of TOL could be proposed as per Fig.\u00a08\n. Under solar irradiation, the Co\u2013TiO2 reach excitation state generating electrons and holes (\n\n\ne\n\nC\nB\n\n\u2212\n\n\n and \n\n\nh\n\nV\nB\n\n+\n\n\n) in reaction 1. Electrons react with O2 or water producing radicals (\n\n\u2022\n\nO\n2\n\n\u2212\n.\n\n\n\n) as per reaction 7, and the \n\n\nh\n\nV\nB\n\n+\n\n\n react with water producing hydrogen peroxide that dissociates generating OH\u2022 (reactions 2 and 3). The presence of Co metal in the photo-catalyst structure facilitates the transfer of electrons and thus enhance the radical generation. Additional OH\u2022 can be generated by reaction with oxygen on the surface of metal On the other hand, photo-generated electrons react with oxygen on titanium dioxide itself producing superoxide radicals (Urase and Kikuta, 2005). All the photo-generated radicals are available to react with TOL and contribute to high %TN\nconv\n.The amount of TOL adsorbed on the TiO2 and Co\u2013 TiO2 did not exceed 3% and 10% of the inlet concentration respectively, suggesting a limited adsorption profile. Regression analysis of experimental data following Eq. (5) showed poor fitting and the model was rejected. As the adsorption of TOL was very low, Eq. (6) can be reduced to Eq. (7), linear form (Almomani and Baranova, 2013):\n\n(7)\n\n\nln\n\n{\n\n\u2212\n\n\nd\nC\n\n\nd\nt\n\n\n\n\n}\n\n=\nln\n\n{\n\n\nK\n\nP\nH\n\n\n\n}\n\n+\nln\n\n(\n\nK\n\nO\nH\n\n\n\nC\n\nO\nH\n\n\n)\n\nC\n\n\n\n\nInformation about the concentration of hydroxyl radicals is required to solve Eq. (7). However, as the hydroxyl radical is self-generated in the solar reactor and highly dependent on the solar energy and gas phase, %RH, Eq. (7) can be rewritten as Eq. (8):\n\n(8)\n\n\nln\n\n{\n\n\u2212\n\n\nd\nC\n\n\nd\nt\n\n\n\n\n}\n\n=\nln\n\n{\n\n\nK\n\nP\nH\n\n\n\n}\n\n+\nln\n\n(\n\nR\nH\n\n)\n\nC\n\n\n\n\nExperimental data of TOL oxidation over TiO2 and Co\u2013TiO2 showed a good agreement with Eq. (8). Table\u00a01 presents the kinetic constants of the solar photo-catalytic oxidation of TOL over TiO2 and Co\u2013TiO2. Based on the experiment results, the photo-catalytic oxidation of TOL depends in the %RH;(1) At low %RH (i.e. low water vapor), the oxidation reaction initiated by electron transfer from Co\u2013TiO2 to O2 and water generating radicals causing the decomposition of TOL. As the concentration of water is low, the available oxygen plays a key role in the generation of more radicals and enhancing TOL conversion. The obtained results suggest that under low humidity, the oxidation of TOL can be improved by increasing the O2 content in the gas stream.(2) At high %RH, the available water plays an important role in the direct formation of OH\u2022 radicals leading to higher %TN\nconv\n.\nTable\u00a01 also presents important information regarding the effect of the gas flow rate on the process kinetic. The general trends showed a decrease in the kinetic constant by increasing the flow rate. The kinetic constant values were 7.6\u00a0\u00b1\u00a00.3, 5.0\u00a0\u00b1\u00a00.2 and 3.3\u00a0\u00b1\u00a00.3 min\u22121 for tests carried out with gas flow rate of 27.5, 42.0 and 82.5\u00a0L\u00a0min\u22121 at RH of 20%. Further investigation will be carried out shortly to study the techno-economic aspects of the process as well as the kinetic data for scale-up purposes.In this study, for the first time, a new photo-catalyst (Co\u2013TiO2) was synthesized with a different mass fraction of Co and tested for the degradation of toluene (TOL). The as-prepared catalyst has improved surface characteristics and visible light absorption, and reduced electron-hole recombination. The enhanced photo-catalytic properties of Co\u2013TiO2 improved the degradation of TOL by reducing bandgap energy and increasing the generation of radicals. The photo-degradation of TOL depends on the mass fraction of Co, inlet TOL concentration, gas flow rate, and relative humidity. The highest toluene conversion (%TN\nconv\n) of 96.5% was obtained using 5\u00a0wt% Co, 150\u00a0ppm toluene concentration, 27.5\u00a0L\u00a0min\u22121 flow rate and 50% relative humidity. The co-doped Co\u2013TiO2 catalysts showed high selectivity (>90%) toward partial oxidation of TOL to produce CO2, benzene and benzaldehyde. The obtained results suggest that adding Co metal to TiO2 displayed excellent solar photo-catalytic properties that can be employed to remove toluene from the gas phase stream at an industrial scale.The publication of this article was funded by the Qatar National Library.", "descript": "\n                  Cobalt (Co) co-doped TiO2 photo-catalysis were synthesized, characterized and tested toward solar photocatalytic oxidation of toluene (TOL). A multi-technique approach was used to characterize and relate the photo-catalytic property to photo-oxidation performance. Adding Co to TiO2 significantly changed crystal size and surface morphology (surface area, pore-volume, and pore size), reduced the bandgap energy of TiO2 and improved the solar photo-oxidation of TOL. Up to 96.5% of TOL conversion (%TN\n                        conv\n                     ) was achieved by using Co\u2013TiO2 compared with 28.5% with naked TiO2. The maximum %TN\n                        conv\n                      was achieved at high hydraulic retention time (HRT)\u00a0\u2265\u00a0100\u00a0s, Co content in the photo-catalyst of 5\u00a0wt% and relative humidity (%RH) of 50%. The mechanism of TOL solar oxidation was related to the concentration of OH\u2022 and \n                        \n                           \u2022\n                           \n                              O\n                              2\n                              \n                                 \u2212\n                                 .\n                              \n                           \n                        \n                      radicals produced from the generated electrons and holes on the surface of Co\u2013TiO2. The products formed during the photo-catalytic oxidation of TOL were mainly CO2 and water, and minor concentration of benzene and benzaldehyde. Overall, the Co\u2013TiO2 could be used as a potential photo-catalyst for the oxidation of toluene in gas-phase streams on an industrial scale.\n               "}
{"full_text": "Since the first industrial revolution, advanced inventions and technologies have enabled us to enjoy warmth in the winter, cool in the summer, brightness at night, and convenient transportation all over the world.\n1\n Most of these technologies depend heavily on our ability to exploit fossil sources of energy, resulting in an increasing demand for fossil fuel and excessive emissions of CO2. As projected by the International Energy Agency, the global annual energy demand will increase to 18 billion tons of oil equivalents, and 43 gigatons of CO2 will be released per year by 2035, which will aggravate energy crisis, increase the global average temperature, and acidify the ocean.\n1,2\n Severe situations have motivated a large number of researchers to pursue reliable and clean energy options. Proton-exchange membrane fuel cells (PEMFCs), especially refueled with hydrogen from renewable energy, are generally considered one of the most promising solutions because of their competitive advantages, such as zero emission, high efficiency, fast refueling, and low upfront cost.\n3\n In a typical PEMFC, fuel molecules (e.g., hydrogen) are oxidized on the anode, and oxygen gas is reduced on the cathode, outputting electric energy with pure water and heat as the only by-products (Figure\u00a01\nA). Unfortunately, the difficulty in O2 activation, O\u2013O bond cleavage, and oxide removal causes sluggish kinetics\u00a0of the oxygen reduction reaction (ORR) on the cathode, thus demanding stringent requirements to the catalysts.\n4\n After a long period of experimental exploration, platinum (Pt) and Pt-based catalysts are generally considered to be the most efficient ORR catalysts. Low-temperature PEMFCs currently adopt Pt nanoparticles (NPs) supported on carbon (Pt/C) or other Pt-rich materials as the cathode catalyst.\n5\n Nevertheless, the high cost of Pt greatly hampers further large-scale adoption of PEMFCs. According to the strategic analysis report, catalyst layers in the PEMFC system amount to US $11.24\u00a0kW\u22121 or over 20% of the total cost, in which over US $10\u00a0kW\u22121 attributes to Pt usage. How to reduce the dosage of Pt or substitute nonprecious metal for Pt without loss of performance is now of the greatest concern.\n3,5\u20137\n\nAt present, improving atom utilization and boosting the intrinsically catalytic activity of Pt by reducing Pt nanostructure sizes, alloying, and constructing specific nanostructures with Pt-rich skin are common strategies for reducing the dosage of Pt.\n4,5,7\n A variety of delicate Pt-based nanostructures have been reported to exhibit significantly enhanced ORR activity. For instance, Li et\u00a0al. fabricated ultrafine Pt jagged nanowires with diameters less than 5\u00a0nm, delivering 33 times the specific activity (catalytic activity normalized by surface area) or 52 times the mass activity (catalytic activity per given mass of Pt) of the commercial Pt/C catalyst.\n8\n The Adzic group deposited Pt monolayers on PdAu NP surfaces by the galvanic displacement method to optimize Pt utilization. The ultra-low Pt content was found to be enough to achieve high ORR catalytic performance.\n9\n In another important work by Chen et\u00a0al., Pt3Ni nanoframes with Pt-skin surfaces were constructed after the interior of polyhedral PtNi3 nanocrystals was dissolved, significantly outperforming the commercial Pt/C catalyst for ORR activity.\n10\n\nHowever, these fine nanostructures typically have a high propensity to agglomerate or deform during the electrochemical process, resulting in an unfavorable deactivation and poor stability during long-term operation.\n11\n Meanwhile, the complicated synthetic procedures cause the manufacture of catalysts to be costly. These disadvantages make Pt-based catalysts still doubtful in further wide adoption. In consideration of the much lower price of nonprecious metals, such as iron (Fe), cobalt (Co), and nickel (Ni), the cost of PEMFCs could be significantly reduced by substituting nonprecious-metal catalysts for Pt-based catalysts. However, the ORR activity of conventional nonprecious NPs is lower than that of Pt counterparts by almost one order of magnitude, preventing them from directly acting as eligible ORR catalysts.\n12,13\n Similar to the case of Pt-based electrocatalysts, regulation of morphological and electronic structure of the nonprecious-metal catalysts is a general strategy for improving their ORR activity. Unfortunately, despite the tremendous efforts, few results achieve satisfactory catalytic activity and durability because of the flagrantly low intrinsic ORR catalytic activity of nonprecious metals.\n4,5\n\nThe size of metal particles is a key factor in determining their catalytic performance given that the specific activity per metal atom generally increases with decreasing size of the particles. Single-atom catalysts (SACs) represent the theoretically ultimate size limit for metal particles, in which metal atoms are dispersed on specific supports and isolated from each other without appreciable interaction between them. Therefore, SACs are supposed to possess relatively high catalytic activity and maximum atom-utilization efficiency.\n14\u201316\n In 2000, Heiz et\u00a0al. prepared a series of Pdn cluster supported on magnesium oxide with the help of mass-selected soft-landing techniques.\n17\n The single palladium (Pd) atom is surprisingly found to exhibit enough catalytic activity in acetylene cyclotrimerization to benzene. In another research by the Zhang group, atomically dispersed Pt atoms supported on Fe oxide (Pt1/FeOx) were successfully synthesized, carefully characterized, and applied in efficient and durable CO oxidation.\n18\n In 2016, Liu and co-workers developed a convenient photochemical strategy to fabricate a stable SAC with Pd atoms supported on ultrathin TiO2 nanosheets.\n19\n Such a Pd SAC exhibited high catalytic activity in hydrogenation of C=C bonds, outperforming commercial Pd/C catalysts and homogeneous H2PdCl4. In the same year, the Li and Wu groups initiated a new method to construct isolated metal atoms anchored on three-dimensional nanostructures, achieving a high ORR activity.\n20\n Other than this significative research, a series of SACs have been reported and showed surprisingly excellent performance in various catalysis processes, such as catalytic reduction of CO2,\n21,22\n electrochemical synthesis of ammonia,\n23\n methane conversion,\n24\n selective acetylene hydrogenation,\n25\n and other important chemical reactions.\n14,15\n Inspired by the powerful achievements by SACs, scientists have made fruitful researches in tuning SACs into active, reliable ORR catalysts as an alternative to expensive Pt-based materials.\n15,16\n In addition, SACs with uniform catalytically active sites provide us a golden opportunity of exploring the relationship between ORR catalytic performance and catalyst structure in an atomic scale, which could spur further research on the atomically rational design of ORR catalysts.\n22\n\nIn this review, we first introduce the mechanism and electrochemical evaluation of ORR. Then, we concisely retrospect the development of Pt-based ORR catalysts and demonstrate how scientists optimize their catalytic performance by controlling their component, morphology, size, and facet exposure. After that, we describe the ORR performance of nonprecious-metal-based catalysts and the following three common strategies of improving their performance: (1) increasing the intrinsic activity by composition modulation, (2) confining metal species into carbon shells to preserve metal from corrosion, and (3) increasing the accessibility by constructing porous or other large-area structures. Further, we briefly review the developments of SACs, summarize recent advances in SACs for ORR catalysis, and demonstrate how the ORR performance on SACs is promoted by support construction and regulation of electronic structures. At last, we also present a brief perspective on the remaining challenges and future directions of SACs for ORR.It is generally accepted that the ORR undergoes either a \u201cdirect\u201d four-electron pathway to generate O2-species (H2O in acidic solutions or OH\u2212 in alkaline solutions) or a \u201cseries\u201d two-electron pathway to generate hydrogen peroxide (H2O2).\n4,5\n The \u201cseries\u201d way has been considered one of the competitive strategies of producing H2O2, and it could replace the energy-intensive anthraquinone process. However, the \u201cdirect\u201d four-electron oxygen reduction pathway is unanimously recognized as the favorable pathway since H2O2 reduces energy-conversion efficiency and accelerates the degradation of the proton-conducting polymer electrolyte in PEMFCs.\n13\n Different intermediates, including oxygenated (O*), hydroxyl (OH*), and superhydroxyl (OOH*) species, could be generated during oxygen reaction under common ORR conditions. Several possible transformations between these intermediates, as schematically shown in Figure\u00a01B, make the ORR process more complicated.\n26\n Despite the tremendous efforts to find the rate-determining step in ORR, there is still\u00a0no definitive conclusion because the reaction pathway depends, to a great extent, on the catalysts and environmental parameters such as solvent, temperature, and applied electrode potential. In the majority of cases, the overall ORR rate is\u00a0determined by one of these three steps: (1) the first electron transfer to adsorbed\u00a0O2 molecule, (2) the hydration of O2, and (3) the final desorption of H2O.\n4\n In\u00a0addition, several studies have supported that oxygen coverage plays a critical\u00a0role\u00a0in ORR mechanisms. A high oxygen coverage causes O\u2013O cleavage posterior\u00a0to OOH* formation (so-called associative mechanism), whereas a low oxygen\u00a0coverage makes O\u2013O cleavage anterior to OH* formation (dissociation mechanism).\n27\n\nFor a rigorous evaluation, a new ORR catalyst is supposed to be employed in a PEMFC and compared with acknowledged benchmarks, such as commercial Pt/C. Nevertheless, the complicated and costly fabrication of a PEMFC makes this approach impractical. In science labs, benefiting from operability as well as inexpensiveness, the rotating disk electrode (RDE) method has been widely adopted to quickly screen the ORR catalytic performance of new materials.\n5,27\n Typically, the catalyst sample is first dispersed into a homogenous ink with mixing water, alcohol (isopropyl in some cases), and Nafion by an optimized ratio, which is then deposited on a glassy carbon RDE. For Pt-based catalysts, especially for Pt NPs, the Pt loading is usually controlled below 50\u00a0\u03bcg/cm2 to avoid mass-transport loss caused by catalyst agglomeration.After electrode preparation, in a typical procedure, a cyclic voltammogram (CV) is first investigated by cyclic voltammetry in inert-gas (N2 or Ar)-saturated acidic solutions (0.1\u00a0M HClO4, 0.5\u00a0M H2SO4, or 0.1\u00a0M KOH). H+ would be reduced and adsorbed on catalytically active sites on the catalyst surface during the cathodic scanning, corresponding to the H adsorption region in current-potential (I-V) curve. In reverse, the regeneration of H+ occurs during anodic scanning and corresponds to the H desorption region in the I-V curve. After mathematic conversion, the electrochemical active surface area (ECSA) can be obtained from the integral of H desorption area in the I-V curve. The ORR polarization curve is measured in an O2-saturated solution (usually 0.1\u00a0M HClO4, 0.5\u00a0M H2SO4, or 0.1\u00a0M KOH) in a potential scanning window between 0.05 and 1.20\u00a0V versus reversible hydrogen electrode (RHE). As the mass transfer largely influences the ORR catalytic performance, the RDE is rotating (usually at a speed of 1,600\u00a0rpm) to mitigate the mass transfer loss during the ORR evaluation. To minimize the contribution from capacitive current, the scan rate is usually controlled below 20\u00a0mV s\u22121. One should note that current PEMFCs adopt cation-exchange membranes to separate the anode and cathode, making RDE tests in an acidic medium more practically significative.After polarization curve measurements, the kinetic current (jk, catalytic current without the loss caused by mass transfer) can be extracted according to the Levich-Koutechy equation:\n\n\n\n\n1\nj\n\n=\n\n1\n\n\nj\nk\n\n\n\n+\n\n1\n\n\nj\n\nl\n,\nc\n\n\n\n\n=\n\n1\n\n\nj\nk\n\n\n\n+\n\n1\n\n0.62\nn\nF\nA\n\nC\n0\n*\n\n\nD\n0\n\n2\n/\n3\n\n\n\n\u03bd\n\n\u2212\n1\n/\n6\n\n\n\n\u03c9\n\n1\n/\n2\n\n\n\n\n,\n\n\n\nwhere j is the apparent current density (extracted from the polarization curve under different applied potentials directly) and jl,c is the diffusion-limited current density (usually the highest current density under relatively negative potentials). Thus, the jl,c is determined by the average number of electron transferred during ORR (n), the faradic constant (F), the geometric area of electrode (A), the concentration of dissolved O2 in catalysis solution (C0*), the diffusion coefficient of O2 (D0), the kinetic of viscosity of the solution (\n\u03bd\n), and the RDE rotation speed (\n\u03c9\n).\n5\n Based on a series of polarization curves under different rotation speeds and measurements of jl,c, the Levich-Koutechy equation allows us to extract jk (under 0.9\u00a0V versus RHE or other specific potentials), which can be further conversed into specific activity and mass activity by normalizing with ECSA and Pt (or other metal) loading. The catalytic capability of a material is generally evaluated by these two parameters. In addition, the half-wave potential (E1/2), which is the required potential to achieve a current that is half that of jl,c, is also widely used to describe the catalytic performance (Figure\u00a01C). A higher E1/2 signifies a lower required overpotential to achieve 1/2 j\n\nl,c\n and thus reflects a higher catalytic activity. It is also noteworthy that the mass activity, especially for Pt-based catalysts, may be more significant to evaluate the ORR activity in consideration of the expensiveness of Pt. Researchers\u00a0usually benchmark obtained ORR performance against the targets set by the United States Department of Energy (DOE). In these targets, a mass activity of 0.44 A mgPGM\n\u20131 (PGM: precious group metal) at 0.900\u00a0V in a PEMFC should be achieved before 2020.\n28\n\nAs a common strategy, the shape and size control has been widely used in numerous catalytic systems to improve the catalytic performance of nanocatalysts. The ORR catalytic performance of different Pt facets, especially low-index planes (planes with low Miller indices), has been extensively studied. In strongly adsorbed electrolyte (such as H2SO4 solution), in which the strong adsorption of anion deactivates the Pt (111) surface dramatically, the ORR activity follows an order of Pt (111)\u00a0< (100). While in weakly adsorbed electrolyte (such as HClO4 solution), the ORR active increases in the order of Pt (100) \u226a Pt (110) \u2248 Pt (111).\n27\n Several high-index planes (planes with high Miller indices) have been demonstrated to exhibit higher ORR catalytic activity. For example, the Xia group prepared Pt concave nanocubes by a synthetic method with Br\u2212 as a capping agent to hinder the growth of the <100> axis.\n29\n With the help of high-resolution transmission electron microscopy (HRTEM), these nanocubes were confirmed to be enclosed mainly by {720} as well as {510} and {830} facets. A substantially enhanced ORR catalytic activity was observed on these Pt nanocubes. These concave Pt nanocubes exhibited approximately three and two times the specific activity at 0.90\u00a0V of the Pt cubes and cuboctahedra (bounded by low-index facets) in ORR, respectively. Unfortunately, despite the achieved high specific activity, the mass activity of Pt concave nanocubes is unsatisfying mainly caused by the relatively large size (>15\u00a0nm). In addition, Pt NPs exposing high-index facets with at least one Miller index being larger than 1 have also been found to show relatively high ORR activity; these include but are not limited to tetrahexahedron (hk0), trapexezohedron (hkk), and trisoctahedron (hhk). The enhanced ORR activity is generally ascribed to dense surface steps, edges, and kinks on high-index facets.\n5\n The main obstacle still lies in the difficulty in stabilization of these thermodynamically unstable shapes during long-term ORR catalysis operation in a more practical and large-scale synthesis.Generally, the activity of heterogeneous catalysts largely depends on the size of the metal particles. Reducing the particle sizes may boost the catalytic performance for multiple reasons. Cutting bulk materials into NPs brings a considerable portion of formerly inner atoms to surfaces where the catalysis reaction occurs. The smaller the NPs are the larger surface ration is. Meanwhile, small particles are more likely to possess dense low-coordinated species such as steps, edges, and kinks, which are more capable of achieving high catalytic performance because of the high surface free energy. Additionally, size reduction enhances metal-support interactions, which may rearrange the electronic structure of metal species and further promote the catalytic process. Thus, size reduction is widely regarded as one of the most effective strategies for improving atomic utilization and catalytic activity. Size effects of Pt in ORR have been deeply explored over the last several decades.\n4,5\n There is now substantial theoretical and experimental research showing that the Pt mass activity is optimized when the size of particles is reduced into the range of 2\u20135\u00a0nm.\n30\n When the size is further reduced, the mass activity is found to decrease with the decreasing of particle size. The reason for this unexpected phenomenon, however, is still in dispute.\n5\n\nBeing blocked by surface atoms, the interior atoms in NPs can be hardly collided by reactants and hence rarely contribute to catalytic activity directly. Therefore, constructing hollow Pt NPs by removing the interior Pt atoms in solid NPs may retain the original catalytic activity and decrease the Pt usage simultaneously. Additionally, a hollow particle offers two sides, the inner surface and outer surface, which may further improve the atom utilization. By a template-removal method, the Adzic group prepared hollow NPs, which were further found to exhibit higher ORR catalytic activity compared with solid Pt NPs with similar sizes.\n31\n The enhanced catalytic performance, after careful investigation with experiments and density functional theory (DFT) calculation, was ascribed to lattice contraction induced by the hollow structure. A compressive strain is usually regarded to shift the Pt d-band center downward, weaken the adsorption of strongly adsorbed oxygenated intermediates, and finally improve the ORR catalytic activity.\n32,33\n Further, Zhang et\u00a0al. prepared Pt nanocages with sub-nanometer thickness and investigated their ORR catalytic performance.\n34\n They first prepared Pd nanocubes with an edge length of ca. 18\u00a0nm and then deposited four atomic layers of Pt on the surface of Pd cubes by reducing Pt salt at 200\u00b0C. Finally, the Pd cube templates were selectively removed, leaving Pt cubic nanocages covered by {100} facets (Figure\u00a01D). This method also allows them to prepare Pt octahedral nanocages covered by {111} facets by using Pd octahedral templates. The specific activity and the mass activity on octahedral nanocages were found to be five and eight times higher than those of commercial Pt/C, respectively (Figure\u00a01E). Unlike previous research, this synthetic method provides the possibility of synthesizing Pt hollow structure with specific facets. He et\u00a0al. prepared icosahedral Pt nanocages via a similar synthetic method, which also achieved outstanding ORR performance.\n35\n\nBenefiting from the large surface area and relative better stability than NPs and nanowire, one-dimensional nanocrystal is considered an ideal structure to exhibit outstanding performance in electrocatalysis. Liang et\u00a0al. prepared a free-standing Pt nanowires membrane through a multistep templating route.\n36\n As expected, the Pt nanowires exhibited both higher mass activity of ca. 0.016 A/mgPt and better durability than the bulk Pt catalyst and commercial Pt/C. The Duan group prepared ultrafine jagged Pt nanowires with an average diameter of ca. 2\u00a0nm via a synthetic method with thermal annealing and electrochemical dealloying (Figure\u00a01F).\n8\n Such Pt nanowires exhibited an extremely high ORR catalytic activity with E1/2 of 0.935\u00a0V versus RHE (more positive than that of Pt/C by ca. 75\u00a0mV), a mass activity as high as 13.6 A/mgPt, and an excellent long-term durability. The enhanced activity is ascribed to the atomic stress as well as ORR-favorable rhombus-configuration on the surface (Figures 1F\u20131H).Tremendous works have demonstrated that alloying two or more metals may empower catalysts with unique properties. As early as 1993, scientists had already realized that the kinetics of ORR could be easily enhanced by at least three times by simply alloying Pt with transition metals, such as Ni, Co, and Mn.\n37\n The atomic and electronic structures of Pt, as generally believed, can be improved by alloying to boost the ORR performance. Further, Markovic et\u00a0al. systematically investigated the function of alloying metals by DFT calculation.\n38\n The specific activities of PtM alloys were found to display a volcano-type relationship with the d-band center (as we discussed before, the d-band center can sever as a descriptor in ORR), elucidating that very strong and very weak oxygen-intermediate adsorption will limit the\u00a0reaction rate by the removal of surface oxides and electron and proton transfer to adsorbed O2, respectively (Figure\u00a02\nA). It is also noteworthy that Pt3Co, Pt3Ni, and\u00a0Pt3Fe alloys dominate the top of the volcano and thus are supposed to exhibit higher ORR activity than that of other PtM alloys or pure Pt materials. Inspired by the above conclusions, scientists have devoted further efforts to tuning diverse PtM alloys into satisfactory ORR catalysts by modulating the component, shape, and size of PtM nanostructures.\n5,10,39,40\n For example, the Yang group prepared Pt3Ni truncated-octahedral nanocatalysts that dominantly expose {111} facets.\n41\n The particles were found to exhibit a 4- and 1.8-fold higher ORR mass activity than commercial Pt/C and normal octahedral Pt3Ni particles, respectively. They then raised a synthetic strategy of preparation of uniform icosahedral nanocrystals of other PtM (M = Au, Ni, and Pd) alloys.\u00a0Their investigation showed that ORR catalytic activity can be further enhanced by using icosahedral Pt3Ni as a catalyst (Figure\u00a02B).\n42\n Given that both octahedral and icosahedral Pt3Ni nanocrystals are bound by {111} facets, they speculated that the enhanced ORR performance may be ascribed to elastic strain. By using molecular dynamics simulations, they found a tensile surface strain on icosahedral particles but a compressive surface strain on the octahedral ones. These surface-strain differences, as they claimed, serve a vital role in the regulation of electronic structure of surface atoms and thus explain the enhanced ORR performance (Figure\u00a02C). Since catalytic performance is sensitive to the surface structure, an in-depth investigation of the component, atomic, and electronic structure on the surface of the catalyst is of great concern. The Markovic group performed surface-sensitive techniques, including low-energy electron diffraction, low-energy ion scattering, Auger electron spectroscopy, surface X-ray scattering (SXS), and synchrotron-based high-resolution ultraviolet photoemission spectroscopy,\n43\n to investigate the Pt3Ni alloy surface. An oscillating structure of Pt3Ni, as shown in Figure\u00a02D, was then proposed: the outermost layer is composed exclusively of Pt; the second layer is Ni enriched (52% of Ni content is larger than 25% of Ni content the bulk), and the third layer possesses a Pt-rich feature (87%). They also found that such a \u201csegregation\u201d surface structure is stable under a potential range from 0.05 to 1.00\u00a0V versus RHE, which was confirmed by in situ SXS measurements. With an unambiguous surface structure in mind, the d-band center and specific activities of different Pt3Ni surface morphology were investigated and summarized (Figure\u00a02E), indicating the superior ORR catalytic activity of Pt3Ni (111). On the basis of the above achievement, the Huang group made a successful attempt to optimize ORR activity of Pt3Ni (111) by a surface-doping strategy.\n44\n They tried a series of transition metals and found that Mo could improve the ORR catalytic performance the best by acting as a doping metal. Their DFT calculation suggests that Mo-doping increases the oxygen-binding energies of the center sites on (111) facet, explaining the enhanced ORR activity (Figure\u00a02F).Another instructive inspiration by the Markovic\u2019s research is that PtM alloy particles may possess a compositional-segregation feature and thus exhibit a different property with a different configuration. Two representatives by the Strasser group have promoted our understanding of segregation on shaped alloy nanocatalysts. In 2013, they followed the morphological and compositional evolution of three octahedral PtxNi1\u2212x alloy NP under electrochemical conditions by employing aberration-corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy.\n45\n A leach in their facet centers and concave octahedral structure were observed. This dealloying and morphological evolution implies the complicacy with shape-selective alloying catalysts under operating conditions. In the following year, they did an intensive study on the element-specific anisotropic growth and degradation of PtM alloy nano-octahedra.\n46\n The results, summarized in Figure\u00a03\nA, forebode that a further enhancement in ORR activity may be achieved by rational synthesis of Pt alloy ORR electrocatalysts.Besides the Pt-Ni system, scientists have also investigated other PtM alloy or bimetallic systems. The Adzic group prepared a Pt/C catalyst stabilized by Au nanoclusters via an under-potential deposition method.\n47\n With in situ X-ray absorption near-edge spectroscopy and voltammetry data, the authors claimed that Au clusters raise the Pt oxidation potential and thus confer the stability during ORR operation. By reducing Pt salt with Pd nanoseeds, the Xia group prepared a Pd-Pt bimetallic nanostructure with Pt branches on a Pd core, which was found to exhibit high ORR activity at both room temperature and 60\u00b0C (Figures 3A and 3B).\n48\n The improved ORR performance was ascribed to the large surface areas and high-index facets exposure of Pt branches. Greeley et\u00a0al. investigated ORR performance on alloys of Pt and early transition metals such as Y and Sc.\n49\n Particularly, polycrystalline Pt3Y showed higher ORR activity than pure Pt by a factor of 6\u201310. The Chorkendorff group synthesized eight Pt-lanthanide and Pt-alkaline earth catalysts and investigated their performance in ORR.\n50\n A volcano-like relationship between the ORR catalytic activity and the bulk lattice parameter was demonstrated (Figure\u00a03C). Thus, the lanthanide contraction was indicated to be capable of modulating strain effects and enhancing the activity and durability of Pt-based catalysts in ORR. Bu et\u00a0al. prepared hierarchical Pt-Co nanowires bounded by high-index and Pt-rich facets, achieving an ORR mass activity approximately 30 times higher than that of commercial Pt/C catalysts.\n40\n\nConstructing Pt-skin structure is also regarded as one of the promising strategies for simultaneously increasing ORR activity and lowering the usage of Pt. The performance of the Pt-skin may be regulated by controlling the composition, size, and shape of intimal metal. Here, we review some representative examples of Pt-skin catalysts with superior ORR catalytic activity. As we discussed in the above section, the component and morphology of Pt-Ni alloys can be protean during growth and catalysis process. Chen et\u00a0al. prepared PtNi3 polyhedra with uniform morphology and size in oleylamine.\n10\n Then, they dispersed the oleylamine-capped PtNi3 polyhedra in nonpolar solvents (e.g., hexane and chloroform) and kept this dispersion at room temperature for 2\u00a0weeks. The PiNi3 polyhedra were found to gradually transform into Pt3Ni nanoframes (Figure\u00a03D). The obtained Pt3Ni nanoframes were then dispersed onto a carbon support and heated to ca. 400\u00b0C in protective argon gas. Most Pt3Ni nanoframes evolved into smooth nanoframes with Pt-skin surface (denoted as Pt3Ni nanoframe/C). Compared with commercial Pt/C catalyst, the Pt3Ni nanoframes/C delivered a factor of 36 and 22 enhancement in mass activity and in specific activity, respectively. The open structure, the sufficient exposure of Pt(111), and the surface strain of Pt atoms were demonstrated to contribute to the enhanced ORR performance. Niu and co-workers synthesized Pt\u2013Ni rhombic dodecahedra (RD) at a lower synthetic temperature than typically reported.\n51\n This lower synthetic temperature allows them to track the growth process during the prolonged synthetic period of time. They found the synthetic solution would turn from green to yellow, brown, and black in about 60\u00a0min. They collected the products at 3, 10, and 30\u00a0min after the solution turned black (denoted as RD-3, RD-10, and RD-30, respectively) and further selectively removed the Ni-rich phase within the collected products by chemical corrosion (denoted as RD-3-cor, RD-10-cor, and RD-30-cor, respectively). Thus, the morphology and dispersion of the Pt-rich phase were tracked. The frame-like morphology of RD-30-cor indicates the Pt migration from internal to external during Pt-Ni alloy growth. The RD-30-cor was also found to exhibit higher ORR activity than RD-3-cor, RD-10-cor, and commercial Pt/C catalyst. The Xia group coated Pd nanocubes with conformal thickness-controllable Pt shell by regulating the injection rate of Pt precursor and synthetic temperature.\n52\n The thickness of the Pt shell could be adjusted to 1, 2, 3, 4, or 6 atomic layers (Figure\u00a03E). All four materials showed higher ORR activity than Pt/C, and as expected, the nanocube with one Pt atomic layer delivered the highest mass activity. Hunt et\u00a0al. demonstrated a self-assembly synthetic approach that allows them to control the particle size, surface Pt layer coverage, and heterometallic composition of the final Pt-M core-shell architecture.\n53\n This synthetic method can be expanded to prepare a variety of core-shell systems. Even though the authors did not investigate ORR performance of the final core-shell particles, this method provides us with a golden opportunity to prepare Pt-skin catalysts with a controllable internal component, Pt-skin thickness and coverage, and doping by other novel metals. Recently, the Huang group reported PtPb/Pt core/shell nanoplates that exhibited superb ORR activity (Figures 3F and 3G).\n54\n At 0.9\u00a0V versus RHE, such a catalyst achieved a specific activity of 7.8 mA cm\u20132 and a mass activity of 4.3 A/gpt (Figure\u00a03H). As revealed by DFT calculation, the edge-Pt and top Pt (110) facets were under large tensile strains, explaining the optimized Pt\u2013O bond strength and the final enhanced ORR performance. Pd, iridium, ruthenium, and other novel metal-based materials have also been studied as ORR catalysts.\n4\n Despite tremendous efforts to optimize their performance, the best performance by them is barely equivalent to that of commercial Pt/C catalysts. In consideration of their expensiveness, it makes little sense to substitute Pt with these materials.Despite the high ORR activity, the cost and durability issues surrounding the Pt-based catalysts severely hamper their widespread commercialization.\n2,5\n The development of low-cost and reliable ORR catalysts is now of great concern. In living organisms, several specific enzymes (e.g., cytochrome C oxidase and laccase) activate oxygen molecules into an electron acceptor that captures the electrons from fuels and thus supplies the energy. Although these enzymes feature nonprecious metals as a catalytic active site, they have demonstrated remarkably reduced overpotential for the oxygen reduction compared with man-made catalysts. This proves that a considerable improvement in ORR activity by nonprecious-metal catalysts is not an illegitimate target.\n12\n During the past decades, various nonprecious-metal-based catalysts have been investigated for substituting expensive Pt-based catalysts in PEMFCs. Among them, nonprecious-metal-nitrogen-carbon composite (M-N-C), nonprecious-metal oxides, chalcogenides, and oxynitrides have been found to be potential candidates.\n4,12\n\nThe premier employment of M-N-C composite as ORR catalysts can be traced back to the research by Jasinski in 1964, in which Co phthalocyanine was found to exhibit ORR catalytic activity.\n55\n After that, tremendous efforts have been devoted to the development of M-N-C ORR catalysts. Generally, there are three common strategies: (1) increasing the intrinsic activity by composition modulation, (2) confining metal species into carbon shells to preserve metal from corrosion (from acidic medium and oxidizing potentials), and (3) increasing the accessibility by constructing porous or other large specific-surface-area structures.Wu et\u00a0al. prepared nonprecious-metal catalysts with Fe and Co confined in multiple C\u2013N shells by using polyaniline (PANI) as a carbon-nitrogen precursor.\n56\n Their best catalyst showed further improved ORR activity that was closer to that of commercial Pt/C (Figure\u00a04\nA) and an enhanced durability in both long-term RDE and PEMFC tests, which was most likely generated from the protection of C\u2013N shells. In 2009, the Dodelet group reported Fe-based catalysts with enhanced ORR activity.\n57\n A mixture of ferrous acetate, carbon black, and phenanthroline was pyrolyzed in argon and ammonia successively after ball-mill. The obtained material was applied in PEMFC as a cathode catalyst and was found to exhibit improved initial ORR activity, which is competitive in comparison with Pt-based catalysts but has unsatisfying stability. Later, this group prepared another Fe-based catalyst by using zeolitic imidazolate framework (ZIF)-8 as a microporous host for phenanthroline and ferrous\u00a0acetate.\n58\n Compared with the previous one, this catalyst achieved an obvious enhancement of ORR activity in H2-O2 PEMFC with a power density of 0.75\u00a0W\u00a0cm\u22122 at 0.6\u00a0V (Figure\u00a04B). They then converted their data into volumetric activity of cathode to compare the obtained activity with that of the US DOE\u2019s targets. The extrapolation of the Tafel slope (at 0.8 V) demonstrated that the volumetric activity by their catalysts was close to the target proposed by the US DOE (300\u00a0A\u00a0cm\u22123 for non-PGM-based catalysts; Figure\u00a04C). Unfortunately, an apparent activity decay of 15% after 100\u00a0h of operation demonstrated the unqualified durability. The Bao group prepared pea-pod-like carbon nanotubes (CNTs) encapsulating Fe NPs (Figure\u00a04D) and found that such a confinement structure can deliver an enhanced durability during long-term PEMFC test.\n59\n They suggested that a charge transfer from the encapsulated Fe cluster to the carbon tube turn the carbon atoms near Fe cluster into ORR active species so that ORR can proceed without the\u00a0immediate contact between O2 and Fe atoms (Figures 4E and 4F). Thus, the enhanced durability can be explained because the carbon shells may preserve the Fe clusters from acid corrosion. The ORR activity of the encapsulated Fe NPs, however,\u00a0still leaves much to be desired. In a typical H2-O2 PEMFC, the encapsulated Fe NPs yielded a voltage of ca. 0.5\u00a0V at a current density of 0.10 A cm\u22122, which was only ca. 60% of that given by Pt/C. Hu et\u00a0al. prepared Fe3C NPs encapsulated by graphitic layers through a high-pressure pyrolysis process. In both acidic and alkaline electrolyte, this catalyst showed remarkable stability.\n60\n The outer graphitic layers were also believed to play a protective role in stabilizing inner particle under corrosive conditions. While in acidic electrolyte, the as-prepared catalysts only achieved E1/2 of ca.\u00a00.73 V, largely underperforming Pt/C.Given that a larger specific surface area usually guarantees a greater accessibility, constructing porous structure is generally considered a remedy for the nonprecious-metal-based catalysts that possess relatively low intrinsic activity. The Qiao group developed CNTs with Fe\u2013N decoration from hierarchically porous carbon.\n61\n Such a material was believed to possess desired merits that included high activity by Fe\u2013N species, facile transportation from large pores, and adequate active-site exposure from large surface area. As expected, an improved ORR performance comparable to that of Pt/C was achieved in alkaline solution. Liang and co-workers prepared a series of mesoporous nonprecious-metal catalysts. Among them, a Co-based mesoporous catalyst, which is fabricated with vitamin B12 as the Co precursor and ordered mesoporous silica SBA-15 as the template, possesses the largest surface area of 568 m2 g\u22121 (Figure\u00a04G).\n62\n This mesoporous catalyst showed an outstanding ORR performance in acidic solution with E1/2 of 0.79\u00a0V versus RHE, an electron-transfer number of ca.3.95, and as excellent durability. The large surface area and the homogeneous distribution of abundant Co\u2013Nx were claimed to contribute to the ORR performance.Selectivity is another noteworthy issue existing in ORR on nonprecious-metal catalysts. Several works have demonstrated the non-negligible and undesired side products, such as H2O2 and O2\n\u2212, in the ORR process on nonprecious-metal catalysts. Recently, the Gewirth group presented their idea that the kinetics of proton transport in the ORR catalysts, to some extent, determine the product distributions.\n63\n They fabricated a hybrid bilayer membrane with Au electrode modified with a self-assembled monolayer of Cu-based ORR catalyst and a monolayer of lipid consisting of proton carrier. Such a hybrid bilayer membrane allowed them to quantitatively regulate kinetics of proton transport to the catalyst by modulating the amount of proton carrier in lipid. As a result, an insufficient proton carrier results in proton transport that is too slow, and thus O2 would be reduced by 1 e\u2212 to O2\n\u2212; an excessive proton\u00a0carrier results in proton transport that is too fast, and the ORR process would\u00a0undergo a 2 e\u2212 pathway with H2O2 as the products (Figure\u00a04H). Thus, a mismatch between proton and electron transport causes unfavorable products (O2\n\u2212 or H2O2), and a commensuration between proton transport and O\u2013O bond breaking rate ensures 4 e\u2212 ORR pathways with H2O as the product.Several nonprecious-metal oxides have also been found to be catalytically active for ORR. Inspired by the biological catalyst in photosystem, the Jaramillo group synthesized Mn oxide thin film through an electro-deposition method.\n64\n This Mn oxide exhibited E1/2 of 0.73 V, which is more negative than that of Pt/C by 130\u00a0mV in alkaline solution. Later, Cheng et\u00a0al. found that the ORR activity on MnO2 can be improved by introducing oxygen deficiency generated by high-temperature treatment in air or argon.\n65\n A modified surface-oxygen interaction and a reduced reaction barrier by oxygen deficiency, as suggested by DFT calculation, were claimed to contribute to ORR performance. Recently, the Dong group reported free-standing tubular monolayer superlattices of hollow Mn3O4 nanocrystal (h-Mn3O4-TMSLs).\n66\n The obtained h-Mn3O4-TMSLs delivered outstanding ORR performance in alkaline solution with an onset potential of ca. 0.91\u00a0V and E1/2 of 0.84\u00a0V (versus RHE), which is about 10\u00a0mV more negative than that of Pt/C. The mesoscale tubular geometry was believed to enhance mass transport, and the monolayer superlattice structure may be beneficial for molecular accessibility. The Dai group grew Co3O4 nanocrystals on reduced graphene oxide (RGO) and found such a hybrid material can sever as an efficient ORR catalyst in alkaline solution.\n67\n The E1/2 by this hybrid material was ca. 0.83\u00a0V versus RHE, similar to that of Pt/C. Because neither Co3O4 nor graphene could hardly catalyze oxygen reduction, the enhanced ORR performance was ascribed to the synergetic chemical coupling effects of the hybrid structure. They further prepared a cobalt-oxide-carbon-nanotube hybrid, which showed an ORR onset potential of 0.93\u00a0V in 1\u00a0M KOH solution.\n68\n This hybrid material was also found to be active and stable in 10\u00a0M NaOH at 80\u00b0C. Several perovskite materials are also active for ORR. The Shao-Horn group investigated ORR activity of a series of perovskites and found that the ORR activity correlates to \u03c3*-orbital occupation and the extent of covalency between B-site metal and oxygen.\n69\n\nThe Dai group fabricated Co1\u2013xS-RGO hybrid material through a solution-phase process followed by solid-state annealing treatment.\n70\n This hybrid material showed an active ORR performance with onset potential of 0.87\u00a0V versus RHE. The small size of Co1\u2013xS NPs and the strong electrochemical coupling between RGO and Co1\u2013xS NPs were believed to promote the ORR performance. Recently, the Xu group constructed honeycomb-like porous carbons with nitrogen and sulfur dual doping and Co9S8 NPs immobilized inside.\n71\n They investigated the ORR activity of a series of such materials after calcination under different temperatures. The best catalyst exhibited an ORR performance with an onset potential of \u22120.05\u00a0V and E1/2 of \u22120.17\u00a0V versus Ag/AgCl. The sufficient accessibility from a honeycomb-like structure and the synergetic interactions between Co9S8 particles and the support may\u00a0explain the enhanced ORR performance. Cao and co-workers prepared CoxMo1\u2013xOyNz supported on carbon, which exhibited onset potentials of 0.918 and 0.645\u00a0V in 0.1\u00a0M KOH and 0.1\u00a0M HClO4, respectively.\n72\n\nOther nonprecious compounds such as MnOOH, TiO2, NbO2, ZrOxNy, TaOxNy, and CoSe2 have also been widely investigated.\n4,5\n In addition, metal-free catalysts, especially N-doped carbon materials, have been reported active for ORR.\n73,74\n Unfortunately, these current nonprecious-metal-based and metal-free catalysts still\u00a0leave much to be desired.\n5\n In particular, because acidic PEMFC is currently much more prevalent than alkaline counterpart, the susceptibility to acid makes nonprecious-metal-based catalysts, especially the oxides, suffer from poor durability in acid medium and limits their widespread adoption in PEMFCs.\n12,13\n\nWith the development of highly advanced characterization techniques, there is growing awareness that the single metal sites with M\u2013N coordination in nonprecious-nitrogen-carbon composite, especially the porphyrin-like FeN4C12 moieties, may be capable of catalyzing the 4 e\u2013 pathway reduction of oxygen to water. In one important work reported by Zitolo et\u00a0al., Fe\u2013N\u2013C catalysts quasi-free of crystallographic Fe were synthesized by a thermal treatment in either Ar or NH3.\n75\n These catalysts had the same Fe-centered moieties but a much higher activity and basicity for NH3-treated Fe\u2013N\u2013C. After a detailed XANES study, two modes of FeN4 porphyrinic different O2 adsorption were identified. The authors noticed that it was difficult to integrate the porphyrinic moieties into graphene sheets, in sharp contrast to Fe-centered species assumed for pyrolyzed Fe\u2013N\u2013C. Such findings not only enlighten bottom-up synthesis strategies of M\u2013N\u2013C SACs but also underline the crucial role of the interaction between single metal atoms and support in ORR catalysis.Meanwhile, SACs have been widely assumed to possess high catalytic activity because of the minimum size of metal species and unique coordination structure. A large number of studies have demonstrated that SACs can exhibit distinctive catalytic performance for a wide variety of catalytic systems. Especially given that SACs may achieve an atomic economy of 100% atom utilization, SACs are logically considered potential ORR catalysts. Though much effort has been devoted, there are still hurdles to overcome before SACs become qualified ORR catalysts in practice. As practical ORR in PEMFC requires high catalytic activity in given geometric area, one challenge is how to improve the metal loading in SACs (typically below 1\u00a0wt\u00a0% for the majority of SACs) or how to promote the accessibility of metal sites to increasing the catalytic activity per given area. Certainly, another challenge is how to improve the intrinsic activity of individual active metal sites. Correspondingly, two strategies are generally used to optimize the ORR performance of SACs. One is to create appropriate supports with a larger specific area to anchor or expose more single-metal sites, with the aim to provide more catalytically active sites. The second one is to modulate the electronic structure of metal sites by tuning the coordination environment or doping heteroatoms with the aim of optimizing the intrinsic catalytic activities. Here, we summarize recent advances in SACs for ORR catalysis and demonstrate how scientists attempt to overcome the above-mentioned issues by constructing rational supports, regulating local coordination environment over single metal sites, or doping heteroatoms.As we discussed before, an ideal support ought to provide dense coordinative sites to anchor sufficient isolated metal atoms, as well as large specific surface areas or porous structures for superior accessibility. Porous ZIFs with intrinsically isolated metal nodes and N-contained organic ligands are thus considered a potential precursor for SACs. In 2016, Yin et\u00a0al. originally reported a facile and effective approach to prepare Co SAC with an extremely high metal loading over 4 wt % via thermal treatment of bimetallic Zn/Co MOFs (Figure\u00a05\nA).\n20\n One significant breakthrough of this work is the elegant use of Zn-Co ZIF for isolating single-metal atoms. Another noteworthy contribution is the greatly enhanced metal loading of the SACs prepared by this approach, highlighting the future direction for practical applications. After this initial disclosure, a variety of protocols for the synthesis of SACs via ZIFs have been reported. For example, Wu and Shao reported a chemical replacement method for constructing Fe-doped ZIF precursors by using an ionic exchange method.\n76\n In this ZIF system, Fe ions can partially substitute Zn ions and bond with imidazolate ligands in three-dimensional frameworks, forming FeN4 complexes. The as-prepared Fe catalyst showed an outstanding ORR activity in acidic media, with E1/2 of 0.85 V, as well as an enhanced stability with a decay of only 20\u00a0mV in E1/2 after 10,000-cycle operation. Recently, Li et\u00a0al. reported a two-step synthetic method to prepare Mn-based SACs with highly dense MnN4 sites. In the first step, a partially graphitized carbon host was prepared by carbonizing Mn-doped ZIF-8 precursors. In the second step, additional Mn and N sources were then adsorbed into the obtained microporous carbon host, followed by a thermal activation, to increase the density of Mn sites. Measured by inductively coupled plasma mass spectrometry (ICP-MS), the Mn content was found to be more than 3\u00a0wt %. In 0.5\u00a0M H2SO4 electrolytes, this catalyst delivered an ORR performance with E1/2 of 0.80\u00a0V and encouraging durability (Figure\u00a05B).\n77\n In 2017, Li et\u00a0al. developed a cage-encapsulated-precursor pyrolysis strategy to generate a highly stable isolated Fe SAC to endow excellent ORR performance.\n78\n In this case, ZIF-8 was used as a molecular-scale cage to isolate and encapsulate the metal precursor Fe(acac)3 because of its special pore size and cavity. After pyrolysis treatment, the ZIF-8 was converted into nitrogen-doped porous carbon, and the Fe(acac)3 was reduced, forming isolated Fe sites anchored on N species. This SAC had a high Fe loading of 2.16 wt % and exhibited a highly efficient activity toward ORR in alkaline media with an E1/2 of 0.900\u00a0V and an exceptional J\nk of 37.83\u00a0mA cm\u2212o at 0.85 V, superior to that of commercial Pt/C. To improve the electron conductivity, the Lin group employed surface-functionalized multiwalled CNTs as a template during the synthesis of Fe-Zn bimetallic ZIFs. After the pyrolysis process, a network of N-doped carbon with atomically dispersed Fe atoms was prepared. The as-prepared Fe SAC gave an ORR performance with E1/2 of 0.81\u00a0V in 0.1\u00a0M HClO4, and a power density of 620 mW cm\u22122 in H2-O2 PEMFC.\n79\n\nThe use of polymers for the synthesis of SACs has also been explored, as the isolated metal atoms can be effectively stabilized by a coordination effect with the nitrogen atoms of N-doped carbon supports In 2018, Li group has offered a polymer encapsulation strategy to prepare SACs supported by porous nitrogen-doped carbon nanospheres.\n80\n In brief, metal precursors were initially encapsulated in polymers by simply mixing the metal acetylacetonate complexes with monomers during the polymerization process. Then, a pyrolysis process was performed at a high temperature to give the polymer-derived porous nitrogen-doped carbon nanospheres with single metal atoms dispersed uniformly. This approach was found to be applicable to both noble and nonprecious metals. They noticed that the Co SAC showed the best performance among other SACs prepared by this approach and delivered a comparable ORR activity (E1/2\u00a0= 0.838 V) and J\nk at 0.83\u00a0V to Pt/C in alkaline media,\u00a0a good methanol tolerance, and an exceptional cycling stability even after 5,000\u00a0cycles. Similarly, a metal-organic polymer supramolecule strategy was introduced by Li and Guo for the construction of a SAC by \u201cself-locking\u201d between metal ions and a natural polysaccharide, sodium alginate (SA).\n81\n SA has a great number of hydrophilic groups (\u2013COOH and \u2013OH) in \u03b1-L-guluronic (G-block) and \u03b2-d-mannuronic (M-block) acid units. It can bond with Fe ions to form a hydrogel, which leads to the formation of atomically dispersed Fe-Nx sites in highly porous, sheet-like structures. The resulting Fe SAC exhibited excellent ORR performance in 0.5\u00a0M H2SO4 and 0.1\u00a0M KOH, along with an admirable durability.Hollow and two-dimensional materials, as a result of high accessibility, are also considered efficient supports for SACs. The Li group reported an interesting template-assisted pyrolysis treatment to access Co SAC dispersed on hollow N-doped carbon spheres (Figure\u00a05C).\n82\n The single Co sites and the hollow carbon spheres collectively contributed to the excellent ORR performance in acid media. Later, they fabricated N-, P-, and S-co-doped hollow polyhedron with embedded single Fe atoms through the Kirkendall effect process (Figure\u00a05D).\n83\n In 0.1\u00a0M KOH solution, this Fe SAC achieved an outstanding ORR performance with E1/2 of 0.912 V, J\nk of 71.9\u00a0mV cm\u22122 at 0.85 V, and a record-level Tafel slope of 36\u00a0mV dec\u22121. In 2017, the Bao group described a simple method to prepare a highly dispersed single Fe catalyst by ball milling of iron phthalocyanine (FePc) and graphene nanosheets (GNs).\n84\n The resultant FeN4/GN showed a high ORR activity in alkaline electrolyte, similar to that of the commercial Pt/C. Importantly, it had a higher stability and resistance to SOx, NOx, and methanol than Pt/C. DFT calculations demonstrated the excellent ORR performance and stability result from the unsaturated Fe centers confined in the graphene nanosheets via four N atoms. Cao et\u00a0al. introduced a surfactant-assisted approach for the preparation of an Fe SAC (Figure\u00a05E) by pyrolyzing the layered-like precursor formed by Fe-loaded water-soluble surfactant F127 and g-C3N4.\n85\n They found that the use of surfactant F127 enabled the uniform dispersion of Fe atoms to form Fe\u2013Nx sites on the support. Additionally, the Fe-doped F127 sheets could strongly anchor on the g-C3N4 so that the Fe clusters could be easily removed by acid treatment. They demonstrated that the ORR activity of Fe SAC stemmed from the Fe-pyrrolic-N4 active sites. Its E1/2 was only 30\u00a0mV less than 20% Pt/C in acidic medium. For the H2-O2 PEMFC testing, it produced a current density of 0.85 A cm\u22122 at 0.6\u00a0V and 3.34 A cm\u22122 at 0.2\u00a0V and achieved the maximum power density of 823 mW cm\u22122. They attribute the good performance of the PEMFC to the accessible and high-density active sites on N-doped carbon nanosheets. Recently, Baek and co-workers synthesized a Cu SAC (up to 20.9 wt %) with isolated Cu atoms distributed in ultrathin nitrogenated carbon nanosheets.\n86\n In their study, the use of L-glutamic was important for high Cu content in SAC because it could introduce additional N species during the synthesis process and effectively trap the metal atoms on the support. Moreover, dicyandiamide could react with the carboxylic acid groups in a trimesic acid to yield a two-dimensional sheet. Because of the synergetic effect between ultrathin nanosheet and high content Cu atoms, a favorable adsorption of O2 and OOH could be obtained. For the ORR testing, it showed over 54 times higher mass activity than Cu NPs at 0.85 V. In addition, it also delivered a lower Tafel slope (37\u00a0mV dec\u22121), higher methanol and carbon monoxide tolerance, and a longer-term stability than commercial Pt/C. The free energy diagrams of ORR pathway on CuN2 and CuN4 were further studied. They showed that under 0.4 V, the reaction on the CuN2 proceeded through a thermodynamically downslope route, implying an excellent electrochemical ORR activity. For CuN4, there was an upslope of 0.75 eV, meaning that the rate-determining factor is the relatively\u00a0weak adsorption of O2 for ORR on CuN4.Several biomaterials are instinctively potential supports for SACs as a result of porosity. Dai and co-workers demonstrated that thermal treatment of unsubstituted phthalocyanine-FePc complexes within micropores of cattle bones can give atomically dispersed Fe atoms on hierarchically structured porous carbon frameworks (Figure\u00a05F).\n87\n The Fe SAC catalyst showed ORR performance comparable to that of the Pt/C in 0.1\u00a0M HClO4 (E 1/2\u00a0= 0.81 V) and an improved long-term durability (7\u00a0mV negative shift after 3,000 cycles). Under alkaline conditions, it outperformed the Pt/C in terms of activity (E 1/2\u00a0= 0.89 V) and long-term durability (1\u00a0mV negative shift after 3,000 cycles).Modulation of the electronic properties of the metal center has also been demonstrated to be an effective route for improving catalytic performance of M\u2013CN catalysts.\n88\n Generally, two approaches can affect the electronic properties of the active metal sites: one is regulating the center metal element, the species, and/or the number of the coordination atoms; another is using long-range interactions between metal sites and doped atoms on the support materials to adjust the electronic structures. For example, doping with heteroatoms (e.g., N, S, and P) also helps to tune the electronic structures to substantially improve the catalytic activity.\n5\n\nFe-based SACs are most commonly employed for ORR, and the catalytic properties are strongly dependent on the types of metal\u2212N2/N4 conformation. However, the problem of the existing FeNx-based catalysts is that the FeN4 is relatively stable sites, which might not be the most active sites based on theoretical predictions due to a strong interaction with O2* and OH*.\n89\n In this regard, Guo and co-workers reported a viable template casting method to access isolated FeN2 species on N-doped ordered mesoporous carbon.\n90\n One significant improvement of this approach is that the Fe precursor can be anchored on the surface of the template (SBA-15). DFT calculations show that the FeN2 outperforms FeN4 because of its lower interaction with OH* and O2* intermediates, along with improved electron transport. Interestingly, another work by Wang and co-workers demonstrated that FeN4 species can also be remarkably active for ORR by mini-trim on the atomic configurations of Fe-N\u2013C moieties.\n91\n The authors constructed atomically isolated Fe atoms on three-dimensional hierarchically porous carbon. In spite of merely 0.20 wt % of Fe metal loading, this SAC is highly efficient for ORR with E1/2 of 0.915\u00a0V in 0.1\u00a0M KOH, exceeding those of Pt/C (E1/2\u00a0= 0.85 V) and most M\u2013N\u2013C catalysts. Importantly, the atom-utilization efficiency in this study was superior to most previous reports. The experimental and DFT results demonstrate that the hierarchical carbon pores can effectively tune the electronic structure of FeN4 by modulating the local coordination of pyridine N. This leads to the selective cleavage of C\u2013N bond near Fe centers, giving edge-hosted FeN4 moieties to lower the ORR barriers to obtain exceptional catalytic activity and durability (Figure\u00a06\nA). Through modulating the electronic properties of the central metal with chlorine ions, the Li group originally developed a FeCl1N4 site catalyst (Fe loading \u223c1.5 wt %) by a thermal-migrating method in 2018.\n92\n The FeCl1N4/CNS showed a superior E1/2 of 0.921\u00a0V in 0.1\u00a0M KOH, 79\u00a0mV more positive than that of Pt/C (Figures 6B and 6C). Moreover,\u00a0it had an excellent J\nk of 41.11 mA cm\u20122 at 0.85\u00a0V and an admirable cycling stability for 10,000 cycles, superior to those of most of the reported nonprecious-metal electrocatalysts. DFT calculations were employed to investigate the effect of chlorine coordination and sulfur doping on the ORR. The catalyst showed a much lower overpotential of 0.44\u00a0V than FeN4/CN but a higher binding energy of O2 (E\nb\u00a0= \u22120.64 eV). They demonstrated a volcano curve showing the relationship between ORR overpotentials and O2 binding energies. That is, a higher O2 binding energy would lead to the difficulty in desorbing OH species, and weaker O2 binding may make the hydrogenation of O2* more sluggish. This suggests that the FeCl1N4/CNS has a moderate charge state and thus shows the highest ORR performance. The results reveal the near-range interaction with chlorine and the long-range interaction with sulfur of the Fe active sites contribute to the modulation of the Fe electronic structure. This work demonstrated the importance of modulation of electronic structure in the design and synthesis of SACs on their catalytic performance, an important future direction for this field.In another case, the Qiao group introduced a variety of graphitic carbon nitride (g-C3N4) coordinated transition metals (M\u2013C3N4) for ORR. The g-C3N4 was used as an efficient support to construct a series of M\u2013C3N4 electrocatalysts. They studied the Co\u2013C3N4 in ORR and oxygen evolution reaction (OER) in alkaline media and found excellent activity stems from CoN2 in the g-C3N4 support, along with an optimal d-band of the catalyst.\n93\n In addition to Fe, Co, Ni, and Mn SACs, carbides of groups have also been investigated as alternatives for precious electrocatalysts in PEMFCs. However, the low density and stability of these active sites greatly hampered their catalytic performance. To address the problem, Chisholm and co-workers originally prepared a new class of single niobium atoms-based carbide catalyst using an arc-discharge approach (Figure\u00a06D).\n94\n Single niobium atoms and ultra-small clusters stabilized in graphitic layers are identified as active sites for catalyzing the cathodic ORR. They found this unique structure greatly enhanced the conductivity to accelerate the exchange rate of ions and electrons, and strongly suppressed the chemical and thermal coarsening of the active species. Importantly, the single niobium atoms stabilized within the graphitic layers can redistribute d-band electrons and therefore significantly facilitate O2 adsorption and dissociation.Heteroatom-doped M\u2013N\u2013C has also shown great potential in improving the ORR performance to substitute precious-metal-based catalysts.\n95\n The S element can be easily introduced to the carbon support by thermal treatment of S-containing species. The relatively large atomic radius of p-block S element may create defects on the support, and the electronegativity of S may tune the electronic properties of M\u2013N sites. In 2018, the Li group described an interesting pyrrole-thiophene copolymer pyrolysis strategy for constructing Fe SAC on S and N-codoped carbon\u00a0with a loading of 0.947 wt %.\n96\n The S and N contents could be changed by controlling the precursors. Interestingly, the catalytic efficiency of the SAC displayed a volcano-type curve with increasing amounts of S employed. The E1/2 of Fe-ISA/SNC was shown to be 0.896 V, and J\nk was 100.7 mA cm\u2212o at 0.85 V, superior to that of Pt/C. X-ray absorption fine structure analysis and DFT showed that the incorporated S could elegantly tune the charges surrounding the Fe active sites. This makes the rate-limiting reductive release of OH* more favorable to give the enhanced ORR activity in alkaline conditions. Overall, this work shows a detailed understanding of the effects of heteroatom doping on the activity of SACs. In another important case, Xie and co-workers achieved isolated Fe\u2013Nx on N- and S-co-decorated hierarchical carbon layers to serve as a bifunctional OER and ORR SAC.\n97\n In their study, vertically aligned CNTs were employed to stabilize catalytic active sites. The isolated single Fe sites on N- and S-co-decorated hierarchical carbon layers were obtained by coating CNTs with 2,2-bipyridine and Fe salt, followed by pyrolysis and acid-leaching steps. Because of the abundance of active Fe sites, three-dimensional conductive networks, and unique hierarchical structure of the support, the Fe SAC catalyst showed an exceptional electrocatalytic performance for ORR and OER in alkaline conditions. By employing this Fe SAC, the polarization curves of Zn-air exhibited an open circuit\u00a0voltage of 1.35\u00a0V and a charge-discharge voltage gap a little lower than that of Pt/C. The maximum power density was as high as 102.7 mW cm\u22122, and the cycling stability was excellent.Among the recent research on SAC for ORR, there are three works worth emphasizing. In the first work by the Zelenay Group in 2017, FeCl3, a polymer (PANI), and a simple organic compound (cyanamide, CM) were deliberately employed as precursors to endow the final Fe SAC (denoted as (CM+PANI)\u2013Fe\u2013C) with pore structures and high activity (Figures 6E and 6F). At voltages higher than 0.75\u00a0V in the H2-air PEMFC test, this Fe SAC achieved almost the same current densities as those obtained with a Pt cathode with a loading of 0.1\u00a0mgpt cm\u22122, underlying the great potential of SAC in practical PEMFCs.\n98\n Later, Wang et\u00a0al. described a double-solvent approach to creating Fe\u2013Co dual sites on an N-doped porous carbon support (Figure\u00a06G).\n99\n The Fe3+ species were reduced and bonded with the adjacent Co atoms. Aberration-corrected high-resolution TEM, X-ray absorption fine structure spectroscopy, and M\u00f6ssbauer spectroscopic measurements were performed to confirm the Fe\u2013Co dual sites coordination environment. The as-obtained (Fe,Co)/N\u2013C catalyst possessed excellent ORR performance (2.842 mA cm\u22122 at 0.9 V) in 0.1\u00a0M HClO4 solution, along with comparable onset potential (1.06 V) and E1/2 (0.863 V) and a superior cycling stability of 50,000 cycles. Remarkably, this catalyst reached maximum power density values of \u223c0.98\u00a0W cm\u2212. in H2/O2 PEMFC and 0.51\u00a0W cm\u2212P in H2/air PEMFC. Moreover, constant-current operation testing showed that the working voltage of this catalyst was maintained even after a 100\u00a0h of operation. This Fe\u2013Co dual-site catalyst outperformed the previously reported Pt-free catalysts in an H2/air PEMFC. DFT calculation showed that the activation of O\u2013O was favored on the (Fe,Co)/N\u2013C dual sites; therefore, the dual sites could greatly decrease the cleavage barrier of O\u2013O bond to give high ORR activity and selectivity to the 4 e\u2212 reduction pathway. This work not only reports an outstanding PEMFC performance by Pt-free catalyst but also demonstrates that the introduction of metal-metal bonds may be a feasible strategy of upgrading SACs into more competitive alternatives to Pt-based catalysts in PEMFCs. The third one reported a convenient synthetic method to prepare SACs from bulk metals. In this approach, ammonia gas first trapped metal atoms in upstream bulk metals by the\u00a0strong Lewis acid-base interaction. Then, the M(NH3)x species were captured by\u00a0the defects on the downstream supports (e.g., pyrolyzed ZIF-8), leaving isolated\u00a0metal sites (Figure\u00a06H). The as-prepared CuN4 SACs exhibited superior ORR performance with an E1/2 of 0.895\u00a0V in 0.1\u00a0M KOH solution. This synthetic strategy may serve as guidance for efficient preparation of SACs directly from bulk metals and demonstrates the potential for scaling up SACs toward industrial applications.\n100\n\nDespite the above achievement we reached, our ultimate goal\u2014a highly active, stable ORR catalyst in PEMFCs with affordable costs\u2014has yet to be achieved. For Pt- and other precious-metal-based catalysts, the primary drawback is still the prohibitively high cost. Although in RDE testing, some advanced catalysts have already achieved high mass activities even outdistancing 0.44 A mgPGM\n\u22121 at 0.9 V,\n10,44,54\n few of them can also deliver such extraordinary performance in a practical PEMFC, leaving the 2020 DOE targets yet to be realized.\n101\n One recent work by the Liu group provides a silver lining. As reported, a Pt-Co catalyst with ultra-low Pt content reached an excellent performance in H2-O2 PEMFC test with a current of 1.77\u00a0A\u00a0mgPt\n\u22121 at 0.9 V, exceeding the DOE 2020 target by approximately four times (Figure\u00a06I).\n102\n Even though the use of extremely expensive Pt precursor and complicated synthetic process lessen the significance, this work demonstrates the feasibility of Pt-based ORR catalysts. However, for nonprecious-metal-based ORR catalysts, it seems that SACs possess greater competitiveness than conventional metal-particle-based materials mainly because of the relatively high activity and, in some cases, the tolerance to acid (while the intrinsic reason has yet to be revealed). Even so, state-of-the-art ORR SACs can still not meet the DOE targets. Thus, to further promote the activity is still the primary research direction for SACs. To fully realize the potential for ORR in PEMFC applications of SACs, more detailed and in-depth research is essential. Detailed structural studies at the atomic level are crucial for us to understand the factors influencing the properties and performance of SACs. So far, the pivotal characterization techniques for SACs are typically based on aberration-corrected HAADF-STEM and synchrotron radiation facility. These techniques are expensive and not easily available for every researcher in the community. Even so, the information we are able to collect by these advanced techniques still leaves much to be desired. In particular, efficient in situ characterization techniques, which enable us to observe catalysts under practical operation, are now urgently needed. In situ TEM,\n103\n XAS,\n104\n and other characterization techniques\n105\n have already been reported to survey the formation process or catalytic behavior under practical conditions.\n106\n With the help of these techniques, we may understand the relationship between catalyst structures and performance on a more profound level and thus prepare improved catalysts accordingly.\n107\n In addition, given that commercialization usually requires low synthetic cost, another future research direction for ORR SACs would be to develop efficient synthetic strategies that allow us facilely prepare SACs on a larger scale. Several works have demonstrated the preparation of SACs directly from metal bulks.\n100,108\n Further research may focus on developing more facile methods that avoid using high temperature or other energy-intensive treatments.This work was supported by the National Key R&D Program of China 2017YFA (0208300 and 0700104) and the National Natural Science Foundation of China (21522107 and 21671180).All authors devised the concept and built the framework of the review. X.W. and Z.L. wrote the manuscript. X.W., Z.L., T.Y., and W.W. organized the figures. X.W., Z.L., Y.W., and Y.L. edited and reviewed the manuscript. All authors approved the final version of the manuscript. X.W. and Z.L. contributed equally.", "descript": "\n                  Platinum (Pt)-based catalysts have been unanimously considered the most efficient catalysts for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). Unfortunately, the exorbitant cost of Pt hampers the widespread adoption and development of PEMFCs. Scientists have devoted tremendous efforts to achieving higher catalytic activity with less Pt usage by constructing delicate nanostructures. Substituting Pt with cheaper metals may be a feasible solution but suffers from a relatively low intrinsic activity. Recently, single-atom catalysts (SACs), which possess the highest metal utilization and excellent activity because of the minimum size of metal and unique coordination structure, are developing rapidly and have been regarded as a potential alternative to Pt-based materials. Here, we review the development of conventional Pt- and nonprecious-metal-based ORR catalysts and summarize recent achievement in SACs for the ORR. A brief perspective on the remaining challenges and future directions of SACs is also presented.\n               "}
{"full_text": "The depletion of fossil reserves in the last century and the high volume of emissions generated have led to search and develop more environmentally friendly alternative energy sources. In this sense, biomass is an attractive alternative since it is the only source from which both energy and chemicals can be obtained, making it the only one with enough potential to replace fossil fuels completely [1]. Even when biomass is widely distributed on Earth, the type of biomass must be carefully selected since, in some cases, the biomass used to obtain energy and chemicals can compete with the food supply [2]. This fact could cause an increase in prices due to the reduction of farmland for food and speculation leading to social imbalances. Among the wide variety of biomass sources, lignocellulosic biomass has emerged as a relevant sustainable source since it does not interfere with the food chain, it is very abundant, and it aims to valorize agricultural residues to produce high value-added chemicals [1,3]. Lignocellulose is generally composed of cellulose, hemicellulose, and lignin, as well as some minor non-structural components, such as proteins, chlorophylls, ash, waxes, tannins (in the case of wood), and pectin (in most fibers) [4]. Focusing on hemicellulose, which displays a disordered and branched structure (with short lateral chains) and low molecular weight, this can be relatively easy to hydrolyze in their respective monomers (pentoses), mainly xylose and arabinose [5].Then, these C5 monomers can be dehydrated through acid catalysis to give rise to furfural (FUR) as the main product [6,7]. This molecule is considered one of the most interesting of the sugar platform in biorefineries, with an annual world production of about 430,000 tons [8,9]. FUR possesses numerous industrial applications, such as fungicide, nematicide, specialized adhesive, flavoring compound, and for the recovery of refinery lubricants, although FUR is mainly employed as a precursor for a wide variety of high value-added chemicals that can be obtained through hydrogenation, oxidation, dehydration, condensation or opening-ring reactions [10\u201312]. Among them, the hydrogenation of furfural is the reaction that possesses the greatest commercial interest. Thus, it has been reported that about 62% of FUR is employed to synthesize furfuryl alcohol (FOL) by hydrogenation [12]. Commercially, this chemical transformation has been carried out by using copper chromite as the catalyst for more than 80\u00a0years [13\u201316]. However, the stringent environmental regulations related to Cr species, together with the high susceptibility of these catalysts to undergo deactivation [17], have led researchers to develop highly active and selective Cr-free catalysts [12]. In this sense, several authors have demonstrated that non-noble metals, such as Cu-, Ni-, and bimetallic-based catalysts [11,12,18], as well as noble metals (Ru, Pt, Pd, Au) [19\u201325] can attain a suitable catalytic performance. As FUR is highly reactive, both the hydrogenating character and the acid/base properties of catalysts need to be modulated. For example, even when Ni-based catalysts are highly active, they are not very selective in many cases because their active centers interact with both the carbonyl group and the aromatic furan ring [26\u201330]. However, Cu-based catalysts have displayed a lower hydrogenating capacity than the Ni ones, which implies that the active sites only interact with the carbonyl group [26,31\u201333], in such a way that the formation of furfuryl alcohol and 2-methylfuran (MF) is favored [26,31,34\u201338]. Fortunately, both compounds are considered valuable FUR derivatives, since FOL is employed in the resin manufacture for the foundry industry and as a starting compound for the synthesis of agrochemicals and bioproducts [12], while MF has potential to be used as additive for biofuels and for the synthesis of heterocycles [12]. On the other hand, it has been reported in the literature that the amount and the strength of acid sites also play an important role in determining the catalytic activity and selectivity. The presence of a high concentration of acid sites favors the FUR polymerization and causes the formation of carbonaceous deposits, leading to the partial blockage of active sites for FUR hydrogenation [17,37]. Generally, the use of Cu-based catalysts in the gas-phase hydrogenation of furfural results in FOL as the main product, while the formation of MF is quite low [33]. However, several papers have reported that the presence of weak acidity and small metal Cu nanoparticles bring about the hydrogenolysis of FOL to MF [35,37].As non-noble metal catalysts are prone to deactivation, most catalysts are prepared with a very high metal loading. One of the greatest challenges for the scientific community is to design catalysts in which textural properties, the proportion of acid and basic sites, and the nanoparticle size of the active phase can be easily modulated. For example, Jiao et al. [39] dispersed several metals, Cu among them, on different supports by using the strong electrostatic adsorption (SEA) method and compared them with others prepared by the conventional incipient wetness impregnation approach. They showed that the SEA method provided better metal dispersion, easier reducibility, and smaller particle size. Recently, Wong et al.\n[40] attained much lower metal particle sizes on silica supports (0.9\u20131.4\u00a0nm), in comparison with metal particles obtained by conventional impregnation (4.2\u201315\u00a0nm), after studying different supported metal catalysts, including Cu-based ones. Regarding the textural properties, the synthesis of porous silica-based solids with ordered structure has grown exponentially [41]. SBA-15 has been the most studied mesostructure in the last 20\u00a0years for its applications in the fields of adsorption and catalysis [42]. This mesostructured silica displays a hexagonal ordering formed by parallel cylindrical channels with a diameter between 4 and 9\u00a0nm [43], although the main advantage is ascribed to the ease of preparation, good thermal and mechanical resistance, and the tunability of the pore size. However, one of the main disadvantages of mesoporous materials is that, in some cases, active metal nanoparticles are greater than the pore diameter of the support, so these nanoparticles remain on their outer surface, leaving a significant fraction of their specific surface area without active phase. On the other hand, the long length of the cylindrical mesochannels facilitates the deactivation, and, in many cases, these pores are too narrow to allow the access of reagents to the active sites. Additionally, these narrow pores can be easily blocked by the strong interaction of the reactants or products with the active centers, or by the formation of coke [44]. To overcome those challenges, the pore diameter of the mesochannels can be enlarged by the incorporation of aromatics [45,46], alkanes [47,48], or amines [49] in the synthesis gel, although this strategy often promotes the loss of ordering in the porous silica [46]. On the other hand, incorporating fluoride species in the synthetic step limits the growth of the mesochannel length, obtaining other structures with poorer ordering denoted as mesocellular foams [45,46,50].Considering these premises, this work aimed to study the effect of the textural properties of several porous silicas on the dispersion of Cu nanoparticles by using strong electrostatic adsorption (SEA) method. Thus, different synthetic strategies were employed to prepare mesoporous silica with different textural properties, and these were compared to commercial fumed silica. Although this study is focused on establishing a correlation between the textural properties and the catalytic behavior of Cu catalysts supported on mesoporous silicas, the effect of the Cu loading on the catalytic activity was also evaluated by supporting highly dispersed Cu nanoparticles on a commercial fumed silica support using the SEA method. The study of these catalysts in the gas-phase hydrogenation of furfural allowed us to establish a correlation between the number of available metal sites and the catalytic activity. Additionally, an analysis of the deactivation processes is performed, and a strategy for the regeneration of the catalysts is put forward.The chemicals used for the synthesis of the porous silica were P123 Pluronic (PEO20PPO70PEO20, average Mn\u00a0\u223c\u00a05800, Sigma-Aldrich) as a template, tetraethylorthosilicate (TEOS) (Aldrich, 98%) as the silicon source, and hydrochloric acid (HCl) (VWR, 37%). For the modification of the textural properties of porous silica, ammonium fluoride (NH4F) (Aldrich, 99%) was employed. Commercial CAB-O-SIL\u00ae EH-5 fumed silica was also used. Before using it as a support, it was washed to remove any potential metal impurities with a 0.1\u00a0M HNO3 aqueous solution for 1\u00a0h at room temperature; then, the solution was filtered and washed with deionized water until neutral pH and dried at 60\u00a0\u00b0C for 24\u00a0h. In all cases, Cu was incorporated by using [Cu(NH3)4]SO4\u00b7H2O (Sigma-Aldrich, 98%) as the metal precursor. Furfural (FUR) (Sigma-Aldrich, 99%) and cyclopentyl methyl ether (CPME) (Sigma-Aldrich, 99.9%) were employed in the hydrogenation reactions. Likewise, the gases utilized were H2 (Air Liquide 99.999%), He (Air Liquide 99.99%), H2/Ar (10% vol. H2, Air Liquide 99.99%), N2 (Air Liquide 99.9999%), and N2O/He (35% vol. N2O, Air Liquide 99.99%).The synthesis of the SBA-15 was carried out by adjusting the temperature of the hydrothermal treatment, following the method previously described by Fulvio et al. [51]. Briefly, Pluronic P123 was dissolved in a 1.7\u00a0M HCl aqueous solution under stirring at 40\u00a0\u00b0C. Once P123 was dissolved, the silicon source (TEOS) was added dropwise to the mother solution to obtain a gel with a molar ratio of 1 P123: 55 SiO2: 350 HCl: 11,100 H2O. This gel was stirred at 40\u00a0\u00b0C for 24\u00a0h and, on the one hand, it was aged at room temperature for 48\u00a0h in order to obtain a porous SBA-15 silica with narrower pore diameter (SBA-LT) and, on the other hand, it was transferred to a Teflon-lined autoclave, where it was aged under hydrothermal conditions at 120\u00a0\u00b0C for 48\u00a0h to get a SBA-15 with a wider pore diameter (SBA-HT).In another synthesis, and in order to modify the textural properties of the SBA-15, NH4F was incorporated into the synthesis step. The synthesis of this modified porous silica was carried out following the synthesis described by Santos et al. [52] in such a way that firstly, both the template (P123) and NH4F were dissolved in a 1.7\u00a0M HCl aqueous solution under stirring at 40\u00a0\u00b0C. After the total dissolution of the template, the silicon source, TEOS, was also incorporated. The final gel had a molar ratio of 1 P123: 1.8 NH4F: 350 HCl: 55 SiO2: 11,100 H2O. Finally, the obtained gel was stirred at 40\u00a0\u00b0C for 24\u00a0h, and then it was aged for 48\u00a0h at room temperature. Santos et al.\n[52] reported that the obtained material displayed a structure with shorter channels, being denoted as mesocellular foam (MFC-LT).In all cases, the obtained gel was washed with distilled water, filtered, and dried overnight at 80\u00a0\u00b0C. Finally, all the solids were calcined at 550\u00a0\u00b0C for 6\u00a0h (heating rate of 1\u00a0\u00b0C\u00a0min\u22121).The physicochemical properties of the obtained porous silicas were compared with those of a fumed silica with submicron particle size provided by Cabot Corporation.Once porous silicas were synthesized and characterized, Cu was incorporated through the strong electrostatic method (SEA). This consists of preparing a dispersion of the support in water and selecting a pH above its point of zero charge (PZC) in order to have a negatively charged surface, thus promoting the interaction with Cu(NH3)4\n2+ species, which results in highly dispersed Cu nanoparticles on silica [39]. For this, 2\u00a0g of porous silica was added to 1 L of deionized water under stirring. In the next step, an aqueous solution of [Cu(NH3)4]SO4\u00b7H2O was added to obtain a final copper loading of 10\u00a0wt%. Then, the pH of the solution was adjusted with diluted NH3 and H2SO4 solutions to pH\u00a0=\u00a09. Later, the solution was stirred for 20\u00a0h at room temperature, and, after that, it was filtered and washed with deionized water, thus obtaining the pertinent catalytic precursors. However, in no case coloration was observed in the washing liquid, so all the Cu(NH3)4\n2+ species were incorporated to the supports.The catalyst precursor was thermally treated to decompose the salt and reduce the metal phase in the same process. To do so, the catalytic precursors were heated with a heating rate of 5\u00a0\u00b0C\u00a0min\u22121 from room temperature up to 300\u00a0\u00b0C under a He flow (60\u00a0mL\u00a0min\u22121). Then, the gas carrier was changed to H2 (60\u00a0mL\u00a0min\u22121) and the temperature was increased up to 400\u00a0\u00b0C (5\u00a0\u00b0C\u00a0min\u22121), which was maintained for 3\u00a0h in order to reduce copper species. Later, the gas was switched back to He again, and the sample was maintained at 400\u00a0\u00b0C for 1\u00a0h before cooling down to room temperature under He flow. Finally, the catalysts were passivated using 0.5\u00a0vol% O2/N2 for 30\u00a0min at room temperature.The samples were labeled as XCu-Y, where\u00a0X\u00a0indicates the theoretical metal Cu loading, expressed as wt.%, and Y the support employed. Thus, the following catalysts were synthesized: XCu-SBA-LT for the Cu-based catalyst supported on SBA-15 aged at room temperature, XCu-SBA-HT for the Cu-based catalyst supported on SBA-15 aged at 120\u00a0\u00b0C, XCu-MCF-LT for the Cu-based catalyst supported on mesocellular foam, and XCu-SiO2 for the Cu-based catalysts supported on a commercial fumed silica. First, the effect of adding different Cu loadings (2.5\u201320\u00a0wt%) was evaluated by using the commercial silica as the support (XCu-SiO2). Then, an intermediate loading of 10\u00a0wt% Cu was chosen to evaluate the influence of each support on the furfural hydrogenation (SBA-LT, SBA-HT, and MCF-LT).Small-angle X-ray scattering (SAXS) measurements were collected on a D8 DISCOVER-Bruker instrument at 40\u00a0mA and 40\u00a0kV. Powder patterns were recorded in capillary transmission configuration by using a LYNXeye detector and a G\u00f6bel mirror (CuK\u03b11 radiation). The powder patterns were performed between 0.2 and 10\u00b0, with a total measuring time of 120\u00a0min.X-ray diffraction (XRD) patterns of the Cu-based catalysts were obtained by using the Bragg\u2013Brentano reflection configuration in a PANanalytical X\u2019Pert Pro automated diffractometer equipped with a Ge (111) primary monochromator (strictly monochromatic CuK\u03b11) and an X\u2019Celerator detector with a step size of 0.0178 (2\u03b8) between 2\u03b8\u00a0=\u00a010 and 70\u00b0, with an equivalent counting time of 712\u00a0s per step. The crystalline particle size (D) was evaluated by using the Williamson\u2013Hall equation, B cos\u03b8 = (K\u03bb/D) + (2\u03b5 sin\u03b8), where B is the full width at half-maximum (FWHM) of the XRD peak, \u03b8 is the Bragg angle, K is the Scherrer constant, \u03bb is the X-ray wavelength, and \u03b5 is the lattice strain [53].A FEI Talos F200X equipment was utilized in order to study the catalyst morphology by transmission electron microscopy (TEM). This equipment combines high-resolution STEM and TEM imaging with energy-dispersive X-ray spectroscopy (EDS) signal detection, and 3D chemical characterization with compositional mapping. Samples were suspended in isopropyl alcohol and dropped onto a perforated carbon film grid.Textural properties were obtained from the N2 adsorption\u2013desorption isotherms at \u2212196\u00a0\u00b0C, as determined by an automatic Micromeritics ASAP 2420 system. Before the adsorption analysis, samples were outgassed overnight at 200\u00a0\u00b0C and 10\u22124 mbar. Surface areas were determined using the BET equation considering a N2 cross-section of 16.2\u00a0\u00c52\n[54], while the microporosity was determined by the t-plot method [55]. The total pore volume was evaluated from the adsorption isotherm at P/P0\u00a0=\u00a00.95, and the average pore size was determined by applying the Barrett\u2013Joyner\u2013Halenda (BJH) method to the desorption branch [56]. On the other hand, a density functional theory (DFT) method was employed to determine the pore size distribution [57].The number of acid sites was determined by temperature-programmed desorption of ammonia (NH3-TPD). For each analysis, 0.08\u00a0g of catalyst was placed in a U-shaped quartz reactor and treated with H2 (60\u00a0mL\u00a0min\u22121) at 300\u00a0\u00b0C with a heating rate of 5\u00a0\u00b0C min\u22121 to remove the passivation layer. Then, the sample was cooled in He (40\u00a0mL\u00a0min\u22121) until 100\u00a0\u00b0C, and once the temperature was reached, the sample was saturated with ammonia for 5\u00a0min, and then, a He flow was employed to remove the physisorbed NH3. Ammonia desorption was performed by heating the sample from 100 to 300\u00a0\u00b0C, with a rate of 5\u00a0\u00b0C min\u22121, while registering the signal using a thermal conductivity detector (TCD).In order to know the reducibility of the catalysts, hydrogen temperature-programmed reduction (H2-TPR) experiments were conducted. In each test, 0.08\u00a0g of the catalyst precursor was employed, being firstly treated under He (60\u00a0mL\u00a0min\u22121) at 350\u00a0\u00b0C (heating rate of 5\u00a0\u00b0C\u00a0min\u22121) for 1\u00a0h to decompose the precursor salt. After that, the sample was cooled to room temperature and the H2 consumption was monitored between 50 and 800\u00a0\u00b0C with a heating rate of 10\u00a0\u00b0C\u00a0min\u22121 by using a 10\u00a0vol% H2/Ar flow (48\u00a0mL\u00a0min\u22121). The consumed H2 was quantified by using an on-line TCD in such a way that the water formed in the reduction process was trapped through a cold finger immersed into a liquid N2/isopropanol slurry (-80\u00a0\u00b0C).The exposed Cu surface and the dispersion of Cu nanoparticles were determined by N2O titration. This methodology is based on the superficial oxidation of Cu0 under a N2O flow, described in previous research [36,52], according to the reaction:\n\n2Cu0\u00a0+\u00a0N2O\u00a0\u2192\u00a0Cu2O\u00a0+ N2\n\n\n\nBefore the analysis, the catalytic precursor was treated under a He flow (60\u00a0mL\u00a0min\u22121) up to 350\u00a0\u00b0C (5\u00a0\u00b0C\u00a0min\u22121) for 1\u00a0h, followed by a reduction step at 300\u00a0\u00b0C for 1\u00a0h, with a heating rate of 5\u00a0\u00b0C\u00a0min\u22121, under a 10\u00a0vol% H2/Ar flow (48\u00a0mL\u00a0min\u22121). Later, the reduced catalyst was cooled to 60\u00a0\u00b0C under a He flow. Then, the mild oxidation of Cu0 to Cu+ was performed by flowing N2O (5\u00a0vol% N2O/He) at 60\u00a0\u00b0C for 1\u00a0h. After that, the sample was cleaned under an Ar flow and cooled to room temperature. Finally, the Cu2O reduction to Cu0 was performed by heating the sample from room temperature to 300\u00a0\u00b0C, under a 10\u00a0vol% H2/Ar flow (48 mL min\u22121) with a heating rate of 5\u00a0\u00b0C\u00a0min\u22121, by using a TCD to quantify the H2 consumption.The metallic surface area was estimated according to the equation proposed by Pakharukova et al.\n[58] (Eq. (1)):\n\n(1)\n\n\n\n\nS\n\n\nCu\n\n\n\n\nN\n\n\n2\n\n\nO\n\n\n(\n\n\nm\n\n\n2\n\n\n\n\n\ng\n\n\nC\nu\n\n\n-\n1\n\n\n)\n=\n\n\n\n\nM\n\n\n\n\nH\n\n\n2\n\n\n\n\n\u00b7\nS\nF\n\u00b7\n\n\nN\n\n\nA\n\n\n\n\n\n\n10\n\n\n4\n\n\n\u00b7\n\n\nC\n\n\nM\n\n\n\u00b7\n\n\nW\n\n\nCu\n\n\n\n\n\n\n\nwhere \n\n\nM\n\nH\n2\n\n\n\n is the number of mol of hydrogen consumed per unit mass of catalyst (\u03bcmol H2 gcat\n\u22121), SF is the stoichiometric factor whose value is 2, NA\n is the Avogadro number, CM\n is the number of copper atoms per surface area unit (1.46\u00b71019 atom m\u22122), and WCu\n is the Cu content (wt%).Considering the spherical morphology of Cu0 nanoparticles, the average size (nm) (\n\n\nd\n\nCu\n\n\n\nN\n2\n\nO\n\n\n\n) was determined from Eq. (2):\n\n(2)\n\n\n\n\nd\n\n\nCu\n\n\n\n\nN\n\n\n2\n\n\nO\n\n\n(\nn\nm\n)\n=\n\n\n6\n\u00b7\n\n\n10\n\n\n3\n\n\n\n\n\n\nS\n\n\nCu\n\n\n\n\nN\n\n\n2\n\n\nO\n\n\n\u00b7\n\n\n\u03c1\n\n\nCu\n\n\n\n\n\n\n\nwhere \u03c1 is the density of copper (8.92\u00a0g\u00a0cm\u22123).The dispersion of the Cu0 nanoparticles was determined by Eq. (3)\n[59]:\n\n(3)\n\n\nD\ni\ns\np\ne\nr\ns\ni\no\nn\n\n\n%\n\n\n=\n\n\n6\n\u00b7\n\n\n\nv\n\n\ncu\n\n\n\n\na\n\n\nCu\n\n\n\n\n\nd\n\n\n\u00b7\n100\n\n\n\nwhere v\nCu is the occupied volume per atom (1.183\u00b710\u221229\u00a0m3 atom\u22121), aCu\n is the occupied surface per atom (6.85\u00b710\u221220\u00a0m2 atom\u22121), and d is the average size of the Cu0 nanoparticles.XPS spectra were obtained in a Physical Electronics PHI 5700 spectrometer with non-monochromatic Mg K\u03b1 radiation (1253.6\u00a0eV, 300\u00a0W, 15\u00a0kV) and multichannel detector. Spectra of catalysts were recorded in the constant-pass energy mode at 29.35\u00a0eV using a diameter analysis area of 720\u00a0\u03bcm. Charge referencing was measured against adventitious carbon (C 1s) at 284.8\u00a0eV as binding energy (BE). The acquisition and data analysis was performed by using the PHI ACCESS ESCA-V6.0F software package. A Shirley-type background was subtracted from the signals. All the recorded spectra were fitted with Gaussian-Lorentzian curves to determine the binding energies of the different element core levels more accurately. As the catalysts were previously reduced, the samples were stored in sealed vials with cyclohexane as inert solvent to avoid their oxidation. Thus, the samples were prepared in a dry box under a N2 flow and analyzed directly without previous treatment, and the solvent was evaporated before the introduction of the samples into the analysis chamber.The gas-phase hydrogenation of FUR was carried out in a \u00bc\u201d tubular quartz reactor. The pelletized catalyst (325\u2013400\u00a0\u03bcm) was placed in the middle section between two layers of quartz wool, discarding diffusion problems through the Weiss-Prater criterion as shown in a previous publication [60]. Prior to the catalytic tests, catalysts were depassivated under a H2 flow (60\u00a0mL\u00a0min\u22121) at 300\u00a0\u00b0C for 1\u00a0h. Then, the reduced catalysts were cooled down to the selected reaction temperature under a H2 flow (10\u00a0mL\u00a0min\u22121). After reaching this temperature, a flow of 3.87\u00a0mL\u00a0h\u22121 of a 5\u00a0vol% FUR solution in cyclopentyl methyl ether (CPME) was continuously injected using a Gilson 307SC piston pump (model 10SC). The temperature of the reaction was controlled with a thermocouple located at the same height of the catalytic bed. FUR was dissolved in CPME to avoid problems related to the use of pure furfural, such as blockage of the lines due to FUR polymerization. CPME possesses interesting physicochemical characteristics, such as that it is an environmentally friendly solvent, its low solubility in H2O in comparison to other ethereal solvents, low boiling point (106\u00a0\u00b0C), low formation of peroxides, and relatively high stability under acid or basic conditions [38]. Liquid samples were collected and kept in sealed vials, being subsequently analyzed by using gas chromatography (Shimadzu GC-14B) with a flame ionization detector (FID) and a CP Wax 52 CB capillary column.In a preliminary test, a Cu-SiO2 catalyst was chosen to evaluate the stability of CPME as solvent in the absence of FUR at 190\u00a0\u00b0C after 5\u00a0h of time-on-stream (TOS). FOL and MF were the only products obtained from the hydrogenation of FUR. These products were quantified from the calibration lines obtained with commercial reagents supplied by Aldrich. The furfural conversion [%], selectivity [%] and yield [%] were calculated as follows (Eqs. (4) to (6)):\n\n(4)\n\n\nConversion\n\n(\n%\n)\n\n=\n\n\nmol\n\no\nf\n\nF\nU\nR\n\nc\no\nn\nv\ne\nr\nt\ne\nd\n\n\nmol\n\no\nf\n\nF\nU\nR\n\nf\ne\nd\n\n\n\u00b7\n100\n\n\n\n\n\n\n(5)\n\n\nS\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n\n\n%\n\n\n=\n\n\nmol\n\nof\n\nproduct\n\n\nmo\nl\n\nof\n\nFUR\n\nconverted\n\n\n\u00b7\n100\n\n\n\n\n\n\n(6)\n\n\nY\ni\ne\nl\nd\n\n\n%\n\n\n=\n\n\nmol\n\nof\n\nproduct\n\n\nmol\n\nof\n\nFUR\n\nfed\n\n\n\u00b7\n100\n\n\n\n\nXRD diffraction patterns of the XCu-SiO2 catalysts were very noisy (Fig. 1\n), even for the catalyst with the largest Cu loading. On 5Cu-SiO2, diffraction peaks located at 2\u03b8\u00a0=\u00a043.6 and 50.6\u00b0, assigned to Cu0 crystallites, can be observed (PDF: 01-077-3038), whereas the signal located at 2\u03b8\u00a0=\u00a036.5\u00b0 was attributed to a crystalline Cu2O phase (PDF: 01-077-0199). The analysis of crystallite sizes by the Williamson-Hall method [53] confirmed that both Cu0 and Cu2O must be highly dispersed as a result of the SEA method since the average crystallite size is lower than 5\u00a0nm, although the intensity of these diffraction peaks increased with the Cu loading.On the other hand, the long-range order of porous silicas was determined by SAXS (Fig. 2\n). In the case of the SBA-15 supports, it can be observed a single peak attributed to the d\n100 reflection, which is shifted towards lower 2\u03b8 values when the aging temperature is increased (from 2\u03b8\u00a0=\u00a01.03\u00b0 to 0.88\u00b0 for SBA-LT and SBA-HT, respectively). This shift can be due to an increase in the pore diameter, wall thickness, or both. It is also noteworthy that the intensity of SBA-LT and SBA-HT signals was similar, which could point out that the ordering of the porous structure does not depend on the synthesis temperature. Similarly, the addition of a modifying pore agent in the synthesis step to form the MFC support also caused a shift of the d\n100 reflection to a lower value (2\u03b8\u00a0=\u00a00.36\u00b0). This fact implies a greater increase in the pore size, although the incorporation of this additive also causes a relevant decrease in the intensity of the d\n100 reflection, suggesting the formation of a structure with a lower ordering, as was previously described by other authors [48,50].Once the ordering of the porous silicas was evaluated by SAXS, these materials were used as supports for the dispersion of Cu nanoparticles, which were incorporated by the SEA method. In order to compare Cu-based catalysts supported on a commercial fumed silica with those supported on mesoporous silicas, a metal loading of 10\u00a0wt% was selected. X-ray diffractograms of the reduced samples (Fig. 3\n) show, in all cases, a broad band at 2\u03b8\u00a0=\u00a023-25\u00b0, which is assigned to the amorphous walls of porous silica. In addition, all samples exhibit intense wide and noisy diffraction peaks at 2\u03b8\u00a0=\u00a043.6 and 50.6\u00b0, which are attributed to the formation of metallic copper crystallites. Additionally, all diffractograms display another small diffraction peak located at 2\u03b8\u00a0=\u00a036.5\u00b0, which is ascribed to the d\n111 reflection of Cu2O crystallites. The presence of Cu2O could be attributed to the partial oxidation of samples, as a result of the handling between the preparation and the XRD analysis or, most likely, to a fraction of CuO nanoparticles interacting more strongly with the support, in such a way that they are only partially reduced, forming Cu2O crystallites. Interestingly, the modification of the support does not modify the intensity of the diffraction peaks of Cu0 and Cu2O. This fact implies that all the catalysts must have a similar crystallite size, which is very interesting in order to evaluate the role of the morphology of the support on the catalytic performance.The morphology of supports and the dispersion of Cu nanoparticles were determined by TEM (Fig. 4\n). The TEM micrograph of 10Cu-SBA-LT (Fig. 4A) evidences an ordered support with a honeycomb morphology and parallel channels. In the case of 10Cu-SBA-HT (Fig. 4B), the support maintains an ordered structure, although there is a greater separation between adjacent silica walls, in agreement with XRD data (Fig. 2). This fact would suggest the formation of a porous structure with a larger pore diameter than SBA-LT. As shown in Fig. 4C1, incorporating a structure modifying agent (MCF-LT) causes a disorder of the porous structure, as the mesochannels detected in other mesoporous silicas cannot be observed here. Regarding copper supported on commercial silica (10Cu-SiO2) (Fig. 4D), small particles with spherical morphology can be seen. From these micrographs, it can be concluded that all the supports exhibit different morphology, which could influence their catalytic behavior. Furthermore, these micrographs also show a high dispersion of Cu nanoparticles on the different supports, with some nanoparticles small enough to enter the porous structure (Fig. 4C2), except for the 10Cu-SBA-HT catalyst where the agglomeration of small Cu crystallites can be observed on the surface of SBA-15 (Fig. 4B). This fact could be due to the hydrothermal synthesis leading to a more hydroxylated external surface than inside the pores, or the existence of regions with different points of zero charge (PZC) within the material, which could promote the strong electrostatic adsorption of Cu particles on the external surface of 10Cu-SBA-HT.In order to determine the textural properties of catalysts, N2 adsorption\u2013desorption isotherms at \u2212196\u00a0\u00b0C were obtained (Fig. S1A). According to the IUPAC classification, 10Cu-SBA-LT, 10Cu-SBA-HT, and 10Cu-MCF-LT catalysts exhibit Type IV isotherms [61], which are ascribed to mesoporous materials; however, the N2 adsorption\u2013desorption profiles are different due to their distinct textural properties. The isotherm of 10Cu-SiO2 shows an increase in the N2 adsorbed at higher relative pressure, and, unlike the other ones, it can be classified as Type II, typical of macroporous materials [61]. In this latter case, the porosity can be ascribed to interparticle voids, and the presence of the hysteresis loop suggests the presence of cavities whose size is greater than 4\u00a0nm [61]. The hysteresis loops of 10Cu-SBA-LT, 10Cu-SBA-HT, and 10Cu-MCF-LT catalysts can be considered as Type H1, found in porous materials with a narrow range of uniform mesopores [61]. However, the hysteresis loop of the 10Cu-SiO2 catalyst resembles the Type H3, which is given by aggregates of spherical silica particles, in agreement with the results obtained by TEM (Fig. 4D).Concerning the BET surface area (SBET, Table 1\n) [54], 10Cu-SBA-LT, 10Cu-SBA-HT, and 10Cu-MCF-LT catalysts show very similar values (356\u2013409\u00a0m2 g\u22121). In contrast, 10Cu-SiO2 possesses a much lower SBET (179 m2g\u22121). However, the pore volume analyses evidence clearer differences than the SBET values. Interestingly, while 10Cu-SBA-LT displays the lowest pore volume (VP), the use of hydrothermal conditions (10Cu-SBA-HT) leads to the largest one. On the other hand, 10Cu-MCF-LT possesses the highest microporous surface area (t-plot, 34 m2g\u22121) and volume (VMP, 0.0139 cm3g\u22121). The pore size distribution was determined by a DFT method (Fig. S1B) [57]. In all cases, the microporosity (<2\u00a0nm) was relatively low, being very similar in all of them, which is in agreement with the t-plot analysis and the micropore volume (VMP). In this sense, it has been reported in the literature that the microporosity of SBA-15 is directly related to the aging temperature. Thus, several authors have affirmed that lower aging temperatures favor the interaction of the P-123 molecules with adjacent micelles, leading to structures interconnected by microchannels [46,62]. The presence of these microchannels has enormous importance in the adsorption and catalysis fields, as they might help to minimize the possible diffusion problems associated with the long mesochannels of SBA-15. However, by increasing the aging temperature, isolated P-123 micelles without connection between them are formed, which results in decreased microporosity after calcination [62]. Despite this, it has been hardly seen any differences between the microporosity of 10Cu-SBA-LT and 10Cu-SBA-HT. In this sense, it has been reported that these micropores are easily blocked when porous silicas are functionalized by grafting or subjected to impregnation [46]. However, regarding the mesoporosity, there are clear differences between supports (Fig. S1B). Even when 10Cu-SBA-LT, 10Cu-SBA-HT, and 10Cu-MCF-LT catalysts present a narrow distribution of pore sizes, these are centered at different pore diameters. It has been previously reported in the literature [62] that an increase in the aging temperature causes a rise of the average pore diameter (dP); therefore, as the pore distribution for 10Cu-SBA-LT is centered at 4.3\u00a0nm and for 10Cu-SBA-HT at 8.6\u00a0nm, our results are consistent with previous studies. In the case of 10Cu-MCF-LT, the pore diameter increases compared to 10Cu-SBA-LT, which was synthesized at the same aging temperature, as a consequence of the addition of fluoride in the synthetic step that modified the growth and ordering of the porous silica, giving rise to a mesocellular structure. Finally, the 10Cu-SiO2 catalyst does not show a homogeneous pore size distribution because its specific surface can be mainly attributed to interparticle voids between silica microspheres.The quantification of the available Cu0 sites was performed by N2O titration at 60\u00a0\u00b0C (Table 2\n), following the methodology described in previous studies [37,38,58]. The incorporation of higher loadings of Cu species over commercial silica led to a higher metallic surface area, reaching the maximum value (25.0 m2\nCu gcat\n\u22121) on 15Cu-SiO2 (Table 2). However, a higher Cu loading (20Cu-SiO2) reduces the available metallic surface area, likely due to the formation of larger Cu crystallites. Regarding the dispersion of the Cu0 sites, a decrease is observed as the Cu content increases.Regarding the influence of the support (10Cu-Y catalysts) (Table 3\n), 10Cu-MCF-LT displays the highest metallic surface area, 25.9 m2\nCu gcat\n\u22121, revealing the highest Cu dispersion and the lowest particle size (2.6\u00a0nm). On the contrary, the 10Cu-SiO2 catalyst shows the lowest metallic surface area and the largest Cu particle size for the same theoretical Cu loading. However, in all cases, the average Cu particle size, estimated by N2O titration, is lower than 5\u00a0nm, which is in agreement with previous studies using the SEA method [39,63,64], thus corroborating that this methodology provides highly dispersed metal particles in comparison with other conventional methods, such as incipient wetness impregnation or precipitation [64].The reducibility of the Cu(NH3)4\n2+ species was elucidated from their H2-TPR profiles (Fig. 5\n). Considering that all catalysts possess the same Cu loading (10\u00a0wt%) and a silica support, the reducibility of the Cu nanoparticles should only be attributed to their size because the interaction should be similar. In this sense, it has been reported in the literature that sometimes it is not feasible to distinguish the different reduction steps for copper species (Cu2+\u00a0\u2192\u00a0Cu+\u00a0\u2192\u00a0Cu0) [37]. As shown in Fig. 5, the maximum of the H2-TPR profiles shifts from 205 to 230\u00a0\u00b0C due to the slight increase in the particle size, as was inferred from the N2O titration (Table 3). Thus, copper nanoparticles are more easily reduced in the case of 10Cu-MCF-LT than in 10Cu-SiO2. With respect to the samples supported on SBA-15, the maximum slightly shifted toward a higher temperature for 10Cu-SBA-HT compared to 10Cu-SBA-LT, which could be attributed to those agglomerated particles detected by TEM (Fig. 4) that would be more difficult to reduce. Additionally, in most H2-TPR profiles, and mainly in the case of 10Cu-SiO2, a shoulder is observed at higher reduction temperatures, which could imply the existence of a small fraction of bigger Cu2+ nanoparticles that require even higher temperatures to be reduced.In order to analyze the chemical composition on the surface, including the oxidation state of the active phase, XPS analyses were also carried out (Fig. 6\n and Table 4\n). The Cu 2p core level spectra analysis for the Cu-based catalysts with a theoretical Cu loading of 10\u00a0wt% (Fig. 6A) shows a band located between 932.6 and 932.9\u00a0eV, which is ascribed to reduced Cu species [65]. However, as it is not possible to differentiate between Cu+ and Cu0 species, it is also necessary to use the CuLMM Auger line (Fig. 6B). The coexistence of Cu0 and Cu+ is observed in all cases, being Cu+ the most abundant (56.6\u201360.2%) (Table 4). The presence of Cu0 and Cu2O agrees with XRD data (Fig. 3), although those showed more intense peaks for Cu0 species. The higher proportion of Cu+ species could be ascribed to partial oxidation of Cu0 species during handling, or to the presence of small Cu2O particles well dispersed on the surface, which would be hardly observed by XRD. On the other hand, in the Si 2p core level spectra, there is a contribution located at about 103.3\u2013103.5\u00a0eV, which is typical of SiO2\n[65], while the O 1\u00a0s core level spectra also show the characteristic band of oxygen in SiO2 at about 532.7\u2013532.8\u00a0eV (Table 4) [65].The surface Cu composition obtained by XPS analysis of the Cu-based catalysts with a 10\u00a0wt% theoretical Cu content is between 1.0 and 2.2% (Table 4). On those catalysts with greater dispersion and consequently smaller particle size, one should expect higher surface copper contents. However, this is not the case when comparing the dispersion obtained by N2O titration (Table 3) and the XPS results (Table 4). In this sense, TEM micrographs have revealed that, in the case of 10Cu-SBA-LT (Fig. 4A) and especially on 10Cu-SBA-HT (Fig. 4B), there is a large proportion of Cu nanoparticles deposited on the external surface of the porous structures, whereas smaller particles are able to penetrate into the pores of 10Cu-MCF-LT (Fig. 4C2). Taking this premise into account and considering that XPS is a surface technique, it is possible to justify the higher surface Cu content on 10Cu-SBA-LT and 10Cu-SBA-HT when compared to 10Cu-MCF-LT.The quantification of the acid sites, performed by NH3-TPD (Table 1), reveals a relatively low concentration between 32 and 57\u00a0\u03bcmol\u00a0g\u22121 for all the 10Cu-Y catalysts. This low acidity is mainly ascribed to the Br\u00f6nsted acid sites associated to silanol groups that have not been involved in the SEA process, whereas the existence of Lewis acid sites can be assigned to Cu+ species, which are more easily formed when small Cu0 particles are present. The acidity is necessary to obtain MF, although the amount and strength of those acid sites need to be modulated to minimize the deactivation in the gas-phase hydrogenation of FUR. It should also be noted that 10Cu-MCF-LT showed the lowest acidity. Regarding the density of acid sites, 10Cu-SiO2 exhibited the highest value, while 10Cu-MCF-LT showed the lowest one, despite having a surface area similar to that of 10Cu-SBA-LT and 10Cu-SBA-HT.These Cu-based catalysts with different textural properties were studied in the gas-phase FUR hydrogenation. In this work, the catalytic conditions were selected to observe the differences between the catalysts prepared with different silica supports, instead of choosing the best experimental conditions to achieve the complete conversion of furfural. In addition, the physico-chemical characterization of fresh and used catalysts also allows to elucidate the causes of the observed deactivation of the catalysts.The first study focused on the influence of the Cu loading on the catalytic behavior. For this purpose, the commercial SiO2 with Cu contents between 2.5 and 20\u00a0wt% was selected (Fig. 7\n). In most cases, it was observed a progressive decrease in the FUR conversion with time-on-stream (TOS) (Fig. 7A), being more pronounced with those catalysts with Cu loadings lower than 15\u00a0wt%. Thus, FUR conversion increases from 47% for 2.5Cu-SiO2 to almost full conversion for 15Cu-SiO2 after 5\u00a0h of TOS at 190\u00a0\u00b0C. It is striking that the catalytic activity slightly decreased for the catalyst with the highest Cu content (20Cu-SiO2) compared to 15Cu-SiO2 under similar experimental conditions. In this sense, previous research studies have shown similar trends with other Cu/SiO2 catalysts with small particle sizes [37]. In that study, it was observed that the increase in the Cu content caused the sintering of metal particles, thus decreasing the metallic surface area (i.e., amount of available active sites).Regarding the product distribution (Fig. 7B-C), the catalysts with lower Cu contents tend to form a larger amount of FOL, even at short reaction times, with the 5Cu-SiO2 catalyst reaching the highest FOL yield after 1\u00a0h of TOS at 190\u00a0\u00b0C (54%), with this slightly decreasing to 49% after 5\u00a0h of TOS. An increase in the Cu loading causes a progressive decrease in the FOL yield, which is accompanied by a rise in the MF yield. This trend is clearly observed in the case of 10Cu-SiO2, although the MF yield decreases from 70% after 1\u00a0h to 12% after 5\u00a0h of TOS at 190\u00a0\u00b0C. On the contrary, the FOL yield increases from 19% after 1\u00a0h to 51% after 5\u00a0h of TOS at 190\u00a0\u00b0C.Considering that all catalysts display similar acidity and metal particle size, these should show similar catalytic behavior. Thus, it is expected that all catalysts are highly selective towards MF at t0; however, the hydrogenolysis sites (FOL\u00a0\u2192\u00a0MF) in the catalysts with lower Cu content (lower metallic surface areas) are poisoned very fast, in such a way that FOL is the main product after the first hour of reaction. In the case of the catalysts with higher Cu contents, their higher metallic surface areas maintain the hydrogenolysis process along the TOS, reaching a MF yield of 82% with the 15Cu-SiO2 catalyst after 5\u00a0h of TOS at 190\u00a0\u00b0C. In any case, the hydrogenolysis sites involved in the FOL\u00a0\u2192\u00a0MF reaction are more susceptible to deactivate than the hydrogenation sites involved in FUR\u00a0\u2192\u00a0FOL, as the increase in the FOL yield with TOS suggests. Interestingly, the 20Cu-SiO2 catalyst provides a lower proportion of MF and larger FOL after 5\u00a0h of TOS compared with the product distribution at the beginning of the reaction. These data are in agreement with a previous study in which it was demonstrated that the incorporation of higher Cu loadings led to the formation of bigger Cu nanoparticles, which favor the hydrogenation reaction (FUR\u00a0\u2192\u00a0FOL) with respect to the hydrogenolysis reaction (FOL\u00a0\u2192\u00a0MF) [37]. In this sense, Shan et al. pointed out that the formation of Cu\u2013O-Si-O- bonds favored the formation of Lewis acid sites, even under H2 flow, which can be involved in the hydrogenolysis step [66]. In this regard, the presence of Cu+ species, identified by XRD (Figs. 1 and 3), may also imply the existence of Lewis acid sites, which may be involved in the FOL\u00a0\u2192\u00a0MF hydrogenolysis process [66,67].The catalytic activity reported in Fig. 7A follows a similar trend to the metallic surface area (m2 gcat\n\u22121) shown in Table 2, in such a way that a higher content of available Cu sites provides a greater catalytic activity and resistance against deactivation. Consequently, the catalyst with the highest metallic surface area (15Cu-SiO2) is the most active. However, the catalyst with the highest Cu content (20Cu-SiO2) displays a selectivity pattern that resembles that of 7.5Cu-SiO2 and 10Cu-SiO2 catalysts, even though the catalytic activity almost stays unaltered, which is likely due to the decreased metallic surface area and the stronger deactivation of the hydrogenolysis sites (FOL\u00a0\u2192\u00a0MF).These results have been compared with those obtained with copper chromite, the commercial catalyst used in the industrial process, under similar experimental conditions [68]. It should be noted that copper chromite reached a FUR conversion of about 75% after 30\u00a0min of reaction, but it was very prone to deactivation, with negligible activity after 5\u00a0h of reaction. Regarding the selectivity, the main product was FOL with a selectivity above 90%. Those data differ from the reported in the present work because deactivation is more limited and the main product, especially at short reaction times, is MF, which indicates that the hydrogenolysis reaction FOL\u00a0\u2192\u00a0MF is promoted due to the presence of a small fraction of weak acid sites.Taking into account that 10Cu-SiO2 is the most prone catalyst to modify its selectivity pattern with TOS (Fig. 7A) and that this catalyst does not maintain a total conversion, a 10\u00a0wt% loading was selected to study the influence of the textural properties and morphology of the porous silica on the catalytic behavior (Fig. 8\n). Considering that all catalysts display similar Cu crystallite size (Table 3) and active site-support interaction (Cu-SiO2), the difference in the catalytic behavior must be ascribed to the textural properties of the catalysts.The data reveal that all the catalysts, except 10Cu-SBA-HT, reach high conversion values at short TOS, with initial values close to 100% (Fig. 8A). While 10Cu-SBA-LT and 10Cu-MCF-LT render FUR conversions higher than 90% after 5\u00a0h of TOS at 190\u00a0\u00b0C, 10Cu-SiO2 deactivates (FUR conversion of 66% after 5\u00a0h of TOS). However, in the case of 10Cu-SBA-HT, the conversion is low from the beginning of the reaction, and the conversion decreases further after 5\u00a0h of TOS, obtaining the poorest FUR conversion (11%).The analysis of the product distribution (Fig. 8B-C) reveals that the most active catalysts (10Cu-MCF-LT, 10Cu-SBA-LT, and even 10Cu-SiO2) show higher MF yields (greater than 70%) at shorter reactions times (1\u00a0h), whereas 10Cu-SBA-HT is more selective towards FOL. As the reaction time progresses, the selectivity towards MF decreases, although the most active catalyst (10Cu-MCF-LT) still maintains a MF yield of 76% after 5\u00a0h of TOS at 190\u00a0\u00b0C. The deactivation process is accompanied by a concomitant increase in the formation of FOL, obtaining the highest FOL yield of 49% with the 10Cu-SiO2 catalyst after 5\u00a0h of TOS. Interestingly, the formation of FOL is more pronounced on the catalysts that deactivate. Therefore, it can be concluded that MF formation occurs through two consecutive reactions, FUR\u00a0\u2192\u00a0FOL on hydrogenation sites, and FOL\u00a0\u2192\u00a0MF on hydrogenolysis sites, which are more likely to be deactivated, as had previously been suggested by other authors [31,35,37]. In this respect, DFT studies have suggested that the first step, the hydrogenation reaction (FUR\u00a0\u2192\u00a0FOL), occurs by the interaction of the Cu nanoparticles with the oxygen atom of the carbonyl group in a top \u03b71(O)-aldehyde binding mode [26]. In the second step, the hydrogenolysis reaction (FOL\u00a0\u2192\u00a0MF), FOL adopts a similar disposition on the copper sites [31] through an interaction with the oxygen atom of the hydroxyl group, while the furan ring is nearly parallel to the surface. Several authors have pointed out that the existence of weak acidity, together with the presence of small Cu particles, favors the hydrogenolysis reaction [35,37]. However, the acidity cannot be high, as this would favor the polymerization and the strong adsorption of FUR on the catalyst surface, blocking the active sites in such a way that the catalysts suffer a drastic deactivation at short reaction times. NH3-TPD data (Table 1) confirmed the low acidity of these catalysts; therefore, using the SEA method for the incorporation of small Cu nanoparticles onto amorphous and porous silica seems to be appropriate to minimize, or slow down, the generation of carbonaceous deposits. Considering that most of the catalysts here synthesized initially produce MF, it could be proposed that the acidity, even if low, would be associated with the silanol groups and partially reduced Cu species (Cu+), which provide Lewis-type acidity. In fact, other authors concluded that small Cu+ particles exhibited Lewis acidity, which promoted hydrogenolysis processes [66,67]. On the other hand, it should be noted that the hydrogenolysis reaction (FOL\u00a0\u2192\u00a0MF) generates H2O as a by-product, which might have an adverse effect on the catalytic performance as it could favor the oxidation of Cu0 to Cu+, thus increasing the Lewis acidity. In this sense, it has been reported in the literature that Cu+ sites can interact more strongly with CO than the Cu0 ones [69]. This fact can be extrapolated to the carbonyl group of the FUR molecule, being able to infer that the presence of a higher proportion of Cu+ sites could favor a stronger interaction of the FUR molecules with the active centers, favoring a faster formation of carbonaceous deposits that block the active sites involved in the FUR hydrogenation. Moreover, previous studies have reported that the adsorption heats of FUR and H2O are very similar (12.3 and 12.4\u00a0kcal\u00a0mol\u22121, respectively) [31,70]; therefore, they could compete for the same active centers, negatively affecting the catalytic behavior. In addition, the interaction of H2O with FUR can also favor its polymerization. Thus, the presence of H2O could promote the partial oxidation of copper species; however, the Cu+/Cu0 ratio detected by XPS was very similar in all cases, discarding its influence on the selectivity pattern. Taking into account all these premises, it seems clear that the sites involved in the second step (FOL\u00a0\u2192\u00a0MF) are more prone to deactivate than those involved in the first step (FUR\u00a0\u2192\u00a0FOL), as shown in Fig. 8B-C, since the decay of the formation of MF is accompanied by larger amounts of FOL. This fact could be explained by the formation of carbonaceous deposits that would mainly block those active sites involved in the hydrogenolysis reaction.Considering that all the 10Cu-Y catalysts possess the same Cu loading (10\u00a0wt%) and all the supports have the same chemical composition (SiO2), a similar catalytic behavior should be expected, although this is not the case (Fig. 8A). As previously stated, 10Cu-Y catalysts displayed different textural properties (Supplementary Information, Fig. S1 and Table 1), and those with the narrowest pore diameter between 3 and 7\u00a0nm (10Cu-MCF-LT and 10Cu-SBA-LT) provided the highest FUR conversions, likely due to the closer proximity of furfural to the active sites within the pores. Moreover, both catalysts exhibited the highest metallic surface area and, consequently, the lowest particle size (Table 3). Thus, a suitable pore size together with the small particle size could favor the reaction of FUR on the Cu sites. Conversely, the lower activity of the 10Cu-SBA-HT catalyst could be attributed to its pore diameter being too large (7\u201312\u00a0nm), which makes difficult the adsorption of FUR on the active sites, in such a way that FUR molecules can go across the SBA-15 mesochannels without interacting with the active sites. That catalyst also presents some agglomeration of the Cu nanoparticles on the outer surface, as observed by TEM (Fig. 4B). This was also observed by N2O titration (Table 3), with 10Cu-SBA-HT and Cu-SiO2 presenting the lowest dispersion values. On the other hand, when comparing the catalysts with narrower pore size distributions (10Cu-MCF-LT and 10Cu-SBA-LT), the 10Cu-SBA-LT catalyst is more prone to modify its selectivity pattern than 10Cu-MCF-LT. In this sense, even when the presence of narrow pores could favor the interaction between FUR and Cu sites, the longer channels could promote the subsequent interaction of FUR, or the reaction products, with the active centers, in such a way that these pores would be blocked. However, with the addition of fluoride in the synthesis step, the length of the silica channels decreases, which facilitates the furfural access and exit of the reaction products from the channels, thus showing a more gradual deactivation. On the other hand, the N2O titration data also show that the 10Cu-MCF-LT catalyst displays the highest metallic surface area, so this catalyst possesses the highest number of available Cu sites for the FUR hydrogenation. It should also be noted that the density of acid sites on 10Cu-SBA-LT is twice that observed for 10Cu-MCF-LT, which could also contribute to a faster deactivation through the formation of carbonaceous deposits. Likewise, it was previously mentioned that the 10Cu-SiO2 catalyst was more prone to deactivation, which agrees with its highest number of acid sites and the lowest metallic surface area. Therefore, it can be concluded that the 10Cu-MCF-LT catalyst possesses suitable textural and acidic properties to provide a high FUR conversion with a low deactivation rate.The following study aims to evaluate the effect of the reaction temperature on the catalytic performance (Fig. 9\n). Considering that the boiling point of FUR is 161.7\u00a0\u00b0C, the catalytic studies were carried out between 170 and 230\u00a0\u00b0C. A volcano-type conversion as a function of temperature has been previously reported in the gas-phase hydrogenation of FUR [17,36,71]. Thus, the FUR conversion increases at lower reaction temperatures because the FUR hydrogenation is thermodynamically favored under those conditions [17]. However, higher temperatures worsen the catalytic performance due to the polymerization processes that take place, which cause a fast deactivation [17]. Furthermore, the hydrogenolysis reaction is favored at high temperatures [17,36]. As expected, the reaction at 170\u00a0\u00b0C provides the lowest conversion values (70% after 5\u00a0h of TOS), although the catalyst is highly selective to FOL (59% yield). Increasing the reaction temperature improves the FUR conversion, but it also modifies the selectivity pattern. Thus, at 190\u00a0\u00b0C, the FUR conversion is higher than 93% and the MF yield is above 75% in all cases, after 5\u00a0h of TOS, confirming that the hydrogenolysis reaction is favored at higher temperatures, as previously reported [36,71]. Surprisingly, a further increase of the reaction temperature did not decrease the conversion, as previously pointed out in the literature [17,36]. This could be due to the small size of the Cu crystallites (<4 nm), which implies that the metallic surface area is very high, so that the catalysts are more resistant to deactivation than others with bigger metal nanoparticles [36\u201338]. It has also been demonstrated that the 10Cu-MCF-LT catalyst possessed a lower density of acid sites and suitable textural properties due to the use of fluoride for the synthesis of this support, which could contribute to slowing down its deactivation.Finally, in order to further evaluate the stability of the 10Cu-MCF-LT catalyst, another experiment was carried out at a longer reaction time (Fig. 10\n). This stability test was performed at 230\u00a0\u00b0C because high conversion and MF selectivity were observed under these experimental conditions, in order to determine if that high MF production could be maintained along with TOS. Even though 10Cu-MCF-LT showed almost complete conversion initially, it suffered gradual deactivation with TOS, obtaining a FUR conversion of 44% after 48\u00a0h of TOS. Regarding product distribution, the main product during the first hours of TOS is MF, resulting from the consecutive hydrogenation-hydrogenolysis reactions taking place. However, after 6\u00a0h of TOS, the MF selectivity decreases likely due to some metal sites involved in the hydrogenolysis reaction being blocked by carbonaceous deposits, as was previously reported [37], so that the FUR\u00a0\u2192\u00a0FOL\u00a0\u2192\u00a0MF process mostly ends in FOL, maintaining a FOL yield of 39% after 48\u00a0h. This fact confirms that the hydrogenation sites are less susceptible to deactivation than the hydrogenolysis ones. As the gas-phase FUR hydrogenation is a reaction that undergoes strong deactivation processes, it is essential to evaluate the regeneration capacity of catalysts. According to previous studies, the carbonaceous deposits involved in the deactivation could be removed by calcination at 500\u00a0\u00b0C [36,37]. After calcination at this temperature, 10Cu-MCF-LT displayed a lower FUR conversion value (80%) than the first cycle (Fig. 10). However, the deactivation follows the same trend to that observed along the first run, obtaining a conversion of 25% after 48\u00a0h of TOS. Nevertheless, the selectivity profile is different to that shown after the first run, with FOL as the main product (62% FOL yield after 1\u00a0h) instead of MF. The change in the selectivity pattern is ascribed to an increase in the Cu particle size during the regeneration process, which is reflected on the loss of the hydrogenolysis sites due to the reduction of the metallic surface area. In this sense, it has been reported in the literature that Cu-based catalyst favor the formation of MF in gas-phase when Cu nanoparticles sizes are lower than 5\u00a0nm [35,37,64], due to the generation of Lewis acid sites [72] besides those associated to the slightly acidic silica support [35,37].In order to elucidate the changes of the active phase throughout the catalytic tests, the 10Cu-MFC-RT catalyst was collected after two cycles of 48\u00a0h of TOS at 230\u00a0\u00b0C. XRD analysis shows that the 10Cu-MFC-RT catalyst hardly suffered any changes (Fig. S2), discarding the transformation of Cu2O, or Cu0, in other crystalline phases. However, the XPS analysis revealed that the proportion of Cu+ remained almost unchanged after two cycles of 48\u00a0h (Fig. S3). While the presence of Cu+ species could favor the hydrogenolysis reaction to produce MF, the modification of the selectivity pattern would indicate that the presence of Cu+ does not have a predominant role in determining the selectivity pattern, as inferred from a higher production of FOL.From the XPS data (Fig. S3 and Table 4), it is observed a decrease in the intensity of the Cu 2p core level spectrum and the Cu AugerLMM line after the two catalytic cycles, which suggests a decrease in the dispersion of Cu species on the catalyst surface. The analysis of the surface chemical composition, before and after the catalytic cycles, also shows an increase in the carbon content after the reaction (Table 4), confirming that a fraction of the Cu species could be partially blocked by the formation of carbonaceous deposits (polymerized FUR and FOL), which seems to agree with the progressive decline of the FUR conversion.A series of Cu-based catalysts was prepared by the strong electrostatic adsorption (SEA) method using several silica supports with different morphologies and textural properties (commercial fumed silica, SBA-15 synthesized at room temperature and under hydrothermal conditions, and mesocellular foam), and these were studied in the gas-phase hydrogenation of furfural, obtaining Cu0 crystallites with a size below 5\u00a0nm. Regarding Cu supported on mesoporous silica catalysts, it can be concluded that the presence of large pores (greater than 7 nm), like those found on the SBA-15 synthesized under hydrothermal conditions, complicated the intimate interaction between the reagents and the active sites along the channels, giving rise to very low catalytic activity. In addition, this catalyst also showed that the agglomeration of the Cu nanoparticles could worsen the activity even more. In contrast, mesoporous silica supports synthesized at room temperature, 10Cu-SBA-LT and 10Cu-MCF-LT catalysts, offered a more suitable pore size for reaction. Moreover, it was demonstrated that the addition of fluoride on the synthesis shortened the length of the silica channels (mesocellular foam), which facilitates the furfural access and exit of the reaction products on 10Cu-MCF-LT, thus providing the highest values of furfural conversion and 2-methylfuran yield. Besides, the high production of 2-methylfuran at the beginning of the reaction could be attributed to the low acidity of this catalyst. However, its selectivity pattern changed along 48\u00a0h of TOS, even after intermediate catalyst regeneration, due to the fact that hydrogenolysis sites are more prone to deactivation processes.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 financial support from the Spanish Ministry of Science, Innovation and Universities (RTI2018\u201094918\u2010B\u2010C44 project), FEDER (European Union) funds (UMA18-FEDERJA-171 and UMA20-FEDERJA-88), Junta de Andaluc\u00eda (P20-00375), and the University of Malaga. C.P.J.G. and C.G.S. acknowledge Junta de Andaluc\u00eda and FEDER funds, respectively, for their postdoctoral contracts. A.C.A.R. is thankful to the University of Toledo for her start-up package. Funding for open access charge was provided by the University of Malaga/CBUA.Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2022.123827.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Cu nanoparticles were incorporated on different porous silica supports (commercial silica, SBA-15 synthesized both at room temperature and under hydrothermal conditions, and mesocellular foam) by the strong electrostatic adsorption method and tested in the gas-phase furfural hydrogenation for the production of furfuryl alcohol and 2-methylfuran, being the latter a promising biofuel. The incorporated copper species provided metal particle sizes lower than 5\u00a0nm in all cases. However, different catalytic behaviors, both in terms of conversion and selectivity, were detected due to the morphology and textural properties of each support. Cu over commercial silica was more prone to suffer from deactivation, and it provided higher furfuryl alcohol yields, probably due to its higher acidity and lower metallic surface area. On the other hand, the agglomeration of Cu nanoparticles together with larger pore sizes complicated the access of furfural to the active sites using the hydrothermal SBA-15 support, thus decreasing the activity. In contrast, the addition of fluoride in the synthesis of mesoporous silica, which shortened the length of the silica channels (mesocellular foam), facilitated the furfural access and provided both higher metallic surface area and lower acidity. This fact led to a more gradual deactivation, still attaining high values of furfural conversion and 2-methylfuran yield (95 and 76%, respectively) after 5\u00a0h at 190\u00a0\u00b0C. However, Cu over mesocellular foam changed its selectivity pattern along 48\u00a0h and after regeneration, increasing the furfuryl alcohol selectivity due to the decreased number of available active sites.\n               "}
{"full_text": "Data will be made available on request.Photocatalysts, projected as one of the most promising catalytic media of producing H2, are key for the development of sustainable energy sources to achieve the goal of averting imminent climate change while sustaining economic growth. The typically used industrial photocatalysts, such as TiO2, or carbon nitrides are active at ultraviolet wavelengths constituting\u00a0<\u00a05\u00a0% of the solar spectrum. Unfortunately, practical strategies often fall short for binary/ternary photocatalysts due to under-explored various interfacial interactions. Two-dimensional (2D) transition metal dichalcogenides, such as MoS2, have recently attracted attention for environmental-friendly applications [1]. However, bulk MoS2 is known to be less efficient for hydrogen evolution reaction (HER) due to photocatalytically inactive basal plane of MoS2, as only the extremely active sulfur terminated edges display robust nature, and slightly more positive conduction band position relative to the redox potential of H+/H2\n[2]. Nevertheless, the physical properties of MoS2 can substantially be modulated by molecular/nanoparticles (NPs) adsorption [3], applied electric field [4], and metallic contacts [5]. In the case of metal-MoS2 contact, the photoactivity not only depends on layered materials\u2019 structure and properties, but also on the interface between the photocatalyst and cocatalyst [2,6]. The well-defined interface between metal and MoS2 can hamper electron-hole recombination and optimize the light absorption of photocatalysts. Moreover, the physical properties MoS2 can effectively be tailored through these interfaces via carrier injection and strain engineering [7,8]. Herein, taking the HER as an example, a combination of synchrotron and lab source-based spectro-microscopic investigations of various interfacial interactions in Ni-Ag-MoS2 (MAN) ternary heterostructure is carried out.Despite the importance, study of the interfacial contact between the layered semiconductor and hetero-junctional sites is not an easy task. As per the previous report, metallic contact between MoS2 and Ni NP happens via Au [9] / Ag [10] nano-buffers as inferred from X-ray absorption (XA)-photoelectron emission microscopy (PEEM), referred to as X-PEEM, a spectro-microscopic full-field, synchrotron-based surface sensitive technique. The high sensitivity of the XA spectrum (XAS) to oxidization states, local structure, and electronic structure makes X-PEEM highly attractive technique. However, X-PEEM fails to probe the effect of interaction between Ag and MoS2 on MoS2 due to dilute concentration of Ag and technical difficulties to record XAS of Ag at the soft X-ray regions.Raman spectroscopy, on the other hand, is an excellent local and non-destructive optical tool to probe structure and local environment in 2D materials. Furthermore, Raman mapping can monitor changes in Raman spectra at spatially different positions [11,12]. In case of metal-MoS2 contact, Raman spectroscopy has been used to probe the effect of metal on the vibrational properties of MoS2. Different observations, such as i) red shift in both the phonon modes due to mechanical strain induced because of metallic contact [13], ii) splitting of both E2g\n1 and A1g modes due to biaxial strain [14], iii) splitting of E2g\n1 mode only due to uniaxial strain [15], and iv) shift in A1g due to charge doping, whereas shift in E2g\n1 due to stress have been made [7,16]. In hybrid structures, dark localized surface plasmon resonances (LSPR) modes can relax non-radiatively by transferring energy to electrons via Landau damping leading to charge carrier (electron) injection in the lattice of the adjacent 2D semiconductor [17,18]. Till date, mainly Raman spectroscopy has been used to probe these interactions. However, this doping and strain distribution can be highly inhomogeneous, depending on the contact between the two, which mandates simultaneous microscopic and spectroscopic determinations.In this work, we have investigated various interfacial interactions in Ni-Ag-MoS2 (MAN) via X-PEEM and Raman mapping. Due to the interaction at Ag-MoS2 interfaces, Raman spectrum of the heterosystem is dominated by compensated charge doping and compressive strain depending upon the laser power used along with intimate contact between MoS2 and Ag. We believe that this study pushes forward the frontier of binary/ternary photocatalyst design towards efficient water splitting.MAN heterostructure was synthesized by utilizing a sonication based wet chemical synthesis following the method reported earlier [9]. Materials for composites include Ni nanopowder (Ningxia Orient Tantalum Industry, Co. Ltd.), MoS2 (99.5\u00a0% assay, were purchased from Nanjing Emperor Nano Material Co. Ltd.) and AgNO3 (0.01\u00a0mol/L, Sigma-Aldrich). MoS2 powder (43.4\u00a0mg) and Ni powder (4\u00a0mg) were mixed in 100\u00a0mL de-ionized (DI) water and shaken vigorously to get evenly dispersed suspension followed by addition of AgNO3 aqueous solution (1.3\u00a0mL, 0.01\u00a0mol/L) to the flask. The material synthesis was carried out by sonication using a Skymen JTS-1018 water bath ultrasonic cleaner. The cleaner was pre-heated to 70\u00a0\u00b0C, and the materials were then sonicated @35\u00a0kHz using maximum power (\u223c 3 A current) for a duration of 4\u00a0h. Post sonication, the samples were kept at room temperature overnight for the sedimentation of particles. Post sedimentation, water was carefully rinsed without losing synthesized particles, followed by decanting the synthesized particles. Drying the remaining water off from the as-prepared samples was carried out by evaporation of the water in an open beaker by utilizing a hot plate to heat samples to approximately 100\u00a0\u00b0C in the ambient air. Dried samples were then scraped off the beaker and stored in sample bottles as a dry powder. The synthesized product was then dispersed in ethanol (Sigma-Aldrich, 95.0\u00a0%) followed by drop cast on pure Si substrate. The same sample was employed for scanning electron microscopy, X-PEEM, and Raman mapping measurements. In a control experiment, Ag-MoS2 binary system was also prepared to compare the HER of MAN with Ag-MoS2.It is worth noting that to reach fundamental aspects down to illustrations at electronic structural levels, the composite was synthesized and treated with the above fractions and conditions so that the compositions are well distinguished in the following spectro-microscopic studies.Zeiss Ultra plus Field emission scanning electron microscope (FESEM) was used to study the morphology of heterostructures. X-PEEM measurements on selected MAN (based on FESEM) was carried out at the AC-SPLEEM end-station of MAXPEEM Beamline at MAX IV laboratory (Lund, Sweden), using a modified SX-700 monochromator equipped with 1220.9 lines.mm\u22121 (high-density) Au/Si grating. The beamline energy resolution was estimated to be 2\u00a0\u00d7\u00a010-4 with a photon flux of 1\u20135\u00a0\u00d7\u00a01012 ph\u22c5s\u22121 (200\u2013900\u00a0eV range). The photon energy was scanned across Ni L2,3 edge with a 0.2\u00a0eV step. More details regarding the PEEM measurements can be found in a previous study [19].Micro-Raman mapping measurements were performed in backscattering geometry at room temperature using an InVia Raman spectrophotometer from Renishaw equipped with an air cooled charged coupled device detector. During the measurements, the confocal mode and a\u00a0\u00d7\u00a0100 long working distance Leica objective with a numerical aperture (NA) of 0.75 were used. With this experimental setup, a spectral resolution of \u223c1\u00a0cm\u22121 and a spatial resolution of about 0.8\u00a0\u03bcm (approximating to the relation 1.22\u00b7\u03bb/NA, where \u03bb is the wavelength of the incoming laser) could be achieved. The mapping measurements were performed with different values of the laser power varying it from 0.5 to 10\u00a0% of the source power (100 mW). The Raman data was collected with 532\u00a0nm laser excitation. Before the measurement, the spectrophotometer was calibrated to the first-order vibrational mode of a Si wafer centered at 520.3\u00a0cm\u22121.The catalytic activity of the pristine and synthesized heterostructure was carried out in a photoreactor to measure any activity differences. Pristine MoS2, Ag decorated MoS2 and MAN heterostructure were used to run experiments in the same conditions, such as identical cells, same volume, an equal amount of DI-water, a total weight of 5\u00a0mg of each sample, and fixed illumination time of 2\u00a0h. A magnetic stirring bar for stirring the suspension was always present during the experiment. The 5\u00a0mg samples were added into a quartz bottle with a total volume of 68\u00a0mL. For each experiment, 3\u00a0s sonication was carried out to provide a consistent starting point in terms of the dispersion of the suspension. The gaseous content was flushed for a fixed repeatable starting point by using argon. In the first round of preparation, we applied careful Ar-flushing, including heating of the water, to minimize the water-dissolved air. The argon flush's primary function was to establish a reliable control method to monitor gas leaks that arose through effusion and, to a lesser extent, diffusion mechanisms, thus detecting potential abnormally high leak rates. By utilizing the available monochromatic wavelengths each at the time, eight experiments per sample were carried out. The repetitive nature of the experiments also provided initial results of the reusability of the samples.For HER measurements, samples were cycled in Perfect Light PCX50B photo reactor through eight monochromatic LED light sources covering near UV regime to visible light (in a range of 365 to 630\u00a0nm) equipped with magnetic stirring. Sampling and analysis of gaseous species were carried out using Agilent Micro 490 GC gas chromatograph equipped with an H2 sensitive column (10MS5A) after the end of each illumination round.Pristine MoS2, Ag decorated MoS2 and MAN heterostructure were used to run HER experiments and it was found that MAN system is superior to the pristine MoS2 and Ag decorated MoS2. Thus, MAN is further investigated by employing spectro-microscopic techniques. In MAN, the MoS2 sheets and Ni NP are bonded via Ag NPs. The diameter of the Ag NPs is found to be \u223c20\u00a0nm, whereas the diameter of Ni NPs is \u223c100\u00a0nm.The SEM image of MAN is shown in Fig. 1\na. Fig. 1b shows the X-PEEM image while the corresponding XAS spectrum is shown in Fig. 1d. By tuning the synchrotron beam energy from 845 to 875\u00a0eV, the XAS covered the main features of Ni L2,3 edges. The main peaks in the Ni 2p XAS are associated with the Ni 2p3/2,1/2\u00a0\u2192\u00a03d dipole transitions, separated by spin\u2013orbit splitting of 17.3\u00a0eV. The general spectral line shape in the MAN displays similar features to Ni metal foil, suggesting the stability of Ni NPs during the wet chemical synthesis. For the XAS spectra collected from MAN, a small peak turns out at the photon energy of 861\u00a0eV. However, its rather weak intensity leaves questions to deduce its origin, e.g., satellite feature of the Ni0, or Ni atoms bonded with S atoms of MoS2, as observed in nickel chalcogenide nanofilms [20]. Even in the latter case, the rather low intensity compared with the main peak show that the bonded Ni atoms are rather few in comparison to that of the Ni-Au-MoS2 system [9]. Nevertheless, a rather low charge transfer between the Ni NPs and the flat MoS2 basal plane beneath and the protruding layer is possible. The metal particle is thus connected to the semiconductor through basal and side contacts. Although X-PEEM uncovers the interaction between MoS2 and Ni, the interaction between Ag and MoS2 and its effect on MoS2 cannot be explored using the same. For this reason, we employed lab source-based Raman mapping.\nFig. 1c shows the Raman image of MAN at the same location (as that for SEM and X-PEEM) and corresponding Raman spectra are plotted at various spatial position within MAN (Fig. 1e). Raman spectra of MAN exhibits mainly-two phonon modes, E2g\n1 and A1g. Along with these two modes, E1g mode at \u223c286\u00a0cm\u22121 and higher intensity of out-of-plane A1g are noted. According to the Raman selection rules, the E1g mode is forbidden in backscattering experiment on the basal plane of bulk MoS2\n[21]. However, when the incident light scatters on the surface of edge terminated MoS2, the corresponding scattering Raman tensor undergoes a rotation transformation, leading to a nonzero differential scattering cross-section and hence the E1g mode can be observed indicating film formation of MoS2. Moreover, the peak intensity of the out-of-plane A1g mode is like that of the in-plane E2g\n1 mode in the bulk MoS2 and 3 times that of the E2g\n1 mode in the MoS2 film. Such preferred excitation of an out-of-plane mode is also consistent with the vertical-aligned crystal texture of the film sample considering the polarization dependence of the Raman scattering cross-section. When measured at spatially different positions, Raman measurements of MAN show variation in the Raman spectra, such as red shift in the phonon frequency of E2g\n1 and A1g modes compared to pristine MoS2 (Fig. 1c & e). The observed variation of optical phonons is further investigated using power dependent Raman mapping to probe the effect of Ag NPs on MoS2.Many sets of power dependent (0.5, 1, 5, and 10\u00a0%) Raman mapping are collected on various sites to make appropriate observations. At a low laser power (0.5\u00a0%), no variation could be seen as E2g\n1 and A1g modes are observed consistently at \u223c384 and 409\u00a0cm\u22121, which is similar to pristine MoS2. However, when the same region is excited with higher laser power (1, 5 and 10\u00a0%), variation in phonon frequency is noted. Representative Raman images for different regions are shown in Figs. 2 & 3\n\n and Fig. S1. All the peak positions are obtained by fitting Raman spectra with Lorentzian line shape (Fig. S2 and Table S1/S2).Increase in the laser power from 0.5 to 1\u00a0% led to either no change or slight red shift in both E2g\n1 and A1g modes (Fig. 4\n). Enhanced red shift in both E2g\n1 and A1g modes is further observed with the increase in laser power to 5\u00a0%. However, further increment in laser power led to both red and blue shift in E2g\n1 and A1g modes at different locations (Fig. 4 & Fig. S1f). Comparison of different Raman mapping data collected at four different laser power shows that increase in the laser power led to red shift as well as blue shift in E2g\n1 and A1g modes at spatially different locations. Moreover, one can see that both E2g\n1 and A1g modes follows the same pattern with the change in the laser power.As a further analysis, Fig. S3 shows the average phonon frequency for both the A1g and the E1\n2g mode, including all the 13 points investigated in Fig. 3. Fig. S3 reveals that, on average, the laser power leads to red shift in the frequencies (note that the vibrational mode of Si maintains a constant value and is hence used as our internal reference). The slightly larger errors visible for higher power of 5 and 10\u00a0% are a direct consequence of the more spread values found while investigating the different spots. The observed spatial variation could be ascribed to i) laser heating, ii) mechanical strain induced due to presence of Ag NPs, and iii) interplay between charge injection and stress.Vasa et al. [16] have reported Raman spectra of MoS2 as a function of excitation power in the range from 0.69 and 21.84 mW. No appreciable peak shift as a function of laser power was noted for either of the two modes. This suggests that MoS2 is quite stable with respect to laser power and observed changes here cannot possibly arise due to increase laser power. Moreover, laser heating give rise to uniform red shift in the phonon frequency and increment in red shift due to increased laser power. However, here we have observed both red and blue shift in E2g\n1 and A1g modes at different laser powers. Moreover, different phonon frequencies are observed at spatially different location. Thus, the effect of laser heating can be negated.Observed changes in the Raman spectra could be due to local mechanical strain due to metal-MoS2 contact [14,15,22]. However, this does not seem plausible here as this strain should be laser power independent, whereas we have not observed any shift in phonon frequency at very low laser power. Moreover, as mentioned in the literature, biaxial strain leads to splits in both the E2g\n1 and A1g modes, whereas uniaxial strain leads to splitting of mainly E2g\n1 mode. However, no splitting is noted here (Fig. 2 and Fig. 3). Moreover, the effects of mechanical strain in thicker MoS2 layers are significantly weaker and dominantly found in monolayer or bilayer. Thus, this possibility can be negated as well.The interplay between charge transfer and compressive stress [13,16] seems to be the most probable cause of the observed power dependent Raman shift in MAN. As per literature, A1g mode corresponding to the structural distortions arising due to the out-of-plane motion of sulphur atoms on either side of Mo atoms preserves the lattice symmetry, whereas E2g\n1 mode corresponding to the collective in-plane motion of two layers of the sulphur atoms in the opposite direction of Mo atoms, does not preserve the lattice symmetry, hence only A1g is sensitive to doping [16]. Whereas, in another work, it has been shown that charge-transfer via external perturbation leads to change in both the E2g\n1 and A1g modes [13]. Herein, Fig. 4 shows that both E2g\n1 and A1g mode follow the same trend indicating that both modes are sensitive to charge transfer and or stress. Since n-doping (p-doping) softens (stiffens) the modes, the observed red shift in the Raman spectra from 0.5 to 5\u00a0% suggests n-doping in the present case. In the current case, the electron transfer to the conduction band of MoS2 can occur via LSPR of Ag NPs excited by the laser excitation. Since ELSPR\u00a0>\u00a0EMoS2\u00a0\u2212\u00a0EAg, the LSPRs can relax non-radiatively via Landau damping transferring the energy to free electrons in Ag. These electrons subsequently get injected into the adjacent MoS2 leading to red shift in phonon modes. Moreover, laser annealing can cause compression in the MoS2 leading to blue shift in the phonons at higher laser power [13].Thus, it is inferred that power dependent laser excitation of Ag NPs in hybrid structure results in optically tunable electron transfer and compressive deformation of MoS2 by electron injection via non-radiative relaxation of LSPRs excited in Ag NPs. The observed shifts of two modes can be related to the extent of electron transfer concentration and strain and by the relations given as follows [13],\n\n(1)\n\n\n\n\u03c9\ni\n\n=\n\nk\nn\n\n\n\n\ni\n\n\n\nn\n\n\n\n\n\n\n(2)\n\n\n\u0394\n\u03c9\n=\n-\n2\n\n\u03b3\ni\n\n\n\u03c9\n\ni\n\n0\n\n\n\n\u03b3\n\u03b5\n\ni\n\n\n\n\n\nEquations (1) and (2) enable the calculation of Raman peak frequencies for constant carrier concentration and strain. Here, \n\n\u03b5\n\n is the lattice strain, n is the charge carrier concentration, \n\n\n\u03b3\ni\n\n\n is the Gr\u00fcneisen parameter corresponding to phonon frequency \n\n\n\u03c9\ni\n\n\n, \n\n\n\u03c9\n\ni\n\n0\n\n\n is the phonon frequency of unstrained and undoped MoS2, and \n\n\nk\nn\n\n\n is \u22120.33\u00a0cm\u22121 per 1013 cm\u22122 e- and \u22122.22\u00a0cm\u22121 per 1013 cm\u22122 e- for E2g\n1 and A1g mode [13], respectively. Average room temperature \n\n\u03b3\n\n value is 0.86 and 0.15 for E2g\n1 and A1g mode, respectively. Since the observed phonon frequencies at 0.5\u00a0% laser excitation is same as that of pristine MoS2, we have chosen E2g\n1\u00a0\u223c\u00a0384\u00a0cm\u22121 (\n\n\n\u03c9\n\n1\n\n0\n\n\n) and A1g\u00a0\u223c\u00a0409\u00a0cm\u22121 (\n\n\n\u03c9\n\n2\n\n0\n\n\n) as the undoped and unstrained value. The inferred laser power dependence of electron transfer concentration and the associated strain averaged over the laser spot size is noted in Table 1\n & Table 2\n. The electron density as well as strain are different at different spatial locations, even though Raman mapping was collected at same laser power.Observed difference in Raman spectrum at different spatial locations can be ascribed to various possible reasons, such as i) non-uniform growth of Ag NPs, ii) larger size of laser spot (0.8\u00a0\u03bcm) compared to Ag NPs (10\u201320\u00a0nm) and iii) intercalation of Ag in MoS2 layers. As per electron microscopy imaging, the deposition of Ag is not continuous, rather they form NPs on the surface of MoS2, which can be interpreted as Volmer-Weber island growth mode [23]. Since the growth of Ag is inhomogeneous, and this inhomogeneity can give rise to two regions, one is the intimate contact between Ag clusters and MoS2 surface while the other region is spaced from MoS2 with a notable separation form Ag. Intimate contact between the two will give rise to larger effect of Ag NPs, whereas the effect decreases with increase in the distance between the two. Moreover, observed no changes in the Raman spectra at certain locations (Fig. 4) can be explained with reported density functional theory calculations, which predict that an interface separation of larger than 6\u00a0\u00c5 between metal and MoS2 is enough to decouple MoS2 from the electronic perturbation of atop metal layers [22]. This further shows that effect of Ni on the Raman spectra of MoS2 can be neglected as Ni is in contact with MoS2\nvia Ag NPs, thus, spaced far enough to have any effect. Since the size of laser spot is much bigger than the size of Ag NPs, laser exposure covering an area might include different number of Ag NPs, when collecting Raman data at different locations, which can give rise to varied doping concentration and strain value. The third possibility can be the intercalation of Ag between MoS2 layers during the growth of Ag from AgNO3. It is known that intercalation of Ag between layered structure MoS2 is possible [24]. Moreover, intercalation can generate stress in the layered structures along with the possibility of charge transfer [25,26]. In the present case, as mentioned above, AgNO3 is added to MoS2 solution and sonicated for 4\u00a0h. Since the growth of Ag on MoS2 follows the Volmer-Weber island growth mode [23], wherein nucleation followed by growth of clusters leads to isolated metal islands on MoS2. The islands keep growing until a continuous and polycrystalline film forms. However, for highly mobile materials, such as Ag, the metal islands change dynamically even near room temperature, where the large islands grow at the expense of the shrinking of small islands. During this process, it can happen that smaller Ag clusters can intercalate within the MoS2 layers. Growth of these clusters will depend on the separation between two adjacent layers, which is around 6\u00a0\u00c5 in the case of MoS2. Thus, this intercalation can lead to varied charge doping and stress as the laser used for excitation has a depth of focus \u223c0.66\u00a0\u03bcm, thus covering many layers of MoS2. However, the intercalation of Ag within MoS2 layers and its effect on electronic and vibrational properties of MoS2 needs further investigation, which will be explored separately.In order to establish the effect of laser-induced electron transfer and strain, we carried out further power dependent measurements in reverse. In this process, Raman mapping was collected at lowest laser power of 0.5\u00a0% after carrying out Raman mapping at 10\u00a0% laser power (Fig. 5\n & S4). The observed reversible effect further establishes the above interpretation of electron transfer and compressive strain in MAN.\nFig. 6\na shows the measured water splitting experiment results for all the samples (bare MoS2 and Ni-Ag-MoS2) under white light illumination. Bare MoS2 show negligible H2 yield, whereas MAN shows significant H2 production. Based on the white light water splitting experiment, we carried out detailed experiment in the visible to UV region for MAN. Fig. 6b shows the H2 production rate of MAN in the light range of 365\u00a0nm to 630\u00a0nm. As can be seen from Fig. 6b, at the excitation wavelength of 485 and 535\u00a0nm, the observed HER performance for MAN is \u223c55\u00a0\u03bcmol\u00a0g\u22121h\u22121. The observed lesser HER at 595\u00a0nm could be due to experimental error.A cyclic test on the MAN system was carried out for 3 cycles with each cycle of 2\u00a0h under white light irradiation. An Agilent 8860 GC was employed to quantify the evolved H2. The catalyst was stably functional in HER as shown in Fig. S5 of the Supplementary material. The SEM was performed on MAN sample after the cycling tests. Depicted in Fig. S6a, the Ni NPs were firmly attached on to the MoS2 through side and basal contacts. The metals of Ni and Ag coexist with these of S and Mo from the semiconductor matrix according to the element mapping of Fig. S6b. A high-resolution transmission electron microscopic evaluation (HRTEM) was further performed on the sample after the cyclic tests. Both the Ni NPs and flakes are well retained in the composite in Fig. S7(a). The selected area electron diffraction (SAED) pattern further proved that the main crystal structures of MoS2 and Ni were kept in Fig. S7(b) and (c). From the above microscopic and microstructural studies, samples are stable after 3 cycles of photocatalysis. The control experiment by using the Ag-MoS2 was also performed and\u00a0<\u00a02\u00a0\u00b5mol/g/h HER yield was found when using the white light as the incident source. Thus, the MAN system is superior to the bare MoS2 and binary Ag-MoS2 systems within the photocatalytic HER abilities. It\u2019s worth noting HER is mainly investigated here and the full water splitting requires dedicated instrumental setups and calibrations along with spectro-microscopic investigations of OER reactive sites to deduce mechanisms. However, a recent work of the similar system [10] showed the oxygen evolution is accompanied with the HER, denoting a full water splitting capability of the MAN system. Beside the same composites, the fraction of Ni used here falls into the range of the MAN system studied previously. Thus, full water splitting capability is also expected on the present sample.The possibility to not only probe locally, but visualize a chemical structure, composition, conformational state, and effect of various components on heterostructures and its catalytic activity has stimulated the development of imaging techniques. Both the spectro-microscopic techniques used here are fingerprint, rich, non-destructive and give information of different interactions (Fig. 7\n). MoS2 being a cheap and abundant mineral has promising applications in transistors, optoelectronics, and UV\u2013vis light convertors. Furthermore, it has photocatalytic abilities in degrading organic pollutants. Despite these achievements, the MoS2 is a poor photocatalysts. However, the chemical robustness could be enhanced by heterostructure engineering. Herein, Ni-Ag-MoS2 contact is established using facile wet chemical synthesis method. Lab-based Raman mapping and synchrotron-based X-PEEM verified the successful bonding of Ni to the layered MoS2 at the nanoscale interface regions via the Ag buffer (Fig. 7). Moreover, power dependent Raman mapping showed the same pattern of downshift or upshift in the phonon frequency of E2g\n1 and A1g modes with the increase in laser power due to various possibilities, such as non-uniform growth of Ag NPs, the contact between MoS2 and Ag, larger laser spot size covering different concentration of Ag NPs, and intercalation of Ag in layered MoS2 (Fig. 7).A side view of ternary structure scheme is illustrated in Fig. 8\na denoting metal/semiconductor contact via the MoS2 basal and side modes. In the first mode, no biaxial strain from the dichalcogenide is needed when joining the Ni NPs onto the MoS2 basal with the Ag as the buffer. This is supported by the absence of splits in both the E2g\n1 and A1g of the Raman spectra in Figs. 2 and 3. In fact, the lattice of Ag (111) buffer well matches the one of the MoS2, providing a metallic contact between the metal and semiconductor [10]. In the second case, the silver is involved in bonding the Ni NPs and the atoms at the MoS2 edge. As a result, the M/S contact was also metallic [9] when the noble metal buffers the Ni and the MoS2\u2032s defective side [27,28]. A band alignment scheme is depicted in Fig. 8b following the spectro-microscopic determinations. The figure adopts the work functions of 5.35\u00a0eV, 4.74\u00a0eV, and 5.20\u00a0eV for Ni, Ag and MoS2, respectively, [10,29] and the metal\u2013semiconductor (M/S) contact model of metal-induced gap state [30]. Therein, the interface dipole was formed by the charge transfer across the bonds at the M/S interfaces. The reference level is set to the charge neutrality level (CNL), similar to Fermi level in semiconductor itself and referring to the highest occupied surface state for the common surface [31]. Here the CNL is very close to the Ag Fermi level due to Ag\u2019s buffer status and a lower work function of the Ag than the MoS2. As per the band alignment scheme when Ni NPs and MoS2 are joined by Ag NPs, free electrons will transfer from Ni to MoS2 via Ag due to the work function difference. This leads to the accumulation of electrons next to the valence band of MoS2 adjacent to the interface region and the decrease of contact resistance which is contributing to the observed higher H2 yield by MAN [32]. The Fermi levels of the Ni, MoS2, and the interface region will be aligned after thermodynamic equilibrium [33], resulting in the band bending of MoS2. During the photocatalysis, electron-hole (e--h+) pairs are first created on the semiconductor MoS2 matrix. Following the aligned bands, the photoexcited electrons from the MoS2 can thus move easily from the valence band of MoS2 to the metal side, as shown in the Fig. 8(b). As a result, the e--h+ recombination is substantially inhibited [34], and a longer time window is left for the water redox than in the pure semiconductor case. Holes in the MoS2 oxide led to formations of the OH\u00b7 and H+ radicals. The protons get reduced by the electrons to form H\u00b7, and later H2. Remaining OH\u00b7 possibly combines to form the H2O2 or partially stack on the metal sides. In the former case, the product gets easily dissolved and emits oxygen under vigorous stirring and light irradiations. The latter one results the increase of the nickel hydroxides along with the native oxides [10] which indeed benefits the oxygen evolution reactions [35].\u201dSpectro-microscopic X-PEEM and Raman mappings have efficiently been used to probe Ni-Ag-MoS2 heterostructure. The interaction of Ni with MoS2 is evidenced through synchrotron X-PEEM, whereas the interaction mechanism at the Ag-MoS2 interface is probed via Raman mapping. The large variation in E2g\n1 and A1g phonon modes in Ni-Ag-MoS2 with the increase in laser power and pristine like behavior during reverse power dependent measurement is observed. These variations are assigned to non-uniform growth of Ag NPs and their intimate contact with MoS2, larger laser spot size covering different concentration of Ag NPs, and Ag intercalated between layered MoS2. These observations further reveal compensation between downshift in E2g\n1 and A1g modes due to charge carrier injection and upshift in E2g\n1 and A1g modes due to laser annealing. The improved photocatalytic activity of Ni-Ag-MoS2 heterostructure (H2 yield\u00a0\u223c\u00a055\u00a0\u03bcmol\u00a0h\u22121 g\u22121) compared to pristine MoS2 (H2 yield\u00a0<\u00a05\u00a0\u03bcmol\u00a0h\u22121 g\u22121) is attributed to successful bonding of Ni, Ag and MoS2. Thus, an interesting possibility of achieving a tunability between electron injection and strain is attained by employing laser induced charge doping and stress. This LSPR-induced electron injection offers unique possibility of spatially localized dynamical electron doping and provides an active manipulation and tuning of 2D semiconductors.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 acknowledge Academy of Finland grant #311934, and The University of Oulu and The Academy of Finland Profi5 - project #326291. W. C. acknowledges funding provided by European Research Council (ERC) under the European Union\u2019s Horizon 2020 research and innovation programme (Grant Agreement 101002219). Authors gratefully acknowledge the Center of Materials Analysis (CMA), University of Oulu for characterizations and Dr. J. Fern\u00e1ndez-Catal\u00e1 and Dr. R. Greco for helping with GC operations. Funding from the Knut and Alice Wallenberg Foundation (Wallenberg Academy Fellowship award, 2016-0220) is kindly acknowledged. We acknowledge MAX IV Laboratory for time on Beamline (MAXPEEM) under Proposal (20200401). Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research council under contract 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2022.09.006.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Despite the boom in catalytic response via constructing interfaces, understanding interfaces\u2019 interaction in heterostructures is still a paradox. In this work, the interaction of Ni with MoS2 in Ni-Ag-MoS2 heterostructure are unveiled through synchrotron X-PEEM and what\u2019s more, the missing interaction mechanism at the Ag-MoS2 interface is probed via Raman mapping. The observed competition between the downshift of the E2g\n                     1 and A1g modes due to charge carrier injection and the upshift of the E2g\n                     1 and A1g modes due to compressive strain during reverse laser power experiment is assigned to the non-uniform growth of Ag nanoparticles, their intimate contact with MoS2, and Ag intercalated layered MoS2. The substantial improvement of the H2 yield of the Ni-Ag-MoS2 (\u223c55\u00a0\u03bcmol\u00a0h\u22121 g\u22121) over the pristine MoS2 and the binary Ag-MoS2 evidence successful bonding of Ni, Ag and MoS2. This study highlights the importance of considering both electronic coupling and strain to optically tune electromechanical properties of MoS2.\n               "}
{"full_text": "Heterogeneous catalysis plays a significant role in synthesizing organic compounds for agrochemicals, pharmaceuticals, and fine chemicals for a sustainable future. These catalysts have contributed to developing technology to produce green chemicals from biomass-derived platform chemicals under environmentally benign processes and avoid using toxic or hazardous materials [1]. However, the design and development of novel eco-friendly solid catalysts require appropriate techniques and strategies for improved recoverability, recyclability, and eco-friendliness [2]. In addition, there is an urgent need to acquire new organic compounds which might find possible applications in diverse areas of study. Hence more environmentally and eco-friendly heterogeneous catalysts are being sought to expedite the synthesis of nitrogen-based heterocycles because of their enhanced biological activities [3].Porous hexagonal boron nitride (h-BN) is a material that demonstrates unique physical and chemical properties, including low density, high specific surface area, high thermal conductivity, oxidation resistance, and chemical durability [4, 5]. These features make h-BN a promising catalyst for their application in various research areas, especially those related to adsorption, like gaseous uptake and pollutant adsorption. Furthermore, h-BN is excellent catalyst support [6] since they possess a hexagonally shaped crystal structure composed of continuous boron-nitrogen bonds, wherein lone pair of electrons on the nitrogen atoms can coordinate with certain metals. The hBN/metal substrates functions as supports for metal clusters with Cr(110), Molybdenum(110), Rhenium(0001), Iron(110), Iridium(111), Copper(111), Gold(111), Silver(111), Nickel(111), and Platinum(111), metal-organic complexes, and organic molecules [7]. Water-soluble and porous boron nitride is usually biocompatible and can effectively employ as a nanocarrier for loading anti-cancer drug doxorubicin [8]. Moreover, several studies supported the use of interactions of h-BN through calcium coordination bonding towards reduced graphene oxide yielded nanocomposites to enhance the mechanical, electrical and thermal properties substantially [9, 10, 11]. Therefore, we aimed to investigate a metal and concoct a novel h-BN metal-based material that could be used as a catalyst to synthesize new nitrogen heterocycles. Hence, we investigated the suitability of a novel zinc boron nitride (Zn-BNT) material to synthesize new benzimidazoles through a simple condensation reaction.Although benzimidazoles are N-based heterocycles are important scaffold for medicinal or pharmaceutical compounds [12, 13], they are\u00a0also display a wide variety of biological activities such as anti-microbial [14], anti-viral [15], anti-cancer [16], anti-protozoal [17], anti-inflammatory [18], and analgesics [19]. They also exhibit significant antiviral activity against different viruses, including HIV (AIDS), HSV-1, influenza, and HCMV [20, 21, 22, 23]. Some selected medicinal compounds containing benzimidazole moiety are presented in Figure\u00a01\n.Benzimidazole scaffold is widely used in the organic synthesis of drugs and drug intermediates [24]. Principally, two protocols are adopted for synthesizing 2-substituted benzimidazoles and their analogs. The first protocol involves a coupling of 1,2-phenylene-diamines with carboxylic acids, whereas the second protocol involves condensation of 1,2-phenylenediamine with aldehydes followed by oxidative cyclodehydrogenation [25, 26]. Benzimidazole moiety can be constructed through reactions involving the use of several types of catalysts such as ammonium chloride [27], alumina [28], sodium metabisulphite [29], lanthanum chloride [30], indium triflate [31], sodium hexafluroaluminate [32], nickel acetate [33], iodine [34], and sodium dodecyl sulfate [35]. At the same time, some biologically active quinoline molecules are synthesized using boron modified sulphonic acid catalyst [36, 37, 38]. However, the methods have several drawbacks, such as the use of hazardous organic solvents, strongly acidic conditions, high moisture-sensitivity or tedious workup conditions, low yields, and difficulty isolating the products from the reaction mixture, and strong oxidizing nature of the reagents employed. Hence our research thrust was to prepare a new catalyst and evaluate its potential to synthesize benzimidazole derivatives. Furthermore, the theoretical calculations are studied for FT-IR and NMR spectra, HOMO-LUMO gap, Mulliken charge analysis, molecular electrostatic potential map, and Fukui function analysis.All the reagents were purchased from commercial sources and used as received. The reactions' progress and the compounds' purity were monitored by thin-layer chromatography (TLC) on pre-coated silica gel plates procured from E. Merck and Co. (Darmstadt, Germany). TLC spots were visualized by UV light and using an iodine-vapor chamber. The melting points of the synthesized compounds were determined using a Stuart SMP 10 melting point apparatus and are uncorrected. The IR spectra were recorded on Varian Scimitar 1000 FT-IR using KBr pellets, and the absorption frequencies are expressed in reciprocal centimeters (cm\u22121). In addition, 1H and 13C NMR were recorded on either a Bruker 400 or 600 MHz spectrometer: DMSO-d\n\n6\n was the solvent while TMS was an internal reference. The chemical shift values are recorded on \u03b4 scale, and the coupling constants (J) are in hertz. The elemental analyses (C, H, N) were obtained from a PerkinElmer precisely 2400 analyzer. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were conducted using TA Instruments. A Carl Zeiss Ultra Plus scanning electron microscope with EDX detector was used. The X-ray diffraction analysis was conducted with a Philips PW 1050 diffractometer set at 1\u00b0/min with a scanning step size of 0.02\u00b0 from 40\u00b0 to 100\u00b0 2\u0472 using monochromated Cuk\u03b1 radiation. Data were captured with a sietonics 122D automated microprocessor linked to the diffractometer [39].To a solution of Zn(OAc)2 (24.9 mg), in acetonitrile (50 ml), was added boron nitride (2.50 mg, 0.1 mmol), and the suspension was stirred at room temperature for seven days under an inert atmosphere. The resulting suspension was filtered, and the solid was washed with aqueous methanol and dried under reduced pressure to give Zn-BNT as a white powder in 95% yield.Equimolar quantities (1 mmol) of o-phenylenediamine (1) and aromatic aldehydes (2a\u2013h) in acetonitrile (6 ml) were transferred to a 50 ml round bottom flask, and Zn-BNT (10 mol%) was added. The mixture was heated under reflux for12 h, and TLC was used to monitor the progress of the reaction mixture. Upon completion of the reaction, the mixture was filtered to recover the catalyst. The solid was washed with chloroform followed by methanol and dried at 120 \u00b0C while the filtrate was purified by silica-gel column chromatography using ethyl acetate and petroleum ether 10:90 (v, v 10%) as an eluent to yield the 2-arylbenzimidazoles 3a\u2013h, quantitatively.The spectra data is presented below:White solid; Yield: 97%; mp: 132\u2013134 \u00b0C; FT-IR (ATR, \u03bd\n\nmax\n, cm\u22121): 3321 cm\u22121 (N\u2013H stretching), 3177 cm\u22121 (=CH), 1625 (C=N), 1551, 1467, 1422 (Ar\u2013C=C); 1H-NMR (400 MHz, DMSO-d\n\n6\n): \u03b4 = 12.91 (1H, s, N\u2013H), 8.20\u20138.22 (2H, m, Ar\u2013H), 7.45\u20137.62 (5H, m, Ar\u2013H), 7.20\u20137.26 (2H, m, Ar\u2013H) ppm. 13C-NMR (100 MHz, DMSO-d\n\n6\n): \u03b4 = 151.18, 130.07, 129.82, 128.92, 128.78, 126.38, 122.09 ppm. Anal. Calc. for C13H10N2: C, 80.39; H, 5.19; N, 14.42%. Found: C, 80.40; H, 5.21; N, 14.44%.Brown solid; Yield: 95%; mp: 296\u2013298 \u00b0C; 1H-NMR (400 MHz, DMSO-d\n\n6\n): \u03b4 = 12.98 (1H, s, N\u2013H), 7.93 (1H, m, Ar\u2013H), 7.60\u20137.62 (1H, d, J = 8 Hz, 1H, Ar\u2013H), 7.47\u20137.49 (1H, d, J = 8 Hz, Ar\u2013H),7.22\u20137.17 (3H, m, Ar\u2013H), 6.71 (1H, m, Ar\u2013H), ppm. Anal. Calc. for C11H8N2O: C, 71.73; H, 4.38; N, 15.2%. Found: C, 71.75; H, 4.40; N, 15.23%.Yellow solid; Yield: 90%; mp: 294\u2013296 \u00b0C; 1H-NMR (400 MHz, DMSO-d\n\n6\n): \u03b4 = 12.82 (1H, s, N\u2013H), 7.96\u20138.03 (2H, m, Ar\u2013H), 7.86\u20137.88 (1H, m, Ar\u2013H), 7.73\u20137.77 (1H, m, Ar\u2013H), 7.59\u20137.62 (2H, m, Ar\u2013H), 7.23\u20137.25 (2H, m, Ar\u2013H), ppm. Anal. Calc. for C13H9N3O2: C, 65.27; H, 3.79; N, 17.56%. Found: C, 65.29; H, 3.81; N, 17.58%.yellow solid; Yield: 88%; mp: 306\u2013308 \u00b0C; FT-IR (ATR, \u03bd\n\nmax\n, cm\u22121): 3321 cm\u22121 (N\u2013H stretching), 3177 cm\u22121 (=CH), 1625 (C=N), 1551, 1467, 1422 (Ar\u2013C=C); 1H NMR (400 MHz, DMSO-d\n\n6\n): \u03b4 13.60 (1H, s, N\u2013H), 8.39 (1H, d, Ar\u2013H), 8.15 (1H, d, 1H, Ar\u2013H), 8.00 (1H, d, 1H, Ar\u2013H), 7.64\u20137.66 (1H, dd, Ar\u2013H), 7.25\u20137.27 (1H, dd, Ar\u2013H) ppm. Anal. Calc. for C13H9N3O2: C, 65.27; H, 3.79; N, 17.56%. Found: C, 65.29; H, 3.81; N, 17.57%.Brown solid; Yield: 89%; mp: 280\u2013282 \u00b0C; 1H-NMR (400 MHz, DMSO-d\n\n6\n): \u03b4 = 12.76 (1H, s, N\u2013H), 8.17 (2H, m, Ar\u2013H), 7.58\u20137.63 (4H, m, Ar\u2013H), 7.20\u20137.22 (2H, m, Ar\u2013H), ppm. Anal. Calc. for C13H9ClN2: C, 68.28; H, 3.97; N, 12.25%. Found: C, 68.30; H, 3.98; N, 12.27%.White solid; Yield: 93%; mp: 186\u2013188 \u00b0C; 1H-NMR (400 MHz, DMSO-d\n\n6\n): \u03b4 = 12.91 (1H, s, N\u2013H), 8.20\u20138.24 (1H, q, Ar\u2013H), 7.74\u20137.78 (1H, q, Ar\u2013H), 7.72 (1H, t, Ar\u2013H), 7.49 (1H, t, Ar\u2013H), 7.34\u20137.42 (1H, m, Ar\u2013H), 7.25 (1H, t, Ar\u2013H), 7.10 (1H, t, Ar\u2013H), 7.00\u20137.03 (1H, q, Ar\u2013H) ppm. Anal. Calc. for C13H9FN2: C, 73.57; H, 4.27; N, 13.20%. Found: C, 73.59; H, 4.29; N, 13.21%.Greyish solid; Yield: 82%; mp: 178\u2013180 \u00b0C; 1H-NMR (400 MHz, DMSO-d\n\n6\n): \u03b4 = 12.82 (1H, s, N\u2013H), 8.04 (2H, m, Ar\u2013H), 7.54\u20137.56 (2H, m, Ar\u2013H), 7.34\u20137.36 (2H, m, Ar\u2013H), 7.16\u20137.20 (2H, m, Ar\u2013H), 2.34 (3H, s, CH3), ppm. Anal. Calc. for C14H12N2: C, 80.74; H, 5.81; N, 13.45%. Found: C, 80.76; H, 5.83; N, 13.47%.White solid; Yield: 94%; mp: 222\u2013224 \u00b0C; 1H-NMR (400 MHz, DMSO-d\n\n6\n): \u03b4 = 12.72 (1H, s, N\u2013H), 9.61 (1H, s, Ar-OH), 8.03\u20138.06 (1H, m, Ar\u2013H), 7.70\u20137.72 (2H, m, Ar\u2013H), 7.27\u20137.36 (2H, m, Ar\u2013H), 6.88 (2H, m, Ar\u2013H), 6.60 (1H, m, Ar\u2013H), ppm. Anal. Calc. for C13H10N2O: C, 74.27; H, 4.79; N, 13.33%. Found: C, 74.29; H, 4.81; N, 13.35%.The new catalyst Zn-BNT (Figure\u00a02\n) was synthesized through a simple reaction between boron nitride, zinc acetate in acetonitrile under an inert atmosphere by stirring at room temperature for seven days. The novel catalyst was characterized using FT-IR, XRD, SEM, SEM-EDX, SEM-mapping, BET, and DSC-TGA.The FT-IR spectrum of Zn-BNT is presented in Figure\u00a0S1 (Supporting Information). The FT-IR revealed the presence of functional bond stretching frequencies N\u2013B\u2013N at 1738 cm\u22121, B\u2013N at 1365 cm\u22121, and B\u2013N\u2013B at 920 cm\u22121, with other additional bands for Zn\u2013N at 1365 cm\u22121 and 1217 cm\u22121 assigned as asymmetric and symmetric stretching frequencies, respectively.The powder XRD diffraction analysis [5] showed the crystal-like Zn-BNT. The characteristic Bragg's XRD peaks at 2\u03b8 boron nitride peaks are observed at 26.67\u00b0, 41.62\u00b0, 55.02\u00b0 and 75.91\u00b0 are indexed to the (002), (100), (004), (220) while the Zn peaks 43.81\u00b0 and 50.15\u00b0 were indexed to (111), (012), respectively (Figure\u00a03a and 3b).Zn-BNT was analyzed by scanning electron microscopy (SEM) for morphological characteristics. The particles appeared to be randomly distributed, spherical and elongated needle-shaped for 1% Zn-BNT (Figure\u00a04\na). An agglomeration of particles was observed, which probably led to the formation of the larger particles for 2% Zn-BNT (Figure\u00a04b). Consistent morphology and clusters of metal particles on the support surface of 4% Zn-BNT was observed in Figure\u00a04c. The size of the rod-like particles of Zn was observed as 50\u2013150 nm (length) and 16 nm (width) on the BN surface for 4% Zn-BNT (Figure\u00a04d). The particles observed in 1% Zn-BNT were small and have a high surface area with a smaller number of active sites compared to 2% Zn-BNT. Similarly, the particles appeared larger and therefore had a smaller surface area in 4% Zn-BNT compared with 2% Zn-BNT.The particles were observed as small and had a high surface area with a smaller number of active sites. Furthermore, the SEM mapping indicated the elements of Zn-BNT (Figure\u00a05\na, b, c, and d) Zn-BNT, boron, nitrogen, and zinc. The morphology of Zn-BNT material was further confirmed by SEM-EDX analysis.The SEM-EDX spectrum of Zn-BNT (Figure\u00a06\n) displayed elements B, N, O, and Zn of weight (%) 20.11, 9.13, 70.72, and 0.04, respectively. Au peaks were due to the sample being coated with Au in the sampling process.Brunauer-Emmett-Teller (BET) specific surface was determined from the nitrogen adsorption data at the relative pressure using a multipoint method. Figure\u00a07\n(a) and (b) shows the isotherms of a 2 and 4% Zn-BNT in N2 adsorption-desorption. The sample exhibited type-III isotherms. The type-III isotherm is closely related to the mesoporous structure of the catalyst and therefore no identifiable monolayer formation; the adsorbent-adsorbate interactions are now relatively weak and the adsorbed molecules are clustered around the most favorable sites on the surface of the solid. 2% Zn-BNT material showed a surface area of 25.42 m2 g\u22121 with a pore volume of 0.119 cm3 g-1 and 4% Zn-BNT material showed a surface area of 23.58 m2 g\u22121 with a pore volume of 0.114 cm3 g-1 and a pore size 188.74 A\u00ba thereby indicating a mesoporous structure and effective as a catalyst for organic reactions.Zn-BNT was analysed for its thermal properties from room temperature to 800 \u00b0C. Figure\u00a08\n presents the DSC-TGA profile of the Zn-BNT material. The DSC curve of the sample indicated a broad exothermic peak. The weight loss occurred in stages; about 1.2% loss up to 200 \u00b0C due to moisture and other volatile materials, whereas a loss of about 1.8% occurred until 800 \u00b0C. These results demonstrated that Zn-BNT has good thermal stability; hence, it can be used effectively as suitable for the catalyst for several organic reactions.Briefly, equimolar quantities of o-phenylenediamine (1) and differently substituted aromatic aldehydes (2a\u2013h) were added in acetonitrile. Zn-BNT was added, and the mixture was subjected to microwave irradiation for 15 min. The crude product was purified by silica-gel column chromatography using ethyl acetate and petroleum ether (10:90) to yield the pure 2-arylbenzimidazoles (3a\u2013h) quantitatively. The recovered catalyst was reused for other reactions. The facile, one-step synthetic route is shown in Scheme 1\n. Notably, various aromatic aldehydes containing both electron-withdrawing and electron-donating groups underwent condensation to afford the target compounds. The results show that aromatic aldehydes bearing electron-donating groups such as hydroxyl and methyl afforded corresponding 2-arylbenzimidazole derivatives in better yields than the aldehydes having electron-withdrawing groups.The effect of different solvents, such as protic and aprotic, with different polarities were investigated (Table\u00a01\n) on the model reaction to synthesize 3a. An initial reaction of ortho-phenylenediamine with benzaldehyde, in acetonitrile under reflux for 12 h produced a 75% yield of 3a. Protic solvents such as methanol and ethanol and aprotic solvents such as toluene, dichloromethane, tetrahydrofuran, and dioxane were investigated. The yield of 3a varied with the nature of the solvents used in the reaction. Since dioxane and acetonitrile gave the highest yield of 3a, we selected these two solvents and used microwave irradiation to decrease the reaction time and increase the yield of 3a. For a reaction time of 15 min, the yield of 3a was 55% and 97% for dioxane and acetonitrile, respectively (Table\u00a01, entry 8\u20139). Published data on benzimidazole synthesis was analyzed and presented in Table\u00a02\n. It was obvious that Zn-BNT outclassed published data; however, further work should be undertaken with named catalysts and under similar reaction conditions. Although the synthesis of the Zn-BNT material was recorded as seven days, the catalyst showed excellent catalytic activity, is environmentally friendly, non-toxic, and produces a high product yield (Table\u00a02).After optimizing the reaction, 3a\u2013h were synthesized using acetonitrile and Zn-BNT under MW conditions at 140 \u00b0C for 15 min. Appropriate ortho-phenylenediamine 1 and different aldehyde (2a\u2013h) were used. The yield of the products (Table\u00a03\n) ranged from 82 to 97%. The products 3a\u2013h were characterized by spectroscopic techniques FT-IR, 1H-NMR, 13C-NMR, and elemental analysis. The spectral confirmation of 3a is discussed as a template analysis. The FT-IR spectrum displayed the characteristic vibrational band as a single sharp peak at 3321 cm\u22121, which indicated the formation of new aromatic secondary amino -N-H stretch, while there was no shoulder peak in the region of 3500\u20133400 cm\u22121, which would strongly indicate the NH2. A strong peak at 1625 cm\u22121 demonstrated the C=N stretch. Also, there was no signal for the C=O stretch in the region of 1680\u20131715 cm\u22121, indicating the absence of the aromatic aldehyde, one of the starting materials. Thus, the new C=N bond and N\u2013H bond confirmed the main functional groups of 3a. Moreover, the characteristic peaks at 1551 cm\u22121, 1467 cm\u22121 and 1422 cm\u22121 indicated the Ar\u2013C=C stretch and a sharp peak at 3177 cm\u22121 represented the = CH stretch.The 1H-NMR spectrum of 3a showed a distinctive N\u2013H proton broad singlet at \u03b4 12.91, and no broad singlets were observed in the region of \u03b4 5\u20137 ppm corresponding to the starting compound. The 13C-NMR displayed the N=C\u2013N signal at \u03b4 151.18 while the remaining 10 aromatic carbons appeared in the range \u03b4 130.07\u2013122.09 ppm. Elemental analysis revealed Anal. Calc. for C13H10N2: C, 80.39; H, 5.19; N, 14.42%. Found: C, 80.40; H, 5.21; N, 14.44%.After optimizing the reaction, 3b\u2013h was synthesized using acetonitrile and Zn-BNT under MW conditions at 140 \u00b0C for 15 min. Appropriate ortho-phenylenediamine 1 and different aldehyde (2a\u2013h) were used. The yield of the products (Table\u00a03) ranged from 82 to 97%. The products 3a\u2013h were characterized by spectroscopic techniques FT-IR, 1H-NMR, 13C-NMR, and elemental analysis.The condensation reaction was conducted using acetonitrile as the solvent at 140 \u00b0C and different amounts of 2% Zn-BNT catalyst in the range of 0.05\u20131 g. The best yield of product 3a was obtained with 0.07 g of the catalyst. Higher amounts of the catalyst did not improve the yield. The catalyst could be quickly recovered by simple filtration of the reaction mixture, followed by washing with chloroform and then methanol: the solid was dried at 105 \u00b0C and then used for subsequent reactions. The reusability potential of Zn-BNT was also investigated in the model reaction to synthesize 3a. Recovery of the catalytic activity of Zn-BNT was better than other catalysts. Zn-BNT could be reused up to 8 times with only 5% loss of catalytic activity (Table\u00a04\n), indicating good potential if undertaken on an industrial scale for synthesizing any benzimidazole derivatives.The ground state (DFT) optimization energy calculations were performed using the hybrid exchange-correlation functional B3LYP [40] with the basis set for the 6\u2013311++G(d,p) [41]. The various possible conformers for compound 3f were optimized in the gas phase. The conformer with the lowest energy with the fundamental frequencies in the ground state was considered and shown in Table\u00a0S1 and Figure\u00a0S13 (Supplementary Information). The total energy and Cartesian coordinates for the optimized minimum energy structure are given in Table\u00a0S2. The infra-red (IR) spectrum and frequency values of 3f from DFT calculations are shown in Figure\u00a0S15 and Table\u00a0S3. Further, the NMR shielding tensors computed 3f with the gauge-independent atomic orbital (GIAO) method is shown in Figure\u00a0S16 and Table\u00a0S4. All the calculations were carried out with the Gaussian09 program package [42].The energy level of the highest-occupied molecular orbital (EHOMO) and lowest-unoccupied molecular orbital (ELUMO) energies, along with the EHOMO-LUMO gap is an important parameter [43] to evaluate the reactivity of 3f. The HOMO-LUMO plot is given in Figure\u00a09\n. The energy gap value of 3f was EH-L = 4.48 eV. The large EH-L energy gap denotes the compound's high stability and lower reactivity.The atomic charge distribution of 3f is obtained using Mulliken charge analysis [44] in Figure\u00a0S14. The nitrogen and fluorine atoms show negative charge distribution due to electronegative properties. Consistently, the hydrogen zones represent a positive charge distribution in compound 3f.The electrostatic potential surface (ESP) [45] ranges an isosurface from \u22120.02 a.u. to 0.02 a.u., as shown in Figure\u00a010\n. The ESP map showed the high electron density for the blue region, which is due to higher electronegativity (nitrogen zones). In contrast, the red region denoted low electron density (hydrogen zones). The results mentioned above indicated that the C\u2013N bond is an active site.Fukui function predicts the most probable sites of the nucleophilic, electrophilic, and radical attack selectivity and chemical reactivity using DFT calculation shown in Figure\u00a0S17 [46]. The Fukui function indices on the i\nth atomic site, for nucleophilic (f\ni\n\n+), electrophilic (f\ni\n\n\u2212), and free radical (f\ni\n\n0) are following equations,\n\n(1)\n\n\n\nf\ni\n+\n\n=\n\nq\ni\n\n\n(\n\nN\n+\n1\n\n)\n\n\u2212\n\nq\ni\n\n\n(\nN\n)\n\n\n\n\n\n\n\n(2)\n\n\n\nf\ni\n\u2212\n\n=\n\nq\ni\n\n\n(\nN\n)\n\n\u2212\n\nq\ni\n\n\n(\n\nN\n\u2212\n1\n\n)\n\n\n\n\n\n\n\n(3)\n\n\n\nf\ni\n0\n\n=\n\n1\n2\n\n\n[\n\n\nq\ni\n\n\n(\n\nN\n+\n1\n\n)\n\n\u2212\n\nq\ni\n\n\n(\n\nN\n\u2212\n1\n\n)\n\n\n]\n\n\n\n\nThe \n\n\nf\ni\n+\n\n\n, \n\n\nf\ni\n\u2212\n\n\nand \n\n\nf\ni\n0\n\n\n is represent the nucleophilic, electrophilic, and free radical attack on the reference compound 3f. From Eqs. (1), (2), and (3), qi denotes the atomic charge at the ith atomic sites of the chemical species are anionic (N + 1), cationic (N \u2212 1), and neutral (N), respectively. Here, the Fukui function for \n\n\nf\ni\n+\n\n\n, \n\n\nf\ni\n\u2212\n\n\nand \n\n\nf\ni\n0\n\n\n attack is calculated using Mulliken charge analysis.The dual descriptor \u0394F(r), the difference between the nucleophilic \n\n(\n\nf\ni\n+\n\n)\n\n and the electrophilic \n\n(\n\nf\ni\n\u2212\n\n)\n\n Fukui function as the given equation is [47],\n\n(4)\n\n\n\u0394\nF\n\n(\nr\n)\n\n=\n\n[\n\n\nf\ni\n+\n\n\u2212\n\nf\ni\n\u2212\n\n\n]\n\n\n\n\n\n\nEq. (4) represents the nucleophilic and electrophilic (electron added and removed from the LUMO and HOMO) sites given in Figure\u00a09. The nucleophilic and electrophilic attack sites are favored, whereas dual descriptor \u0394F(r) > 0 and \u0394F(r) < 0. Here, the dual descriptor \u0394F(r), is the clear distinction between nucleophilic and electrophilic attacks at a particular site with positive/negative values. The dual descriptor values are presented in Table\u00a05\n which indicated that the nucleophilic reactivity of 3f was in the following order N7 > H12 > C1> H13 > C5 > F24 > H10 > H11 > C23 > H22 > H21 > C4 > C19 > H18 > H25 > C8 > H17 > C14 > N9 > C20 > C3 > C16 > C6 > C2 > C15 and electrophilic reactivity in the order C16 > C15 > C1 > C23 > N7 > C4 > H22 > H21 > C8 > H12 > H13 > F24 > H10 > H11 > H18 > H17 > C5 > H25 > C19 > N9 > C20 > C6 > C3 > C2 > C14 while the free radical attack reactivity order was C1 > N7 > C16 > H12 > C23 > H13 > F24 > H10 > H22 > H11 > H21 > C4 > C5 > C8 > C15 > H18 > C17 > C19 > C25 > N9 > C20 > C6 > C3 > C2 > C14. Furthermore, the condition of dual descriptor, the positive values denoted nucleophilic site (F(r) > 0) for C2, C3, C5, C6, N7, N9, H10, H11, H12, H13, C14, C19, C20, and F24. Similarly, the electrophilic site was negative values (F(r) < 0) are C1, C4, C8, C15, C16, H17, H18, H21, H22, C23, and H25. Finally, behavior for the nucleophilic, electrophilic, and free radical attack indicated the highly reactive sites were N26 (0.096), C8 (1.611), and C8 (0.801).In this work, we developed a facile, one-pot synthesis of 2-substituted benzimidazole derivatives through a reaction between o-phenylenediamine and different aromatic aldehydes in the presence of a novel zinc-boron nitride catalyst. The advantages of the developed method include the environmental-friendly reaction conditions, simple operation, broad substrate scope, satisfying yields, easy isolation of the product, and reusability of the catalyst. Furthermore, 2-(4-fluorophenyl)-1H-benzo[d]imidazole (3f) was selected for a computational study of the IR and NMR spectrum, which matched the experimentally generated spectra. The HOMO-LUMO gap was calculated as 4.48 eV. Moreover, the novel catalyst could be employed to synthesize other heterocyclic compounds, and their biological activity can be evaluated.Sureshkumar Mahalingam, Arul Murugesan: Performed the experiments; Contributed reagents, materials, analysis tools or data; Wrote the paper.Thangaraj Thiruppathiraja, Senthilkumar Lakshmipathi: Performed the experiments; Wrote the paper.Talent Raymond Makhanya: Performed the experiments; Analyzed and interpreted the data.Robert Moonsamy Gengan: Conceived and designed the experiments; Analyzed and interpreted data.Sureshkumar Mahalingam was supported by National Research Foundation (NRF) of South Africa (Grant No. 98725).The data that has been used is confidential.The authors declare no conflict of interest.Supplementary content related to this article has been published online at [URL].Supplementary content related to this article has been published online at https://doi.org/10.1016/j.heliyon.2022.e11480.The following is the supplementary data related to this article:\n\nSupplementary information\nSupplementary information\n\n\n\n", "descript": "\n                  A new zinc-based boron nitride (Zn-BNT) material was synthesized from boron nitride and zinc acetate in 95% yield. The morphological and spectroscopic properties of Zn-BNT were elucidated by SEM, XRD, BET, DSC-TGA, and FT-IR. Zn-BNT catalyzed the synthesis of benzimidazoles (3a\u20133h) through a reaction between o-phenylenediamine and different aromatic aldehydes under microwave conditions for 15 min. The compounds were purified by silica-gel chromatography. The synthesized compounds were characterized by FT-IR, 1H-NMR, 13C-NMR, and elemental analysis. Zn-BNT was reused eight times with only a 5% loss of catalytic activity. Furthermore, 2-(4-fluorophenyl)-1H-benzo[d]imidazole (3f) was selected for a computational study of the IR and NMR spectrum, which matched the experimentally generated spectra. The HOMO-LUMO gap was 4.48, and the Fukui function analysis showed high activity in the reactive sites.\n               "}
{"full_text": "Data will be made available on request.An important class of ligands known as Schiff bases has found wide applications in the coordination chemistry of inner transition, transition and main group elements [1\u20135]. Generally, aromatic or aliphatic amines and carbonyl compounds are condensed by nucleophilic addition to produce Imine bases [6\u201312]. All the metal complexes bind to DNA in a non-covalent manner on the groove through electrostatic bonding and intercalation [13\u201318]. Designing less harmful, more affordable and non-covalently bound novel chemotherapeutic medicines is urgently needed to solve the challenge in anticancer treatment [18\u201325]. The synthesis of a novel series of benzothiazole Schiff base metal complexes is what we have concentrated on due to their diverse range of pharmacological actions, including antibacterial, anticancer[26], anti-inflammatory[27], anti-tuberculosis[28], antioxidant[29], anticonvulsant[30] and anthelmintic[31]. Schiff bases are among the compounds most thoroughly explored in medicinal chemistry. Schiff bases are employed as stabilisers for polymers, catalysts, dyes and organic synthesis intermediates [32\u201334]. The benzothiazole-derived Schiff base is used in a wide range of analytical, biological, inorganic, medical and pharmaceutical applications [35\u201338].Synthesis of 2-EBTMCP ligand and their metal complexes is presented in this research article. All the Metal complexes of Co(II), Ni(II), Cu(II) and Zn(II) prepared from 2-EBTMCP ligand. The Physical-chemical spectrum techniques and their analytical data have been used to characterise the spectral properties of the 2-EBTMCP ligand and associated 3d series transition metal complexes. The biological activities of DNA binding studies and MMT test was used to screen the cytotoxicity investigations against the cervical carcinoma (HeLa) and breast adenocarcinoma (MCF-7) cell lines.TLC (Thin Layer Chromatography) method was used to check the purity of the newly prepared 2-EBTMCP ligand and their metal complexes. On a Polmon MP-96 model instrument, measurements of the melting points of the ligands and their metal complexes were obtained.The SHIMADZU Perkin-Elmer Infrared model was used to record FT-IR spectral analysis of ligand and metal complexesin a range of 550 to 4000\u00a0cm\u22121.On a Bruker 400\u00a0MHz NMR instrument, the 1H NMR/13C NMR of the ligand and metal complex was recorded using DMSO\u2011d\n6 as the solvent and TMS as the internal reference.SEM was used to examine the surface morphology of the CT complex (scanning electron microscope on Zeiss evo18). EDX spectra were used to analyze the elemental composition of the ligand and metal complexes (SEM, Quanta FEG 250). The device was powered by a 20\u00a0kV acceleration voltage.A Rigaku MiniFlex 600 X-ray diffractometer was used to perform powdered XRD analysis. Cu-K\u03b1 radiation (\u03bb\u00a0=\u00a01.5406 A\u00b0) in the range 2\u03b8 from 5 to 80\u00b0 with a step size of 0.02\u00b0 and a scan step time of 0.15\u00a0s.6-ethoxy-2-amino benzothiazole and 5-Chloro-2-hydroxy benzaldehyde was purchased by Sigma-Aldrich. The HPLC solvents, which included acetone, chloroform, n-hexane, ethyl acetate methanol, Dichloromethane, acetic acid, Hydrochloric acid (32%), Triethylamine and aqueous ammonia (25%) solutions, were procured from SD fine AVRA company. The metal chloride salts CoCl2\u00b74H2O, NiCl2\u00b74H2O, CuCl2\u00b74H2O and ZnCl2\u00b74H2O were purchased from AVRA Company. MMT dye purchased from Bangalore Genei in Bengaluru India and kept at \u221220\u00b0 C.To an equimolar quantity of 5-Chloro-2-hydroxybenzaldehyde (0.279\u00a0g, 3\u00a0mmol) dissolved in hot methyl alcoholcarbinol (10\u00a0ml) solution, 6-ethoxy-2-aminobenzothiazole (0.194\u00a0g, 3\u00a0mmol) was added. The final mixture was refluxed for five-six hours at 75\u201380\u00a0\u00b0C. After the reaction was finished, a solid precipitate with a yellow tint emerged. This was filtered to separate it (using a suction pump), then washed with cold methyl alcoholcarbinol before being dried in a vacuum (Scheme 1\n).Colour: Yellow. Yield: 83%. Melting point: 243\u2013245. ESI-Mass spectra (m/z): Calculated: 332 Obtained: 333[M\u00a0+\u00a0H]+. Infrared spectra (cm\u22121, KBr): \u03bd(HC=N) 1649, \u03bd(OH/H2O) 3032, \u03bd(C-O) 1267. Elemental analysis: C16H13ClN2O2S for Calculated % (Found %): C, 57.63; H, 3.854; N, 8.35 (C, 57.74; H, 3.94; N, 8.42). 1H NMR in Fig. 1\n (400\u00a0MHz, chloroform-d) \u03b4 9.19 (s, 1H, OH), 7.85 (d, J\u00a0=\u00a08.8\u00a0Hz, 1H), 7.60 (d, 1H), 7.52 (s, J\u00a0=\u00a08.4\u00a0Hz, 1H), 7.10 (s, 1H), 7.85 (d, J\u00a0=\u00a08.2\u00a0Hz, 1H), 7.87 (s, J\u00a0=\u00a08.7\u00a0Hz, 1H), 4.12 (q, 2H), 1.42 (T, 3H). \n13C NMR in Fig. 2\n (400\u00a0MHz, CDCl3): \u03b4 14.40(C1), 64.05(C2), 77.01(t, CDCl3), 104.80(C4), 109.12(C13), 116.21(C11), 119.03(C8), 119.84(C7), 123(C15), 124.27(C16), 127.68(C14), 128.73(C5), 129.40 (C6), 137.34 (C3), 156.96 (C12), 160.90 (C10) and 165.86 (C9).The hot methyl alcohol Carbinol (10\u00a0ml) solution containing the 2-EBTMCP (0.332\u00a0g and 3\u00a0mmol)) ligand was slowly added to the methyl alcohol Carbinol (10\u00a0ml) solutions of the corresponding metal chloride ions such as CoCl2\u00b74H2O (0.201\u00a0g), NiCl2\u00b74H2O (0.201\u00a0g), CuCl2\u00b74H2O (0.206\u00a0g) and ZnCl2\u00b74H2O (0.208\u00a0g). The reaction mass was heated for 3\u20134\u00a0hrs at the same temperature while being refluxed at 70\u201375\u00a0\u00b0C. Following the completion of the initial material, solid precipitates of various colours, including Green, Maroon, Blue and Yellow were obtained. These precipitates were then washed in cold methyl alcohol Carbinol (CH3OH), n-hexane and dried in vacuum (\nScheme 2\n\n).\n\nAnalytical data: Colour: Light orange. Yield: 66%. Melting point: 266\u2013268. ESI-Mass spectra (m/z): Calculated: 720 Obtained: 722 [M\u00a0+\u00a02H]+. Infrared spectra (cm\u22121, KBr): \u03bd(HC=N), \u03bd(OH/H2O), \u03bd(C-O), \u03bd(M\u2212O) and \u03bd(M\u2212N) are shows at 1602, 3107, 1249, 707 and 410 respectively. Elemental analysis: C32H24Cl2CoN4O4S2 for Calculated % (Found %): C, 53.27; H, 3.24; N, 7.66 (C, 53.19; H, 3.35; N, 7.75).\nAnalytical data: Colour: Light orange. Yield: 68%. Melting point: 259\u2013261. ESI-Mass spectra (m/z): Calculated: 721 Obtained: 722 [M\u00a0+\u00a0H]+. Infrared spectra (cm\u22121, KBr): \u03bd(HC=N) 1602, \u03bd(OH/H2O) 3109, \u03bd(C-O) 1251, \u03bd(M\u2212O) 765, \u03bd(M\u2212N) 414. Elemental analysis: C32H24Cl2N4NiO4S2 for Calculated % (Found %): C, 53.14; H, 3.38; N, 7.64 (C, 53.21; H, 3.35; N, 7.76).\nAnalytical data: Colour: Dark green. Yield: 71%. Melting point: 273\u2013275. ESI-Mass spectra (m/z): Calculated: 724 Obtained: 725 [M\u00a0+\u00a0H]+. Infrared spectra (cm\u22121, KBr): \u03bd(HC=N) 1606, \u03bd(OH/H2O) 3197, \u03bd(C-O) 1238, \u03bd(M\u2212O) 767, \u03bd(M\u2212N) 455. Elemental analysis: C32H24Cl2CuN4O4S2 for Calculated % (Found %): C, 52.75; H, 3.38; N, 7.63 (C, 52.86; H, 3.33; N, 7.71).\nAnalytical data: Colour: Dark orange. Yield: 66%. Melting point: 271\u2013272. ESI-Mass spectra (m/z): Calculated: 725 Obtained: 726 [M\u00a0+\u00a0H]+. Infrared spectra (cm\u22121, KBr): \u03bd(HC=N) 1595, \u03bd(OH/H2O) 3182, \u03bd(C-O) 1247, \u03bd(M\u2212O) 748, \u03bd(M\u2212N) 410. Elemental analysis: C32H24Cl2N4O4S2Zn for Calculated % (Found %): C, 52.63; H, 3.24; N, 7.62 (C, 52.72; H, 3.32; N, 7.69).At a temperature of 25\u00a0\u00b0C, the DNA stock solution had a composition of 5\u00a0mM Tris-HCl and 50\u00a0mM NaCl in double-distilled water. The solution became clear after being stirred continuously all night. HCl was utilised in order to get the pH of the homogeneous transparent buffer solution up to 7.2. DNA solution quality was examined using the electronic absorbance ratio of 1.8\u20131.9 at 260 and 280\u00a0nm. Ct-DNA has no impurity protein [38]. Using absorption spectroscopy with a molar absorptivity of 6600\u00a0M\u22121 cm\u22121 and absorbance wavelength of 260\u00a0nm, the proportion of Ct-DNA was determined. The generated solutions were stored at a lower temperature (3\u20134\u00a0\u00b0C) and utilised for three to four days. The compound was dissolved in a buffer solution consisting of 50% Tris-HCl and 50% acetonitrile throughout the whole experiment. DNA concentrations from 0 to 10\u00a0\u03bcM were used in the titration experiments, with the complex concentration held constant.The synthesised ligands and their metal complexes were evaluated for cytotoxicity in vitro using the MTT assay. At DMSO, the compounds were dissolved in concentrations ranging from 1 to 100\u00a0\u03bcM. The cells were seeded into a 96-well plate, and then they were cultured for 48\u00a0h at 5% CO2. The cell was then incubated for 24\u00a0h after being treated with various concentrations DMSO solutions of the metal complexes. The culture media was removed, and then 15\u00a0\u03bcL of MTT dye solution was added to each well, before being re-incubated for another 4\u00a0h in the dark. The use of MTT was discarded, and dimethyl sulfoxide (DMSO) was used for product solubilisation. With an Elisa reader, check the absorbance at 620\u00a0nm in each sample well. The IC50 values were determined by plotting the absorbance of the dosage response curves[39\u201342].\nTable 1\n and Fig. 3\n displays the chemical and physical characteristics of the ligand (LH) and its metal complexes. The mass spectra match up with what would be predicted for each system. According to the analytical results, the molar ratio of metal to the ligand in all of the complexes is 1:2.The useful information about functional groups is provided by the IR spectral data in Fig. 4\n. Table 2\n provides a summary of 2-EBTMCP ligand and its metal complexes primary IR characteristic stretching frequencies. The shift in stretching frequency values can be understood by comparing the IR spectra of the free ligand and its metal complexes. The strong and distinct band seen at 1649\u00a0cm\u22121 in the free ligand is due to the azomethine's HC\u00a0=\u00a0N stretching vibration. This band is shifted to a lower wave number by 47\u00a0cm\u22121 to 54\u00a0cm\u22121 in metal complexes. These changes provide evidence that the ligand imine nitrogen group participates in the binding of metal ions. Additionally, a prominent peak at 1267\u00a0cm\u22121 in the ligand IR spectra was identified as being associated with the phenolic C-O stretching frequency, which is slightly shifted to a lower wave number in metal complexes. These changes confirm that the phenolic \u2013OH of the ligand participates in the binding of the Co(II), Cu(II), Ni(II), and Zn(II) ions, as well as the elimination of the \u2013OH stretching frequency. Peak in complexes provides further proof that phenolic oxygen participated in metal binding via proton dissociation. Additional novel frequencies are seen in metal complexes in the ranges of 707\u00a0cm\u22121, 767\u00a0cm\u22121 and 410\u00a0cm\u22121, 455\u00a0cm\u22121 respectively, confirming the development of M\u2212O and M\u2212N.To ascertain the crystalline size and structure of the produced Schiff base ligand and metal complexes, powder X-ray diffraction analysis was performed. Fig. 5\n shows the X-ray powder patterns for ligand and metal complexes. The semi crystalline nature of the newly synthesised metal complexes is attested to by the sharp and well-defined Bragg Peaks at certain 2\u03b8 angles, as determined by this study. The particle size of the complexes was calculated using the Debye-Scherer formula based on the intensity of the highest intensity line in comparison to the other lines.\n\nD\u00a0=\u00a00.94 \u03bb/\u03b2 cos \u03b8.\n\n\nwhere, \u03bb is the wavelength of the X-ray used, D is the crystalline size in nm, 0.94 is the sheerer constant, \u03b2 is the full width at half maximum (fwhm) and \u03b8 is the position of the particular diffraction peak. Calculations for the size distribution of ligand and metal complex particles using these diffraction peaks provide values of 19.26\u00a0nm for ligand, 24.13\u00a0nm for Cu, 29.76\u00a0nm for Ni, 31.26\u00a0nm for Co, and 41.58\u00a0nm for Zn.SEM examination was used to examine the surface morphology of the Schiff base ligand and their metal complexes and energy dispersive X-ray analysis was used to examine the elements of the compounds (EDX). The Schiff base ligand showed sheet like structures observed in Fig. 6\n. The Co metal complex shows gross like structure, the Ni metal complex has spherical shape, the Cu metal complex has long flakes, and the Zn metal complex rod shaped particles are observed. The surface morphology of metal complexes differs from their ligands and from one another, as shown by the SEM micrographs, as a result of complexation and metal ion displacement.One of the most well-known techniques for examining the interaction of DNA with metal complexes is UV\u2013vis electronic absorption spectroscopy. In the absence and presence of Ct-DNA, the electronic spectra of all metal complexes Co(II), Ni(II), Cu(II) and Zn(II) were recorded (Fig. 7\n). Due to the strong contact between the aromatic chromophore and the DNA base pairs in the intercalative mode, compound binding through intercalation typically results in hypochromism with or without a slight red or blue shift. The absorbance of the metal complexes and the shift in wavelength due to the increase in DNA concentration has been used to assess their binding capacity[43,44]. A considerable hypochromism is detected when increasing amounts of CT-DNA are added. This is due to a strong association between DNA and complexes, and it is also suggested that these complexes bind to the DNA helix by intercalation. The absorption data were analysed to calculate the intrinsic binding constant (Kb) via the Wolfe-Shimmer equation.\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n[\nD\nN\nA\n]\n\n/\n\n(\n\n\u03b5\na\n\n-\n\n\u03b5\nf\n\n)\n\n=\n\n[\nD\nN\nA\n]\n\n/\n\n(\n\n\u03b5\nb\n\n-\n\n\u03b5\nf\n\n)\n\n+\n1\n/\n\nK\nb\n\n\n(\n\n\u03b5\nb\n\n-\n\n\u03b5\nf\n\n)\n\n\n\n\n\nKb is the intrinsic binding constant; [DNA] is the concentration of CT-DNA; \u03b5a is the apparent coefficient; \u03b5f and \u03b5b represent the extinction coefficients for unbound and bound DNA, respectively; Kb can be calculated from a plot of DNA/(\u03b5a-\u03b5f) vs [DNA] by dividing the slope by the intercept. The binding constants Kb is calculated from spectral data found to be 8.78\u00a0\u00d7\u00a0106 M \u22121 (Zn), 7.62\u00a0\u00d7\u00a0106 M \u22121 (Ni), 6.99\u00a0\u00d7\u00a0105 M \u22121 (Co), and 4.39\u00a0\u00d7\u00a0104 M \u22121 (Cu). From the above values, it is clear that Zn(II) and Ni(II) complexes bound strongly to DNA than the other metal complexes.Using the MTT assay, the Schiff base ligands and metal complexes were tested for in vitro cytotoxicity against the cell lines HeLa and MCF-7. The Percentage inhibition of cancer cell development was evaluated after treating two cell lines with CT complex in a range of concentrations (12.5\u2013100\u00a0\u03bcM) for 24\u00a0h. The cell viabilities (%) vs concentrations obtained with continuous exposure for 24\u00a0h were depicted in Fig. 8\n. Cisplatin was used as control. The complexes cytotoxicity was determined to be concentration dependant. The screening results indicated that all of the metal complexes had significant anticancer activity (Table 3\n). The order of IC50 values of the metal complexes against both cell lines as Zn(II)\u00a0>\u00a0Ni(II)\u00a0>\u00a0Co(II)\u00a0>\u00a0Cu(II)\u00a0>\u00a0Ligand (Fig. 8).In this study, the coordination capabilities of a new ligand were investigated utilising computational and equilibrium techniques. The complexes of solid metals of the substances involving Co(II), Ni(II), Cu(II) and Zn(II)were developed, then characterised by different spectrum analytical methods FT-IR, ESI-Mass spectra, XRD and SEM-EDX. Co(II), Ni(II), Cu(II) and Zn(II) complexes analytical spectrophotometric investigation revealed that metal and ligand formed stoichiometric in a 1:2 ratio. The DNA binding testing for electronic absorption showed that the intercalation mode was used by all complexes to engage with CT-DNA. Following cytotoxic screening, all metal complexes displayed more potency than that of the corresponding ligand. Zn complex was the most active of all the complexes when compared to other complexes.\nKamble Gopichand: Conceptualization, Data curation, Funding acquisition, Investigation, Methodology, Writing \u2013 original draft. Varukolu Mahipal: Writing \u2013 review & editing, Resources, Software, Validation, Visualization. N. Nageswara Rao: Methodology. Abdul Majeed Ganai: . P. Venkateswar Rao: Supervision, Conceptualization, 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.GK thanked the Head of the Chemistry Department at Osmania University in Hyderabad for his help with my research.Supplementary data to this article can be found online at https://doi.org/10.1016/j.rechem.2023.100868.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  2-((E)-(6-ethoxybenzo[d]thiazol-2-ylimino)methyl)-4-chlorophenol (2-EBTMCP) of Schiff base (HL) which would be produced from 6-ethoxy-2-amino benzothiazole and 5-Chloro-2-hydroxybenzaldehyde was prepared and characterised using 1H NMR, Infrared, ESI-mass spectra, elemental analysis, SEM, EDX and powder XRD spectroscopic methods. Its complexes with Ni(II), Co(II), Zn(II) and Cu(III) were made, isolated as solid compounds, and identified by various spectroscopic methods. The research of metal complexes here includes the intercalative form of DNA binding studies with CT (Calf thymus) DNA. The UV\u2013Vis electron absorption spectroscopy approach has been used to explore the DNA-binding capabilities of transition metal complexes. Additionally, a MTT assay was performed to investigate their in vitro cytotoxic potential.\n               "}
{"full_text": "Depletion of natural energy resources (crude oil, natural gas, solid fuels) has prompted a great interest in the use of renewable energy sources, mainly biomass [1]. An increasing interest in the biomass conversion is not only because of the energy related reasons but also ecological reasons, mainly the need to restrict the emission of greenhouse gases [2,3]. Biomass is composed of cellulose, hemicellulose and lignin units [4,5]. Because of its chemical decomposition and processing abilities it is a valuable raw material that can be processed into a number of useful products, e.g., substitutes of crude oil fuels [6,7]. This aspect is very important because at present transport uses up to 1/5 of energy on the global scale [8,9].One of the technologies used for transformation of biomass is fast pyrolysis, which is thermal decomposition of high-molecular chemical compounds to simpler components under anaerobic conditions [10,11]. At these conditions, the building blocks of biomass undergo degradation leading to the formation of a liquid fraction known as biooil or pyrolytic oil [12]. It is a mixture of over 300 different chemical compounds containing a significant number of oxygen atoms [13], mainly in the form of oxygen lignin derivatives, aldehydes (including heterocyclic furfurals), alcohols, phenols, ketones, carboxylic acids or carbohydrates [6,14]. Biooil has a high energy potential as it maintains up to 70 % of the initial biomass energy [5]. However, it cannot be applied directly as a fuel because of a too high content of water (up to 25 wt.%) and oxygen (up to 40 wt.%), which finally results in a low calorific value [5,9]. In order to make its properties more resembling those of crude oil, biooil must be subjected to physicochemical processing [15].A promising pathway to get high quality hydrocarbon fuel from biooil is the catalytic hydrodeoxygenation (HDO) [16\u201318]. This process is based on the reaction of chemical compounds contained in biooil with gaseous hydrogen, under elevated pressure, elevated temperature and in the presence of a catalyst. Under these conditions the reagents are transformed to simpler compounds, mainly hydrocarbons [18,19]. This process permits a removal of oxygen atoms from chemical compounds of biooil but also leads to the saturation of multiple bonds of the reagents, which improves the system stability. Finally, the process causes a decrease in the O/C ratio at the simultaneous increase in the H/C ratio, which means that the improved biooil, called a biofuel, can be used as a motor vehicle fuel [9]. The HDO process is usually carried out at a temperature between 200 and 500 \u00b0C under hydrogen at a pressure reaching 200 bar [20]. Under such conditions, the majority of chemical compounds are able to convert into their deoxygenated analogues [6]. In the laboratory scale the mechanism of HDO is studied using the so-called model chemical compounds that are identical to those contained in biooil [21]. Very often anisole is selected for investigation because it contains an isolated methoxyl group (OCH3) which is the most often present in the chemical structure of biooil [22\u201324].High yield of HDO process depends on the catalyst which should by characterized by high activity even at lower pressure and temperature [21,25]. The most effective catalysts for this process are those with transition metal ions (Pt, Pd, Ru, Rh, Ir, Co, Ni) as the active components because they show high activity in the processes involving hydrogen [26,27]. Because of their high cost [28], they are usually deposited on supports in order to enlarge the surface area of the active phase and improve the mechanical strength and thermal stability of the system [29]. A particularly interesting group of supports are ordered mesoporous silicas [30\u201332], characterized by well-developed system of pores with diameters between 2 and 50 nm, high pore volume (\u223c0.7 cm3/g) and large surface area reaching even 1000 m2/g [29,33]. Very attractive silicas are those of SBA (Santa Barbara Amorphous) type materials because they are non-toxic, possess ordered mesoporous structure with interconnected micropores in the mesopore walls and can be easily synthesized by block copolymer soft templating [30,34].The best-known representative of the SBA class of materials is SBA-15 silica with two-dimensionally ordered mesopores. Below are some examples from the literature where this material was used as catalyst in the HDO reaction.A very interesting silica from the SBA group is SBA-12, which has a hexagonal distribution of channels with the p63/mmc space group. The specific surface area of this silica often exceeds 1000 m2/g [35]. The pores of the SBA-12 material have a three-dimensional architecture that essentially facilitates the transport of larger reagent molecules, which in turn provides easier access to active sites. The material in question has not yet attracted much attention, although there is a good chance that it will because, like the SBA-15 silica, it has thick pore walls, thanks to which it is thermally and hydrothermally stable.Catalytic systems based on mesoporous silicas have been explored by many authors in hydrodeoxygenation of anisole. Yang et al. [36] have studied the mechanism of HDO of anisole catalyzed by a catalyst having nickel atoms on SBA-15 type mesoporous silica. The process was performed at 310 \u00b0C under hydrogen pressure of 30 bar for 6 h. The products formed in large amounts at 100 % anisole conversion were hexane (selectivity 26 %), cyclohexane (30 %) and benzene (26 %). Results of this experiment proved high activity of the catalyst used because it was possible to obtain a high degree of deoxygenation and anisole hydrogenation.Similar studies have been carried out by Sankaranarayanan et al. [5] who investigated the catalytic activity of ordered mesoporous silica, specifically SBA-15 with deposited cobalt atoms as the active phase. The process was performed at 220 \u00b0C under hydrogen pressure of 50 bar for 2 h. These authors achieved 99 % anisole conversion to the following products: methoxycyclohexane (selectivity 72 %), cyclohexane (12 %), cyclohexanone (1\u2009%) and cyclohexene (0.5 %). The catalytic system was also very active, although the highest selectivity was achieved for methoxycyclohexane, the compound with single bonds between carbon atoms but still having oxygen in the structure. Although the results of hydrodeoxygenation of chemical compounds of biooil are good, the search for catalytic systems capable of providing high yields but at possibly lower temperatures and pressures is continued [26]. According to literature, the HDO process is the most effective at elevated temperatures and pressures, but at these conditions it is an energy consuming process, uneconomical from industrial viewpoint [37].Previously only SBA-15 among the SBA-family materials was used as a support of the HDO catalyst. Here we report data for hydrodeoxygenation of anisole over the ruthenium catalyst deposited on SBA-12 silica at relatively low temperature (90\u2212130 \u00b0C) and under low pressure of gas hydrogen (25\u221260 bar). We studied the effect of the reaction time, the amount of catalyst and the active phase loading on the conversion process. The impact of these parameters on the degree of anisole conversion, types and amounts of the reaction products is discussed. It is the first attempt of using SBA-12 silica with transition metal catalysts in the HDO conversion. Such ordered mesoporous silica with symmetry group p63/mmc can be of consideration when outstanding stability and three-dimensional mesopore structure is desired for the proper adsorption/diffusion necessary in heterogeneous catalysis. A comparison with other catalysts is assessed.SBA-12 silica was synthesized by adding structure-directing agent BRIJ 76 surfactant (Sigma Aldrich; 8 g) to a mixture of water (40 g) and 0.1 M hydrochloric acid (160 g) and stirring for 2 h. Then, 17.6 g of tetraethyl orthosilicate (TEOS, Fluka, 99 % purity) was added and the mixture was stirred for 20 h. After this time, the mixture was poured to PP bottles and placed in a drying apparatus at 80 \u00b0C for 24 h. The solid product was filtered, dried and calcined in order to remove the template at 550 \u00b0C for 6 h. The resulting silica was subjected to wet impregnation in order to introduce different amounts of ruthenium to obtain catalysts with different amount (wt.%) of Ru with respect to the mass of the support (the incremental dosage was 0.5 wt.% up to maximum 3 wt.%, i.e., catalysts with 0.5; 1; 1.5; 2; 2.5 and 3 wt.% were prepared). Ethanol solution of ruthenium(III) chloride was introduced dropwise to the dry SBA-12 support in the amount sufficient to fill the pores and wet the external surface of particles. The content of the beaker was stirred. The beaker was tightly closed with a polyolefin-paraffin foil Parafilm\u00ae and left to rest for 24 h to allow penetration of the solution into the pores of SBA-12. Afterwards, the foil was removed and the beaker was placed under fume hood at room temperature until the entire solvent was evaporated under stirring the content from time to time. Next, the catalysts were dried for 1 h at 30 \u00b0C, 1 h at 40 \u00b0C, and 18 h at 60 \u00b0C, followed by reduction in a tube furnace in hydrogen atmosphere. To remove air from the system argon of N5.0 purity (Linde Gas Poland) (50 cm3/min) was blown to the furnace, then hydrogen (50 cm3/min) was introduced and after 0.5 h the furnace was switched into the heating mode. The catalysts were reduced at 250 \u00b0C for 3 h (the temperature was established based on the TPR results). The catalysts studied were labelled as y RuSBA-12, where y is the wt.% content of ruthenium in relation to the support.The process of anisole hydrodeoxygenation was carried out in a high-pressure reactor (CAT 24 HEL) placed on a magnetic stirrer and equipped with a thermocouple and manometer to measure the initial pressure in the reactor. The catalysts dispersed on SBA-12 silica support and containing 1 or 3 wt.% ruthenium (labelled as 1% RuSBA-12, 3% RuSBA-12) with respect to the support, were placed in a furnace at 250 \u00b0C for 3.5 h under dry argon flow prior to the hydrodeoxygenation in order to remove traces of water. An appropriate amount of the catalyst (0.025, 0.05 or 0.1 g) together with anisole (1 g) was placed in a glass vessel equipped with a magnetic stirrer. No solvent was added as anisole was in the liquid state. The glass vessel with the reaction mixture was placed in a high-pressure reactor, which was tightly closed and flushed with argon of N5.0 purity in order to remove air from the reactor. Then hydrogen of N5.0 purity was introduced to the reactor and then the reactor was filled with hydrogen up to the pressure of 25, 40 or 60 bar. The reaction was conducted at three temperatures 90, 110 or 130 \u00b0C, for the time of 1, 2.5 or 4 h. The reaction mixture was stirred at 700 rotations per minute. After the reaction time, the reactor was cooled to room temperature. The reaction mixture was centrifuged (VWR) in order to separate the liquid reaction products from the solid catalyst.The reaction substrates and products were analyzed using a gas chromatograph made by VARIAN 3900 GC equipped with a capillary column CPWAX57CB (length 25 m, diameter 0.32 mm, film thickness 1.2 \u03bcm) and a flame-ionization detector. The products were identified on the basis of comparison of retention times of the compounds obtained with those of the standards. The results were confirmed by the results of gas chromatography analysis using a chromatograph coupled with a mass detector made by VARIAN 4000 GC and a capillary column VF-5MS (length 30 m, diameter 0.25 mm, film thickness 0.25 \u03bcm).The Ru content in all synthesized catalysts was evaluated by inductively coupled plasma (ICP) analysis. In addition, these catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), low-temperature nitrogen adsorption/desorption (N2 ads/des). The XRD diffractograms were recorded on a diffractometer Bruker AXS D8 Advance, using CuK\n\u03b1 (0.154 nm) radiation, in the range 2\u019f = 0.6\u201310\u00b0 with 0.02\u00b0 step (small-angle range) and 2\u019f = 4\u201360\u00b0 with 0.05\u00b0 step (wide-angle range). TEM images were recorded on a JEOL-2000 TEM microscope at 80 kV.Low-temperature nitrogen adsorption/desorption isotherms were recorded on Nova Quantachrome instrument at 176 \u00b0C, after prior degassing of the sample in vacuum at 350 \u00b0C for 24 h. The surface area of the catalysts was determined by the BET (Brunauer \u2013 Emmet \u2013 Teller) method. The pore volume was determined using the KJS-BJH method based on the BJH (Barret \u2013 Joyner \u2013 Halenda) algorithm [38,39].All the materials studied were characterized by using some of the techniques described in chapter 3. This allowed selection two materials for detailed characterization. Additionally, due to the lack of space, only data for 1 and 3 wt.% are discussed here. Fig. 1\n presents the diffractograms recorded for pristine SBA-12 and SBA-12 support with deposited ruthenium, in the small-angle range (Fig. 1a) and wide-angle range (Fig. 1b). The small-angle diffractograms show one main reflection at 2\u019f = 1.8\u00b0 corresponding to the plane [002] and a lower-intensity reflection at 2\u019f = 1.27\u00b0, corresponding to the plane [100]. The material of SBA-12 type structure belongs to the space group p63/mmc and has hexagonal symmetry [40,41] so the elementary cell parameters were calculated for hexagonal symmetry.The a and c lengths of the elementary cell for hexagonal RuSBA-12 sample vary in the range 8.0\u20139.6 nm, as shown in Table 1\n. The unit cell parameters of the SBA-12 (a = 8.0\u20138.4 nm, c = 8.9\u20139.1 nm, c/a = 1.63) are in good agreement with previously reported results [42,43]. After introduction of ruthenium in the amounts smaller than 1 wt.% the values of a and c lengths decrease and then increase when the content of ruthenium increases from 1 to 3 wt.%.The nitrogen adsorption/desorption isotherms recorded for samples 1% RuSBA-12 and 3% RuSBA-12 (Fig. 2\n) are of type IVa, characteristic of mesoporous materials. Also, the type of hysteresis loop is typical for mesoporous materials [40,44,45]. As was mentioned above, the isotherms obtained for the SBA-12 samples studied match the patterns reported for other mesoporous materials with p63/mmc symmetry [40]. The additional hysteresis loop recorded for samples with 1 wt.% of Ru can be interpreted as a result of the presence of voids between particles [42,46]. Surface area of the samples is close to 1000 m2/g, while the average pore size is 5 nm, as shown in Table 1.The TEM images presented in Fig. 3\n, confirm the ordered mesoporous structure of the materials studied. They have hexagonal 3D structure with densely packed mesopores (hcp) [47]. No crystalline ruthenium oxide species were observed (wide-angle XRD, TEM), which can suggest that agglomerates at the outer surface were not formed. Thus, Ru probably exists in the catalyst in the form of isolated Ru atoms The results of SEM/EDX analysis (not shown here) confirm homogeneous distribution of ruthenium species.Hydrodeoxygenation of organic compounds is the process of obtaining hydrocarbons in the presence of gaseous hydrogen supplied under elevated pressure that takes place at elevated temperature and in the presence of a catalyst, as shown in Fig. 4\n.In this study a series of hydrodeoxygenation reactions of anisole were performed using SBA type silica loaded with different amounts of ruthenium catalyst. The reaction does not proceed without a catalyst and proceeds with low conversion with unmodified SBA-12 catalyst (Table 2\n, entry 19). The effects of the reaction time, the amount of catalyst, hydrogen pressure and temperature were evaluated. According to literature data, the sequence of transformations of anisole molecules under elevated pressure and temperature includes a few reactions such as hydrogenation, dehydration, demethoxylation and demethylation [12,48]. The type and amounts of products of HDO of anisole depend on many parameters, including the type of catalyst and conditions of the process.The main products of HDO of anisole identified by GC or GC\u2013MS, irrespective of the conditions, are methoxycyclohexane and 1-methoxycyclohexene, as shown in Figs. 6\u20139. Besides the above main products, the formation of cyclohexane, cyclohexene, benzene, 1,1\u2019-dimethoxycyclohexane, cyclohexanone, cyclohexanol, was confirmed. The selectivities to these products are listed in Table 2. However, these products are not presented on the plots as their total content did not exceed 20 %. Based on the identified products of the reaction, three pathways of HDO of anisole to a fully hydrodeoxygenated compound - cyclohexane - are proposed (Fig. 5\n\n\n\n\n). The first pathway involves hydrogenation of the aromatic ring, leading to 1-methoxycyclohexene and subsequently to methoxycyclohexane, then the CO bond between the carbon from the aromatic ring and the oxygen from methoxyl group is broken, which leads to cyclohexane formation. The formation of undesirable product of 1,1\u2019-dimethoxycyclohexane (that contains more oxygen atoms than the initial compound) as a result of methanol attachment to 1-methoxycyclohexene is also possible.In the second pathway, cyclohexane is obtained via an intermediate product benzene. This pathway starts with direct demethoxylation of anisole to the aromatic structure and then the aromatic ring is hydrogenated.In the third pathway, the first reaction is demethylation of the substrate, leading to the formation of phenol, which is converted to cyclohexanol via total hydrogenation of the aromatic ring. Cyclohexanol can undergo isomerization to cyclohexanone, dehydration of which gives cyclohexene. Cyclohexene having a double bond in its structure can easily undergo hydrogenation to cyclohexane. Because of possible cyclohexanol dehydration, the reaction can also give dicyclohexyl ether.Analysis of Figs. 6\u20139 and Table 2 data suggests that in this study the main pathway seems to be the first one, involving direct hydrogenation of aromatic ring: anisole \u2192 1-methoxycyclohexene \u2192 methoxycyclohexane \u2192 cyclohexane. The domination of this pathway may be related to the character of transition metal used as a catalyst that prefers hydrogenation of aromatic ring [37] and to the fact that the reactions were run at low temperatures and low pressures, at which the energy needed for hydrogenation of aromatic ring is lower than that needed to break the bond between the carbon atom from the aromatic ring and the methoxyl group or the oxygen atom and carbon from the methyl group [5]. A general conclusion is that in the HDO reaction of anisole catalyzed by SBA-12 silica with deposited ruthenium atoms the hydrogenation of aromatic ring is preferred over demethylation and demethoxylation. The product forming in the largest amount is methoxycyclohexane with higher H/C ratio to be achieved in the process. The teams of Shi [49], Khromov [50] and Sankaranarayanan [5] studied also HDO of anisole and reported the highest selectivity to methoxycyclohexane.The plots illustrating the influence of the reaction time, the amount of catalyst, catalyst loading, pressure and temperature of the reaction are shown below.Preliminary studies of hydrodeoxygenation of anisole were carried out to establish the optimum time of the process. The reaction was carried out for 1, 2.5 or 4 h at 110 \u00b0C, under hydrogen pressure of 60 bar and in the presence of 0.05 g SBA-12 containing 1 or 3 wt.% of ruthenium. Decomposition of the main products of the reaction and the degree of anisole conversion as a function of time are presented in Fig. 6, while the selectivities to the side products are given in Table 2 (entries no. 1, 3, 5 for the catalyst 1 wt.% RuSBA-12, and 2, 4, 6 for the catalyst 3 wt.% RuSBA-12). The results unambiguously show that the quantitative composition of the products and anisole conversion depend significantly on the time of the HDO process. In the experiment with 1% RuSBA-12 catalyst the time of reaction had a great effect on the anisole conversion, which increased from 24 % after 1 h to 93 % after 2.5 h, to 100 % after 4 h (Fig. 6a). Moreover, the catalyst became increasingly selective towards methoxycyclohexane with time of the reaction because the amount of this compound increased. A similar correlation was observed for the process with 3% RuSBA-12, for which after 4 h of the process the selectivity towards methoxycyclohexane increased to 96 % (Fig. 6b). Using the 3% RuSBA-12 catalyst, the degree of anisole conversion was smaller than in the presence of 1% RuSBA-12. The conversion of anisole was already high - 93 % after 1 h of the process, then after 2.5 h it increased to 95 % and after 4 h it was 100 %. Greater activity of 3% RuSBA-15 catalyst after 1 h of the process was a direct consequence of a greater content of metal with respect to the substrate. Based on the results of these experiments, we decided to carry out further experiments for 4 h.At the next stage, our aim was to establish the optimum amount of the catalyst used in the process. The HDO of anisole was performed at 110 \u00b0C under a hydrogen pressure of 60 bar for 4 h, in the presence of 0.025, 0.05 or 0.1 g of samples 1% RuSBA-12 or 3% RuSBA-12. Analysis of HDO of anisole in the presence of sample 1% RuSBA-12 (Fig. 7a) (Table 2, items 5, 7, 9) reveals that the greatest catalytic activity was obtained for the catalyst amount of 0.05 g. The use of catalyst in this amount gave the maximum conversion of anisole to 94 % of methoxycyclohexane. The reaction in the presence of 0.025 or 0.1 g of the catalyst led to the formation of noticeable amounts of 1-methoxycyclohexene \u2013 a compound of a lower content of hydrogen than methoxycyclohexane \u2013 and the degree of anisole conversion was lower than when an intermediate amount of catalyst was used, i.e., 0.05 g. When 0.1 g catalyst was used in the reaction, a decrease in the anisole conversion and a decrease in the selectivity to methoxycyclohexane were observed compared to the reaction catalyzed by 0.05 g of silica. A decrease in the catalyst activity can be attributed to the physico-chemical properties of the catalyst, and of course the reaction conditions according the Sabatier principle stating that the bond strength between reactants and catalyst should be intermediate (i.e., not too weak, so that the reactants adsorb on the catalyst surface but not too strong to avoid, its poisoning).For the processes in the presence of 3% RuSBA-12 (Fig. 7b) (Table 2, entries no. 6, 8, 10) no significant effect of the amount of the catalyst was observed as in all reactions the selectivity to methoxycyclohexane was greater than 90 % and the anisole conversion was maximum. Based on these results, we decided to use in the further study the catalyst in the amount of 0.05 g.The studies aimed at establishment of the optimum reaction time and optimum amount of the catalyst also provided information on the activity of the catalysts used, 1% RuSBA-12 or 3% RuSBA-12, differing in ruthenium loading. The more selective catalyst with higher anisole conversion was 3% RuSBA-12. Its higher activity can be directly related to the greater content of metal on the support surface. Therefore, further studies on the effect of pressure and temperature on HDO of anisole were performed with the catalyst 3% RuSBA-12.The influence of hydrogen pressure on the distribution of the reaction products and the degree of anisole conversion was studied for three pressure values, 25, 40 or 60 bar at three temperatures 90 \u00b0C (Fig. 8a), 110 \u00b0C (Fig. 8b) or 130 \u00b0C (Fig. 8c). Table 2 gives the amounts of products of HDO of anisole besides the main ones (entries no. 6, 11\u201318). The reaction was performed for 4 h using 0.05 g of 3% RuSBA-12 catalyst. The results imply a significant effect of hydrogen pressure on the character of HDO of anisole. For each temperature, the conversion of anisole increased with increasing hydrogen pressure. Under the hydrogen pressure of 60 bar the aromatic ring hydration occurred in the highest degree. It resulted in the formation of a large amount of methoxycyclohexane and small amount of 1-methoxycyclohexene, which is particularly evident for the processes carried out at 110 and 130 \u00b0C. For the reactions performed at 90 \u00b0C, the change in hydrogen pressure had practically no effect on the distribution of the main reaction products.The influence of temperature on HDO of anisole on its conversion and selectivity of the reaction towards individual reaction products is illustrated in Fig. 9a\u2013c. The experiments were performed at 90, 110 or 130 \u00b0C under hydrogen pressures of 25 bar (Fig. 9a), 40 bar (Fig. 9b) or 60 bar (Fig. 9c). Table 2 presents the amounts of the reaction products other than the main ones (entries no. 6, 11\u201318). These data show that the reaction temperature, similarly as pressure, has a significant influence on the character of the process. At each pressure an increase in temperature caused greater conversion of anisole. However, temperature changes had no effect on the amounts of the particular reaction products. The catalyst became more selective with increasing temperature, which is well visible for the processes at 130 \u00b0C.As can be seen from Fig. 9a\u2013c, anisole conversion at 90 \u00b0C is very low and the main products are 1-methoxycyclohexene and methoxycyclohexane. At higher temperature, 110\u2212130 \u00b0C, the amount of generated cyclohexane increases (Table 2), while the amount of 1-methoxycyclohexene decreases (Fig. 9a\u2013c). In other words, the Caryl\nO bonds cleave and anisole is converted preferentially to cyclohexane at higher temperature, which indicates the occurrence of the reaction according to the pathways I and II in Fig. 5.The main reaction product in our experiment was methoxycyclohexane - chemical compound with a higher H/C atom ratio than the starting substrate. According to the literature, methoxycyclohexane is very often obtained in this reaction regardless of the type of catalyst used. In accordance with the data collected in the Table 3\n (examples 2 and 6), scientists also reported methoxycyclohexane as the main product, however the reaction temperature used was much higher. Moreover, in our experiments, methoxycyclohexane was obtained with higher selectivity and higher conversion of anisole. Some research groups have reported deoxygenated chemicals (Table 3, examples 1, 3, 4, 7, 8), however, several factors contributed to this, such as harsher process conditions and the presence of acid centers. The table below collects more examples from the literature, where anisole was subjected to the hydrodeoxygenation reaction.The reaction of hydrodeoxygenation of chemical compounds obtained from biomass degradation is generally considered as a new and attractive method for production of biofuels. Further research needs to be conducted to optimize the process and make it energetically efficient. Studies of hydrodeoxygenation of anisole, chosen as a representative of organic compounds present in biooil, catalyzed by RuSBA-12 at varying pressures (25\u221260 bar) and temperatures (90\u2212130 \u00b0C), permitted the following conclusions: (I) ordered mesoporous silica such as SBA-12 with ruthenium deposited on their surface are highly active in HDO of anisole, (II) larger content of ruthenium on the support surface is directly related to a higher conversion of anisole, (III) the product obtained in the highest amount, irrespectively of the reaction conditions, was methoxycyclohexane, (IV) the pressure and temperature of the process have significant effect on the degree of anisole conversion and the selectivities to the reaction products. The yield of the process increased with increasing pressure and temperature.Briefly, Ru catalyst catalyzed aromatic CO bonds to produce aromatic hydrocarbons due to its oxophilicity. The strong metal oxophilicity of Ru favors the direct Caryl\nO bond cleavage to benzene, while the weak oxophilic catalyst (e.g. Pt/SiO2) favors the aliphatic Calkyl\nO bond breaking to phenol. Both demethylation and demethoxylation occur over the moderately oxophilic Ru catalysts.To Maria, a very outstanding scientist, sincere thanks for all she has done and will continue to do in the future in the field of heterogeneous catalysis. Thank you, for being always ready to share your knowledge.The authors wish to thank the National Centre for Science for financial support of the studies reported within the research project HARMONIA-5 (no. DEC-2013/10/M/ST5/00652).", "descript": "\n                  Hexagonally ordered mesoporous silica SBA-12 catalysts containing various amounts of Ru (1 or 3 wt.%) were obtained by wet impregnation. These catalysts were thoroughly characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and low-temperature nitrogen adsorption/desorption (N2 ads/des). Anisole conversion was measured over catalysts at different process conditions, where the process temperature was 90\u2212130 \u00b0C and the hydrogen pressure \u2013 25\u221260 bar. Prior the experiments the process was optimized, i.e., the amount of catalyst used in the reaction and the time of reaction were adjusted. This study shows a significant effect of hydrogen pressure and process temperature on the hydrodeoxygenation of anisole, the conversion of which increased (16\u2013100 %) with both increasing parameters. For all the catalysts studied, the highest selectivity was obtained for two main reaction products, methoxycyclohexane and 1-methoxycyclohexene. Along with increasing conversion of anisole, the selectivity to the main reaction product - methoxycyclohexane generally increased (65\u201396 %) reducing the amount of 1-methoxycyclohexene (0\u201327 %).\n               "}
{"full_text": "Diesel engines commonly have higher thermal efficiency than gasoline engines due to their higher compression ratio, without throttle loss and low pump gas loss characteristics. Additionally, since diesel engines have low-speed torque characteristic advantages, they are widely used in non-road mobile machinery such as engineering machinery, agricultural machinery, generating set, ship and train moving mechanical equipment (Pirjola et al., 2017; Hu et al., 2021). It is well known that increasing the compression ratio raises the combustion temperature, which eventually results in serious emissions (Mohiuddin et al., 2021; Saxena and Maurya, 2017). It's much worth noting that non-road mobile machinery has a large amount of ownership (Feng et al., 2022), covers a wide range of industries, the working conditions are complex and harsh environment, and there are mobile operation and trans-regional operation characteristics, all of which will considerably exacerbate the emissions. The main pollutants discharged include CO, NOX, solid PM, and HC, which will cause serious air pollution (Kumar et al., 2021). The emissions will pose a major threat to human health and the climatic environment. Moreover, the emissions will induce extremely significant ailments in individuals, such as immune system destruction, influence the blood supply and impair the respiratory tract (Margaryan, 2021; Marco et al., 2019; Dong et al., 2021). World Health Organization (WHO) shows that air pollution kills an estimated seven million people worldwide every year and 9 out of 10 people breathe air that exceeds WHO guideline limits containing high levels of pollutants (World Health Organization, 2020). Carbon dioxide (CO2) was not considered as a pollutant in the past, but the excessive use of carbon-containing fossil fuels has unbalanced the earth's carbon cycle and intensified the \u201cgreenhouse effect\u201d. So CO2 emissions have also drawn much global attention. For example, the United States (US) Energy Information Administration (EIA) estimates that diesel fuel consumption resulted in 456 million metric tons of CO2 emissions in 2019 (Energy Information Administration, 2021). This amount was equal to about 24% of total US transportation sector CO2 emissions and nearly 9% of total US energy-related CO2 emissions in 2019, and diesel-fueled machinery is a major source of harmful pollutants. Similarly, according to 2014 statistics from the US Environmental Protection Agency (EPA) (United States Environmental Protection Agency, 2021), non-road diesel machinery in the US contributes over 35% of NOX and 44% of PM emissions from mobile sources. According to the European Environment Agency (EEA) (European Environment Agency, 2021), non-road PM emissions account for 17% of total emissions. In China, the number of non-road diesel engines is far less than on-road engines. The emission standards of road diesel and gasoline engines are much higher than those of non-road diesel engines. Non-road pollution sources have gradually become an important source of air pollution and have become more and more serious (China Mobile Source Environmental Management Annual Report (2020), 2021). According to the Ministry of Ecology and Environment released \u201cChina Mobile Source Environmental Management Annual Report (2020)\u201d (Ministry of Ecology and Environment, 2020), 2019 in China, the HC, NOX, and PM emissions of non-road diesel engines were 43.50 million tons, 4.93 million tons and 240 thousand tons, respectively. The NOX emissions of non-road diesel engines were the same as those of on-road motor vehicles. As a result, non-road pollutant emissions have become a major global problem.More and more countries in the world attach importance to pollution and formulate more stringent emission regulations for non-road diesel engines (Olabi et al., 2020; Ni et al., 2020). The European Commission Stage V and the US EPA Tier 4 Final standards have the same emissions limits (European Commission, 2016). China has formulated the national standard GB 20891-2014 \u201cLimits and Measurement Methods for Exhaust Pollutants from Diesel Engines of non-road Mobile Machinery (CHINA III, IV)\u201d for non-road diesel engines (GB 20891-2014, 2014). And the China IV emission standards will be implemented after December 1, 2022.In order to cope with the environmental pollution pressure and increasingly strict emission regulations, various energy conservation and emission reduction technologies are inevitably applied to non-road diesel engines (Venu et al., 2019; Hamedi et al., 2021; Datta and Mandal, 2016). In recent years, non-road diesel engine emissions have been alleviated to a certain extent through the research of experts and scholars (Boccardo et al., 2019; Duraisamy et al., 2019; Frosina et al., 2015; Ganesh et al., 2019). Like the road diesel engine, the exhaust emission reduction technologies of non-road diesel engines include internal purification and external purification technology. The internal purification technology mainly improves the engine combustion efficiency to reduce the harmful gas composition in the tail gas (Wu et al., 2021; Agrawal et al., 2019; You et al., 2020). At present, the main internal purification technologies include intake management technology, combustion technology (Wang et al., 2020), electronic controlled combustion injection technology, multi-valve technology and exhaust gas recirculation (EGR) (Wang et al., 2019). In recent years, dual-fuel diesel engines have made great progress. Ning et al. (2020) showed that the addition of primary alcohol fuels in dual-fuel mode reduces CO and soot emissions, but the total hydrocarbon (THC) and NOX emissions increase (Lee et al., 2020). Nag et al. (2022) investigated the co-combustion characteristics of hydrogen (H2)-diesel dual-fuel with EGR, and found that the synergistic action of H2 and EGR has a higher emission reduction potential, which can reduce NOX by more than 38% (Nag et al., 2019). However, adding H2 to a diesel engine will also face overwhelming challenges. For example, Kumar et al. (2021) found that adding H2 to the diesel engine was conducive to reducing carbon-based emissions, while Karthic et al. (2020) reported that the engine is prone to knocking at a higher H2 level. Kim et al. (2021) found that natural gas substitution significantly reduces PM and NOX, but at the same time increases CO and unburned hydrocarbon (UHC) emissions. As we all know, EGR can effectively reduce NOX emissions (Ayhan et al., 2020), but engine combustion is easy to deteriorate, which will increase HC, CO, and soot emissions (Pradelle et al., 2019). Zhang et al. (2021) found that EGR can reduce oxygen (O2) and hydroxide radical (OH) concentration and flame temperature, thus reducing O2 and OH oxidation rate on the soot surface, so it will increase soot emissions, especially when the EGR rate is high. Recently, homogeneous charge compression ignition (HCCI) and reactivity controlled compression ignition (RCCI) had been introduced into non-road diesel engines due to the advantages of higher thermal efficiency, lower NOX and lower PM (Duan et al., 2021; Reitz and Duraisamy, 2015).Although internal purification technologies can effectively reduce the emissions of non-road diesel engines to a certain extent, the exhaust aftertreatment technology is widely used in non-road diesel engines because the internal purification technologies cannot meet the requirements of relevant emission regulations. The exhaust aftertreatments mainly include SCR (Jiang and Li, 2016), DOC (Tan et al., 2019), DPF (Kang et al., 2018; Zhang et al., 2020), POC and ASC. NOX emissions can be significantly removed by SCR (Lauren et al., 2020), while DOC is commonly used to decrease CO and HC emissions as well as small amounts of particles. PM is a very harmful exhaust emission and most of it can be purified by DPF or POC. The DPF is a wall-flow filter that forces airflow through the porous wall to capture particles by alternately blocking the inlet and outlet of the current-carrying hole so that a large number of particles can accumulate during the operation of the trap. Note that the PM purification efficiency of DPF can reach 90% or higher. However, a large amount of PM accumulation will block the passage and cause high back pressure in the exhaust pipe (Fang et al., 2019), finally resulting in lower engine efficiency and increased emissions. Therefore, the DPF needs continuous regeneration (Jang et al., 2017). DPF regeneration requires more fuel or electricity consumption, so diesel engines equipped with DPF are less economical. In addition, the fuel also has high requirements, because DPF is very sensitive to the sulfur in the fuel and prone to form sulfuric acid at high temperatures, high sulfur content is also easy to block DPF (Rounce et al., 2019). Unlike DPF, POC has no risk of clogging because it mainly uses a cellular channel structure with multiple folds, so there is no blockage in and out of the carrier hole, and most PM can be captured (Yao et al., 2011; Liu et al., 2012). What is more, POC has the advantages of lower back pressure and lower cost compared to DPF (Lehtoranta et al., 2007). However, POC capture efficiency of PM in emissions is only 40\u201370% and it is mainly influenced by soot loading density, the structure of POC, PM to nitrogen dioxide (NO2) ratio and exhaust temperature (Guan et al., 2016; Zhan et al., 2012).In recent years, DPF and POC have been widely used in diesel engines, usually combined with other aftertreatments. The general aftertreatment (DOC+DPF+SCR) technical line can effectively reduce the emissions of PM and NOX (Jung et al., 2019\n;\nKo et al., 2019). Guan et al. (2016) demonstrated that diesel engine emissions can be significantly reduced by using POC under different fuel injection strategies. The optimized injection strategy can further improve the POC removal rate. Rounce et al. (2019) found that using a catalytic partial flow filter as a POC, combining oxidation and filtration functions in a single unit, helped achieve greater catalytic pollutant removal capacity and significant PM filtration efficiency, reducing the pressure increase across the DPF. Feng et al. (2014) showed that POC significantly increased the NO2/NOX ratio. It can be increased by 4.5 times on average in all modes. The average reduction rate of particle number (PN), which is another display method of PM, was 61%. When the engine speed was set at 1400\u00a0rpm, the PN decreased with the increasing particle size. Liu et al. (2012) reported that POC can remove organic components from the total PM, but can only partially remove carbonaceous particles with the particle size less than 30\u00a0nm due to the honeycomb structure of POC and insufficient time to oxidize or capture solid particles. Geng et al. used a double diesel oxidation catalyst (DDOC) and a DOC closely coupled with a POC in series (DPOC) to research the PM. They found that after DDOC and DPOC treatment, when the exhaust gas temperature is high enough, the quantity and mass concentration of particles, especially nuclear particles, can be significantly reduced. However, the filtering effect of POC on PM is much lower than that of DPF (Geng et al., 2015). Rossomando et al. have shown that the removal efficiency of DPF particles below 23\u00a0nm was higher than 90%, and the highest efficiency was 99% in the range of 20\u201340\u00a0nm. Due to the high removal efficiency of DPF within the study range, the engine operating conditions had little effect on particle emissions (Rossomando et al., 2021). Ko et al. found that above 300\u00a0\u00b0C, nitric oxide (NO) was converted to NO2 by Pt catalytic oxidation reaction. The highest conversion rates appeared at 450\u00a0\u00b0C for DOC and 350\u00a0\u00b0C for DPF (Ko et al., 2019). Chen et al. (2020) demonstrated that at high DOC inlet temperatures (> 190\u00a0\u00b0C), the diesel methanol dual fuel (DMDF) model combined with DOC achieved higher DPF inlet temperature and higher NO2/PM ratio, which were conducive to passive regeneration. The evolutionary mechanism of the soot layer has an important effect on the regeneration process of DPF (Ou et al., 2021). Temperature also has an important effect on DPF regeneration (Meng et al., 2020) For instance, the soot load of 8\u00a0g/L and the regeneration temperature of 575\u00a0\u00b0C resulted in a large total emission of particulate matter downstream of DPF. When the regeneration temperature is 550\u00a0\u00b0C, the regeneration efficiency of 4\u00a0g/L and 8\u00a0g/L DPFs is moderate, and the total emission of particles is relatively low.From the above discussion, although lots of work has been done in both in-cylinder purification and aftertreatment technology, most of the studies on the emission characteristics of non-road diesel engines mainly focus on steady state conditions and separately study the emission reduction effects of DPF or POC, and most of the researches have been done on the microstructure and regeneration methods of the aftertreatment processors. However, there are few studies on coupling several aftertreatment processors together, and the coupled aftertreatment processors mainly focus on the emission of the vehicle or engine under steady-state working conditions, compared with transient working conditions. It is necessary to study the emissions of aftertreatment processors under transient conditions. What's more, most of the research on this problem focuses on on-road diesel engines. Although the proportion of non-road diesel engines is small, the emissions from these engines are very serious. Accordingly, the effects of the DOC +\u00a0DPF/POC +\u00a0SCR +\u00a0ASC integrated after-treatment system on non-road diesel emissions are contrastively studied under NRSC and NRTC simultaneously in this paper. And the difference between POC and DPF is fully compared and analyzed. The main goal of the current study is to explore the possibility and implementation of POC to replace DPF and provide valuable guidance for the emission reduction of non-road diesel engines.The experiments were performed on an in-line six-cylinder, four-stroke, turbocharging, water-cooled, non-road diesel engine, and the main parameters are listed in \nTable 1. The Chinese #0 diesel was used throughout the experiments. The fuel's density (20\u00a0\u00b0C) and kinematic viscosity (20\u00a0\u00b0C) were 829.20\u00a0kg/m3 and 4.68\u00a0mm2/s, respectively. The fuel sulfur content was <\u00a01\u00a0mg/kg, the heat value and cetane number were 43.44 MJ/kg and 52.30, respectively.In this investigation, two kinds of experiments were conducted, which were named eight-condition NRSC and NRTC tests, with two kinds of aftertreatment combinations of POC or PDF coupled with DOC, SCR and ASC. The aftertreatment combinations with POC or DPF were numbered as POC and DPF, respectively. Specifications of aftertreatments are shown in Table S1 of the Supplemental materials, and the diagram of the experimental setup is shown in \nFig. 1. The specific process was strictly regularized by China's National Standard GB 20891-2014, and the detailed test process is described as follows (GB 20891-2014, 2014):\n\n1)\nThe ambient conditions of the test were kept stable at normal temperature and pressure, namely the ambient temperature was 25\u00a0\u00b1\u00a01\u00a0\u00b0C, the pressure was 101.30\u00a0\u00b1\u00a00.10 kPa, and the ambient humidity was between 50% and 70%. Laboratory atmospheric factor fa met the condition of 0.96\u00a0\u2264\u00a0fa \u2264\u00a01.06. The intercooler temperature should be within the manufacturer\u2019s specified range \u00b1\u00a05\u00a0\u00b0C under the rated net power point, and it should not be lower than 20\u00a0\u00b0C. The intercooler\u2019s outlet temperature should be between 45\u00a0\u00b0C and 55\u00a0\u00b0C at the rated power. The fuel temperature at the inlet of the fuel injection pump should be 33\u201343\u00a0\u00b0C.\n\n\n2)\nAll laboratory instruments and sensors were properly connected. The main test instruments/sensors and their parameters are reported in Table S2 of the Supplemental materials. The exhaust sampling system and gas analyzer needed to preheat to normal operating conditions. During the test, the speed and load of the diesel engine, inlet air temperature, fuel flow rate, intake and exhaust flow rates, POC or DPF inlet temperature and outlet temperature and pressure were measured when the diesel engine was working steadily. All the volume and flow rates were converted to the standard atmospheric state of 273\u00a0K (0\u00a0\u00b0C) and 101.30 kPa. And various pollutants (PM, NOX, HC, CO, CO2) were collected by sampling and corresponding gas analyzers at the frequency of 0.10\u00a0s. Gaseous pollutants were carried out by AVL AMA I60 gas analyzer, and the smoke was measured with AVL 439 opacimeter smoke meter and AVL415S filter-type smoke meter. The opacimeter smoke degree was used as PM indicator and can be estimated by calculating the light absorption coefficient in Eq. (1)\n\n\n(1)\n\n\nk\n=\n\n\n\u2212\n1\n\n\n\n\nL\n\n\nA\n\n\n\n\nln\n\n\n\n1\n\u2212\n\n\nN\n\n\n100\n\n\n\n\n\n\n\n\nwhere, \nk\n is the absorption coefficient (m-1), N is the opacity of the smoke meter (%), \n\n\nL\n\n\nA\n\n\n is the effective optical path length (m). Specific emissions of PM can be evaluated indirectly by \nk\n.\n\n\n3)\nUnder the NRSC test, every steady operating condition ran for 10\u2009min, so that the diesel engine could run fully and stably. The exhaust gas should pass through the analyzer during the last 3\u2009min of each working condition and the analyzer output should be recorded by a tape recorder or the equivalent data acquisition system. The eight-condition points and weighting coefficient under the NRSC test are shown in \nFig. 2b and Table S3 of the Supplemental materials. According to GB 20891-2014, in Table S3, four points were selected for the rated speed. The corresponding loads were 100%, 75%, 50% and 10%. Three points were selected for the intermediate speed, and the corresponding loads were 100%, 75% and 50% respectively. The idle load is 0. And the eight-condition points correspond to the serial numbers of the NRSC in Fig. 2b.\n\n\n4)\nNRTC test baseline cycle conditions are shown in Fig. 2a. The transient test cycle lasted 1238\u2009s, and each 1\u2009s was a working condition. In order to fully study the emission characteristics of various species in transient working conditions, the computer recorded data every 0.10\u2009s, where \"H\" represents the hot start state in this study. The cold start cycle test was initiated when the temperature of the engine coolant, oil, engine treatment system and auxiliary equipment was maintained between 20\u2009\u00b0C and 30\u2009\u00b0C. After the end of the cold start, the hot immersion period of 20\u2009min was carried out immediately, and then the hot start cycle test was carried out. The final specific emission results were 90% weight of the hot-start cycle and 10% weight of the cold-start cycle.\n\n\nThe ambient conditions of the test were kept stable at normal temperature and pressure, namely the ambient temperature was 25\u00a0\u00b1\u00a01\u00a0\u00b0C, the pressure was 101.30\u00a0\u00b1\u00a00.10 kPa, and the ambient humidity was between 50% and 70%. Laboratory atmospheric factor fa met the condition of 0.96\u00a0\u2264\u00a0fa \u2264\u00a01.06. The intercooler temperature should be within the manufacturer\u2019s specified range \u00b1\u00a05\u00a0\u00b0C under the rated net power point, and it should not be lower than 20\u00a0\u00b0C. The intercooler\u2019s outlet temperature should be between 45\u00a0\u00b0C and 55\u00a0\u00b0C at the rated power. The fuel temperature at the inlet of the fuel injection pump should be 33\u201343\u00a0\u00b0C.All laboratory instruments and sensors were properly connected. The main test instruments/sensors and their parameters are reported in Table S2 of the Supplemental materials. The exhaust sampling system and gas analyzer needed to preheat to normal operating conditions. During the test, the speed and load of the diesel engine, inlet air temperature, fuel flow rate, intake and exhaust flow rates, POC or DPF inlet temperature and outlet temperature and pressure were measured when the diesel engine was working steadily. All the volume and flow rates were converted to the standard atmospheric state of 273\u00a0K (0\u00a0\u00b0C) and 101.30 kPa. And various pollutants (PM, NOX, HC, CO, CO2) were collected by sampling and corresponding gas analyzers at the frequency of 0.10\u00a0s. Gaseous pollutants were carried out by AVL AMA I60 gas analyzer, and the smoke was measured with AVL 439 opacimeter smoke meter and AVL415S filter-type smoke meter. The opacimeter smoke degree was used as PM indicator and can be estimated by calculating the light absorption coefficient in Eq. (1)\n\n\n(1)\n\n\nk\n=\n\n\n\u2212\n1\n\n\n\n\nL\n\n\nA\n\n\n\n\nln\n\n\n\n1\n\u2212\n\n\nN\n\n\n100\n\n\n\n\n\n\n\n\nwhere, \nk\n is the absorption coefficient (m-1), N is the opacity of the smoke meter (%), \n\n\nL\n\n\nA\n\n\n is the effective optical path length (m). Specific emissions of PM can be evaluated indirectly by \nk\n.Under the NRSC test, every steady operating condition ran for 10\u2009min, so that the diesel engine could run fully and stably. The exhaust gas should pass through the analyzer during the last 3\u2009min of each working condition and the analyzer output should be recorded by a tape recorder or the equivalent data acquisition system. The eight-condition points and weighting coefficient under the NRSC test are shown in \nFig. 2b and Table S3 of the Supplemental materials. According to GB 20891-2014, in Table S3, four points were selected for the rated speed. The corresponding loads were 100%, 75%, 50% and 10%. Three points were selected for the intermediate speed, and the corresponding loads were 100%, 75% and 50% respectively. The idle load is 0. And the eight-condition points correspond to the serial numbers of the NRSC in Fig. 2b.NRTC test baseline cycle conditions are shown in Fig. 2a. The transient test cycle lasted 1238\u2009s, and each 1\u2009s was a working condition. In order to fully study the emission characteristics of various species in transient working conditions, the computer recorded data every 0.10\u2009s, where \"H\" represents the hot start state in this study. The cold start cycle test was initiated when the temperature of the engine coolant, oil, engine treatment system and auxiliary equipment was maintained between 20\u2009\u00b0C and 30\u2009\u00b0C. After the end of the cold start, the hot immersion period of 20\u2009min was carried out immediately, and then the hot start cycle test was carried out. The final specific emission results were 90% weight of the hot-start cycle and 10% weight of the cold-start cycle.It is necessary to convert the gaseous emission concentration (ppm) into specific emission [g/(kW\u00b7h)] at each operating condition. The exhaust gas flow was measured with air flow meters. Then, a tail gas analyzer was used to detect the concentration of exhaust emissions under various working conditions. After that, the mass flow rate of each exhaust emission was calculated by formulas (2)\u2013(4). The emission calculation method (formula) in this paper came from China's National Standard GB 20891-2014 (GB 20891-2014, 2014).\n\n(2)\n\n\n\n\n\n\n\n\nNO\n\n\nx\n\n\n\n\n\n\nmass\n,\ni\n\n\n=\n0.001587\n\u00d7\n\n\nK\n\n\nH\n\n\n\u00d7\n\n\n\n\n\n\nNO\n\n\nx\n\n\n\n\n\n\nconc\n,\ni\n\n\n\u00d7\n\n\nG\n\n\nEXHW\n,\ni\n\n\n\n\n\n\n\n\n(3)\n\n\n\n\nHC\n\n\nmass\n,\ni\n\n\n=\n0.000479\n\u00d7\n\n\nHC\n\n\nconc\n,\ni\n\n\n\u00d7\n\n\nG\n\n\nEXHW\n,\ni\n\n\n\n\n\n\n\n\n(4)\n\n\n\n\nCO\n\n\nmass\n,\ni\n\n\n=\n0.000966\n\u00d7\n\n\nCO\n\n\nconc\n,\ni\n\n\n\u00d7\n\n\nG\n\n\nEXHW\n,\ni\n\n\n\n\n\nwhere, \n\n\nG\n\n\nEXHW\n,\ni\n\n\n is the exhaust flow rate (kg/h) at the \ni\n working condition; \n\n\nK\n\n\nH\n\n\n is the humidity correction coefficient of \n\n\nNO\n\n\nx\n\n\n. \n\n\n\n\n\n\nNO\n\n\nx\n\n\n\n\n\n\nconc\n,\ni\n\n\n, \n\n\nHC\n\n\nconc\n,\ni\n\n\n and \n\n\nCO\n\n\nconc\n,\ni\n\n\n are the concentrations (ppm) of \n\n\nNO\n\n\nx\n\n\n, \nHC\n and \nCO\n at the \ni\n working condition respectively;\n\n\n\n\n\n\n\n\nNO\n\n\nx\n\n\n\n\n\n\nmass\n,\ni\n\n\n\n, \n\n\nHC\n\n\nmass\n,\ni\n\n\n and \n\n\nCO\n\n\nmass\n,\ni\n\n\n are the mass flow (g/h) of \n\n\nNO\n\n\nx\n\n\n, \nHC\n and \nCO\n at the \ni\n working condition, respectively.Then, the mass flow rates of exhaust pollutants which calculated by Eqs. (2)\u2013(4) were substituted into Eq. (5) to calculate the weighted specific emissions of each component under various working conditions.\n\n(5)\n\n\n\n\nA\n\n\ni\n\n\n=\n\n\n\n\nA\n\n\nmass\n,\ni\n\n\n\u00d7\n\n\nWF\n\n\ni\n\n\n\n\n\n\u2211\n\ni\n=\n1\n\n\nn\n\n\n\n\n\n\n\nP\n\n\n(\nn\n)\ni\n\n\n\u00d7\n\n\nWF\n\n\ni\n\n\n\n\n\n\n\n\n\n\nwhere, \n\n\nA\n\n\nmass\n,\ni\n\n\n is the mass flow rate (g/h) of A species under the \ni\n working condition; \n\n\nWF\n\n\ni\n\n\n is the weighting coefficient of the \ni\n working condition; \n\n\nP\n\n\n(\nn\n)\ni\n\n\n is the net power of \ni\n working condition;\n\n\n\n\nA\n\n\ni\n\n\n\n is the specific emission [g/(kW\u2009h)] of some gaseous pollutant under the \ni\n working condition.The specific emission of PM was calculated by the filter paper weighing method. For each operating point of the engine, the exhaust gas was passed through the filter paper for a period at a specific engine power. Then, the mass of the filter paper was weighed and the value was recorded. The unit of mass was \u03bcg, which should be converted to g/(kW\u2009h).The experimental error and validity were accessed. The test error was examined through the original emission measurement (without an aftertreatment system). The error came from the devices and random during the experiment. The error estimation method was adopted from Wu et al. (2020). The detailed accuracy of devices can be found in Table S2 of the Supplemental materials in the appendix. The random error was estimated by repeated tests, three times for the original emission measurement. The relative errors of CO, HC, NOX, and PM are 4.3%, 2.4%, 2.4% and 3.7%, respectively. And the error bar was added into the figures. Note that error bars have not been added to the transient result curves for clearer readability.The conversion rate and specific emissions between POC and DPF under the NRSC test are shown in \nFig. 3. It can be seen from the figure that being equipped with an aftertreatment system can effectively reduce emissions. The overall emissions of PM, NOX, HC and CO are all lower than the China IV, US, and European Union (EU) standards emission limits. So, it is very necessary to add aftertreatment technology to non-road diesel engines. From the perspective of conversion rate, the DPF conversion efficiency of PM, NOX, HC, and CO emissions are 87%, 98.50%, 98%, and 75.50%, respectively. While the POC conversion efficiency of PM, NOX, HC, and CO emissions are 60%, 96.74%, 95.27% and 77.78%, respectively. This is mainly related to the internal structure and operating conditions of POC and DPF. In the test, we just did the same test after replacing POC with DPF (operating for 10\u2009min in each operating condition and collecting data in the last three minutes). The catalyst and the catalyst carrier have not been changed, but the plugging of the POC outlet end has been added. The emissions of NOX and HC are very close and their conversion rate exceeds 95%, almost achieving zero emissions. However, the total CO emissions of POC are lower than DPF, and the conversion efficiencies of POC are higher than DPF. Noble metal catalysts (such as Pt and Pd) have priority to oxidize CO. The affinity of DOC for HC and CO is different. The surface catalyst of DOC can adsorb CO and HC, but CO reacts before HC, and the reaction rate of HC is lower than that of CO (Wang et al., 2008), and POC can further oxidize CO and HC to a certain extent. At the same time, the working conditions of the system also have an important impact on the emission of pollutants, as shown in \nFig. 4. The detailed emission process will be discussed in detail in the next section.Non-road diesel vehicles are an important source of PM emissions (Dhital et al., 2021; Ganesh et al., 2019). The main components of PM include dry soot (DS), soluble organic fraction (SOF) and sulfate. The main reason is that oxygen-deficient fuel will undergo cracking and dehydrogenation under high temperature and pressure environment, and finally form soot particles. These soot particles will absorb various unburned or incompletely burned heavy HC and other condensed-phase substances during the cooling process and constitute the PM. Therefore, reducing PM emissions has always been a focus of research. Fig. 4a shows that the filtration effect of POC and DPF is affected by engine speed and load. The PM concentration at 2200\u2009rpm is higher than that at 1400\u2009rpm, and PM increases with the decrease of load, and the maximum occurs at 50% load. At the same time, in the third working condition (2200\u2009rpm and 50% load, see Fig. 2b), both POC and DPF have the largest contribution rate (the proportion of each working condition), and they are 26.07% and 27.80%, respectively. At the rated speed, the PM contribution rate decreases with the increase of the load. At medium speed, the PM contribution rate increases with the increase of the load. This is mainly due to the low combustion temperature of low load and the richer mixture of high load, which results in fuel incomplete combustion and reduces soot oxidation rate. Meanwhile, as can be seen from Fig. 4a, the line of POC is always above DPF. That is to say, the purification conversion efficiency of DPF is higher than that of POC in each working condition. So DPF has more advantages in reducing PM than POC. For example, at 1400\u2009rpm, the purification effect of DPF is greatly improved compared to POC, and the maximum increase is 76.51% at 100% load. That means that DPF is more conducive to reducing PM at medium speed and high load. The filtration efficiency is mainly related to pressure drop. The highly porous wall will show low pressure drop and thus low filtration efficiency (Konstandopoulos et al., 2012). The exhaust temperature has a similar trend relative to the pressure drop (Zheng and Banerjee, 2009), as shown in Fig. 4e and f, the pressure drop increases with the increase of exhaust temperature in DPF and POC channels, so the PM purification efficiency of fifth working condition (1400\u2009rpm and 100% load) is greater than that of other working conditions.NOX is an important environmental pollutant. NOX undergoes a series of chain photochemical reactions under strong sunlight to generate ozone (O3) (Li and Cocker, 2018) and peroxyacetyl nitrate (PAN) (Sun et al., 2021), that is, photochemical smog of secondary pollutants (photochemical smog) (Sun et al., 2020). Therefore, it is also crucial to reduce NOX emissions. SCR is currently recognized as the most effective and promising technology for reducing NOX (Kang et al., 2018), and the purification efficiency of SCR on NOX is as high as 91.6% (Hu et al., 2021). At present, SCR is based on V2O5/WO3/TiO2 as a catalyst, and urea aqueous solution as a reducing agent is widely used to reduce NOX (Jung et al., 2017). Fig. 4b shows that NOX emissions decrease with load decreasing, and the NOX emissions of DPF are always lower than those of POC at the same speed and load. For example, NOX emissions of working condition 1 (2200\u2009rpm and 100% load) and working condition 5 (1400\u2009rpm and 100% load) are much higher than that of other working conditions. This is because the average air-fuel ratio (A/F) of the combustible mixture reduces as the engine load increases, causing an increase of the maximum combustion temperature (Nabi et al., 2021). At low engine load, although the excess air coefficient is larger, the lower combustion temperature is not conducive to the production of NOX (Deng et al., 2019). So, high load and high temperature are more conducive to the production of NOX, while the influence of speed on NOX is smaller than that of load.CO is an intermediate product produced in the combustion reaction process. It is mainly produced by partial lack of oxygen due to uneven mixing of mixed gases (Zhu et al., 2018). And too low temperature will also lead to incomplete combustion and increase CO emission. According to the latest study by Sun et al., they found that the HC and soot emissions of natural gas (NG) SI engine were much lower with hydrogen addition. In addition, the energy efficiency and fuel economy of the NG SI engine were improved with hydrogen addition (Sun et al., 2022). This provides a new idea for reducing CO in non-road diesel engines. Fig. 4c shows the CO emission characteristics under NRSC. It can be seen that CO emissions increase with load increases. The smallest CO emissions occur at idle speed, while the largest CO emissions occur at 2200\u2009rpm and full load. The difference in CO purification effects between POC and DPF is minor, and the change trend is consistent. It is important to note that the original CO emissions are very low. Although the CO conversion efficiency of POC and DPF is between 75% and 80%, the influence of load is not obvious due to the very low original CO emissions. Combustion temperature and local hypoxia are still the key factors.Diesel engines produce HC mainly due to two reasons: uneven fuel mixing and poor fuel atomization quality. As shown in Fig. 4d, HC decreases with the increase of load at 2200\u2009rpm (50\u2013100% load). The load increase makes the engine burn more thoroughly, leading to an increase in exhaust temperature (see Fig. 4e) and ultimately a reduction of HC emissions. The HC purification effect of DPF is better than that of POC. However, it should be noted that the original emission of HC is very low. The emission of HC is mainly concentrated at a higher speed (2200\u2009rpm), and the changing trend of the contribution rate is akin to the specific emission. This is due to the uneven mixture of fuel and surrounding air when at high speed in the spray area, the mixture becomes too rich and then becomes leaner gradually. And a too lean mixture is formed at the periphery of the spray before it catches fire, causing the fuel to not burn completely.\n\nFig. 5 shows the overall value-specific emissions obtained from the NRTC test. It is a weighted value of 90% hot start cycle and 10% cold start cycle. PM, NOX, HC and CO all meet the China IV emission requirements. And the specific emissions of DPF are lower than those of POC. However, from the perspective of conversion efficiency, except for the greater difference in PM conversion efficiency, other emissions conversion efficiencies are relatively close. For example, the conversion efficiencies of POC to PM, NOX, HC, and CO are 60.12%, 95.45%, 92.82%, and 79.51%, respectively, and those of DPF to PM, NOX, HC, and CO are 92.83%, 96.99%, 96.86% and 81.45%, respectively. Compared with NRSC, the conversion efficiency of NOX and HC under NRTC has a small decrease, and the conversion efficiency of CO and PM under NRTC has a small increase, which is mainly affected by the operating conditions. The original emissions of HC and CO in the steady state and transient state are relatively low and have met the emission regulation limit, so PM and NOX are our key research objects.However, it is worth noting that for the POC purification system, PM has not reached the latest emission limits of the US and EU, but is very close to them, especially the US limit. The main reason is that the China IV standards mainly refer to the EU IV standards, which are equivalent to the Tier 4 emission regulations of the US. Therefore, China's emission regulations are one or two stages later than those of the US and EU. In order to meet stricter emissions regulations, the mainstream research route for the non-road diesel engine is still carried out in two aspects: internal purification and external purification technology. The internal purification technologies mainly include low-emission combustion systems, adding EGR (Yin et al., 2022; Zhao et al., 2022), low temperature combustion technology (Xu et al., 2022), fuel injection system optimization (Zhang et al., 2018), variable intake system, variable turbocharging compressor, homogeneous compression ignition technology (Zhou et al., 2022), alternative fuels (Liu et al., 2022a; Sun et al., 2022), dual-fuel (Atelge et al., 2022; Liu et al., 2022b; Tripathi et al., 2022) including adding H2 (\u015eanl\u0131 et al., 2021; Bose and Banerjee, 2012); In our another dedicated study, injection strategies (including the smoke limit strategy) were discussed. External purification technology is mainly related to the catalyst (E et al., 2020a), such as honeycomb microstructure optimization and/or improvement of catalyst dispersion, catalyst precious metal exploitation, catalyst structure optimization and continuous regeneration (E et al., 2020b; Zhao et al., 2021). Actually, the POC microstructure effect on PM reduction was discussed in our another dedicated study.In the NRTC test, we found that the original emissions of HC and CO were lower than the China IV standards emission limits, and the emissions were lower after being equipped with an aftertreatment system. So, we focused on the PM and NOX emissions, and only described the HC and CO emissions as a whole. The average value of CO during the cold start of POC is 30\u2009ppm, and the peak value is 502\u2009ppm at 45\u2009s. While the average value of CO during the cold start of DPF is 20\u2009ppm, and the peak value is 605\u2009ppm at 51\u2009s. DPF efficiency is increased by 33.33% during cold start and only 6.72% during hot start. That is mainly due to the emissions of cold start is higher than the hot start. It is caused by lower aftertreatment temperature and higher air-fuel ratio (Iodice and Senatore, 2016). At the same time, the CO reduction rates of POC and DPF during hot start are 81.50% and 82.74%, respectively. Hu et al. found that the CO oxidation efficiency of DOC during hot start is 55% (Hu et al., 2021), so POC and DPF can further improve the CO reduction efficiency. HC decreases sharply with the increase of air-fuel ratio. When the air-fuel ratio exceeds \n\n\n\u03d5\n\n\na\n\n\n=\u20091, HC will drop to the lowest value. However, if the air-fuel ratio is too large, HC will rebound again for unstable combustion. The HC fluctuating range during most time of cold start and hot start is relatively small, and the average value of the two start modes is also relatively close. However, compared with POC, the HC purification effect of DPF at cold start is increased by 71.16% on average, but it is only increased by 47.60% at hot start.Opaque smoke can evaluate PM emissions to a certain extent, and hot start accounts for 90% of the weighting ratio, so we focused on the hot start opaque smoke and NOX emissions, as shown in \nFig. 6. During cold start, DPF can keep the opaque smoke emission at a low level with small fluctuations. The change trend is akin to that of hot start and the emission is close to zero, but the POC has greater fluctuations at higher speed. Compared with cold start, the emission of hot start is reduced, the emissions of opaque smoke in hot start are shown in Fig. 6a. POC changes frequently from 0\u2009s to 300\u2009s, which is related to changes in engine speed and load. In addition, the conversion efficiency of POC is low, but DPF always has higher conversion efficiency. The main reason is that the filtration principles of ceramic POC and ceramic DPF are different. POC relies on the pressure difference between the direct flow channel and the closed-end channel to achieve the flow of airflow, and captures the particles through the wall. Unlike the previous plugging method, we added ceramic plugging at the outlet port of the POC (see Fig. 1a), and the inner wall is coated with a layer of catalyst containing Pt and Pd, which can oxidize PM into CO2, and reduce HC and CO at the same time. The strong oxidation performance of POC is mainly derived from residual oxygen and NO2. DOC can convert NO into NO2 and increase the content of NO2 in POC intake flow. NO2 has strong oxidizing properties and can oxidize soot. Both upstream and downstream of the DPF are plugged alternately and asymmetrically by ceramics (see Fig. 1a). Exhaust gas enters from the inlet port and then exits from the outlet port. The carrier is coated with a layer of catalyst containing precious metals (such as Pt, Pd), which is conducive to the adsorption of DPF.It can be seen from Fig. 6b that the opaque smoke of POC and DPF varies greatly from 725\u2009s to 760\u2009s, which is very similar to cold start. Pressure drop can well characterize the filtration efficiency (Konstandopoulos et al., 2012). So, let\u2019s look into pressure drop evolution, the pressure drop of POC and DPF during 725\u2013760\u2009s has a significant decrease (see Fig. 6f), which indicates that the exhaust resistance has decreased, which is mainly related to the exhaust flow rate and soot layer thickness. Usually, PM is filtered and regenerated in POC at the same time. If the efficiency of capture and regeneration are in balance, then the POC will work normally. But with the engine speed and load increase, PM will increase (see Fig. 6b, at high speed and load). The accumulation phenomenon will cause the catalytic efficiency of the catalyst coating to decrease, and the regeneration process will be blocked. So, PM will be discharged along the channel, and the POC conversion efficiency will decrease. When the accumulated soot is oxidized, the POC conversion efficiency will increase again. So, the catalyst coating has a substantial effect on distribution of soot on the wall. The accumulation of soot will also change the porosity and soot distribution of DPF, which will result in increase of pressure drop. Therefore, the filtration efficiency of DPF will also change, but the DPF efficiency is not significantly reduced due to two-end alternated blocking in the DPF structure. Fig. 6e also finds that after 820\u2009s, the overall pressure drop is lower, which indicates that the filtration efficiency of POC and DPF is reduced. It can be seen from Fig. 6a (within the black circle area) that the opaque smoke of DPF has risen slowly. In other words, the filtration efficiency of DPF is reduced, however, the filtration efficiency of POC remains constant. This may be related to the particle size of PM, which is discharged from the channel as the particle size becomes smaller, resulting in an increase in PM emissions. This phenomenon requires further work to continue to explore.In the combustion process of diesel engines, NO is mainly produced, and NO accounts for up to 95% of NOX, which is mainly converted into NO2 through DOC and DPF (Liang et al., 2019; Salman et al., 2018). Due to diesel engines\u2019 large excess air coefficient (\n\n\n\u03d5\n\n\na\n\n\n), the NO2 content is generally between 5% and 15%, NO2 will affect the reduction ability of SCR and the regeneration process of PM in DPF (Tighe et al., 2012; Jiao et al., 2017). Although POC and DPF can reduce NOX emissions to a certain extent, the conversion rate is low. Note that the former DOC catalyzer oxidizes part of NO into NO2, but DOC does not reduce NOX emissions, and that only changes the NOX composition. After NO2 enters POC and DPF (Reijnders et al., 2016), the molecular bond of NO2 can be broken at a lower temperature (about 250\u2009\u2103), and the resulting O2 can be burned with the captured PM to generate CO2. Thus, PM can be effectively removed. NOX is mainly removed by SCR. The SCR catalyst used in this experiment is mainly V2O5/WO3/TiO2, which is characterized by better selectivity, wide temperature window and resistance to sulfur poisoning to NOX (Kang et al., 2019). It should be noted that the reducing agent in SCR catalytic reaction is ammonia (NH3). Since more than 90% of NOX emissions are NO, the main principle of reduction is shown as Formula (R-1). This reaction is also known as the \"standard SCR reaction\", and O2 is essential. The reaction rate of Formula (R-2) is 17 times faster than that of formula (R-1) at low temperature, which is called \"fast SCR reaction\". It is beneficial to improve the low temperature activity of catalysts and solve the problem of low exhaust temperature of the diesel engine. Practical studies show that when the NO/NO2 value is 1 (Nova et al., 2006; Ciardelli et al., 2007), the optimal NOX conversion efficiency can be achieved. When the NO2 ratio is too high, the conversion efficiency will decrease. That is mainly due to the increase of (R-3) reaction, but the reaction rate is very slow (Hu et al., 2018). Ko et al. (2019) similarly found that when the ratio of NH3/NOX is 1, SCR has the highest efficiency. Nevertheless, too high NO2 in POC and DPF would worsen the reduction of SCR.\n\n(R-1)\n4NO+4NH3+O2=4N2+6H2O\n\n\n\n\n(R-2)\n2NH3+NO+NO2=2N2+3H2O\n\n\n\n\n(R-3)\n6NO2+8NH3=7N2+12H2O\n\n\nThere is a clear cutoff point for NOX emissions during the cold start. From 0 to 400\u2009s, there is a large NOX emission, and the NOX emissions from 400\u2009s to the end of the test are very low, almost reaching zero emission. This is mainly related to the inlet temperature of SCR. The best working temperature of SCR is 250\u2009\u00b0C\u2013450\u2009\u00b0C, and the inlet temperature of SCR is only close to 250\u2009\u00b0C at 300\u2009s during cold start. The hot-start emission of NOX is shown in Fig. 6c. The NOX conversion efficiency of DPF and POC is very close, and the trend of change is also very similar. For example, after 300\u2009s, the NOX of DPF always remains at a low level, and the NOX of POC fluctuates frequently, but the peak value is relatively small. In order to have a clearer understanding of NOX emissions, we have partially enlarged the view of Fig. 6c, as shown in Fig. 6d. It can be seen that the NOX peak values of POC and DPF during hot start are 361\u2009ppm at 96\u2009s and 345\u2009ppm at 94\u2009s, respectively, and that they are much smaller than the peak emission of cold start. That is mainly due to the engine has a 20-minute hot soak period after cold start. Therefore, the SCR inlet temperature decreases, and the SCR purification ability decreases. So there will be a large NOX emissions peak. After the temperature gradually rises, NOX emissions have a significant drop, and the SCR plays a major role in purification. However, the engine load has an important influence on NOX emissions. In the black part of Fig. 6d, the large fluctuation of NOX is related to the sharp decrease of engine load. For example, in the red circle area, both cold start and hot start have similar changes, but when the load is dropped quickly from 97% to 0%, the speed is only slightly reduced, and the NOX emissions of POC and DPF both increase slightly, but POC has more NOX emissions.In this paper, the DOC +\u2009DPF/POC +\u2009SCR +\u2009ASC coupling aftertreatment system is adopted to reduce non-road emissions in view of the advantages of non-road diesel engines and increasingly strict non-road emissions regulations. It is necessary to save costs while improving the emissions of non-road diesel engines in order to reduce the pollution of non-road diesel engines. Therefore, the influence of a coupled aftertreatment system on non-road diesel engine emissions under NRSC and NRTC (including hot start cycle and cold start cycle) conditions was discussed in detail, and the performance and feasibility of aftertreatment were evaluated and analyzed. Some important findings are summarized below:\n\n1)\nDOC +\u2009DPF/POC +\u2009SCR +\u2009ASC can effectively reduce the emissions of non-road diesel engines, but the two aftertreatment systems are significantly affected by engine speed and load. Under steady-state conditions, DPF can reduce PM, NOX, and HC emissions more than POC, without producing more CO2. At high speed and high load, more PM will be generated. DPF is more conducive to reducing PM than POC, and the conversion rate is 87% and 60% respectively. NOX and HC conversion rates of DPF and POC are relatively close, both are above 95%. DPF and POC also have higher CO conversion rates which are more than 75%, and POC conversion rate of CO is slightly higher than DPF.\n\n\n2)\nUnder transient conditions, the cold start cycle emissions of PM, NOX, HC, and CO are much larger than the hot start cycle emissions, and the engine load is the main influencing factor. DPF and POC can effectively reduce PM emissions, and the PM conversion rate is 92.83% and 60.12%, respectively. The transient PM conversion rate of DPF is 5.83% higher than that of steady state, but NOX and HC are reduced by about 3%. For HC emission, from 400\u2009s to 1238\u2009s, the DPF purifying effect of the cold start is improved by 71.16% compared with POC. But the DPF purification effect of hot start is improved by 47.60% compared with POC, and the peak of NOX emission usually occurs at the position of load mutation. The DPF conversion of CO is slightly higher than POC.\n\n\n3)\nPOC and DPF have similar filtering effects on NOX, HC, and CO emissions, and can meet the China IV standards emission limits. For PM, although the filtration efficiency of POC is lower than that of DPF, it can still meet the China IV standards emission limits. Moreover, DPF needs to accurately control the regeneration conditions and regeneration frequency, which are prone to increase back pressure, long development period, and high operating cost. Therefore, POC can save costs while reducing emissions, and POC has great potential to replace DPF.\n\n\nDOC +\u2009DPF/POC +\u2009SCR +\u2009ASC can effectively reduce the emissions of non-road diesel engines, but the two aftertreatment systems are significantly affected by engine speed and load. Under steady-state conditions, DPF can reduce PM, NOX, and HC emissions more than POC, without producing more CO2. At high speed and high load, more PM will be generated. DPF is more conducive to reducing PM than POC, and the conversion rate is 87% and 60% respectively. NOX and HC conversion rates of DPF and POC are relatively close, both are above 95%. DPF and POC also have higher CO conversion rates which are more than 75%, and POC conversion rate of CO is slightly higher than DPF.Under transient conditions, the cold start cycle emissions of PM, NOX, HC, and CO are much larger than the hot start cycle emissions, and the engine load is the main influencing factor. DPF and POC can effectively reduce PM emissions, and the PM conversion rate is 92.83% and 60.12%, respectively. The transient PM conversion rate of DPF is 5.83% higher than that of steady state, but NOX and HC are reduced by about 3%. For HC emission, from 400\u2009s to 1238\u2009s, the DPF purifying effect of the cold start is improved by 71.16% compared with POC. But the DPF purification effect of hot start is improved by 47.60% compared with POC, and the peak of NOX emission usually occurs at the position of load mutation. The DPF conversion of CO is slightly higher than POC.POC and DPF have similar filtering effects on NOX, HC, and CO emissions, and can meet the China IV standards emission limits. For PM, although the filtration efficiency of POC is lower than that of DPF, it can still meet the China IV standards emission limits. Moreover, DPF needs to accurately control the regeneration conditions and regeneration frequency, which are prone to increase back pressure, long development period, and high operating cost. Therefore, POC can save costs while reducing emissions, and POC has great potential to replace DPF.\nRenhua Feng: Writing \u2013 review & editing. Xiulin Hu: Validation. Guanghua Li: Writing \u2013 review & editing. Zhengwei Sun: Resources. Banglin Deng: Conceptualization, Methodology.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 work is jointly sponsored by the Guangdong Basic and Applied Basic Research Foundation (Grant No. 2021A1515010481). The authors appreciate the anonymous reviewers and the editor for their careful reading and many constructive comments and suggestions on improving the manuscript.Supplementary data associated with this article can be found in the online version at doi:10.1016/j.ecoenv.2022.113576.\n\n\n\nSupplementary material.\n\n\n\n.", "descript": "\n                  Non-road emission regulations are becoming increasingly rigorous, which makes it necessary for non-road engines to adopt aftertreatment systems. The commonly used aftertreatments mainly include diesel oxidation catalytic (DOC), diesel particulate filter (DPF), particle oxidation catalyst (POC), selective catalytic reduction (SCR) and ammonia purification catalyst (ASC). The purpose of this study is to investigate the effects of using an integrated system (DOC + DPF/POC + SCR + ASC) on non-road diesel engine emissions under steady-state and transient operating conditions, respectively. The major works are the comparison between POC and DPF from the viewpoint of emission reduction. The results show that both POC and DPF can effectively reduce particulate matter (PM) and nitrogen oxide (NOX) emissions under steady-state conditions, and DPF has better purification effect than POC, especially for PM. The PM conversion rate of DPF is up to 87%, while that of POC is only 60% under the non-road steady-state test cycle (NRSC). Both NOX and hydrocarbon (HC) conversion rates are high, exceeding 95%. The conversions of PM, NOX, HC, and carbon monoxide (CO) of DPF in the non-road transient test cycle (NRTC) are 92.83%, 96.99%, 96.86% and 81.45%, respectively, while those of POC are 60.12%, 95.45%, 92.82% and 79.51%, respectively. Both the POC and DPF systems can meet the emission regulation limits. As a result, POC has the potential to substitute DPF in non-road engines due to its lower product and maintenance costs. We hope that the comparison study will provide useful guidance for improving the emissions performance of non-road diesel engines.\n               "}
{"full_text": "\n\n\nData will be made available on request.\n\n\nData will be made available on request.Hydrogen is considered to be a fuel of the future. Pure H2 and the product of its burning are environmentally safe. H2 exhibits the highest energy capacity per weight unit among all known fuels (121 kJ/g). Nowadays, 90% of the H2 is produced via natural gas reforming or light oil fractioning during crude oil conversion [1\u20133]. However, these processes are not sustainable. The decrease of fossil fuel stocks along with the reduction of atmospheric pollution requires exploiting alternative feedstocks. Among the possible alternatives, ethanol has several advantages as a chemical source of H2. Ethanol exhibits extremely low toxicity and can be produced via fermentation of renewable biomass feed [4\u20136]. To emphasize the fact that ethanol is obtained from biomass sources, the term \u201cbioethanol\u201d is often used. A typical ethanol concentration in crude bioethanol is 7 \u2013 12 vol.% [7].Ethanol steam reforming (ESR) is an attractive, sustainable, and environmentally friendly approach to H2 production. Based on the stoichiometry principle, the ESR reaction may be represented as follows:\n\n\n\n\nC\n2\n\n\nH\n5\n\nO\nH\n+\n3\n\u00b7\n\nH\n2\n\nO\n=\n2\n\u00b7\nC\n\nO\n2\n\n+\n6\n\u00b7\n\nH\n2\n\n\n\n\n\nThe ESR reaction is endothermic. In a gas phase, enthalpy change of the reaction per mole of ethanol equals \u2206H298\u00a0=\u00a0173.4 kJ/mol, whereas \u2206H298\u00a0=\u00a0347.4 kJ/mol for the reaction in a liquid phase [8,9]. It is worth noting that the expensive distillation process, which is an essential part of the ESR-based technology, may be avoided if the crude bioethanol is directly converted into H2 by steam reforming. Highly diluted ethanol (12 vol.% of ethanol) offers a high steam/carbon ratio appropriate for the steam reforming process [10].The ESR reaction allows extracting H2 from both, ethanol (50%) and water (50%). Moreover, H2 production from bioethanol by ESR reaction does not affect the concentration of carbon dioxide in the atmosphere [11\u201313]. Considering the increasing demand for renewable energy, the industrial implementation of bioethanol steam reforming (BESR) as an alternative source of H2 is highly realistic [14]. H2 produced via steam reforming from the renewable feed may be efficiently utilized for electric energy production, especially for small-scale electricity supply. Coupling the steam reforming with a fuel cell establishes a new kind of technology for power generation. Using portable power systems based on fuel cell application simultaneously resolves the two issues of H2 use, particularly, safe hydrogen storage and its transportation [15\u201317]. In this respect, bioethanol is a promising feedstock for alternative energy production. Using diluted bioethanol significantly reduces the energy costs compared to pure ethanol reforming.In the catalytic ESR process, the choice of the appropriate catalyst is a key factor for ethanol conversion and high H2 selectivity. A wide range of catalysts was explored to obtain optimum reaction conditions [8,11,18\u201321]. Most studies have been performed over supported noble metals (Pt, Pd, Rh, Ru, Ir) and non-noble metals, e.g. Ni and Co. However, limited attention was paid to the oxide catalysts. Nevertheless, the spinel-type MnFe2O4 belongs to the best catalysts toward H2 yield (> 90%) [20]. This catalyst is relatively cheap and exhibits no transformation under reaction conditions in contrast to many other oxide catalysts.Various hydrogen generators, including portable devices, have been introduced in recent years. A small-size generator based on n-dodecane steam reforming for naval use with the H2 productivity of 1.5 Nm3/h has been reported [22]. A combined all-in-one portable generator with a PEMFC has been developed using sodium borohydride as a source of H2\n[23]. An autonomous setup for H2 production by methane steam reforming coupled to catalytic methane combustor has been developed [24]. However, the reported devices imply the use of chemical hydrides, ammonia boran [25], and hydrocarbons as a feed for H2 generation. These chemicals are typically expensive and scarcely can be produced in large amounts suitable for large-scale application of the corresponding H2 generators. In this respect, creating a portable H2 generator based on bioethanol feed is a timely topic in the development of green technologies.Previously, we have studied the ESR over the MnFe2O4 catalyst [26]. However, the raw bioethanol typically contains small amounts of other alcohols, e.g. propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol, pentan-1-ol. Due to the presence of the impurities, the peculiarities of the ESR process may be quite different compared to pure ethanol reforming. Herein, we investigate the steam reforming of the mixtures of ethanol and higher alcohols over the MnFe2O4 catalyst. The optimum parameters of the ESR process are highlighted to create a conceptual design of an autonomous catalytic H2 generator. To this end, the goal of the present paper is to introduce a conceptual design of a portable autonomous catalytic H2 generator based on steam reforming of either diluted ethanol or ethanol/higher alcohols mixture over the designed noble metal free MnFe2O4 catalyst. Taking that into account, we used water/ethanol molar ratio equal to 19/1. On one hand, that ratio is close to the water/ethanol ratio in bioethanol obtained via fermentation of biofeed [7]. On the other hand, this ratio falls within the range of water/ethanol ratios typically utilized for hydrogen production by steam reforming of diluted ethanol because using high molar excess of water in the feedstock prevents coke formation [10].The preparation of the MnFe2O4 catalyst has been performed by two different procedures. According to the first procedure, MnFe2O4 was synthesized by solvothermal decomposition of the heteronuclear complex [MnFe2O(CH3COO)6(H2O)3]\u22192H2O. A detailed description of the synthesis technique is presented in [27]. The use of this complex allows the formation of ferrite MnFe2O4 with the exact stoichiometric ratio of Fe/Mn\u00a0=\u00a02. The obtained ferrite was calcined at 250\u00b0C for 5 h and denoted as MnFe2O4-HC. Ferrite MnFe2O4 forms a stoichiometric chemical compound. The formation of an alloy during the synthesis of the catalyst and after the catalytic test was not observed, according to the XRD analysis [27]. The method of decomposition of the heteronuclear complex yields oxides of a given stoichiometric composition because the stoichiometry is a priori defined by the ratio of the metal ions in the framework of the conventional trinuclear complex with molar ratio Fe/Mn\u00a0=\u00a02, which is confirmed by atomic absorption spectroscopy [27].Another procedure admits the manganese ferrite, further denoted as MnFe2O4-CP, preparation by chemical co-precipitation. Accordingly, the ammonia-water solution was added to a solution of Fe(III) and Mn(II) nitrates with a molar ratio of 2:1 under stirring. The obtained reaction mixture was kept at 90\u00b0C for 5 h under stirring. The final precipitate was separated by magnetic decantation, washed with deionized water, ethanol, and diethyl ether, dried at room temperature and calcined at 400\u00b0C for 2 h. Commercial chemically pure grade and analytical grade reagents were used for catalyst synthesis without additional purification.X-ray diffraction measurements (XRD) were carried out with a Bruker D8 Advance diffractometer, with a Cu-anode, \u03bb\u00a0=\u00a00.154 nm, step 2\u03b8\u00a0=\u00a00.05 exposition time 5 s/step. The identification of crystalline phases was performed by matching with the ICDD files in the PDF-2 Version 2.0602 (2006) database. BET surface areas were measured by using a Sorptomatic 1990 instrument by adsorption of nitrogen at 77 K. Electron diffraction analysis (EDA) was conducted on a PEM-125K transmission electron microscope (Selmi, Ukraine) at 100 kV acceleration voltage.The ESR reaction was carried out in a fixed-bed tubular quartz reactor at atmospheric pressure. The procedure of catalytic tests and reaction mixture analysis were described elsewhere [28]. A catalyst, approx. 1 g, with a particle size between 1 and 2 mm, was placed in a quartz reactor between two layers of quartz grains of the same diameter. It was held in a reaction mixture at each temperature for 1 h, followed by gas chromatography analysis. Before a chromatographic analysis, condensable vapors, e.g. alcohols, aldehydes, ketones, and water, were trapped using a condenser at -15\u00b0C (258 K).The product analysis included three gas chromatographs (GC) equipped with three thermal conductivity detectors (TCD), a flame ionization detector (FID), and four kinds of packed columns: (1) Molecular sieve 5A, (2) Polysorb, (3) tris-cyano-ethoxypropane/Polysorb, and (4) Separon-BD. Analysis of H2 and CO was carried out using column (1), TCD, and Ar as a gas carrier. Column (2) was used for CO2 and N2 analysis (TCD, He as a gas carrier). The liquid samples were analyzed with column (3) (TCD, He as a gas carrier). Hydrocarbons were analyzed using column (4) with FID detector and He as a gas carrier.The sensitivity of the TCD and FID detectors to each analyzed compound (response factor) was determined periodically by their calibration against standards of gas and liquid mixtures of known compositions.The calculation procedure was as follows. The inlet flow rates of the components of the initial reaction mixture (alcohol, water, nitrogen), mol/h, were determined. Knowing the volume of the liquid sample accumulated over a certain time, it is possible to calculate the outlet feed of liquid products of the reaction, l/h. The flow rates of the liquid components (alcohols, aldehydes, ketones, water) of the reaction products mixture (mol/h) were determined using the results of the chromatographic analysis (in terms of the component molar concentration, mole/l) and outlet feed of liquid products. The flow rates of the gaseous components of the reaction products mixture (mol/h) were calculated using the results of the chromatographic analysis (in terms of component mole fraction) and outlet feed of gaseous products (nitrogen was the internal standard). For the reported runs, the carbon balance is defined as the ratio of the product moles to the consumed moles of ethanol, accounting for stoichiometry. The carbon balance error did not exceed 5%.The following experimental conditions were used: temperature 500 \u2013 700 \u043e\u0421, inert gas-carrier (nitrogen) rate Fg\n\u00a0=\u00a08.0\u22c510\u22122 mol/h, liquid feed rate Fl\n\u00a0=\u00a08.9\u00b710\u22122 mol/h, which corresponds to WHSV equal to 4000 h\u22121, molar ratio H2O/C2H5OH\u00a0=\u00a019 (2.7 mol.% C2H5OH, 50 vol.% H2O, N2 balance). This ratio is close to the water/ethanol ratio in crude bioethanol and widely utilized for hydrogen production by BESR. To compare the catalytic performance towards different alcohol/water mixtures, the following diluted alcohols were used: propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol, pentan-1-ol (1% (mol.) of each alcohol), and 4.75% (mol.) of ethanol. After a catalytic test, the catalyst was cooled in the N2 atmosphere to room temperature and stored for characterization.Ethanol conversion, X, and selectivity of carbon-based reaction products, SCn\n, were evaluated according to the following expressions:\n\n(1)\n\n\nX\n=\n\n\nF\n\nC\nn\n\n\n\n\n\nF\n\nE\nt\n,\ni\nn\n\n\n\n\n\u00b7\n100\n%\n\n\n\n\n\n\n(2)\n\n\n\nS\n\nC\nn\n\n\n=\n\n\nn\n\u00b7\n\nF\n\nC\nn\n\n\n\n\n\n\u2211\nn\n\u00b7\n\nF\n\nC\nn\n\n\n\n\n\u00b7\n100\n%\n\n\n\nwhere n is the number of C-atoms in a product Cn; FEt,in\n is an inlet feed of ethanol, mol/h; FCn\n is a feed of corresponding product, mol/h.H2 selectivity was defined as 100% under the assumption that 6 moles of H2 are formed per 1 mole of converted C2H5OH. H2 selectivity was calculated as follows:\n\n(3)\n\n\n\nS\n\nH\n2\n\n\n=\n\n\nF\n\nH\n2\n\n\n\n6\n\u00b7\n\nF\n\nE\nt\n,\ni\nn\n\n\n\n\n\u00b7\n100\n%\n\n\n\nwhere \n\nF\n\nH\n2\n\n\n is an outlet feed of H2, mol/h.H2 yield, \n\nY\n\nH\n2\n\n\n, was calculated as follows:\n\n(4)\n\n\n\nY\n\nH\n2\n\n\n=\nX\n\u00b7\n\nS\n\nH\n2\n\n\n\n\n\n\nIt should be also emphasized that the experimental analysis of BESR over the developed catalysts revealed that ethanol decomposition, carbon formation reaction, and methanation reactions do not occur.After the BESR process, the catalyst samples obtained by decomposition of heteronuclear complexes and by co-precipitation are the manganese ferrite MnFe2O4 with a spinel structure, according to XRD analysis (Fig.\u00a01\na and 1b). Fig.\u00a01c presents a TEM image of the as-synthesized catalyst MnFe2O4-HC. The particle size distribution is rather sharp and centered at 8 nm. For the as-synthesized catalyst MnFe2O4-CP, reflections were not observed on the XRD pattern and this may be explained by the small size of the catalyst nanoparticles (< 3 nm). This suggestion is confirmed by the presence of ring reflections in the electron diffraction patterns of the sample, from which the crystalline phase of ferrite with a lattice of the cubic spinel-type was identified [26]. Therefore, the catalysts mainly consist of crystalline particles. BET surface area of as-synthesized MnFe2O4-HC and MnFe2O4-CP samples was 56 m2/g and 140 m2/g, respectively. A comparison of the surface area and the crystallite size for MnFe2O4-HC and MnFe2O4-CP indicate that a difference in the surface area for these two solids is mainly associated with a difference in a crystallite size that is strongly affected by a synthetic procedure.To design an autonomous H2 generator an appropriate catalyst should be used or developed. Investigation of the optimal conditions of the catalytic reaction is also a key feature for the successful implementation of the process of H2 synthesis into a portable power generator. Herein, we provide a theoretical analysis of the optimal BESR reaction condition and develop an effective catalyst for the corresponding process.The detectable products of the BESR are CO2 and H2 only. However, depending on the reaction conditions and catalysts used, the reaction network may be very complex. The following reactions are suitable for thermodynamics analysis of BESR over catalysts used in this study [3]. This set of reactions accounts for all possible products that are typically observed in various experimental studies over different catalysts [2,3,8,10\u201313].\n\n(R1)\nC2H5OH \u21c4 CH3CHO\u00a0+\u00a0H2\n\n\n\n\n\n(R2)\nCH3CHO\u00a0+\u00a0H2O \u21c4 2 CO\u00a0+\u00a03 H2\n\n\n\n\n\n(R3)\nCO\u00a0+\u00a0H2O \u21c4 CO2\u00a0+\u00a0H2\n\n\n\n\n\n(R4)\nCH3CHO \u21c4 CO\u00a0+\u00a0CH4\n\n\n\n\n\n(R5)\nCH4\u00a0+\u00a0H2O \u21c4 CO\u00a0+\u00a03 H2\n\n\n\n\n\n(R6)\n2 CH3\u0421\u041d\u041e \u21c4 \u0421\u041d3\u0421\u041e\u0421\u041d3\u00a0+\u00a0\u0421\u041e\u00a0+\u00a0H2\n\n\n\n\n\n(R7)\n\u0421\u041d3\u0421\u041e\u0421\u041d3\u00a0+\u00a02 \u041d2\u041e\u00a0=\u00a03 \u0421\u041e+5 \u041d2\n\n\n\n\n\n(R8)\nC2H5OH \u21c4 C2H4\u00a0+\u00a0H2O\n\n\nThese reactions are typical for the most of experiments presented in the literature. A linear combination of these reactions allows one to obtain other reaction equations that are used in the literature for a description of the ESR process.Equilibrium yields of the reaction products may be calculated based on the thermodynamic approach. Using this approach is especially essential because the optimal reaction conditions may be theoretically estimated. Thermodynamic calculations are based on using the equilibrium constant, K\np, as a function of temperature, T, for reactions (R1) \u2013 (R8):\n\n(5)\n\n\nln\n\nK\np\n\n=\nA\n\u00b7\n\n\nT\n\n\n\u2212\n1\n\n\n+\nB\n\u00b7\nln\nT\n+\nC\n\u00b7\nT\n+\nD\n\u00b7\n\n\nT\n\n2\n\n+\nE\n\n\n\n\nNumerical values of A, B, C, D, and E are presented in Table\u00a01\n. These values were calculated using thermodynamic parameters at 500, 700, and 900 K [29].Reactions (R5) and (R7) are not independable. They may be obtained as a sum of other reactions from that reaction network:\n\n(R5)\u00a0=\u00a0(R2) \u2013 (R4)\n\n\n\n\n(R7)\u00a0=\u00a02(R2) \u2013 (R6)\n\n\nTherefore, these reactions were not taken into account in the thermodynamic analysis.It is worth noting that some catalysts may exhibit coke deposition. Typical reactions leading to coke deposition are the Boudouard reaction, methane cracking, and reverse gasification reaction:\n\n2 \u0421\u041e\u00a0=\u00a0CO2\u00a0+\u00a0\u0421\n\n\n\n\n\u0421\u041d4\u00a0=\u00a02 \u041d2\u00a0+\u00a0\u0421\n\n\n\n\n\u0421O\u00a0+\u00a0H2\u00a0=\u00a0\u041d2O\u00a0+\u00a0\u0421\n\n\nNo coke deposition was found for the catalysts used in our study for the selected conditions. Therefore, these reactions were not taken into account.Ethanol conversion is not thermodynamically limited at any temperature. Thermodynamic calculations provide no information regarding the product distribution of ESR reaction under kinetic control. Nevertheless, a thermodynamic analysis may be useful to choose the optimal parameters of the process. These parameters allow preserving the kinetically controlled reaction regime under thermodynamically favorable conditions. Therefore, the catalyst for the ESR process should simultaneously exhibit high activity in reaction (R1), as well as low catalytic activity in methanation, WGS, and coke deposition reactions. Both catalysts used in the present study satisfy these conditions. Therefore, the thermodynamic analysis was performed using reaction network (R1) \u2013 (R4), (R6), and (R8) using Eq.\u00a0(5) with parameters presented in Table\u00a01 to calculate equilibrium constants at different temperatures.\nFig.\u00a02\n presents the equilibrium H2 yield versus temperature. Based on the temperature for which the equilibrium H2 yield achieves YH2eq\n\u00a0=\u00a050% during the steam reforming, higher alcohols may be placed in the following order: ethanol < propan-1-ol < propan-2-ol < butan-1-ol < pentan-1-ol < 2-methylpropan-2-ol. For temperatures higher than approximately 270\u00b0C, the equilibrium H2 yield achieves 100% for each alcohol. Therefore, the appropriate reaction temperature is above 270\u00b0C.\nFig.\u00a03\n demonstrates the temperature dependence of the ethanol conversion and hydrogen selectivity during water/ethanol steam reforming over MnFe2O4-CP and MnFe2O4-\u041dC catalysts. The selectivity toward each reaction product is presented in Table\u00a02\n.During BESR over MnFe2O4-\u0421\u0420 catalyst, almost 100% ethanol conversion is achieved in the temperature range between 500 \u043e\u0421 and 550 \u043e\u0421. Selectivity toward \u0421\u041e2 increases from 57% to 81% with the temperature increase from 500 \u00b0C to 600 \u043e\u0421, whereas selectivity toward H2 reaches the maximum (SH2\n\u00a0=\u00a079.7%) at 600 \u00b0C. The main reaction products are hydrogen and carbon dioxide. Insignificant amounts of oxygenates, e.g. acetic aldehyde (0.1 \u2013 1.4%), acetone (0.3 \u2013 6.1%), and hydrocarbons, e.g. methane (1.8 \u2013 3.1%), \u04212 \u2013 \u04213 hydrocarbons (0.9 \u2013 1.3%) were also detected among the products. No detectable amount of carbon monoxide was found among the reaction products at 500 \u2013 600 \u043e\u0421. However, at 650 \u043e\u0421 carbon monoxide formation was detected due to the reverse reaction of the water-gas shift (WGS).The ethanol conversion over the MnFe2O4-\u041dC catalyst in the temperature range of 500 \u2013 650 \u00b0\u0421 is 90 \u2013 100%. This catalyst demonstrates higher selectivity toward H2 at 600 \u2013 650 \u00b0\u0421. The selectivity reaches 94.6%. The formation of the carbon monoxide on the MnFe2O4-\u041dC catalyst is observed only at 700\u00b0C. Therefore, the MnFe2O4 catalysts do not support CO formation at low temperatures. Perhaps, that is related to the peculiarities of the surface reactions when CO2 becomes the final product of the ethanol transformations over the surface of the catalyst while the probability of the CO formation on the surface is miserable due to high oxygen surface concentration. Moreover, our results indicate that both MnFe2O4-CP and MnFe2O4-\u041dC do not catalyze the water gas shift reaction at low temperatures due to kinetic limitations. The water gas shift reaction becomes appreciable only at 650\u043e\u0421 for MnFe2O4-CP and 700 \u043e\u0421 for MnFe2O4-\u041dC. Selectivity for CO over these two catalysts is lower compared to the thermodynamics (15.6% at 650 \u00b0C and 18.8% at 700 \u043e\u0421) which is an indication of kinetic limitations.The results of thermodynamic calculations regarding equilibrium product selectivity are given in Table\u00a03\n. Under equilibrium conditions, the formation of H2 and CO2 is the most favorable, as follows from the data presented in Table\u00a03 where selectivity for each product is given at equilibrium. A comparison between the results of catalytic steam reforming of bioethanol (Table\u00a02) and thermodynamic equilibrium (Table\u00a03) shows the following. With increasing temperature, the reaction system gradually approaches equilibrium, while on the MnFe2O4-HC catalyst, the equilibrium and experimental hydrogen selectivity at 650 and 700\u00b0C are almost identical. At low temperatures, the equilibrium selectivity for hydrogen significantly exceeds the selectivity observed in experiments, especially in the case of the MnFe2O4-CP catalyst. That may be caused by a low rate of acetaldehyde and acetone conversion under experimental conditions. As a result, the selectivity observed in experiments for acetaldehyde and acetone far exceeds their equilibrium values leading to a decrease in the selectivity for hydrogen.Other features of the MnFe2O4-HC catalyst are associated with a high selectivity toward CO2 that reaches 92.5% at 650 \u00b0C and the absence of CO among the products at temperatures up to 650 \u00b0C. Carbon monoxide content may be affected by WGS reaction (R3). The data presented in Tables\u00a02 and 3 demonstrate that the WGS reaction (R3) occurs at a relatively low rate on ferrite catalysts (CO and CO2 concentrations are far from equilibrium). As a result, carbon dioxide, not CO, is a primary product of a sequence of ethanol transformations over ferrite. The concentration of CO is either significantly lower than the equilibrium value or CO is absent in the reaction products as it occurs in the temperature range of 300 \u2013 600 \u00b0C. Seemingly, carbon monoxide does not desorb into a gas phase during catalysis and reacts with the oxygen located on the surface of MnFe2O4 yielding in CO2 formation. The reacted surface oxygen is balanced by the dissociative adsorption of water.Our results are in good agreement with the experimental studies of the bio-ethanol catalytic steam reforming over supported metal catalysts that show zero selectivity for CO over various catalysts [1]. Moreover, a comparison of the present study with the studies presented in the literature indicates that not only metal-containing catalysts but also metal oxides may show zero selectivity for CO in BESR.\nFig.\u00a04\n shows the H2 yield versus temperature for MnFe2O4-HC and MnFe2O4-CP catalysts. MnFe2O4-\u0421\u0420 catalyst exhibits higher H2 yield in the temperature range between 450 \u00b0C and 550 \u00b0C compared to MnFe2O4-\u041dC catalyst, as it follows from Fig.\u00a04. In contrast, the highest H2 yield is observed on the MnFe2O4-\u041dC catalyst at \u0422 > 600 \u00b0C. The MnFe2O4 catalyst productivity in the temperature range between 550 \u00b0C and 600 \u043e\u0421 is 300 \u2013 600 ml\u00a0\u041d2/(h\u2219gcat.).The major impurities in raw ethanol are C1, C3, and C4 alcohols, which represent up to 87% of all impurities, C3 and C4 aldehydes, acetone, acetic acid, glycerin, ethers, and nitrogen-containing chemicals. The presence of these impurities complicates the steam reforming process and results in either an increase or decrease of the H2 yield and catalyst lifetime. The results of the steam reforming investigations carried out using model mixtures of raw bioethanol and real bioethanol are summarized in ref. [8].The temperature dependence of the alcohol conversion and H2 selectivity for C3 \u2013 C5 alcohols steam reforming over the MnFe2O4-CP catalyst is presented in Fig.\u00a05\n. Temperature increase from 500 \u00b0C to 650 \u00b0C leads to the conversion increase from 88% to 100% and from 80% to 98% for propan-1-ol and propan-2-ol, respectively. Butan-1-ol and pentan-1-ol conversion is higher compared to propan-1-ol and remains 100% in the whole range of the used temperature.Selectivity toward H2 during steam reforming of C3 \u2013 C5 alcohols in the temperature range between 500 and 650 \u00b0C typically overreaches 90%. Moreover, a 150 \u00b0C temperature increase results in a decrease of the selectivity from 98% to 94% and from 98% to 90% for propan-1-ol and butan-1-ol, respectively. For pentan-1-ol, SH2\n\u00a0=\u00a094% \u2013 96%, whereas for propan-2-ol SH2\n increases from 89% to 96%. Considerable decrease of the H2 selectivity during butan-1-ol steam reforming is associated with the presence of carbon monoxide in the reaction mixture, which enhances the water gas shift reaction.For the steam reforming of the ethanol and C3 \u2013 C4 alcohols mixture, the ethanol conversion reaches 97% at 500 \u00b0C. For other alcohols in the mixture, conversion varies between 83% and 90% depending on the alcohol nature, and H2 selectivity is approximately 80% (Fig.\u00a06\n). Based on the conversion value, the alcohols may be placed in the following order: butan-1-ol < ethanol < propan-1-ol \u2248 butan-2-ol < propan-2-ol (at 300 \u043e\u0421) and butan-1-ol < butan-2-ol < propan-2-ol < propan-1-ol < ethanol (at 500 \u043e\u0421). At 600 \u043e\u0421, ethanol conversion and H2 selectivity decrease.Reforming product distributions over the MnFe2O4 catalyst is quite different at 300\u00b0C and 400\u00b0C. Particularly, at a lower temperature, the only reaction products are H2 (92.6 mol. %), \u0421\u041d3\u0421\u041d\u041e (7.1 mol. %), and C2 \u2013 C3 hydrocarbons (0.3 mol. %). The reaction temperature increases up to 400\u00b0C leads to an increase in higher hydrocarbons yield, as a consequence of an increase in the alcohol dehydration rate. The reaction products at 400\u00b0C are as follows: H2 (63.8 mol. %), CH3CHO (0.8 mol. %), CO2 (32.7 mol. %), CH4 (0.2 mol. %), and C2 \u2013 C3 hydrocarbons (2.5 mol. %).By comparing the results of the ethanol-C3/C4 alcohols mixture reforming and individual C2 \u2013 C4 alcohols reforming, the following peculiarities are obtained. For ethanol-higher alcohols mixture reforming, an insignificant decrease in the ethanol conversion (\u0425EtOH\n\u00a0=\u00a097.2%) is observed at 500 \u043e\u0421 in contrast to water-ethanol mixture conversion (\u0425EtOH\n\u00a0=\u00a098.2%). The butan-1-ol conversion in the alcohol mixture is only 83.3%, which is considerably lower compared to the conversion of the individual alcohol. For the propan-1-ol, the conversion is identical for both, mixture reforming and pure propan-1-ol reforming. Propan-2-ol exhibits an increased conversion in the mixture compared to pure alcohol reforming. The main reaction products for alcohol mixture reforming are H2 and CO2. At 500 \u00b0C, productivity toward H2 of the steam reforming process is higher for the alcohol mixture in contrast to the water-ethanol mixture without higher alcohols. This difference in productivity is governed by an effective steam reforming of higher alcohols with the utilization of the water vapor on the developed catalyst.The design of the autonomous catalytic H2 generator was created under ISO 16110 Hydrogen generators using fuel processing technologies and IEC 62282-5-100 Portable fuel cell power systems requirements. In general, the essential parts of a portable hydrogen generator are fuel processing system, fuel cell module, fuel supply system, onboard energy storage system, and water treatment system. The fuel processing system provides fuel conversion into H2. In the fuel cell module, H2 is converted into electric energy and heat in an electrochemical way [30,31]. The obtained heat and electricity are further integrated into the energy generation system. The fuel supply system may be either built-in or as a separate container that is refueled on demand. The onboard energy storage system provides energy for the correct work of the fuel cell module. The water treatment system improves the quality of either regenerated or freshwater to make it suitable for use in a compact power plant. Thereafter, to develop a portable autonomous generator, all these parts should be as compact as possible. Also, the profound efficiency of the catalyst is required to reduce the reactor volume.For a successful design of an autonomous catalytic H2 generator, the following items should be carefully considered: the type of the fuel cell, its characteristics which define the chemical reactions in the fuel cell, catalysts, temperature range, and the required feed. In this paper, we treat the proton-exchange membrane fuel cell (PEMFC). This cell works at low temperature (< 80\u00b0C) and is not sensitive to H2 purity, e.g. CO2 additives [32]. For the calculations of the generator efficiency, we use the characteristics of the commercially available PEMFC produced by Horizon Fuel Cell Technologies Company. Particularly, to produce 1 kW of electricity PEMFC requires 13 l of technical H2 per minute. These values were utilized to evaluate the technological and financial aspects of the portable autonomous catalytic H2 generator design.The design of the fuel supply system is defined by the reaction pressure of BESR. Steam reforming reaction results in an increase in the overall number of moles in the reaction mixture. Therefore, the higher is the reaction pressure, the lower is the reaction rate in equilibrium conditions. In a compact power station, PEMFC is operated under atmospheric pressure in contrast to apparatus of the natural gas steam reforming that is typically operated at relatively high pressure (15 \u2013 30 bar) [33]. As a consequence, BESR should be performed at atmospheric pressure. The low pressure enhances the H2 yield at a lower temperature. Therefore, the fuel supply system should provide the component dosage under the pressure required for homogeneous feed flow.According to the overall BESR reaction scheme, the stoichiometric molar ratio water/alcohol equals 3, which corresponds to 46% of alcohol by weight. The initial bioethanol (grout) contains approximately 9% (vol.) of alcohol, which corresponds to the molar ratio of water/alcohol equal to 19 [34]. For a three-column scheme of the bioethanol synthesis, the grout distillate with an average alcohol content of approximately 40% (vol.) is obtained in the first column. This alcohol content is equivalent to the water/alcohol molar ratio of 3.5. The higher is the water/alcohol ratio, the higher is H2 yield [35]. The water excess also prevents the catalyst deactivation by coke deposition. However, increasing the water/alcohol ratio results in a higher heat amount required for water vaporization. In this study, the water/alcohol ratio identical to 3.5 was used. This composition may be easily obtained using partial purification of bioethanol grout by rapid water vaporization.For the portable H2 generator, the fuel processing system should exhibit a maximum of technological simplicity. All of the existing BESR technologies imply several reactors combined differently, e.g. BESR reformer, HT-WGS, LT-WGS, and Met \u2013 purification from CO [36,37]. This approach is reasonable for large-scale H2 production because many sub-processes (methane steam reforming, WGS, CO removal) are well-known and applied in industry in different operating regimes. In this respect, H2 synthesis by the BESR method in various reactors allows optimization of the whole process technology. However, these complicated schemes are unsuitable for a portable autonomous generator. In this paper, we propose an approach to carry out a BESR reaction in a single fixed bed reactor using the developed ferrite catalyst.The portable autonomous H2 generator is operated under low temperature and requires no strict specifications of the feed content. A principal technological scheme of the H2 generator is shown in Fig.\u00a07\n. The applicability of a single-reactor scheme is defined by the developed MnFe2O4-\u041dC catalyst. Using this catalyst in the BESR reaction, almost 100% bioethanol conversion may be achieved at a relatively low reaction temperature (650\u00b0C). Very small amounts of the reaction byproducts e.g. oxygenate and CO, are obtained. The amount of CO is only 50 \u2013 60 ppm, which is suitable for PEMFC stable functioning [38]. The unreacted bioethanol and reaction byproducts (oxygenates) do not affect the PEMFC operating. No CO2 poisoning the Pt-Ru catalyst in the fuel cell occurs. Water is required for stable fuel cell functioning.Consider 1 kW PEMFC fuel cell module. The latter requires 13 l of H2 per minute or 70 grams of H2 per hour [39]. For the technical evaluation of the autonomous H2 generator concept, the BESR reactor temperature was considered to be 650\u00b0C. Using the experimental data obtained for the developed catalysts, the mass balance of the proposed technological scheme is calculated (Table\u00a04\n).The experimentally verified average productivity of the catalytic BESR process is 450 ml H2/(gcat\u2219h). Therefore, to produce 13 l of H2 per minute, 1.3 kg of the catalyst is required. The bulk density of the catalyst is approximately 1 g/cm3. The reactor volume for this BESR process equals 2.5 l.With the knowledge about the mass balance, the energy consumption of the technological scheme may be evaluated (Table\u00a05\n). At the first stage, 627.05 g/h of the water-alcohol mixture is heated to 650 o\u0421. This requires 1.96 MJ/h of thermal energy. To initiate the endothermic ESR reaction, an additional 1.19 MJ/h of heat is needed. Therefore, for the production of 13 l/h of hydrogen, the overall heat consumption is 3.15 MJ/h. This amount of thermal energy is consumed to produce 3.6 MJ/h of electricity using 1 kW PEMFC. The energy recovery under given reaction conditions equals 0.45 MJ/h. The water/alcohol ratio equal to 5 is recommended by Ref. [40] to prevent coke deposition, as well as to increase H2 yield. However, for water/alcohol ratio\u00a0=\u00a05, 3.74 MJ/h thermal energy is required to produce 3.6 MJ/h electricity, which is economically unreasonable. Energy consumption for H2 production may be reduced only by optimization of the technological scheme and reaction heat utilization. The reaction products are cooled from 650 o\u0421 to the temperature of the fuel cell module (60 o\u0421). The optimized technological scheme is demonstrated in Fig.\u00a08\n.The presented scheme admits the utilization of thermal energy of the reaction mixture for vaporization and heating the input water-alcohol mixture. According to this scheme, the heat exchanger is placed between the reactor and fuel cell module and serves as a pinch zone. Using the pinch methods [41,42] allows evaluating the energy balance of the presented scheme. The calculated amount of energy consumed by the system is 2 MJ/h. The thermal energy required for the synthesis of 13 l/h of H2 is compensated by the heat recuperation of the reaction products. Therefore, the difference between the produced energy (3.6 MJ/h) and consumed energy (2 MJ/h) equals 1.6 MJ/h, which is 3 times higher compared to the use of the ordinary linear technological scheme.The energy saving of the proposed design of the hydrogen generator for PEMFC is highlighted by the pinch analysis that has been performed for the heat exchanger. The results of the pinch analysis are presented in Fig.\u00a09\n. The obtained results give the value of thermal energy QREC =1 MJ/h that can be recovered in this process at the pinch point for the technological parameters used. Pinch analysis also indicates no need for external cooling for the proposed design of the hydrogen generator at temperatures above the pinch point that support the autonomous operation of the generator.Also, the pinch zone (Fig.\u00a08) provides the flexibility of the water/alcohol ratio in the input mixture. The role of the water/alcohol ratio is crucial because this ratio significantly affects the alcohol conversion, H2 yield, and catalyst lifetime. Heat recuperation of the reaction products allows using the water/alcohol mixture with arbitrary alcohol dilution because the water excess serves as an ordinary heat carrier. The energy required for vaporization and heating the excessing water is recovered in the second recuperative heat exchanger.2 MJ/h energy required for BESR operation may be obtained in 3 different ways: (i) electric heating using the produced electricity; (ii) burning the input water/alcohol mixture (however, a major issue with this approach is associated with the fact that diluted alcohol mixture is inflammable); (iii) the use of different fuel, e.g. natural gas. For a compact power plant, we believe that the simplest and most effective approach admits the use of electric heating.Furthermore, the energy conversion efficiency (\u03b7) of the developed power plant is evaluated. The energy conversion efficiency is calculated as the ratio between the produced electricity and the energy which may be obtained by burning the alcohol amount used for electricity production:\n\n\n\n\n\u03b7\n\n=\n\nE\nQ\n\n\u00b7\n100\n%\n,\n\n\n\n\nWhere \u0415 is the electricity amount produced by the fuel cell in a time, Q is the amount of thermal energy obtained by burning the equivalent alcohol amount at the same time. The heat of the thermal ethanol burning is 30.6 MJ/kg. 267 g of ethanol feed is required to produce 13 l of hydrogen and 3.6 MJ of electricity. The heat of 267 g of ethanol burning is 8.1 MJ. Therefore, the energy conversion efficiency toward ethanol equals 44%.The obtained value of the energy conversion efficiency is higher compared to other approaches based on the utilization of renewable energy sources. For instance, the solar cells exhibit the maximum energy conversion efficiency of almost 39% [43]. The energy conversion efficiency obtained during biomass fermentation is only 26.6 % [44].The presented calculations concern the conceptual design of the hydrogen generator. To construct a physical device, knowledge about the size of the main components and the specific size and volume of the whole system with respect to the targeted applications is required. The ready-to-use generator should also contain pressure control, temperature control, and the controller for the content of the water-gas mixture. However, the engineering aspects of the relevant construction lie outside the scope of the paper. To start the generator, any power source is appropriate, e.g. electricity, battery, input alcohol, etc. The power source is chosen based on the generator's construction, power output, and operating conditions. For this purpose, electrocatalysis or photocatalysis may be also applied. However, in this case, the construction of the generator will be too complicated.Hydrogen is a perspective fuel that may remove traditional fossil fuels in the future. H2 may be produced from renewable feedstock, e.g. bioethanol derived from biomass, using steam reforming. In this case, H2 yield significantly depends on the catalyst used and process conditions. Therefore, to investigate the optimal parameters for the conceptual design of the autonomous catalytic H2 generator, the process of the steam reforming of either diluted C2 \u2013 C4 alcohols or ethanol/higher alcohols mixtures is studied that reflects the composition of the raw bioethanol. Steam reforming is performed over the noble metal free MnFe2O4 catalyst with spinel structure. Using this catalyst 98 \u2013 100% ethanol conversion is achieved in the temperature range between 500 and 650 \u00b0C, whereas H2 yield reaches up to 94.6%. For the ethanol/higher alcohol mixture, ethanol conversion and higher alcohol conversion at 500 \u00b0C are 97% and 83 \u2013 90%, respectively. Selectivity toward H2 is \u223c80%. The catalyst productivity is 0.3 \u2013 0.6 l of H2/gcat\u00b7h in the temperature range between 550 and 600 \u043e\u0421.H2 obtained by steam reforming may be converted into electricity via the application of the fuel cells. Existing H2 power plants are inappropriate for small-scale electricity production. To this end, a conceptual design of a single-stage autonomous catalytic hydrogen generator is introduced. The developed generator concept contains no H2 purification equipment and utilizes the heat of the reaction products. For a stable functioning, 1 kW fuel cell consumes 0.63 kg of water/alcohol mixture per hour with 50% ethanol content. This fuel cell consumes 780 l of H2 per hour resulting in an energy conversion efficiency of 44%.The authors declare no conflict of interests.This research is partially supported by the Target Program of the National Academy of Sciences of Ukraine \u201cDevelopment of scientific grounds for hydrogen production, storage, and use in autonomous energy supply systems\u201d. This work was completed despite the unprovoked invasion of Ukraine by Russia, supported by Belarus. The authors are thankful to the Armed Forces of Ukraine for serving our country and protecting our freedom.", "descript": "\n                  The conceptual design of a portable autonomous catalytic hydrogen generator is introduced. The generator is based on the bioethanol steam reforming over the developed ferrite catalyst. The generator admits the utilization of thermal energy of the reaction mixture for vaporization and heating the input water-alcohol mixture. Moreover, the generator is characterized by a simple single-stage design without a stage for hydrogen purification. The generator is capable to produce 1 kW/h of electricity with 0.63 kg/h water/alcohol mixture (50% ethanol) consumption. The energy conversion efficiency of the developed generator is 44%. The proposed hydrogen generator is suitable for various applications related to on-site hydrogen production.\n               "}
{"full_text": "Composite solid propellants are extensively used as one of the most important propulsion energy sources in the field of rocket launching and space vehicles carrying [1\u20133]. With the rapid development of aerospace technology and the increasing competition among different nations, higher requirements have been put forward for the performance of composite solid propellants. Developing composite solid propellants with high energy characteristics, high firing range and high survivability have become the mainstream research directions. It is known that composite solid propellants are mainly composed of fuel, oxidizing agent, polymer binder and other functional components. Ammonium perchlorate (AP), which is a kind of strong oxidizer [4,5], presents some unique characteristics, including high density, high oxygen content, high heat generation, large gas production rate and high stability. Owing to these excellent features, AP has been widely used as oxidizing agent in composite solid propellants [6]. In addition, AP accounts for 65\u201370\u00a0wt percent of the overall propellant, and in some formulations, the content can even be as high as 90%. It can be seen that the characteristics of AP have a decisive impact on the property of composite solid propellant [7\u20139]. Burning rate and energy performance, which can directly dominate the ballistic property of missiles and rockets, are two key factors in evaluating the property of composite solid propellant. The improvement of burning rate and energy performance of propellant can be gained by enhancing the thermal decomposition of AP.Therefore, it is necessary to take feasible technical measures to modify the thermal decomposition behavior of AP. Generally, there are two major methods to modify the thermal decomposition of AP, including physical method (super-refining treatment of AP), and chemical method (utilizing various catalysts). Super-fining treatment of AP is one of measures currently taken to promote the thermal decomposition of AP, which can be ascribed to the increased specific surface area and active contact sites by decreasing the particle size of AP. Nevertheless, the superfine particles tend to aggregate, which will reduce the effectiveness in practical use. Besides, the super-fining process of AP should be carried out under severe conditions to ensure safety [10,11]. Hence, many research works are concentrated on different catalysts on AP decomposition and the thermal decomposition of AP can be accelerated by adding a small amount of catalysts [12\u201316]. Utilizing a reasonable catalyst can reduce the thermal decomposition temperature of AP, and increase the thermal decomposition rate and the amount of heat release, which are beneficial to shortening the ignition time and increasing the combustion rate of propellant. Moreover, the pressure index of propellant can also be adjusted by rational design of catalyst. Consequently, designing and constructing of different catalytic materials with complex microarchitectures have raised a wide concern in recent years.In the last few decades, a variety of catalysts, such as metal powders [17], metal alloys [18], metal oxides [19\u201324], metal hydroxide [25\u201327], metal-organic chelates [28\u201331], carbon-supported composites [32\u201335], and so forth, have been extensively researched and demonstrated to be effective in modifying thermal decomposition behavior of AP. In the past five years, most research work has focused on transition metal oxides and carbon-supported transition metal oxides, due to their high reactivity, versatile structure, low cost and natural abundance. Although extensive research on the catalytic effect for thermal decomposition of AP in the presence of transition metal oxide and carbon-supported transition metal oxides has been performed, the high thermal decomposition (HTD) temperature, the amount of heat release, and kinetic parameters, remain as key factors to evaluate the catalytic activity. This paper provides a comprehensive summary on transition metal oxides and carbon-supported transition metal oxides as catalysts for thermal decomposition of AP in recent five years.It is well known that transition metal oxides (TMOs) can serve as active catalysts for AP decomposition [36\u201338]. When a small amount of TMOs are introduced, the thermal decomposition performance of AP can be regulated. Up to now, a variety of TMOs with different morphologies and versatile composition have been explored to catalyze AP [39]. Here, we classify TMO catalysts into three categories, including single transition metal oxide catalysts, binary transition metal oxide catalysts and composite transition metal oxide catalysts. The related reports on catalytic performance for the thermal decomposition of AP are summarized.Nowadays, single transition metal oxide catalysts, such as ferric oxides, cobalt oxides, nickel oxides, zinc oxides, and copper oxides and so on, have been extensively researched, due to their facile fabrication, tunable structure and high catalytic activity.The catalytic performance of ferric oxide is closely related to its morphology and average particle size. When the particle size decreases to nano size, the catalytic activity of ferric oxide will be greatly improved. Cao et\u00a0al. [40] investigated the catalytic performance of nano-sized \u03b1-Fe2O3 with four different particle sizes (127\u00a0nm, 115\u00a0nm, 86\u00a0nm and 84\u00a0nm) using differential scanning calorimetric (DSC) method. DSC tests indicated that the temperature for high-temperature decomposition (HTD) of AP decreased by 40.7\u00a0\u00b0C, 42.9\u00a0\u00b0C, 50.6\u00a0\u00b0C and 53.4\u00a0\u00b0C with the addition of 2\u00a0wt% of four different \u03b1-Fe2O3, implying the catalytic activity of \u03b1-Fe2O3 on the thermolysis of AP increased when the average particle size of \u03b1-Fe2O3 decreased. For pure AP, the released heat (\u0394H) during the process of thermal decomposition was calculated to be 864\u00a0J/g. When AP was mixed with 2\u00a0wt% of 127\u00a0nm and 84\u00a0nm \u03b1-Fe2O3, the values of released heat were increased to 984\u00a0J/g and 1235\u00a0J/g, which indicated that the thermal decomposition of AP could be improved by nano-sized \u03b1-Fe2O3. The authors also studied the kinetic analysis and the results further illustrated the decreased particle size of \u03b1-Fe2O3 could increase the efficiency of catalytic action. They thought that more active sites were exposed on the surface of smaller particles, which would result in higher catalytic activity. Mechanism was not proposed here.Hossein and his co-workers studied the thermal decomposition behavior of AP catalyzed by nano-sized \u03b1-Fe2O3 with spherical morphology [41]. They found that both the particle size and the content of \u03b1-Fe2O3 can affect the decomposition of AP. Small average particle size and high content of \u03b1-Fe2O3 can lead to low decomposition temperature and high decomposition enthalpy of AP. Authors also further investigated the variation tendency of kinetic and thermokinetic parameters, the apparent activation energy (Ea) and the activation enthalpy (\u25b3H\n\u2260) are remarkably decreased in the presence of \u03b1-Fe2O3 NPs. Activation energy can be defined as the minimum energy that is required from the reactant molecule to the activated molecule in a chemical reaction. The smaller the activation energy is, the higher the reactivity is. The activation enthalpy (\u25b3H\n\u2260) represents the reaction heat that the molecules absorbing or releasing from stable state to activated state. The decreased values of \u25b3H\n\u2260 imply less energy is needed during the reaction process. Hence, the reactant activity of AP is improved in the presence of \u03b1-Fe2O3 NPs.Sharma et\u00a0al. [42] investigated the catalytic performance of hexagonal cones structural \u03b1-Fe2O3 with average particle size around 400\u2013500\u00a0nm. Adding 2% of \u03b1-Fe2O3 to AP can remarkably decrease the LTD and HTD temperature by 20\u00a0\u00b0C and 75\u00a0\u00b0C, respectively. A possible mechanism has been proposed by the authors according to electron transfer mechanism, as shown in Fig.\u00a01\n.Generally, the decomposition of AP undergoes three primary steps, including endothermic low-temperature crystal transformation, exothermic low temperature decomposition (LTD) and exothermic high-temperature decomposition (HTD). The LTD process acts as a controlling step and electrons transfer from ClO4\n- to NH4\n+ during this process, while for HTD, the main reaction can be attributed to the transformation from oxygen (O2) to superoxide ion (O2\n-). Due to the distinct morphology, remarkable photoelectric and conductive performance of \u03b1-Fe2O3 HCs, electron movement can be enhanced, which might accelerate electron transmission from ClO4\n- to NH4\n+. Besides, the accelerated electron flow would facilitate the translation of O2 into O2\n- . Hence, the thermal decomposition of AP is considerably enhanced.Researchers are paying great attention to fabricating cobalt oxide (Co3O4) on account of its variety of application in the fields of catalysts [43], sensors [44], lithium sulfur batteries [45], super-capacitors [46] and so on. As an important member of transition metal oxide, Co3O4 present outstanding catalytic activity towards the thermal decomposition of AP [47]. Li and co-workers [48] introduced Co3O4 spherical microspheres to catalyze AP, the thermal decomposition of AP presents a quite different feature in comparison with that of pure AP. There is only one strong exothermic peak located at 325.4\u00a0\u00b0C with the addition of 2\u00a0wt% of Co3O4 microspheres. The thermal decomposition temperature of AP was 111\u00a0\u00b0C lower than that of pristine AP, indicating Co3O4 microspheres show outstanding catalytic activity on thermolysis of AP. Moreover, the heat release of the mixture was estimated to be 1312.9\u00a0J/g, which is 3.76 times higher than that of pure AP (349.0\u00a0J/g). The activation energy (Ea) and the pre-exponential factor (lnA) of the mixture were calculated to be 121.9\u00a0\u00b1\u00a02.87\u00a0kJ/mol and 4.40\u00a0\u00b1\u00a00.02 min\u22121, respectively. Whereas, the values of Ea and lnA for pure AP were calculated to be 280.5\u00a0\u00b1\u00a011.8\u00a0kJ/mol and 26.40\u00a0\u00b1\u00a00.04 min\u22121, respectively. With the catalytic effect of Co3O4 microspheres, the values of Ea and lnA for AP were significantly decreased, indicating the as-prepared Co3O4 microspheres possess highly catalytic efficiency in AP thermal decomposition.The performance of nano-sized materials is highly related to their average particle size (APS) and specific surface area (SSA). Hossein and his cooperator [49] systematically researched the effect of nano-sized Co3O4 with various APS and SSA on thermolysis temperature of AP. Solvent and non-solvent methods were utilized to fabricate AP/Co3O4 nanocomposites (2 or 5% of Co3O4 NPs in weight percentage). The specifications of three kinds of commercial Co3O4 nanoparticles (marked as A, B and C) with different APS and SSA are summarized in Table\u00a01\n and their catalytic performance on AP are listed in Table\u00a02\n.APS, SSA and the content of Co3O4 can directly affect the decomposition behavior of AP according to the results listed in Table\u00a0.2. With the decrease of APS and the increase of SSA, the catalytic efficiency, including decreased decomposition temperature and enhanced decomposition heat, is remarkably improved. Authors have also illustrated that catalytic performance can be improved by increasing the content of Co3O4 on the thermal decomposition of AP.It is well known that the property of materials can be adjusted by tuning their microstructures. It is of great importance to design and construct various micro/nano materials with complex microstructures. Low-dimensional micro/nano structures, including zero-dimensional (0D) nanoparticles [50], one-dimensional (1D) nanowires [51], and two-dimensional (2D) nanosheets [52], have been extensively researched on account of excellent property such as small grainsize, exposed active sites, high specific surface area and shortened mass transfer distance. However, the nano structures tend to aggregate due to their high surface energy, which will inhibit practical applications. Some achievements have been made for the preparation of nano transition metal oxides, but the aggregation still remains a challenge for developing catalysts with high activity. In order to inhibit the aggregation of low-dimensional nanomaterials, an efficient measure can be taken by designing three-dimensional (3D) hierarchical micro/nanostructure. Investigations by Miao et\u00a0al. [53] prove that different morphologies of Co3O4 have different impact on the thermal decomposition of AP. Different morphological 3D hierarchical Co3O4 micro/nanostructures (Fig.\u00a02\n, Sample 1#-5#) are introduced as catalyst for AP decomposition.DTA results indicate that different morphologies of Co3O4 micro/nano structures present different activity on thermal decomposition of AP. The thermolysis of pure AP undergoes two weight loss procedures, the initial decomposition temperature was about 283\u00a0\u00b0C and the final decomposition temperature was around 443\u00a0\u00b0C. When hierarchical Co3O4 micro/nano structures (Sample 1#-5#) are employed, the related initial decomposition temperatures are decreased to 235, 233, 234, 232 and 232\u00a0\u00b0C, and the final decomposition temperatures are decreased to 306, 308, 315, 296 and 301\u00a0\u00b0C, respectively. DSC tests confirm the LTD and HTD process are merged into one exothermic process and the decomposition temperature decreased significantly compared with that of pure AP. Whereas, the exothermic heat is enhanced and the values are increased to 1197, 994, 1228, 933 and 1123\u00a0J/g for AP with the addition Co3O4 catalyzers (Sample 1#-5#), respectively. These results clearly imply that Co3O4 present good catalytic activity and the catalytic performance can be controllably tuned by regulating the morphologies of Co3O4 nanoparticles.Besides, a multiple of Co3O4 with different morphologies and particle sizes have been fabricated, and the detailed information including method of synthesis, morphology, particle size, surface area, HTD temperature of AP with and without catalysts, have been summarized in Table\u00a03\n.According to the data in Table\u00a03, it can be seen that HTD temperatures for pure AP utilized in different papers are varied. This phenomenon can be ascribed to the difference of the physical properties of AP utilized in different papers, such as particle size, particle size distribution, and morphology of AP. Besides, test conditions during the DSC period, such as types of gaseous condition (nitrogen atmosphere or oxygen atmosphere, gas flow et\u00a0al.) and types of crucible (aluminum crucible or alumina crucible). All the above mentioned factors can affect the decomposition temperature of AP. Moreover, it can be also observed that morphology, particle size, surface area, and the contents of catalysts have great influence on the catalytic performance on Co3O4.Nickel oxide (NiO), as a p-type transparent semiconductor, has been widely used in electronic, magnetic and catalytic aspects. For thermal decomposition of AP, NiO nanoparticles have been drawn great attention due to their apparent catalytic activity. A comparative research on catalytic effects of two different morphologies of NiO was performed by Zhao and his co-workers [66]. NiO microflowers present higher catalytic activity than that of NiO nanorods, which could be attributed to the difference of specific surface area. Based on the experimental results in this paper, the specific surface area for NiO microflowers is calculated to be 41.725\u00a0m2/g, which is higher than that of NiO nanorods (38.077\u00a0m2/g). This means more effective and active sites of NiO microflowers would be exposed on the surface, which is helpful to gas adsorption reaction. Therefore, the catalytic activity of NiO micro flowers is better than NiO nanorods.Sharma et\u00a0al. [67] reported a green and eco-friendly biosynthetic strategy to fabricate NiO nanoparticles (NPs) by using leaf extract of plant calotropis gigantea. The as-obtained NiO NPs present spherical morphology with uniformly distributed particle size about 20\u201350\u00a0nm. The catalytic results indicate NiO NPs prepared by biosynthetic method possess better catalytic activity than the NPs fabricated by chemical routes. Authors also studied the dependence of Ea on different extent of conversion (\u03b1) for AP and mixtures of AP with NiO NPs (Fig.\u00a03\n). According to the results, Ea for pure AP are higher than those for AP mixed with NiO NPs at all values of \u03b1. The variation tendency between Ea and \u03b1 manifests that themolysis of AP is a complicated interaction effect of multiple, competing process and the rate limiting process varies with the extent of conversion. At the initial stage of \u03b1, the high values of Ea may be dominated by nucleation and growth of nuclei. Whereas, the lessening of Ea observed at \u03b1\u00a0>\u00a00.15 is attributed to the transition from kinetically controlled decomposition process to the mass transfer controlled decomposition process [68].Among various transition metal oxides, zinc oxides are also active catalysts in the thermal decomposition of AP. Tian and his co-workers prepared hierarchical porous ZnO hollow microspheres by a facile template-free method in mild experimental conditions [69]. The as-obtained ZnO hollow microspheres were assembled by ZnO nanorods and exhibited exposed (001) facets on the external surface. Both ZnO hollow microspheres and ZnO nanorods show catalytic activity towards the thermal decomposition of AP. In the presence of ZnO hollow microspheres, the decomposition temperature of AP is reduced to 308\u00a0\u00b0C and the decomposition heat release can reach up to 1174\u00a0J/g. The maximum decomposition temperature and the decomposition heat for AP are estimated to 321\u00a0\u00b0C and 959\u00a0J/g, when ZnO dispersed nanorods are added. Kinetic study indicates the values of Ea are remarkably decreased to 63\u00a0\u00b1\u00a07\u00a0kJ/mol and 90\u00a0\u00b1\u00a011\u00a0kJ/mol with the catalytic effect of ZnO hollow microspheres and ZnO dispersed nanorods, respectively. Compared with that of ZnO dispersed nanorods, the catalytic activity of ZnO hollow microspheres is prominent in the thermal decomposition of AP. This may be caused by the structural difference between microspheres and nanorods, such as specific surface area, crystallinity and exposed facets. ZnO hollow microspheres possess a larger specific surface area than ZnO dispersed nanorods, which is beneficial for the adsorption and diffusion process of gaseous HClO4 and NH3 (Fig.\u00a04\n (b)). The exposed (001) facets positioned at the external surface of ZnO hollow microspheres can also accelerate the generation of active oxygen species from the adsorbed HClO4 which will further oxidize NH3 gas. Hence, the absorbed gases will be decomposed. While for ZnO dispersed nanorods, all of the (100) facets, (101) facets and (001) facets are exposed to the gaseous HClO4 and NH3 (Fig.\u00a04 (a)). Although most gases were absorbed by the (100) facets, they will not be decomposed [70], which will affect catalytic performance. Therefore, the catalytic activity of ZnO hollow microspheres are enhanced compared with ZnO dispersed nanorods.Oxides of copper, as important transition metal oxides, have been extensively researched in the aspect of thermal decomposition of AP because of their prominent catalytic performance. Ke et\u00a0al. [71] prepared three-dimensionally ordered microporous (3DOM) CuO and investigated its catalytic performance for the thermal decomposition of AP. DTA results illustrated that with the effort of 2\u00a0wt% 3DOM CuO, the HTD temperature decreased to 354.9\u00a0\u00b0C, and the heat-release of the apparent decomposition of AP increased from 950\u00a0J/g to 1453\u00a0J/g. The excellent catalytic activity can be ascribed to large surface area and good mass transfer performance of 3D unique structure. Xie and his co-workers [72] fabricated one-dimensional CuO nanofibers by electrospinning method. They investigated the catalytic performance on the thermal decomposition of AP by TG and DTA. The HTD temperature of AP/CuO nanofibers were decreased by 101.9\u00a0\u00b0C compared with pure AP, indicating CuO nanofibers possess excellent catalytic activity. They ascribed this phenomenon to the higher surface to volume ratio of beaded CuO nanofibers. Hossein et\u00a0al. [73] prepared uniformly distributed CuO nano particles by calcination of copper carbonate. With the addition of 0.5%, 2% and 5% CuO NPs, the HTD temperature of AP was reduced by 69.8, 66.9 and 104.5\u00a0\u00b0C. The decomposition heat increased to 1356, 1512 and 1588\u00a0J/g with the catalytic effect of 0.5%, 2% and 5% CuO NPs, whereas, the decomposition heat for pure AP was only 728\u00a0J/g. The activation energy (Ea) was also decreased remarkably when CuO NPs were employed. With the addition of 5% CuO NPs, the value of Ea decreases to 178.9\u00a0kJ/mol, which is approximately 65% of the value for pure AP (280.3\u00a0kJ/mol). Authors explained the catalytic performance by electron transfer mechanism. In their opinion, metal oxides act as a bridge for the transportation of electrons, which speed up the electron transferring from ClO- 4 to NH+ 4, thus, the decomposition behavior of AP was enhanced.Luo et\u00a0al. [74] investigated three different morphologies of Cu2O cubes (cubic aggregate, mono-dispersed cube and {100} planes etched cube) for the thermal decomposition of AP. According to the calculated kinetic parameters, the numerical values of E arrange in an ascending order of {100} planes etched cube (92.6\u00a0J/mol), mono-dispersed cube (103.1\u00a0J/mol), cubic aggregate (110.4\u00a0J/mol). These results indicate the average E for AP mixed with Cu2O cubes are less than half the average E of pure AP (280.2\u00a0J/mol), implying Cu2O cubes possess outstanding performance in catalyzing AP decomposition. Besides, the {100} planes etched cube presents the highest activity in the aspect of decreasing the apparent activation energy. They also investigated the complete decomposition time of AP mixed with Cu2O cubes varies with temperature. The catalytic activity for the three types of Cu2O cubes can be easily distinguished in predicting the isothermal decomposition of AP. The results disclose that {100} planes etched cube shows better catalytic performance in AP isothermal decomposition than the other two.Besides single transition metal oxides, binary transition metal oxides with spinel structures have drawn great attention for catalyzing AP decomposition, due to their superb catalytic activity caused by the synergistic effect between two different constituent parts [75,76]. Spinel crystal structures usually can be expressed by the formula of AB2O4, where A and B represent di- and trivalent metal cations, respectively.Xiao et\u00a0al. prepared mesoporous ZnCo2O4 rods through oxalate co-precipitation combined with controlled thermal decomposition method without any template [77]. The oxalates precursor was calcined at settled temperature under a slow heating rate and the nano-scaled ZnCo2O4 crystallites were automatically gathered to generate mesoporous ZnCo2O4 rods. They found that the calcination temperature could not change the ultimate structures of ZnCo2O4 rods, but the specific surface areas are greatly influenced by the calcination temperature. ZnCo2O4 nano crystallites will grow rapidly and the pore network will collapse under high calcination temperature. The specific surface area of ZnCo2O4 will be decreased as calcination temperature arises. Authors have also demonstrated the effect of increasing specific surface areas on the thermal decomposition of AP which possessed accelerated catalytic activity by increasing the specific surface areas. ZnCo2O4 rods calcined at 250\u00a0\u00b0C possess the largest surface area (102.34\u00a0m2/g) and highest catalytic performance, which can significantly reduce AP pyrolysis temperature by 162.1\u00a0\u00b0C. The catalytic activity of ZnCo2O4 rods can be explained by electrons transferring mechanism. Briefly, ZnCo2O4 rods act as a bridge for electrons transferring from ClO- 4 to NH+ 4 and from O2 to O- 2. Owing to high specific surface area, great adsorption of the mesoporous ZnCo2O4 rods and positive synergistic catalytic effect of binary oxide, the decomposition behavior of AP will be enhanced with the addition of ZnCo2O4 rods.A comparative investigation on catalytic performance of spinel MnCo2O4 nanoparticles and unclaimed MnCo2O4 precursor on the thermal decomposition of AP was done by Juibari and his co-workers [78]. The results indicate MnCo2O4 NPs present promising catalytic activity in decomposing of AP, while, the unclaimed MnCo2O4 precursor has little effect on thermolysis of AP. When 2, 3, 4\u00a0wt% of MnCo2O4 NPs are employed, the released heat of AP increase to 1350, 1410 and 1480\u00a0J/g, meanwhile, the HTD temperature shift downwardly to 308, 297 and 293\u00a0\u00b0C, respectively. The results illustrate the catalytic performance can be tuned by changing the content. The kinetic parameters of thermal decomposition of AP further indicate the reaction rate increases with the effort of MnCo2O4 NPs. As a p-type semiconductor, MnCo2O4 possess active d orbital of Co3+ (3d\n5) and Mn2+ (3d\n5) [79], which can be contemporaneously involved in the process of electron transfer and speed the process by simultaneous exposure to NH4\n+ and ClO4\n-:\n\nCo3+\u00a0+\u00a0ClO4\n-\u2192 Co2+\u00a0+\u00a0ClO4\n\n\n\nThe bivalent cobalt cation (Co2+) are unstable and will transform Mn2+ (3d\n5) into the Mn3+ (3d\n6) during another electron transfer process:\n\nCo2+\u00a0+\u00a0Mn2+ \u2192 Co3+\u00a0+\u00a0Mn3+\n\n\n\nA synergistic effect is probable to take place between Co3+ and Mn3+, which in turn promotes the formation of active sites of Mn+ and Co4+. The active sites play an important role in accelerating the catalytic process.Copper chromite is also an active catalyst for modifying thermal decomposition behavior of ammonium perchlorate. Hosseini and his co-workers [80] prepared a pure phase of spinel copper chromite by a sol-gel method. Authors investigated the catalytic performance of different Cu\u2013Cr\u2013O. The results indicated that different samples with various morphologies presented different catalytic activity. With the catalytic effect, all of the exothermic peaks of AP decreased. Among these, the sphere-like CuCr2O4 NPs presented the highest catalytic activity in reducing the decomposition temperature of AP. The sphere-like morphology of CuCr2O4 can effectively prevent nanostructures from aggregating, resulting in decreased particle size and uniform distribution. Moreover, the more crystallization makes sphere-like CuCr2O4 NPs a pure phase. All these factors endow sphere-like CuCr2O4 NPs with the highest catalytic activity.Nano-structured composite materials (or hybrid materials) with extraordinary physico-chemical performance have been widely researched and applied in versatile fields, ascribing to synergistic effect among different composite components. Inspired by this, extensive studies are focus on designing and synthesis of various nano-sized composite transition metal oxide to enhance catalytic activity toward AP decomposition [81].\n\u03b2-AgVO3/ZnFe2O4 nanocomposites were employed as catalyst for thermal decomposition of AP by Abazari and co-workers [82]. As a comparison, \u03b2-AgVO3 and ZnFe2O4 were also prepared, respectively. According to the DSC tests for pure AP and AP mixed with 3\u00a0wt% of \u03b2-AgVO3, ZnFe2O4, and \u03b2-AgVO3/ZnFe2O4 nanocomposites, the HTD temperatures shift from 432 to 402, 367 and 339\u00a0\u00b0C, respectively. Moreover, the heat release (\u0394H) for pure AP, AP\u00a0+\u00a0ZnFe2O4, and AP+\u03b2-AgVO3/ZnFe2O4 were estimated to be 764.8, 1169, and 1487.3\u00a0J/g, respectively. The results indicate that \u03b2-AgVO3/ZnFe2O4 nanocomposites are more active than \u03b2-AgVO3 and ZnFe2O4.Paulose et\u00a0al. [83] prepared copper oxide alumina composite by using block copolymer template assisted sol-gel method. Mesoporous copper oxide dispersed on alumina (MCO) with a series of rations of copper oxide and alumina were synthesized. When introducing MCO, the crystallographic phase transition temperature of AP remained unchanged and the LTD temperatures were not remarkably reduced, indicating mesoporous CuO\u2013Al2O3 have a slight impact on the primary decomposition of AP into ammonia and perchloric acid. Whereas, all the samples of as-obtained CuO\u2013Al2O3 can significantly influent the HTD temperature. The exothermic temperature in HTD process declined, illustrating MCO samples can accelerate the decomposition of AP at a lower temperature.Nanoparticles are easily agglomerated, which remarkably decreases their specific surface area and catalytic activity. In order to overcome this problem, carbon materials, such as graphene, nitrogen-doped graphene, graphitic carbon nitride, carbon nanotubes, carbon black and so on, can be employed as a substrate to decorate nano-sized transition metal oxides. Carbon-supported nanocomposites present the combinative merits of nano-sized transition metal oxides and carbon based materials to produce excellent catalytic performance.The catalytic activity can be remarkably enhanced when the particle size is in nanometer-scale. Bare Fe2O3 nanoparticles tend to aggregate and fewer active sites are exposed, resulting in the decrease of catalytic activity. Graphene, owing to its unique structure and performance, can be a promising substrate to disperse and stabilize nanoparticles. With this in mind, the catalytic activities of Fe2O3 nanoparticles have been improved considerably by utilizing graphene as substrate. Lan [84] and co-workers synthesized graphene/Fe2O3 aerogel via a facile sol-gel and supercritical carbon dioxide drying method, as shown in Fig.\u00a05\n.The Fe2O3 nanoparticles in graphene/Fe2O3 are spherical and well dispersed on the graphene sheets. The specific surface area of graphene/Fe2O3 aerogel (101\u00a0m2/g) is much larger than that of pure Fe2O3 nanoparticles (13\u00a0m2/g), which confirms that graphene could prohibit the aggregation of Fe2O3 particles. The exothermic peaks for low temperature and high-temperature shift to a lower position with the addition of graphene/Fe2O3 aerogel. The exothermic heat shows a rising trend with the increased contents of graphene/Fe2O3 aerogel. Yuan et\u00a0al. [85] synthesized Fe2O3/graphene nanocomposite by hydrothermal method. Fe2O3 nanoparticles are homogeneously distributed on the wrinkled graphene sheets and the particle sizes are ranged from 50\u00a0nm to 80\u00a0nm. DSC tests indicate that both Fe2O3/graphene and Fe2O3 show high catalytic activity in the thermal decomposition of AP, and Fe2O3/graphene show higher catalytic activity than pure Fe2O3, which is related to the high theoretical surface area and good conductivity of graphene. Graphene could not only prevent the agglomeration of Fe2O3 but also provide accelerated electrons to enhance the decomposition of AP. Hence, the catalytic performance of Fe2O3/graphene is superior to pure Fe2O3. So, the support of graphene can effectively improve the catalytic properties of Fe2O3 nanoparticles.To improve the dispersity of CuO nanoplates in the graphene nanosheets, a facile one-step in situ method was employed to fabricate G/CuO nanocomposite according to Fertass and co-worker\u2019s report [86]. On the basis of G/CuO nanocomposite, Al/G/CuO (Al: G/CuO\u00a0=\u00a082.18: 17.82) composite was also obtained by physical mixing of aluminum powder and G/CuO nanocomposite. SEM images show that some CuO nanocomposites are decorated on the surface of graphene nanosheets, while others are wrapped within the graphene nanosheets. For Al/G/CuO composite, the whole surface of aluminum powder is covered by G/CuO nanocomposites. In the presence of CuO, G/CuO and Al/G/CuO additives, the LTD and HTD peaks of AP all merged into one decomposition peak, which is consistent with the observed result in TG curves. The high decomposition temperature of AP blended with G, CuO, G/CuO and Al/G/CuO declined from 432\u00a0\u00b0C to 400\u00a0\u00b0C, 350\u00a0\u00b0C, 325\u00a0\u00b0C and 315\u00a0\u00b0C, respectively. Meanwhile, the activation energy are decreased from 129\u00a0kJ/mol to 123.41\u00a0kJ/mol, 85.12\u00a0kJ/mol, 71.47\u00a0kJ/mol and 56.18\u00a0kJ/mol, respectively. The order of catalytic performance for AP thermal decomposition is ranked as Al/G/CuO\u00a0>\u00a0G/CuO\u00a0>\u00a0CuO\u00a0>\u00a0G. The enhancement of AP decomposition is connected with the inherent characteristics of nano additives. Graphene nanosheets present large surface area and high electron transfer, which can accelerate the decomposition of AP. As a transition metal oxide, the d-orbitals of Cu2+ cations are partially filled, which can accept electrons generated from AP ions, thus, the electron mobility is promoted and the thermal decomposition of AP is accelerated. The as-prepared G/CuO presents higher catalytic activity than that of pure CuO, which can be attributed to the increased dispersity of CuO nanoplates in graphene nanosheets and more exposed active sites. The substrate of highly conductive graphene decreases the aggregation of CuO, whereas the highly active surface area of graphene remarkably improves the catalytic activity of G/CuO. The catalytic performance of Al/G/CuO is better than G/CuO, indicating the aluminum powder can increase the catalytic activity of G/CuO. The aluminum powder can improve the heat transfer and therefore enhance the chemical reaction process. Moreover, the Al/G/CuO composite can provide a large number of active sites to absorb the gases generated from the initial decomposition process of AP, sequentially, the second decomposition process of AP can be accelerated. Hence, Al/G/CuO shows the best catalytic activity among these additives.In addition to graphene, nitrogen-doped graphene is also attractive in the field of catalyst due to the combination of the three dimensional frameworks and the prominent performance of graphene.Hosseini et\u00a0al. [87] reported a promising catalyst for AP decomposition, which contains CuO nanoparticles and nitrogen-doped graphene. CuO nanoparticles are uniformly distributed and directly decorated on three dimensional graphene-based frameworks (3D-GFs) with particle size around 20\u201330\u00a0nm. The catalytic properties of as-obtained CuO@3D-(N)GFs nanocomposite are related to its specific surface area. By using nitrogen adsorption-desorption analysis, the value of specific surface area for CuO@3D-(N)GFs nanoparticles is calculated to be 124.6\u00a0m2/g, while for CuO, the value is 15.6\u00a0m2/g. When 4% 3D-(N)GFs are employed, there is only slight effect on the thermal decomposition of AP. Whereas, with the addition of 1, 2 and 4% CuO@3D-(N)GFs, remarkably decrease of HTD temperatures of AP can be observed, which may be attributed to large specific area and more exposed active sites of CuO nanoparticles. Owing to the synergistic effect between 3D-(N)GFs and CuO, the enhanced exothermic heat for AP mixed with CuO@3D-(N)GFs is significantly improved compared with pure AP. The catalytic mechanisms are proposed according to electron transfer theory and proton transfer theory, respectively. On the basis of electron transfer theory, 3D-(N)GFs could provide accelerated electrons to promote the electrons transfer from ClO- 4 to NH+ 4 and the generation of superoxide (O2\n-) from oxygen (O2). Besides, the positive hole provided by partially filled 3d orbit in Cu2+ can act as electron acceptor to decompose AP. Under the combined action of 3D-(N)GFs and CuO, CuO@3D-(N)GFs present excellent catalytic activity. When referring to proton transfer theory (Fig.\u00a06\n), proton transfer happens between NH4\n+ and ClO4\n-, the superoxide ions (O2\n-) generated from AP decomposition or located on the surface of CuO nanoparticles can capture protons during the process [88]. As depicted in Fig.\u00a06, the advantageous performance of 3D-(N)GFs, including large specific surface area and high thermal conductivity can facilitate the proton transfer from NH4\n+ to ClO4\n- and adsorb more intermediate gas of HClO4 and NH3. As the temperature goes up, the adsorbed NH3 and HClO4 will desorb and react with each other in the gas phase. Moreover, the graphene based substrate can also participate in combustion reaction with HClO4, which will produce more CO2 and more exothermic heat will be produced. Furthermore, the 3D-(N)GFs as a substrate can inhibit the aggregation of CuO NPs, resulting in an increase of specific surface area and more active sites, which will further promote the catalytic process.Ni\u2013Mn bimetallic nanoparticles decorated on three dimensional nitrogen-doped graphene-based frameworks by chemical co-reduction method has been also reported by Hosseini and his group [89]. They studied the catalytic performance of molar ratio of Ni: Mn, the weight ratio of Ni1Mn2@3D-(N)GFs, 3D-(N)GFs support and synergistic effect of Ni and Mn metals on thermal decomposition of AP in detail. The molar ratio of Ni and Mn contained in NiMn@3D-(N)GFs nanocomposites were 2:1, 1:1 and 1:2, respectively. The results indicate Ni and Mn with molar ratio of 1: 2 in NiMn@3D-(N)GFs nanocomposites present the best catalytic activity. The effect of the different weight ratio (3, 5, and 7\u00a0wt%) of Ni1Mn2@3D-(N)GFs nanocomposites toward AP decomposition were also studied. When 3\u00a0wt% of Ni1Mn2@3D-(N)GFs was employed, the LTD and HTD peaks shift downwardly from 389 to 430\u00a0\u00b0C to 281 and 335.14\u00a0\u00b0C, respectively, and the heat release increase from 509\u00a0J/g to 1411.78\u00a0J/g. With the addition of 5 and 7\u00a0wt% of Ni1Mn2@3D-(N)GFs nanocomposites, the LTD and HTD exothermic peaks were combined into one peak, which are centered at 329.43 and 287\u00a0\u00b0C, respectively. The overall heat release estimated for samples with 5 and 7\u00a0wt% additives were 1744.92 and 1331.17\u00a0J/g. The above results indicate the samples of 5 and 7\u00a0wt% show better catalytic performance than samples of 3\u00a0wt%. Compared with the catalytic effect of Ni1Mn2 NPs and Ni1Mn2@3D-(N)GFs nanocomposite, three-dimensional nitrogen-doped graphene act as an efficient support in improving the catalytic performance, which may be caused by the synergistic effect between 3D-(N)GFs and Ni1Mn2. Moreover, synergistic effect also exists in Ni and Mn metals. With the addition of Ni@3D-(N)GFs and Mn@3D-(N)GFs nanocomposites, there are two exothermic peaks ascribing to LTD and HTD process, respectively. When adding Ni1Mn2@3D-(N)GFs nanocomposites, there is a single exothermic peak, indicating the synergistic effect between two metals improves the catalytic performance.To date, two dimension graphitic carbon nitride (g-C3N4) as a narrow band gap semiconductor has been extensively concerned owning to its unique physical and chemical performance, such as high nitrogen content, excellence chemical and thermal stability, controllable electronic structure and eco-friendly. All these characteristics make g-C3N4 a prospective candidate for catalyst and catalytic substrate [90\u201392].Li et\u00a0al. [93] reported g-C3N4 as an efficient and eco-friendly catalyst for thermal decomposition of AP by calcining the dicyandiamide. Bulk g-C3N4 displays 2D layered structures, which consists of several graphitic stacking layers. When g-C3N4 was introduced, the LTD and HTD process of AP were combined into a sole procedure with the exothermic temperature ranging from 384.4 to 390.1\u00a0\u00b0C. The result shows that g-C3N4 can accelerate thermal decomposition rate of AP. In the presence of 10\u00a0wt% g-C3N4, the decomposition temperature and activation energy (Ea) of AP are reduced by 70\u00a0\u00b0C and 119.8\u00a0kJ/mol, respectively. With the catalytic effect of 10\u00a0wt% g-C3N4, the exothermic heat of AP has a remarkable increase and the value can reach up to 1362.6\u00a0J/g, which is much higher than pure AP. The instinct of g-C3N4 is made up of triazine units linked by planar amino groups, which can be regarded as a Lewis base. Lewis acid-base interaction will be formed when HClO4 is absorbed on the surface of g-C3N4. The activation energy of AP decomposition can be decreased by the Lewis acid-base interaction, resulting in the enhancement of AP decomposition. Moreover, g-C3N4 is a kind of polymer semiconductor with a band gap and conduction band potential at 2.7\u00a0eV and\u00a0\u22121.3\u00a0eV vs. RHE, respectively, which can be easily stimulated by external heat. When the energy of external heat surpasses the band gap energy, g-C3N4 will be excited to generate conduction-band electrons (e\u2212) and valence band holes (h+) on the surface. In the decomposition process, HClO4 could be reduced by the conduction-band electrons to create a superoxide radical anion \u2022O2\u2212. Meanwhile, \u2022O2\u2212 and h+ would further oxidize NH3 to produce H2O, NO2 and N2O. Thus, g-C3N4 presents catalytic activity on thermal decomposition of AP.On this basis of bare g-C3N4, Li also [94] successfully fabricated SnO2/g-C3N4 hybrids via one-pot calcining method. The catalytic results indicate SnO2 NPs/g-C3N4 hybrids display the best catalytic activity compared with SnO2 and g-C3N4, which may be ascribed to the synergistic effect between SnO2 NPs and g-C3N4. As stated above, e\u2212 and h+ could be formed on the surface of g-C3N4 under heat irradiation. Based on the synergistic effect of SnO2 NPs, the generated electrons on g-C3N4 would transfer to SnO2 (Fig.\u00a07\n), thus increasing the separation efficiency and stabilization of the electron-hole pairs. Therefore, the synergistic effect of SnO2 NPs and g-C3N4 lead to the best catalytic performance among all the counterparts.Tan et\u00a0al. [95] reported a direct precipitation method to prepare (g-C3N4/CuO) nanocomposites. The well-dispersed CuO nanorods with length of 200\u2013300\u00a0nm and diameter around 5\u201310\u00a0nm were directly anchored on g-C3N4 by the ion-dipole interaction between cupric ions and long pair electrons on the nitrogen atoms of g-C3N4. In the presence of different catalysts, including pure g-C3N4, CuO, and g-C3N4/CuO (various content of CuO from 5 to 50\u00a0wt % versus g-C3N4/CuO), the catalytic performance is varied. The catalytic activities are ranked in ascending sequence of g-C3N4<CuO\u00a0<\u00a0g-C3N4/CuO, which imply the existence of synergistic effect between g-C3N4 and CuO nanorods.Rice-shaped manganese dioxides (MnO2) nanoparticles with width of around 5\u201310\u00a0nm and 10\u201330\u00a0nm in the length were successfully anchored on the surface of carbon nanotubes (CNTs) using redox reaction between potassium permanganate and CNTs according to Ling\u2019s research [96]. The catalytic performance indicates the activity of rice-shaped MnO2/CNTs composite are superior compared with that of the mixture for MnO2 nanorods and CNTs, indicating the well distributed rice-shaped MnO2 with a larger surface area can expose more catalytic active sites to promote the thermal decomposition reaction of AP.Cui et\u00a0al. [97] prepared CNTs/CuO composites by a complex-precipitation method. They also made a comparison between CNTs/CuO composites and the mixture of CuO-CNTs. With the addition of the mixture of CuO-CNTs, the high decomposition temperature decreases by 135\u00a0\u00b0C, while CNTs/CuO composites were employed, the high decomposition temperature decreases by 145\u00a0\u00b0C, demonstrating that the catalytic performance can be enhanced when CuO nanoparticles are deposited on CNTs.Carbon-coated metal nanoparticles possess distinct performance, such as core-shell structure, large specific surface area, good adsorption capacity, excellent conduction and heat transfer, which makes it a promising catalyst for the thermal decomposition of AP. An et\u00a0al. [98] fabricated carbon-coated copper nanoparticles (CCNPs) via a detonating method, by which the mixture of microcrystalline wax, RDX, and copper nitrate hydrate were initiated through an electric detonator under nitrogen atmosphere and normal pressure. The as-obtained detonation products were spherical morphology with particle size ranging from 25\u00a0nm to 40\u00a0nm. The nanoparticles consist of a darker copper nanocrystal core and a lighter carbonous shell (3\u20135\u00a0nm thickness). With the catalytic effect of CCNPs, the peak at HTD stage decreases from 424.07\u00a0\u00b0C to 327.08\u00a0\u00b0C and the activation energy of AP are decreased by 25%, indicating CCNPs can accelerate the decomposition of AP under thermal stimulus.It is known that catalytic performance is related to conduction-band electrons from the irradiated ZnO. However, the electrons and holes tend to reunite, which will decrease the catalytic activity. Wang and co-workers decorated ZnO on carbon black (CB) via atomic layer deposition (ALD) method [99]. When ZnO nanoparticles are decorated on carbon black (CB), the electrons in conduction band will migrate to CB, thus, the recombination of electrons and holes will be prevented, which can further improve the catalytic performance. The ALD ZnO nanoparticles with mean particle size of 10\u00a0nm are deposited on the surface of carbon black through stable C\u2013O\u2013Zn bond, which were formed between oxygen-containing functional group on carbon black and Zn2+ ion. The catalytic effect of ZnO/CB hybrid was performed and the results indicate ZnO/CB hybrid present outstanding catalytic activity for the thermal decomposition of AP. The exothermic peak of AP catalyzed by ZnO/CB hybrids is located at 295\u00a0\u00b0C, which is lower than that of bare ZnO (311\u00a0\u00b0C). Moreover, the exothermic heat of AP catalyzed by ZnO/CB hybrids is increased from 376 to 1692\u00a0J/g, indicating ZnO/CB hybrid possess admirable catalytic performance. This phenomenon is caused by the appearance of C\u2013O\u2013Zn bond, which can promote electron transport from irradiated ZnO to CB, leading to high utilization rate of the electron than bare ZnO. Thus, the catalytic performance of ZnO/CB hybrids is enhanced.In the last few decades, numerous researches have been concentrated on the decomposition mechanisms of ammonium perchlorate and a number of possible decomposition mechanisms of AP have been proposed, but the decomposition mechanism remains a debatable issue. In this review, two possible mechanisms are briefly summarized.Bicromshaw and Newman proposed electron transfer mechanism in 1955 [100]. According to this theory, electrons transfer from ClO- 4 to NH+ 4, the decomposition procure occurs:\n\nClO4\n-\u00a0+ NH4\n+ \u2192ClO4\u00a0+ NH4\n\n\n\nAfter harvesting an electron, decomposition of the ammonium radicals into ammonia and hydrogen atom takes place:\n\nNH4 \u2192 NH3\u00a0+ H\n\n\nHydrogen atom transfers over the lattice. Electron moves accurately in the same way over the anion sublattice:\n\nClO4\u00a0+ ClO4\n- \u2192ClO4\n-\u00a0+ ClO4\n\n\n\nHClO4 is generated owing to the reaction between ClO4 radical and H. The resultant HClO4 may continue reacting with H, hence:\n\nHClO4\u00a0+H \u2192 H2O\u00a0+\u00a0ClO3\n\n\n\nThe ClO3 radical acts as acceptors of electrons which can capture electrons. After harvesting an electron, the ClO3 radical is transformed into ClO3\n\u2212 ion. Then, chlorite ion and ClO4 radical are decomposed and the products can react with NH+ 4 ions. Accordingly, secondary products including chlorine, nitrogen hemioxide and water are produced.In our previous study [101], we investigated the decomposition mechanism of AP catalyzed by CoFe2O4/RGO hybrids on the basis of electron transfer mechanism (Fig.\u00a08\n). The catalytic performance of CoFe2O4/RGO indicates the hybrids can promote both the LTD and HTD process. The lattice of AP is usually assigned to the pair of ions (NH4\n+ and ClO4\n-). As a controlling step, electrons transfer from ClO4\n- to NH4\n+ 4 in LTD process. Meanwhile, oxygen (O2) would transform into superoxide (O2\n-) in the HTD process. The partially filled 3d orbits in Fe or Co atom are beneficial for electron transferring. Moreover, positive hole in Fe or Co atom acts as electron acceptor for AP ion and its intermediate products, which can enhance the thermal decomposition of AP. Owing to the unique structure and excellent performance of graphene, the movement of an electron in graphene is fairly faster than in metal atoms and travel much longer distance without being scattered. CoFe2O4/RGO hybrids could provide accelerated electron flow to promote the controlling step. With the efforts of more active and accelerated electron flow, NH4\n+ and ClO4\n- 4 can be easily transformed to NH3 and HClO4. Furthermore, HClO4 decompose into O2, which is subsequently transformed into superoxide (O2\n-). The superoxide could help NH3 decompose into N2O, O2, Cl2, H2O and a little NO, completely.Jackobs [102] put forward proton transfer mechanism of AP thermal decomposition and this mechanism can be described as follows:\n\nNH4ClO4 \u2194 NH4\n+(a)\u00a0+\u00a0ClO4\n-(a) \u2194NH3 (g)\u00a0+\u00a0HClO4 (g)\n\n\nThis mechanism comprises three steps. In step I, the pair of ions (NH4\n+ and ClO4\n-) are involved in NH4ClO4 lattice. Step II involves proton transfer from the cation NH4\n+ to the anion ClO4\n- via a molecular complex. In Step III, the molecular complex breaks down into ammonia and perchloric acid. The dissociation products of AP, NH3 and HClO4 molecules, either react in the absorbed layer over the surface of AP or interact by desorption and sublimation in the gas phase. The gaseous phase of NH3 and HClO4 react quickly, generating O2, N2O, Cl2, NO, and H2O as accessory products at a low temperature (<350\u00a0\u00b0C).During the reaction happening in the absorbed layer, the perchloric acid desorbed more quickly compared to ammonia, hence, resulting in incomplete oxidation of ammonia and forming a saturated atmosphere of NH3. As a result, the reaction in HTD process will decelerate and transform incompletely, thus forming NO, O2, Cl2, and H2O in the second exothermic decomposition procedure. Both LTD and HTD begin with proton transfer from NH4\n+ to ClO4\n-. The difference between LTD and HTD is that the reaction in a low temperature takes place on the defects of AP crystal, whereas the slow reaction in a high temperature occurs in the lattice of the unreacted normal crystal. Ammonia and chloric acid will be absorbed and desorbed in HTD. Thus, proton transfer in the primary process plays an important role. When additives are introduced, the concentration of the protons will be changed, which will further affect ammonia.Zhang et\u00a0al. [103] investigated the thermal decomposition behaviors of AP catalyzed m-g-C3N4/CuO and explained the reaction mechanism on the basis of proton transfer mechanism [98]. A solid-gas heterogeneous reaction happens in the LTD process, during which protons transfer from NH4\n+ to ClO4\n-. Subsequently, NH3 and HClO4 are formed and then HClO4 will oxidize NH3 in the gas phase. In the primary stages, the adsorbed HClO4 located in the surface and pores of AP lattice act as a crucial chain carrier for AP decomposition, which can further accelerate the decomposition of AP. Besides, m-g-C3N4 can be classified into Lewis base, owing to its distinct morphologies and surface features. According to the Lewis acid-base theory, HClO4 absorbed on the surface of g-C3N4 would possess decreased activation energy to enhance the thermal decomposition of AP.As a semiconductor, the band gap for m-g-C3N4 is about 2.70\u00a0eV, which is beneficial for thermal decomposition of AP. When the excitation energy surpasses the banding gap energy, the valence band holes (h+) and conduction-band electrons (e\u2212) can be formed on the surface of m-g-C3N4. The unique properties of m-g-C3N4, such as large specific surface area, high separation efficiency of electrons and pores, are beneficial for adsorbing HClO4 and NH3 on the surface. The gaseous HClO4 react with e\u2212 to form super-oxide radical anion (O2\u2212), which could further react with h+ and NH3 to generate N2O, H2O and NO2. On the other hand, the low banding gap (1.68\u00a0eV) and band potential (0.46\u00a0eV) of CuO make it to be easily activated by heating. The excited electrons on CB of m-g-C3N4 could transfer to the CB of CuO and the h+ on the VB of CuO could transfer to the VB of m-g-C3N4, which inhibited the recombination of charge carrier. The high separation efficiency of e\u2212 and h+ improves the catalytic activity (Fig.\u00a09\n).Although many efforts have been made to investigate the mechanism of thermal decomposition of AP catalyzed by different catalysts, the primary thermal decomposition mechanism is still not fully understood, so, there is still a long way to further investigate the mechanism by various technologies.In recent five years, there has been comprehensive research on preparation, modifications, characterization and performance for various kinds of catalysts in the thermal decomposition of AP. The easily tunable morphology of transition metal oxide and the unique structure of carbon-supported materials, as well as the excellent catalytic performance, makes transition metal oxide and carbon-supported transition metal oxide materials a promising candidate for AP decomposition. This review gives a summary of general strategies and recent process in developing novel and high efficient catalyst in thermal decomposition of AP, including transition metal oxide and carbon-supported transition metal oxide. The as-stated materials in this review present tunable catalytic performance by varying metal elements, adjusting morphology and compounding with carbon-supported analogue. This progress has illustrated that transition metal oxide and carbon-supported transition metal oxide are playing and will continue to play a significant part in modifying the decomposition behavior of AP.Although a remarkable improvement has been made for the development of high catalytic activity of transition metal oxide and carbon-supported transition metal oxide, there are still considerable challenges for further investigation. Most research works focus on the decomposition behavior of AP mixed with versatile catalysts, however, few works are concentrated on the combustion behavior of solid composite propellants when the novel nano-sized catalysts are employed. The combustion process of solid composite propellants is complicated and the internal ballistic property of solid motor related with the mechanisms is still not fully understood. Moreover, batch manufacturing of nano-sized catalysts have not been implemented, which hinders their practical application. So, it\u2019s necessary to do some research about the catalytic effect on solid composite propellants in practical formulation and develop manufacturing technology for large scale production of catalysts. Motivated by the increasing demand of solid composite propellants coupled with critical performance, it can be envisioned that the utilization of novel nano-sized catalysts will be increased in the near 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.This work was financially supported by the Science and Technology project of Jiangsu province (BN2015021, XZ-SZ201819).", "descript": "\n                  As a main oxidizer in solid composite propellants, ammonium perchlorate (AP) plays an important role because its thermal decomposition behavior has a direct influence on the characteristic of solid composite propellants. To improve the performance of solid composite propellant, it is necessary to take measures to modify the thermal decomposition behavior of AP. In recent years, transition metal oxides and carbon-supported transition metal oxides have drawn considerable attention due to their extraordinary catalytic activity. In this review, we highlight strategies to enhance the thermal decomposition of AP by tuning morphology, varying the types of metal ion, and coupling with carbon analogue. The enhanced catalytic performance can be ascribed to synergistic effect, increased surface area, more exposed active sites, and accelerated electron transportation and so on. The mechanism of AP decomposition mixed with catalyst has also been briefly summarized. Finally, a conclusive outlook and possible research directions are suggested to address challenges such as lacking practical application in actual formulation of solid composite propellant and batch manufacturing.\n               "}
{"full_text": "Concentration of the component i (mmol cm\u22123)Diffusion coefficient (cm2 s\u22121)Activation energy named i (J mmol\u22121)Molar flow at the reactor inlet of the component i (mmol min\u22121)Molar flow at the reactor outlet of the component i (mmol min\u22121)Kinetics constant number i (see reference [26])Preexponential factor number i (J mmol\u22121)CO2 hydrogenation equilibrium constant (atm\u22122)Adjust parameter (dimensionless)Reaction rate of the component i (mmol g\u22121 s\u22121)Average formation rate of CH4 (mmol g\u22121 s\u22121)Ideal gas constant (J K\u22121 mmol\u22121)Time (s)Time of the storage step (s)Time of the hydrogenation step (s)Temperature (K)Axial position (cm)CO2 conversion (%)H2 conversion (%)CH4 production (\u03bcmol g\u22121)Correction factor (dimensionless)Porosity (dimensionless)Covering factor of the component i (dimensionless)Density (g cm\u22123)Adsorption capacity (mmol g\u22121)Maximum adsorption capacity (mmol g\u22121)Dual Function MaterialPartial Differential EquationSynthetic Natural GasThe CO2 methanation reaction (Eq. 1), also known as Sabatier reaction, originates in 1902 [1]. The scientific interest on the Sabatier\u2019s reaction has grown in recent years in the context of a massive implementation of renewable energies. One of the main drawbacks of renewable energies is their intermittent nature due to their dependence on atmospheric conditions. In an energy system based on renewable energies, periods of energy shortage or surplus can occur. Thus, the storage of energy to balance the energy demand is essential. In this context, the CO2 methanation has a practical application. In periods of energy surplus, the electric energy produced by renewables energies would be used to produce hydrogen by electrolysis, which then reacts catalytically with CO2 (captured from an industrial effluent) to produce CH4 named as synthetic natural gas (SNG). This process is also known as Power to Gas (PtG) technology, which aims to connect the electric grid and the gas grid to make the future energy system more robust [2].\n\n(1)\nCO2+4H2 \u21c6\u00a0CH4+2H2O\n\n\nSabatier\u2019s reaction is characterized by being strongly exothermic, and therefore equilibrium is favored at low temperatures. On the other hand, the number of moles of products is less than that of reagents, so the thermodynamic equilibrium is favored at high pressures. However, working at high pressure implies a high economic cost, so it is more convenient to work at atmospheric pressure [3]. In addition, it should be noted that the complete reduction of CO2 (oxidation state C+4) to methane (oxidation state C\u22124) implies the transfer of eight electrons, which implies to overcome a high kinetic barrier. Therefore, the use of catalysts is essential [4].In general, the catalysts used for this reaction must have thermal stability in the operating temperature range of 200\u2013400\u2009\u00b0C. Catalysts with different active phases (Ru, Ni, Fe, Co.) [5\u20139], and different supports (Al2O3, zeolites, SiO2, TiO2.) [10\u201314] have been used in recent years. Among the active phases, nickel shows high CO2 conversion and is one of the most used metals due to its abundance and low cost. However, nickel tends to sinter, and therefore, deactivate. On the other hand, ruthenium is very active, selective and stable towards methane formation even at low temperatures. Although Ru is more expensive than Ni, it has been reported in a large number of publications [9,15]. On the other hand, the support can influence the dispersion of the active phases, its reducibility and the formation of spinels that can reduce the activity of the catalyst [16]. Generally, basic mesoporous solids are used; in particular, alumina has been the most used support to disperse the active phase.In the early 2020 there were 38 methanation plants with a total capacity of 14.5\u2009MW and that number is growing exponentially [17]. One of the main drawbacks of PtG technology is the high costs associated with the CO2 purification. One cost effective alternative is the utilization of a dual-function material (DFM) as catalyst. The DFM contains an alkaline or alkaline earth element that acts as an adsorbent and a noble metal that assists the methanation reaction [18,19]. The DFM allows the capture of CO2 and its direct conversion to methane, without the need of intermediate CO2 sequestration processes, which are energy intensive. The operation is carried out cyclically alternating steps of CO2 storage and hydrogenation. This novel operative strategy has recently been proposed by Duyar et al. [20]. CO2 is first captured onto the basic element of the catalyst until saturation. Afterwards, H2 is injected and favors a spillover phenomenon that conducts the chemisorbed CO2 to the metal site where the methanation takes place. Both the CO2 capture process and the CH4 production process can operate at a temperature of 250\u2013400\u2009\u00b0C. An effluent of a combustion process can easily reach this temperature. Thus, the PtG technology using DFMs can be directly applied, without the need for an external heat input [21,22].This novel cyclic operation strategy is very promising. However, it still needs further development for industrial implementation. Advances in the formulation and physyco-chemical properties of DFMs are required to boost the adsorption capacity and hydrogenation activity of the samples. In this sense, an intimate contact between the adsorbent and the metal is crucial [22]. On the other hand, the influence of the operational variables on the CO2 adsorption and hydrogenation performance has to be addressed. This experimental work is usually a very time consuming step if the number of studied variables is so large to cover a wide range study. One possibility to predict the influence of operational variables on the catalytic behavior is simulation. For that, it is required to build first a robust model able to predict accurately the evolution of reactants and products under a wide range of operational conditions.In own previous work [23\u201325], we reported a complete reaction scheme able to describe the CO2 adsorption and hydrogenation using DFMs with formulation x-Na2CO3/Al2O3 (x\u2009=\u2009Ru/Ni). Briefly, CO2 and H2O compete for the adsorption sites (Na2O), forming the corresponding carbonate (Na2CO3) or hydroxide (NaOH), respectively. During the adsorption step, a CO2 molecule can displace a previously adsorbed H2O molecule, forming the carbonate and releasing H2O to the gas phase. During the hydrogenation step, the as-formed carbonates are decomposed and hydrogenated on the metal site producing CH4 and H2O. Some fraction of the as-formed H2O is adsorbed onto the storage sites forming the hydroxide.Based on the elemental reactions that govern the process, we proposed a kinetic model, which accurately predicts the evolution of CO2, CH4 and H2O [26]. The kinetic equations of the model rely on the concentration of reactants and products and on the surface coverage of CO2 and H2O. The model was validated in a wide range of reactants concentrations, i.e. 1.4\u201310.9% CO2 during the adsorption step and 1.4\u201310.9% H2 during the hydrogenation step, and in the 250\u2013400\u2009\u00b0C temperature range.In this work, we shall use the previously developed and validated model to optimize by simulation modeling the duration of the CO2 storage step and the duration of the hydrogenation step, i.e. the CO2 adsorption and hydrogenation cycles timing (t\nCO2\n/t\nH2). First, different simulations are performed with different t\nCO2\n/t\nH2 to qualitatively observe how the pair of times influences on the temporal evolution of CO2, CH4 and H2O. The surface coverages of CO2 and H2O are also analyzed at this point. Then, catalytic parameters are defined with which the global performance of the catalyst can be evaluated at any given t\nCO2\n/t\nH2. The conversion of CO2 and H2, CH4 yield and the average CH4 formation rate are analyzed in order to define an optimum CO2 adsorption and hydrogenation cycle timing (t\nCO2\n/t\nH2). Based on the optimal cycle timing, a reactor configuration is proposed for the industrial application.A dual function material with formulation 4%Ru-10%Na2CO3/Al2O3 was prepared by wet impregnation. A detailed description of the preparation procedure and characterization of the catalyst can be found elsewhere [26]. Reactor tests were performed in a stainless steel tube placed in a vertical furnace. 3\u2009g of pelletized (0.3\u20130.5\u2009mm) catalyst was housed in the reactor. The catalyst was pre-treated with a gas stream composed of 10% H2/Ar at 350\u2009\u00b0C for 45\u2009min to favor the reduction of Ru.The CO2 storage and hydrogenation is carried out with cyclic feeding. During the CO2 storage step, a gas stream composed of 5.7% CO2/Ar was fed for 2.5\u2009min. During the hydrogenation step, a gas stream composed of 5.7% H2/Ar was fed for 5\u2009min. A purging step with argon was fed between adsorption and hydrogenation cycles to avoid mixing of both gas streams. The operation was carried out at 350\u2009\u00b0C and the total flowrate was set at 1200\u2009ml\u2009min\u22121, which corresponds to a space velocity of 15,000\u2009h\u22121. The composition of the gas stream leaving the reactor was analyzed by FTIR (MKS MultiGas 2030) for quantitative determination of CO2, CH4 and H2O concentration.The CO2 adsorption capacity (\u03a9) during the storage period is calculated by Eq. (2). CH4 and H2O productions are calculated by Eqs. (3) and (4), respectively.\n\n(2)\n\n\n\u03a9\n\n\nmmol\n\n\n\ng\n\n\n\u2212\n1\n\n\n\n\n=\n\n1\n\nW\n\n\n\n\n\u222b\n\n\n0\n\n\n\nt\n\nCO\n2\n\n\n\n\n\n\n\nF\n\nCO\n2\n\nin\n\n\nt\n\n\u2212\n\nF\n\nCO\n2\n\nout\n\n\nt\n\n\n\nd\nt\n\n\n\n\n\n\n(3)\n\n\n\nY\n\nCH\n4\n\n\n\n\nmmol\n\n\n\ng\n\n\n\u2212\n1\n\n\n\n\n=\n\n1\nW\n\n\n\u222b\n0\n\nt\n\nH\n2\n\n\n\n\nF\n\nCH\n4\n\nout\n\n\nt\n\nd\nt\n\n\n\n\n\n\n(4)\n\n\n\n\nY\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\n(\nmmol\n\n\n\ng\n\n\n\u2212\n1\n\n\n)\n=\n\n1\nW\n\n\n\u222b\n0\n\n\n\nt\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\nO\n\n\nout\n\n\n\n(\nt\n)\n\nd\nt\n\n\n\n\n\nt\nCO2 and t\nH2 correspond to the duration of the CO2 storage and hydrogenation periods, respectively. \n\n\nF\n\n\n\n\nCO\n\n\n2\n\n\n\n\nin\n\n\nand \n\n\nF\n\n\n\n\nCO\n\n\n2\n\n\n\n\nout\n\n\n correspond to the CO2 molar flow at the reactor inlet and outlet streams, respectively. \n\n\nF\n\n\n\n\nCH\n\n\n4\n\n\n\n\nout\n\n\n and \n\n\nF\n\n\n\n\nH\n\n\n2\n\n\nO\n\n\nout\n\n\n are the molar flows of CH4 and H2O at the reactor outlet stream, respectively. W is the weight of the catalyst housed in the reactor.Two additional parameters will be used to evaluate the catalytic performance, i.e. the conversion of CO2 and the conversion of H2 during the hydrogenation period.\n\n(5)\n\n\n\n\nX\n\n\n\n\nCO\n\n\n2\n\n\n\n\n=\n\n\n\n\u222b\n0\n\n\n\nt\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\n\n\n[\n\n\n\nF\n\n\n\n\nCO\n\n\n2\n\n\n\n\nin\n\n\n\n(\nt\n)\n\n\u2212\n\n\nF\n\n\n\n\nCO\n\n\n2\n\n\n\n\nout\n\n\n\n(\nt\n)\n\n\n]\n\nd\nt\n\n\n\n\n\u222b\n0\n\n\n\nt\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\n\n\n\nF\n\n\n\n\nCO\n\n\n2\n\n\n\n\nin\n\n\n\n(\nt\n)\n\nd\nt\n\n\n\n\u22c5\n100\n=\n\n\n\n\u222b\n0\n\n\n\nt\n\n\n\n\nH\n\n\n2\n\n\n\n\n\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\u222b\n0\n\n\n\nt\n\n\n\n\nCO\n\n\n2\n\n\n\n\n\n\n\n\n\nF\n\n\n\n\nCO\n\n\n2\n\n\n\n\nin\n\n\n\n(\nt\n)\n\nd\nt\n\n\n\n\u22c5\n100\n\n\n\n\n\n\n(6)\n\n\n\nX\n\nH\n2\n\n\n=\n\n\n\n\u222b\n0\n\nt\n\nH\n2\n\n\n\n\n\n\nF\n\nH\n2\n\nin\n\n\nt\n\n\u2212\n\nF\n\nH\n2\n\nout\n\n\nt\n\n\n\nd\nt\n\n\n\n\u222b\n0\n\nt\n\nH\n2\n\n\n\n\nF\n\nH\n2\n\nin\n\n\nt\n\nd\nt\n\n\n\u00b7\n100\n=\n\n\n4\n\n\u222b\n0\n\nt\n\nH\n2\n\n\n\n\nF\n\nCH\n4\n\nout\n\n\nt\n\nd\nt\n\n\n\n\u222b\n0\n\nt\n\nH\n2\n\n\n\n\nF\n\nH\n2\n\nin\n\n\nt\n\nd\nt\n\n\n\u00b7\n100\n\n\n\n\nAll the CO2 stored reacts to form CH4, i.e. unreacted CO2 is not experimentally observed during the hydrogenation period, as will be seen later. Thus, the amount of CO2 stored can be also evaluated as the amount of CH4 produced during the hydrogenation step: \n\n\n\u222b\n0\n\n\n\nt\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n\n\n[\n\n\n\nF\n\n\n\n\nCO\n\n\n2\n\n\n\n\nin\n\n\n(\nt\n)\n\u2212\n\n\nF\n\n\n\n\nCO\n\n\n2\n\n\n\n\nout\n\n\n(\nt\n)\n\n]\n\nd\nt\n\n=\n\n\u222b\n0\n\n\n\nt\n\n\n\n\nH\n\n\n2\n\n\n\n\n\n\n\n\n\nF\n\n\n\n\nCH\n\n\n4\n\n\n\n\nout\n\n\n(\nt\n)\nd\nt\n\n\n, provided that Sabatier\u00b4s reaction stoichiometry states 1\u2009mol CO2:1\u2009mol CH4. On the other hand, H2 conversion can be calculated based on methane formation. For that, it is considered that hydrogen consumption quadruples methane formation, following again the stoichiometry of the Sabatier\u00b4s reaction (Eq. 1).Dynamic one dimensional isothermal heterogeneous plug flow reactor model with axial dispersion is considered for the modeling of the CO2 capture and hydrogenation. The evolution of the concentration of CO2, CH4 and H2O is calculated by solving jointly the partial differential equation (PDE) for the gas phase (Eq. 7) and the ordinary differential equation (ODE) for the solid phase (Eq. 8).\n\n(7)\n\n\nGas\n\nphase\n:\n\n\n\n\u2202\n\n\nC\n\n\ni\n\n\n\n\n\u2202\nt\n\n\n=\n\u2212\n\nu\n\u03b5\n\n\n\n\u2202\n\n\nC\n\n\ni\n\n\n\n\n\u2202\nx\n\n\n+\n\nD\n\u03b5\n\n\n\n\n\n\u2202\n\n2\n\n\n\nC\n\n\ni\n\n\n\n\n\u2202\n\n\nx\n\n2\n\n\n\n+\n\n\n\u03c1\n\n\nr\n\n\ni\n\n\n\n\u03b5\n\n\n\n\n\n\n\n(8)\n\n\nAdsorbent\n\nphase\n:\n\n\n\u2202\n\n\n\u03b8\n\n\nj\n\n\n\n\n\u2202\nt\n\n\n=\n\n\n\n\nR\n\n\ni\n\n\n\n\n\n\n\u03a9\n\n\nmax\n\n\n\n\n\n\n\nwhere \u03b5 is the void fraction, C\n\ni\n the concentration of the gas phase of species i, \u03b8\n\nj\n is the surface coverage of species j, u the linear velocity of the gas, D the diffusion coefficient, \u03c1 the density of the bed, x the axial coordinate of the reactor, \u03a9max the maximum CO2 adsorption capacity of the catalyst, and R\n\ni\n the rate of formation of species i, calculated according to:\n\n(9)\n\n\n\n\nR\n\n\ni\n\n\n=\n\n\n\u2211\n\nk\n=\n1\n\n\n\n\n\nr\n\n\nk\n\n\n\n\n\u03c5\n\n\ni\n,\nk\n\n\n\n\n\n\n\nwhere i is the index of the species considered, r\n\nk\n the intrinsic velocity of reaction k and \u03bd\n\ni,k\n the stoichiometric coefficient of species i in reaction k.We reported in a previous work [24] the mechanism of the CO2 storage and hydrogenation to CH4 using a dual function material that operates in alternate cycles. The main reactions occurring during adsorption period are:\n\n(10)\nNa2O+CO2 \u21c6\u00a0Na2CO3\n\n\n\n\n\n(11)\n2NaOH+CO2 \u21c6\u00a0Na2CO3+H2O\n\n\n\n\n(12)\nNaOH+CO2 \u21c6\u00a0NaHCO3\n\n\nand during methanation period:\n\n(13)\nNa2CO3 \u21c6\u00a0Na2O+CO2\n\n\n\n\n\n(1)\nCO2+4H2 \u21c6\u00a0CH4+2H2O\n\n\n\n\n(14)\nNa2O +H2O\u00a0\u21c6\u00a02NaOH\n\n\nCheck out reference [24] for more details regarding the CO2 adsorption and hydrogenation mechanism. The kinetic equations used by the model are based on those reactions. A detailed discussion about the kinetic expressions adopted by the model can be also found in our previous work [26]. The kinetic expressions used to predict the reaction rates of CO2, CH4 and H2O during the adsorption, purge and hydrogenation period are collected in \nTable 1.The temporal evolution of the concentration of CO2, CH4 and H2O predicted by the model was obtained by integrating the mass balance for the gas phase (Eq. 7) and for the solid phase (Eq. 8). The model considers that CO2 and H2O can be adsorbed onto the storage sites (Na2O) leading to the formation of carbonates (Na2CO3) and hydroxides (NaOH). The presence of carbonates and hydroxides in the surface of catalysts containing a basic element, such as Na2O or CaO, has been confirmed by FTIR when exposed to gas phase CO2 or H2O [27]. The kinetic expressions adopted in the model for the estimation of the reaction rates rely on the surface coverage of CO2 (\u03b8\nCO2) and H2O (\u03b8\nH2O). The surface coverage of CO2 is defined as the amount of CO2 adsorbed in the storage sites (\u03a9) with respect to the maximum CO2 storage capacity (\u03a9max). Thus, if the catalyst is saturated with CO2, the surface coverage of CO2 would be 1 (\u03b8\nCO2=1). On the contrary, if the catalyst is fully regenerated, the surface coverage of CO2 would be 0 (\u03b8\nCO2=0). Depending on the state of the catalyst, the covering factor takes values comprised between 0\u2009\u2264\u2009\u03b8\nCO2 \u2264\u20091. The surface coverage of H2O is defined as the amount of H2O adsorbed in the storage sites with respect to the maximum CO2 storage capacity (\u03a9max). As observed for \u03b8\nCO2, depending on the state of the catalyst, the covering factor of H2O takes values comprised between 0\u2009\u2264\u2009\u03b8\nH2O \u2264\u20091. Taking into account the adsorption stoichiometry of CO2 and H2O (Eq. 10 and Eq. 14) one molecule of CO2 or one molecule of H2O is adsorbed onto one molecule of the storage site (Na2O). As CO2 and H2O compete for the same adsorption sites, at any time 0\u2009\u2264\u2009\u03b8\nCO2 +\u2009\u03b8\nH2O \u2264\u20091. Besides, the model also considers the formation of bicarbonate type species (Eq. 12) when CO2 and H2O coexist in the gas phase. Note that formation of bicarbonates means that one additional molecule of CO2 and H2O are adsorbed onto and already carbonated adsorption site (Na2CO3). A global reaction scheme for bicarbonates formation was proposed in our previous work [26], in which CO2 is delivered by a neighborhood adsorption site and H2O is adsorbed from the gas phase. Thus, the formation of bicarbonates is considered as an unstable reservoir for the storage of H2O without implication in the storage of CO2. The surface coverage of bicarbonates is defined as \u03b8\nH2O/CO2. In the presence of bicarbonates, the sum of the surface coverages of CO2 (\u03b8\nCO2), H2O (\u03b8\nH2O) and (\u03b8\nH2O/CO2) could exceed 1, as a H2O molecule is adsorbed onto an already carbonated site.As defined in the previous paragraph, the surface coverages of CO2 (\u03b8\nCO2), H2O (\u03b8\nH2O) and (\u03b8\nH2O/CO2) are all defined as the amount of adsorbed specie divided by the maximum CO2 storage capacity (\u03a9max). In order to experimentally calculate \u03a9max, CO2 adsorption and hydrogenation cycles are carried out provided that the regeneration period is extended until complete regeneration of the catalyst. We consider full regeneration of the catalyst when carbon containing products (CH4) are not observed at the reactor outlet stream, i.e. concentration is below 5\u2009ppm. The subsequent CO2 adsorption period is extended until the catalyst is saturated with CO2. Under this experimental conditions, i.e. full regeneration during the hydrogenation period and full saturation during the adsorption period, the maximum CO2 storage capacity (\u03a9max) is calculated by Eq. (2).Due to the cyclic nature of the operation, which alternates among different feeding compositions during the adsorption or hydrogenation periods, it is important to model how the feed enters the reactor. We showed in our previous work [26], that a first order transfer function was able to describe the evolution of CO2 concentration at the reactor entrance when the CO2 concentration is changed in step mode from 0% to 5.7% and from 5.7% to 0%. A first order transfer function was also applied to model the feeding of hydrogen at the beginning and at the end of the hydrogenation period.To solve the PDE system, the axial coordinate of the reactor was discretized based on finite differences in 19 equidistant elements. Backward and central differences were applied for the evaluation of the first and second derivatives of the concentration with respect to the reactor length, respectively. To solve the system of resulting ordinary differential equations, a program was developed in Matlab.The model has already been validated in our previous work [26] in a wide range of reactants concentration and temperature. Even though, as an example, Fig. S1 shows the temporal evolution of experimental gas phase CO2, H2O and CH4 concentration together with that simulated by the model. The operation is carried out at 350\u2009\u00b0C feeding a gas stream composed of 5.7% CO2/Ar during the storage period and 5.7% H2/Ar during the hydrogenation period. The duration of the storage period is 2.5\u2009min and an Ar purge is continued for 2\u2009min. Then the hydrogenation period is extended for 5\u2009min and finally another Ar purge is performed for 1\u2009min before starting a new cycle. As can be observed, the model accurately predicts the experimental evolution of gas phase CO2, H2O and CH4 during the CO2 adsorption and hydrogenation. Besides, the evolution of the covering factors at the reactor exist are also shown.The kinetic parameters that best fit the experimental data were calculated based on the least squares method, using the concentrations of gaseous species (CO2, CH4 and H2O) as experimental responses and the values are collected in the Table S1.\n\nFig. 1 shows the temporal evolution of gas phase CO2, CH4 and H2O predicted by the model when the operation is carried out at 350\u2009\u00b0C. A gas stream composed of 5.7% CO2/Ar is fed to the reactor during the CO2 adsorption period, which is extended for 80\u2009s. After the purging period with Ar for 120\u2009s, a gas stream composed of 5.7% H2/Ar is fed to the reactor. The hydrogenation period is extended for 160\u2009s. The covering factors for the last axial coordinate (in which the reactor has been discretized for the integration of the system of ODEs) are also collected in Fig. 1d. The dynamics of the CO2 capture and hydrogenation can be explained based on the main reactions that govern the process [23,24].During the adsorption period, CO2 is stored onto the basic sites of the catalyst (Na2O) in the form of carbonates (Na2CO3) through Eq. (10). Alternatively, CO2 can be also stored onto the hydrated form of the adsorption sites (NaOH) to form the carbonate through Eq. (11). Note that this route implies the release of H2O to the gas phase, i.e. CO2 displaces adsorbed H2O to form the carbonate. Eqs. (10) and (11) are the main reactions describing the CO2 storage process. The CO2 storage onto the catalyst can be evidenced by comparing the CO2 concentration signal at the reactor inlet (dotted line in Fig. 1a) and outlet streams (red line). Note that the CO2 concentration is lower at the reactor outlet with respect to that observed in the inlet stream, which highlights the CO2 adsorption capacity of the catalyst. In fact, the area comprised between the CO2 concentration signal at the reactor inlet and outlet streams can be directly related with the CO2 storage capacity of the catalyst. See Fig. S2 in supplementary material for detailed mathematical procedure to calculate the CO2 storage capacity of the catalyst.At the beginning of the CO2 storage period, the adsorption of CO2 occurs through Eq. (10). Afterwards, once the sodium oxide (Na2O) sites have been completely carbonated, the storage of CO2 can proceed through Eq. (11). Following the reaction stoichiometry, one molecule of H2O is released to the gas phase when one molecule of CO2 is stored. Thus, the storage of CO2 through Eq. (11) can be evidenced by the presence of gas phase H2O. As can be observed in Fig. 1c, water concentration breakthrough is detected after 0.25\u2009min (15\u2009s) of the storage period. Thus, for storage times lower than 15\u2009s, the storage of CO2 proceeds through Eq. (10) (without the release of H2O), and afterwards through Eq. (11), as evidenced by the increase of the H2O concentration. As the storage period proceeds, the covering factor of CO2 (\u03b8\nCO2) increases and the covering factor of H2O (\u03b8\nH2O) decreases, as can be observed in Fig. 1d. Eventually, all the storage sites of the catalyst become carbonated. Hence, the concentration of CO2 at the reactor outlet matches that of the inlet (Fig. 1a) and the CO2 covering factor reaches the value of 1 (Fig. 1d). When the catalyst is saturated with CO2 no additional H2O is released to the gas phase and the concentration of H2O progressively decreases (Fig. 1c). When H2O and CO2 coexist in the gas phase, formation of bicarbonates is possible through Eq. (12). Indeed, bicarbonates covering factor (\u03b8\nCO2/H2O) describes a maximum and then decreases following the decreasing trend observed for H2O concentration. The evolution of CO2 and H2O concentration during the storage period is governed by the kinetic equations of the model.After the storage period, CO2 is removed from the feed stream and the catalyst is purged with Ar for two minutes, observing that the CO2 concentration decreases progressively to practically zero. A slight decrease in \u03b8\nCO2 is observed (Fig. 1d), due to the desorption of part of CO2 that is weakly adsorbed. This process has been described by Eq. (13) and is modeled by Eq. (18) using a Temkin-type desorption kinetics. At the beginning of the purging period, the covering factor of water (\u03b8\nH2O) is zero because water has been completely displaced from the adsorption sites due to CO2 adsorption. Meanwhile, bicarbonates decomposition is accelerated by the elimination of gas phase CO2, which reduces their stability. Thus, the covering factor of bicarbonates (\u03b8\nCO2/H2O, Fig. 1d) is rapidly reduced to zero. The H2O formation rate during the CO2 storage period is also valid for the purging period (Eq. 20).Finally, the hydrogenation period begins admitting 5.7% H2 in the feed. The inclusion of hydrogen provokes the decomposition of adsorbed CO2, which is represented by Eq. (13) and is modeled by the first term of Eq. (21). This reaction pathway can be facilitated by the lower stability of carbonates in the presence of H2 or by a catalytic process involving the spillover of hydrogen ad-atoms to the adsorption sites [28]. In the presence of gas phase CO2 and H2, the Sabatier\u00b4s reaction (Eq. 1) proceeds and CH4 and H2O are produced. Thus, just from the beginning of the hydrogenation period CH4 formation is detected. The formation of CH4 is modeled with a potential kinetic equation recently reported by Falbo et al. [29] (Eq. 22). Note that during the whole hydrogenation period, gas phase CO2 is not observed, which highlights that the CO2 methanation rate (r\nCH4) is higher than the CO2 decomposition rate (expressed by the first term of Eq. 21). As the hydrogenation period proceeds, CH4 formation is observed while the covering factor of CO2 (\u03b8\nCO2, Fig. 1d) decreases progressively. The progressive diminution of the CO2 covering factor indicates that the adsorption sites of the catalyst are being regenerated. Due to the progressive reduction of the CO2 covering factor, the carbon source to be hydrogenated is reduced, and consequently, CH4 formation progressively decreases in the last section of the hydrogenation period.Water formation is also observed at the outlet of the reactor during the hydrogenation period (Fig. 1c). According to the Sabatier\u00b4s reaction (Eq. 1), water formation should double CH4, and should present a similar concentration profile. However, this is not observed in Fig. 1. The reason is that water interacts with the adsorption sites and is adsorbed, as described by Eq. (14). The consequence is that water formation in the gas phase is retarded with respect to CH4. Due to water adsorption on the storage sites, the covering factor of water (\u03b8H2O) increases progressively during the hydrogenation period. Water formation during the hydrogenation period is modeled by Eq. (23).Up to now, the mechanism of the CO2 storage and hydrogenation has been presented together with the kinetic equations used to model the operation. Now, we will focus on the influence of the adsorption and hydrogenation periods timing on the dynamics of the dual operation. For that, we will use the temporal evolution of the concentration of CO2, H2, CH4 and H2O as predicted by the model.\n\nFig. 2 shows the evolution of reagents and products concentration, together with the covering factors, for different adsorption and hydrogenation periods timing, i.e. t\nCO2 and t\nH2, respectively. The concentration of CO2 and H2 at reactor inlet (dotted line) is also displayed in the corresponding charts. We have selected three scenarios to understand the influence of the adsorption and hydrogenation periods timing. In Fig. 2a we have selected t\nCO2=\u200945\u2009s and t\nH2=\u2009300\u2009s as representative of a short adsorption period and long hydrogenation period. In Fig. 2b we have selected t\nCO2=\u2009150\u2009s and t\nH2=\u2009300\u2009s as representative of long adsorption and hydrogenation periods. Finally, in Fig. 2c we have selected t\nCO2=\u2009150\u2009s and t\nH2=\u200960\u2009s as representative of long adsorption period and short hydrogenation period. Depending on the adsorption and hydrogenation periods timing, large differences are observed in the evolution of gas phase CO2, H2, CH4 and H2O concentrations, which we will explain in detail below. Note that the evaluation of the CO2 adsorption and hydrogenation performance has to be done in the whole operation, considering the adsorption and hydrogenation performances. As this is a cyclic operation, alternating consecutive adsorption and hydrogenation periods, the state of the catalyst at the beginning of a given period depends on the state of the catalyst at the end of the previous period. For example, the CO2 adsorption performance will be dependent on the state of the catalyst at the end the previous hydrogenation period. The same is applied for the hydrogenation period, which performance also depends on the state of the catalyst at the end of the previous adsorption period.As can be observed in the upper chart of Fig. 2a, an adsorption period of 45\u2009s does not achieve the saturation of the catalyst with CO2. Note that at the end of the adsorption period, the CO2 concentration at the reactor outlet is notably below the CO2 concentration at the reactor inlet. Besides, the CO2 covering factor depicted in the lower chart shows a value below 1, i.e. \u03b8\nCO2\n=\u20090.6. At this point, it is important to emphasize that the CO2 adsorption takes place following an adsorption front, which moves forward along the reactor length as the adsorption sites are spent or carbonated. To illustrate the previous statement, \nFig. 3a shows the evolution of the CO2 covering factor (\u03b8\nCO2) along the reactor length during the adsorption period. As can be observed, at the beginning of the adsorption period, the CO2 covering factor is 0 along the reactor length, which highlights that the catalyst has been fully regenerated in the previous regeneration period. Then, as the adsorption period proceeds, CO2 is captured by the catalyst and thus \u03b8\n\nCO2\n increases. Note that the CO2 is preferentially captured in the reactor entrance, leaving the adsorption sites located downstream empty and available for the CO2 capture. As the adsorption period continues, the covering factor at the reactor entrance gets more and more saturated, and thus, adsorption sites located downstream start to be filled. At the end of the adsorption period, the CO2 covering factor at the reactor entrance is 1 (meaning a complete saturation) but the CO2 covering factor at the reactor outlet is 0.6 (as can be also observed in Fig. 2). Thus, under this operating conditions (t\nCO2=45\u2009s and t\nH2=300\u2009s), the catalyst is not fully saturated at the end of the adsorption period.There is another phenomenon that should be studied during the CO2 adsorption period, i.e. the release of water displaced by the CO2 adsorption onto the storage sites to the gas phase. As already reported in the previous section, first CO2 is adsorbed onto the free adsorption sites (Eq. 10), and once those sites are occupied, the storage of CO2 proceeds with the displacement of water (Eq. 11). This is the reason why water detection (Fig. 2c) is retarded with respect to the beginning of the adsorption period. Then, water concentration starts to increase but the adsorption period finishes before reaching the maximum value. Again, we will rely on the evolution of the H2O covering factor along the reactor length during the adsorption period (Fig. 3b) to better understand the state of the catalyst. As can be observed, the H2O covering factor (\u03b8\nH2O\n) is not zero at the beginning of the adsorption period, because some of the adsorption sites are hydrated at the end of the previous hydrogenation period. Note that \u03b8\nH2O is higher at the reactor outlet due to the dynamics of the regeneration, which will be explained later. As can be observed, \u03b8\nH2O is hardly affected in the first 15\u2009s of the adsorption, because the adsorption of CO2 is being taken place in the free adsorption sites. Afterwards, \u03b8\nH2O starts to decrease at the reactor entrance, where the occupation of the adsorption sites by CO2 is higher (Fig. 3a). At the end of the adsorption period, water has been completely removed from the adsorption sites (\u03b8\nH2O=0) at the reactor entrance but there is still water adsorbed at the rear of the reactor. This is because the CO2 adsorption front does not reach the rear of the reactor and consequently does not displace adsorbed water.During the hydrogenation period, the evolution of H2, CH4 and H2O is also observed (Fig. 2a). As can be observed, H2 concentration at the reactor outlet is, at any time, lower than that fed to the reactor (dotted line). This fact indicates that H2 is being consumed through the Sabatier\u00b4s reaction (Eq. 1) to produce CH4 and H2O. CH4 is immediately detected after the beginning of the hydrogenation period. As the hydrogenation period proceeds, the covering factor of CO2 is progressively reduced. Eventually \u03b8\nCO2 reaches a value near 0 at the end of the regeneration period, which reveals a complete regeneration of the catalyst. In line with the complete regeneration of the catalyst, CH4 concentration is insignificant at the effluent of the reactor at the end of the regeneration period. As already explained in the previous section, water detection in the gas phase is retarded with respect to CH4 because water is adsorbed onto the storage sites. Thus, \u03b8\nH2O increases with the hydrogenation time. In order to clarify the state of the catalyst during the hydrogenation, we will comment on the evolution of \u03b8\n\nCO2\n and \u03b8\n\nH2O\n along the reactor length during the hydrogenation period (Fig. 3c and d). The first interesting phenomena to highlight is that the regeneration of the catalyst (decrease \u03b8\n\nCO2\n) does not occur following a regeneration front. Opposite to that observed for the CO2 adsorption in Fig. 3a, the decrease of \u03b8\nCO2 occurs homogeneously along the reactor length. At the end of the regeneration period, the occupation of the adsorption sites by CO2 is insignificant along the reactor length. With respect to \u03b8\nH2O, it can be observed that the occupation of the storage sites with H2O is null in the whole reactor length at the beginning of the hydrogenation period. Afterwards, \u03b8\nH2O increases due to the adsorption of water (produced through the Sabatier\u2019s reaction). Water adsorption explains the retard observed in the detection of water in the gas phase with respect to CH4 (Fig. 2a). Water preferentially occupies the positions of the rear of the reactor because water produced at the reactor entrance is adsorbed in subsequent positions of the reactor axial coordinate. At the end of the regeneration period, the occupation of the adsorption sites by H2O is significant, specifically at the rear of the reactor.To sum up, under this operating conditions (t\nCO2=45\u2009s and t\nH2=300\u2009s), the catalyst is not fully saturated with CO2 at the end of the adsorption period and some water remains adsorbed, specifically in the adsorption sites located at the rear of the reactor. On the other hand, the catalyst is fully regenerated at the end of the hydrogenation period, i.e. almost no CO2 is adsorbed at any position of the reactor axial coordinate. However, a significant fraction of the storage sites is occupied by H2O.In Fig. 2b we will explain the performance of the catalyst when the adsorption and hydrogenation periods timing is t\nCO2/t\nH2=\u2009150/300, representative of long storage and hydrogenation periods. As can be observed, the main difference during the CO2 adsorption period (with respect to a shorter storage period of 45\u2009s, Fig. 2a) is that the CO2 concentration at the reactor outlet matches that of the inlet at the end of the adsorption period. This fact reveals a total saturation of the catalyst with CO2, which can be corroborated by the fact that \u03b8\nCO2 reaches a value of 1. The evolution of the CO2 covering factor along the reactor length during the adsorption period (Fig. S3a) shows the same trend as in the shorter storage period (Fig. 3a). The unique difference is that a longer storage period of 150\u2009s results in the total saturation of the catalyst with CO2 in the whole reactor length, as opposite to the partial saturation observed with a shorter duration of 45\u2009s. The second difference is that water concentration peak is totally developed. In fact, at the end of the adsorption period, water concentration is negligible after peaking at 1.1% at 1\u2009min of storage time. Besides, the covering factor of water is also zero. The evolution of the H2O covering factor along the reactor length during the adsorption period (Fig. S3b) shows that water is totally removed before the adsorption period is finished. No adsorbed water remains at the catalyst surface irrespective the reactor length.During the hydrogenation period, the performance of the catalyst running with t\nCO2/t\nH2=\u2009150/300\u2009s is similar to that shown in Fig. 2a (t\nCO2/t\nH2=45/300). The only difference is that with a longer storage time of 150\u2009s the catalyst is fully saturated with CO2, and thus, during the hydrogenation period CH4 and H2O formation is slightly enhanced. Apart from that, the state of the catalyst at the end of the hydrogenation period is almost similar with both timings (t\nCO2/t\nH2=45/300 or t\nCO2/t\nH2=150/300). A long hydrogenation period of 300\u2009s enables almost a total regeneration of the catalyst, and \u03b8\n\nCO2\n is almost zero irrespective the reactor length (Fig. S3c). As previously explained, the storage sites of the catalyst are partially occupied with H2O as can be observed in Fig. S3d.To sum up, under this operating conditions (t\nCO2=150\u2009s and t\nH2=300\u2009s), the catalyst is completely saturated with CO2 at the end of the adsorption period and no water remains adsorbed irrespective the reactor length. On the other hand, the catalyst is fully regenerated at the end of the hydrogenation period. No CO2 is adsorbed at any position of the reactor axial coordinate but a significant fraction of the storage sites is occupied by H2O.Finally, Fig. 2c shows the performance of the catalyst when the adsorption and hydrogenation periods timing is t\nCO2/t\nH2=\u2009150/60, representative of a long storage period and a short hydrogenation period. Due to a shorter regeneration period, the catalyst is not completely regenerated (as will be seen later) and some CO2 remains adsorbed in the storage sites at the beginning of the adsorption period. In fact, the CO2 covering factor for the last axial position of the reactor is 0.5 at the beginning of the adsorption period. Consequently, the CO2 adsorption capacity of the catalyst is limited and the CO2 breakthrough is earlier detected with respect to the previous t\nCO2/t\nH2 timings of 45/300 or 150/300. The evolution of the CO2 covering factor along the reactor length during the adsorption period (Fig. S4a) reveals that \u03b8\nCO2 is around 0.5 irrespective the reactor length. Afterwards, the CO2 adsorption front evolves (as observed in Figs. 3a and S3a). However, as the CO2 adsorption period begins with the catalyst partially occupied by CO2, the saturation is achieved at earlier adsorption times.\nFig. 2c shows that the hydrogenation period finishes before CH4 concentration peak is totally developed. This information, together with the fact that \u03b8\nCO2 is not cero, points out that only a partial regeneration of the catalyst has been achieved. The evolution of the CO2 covering factor along the reactor length (Fig. S4c) shows that 60\u2009s of hydrogenation is not enough to complete the regeneration. All the positions of the reactor show a rather homogeneous occupation of CO2, with a slight tendency to increase \u03b8\nCO2 with the reactor length. This means that the reactor entrance achieves a slightly higher regeneration. Due to the lower regeneration of the catalyst, less H2O is also produced through the Sabatier\u00b4s reaction, and thus, less H2O is adsorbed onto the catalyst (as can be observed in Fig. S4d).To sum up, under this operating conditions (t\nCO2=150\u2009s and t\nH2=60\u2009s), the catalyst is not fully regenerated and some CO2 together with H2O remain adsorbed onto the storage sites at the end of the regeneration period. This fact limits the CO2 adsorption capacity of the subsequent storage period and the CO2 breakthrough is earlier detected.Once the dynamics of the dual process have been analysed, now the global performance of the catalyst is evaluated based on the following parameters: CO2 conversion (Eq. 5), H2 conversion (Eq. 6) and CH4 production (Eq. 3). First, we will calculate the catalytic parameters for the CO2 adsorption and hydrogenation timings defined in the previous section. Then, we will extend the analysis for CO2 adsorption periods ranging from 10 to 150\u2009s and hydrogenation periods ranging from 20 to 300\u2009s. The evolution of CO2 conversion (X\nCO2, %), H2 conversion (X\nH2, %) and CH4 production (Y\nCH4, mmol g\u22121 cycle\u22121) will be shown as a function of the CO2 adsorption and hydrogenation periods timing in a 3D picture (\nFig. 4).The conversion of CO2 (Eq. 5) relates the percentage of CO2 stored onto the catalyst with respect to the amount of CO2 fed. When the operation is carried out with a CO2 adsorption and hydrogenation periods timing of 45/300 (Section 3.2.1, Fig. 2a) the CO2 conversion results in 57%. This high CO2 conversion is the result of a deep regeneration, which fully regenerates the adsorption sites of the catalyst. This fact enables a high CO2 adsorption performance at the beginning of the adsorption period. Besides, due to the short CO2 storage period, the catalyst does not reach saturation and the amount of CO2 leaving the reactor is limited. The CO2 conversion is significantly reduced to 12% when the CO2 adsorption and hydrogenation periods timing is 150/300 (Section 3.2.2, Fig. 2b). The longer duration of the CO2 storage period, results in the complete saturation of the catalyst. Extending the length of the adsorption period after catalyst saturation penalizes the CO2 conversion, as no CO2 is further adsorbed and all the CO2 fed to the reactor is emitted in the effluent. Finally, the CO2 conversion is further reduced to 4% when the CO2 adsorption and hydrogenation periods timing is 150/60 (Section 3.2.3, Fig. 2c). The short regeneration period does not obtain the full regeneration of the catalyst. Consequently, the catalyst is earlier saturated (the CO2 breakthrough is earlier detected) and the amount of CO2 emitted in the effluent is enhanced. The result is a further reduction of the CO2 conversion.\nFig. 4a shows the evolution of the CO2 conversion (X\nCO2, %) as a function of the CO2 adsorption and hydrogenation periods timing. For a given t\nCO2, the CO2 conversion increases with t\nH2 due to a deeper regeneration of the catalyst. For a given t\nH2, the CO2 conversion decreases with t\nCO2 due to a higher fraction of CO2 emitted after the saturation of the catalyst. Maximum CO2 conversion of 95% is obtained with t\nCO2/t\nH2 of 10/300, i.e. very short adsorption period and long hydrogenation period.The conversion of H2 is defined by (Eq. 6). When the operation is carried out with a CO2 adsorption and hydrogenation periods timing of 45/300 (Section 3.2.1, Fig. 2a) the H2 conversion results in 23%. First, it should be noted that unreacted H2 is observed at the reactor outlet from the very beginning of the hydrogenation period, which reveals a slow CO2 desorption and hydrogenation kinetics. In order to fully regenerate the catalyst, long hydrogenation periods are required, as observed in the previous section. However, due to the slow hydrogenation kinetics, high amounts of hydrogen are emitted without being converted, which reduces H2 conversion. The hydrogen conversion is hardly affected when the CO2 adsorption time is extended from 45 to 150\u2009s, i.e. t\nCO2/t\nH2 of 150/300 (Section 3.2.2, Fig. 2b). The only difference among those operating conditions is that a longer adsorption time of 150\u2009s fully saturates the catalyst. Consequently, slightly higher amounts of carbonates are adsorbed on the catalyst surface, which enhances somewhat H2 conversion to 25%. Finally, hydrogen conversion is significantly promoted to 51% when the operation is carried out with a CO2 adsorption and hydrogenation periods timing of 150/60 (Section 3.2.3, Fig. 2c). Due to the short regeneration period of 60\u2009s, the hydrogenation of carbonates occurs with a high local concentration of carbonates, which enhances hydrogenation kinetics, and thus, results in higher H2 conversion. Note that under these operating conditions, high H2 conversion is obtained but at the expense of a low CO2 conversion due to an incomplete regeneration of the catalyst.\nFig. 4b shows the evolution of the H2 conversion (X\nH2, %) as a function of the CO2 adsorption and hydrogenation periods timing. For a given t\nCO2, the H2 conversion decreases with t\nH2 due to the progressive inefficient usage of hydrogen, as already explained. For a given t\nH2, the H2 conversion is promoted up to t\nCO2=\u200960\u2009s, and afterwards, is maintained unaltered. H2 conversion is promoted in the t\nCO2 range (0\u201360\u2009s) where a progressive extension of the storage period results in a higher amount of CO2 stored. Therefore, a higher population of carbonates promotes hydrogen consumption. Storage times longer than 60\u2009s do not change the amount of CO2 stored (since the catalyst is already fully saturated) and consequently do neither modify H2 conversion. Maximum H2 conversion of 56% is obtained for t\nCO2/t\nH2 of 60/20, i.e. a storage time leading to a complete saturation of the catalyst and a very short hydrogenation period.The production of CH4 is defined by (Eq. 3). When the operation is carried out with a CO2 adsorption and hydrogenation periods timing of 45/300 (Section 3.2.1, Fig. 2a) the production of CH4 results in 219 \u03bcmol g\u22121. The long hydrogenation period guarantees the complete regeneration of the catalyst, and thus, CH4 production is promoted. A slightly higher amount of CH4 is produced (232 \u03bcmol g\u22121) when the CO2 adsorption and hydrogenation periods timing is set at 150/300 (Section 3.2.2, Fig. 2b). The extension of the adsorption period leads to the complete saturation of the adsorption sites, and consequently, CH4 production is slightly enhanced during the hydrogenation period. Finally, CH4 production is significantly reduced to 73 \u03bcmol g\u22121 when the CO2 adsorption and hydrogenation periods timing is set at 150/60 (Section 3.2.3, Fig. 2c). Due to a short regeneration period, the catalyst is not completely regenerated and CH4 concentration peak is not totally developed, as observed in Fig. 2c.\nFig. 4c shows the evolution of the CH4 production (Y\nCH4, \u03bcmol g\u22121) as a function of the CO2 adsorption and hydrogenation periods timing. Maximum CH4 production of 232 \u03bcmol g\u22121 is obtained for t\nCO2/t\nH2 of 60/300, i.e. a storage time leading to a complete saturation of the catalyst and a very long hydrogenation period to promote the complete decomposition of adsorbed carbonates and their hydrogenation to CH4.It is obvious that during the CO2 adsorption and hydrogenation, conversion of CO2 and H2 along with CH4 production should be maximized. However, as already observed in the previous section, it is not possible to look for a unique CO2 adsorption and hydrogenation period timing (t\nCO2/t\nH2) to maximize jointly three catalytic parameters. In principle, results in Fig. 4 suggest adsorption times around 60\u2009s (close to catalyst saturation) and moderate hydrogenation times, which produce a high amount of CH4 per cycle with a reasonable H2 conversion. To better select the optimal hydrogenation time, a more appropriate catalytic parameter should be the average formation rate of CH4 (\n\n\n\nr\n\u00af\n\n\n\n\n\nCH\n\n\n4\n\n\n\n\n, \u03bcmol g\u22121 s\u22121). \nFig. 5 shows the production of CH4 and the average formation rate of CH4, as function of the hydrogenation time. The adsorption time has been set at 60\u2009s in Fig. 5. As explained above (Fig. 4c), the amount of CH4 produced increases with the hydrogenation time, having a greater slope for low times. On the other hand, the average formation rate has a maximum between 80 and 100\u2009s of hydrogenation. However, 120\u2009s is selected as the optimal hydrogenation time, because also present a high average formation rate of CH4 and would allow working with three identical beds in parallel, one operating in adsorption and two regenerating producing methane. Thus, under the optimum CO2 adsorption and hydrogenation periods timing of 60/120 the production of CH4 results in 148 \u03bcmol g\u22121 cycle\u22121 (1.2 \u03bcmol g\u22121 s\u22121) and a CO2 and H2 conversion of 25% and 43%, respectively.This operation strategy, with 3 catalytic reactors, one working in adsorption and the other two regenerating producing SNG, is shown in \nFig. 6. In Scheme 1 of Fig. 6, the first reactor operates in adsorption and the second and third in hydrogenation. However, the reactors that operate in hydrogenation are out of phase. When the hydrogenation begins in the second reactor, it is fully saturated, while the third reactor is partially regenerated, having completed half of the period. In parallel, the first reactor begins the adsorption step fully regenerated. Next, Scheme 2 shows the period change in the first reactor (adsorption to hydrogenation) and in the third reactor (hydrogenation to adsorption), while the second continues to hydrogenation. Subsequently, in Scheme 3, the second reactor changes to adsorption and the third to hydrogenation. Once again, the reactors that work in hydrogenation, both in Schemes 2 and 3, are out of phase. Finally, from Scheme 3 it is changed to Scheme 1 and the operation continues cyclically alternating the schemes.The model used in this work allows predicting the temporal evolution of reagents and products during the dual operation of CO2 adsorption and methanation, considering that the adsorption sites can be occupied by CO2, H2O or simultaneously by both forming a weakly adsorbed bicarbonate. The evaluation of the CO2 adsorption and hydrogenation yield is carried out in the whole operation, considering the adsorption and hydrogenation performances. As this is a cyclic operation, the state of the catalyst at the beginning of a given period depends on the state of the catalyst at the end of the previous period. In simulations with a short adsorption period and a long hydrogenation period, the catalyst is not fully saturated with CO2 at the end of the adsorption period and some water remains adsorbed, specifically in the adsorption sites located at the rear of the reactor. On the other hand, the catalyst is fully regenerated at the end of the hydrogenation period and a significant fraction of the storage sites are occupied by H2O. In simulations with a long adsorption and hydrogenation periods, the catalyst is completely saturated with CO2 at the end of the adsorption period and no water remains adsorbed. By last, in simulations with a long adsorption period and a short hydrogenation period, the catalyst is not fully regenerated and some CO2 together with H2O remain adsorbed onto the storage sites at the end of the regeneration period. This fact limits the CO2 adsorption capacity of the subsequent storage period.The global performance of the catalyst is evaluated based on the CO2 conversion, H2 conversion and CH4 production. Maximum CO2 conversion of 95% is obtained with t\nCO2/t\nH2 of 10/300, i.e. very short adsorption period and long hydrogenation period. Maximum H2 conversion of 56% is obtained for t\nCO2/t\nH2 of 60/20, i.e. a storage time leading to a complete saturation of the catalyst and a very short hydrogenation period. In addition, maximum CH4 production of 232\u2009\u03bcmol\u2009g\u22121 is obtained for t\nCO2/t\nH2 of 60/300, i.e. a storage time leading to a complete saturation of the catalyst and a very long hydrogenation period to promote the complete decomposition of adsorbed carbonates and their hydrogenation to CH4. Therefore, it is not possible to define a unique CO2 adsorption and hydrogenation period timing (t\nCO2/t\nH2) to maximize all the above catalytic parameters. Adsorption times around 60\u2009s (close to catalyst saturation) and moderate hydrogenation times, which produce a high amount of CH4 per cycle with a reasonable H2 conversion, are appropriate. To better select the optimal hydrogenation time, a new catalytic parameter is set, the average formation rate of CH4 (\n\n\n\nr\n\u00af\n\n\n\n\n\nCH\n\n\n4\n\n\n\n\n, \u03bcmol g\u22121 s\u22121). 120\u2009s is selected as the optimal hydrogenation time, which enable to work with three identical beds in parallel, one operating in adsorption and two regenerating producing methane, with a high average formation rate. Thus, under the optimum CO2 adsorption and hydrogenation periods timing of 60/120 the production of CH4 results in 148 \u03bcmol g\u22121 cycle\u22121 (1.2 \u03bcmol CH4 g\u22121 s\u22121) and a CO2 and H2 conversion of 25% and 43%, respectively.By last, we are now doing further research to readjust the model to predict the operation in the presence of O2 and H2O during the adsorption period and simulate the new optimal operating conditions, on which we will report shortly.\nAlejandro Bermejo-L\u00f3pez: Validation, Methodology, Investigation, Writing \u2013 original draft. Be\u00f1at Pereda-Ayo: Conceptualization, Methodology, Visualization, Writing \u2013 review & editing. Jos\u00e9\u00a0A. Gonz\u00e1lez-Marcos: Methodology, Software, 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 Economy and Competitiveness Spanish Ministry (CTQ2015-67597-C2-1-R and 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 (ABL) also acknowledges the Economy and Competitiveness Spanish Ministry for his PhD grant (BES-2016-077855).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.cattod.2021.08.023.\n\n\nFigure S1\n\nSupplementary material\n\n\n\n.", "descript": "\n                  CO2 methanation could play a significant role in the future energy system. The excess of renewable electric energy can be transformed into storable methane to balance the energy demand when required. Moreover, the CO2 methanation can be performed alternating steps of CO2 storage and reduction, avoiding expensive CO2 purification steps. In this work, we will use a previously developed and validated model to optimize by simulation the CO2 adsorption and hydrogenation cycles timing (t\n                     CO2\n                     /t\n                     H2). The performance of the catalyst is quantified by the CO2 conversion (X\n                     CO2, %), H2 conversion (X\n                     H2, %) and CH4 production (Y\n                     CH4, mmol g\u22121 cycle\u22121). Long adsorption and hydrogenation times result in high CH4 productions per cycle, however, low CO2 and H2 conversion. Therefore, adsorption times close to the catalyst saturation (t\n                     CO2=60\u00a0s) and moderate hydrogenation times are preferable. To better select the optimal hydrogenation time, a new catalytic parameter is set, the average formation rate of CH4 (\n                        \n                           \n                              \n                                 r\n                                 \u00af\n                              \n                           \n                           \n                              \n                                 \n                                    CH\n                                 \n                                 \n                                    4\n                                 \n                              \n                           \n                        \n                     , \u03bcmol g\u22121 s\u22121). The optimal hydrogenation time is set at 120\u2009s. In addition to having a high average formation rate of CH4, t\n                     CO2\n                     /t\n                     H2=\u200960/120 cycle timing would allow to work with three identical beds in parallel, one in adsorption mode and two in regenerating mode. With the optimum cycle timing of 60/120 the production of CH4 results in 148 \u03bcmol g\u22121 cycle\u22121 (1.2 \u03bcmol CH4 g\u22121 s\u22121) and a CO2 and H2 conversion of 25% and 43%, respectively\n               "}
{"full_text": "Data will be made available on request.Recently, with the increasing depletion of fossil fuels and growing awareness of global warming, biomass has received increasing attention as an abundant and CO2-neutral renewable energy source [1]. Gasification is one of the most promising technologies for converting biomass waste into fuel gases comprising H2, CO, CH4 and CO2, given the high volatile matter content and low N, S and ash content of biomass feedstocks, as well as its operational simplicity and flexibility of use [2,3]. The fuel gas can be fed into gas engines or turbines to generate electricity and/or heat, or it can be further processed to produce hydrogen or syngas for fuel cells and Fischer-Tropsch synthesis, respectively [3]. However, during the gasification process, undesirable byproducts such as NH3, NO\nx\n, tar and fly ash are inevitably generated, with tar being one of the most troublesome contaminants [4]. Tar is typically a complex mixture of condensable hydrocarbons containing monocyclic to polycyclic aromatic hydrocarbons and oxygen-containing compounds that can condense on the available surface as operating temperatures fall below its dew point, resulting in clogging and corrosion problems [5,6]. Additionally, the tar content is highly influenced by the gasifier type and operating conditions, ranging from 0.5 to 100\u00a0g/m3, which usually exceeds the permissible maximum of various downstream devices [7]. For example, the acceptable tar content of gasification fuel gas fed into internal combustion engines and gas turbines is typically below 100\u00a0mg/m3 and 5\u00a0mg/m3, respectively [8]. Therefore, efficient removal or conversion of tar is critical for the use of the gasification fuel gas.Several methods for tar removal have been proposed, including mechanical separation, thermal cracking, and catalytic reforming. The use of mechanical separation could result in secondary environmental pollution and the loss of energy contained in tar [6]. In thermal cracking processes, high operating temperatures (>1000\u00a0\u00b0C) are typically required to achieve desirable performance, resulting in high energy consumption [9]. Although catalytic reforming can convert tar into valuable products at relatively low temperatures, its industrial application still faces two major challenges: firstly, high reaction temperatures (>600\u00a0\u00b0C) are still required, resulting in high energy costs; and second, rapid deactivation of the catalysts due to sintering and carbon deposition compromises processing stability [10].In addition to the aforementioned approaches, non-thermal plasma (NTP) technology is receiving increasing interest as a potential alternative for tar removal due to its ability to activate reactants under mild conditions [11,12]. In NTPs, the energetic electrons have a typical temperature of 1\u201310\u00a0eV, which is high enough to initiate chemical reactions, while keeping the gas temperature low [13]. Several types of NTP have been used for tar removal, including dielectric barrier discharge (DBD) [14\u201317], corona discharge [18,19], gliding arc discharge [20\u201322] and microwave discharge [23,24]. According to the literature, the advantages of using NTP for tar removal include high tar conversion, mild reaction conditions, and operational simplicity and convenience. However, the relatively high energy consumption and low selectivity toward the desired products may limit its industrial applications.The introduction of heterogeneous catalysis into NTP, known as plasma catalysis, provides a promising approach to addressing the aforementioned issues through catalyst functionalities such as lowering activation energy and tuning product selectivity [25,26]. There are two configurations for the combination of NTP and solid catalysts: one-stage and two-stage. The one-stage configuration means that the catalyst is placed directly in the discharge zone, partially or completely filling the discharge gap, whereas the two-stage configuration means that the catalyst bed is typically placed next to the plasma reactor, with the one-stage configuration being the most common for plasma-catalytic tar removal [11]. Great efforts have been made to couple different types of NTP with catalysts for tar removal, such as corona discharge coupled with Ni/SiO2\n[27], DBD coupled with Ni/Al2O3\n[28], Ni/ZSM-5 [29], Fe/Al2O3\n[30], Mn@13X [17] and NiFe/(Mg, Al)O\nx\n\n[31], and gliding arc discharge coupled with Ni/Al2O3\n[20] and Ni-Co/Al2O3\n[32], among others. When compared to the plasma-only system, the coupling process significantly improves tar conversion, selectivity and yield of target products and energy efficiency. Moreover, comparable performance can be achieved at low temperatures when compared to catalyst-only cases. Clearly, plasma catalysis is a promising alternative for achieving effective tar conversion under mild conditions.The most attractive advantage of plasma catalysis is the potential to generate a synergistic effect by integrating NTP and catalyst, whereby the reaction performance achieved in the coupling system is better than the sum of those achieved in plasma-only and catalyst-only modes. Mei et al. combined gliding arc discharge and Ni-Co catalysts for steam reforming of mixed tar model compounds (toluene and naphthalene), and a synergistic effect was successfully obtained in terms of tar conversion, energy efficiency, and yield and selectivity for H2, CO2 and CH4\n[32]. In our previous study, toluene removal was carried out in a DBD reactor coupled with Ni catalysts using a simulating gasification gas. At 400\u00a0\u00b0C, the highest toluene removal of 91.7% was achieved, which was significantly higher than the sum (58.1%) of those obtained in the catalyst-only and plasma-only processes [33]. The synergistic effect is resulted from the complicated interactions between NTP and solid catalysts, and a detailed understanding of the synergistic effect is critical to facilitate the design and optimization of plasma reactors and catalysts, thus achieving better performance at a lower energy consumption. However, there has been very little research into the synergistic effect of plasma catalysis, particularly for tar removal. For instance, the relationship between the synergistic effect and key factors such as operating conditions and catalysts is unclear.In this work, plasma-catalytic steam reforming of tar was carried out using a DBD reactor. Toluene was selected as a model tar compound as it is one of the main compounds with high thermal stability in tar products [34], and Ni/\u03b3-Al2O3 was used as a catalyst because of its high activity and low cost. The effects of three key factors (reaction temperature, calcination temperature of catalysts and relative permittivity of packing materials) on reaction performance and synergistic effect were investigated. Moreover, the characteristics of both the discharge and the catalyst were investigated using various approaches to gain a better understanding of the synergistic effect in the plasma-catalytic reforming of tar.The Ni/\u03b3-Al2O3 catalysts used in this work were prepared using a wetness impregnation method. Before use, the commercial strip-shaped \u03b3-Al2O3 support (diameter\u00a0\u00d7\u00a0length: 3 \u00d7 (4 \u2013 10) mm, specific surface area: 169\u00a0m2/g, Jiangsu Jingjing New Materials Co., Ltd, China) was calcined in air at 550\u00a0\u00b0C for 3\u00a0h before being crushed and sieved to particle sizes of 40\u201360 mesh. An appropriate weight of \u03b3-Al2O3 was added to an aqueous solution of Ni(NO3)2\u00b76H2O and impregnated overnight at room temperature. Following impregnation, the catalyst precursor was dried at 120\u00a0\u00b0C for 10\u00a0h and before being calcined in an air atmosphere for 4\u00a0h at different temperatures (450, 500, 550 and 600\u00a0\u00b0C). The as-prepared catalysts were labeled as NA(x), where\u00a0x\u00a0represents the calcination temperature. The accurate Ni loading was determined to be 8.9\u00a0wt% using the inductively coupled plasma optical emission spectroscopy (ICP-OES).The specific surface area and pore volume of the catalysts were determined by N2 adsorption/desorption isotherms at \u2212196\u00a0\u00b0C using a surface area analyzer (ASAP 2010, Micromeritics). Prior to the measurement, the samples were degassed at 200\u00a0\u00b0C for 10\u00a0h under vacuum.Powder X-ray diffraction (XRD) measurements were performed on an X-ray diffractometer (PANalytical, X\u2019pert Pro MPD) equipped with a Cu K\u03b1 (\u03bb\u00a0=\u00a00.154\u00a0nm) radiation source (40\u00a0kV and 40\u00a0mA) in the scanning range of 10-80\u00b0. The average crystallite size of Ni nanoparticles (NPs) was calculated by Scherer\u2019s equation [35]:\n\n(1)\n\n\nD\nNi\n\n=\nK\n\u00d7\n\u03bb\n/\n(\n\u03b2\n\u00d7\ncos\n\u03b8\n)\n\n\nwhere the dimensionless shape factor K is 0.9, and \u03b2 is the full width at half maximum of the Ni (200) peak at 51.7\u00b0.H2-temperature programmed reduction (H2-TPR) measurements were carried out on a TPR instrument (ChemStar, Quantachrome). Before the measurement, 50\u00a0mg of sample was preheated in a He stream at 300\u00a0\u00b0C for 30\u00a0min before being cooled to room temperature. The reaction chamber was then filled with 50\u00a0mL/min of 10\u00a0vol% H2/Ar gas while the temperature was raised from 40 to 900\u00a0\u00b0C at a heating rate of 10\u00a0\u00b0C/min.The CO2 and NH3 temperature programmed desorption (CO2-TPD and NH3-TPD) were performed on a TPD instrument (ChemStar, Quantachrome). Prior to the adsorption, 150\u00a0mg of sample was reduced at 650\u00a0\u00b0C in 10\u00a0vol% H2/Ar (50\u00a0mL/min) for 1\u00a0h and then cooled to 50\u00a0\u00b0C in He flow. The sample was subsequently heated to 50\u00a0\u00b0C or 100\u00a0\u00b0C for CO2 or NH3 adsorption, respectively. The adsorption of CO2 or NH3 was conducted by flowing pure CO2 or 7.9\u00a0vol% NH3/He (50\u00a0mL/min) through the sample, respectively. After adsorption for 1\u00a0h, the sample was purged with the He flow until baseline stabilization, and then heated up to 800\u00a0\u00b0C with a heating rate of 10\u00a0\u00b0C/min in the He flow (50\u00a0mL/min). The corresponding TPD spectra were obtained by monitoring the desorbed CO2 or NH3 using a thermal conductivity detector.The pulse chemisorption of CO was carried out on a chemisorption apparatus (AutoChem II 2920, Micromeritics). Prior to the measurement, 0.5\u00a0g of sample was reduced at 650\u00a0\u00b0C for 1\u00a0h in a 10\u00a0vol% H2/Ar (50\u00a0mL/min) atmosphere and then cooled to 50\u00a0\u00b0C in He flow. The CO chemisorption was operated by injecting 0.5\u00a0mL of 8\u00a0vol% CO/He and repeating the procedure every 6\u00a0min until the CO peaks became identical. The CO uptake was measured by a thermal conductivity detector and used for the calculation of the Ni metal surface area using the following equation [36]:\n\n(2)\n\nS\n\nA\nNi\n\n\n\n\nm\n2\n\n/g-catal.\n\n\n=\nX\n\u00d7\nSF\n\u00d7\nN\n\u00d7\nRA\n\n\nwhere\u00a0X\u00a0is the CO uptake in moles per gram of catalyst (mol/g-catal.), SF is the stoichiometric factor (1), N\u00a0=\u00a06.023\u00a0\u00d7\u00a01023 Ni atoms/mol, and RA is the atomic cross-sectional area of Ni (0.0649\u00a0nm2).In addition, the dispersion degree (%D) and the average particle size (d\n\nNi\n) of Ni were calculated by the following equations [37]:\n\n(3)\n\n%\nD\n=\n1.17\n\u00d7\nX\n/\n\n\nW\n\u00d7\nf\n\n\n\n\n\n\n\n(4)\n\n\nd\n\nN\ni\n\n\n\n\nn\nm\n\n\n=\n97.1\n/\n%\nD\n\n\nwhere W is the weight percentage of nickel, and f is the reduction degree.Thermogravimetric analysis (TG, STA409PC, NETZSCH) combined with a mass spectrometry (MS, QMS403, NETZSCH) was used to characterize the spent catalysts. The samples were heated from 40\u00a0\u00b0C to 900\u00a0\u00b0C at a heating rate of 10\u00a0\u00b0C/min and an air flow rate of 30\u00a0mL/min.The FTIR spectra of spent catalysts were recorded by an infrared spectrometer (INVENIO-S, Bruker) in the range of 400\u20134000\u00a0cm\u22121 with a resolution of 4\u00a0cm\u22121. Before the measurement, 1\u00a0mg of each sample was mixed with 100\u00a0mg of KBr (purity\u00a0>\u00a099%, Aladdin), and the mixtures were pressed into wafers with a diameter of 13\u00a0mm.Catalyst surface analysis was performed on an XPS instrument (ESCALAB 250Xi, Thermo Fisher) equipped with an Al (K\u03b1) (hv\u00a0=\u00a01486.6\u00a0eV) X-ray radiation source. All binding energies were calibrated based on the C1s hydrocarbon peak at 284.6\u00a0eV.\nFig. 1\n shows a schematic diagram of the experimental setup. The DBD reactor consists of a cylindrical corundum ceramic tube (i.d. 19\u00a0mm, o.d. 25\u00a0mm) wrapped with a 50-mm-long stainless-steel mesh as the outer electrode. A stainless-steel rod (diameter 16\u00a0mm) is placed along the axis of the tube as the inner electrode. Hence, the discharge gap is 1.5\u00a0mm with a corresponding discharge volume of\u00a0\u223c4.1\u00a0mL. The catalysts were held in place by a stainless-steel sieve attached to the end of the inner electrode, and quartz sand (40\u201360 mesh) was used to fill the region between the lower edge of the discharge zone and the stainless-steel sieve. In the experiments, 0.4\u00a0g of catalyst (about 0.8\u00a0mL), 1\u00a0mL of packing material (quartz, corundum, zirconia ceramics or silicon carbide) and 3\u00a0mL of quartz sand, all having the same particle size of 40\u201360 mesh, were placed at the discharge zone after being fully mixed. Then, plasma catalysis and catalyst-only modes can be achieved by turning the plasma on and off, respectively. In addition, the catalyst can be replaced by quartz sand with the same particle size to evaluate the performance of a plasma-only mode. The DBD reactor was placed inside a tubular furnace with a temperature range of room temperature to 750\u00a0\u00b0C. The reaction temperature was measured using a K-type thermocouple located on the outside reactor tube wall at the midpoint of the discharge zone after the reaction reached a stable stage. The catalysts were reduced in situ in a flowing 10\u00a0vol% H2/N2 at 650\u00a0\u00b0C for 1\u00a0h before the experiments. After each experiment, the reactor was cleaned by heating it to 700\u00a0\u00b0C for 1\u00a0h in an air atmosphere to remove carbon deposition and other contaminants formed during the reactions.Toluene and H2O were pumped into the mixing chamber by two syringe pumps (LSP01-1A, Longer Pump) with a flow rate of 5.196\u00a0\u03bcL/min and 11.64\u00a0\u03bcL/min, respectively, to attain a constant steam/carbon (S/C) molar ratio of 2. Subsequently, toluene and H2O were vaporized and mixed with 133\u00a0mL/min carrier gas (N2) in a mixing chamber with a temperature of 250\u00a0\u00b0C before being fed into the DBD reactor. The produced gas stream passed through two absorption bottles, which were connected in-line and placed in an ice water bath. The former one contained 50\u00a0mL of n-hexane or isopropanol solvent to collect unconverted toluene or condensable byproducts, respectively, while the latter was left empty to collect entrained droplets. To avoid condensation of water vapor, toluene and liquid products, the pipeline between the mixing chamber and the inlet, as well as the pipeline connecting the outlet to the absorption bottle and the vent were heated to 200\u00a0\u00b0C during the experiments.The plasma was generated by an AC high voltage power supply (CTP-2000\u00a0K, Nanjing Suman) with a peak voltage of 30\u00a0kV and a frequency of 5\u201320\u00a0kHz. The frequency was kept at 7.5\u00a0kHz in this work. The applied voltage (V) of the DBD reactor was measured by a high voltage probe (P6015A, Tektronix). The charge (Q) and current were obtained by measuring the voltage drops on a capacitor (0.1 \u03bcF) and a resistor (200\u00a0\u03a9), respectively. These signals were recorded by a digital oscilloscope (DPO2024B, Tektronix). In this work, the discharge power was determined by multiplying the area of the V-Q Lissajous diagram with the frequency and was fixed at 13\u00a0\u00b1\u00a00.5\u00a0W.The unconverted toluene and by-product (benzene) collected by the n-hexane-containing bottle in 5\u00a0min were analyzed using gas chromatography (GC, GC-2014, Shimadzu) equipped with a capillary column (AE-PEG-20\u00a0M, ATEO) and a flame ionization detector. The gas products were analyzed by an online gas chromatography system (GC, Micro GC490, Agilent) equipped with two thermal conductivity detectors, as well as a Molsieve 5A and PoraPLOT Q column. After a 1\u00a0h reaction, liquid products were collected by the isopropanol-containing bottle and analyzed by an off-line gas chromatography-mass spectrometry instrument (GC\u2013MS, Thermo Fisher, Trace 1300-ISQ) equipped with a DB-5\u00a0ms column (Agilent). Further details on the GC and GC\u2013MS measurements are presented in Table S1.The toluene conversion X\ntoluene and energy efficiency E were determined by following equation:\n\n(5)\n\n\nX\ntoluene\n\n\n\n(\n%\n)\n\n\n=\n\n\n\n\n\n[\nT\n]\n\nin\n\n\n-\n\n\n\n[\nT\n]\n\nout\n\n\n\n\n[\nT\n]\n\nin\n\n\n\n\u00d7\n\n100\n\n\n\n\n\n(6)\n\nE\n\n\n(\ng\n/\nkWh\n)\n\n\n=\n\n\n\n\n[\nm\n]\n\nremoval\n\n\nP\n\n\u00d7\n\n60\n/\n3600000\n\n\n\n\nwhere [T]in and [T]out represent the molar concentration of toluene at the inlet and outlet, respectively, and P represents the discharge power in watt and [m]removal represents the grams of toluene removed per minute.Note that the external heat power was not taken into account in the calculation of energy efficiency, in consistence with previous works [29,30,38].The yield Y and selectivity S of the products, and the total gas yield Y\nT were calculated by equations 7\u201312. As we cannot measure the conversion of H2O in this study, the selectivity of H2 cannot be determined.\n\n(7)\n\n\nY\n\nH\n2\n\n\n\n\n(\n%\n)\n\n\n=\n\n\n\n\n\n\n\n[H\n2\n\n]\n\nout\n\n\n\n4\n\n\u00d7\n\n\n\n[\nT\n]\n\nin\n\n\n+\n\n\n\n[\n\nH\n2\n\nO\n]\n\nin\n\n\n\n\n\u00d7\n\n100\n\n\n\n\n\n(8)\n\n\nY\n\nCO\nx\n\n\n\n\n(\n%\n)\n\n\n=\n\n\n\n\n\n\n[\n\nCO\nx\n\n]\n\nout\n\n\n\n7\n\n\u00d7\n\n\n\n[\nT\n]\n\nin\n\n\n\n\n\u00d7\n\n100\n\n\n\n\n\n(9)\n\n\nY\n\n\nC\nx\n\n\nH\ny\n\n\n\n\n\n(\n%\n)\n\n\n=\n\n\n\nx\n\n\u00d7\n\n\n\n[\n\nC\nx\n\n\nH\ny\n\n]\n\nout\n\n\n\n7\n\n\u00d7\n\n\n\n[\nT\n]\n\nin\n\n\n\n\n\u00d7\n\n100\n\n\n\n\n\n(10)\n\n\nS\n\nCO\nx\n\n\n\n\n(\n%\n)\n\n\n=\n\n\n\n\n[\n\nCO\nx\n\n]\n\nout\n\n\n7\n\n\u00d7\n\n(\n\n\n[\nT\n]\n\nin\n\n\n-\n\n\n\n[\nT\n]\n\nout\n\n)\n\n\n\n\u00d7\n\n100\n\n\n\n\n\n(11)\n\n\nS\n\n\nC\nx\n\n\nH\ny\n\n\n\n\n\n(\n%\n)\n\n\n=\n\n\n\nx\n\n\u00d7\n\n\n\n[\n\nC\nx\n\n\nH\ny\n\n]\n\nout\n\n\n\n7\n\n\u00d7\n\n(\n\n\n[\nT\n]\n\n\nin\n\n\n\n-\n\n\n\n[\nT\n]\n\nout\n\n)\n\n\n\n\u00d7\n\n100\n\n\n\n\n\n(12)\n\n\nY\nT\n\n\n\n(\n%\n)\n\n\n=\n\n\n\n\n\n\n[\n\nH\n2\n\n]\n\nout\n\n\n+\n\n\n\n\n[\nCO\n]\n\nout\n\n\n+\n\n\n\n[\n\nCO\n2\n\n]\n\nout\n\n+\n\n\n\n\n[\n\nCH\n4\n\n]\n\nout\n\n\n+\n\n\n\n\n\n[C\n2\n\n]\n\nout\n\n\n+\n\n\n\n[\n\nC\n3\n\n]\n\nout\n\n\n\n\n\n[\nT\n]\n\n\nin\n\n\n\n+\n\n\n\n[\n\nH\n2\n\nO\n]\n\nin\n\n\n\n\n\u00d7\n\n100\n\n\nwhere [H2]out, [CO\nx\n]out, [C\nx\nH\ny\n]out are the molar amounts of H2, CO\nx\n (CO and CO2) and C\nx\nH\ny\n (CH4, C2H2, C2H4, C2H6, C3H6, C3H8 and C6H6) at the outlet, respectively, and [H2O]in is the molar amount of H2O at the inlet. C2 contains C2H2, C2H4 and C2H6, and C3 includes C3H6 and C3H8.The carbon balance B\nC of the plasma catalytic process was determined by equation (13).\n\n(13)\n\n\nB\nC\n\n\n\n(\n%\n)\n\n\n=\n\n\u2211\n\nS\n\n\nC\nx\n\n\nH\ny\n\n\n\n\n\n(\nx\n\n=\n\n1\n,\n\n2\n,\n\n3\n,\n\n6\n)\n\n\n\n(\n%\n)\n\n\n+\n\n\nS\n\nCO\nx\n\n\n\n\n(\nx\n\n=\n\n1\n,\n\n2\n)\n\n\n\n(\n%\n)\n\n\n\nThe synergistic capacity SC was used to evaluate the intensity of the synergistic effect between plasma and catalysts and calculated by equation (14).\n\n(14)\n\nS\n\nC\n\u03be\n\n\n(\n%\n)\n\n\n=\n\n\n\n\n\u03be\np+c\n\n\n-\n\n\n\u03be\np\n\n\n-\n\n\n\u03be\nc\n\n\n\n\n\u03be\np\n\n\n+\n\n\n\u03be\nc\n\n\n\n\n\u00d7\n\n100\n\n\n\nWhere \u03be can be the toluene conversion, and the yield and selectivity of gas products. The subscripts, p\u00a0+\u00a0c, p and c, represent the performances obtained by plasma catalysis, plasma-only and catalyst-only, respectively.\nFig. 2\na shows the toluene conversion obtained in the plasma-only, catalyst-only and plasma catalysis modes at different reaction temperatures. Quite different removal behaviors are observed among these processes. In the catalyst-only system, the conversion of toluene increased progressively with increasing reaction temperature, reaching a maximum of 45.4% at 450\u00a0\u00b0C. However, in the plasma-only system, the toluene conversion gradually decreased from 96.5% at 200\u00a0\u00b0C to 67.1% at 450\u00a0\u00b0C. In the plasma catalysis process, the conversion of toluene reached a maximum of\u00a0\u223c100% at 200\u00a0\u00b0C, then remained at\u00a0\u223c95% in the temperature range of 250\u00a0\u00b0C to 350\u00a0\u00b0C, then declined to 79.4% at 400\u00a0\u00b0C, followed by a rise to 87.1% at 450\u00a0\u00b0C. Consistent with the change trend of the conversion, the energy efficiency of plasma catalysis first decreased and then increased with increasing temperature, ranging from 16.6 to 20.8\u00a0g/kWh (Fig. 2b). Fig. 2c shows the effects of reaction temperature on total gas yield. In contrast to the decreasing trend observed in the plasma-only mode, an increase in total gas yield with temperature is observed in both catalyst-only and plasma catalysis modes, especially above 350\u00a0\u00b0C, indicating that the efficient production of gas products is strongly dependent on catalysts. At 450\u00a0\u00b0C, the maximum gas yields for catalyst-only and plasma catalysis were 39.2% and 72.6%, respectively. As a result, the combination of NTP and catalysts at 450\u00a0\u00b0C achieved 87.1% toluene conversion, 72.6% total gas yield and 18.2\u00a0g/kWh energy efficiency, outperforming the plasma-only and catalyst-only modes.The effect of reaction temperature on the selectivity and yield of gas products and benzene is presented in Figs. S1, S2 and S3. In the plasma-only system, the selectivity and yield of gas products and benzene maintain below 14% within the temperature range of 200\u2013400\u00a0\u00b0C, with CO and H2 being the main products. When the temperature increased to 450\u00a0\u00b0C, the selectivity and yield of CO, H2, and C2 dropped significantly to 2%, becomes the dominating component in produced gases. In the catalyst-only case, due to the increasing catalytic activity, at higher temperatures all gas products obtain higher selectivity and yield values, which are much higher than those in the plasma-only mode. H2, CO, CO2 and benzene were the major products, and particularly, the selectivity of benzene was even up to 45%, which is an unsatisfactory result considering the steam reforming pursuing the production of combustible gases. In plasma catalysis, the selectivity and yield of CO, CO2 and H2, at temperatures below 350\u00a0\u00b0C, maintain less than 10%, and then increase markedly with rising temperature, obtaining the maximums, most of which are higher than those obtained by catalyst-only. Meanwhile, the plasma catalytic process greatly lowers the selectivity of benzene, CO, H2 and CO2 being the main products. For instance, at 450\u00a0\u00b0C, the CO selectivity is up to nearly 50% with a corresponding yield of about 45%, together with the selectivity of benzene of less than 10%.At temperatures below 350\u00a0\u00b0C, plasma catalysis shows satisfactory results in converting toluene (Fig. 2), but its poor performance in generating gas products indicates that most of the toluene is converted into liquid products and/or carbon deposition. This carbon deposition is an unwanted byproduct that can reduce the catalytic activity by blocking active sites and lowering the discharge power, leading to a decline in plasma catalysis performance [28]. To gain a better understanding of reaction products, liquid products and carbon deposition produced at temperatures of 200\u00a0\u00b0C and 450\u00a0\u00b0C were analyzed. Researchers conducted GC\u2013MS analysis of liquid products and TG-MS, FTIR, and XPS analysis of spent catalysts, and the results are shown in Fig. S5, Table S2, and Fig. S6. At 200\u00a0\u00b0C, most of the toluene was converted into carbon deposition, which was mainly composed of aliphatic carbon and easier to eliminate. At 450\u00a0\u00b0C, the increasing catalytic activity improved the oxidation reactions of carbonaceous species, leading to a decrease in the amount of carbon deposits and a corresponding increase in CO and CO2 production. Additionally, at both temperatures, a small portion of the removed toluene was converted into liquid products with a molecular weight greater than benzene. The number and relative area of O- and N-containing compounds were higher at 200\u00a0\u00b0C than at 450\u00a0\u00b0C. These liquid products were formed through reactions between intermediates and fragments or radicals, such as CN and OH radicals [39].Previous studies have shown that changes in reaction temperature can affect the physical properties of plasma and influence its chemistry [40,41]. Thus, we investigated the discharge characteristics of the DBD at different temperatures. The packed-bed effect in this study resulted in a combination of filamentary discharge and surface discharge. The introduction of packing material pellets reduced the available discharge volume, leading to the formation of filaments only in the void between the pellets and the reactor wall. Furthermore, an increased electric field, due to polarization effects and charge accumulation, was found to be mainly located around the contact points between the pellets, where surface discharge was formed and propagated along the surface [42,43]. Fig. 3\n shows the electrical signals of the DBD operating at different temperatures with a fixed discharge power. The current signal of the discharge was quasi-sinusoidal with multiple superimposed current pulses per half-cycle of the applied voltage. When the reaction temperature increased from 200\u00a0\u00b0C to 450\u00a0\u00b0C at the same discharge power, the magnitude of the current pulses decreased, but the displacement current increased correspondingly, as well as the applied voltage decreasing from 10.4 kVpk-pk to 8.8 kVpk-pk (as shown in Fig. 3a and b). These changes indicate that filamentary discharge weakened while the component of surface discharge increased at high temperatures [42,44].\nFig. 3c shows the Lissajous curves of the DBD at different reaction temperatures while keeping the discharge power constant. As the temperature increases, the shape of the curve transforms from a parallelogram to an oval, indicating changes in the discharge characteristics. The Lissajous curve enables us to determine the the onset voltage (Uon) and the effective capacitances of the dielectric barrier (Cd) and the total system (Ctot). Using these parameters, we can calculate the capacitance of the gas (Cg), the breakdown voltage in the gas gap (Ub), and the average reduced electric field (E/n). The calculation process is detailed in section 6 of the Supporting Information. Table S3 summarizes the estimated parameters at different temperatures. Fig. 3d shows the E/n decreases from 96.8 Td to 80.7 Td as the temperature increases from 200\u00a0\u00b0C to 450\u00a0\u00b0C. This phenomenon has also been observed in plasma-assisted cellulose reforming [40] and plasma CH4 reforming [41].Furthermore, the mean electron energy at different E/n values can be calculated using the Boltzmann equation and BOLSIG+ [45\u201347], as shown in Fig. 4\na. The mean electron energy increases with rising E/n, however, as temperature increases in the range of 200\u2013450\u00a0\u00b0C, it progressively declines from 1.95\u00a0eV to 1.50\u00a0eV. This decline in mean electron energy weakens the plasma chemistry trigger, which negatively impacts the reaction performance of plasma. In the plasma toluene steam reforming process, important active species such as excited N2 molecules, OH and O radicals initiate and drive reactions [38,48]. Hence, the rate coefficients of the electron impact reactions leading to the formation of these species were calculated using BOLSIG+, and are shown in Fig. 4b. The rate coefficient of all reactions increases with increasing E/n, implying that higher rate coefficients can be achieved at lower temperatures. This result suggests that higher reaction temperatures are not favorable for generating excited species and radicals that can effectively decompose toluene.The analysis presented above allows us to draw several conclusions regarding reaction performance. In plasma catalysis, the destruction of toluene depends heavily on plasma intensity at low temperatures. The decreasing E/n caused by rising temperatures lowers the mean electron energy, reducing the production of active species, and subsequently leading to a decrease in conversion. The higher E/n values are more favorable for ring cleavage of aromatic intermediates and toluene, which is mainly initiated through reactions with energetic electrons and excited N2\n[49,50]. This could explain the higher aliphatic nature of the carbon deposits formed at low temperatures. At high temperatures, the increased catalytic activity plays a crucial role in toluene destruction, reversing the declining trend in conversion. Furthermore, the efficient formation of gaseous products is strongly dependent on catalysis, and a significant increase in gas production can only be observed at high temperatures where catalytic activity has notably increased, accompanied by a correspondingly significant decrease in carbon deposits.The synergistic effect of the process was evaluated by analyzing the toluene conversion and total gas yield. Fig. 5\na displays the values of synergistic capacities at different reaction temperatures. Synergistic capacities were calculated to evaluate the synergistic effect for toluene conversion and gas production. The results showed that the synergistic effect can only be achieved at temperatures below 350\u00a0\u00b0C for toluene conversion with a capacity of about 4%. However, the synergistic capacity decreases with increasing temperature from 350 to 450\u00a0\u00b0C. On the other hand, for gas production, the synergistic capacities remained negative at around \u221250%, at temperatures between 200 and 350\u00a0\u00b0C. The synergistic effect increased with temperature, reaching about 65% at 450\u00a0\u00b0C. Fig. S8 presents the synergistic capacities calculated using selectivity and yield of gas products. No synergistic effect was observed in terms of both selectivity and yield of all gas products at temperatures lower than 350\u00a0\u00b0C. At higher temperatures, the synergistic effect was concentrated in the yield of the main gas products (H2, CO and CO2), and the synergistic capacity significantly increased with temperature.The relationship between reaction temperature and the synergistic capacity in terms of toluene conversion and gas production is shown in Fig. 5b. The temperature dependence curve can be divided into two parts based on a threshold temperature of 350\u00a0\u00b0C. Below 350\u00a0\u00b0C, although the linear fitting method cannot achieve a satisfactory result, the low slope of the fitted straight line suggests a weak temperature dependence of the synergistic capacity in this temperature range. Above 350\u00a0\u00b0C, however, the relationship between temperature and synergistic capacity is linear and significant. The synergistic capacity in toluene conversion and gas production shows negative and positive temperature dependence, respectively.The catalytic performance of the catalyst and discharge characteristics of the DBD suggest that increasing the temperature from 200\u00a0\u00b0C to 450\u00a0\u00b0C enhances the formation of surface discharge and decreases the E/n. By contrast, the activity of the catalyst increases significantly above a threshold temperature of 350\u00a0\u00b0C (Figs. 2 and S2). The threshold temperature for the rapid increase in catalytic activity is consistent with the threshold temperature for the change in synergistic capacity. This suggests that the variation in catalytic activity plays a more important role in the generation of the synergistic effect compared to the discharge characteristics.In summary, the synergistic effect of plasma catalysis in the steam reforming of toluene is largely determined by the catalytic activity of the catalyst and is therefore greatly influenced by the reaction temperature. The synergistic effect is most pronounced at temperatures above 350\u00a0\u00b0C and is particularly noticeable in terms of gas production. Above this threshold temperature, there is a strong linear relationship between the synergistic capacity and the reaction temperature, with a negative correlation in toluene conversion and a positive correlation in gas production.The effect of calcination temperature on the performance of catalysts and the synergistic effect in steam reforming of toluene is discussed in this section. The experiments were conducted at 450\u00a0\u00b0C, which is the most suitable operating temperature for the plasma-catalytic process in this study. It is worth mentioning that the results obtained in the plasma-only mode in this section are equivalent to the results obtained at 450\u00a0\u00b0C in the previous section. This was achieved by replacing the catalyst with quartz sands to create a plasma-only mode in the study. Fig. 6\na shows the effect of different calcination temperatures on toluene conversion. It can be seen that an increase in calcination temperature leads to a decrease in both the catalyst-only and plasma-catalytic modes of conversion. For example, when the packing material is changed from NA(450) to NA(600), the conversion in the catalyst-only mode drops from 84% to 27% and in plasma catalysis it decreases from 100% to 43%. This is accompanied by a significant decrease in energy efficiency, from 20.9\u00a0g/kWh to 9.0\u00a0g/kWh (Fig. 6b). The 100% toluene conversion and 20.9\u00a0g/kWh energy efficiency achieved with NA(450) as a catalyst is a competitive result, especially in DBD systems, compared to similar works listed in Table S5. The total gas yield is presented in Fig. 6c. In the catalyst-only mode, the yield ranges from 25% to 40% and NA(500) and NA(600) give the maximum and minimum, respectively. In plasma catalysis, the total gas yield decreases significantly with the increase in the calcination temperature of the catalyst, from 85% with NA(450) packing to 35% with NA(600) packing. The results suggest that the use of a catalyst with a low calcination temperature is favorable for both toluene conversion and gas production in plasma catalysis.The results show that the selectivity and yield of gas products and benzene are influenced by the calcination temperature of the catalysts used (Figs. S11 and S12). As the calcination temperature of the catalysts increases, the selectivity of CO and benzene increases (Fig. S11), while the selectivity of CO2 achieves the highest value with NA(500). The yield of CO, CO2 and H2 decreases with the increasing calcination temperature of the catalysts used. The selectivity and yield of CH4, C2, and C3 is less than 2% and is not significantly influenced by the change of catalysts packed. In plasma catalysis, the selectivity of CO is kept at 50\u201360% and the benzene selectivity is significantly reduced compared to the catalyst-only process. The yield of the main gas products decreases with increasing calcination temperature of the catalysts.Apparently, the calcination temperature of catalysts strongly influences the reaction performance of plasma catalysis in terms of removal capacity and gas production. To better understand this effect, various characterization techniques, such as BET, XRD, H2-TPR, CO2\u2013/NH3-TPD and CO pulse chemisorption, were employed. As shown in Table 1\n, with increasing calcination temperature from 450\u00a0\u00b0C to 600\u00a0\u00b0C, the surface area of the catalyst decreases slightly from 141.0\u00a0m2/g to 134.8\u00a0m2/g, while the mean pore size rises from 11.5\u00a0nm to 11.9\u00a0nm. The XRD patterns of the reduced catalysts are presented in Fig. 7\na. The main peaks at 44.3\u00b0, 51.6\u00b0 and 76.3\u00b0 correspond to metallic nickel, and the calculated nickel particle sizes (6.1 and 6.8\u00a0nm) are slightly influenced by the calcination temperature. Fig. 7b shows the H2-TPR profiles of the catalysts calcined at different temperatures. The three main peaks, the low-, medium- and high-temperature peak, correspond to free NiO species, the NiO specie with stronger interactions with the support, and stable nickel aluminate with a spinel structure, respectively [51]. The increase of calcination temperature results in a shift of the low- and medium-temperature peaks to higher temperatures, as well as an increase in the intensity of the high-temperature peak. It indicates that a high calcination temperature strengthens the interaction between NiO species and the support, which is unfavorable for the reduction of NiO species during the activation treatment and leads to a decreased reduction degree of the catalysts. The basic and acidic properties of the catalysts were characterized by CO2\u2013 and NH3-TPD, and the results are shown in Fig. 7c and d. Clearly, the CO2 desorption curves show two broad peaks, corresponding to the desorption of weakly and strongly adsorbed CO2. Similarly, the NH3 desorption curves show three peaks, associated with weak and medium/strong acid sites [52]. Table 2\n summarizes the base/acid site distribution and density of the catalysts after the curves were deconvoluted. Interestingly, the variation in calcination temperature did not significantly affect the acidic and basic properties of the catalysts. This is likely because the basicity and acidity of the Ni/Al2O3 catalysts primarily arise from the alumina support [35,53]. As the \u03b3-Al2O3 support used in this study was already calcined at 550\u00a0\u00b0C before use, calcining the precursor within the 450\u2013600\u00a0\u00b0C range did not induce a notable or regular change in the basic and acidic properties of the catalysts. CO pulse chemisorption analysis was used to determine the Ni surface area, dispersion and particle size, and the results are presented in Table 3\n. The Ni surface area decreased with increasing calcination temperature, from 1.22\u00a0m2/g-catal. of NA(450) to 0.55\u00a0m2/g-catal. of NA(600). Notably, the lowest dispersion was obtained with NA(500) instead of NA(450). Despite this, increasing the calcination temperature appeared to enhance the metallic dispersion. On the other hand, the Ni particle sizes had an opposite trend to the dispersion with sizes ranging from 21.1 to 28.9\u00a0nm.As previously mentioned, increasing the calcination temperature resulted in only minor variations in pore structure, base and acid properties, with a linear decrease in Ni surface area and nonlinear changes in dispersion and Ni particle size. In the catalyst-only case, the progressively decreasing conversion observed with increasing calcination temperature can be attributed to the decrease in Ni surface area, which leads to a reduction in available active sites, limiting toluene destruction. On the other hand, the change in total gas yield induced by the calcination temperature of the catalysts can be explained by Ni particle size (or dispersion), given the similar change trend. It is well-known that the size or dispersion of metal particles significantly influence catalyst selectivity and, therefore, the product distribution [54,55]. Additionally, in the plasma-catalytic process, the similar and decreasing trend in both conversion and total gas yield implies that the Ni surface area plays a crucial role in determining the reaction performance of plasma catalysis.\nFig. 8\na shows the synergistic capacities calculated using toluene conversion and total gas yield for the different catalysts. No synergistic effect is observed in terms of toluene conversion. Catalysts calcined at higher temperatures tend to have lower synergistic capacities, except for NA(450). However, a clear synergistic effect is achieved in gas production regardless of the catalyst, with the synergistic capacity decreasing in the order of NA(450)\u00a0>\u00a0NA(500)\u00a0>\u00a0NA(550)\u00a0>\u00a0NA(600). The synergistic capacities for gas product selectivity and yield are presented in Fig. S9, showing that the synergistic effect in selectivity is mainly concentrated in NA(450), while for yield, a noticeable but weakening synergistic effect is observed at higher calcination temperatures. No synergistic effect is obtained for benzene and light hydrocarbons, except for CH4.As discussed earlier, the intensity of the synergistic effect appears to be closely related to the Ni surface area, which is supported by the negative correlation observed between the synergistic capacity in gas production and the calcination temperature of the catalysts. To investigate this relationship further, the correlation between Ni surface area and the synergistic capacity was analyzed for both toluene conversion and gas production, and the results are presented in Fig. 8b. Notably, the synergistic capacities obtained using NA(450) were not considered, as toluene was not detected at the outlet, making it difficult to estimate the actual values of toluene conversion and total gas yield. The results show a clear positive and linear correlation between Ni surface area and the synergistic capacity, suggesting that a higher Ni surface area is associated with a stronger synergistic effect. This finding can be explained by the fact that a higher Ni surface area provides more active sites, which increases the probability of generating a synergistic effect.The effect of the relative permittivity of packing materials was also investigated. Four packing materials were employed: quartz, corundum, zirconia ceramics, and silicon carbide, all of which were calcined at 950\u00a0\u00b0C for 6\u00a0h prior to use. Table S6 lists the composition and relative permittivity of these packing materials. The relative permittivity increases in the order of quartz\u00a0<\u00a0corundum\u00a0<\u00a0zirconia ceramics\u00a0<\u00a0silicon carbide, with silicon carbide having the highest relative permittivity of 200.3, which is much greater than that of the other materials.The conversion obtained with different packing materials is presented in Fig. 9\na. In catalyst-only experiments, the conversion remains at about 45% regardless of the packing material used, indicating that these materials have little thermal catalytic activity for toluene steam reforming. In the plasma-only mode, the use of high relative permittivity packing materials results in a decrease in conversion from about 67% with quartz packing to about 40% with silicon carbide packing. However, packing high relative permittivity materials in the plasma catalysis system leads to a slight increase in toluene conversion, and accordingly, the energy efficiency slightly increases from 18.2\u00a0g/kWh with quartz packing to 20.0\u00a0g/kWh with silicon carbide packing, as shown in Fig. 9b.Taking into consideration the limited catalytic activity of the packing materials in the catalyst-only process, their effect on product generation is minimal and not considered significant. The impact of packing materials on the total gas yield is shown in Fig. 9c. In plasma-only experiments, the use of silicon carbide as the packing material results in the lowest total gas yield, while the other materials have similar values. However, under plasma catalysis, a noticeable difference in gas production is observed, particularly in the case of silicon carbide, which exhibits the highest total gas yield of about 90%, compared to the values of less than 80% obtained with other materials. Overall, the use of packing materials with high relative permittivity has both detrimental and promoting effects on the reaction performance of the plasma-only and plasma catalysis modes, respectively.The effects of packing materials on the selectivity and yield of gas products and benzene in the plasma-only and plasma catalysis modes are illustrated in Figs. S14 and S15. In the plasma-only mode, there is only a slight variation in the selectivity and yield of gas products and benzene among quartz, corundum, and zirconia ceramics, while the use of silicon carbide leads to a significant decrease in the selectivity and yield of produced gases, along with a corresponding increase in benzene selectivity and yield. In the case of plasma catalysis, the differences in gas production among packing materials are more pronounced, particularly for silicon carbide, which results in higher CO and CH4 selectivity, as well as higher H2, CO, CO2, and CH4 yields, while also lowering the benzene selectivity and yield.To further understand the effect of the relative permittivity of the packing materials, the discharge characteristics of the DBD packed with different materials are also characterized. Fig. 10\n shows the electrical signals of the DBD packed with different materials and operated at a fixed discharge power. With the exception of silicon carbide, which shows the highest applied voltage of 9.2 kVpk-pk, other materials have values of around 8.8 kVpk-pk. As the relative permittivity of the packing materials used increases, the magnitude of current pulses gradually decreases, but the displacement current increases accordingly, especially in the case of silicon carbide. This phenomenon indicates that the use of packing materials with high relative permittivity could increase the component of surface discharge in DBD [42]. Higher relative permittivity materials are more effectively polarized, resulting in a stronger locally enhanced electric field, especially around the contact points between pellets [56]. Thus, surface discharge on the surface of pellets is easily ignited when using high relative permittivity packing material. Fig. 10c exhibits Lissajous curves of the DBD with different materials packing at a constant discharge power. Quartz, corundum, and zirconia ceramics have almost identical Lissajous curves, but that of silicon carbide is quite different. Table S4 summarizes discharge parameters calculated through different Lissajous curves, and the E/n is shown in Fig. 10d. Quartz, corundum, and zirconia ceramics have almost the same E/n values, which are much higher than that obtained with silicon carbide. Obviously, the three materials that possess close values of relative permittivity have close discharge parameters, but due to the large difference in relative permittivity, silicon carbide gets quite different values.Based on the discharge characteristics results presented, it appears that in the plasma-only mode, the decrease in toluene conversion and total gas yield when using silicon carbide can be attributed to a decline in E/n. However, in the plasma catalysis mode, the results show that the use of silicon carbide leads to higher toluene conversion and total gas yield. This improvement is likely due to the increased surface discharge in the presence of silicon carbide.\nFig. 11\na shows the synergistic capacities on conversion and total gas yield using different packing materials. The results indicate that all packing materials exhibit a synergistic effect in terms of gas production, with silicon carbide showing the highest synergistic capacity at around 120%, while the other materials have values around 70%. However, for toluene conversion, the synergistic effect is only observed in the case of silicon carbide, while the other materials show negative synergistic capacities around \u221215%.We also investigated the effect of packing material on the synergistic capacities calculated based on the selectivity and yield of gas products, as shown in Fig. S10. The results indicate that the synergistic effect is observed in the selectivity of CO, as well as in the yield of H2, CO, and CO2. In particular, silicon carbide exhibits higher and lower synergistic capacities in the yield of main gas products and benzene, respectively, compared to other packing materials. Moreover, for light hydrocarbons the synergistic effect is mainly observed in the selectivity and yield of CH4 and C3 hydrocarbons.We further examined the correlation between the relative permittivity of packing materials and the synergistic capacity in terms of toluene conversion and gas production, as shown in Fig. 11b. The results indicate that the synergistic capacity exhibits a positive and strong linear correlation with the relative permittivity of the packing materials. This finding suggests that the use of a packing material with high relative permittivity could enhance the synergistic effect.The observed correlation may be attributed to the increased surface discharge in the case of packing materials with high relative permittivity. The greater surface discharge component implies that more area on the catalyst surface is covered by the discharge, as reported in previous studies [43]. Additionally, the active species generated by discharge and involved in surface reactions through Langmuir-Hinshelwood or Eley-Rideal mechanisms are considered key drivers of the synergistic effect [26]. However, most of the active species produced by discharge activation have a short lifetime [57]. Therefore, increasing the discharge-covered area on the catalyst surface could increase the probability of active species participating in surface reactions, thereby intensifying the synergistic effect.In summary, the choice of packing material has a significant impact on the synergistic effect in plasma-catalytic processes, affecting the selectivity and yield of gas products. Moreover, using a packing material with high relative permittivity could lead to a stronger synergistic effect due to the increased surface discharge and higher probability of active species participating in surface reactions.In this study, we investigated the performance of plasma-catalytic steam reforming of toluene in a DBD plasma reactor combined with Ni/\u03b3-Al2O3 catalysts. The results showed that the toluene conversion and gas production were affected by the reaction temperature, catalyst calcination temperature, and packing material relative permittivity. At low reaction temperatures, the toluene conversion mainly depended on the intensity of the plasma, while gas production was limited. However, at high reaction temperatures, the increased catalyst activity promoted toluene conversion and enhanced the oxidation of carbonaceous species, leading to a greater production of gas products. The process achieved a high toluene conversion of 87.1%, a total gas yield of 72.6%, and an energy efficiency of 18.2\u00a0g/kWh at 450\u00a0\u00b0C. Furthermore, we found that the synergistic capacity of plasma catalysis was positively correlated with the metal surface area and relative permittivity of the packing materials, and negatively correlated with the reaction temperature in terms of toluene conversion. However, gas production had a positive correlation with reaction temperature. These findings suggest that using catalysts with lower calcination temperatures and packing materials with higher relative permittivity can improve the process efficiency. Overall, this work highlights the potential of plasma-catalytic steam reforming of toluene for sustainable hydrogen production and provides insights into optimizing the process parameters.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 National Natural Science Foundation of China (Grant No. 52106282), the National Key R&D Program of China (Grand No. 2019YFB1503902), the Strategic Priority Research Program of Chinese Academy of Sciences (Grand No. XDA21060600), the Science and Technology Program of Guangzhou (Grant No. 202102020292, 201904010098 and 202002030126) and the Natural Science Foundation of Guangdong Province of China (Grant No. 2019A1515011535). X. Tu thanks the support of the British Council Newton Fund Institutional Links Grant (No. 623389161). N. Wang thanks the University of Liverpool and the Chinese Scholarship Council for funding his PhD.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2023.142696.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  In this study, steam reforming of toluene was carried out in a dielectric barrier discharge (DBD) plasma reactor combined with Ni/\u03b3-Al2O3 catalysts. The effect of reaction temperature, calcination temperature of catalysts, and relative permittivity of packing materials, on the reaction performance and synergistic effect of plasma catalysis was investigated. The results showed that toluene conversion decreased initially and then increased with increasing temperature, due to a decreasing average reduced electric field and increasing catalytic activity at higher temperatures. At 450\u00a0\u00b0C, the process achieved a high toluene conversion of 87.1%, a total gas yield of 72.6%, and an energy efficiency of 18.2\u00a0g/kWh, demonstrating the potential of this approach for sustainable hydrogen production. Catalysts prepared at lower calcination temperatures or with higher relative permittivity packing materials perform better, owing to the larger Ni surface area available for catalytic reactions and the higher surface discharge facilitating the occurrence of surface reactions. In addition, the synergistic capacity in terms of toluene conversion and gas production exhibited a positive relationship with the metal surface area of catalysts and the relative permittivity of packing materials, while the relationship between reaction temperature and toluene conversion was negative.\n               "}
{"full_text": "Energy crisis and environmental pollution have increasingly limited the development of human society, so seeking and developing high-efficiency, eco-friendly and recycling new energy has been imminent [1\u20135]. The natural-born merits of high energy density, strong renewable ability, convenient transportation, carbon-free emission make hydrogen as a ''Holy Grail'' of new energy to replace the traditional fossil energy which is imminent depletion and nonrenewable [6,7]. Because of the relatively low energy consumption, green and clean production technology, safe and simple operation process, the new way for acquiring hydrogen by water electrolysis has received extensive attention [8,9]. Water splitting cannot occur spontaneously, but in theory, it can be achieved by applying a voltage of 1.23\u00a0V [10]. However, the limitation of the slow OER dynamics occurring at anode, the efficiency of water electrolysis is not ideal. The required driving potential for the formation of O\u2013O bond in actual conditions is much higher than that in theoretical conditions [11,12]. Therefore, it is of great significance to find appropriate means to reduce the anode reaction potential for improving the economic benefits of electrolytic water and promoting industrial application technology. In general, there are two feasible methods that have been widely accepted for reducing the driving potential. On the one hand, change the type of anodic oxidation reaction, which means replacing OER with other feasible anodic reactions with much lower theoretical oxidation potentials; on the other hand, explore and prepare robust, stable and low-cost oxygen-evolving electrocatalysts to accelerate the reaction rate and improve the reaction efficiency [13\u201316].The type of anodic electrooxidation reaction depends mainly on the choice of electrolyte. KOH solution is usually served as the electrolyte for OER, and rich OH\u2212 environment is conducive to accelerating the formation of O2. Recently, some readily oxidized nucleophile reagents are suitable candidates for replacing OER because of their superior oxidation thermodynamics, such as alcohols, aldehydes, amines and urea [17\u201320]. Among these nucleophiles, urea stands out for its superior stability, high energy density, low toxicity and abundant storage. Moreover, hydrogen production and wastewater degradation can be realized simultaneously by urea electrolysis [21,22]. Compared with the single KOH electrolyte, when a certain proportion of urea is added in the process of water electrolysis, HER (6H2O\u00a0+\u00a06 e\n-\n \u2192 3H2\u00a0+\u00a06 OH\n\u2212\n) still occurs at the cathode, while UOR (CO(NH2)2\u00a0+\u00a06 OH\n\u2212\n \u2192 N2\u00a0+\u00a05H2O\u00a0+\u00a0CO2\u00a0+\u00a06 e\n-\n) occurs at the anode (seek supporting information for the specific reaction process of urea electrolysis) [23]. Notably, the UOR process shows more favorable chemical reaction kinetics compared with the sluggish OER process, of which theoretical oxidation potential (0.37\u00a0V) is much lower than that (1.23\u00a0V) of the OER [24]. Nevertheless, the complicated steps of six-electron transfer make the gas release process difficult, so, the actual UOR process needs to be further optimized [25,26].Precious metals and their derivatives are excellent catalysts for water electrolysis, while they subjected to high price, scarce storage and weak stability [27,28]. In recent years, researches have made some achievements on non-precious metal-based electrolytic water catalysts. For example, transition metal oxides [29,30], hydroxides [31,32], sulfides [33,34], phosphides [35,36], selenides [37,38] and nitrides [39,40], which not only avoids the defects of precious metal-based catalyst, but also gain on the former level in performance constantly. Among them, transition metal sulfides (TMSs) such as MxSy (M\u00a0=\u00a0Ni, Co, Mo, Cu, Zn, etc.) are well-known for the outstanding electrocatalytic activity, resulting from its rich valence states and certain structural defects [41,42]. In particular, the complex of two or more TMSs (e.g., Co9S8@Ni3S2 [43,44], MoS2@Ni3S2 [45,46], Cu2S@Co9S8 [47], Ni3S2@Co9S8@MoS2 [48], CdS@Co9S8@Ni3S2 [49], etc.) possesses higher conductivity, larger specific surface area and more active sites due to the metal synergistic effect, defective heterointerface and hierarchical structure, which are not available in single-component TMSs. Although much progress has been made in the exploration of the mixed TMSs, it is still important but also challenging to further improve their catalytic activity for water electrolysis. Elaborate design and synthesis of mixed TMSs with appropriate structures is an effective strategy to improve their electrocatalytic property. As a new star in the family of porous crystal materials, the metal\u2013organic framework (MOF) has attracted great attention since its appearance in 1995 [50]. On the one hand, the adjustable framework structures, unique porous characteristics and clear crystal distributions contribute to abundant active sites; on the other hand, the in-situ growth of the material eliminates the use of adhesive, which significantly reduces contact resistance [51,52]. Therefore, MOF materials are widely worked as various advanced electrodes, for instance, Tang et\u00a0al. synthesized a novel hybrid nanostructure of CeOx nanoparticles dotted the Zeolitic imidazolate framework (ZIF) derived hollow CoS (CeOx/CoS) by means of interfacial engineering strategy for boosting the alkaline oxygen evolution, which only requires a low overpotential of 269\u00a0mV to delivers the current density of 10\u00a0mA\u00a0cm\u22122 [53]. Zhou et\u00a0al. carbonized MOFs on conductive support nickel foam (NF) in a few minutes by advanced laser-induced annealing technology to obtain an excellent water electrocatalyst, of which the high activity results from the remarkable adsorption of intermediates by the nickel-doped Fe3O4 overlayer formed during laser treatment [54].Intrigued by these above-mentioned studies, in this work, we firstly successfully synthesized MOF-derived ZCNS with hollow NSAs on NF through a facile two-step hydrothermal method. Keeping the total moles of Zn2+ and Co2+ ions constant, a set of parallel samples ZCNS-r (r\u00a0=\u00a01/3, 1/2, 1) were obtained by adjusting the molar ratio of Zn2+ and Co2+ ions to optimize the catalytic performance of materials. The introduction of Zn2+ ion in the first hydrothermal process directly created the unique sword-like MOF structure and the Ni\u03b4+ (\u03b4\u00a0=\u00a02 or 3) ion released from the corroded NF during the second sulfuration process also generated the Co9S8@Ni3S2 heterostructure unexpectedly. Meanwhile, in order to study the effect of Zn ion on the configuration and activity of the catalyst, a control sample (without Zn ion, called CNS) was synthesized by the same method. Finally, the close-knit and hollow ZCNS-1/2 NSAs were obtained, the material not only have an optimum activity but also display an excellent stability for catalyzing both water and urea electrolysis. It's worth noting that the ZCNS-1/2 material display superior electrocatalytic performance to deliver a certain current density for HER (97\u00a0mV@10\u00a0mA\u00a0cm\u22122, 215\u00a0mV@100\u00a0mA\u00a0cm\u22122), OER (1.463\u00a0V@20\u00a0mA\u00a0cm\u22122, 1.537\u00a0V@100\u00a0mA\u00a0cm\u22122), UOR (1.264\u00a0V@20\u00a0mA\u00a0cm\u22122, 1.316\u00a0V@100\u00a0mA\u00a0cm\u22122), water electrolysis (1.522\u00a0V@10\u00a0mA\u00a0cm\u22122, 1.721\u00a0V@100\u00a0mA\u00a0cm\u22122) and urea electrolysis (1.314\u00a0V@10\u00a0mA\u00a0cm\u22122, 1.506\u00a0V@100\u00a0mA\u00a0cm\u22122). This research will provide certain reference to design and synthesize MOF-derived trimetallic sulfides as efficient and\u00a0stable electrocatalyst for enhanced water and urea electrolysis.Concentrated hydrochloric acid (HCl, 12\u00a0mol/L), acetone (CO(CH3)2, >99%), Cobalt nitrate hexahydrate (Co(NO3)2\u00b76H2O, >99%), Zinc nitrate hexahydrate (Zn(NO3)2\u00b76H2O, >99%), 2-Methylimidazole (C4H6N2, >98%), Thioacetamide (TAA, CH3CSNH2, >99%), urea (CO(NH2)2, >99%), ethanol (CH3CH2OH, >99%) and potassium hydroxide (KOH, >99%) were bought from Sinopharm Chemical Reagent Ltd and no further purification was required before use. Nickel foam (NF, 1.0\u00a0mm in thickness) was worked as substrates as well nickel source of materials with pretreatment before use. Furthermore, sufficient deionized water (DIW) was prepared throughout the experiments. The dosages of relevant reagents were recorded in Table S1.i) Pretreatment of NF. NF (3\u00a0cm\u00a0\u00d7\u00a07\u00a0cm) was ultrasonically treated in 100\u00a0mL 3.0\u00a0M HCl solution and 100\u00a0mL acetone for 20\u00a0min, respectively, to remove the oxide layer and oil stain from its surface. Then, it was rinsed subsequently with ethanol and DIW several times and dried under vacuum at 50\u00a0\u00b0C for 6\u00a0h to ensure a clean and dry surface. ii) Preparation for the mixed solutions of zinc nitrate and cobalt nitrate (denoted as solution A). The total moles of Zn2+ and Co2+ ions were kept at 2\u00a0mmol, and then different masses of zinc nitrate and cobalt nitrate were dissolved in 40\u00a0mL DIW respectively according to the molar ratio (1/3, 1/2 and 1/1) of Zn2+ and Co2+ ion. iii) Preparation of 2-methylimidazole solution (denoted as solution B). 1.25\u00a0g 2-methylimidazole was dissolved in 40\u00a0mL DIW with vigorous stirring to obtain an orange transparent solution. The solutions A and B were poured into a 100\u00a0mL Teflon-lined stainless-steel autoclave to form a blue-violet mixed solution, and then a piece of pretreated NF was transferred into the resulting solution. After a tight sealing, the autoclave was heated at 70\u00a0\u00b0C for 4\u00a0h. When the reaction was completed, cooled it to room temperature naturally. Taking the materials out and washed subsequently with DIW and ethanol several times, and then dried under vacuum at 50\u00a0\u00b0C for 6\u00a0h to obtain a series of ZnCo MOFs materials. If zinc nitrate was removed from solution A, Co MOF would be synthesized as a control sample by the same steps.First, 200\u00a0mg TAA was added into 80\u00a0mL ethanol and stirred vigorously for 30\u00a0min to form a clarified solution C. Then, all the MOFs and four copies of solution C were transferred to a 100\u00a0mL Teflon-lined stainless-steel autoclave respectively, sealed and kept at 150\u00a0\u00b0C for 2\u00a0h. Finally, take out the materials, washed it repeatedly and dry thoroughly to obtain the parallel group samples Zn\u2013Co\u2013Ni\u2013S-r (ZCNS-r, r represents molar ratio of Zn2+ and Co2+ ion, r\u00a0=\u00a01/3, 1/2, 1) and controlled sample Co\u2013Ni\u2013S (CNS).The hollow ZCNS-1/2 NSAs were prepared by a simple and facile two-step hydrothermal method, involving coordination precipitation reaction and sulfuration process, respectively (Fig.\u00a01\n). The first reaction was a low temperature hydrothermal process, involving the trapping of metal ions by organic ligands and the growth of ZnCo MOFs on the substrate. In the first step, 2-methylimidazole was acted as an organic ligand, zinc nitrate and cobalt nitrate provided Zn and Co sources, respectively. NF was selected as the substrate due to its foam-like 3D porous structure and the innate high electrical conductivity. A blue-violet solution was formed swiftly after the pink solution A mixed with the orange solution B, which was the result of the rapid and sufficient capture of metal ions by the ligand reagent. When the reaction was completed, the color of the NF changed from silver-gray to blue-violet and the color gradually deepened with the increase of the molar ratio of Zn2+ and Co2+, demonstrating that the ZnCo MOFs material were successfully grown on the NF. In the second step, the ethanol solution of TAA provided the S source in the sulfuration process, it's worth noting that it would cause some etching on the NF substrate to form nickel sulfide [43,55]. While the ligand in the ZnCo MOFs were gradually substituted with S2\u2212 during the sulfuration process, leading to the formation of MOF-derived Zn\u2013Co\u2013Ni sulfides (ZCNS) with smaller solubility product and the color of NF also turned black by degrees.The chemical composition and crystal information of the material were obtained by X-ray diffraction (XRD) analysis. Under the influence of the material substrate, the three strong peaks located at 44.51\u00b0, 51.85\u00b0 and 76.37\u00b0 are ascribed to NF (JCPDS #04\u20130850) (Fig.\u00a02\na). The peaks seated at 28.32\u1d52, 47.99\u1d52 and 56.29\u1d52 are assigned to (111), (220) and (311) crystal plane of ZnS (JCPDS #05\u20130566), while the peaks located at 29.91\u1d52 and 73.38\u1d52 are corresponding to (311) and (731) crystal plane of Co9S8 (JCPDS #19\u20130364). Notably, the peaks at 21.75\u1d52, 31.11\u1d52, 37.78\u1d52, 49.73\u1d52, 50.12\u1d52, 55.21\u1d52, 55.46\u1d52 can be attributed to (101), (110), (003), (113), (211), (122) and (300) crystal plane of Ni3S2 (JCPDS #44\u20131418), which results from the corrosion effect on NF during the sulfuration process. Moreover, the enlarged XRD patterns shows that ZnS only existed in the parallel samples rather than the control sample (Fig.\u00a02b), indicating that the synthesized parallel samples were ternary Zn\u2013Co\u2013Ni sulfides (ZCNS) and the control sample was binary Co\u2013Ni sulfides (CNS). Both scanning electron microscope (SEM) and transmission electron microscope (TEM) can be used to analyze the morphology of materials and the detailed crystal information of the single hollow ZCNS-1/2 NS can be further obtained by high-resolution TEM (HR-TEM). The morphology of CNS shows a simple combination of dispersed particles and irregular clumps (Fig.\u00a0S1a), which are speculated to be Co9S8 and Ni3S2, respectively. With the introduction of Zn, the metal skeleton structure is basically formed and ZCNS generally displays a hollow nanosword (NS) structure, of which the inner wall thickens with the increase of molar ratio of Zn2+ and Co2+ ion. Furthermore, the inner wall of ZCNS-1/3 is too thin to maintain the structure, resulting in a certain degree of fragmentation (Fig.\u00a0S1b1-4). It's worth noting that the inner wall of ZCNS-1 is so thick that causes the longitudinal growth of the surface and the combined growth of the bottom (Fig.\u00a0S1d1-4). Only ZCNS-1/2 equips with an array structure (Fig.\u00a03a1-4\n and Fig.\u00a0S1c1-4) and the uniform distribution results in a larger specific surface area, which is conducive to the catalytic reaction. In addition, TEM images from different angles (Fig.\u00a03b1-4 and c1-4) reveals the unique hollow sword-like nanostructure of ZCNS-1/2, which is in line with the SEM observation (Fig.\u00a03a1-4). Fig.\u00a03b4 shows the HR-TEM of a single NS from the front view, where the three distinguished lattice fringes are assigned to the (311) crystal plane of Co9S8 with 0.298\u00a0nm, (110) crystal plane of Ni3S2 with 0.287\u00a0nm and (200) crystal plane of ZnS with 0.271\u00a0nm. It's worth noting that the heterostructure of Co9S8(111)@Ni3S2(101) and the (111) crystal plane of ZnS with 0.312\u00a0nm simultaneously exist in the HR-TEM from top view (Fig.\u00a03). The existence of heterostructure is beneficial to the rearrangement of local positive and negative charges, thus accelerating the charge transfer rate of the material [56,57]. Considering that the top of the NS is the intersection of all planes, where the overall elements distribution can be better observed, so the element mapping of the NS is emphasized (Fig.\u00a03d and e). The detected elements of Zn, Co, Ni and S mainly concentrates at the junction of each surface while less distributes on the surface, which may explain why some NSs have holes in certain planes. Energy-dispersive X-ray (EDX) spectrum (Fig.\u00a03f) demonstrates that Zn, Co, Ni and S elements coexist in ZCNS-1/2 and their specific weight distribution is about 1.46: 1: 1.24: 1.77. The catalyst powder scraped from NF surface was analyzed by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) to obtain the real content and proportion of each metal element, and the measured molar ratio of Zn and Co is approximate to the added counterpart (Table S2).X-ray photoelectron spectroscopy (XPS) is usually used to obtain the information of surface elemental valence state from the as-prepared samples. As indicated in the XPS survey spectra (Fig.\u00a0S2a), Co, Ni and S elements simultaneously exist in CNS and ZCNS-1/2 with almost the same peak position except for the extra Zn element in ZCNS-1/2, further verifying that the Zn2+ was successfully introduced into CNS to form ZCNS. As shown in Fig.\u00a04\na, the XPS spectrum of Zn 2p is mainly fitted with two peaks, among which the peak at 1047.6\u00a0eV is Zn 2p1/2 orbit and the other peak at 1023.3\u00a0eV is Zn 2p3/2 orbit [58,59]. The XPS spectra of Co 2p and Ni 2p can be well deconvoluted into two spin\u2013orbit doublets and two shakeup satellites. Because of the existence of the electron transfer and electronic coupling between ZnS and Co9S8 as well between ZnS and Ni3S2, both Co 2p and Ni 2p in ZCNS-1/2 have a slight negative deviation compared with that of CNS. For Co 2p spectrum of ZCNS-1/2 (Fig.\u00a04b), the peaks located at 797.5\u00a0eV and 782.1\u00a0eV are assigned to the Co2+ of Co 2p1/2 and Co 2p3/2, while those located at 795.8\u00a0eV and 780.1\u00a0eV are ascribed to the Co3+ [60,61]. Likewise, for Ni 2p spectrum of ZCNS-1/2 (Fig.\u00a04c), the peaks seated at 874.2\u00a0eV and 856.6\u00a0eV come down to the Ni3+ of Ni 2p1/2 and Ni 2p3/2, while those seated at 872.4\u00a0eV and 854.9\u00a0eV are boiled down to the Ni2+ [45,62]. Significantly, compared to CNS, the ratio of Co2+/Co3+ increases while the ratio of Ni2+/Ni3+ decreases in ZCNS-1/2, indicating that charge transfer and electron rearrangement may occur in the heterostructure of Co9S8(111)@Ni3S2(101) after the addition of Zn2+. What is more, the increase of Co2+ content can promote the generation of more active intermediates (CoOOH) as efficient active sites during the catalytic process, which is beneficial to improving the capture of OH\u2212 ions and the release of O2, giving rise to an accelerated reaction kinetics [53]. The XPS spectra of S 2p and O 1s can be well deconvoluted into three set of peaks. Contrary to Co 2p and Ni 2p, both S 2p and O 1s in ZCNS-1/2 have a slight positive deviation compared with that of CNS. For S 2p spectrum in ZCNS-1/2 (Fig.\u00a04d), the peak at 168.5\u00a0eV, 161.6\u00a0eV and 162.4\u00a0eV are corresponding to the S\u2013O bond, S 2p1/2 and S 2p3/2, respectively [41,55]. For O 1s spectrum in ZCNS-1/2 (Fig.\u00a0S2b), the peak Oa located at 532.9\u00a0eV is corresponding to physicochemical water adsorbed on the surface of catalysts, the peak Oi seated at 531.5\u00a0eV is matching to oxygen ions, the peak Om situated at 529.7\u00a0eV is referring to the metal\u2013oxygen bond [43,63]. When Zn2+ is introduced and the total amount of metal ions remains unchanged, the adsorption of water in ZCNS-1/2 increases, while oxygen ions and metal\u2013oxygen bond are relatively reduced, suggesting that MOF-derived hollow NSAs endow ZCNS-1/2 with relatively stable structure, so its surface is less susceptible to oxidation relative to CNS.The performance parameters of all samples in HER process were mainly measured by using a three-electrode system in 1\u00a0M KOH solution at room temperature and recorded in Table S3. Linear sweep voltammetry (LSV) curves in HER process show that the performance of the parallel group samples (ZCNS-r, r\u00a0=\u00a01/3, 1/2, 1) is generally better than that of the control group sample (CNS). The ZCNS-1/2 performs best in the parallel group samples (Fig.\u00a05\na), suggesting that the addition of Zn2+ ion can greatly enhance the catalytic activity of CNS and achieve the best electrocatalytic performance by adjusting the molar ratio of Zn2+ and Co2+ ions. HER activity of ZCNS-1/2 is also significantly better than that of NF, it's worth noting that a certain gap exists at low current density while surpasses at high current density electrode compared to the Pt/C (Fig.\u00a0S3a), far beyond other HER catalysts recently reported (Table S4). The LSV curve of ZCNS-1/2 are almost the same with or without urea, illustrating that urea may have little effect on cathodic hydrogen evolution (Fig.\u00a0S3b). Double-layer capacitance (C\n\ndl\n) is obtained by fitting the data on the cyclic voltammetry curves (CV), and is applied to subsequently estimating the ECSA of materials. CV curves of all samples for HER progress was plotted in Fig.\u00a0S4. As expected, the C\n\ndl\n value of ZCNS-1/2 (54.69\u00a0mF\u00a0cm\u22122) overtops that of ZCNS-1 (45.26\u00a0mF\u00a0cm\u22122), ZCNS-1/3 (32.81\u00a0mF\u00a0cm\u22122) and CNS (21.64\u00a0mF\u00a0cm\u22122), suggesting that ZCNS-1/2 possess the maximal ECSA resulted from the hollow NSAs (Fig.\u00a05b). The reaction kinetics of catalysts in HER process can be well revealed by analyzing the corresponding Tafel slopes. There are generally two main steps for the HER in alkaline solutions, that is, the Volmer electrochemical hydrogen adsorption: H2O\u00a0+\u00a0e \u2192H ads\u00a0+\u00a0OH\u2212, and the Tafel reaction (chemical desorption: Had\u00a0+\u00a0Had\u2192H2) or Heyrovsky process (chemical desorption: Had\u00a0+\u00a0H2O\u00a0+\u00a0e\u2192H2+OH\u2212). As shown in Fig.\u00a05c, a lower Tafel slope of 42.24 mV/dec indicates that ZCNS-1/2 undergo the Volmer\u2013Heyrovsky reaction process with a fast catalytic kinetics (H2O\u00a0+\u00a0Hads\u00a0+\u00a0e \u2192 H2\u00a0+\u00a0OH\u2212), while a higher Tafel slope of 190.51 mV/dec indicates that CNS undergo the Volmer reaction process with a sluggish reaction kinetics (H2O\u00a0+\u00a0e \u2192 Hads\u00a0+\u00a0OH\u2212) [64]. The resultant elertrocatalysts display different Tafel slopes for the HER, demonstrating the rate-determining steps and reaction pathways was different for water reduction reactions over these catalysts, which is in accord with previously reported Ni-based HER catalysts. Impressively, ZCNS-1/2 requires the smallest overpotential of 97\u00a0mV and 215\u00a0mV to drive the same current density of 10\u00a0mA\u00a0cm\u22122 and 100\u00a0mA\u00a0cm\u22122, respectively (Fig.\u00a05d). In order to better evaluate the activity of catalyst, the turnover frequency (TOF) was also considered (see supporting information for details). The TOF of ZCNS-1/2 reaches up to 0.021\u00a0s\u22121\u00a0at the overpotential of 250\u00a0mV, which is over 35-fold larger than that of the CNS, explicitly evidencing the prominent intrinsic activity of ZCNS-1/2 (Fig.\u00a0S5). Electrochemical impedance spectroscopy (EIS) was measured at an open circuit potential of \u22121.2\u00a0V in the high frequency range from 10\u22121 to 105\u00a0Hz to evaluate the charge transfer rate of materials. A smaller nyquist semicircle endows ZCNS-1/2 with better electrical conductivity, which in turn leads to the excellent electrocatalytic HER activity (Fig.\u00a05e). Stability test of ZCNS-1/2 for cathodic hydrogen evolution in 1\u00a0M KOH solution was measured by chronopotentiometry method and the result shows that ZCNS-1/2 can catalyze stably for more than 15\u00a0h under a constant current of 100\u00a0mA\u00a0cm\u22122 without drastic voltage fluctuations (Fig.\u00a05f). The difference of physicochemical properties of ZCNS-1/2 before and after the reaction was researched by means of XRD characterization (Fig.\u00a0S6). All the above analyses demonstrate that the ZCNS-1/2 material can be used as advanced electrode to catalyze efficiently and steadily HER process.The performance parameters of all samples in OER process were mainly measured by using a three-electrode system in 1\u00a0M KOH solution at room temperature and recorded in Table S5. LSV curves was measured by the reverse scanning method to eliminate the influence caused by the forward oxidation peak (Ni2+ to Ni3+) at low current density, and the negative reduction peak near 1.3\u00a0V caused by reverse scanning corresponds to the conversion of Ni3+ to Ni2+ (Fig.\u00a06\na) [23,65]. In addition, under an identical current density, the driving potential of ZCNS-1/2 is always smaller than that of ZCNS-1/3, ZCNS-1 and CNS, which is also much less than the counterpart of NF and RuO2/NF (Fig.\u00a0S7), naturally making it become one of the best OER catalysts recently reported (Table S6). CV curves of all samples for OER process were plotted in Fig.\u00a0S8 and the resulted C\n\ndl\n value of ZCNS-1/2 achieves the largest (42.84\u00a0mF\u00a0cm\u22122), which is more than twice that of CNS (19.85\u00a0mF\u00a0cm\u22122) (Fig.\u00a06b). As can be seen in Fig.\u00a06c, a lower Tafel slope of 100.69 mV/dec demonstrates that the ZCNS-1/2 material suffers from faster reaction kinetics for anodic oxygen evolution. For OER, ZCNS-1/2 still applies the smallest overpotential of 233\u00a0mV and 307\u00a0mV to attain the same current density of 20\u00a0mA\u00a0cm\u22122 and 100\u00a0mA\u00a0cm\u22122, respectively (Fig.\u00a06d). As indicated in Fig.\u00a0S9, ZCNS-1/2 attains the largest TOF of 0.014\u00a0s\u22121\u00a0at the overpotential of 300\u00a0mV, which easily exceeds those of ZCNS-1/3 (0.0063 s\u22121) and ZCNS-1 (0.0056 s\u22121), let alone the feeble CNS (0.0035 s\u22121). Nyquist curves are not perfect semicircles, but the fitted radius of ZCNS-1/2 remains minimum (Fig.\u00a06e), manifesting that it still equips with a preferable conductivity in catalyzing the OER process. The anodic oxidation potential of ZCNS-1/2 increased from 1.546\u00a0V to 1.564\u00a0V after chronopotentiometry measurement 15\u00a0h under a constant current of 100\u00a0mA\u00a0cm\u22122 (Fig.\u00a06f). All the above analyses reveal that the ZCNS-1/2 material can be used as advanced electrode to catalyze efficiently and steadily OER process.The performance parameters of all samples in UOR process were mainly measured by using a three-electrode system in the electrolyte of 1\u00a0M KOH with 0.5\u00a0M urea at room temperature and recorded in Table S7. Compared with OER process, the distribution trend of measured LSV curves in UOR process behaves almost the same, but the catalytic activity of each sample was enhanced dramatically. Taking the most active ZCNS-1/2 as an example, the improved activity is digitized to a reduced potential of 221\u00a0mV at the current density of 100\u00a0mA\u00a0cm\u22122 (Fig.\u00a07\na), which possesses remarkable UOR activity and surpass most UOR catalysts reported so far (Table S8). CV curves of all samples for UOR process were plotted in Fig.\u00a0S10 and the resulted C\n\ndl\n value of each sample enlarges obviously relative to the counterpart in OER process (Fig.\u00a07b), inferring a larger ECSA may be obtained with the addition of urea. On the contrary, the Tafel slope of catalyst for urea oxidation is always lower than that of water oxidation under a similar logarithmic current gradient range, further verifying the robust reaction kinetics of UOR process (Fig.\u00a07c). As demonstrated in Fig.\u00a07d, the potentials of as-prepared samples was required for UOR at different current densities of 20\u00a0mA\u00a0cm\u22122 and 100\u00a0mA\u00a0cm\u22122: CNS (1.315\u00a0V, 1.358\u00a0V), ZCNS-1/3 (1.299\u00a0V, 1.343\u00a0V), ZCNS-1/2 (1.264\u00a0V, 1.316\u00a0V), ZCNS-1 (1.285\u00a0V, 1.332\u00a0V), and the improved activity compared to OER was recorded in Fig.\u00a0S11: CNS (218\u00a0mV, 237\u00a0mV), ZCNS-1/3 (216\u00a0mV, 231\u00a0mV), ZCNS-1/2 (199\u00a0mV, 221\u00a0mV), ZCNS-1 (196\u00a0mV, 229\u00a0mV), concluding that the higher the current density, the higher the activity promotion. Not surprisingly, the TOF of ZCNS-1/2 is still the most prominent at the same potential of 1.35\u00a0V, and is 7 times higher than that of urea-free counterpart (Fig.\u00a0S12). Nyquist curves on the positive Y-axis are not a semicircle but a semi-ellipse in alkaline urea solution. Meanwhile, under the same open-circuit voltage of 0.4\u00a0V, the EIS of ZCNS-1/2 from urea oxidation can even be surrounded that from water oxidation (Fig.\u00a07e), speculating that the introduction of urea may accelerate the charge transfer rate of the catalyst. After chronopotentiometry measurement of 15\u00a0h under a constant current of 100\u00a0mA\u00a0cm\u22122, the potential fluctuation caused by urea oxidation (10\u00a0mV) is much smaller than that caused by water oxidation (18\u00a0mV), which is mainly affected by the type of anodic oxidation and applied voltage. All the above analyses demonstrate that the ZCNS-1/2 material has better activity and stability as an advanced electrode for UOR process.Given that remarkable activity of ZCNS-1/2 in HER, OER and UOR process, it can be assembled as a favorable electrode couple (ZCNS-1/2//ZCNS-1/2) to catalyze both water electrolysis and urea electrolysis. Relevant tests were carried out with a two-electrode system in the electrolyte of 1\u00a0M KOH with or without 0.5\u00a0M urea at room temperature (Fig.\u00a0S13). As illustrated in Fig.\u00a08\na, CV curve was split into two LSV curves, among which the solid line represents the LSV curve with obverse scanning, while the dashed line represents the LSV curve with reverse scanning. It should be noted that the LSV curves of urea electrolysis almost coincide and the result of reverse scanning is better than obverse scanning, while the LSV curves of water electrolysis varies distinctly and the result of reverse scanning is worse than obverse scanning. The cell voltages of ZCNS-1/2//ZCNS-1/2 was regularly read from the relatively poor LSV curve and plotted in Fig.\u00a08b. To reach different current densities, the required cell voltages for water electrolysis: 1.522\u00a0V@10\u00a0mA\u00a0cm\u22122, 1.721\u00a0V@100\u00a0mA\u00a0cm\u22122, 1.788\u00a0V@200\u00a0mA\u00a0cm\u22122 and urea electrolysis: 1.314\u00a0V@10\u00a0mA\u00a0cm\u22122, 1.506\u00a0V@100\u00a0mA\u00a0cm\u22122, 1.567\u00a0V@200\u00a0mA\u00a0cm\u22122, sequentially concluding that the difference value of cell voltage between the water electrolysis and urea electrolysis becomes larger with current density increases. Comparisons of the catalytic ability of ZCNS-1/2//ZCNS-1/2 with some representative recently reported electrode couples highlights its superior activity for both urea electrolysis and water electrolysis (Tables S9\u201310). The difference value (\u0394V) of potential between independent anodic oxidation and independent cathodic reduction was contrasted with the cell voltages of assembled electrode couple in the same electrolyte. Whether in water electrolysis or urea electrolysis, the two are very similar at different current densities (Fig.\u00a08c), which preliminarily verifies the stability of ZCNS-1/2. Chronoamperometry measurement ran for 15\u00a0h at 1.5\u00a0V for urea electrolysis and 1.6\u00a0V for water electrolysis without apparent current attenuation (Fig.\u00a08d), further testifying the stability of target catalyst under a constant cell voltage. Apart from that, multi-current steps measurement ran for 5\u00a0h at a rising current density from 25\u00a0mA\u00a0cm\u22122 to 125\u00a0mA\u00a0cm\u22122 with a gradient of 25\u00a0mA\u00a0cm\u22122 for urea electrolysis and from 10\u00a0mA\u00a0cm\u22122 to 50\u00a0mA\u00a0cm\u22122 with another gradient of 10\u00a0mA\u00a0cm\u22122 for water electrolysis without obvious voltage fluctuation (Fig.\u00a08e), as always proving the stability of target catalyst under different current densities.The difference of physicochemical properties of ZCNS-1/2 before and after the reaction was researched by means of some characterization methods, such as XRD, SEM and XPS. The reaction here mainly refers to the anodic oxidation reaction in the two-electrode system with chronoamperometry measurement for 15\u00a0h, including the UOR in urea electrolysis at 1.5\u00a0V and OER in water electrolysis at 1.6\u00a0V. After the reaction, the peaks of ZnS, Co9S8 and Ni3S2 are still appeared in the XRD patterns. The difference is that all the three sulfide peaks in ZCNS-1/2 after OER 15\u00a0h are almost consistent with that in fresh ZCNS-1/2, while the peak intensity of ZnS in ZCNS-1/2 after UOR 15\u00a0h decreases significantly (Fig.\u00a0S14), speculating that the catalyst has been corroded during the long time UOR process. In terms of the SEM images, the hollow NSAs from ZCNS-1/2 after OER 15\u00a0h are still maintained (Fig.\u00a09\nb), of which the surface becomes rough due to corrosion, while the NSAs from ZCNS-1/2 after UOR 15\u00a0h collapses to certain degrees and the single NS shrinks in size (Fig.\u00a09c). Combined with ICP-OES result analysis (Table S11), it may be ZnS that suffers from the corrosion effect, which is the core element of the hollow NSAs structure. The loss of ZnS in the material after UOR 15\u00a0h outdistance that after OER 15\u00a0h, leading to different changes of morphology, which all are assigned to the varying degree of current erosion on ZCNS-1/2. Although the setting value of cell voltage in urea electrolysis process is lower than that in water electrolysis process, the corresponding current density of the former (100\u00a0mA\u00a0cm\u22122) is four times that of the latter (25\u00a0mA\u00a0cm\u22122). In view of this, the XPS spectrum of ZCNS-1/2 after UOR 15\u00a0h is the focus of study. All the peaks in recovered survey are almost the same as fresh survey (Fig.\u00a0S15a). It can be seen from the XPS fine spectrum that the main change exists in Zn 2p, S 2p and O1s peaks rather than Co 2p and Ni 2p peak before and after the reaction. The peak area of recovered Zn 2p only accounts for one third of that of fresh Zn 2p (Fig.\u00a09d). The subtle change in Co 2p and Ni 2p peaks result from the charge transfer at the heterointerface, not only in microscopic view leads to the slight conversion of corresponding element valence states but also in macroscopic view alleviates the erosion of current to a certain extent (Fig.\u00a09e and f). For S 2p orbital peak (Fig.\u00a09g), the extended area of S\u2013O peak accompanied with the reduced area of S 2p3/2 peak reveals that a small amount of S2\u2212 turned into SO caused by surface oxidation. For O 1s orbital peak, the adsorbed water molecules on the catalyst surface decreases while the metal\u2013oxygen bond increases (Fig.\u00a0S15b), suggesting that part of the metal sulfides were oxidized to metal oxides. To sum up, the loss of ZnS and surface oxidation bring about a certain collapse in the morphology of ZCNS-1/2 after the reaction, but ZCNS-1/2 still shows excellent performance in catalyzing water electrolysis at low current density and meets the needs to catalyze urea electrolysis at high current density as well.Density functional theory (DFT) calculation was performed to figure out the adsorption of water by each sulfide in ZCNS-1/2, which contributes to identify the real active site and better estimate the water electrolysis performance of target catalyst. The optimize ball-and-stick model of ZnS, Co9S8 and Ni3S2 is plotted in Fig.\u00a010 a-\nc with single H2O molecular absorbed on their surface. The calculated water adsorption energy (\u0394GH2O) of Co9S8, ZnS and Ni3S2 are \u22120.48\u00a0eV, \u22120.53\u00a0eV, \u22120.56\u00a0eV, respectively (Fig.\u00a010d). It can be deduced that most water molecules are adsorbed on the surface of Ni3S2 in the catalytic process because of the superior adsorption feature as well as the larger proportion compared to Co9S8 and ZnS in ZCNS-1/2. As illustrated in Fig.\u00a010e, the density of states (DOS) of Co9S8 near Fermi level reaches the maximum (8.57), which is 1.3 times that of Ni3S2 (6.51) and 2.4 times that of ZnS (3.63), indicating that Co9S8 equips with a better intrinsic metallic property. The existence of heterostructure realizes the strong combination between the larger \u0394GH2O of Ni3S2 and the preferable metal activity of Co9S8, which greatly enhances the integral catalytic performance of ZCNS-1/2. Moreover, the partial electronic DOS (PDOS) of each element for ZnS, Co9S8 and Ni3S2 was plotted in Figs. S16\u201318. In particular, the distribution of the total state density mainly comes from the p orbitals of S and the d orbitals of Zn, Co, Ni for Co9S8, ZnS and Ni3S2. The formation of Zn\u2013S, Co\u2013S, Ni\u2013S bonds stem from overlapping of p-orbital of Zn, Co, Ni, and the p-orbital of S (i.e., p\u2013p hybridization). Combined with the previous morphology and stability analysis, small doses of ZnS are mainly used to construct and stabilize the hollow NS structure, while large doses of Co9S8 and Ni3S2 play the key role in catalytic activity and the introduction of ZnS can improve the performance of ZCNS-1/2 to a certain extent.Nevertheless, under the OER conditions, the surface composition of MOF-derived Zn\u2013Co\u2013Ni sulfides would be changed to amorphous oxide. The water adsorption energy of NiOOH and Zn\u2013Co\u2013NiOOH have also been provided (Fig.\u00a0S19).We firstly reported the synthesis of a MOF-derived hollow ZCNS-1/2 NSAs on 3D porous nickel foam by dint of a facile two-step hydrothermal method. The as-obtained target catalyst affords outstanding activity and stability in HER and OER process. The water electrolysis process can drive current densities of 10, 100 and 200\u00a0mA\u00a0cm\u22122 with cell voltages of 1.522, 1.721 and 1.788\u00a0V, respectively. Furthermore, ZCNS-1/2 also exhibits excellent UOR activity, achieving the improved activity of 199 and 221\u00a0mV at current densities of 20 and 100\u00a0mA\u00a0cm\u22122, respectively, compared to OER process. The corresponding urea electrolysis process requires a very low cell voltage of 1.506\u00a0V to drive 100\u00a0mA\u00a0cm\u22122, which is 215\u00a0mV less than that of water electrolysis process. Morphology and stability analysis reveals that the formation and maintenance of MOF structure mainly depend on the introduction of ZnS, while DFT calculation demonstrate that the overall electrocatalytic activity largely rely on the synergy between Co9S8 and Ni3S2. The structure design and performance optimization of ZCNS-1/2 in this experiment play an exemplary role in exploring efficient and stable catalyst for both water electrolysis and urea electrolysis.There are no conflicts to declare.This work was financially supported by the National Science Foundation of China (Grant No. 21802126).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.2021.09.007.", "descript": "\n                  Water electrolysis is a promising technology to produce hydrogen but it was severely restricted by the slow oxygen evolution reaction (OER). Herein, we firstly reported an advanced electrocatalyst of MOF-derived hollow Zn\u2013Co\u2013Ni sulfides (ZnS@Co9S8@Ni3S2-1/2, abbreviated as ZCNS-1/2) nanosword arrays (NSAs) with remarkable hydrogen evolution reaction (HER), OER and corresponding water electrolysis performance. To reach a current density of 10\u00a0mA\u00a0cm\u22122, the cell voltage of assembled ZCNS-1/2//ZCNS-1/2 for urea electrolysis (1.314\u00a0V) is 208\u00a0mV lower than that for water electrolysis (1.522\u00a0V) and stably catalyzed for over 15\u00a0h, substantially outperforming the most reported water and urea electrolysis electrocatalysts. Density functional theory calculations and experimental result clearly reveal that the properties of large electrochemical active surface area (ECSA) caused by hollow NSAs and fast charge transfer resulted from the Co9S8@Ni3S2 heterostructure endow the ZCNS-1/2 electrode with an enhanced electrocatalytic performance.\n               "}
{"full_text": "The sources of fossil fuels such as oil are gradually exhausted due to large consumption in field of energy over the years. Meanwhile the human need for energy is increasing continuously with the passage of time, therefore the development of new energy sources is utmost need to meet this requirement [1,2]. Most of the energy is obtained from fossil fuels, which led to the rising concentration of carbon dioxide in the atmosphere and thus resulted into global warming and serious climate change [3,4]. Hydrogen has the advantage of high energy and no pollution to the environment over other energy sources. Therefore, it has great potential to replace existing fossil fuels [5\u20139]. At the same time, the storage of hydrogen is a challenging task for researchers [10,11]. Ammonia borane (AB), a chemical compound, has high chemical hydrogen storage potential. It is solid under environmental conditions with excellent stability and ultra-high hydrogen content (19.6 wt%) [12\u201314]. Recently, a lot of research works have been done in hydrolyzing and dehydrogenation of AB to obtain free hydrogen from it [15\u201317]. The hydrolysis of AB is expressed as follows:\n\n(1)\nNH3BH3\u00a0+\u00a02H2O\u00a0\u2192\u00a0NH4\n+\u00a0+\u00a0BO2\n\u2212\u00a0+\u00a03H2\u2191\n\n\nThe hydrolysis and dehydrogenation of AB is a slow process in absence of any catalyst, so the selection of appropriate catalyst can boost the release of hydrogen from it. Metal catalysts including transition metal nanoparticles, such as Pt, Rh, Ru, Pd, Co and Ni, etc. are appropriate options to enhance the hydrogenation process of AB. The noble metals such as Pt [18], Rh [19], Pd [20] and Ru [21\u201323] catalysts show relatively high catalytic hydrogen generation rate but its high cost and scarcity make it limiting for a wide range of practical applications. The non-noble metal such as Fe [24,25], Co [26,27], Ni [28,29], Cu [30,31], etc. can be used as an alternate source of precious metals. In recent years, the hydrogen production from AB by use of Co based catalysts has attracted great attention of researchers. Among the non-noble metal catalyst system, the Co catalysts express the best catalytic activity under the same preparation conditions as used for Ni and Cu based catalysts [32]. However, the aggregation of cobalt nanoparticles reduces their catalytic activity. Carbon is the best carrier for active catalysts due to its high chemical interactivity, especially in high alkaline and acidic environments and good interactions with active metals [33]. Wang's group has reported the one-step synthesis of Co nanoparticles in porous N-doped carbon (Co@N-C) and the catalytic stability of AB hydrolysis [34]. Meanwhile Zhang's group has reported a simple and efficient in-situ mosaic strategy for the preparation of mesoporous carbon catalysts co-doped with non-noble metals and nitrogen [35]. The study of Lin's group described the rapid synthesis of a catalyst encapsulated into graphitized nitrogen-doped carbon nanotubes by cobalt pyrolysis by one-pot pyrolysis [36]. Catalyst support affects the catalytic activity and stability of metal NPs [37]. Titanium dioxide (TiO2) has good photocatalytic performance, with its non-toxic nature and low cost. TiO2 is one of the catalysts suitable for environmental applications because of its strong oxidation capacity and high corrosion resistance [38]. TiO2 can be used as the carrier matrix of metal nanoparticles to improve the catalytic activity or stability of metal NPs [39].In this article, we propose an effective strategy for highly dispersing active component by the carrier and then encapsulating it in the carbon layer. The catalyst is treated in air to regulate its active components. A series of catalysts are obtained by adjusting the molar amount of Co. COTC-II exhibits the best performance in the production of hydrogen from AB. The prepared catalyst in our study has advantages of low cost, high activity and stability over other previous reported catalysts. Its magnetic property facilitates its recycling. The interaction between the support and used metal resulted in synergistic effect of Co and Co3O4, exhibiting the significant catalytic activity during the hydrolysis of AB.Resorcinol (0.64\u00a0g) was dissolved in anhydrous ethanol (3\u00a0mL), above solution was added into another anhydrous ethanol (60\u00a0mL), then ethylenediamine (0.58\u00a0g) and titanium butoxide (3.04\u00a0g) were added in order and stirred to form the mixture A. Formaldehyde (1.04\u00a0g) was added to deionized water (120\u00a0mL) and stirred to form mixture B. Mixture A was added to mixture B drop by drop to form solution C. Cobalt nitrate (2.6\u00a0g) was mixed into deionized water (20\u00a0mL) and added to solution C. Solution C was sonicated for 30\u00a0min, the mixture was stirred for 24\u00a0h in a 30\u00a0\u00b0C constant temperature water bath. The formed sol\u2013gel was directly dried at 50\u00a0\u00b0C to obtain powder products. Powder products was heated to 800\u00a0\u00b0C at 2\u00a0\u00b0C\u00a0min\u22121 under the protection of N2 for 1\u00a0h, and cooled naturally to room temperature. A sample of black powder was obtained. The sample was recorded as CTC-n (n\u00a0=\u00a0I, II, III) by adding 1.3\u00a0g, 2.6\u00a0g and 5.2\u00a0g cobalt nitrate respectively. COTC-n (Co-CoOx/TiO2@N-C) (n\u00a0=\u00a0I, II, III) was obtained by activating the above materials in air at 250\u00a0\u00b0C for 22\u00a0h. Co@N-C was obtained without the addition of titanium butoxide in a similar preparation process of CTC-II. COC (Co-CoOx@N-C) was obtained after air treatment of Co@N-C in air at 250\u00a0\u00b0C for 22\u00a0h.Resorcinol (0.64\u00a0g) was dissolved in anhydrous ethanol (3\u00a0mL), above solution was added into another anhydrous ethanol (60\u00a0mL), then ethylenediamine (0.58\u00a0g) and titanium butoxide (3.04\u00a0g) were added in order and stirred to form the mixture A. Formaldehyde (1.04\u00a0g) was added to deionized water (140\u00a0mL) and stirred to form mixture B. Mixture A was added to mixture B drop by drop to form solution C. Solution C was filtered and dried (50\u00a0\u00b0C) after stirring at 30\u00a0\u00b0C for 24\u00a0h. The powder obtained after drying was heated to 800\u00a0\u00b0C at 2\u00a0\u00b0C\u00a0min\u22121 under the protection of N2 for 1\u00a0h. TiO2@N-C was prepared after natural cooling to room temperature.The crystalline phases of the prepared materials were characterized by X-ray powder diffraction (XRD, Bruker/D8-Advance, Cu K\u03b1, \u03bb\u00a0=\u00a01.5418\u00a0\u00c5) in the 2\u03b8 range from 5\u00b0 to 80\u00b0. The Raman spectrum was recorded on an HR Evoltion Raman Spectrometer (Horiba Scientific, France) with excitation from the 514\u00a0nm line of the Ar-ion laser at a power of about 5\u00a0mW. X-ray photoelectron spectroscopy (XPS) is recorded on a PHI Quantum SXM spectrometer (with Al K\u03b1\u00a0=\u00a01486.6\u00a0eV excitation source), and the binding energy is calibrated by reference to the C 1s peak (284.8\u00a0eV) to reduce the charge effect of the sample. The morphology of catalysts was studied by using transmission electron microscope (HRTEM, FEI Tecnai G2 F20\u00a0S-TWIN electron microscope, operating at 200\u00a0kV). The N2 sorption isotherms were measured on surface area and pore size analyzer (ASAP2420-4MP, Micromeritics, USA) at 77\u00a0K. From the adsorption branch of isotherm curves in the P/P0 range between 0.05 and 0.35, the specific surface areas (S\nBET) of COTC-n (n\u00a0=\u00a0I, II, III) were calculated by the multi-point Brunauer\u2013Emmett\u2013Teller (BET) method. The pore size distribution was evaluated by the non-localized density function theory (NLDFT).Hydrogen generation was studied with the typical water displacement method. Catalyst (20\u00a0mg) is placed in a round-bottom glass flask. Then the aqueous NaOH (1\u00a0M, 10\u00a0mL) solution of AB (86\u00a0mg) was injected through constant pressure drop funnel. The flask was placed on a magnetic stirrer. In self-stirring mode, only catalyst and reaction mixture were loaded in the flask. The stirring rate was fixed at 500\u00a0rpm. An inverted and water-filled gas burette in a water-filled vessel was used to monitor the volume of the evolved H2. The H2 generation specific rates were calculated using the information in the initiating and stabilizing stages (80\u00a0mL of hydrogen generated) according the following formula:\n\n(2)\n\n\n\nr\nB\n\n=\n\n\n80\n\n(\nmL\n)\n\n\n\n\n[\n\n\nt\n140\n\n\u2212\n\nt\n60\n\n\n]\n\n\n(\nmin\n)\n\n\u00b7\n\nw\nc\n\n\n(\ng\n)\n\n\n\n\n\n\nhere, r\nB is denoted the hydrogen generation specific rate, t\n140 represents the time for 140\u00a0mL of hydrogen generation, and t\n60 for 60\u00a0mL, w\nc is the Co weight in catalyst.The designed composite material is synthesized by high temperature calcination using sol\u2013gel method, as shown in (Fig.\u00a01\na). In synthesis process, the resorcinol is completely blended into the ethanol, then ethylenediamine and titanium butoxide are added dropwise to form an intermediate, which resulted in the solution change from clarification to yellow turbid liquid. The intermediate is polymerized with formaldehyde to form a phenolic resin with the addition of cobalt salt to above solution. Finally, the titanic acid and Co ions are converted by calcining in nitrogen atmosphere to prepare the N-doped carbon from phenolic resin. The growth of titanium dioxide and the aggregation of cobalt nanoparticles are effectively limited by carbon [40]. It resulted in the preparation of CTC catalysts then follow its activation in air at 250\u00a0\u00b0C for 22\u00a0h to obtain COTC.\nFig.\u00a01b revealed the XRD pattern of the prepared samples. All peaks in the XRD pattern are consistent with the data reported in the literature. It can be observed that TiO2@C-N without Co NPs elaborate a clear diffraction peak at 2theta values of 25.3\u00b0 and 27.4\u00b0, indicating two phases of TiO2 (rutile and anatase). The diffraction peak of CTC-II indicates the accelerated transformation from anatase to rutile due to Co NPs, and only the diffraction peak of TiO2 rutile phase is appeared [41]. X-ray diffraction patterns utter that titanium dioxide nanoparticles at 2\u03b8\u00a0=\u00a027.4\u00b0, 36.0\u00b0, 41.2\u00b0, 44.0\u00b0, 54.3\u00b0 and 65.4\u00b0 with typical peak rutile phases, corresponding to crystal surfaces of (110), (101), (111), (210), (211) and (221) (JCPDS Card No. 21-1276). The XRD patterns of Co@N-C clearly confirm the metal Co phase, with the diffraction peak at 44.2\u00b0, 51.5\u00b0 and 75.8\u00b0 respectively corresponding to Co (111), (200) and (220) (JCPDS No. 15-0806). The diffraction peaks of COTC-II can be observed at 2\u03b8\u00a0=\u00a019.0\u00b0, 31.2\u00b0, 36.8\u00b0, 38.5\u00b0, 55.6\u00b0, 59.3\u00b0, 65.2\u00b0, 77.3\u00b0 and 78.4\u00b0 corresponding to (111), (220), (311), (222), (422), (511), (440), (533) and (622) lattice planes of Co3O4 (JCPDS Card No. 42-1467) respectively. More information about the material is obtained by Raman spectral analysis. Raman spectral measurements shown in Fig.\u00a01c elaborate that the information about prepared catalysts. The D-peak and G-peak, the characteristic Raman peaks of C atomic crystal, are obtained around 1361\u00a0cm\u22121 and 1591\u00a0cm\u22121 respectively [42]. The D-peak represents the defects in the lattice of C atom, while the G-peak in-plane stretching vibration of sp2 hybridization of C atom. I\nD/I\nG (I\u00a0=\u00a0intensity) calculates the intensity ratio between D-peak and G-peak which indicate the state of C atom [43]. Two broad peaks (D and G) of carbon are identified at about 1361\u00a0cm\u22121 and 1591\u00a0cm\u22121. The I\nD/I\nG intensity ratio of the COTC-II catalysts is calculated to be 0.86, indicating that the large proportion of graphite carbon in the sample.The microstructure of CTC-II composite materials is examined by TEM images (Fig.\u00a02\na and b), which expresses the granular structure of CTC-II catalyst. The size of single carbon spheres appears about 15\u201318\u00a0nm, and the particle size of Co NPs is observed at about 5\u20138\u00a0nm by HR-TEM image (Fig.\u00a02c). COTC-II obtained by controllable oxidation expresses as spherical particles (Fig.\u00a02d and e). Small particles have a positive effect on hydrolysis of AB [44]. As shown in Fig.\u00a02f, the metal particles clearly exist in the carbon layer which prove that the core\u2013shell structure of COTC-II. The lattice fringe spacing of 0.205\u00a0nm is matched with the (111) plane of Co (JCPDS Card No. 15-0806). The lattice fringe spacing matches with Co3O4 (311) crystal face (JCPDS Card No. 42-1467) is 0.24\u00a0nm [45]. The spacing of lattice stripes matches with rutile phase TiO2 (110) crystal face (JCPDS Card No. 21-1276) to be 0.32\u00a0nm. The element mapping images (Fig.\u00a02g) express the even distribution of Co and Ti in the composite material, and carbon doping with N is also determined.The chemical element composition and chemical valence of typical samples CTC-II and COTC-II surface can be determined by measuring X-ray photoelectron spectroscopy (XPS). Ti, Co, O, C and N elements in the samples are easy to determine. The peaks at 796\u00a0eV (780\u00a0eV), 531\u00a0eV (530\u00a0eV), 465\u00a0eV (472\u00a0eV), 400\u00a0eV (399\u00a0eV) and 285\u00a0eV (285\u00a0eV) correspond to Co 2p, O 1s, Ti 2p, N 1s, and C 1s in CTC-II (COTC-II), respectively have been expressed in Fig.\u00a0S1. The increase of O in COTC-II proves the oxidation of Co. The spectrum of Ti 2p of CTC-II is fitted as 458.3\u00a0eV and 464.0\u00a0eV. The Ti 2p spectra of COTC-II has a negative shift due to the activation (Fig.\u00a03\na) [46]. The Co 2p spectrum of CTC-II is shown in Fig.\u00a03b. Peak pairs at 778.7\u00a0eV (Co 2p3/2) and 793.7\u00a0eV (Co 2p1/2) are attributed to Co0 [47]. The peaks at 780.2\u00a0eV (Co 2p3/2) and 795.6\u00a0eV (Co 2p1/2) are assigned to Co2+, and the formation of Co2+ is due to oxidation of the sample surface. The peaks at 786.0\u00a0eV and 802.7\u00a0eV are shake-up satellite peaks. We can observe that the peak of Co3+ (781.8\u00a0eV) in the Co 2p spectra of COTC-II, which is accompanied by Co2+ (780.2\u00a0eV for Co 2p3/2,795.2\u00a0eV for Co 2p1/2), and the satellite peak (787.3\u00a0eV and 803.9\u00a0eV). The extent of shift in peaks express that Co element is converted to cobalt oxide in the activation process at 250\u00a0\u00b0C. The C 1s of CTC-II can be divided into three peaks of 284.8\u00a0eV\u00a0(CC/C\u2013C), 286.2\u00a0eV (C\u2013N) and 289.2\u00a0eV (O\u2013CO), respectively (Fig.\u00a0S2) [48,49]. The C 1s spectrum of COTC-II is similar to that of CTC-II. The O 1s spectrum of CTC-II can be determined into the two peaks as \u2013OH (530.6\u00a0eV) and absorbed water (532.1\u00a0eV) respectively [50] (Fig.\u00a0S3). The N 1s peaks of CTC-II and COTC-II are decomposed into pyridine nitrogen (398.1\u00a0eV/398.1\u00a0eV), pyrrole nitrogen (399.2\u00a0eV/398.1\u00a0eV) and graphite nitrogen (400.3\u00a0eV/398.1\u00a0eV) [51,52] (Fig.\u00a0S4). Table S1 shows the element information from XPS. Due to the existence of Co-NPs, ferromagnetic behavior of catalyst is determined by magnetic testing which indicates its easy recovery by external magnetic field and advantage for the catalytic reaction (Fig.\u00a0S5).The nitrogen adsorption\u2013desorption isotherms provide us more detailed sample structure information. As shown in Fig.\u00a03c, COTC-II samples show type-IV isotherm and type-H1 hysteresis loop, and S\nBET values with 54\u00a0m2\u00a0g\u22121. COTC-II expresses mesoporous structure (Fig.\u00a03d) for smooth hydrogen production.The hydrogen production equipment for AB hydrolysis is shown in Fig.\u00a04\na. Firstly, hydrogen production is first carried out with different catalysts in the absence of magneton at 500\u00a0rpm and 298\u00a0K (Fig.\u00a04b). Under the same reaction conditions, the experimental results show that TiO2@N-C has no catalytic activity for the hydrogen production of AB hydrolysis, and there is still no hydrogen production after 30\u00a0min of AB hydrolysis reaction (Fig.\u00a0S6). COTC-II obtained after oxidation treatment and show relatively good catalytic activity. Among the catalysts with different proportions of Co and Ti, COTC-II expresses the highest catalytic activity (Fig.\u00a04c). The results show that TiO2 effectively improves the performance of the catalyst (Fig.\u00a0S7), samples with TiO2 show higher performance compared to those with samples without TiO2. Based on the synergistic effect of Co and Co oxide as well as TiO2 as carrier, the catalytic activity of COTC-II appeared to be better than other samples. The catalysts are obtained after calcination at different temperatures are further studied (Fig.\u00a0S8). At the temperature below 700\u00a0\u00b0C, the hydrogen production performance is limited due to the low crystallinity of the catalyst. The rise in temperature of calcination causes the increase in crystallinity and catalytic activity of the prepared catalyst. The highest hydrogen production resulted at 800\u00a0\u00b0C. In order to study the catalytic performance of the sample, the CTC-II is treated in air atmosphere at 200\u00a0\u00b0C with no significant improvement in hydrogen production from AB. The performance of the catalyst is greatly improved at 250\u00a0\u00b0C after 22\u00a0h (Fig.\u00a0S9). The formation of hydrogen gas at different temperatures, 25\u00a0\u00b0C\u201345\u00a0\u00b0C, is studied (Fig.\u00a04d), with increases in the hydrogen production rate of COTC-II from 5905 to 15,957\u00a0mL\u00a0min\u22121\u00a0g\u22121, as the temperature increases, ions and water molecules become more active which lead to a rise in the catalytic activity of catalyst. At the temperature range of 298\u2013328\u00a0K, the Arrhenius plot of lnk versus the reciprocal absolute temperature (1/T) is obtained as straight line. The apparent activation energy (E\na) of catalytic reaction is calculated by following Arrhenius equation:\n\n(3)\n\n\nln\nk\n=\nln\nA\n\u2212\n\n\n\nE\na\n\n\n/\n\nR\nT\n\n\n\n\n\n\nIn the equation, k represents the rate constant, R is the ideal gas constant, the exponential factor is denoted as A, and T is the reaction temperature. According to the slope of Arrhenius, the calculated activation energy of COTC-II is 38.5\u00a0kJ\u00a0mol\u22121 substantially the same or lower than that of other non-noble metal catalysts (Fig.\u00a04e). The detailed comparison is shown in Table S2. In our study, the prepared catalysts retain 85% of its initial catalytic activity after its use in five cycles of hydrogen production from AB (Fig.\u00a04f). The results prove the stability and good ability of catalyst to recycle it with high catalytic activity for many times in hydrogen production reaction from AB. The decrease in the catalytic activity to some extent after many cycles may be due to the deformation, aggregation and other surface changes occur during catalytic reaction on the surface of a catalyst.In order to study the catalytic effects under different conditions, the effects of different concentrations of AB on the catalyst are further studied (Fig.\u00a05\na). The experimental results confirm that the hydrogen production rate is positively correlated with the reactant concentration (0.136\u00a0M, 0.272\u00a0M, 0.544\u00a0M), as the concentration of AB increases, the rate of hydrolytic dehydrogenation increases. Further analysis of factors affecting hydrolysis kinetics, hydrogen production from AB at different NaOH concentrations has been studied. The hydrogen production rate increases with the rise of NaOH concentration (Fig.\u00a05b). Therefore, NaOH is considered as a co-catalyst and has a positive effect on hydrogen production. In order to verify the positive co-relation of hydrogen production rate with catalyst concentration, AB hydrogen production is performed at different catalyst concentrations (10\u00a0mg, 20\u00a0mg, 40\u00a0mg), as shown in Fig.\u00a05c. As the catalyst concentration increases, the active component also increases which causes improvement in rate of hydrogen production by hydrolysis of AB. Under the same conditions, hydrogen production of NaBH4 is carried out a r\nB of 2667\u00a0mL\u00a0min\u22121\u00a0gCo\n\u22121\u00a0at 298\u00a0K (Fig.\u00a05d). The catalyst appeared to be negative for the hydrogen production of NaBH4 compared to AB.Based on the above evaluation and analysis of the catalyst, we propose a simple synergistic mechanism based on the catalyst (Fig.\u00a06\n). First, NH3BH3 molecule and H2O molecule are adsorbed and activated on the catalyst surface. Then the B\u2013H bond in NH3BH3 and the O\u2013H bond in H2O are broken into radicals respectively. Finally, two adsorbed H\u2217 atoms form an H2 molecule and then the H2 desorbs from the catalyst surface. The remaining OH\u2217 radicals from H2O molecule react with NH3BH2\u2217 to form NH3BH2OH\u2217. Secondly, NH3BH2OH\u2217 and another H2O molecule are activated again on the catalyst surface to release two H atoms to form H2. Similar to the previous step, the remaining OH\u2217 from the H2O molecule and NH3BHOH\u2217 form NH3BH(OH)2\u2217 is adsorbed on the catalyst surface. Because NH3BH(OH)2\u2217 is unstable, one H2O molecule is released. The remaining NH3BHO\u2217 is activated to break the B\u2013H bond, the O\u2013H bond in H2O molecule is broken, and the last H2 is released. At the same time NH3BOOH\u2217 is formed. Due to the attraction of NH3\u2217 to H, NH3BOOH\u2217 is converted into NH4\n+ and BO2\n\u2212. Synoptically, during the hydrolysis of NH3BH3, the metallic active components activate both the H2O molecule and the NH3BH3 molecule at the same time, causing the O\u2013H bond and the B\u2013H bond to be dissociated. Two H\u2217 atoms form one H2 to desorb from the catalyst surface. The synergy effect between Co and Co3O4 enhances the intrinsic catalytic activity. The presence of TiO2 also has a positive effect on the catalytic activity. Rational design strategy greatly improves the catalytic performances of hydrogen production.In conclusion, a core\u2013shell catalyst with good stability and catalytic activity for the hydrolysis of AB are reported. Carbon encapsulation limits the growth of TiO2 and increases the dispersion of Co. Catalysts with different ratios of Co and Ti are studied for hydrogen production. The discussed results express that COTC-II has the best catalytic activity in the hydrogen production of AB. The activity and stability of the catalyst are enhanced by the interaction between the metal and the support, as well as the synergy between the metal and the metal oxide. The synthesized catalyst also delivers its good catalytic activity up to 85% of its initial catalytic performance after 5 cycles of hydrogen production. The separation of catalyst from reaction mixture is quite easy due to its ferromagnetic property. In addition, the good performance of COTC-II catalyst provides a new opportunity for non-precious metal catalysts in the catalytic field for clean energy sources. At the same time, our study has developed the process of hydrogen production for new, economic and ecofriendly source to meet dire need of energy for human beings of recent era.The authors declare no competing financial interests.Financial supports from the National Natural Science Foundation of China (No. 51871090, U1804135, 51671080, 21401168 and 51471065), and Plan for Scientific Innovation Talent of Henan Province (No. 194200510019) are acknowledged.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.2020.03.012.", "descript": "\n                  Ammonia borane (AB) can be catalytically hydrolyzed to provide hydrogen at room temperature due to its high potentaial for hydrogen storage. Non-precious metal heterogeneous catalysts have broad application in the field of energy catalysis. In this article, catalysts precursor is obtained from Co-Ti-resorcinol-formaldehyde resin by sol\u2013gel method. Co/TiO2@N-C (CTC) catalyst is prepared by calcining the precursor under high temperature conditions in nitrogen atmosphere. Co-CoOx/TiO2@N-C (COTC) is generated by the controllable oxidation reaction of CTC. The catalyst can effectively promote the release of hydrogen during the hydrolytic dehydrogenation of AB. High hydrogen generation at a specific rate of 5905\u00a0mL\u00a0min\u22121\u00a0gCo\n                     \u22121 is achieved at room temperature. The catalyst retains its 85% initial catalytic activity even for its fifth time use in AB hydrolysis. The synergistic effect among Co, Co3O4 and TiO2 promotes the rate limiting step with dissociation and activation of water molecules by reducing its activation energy. The applied method in this study promotes the development of non-precious metals in catalysis for utilization in clean energy sources.\n               "}
{"full_text": "The current increasing environmental awareness and the inevitable depletion of fossil fuel reserves have prompted the growth of search for renewable energy sources, with a greater increment in the biodiesel industry [1,2]. Biodiesel has been typically produced by the alkaline transesterification of triglycerides present in the vegetable oils, resulting in the co-production of glycerol in 10\u00a0wt% [3]. The actual market is still unable to consume this large surplus of glycerol, mainly because the processes of purification are expensive to be performed at a large scale [4,5]. Therefore, to develop processes to convert crude glycerol to high value-added products arising as an attractive option to consumption the glycerol generated by the biodiesel industry [6].Considering this current trend, several routes to obtain valuable chemicals such as the bio-solvents, surfactants, polymers, and the more highlighted glycerol derivatives have been proposed [7\u201310]. Among them, glycerol derived bioadditives has attracted very attention, mainly because these compounds can be blended to the diesel or gasoline, reducing the emission of particulate material, and improving the physicochemical properties of these liquid fuels [11\u201315].The reactions of glycerol with acetic acid or tert-butyl alcohol give mono-, di- and tri-substituted glycerol derivatives, which are highly valuables compounds due to their properties as fuel bioadditives [16\u201319]. Recently, another glycerol derivative has also attracted attention as a compound able to improve the properties of diesel fossil and gasoline; the solketal (i.e., 2,2-dimethyl-1,3-dioxolane-4-methanol). It can be used to decrease gum formation, increase the octane index, diminish viscosity, improve the flashpoint, and oxidation stability [15,20\u201322].Solketal, as well as ethers and esters of glycerol, are compounds synthesized through the acid-catalyzed reactions, however, some of these processes involve the use of homogeneous catalysts which are highly corrosive in nature and environmentally dangerous [23]. Some works have demonstrated being possible use homogenous acid catalysts as sulfuric acid to promote the hydrolysis of solketal to generate glycerol with high level of purity, as the requirements of food and pharmaceutical industry [24]. Nonetheless, these soluble catalysts have still serious drawbacks. The use of a homogeneous catalyst involves a tedious workup procedure to be separated from products, which result in a large generation of effluent and neutralization residues, which should be disposed into the environment [25].Acidic solids can circumvent the drawbacks of the homogeneous catalysts, therefore, they have been used in newer cost-effective and selective processes to convert glycerol to solketal; sulfonic acid resins, metal oxides, zeolites, and solid supported catalysts are only some examples [26\u201328]. The greater challenge of most of the heterogeneous processes is to overcome the leaching of the active phase and the consequent deactivation of solid-supported catalysts. On this sense, several works have assessed the tolerance of acidic catalysts to these challengers; while Amberlyst-36 resin was few effective and quickly deactivated, hydrate aluminum fluoride demonstrated to be a more cost effective and efficient catalyst [29]. Similarly, zeolite Beta was significantly more resistant to the presence of water than Amberlyst-15 resin [30].Keggin heteropolyacids (HPAs) are attractive catalysts and are highly active in oxidative processes or acid-catalyzed reactions [31]. Phosphotungstic acid is the strongest Br\u00f8nsted acid among the Keggin HPAs; it is soluble in solvent polar and has been successfully used as a homogeneous catalyst in reactions to converting glycerol to bioadditives [32]. Nonetheless, when solid, it has a low surface area, hampering its use in conditions of heterogeneous catalysis. Therefore, they have been used as solid supported catalysts in several acid-catalyzed reactions [33,34].To circumvent the undesirable problem of leaching, the unique chemical-physical properties of HPAs can be easily manipulated with suitable tailoring of their constitution [35]. For instance, the protons exchanging by larger radium metal cations make HPA salts insoluble in polar solvents [36]. Moreover, when other Lewis acid metal cations replace the protons, their catalytic properties can be significantly enhanced toward goal-process [37,38]. Recently, Sn(II)-exchanged phosphotungstic acid salts demonstrated to be an efficient catalyst in glycerol esterification reactions, achieving high ester yielding [39,40]. The same was verified when it was used to synthesize tert-butyl ethers of glycerol [41].In this work, the protons of silicotungstic acid were exchanged by Sn2+ cations, and the salt formed was used as a catalyst in reactions of glycerol condensation with acetone to produce solketal. The catalytic activity of Sn2SiW12O40 was compared to the Lewis and Br\u00f8nsted acid catalysts including other Keggin HPAs. The impacts of the main reaction variables were investigated. Insights on the reaction mechanism were performed. The reusability of the catalyst was successfully evaluated.All chemicals were acquired from commercial sources and utilized without prior handling as received. Glycerol (99.5\u00a0wt%), acetone (99\u00a0wt%), and dodecane (99\u00a0wt%) were purchased from Vetec. SnCl2 and H4SiW12O40\u2219n H2O, all 99.9\u00a0wt%) were purchased from Sigma Aldrich.Sn2SiW12O40 salt was prepared through a procedure adapted from literature [41]. Usually, a SnCl2 solution at a stoichiometric amount was slowly added to an aqueous solution containing the solved H4SiW12O40. The mixture obtained was magnetically stirred by 3\u00a0h at 333\u00a0K, followed by the evaporation to dryness to releasing the gaseous HCl. The solid was dried in an oven at 423\u00a0K/ 3\u00a0h.Aiming a comparison, all the characterization data of pristine silicotungstic acid were also analyzed. The infrared spectroscopy analyses were performed in a Varian 660-IR spectrometer coupled to the attenuated total reflectance accessory (FT-IR/ATR). The patterns of X-rays diffraction (XRD) of the silicotungstic catalysts were recorded in an XRD-rays diffraction system model D8-Discover Bruker using Ni filtered Cu-k\u03b1 radiation (\u03bb\u00a0=\u00a01.5418\u00a0\u00c5), at 40\u00a0kV and 40\u00a0mA, with time counting 1.0\u00a0s, with diffraction angle (2\u03b8) varying from 5 to 80\u00b0.The H2 adsorption/ desorption isotherms were obtained in a NOVA 1200e High Speed, automated surface area and pore size analyzer Quantachrome instrument. Prior to the analysis, the sample was degassed 5\u00a0h. The surface area was calculated by the Brunauer-Emmett-Teller equation applied to the isotherms.The strength of acidity of the silicotungstic catalysts was estimated measuring the initial electrode potential (i.e. Bel, model W3B) of a CH3CN solution containing the soluble or suspended sample. The acidic sites number was determined by potentiometric titration as follows; typically, the sample (ca. 50\u00a0mg) was magnetically stirred in CH3CN by 3\u00a0h and then titrated with an n-butylamine solution in toluene (ca. 0.025\u00a0mol L-1) [42].Thermal analyses (TG) were performed in a Perkin Elmer Simultaneous Thermal Analyzer (STA) 6000. Typically, a sample (ca. 10\u00a0mg) was heated at a rate of 10 Kmin\u22121 under nitrogen flow. The temperature of the TG curves varied from 303 to 973\u00a0K.The elemental composition of silicotungstate salt was confirmed in an energy dispersive spectrometry system (EDS). The scanning electron microscopy (SEM) images were taken in a JEOL JSM-6010/LA microscope. A working distance of 10\u00a0mm and 20 KV acceleration voltage were used to acquire SEM images and EDS spectra.Catalytic runs were carried out using glycerol, acetone, and an adequate catalyst. In a typical reaction, glycerol and acetone at an adequate proportion were magnetically stirred until complete solubilization at room temperature at an adequate molar ratio. After adding the catalyst, the reaction was started and carried out 4\u00a0h.The reaction progress was periodically monitored collecting aliquots at regular intervals of time, and analyzing them by gas chromatography (Shimadzu 2014, FID, Carbowax 20\u00a0M capillary column). Prior to the analysis, the samples were diluted in methanol. The temperature profile was as follows: 80\u00a0\u00b0C/ 3\u00a0min; 10\u00a0\u00b0C / min; final temperature 210\u00a0\u00b0C/ 3\u00a0min. Injector and detector were both kept at 250\u00a0\u00b0C. The glycerol conversion was calculated according to the Equation (1):\n\n(1)\n\n\nC\no\nn\nv\ne\nr\ns\ni\no\nn\n\n\n\n%\n\n\n\n=\n\n\n\n\n\nA\n\ns\no\nl\nk\nc\no\nr\nr\ne\nc\nt\ne\nd\n\n\n\n\nA\n\ng\nl\ny\nc\ne\nr\no\nl\n\n\n+\n\nA\n\ns\no\nl\nk\nc\no\nr\nr\ne\nc\nt\ne\nd\n\n\n\n\n\n\n\nx\n100\n\n\n\nwhere Aglycerol, is the unreacted glycerol chromatographic peak area and Asolketal corrected is the corrected chromatographic peak area of solketal, obtained by the ratio of glycerol chromatographic peak area/solketal chromatographic peak area, injected with same concentration.The reaction products were identified by mass spectrometry analysis, performed in a Shimadzu GC 2010 gas chromatograph coupled with a MS-QP 2010 Ultra, with a carbowax capillary column (0.25\u00a0\u03bcm\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a030\u00a0m), and He as the carrier gas at 2mLmin\u22121. The temperature program was equal to the GC analyses. The GC injector and MS ions source temperatures were 523 and 533\u00a0K, respectively. The MS detector operated in the EI mode at 70\u00a0eV, with a scanning range of m/z 50\u2013400.The Sn2SiW12O40 catalyst was reused after a simple procedure. At the end of the reaction, the solution was vapored under vacuum to remove the excess acetone, which was recovered to be used in another reuse cycle. The remaining liquid contains solketal, catalyst, and a small amount of unreacted glycerol (i.e., quantified by GC analysis of aliquot when the reaction was stopped). After three steps of liquid\u2013liquid extraction with ethyl acetate, the solketal was removed. A simple distillation provided the ethyl acetate, which was recycled, and the pure solketal. To the reactor containing catalyst and the unreacted glycerol, recovered acetone and fresh glycerol were added, to start another cycle of reuse. This procedure was four times repeated.The characterization of the Sn(II) silicotungstate catalyst was previously discussed in another work, where they were used in an one-pot synthesis of alkyl levulinates from biomass derivative carbohydrates [43]. Notwithstanding, all the important data obtained in characterization (i.e., infrared spectra, powder XRD patterns, porosity properties, BET surface area, EDS analyses, and measurements of the strength of acidic sites are shown in the supplemental material (Fig. 1\nSM-6SM). The main characterization data and the respective conclusions are summarized as follow:\n\n\u2022\nThe integrity of the primary structure of HPAs catalysts were confirmed by infrared spectroscopy analysis, after to check the fingerprint region that should contain the characteristic absorption bands of Keggin anion [44]. A comparison of the infrared spectra of tin(II) silicotungstate salt and precursor HPA clearly showed that the primary structure of catalyst (i.e., Keggin heteropolyanion) was kept almost untouched after the synthesis (Fig. 1SM).\n\n\n\u2022\nPowder XRD patterns analysis can be useful to verify if any changes happened on the secondary structure of HPA when the protons are exchanged by other cations. X-rays diffractograms of Sn(II) silicotungstate and its synthesis precursors (i.e. SnCl2 and H4SiW12O40) evidenced that secondary structure of HPA presented only a little bit changes (Fig. 2\nSM); although new diffraction lines were noticed at low angle region (ca. 10\u00b0, 2 \u03b8) of diffractogram of the salt, in general, their profile was very similar to the acid. These changes are assigned the difference between the ionic radius of hydrate protons (i.e., H3O+, H2O5\n+) and the Sn2+ ions, that may affect the packaging of the heteropolyanions on the secondary structure [41].\n\n\n\u2022\nThe crystallite size was measured of silicotungstic catalysts was measured applying the Scherrer to the most intense XRD peaks. Values varied from 24 to 42\u00a0nm for the salts, while the acid presented values of 26\u00a0nm.\n\n\n\u2022\nThe strength of acidic sites belonging to the H4SiW12O40 and Sn2SiW12O40 was estimated measuring the initial electrode potential of their acetonitrile solutions; values of 713\u00a0mV and 685\u00a0mV, were obtained for the acid and silicotungstic salt, respectively (insert on Fig. 3\nSM). It allowed us to classify the acidic sites of both catalysts as very strong [42]. Through the potentiometric titration curves was possible to calculate the acid sites number of silicotungstic catalysts; 1.3 and 1.2\u00a0meq\u00a0g\u22121 were the values obtained for the acid and slat respectively (Fig. 3SM). Literature has explained the Br\u00f8nsted acidity is due to hydrolysis undergone by the metal cations coordinated to the water molecules, which result in releasing of H+ cations [39\u201341].\n\n\n\u2022\nLarger metal cation salts with an ionic radius >1.3\u00a0\u00c5, like Cs+ ions, make water-insoluble the HPA salts, in addition, they increase its surface area [43]. Nonetheless, the Sn2+ ions included in the H4SiW12O40 have a small radius (\u2248 1.3\u00a0\u00c5). The Sn(II) silicotungstate salt almost insoluble in acetone. During the reaction, the water formed favor the solubility of salt; however, at the end of the process, the suspension should be centrifugate to give solid at side bottom of the reactor.\n\n\n\u2022\nThe MEV images reveled that silicotungstic acid has like rice grains, while the salt presented particles with greater size (Fig. 4\nSM). The EDS analyses confirmed the elemental composition of metal silicotungstate salts (Fig. 5\nSM).\n\n\n\u2022\nThe hydration level of silicotungstic catalysts was determined by TG/ DTG curves. While the silicotungstic acid had 14\u00a0mol of H2O/ mol of catalyst, their salt presented 7 water mol per mol catalyst (Fig. 6\nSM).\n\n\nThe integrity of the primary structure of HPAs catalysts were confirmed by infrared spectroscopy analysis, after to check the fingerprint region that should contain the characteristic absorption bands of Keggin anion [44]. A comparison of the infrared spectra of tin(II) silicotungstate salt and precursor HPA clearly showed that the primary structure of catalyst (i.e., Keggin heteropolyanion) was kept almost untouched after the synthesis (Fig. 1SM).Powder XRD patterns analysis can be useful to verify if any changes happened on the secondary structure of HPA when the protons are exchanged by other cations. X-rays diffractograms of Sn(II) silicotungstate and its synthesis precursors (i.e. SnCl2 and H4SiW12O40) evidenced that secondary structure of HPA presented only a little bit changes (Fig. 2\nSM); although new diffraction lines were noticed at low angle region (ca. 10\u00b0, 2 \u03b8) of diffractogram of the salt, in general, their profile was very similar to the acid. These changes are assigned the difference between the ionic radius of hydrate protons (i.e., H3O+, H2O5\n+) and the Sn2+ ions, that may affect the packaging of the heteropolyanions on the secondary structure [41].The crystallite size was measured of silicotungstic catalysts was measured applying the Scherrer to the most intense XRD peaks. Values varied from 24 to 42\u00a0nm for the salts, while the acid presented values of 26\u00a0nm.The strength of acidic sites belonging to the H4SiW12O40 and Sn2SiW12O40 was estimated measuring the initial electrode potential of their acetonitrile solutions; values of 713\u00a0mV and 685\u00a0mV, were obtained for the acid and silicotungstic salt, respectively (insert on Fig. 3\nSM). It allowed us to classify the acidic sites of both catalysts as very strong [42]. Through the potentiometric titration curves was possible to calculate the acid sites number of silicotungstic catalysts; 1.3 and 1.2\u00a0meq\u00a0g\u22121 were the values obtained for the acid and slat respectively (Fig. 3SM). Literature has explained the Br\u00f8nsted acidity is due to hydrolysis undergone by the metal cations coordinated to the water molecules, which result in releasing of H+ cations [39\u201341].Larger metal cation salts with an ionic radius >1.3\u00a0\u00c5, like Cs+ ions, make water-insoluble the HPA salts, in addition, they increase its surface area [43]. Nonetheless, the Sn2+ ions included in the H4SiW12O40 have a small radius (\u2248 1.3\u00a0\u00c5). The Sn(II) silicotungstate salt almost insoluble in acetone. During the reaction, the water formed favor the solubility of salt; however, at the end of the process, the suspension should be centrifugate to give solid at side bottom of the reactor.The MEV images reveled that silicotungstic acid has like rice grains, while the salt presented particles with greater size (Fig. 4\nSM). The EDS analyses confirmed the elemental composition of metal silicotungstate salts (Fig. 5\nSM).The hydration level of silicotungstic catalysts was determined by TG/ DTG curves. While the silicotungstic acid had 14\u00a0mol of H2O/ mol of catalyst, their salt presented 7 water mol per mol catalyst (Fig. 6\nSM).Aiming to investigate the effect of Keggin anion on activity and selectivity of tin salt catalysts we carried out reactions shown in Fig. 1. Additionally, liquid Br\u00f8nsted acid catalysts (i.e., HCl, H2SO4 and p-toluenesulfonic acid (PTSA)) were also assessed.Regardless of acetone excess (ca. 1: 4 glycerol to acetone), without catalyst, no conversion of glycerol was noticed. Conversely, despite the low catalyst concentration used, in the presence of Lewis or Br\u00f8nsted acid catalysts the overall selectivity\u2013defined as the percentage ratio of the desired acetalization products (i.e., a total of isomers I and II) with respect to the conversion\u2013was always\u00a0>\u00a097% for solketal (Scheme 1\n).It is important to note that the reaction conditions were not optimized to achieve the maximum yield. Another important point is that the Br\u00f8nsted acid catalysts were used at the same H+ ions concentration; similarly, the metal salts were used with the same Sn2+ ions concentration (i.e., 0.01\u00a0mol %). Fig. 1 shows that the Sn(II) heteropoly salts had superior performance to that of Br\u00f8nsted acids. Moreover, amongst them, Sn2SiW12O40 was the most active catalyst.Previously, we have exploited the activity of different catalysts (i.e., SnCl2, SnF2, H3PW12O40) in condensation reactions of glycerol with acetone at the similar reaction conditions used herein (ca. 298\u00a0K, 1:4 glycerol to acetone). The main results are summarized in Fig. 2. It is known that in general a catalyst does not affect the thermodynamic equilibrium but only changes the kinetics. Nonetheless, our intention was to compare the conversions achieved within a specific time interval (see footnote of Fig. 2), independent of equilibrium has been reached or not. After comparison, it is possible to realize that even used at the higher loads, the tin halides (i.e. (soluble) SnCl2, (solid) SnF2)), and the (soluble) phosphotungstate acid, achieved lower conversions (Fig. 2) [45\u201347].When the Br\u00f8nsted acid catalysts are totally soluble, their activity on glycerol acetalization can be linked to their strength of acidity, which was estimated by pKa measurements in different solvents; H3PW12O40\u00a0>\u00a0H4SiW12O40\u00a0>\u00a0H3PMo12O40\u00a0>\u00a0H2SO4\u00a0>\u00a0HCl [48,49]. On this regard, we had evaluated the activity of soluble Keggin HPAs and verified that a using 1:20\u00a0M ratio of glycerol to acetone and 3.0\u00a0mol % of catalyst load, the reactions in the presence of H3PW12O40, H4SiW12O40 or H3PMo12O40 achieved conversions of 91, 39 and 41% after 2\u00a0h reaction [47]. Comparing the conversions of HPA-catalyzed reactions and those in the presence of their salts we conclude that the presence of Sn(II) ions remarkably improves the performance of acid catalysts.This synergic effect between the heteropolyanion and the tin cation was previously reported in other acid-catalyzed reactions such as glycerol and glycol etherification reactions [48,49]. The higher softness of silicotungstic anion makes him more efficient to stabilize positively charged intermediates, an aspect that besides the high Lewis acid of Sn2+ ions may be useful to explain the highest activity of Sn2SiW12O40 catalyst [41,48,49]. Moreover, the Sn2+ ions can undergo hydrolyzes, reacting with residual water or generated along the process; consequently, the H+ ions produced may itself catalyze the reaction. A possible reaction pathway is depicted in Scheme 2\n.The literature has described that to be condensate with glycerol, the acetone should have their carbonyl group activated through the protonation step or polarization by the coordination to the metal cation. In this work, these two mechanisms (i.e., Br\u00f8nsted and Lewis acid-catalyzed) may be operating. Recently, we demonstrated through pyridine adsorption measurements by infrared spectroscopy that the Sn2SiW12O40 catalyst has these two types of acidic sites [48,49]. Therefore, we suppose that both mechanisms may be operating (step I, Scheme 2); the activation of the carbonyl group may be triggered by protonation and or coordination to the Sn2+ ions of silicotungstic salt catalyst.Another key aspect is the regioselectivity of process; although six-membered ring compounds are thermodynamically more stable than five-membered ones, the solketal (I) was always the most selectively formed product herein. We assigned this preferential formation to an easier attack of the secondary hydroxyl group on the charged positively carbonyl group (i.e., 1a intermediate, Scheme 2) if compared to the attack of the primary hydroxyl group (i.e., 1b intermediate, Scheme 2). Moreover, Mota et al., demonstrated that the methyl group in the axial position dioxane isomer repulsively interacts with two axial hydrogens of six members ring, make him less stable than dioxolane (i.e., solketal) [52]. Regardless of the catalyst, all the reactions in Fig. 2 provided solketal with an average selectivity of 97%.The effect of the variation in the stoichiometry of the reactants was also investigated and the main results are displayed in Fig. 3. It is important to note that the effects of diffusional limitations were also assessed, performing reactions with different molar ratios at different stirring rates (Table 1SM). No significant changes in conversions of reactions were observed using three distinct levels of the stirring speed rate.Since that, the acetalization of glycerol is a reversible reaction, an increase of acetone load shifted the equilibrium toward the products, increasing both initial rate and final conversion of the reactions. Conversely, no significant changes in the selectivity were observed, and solketal was always the main product (ca. 89\u201397% selectivity), regardless of the excess of acetone.Nanda et al. performed a comparison of impacts of the catalyst one yield of the glycerol ketalization processes developed in both batch and continuous reactors [28]. They conclude that continuous-flow processes are more promising in large scale than batch processes. Those authors verified that since the equilibrium constant of this reaction is low, the best yields are achieved in continuous systems, where an excess of acetone was used, or the water generated is continuously removed. The best performance was achieved in the Amberlyst 36-catalyzed reactions, at a proportion of 4:1 acetone to glycerol, 298\u00a0K; nonetheless, it was achieved at continuous-flow conditions (ca. WHSV of 2\u00a0h-) and high pressures (ca. 500 psi \u2248 34\u00a0atm) [28].Due to slight enhancement obtained on the conversion with reactions at higher proportions, 1:12 was the molar ratio selected to assess the other reaction variables. The effect of catalyst load was then evaluated using this proportion at room temperature and the kinetic curves are displayed in Fig. 4.Different from the observed assessing the effect of the reactant stoichiometry, a variation on the catalyst load had a noticeable impact on the reaction selectivity. Independent of the catalyst load, the conversion and the selectivity of reactions were almost similar after 1\u00a0h run (Fig. 4).Regardless of the catalyst load, solketal was always obtained with selectivity equal to or higher than 82%. Nonetheless, during the reactions was verified that the less stable product (i.e., dioxane), even being always the minor product, had its formation more favored in the initial period of reactions, mainly when the catalyst was present in lower load (Fig. 5). It means that the reaction selectivity was under kinetic control, which was impacted by the catalyst load. When a high catalyst load was used, the most stable product was always more selectively formed since the reaction beginning. Whereas, when the catalyst was present at low loads, the reaction becomes slower, and the product kinetically favorable although always the minority, had its formation enhanced, mainly in the initial period of the process.In Fig. 6, a quick survey of literature highlights the main results achieved in HPA salts-catalyzed glycerol acetalization reactions with acetone [53-55]. The physical properties of heteropoly salt depend on the cation nature used to replace the protons of Keggin acids. In special, the solubility of the salt impacts the workup needs to separate the catalyst from the medium of reaction. For these reasons, although the HPA salts containing cations with large ionic radius are almost insoluble in the reaction, sometimes it is difficult to separate or recovery the catalyst. The cesium phosphotungstate is an example, and consequently, it has been used solid-supported [54].Although a better comparison requires to take into account the amount of supported catalyst used in the reactions and its loading in the support, it is possible realize that the Sn2SiW12O40 catalyst has a performance equal or superior to the majority of the solid heteropoly salts or solid supported shown in Fig. 6.The reusability of Sn(II) silicotungstate was also assessed. The suspension formed by the catalyst in the reaction solution requires that it should be centrifugated to be recovered and reused. To circumvent this drawback, we envisaged a simple process to recover the catalyst. In this procedure, the excess acetone is removed and reused in another run. The reactor contains unreacted glycerol, solketal, and catalyst. The addition of ethyl acetate solubilizes the solketal, which is then extracted. To the reactor containing unreacted glycerol and the catalyst, recovered acetone is added and then a new run is carried out (Scheme 3\n).It is noteworthy that even if the catalyst had been recovered by filtration, the steps of recovery of the acetone and purification of the solketal should be also performed, therefore, they are not additional but obligatory steps of the process.Therefore, following the procedure in Scheme 3, we successfully recovered and reused the Sn2SiW12O40 salt in 4 catalytic runs. No decrease in the catalytic performance was noticed. Infrared analysis of catalyst after the last recycle showed that no significant change was observed at the fingerprint region of the Keggin anion spectrum.In this work, the activity of Sn2SiW12O40 salt was assessed on the acetalization of glycerol with acetone. The catalyst was spectroscopically characterized and demonstrated that after the exchange of the protons by Sn(II) cations, no modification in the primary structure (i.e., Keggin anion) was detected. In all the runs, glycerol was majority converted to solketal. The effects of the main reaction variables were investigated. We have found that catalyst concentration affects the reaction selectivity; when lower loads are used, the six-membered ring dioxane has its formation favored, although the solketal remains even as the main product. The Sn(II) cation showed to be a key constituent of catalyst. On the other hand, among the three catalysts with different Keggin anions, that containing the silicotungstic anion was the more efficient. Finally, the catalyst was recovered and reused without loss of activity.\nM\u00e1rcio Jose da Silva: Conceptualization, Methodology, Data curation, Investigation, Writing - original draft, Project administration, Funding acquisition. Milena Galdino Teixeira: . Diego Morais Chaves: . Lucas Siqueira: .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 the financial support from CNPq and FAPEMIG (Brasil). 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.Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2020.118724.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  In this work, Sn(II)-exchanged silicotungstic acid salt (i.e., Sn2SiW12O40) was synthesized and evaluated as the catalyst on the acetalization of glycerol with acetone to produce solketal, a versatile bioadditive of fuel. The Sn2SiW12O40 salt was compared to the other solid Keggin heteropoly salts (i.e., Sn3/4PMo12O40, Sn3/4PW12O40), and liquid (i.e., HCl, H2SO4 and p-toluenesulfonic acid) catalysts. Amongst the catalysts assessed, it was the most active, achieving a high conversion (ca.\u00a0>\u00a099%, after 1\u00a0h reaction at room temperature) and selective (ca. 97%) toward the formation of solketal. Moreover, the Sn2SiW12O40 salt demonstrated to be more active than acid and precursor Tin (II) chloride salt, as well as other heteropoly salts and solid supported catalysts. The effects of the main reaction variables were assessed. The Sn(II) cation, as well as the silicotungstate anion, showed being essential to convert glycerol to solketal. Insights on the reaction mechanism were performed. In a simple recycle procedure, the solketal was purified, the acetone excess recycled, and the catalyst was reused without loss activity.\n               "}
{"full_text": "Selective hydrogenations of aromatic ketones into the corresponding alcohols are commonly performed by homogeneous catalysis. However, these processes have economic and environmental disadvantages. Taking into account the key role of this type of alcohols in the fine chemical industries (and especially pharmaceuticals), great efforts are performed in order to replace homogeneous by heterogeneous catalysis, achieving an adequate activity and selectivity. In order to produce the hydrogenation of aromatic ketones, noble metals as Pd, Pt, Rh and Ru are commonly used as catalysts. Several items, such as metal and support selection, metal precursor, catalyst preparation and activation methods, particle size, etc., have strong influence on activity and selectivity in these types of reactions [1, 2, 3, 4]. However, these metals have several disadvantages: high cost, low abundance and low selectivity produced by different kind of side reactions such as the aromatic ring hydrogenation, as well as hydrogenolysis of our valuable product, the intermediate aromatic alcohol. With the purpose of increasing selectivity, a second metal is added. These promoters can modify the noble metal either electronically and/or geometrically [5]. Specifically, a decrease in their hydrogenating capacity is required to achieve an increase in selectivity towards the desired product. The promoters used such as Ni, Cr and Sn, are more electropositive than the noble metal [6, 7, 8, 9]. They produce a double effect: the noble metal gets a negative charge density, Me\u03b4-, and the active sites are diluted, consequently the hydrogenating capacity is decreased. Besides, these electronic and geometric rearrangements in the active site could produce changes in the adsorption mode of the substrate increasing the selectivity towards the desired product. However, this methodology normally leads only to an enhancement, but not a complete change in selectivity.More recently, with the advancement of nanotechnology, the chemoselective hydrogenations with nanomaterials of transition metals, such as Ni and Fe (cheaper, abundant and less toxic metals), have begun to be explored [10]. Primarily, these reports have analyzed the hydrogenation of \u03b1,\u03b2-unsaturated compounds such as acrolein and cinnamaldehyde but not of aromatic ketones. Besides, the use of pure nickel as catalyst in hydrogenation of cinnamaldehyde, both in gas and liquid phases, leads to production of hydrocinnamaldehyde [11, 12, 13]. It is well known that the hydrogenation of C=C bond is both thermodynamically and kinetically favored over the C=O bond, due to the lower C=C dissociation enthalpy (611 kJ/mol) than for C=O bond (737 kJ/mol) [14]. Therefore, these results do not represent examples of chemoselective hydrogenations in order to obtain the more interesting product: the cinnamyl alcohol. Subsequently, Malobela et\u00a0al. studied the effect of nickel dispersion and they found that the turnover frequency and the selectivity to unsaturated alcohol increased when the nickel crystal size decreased in the following order: 14.5 nm < 7.8 nm < 2.8 nm [15]. In agreement with these results, Viswanathan et\u00a0al. reported the cinnamyl alcohol production using Ni/TiO2 catalysts prepared by four different methods. The catalysts with the smallest nickel particle size, showed the higher cinnamaldehyde conversion and selectivity to cinnamyl alcohol [16]. On the other hand, there are only reports of hydrogenation of aromatic aldehydes and imides, which were transformed into the corresponding alcohols or amines, using iron nanoparticles supported on polymers and molecular H2 as hydrogenating agent [17].With the aim to prepare catalysts based on nickel, some compounds could be more appropriate to perform chemoselective hydrogenation of aromatic ketones, than the pure nickel. These substances should preserve nickel metallic characteristics, have moderate hydrogenation capacity and catalytic sites with particular geometries. Therefore, the spatial configuration of the adsorbed molecules and the structural characteristics of the catalytic sites would allow to \u201ctune\u201d the proper arrangement to reach the desired hydrogenation. Taking into account these concepts and considering that in last years, nickel phosphides catalysts have emerged as excellent hydrotreating catalysts [18, 19, 20, 21], they can have good activity in hydrogen transfer reactions, such as chemoselective hydrogenations. Nickel phosphides have metallic properties, the phosphorus presence produces a diluting effect on the Ni atoms, and they have a wide range of stoichiometries, from Ni3P to NiP3. Because of these different compositions, they have a great diversity of crystallographic structures, which produce surface sites with very diverse geometries. Therefore, there could be catalytic sites, with geometries that could be able to hydrogenate different aromatic ketones to obtain the desired products.The use of nickel phosphides as chemoselective hydrogenation catalysts in the fine chemical area is scarce and it has not been applied for hydrogenation of aromatic ketones. As an example of chemoselective hydrogenation of other type of molecules using nickel phosphide as catalyst, Carenco et\u00a0al. [22] reported good conversion of terminal and internal alkynes to cis-alkenes with high selectivity using nanoparticles of Ni2P.From a complete analysis of the previous topics present work explores the possible promoter effect produced by the presence of phosphorus atoms in nickel phosphides when they are used in chemoselective hydrogenation of an aromatic ketone. We decide to study this effect on the activity and selectivity in the chemoselective hydrogenation of acetophenone (AP) to obtain 1-phenylethanol. This is a very important intermediate aromatic alcohol in the fine chemical industry and is conventionally produced by this reaction [23]. Is interesting to remark that the promoting effect on the noble metals is achieved adding a second more electropositive metal (Ni, Cr, Sn). In this case, if nickel phosphides are used as catalysts, phosphorus atoms are more electronegative than Ni. Therefore, a positive charge density on Ni atoms (Ni\u03b4+) should be expected. As a consequence, a contrary effect with respect to previous studies would be awaited. Recently, we have published results indicating that nickel phosphides nanoparticles of 9 nm are active as chemoselective hydrogenation catalysts of an aromatic ketone [24]. To our knowledge, this is the first report on this application until now. In the present work, the hydrogenation results obtained with nanoparticles of pure metallic nickel and nickel phosphides of very similar crystal size (\u2245 20 nm) are compared.Considering that these type of reactions are structure sensitive [16, 17] both catalysts were prepared with monodisperse nanoparticles (NPs) pre-synthesized with the same average diameter. In this way, the catalytic results will show the specific effect of the electronic and structural differences between the metallic nickel and the nickel phosphides, without the influence of different sizes of active NPs. After the obtaining and purification of the NPs, they are supported on mesoporous silica nanospheres of about 500 nm and are used as catalysts in the hydrogenation of AP in liquid phase.In a one-pot synthesis to obtain Ni0 NPs, determined amounts of nickel (II) acetylacetonate (Ni(acac)2, 1 mmol, Sigma-Aldrich, 98%), trioctylphosphine as ligand (TOP, 0.8 mmol, Sigma-Aldrich, 97%), oleylamine as solvent and reducing agent (OA, 10 mmol, Sigma-Aldrich, 70%) were directly added into a three-neck round bottom flask fitted with a condenser and magnetic stirring. The two remaining necks were used to introduce a thermocouple with a glass sheath and Ar flow, respectively. The mixture was heated at 220 \u00b0C for 2 h. Finally, the NPs were purified and isolated precipitating the suspension with acetone and re-dispersed in n-hexane.In order to obtain nickel phosphide NPs, the same procedure was used but TOP was replaced by triphenylphosphine, acting as ligand and phosphorus source (Ph3P, 0.4 mmol, Sigma Aldrich, 99%).The NPs were characterized by X-ray diffraction (XRD), diffuse light scattering (DLS), transmission electron microcopy (TEM), selected area electron diffraction (SAED) and Fourier transformer infrared spectroscopy (FT-IR). Atomic absorption spectroscopy (AA) was used to determine the Ni content in the suspension and in catalysts. In this work only the characterization of Ni0 NPs will be reported, because nickel phosphides characterizations were previously described [24].Ordered mesoporous silica nanospheres (MSNS) were prepared following the methodology proposed by Gr\u00fcn et\u00a0al. [25] using tetraethyl orthosilicate (TEOS \u226599 %, Aldrich) as silica source, n-hexadecyltrimethylamonnium bromide (CTMABr \u226598 %, Sigma) as template agent, NH4OH (28 % p/p, Merck) to generate an alkaline medium, absolute ethanol (Cicarelli, 99.5 %) and distilled water. All reactants were mixed under vigorous magnetic stirring using the following molar composition: 1TEOS: 0.3CTMABr: 11NH4OH: 58 EtOH: 144H2O. The reaction mixture was kept under stirring at 30 \u00b0C for 2 h. The precipitate was collected by vacuum filtration and washed with distilled water. The sample was calcined up to 550 \u00b0C in air atmosphere for 2 h, with a heating rate of 10 \u00baC/min in order to remove the CTMABr. The solid was characterized by N2 adsorption at -196 \u00b0C, scanning electron microscopy (SEM) and TEM.The Ni0 and nickel phosphides NPs catalysts were prepared by impregnation of silica nanospheres with the corresponding pre-synthetized NPs suspensions and dried at 60 \u00b0C in air during 2 h. They were called Ni-MSNS and NiP-MSNS, respectively. The volume suspensions was fixed in order to obtain a nominal Ni loading of 5 % wt/wt.With the purpose to eliminate the surfactants (TOP or Ph3P) from the surface of the NPs, both solids were washed three times with CHCl3. The catalysts were characterized by TEM. Before their use in the reaction, they were reduced in H2 flow, heating at 10 \u00baC/min up to 500 \u00b0C and maintaining at this temperature during 2h. Afterwards, TEM micrographs were obtained in order to verify that the reduction treatment did not produce sintering of the NPs.The number of the catalytic surface sites of Ni-MSNS and NiP-MSNS was titrated by H2 and CO chemisorption, respectively. Besides, the volumetric oxidation technique was used to evaluate the oxygen total quantity needed to re-oxidize the reduced Ni-MSNS catalyst.XRD patterns were recorded using a standard automated powder X-ray diffraction system (Philips PW170, the Netherlands) with diffracted-beam graphite monochromator, using Cu K\u03b1 radiation (\u03bb = 0.15406 nm) in the range 2\u03b8 = 30-80\u00b0 with steps of 0.05\u00b0 and counting time of 6 s/step. Besides, a diffractogram of MCM-41 nanospheres at low angles (2\u03b8 = 1.0\u201310.0\u00b0) was obtained with laboratory beamline Xenocs (Model Xeuss 2.0, France). This equipment has the capability to run simultaneous small and wide-angle X-ray scattering measurements (SAXS-WAXS). The present test was made in WAXS mode.The size distribution of the Ni NPs in suspension was obtained with a Zetasizer Nano (Nano ZSizer-ZEN3600, Malvern, U.K.) commercial equipment at room temperature. The light source was a helium/neon laser (\u03bb = 632.8 nm) and the light scattering was measured at scattering angle of \u03b8 = 173\u00b0.A Philips CM 200 UT microscope (the Netherlands) equipped with an ultra-twin objective lens was used to obtain the TEM and HRTEM (high resolution transmission electron microscopy) images. A LaB6 filament operated at 200 keV was the electron source. In the high-resolution mode, the nominal resolution was of 0.2 nm. The micrographics were acquired with a CCD digital camera. A commercial program for image treatment was used to adjust linearly the illumination and contrast. Besides, electron diffraction of selected area (SAED) was obtained. Statistics on particles were done with the program Image J 1.43U. Particle size is given as the geometric average size \u00b1standard deviation of the largest particle dimension. In all cases, more than 100 measurements were averaged, sampling in different regions of the sampler holder.A Philips 505 (the Netherlands) microscope was used for SEM analysis.A FT/IR Jasco spectrometer (model 4200, Japan) equipped with a PIKE diffuse reflectance IR cell with a resolution of 1 cm\u22121 was used. From 200 to 400 scans were accumulated in each case.Textural properties as specific surface area (Sg), specific pore volume (Vp) and pore diameter (Dp) of MSNS were measured with a Micromeritics ASAP 2020 V1.02 E device (U.S.A.).Measurements of the surface Ni atoms were made on the catalysts in conventional static volumetric handmade equipment. Both isotherms were measured at 50 \u00b0C, with H2 as titration reactant for Ni-MSNS and CO for NiP-MSNS. Before acquisition of isotherms, the catalysts were reduced in situ as it was previously described. After H2 chemisorption test, Ni-MSNS catalyst was degassed and completely oxidized with a known amount of pure O2.AP hydrogenation reaction was carried out in a stirred autoclave reactor at 1MPa H2 pressure and 80 \u00b0C, using 0.25 g catalyst and n-heptane as solvent. Before the catalytic test, the solids were activated in pure H2 following the procedure already described. The operative conditions for the catalytic tests were specifically chosen to avoid mass transfer control. The reaction evolution was followed by gas chromatography in a GC Varian 3400 chromatograph (the Netherlands) equipped with a capillary column of 30 m CP wax 52 CB and FID. The identification of reaction products was accomplished by GC/MS using Shimadzu QP5050 equipment (Japan).The characterization of nickel phosphide NPs was previously reported [24]. Briefly, a mixture of Ni12P5 and Ni2P NPs was obtained, where each NP is monophasic. The major phase is Ni12P5. About of 87% molar of the mixture corresponds to this compound. In this article the geometric average size of the NPs, determined by TEM, was of 9.6 \u00b1 0.2 nm. In the present work the molar ratio of reactants: Ni(acac)2: OA: Ph3P was modified, decreasing the Ph3P quantity from 0.8 to 0.4 mmol, while quantities of Ni(acac)2 and OA were maintained equal. This change was performed in order to obtain NPs with an average diameter similar to that of the Ni0 NPs. In this way, an average value of 15.1 \u00b1 0.6 nm was obtained by TEM. As it will be explained below, this size is nearly equal to the diameter of the Ni0 NPs. Therefore, possible effects of the NPs sizes on activity and selectivity of the catalysts should be avoided [2].In Fig.\u00a01\n XRD diffractogram of the Ni NPs, with peaks at 2\u03b8 = 44.3, 51.3 and 76.4\u00b0 is shown. The position of the three peaks are in good agreement with (1 1 1) (2 0 0) and (2 2 0) crystallographic planes of a face-centered cubic (f.c.c.) unit cell, typical of metallic Ni [PDF 88\u20132326]. The broadening of the diffraction peaks is characteristic of very small NPs. At first sight, the presence of nickel oxide phase cannot be ruled out completely because the strongest peak of NiO appears at 2\u03b8 = 43.3\u00b0 [PDF 89\u20137390]. Due to the considerably broadening of the Ni0 peaks, could be possible that the left side of 44.3\u00b0 signal hidden a very small peak of NiO. However, the absence of any distinguishable peak at other characteristic positions of nickel oxide, such as 2\u03b8 = 37\u00b0, indicates that the presence of this phase is negligible. Therefore, we can conclude that the synthesis procedure used led to the obtaining of Ni0 NPs.In order to obtain information about the average size diameter and the monodisperse character of the Ni0 NPs distribution, DLS measurements were performed. They are easily and quickly performed and provide significant statistical information. However, this technique measures the hydrodynamic diameters, this mean, the real NPs diameters plus the thickness of the NPs coverage. Depending on the difference of the refractive indexes between the solvent used in the suspension and the NPs surfactant, a significant or a negligible disagreement, in comparison with the real diameter, can be observed [26]. The results obtained by this technique show that the Ni0 NPs suspension is monodisperse (polydispersity index <0.06) and they have an average diameter of 19 \u00b1 6 nm.The Ni0 NPs were characterized using TEM and SAED techniques (Fig.\u00a02\n A and B). Fig.\u00a03\n show the histogram obtained counting 115 NPs. It was fitted using a log-normal distribution considering that particles lower than 20 nm present this class of size distribution [27]. The statistical parameters obtained from the fitting showeda geometric average diameter of 16.0 \u00b1 0.2 nm. Comparing this value with that obtained by DLS, and considering the standard error, there is a good agreement between both techniques. In order to confirm that the NPs obtained are of Ni0, the SAED was acquired and analyzed. The lattice spacings measured from the rings of the diffraction pattern (Fig.\u00a02B) were: 0.200, 0.167 and 0.118 nm. They are in very good agreement with the known lattice spacings for Ni0 bulk: 0.199, 0.173 and 0.122 nm. Therefore, this technique confirms the assignment performed by XRD.To determine if the OA and TOP remain on the NPs surface, the FT-IR spectrum was obtained (Fig.\u00a04\n). The NPs suspension was mixed with KBr mechanically and then dried. This procedure was performed in order to avoid the overlapping of the support bands with the ones belonging to OA and TOP. In Table\u00a01\n are shown the detected peaks and its assignations. The bands corresponding to symmetric and asymmetric stretching vibrations of (C\u2013H) and bending vibration of (CH3) can be assigned to the alkyl groups of OA and TOP [28, 29, 30]. It is no possible to distinguish between both compounds from these signals. However, the bending vibrations of (=C\u2013H) (-C=C) and (-N-H) are exclusive of OA, and their presence on the NPs surface could be confirmed [28, 29, 30, 31]. On the other hand, the presence of other bending bands (CH3) and various stretching (C\u2013P) bands in the range of 1159\u20131023 cm\u22121 indicates that TOP is also adsorbed on the surface of Ni NPs [29, 30]. The stretching peak of the carbonyl group at 1724 cm\u22121 appears as a consequence of the reaction between OA and acetylacetonate groups [32]. Finally, the band at 1080 cm\u22121 could be assigned to the stretching -P=O bonded at a surface nickel atom [33]. These species would be produced by TOP oxidation during the purification process of the NPs and its handling to prepare the sample to obtain the FTIR spectrum, because these steps were performed in air atmosphere.A similar result was found with the nickel phosphide NPs, but instead of TOP, Ph3P was detected on the surface [24].The silica support obtained is made of nanometric spheres of an average diameter of 530 \u00b1 8 nm with interparticular channels. In Fig.\u00a05\nA a SEM micrograph is shown. In order to estimate the average value of these channels, the textural properties of MSNS before calcination were measured. The values obtained were: BET specific surface area: 17 m2/g, average pore diameter, from BJH method: 10 nm and pore volume: 0.02 cm3/g. After calcination these values were: BET specific surface area: 1067 m2/g, average pore diameter: 2 nm and pore volume: 0.6 cm3/g. The textural values of the MSNS after calcination were typical of a mesoporous ordered silica (MCM-41). The hexagonal ordering was checked by XRD at low angles (Fig.\u00a05B). Besides, in TEM image (Fig.\u00a05C) this arrangement can be observed. It must be highlighted that the presence of the mesopores typical of MCM-41 (about of 2 nm of diameter) are not useful to locate Ni0 and nickel phosphides NPs inside of them because of steric hindrance. However, the CTMABr must be added within the gel synthesis in order to obtain the SiO2 nanospheres. On the other hand, as it can be seen in the SEM micrograph, there are interparticular macropores with different sizes depending on the nanospheres packing. The sizes of these macropores change from 90 \u00d7 80 nm to 450 \u00d7 400 nm. The nanospheres and the macropores between them afford the adequate support to anchorage the NPs, as we will describe below.The silica support was impregnated with Ni0 and nickel phosphides pre-synthetized NPs suspensions, respectively. As it can be seen in TEM micrographs neither agglomeration nor changes in NPs size were detected (Fig.\u00a06\nA and reference [24]). As it can be seen in Fig.\u00a06A, Ni0 NPs are located preferentially on the surface of the SiO2 nanospheres, but some of them are placed inside the interparticular pores (one of them is highlighted with a red circle). The Ni loadings of both systems, determined by AA, are shown in Table\u00a02\n.The HRTEM analysis of the supported Ni0 NPs shows that they have a \u201ccore-shell\u201d structure (Fig.\u00a06B). A \u201cshell\u201d thickness of about 3.5 nm was measured in the micrograph. On the other hand, using the inverse Fourier transform of HRTEM images of this \u201cshell\u201d, an average lattice spacing of 0.27 nm was obtained (Fig.\u00a06C). As a consequence of the small thickness of this \u201cshell\u201d, few diffraction points can be selected to produce the inverse Fourier transform. Therefore, not many crystalline distances can be measured in order to obtain an average value. In spite of these constraints, clearly the value of 0.27 nm cannot be assigned to Ni0. Instead, it has a good coincidence with (1 1 1) diffraction plane of NiO with f. c.c. crystalline structure. This is an interesting result because we must remember that the surface of the NPs is covered with OA and TOP. Therefore, during solvent elimination (hexane) at 60 \u00b0C in air, this layer of organic molecules cannot inhibit the approach of atmospheric O2, leading to the NiO \u201cshell\u201d production.Considering the previous results, before using the supported NPs as catalysts, two processes were necessaries. In both catalysts the organic layer (phosphorous oxidized species -P=O) was eliminated by washing and reducing with hydrogen flow. This last treatment also eliminated the NiO \u201cshell\u201d in the catalyst with Ni0 NPs.To perform the first step, three washes of the supported catalysts with CHCl3 were done, following the method proposed by Senevirathne et al [33]. In order to check the efficiency of the procedure, the same mixture of (Ni0 NPs + KBr) used to obtain the FT-IR spectrum of Fig.\u00a04, was washed with CHCl3 and a new FT-IR spectrum was obtained. In Fig.\u00a04 it can be seen that, this treatment partially eliminates the OA. In this way, the \u03b4(-C=C) band have disappear completely, but the other bands are visible yet. On the other hand, all bands assigned to TOP were clearly detected. This result proves that the adsorption of TOP is stronger than that of OA on the Ni0 NPs surface.Considering that we used the same sample, the peak intensities between non-washed and washed sample can be qualitatively compared. As consequence, it can be concluded that the band assigned to the stretching -P=O, bonded at a surface nickel atom, increase its intensity in a significant way after washing with CHCl3. Therefore, handling the sample in air atmosphere, increase TOP oxidation.After the partial elimination of the organic layer both systems: Ni-MSNS and NiP-MSNS were reduced as it was previously described. In Fig.\u00a07\nA, the micrograph reveals that Ni0 NPs sintering did not occur during this treatment. On the other hand, in Fig.\u00a07B and C HRTEM image of one Ni0 NPs and their corresponding inverse Fourier transform are shown. Because of the reduction treatment is evident that the \u201ccore-shell\u201d structure disappeared and the crystalline spacing determined by inverse Fourier transform (Fig.\u00a07C) is of 0.23 nm. Clearly, this value is lower than that assigned to (1 1 1) diffraction plane of NiO with f. c.c. crystalline structure (Fig.\u00a06C).On the other hand, it is higher than Ni0 spacing assigned to (1 1 1) planes. There are several reports in which an increasing of the interplanar distances in NPs, in comparison with the bulk value, has been detected. Thus, Winnischofer et\u00a0al. [34] found a shifting to higher value crystalline spacing for the (1 1 1) plane in Ni0 NPs with f. c.c. crystalline structure. These authors considered that, most of the metals with f. c.c. crystalline structure and nanometric sizes, exhibit axes with five-fold symmetry. This kind of structure is forbidden in bulk crystals and led to NPs with icosahedral or decahedral shapes. These types of particles are known as \u201cmultiply-twinned particles\u201d, and this structural distortion would produce the increasing of the interplanar distances.On the other hand, the same mixture of (Ni0 NPs + KBr), used to obtain the FT-IR spectrum after three washes with CHCl3, was reduced using identical conditions to Ni-MSNS. In Fig.\u00a04, the FT-IR spectrum is shown. Weak bands at 1500 and 1460 cm\u22121 -corresponding to bending vibrations of (CH3) groups of TOP- and a wide signal in the range of 1159\u20131023 cm\u22121 -assignable to stretching of (C\u2013P) bonds of TOP- were detected.Also, the band at 1080 cm\u22121, corresponding to stretching of -P=O, bonded to a superficial nickel atom, was observed. Strikingly, we can conclude that, after the treatment in pure H2 flow during 2 h at 500 \u00b0C, there are remains of TOP and oxidized TOP on the surface of the Ni0 NPs. This fact is undesired because both species will block a certain number of active sites.\nTable\u00a02 lists the H2 and CO chemisorption results for the Ni-MSNS and NiP-MSNS, respectively. Besides, the metallic dispersions and the crystallite sizes obtained from the corresponding chemisorption measurements are reported. Assuming that the Ni0 NPs have a spherical geometry, the particle size can be estimated from the equation d\n\nAV\n = 101/D, where d\n\nAV\n represents surface-weighted average crystallite diameter in nm and D the metal dispersion, in % [35]. Using this equation and the metallic dispersion calculated from the H2 chemisorption value by assuming a stoichiometry of one H atom per surface metallic atom, the Ni NPs size was calculated (Table\u00a02). As it can be seen, this value is approximately five times larger than the size determined by TEM (101 nm vs 19 \u00b1 6 nm, respectively). Clearly, this discrepancy has its origin in the very low H2 chemisorption value obtained. This experimental fact is coherent with the presence of TOP fragments that remain on the Ni0 NPs surface, as was detected from FT-IR spectrum. A similar procedure was followed to evaluate the average size of the nickel phosphides NPs from the CO chemisorption result, assuming spherical geometry and applying the equation d\n\nAV\n\n= 6nf/\u03c1L. Here, f is the weight fraction of the nickel phosphides in the catalyst, n is the average surface metal atom density (atoms/cm2), \u03c1 is the nickel phosphide density (g/cm3) and L is the metal site concentration obtained from CO chemisorption by assuming one CO chemisorbed molecule per surface metal atom (atoms/gcatalyst) [36]. Bearing in mind that the NPs suspension used to impregnate the support is a mixture of Ni12P5 and Ni2P, where each NPs is monophasic, we used weighted averages \u03c1 and L values considering the molar composition previously mentioned. The density values used were: \u03c1Ni12P5 = 7.53 g/cm3 and \u03c1Ni2P = 7.35 g/cm3 [37] and the L values were: LNi12P5 = 1.21 \u00d7 1015 atoms/cm2 and LNi2P = 1.01 \u00d7 1015 atoms/cm2 [38]. As it can be seen in Table\u00a02 there is an excellent agreement between the calculated d value from the CO chemisorption test and that obtained by TEM. From this result, we can conclude that the surface of the nickel phosphides NPs was properly cleaned during the reduction step and the estimation of the surface metallic atoms obtained from CO chemisorption would produce a reliable TOF number for the catalytic reaction.On the other hand, we can determine from volumetric oxidation test that a very high percentage of Ni reduction was reached in Ni-MSNS catalyst (Table\u00a02). Probably, the small non-reduced quantity remains as Ni2+ diffused inside the walls of the SiO2 support. This process could occur as consequence of the strong interaction between the NiO \u201cshell\u201d of the NPs and the SiO2 support. During the reduction step two parallel and competitive processes could take place: the NiO reduction and the diffusion of Ni2+ ions inside the SiO2 lattice. The first step would be predominant and a 91 % of the total Ni loading is reduced to metallic state.\nTable\u00a02 shows that, Ni-MSNS catalyst reached the higher AP conversion at 300 min of reaction time with a value of 31 %. At this time, NiP-MSNS only reach 17 % of conversion. Notwithstanding, after 420 min the conversion value of this catalyst is 27 %. Therefore, the AP hydrogenation process takes place more slowly in the catalyst with nickel phosphide NPs. The phosphorus atoms that surround the nickel atoms would produce a diluting effect on the nickel assembly, decreasing the hydrogenation velocity. The corresponding TOF numbers evaluated at 300 min (Table\u00a02) reflects this experimental fact. Is important to remark two aspects about the TOF of Ni-MSNS:\n\n-\nthe presence of TOP fragments on the surface of the Ni0 NPs could block active sites. Therefore, it is possible that higher conversion could be obtained if a complete elimination of the surfactant could be achieved,\n\n\n-\nby the same reason, the quantity of the H2 chemisorbed would be underestimated. As consequence, TOF number would be overestimated.\n\n\nthe presence of TOP fragments on the surface of the Ni0 NPs could block active sites. Therefore, it is possible that higher conversion could be obtained if a complete elimination of the surfactant could be achieved,by the same reason, the quantity of the H2 chemisorbed would be underestimated. As consequence, TOF number would be overestimated.In order to determine if these catalysts are chemoselective to hydrogenate the carbonyl group of the AP to produce 1-phenylethanol, the selectivities at the same level conversions (about 30 %) were evaluate (Fig.\u00a08\n). Both catalysts have a very high selectivity to this product. We will analyze these results taking into account the two possible adsorption modes of carbonyl groups on the surface of metal transitions: \u03b71(O) and \u03b72(C,O). For AP hydrogenation with a Pt/SiO2 catalyst, Chen et\u00a0al. [39] proposed that in \u03b71(O) mode the coordination happens between the oxygen of the carbonyl group and one metallic site and the aromatic ring remains parallel to the metal surface. Instead, in \u03b72(C,O) mode, the coordination takes place between \u03c0-electrons of C=O and two neighbors surface metallic sites [39]. Considering that the carbon atom of the carbonyl group has sp2 hybridization, the aromatic ring is tilted with respect to the metallic surface. This configuration would inhibit the phenyl group hydrogenation and high 1-phenylethanol selectivity could be obtained. A similar process would occur with the Ni-MSNS catalyst. Comparing the interatomic Ni\u2013Ni distance (0.249 nm) with the length of the double bond C=O (0.120 nm), in order to get bridge adsorption, the carbonyl bond would be weakened and could be hydrogenated easily.When nickel phosphides are used as a catalyst to hydrogenate AP some important differences respect to pure metal must be considered. Thus, in these phases, P atoms have higher electronegativity than Ni atoms. As consequence, they can be represented as: P\u03b4\u2212 and Ni\u03b4+, respectively. The Ni\u03b4+ surface atoms behave as Lewis acid sites, attracting the atoms with negative charge density of the AP. Besides they work as metallic sites for hydrogenation [40]. Other difference between these compounds and Ni0, is the presence of charge accumulations along several bonds within the Ni coordination polyhedron surrounding the P atoms as it was demonstrated using the density functional theory [37]. Therefore, it should be unlikely that the AP can be adsorbed in \u03b72(C,O) mode because charge accumulations would repel the \u03c0-electrons of C=O. In bibliography has been proposed that the only intermediate of adsorption to produce the hydrogenation of carbonyl molecules, when transition metals are used as catalysts, is \u03b72(C,O) mode [39]. However, considering our experimental results, we assume that when nickel phosphides are used as hydrogenation catalysts, the Ni\u03b4+ surface atoms attract the oxygen of the C=O group and, at the same time, a strong repulsion is produced between the aromatic ring and the negative charge density, accumulated on the surface by the P atoms. As consequence, the bond C=O is weakened, and the hydrogenation is possible but through \u03b71(O)-like mode as intermediate. Following this train of thought, nickel phosphides could change the mechanism of the chemoselective hydrogenation of the carbonyl group in AP and the intermediate similar to \u03b71(O) would be reactive to produce 1-phenylethanol. However, this adsorption mode would be less reactive in comparison with \u03b72(C,O) mode and this would be a second reason (besides the diluting effect produced by phosphorus atoms presence) that would explain the slower hydrogenation with nickel phosphides catalysts with respect to metallic nickel.Monodisperse pre-synthetized NPs of Ni0 and nickel phosphides with the same average diameter (16.0 \u00b1 0.2 nm vs. 15.1 \u00b1 0.6 nm respectively) were used to prepare two \u201cquasi-model\u201d catalysts. Both NPs species were deposited on nano-spheres of MCM-41 with an average diameter of 530 \u00b1 8 nm. The textural properties of this support were adequate to inhibit agglomeration and sintering processes during impregnating, washing and reduction steps. In this way, we have obtained two supported and activated catalysts with the same average NPs diameter. This structural characteristic allowed performing the comparison of the catalytic results without misleading produced by crystal size effects of the active species.Both catalysts were tested in hydrogenation of AP and they showed a very similar final conversion of this compound (\u2245 30 %) but nickel phosphides present a lower reaction velocity than Ni0. On the other hand, when the selectivities were compared at similar conversion levels (\u2245 30 %), in order to avoid some influence of this parameter, both catalysts showed a very high selectivity to 1-phenylethanol (the desired product) of about 95 %. Therefore, we can conclude that the only catalytic difference between both systems would be the hydrogenation reaction velocity. It is necessary to remark that an optimization of the reaction operative conditions was not performed. Thus, it could be possible to get similar reaction velocities if higher reaction temperature is used.On the other hand, if geometric and electronic surface properties of Ni0 and nickel phosphides are compared, important differences appear. Thus, the situation with metallic nickel would be similar to other transition metals: AP could be adsorbed through \u03b72(C,O) mode and the chemoselective hydrogenation occurs successfully. Instead, in nickel phosphides surface there are zones with great negative charge accumulations along some Ni\u2013P bonds. Besides, the electronegativity differences between Ni and P produce charge densities on both atoms: P\u03b4\u2212 and Ni\u03b4+. This complex electronic distribution would produce a strong electrostatic repulsion between some areas of the surface of nickel phosphides and the phenyl group and \u03c0-electrons of C=O of the AP. As consequence, we propose that AP only could be adsorbed on top Ni\u03b4+ atoms through the oxygen atom of the carbonyl group. That means, the AP would be adsorbed with a mode similar to \u03b71(O) as intermediate. Previous results have shown that through this intermediate, the hydrogenation of the carbonyl group cannot occur if transition metals are used as catalysts. Instead, if nickel phosphides are used, we suppose that the AP would be adsorbed through its oxygen to a Ni\u03b4+ atom but, at the same time, the molecule would be repelled far away from the surface due to the strong electrostatic repulsion generated between them, as it was previously described. In this situation, the C=O bond would be weakened (by a different reason that in the transition metals case) and it could be hydrogenated.Finally, is interesting to emphasize that nickel phosphides have a very wide range of compositions from Ni3P to NiP3. Among them there are great structural and electronic differences which will produce very diverse catalytic sites. Therefore, we could assume that there would be many different organic substrates, with more than one functional group, on which a chemoselective hydrogenation could take place if nickel phosphides, with different stoichiometries, are used. As consequence of these results, we can infer that due to the great versatility of these phases, they appear to be new potential chemoselective hydrogenation catalysts and new attempts to study different compositions and substrates are justified.The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.Virginia Vetere: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper.Dolly Costa: Performed the experiments.Analia Soldati: Performed the experiments; Contributed reagents, materials, analysis tools or data.Jose Fernando Bengoa: Performed the experiments; Analyzed and interpreted the data.Sergio Marchetti: Analyzed and interpreted the data; Wrote the paper.This work was supported by ANPCyT (PICT N\u00b0 00549 and 0148) and Universidad Nacional de La Plata (Projects X757 and X710).The authors declare no conflict of interest.No additional information is available for this paper.", "descript": "\n                  Two catalysts were prepared using monodisperse pre-synthetized nanoparticles of metallic nickel and nickel phosphides with the same average diameter. Both nanoparticles species were deposited on the same support: mesoporous silica nano-spheres of MCM-41. This support is suitable to inhibit agglomeration and sintering processes during preparation steps. Therefore, two supported and activated catalysts with the same average nanoparticles diameter were obtained. They differ only in the nature of the active species: metallic nickel and nickel phosphides. The effect of the presence of a second element (phosphorus), more electronegative than nickel, on the activity and selectivity in the chemoselective hydrogenation of acetophenone was studied. The reaction conditions were: H2 pressure of 1 MPa, 80 \u00b0C using n-heptane as solvent. With the aim to understand the catalytic results, nanoparticles, support and catalysts were carefully characterized by X-ray diffraction, diffuse light scattering, transmission electron microcopy, high resolution transmission electron microcopy, selected area electron diffraction, scanning electron microcopy, Fourier transformer infrared spectroscopy, N2 adsorption at -196 \u00b0C, atomic absorption, H2 and CO chemisorption and volumetric oxidation. Considering these results and geometric and electronic characteristics of the surface of both active species, a change in the adsorption intermediate state of acetophenone in presence of phosphorus is proposed to explain the hydrogenation chemoselectivity of nickel phospides.\n               "}
{"full_text": "With the increasing concern on the emission control of carbon dioxide, the catalytic CO2 conversion has attracted worldwide attentions. Among various options for CO2 conversion, the hydrogenation of CO2 to methanol is very promising with the rapid development of renewable energy. It can convert carbon dioxide in a large scale. The product, methanol, has broad applications as chemical intermediate and fuel. The catalyst with high activity and selectivity for CO2 hydrogenation becomes the key for the further applications. From the reported works, the copper-based catalysts are the mostly investigated ones for CO2 hydrogenation to methanol [1\u20134]. Noble metals like palladium, platinum, gold, rhodium, and iridium have been frequently used either as the catalyst (including the bimetallic catalyst) or as the promoter. However, a few works can be found in the literature with silver as the principal catalyst or as the promoter for CO2 hydrogenation to methanol. Silver has a relatively low price. Silver based catalysts have indeed attracted significant attentions. However, most of the reported silver catalysts are for oxidation reactions. According to the Web of Science, 16,986 papers was found with topics of Ag and catalyst since 2000 (by January 25, 2021). Among them, 6254 were for oxidation and 1255 for hydrogenation. A well-known example is the epoxyethane production from oxidation of ethylene over Ag catalyst. 33 papers were found with topics of \u2018Ag\u2019, \u2018catalyst\u2019, \u2018hydrogenation\u2019, \u2018CO2\u2019 and \u2018methanol\u2019. However, only a few papers are directly related with the supported Ag catalysts for hydrogenation of carbon dioxide to methanol [5\u201312] with some other papers using silver as the promoter [13,14]. Ag/ZrO2 presents a higher methanol selectivity from CO2 hydrogenation than Cu/ZrO2 [6\u20138], although its activity is lower than Cu/ZrO2. An increase of the t-ZrO2 phase and Ag+\u00a0content causes an increase in the rate of methanol formation [11]. A high dispersion or a small size of the silver catalyst is also favored for the selective hydrogenation of CO2 to methanol [10]. In general, the activity of the reported silver catalysts is not high for CO2 hydrogenation to methanol. Different from palladium and platinum, silver is lack of affinity toward hydrogen because of the filled d-band [15]. The further improvement in the catalyst preparation is needed in order to improve the activity of silver catalysts for CO2 hydrogenation to methanol [15].Very recently, we found the In2O3 has an intense interaction with palladium [16\u201319], nickel [20], rhodium [21], platinum [22,23] gold [24,25] and iridium [26,27]. This intense interaction leads a high metal dispersion and high activity towards selective hydrogenation of carbon dioxide to methanol, which has been confirmed as well by other groups [16,19,23,28]. This is especially unusual for Ni, Rh, Pt, Au and Ir catalysts, which have normally poor activity for CO2 hydrogenation to methanol. In this work, we attempt to load the silver catalyst onto In2O3. We confirm the In2O3 supported Ag catalyst is active towards the selective hydrogenation of CO2 to methanol.All the theoretical calculations were carried out using the Vienna ab initio simulation package (VASP) using a plane-wave basis set [29,30]. The projector augmented wave (PAW) method is used to describe the interaction between the valence electrons and the atomic cores [31]. The exchange and correlation energies were calculated using the Perdew-Burke-Ernzerhof (PBE) functionals [32]. Based on the results of XRD, the (111) facet was chosen because it is the most thermodynamically stable facet of this phase [33]. The In2O3(111) surface was modeled as a periodically repeated slab consisting of 72 O atoms and 48 In atoms distributed in three atomic layers and is separated by a vacuum layer of 12\u00a0\u00c5. The oxygen vacancy is created by removing one oxygen atom from the perfect In2O3(111) surface. The supercell has a dimension of 14.56\u00a0\u00c5\u00a0\u00d7\u00a012.61\u00a0\u00c5\u00a0\u00d7\u00a020.04\u00a0\u00c5. A plane-wave basis set with a cutoff energy of 400\u00a0eV and a (3\u00a0\u00d7\u00a03\u00a0\u00d7\u00a01) k-point grid generated with the Gamma-Centered scheme was found to give the converged results. The geometry optimization and self-consistent field convergence criterion were set to 0.03\u00a0eV \u00c5\u22121 and 10\u22125\u00a0eV, respectively. The top two tri-layers were allowed to fully relax while the bottom tri-layer was fixed at the equilibrium position. The Ag4 cluster in the gas phase was calculated and optimized in an a\u00a0=\u00a0b\u00a0=\u00a0c\u00a0=\u00a020\u00a0\u00c5 lattice. The Ag/In2O3 model is established by placing the optimized Ag4 cluster on the defective In2O3(111) surface.The adsorption energies of intermediates M were calculated as:\n\n\n\n\u0394\n\nE\n\na\nd\n\n\n\n(\nM\n)\n\n=\n\nE\n\nM\n/\n\n(\n\n\n\nA\ng\n\n4\n\n/\n\n\n\nI\nn\n\n2\n\n\nO\n3\n\n\n\n)\n\n\n\n\u2212\n\nE\n\n(\n\n\n\nA\ng\n\n4\n\n/\n\n\n\nI\nn\n\n2\n\n\nO\n3\n\n\n\n)\n\n\n\u2013\n\nE\n\n(\nM\n)\n\n\n\n\n\nwhere \n\n\nE\n\nM\n/\n\n(\n\n\n\nA\ng\n\n4\n\n/\n\n\n\nI\nn\n\n2\n\n\nO\n3\n\n\n\n)\n\n\n\n\n, \n\n\nE\n\n(\n\n\n\nA\ng\n\n4\n\n/\n\n\n\nI\nn\n\n2\n\n\nO\n3\n\n\n\n)\n\n\n\n and \n\n\nE\n\n(\nM\n)\n\n\n\n represent the total energies of the Ag4/In2O3 model with the adsorbate and the clean Ag4/In2O3, the free molecule, respectively. As defined above, the negative values of the adsorption energy indicate that the process is exothermic whereas the positive values mean that the process is endothermic. The climbing image nudged elastic (CI-NEB) band method with 4\u20138 images was used to locate the likely transition state firstly. Then, the likely transition state was relaxed via the Dimer method. The relaxed transition state was confirmed through frequency analysis.The In2O3 support was prepared via the precipitation method. The desired amount of In(NO3)3\u00b74H2O (2.41\u00a0g, HWRK Chem, 99.99%) was dissolved into the deionized water as the precursor solution (0.15\u00a0mol L\u22121). 3.50\u00a0g of sodium carbonate hydrate (Tianjin Kermel Chemical Reagent, 99%) was also dissolved into the deionized water as the precipitant solution (0.2\u00a0mol L\u22121). Firstly, the precipitant solution was dropped into the precursor solution with vigorous stirring under 80\u00a0\u00b0C until the pH value reached 7. The mixture was aged for another 3\u00a0h under the same condition. The precipitate was then washed with deionized water several times. After dried at 80\u00a0\u00b0C overnight, the as-prepared solid was calcined in static air at 450\u00a0\u00b0C for 3\u00a0h.The Ag/In2O3 catalyst was prepared via the deposition-precipitation method. Silver nitrate (Aladdin Industrial Corporation, Shanghai, 99.99%, metal basis) was dissolved into 50\u00a0mL deionized water. Then the as-prepared In2O3 support was added into the solution, followed by vigorous stirring for 1\u00a0h at room temperature. An excessive amount of sodium carbonate solution (1.0\u00a0g in 10\u00a0mL of the deionized water) was then added into the mixture and the formed precipitate was continuously stirred at 80\u00a0\u00b0C for 3\u00a0h. After washed and filtered with 1\u00a0L deionized water, the mixture was dried in a vacuum at 60\u00a0\u00b0C overnight. The actual loading weight of Ag species was determined by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) measurements.77\u00a0K N2 adsorption/desorption isotherms of the samples were measured on an Autosorb-1-C instrument (Quantachrome). The specific surface area (SBET) was calculated using the Brunauer\u2013Emmett\u2013Teller (BET) model. The bulk analytical composition of the samples was determined by inductively coupled plasma optical emission spectrometer (ICP-OES) measurements, using a Perkin Elmer Optima 5300DV system. Powder X-ray diffraction (PXRD) was conducted to characterize the textural structures of the samples, using a Rigaku D/max 2500v/pc diffractometer with Cu K\u03b1 radiation (40\u00a0kV, 200\u00a0mA). The scanning rate was set to 8\u00b0 min\u22121 within the 2\u03b8 range of 10\u00b0\u201380\u00b0. The phase identification was made by comparison with the Joint Committee on Powder Diffraction Standards (JCPDSs). Transmission electron microscopy (TEM) measurements were performed on a JEOL JEM-2100F system equipped with an energy-dispersive X-ray spectrometer (EDX) operated at 200\u00a0kV. The sample powder was firstly suspended into ethanol and then dispersed ultrasonically for 5\u00a0min. A drop of the suspension was deposited on a copper grid coated with carbon. Temperature programmed desorption of carbon dioxide (CO2-TPD) experiments were conducted on a Micromeritics Autochem II 2920 chemisorption analyzer equipped with a Hiden HPR-20 EGA mass spectrometer (MS). 100\u00a0mg of the sample was loaded into a U-shaped quartz tube and reduced with a gaseous mixture of 10% H2 in Ar for 1\u00a0h at 200\u00a0\u00b0C and then cool down to 50\u00a0\u00b0C under flowing helium, followed by the CO2 adsorption at the same temperature for 1\u00a0h. After purged by flowing helium for 1\u00a0h to remove the physically absorbed CO2, the sample was then heated to 700\u00a0\u00b0C at a rate of 10\u00a0\u00b0C min\u22121. The signals of m/z\u00a0=\u00a044 and 28 were collected by the mass spectrometer. Electron paramagnetic resonance (EPR) spectra of the samples were collected at room temperature using a Bruker A300 EPR spectrometer operated at the X-band frequency. Raman spectra of the samples were collected using an inVia Reflex Renishaw Raman Spectroscopy System. The scan range is 200\u2013800\u00a0nm with 532\u00a0nm laser as the excitation source. The laser power was set at 5\u00a0mW and the integration time is 5\u00a0s. The UV\u2013vis absorption spectra of the samples were recorded at room temperature, using a UV-2600 UV\u2013vis spectrophotometer (Shimadzu Corporation).The catalytic tests for methanol synthesis from CO2 hydrogenation over In2O3 and the Ag/In2O3 catalysts were performed in a vertical bed reactor. 0.2\u00a0g of the sample was diluted with 1.0\u00a0g SiC before being loaded into the reactor. Firstly, the reactor with the sample was purged by flowing nitrogen for 0.5\u00a0h at room temperature. The Ag/In2O3 catalyst was then pre-reduced by a gaseous mixture of 10% H2 in N2 at\u00a0200\u00a0\u00b0C for 1\u00a0h. The reactant mixture was introduced into the\u00a0reactor until the pressure reached 5\u00a0MPa at the same temperature. The reaction was performed with a gas hourly space velocity (GHSV) of 21,000\u00a0cm3 gcat\n\u22121 h\u22121 and a temperature range from 200\u00a0\u00b0C to 300\u00a0\u00b0C. The products were analyzed using an online gas chromatograph (Agilent 7890A), equipped with a thermal conductivity detector (TCD) and a flame ionized detector (FID). To prevent the condensation of methanol, all the valves and lines between the reactor outlet and the GC inlet were maintained at 140\u00a0\u00b0C.The CO2 conversion (\n\n\nX\n\nCO\n2\n\n\n\n), methanol selectivity (Smethanol), and space-time yield (STY) of methanol were calculated according to the following equations:\n\n\n\n\nX\n\n\nC\nO\n\n2\n\n\n=\n\n\n\nF\n\n\n\nC\nO\n\n2\n\n,\n\ni\nn\n\n\n\n\u2212\n\nF\n\n\n\nC\nO\n\n2\n\n,\n\no\nu\nt\n\n\n\n\n\nF\n\n\n\nC\nO\n\n2\n\n,\n\ni\nn\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n\n\nS\n\nm\ne\nt\nh\na\nn\no\nl\n\n\n=\n\n\nF\n\nm\ne\nt\nh\na\nn\no\nl\n,\no\nu\nt\n\n\n\n\nF\n\n\n\nC\nO\n\n2\n\n,\n\ni\nn\n\n\n\n\u2212\n\nF\n\n\n\nC\nO\n\n2\n\n,\n\no\nu\nt\n\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n\n\n\nM\ne\nt\nh\na\nn\no\nl\n\n\n\nS\nT\nY\n\n=\n\n\n\nF\n\n\n\nC\nO\n\n2\n\n,\n\ni\nn\n\n\n\n\u00d7\n\nX\n\n\nC\nO\n\n2\n\n\n\u00d7\n\nS\n\nm\ne\nt\nh\na\nn\no\nl\n\n\n\nW\n\n\u00d7\nM\n\n\n\nwhere F is the molar flow rate, M is the molar mass of methanol and W is the weight of the catalyst.To investigate the electronic interaction between the Ag species and the In2O3 support, DFT calculations were performed with a model catalyst of the Ag4 cluster on the In2O3(111) with an oxygen vacancy. The loading of Ag species on the In2O3 support was firstly investigated. As shown in Fig.\u00a01\na, the Ag4 cluster is a typical structure of the Ag cluster. The average length of the Ag\u2013Ag bonds is 2.71\u00a0\u00c5, which is well consistent with the literature [34,35]. Fig.\u00a01b and c show the optimized structures of the perfect and the defective In2O3(111), respectively. The optimized structure of the Ag4 cluster supported on the defective In2O3(111) model is named \u201cAg4/In2O3_D\u201d as shown in Fig.\u00a01d. The adsorption energy of the Ag4 cluster on the Ag4/In2O3_D is \u22121.42\u00a0eV, which is closed to that of the Ag4 cluster on the m-ZrO2(111) surface (\u22121.49\u00a0eV) [35]. The average length of Ag\u2013Ag bonds is increased to 2.75\u00a0\u00c5 due to the interaction between the Ag4 cluster and the defective In2O3 surface. To gain the insight into the electronic structure of the supported Ag4 cluster, Bader charge analysis was conducted. As shown in Table 1\n, the total charge of the free Ag4 cluster is zero, indicating electrical neutrality. Due to the presence of the strong metal-support interaction (SMSI) between the Ag4 cluster and the defective In2O3(111) surface, the total charge of the Ag4 cluster is increased to +0.34 |e| [25,36,37]. This result confirms the presence of the strong electron transfer from the Ag4 cluster to the In2O3 support.The oxygen vacancies over In2O3 play a very important role in CO2 adsorption as well as CO2 activation [25]. Therefore, the CO2 adsorption on the interfacial site between the positively charged Ag4 cluster and the surface oxygen vacancy of the Ag4/In2O3_D model is studied. As shown in Fig.\u00a02\na, the CO2 molecule can be activated on such interfacial site of the Ag4/In2O3_D model through R1 (CO2(g) + \u2217\u2192CO2\u2217), with the adsorption energy of \u22120.50\u00a0eV. The bond length of C-Oa and C-Ob are increased to 1.34\u00a0\u00c5 and 1.24\u00a0\u00c5, respectively. It is 1.17\u00a0\u00c5 in the free CO2 molecule. In addition, the O\u2013C\u2013O angle becomes 123.1\u00b0. The Bader charge analysis shows that the CO2 molecule is negatively charged due to the interaction with the interfacial site of the Ag4/In2O3_D model as shown in Table 2\n. The total charge of the adsorbed CO2 is \u22120.95 |e|. This is also confirmed by deformation charge density, shown in Fig.\u00a02c, where the yellow iso-surface around the adsorbed CO2 is clearly seen and is assigned to the accumulation of electrons. As shown in Fig.\u00a02b, hydrogen can be activated by the positive charged Ag species through R2 (H2(g) + \u2217 + \u2217\u21922H\u2217), with the adsorption energy of \u22120.71\u00a0eV. The average H\u2013Ag bond length is 1.76\u00a0\u00c5. The active H adatoms can spill over and react with the activated CO2 intermediates on the\u00a0interfacial site of the Ag4/In2O3 model. All the results indicate that the electron transfer induced by the interaction between the Ag species and the In2O3 support promotes the formation of the positively charged Ag species, which can facilitate the activation of CO2 and hydrogen.Wang et\u00a0al. [38] and Ye et\u00a0al. [27] reported that the oxygen vacancies over In2O3 can facilitate the CO2 dissociation via the experimental and the theoretical studies, respectively. Herein, the dissociation of CO2 over the interfacial site of the Ag4/In2O3_D model through R3 (CO2\u2217 + \u2217\u2192CO\u2217 + O\u2217) was also calculated. The potential energy surface is shown in Fig.\u00a03\n. The distance of C atom and Ag atom is 3.08\u00a0\u00c5 in the structure of CO2\u2217, while it is shortened to 2.55\u00a0\u00c5 due to the formation of C\u2013Ag bond in the transition state (TS1). Meanwhile, the C-Oa bond is slightly increased to 1.38\u00a0\u00c5 whereas the length of the C-Ob bond is decreased to 1.21\u00a0\u00c5. The reaction is slightly endothermic by +0.10\u00a0eV with an activation barrier of 0.41\u00a0eV. The low barrier indicates that the CO2 dissociation on the interfacial site of the Ag/In2O3 catalyst is kinetically favorable, which also implies that the methanol synthesis on the Ag/In2O3 catalyst can undergo the CO hydrogenation route. Therefore, the methanol synthesis via the CO hydrogenation route on the interfacial site of the Ag4/In2O3_D model is investigated.\nFig.\u00a04\n shows the potential energy surface of methanol synthesis from CO2 hydrogenation through the CO hydrogenation route on the interfacial site of the Ag4/In2O3_D model, with comparison of the RWGS reaction. All the structural parameters and adsorption energies of the reaction intermediates involved are summarized in Table S1. The details of these structures are shown in Fig.\u00a0S1. The structural details of initial states (IS), transition states (TS) and finial states (FS) are also shown in Fig.\u00a0S2. The reaction energies and the activation barriers of all elementary steps involved methanol synthesis on the Ag4/In2O3_D model are listed in Table 3\n.After the CO2 dissociation through R3, the produced O\u2217 can react with the H adatom activated on the Ag4 cluster and form the surface OH\u2217 through R4 (CO\u2217 + O\u2217 + H\u2217 \u2192CO\u2217 + OH\u2217\u00a0+ \u2217). The transition state is TS2. This reaction is exothermic by \u22120.23\u00a0eV with an activation barrier of 0.99\u00a0eV, indicating that this process is both thermodynamically and kinetically favorable. In particular, the direct hydrogenation of CO2 to HCOO\u2217 and COOH\u2217 are also examined to investigated the feasibility of the formate route and the RWGS route, respectively. As shown in Fig.\u00a0S3, the hydrogenation of CO2 to HCOO\u2217 (R14: CO2\u2217 + H\u2217\u2192HCOO\u2217 + \u2217) is exothermic by\u00a0\u22120.19\u00a0eV with a huge barrier of 1.65\u00a0eV. The hydrogenation of CO2 to COOH\u2217 (R15: CO2\u2217 + H\u2217\u2192COOH\u2217 + \u2217) is endothermic by +0.85\u00a0eV with an activation barrier of 1.35\u00a0eV. These results indicate that the direct hydrogenation of CO2 on the interfacial site of the Ag4/In2O3_D model is not feasible, compared to R4 of the CO hydrogenation route.The H adatom activated on the Ag4 cluster can react with\u00a0the C atom of CO\u2217 through R5 (CO\u2217 + OH\u2217 + H\u2217\u2192HCO\u2217 + OH\u2217 + \u2217). The transition state is TS3. This reaction is endothermic by +0.32\u00a0eV with an activation barrier of 0.81\u00a0eV. Moreover, the activated H adatom can also react with OH\u2217 and produce H2O\u2217 through R6 (CO\u2217 + OH\u2217 + H\u2217\u2192CO\u2217 + H2O\u2217 + \u2217). The transition state is TS4. This reaction is endothermic by +0.72\u00a0eV with an activation barrier of 1.27\u00a0eV. The CO\u2217 and H2O\u2217 proceed to desorb from the interfacial site with an overall energy cost of 1.66\u00a0eV to complete the RWGS reaction. These confirm that the activated H adatom prefers to react with the C atom of CO\u2217 due to the lower activation barrier.Subsequently, the production of H2O\u2217 from the hydrogenation of OH\u2217 undergoes R7 (HCO\u2217 + OH\u2217 + H\u2217\u2192HCO\u2217 + H2O\u2217 + \u2217). The transition state is TS5. The reaction is endothermic by +0.39\u00a0eV with the activation barrier of 0.98\u00a0eV. Based on the adsorption energies of HCO\u2217 (\u22122.26\u00a0eV) and H2O\u2217 (\u22120.97\u00a0eV), H2O\u2217 can desorb from the interfacial site more easily than HCO\u2217.H2CO\u2217 is one of the important intermediates in methanol synthesis from CO2 hydrogenation. H2CO\u2217 can be produced from the hydrogenation of HCO\u2217 through R8 (HCO\u2217 + H2O(g) + H\u2217\u2192H2CO\u2217 + H2O(g) + \u2217). The transition state is TS6. The reaction is exothermic by \u22120.71\u00a0eV with an activation barrier of 0.98\u00a0eV, which indicates that this process is both thermodynamically and kinetically favorable.The hydrogenation of H2CO\u2217 produces either H3CO\u2217 or H2COH\u2217. The production of H3CO\u2217 (R9: H2CO\u2217 + H2O(g) + H\u2217\u2192H3CO\u2217 + H2O(g) + \u2217, \u0394E\u00a0=\u00a0\u22120.66\u00a0eV, Ea\u00a0=\u00a01.07\u00a0eV) is much more kinetically and thermodynamically favorable than the production of H2COH\u2217 (R10: H2CO\u2217 + H2O(g) + H\u2217\u2192H2COH\u2217 + H2O(g) + \u2217, \u0394E\u00a0=\u00a0+1.18\u00a0eV, Ea\u00a0=\u00a02.52\u00a0eV) due to the lower activation barrier and exothermic nature. The transition states of R9 and R10 are TS7 and TS8, respectively.Finally, H3COH\u2217 can be produced via H3CO\u2217 protonation (R11: H3CO\u2217 + H2O(g) + H\u2217\u2192H3COH\u2217 + H2O(g) + \u2217, \u0394E\u00a0=\u00a0+0.45\u00a0eV, Ea\u00a0=\u00a01.01\u00a0eV) or the hydrogenation of H2COH\u2217 (R12:\u00a0H2COH\u2217 + H2O(g) + H\u2217\u2192H3COH\u2217 + H2O(g) + \u2217, \u0394E\u00a0=\u00a0\u22120.48\u00a0eV, Ea\u00a0=\u00a01.30\u00a0eV). The hydrogenation of H2COH\u2217 is more thermodynamically but less kinetically favorable than the protonation of H3CO\u2217. The transition states of R11 and R12 are TS9 and TS10, respectively. CH3OH\u2217 proceeds to desorb from the interfacial site with an energy cost of 0.79\u00a0eV and 0.67\u00a0eV for the CH3O pathway and the H2COH pathways, respectively. Furthermore, the potential energy surface of the CH3O pathway is always below the H2COH pathway. Therefore, methanol will mainly be produced through the CH3O pathway in the CO hydrogenation route.\nFig.\u00a05\n shows the changes of CO2 conversion and methanol selectivity on Ag/In2O3 with the reaction temperature. The catalytic activity of In2O3 is shown as well for the comparative purpose. The activity data in Fig.\u00a05a\u2013c were collected when the reaction was carried out for 30\u00a0min. The carbon balances over both In2O3 and Ag/In2O3 catalyst are better than 98%. Only trace methane can be detected when the reaction temperature is beyond 275\u00a0\u00b0C. As shown in Fig.\u00a05a and b, enhanced activity is achieved for CO2 hydrogenation to methanol by the loading of Ag. The CO2 conversion and the space-time yield (STY) of methanol on Ag/In2O3 are higher than those on In2O3. The STY reaches the highest value of 0.453 gmethanol gcat\n\u22121 h\u22121 on Ag/In2O3 at 300\u00a0\u00b0C and 5\u00a0MPa, whereas it is 0.335 gmethanol gcat\n\u22121 h\u22121 on In2O3 at the same condition. The STY of Ag/In2O3 is around 4 times higher than the reported STY on Ag/ZrO2 at 300\u00a0\u00b0C [11]. The product distribution of CO2 hydrogenation to methanol over the Ag/In2O3 catalyst at 300\u00a0\u00b0C and 5\u00a0MPa is shown in Table S2. Apparent activation energies for In2O3 and Ag/In2O3 were calculated based on the Arrhenius equation. As shown in Fig.\u00a05c, the apparent activation energy of CO2 conversion of In2O3 is 101.98\u00a0kJ mol\u22121, which is in line with the literature [16,22,39]. The Ag loading significantly reduces the apparent activation energy of CO2 conversion to 86.44\u00a0kJ mol\u22121. This confirms that the Ag loading is favorable for the CO2 hydrogenation. Fig.\u00a05d presents the stability test results of In2O3 and Ag/In2O3. The methanol STY on Ag/In2O3 maintains over 90% of its initial value after the 10-h reaction at 300\u00a0\u00b0C and 5\u00a0MPa whereas the pure In2O3 catalyst loses near 20% of its initial methanol STY after 10-h in the reaction stream. This result indicates that the addition of Ag species significantly improves the stability of In2O3. Table S3 summarizes the comparison of the catalytic activities of Ag/In2O3 with some typical catalysts.Based on the results of N2 adsorption, the specific surface area of In2O3 and Ag/In2O3 was 82.81\u00a0m2 g\u22121 and 89.34\u00a0m2 g\u22121, respectively. The Ag loading has not a significant effect on the specific surface area. Fig.\u00a06\n shows the X-ray diffraction (XRD) patterns of In2O3 and Ag/In2O3. In the following discussions, the pristine Ag/In2O3 catalyst is named as \u2018Ag/In2O3\u2013P\u2019. The samples after hydrogen reduction are named as \u2018Ag/In2O3\u2019 and \u2018In2O3-R\u2019. The samples after the reaction at 300\u00a0\u00b0C and 5\u00a0MPa are assigned to \u2018Ag/In2O3-AR\u2019 and \u2018In2O3-AR\u2019. According to PDF#06-0416, the diffraction peaks centered at 21.5\u00b0, 30.7\u00b0, 35.5\u00b0, 45.7\u00b0, 51.0\u00b0 and 60.7\u00b0 are assigned to the diffractions from (211), (222), (400), (431), (440) and (622) facets of In2O3. This is in accordance with the results of Pd/In2O3 [16,17], Pt/In2O3 [22,23], Rh/In2O3 [21], Ni/In2O3 [20], Au/In2O3 [24] and Ir/In2O3 [26] catalysts. No diffraction peaks of metallic Ag or Ag2O can be observed due to the low loading weight of Ag species (0.33\u00a0wt% according to the analysis of ICP-OES) with the high dispersion over the In2O3 support. To further clarify the dispersion of Ag species, TEM analyses with the corresponding EDX elemental mapping were conducted. As shown in Fig.\u00a07\n, the EDX elemental maps show the extremely high dispersion of Ag species on the In2O3 support. The Ag/In2O3 catalyst after H2 reduction (H2/N2\u00a0=\u00a01/9, molar ratio, at 200\u00a0\u00b0C for 1\u00a0h) and after reaction (300\u00a0\u00b0C, 5\u00a0MPa) remains the high dispersion of Ag species as shown in Fig.\u00a07b and c, respectively. This result confirms that the interaction between the Ag species and the In2O3 support facilitates the dispersion of Ag species, which further provides much more active sites for the reaction.The previous studies have confirmed that CO2 adsorption occurs on the oxygen vacancies of In2O3. The CO2 molecule can be considered as a probe to characterize the oxygen vacancies of In2O3-based catalysts as well [22,40]. However, CO2 can be dissociated to CO on oxygen vacancies as reported recently by Wang et\u00a0al. [38]. Therefore, we use a mass spectrometer (MS) to analyze the signals of temperature programmed desorption of carbon dioxide (CO2-TPD). The signals of m/z\u00a0=\u00a044 were recorded as the intensities of CO2 desorption whereas the signals of m/z\u00a0=\u00a028 were recorded as the intensities of CO desorption. As shown in Fig.\u00a08\n, the CO2-TPD-MS profiles can be distinguished into four regions. In the region (I), the CO2 desorption peaks located at the temperature below 100\u00a0\u00b0C can be assigned to the desorption of physically absorbed CO2 and CO [17,20,22,38,41]. The CO signals can be attributed to the CO2 dissociation under the process of CO2 adsorption. The CO2 desorption peaks at ca. 240\u00a0\u00b0C in the region (II) can be assigned to the surface oxygen vacancies of In2O3 induced by hydrogen reduction. Also, the intensity of the CO2 desorption of In2O3 is stronger than that of Ag/In2O3. Furthermore, the CO2 desorption peak of In2O3 at ca. 430\u00a0\u00b0C in the region (III) is attributed to the thermally induced oxygen vacancies [41], whereas the peak of Ag/In2O3 can be attributed to the CO2 desorption on the interfacial oxygen vacancy site created by the Ag\u2013In2O3 interaction under hydrogen reduction. The CO2 desorption peak of Ag/In2O3 at ca. 410\u00a0\u00b0C exhibits a much stronger intensity. This indicates a much stronger CO2 adsorption on the interfacial site of Ag/In2O3 than In2O3, which is well consistent with the DFT calculations above. A broad CO2 desorption peak of the Ag/In2O3 sample centered at ca. 560\u00a0\u00b0C can be attributed to the thermally induced oxygen vacancies in the region (IV), which confirms the presence of the interaction between Ag species and the In2O3 support. The total amount of oxygen vacancies of Ag/In2O3 is higher than that of In2O3 according to the much stronger CO2 desorption on the interfacial oxygen vacancy site created by the addition of Ag species. More importantly, the CO desorption peak of the Ag/In2O3 sample at ca. 410\u00a0\u00b0C can be observed in the region (III). The onset temperature of CO desorption is ca. 350\u00a0\u00b0C. This result indicates that the new oxygen vacancy site created by the addition of Ag species facilitates the activation of CO2. This is also consistent with the result of DFT calculation discussed above.To gain the insight into the oxygen vacancies influenced by the addition of Ag species, the electron paramagnetic resonance (EPR) analysis was performed. As shown in Fig.\u00a09\n, all samples exhibit a signal near 3510\u00a0G with a g factor of 2.003. According to the literature, singly ionized oxygen vacancies (F-centers) are paramagnetic and are expected to yield a single EPR signal centered near the free electron g-value (2.0023) [42\u201344]. The trapped electron on or near an oxygen vacancy (F-center) can determine the visible light activity because the F-center provides a unique energy level [42,43], which can be characterized by the UV-vis absorption spectra below. Therefore, the stronger intensity of Ag/In2O3 affirms that the increasing oxygen vacancy sites are created by the addition of Ag species, which is consistent with the results of CO2-TPD-MS.Typically, the oxygen vacancies of In2O3 can be characterized by Raman spectra [45,46]. As shown in Fig.\u00a010\n, all the scattering features are attributed to the vibrations of the InO6 structural unit [46\u201348]. The peaks at around 306\u00a0cm\u22121 (I1) and 367\u00a0cm\u22121 (I2) are assigned to the bending vibration and the stretching vibration of In\u2013O\u2013In in InO6 octahedra, respectively. To quantify the oxygen vacancies, the integrated areas of the I1 peak and the I2 peak were calculated. The ratio of I2/I1 can be used to characterize the amount of oxygen vacancies on In2O3 [22,49,50]. For the pristine samples, Ag/In2O3 shows a higher ratio of I2/I1. This implies that the addition of Ag species can promote the formation of the oxygen vacancies of In2O3. Furthermore, the increasing I2/I1 ratios of both In2O3 and Ag/In2O3 after hydrogen reduction indicates that the oxygen vacancies can be created via hydrogen reduction. It can be seen that the difference of the I2/I1 ratio between In2O3 and Ag/In2O3 decreases after hydrogen reduction. This result indicates that Ag/In2O3 exhibits the higher stability under hydrogen reduction. Combined with the results of CO2-TPD-MS, the increasing oxygen vacancies of Ag/In2O3 can be attributed to the interfacial oxygen vacancy site created by the Ag\u2013In2O3 interaction under hydrogen reduction. Furthermore, the amount of oxygen vacancies on In2O3 exceeds that on Ag/In2O3 after reaction at 300\u00a0\u00b0C and 5\u00a0MPa. As reported by Tsoukalou et\u00a0al. [51], In2O3 itself is not very stable under the reaction conditions of CO2 hydrogenation to methanol. They claimed that the formation of metallic indium as a result of the partial reduction of In2O3 results in the deactivation, which typically occurs at elevated temperatures. Therefore, the addition of the Ag species stabilizes the structure of the catalyst under the reaction conditions.To further understand the oxygen vacancies of Ag/In2O3 induced by the addition of Ag species and hydrogen reduction, UV-vis absorption spectra was analyzed. As shown in Fig.\u00a011\na, a strong UV absorption is observed in all samples due to the semiconductor nature of In2O3 [22]. The band gap can be determined by the absorption using the Tauc relationship. As the results shown in Fig.\u00a011b, the band gap is 2.79\u00a0eV, 2.56\u00a0eV and 2.47\u00a0eV for In2O3, Ag/In2O3\u2013P and Ag/In2O3, respectively. The narrowed band gap for the Ag/In2O3\u2013P can be attributed to the oxygen vacancies created by the addition of Ag species [45,46], which is consistent with the results of EPR above. In addition, the band gap of Ag/In2O3 is further narrowed, corresponding to the formation of interfacial oxygen vacancy site created by the Ag\u2013In2O3 interaction under hydrogen reduction [45,46]. This confirms the results of CO2-TPD-MS discussed above.In conclusion, the DFT and experimental studies confirmed the methanol synthesis from CO2 hydrogenation over the Ag/In2O3 catalyst is feasible. The intense interaction between the Ag4 cluster and the defective In2O3(111) surface makes the Ag4 cluster positively charged. The interfacial site between the positively charged Ag4 cluster and surface oxygen vacancy of the Ag4/In2O3_D model facilitates the activation and the dissociation of CO2, resulting in methanol synthesis via the CO hydrogenation route. The methanol selectivity of Ag/In2O3 reaches 100.0% at reaction temperature of 200\u00a0\u00b0C. It remains more than 70.0% between 200 and 275\u00a0\u00b0C. The CO2 conversion reaches 13.6% with the methanol selectivity of 58.2% at 300\u00a0\u00b0C and 5\u00a0MPa. The methanol STY is 0.453 gmethanol gcat\n\u22121 h\u22121 under the same condition. This is the highest methanol STY ever reported on Ag catalysts for CO2 hydrogenation to methanol. The catalyst characterization indicates a high Ag dispersion on In2O3. The intense Ag\u2013In2O3 interaction promotes the formation of interfacial oxygen vacancy site with increasing amount of oxygen vacancies. The enhanced activity is thereby achieved.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 (2016YFB0600902).The following is the Supplementary data 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 5\nMultimedia component 5\n\n\n\n\n\nMultimedia component 6\nMultimedia component 6\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2021.05.004.", "descript": "\n                  Silver catalyst has been extensively investigated for photocatalytic and electrochemical CO2 reduction. However, its high activity for selective hydrogenation of CO2 to methanol has not been confirmed. Here, the feasibility of the indium oxide supported silver catalyst was investigated for CO2 hydrogenation to methanol by the density functional theoretical (DFT) study and then by the experimental investigation. The DFT study shows there exists an intense Ag\u2013In2O3 interaction, which causes silver to be positively charged. The positively charged Ag species changes the electronic structure of the metal, facilitates the formation of the Ag\u2013In2O3 interfacial site for activation and dissociation of carbon dioxide. The promoted CO2 dissociation leads to the enhanced methanol synthesis via the CO hydrogenation route as CO2\u2217\u2192CO\u2217\u2192HCO\u2217\u2192H2CO\u2217\u2192H3CO\u2217\u2192H3COH\u2217. The Ag/In2O3 catalyst was then prepared using the deposition-precipitation method. The experimental study confirms the theoretical prediction. The methanol selectivity of CO2 hydrogenation on Ag/In2O3 reaches 100.0% at reaction temperature of 200\u00a0\u00b0C. It remains more than 70.0% between 200 and 275\u00a0\u00b0C. At 300\u00a0\u00b0C and 5\u00a0MPa, the methanol selectivity still keeps 58.2% with a CO2 conversion of 13.6% and a space-time yield (STY) of methanol of 0.453 gmethanol gcat\n                     \u22121 h\u22121, which is the highest methanol STY ever reported for silver catalyst. The catalyst characterization confirms the intense Ag\u2013In2O3 interaction as well, which causes high Ag dispersion, increases and stabilizes the oxygen vacancies and creates the active Ag\u2013In2O3 interfacial site for the enhanced CO2 hydrogenation to methanol.\n               "}
{"full_text": "Biomass tar contains a variety of organic compounds (mainly aromatic and oxygen containing compounds) which have many disadvantages because of their toxicity, therefore, its removal is highly desirable [1,2]. Among the many methods for tar removing, catalytic steam reforming can convert tar into valuable products at low temperatures. In order to study the reaction mechanism, due to the complexity of real tar, most researchers are using a model compound of tar such as benzene, toluene or naphthalene. In the steam reforming of tar, dehydrogenation of hydrocarbon components and carbon formation take place on the same active sites. Subsequently, for the catalyst activity maintaining, the carbon deposited on the site is necessary to react with steam to generate CO [3].For this reason, the development of catalysts with high activity, long-time stability and reusability, with a low-cost still remains a challenge. Many catalysts were studied and each of them presents advantages and disadvantages.Ni-based catalysts are studied extensively for tar conversion because of their good activity and stability. However, the rapid deactivation by coking is a major limitation. A series of oxides such as MgO, \u03b1-Al2O3, \u03b3- Al2O3, SiO2, ZrO2 were studied as support for Ni in the steam reforming of toluene and the activity of catalyst greatly depended on Ni particles size and on the type of interaction between Ni and support [4]. The best catalytic performance was observed on Ni/MgO due to the strong interaction between NiO and MgO. For Ni-Fe alloy supported over iron-alumina catalysts it was observed that the presence of iron plays a role of cocatalyst by increasing the oxygen species [5].Ni/olivine catalysts were studied for steam reforming using toluene as a model compound for tar and exhibited high activity, selectivity to H2 and CO and furthermore, a good resistance to deactivation due the strong metal-support interaction [6]. For example, for improving the stability of Ni/olivine catalyst a second metal such as CeO2 was added as additive, which due to its redox properties could adsorb and dissociate water and the resulting groups react with carbon deposed on Ni sites generating CO and CO2\n[7]. Other promoters studied for Ni/olivine catalysts were Ca, K, Mn and it was observed that Mn exhibited the highest catalytic activity (63% conversion) but unfortunately at a high reaction temperature of 800\u202f\u00b0C [8].The catalyst based on NiMn/Al2O3 doped with Ru to improve the redox properties was active at low temperature, 600\u202f\u00b0C, achieving 100% conversion, but the use of noble metal makes catalyst more expensive [9]. Among others, perovskite-type oxides (Ni/LaSrAl) were investigated as support for Ni, and it was found that larger specific surface area of Ni/LaSrAl produces higher lattice oxygen release rate, high catalytic activity, and low amount of coke deposition on the surface [10]. Another studied metal was Co on the same perovskite-type oxide support emphasizing that lattice oxygen suppresses coke formation activating toluene for a redox mechanism, this catalyst showed higher redox properties than Ni and consequently higher conversion of toluene (61% compared with 50% for Ni) [11].Hydrotalcite type materials represent a novel class of supports which, due to their structure could be excellent precursors of very well dispersed metallic oxide catalysts. La-promoted Ni-hydrotalcite-derived catalysts were studied in the dry reforming of methane [12], doped with Cu were studied for steam reforming of 1-methylnaphthalene [13], and doped with noble metals for Cedar wood steam reforming [14].The identification of highly active heterogeneous catalysts for steam reforming is an open challenge. Mixed oxides MgAlMo derived from hydrotalcites are very complex materials with an important role in heterogeneous catalysis and the recent achievements recommended them as suitable for many catalytic reactions. Because there is no literature reports in terms of use Mo-HT catalysts for toluene steam reforming, the aim of this work was to investigate the influence of molybdenum content on the structure and to study the catalytic performance of these samples in the steam reforming of toluene.The MgAl LDH with molar ratio Mg/Al\u202f=\u202f3/1 was prepared by coprecipitation at constant pH\u202f=\u202f10 (controlled with a TitraLab TIM 854 apparatus), at a temperature of 60\u202f\u00b0C, using Mg(NO3)2\u00b76H2O (from Merck), Al(NO3)3\u00b79H2O (from Tunic), Na2CO3 and NaOH (from Lach-Ner). Two solutions were prepared: a solution of NaOH (1\u202fM) and a second solution containing Mg and Al nitrates (0.4\u202fmol of Mg and 0.13\u202fmol of Al) dissolved in distilled water. The two solutions were simultaneously added dropwise, at a steady rate with vigorous stirring, to an aqueous solution of Na2CO3\u00b710H2O 0.1\u202fM (Mg/Na\u202f=\u202f3.33). The resulting gel was aged for 24\u202fh, then separated by filtration, washed up with abundant distilled water until reaching a neutral pH, dried at 100\u202f\u00b0C for 10\u202fh and calcined at 200\u202f\u00b0C for 2\u202fh, 400\u202f\u00b0C for 2\u202fh and finally at 550\u202f\u00b0C for 2\u202fh.Molybdenum was introduced in the LDH structure by impregnation, so that the molar ratio Mg:Al:Mo ranged between 3:1:0.04, 3:1:0.08 and 3:1:0.12. The corresponding amount of (NH4)6Mo7O24\u00b74H2O had been calculated so that Mo was 1.5, 3 and 4.5% (wt) respectively, and the catalysts\u2019 abbreviation was Mo1.5MgAl, Mo3MgAl and Mo4.5MgAl. The slurry was maintained under stirring at constant temperature (80\u202f\u00b0C) until water was evaporated. Then, the procedures for drying and calcination were followed as described above.Powder X-ray diffraction (PXRD) patterns for prepared catalysts were recorded on a Siemens D5000 diffractometer using Cu K\u03b1 radiation (\u03bb\u202f=\u202f1.54\u202f\u00c5), operating at 50\u202fkV and 40\u202fmA. They were recorded over the 5\u201370\u00b0, with a step size of 0.0403\u00b0 and a scan time of 0.75sec/step. The average crystallite size was evaluated according to the Scherrer equation with formula: D(hkl)\u202f=\u202f0.9\u03bb/\u03b2\u00b7cos\u03b8, in which (\u03bb) is the wavelength of Cu K\u03b1, (\u03b2) is the full width at half maximum intensity peak, and (\u03b8) is Bragg\u2019s diffraction angle. For the hydrotalcite type samples cell parameter c of the rhombohedral structure was determined from the positions of the (0\u202f0\u202f3) and (0\u202f0\u202f6) diffraction lines. In this case \u201cc\u201d was calculated from two diffraction lines using equation c\u202f=\u202f3/2 (d003\u202f+\u202f2d006) [15]. The \u201ca\u201d parameter was calculated from the direction (1\u202f1\u202f0) with equation a\u202f=\u202f2\u00b7d110 and represents the cation\u2013cation distance in the brucite-like sheets [16].The N2 adsorption\u2013desorption isotherms at 77\u202fK were measured by the static method in an automatic volumetric Micromeritics ASAP 2020 Surface Area and Porosity Analyzer at 77\u202fK. Prior to the measurements, samples were degassed under vacuum at 200\u202f\u00b0C for 8\u202fh. To calculate the surface area, the Brunauer\u2013Emmett\u2013Teller (BET) model was applied. Desorption branch was analyzed by applying the Barrett\u2013Joyner\u2013Halenda (BJH) model using the Halsey thickness curve. The total quantity of gas adsorbed at the data point closest to P/Po\u202f=\u202f0.98 by desorption branch was used to approximate the total pore volume. The pore size distribution was calculated from the desorption branches of the isotherms.Scanning electron microscopy (SEM) studies, including imaging and electron dispersive X-ray spectra (EDX), were performed using an AMRAY 1910 field emission SEM. The analyses were performed using an accelerating voltage of 15\u202fkeV, on dry-ground samples.The surface density of molybdenum was expressed as:\n\n\n\nS\nu\nr\nf\na\nc\ne\n\nd\ne\nn\ns\ni\nt\ny\n\n\n(\n\n\nM\no\n\n\na\nt\no\nm\ns\n/\nn\nm\n2\n\n\n)\n\n=\n\n\nwt\n%\n\nM\no\nl\ny\nb\nd\ne\nn\nu\nm\n\nl\no\na\nd\ni\nn\ng\n\u2219\n6.023\n\u2219\n\n\n10\n\n23\n\n\n\nMolecular\n\nW\ne\ni\ng\nh\nt\n\no\nf\n\nM\no\nl\ny\nb\nd\ne\nn\nu\nm\n\u2219\n100\n\u2219\nS\nu\nr\nf\na\nc\ne\n\nA\nr\ne\na\n\n\n\n\n\n\nFor transmission electron microscopy (TEM) studies, dry samples were triturated and scattered on to TEM grids. High resolution TEM images were collected on a CM200-FEG at an accelerating voltage of 200\u202fkV.The Fourier transform infrared (FT-IR), Bruker IFS 66\u202fV/S spectrometer equipped with a diamond attenuated total reflectance (ATR) accessory was used to record the spectra from 400 to 2000\u202fcm\u22121 with a resolution of 4\u202fcm\u22121.The UV\u2013VIS spectra were recorded in the range 220\u2013600\u202fnm (using a UV3600 UV\u2013vis spectrophotometer) with a wavelength step of 2\u202fnm, having a slit width of 8\u202fnm.Steam reforming of toluene was performed at atmospheric pressure using a conventional fixed bed flow reactor (i.d 9\u202fmm) placed in a furnace in which the heating is monitored by a thermocouple. The 0.1\u202fg of catalyst was charged in a reaction tube made of quartz. Water and toluene (S/C molar ratio 0.5\u20132) were introduced by syringe pumps into a vaporization furnace (300\u202f\u00b0C) and then were carried to the reactor by a flow of nitrogen (carrier gas). The reaction temperature was varied from 400 to 500\u202f\u00b0C, and the nitrogen flow rate was 0.3 L/h.Liquid products were collected by an ice water bath located downstream of the reactor, and the gas and liquid compositions were analyzed using gas chromatography with a thermal conductivity detector (TCD) and a flame ionization detector (FID), respectively.The gaseous product was composed of H2, CO, and CO2, and the main reactions are as follows:Steam reforming\n\n(1)\n\n\n\nC\n7\n\n\nH\n8\n\n+\n14\n\nH\n2\n\nO\n\u2192\n7\n\nCO\n2\n\n+\n18\n\nH\n2\n\n\n\u0394\n\n\nH\n\n\u2218\n\n=\n647\n\nkJ\n/\nmol\n\n\n\n\n\n\n(2)\n\n\n\nC\n7\n\n\nH\n8\n\n+\n7\n\nH\n2\n\nO\n\u2192\n7\nCO\n+\n11\n\nH\n2\n\n\n\u0394\n\n\nH\n\n\u2218\n\n=\n870\n\nkJ\n/\nmol\n\n\n\n\nWater\u2013gas shift\n\n(3)\n\n\nCO\n+\n\nH\n2\n\nO\n\u2194\n\nCO\n2\n\n+\n\nH\n2\n\n\n\u0394\n\n\nH\n\n\u2218\n\n=\n-\n41\n\nkJ\n/\nmol\n\n\n\n\nDry reforming\n\n(4)\n\n\n\nC\n7\n\n\nH\n8\n\n+\n7\n\nCO\n2\n\n\u2192\n14\nCO\n+\n4\n\nH\n2\n\n\n\n\n\nThe XRD diffractogram of MgAl catalyst, Fig. 1\n, reveals a typical pattern of a pure layered double hydroxide structure with lines at 2\u03b8\u202f\u2248\u202f11.2\u00b0, 22.4\u00b0, 34.4\u00b0, 38.8\u00b0; 46.1\u00b0 and 60.5\u00b0 corresponding to (0\u202f0\u202f3), (0\u202f0\u202f6), (0\u202f1\u202f2), (0\u202f1\u202f5), (0\u202f1\u202f8) and (1\u202f1\u202f0). Periclase (MgO) structure corresponding to (2\u202f0\u202f0) and (2\u202f2\u202f0) planes is located at 2\u03b8\u202f\u2248\u202f42.5\u00b0 and 62.5\u00b0. Three major changes in the structure were observed at molybdenum introduction. The first modification in structure consists in decrease, until disappearance of line (0\u202f0\u202f3) corresponding to layered double hydroxide structure. The second change consists in the apparition of a new phase (MgAl2O4 spinel) [17] associated with the line from 65\u00b0. And finally, the interaction between molybdate anions and magnesium leads to the appearance of small tetrahedral crystallites of MgMoO4. Widening of line (0\u202f0\u202f3) with the entry of molybdate indicates a more disorganized structure or a decrease in crystallinity [18].The average crystal size for the direction perpendicular to the plane (0\u202f0\u202f3) and the lattice parameters are shown in Table 1\n. The cell parameter a represents cation\u2013cation distance inside the brucite layer, while parameter c represents interlayer distance and thickness of the brucite layer [19].The full width at half maximum (FWHM) of the basal reflection plane (0\u202f0\u202f3) is used for the assessment of crystallinity in the layering direction [20]. It can be noticed that Mo4.5MgAl has the highest FWHM value (1.4) and consequently, lower crystallinity than others.The basal spacing d003 for sample without molybdenum is 7.8\u202f\u00c5 and corresponds to hydrotalcites with carbonate interlayer. This basal spacing was not affected very much when low amounts of Mo were introduced, which suggests that the molybdate anions could be adsorbed on the surface with formation of layered double hydroxide. However, when higher amounts of Mo were used, an increase in d spacing was observed. For example, the Mo4.5MgAl sample has a basal spacing of 8\u202f\u00c5 suggesting that a part of molybdate ions have been incorporated into the structure of MgAl layered double hydroxides. This is in concordance with the higher cell parameter \u201cc\u201d for this catalyst. The intercalation of MoO4\n2\u2212 anions into the interlamellar domain induced an increase in the interlayer distances since the molybdate anion have anionic radius higher than CO3\n2\u2212. The parameter \u201ca\u201d was not influenced very much.The average size of crystallites decreased from 8.3 on MgAl to 5.7 on Mo4.5MgAl.The specific surface areas and specific total pore volumes are summarized in Table 2\n. The MgAl sample exhibited the highest surface area and pore volume. Both surface area and pore volume decreased drastically with incorporation of molybdenum, resulting in dense phase and blocking of pores. The highest surface of Mo4.5MgAl compared with others is attributed to intercalation of a part of molybdate anions into the interlamellar domain and release of the pores. From the pore size distribution it can be seen that, MgAl has a broader distribution with a maximum at 22.8\u202fnm and a shoulder at 15.8\u202fnm. Mo1.5MgAl has a bimodal pore size distribution with two maxima at 6.8\u202fnm and 22\u202fnm, Mo3MgAl has a maximum at 19.2\u202fnm and a small shoulder at 10.5\u202fnm. By contrast, Mo4.5MgAl sample exhibits narrow unimodal pore size distribution centered at 15.5\u202fnm. The addition of molybdenum decreases the pore size, shifting the pore size distribution to lower values, the pore dimension being higher for HT without molybdenum as expected.The results obtained from EDX analysis are presented in Table 2 and it is noticeable that, with the exception of Mo1.5MgAl for all others samples the Mg to Al ratio is slightly less than the calculated value of 3:1, and for the catalysts with molybdenum this ratio decreases with increasing Mo loading.The SEM micrograph of Mo hydrotalcites presented in Fig. 2\n reveals a compact angular shape with smooth faces and irregular sizes and some large interparticle cavities. Also, a very homogeneous distribution of crystal aggregates was observed.The TEM micrograph shown in Fig. 3\n confirms the characteristics of LDH platelet structure with the platelets placed one above the other and some regions with fibrous morphology. The micrograph of Mo-hydrotalcites shows the tendency of platelets to cluster together in large conglomerates.The FT-IR spectra of Mo-hydrotalcites are shown in Fig. 4\n. All samples present a broad absorption band in the range 3400\u20133500\u202fcm\u22121 and another band located at 1650\u20131660\u202fcm\u22121 that correspond to the O\u2013H stretching vibration and bending vibration of interlayer water molecules [21], respectively. The band at 1390\u20131400\u202fcm\u22121 is assigned to the stretching vibration of CO3\n2\u2212 and the intensity of these peaks decrease with increasing molybdenum content, indicating the intercalation of less carbonate ions inside the layer and introduction of molybdate ions interlayer. The band located at 1500\u202fcm\u22121 corresponds to carbonate species adsorbed on the surface. The characteristic band of antisymmetric vibration of Mo-O-Mo in MoO4\n2\u2212 is located in the range 790\u2013810\u202fcm\u22121\n[22] and is more pronounced for Mo3MgAl and Mo4.5MgAl samples. The band located at 590\u2013600\u202fcm\u22121 was attributed to the presence of aluminum cations in tetrahedral sites [23], and is more pronounced for catalyst without molybdenum. The intensity of this band decreases with addition of molybdenum (for Mo4.5MgAl it almost disappears) which proves that molybdenum not only gets into the layer but also replaces a part of aluminum in the structure.The presence of tetrahedral coordination of molybdenum (MoO4 species) was also proven by the UV\u2013VIS spectra displayed in (Fig. 5\n).The catalytic performance of MoMgAl catalysts in steam reforming of toluene as a biomass tar model was carried out at 400\u2013500\u202f\u00b0C with a ratio of S/C 0.5\u20132. The major components of produced gas are H2, CO and CO2.The conversion of toluene, expressed in terms of carbon conversion, was calculated with the formula:\n\n\n\n\nX\n\ntoluene\n\n\n\n\n\n\n%\n\n\n\n=\n\n\n\nn\n\nCO\n\n\n+\n\nn\n\nC\nO\n2\n\n\n\n\n\n7\n\u2217\nn\n\n\nC\n7\nH\n8\n\n\n\n\u2219\n100\n\n\n\n\nThe conversion of toluene and product distribution as function of temperature and molybdenum loading are shown in Fig. 6\n. Toluene conversion and H2 amount increase with increasing temperature. Also the toluene conversion increases with molybdenum content, while the H2 is directly proportional to molybdenum surface density (Fig. 6d) and to the presence of both MoO4\n2\u2212 anions and aluminum cations in the tetrahedral sites on the surface (Mo3MgAl). With temperature increasing from 400\u202f\u00b0C to 500\u202f\u00b0C, CO increases while CO2 decreases and this could be explained by the fact that, lower temperatures favor the water\u2013gas shift reaction [24]. The effects of metal loading on toluene conversion are similar with those reported by Yue [25] over hydrotalcites with Ni, noticing the increase in conversion with metal content. Our previous works [26,27] on glycerol steam reforming over molybdenum and molybdenum-cerium catalysts also showed an increase in glycerol conversion with metal loading.In this study the following conversions of toluene were obtained: 17% on MgAl, 37.1% on Mo1.5MgAl, 53.9% on Mo3MgAl and 64.3% on Mo4.5MgAl catalyst. The hydrogen amount was 41% on MgAl, 52% on Mo1.5MgAl, 66.2% on Mo3MgAl and 60.7% on Mo4.5MgAl at 500\u202f\u00b0C, respectively. These results are in concordance with those obtained by Josuinkas [28] over nickel catalysts derived from hydrotalcite-like compounds (28\u201360% toluene conversion), but the hydrogen amount was lower (10\u201315%). \u0141amacz [29] in steam reforming of toluene over ceria zirconia based Ni catalysts obtained a conversion between 38 and 65%, with a hydrogen yield around 70% at 500\u202f\u00b0C, S/C ratio 2.4.In Fig. 6c H2/CO ratio decreases with increasing temperature while H2/CO2 ratio increases with the temperature. The water\u2013gas shift exothermic reaction could be favored at low temperatures and consequently low selectivity of CO and high molar ratio H2/CO (3.2) are obtained. At 500\u202f\u00b0C the H2/CO2 ratio is higher on Mo3MgAl (6.8) and Mo4.5MgAl (5.3), so, over these catalysts both reaction (2) through which CO is obtained and dry reforming (reaction (4)) are favored. The CO and CO2 concentrations are different from equilibrium values, the only catalyst that has CO2 concentration very close to 2.6 is Mo3MgAl at 450\u202f\u00b0C (2.5).The Arrhenius plot of MoMgAl samples are shown in Fig. 7\n, and for these catalysts the activation energy, Ea, is 53.2 (MgAl), 40.6 (Mo1.5MgAl), 39.1 (Mo3MgAl) and 36.8 (Mo4.5MgAl) kJ/mol.The steam to carbon ratio plays an important role in toluene reforming. In this study ratio varied from 0.5 to 2 at 500\u202f\u00b0C and its effect on the conversion of toluene and gas product components is shown in Fig. 8\n. It is noticeable that, the conversion of toluene is directly proportional to S/C ratios for all catalysts. Also, the hydrogen amount increases with the increasing steam to carbon ratio, while CO decreases. In the same time CO2 amount reached a maximum at S/C ratio 2, proving that a steam excess favors reforming and water\u2013gas shift reaction [30]. Fig. 8d shows ratio between hydrogen and carbon monoxide, respectively between hydrogen and carbon dioxide. The H2/CO ratio increases with increasing steam and the values of S/C\u202f=\u202f0.5 is very close to stoichiometric ones (1.6). The values obtained are 1.7 on Mo1.5MgAl, 1.8 on Mo3MgAl and 1.3 on Mo4.5MgAl. The H2/CO2 ratio is inversely proportional to S/C ratio and is very large for Mo3MgAl and Mo4.5MgAl compared with Mo1.5MgAl (2.4\u20132.5) over that H2/CO2 ratio is close to the stoichiometric conditions (2.6).The stability of the catalysts in time was carried out over all samples for 15\u202fh of reaction at 500\u202f\u00b0C and S/C ratio of 2. The results are shown in Fig. 9\n. The results reveal a good stability for all catalysts. Furthermore, a good thermal stability was proven by the XRD pattern after the reaction (Fig. 1). The catalyst stability could be associated with a good interaction between Mo and Mg that favors the formation of Mo particles with small sizes.Layered double hydroxides (LDH) with intercalated molybdate anions have been prepared by co-precipitation and impregnation routes. The XRD results showed that, the molybdate anions react with magnesium leading to small tetrahedral crystallites of MgMoO4, tetrahedral coordination of molybdenum (MoO4 species) was also evidenced from UV\u2013VIS spectra. The FT-IR spectrum reveals that molybdenum not only gets into the layer but also replaces a part of aluminum in the structure. The toluene steam reforming reaction was carried out on these catalysts and it was observed that the conversion of toluene and the H2 amount are directly proportional to the temperature. Furthermore, the toluene conversion increases with molybdenum content, while the hydrogen depends on two factors: the presence of molybdate species on the surface and the presence of Al in tetrahedral coordination (Mo3MgAl sample).", "descript": "\n                  This study evaluated the catalytic activity of Mo catalysts derived from hydrotalcite-like compounds for steam reforming of toluene as a model compound for tar. The catalysts with 1.5, 3 and 4.5 Mo loadings (wt%), denoted as Mo1.5MgAl, Mo3MgAl and Mo4.5MgAl respectively, were prepared by coprecipitation and characterized by BET, XRD, SEM, TEM, FT-IR and UV\u2013VIS. The results showed that toluene conversion increased with increasing molybdenum content. The hydrogen amount depended on two factors: the presence of molybdate species on the surface and the presence of aluminum cations in tetrahedral sites (Mo3MgAl), with molybdenum influence being more pronounced. The H2/CO ratio decreased at increasing temperature while, the H2/CO2 ratio increased proportionally with temperature. Mo1.5MgAl catalyst was more selective for CO2 and H2, while, Mo3MgAl and Mo4.5MgAl were more selective for CO and H2.\n               "}
{"full_text": "Recently, society has been concerned by the environmental pollution of soils, atmosphere and water. Specifically, about the water pollution, continuous technological advance and consumption growth in today\u2019s society increase worryingly contamination of the aqueous environment [1], and currently, the effect of the contaminants of emerging concern (CECs) has taken a great interest [2\u20135]. These pollutants are chemicals with high complexity (chemical and physical) that resist photolytic, biological and chemical degradation. Some examples of these contaminants are the pharmaceutical and personal care products, pesticides, etc. They present a high toxicity in the aquatic medium, even at low concentration, and a high diffusion by air and water, which leads to a more harmful effect on the environment [3,5,6]. Thus, the presence of these compounds in water can affect flora and fauna and, consequently, human health. Specifically, they can cause different types of cancer (breast, ovary, prostate, testes, etc.), disorders in endocrine and neurological systems, reproductive capabilities and hormonal control [7\u201310].This problem increases because of conventional treatments of water and wastewater treatment plants failed to completely remove these pollutants. Specifically, ofloxacin (OFX), a widely used antibiotic today, cannot be completely removed by these treatments and it can be found in rivers and lakes [11\u201313]. Its presence in the aquatic medium could pose low to medium risks to aquatic organisms, and the occurrence and distribution of antibiotic resistance genes can take place [14\u201319].Thus, it is necessary to look for alternative treatments that improve the CECs removal. In this context, it is possible to find non destructive and destructive methods. In the first group, adsorption and solvent extraction stand out in research and industry areas, however, the efficiency of these technologies to remove CECs is low [5,19]. Within the second group, incineration, oxidation process, wet oxidation and supercritical oxidation present a good efficiency in the degradation of different organic compounds from water and wastewater, but not for the removal of CECs [20,21]. However, advanced oxidation processes, based on the generation of hydroxyl radicals and other oxidizing species are considered by many authors as a good option for the treatment of water and wastewater for the degradation of CECs, with a high efficiency and environmental compatibility [1,22\u201326]. Among them, it is worth highlighting photocatalytic processes and, specifically, the heterogeneous photocatalysis with supported TiO2 nanotubes, which allows, on the one hand, to remove a final stage to take the photocatalyst from the medium after the treatment, significantly lowering the cost of operation. On the other hand, knowing that photocatalysis is a process that takes place mainly on the catalyst surface [27,28], the use of TiO2 nanotubes in the anatase phase (photocatalytically active phase) entails a high activity, because of the higher active surface on the nanotubes able to generate hydroxyl radicals and other oxidants of organic compounds.The next reactions can take place during the water treatment by photocatalysis, where ultraviolet photons reach the catalyst surface and form excited radicals, which can improve the treatment efficiency [24,26].\n\n(1)\nH2O \n\u2192\n \u2022OH + H+ + e-\n\n\n\n\n\n(2)\nOH- \n\u2192\n \u2022OH + e-\n\n\n\n\n(3)\n2 \u2022OH \n\u2192\n H2O2\n\n\n\n\n(4)\nOH + O2 \n\u2192\n O3 + H+\n\n\n\n\n(5)\nMOx(\u2022OH) \n\u2192\n MOx(\u2022OH)*\n\n\nIn addition, it is possible to find an uniform distribution and optimal size for the nanotubes with a good transmission in the UV region, stability and resistance to degradation and durability [9,28,29]. In this context, anodization process, carried out for the formation of the TiO2 nanotubes, can be studied in order to look for the optimal conditions for the most active photocatalyst.With this background, the goal of this work is to study the anodization process for the formation of TiO2 nanotubes in the photocatalyst, paying special attention to the electrolyte used (H2SO4 in water or NH4F/H2O in ethylene glycol, chosen according to literature [9,27\u201330]), the maximum potential applied (20\u201360\u2009V), the potential ramp (2\u20134\u2009V\u2009min\u22121) and the subsequent heat treatment (maximum temperature of 450 \u00baC), in order to transform the amorphous phase into crystalline-phase TiO2 nanotubes, to increase their catalytic capacity after crystallizing in their anatase form [31,32]. The optimization of the process was conducted through the application of factorial design of experiments and surface response analysis [33]. The efficiency of the process will be evaluated by the treatment of synthetic wastewater polluted with OFX, as an antibiotic model of CEC. Moreover, wastewater treatment experimental conditions will be also statistically analyzed (UV wavelength, irradiance and initial concentration of OFX), in order to find the optimal operating conditions.Titanium plates (5\u2009\u00d75\u2009cm; 1\u2009mm thickness; 99.2% purity) were supplied by Alfa Aesar (Ward Hill, Massachusetts, United States of America). Ofloxacin (OFX), nickel plate (used as cathode for the anodization process), ammonium fluoride, ethylene glycol and acetone (for plate cleaning) were supplied by Sigma-Aldrich (Steinheim, Germany) with >\u200999.0% purity. Sulfuric acid (97%) and acetonitrile, with analytical grade, were supplied by Panreac Qu\u00edmica S.A. (Barcelona, Spain).The anodization process and the subsequent heat treatment were conducted according to Mart\u00edn de Vidales et al. [34], being the electrolyte used not only 250\u2009mL of NH4F 0.1\u2009M in a solvent formed by 20% v/v of water in ethylene glycol, but also H2SO4 0.5% w/w in water, depending on the studied conditions. Therefore, a Ti plate is immersed in an electrolyte in front of a Ni plate, and a potential ramp is applied up to a maximum potential that is maintained for 90\u2009min. Then, a heat treatment (Energon oven) can be applied up to a maximum temperature of 450 \u00baC (a ramp of 2 \u00baC min\u22121 up to 350 \u00baC, which is maintained for 30\u2009min, another ramp of 2 \u00baC min\u22121 up to 450 \u00baC, maintained for 150\u2009min, and final drop to room temperature) [27]. Before anodization, the titanium plate requires a previous cleaning treatment with clean water and drying it with paper; and after that, acetone and water ultrasonic baths for 15\u2009min.The formation of TiO2 nanotubes in Ti plates was analyzed by scanning electron microscopy (SEM) with a JEOL JSM-820 analyzer, 1000 \u2013 30,000\u2009V. Image resolution at 25 KV: 3.5\u2009nm (at 8\u2009mm working distance), 10.0\u2009nm (at 39\u2009mm working distance).The maximum absorption wavelength of OFX solution is 332\u2009nm, obtained from the scanning plot measured in a UV\u2013VIS spectrophotometer (UVIKON 941 plus). OFX concentration was measured using a HPLC Jasco MD-2010/2015 with a 5\u2009\u00b5m C18 analytical column (4.6\u2009mm\u2009\u00d7\u2009250\u2009mm), using a mobile phase of acetonitrile/water (50/50\u2009v/v %) at the flow rate of 1\u2009mL\u2009min\u22121. The column temperature was 25\u2009\u2103 and samples of 5\u2009\u00b5L were injected. Before analysis, the samples were filtered by 0.45\u2009\u00b5m cellulose filter.Bench-scale photocatalysis treatments were conducted under batch-operation mode (\nFig. 1). Wastewater was stored in a jacketed glass tank (volume = 500\u2009mL) with agitation acting as a reactor, where the photocatalyst (Ti plate with TiO2) was put into it, near the liquid surface, on a metal mesh. An ultraviolet lamp was placed over the Ti plate and the wastewater (the Ti plate is submerged in the wastewater 1\u2009cm from the surface, leaving only the anodized side irradiated), in order to activate the electron leap from the valence to the conduction band, and improve the generation and/or activation of oxidizing agents, which are mainly found on the catalytic surface [9]. UV wavelength was 365\u2009nm (UV-A), 311\u2009nm (UV-B) or 254\u2009nm (UV-C), and the irradiance on the photocatalyst surface was measured with a HD 2102.1 radiometer, with measurement probes LP 471 UVA, LP 471 UVB and LP 471 UVC, supplied by PCE Instruments S.L. (Albacete, Spain). The ultraviolet lamps were supplied by BCB S.L. (Barcelona, Spain).The initial concentration of OFX was of 15\u201335\u2009mg\u2009dm\u22123 (synthetic wastewater), a typical concentration found in Wastewater Treatment Plants [2]. The jacket was coupled with a controlled thermostatic bath (Digiterm 100, JP Selecta, Barcelona, Spain) pumping water to maintain the temperature at the desired set point (25 \u00baC).The statistical analysis was performed using Statgraphics v.17.2.00 software (Statgraphics Technologies Inc.).\nIn order to optimize the anodization process, the influence of the electrolyte used (H2SO4 in water or NH4F/H2O in ethylene glycol), the maximum potential applied (20, 40 or 60\u2009V), the potential ramp (2, 3 or 4\u2009V\u2009min\u22121) and the subsequent heat treatment (to apply it or not) were evaluated using the factorial design and response surface methodology.In a first stage of the experimental design, the effect of the electrolyte used and the possibility of applying a heat treatment after the anodization were evaluated using a two-level factorial design. As defined in the methodology of the factorial design of experiments [33], the main effect of a factor is defined as the mean change of the response variable obtained varying a factor among the higher and the lower level. Accordingly, the effect of the electrolyte factor (EEL) on the kinetic constant (k) as response variable is defined in Eq. 6:\n\n(6)\n\n\n\n\nE\n\n\nEL\n\n\n=\n\n\n\n\u2211\n\n\n\n\nk\n\n\nEL\n+\n\n\n\u2212\n\n\u2211\n\n\n\n\nk\n\n\nEL\n\u2212\n\n\n\n\n\n\n\n\nN\n\n/\n\n2\n\n\n\n\n=\n\n\u2211\n\n\n\n\n\n\nk\n\n\nEL\n+\n\n\n\n\u0305\n\n\u2212\n\n\u2211\n\n\n\n\n\n\nK\n\n\nEL\n\u2212\n\n\n\n\u0305\n\n\n\n\n\n\n\nThe interaction effects are calculated with a similar equation taking into account the value of the response variable in the experiments with both factors at higher and lower level. Thus, the interaction between the electrolyte factor and heat treatment factor EEL-HT is defined in Eq. 7:\n\n(7)\n\n\n\n\nE\n\n\nEL\n\u2212\nHT\n\n\n=\n\n\n\n\u2211\n\n\n\n\nk\n\n\n(\nEL\nand\nHT\n)\n+\n\n\n\u2212\n\n\u2211\n\n\n\n\nk\n\n\n(\nEL\nand\nHT\n)\n\u2212\n\n\n\n\n\n\n\n\nN\n\n/\n\n2\n\n\n\n\n=\n\n\u2211\n\n\n\n\n\n\nk\n\n\n(\nEL\nand\nHT\n)\n+\n\n\n\n\u0305\n\n\u2212\n\n\u2211\n\n\n\n\n\n\nK\n\n\n(\nEL\nand\nHT\n)\n\u2212\n\n\n\n\u0305\n\n\n\n\n\n\n\nAccording to the methodology of the experimental design [33], if the value of one effect is out of the confidence interval then this factor has a significant effect of the response variable. The same can be said about the value of the interaction effect.The confidence interval, at 95% of significance level, was calculated using the Eq. 8:\n\n(8)\n\n\nCI\n\n\n\n95\n%\n\n\n\n=\n\u00b1\n\n\nt\n\n\n0\n,\n05\n,\n8\n\n\n\u00b7\n\n\n\u03c3\n\n\nt\n\n\n\u00b7\n\n\n\n1\n\n/\n\nn\n\n\n\n\n\n\nWhere \u201ct\u201d es the value of the Student\u00b4s test statistics calculated at 95% of significance level and 8 degrees of freedom (2 degrees of freedom for each 3-replicated experiment and 4 experiments); n is the total number of samples (12 samples accounting for 3 replicates in each of the 4 experiments), and \u03c3t is the standard deviation of the 12 values of kinetic constant, calculated by the Eq. 9 with the standard deviation of the kinetic constant of each experiment \u03c3:\n\n(9)\n\n\n\n\n\u03c3\n\n\nt\n\n\n=\n\n\n\n\u2211\n\n\n\n\n\n\n\u03c3\n\n\ni\n\n\n\n\n2\n\n\n\n\n\n\n\n\n\n\n\nTable 1 shows the experimental matrix for a two-level factorial design carried out in order to study these two factors. The experiments were run at random to minimize errors due to possible systematic trends in the variables. Columns 2 and 3 present the two factors on a natural scale while maintaining constant the potential ramp and maximum potential in the central values of 3\u2009V\u2009min\u22121 and 40\u2009V, respectively. Columns 4 and 5 represent the dimensionless coded levels of the factors (+1 and \u22121 for the higher and lower level, respectively). As can be observed, plate anodization was conducted in triplicate, in order to evaluate the experimental error. The kinetic constant for the degradation of ofloxacin in water ([OFX]0 = 25\u2009mg\u2009dm\u22123. UV-A: \u03bb\u2009=\u2009365\u2009nm) was chosen as response variable to assess the efficiency of the different operating conditions of the anodizing process. Columns 6, 7 and 8 present the kinetic constant (calculated as described below) for the three replicates of each experiment, whereas column 9 presents the mean value for the kinetic constant of each experiment.The results in the photocatalytic tests are shown in \nFig. 2. Experiments were carried out in triplicate with each plate, under the same operating conditions, in order to minimize the experimental error. The dots show the average values, and the error bars the standard deviation of the three replicates under the conditions 1, 2, 3 and 4. The y-axis show the dimensionless normalized concentrations of OFX for a better comparison.It is important to highlight that photolysis process was also carried out, and a nil degradation of OFX was observed. Thus, as shown in Fig. 2, photocatalytic process improves the pollutant degradation in all cases. When a heat treatment after the anodization process is not applied, the process efficiency is lower. This can be explained taking into account that the heat treatment improves the disposition and arrangement of the nanotubes [9,27\u201329,31], transforming the amorphous phase into crystalline-phase TiO2 nanotubes, and increasing the catalytic capacity after crystallizing in their anatase form [31,32]. On the other hand, to use an electrolyte of H2SO4 in water only entails the formation of a film of TiO2 on the Ti plate [30], while NH4F/H2O in ethylene glycol allows the formation of TiO2 nanotubes in its anatase form, as Mart\u00edn de Vidales et al. observed by X-ray diffraction [34], since the formation and growth of TiO2 nanotubes during anodizing is due to the competition that exists between the formation of TiO2 itself on the surface of the plate and the formation/dissociation of complexes formed by Ti-F, according to the following reactions [27\u201329]:\n\n(10)\nH2O \u2192 2\u2009H+ + O2-\n\n\n\n\n\n(11)\nTi + 2\u2009O2- \u2192 TiO2 + 4 e-\n\n\n\n\n\n(12)\nTiO2 + 6\u2009F- + 4\u2009H+ \u2192 TiF6\n2- + 2\u2009H2O\n\n\nThese reactions, starting with the electrolytic dissociation of water due to the passage of current, continuing with the formation of the TiO2 layer and, finally, with the formation of the TiF6 complex allows the formation and growth of nanotubes. Specifically, the fluorinated complex causes the chemical dissolution of the oxide layer, creating small pores that allow the passage of current and the formation of nanotubes in a direction perpendicular to the surface of the titanium sheet, which acts as an electrode during the anodizing process. Thus, the formation of TiO2-anatase nanotubes improves the active surface for a higher process efficiency.In order to check this result, SEM was carried out to analyze the surface of these plates. \nFig. 3 shows some examples of plates anodized with both electrolytes and with or without heat treatment.As observed, when H2SO4 is used as electrolyte, TiO2 nanotubes are not formed and only a film of TiO2 is found, with a much smaller photocatalytically active surface. In this context, it is important to highlight that the anodization process conducted with H2SO4 in water as electrolyte, which does not entail the formation of TiO2 nanotubes and only a layer of TiO2 is observed on the catalyst surface, was evaluated because plates with different colors can be obtained and they can be used in exterior walls of buildings to treat polluted atmospheres, or in the hull of ships to remove pollutants from the ocean [30].In addition, when a heat treatment is applied after the anodization process using NH4F/H2O electrolyte, a better disposition of the nanotubes is observed, and it seems that a thin film of TiO2 that could hinder the access of the pollutant to the surface of the nanotubes, disappears. Therefore, results obtained by SEM analysis corroborates what was said before. On the other hand, it is observed that the heat treatment allows the formation of a more uniform TiO2 film after the anodization with H2SO4 as electrolyte, which can improve catalyst-contaminant contact.Kinetic constants were calculated for all experiments conducted, taking into account a pseudo-first order kinetic [35\u201337], and results are shown in Table 1 (kinetic constant of each plate was calculated with the medium value of the three experiments conducted under the same operating conditions).A statistical study of the results has been conducted to corroborate these conclusions. \nTable 2 presents the main effects and interactions of the two studied factors on the kinetic constant as response variable. The confidence interval at 95% of significance level was \u00b1\u20091.16\u00b710\u22124 min\u22121.As it can be seen from the table, the main effects of both factors as well as the interaction effect between them are higher than the confidence interval at 95% of significance level being all of them positive. It indicates that the use of NH4F/H2O in ethylene glycol as electrolyte, the heat treatment after the anodizing process and the interaction between both factors present a statistically significant influence on the kinetic constant. Specially, the influence of the heat treatment is very high indicating that its application after the anodizing process notably increase the decontamination rate of OFX. The use of NH4F/H2O in ethylene glycol as electrolyte and the interaction with the heat treatment is also positive for the photocatalytic activity but in a lesser extension.Takin into account these results, the optimal conditions found for the first stage of the experimental design are the use of NH4F/H2O in ethylene glycol as electrolyte, and a heat treatment after the anodization process.In a second stage the influence of the potential ramp and the maximum potential reached in the anodizing process was studied over the treatment of a water polluted with 25\u2009mg\u2009dm\u22123 of ofloxacin, where kinetic constant is chosen as response variable using a face-centered central composite design (\nTable 3). Taking into account the results of the first stage of the experimental design, electrolyte and heat treatment factors were maintained in their high values. Table 3 presents the experimental design matrix for this second set of experiments, where columns 2 and 3 present the values of the factors (maximum potential and potential ramp) on a natural scale, maintaining NH4F/H20 in ethylene glycol as electrolyte and heat treatment applied in all experiments. Columns 4 and 5 represent the dimensionless coded value for the maximum potential and potential ramp factors, labeled as +\u20091 in the upper level of the factor and \u2212\u20091 in the lower level, being the label 0 for the central value of the factor. Kinetic constant as response variable ([OFX]0 = 25\u2009mg\u2009dm\u22123. UV-A: \u03bb\u2009=\u2009365\u2009nm) was calculated for all experiments conducted, also taking into account a pseudo-first order kinetic [34\u201336], and results are shown in column 7.The experiments 5\u20138 constitute the four points of the experimental design. The experiment 13 constitutes the central point of the design and allows calculate the error in order to carry out the statistical analysis of the results. The experiments 9\u201312 constitute the star points (face-centered points), corresponding to anodizing conditions resulting of the combination of points of the factorial design.The ANOVA analysis for these results is presented in \nTable 4, where the influence of factors and its squares and the interactions between factors is reflected. According to ANOVA analysis, those parameters with F-value higher than 1 are significant. Moreover, the p-value is related to the significance level and a p-value less than 0.05 indicates that this parameter has statistical significance with a 95% significance level.According to these results, the maximum potential is not significant whereas the potential ramp and the square of both factors are significant. Thus, the response variable has a linear relationship with the potential ramp and a quadratic relationship with both studied factors.Among the significant factors, the square of the potential ramp is significant at 93% of significance level being this p-value the minor of those corresponding to significant factors. Therefore, this percentage is selected as a limit, and the effects with p-value less than 0.07 will be considered as significant. According to this, the potential ramp and the square of maximum potential and potential ramp will be taking into account to obtain the mathematical model that relates these parameters with the response variable.\nEq. 13 present the statistical fitting model that express the relationship among the response variable (kinetic constant) and the codified values of the maximal potential and potential ramp factors, within the experimental range considered.\n\n(13)\n\n\nk\n\n\n\n\n\nmin\n\n\n\u2212\n1\n\n\n\n\n\n=\n1.11\n\u00b7\n\n\n10\n\n\n\u2212\n3\n\n\n\u2212\n1.2\n\u00b7\n\n\n10\n\n\n\u2212\n3\n\n\n\u00b7\n\n\nX\n\n\nR\n\n\n\u2212\n1.75\n\u00b7\n\n\n10\n\n\n\u2212\n4\n\n\n\u00b7\n\n\nX\n\n\nV\nmax\n\n\n2\n\n\n+\n1.65\n\u00b7\n\n\n10\n\n\n\u2212\n4\n\n\n\n\nX\n\n\nR\n\n\n2\n\n\n\n\n\n\nThe mathematical model allows to predict the response in the experimental region and to obtain the optimum value for the kinetic constant, that would correspond to the central value for the maximum potential (40\u2009V corresponding to a coded value of XVmax = 0) and the lower value for the potential ramp (2\u2009V\u2009min\u22121 corresponding to a coded value of XR = \u22121).SEM images of plates corresponding to the second part of the experimental design, i.e. experiments 6\u201313 (Figs. S1 and S2), show that an array of TiO2 nanotubes has been obtained in all cases. However, the authors have not found a clear relationship between the structural/morphological properties of the nanotubes film and the experimental conditions in the anodizing process, i.e. the maximum potential and potential ramp.According to the literature [28,38], the growth and dimensions of the nanotubes are affected by several parameters of the anodizing process such as maximum potential, profile potential-time or number of steps with different maximum potential. The results obtained in the statistical analysis suggest that a low potential ramp allows the growth of an uniform array of nanotubes that provides higher surface and, consequently, a better photocatalytic performace. A high potential ramp could favour a less uniform nanotubes array, as well as the breaking of the nanotubes or/and prevent their growing. In addition, acording to the results of the factorial design, the influence of the maximum potential applied during the anodizing process, at least in the experimental interval tested in this work, would not be a critical parameter for controlling the structural morphology of the nanotubes array and, hence, the photocatalytic activity.\n\nFig. 4 shows the surface response of kinetic constant based on coded factors of maximal potential and potential ramp.The obtained surface has the shape of a saddle, and the maximum is obtained for the central point of the maximum potential and for the minimum value of the potential ramp. The surface is projected on the lower plane of the figure and in the side bar, coloring the optimal zone of the response variable in red and the zones where the minimum is reached in blue.Finally, a two factors three-level factorial design has been applied for the third stage of the experiments design, where wastewater treatment conditions were studied, in order to find the optimal operating conditions. The studied factors were irradiance and initial concentration of OFX at different UV wavelength. To do this, different experiments were carried out applying UV-A, UV-B or UV-C in the photocatalytic process, with an irradiance of 5, 10 or 15\u2009W\u2009m\u22122, to treat a synthetic wastewater polluted with 15, 25 or 35\u2009mg\u2009dm\u22123 of OFX. All experiments were carried out using a new Ti plate anodized with the optimal conditions obtained in the previous section (NH4F/H2O as electrolyte, 2\u2009V\u2009min\u22121, 40\u2009V and heat treatment). \nTable 5 shows the experimental matrix design with the natural value of the factors in columns 2 and 3, and the coded value in columns 4 and 5. The first-order kinetic constant of each experiment for the different UV lamps are in columns 6, 7 and 8.The ANOVA analysis of these results is shown in \nTable 6 for UVA-B, and Tables S1 and S2 (see supplementary information) for the UV-A and UV-C radiation, respectively. Eqs. 14, 15 and 16 present the statistical fitting model that express the relationship among the response variable (kinetic constant) and the codified values of irradiance and initial concentration of OFX, within the experimental range considered, for UV-B, UV-A and UV-C, respectively.\n\n(14)\n\n\nk\n\n\n\n\n\nmin\n\n\n\u2212\n1\n\n\n\n\n\n=\n7.88653\n\u00b7\n\n\n10\n\n\n\u2212\n4\n\n\n+\n4.25113\n\u00b7\n\n\n10\n\n\n\u2212\n4\n\n\n\u00b7\n\n\nX\n\n\nI\n\n\n\u2212\n3.29613\n\u00b7\n\n\n10\n\n\n\u2212\n4\n\n\n\u00b7\n\n\nX\n\n\n\n\n\n\nOFX\n\n\n\n\n0\n\n\n\n\n\n\n\n\n\n\n(15)\n\n\nk\n\n\n\n\n\nmin\n\n\n\u2212\n1\n\n\n\n\n\n=\n6.98895\n\u00b7\n\n\n10\n\n\n\u2212\n4\n\n\n+\n3.39167\n\u00b7\n\n\n10\n\n\n\u2212\n4\n\n\n\u00b7\n\n\nX\n\n\nI\n\n\n\u2212\n1.93667\n\u00b7\n\n\n10\n\n\n\u2212\n4\n\n\n\u00b7\n\n\nX\n\n\n\n\n\n\nOFX\n\n\n\n\n0\n\n\n\n\n\n\n\n\n\n\n(16)\n\n\nk\n\n\n\n\n\nmin\n\n\n\u2212\n1\n\n\n\n\n\n=\n6.54895\n\u00b7\n\n\n10\n\n\n\u2212\n4\n\n\n+\n3.03333\n\u00b7\n\n\n10\n\n\n\u2212\n4\n\n\n\u00b7\n\n\nX\n\n\nI\n\n\n\u2212\n3.48167\n\u00b7\n\n\n10\n\n\n\u2212\n4\n\n\n\u00b7\n\n\nX\n\n\n\n\n\n\nOFX\n\n\n\n\n0\n\n\n\n\n\n\n\n\n\nTable 6 shows that the main effects of both factors, irradiance and the initial concentration of OFX, are significant at 95%. The Eq. 14 shows that kinetic constant increases with the applied irradiance (positive coefficient in the fitting equation), because more radiation is supplied to the photocatalyst and more oxidants can be formed [32]. On the other hand, the constant decreases when the initial concentration of the pollutant increases in the reaction medium (negative coefficient in the fitting equation). This can be explained taking into account that a higher concentration of the pollutant can negatively affect to the radiation that can reach the catalytic surface [39]. In this context, it is important to highlight that photocatalysis is a surface process where the contact between pollutant, radiation and photocatalyst is necessary [40].For UV-A and UV-C experiments the same conclusions are obtained (Tables S1 and S2, Eqs. 15 and 16), and therefore the irradiance and initial concentration of OFX are significant factors at 95%, with a positive effect of the irradiance and negative effect of the initial concentration of OFX on the kinetic constant.The coefficients of the mathematical fit for both factors, irradiance and initial concentration of OFX (Eqs. 10, 11 and 12), are higher for the experiments with UV-B lamp. This means that the effect of both factors is higher with this wavelength and therefore, UV-B seems be the most efficient radiation, which agrees with what was observed by McMurray et al. [41], because of this is the optimal radiation when TiO2-anatase is used as photocatalyst.It is important to highlight that this is an initial study and next works are necessary in order to find more efficient processes with a higher degradation of the contaminants. Nevertheless, this study opens the door to a promising technology for the treatment of water and wastewater polluted with CECs. In addition, it is possible to use sunlight as radiation source, obtaining a low cost technology, with easy implantation, high efficiency and environmental compatibility.From this work, the following conclusions can be drawn:\n\n-\nHeterogeneous photocatalysis with TiO2-anatase has been applied for the removal of contaminants of emerging concern from wastewater, specifically, ofloxacin degradation has been tested.\n\n\n-\nSynthesis of the photocatalysts has been carried out by anodization of Ti plates. Anodization conditions and the possibility of to apply a heat treatment after the anodization process have been studied by experimental design, evaluating the electrolyte used (NH4F/H2O in ethylene glycol or H2SO4 in water), the potential ramp applied (2\u20134\u2009V\u2009min\u22121), the maximal potential (20\u201360\u2009V) and the heat treatment after anodizing (maximal temperature of 450 \u00baC). The influence of these variables has been evaluated using the kinetic constant of the OFX degradation experiments as response variable, and results show that NH4F/H2O as electrolyte, 2\u2009V\u2009min\u22121, 40\u2009V and a heat treatment are the optimal parameters for the formation of TiO2-anatase nanotubes, with a good size and distribution. However, a statistical study shows that the maximum potential is not a determinant factor.\n\n\n-\nCharacterization of the prepared photocatalysts is carried out by SEM, and results show that, when NH4F/H2O is used as electrolyte and a heat treatment is applied, TiO2 nanotubes are formed with better disposition and arrangement, in the anatase form (active phase for the photocatalysis process).\n\n\n-\nThe operating conditions for the wastewater treatment (UV radiation, irradiance and initial concentration of the pollutant) also have been evaluated. Results of the experimental design show that UV-B, the maximal irradiance and the minimal initial concentration of the pollutant are the optimal conditions, because of a higher formation of oxidants of organic matter.\n\n\nHeterogeneous photocatalysis with TiO2-anatase has been applied for the removal of contaminants of emerging concern from wastewater, specifically, ofloxacin degradation has been tested.Synthesis of the photocatalysts has been carried out by anodization of Ti plates. Anodization conditions and the possibility of to apply a heat treatment after the anodization process have been studied by experimental design, evaluating the electrolyte used (NH4F/H2O in ethylene glycol or H2SO4 in water), the potential ramp applied (2\u20134\u2009V\u2009min\u22121), the maximal potential (20\u201360\u2009V) and the heat treatment after anodizing (maximal temperature of 450 \u00baC). The influence of these variables has been evaluated using the kinetic constant of the OFX degradation experiments as response variable, and results show that NH4F/H2O as electrolyte, 2\u2009V\u2009min\u22121, 40\u2009V and a heat treatment are the optimal parameters for the formation of TiO2-anatase nanotubes, with a good size and distribution. However, a statistical study shows that the maximum potential is not a determinant factor.Characterization of the prepared photocatalysts is carried out by SEM, and results show that, when NH4F/H2O is used as electrolyte and a heat treatment is applied, TiO2 nanotubes are formed with better disposition and arrangement, in the anatase form (active phase for the photocatalysis process).The operating conditions for the wastewater treatment (UV radiation, irradiance and initial concentration of the pollutant) also have been evaluated. Results of the experimental design show that UV-B, the maximal irradiance and the minimal initial concentration of the pollutant are the optimal conditions, because of a higher formation of oxidants of organic matter.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Maria Jose Martin de Vidales reports financial support was provided by Polytechnic University of Madrid.The authors acknowledge the acceptation of this paper to be published in Catalysis Today - Special Issue Selected Contributions of the 11th European Meeting on Solar Chemistry and Photocatalysis: Environmental Applications (SPEA11).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.cattod.2023.01.002.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  In this work, the treatment of wastewater polluted with contaminants of emerging concern is evaluated by heterogeneous photocatalysis with TiO2. Titanium plates are anodized by different experimental conditions, and the influence of these conditions on the catalytic performance is evaluated by factorial design of experiments. The formation of TiO2 nanotubes in the anatase form is sought for photocatalysts with high active surface. Thus, electrolyte used in the anodization process (NH4F/H2O in ethylene glycol or H2SO4 in water), potential ramp (2 \u2013 4\u00a0V\u00a0min\u22121), maximal applied potential (20 \u2013 60\u00a0V) and heat treatment are evaluated as influencing factors of the experimental design. The activity of these catalysts is tested by ofloxacin (pharmaceutical compound used as model of contaminant of emerging concern) degradation in wastewater treatment, obtaining the kinetic constant of the process, parameter chosen as response variable of the experimental design. Results show that NH4F/H2O as electrolyte, 2\u00a0V\u00a0min\u22121, 40\u00a0V and a heat treatment are the optimum conditions for the formation of TiO2\u2013anatase nanotubes with a good disposition and arrangement, observing the higher efficiency in the wastewater treatment process. In addition, wastewater treatment conditions are evaluated (UV wavelength, irradiance and initial concentration of ofloxacin), and it is found that UV-B, maximal irradiance and minimal initial concentration of the pollutant are the optimal factors for a higher process efficiency. This is a work that opens the door to the removal of contaminants of emerging concern from wastewater with a technology of low cost, with easy implantation and environmental compatibility, making possible to use only sunlight as a reagent.\n               "}
{"full_text": "Ammonia is regarded as a safe and sustainable energy carrier due to its high hydrogen content and narrow flammable range [1,2] enabling the long term (days to months) energy storage in chemical bonds versus the short-term storage (seconds to hours) offered by electrochemical storage (i.e. batteries). In this way, the use of ammonia as an energy vector could facilitate the balance of seasonal energy demands and intermittent renewable energy production (e.g. solar, tidal and wind) in a carbon-free society [3\u20135]. Established safety protocols and existing transportation and distribution networks applicable for ammonia [6] make it also an effective possible solution in comparison to hydrogen due to the current lack of viable methods to store hydrogen in a compact, safe and cost-effective manner [7]. Despite its potential, the implementation of ammonia in the energy landscape relies on the capability of releasing hydrogen on-demand, preferably at temperatures aligned to those of fuel cells [8]. A considerable scientific effort is currently focused on the design of catalysts for the low temperature activation of ammonia for the production of hydrogen [2,9,10]. The most active catalysts reported in the literature are ruthenium-based [11\u201316]. The optimum properties of ruthenium actives sites are associated with optimum N-adsorption energy [17] which enables activation of the ammonia molecule while avoiding poisoning by N-adatoms at low temperature (known to be the limiting step at such conditions). Enhancement of the ruthenium activity can be achieved by the use of electron donating promoters [11,13,18] and highly conductive supports, such as graphitised carbon nanotubes [19]. However, there is much interest in the identification of alternative catalysts which rival, if not exceed, the performance of ruthenium. Our recent review on the subject identifies cobalt as an attractive alternative, however studies show it to possess poor activity compared to ruthenium-based systems, especially at low temperatures [2,20\u201323]. The work reported within this manuscript demonstrates a systematic approach to replicate or improve upon the ammonia decomposition activity of ruthenium-based catalysts. The strategy employed by us has been the development of bi-metallic systems combining metals possessing different N-adatom adsorption energies following the DFT simulations by Hangsen et al.[17] to achieve an optimum binding energy for catalytic performance. Within our studies, cobalt-rhenium systems present activity at conditions comparable to Ru/CNT catalysts [11] with very high stability under consecutive runs and no observed formation of nitrides (Co-N and Re-N) under the ammonia atmosphere. Even though we recognise the scarcity and cost of Re, the knowledge provided in this study is useful for the development of catalysts of enhanced activity. The low temperature activity is directly related to the intimate Co-Re interaction with the activity onset related to the contraction of the Re-Co bond distance.Cobalt rhenium materials were prepared to yield different Co/Re ratios, by mixing varying amounts of ammonium perrhenate (NH4ReO4, Sigma Aldrich, >99%) in deionized water with cobalt nitrate (Co(NO3)2.6H2O, Sigma Aldrich, >98%). The solutions were stirred for 1\u202fhour then dried in an oven at 125\u202f\u00b0C for 12\u202fhours. After drying, the materials were ground by hand and calcined in air at 700\u202f\u00b0C (using a 10\u202f\u00b0C\u202fmin-1 ramp rate) for 3\u202fhours. Ruthenium supported on carbon nanotube (Ru/CNT) catalysts were prepared by incipient wetness impregnation using Ru(NO)(NO3)3 (Alfa Aesar). Multi-walled carbon nanotubes (Sigma Aldrich, OD 6-9\u202fnm, length 5\u202f\u03bcm, SBET 253\u202fm2\u202fg-1) were used as support. After impregnation of the aqueous solutions, the catalysts were dried at 100\u202f\u00b0C under vacuum for 3\u202fhours and then reduced under hydrogen at 230\u202f\u00b0C for 1.5\u202fhours.Following degassing the materials, nitrogen physisorption isotherms were measured at \u2212196\u202f\u00b0C using a Micromeritics ASAP 2020 instrument. The surface area was calculated using the Brunauer, Emmett and Teller (BET) method. Temperature programmed reduction (TPR) experiments were carried out in a Micromeritics Autochem 2920 instrument equipped with a thermal conductivity detector (TCD). The samples characterised from room temperature to 900\u202f\u00b0C using a temperature ramp rate of 10\u202f\u00b0C\u202fmin-1 under 50\u202fmL min-1 flow of 5 % H2/Ar. CO pulse chemisorption analyses at 35\u202f\u00b0C were carried out using the Micromeritics Autochem 2920 instrument equipped with a TCD. Samples were pre-treated at 250\u202f\u00b0C under a helium flow for 1\u202fhour to ensure desorption of water.Cobalt K-edge and rhenium LIII-edge XAS data were collected at the Swiss-Norwegian Beamline (SNBL, BM1B) at the European Synchrotron Radiation Facility (ESRF) in transmission mode. The data was collected in the 16-bunch filling mode, providing a maximum current of 90\u202fmA. A bending magnet collected the white beam from the storage ring to the beamline. The SNBL is equipped with a Si(111) double crystal monochromator for EXAFS data collection. The incident and transmitted intensities (I0 and It + I2) were detected with ion chambers filled with, I0 (17\u202fcm) 50 % N2 + 50 % He, and It and I2 (30\u202fcm) with 85 % N2 + 15 % Ar at the cobalt edge. Cobalt references (Co-foil, CoO, Co3O4) and rhenium references (Re-foil, NH4ReO4) were also collected. The cobalt K-edge XAS data were measured in continuous step scan mode from 7600\u202feV to 8300\u202feV with a step size of 0.5\u202feV and counting time of 300\u202fms per step. The rhenium L-III data were collected in transmission mode, using ion chambers fillings of 100 % N2 (I0, 17\u202fcm), 50 % N2 + 50 % Ar (It, 30\u202fcm). Step scans were collected between 10350\u202feV to 11800\u202feV, with a step size of 0.5\u202feV and counting time of 200\u202fms per step.For all in situ measurements, great care was taken to ensure similar conditions were applied for both edges, hence sample weight, cell thickness and gas flow were kept constant. The CoRe catalysts were mixed with boron nitride, pressed to wafers and sieved fractions (above 375\u202f\u03bcm) were then placed inside 0.9\u202fmm quartz capillaries with quartz wool on either side. The capillary was heated by a blower placed directly under the sample, and the exhaust was continuously sampled using a Pfeiffer Omnistar Mass Spectrometer. The protocol for the ammonia decomposition includes pre-treatment in 75% H2 in argon at 600\u202f\u00b0C for one hour using a 5\u202f\u00b0C\u202fmin-1 ramp rate using a total flow of 10\u202fmL min-1. EXAFS step scans were collected continuously, with XRD patterns being collected at end points. After the pre-treatment, samples were cooled to 200\u202f\u00b0C before switching to 5% NH3 in helium and heating to 700\u202f\u00b0C using a ramp rate of 2\u202f\u00b0C min-1. EXAFS step scans were collected continuously, and the exhaust was continuously analysed using the mass spectrometer.The XAS data were binned (edge region \u221230\u202feV to 50\u202feV; pre-edge grid 10\u202feV; XANES grid 0.5\u202feV; EXAFS grid 0.05 \u00c5-1) and background subtracted, and the EXAFS part of the spectrum extracted to yield the \u03c7i\nexp(k) using Athena software from the IFFEFITT package. [24] The XANES spectra were normalised from 30 to 150\u202feV above the edge, while the EXAFS spectra were normalised from 150\u202feV to the end point. The data were carefully deglitched and truncated when needed. For cobalt the threshold energy (E0) was set to be at the mid-point (0.5) of the normalised absorption edge step ensuring it was chosen after any pre-edge or shoulder features. For rhenium samples E0 was determined to be the first inflection point in the first derivative spectra, as there are no pre-edges or shoulder features. All XANES spectra were energy corrected against the corresponding reference foil (Co\u202f=\u202f7709\u202feV, Re\u202f=\u202f10535\u202feV).Due to the bimetallic nature of the CoRe-catalyst, as reported previously [25], obtaining accurate comparable results from linear combination of XANES using known references was difficult. For this reason, reduction and reaction profiles were obtained using multivariate curve resolution (MCR). MCR using the alternating least-square (ALS) mathematical algorithm is a chemometric method which is well-known for its ability to provide the pure response profile of the chemical constituent (species) of an unresolved mixture. Nowadays, MCR is heavily used as a blind source separation method (no reference spectra) to process large data-sets generated in labs and synchrotron facilities all over the world. For a detailed description of the method employed, software and usage, the reader is directed to literature from Jaumot et al. [26,27] and Ruckebush et al. [28] MCR-ALS was used to analyse the operando time-resolved (TR) XANES data-sets for Co-Re bimetallic catalysts during the pre-reduction step. For the assessment of the minimum number of principal components that describe the system, i.e. rank analysis, a built-in method based on the singular value decomposition (SVD) was used [29]. The SVD results display the calculated eigenvalues of the data versus the component number (a so-called scree plot), which allows understanding how much variance each component can explain. A break in the slope of such a plot is generally associated to the minimum number of components able to simulate the initial mixture. The MCR-ALS graphical user interface (GUI) for Matlab\u00ae used in the present manuscript (http://www.mcrals.info/) was applied on the XANES data-sets for both Co and Re edges (Co: 7600-8000\u202feV and Re: 10450-10800\u202feV). Positive constraints were utilised for both concentration and spectra profiles and closure constraints for the concentration (i.e. no mass transfer; constant concentration of the absorber throughout the experiment).The peak fitting feature in the Athena software was used to calculate the area for the white line intensity at the Co K-edge for CoRe1.6 during NH3-treatment. A Gaussian curve was used to calculate the area. Difference spectra were made with the Athena software.EXAFS least-squares refinements were carried out using DL-EXCURV [30], which conducts the curve fitting of the theoretical \u03c7calc(k) to the experimental \u03c7exp(k) using the curved wave theory. The fit parameter reported for each refinement procedure is given by the statistical R-factor, defined as:\n\n\nR\n=\n\n\n\u2211\ni\n\n\n\n\n\n\n\n\n\n\u03c7\ni\n\ne\nx\np\n\n\n-\n\n\u03c7\ni\n\nc\na\nl\nc\n\n\n\n\n\nk\n\nW\nT\n\n\n\n\n\n2\n\n/\n\n\n\u2211\ni\n\n\n\n\n[\n\n\n\n\u03c7\ni\n\ne\nx\np\n\n\n\n\n\nk\n\nW\nT\n\n\n]\n\n2\n\n\n\n\n\nx\n100\n%\n\n\nkWT is the weight factor and a k3 -weighting was used for the analysed data. Ab initio phase shifts were also calculated within DL-EXCURV and verified using reference compounds. The least-squares refinements were carried out in typical wave number k range 2-8.5\u202f\u00c5-1 for cobalt and k range 3.5-9.5\u202f\u00c5-1 for rhenium using a k3 weighting scheme.Bimetallic fractions were calculated from the multiplicities of the Re LIII-edge from the EXAFS analysis after the method of Shibata et al. [31] The coordination number of the bimetallic contribution (NRe-Co) was used to determine the ratio of bimetallic phase compared to the total coordination number (NRe-Re + NRe-Co).Ammonia decomposition reactions were carried out in a continuous differential packed bed reactor using 25\u202fmg of catalyst diluted in a silicon carbide bed. The reactor system was equipped with mass flow and temperature controllers. The reaction tubing was heated to 60\u202f\u00b0C to avoid ammonia condensation and consequent corrosion. Prior to each catalytic reaction study, the catalysts were pre-reduced in situ under a H2 flow at 600\u202f\u00b0C for 1\u202fhour (unless otherwise stated). After pre-reduction, the temperature was returned to ambient under the H2 flow. Following this, the reaction temperature was ramped from room temperature to 600\u202f\u00b0C using a Carbolite tube furnace equipped with a PID controller. A 2.6\u202f\u00b0C min-1 ramp under 2.5\u202fmL min-1 NH3 and 6\u202fmL min-1 He (GHSV of 6000 mLNH3\u00b7gcat\n-1\u202fh-1) was applied. The reactor exit gas was analysed using an on-line gas chromatograph fitted with a Porapak column and employing a thermal conductivity detector. Mass balance calculations were carried out to account for the molar expansion occurring as a result of the reaction. Mass balance was achieved within a \u00b1 10 % error.A number of unsupported bimetallic combinations including CoRe1.6, Ni2Mo3N, Co3Mo3N were tested for hydrogen production from ammonia decomposition (Fig. 1\na) with the aim of achieving similar activities than the state-of-the-art ruthenium-based catalysts by combining transition metal catalysts with respectively higher and lower N-adatom adsorption energy than ruthenium. In addition to metal based catalysts, nitrides have also been investigated for ammonia decomposition, for example. [32,33] While Co-Mo alloy has been previously predicted as an optimum bimetallic combination [17,34,35], a considerably higher activity and, most importantly, an onset of activity at lower temperatures was achieved by the Co-Re alloy. Indeed, CoRe1.6 has an activity comparable to 7\u202fwt.% Ru/CNT [11]. 7\u202fwt.% Ru/CNT is considered to be one of the optimum catalysts for this reaction due to the high concentration of B5 sites (an arrangement of three Ru atoms in one layer and two further Ru atoms in the layer directly above) expressed in the 3.5-5\u202fnm Ru nanoparticles present which are promoted by the CNT support [36]. While 3-5\u202fnm supported Ru nanoparticles (7\u202fwt.% Ru/CNT) present a considerably higher activity than the unsupported CoRe1.6 per mol of metal (Fig. 1b), a rate a few orders of magnitude higher is shown by the CoRe1.6 when activity is normalised by metallic surface area. It is important to highlight that the surface area of the unsupported CoRe1.6 catalyst is only 0.2 m2 g-1 as determined from the BET measurement (although the limitations in this regard with such a low surface area must be recognised) while the metallic surface area of the 7% Ru/CNT catalyst is 10 m\u00b2 g-1 as measured by CO chemisorption. Pre-reduction in a H2 flow was undertaken prior to surface area determination in both cases. Whilst the limitations of the BET method applied to CoRe1.6 in view of its very low surface area are acknowledged, triplicate analyses confirmed the absence of micro- and meso-porosity. Considering the surface catalysed nature of ammonia decomposition, these results imply that the active sites in the Co-Re system might be considerably more active than those in their ruthenium counterparts. Both catalysts show a similar activation energy with values of 91\u202fkJ mol-1 and 85\u202fkJ mol-1 for 7\u202fwt.% Ru/CNT and unsupported CoRe1.6 respectively (the Arrhenius plots are available in the SI, Figure S1). These values are in agreement with those previously reported for Ru-based catalysts [11,19] and may possibly suggest similarities in the nature of the active sites in both systems, thereby confirming the potential of the design principle adopted, although this is a matter for further exploration.As shown in Fig. 2\n, cobalt only and rhenium only counterpart catalysts show limited activity for ammonia decomposition demonstrating that the high activity of the CoRe1.6 material is due to the synergetic effect achieved by the alloy formation [25].The activity of the CoRe1.6 catalyst was significantly increased as the pre-reduction temperature of the catalyst was increased (Fig. 3\n) from 400 to 600\u202f\u00b0C. As the catalysts have been calcined at 700\u202f\u00b0C under air, the role of pre-reduction is most likely not related to the promotion of the thermal interaction between the Co and the Re atoms but rather the oxidation state of the active species. However, temperature programmed reduction (TPR) of the CoRe1.6 catalyst (Figure S2 in the SI) reveals full reduction of both Co and Re components at 400\u202f\u00b0C under hydrogen indicating that the apparent increase in catalytic activity at higher pre-reduction temperatures is likely to be associated with thermally-induced modifications in the bi-metallic material as evidenced below. For comparison, Figure S2 also shows the TPR of the calcined cobalt and rhenium precursors where full reduction requires temperatures of \u223c 450-500\u202f\u00b0C.To gain a better understanding of the promotion of the interaction between the Co and Re during the pre-reduction of the CoRe1.6 material, in situ X-ray absorption spectroscopy was applied. It is particularly applicable to the current study where there could be concerns that the ex situ nature of the material may differ from that under reaction conditions. Attention was particularly directed to the near edge (XANES) region as it is highly sensitive to the local environment and oxidation state. Fig. 4\n reveals that a cobalt intermediate state is formed during reduction at temperatures above 100\u202f\u00b0C comprising an apparent mixture of Co2+ and Co0 species. Full reduction to Co0 occurs over a narrow temperature window starting at 350\u202f\u00b0C with the final reduced state being complete at 400\u202f\u00b0C. Reduction of rhenium occurs abruptly and in one step initiating at 300\u202f\u00b0C. Fig. 5\n presents spectra determined at different stages of the activation and reaction process.During heating of the CoRe1.6 catalyst in 5% NH3, partial oxidation of cobalt is evident by the increased white line intensity observed between 200-400\u202f\u00b0C [37,38](Fig. 5c), as in agreement with supported CoRe in a silica aerogel. [39] Even if changes were observed in the more sensitive XANES for cobalt, EXAFS analysis (Table 1\n) revealed no light atom scattering pairs (i.e. Co-N) formed during the low-temperature ammonia treatment. Hence, any oxidation must be limited to the surface which would be difficult to distinguish in the EXAFS analysis of CoRe1.6. Starting at 400\u202f\u00b0C during ammonia treatment, the white line intensity (Fig. 5d) gradually decreased reaching a similar intensity at 600\u202f\u00b0C as observed for CoRe1.6 during pre-treatment. This corresponds to a partial reduction of cobalt coinciding with CoRe1.6 becoming active for ammonia decomposition. Interestingly, no changes were observed in the XANES region (Figure S3 in the SI) for rhenium during ammonia decomposition. These observations are in agreement with the widespread use of Re as a promoter in cobalt-based catalysts for the Fischer-Tropsch reaction where it facilitates not only Co reduction but also retards its sintering [40].Results from EXAFS analysis (Table 1) and average coordination numbers for the unsupported CoRe1.6 catalysts from pre-reduction between 400-600\u202f\u00b0C show an average multiplicity of the Co-Co pair between 3.5 and 4 at around 2.45\u202f\u00c5, while the first Re-Re shell at 2.64 -2.69\u202f\u00c5 contains on average 6-7 neighbours. While no Co-Re contribution could be fitted, a small Re-Co contribution could refined between 2.55-2.57\u202f\u00c5 with an average multiplicity between 1.2-1.4. Attempts were made to refine the corresponding Co-Re shell, however they were not successful. This can be explained by Co-Co and Co-Re backscattering pairs having similar bond distances which were not resolved by our limited \u0394k-window. Attempts were also made to include mixed site option in the EXCURV software, essentially forcing Co-Co and Co-Re shells to have the same bond length, however, this did not improve the fit-factor and therefore the Co-Re contribution was removed. It is believed that the refinement of the Co-Re contribution is limited by the experimental constraints, i.e. filling mode (16-bunch), high rhenium content and elevated temperatures. EXAFS refinements indicated a complete reduction of CoRe1.6 from 400\u202f\u00b0C in 75% H2 as no metal-O pairs could be fitted, in agreement with the TPR results described above. The addition of a metal-O pair in the fitting model for Fig. 6\na) and Fig. 7\na) produced no valid results and did not improve the fit. This reveals that the major contributing species formed during pre-treatment of CoRe1.6 are Co-Co and Re-Re with only \u223c20% of the bimetallic Re-Co pair.After cooling and switching to 5% NH3, partial oxidation of Co was observed in the XANES, however this was not reflected in the EXAFS (at 300\u202f\u00b0C) where four Co-Co pairs at 2.45\u202f\u00c5 were found similarly to the pre-treatment. A better fit was obtained at the Re LIII-edge at lower temperatures yielding an average of 3.3 Re-Re backscattering pairs at 2.65\u202f\u00c5. We believe the lowered multiplicity originates from better accuracy and is not a reflection of reduction in particle size. The Re-Co bimetallic pair remained but at an elongated bond distance of 2.61\u202f\u00c5 with a slightly increased average multiplicity to 1.6. Hence, there were minor structural changes observed in the EXAFS for CoRe1.6 when switching from a hydrogen to an ammonia atmosphere (Fig. 6 and Fig. 7)When increasing the temperature from 300\u202f\u00b0C to 600\u202f\u00b0C under the ammonia atmosphere, the Co-Co backscattering pair was surprisingly stable (Fig. 8\n and Fig. 9\n) despite the observed partial oxidation and re-reduction observed in the XANES. These observations align with the excellent stability of the CoRe1.6 catalysts under consecutive reaction runs as shown below. The average Co-Co multiplicity remained at 4 with a bond distance of 2.45\u202f\u00c5 throughout the ammonia treatment. This contrasts with the Re-Re backscattering shell where significant elongation of bond distance from 2.65\u202f\u00c5 to 2.73\u202f\u00c5 was observed (Fig. 9) during ammonia decomposition. Great care must be taken when comparing coordination shell distances from EXAFS at different temperatures, as other effects, such as thermal expansion, may contribute to the observed changes. [41] While multiplicities of Re-Re, Re-Co and Co-Co remained constant after the reduction stage, we observed variations in bond distances for these coordination shells as mentioned above. Fig. 9 illustrates these changes, \u0394R, for these shells during pre-treatment and ammonia decomposition. During the pre-treatment step, relative changes of the first metal coordination shells were apparent relative to their initial appearance (400\u202f\u00b0C), while relative changes during ammonia decomposition were shown in relation to the pre-treated sample. While the Co-Co bond distance remained unchanged during pre-treatment, an observed elongation of the Re-Re bond (2.65-2.69\u202f\u00c5) concurs with a slight contraction of the Re-Co bond (2.57-2.55\u202f\u00c5). This strongly indicates that local restructuring of both monometallic and bimetallic particles occurs between 400-600\u202f\u00b0C, after TPR and MCR data suggest CoRe1.6 is fully reduced. After switching to ammonia at 200\u202f\u00b0C the Re-Re bond was significantly shortened to 2.65\u202f\u00c5 and the Re-Co distance is elongated (2.61\u202f\u00c5). During ammonia decomposition however these two distances were separated as Re-Re increased to 2.73\u202f\u00c5, while Re-Co was shortened (2.56\u202f\u00c5) coinciding with CoRe1.6 becoming active. While the observed elongation in bond distance approaches that of Re-foil (2.74\u202f\u00c5), it did not seem to be associated with sintering of a pure Re-Re phase in CoRe1.6 as the average multiplicity remained between 6-7 similarly to that observed in the H2 pre-treatment. Similar bond distances (2.56-2.57\u202f\u00c5) were found for rhenium promoted Co/Al2O3 during CO oxidation by in situ XAS. [42]It should be noted that higher order cumulants [43] (C3 and C4) were introduced during EXAFS refinements, however they did not improve the fits for either rhenium or cobalt. Somewhat ordered higher coordination shells are observed at the Co K-edge (Fig. 6) at low temperatures during pre-treatment, but these could not be fitted. However, these shells seem to disintegrate at higher temperatures and during ammonia treatment. Higher coordination shells were not observed at the Re LIII-edge (Fig. 7) at any reaction stage. This indicates a high degree of disorder in the CoRe1.6 during ammonia treatment. This is also the case for the material for ammonia synthesis. [25]It is of interest to monitor the bimetallic Re-Co contribution during the reaction stages and we have included two methods where we utilise information from XANES and EXAFS. Bimetallic fractions calculated from EXAFS (Fig. 10\n, bottom) are somewhat stable between 0.1 and 0.3 through the reaction stages. However, due to high uncertainties of the Re-Re multiplicities especially, the XANES has been examined to look for features associated with the Re-Co bimetallic pair. The difference spectra (Fig. 10, top, black line) between Co-foil and the pre-treated CoRe1.6 show several features, one at 7712\u202feV associated with the pre-edge and a negative peak at 7722\u202feV together with a peak at 7730\u202feV related to the white line. Of these, the feature at 7712\u202feV would be more characteristic for assigning Re-Co bimetallic species, as the latter two can also be affected by oxidation and reduction processes. The feature at 7760\u202feV will be affected by temperature and particle size and is therefore less reliable for this purpose. The difference spectra for CoRe1.6 during ammonia treatment (Fig. 10, top, green line) monitor possible changes in the XANES compared to the final state at 600\u202f\u00b0C in ammonia. It is evident that there are no significant changes in bimetallic Re-Co contribution during the process, rather the observed changes are attributed to partial oxidation and re-reduction.These results are similar to the behaviour of CoRe1.6 during ammonia synthesis where the Ar/H2 pre-treated material shows a stable bimetallic fraction throughout the process. [25] The complete reduction observed from the lack of Co-O and Re-O interactions in the EXAFS confirms that all of the metal is in the metallic state, and while the system is made up of Co-Co, Re-Re and Co-Re/Re-Co coordination pairs, the local structure of each particle is more difficult to predict.Results from XAS reveals that the CoRe1.6 catalyst remains largely unchanged during the ammonia treatment. This also precludes nitride formation as no Co-N/Re-N coordination shells were found during any stage of the process with the interaction with reactants and productsThe stable Co-Re contributions are in agreement with the excellent stability of the unsupported CoRe1.6 catalysts in at least 6 consecutive ammonia decomposition runs up to 500\u202f\u00b0C where the temperature is returned to ambient values after each run before being increased again (Fig. 11\n).This paper provides new information for the development of active ammonia decomposition catalysts using bi-metallic combinations. Unsupported CoRe catalysts present an ammonia decomposition activity comparable to the state-of-the-art Ru-based systems with high stability under consecutive runs. The high activity is related to the bimetallic Co-Re contribution with no significant change across the studied temperatures with the exception of partial oxidation and re-reduction of the cobalt species. The re-reduction in the presence of the Re component coincides with the onset of ammonia decomposition activity. In addition, no Co-N or Re-N backscattering shells are found during EXAFS analysis under ammonia atmosphere. In terms of the CoRe catalyst, future attention should be directed towards the application of supports to increase the dispersion of the active phase and also to the application of potential promoters, whilst recognising the relative high cost of the Re component.\nKarsten G. Kirste: Investigation, Data curation, Writing - original draft, Writing - review & editing. Kate McAulay: Investigation, Writing - review & editing. Tamsin E. Bell: Investigation, Data curation, Writing - original draft, Writing - review & editing. Dragos Stoian: Investigation. Said Laassiri: Investigation, Writing - review & editing. Angela Daisley: Investigation, Writing - review & editing. Justin S.J. Hargreaves: Supervision, Funding acquisition, Writing - review & editing. Karina Mathisen: Supervision, Funding acquisition, Writing - original draft, Writing - review & editing. Laura Torrente-Murciano: Supervision, Funding acquisition, Writing - original draft, Writing - review & editing.The authors report no declarations of interest.The authors would like to acknowledge UK Engineering and Physical Science Research Council (EPSRC grant numbers EP/L020432/2, EP/N013778/1, EP/L02537X/1 and EP/L026317/1) for funding. The Norwegian University of Science and Technology and the Norwegian Resource Council is acknowledged for grants supporting the Swiss-Norwegian Beamlines (SNBL) and K.G. Kirste and K. Mathisen acknowledge the grants from the Anders Jahre fund for promotion of science. The assistance of beamline scientists M. Brunelli and W. van Beek is very much appreciated.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2020.119405.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  On-demand production of hydrogen from ammonia is a challenge limiting the implementation of ammonia as a long term hydrogen vector to overcome the difficulties associated with hydrogen storage. Herein, we present the development of catalysts for the on-demand production of hydrogen from ammonia by combining metals with high and low N-adatom adsorption energies. In this way, cobalt-rhenium (Co-Re) catalysts show high activity mimicking that of ruthenium. EXAFS/XANES analyses demonstrate that the bimetallic Co-Re contribution is responsible for the activity and the stability of the catalysts in consecutive runs with no observable formation of nitrides (Co-N and Re-N) occurring under the ammonia atmosphere. While cobalt is partially re-oxidised under ammonia, re-reduction in the presence of rhenium is observed at higher temperatures, coinciding with the on-set of catalytic activity which is accompanied by minor structural changes. These results provide insight for the development of highly active alloy based ammonia decomposition catalysts.\n               "}
{"full_text": "Dichromate is an essential additive in the chlorate process (the electrolytic production of NaClO3 from NaCl), to catalyze the chlorate formation as well as to inhibit side reactions. Thus, no chlorate production is possible in the current technology without adding chromium(VI) (Colman and Tilak, 1995). However, chromium(VI) compounds are in the REACH annex XIV list which means that a special authorization is required to use them within the EU after 2017. Chlorate is produced by electrolysis of sodium chloride in an undivided cell, where chlorine is produced at the anode and hydrogen at the cathode (Cornell, 2014a; Cornell, 2014b). Chlorine is immediately hydrolyzed in the electrolyte forming hypochlorite/hypochlorous acid (their ratio is defined by the pH) which disproportionate into chlorate and chloride ions. The latter reaction is catalyzed by chromium(VI) in the electrolyte. The chromium(VI) additive has further functions in the process and large efforts have been spent to find alternative components for replacing it in all its roles in the chlorate process (Endr\u0151di et al., 2017; Endr\u0151di et al., 2019).The uncatalyzed decomposition of hypochlorite has been thoroughly investigated over the years and is known to follow overall third order kinetics (Adam et al., 1992). Furthermore, the maximum rate of the reaction occurs at a pH where the ratio of HOCl and OCl\u2212 is 2:1. A deeper understanding of the mechanism of the uncatalyzed reaction was recently achieved in a theoretical study showing that the reaction is initiated by a fast equilibrium between HOCl, OCl\u2212, Cl2O and Cl3O2\n\u2212 and the subsequent abstraction of Cl\u2212 to form Cl2O2 is the rate determining step in chlorate formation (Szab\u00f3 et al., 2018). The catalytic effect of chromium(VI) in the chlorate process has been considered for a long time, and lately this effect has been explored in detail (Endr\u0151di et al., 2019).Replacing chromium(VI) as a catalyst in the chlorate process is a particularly challenging task due to the extreme operating conditions applied in the corresponding technology (Endr\u0151di et al., 2017; Sandin et al., 2015; Busch et al., 2019). In our previous work, the chromium(VI) catalyzed decomposition of HOCl was thoroughly studied, and it was concluded that the catalytically active species is CrO4\n2\u2212. Such an effect was not observed with the structurally analogous phosphate ion, thus it was concluded that the catalytic activity of CrO4\n2\u2212 is associated with a partial electron transfer process in the transition state. This enhances the conversion of Cl2O into HCl2O2\n\u2212 (Kalm\u00e1r et al., 2018). The main objective of this study was to find efficient alternative catalysts for the conversion of hypochlorous acid into chlorate ion. We mainly focused on compounds which were expected to feature the noted partial electron transfer phenomenon. The experiments were performed at elevated temperature (80\u00b0C) in order to mimic the process conditions.All chemicals were of analytical reagent grade, purchased from commercial sources and used as received, without further purification. Doubly-deionized and ultrafiltered (ELGA Purelab Classic system) water was used to prepare the stock solutions and samples. Sodium hypochlorite (NaOCl) solution was prepared by bubbling gaseous chlorine into sodium hydroxide solution. The stock solution of NaOCl was standardized by iodometric titration. A Metrohm 785 DMP Titrino automatic titrator equipped with a Metrohm 6.0451.100 combination platinum electrode was used. The excess NaOH concentration was determined by pH-metric titration with standard HClO4. In this case, a Metrohm 6.0262.100 combination glass electrode was used.The decomposition reaction was triggered by simultaneous addition of NaOCl and the catalyst to well stirred aqueous perchloric acid solution. In the case of heterogeneous systems, the progress of the reaction was monitored by taking individual samples from the reaction mixture at different reaction times, and the nominal concentration of the catalyst is given, i.e. the weighted amount of catalyst divided by the volume of the reaction mixture. Obviously, the heterogeneous catalysts were not dissolved, and the catalytic process occurred predominantly on the surface of the catalyst. The decomposition reaction was studied at 80\u00b10.1\u00b0C and the samples were stirred with a magnetic stirrer. High ionic strength was not set in these experiments because it would have saturated the ion chromatographic column in the ionchromatographic experiments. Thus, the ionic strength was always defined by the ionic forms of the reactants and the catalyst in the samples. Since some of the species are involved in acid-base equilibria the actual total concentration of the ions was also affected by the pH. It follows, that constant ionic strength could not be used in these studies, it was somewhere between 0.10 and 0.15M (estimated value). Within this range, significant ionic strength effects are unlikely on the kinetics and stoichiometry, and the corresponding results are directly comparable.To measure the pH of the inhomogeneous reaction mixtures, a special, Metrohm Unitrode Pt 1000 (6.0258.010) electrode was used equipped with a temperature sensor unit. Before use, it was confirmed that the electrode is reliable and reproducible in heterogeneous model systems. The electrode was calibrated every day at 80\u00b0C using KH-phthalate (c\n=0.05M, pH=4.159) and borax (c\n=0.01M, pH=8.910) standard solutions (Covington et al., 1985). In this study, the pH readout was not converted into log[H+] to correct for the ionic strength effect as recommended by Irving et al. (1967). Accordingly, the readout of the pH meter is plotted as pH in the corresponding figures. It needs to be emphasized that the actual correction factor (Irving factor) is electrode specific and may exceed \u00b11 pH unit at 80\u00b0C. In principle, the correction should have been made point by point due to the lack of constant ionic strength even within a kinetic run and the presence of the heterogeneous phase. Obviously, such a procedure is not feasible. In general, the correction of the pH readout could have varied within 0.1\u20130.2 pH unit only.During a kinetic run, 1mL sample was retracted from the reaction mixture in every 5min. This aliquot of the sample was immediately cooled to 25\u00b0C in an ice bath and filtrated with regenerated cellulose membrane-filter (pore size 0.45\u03bcm). The filtered solution was diluted using NaOH as quenching agent. The final concentration of NaOH was 0.1M. In the case of YCl3, a small amount of hydroxo precipitate formed which was removed by a second filtration. UV-Vis spectra of these solutions were recorded in the 200\u2013400nm wavelength range on an Agilent-8453 diode array spectrophotometer. It was confirmed that unwanted photochemical side reaction did not occur during the measurements (F\u00e1bi\u00e1n and Lente, 2010). The amount of NaOCl was quantified at the absorption band of OCl\u2212 (\u03bb\n\nmax\n\n=292nm, \u03b5\n=339.5M\u22121\ncm\u22121).The formation of chlorate as a function of time was monitored with a Thermo Scientific Dionex ICS-5000\n\n+\n ion chromatographic system by using a 25\u03bcL injector loop. Isocratic elution was carried out using NaOH solution (0.020M). The method was calibrated by a dilution series of chlorate solutions. In each system, the concentration of the product chlorate ion as well as the concentration change of hypochlorous acid was measured as a function of time.The protonation constants (pK\na) of telluric acid were determined by pH-potentiometric titration method using a carbonate free NaOH solution (ca. 1M). The carbonate contamination was determined using the appropriate Gran functions (Gran, 1952). In this titration, 45mL aliquots of telluric acid (ca. 0.018M) were titrated using NaCl (I\n=1.7M) and NaClO3 (I\n=4.7M) as background electrolytes. The headspace over the sample was purged with argon to ensure the absence of oxygen and carbon dioxide. The pH measurements were made using a Metrohm Unitrode Pt 1000 (6.0258.010) electrode. In this case, the pH reading was converted to hydrogen ion concentration as described by Irving et al. (1967). The protonation constants were calculated by using the designated computational program, SUPERQUAD (Gans et al., 1985).The rate of the decomposition reaction of hypochlorous acid depends on various parameters, especially on the pH. In the present study, it is assumed that the actual pK\na of HOCl is around 6.79 obtained at 80\u00b0C, 0.5M NaCl (Wanng\u00e5rd and Wildlock, 2017). No attempt was made to determine the exact pK\na for the conditions applied here for the following reasons. The main goal of this study is to establish how various substances catalyze the decomposition of HOCl as a function of pH, but it is not studied which form of a given species is active in these reactions. Because the pH decreases steadily, the [OCl\u2212]/[HOCl] ratio always changes significantly over the course of the reaction. Thus, the interpretation of the general trends and the comparison of the results at different pH values does not require the exact knowledge of the pK\na. A more quantitative approach would require a different set of experiments at constant ionic strength which would not make possible the stoichiometric measurements with the method used here.As it was reported in the case of Cr(VI) catalyzed decomposition of HOCl, the catalytic reaction path is also pH dependent (Kalm\u00e1r et al., 2018). This was interpreted by considering that only one form of Cr(VI) is catalytically active and the speciation of Cr(VI) is controlled by the pH (Szab\u00f3 et al., 2018). The very same features are expected in any system where the potential catalyst is involved in pH dependent equilibrium processes. The practical consequence of this is that the catalytic activity needs to be tested in a broader pH range. Thus, test experiments were run by varying the initial pH.The kinetic traces (HOCl decay) in the presence of the potential catalyst were compared to those obtained in the Cr(VI) catalyzed reaction. The results of control experiments are also presented under identical initial conditions in either non-buffered and phosphate buffered solutions in the absence of the catalyst. The comparison of these traces revealed significant differences in the pH profiles. This is quite reasonable if we consider that different acid\u2013base reactions and, as a consequence, different buffering effects take place in the compared systems.Earlier it was established that the decomposition of hypochlorous acid may proceed via two distinct reaction paths (Busch et al., 2019), called the chlorate (Eq. (1)) and the oxygen (Eq. (2)) paths.\n\n(1)\n\n\n3\nHOCl\n=\nCl\n\nO\n3\n\u2212\n\n+\n2\n\nCl\n\u2212\n\n+\n3\n\nH\n+\n\n\n\n\n\n\n\n(2)\n\n\n2\nHOCl\n=\n2\n\nCl\n\u2212\n\n+\n2\n\nH\n+\n\n+\n\nO\n2\n\n\n\n\n\nIt is noteworthy to mention that only the chlorate path is preferable for the industrial process and the oxygen path needs to be avoided as much as possible. We characterize the relative significance of the two decomposition paths by the stoichiometric ratio of the reactant consumed and product formed as follows: R\n=\u0394[OCl\u2212]/[ClO3\n\u2212]. When only the chlorate path is operative R\n=3.0. Higher ratios indicate that the oxygen path also contributes to the overall process. Accordingly, the main goal is to achieve R\n=3.0 in a catalytic system. At this point, it needs to be emphasized that the presence of chlorate ion impurities in the reactants may have significant effect on the value of R. It was noticed that a small initial amount of chlorate ion was always present as impurity in the reaction mixtures. It could originate either from the hypochlorite stock solution or from other reagents such as perchloric acid. This problem was circumvented by fitting the kinetic traces with a polynomial function using a non-linear least squares routine to estimate the initial concentrations of HOCl and ClO3\n\u2212 (ORIGIN, 2014). In the case of HOCl, the estimated values were in excellent agreement with the values calculated by considering the dilution of the stock solutions. During the calculation of R, the measured concentration of chlorate ion was corrected by its initial concentration in each point. The uncertainties associated with this procedure are small and coherent results were obtained which are useful to establish the major trends in these reactions.In agreement with the above considerations, the following requirements need to be satisfied by an ideal catalyst. It must accelerate the decay of HOCl and the catalytic process should solely progress via the chlorate path.Preliminary experiments have demonstrated slight catalytic activity with Y2O3, therefore, we explored the role of this species and the related YCl3 in more detail. Typical chromatographic peaks of chlorate ion in the spent reaction mixtures in the absence and presence of the catalysts are shown in Fig. S1. It is quite obvious that approximately the same amounts of chlorate ion are formed in the control experiment and in the presence of Y2O3. The concentration of chlorate ion is considerably smaller with YCl3 which is already a strong indication that this compound promotes O2 formation.Kinetic traces for the decomposition of HOCl at initial pH=7.2 are shown in Fig. 1\n. In the presence of YCl3, the reaction is faster than in the control experiment or with Y2O3 (Fig. 1a). In accordance with Eqs. (2) and (3), the decomposition of hypochlorous acid always generates hydrogen ion and the pH profile as a function of time strongly depends on the acid\u2013base side-reactions in the reaction mixture (Fig. 1b). The pH drops suddenly in the control experiment at around 3000 s because the buffer capacity of the system diminishes when sufficient amount of HOCl decomposes. In the presence of Y2O3 or YCl3, the reaction mixtures \u201cself-buffer\u201d themselves to the pH 5.5\u20136.0 region. In the case of YCl3, the pH decreases below 6.0 much earlier than in the other systems. This is the consequence of the hydrolysis of the catalyst which generates substantial amounts of proton. The kinetic traces in the control experiment and in the presence of Y2O3 are identical for all practical purposes (see also Fig. S2). This confirms the lack of catalytic effects under such conditions. Phosphate ion exerts significant buffer capacity and the pH cannot decrease to the optimum range for the decomposition in the first hour. As a consequence, the reaction proceeds slower than in the control experiment. As expected on the basis of our previous studies, CrO4\n2\u2212 has a marked catalytic effect (Kalm\u00e1r et al., 2018).As shown in Fig. 2\n, the final R is around 4.0 in the control experiment and in the presence of Y2O3, PO4\n3\u2212 and CrO4\n2\u2212. In the case of YCl3, R is above 40 confirming that the O2-path is dominant in the decomposition of hypochlorous. Very similar results were obtained over the entire studied pH range, i.e. when the initial pH was systematically varied from pH 7.5 to 6.15.The actual form of Y(III) has a significant role on the overall process. When an YCl3 solution is added to the neutral or slightly alkaline reaction mixture, a gelous hydroxo precipitate forms immediately. This precipitate is presumably Y(OH)3 or some sort of an oxo\u2013hydroxo precipitate with unknown stoichiometry. The solution becomes opaque indicating the presence of a colloidal system. Accordingly, the precipitate is expected to have a relatively large specific surface area. The morphology and the surface of this precipitate are very different from that of Y2O3. Apparently, the surface of this precipitate is an excellent catalyst of the O2 path, similarly to the hydroxides of other metal ions such as Ni2+, Co2+, Cu2+, etc. In contrast, Y2O3 is a well-defined compound with well-defined surface structure. This may explain the difference between the catalytic activities of these compoundsWhile Y2O3 is not an active catalyst of O2 formation, it slightly catalyzes the chlorate path under slightly acidic conditions\n.\n As shown in Fig. 3\n, the decomposition becomes somewhat faster in the presence of Y2O3 when the pH is decreased.In the control experiments, the usual pH profiles were observed (Fig. S3a). As expected, the sudden pH change occurs at shorter reaction times when the initial pH is lower. The final pH is between 3 and 3.5 in all cases. In contrast, the final pH is between 5.5 and 6.0 in the presence of Y2O3 regardless of the starting pH (Fig. S3b). This implies that Y2O3 consumes the proton formed in the decomposition process in an acid-base reaction.The titration of an aqueous suspension of Y2O3 with HClO4 confirms the existence of a protonation process and, as a consequence, the appearance of a buffered region in the titration curve (Fig. S4). It should be added that the shape of the titration curve is highly dependent on the duration of the titration. Slower dosing of HClO4 results in a longer buffered region in the titration curve indicating that the acid consumption process is relatively slow. In any case, these titrations prove the origin of the buffering effect of Y2O3 in the corresponding experiments. It is possible that the noted catalytic effect (Fig. 3) is a buffering effect in reality. It is well known that the rate of decomposition decreases by decreasing the pH below pH \u223c7.2. Thus, the reaction proceeds at higher rate in the presence of Y2O3 because it does not allow the pH to drop suddenly.The comparison of Fig. 4a and b confirms that the oxygen path is somewhat more pronounced in the control experiments than in the presence of Y2O3. It is quite apparent that the value of R increases more significantly when the starting pH of the control experiments is decreased. This leads to the conclusion that acidic conditions promote the decomposition of HOCl into O2.In the case of telluric acid (Te(OH)6) remarkable results were obtained under slightly acidic conditions (Fig. S5). The kinetic traces of decomposition are compared in the presence of Y2O3, telluric acid and the control experiments. The results in the presence of phosphate ion are not included in the comparison because the buffer capacity of phosphate ion vanish at this relatively low pH. In the presence of telluric acid, about 80% of the initial HOCl decomposes in about 2h. Furthermore, the stoichiometric ratio (R) is about 3 (Fig. 5\n). This excludes that the oxygen path has a significant role in this system. As shown in Fig. S6, telluric acid has a considerable buffer capacity at the studied pH, because the pH decreases only slightly compared to the control experiment. Furthermore, the acceleration of the decomposition is more pronounced with Te(OH)6 than with Y2O3 although the pH converge to about the same value at about 2h reaction time in both cases. Thus, buffering effect alone cannot be responsible for the faster reaction and it is reasonable to assume that a real catalytic process is observed in the Te(OH)6 system.In order to explore whether decomposition is catalyzed or the oxidation of HOCl by telluric acid occurs, the following experiment was designed. First, the decomposition reaction was monitored for 2h. After this period, an aliquot of HOCl was added to the reaction mixture bringing the HOCl concentration and the pH to the initial values. Then, the decay of HOCl was monitored again. The consecutively kinetic traces were very similar (Fig. 6\n), confirming that a real catalytic process takes place.A systematic pH dependent study reveals that the optimum pH of decomposition is at somewhere around 6.7\u20136.9 (Fig. S7).The noted pH effect is most likely associated with the acid\u2013base equilibria of telluric acid. In accordance with literature results, the titration curve of acidic telluric acid solution with NaOH is consistent with two acid dissociation steps (Fig. S8). The corresponding pK\na-s are strongly dependent on the conditions applied. The speciation diagram as a function of pH was calculated by using pK\n1\n=7.5\u00b10.1 and pK\n2\n=9.3\u00b10.1 (Fig. 7\n). The estimated pH values are in good agreement with those available in the literature (Filella and May, 2019; McPhail, 1995). The results confirm that the first deprotonation step coincides with the optimum pH region of the decomposition. This implies that somehow the Te(OH)6 and TeO(OH)5\n\u2212 forms are active in the catalytic process.The comparison of the results obtained in the presence of chromium(VI) and telluric acid reveals that the former is more active catalyst (Fig. S9), but the stoichiometries of the reaction are very similar with both species (Fig. 8\n).The results presented here reveals that the decomposition of HOCl is clearly accelerated by YCl3, Y2O3 and Te(OH)6. By comparing the stoichiometries and relative rates of the catalytic reactions, only telluric acid seems to be a useful candidate to replace Cr(VI). The comparison of the results obtained in the presence of chromium(VI) and telluric acid reveals that the former is more active catalyst, however, its adversary impact on workers health may justify the introduction of Te(OH)6 as an alternative catalyst in the chlorate process. Further studies should be directed toward understanding the details of telluric acid catalysis and exploring effects of this catalyst on the electrochemistry of the industrial process. It is important to note that the higher price of the latter catalyst is an important issue regarding 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.N.L. is indebted to the New National Excellence Program of the Ministry for Innovation and Technology from the source of the National Research, Development and Innovation Fund (\u00daNKP-20-4-II).Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cherd.2021.03.010.The following are the supplementary data to this article:\n\n\n\n\n", "descript": "\n                  By pursuing the aim of identifying new types of catalysts in the electrolytic production of NaClO3 from NaCl (chlorate process), we report a detailed study on the decomposition of hypochlorous acid accelerated by yttrium(III) chloride (YCl3), yttrium(III) oxide (Y2O3) and telluric acid (Te(OH)6) at 80\u00b0C. The results were compared with those obtained in the uncatalyzed and chromium(VI) catalyzed reactions. In general, the decomposition of HOCl occurs via two competing paths toward the formation of ClO3\n                     \u2212 or O2. In the case of YCl3, the decomposition proceeds via the oxygen path over the entire studied pH range. Y2O3 slightly catalyzes the chlorate path under acidic conditions, however the noted catalytic effect is probably due to the \u201cself-buffering\u201d of the reaction mixture (Y2O3 suspension). Although, a real catalytic process takes place in the presence of Te(OH)6, a significant pH-effect is also observed which is most likely associated with the acid\u2013base equilibria of telluric acid. pH dependent studies demonstrate that the optimum pH of decomposition is at around 6.7\u20136.9 in this case. The comparison of the results obtained in the presence of chromium(VI) and Te(OH)6 reveals that the former is a more active catalyst. On the basis of kinetic and stoichiometric results, it is reasonable to assume that Te(OH)6 may be utilized as an alternative catalyst in the chlorate process.\n               "}
{"full_text": "The indole structure is one of most important heterocycles appears in many popular drugs, agrochemicals, advanced organic materials as well as bioactive alkaloids [1\u20137]. Notably, the indole scaffold has been known as an important pharmacophore in medicinal chemistry present in over 3000 natural products and 40 pharmaceutical compounds [1]. Especially, bis(3-indolyl)methane derivatives (BIMs) are highly important molecules due to their presence in the core structure of many bioactive natural alkaloids (arundine, vibrindole A, arsindoline A, barakacin, etc.). They also play an important role in develoment of novel bioactive compounds (anti-inflammatory, anticancer and antimetastatic etc.) [3\u201310].The construction of indole structure from simple building blocks based on the cyclization reactions in the absence or presence of metal catalysts has been well established [11,12]. However, the construction of large molecules containing more than one indole moieties in the structure from simple building blocks is a challenging issue. Due to the importance of BIM derivatives in the development of novel bioactive molecules, many new synthetic methods to synthesize BIMs by using indole derivatives as starting materials have been disclosed [3,6,12].Most of reports based on the direct alkylation of indoles with aldehydes or ketones in the employment of Lewis or Bronsted acids [6,12]. In 2013, Bhaumik et al. reported the preparation of porous organic polymer bearing built-in-CO2H groups and described its utility an efficient heterogeneous carbocatalyst for the alkylation of indoles with benzaldehyde and secondary benzylic alcohol derivatives to form BIMs under room temperature [13]. As a result of developing green and sustainable processes, several new procedures for the preparation of BIMs by the direct transition metals-catalyzed coupling of indoles with a variety of alcohols (including aliphatic alcohols) have been demonstrated [14\u201319]. Grigg and coworkers reported the first isolation of BIM as a side product in the Ir-catalyzed alkylation reaction of indoles with alcohols [14]. In 2012, Liu group disclosed the convenient synthesis of BIM derivatives relied on the Ru-catalyzed reaction of indoles with benzylic alcohols [15]. One year later, Ohta et al. developed a practical Ru-catalyzed alkylation of indole with benzylic alcohols for 24\u00a0h at 110\u00a0\u00b0C [16]. In 2020, Srimani group have just disclosed the ruthenium pincer complex catalyzed transformations of indoles with alcohols to give either C3-alkylated indoles or BIMs through modifying reaction conditions [17]. Hikawa and Yokoyama reported a Pd-catalyzed domino process for the preparation of BIM derivatives involving C3H benzylation of indoles and benzylic CH functionalization in water [18]. In the recent development of cheaper and greener catalysts relying on base metals for the synthesis of BIMs, in 2014, Sekar group reported the first FeCl2/BINAM catalyst in the use of dicumyl peroxide as an oxidant for the synthesis of BIMs in moderate yields [19]. Even though these homogeneous metals catalysts often offer higher yield and selectivity, their practical applications in industrial processes are limited by the difficulty in separation and reuse of catalysts after reactions [20,21]. In addition, the transition metal contamination in desired products could be a serious issue in the pharmaceutical and fine chemical industries [20,21]. To overcome these drawbacks, Babazadeh and coworkers demonstrated the preparation of BIMs under air using Ni nanoparticles supported on ionic liquid-functionalized magnetic silica as a recyclable heterogeneous catalyst [22]. Very recently, a useful method for the synthesis of BIMs by the alkylation reaction of alcohols and indoles in the employment of Fe3O4@SiO2@TPP-Cu as the photocatalyst under blue LED light was described [23]. In general, the procedures for the synthesis of BIM derivatives often require utilization of well-designed catalysts or special operating conditions which are inconvenient in practical applications. In addition, these heterogeneous catalysts only can work well with benzylic alcohols, and aliphatic alcohols remains as challenging substrates in the formation of desired BIM products.In recent years, magnetic nanoparticles have been considered as efficient heterogeneous catalysts in importantly organic transformations due to their outstanding properties, for example, easy recyclability, very low amount of metals leaching and high catalytic activities [24]. Copper ferrite nanoparticles have been known as a practical heterogeneous catalyst for several important organic transformations such as hydrogen transfer, CN, CO, CS coupling, Sonogashira and Click reactions [25\u201329]. An additional advantage of using this catalyst is an easy recyclability by applying of an external magnet [24\u201329]. Therefore, we put our effort in exploring new interesting properties of this catalyst in alkylation reaction of indoles with alcohols as alkylating reagents. Herein, we are reporting an air stable and highly efficient CuFe2O4 catalyst system for alkylation of indoles with alcohols (including challenging aliphatic alcohols) to give BIM derivatives in high yields under mild condition. In addition, this CuFe2O4 catalyst could be considering as a sustainable catalyst due to the non-toxic, easy preparing and recyclable properties which are promising for finding potential applications in pharmaceutical and fine chemical industries.A mixture of Indole (0.3\u00a0mmol), Alcohol (1.2\u00a0mmol), CuFe2O4 (7.2\u00a0mg, 10\u00a0mol%) and LiOtBu (24\u00a0mg, 1 equiv.) were charged in a pressure tube. The pressure tube was immersed in a pre-heated oil bath at 80\u00a0\u00b0C and stirred for 24\u00a0h. After cooling, the reaction mixture was filtered over a plug of Celite with hot water to eliminate the excess of alcohol. The organic phase was washed by Ethyl acetate, then dried by sodium sulfate (Na2SO4). The concentrated residue was purified by column chromatography (Hexane/Ethyl acetate).A mixture of indole (0.3\u00a0mmol), alcohol (2\u00a0mmol), CuFe2O4 (7.2\u00a0mg, 10\u00a0mol%) and LiOtBu (48\u00a0mg, 2 equiv.) were charged in a pressure tube. The pressure tube was immersed in a pre-heated oil bath at 120\u00a0\u00b0C and stirred for 24\u00a0h. After cooling, the reaction mixture was filtered over a plug of Celite with hot water to eliminate the excess of alcohol. The organic phase was washed by Ethyl acetate, then dried by sodium sulfate (Na2SO4). The concentrated residue was purified by column chromatography (Hexane/Ethyl acetate).Our first optimizations for the alkylation of indole with benzyl alcohol were carried out. After screening several conditions using homogeneous copper catalysts (Cu(OAc)2, CuCl2, CuCl), we only obtained a mixture of hydrogen transferred product and bis(3-indolyl)phenylmethane (BIM). Indeed, BIM could be regioselectively formed under lower temperature. In order to maximize the formation of bis(3-indolyl)phenylmethane, other copper sources were examined. Interestingly, in the presence of CuFe2O4 nanocatalyst (10\u00a0mol%), we can achieve 95% yield of BIM product under mild condition (80\u00a0\u00b0C). Then, several nanoparticle oxides (CuO and Fe3O4) were also employed as the catalysts for this reaction, only low yields of the desired product were observed (Table 1\n, entries 1, 2). Therefore, a synergistic effect in the cooperation of iron and copper metals in spinel structure would probably be counted for the high catalytic activity of CuFe2O4 nanocatalyst. In order to investigate the role of base, a series of bases have been examined. While KOtBu, KOH, K2CO3 bases did not give a satisfactory yield, LiOtBu was found to be the most suitable base for the formation of BIM product in highest yield (Table 1, entry 3). In the absence of any bases, only trace amount of BIM product could be formed after 24\u00a0h reaction (Table 1, entry 8). Interestingly, up to 84% yield of BIM product was formed in the presence of only 5\u00a0mol% of CuFe2O4 catalyst under the optimized reaction (Table 1, entry 7). This alkylation reaction was carried out at lower temperature, i.e. 60\u00a0\u00b0C, only a trace amount of BIM was observed. Interestingly, up to 93% isolated yield of BIM product was formed within only 12\u00a0h reaction. In this research, we believe that oxygen should be the oxidant in the oxidation step of alcohols to corresponding aldehydes. A control experiment was carried out in argon which being resulted in a very low yield of BIM product (Table 1, entry 12).With the optimized condition in hand, we like to explore the scope of this reaction. First, a series of benzylic alcohol derivatives were used in the alkylation of indole. In general, BIM derivatives with the tolerance of both electron donating and withdrawing groups were prepared in good to excellent isolated yields (Table 2\n, compound 1\u20139). Interestingly, among these products, turbomycin B alkaloid and an anticancer agent were easily prepared in 95% and 78%, respectively (compounds 1, 4). The reactions of indole with p-trifluoromethylated benzylic alcohol and bulky 2-napthyl alcohol also afforded to corresponding products in 85% and 90% yields, respectively (compounds 10, 11). Notably, we could extend the scope of this reaction with aliphatic alcohols as well. In fact, aliphatic alcohols are challenging alkylating agents compared to benzylic alcohols due to the difficulty in their in-situ oxidation to the corresponding aldehydes. To the best of our knowledge, so far, there are no heterogeneous catalyst systems which can work properly in the alkylation reaction of indoles with aliphatic alcohols. Remarkably, up to 87% yield of the desired products (Table 2, compounds 12\u201315) containing aliphatic alkyl groups were obtained under harder condition (120\u00a0\u00b0C, 24\u00a0h). In addition, the CuFe2O4 catalyst also showed very good perfomance in the alkylation of indole derivatives with benzylic alcohol. Indoles bearing with methyl, bromine, chlorine and flourine groups were very well tolerated which gave high yields of the desired products (up to 93% yield) (Table 3\n). Attractively, an antileukemic agent was successfully prepared in 72% isolated yield (compound 20).Due to i) control experiments in Table 1 (entries 8, 12) demonstrated the essential role of base and oxygen in the transformation and ii) large amount of aldehyde was formed in the course of reaction, we would like to proposed a plausible mechanism as presented in Fig. 1\n. The conversion is initiated by the adsorption of the alcohol on the surface of CuFe2O4 nanoparticles via the lone pair electron on oxygen atom of alcohol (structure A in Fig. 1) [30]. However, due to the weak interaction of this coordination, high activation barriers are usually required to activate the OH bond, as reported for this type of reaction on copper-based catalyst [31]. Therefore, this step is more feasibly facilitated via the proton-transfer reaction with the LiOt-Bu base. After the initial structure A has been activated, the generated alkoxy could be easily converted on metal oxides surfaces (intermediate B in Fig. 1) [32], generating the key aldehyde intermediate (II) and metal hydride species C. The hydride strongly bound to the surface of CuFe2O4, but subsequently is converted to metal hydroxy (M-OH (M\u00a0=\u00a0Cu or Fe)) by combining with molecular oxygen [33]. This step is very important in stabilizing the structure of metal oxide during the reaction, since the accumulation of surface hydrogen on the surface of metal oxides usually result in the fast reducing of metal oxides to metallic phases and lost the activity as were reported in our earlier integrated theoretical and isotope-labelling studies [30,32]. Furthermore, the surface hydroxyl group of those metal hydroxy can also facilitate the OH bond activation of the starting alcohol material via the low barrier hydrogen-abstraction reaction (intermediate E in Fig. 1), regenerating the metal alkoxy intermediate and H2O [30,31]. Indeed, the generation of H2O was also proposed by several previous reports. When aldehyde (II) was formed, it easily reacts with indole to give intermediate III which is converted to give intermediate IV via dehydration process. Finally, the most important step for the overall conversion is the 1,4-addition reaction between the intermediate IV and the indole to form BIM product (via intermediate F). Without the catalyst, the barriers of these type of reaction were reported as extremely high of ~46\u00a0kcal/mol in our earlier studies [32], hindering those reaction to be processed. The metal oxides surface therefore could stabilize the transition state of this reaction via coordinating with the lone pair electron of N atom and reduce the barrier of this step by 20\u201330\u00a0kcal/mol, making this reaction much more feasible [32]. Then, the formation of intermediate G is possible when the activated intermediate IV reacts with another indole molecule via 1,4-addition reaction. The last step is the desorption of BIM product from the surface of catalyst. Therefore, the crucial role of the bifunctional catalyst of CuFe2O4 is to facilitate the formation of key intermediate aldehyde and then drive the 1,4-addition reaction via the lower barrier pathway.The main advantage of heterogeneous catalysts to homogeneous catalysts is the stability and recyclability. Notably, by applying of an external magnet, the CuFe2O4 catalyst could be recycled at least five times without losing significant catalytic activity for the standard alkylation of indole using benzyl alcohol (Fig. 2\n). However, the catalytic activity of CuFe2O4 was gradually decreased after each run due to the aggreation of nanoparticles, as was evidenced by the TEM analysis. The TEM image of the fresh catalyst showed the presence of spherical CuFe2O4 nanoparticles with an average particle size of 27.56\u00a0\u00b1\u00a01.11\u00a0nm (Fig. 3A). The inter planar spacing of 0.258\u00a0nm was visualized by its HRTEM image which was characterized for the (311) crystal planes of cubic spinel CuFe2O4 structure (Fig. 3A, inset) [34]. However, the TEM image of the reused catalyst indicated that these CuFe2O4 nanoparticles were aggregated to much larger particles of 164.41\u00a0\u00b1\u00a04.82\u00a0nm (Fig. 3B). Furthermore, beside the presence of (311) crystal plane on the reused catalyst (corresponding to the inter planar spacing of 0.256\u00a0nm) [34], there was a new appearance of (220) crystal plane in the HRTEM image of the reused catalyst which was characterized by the inter planar spacing of 0.294\u00a0nm [35] (Fig. 3B, inset). This observation might imply that there were structural changes during the aggregation, resulting in the formation of less active (220) surface on the reused catalyst. As reported earlier in the study of Singuru et al. [32], the barriers for both CH and NH bonds activations on the (220) facet of metal oxides are significantly higher than on the (311) facet. Thus, two reasons for the gradual decrease of activity from the TEM analysis: (i) the nanoparticles were aggregated to larger particles during the reaction, resulting in the decrease of active areas and (ii) structural changes occurred during the nanoparticle aggregation, inducing the formation of less active (220) facet and consequently the catalytic activity.The XRD results were in good agreement with the TEM analysis. As shown in Fig. 1S (see supplementary material), the fresh CuFe2O4 catalyst displayed typical, resolved peaks at 2\u03b8\u00a0=\u00a030.1\u00b0, 35.6\u00b0, 43.1\u00b0, 57.1\u00b0, 62.7\u00b0 which can be attributed to Miller indices (220), (311), (400), (511) and (440) reflections of cubic spinel structure of CuFe2O4 [25\u201329]. After five cycles, the reused CuFe2O4 catalyst showed identical reflections compared to the fresh one, suggesting that the crystalline spinel structure was well preserved. Moreover, the reflections of the reused catalysts looked more pronounced with respect to those of the fresh one, confirming the formation of larger CuFe2O4 nanoparticles by the aggregation. The SEM and EDX images of the fresh and reused CuFe2O4 samples were also taken (Fig. 2S, supplementary material). The fresh CuFe2O4 sample exhibited fine, regular particles while the reused sample showed bigger and irregular ones, further confirming the occurrence of particle aggregations. However, the EDX images of both fresh and reused CuFe2O4 catalysts showed well-dispersion of inherent elementals, i.e. Cu, Fe and O in the studied catalysts, suggesting that the elemental composition was not changed during the reaction.We have reported the application of an active and stable heterogeneous CuFe2O4 nanoparticles catalyst for the efficient alkylation of indoles with a series of alcohols to form BIM derivatives under mild operating conditions in air. Especially, we successfully figured out a good solution for the utilization of challenging aliphatic alcohols as alkylating reagents in the alkylation reaction of indoles which resulted in high yields of the desired products. O2 in air was confirmed to be the oxidant in the oxidation of alcohol to aldehyde. The CuFe2O4 catalyst is highly stable and can be recycled at least five times without losing the significant catalytic activity. Several applications of the CuFe2O4 catalyst were described for the synthesis of various important products, such as turbomycin B alkaloid, anticancer and antileukemic agents in high yields. This procedure would be considerable interest for exploring synthetic applications in medicinal chemistry. Further studies to understand the mechanistic insights of this reaction and actual role of CuFe2O4 catalyst are in progress.Ngoc Khanh Nguyen, Ha Minh Tuan and Bui Hoang Yen: synthesis and investigation; Quang Thang Trinh, Bich Ngoc Tran: characterization, data analysis and writing; Van Tuyen Nguyen: supervision and reviewing; Tran Quang Hung, Tuan Thanh Dang and Xuan Hoan Vu: conceptualization, supervision, data analysis, 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 is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.01-2017.320. The authors would like to thank Dr. Duc-The Ngo, University of Manchester (UK) for his helpful instruction and discussion in performing the HRTEM measurement and analysis.\n\n\n\nSupplementary material\n\nImage 2\n\n\n\nSupplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2020.106240.", "descript": "\n                  \n                     Bis(3-indolyl)methanes (BIM) are highly valuable and appear in the core structure of many natural products and pharmacologically active compounds (anticancer, anti-inflammatory, antiobesity, antimetastatic, antimicrobial, etc.). Herein, we have disclosed an air stable and highly efficient CuFe2O4 heterogeneous catalyst for alkylation of indoles with alcohols to give bis(3-indolyl)methanes in very good yields. The CuFe2O4 catalyst has been found to be magnetically recycled at least five times without losing significant catalytic activity.\n               "}
{"full_text": "Hydrogen, as an energy carrier, has potential to provide inexhaustible energy because it has massive reserves on Earth [1\u20133]. In addition, because hydrogen has a series of outstanding advantages such as environmental friendliness and three times higher calorific value than oil, it is considered to be an attractive alternative to fossil fuels in the field of transportation [4\u20136]. Major industrial countries have made detailed plans for the wide use of hydrogen fuel cell vehicles. It appears that one day our environmental problems will be solved and we will no longer worry about exhausting our limited fossil fuels. However, the low volumetric energy density of gaseous hydrogen is unacceptable [7]. The main technical hurdle in automotive and portable electronic devices is the design of hydrogen storage systems with high efficiency, non-hazardous and low cost [8\u201312]. There are three storage states of hydrogen, namely gaseous, liquid and solid [13]. At present, to obtain 4.8 wt.% of hydrogen storage capacity, the needed hydrogen pressure is as high as 70\u00a0MPa [14]. There are two disadvantages in the storage of gaseous and liquid hydrogen: short storage time and high evaporation loss [15]. Obviously, both of them have safety issues and technical challenges. Solid hydrogen storage is widely studied for its high energy storage density and safety, and is considered to be the ideal solution to the problem of hydrogen storage and transportation [16\u201318]. A large number of solid hydrogen storage materials have been developed in recent decades. However, the capacity of the currently developed hydrogen materials is too low and the cycle performance is too poor to satisfy the commercial requirements of vehicle fuel cells. MgH2 is seen as a front-runner in the commercialization of hydrogen storage materials, in particular with coming up with hydrogen absorption and desorption capacities, abundant reserves and non-toxicity [19\u201322]. Unfortunately, the application prospect is severely hampered because of its slow hydrogen absorb/desorb rate, activation problem and high hydrogen releasing temperature [23,24]. In recent years, the decomposition temperature and kinetic properties of MgH2 have been studied extensively by scientists due to many attractive advantages, and some achievements have been obtained [4,25\u201331].In terms of preparation techniques, high energy ball milling and introducing additives have been testified to be a beneficial method to dramatically improve the hydrogen storage performance of Mg, which is owing to the reduction of particle size, the increase of active sites and crystal defects [32,33]. In the last decades, lots of literatures have reported the strong improvement effects of introducing additives on the kinetics of MgH2. Common additives include transition metals [34,35] and their oxides [36,37], halides [38,39] and hydrides [40], and rare earth elements [41,42] and their compounds [43,44]. Improvements in hydrogen storage properties are found to be associated with the decreased dissociation energy of hydrogen molecule and the weakened Mg-H bond energy [45]. After introducing additives in MgH2, it can be achieved successfully that very fast hydrogen absorption/desorption kinetics behavior (around minutes or seconds) and a significant reduction on dehydrogenation temperature (around 523\u00a0K) [46].Ti, Ni and their compounds often are used as the additives. Research showed that adding the additives can improve the hydrogen storage properties of the alloys. For example, Ti [47], TiH2\n[48], TiO2\n[49] and TiF3\n[50] can improve the hydrogenation kinetics significantly. Wronski et\u00a0al. [51] found that doping 5 wt.% of nano-nickel in MgH2 can reduce the hydrogen absorption temperature of MgH2. The alloy can be hydrogenated at 523\u00a0K. Compared with the original alloy, the reaction temperature dropped by 90 K. Jin et\u00a0al. [52] reported that NbF5 and TiF3 were better additives. MgH2 reacted with the transition metal fluorides to form corresponding hydrides, which catalyzed the subsequent reaction of MgH2. In addition, fluorine has high electronegativity, which is beneficial to weakening the strength of Mg-H bond.Our previous studies have shown that adding rare earth elements can improve the dynamic properties of magnesium alloys [53,54]. Therefore, in this study, the property of magnesium alloys is expected to be improved by partly replacing Mg in the alloy with Ni, Ce and ball milling with TiF3. At present, there are few relevant studies. In addition, it has been reported that temperature has important effect on the performance of the function materials [55\u201357]. Therefore, the hydrogen storage performances of the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys at different temperature also have been studied.The Ce5Mg85Ni10 alloy was prepared through induction melting with cerium, magnesium and nickel (purity \u2265 99.9%) as raw materials. Metals Ce and Mg will volatilize during melting. Therefore, extra 5 wt.% Ce and 8 wt.% Mg were added to compensate for the loss. The composition of the alloys was examined by an inductively-coupled plasma system (ICP) (Agilent ICPOES730). Before ball milling, the ingots were crushed and ground mechanically to powders of 200\u2013400 mesh. The received powders and 3 wt.% TiF3 were milled together in a planetary ball mill (QM-3SP2 type; produced by Nanjing Chi Shun Technology Development Co., Ltd). The weight ratio of ball to powder was 40:1 and the rotating speed was set at 350\u00a0rpm. In the milling process, the mill was interrupted for 30\u00a0min after running every 30\u00a0min for dissipating heat and reducing clustering of powders. After milling for 5 h, the mill was stopped for scraping the powders adhered to the mill chamber walls and grinding balls (material: stainless steel). A glove box (lab2000 type; produced by Etelux Inert Gas System (Beijing) Co., Ltd) full of a protective atmosphere of argon was used for all these operations. The oxygen and moisture in the glow box were less than 1\u00a0ppm.The phase compositions of the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10\u00a0+\u00a03 wt.% TiF3 (named as Ce5Mg85Ni10+3TiF3) alloys before absorbing hydrogen and after releasing hydrogen were examined by X-ray diffraction (XRD) (D/max/2400). The morphology of the alloy particles was observed by scanning electron microscope (SEM) (QUANTA 400). High-resolution transmission electron microscopy (HRTEM) (JEM-2100) was used to characterize the crystal structure.Isothermal kinetics of the hydrogenation and dehydrogenation of the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys were tested through an automatic Sieverts apparatus with \u00b11 K of the temperature accuracy. The mass of each sample was 300 mg. Before the experiment, the samples were activated four times under the condition of 3\u00a0MPa H2 for hydrogenation and 1\u00a0\u00d7\u00a010\u22124MPa H2 for dehydrogenation at 633 K. The hydrogenation was conducted at 423, 473, 533, 553, 573, 593, 613 and 633 K at 3\u00a0MPa H2, and the dehydrogenation was conducted at 533, 553, 573, 593, 613 and 633 K at 1\u00a0\u00d7\u00a010\u22124MPa H2, respectively. In general, the heating rate has an effect on the nonisothermal dehydrogenation performance of the samples. Therefore, with the heating rates of 5, 10, 15 and 20\u00a0K min\u22121, the thermogravimetry (TGA) and differential scanning calorimetry (DSC) (SDTQ600) were used to study the dehydrogenation performances of the samples.The constituents and evolutions of the un-hydrogenated, hydrogenated and dehydrogenated alloys (as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys) are subject to identify by XRD, as illustrated in Fig.\u00a01\n. The hydrogenated and dehydrogenated samples are prepared at 633\u00a0K in 3 and 1\u00a0\u00d7\u00a010\u22124MPa H2 respectively. The incisive and narrow diffraction peaks of the as-cast Ce5Mg85Ni10 alloy show it is a typical crystal structure, in which three phases CeMg12, Mg and Mg2Ni can be detected. After mechanical milling, the diffraction peaks become wider caused by the increase of internal strain and decrease of grain size, suggesting the samples belong to a nanocrystalline structure [58]. The diffraction peaks have significant changes after hydrogenation. Three hydrides, viz. MgH2, Mg2NiH4 and CeH2.73, are viewable in the as-cast and milled Ce5Mg85Ni10 alloys. We have carried on a suitable conjecture of the following path after the XRD analysis:\n\n(1)\nCeMg12+Mg+Mg2Ni+H2\u2192MgH2+Mg2NiH4+CeH2.73\n\n\n\nThe XRD results show that there are five phases in the TiF3-added alloy after hydrogenation: MgH2, Mg2NiH4, CeH2.73, TiH2 and MgF2. The way to produce these substances can be described as:\n\n(2)\nCeMg12+Mg+Mg2Ni+TiF2+MgF2+H2\u2192MgH2+\u202fMg2NiH4+CeH2.73+TiH2+MgF2\n\n\n\nAfter dehydriding, three phases appear in the as-cast sample, viz. Mg, Mg2Ni and CeH2.73, which may be formed by the following reaction:\n\n(3)\nMgH2+Mg2NiH4+CeH2.73\u2192Mg+Mg2Ni+CeH2.73+H2\n\n\n\nFive phases appear in the as-milled Ce5Mg85Ni10+3TiF3 alloy after dehydriding, viz. Mg, Mg2Ni, CeH2.73, TiH2 and MgF2, which can be described by the following reaction:\n\n(4)\nMgH2+Mg2NiH4+CeH2.73+TiH2+MgF2\u2192Mg+Mg2Ni+CeH2.73+TiH2+MgF2+H2\n\n\n\nIt is obvious that CeH2.73, TiH2 and MgF2 phases keep unchanged. The reason for this phenomenon is their high thermostability [59]. It allows us to conclude that the reversible reactions occurring in the alloys are Mg+H2\u2194MgH2 and Mg2Ni+H2\u2194Mg2NiH4.The microstructure evolutions of the un-hydrogenated, hydrogenated and dehydrogenated alloys (as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys) are examined by HRTEM, as illustrated in Fig.\u00a02\n. Nanocrystalline and some crystal defects including grain boundary, phase boundary, lattice distortion zone and dislocation can be seen clearly. The addition of TiF3 significantly changes the microstructure of the alloys. The interplanar spacing of Mg grains is measured using Digital Micrograph software, which is 0.167 nm (the average value of five adjacent atomic planes), as marked in Fig.\u00a02(a). By comparing the data in PDF cards, it can be determined that the adjacent atomic planes belong to (110) crystal plane of Mg phase. The other crystalline phases also are identified using the same method. It is found that there are three phases of CeMg12, Mg and Mg2Ni in the as-cast and milled Ce5Mg85Ni10 alloys, and the addition of TiF3 creates two phases of TiF2 and MgF2. After hydrogenation, the density of crystal defects (especially the lattice distortion region) increases significantly, which is assigned to the enlarged lattice caused by absorbing hydrogen atoms. With the aid of the index of ED rings, three hydrides (MgH2, Mg2NiH4 and CeH2.73) can be easily found in the hydrogenated as-cast and milled Ce5Mg85Ni10 alloys, and a new hydride TiH2 can be found in the hydrogenated Ce5Mg85Ni10+3TiF3 alloy. After dehydrogenation, Mg, Mg2Ni and CeH2.73 phases appear in the as-cast and milled Ce5Mg85Ni10 alloys. The result is consistent with that of XRD. The hydrides CeH2.73 and TiH2 in those alloys remain unchanged during the dehydrogenation process. The CeH2.73 (or TiH2)/Mg (or MgH2) interfaces provide special channels for the rapid diffusion of hydrogen atoms, which is beneficial to improving the kinetics. Besides, the containing-Ti additives can reduce the thermodynamic stability of MgH2, which is mainly due to the fact that the bond energy of Mg-H is able to be weakened under the action of the stronger interaction of Ti-H [60]. It is reported that the dehydrogenation enthalpy of the formed MgH2-0.1TiH2 system is 68 kJ mol\u22121 H2, which is lower than that of pure MgH2\n61.It is generally believed that the surface state of the alloy has great influences on the decomposition of hydrogen molecules [62]. A cracked and coarse surface is deemed to be beneficial in imparting faster hydrogen absorption rate [63]. Fig.\u00a03\n shows the morphology of the un-hydrogenated, hydrogenated and dehydrogenated alloys (as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys). The alloy particles produced by mechanical crushing have a very smooth surface and the particle size ranges from 20 to 80\u00a0\u00b5m. Ball milling makes the particle dimension decreased significantly. The addition of TiF3 has an insignificant effect on the particle size, but it engenders an obvious change in the surface state of the as-milled particles. It can be seen that there are much more cracks on the particles with TiF3 additive and their surfaces become more rough.After absorbing hydrogen, the morphology of the particles has undergone tremendous changes, as shown in Fig.\u00a03(b), (e) and (h). Numerous cracks appear on the surface. They are caused by the increase of lattice stress. As discovered by Antisari et\u00a0al. [64], the cell volume of magnesium is about 33% smaller than that of MgH2. The hydrogen storage materials will inevitably undergo lattice distortion [65,66], expansion and contraction of the unit volume, thereby introducing lattice stress and lattice defects in the alloys. The accumulation of lattice stress rapidly grows with the increase of hydrogen atoms in the space lattice. Alloy pulverization will occur when it exceeds the maximum value that the lattice can withstand. The morphology of the dehydrogenated alloy particles is similar with that of the un-hydrogenated alloy particles except the increased cracks on the surfaces, which indicates that most of the hydrides in the hydrogenated alloys have been decomposed.Usually, the alloy powders prepared by conventional mechanical crushing and grinding have difficulty in absorbing hydrogen at the beginning stage. This happens because the alloys tend to form an oxide film on the sample surface when they are exposed to air. The oxide film hinders the dissociation process of hydrogen molecules. Fortunately, it has been found that the oxide film can be destroyed when the alloys are subject to the proper temperature and hydrogen pressure. During the process, some fresh surface will appear and the alloys obtain the hydrogen absorption ability.The activation curves of the experimental alloys are demonstrated in Fig.\u00a04\n. In order to achieve the complete activation state, the alloys are hydrogenated and dehydrogenated four cycles. It is evident that there is no obvious incubation period for the first cycle under the activation conditions, suggesting that the experimental alloys have good activation property. The first hydriding curves of the experimental alloys are presented in Fig.\u00a04(d). It reveals that ball milling and adding TiF3 have apparent effects on the hydrogenation rate of the alloys in the activation process. The needed time for absorbing 4 wt.% hydrogen is 8944, 6163 and 2852 s for the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys, respectively.Research showed that it was difficult for pure Mg to react with hydrogen at low temperature, even with clean Mg surface [67]. It is because that the dissociation energy of hydrogen molecules on magnesium surface is very high. For the as-cast Ce5Mg85Ni10 alloy, the distributed Ni and RE elements have good catalytic activity, which can greatly reduce the dissociation energy mentioned above and promote the hydrogenation reactions [68,69]. For the as-milled alloys, the improved activation ability is definitely ascribed to the modification of surface state caused by ball milling and adding TiF3. The microstructure analysis has proved that ball milling can reduce the particle size, produce reactive clean surfaces and form crystal defects. The reduction of particle size means the increase of the reaction interfaces between hydrogen molecules and the alloy particles. In addition, hydrides are easier to nucleate at the positions of crystal defects. Therefore, the surface activity of the alloys becomes better after ball milling and adding TiF3. The formed MgF2, TiH2, and CeH2.73 nanoparticles strengthen the above effect [70].The dehydriding activation curves of the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys are given in Fig.\u00a05\n. The curves for the last three cycles almost overlap, suggesting that the alloys have been completely activated after the first hydrogen absorb and desorb cycle. The evolution of the first desorption curves of the experimental alloys is given in Fig.\u00a05(d), from which the time required for desorbing 3 wt.% hydrogen can be obtained. It is 90, 60 and 49 s for the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys, respectively. It suggests that ball milling and adding TiF3 can evidently improve the hydrogen desorption kinetics of the alloys because of the following factors. First, TiF3 and TiF2 can promote the decomposition of MgH2. XRD analysis has proved that TiF2 is generated in the process of ball milling the as-cast alloy with TiF3. It should be noted that there are probably a small number of TiF3 in the alloy, which cannot be found in the XRD analysis due to the less content. The roles of TiF3 and TiF2 are similar. Lu et\u00a0al. [61] also mentioned that the Mg-H bond energy in the Mg-Ti-H system was relatively weak. Song et\u00a0al. [71] published a theoretical prediction about the reaction enthalpy of MgH2-Ti system. This prediction supported that the above-mentioned thermodynamic change resulted from the weakened Mg-H bond energy. Second, the result in Fig.\u00a03 indicates that hydrogen atoms enter the alloy will cause new cracks, resulting in the increase of specific surface area of the particles and prompting the dehydrogenation kinetics of the alloy. Third, the dispersed MgF2 and TiH2 can provide some sites for the recombination of hydrogen atoms [72].The hydrogen absorption kinetics of the activated as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys are tested at temperatures ranging from 423 to 633\u00a0K under 3\u00a0MPa H2 pressure, as illustrated in Fig.\u00a06\n. It is clear that the hydriding rate is quite fast at the initial stage, and hydrogen absorption capacity can reach over 85% of the saturated capacity in less than 200\u00a0s except for 423\u00a0K. After that, it takes a long time to reach saturation. The hydrogenation curves at 423\u00a0K are compared and the needed time for absorbing 4 wt.% hydrogen is 984, 517 and 228 s at 423\u00a0K for the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys, respectively. It is found that ball milling and adding TiF3 effectively heighten the hydrogen absorption kinetics of the experimental alloys. The phase structure change during the hydrogenation process (3\u00a0MPa H2, 633\u00a0K) is analyzed through XRD pattern shown in Fig.\u00a06(d). The result shows that the hydrogenated alloy includes MgH2, Mg2NiH4 and CeH2.73 phases, and the relative content is 59.8%, 24.1% and 16.1%, respectively. The results show that the alloy is to be hydrogen-saturated.There are three steps to generate MgH2, viz. (a) hydrogen molecule decomposes into atoms on the surface of the alloy; (b) hydrogen atoms enter the alloy through defects on the surface of the alloy; (c) Mg converts to MgH2 at the interfaces additive/magnesium. The surface of the alloy is one of the main keys to hydrogen absorption kinetics. The schematic diagram in Fig.\u00a07\n shows the mechanism for hydrogenation of the alloy with TiF3. On the one hand, hydrogen molecules are inclined to adsorb on the TiF3 and TiF2 sites with high activity, which reduces the dissociation energy of hydrogen molecule and speeds up the splitting process. On the other hand, adding TiF3 during the period of ball milling makes the grain of the alloy considerably refined and the crystal defect density enormously increased, which has a strong promoting effect on the diffusion of hydrogen atoms. As hydrogen molecules decompose and hydrogen atoms diffuse, the hydrogen concentration quickly reaches the critical concentration required to the formation of metal hydrides. The formed hydride layer hinders the diffusion of hydrogen in the alloy [59]. Hydrogen diffusion in the hydride layer will stop when the thickness of hydride layer is more than 30\u201350\u00a0\u00b5m [73]. MgH2 phase only grows slowly through the interfaces of Mg and Mg hydride [74].The TGA and DSC curves of the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys are used to study the stability of the generated hydrides, as provided in Fig.\u00a08\n. The hydrogenated alloys (at 633\u00a0K and 3\u00a0MPa H2) are tested in a closed space with the heating rate of 5\u00a0K min\u22121. It is found that ball milling with TiF3 engenders a significant impact on the onset dehydrogenation temperature, which is 544.6, 541.3 and 525.6\u00a0K for the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys, respectively. The initial dehydrogenation temperature is usually used to show the stability of hydrides. The experimental results reveal that ball milling with TiF3 decreases the initial dehydrogenation temperature of the alloys distinctly. It is found by Berube et\u00a0al. [75] that ball milling can lower the stabilization of the hydrides, the premise of which was that the alloy particle size was small enough. Whereas, the positive contribution of ball milling to the reduction of the stability of hydride is very limited in this experiment since the particle size is much greater than nanometer scale. As for the positive contribution of adding TiF3 to lessen the steadiness of hydrides, it is believed to be relevant to MgF2 and TiH2 phases that originated from TiF3 phase and Mg phase, which can act as the nucleation centers of dehydrogenation phases. Proper additives can facilitate surface reactions, and some researchers believed that the dehydrogenation reaction was limited by surface reactions [76]. The unsaturated d/f electron shells of transition metals can react with valance electrons of H so as to weaken the Mg-H bond energy [77].The hydrogen desorption kinetics of the hydrogenated as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys are tested at temperatures ranging from 533 to 633\u00a0K under 1\u00a0\u00d7\u00a010\u22124MPa H2, as illustrated in Fig.\u00a09\n. The experimental results show that the alloy can achieve complete dehydrogenation state even at 533\u00a0K and has good kinetic property. The reasons have been given in the section of activation behaviors (see the description and explanation about Fig.\u00a05). Meanwhile, the experimental results show that temperature has a great influence on the kinetics of hydrogen desorption, and higher temperature is conducive to the process of desorbing hydrogen. The time required for desorbing 3 wt.% hydrogen at 533, 553, 573, 593, 613 and 633\u00a0K is 1410, 390, 185, 108, 72 and 48\u00a0s for the as-cast Ce5Mg85Ni10 alloy, and 906, 324, 180, 102, 66 and 42\u00a0s for the as-milled Ce5Mg85Ni10 alloy and 724, 286, 161, 89, 58 and 36\u00a0s for the as-milled Ce5Mg85Ni10+3TiF3 alloy, respectively. The results show that ball milling and the addition of TiF3 can accelerate the hydrogen desorption kinetics of the alloy at the same temperature, but as the temperature increases, this positive effect weakens gradually. The phase structure change during the dehydrogenation process (1\u00a0\u00d7\u00a010\u22124MPa H2 and 633\u00a0K) of the dehydrogenated as-cast Ce5Mg85Ni10 alloy is analyzed through XRD pattern showed in Fig.\u00a09(d). It can be seen that there are Mg, Mg2Ni and CeH2.73 phases in the dehydrogenation alloy. The mass percentage of each phase is 59.2%, 25.1% and 15.7%, respectively. Obviously, the rare earth hydride CeH2.73 cannot decompose under the experimental temperature and pressure.The dehydrogenation of MgH2 can be divided into three steps as well: (a) Mg nucleates and grows at the defect sites; (b) hydrogen atoms diffuse from the decomposed MgH2 to the particle surface; (c) the combination of two adjacent hydrogen atoms into one hydrogen molecule [78]. As mentioned above, the main reason for the choice of ball milling method is that ball milling can reduce the size of alloy particles, increase specific surface area and form micro nanostructure and many defects. Adding TiF3 can weaken the Mg-H bond energy, thus enhancing the dehydrogenation behavior of MgH2\n[79]. The schematic diagram of the action mechanism of TiF3 for dehydrogenation is described in Fig.\u00a010\n. Firstly, the electronegativity of Ti is 1.8, which is between that of H (2.2) and Mg (1.3). Therefore, adding TiF3 can weaken the Mg-H bond energy sufficiently. Ti ions gain electrons more easily than Mg2+. Compared with H\u22121, Ti ions are more likely to lose electrons. Secondly, Ti ions with different valence states are prone to transformation. Hence, the effect of TiF3 on hydrogen desorption of the metal is as follows: (a) The H\u2212 on the Mg-H bond passes one electron to Ti3+, and H\u2212 loses one electron and goes to the free state H0, then Ti3+ gets one electron and turns into low valency Ti2+. (b) SinceTi ions in different valence states are easy to be transformed, Ti2+ can spontaneously lose one electron and transform into Ti3+. At the same time, Mg2+ obtains two electrons and form Mg; (c) free state H0 diffuses to the surface of the alloy and forms H2\n[80]. This catalytic electron transfer process is much easier than the direct transfer of electrons from H\u2212 to Mg2+. Therefore, the activation energy of dehydrogenation is diminished.To verify the above conclusions, apparent activation energies of the dehydrogenation reactions of the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys are evaluated by the Arrhenius and Kissinger method. Apparent activation energy is usually used to represent the energy hurdle that needs to be overcome in a gas-solid reaction. Therefore, the apparent activation energy can be used to represent the minimum level of the reactions that occur in the system. The main driving mode of dehydrogenation of Mg-based alloys is nucleation and growth [42]. Generally, these solid-state reactions can be simulated by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory [81] through the following equation:\n\n(5)\n\n\nl\nn\n\n[\n\n\u2212\nl\nn\n\n(\n\n1\n\u2212\n\u03b1\n\n)\n\n\n]\n\n=\nh\nl\nn\nk\n+\nh\nl\nn\nt\n\n\n\n\nIn this equation, the proportion of MgH2 transformed into Mg in fixed time t is represented by \u03b1, k and \u03b7 are the kinetic parameter and Avrami exponent respectively. Referring to the data in Fig.\u00a09, the relationships between ln [-ln(1-\u03b1)] and lnt at 573, 593, 613 and 633 K are plotted in Fig.\u00a011\n. The apparent activation energy E\na can be derived from the following equation 42:\n\n(6)\n\n\nk\n=\nA\ne\nx\np\n\n[\n\n\n\u2212\n\nE\na\n\n\n/\n\n(\n\nR\nT\n\n)\n\n\n]\n\n\n\n\n\nThe above formula is the Arrhenius equation, in which A, R and T respectively represent temperature-independent coefficient, gas constant and absolute temperature of the reaction, and k has the same definition as above. The obtained apparent activation energy (E\na) of the experimental alloys is provided in Fig.\u00a011(d). It is obvious that ball milling and adding TiF3 markedly decrease the apparent activation energy E\na of the alloys. Data analysis shows that E\na of MgH2 is much higher than that of the experimental samples, because the addition of Ni in the sample improves the catalytic activity of the alloy surface. Therefore, the in-situ formed Mg2Ni/Mg2NiH4 has a good catalytic effect on Mg-based alloys [82]. There are also sources that Ce has a good effect on MgH2 dehydrogenation. Mustafa [83] reported that the reduction of E\na significantly promotes the decomposition of MgH2. Hou et\u00a0al. [84] found that adding a suitable catalyst is an effective strategy to reduce the E\na of MgH2.The DSC curves at different heating rates are also tested for comparison with the JMAK model. The DSC curves are shown in Fig.\u00a012\n and the apparent activation energy (E\nk) of hydride decomposition is determined by Kissinger equation [85]:\n\n(7)\n\n\n\n\nd\n\n[\n\nl\nn\n\n(\n\n\u03b2\n/\n\nT\np\n\n/\n\nT\np\n\n\n)\n\n\n]\n\n\n/\n\nd\n\n(\n\n1\n/\n\nT\np\n\n\n)\n\n\n\n=\n\u2212\n\n\nE\nk\n\n/\nR\n\n\n\n\n\nIn Eq.\u00a0(7), \u03b2 and T\np are heating rate and absolute temperature corresponding to the peak of DSC curves, respectively. R has the same definition as Eq.\u00a0(6). The graphs of ln(\u03b2/T\np/T\np) vs. 1/T\np can be charted through Eq.\u00a0(7), as illustrated in Fig.\u00a012. E\nk values of the alloys can be obtained by the slope of the Kissinger diagram, as presented in Fig.\u00a012(d), which is very close to the results in Fig.\u00a011(d). The above results allow us to believe that the heightened dehydrogenation kinetics caused by ball milling and adding TiF3 is assigned to the decrease in dehydrogenation activation energy.The thermodynamic parameters of the hydrides are important indicators to assess their stability. The main problem of practical application is the high thermal stability of MgH2. Here, P-C-T curves of the experimental alloys are gauged at 573, 593, 613 and 633\u00a0K in order to inspect the thermodynamics of the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys, as presented in Fig.\u00a013\n. Each P-C-T curve has two platforms. The high platform corresponds to the formation and decomposition of Mg2NiH4, and the low platform corresponds to the formation and decomposition of MgH2. Moreover, it is found that the hysteresis coefficient (H\nf=ln(P\na/P\nd)) of the plateau pressures of Mg/MgH2 is very small, while that of Mg2Ni/Mg2NiH4 is quite large, which may be due to the lattice stress caused by hydrogen atoms enter and overflow the alloys [86]. The pressure values corresponding to the longer and lower plateaus are adopted to calculate the hydrogenation and dehydrogenation thermodynamic parameters of Mg/MgH2 in the alloys. For the Mg2Ni/Mg2NiH4 platform, the thermodynamic parameters are not calculated because the equilibrium pressure is too short. The enthalpy change (\u0394H) and entropy change (\u0394S) of the formation and decomposition of MgH2 can be calculated by Van't Hoff equation. The calculation is based on the pressure value at the center point of the platform at equilibrium [87]:\n\n(8)\n\n\nl\nn\n\n(\n\n\nP\nH\n\n/\n\nP\n0\n\n\n)\n\n=\n\n\u0394\n\nH\n/\nR\nT\n\u2212\n\u2212\n\n\u0394\n\nS\n/\nR\n\n\n\n\nIn Eq.\u00a0(8), P\nH and P\n0 are hydrogen equilibrium pressure and standard atmospheric pressure respectively. An explanation of R and T has been given before. By fitting the linear relationship between ln(P\nH/P\n0) and 1/T, the Van't Hoff diagram of Mg/MgH2 can be obtained as shown in Fig. 13. Therefore, the thermodynamic parameters can be calculated by the slope and intercept of the Van't Hoff diagram, as presented in Fig. 13(d). It is evident that ball milling and adding TiF3 result in a slight decrease of the absolute values of the hydrogenation enthalpy change (\u0394H\nab) and dehydrogenation enthalpy change (\u0394H\nde). The \u0394H\nde value is 77.24, 75.28 and 75.16\u00a0kJ mol\u22121 H2 for the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys, respectively. The change in microstructure caused by introducing TiF3 during ball milling has a positive effect on the reduction of enthalpy change. As has been reported in references [77,88], sample particles of magnesium-based alloys can be reduced to the nanoscale by ball milling, and its thermal stability will be greatly reduced. Since the size of the as-milled particles in this experiment is much bigger than nanometer scale, the improvement of thermodynamic properties of the experimental alloys caused by ball milling is very limited. Agarwal et\u00a0al. [89] considered that the small change of enthalpy may be related to the defects in the alloys. In addition, the added TiF3 and the reaction products (TiF2, MgF2 and TiH2) are beneficial to promoting the reduction of Mg-H bond energy and the decomposition of MgH2.The thermodynamics and kinetics of storing hydrogen in the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10+3TiF3 alloys have been studied in detail in this paper and some conclusions can be reached:\n\n(1)\nThe as-cast and milled Ce5Mg85Ni10 alloys are composed of CeMg12, Mg and Mg2Ni phases. The addition of TiF3 creates new phases of MgF2 and TiF2. The as-milled alloys exhibit a nanocrystalline structure. Ball milling engenders an obvious refinement of the grains and increment of the density of crystal defects. The addition of TiF3 makes the surface of the milled particles rough and irregular.\n\n\n(2)\nThe experimental as-cast alloy exhibits a good activation property, and ball milling and adding TiF3 further facilitate the activation property. The crystal lattice expansion and shrinkage caused by hydrogen entry and release during the activation process results in the severe pulverization of the alloy particles along with the generation of numerous new surfaces and crystal defects, which improve the hydrogen storage kinetics of the alloys.\n\n\n(3)\nBall milling and addition of TiF3 has slightly beneficial effects on the thermodynamics of the experimental samples, which is most likely assigned to the reduction of the particle size and the generation of numerous crystal defects and reaction products (TiF2, MgF2 and TiH2).\n\n\nThe as-cast and milled Ce5Mg85Ni10 alloys are composed of CeMg12, Mg and Mg2Ni phases. The addition of TiF3 creates new phases of MgF2 and TiF2. The as-milled alloys exhibit a nanocrystalline structure. Ball milling engenders an obvious refinement of the grains and increment of the density of crystal defects. The addition of TiF3 makes the surface of the milled particles rough and irregular.The experimental as-cast alloy exhibits a good activation property, and ball milling and adding TiF3 further facilitate the activation property. The crystal lattice expansion and shrinkage caused by hydrogen entry and release during the activation process results in the severe pulverization of the alloy particles along with the generation of numerous new surfaces and crystal defects, which improve the hydrogen storage kinetics of the alloys.Ball milling and addition of TiF3 has slightly beneficial effects on the thermodynamics of the experimental samples, which is most likely assigned to the reduction of the particle size and the generation of numerous crystal defects and reaction products (TiF2, MgF2 and TiH2).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.It is sincere thanks to the National Natural Science Foundation of China (Nos. 51871125, 51761032, 52001005 and 51731002) and Major Science and Technology Innovation Projects in Shandong Province (No. 2019JZZY010320) for financial support of the work.", "descript": "\n                  Mg-based hydrides are too stable and the kinetics of hydrogen absorption and desorption is not satisfactory. An efficient way to improve these shortcomings is to employ reactive ball milling to synthesize the nanocomposite materials of Mg and additives. In this experiment, TiF3 was selected as an additive, and the mechanical milling method was employed to prepare the experimental alloys. The alloys used in this experiment were the as-cast Ce5Mg85Ni10, as-milled Ce5Mg85Ni10 and Ce5Mg85Ni10\u00a0+\u00a03 wt.% TiF3. The phase transformation, structural evolution, isothermal and non-isothermal hydrogenation and dehydrogenation performances of the alloys were inspected by XRD, SEM, TEM, Sievert apparatus, DSC and TGA. It revealed that nanocrystalline appeared in the as-milled samples. Compared with the as-cast alloy, ball milling made the particle dimension and grain size decrease dramatically and the defect density increase significantly. The addition of TiF3 made the surface of ball milling alloy particles markedly coarser and more irregular. Ball milling and adding TiF3 distinctly improved the activation and kinetics of the alloys. Moreover, ball milling along with TiF3 can decrease the onset dehydrogenation temperature of Mg-based hydrides and slightly ameliorate their thermodynamics.\n               "}
{"full_text": "The depletion of fossil resources together with a strong drive to limit greenhouse gas emissions has led to an increasing effort in the development of sustainable and green transportation fuels. Well known examples are ethanol from sugars using fermentative approaches [1] and biodiesel from vegetable oils [2], which have both been commercialized in the last decades. When considering ethanol, some disadvantages have been identified, including a low energy density, high vapor pressure and high water solubility, which cause corrosion issues when using ethanol-rich ethanol-gasoline blends [3]. These disadvantages may be alleviated by using C3+ alcohols, which have superior fuel properties, such as higher energy density, lower volatility and better solubility in hydrocarbons (HC), while at the same time possessing comparable octane numbers as found for gasoline [4].When considering chemo-catalytic routes to higher alcohols, syngas appears an interesting feed [5]. Various catalytic systems have been identified for this purpose [6]. Among them, molybdenum sulfide-based catalysts are of particular interest due to their low cost, high water-gas shift activity and high resistance to sulfur poisoning [7], thus avoiding the need for water separation and deep desulfurization units. MoS2 alone mainly produces CO2 and hydrocarbons (HC) from syngas, while alkali metals, especially potassium (K) modified MoS2 catalysts are commonly used to achieve good selectivity for alcohols [8]. K promotion suppresses hydrogenation of metal-alkyl species to HCs and enhances the rate of CO insertion in the M-alkyl bond to form metal-acyl species, which are subsequently converted to alcohols [9]. It is proposed that KMoS2 phases, formed by the intercalation of K into the MoS2 structure, are responsible for the higher selectivity to alcohols when compared to MoS2 alone [10\u201313].However, K modified MoS2 catalysts normally suffer from low activity [6], leading to relatively low CO conversion and thus a low yield of alcohols. Efforts have been undertaken on tailoring the structure of the K modified MoS2 catalysts to enhance the selectivity to C3+ alcohols [14,15]. In previous work from our groups, we prepared multilayer K modified MoS2 catalysts with well-contacted MoS2 and KMoS2 phases and showed that these catalysts lead to improved alcohol selectivities [16]. Another approach involves promotion by group VIII metals, such as Co and Ni [7,17\u201319]. Especially cobalt is known to promote carbon chain growth, leading to higher selectivities to higher alcohols [20,21], though often ethanol is the major product.Co promoted MoS2 catalysts are widely used in hydrodesulfurization (HDS) reactions and the promoting effect of Co is attributed to the formation of a Co-Mo-S phase [22], formed by partial substitution of Mo atoms at the edge of MoS2 slabs by Co atoms [23]. This particular phase has also been observed in K modified, Co promoted MoS2 catalysts for alcohol synthesis [18,20,24\u201327]. To elucidate the function of cobalt, Mo free, K modified cobalt sulfide catalysts were employed for the reaction. In this case, the amount of higher alcohols was low and C1-C4 alkanes were prevailing [20], indicating that K-CoSx phases are not suitable for higher alcohol synthesis. It also has been shown that, the number of active Co-Mo-S species decreases at high Co loadings due to the formation of Co9S8 phases, which are stable under typical reaction conditions and have a low activity for higher alcohols [28\u201331].Thus, literature data imply that a Co-Mo-S phase in Co promoted MoS2 catalysts is the active phase, [20,28\u201333], though the exact mechanism to promote carbon chain growth is still under debate. However, the role of both K and Co in K modified CoMoSx catalysts has not been explored in detail. We therefore performed a systematic investigation on the effect of these promotors on the performance of MoS2 catalysts for higher alcohol synthesis from syngas. For this purpose, a series of K modified Co promoted molybdenum sulfide catalysts with different Co contents and a fixed K content were prepared, characterized in detail and tested for the conversion of syngas to higher alcohols. The results were compared with a Mo free catalyst in the form of K-CoSx and a K-free catalyst (CoMox-0.13). In addition, for the optimized catalyst regarding Co content, the effect of process conditions, such as temperature (T), pressure (P), gas hourly space velocity (GHSV) and H2/CO ratio was explored. The results were quantified using statistical approaches allowing determination of the optimal process conditions for higher alcohol selectivity and yield.The cobalt-molybdenum sulfide was prepared by sulfurization of the cobalt-molybdenum oxide precursor with KSCN according to a method reported in the literature [34] with some modifications. The cobalt-molybdenum oxide precursor was typically synthesized by mixing Co(NO3)2\u00b76H2O and (NH4)6Mo7O24\u00b74H2O (20\u202fg in total, Sigma-Aldrich) in 50\u202fmL of deionized water. The resulting suspension was heated and maintained at 120\u202f\u00b0C for 3\u202fh, during which most of the water evaporated. The resulting mixture was calcined in air at 500\u202f\u00b0C for 3\u202fh to form the cobalt-molybdenum oxide. The amount of Co(NO3)2\u00b76H2O and (NH4)6Mo7O24\u00b74H2O was varied to adjust the atomic ratio Co/(Co\u202f+\u202fMo) between 0 and 0.7.For sulfurization, the cobalt-molybdenum oxide (0.648\u202fg), KSCN (0.875\u202fg, Sigma-Aldrich), and deionized water (35\u202fmL) were mixed in an autoclave, which was kept at 200\u202f\u00b0C for 24\u202fh. Then the autoclave was rapidly cooled with ice, and the resulting precipitate was filtered and washed with deionized water (total 500\u202fmL). The product was obtained after drying at ambient conditions overnight. The molybdenum sulfide is labelled as MoSx and the mixed metal sulfide catalysts are labelled as Co-MoSx-R, where R represents the actual Co/(Co\u202f+\u202fMo) ratio as obtained from ICP-OES. The elemental composition of the sulfurized catalysts is shown in Table 1\n.The K promoted K-Co-MoSx-0.13 catalyst, used for detailed analyses by XRD, HRTEM and STEM with EDS mapping, was prepared by physically mixing Co-MoSx-0.13 with K2CO3 followed by a treatment under hydrogen (1\u202fbar, 8\u202fh, 400\u202f\u00b0C) and subsequent passivation (1% O2/N2, 4\u202fh, 25\u202f\u00b0C).The K promoted CoS2 catalyst was prepared by physically mixing a CoS2 sample (Sigma Aldrich) with K2CO3 followed by a reduction procedure as described above.The cobalt-molybdenum sulfide samples were characterized with ICP-OES (Spectroblue, Germany) to quantify the elemental composition.The specific surface area and pore parameter were determined using N2 physisorption, which was conducted at 77\u202fK using an ASAP 2420 system (Micromeritics, USA). Prior to analysis, the samples were degassed at 150\u202f\u00b0C under vacuum for 12\u202fh. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method in the P/P0 range of 0.05\u20130.25. The total pore volume was estimated from the single point desorption data at P/Po\u202f=\u202f0.97. The pore diameter was obtained from the desorption branch according to the Barrett-Joyner-Halenda (BJH) method.X-ray diffraction (XRD) patterns of the sulfurized samples were collected for a 2\u03b8 scan range of 5\u201380\u00b0 on a D8 Advance powder diffractometer (Bruker, Germany) with CuK\u03b1 radiation (\u03bb\u202f=\u202f1.5418\u202f\u00c5) operated at 40\u202fkV and 40\u202fmA. XRD spectra of the K modified sample (K-Co-MoSx-0.13) were recorded in the same way.H2-TPR measurements were conducted using 10\u202fvol.% H2 in He (30\u202fml min-1) and the samples were heated from room temperature to 900\u202f\u00b0C at a temperature ramp of 10\u202f\u00b0C/min using an AutoChem system (Micromeritics, USA) equipped with a thermal conductivity detector (TCD). Raman spectroscopy was measured using a WITec Alpha 300R microscope with a 532\u202fnm excitation laser.The micro-structure of the sulfurized samples was examined with high-resolution transmission electron microscopy (HRTEM, JEOL 2010 FEG, Japan) operating at 200\u202fkV. The samples were first ultrasonically dispersed in ethanol and then deposited on a carbon-coated copper grid. Processing of the HRTEM images was accomplished using DigitalMicrograph software.High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the K-Co-MoSx-0.13 sample were obtained using a probe and image aberration corrected Themis Z microscope (Thermo Fisher Scientific) operating at 300\u202fkV in STEM mode with a convergence semi-angle of 21\u202fmrad and a probe current of 50 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).Reactions were performed in a continuous fixed-bed reactor (stainless steel) with an internal diameter of 10\u202fmm. Typically, the cobalt-molybdenum sulfide catalyst (0.35\u202fg) was physically mixed with K2CO3 (0.05\u202fg, Sigma-Aldrich) and SiC (2.0\u202fg, Sigma-Aldrich) and then loaded to the reactor. Before reaction, the catalyst was reduced in situ using a flow of H2 (50\u202fml min-1) at 400\u202f\u00b0C for 8\u202fh. After cooling to room temperature under a N2 stream, the reaction was started by switching to a gas mixture of H2/CO (molar ratio ranging from 1.0 to 2.0) with 6% N2 (internal standard). Typical reaction conditions are pressures between 8.7 and 14.7\u202fMPa and temperatures between 340 and 380\u202f\u00b0C. The gas hourly space velocity (GHSV) was varied from 4500 to 27000\u202fmL g-1 \u202fh-1 by adjusting the flow rate of the feed gas. The reactor effluent was cooled and the liquid product was separated from the gas phase by using a double walled condenser at \u22125\u202f\u00b0C. Details regarding product analysis are described in a previous publication from our groups [16]. The CO conversion (XCO), the product selectivity (Si) and yield (Yi) were calculated using Eqs. (1)\u2013(3).\n\n(1)\n\n\nX\n\nC\nO\n\n\n=\n\n\nm\no\nl\ne\ns\n\u2009\no\nf\n\u2009\n\n\nC\nO\n\n\ni\nn\nf\nl\nu\ne\nn\nt\n\n\n-\nm\no\nl\ne\ns\n\u2009\no\nf\n\u2009\n\n\nC\nO\n\n\ne\nf\nf\nl\nu\ne\nn\nt\n\n\n\n\nm\no\nl\ne\ns\n\u2009\no\nf\n\u2009\n\n\nC\nO\n\n\ni\nn\nf\nl\nu\ne\nn\nt\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n(2)\n\n\n\nS\ni\n\n=\n\n\nm\no\nl\ne\ns\n\u2009\no\nf\n\u2009\np\nr\no\nd\nu\nc\nt\n\u2009\ni\n\u00d7\nn\nu\nm\nb\ne\nr\n\u2009\no\nf\n\u2009\nc\na\nr\nb\no\nn\ns\n\u2009\ni\nn\n\u2009\np\nr\no\nd\nu\nc\nt\n\u2009\ni\n\n\nm\no\nl\ne\ns\n\u2009\no\nf\n\u2009\n\n\nC\nO\n\n\ni\nn\nf\nl\nu\ne\nn\nt\n\n\n-\nm\no\nl\ne\ns\n\u2009\no\nf\n\u2009\n\n\nC\nO\n\n\ne\nx\nf\nl\nu\ne\nn\nt\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(3)\n\nY\n=\n\nX\n\nC\nO\n\n\n\u00d7\n\nS\ni\n\n\n\n\nThe activity data given in this study are the average for at least 6\u202fh runtime and collected after 20\u202fh, to ensure stable operation of the reactor. The selectivity of all products is carbon based and only data with carbon balances higher than 95% are reported here.The chain growth probability \n\u03b1\n was determined from the experimental data assuming an ASF distribution for the alcohols (Eq. (4)).\n\n(4)\n\n\n\n\nS\nn\n\n\nn\n\n=\n\n\u03b1\nn\n\n\u00d7\n\n\n(\n1\n-\n\u03b1\n)\n\na\n\n\n\nHere, \n\nS\nn\n\n is the selectivity of the alcohols with a carbon number of \nn\n, \nn\n is the carbon number, and \n\u03b1\n is the chain growth probability. The value of \n\u03b1\n was determined by plotting \nl\nn\n(\n\n\n\nS\nn\n\n\nn\n\n)\n against n.Multivariable regression was used to quantify the effect of process conditions (T, P, GHSV and H2/CO ratio) on catalytic performance (Eq. (5)).\n\n(5)\n\nY\n=\n\na\n0\n\n+\n\n\u2211\n\n\na\ni\n\n\nx\ni\n\n\n\n+\n\n\u2211\n\n\na\n\ni\ni\n\n\n\nx\ni\n2\n\n\n\n+\n\n\u2211\n\n\na\n\ni\nj\n\n\n\nx\ni\n\n\n\n\nx\nj\n\n\n\nHere x is independent variable (T, P, GHSV and H2/CO ratio) and Y is a dependent variable (selectivity and yield of C3+ alcohol), ai, aii, and aij are the regression coefficients and a0 is the intercept. The regression coefficients were determined using the Design-Expert (Version 7) software by backward elimination of statistically non-significant parameters. The significant factors were selected based on their p-value in the analysis of variance (ANOVA). A parameter with a p-value less than 0.05 is considered significant and is included in the response model.The cobalt-molybdenum sulfide catalysts with different Co contents were prepared by sulfurization of the corresponding cobalt-molybdenum oxide precursors using KSCN. The actual Co/(Co\u202f+\u202fMo) molar ratio was determined by ICP-OES and ranged from 0 to 0.63 (Table 1). The textural properties of the sulfurized catalysts (without K addition) are depending on the Co content, see Table 1 for details. When considering the specific surface area, a maximum was found for Co-MoSx-0.13, with a value of 11.5\u202fm2\u202fg-1. This value is in the broad range reported in the literature for Co-MoSx catalysts (from single digit values to several hundred square meters per gram [35]), rationalized by differences in the Co and Mo precursors used and synthesis conditions. The observed reduction at higher Co amounts may be due to the formation of a segregated Co sulfide phase [36]. Similar trends were observed for the pore volume and pore diameters of the catalysts, viz. the highest value was found for catalyst Co-MoSx-0.13.The XRD patterns of the catalyst (without K addition) are shown in Fig. 1\n. The MoSx catalyst shows broad diffractions at 2\u03b8 values of about 14\u00b0, 33\u00b0, 36\u00b0 and 58\u00b0, which are associated with the (0 0 2), (1 0 0), (1 0 2) and (1 1 0) planes, respectively, of the 2H-MoS2 phase (JCPDS card No. 00-037-1492). Upon the addition of Co, the reflexes of the crystalline MoS2 phase disappear and new signals arise. These were identified as cobalt-containing species like CoS2 (JCPDS card No. 01-089-3056), CoMoS3.13 (JCPDS card No. 00-016-0439) and CoMoO4 (JCPDS card No. 00-021-0868). Of interest is the presence the CoMoS3.13 phase, which is known to be formed by partial substitution of Mo atoms at the edges of MoS2 sheets by Co. Mixed Co-Mo-S phases are generally thought to be active for higher alcohol synthesis by promoting carbon chain growth [6]. At high Co loadings, sharp reflexes from crystalline CoS2 and CoMoO4 are present, suggesting a higher abundance and larger nanoparticle sizes. Reflexes attributed to a Co9S8 phase, reported to be present at higher Co loadings, were not detected [30].H2-TPR measurements were performed for all sulfided Co-Mo catalysts and the profiles are given in Fig. 2\n. The Co free MoSx catalyst displays two H2 peaks, a small one at 310\u202f\u00b0C and a larger one at about 720\u202f\u00b0C. The first peak is ascribed either to the presence of over-stoichiometric Sx species or to weakly bonded sulfur anions along MoS2 edges [37]. The high temperature peak is associated with more strongly bound sulfur anions located at the edges [38]. Another possibility is a phase formed by desulfurization of the MoS2 phase by elimination of basal sulfur, though not likely as temperatures higher than 830\u20131030\u202f\u00b0C are required for this transition [39]. Upon the addition of Co, additional peaks become visible. The low temperature peak is shifted to lower temperatures (about 220\u202f\u00b0C), indicating that the presence of Co leads to a weakening of the Mo-S bond [40]. A similar low temperature peak was also observed during H2-TPR measurements on supported Co-MoS2/Al2O3 catalysts for HDS reactions and associated with the presence of a Co-Mo-S phase [41]. The area of the first peak is reduced when adding more Co in the catalyst formulation. Besides, a new peak at an intermediate temperature (370\u2013470\u202f\u00b0C) appears, which is ascribed to a cobalt sulfide phase [41]. In line with this explanation is the observation that the area of this particular peak increases with increasing Co content. This suggests that for low Co/(Co\u202f+\u202fMo) ratios, the Co atoms are dispersed at the edge of a MoS2 phase to form a Co-Mo-S phase, whereas higher Co amounts lead to the formation of Co sulfide species. These may be present as a single phase or closely interact with Co-Mo-S and MoS2 phases.The Raman spectra of the sulfided Co-Mo catalysts (without K) are shown in Fig. 3\n. The unpromoted MoSx catalyst exhibits two peaks at 380\u202fcm-1 and 405\u202fcm-1, which are ascribed to the in-plane E1\n2g and out-of-plane A1g vibration mode of the MoS2 layer structure [42]. These two bands are also detected in Co-MoSx-0.13, and the distance between the two bands, which is an indicator for the interlayer distance between the MoS2 stacked layers [15,43], is similar to that for the unpromoted MoSx catalyst. This suggests that, different with K [12], Co is not intercalated in the interlayer space of MoS2 phase, which is consistent with the H2-TPR result. For the catalysts with high Co contents, the two peaks disappear, and a new peak at 931\u202fcm-1 emerges, associated with the formation of a \u03b2-CoMoO4 phase, which is in consistent with the XRD results. The intensity of the peak increases with increasing Co content.HRTEM was used to determine the morphology and microstructure of the catalysts. Representative images are displayed in Fig. 4\n. The MoSx catalyst without Co shows a multilayer structure with a lattice spacing of 0.63\u202fnm, corresponding to the (0 0 2) plane of the MoS2 phase (Fig. 4a) [44]. After the addition of Co, various Co-containing species were identified based on their specific lattice fringes. Examples are Co-MoSx, CoSx and CoMoO4 phases (Fig. 4b\u2013f). The lattice fringe with a lattice spacing of 0.25\u202fnm corresponds to the (2 1 0) plane of CoS2.Of interest is the observation of close contacts between the CoS2 and MoS2 phase for Co-MoSx-0.13 (Fig. 4b\u2013c), indicating the presence of a CoS2/MoS2 interface. The presence of this interface has been reported to be beneficial for higher alcohol formation [45]. The phase with a lattice spacing of 0.63\u202fnm may be either from MoSx or a CoMoS3.13 species. For catalysts with a higher Co content (e.g. Co-MoSx-0.37), a CoMoO4 phase is present (lattice fringe with a spacing of 0.68\u202fnm (Fig. 4d)), consistent with the XRD analysis.With the catalyst characterization data available, the effect of the amount of Co on catalyst structure may be assessed. Unpromoted MoSx reveals a multilayer structure with long-range ordered MoS2 domains, in line with the literature data. After promotion with Co, Co-Mo-S and CoS2 phases are formed, which are considered possible active phases for higher alcohol synthesis (Co-MoSx-0.13). At higher Co contents, higher amounts of CoS2 and CoMoO4 species are present, which may have a negative effect on catalyst performance (vide infra).Finally, the K promoted version of Co-MoSx-0.13 (K-Co-MoSx-0.13), which is the best catalyst in terms of performance for higher alcohol synthesis (vide infra), was characterized in detail using XRD, HRTEM and STEM with EDS mapping to gain insights in changes in the structure upon the addition of K. The sample was prepared by physically mixing Co-MoSx-0.13 with K2CO3 followed by reduction with hydrogen and passivation (see experimental section).XRD spectra of K-Co-MoSx-0.13, together with MoSx and Co-MoSx-0.13 for comparison, are given in Fig. 5\na. The (002) reflex of K-Co-MoSx-0.13 at 13.3\u00b0 is slightly shifted downfield compared to that of MoS2 (14.1\u00b0), indicating an expanded interlayer spacing due to the incorporation of K. A HRTEM image (Fig. 5b) of K-Co-MoSx-0.13 confirms the expanded interlayer spacing (0.77- 0.81\u202fnm vs 0.63\u202fnm for Co-MoSx-0.13, Fig. 4c) after K addition. The intercalation of K into the MoS2 structure leads to the formation of a KMoS2 phase, which was discussed in detail in our previous work [16] and is suggested to be essential for alcohol synthesis.The reflexes of CoS2, clearly visible in Co-MoSx-0.13, are absent in the XRD spectrum of K-Co-MoSx-0.13. New reflexes at 30.1\u00b0, 31.2\u00b0 and 39.7\u00b0, identified as Co9S8 species (JCPDS card No. 00-003-0631) are present. The Co9S8 species are likely formed by reduction of CoS2, which is consistent with the H2-TPR results (Fig. 2). Representative reflexes of crystalline CoMoS3.13 are also present in K-Co-MoSx-0.13. The presence of both Co9S8 and CoMoS3.13 species in K-Co-MoSx-0.13 is confirmed by HRTEM images (Fig. 5c\u2013d). Close contacts between the Co9S8 and K promoted (Co)MoSx phase were observed (Fig. 5b\u2013d), in agreement with the observation of CoS2/(Co)MoSx interfaces in the unpromoted Co-MoSx-0.13 catalyst (Fig. 4b\u2013c).A STEM dark field image combined with EDS mapping (Fig. S1) of K-Co-MoSx-0.13 shows that K, Co, Mo and S are uniformly dispersed in the catalyst. Such a homogeneous distribution is indicative for the presence of abundant Co9S8/K-(Co)MoSx interfaces in K-Co-MoSx-0.13.Benchmark experiments with all catalysts were performed at 360\u202f\u00b0C, 8.7\u202fMPa, a GHSV of 4500\u202fmL g-1\u202fh-1 and a H2/CO ratio of 1 in a continuous packed bed reactor set-up. These conditions were selected based on previous experience in our group on the use of MoS2 catalysts for higher alcohol synthesis [16]. Prior to reaction, the catalysts were promoted with K using a physical mixing method followed by an in situ treatment with H2. The same amount of K was used for all catalyst formulations. The experiments were performed for at least 6\u202fh and the performance of the catalyst was the average over the time period from 20\u202fh to final runtime and thus taken at steady state conditions in the reactor (Table 2\n).A typical example of the product selectivity and CO conversion versus the runtime is given in Fig. 6\n (340\u202f\u00b0C, 11.7\u202fMPa, GHSV of 4500\u202fmL g-1\u202fh-1 and H2/CO ratio of 1.5 using the K-Co-MoSx-0.13 catalyst). It also shows the catalyst is stable for at least 100\u202fh without co-feeding of sulfur.Typical reactions products are alcohols (methanol, ethanol, and C3+ alcohol), hydrocarbons (methane and higher ones) and CO2. The latter is formed by the water-gas shift reaction involving CO and water. The unpromoted K-MoSx catalyst provides a selectivity of 40.8% to alcohols and 24.8% to hydrocarbons at a CO conversion level of 25.6% (Fig. 7\n), which is typical for Mo-based catalysts [6]. Upon the addition of Co to the catalyst formulation, the CO conversion decreases, which may be due to the reduced availability of the active sulfided Mo-Co species by coverage with inactive CoMoO4 species and/or the presence of less active CoS2 species, as observed from XRD and HRTEM results.The selectivity is a clear function of the Co content. Alcohol selectivity reaches a maximum (47.1%) for the K-Co-MoSx-0.13 catalyst and decreases with higher Co loadings, see Fig. 7 for details. The selectivity to hydrocarbons (mainly CH4), shows a reverse trend, whereas the CO2 selectivity is about constant. The product selectivity at two other temperatures (340 and 380\u202f\u00b0C) also shows a similar trend regarding the Co content in the catalyst formulation (Table S2).The effect of Co addition on the carbon distribution of the alcohols is given in Fig. 8\na. It shows that the amount of C3+ alcohols reaches a maximum at 59.0% for the K-Co-MoSx-0.13 catalyst and decreases at higher Co amounts. The individual distribution of alcohols for the unpromoted K-MoSx, K-Co-MoSx-0.13 and K-Co-MoSx-0.63 catalyst are depicted in Fig. 8b (the distributions for other catalysts are shown in Fig. S2) as Anderson-Schulz-Flory (ASF) plots. The unpromoted K-MoSx catalyst shows a large deviation for particularly methanol when considering an ideal linear ASF distribution. This is in line with previous findings of our group, rationalized by assuming an enhanced chain growth mechanism for C3+ alcohol using these types of catalysts [16]. After loading with Co, an even larger deviation for methanol and also for ethanol is observed for the K-Co-MoSx-0.13 catalyst. However, the deviation is less pronounced when further increasing the Co content (Fig. S2) and the K-Co-MoSx-0.63 catalyst shows an almost perfect linear distribution for the mixed alcohols including methanol. The carbon chain growth probability was calculated for the C2+ alcohols, showing a volcano-shaped curve with a peak for the K-Co-MoSx-0.13 catalyst (Fig. S3). Thus, alcohol selectivity and carbon chain growth are best for the K-Co-MoSx-0.13 catalyst, whereas higher Co contents lead to a higher hydrocarbon selectivity and a lower carbon chain growth for the alcohols.For comparison, and also to determine the role of Mo in the catalyst formulation, the catalytic performance of a K promoted CoS2 catalyst was also investigated. We first attempted to prepare the CoS2 catalyst by a similar procedure as used for the Co-MoSx samples (viz. sulfurization of the cobalt-oxide precursors using KSCN). However, Co3O4 instead of CoS2 was obtained (Fig. S4), indicating that Co-oxides are difficult to sulfurize using KSCN at the prevailing conditions. Therefore, CoS2 (Sigma-Aldrich) was used as the catalyst precursor, and after K addition and pretreatment (in situ reduction with H2 at 400\u202f\u00b0C for 8\u202fh) tested for higher alcohol synthesis (360\u202f\u00b0C, 8.7\u202fMPa, GHSV of 4500\u202fmL g-1\u202fh-1 and H2/CO molar ratio of 1). A very high hydrocarbon selectivity of 63.1% was achieved at a CO conversion of 1.3% (Table S3). Higher alcohols could not be detected in the liquid phase. The low CO conversion might be due to the presence of large crystallites (76\u202fnm, from XRD data using Scherrer equation) and the lack of structural defects (Fig. S5). These findings are in line with experiments by Li et al., who reported that only C1-C4 alkanes and no alcohols were formed when using a K-CoSx on activated carbon catalyst (in which Co is present in the form of Co9S8 crystallites) [20]. Co9S8 species, formed by reduction of CoS2 were indeed detected after reaction (Fig. S5), in line with literature data [20].The unpromoted Co-MoSx-0.13 catalyst (without K) showed high CO conversion and very low selectivity for alcohols (< 2%) in comparison with that of K-Co-MoSx-0.13 (Table S4), indicating the important role of K for alcohol synthesis. Specifically, the presence of a KMoS2 phase (Fig. 5) is considered to be essential for alcohol synthesis, see also previous work from our group [16]. This is also in agreement with literature data revealing that the addition of K in MoS2 catalysts leads to lower hydrogenation rates while maintaining good CO insertion rates [8,9,46]. The obtained higher alcohols over the K modified catalyst are mainly composed of linear primary alcohols as well as branched alcohols like 2-methyl-1-propanol, 2-methyl-1-butanol, and 2-methyl-1-pentanol (Figs. S6\u20139). These branched alcohols were suggested to be formed via a \u03b2-addition process [47,48]. We have recently proposed that the linear primary alcohols are formed through CO insertion, while the branched alcohols are formed by CO insertion and CHx \u03b2-addition [16,49], see Schema 1 for details. n-Propanol is formed through both routes, supported by the high amount (> 97%) of n-propanol in total propanol fraction (Fig. S6) (Scheme 1\n).In the current investigation, the role of Co on product selectivity was investigated. Upon Co addition, the CH4 selectivity is lowered slightly from 17.7% for K-MoSx to 16.8% for the K-Co-MoSx-0.13 catalyst. A further increase in Co in the catalyst formulation leads to a gradual increase in CH4 selectivity (Fig. 7), suggesting a somewhat higher hydrogenation ability. The latter may be due to the presence of higher amounts of (K promoted) CoS2 species (Figs. 1, 2 and 4) in the catalysts at higher Co contents.The selectivity to alcohols in general and C3+ alcohols in particular shows an optimum for the K-Co-MoSx-0.13 catalyst and decreases with higher Co loadings (Figs. 7 and 9\n). These findings are rationalized by considering that the amounts of Co-Mo-S and CoS2 phases in the Co-MoSx-0.13 catalyst are highest and that these are preferred for higher alcohol synthesis. At higher Co contents, considerable amounts of CoMoO4 species are present which result in lower higher alcohol selectivity.The trend as given in Fig. 9 holds for the unpromoted (no K) catalysts. Analyses of a K-promoted catalyst (K-Co-MoSx-0.13) by XRD and HRTEM shows that the CoS2 phase, is reduced to Co9S8 (Fig. 5). Based on these findings, we propose that the catalytic performance of the K-MoSx catalyst is enhanced by the addition of Co due to the formation of cobalt sulfides (mainly Co9S8) and a K-promoted (Co)MoSx phase in close proximity. This assembly is given in Scheme 2\n and shows a (K promoted) Co9S8 phase sandwiched between two K-promoted (Co)MoSx phases. The (K-)Co9S8 phase gives mainly hydrocarbons for syngas conversions, see results for the Mo free K-Cox provided in this manuscript and literature data [20]. This implies the presence of significant amounts of adsorbed CHx* (and higher carbon number analogs) on the surface of the Co9S8 phase. We assume that efficient transfer of such CHx* species from the Co9S8 phase to adsorbed CH3CHCH2O* species on the K-(Co)MoSx phase occurs, leading to branched alcohols (CHx \u03b2-addition mechanism). In addition, linear alcohols are formed by transfer of adsorbed CH3CH2CH2* on the Co9S8 phase to adsorbed CO on the K-(Co)MoSx phase.To determine the effects of process conditions on CO conversion and product selectivity (particularly C3+ alcohols), a total of 44 experiments were performed in the continuous set-up at a range of 340\u2013380\u202f\u00b0C 8.7\u201314.7\u202fMPa, GHSV of 4500\u201327000\u202fmL g-1\u202fh-1 and H2/CO ratio of 1.0\u20132.0 for the best catalyst (K-Co-MoSx-0.13) based on the benchmark experiments. In the initial stage, one variable was changed within the range while the other variables were kept constant (Figs. S10\u201318). This allows for determination of the individual effects of a variable on the CO conversion and product selectivity. In a later stage all experimental data (Table 3\n) were used simultaneously to develop multivariable nonlinear regression models of the form given in Eq. (5). This approach allowed the identification of interactions between the variables (T, P, GHSV and H2/CO ratio) on the selectivity and yield of C3+ alcohol.The yield (%) and selectivity (%) of C3+ alcohol as a function of reaction conditions were successfully modeled and the results are given in Eqs. (6) and (7), respectively.\n\n(6)\n\nY\ni\ne\nl\nd\n=\n2.05\n\u00d7\nP\n+\n0.13\n\u00d7\nT\n+\n0.00034\n\u00d7\nG\nH\nS\nV\n-\n1.27\n\u00d7\nR\na\nt\ni\no\n-\n0.000021\n\u00d7\nP\n\u00d7\nG\nH\nS\nV\n-\n0.088\n\u00d7\n\nP\n2\n\n-\n3.91\n\u00d7\n\n10\n\n-\n9\n\n\n\u00d7\n\n\nG\nH\nS\nV\n\n2\n\n-\n51.51\n\n\n\n\n\n(7)\n\nS\ne\nl\ne\nc\nt\ni\nv\ni\nt\ny\n=\n-\n11.45\n\u00d7\nP\n+\n0.20\n\u00d7\nT\n+\n0.0037\n\u00d7\nG\nH\nS\nV\n-\n5.14\n\u00d7\nR\na\nt\ni\no\n-\n0.00017\n\u00d7\nP\n\u00d7\nG\nH\nS\nV\n-\n0.00029\n\u00d7\nG\nH\nS\nV\n\u00d7\nR\na\nt\ni\no\n+\n0.41\n\u00d7\n\nP\n2\n\n-\n1.86\n\u00d7\n\n10\n\n-\n8\n\n\n\u00d7\n\n\nG\nH\nS\nV\n\n2\n\n+\n20.77\n\n\n\nThe high F-value of both models (Tables S5\u20136) implies that the models are significant and adequate to represent the actual relationship between the response and the variables [50]. The models also reveal that interactions between parameters are significant (e.g. P\u202f\u00d7\u202fGHSV and GHSV\u202f\u00d7\u202fRatio). The predicted values of C3+ alcohol yield and selectivity match well with the experiment data (Fig. S19\u201320, R2\u202f=\u202f0.92 for yield and R2\u202f=\u202f0.91 for selectivity).The effect of the pressure and GHSV on C3+ alcohol yield (Fig. 10\n) and selectivity (Fig. S21) are represented in response surface plots. It shows that intermediate pressure and GHSV are best for highest C3+ alcohol yield. This is confirmed by experiments in this regime, viz. a C3+ alcohol yield of 9.2% at 11.7\u202fMPa, GHSV of 13500\u202fmL g-1\u202fh-1 (380\u202f\u00b0C, H2/CO ratio of 1, Table 3, entry 11). The model also predicts that a relatively high temperature and low H2/CO ratio are also best for higher alcohol synthesis (surface plots not shown for brevity).The experimentally obtained C3+ alcohol selectivity at different CO conversion over the best catalyst (K-Co-MoSx-0.13) in this study is given in Fig. 11\n, together with literature data for other Mo based catalysts. Details regarding reaction conditions are shown in Table S7. Literature sources providing alcohol selectivity only on a CO2-free basis were excluded since this leads to an overestimation of the actual C3+ alcohol selectivity and thus does not enable a fair comparison. The majority of the KMoS2-based catalyst reported in the literature are promoted by Co or Ni and are supported on activated carbon (AC), carbon nanotubes (CNT), mixed metal oxides (MMO) and Al2O3.It is clear that the best catalysts identified in this work (K-Co-MoSx-0.13) outperforms all existing Mo-based catalysts. In comparison with the Co free K-MoS2 catalyst reported previously by our groups (Table S7, entry 5), promotion with the appropriate amount of Co leads to higher selectivity and yield for C3+ alcohol.We have prepared a series of K-Co-MoSx catalyst with different Co contents to investigate the effect of Co promotion on product selectivity and particularly C3+ alcohol formation from syngas. The preparation of the Co-MoSx samples through sulfurization of cobalt-molybdenum oxide precursors leads to among others the formation of Co-Mo-S and CoS2 phases, the actual amounts being dependent on the Co amount in the catalyst formulation. The best performance was obtained using the K-Co-MoSx-0.13 catalyst. This catalyst contains the highest amounts of Co-Mo-S and Co9S8 phases, implying that these are preferred for higher alcohol synthesis. It is speculated that close contact between a potassium modified Co9S8 phase and a Co promoted Mo-S phases is beneficial for higher alcohol synthesis due to facile transfer of adsorbed CHx* species (and higher analogs) on the Co9S8 phase to oxygenated species on the Co promoted Mo-S phase to give branched higher alcohols and transfer of adsorbed CH3CH2CH2* on the Co9S8 phase to adsorbed CO on the K-(Co)MoS phase to give linear alcohols. Reaction conditions (T, P, GHSV and H2/CO ratio) were varied to study the effect on catalytic performance and models with high significance were developed. Highest C3+ alcohol yields of 7.3\u20139.2% and selectivities between 31.0\u201337.6% were obtained at a temperature of 380\u202f\u00b0C, a pressure of 11.7\u202fMPa, a GHSV of 13500\u201327000\u202fmL g-1\u202fh-1 and H2/CO ratio of 1 over the optimized K-Co-MoSx-0.13 catalyst. These results are the highest reported in the literature so far, and indicate the potential of such catalysts for further scale up studies.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.\nXiaoying Xi: Investigation, Data curation, Formal analysis, Writing - original draft. Feng Zeng: Investigation, Data curation, Formal analysis, Writing - original draft. Huatang Cao: Investigation, Data curation, Formal analysis. Catia Cannilla: Investigation, Data curation, Formal analysis, Writing - review & editing. Timo Bisswanger: Data curation, Formal analysis, Writing - review & editing. Sytze de Graaf: Investigation, Data curation, Formal analysis. Yutao Pei: Supervision, Validation, Writing - review & editing. Francesco Frusteri: Supervision, Validation, Writing - review & editing. Christoph Stampfer: Data curation, Formal analysis. Regina Palkovits: Conceptualization, Supervision, Validation, Writing - review & editing. Hero Jan Heeres: Conceptualization, Funding acquisition, Supervision, Validation, Writing - review & editing.Xiaoying Xi and Feng Zeng acknowledge the China Scholarship Council (CSC) for financial support.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2020.118950.The following is Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  K-Co-MoSx catalysts varying in Co content were prepared to investigate the role of Co in this catalyst formulation for the synthesis of C3+ alcohols from syngas. The Co-MoSx precursors and the best performing K-doped version were characterized in detail and the amount of active cobalt sulfide and mixed metal sulfide (Co-Mo-S) phases were shown to be a function of the Co content. The catalysts were tested in a continuous set-up at 360\u202f\u00b0C, 8.7\u202fMPa, a GHSV of 4500\u202fmL g-1\u202fh-1 and a H2/CO ratio of 1. The highest alcohol selectivity of 47.1%, with 61% in the C3+ range, was obtained using the K-Co-MoSx catalyst with a Co/(Co\u202f+\u202fMo) molar ratio of 0.13. These findings were rationalized considering the amount and interactions between cobalt sulfide and Co-Mo-S or MoS2 phases. Process studies followed by statistical modeling gave a C3+ alcohol selectivity of 31.0% (yield of 9.2%) at a CO conversion of 29.8% at optimized conditions.\n               "}
{"full_text": "O2/H2 fuel cells were invented by Sch\u00f6nbein [1] and Grove [2] in 1839 and they are still not widely used. The 4-electron oxygen reduction reaction (ORR) is a very spontaneous reaction in H2/O2 fuel cells (O2\u00a0+\u00a02H2 \u2192 2H2O), but its kinetics are sluggish on most electrode surfaces because it involves several electron transfer steps and consequently several activation barriers [3]. The ORR at the cathode represents the bottleneck in fuel cell performance. The cathode requires the presence of expensive catalysts. Most active catalytic materials contain Pd or Pt-based catalysts (for alkaline and proton exchange membrane fuel cells, respectively) [4\u20138]. Catalysts containing very low amounts of highly dispersed Pt group metals have been developed, but they still contribute substantially to the cost of most fuel cells. For this reason, the search for less expensive catalysts using earth abundant elements has been active for five decades [9\u201315]. Earlier catalytic materials belonged to the MN4 or MNx class involving metal-phthalocyanines, metal-porphyrins, and molecules alike (refer Figure\u00a01\n). These molecular catalysts are cheaper than Pt group metals and exhibit rather high activities when immobilized on graphitic and carbon surfaces, but most of them do not have long-term stability in fuel cells electrolytes, especially in acid [16\u201320]. However they have served as models to establish reactivity descriptors that essentially indicate that the metal centers need to have a rather positive M(III)/(II) redox potential (ideally close to the O2/H2O reversible potential) and moderate M\u2013O2 binding energies [21,22], essentially similar to those for Pt. On the other hand, their low stability has been improved by heat-treatment of intact MN4 complexes or by pyrolysis using ingredients containing the necessary elements C, N, and a metal, generally Fe. Most MN4 complexes have pyrrolic inner ligands, but pyrolyzed materials have pyridinic inner ligands, and they have been modeled recently using Fe(phen)2N2 chelates [23]. Many procedures have been reported to prepare these pyrolyzed materials involving heat treatments up to 1000\u00a0\u00b0C, and stability is linked to the method of preparation. Even though stability is crucial for practical applications, it is an issue that has not been addressed with the same emphasis compared with that for achieving high activity [24\u201327]. Hence, it is important to develop unified stability\u2013activity relationships as guidelines for the development of realistic catalysts for fuel cells.Recently, the attention has been focused on two reactivity descriptors, the active metal site density (\n\nS\nD\n\n\n[\n\nm\no\n\nl\n\ns\ni\nt\ne\n\n\n\nc\n\nm\n\n\u2212\n2\n\n\n\n]\n\n\n, or \n\nS\n\nD\n\nm\na\ns\ns\n\n\n\n\n[\n\ns\ni\nt\ne\n\n\ng\n\nc\na\nt\n\n\n\u2212\n1\n\n\n\n]\n\n\n, or \n\n\nS\n\nD\n\nv\no\nl\n\n\n\n\n[\n\n\ns\ni\nt\ne\n\nc\n\nm\n\n\u2212\n3\n\n\n\n]\n\n\n) and the catalytic turn-over frequency (\n\nT\nO\nF\n\n\n[\n\ns\n\n\u2212\n1\n\n\n]\n\n\n or \n\n\n[\n\ne\nl\ne\nc\nt\nr\no\nn\n\ns\ni\nt\n\ne\n\n\u2212\n1\n\n\n\n\ns\n\n\u2212\n1\n\n\n\n]\n\n\n), that is the electrons transferred per active site per second [28]. Increasing the value of one, or both of these descriptors, predicts highly active electrocatalysts. The combination of TOF and SD, together with the Faraday constant F, the number of electrons involved in the reaction, n, and \u03c4\n\nCL\n the thickness of the catalyst layer, provide the kinetic activity of platinum group metal-free (PGM-free) catalysts at a specific potential [29,30]:\n\n\n\n\nJ\n\nk\ni\nn\n\n\n\n\n[\n\nA\n\nc\n\nm\n\n\u2212\n2\n\n\n\n]\n\n=\nn\n\u00b7\nF\n\n\n[\n\nC\n\nm\no\n\nl\n\n\u2212\n1\n\n\n\n]\n\n\u00b7\nT\nO\nF\n\n\n[\n\ns\n\n\u2212\n1\n\n\n]\n\n\u00b7\nS\nD\n\n\n[\n\nm\no\n\nl\n\ns\ni\nt\ne\n\n\n\nc\n\nm\n\n\u2212\n2\n\n\n\n]\n\n\n\n\nor\n\n\n\n\nJ\n\nk\ni\nn\n,\nm\na\ns\ns\n\n\n\n\n[\n\nA\n\n\ng\n\n\u2212\n1\n\n\n\n]\n\n=\nT\nO\nF\n\n\n[\n\ne\nl\ne\nc\nt\nr\no\nn\n\ns\ni\nt\n\ne\n\n\u2212\n1\n\n\n\ns\n\n\u2212\n1\n\n\n\n]\n\n\u00b7\nS\n\nD\n\nm\na\ns\ns\n\n\n\n\n[\n\ns\ni\nt\ne\n\n\ng\n\n\u2212\n1\n\n\n\n]\n\n\u00b7\ne\n\n\n[\n\nC\n\ne\nl\ne\nc\nt\nr\no\n\nn\n\n\u2212\n1\n\n\n\n]\n\n\n\n\nor\n\n\n\n\nJ\n\nk\ni\nn\n,\nv\no\nl\n\n\n\n\n[\n\nA\n\nc\n\nm\n\n\u2212\n2\n\n\n\n]\n\n=\nT\nO\nF\n\n\n[\n\ne\nl\ne\nc\nt\nr\no\nn\n\ns\ni\nt\n\ne\n\n\u2212\n1\n\n\n\ns\n\n\u2212\n1\n\n\n\n]\n\n\u00b7\nS\n\nD\n\nv\no\nl\n\n\n\n\n[\n\ns\ni\nt\ne\n\nc\n\nm\n\n\u2212\n3\n\n\n\n]\n\n\u00b7\ne\n\n\n[\n\nC\n\ne\nl\ne\nc\nt\nr\no\n\nn\n\n\u2212\n1\n\n\n\n]\n\n\u00b7\n\n\u03c4\n\nC\nL\n\n\n\n\n[\nc\nm\n]\n\n\n\n\n\nThe combination of TOF and SD can be considered as a rigorous comparison between catalysts [28]. TOF can be estimated in different ways and it is very important to know how they are estimated. A typical technique to measure both SD, or better the gravimetric site density \n\nS\n\nD\n\nm\na\ns\ns\n\n\n\n\n[\n\ns\ni\nt\ne\n\n\ng\n\nc\na\nt\n\n\n\u2212\n1\n\n\n\n]\n\n\n, and TOF, is represented by the ex situ low temperature (or cryo) CO chemisorption/desorption at\u00a0\u221280\u00a0\u00b0C. CO rapidly and strongly adsorbs on oxygen-free Fe(II)-Nx sites [31]. Moreover, the amount of CO adsorbed is monotonic proportional to the ORR activity [32,33], i.e. one adsorbed CO molecule corresponds to one Fe(II)-Nx moiety at the surface of the catalyst. Thus, the measurement of the CO uptake, n\n\nCO\n, allows calculating the SD\n\nmass\n, which can be further used in combination with J\n\nkin\n to calculate the TOF.\n\n\n\nS\n\nD\n\nm\na\ns\ns\n/\nC\nO\n\n\n\n\n[\n\ns\ni\nt\ne\n\n\ng\n\nc\na\nt\n\n\n\u2212\n1\n\n\n\n]\n\n=\n\nn\n\nC\nO\n\n\n\n[\n\nm\no\nl\n\n\ng\n\nc\na\nt\n\n\n\u2212\n1\n\n\n\n]\n\n\u00b7\n\nN\nA\n\n\n[\n\ns\ni\nt\ne\n\nm\no\n\nl\n\n\u2212\n1\n\n\n\n]\n\n\n\n\n\n\n\n\n\nT\nO\n\nF\n\nC\nO\n\n\n\n\n[\n\n\n\ns\n\n\u2212\n1\n\n\n\n]\n\n=\n\n\n\nJ\n\nk\ni\nn\n,\nm\na\ns\ns\n\n\n\n\n[\n\nA\n\n\ng\n\nc\na\nt\n\n\n\u2212\n1\n\n\n\n]\n\n\u00b7\n\nN\nA\n\n\n[\n\ns\ni\nt\ne\n\nm\no\n\nl\n\n\u2212\n1\n\n\n\n]\n\n\n\nF\n\n\n[\n\nC\n\nm\no\n\nl\n\n\u2212\n1\n\n\n\n]\n\n\u00b7\nS\n\nD\n\nm\na\ns\ns\n/\nC\nO\n\n\n\n\n[\n\ns\ni\nt\ne\n\n\ng\n\nc\na\nt\n\n\n\u2212\n1\n\n\n\n]\n\n\n\n\n\n\n\nA strip protocol of cleaning the catalyst surface of oxygen, followed by a series of CO pulses to reach saturation of the active centers and the temperature program desorption allows a very precise measurement [32,33].\nSD can also be estimated with an in situ electrochemical technique based on the adsorption of nitrite and electrostripping of NO through a 5-electron reaction on Fe(II)-Nx active sites, assuming that one NO molecule poisons one site. The determination of Q\n\nstrip\n, that is the excess coulometric charge associated with the stripping peak, together with the number of electrons necessary to reduce the nitrite ion, n\n\nstrip\n, and the specific surface area of the catalyst, S\n\nBET\n, allows determining the SD. NO2\n\u2013 anions adsorption largely affects the ORR activity of the electrocatalyst, by poisoning the active sites. Thus, it is also possible to evaluate the TOF at a certain potential by measuring the kinetic mass current of the catalyst as the difference of the unpoisoned and poisoned kinetic mass current values, \u0394J\n\nkin\n (\n\n\nJ\n\nk\ni\nn\n,\nm\na\ns\ns\n\n\nu\nn\np\no\ni\ns\no\nn\ne\nd\n\n\n\u2212\n\nJ\n\nk\ni\nn\n,\nm\na\ns\ns\n\n\np\no\ni\ns\no\nn\ne\nd\n\n\n\n). This technique has been developed by the group of Kucernak et\u00a0al. [29,30,34], it requires a series of subsequent steps of cleaning, poisoning, and stripping of the reaction products from the catalyst layer for the determination of Q\n\nstrip\n and \u0394J\n\nkin\n.\n\n\n\nS\n\nD\n\nm\na\ns\ns\n/\nN\n\nO\n2\n\u2212\n\n\n\n\n\n[\n\nm\no\nl\n\u00a0\ns\ni\nt\ne\n\n\ng\n\n\u2212\n1\n\n\n\n]\n\n=\n\n\n\nQ\n\ns\nt\nr\ni\np\n\n\n\n\n[\n\nC\n\n\ng\n\n\u2212\n1\n\n\n\n]\n\n\n\n\nn\n\ns\nt\nr\ni\np\n\n\n\u00b7\nF\n\n\n[\n\nC\n\nm\no\n\nl\n\n\u2212\n1\n\n\n\n]\n\n\n\n\n\n\n\n\n\n\n\nT\nO\n\nF\n\nN\n\nO\n2\n\u2212\n\n\n\n\n\n[\n\ns\n\n\u2212\n1\n\n\n]\n\n=\n\n\n\u0394\n\nJ\n\nk\ni\nn\n\n\n\n\n[\n\nA\n\n\ng\n\n\u2212\n1\n\n\n\n]\n\n\n\nF\n\n\n[\n\nC\n\nm\no\n\nl\n\n\u2212\n1\n\n\n\n]\n\n\u00b7\nS\n\nD\n\nm\na\ns\ns\n/\nN\n\nO\n2\n\u2212\n\n\n\n\n\n[\n\n\nm\no\nl\n\u00a0\ns\ni\nt\ne\n\n\ng\n\n\u2212\n1\n\n\n\n]\n\n\n\n\n\n\n\nDouble layer (DL) capacitance can be used to estimate the area of electrodes. For pyrolyzed catalysts, the wetted surface area can be estimated from Electrochemical Impedance Spectroscopy (EIS) measurements [35], but it is very dependent on the type of carbon or graphitic materials, graphitic edges, and presence of carbon functionalities [36]. Thus, DL capacitances can vary from as low as 4\u00a0\u03bcF\u00a0cm\u22122 (defect-free basal plane graphite) to 60\u00a0\u03bcF\u00a0cm\u22122 for more heterogeneous carbon/graphitic materials and in the average, most materials exhibit DL-capacitances around 20\u201325\u00a0\u03bcF\u00a0cm\u22122 [36].It is important to know how TOF values are estimated. A correct TOF estimation should only consider the active sites available for the reaction. An overestimation will lead to lower TOF values. For example, a catalyst containing Fe as active sites, where Fe bulk is not effective for several reasons: oxygen has no access to those sites but they are considered in the TOF calculation. There could be another reason for those sites not be active: they are partially covered or occupied by adsorbed intermediates resulting from the reaction and another: some of those active sites are in the wrong oxidation state at the particular electrode potential. It is widely accepted that only M(II) is active.This is illustrated in Figure\u00a02\na and shows a clear volcano correlation for several MN4 molecular catalysts adsorbed on a smooth graphite surface. Hypothetically, it is assumed that all active sites are accessible to oxygen as the adsorbed MN4 molecules are lying flat on the graphite surface, and there are no MN4 molecules imbedded in the graphite. Those electrodes were made of graphite crystals, so porosity is very low or absent. NiPc shows low activity because the interaction between Ni and O2 is too weak. The opposite is true for CrPc. The classical arguments for the low activity of CrPc would be that most active sites are occupied by strongly bound oxygen intermediates. However, the most important factor contributing to the low activity of CrPc is that its Cr(III)/(II) redox potential is too negative compared to the potential at which ORR currents are compared. Thus, most Cr sites are in the inactive state Cr(III). However, if the currents are divided by the fraction of sites \u03b8\nM(II) in the active state M(II) for all MN4 catalysts, the volcano correlation becomes a linear correlation and the activity per active sites increases going from weak oxygen binding catalysts (right) to strong binding catalysts (left). The same happens if the TOF values are plotted versus the M\u2013O2 binding energy in Figure\u00a02b, as log(i/\u03b8\nM(II))E values are directly proportional to TOF values but with the slight difference that for strongly binding catalysts n\u00a0=\u00a04 (O2 reduction to OH\u2013) and for weak binding catalysts n\u00a0=\u00a02 (O2 reduction to peroxide).In Figure\u00a02a CrPc shows very low activity but exhibits the highest TOF in Figure\u00a02b. The best catalyst illustrated in Figure\u00a02a is FePyPz that shows a TOF value almost 5 orders of magnitude lower than the poor CrPc catalyst. Yang et\u00a0al. [37] reported a series of TOF values for the activity of FePc for ORR in 0.1\u00a0M KOH using several carbon substrates and they report values that vary in the range from 0.5 to 2.8\u00a0at\u00a0E\u00a0=\u00a00.868\u00a0V vs RHE. However, these values are underestimated because that particular potential is the formal potential of the Fe(III)/(II) couple of FePc. At this potential \u03b8\nFe(II)\u00a0=\u00a00.5, consequently the TOF values are underestimated by a factor of 2. Therefore, errors can be introduced if the potential for comparing ORR currents is close to the potential of that redox couple. For pyrolyzed materials, \u03b8\nM(II) cannot be estimated easily as most of these materials exhibit no clear redox signals that can be attributed to the M(III)/(II) redox couple.The two in situ and ex situ techniques mentioned before let the estimation of reactivity descriptors of PGM-free catalysts with a high degree of precision, and with comparable results. Usually, TOF values estimated from CO chemisorption result slightly lower that those calculated by nitrite stripping\u00a0because of the overestimation of the related SD, or different way to calculate the kinetic current in the two methods [33]. Moreover, also the difference between gas-phase accessibility (CO chemisorption) and electrochemical accessibility (nitrite reactivity) plays a role in the different values obtained by measuring the SD by in situ or ex situ methods. In fact, the electrochemical surface can match rather well the gas-phase surface for Fe\u2013N\u2013C materials with low S\n\nBET\n (specific surface area, that is, relatively low amount of micropores), although the two values can be quite different in the case of high S\n\nBET\n, where the micropore area is prevalent [33]. However, Fe\u2013N\u2013C materials have modest intrinsic catalytic activity, lower than Pt, especially in acid, obliging increasing the catalytic loading at the cathode to compensate for the overall activity [28,38\u201340]. In addition, thicker catalytic layers represent a limitation in terms of mass transport resistance, electronic resistance, and proton resistance [38], not only at RDE level\u00a0but especially at MEA (membrane electrode assembly) level, when testing polymer electrolyte fuel cells [40\u201342]. In fact, when working at high power density, thin electrodes are required to limit mass-transport\u2013related voltage losses [43]. Thus, enhanced proton transport properties of the active site are essential for a high TOF in acid [44]. Just as an example, Pt-based catalysts can easily reach TOF values ranging between 10 and 42 e\u2013 site\u22121 s\u22121 at reported conditions [28,43], whereas Fe\u2013N\u2013C materials are at much lower order of magnitude, not overcoming 2 e\u2013 site\u22121 s\u22121 at reported conditions [33,45].Different groups have investigated MN4 macrocyclic complexes not subjected to any heat treatment. They provide simple models to identify some reactivity descriptors because active sites are clearly identified, especially the metal centers in the MN4 moiety [21] as showing well-defined CV redox peaks attributed to metal-centered redox processes. The charge under those reversible peaks allows the accurate determination of the amount or electroactive catalysts present as \n\nS\nD\n\n\n[\n\nm\no\n\nl\n\ns\ni\nt\ne\n\n\n\nc\n\nm\n\n\u2212\n2\n\n\n\n]\n\n\n. Most reports agree that they lack the long-time stability/durability required for fuel-cell performance, especially in acid. However, Cao et\u00a0al. [46] reported that FePc can exhibit higher activity than Pt/C in alkaline media and a stability higher than 1000 cycles, when anchored on carbon nanotubes via a pyridine axial ligand (FePc-Py-SWCNT) (refer Figure\u00a01). Yang et\u00a0al. [37] have studied FePc as well and directly deposited on different nanocarbons. The activity depends strongly on the type of carbon substrate used. Figure\u00a03\na illustrates the TOF values for FePc on different carbon supports and the highest TOF value is observed when C450, a 3D nanoporous C with macropores of 450\u00a0nm, is used [37]. The low stability of FePc-based catalysts having high TOF values could be due to the fact that highly reactive sites for ORR can also be highly reactive to other species and/or intermediates such as peroxide and OH radicals that are generated faster than less active catalysts attacking those sites, besides carbon corrosion [35]. Similar mechanisms have been described for pyrolyzed metal\u2013nitrogen\u2013carbon catalysts [33,46].Yang et\u00a0al. [37] calculated these TOF values according to\n\n\n\nT\nO\nF\n\n\n[\n\ns\n\n\u2212\n1\n\n\n]\n\n=\n\n\ni\n\n[\n\nA\n\nc\n\nm\n\n\u2212\n2\n\n\n\n]\n\n\n\n4\n\u00b7\n\nQ\n\nF\ne\n,\na\nc\nt\ni\nv\ne\n\n\n\n[\n\nC\n\nc\n\nm\n\n\u2212\n2\n\n\n\n]\n\n\n\n\n\n\nwhere i is the current at 0.868 VRHE and Q\n\nFe,active\n represents the amount of electrochemically active centered Fe ion, estimated from the area of the Fe(III)/(II) redox peak determined by cyclic voltammetry under N2-saturated 0.1 KOH. Q\n\nFe,active\n depends on the morphology of the carbon support (specific surface area, pore structure, and roughness of the surface of the carbon), which affects somehow the amount of FePc deposited on the support itself at equal deposition procedures [37].As explained before, those TOF values in Fig.\u00a03 are probably underestimated because the number of Fe(II) active sites could be lower than the total sites N (mol cm\u22122). In fact, at that particular potential, a fraction of the sites are in the oxidation state Fe(III) as 0.868 VRHE is too close to the Fe(III)/(II) formal potential of the catalyst. A rough estimation using a formal potential of 0.868 VRHE using the Nernst equation (estimated from the peak of CV curves reported, which coincides with the potential used for comparing activities) indicates that only ca. 50% of the catalyst is active as Fe(II). Fe(III) in alkaline media does not catalyze ORR as those sites are strongly binding OH\u2013 ions [21]. However, the order of activity totally changes after a chronoamperometry at 0.868 VRHE, showing that the most durable catalyst (that is the one that lasted longer before reaching the 50% current loss) was the one with the lowest TOF value (FePc/KBC) [37]. Interestingly, the results denote a shorter durability with TOF increasing. This effect can be explained considering that a low TOF value implies a slower formation of reaction intermediates during ORR, such as HO2\n\u2013, which can partially hinder the active site. Thus, degradation is limited compared with catalysts with higher TOF values.Cao et\u00a0al. [46] did not provide any TOF, SD values, or CV profiles that could allow the estimation of Q. The enhancement of the activity of FePc when using a pyridine back-ligand seem to be associated to the electron-withdrawing effect of pyridine that would shift the Fe(III)/(II) redox potential in the positive direction and push the catalyst up towards the top of the volcano correlation of (logi)\nE\n versus the E\u00ba\u2032Fe(III)/(II) of the catalyst [21]. The beneficial effect of an electron-withdrawing axial ligand on Fe phthalocyanines has been demonstrated using several FePcs attached to carbon nanotubes [48,49] substrates and gold (111) [50]. These axial ligands seem to mimic the action of similar ligands in enhancing the catalytic activity for ORR of cytochrome-c in the respiratory chain of aerobic life [47,51].C\u2013N materials show ORR activity even in the absence of any transition metal. Chakraborty et\u00a0al. [52] have proposed a method for evaluating the active SD of metal-free nitrogen-doped carbon using catechol as an adsorbate. These catechol provide well-defined redox peaks that facilitate an indirect estimation of the mass-specific active SD (SD\n\nmass\n) evaluated from the electrical charge involved in these redox processes according to:\n\n\n\nS\n\nD\n\nm\na\ns\ns\n\n\n\n\n[\n\nactive\u00a0site\u00a0\u00a0\n\ng\n\n\u2212\n1\n\n\n\n]\n\n=\n\n\nIntegrated\u00a0CV\u00a0area\u00a0\n\n[\n\nA\n\nV\n\n]\n\n\u00b7\n\nN\nA\n\n\n\n[\n\ns\ni\nt\ne\nm\no\n\nl\n\n\u2212\n1\n\n\n\n]\n\n\n\nn\n\u00b7\ns\nc\na\nn\nr\na\nt\ne\n\n[\n\nV\n\ns\n\n\u2212\n1\n\n\n\n]\n\n\u00b7\nF\n\n[\n\nC\n\nm\no\n\nl\n\n\u2212\n1\n\n\n\n]\n\n\u00b7\nm\n\n[\ng\n]\n\n\n\n\n\n\nwhere N\n\nA\n is the Avogadro number, n the number of electrons, m is the catalyst's mass. The rest of terms have the usual meaning. The electrochemical surface area can be estimated according to (using the normalizing factor of 611\u00a0\u03bcC\u00a0cm\u22122 as charge of unit surface area on graphite [53]):\n\n\n\nE\nS\nA\n\n[\n\nc\n\nm\n2\n\n\ng\n\n\u2212\n1\n\n\n\n]\n\n=\n\n\nC\na\n\nt\n\na\nd\ns\n\n\nc\nh\na\nr\ng\ne\n\n[\nC\n]\n\n\n\n2\n\u00b7\nm\n\n[\ng\n]\n\n\u00b7\n611\n\n[\n\n\u03bc\nC\n\nc\n\nm\n\n\u2212\n2\n\n\n\n]\n\n\n\n\n\n\n\nChakraborty et\u00a0al. [52] double checked their electrochemical surface area estimation using the equation mentioned previously using the BET-specific surface area (S\n\nBET\n) and multiplied it by the total pyridinic nitrogen percentage, as reported by Guo et\u00a0al. [53] and found comparable results suggesting the accuracy of the equation aforementioned. This is important for intact and pyrolyzed catalysts as nonmetallic sites can contribute to the catalytic process.Most intact MN4 complexes do not exhibit long-term stabilities that can be compatible with fuel performance. This is less critical in alkaline media, and there are some reports that might promise some success in this sense if the FeN4 complexes are attached to carbon nanotubes directly or linked via pyridinic axial ligands.The combination of TOF and SD can be considered as a rigorous comparison between catalysts [28]. There are different ways of estimating TOF values and the values obtained depend on the method used. This is relevant to both intact and pyrolyzed materials. SD should consider only active sites that are available for ORR. Bulk sites do not count. When using TOF values, special care needs to be taken in estimating the amount of M(II) active sites present under operating conditions (electrode potential) because some complexes could exhibit M(III)/(II) redox potentials close to the operating potential.Finally, it has been suggested recently that intact complexes, which possess pyrrolic N inner nitrogens are not representative of real active sites present in MNx pyrolyzed catalysts, which predominantly pyridinic N inner ligands. In this case, complex having phenanthroline inner ligands can serve as better models for future studies.The authors contribute equally to the work by discussing and writing the manuscript, drawing the figures, along with approving its final version.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.J.H.Z. acknowledges the funding from Anillo Project ACT 192175 Chile and Fondecyt Project 1181037. S.S. and P.A. acknowledge the funding provided by the Staff Mobility for Training & Teaching between Programme and Partner Countries within the Program Erasmus+/KA1 Higher Education action KA107 (International Credit Mobility, years 2017 and 2018) for the reciprocal visits @ UCI (in 2019) and POLITO (in 2020), respectively.", "descript": "\n                  There has been a significant progress toward\u00a0the development of highly active and stable platinum group metal-free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells, promising a low-cost replacement for Pt group electrocatalysts. However, the success of such developments depends on the implementation of PGM-free electrocatalysts that are not only highly active but importantly, they also exhibit long-term durability under polymer electrolyte fuel cell operating conditions. This manuscript is an overview of the current status of a specific, most advanced class of PGM-free electrocatalysts: transition metal\u2013nitrogen\u2013carbon\u00a0ORR catalysts. We present an overview for the understanding of catalysts\u2019 performance descriptors for metal\u2013nitrogen\u2013carbon materials.\n               "}
{"full_text": "Isobutene (2-methylpropene) is a vital base chemical extensively used as a building block for the synthesis of a vast number of intermediates in the chemical industry. It is an essential precursor for the synthesis of various oxygenates like methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and methacrylates which are used as octane enhancing additives in gasoline [1]. Isobutene is also widely used in the polymer industry for the production of butyl rubber [2]. The estimated increasing demand for isobutene soon insists on the need for alternative synthesis pathways other than conventional naphtha steam cracking and crude oil fluid catalytic cracking methods. In this regard, the catalytic dehydrogenation of isobutane acquires much significance due to the available low-cost feedstocks [3]. However, the endothermicity of this process requires an elevated operational temperature to obtain high yield of isobutene.Chromium and platinum-based catalysts have been extensively studied for the dehydrogenation of isobutane (DHisoB). Pt-Sn/Al2O3 and CrOx/Al2O3 systems are well explored and already implemented in the industry a few decades back. Even though these catalytic systems are giving satisfactory results, they are suffered from some disadvantages. A part of chromium ions that exist in carcinogenic Cr6+ in alumina supported chromium catalyst causes severe environmental threats [4]. Also, possible sintering of Pt nanoparticles and high-cost limits Pt-based catalysts to some extent [5]. Moreover, the catalyst deactivation due to coke deposition is unpreventable at stringent reaction conditions. Hence, the development of an environmentally friendly and cost-effective promising catalyst for the non-oxidative dehydrogenation of isobutane becomes imperative.Catalyst support has a significant role in defining the activity performance. Especially for a dehydrogenation reaction, the support should be thermally stable to survive the rigorous reaction conditions. Moreover, the limited acidity can evade undesired C-C cracking and alkane isomerization reactions. High surface area and uniform pore size distribution of the support will enhance homogeneous metal dispersion [6]. While considering all these aspects, the nonreducible metal oxide, alumina, serves as an excellent candidate for the dehydrogenation reaction owing to its high thermal stability as well as mechanical strength. Moreover, state of the art drawn from the existing literature evinced alumina as the most preferred support for DHisoB, including various industrial applications [7\u20139]. Meanwhile, the commercialized catalysts have been modified to overcome the current difficulties and to improve activity by adding different metal oxides as active components [10]. Recently, Uwe et al. have proved that bare alumina itself is active for the reaction with a 30% yield towards isobutene due to the coordinatively unsaturated Al sites on the surface [11]. The supported VOx, GaOx, and MoOx materials on alumina have improved the conversion and selectivity for DHsioB [6].Alumina is acidic, and hence it should be modified to control the acidity. Researchers have successfully attempted to reduce the acidic properties of alumina by combining with ZnO and MgO, which has improved the dehydrogenation activity and catalyst stability [12]. The addition of alkali metals can also contribute to this by selectively poisoning the acidic sites and thus hinder the coke formation [13,14]. Shingo et al. have established that the addition of a small amount of iron can improve the activity, selectivity, and stability of Pt/Al2O3 catalyst for the DHsioB. NH3-TPD studies have proved a considerable reduction in influential acid sites after Fe addition [8], which is in good agreement with the investigations of Kania et al. [15]. These prominent acidic sites prompt alkene hydrogenolysis and decrease isobutene selectivity [16]. Apart from these, iron oxide-containing activated carbon has also served as a suitable catalyst for the dehydrogenation of C4-C5 hydrocarbons [17].Inspired by these described observations, in this contribution, we illustrated the synthesis of chromium-free catalytic system based on iron-doped mesoporous alumina system. To solve the above-discussed problems, the catalyst is separately modified with an alkali metal, phosphorous as well as noble metal via dry impregnation method. The synthesized materials are characterized for their topology, morphology and chemical properties by an array of instrumentation techniques including powder X-ray diffraction (PXRD), N2 physisorption analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (H2-TPR), temperature-programmed desorption (NH3-TPD) and thermogravimetric analysis (TGA). The materials displayed promising catalytic activity towards the non-oxidative dehydrogenation of isobutane. Iron doped alumina based catalyst systems were explored for different reactions. However, in the current study Ag, K and P are used as promoters to enhance the catalytic activity of isobutane dehydrogenation and is reporting for the first time up to our knowledge.The chemicals with more than 99% purity were used as received from the respected company for material synthesis. Fe doped mesoporous alumina catalyst was prepared via method reported by Bing Yan et al. [18]. In a typical synthesis, roughly 1.5\u00a0g of F-127 (Sigma Aldrich) and 0.36\u00a0g Fe(NO3)3.9H2O (Merck) was added to 20\u00a0mL anhydrous ethanol (99.9%, CSS) and vigorously stirred for 4\u00a0h and labeled as \u2018A.\u2019 Meanwhile, to a stirring mixture of 15\u00a0mL anhydrous ethanol and 1.6\u00a0mL concentrated nitric acid (69\u201370%, Thomas baker), 12\u00a0mmol aluminum isopropoxide (Aldrich) was added and labeled as \u2018B.\u2019 Both A and B were combined using 3.0\u00a0mL ethanol to transfer the solution B. This final mixture was stirred continuously for 8\u00a0h and the obtained residue was dried in the oven at 60\u00a0\u00b0C for 48\u00a0h. Further, the solid was ground well and calcined at 700\u00a0\u00b0C for 5\u00a0h with 1\u00a0\u00b0C\u00a0min\u22121 ramping rate. The final powder was represented as MesoFeAl.For the synthesis of promoted catalysts, 1\u00a0wt% of K, P, and Ag were dry impregnated over 2\u00a0g MesoFeAl from a concentrated solution of the precursors KNO3 (Merck), orthophosphoric acid (88%, Merck) and AgNO3 (99.995%, Alfa Aesar) respectively. These solids were sintered at 550\u00a0\u00b0C for 2\u00a0h with 1\u00a0\u00b0C\u00a0min\u22121 and denoted as KMesoFeAl, PMesoFeAl, and AgMesoFeAl. For the comparison study, the pristine support mesoporous alumina (MesoAl) was also synthesized by following the above procedure without adding the Fe precursor. Later, Fe was impregnated from the nitrate precursor over MesoAl and represented as FeMesoAl.The catalytic activity of as-synthesized mesoporous alumina catalysts was measured for the non-oxidative dehydrogenation of isobutane in a continuous flow mode fixed bed reactor system equipped with an Inconel (nickel\u2011chromium based alloy) made reactor tube of 8*11*480 (ID:OD:L) mm dimension. The reactor tube was heated in a furnace fitted with two zones maintained at the same reaction temperature. In a single measurement, 300\u00a0mg catalyst sieved into 1.2 -1.7\u00a0mm grain size pellets were loaded at the center of the reactor tube in between quartz wool to form a catalyst bed. High temperature stable ceramic beads were used to fill the remaining space. The bed temperature was monitored continuously with K type thermocouple. A mixture of isobutane and Ar taken in an equal ratio was continuously passed into the catalyst bed using 5890E series Brook's make mass flow controllers at 400\u00a0h\u22121 GHSV. The reaction temperature was varied between 400 and 600\u00a0\u00b0C for the study in different runs. The dehydrogenated products were analyzed in every 30\u00a0min time interval with a Thermo Scientific Trace 1110 gas chromatograph equipped with both FID coupled with Alumina plot column and TCD coupled with Porapak Q as well as Molecular sieve columns. The major components in the effluents were n-butane, 2-butenes, 1,3-butadiene, propene, propane, ethene, ethane, methane and hydrogen. No carbon oxides were detected. Conversion of isobutane, and selectivity for isobutene, werecalculated using the equations reported elsewhere [10].Crystalline features of the synthesized materials were collected by powder X-ray diffraction analysis in PANalytical X'pert Pro dual goniometer diffractometer. The data were collected with a step of 0.008\u00b0 (2\u03b8) and a scan rate of 0.5\u00b0 min\u22121 at room temperature. The radiation applied was Cu K\u03b1 (1.5418\u00a0\u00c5) with a Ni filter, and the data was obtained using a flat holder in Bragg-Brentango geometry. Nitrogen physisorption was analyzed for the textural properties of the materials from Quantachrome Quadrasorb SI using the Brunauer-Emmett-Teller (BET) model. N2 sorption data were examined at \u2212196\u00a0\u00b0C after degassing the samples at 300\u00a0\u00b0C and the relative pressure of P/P0\u00a0=\u00a00.05\u20130.3 was selected to calculate surface area. Horiba JY LabRAMHR800 Raman spectrometer coupled with a microscope in reflectance mode was used with a 628\u00a0nm excitation wavelength for Raman studies. X-ray photoelectron spectra were acquired from the pelletized materials on the Thermo Scientific K-Alpha+ instrument using micro-focused and monochromated Al K\u03b1 radiation with energy 1486.6\u00a0eV.Temperature programmed reduction experiments were carried out in a Micromeritics Autocem II 2920 chemisorption analyzer (USA). In a single TPR analysis, 0.05\u00a0g catalyst was pre-treated at 400\u00a0\u00b0C inside the furnace coupled with the instrument with 30\u00a0cm3\u00a0min\u22121 of 10% O2/He controlled by Brooks make mass flow controllers. Then, the sample was reduced with 5% H2/Ar at a 30\u00a0cm3\u00a0min\u22121 flow rate while ramping the sample temperature from 50 to 1000\u00a0\u00b0C at 10\u00a0\u00b0C\u00a0min\u22121. Hydrogen consumption was quantified using a thermal conductivity detector. Acidity of the materials were determined in the same model instrument. In a typical TPD study, the samples were degassed at 400\u00a0\u00b0C under Helium at 30\u00a0cm3\u00a0min\u22121 flow before each run. Afterwards, the sample temperature was brought down to 50\u00a0\u00b0C to adsorb the probe molecule at 10% NH3/He (30\u00a0cm3\u00a0min\u22121) for the acidic site evaluation. The desorbed gas was analyzed from 100 to 1000\u00a0\u00b0C with 10\u00a0\u00b0C\u00a0min\u22121 ramp rate. The quantification was made from the resulting TPD profiles.A dual-beam scanning electron microscope FEI company made and Quanta 200 3D model operated at 30\u00a0kV was used to collect scanning electron microscopy images. A high-resolution transmission electron spectroscopy study was performed in JEM2100 multipurpose instrument operated at 300\u00a0kV. The sample was prepared by dispersing the powder in isopropyl alcohol followed by drop-casting on carbon\u2011copper mesh (200\u00a0\u03bcm size) and silicon wafer respectively for TEM and SEM study. Coke deposition over the spent catalyst was determined with the help of the Perkin Elmer instrument. TG analysis was performed under the air atmosphere and the carbon combustion temperature was taken from the exothermic event in DTA.A conventional fixed bed reactor was used to study the catalytic activity of the mesoporous alumina catalysts. The effect of different promoters, as well as reaction parameters like temperature on the activity performance, was investigated through various experiments. The main products were obtained from the dehydrogenation, isomerization, and cracking processes occurred during the reaction (Scheme 1\n). Coke deposition was also detected near the catalyst bed at the end of each experiment.Catalyst promoters have been utilized on an existing catalyst to improve the product yield. Therefore, the effect of different promoters on the selective dehydrogenation of isobutane to isobutene was performed at 600\u00a0\u00b0C. All experiments were conducted at 400\u00a0h\u22121 GHSV concerning isobutane and argon mixture taken in 1:1 ratio. The results are depicted in Fig. 1\n as well as in Table S1. For the comparison study, the reaction was carried out over MesoAl also. Interestingly, as can be seen, the bare support exhibited appreciable activity under the given reaction conditions with the highest selectivity towards total dehydrogenating products (STDP), which include isobutene, 1-butene, 2-butenes, propene, and ethene. However, the final conversion of isobutane (Xisobutane) and selectivity for isobutene (Sisobutene) were low. The incorporation of Fe simultaneously decreases STDP and increases isobutane conversion. It can be realized from the activity performance given in Fig. 1 that the addition of small amount of promoters only slightly affected the dehydrogenation reaction pattern at 600\u00a0\u00b0C. Potassium promoted catalyst (KMesoFeAl) showed excellent selectivity towards the dehydrogenation reaction while conversion was comparatively low. Conversely, the acid-treated material (PMesoFeAl) was less selective for isobutene with a better isobutane conversion. Surprisingly, the noble metal promoted AgMesoFeAl displayed slightly higher yield of isobutene, Yisobutene (Table S1) at the studied reaction parameters. Isobutane conversion achieved was 36%, with 32% selectivity towards isobutene. It is essential to highlight the considerable STDP over the same catalyst.To investigate the effect of the synthesis method adopted for Fe incorporation in MesoAl support on the non-oxidative dehydrogenation reaction, FeMesoAl was prepared by dry impregnation method. Moreover, the activity performance of this catalyst was compared with the in situ prepared MesoFeAl catalyst. The activity results represented in Fig. 1 clearly illustrates that FeMesoAl shows the least selectivity towards the dehydrogenation products. The reason for this observation was analyzed with the material properties and explained in the respective sections. It may be interpreted in terms of the amount of coke deposited on the surface, which is quantified from TG analysis. From the above results, AgMesoFeAl was selected as a representative catalyst for further analysis.Fig. S1 shows isobutane conversion and isobutene selectivity as a function of reaction temperature over AgMesoFeAl catalyst. At 400\u00a0\u00b0C, Xisobutane was insignificant with the least isobutene selectivity due to the formation of other dehydrogenated products. The main process that occurred at this temperature would be isomerization of the formed alkene to n-butenes. On further increasing the reaction temperature up to 500\u00a0\u00b0C, conversion slightly improves accompanied by the steady increase in the product selectivity. A sharp increase in reactant conversion is observed above 500\u00a0\u00b0C. Contrastively, the selectivity slowly increases with the reaction temperature and remained constant after 550\u00a0\u00b0C. Thus, Fig. S1 illustrates that 36% isobutane is converted with 32% selectivity towards isobutene at 600\u00a0\u00b0C. The formation of other alkenes is also in line with the isobutene selectivity pattern when plotted against reaction temperature (Fig. S1). At higher temperatures, isobutene was further cracked into lighter hydrocarbons. This results in coke formation which is deposited on the catalyst surface and blocks the active sites. Post characterization of the spent catalysts supports this observation. This activity trend with temperature may also be correlated to the particle size distribution over the alumina support during the TOS analysis.The stability of a catalyst under the reaction conditions has a major significance while approaching commercialization. Hence, the time on stream study of AgMesoFeAl was executed at an optimized reaction temperature; 600\u00a0\u00b0C. Fig. 2\n further depicts the long term performance of AgMesoFeAl. It was perceived from the study that at an early time on stream, the conversion is 30% with appreciable selectivity for dehydrogenated products. Gradually, Xisobutane improves during this initial 4\u00a0h phase and correspondingly Sisobutene decreases. The most top performance over this catalyst was achieved as 32% isobutene selectivity and 36% conversion of isobutane under the studied parameters. After 4\u00a0h, a steady reduction in conversion (30%) as well as selectivity (30%) is observed. Subsequently, this activity remained constant above 10\u00a0h. Followed by this initial phase, nearly stable activity was maintained up to 60\u00a0h.The main significance of the present work is the maintenance of overall product yield over 60\u00a0h under the drastic reaction conditions. Additionally, here dehydrogenation is practiced with low space velocity. Major reasons for the decrease in activity after 10\u00a0h and the maintenance of isobutene yield in TOS are explained in the material characterization section based on the particle distribution and carbon deposition on the catalyst.All synthesized mesoporous alumina catalysts were well studied for the structural-textural properties, and surface morphology. The acivity results presented in the previous section are correlated to establish the relationship between the performance and material characteristics.N2-sorption was used to characterize the textural features of the calcined promoted catalysts and pure MesoAl support. Total surface area (SBET), pore-volume (Vp), and average pore-size values were calculated applying the BET method and presented in Table 1\n. According to the IUPAC classification system, the N2-sorption isotherms given in Fig. 3\n indicate type IV with H1 hysteresis loop typical of mesoporous materials. However, FeMesoAl possesses slight features of the H2 loop with wide pore size distribution. These are characteristic of bottlenecked pores [19]. A close analysis of isotherms of the promoted catalysts proves that even after the impregnation process, the materials maintained mesoporosity. Pore size distribution curves derived from the N2-desorption process shown in Fig. S2 also supports the presence of mesopores. Additionally, it may be mentioned here that small pores are unfavorable for the migration of coke deposited on the active sites towards the acidic sites [20].All mesoporous materials exhibited an appreciable specific surface area. Surprisingly, the total surface area of MesoAl increases after Fe doping. However, iron impregnation on mesoporous alumina support may block the pores of FeMesoAl in the rich presence of iron oxide resulting in a slight decrese in surface area from 162 to 153 m2/g. The average pore diameter (Dp) considerably reduces in the synthesized catalysts compared to the pristine support MesoAl due to the entering of the active metal species into the pore channels. The uniform mesoporous structure and noticeably large pore volume of these materials can significantly contribute towards catalytic activity. AgMesoFeAl also possess remarkable surface area with comparable pore dimensions with other samples.\nFig. 4\n presents the X-ray diffractions of the synthesized mesoporous alumina-based catalysts. The diffraction pattern of all samples indicates the formation of \u03b3-Al2O3 phase with JCPDS card no. 10\u20130425. The peaks observed at 31.9, 37, 39.5, 45.9, 60.5 and 67\u00b0 are attributed to the diffraction planes 220, 311, 222, 400, 440 and 511 respectively [21]. For MesoFeAl, no peak linked to iron is found suggesting high dispersion of Fe during the in situ synthesis, and the formed iron oxide domains are too small to be detected. However, Fe has indeed detected in XPS and elemental mapping analysis. Moreover, Kobayashi et al. reported that at low content, Fe2O3 could form a solid solution with \u03b3-Al2O3 [8]. Although, the iron impregnated sample FeMesoAl exhibited peaks at 2\u03b8 values 24.3, 33.3, 35.9, 41.1, 49.6, 54.3, 57.6, 57.9, 62.7, and 64.4\u00b0 corresponding to \u03b1-Fe2O3 phase (JCPDS card no. 33\u20130664). This observation stipulates that only a diminutive amount of Fe is doped in the alumina framework and also, iron oxide species are poorly dispersed on mesoporous alumina support due to the ex situ synthesis method. The excess amount of Fe in FeMesoAl would oxidize in the air during the calcination process to form iron oxide and separately crystallizes on the surface.When promoters are added in the MesoFeAl sample, the corresponding peaks are not identified due to the high dispersion of these species on the alumina matrix. Except in FeMesoAl, a small shift in the diffraction angle to lower 2\u03b8 values are noticed in other solids compared to the pristine support MesoAl. This could be because of the changes that occurred in the lattice parameters, which might arise from the proper doping of the elements into aluminum site [22]. The difference in ionic radii of Fe3+ (0.645\u00a0\u00c5) and Al3+ (0.535\u00a0\u00c5) might cause the broadening of \u03b3-Al2O3 reflections. Compared to other materials, AgMesoFeAl best resembled in crystalline structural features with MesoAl proving the perfect incorporation of Fe and Ag. This could be a reason for the observed faintly higher activity of AgMesoFeAl. No silver oxide peaks were found in this sample, because of the minimal size and greater dispersion in the MesoFeAl matrix. Hence it can be concluded that stable phases of Fe2O3 and Al2O3 are formed in the studied catalysts after sintering at high calcination temperature.Further examination of the structural and crystalline properties was conducted with Raman analysis of the annealed samples. Fig. S3 shows the Raman spectra of FeMesoAl, MesoFeAl, and AgMesoFeAl samples. FeMesoAl revealed peaks at 228 (A1g), 504 (A1g), 247(Eg), 297 (Eg), 416 (Eg), 620 (Eg) cm\u22121 corresponding to the Raman modes of \u03b1-Fe2O3. Additionally, some rare Raman peaks reported by a few researchers for the hematite phase are also identified at 669 and 836\u00a0cm\u22121 [23]. Surprisingly, MesoFeAl does not exhibited a single peak attributing to iron oxide, demonstrating either high dispersion of Fe in alumina matrix or it is undetectable in Raman analysis. This observation further supports PXRD and HRTEM studies.Material morphology and proper dispersion of the elements contribute to a great extent towards the catalytic performance. The selected AgMesoFeAl catalyst was analyzed with TEM, and the images collected are shown in Fig. S4. The results exhibit a mesoporous structure which are in consistent with the type of isotherms obtained from the N2 sorption analysis. A sheet-like morphology is visible for the fresh AgMesoFeAl catalyst with evenly distributed metal nanoparticles. High-resolution TEM analysis provided well-resolved lattice fringes with 0.2\u00a0nm spacing characteristic of the interplanar distance corresponding to (104) plane of the \u03b1-Fe2O3 phase. The SAED pattern of this sample with concentric rings and bright spots represented typical of a nanocrystalline material with an average particle size of 5.6\u00a0nm. Small clusters of iron oxide particles on the surface are observed in FeMesoAl, as represented in Fig. S5 demonstrating agglomeration and poor dispersion. The segregated \u03b1-Fe2O3 crysatallites are clearly emerging in the PXRD pattern shown in Fig. 4. This could be due to the different synthesis strategies adopted for the material. Therefore, all other catalysts are superior to FeMesoAl in the studied selective dehydrogenation reaction. For example, AgMesoFeAl exhibited better dispersion of metal nanoparticles than FeMesoAl, which is contributing to the catalytic activity.Elemental mapping of fresh AgMesoFeAl as a respect for its best catalytic properties was performed using HRTEM, and the results are given in Fig. S6. The images revealed uniform distribution of the elements over the surface. This further indicates fine dispersion of the catalyst supporting PXRD as well as Raman results. SEM analysis was used to determine the surface morphology of the catalyst, and it does not exhibited any particular morphology as represented in Fig. S7. The particle shape remained the same even after the non-oxidative dehydrogenation reaction. The EDX data also confirms the presence of all elements in the catalyst. No elements other than Ag, Fe, Al, and O were present, which signify the excellent purity of the sample. Therefore, it may be perceived that the proper dispersion of active metal species on the mesoporous alumina matrix can facilitate the dehydrogenation activity. These observations declare the appropriate incorporation of the metal species in alumina consistent with the structural features of the optimized catalyst. Time-dependent DHsioB experiments are carried out over the AgMesoFeAl catalyst to get insight into the morphology changes happening on its surface. The results are depicted in Fig. S4 and explained in the post reaction analysis.Acidic-basic properties of the materials are crucial in defining the catalytic activity. In view of this, temperature-programmed desorption experiments were conducted with NH3 as a probe molecule to measure the number of acid sites present in the catalysts. The uptake of NH3 is propotional to this value. The TPD profiles are given in Fig. S8 and the acidity values measured are presented in Table 1. According to the existing works from the literature, the samples exhibit two significant peaks at around 150, and 450\u00a0\u00b0C corresponding to NH3 desorbed from the weak and medium Lewis acidic sites, respectively [24]. It is observed from the TPD profile and the acidic strength values shown in Table 1 that bare support MesoAl (0.02\u00a0mmol/g) and FeMesoAl (0.07\u00a0mmol/g) are least acidic in nature. The in situ incorporation of metal ions enhanced the acidity of the catalysts. Iron doping to MesoAl has considerably improved the acidity to 0.34\u00a0mmol/g. As reported by Carvalho et al., strong acid sites cause hydrogenolysis of alkenes [16], which are absent in the studied materials. Isobutane mainly activates on the acidic centers and promotes the conversion. On the other hand, desorption of isobutene may inhibit at the medium and strong acidic centers, which can increase the cracking reactions [20]. The in situ iron-doped catalysts show a comparable amount of medium acidic sites, which matches well with the activity trend proving the role of catalyst acidity. The ratio between the number of active metal sites to the number of acidic sites is significant in defining catalytic performance.The redox properties of the catalysts are another factor affecting the catalyst performance. Therefore, the materials were measured with H2 probed temperature-programmed reduction analysis. Under the reducing atmosphere, the amount of consumed hydrogen for all sintered catalysts is quantified from the TPR profiles (Fig. S9) and tabulated in Table 1. AgMesoFeAl exhibits the highest H2 consumption among the promoted catalysts (1.49\u00a0mmol/g) due to the additional reduction of silver oxide, which occurs at low temperature. Except in FeMesoAl, no other materials demonstrated reduction peaks correspond to the iron oxides proving the high dispersion of these species on alumina. On the contrary, the iron oxide species in FeMesoAl is reduced in different steps as Fe2O3\u00a0\u2192\u00a0Fe3O4\u00a0\u2192\u00a0FeO\u00a0\u2192\u00a0Fe [25]. Hence, TPR further corroborates the poor dispersion of Fe over mesoporous alumina in FeMesoAl unlike other catalysts. Peak broadening was detected due to the multistep reduction of iron oxide. The highly exposed iron oxide crystallites can lead to further cracking of formed alkenes which result in low selectivity towards the desired isobutene as well as other dehydrogenated products (Fig. 1). Therefore, the highest coke deposits are found on this catalyst (26.2%) as shown in Table 1. Quickly reducible silver oxides in AgMesoFeAl might make it favorable for high-temperature dehydrogenation.To understand the surface species involved, XPS analysis was conducted ex situ for the fresh and spent catalysts, and the results are represented in Fig. S10. All spectra were binding energy (BE) shifted by carbon correction using the C 1\u00a0s core level value (284.8\u00a0eV) of adventitious carbon on the surface. Spectra recorded for Ag 3d exhibited (Fig. S10a) main peaks at 367.8 and 373.7\u00a0eV constitute for 3d5/2 and 3d3/2 of Ag2O in Ag+ state [26]. The presence of AgO cannot be rejected or confirmed owing to its high instability under ultra-high vacuum [27]. Al 2p spectra, given in Fig. S10c contained 2p3/2 peak at 74.5\u00a0eV assigned to Al3+ in mesoporous alumina. XPS peak appeared at 78\u00a0eV is attributed to anhydride Al2O3 [28]. The doping of metal into alumina lattice can affect the chemical states of Al and O, and the BE values will be changed [29]. O 1\u00a0s core level XPS spectra recorded is shown in Fig. S10d for the samples before and after the reaction. A major broad peak at lower BE value (530\u00a0eV) represents the lattice oxygen collectively contributed from all metal oxides. The surface Al-O species will be present at around 531.4\u00a0eV. Subsurface O species form silver may also contribute towards this peak [30]. The isolated \u2013OH groups give rise to a peak at 532.7\u00a0eV. It is well known that any chemisorbed oxygen species like water molecules appear at higher BE. These values fall in slightly higher regions compared to the reported data. A close inspection of Fe 2p core-level spectra of fresh AgMesoFeAl in Fig. S10b hints the presence of 2p3/2 of Fe3+ state at BE, 712\u00a0eV accompanied by satellite peak located at 720\u00a0eV [31]. A small peak shift observed indicate that the catalyst surface is reduced after the reaction.Silver oxide can be reduced to metallic silver under the high-temperature dehydrogenation atmosphere in the presence of isobutane. Probably the Ag0 would drive the reaction. However, no peaks correspond to Ag metal are detected in XPS may be due to its high susceptibility for atmospheric oxidation. Interestingly, a well-known phenomena called voltage induced differential charging [32] might happened in the core level spectra of the studied samples. Consequently, distortion of peaks and shift of the measured position of the peaks corresponding to the core levels could occur. This is usually observed on the non-uniformly conducting sample surface [32]. The splitting of peaks observed in the current study may be attributed to this phenomenon. The catalytic activity is not directly correlated to the XPS analysis although gives the idea about the surface composition as well as oxidation states of each element in the catalyst.Post reaction analysis of the catalysts was carried out to discover the structural and chemical changes that occurred during the high-temperature isobutene synthesis. The X-ray diffraction patterns of all catalysts after the reaction were collected and given in Fig. S11. The crystallinity of the samples remained intact even after the reaction at 600\u00a0\u00b0C. Crystalline phases are analyzed after 6\u00a0h as well as 60\u00a0h of DHsioB reaction at the optimized reaction temperature and presented in Fig. 5\n.TEM images of spent AgMesoFeAl catalyst are recorded to study the changes that occurred in the catalyst morphology. The material retained sheet-like mesoporous structure even after 6\u00a0h of dehydrogenation reaction at 600\u00a0\u00b0C with a small increment in particle size to 7.7\u00a0nm. It is clearly seen as slightly bigger particles in the TEM image represented in Fig. S4. However, after 60\u00a0h of TOS study, more agglomerated metal particles are observed (Fig. S4), and the average particle size increases to 16.9\u00a0nm. The SAED pattern given in the inset corroborated the separation of crystalline metal oxides and is also distinctly perceived from the XRD pattern of AgMesoFeAl after 60\u00a0h as presented in Fig. 5. Moreover, thermogravimetric analysis performed in the presence of air quantified 14.4% coke deposition on the catalyst after on-stream analysis. It is observed as carbon nanotubes in the TEM image depicted in Fig. S4 after 60\u00a0h reaction. Close analysis of the TEM images illustrates some of the active metal particles are entrapped in the nanotubes, which would also be a reason for the activity decline. The Raman spectrum of this sample shown in Fig. S12 confirms the formation of both sp2 and sp3 hybridized carbon formed during the high-temperature dehydrogenation process. This coke formation mainly leads to slight deactivation of AgMesoFeAl over 60\u00a0h of stability test. However, the material maintained mesoporosity throughout the reaction.Coke formation is one of the chief concerns involved in the isobutane dehydrogenation at high temperatures. The type of coke formed and the blocking of active sites by coke deposition can cause severe catalyst deactivation. Hence, AgMesoFeAl spent catalyst after 60\u00a0h of on stream study was investigated with Raman spectroscopy to study the nature of coke. The spectrum given in Fig. S12 proved the presence of D and G bands at 1330 and 1590\u00a0cm\u22121, respectively which are attributed to the disordered and pristine graphene bands. In addition to this, peaks correspond to Fe2O3 are also emerged, which were not visible in the fresh catalyst. These observations could be correlated to PXRD and TEM results evidencing phase separation as well as carbon deposition happening on the catalyst surface that leads to the deactivation during TOS analysis.Since carbon formation is observed as one of the main reason for catalyst deactivation, the quantification of coke deposited over the spent catalyst becomes inevitable. Therefore, thermogravimetric analysis was carried out for all spent catalyst and depicted in Fig. S13. The thermogram of AgMesoFeAl-TOS after on stream DHsioB result is also represented. The amount of coke for each catalyst was quantified and tabulated in Table 1. The pristine support MesoAl exhibits the least coke deposition, which might be due to the lowest isobutane onversion and less C-C cracking reactions occurred during the dehydrogenation reaction. The table clearly shows PMesoFeAl (12%), as well as AgMesoFeAl (12.7%), have lower coke deposits compared to other promoted catalysts. The maximum C-C cracking occurred on FeMesoAl (26.6%), which is higher than both AgMesoFeAl after 60\u00a0h on stream study (14.4%) and MesoFeAl (16.2%). This observation illustrates the importance of in situ Fe doping in the mesoporous alumina. Moreover, the results obtained from TG analysis can be directly correlated to the dehydrogenation reaction trend. Coke formation continues even after the initial period which is evidenced from the thermogravimetric analysis of spent catalyst after 6\u00a0h (12.7%) and 60\u00a0h (14.4%). But the coke deposition process is very slow. This might be a reason for the stable activity of AgMesoFeAl after the initial period. In addition, FeMesoAl exhibited a meager isobutene yield due to the cracking of formed alkene under severe reaction conditions. Therefore, maximum carbon deposition was observed for this sample. Additionally, sometimes carbon deposition can also contribute to maintain the alkene selectivity through deactivating the extreme active sites. McGregor et al. have studied butane dehydrogenation over VOx/Al2O3 and observed coke encapsulation over the catalyst after the reaction at 700\u00a0\u00b0C. Unlike typical catalyst deactivation, in this case the catalytic activity remained the same indicating that carbon species can catalyze the reaction [33]. Appreciable activity towards carbon-based systems are also reported for the catalytic dehydrogenation of propane by Liu et al [34]. Interestingly, nitrogen-doped carbon nanotubes exhibited efficient isobutene yield for the conversion of isobutane without any oxidant [35]. These results can be correlated to the current study and the possibility of carbon nanotubes to catalyze the dehydrogenation process cannot be ruled out. This could also be a reason for the stable activity of AgMesoFeAl in the TOS analysis.In summary, mesoporous alumina-based catalysts synthesized by the sol-gel method along with the material characterization correlated to their activity performance for non-oxidative dehydrogenation delivered essential insights to the synthesis of isobutene. The overall yield of isobutene over MesoAl was improved after metal incorporation. The proper doping of iron in the alumina matrix and the optimum surface acidity are viewed as the chief properties to inhibit cracking reactions and thus affecting isobutene selectivity. AgMesoFeAl was selected for further studies as a representative of the metal incorporated material. The catalyst surface was reduced after the reaction. Particle agglomeration followed by carbon nanotube development occurred over the continuous gas phase high-temperature dehydrogenation reaction, and isobutene formation are slightly hindered. To conclude, the promoted materials exhibited nearly the same activity and can be a promising catalyst for the selective dehydrogenation of isobutene. AgMesoFeAl catalyst provided a stable yield of isobutene during the long time reaction under the drastic reaction conditions with low space velocity.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 thank the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for fellowship. Financial support from Mission Mode Project (HCP-0009) on catalytic methodologies for dimethyl ether funded by CSIR, India, and GAP-324426 project funded by the Department of Science and Technology-Science and Engineering Research Board (DST-SERB), India are greatly 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.2020.106263.", "descript": "\n                  Production of isobutene is commercially consequential and highly demanding from the end-use industries being a key platform molecule as well as an intermediate for a variety of value-added chemicals. Traditionally, isobutene is prepared via steam cracking and fluid catalytic cracking methods. However, the catalysts used in these conventional methods have disadvantages like coke formation, sintering, etc. In this study, the catalytic non-oxidative dehydrogenation of isobutane over acidic, alkaline, and noble metal promoted mesoporous iron-doped catalysts was investigated. Iron doping has a significant function in controlling isobutene selectivity. The synthesis method is crucial to achieve successful metal doping in the mesoporous alumina matrix. Promoted catalysts exhibited a notable difference in isobutane conversion with a marginal change in dehydrogenation selectivity. Silver promoted catalyst showed slightly higher isobutene yield due to the optimal catalytic properties. Thiscatalyst was stable for a considerable duration, and coke deposition, as well as particle agglomeration, were observed to faintly inhibit the catalytic activity.\n               "}
{"full_text": "In the past centuries, human society has made tremendous technological and economical progress, which were accompanied by the rapid growth in world population [1]. The resulting challenge of fulfilling the increasing demand in food supply has always been a universal concern. As an efficient solution to this problem, intensive agriculture strongly relies on the industrial manufacturing of synthetic fertilizers, which require a large-scale supply of ammonia as feedstock. Fritz Haber [2\u20134] demonstrated the possibility of a catalyzed synthesis of ammonia from hydrogen and nitrogen under high temperature and pressure. Later Carl Bosch realized the industrial scale production of ammonia from its elements through the Haber\u2013Bosch process [5,6].Although many catalyst systems have been proven to be active for ammonia synthesis [7,8], only two of them showed a potential high enough for application in the industrial production of ammonia. One of these catalyst systems is the promoted Ru-based catalyst supported on carbon materials [9\u201311] or metal oxides [12\u201314]. However, the high manufacturing costs make Ru-based catalysts hard to compete with the other catalyst system [15,16], i.e. the industrially widely applied iron catalyst found by Alwin Mittasch. The catalyst precursor of the iron-based catalysts is routinely prepared by fusing iron oxides with other oxide additives consisting of \u201cstructural\u201d promoters like Al2O3 and CaO, and \u201celectronic\u201d promoters like K2O [17\u201320]. Prior to ammonia synthesis, the fused iron oxide, which contains additional oxide promoters, is activated by a multistep-reduction process in a H2/N2 mixture [21\u201323]. The iron oxide phase in the catalyst precursor has proven to be a crucial factor that influences the phase composition and the morphology of the reduced catalyst and determines the performance of the catalyst in ammonia synthesis [24,25]. Besides the conventional fused magnetite, wuestite has shown promising potential as an alternative catalyst precursor [25\u201328]. In the late 1980s, Liu et al. [27,29\u201332] have reported that catalysts based on non-stoichiometric wuestite Fe1-xO as precursor show superior activity in ammonia synthesis compared to the magnetite-based catalysts. Please note, that although magnetite and wuestite are the typical industrial oxide precursors for Fe-based ammonia synthesis catalysts, Fe2O3-derived ammonia synthesis catalysts are widely applied in academic research [33\u201335].In addition to the iron oxide precursor, the oxide promoters also play a decisive role not only in the formation of the active catalyst but also on the performance. Alumina is considered as one of the important structural promoters for ammonia synthesis catalyst [18,36]. For magnetite-based catalysts it is reported, that alumina is present in the reduced catalyst in the form of a thin Al2O3 layer [37,38] on the surface or segregated as both, FeAl2O4 and Al2O3 [39]. The presence of alumina or/and a Fe-Al solid solution [40] can prevent the activated iron from sintering, which would otherwise lead to the formation of larger crystals [18]. In the wuestite precursor, FeAl2O4 can also be formed via the solid reaction between FeO and Al2O3 [41]. However, due to the different crystal structure of FeO and FeAl2O4, the distribution of Al2O3 in the wuestite precursor is not as uniform as in magnetite precursors [25]. Consequently, it is assumed that alumina is not the only structural promoter and other oxide promoters such as SiO2 and ZrO2 are required [25]. Nevertheless, for wuestite-based precursor alumina is believed to participate in the restructuring of the surface of the reduced catalyst [25,42,43]. CaO is another structural promoter for catalysts based on magnetite as precursor [44,45]. In the reduced catalyst CaO segregates to the space between the Fe crystallites [46,47]. Furthermore, CaO increases the surface area and activity of the activated catalyst as well as promotes its resistance against impurities in the reactant gas [20]. For the wuestite precursor, CaO additionally inhibits the disproportionation reaction at low temperature of Fe1-xO which would form magnetite and metallic iron [25].Besides the structural promoters, K2O is the most important electronic promoter for ammonia synthesis catalysts based on both, magnetite or wuestite precursors [18,25]. Here, an enrichment of potassium on the surface during the reduction of the iron oxide precursors is taking place [25,46,48]. K2O is hydrolyzed to strongly basic potassium hydroxide during the reduction, which enables the formation of amphoteric metal oxohydroxides with alumina and iron oxide [20]. The basic iron oxides act as a binder to the other oxide promoters as well as positively influence the reduction kinetics. In the activated catalyst K exists in the form of anhydrous KOH rather than a metallic adsorbate [20]. The promoting effect is attributed to the ability of the active anhydrous KOH species to enhance the dissociative adsorption energy of nitrogen, which is described as rate determining step of ammonia synthesis [20,49]. Furthermore, the basic KOH species reduces the adsorption energy of ammonia and, therefore, prevents the catalyst from self-poisoning caused by adsorption of the formed ammonia, especially at high reaction pressures.In the present work, we focus on wuestite-based ammonia synthesis catalysts. To investigate the effect of different promoters on the performance of the catalyst, a series of wuestite-based catalysts containing different oxide promoters were synthesized, characterized and tested in ammonia synthesis. An industrial wuestite-based catalyst was used as reference. As we will show, disproportionation is the dominant chemical feature during the reductive activation at lower pressures and it can be strongly influenced by the use of promoters. Furthermore, the promoters as well as elevated pressure let all reductive events collapse into one reduction process that retains the phase disposition generated during fusion. This allows for the complete reduction of wuestite and the formation of \u201cammonia iron\u201d below 500 \u00b0C.The catalyst samples (Table 1\n) were prepared according to the recipe of the applied industrial catalyst in a lab-scale electric arc furnace. Raw materials were mixed together as fine powders, and the mixture was placed inside a melting pot. The furnace chamber was evacuated and the synthetic air pressure was set to the desired value. A voltage was applied, creating an electric arc. When the melting process was finished, the melt was cooled down and crushed by jaw crusher. Finally, the granules were sieved in order to obtain the desired size fraction.For the TEM investigation, the wuestite grains were crushed and the resulting powders were dispersed in ultrapure ethanol and sonicated for 5 min. The experiments were performed with a transmission electron microscope (TEM) JEOL ARM 200 F operating at 200 kV, equipped with a double spherical aberration correctors, and GATAN Oneview and Orius cameras. For scanning TEM (STEM) studies, a high\u2010angle annular dark\u2010field (HAADF) detector was used, which maximized the collection of incoherent scattered electrons.SEM investigations were conducted using Hitachi microscope, operating at 15 kV, and which is equipped with a secondary ion and electron detectors.The BET surface areas and BJH pore size distributions were determined by measuring N2-physisorption isotherms at \u2212196 \u00b0C with a Quantachrome QUADRASORB evo MP set-up. For regular measurements of the air-stable precursors the sample surfaces were cleaned from water and other potential adsorbates by degassing them at 100 \u00b0C for 12 h in vacuum. The air sensitive catalysts after reduction were transferred without air contact and directly measured without any thermal pretreatment. BET surface areas were calculated from data collected in a p/p0 range between 0.05 and 0.3. Adsorption and desorption isotherm were measured at a p/p0 range between 0.05 and 0.95 and used for the determination of BJH pore size distributions.The quasi in situ XRD data were collected in Bragg-Brentano geometry using a STOE Theta/theta X-ray diffractometer (CuK\n\u03b11+2 radiation, secondary graphite monochromator, scintillation counter) equipped with an Anton Paar XRK 900 in situ reactor chamber. The samples were reduced in the in situ chamber with a heating rate of 3 \u00b0C min\u22121 until the desired target temperature was reached, followed by rapid cooling (20 \u00b0C min\u22121) and XRD measurement at 25 \u00b0C. Subsequently, the sample was heated again with 20 \u00b0C min\u22121 until reaching the previous target temperature, where the original TPR ramp of 3 K min\u22121 was resumed until the final reduction temperature of 850 \u00b0C was reached. The gas feed was mixed by means of Bronkhorst mass flow controllers, using 20 % H2 in helium at a total flow rate of 100 N mL min\u22121. The effluent gas composition was monitored with a Pfeiffer OmniStar quadrupole mass spectrometer. Ex situ XRD measurements of post mortem samples were performed in Bragg-Brentano geometry on a Bruker AXS D8 Advance II theta/theta diffractometer, using Ni filtered CuK\n\u03b11+2 radiation and a position sensitive energy dispersive LynxEye silicon strip detector. The diffraction patterns were analyzed by whole powder pattern fitting using the TOPAS software (version 5, \u00a91999\u22122014 Bruker AXS).Temperature-programmed reduction (TPR) experiments were performed in a custom-designed set-up equipped with stainless-steel tubes, a fixed bed reactor (quartz glass, U-tube) and an on-line thermal conductivity detector (TCD) for monitoring the H2 consumption. The TCD (Emerson X-stream) was calibrated by reducing a known amount of CuO. A molecular sieve containing tube was installed ahead of the detector as water trap.For a measurement 100 mg of catalyst precursor (particle fraction 250\u2212425 \u03bcm) were reduced by heating it to 900 \u00b0C in a total gas flow of 75 N mL min\u22121 (20 % H2, 80 % Ar) applying a linear heating rate of 3 \u00b0C min\u22121. The Monti-Baker criterion was in a range of 100\u2013125 depending on the sample [50]. The Mallet-Caballero criterion was in a range of 5\u20136.5 K depending on the sample [51].The ammonia synthesis tests were conducted in a commercial all stainless-steel flow set-up (Integrated Lab Solutions Gmbh) equipped with a guard reactor, a synthesis reactor, and an on-line IR-detector for NH3 and H2O (Emerson X-stream) for quantitative product gas analysis. For a detailed description of the set-up see [52].For a measurement 3 g of Fe-based precursor (particle fraction 425\u2212560 \u03bcm) were diluted with 3.9 g SiC (average particle size 154 \u03bcm). The catalyst bed was placed in the synthesis reactor between pure SiC and held in position by glass wool plugs at the entrance and exhaust of the reactor. After intensive purging of the reactor until water content was stable at almost zero, the sample was reduced by heating it in a gas flow of 858 N mL min\u22121 (75 % H2, 25 % N2) with a temperature program up to 500 \u00b0C at an elevated pressure of 30 bar. The precursor was reduced in three temperature steps with different heating rates: from room temperature to 250 \u00b0C with 1.2 \u00b0C min\u22121, 250\u2212400 \u00b0C with 0.3 \u00b0C min\u22121 and 400\u2212500 \u00b0C with 0.2 \u00b0C min\u22121. Afterwards, the conditions were kept constant for ca. 4.5 h. In total the reduction procedure took 24 h.For catalytic testing, the temperature was kept at 500 \u00b0C while the total gas flow was adjusted to 357 N mL min\u22121 (75 % H2, 25 % N2). The pressure was increased from 30 bar up to 90 bar in three steps of 20 bar. Each step was performed with a pressure ramp of 1 bar min\u22121 (1 h per step) and after reaching the elevated pressure it was kept constant for 40 min before starting the next step. After reaching 90 bar, the temperature was reduced to 400 \u00b0C with a rate of 1 \u00b0C min\u22121 and kept constant for 22 h. Afterwards the catalyst was heated again with 1 \u00b0C min\u22121 to 500 \u00b0C and measured for 14 h before cooling it back to 400 \u00b0C and measuring it for 22 h. This was repeated in total two times. At the end of the measurement the pressure was released and the catalyst was cooled down to room temperature, while the reactor was flushed with nitrogen. The tested sample was removed inside of a glovebox to allow further characterization of the catalyst in a reduced form. The activity of the catalysts is given as relative NH3 synthesis activity, where the effluent mole fraction of NH3 of the different catalysts was normalized to the initial effluent mole fraction of NH3 of the industrial catalyst FeO-04.Four different Fe1-xO-based precursors were investigated (Table 1) including three laboratory produced samples (FeO-01, FeO-02, FeO-03) and one industrially applied catalyst (FeO-04). The samples differ in their degree of promotion (Table 1). One laboratory sample (FeO-01) is unpromoted. FeO-02 is promoted with K and Al which reflect the most common promotors for all Fe-based ammonia synthesis catalysts [18]. FeO-03 has the same K and Al content as FeO-02 and is additionally doped with Ca as it is known to be one of the most important promoters for wuestite-based precursors [25]. The industrial FeO-based catalyst (FeO-04) contains a package of different promoters that are present in different amounts. The amounts of K, Al and Ca in FeO-04 are similar to FeO-03.The precursors were activated by a reductive pretreatment as described in the experimental section to form the actual catalyst. The reduction of the precursors is accompanied by water formation and initiates the production of ammonia (Figure S1). Following the reductive activation, the catalytic activity towards ammonia synthesis was measured at a pressure of 90 bar at two different temperatures (400 \u00b0C and 500 \u00b0C) (Figure S2).The reduction behavior of the Fe1-xO precursors varies strongly with the applied reduction conditions (Fig. 1\na). Under the pressure of 30 bar the samples exhibit quite similar reduction profiles. The peak shape during reduction is asymmetric for all catalysts indicating overlaying reduction steps and/or higher order kinetics. Furthermore, the rate maximum is shifted to higher temperatures with increasing degree of promotion. Temperature programmed reduction (TPR) analysis of these catalysts at atmospheric pressure reveals kinetic resolution and a splitting of the reduction profile and for the catalysts with lower degree of promotion intermediate phases between reduction steps become stable. Due to the lower pressure and the lower H2 amount in the gas phase the reduction potential is lower compared to the activation procedure at higher pressures as evidenced by the TPR measurements. In addition, the position of all reduction peaks is shifted to higher temperatures at lower hydrogen partial and total pressures. Although for pure wuestite only one reduction signal would be expected (Fe1-xO \u2192 Fe), the reduction profile of the unpromoted sample FeO-01 is split into three overlapping peaks. This arises from the consecutive reduction of wuestite and the disproportionation products. With increasing promotion the peak splitting decreases. While FeO-02 that is doped with K and Al shows two overlapping peaks, FeO-03 and FeO-04 exhibit only one visible reduction peak. This observation highlights that phase formation during reduction is strongly affected by the promoters acting on the catalysts synthesis as well as on the ammonia synthesis.As expected, the comparison of the catalytic activity of the samples that is presented in Fig. 1b displays an enhancement of the activity with an increasing degree of promotion. The addition of K and Al (FeO-02) almost doubles the activity of the wuestite-based catalyst in comparison to the unpromoted sample (FeO-01). A further significant activity boost is achieved by the additional presence of Ca (FeO-03), which leads to an even three times higher activity compared to the unpromoted sample. With the addition of several different promoters the multi-promoted industrial catalyst (FeO-04) exhibits by far the highest activity and still leaves a significant gap to the sample with 3 promoters (FeO-03). The massive influence of promotion is especially visible by comparing the multi-promoted industrial catalyst (FeO-04) to the unpromoted sample (FeO-01). The multi-promoted Fe1-xO-based catalyst is more than 5 times as active as the unpromoted analogue.A major contribution to this increase in activity can be assigned to structural promotion as can be seen by the BET surface areas and mesoporous pore volumes of the reduced and tested catalysts (Fig. 1b, Table S1). The addition of K and Al (FeO-02) and the subsequent introduction of Ca (FeO-03) leads to an increasing surface area and pore volume quite comparable to the increase of the overall catalytic activity. It should be noted that this behavior does not exclude an influence of other promoting effects like the improvement of the reaction kinetics, where K is known to be important. In general, Al and Ca are known as structural promoters, which preserve the Fe nanostructures from sintering [27,32]. Furthermore, they cause an increase of the surface area of the catalyst and, therefore, an increase of the total number of active centers. By comparing the BJH pore size distribution of the samples after ammonia synthesis the difference in the effect of structural promotion becomes visible (Figure S3). While FeO-01 exhibits only little mesoporosity the addition of K and Al (FeO-02) leads to the formation of a small mesoporous pore volume with a broad pore size distribution. A significant effect can be seen by the addition of Ca (FeO-03). A clear mesoporous structure can be observed with a defined maximum centered around 15 nm. The pore size distributions of FeO-03 and FeO-04 are almost identical. Thus, Ca seems to have a major role as a structural promoter and leads to the formation of a mesoporous network within high performance ammonia synthesis catalysts. Besides structural promoting, the addition of further promoters in FeO-04 may have a primary effect on the ammonia reaction kinetics, while the combination of K, Al and Ca in FeO-03 are believed to be the major constituents that lead to the total surface area and mesoporous nanostructure of the industrial catalyst.As mentioned before the four samples vary in their reduction behavior during the TPR at 1 bar. In order to understand associated changes of the iron phases, XRD measurements were performed before the reduction, after the reduction and in the middle of the reduction process at 500 \u00b0C in a hydrogen-containing atmosphere. Due to the low time resolution (ca. 20 h per scan), all XRD measurements were performed at room temperature to avoid ongoing reduction of the sample during the data collection, which renders this technique quasi in situ. The intermediate target temperature of 500 \u00b0C was chosen according to the minima of the TPR profile of the sample FeO-01, while 850 \u00b0C was the maximum temperature accessible with the setup. At the minima of the TPR profile, the reduction rate is the lowest and thus the necessary interim cooling/re-heating phases should have the smallest possible impact on the reduction profile. Furthermore, it may be expected that potential intermediate phases have their maximum concentrations at these points. Although the minima are less resolved for FeO-02 or no TPR minima could be found for FeO-03, the same temperature program was applied to all samples for the sake of comparability.Before reduction, the XRD patterns of all samples exhibit a distinct wuestite (Fe1-xO) phase with a small amount of \u03b1-Fe, while after reduction only an \u03b1-Fe phase is present for all samples (Fig. 2\n). A difference for the samples can be observed with the XRD patterns in the middle of the reduction progress at 500 \u00b0C. It is possible to see the formation of a magnetite (Fe3O4) phase in different amounts for all samples before iron is fully reduced at higher temperatures. It should be noted that the XRD pattern of sample FeO-03 after reduction to 500 \u00b0C is peculiar in showing an unusually strong 220 reflection (60.5\u00b0) of the wuestite phase. Peak shape and broadening exclude the possibility of bad sampling statistics, which could cause significant intensity deviations in the case of highly crystalline phases. Furthermore, the cubic crystallographic symmetry and the lack of directing mechanical forces during the experiment rule out preferred orientation effects as a possible explanation. Thus, we interpret this surprising change of the relative intensities as a true structural effect. A Rietveld refinement was obtained after allowing the occupation of tetrahedral interstitial sites in the wuestite crystal structure by Fe atoms [53]. In the refined model, about one third of the iron atoms resided on the new tetrahedral positions, while the rest occupied the normal octahedral sites. Whether this unusual, modified wuestite phase is directly stabilized by the promoters in FeO-03, or whether it occurs generally as an intermediate during reduction and was only accumulated into noticeable amounts due to the delayed disproportionation/reduction kinetics, remains open.The formation of magnetite is caused by thermal disproportionation of the wuestite into \u03b1-iron and magnetite. When the amounts of the different iron oxide phases are compared (Table 2\n) it can be seen that the unpromoted sample FeO-01 exhibits a much higher amount of magnetite at 500 \u00b0C compared to the two other promoted samples at this temperature. This shows that the promoters are stabilizing the metastable wuestite phase and inhibit thermal disproportionation (Fig. 2), i.e. minimizing the amount of magnetite at the final reduction temperature such that the individual events occur at lower temperature and coincide.The thermal disproportionation can explain the peak splitting of the reduction peaks, which is strong in the unpromoted sample. Due to the disproportionation into \u03b1-Fe and Fe3O4 the precursor turns into phase mixtures of Fe1-xO, \u03b1-Fe and Fe3O4. This leads to a reaction network during reductive activation (Fig. 3\n). Hence, parts of the Fe1-xO phase are directly reduced, while other parts of this phase disproportionate into iron and magnetite. The newly formed Fe3O4 is also reduced at higher temperatures compared to the original wuestite phase. It can be speculated that Fe3O4 forms during its reduction a new Fe1-xO phase as an intermediate step, which is subsequently reduced or disproportionated. It is further possible that Fe1-xO phases with different x values are formed. This could also explain the change of their reduction rate. At the applied higher temperatures all reactions can happen simultaneously as indicated by the overlaying TPR profiles.Peak splitting in TPR decreases with increasing degree of promotion until only one reduction peak is present. However, disproportionation can still not be fully excluded as even the most promoted model sample FeO-03 still exhibits a detectable amount of magnetite as shown by the XRD patterns in Fig. 2f. Thus, the single reduction peak of the FeO-03 precursor can result from an overlap of different reduction events.Please note, the quasi in situ XRD analysis was limited to a maximum measurement angle of 90\u00b0 2\u03b8. Due to this limitation only three accessible reflections that can be assigned to bcc Fe could be obtained. The absence of further reflections has limited the peak profile analysis to consider only isotropic size broadening. Nonetheless, this analysis hints to a slight anisotropy in the peak profiles corresponding to \u03b1-Fe (Fig. 4\n) [54]. The peak width misfit is most pronounced for the 200 reflection. As opposed to the quasi in situ analysis, this effect of anisotropic peak broadening is more obvious in the diffraction patterns that were measured of all FeO catalysts after ammonia synthesis. However, these catalysts have been investigated ex situ. To illustrate the effect of anisotropic peak broadening, the XRD results of FeO-03 after ammonia synthesis are presented as a structural example in Figure S4a. The data was fitted with an isotropic profile in the absence of a crystal structure model. Furthermore, a fit that includes a crystal structure model (i.e. Rietveld refinement), reveals an additional mismatch of the calculated relative intensities for the spent FeO-03 catalysts (Figure S4b). Since the bcc crystal structure of \u03b1-Fe has no internal degrees of freedom, i.e. it accommodates only one atomic site that is fixed to a special position, except the thermal displacement parameter, this result can be taken as evidence that the real structure of the iron phase must be more complex than simple \u03b1-Fe. Phenomenologically, the observed intensity distribution could be approximated either by allowing the thermal displacement parameter to take physically implausible negative values or by assuming that additional electron density is residing on interstitial positions. It should be noted here that diffractometer misalignment or beam spill effects were explicitly ruled out as potential causes for the observed intensity mismatch.From the applied Rietveld analysis of the experimental data of the wuestite precursors that have been measured by the quasi in situ XRD approach lattice parameters a and domain size values L\nVol-IB were extracted which are presented in Table 3\n. However, due to limitations to the isotropic fit model, these values should be interpreted only in terms of a trend rather than absolute values. The domain size values suggest that the addition of K, Al reduce sintering, while the addition of Ca seems to increase the Fe lattice parameter after reduction at 500 \u00b0C. It should be noted that the peculiarity of the lattice parameter occurs in the same scan as the \u201cinterstitial wuestite\u201d phase occurred and vanishes with higher reduction temperatures.These different possible reduction pathways render any detailed analysis difficult. While the formation of magnetite can only originate from the disproportionation of wuestite, the \u03b1-Fe can come from the disproportionation of wuestite as well as from the reduction of magnetite. In addition, it is possible that the full amount of all Fe phases cannot be detected by XRD due to the absence of translational symmetry or the formation of too small crystalline domains. For example, for sample FeO-01 (4 FeO \u2192 Fe3O4 + Fe) that exhibits a large amount of magnetite a \u03b1-Fe to magnetite weight-% ratio of 1\u20134 it could be stoichiometrically expected. However, the actual ratio is only around 1\u20139. This indicates that probably not all \u03b1-Fe in the sample is detected. Despite these uncertainties it is still possible to conclude on a few trends, in particular, when the amount of phases is strongly changing as it does for the aforementioned amount of magnetite. It is also possible to observe that the FeO-02 sample is at 500 \u00b0C more reduced indicated by a higher amount of \u03b1-Fe compared to FeO-03 sample. This can be explained by the presence of Ca in the FeO-03 sample, which slows down the reduction process of wuestite [55]. CaO crystallizes in the same crystal structure than wuestite and, therefore, it can be incorporated into the wuestite lattice [25]. This could form CaxFe3-xO4 solid solutions, which inhibits the disproportionation process. This is in agreement with the findings of Li et al. [55], who have shown the positive effect of CaO on thermal stability of wuestite structure under base pressure of 1.33 Pa. Meanwhile, the formation of CaxFe3-xO4 species will hinder the reduction rate in comparison to the wuestite precursor without Ca. A similar hindering effect of Ca on the magnetite-based precursor reduction has been reported by Liu et al. [32]. As the Ca content is sufficiently low the precipitation of the spinel phase could be avoided. It has been highlighted that phase purity of the oxide precursor is a necessary requirement for a good ammonia synthesis catalysts [25].To investigate how the early stages of disproportionation influences the structure on the local scale transmission electron microscopy investigations were conducted. Transmission electron microscopy allows to investigate the morphology and structure of the catalysts at the (sub)-nanometric scale and is thus complementary to bulk averaging characterization techniques. In order to study the morphological and structural changes that occur during the reduction on the individual particles, quasi in situ TEM experiments were conducted [56,57]. This later allows to investigate catalyst particles before and after activation at elevated pressure and temperature, which are relevant for ammonia synthesis catalysts.The reduction of the wuestite was first followed for the FeO-03 precursor to exemplify the presence of local disproportionation events for a fully promoted sample by submitting the sample to a H2/Ar mixture of 3 to 1 at 10 bar and 365 \u00b0C. Fig. 5\n shows typical and identical wuestite particles before (Fig. 5a) and after the quasi in situ experiment (Fig. 5b). Before quasi in situ activation (Fig. 5a), surface near diffraction contrast indicates the formation of defective structures within the particles which are characteristic of wuestite-type materials. After reductive treatment (Fig. 5b), a cracking of the identical particle occurred that can be best described by the formation of a hedgehog-like structure, which is accompanied by the formation of porosity and the outgrowth of multiple nanoplatelets [58,59]. High-resolution (HR)-TEM imaging and corresponding Fast-Fourier transform (FFT) analysis (Fig. 5c-d) of the nanoplatelets denote the growth of polycrystalline particles with a nanoparticular structure. After treatment at 365 \u00b0C the wuestite structure is still present in the sample. In addition, \u03b1-iron and magnetite are formed. Thus, this observation corroborates the thermal disproportionation in the early stage of the reduction process, as previously established by XRD and TPR even for the fully promoted samples.Additional areas of the FeO-03 sample after reduction were investigated, which show the formation of complex structures and the local inhomogeneity of the sample. Fig. 6\n presents one example of a detailed HRTEM analysis. Polycrystalline particles are observed including the presence of Fe3O4 (Fig. 6b\u2013d) and Fe (Fig. 6e\u2013g) phases. In addition, Moir\u00e9 patterns (Fig. 6a and e) as a result of overlapping lattice planes indicate the presence of turbostratic disordered layers, which can also highlight the formation of defective phases. The complexity of the atomic structure is further corroborated by analyzing the surface layer of such a particular aggregate, including amorphous layers (Fig. 6h), short-range ordered surface structures (dashed ellipse in Fig. 6i) and surface roughness (Fig. 6i). The observation of amorphous and short-range ordered phase may also be in line with the missing Fe content calculated from Rietveld analyzed XRD patterns. Elemental analysis indicates that the promoters (Al, K, Ca) are in close contact with iron phases (Figure S5).From the above results, it is apparent that the temperature of 365 \u00b0C is insufficient to fully reduce the wuestite precursor even at elevated pressure. In order to corroborate the TPR results showing that at elevated pressure and higher temperature (above 400 \u00b0C) also the unpromoted sample can be fully reduced quasi in situ investigations of FeO-01 were conducted. Fig. 7\n shows TEM images on the FeO-01 sample before (Fig. 7a) and after (Fig. 7b) quasi in situ reduction experiment at 10 bar of H2/N2 (3/1) mixture at 470 \u00b0C showing that at elevated pressure the unpromoted sample is fully reduced even at the local scale. The wuestite precursor (Fig. 7a) indicates a particle-like morphology with a length of about 500 nm. Similar to the pristine FeO-03 sample, localized diffraction contrast is observed which is distributed all over the particle. After the exposure of the precursor to a H2/N2 (3/1) mixture at 10 bar and 470 \u00b0C (Fig. 7b), a drastic morphological change occurred which is expressed by the formation of elongated polycrystalline iron phases. This is confirmed by HRTEM imaging (Fig. 7c), which shows the presence of (011) \u03b1-Fe planes of different crystals. The HRTEM also shows the formation of Moir\u00e9 patterns between the agglomerates of particles, as a result of the interference of overlapping (011) lattice planes indicating thin layers. Furthermore, the particles exhibit a dark contrast (see arrows in the Fig. 7c), which is indicative of the formation of strains on the particles.In addition, the structure of the sample before and after the quasi in situ reduction was investigated using selected area electron diffraction (SAED). Ring patterns were observed in both before (Fig. 7d) and after (Fig. 7e) quasi in situ reduction, indicating a polycrystalline structure with a small crystallite size. The analysis confirms that the full reduction of wuestite precursors to Fe occurs at elevated pressures even for the unpromoted sample at lower temperatures which corroborates the TPR results presented in Fig. 1a.It should be noted that we have conducted similar reduction experiments for FeO-02 by quasi in situ TEM (see Figure S7). As demonstrated for FeO-01 (Fig. 7) the sample is reduced and similar morphological and structural features were detected.Additionally, the microstructure of the FeO-03 sample before reduction (Fig. 8\na) as well as after reduction and ammonia synthesis (Fig. 8b-c) was investigated by scanning electron microscopy (SEM). A complex microstructure is observed with the presence of different grain sizes (Fig. 8a). Furthermore, grain boundaries can be observed which indicate the presence of a defective structure. The bulk morphology of the wuestite precursor is similar to the one after ammonia synthesis. However, a sponge-like structure is formed with voids distributed all along (see areas highlighted by arrows in Fig. 8c). This is in line with the BJH results, which showed a clear mesoporous structure for the wuestite sample promoted with K, Al and Ca. This may be related to the reduction of the particles, which generates the porosity and increases the surface area of the particles as confirmed by the BET surface area.In order to investigate the microstructure and chemical composition on the sponge-like structure formed after NH3 synthesis, SEM-EDX mapping were conducted (Fig. 8d\u2013I and Figure S6). The EDX mapping has revealed the presence mainly of Fe. However, O, Al, K, Ca were also detected, and are more concentrated into the grain boundaries. Similar localization of the promoters in the wuestite grains before reduction were observed (see Figure S8). Therefore, the promoters could act as binders between the larger grains and thus help to stabilize the sample morphology during the reduction/activation steps.In summary, the reduction of wuestite-based precursors occurs in a complex reaction network involving reduction and disproportionation events. Our results show positive effects of K, Al and Ca as the main promoters on the performance and reduction behavior of wuestite-based ammonia synthesis catalysts. They have a significant influence on improving and stabilizing the catalyst nanostructure that is defined during the fusion and retained during the reduction of the precursors, whereas the promoters seem to be concentrated in the grain boundaries of the formed \u03b1-iron bulk crystals. Furthermore, they narrow the reduction and disproportionation events of metastable wuestite which allows a more direct reduction with less amounts of magnetite at the final activation temperature. The presented results support the formation of a structure of defective and disordered nanoplatelets within a mesoporous network as the origin of \u201cammonia iron\u201d in comparison to bulk \u03b1-iron. This enhances by far the surface area of the catalyst. These global improvements from the promoters lead to a more active catalyst for ammonia synthesis. Especially Ca seems to play a major role on defining the reduction process and, therefore, also the mesoporous network and the surface area of the resulting catalyst. We note that the difference between normal iron and ammonia iron mostly occurs on a mesoscopic scale. Typical spectroscopies as M\u00f6ssbauer or EXAFS would not detect such differences [60]. XRD is slightly sensitive in its line shapes [61] being peculiar in active catalysts. The information content of these anomalies precludes a distinction of mesoscopic defect from possibly present additional local defects for which TEM gave some hints.\nJan Folke: Writing - original draft, Investigation, Validation, Formal analysis, Visualization. Kassiog\u00e9 Demb\u00e9l\u00e9: Writing - original draft, Investigation, Validation, Formal analysis, Visualization. Frank Girgsdies: Investigation, Formal analysis, Visualization, Writing - review & editing. Huiqing Song: Investigation, Validation, Formal analysis, Writing - original draft. Rene Eckert: Investigation, Writing - original draft, Writing - review & editing, Project administration. Stephan Reitmeier: Writing - review & editing, Resources, Supervision. Andreas Reitzmann: Writing - review & editing, Resources, Supervision. Robert Schl\u00f6gl: Conceptualization, Methodology, Writing - review & editing, Resources, Supervision. Thomas Lunkenbein: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Holger Ruland: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Project administration.The authors declare no conflict of interest.The authors would like to thank the Max Planck Society for financial support. Furthermore, we thank Birgit Deckers for the graphical artwork.Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2021.03.013.The following are Supplementary data to this article:\n\n\n\n\n", "descript": "\n                  Ammonia synthesis remains one of the most important catalytic processes since it enables efficient hydrogen storage and provides the basis for the production of fertilizers. Herein, complementary bulk and local analytical techniques were combined to investigate the effect of selected promoters (Al, K, Ca) on the reduction of wuestite into \u03b1-iron and their catalytic performance for ammonia synthesis. The use of promoters appears to have a positive effect on the wuestite-derived catalyst in ammonia synthesis. The promoters seemingly act as a binder for wuestite grains and impede the reduction and disproportionation events of wuestite precursors resulting in an increased catalytic performance. This effect is associated with an increase of surface area and mesoporosity. The study delivers new insights into the interplay of structure and promoters in wuestite-based catalysts.\n               "}
{"full_text": "Data will be made available on request.The industrial revolution has made life very convenient by regularly upgrading the technologies, however, it\u2019s still dependent on natural fuels such as natural gas, coal and petroleum [1\u20133]. On one hand, the combustion of these fuels causes serious environmental pollution, while on the other hand, its reservoirs are limited, which compels researchers to explore new energy sources [4,5]. In this regard, ideas have been projected to overcome the said problem and the solution seems to be in fuel cells, metal-air batteries, and alkaline water electrolysis [6]. Complicated modules have been designed, but their basics are quite simple, such as two electrodes assembly where oxygen evolution reaction (OER) takes place at the anode, while hydrogen evolution reaction (HER) at the cathode [7,8]. Sluggish kinetics is the big hurdle keeping away the process from practical application. OER is a four-electron transfer reaction, while HER is 2e\u2212 a transfer reaction, so OER needs a higher overpotential [9]. In the previous few decades, serious efforts have been made to design such a catalyst that enhances kinetics as well as stability in different media. Benchmark for OER is RuO2 / IrO2, which shows high activity in any media [10]. However it is highly unstable in acidic/basic media and oxidized to RuO4 / IrO3 and gets dissolved in the solution [11]. The comparative stability of IrO2 is higher than RuO2 but still both are precious and far away for large\u2013scale usage [12]. Therefore, researchers investigated alternatives of RuO2 and IrO2. The possible candidates are those species that mimic the electronic environment of those species. The closer ones to those are transition metal oxides/hydroxides and doped transition metal oxides and hydroxides.Remarkable efforts have been made to study transition metal oxides (TMO) such as perovskite, spinel and layer structures having remarkable OER activity [13]. The easy synthetic approach, low capital cost, environment-friendly, high OER activity and stability in alkaline media make them an attractive choice for researchers. TMO has a rich blend of TM with variable oxidation state and coordination environment, which is fine-tuneable for OER [14]. Layer structures have also been given considerable attention, especially for their boosting activity upon doping. Recently, there has been noteworthy interest in 2D and layered 2D materials, particularly for their electrocatalytic application [15].Non\u2013oxide electrocatalysts such as CoC, CoP, Co2N, Co2P, CoC2, Co3N, Co3S4, Co4N, Co9S8, Ni3N, Ni3C2 (C\u00a0=\u00a0Se, S and Te) and organometallics have also been reported for having high OER or HER activity and stability, but overall they are suffering from high over potential [16,17].Layer structures are usually transition metal hydroxide M(OH)2 and oxyhydroxides MOOH, and almost both have significant OER activity [18]. In a typical structure, TM is located in the center and oxygen anions at the corner of octahedrons forming [MO6] as a repeating subunit, while H+ is sandwiched between layers. Metal oxyhydroxides are of many kinds. Subbaraman et al. reported the same trend (Ni\u00a0>\u00a0Co\u00a0>\u00a0Fe\u00a0>\u00a0Mn) for LDH especially 3d TM such as Co, Mn, Ni, Fe hydr(oxy)oxide as in perovskite [19]. According to their analysis, the Ni\u2013OH optimal bonding is the causative agent for high OER activity. Contrary to this statement, Corrigan et al. observed that introducing Fe and increasing Fe content boosted the OER performance of NiOOH. Trotochaud et al. also thoroughly studied Fe influence in NiOOH, and concluded that 25\u00a0% addition of Fe caused a 30-fold increase in its conductivity, which consequently enhanced its OER activity reflected from dropping overpotential to 200\u00a0mV. But still, the high robust activity cannot solely be explained by co-precipitation and a slight increase in conductivity [20]. Bode et al. proposed the oxidation of Ni+2 to Ni+3 by deprotonation during charging/discharging in basic media [21].Compiling all the reported data in the literature, it can be inferred that incorporating Fe into the NiOOH increases its OER activity because Fe acts as an active site for the reaction. In the current work, we incorporated different amounts of Fe such as 10, 20, 30 and 40\u00a0% and successfully doped it in NiOOH sheets. The electrochemical analysis indicates that the OER activity increases with an increase in doping amount and reaches to peak position upon 30\u00a0% doping and then drops down upon further incorporation of Fe. This may be explained based on exposed active sites to OER which are gradually covered upon further addition consequently giving a volcano plot.Nickel (II) acetate tetrahydrate. (C4H6NiO4\u00b74H2O), Iron (III) nitrate nonahydrate (Fe(NO3)3\u00b79H2O), Nafion\u00ae (5\u00a0wt% in lower aliphatic alcohols and water, contains 15\u201320\u00a0% water), isopropyl alcohol (C3H8O), ethanol (C2H5OH) and N,N-Dimethy formamide (C3H7NO) were bought from Sinopharm Chemical Reagent Co. Ltd. Throughout the course of experimental work the chemicals were used as collected without subjecting to any purification. The ultrapure Millipore water (18.2 M\u03a9) were used in all experiments.Typically, NixFexOOH sheets were prepared via a single-step approach. C4H6NiO4\u00b74H2O and Fe (NO3)3\u00b79H2O with different molar ratios were dissolved in a mixture of H2O and DMF and sonicated for 30\u00a0min then transferred to Teflon Lined Autoclave (TLA) and heated at 150\u00a0 degreesC for 12\u00a0h. The prepared catalyst is cooled down at room temperature and washed several times with DI water. The synthetic detail is given in the Supplementary Table 1 (S1, ESI\u2020). According to our knowledge, this synthetic route for NixFexOOH sheets is novel.The electrochemical analysis was made using an IM6 electrochemical workstation (Zahner, Germany) with a three-electrode system. The prepared NiOOH, Ni0.9Fe0.1OOH, Ni0.8Fe0.2OOH, Ni0.7Fe0.3OOH and Ni0.6Fe0.4OOH nanosheets/C were used as the working electrodes. Platinum wire was used as the counter electrode, and Ag/AgCl electrode was used as the reference electrode. All potentials were referenced to the reversible hydrogen electrode (RHE) through RHE calibration. Before any analysis, the electrolyte cell was purged with O2 gas for 30\u00a0min to keep a saturated vapour pressure. During analysis, the mild flow of O2 was continued to ensure H2O/O2 equilibrium at 1.23 vs RHE. Approximately 4\u00a0mg of the catalyst was dispersed in a mixture containing 0.75\u00a0mL of water, 0.25\u00a0mL of isopropanol alcohol, and 20\u00a0\u03bcL of 5\u00a0wt% Nafion (Aldrich). After ultrasonication for 1\u00a0h, suitable microliters of the ink were decorated on a glassy carbon rotating disk electrode (Pine Instruments). Polarization curves were recorded in 1\u00a0M KOH solution with a rotation rate of 1600\u00a0rpm and a scan rate of 5\u00a0mV\u00a0s\u22121. Electrochemical impedance spectroscopy was performed over a frequency range from 100\u00a0kHz to 0.5\u00a0Hz with a sinusoidal voltage amplitude of 5\u00a0mV. Accelerated stability tests were conducted by applying cyclic sweeps between 1.2 and 1.7\u00a0V versus the RHE in 1\u00a0M KOH electrolyte at a scan rate of 100\u00a0mV\u00a0s\u22121. Polarization curves were recorded at the end of cycling with a rotation rate of 1600\u00a0rpm and a scan rate of 5\u00a0mV\u00a0s\u22121.Transmission electron micrographs (TEM) images were obtained by using Hitachi H-7650 instrument with an acceleration voltage of 100\u00a0kV. The composition of the samples was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES, Atomscan Advantage, Thermo Jarrell Ash, USA). The surface valance band spectra were collected by high-resolution X-ray photoemission spectroscopy (XPS) on the nanosheet monolayers. These were recorded on an ESCALAB-250 spectrometer having a monochromatic Al K\u03b1 X-ray source (h\u03bd\u00a0=\u00a01486.6\u00a0eV), with a spot size of 500\u00a0\u03bcm. X-ray diffraction (XRD) characterization was performed using a Philips X\u2019Pert Pro X-ray diffractometer with a monochromatized Cu K\u03b1 radiation source and a wavelength of 0.1542\u00a0nm.Typically, the NixFexOOH nanosheets were synthesized via a single-step novel approach, to the best of our knowledge, the method has not been reported yet. The whole process is schematically represented in Fig. 1\n. The full detail is given in Table S1\u2020. The nanosheet powder was vacuum dried and then dispersed in ethanol via ultrasonication. The nanosheet powder dispersion was dropped onto the copper grid coated with a carbon layer and air-dried for TEM measurements.Both TEM and SEM reveal that NiOOH nanosheets and Fe-doped sheets are of average size 400\u2013500\u00a0nm Fig. 1-A, B, C, D and E. Additionally, the doping didn\u2019t change the morphology as well as the sizes of the sheets. The retention of morphology is due to the fact that Ni\u00a0\u2212\u00a0O and Fe\u00a0\u2212\u00a0O are approximately the same and comparable to that of NiOOH as reported by Nilsson and Bill et al. in their computational study. STEM-energy dispersive X-ray (STEM-EDX) elemental mapping images of the Ni0.7Fe0.3OOH nanosheets clearly indicate that Ni, Fe and O are homogeneously distributed (Fig. 1-F). The composition of the samples was analysed by inductively coupled plasma atomic emission spectrometry, the data shows that the molar ratio of the prepared catalyst was 0.9:0.1, 0.8:0.2, 0.7:0.3 and 0.6:0.4 for Ni and Fe, respectively.The XPS data confirms the presence of Fe in NiOOH sheets. As shown in Fig. 1-G, H the peak at 856.1 and 873.7 refers to Ni 2P3/2 and 2P1/2 respectively, while the peak at 709.6 refers to Fe2p3/2. Zhenzhi Yin et al & Runze He et al finding is relatable with our analysis. [22,23] All the XPS peaks show that both Ni and Fe were in the oxidized form, the comparative XPS full range plot is given as Fig. S2 ESI\u2020. XRD pattern of the samples (Fig. S3 ESI\u2020) obtained using the standard procedure shows the crystallinity of the material before and after doping it also indicates clear peak shifting after Fe doping. Although the peak left shifting due to doping is\u00a0\u223c\u00a00.5 degree, it still cannot be ignored because it strengthens the incorporation of Fe to NiOOH sheets and does not undergo phase changes. It may act as a substituting agent which may also be confirmed from the TEM and SEM images. The representative SEM images and EDX of Ni0.7Fe0.3OOH are given in Fig. S4 ESI\u2020 [24,25].Before the electrochemical analysis, the prepared catalyst is dispersed in a mixture of isopropanol, water and Nafion. A typical mass ratio among the Nafion, catalyst and water is 1:1:1000. The mixture is then sonicated to get a uniform catalyst ink. The catalyst ink is then dropped on the glassy carbon electrode (GCE). The electrode is then subjected to get dried under an ambient environment. The electrocatalytic OER activity of the Ni0.7Fe0.3OOH nanosheets were analysed in 1.0\u00a0M KOH solution at room temperature using a standard three-electrode system. Before analysing the polarization curve the working electrode was stabilized by repeated cyclic voltammograms. Then the polarization curve was recorded (LSV). The OER activity of all the prepared catalysts was compared. Fig. 2\n-A shows polarization curves recorded by linear sweep voltammetry (LSV) at a slow scan rate of 5\u00a0mV\u00a0s\u22121. The ohmic potential drop (iR) losses that were created from the electrolyte solution resistance were all corrected before comparison.Interestingly, the Ni0.7Fe0.3OOH nanosheets revealed a much lower onset potential and higher current density relative to the NiOOH, Ni0.9Fe0.1OOH, Ni0.8Fe0.2OOH and Ni0.6Fe0.4OOH nanosheets. The first peak at 1.38\u00a0V versus RHE (observed for all catalysts) is attributed to the characteristic interconversion of Ni2+/Ni3+mediated by OH\u2212. OER performance can be approached by checking the Tafel slope, which holds a paramount position. Generally, the smaller value of the Tafel slope indicating high OER activity provided that the data is at the same overpotential increment. The Tafel plot for Ni0.7Fe0.3OOH gave a value of 44.8\u00a0mV/decade (Fig. 2.B), which was much smaller than that determined for NiOOH (118.1\u00a0mV/decade), Ni0.9Fe0.1OOH (72.2\u00a0mV/decade), Ni0.8Fe0.2OOH (66.8\u00a0mV/decade), and Ni0.6Fe0.4OOH (68.8\u00a0mV/decade). The Tafel slope value for Ni0.7Fe0.3OOH is comparable to the data published by Jiawang Li et al (35.3\u00a0mV/decade) [26] and Meng Li et al 41.9\u00a0mV/decade for Fe based electro catalyst [27]. A low value of the Tafel plot (Fig. 2-B) specifies a superior OER ability with lowering overpotential [28].For most of the catalysts, the electrocatalytic performance is usually a function of exposed electrochemical active surface area (ECSA) [29]. ECSA was not directly measured here due to the difficulties in measuring. Instead, the double layer capacitance (C\ndl\n) is implemented to imitate the ECSA, because they are linearly related. To explain the difference among the prepared material, C\ndl\n was calculated for each sample by CV method in the non-Faradic region, which is a voltage region commonly positioned at the open circuit potential (OCP) of an electrode, assuming that obtained current is specifically from DL charging-discharging. C\ndl\n was calculated by using equation (1).\n\n(1)\n\n\n\ni\nc\n\n=\n\u03c5\n\nC\n\ndl\n\n\n\n\n\n\nWhere i\nc stands for charging current and \u03c5 for the scan rate. Fig. 3\n indicates the ECSA of all the prepared materials, which clearly reveals that there is a negligible change that occurred in the ECSA of the samples after incorporating the Fe. The electrocatalytic performance at fixed overpotential (300\u00a0mV) of all the catalysts was directly compared and the current densities are summarized and given on the left side of Fig. 2-C. It is obvious that the Ni0.7Fe0.3OOH nanosheets showed the highest current density of 56.5\u00a0mA\u00a0cm\u22122 which was 22.6, 6.27, 4.18 and 5.13 times higher than those of NiOOH, Ni0.9Fe0.1OOH, Ni0.8Fe0.2OOH and Ni0.6Fe0.4OOH nanosheets /C, respectively. The high activity of the NiFe DLH is due to its H2O and hydroxide permeable nature so that these species reach layers below the electrode/electrolyte interface. This free moment of a hydroxide ion and water species is the key factor for high OER activity. Electrical impedance spectroscopy (EIS) analysis of all the synthesized catalysts was conducted to probe into the deep insight of the OER kinetics. Fig. 2-D indicates that the samples experience a semicircle in the high-frequency range related to charge transfer resistance (Rct). In comparison to NiOOH, Ni0.9Fe0.1OOH, Ni0.8Fe0.2OOH and Ni0.6Fe0.4OOH nanosheets /C, the Ni0.7Fe0.3OOH nanosheets show the minimum Rct of 27\u00a0\u03a9, which signify the enhanced charge transfer kinetics due to incorporation of Fe in the NiOOH sheet. Mohammad Etesami et al. find the same trend for NiFe-based electrocatalysts compared with commercial Pt/C [30]. The sample without Fe shows the highest Rct of 174\u00a0\u03a9. The graph reveals that Rct values decrease as the Fe content increases reaching a minimum value and then increasing following the volcano plot [31].Stability is another key indicator to estimate the performance of an electrocatalyst. CV cycling is generally used to estimate the stability of supercapacitor materials because their real working condition is dependent on charging and discharging [32]. However, in water splitting, the OER is usually performed at a constant current or voltage. In order to imitate the more likely real working condition, chronopotentiometric analysis was used to estimate the stability of the Ni0.7Fe0.3OOH. Fig. 4\n-A shows that at 10\u00a0mA\u00a0cm\u22122 current density, the catalyst was quite stable for almost 15\u00a0h. Jiawang Li reported a nitrogen-doped FeNiOOH electrocatalyst for OER and showed comparable chronopotentiometry analysis. Polarization curves of the Ni0.7Fe0.3OOH before chronopotentiometric analysis and after are given as inset of Fig. 4-A. The comparative histogram of overpotential of all the prepared catalysts as a function of different Fe content at fixed current density (10\u00a0mA\u00a0cm\u22122) indicates that Ni0.7Fe0.3OOH has the best performance among all the catalysts given in Fig. 4-B.In summary, Fe-doped NiOOH nanosheets were successfully synthesized by a single-step approach. Ni0.7Fe0.3OOH nanosheets are represented as a model OER catalyst with significantly enhanced OER activity. It requires an overpotential of 265\u00a0mV to generate 10\u00a0mA\u00a0cm\u22122 current density. The robust OER activity is due to the incorporation of Fe, which increases the material's conductivity and provides active sites for OER. The current density produced at fixed overpotential (300\u00a0mV) for Ni0.7Fe0.3OOH was 56.5\u00a0mA\u00a0cm\u22122 which was 22.6, 6.27, 4.18 and 5.13 times higher than those of NiOOH, Ni0.9Fe0.1OOH, Ni0.8Fe0.2OOH and Ni0.6Fe0.4OOH nanosheets /C, respectively. We believe that this approach would promote the encouraging possibilities for designing highly efficient catalysts based on engineering the active sites.\nFawad Ahmad: Writing \u2013 original draft, Conceptualization, Investigation. Asad Ali: Writing \u2013 review & editing, Supervision, Project administration. Jiaqian Qin: 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 Collaborative Innovation Center of Suzhou Nano Science and Technology, MOST of China (2014CB932700), NSFC (21603208, 21573206), Key Research Program of Frontier Sciences of the CAS (QYZDBSSW- SLH017), Anhui Provincial Key Scientific and Technological Project (1704a0902013), Major Program of Development Foundation of Hefei Center for Physical Science and Technology (2017FXZY002), and Fundamental Research Funds for the Central Universities. Fawad is also grateful for a generous CAS-TWAS president\u2019s fellowship.This manuscript accompanies supporting information (Table S1\u2020, XPS Fig. ES2\u2020, XRD Fig. ES3\u2020, SEM images of Ni0.7Fe0.3OOH Fig. S4 ESI\u2020) available free of cost. Supplementary data to this article can be found online at https://doi.org/10.1016/j.rechem.2023.100808.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Electrochemical water splitting to generate oxygen and hydrogen is a key process for several energy storage and conversion devices. Developing low-cost, robust, efficient, and earth-abundant electrochemical catalysts for the oxygen evolution reaction (OER) is therefore holding a paramount position. Herein, we report the doping process to prepare two-dimensional Fe-doped NiOOH nanosheets with the tuneable molar ratio of Fe ranging from 0 to 0.4 by using a single-pot synthetic approach. Among the obtained nanomaterials, the Ni0.7Fe0.3OOH nanosheets/C exhibited greatly enhanced electrocatalytic performance toward OER in alkaline media (1.0\u00a0M KOH), with an overpotential of 265\u00a0mV to afford 10\u00a0mA\u00a0cm\u22122 current density. The current density produced at fixed overpotential (300\u00a0mV) for Ni0.7Fe0.3OOH was 56.5\u00a0mA\u00a0cm\u22122 which was 22.6, 6.27, 4.18 and 5.13 times higher than those of NiOOH, Ni0.9Fe0.1OOH, Ni0.8Fe0.2OOH and Ni0.6Fe0.4OOH nanosheets /C, respectively. Moreover, the nanosheets were able to retain excellent performance for over 15\u00a0h without obvious degradation. The Tafel slope for Ni0.7Fe0.3OOH was 44.8\u00a0mV/decade. Therefore, this approach has opened a new possibility for designing highly efficient catalyst-based active sites.\n               "}
{"full_text": "Proton exchange membrane fuel cell (PEMFC) is a fascinating sustainable energy technology that converts the chemical energy of hydrogen into electricity to power clean electric vehicles [1\u20133]. Massive Platinum group metals (PGM) are needed to catalyze the sluggish oxygen reduction reaction (ORR) at the cathode, which constitutes the major cost barrier in the large-scale application of PEMFC [4,5]. Hence, many efforts have been aimed at seeking PGM-free catalysts with decent ORR activity and stability as low-cost alternatives [6\u20138].Among PGM-free catalysts that have been developed, metal\u2013nitrogen\u2013carbon catalysts (M\u2013N\u2013C, M\u00a0=\u00a0Fe, Co, Mn, etc.) stand out because of their most promising fuel cell performance [9\u201315]. Their initial fuel cell performances could currently approach those of Pt-based cathodes at low loadings. For example, Shui and co-workers reported a Fe\u2013N\u2013C catalyst with highly dense Fe\u2013N4 active sites achieving the highest peak power density (P\nmax) of 1.18\u00a0W\u00a0cm\u22122 under 2.5\u00a0bar\u00a0H2\u2013O2 [15]. However, the stability of these highly active PGM-free catalysts in real fuel cell conditions is far from satisfactory. Specifically, their performance loss following the first 100-h test typical reaches 40\u201380%. The limited stability must be addressed to make PGM-free catalysts commercially viable. Therefore, the focus of continued research and development is turning from activity enhancement to the degradation mechanisms and targeted mitigation strategies.An in-depth understanding of the nature of the active sites is a prerequisite for diagnosing the cause of instability. M\u2013N\u2013C catalysts are generally synthesized by pyrolyzing a precursor containing iron, nitrogen, and carbon elements. The obtained catalysts might possess three types of active sites including nitrogen-doped carbons (denoted as NxCy), nitrogen\u2013carbon encapsulated metal nanoparticles (denoted as M@NxCy), and nitrogen coordinated metal atoms embedded in carbon substrate (denoted as M\u2013Nx or MNxCy) [16\u201324]. After extensive experimental and computational studies [25\u201329], a consensus has been reached that M\u2013Nx sites are most active in acid media, though some studies showed that M@NxCy structures also exhibited exceptional ORR activities. The overall fuel cell performance depends on the active site density (SD), the intrinsic activity of a single site (turnover frequency, TOF), as well as the accessibility to the reactants [30,31]. The decrease of any would result in the performance decay. In this context, four main suspicious degradation mechanisms of PGM-free catalysts were proposed previously: 1) carbon oxidation, 2) demetalation, 3) protonation and 4) micropore flooding [32\u201334]. The root-cause of instability is under extensive debate partially due to the structural complexity of M\u2013N\u2013C catalysts. In addition, the protocols to study stability vary with labs, which may also bring different results. For example, the rotating disc electrode (RDE) technique is often used for simplicity, which is different from the fuel cell environment [35]. The atmosphere, temperature, potential range also affect the conclusions.Recently, with the advancement of synthesis of ultra-pure materials [36\u201338] and operando characterization techniques [39\u201341], the research community is active and fruitful to identify the most likely degradation mechanisms by well-designed experiments. In this review, a survey of the increasing understanding of the degradation mechanisms of PGM-free catalysts is provided, with the major advances highlighted. Based on the mechanism understanding, the strategies for improving the stability of PGM-free catalysts are also summarized.The automotive applications of PEMFC involve various working conditions, including routine steady-state operation, rapid cycling, and frequent starts and stops. The term stability refers to the ability to maintain performance at constant current/voltage conditions while durability refers to the ability to maintain performance following a voltage cycling accelerated stress test [32]. Most fuel cell level investigations have focused on stability, while only limited studies on voltage cycling durability. As shown in Fig. 1\n, the stability was typically measured by chronoamperometry at the cell voltage around 0.4\u20130.6\u00a0V [15,42]. Two decay rates may be observed: a fast decay lasting about 15\u201320\u00a0h accounting for the major performance loss, followed by a much slower decay lasting up to the end of test. The major task of stability studies is to identify the cause of the rapid initial performance loss.The oxidation of the carbon support leads to the modification of the carbon surface and even worse the disintegration of active sites, which can be divided into two types: electrochemical oxidation and chemical oxidation. Electrochemical oxidation of the carbon is triggered by electrochemical potential and thermodynamically possible above 0.207\u00a0V versus standard hydrogen electrode (SHE) [39]:\n\n(1)\nC\u00a0+\u00a02H2O \u2192 CO2\u00a0+\u00a04H+\u00a0+\u00a04e\u2212, E\n0\u00a0=\u00a00.207\u00a0V\n\n\n\n\n(2)\nC\u00a0+\u00a0H2O \u2192 CO\u00a0+\u00a02H+\u00a0+\u00a02e\u2212, E\n0\u00a0=\u00a00.518\u00a0V\n\n\nHowever, these reactions generally require an overpotential of at least hundreds of millivolts. Such high potentials usually appear in the case of uncontrolled and transient starts/stops (ST/ST) of PEMFCs [43]. Mayrhofer and co-workers observed intensified ORR activity decay of a Fe\u2013N\u2013C catalyst with positively shifted potential range and elevated temperature by RDE tests [39]. They further made in situ observation of carbon oxidation of the catalyst in a modified scanning flow cell (SFC) system with differential electrochemical mass spectroscopy (DEMS) [39]. They found the onset potentials of carbon oxidation to CO2 and CO were about 0.9 and 1.2\u00a0V, respectively (Fig. 2\n). Using identical location-scanning transmission electron spectroscopy (IL-STEM) and IL-energy dispersive X-ray (EDX) spectroscopy analysis, they revealed a 2D shrinkage of 5\u201315% of the catalyst particle dimensions and a 20% reduction in iron content after 5000 cycles performed between 1.2 and 1.5\u00a0V\u00a0at 50\u00a0\u00b0C in 0.1\u00a0M HClO4 (Fig. 2b\u2013d). The Fe dissolution signal was directly proportional to the rate of carbon oxidation (Fig. 2e). This study demonstrated the direct correlation between and carbon oxidation/Fe leaching and ORR activity decay at high electrochemical potential. Note that the above experiments were performed in a de-aerated electrolyte. The presence of O2 is not expected to play a direct role in electrochemical carbon corrosion since the reactant is water rather than O2. However, the aerobic environment may trigger the formation of hydrogen peroxide (H2O2), an undesirable byproduct of ORR process.In addition to electrochemical carbon oxidation at high potentials, chemical oxidation by H2O2 is another widely accepted cause of instability of PGM-free catalysts. There might be two paths of H2O2 attack: 1) direct oxidation by H2O2; 2) H2O2 decomposes into reactive oxygen species (ROS) via Fenton reaction, and then ROS attack the catalysts. A proposed mechanism for the direct attack was provided by Schulenburg and co-workers [44]. In this work, it was suggested that the H2O2 can directly attack the N ligands to which the metal center is bound. While this mechanism could explain some parameter changes in M\u00f6ssbauer spectra of the degraded catalysts, the follow-up investigations were rare.The attack by ROS was firstly suggested by Dodelet and co-workers [45]. These ROS, in particular hydroxyl free radical (\u00b7OH), readily add to unsaturated aliphatic or aromatic compounds on the carbon support thus forming oxygenated surface groups, which were believed to be detrimental to the activity. It should be noted that the ROS would also induce the oxidative degradation of the Nafion membrane, such as thinning, pin-hole formation and ionic conductivity loss, which might ultimately contribute to the stack failure [46,47]. The mild surface oxidation was systemically investigated by Choi and co-workers with RDE technique (Fig. 3\n) [48]. They showed that ex situ exposure to H2O2 in the acid medium would selectively oxidize a fraction of top-surface carbon atoms via Fenton-like reactions in the presence of surface iron sites, without the formation of volatile CO or CO2. Such a mild surface oxidation would not change the structure of FeNxCy moieties but weaken the O2-binding, thus decreasing their single-site activity and 4e\n\u2212 selectivity. This type of deactivation was reversible, as the activity and selectivity could be partially recovered after electrochemical reduction to remove some oxygen groups. It was also found that peroxide-treatment in alkaline medium did not modify the ORR activity nor selectivity of the catalyst. This discrepancy suggested the formation of ROS from peroxide and surface iron sites was pH-dependent and primarily supported the pathway of indirect attack by ROS.Although only mild surface oxidation occurred with ex situ exposure to H2O2, the fate of the catalysts during PEMFC operation would be rather complex. Recently, Maillard and co-workers reported unexpected carbon corrosion coupled with significant demetalation at low potential of 0.3\u20130.7\u00a0V in the presence of O2 [49]. Compared with load cycling in Ar-saturated electrolyte, the activity loss was four times higher in O2-saturated electrolyte. This type of carbon corrosion was not controlled by classical electrochemical carbon corrosion but due to ROS produced between H2O2 and Fe sites via Fenton reactions. However, different from the only mild oxidation of the surface from ex situ H2O2 treatment, additional irreversible carbon corrosion, i.e. forming volatile products CO and CO2, also occurred. A significant fraction of the atomic Fe\u2013Nx sites was leached from the catalyst, which either exited the cathode layer at 25\u00a0\u00b0C or precipitated as iron oxide particles at 80\u00a0\u00b0C. Although the above studies were conducted using RDE technique, the iron oxide formation from some Fe\u2013Nx sites could also be transposable to the PEMFC operating conditions at 80\u00a0\u00b0C. In this report, the mechanism of iron leaching was left quite vague but strongly related to the irreversible carbon corrosion. The overall decay of the activity was caused by both the decreased active site density (SD) and the decreased single-site activity (TOF), with the latter due to the mild surface carbon oxidation. However, it was currently difficult to quantitatively distinguish the contributions of these two factors, because of the technical barrier of accurate measurement of SD under the interference of iron oxide.The metal leaching from PGM-free catalysts has been investigated for a long time [50,51]. The main challenge lies in the complex heterostructure of the catalyst and the lack of operando characterization methods. Therefore, it is difficult to identify the type of metal moieties that are easily leached out and whether it is responsible for performance degradation.The metallic Fe or Co particles without the protection of graphitic layers are generally believed with negligible ORR activity and are readily dissolved in the acid PEMFC environment according to the Pourbaix (E-pH) diagrams (Fig. 4\n), which illustrate the thermodynamic stability of metals in aqueous solutions [52,53]. Contrary to the common belief, Mayrhofer and co-workers showed that the operando Fe leaching from iron particles was potential-dependent in the SFC system mentioned in Section 2.1 [39]. Fe ions were released from Fe\u2013N\u2013C at low potential (<0.7\u00a0V) while no No Fe signal was detected above 0.8\u00a0V. They further elucidated that the ex situ acid-wash could not completely remove the iron particles due to a relatively high open circuit potential (~0.9\u00a0V) leading to the formation of insoluble ferric species, whereas these particles dissolved at the low potential due to operando reduction to soluble ferrous cations [40]. The Fe leaching from inactive iron particles will not cause a decrease in activity, and it may even improve the activity as new mesopores are created to expose more active sites [54]. However, the leached metal ions would have negative effects on the PEMFC performance, such as the carbon oxidation via Fenton reaction and the deterioration of the membrane and ionomer in the catalyst layer [55].The demetalation of active Fe\u2013Nx sites directly decreases the performance. Besides the destruction of Fe\u2013Nx sites via irreversible carbon corrosion as discussed above, there were very few reports discussing the direct demetalation of the Fe\u2013Nx sites. In 2005, Coutanceau and co-workers reported the deactivation of a FePc-based catalyst during the ORR was due to a substitution of the central iron by two protons, leading to inactive H2Pc [56]:\n\n(3)\nFePc\u00a0+\u00a02H+ \u2192 H2Pc\u00a0+\u00a0Fe2+\n\n\n\nIt was suggested that Fe2+ was oxidized to Fe3+ in the presence of O2, which reduced its ionic radius thus making it less stable in the macrocycle. More recently, Dodelet and co-workers proposed that the specific demetalation of Fe\u2013N4 catalytic sites located in the micropores was at the origin of the initial activity loss based on a systematic study at fuel cell level [57]. The Fe\u2013N4 sites were calculated thermodynamically stable in stagnant acidic conditions, but those in the micropores would demetalate under the quick water flow running into the micropores. The flux of water, which was driven by the humidified air streaming through the working cathode, would continuously shift the thermodynamic equilibrium between Fe ions and Fe\u2013N4 sites towards the direction of demetalation according to Le Chatelier principle. The degree of demetalation was measured by M\u00f6ssbauer spectroscopy and showed a similar trend to the relative current density measured at 0.6\u00a0V (Fig. 5\na), which strongly supported this degradation mechanism. The electro-oxidation of the catalyst surface was ruled out to play a major role in the initial fast performance decay based on the following two reasons. First, the activity decay was found irrespective of the value of the potential applied during chronoamperometry (Fig. 5b), while a large difference in the carbon support oxidation currents was observed (Fig. 5c). More importantly, electro-oxidation would only result in more hydrophilic micropores and less accessible active sites, thus the loss of Fe\u2013N4 sites would not be detected by M\u00f6ssbauer spectroscopy. It was also noticed that a fraction of Fe\u2013N4 sites survived after 50\u00a0h, which were recognized as Fe\u2013N4 sites located in the mesopores. The dynamic equilibrium in the mesopores could be established and a longer stability term was expected. These findings suggest that increasing the proportion of active sites located in mesopores might be a way to improve the stability of PGM-free catalysts. Wu and co-workers provided electron microscopic evidence for the demetalation of Fe\u2013N4 sites (Fig. 5d\u2013g) [58]. Using high angle annular dark field STEM (HAADF-STEM) and electron energy loss spectroscopy (EELS), they observed the formation of iron clusters and the break of Fe\u2013N bonds on a single-atom Fe\u2013N\u2013C catalyst that had worked for 100\u00a0h.There were several Fe@NxCy structures also reported highly active in acid medium. Although their activities were still inferior to the atomic Fe\u2013Nx sites, their fuel cell stabilities seemed better. For example, in 2011, Zelenay and co-workers reported a series of PGM-free catalysts featuring metal-containing particles encapsulated in onion-like graphitic carbon nanoshells [59]. The most active catalyst could serve stably for 700\u00a0h at a cell voltage of 0.4\u00a0V. Bao and co-workers produced a catalyst of Fe nanoparticles encapsulated within the compartments of pea-pod like CNTs that also demonstrated long-term stability in PEMFC [60]. More recently, Mukerjee and co-workers synthesized a Fe\u2013N\u2013C catalyst with exclusive Fe@NxCy structure and devoid of any Fe\u2013Nx sites that showed high activity (half-wave potential E\n1/2\u00a0=\u00a00.77\u00a0V) and a 4e\n\u2212 selectivity in acidic media [61]. The membrane electrode assembly (MEA) made with this catalyst was even more durable compared with the state-of-the-art Pt/C MEA. The exact origin of the durability of M@NxCy was still under investigation. A possible advantage of this structure was that Fe does not directly participate in the reaction, thus potentially eliminating any Fenton reactions involving exposed iron ions. Nevertheless, Jaouen and co-workers compared the stability of M\u2013Nx sites and M@NxCy structures (M\u00a0=\u00a0Fe, Co) and found the former were more robust to both demetalation and carbon corrosion than the latter during load-cycling (0.6\u20131.0\u00a0V) or ST/ST process (1.0\u20131.5\u00a0V) [62]. However, this study was performed in an Ar-saturated electrolyte. Further clarification of the operando stability of these two types of active sites requires more intensive investigation.The mechanism of protonation was first proposed by Popov and co-workers to explain the difference in stability between the active sites of pyridinic nitrogen and graphitic (quaternary) nitrogen [63]. Two catalysts were synthesized with different pyrolysis temperatures. One pyrolyzed at 800\u00a0\u00b0C containing both pyridinic and graphitic nitrogen showed higher initial activity but much lower stability. The other pyrolyzed at 1100\u00a0\u00b0C only containing graphitic (quaternary) nitrogen showed the opposite. It was hypothesized that the lone electron pair of pyridinic-N could be protonated in the acidic PEMFC environment and thus formed an inactive pyridinic-N\u2013H group. In contrast, the graphitic-N possessed no extra electron to be protonated, explaining its higher stability. The protonation mechanism provided a plausible explanation for the initial rapid performance loss of PGM-free catalysts in PEMFC. However, this degradation mechanism was criticized by Banham and co-workers [32]. Based on the protonation hypothesis, the pyridinic-N would be protonated very fast in the RDE test given the more available protons in the liquid electrolyte. This should have resulted in very similar activities of these two catalysts, which had not been observed in the experiment.Afterward, in 2011, Jaouen and co-workers provided a variant of the protonation mechanism: N protonation and anion binding [64]. It was proposed that the TOF of Fe\u2013N4 sites could be tuned by the chemical state of the adjacent basic N-groups on the catalytic surface. The protonation of the basic N-groups did not decrease the TOF of the Fe\u2013N4 sites, while the subsequent anion binding on the protonated N sites neutralized their basicity and ultimately resulted in activity decay. The activity could be restored after the anions were removed thermally or chemically. It was further proposed that protonation of basic N-groups was rapid but the anion binding was delayed in PEMFC due to restricted mobility of the sulfonate groups. Therefore, slow anion binding by polymeric anions was believed to be an indispensable reason for the initial rapid decay of PGM-free catalysts in PEMFC. However, in a report published in 2016, the authors abandoned this anionic neutralization hypothesis after several unsuccessful attempts to modify the first decay behavior by tuning the properties of the proton providers, such as replacing part of Nafion with hydrous ruthenium oxide [42].Based on intensive studies [65,66], Dodelet and co-workers had demonstrated that the active M\u2013Nx sites were mainly located in the micropores of carbon matrix. Therefore, the water flooding of micropores would be detrimental to fuel cell performance by impeding the transport of oxygen. In 2015, micropore flooding was proposed by Dodelet and co-workers to explain the superior stability of a catalyst obtained after the pyrolysis in Ar at a high temperature of 1150\u00a0\u00b0C [67]. This catalyst possessed the best degree of graphitization of the carbon support and loss of heteroatoms (N and O), which was proposed to render the carbon support highly hydrophobic and would, therefore, reduce the possibility of water flooding. This mechanism also corroborated the negative correlation between stability and the micropore percentage in a catalyst. In a follow-up report, they further proposed that the micropore flooding was mainly responsible for the initial performance loss of PEMFC [42]. The rate of flooding depended on the hydrophilicity of the micropore walls, which would gradually change from hydrophobic to hydrophilic on the first 15\u00a0h of PEMFC operation due to the slow carbon electro-oxidation. The surface oxidation not only decreased the current density at 0.6\u00a0V by inducing mass transfer problems but also impeded the kinetics of electron transfer at 0.9\u00a0V. The Fenton reaction via H2O2 and iron ions was excluded as the major cause of the rapid initial performance decay, because they produced a series of catalysts with different level of iron content and found their decay patterns were similar.It is also important to clarify the difference between the micropore flooding and catalyst layer flooding. The latter can occur at high relative humidities (RH)/current densities due to the accumulation of water in the catalyst layer, which blocks mass transport pathways thus leading to performance loss. This type of degradation is reversible with periodic water drainage but not intrinsic to the catalyst. Recent work showed that the drainage ability of the catalyst layer had a negligible influence on the stability of Fe\u2013N\u2013C fuel cells [68].More recently, Chen and co-workers conducted a systematic study to investigate micropore flooding in situ before and after fuel cell stability tests (Fig. 6\n) [69]. The results cast doubt on micropore flooding as a major contributor to instability. The degree of micropore flooding was monitored by the changes in the double layer capacitance obtained by cyclic voltammetry (CV). It was analyzed that the micropores are partially wetted at beginning of life (BOL). After the stability test, a small (~8%) degree of additional catalyst layer wetting was observed, which could not account for the significant performance loss (\u00a0\u00d7\u00a010 reduction in activity). The micropore flooding hypothesis was rejected primarily because the performance decay was largely kinetic, rather than from mass transport loss due to flooded micropores. The reversible catalyst layer flooding was also ruled out as no performance recovery was achieved after the dry-out protocol. Later, Dodelet and co-workers agreed that the micropore flooding mechanism should be abandoned and proposed the new hypothesis of specific demetalation, as discussed in section 2.2.As discussed above, carbon oxidation by Fenton-like reaction and demetalation might be the most likely degradation mechanisms of PGM-free catalysts. Therefore, effective improvement of the stability may be achieved by eliminating Fenton reaction, increasing the corrosion resistance of carbon support, as well as mitigating demetalation.The iron ions are criticized for their catalysis of the decomposition of H2O2 into ROS. Other transition metals, such as Cr, Mn, Co, Ni, Cu, Zn, are not powerful Fenton's reagents [70]. Therefore, the M\u2013N\u2013C catalysts based on these non-Fe metals hold the potential to alleviate Fenton reaction [71\u201374]. However, the ORR activities of the PGM- and Fe-free catalysts are generally poorly competitive with the iron counterparts, which restricts their application in real fuel cells. Given the ORR activity is a product of SD and TOF, increasing the density of M\u2013Nx active sites in a Fe-free catalyst is a pathway to enhance the activity.Shui and co-workers optimized the Co content of a series Co\u2013N\u2013C catalysts to achieve the highest density of Co\u2013N4 active sites [75]. The best catalyst exhibited a P\nmax of 0.83\u00a0W\u00a0cm\u22122, approaching that of Fe\u2013N\u2013C. As shown in Fig. 7\na, the higher stability of Co\u2013N\u2013C over Fe\u2013N\u2013C was observed. Wu and co-workers developed a surfactant-assisted confinement pyrolysis strategy to fabricate a Co\u2013N\u2013C catalyst with doubled Co\u2013N4 site density, which showed fuel cell performance comparable to Fe\u2013N\u2013C [76]. A 100-h stability test at a cell voltage of 0.7\u00a0V using 1\u00a0bar\u00a0H2-air is shown in Fig. 7b. Although significant performance decay was observed, the authors wrote that the stability was commendable at such a relatively high voltage, compared to other PGM-free catalysts. They also reported a single-atom Mn\u2013N\u2013C catalyst with increased active site density, achieving encouraging activity and stability [70]. Xing and co-workers synthesized a novel Cr\u2013N\u2013C catalyst with atomic Cr\u2013N4 sites and investigated its catalytic ability toward Fenton reaction [77]. This catalyst exhibited a considerable half-wave potential of 0.74\u00a0V, and more impressively its stability was superior to the Fe\u2013N\u2013C counterpart. Although the Cr\u2013N\u2013C catalyst showed high H2O2 production, the ROS formation was suppressed according to the color reaction with ABTS (2, 20-azinobis (3-ethylbenzthiazoline-6-sulfonate) (Fig. 7c and d). The low catalytic ability toward Fenton reaction could thus remedy the shortcoming of high H2O2 production. In addition, PGM-based M\u2013N\u2013C catalysts with atomic M\u2212N4 (M\u00a0=\u00a0Ir, Rh, Pt and Pd) were successfully synthesized by Shui and co-workers [78]. The Ir\u2013N\u2013C catalyst also exhibited higher durability than typical Fe\u2013N\u2013C catalysts.It is worth noting that some metal-free catalysts have shown sufficient stability in PEMFC [79,80]. However, their performances were still at a relatively low level. Further improvements may render them applicable in certain scenarios.As discussed in Section 2.2, Fe@NxCy structures have the advantages of indirect participation of iron and thus the potential to eliminate Fenton reactions. The iron nanoparticles also catalyze the graphitization of surface carbon layers thus increasing their resistance to oxidation. Recently, a growing number of researchers resort to this type of catalyst to achieve a stable PGM-free fuel cell cathode with acceptable performance [81\u201384]. The density of metal nanoparticles and the number of graphitic layers on their surface have a great impact on the performance. However, this type of catalyst is usually produced by pyrolysis, hence the controllable synthesis of this material with high-density nanoparticles and well-defined graphitic layers remains a challenge [85].Because H2O2 is the source of ROS, decreasing the production of H2O2 or breaking down the produced H2O2 might be effective to mitigate the attack of ROS. Liu and co-workers made a combination of Pt\u2013Co nanoparticles and Co\u2013N\u2013C catalyst and found the interaction between Pt\u2013Co and Co\u2013N4 sites improved ORR activity and durability [86]. This combination led to a 4-electron transfer, which suggested the completed conversion from O2 to H2O instead of H2O2. It was speculated that intermediate H2O2 was transferred from the Co\u2013N4 site to the Pt\u2013Co nanoparticles through a reverse spillover. As shown in Fig. 8\n, theoretical calculations revealed the subsequent thermodynamic favorable breakdown of H2O2 over the strained Pt (111) surface, which therefore served as a powerful H2O2 scavenger. Similarly, Jaouen and co-workers reported the addition of 1\u20132\u00a0wt% Pt to a Fe\u2013N\u2013C catalyst could significantly improve its fuel cell stability [87]. Introducing ultralow-loading of Pt to PGM-free catalysts may provide a way to achieve a balance of cost and performance [88]. The remaining challenge is to find applicable low-cost H2O2 scavengers. Eliminating ROS might be another way to circumvent Fenton reaction. Ramani and co-workers previously demonstrated that the addition of CeO2 nanoparticles to the proton exchange membrane effectively scavenge ROS thus mitigating their degradation [89]. The utility of CeO2 as a ROS scavenger to improve the stability of PGM-free catalysts, however, remains to be further explored [90].To date, most well-performing PGM-free catalysts were built on highly microporous carbon supports with low degree of graphitization, which resulted in the drawbacks of low conductivity and poor corrosion resistance. In this regard, growing active sites on highly graphitic carbons such as carbon nanotubes (CNTs) and graphene may have the chance to improve stability [91\u201394]. Chen and co-workers fabricated a catalyst with abundant 3D porous graphene-like structures, which hosted a dense population of accessible active sites [95]. After 5000 load cycles in N2 environment, this catalyst retained 90% of its initial fuel-cell performance, attesting its high resistance to carbon corrosion. Kang and co-workers successfully embedded high-density Fe\u2013Nx active sites into CNTs (Fig. 9\n) [96]. When compared with a state-of-the-art ZIF-derived Fe\u2013N\u2013C catalyst, the new Fe\u2013N/CNT catalyst demonstrated much enhanced stability.Fluorination of the carbon is also known to be beneficial to the stability towards oxidation [97]. Zhou and co-workers developed a surface fluorination strategy to boost the stability of the Fe\u2013N\u2013C cathode (Fig. 10\n) [98]. The catalyst surface was covalently grafted of a hydrophobic trifluoromethylphenyl (Ar-CF3) group, which could effectively prevent the catalyst layer flooding. More importantly, the electron-withdrawing property of Ar-CF3 group lowered the Fermi level of carbon matrix, thus increasing the reaction energy barrier and decreasing the rate of carbon oxidation. The intrinsic hydrophobicity and timely removal of excess water were also suggested to decrease the H2O-involved carbon oxidation rate. The demetalation was suppressed, which might be attributed to the decrease of carbon corrosion. As a result, the fluorinated catalyst could deliver a stable current density of 0.56\u00a0A\u00a0cm\u22122\u00a0at cell voltage of 0.5\u00a0V up to 120\u00a0h. Note that the surface fluorination had an adverse effect on the BOL performance, however, this strategy was laudable as the final current density was much higher than that of original Fe\u2013N\u2013C.The primary strategy is to remove unprotected free metals in the catalysts. It could be achieved by post-acid wash or by fabricating pure-phase materials. As the mechanism of direct demetalation from the atomic M\u2013Nx site was proposed most recently, it is not surprising there was little work discussing the targeted mitigation strategies. A survey of the literature suggests a possible way is to redesign the atomic structure of the active site, which should exert atomic-level protection to the metal center or increase the strength of the metal-N bonds [99]. Shui and co-workers grafted of a Pt atom onto the iron center through a bridging oxygen molecule, creating a new active moiety of Pt1\u2013O2\u2013Fe1\u2013N4 (Fig. 11\na) [100]. This structure indeed exhibited considerably improved fuel cell performance and stability compared with the untreated Fe\u2013N\u2013C (Fig. 11b). The Pt1\u2013O2\u2013 cap was speculated to avoid Fenton-like reaction and strengthen the Fe\u2013N coordination. Another hotspot of the electrocatalysis is the double-atom catalyst, which features a N-coordinated double-atomic metal center. Li and co-workers designed and synthesized a catalyst with Fe\u2013Co dual sites embedded in nitrogen-doped carbon [101]. This catalyst exhibited state-of-the-art ORR activity (E\n1/2\u00a0=\u00a00.863\u00a0V) and fuel cell performance (P\nmax\u00a0=\u00a00.98\u00a0W\u00a0cm\u22122\u00a0at 2\u00a0bar\u00a0H2\u2013O2). Surprisingly, the catalyst showed negligible performance loss after 100-h H2-air single cell operation. Several follow-up studies reporting similar diatomic structures such as Co\u2013Zn atomic pairs also pointed to their enhanced stability [102,103]. However, the exact reason for the superior stability of double-atom catalysts was seldomly discussed. If these results were convincing, we speculated the larger radius of the diatomic center might decrease the tendency of demetalation.Several possible mechanisms have been previously proposed to account for the instability of PGM-free catalysts in PEMFC, including 1) carbon oxidation, 2) demetalation of metal sites, 3) protonation of active sites with possible subsequent anion binding, and 4) micropore flooding. After decades of extensive research, carbon oxidation and demetalation are generally accepted as primary degradation mechanisms, while the latter two mechanisms are strongly challenged. Carbon oxidation can be either triggered by high electrochemical potential >0.9\u00a0V or caused by oxidative attack by H2O2/ROS in a wider potential range. Mild surface oxidation may not destruct the nearby M\u2013Nx active site but decrease its TOF via weakening the O2-binding. More severe carbon corrosion with the formation of volatile CO or CO2 will lead to the disintegration of active sites. Another type of specific demetalation of micropore-hosted M\u2013N4 sites straightforward degrades the activity by decreasing the number of active sites. The leached metal ions, particularly iron ions, could in turn catalyze the ROS formation from H2O2 via Fenton reaction.With the deepening understanding toward the degradation mechanisms, the researchers are seeking solutions to the poor stability with increasing vigor. The mitigation strategies include eliminating Fenton reaction, enhancing carbon corrosion resistance, and alleviating demetalation. To eliminate Fenton reaction, the direct participation of iron should be avoided. In this regard, the researchers are developing PGM- and Fe-free catalysts (such as Co\u2013N\u2013C, Mn\u2013N\u2013C) and the catalysts of encapsulated M@NxCy structures with their performances comparable to the Fe\u2013N\u2013C counterparts. Introducing H2O2/ROS scavengers is also a promising strategy to alleviate Fenton reaction. To enhance carbon corrosion resistance, highly graphitic CNTs and graphene could be used as robust supports for active sites. Fluorination appears also beneficial the stability towards carbon oxidation. The mitigation of direct demetalation has been seldomly reported in the literature. A direction is atomic-scale redesign of the active site with stronger M\u2013N bonds.Despite the significant progress, the long lifetime of high-performance PGM-free catalysts has remained an elusive target. Further understanding the degradation mechanisms at fuel cell levels requires advanced in situ characterization techniques with more localized resolutions. Several key questions remain to be answered and the efforts may focus on 1) identification of the durable and non-durable sites among FeNxCy moieties, 2) exploration of the factors affecting demetalation and remediation strategies, 3) investigation of the reason why some catalysts (for example, Fe\u2013Co double-atom catalyst) are so stable in fuel cell 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 Thousand Talents Plan of China, the National Natural Science Foundation of China (Grant No. 21673014 and 21975010).", "descript": "\n                  While Platinum group metals (PGM) free catalysts are promising alternatives to expensive Pt as the cathode catalyst in proton exchange membrane fuel cells, their rapid degradation must be addressed for the commercial feasibility. This review provides a historical survey of the possible degradation mechanisms of PGM-free catalysts. Decades of extensive studies confirm that carbon oxidation and demetalation are primarily responsible for the instability, whereas the mechanisms of protonation and micropore flooding are strongly criticized. Based on the mechanism understanding, the mitigation strategies for improving stability are discussed in detail. Finally, some directions to achieve high-performance and durable PGM-free catalysts are proposed.\n               "}
{"full_text": "Water still remains an essence of life, however with continuous discharge of waste into water bodies, access to clean and potable water has continued to dwindle. The detection of various waste organic contaminants such as pharmaceutical, dyes, pesticides, personal care products in surface, ground and drinking water is of major challenge globally due to their detrimental effects (Du and Zhou, 2021). For example, the consumption of water containing organic dyes like methylene blue (MB), rhodamine B (RhB) and methyl orange (MO) can cause eye irritation, bladder cancer and respiratory problems (Fern\u00e1ndez et al., 2010; Rai et al., 2005; Tan et al., 2015). With the rapid growth of textile industries, one of the major industrial sources of organic dyes owing to>100 000 tons of production of dyes per year, thus it is vital to monitor and treat these industrial waste dyes before reaching the environment (Abdi et al., 2017; Gupta et al., 2013; Gupta and Suhas, 2009; Holkar et al., 2016; Katheresan et al., 2018). Of major concern, are pharmaceutical products that have been widely used in various fields including households, agriculture and medicine. In medicine, products such as penicillin, ciprofloxacin, tetracycline and sulfamethoxazole are used as antibiotics to treat bacterial infections (Huang et al., 2021; Mo et al., 2017). The production of antibiotics increases daily due to their high demand for bacterial infection prevention or to cure diseases. The production of penicillin per year was reported to be approximately 28,000 tons, thus making it 68\u00a0% of the global consumption of antibiotics (An et al., 2015; Du and Liu, 2012). The presence of some of these antibiotics are of great concern due to their serious health effects such as vomiting, nausea, acute renal failure and diarrhea (Orimolade et al., 2020).Other water contaminants which have been detected in different water bodies include pathogenic bacteria such as escherichia coli (E.coli), staphylococcus aureus (S. aureus), pseudomonas aeruginosa (P.aeruginosa), enterococcus faecalis (E.facelis) and other microbes. Inappropriate disposal of sewage and animal waste are the most common sources of faecal matter in the environment. The discharge of these waste materials from the environment into different water bodies such as rivers, lakes, oceans and streams does not affect only chemical oxygen demand (COD), biological oxygen demand (BOD) and turbidity of the surface water but also increases the number of various pathogenic pollutants (viruses and bacteria) existing in them (Pandey et al., 2014). In 2020, approximately>12\u00a0% of the global population was reported to be drinking water containing a substantial amount of unsafe pathogens. Drinking water containing these harmful pathogens can be lethal and cause some serious health issues and waterborne diseases such as diarrhoea, polio, typhoid and malaria etc (Pandey et al., 2014). According to the World Health Organisation (WHO) and standards, the allowed recommendable limitation concentration of organic dyes and bacteria should be below 1\u00a0ppm and 0\u00a0CFU/100\u00a0ml in drinking water, respectively (Katheresan et al., 2018; Masekela et al., 2020; Mahlaule-Glory et al., 2019). Thus, it is crucial to maintain the level of organic dyes, pharmaceutical and pathogenic bacterial within permissible limit so as to provide clean drinking water to humans and protect the environment.Several water technologies and bacterial disinfection techniques including chlorination, chlorine dioxide, ozonation, ultraviolet light (UV), adsorption, membrane filtration and coagulation have been developed to maintain the level of waste water pollutants (organic dyes, bacteria and pharmaceuticals) within a safe level (Hassan et al., 2012; Masekela et al., 2022b; Saucier et al., 2017; Sir\u00e9s et al., 2014; Sir\u00e9s and Brillas, 2012a). However, these methods suffer from several limitations including generation of secondary toxic waste, high cost maintenance, incomplete removal of wastewater pollutants, poor recyclability and the use of toxic chemicals. Chlorination is one of the most popular and inexpensive bacterial disinfection techniques for the removal of all micro-organisms present in drinking water. Even though this technique is relatively less expensive, it produces harmful toxic by products such as trihalomethanes (THMs), haloacetonitriles (HANs), haloacetic acids (HAAs) etc (Xiang et al., 2018). These disinfection by products (DBPs) have negative impacts on human health as they can cause intestinal cancer. Additionally, chlorination with other methods like adsorption and filtration partially removes pharmaceuticals from wastewater, since 60\u00a0% of pharmaceutical residues remain even after treatment (Orimolade and Arotiba, 2020; Sir\u00e9s and Brillas, 2012b; Xiao et al., 2015). Furthermore, adsorption and membrane filtration generate secondary toxic waste pollutants, thus require additional treatment which is very expensive (Gupta et al., 2012). Therefore, it is very important implement methods which are highly effective, economical and can completely degrade a majority of the wastewater pollutants into less harmful by products.Advanced oxidation processes (AOPs) such photocatalysis and piezocatalysis have been used as effective methods for complete destruction of organic waste pollutants into less harmful by products. Photocatalysis and piezocatalysis uses generated strong oxidants such hydroxyl radicals (\u2022OH) and superoxide anion (\u2022O2\n\u2013) to completely decompose organic pollutants under the influence of visible light and ultrasonic vibration, respectively (Chen et al., 2020; Koe et al., 2020; Li et al., 2019; Liang et al., 2018; Wu et al., 2018a). Unlike other conventional methods, AOPs completely oxidise organic waste pollutants into less harmful by products such as carbon dioxide (CO2) and water (H2O). In the photocatalytic degradation process, one main disadvantage is the fast recombination of electrons and holes (X. Liu et al., 2020). Over the past years, several methods such as metallic or non-metallic doping, formation of heterojunction and composites have been employed to enhance their photocatalytic activity, however effective electron-holes separation still remains a problem (Alex et al., 2019; Kanhere et al., 2014; Qi et al., 2017; Wang et al., 2017; Yong Zhang et al., 2019). Consequently, a piezo-electric field that is built within semiconductors has been shown to effectively separate charge carriers (electron and holes) to prevent recombination reactions. Recently, piezoelectric perovskites (ABO3) structure materials have been employed as an alternative way for better separation of charge carriers (e- and h+) (Y. Feng et al., 2018; Fu et al., 2021; X. Li et al., 2021, 2021; Liu et al., 2021; Y. Liu et al., 2020; J. Wu et al., 2020; J. Zhang et al., 2019).Piezoelectric materials are known as smart materials which produce electric charges under the influence of applied mechanical vibration. These smart materials also tend to exhibit inverse piezoelectric effect, like the generation of mechanical stress under the influence of applied electric field (Xu et al., 2018). The generated electric charges on the opposite site of piezoelectric materials tends to form an electric field across the material. The built in electric field significantly separates the charge carrier (e- and h+) which further reacts with dissolved water and oxygen molecules to generate reactive oxygen species (hydroxyl and superoxide radicals), which are responsible for the breakdown of organic waste water pollutants (Y. Feng et al., 2018; Mushtaq et al., 2018; J. Wu et al., 2020).Among the numerous piezoelectric materials which have been used as piezocatalysts for catalytic degradation of organic waste pollutants present in wastewater, (BaTiO3) has grabbed more attention as a piezocatalyst due to its excellent piezoelectric properties and biocompatibility (Kumar et al., 2019a; Ray et al., 2021). Besides that, it is a lead-free piezoelectric material thus making it more appropriate to be applied in environmental applications. Previously, BaTiO3 as a lead free piezocatalyst has been widely used in sensors. However, recently piezo-photocatalytic applications of BaTiO3 as a piezo-photocatalyst has attracted more attention in environmental wastewater treatment (Aksel and Jones, 2010; Ray et al., 2021). Therefore, this review article gives an overview of the recent applications of BaTiO3 as a piezo-photocatalyst for the catalytic breakdown of organic dyes, bacteria and pharmaceutical pollutants. Moreover, the concept of piezocatalysis, photocatalysis, different fabrication methods, relevant piezoelectric properties and modification methods of BaTiO3 are discussed in detail.Photocatalysis and piezocatalysis processes are regarded as advanced oxidation processes. These two processes have been widely used in many applications including water splitting, bacterial disinfection, degradation and wastewater treatment (Mengying et al., 2017). The concept of piezocatalysis is similar to that of photocatalysis, the only difference lies on the triggering source to generate reactive oxygen species (ROS) which participate in redox reactions to degrade organic pollutants. In photocatalysis, a light source is usually utilized in the presence of a semiconductor (photocatalyst) to generate electron-holes pairs. The semiconductor absorbs the irradiated UV light with high energy thus resulting in electron excitation from valence band (VB) to conduction band (CB) leaving holes behind as displayed in Fig. 1\n.As shown in Fig. 1, the photo-generated electron-hole pairs move on separate active sites of the semi-conductor to initiate redox-reactions which generates reactive oxygen species (ROS) responsible for the decomposition of organic waste pollutants. Unfortunately, the rate of electron-holes recombination is very fast which limits the application of semiconductors for photocatalysis. However, in piezo-photocatalysis, an internal voltage is generated under ultrasonic vibration with a built-in-electric field within the semiconductor. The in-built electric field piezo-semiconductors assists in the separation of photo-generated charge carries thus improving the photoactivity of the semiconductor. As shown in Fig. 2\n(a) and (b), the separated charge carries due to piezoelectric effect at the opposite surfaces generates free radicals through redox reactions.Piezo semiconductor materials under the influence of applied pressure have been shown to behave like electrocatalytic reactors. The free electric charges (electrons and holes) at opposite sides of these materials tend to act as anode and cathode (Liang et al., 2018). The reaction (1) and (3) shows the formation of reactive oxygen species from free electric charges on the opposite sites of piezoelectric semiconductor materials. As shown in equation (2) and (3), the free positive charges react with water to form hydroxyl radicals (\u2022OH), whereas negative charges react with free oxygen molecules to form superoxide radicals (\u2022O2\n\u2013). These reactive oxygen species (\u2022O2\n\u2013 and \u2022OH) are regarded as strong oxidants and are responsible for the degradation of organic dyes and bacterial disinfection.\n\n(1)\nBaTiO3 (Piezoelectric material)\u00a0+\u00a0ultrasonic vibration\u00a0\u2192\u00a0BaTiO3 (e- + h+)\n\n\nNegatively charged surface of piezoelectric material\n\n(2)\nh+ + H2O \u2192 \u2022OH\u00a0+\u00a0H+\n\n\n\n\n\n(3)\nh+ + OH\u2013 \u2192 \u2022OH\n\n\nPositively charged surface of piezoelectric material\n\n(4)\ne- + O2 \u2192 \u2022O2\n\u2013\n\n\n\nUnlike photoelectrocatalysis which is another type of electrochemical advanced oxidation for wastewater treatment, this process requires an external high voltage to reduce the rate of electron-holes recombination. Instead of using an external voltage, piezoelectrics materials are used to produce an internal voltage under mechanical vibration. The most extensively used are lead based piezoelectric materials such as lead zirconate (PZT). However, PZT contains about 80\u00a0% of the lead (Pb) content thus limiting their use in various applications (Panda and Sahoo, 2015). Due to lead being toxic, it is very important to develop piezoelectric materials which are lead free. Over the past few years, barium titanate (BaTiO3) has been given more attention as a lead free piezoelectric material for the production of piezoelectricity under mechanical vibration. Furthermore, recently BaTiO3 has been widely used as one of the piezo semiconductors in piezo-photocatalytic wastewater treatment applications.Piezo-photocatalytic experiments using powder catalyst involves dispersing a certain amount of the catalyst into a contaminated solution. Since powder catalyst offers high surface to volume ratio, the solution mixture consisting of the catalyst is stirred for a certain time, normally for 30\u00a0min to reach an adsorption\u2013desorption equilibrium in the dark. Thereafter, the solution mixture gets exposed to a light source. Some of the important parameters which need to be considered during conducting piezo-photocatalytic experiments in suspension systems includes the type of the light source (solar or UV light), UV light power, Ultrasonic power, UV intensity, the amount of the material used (dosage), reaction time and pH of the solution. Recently, a majority of the piezoelectric semiconductors such as BaTiO3 have been modified to convert their absorption from UV region to visible region (reduce their band gap) to utilize the visible light as a source of light, which constitutes of 43\u00a0% of the solar energy. For instance, the activity of piezoelectric semiconductors like ZnO and BaTiO3 were tested under different light sources such as sunlight and artificial visible light (Xenon lamp 1000\u00a0W, which emits visible light in the wavelength between 400 and 800\u00a0nm). Under solar light irradiation, the total organic carbon (TOC) results showed complete mineralization of phenol at lower concentrations as compared to artificial visible light irradiation (Pardeshi and Patil, 2008).The type of the vibration normally employed in piezo-photocatalysis process is ultrasonic vibration. Ultrasonic excitation, can be used to induce piezoelectric materials to produce piezoelectric potential, which can effectively encourage the deterioration of organic dyes. However, under stress the generated free carriers will move in a specific direction to their end two poles and shield the piezo-potential, reducing the driving force. As a result, to maintain the electric field during the piezocatalysis process, continual oscillation is necessary. The ultrasound has the capacity to deliver continuous stress as a physical expression of mechanical energy (Lu et al., 2022). It is important to note that prolonged ultrasonic vibration will have both sonochemical and piezoelectric effects on materials that are made of piezoelectric components (Torres et al., 2008). The sonochemical effect can also help in the degradation of organic or inorganic wastewater pollutants.The issues associated with powder piezo-photocatalyst such as low separation efficiency, poor recovery and regeneration ability, could be resolved by fabricating piezo-photocatalysts supported on the substrate to produce thin film electrodes. Typically, powder catalysts are separated from aqueous solution via filtration and centrifugation process, thus time consuming and some of the catalyst residue might remain in the solution and lead to secondary pollution. Piezo-photocatalyst thin film electrodes offer a good recoverability and recyclability, unlike powder catalysts. However, thin film electrodes during degradation process do not offer the full contact with the solution as compared to powder catalyst. Due their limited contact with the solution or low surface area, thin films exhibit slow degradation rate and low degradation efficiency. Besides that, growing interest is being shown in thin films with nanostructures that are directly formed on the surface of the substrate and are particularly susceptible to exposure to the dye solution. The degradation of organic pollutants via piezo-photocatalytic processes can be illustrated in Fig. 3\n\n. As shown in the Fig. 3 experiment, the prepared piezo-photocatalyst thin film is dispersed into a solution containing organic pollutants, thereafter exposed to light and ultrasonic irradiation. Just like piezo-photocatalytic experiment in suspension system, the parameters such as; the distance between the thin film electrode and light source, distance between thin film electrode and ultrasonic probe, ultrasonic power and light source need to be considered. Recently, floatable thin films are designed, which freely moves atop the water offering better utilization of sunlight. Unlike, steady thin film which requires photoreactor and external light source. Furthermore steady thin film requires a specific platform to control the distance between the light source and thin film electrode, which obviously raises the cost of scalable water purification (Yaozhong Zhang et al., 2019).Another form of using thin film electrode is via sono (piezo)-photoelectrocatalytic degradation processes. Sono(piezo)-photoelectrocatalytic processes is a combination of sonocatalysis/piezocatalysis, photocatalysis and electrocatalysis. These processes have not yet been extensively investigated. In this experiment, light irradiation, ultrasonic vibration and bias voltage is applied on the surface of the thin film electrode. The degradation experiment is conducted using potentiostat/galvanostat, the prepared piezo-photocatalyst thin film is employed as a working electrode in the presence of a reference (Ag/AgCl) and counter electrode (platinum wire). Generally, the fabricated thin film electrode is positioned vertically opposite to the ultrasonic probe and light source (Fig. 4\n).BaTiO3 is one of the highly applied ferroelectric materials which exhibit piezoelectricity under any form of mechanical vibration. It belongs to the perovskites family (ABO3), whereby A denotes a Barium (Ba) atom and B is a Titanium (Ti) atom. The crystal structure of BaTiO3 consists of Ti4+ atoms co-ordinated to six oxygen atoms to produce octahedral cluster\u2019s (TiO6) and Ba2+ co-ordinated to twelve oxygen atoms to form (BaO12) clusters (Fig. 5\n). As shown in Fig. 5, Ba atoms are situated at every corner position, O atoms at face centred positions and Ti atoms at the centre.Barium titanate can exist in different crystal structures such as cubic, tetragonal, orthorhombic and rhombohedral depending on the theta angles and phase transition temperature. The major distinction between cubic and tetragonal phases of BaTiO3 lies on the slight shift of theta angles of octahedral (TiO6) clusters from 90\u00b0 to \u2248 93.3\u00b0, whereas the orthorhombic and rhombohedral phase occurs in the theta angles from approximately 89.9\u00b0 to\u00a0\u223c\u00a085.7\u00b0(Itoh et al., 1985). The BaTiO3 crystal structures undergoes three different phase transitions under different temperatures. At temperature between 26.85\u00a0\u00b0C and 46.85\u00a0\u00b0C, cubic crystal structures transform into tetragonal structures, and to orthorhombic at approximately \u201323.15\u00a0\u00b0C to 6.85\u00a0\u00b0C, then ultimately to rhombohedral at temperatures around \u221273.15\u00b0 C and \u201333.15\u00a0\u00b0C (Acosta et al., 2017; Oliveira et al., 2020). The band energy gap of each crystal structure of BaTiO3 plays a significant role in the photocatalysis process. The cubic crystal structure of BaTiO3 has a theoretical direct band energy gap of 4.68\u00a0eV, while orthorhombic, tetragonal and rhombohedral exhibit an indirect band energy gap of 5.06, 4.73 and 5.06\u00a0eV, respectively (Oliveira et al., 2020; Piskunov et al., 2004). Amongst these crystal structures, tetragonal-BaTiO3 (t-BaTiO3) has the lowest band energy gap than other phases (orthorhombic and rhombohedral). Owing to its low energy band gap (t-BaTiO3) when compared to other phases, this makes it a suitable piezo semiconductor for photocatalytic degradation of organic waste pollutants present in wastewater. Moreover, due to its well-positioned valence band, it also an important material in water splitting for hydrogen production.Furthermore, BaTiO3 has a wide band energy gap just like other metal oxides such as TiO2, ZnO, SnO2, WO3 and BiVO4 etc., and its photoactivity is limited by the recombination of photogenerated charge carriers (e- and h+), which occurs rapidly (Demircivi and Simsek, 2019). However, unlike normal semiconductors, BaTiO3 is considered also as a piezo semiconductor which produces an internal piezo electric field under mechanical vibration. The induced built-in piezoelectric field separates the photogenerated charge carries, thus reducing their rate of recombination (X. Liu et al., 2020).Several methods have been proposed to improve the photocatalytic performance of other metal oxide semiconductors such as metal/non-metal doping, formation of several metal oxide based composites, synthesis tailoring to attain certain morphology with improved photocatalytic activity and heterojunction formation with other semiconductors (Ray et al., 2021). These modification methods have been reported to help spatial charge separation and mitigate against fast recombination of photogenerated holes. However, to achieve effective degradation performance, it is proposed that the surface of the semiconductor must be loaded with a reduction cocatalyst and oxidation cocatalyst to achieve long live charge separation and speed up photogenerated hole transfer (G. C. Zhang et al., 2019). The next section, hence discusses the method of preparation and other modification strategies researchers have adopted to improve the performance of BaTiO3.\nSince the discovery of BaTiO3 during World War II (1941\u20131944) (Bouzidi et al., 2019), there has been a progressive development of BaTiO3 using different preparation approaches including sol\u2013gel, hydrothermal/solvothermal, co-precipitation, mechanochemical and solid-state method. These synthesis methods have an impact on the physical and chemical characteristics of BaTiO3. Thus, it is critical to select appropriate preparation methods, since piezo-photocatalytic activity greatly depends on the physical and chemical properties of BaTiO3.Hydrothermal synthesis is one of the popular methods for the fabrication of powdered BaTiO3 since it is not expensive and can form stable and pure materials. This method involves a reaction between Barium (Ba) and Titanium (Ti). During their synthesis the most widely used precursors include barium chloride (BaCl2), barium hydroxide (Ba(OH)2), TiCl4 and TiO2 materials. The hydrothermal reactions take place in an autoclave at temperatures above 100\u00a0\u00b0C. Several parameters including reaction time, temperature, solvents and solution pH can influence the morphology, particle size and crystal structure of BaTiO3. Xia et al. prepared BaTiO3 nano/microcrystals using commercial titanium dioxide (TiO2) and Ti(OH)4 as a titanium (Ti) precursor mixed with Ba(OH)4 as barium (Ba) precursor (Xia et al., 1996). A very well crystalline and dispersed BaTiO3 with a crystallite size\u00a0<\u00a0100\u00a0nm was formed when Ti(OH)4 gel and Ba(OH)2 solution were used as precursors. The results showed that the starting precursors also had a strong impact on the morphology and crystallite size of the prepared BaTiO3. Furthermore, the hydrothermal reaction temperature had a strong influence on the crystal structure. As shown in Table 1\n, the lattice constant \u201ca\u201d slightly decreased with an increase in reaction temperature. According of the study conducted by Wen et al., it was found that lattice parameter a can affect photocatalytic activity of the semiconductor (Wen et al., n.d.). The photocatalyst (TiO2) anatase material with same composition, morphology, phase, and surface states but different lattice parameter \u2018\u2019a\u2019\u2019 were employed for photocatalytic degradation and photo-reduction of toluene and Cr(VI), respectively. However, greater catalytic activity was achieved by TiO2 with the extended lattice parameter than standard TiO2. Increasing in the length of the lattice parameter \u2018\u2019a\u2019\u2019 caused the bottom of the TiO2 conduction band to move higher, thus improving its photocatalytic activity.Moreover, Habib et al. showed that the structural morphology of the powdered BaTiO3 was temperature dependent (Habib et al., 2008). According to their result, the hydrothermal BaTiO3 obtained at low temperature (90\u00a0\u00b0C) had less pores compared to those attained at 120 and 150\u00a0\u00b0C. The study involving the relationship between photocatalytic activity of BaTiO3 thin film with porosity and surface area was conducted by Augurio et al.(Augurio et al., 2022). The porous BaTiO3 thin films exhibited higher photocurrent response than non-porous BaTiO3 thin film, indicating that porosity is beneficial in photocatalysis. This suggested that porous BaTiO3 can enhance interfacial charge transfer whilst lowering the charge carrier recombination rates, thus improving the photocatalytic activity. In another study, Zhan et al. controlled the hydrothermal reaction time (from 15\u00a0min to 480\u00a0h) to obtain BaTiO3 nanoparticles (Zhan et al., 2012). The XRD results showed no diffraction peaks after 15\u00a0min of hydrothermal reaction, demonstrating that the material lacked crystalline phases. However, longer hydrothermal reaction times (20\u00a0min to 48\u00a0h) led to the appearance of diffraction peaks in the XRD patterns that were attributed to the cubic BaTiO3. A continuous increase in the intensity of the diffraction peaks was observed with an increase in reaction time, demonstrating a persistent rise in the crystallinity and size of the crystals. Surmenev et al. produced BaTiO3 nano and micro rods via the hydrothermal method. The BaTiO3 nano- and micro rods were obtained at a temperature of 160\u2013210\u00a0\u00b0C, using 0.02 and 0.15\u00a0M (NaOH) concentration and within 45\u201390\u00a0min (Surmenev et al., 2021). The XRD results showed that BaTiO3 purity drastically increased as NaOH concentration increased from 0.025 to 0.15\u00a0M. Furthermore, BaTiO3 tetragonal phase was clearly visible after 6 hrs of hydrothermal synthesis at 210\u00a0\u00b0C and varied alkalinities (from 0.025 to 0.15\u00a0M), whereas 45 and 90\u00a0min produced a combination of cubic or tetragonal phases. The results showed that the hydrothermal reaction conditions such as temperature, alkalinity and time, have a great impact on the formation of BaTiO3 structures with different morphologies. Wei et al. controlled the size of BaTiO3 nanoparticles via hydrothermal approach with Fe doping and ethylenediamine (en) addition (Wei et al., 2008). The crystal size of the synthesized BaTiO3 were investigated by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HR-TEM). The results showed that BaTiO3 crystal size decreased as it was doped with Fe, indicating that Fe-doping suppress the crystal growth. It was further noticed that as Fe doping concentration increases, the average particle size also decreases (Fig. 6\n). Additionally, the addition of en, which served as both a solvent and a capping agent, may inhibit particle growth and cause a contained effect that changed the shape of the particles from spherical to cubic. It has been reported that semiconductors with smaller particle sizes have excellent photocatalytic activity as compared to those with large particles.BaTiO3 heterostructures are easily fabricated using the hydrothermal method. Li et al. prepared BaTiO3/TiO2\nheterostructure nanotube arraysusing a straight forward hydrothermal process, the hydrothermal reaction was carried out at different reaction times, temperature and concentration of Ba(NO3)2 (R. Li et al., 2013). Zhao et al. and Kappadan et al. demonstrated the preparation of Ag2O/BaTiO3 and BaTiO3/ZnO heterostructures, respectively, using hydrothermal method (Zhao et al., 2020)(Kappadan et al., 2020a). Based on their experimental results, BaTiO3\nnanoparticleswere anchored on hexagonal rod-shaped ZnO (Kappadan et al., 2020a). The combination of hydrothermal and microwave method could be used to fabricate BaTiO3 (Sun et al., 2006). For example, Amaechi et al. prepared Fe-doped BaTiO3 via ultrafast microwave-assisted hydrothermal method (Amaechi et al., 2021). Furthermore, the hydrothermal approach could be used with the electrospinning method. By combining an electrospinning and a hydrothermal technique, Ren et al. developed ZnO/BaTiO3 nanofiber heterostructures (Ren et al., 2012). As seen in Fig. 7\n(a), BaTiO3 nanofibers had a rather smooth surface and formed a network topology. BaTiO3 nanofibers ranged from 300 to 400\u00a0nm in diameter and up to several micrometers in length. The ZnO nanoparticles were uniformly dispersed on the rough surface of BaTiO3 nanofibers (Fig. 7(b)). The elemental composition of the pure BaTiO3 nanofibers and ZnO/BaTiO3 nanofiber heterostructures showed the presence of Ba, Ti, O and Zn. The detected Al element was from aluminium foil.The sol\u2013gel method is another simple method that is used to prepare barium titanates such as BaTiO3, BaTi4O9, Ba2TiO4 and BaTi2O5. Normally, barium acetate (Ba(OAc)2 and titanium (VI) isopropoxide (C12H28O4Ti) are used as barium and titanium precursors, respectively. The mixture of titanium (VI) isopropoxide and barium acetate solution tends to form BaTiO3-gel which is further dried and calcined at high temperatures (400\u20131200\u00a0\u00b0C) (Kavian and Saidi, 2009). The synergic sol gel and template method was used to fabricate BaTiO3 nanotubes (Cao et al., 2006). The formation of BaTiO3-gel was attained via mixing (Ba(OAc)2 and titanium isopropoxide, thereafter the nanostructured BaTiO3-gel grew on the porous alumina membrane (pore size 200\u00a0nm). The resultant alumina template covered with BaTiO3-gel was calcined at 700\u00a0\u00b0C to form BaTiO3 nanotubes with 50\u00a0\u00b5m length. The calcination temperature had a significant impact on the BET surface area of the BaTiO3. Pffaf (Pfaff, 1992) indicated that the BET surface area of BaTiO3 prepared via sol\u2013gel method decreased as the calcination temperature increased (Table 2\n). The high specific surface was obtained when BaTiO3-gel was calcined at low temperature (200\u00a0\u00b0C). It has been reported that at elevated temperatures, nanoparticles tend to agglomerate extensively thus resulting in a significant reduction in BET surface area and pore diameter (Zhang et al., 2015).However, XRD diffraction showed highly pure crystalline BaTiO3 obtained at higher temperature. At high calcination temperatures above 800\u00a0\u00b0C, the crystal structure of BaTiO3 transformed from cubic to tetragonal structure. This crystal structure transformation (cubic to tetragonal) was depicted by XRD peak splitting at 2\u03b8 value approximately 45\u00b0 (Fig. 8\n) [70]. From this study, it can be noted that crystallinity did have an influence on the optical properties of the semiconductors. According to literature, the optical band energy gap of the semiconductors decreases with an increase in crystallinity. Nishioka and Maeda (Nishioka and Maeda, 2015) studied the influence of the post heating of the hydrothermally synthesized Rhodium-doped barium titanate (BaTiO3:Rh) which could increase crystallinity and further improve photocatalytic activity. The XRD patterns were stronger and narrower after post heating 900\u00a0\u00b0C, thus confirming crystallization. However, the specific surface area was reduced from 8 to 4\u00a0m2 g\u22121. UV\u2013vis diffuse reflectance spectroscopy (DRS) was employed to investigate the optical properties of BaTiO3:Rh. Upon post heating, DRS exhibited a series of changes as the temperature increased (with exception of the sample at 1150\u00a0\u00b0C). At higher temperatures, Rh4+ species induced greater absorption at longer wavelengths. This would make sense because at a high-temperature heat treatment increased the oxidation of Rh3+ to Rh4+ in BaTiO3:Rh. In terms of photocatalytic activity, unheated samples tend to show low activity, while on the other hand the activity increased with an increase in post heating temperature until 1000\u00a0\u00b0C. At elevated temperature above 1000\u00a0\u00b0C, the photocatalytic activity of BaTiO3:Rh decreased drastically.Sol gel was combined with low temperature hydrothermal reaction procedure to prepare BaTiO3 nanopowder (Wang et al., 2013). In the study reported by Wang et al., it was found that experimental conditions such as potassium hydroxide concentration (KOH), reaction temperature and time had a significant role on the crystallinity and morphology of BaTiO3 powder (Wang et al., 2013). A highly crystalline pure BaTiO3 with a cubic structure was obtained at 120\u00a0\u00b0C (after 2\u00a0h of reaction time) with KOH concentration over 1.0\u00a0M. The hydrothermal and reaction time showed less effect on the crystallinity and morphology, whereas KOH concentration showed a significant impact on the crystallinity and morphology. It was observed that, when the KOH concentration rised from 1.0\u00a0M to 8.0\u00a0M, the average size of the BaTiO3 particles decreased from 370\u00a0nm to 100\u00a0nm.Solid state synthesis is a common method which is usually employed to produce polycrystalline materials such as barium titanate (BaTiO3). This method requires a very high temperature, however its benefits include simplicity and high yield production. The main factors which affect solid state reaction include reaction temperature, pressure, chemical and morphological properties of the starting reagents/materials. The solid state synthesis of BaTiO3 nanoparticles was reported by Qi et al. (Qi et al., 2020). In their studies, different molar ratios of Barium nitrate (Ba(NO3)2) and Ti powder (Ba/Ti) as starting materials were varied and calcined at different temperatures (500, 550 and 600\u00a0\u00b0C). The calcination temperature played a crucial role in the formation of BaTiO3. This was confirmed by the XRD pattern which showed that there was no formation of BaTiO3 at temperatures below 500\u00a0\u00b0C, since only XRD peaks for starting materials (Ba(NO3)2 and Ti) were revealed (Fig. 9\n). At high temperature (600\u00a0\u00b0C), the peaks were almost indexed to BaTiO3 material, thus now confirming the effect of calcination temperature on these materials. Other studies reported the thermal decomposition reaction of barium carbonate (BaCO3) and titanium dioxide (TiO2) for the formation of BaTiO3 (Pithan et al., 2005). Since the rate of reaction is controlled by the diffusion rate of Ba ions into Titanium dioxide (TiO2) lattice, the shape and size of the BaTiO3 produced was more influenced by the TiO2 morphology. The formation of titanates were explained in detail in the literature (Beauger et al., 1983). Trzebiatowski et al. reported that the formation of barium titanate (BaTiO3) and barium orthotitanate (Ba2TiO4) occurs simultaneously via the below chemical reaction (Brdi et al., 1950);\n\n(1)\nBaCO3\u00a0+\u00a0TiO2\u00a0\u2192\u00a0BaTiO3\u00a0+\u00a0CO2\n\n\n\n\n\n(2)\n2BaCO3\u00a0+\u00a0TiO2\u00a0\u2192\u00a0Ba2TiO4\u00a0+\u00a02CO2\n\n\n\nIn other studies they have reported that Ba2TiO4 forms when BaTiO3 reacts with TiO2 as shown in equation (4), thereafter the formed Ba2TiO4 reacts with the remaining TiO2 to produce meta titanate as shown in equation (5) (Beauger et al., 1983).\n\n(4)\nBaTiO3\u00a0+\u00a0BaCO3\u00a0\u2192\u00a0Ba2TiO4\n\n\n\n\n\nO3\nBa2TiO4\u00a0+\u00a0TiO2\u00a0\u2192\u00a02BaTi\n\n\nSolid state reaction method could be combined with sol\u2013gel method. Mi et al. prepared nano BaTiO3\nceramics using TiO2\nprecursor gel and BaCO3 as starting raw materials (Mi et al., 2020). The XRD results showed the initial formation of BaTiO3 at calcination temperatures of 600\u00a0\u00b0C. A cubic BaTiO3 structure was formed when the calcination temperature reached 800\u00a0\u00b0C. At 900\u00a0\u00b0C calcination temperature, the diffraction peak of (200) separated into peaks of (002) and (003), thus suggesting phase transition from cubic to tetragonal phase. In another experiment, Ren et al. used a solid state method to fabricate Bi2O3/BaTiO3\nheterostructure (Ren et al., 2013). Firstly, BaTiO3 was prepared from Ba(CH3COO)2 and TiCl4\nvia the hydrothermal treatment process. Thereafter, Bi2O3/BaTiO3\nheterostructure were prepared through ball milling and calcination process using the prepared BaTiO3\nand commercial Bi2O3 (mass ratio BaTiO3: Bi2O3\u00a0=\u00a04:1). After the calcination procedure, it was discovered that Bi3+ had dissolved in the BaTiO3 lattice and that a chemical connection had been created at the interface between Bi2O3 and BaTiO3.Recently sound energy has been utilized to prepare different metal oxide semiconductors such as BaTiO3 for different applications. In contrast to basic reactions, ultrasound-assisted reactions actually have a lot of advantages. High pressure, low pressure, and localized boiling zones are all produced by ultrasound in the reaction mixture. This shortens the reaction period and makes room-temperature synthesis possible. It has been noted that ultrasonography facilitates the uniform dispersion of reactants in a reaction mixture. Dang et al. reported sonochemically synthesized BaTiO3\nnanoparticles (Dang et al., 2011). In their study, mixtures of ethanol and distilled water were prepared with different volume ratios. Thereafter, BaCl2 and TiCl4 (molar ratio Ba:Ti\u00a0=\u00a01:1 were added to the above solution mixture, followed by addition of NaOH. The solution suspension was exposed to ultrasonic irradiation for 40\u00a0min at low temperature (50\u00a0\u00b0C). The applied ultrasonic energy was 150\u00a0W/cm2. Following synthesis, the precipitate was centrifugally separated, twice washed with deionized water, and then dried for two hours in a vacuum at 100\u00a0\u00b0C. In another study, BaTiO3 submicronic particles were prepared following multiple procedures such ultrasonication, microwave drying and thermal treatment (Rotaru et al., 2017). Mixture of BaCO3 and TiO2 as raw materials were ultrasonicated (ultrasonic frequency: 20\u00a0kHz, 750\u00a0W nominal electric power) in milli-Q ultrapure water. After 30 and 60\u00a0min of ultrasonication, the prepared samples were dried in the microwave furnace for 10\u00a0min. The last procedure was thermal treatment of the samples at different temperatures (780\u20131300\u00a0\u00b0C) for 3 hrs. Ashiri et al. reported similar approach to obtain BaTiO3 nanocrystals via rapid ultrasound-assisted wet chemical method (Ashiri et al., 2015). Utara and Hunpratub synthesized cubic structure of BaTiO3 nanoparticles using ultrasonic method at room temperature without thermal treatment step (Utara and Hunpratub, 2018a). The starting precursors were barium hydroxide (BaOH)2 and diisopropoxytitanium bis(acetylacetonate) (C12H28O6Ti). The effect of ultrasonic reaction time on the morphology of BaTiO3 nanoparticles (NPs) was investigated using TEM micrographs. It was found that the particle sizes of the BaTiO3 NPs decreased with increase in ultrasonic reaction time. The average particle size reduced from 56.69\u00a0\u00b1\u00a030.14\u00a0nm (30\u00a0min of ultrasonic irradiation) to 32.72\u00a0\u00b1\u00a011.83\u00a0nm (4 hr of ultrasonic irradiation). Similar observations were reported by Moghtada and Ashiri (Moghtada and Ashiri, 2016). It was concluded that smaller particles were produced as a result of ultrasonic irradiation at 50\u00a0\u00b0C.Co-precipitation method is the most frequently utilized synthesis approach for metal oxides (Rao et al., 2017). This method involves dissolving of metals salts in an appropriate solvent, followed by the addition of a precipitating agent. The most widely used precipitating agents include sodium hydroxide (NaOH), ammonium hydroxide (NH4OH) and potassium hydroxide (KOH). In case of preparing BaTiO3 using oxalate co-precipitation, it is challenging to obtain optimal conditions where both Barium (Ba) and Titanium (Ti) precipitates at the same time. Since Titanium (Ti) precipitates as titanly oxalate in the presence of alcohol at pH \n\n\u2264\n\n 2 whereas Barium (Ba) precipitates as BaC2O4 at pH \n\n\u2265\n\n 4. Titanium generates soluble anionic species such as TiO(C2O4)2\n2\u2013 in the pH between 2 and 4, thus influencing the stoichiometry ratio of Ba/Ti simultaneously (Geetha et al., 2016). It has been reported that through manipulation of several chemical conditions such as pH, reactants, and reaction medium, it is possible to make Ba and Ti to precipitate at the same time. Prasadarao et al. investigated the influence of pH (range 2\u201310) on the synthesis of BaTiO3 from barium chloride (BaCl2) and potassium titanyl oxalate (KTO) (Prasadarao et al., 2001). The formation of barium titanyl oxalate was obtained at pH 2.5 and an increase in pH to 5 led to the formation of barium titanyl hydroxy oxalate. At higher pH values (7\u20139), precipitation reactions yielded a mixture of titanium dioxide (TiO2) and barium oxalate (BaC2O4). He et al. also prepared BaTiO3 powder via the co-precipiation of BaCl2 and TiOCl2 in an highly-alkaline environment (He et al., 2014). The pH solution and concentration of the starting precursors (BaCl2 and TiOCl2) had an effect on the particle grain size and homogeneity of the BaTiO3 powder. An average particle size of approximately 80\u00a0nm was obtained at pH 14 and reaction temperature of 80\u00a0\u00b0C. In another study, Zhang et al. used BaCl2, TiCl4 as starting raw materials and tartaric acid as a precipitant agent for the preparation of tetragonal BaTiO3 nano-powder (X. Zhang et al., 2021). The white precipitated were formed by adding slowly a solution of ammonium hydroxide solution. Followed by thermal treatment at different calcination temperatures (750\u20131050\u00a0\u00b0C) for 4 hrs. The microwave assisted co-precipitation was reported to produce BaTiO3@rGO nanocomposite (Khan et al., 2021a). BaTiO3 and GO as starting materials were prepared separately via the sol\u2013gel method and modified Hummers method, respectively. Thereafter, a certain amount of BaTiO3 and rGO were added to 50\u00a0ml of deionised water and stirred for 1 hr at room temperature. Then, NaOH solution was slowly added to the above mixture solution, and heated for 1 hr in a microwave oven. The reduction of GO into rGO was confirmed by color change of the solution from brown to black. The co-precipitated nanocomposite was washed with mixture of ethanol/water and dried at 60\u00a0\u00b0C in an oven for 12 hr. The TEM images of pure BaTiO3 and BaTiO3@rGO are shown in Fig. 10\n\n(a-b). As shown in Fig. 10(a), pure BaTiO3 exhibits spherical nanoparticles with a size distribution of 10\u201330\u00a0nm, whereas Fig. 10(b) shows spherical BaTiO3 nanoparticles with an average particle size range of 15\u201334\u00a0nm, which are uniformly distributed on the surface of rGO sheet. Table 3\n highlights the summary of some of the synthetic methods for BaTiO3 powder.These techniques are mostly applied to prepare powdered BaTiO3, however powdered piezo-photocatalyst are difficult to be recycled in practical applications. For instance, after the degradation process, some parts of the powdered catalyst may persist in the aqueous solution, thus leading to secondary pollution. Therefore, recently piezo-photocatalyst based thin films are being developed for better recoverability, thus in the next section some common techniques that are used to produce BaTiO3 based thin films will be highlighted.There are various methods implemented for the preparation of BaTiO3 thin films, these include physical and chemical techniques. The physical methods include sputtering deposition, pulsed laser deposition (PLD), spin coating, dip coating and the Dr Blade method (Asadzadeh et al., 2021; Cernea, 2004). Chemical methods include chemical vapour deposition (CVD), sol\u2013gel method and hydrothermal method. All of these have their own advantages and disadvantages. Cernea et al. explained most of these methods basic principle and their own benefits (Cernea, 2004). In this review, a few common physical and chemical methods are discussed below.Dip coating is one of the most popular liquid-phase deposition methods for the fabrication of thin-films. This method involves dipping a substrate in the solution containing a starting material/ceramic powder, binder, solvent and dispersant. Once the material of interest has been deposited, the substrate is removed slowly from the solution and dried at ambient temperature. Several parameters such as immersion period, withdrawal rate, number of immersion cycles, solution composition, concentration and temperature tends to affect the film characteristics, smoothness and thickness (Schneller et al., 2013). This method has been used for the production of numerous piezoelectric thin-films including Pb (Zr, Ti)O3, CaBi4Ti4O15, ZnO, PVDF and BaTiO3. Ashiri et al. reported a crack-free nanostructured BaTiO3 produced from a modified sol\u2013gel dip coating method (Ashiri et al., 2014). The silica substrate was immersed into a sol prepared from barium acetate, glacial acetic acid, titanium tetraisopropyl alkoxide and 2-propanol. After deposition, the substrate with coated BaTiO3 was taken out from the sol\u2013gel solution with a withdrawal rate of 1\u00a0cm/min and dried at 100\u00a0\u00b0C. The resultant substrate coated with BaTiO3 was further calcined at 800\u00a0\u00b0C for 1 hr (heating rate 5\u00a0\u00b0C/min) to produce a thin film with a thickness of approximately 2\u00a0nm.In the spin coating process, the coating material is firstly dissolved in an appropriate solvent and the solution is dropped at the centre of the solid substrate surface. The solid substrate is then spun at controlled high speed. During this process, the solid substrate is rotated around an axis which is perpendicular to the coated region. The thickness and other properties of the final thin film depends greatly on the spinning rate of the substrate, viscosity of the solution, solvent evaporation rate, spinning time and surface wettability. This method is suitable and can be used for fabrication of several ceramics, including barium titanates such as BaTiO3 (Aminirastabi et al., 2020).Chemical vapour deposition is a widely used method to produce 2D nanomaterials and thin films. In this process, a solid material is deposited from the vapour by a chemical reaction occurring on or in the vicinity of a typically heated substrate (Mittal et al., 2021). The thin film nanostructures and thickness can be tuned by controlling the deposition conditions and the CVD system key factors. These include the substrate material and precursors, composition of reaction gas mixture, total pressure gas flows and temperature. Suzuki and Kijima (Suzuki and Kijima, 2006) prepared nanostructured BaTiO3 thin film from bis-dipivaloylmethanate barium (Ba(DPM)2) and titanium (IV) isopropoxide (Ti(OiPr)4 deposited on platinum/alumina/silica/silicon (Pt/Al2O3/SiO2/Si) substrate using the CVD technique assisted with Inductively Coupled Plasma (ICP). The size of the resultant nanostructured BaTiO3 thin film was greatly influenced by substrate temperature. The single phase BaTiO3 structure and particles sizes of approximately 30\u00a0nm were successfully obtained at temperature of 500\u00a0\u00b0C (Fig. 11\n(a)). The deposited spherical BaTiO3 nanoparticles on the surface of the substrate were more agglomerated with less pores. Fig. 11(b) shows a cross section image of the deposited dense nanoparticles on the substrate surface, however the thickness of the thin film was not determined. At substrate temperatures above 600\u00a0\u00b0C, the deposited BaTiO3 nanoparticles fused into a columnar form as shown in Fig. 11(c)-(d). The bottom of the thin films formed, exhibited a columnar structure, whereas the structure surrounding the surface was made of nanoparticles.Tape casting known as the Dr.Blade method has been widely used for the production of ceramic thin films. This technique is usually used to obtain thin films with a thickness ranging from 10 to 1000\u00a0\u03bcm. In this method, the powdered starting materials are mixed with appropriate solvents and binders to form a homogeneous mixture which is tape casted on the solid substrate (Asadzadeh et al., 2021). Thereafter, the tape casted substrate is dried at certain temperatures. The drying rate and temperature tend to be the most important factors which control the crack free of the thin film. Other factors which can affect the thin film properties and thickness include relative content of ceramic powder (starting materials), solvent and binder. Lilge et al. hydrothermally synthesized BaTiO3 powder from BaCl2.8H\u2082O and Ti [OCH(CH3)\u2082]\u2084 (Lilge et al., 2020). The hydrothermal reaction place was performed in a microwave for 120\u00a0min at 140\u00a0\u00b0C. The resultant BaTiO3 powder was further used to prepare a photoanode electrode using the Dr Blade method. For the preparation of the photoanode electrode, the powdered BaTiO3 was mixed with ethylene glycol Triton X-100 and ethanol. The slurry mixture was then taped casted on the FTO substrate (area of 1\u00a0cm2) to form a thin film. The BaTiO3 pasted on the surface of FTO appeared to be spherical in shape and agglomerated (Fig. 12\n).Despite the fact that BaTiO3 as a semiconductor has received a lot of attention for piezo-photocatalytic applications due to its incredible ferroelectric/piezoelectric properties and accessibility in a wide assortment of sizes and morphologies, it has significant limitations, most which are linked to its photocatalytic activity (X. Liu et al., 2020; Ray et al., 2021). Owing to its wide energy band gap of approximately 3.2 and 3.4\u00a0eV, it is associated with rapid recombination of photogenerated electron-holes which reduces its photocatalytic activity. Recently, various strategies have been explored including tailoring the morphology and particle sizes, doping and fabrication of heterojunction/composite photocatalyst to prevent some of these limitations (Scheme 1\n).The surface morphology and particles size of the semiconductor photocatalyst/piezocatalyst plays an important role in the catalytic degradation of organic waste pollutants. It has been reported that BaTiO3 with different morphological structures including nanowires, nanofibers, nanorods, nanotubes, nanocubes and nanoparticles exhibits different piezoelectricity. For example, 1-D fiber/wire piezoelectric materials show a superior piezocatalytic response as compared to spherical particles. Whereas, thin sheet-like 2-D structures also generate more piezoelectricity under mechanical vibration (Mondal et al., 2022). As piezocatalyst, Liu et al. explored different nanostructures (nanocubes (NCs), nanoparticles (NPs) and nanofibers (NFs)) of BaTiO3 for piezocatalytic degradation of Rhodamine B (Rh B) (D. Liu et al., 2020). BaTiO3 nanofibres showed greater piezocatalytic performance as compared to nanocubes (NCs) and nanoparticles (NPs) due to a higher surface area, easy deformation structure and fine crystal size. Moreover, Jiao et al. prepared different BaTiO3 nanostructures via the hydrothermal route at different reaction times (starting from 4 to 16 hr) (Jiao et al., 2017). Spherical BaTiO3 nanoparticles formed at 4\u20138 hrs were more effective for photocatalytic degradation of Rh B dye than other morphological nanostructures such as bowl like and agglomerated spherical particles. To further understand the enhancement in photocatalytic activity due to morphological-tuning of the semiconductor photocatalyst, different analyses including BET surface area, photon energy, electrochemical Impedance Spectroscopy (EIS), photoluminescence (PL) and photocurrent also need to be conducted. Since the method of preparation has an impact on the final morphological product, various authors have also synthesized these piezomaterials using varying methods Xiong et al. fabricated BaTiO3 nanocubes, since cubic structures are known to have the best ability to reduce crystal defects and increase the surface-to-volume ratio (Xiong et al., 2015). The effect of reaction time (24, 48, 74 hr) using the hydrothermal method was employed to produce the cubic like BaTiO3 structure (Fig. 13\n(a)-(f)). The particle size increased with an increase in hydrothermal synthesis duration. Furthermore, the edges of the cube got sharper as the reaction time increased, showing an increase in the cubic phase's crystallinity. The BaTiO3 nanocubes formed over period of 48 hrs exhibited impressive photocatalytic performance under light irradiation. This better performance was due to more uniform morphological distribution, higher crystallinity, small particle size and higher surface area which lead to more active sites, reduction in migration path of charge carriers, narrowing the energy band gap and reducing the rate of charge carrier\u2019s recombination.The optical properties of the BaTiO3 nanocubes calcined at different temperature were investigated by photoluminescence (PL) and UV vis spectrophotometer. The calcination temperature had an effect on the optical properties of the hydrothermally synthesized BaTiO3 reported by Hasbullah et al.(Hasbullah et al., 2019). The energy band gap calculated from tauc\u2019s plot (\nFig. 14\n\n) for BaTiO3 calcined at 500, 600, 700 and 1000\u00a0\u00b0C were 3.18, 2.87, 2.83 and 2.74\u00a0eV, respectively. Upon increasing the calcination temperature, the energy band gaps of BaTiO3 were expected to increase due to their high crystallinity. However, BaTiO3 resulted in lower energy band gap than expected. This could be due to inadequate oxygen delivery during the calcination process in ambient air resulted in oxygen deficiency in BaTiO3 structures (Orhan et al., 2005). As a result, the calcined BaTiO3 had a greater density of oxygen vacancy or non-bridging oxygen. The oxygen vacancy has the potential to change the BaTiO3 structure and cause localized electronic states. Therefore, resulting in reduction of energy band gaps for extremely crystalline BaTiO3 structures.Moreover, photoluminescence (PL) was employed to study the rate of photogenerated electrons and holes recombination. As seen in Fig. 15\n(a)-(f), the PL intensity was expected to decrease with increase in crystallinity. However, in this study the PL intensity reached its highest peak as the calcination temperature was elevated to 1000\u00a0\u00b0C. It was hypothesized that the increase in photoluminescence intensities is due to the presence of a localized state within BaTiO3 structures. With sufficient stimulation, the localized state effectively lowers the band gap of BaTiO3 structures, hence resulting in strong photoluminescence intensity.Metal or non-metal doping is one of the most popular methods used to modify semiconductor photocatalysts to improve their optical properties such as a reduction of band gap and photogenerated charge carriers (electron-holes), increase in photocurrent response and interfacial charge carries. The improvement of these properties tends to enhance the photocatalytic and piezo-photocatalytic degradation of organic wastewater pollutants. It has been reported that the modification of photocatalyst semiconductors such as TiO2 (Khairy and Zakaria, 2014), ZnO (Kaur and Singhal, 2014), WO3 (Peleyeju and Viljoen, 2021), BiVO4 (Orimolade and Arotiba, 2020)and BaTiO3 (Ray et al., 2021) by metal ion doping can successfully shift their optical absorption to the visible light region, thus narrowing their band gaps. Recently, a lot of research has shifted towards metal doping rather than no-metal doping since metal doping synthesis is easily achievable. To date, many transition metals including copper (Cu), Iron (Fe), Manganese (Mn), Tungsten (W) and Cerium (Ce) to mention a few have been explored as BaTiO3 photocatalyst dopants for their improved break down of several organic contaminates like methylene blue (MB), tetraclycline (TC), methyl orange (MO), and atrazine. Among these transition metal dopants, Cu has been shown to be the most efficient BaTiO3 dopants due to the fact it has shown greater improvement in degradation of organic pollutants as compared to Mn-, Fe-, Ce-, W- and Cr doped BaTiO3 (I. C. Amaechi et al., 2019; Ifeanyichukwu C. Amaechi et al., 2019; Basaleh and Mohamed, 2020; Nageri and Kumar, 2018; Senthilkumar et al., 2019). Basaleh and Mohamed (Basaleh and Mohamed, 2020) investigated the degradation activity of undoped and cu-doped BaTiO3 for the removal atrazine from wastewater. According to their outcomes, 5\u00a0wt% Cu/BaTiO3 showed the highest degradation efficiency of 100\u00a0% after 60\u00a0min, which was 33 times better compared to the undoped BaTiO3. The addition of Cu to the BaTiO3 surface reduced the band gap of undopoed BaTiO3 sample from 3.28 to 2.77\u00a0eV, thus improving the photocatalytic activity of the Cu-doped BaTiO3 sample.Noble metals including gold (Au), silver (Ag), platinum (Pt) and palladium (Pd) have been shown to improve the BaTiO3 piezo-photocatalyst sensitivity either under visible light or ultrasonic vibration. These noble metals are receiving more attention from researchers because of their superb utilisation of the solar spectrum, from visible to infrared, through the SPR effect (Chao et al., 2020; Cui et al., 2013). The BaTiO3 plasmonic photocatalyst have been fabricated from doping these noble metals with pure BaTiO3 sample. Under solar irradiation, the plasmonic photocatalyst generates an internal electric field which causes the photogenerated charge carriers to move in opposite directions. According to the charge transfer process in plasmonic photocatalysts, electrons from noble metal NPs can travel to the photocatalyst's CB and vice versa. Therefore, resulting in improved separation of charge carriers of plasmonic photocatalyst for better photocatalytic performance. In a study conducted by Xu et al., plasmonic piezo-photocatalyst (Ag/BaTiO3) compared to pure BaTiO3 showed an improved absorption under simulated solar irradiation (Xu et al., 2019). Due to this improvement, they discovered that Ag/BaTiO3 had a greater photocatalytic effectiveness than pure BaTiO3. The SPR of silver (Ag) nanoparticles resulting from internal band transitions from the 5d band to and within the 6sp band of the noble metal resulted in an increase in the piezo-photocatalytic activity of the modified BaTiO3.Another way of adjusting the band gap and improving the photocatalytic performance of the semiconductor is via non-metal doping. Unlike noble metals which are very expensive, non-metal materials are less expensive and can be applied as dopants for several photocatalyst to be used in wastewater treatment. Carbon based materials have been widely used as non-metal dopants to improve piezo-photocatalytic performance of BaTiO3 since they can improve the rate of electron transfer and also reduce the electron-hole recombination rate. Some of the widely used carbon-based materials include carbon nanotubes (CNTs), activated carbon nanofibres (ACFs), graphene oxide (GO), Biochar, and Carbon nanodots (CNDs) to mention a few (Orimolade et al., 2021a). These distinct carbonaceous materials have different morphologies (surface area and pore size) and surface chemical characteristics (functional groups, hydrophobicity, and hydrophilicity) which all have a significant role in photocatalytic degradation of waste pollutants. Over past years, several few types of these carbonaceous materials have been employed to modify BaTiO3 structure. However, the utilization of graphene oxide (GO) has been shown to be the most effective strategy. Unlike other carbons, GO offers a variety of benefits including high UV\u2013visible light transmittance, quick electrical and thermal conductivity, superior mechanical and tribological characteristics, and corrosion resistance. In addition, the delocalization of pi (\u03c0) network of the layers effectively suppresses electron-hole recombination thus resulting in improved photocatalytic performance (Zou et al., 2019). For instance, Zhao et al. showed an improved photocatalytic performance of BaTiO3 after loading it with different mass ratios of graphene oxide (Zhao et al., 2018). Firstly, the graphene oxide was prepared from the oxidation of graphite powder using the Hummer\u2019s method and, later the freeze drying method was employed for the preparation of graphene oxide-BaTiO3 hybrid photocatalyst (Fig. 16\n(a)). Under light exposure, the hybrid material had superior photocatalytic performance than the unmodified BaTiO3, as shown in Fig. 16\n(b-c). Similar observations were reported by Rastogi et al. and Wang et al., whereby the introduction of graphene oxide (GO) into BaTiO3 lattice structure resulted in a higher photoresponse under the ultraviolet region or in the visible region than BaTiO3 pristine (Rastogi et al., 2016b) (Wang et al., 2015).The combination of BaTiO3 with other several semiconductors to form BaTiO3 based heterojunction photocatalyst is another approach of improving piezo-photocatalytic efficiency of the BaTiO3 pristine. Mostly metal oxide semiconductors such as TiO2, ZnO, SnO2, Bi2O3, Bi2WO6 and Cu2O which have an unequal band gap as BaTiO3 are used to form heterojunctions (Mengying et al., 2017; Ray et al., 2021; Sharma et al., 2016; Wang et al., 2021). The formation of a heterojunction results in a band alignment which promotes the extension lifetime of the photoexcited holes and electrons within the heterostructured catalyst, thus reducing the rate of electron and holes recombination. Heterojunctions such as p-n (between a p-type semiconductor and an n-type semiconductor), n-n (between two n-type semiconductors), and p-p (between two p-type semiconductors) can be created depending on the kind of semiconductors that are combined. Furthermore, the band alignment of the heterostructured catalyst can be classified as Type I (straddling), Type II (staggered), and Type III (broken). In semiconductors, type II (including Z scheme) have been reported to efficiently improve electrons and holes separation (Orimolade and Arotiba, 2020). The charge transfer mechanism of type II was explained more in detail by Orimolade et al., Zhang et al. and Peleyeju et al. (Orimolade and Arotiba, 2020)(Zhang and Jaroniec, 2018)(Peleyeju and Arotiba, 2018). As shown in Fig. 17\n, electron transfer within a heterojunction interface commonly follows a two-step pathway depending on the Femi energy level of the coupled semiconductors. On the first pathway mechanisms (Fig. 17(a)), when the Fermi energy level of SC-1 (p-type semiconductor) is smaller than that of SC-2 (n-type semiconductor), electrons (e-) migrate from SC-1 conduction band (CB) to SC-2 conduction band (CB), while holes (h+) migrate from SC-2\u2032s valence band (VB) to SC-1\u2032s valence band (VB). However, when the Fermi energy level of SC-1 is greater than that of SC-2, electrons (e-) from SC-2 merge with the holes (h+) from SC-1 following band alignment in the heterojunction, thus resulting in electrons and holes separation from SC-1 and SC-2 in Fig. 17(b). The accessible separated holes (h+) in SC-2 and electrons (e-) in SC-1 are responsible for piezo-photocatalytic breakdown of organic waste pollutants. This type mechanism pathway of electrons and holes separation is known also as Z-scheme (Fig. 17(b)).Several BaTiO3 based heterojunctions have been fabricated using different synthetic methods such as hydrothermal, sol\u2013gel, solid state method and co-precipitation method for various applications, including wastewater treatment. In water and wastewater treatment, BaTiO3 have been coupled with several metal oxide semiconductors including ZnO, SnO2, TiO2, Bi2O3, Fe2O3 and MnO2 for better piezocatalytic/photocatalytic removal performance. Other non-metal oxides including g-C3N4, Ag3PO4 and AgBr have also been coupled with BaTiO3 for improved photocatalytic/piezocatalytic activity (Mengying et al., 2017; Ray et al., 2021). Feng et al. synthesized BaTiO3/SnO2 hybrid heterostructured catalyst using the hydrothermal method for piezocatalytic degradation of organic contaminates (Feng et al., 2020). The effect of SnO2 loading on BaTiO3 had a huge impact of piezo-current response, as shown in Fig. 18\n(a), BaTiO3 loaded with SnO2 and SnO2-Sb generated greater piezoelectrochemical current response than pure BaTiO3. Furthermore, electrochemical impedance measurements showed the evidence of improved electron mobility via reduction in charge transfer resistance (Rct) of the composites (BaTiO3/SnO2 and BaTiO3/SnO-Sb) (Fig. 18(b)).Over the past decades, several wastewater technologies including non-destructive and destructive methods as shown in Fig. 19\n, have been employed to remove toxic organic contaminates and pathogenic bacteria. Among them, advanced oxidation methods have been extensively applied as the most effective methods that accelerates the oxidation and degradation of a wide range of organic and inorganic chemicals that are resistant to traditional treatment methods. Piezocatalaysis is one of the emerging AOPs which uses energy harvesting materials called piezoelectric materials to convert mechanical energy into electrical energy. Recently, pieozocatalysis has gained much attention in several electrochemical applications including bacterial disinfection (Kumar et al., 2019a), hydrogen production (Hong et al., 2010), wastewater treatment and degradation of water pollutants (Mengying et al., 2017). In bacterial disinfection and degradation of pollutants, the piezoelectric materials (piezocatalyst) generate negative and positive electric charges under the influence of mechanical vibration at opposite surfaces. These free electric charges are responsible for redox reactions resulting in highly reactive species (ROS) such as \u2022O2\u2013 and \u2022OH. These strong reactive oxygen species (ROS) are capable of breaking down toxic organic compounds into less toxic compounds, hydroxyl radicals (\u2022OH) have been recognized as secondary oxidants (after the strongest fluorine) due to their high standard reduction potential (Eo \u2022OH/H2O) of roughly 2.8\u00a0V versus SHE. Coupling piezocatalysis with photocatalysis can enhance the degradation performance of piezo-photocatalyst and supress the rate of electron and holes recombination in photocatalytic degradation processes. Therefore, this article intends to review enhanced piezo-photocatalytic degradation of organic dyes (section 6.1), pharmaceuticals (section 6.2) and bacteria (section 6.3) using BaTiO3 based catalysts.In the past, BaTiO3 based catalysts have been extensively investigated for their piezocatalytic and piezo-photocatalytic removal ability of several wastewater pollutants including organic dyes. For example, Wu et al. synthesized BaTiO3 nanoparticles and nanowires using a two-step hydrothermal method for piezocatalytic removal of methyl orange (MO) from wastewater (Wu et al., 2018b). Under ultrasonic vibration, BaTiO3 nanowires (NWs) were easily deformed therefore showed better piezocatalytic performance as compared to BaTiO3 nanoparticles (NPs) with poor deformability. The highest piezocatalytic efficiency obtained by BaTiO3 NWs under ultrasonic vibration (power 80\u00a0W) was about 92\u00a0% within 160\u00a0min, with reaction processes following pseudo-first order kinetics model (Fig. 20\n(a)). Scavenger studies were conducted to investigate the reactive oxygen species that were more effective in breaking down of MO into CO2 and H2O. Various trapping agents such as tert-butyl alcohol (TBA), benzoquinone (BQ) and disodium ethylene diamine tetra-acetate dehydrates (EDTA-2Na) were used to supress hydroxyl radicals (\u2022OH), superoxide (\u2022O2\u2013) and holes (h+), respectively. As shown in Fig. 20(b), upon the addition of TBA, the degradation efficiency reduced dramatically thus confirming that hydroxyl radicals (\u2022OH) were the most effective ROS species for MO break down, followed by superoxide radicals ((\u2022O2\u2013). Another study was conducted by Hong et al., were BaTiO3 as a piezocatalyst generated more hydroxyl radicals (\u2022OH) and superoxide radicals ((\u2022O2\n\u2013) to break down Acid orange 7 (AO7) dye into CO2 and H2O (Hong et al., 2012). Under piezoelectric effect, the strained BaTiO3 dendrites decomposed about 80\u00a0% of the AO7 dye after 90\u00a0min. Several factors including influence of pH, catalyst dose and initial concentration which can affect the piezocatalytic process were investigated. In case of catalyst loading, the piezocatalytic efficiency increased with an increase in catalyst dosage until reaching a plateau region with 0.025\u00a0g of BaTiO3 catalyst. This was due to more available strained induced charges on the surface of BaTiO3 as its total surface-active sites increased with an increase in the amount of catalyst dosage. Therefore, resulting in enhancement of piezocatalytic degradation efficiency. The pH solution and initial AO7 concentration significantly influenced piezocatalytic processes, the piezocatalytic efficiency decreased with an increase in intial AO7 concentration. It was speculated that as initial concentration increases, more AO7 molecules increases which cover less active surface sites of the catalyst thus leading to a decrease in piezocatalytic efficiency. The highest piezocatalytic efficiency was slightly reduced in alkaline media and enhanced in acidic media. In acidic conditions, the surface of the BaTiO3 dendrites is protonated (positively charged) thus enlarges electrostatic interaction between anionic AO7 dye (negatively charged) and positively charged BaTiO3 surface, and resulting in higher piezocatalytic removal of AO7.To improve the photocatalytic activity of BaTiO3, Li et al. fabricated new hybrid composites (Ag2O-BaTiO3) by combining BaTiO3 ferroelectric with Ag2O semiconductor. Under ultrasonic vibration, an internal electric field was generated by ferroelectric BaTiO3 nanocrystal to reduce the rate of electrons and holes recombination thus enhancing the photocatalytic performance of the hybrid composite (Ag2O-BaTiO3) (Li et al., 2015). Fig. 21\n shows the effect of ultrasonic vibration on the photocatalytic degradation of Rh B dye using hybrid Ag2O-BaTiO3 photocatalyst. Four photocatalyst materials such as commercial P25 nanoparticles, Ag2O, BaTiO3, mixture of BaTiO3 and Ag2O were used for photocatalytic degradation comparison study. As depicted in Fig. 21(a), P25, BaTiO3 nanocubes, or Ag2O nanoparticles were not effective in the degradation of Rh B under ultrasonic irradiation only. However, the physical mixture of Ag2O and BaTiO3 as well as the Ag2O-BaTiO3 hybrid composite showed a slight deterioration of Rh B. These results shows that the combination of ferrolectric BaTiO3 nanoctrystal and Ag2O semiconductor can improve piezocatalysis/sonocatalysis performance of the Ag2O-BaTiO3 hybrid piezocatalyst. Fig. 21(b) illustrates the photocatalytic degradation of Rh B with all four samples in the absence of an ultrasonic irradiation. The synthesized BaTiO3 showed no photocatalytic degradation towards Rh B, whereas Ag2O, Ag2O-BaTiO3 and their physical mixtures showed higher photocatalytic degradation performance towards the removal of Rh B. Under both UV light and ultrasonic irradiation, Ag2O-BaTiO3 hybrid photocatalyst completely degraded all Rh B within a short space of time (1.5\u00a0h) (Fig. 21\n(c-d)). Their piezo-photocatalytic or sono-photocatalytic mechanisms were explained as follows: under UV light irradiation, the Ag2O surface generates electrons (e-) and holes (h+), these charge carriers (e-, h+) are required to be separated in order to produce reactive oxygen species (ROS) such as hydroxyl and superoxide radicals for deterioration of Rh B. Under ultrasonic vibration, the ferroelectric BaTiO3 nanocrystal in the Ag2O-BaTiO3 hybrid composite generated an internal piezo-electric field which acted as a driving force for the separation of charge carrier\u2019s (electrons and holes) (Fig. 21\n(e-f)). The suppression of rapid electrons and holes recombination separation led to an improved photocatalytic activity of the hybrid composite (Ag2O-BaTiO3).In addition, BaTiO3 photocatalytic performance was further improved by modifying it with several co-catalysts such as ZnO, SnO2, Fe2O3, TiO2 and Bi2O3. Heterostructured BaTiO3/ZnO composites were prepared using the sol\u2013gel and hydrothermal methods for the degradation of MB, MO, and RhB (Kappadan et al., 2020b; Karunakaran et al., 2014; L. Wang et al., 2019). In the study conducted by Kappadan et al., hydrothermal method was employed to prepare n-n heterojunction photocatalyst of BaTiO3/ZnO using Barium acetate, titanium tetra isopropoxide and Zinc nitrate hexahydrate as precursors (Kappadan et al., 2020b). The scanning electron microscopic images of the composite (BaTiO3/ZnO) clearly showed spherical BaTiO3 nanoparticles anchored on hexagonal rod-shaped ZnO. The formation of n-n heterojunction resulted in improved photocatalytic performance, as evidenced by the reduction of band gap energy from 3.1 to 2.97\u00a0eV and charge transfer resistance from 871 to 745\u00a0\u03a9. The mechanisms, as shown in Fig. 22\n(a), revealed that an improved charge carriers separation was achieved by forming a typical type II band alignment within the BaTiO3/ZnO heterojunction interface, which allowed BaTiO3 photogenerated electrons to migrate into the conduction band (CB) of ZnO, whereas ZnO photogenerated holes were transported into the valence band (VB) of BaTiO3. The prepared heterostructured BaTiO3/ZnO (BTZ) photocatalyst showed good stability since even after the 3rd cycle the degradation efficiency was above 91\u00a0% for methylene blue (MB) (Fig. 22(b)). However, after the 4th cycle the photocatalytic degradation efficiency slightly reduced from 91 to 86\u00a0%. The total mineralisation of MB dye was calculated using TOC, under UV light irradiation, the BTZ heterostructure recorded a TOC removal of 87.1\u00a0% after 60\u00a0min. A recent report by Liu et al. modified BaTiO3 with TiO2 via hydrothermal process to form BaTiO3-TiO2 core\u2013shell heterostructures (Liu et al., 2019). The heterostructured photocatalyst was applied for photocatalytic deterioration of Rh B dye from wastewater. The ratio of BaTiO3:TiO2 played a significant role in the photocatalytic degradation of Rh B. All BaTiO3-TiO2 core\u2013shell heterostructures with different molar ratios exhibited better photocatalytic removal towards Rh B as compared to pure BaTiO3. The BaTiO3-TiO2 core\u2013shell heterostructures (1.2:1) showed the greatest photocatalytic performance compared to other samples, its performance was 1.8 times greater than pure TiO2. The improved photodegradation activity was due to the fact that BaTiO3-TiO2 core\u2013shell heterostructures (1.2:1) exhibited the lowest photoluminiscent (PL) intensity thus lowering the rate of electrons and holes recombination. Wu et al. boosted the photocatalytic activity of heterostructured BaTiO3/TiO2 nanocomposites with piezotronic effect under ultrasonic vibration (J. Wu et al., 2020). Under ultrasonic activation, the built-in electric field exhibited by ferroelectric BaTiO3 facilitated the charge transfer and separation within BaTiO3/TiO2, thus improving its photocatalytic activity. Comparing with other BaTiO3 based metal oxides (Alex et al., 2019; Cui et al., 2017; Fan et al., 2012; Karunakaran et al., 2014; Lin et al., 2007; Liu et al., 2019; Selvarajan et al., 2017; Zhou et al., 2019), BaTiO3/TiO2 composite outperformed them in terms of photocatalytic performance due to excellent suppression of electrons and holes recombination.Plasmonic photocatalysts have also attracted a lot of attention in the photocatalytic removal of organic pollutants in wastewater. Several noble metals including platinum (Pt), gold (Au), rhodium (Rh) and silver (Ag) have been doped with BaTiO3 to promote chemical redox reaction under UV light and ultrasonic irradiation. For example, chao et al. fabricated a heterostructured Au@BaTiO3 photocatalyst using the hydrothermal method for the breakdown of Rh B under UV light exposure (Chao et al., 2020). The plasmonic heterostructured photocatalyst showed an enhanced photocatalytic activity towards the removal of Rh B. The heterostructured composite (Au@BaTiO3) nearly degraded about 100\u00a0% of Rh B after 36\u00a0min and the reaction process followed Langmuir-Hinshelwood model. The degradation rate constant (k) obtained from Langmuir-Hinshelwood model were 0.05446 and 0.01118\u00a0min\u22121 for Au@BaTiO3 and pure BaTiO3, respectively. The apparent rate constant (k) for Au@BaTiO3 was 4.9 times than that of pristine BaTiO3. These results confirmed an enhancement in the photocatalytic properties of heterostructured Au@BaTiO3 photocatalyst. Several studies investigated the photocatalytic performance of silver (Ag) doped BaTiO3 photocatalyst for the catalytic degradation of hazardous organic dyes (Rh B and MO) (Cui et al., 2013; Lin et al., 2021; Nithya and Devi, 2019; Xu et al., 2019). For instance, Khan et al. reported Ag-doped BaTiO3 prepared via sol\u2013gel method for photodegradation of Rh B (Khan et al., 2021b). The XRD and HR-TEM results confirmed their tetragonal phase and crystallite size range of 46\u201354\u00a0nm, respectively. The incorporation of silver (Ag) dopants into BaTiO3 reduced its band gap from 3.87 to 3.47\u00a0eV. Furthermore, they observed a reduction in photoluminescent peak intensity upon addition of silver ions into tetragonal BaTiO3, confirming restrain in electrons and holes recombination. The BET surface of Ag (5\u00a0%)-doped BaTiO3 had a higher surface area of 20.1\u00a0m2.g\u22121 as compared to pure BaTiO3 (15.4\u00a0m2.g\u22121). Therefore, due to Ag-doped BaTiO3 having a higher surface area and lower band gap, their photocatalytic performance was improved. Under light irradiation (400\u00a0W sodium lamp), 1\u00a0%Ag@ BaTiO3, 3\u00a0%Ag@BaTiO3 and 5\u00a0%Ag@BaTiO3 showed better photocatalytic performance than pure BaTiO3. The improved photocatalytic activity of Ag-doped BaTiO3 could be due to the reduced rate of electrons and holes recombination, surface area increase and reduction in band gap energy after Ag-deposition. The highest photocatalytic removal for Rh B was 79, 58, 46 and 51\u00a0% when 5\u00a0%Ag@BaTiO3, 3\u00a0%Ag@ BaTiO3, 1\u00a0%Ag@ BaTiO3 and BaTiO3 were applied as photocatalysts, respectively. Niu and Xu observed the same photocatalytic enhancement when Ag-BaTiO3 was further co-doped with other metal dopants such as Ni, Pd and Pd\u2013Sn\u2013Ni through a one-step ball milling process (Niu and Xu, 2019). The co-doped Ag@BaTiO3 rate constant was 4.5 times greater than undoped BaTiO3.\nTable 4\n summarizes an overview application of BaTiO3-based piezo/photocatalyst for catalytic degradation of organic dyes in water and wastewater.In recent decades, antibiotics have been identified as emerging contaminants owing to their endurance in aquatic ecosystems. They are widely used for treating numerous bacterial infections. However, they find their way into several waterbodies through incorrect disposal of unused or expired medications and human excretion. In ground and surface water, antibiotics are found in lower concentrations ranging from \u03bcg/L to mg/L (Cabeza et al., 2012; Orimolade et al., 2021b; Wang and Wang, 2016). Unfortunately, the detection of these pharmaceuticals in aquatic environment can lead to several harmful biological and economic impacts. For example, continuous ingestion of drinking water contaminated with pharmaceuticals can result in the formation of drug-resistant bacterium strains to humans and animals. The elimination of several emerging pharmaceuticals by BaTiO3 based ferroelectric/piezo-photocatalyst system have attracted so much attention recently. Kurniawan et al. investigated the photocatalytic performance of BaTiO3 and BaTiO3/TiO2 composites for the removal of acetaminophen (Ace) from distilled water under UV\u2013vis irradiation (Kurniawan et al., 2018). The XRD patterns confirmed the cubic phase structure of BaTiO3 nanoparticles. The BaTiO3/TiO2 composites as compared to individual pristine (BaTiO3 and TiO2) performed better, thus confirming that the synergetic effect of the two semiconductors can improve visible light absorption and efficient charge separation. Furthermore, it was found that the photocatalytic performance of the composites (BaTiO3/TiO2) can be enhanced by varying the molar weight ratios (w/w) of BaTiO3 and TiO2. Under optimal conditions, dosage of 1\u00a0g/L, pH 7, 4 hrs reaction time and initial Ace concentration of 5\u00a0mg/L, BaTiO3, TiO2 and BaTiO3/TiO2 photocatalytically degraded about 18, 33 and 95\u00a0% of Ace respectively. As shown in Fig. 23\n, initial degradation pathways for Ace were through hydroxylation and photolysis. The intermediates formed during this route included hydroquinone and 1,4-benzoquinone, which were consistent with the results obtained by Zhang et al. and Aguilar et al. (Zhang et al., 2008) (Aguilar et al., 2011). The hydroxyl radicals (\u2022OH) further attacked these intermediates to form hydroxylation products. The first detected intermediate was quinoneimine which was effortlessly hydrolized to 1,4-benzoquinone. The intermediates were then oxidized further into carboxylate acid and carbon dioxide by destroying their aromatic structures. The total mineralization via photocatalysis process generally does not occur quickly, however after some few hours these organics can be totally mineralized. Demircivi and Simsek reported tungsten-doped BaTiO3 (W-BaTiO3) for enchanted photocatalytic removal of tetracycline under and visible light-driven and UV-A light irradiation (Demircivi and Simsek, 2019). The composite was prepared through a simple hydrothermal method using a Teflon-lined stainless-steel autoclave at 200\u00a0\u00b0C, followed by washing, drying and calcination for 2 hrs at 700\u00a0\u00b0C. The effect of tungsten (W) loading on BaTiO3 was investigated for tetracycline degradation. It was found that BaTiO3 doped with low amounts of tungsten (W) exhibited higher photocatalytic activity than pure BaTiO3 and BaTiO3 doped with higher amount of tungsten (W). According to this study, pH played a significant role in the photocatalytic degradation of tetracycline. The photocatalytic degradation removal percentage increased with an increase in pH solution. The degradation efficiency was recorded to be 3, 80 and 90\u00a0% at pH 3, 5.60 and 10, respectively. In addition, tungsten-doped BaTiO3 (W-BaTiO3) was used to treat spiked real water samples.(tap and drinking water). Under light irradiation (after 3 hrs), the composite achieved a degradation efficiency of 74 and 76\u00a0% in drinking and tap water, respectively. The decrease in the degradation efficiency from tap and drinking water was due to other various pollutants or ions detected in the water matrices, which has some negative effect on the photocatalytic process (Bilgin Simsek, 2017). In a study conducted by Demircivi et al., decorated BaTiO3 with carbon fibers (CFs) were synthesized for enhanced photocatalytic degradation of tetracycline (Demircivi et al., 2020). The incorporation of CFs to BaTiO3 reduced the band gap of the composite and showed enhancement in photocatalytic activity of BaTiO3/CF. Under UV and visible light irradiation, BaTiO3/CF showed the highest degradation efficiency of 96\u00a0% as compared to BaTiO3 (W-BaTiO3). According to re-usability studies, the composite showed the highest stability within 5 cycles. However, after 6 cycles, a sharp decline in degradation efficiency was observed from 96 to 77\u00a0%. This could be due to the loss of tiny photocatalyst powder with an increase in recyclability. Almost a 100\u00a0% degradation efficiency was reported when Ti32-oxo-cluster/BaTiO3/CuS p-n heterojunction was employed for wastewater treatment under both visible light irradiation and mechanical vibration (Piezo-photocatalysis) (Zhou et al., 2021).The piezoelectric/ferroelectric materials including ZnSnO3, MoS2, NaNbO3, BiFeO3, KNbO3, BiOCl, Bi4 Ti3O12 and BaTiO3 nanoparticles in powder forms have been utilized as promising piezocatalysts for several applications. However their applications are limited in cleaning water due to the inability of being recovered from aqueous solution (Lin et al., 2020). Sharma et al. investigated the piezocatalytic activity of cement-based BaTiO3 composites for the removal of several pollutants in water such as pharmaceutical (paracetamol) and dyes ((Rh B), (MO) and (MB)) (Sharma et al., 2020). Sharma and co-workers, combined powdered BaTiO3 nanoparticles with cement to form cement-ferroelectric composites which can be easily recovered from aqueous solution after wastewater treatment. Under ultrasonic vibration, the poled BaTiO3 cement composites showed significant piezocatalytic removal of all organic pollutants. The poled composite exhibited the highest piezocatalytic degradation of approximately\u00a0>\u00a090, 86, 85, and 79\u00a0% for Rh B, MB, MO and paracetamol, respectively. The composites could be reused up to the 5th cycle of piezocatalysis under ultrasonic vibration. Another antibiotic drug which is in the class of fluoroquinolone antibiotics is Norfloxacin (NFX). This antibiotic drug is not easily degradable and can further contribute to antibiotic resistance when used. Therefore, the double Z-scheme of BiFeO3/CuBi2O4/BaTiO3 was fabricated by Zhang et al., and employed for photocatalytic degradation of Norfloxacin (NFX) under solar-light irradiation (Zhang et al., 2020). The combination of BiFeO3, CuBi2O4 and BaTiO3 to form the composites, extended the light absorption to UV\u2013visible and near-infrared (NIR) light to allow efficient use of the whole solar light spectrum. As shown in Fig. 24\n(a), the nanocomposites Z-scheme of BiFeO3/CuBi2O4/BaTiO3 strongly absorbed in the 200\u2013800\u00a0nm range. The calculated optical band energy gap from the tauc\u2019s plot were approximately 3.30, 1.76, 2.29 and 2.20\u00a0eV for BaTiO3, CuBi2O4, BiFeO3 and BiFeO3/CuBi2O4/BaTiO3, respectively (Fig. 24(b)). The effect of irradiation time, catalyst amount and initial NFX concentration were investigated on the photocatalytic performance of the Z-scheme composites. It was noticed that as irradiation time and catalyst dosage increased, the degradation efficiency also increased (Fig. 24\n(c-d)). The degradation rate increased with an increase in initial NFX concentration within a particular concentration range (Fig. 24(e)), this was due to the fact that more NFX molecules remained in aqueous solution at higher concentrations. Under the same optimal parameters (dosage of 1.0\u00a0g/L, within 60\u00a0min, and NFX concentration of 2.5\u00a0mg/L), BiFeO3/CuBi2O4/BaTiO3 nanocomposites had a better photocatalytic activity than individual samples of BaTiO3, CuBi2O4, BiFeO3. The composite reached its highest degradation of 93.5\u00a0% which could be attributed to a better separation of electrons (e-) and holes (h+) to improve the redox ability thus enhancing the photocatalytic activity of the catalyst. The electrons and holes separation were confirmed by photoluminescence (PL) spectrum (Fig. 24(f)). Generally, a low PL intensity reveals greater electrons (e-) and holes (h+) separation efficiency. As shown in Fig. 24(f), BiFeO3/CuBi2O4/BaTiO3 had the lowest PL intensity than BaTiO3, CuBi2O4 and BiFeO3, thus indicating better enhancement in photocatalytic activity of the composite. The extent of mineralization of NFX (10.0\u00a0mg/L) obtained from TOC was 3.9, 6.8, 40.9 and 60.3\u00a0% for BaTiO3, CuBi2O4, BiFeO3 and BiFeO3/CuBi2O4/BaTiO3, respectively. However, the extent of mineralization for NFX (2.5\u00a0mg/L) reached up to 93.5\u00a0% within 1 hr. According to scavenger experiments conducted, it was found that hydroxyl radicals (\u2022OH) and holes (h+) played an important role in the deterioration of NFX than superoxide radicals (\u2022O2\n\u2013).Pharmaceuticals used in aquaculture are also a source of pollution that is delivered directly into surface water. For example, atrazine is a well-known herbicide that is used to control broadleaf and grassy weeds in water. It was banned in most countries because of its negative impact on aquaculture and humans (Cavas, 2011). Due to that, Basaleh and Mohamed (Basaleh and Mohamed, n.d.) developed Copper (Cu)-doped BaTiO3 photocatalyst for the removal of this toxic herbicide (atrazine) from wastewater so as to provide clean water to the environment. The photocatalyst (Cu-BaTiO3) was prepared through the hydrothermal and photo-assisted deposition method. The prepared photocatalyst was added into 300\u00a0ml atrazine solution (50\u00a0ppm) and irradiated with Xenon lamp. The photocatalytic removal for atrazine using 0.5 Cu/BaTiO3, 1.0 Cu/BaTiO3, 3.0 Cu/BaTiO3 and 5.0 Cu/BaTiO3 were recorded to be 45, 65, 100 and 100\u00a0%, respectively. As for pure BaTiO3, the removal efficiency was low as 3\u00a0% due to the high electron and holes recombination. These results confirmed that doping BaTiO3 with Cu can suppress the rate of electrons and holes recombination, thus increasing photocatalytic performance of Cu-BaTiO3. The suppression of electrons and holes recombination was confirmed by photocurrent response and photoluminescence (PL) spectrum. The Cu/BaTiO3 had a greater photocurrent response of 12.8\u00a0mA\u00a0cm\u22122 than pure BaTiO3 (2.8\u00a0mA\u00a0cm\u22122). In another study, BaTiO3 was co-loaded with two electrocatalyst (Pt and RuO2) to promote sufficient redox ability for piezocatalytic degradation of tricyclazole under mechanical vibration (Feng et al., 2019). The platinum (Pt) was selected since it has been considered as the best catalyst for oxygen reduction, whereas RuO2 can produce large amounts of hydroxyl radicals and facilitate protons transport during electrocatalytic reaction. The loading of co-catalysts to BaTiO3 resulted in surface area increment from 25.5 to 28.8 m2g\u22121. Specific surface area of the materials is another factor which can influence the photocatalytic performance of the photocatalyst. Under ultrasonic vibration (40\u00a0kHz, 110\u00a0W), the composite achieved the highest removal percentage of 86\u00a0% which was higher than the values obtained using RuO2/t-BaTiO3 (51.0\u00a0%) and Pt/t-BaTiO3 (75.9\u00a0%). According to apparent rate constant values (k) obtained from pseudo-first order kinetics, the piezocatalytic reaction rate for the composites (k\u00a0=\u00a00.0320\u00a0min\u22121) was 3.11 times greater than pure BaTiO3 (k\u00a0=\u00a00.0103\u00a0min\u22121) and the sum of k values of Pt/t-BaTiO3 (0.0125\u00a0min\u22121) and RuO2/t-BaTiO3 (0.0124\u00a0min\u22121). The outstanding performance of the composite confirmed that coupling ferroelectric materials with a good catalyst can yield a synergistic enhancement effect on the piezocatalytic degradation. Another BaTiO3 based heterostructured catalyst which has been employed for piezocatalytic/photocatalytic and piezo-photocatalytic removal of pharmaceutical pollutants is BaTiO3/La2Ti2O7 heterojunction. Li et al. prepared BaTiO3/La2Ti2O7 composites via a two-step hydrothermal and microwave hydrothermal synthesis for piezo-photocatalytic degradation of ciprofloxacin (Y. Li et al., 2021). After 90\u00a0min of photocatalytic degradation, BaTiO3/La2Ti2O7 recorded a degradation efficiency of 16.5 and 22.7\u00a0% greater than that of BaTiO3 and La2Ti2O7, respectively, thus indicating that the formation of a heterojunction improved the photocatalytic degradation process. Under ultrasonic vibration (piezocatalysis), the degrading efficiency of CIP over BaTiO3/La2Ti2O7 (37.7\u00a0%) was roughly 19\u00a0% greater than that of BaTiO3 and La2Ti2O7. The degradation rate constant (k value) obtained from pseudo first-order kinetics model was 0.00593\u00a0min\u22121 for BaTiO3/La2Ti2O7, which is roughly 2.8 and 2.3 times greater than BaTiO3 and La2Ti2O7, respectively. The higher degradation efficiency of 50.2\u00a0% was achieved when piezocatalysis and photocatalysis were merged. For all samples, the performance of piezo-photocatalysis was superior to that of photocatalysis or piezocatalysis. This might be due to photocatalysis's low visible-light absorption and low carrier separation efficiency. The catalytic performance was likewise low during the piezocatalytic procedure due to the restricted amount of free charge carriers created. Photogenerated charge carriers were efficiently separated under the combined action of visible light and ultrasound, and CIP degradation efficiency was greatly increased when compared to sole-ultrasound and sole-visible-light irradiation. The composites showed higher catalytic activity than individual samples, thus indicating that the BaTiO3/La2Ti2O7 heterojunctions boost the catalytic process. The photocatalytic degradation of tetracycline and Rh B was reported by Zheng et al., using a Z-type BaTiO3/\u03b3-Bi2O3 heterojunction which was prepared via hydrothermal, co-precipitation, and calcination (Zheng et al., 2022). The morphological structure of the prepared samples appeared to be of a tetrahedron shape, irregular nano-particles and a mixture of both shapes (tetrahedron and nanoparticles) for \u03b3-Bi2O3, BaTiO3 and BaTiO3/\u03b3-Bi2O3 heterojunction, respectively (Fig. 25\n\n(a-c)). It was found that the calcination temperature had no effect on the morphology, the obtained average particles for \u03b3-Bi2O3, BaTiO3 and BaTiO3/\u03b3-Bi2O3 (HS3) were roughly around 5.9\u00a0\u03bcm, 425.9\u00a0nm, and 1.9\u00a0\u03bcm, respectively. The effect of catalyst dose, pH of the solution and different water bodies on photodegradation of tetracycline was explored, however these parameters had a little impact on the photocatalytic degradation process (Fig. 25\n(d-g)). The degradation efficiency for both pristine (\u03b3-Bi2O3 and BaTiO3) were below 67\u00a0%, for \u03b3-Bi2O3 and BaTiO3 were found to be 59.65 and 66.28\u00a0%, respectively. However, for all Z-type BaTiO3/\u03b3-Bi2O3 (HS) heterojunctions with different molar ratio, reaction time and calcination temperature, the degradation removal percentages were above 93\u00a0%, with HS3 exhibiting the highest degradation efficiency of 97.95\u00a0% for tetracycline. Even for Rh B dye, the degradation efficiency for single \u03b3-Bi2O3 and BaTiO3 were below 67\u00a0%. It was found that \u03b3-Bi2O3 and BaTiO3 degraded about 63.34 and 45.35\u00a0%, respectively. The photocatalytic degradation efficiencies for all HS heterojunction samples were above 73\u00a0%, with HS3, HS4, and HS5 breaking down the Rh B molecule entirely. The enhanced photocatalytic activity was due to electrons transferred via Z-type from Bi2O3 conductor band (CB) to BaTiO3 valence band (VB) by work function and charge density difference which resulted in charge separation.Another type of hydrothermally synthesized Z scheme heterojunction of La(OH)3@BaTiO3 (LB) composite was investigated for deterioration of an A-ring of tetracycline (Zheng et al., 2022). The reactive oxygen species (ROS) such as hydroxyl radicals (OH), superoxide (O2\n\u2013), holes and electrons completely degraded 100\u00a0% of tetracycline. The scavenger studies confirmed that four active species played a role in their photocatalytic degradation, as follows: h+\u00a0=\u00a0\u2022O2\n\u2212 >\u2022OH\u00a0>\u00a0e- (before 20\u00a0min reaction) and h+ > \u2022OH >\u2022O2\n\u2013 > e- (after 20\u00a0min reaction). According to these results, it means that holes (h+) played a major role in the photocatalytic degradation whereas electrons (e-) showed the least contribution during the photocatalytic degradation of tetracycline. The application of BaTiO3-based catalyst for removal of pharmaceuticals is summarized in Table 5\n.Despite using BaTiO3-based catalyst for piezo-photocatalytic degradation of organic dyes and pharmaceuticals, there are some factors which affects the piezo-photocatalytic degradation process negatively. Some of these factors include nature of the semiconductor, amount of the catalyst, solution pH, reaction time, light intensity, dissolved reactive oxygen species and temperature. For examples, some organic pollutants showed maximum adsorption removal and piezo-photodegradation at lower (acidic media) or higher pH (basic media) due their complex structure. Therefore, limiting their applications in real wastewater treatment because it means prior to degradation processes, the pH of the real wastewater samples needs to be adjusted. Another critical factor is the surface area of the piezo-photocatalyst, since the piezo-photodegradation efficiency increases with an increase in surface area of the catalyst. It is very important to select piezo-photocatalyst with very high surface area because of more active sites, which assist in the enhancement of piezo-photodegradation. Furthermore, this processes requires reactive high amount of reactive oxygen species (ROS) such as hydroxyl radicals and superoxide to completely oxidize organic dyes and pharmaceutical into less harmful by-products such as carbon dioxide (CO2) and water (H2O). Therefore, lower levels of reactive oxygen species would result in incomplete oxidation of organic pollutants.Piezocatalytic, photocatalytic and piezo-photocatalytic disinfection has attracted more attention in elimination of pathogenic bacteria. These processes usually use reactive oxygen species (ROS) such as hydroxyl radicals, superoxide, hydrogen peroxide and h+, e- generated when piezo-photocatalyst is exposed under light irradiation or mechanical vibration to kill bacteria (J. He et al., 2021). Generally, the idea of photocatalytic disinfection is to first remove each bacterium's cell wall, removing its protection, and then to damage its cytoplasmic membrane, causing the cellular material inside the newly torn cell envelope to degrade (Fig. 26\n). Ferroelectric based catalysts such as barium titanate (BaTiO3) have been used as piezocatalyst/photocatalysts for bacterial disinfection due to its exceptional optical and piezoelectric/ferroelectric properties. For example, Zhao et al. fabricated a novel p-n type Cu2MgSnS4/BaTiO3 (CMTS@BaTiO3) heterojunction for the degradation of organic dyes and bacterial disinfection (Ali et al., 2021). In terms of bacterial disinfection, CMTS@BaTiO3 obtained 72\u201376\u00a0% and 84\u201390\u00a0% inhabitation for E.coli and S.aureus, respectively, which was three times greater than pure CMTS and BaTiO3. Furthermore, CMTS@BaTiO3 heterojunction was tested for inactivation of E.coli and S.aureus in the presence of wastewater containing dyes (MG and MB). When compared to as-grown bacteria, inactivated levels of bacterial percentages employing CMTS@BaTiO3 composites were around 94.22\u2013101.24\u00a0% with MB and MG, respectively, against E.coli and 97.59 to 87.96\u00a0% for S.aureus. In comparison to CMTS@BaTiO3, Kumar et al. demonstrated piezocatalytic, photocatalytic and piezo-photocatalytic disinfection of E.coli using unpoled and poled BaTiO3 ceramic under UV light and ultrasonic vibration (Kumar et al., 2019a). During the piezocatalytic process (under ultrasonic vibration), unpoled BaTiO3 showed a 56\u00a0% of bacterial disinfection within 30\u00a0min. Under light exposure and ultrasonic vibration, piezo-photocatalysis improved the bacterial degradation rate and the piezo-photocatalytic degradation of E.coli increased from 56 to 70\u00a0% within the same reaction time. When the poled BaTiO3 ceramic were employed for piezocatalytic degradation (under ultrasonic vibration), it was found that about 97\u00a0% of bacteria were killed. Under the piezo-photocatalysis process (ultrasonic vibration and light exposure), poled BaTiO3 ceramic showed some catalytic enhancement towards catalytic degradation of bacteria (99.99\u00a0% recorded within 20\u00a0min). From these results, it can be concluded that poled BaTiO3 ceramic have better photocatalytic activity than unpoled BaTiO3 ceramic and coupling piezocatalysis with photocatalysis further enhanced the catalytic degradation activity of BaTiO3.Kumar et al. investigated the photocatalytic and antibacterial activity of poled BaTiO3 prepared via a solid state method (Kumar et al., 2019b). From the FESEM analysis, it was found that BaTiO3 consisted of large dense grains with clear grain boundaries (Fig. 27\n(a)). The calculated average grain size of BaTiO3 sample ranged from 40 to 60\u00a0\u03bcm (Fig. 27(b)). Both unpoled and poled BaTiO3 were shown to respond under UV irradiation. The poled BaTiO3 exhibited the highest photocurrent response of about 0.006 \u03bcA, which was>100 times greater than that of unpoled BaTiO3. Thus, indicating that poled BaTiO3 had a higher photocatalytic activity (Fig. 27\n(c-d)). This was due to its remnant polarization which inhibited recombination of photogenerated charges. However, under dark conditions both samples (poled and unpoled BaTiO3) showed no response to UV irradiation.The colony forming unit (CFU) method was employed to study the effect of poled BaTiO3 samples on absolute bacterial mortality. As shown in Fig. 27(e), the results display that there was no substantial antibacterial activity with unpoled BaTiO3 and negative side of poled samples in the dark. However, the positive side of poled BaTiO3 exhibited strong antibacterial activity with about 90\u00a0% bacterial destruction after 60\u00a0min when UV light was irradiated (Fig. 27(f)). In the absence of a catalyst (BaTiO3 poled or unpoled), the UV light alone showed antibacterial activity since it can prevent bacterial growth by destroying their structural DNA. Kushwana and co-workers reported (Kushwaha et al., 2015) Li-Doped Bi0.5Na0.45\u00a0K0.5TiO3\u2013BaTiO3 (BNKLBT) for antibacterial activity against E.coli and A.flavus using standard disc diffusion method. This method involves bacterial growth using Luria\u2013Bertani (LB) broth\u2013agar in a petridish disc. Three different concentrations (10, 50 and 100\u00a0\u03bcg) of BNKLBT were tested against bacteria and compared with commercial antibiotics (kanamycin (k30)). Based on their results, the zone of inhibition increased as BNKLBT concentration increased on the disc. The reason for this trend was not explained in this work. In another report, BaTiO3 nanoparticles were tested for antibacterial efficacy against S.aureus and P. aeruginosa by Shah et al.(Shah et al., 2018). The diffusion method was employed to investigate the antimicrobial activity of BaTiO3 nanoparticles. The optimal concentration of 100\u00a0\u03bcg/ml (BaTiO3) achieved bacterial inhibition of 85\u00a0\u00b1\u00a03.5\u00a0% and 80\u00a0\u00b1\u00a03\u00a0% against S.aureus and P.aeruginosa biofilms, respectively.Shuai et al. prepared PVDF/xAg-pBT composites via laser sintering method for bacterial disinfection (Shuai et al., 2020a). According to their studis, PVDF/xAg-pBT composites generated piezoelectric potential/voltage. Furthmore, it was found that with an increase in Ag concentration in the PVDF/pBT composites, the piezoelectrical current and voltage firstly rose then dropped. Babu et al. and Parl et al. observed comparable enhancement after adding conductive fillers to polymer-ceramic composites (Babu and de With, 2014) (Park et al., 2012). Their assumptions on the output voltage and current improvement were based on the conductivity enhancement. The antibacterial activity of the prepared scaffolds (PVDF/4Ag-pBT and PVDF/pBT) were evaluated using zone of inhibition method. Fig. 28\n\n(a-b) shows the bacterial inhibition zones of the scaffolds. PVDF/4Ag-pBT was shown to be more successful in inhibiting the growth of E.coli as compared to PVDF/pBT scaffold. Further test including Turbidimetric test were conducted to verify antibacterial activity of the scaffolds. The transparent vials including and excluding E.coli suspensions were labelled as control and blank group, respectively. The turbidity was the same for vials containing PVDF/4Ag-pBT and control. Surprisingly, the vial containing E.coli solution incubated with PVDF/4Ag-pBT was clear as a blank group, thus indicating that the scaffold inhibited bacterial growth of E.coli (Fig. 28(c)). The SEM images also confirmed that PVDF/4Ag-pBT scaffold raptured and destroyed the whole rod-shape structure of E.coli, whereas PVDF/pBT scaffold had minimal impact on the smooth rod-shape of E.coli (Fig. 28\n(d-e)). Fig. 28\n(f-g) displays bacterial inhibition rate of scaffolds and cumulative or non-cumulative of silver ion concentration released by PVDF/4Ag-pBT. The PVDF/4Ag-pBT scaffold inhibited bacteria at a rate of over 81\u00a0%, while the PVDF/pBT scaffold had no antibacterial activity.Overall, BaTiO3-based catalyst have been shown to be appropriate for applications involving the combination of photocatalysis, piezocatalysis and other catalysis processes. An overview of the previous studies addressing the use of BaTiO3-based catalyst for water disinfection is summarized in Table 6\n. The use of piezo-photocatalyst based materials for bacterial disinfection is still in its infancy. The process of piezo-photocatalysis has been widely applied including hydrogen production (H2), degradation of organic pollutants in wastewater, carbon dioxide (CO2) conversion and hydrogen peroxide (H2O2) production. In piezo-photocatalytic disinfection, it is challenging to develop a general mechanism for bacteria inactivation because of a wide range of pathogens and their cell complexity. Therefore, deeper understanding of the mechanisms the piezo-photocatalytic process is still required since suggested mechanisms highly rely on the generated reactive oxygen species during redox reactions. It is still debatable whether polarized charge carriers participate in the redox catalytic process. According to certain studies, photogenerated electrons and holes play a crucial part in redox processes, and the polarization potential of piezoelectric materials merely helps to separate photogenerated charge carriers (Fu et al., 2022).Barium titanate (BaTiO3) offers a wide range of applications in the energy and environmental fields. In comparison to certain applications like water hydrogen production, its usage in the piezo-photocalaytic wastewater treatment and bacterial disinfection is limited and recent. This review article has presented a complete summary of current developments in the use of barium titanate-based catalysts in photocatalytic, piezocatalytic and piezo-photocatalytic decomposition of organics and bacterial disinfection in water and wastewater. The selection of BaTiO3 as a suitable piezo-photocatalyst was due to its outstanding dielectric/ferroelectric/piezoelectric characteristics, low toxicity, low cost, environmental friendliness, existence in broad range of sizes and morphologies, multiple crystal structures and good stability. These characteristics are undoubtedly being used in the piezo-photocatalytic elimination of a variety of organic contaminants. Since, BaTiO3 as a photocatalyst suffers from poor conductivity and rapid recombination of photogenerated charge carriers. In this review, we have covered several strategies to circumvent these restrictions including morphology control, doping, metal loading, and heterojunction construction with appropriate semiconductors. Another way of improving electrons and holes separation is through coupling photocatalysis with other catalysis processes such as piezocatalysis and electrocatalysis. These strategies have achieved excellent results while conserving energy. The effectiveness of BaTiO3-based catalyst for piezo-photocatalytic degradation of organic pollutants depends mainly on several factors such as pH, reaction time, amount of catalyst, ultrasonic and light power. The fundamental benefit of heterogeneous photocatalysis is its capacity to use solar energy in the form of solar photons, which gives the degradation process a large boost in environmental value. Particularly for large-scale aqueous-phase applications, solar light-assisted photodegradation of wastewater contaminants can make it an economically viable method.Even though several reports have shown some strategic ways to improve photocatalytic activity of barium titanate, however some few other limitations and challenges needs to be highlighted for its success in wastewater treatment.\n\n1.\nThere is limited research on the structure of the barium titanate after being exposed to ultrasonic vibration and light irradiation. It is very important to investigate the stability of the barium titanate structure using TEM, SEM and XRD before and after wastewater treatment.\n\n\n2.\nEven though doping barium titanate with plasmonic metals have shown significant improvement in photocatalytic activity, the practical applicability of plasmonic BaTiO3 piezo-photocatalyst is limited due to the high cost and photo-corrosion associated with noble metal nanoparticles.\n\n\n3.\nSince the size and morphology of the doped noble metals can affect the photocatalytic activity of the BaTiO3, there are no reports in the literature on the size and morphology of the noble metals nanoparticles doped on the surface of BaTiO3 piezo-photocatalyst. To establish the ideal condition of noble metal nanoparticles needed for photocatalytic enhancement, it is therefore necessary to assess the surfaceplasmonresonance (SPR) impact from different sizes and shapes.\n\n\n4.\nFrom our analysis, there\u2019s limited understanding of piezo-photocatalytic mechanism for BaTiO3-heterojunction piezo-photocatalyst. Therefore, for future studies it is very important to study reaction mechanisms for piezo-photocatalytic degradation of organic pollutants in detail.\n\n\n5.\nFurthermore, many reports in literature have given limited information regarding how synthetic methods and structural properties of BaTiO3 piezo-photocatalyst affects the efficiency of the process. As a result, future research should pay close attention to how these factors impact piezo-photocatalyst effectiveness.\n\n\n6.\nThe amount of organic pollutant degradation is determined by the photocatalysts' mineralization capacity. However, the majority of earlier studies on BaTiO3-based materials did not identify the total organic carbon (TOC) in the mineralization process, which should be taken into account for an accurate assessment of the whole degradation of organic contaminants.\n\n\nThere is limited research on the structure of the barium titanate after being exposed to ultrasonic vibration and light irradiation. It is very important to investigate the stability of the barium titanate structure using TEM, SEM and XRD before and after wastewater treatment.Even though doping barium titanate with plasmonic metals have shown significant improvement in photocatalytic activity, the practical applicability of plasmonic BaTiO3 piezo-photocatalyst is limited due to the high cost and photo-corrosion associated with noble metal nanoparticles.Since the size and morphology of the doped noble metals can affect the photocatalytic activity of the BaTiO3, there are no reports in the literature on the size and morphology of the noble metals nanoparticles doped on the surface of BaTiO3 piezo-photocatalyst. To establish the ideal condition of noble metal nanoparticles needed for photocatalytic enhancement, it is therefore necessary to assess the surfaceplasmonresonance (SPR) impact from different sizes and shapes.From our analysis, there\u2019s limited understanding of piezo-photocatalytic mechanism for BaTiO3-heterojunction piezo-photocatalyst. Therefore, for future studies it is very important to study reaction mechanisms for piezo-photocatalytic degradation of organic pollutants in detail.Furthermore, many reports in literature have given limited information regarding how synthetic methods and structural properties of BaTiO3 piezo-photocatalyst affects the efficiency of the process. As a result, future research should pay close attention to how these factors impact piezo-photocatalyst effectiveness.The amount of organic pollutant degradation is determined by the photocatalysts' mineralization capacity. However, the majority of earlier studies on BaTiO3-based materials did not identify the total organic carbon (TOC) in the mineralization process, which should be taken into account for an accurate assessment of the whole degradation of organic contaminants.Overall, the use of BaTiO3-based piezo-photocatalysts for real-world remediation of pharmaceutical, organic dyes and microbes from wastewater has a bright future. Therefore, it is highly recommended that advanced oxidation processes such as photocatalysis and piezocatalysis should be used for wastewater treatment in the future instead of traditional techniques.This work was supported by GES 4.0 (Global Excellence Structure 4.0), the Centre for Nanomaterials Science Research (CNSR) University of Johannesburg), National Research Foundation of South Africa (SRUG200326510622), National Research Foundation of South Africa (NRFTTK117999) and the Faculty of Science University of Johannesburg (UJ), South Africa for financial support.", "descript": "\n                  The coupling of piezocatalysis and photocatalysis known as piezo-photocatalysis has attracted a lot of attention as one of the most effective advanced oxidation process (AOPs) for wastewater treatment, especially for the degradation of organic pollutants and disinfection of microbes. To advance this technology, there\u2019s a need to develop lead free piezoelectric materials to drive both piezocatalytic and photocatalytic process to prevent secondary pollution due to lead toxicity. Hence, barium titanate (BaTiO3) has been widely used as lead free piezoelectric material for several applications including water splitting, bacterial disinfection, and wastewater treatment due to its exceptional optical and piezoelectric properties. This work presents a comprehensive review on the application of BaTiO3 as a promising lead-free piezo-photocatalyst for the catalytic degradation of organic pollutants and bacterial disinfection from aqueous solution. This review article details the optical and piezoelectric properties, modification strategies, and synthetic methods of BaTiO3. Furthermore, the application of BaTiO3 as a preferred piezo-photocatalyst for wastewater treatment and a future perspective is presented.\n               "}
{"full_text": "Fuel cells have attracted a great deal of attention as clean energy conversion devices because of their high efficiency and low emissions [1,2]. Proton exchange membrane fuel cells (PEMFCs) have already reached the level of practical use but remain costly, because they require catalysts that include acid-resistant platinum group metals (PGM). Currently, anion exchange membrane fuel cells (AEMFCs) have been investigated as low-cost alternatives to PEMFCs due to their potential use of non-PGM catalysts and their enhanced oxygen reduction kinetics under alkaline conditions [3\u201310].The main technical challenges for the practical application of AEMFCs involve the achievement of both high performance and high durability. Improvements in the chemical/mechanical stability and anion conductivity of the anion exchange membranes (AEMs), crucial components of AEMFCs, have been continuing as part of the effort to meet these challenges [11\u201317]. Pan et al. reported that a self-cross-linked quaternary ammonia polysulfone (QAPS) developed a hydrophilic/hydrophobic phase-separated structure, and the conductivity exceeded 100\u00a0mS\u00a0cm\u22121 at 80\u00a0\u00b0C, similar to that of Nafion [11]. Hassan et al. reported a current density close to 10 A cm-2, which they achieved due to their use of a high IEC membrane, in which they suppressed the swelling by use of cross-linking [12]. Ponce-Gonzalez et al. reported that lengthening the spacers of the side chains improved alkaline stability [13]. In our previous reports, we described the characteristics of an in-house-developed AEM called quaternized poly(arylene perfluoroalkylene), i.e., QPAF-4, which has high anionic conductivity similar to that of QAPS, high mechanical strength, due to the introduction of perfluoroalkylene groups, and high alkali stability, due to the introduction alkylene spacers [14].The development of effective non-PGM catalysts is also crucial. For the anode, primarily non-PGM catalysts based on Ni have been reported [5,7,18\u201321]. For the cathode, non-PGM catalysts based on Fe, Co, etc. have been reported [7,22\u201324]. Among these, Hossen et al. reported the remarkable result that an Fe\u2013N\u2013C catalyst had the same performance as that of Pt/C, due to the combination of the N\u2013C material used to synthesize the catalyst and the optimization of the ionomer content of the cathode catalyst layer (CL) [24].In AEMFCs, water management is also extremely important, for both the anode and cathode, because water is produced at the anode via the hydrogen oxidation reaction (HOR) and is consumed at the cathode via the oxygen reduction reaction (ORR). The water moves from the cathode to the anode, due to electro-osmotic drag associated with the movement of OH\u2212, and also moves from the anode to the cathode, i.e., back-diffusion of generated water. AEMFCs must provide enough water to maintain hydration of the AEM and electrodes without flooding or drying of the CLs [25\u201329]. Kasepar et al. reported improvement of flooding and drying for CLs by controlling the use of microporous layers (MPLs) associated with the gas diffusion layers (GDLs) and the humidification of the feed gases [29]. In another recent study, Mustain et al. reviewed the water management of AEMFCs in detail [30]. Peng et al. reported suppression of flooding by lowering the humidification temperature to the extent that ionomer decomposition did not occur and increasing the hydrophobicity of the GDL and CL [31]. Dekel et al. also reported, by means of simulation, that the water required for the cathodic reaction increases with increasing current density and that the lack of water shortens the AEMFC life [32,33]. Recently, Omasta et al. reported that back-diffusing water is the main source for maintaining the hydrated state of AEMs during cell operation, in addition to being an important water source for the cathodic reaction [34,35]. A limitation of these studies on water management has been that the cells were evaluated under unrealistic conditions involving the use of high gas flow rates (e.g., 1\u00a0L\u00a0min\u22121 for 4\u00a0cm2 cell) in order to achieve maximum power density [35\u201340]. These evaluation conditions result in low gas utilization rates (3% or less at 1\u00a0A\u00a0cm\u22122 for hydrogen), far from the specifications of practical AEMFC systems. In the present work, we make use of practical gas flow conditions and focus on these water management challenges, reporting improvements of cell performance for membrane-electrode assemblies (MEAs) using a commercial non-PGM catalyst (Fe\u2013N\u2013C) for the cathode and a novel anion exchange ionomer (QPAF-4, see Fig. 1\n) for both the membrane and the CL binder.QPAF-4 was synthesized based on the synthetic procedure of Ono et al. [14]. Further details of the synthesis and characterization can be found in the Supporting Information (Scheme S1, Fig. S8 and Table S2).The catalyst inks for the anodes were prepared with Pt catalyst supported on carbon black (Pt/CB: TEC10E50E, Tanaka Kikinzoku Kogyo, K. K.), methanol and pure water by use of a planetary ball mill for 30\u00a0min. Subsequently, 5\u00a0wt% QPAF-4-MeOH (ion exchange capacity (IEC)\u00a0=\u00a02.0 meq g\u22121) binder solution was added to the slurry, and the mixture was further stirred with a planetary ball mill for 30\u00a0min. The weight ratio of QPAF-4 binder to support carbon was adjusted to 0.8. In the same way, the catalyst inks for the cathodes were prepared with the Fe\u2013N\u2013C catalyst (XPMF2000E, Pajarito Powder), 5\u00a0wt% QPAF-4-MeOH binder solution (IEC\u00a0=\u00a02.0 meq g\u22121), methanol and pure water by use of a planetary ball mill. The weight ratio of QPAF-4 binder to support catalyst was adjusted to 0.43. These catalyst inks were directly sprayed onto the microporous layers (MPL) of the gas diffusion layer (GDL) as the anode (Carbon cloth GDL, GDL with MPL formed after water repellent treatment of PANEX30 PW03 from Zoltek) and cathode (29BC, SGL Carbon Group Co., Ltd.) by the pulse-swirl-spray (PSS, Nordson Co. Ltd.) technique in order to prepare the gas diffusion electrodes (GDEs). The electrode areas were 4.41\u00a0cm2, the Pt loading of the catalyst layers (CLs) was 0.50\u00a0\u00b1\u00a00.02\u00a0mg\u00a0cm\u22122, and the Fe\u2013N\u2013C loading of the CLs was 0.50\u00a0\u00b1\u00a00.05\u00a0mg\u00a0cm\u22122. The prepared GDEs were immersed in 1\u00a0M KOH 80\u00a0\u00b0C for 2 days before measurement in order to ion-exchange to the OH\u207b form. Similarly, the QPAF-4 electrolyte membranes (IEC\u00a0=\u00a02.0 meq g\u22121, average thickness\u00a0=\u00a030\u00a0\u03bcm) were also immersed in 1\u00a0M KOH at 80\u00a0\u00b0C for 2 days before measurement. Excess aqueous KOH and water were removed from the GDEs and membranes with a laboratory cloth prior to assembly. Each set of GDEs and QPAF-4 membrane was pressed together in-cell to form the membrane electrode assembly (MEA) without hot pressing. The MEAs were sandwiched between two single serpentine flow graphite plates and 200\u00a0\u03bcm silicone/poly(ethyl benzene-1, 4-dicarboxylat/silicone gaskets (SB50A1P, Maxell Kureha Co., Ltd.) and were fastened to 10 kgf cm\u22122 with four springs.The cell voltages (V) as a function of current density (I) were measured with hydrogen and oxygen at 60\u00a0\u00b0C at various pressures. The back-pressure (BP) was controlled at 0\u2013100\u00a0kPa (gauge: kPag). Hydrogen and oxygen gases were supplied to the anode and the cathode at a flow rate of 100\u00a0mL\u00a0min\u22121. The \ufb02ow rates of all gases were controlled by mass \ufb02ow controllers. These gases were humidi\ufb01ed at 80\u2013100% relative humidity (RH) by bubbling through a hot water reservoir. The I\u2013V curves were galvanostatically measured under steady-state operation by use of an electronic load (PLZ664WA and KFM2150, Kikusui Electronics Corp.) controlled by a measurement system (fuel cell characteristic evaluation device, Netsuden Kogyo Corp.). The measurement times in the direction of increasing current were 1\u00a0min up to 0.02\u00a0A\u00a0cm\u22122, 3\u00a0min up to 0.1\u00a0A\u00a0cm\u22122, 5\u00a0min up to 0.2\u00a0A\u00a0cm\u22122, 7\u00a0min up to 0.3\u00a0A\u00a0cm\u22122, and 10\u00a0min up to 1.0\u00a0A\u00a0cm\u22122. The measurement times in the direction of decreasing current were just half those used for increasing current. Also, since resistances are difficult to measure with alternating current (AC) impedance at current densities below 0.1\u00a0A\u00a0cm\u22122\u00a0(KFM2150, Kikusui Electronics Corp.), they were measured with a 1\u00a0kHz external resistance meter (MODEL 3566, Tsuruga Electric Corp.) For current densities of 0.1\u00a0A\u00a0cm\u22122 or more, the membrane resistance was measured by AC impedance. In the hydrogen pump test, hydrogen was flowed through both electrodes at 60\u00a0\u00b0C 100% RH and 100\u00a0mL\u00a0min\u22121, and the anode overpotential was measured by use of an Automatic Polarization System (HZ-5000, Hokuto Denko Co.) at the same current density positions and stabilization times as those used in the I\u2013V measurements.For a more detailed comparison of the CLs using Fe\u2013N\u2013C and Pt/CB, they were investigated by use of various analytical methods, as follows. The cross-sections of the CLs on GDEs were observed by FIB-SIM (FB2200 and SU3500, HITACHI High-Tech Corp.). The wettability of the CL surfaces was investigated by contact angle measurement (DM-501, Kyowa Interface Science Co., Ltd.). Reagents (wetting tension test mixture, Kanto Chemical Co., Inc.) having different surface tensions of 30, 40, 50, and 73\u00a0mN\u00a0m\u22121 were pipetted on the CL surfaces, and the contact angles were measured. The above reagent was pipetted on each CL formed on 29BC GDL, and the contact angle between the reagent and the CLs was measured with analysis software (FAMAS, Kyowa Interface Science Co., Ltd.).We applied N2 adsorption in order to investigate the pore structures of the CLs. The N2 physisorption experiments were measured at 77\u00a0K by use of an automated gas sorption analyzer (Autosorb iQ, Anton-Paar GmbH). All of the samples (0.1\u00a0g or more) were degassed at 60\u00a0\u00b0C for 24\u00a0h in an onboard degassing port, prior to the adsorption experiments. The N2 adsorption measurements were conducted in the P/P0 range 0.025\u20130.997, where P represents the gas pressure and P0 the saturation pressure. The specific surface areas and pore volume distributions were calculated by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. In the case of the catalyst powders, the powders were directly filled in the spherical cells. The N2 adsorption values of the GDLs were larger than those of the CLs. In order to obtain precise measurements of the values of the CLs and avoid the influence of the values of GDLs, catalyst-coated membranes (CCMs) were prepared by coating the catalysts on the QPAF-4 electrolyte membrane by the PSS method. The CCMs (5\u00a0cm\u00a0\u00d7\u00a05\u00a0cm) were divided into three parts and placed in the measurement cell. The specific surface area and pore size distribution were calculated from the obtained adsorption isotherm curves.We also applied water vapor adsorption in order to investigate the pore structures of the CLs. The experiments of water vapor physisorption were measured at 60\u00a0\u00b0C with water vapor sorption analyzers (Vstar, Anton-Paar GmbH). All of the samples (0.1\u00a0g or more) were degassed at 60\u00a0\u00b0C for 24\u00a0h in an onboard degassing port prior to the adsorption experiments. The values of water vapor adsorption were measured in the P/P0 range 0.05\u20130.95. The catalyst powders were directly filled in the spherical cells. In the case of the CLs, these were formed on a PP film by the PSS method and were removed and filled into the cell.In Fig. 2\n, the polarization curves and ohmic resistance changes of the cell using Fe\u2013N\u2013C CL and Pt/CB CL (as reference) as the cathode CL are shown. Fig. 2a shows time courses of voltage and current density with increasing and decreasing current for each cathode CL. The polarization curves and Tafel plots shown in Fig. 2b and d were drawn using the quasi-steady-state voltage data, i.e., the final values observed during each period of current density in Fig. 2a. The cell using the Fe\u2013N\u2013C CL exhibited large hysteresis in the I\u2013V curve, i.e., a large difference in potential between the increasing and decreasing current curves. In the case of the Pt/CB CL, the hysteresis was very small. The ohmic resistance of the cell using the Fe\u2013N\u2013C CL increased with increasing current density; however, in the low current density region, the change with decreasing current density decreased, and hysteresis was observed in this region. The Tafel slopes of the cell using Fe\u2013N\u2013C CL were very different for increasing and decreasing current density. The change of the slope with increasing current density was larger than that with decreasing current density.The Tafel slopes were analyzed with a component analysis technique developed in our laboratory (Fig. 2d and e) [41]. This technique involves fitting the I\u2013V curves with a primary Tafel slope, typically corresponding to a transfer coefficient \u03b1 of 1.0, which would be 66\u00a0mV dec\u22121 at 60\u00a0\u00b0C, typically together with doubled (132\u00a0mV) and quadrupled (264) slopes. Perry et al. have shown that either gas mass transport or ionic transport limitations can lead to Tafel slope doubling, and the combination of the two can lead to quadrupling [42]. That analysis is based on the hydrogen anode being essentially nonpolarized under acidic conditions. For the AEMFC, it is well known that the hydrogen anode is significantly polarized [5,30,35]. As shown in the Supporting Information, an MEA operated under hydrogen pump conditions exhibited a rather small polarization at low current densities, which would not perturb the low current density region of the H2\u2013O2 cell. However, at high current density, the Tafel slope was 476\u00a0mV, corresponding to quadrupling, approximately half of which (238\u00a0mV) can be assigned to the hydrogen anode. Thus, an approximate I\u2013V curve for the hydrogen anode can be generated and used to correct the observed cell voltages (see Fig. S1-S6 for further details). The corrected I\u2013V curves for the Pt/CB (Fig. 2d) and Fe\u2013N\u2013C (Fig. 2e) CLs are shown for increasing and decreasing current density. As shown in Fig. S2, the apparent (uncorrected) slope of 532\u00a0mV for the Pt/CB H2\u2013O2 cell was decreased to approximately 294\u00a0mV (532\u2013238\u00a0=\u00a0294), i.e., more consistent with a quadrupled slope (264\u00a0mV). The additional 30\u00a0mV polarization is small but might possibly be due to the additional coupling of water mass transport with gas and ionic transport. The precisely quadrupled slopes, although not observed, are also shown in Fig. 2d and e for reference. For both Pt/CB and Fe\u2013N\u2013C catalysts, the low current density region, with an initial Tafel slope of 56.0\u00a0mV, can be assigned to pure kinetic control. Even though there is curvature, the curve-fitting allows us to clearly determine the slopes precisely. For Pt/CB, with both increasing and decreasing current density, the behavior transitioned directly to the quadrupled slope, with additional polarization, bypassing the doubled slope. For Fe\u2013N\u2013C during increasing current density, the I\u2013V curve increased to a significantly higher value, 448\u00a0mV, corresponding precisely to slope octupling, most likely due to a strong effect of limited water transport. At high current density, the behavior became unstable, with the potential increasing chaotically, presumably due to the influx of generated water from the anode, giving rise to a deviation from the slope of 448\u00a0mV (Fig. 2e); this behavior was time-dependent, as seen from Fig. 2a. During the decreasing-current portion, the I\u2013V curve for the Fe\u2013N\u2013C CL became less steeply sloped (269\u00a0mV), i.e., consistent with slope quadrupling, which is most likely due to the relaxation of one of the three types of transport limitation, which we propose would be principally water transport. The precise assignments for these components will be taken up in ongoing work. Further details of the analysis can be found in the Supporting Information, including a summary of the calculated Tafel slopes in Table S1.In order to investigate the factors controlling the I\u2013V hysteresis, the effect of the BPs of the supply gases on performances were evaluated (Fig. 3\na\u2013d). In the case of the cell using Fe\u2013N\u2013C CL, the degree of I\u2013V hysteresis decreased as the BPs on both electrodes increased from 0 to 50, and 100 kPag (Fig. 3a). Under 100 kPag BP on both electrodes, the cell performances for the Fe\u2013N\u2013C CL and Pt/CB CL were comparable, and there was negligible I\u2013V hysteresis (Fig. 3b). Fig. 3c and d show I\u2013V curves with BP applied to only one side, i.e., anode and cathode, respectively. Despite the presence or absence of BP at the anode, I\u2013V hysteresis was observed but was not observed when only the cathode was pressurized to 100 kPag. These results indicate that the I\u2013V hysteresis occurs only in the cell using Fe\u2013N\u2013C CL as the cathode, and the degree of the hysteresis is reduced by applying BP to the cathode.With increasing BP, the amount of water vapor in the gas decreases, the oxygen partial pressure increases, and the amount of liquid water also increases in the CL. The effects of oxygen partial pressure and water vapor pressure are shown in Fig. 3e and f, respectively. The performance of the cell under air was lower than that under O2, but the I\u2013V hysteresis was hardly observed (Fig. 3e). The I\u2013V hysteresis increased with lowering the relative humidity of the gas supplied to the cathode, namely, lowering the water vapor pressure (Fig. 3f).\nFig. 4\n shows Tafel plots and ohmic resistances with 0 kPag at both electrodes, 100 kPag and 100% RH at the cathode, 100 kPag and 80%RH at the cathode. In the part deviating from the Tafel slope in Fig. 4a, the voltages for both increasing and decreasing current decreased in the order of 100 kPag + 100% RH at the cathode >100 kPag + 80% RH at the cathode >0 kPag, and the I\u2013V hysteresis decreased in that order. On the other hand, for the ohmic resistance (Fig. 4b), the values decreased in the order of 100 kPag\u00a0+\u00a080% RH at the cathode\u00a0>\u00a0100 kPag\u00a0+\u00a0100% RH at the cathode\u00a0>\u00a00 kPag. The results of Fig. 4 show that the I\u2013V hysteresis occurred in the mass transport region at high current density, but little ohmic resistance hysteresis was observed in this region. In the cathode, a decreasing amount of liquid water due to a decrease of BP and relative humidity increased the ohmic resistance because of decreasing water content in the cathode ionomer and membrane. These results suggest that the I\u2013V hysteresis is caused by a deficiency of liquid water at cathode reaction sites with increasing current density.In the reaction in the cathode of the AEMFC, water is also essential to the reaction. These results indicate that the key factor controlling the hysteresis is water concentration in the cathode, not oxygen concentration. The lack of water supply in the cathode led to the hysteresis and also the increased resistance of the electrolyte membrane. The results of the BP changes also suggest that the increased amount of liquid water associated with increasing BP suppressed the hysteresis. During increasing current density, the void volume of the CL is large but decreased at high current density due to the voids becoming occupied with liquid water, which is supplied by both the cathode gas and back-diffusing water generated in the anode. These hysteresis phenomena were highly significant in the Fe\u2013N\u2013C CL but were hardly observed in the Pt/CB CL.Therefore, these results suggest that the difference of these hysteresis phenomena might arise from a difference in the absorption capacity of liquid water for both CLs, thus affecting the supply of water at reaction sites in the cathode. In order to suppress the hysteresis in the cell using Fe\u2013N\u2013C at the cathode, it is necessary to find ways to better manage the water supply so as to optimize the trade-off between the number of effective reaction sites and the void volume, which accompany the appropriate types of mass transport at the various current densities.In the contact angle changes for various reagents with different surface tensions, using both CLs (Fig. 5\n), the values for the Fe\u2013N\u2013C CL were approximately 10\u00b0 lower than those for the Pt/CB CL at each measurement point. This result indicates that the Fe\u2013N\u2013C CL was more hydrophilic than the Pt/CB CL. In a comparision between the Fe\u2013N\u2013C catalyst and the Fe\u2013N\u2013C CL coated with QPAF-4 (Fig. 6\na), the volume for pores below 20\u00a0nm decreased 84% as a result of QPAF-4 addition. In a comparison between thePt/CB catalyst and the Pt/CB CL coated with QPAF-4 (Fig. 6b), the volume for pores below 100\u00a0nm decreased 85% as a result of QPAF-4 addition. These results show that the Fe\u2013N\u2013C CL pores in the range of 20\u2013100\u00a0nm were relatively empty compared to those in the Pt/CB CL, despite the addition of QPAF-4. In comparing the water vapor adsorption isotherms of Fe\u2013N\u2013C and Fe\u2013N\u2013C CL (Fig. 7\na) and Pt/CB and Pt/CB CL (Fig. 7b), the total volume of water vapor adsorption for Fe\u2013N\u2013C was approximately a factor of two larger than that for the Pt/CB, irrespective of the presence or absence of QPAF-4. These results regarding the microstructure and hydrophilicity indicate that the Fe\u2013N\u2013C CL had many hydrophilic voids in the range of 20\u2013100\u00a0nm and that the capacity for water absorption was much higher than that for Pt/CB. Both MEAs used the same fabrication technique, binder, and membrane; however, the catalysts differed. Thus, the differences in the porosity characteristics in the CL are due to the differences in the carbon structures of Fe\u2013N\u2013C and Pt/CB.Consequently, we can suggest as a major difference between Fe\u2013N\u2013C and Pt/CB from the morphology analysis that the void volume of the Fe\u2013N\u2013C catalyst absorbed the water generated during increasing current density, and this led to the lack the reactant water at the cathode reaction sites, thus contributing to the I\u2013V hysteresis.The comparison of the cross-sectional structure images in Fig. 8\na and b shows that the Fe\u2013N\u2013C CL was about 15\u00a0\u03bcm in thickness and had larger pores than the Pt/CB CL, and the Fe\u2013N\u2013C CL contained micrometer-order and submicrometer-order pores. On the other hand, the thickness of the Pt/CB CL was approximately 5\u00a0\u03bcm and contained fine pores of 1\u00a0\u03bcm or less.\nFig. 8c and d schematically depicts the behavior of the water distribution in the Fe\u2013N\u2013C CL during increasing current density, together with the corresponding Tafel plots. We divided the regions acording to the values of current density in the Tafel plots. Region a' corresponds to reaction kinetics control (56\u00a0mV), and the cell voltage undergoes abrupt changes in the degree of decrease with increasing current density in regions a to c. In region a (low current density, Fig. 8c), the potential drops as the supply of water starts to become a limiting factor. The reactant water supplied from Fe\u2013N\u2013C CL pores, originating from water supplied by back-diffusion from the anode, is relatively balanced with the reaction. At that time, the ohmic resistace of cell is maintained at low values, as shown in Fig. 4b (100% RH, 0 kPag). In region b (mid-range current density, Fig. 8d), the cell voltage enters the high-slope regime due to the coupling of all three types of mass transport, i.e., gas, ions, and water, with the latter being the most important. We consider that the reactant water supply from Fe\u2013N\u2013C CL pores becomes a limiting factor for the reaction, due to the insufficiency of water generated at the anode. At that time, the ohmic resistace of the cell also increases due to decreasing membrane water content, as shown in Fig. 4b (100% RH, 0 kPag). In region c (high current density, Fig. 8e), the slope deviates from 448\u00a0mV and decreases markedly at current densities over 0.2\u00a0A\u00a0cm\u22122. The reactant water supplied from Fe\u2013N\u2013C CL pores becomes sufficient, so that water mass transport becomes less of a limiting factor for the cathode reaction, due to the large amount of water generated at the anode. At that time, the increase of the ohmic resistance is reversed due to the rehydration of the membrane from the water generated in anode, as shown in Fig. 4b (100% RH, 0 kPag). In the case of the decreasing current density, going back from region c to a, there was no sudden slope increase of the cell voltage. We consider that the water transport, although still a factor, was less limiting, due to a sufficient supply at reaction sites, because the balance between the water supply from back-diffusion and the reaction demand was maintained as a result of the large water volume in the Fe\u2013N\u2013C CL. These results also show that the cell potentials in region a' for both increasing and decreasing current density did not change. These indicate that there were negligible changes in the number of reaction sites, and basically, the behavior in regions a, b and c can be explained by combinations of various types mass transport, with water playing a major role. In the case of the Pt/CB cathode CL, the ohmic resistance in the current density range over 0.2\u00a0A\u00a0cm\u22122 hardly changed with increasing current density, as shown in Fig. 2c. In the case of the Fe\u2013N\u2013C cathode CL, however, the values of the ohmic resistances were larger than those of the Pt/CB cathode CL and increased with increasing current density. These results also suggest that the water content of the membrane with the Fe\u2013N\u2013C cathode CL was lower than that of the Pt/CB cathode CL and continued to decrease, and thus, the water permeability in the membrane was also insufficient. More specifically, in the case of Fe\u2013N\u2013C, the total water consumption of the cathode used for the water absorption and reaction of the CL is larger than that of the water back-diffusing from the anode. The large void volume of Fe\u2013N\u2013C is concluded to lead to a decreased number of contact points at the interface between the membrane and the CL, and also a decreased number of supply pathways for back-diffusing water from the anode. These conditions lead to an increased cathode-ionomer interfacial resistance in the high current density region. These results indicate the importance of providing the cathode reactant water with the back-diffusing water from the anode [28,35], while maintaining liquid water at a level that does not cause flooding. We also found that lowering the humidity of the anode increased the hysteresis, not only in Fe\u2013N\u2013C but also in Pt/CB (Fig. S7). This proves that the supply of water to the reaction sites of the cathode via the back-diffusing water from the anode is essential. In order to reduce the hysteresis, it is necessary to enhance the membrane water permeability, thus increasing the amount of back-diffusing water and forming a homogeneous interface between the membrane and the CL.In order to suppress the hysteresis in the cell using Fe\u2013N\u2013C and the higher ohmic resistance of the cell, we must find ways to control the water management conditions. We should optimize the trade-off between the number of effective reaction sites and the void volume, improve the interface between the membrane and the CL, and improve the membrane water diffusivity, which accompany the appropriate types of mass transport in the various current density ranges. By addressing these challenges, the development of the AEMFC will progress.We investigated the cell performance for the AEMFC using a non-PGM catalyst (Fe\u2013N\u2013C) for the cathode and an in-house-developed anion exchange ionomer (QPAF-4) for both the membrane and the CL binder under practical gas flow conditions, i.e., high utilization. The cell using the Fe\u2013N\u2013C CL exhibited large hysteresis in the I\u2013V curve under ambient pressures for both electrodes. Irrespective of the presence or absence of BP in the anode, the I\u2013V hysteresis occurred but was not observed when only the cathode was pressurized to 100 kPag. In the cathode, decreasing amounts of liquid water due to a decrease of BP and relative humidity increased the ohmic resistance because of decreasing water content in the cathode ionomer and membrane. These results suggest that the difference of these hysteresis phenomena arises from the difference in the absorption capacity of liquid water for both CLs, affecting the supply of water at reaction sites in the cathode. We were able to clarify differences between Fe\u2013N\u2013C and Pt/CB also based on morphology analysis. The void volume of the Fe\u2013N\u2013C absorbed the generated water during increasing current density and led to an insufficiency of reactant water at the cathode reaction sites, so that water mass transport became a major limiting factor, causing the I\u2013V hysteresis. The Tafel slope component analysis revealed that the I\u2013V behavior in this region can be characterized as a direct transition from kinetic control (56\u00a0mV slope) to combined gas-ion-water transport control, with a unique slope octupling behavior (448\u00a0mV slope). These results also support the importance of back-diffusing water from the anode for the rate of the cathode reaction.\nKanji Otsuji: Formal analysis, Investigation, Writing - original draft. Naoki Yokota: Formal analysis, Investigation. Donald A. Tryk: Tafel slope component analysis, Writing - review & editing. Katsuyoshi Kakinuma: Validation, Investigation. Kenji Miyatake: Conceptualization, Validation, Resources. Makoto Uchida: Conceptualization, Methodology, Validation, Resources, Writing - 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.This project was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) Japan through funds for the \u201cAdvanced Research Program for Energy and Environmental Technologies,\u201d by the Japan Society for the Promotion of Science (JSPS) and the Swiss National Science Foundation (SNSF) under the Joint Research Projects (JRPs) program, and by the Japan Science and Technology (JST) through Strategic International Collaborative Research Program (SICORP).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.jpowsour.2020.229407.", "descript": "\n                  We focus on the water management challenges and report on the improvements of cell performance for anion exchange membrane fuel cells (AEMFCs) using a non-PGM catalyst (Fe\u2013N\u2013C) for the cathode and an in-house-developed anion exchange ionomer (quaternized poly(arylene perfluoroalkylene), QPAF-4) for both the membrane and the catalyst layers (CLs) binder under practical gas flow rates conditions. The cell using the Fe\u2013N\u2013C cathode exhibited similar current-voltage (I\u2013V) performance compared with those using Pt catalyst supported on carbon black. The cell using the Fe\u2013N\u2013C catalyst showed I\u2013V hysteresis between increasing and decreasing current. The hysteresis decreased with increasing back-pressure. Based on the results of various I\u2013V measurements, we conclude that the hysteresis is related to water supplied to the cathode using the Fe\u2013N\u2013C catalyst. Tafel slope component analysis revealed that a severe polarization occurred, amounting to slope octupling, with increasing current density, most likely due to the addition of water transport to the usual combination of gas and ionic transport. This severe polarization was alleviated after the cathode layer became sufficiently hydrated. We found from these results that water management is essential, due to the role of water as a reactant in the cathode reaction, for high-performance AEMFCs.\n               "}
{"full_text": "The harsh reduction in fossil fuel reserves and the increasing concern about the environmental pollution have boosted the development of new strategies for the valorization of alternative sustainable sources in order to mitigate the problems associated with CO2 emissions and global warming [1\u20133]. In this regard, biomass has been considered one of the most suitable renewable alternatives for future energy and fuels due to its availability and carbon neutral emissions. Thus, in recent years, there is an increasing interest in the development of new strategies for the production of value-added and sustainable biofuels, chemicals, and bioproducts [4\u20137], in which biomass is used as a raw material. Amongst them, thermochemical processes have deserved a remarkable attention in the literature [1,5,6], particularly biomass steam gasification [8\u201311], biomass fast pyrolysis [12\u201315], and the steam reforming of the bio-oil produced in the pyrolysis process [16,17].Biomass steam gasification is one of the most studied and developed technologies for the production of H2 rich syngas. However, the excessive tar content in the syngas is currently a challenge to be overcome [8,18].Alternatively, the bio-oil produced in the pyrolysis reaction has attracted increasing attention for the production of other high value-added products, such as H2, automotive fuels and chemicals, by means of several catalytic and thermochemical routes [19\u201321]. The bio-oil is a complex mixture of oxygenated compounds and water, and is mainly composed of small carboxyl and carbonyl molecules (acids, ketones, aldehydes), sugar-derived compounds (furans, anhydrosugars), and lignin-derived compounds (phenols, aromatic oligomers) [22,23]. Nevertheless, its direct application involves several drawbacks associated with its properties (low heating value, low volatility, thermal instability and strong corrosiveness [24,25]), along with the difficulties involving its feeding (due to incomplete vaporization and re-polymerization of unstable compounds). Accordingly, bio-oil stabilization (for its further utilization as fuel or raw material in other catalytic processes) has deserved a remarkable attention in the literature. Thus, a wide range of strategies (physical, thermal or catalytic treatments) have been extensively analyzed for bio-oil conditioning, as are: esterification, aldol condensation, ketonization, in situ cracking, and mild hydrodeoxygenation [19,26\u201329].In spite of the aforementioned drawbacks, the bio-oil can be used as fuel by mixing with diesel (with the bio-oil content being of up to 75\u202fwt%), or for the production of several feedstocks based on bio-oil compounds, i.e., synthesis of wood adhesives or resins from phenolic compounds [30]. Besides, the bio-oil can be catalytically transformed downstream by the following routes: i) deep hydrodeoxygenation (HDO) in order to produce fuels, ii) ex-situ catalytic cracking for the production of olefins and BTX aromatics or for vapour upgrading by carrying out a second step, and iii) steam reforming aimed at the production of H2\n[6,19,20,31].In all these processes, the selection of suitable catalytic materials plays a key role for their industrial scale viability. Thus, a wide range of catalysts have been used in order to attain the desired purity of the products obtained, decrease the severity of reaction conditions, attenuate catalyst deactivation and/or reduce catalyst costs [32]. In the hydrodeoxygenation (HDO) process, bifunctional catalysts are considered the most promising ones, since they strike a suitable balance between catalysts activity (provided by the acid sites) and catalyst deactivation by coke deposition [33]. In the catalytic cracking process, catalysts with acidity and shape selectivity, such as zeolite based ones (HZSM-5, Y-type zeolite, H-mordenite and so on), are preferred in order to produce olefins and BTX aromatics [34]. Concerning the steam reforming process, metal supported catalysts (particularly Ni and noble metals based catalysts supported on Al2O3) are commonly used, and extensive research has been made in order to improve their activity and stability [6,16,35].Nevertheless, the fast catalyst deactivation by coke deposition in the aforementioned routes is still the main challenge to overcome [16,34,36]. It is well-established that the mechanisms of catalyst deactivation and coke formation are greatly influenced by the feed composition [37]. Thus, certain bio-oil compounds, namely, aldehydes, saccharides (mainly levoglucosan) and phenolic compounds, such as the guaiacols produced from the thermal degradation of lignin, are considered the main responsible for coke formation in the catalytic pyrolysis process [38\u201340]. Within this scenario, the upgrading of the bio-oil by catalytic cracking prior to its valorization in a second step may help to overcome the fast catalyst deactivation. Accordingly, different catalytic materials have been widely investigated, as are acid metal oxides (mainly Al2O3), basic materials (such as MgO and CaO), or other transition metal oxides (such as ZrO2, ZnO, TiO2, Fe2O3) [27,41]. Besides, the use of inexpensive catalysts, waste products or natural minerals is gaining increasing attention due to its low cost and availability [24,38,42]. Accordingly, Ro et al. [38] analyzed the product selectivity of lignin pyrolysis in a fixed bed reactor by using low-cost additives (bentonite, olivine, and spent FCC catalyst) as in-situ catalysts, and HZSM-5 catalysts placed downstream in a fixed bed reactor. They reported higher catalytic activity and lower coke deposition when bentonite was tested. Valle et al. [32] approached the modification of the raw bio-oil by its continuous catalytic upgrading over dolomite in a low-cost reaction system. They concluded that the composition of the upgraded bio-oil is suitable for downstream valorization processes, such as the production of H2 by steam reforming or aromatic hydrocarbons by a two-step hydrogenation-cracking process. However, no studies have been reported in the literature on the joint process of continuous biomass pyrolysis and in-line catalytic modification of the volatile stream; that is, there are no detailed studies aimed to ascertaining the main bio-oil compounds responsible for the catalyst activity decay in the steam reforming reactions. The aim of this paper is therefore to analyze the feasibility of continuous bio-oil upgrading for its further transformation into high value-added products. Accordingly, continuous pinewood sawdust pyrolysis and in-line catalytic conditioning by means of different low cost materials (inert sand, olivine, spent FCC catalyst and \u03b3-Al2O3) has been analyzed by paying special attention to product yields and compositions. Thus, a detailed knowledge of the modified stream will allow ascertaining its suitability for further valorization in other catalytic routes (this study focuses on steam reforming). Furthermore, it will also allow understanding catalyst deactivation in order to improve the catalyst performance. The main mechanisms of bio-oil transformation on these catalysts will also be analyzed in this study.The biomass used is forest pine wood (pinus insignis), which has been crushed, ground and sieved to a particle size in the 1\u20132\u202fmm range. This particle size eases continuous feeding operation. Table 1\n summarizes the most important properties (ultimate analysis, proximate analysis and the higher heating value) of the biomass used in this study, whose empirical formula is CH1.47O0.67. The ultimate analysis has been determined in LECO CHN-932 and VTF-900 elemental analyzers. An ultra-microbalance SARTORIOUS M2P is on-line with a computer for the processing of the data provided by the analyzers. The proximate analysis (volatile matter, fixed carbon and ashes) has been determined in a thermogravimetric analyzer (TA Instrument TGA Q5000IR). The higher heating value (HHV) has been measured in a Parr 1356 isoperibolic bomb calorimeter.Inert silica sand, olivine, \u03b3-Al2O3 and spent FCC catalysts have been tested in order to ascertain their capacity for improving the composition of the biomass pyrolysis volatile stream for its subsequent reforming for H2 production. Silica sand and olivine have been supplied by Minerals Sibelco, \u03b3-Al2O3 by Alfa Aesar and the FCC spent catalyst is the one used in the FCC unit at Petronor Refinery in Muskiz, Spain. Thus, materials with different features have been selected: i) inert silica sand, ii) olivine with basic character and activity for reforming oxygenate compounds [43,44], iii) \u03b3- Al2O3 and spent FCC catalyst of moderate acidity and adequate for promoting oxygenate cracking [45\u201347]. Apart from their suitable catalytic activity, the selection of the materials was based on their low cost, and bearing in mind their possible application as guard beds prior to the stream valorization in a second catalytic step of steam reforming. Furthermore, the use of a spent FCC catalyst involves reusing and therefore valuing a refinery waste material.Prior to use, the spent FCC catalyst was agglomerated with bentonite (50%) in order to increase mechanical strength as well as provide meso and macropores to the catalyst to avoid the blockage of the zeolite external pores by coke deposition [48,49]. Firstly, the spent FCC catalyst was regenerated by calcination with air at 575\u202f\u00b0C for 1\u202fh for burning all the coke deposited in the refinery unit. It was then agglomerated by wet extrusion with bentonite, and dried overnight. Finally, the catalyst was calcined at 575\u202f\u00b0C for 2\u202fh. All the catalysts were ground and sieved to a particle size in the 0.8\u20131.6\u202fmm range.The physical properties of the catalysts were determined by N2 adsorption\u2013desorption (Micromeritics ASAP 2010). The chemical composition was measured by X-ray Fluorescence (XRF) spectrometry. This analysis was carried out under vacuum using a sequential wavelength dispersion X-ray fluorescence spectrometer (WDXRF), PANalytical AXIOS, equipped with Rh tube and three detectors. The samples were prepared mixing flux Spectromelt A12 from Merck (ref. No. 11802) with powder catalyst in a ratio of approximately 20:1. Before the chemical analysis, the samples were melted in an induction micro-furnace.The total surface acidity of all materials was analyzed by NH3-TPD in an AutoChem II 2920 Micromeritics equipment. Thus, the procedure was as follows: i) Removal of the possible impurities adsorbed on the sample with a He stream following a ramp of 15\u202f\u00b0C\u202fmin\u22121 to 550\u202f\u00b0C, ii) NH3 adsorption (150\u202f\u03bcL\u202fmin\u22121) until reaching sample saturation; (iii) desorption of the physisorbed NH3 with a He stream at 150\u202f\u00b0C, and (iv) continuous signal recording by TCD of the chemisorbed NH3 following temperature programmed desorption from 150 to 550\u202f\u00b0C.The experimental equipment used in this study is shown in Fig. 1\n, which is composed of a conical spouted bed reactor (CSBR) and an in-line fixed bed reactor. Continuous biomass pyrolysis (500\u202f\u00b0C) was carried out in a conical spouted bed reactor, whose suitable performance for biomass pyrolysis and gasification has already been proven [22,50\u201353]. The main dimensions of the CSBR are as follow: height of the conical section, 73\u202fmm; diameter of the cylindrical section, 60.3\u202fmm; angle of the conical section, 30\u00b0; diameter of the bed bottom, 12.5\u202fmm, and diameter of the gas inlet, 7.6\u202fmm. These dimensions were selected based on previous hydrodynamic studies, and ensure a stable operation in a wide range of gas flow rates [54\u201357]. 50\u202fg of silica sand (0.3\u20130.35\u202fmm) were used as bed material in the CSBR. In addition, the unit was provided with a lateral outlet pipe located above the bed surface to continuously remove the char particles from the CSBR.The biomass was continuously fed (0.75\u202fg\u202fmin\u22121) into the CSBR by means of a solid feeding system that allowed continuous feeding in the range from 0.5\u202fg\u202fmin\u22121 to 5\u202fg\u202fmin\u22121. The feeding system consisted of a vessel equipped with a vertical shaft connected to a piston placed below the bed material. As the piston rised, the biomass was fed into the reactor helped by a vibration system.The gas feeding system is provided with three mass flow meters, which allow feeding N2 (used as fluidizing agent during the heating process), H2 (for the reduction of metal-based catalysts in further reforming studies), and air (used for coke combustion). Besides, the water to generate the steam used as fluidizing agent in the pyrolysis step was fed by a high precision Gilson 307 pump. A water flow rate of 3\u202fmL\u202fmin\u22121 was used in all runs, with the steam/biomass weight ratio being 4. Before entering the reactor, the water was vaporized and the steam preheated to 500\u202f\u00b0C. The CSBR and the preheater were placed inside a radiant oven of 1250\u202fW.The biomass pyrolysis volatiles formed in the CSBR circulate through a fixed bed reactor (600\u202f\u00b0C) connected in-line, where the inert or catalyst (silica sand, olivine, \u03b3-Al2O3 or spent FCC) were placed. It is to note that, given the low activity of these low-cost materials, the masses of the catalysts used correspond to the same bed length in all the runs. Thus, the significant differences in the densities of the materials were considered (sand, 2600\u202fkg\u202fm\u22123; olivine, 3300\u202fkg\u202fm\u22123; FCC, 1246\u202fkg\u202fm\u22123, and \u03b3-Al2O3, 1666\u202fkg\u202fm\u22123), and so the bed masses were 44.2\u202fg of silica sand, 46.2\u202fg of olivine, 17.3\u202fg of spent FCC catalyst and 19.9\u202fg of \u03b3-Al2O3. Accordingly, all runs were carried out with a gas hourly space velocity (GHSV) of 3100\u202fh\u22121. The fixed bed reactor was placed inside a radiant oven of 550\u202fW.Both reactors were placed inside a convection oven kept at 270\u202f\u00b0C in order to avoid the condensation of the volatiles formed in the pyrolysis step, which were fed into the catalytic step. The outlet stream was fed into the product condensation device prior to its analysis.The product stream leaving the fixed bed reactor was analyzed in-line by a GC Agilent 6890 provided with a HP-Pona column and a flame ionization detector (FID). The sample was injected to the GC by means of a thermostated line kept at 280\u202f\u00b0C to avoid the condensation of heavy oxygenated compounds. Cyclohexane (not formed in the process) was used as an internal standard to validate the mass balance closure, which was fed into the product stream at the outlet of the catalytic reactor. Furthermore, the non-condensable gases were analyzed by means of a micro GC Varian 4900, which allowed detailed quantification of the product stream. The liquid compounds (dissolved in acetone to avoid the clogging of the GC\u2013MS injector) were identified by means of a GC\u2013MS spectrometer (Shimadzu 2010-QP2010S) provided with a BPX-5 (50\u202fm\u202f\u00d7\u202f0.22\u202fmm\u202f\u00d7\u202f0.25\u202f\u00b5m). The temperature sequence of the oven was as follows: steady heating from 45\u202f\u00b0C to 290\u202f\u00b0C following a ramp of 3\u202f\u00b0C\u202fmin\u22121 for separating the volatile products, with this temperature being kept for 5\u202fmin in order to ensure total removal of all products from the column. The column was connected to a mass spectrometer, which operated under the following conditions: ion source and interface temperatures 200\u202f\u00b0C and 300\u202f\u00b0C, respectively, operating in the 40\u2013400\u202fm/z range.The physical and chemical properties as well as the acidity of the materials used are shown in Table 2\n. It can be seen that the materials selected for biomass pyrolysis volatile stream modification have several differences in their main properties. Regarding physical properties, it can be observed that silica sand and olivine have a very low surface area, whereas FCC and \u03b3-Al2O3 have BET surface areas of 81 and 100\u202fm2 g\u22121, respectively. Accordingly, sand and olivine are not porous materials, and spent FCC and \u03b3-Al2O3 catalysts are mesoporous materials with an average pore diameter of 168\u2013169\u202f\u00c5. It should be noted that the spent FCC catalyst is based on HY zeolite; however, it has been agglomerated with bentonite to increase its mesoporous structure for facilitating the diffusion of bulky molecules and so avoid the blockage of zeolite external pores by coke deposition [58]. Thus, mesoporous materials with a uniform pore size promote the interaction of large organic molecules with the active sites [27]. The spent FCC catalyst has a microporous surface area of 57\u202fm2 g\u22121, which is evidence of the presence of a zeolite on its structure.Concerning the chemical composition of each material, sand and \u03b3-Al2O3 contain small amounts of impurities. The spent FCC catalyst is mainly composed of SiO2 and Al2O3, as well as various metal oxides, which are accumulated in the catalyst in the consecutive reaction-regeneration cycles in the refinery unit. The high amount of Fe2O3 (7.68\u202fwt%) in the olivine is noteworthy, as it plays an important role in its catalytic activity by promoting the reforming of oxygen compounds [59]. Olivine has been widely used in biomass gasification for tar reduction due to its activity for cracking and reforming reactions and its low cost compared to metal catalysts [43,60]. Several researches stated that the catalytic activity of olivine depends on the amount of Fe present on its composition, as well as on its oxidation state, with Fe being more active as its reduction state is increased [43,61].Moreover, the total acidity of \u03b3-Al2O3 is higher than the one of the spent FCC catalyst, 106 and 47 \u00b5molNH3 gcat\n\u22121, respectively. The low acidity of the spent FCC catalyst is attributed to the fact that it has been agglomerated with bentonite, which decreased the amount of HY zeolite to 8\u202fwt%. Acid catalysts enhance dehydration and decarbonylation of oxygen components to form carbon monoxide and water as primary products in the deoxygenation reaction [62], as well as cracking, oligomerization, alkylation, isomerization, cyclization and aromatization via a carbonium ion mechanism [27]. It is to note that the moderate acidity of the catalysts used in this case lead to lower deoxygenation activity, but also to lower coke formation by secondary cyclization and condensation reactions [63,64]. Conversely, basic catalysts, such as olivine, enhance ketonization and aldol condensation reactions, leading mainly to the formation of carbon dioxide and water [62].Continuous biomass fast pyrolysis has been carried out in a CSBR at 500\u202f\u00b0C with the aim of maximizing the bio-oil yield. Previous studies have shown the good performance of this reactor for biomass pyrolysis [22,51,65]. Thus, this reactor provides several advantages compared with other reactor configurations, namely: i) short residence time of the volatiles in the reactor (of around 20\u202fms due to the high velocity of the gas, thus minimizing volatile transformation by secondary reactions, and so maximizing the bio-oil yield in the biomass pyrolysis), ii) high heat and mass transfer rates, i.e., the high velocity of both gas and solid phases and their countercurrent contact improve heat and mass transfer rates, and iii) rapid removal of the char from the reactor by the segregation of char from sand in the fountain, which allows continuous operation. Besides, its simple design eases the scalability of the pyrolysis process.According to the previous biomass pyrolysis studies, a moderate temperature of 500\u202f\u00b0C minimizes secondary reactions, which is a promising fact to decrease the gas yield from bio oil cracking [22,65]. In fact, the most important parameters for maximizing bio oil production in the biomass pyrolysis are [66]: i) very high heating rates, ii) high heat and mass transfer rates; iii) moderate temperatures (of around 500\u202f\u00b0C), iv) very short residence times, and, v) rapid char removal from the reactor.\nTable 3\n shows the yields of the main products obtained in the biomass steam pyrolysis at 500\u202f\u00b0C in a CSBR. The inert nature of steam in the biomass pyrolysis has been previously verified, i.e., product distribution is the same as when N2 is used as fluidizing agent [67]. Under the conditions studied, the char yield is 17.3\u202fwt%, and is continuously removed from the pyrolysis reactor by means of a lateral outlet. This char is suitable for the production of diverse products, such as adsorbents, fertilizers, catalyst supports and soil amenders [68\u201370].As observed, the gas fraction is mainly composed of carbon monoxide (2.3\u202fwt%) and carbon dioxide (4.7\u202fwt%). The low yield of methane (0.2\u202fwt%) and light hydrocarbons (almost negligible) is indicative of the low extent of secondary cracking reactions in the volatile stream [71]. Thus, a high yield of bio-oil is obtained (75.4\u202fwt%), and so the overall yield of volatile compounds fed into the next step is 82.7\u202fwt%.Regarding the bio-oil, more than 100 compounds have been identified in this fraction, and therefore the yields of only the functional groups and main compounds have been included in Table 3. It can be observed that bio-oil is composed of acids (3.1\u202fwt%), aldehydes (2.5\u202fwt%), alcohols (1.8\u202fwt%), ketones (7.3\u202fwt%), phenols (16.6\u202fwt%), furans (2.3\u202fwt%) and saccharides (4.5\u202fwt%). Among the different functional groups, phenols are the most abundant ones, which are formed from the degradation of lignin [72]. The high yield of saccharides is noteworthy, mainly levoglucosan (4.5\u202fwt%), which is the major individual compound in the bio oil, and is obtained as a primary product by cellulose depolymerization.The bio-oil contains a significant amount of water coming from the raw biomass moisture (10\u202fwt%) and also from dehydratation reactions involving cellulose and hemicellulose [73,74]. The high water and oxygen content in the bio-oil, as well as the low pH and low heating value, make bio-oil upgrading to be essential for subsequent use [75,76].The catalysts used for the steam cracking of the biomass pyrolysis volatile stream are \u03b3-Al2O3, spent FCC and olivine. The pyrolysis step has been carried out at 500\u202f\u00b0C and the cracking step at 600\u202f\u00b0C. Inert silica sand has also been used in order to assess the effect of thermal cracking on the volatile stream. The overall product distribution obtained in the two-step pyrolysis-cracking process is a complex mixture of many compounds, and so the products have been grouped into three fractions: i) the gas fraction composed mainly of CO and CO2, as well as low amounts of H2 and C1-C4 hydrocarbons; ii) the bio-oil, which is a complex mixture of oxygenated compounds and water; iii) the char fraction, which is the non-volatilized biomass fraction. Fig. 2\n shows the effect of each catalyst on the product fraction yields. The yields of each fraction obtained in the pyrolysis step at 500\u202f\u00b0C have also been included. The char fraction is continuously removed from the pyrolysis reactor, and is not therefore fed into the second catalytic step. Accordingly, the yield of char remains constant (17.3\u202fwt%) in all the runs, independently of the catalyst used.As observed, all the catalysts are active for cracking, as they increase the yield of the gas fraction in detriment of that of the bio-oil. Furthermore, bio-oil cracking is more severe as the acidity of the catalyst is higher. Thus, when the spent FCC catalyst is used, the gas yield increases from 7.3\u202fwt% to 26.1\u202fwt%, and when \u03b3-Al2O3 is used to 32.5\u202fwt%. However, a basic catalyst, such as olivine, has lower cracking activity, as it only increases the gas yield to 18.9\u202fwt%. Apart from their different character, the catalysts have significant differences in their physical properties, with \u03b3-Al2O3 and spent FCC catalyst being mesoporous materials and olivine a non-porous material. Thus, the limited porous structure hinders the diffusion of bulky oxygenated compounds into the bed material, leading to a lower extension of cracking and deoxygenation reactions [77].As observed in Fig. 2, thermal cracking is significant when inert silica sand is used. Thus, the bio-oil yield decreases from 75.4\u202fwt% to 68.3\u202fwt% and the gas yield increases from 7.3\u202fwt% to 14.4\u202fwt%. Therefore, apart from the effect of the catalyst acidity/basicity, the fact that the cracking step is performed 100\u202f\u00b0C above that of pyrolysis leads to bio-oil thermal cracking reactions in parallel to catalytic ones.\nFig. 3\na and 3b show the effect of the catalyst on the gas fraction composition and on the yields of the individual components, respectively. As observed in Fig. 3a, CO2 is the main compound in the gas fraction at the inlet of the cracking reactor (obtained by pyrolysis at 500\u202f\u00b0C). However, when inert sand is used, a sharp increase in CO and CH4 concentrations (45 and 12\u202fvol%, respectively) is observed, at the expense of decreasing that of CO2 (28\u202fvol%), which is due to the thermal cracking reactions.Moreover, the particular features of FCC and \u03b3-Al2O3 catalysts, especially the total acidity, modified significantly the gaseous product composition, leading to the highest concentrations of CO and HCs, which is evidence of the higher extension of the cracking reactions as when compared with the use of olivine or inert sand. Accordingly, when \u03b3-Al2O3 and FCC catalysts are used, the CO concentration accounts for almost 50\u202fvol% of the gas fraction when any one of these catalysts is used, with that of CO2 being 20.0% and 29.1\u202fvol% respectively. Moreover, the use of \u03b3-Al2O3 and FCC catalysts also leads to an increase in CH4 and light hydrocarbon concentrations, particularly that of the olefin fraction, which stem from the decarbonylation of oxygenated intermediates or alkyl aromatics [78]. The higher concentration of CO than CO2, as well as the increase in the yields of aromatics and olefins, was also observed by Ro et al. [38], who analyzed the upgrading of the lignin-derived bio-oil using different catalysts (bentonite, olivine, spent FCC catalyst and HZSM-5). The promotion of decarbonylation over decarboxylation reactions was also reported by Wang et al. [79] in the catalytic pyrolysis of hybrid poplar wood.Significant features are also observed in the composition of the gas fraction obtained using olivine: 33.7\u202fvol% CO, 29.2\u202fvol% CO2, 26.4\u202fvol% H2, 7.1\u202fvol% CH4 and 3.5\u202fvol% C2-C4 hydrocarbons. Thus, apart from the deoxygenation reactions of dehydratation, decarbonylation and decarboxylation, olivine also enhances oxygenate compound reforming and the water gas shift reaction, which lead to the formation of CO, CO2 and H2. This is mainly due to the chemical composition of olivine, with contains Fe0 on its surface, promoting reforming and WGS reactions [80,81].Given the differences observed in the overall gas yield obtained depending on the catalytic material (Fig. 2), the yields of individual gaseous products have been displayed in Fig. 3b. As observed, \u03b3-Al2O3 and spent FCC catalyst, who account for a gas yield of 32.5 and 26.1\u202fwt%, respectively, showed the highest yields of CO, CH4 and light hydrocarbons. Thus, the yields of these compounds increase with catalyst acidity, which is clear evidence that acidity promotes cracking reactions. Besides, these acid catalysts enhance decarbonylation reactions rather than decarboxylation ones, with CO being the main compound in the gaseous stream.Moreover, it is noteworthy that CO2 yield was higher than that of CO and remaining gaseous compounds only when olivine was used. The basic nature of olivine enhances ketonization and aldol condensation reactions, which involve the formation of CO2 and water [62].\nTable 4\n shows the bio-oil composition once the pyrolysis volatiles have passed through each catalyst bed. As aforementioned, the products identified have been grouped based on their functional groups, and the composition of the main compound families is shown in Table 4.As observed in Table 4, all the catalysts significantly modify the composition of the bio-oil. It is to note that similar trends were observed for both the yields and the concentrations of individual bio-oil compounds. Furthermore, temperature has also a significant influence when these catalysts are used. As aforementioned, the first step of biomass pyrolysis is conducted at 500\u202f\u00b0C, whereas the second catalytic step is carried out at 600\u202f\u00b0C. Accordingly, the amount of phenols, which are formed from the depolymerisation of lignin macromolecules [82], was substantially reduced with all the catalysts used, with this decrease being especially noteworthy in the fraction of catechols and guaicols.When the inert sand was used, a decrease in the phenolic concentration was observed due to the sharp reduction in the catechol fraction (from 11.0 to 5.6\u202fwt%), with alkyl-phenols and guaicols being hardly affected by thermal cracking. Moreover, the saccharide fraction, which is formed from the depolymerisation of cellulose and hemicellulose and is mainly composed of levoglucosan [74,83], decreased from 6.0 to 4.1\u202fwt% due to the poor thermal stability of the other saccharide compounds [22]. The fraction of acids, ketones and furans, which are formed from the decomposition of cellulose and hemicellulose in the biomass [82,83], did not undergo a substantially modification, whereas the aldehyde concentration increased from 3.3 to 5.4\u202fwt%, mainly by enhancing the formation of benzaldehyde instead of lighter species, such as formaldehyde and acetaldehyde [82]. A considerable reduction in the alcohol fraction was observed as opposed to that of polycyclic aromatic alcohols, which increased from 0.3 to 2.5\u202fwt%. In fact, olefinic alcohols may undergo aromatization reactions leading to heavier polycyclic aromatic alcohols.As mentioned before, the use of spent FCC and \u03b3-Al2O3 catalysts led to low bio-oil yields (56.6 and 50.2\u202fwt% respectively) due to the features of these materials, especially the total acidity (see Table 2), which promoted cracking reactions [27]. The bio-oil composition obtained with these materials is also a consequence of secondary reactions leading to a substantial increase in the hydrocarbon concentration (6.1 and 8.5\u202fwt%, over FCC and Al2O3, respectively). Accordingly, the higher acidity of these catalysts compared to olivine promotes hydrocarbon formation [38].Concerning the phenol functional group, although similar concentrations were obtained (17.9 and 15.9\u202fwt% for FCC and \u03b3-Al2O3 catalysts, respectively), significant differences were observed in the distribution of catechols, guaiacols and alkyl-phenols. Thus, while catechols are the main phenolic compounds in the bio-oil obtained with the FCC catalyst (11.6\u202fwt%), followed by alkyl-phenols (5.2\u202fwt%), the phenolic fraction obtained with Al2O3 catalyst was only composed of alkyl-phenols, which is due to the secondary recombination and cyclization reactions via Aldol condensation [22,74]. The higher selectivity of Al2O3 catalyst to alkyl-phenols revealed the effective dealkoxylation of guaiacols and cathecols [84]. Thus, the guaicol fraction in the volatiles derived from biomass pyrolysis at 500\u202f\u00b0C may undergo oxygen-aromatic carbon bond cleavage to form phenol/aromatic hydrocarbons or undergo oxygen-alkyl carbon bond cleavage to form benzenediols or benzenetriols (catechols). This catechol fraction may then be converted into alkyl-phenols by deoxygenation reactions. Guaiacol cracking can be initiated by homolytic cleavages of CH3\u2013O or O\u2013H bonds leading to the formation of methane, dihydroxybenzene (catechols), o-cresol (alkyl-phenol), and 2-hydroxybenzaldehyde, among others [27,85].It is noteworthy that FCC and \u03b3-Al2O3 catalysts led to full disappearance of acids, light alcohols, and saccharides, and to a significant reduction in the concentration of aldehydes and ketones. Thus, the small oxygenate and olefin molecules in the volatile stream formed in the biomass fast pyrolysis may be converted into aromatics via aromatization, with oxygen being released as CO, CO2, and H2O [27,86]. As aforementioned, the FCC catalyst has a microporous structure due to the presence of HY zeolite. The shape-selectivity of this zeolite promotes the diffusion of the mentioned compounds (acids, aldehydes, alcohols, ketones, and furans) into the zeolite channels and the reactions of deoxygenation ending up in the formation of aromatic hydrocarbons [87]. Moreover, the basic properties of the bentonite, which is the binder to agglomerate the spent FCC catalyst, promote ketonization reactions involving carboxylic acids and carbonyl compounds [27]. In the case of Al2O3, its better textural properties, as well as its higher acidity, enhance further decomposition on the acid sites of the catalyst, and therefore increase the hydrocarbon fraction. Furthermore, there is a higher concentration of water in the bio-oil stream treated with FCC and \u03b3-Al2O3 catalysts (45.7 and 50.1\u202fwt%, respectively) due to secondary cracking-dehydration reactions.Regarding olivine, a decrease in the amount of acids, aldehydes, and furans was observed compared to inert sand. The ketone fraction remained almost constant (at around 9\u202fwt%), although longer chain ketones were formed when olivine was used. Therefore, basic catalysts promote, on the one hand, ketonization of acids and, on the other hand, aldol condensation of small ketone and aldehyde molecules to larger chain ketones by carbon\u2013carbon coupling reactions [27,62]. A more detailed analysis is hindered by the complexity of the reactions occurring when the pyrolysis volatiles cross the catalyst bed and the fact that several reactions may occur simultaneously and lead to opposite effects. The concentration of phenols also decreased from 17.5 to 13.1\u202fwt% (mainly guaicol compounds), and the yield of hydrocarbons (mainly naphthalene compounds) increased to 1.1\u202fwt% as a result of secondary cracking reactions. Besides, the presence of Fe0 metal in the olivine chemical composition promotes deoxygenation reactions, leading to an increase in the production of these aromatic hydrocarbons [38].The results shown in Table 4 are evidence of a less oxygenated nature of the bio-oil obtained when acid catalysts were used. Accordingly, the yields of the oxygenated compounds, i.e., all the functional groups shown in Table 4, except the one of hydrocarbons, decreased as follows: pyrolysis 500\u202f\u00b0C (67.5\u202fwt%)\u202f>\u202finert sand (54.5\u202fwt%)\u202f>\u202folivine (46.9\u202fwt%)\u202f>\u202fspent FCC catalyst (37.4\u202fwt%)\u202f>\u202f\u03b3-Al2O3 (28.8\u202fwt%).The treatment described in this study pursues the production of an upgraded volatile stream for its further in-line catalytic valorization in a third step for the production of H2 in a steam reforming process or the production of fuels, chemicals and aromatic hydrocarbons by other catalytic routes.As aforementioned, the main challenge to overcome in these processes is the fast catalyst deactivation by coke deposition. Several researches have reported that certain bio-oil compounds, such as phenolic ones, are the main precursors of coke formation [38,39]. However, amongst the different compound lumps contained in the phenolic fraction (alkyl-phenols, catechols and guaicols), it is not clear which is the main responsible for catalyst deactivation. Moreover, other authors have emphasized the relevance of removing the acids from the pyrolysis volatile stream in order to avoid operational problems in further catalytic valorization processes [32]. Thus, Gayubo et al. [40] attributed the formation of deactivating coke to mainly phenols and aldehydes, whereas Rem\u00f3n et al. [88] reported that, apart from guaicol phenolic compounds, furfural (aldehyde) and levoglucosan (saccharide) have high tendency to produce coke in steam reforming reactions.In this study, several low-cost materials have been used downstream the pyrolysis process in order to condition the volatile stream by removing and/or reducing undesirable compounds. The significant differences in the catalysts used led to volatile streams of considerable compositional diversity, which allowed delving into the understanding of the relationship between the composition of the feed into the reforming step and catalyst deactivation.Accordingly, the use of olivine led to a significant removal of acids and phenols, with the latter due mainly to the reduction in the guaicol fraction. Besides, the chemical composition of olivine, with Fe0 on its surface, plays a positive role in the bio-oil oxygenate decomposition and reforming reactions.The use of acid catalysts (FCC and \u03b3-Al2O3) results in a bio-oil composition with a considerable reduction in the aldehyde fraction, and free of acids, alcohols and saccharides at the expense of hydrocarbon formation. The phenolic fraction was considerably reduced compared to the pyrolysis conducted at 500\u202f\u00b0C (from 22.0\u202fwt% to 17.9 and 15.9\u202fwt% for FCC and Al2O3 catalysts, respectively) as a consequence of thermal and catalytic cracking. Moreover, the higher acidity of \u03b3-Al2O3 catalyst promoted the conversion of heavy oxygenated compounds into alkyl-phenols, whereas catechols were the major fraction when the FCC catalyst was used.Future studies will be conducted using these low-cost materials (inert sand, Al2O3, spent FCC catalysts and olivine) as guard beds in order to attenuate the fast catalyst deactivation in the in-line biomass pyrolysis-steam reforming process. Thus, the stability of reforming catalysts and their deactivation will be analyzed with the aim of understanding the role played by the volatile composition, and knowledge will be acquired about the main species responsible for the deactivation of the reforming catalyst.The conical spouted bed reactor allows attaining a reproducible volatile stream for its in-line catalytic pyrolysis. The modification of the pyrolysis volatile stream composition by catalytic cracking was analyzed by using different low cost catalysts and inert sand placed downstream in a fixed bed reactor. The features characterizing each material (physical properties, chemical composition and acidity) play a key role in the transformation of the volatile stream, leading to remarkable differences in the distribution and composition of the gaseous stream.The biomass pyrolysis conducted at 500\u202f\u00b0C in a CSBR led to a gas yield of 7.3\u202fwt% (with CO and CO2 being the main products), and a bio-oil yield of 75.4\u202fwt%, which was composed of mainly phenols, ketones, and saccharides. At 600\u202f\u00b0C, thermal cracking was evidenced when inert sand was used, increasing the yield of the gas to 14.4\u202fwt%, and so reducing that of the bio-oil to 68.3\u202fwt%. Thus, thermal cracking reactions occurred in parallel to the catalytic ones with all the catalysts tested.Bio-oil cracking was more severe as catalyst acidity was increased, i.e., olivine\u202f<\u202fspent FCC catalyst\u202f<\u202fAl2O3. Besides, acid catalysts enhanced decarbonylation over decarboxylation reactions, with CO being the main compound in the catalytic cracking. The chemical composition of olivine, with Fe phase on its structure, also promoted reforming and water gas shift reactions, leading to the formation of CO, CO2 and H2.The bio-oil composition was affected when either inert sand or any catalyst was used. Alcohols, saccharides, and especially the phenolic fraction were substantially reduced due to thermal cracking when the inert sand was used. This significance of this drop depended on the catalyst used. The basic character of olivine promoted ketonization of acids, and aldol condensation of ketones and aldehydes, leading to the formation of CO2 and water. Concerning FCC and \u03b3-Al2O3 catalysts, both led to a substantial increase in the hydrocarbon fraction (6.1 and 8.5\u202fwt%, respectively). Accordingly, the acidity of these catalysts played a key role in the cracking of pyrolysis volatile oxygenates, since the acid sites promoted deoxygenation reactions, as well as cracking, oligomerization, alkylation, isomerization, cyclization and aromatization, which greatly increased the hydrocarbons fraction. The phenolic fraction was influenced by the type of catalyst employed by promoting the formation of catechols and alkyl-phenols when FCC and Al2O3 catalyst, respectively, were used.The results provided in this study are of special relevance for further studies wherein the production of H2 will be approached by feeding the bio-oil stream leaving the catalytic process into the two step biomass pyrolysis-steam reforming strategy.\nEnara Fernandez: Investigation, Visualization, Writing - review & editing. Laura Santamaria: Writing - original draft, Visualization, Writing - review & editing. Maite Artetxe: Writing - original draft, Conceptualization, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Maider Amutio: Conceptualization, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Aitor Arregi: Validation, Visualization, Writing - review & editing. Gartzen Lopez: Conceptualization, Validation, Writing - review & editing, Visualization, Supervision, Project administration. Javier Bilbao: Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Martin Olazar: Writing - 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 from Spain\u2019s ministries of Science, Innovation and Universities (RTI2018-101678-B-I00 (MCIU/AEI/FEDER, UE)) and Science and Innovation (PID2019-107357RB-I00 (MCI/AEI/FEDER, UE)), the European Union\u2019s Horizon 2020 research and innovation programme under the Marie Sk\u0142odowska-Curie grant agreement No. 823745, and the Basque Government (IT1218-19 and KK-2020/00107).", "descript": "\n                  Biomass pyrolysis and the in-line catalytic cracking of the pyrolysis volatile stream has been approached in this study. The pyrolysis step was carried out in a conical spouted bed reactor at 500\u202f\u00b0C, whereas the inert sand or the cracking catalysts (\u03b3-Al2O3, spent FCC and olivine) were placed in a fixed bed reactor at 600\u202f\u00b0C. Product analysis was carried out on-line by means of chromatographic methods, and the distribution and composition of the main products obtained have been related to the features characterizing each catalyst (physical properties, chemical composition and acidity).\n                  Decarbonylation reactions were favoured over decarboxylation ones when acid catalysts (spent FCC and \u03b3-Al2O3) were used, whereas olivine promoted ketonization and aldol condensation reactions. The Fe species in the olivine structure enhanced reforming and WGS reactions. Bio-oil cracking was more severe as catalyst acidity was increased, leading to an increase in the hydrocarbon fraction. The Al2O3 derived bio-oil was substantially deoxygenated, with a considerable reduction in the phenolic fraction, which accounted mainly for alkyl-phenols. The three materials tested led to a significant decrease in acid and phenolic compounds in the volatile stream, making it suitable for further catalytic valorization for the production of H2, fuels and chemicals.\n               "}
{"full_text": "The structure and nature of the active sites of hydrogenation catalysts based on copper has not been the focus of scientific research since the early 2000\u2032s, despite previous debate regarding the catalytic mechanism. This subject has recently become relevant again due to the demand for stable, Cr-free industrial catalysts in light of the sunset date for CrVI in 2017, rendering the established copper chromite Adkins catalyst obsolete [1\u20134]. This catalyst consists of a copper oxide phase distributed over a copper chromite spinel phase (CuO\u00b7CuCr2O4) and exhibits excellent stability and activity [3]. Investigated as early as 1991, copper aluminate spinel based catalysts (CuO\u00b7CuAl2O4) offer an analogous structure to the Adkins catalyst, where Cr is replaced by Al in the spinel phase [5\u20137]. These alternative catalysts often contain other metals as additional components for industrial hydrogenation processes [5,8]. Several examples of manganese as component in copper aluminate catalysts are available in the patent literature, though an explanation of the role that manganese plays in these catalysts is lacking [6,9,10]. Manganese possibly increases the stability of the catalysts (e.g. against acidic impurities), and it is claimed that manganese is a necessary part of the catalyst. Recently, also (non-spinel type) Cu/Zn oxides were reported as interesting alternative to Adkins catalysts for hydrogenolysis reactions [11].The normal spinel structure, found in copper chromite (CuCr2O4), contains tetrahedrally coordinated CuII cations in A-sites and octahedrally coordinated CrIII cations in B-sites, following the general formula:\n\n\n\n\nA\n\n(\nt\ne\nt\n)\n\n\nII\n\n\n\nB\n\n2\n(\no\nc\nt\n)\n\n\nIII\n\n\n\nO\n4\n\n\n\n\n\nCopper aluminate spinel (CuAl2O4) is partially inverse, with CuII\nand AlIII each found in both tetrahedral and octahedral sites [12]. In manganese aluminate spinel (MnAl2O4), the degree of inversion is dependent on the method of preparation and oxidative transfer between sites can occur, with MnII in tetrahedral sites converting to MnIII in octahedral sites and the formation of an Al2O3 phase with the displaced AlIII\n[13,14]. Fast redox processes within manganese oxides are considered key for their catalytic activity and Mn is therefore often used as an oxophilic redox promotor and electron scavenger to improve selectivity or activity of metal oxide catalysts [15].The oxidic precursors require an activation step to become hydrogenation catalysts, involving reduction of the copper species under hydrogen flow at elevated temperature. Hydrogen is thought to penetrate the mixed metal oxide bulk to react with CuII ions, yielding H+ and Cu0\n[16,17]. Copper atoms migrate to the surface of the catalyst particle and form hemispherical copper nanoparticles in close contact with the residual bulk spinel [18,19]. The protons remain sequestered in the resulting cation deficient lattice in tetrahedral sites previously occupied by Cu2+, covalently bonded to one lattice oxygen and stabilising the structure in the active state [18,20]. XPS measurements combined with XRD showed CuII in tetrahedral sites are reduced to CuI (at 150\u00a0\u00b0C), which migrate to octahedral sites, and to Cu0 (at 250\u00a0\u00b0C), whereas CuII in octahedral sites are reduced at higher temperature (300\u00a0\u00b0C) to Cu0 nanoparticles and CuI, which remain stabilised in the octahedral sites of the spinel [18,21]. In-situ XANES investigations of CuAl2O4 (formed by impregnation of Al2O3 with 5\u00a0wt% Cu) showed the final copper oxidation states as 70% Cu0 and 30% CuI, in a spinel-like environment [22].The role of the reduced spinel and the copper nanoparticles in the catalytic mechanism of hydrogenation reactions of CC and CO double bonds is the subject of some debate. Three possible mechanisms are described in previous research, in which the different copper species are assigned different roles. The first possible mechanism was championed by Bechara et al. in 1985, where they found the catalytic activity of isoprene and 1,3-pentadiene hydrogenation to correlate with the amount of CuI in octahedral sites of a copper chromite spinel, as well as with H species occluded in the spinel [23]. Bechara et al. concluded that the oxidisable part of the reduced spinel (surface) was therefore more important than the surface area of the metallic copper and identified the active site as a CuI-H pair [23]. Hubaut et al. extended this research to the selective 1,2-hydrogenation of \u03b1,\u03b2-unsaturated aldehyde or ketone to the allylic alcohol [24]. This mechanism requires the catalytic activity to be controlled by the amount of CuI present in the activated spinel.The second proposed mechanism identified Cu0 nanoparticles as the site for hydrogen activation under reaction conditions, similar to Group VIII (Pt, Pd or Ni) metal catalysts, but with higher activation energy [25]. Gudkov et al. reported the dependency of the rate equation for the hydrogenation of butyraldehyde on atomic hydrogen. Dissociatively adsorbed hydrogen was confirmed to participate in the reaction mechanism by isotope studies of irreversible butyraldehyde hydrogenation using adsorbed deuterium, where deuterium/hydrogen exchange rates increased with increasing copper content [25]. The capability of metallic copper to dissociatively adsorb hydrogen was reported to be dependent on particle size and the presence of high index faces (211), (311) and (755) [26,27]. It is expected that a mechanism where the copper nanoparticles supply atomic hydrogen and are the active site for hydrogenation would result in catalytic activity correlating with larger metal surface area, which is affected by both particle size and shape.The third mechanism, described by Yurieva et al. for the hydrogenation of acetone by a reduced copper chromite spinel catalyst, depends on both the surface of Cu0 nanoparticles and the bulk spinel lattice [28]. According to the authors, acetone is adsorbed on the surface of the Cu0, which supplies two electrons to the \u03c0* orbital of the carbonyl group, giving the carbon a negative charge. Simultaneously, a proton from the spinel lattice transfers to the oxygen to form an alcohol group. This mechanism then describes the migration of the resulting oxidised Cu2+ back into the spinel lattice to occupy a previously vacated cation site, whilst a second proton migrates in the reverse direction to the anionic carbon, allowing the alcohol to desorb from the nanoparticle surface. In this way, the reduced spinel lattice behaves as a Br\u00f8nsted acid, supplying protons, and the nanoparticle acts as a CuII\n\n\n\n\u2194\n\n\n\ne\n\n-\n\n\n\n Cu0 switch, supplying electrons. However, the migration of copper between the spinel and nanoparticle is likely mass transport limiting to the catalytic rate of reaction, making this aspect of the mechanism debatable.In order to determine which, if any, of these three mechanisms proposed for the traditional chromate system is most likely to be correct in the contemporary copper aluminate spinel-based catalyst (CuO\u00b7CuAl2O4) the structure of the activated copper aluminate was investigated. Detailed insights were obtained by characterising the bulk catalysts using XRD, TPR, XANES and EXAFS after synthesis by co-precipitation and calcination to give the spinel structure, and during and after activation in hydrogen forming the final catalyst. The catalytic performance in the reduction of model substrate butyraldehyde by copper aluminate catalysts with varying copper metal surface area and a pure copper spinel model CuAl2O3 was studied to deduce structure\u2013activity relationships. In particular, the effect of manganese on the structure and redox properties of copper aluminate spinel-based catalysts, which has not been previously investigated, was used as a tool to probe the active structure. Using a combination of techniques, this work attempts to investigate the role of manganese in structure formation, activation and catalytic behaviour of these industrially relevant catalysts and contribute to the discussion on the reaction mechanism.Copper aluminate (CuO\u00b7CuAl2O4) catalysts were synthesised via co-precipitation of the metal nitrates with sodium carbonate. A metal nitrate feed solution (0.6\u00a0M Cu(NO3)2\u00b73H2O, 0.6\u00a0M Al(NO3)3\u00b79H2O and 0.1\u00a0M Mn(NO3)2\u00b74H2O in Mn including catalyst synthesis) and Na2CO3 (2\u00a0M) were co-fed into a precipitation vessel containing warm water (50\u00a0\u00b0C, stirring at 400\u00a0rpm) at a rate of 5\u00a0mL/min (0.2\u00a0M final Cu concentration). The pH was kept at 6.5 by small adjustments to the rate of addition and the precipitates were aged for 1\u00a0h (50\u00a0\u00b0C). The precipitates were subsequently filtered and washed by re-suspending in deionised water until the spent wash fluid had a conductivity\u00a0\u2264\u00a00.5 mS. The obtained solids were dried at 120\u00a0\u00b0C overnight and subsequently calcined at 750\u00a0\u00b0C for 2\u00a0h (2\u00a0K/min). Catalysts were then activated under H2 flow, heating at 1\u00a0K/min to 300\u00a0\u00b0C. The final temperature was then held for 1\u00a0h before flushing with argon and allowing to cool.Pure copper aluminate spinel (CuAl2O4) was prepared in a similar co-precipitation method as described above, using a modified molar ratio of Cu:Al\u00a0=\u00a01:2. The calcination step was also modified to 800\u00a0\u00b0C for 2\u00a0h (5\u00a0K/min). Residual CuO was removed using saturated (NH4)2CO3 solution in an ultrasonic bath for 2\u00a0h. Thereafter, the leached spinel was stirred at 50\u00a0\u00b0C for 30\u00a0min, filtered, washed with deionised water and dried at 120\u00a0\u00b0C overnight.\nElemental Analysis (copper and manganese). The samples were digested by treatment with concentrated acids and the metal contents were analysed by photometry.\nCatalytic tests. Catalytic hydrogenation was carried out on butyraldehyde as a model compound in a 300\u00a0mL stainless steel autoclave (Parr) equipped with a heater and overhead stirrer. The reactor was loaded with butyraldehyde (8\u00a0g), hexane (100\u00a0mL), and n-dodecane (1.2\u00a0g) as a GC internal standard. Before addition of the activated catalyst, the liquid phase was de-gassed using argon (Westfalen, 5.0) for 5\u00a0min. The autoclave was pressurized with H2 (60\u00a0bar, Westfalen, 5.0) and subsequently heated to 120\u00a0\u00b0C. After reaching the desired temperature, stirring was commenced (750\u00a0rpm) and the reaction time started. After 1\u00a0h, the reaction was quenched by stopping the stirring and cooling with an ice bath to 17\u00a0\u00b0C, upon which the pressure was released. The liquid-phase composition was determined by gas chromatography.Liquid-phase composition of the batch reaction products was determined by gas chromatography (HP6890 gas chromatograph) equipped with a HP-1 methyl siloxane capillary column (60\u00a0m\u00a0\u00d7\u00a00.25\u00a0mm\u00a0\u00d7\u00a00.25\u00a0mm) and a flame ionisation detector (FID). The oven program started at 40\u00a0\u00b0C with a hold time of four minutes which was then heated to 280\u00a0\u00b0C with 30\u00a0K/min. Helium was used as a carrier gas with 1.2\u00a0mL/min and the chromatograph was set to operate at constant pressure. Characteristic retention times and response factors (fx\n) were determined using calibration standards. The relationship between fx\n, mass (m) and GC peak integral (A) referenced to the internal standard n-dodecane for component identification and quantification.Catalysts were characterised by X-ray diffraction (XRD) to determine crystallite size and phase composition, temperature programmed reduction (TPR) for reduction behaviour, N2O-chemisorption for copper surface areas, and finally X-ray absorption spectroscopy for oxidation state (XANES) and local structure (EXAFS).\nXRD. X-ray diffractograms were measured on an X\u2019Pert Pro (PANalytic) with a Bragg-Brentano geometry or a Rigaku desktop X-ray Diffractometer with a Miniflex2 counter detector using Cu K\u03b1 radiation in the 2\u03b8 range 20-70\u00b0 and step size of 0.01\u00b0 2\u03b8. In-situ XRD was performed on an X'Pert Pro PW 3040/60 by PANalytical with Bragg-Brentano geometry and Cu K\u03b1 radiation (\u03bb\u00a0=\u00a01.54\u00a0\u00c5, 45\u00a0kV, 40\u00a0mA). The instrument was equipped with a Ni-K\u03b2 filter and a solid state detector (X'Celerator). Scanning range was 5\u201370\u00b0 2\u03b8 with a step size of 0.017\u00b0. The in-situ measurements were conducted in a HTK 1200 sample chamber by Anton Parr on a special sample holder equipped with heating. Reduction gas (5% H2 in N2) was mixed with Bronkhorst MFCs from the pure gases to obtain a flow of 10\u00a0mL/min. Heating rates were set to 2\u00a0K/min and samples were allowed to equilibrate after reaching the desired temperatures for 15\u00a0min.\nTemperature programmed reduction experiments were conducted on a Micromeritics AutoChem 2910. 20\u00a0mg of the sample to be analyzed were fixed in the quartz reactor (\u00d8= 9\u00a0mm) with quartz wool. To account for effects such as varying storage time or air humidity, samples were pre-dried and flushed with helium prior to reduction at 120\u00a0\u00b0C (10\u00a0K/min) for 30\u00a0min in helium (15\u00a0mL/min). After cooling back to room temperature, the cooling trap was equipped with a LN2/isopropanol cooling bath before the TPR experiments were started. Heating rate was set to 10\u00a0K/min with 75\u00a0mL/min 2.5% H2 in argon. TPR experiments were concluded usually at 600\u00a0\u00b0C and never exceeded 800\u00a0\u00b0C to avoid damage on the equipment. Removal of hydrogen from the reduction gas was observed with a thermal conductivity detector (TCD) detector. By calibrating the TCD integral of the TPR measurement with hydrogen consumption of a known quantity of pure CuO (see SI Fig. 1), the degree of reduction was calculated by relating hydrogen consumption stoichiometrically to copper content in the catalyst. For this calculation it was assumed that CuII is completely reduced to Cu0, as shown in Eq. (2):\n\n(2)\nCuIIO\u00a0+\u00a0H2\u00a0\u2192\u00a0Cu0\u00a0+\u00a0H2O\n\n\n\nN2O chemisorption. Surface area of metallic copper was determined with the N2O pulse chemisorption method (N2O gas purity 99.5%, Westfalen) on a Micromeritics AutoChem 2910. The loop volume was calibrated with nitrogen gas as well as the pulse size for complete conversion with the aid of manual injections. Prior to chemisorption, 200\u00a0mg of the sample were fixed in the U-tube quartz reactor and subsequently flushed with helium, followed by the activation procedure described above for the catalysts with pre-drying (120\u00a0\u00b0C, 30\u00a0min, 10\u00a0K/min). Flows were adjusted to 20\u00a0mL/min to account for the reduced amount of catalyst. After activation, the cool trap was supplied with liquid nitrogen. lt was verified that flowing nitrogen, product of copper oxidation with nitrous oxide, was not affected and only leftover N2O was removed by the cooling trap. Via the TCD-integrals calibrated with pure CuO, a stoichiometric factor of two and a surface density of copper of 1.47\u00a0\u00b7\u00a010\u221219 m\u22122 were determined. The relative error of the copper surface area determination was estimated to be \u00b12%.\nIn situ-IR with CO chemisorption. For the infrared spectroscopic measurements, a FTS-175 by BioRad was used. A self-made gas cell with a tablet holder as well as heating unit were used. A 2416 by Eurotherm was used as a controller for heating. Gas supplies were connected to the cell with Swagelok tubing. Helium, 1% CO in He and hydrogen gas for the reduction of catalyst tablets were connected and steered by Bronkhorst mass flow controllers. Catalysts were pressed into self-supporting tablets for measurements. After closing of the cell, it was put into the IR spectrometer and connected to the gas supply. The IR was then flushed with N2 for at least 20\u00a0min prior to the first measurement. Activation/treatment procedures were set at 5\u00a0mL/min gas flows with heating rates of 1\u00a0K/min. After activation, the catalyst tablets were cooled to room temperature in helium before CO-adsorption was begun with varying measurement intervals.\nXAS. X-ray absorption spectroscopy experiments were carried out at several beamlines at Diamond Light Source (UK). Measurements Mn K-edge (6539\u00a0eV) were carried out at the B18 beamline, using a Si(111) monochromator with Pt coated optics and harmonic rejection and argon filled ion chambers. A Mn0 foil spectrum was simultaneously obtained with each measurement for energy calibration. However, due to severe oxidation a MnO2 spectrum was used to determine the amplitude correction. Samples were prepared by diluting the oxidic and reduced catalysts in boron nitride and mixing to homogeneity before pressing into pellets (catalyst depth of approximately 1 unit edge step). The reduced catalyst pellets were sealed in an airtight cell with polyimide (Kapton) film windows before placing in the beam. XANES and EXAFS spectra were collected up to k\u00a0=\u00a012 in transmission and fluorescence mode.In-operando measurements at the Cu K-edge (8979\u00a0eV) during catalyst reduction were carried out at the I20 beamline, using a Si(111) monochromator with Rh coated optics and harmonic rejection mirrors. A Cu0 foil spectrum was obtained before measurements were carried out for energy calibration and to determine the amplitude correction. Samples were prepared by diluting the oxidic catalyst in boron nitride and mixing to homogeneity. The material was pressed into pellets and pushed through a metal sieve. Material of a 125\u2013250\u00a0\u03bcm diameter sieve fraction was transferred to a quartz capillary flow cell (OD 3\u00a0mm, wall thickness 0.05\u00a0mm) and held in place with quartz wool (catalyst depth of approximately 1 unit edge step). A thermocouple was placed inside the quartz capillary, which was mounted above a hot air blower and attached to a gas flow controller. Hydrogen gas was then flowed through the capillary (15\u00a0mL/min) and temperature was ramped to 300\u00a0\u00b0C (1\u00a0K/min). XANES and EXAFS spectra were collected continuously up to k\u00a0=\u00a016 in transmission mode, with a 26\u00a0\u00b0C temperature change during each spectrum. The final temperature was then held for 1\u00a0h before allowing the system to cool rapidly under He flow (15\u00a0mL/min).\nXANES. X-ray absorption near edge structure measurements were processed in Athena (Demeter 0.9.25, using Ifeffit 1.2.12) for background subtraction and normalisation [29]. A Linear Combination Fit (LCF) of the experimental data was carried out with standard reference spectra (Cu0 foil, Cu2O, CuO, CuAl2O4). The weighting of the fitting standards was forced to sum to 1 and the fits were evaluated with an R-factor (SI Equation 1) and reduced \u03c72. These values and the accompanying linear combination fits are shown in the supporting information (SI Table 1 and SI Fig. 3)\nEXAFS. Extended x-ray absorption fine structure measurements were processed in Artemis (Demeter 0.9.25, using Ifeffit 1.2.12) for summation of FEFF computed scattering paths and evaluation of the EXAFS equation (SI Equation (2)) for each path. Scattering paths were generated by FEFF calculations using CIF input files of Cu0, MnO2 and MnAl2O4 (modified to remove positions of multiple occupancy) [30\u201332]. EXAFS fitting of the experimental data (see Supporting Information, SI Table 3 and 4 for fit details) allowed estimation of the average coordination number (n) of the first Cu-Cu scattering shell. This is possible because the amplitude of oscillations in k-space that result in the first peak after Fourier transformation to R-space is proportional to the average number of neighbouring atoms in the first coordination shell. Nanoparticle diameters (D) and numbers of atoms (N) were in turn estimated from n using SI Equation 3 (correlation plot shown in SI Fig. 4), following the method developed by Beale and Weckhuysen [33]. The relationship between n and nanoparticle size becomes stronger with fewer numbers of atoms, where a greater proportion of the atoms are undercoordinated at the surface and n decreases rapidly.A series of catalysts with including manganese were prepared and compared to the CuO\u00b7CuAl2O4 benchmark catalyst. The catalysts were synthesised via controlled co-precipitation in sodium carbonate solutions (with or without addition of manganese). After separation and drying, thermal treatment at 750\u00a0\u00b0C was necessary for the formation of the spinel structure. Additional catalysts were prepared for comparison in the activity study: Pure (\u201cstoichiometric\u201d) copper aluminate spinel (CuAl2O4; without additional CuO) and CuO\u00b7CuAl2O4 catalysts with varying copper metal surface area were obtained by variation of synthesis parameters. The catalytic performance relative to surface area (reported below) demonstrated substantial and complex differences due to the inclusion of Mn. The structures of the (2, 4 and 6\u00a0wt%) Mn containing CuO\u00b7CuAl2O4 catalysts were therefore studied in detail in comparison to the (0\u00a0wt% Mn) benchmark CuO\u00b7CuAl2O4 catalyst as oxidic pre-catalysts, and during and after activation by reduction with hydrogen.To explore the catalytic activity of the spinel type CuO\u00b7CuAl2O4 catalysts, the batch liquid-phase hydrogenation of butyraldehyde was used as a model reaction. The reaction was performed in an autoclave at 120\u00a0\u00b0C and 60\u00a0bar hydrogen pressure. Before the reaction, the oxidic pre-catalysts were activated in hydrogen at 300\u00a0\u00b0C for one hour. The yield of butanol is represented as a function of copper nanoparticle surface area in Fig. 1\n. Two series of CuO\u00b7CuAl2O4 catalysts were measured. In the first series (blue \u25a0), copper nanoparticle surface area was varied by modifying the synthesis conditions, see Section 3.2. In the second series (black \u25b4), nanoparticle surface area was kept constant with increasing Mn content from 0% to 6. In the first series, a linear correlation of activity with nanoparticle surface area is observed. However, presence of 6\u00a0wt% Mn results in significantly lower activity (~48% yield) than the Mn free catalyst (~67% yield) with similar copper surface area (~15\u00a0m2/g). In the second series, the yield likewise decreases with increasing manganese content, from 56% at 0\u00a0wt% Mn to a minimum of 21% yield at 4\u00a0wt% Mn with a slight recovery to 29% yield at 6\u00a0wt% Mn. However, while the butanol yield undergoes drastic changes, copper surface area hovers around 9\u00a0m2/g, regardless of manganese content. As the activity of catalysts containing Mn is shown not to correlate to Cu0 metal surface area, another factor may be expected also to control catalyst activity. The inclusion of manganese therefore offers a possible additional tool to investigate the active state. Finally, despite exhibiting the lowest metallic copper surface area (6.4\u00a0m2/g), the pure spinel phase CuAl2O4 (red \u25cf) prepared as model system yields more butanol than almost all other catalysts. This indicates that the spinel itself, as well as the nanoparticle surface, is important in the reaction mechanism.Manganese has a strong influence on the phase composition and crystallite size of the oxidic pre-catalysts, as shown by XRD analysis (Fig. 2\na and b). Increasing manganese content leads to increased crystallinity of CuAl2O4 spinel (indicated by \u25cf) and CuO phases, indicated by sharper and more intense diffraction peaks. No additional Mn containing phase is formed, but the spinel phase becomes more pronounced after inclusion of Mn, relative to the Mn free catalyst. It is therefore likely that Mn is incorporated into the CuAl2O4 phase which exhibits higher crystallinity than the Mn free catalyst. The increased peak intensity at higher Mn doping is also indicative of the growth of both spinel and copper oxide crystallites. Crystallite diameters for both phases were determined by the Scherrer equation (indicated by \u25a0 and \u25cf in Fig. 2b and later also summarised in Table 3). In the Mn free catalyst, crystallites as small as 9.4\u00a0nm (CuO) and 7.3\u00a0nm (CuAl2O4) are formed; although the error in spinel particle size determination is large as the diffraction peaks are narrow. At 6\u00a0wt% Mn, the crystallite sizes of both phases increase to 16.2\u00a0nm (CuO) and 12\u00a0nm (CuAl2O4).The oxidic pre-catalyst is reduced with H2 to the active structure, resulting in the formation of Cu0 nanoparticles on the surface of the spinel. The surface area of these nanoparticles was determined by N2O pulse chemisorption for each catalyst (indicated by \u25be in Fig. 2b). Despite the changes in oxidic crystallite sizes, the surface area of copper nanoparticles after catalyst activation remains surprisingly constant (~9 m2/g) with increasing Mn content. To further explore the effect Mn has on the reduction behavior to the active catalyst, TPR, XRD, XANES and EXAFS was carried out.The effect of manganese content on reduction behaviour is very pronounced. TPR studies are shown in Fig. 2c and summarised in Table 3, where the reduction temperature (TM, given by the TPR peak position) is shifted from 319\u00a0\u00b0C for Mn free CuO\u00b7CuAl2O4 to 257\u00a0\u00b0C for the 6\u00a0wt% Mn containing catalyst. The lower temperature of reduction is not only observed in TM, but also in the onset of reduction, which is shifted to lower temperatures too and undergoes a faster rate of reaction with Mn presence, as indicated by the steeper slope of the TCD signal. The degree of reduction of CuII species, determined from the H2 consumption, assuming stoichiometric CuII reduction, is presented in Fig. 2d (\u25b4), alongside the reduction temperature (TM, \u25cf). Up to 97% of total copper in a Mn free CuO\u00b7CuAl2O4 catalyst was reduced, whereas 6\u00a0wt% Mn decreased the degree of reduction to just 66%. These results suggest that reduction of copper is inhibited in the Mn incorporated spinel. However, the decrease in reduction temperature (TM) suggests that a certain fraction of CuII is simultaneously more easily reduced. It is possible that the presence of Mn results in competing effects which account for this contradictory behaviour.The reduction of an Mn free CuO\u00b7CuAl2O4 oxidic catalyst in hydrogen (5% in N2) was studied further using in-situ XRD in order to track the changes in the crystalline phases of the bulk catalyst. The results can be seen in Fig. 3\na, where copper oxide reflections decrease in intensity below 150\u00a0\u00b0C, but the XRD pattern does not change substantially. Above 150\u00a0\u00b0C, metallic copper appears while spinel and copper oxide reflections remain visible. At 200\u00a0\u00b0C all crystalline CuO is reduced and metallic copper dominates the diffractogram. At temperatures\u00a0>\u00a0200\u00a0\u00b0C the spinel peaks shift to higher angles but remain present up to 600\u00a0\u00b0C. The shift of the pure spinel (CuAl2O4) reflexes to higher angles is described by Plyasova et al. to be indicative of a contraction of the spinel unit cell caused by reduction of the spinel phase itself [18]. Reduction of pure CuAl2O4 at 300\u00a0\u00b0C is known to yield metallic copper and a cation deficient spinel phase, into which protons are incorporated that offset the lost positive charge. Intermediate CuI species, Cu2O and CuAlO2, are also known to form [22,34]. The contraction of the spinel lattice is therefore likely due to the replacement of large CuII ions by much smaller protons.For comparison with the Mn free catalyst, 6\u00a0wt% Mn-CuO\u00b7CuAl2O4 was activated by reduction in hydrogen at 300\u00a0\u00b0C and also studied using XRD (see Fig. 3b). The CuO reflections are similarly absent in the reduced Mn containing catalyst pattern (grey line), but unlike the Mn free CuO\u00b7CuAl2O4, the spinel phase remains distinctive alongside sharp metallic copper peaks. One further essential difference is observed: the spinel peaks are not shifted compared to the oxidic pattern (black line), indicating that the spinel phase with Mn does not undergo a lattice contraction. The absence of a lattice contraction suggests that less CuII is reduced and removed from the Mn containing spinel phase than in the Mn free CuO\u00b7CuAl2O4. The reduced lattice seems to be stabilised by the presence of Mn.Since XRD only probes crystalline phases, the reduction was studied using in-situ XAS to investigate the bulk catalyst, including possible amorphous phases. The Cu K-edge XANES spectra of the Mn free CuO\u00b7CuAl2O4 can be seen in Fig. 4\na, where the oxidic CuO\u00b7CuAl2O4 (black line) exhibits the 1\u00a0s\u00a0\u2192\u00a04p transition at an edge position of 8983.8\u00a0eV, (edge position is defined as the maximum point of the first derivative) which lies closer to the edge positions of pure CuO than CuAl2O4 (8983.5\u00a0eV and 8986.6\u00a0eV respectively, shown in SI Fig. 2). A weak pre-edge at 8976.5\u00a0eV is due to the 1\u00a0s\u00a0\u2192\u00a03d transition, forbidden by dipole selection rules, but observed due to 3d/4p orbital mixing [35,36]. This dipole-forbidden transition is characteristic of CuII and is not observed in CuI due to its closed shell d10 configuration [37]. Below 200\u00a0\u00b0C the peak intensity of the white line and oscillations of the multiple scattering region dampen slightly, but the edge position does not change. The reduction of CuII in the oxidic catalyst occurs between 200\u00a0\u00b0C and 250\u00a0\u00b0C, as indicated by the edge shift from 8983.8\u00a0eV to 8979\u00a0eV, the disappearance of the weak pre-edge feature at 8976.5\u00a0eV and the change in oscillations in the multiple scattering region above the edge. At higher temperatures (>300\u00a0\u00b0C), the catalyst is reduced, with no further spectral changes observed by XAS.Linear combination fitting of the oxidic and reduced catalysts gives more detailed information on the relative contributions to the observed spectra by different copper species. The Mn free CuO\u00b7CuAl2O4 is compared to the 6\u00a0wt% Mn-catalyst in Fig. 4b with contributions of reference components (CuIIO, CuIIAl2O4, CuI\n2O and Cu0, see SI Fig. 3 for fitted spectra) quantified by linear combination fitting of XANES data. Before reduction, the oxidic catalysts are shown to contain CuII in CuAl2O4 spinel and CuO species only, with no CuI contributions to the observed spectra at all. However, the percentage of CuII in the Mn containing pre-catalyst is higher in the spinel phase than in CuO (58:42%), compared to the Mn free catalyst (51:49%). After reduction, CuII is almost completely removed from both CuO and CuAl2O4 phases in the Mn free catalyst, whereas 21% of copper is retained as CuII in the Mn containing CuAl2O4 spinel. This results in 70% of copper species forming Cu0 in the Mn free catalyst, but only 50% Cu0 when 6% Mn is present. The XANES analysis therefore definitively shows that Mn presence results in greater retention of CuII in spinel and lower reduction to Cu0.To assess the role of Mn in the structure, the oxidation state and local coordination of Mn within the catalyst was also investigated using XAS. The XANES of the oxidic and reduced catalysts are shown in Fig. 5\na, alongside three manganese oxide references, with the pre-edge feature A and edge features labelled B1/2, and the white line feature C. The pre-edge feature A is due to the 1\u00a0s\u00a0\u2192\u00a03d transition and becomes sharper and more intense for non-centro-symmetrically coordinated Mn, for example in tetrahedral sites [38]. The pre-edge feature is sharper in the catalyst XANES spectrum than the octahedrally coordinated manganese oxide reference spectra, indicating that the catalyst contains Mn in tetrahedral sites. Comparison with literature examples of Mn K-edge XANES analysis suggests that the reduced catalyst corresponds to a MnAl2O4 spinel, identified by the features A-C which are characteristic of Mn spinel structures [13]. The feature labelled B1/2 are therefore attributed to the symmetry allowed 1\u00a0s\u00a0\u2192\u00a03p transition, which is divided over two peaks due to the distribution of Mn over two sites in the spinel lattice [39].The first derivative is shown in Fig. 5b, from which the first peak (B1) after the pre-edge (A) gives the edge energy E. The K-edge energy shifts are calculated from E-E\n0 (where E\n0\u00a0=\u00a06539\u00a0eV) are listed in Table 1\n, with the corresponding oxidation state of Mn for each compound. The oxidic Mn containing catalyst has an edge shift of 7.7\u00a0eV, indicating an oxidation state between 2 and 3, which does not change after reduction to the activated catalyst. However, although all features A-C are present in the reduced catalyst, the shape of the XANES does undergo a change, with a shift in the position and size of feature C. Although the energy shift of the oxidic catalyst is not higher than the reduced catalyst, based on the position of feature B1, the differences in the spectrum at higher energy indicates contributions from Mn in a higher oxidation state. Linear combination fitting, shown in Fig. 5c, of standard components to the oxidic 6\u00a0wt% Mn - catalyst data resolves two 50:50 contributions of MnAl2O4 spinel and MnIVO2 to the measured XANES spectrum.EXAFS fitting was carried out in order to confirm the MnAl2O4 structure identified in the activated catalyst. Fig. 6\n shows the k\n2-weighted Mn K edge EXAFS data of the 6\u00a0wt% Mn - catalyst in k-space (Fig. 6a) and the corresponding k\n2-weighted Fourier transformation to R-space (Fig. 6b). The oscillations of the reduced catalyst show a shift to lower wavenumbers which is more pronounced at higher k and the number of contributions also increases. In R-space, the reduced catalyst results in a first shell peak of lower amplitude than the oxidic catalyst, as well as changes is the second shell. The fits for both the oxidic and reduced 6\u00a0wt% Mn - catalyst are indicated by the dotted lines in Fig. 6 and the scattering paths and parameters used to generate the fits are summarised in Table 2\n. The fit to the oxidic catalyst data was generated using scattering paths from two crystal structures (MnAl2O4 and MnO2) each given a 50% weighting, based on the LCF results, resulting in an R-factor of 0.01. The fit of the reduced catalyst was generated using only MnAl2O4 scattering paths, giving an R-factor of 0.02.In summary, doping the CuO\u00b7CuAl2O4 catalyst with 6\u00a0wt% Mn results in incorporation of approximately half the Mn into tetrahedral sites of the spinel lattice with oxidation states between 2 and 3. The remaining Mn is located in octahedral sites, either within the spinel lattice or in MnO2 type structure, with an oxidation state of 4. Activation results in reduction of the MnIV to an average oxidation state of 2.5 accompanied by a decrease in the average coordination number (n) from 5 to 4. One explanation is that MnIV in octahedral sites also migrates to tetrahedral sites within the spinel upon reduction.In order to investigate the copper nanoparticles formed on the reduced catalysts, EXAFS analysis of in-situ activated catalysts was carried out. Fig. 7\n shows the k\n2-weighted Cu K edge EXAFS data of the Mn free CuO\u00b7CuAl2O4 and 6\u00a0wt% Mn - catalyst in k-space (Fig. 7a) and the corresponding k\n2-weighted Fourier transformation to R-space (Fig. 7b). The oscillations of the Mn containing catalyst in k-space are noticeably dampened compared to the Mn free CuO\u00b7CuAl2O4. This produces less intense peaks in R-space and is indicative of smaller nanoparticles as a result of Mn presence of the spinel [33]. Assuming hemispherical copper particles, as previously reported by Plyasova et al., with face centered cubic (fcc) structure, the average nanoparticle sizes are quantified Table 3\n in terms of diameter and number of atoms [18]. Data fitting to correlate the average coordination number of atoms in the nanoparticles to the size of the nanoparticle is shown in the supporting information (SI Equation 3 and SI Fig. 4). The Mn containing catalyst exhibits smaller nanoparticles, containing just 70\u00a0\u00b1\u00a020 atoms with a hemispherical diameter of 1.3\u00a0\u00b1\u00a00.2\u00a0nm, compared to the Mn free catalyst nanoparticle size of 210\u00a0\u00b1\u00a050 atoms and diameter of 2.6\u00a0\u00b1\u00a00.3\u00a0nm. The presence of 6\u00a0wt% Mn therefore results in nanoparticles that are on average half the diameter of those of the Mn free catalyst, with approximately one seventh of the number of atoms.To further investigate the role of spinel, pure CuAl2O4 was prepared (Materials and Methods), reduced and analysed in-situ with IR-spectroscopy coupled with CO-chemisorption, to probe the outermost catalyst surface. The copper aluminate spinel exposed to CO exhibited the IR bands shown in SI Fig. 5 (descriptions of experiments and assignment of IR bands also in SI) [40]. As expected, an activated spinel consists of Cu0 as well as of CuI/CuII ions on its surface. Unfortunately, the oxidized species are not distinguishable from each other by this technique. The experiments indicate reduction of CuI/II ions to metallic copper by CO. The redox reaction with CO at room temperature shows that the surface is highly labile, with multiple oxidation states of copper indicating availability and activity of these species also at the outermost surface.As described in the introduction, the mechanism of hydrogenation over copper spinel catalysts is disputed in the literature. Several different possible active species in the reduced spinel have been proposed: a CuI species, the Cu0 nanoparticles, and a combination of both nanoparticles and the spinel support [17,23\u201325]. Several accessible oxidation states of copper have been identified during reduction of the co-crystallised CuO\u00b7CuAl2O4 mixed phase catalysts. Firstly, the CuO phase appears to be directly transformed into crystalline Cu0, with no intermediate steps or formation of other crystalline species. Previous research on reduction of CuO under industrially relevant conditions also showed that the reduction of CuO to Cu0 occurs in one step [41,42]. However, CO-probing the pure spinel surface with IR measurement indicates the presence of either CuI or CuII after reduction (SI Fig. 5). Plyasova et al. also suggested that not all spinel copper is reduced completely to Cu0 but that some CuII ions are retained within the lattice [43].The more detailed linear combination fitting of XANES data of reduced catalysts definitively showed the formation of a CuI species, likely located within the spinel, as well as significant amounts of CuII remaining in the reduced Mn containing spinel phase (Fig. 4). The reaction mechanism suggested by Bechara et al. involves CuI stabilised in octahedral positions as the active species, as they were able to correlate the amount of CuI ions to activity [23]. However, in this research the amount of CuI species in the reduced catalysts increases slightly with Mn presence (Fig. 4b), whereas activity was shown to decrease (Fig. 1). This indicates that CuI cannot be crucial to the active state, so the research mechanism proposed by Bechara et al. is unlikely to be correct for the catalysts and reaction studied here [23].The cation deficient reduced spinel lattice forms the bulk of the reduced catalyst, with copper metal nanoparticles growing upon its surface. It is also known that copper nanoparticles can be active sites for the dissociative adsorption of H2\n[25]. Yurieva et al. went further to suggest that metallic copper provides electrons for the hydrogenation, which leads to the expectation that activity must correlate with copper metal surface area [17]. This is not the case, as catalytic activity is reduced by Mn presence whereas the metal surface area remains constant (Fig. 1, Table 3). A mechanism only controlled by dissociative adsorption of H2 on metallic copper cannot fully explain the observed trend. Furthermore, pure CuAl2O4 with a smaller copper metal surface area yielded significantly more butanol than expected based on a linear trend between surface area and activity (Fig. 1). This indicates that the reaction mechanism does not only involve the copper metal nanoparticle surface, but is also affected by the structure of the spinel and thus more complex.Incorporation of Mn into the spinel lattice has a profound effect on the spinel structure. The lattice is stabilised by raising the relative amount of spinel to copper oxide and increasing the spinel crystallite size. This appears to inhibit the reduction of CuII, causing copper to be retained within the lattice which therefore does not undergo contraction, unlike the Mn free catalyst. At the same time, the reduction of CuII is facilitated in terms of the lower reduction temperature (Tm, Fig. 2c, Table 3). A variety of mechanisms to explain how Mn simultaneously lowers Tm and prevents a significant percentage of CuII from being reduced are possible. Mn is a known redox catalyst with several possible oxidation states and could therefore modify the reduction behaviour of the CuII spinel due to its different redox potential. The spinel structure contains tetrahedral and octahedral cationic sites and Mn is known to occupy both, as either MnII or MnIII in different types of spinels [14]. XANES and EXAFS measurements of the reduced catalyst showed MnII/III in predominantly tetrahedral sites, which are known to be vacated by CuII upon reduction to Cu0\n[18]. The MnAl2O4 type local structure identified is known to undergo oxidative transfer of MnII in tetrahedral sites to MnIII in octahedral sites under certain conditions, which are not reduced under 800\u00a0\u00b0C in 5%H2/Ar [14,44]. It is therefore possible that MnII, in tetrahedral sites, could act as an electronic promoter within the spinel, increasing the availability of electrons to CuII and resulting in a lower temperature of copper reduction.The decrease in reducible copper could be explained by two further effects in the Mn containing catalyst. The simple increase in crystallite size could result in pathways to the spinel surface that are simply too long for copper to migrate. In addition, blocking of Cu migration pathways by MnII/III ions incorporated into tetrahedral sites of the spinel structure is also possible. It is considered likely that a combination of all three mechanisms is at play. A lowered redox potential gives rise to a spinel more active towards reduction and accounts for the earlier onset and lower temperature of CuII reduction in Mn containing catalysts. However, inhibited migration due to larger distances and blocking of pathways could account for the large fractions of CuII that are not reduced in the manganese containing spinel.Yurieva et al. championed an interfacial reaction mechanism where protons are supplied by the lattice and electrons are supplied by Cu0 for the reduction of the reactant molecules, after which the resulting CuII ions migrate back into the lattice [17]. This type of mechanism, involving the redox reaction of CuII\n\n\n\n\u2194\n\n\n\ne\n\n-\n\n\n\n Cu0, requires access to multiple copper oxidation states in the activated catalysts, which we have shown to be the case. However, in order for this mechanism to be catalytically relevant, unrealistically high rates of ion migration in and out of the lattice would be required per cycle. We therefore propose a modified mechanism: a predominantly interface-based reaction mechanism, involving both the spinel and copper nanoparticle surface (illustrated in Fig. 8\n). In this mechanism, the electron reservoir of the metallic copper particle is refreshed by the dissociative adsorption and oxidation of H2 on the nanoparticle surface. The resulting protons are then stored inside the spinel phase and are able to migrate quickly through the outermost layers of the lattice to active sites on the surface. Besides the lower reduction degree of copper in the manganese containing catalyst, an additional effect of Mn in such a mechanism could be to reduce proton capacity of the lattice due to occupation of cation sites by MnII/III and non-reduced CuII, which have been shown to be retained in the spinel by XANES analysis (Figs. 4b and 5). The incorporation of Mn ions in the lattice could also block or influence proton migration pathways through the lattice, slowing the resupply of protons to the active sites on the spinel surface. Both, or either, of these effects could account for the reduced activity of the Mn containing catalyst.Multiple techniques have been used to investigate the structure of the active Mn containing and Mn free CuO\u00b7CuAl2O4 catalysts for hydrogenation of butyraldehyde. Mn presence results in a stabilisation of the spinel towards reduction of CuII to Cu0 by occupation of tetrahedral sites within the spinel by Mn cations, causing decreased catalytic activity. This has implications regarding the catalytic mechanism and highlights the role of the spinel lattice as a proton reservoir. A modified mechanism is therefore proposed, where the copper nanoparticle acts as a site for H2 dissociation and supplies electrons and the spinel lattice stores and delivers protons, with the catalytic reaction likely occurring at the interface.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 Clariant for funding (to C.D. and K.K.), and NWO and Clariant for funding (NWO LIFT 731.015.407, Launchpad for Innovative Future Technology, PreCiOuS, to M.T. and M.H.). K.K. thanks the University of Amsterdam for an HRSMC (Holland Research School of Molecular Chemistry) fellowship. The authors thank the staff of the I20 and B18 beamlines at Diamond Light Source in Didcot, UK (proposal number SP16558-1) for support and access to their facilities. The authors thank Bas Venderbosch, Jean-Pierre Oudsen and Lukas Wolzak for assistance during synchrotron measurements. The authors would like to thank Ulrike Ammari, Petra Ankenbauer and Bircan Dilki from the microanalytical laboratory at Technical University of Munich for the conduct of elemental analyses.Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2020.12.017.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  Copper aluminate spinel (CuO\n                        .\n                     CuAl2O4) is the favoured Cr-free substitute for the copper chromite catalyst (CuO\n                        .\n                     CuCr2O4) in the industrial hydrogenation of aldehydes. New insights in the catalytic mechanism were obtained by systematically studying the structure and activity of these catalysts including effects of manganese as a catalyst component. The hydrogenation of butyraldehyde to butanol was studied as a model reaction and the active structure was characterised using X-ray diffraction, temperature programmed reduction, N2O chemisorption, EXAFS and XANES, including in-situ investigations. The active catalyst is a reduced spinel lattice that is stabilised by protons, with copper metal nanoparticles grown upon its surface. Incorporation of Mn into the spinel lattice has a profound effect on the spinel structure. Mn stabilises the spinel towards reduction of CuII to Cu0 by occupation of tetrahedral sites with Mn cations, but also causes decreased catalytic activity. Structural data, combined with the effect on catalysis, indicate a predominantly interface-based reaction mechanism, involving both the spinel and copper nanoparticle surface in protonation and reduction of the aldehyde. The electron reservoir of the metallic copper particles is regenerated by the dissociative adsorption and oxidation of H2 on the metal surface. The generated protons are stored in the spinel phase, acting as proton reservoir. Cu(I) species located within the spinel and identified by XANES are probably not involved in the catalytic cycle.\n               "}
{"full_text": "The development of the Haber Bosch Process was undoubtedly a major achievement of the 20th Century. Through provision of a route to access synthetic fertiliser, it can be credited with sustaining 40% of the global population. It has been estimated that it is responsible for 2% of the global commercial energy requirement [1]. This figure takes into account the production of reactants for which the hydrogen required is generally prepared via fossil based feedstocks. As a whole the process, which currently runs on a production scale of ca 174 million tonnes per annum, accounts for 670 million tonnes of CO2 emissions per year equating to around 2.5% of worldwide fossil fuel based CO2 emissions [1]. It is therefore a very important major process for which a number of superlatives apply. On the industrial scale, ammonia synthesis is conducted at high pressure (>100 atmospheres) and moderate temperatures, ca. 400\u202f\u00b0C. The catalysts employed are either based upon iron or ruthenium. Although thermodynamically ammonia synthesis is favoured by lowering reaction temperature, the conditions employed are dictated by the requirement for acceptable process kinetics. In current commercial application, the process is highly integrated and very efficient. However, with the increasing availability of renewable electricity it is becoming more and more practical to produce the hydrogen required for localised ammonia synthesis via, for example, electrolysis. The possibility for localised sustainable \u201cgreen\u201d ammonia synthesis which could, for example, be conducted on farmland for provision of on-demand fertiliser is a strong driver for the discovery of more active catalysts which can be applied more easily under such conditions. There is also increasing interest in the application of ammonia as a fuel or hydrogen carrier. There are a number of alternative directions currently being investigated in terms of sustainable ammonia production and these include electrocatalytic approaches [2,3], photocatalytic ammonia synthesis [4,5] and chemical looping routes [6,7].In terms of investigation of catalysts for ammonia synthesis, it is generally the case that individual studies in the literature tend to focus in detail on small subsets of related materials and it is often difficult to benchmark such materials against different material classes reported by other groups due to issues such as differences in experimental procedure. This is a driver for the present manuscript in which we present an empirical overview of the performance of a wide range of supported catalysts which have been selected on the basis of their expected activity. Relevant considerations which have been documented in the literature are highlighted presenting a structured overview of some of the previous literature in relation to the materials screened. Further investigation would involve performance evaluation under conditions of greater relevance to application, such as operation at higher pressure and inclusion of ammonia in the feedstream. The conditions which have been selected correspond to those reported in a number of studies and which we have applied in our previously published studies to discern structure-composition and activity relationships which might otherwise be obscured by operation at higher reaction pressure and/or the inclusion of a low level of ammonia in the reactant feed. Accordingly, the study is intended to be a starting point for further development of active catalytic materials and the systems we have screened have neither been optimised nor characterised in great detail. The data reported suggest potential directions for further investigation where further detailed systematic investigation may, for example, provide the basis for computationally aided design [8].The materials screened within this study were prepared as follows.\n5% Ru/Al2O3\n was used as commercially obtained (Sigma Aldrich, Ru 5\u202fwt. % on alumina, powder, reduced, dry). The material was pretreated at 500\u202f\u00b0C under 60\u202fmL/min of 3:1 H2/N2 (BOC, H2 99.998%, N2 99.995%).5% Ru/Al2O3 + 1% KOH - approximately 1\u202fg of Ru/Al2O3 (Sigma Aldrich, Ru 5\u202fwt. % on alumina, powder, reduced, dry) was impregnated by dropwise addition of an aqueous solution of 1\u202fwt precent KOH (Sigma Aldrich, reagent grade, 90%, flakes). The material was then dried in air overnight at 110\u202f\u00b0C and was pretreated at 500\u202f\u00b0C under 60\u202fmL/min of 3:1 H2/N2 (BOC, H2 99.998%, N2 99.995%).\n5% Os3(CO)12/SiO2 and 5% Os3(CO)12/\u03b3-Al2O3\n - triosmium dodecacarbonyl (Os3(CO)12, Sigma Aldrich, 98%) was supported onto silica (amorphous, precipitated, Sigma Aldrich) or alumina (\u03b3-alumina, Condea Chemie, alumina extrudates) using the method outlined by Collier et al. [9]. The support was impregnated with a solution of Os3(CO)12 in dichloromethane. The volume of dichloromethane required was determined by point of wetness for each support. The material was then dried at 40\u202f\u00b0C to remove the dichloromethane to produce a yellow powder. The material was prepared to achieve a 5% loading by weight of osmium. The material was pretreated at 500\u202f\u00b0C under 60\u202fmL/min of 3:1 H2/N2 (BOC, H2 99.998%, N2 99.995%).\n5% Os3(CO)12 /SiO2 + 1% KOH - approximately 1\u202fg of 5% Os3(CO)12/SiO2 was impregnated by dropwise addition of an aqueous solution of 1\u202fwt percent of KOH (Sigma Aldrich, reagent grade, 90%, flakes). The material was then dried in air overnight at 90\u202f\u00b0C. The material was pretreated at 500\u202f\u00b0C under 60\u202fmL/min of 3:1 H2/N2 (BOC, H2 99.998%, N2 99.995%).\n5% Os3(CO)12/SiO2 dehydroxylated - silica (amorphous, precipitated, Sigma Aldrich) was dried following the method detailed by Collier et al. [9]. The support was dried at 500\u202f\u00b0C for 16\u202fh under a flow of nitrogen at 60\u202fmL/min. The ramp rate for heating was 10\u202f\u00b0C/min. After 16\u202fh, the material was cooled under nitrogen. Triosmium dodecacarbonyl (Os3(CO)12, Sigma Aldrich, 98%) was then supported onto the dried silica. The support was impregnated with a solution of Os3(CO)12 in dichloromethane. The volume of dichloromethane required was determined from the point of incipient wetness for silica. The material was then dried at 40\u202f\u00b0C to remove the dichloromethane to produce a yellow powder. The material was pretreated at 500\u202f\u00b0C under 60\u202fmL/min of 3:1 H2/N2 (BOC, H2 99.998%, N2 99.995%).\n10% CoRe supported on silica, alumina and zirconia - 5\u202fg of silica (amorphous, precipitated, Sigma Aldrich), alumina (\u03b1-alumina, Fisher Chemicals, aluminium oxide-calcined) or zirconia (monoclinic zirconium (IV) oxide, Sigma Aldrich, powder, < 5\u202f\u03bcm, 99% metal basis) were impregnated simultaneously with an aqueous solution of ammonium perrhenate (NH4ReO4, Sigma Aldrich, assay, form, powder or crystals, \u2265 99%, 0.55\u202fg) and cobalt nitrate hexahydrate (Co(NO3)2\u00b76H2O, Sigma Aldrich, ACS reagent, \u2265 98%, 0.60\u202fg). The material was stirred for 1\u202fh at room temperature. The material was then dried at 125\u202f\u00b0C overnight. Following this, the material was calcined in air at 700\u202f\u00b0C (applying a 10\u202f\u00b0C/min ramp rate) for 3\u202fh. Prior to reaction, 0.5\u202fg of the sample was pre-treated for 2\u202fh at 600\u202f\u00b0C under a 60\u202fmL/min flow rate of 3:1 H2/N2 (BOC, H2 99.998%, N2 99.995%). The material was prepared to give a 10\u202fwt% of CoRe on support with a 1:1\u202fwt ratio of cobalt to rhenium.\n5% Re/SiO2\n - 5\u202fg of silica (amorphous, precipitated, Sigma Aldrich) was impregnated simultaneously with an aqueous solution of ammonium perrhenate (NH4ReO4, Sigma Aldrich, assay, form, powder or crystals, \u2265 99%, 0.55\u202fg). The material was stirred for 1\u202fh at room temperature. The material was then dried at 125\u202f\u00b0C overnight. Following this, the material was calcined in air at 700\u202f\u00b0C (applying a 10\u202f\u00b0C/min ramp rate) for 3\u202fh. Prior to reaction, 0.5\u202fg of the sample was pre-treated for 2\u202fh at 600\u202f\u00b0C under a 60\u202fmL/min flow rate of 3:1 H2/N2 (BOC, H2 99.998%, N2 99.995%). The material was prepared to target a loading of 5 percent by weight of rhenium.\n10% CoRe/MgO - 5\u202fg of magnesium oxide (Sigma Aldrich, 325 mesh 99%+ metals basis) was impregnated simultaneously with aqueous solutions of precursors (5\u202fwt.% loading of Co and 5\u202fwt.% loading of Re), then stirred for 60\u202fmin at room temperature. The material was then dried at 125\u202f\u00b0C for 12\u202fh and then calcined in air at 700\u202f\u00b0C using a ramp rate of 10\u202f\u00b0C/min for 3\u202fh. 0.5\u202fg of this material was placed in a quartz glass reactor tube and charged to the reactor, it then underwent pre-treatment by flowing gas mixture of 60\u202fmL/min N2:H2 (BOC, H2 99.998%, N2 99.995%) (1:3) at 600\u202f\u00b0C for 2\u202fh using a ramp rate of 10\u202f\u00b0C/min.5% Ni2Mo3N/SiO2\n - 5\u202fg of silica (amorphous, precipitated, Sigma Aldrich) was impregnated simultaneously with aqueous solutions of precursors (2\u202fwt.% loading of Ni and 3\u202fwt.% loading of Mo) and stirred for 60\u202fmin at room temperature. The material was then dried at 150\u202f\u00b0C overnight. 0.6\u202fg of this material was then calcined in 5\u202fmL/min flowing N2 at 700\u202f\u00b0C using a ramp rate of 10\u202f\u00b0C/min for 6\u202fh. 0.4\u202fg of this material was placed in a quartz glass reactor tube and charged to the reactor, it then underwent pre-treatment by flowing gas mixture of 60\u202fmL/min N2:H2 (BOC, H2 99.998%, N2 99.995%) (1:3) at 700\u202f\u00b0C for 2\u202fh using a ramp rate of 10\u202f\u00b0C/min.\n10% Co/SiO2\n - 5\u202fg of silica (amorphous, precipitated, Sigma Aldrich) was impregnated with an aqueous solution of precursor (10\u202fwt.% loading for Co) and stirred for 10\u202fmin at room temperature. The material was then dried at 110\u202f\u00b0C overnight and then calcined in air at 600\u202f\u00b0C using a ramp rate of 10\u202f\u00b0C/min for 4\u202fh. 0.5\u202fg of this material was placed in a quartz glass reactor tube and charged to the reactor, it then underwent pre-treatment by flowing gas mixture of 60\u202fmL/min N2:H2 (BOC, H2 99.998%, N2 99.995%) (1:3) at 600\u202f\u00b0C for 3\u202fh using a ramp rate of 10\u202f\u00b0C/min.\n10% Co/\u03b1-Al2O3\n - 5\u202fg of \u03b1\u2013alumina (\u03b1\u2013Al2O3, Fisher Chemicals, aluminium oxide-calcined) was impregnated with an aqueous solution of precursor (10\u202fwt.% loading for Co) and stirred for 10\u202fmin at room temperature. The material was then dried at 110\u202f\u00b0C overnight and then calcined in air at 600\u202f\u00b0C using a ramp rate of 10\u202f\u00b0C/min for 4\u202fh. 0.5\u202fg of this material was placed in a quartz glass reactor tube and charged to the reactor, it then underwent pre-treatment by flowing gas mixture of 60\u202fmL/min N2:H2 (BOC, H2 99.998%, N2 99.995%) (1:3) at 600\u202f\u00b0C for 3\u202fh using a ramp rate of 10\u202f\u00b0C/min.\n10% Mo2N0.78/SiO2\n - 5\u202fg of silica (amorphous, precipitated, Sigma Aldrich) was impregnated with an aqueous solution of ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24.4H2O, Sigma-Aldrich (Germany) Puriss p.a., ACS reagent \u226599.0% (T)) corresponding to 10\u202fwt.% loading for MoO3 and stirred for 10\u202fmin at room temperature. The material was then dried at 110\u202f\u00b0C overnight and then calcined in air at 450\u202f\u00b0C for 2\u202fh using a ramp rate of 10\u202f\u00b0C/min. 0.5\u202fg of this material was placed in a quartz glass reactor tube and charged to the reactor, it then underwent pre-treatment by flowing gas mixture of 60\u202fmL/min N2:H2 (BOC, H2 99.998%, N2 99.995%) (1:3) at 700\u202f\u00b0C for 2\u202fh using a ramp rate of 5\u202f\u00b0C/min.\n10% MoPOMSi/\u03b1-Al2O3 and MoPOM/\u03b1-Al2O3\n - 5\u202fg of \u03b1\u2013alumina (\u03b1\u2013Al2O3, Fisher Chemicals, aluminium oxide-calcined) was impregnated with an aqueous solution of precursor (either phosohomolydic acid (H\u202f+\u202fPMo12O40, Sigma-Aldrich USA) or 12-molydosilicic acid (H4Mo12O40Si, Strem Chemicals USA) corresponding to 10\u202fwt.% loading for MoO3) and stirred for 10\u202fmin at room temperature. The material was then dried at 110\u202f\u00b0C overnight and then calcined in air at 450\u202f\u00b0C for 2\u202fh using a ramp rate of 10\u202f\u00b0C/min. 0.5\u202fg of this material was placed in a quartz glass reactor tube and charged to the reactor, it then underwent pre-treatment by flowing gas mixture of 60\u202fml/min N2:H2 (BOC, H2 99.998%, N2 99.995%) (1:3) at 700\u202f\u00b0C for 2\u202fh using a ramp rate of 5\u202f\u00b0C/min. In relation to terminology, \u201cPOMSi\u201d refers to the material prepared using the 12-molybdosilicic acid precursor and \u201cPOM\u201d refers to that derived from phosphomolybdic acid. The corresponding silica and zirconia analogues were prepared analogously using silica (amorphous precipitated, Sigma Aldrich) and monoclinic zirconia (Sigma\u2013Aldrich (United Kingdom) zirconium (IV) oxide, powder, <5\u202f\u03bcm, 99% metal basis) respectively as supports. The same sample abbreviation indicating the precursor is used as for the case of \u03b1\u2013Al2O3.Catalytic activity was evaluated using a fixed bed microreactor operating at ambient pressure. 0.3 \u2013 0.5\u202fg of material was loaded into a quartz reactor tube, held between quartz wool plugs and heated using a Carbolite furnace. Brooks mass flow controllers were used to deliver 60\u202fmL/min of 75\u202fvol % H2/N2 (BOC, 99.98%) reactant gas through the reactor bed at the specified reaction temperatures. Ammonia production was determined by observing the decrease in conductivity of 200\u202fmL of a 0.0018\u202fM solution of H2SO4 which the exit stream of gas flowed through. The rates reported in the present study correspond to steady state reaction conducted over a minimum period of 8\u202fh.\nFig. 1\n presents the mass normalised rates for ammonia synthesis determined at 400\u202f\u00b0C for various materials. All materials were screened under this reaction condition but only a few were found to exhibit activity. It is notable that the K+ promoted Ru/Al2O3 presents the highest activity at 400\u202f\u00b0C, with a pronounced enhancement due to K+ doping being evident. This is consistent with the literature in which Ru is considered to be a close to optimum catalyst [10] and which can be further promoted by the addition of alkali metals [11]. Such promoters, which include K+, are believed to function via donating electron density to the Ru surface, an effect which can be seen indirectly in the infra-red spectra of adsorbed N2 molecules [11]. Ru is also known to be a strongly structure sensitive catalyst for the reaction with activity being related to the B5 step site which gives a pronounced particle size dependence [12], although it has been argued that mixed particle size distribution is most effective with larger particles activating mobile hydrogen species which migrate to small Ru particles promoting NHx hydrogenation in the case of Ru/Al2O3 catalysts [13]. Modification of the support is also known to be of importance in the case of Ru catalysts with MgO [14], BN [15] and electrides [16] being reported to enhance performance. In the current study, we have applied a commercial Ru/Al2O3 reference as a benchmark and as such the material is not optimised. Whilst Ru, which forms the basis of the commercial KAAP catalyst [17], has attracted a lot of interest in the literature due to its high activity the Co-Re material which possesses the next highest activity at 400\u202f\u00b0C in our study has been seldom studied. In fact, to our knowledge, this is the first report of the performance of MgO supported Co-Re. Previous reports of performance have focussed upon bulk CoRe4 systems [18,19]. Active materials were originally prepared by ammonolysis and the suggestion had been made that the presence of Co stabilised an active rhenium nitride phase [18]. Subsequent studies have shown that catalysts of enhanced activity can be prepared by replacement of the ammonolysis step by 3:1 H2:N2 pre-treatment [19]. When the 3:1 H2:N2 pretreatment mixture is replaced by 3:1 Ar:H2 activity develops following a short induction period [19]. Rhenium nitride has been reported to be an active catalyst which decomposes to yield lower activity rhenium metal during the course of reaction [18,20]. In\u2013situ XAS based studies of bulk CoRe catalysts prepared via the H2:N2 and Ar:H2 pretreatment steps shows that the active state of the catalyst is a complex mixture of bimetallic and monometallic species with no definitive evidence for a nitride phase being present [21]. In that study activity development was related to Co-Re mixing. In the context of the current study, attention is drawn to a previous report centring upon the activity of Al2O3 supported rhenium in which promoted with Cs+ is related to the removal of hydrogen inhibition [20]. Indirectly, this may suggest that application of a basic support may enhance performance and this was the basis for the selection of the MgO support. As a benchmark, we have previously reported an ammonia rate of 943 +/- 44\u202f\u03bcmol\u202fh\u22121\u202fg\u22121 at 400\u202f\u00b0C for bulk CoRe pretreated with N2:H2 and run in the same reactor system [19].The fourth most active catalyst under our conditions was formed from SiO2 supported Os3(CO)12. We were interested in inclusion of Os containing systems as historically Os had been identified as an active catalyst. In addition, Os can be found in the same group of the periodic table as Fe and Ru, the two elements on which different commercial ammonia synthesis catalysts are based. Indeed, the activity for ammonia synthesis is found to increase from Fe to Ru and, with Os lying below Ru, it was of interest to further compare Ru and Os despite obvious limitations such as the nature of the precursor etc. Concerns with the application of Os relate to potential toxicity arising from the formation of OsO4 and, historically, element scarcity. To the authors\u2019 knowledge, there have not been very many studies which have reported the performance of Os based systems. One such study, reported a cyclical approach to ammonia synthesis which involved separate pulse sequences of N2 and H2 as a means to obtain high yield at reduced reaction pressure [22]. More recently, a DFT based investigation has been published in which a similar N2 activation barrier over Ru and Os nanoparticles was reported [23] with Ru being the better catalyst due to satisfying the requirements of activation energy, surface vacancy sites and number of step sites for particles of 2\u20134\u202fnm diameter. In the present study, the continuous feed ammonia synthesis activity of the supported Os based system is interesting and can be directly compared to the 78.5 +/- 0.5\u202f\u03bcmol\u202fh\u22121\u202fg\u22121 which we have measured at 400\u202f\u00b0C on bulk osmium powder. Whilst there could be some loss of osmium through volatilsation, decomposition of the supported cluster via an intermediate hydride might occur [24]. The decomposition of supported osmium carbonyl clusters with respect to retention of nuclearity has been controversial [25\u201327] especially since the Os\u2212Os and Os\u2212CO bond strengths are similar. Collier et al. have argued that the cluster structure may be retained in their study which employed extensively dehydroxylated supports [10]. Hence, we have compared hydroxylated and dehydroxylated SiO2 supports (see Figs. 2 and Fig 3 \n) in the current study and observe a relatively small enhancement of rate on the dehydroxylated system at 500\u202f\u00b0C with the hydroxylated SiO2 supported system apparently being more active at 400\u202f\u00b0C. Further investigation would be required to draw firm conclusions concerning the nature of the active phases. However, this preliminary screening exercise suggests that such further studies to both elucidate structure sensitivity and potentially optimise this catalytic system might be useful avenues of further exploration. If nuclearity could be preserved by judicious choice of preparation route, the application of osmium carbonyl precursors provides a potential route to control ensemble size since a wide range of osmium carbonyl cluster sizes are documented [28]. In addition, the potential application of mixed metal osmium cluster precursors provides a potential route to systematic tuning of the activity of dispersed metal particles.The fifth material to display measureable activity at 400\u202f\u00b0C under the reaction conditions employed has been labelled as Ni2Mo3N/SiO2, although we have not established the definite formation of the ternary nitride phase. This phase was targeted in view of the reported high catalytic activity of ternary nitrides [29\u201333]. Bulk Co3Mo3N, particularly when promoted with low levels of Cs+, has been widely recognised to be a very active catalyst for ammonia synthesis which has been variously ascribed to the result of a scaling relationship relating to N2 adsorption enthalpy whereby the combination of Co and Mo yields an enhanced activity material comparable to the performance of Ru [10], or a N-based Mars-van Krevelen mechanism [34,35] possibly involving an associative mechanism [36]. In this case, Ni2Mo3N was targeted rather than Co3Mo3N due to the fact that it can be more easily prepared without an ammonolysis step (ammonolysis was not employed at all in the present study due to concerns of NH3 retention on the various supports and its subsequent release complicating reaction rate analyses) just employing the 3:1 H2:N2 reaction mixture alone [37]. In addition, by application of a Pechini based route, it has been shown that Ni2Mo3N with comparable performance to Co3Mo3N can be prepared [38]. With this in mind, the activity of the supported material is not surprising although additional studies would be necessary to establish its nature.\nFig. 2 presents the mass normalised rates for ammonia synthesis determined at 500\u202f\u00b0C for various materials. It is apparent that a wider range of materials exhibit activity at this temperature than at 400\u202f\u00b0C, although the ammonia synthesis reaction is less thermodynamically favourable with increasing temperature (the thermodynamically limited yields are 0.44% and 0.129% at 1 atmosphere pressure and 400\u202f\u00b0C and 500\u202f\u00b0C respectively). Once again, it can be seen that the Ru based systems are the most active. However, the extent of K+ promotion is lost with respect to the lower temperature. This may correspond to loss of K+ as the reaction temperature is increased due to enhanced mobility, but this would have to be established by elemental analysis. Cs+ is acknowledged to be a better promoter than K+ [11] but we did not explore this as in other studies on different systems we have found it to be highly mobile and easily lost from the catalytic phase at elevated reaction temperature. It can also be seen that the Os3(CO)12 derived catalysts are prominent amongst the higher activity materials (for reference the activity of bulk osmium powder was measured to be 282\u202f\u03bcmol\u202fh\u22121\u202fg\u22121 at 500\u202f\u00b0C in the same reactor set up). An additional observation to be made in the present study is that there is no evidence of promotion by K+ for the Os system. As discussed earlier, it is also possible that there are some support effects amongst these materials (Fig. 3), although when the error bars in relation to activity data are taken into account, the effect seems relatively small overall. This general observation is in marked contrast to the CoRe systems. In order to facilitate comparison, they are presented in Fig. 4\n where a pronounced dependence of activity upon the support identity can be seen. MgO is found to be the best support of those investigated for this system and the origin in this observation may relate to its basic nature as discussed previously. Silica is found to be reasonably good as a support whereas \u03b1-Al2O3 and particularly ZrO2 are found to be much less effective with Re/SiO2 exhibiting higher performance. The origin of these differences is not yet apparent and could relate to particle dispersion and/or mixing effects. In view of the relative performance of the CoRe systems it appears that they are worthy of further attention. To date, as for Os based catalysts, they have not been the subject of extensive investigation. As stated previously, an ammonia synthesis rate of 943 +/- 44\u202f\u03bcmol\u202fh-1\u202fg-1 at 400\u202f\u00b0C has been reported for the bulk material [19]. This material is known to possess very low surface area (< 1 m2\u202fg-1) and so on a surface area normalised basis CoRe is a comparatively highly active which warrants further investigation into supported CoRe systems. Unlike the case for the 400\u202f\u00b0C tests, at 500\u202f\u00b0C the supported Co systems are active. Co is fairly frequently found to be a component of active materials with CoRe being investigated in the present study, and being found to be comparatively active, and the activity of Co3Mo3N being referred to. Within the literature, LaCoSi has been reported to be an effective catalyst [39]. Indeed when a comparison was made between CoMo/CeO2, which is believed to form the supported active Co3Mo3N phase, and Co/CeO2 at 400\u202f\u00b0C and 0.9\u202fM\u202fP, the Co/CeO2 is initially observed to be significantly more active (4\u202fmmol h-1\u202fg-1 versus < 3\u202fmmol h-1\u202fg-1) although with time on stream the CoMo system maintains performance and the Co system significantly deactivates over the first 100\u202fh on stream to ca, 2\u202fmmol h-1 \u202fg-1 [40]. The application of Co/CeO2 has been discussed in terms of low-pressure ammonia synthesis and issues relating to deactivation via sintering of Co nanoparticles has been detailed in the literature very recently [41]. Incorporation of dopamine into the synthesis procedure and the associated removal of the resultant carbon layers has been reported to enhance the activity of a Co/CeO2 catalyst from 3.81\u202fmmol h-1\u202fg-1 to 19.12\u202fmmol h-1\u202fg-1 at 425\u202f\u00b0C and 1\u202fMPa with stability being maintained for at least 50\u202fh on stream [41]. This enhancement has been attributed to smaller resultant Co crystallite size, enhanced metal-support interaction and lowered N2 activation energy. In the present study, the pH of the impregnating solution is 3 which is anticipated to be significantly below the point of zero charge of the supports [42]. This will result in a net surface charge and, given that the impregnating solution comprises [Co(H2O)6]2+, it can be anticipated that the Co dispersion would be poor leaving room for potential further optimisation.The final sub-set of materials to compare, relate to those comprising molybdenum. Molybdenum oxide precursors are known to nitride under 3:1 H2:N2 under the pretreatment conditions employed within this study [43]. MoO3 can be nitrided to produce the \u03b2-Mo2N0.78 phase which, as reported elsewhere is active for NH3 synthesis (a rate of 35\u202f\u03bcmol\u202fh\u22121\u202fg\u22121 at 400\u202f\u00b0C and ambient pressure using a 3:1 H2:N2 reaction mixture has been reported [43]). \u03b3-Mo2N prepared by ammonolysis of the same precursor reportedly exhibits a very similar are which is insensitive to morphology (the pseudomorphic nature of ammonolysis can be used to good effect here with MoO3 precursors of different morphology) [43], although structure sensitivity for ammonia synthesis at ambient pressure and 400\u202f\u00b0C has been reported for Mo2N where site time yield ratios of 40:25:1 have been reported for 63, 13 and 3\u202fnm diameter particles respectively [44]. In the present study polyoxometallates have been explored as potential precursors to highly dispersed MoNx phases for which controlled dopant levels (as achieved by the heteroatom) and size and composition could be achieved. Our initial studies have concentrated upon employing phosphomolybdic acid and silicomolybdic acid as Keggin structured precursors containing controlled levels of P and Si \u201cdopant\u201d respectively. As can be seen in Fig. 5\n, there is very limited influence of both MoNx precursor and also dopant (in terms of the latter point, the activities are comparable to that of the Mo2N0.78/SiO2 sample, which employs ammonium heptamolybdate as precursor). The composition Mo2N0.78 has not been directly verified and is assumed based upon the anticipated binary molybdenum nitride phase which would result from the nitridation conditions employed within the current study [43]. The significant promotional effect of the inclusion of Ni, and suggested formation of the supported ternary nitride, is readily apparent in this figure as was discussed previously. In this context, it is important to establish that supported Ni is not expected to exhibit ammonia synthesis activity. In terms of benchmarking, the ammonia production rate measured with Co3Mo3N under comparable reaction conditions applying the same reactor is 489 +/- 17\u202f\u03bcmol\u202fh\u22121\u202fg\u22121 [45].In this manuscript, we have undertaken an empirical screening of a wide range of supported ammonia synthesis catalysts. The systems selected have been based on the known previous activity of component phases. Whilst, as might have been expected in terms of the literature, the Ru based systems we have studied were observed to exhibit the highest activity, there are a number of other potentially interesting observations. The activity of osmium based systems seems worthy of further investigation, perhaps employing carbonyl cluster precursors to control ensemble size and composition associated with preparation methods developed to retain cluster nuclearity. Supported CoRe systems, which demonstrate pronounced dependence upon the identity of the support, are also interesting candidates for further investigation, as are supported ternary nitrides.We wish to acknowledge funding in the area of ammonia synthesis from the Engineering and Physical Sciences Research Council through grant EP/L02537X/1.", "descript": "\n                  The present study presents an empirical screening study of the catalytic performance of a variety of supported materials for ammonia synthesis at 400 and 500\u202f\u00b0C. Amongst the materials tested, those derived from Ru/Al2O3 exhibited the best performance. Supported Os and CoRe catalysts also demonstrated comparatively high activities indicating them to be potentially worthy of further investigation.\n               "}
{"full_text": "Nowadays, hydrogen is used in several industrial applications. Primally it is an essential material required in a variety of chemical processes such as the oil refining, the production of ammonia and methanol, as well as the synthesis of many polymers. Additionally, hydrogen is also used in other industrial sectors, namely: glass and electronic production, metallurgy and also food industry [1,2]. Because of its properties, hydrogen is also recognized as one of the most important energy sources, that can be an alternative to the carbon-based fuels. Presently, most of the hydrogen is produced from fossil fuels, which is responsible for the emission of significant amounts of CO2 to the atmosphere [2\u20134]. Therefore, it is so important to develop alternative methods of hydrogen production. Among a variety of different processes, water electrolysis is considered as an environment-friendly method. If the electrical energy needed to carry out the electrolysis, originate from the renewable sources (wind or solar energy), the process meets the \u201czero-emission\u201d criteria. In this case the obtained hydrogen is the cleanest energy carrier, which can be used to store the excess of the electricity. Moreover, water electrolysis allows to produce the high purity H2.Three types of electrolysis technologies are currently commercially available namely: alkaline water electrolysis (AWE), proton exchange membrane (PEM) and solid oxide electrolysis cells (SOEC) [2,4]. Since the alkaline technique was developed first, it is considered as one of the most mature methods of hydrogen production. However, the high energy consumption and the low energy conversion efficiency results in the high production costs of the alkaline water electrolysis. Because of that, the contribution of AWE and other electrolysis techniques to the total H2 production technologies, remains only at the level of about 5% [4]. Developing of new electrode materials, characterized by low overpotential for the hydrogen evolution reaction (HER) can allow to decrease the production costs and improve the cost-effectiveness of the alkaline process. That is why, currently, a lot of research concerns to investigate the catalytic activity for the HER of various materials [5\u201312].It is well known that Pt and the Pt-based materials are characterized by the superior catalytic activity for the HER. However, their utilization in the large-scale production is impossible because of their high costs and scarcity. In the literature one can find many reports on the electrocatalytic activity of the less noble metals e.g. Ni, Co, Fe. It was found that materials composed of the mixture of two or more metallic components possess better catalytic properties compared to the electrodes made of pure metals. Therefore, many scientific reports concern the investigation of the HER performance on the various binary and ternary alloys cathodes, as for example: Ni\u2013Co, Ni\u2013Fe, Ni\u2013Mo, Co\u2013Mo, Ni\u2013W, Ni\u2013Mo\u2013Cu, Ni\u2013Co\u2013Cu, Co\u2013Ni\u2013Mo and Ni\u2013Mo\u2013W [13\u201319]. The improvement in the electrocatalytic performance of the alloy electrodes may be caused by both: the increase of the real surface area and the increase of the intrinsic activity of the alloy material. Another way to enhance the electrocatalytic activity of the non-noble metal electrodes is to introduce solid particles into the metal matrix. The literature data confirm enhanced electrocatalytic performance of different composite material e.g.Co\u2013W/CeO2, Ni\u2013CeO2, Ni\u2013W/TiOx, Co/Ni\u2013MoO2, and Co\u2013Ni-graphene [20\u201329].Ni and Ni-based materials are widely used as cathodes in the alkaline water electrolysis. This is a consequence of their high corrosion resistance in the concentrated alkaline media [13,30]. Due to low hydrogen evolution overpotential [31], molybdenum is the willingly used alloy component of electrode materials for the HER.The aim of the present paper was to investigate the electrocatalytic properties for the HER of Ni\u2013Mo/WC composite coatings. It was demonstrated that tungsten carbide is characterized by the \u201cplatinum-like\u201d catalytic properties. This phenomenon is explained as the result of the modification of the tungsten lattice by carbon, in the way that the surface electronic properties of WC resemble those of Pt [32,33]. Therefore, materials containing WC are consider as potential alternatives to Pt for the HER processes [29,32\u201334]. In our research the influence of the addition of WC nanoparticles into the Ni\u2013Mo alloy matrix on the electrocatalytic performance for the HER in alkaline media was thoroughly investigated. The Ni\u2013Mo/WC composites were obtained by the electrochemical deposition. This method is a convenient technique that allows to prepare the nanocrystalline alloys and gives the possibility to regulate their composition by fixing a proper electrolyte composition and process parameters. The structure, composition and surface morphology of the studied coatings were investigated using XRD, SEM and EDS methods. XPS technique was used to examine the surface composition of the Ni\u2013Mo/WC composites. The electrocatalytic activity for the HER was evaluated based on the cathodic polarization measurements and the electrochemical impedance spectroscopy (EIS). The cyclic voltammetry (CV) and chronopotentiometry (CP) technique was used to determine the long-term behaviour and the stability of the obtained composite coatings during the HER process.Ni\u2013Mo/WC composite coatings with different Mo and WC content were electrodeposited from the solutions with the composition presented in Table 1\n. The value of the electrolyte pH was adjusted to 4.5 with the solution of H2SO4. WC with the average grain size of 150\u2013200\u00a0nm was obtained from Sigma-Aldrich. The amount of WC nanopowder that was added to the plating solution was fixed at 2.7\u00a0g\u00a0dm\u22123. Such concentration of the nanopowder allowed to obtain composite coatings with significant WC content. Higher concentrations of WC nanoparticles resulted in their agglomeration and accumulation of the nanopowder at the vessel bottom. In order to ensure the homogenous dispersion of the solid particles in the electrolyte, the ultrasonic treatment (30\u00a0kHz, 50\u00a0W) and mechanical agitation (500\u00a0rpm) was used for 1\u00a0h before starting and during the electrodeposition process. The studied coatings were obtained in the galvanostatic regime at a current density of 4.5\u00a0A\u00a0dm2. The electrodeposition was carried out at a temperature of 25\u00a0\u00b0C. The coatings were deposited on carbon steel disc electrodes with the surface area of 1.7\u00a0cm2. The Pt plate was used as an anode. During the deposition process the electrodes were placed horizontally in the vessel, parallel to each other. Before the deposition process, the cathode surface was mechanically polished with successive grades of abrasive paper and finally with a diamond paste. Then the samples were degreased with acetone, rinsed with doubly distillate water and dried. The deposition time was set at 30\u00a0min.In the investigation of the electrocatalytic performance for the HER of the Ni\u2013Mo/WC composites, Ni\u2013Mo alloy coating was a reference sample. The Ni\u2013Mo deposit was obtained under similar conditions as the composite coatings, from the solution without the addition of WC nanopowder.Scanning electron microscopy (SEM) method (Quanta 3D 200i Microscope and FEI Helios G4 PFIB CXe DualBeam Microscope) was used to investigate the surface morphology of the studied composite and alloy coatings. The composition of the obtained deposits was analyzed by the Oxford Energy Dispersive X-ray Spectrometer (EDS) coupled to the scanning microscope. The element distribution map was obtained using Bruker XFlash 630 EDS Detector. Coatings cross-sections were prepared using Helios G4 PFIB CXe DualBeam Microscope. The place of investigation was protected with thin Pt layer obtained by focused ion beam induced deposition. By ion milling the material in front of the protective layer was removed and then the obtained cross-sections were polished with ion beam.X-ray diffraction technique was adopted to examine the structure of the investigated coatings. Siemens D5000 diffractometer with Cu K\u03b11 radiation (\u03bb\u00a0=\u00a00.15406\u00a0nm) was applied. The measurements were performed in the 2\u03b8 range of 20\u2013100\u00b0 with a step of 0.02.X-ray photoelectron spectroscopy (XPS) studies were carried out using a SPECS PHOIBOS-100 hemispherical spectrometer equipped with a Mg source (1253.6\u00a0eV) operating at 250\u00a0W for high resolution spectra. The analyzed area of the sample was approx. 1\u00a0cm in diameter. The surfaces of catalyst samples were analyzed in the \u201cas received\u201d form and after gently Ar+ ion beam sputtering (1\u00a0keV, 1.3\u00a0\u03bcA/cm2). Spectra were processed and fitted by SPECLAB 2 and CasaXPS ver. 2.19 software using Gaussian-Lorentzian curve profile and Shirley baseline. The C 1s peak at 284.8\u00a0eV was used as the reference.The electrocatalytic properties for the HER of the Ni\u2013Mo/WC and Ni\u2013Mo coatings were investigated in 1\u00a0M KOH solution at a temperature of 25\u00a0\u00b0C. The measurements were performed with the Gamry Reference 600 potentiostat. A standard three-electrode cell configuration with Pt counter electrode and Ag/AgCl reference electrode was applied. During the electrochemical tests, the Pt counter electrode was separated from the rest of the system by the Nafion\u2122 117 membrane. The measured values of the potential were converted to a reversible hydrogen electrode (RHE) using the following formula: E (V vs RHE)\u00a0=\u00a0E (V vs Ag/AgCl)\u00a0+\u00a01.023\u00a0V.The potentiodynamic polarization test were performed at a scan rate of 1\u00a0mV\u00a0s\u22121. The curves were collected in the potential range of \u22120.58\u00a0V vs RHE up to the open circuit potential. The obtained data were corrected on the IR ohmic drop. The solution resistance values were determined based on the EIS results. Before the experiment, each sample was held for 10\u00a0min in 1\u00a0M KOH solution at a potential of \u22120.58\u00a0V vs RHE. This was to reduce the metal oxides that might formed on the metal surface after the deposition process.EIS measurements were conducted in the frequency range of 10\u00a0mHz to 10\u00a0kHz. A sinusoidal signal of 5\u00a0mV amplitude was used. The experiments were performed at a selected cathodic overpotential. Before the measurement, each sample was held for 10\u00a0min in 1\u00a0M KOH solution at a potential of \u22120.58\u00a0V vs RHE. The obtained data were analyzed with Gamry Echem Analyst software.CV curves were collected in a potential range of 0.12 to \u22120.58\u00a0V vs RHE. The measurements were performed for 100 cycles, at a scan rate of 50\u00a0mV\u00a0s\u22121.Chronopotentiometry measurements were conducted at a constant current density value of 100\u00a0mA\u00a0cm\u22122. The CP curves were recorded for 60\u00a0h.\nFig.\u00a01\n presents the SEM images of the studied Ni\u2013Mo/WC and Ni\u2013Mo coatings. The surface of Ni\u2013Mo alloy coating obtained from the solution without an addition of WC nanoparticles, is compact and homogenous. The coating is characterized by globular and cauliflower-like structure (Fig.\u00a01a). According to EDS analysis, the alloy deposit contained about 22,8\u00a0wt% of Mo (Table 2\n). The Ni-Mo-WC composite coatings obtained from solution containing 2.7\u00a0g\u00a0dm\u22123 of WC nanopowder, also show globular structure, however they are less compact and regular in comparison to the alloy deposit. The surface of the composite coatings with higher WC content (Fig.\u00a01b and c) is rough and highly developed. Ni\u2013Mo/WC 3 deposit obtained from the solution with the highest molybdate concentration is characterized by smother surface, however globular grains are also visible (Fig.\u00a01d). Fig.\u00a02\n presents the distribution of the individual elements (Ni, Mo and W) on the surface of Ni\u2013Mo/WC 2 and Ni-Mo-WC 3 composites. For both coatings the Ni and Mo atoms are rather uniformly distributed on the composite surface. However, it can be observed, especially on the surface of Ni\u2013Mo/WC 2 coating with higher WC content, that the intense signal originating from W atoms is followed by more intense signal of Mo and less intense signal of Ni. This phenomenon is probably the result of the adsorption of molybdate ions on the surface of the WC nanopoarticles in the plating solution.The obtained EDS results reveal that the concentration of MoO4\n2\u2212 ions in the plating solution affects not only the amount of codeposited Mo, but also the amounts of WC nanoparticles incorporated into the coating. As one might expect, an increase in the amount of the molybdate ions in the solution led to a rise in Mo content in the composite coating from 7.1\u00a0wt% for MoO4\n2\u2212 concentration of 0.009\u00a0mol\u00a0dm\u22123 to 27.7\u00a0wt% for MoO4\n2\u2212 concentration of 0.025\u00a0mol\u00a0dm\u22123 (Table 2). On the other hand, higher amounts of molybdate ions in the plating solution resulted in lower WC content in the composite coating. According to EDS analysis, the W content amounted to 21.2 and 8.7\u00a0wt% for the coating deposited from the solution with the lowest (0.009\u00a0mol\u00a0dm\u22123) and highest (0.025\u00a0mol\u00a0dm\u22123) MoO4\n2\u2212 concentration, respectively (Table 2). A negative influence of high molybdate concentration on the incorporation of nanoparticles into the metallic matrix, was observed also in our earlier research [35]. This phenomenon may be caused by a decrease in the surface positive charge of WC nanoparticles due to the adsorption of MoO4\n2\u2212 ions on their surface.The Mo/Ni ratio calculated for the Ni\u2013Mo/WC 3 composite coating and Ni\u2013Mo alloy deposit obtained from solution with the same molybdate concentration (0.025\u00a0mol\u00a0dm\u22123) was 0.38 and 0.29 respectively. This indicates, that the Mo codeposition was facilitated by the presence of WC nanoparticles in the electrolyte.The local cross-sections of the studied coatings were presented in Fig.\u00a03\n. Despite the same deposition time (30\u00a0min) a significant differences in the coatings thickness was observed. These changes are related to the Mo content in the Ni\u2013Mo matrix. An increase in the amount of codeposited Mo resulted in the smaller coating thickness due to lower current efficiency of the deposition process. A similar effect was observed in our earlier research of Ni\u2013Mo\u2013ZrO2 composite coatings [35]. The cross-section obtained for Ni\u2013Mo alloy containing ~22.8\u00a0wt% Mo (Fig.\u00a03a) revealed a compact and homogenous structure. The coating thickness was about 7\u00a0\u03bcm. The thickness of the composites with lower Mo content was grater - about 17\u00a0\u03bcm for Ni\u2013Mo/WC 1 (~7.1\u00a0wt% Mo) and 13\u00a0\u03bcm for Ni\u2013Mo/WC 2 (15.4\u00a0wt% Mo). Moreover, the cross-sections of Ni\u2013Mo/WC 1 and Ni\u2013Mo/WC 2 composites containing high amounts of incorporated WC nanoparticles, demonstrate less compact structure with empty gaps occurring in the coatings volume. Fig.\u00a04\n shows the distribution map of the individual elements in the cross-section of Ni\u2013Mo/WC 2 composite.It can be seen, that WC nanoparticles are distributed in the whole coating volume, however they also tend to accumulate into bigger aggregates. The cross-section obtained for Ni\u2013Mo/WC 3 composite revealed a compact, however, significantly thinner coating - thickness of ~1\u00a0\u03bcm. The low thickness is related to high Mo content (~27.7\u00a0wt %) in the Ni\u2013Mo matrix. According to the literature, the first stage of Mo deposition is an incomplete reduction of molybdate ions to Mo oxides of lower valence, which are characterized by low overpotential for the hydrogen evolution reaction [36]. Therefore, their presence on the cathode surface results in the intensification of the hydrogen evolution during the deposition process and causes the decrease of the current efficiency. Moreover, intensification of the hydrogen evolution process leads to formation of higher amounts of gas bubbles on the cathode surface, which hinder the adsorption and reduction of metal ions.XRD patterns of the studied coatings are presented in Fig.\u00a05\n. In the pattern obtained for the Ni\u2013Mo alloy coating one broad peak at the 2\u03b8 angle of about 42\u00b0 is visible. The peak corresponds to the (111) plane of nickel FCC structure (ICDD no. 01-071-4654). However, its maximum is shifted to the lower 2\u03b8 value in comparison to the pure nickel pattern. This phenomenon is characteristic for the formation of Ni\u2013Mo solid solution [37]. The small peak at the 2\u03b8 of about 82.4\u00b0 originating from the Fe phase of the steel substrate is also visible in the Ni\u2013Mo pattern. In the diffractograms of Ni\u2013Mo/WC composite coatings, the peaks corresponding to Ni\u2013Mo solid solution are broadened and their intensity decreased. This indicates smaller size of crystallites of composite coatings. A decrease in the crystallite size due to the incorporation of solid particle into the metal matrix was observed also by other researchers. This phenomenon was explained as the result of inhibition of the crystals growth and enhancing of crystals nucleation by solid particles adsorbed on the metal matrix during the deposition process [38,39]. For the studied Ni\u2013Mo/WC composites the crystalline size is determined by two factors, namely the amount of incorporated WC nanoparticles and Mo content in the Ni\u2013Mo matrix. It has been demonstrated in the literature, that an increase in the Mo content results in the crystallite size refinement of Ni\u2013Mo alloy [37,40]. In the XRD pattern of Ni\u2013Mo/WC 3 coating, low intensity of the peak corresponding to Ni\u2013Mo solid solution and high intensity of the peak originating from the Fe phase of the steel substrate (at the 2\u03b8 of about 44.7 and 82.4\u00b0), is also the consequence of low deposit thickness (~1\u00a0\u03bcm). In the diffractograms recorded for Ni\u2013Mo/WC composites additional peaks corresponding to WC phase (ICDD no. 00-025-1047) also appeared.XPS method was used to analyse the surface composition of the studied composite and alloy coatings. Table 3\n presents contents of the individual elements on the as-received surface (without Ar+ ions sputtering) of the Ni\u2013Mo/WC and Ni\u2013Mo coatings. The as-received surface of all coatings was covered with the layer of carbon adsorbed from the air. This contamination carbon appeared in the most outer surface layer with the thickness less than 1\u00a0nm. As it can be seen in Table 3, the oxygen content is significantly higher for the composite coatings than for the Ni\u2013Mo alloy. This indicates, that the Ni\u2013Mo/WC coatings were characterized by more oxidized surface in comparison to Ni\u2013Mo deposit. The oxygen content in the surface layer increased with the rise of molybdate concentration in the plating solution. The surface of Ni\u2013Mo/WC 3 composite contained the highest oxygen content (38.1\u00a0at. %). For Ni\u2013Mo alloy coating the Mo/Ni ratios calculated based on XPS and EDS results are the same (around 0.3). In case of composite coatings Mo/Ni ratios calculated based on XPS results are higher than the ratios resulting from the EDS analysis. This shows that the surface of Ni\u2013Mo/WC composites was enriched in Mo in comparison to the coating bulk.The percentage shares of individual forms of the elements in the studied coatings was evaluated based on the deconvolution of Ni 2p, Mo 3d and W 4f spectra. Fig.\u00a06\n presents the Ni 2p3/2/spectra obtained for the Ni\u2013Mo/WC and Ni\u2013Mo coatings. The spectrum of metallic nickel can be fitted using three peaks: the main asymmetric peak at 852.6\u00a0eV and two broader satellites peaks at about 3.7 and 6.1\u00a0eV above the main contribution [41\u201343]. For a better fit, in our work, the surface of Ni\u2013Mo coating after Ar+ sputtering was used as a reference spectrum of metallic Ni. In this case the maximum of the main peak was located at 852.3\u00a0eV and the maxima of the satellite peaks were shifted by 3.7 and 6.5\u00a0eV above (Fig.\u00a06a). Based on our earlier research [35,44], we expected that besides metallic Ni also Ni(OH)2 would appear on the surface of the studied coatings. Therefore the part of the spectrum above the binding energy of 854\u00a0eV was analyzed assuming the presence of Ni(OH)2. A two-component deconvolution procedure was used to fit the Ni 2p3/2 core level spectra with Ni(OH)2 envelope. The major peak attributed to Ni(OH)2 component was found at the binding energy of 859.9\u00a0eV and a satellite peak appeared 5.8\u00a0eV above. This results agreed well with the data presented in the literature [45]. The obtained data revealed that the surface layer of the as-deposited Ni\u2013Mo alloy coating contained mainly metallic Ni (90.8\u00a0at.% of the total Ni) (Table 4\n). Different situation was observed for Ni\u2013Mo/WC composites. In case of Ni\u2013Mo/WC 2 and Ni\u2013Mo/WC 3 deposit, the oxidized Ni was the main component of the coatings surface layer. An increase in the molybdate concentration in the plating solution resulted in higher Ni(OH)2 contents on the composite surface. For the as-deposited Ni-Mo-WC 3 coating 90.4% of the total Ni were oxidized in the surface layer.Fitted high-resolution spectra recorded for Mo 3d regions were presented together with Shirley background line in Fig.\u00a07\n. The spectra were deconvoluted assuming the presence of the following components: Mo(0), MoO2, Mo2O5 and MoO3. Deconvolution was performed using the procedure presented by Baltrusaitis et\u00a0al. [46]. And applied already in our previous research [35]. The binding energies of 227.45, 229.30, 230.60 and 232.2\u00a0eV were ascribed to the Mo(0), Mo(IV), Mo(V) and Mo(VI), respectively. Similarly to Ni(0), for a better fit, the surface of Ni\u2013Mo coating etched with Ar+ ions was used as a reference spectrum of metallic Mo (Fig.\u00a07a). The XPS data obtained based on Mo 3\u00a0d spectra revealed similar relationship as for those observed for nickel. On the as-deposited surface of Ni\u2013Mo alloy coating the share of metallic Mo amounted to 73.5%. The content of metallic Mo on the as-deposited surface of the composite coatings was lower and it decreased with an increase in the molybdate concentration in the plating solution \u2013 from 53.1% for the Ni\u2013Mo/WC 1\u20135.4% for the Ni\u2013Mo/WC 3. Mo2O5 was the main oxidized form of Mo in the surface layer of the studied coatings. The exception was the Ni\u2013Mo/WC 1 composite, for which MoO3 was the main form of the oxidized Mo.\nFig.\u00a08\na shows the W 4f spectra obtained for the WC nanopowder. Since, according to the literature data, WC can oxidize in the air [47] the spectrum was deconvoluted into three components: WC (32.0 and 34.0\u00a0eV), WO2 (33.0 and 35.0\u00a0eV) and WO3 (35.91 and 38.11\u00a0eV). The spectra recorded for the Ni\u2013Mo/WC coatings were presented in Fig.\u00a08b \u2013 d. It can be seen from Fig.\u00a08 that for the composite coatings, the maximum of the highest peak appears at lower values of binding energy than in case of WC nanopowder. In Fig.\u00a08e a reference W 4f spectrum fitted with consideration of metallic W, WC and W oxide was presented [48]. Comparing the reference spectrum with the spectra recorded for the studied composites, it might be concluded that metallic W was present on the surface of Ni\u2013Mo/WC coatings. This indicates that during the deposition process, some of W atoms was reduced to metallic tungsten. This assumption seems to be reasonable, since during the deposition process reducing conditions occur at the cathode surface. Tran et\u00a0al. [49], in their research also observed the reduction of W atoms to metallic tungsten in Mo\u2013W bimetallic carbide. The highest percentage share of metallic W was calculated for the Ni\u2013Mo/WC 1 composite (93.5\u00a0at.%) (Table 4). Ni\u2013Mo/WC coatings obtained from the solutions with higher molybdate concentration were characterized by lower contents of W0 in the surface layer (76.9 and 74.1\u00a0at. % for the Ni\u2013Mo/WC 2 and Ni\u2013Mo/WC 3 deposit, respectively). Opposite relationship was observed for the oxidized W. Similarly as for Ni and Mo, the share of the Wox rises with an increase in the molybdate concentration in the plating solution.The obtained XPS results indicates that the incorporation of WC nanoparticles significantly influences the surface of the Ni\u2013Mo alloy coating. The surface of Ni\u2013Mo/WC composites is more oxidized in comparison to Ni\u2013Mo deposit. This effect becomes particularly apparent for the composites with high Mo content in the Ni\u2013Mo matrix.It was proposed in the literature [50\u201353] that the hydrogen evolution process in alkaline media proceed through the following reactions:\n\n(1)\nM\u00a0+\u00a0H2O\u00a0+\u00a0e\u2212 \u2192 MHads\u00a0+\u00a0OH\u2212\n\n\n\n\n\n(2)\nMHads\u00a0+\u00a0H2O\u00a0+\u00a0e\u2212 \u2192 H2\u00a0+\u00a0M\u00a0+\u00a0OH\u2212\n\n\n\n\n\n(3)\n2MHads \u2192 2M\u00a0+\u00a0H2\n\n\n\nIn the presented mechanism the proton discharge electrosorption - Volmer reaction (1) is followed by electrodesorption - Heyrovsky reaction (2) and/or H recombination - Tafel reaction (3). The value of Tafel slope (bc) is determined by the rate-limiting reaction, so it can provide some insight into the mechanism of the hydrogen evolution process. According to the general model of the HER mechanism, the Volmer reaction control the process speed if bc is around 120\u00a0mV dec\u22121. The Heyrovsky or Tafel step determines the HER rate if Tafel slope is 30 or 40\u00a0mV dec\u22121, respectively.The potentiodynamic curves recorded for the studied Ni\u2013Mo/WC and Ni\u2013Mo coatings are shown in Fig.\u00a09\n. The kinetics parameters determined from the obtained data are presented in Table 5\n. In case of the composite with the lower Mo content (Ni\u2013Mo/WC 1) and the Ni\u2013Mo alloy, two regions with different slopes can be distinguished on the obtained curves. In both cases, in the lower potential range (region 1 in Fig.\u00a09) the higher values of bc were calculated. This phenomenon is most probably caused by the oxide layer on the coatings surface. Other authors also observed the influence of the surface oxide layer on the hydrogen evolution process on metal electrodes [54\u201356]. XPS results confirmed the presence of oxides on the surface of all studied coatings. However, Ni\u2013Mo alloy and Ni\u2013Mo/WC 1 composite were characterized by lower thickness of the oxide film (lower oxide content \u2013 Table 3) in comparison to other Ni\u2013Mo/WC coatings. It is possible that thinner oxide layer, especially in case of Ni\u2013Mo alloy was also more compact and could inhibit the electron transfer in the low potential range. This resulted in very high value (590\u00a0mV dec\u22121) of Tafel slope determined for Ni\u2013Mo alloy for the region 1 of the polarization curve. For the region 2 the bc value was 130\u00a0mV dec\u22121, which suggest that Volmer reaction determined the hydrogen evolution rate on the Ni\u2013Mo alloy in the higher potential range. In case of Ni\u2013Mo/WC 1 coating, the determined value of Tafel slope was 208 and 104\u00a0mV dec\u22121 for the region 1 and 2 (Fig.\u00a09), respectively. The lower value of bc in the low potential range in comparison to Ni\u2013Mo alloy may indicate the less compact oxide layer on the surface of the composite coating. The value of Tafel slope obtained for the region 2 indicates that, similarly to the alloy coating, the Volmer reaction determined the rate of the hydrogen evolution on the Ni\u2013Mo/WC 1 composite in the higher potential range. Different behaviour was observed for the composites with higher Mo content. The polarization curves recorded for Ni\u2013Mo/WC 2 and Ni\u2013Mo/WC 3 coatings were characterized by one value of Tafel slope in the whole potential range. According to XPS data the surface oxide layer of these composites was characterized by greater thickness and different composition in comparison to Ni\u2013Mo and Ni\u2013Mo/WC 1 coatings. In this case, the region with significant limitations of the electron transfer for low potentials was not observed. The calculated Tafel slopes are 153\u00a0mV dec\u22121 for Ni\u2013Mo/WC 2 and 163\u00a0mV dec\u22121 for Ni\u2013Mo/WC 3. These values are greater than the expected value of ~120\u00a0mV dec\u22121. Similar results were also observed for other composite materials and explained by the presence of the outer oxide film, which could affect the charge transfer on the electrode surface [22,57,58]. Considering the XPS results, which confirmed the high oxygen content on the surface of Ni\u2013Mo/WC 2 and Ni\u2013Mo/WC 3 coatings, this explanation seems reasonable. The results presented in Fig.\u00a09 reveal different catalytic activity for the hydrogen evolution reaction of the studied composite and alloy coatings. Additionally the obtained results indicate that the presence of the surface oxide layer (with different thickness and composition for different coatings) significantly affects the electron transfer process on the electrode surface/solution interface influencing at the same time the course of the recorded potentiodynamic curves.The exchange current density (i0) is the parameter that provides the information about the catalytic activity of the electrode material. It can be seen from Table 5 that the Ni\u2013Mo/WC composite coatings are characterized by significantly higher values of i0 in comparison to Ni\u2013Mo alloy deposit. This confirms the beneficial effect of the WC particles on the catalytic activity of Ni\u2013Mo alloy deposit. Moreover, it can be observed that the determined i0 value increased with the rise in the Mo content in the composite coating. The highest value of the exchange current density was calculated for Ni\u2013Mo/WC 3 composite indicating its superior catalytic activity to the other studied coatings. In order to hydrogen evolution proceed with a measurable rate a certain overpotential is required. Therefore, a good method of comparing the electrocatalytic activity of different materials is to determine the overpotential needed to achieve a fixed value of current density (i.e. a hydrogen production rate). This gives an information about the amount of the energy required to produce a fixed amount of hydrogen for each catalysts. In Table 5, for each studied coating, the overpotential needed to achieve the current density of 10\u00a0mA\u00a0cm2 (\u03b710) was presented. The \u03b710 values reveal the similar trend in the electrocatalytic activity as the trend observed based on the j0 values. The required overpotentials were lower for Ni\u2013Mo/WC composites than for Ni\u2013Mo alloy coating. Ni\u2013Mo/WC 3 electrode was characterized by the lowest value of \u03b710, which confirms its highest catalytic activity for the HER among the studied coatings.In order to further investigate the hydrogen evolution process on the studied coatings and evaluate their electrocatalytic activity, the electrochemical impedance technique was applied. For each coating the EIS spectra were recorded at three different values of cathodic overpotential, which corresponded to different hydrogen evolution rates (current densities range from ~5 to\u00a0~\u00a020\u00a0mA\u00a0cm\u22122). In order to receive a physical picture of the processes occurring at the electrode/solution interference the obtained EDS must be fitted with a proper model. The literature data reveal that three different equivalent circuits have been mostly proposed to model the processes that occur during the HER [54,55,59\u201364]. The one time constant model (1\u00a0TC) presented in Fig.\u00a010\na describes the hydrogen evolution process when the response of hydrogen adsorption is not manifested on the spectra. In this model, R\n\ns\n is the solution resistance, R\n\n1\n is the charge transfer resistance and CPE\n\n1\n is the parameter associated with the double layer capacitance. If the system is characterized by two time constants, the two time constant parallel model (2TCP) or the two time constant serial model (2TCS) model have been used to describe the HER. The 2TCP model (Fig.\u00a010b) reflects the response of a system in which both time constants are related to the HER kinetics. It is assumed that the high frequency (HF) time constant \u03c41 (CPE1 \u2013 R1) corresponds to the HER charge transfer kinetics, while the low frequency (LF) time constant \u03c42 (CPE2 \u2013 R2) is related to the hydrogen adsorption. Both time constants depend on the applied overpotential. In the 2TCS model (Fig.\u00a010c) only the LF time constant is related to the HER kinetics and it change with overpotential. The HF time constant corresponds to the surface porosity and it is potential independent.In our research, due to ensure the more accurate fit of the collected data, in all adopted models the constant phase element (CPE) was used instead of the capacitor. The use of the CPE is often required because of the distribution of the relaxation times as a result of the surface inhomogeneities such as the surface roughness and porosity or when the adsorption or diffusion takes place [45]. The impedance of the CPE is defined as:\n\n(4)\n\nZ\n\nCPE\n = Y\n\n0\n\n\n\u22121\n\n(j\u03c9)\n\n-n\n\n\n\nwhere Y\n\n0\n is a time constant parameter (\u03a9\u22121 s-n cm\u22122), \u03c9 is the angular frequency of the AC signal and n is the CPE exponent.\nFig.\u00a011\n presents the EIS spectra collected for the studied coatings. The one time constant respond was recorded only for Ni\u2013Mo/WC 1 composite for the data collected at the overpotential of \u2212336 and \u2212386 mV (Fig.\u00a011a). In this case the data were fitted with the model presented in Fig.\u00a010a. The other recorded spectra were characterized by the presence of two time constants. Since applying the 2TCP and the 2TCS models resulted in almost identical goodness of fit expressed by the Chi-squared value (\u03c72), deciding which model is correct required a careful analysis of the obtained data.The experimental data recorded for Ni\u2013W/WC 1 composite at the lowest overpotential (\u2212286\u00a0mV) were fitted with the 2TCP model. The same model was also apply for the all EIS data collected for Ni\u2013Mo/WC 2 coating (Fig.\u00a011b). The resulting EIS parameters are presented in Table 6\n. It can be seen, that in case of data fitted with the 2TCP model, the value of Y\n\n01\n parameter is relatively constant and the values of R\n\n1\n decrease with an increase in the overpotential. This is a characteristic behaviour of the time constant related to the HER charge transfer kinetics. Moreover, the Y\n\n02\n values are higher than Y\n\n01\n and they increase with overpotential. At the same time, the values of R\n\n2\n decrease with an increase in the overpotential. This observations confirms that the low frequency time constant (CPE\n\n2\n\n\u2013 R\n\n2\n) is related to the response of the hydrogen adsorbed on the electrode surface [54,55].Different behaviour was observed for the Ni\u2013Mo/WC 3 composite. It can be seen from Fig.\u00a011c that the diameter of the high frequency semicircle does not change with overpotential. This suggests that the HF time constant is related to the surface porosity and the LF time constant corresponds to the charge transfer kinetics of the HER. Similar tendency was also observed for Ni\u2013Mo alloy coating (Fig.\u00a011d). Therefore the EIS data collected for Ni\u2013Mo/WC 3 composite and Ni\u2013Mo alloy were fitted with the 2TCS model. Similar behaviour of Ni\u2013Mo/WC 3 and Ni\u2013Mo electrodes might be the result of the relatively high Mo content in both coatings (~28 and ~23\u00a0wt% for Ni\u2013Mo/WC 3 and Ni\u2013Mo, respectively), which had influenced the structure of the Ni\u2013Mo alloy matrix. Additionally, in case of Ni\u2013Mo alloy, in the spectrum recorded at the highest overpotential value (\u2212625\u00a0mV), a characteristic inductive loop is visible in the low frequency region (Fig.\u00a011d). This phenomenon was also observed by other researchers and explained as the result of adsorption of large amounts of hydrogen on the cathode surface [21,59,62,65]. In this case the experimental data were fitted with the equivalent circuit presented in Fig.\u00a010d (2TCSI), containing two additional elements related to the adsorption process \u2013 inductance L and resistance R\n\nL\n. The resulting EIS parameters calculated for Ni\u2013Mo/WC 3 composite and Ni\u2013Mo alloy are also presented in Table 6. The EIS spectra of each studied coating were collected at different cathodic overpotentials, so the obtained values of\u00a0resistance cannot be directly compared. However the obtained results allow to conclude, that an increase in the Mo content in composite coating resulted in a decrease in the value of the charge transfer resistance. Even though the EIS spectra of Ni\u2013Mo/WC 3 coating were recorded at lower cathodic overpotentials in comparison to other studied coatings, this composite was characterized by the lowest values of the charge transfer resistance. This proves its highest catalytic activity among the studied materials. These results are consistent with the trend observed based on the cathodic polarization measurements. The relatively low values of charge transfer resistance of Ni\u2013Mo alloy coating were the results of high cathodic overpotentials for which the EIS spectra were collected.The obtained EIS parameters that corresponds to the HER charge transfer kinetics, gives the possibility to evaluate the catalyst real surface area, which is the true electrochemically accessible area for the hydrogen to adsorb. The real surface area can be estimated from the value of double layer capacitance (C\n\ndl\n). Assuming that the average double layer capacitance of a smooth metal surface is 20\u00a0\u03bcF [54,55,57,59,62], the real surface area can be calculated as A\n\nreal\n = C\n\ndl\n\n/20\u00a0cm2. The double layer capacitance can be determined by the following relation [66]:\n\n(5)\n\n\n\nC\ni\n\n=\n\n\n[\n\n\nY\n\n0\ni\n\n\n/\n\n\n(\n\n\nR\ns\n\n\u2212\n1\n\n\n+\n\nR\ni\n\n\u2212\n1\n\n\n\n)\n\n\n(\n\n1\n\u2212\n\nn\ni\n\n\n)\n\n\n\n]\n\n\n1\n/\n\nn\ni\n\n\n\n\n\n\n\nThe real electrochemically active area of the material allow to calculate the roughness factor (R\n\nf\n) of its surface. This parameter characterize the ratio of the real surface area to the geometric surface are, so it can be calculated as R\n\nf\n = A\n\nreal\n\n/A\n\ngeometric\n. Knowing the surface roughness factor it is possible to evaluate the intrinsic catalytic activity (defined as i/R\n\nf\n) of the catalyst. The calculated values of double layer capacitance and R\n\nf\n parameter for the studied coatings are presented in Table 7\n. The obtained results reveal that Ni\u2013Mo/WC composite coatings are characterized by higher values of the surface roughness factor in comparison to Ni\u2013Mo alloy. The highest value of R\n\nf\n was obtained for Ni\u2013Mo/WC 3 composite. This was the result of the presence of WC nanoparticles as well as the high Mo content in the Ni\u2013Mo matrix. Table 7 also illustrates the comparison of the intrinsic catalytic activity of the studied coatings. The ratios i/R\n\nf\n were calculated for the current densities determined from the polarization curves at a set overpotential of 150\u00a0mV. This is a value of the overpotential that can be expected during the hydrogen production in the electrolyser. The highest i/R\n\nf\n ratio was also calculated for Ni\u2013Mo/WC 3 coating. This indicates that the superior catalytic activity of the Ni\u2013Mo/WC 3 electrode is the result of both high real surface area and high intrinsic catalytic activity of the composite coating. The other studied coatings were characterized by slightly lower i/R\n\nf\n ratios than Ni\u2013Mo alloy. The presented considerations indicate that both factors, namely, the presence of WC nanoparticles as well as Mo content in Ni\u2013Mo alloy matrix affected the catalytic activity of the studied composite materials.In order to evaluate the long term behaviour and stability of the studied materials during the HER, two electrochemical techniques have been applied, namely, cyclic voltammetry (CV) and chronopotentiometry.The CV measurements allowed to investigate the electrocatalytic behaviour of the studied coatings under the influence of the cyclically changing potential in the range of 0.12 to \u22120.58\u00a0V vs RHE. The results of CV investigations were presented in Fig.\u00a012\n and Table 8\n. For the Ni\u2013Mo/WC 1 and Ni\u2013Mo/WC 2 composites the electrochemical behaviour was similar for the initial and final cycle (Fig.\u00a012a and b). In case of the Ni\u2013Mo/WC 1 coating the value of current density recorded at the potential of \u22120.58\u00a0V for the 100th cycle slightly decreased in comparison to the 1st cycle (Table 8). For Ni\u2013Mo/WC 2 the measured current densities were similar for the 1st and 100th cycle. Higher values of current densities recorded for Ni\u2013Mo/WC 2 coating in comparison to Ni\u2013Mo/WC 1 deposit confirmed better electrocatalytic properties for the HER of composite with higher Mo content. Different electrochemical behaviour was observed for Ni\u2013Mo/WC 3 composite and Ni\u2013Mo alloy. In this case the electrochemical performance changed clearly with the increase in the number of cycles (Fig.\u00a012c and d).For Ni-Mo-WC 3 composite the current density recorded at \u22120.58\u00a0V vs. RHE increased from 74.6\u00a0mA\u00a0cm\u22122 for the 1st cycle to 89.5\u00a0mA\u00a0cm\u22122 for the 100th cycle. This indicates that the changes occurring on the coating surface as a result of the subsequent CV cycles positively affected the electrocatalytic properties for the HER of the Ni-Mo-WC 3 composite. Opposite relationship was observed for the Ni\u2013Mo alloy coating. In this case the recorded current densities decreased significantly when increasing the number of cycles (from 52.2\u00a0mA\u00a0cm\u22122 for the 1st cycle to 22.8\u00a0mA\u00a0cm\u22122 for the 100th cycle), which proves the deterioration of the catalytic activity.In order to understand the observed behaviour, the surface of Ni\u2013Mo/WC 3 and Ni\u2013Mo coating after 100 cycles of CV measurement was analyzed using the XPS method. The recorded Ni\u00a02p3/2, Mo 3d and W 4f spectra (Fig.\u00a013\na\u2013c) were deconvoluted according to the procedures described in the previous chapter. Based on the obtained results the percentage shares of the individual forms of the elements were calculated. In Table 9\n the surface composition of the Ni\u2013Mo/WC 3 and Ni\u2013Mo coatings in the as-deposited form and after 100 cycles of CV measurements was compared. In both cases, a decrease in the content of metallic Ni and a rise in the amount of Ni(OH)2 was observed on the coating surface after the 100 cycles of CV. According to the literature data the formation of \u03b1-Ni(OH)2 may occur at the potential of about \u22120.9, \u22120.8\u00a0V vs SCE (0.1, 0.2\u00a0V vs RHE) [67,68]. Since our experiment was performed at the potential range of 0.12 to \u22120.58\u00a0V vs RHE, Ni oxidation to Ni(OH)2 could appear during the subsequent CV cycles. The obtained XPS results indicate the irreversible nature of this process. At the same time, an increase in the content of metallic Mo on the surface of both kinds of coatings after 100th cycles was observed. In case of Ni\u2013Mo alloy coating, the Mo0 content amounted to 98.8% of the total Mo. The hole oxidized Mo was in the form of MoO2.The observed changes on the surface of Ni\u2013Mo alloy coating led to the deterioration of the catalytic activity for the HER. For Ni-Mo-WC 3 composite the content of Mo0 in the surface layer increased from 5.4% for the as-deposited coating to 12.7% for the deposit after 100 of CV cycles. Additionally, it was observer that Mo2O5 was the predominant oxide on the surface of as-deposited coating (51% of the total Mo), while on the surface of the coating after CV measurements 66.6% of the total Mo was in the form of MoO3. Moreover the subsequent CV cycles resulted in a complete reduction of the oxidized W. The literature data reveal that MoO3 possess interesting catalytic properties and has been widely used as a catalytic material for the hydrogen evolution reaction [69\u201375]. Therefore the observed improvement of the catalytic activity of Ni-Mo-WC 3 composite coating after subsequent CV cycles may be the result of an increase in the MoO3 content in the electrode surface layer.The long term catalytic activity and stability during the HER was evaluated based on the chronopotentiometry method. The measurement was performed for Ni\u2013Mo/WC 3 composite and for comparison for Ni\u2013Mo alloy electrode. CP curves were recorded for 60\u00a0h at a set current density of 100\u00a0mA\u00a0cm\u22122. The obtained results are presented in Fig.\u00a014\n. For both tested coatings the measured potential, after an initial slight increase, remained at a constant level throughout the entire experiment. The potential required to provide the fixed current density was significantly lower for the Ni\u2013Mo/WC 3 electrode (~-0.66\u00a0V vs RHE) than for Ni\u2013Mo alloy(~-0.91\u00a0V vs RHE). These results confirm the improvement of catalytic activity due to WC nanoparticles incorporation into Ni\u2013Mo alloy matrix.The obtained results indicate that both factors, namely, the presence of WC nanoparticles as well as Mo content in Ni\u2013Mo alloy matrix affected the catalytic activity of the studied composite materials. Both parameters strongly influenced the surface composition and surface roughness factor of the Ni\u2013Mo/WC catalysts. The composites containing higher amounts of WC nanoparticles and at the same time with lower Mo content (Ni\u2013Mo/WC 1 and Ni\u2013Mo/WC 2) were characterized by slightly lower values of the intrinsic catalytic activity in comparison to Ni\u2013Mo alloy. This indicates that their improved catalytic activity is the consequence of an increase in the surface roughness factor due to incorporation of WC nanoparticles. The XRD results revealed more nanocrystalline structure of the composite coatings in comparison to Ni\u2013Mo deposit. An increase in Mo content in the Ni\u2013Mo alloy matrix also results in the crystallite size refinement, therefore both parameters \u2013 WC incorporation and codeposition of Mo affected the structure and the value of roughness factor of the studied composites. The highest value of R\n\np\n parameter was calculated for Ni\u2013Mo/WC 3 catalyst. This was the consequence of the WC incorporation (WC content was lower than in other composites, but still relatively high - 8.7\u00a0wt% of W) and the highest content of Mo in the Ni\u2013Mo matrix among the studied coatings. On the other hand the presence of WC nanoparticles resulted in more oxidized surface of composite coatings in comparison to Ni\u2013Mo alloy. The thickness of the oxide layer on the composite surface was also enhanced by higher Mo content in Ni\u2013Mo matrix. Ni\u2013Mo/WC 3 catalyst was characterized by the most oxidized surface, which was the result of both the presence of WC nanoparticles and the highest Mo content in the Ni\u2013Mo matrix. High content of oxides on the surface of Ni\u2013Mo/WC 3 composite resulted in its higher intrinsic catalytic activity in comparison to other studied coatings. The CV and XPS results suggest that this effect was the consequence of high concentration of MoO3 on the composite surface.The catalytic activity for the HER of Ni\u2013Mo/WC composites with different Mo and WC content and, for comparison, of Ni\u2013Mo alloy electrode was investigated in 1\u00a0M KOH solution. The SEM analysis revealed that the incorporation of WC nanoparticle into Ni\u2013Mo alloy matrix resulted in less compact and regular surface of the composite coatings. The amount of incorporated nanoparticles was affected by the molybdate concentration in the plating solution. All studied coatings were composed of nickel FCC phase. However, lower intensity and broadening of the peak corresponding to Ni\u2013Mo matrix in the patterns of Ni\u2013Mo/WC coatings, indicates smaller crystallite size of the composites in comparison to Ni\u2013Mo alloy. The XPS results revealed that the surface of Ni\u2013Mo/WC composite electrodes was more oxidized than the surface of Ni\u2013Mo alloy. This indicates, that the thickness of the outer oxide layer was greater for the composite coatings. Moreover it was discovered, that the oxide layer thickness increased with the rise in the Mo content in Ni\u2013Mo metal matrix.The cathodic polarization measurements revealed that, in case of the all studied electrodes, the hydrogen evolution rate was determined by the Volmer reaction. Moreover, the process was affected by the outer oxide layer presented on the coatings surface. The performed electrochemical tests proved that the incorporation of the WC nanoparticles into the Ni\u2013Mo matrix resulted in the improvement of the electrocatalytic properties for the HER. The catalytical activity increased with an increase in the Mo content in the alloy matrix of the Ni\u2013Mo/WC composites. The highest catalytic activity was identified for the Ni\u2013Mo/WC 3 composite, characterized by the highest Mo content and the most oxidized surface among the studied coatings. According to EIS results this improvement was the consequence of both high real surface area and high intrinsic catalytic activity of Ni\u2013Mo/WC 3 electrode.It was discovered that the catalytic activity of Ni\u2013Mo/WC 3 composite increased after subsequent CV cycles. XPS analysis revealed that this was a consequence of an rise in the MoO3 content in the oxide layer on the electrode surface.The obtained results allow to conclude that the catalytic activity of Ni\u2013Mo/WC composites was determined by two factors, namely, the presence of WC nanoparticles and the Mo content in the Ni\u2013Mo metallic matrix. The presence of WC nanoparticles and high content of Mo in Ni\u2013Mo/WC 3 composite resulted in highly oxidized surface and led to higher catalytic activity for the 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 co-financed by statutory activity subsidy from the Polish Ministry of Education and Science for the Faculty of Production Engineering of Wroclaw University of Economics and Business (Department of Inorganic Chemistry; No 501-110-0310005000) and for the Faculty of Chemistry of Wroc\u0142aw University of Science and Technology (Department of Advanced Material Technologies (K26W03D05); grant number 8211104160).", "descript": "\n                  The catalytical activity for the hydrogen evolution reaction (HER) of the electrodeposited Ni\u2013Mo/WC composites is examined in 1\u00a0M KOH solution. The structure, surface morphology and surface composition is investigated using the scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The electrocatalytic properties for the HER is evaluated based on the cathodic polarization, electrochemical impedance, cyclic voltammetry and chronopotentiometry methods. The obtained results prove the superior catalytic activity for the HER of Ni\u2013Mo/WC composites to Ni\u2013Mo alloy. The catalytic activity of Ni\u2013Mo/WC electrodes is determined by the presence of WC nanoparticles and Mo content in the metallic matrix. The best electrocatalytic properties are identified for Ni\u2013Mo/WC composite with the highest Mo content and the most oxidized surface among the studied coatings. The impedance results reveal that the observed improvement in the catalytic activity is the consequence of high real surface area and high intrinsic catalytic activity of the composite.\n               "}
{"full_text": "The development of industrial fields has been progressing since the industrial revolution in the eighteenth century. The discharged wastewaters from industrial activities contain many organic and inorganic pollutants that are highly toxic and persistent [1,2]. Phenolic compounds are one of the major organic water pollutants produced from different industries, including petrochemicals, polymers, dyes, and pharmaceuticals. Particularly, phenol is a persistent organic pollutant with toxic and carcinogenic properties [3]. The levels of phenol are restricted by the environmental protection agency (EPA) to less than 0.1\u00a0ppm in wastewater discharges. Consequently, significant attention has been paid to removing phenol from industrial wastewater, and many methods have been developed in this regard [4].Advanced Oxidation Processes (AOPs) are widely used for the effective removal of organic pollutants in wastewater. AOPs have the potential to degrade various types of organic pollutants in wastewater. AOPs are based on the generation of active free radicals, such as hydroxyl radicals (HO), that degrade organic pollutants to nontoxic products [5]. The use of heterogeneous catalysts in AOPs, with the involvement of a strong oxidizing agent such as hydrogen peroxide (H2O2), is commonly studied by researchers to improve the degradation of organic pollutants [6].Spinel ferrites MFe2O4 (M = Co, Ni, and Cu, etc.) are interesting materials with unique physical and chemical properties. They are widely used in various applications, including biomedical, sensors, supercapacitors, microwave absorption, and catalysis [7]. Among various ferrites, cobalt ferrite (CoFe2O4) has received considerable attention for its excellent magnetic properties, narrow optical bandgap, good chemical stability, ease of preparation, nontoxicity, and good catalytic performance [8]. Cobalt ferrites have been used as catalysts for various reactions [9].The present study investigated the application of cobalt-based ferrite nanoparticles as catalysts for phenol degradation at neutral pH. Various cobalt-based ferrites were prepared, namely: pure ferrites (CoFe2O4), mixed ferrites (CoxNi1-xFe2O4), and P-modified ferrites (P-CoFe2O4). In the preparation of these ferrites, various metal precursors and phosphorus sources were used and the differences among the ferrites produced were revealed. To the best of authors' knowledge, there are no reports available in the literature on the preparation and catalytic performance of P-modified ferrites.In this study, the catalysts were prepared by the sol-gel auto-combustion method and characterized using FTIR, XRD, TEM, SEM, DR/UV\u2013Vis and H2-TPR techniques. The ferrites activities for phenol degradation under catalytic and photocatalytic reaction conditions were examined and compared.The nitrate precursors of Co(NO3)2\u02d66H2O and Fe(NO3)3\u02d69H2O were purchased from Merck, whereas the chloride precursors of CoCl2\u02d66H2O and NiCl2\u02d66H2O were obtained from ACROS. The phosphorus precursors were ortho-phosphoric acid (H3PO4, Scharlau) and diammonium hydrogen phosphate ((NH4)2HPO4, Sigma-Aldrich).A series of cobalt-based ferrite catalysts were synthesized using the previously reported sol-gel auto-combustion method of ferrite nanoparticles [5]. As presented in Table S1 (Electronic Supplementary Information, ESI), three different sets of the catalysts were synthesized, namely: pure-, mixed-, and P-modified cobalt ferrite nanoparticles. The pure CoFe2O4 ferrites were synthesized using either chloride (CoCl2.6H2O) or nitrate (Co(NO3)2\n.6H2O) metal precursors. The chemical formula of the mixed ferrites is CoxNi1-xFe2O4, where x= 0.3, 0.5, and 0.7. These three mixed ferrite samples were synthesized using chloride precursors of cobalt and nickel. The P-modified (0.7 and 1.5\u00a0wt%) ferrites (P-CoFe2O4) were synthesized using either H3PO4 or (NH4)2HPO4 as phosphorus sources.A stoichiometric amount of the divalent metal ion (M = Co and Ni) precursor and the iron (Fe) precursor were dissolved separately in distilled water. The two solutions were then mixed, and a certain amount of citric acid (CA) was added to the solution. The molar ratio of M:Fe:CA was 1:2:3. The solution was heated up to 80\u00a0\u00b0C with continuous stirring, followed by the addition of ammonium hydroxide to adjust the solution's pH to 8. The mixture was heated to form a gel that was then combusted to form an ash-like powder. Finally, the obtained catalyst was easily ground to a fine homogeneous powder using a mortar and pestle. The P-modified ferrites were also prepared using the same sol-gel synthesis method. Four CoFe2O4 catalysts were synthesized with 0.7\u00a0wt% or 1.5\u00a0wt% loading of phosphorus using either H3PO4 or (NH4)2HPO4 precursors. In the same solution of the metal precursors, the corresponding volume of phosphorus precursor was used and the same synthesis process was followed.The prepared catalysts' purity and structure were analyzed by X-ray diffraction (XRD) using PANalytical Powder Diffractometer (X'Pert PRO) with Cu-K\u03b1 radiation at 40\u00a0kV, \u03bb= 1.5406\u00a0\u00c5 and 40\u00a0mA. FTIR spectra were collected in the range of 400\u20134000\u00a0cm\u22121 using Bruker ALPHA-Platinum ATR. The scanning electron microscopy (SEM) technique was used to characterize the prepared catalysts' morphological features by FEG QUANTA 250 operated at 30\u00a0kV. Transmission electron microscopy (TEM) measurements were conducted with FEI Tecnai 20 operated at 200\u00a0kV. Diffuse reflectance (DR) spectra were recorded using Cary 5000 UV-VIS-NIR Spectrophotometer. H2 temperature-programmed reduction (H2-TPR) measurements were performed using a Quantachrome ChemBET-TPR/TPD instrument.The catalytic activities of the various ferrites solids were assessed for the degradation of phenol in water at room temperature. A typical catalytic reaction was performed in a 150\u00a0mL beaker containing 95\u00a0mL of 200\u00a0ppm of phenol solution, 5\u00a0mL of 30% H2O2, and 60\u00a0mg of the catalyst under stirring at room temperature. During the reaction, samples of 1\u00a0mL of the reaction mixture were withdrawn at specific time intervals and filtered through 0.2\u00a0\u03bcm nylon membrane filters. The collected filtrates were analyzed by a HPLC system (Shimadzu) to monitor the changes in phenol concentration at \u03bb = 280\u00a0nm. The separation was performed using a C18 column (Restek, 150 \u00d7 4.6\u00a0mm) with a flow rate of 1\u00a0mL/min, and a mobile phase composed of 64% water, 35% methanol and 1% acetic acid. The same experimental set-up was used to test phenol's catalytic degradation under photocatalytic conditions using a photoreactor with an Osram-metal halide lamp (400\u00a0W, 350\u2013750\u00a0nm). The following Eq. (1) was used to calculate the % of phenol degradation:\n\n(1)\n\n\nDegradation\n\n\n%\n\n=\n\n\n\n\nC\no\n\n\u2212\nC\n\n\n\nC\no\n\n\n\u00d7\n100\n\n\nwhere C\n\no\n is the initial concentration of phenol and C is the concentration of phenol after a certain time of the reaction. The kinetics of degradation reactions were studied using the following first-order expression described by Eq. (2):\n\n(2)\n\n\nln\n\n\nC\n\nC\no\n\n\n\n=\n\u2212\n\nk\napp\n\nt\n\n\n\nwhere C\n\no\n is the initial concentration of phenol, C is the concentration of phenol after a certain time of the reaction, t is the time (min), and k (min\u22121) is the rate constant of the reaction.To get an initial insight into the degradation reaction, control catalytic experiments were carried out as described above using the prepared CoFe2O4 catalysts only (without H2O2). The results are displayed in Fig. S1, which clearly shows that no degradation of phenol occurred in the absence of H2O2. This indicates that phenol does not directly react onto the CoFe2O4 surface. However, as discussed in Section 3.2, phenol degradation is caused by CoFe2O4 catalyst in the presence of H2O2.The phase and crystallinity of the CoFe2O4 nanoparticles synthesized from nitrate and chloride precursors were determined using powder XRD. The results are presented in Fig. 1(A), which shows diffraction peaks at 2\u03b8 = 18.5, 30.4, 35.8, 37.4, 43.4, 53.8, 57.3, 62.9 and 74.4\u00b0. The diffraction peaks can be assigned to (111), (220), (311), (222), (400), (422), (511), (440) and (533) reflection facets that confirmed the spinel cubic structure [10\u201312]. The observed XRD patterns agreed with the PDF card 22\u20131086 of CoFe2O4 and with literature data [9]. The XRD patterns of both samples demonstrated their high purity and crystallinity. However, the nitrate-derived CoFe2O4 sample peaks are more sharp and intense, which indicates a higher crystallinity for this sample than the chloride derived CoFe2O4. An average primary crystallite size was calculated using the Scherrer formula given by the following Eq. (3):\n\n(3)\n\nL\n=\n\nK\u03bb\n\nB\n\ncos\u03b8\n\n\n\n\n\nwhere K is the Scherrer constant, which is set to 0.89 (spherical shape), \u03bb is the wavelength (nm) of the X-ray source, B is the peak width at half maximum (radian), \u03b8 is the diffraction angle, and L is the crystallite size (nm). The calculated crystallite sizes using Eq. (3) were 24.8 and 34.0\u00a0nm for the chloride and nitrate derived CoFe2O4 samples, respectively.Two vibrational metal-oxygen modes at about 400 and 600\u00a0cm\u22121 are typically observed for ferrites samples [5,13]. The absorption band located at ~ 600\u00a0cm\u22121 (v\n\n1\n) corresponds to the stretching vibrational mode at the tetrahedral sites in the lattice, whereas that at ~ 400\u00a0cm\u22121 (v\n\n2\n) corresponds to the vibrational mode at the octahedral site [12]. For the prepared CoFe2O4 samples, v\n\n1\n band was detected at 540\u00a0cm\u22121, but v\n\n2\n band was not observed due to instrumental limitations, as shown in Fig. 1(B), which compares the FTIR spectra of the synthesized samples from nitrate and chloride precursors. Remarkable differences were observed in the FTIR spectra of the two CoFe2O4 samples. The CoFe2O4 sample obtained from the nitrate precursor mainly showed the v\n\n1\n IR band, whereas additional bands at ~ 1400, 1560 and 3200\u00a0cm\u22121 were observed in the spectrum of the CoFe2O4 sample prepared from the chloride precursor. The first two bands (1400 and 1560\u00a0cm\u22121) can be assigned to O-H bending vibrational modes, and the third broad IR absorption band (3200\u00a0cm\u22121) can be associated with the O-H stretching vibrational modes of water molecules. Such structural differences are expected to induce different physical and chemical properties to the CoFe2O4.\nFig. 2\n shows high-resolution SEM images of CoFe2O4 samples prepared from the nitrate and chloride precursors. These images clearly show significant differences in the morphological features of the CoFe2O4 ferrites prepared using different precursors. The morphology of the CoFe2O4 sample, synthesized from a chloride precursor, consisting of large agglomerates with cubic and polyhedral structures, is represented in Fig. 2(A) [14,15]. On the other hand, the CoFe2O4 particles prepared from the nitrate precursor consists of regular spherical shapes, as presented in Fig. 2(B). Comparing the SEM images, it appears that the chloride derived particles are more dispersed and larger than the nitrate derived particles. The different morphologies obtained from these CoFe2O4 samples agree with the reported findings of Sink\u03cc et al. [15], which showed that different morphologies were present in synthesized CoFe2O4 samples from different precursors. Nitrate salts have been proposed to be more powerful precursors for the sol-gel phase than chloride salts. In addition, condensation reactions in the nitrate solution are more intensive.The powder XRD patterns of the mixed CoxNi1-xFe2O4 ferrites are presented in Fig. S2(A). The patterns exhibited distinctive diffraction peaks that confirm the cubic structure of the prepared ferrites [16]. The observed peaks correspond to (111), (220), (311), (222), (400), (422), (511), (440) and (533) reflections. Using the Scherrer formula, the crystallite sizes were found to be 27.2, 31.0, and 23.7\u00a0nm for Co0.3Ni0.7Fe2O4, Co0.5Ni0.5Fe2O4, Co0.7Ni0.3Fe2O4, respectively. Fig. S2(B) displays the FTIR spectra of the mixed ferrites (CoxNi1-xFe2O4), which are similar to the FTIR spectra of the pure CoFe2O4 sample as shown in Fig. 1(B). In addition to the v\n\n1\n characteristic IR band at ~ 560\u00a0cm\u22121, other bands were observed at 1400 and 1560\u00a0cm\u22121 due to to O-H bending vibrational modes, and at ~ 3200\u00a0cm\u22121 attributable to the O-H stretching vibrational modes of water molecules [17].The SEM images of the three CoxNi1-xFe2O4 samples are depicted in Fig. S3, which clearly show differences in the morphological features of the synthesized mixed ferrites due to the variation in their chemical composition. The Co0.7Ni0.3Fe2O4 particles showed irregular flakes. At the same time, agglomerated cubic and polyhedral-like particles were observed in Co0.5Ni0.5Fe2O4 and Co0.3Ni0.7Fe2O4 microimages. A similar observation was reported for CoxNi1-xFe2O4 synthesized from chloride precursors via a hydrothermal process [16].High-resolution TEM analysis was also carried out for the CoxNi1-xFe2O4 nanoparticles. The TEM images showed that Co0.5Ni0.5Fe2O4 and Co0.3Ni0.7Fe2O4 particles possessed cube-like structures, as observed in Fig. S4(c) and (e), while Co0.7Ni0.3Fe2O4 particles exhibited spherical structures as shown in Fig. S4(a). Co0.3Ni0.7Fe2O4 nanoparticles were well-dispersed, while Co0.5Ni0.5Fe2O4 and Co0.7Ni0.3Fe2O4 nanoparticles were agglomerated. The TEM measurements revealed that the average nanoparticle size was ~ 24, 28, and 21\u00a0nm for Co0.3Ni0.7Fe2O4, Co0.5Ni0.5Fe2O4, and Co0.7Ni0.3Fe2O4 samples, respectively. The TEM images of these ferrites possessed well-defined lattice fringes. The d-spacing values were estimated and found to be in the range of 0.262\u20130.266\u00a0nm, and correspond to the 311 plane, which agree with the discussed XRD data, and confirm the spinel phase present in the mixed ferrites.A large number of literature reports is available on the catalytic properties of P-modified oxides, but there are no reports on the preparation and catalytic properties of P-modified CoFe2O4 ferrites to the best of our knowledge. Powder XRD was used to analyze the phase structure of synthesized P-CoFe2O4 catalysts, and the findings are presented in Fig. 3\n. Typical peaks of the spinel phase confirming the cubic structure of these ferrites are indicated and the intensities of the XRD peaks of P-CoFe2O4 samples are reduced compared to the pure CoFe2O4, which indicates that P-addition affected the crystallinity of the CoFe2O4. Using the Scherrer formula, the crystallite sizes were found to be 20.9 and 24.6\u00a0nm for the 0.7\u00a0wt% and 1.5\u00a0wt% P-CoFe2O4 using (NH4)2HPO4 as a phosphorus source. On the other hand, the crystallite sizes were found to be 22.9 and 22.8 for the 0.7\u00a0wt% and 1.5\u00a0wt% P-CoFe2O4 using H3PO4 as a source of phosphorus. It is worth noting that in the latter material, 1.5\u00a0wt% P-CoFe2O4 using H3PO4, two peaks were observed ~ 28\u00b0 and 33\u00b0 which can be attributed to a cobalt phosphate crystal phase [18,19].Fig. S5 shows the FTIR spectra of four P-CoFe2O4 ferrite samples. The ferrite characteristic v\n\n1\n IR band was observed in all spectra at ~ 560\u00a0cm\u22121. The vibrational bands corresponding to the O-H bending modes were detected at 1400 and 1560\u00a0cm\u22121, while the broad O-H stretching vibrational band was observed at ~ 3200\u00a0cm\u22121. In addition, there is an absorption band ~ 1025\u00a0cm\u22121, which can be attributed to the phosphate group (PO4\n3\u2212) vibrational stretching mode [20\u201322]. The intensity of this band increased with increasing the phosphorus content from 0.7 to 1.5\u00a0wt%.The SEM images of the P-CoFe2O4 synthesized using different phosphorus precursors are shown in Fig. S6. The agglomeration of the cubic-like crystals is clearly shown in these images. A remarkable change can be seen in Fig. S6 as the morphology of P-CoFe2O4 became less homogenous, more porous, and possessed a rougher surface compared to the pure CoFe2O4 sample (Fig. 2(A)).The photocatalytic activity of semiconducting materials is related to their optical absorption ability, which plays an important role in the determination of photocatalytic efficiency. Therefore, optical studies of all the synthesized ferrite catalysts were performed by using UV\u2013Vis diffuse reflectance spectroscopy. The Tauc transformation of DR/UV\u2013Vis spectra allows determining the optical band gap energies (Eg) using the following Eq. (4):\n\n(4)\n\n\n\n\n\n\u03b1\nh\n\u03bd\n\n\n\n1\nn\n\n\n=\nA\n\n\nh\n\u03bd\n\u2212\n\nE\ng\n\n\n\n\n\nwhere \u03b1 is the absorption coefficient, h is Planck's constant, v is the light's frequency, A is a constant \u201cband tailing parameter\u201d, E\n\ng\n is the band gap in electron volt, and n is a number that represents the type of transition. The DR/UV\u2013Vis spectra of the prepared ferrites are presented in Fig. S7. The band gap energy values (Table S2) were estimated from the specta by extrapolating each curve's linear region to intersect the energy axis. The prepared ferrite samples were found to exhibit E\n\ng\n values in the range of 1.78\u20131.84\u00a0eV, which agreed with band gaps energies previously reported [23]. Both CoFe2O4 samples prepared from nitrate and chloride precursors showed the same band gap value of 1.78\u00a0eV, implying that the used precursor does not affect the band gap energy CoFe2O4. On the other hand, a slight increase in the E\n\ng\n values was observed when Ni and P atoms were introduced into the CoFe2O4 ferrite. As presented in Table S2, increasing the Ni content in the mixed CoxNi1-xFe2O4 ferrites increased the E\n\ng\n values, demonstrating blue shift as a function of Ni substitution.Catalytic and photocatalytic reactions were conducted at room temperature and neutral pH using pure, mixed and P-modified cobalt ferrite catalysts. Fig. 4\n shows the variations in the catalytic activities toward phenol degradation observed for the pure CoFe2O4 ferrites prepared from nitrate and chloride precursors. Interestingly, the various precursors have caused entirely different catalytic activity against the degradation of phenol. The catalytic activity of CoFe2O4 prepared using the chloride precursor is very high, revealing a complete degradation of phenol within 80 and 15\u00a0min under catalytic and photocatalytic conditions, respectively. On the other hand, the CoFe2O4 synthesized from the nitrate precursor is catalytically inactive but shows a gradual increase in phenol photodegradation during the reaction time, reaching nearly 39% after 5\u00a0h of reaction. The rate of phenol degradation reaction followed the first-order kinetic model (Eq. 3), and calculated k values of these reactions are presented in Table S3.The higher catalytic activity of the CoFe2O4 prepared from the chloride precursor is possibly attributed to an increased concentration of surface active sites compared to the CoFe2O4 prepared from the nitrate precursor. Such differences in the catalytic activity can also be related to the different structural and morphological properties of the CoFe2O4 samples, as shown in Fig. 2, which in turn are linked to site activity. Cubic-like structures were observed for the CoFe2O4 prepared from the chloride precursor (Fig. 2(A)), while a completely different structure characterized the CoFe2O4 prepared from the nitrate precursor (Fig. 2(B)). Previous reports demonstrated that ferrites' morphologies play an important role in enhancing their properties and expanding their applications [24,25]. For instance, different morphologies were obtained using other precursors to prepare ZnFe2O4 ferrite, which was used as a catalyst to degrade safranine-O and remazol brilliant yellow dyes [25]. Nanorods samples achieved the highest catalytic activity compared to nanoflowers and hollow microspheres samples [25].In addition, the difference in the catalytic activity of CoFe2O4 ferrites prepared from nitrate and chloride precursors can be due to variations in their redox properties. Measurements of H2-TPR were carried out to examine the reducibility of CoFe2O4 catalysts produced from chloride and nitrate precursors. The findings are shown in Fig. S8, which typically indicate that these ferrites have been reduced at a high-temperature range. The nitrate precursor H2-TPR profile of CoFe2O4 shows two peaks of reduction: the main peak at 534\u00a0\u00b0C due to the production of metallic cobalt (Co0) and Fe3O4, while the second peak at ~ 658\u00a0\u00b0C could be correlated with the production of FeO and Fe0 [26]. On the other hand, the TPR profile of CoFe2O4 from the chloride precursor is considerably different. It starts at 442\u00a0\u00b0C and is followed by a broad peak at 658\u00a0\u00b0C and another peak at 860\u00a0\u00b0C. The distinct TPR profiles demonstrates the difference in the reduction ability (related to the metal cation \u2013 oxygen anion bond strength) of these ferrites, and hence the difference in their catalytic activity. The first H2 reduction peak maximum temperature of the CoFe2O4 prepared from the chloride precursor catalyst appears lower than that of the nitrate precursor. This suggests that the redox ability of the CoFe2O4 prepared from the catalyst of the chloride precursor is higher, which led to its higher catalytic activity compared to the CoFe2O4 from the nitrate precursor.The influence of incorporating nickel into the CoFe2O4 nanoparticles on phenol degradation is demonstrated in Fig. S9(C-D). Clearly, the three mixed CoxNi1-xFe2O4 (x= 0.3, 0.5, or 0.7) ferrites were very active compared to the performance of the pure cobalt CoFe2O4 ferrite for both catalytic and photocatalytic reactions. It seems that Co0.3Ni0.7Fe2O4 is slightly more active than the other two mixed ferrites. Complete degradation of phenol was achieved in less than 30\u00a0min using Co0.3Ni0.7Fe2O4, while pure CoFe2O4 removed phenol in about 80\u00a0min. A possible explanation for this result is that incorporating Ni into the CoFe2O4 structure improved the ferrite's redox properties, which consequently enhanced its catalytic activity. In the case of photoassisted conditions, the three mixed CoxNi1-xFe2O4 (x= 0.3, 0.5, or 0.7) solids exhibited high catalytic activities, showing a complete degradation of phenol within less than 10\u00a0min. It is worth mentioning that Co0.3Ni0.7Fe2O4 completely degraded phenol within less than 5\u00a0min. Table S4 presents the rate constants of the catalytic reactions of the mixed CoxNi1-xFe2O4 (x= 0.3, 0.5, or 0.7) ferrites.The P-CoFe2O4 ferrites were prepared using (NH4)2HPO4 and H3PO4 precursors with two phosphorus loadings, 0.7 and 1.5\u00a0wt%. The catalytic activities of these samples toward the degradation of phenol were compared, and results are displayed in Fig. S9(E-F) and Table S5. The catalytic activities of the P-CoFe2O4 appear higher than the pure CoFe2O4 under both catalytic and photocatalytic conditions. The phosphorus modification significantly enhanced the catalytic activity towards phenol degradation. No reports are available on the preparation and catalytic properties of P-modified CoFe2O4 ferrites. However, the effect of P-modified cobalt oxide (CoO) was investigated to enhance the catalytic oxidative dehydrogenation of propane [27]. Recently, the post-treatment of copper ferrite (CuFe2O4) using H3PO4 showed an improvement in its catalytic activity towards phenol degradation [22]. Obviously, the catalytic activities of P-CoFe2O4 were significantly improved under photoassisted conditions. The highest photocatalytic activity towards phenol degradation was achieved by 1.5\u00a0wt% P-CoFe2O4 prepared using (NH4)2HPO4.The catalytic performance of copper ferrite (CuFe2O4) catalysts was demonstrated as highly efficient Fenton-like reagents to treat water from organic pollutants [5]. The degradation of pollutants proceeded by the decomposition of H2O2 to HO\u2022 radicals in the presence of CuFe2O4 catalyst. In the current work, CoFe2O4 catalyst may promote similar Fenton-like degradation reactions in which HO\u2022 radicals attack phenol molecules, producing new radicals (PhO\u2219) that may further decompose into smaller molecules such as CO2 and H2O according to the following reactions (5)\u2013(7):\n\n(5)\n\n\nH\n2\n\n\nO\n2\n\n+\n\nCoFe\n2\n\n\nO\n4\n\n\u2192\n\n2HO\n\u2022\n\n\n\n\n\n\n(6)\n\n\nHO\n\u2022\n\n+\nPhOH\n\u2192\n\nPhO\n\u2022\n\n+\n\nH\n2\n\nO\n\n\n\n\n\n(7)\n\n\nPhO\n\u2022\n\n+\n\nH\n2\n\n\nO\n2\n\n\u2192\nintermediates\n\u2192\n\nCO\n2\n\n+\n\nH\n2\n\nO\n\n\n\nUnder photocatalytic conditions, the reaction is initiated when CoFe2O4 catalyst absorbs a photon, which leads to the promotion of an electron in the conductive band (eCB\n\u2212) and a positive hole in the valence band (hVB\n+). In addition to HO\u2022 radicals, the produced eCB\n\u2212 and hVB\n+ enable the reduction\u00a0and oxidation reaction, respectively, that degrade phenol molecules:\n\n(8)\n\n\nCoFe\n2\n\n\nO\n4\n\n+\nhv\n\u2192\n\n\ne\nCB\n\n\u2212\n\n+\n\n\nh\nVB\n\n+\n\n\n\n\n\n\n(9)\n\n\nH\n2\n\n\nO\n2\n\n+\n\n\ne\nCB\n\n\u2212\n\n\u2192\n\nOH\n\u2212\n\n+\n\nHO\n\u2022\n\n\n\n\n\n\n(10)\n\nPhOH\n+\n\n\nh\nVB\n\n+\n\n\u2192\nintermediates\n\u2192\n\nCO\n2\n\n+\n\nH\n2\n\nO\n\n\n\n\n\n(11)\n\nPhOH\n+\n\n\ne\nCB\n\n\u2212\n\n\u2192\nintermediates\n\u2192\n\nCO\n2\n\n+\n\nH\n2\n\nO\n\n\n\n\n\n(12)\n\nPhOH\n+\n\nHO\n\u2022\n\n\u2192\nintermediates\n\u2192\n\nCO\n2\n\n+\n\nH\n2\n\nO\n\n\n\nIn this study, AOP was utilized for phenol degradation using different cobalt-based ferrite catalysts. Pure CoFe2O4 and mixed CoxNi1-xFe2O4 (x= 0.3, 0.5, and 0.7) cobalt ferrites were successfully synthesized using the sol-gel auto-combustion method. The ferrite catalysts were prepared using chloride and nitrate precursors. Phosphorus modified cobalt ferrites (P-CoFe2O4) were also synthesized using different phosphorus sources: H3PO4 and (NH4)2HPO4. The prepared ferrites were characterized using powder XRD, FTIR, SEM, TEM, DR/UV-Vis and H2-TPR techniques. The rates of reactions of the pure ferrites prepared from chloride precursors were higher than those prepared using nitrate precursors. The catalytic degradation of phenol was successfully achieved at ambient conditions. Mixed ferrites (CoxNi1-xFe2O4) and P-modified ferrites (P-CoFe2O4) showed higher catalytic performance than pure CoFe2O4 ferrites for the degradation of phenol. The rate of photocatalytic reactions of phenol degradation was comparatively higher than the catalytic reactions for all of the prepared catalysts. Some catalysts achieved complete degradation of phenol under photo-induced conditions in less than 5\u00a0min.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 to the Research Office at Khalifa University for funding under the project No. LTR14013. The authors would also like to thank Prof. Abbas Khalil from the United Arab Emirates University (UAEU) for the H2-TPR measurements.\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.2020.106267.", "descript": "\n                  Nowadays, cobalt-based catalysts are becoming increasingly attractive for various reactions. In this work, the catalytic and photocatalytic activities of different cobalt ferrites were investigated for the degradation of phenol in water at neutral pH. Pure CoFe2O4 and mixed CoxNi1-xFe2O4 ferrites were synthesized using different precursors (chloride and nitrate). CoFe2O4 catalysts derived from chloride precursors showed a higher catalytic efficiency compared to those derived from nitrate precursors. Significant enhancement in the catalytic activities was observed when Ni was incorporated into CoFe2O4. Furthermore, the catalytic activities of phosphorus-modified cobalt ferrites (P-CoFe2O4) significantly improved the photocatalytic degradation of phenol.\n               "}
{"full_text": "Ammonia nitrogen, which is a collective term for NH3 and NH4\n+, is a common nitrogen compound. In nature, high concentrations of ammonia nitrogen found in water and soil come mainly from the improper disposal of human waste and livestock excretion, industrial effluents and the excessive use of fertilizers. Although ammonia nitrogen is necessary for plant growth, in excess, it causes eutrophication of rivers, lakes and inland seas, causing serious damage to the environment [1]. Thus, purification of water with high concentrations of ammonia nitrogen is required to ensure environmental sustainability [2].Catalytic wet air oxidation (CWAO) over supported precious metal catalysts and mixed metal oxides is a promising technique for purification of water containing ammonia nitrogen because ammonia nitrogen is selectively oxidized to harmless gaseous compounds [3,4]. However, temperatures far greater than the boiling point of water and high pressures are needed for the reaction to occur [5,6], and this is a major disadvantage.Recently, oxidative decomposition of organic pollutants with O3 in water has gained much attention [7,8] because the reaction proceeds even at low temperatures and ambient pressure. To enhance the decomposition efficiency, the reaction is often performed in the presence of catalysts, which is called catalytic ozonation [9\u201311]. Catalytic ozonation is applicable to the decomposition of ammonia nitrogen in water, but there are only a few reports. Our group has previously reported the catalytic ozonation of ammonia nitrogen in water [12]. We have tested eight metal oxides (MOx, M\u00a0=\u00a0Co, Ni, Fe, Sn, Mn, Cu, Mg, and Al) and found that Co3O4 is the best catalyst in terms of selectivity to gaseous compounds and stability in the reaction solution. However, the decomposition rate of ammonia nitrogen over Co3O4 is only the third best among the tested metal oxides [12]. Subsequently, Co3O4-MgO [13] and MgO [14] have been reported to be more active than Co3O4. However, they have relatively high selectivity to NO3\n\u2212, which is a problem in wastewater treatment. In addition, we recently reported that MgO acts as a reactant but not as a catalyst for the decomposition of ammonia nitrogen with O3 [15]. MgO dissolves in the reaction solution, maintaining weakly basic conditions (pH\u00a0\u2248\u00a09.5) by neutralizing the H+ formed during the decomposition of ammonia nitrogen (NH3\u00a0+\u00a04O3\u00a0\u2192\u00a0NO3\n\u2212 +\u00a0H2O\u00a0+\u00a0H+ + 4O2) with OH\u2212, which is formed upon dissolution of MgO [15]. Thus, the development of catalysts that are more active, selective and stable is necessary for realizing commercial applications.Supported noble metal catalysts have been applied for various total oxidation reactions with O2 [16]. In catalytic ozonation, some supported noble metal catalysts have been used for decomposition of organic pollutants in water [9,10]. Li et al. [17] have found that Pd/CeO2 promotes the decomposition of pyruvic acid in water via catalytic ozonation. Gomes et al. [18] have reported enhancement of the decomposition rate for 4-hydroxylbenzoic esters as well as improvement of chemical oxygen demand (COD) removal by modifying TiO2 with noble metals, especially Pt and Pd. However, to the best of our knowledge, the application of heterogeneous noble metal catalysts for the catalytic ozonation of ammonia nitrogen in water has not been reported yet.In the present study, we tested six Al2O3-supported noble metal catalysts (Ru, Rh, Pd, Ag, Ir, and Pt) for the catalytic ozonation of ammonia nitrogen in the presence of Cl\u2212. We found that Pd was the best metal in terms of activity and selectivity and then further investigated the influence of support on the catalytic performance. In addition, factors controlling the catalytic performance of the supported noble metal catalysts are discussed in relation to the electronic structure of the metals, namely, the d-band center relative to the Fermi level, which is known as the Hammer-N\u00f8rskov d-band model.All materials were of analytical grade and were used without further purification. RuCl3\u00b7nH2O, RhCl3, PdCl2, IrCl3, AgNO3, H2PtCl6 and activated carbon (AC) were purchased from FUJIFILM Wako Pure Chemical Co. Al2O3 (AEROXIDE\u00ae Alu C), SiO2 (AEROSIL\u00ae 300) and TiO2 (AEROXIDE\u00ae TiO2 P25) were provided by Nippon Aerosil Co., Ltd., and CeO2 (type-A) was provided by Daiichi Kigenso Kagaku Kogyo Co., Ltd.All catalysts were prepared by using a conventional impregnation method. To 10\u00a0mL of an aqueous solution of noble metal salt with a concentration of 10\u00a0g\u00a0L\u22121 (as metal), 2\u00a0g of Al2O3 were added. The resulting suspension was vigorously stirred at 353\u00a0K for 2\u00a0h, and then the water was completely removed from the suspension in an oven at 373\u00a0K overnight. The solid was then calcined at 523\u00a0K for 3\u00a0h and then reduced in an H2 gas flow (20\u00a0mL\u00a0min\u22121) at 723\u00a0K for 2\u00a0h. The loading amount of the noble metal was fixed at 5\u00a0wt%. For the supported Pd catalysts, SiO2, TiO2, CeO2, and AC were used as supports in addition to Al2O3. Details of the characterization of the catalysts can be found in the Electronic Supporting Information (ESI).The reaction solution containing 10\u00a0mmol\u00a0L\u22121 of NH4\n+ was prepared using NH4Cl. Although the initial pH of the reaction solution was adjusted to 7 by adding an aqueous KOH solution, the pH was always ~3 after the reaction. Since the pK\na of NH4\n+ is 9.25, NH4\n+ was the predominant species in the reaction solution during the reaction. Thus, NH4\n+ will be used to express ammonia nitrogen throughout the paper.Catalytic ozonation of NH4\n+ in water was performed in a flask connected to a gas flow line and traps (Fig. S1). Typically, 0.1\u00a0g of the catalyst was added to the reactor containing 100\u00a0mL of the reaction solution, and then the suspension was heated in an O2 gas flow with vigorous stirring. After the temperature of the suspension reached 333\u00a0K, the gas was changed to a mixture of O3/O2 (O3 concentration, 1.88\u00a0mmol\u00a0L\u22121 and total flow rate, 100\u00a0mL\u00a0min\u22121) to start the reaction. O3 was generated from O2 using an ozone generator (Tokyu Car Co. SO-03UN-OX). A small portion of the reaction solution was withdrawn at regular intervals and analyzed using two ion chromatographs (Tosoh Co. Ltd., IC-2001) to determine the concentrations of NO3\n\u2212 and NH4\n+. The details of the analytical conditions are described in the ESI.Selectivity to NO3\n\u2212 (SNitrate) was calculated using Eq. (1):\n\n(1)\n\n\nS\nNitrate\n\n\n%\n\n=\n\n\nFormed\n\n\n\nNO\n3\n\n\u2212\n\n\n\nConsumed\n\n\n\nNH\n4\n\n+\n\n\n\n\u00d7\n100\n\n\n\nSince NO2\n\u2212 was not detected in the reaction solution, and the gaseous products (GasN) were not analyzed in this study, based on N-material balance the selectivity for Gas-N (SGas-N) was calculated by subtracting SNitrate from 100% (Eq. 2):\n\n(2)\n\n\nS\n\nGas\n\u2212\nN\n\n\n\n%\n\n=\n100\n\u2212\n\nS\nNitrate\n\n\n\n\nThe catalytic performance of the Al2O3-supported noble metal catalysts is shown in Fig. 1\n, where the catalytic activity was evaluated in terms of the initial reaction rate per unit amount of metal (r\n0, mol h\u22121 molmetal\n\u22121) and the selectivities were obtained after 6\u00a0h of reaction. Ir/Al2O3, Rh/Al2O3 and Pt/Al2O3 catalysts show moderate activity for the reaction, whereas the activities of Ag/Al2O3 and Rh/Al2O3 are negligible. On the other hand, Pd/Al2O3 exhibits high activity (r\n0\u00a0=\u00a020.4\u00a0mol\u00a0h\u22121 molmetal\n\u22121) and selectivity to Gas-N (90%) for the reaction.To determine the intrinsic activity of each noble metal, the catalytic activities were compared on the basis of the turnover frequency (TOF), which is defined as the initial decomposition rate of NH4\n+ per metal atom exposed on the surface (Eq. (3)):\n\n(3)\n\nTOF\n\n\n\nh\n\n\u2013\n1\n\n\n\n=\n\n\nInitial decomposition rate of\n\n\n\nNH\n4\n\n+\n\n\n\n\nmol\n\n\nh\n\n\u2013\n1\n\n\n\n\n\ng\ncatalyst\n\n\n\u2013\n1\n\n\n\n\n\n\n\nNumber of metal atoms exposed\n\non\n\nthe surface\n\n\n\nmol\n\n\n\ng\ncatalyst\n\n\n\u2013\n1\n\n\n\n\n\n\n\n\n\nThe amount of chemisorbed CO is commonly used to estimate the number of atoms exposed on the surface of supported noble metal catalysts. However, this method was inapplicable to some catalysts tested in this study because little or no CO was chemisorbed [19]. Thus, the particle size distribution obtained from TEM images (Fig. S2) was used to estimate the number of metal atoms exposed on the surface. Details of the estimation are given in the ESI. As seen in Fig. S3, the TOF for Pd/Al2O3 is much higher than that of the other catalysts, and is more than 3-times higher than those of the second-best catalysts (Rh/Al2O3 and Pt/Al2O3), indicating that Pd is highly active for the reaction.In some catalytic reactions over supported noble and transition metal catalysts, a trend in the catalytic activity is associated with the position of the d-band center (\u03b5\nd) relative to the Fermi level (E\nF), called the Hammer-N\u00f8rskov d-band model, because d-electrons of metals often play an important role in chemisorption. Also, the state of the d-electrons determines the stability of an intermediate in a transition state [20,21]. Thus, the TOF for the catalytic ozonation of NH4\n+ over the supported noble metal catalysts was plotted as a function of the position of the d-band center relative to the Fermi level (\u03b5\nd \u2013 E\nF) calculated by N\u00f8rskov and co-workers [21]. As seen in Fig. 2\n, the TOF has a volcano-type dependence on the \u03b5\nd \u2013 E\nF values. It has been reported that the bond energies of metal\u2013hydrogen (M-H) and metal\u2013oxygen (M-O) bonds correlate with the value of \u03b5\nd \u2013 E\nF and that the larger \u03b5\nd \u2013 E\nF is, the lower the bond energies of the M-H and M-O bonds are [22]. Although a detailed reaction mechanism for the catalytic ozonation of NH4\n+ over the supported noble metal catalysts is still unknown, dissociation of the NH bond of NH4\n+ and activation of O3 to give M-H and M-O, respectively, on the metal particle must be involved in the reaction. Since Pd has a moderate value of \u03b5\nd \u2013 E\nF in comparison with those of other noble metals tested in this study, activation of the reactants to give PdH and PdO occurs relatively smoothly, and the formed intermediate does not cause poisoning of the active Pd sites. Thus, the moderate nature of Pd may be the reason for the high catalytic activity for the catalytic ozonation of NH4\n+.To improve the performance of the supported Pd catalyst, the effects of the support were investigated using SiO2, TiO2, CeO2 and activated carbon (AC), in addition to Al2O3 (Fig. 3\n). The catalytic activity significantly changes depending on the support, whereas the selectivity is barely affected. Pd/AC shows only a negligible activity, and Pd/TiO2 is less active than Pd/Al2O3. It should be noted that Pd/SiO2 and Pd/CeO2 are much more active than Pd/Al2O3 is. Pd/CeO2 shows a slightly higher selectivity for Gas-N than Pd/SiO2. High selectivity for Gas-N is advantageous for the purification of wastewater containing ammonia nitrogen. Thus, Pd/CeO2 is preferable to Pd/SiO2. In addition, the Pd in Pd/CeO2 was less soluble than Pd in Pd/SiO2 was (Table S1), which indicates that Pd/CeO2 is highly stable under the reaction conditions. In fact, Pd/CeO2 could be reused for the reaction without significant reduction in the catalytic performance (Fig. S4), and the reaction stopped after the catalyst was removed from the reaction solution (Fig. S5). Thus, we conclude that Pd/CeO2 is the best catalyst for the ozonation of NH4\n+ in water.As described so far, Pd/CeO2 exhibits high activity and selectivity to Gas-N as well as high stability for the catalytic ozonation of NH4\n+ when the reaction is performed in the reaction solution prepared using NH4Cl. However, the catalyst was completely inactive in the reaction solution prepared using (NH4)2SO4. This fact implies that Cl\u2212 is involved in the reaction over Pd/CeO2. To understand the role of Cl\u2212 in the reaction, changes in the concentration of Cl\u2212 in the solution during the catalytic ozonation of NH4\n+ over Pd/CeO2 were investigated. The reaction was performed in the reaction solution prepared using NH4Cl. As shown in Fig. S6, the concentration of Cl\u2212 decreased as that of NH4\n+ decreased. A similar behavior has been reported for the catalytic ozonation of NH4\n+ over Co3O4 [11,15]. In the reaction over Co3O4, Cl\u2212 is consumed to form chloramines (NH3\u2013x\nCl\nx\n, x\u00a0=\u00a01\u20133) due to the reaction of NH4\n+ with ClO\u2212, which forms from the reaction of Cl\u2212 with O3 (Cl\u2212 +\u00a0O3\u00a0\u2192\u00a0ClO\u2212 +\u00a0O2). Thus, it is plausible that the formation of chloramines occurred over Pd/CeO2. A similar reaction has been reported for the ozonation of ammonia nitrogen in the presence of Br\u2212 but in the absence of any catalysts [23,24]. However, the reaction rate is slower in the presence of Cl\u2212 than it is in the presence of Br\u2212 [24].To confirm the formation of chloramines, we performed the following experiments. Catalytic ozonation of NH4\n+ over Pd/CeO2 was performed in a reactor connected to three traps containing a strongly basic KOH solution (pH\u00a011), in which a fine powder of MgO was dispersed, placed at the outlet of the reactor (Fig. S1). Since chloramines are less soluble in water and easily undergo decomposition in strongly basic solutions to give Cl\u2212 [25], the chloramines formed during the reaction come out of the reactor along with the flow of the O3/O2 mixture and undergo decomposition in the trap solution. Thus, we checked whether or not Cl\u2212 was present in the trap solution. After 6\u00a0h, Cl\u2212 which would have a concentration of 7.3\u00a0mmol\u00a0L\u22121 if it was present in the reaction solution, was found in the trap solution. Thus, chloramines were formed during the reaction.As mentioned above, NO3\n\u2212 was the only ionic product found in the reaction solution, and therefore, the rest of the products must be GasN, which includes chloramines and likely other nitrogen compounds (N2, N2O, NO, and NO2). Although the decomposition of NH4\n+ to Gas-N is desirable for the purification of wastewater, we performed quantitative evaluation of the selectivities to chloramines and other Gas-N for the reaction over Pd/CeO2.To determine the quantity of chloramines remaining in the reaction solution after 6\u00a0h, at which the conversion of NH4\n+ was 99% or more (Fig. 4\n), the pH of the reaction solution was increased to 11 by adding an aqueous KOH solution after the reaction. After increasing the pH, 0.7\u00a0mmol\u00a0L\u22121 of Cl\u2212 was present in the reaction solution. As mentioned above, 7.3\u00a0mmol\u00a0L\u22121 of Cl\u2212 was present in the trap solution. Since the initial concentration of NH4\n+ was 10\u00a0mmol\u00a0L\u22121 and 1.0\u00a0mmol\u00a0L\u22121 of NO3\n\u2212 formed in the reaction solution after 6\u00a0h, the selectivities to chloramines, other Gas-N and NO3\n\u2212 were calculated to be 80%, 10%, and 10%, respectively, as only monochloramine (NH2Cl) of the three possible chloramines formed (Fig. 4). If dichloramine (NHCl2) and trichloramine (NCl3) had formed, the selectivity to chloramines would have been lower, and that to other Gas-N would have then been higher.Co3O4 is one of the most active and selective metal oxide catalysts ever reported for the catalytic ozonation of NH4\n+ in the presence of Cl\u2212 [11]. Thus, we compared the catalytic performance of Pd/CeO2 with Co3O4 under the same reaction conditions (Fig. 4). As seen in Fig. 4, the activity of Pd/CeO2 is about four times higher than that of Co3O4 at 333\u00a0K. Even when the reaction was performed at 293\u00a0K, the decomposition rate over Pd/CeO2 was still faster than that over Co3O4 at 333\u00a0K (Fig. 4). In addition to the catalytic activity, it should be noted that the selectivity of Pd/CeO2 to Gas-N is higher than that of Co3O4 (90% and 83% for Pd/CeO2 and Co3O4, respectively). Therefore, we may conclude that Pd/CeO2 must be the best heterogenous catalyst for the catalytic ozonation of NH4\n+ so far reported to the best of our knowledge.Among the noble metals tested in this study, which included Ru, Rh, Pd, Ag, Ir and Pt, Pd showed the highest activity and selectivity to gaseous compounds in the decomposition of NH4\n+ by O3 in water and in the presence of Cl\u2212. The moderate value of the d-band center relative to the Fermi level (\u03b5\nd \u2013 E\nF) for Pd is favorable for the activation of the reactants, leading to the high catalytic performance of supported Pd. The support for Pd had a large impact on the catalytic performance, and CeO2 was the best support to give a catalyst with high activity, selectivity and stability. Pd/CeO2 was more active and selective to gaseous compounds, which included predominantly chloramines, than Co3O4, which is one of the best catalysts so far reported to the best of our knowledge.Philip Anggo Krisbiantoro: Conceptualization; Data curation; Formal analysis; Writing-original draft.Tomokazu Togawa: Conceptualization; Data curation; Formal analysis.Koki Kato: Conceptualization; Data curation.Jiequiong Zhang: Data curation.Ryoichi Otomo: Writing-review & editing.Yuichi Kamiya: Formal analysis; Investigation; Project administration; 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.P.A. Krisbiantoro received a scholarship from Indonesia Endowment Fund for Education (LPDP scholarship) from the Ministry of Finance, Republic of Indonesia. The analysis of TEM was carried out with JEM-2010 microscope (JEOL) at Faculty of Engineering, Hokkaido University, supported by Material Analysis and Structure Analysis Open Unit (MASAOU).\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.2020.106204.", "descript": "\n                  Al2O3-supported precious metal catalysts were tested for the oxidative decomposition of NH4\n                     + in water with O3 and the effects of support on the catalytic activity of Pd were investigated. Pd/CeO2 was found to be the best catalyst in terms of activity, selectivity and stability and was more active and selective than Co3O4, which is one of the best catalysts reported so far. The reaction proceeded only in the presence of Cl\u2212. The moderate value of the d-band center relative to the Fermi level of Pd was found to be favorable for the activation of reactants, leading to a high catalytic performance.\n               "}
{"full_text": "Commonly used catalysts include various metal complexes, salts and metal oxides, as well as transition metals. All of these compounds belong to groups of noble or critical metals [1\u20133]. Currently, they are widely applied in the developing technologies of wind, nuclear, geothermal and biomass power plants, solar photovoltaics, electric vehicles, batteries, fuel cells, and carbon capture and energy storage systems, and may be used in both homogeneous and heterogeneous catalysis [3]. Therefore, having regard to sustainable development and the rational use of resources, the replacement of metals such as platinum, palladium, ruthenium, silver and gold with other types of catalytically active species is an important issue [4]. In the development of new catalytic materials, there is growing interest in the application of abundant elements such as iron, nickel, cobalt, aluminum, or copper, which are considered reasonable choices in both academic and industrial settings.The use of cobalt in homogeneous and heterogeneous catalysis is attracting great interest, because pure cobalt, as well as its oxides CoO, Co2O3 and Co3O4, exhibit good reactivity and stability [5\u20138]. On the other hand, from the point of view of sustainable development, cobalt-based catalysts are an interesting alternative because of the low price of cobalt and abundant sources of its minerals [9]. The easy accessibility of cobalt precursors, the predictable redox mechanisms of prepared catalysts, and their resistance to autooxidation \u2013 similar to the resistance of second- or third-row transition metals \u2013 are reasons for the growing interest in the preparation and use of cobalt-based catalysts in oxidation-reduction and coupling reactions [5,6,10\u201312].To increase catalytic efficiency, the improvement of activity, reusability, and selectivity is essential for the design of catalysts. In the case of heterogeneous materials, it is possible to design a material with the desired structure and suitable dispersion of active sites [4,13,14]. An attractive new field of study, enabling the provision of such materials in a more economically feasible manner, is biomimetics. The term biomimetics describes the emulation of nature's systems, elements and models to solve the complex human problems [15\u201317]. A good example of organisms which serve as inspiration for materials science is the marine sponges. These simplest animals possess three-dimensional skeletons made of spongin, chitin, calcium carbonate, or silica, depending on the species [18]. Currently, the most interesting for materials science purposes \u2013 especially for catalysis \u2013 are those sponges with spongin-based and chitin-based three-dimensional fibrous skeletons [19\u201324]. These open up new possibilities of creating naturally prefabricated supports for catalytic materials, which are comparatively cheap and renewable. Besides, the carbonization of these scaffolds broadens the range of their potential applications.As a naturally produced biomaterial, spongin represents a renewable source, which gives it an advantage over other commercially available materials. Their unique chemical and structural properties eliminate the necessity of molding and functionalizing the structure before application. Interestingly, the carbonization of spongin-based skeletons is still poorly described in the literature [25,26]. There are only two papers considering applications of carbonized spongin-based scaffolds functionalized with various metal oxides. In previously published work by Szatkowski et al. [25], they carbonized spongin-based skeletons at 650\u00a0\u00b0C, and then functionalized the resulted biocarbon with manganese(IV) oxide. In another study, by Petrenko et al. [26], carbonization of the spongin-based skeleton took place at a higher temperature of 1200\u00a0\u00b0C, leading to turbostratically disordered carbon in the form of graphite, which was functionalized with copper(I) oxide to obtain a catalyst for the reduction of 4-nitrophenol.These investigations encourage us to expand the existing knowledge by applying spongin-based skeletons in the development of novel materials for oxidation-reduction reactions. In that event, the main aim of this study was the preparation of biocarbons in low carbonization temperatures using spongin-based skeletons as a biocarbon source and evaluation of their functionalization ability with cobalt oxides. The physicochemical properties of the obtained composites were investigated in detail and then compared to the properties of biocarbons before functionalization. Moreover, prepared composites have been tested as potential catalysts in oxidation-reduction reactions, namely the oxidation of styrene, the decolourization of rhodamine B, and 4-nitrophenol reduction. Catalytic tests included reusability and kinetic studies.Commercial sponges were obtained from INTIB GmbH (Germany). 4-Nitrophenol, styrene, acetonitrile, sodium borohydride, rhodamine B and tert-butyl hydroperoxide were purchased from Sigma Aldrich (Reagent Plus standard). Cobalt nitrate hexahydrate and hydrogen peroxide were purchased from VWR (Germany). All reagents were used without any additional purification.First, the spongin-based skeletons were washed and cleaned as described in previous publications [21,22]. Then the cleaned skeletons were subjected to a carbonization process. The carbonization was carried out at various temperatures (400, 500, and 600\u00a0\u00b0C) using an R 50/250/13 tube furnace (Nabertherm, Germany) with a heating rate of 10\u00a0\u00b0C/min; heating was followed by a one-hour plateau and then cooling by thermal inertia to 50\u00a0\u00b0C. The whole procedure was carried out under a nitrogen atmosphere with a flow of 10\u00a0mL/min. In the next step, carbonized skeletons were functionalized with cobalt using the sorption-reduction method. Sorption was carried out by immersing 100\u00a0mg of the selected carbon material in 50\u00a0mL of an aqueous solution of cobalt nitrate hexahydrate with a cobalt ion concentration of 0.085\u00a0M. Sorption was conducted for one hour under continuous stirring of the reagents. Afterwards, 50\u00a0mL of 0.1\u00a0M water solution of sodium borohydride was added to this mixture, initiating the reduction, which was carried out for one hour. Then the solid material was recovered from the mixture by filtration, washed several times with water, and dried in a dryer at 100\u00a0\u00b0C overnight. The sorption-reduction procedure was repeated three times. After the procedure, the resulted materials were submitted to ultrasonic treatment (15\u00a0min) to check the stability of the metal-containing phase.The morphology and microstructure of the functionalized solids were investigated using a Zeiss EVO40 scanning electron microscope (Germany). The surface composition of the prepared materials was analysed by energy-dispersive X-ray spectroscopy (EDS) using a Princeton Gamma-Tech unit equipped with a digital prism spectrometer. Energy-dispersive X-ray fluorescence spectrometry has been carried out to evaluate the chemical composition of prepared samples using Epsilon 4 spectrometer equipped with a high-resolution silicon drift detector (SDD), typically 135\u00a0eV@ Mn-K\u03b1 (Malvern Pananalytical). The surface area, pore volume and average pore size were determined using an ASAP 2020 instrument (Micromeritics Instrument Co.). All samples were degassed at 120\u00a0\u00b0C for 24\u00a0h in a vacuum chamber before measurement. The surface area was determined by the multipoint BET (Brunauer\u2013Emmett\u2013Teller) method using adsorption data under relative pressure (p/po). The crystalline structure of the obtained materials was determined by a wide-angle X-ray diffraction method. A Rigaku Miniflex 600 analyser (Rigaku, Tokyo, Japan) operating with Cu K\u03b1 radiation (\u03b1\u00a0=\u00a01.5418\u00a0\u00c5) was used. The patterns were obtained over an angular range of 10\u201380\u00b0, with measurement steps of 0.02\u00b0 and 1\u00a0s dwell time. The analysis was based on the International Centre for Diffraction Data (ICDD) database. The XPS spectroscopy has been performed using a Prevac spectrometer (Prevac Ltd). Spectra were collected using a hemispherical Scienta R4000 electron analyser. Scienta SAX-100 x-ray source (Al K\u03b1, 1486.6\u00a0eV, 0.8\u00a0eV band) equipped with the XM 650 X-Ray Monochromator (0.2\u00a0eV band) were used as a complementary equipment. The pass energy of the analyser was set to 200\u00a0eV for survey spectra (with 500\u00a0meV step) and 50\u00a0eV for regions (high-resolution spectra): Co 2p, O 1s and C 1s (with 50\u00a0meV step). The base pressure in the analysis chamber was 5\u00b710\u22129 mbar. During the spectra collection, it was not higher than 3\u00b710\u22128 mbar. Fourier-transform infrared spectroscopy (FTIR) was carried out in ATR mode (Vertex 70, Bruker, Germany). The analysis was performed over a wavenumber range of 4000\u2013400\u00a0cm\u22121 (at a resolution of 0.5\u00a0cm\u22121). The influence of pH on the zeta potential was measured using a Zetasizer Nano ZS instrument equipped with an autotitrator (Malvern Instruments Ltd.) by analysing 0.01\u00a0g of catalyst in 25\u00a0mL of 0.001\u00a0mol/L NaCl solution at 25\u00a0\u00b0C. The suspensions' pH was automatically adjusted by an automatic titrator using sodium hydroxide (0.2\u00a0mol/L) or hydrochloric acid (0.2\u00a0mol/L). The zeta potential was obtained from the electrophoretic mobility by the Smoluchowski equation [27].The prepared materials were used as catalysts to treat the organic contaminants styrene, rhodamine B and 4-nitrophenol, in oxidation, decolourization and reduction, respectively. The reaction conditions are shown in \nTable 1. Detailed information about the catalytic tests is given in Supplementary note 1.The chemical composition and physicochemical properties of the prepared materials were investigated using SEM-EDS and XRF. In \nFig. 1, SEM images with EDS maps and the corresponding numerical data are shown.The cobalt percentage by weight varies with the support used; it takes the values 15.64%, 36.93% and 21.84% for Co3O4@C400, Co3O4@C500 and Co3O4@C600, respectively. The variation in cobalt content is related to differences in the elemental composition of carbonized supports as shown in Supplementary note 2, the increase in carbonization temperature results in an increase of carbon content and the changes of the content of other heteroatoms, which can impact the affinity of carbonized biocarbons towards cobalt [28,29]. Taking into consideration the conditions of the functionalization process and properties of used biocarbons, the general conclusion has been found that the chemical composition of C_500\u2009\u00b0C support and its highest surface area results in the best affinity to bind the cobalt species during the functionalization process.The presence of the heteroatoms may be explained by the spongin-based skeleton's chemical composition, which is built from amino acids (for more detail, please see Supplementary note 2). The existence of silicon in the form of silicon dioxide has been noticed either in biocarbons and materials after functionalization. This small amount of SiO2 is related to the presence of this element in the sponge structure [26]. On the other hand, aluminum and iron may be incorporated into the skeleton during the sponge's growth [30]. Moreover, the elemental composition was also analysed using the XRF spectroscopy, and corresponding data are gathered in Fig. 1E. The difference in the measured elemental content can be ascribed to the methodological approach of each technique, where during XRF analysis, the X-ray beam penetrates deeper into the sample. Likewise, XRF analysis results show the existence of iodine and bromine (the presence of these elements was not shown using the EDS method) and iron in all tested catalysts. The existence of iodine and bromine is a consequence of bromo- and iodothyronines presented in the amino acid chains of spongin-based material [18]. These results provide evidence about the diversity of elements, which will affect the variety of functional groups and enhance catalytic ability. The variation in the percentages by weight of sulfur, nitrogen and silicon dioxide in prepared composites is related to the different coverage of fibers with the metal-containing phase and the different carbonization temperature of support used to functionalization. Thus, differences in pyrolysis conditions affect the number, type and variety of chemical moieties existing on the surface of the carbonized scaffolds and then final catalysts [26].From the presented SEM results (Fig. 1), it is visible that the fibers are mostly tightly covered by the metal-containing phase. However, this phase is absent on some parts of the fibers, probably due to the irregular diffusion of the substrates and the process of mechanical cutting of the materials. The EDS mapping shows that the deposition of cobalt is uniform. A similar observation applies to the oxygen distribution, which is related to the chemical nature of the metal-containing phase consisting of cobalt oxides, as will be discussed in the XRD analysis. Moreover, to investigate the structure and morphology of the prepared catalyst, SEM images of the supports and functionalized catalysts were recorded at higher magnification, as shown in \nFig. 2.From the SEM images in Figs. 1 and 2, it is apparent that the carbonized spongin-based skeleton can be expected to be good support due to its unique fibrous organization. The fibers create a system of open porous channels with different shapes (such as rectangular, pentagonal, hexagonal), and the mesh diameter varies from several tens of micrometers to approximately 300\u2009\u00b5m. From the observation of the fibers' morphology before and after carbonization (Fig. 2), it is visible that the superficial microfibrillar arrangement \u2013 which is typical of non-carbonized spongin-based scaffolds obtained from commercial sponges (Fig. 2A) \u2013 has vanished. However, based on an HRTEM study, it was previously shown that even after carbonization at 1200\u2009\u00b0C the triple helices of collagenous origin in spongin are preserved. The development of a mesoporous surface is not yet observed, as described elsewhere [26].Changes in the morphology of fibers after functionalization can be observed. Examining more closely the morphology of the surface of the fibers (Fig. 2C, E, G), it is seen that each catalyst exhibits a different form of the metal-containing phase. Independently of the support used, the tendency to form rod-like and round-shaped aggregates are well visible. For the material Co3O4@C400, the fibers are covered with rod-like structures (marked with arrows in Fig. 2C) with a length of approximately 1\u2009\u00b5m. The surface of the Co3O4@C500 catalyst is characterized by the presence of sizable agglomerates with uneven shapes. For this material, similar rod-like structures can also be distinguished (marked with arrows in Fig. 2E), but they are significantly more prolonged and larger. Fig. 2G depicts the surface of the fibers of the Co3O4@C600 catalyst, which is covered with a phase containing two distinct structures: tiny rods (marked with an arrow) and aggregations of round-shaped particles (also marked with an arrow). These intriguing differences can be explained by the differences in physicochemical properties of applied supports (for comparison, see Supplementary note 2) as well as differences in the content of the cobalt-containing phase. With the increasing content of cobalt, the aggregation of cobalt oxide particles is apparent. Thus, the rod-like structures form larger aggregates, and round-shaped particles start to be formed and subsequently become a majority within crystallites.Besides the catalysts' morphology, the well-developed surface is crucial to ensure the proper diffusion and mass transfer. Therefore, the surface area analysis was carried out based on the nitrogen adsorption isotherms, and the obtained results are shown in \nFig. 3.BET analysis results reveal that the prepared catalysts possessed a relatively small surface with an average mesopore size varied from 19 to 26\u2009nm. The value of surface area differs with catalyst, and it is the smallest for Co3O4@C600 and the highest for Co3O4@C500 catalyst. Nevertheless, owing to the fact that the most trustworthy value of BET surface area of the precursor of carbonized material \u2013 spongin-based scaffolds \u2013 is equalled 3.5\u2009m2/g (r2 = 0.9998) [18], it is visible that carbonization followed by functionalization does not lead to the collapse of the structure and reduction of porosity. Moreover, comparing the results with data obtained for biocarbons, it is visible that functionalization results in decreasing the surface area of resulted materials, which is a consequence of the type of the functionalization process. The prepared catalysts possess a bigger pore size than corresponding biocarbons, but the volume of pores decreased. Observed for these materials isotherm type of IVa with hysteresis loop H3 is typical for plate-like aggregates where the pore structure mainly consists of macropores which are not entirely filled with pore condensate.Evaluation of the chemical nature of the metal-containing phase of the prepared catalysts is crucial to considerations of their catalytic ability. For this purpose, XRD analysis was carried out, and the corresponding diffraction patterns are shown in \nFig. 4, and the diffractograms of the biocarbons are present in Supplementary note 2.As was confirmed by the SEM\u2009+\u2009EDS studies, the results of XRD analysis show, as expected, a characteristic pattern corresponding to cobalt oxides (Fig. 4). The metal-containing phases of the prepared catalysts consist mostly of Co3O4 and also of CoO, which is confirmed by the presence of diffraction patterns characteristic for various phases of Co3O4 \u2013 such as Co3O4 (112), Co3O4 (211), and Co3O4 (004) (ICDD no. 1285798). Also, only for the Co3O4@C600 catalyst, the diffraction peak characteristic for CoO2 (312) (ICDD no. 14149) is observed, while for Co3O4@C400 and Co3O4@C500 an additional diffraction pattern corresponding to the CoO (200) phase (ICDD no. 22408) can be distinguished. Moreover, the intensity of diffractograms pattern varied with catalyst and is the highest for Co3O4@C500 catalyst and the lowest for Co3O4@C400. Such results are in agreement with the content of the metal-containing phase. It should be noted that the differences between the diffractograms of the biocarbons and functionalized materials are significant (for comparison, see Supplementary note 2). The decrease in the intensity of the broad peak corresponding to C(002) (ICDD no. 9011577) after cobalt oxide capping can be attributed to the heavy atom effect of cobalt [31,32].Considering that the surface composition of prepared catalysts has a crucial impact on the overall catalytic properties, the surface of the catalysts was investigated by XPS. The surface composition of prepared materials supports the results of EDS and XRF methods (for comparison, see Supplementary note 3, Fig. S5). The spectra for Co 2p and C 1s are shown in \nFig. 5.The cobalt signal has a typical profile with Co 2p3/2 and Co 2p1/2, showing the satellite peaks associated with Co2+ ions. This magnetic effect of Co2+ is typical for the cobalt oxide-based materials, and it is well described in the literature [33\u201335]. Either Co 2p3/2 and Co 2p1/2 profiles indicate that the surface species are a mixture of cobalt oxide species, mostly Co3O4 based on XRD results and the addition of cobalt hydroxide. Interestingly, the presence of cobalt hydroxide was not confirmed using the XRD analysis. It can be assumed that cobalt hydroxide is a surface feature related to moisture adsorption from the atmosphere.Based on the XPS deconvoluted spectra, it is apparent that the Co3O4@C600 catalyst is characterized by the lowest content of surface cobalt hydroxide and higher cobalt oxide. The catalytic activity of cobalt oxide is beyond doubt; therefore, a higher content of surface cobalt oxide could be related directly to higher catalytic activity. For C 1s profile, an asymmetric spectrum was obtained, whose deconvolution led to three secondary peaks: the principal peak is ascribed to C\u2012C bond with the energy around 285\u2009eV for all materials. The other two peaks at higher binding energies: 286\u2013287\u2009eV and 289\u2009eV; are referred as C\u2012O and C\u2012O\u2012O bonds, respectively, and are associated with carbonates species adsorbed from the atmosphere on the sample surface. Despite the similarities, for the Co3O4@C600 catalyst, the peak assigned for C\u2012O\u2012O bond is well separated from the principal peak due to a very low signal corresponding to C\u2012O (blue subpeak in Fig. 5D). That coincides with the highest content of surface cobalt oxide observed in Co 2p spectra (Fig. 5C) and could be evidence of cobalt-support interactions.As was mentioned before, higher temperatures of support preparation result in the preparation of different biocarbons that interact differently with cobalt. Nevertheless, it should be pointed out that the prepared solids differ from each not in the composition and oxidation state of cobalt-containing phases, but only in the content of these particular species, which is in agreement with the results of other performed analysis. The values of binding energies slightly vary between the catalysts, which can be explained by variation in elemental composition.The surface functional moieties of the catalysts were investigated using ATR-FTIR analysis. The spectra obtained are shown in \nFig. 6.An intense broadband characteristic for stretching vibrations of O\u2012H and N\u2012H groups (wavenumbers in the range 3500\u20133450\u2009cm\u22121) was observed. From a comparison with the FTIR spectrum of the pure spongin-based skeleton (for more details see [23,36]), it is evident that in these regions three peaks corresponding to O\u2012H and N\u2012H vibrations (3200\u20133000\u2009cm\u22121) together with peaks derived from C\u2012H (2950\u20132850\u2009cm\u22121) should be present. The observed strong overlapping of broad peaks characteristic for O\u2012H groups indicates the existence of cobalt-bound hydroxyl groups and probably hydrogen-bound water molecules. The peaks at 1440\u20131435\u2009cm\u22121 visible for all examined solids can be assigned to the O\u2012H bending vibrations characteristic for carboxylic groups. The peaks in the wavenumber range 1380\u20131370\u2009cm\u22121 presumably result from bending vibrations of phenolic O\u2012H bonds. Bands characteristic for stretching vibrations of S\u02edO bonds are well visible in the wavenumber range 1340\u20131350\u2009cm\u22121. These peaks provide additional confirmation of the presence of sulfur within the structure of the prepared solids. Bands characteristic for stretching vibrations of carbonyl groups (wavenumber 1770\u20131760\u2009cm\u22121) and bending vibrations of aromatic C\u2012H groups (wavenumbers in the range 800\u2013700\u2009cm\u22121) are present in each of the presented spectra. Besides, the FTIR analysis confirms the presence of cobalt oxide in the structure of the catalysts, based on a band characteristic for Co\u2012O vibrations at 540\u2009cm\u22121, and another at 640\u2009cm\u22121 associated with O\u2012Co\u2012O vibrations. The existence of these bands is a consequence of the influence of phenolic groups, which stabilize the process of the formation of Co3O4 particles during the reduction [12,13,37,38].Interestingly, although the superficial techniques (EDS, FTIR) reveal the absence of silica or its presence in only small amounts (less than 0.55\u2009wt%), in the XRD pattern, peaks corresponding to silica compounds are well distinguished. This suggests that silica moieties are located in the bulk of the prepared catalysts.Electrokinetic behavior was analysed to better and indirectly understand the colloidal properties of surface activity by measuring the zeta potential in NaCl solution as an electrolyte at various pH values, as illustrated in \nFig. 7.The zeta potential of Co3O4@C400, Co3O4@C500, and Co3O4@C600 catalysts is negative in the pH range 8\u20134 and becomes positive when the pH decreases below 4. The measured isoelectric points are 3.00 for Co3O4@C400, 3.40 for Co3O4@C500 and 3.50 for Co3O4@C600. This increase in the value of the isoelectric point for biocarbons with higher carbonization temperatures might be explained by changes in the content of electron-donating groups such as hydroxyl, amino or alkyl groups. The tendency can be observed that for Co3O4@C400 material, which has a significantly lower oxygen content, the isoelectric point has been measured at the lowest pH. Interestingly, comparing the curves of zeta potential vs pH of prepared catalysts with results obtained for biocarbons \u2013 the shift of isoelectric point towards higher values is observed (see Supplementary note 2 Fig. S4). Moreover, with increasing carbonization temperature, the IEP of prepared composites is shifted towards lower values, showing the opposite tendency when comparing the results presented for prepared catalysts to those obtained for biocarbons. Such results might be a consequence of the change of surface functional groups after the functionalization process, which is confirmed when FTIR spectra are compared.The dependence of the zeta potential on pH results from the protonation/deprotonation of the surface groups. In an aqueous solution, the surface of cobalt oxide is covered with hydroxyl groups, as was also shown by the FTIR measurements. Therefore, with the cobalt oxide surface denoted as \u2261M\u2012OH, the protonation (1) and deprotonation (2) reactions can be written as follows [39,40]:\n\n(1)\n\u2261M\u2012OH + H+ \u2194 \u2261M\u2012OH2\n+\n\n\n\n\n\n(2)\n\u2261M\u2012O\u2013 \u2194 \u2261M\u2012OH + H+\n\n\n\nIn general, the metal-containing phase of the prepared catalysts is hydrated in an aqueous environment. Thus at acidic pH (up to the isoelectric point), the surface of the catalyst is positively charged (together with the hydroxyl, carboxyl and amide groups of the support). However, with increasing pH, the deprotonation reaction occurs, and the surface functional groups then become negatively charged.Styrene oxidation was chosen as a model reaction to test the catalytic activity of the prepared solids. According to the literature, the solvent has an essential role during the catalytic oxidation of styrene and can influence the mechanism of oxidation [41,42]. Acetonitrile was chosen as a solvent because of its polarity. At the beginning of the investigations, hydrogen peroxide was applied as an oxidant. The results obtained are shown in Supplementary note 4. After these unsuccessful attempts, the oxidant was changed to tert-butyl hydrogen peroxide. The results are presented in \nTable 2.From the data in Table 2, it is apparent that the conversion of styrene ranges from 86.75% for the Co3O4@C400 catalyst to 93.2% for Co3O4@C600. Interestingly, the change of oxidizing agent led to a change of mechanism of the reaction because the epoxide is the main product, formed with a selectivity depending on the catalyst used (69.7% for Co3O4@C500, 74.6% for Co3O4@C400 and 72.4% for Co3O4@C600). Benzaldehyde was not observed as a product. However, from the GC results, it is evident that benzoic acid and phenyl acetyl aldehyde are the main minor products. The fact that Co3O4@C400 has the lowest oxidation activity of all investigated catalysts may be related to the fact that it has the lowest content of cobalt oxide within its structure. However, the most surprising result is that the highest oxidation activity was produced by the Co3O4@C600 catalyst, which has a lower content of cobalt oxide than Co3O4@C500. It appears that the content of cobalt oxide is not the main factor affecting the catalytic activity. One of the possible explanations for this is the structure of the two catalysts: the Co3O4@C500, despite having the highest content of cobalt, the metal-containing phase forms huge agglomerates, which can hamper the diffusion of the substrate and decrease the contact area between the reagents and the surface of the catalyst. In contrast, the Co3O4@C600 catalyst, with a lower cobalt oxide content, is characterized by diversified morphology, the highest content of surface cobalt oxide, and the largest average pore size. Thus, it seems to offer better access to the catalyst's surface.Because the Co3O4@C600 catalyst possesses the highest catalytic activity, this material was used to evaluate reusability. The same catalyst was used repeatedly five times. After each catalytic cycle, the catalyst was washed with acetonitrile and acetone and dried for 12\u2009h. The results indicate decreasing selectivity of styrene oxide formation, from 72.4% in cycle 1\u201333.6% in the fifth cycle (Table 2). The conversion of styrene also decreases, from 93.2% in the first cycle to 54.1% in the last catalytic run. Detailed data related to styrene and TBHP conversion and selectivity for the formation of styrene oxide and the respective TON values, are presented in Table 2.The most striking observation to emerge from the data concerns the changes in the conversion of TBHP with the catalysts used. The highest conversion is observed for the Co3O4@C600 catalyst, which also gives the highest styrene conversion. Co3O4@C400 and Co3O4@C500 produced similar TBHP conversion values (approximately 50%). For the Co3O4@C600 catalyst, both styrene and TBHP conversion decreased during the reusability tests. The variation in TBHP conversion may be the main reason for the decrease in selectivity for styrene oxide as well as the general decrease in the conversion of styrene. The decline of activity during the catalytic cycle is apparent. However, in the case of the TON value, a decrease occurred after the third cycle. Surprisingly, the TON value for the Co3O4@C500 catalyst is the lowest of all, which agrees with the proposed finding that the morphology of the catalyst surface significantly impacts the catalytic ability of the tested material.While the calculated rate constants are compared, it is apparent that in the reaction with Co3O4@C600 catalyst, the calculated rate constant exhibits the highest value \u2013 which is in accordance with all previously mentioned results. Moreover, during catalytic cycles, the value of the rate constant decreases by approximately 30% compared to the rate constant calculated for the first cycle. Important to note is the fact that when the values of rate constants calculated for other catalysts are compared, it is visible that the k value obtained after the fifth reaction cycle is still higher than the value calculated for the reaction with Co3O4@C400 catalyst. On the other hand, the value of the rate constant calculated for the reaction with Co3O4@C500 is significantly higher than for Co3O4@C400 catalysts, resulting from high styrene and TBHP conversions. Therefore, despite the high cobalt-containing phase and the highest BET surface area of Co3O4@C500, the best catalytic activity in the styrene oxidation was achieved by Co3O4@C600. That could be explained by the synergistic effect of diverse morphology of the surface's fibers with the highest content of surface Co3O\n4\n and the largest average pore size that impacts the catalytic activity.Furthermore, to evaluate the possible effect of the supports on the catalytic activity, the carbonized materials were also applied as catalysts. The results showed that spongin-based scaffolds carbonized at different temperatures exhibit activity in styrene oxidation. Interestingly the C_600 material possesses 90% conversion and 60% selectivity in styrene oxidation. While comparing results of Co3O4@C600 in the same reaction, they are slightly higher in both cases. Therefore, it could be concluded that functionalized materials are responsible for the high catalytic activity performance where improvement seems to be associated with selectivity. For biocarbons obtained at lower temperatures, the functionalization with cobalt species provides enhancement of catalytic properties in both conversion and selectivity; therefore, the role of cobalt cannot be denied. For more information, see Supplementary note 5.Based on the currently available literature [43\u201347], a mechanism for the oxidation of styrene by carbonized spongin-based skeletons functionalized with cobalt oxides is proposed, as shown in \nFig. 8.Firstly, TBHP molecules coordinate the Co sites, which results in the formation of CoIII-oxo species. At the next stage, those species are transformed into active CoIII-peroxo species due to the presence of TBHP molecules. Then the interaction between the CoIII-peroxo species and the styrene C\u02edC bonds results in the formation of peroxo metallocycles. At the last stage, the styrene oxide is formed due to the breaking of these peroxo metallocycle species, with simultaneous regeneration of CoIII sites. The lack of benzaldehyde as a reaction product implies that direct cleavage of the C\u02edC bond of styrene does not occur.After the successful oxidation of styrene, the prepared catalysts were applied to the decolourization reaction of rhodamine B in water to check the catalytic behavior of the prepared materials in an aqueous environment. Rhodamine B is a model dye frequently described in the literature [10,21,48,49]. The UV\u2013Vis spectra recorded for the reactions carried out with the catalysts Co3O4@C400, Co3O4@C500, and Co3O4@C600 are shown in \nFig. 9.First, it is important to note that the sorption process was not considered because, at the pH at which the reaction occurs, the surface groups of each catalyst are positively charged, as is the rhodamine B molecule. Consequently, Fig. 9 shows that, at acidic pH, after two hours of reaction, the \u03bbmax value typical for rhodamine B decreases gradually; only for the Co3O4@C400 catalyst is a peak still weakly visible after two hours (Fig. 9A). The Co3O4@C500 catalyst exhibits the best decolourization ability; this seems to be related to the fact that its structure contains the highest percentage by cobalt weight. Surprisingly, issues related to the impairment of catalytic activity by forming large crystallites and their agglomeration do not seem to play an important role for this catalyst in this reaction.Additionally, the acidic pH of the solution prevents the aggregation of rhodamine B molecules by the formation of electrostatic interactions between the xanthenes and the carboxyl groups of the dye molecules. On the other hand, it has to be noted that the repelling forces between positively charged functional groups of the catalysts (both the support and metal-containing phase) and the cationic form of the rhodamine B molecules do not affect the decolourization efficiency. Therefore, it can be assumed that the prepared catalysts serve as a place of formation of active radicals, which attack the dye molecules. The formation of hydroxyl radicals follows a pattern similar to the Fenton process, in which the Co2+ cations react with hydrogen peroxide to form hydroxyl radicals, as part of the reaction scheme outlined by the reactions (3)\u2013(5)\n[50,51]:\n\n(3)\nCo2+ + H2O2 + H+ \u2194 Co3+ + OH\n\u2219\n + H2O\n\n\n\n\n(4)\nOH\n\u2219\n + H2O2 + \u2194 HOO\n\u2219\n + H2O\n\n\n\n\n(5)\nCo3+ + HOO\n\u2219\n \u2194 Co2+ + O2 + H+\n\n\n\nThis finding is supported by the fact that when the decolourization reaction was carried out in the absence of a catalyst, the application of hydrogen peroxide did not lead to significant decolourization even after six hours of reaction (Fig. 9D), due to the formation of hydroxyl ions instead of hydroxyl radicals. Thus, it may be assumed that the oxidation mechanism involves a reaction between the rhodamine B molecule and hydroxyl radicals, which leads to the formation of intermediate products, followed by decomposition to final degradation products. The products of rhodamine B oxidation are discussed in detail in references [52\u201355].Similarly to the case of styrene oxidation, the Co3O4@C600 catalyst was subjected to reusability tests, as described in Supplementary note 6. These promising results show that a cobalt oxide-based catalyst can be successfully applied in the decolourization reaction several times with an eco-friendly oxidant without losing activity. This is especially important considering that in the existing literature, the removal of rhodamine B has usually been achieved using complicated oxidation systems or photocatalytic processes [21,56,57].The successful application of the prepared catalysts in oxidation reactions motivated us to investigate the possibility of using them for reduction reactions. Therefore, the prepared materials were applied in the standard reduction of 4-nitrophenol to 4-aminophenol in water. The changes in the absorbance in time are shown in \nFig. 10. Interestingly, this reaction does not occur without the addition of a catalyst due to kinetic restrictions. Thermodynamically, the reaction between 4-NP and sodium borohydride is possible (E\n\n0\n is \u22120.76\u2009V for 4-NP/4-AP and \u22121.33\u2009V for H3BO3/BH4) [26,58,59].All of the prepared catalysts exhibited similar catalytic activity in the reduction of 4-nitrophenol. Independently of the catalyst used, 100% reduction was achieved after five minutes of reaction. Therefore, to investigate which material has the best catalytic activity, the reduction's kinetics was calculated based on a pseudo-first-order model (see Supplementary note 1). The calculated rate constants are similar for each catalyst; the values are 0.642\u2009min\u22121 for Co3O4@C400, 0.742\u2009min\u22121 for Co3O4@C500, and 0.755\u2009min\u22121 for Co3O4@C600. However, only for the Co3O4@C600 material is there a visible induction period. This may be a consequence of the change of the local surface charges of the catalyst, as well as slower diffusion of the substrate towards the catalyst surface. However, considering the preparation of the support, where an increase in carbonization temperature led to the enhancement of the skeletal structure of the support [26], the size of fibers and diameter of channels do not change significantly with the support used. Thus, the diffusion of the substrate should not be a limiting step. Therefore, the rearrangements of the surface charges are believed to impact the existence of an induction period. This view agrees with the zeta potential measurements (compare Fig. 6), which demonstrate that at basic pH, the surface of the catalyst is deprotonated and negatively charged. Nevertheless, the 60\u2009s induction period does not affect the reaction time and has essentially no impact on the rate constant. This catalytic behavior of the Co3O4@C600 material is in line with the previously presented results for the oxidation of styrene and rhodamine B dye (see Fig. 9 and Supplementary note 6). For comparison, the results of 4-nitrophenol reduction using biocarbons are presented in Supplementary note 7.Because the Co3O4@C600 material exhibits the best catalytic ability to reduce 4-nitrophenol, this catalyst was chosen to investigate reusability. The results are presented in \nFig. 11 in the form of a plot of C\n\nt\n/C\n\n0\n vs time.The findings indicate the prolongation of the reaction time with each cycle, from 5\u2009min for the first run to 13\u2009min for the fifth run. It is important to note that the induction period obtained with the Co3O4@C600 catalyst in the first run was not observed in the succeeding catalytic cycles. Interestingly, these results can be correlated with the irreversible change in the catalyst's surface charges after the first reaction cycle. Also noteworthy is the significant decrease in the rate constant after each run, which is a consequence of the prolonged reaction time (for comparison see Fig. 11B).Considering the shape of the curves of C\nt\n/C\n0\n vs. time (Fig. 11A), the rate constant for cycles 2\u20135 was calculated for the time 4\u201313\u2009min. It was also found that the calculated reduction efficiency in the 5th minute for cycles 2\u20135 was high, at approximately 90%. This results from the rapid conversion within the 5\u2009min period (well visible in Fig. 11A, from the steep C\n\nt\n/C\n\n0\n vs. time curve for each cycle). The increase in the time taken for the reaction to terminate may be explained by blocking the active sites of the catalyst and partial deactivation of the catalyst surface and loss of catalyst during recovery and filtration.Finally, a reaction mechanism is proposed. According to the literature, there are two main routes of 4-NP reduction using metal particles. The first is related to surface-mediated hydrogen transfer [60], and the second occurs by electron transfer via metal particles [61]. In both mechanisms, the sorption of the reactants occurs before the hydrogen or electron transfer. The reduction occurs due to electron relocation from a BH4\n\u2212 anion to a 4-NP molecule via the catalyst surface, in so-called surface-mediated electron transfer. In this case, the metal-containing phase is considered as an electron reservoir, and thus the size of the cobalt oxide grains has a significant effect on the catalytic activity [62,63]. This is in line with the results obtained for the Co3O4@C500 catalyst. However, the other reduction path should also be taken into consideration. Therefore, based on the kinetic study and current literature [60,62\u201364], the authors suggest that the overall reduction mechanism over cobalt-based catalysts involves firstly the production of hydrogen radicals by electron transfer from borohydride, and then the addition of hydrogen species to 4-NP molecules (see Fig. 11C).Furthermore, the catalytic properties of the prepared materials in styrene oxidation and 4-nitrophenol reduction were compared with other cobalt-based catalysts. Data are presented in \nTable 3.In summary, three different cobalt oxide-based catalysts were prepared by the carbonization of spongin-based skeletons at temperatures of 400, 500, and 600\u2009\u00b0C, followed by metallization via the sorption-reduction method. Such an approach of low-temperature spongin carbonization to obtain functional catalysts has been presented for the first time. The spongin-based skeletons are a good and stable source of a renewable, fibrous 3D material with predictable morphology and properties. Moreover, the utilization of spongin-based skeletons fits well into the biomimetic approach, thus generating promising possibilities in designing novel organic and inorganic materials suitable for a wide range of applications. The exhaustive physicochemical analysis of prepared materials has been shown with the comparison to properties of biocarbons. The presence of cobalt oxide as the main cobalt species on the surface of these materials has been proved. Consequently, these composites have been successfully tested as potential catalysts in oxidation-reduction reactions in different conditions. It has been shown that all of these materials possess superior catalytic properties in the oxidation of styrene, decolourization of rhodamine B and reduction of 4-nitrophenol \u2013 comparable to other materials described in the literature. The promising catalytic activity of the presented materials can be ascribed to synergistic effects of the support and the metal-containing phase. However, the results show that the temperature of the carbonization process affects the chemical composition and structure of the final product. The catalytic study demonstrated that H2O2 is an inactive oxidant during the oxidation of styrene, and that changing the oxidant to TBHP results in successful oxidation of styrene to styrene oxide, with high selectivity. The reusability tests carried out for the Co3O4@C600 catalyst showed the possibility of its repeated application without significant loss of activity in each tested reaction. The study demonstrates the possible application of carbonized spongin-based scaffolds functionalized with cobalt oxide as catalysts in different types of oxidation and reduction reactions.\nSonia \u017b\u00f3\u0142towska: Conceptualization, Investigation, Writing - original draft, Writing - review & editing, Visualization. Juan F. Minambres: Investigation, Writing - review & editing. Adam Piasecki: Investigation. Florian Mertens: Investigation, Resources. Teofil Jesionowski: Supervision, Writing - 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, and the Ministry of Education and Science, Poland. Sonia \u017b\u00f3\u0142towska and Teofil Jesionowski would like to thank Professor Monika Mazik of TU Bergakademie Freiberg for assistance with the catalytic tests. The XPS research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-06024/09 \u2013 Centre of Functional Nanomaterials).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2021.105631.\n\n\n\nSupplementary material\n\n\n\n.", "descript": "\n                  This study concerns an application of spongin-based scaffolds of commercial sponge origin as a naturally structured precursor of carbon material. Further functionalization with cobalt via a simple sorption-reduction method resulted in the preparation of novel catalysts tested in oxidation-reduction reactions. The structure and chemical composition of the prepared materials were investigated in detail, demonstrating the presence of carbonized fibers tightly covered with a metal-containing phase mainly composed of Co3O4. The fibrous structure with open porous canals provides good accessibility for substrates to the surface of the catalysts. Biocarbon material obtained at 600\u00a0\u00b0C exhibited good catalytic ability in the oxidation of styrene (with high selectivity for the formation of styrene oxide) and rhodamine B compared with other prepared catalysts and biocarbons. Interestingly, all of the prepared materials exhibit favorable activity in the reduction of 4-nitrophenol. A reusability study showed good activity even after the fifth catalytic cycle in both oxidation and reduction reactions. The study proved the adaptability of spongin-based scaffolds to prepare biocarbons with high potential to be used as a support for various catalytic applications.\n               "}
{"full_text": "Most of the chemical reactions for the production of the molecules and materials, such as in the fields of chemical manufacturing, energy conversion, environmental remediation, and human health care, make use of catalysts, either in a homogeneous or heterogeneous process.\n1\n Generally speaking, the homogeneous operation mode suffers from catalyst reusability and severe catalyst losses. Heterogeneous catalysis offers a much more convenient separation of products from catalysts and is thus vital to many chemical industries.\n2\u20135\n Modern society has an ever-increasing demand for environmentally friendly catalytic processes to improve the efficiency of the chemical industry, to reduce emission of pollutants, and to insist sustainable development strategy. In the face of present and future challenges of green chemistry, one of the most important tasks is to develop new catalysts.\n6\n Metal materials are the most widely used heterogeneous catalysts, with catalytic performances strongly dependent on their composition, size, morphology, and structure.\n7\n Significant development of modern surface science and computational methods in recent years has made it possible to understand the fundamental factors that govern catalytic performances of metals.\n8\n Up until now, extensive effectors have been made to improve the catalytic performances of metallic catalysts via rational and precise design of powerful metallic catalysts. In addition, several review articles dealing mainly with the principle of preparation of metallic materials and the correlation between the catalytic performances and their fundamental aspects (structural and electronic properties) have been published.\n9\u201314\n\nInspired by natural materials with special functions resulting from their unique composition and/or size, morphology, and structure, the design of catalysts with a controllable composition and/or morphology has been the subject of great attention because of their fascinating functions and enhanced properties. In terms of heterogeneous metallic catalysts, extensive in-depth studies have revealed that metals have specific catalytic activity for different kinds of reactions as a result of the diverse radial expansion of d bands of metals and thus the different adsorption strengths of substances on their surfaces. Besides the composition, control over their size, morphology, and structure is essential and necessary for developing superior catalysts, which will result in enhanced intrinsic activity, increased surface-active area, and an improved adsorption model of substances on the surface of catalysts.\n15,16\n On the basis of such concepts, herein we summarize the recent developments in designing highly efficient metal catalysts with a particular focus on their structure (Figure\u00a01\n). Meanwhile, some novel metal-catalytic reaction systems are also discussed.As we know, the catalytic performances of metal catalysts depend strongly on their surface properties. Therefore, their activity and even selectivity can be controlled by tuning the morphology because it can determine the number and nature of the exposed surface atoms serving as active sites.\n14\n Recent breakthroughs in the synthesis of nanostructured materials have achieved control of the morphology of materials that are relevant for catalyst design. Many groups have paid intensive attention and made great progress in exploiting metal catalysts with controllable morphology. In the following section, we will discuss some salient features of morphology-controllable synthesis of metallic catalysts using examples mainly from our research.Heterogeneous catalysis is a phenomenon that is exclusively dependent on the reactivity of surface atoms. As a result, a high surface-area-to-volume ratio is desired, that is, more surface-active atoms exposed to reactants. In term of metal catalysis, this demand can be equated with a high degree of dispersion of the metal or a very small metal particle size. The most frequently applied method of metal catalyst preparation is solution-based reduction of metallic precursors. Because of the isotropic structures of metals, they are prone to grow into bigger nanoparticles (NPs). Because the reaction between metallic precursors and reductants is usually highly exothermic, particle aggregation inevitably occurs owing to the high local temperature. Consequently, the metal catalysts prepared by the traditional solution-phase synthetic method usually have low surface areas and broad size distributions, which are harmful to activity, selectivity, and even thermal stability. The ability to synthesize monodispersed metal catalysts permits one to improve their catalytic properties. There are a number of examples demonstrating the influence of catalyst particle size on the reaction performances and discovering the aspects of particle-size-dependent phenomena, which can hitherto be helpful in interpreting structure-sensitive reactions. Therefore, size-controllable synthesis of uniform metal NPs enables the study of size effects on the properties of metal catalysts. Meanwhile, size-controllable synthesis of monodispersed metal catalysts by modified chemical reduction methods is also highly desirable.Abundant publications have already highlighted the benefits of using stabilizers such as surfactants and polymers, which can bind strongly to metallic precursors for precisely tuning the metal particle size.\n17\n However, the resultant metal NPs always present very limited catalytic activities because of the lack of a \u201cclean\u201d NP surface because of the presence of strongly binding capping agents. Usually, high-temperature treatments are required to remove those organic capping agents, which often inevitably cause deformation and aggregation of metal NPs, leading to the deterioration of catalytic performances. Oleylamine-capped Pd NPs can be synthesized by reduction of palladium (II) acetylacetonate [Pd(acac)2] with tert-butylamine borane in the presence of oleylamine.\n18\n The as-prepared oleylamine-capped Pd was present in the form of uniformly ultrafine NPs with an average particle size of about 2.8\u00a0nm (Figure\u00a02\nA). Recovery of these oleylamine-capped Pd NPs through subsequent reactions was rather challenging because of their colloidal properties. To utilize the activity of the Pd NPs while allowing easy reuse, the oleylamine-capped Pd NPs were immobilized into channels of mesoporous silica such as SBA-15. After that, the supported Pd@SBA-15 was subjected to acetic acid treatment for removing oleylamine molecules, leading to a stabilizer-free \u201cclean\u201d Pd surface. One-pot dynamic kinetic resolution (DKR) of racemic alcohols to chiral acetates is a powerful tool to synthesize enantiomerically pure alcohols. However, improving the matchability of metal-catalyzed racemization and enzymatic resolution is still a difficult task.\n19\n The size-controlled 2.8\u00a0nm Pd NPs can deliver a sufficiently high racemization rate for racemic secondary alcohols to ensure the continuous feed of the faster-reacting enantiomer to the immobilized lipase B, leading to an optically pure product with an excellent yield under microwave irradiation (Figure\u00a02B).Control of metal precursors reduction kinetics is a key process for achieving metal shape control. To tackle the metal surface pollution problem, we developed an effective capping-agent-free approach to synthesize size-controllable metal NPs by ultrasound-assisted reduction of M(NH3)6\n2+ (M\u00a0= Co, Ni) with borohydride.\n20\n The particle size can be controlled by adjusting either the ultrasound power or the ultrasonication time. Because of the strong coordination of NH3 to metallic ions, the reduction process is very slow, resulting in relatively large metal particles (>100\u00a0nm), which is unfavorable for the catalytic activity. Taking into account the weaker coordination between halide ligands and metallic ions than that between NH3 and metallic ions, [CoX4]2\u2212 formed in the presence of KCl and Bu4PBr was used as a metallic precursor for the synthesis of Co-B amorphous alloy.\n21\n The simple reduction of [CoX4]2\u2212 with borohydride resulted in monodispersed and uniformly spherical NPs (Co-B-X) with an average particle size about 55\u00a0nm (Figures 3A and 3B). A series of controlled experiments demonstrated that both KCl and Bu4PBr play a key role in fabricating such monodisperse Co-B NPs with crack channels (inset in Figure\u00a03A) and higher surface B content. On the one hand, KCl and Bu4PBr provide halide anions (Cl\u2212 and Br\u2212) to form [CoX4]2\u2212 by coordinating with Co2+. On the other hand, the stabilizing effect of Bu4P+ ions can prevent the agglomeration of Co-B clusters. The as-synthesized Co-B-X amorphous alloy was subjected to Heck-olefination of iodobenzenes (Table 1\n) under ligand-free conditions in a mixed solution containing dimethylformamide (DMF)/water\u00a0= 1/1 as the solvent and K2CO3 as the base, which exhibited the enhanced activity up to two times with the conventionally prepared Co-B-C (Table 1, entries 1 and 2). Besides the larger number of Co active sites, the more electron-enriched Co in the Co-B-X resulting from electron-donation of\u00a0B\u00a0also plays a promoting effect, which allows a more favorable oxidative addition of the metallic Co to the carbon-halogen bond. Such a catalyst can be used repetitively 11 times with only a slight loss of activity (8%) for butyraldehyde hydrogenation to n-butanol (Figure\u00a03C). Both the enhanced surface B content and the uniform particle size of Co-B-X led to a prominent increment in its thermal stability, leading to a higher durability than the conventionally prepared Co-B-C (Figure\u00a03C).We have synthesized ultralong single-crystalline Cu nanowires (CuNWs) with excellent dispersibility via thermal reduction of copper acetylacetonate (Cu(acac)2) in a liquid-crystalline medium.\n22\n In a typical run of synthesis, hexadecylamine (HAD) and cetyltriamoninum bromide (CTAB) were mixed at an elevated temperature to form an ordered liquid crystal (Figure\u00a04\nA). Then, Cu(acac)2 rapidly coordinated with Br\u2212 from CTAB and HAD, leading to the metal moieties enriched within the tubular channels. Subsequently, metal clusters and particles were built in these channels after reduction. Cu could grow up along one direction to form the ultralong nanowires morphology as shown in Figures 4B and 4C. The average diameter of the nanowires was \u223c78\u00a0nm, and their lengths varied from tens to hundreds of micrometers. On the basis of the superconductivity of CuNWs, we developed some composite materials with enhanced performances in both catalysis and energy storage, such as CuNWs-ZIF-8 with a core-shell structure, CuNWs-TiO2 with direct growth of rutile nanorods, and a CuO nanotube-graphene sandwich structure. All of these unique architectures could be synthesized by the microwave-assisted thermal reaction because CuNWs were induced as super-hot surfaces by microwave irradiation, which facilitated the nucleation and growth of crystals. Moreover, the strong interfaces created by microwave synthesis between two components could favor the charge transfer during a catalytic reaction or electrochemical reaction. For example, uniform ZIF-8 NPs were prepared by direct growth and coverage on CuNWs surfaces, which exhibited high catalytic activity and stability in H2 production via NH3BH3 hydrolysis. Such one-dimensional CuNWs could offer a rapid electron transfer channel. Meanwhile, those ZIF-8 NPs rapidly transferred H\u2212 and H+ ions toward Cu active sites. By a similar process, TiO2 nanorods could also assemble and grow directly on the surface of CuNWs to form a nanorod-nanowire structure; see the field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images and scheme in Figures 4D\u20134F.\n23\n In this composite, TiO2 nanorods promoted light harvesting via multiple reflections to generate more and more photoelectrons. The one-dimensional CuNWs facilitated the transfer and gathering of the photoelectrons as a result of the Schottky barrier at the interface illustrated in Figure\u00a04G, leading to a high H2 production rate during photocatalytic water splitting. The H2 production rate reached up to 5,104\u00a0\u03bcmol h\u22120 g\u2212g with an apparent quantum yield (AQY) of 17.2%, which was remarkably high among the noble-metal-free TiO2-based photocatalysts and even exceeded the activity of Pt loaded TiO2 nanorods.Recently, triangular metals have attracted increasing attention owing to their excellent properties from the extremely high anisotropy.\n24\u201327\n However, fabrication of a highly anisotropic structure is not thermodynamically favorable since most of the bulk metals display a face-centered cubic shape. Controlled colloid chemistry reaction is one of the main techniques for synthesizing triangular metals in which the capping ligand has proven to be critical. However, the capping ligand alone cannot guarantee the symmetry breaking.Chen et\u00a0al. developed a rapid seedless growth process to synthesize monodisperse triangular gold nanoplates with sharp tips and tailored edge lengths (Figures 5A and 5B).\n28\n Systematic studies reveal that iodide ions can promote the formation of triangular gold nanoplates by both selective binding onto the Au {111} facets and oxidative etching to remove other less stably shaped impurities by forming tri-iodide ions (I3\n\u2212), leaving behind dominant planar-structured nuclei (Figure\u00a05C).Soon afterward, Gangishetty et\u00a0al. synthesized AuPd bimetallic nanotriangles through reduction of K2PdCl4 onto the as-prepared Au nanotriangles using ascorbic acid as the reducing agent (Figure\u00a06\nA).\n29\n During Suzuki coupling reactions, the plasmonic Au nanotriangles were used to harvest light to improve the catalytic activity of Pd (Figure\u00a06B). Upon exciting the surface plasmons in AuPd nanotriangles using green LEDs with wavelengths near the maximum of the plasmon band, a significant improvement in the reaction rate of Suzuki was observed (Figure\u00a06C). The nanotriangular structure can initiate strong plasmonic heating effects because of the sharp features and can also provide a much greater number of active sites for catalytic reactions because of the plate-like morphology. Despite the efficiency, the sharp-featured triangles are unstable since severe deactivation was observed after two cycles of Suzuki reactions.Wang et\u00a0al. successfully prepared well-defined CuNi nanocubes by co-reduction of Cu and Ni metallic precursors (Figure\u00a07\nA).\n30\n The key process was supposed to apply borane morpholine as a reducing agent, resulting in the explosive generation of H2 molecules. More interestingly, the obtained CuNi nanocrystals transformed from octahedral to cubic morphology with increasing borane morpholine. Some chemicals derived from borane morpholine in the synthetic systems seemed to act as capping agents and selectively adsorb onto the (100) facets, making these facets thermodynamically more favorable by reducing their interfacial free energies through chemisorption. A3 coupling reaction with alkyne, aldehyde, and amine was chosen as a model reaction to investigate the dependence of catalytic activity on the facets. Under the same reaction conditions (4 h, 120\u00b0C), the catalytic activity of Cu50Ni50 nanocubes was 2.2 times that of Cu51Ni49 octahedra (Figure\u00a07B). Density functional theory (DFT) calculations were performed to elucidate the different catalytic activities of CuNi nanocrystal among different exposed facets. It was found that the surface energy of the (100) facet is always higher than that of the (111) facet at the same molar ratio, which could account for the higher catalytic activity of Cu50Ni50 nanocubes than Cu51Ni49 octahedra.In comparison with a convex structure, the nanocatalysts with an excavated structure expose a larger accessible surface area and higher density of low-coordinated atoms (e.g., edges, corners, and kinks), which provide more active sites and thus promote the activity. Very recently, an approach to the synthesis of Pt3Co nanocubes was developed, which allows the control of the concavity of their surface by just altering the feeding amount of deionized water (Figures 8A\u20138C).\n31\n The formation of deeply excavated Pt3Co nanocubes was attributed to the combination of facet-selective capping and oxidative etching (Figure\u00a08D). For both methanol and formic acid oxidation reactions, such a nanocube catalyst with a high degree of concavity exhibited high electrochemical surface area (ECSA) and catalytic activity (Figure\u00a08E), which was attributed to the large surface area, the high energy facets, and low-coordinated atoms.Most of the metals prepared traditionally by solution-phase synthesis are present in the form of solid particles. In general, the particle size should be reduced to be as small as possible to obtain more efficient metal catalysts. However, very tiny metal particles usually add problems in catalyst separation and also induce agglomeration because of high surface energy, leading to a decrease in catalytic efficiency. Porous materials represent a new class of powerful catalysts because they offer advantages over their dense counterparts in terms of increased surface area, easy recovery, and controlled porosity. In the upcoming section, we will discuss the recent progress in the synthesis of porous metals and the studies of their catalytic performances.Nanoporous Au can be synthesized by means of the dealloying of Au-Ag alloys with nitric acid.\n32\n The monolithic Au consists of a three-dimensional network of ligaments with diameters from 10 to 50\u00a0nm (Figure\u00a09\nA), depending on the preparation conditions. Meanwhile, it contains a large fraction of low-coordinated Au on the surface of the porous material. As a result of a porous structure permeable for reactants and stable without any support, such Au catalysts delivered selectivities above 97% and high turnover frequencies at temperatures below 80\u00b0C in gas-phase selective oxidative coupling of methanol to methyl formate (Figures 9B and 9C). The surprising reactivity of nanoporous Au was ascribed to the unaltered selective surface chemistry of Au and the efficient dissociation of O2.Skeletal rapidly quenched (RQ) metals can be synthesized by alkali leaching of RQ alloys, a similar route to that for the preparation of Raney nickel. Qiao\u2019s group reported a skeletal RQ Fe dealloyed from the RQ Fe50Al50 alloy, in which the Fe-Fe coordination number (CN) is 4.0, only half of the standard Fe-Fe CN in body-centered-cubic (bcc) Fe. Moreover, the Fe-Fe distance (R) is expanded to 2.50\u00a0\u00c5, significantly longer than 2.48\u00a0\u00c5 in the bcc Fe (Figure\u00a010\nA).\n33\n The skeletal RQ Fe is highly reactive and can be used to synthesize a \u025b-Fe2C-dominant catalyst in low-temperature Fischer-Tropsch synthesis (LTFTS) at 423\u2013473 K (Figure\u00a010A). The structural peculiarities of this skeletal RQ Fe (nanocrystalline dimensions, low CN, and expanded lattice) are essential to overcome the seemingly insurmountable hindrance so that the carbidation of metallic Fe to \u025b-Fe2C is kinetically limited at a low temperature, taking into account that the \u025b-Fe2C phase is stable only at low temperature. The as-prepared \u025b-Fe2C-dominant catalyst exhibits superior activity for LTFTS in comparison with the reported Fe and Co catalysts. Moreover, this catalyst displayed activity comparable to that of the noble Ru catalyst, together with the high selectivity to liquid fuels and robustness without the aid of electronic or structural promoters (Figures 10B and 10C). By means of the same approach, the skeletal RQ Ni with peculiar undercoordinated site (UCS) abundant and tensile-strained structural characteristics can also be synthesized.\n34\n This catalyst has superior activity in the low-temperature COx methanation, in which the turnover frequency (TOF) of CO2 is about eight times that of the highest TOF of CO2 ever reported at 473 K. The DFT calculations reveal that the CO activation barrier decreases when the Ni\u2013Ni distance expanded from 2.49 to 2.51\u00a0\u00c5 with tensile strain on the Ni (111) surface. The superior activity confirms that the UCSs are the active centers for COx methanation and the tensile-strain effect can further accelerate the rate-limiting CO dissociation step. Therefore, more efforts should be aimed at fabricating undercoordinated catalytic materials.Since the first discovery of mesoporous Pt in 1997 by Attard and coworkers,\n35\n mesoporous metal catalysts have attracted much interest because of their high porosities, large surface areas, tunable pore sizes, narrow pore-size distributions, high electroconductivities, and excellent activity-structure relationships. To date, various mesoporous metals in bulk, thin film, and powder forms have been synthesized based on different strategies including electrochemical depositions, galvanic replacement reactions (GRRs), as well as soft- and hard-templating techniques.\n36\u201343\n Besides enhanced surface areas, mesoporous metals possess a concave inner surface, which can improve catalytic selectivity through changing the adsorption model of reductants.\n20\n\nRecently, Yamauchi\u2019s group developed a robust, scalable synthetic strategy to generate mesoporous noble metals (e.g., Pt and Rh) via chemical reduction on general polymeric micelle templates, which is different in concept from traditional soft-templating and hard-templating approaches.\n39\u201341\n Preparing high-surface-area Rh at mild conditions is extremely challenging because its surface energy is larger than that of other noble metals, such as Pt, Au, and Pd. The first synthesis of mesoporous Rh was achieved successfully a Rh precursor salt (Na3RhCl6) on self-assembled polymeric PEO-b-PMMA micelle templates (Figures 11A and 11B).\n41\n Ascorbic acid is used as a reducing agent, and DMF/H2O is selected as a mixed solvent. This synthesis strategy generally involves five steps (Figure\u00a011C): (1) the addition of water causes the PEO-b-PMMA to self-assemble into spherical micelles with a PMMA core and a PEO shell; (2) Na3RhCl6 is dissolved in the solution containing Na+ and [Rh(H2O)3-xCl6-x](3-x)\u2212 and then interacts with the aqua complexes with the PEO moieties to form PEO-b-PMMA/[Rh(H2O)3-xCl6-x](3-x)\u2212 composite micelles; (3) the Rh ions are reduced to form solid Rh nuclei; (4) Rh nuclei coalesce and further grow into mesoporous Rh nanostructures over the templates; and (5) the templates are removed by a solvent extraction. The as-prepared mesoporous Rh NPs display high surface area with abundant low-coordination atoms, which exhibit great thermal stability. During the electrocatalytic methanol oxidation reaction (MOR), the mesoporous Rh NPs exhibit \u223c2.6 times higher activity than that of commercial Rh catalysts (Figure\u00a011D). The Rh NPs also exceed the performance of commercial Rh catalysts for the remediation of nitric oxide (NO) in lean-burn exhaust containing high concentrations of O2 (Figure\u00a011E).In recent years, hollow structures with uniform morphology and good stability have become attractive because of their widespread applications in nanoreactors, adsorption, drug delivery, microelectronics, photonics, and catalysis.\n44\n In the domain of catalysis, hollow-structured metals with permeable shells represent a new class of efficient catalysts because they offer advantages of high surface area, light density, easy recovery, self-supporting capacity, low cost, and good surface permeability.\n45\n More importantly, the presence of a concave inner surface in hollow metal nanospheres can exhibit improved catalytic performances relative to that of the NPs exposing only a convex surface, as found on the abovementioned mesoporous metal. Generally, the synthesis of hollow-structured metals can be classified into two categories: soft-templating and hard-templating techniques.The soft-templating strategy is generally used to coat metals onto the surface of \u201csoft\u201d templates by an interfacial reduction reaction. Many endeavors are being devoted to synthesizing hollow metals through a simple vesicle-assisted chemical reduction approach in our laboratory.\n20\n On the basis of metallic ion-Bu4PBr composite vesicle template, we successfully synthesized a series of Pd-based nanospheres with a hollow chamber, which showed much higher activity than the dense counterpart NPs during liquid-phase hydrogenation or C-C coupling reactions. However, most transition-metal compounds, such as FeCl3, CoCl2, and NiCl2, cannot induce vesicle formation in this way; the hollow-structured metal catalysts are only limited in noble metals. Very recently, we fabricated hollow Ni-Co-B amorphous alloy nanospheres through a vesicle-assisted chemical reduction method.\n46\n The basis for this synthesis is the use of a Bu4P+/[MX4]2\u2212 (M\u00a0= Ni and Co) composite vesicles templates (Figure\u00a012\nA), where spherical vesicle precursors are first formed by the electrostatic interaction between [MX4]2\u2212 and Bu4P+. The addition of borohydride induces chemical reduction of the confined [MX4]2\u2212 ions into Ni-Co-B clusters, which then develop into particles coating the vesicles to construct a thin shell surrounding the vesicle templates, leading to the hollow spheres composed of uniform NPs (Figure\u00a012B). Coexistence of NiII and CoII species plays an important role in fabricating hollow nanospheric structure because only solid NPs can be obtained in the presence of a mono-metallic precursor. Co-reduction of mixed [NiX4]2\u2212 and [CoX4]2\u2212 with a Ni/Co molar ratio of 1:1 forms hollow nanospheres in high yield. Previous studies revealed that the chemical preparation of M-B is an autocatalytic reaction, and Ni-Co-B with a Ni/Co molar ratio of 1:1 possesses the highest catalytic reactivity. As a result, co-reduction of a mixture containing the same amount of [NiX4]2\u2212 and [CoX4]2\u2212 affords a highly active Ni-Co-B catalyst with a stable wall thickness. Such a hollow Ni-Co-B displays a surface-active area of 30 m2 g\u2212g, much higher than 16\u00a0m2 g\u2212g of the dense Ni-Co-B. During liquid-phase hydrogenation of 2-ethyl-2-hexenaldehyde (EHEA), this hollow Ni-Co-B catalyst exhibits much higher activity and selectivity than the dense Ni-Co-B catalyst prepared by direct reduction of the mixture of nickel ions and cobalt ions with borohydride (Figure\u00a012C). Additionally, this catalyst is also easily handled in liquid-phase reactions because of its lower density and magnetic property, which allow the reuse for more than seven times with a significant decrease in activity (Figure\u00a012D). This work opens a new avenue for the development of hollow non-noble metal catalysts.The fabrication of hollow metal catalysts with tunable inner and outer diameters is not easy because of the \u201csoft\u201d nature of the structuring units and the difficulty in controlling the phase behavior of surfactants. Therefore, the hard-templating technique is used to deposit a metallic shell onto the surface of \u201chard\u201d templates (e.g., polymer colloid beads and silica spheres) via a layer-by-layer technique and the subsequent template removal. Such a process shows advantages in transcriptive imprinting of the template morphology and finely tuning the chamber size and shell thickness. However, this method is relatively complicated because multiple steps are often necessary. Strategies based on the mechanisms of galvanic replacement have also been developed to fabricate hollow metal catalysts, which are more facile and cost-effective since the templates could also act as reducing agents to produce metal shells, and thus no additional steps are needed to remove the templates. By using Ni NPs with a tunable size as sacrificial templates, we synthesize hollow Pt-Ni alloy nanospheres with controllable chamber size (7\u2013350\u00a0nm) and shell thickness (1.8\u201322\u00a0nm) through a modified galvanic replacement approach (Figures 13A\u201313D).\n47\n First, Ni NPs templates are fabricated through chemical reduction of Ni2+ ions with borohydride. The surface of the as-made Ni NPs is wrapped with poly(vinylpyrrolidone) (PVP), which acts as a stabilizer to protect Ni NPs from agglomerating. When PtCl6\n2\u2212 aqueous solution is added, a portion of PtCl6\n2\u2212 ions are reduced by the excess borohydride. The produced Pt atoms will preferentially adsorb on the surface of PVP to reduce their surface energy. Meanwhile, partial PtCl6\n2\u2212 ions can diffuse onto the surface of Ni NPs, followed by immediate reduction to Pt atoms by Ni. Once the Ni NPs are sacrificially dissolved, the produced Ni2+ ions are trapped by the PVP surrounding the Ni NPs and re-reduced to Ni atoms by borohydride. Because Pt and Ni nucleation sites are highly reactive, they diffuse quickly to form homogeneous bimetal in an alloy phase, leading to a shell covering the original Ni core (Figure\u00a013E). During the liquid-phase p-chloronitrobenzene (p-CNB) hydrogenation to p-chloroaniline (p-CAN), such a hollow alloy exhibits much higher activity and selectivity than the solid Pt NPs, together with the excellent durability (Table 2\n). The higher activity could be attributed to the enhanced dispersion degree of Pt active sites (S\nPt) and the increased intrinsic activity (TOF). The larger S\nPt of Pt-Ni-x(H) compared to Pt(S) is due to the promotional effect of both the chamber structure and the alloying Ni. The increased intrinsic activity could be due to electron-deficient Pt active sites in Pt-Ni alloys, which favors the dissociative adsorption of hydrogen molecules to form H\u2212, leading to the enhanced activity because the nucleophilic attack of H\u2212 on the nitrogen atom of the nitro group is a rate-determining step in a p-CNB hydrogenation reaction. In addition, the electron-deficient Pt active sites can \u201crecognize\u201d and preferentially adsorb the electron-enriched nitro group, which can account for the improved selectivity toward p-CAN. Besides, the controllable chamber structure with alloying shell also has a positive influence on\u00a0the catalytic behavior in the liquid-phase p-CNB hydrogenation to p-CAN (Figure\u00a013F).Tubular nanostructures have well-defined structures in terms of hollow interiors, which stimulate extensive research efforts in recent years because of their unique physical properties and potential applications in advanced electronic or magnetic devices, gas and fluid paths or reservoirs in catalysis, fuel cells, sensors, and separation systems.\n48,49\n Some catalytic reactions confined within nanotubular materials have been reported owing to the enhanced activities.\n50\u201352\n In spite of their importance in catalysis and other nanotechnological fields, only a few reports on\u00a0their fabrication can be found, possibly because of their extremely difficult processes.Ding et\u00a0al. synthesized porous Pt-Ni-P composite nanotube arrays (NTAs) with low Pt content through highly efficient template-assisted electrodeposition (Figure\u00a014\nA).\n53\n The as-prepared Pt-Ni-P composite NTAs (Figure\u00a014B) display a unique hollow nanostructure, porous structure, anisotropic nature, and multicomponent effect, which shows high electrochemical activity and long-term stability in methanol electrooxidation (Figure\u00a014C). The synergistic effect allows a homogeneous nanocrystal size distribution and an enormous increase of the electrochemically active surface areas of Pt-Ni-P NTAs, which significantly improves the relative content of Pt (0) and the 5d electron density of Pt in Pt-Ni-P NTAs. The Pt-Ni-P nanotubes display a rough surface and a length of \u223c2\u00a0\u03bcm with an inner diameter and wall thickness of \u223c400 and 70\u00a0nm, respectively. Thus, the high void volume in Pt-Ni-P NTAs can provide a 3D space for mass transfer of reactant and resultant molecules. These favorable characteristics could sufficiently account for an enormous increase in electrocatalytic activity. The unique hollow tubular structure with porous walls imparts an advantage for the full oxidation of the carbonaceous species generated during methanol electrooxidation, which could effectively reduce the poisoning effect from carbonaceous species (Figure\u00a014D). In addition, well-aligned NTAs can provide a continuous charge-carrier transport pathway without dead ends. By using the similar ZnO-template-assisted electrodeposition method, PdCo NTAs supported on carbon fiber cloth (CFC) (PdCo NTAs/CFC) were also fabricated by Wang et\u00a0al., which could be used as a high-performance and flexible electrocatalyst in ethanol oxidation for direct ethanol fuel cells (DEFCs).\n54\n The PdCo NTAs/CFC provide a large surface area and fast electrolyte penetration and diffusion because of the hollow and porous structures. Moreover, because the CFC is highly flexible, the PdCo NTAs/CFC display excellent flexibility. The electrochemical measurements demonstrate that the PdCo NTAs/CFC exhibit significantly improved electrocatalytic activity and durability compared with those of Pd NTAs/CFC and commercial Pd/C catalysts. Most importantly, the PdCo NTAs/CFC exhibit excellent flexibility, high electrocatalytic activity, strong durability, and CO stripping ability, indicating a promising prospect for developing flexible fuel cell devices. As a continuation of the former research, Co nanosheet nanotubes (NSNTs) decorated with TiO2 nanodots (NDS) were supported onto carbon fibers (CFs) (TiO2 NDs/Co NSNTs-CFs),\n55\n which can promote water adsorption and optimize the free energy of hydrogen adsorption, leading to high catalytic performance toward hydrogen evolution reaction (HER) in alkaline solution.Hierarchically nanostructured materials generally comprise integrated molecular units or their aggregates embedded in or intertwined with other units or aggregates.\u00a0Learning from nature, the hierarchical nanostructures enable the assembled architectures to obtain unique properties and functionalities. Thus, the hierarchical nanostructures can be considered as advanced materials for their promising applications in various areas including energy storage and conversion, adsorption, catalysis, sensing, and so on.\n56\u201359\n The development of such hierarchical nanostructures is an important task in generating advanced nanotechnology to realize better materials performance by a rational combination of multiple components. In this section, we present some examples that highlight the factors related to the hierarchical shape and catalytic properties.The growth of highly anisotropic one-dimensional nanostructures from NPs shows considerable improvement on catalytic, electronic, optoelectronic, and magnetic properties.\n60\n Because of efficient charge transfer, one-dimensional nanochains are widely used in electrocatalysis. We developed a facile approach to preparing chain-like Co-B amorphous alloy by the chemical reduction of cobalt ions with borohydride in a dodecanethiol-water biphasic system (Figure\u00a015\nA).\n61\n The as-prepared Co-B nanochain comprised uniform spherical NPs with an average size around 29\u00a0nm (Figure\u00a015B). These Co-B NPs were connected in one dimension by metal bonding. It was found that dodecanethiol is essential for the formation of Co-B nanochains. Dodecanethiol constructs a biphasic system with an aqueous solution, acts as a stabilizer for Co-B NPs, and also induces the dipoles on Co-B NPs. Because of the resonant enhancing effects from the linearly ordered array of both magnetic moments and electric dipoles, such Co-B nanochain catalysts exhibited stronger ferromagnetic property and higher electrochemical activity (Figure\u00a015C) than the conventionally prepared Co-B NPs catalysts. The ordered array of Co-B NPs in the Co-B nanochain catalyst facilitated the electron transfer, leading to the enhanced electrochemical activity.Very recently, Yuan et\u00a0al. synthesized a series of Co-based alloy nanochains by a direct current arc-discharge method.\n62\n The schematic synthesis of Co-Fe alloy nanochains is presented in Figure\u00a016\nA. Because of the high arc temperature, the anode evaporated into a homogeneous atomic mixture. Once the mixture cooled down, the Co-Fe alloy was formed; it displayed uniform nanochains with a diameter around 40\u201350\u00a0nm, which could couple to each other and ranged up to several micrometers (Figure\u00a016B). The nanochains with different Co/Fe ratios were synthesized; among them, Co7Fe3 exhibited the optimal performance in an oxygen evolution reaction (OER) with an onset potential of 1.50\u00a0V (versus reversible hydrogen electrode [RHE]) and an overpotential of 365\u00a0mV at 10 mA cm\u2212m, much higher than that of the Co and Fe nanochains (Figure\u00a016C). The good OER activity of the Co7Fe3 nanochains can be mainly attributed to two aspects. First, the metal can inject electrons into the surface oxide, which manipulates the work function of the oxide and improves its oxygen evolution efficiency. Second, Fe doping can introduce defects into the surface oxide, which could increase the active sites and ECSA. The Co7Fe3 nanochains also exhibit excellent stability with 92.0% current retention after a long-term chronoamperometry test. Cobalt-based alloys with other metals (Ti, Nb, and Mo) have also been synthesized by the same method, which shows promising applications in the OER.Onion-structured materials, generally called multishell spheres, represent those spheres of multiple concentric shells with different diameters. It can be regarded as a nestification of several hollow spheres.\n63\n Metals in onion-like structures are often synthesized through a hard-templating technique and self-assembly method. The hard-templating technique usually uses a hollow-structured template instead of a solid one and coats metals onto both the inner and the outer surface. This strategy is comprehensible in concept and difficult in practical applications.Composite vesicle systems are fascinating self-assembled structures that can be applied as efficient directors for the rapid synthesis of multishell materials. Currently, we successfully fabricated onion-structured Pd nanospheres by using a simple self-assembly template (unpublished data). The basis for this synthesis is the use of a composite multishell vesicle system that comprises didodecyldimethylammonium bromide, cyclohexane, and water. The region near the surfaces of the vesicle layers can gain a higher concentration of Pd ions. Once chemical reduction of Pd ions with sodium hypophosphite occurs, Pd clusters are produced and develop gradually into several concentric hollow spheres (Figure\u00a017\nA), leading to onion-like spheres with an average shell thickness of about 2.2\u00a0nm (Figure\u00a017B). The shell layers can be facilely adjusted by changing the amount of cyclohexane. The onion-like Pd nanospheres delivered much higher electrocatalytic activity toward ethanol oxidation for direct alcohol fuel cells than Pd NPs and commercial Pd/C (Figure\u00a017C), mainly attributed to their larger active surface area and the unique multishell configuration that accelerates the electron transfer.Jia et\u00a0al. developed a low-temperature interface-induced assembly approach to synthesizing onion-like Pt-Cu alloy nanocrystals.\n64\n A two-phase reaction system comprising HAD and water was designed to form multilamellar micelles. With the assistance of ascorbic acid, Pt-Cu NPs were formed and can be controlled by\u00a0assembling at a low temperature in the interlayer regions of multilamellar micelles (Figure\u00a018\nA). The obtained products consist of superparticles with diameters of 50\u00a0\u00b1 10\u00a0nm (Figure\u00a018B). High-resolution TEM (HRTEM) (Figure\u00a018C) images reveal that each particle is composed of nested multilayers characteristic of an onion-like structure. Their three-dimensional nanostructure was certified by tilting the sample along the x and y axes from \u221230\u00b0 to\u00a0+30\u00b0. The high curvature of each layer makes the onion-like Pt-Cu alloy nanocrystals expose a high density of defects, which acts as a way of releasing strain caused by the bending process. The unique structure with a confined interior as well as the existence of twin defects makes the onion-like Pt-Cu alloy nanocrystals exhibit excellent catalytic properties in the electro-oxidation of methanol and ethanol (Figure\u00a018C) and cycloaddition reactions.Core-shell nanomaterials represent unique spheres containing a middle core and an outer shell. Due to the synergism between two or more components, core-shell nanomaterials exhibit extraordinary properties in many areas, such as electronics, biomedicine, pharmaceuticals, optics, and catalysis.\n65\u201367\n For traditional core-shell materials, the core and shell components are compactly attached without any interspaces.\n68\n In the synthesis of multi-metallic materials from the mixed-metal salt precursors, it is difficult to ensure that different kinds of metal atoms contribute equally to the metal-metal bond formation because of their diverse reaction kinetics. For example, when more than one metal salt precursor is co-reduced in a homogeneous reaction solution, it is difficult to simultaneously control the reduction and nucleation process of different kinds of metals because of the difference in their redox potentials and chemical behaviors. Generally speaking, the expensive noble-metals like Pd and Pt with higher standard reduction potentials are more reactive than non-noble metals such as Fe, Co, and Ni during the co-reduction process. As a result, co-reduction of those mixed-metal salt precursors will be anticipated to lead to noble-metal-enriched cores and non-noble-metal-enriched shell particles. Considering that heterogeneous reactions take place on the surface of catalysts, having a large fraction of expensive noble metals in the core of the catalysts is undesirable. Thus, the design of new core-shell catalysts with non-noble-enriched cores and noble-metal-enriched shells represents a promising way to improve the activity at a low cost. A GRR is a facile and cost-effective approach to preparing noble-metal catalysts by using non-noble metals as the sacrificial templates.\n69\n Recently, we prepared a core-shell Pd@Co-B with high dispersion of Pd onto Co-B amorphous alloy nanospheres by GRR between Co-B uniform nanospheres with a diameter around 55\u00a0nm and Na2PdCl4 (Figure\u00a019\nA).\n70\n During the reduction Pd ions, Pd NPs gradually produced and deposited onto the Co-B core, corresponding to the formation of the Pd outer shell and the size decrease of the Co-B core (see Figure\u00a019B). HRTEM images of Pd@Co-B (Figure\u00a019C) clearly illustrate that the actual incorporation of Pd secondary nucleation on the surface of Co-B. Pd@Co-B with tunable Pd content could be achieved by adjusting the amount of Na2PdCl4 in the reaction mixture. Hydrogenation of EHEA to 2-ethyl-1-hexanol (EHO) is of great industrial importance since EHO is a valuable synthetic alcohol used as a synthon for the manufacture of ester plasticizers, coating materials, adhesives, printing inks, and impregnating agents or as an additive in foods and beverages as a volatile flavor. In industry, two-step hydrogenation of EHEA is necessary to produce EHO because one critical issue associated with that process is the partial hydrogenation that leads to a mixture of EHO, 2-ethyl-hexanal (EHA), and 2-ethyl-2-hexenol (EHEO). In general, the unsaturated alcohol, EHEO, is particularly undesirable because of the great difficulty in separating EHO by distillation. The as-prepared Pd@Co-B catalyst allowed the production of pure EHO via one-step hydrogenation of EHEA in liquid phase, which exhibited extremely higher activity and selectivity than either the Co-B amorphous alloy or the Pd catalyst. Results from the catalytic evaluation demonstrated that both metals in Pd@Co-B play important roles in promoting the reaction. The Pd highly exposed on the surface of Co-B amorphous alloy nanospheres is largely responsible for the hydrogenation of C=C bonds in the EHEA molecule, although Pd is relatively inactive for the hydrogenation of C=O bonds in EHEA. The incorporation of Pd can greatly increase hydrogenation ability associated with Co-B for C=O bonds (Figure\u00a019D). Moreover, the core-shell-structured Pd@Co-B exhibited much greater efficiencies than the classical systems generally used in industry, including Cu-Zn-Al, Cu-Cr, and Cu-Ni (Figure\u00a019E). A synergetic effect between Pd and Co\u00a0can be demonstrated by hydrogen temperature-programmed desorption (H2-TPD) results, which confirmed that the dispersion of Pd on the surface of the Co-B core could provide a much higher concentration of active hydrogen via hydrogen spillover from Pd to Co. In terms of heterogeneous catalysis, hydrogen spillover is a well-documented phenomenon, which can enhance H2 activation ability and thus improve the hydrogenation activity.Yolk-shell structures can be regarded as a variation of the ordinary core-shell structures in which one or several movable cores are encapsulated inside a hollow shell and can move freely in the void space. They are also termed as movable core-shell or rattle-structured materials.\n71\n The inside core and the outer shell can be either an identical material or two kinds of different materials. As an interesting family of complexes with new nanoarchitectures, the yolk-shell metal nanostructures have attracted tremendous attention because they bring the advantages of two classes of morphologies of the core shell and hollow together. To create high-surface-area amorphous alloys with tunable chamber structures, we developed a novel hard-soft co-templating method of catalyst synthesis.\n72\n Several mesoporous M-B (M\u00a0=\u00a0Fe, Co, and Ni) amorphous alloys with tunable chambers including yolk-shell or hollow structures were prepared by syringe-squeezing a solution composed of micelles containing Brij-76 [C18H37(OCH2-CH2)10OH] surfactant and metallic ions into borohydride solution to form oil droplets (Figure\u00a020\nA), followed by an ecto-entad stepwise reduction of metallic ions with borohydride due to a Kirdendal diffusion process. Reaction temperatures played important roles in determining the morphology of Ni-B amorphous alloys. Only random dispersed Ni-B NPs were obtained at a very low temperature. With an increase in reaction temperature, the morphology changed gradually to nanochains, yolk shell (Figure\u00a020B), and hollow (Figure\u00a020C). Moreover, the Brij-76 surfactant also played a key role in determining the M-B amorphous alloy morphology. On the one hand, it could stabilize the syringe-pinhole templated oil droplets in the KBH4 aqueous solution. On the other hand, it also acted as a soft template to assemble micelles, leading to a mesoporous structure in the M-B NPs. During liquid-phase hydrogenation of p-CNB to p-CAN, the yolk-shell Ni-B amorphous alloy exhibited the highest activity and selectivity. The improved catalytic performances could be mainly attributed to the high surface area and the unique yolk-shell chamber which might enrich hydrogen and act as a microchemical reactor.The lifetime is one of the most important factors for evaluating industrial catalysts. In general, the catalyst lifetime depends on the stability against corrosion, leaching, gathering, phase transformation, structure change and/or collapse, and poisoning, etc. Highly stable metal catalysts can be achieved by encapsulating active metal NPs by a protective matrix or shell. Such an encapsulation structure not only promotes the catalytic performances of metal NPs as a result of the unique collective and synergetic effects but also protects metal NPs from gathering and leaching as well as poisoning during the catalyst synthesis and its application in reactions.\n73\u201376\n\nPorous materials have been demonstrated to be suitable host matrices for the encapsulating metal NPs. The window size of the cavity is a key factor for achieving the efficient catalysts. On one hand, the window entrance can offer the possibility for encapsulating metal NPs and preventing their leaching. On the other hand, the window channels allow the diffusion of reactants and products. Additionally, the environments of the pores also have influences on the host-guest interaction and even the catalytic reaction. In the past few decades, metal NPs supported on metal oxides, zeolites, mesoporous materials, and activated carbons have been widely studied as catalysts. SBA-15 and MCM-41 have been well examined among the silica-based mesoporous materials.\n77\n However, supported metal catalysts on mesoporous silica prepared by a direct impregnation method usually show a non-uniform distribution of active sites and blockage of the pore channels, leading to the decrease in activity and even selectivity. To solve those problems, we developed a simple synthesis approach to confine Ru-B NPs within the mesopores by ultrasound-assisted incipient wetness infiltration of (NH4)2RuCl6 into SBA-15 channels, followed by reduction with borohydride.\n78\n During liquid-phase hydrogenation of maltose to maltitol, this catalyst delivered high activity up to seven times higher than that of the unsupported Ru-B catalyst because of the small active sites and cooperative effects from SBA-15 support. It also showed excellent durability owing to the confinement effect of mesoporous silica, which protected the metal active sites from agglomeration and leaching. Besides improving the synthesis method, other efforts have also been made to promote the dispersion of Ru-B NPs and\u00a0enhance the metal-support interaction, including the change in the silica morphology and the modification of the silica surface with inorganic and/or organic groups. For example, mesoporous silica nanospheres (MSNSs) with an average diameter around a few tens of nanometers are attractive candidates for catalyst carriers because of their large surface area, short pore channels, and regular nanospherical morphologies. Moreover, the surface properties of mesoporous materials can be modified through the region-selective immobilization of diverse organic groups. MCM-41-type MSNSs externally covered by methyl groups (\u2212CH3) but internally grafted by aminopropyl groups (\u2212NH2) were used as a host matrix for loading Ru-B NPs.\n79\n The \u2212NH2 and the \u2212CH3 groups served synergistically as effective functionalities for highly dispersing Ru-B NPs within the pore channels of the mesoporous host. Such a catalyst exhibited very high activity in liquid-phase d-glucose hydrogenation, much better than industrial Raney Ni and the commercially available Ru/C catalysts.As a new kind of highly ordered porous material with tunable size, shape, and microenvironment of the pores, metal-organic frameworks (MOFs) are promising platforms for preparing supported metal catalysts by embedding metal NPs into MOF pores to limit the migration and aggregation of metal clusters and/or NPs. Very recently, Pd NPs were encapsulated inside the cages of ethylenediamine-grafted MIL-101 (ED-MIL-101), giving high catalytic efficiencies for the racemization of primary amines.\n80\n The racemization can be combined with a lipase-catalyzed resolution in a one-pot DKR of rac-1-phenylethylamine, leading to an optically pure product (>99% enantiomeric excess [ee]) with an excellent conversion and selectivity up to 99% and 93%, respectively, which are remarkably superior to those of Pd/MIL-101, Pd/MCM-41, and commercially available Pd/C. Such a catalyst can be easily recovered and reused more than eight times without significant loss of activity. The enhanced catalytic performances were mainly attributed to the amine modification of MIL-101, which endows the MOF cage surface with basic properties and also enables efficient confinement of Pd NPs in the cages to inhibit their gathering. Metal NPs encapsulated in channels of mesoporous materials can be further encapsulated by a protective shell to fabricate unique yolk-shell nanoarchitectures, which could be used for those reactions conducted under harsh conditions. Recently, we applied the combination of such a yolk-shell-structured catalyst and enzyme to act as a powerful platform for one-pot biomass conversion via sequential enzyme-catalyzed hydrolysis of biomass materials to glucose and the subsequent metal-catalyzed hydrogenation of glucose to sorbitol.\n81,82\n Preliminary studies revealed that the enzyme is easily poisoned when contacting Ru-based catalysts. Meanwhile, the metallic Ru active sites would be covered by the enzyme and the colloidal substances originated from dextrin hydrolysis, leading to a rapid deactivation for the subsequent glucose hydrogenation to sorbitol. To solve these problems, Ru-B NPs encapsulated in yolk-shell silica (Ru-B/mSiO2@air@SiO2) (Figure\u00a021\nA) and a yolk-shell-structured material composed of a Ru-B/mCarbon core and a mesoporous silica shell (Ru-B/mCarbon@air@mSiO2) (Figure\u00a021B) were prepared. Assisted by amyloglucosidase, these are essentially binary catalytic systems for one-pot conversion of dextrin to sorbitol. (Figures 21C and 21D) More specifically, the permeation-selective outer silica shell inhibited the diffusion of amyloglucosidase with a large molecular size into the chamber to contact Ru-B on the core, which avoided the poisoning effect on each other. Meanwhile, it also prevented the diffusion of dextrin with a big molecular size into the chamber but allowed the diffusion of glucose and product sorbitol inside the chamber owing to their small molecular sizes. As a result, the enzymatic dextrin hydrolysis to glucose occurred in bulk solution, followed by glucose hydrogenation to sorbitol catalyzed by Ru-B on the core, leading to high reaction efficiency. The enzyme integrates into yolk-shell nanostructure containing Ru-B to form a Ru-B/af-mCarbon@air@af-mSiO2 bifunctional biochemical nanoreactor.\n83\n The synthesis of such a composite is achieved through a stepwise crosslinking method that involves the covalent attachment of yolk-shell-structured catalyst onto amyloglucoamylase with glutaraldehyde and the subsequent coupling of the composite in the presence of modified dextran. The biochemical composite enables the efficient synthesis of sorbitol through one-pot reactions from dextrin, cellobiose, and even cellulose.The incorporation of metal NPs in the framework of porous materials for heterogeneous catalysis may avoid particle aggregation, movement, and leaching. A multicomponent assembly approach was used to cooperatively assemble surfactant, titania, and nickel precursors in a one-pot process. Ni NPs with sizes ranging from 1 to 6\u00a0nm were homogenously embedded within the framework of mesoporous TiO2.\n84\n During gas-phase hydrodechlorination of the chlorobenzene reaction, the as-prepared 0.23%-Ni/TiO2-DS catalyst delivered a TOF up to 1.5 times greater than that of the Ni/TiO2 catalyst prepared by conventional incipient-wetness impregnation method. Meanwhile, such a catalyst also exhibited much improved stability in a continuous reaction. An enhanced hydrogen spillover effect is believed to play an important role in promoting a hydrodechlorination reaction as a result of the intimate interfacial contact between Ni NPs and the TiO2 support. Meanwhile, 3\u00a0nm Pd NPs encapsulated homogenously within 10-nm-thick porous silica shells were synthesized by conducting silica polymerization around oleylamine-capped Pd NPs in a water-in-oil microemulsion system, followed by removing oleylamine molecules via calcination. During the CO oxidation reaction, the core-shell-structured Pd@SiO2 delivered a TOF up to 33 times greater than that of the Pd catalyst prepared by a conventional immobilization method and also showed excellent durability. Detailed studies demonstrated that core-shell configuration played a key role in promoting catalytic performance. The Pd cores with a small size of around 3\u00a0nm could efficiently activate CO molecules by weakening the strength of CO adsorption, and the core-shell structure could inhibit Pd gathering during reactions. Meanwhile, the porous silica shells allow the reactants to penetrate into the core-shell-structured Pd@SiO2 and thus increase their accessibility with the Pd cores. Furthermore, the porous silica shell also protected Pd NPs from agglomeration, leading to enhanced thermal stability.\n85\n By a similar reverse microemulsion method, core-shell-structured M@SiO2 (M\u00a0= Ni, Co, and NiCo) catalysts were also synthesized.\n86\n Each SiO2 nanosphere contained a single Ni-, Co-, or NiCo-alloy NP as the core (Figures 22A\u201322F). During the dry reforming of CH4 with CO2 (DRM) reaction, NiCo@SiO2 delivered high activity and selectivity, superior over either the Ni@SiO2 or the Co@SiO2. The CO2 and CH4 (1:1) could be absolutely converted into CO and H2 with a molar ratio around 1:1 (Figure\u00a022G). More importantly, those catalysts displayed very good durability at a high reaction temperature. For example, the Ni@SiO2 could be used continuously for more 1,000\u00a0h without a significant efficiency decrease in the DRM reaction at 800\u00b0C (Figure\u00a022H). Kinetic studies revealed that the catalyst with a small metal NP size exhibited higher activity and inhibited carbon deposition, which would poison the active sites. The encapsulation of metal NPs by a SiO2 shell could effectively inhibit the agglomeration of active sites, corresponding to high activity and the long lifetime. An approach to coating Al2O3-supported Ni NPs with a porous Al2O3 thin film through atomic layer deposition was developed.\n87\n During the DRM to synthesis gas at 800\u00b0C, the as-prepared Al2O3/Ni/Al2O3 sandwiched catalyst could effectively protect Ni NPs from gathering owing to the double strong interactions of the Ni active sites with the \u03b3-Al2O3 support and the Al2O3 film. Such a sandwiched Al2O3/Ni/Al2O3 catalyst with 80 layers of Al2O3 thin films exhibited the highest activity. Both CO2 and CH4 conversions reached nearly 100% with absolute selectivities toward CO and H2. More importantly, this catalyst displayed excellent stability and could be used for more than 400\u00a0h in the DRM reaction at 800\u00b0C without significant deactivation.Metal catalysts are always attractive because of their wide applications in chemical industries and many other areas. Their reactivities and selectivities, as well as stabilities, can be tuned by controlling the morphology because the exposed surfaces of the metals have distinct crystallographic planes depending on the shape. This review provides an overview of recent advances in the synthesis of\u00a0metallic materials and their fascinating catalytic performances in many reactions.\u00a0Versatile chemical reduction approaches combined with desired structure templates can be conducted in different systems to fabricate metal catalysts with well-controlled shapes and morphologies, leading to excellent catalytic properties.Industrial application of metal catalysts greatly relies on their stability. In an effort to favor the stability of metal catalysts, the applied reaction type should be considered carefully. For example, under some harsh reaction conditions, such as elevated temperature or pressure, the porous metallic materials usually display an irreversible shrinkage caused by pore collapse. As a result, the porous metallic materials are unstable relative to metal NPs loaded on supports for some reactions, especially strongly exothermic reactions. As electrocatalysts, however, the porous metallic materials are always stable compared with metal NPs supported on carbon. The poor stability of metal NPs supported on carbon is mainly ascribed to the corrosion of carbon supports, which further results in migration, aggregation, and Ostwald ripening of metal NPs owing to their high surface energy and isotropic zero-dimensional structural features. Because of the inherent anisotropic morphology and unique structure, the nanoarchitectured metal catalysts in the absence of carbon supports possess superior durability when used as electrocatalysts even in harsh media.We expect that with further development of chemical reaction engineering and nanomaterial synthesis technologies, more and more new metal catalysts will be\u00a0developed, which will offer more opportunities for their industrial applications.\u00a0Obviously, fundamental research is necessary for such new materials to uncover the key factors that greatly regulate their nanostructures and finally determine their catalytic performances. One of the most challenging problems is\u00a0the underlying mechanism responsible for the trajectories to form metal catalysts\u00a0with controllable morphology, which should be helpful to achieve the goal of rationally designing catalysts. On the other hand, further efforts must be\u00a0made to achieve the large-scale assembly of these nanoscale catalysts to realize\u00a0their practical applications. Moreover, we anticipate that a variety of unique\u00a0nanostructured metals to allow molecular-level fine-tuning of catalytic performances will emerge soon as a new progress in metal catalyst design. In\u00a0particular, the development of methodology to control the nanostructures of\u00a0metal catalysts for further improvement of catalytic activity and selectivity and\u00a0to fabricate novel efficient catalysts will be the keystone for the future industries.We would like to thank the programs supported by the National Natural Science Foundation of China (21761142011), Singapore National Research Foundation\n(NRF2017NRF-NSFC001-007), Ministry of Education of China (PCSIRT_IRT_16R49), and Shanghai Government (18JC1412900, 15520711300, and 18DZ2254200).", "descript": "\n                  Metal catalysts have been widely employed in chemical production, medicine manufacture, organic synthesis, and environmental cleaning, etc. Catalyst design plays a key role in enhancing efficiencies including activity, selectivity, and durability. Both theoretical predictions and experimental research has demonstrated that besides the composition, the morphology and/or the porous structure play key roles in determining catalytic performances. This review highlights the recent progress in the controlled synthesis of new metal catalysts with hierarchical structures mainly based on our research. First, we describe the metal catalysts with unique morphologies. Second, we show the metal catalysts with different porous structures. Finally, we summarize the metal catalysts with hierarchical structures. The catalytic performances of different catalysts are also included, and their correspondence to the catalyst structure is explored. This review might supply guidance for designing new and powerful metal catalysts for industrial applications.\n               "}
{"full_text": "Hydrodesulfurization (HDS) has become an increasingly important research topic over the past decades due to the growing demand for transportation fuels [1\u20137]. Environmental regulations in many countries worldwide demand the production of clean fuels with an ultra-low sulfur content of 10\u00a0ppm or less [8\u201310]. As the traditional HDS catalysts, alumina-based Ni(Co)\u2013W(Mo) catalysts are widely used in industrial processes due to their satisfying activity and low cost [11\u201316]. However, in order to meet the requirements of deep HDS conversion, new efficient catalysts with high activity and selectivity should be developed to remove the refractory sulfur-containing compounds, e.g., DBT and 4,6-DMDBT, from transportation fuels [17\u201321].Compared to other transition metals, noble metals and the corresponding alloys (Pt or PtPd alloy) show better hydrogenation properties in the DBT and 4,6-DMDBT HDS reactions [22\u201326]. Although noble metal catalysts display high catalytic performance in the hydrotreating process, they are relatively expensive and susceptible to sulfur poisoning even at very low sulfur contents feedstocks, which limit their practical applications in HDS reactions [27,28]. Therefore, the research of noble catalysts with sulfur resistance properties arouses more interests of scientists in different research fields. Many preparation strategies, such as constructing a core\u2013shell structure or alloying with a second metal (Pd, Ru or Ir, etc.), have been performed to enhance the sulfur tolerance of noble metal catalysts [29\u201331]. For example, Pt noble metal can be encapsulated in the pores (around 0.5\u00a0nm) of zeolite only to allow the migration and dissociation of small hydrogen molecules (0.289\u00a0nm) and exclude the large sulfur-containing compounds [32\u201335]. This method protected the noble metal catalyst from sulfur poisoning and provided activated hydrogen for the HDS reaction [36]. Unfortunately, a fatal disadvantage of poisoning caused by the smaller hydrogen sulfide molecule (H2S, 0.362\u00a0nm) poisoning cannot be prevented [37]. Therefore, a critical problem to be solved is to isolate the H2S from the noble metal center, to achieve excellent sulfur-resistance performance in the presence of H2S. Besides, the noble metal catalyst with a high loading of noble metals will increase the production cost of the catalyst compared with those of the transition metal sulfide catalysts. As for noble metal catalysts, the approach to realize high HDS activity but cheap price would be the focus to explore.Herein, a new dendritic MoS2/Pt@TD catalyst consisting of sulfur-resistance Pt@TS-1 and MoS2 active phases was prepared. The pore-modified Pt@TS-1 seed with appropriate micropore sizes (between 0.289\u00a0nm and 0.362\u00a0nm) not only created activated H protons over the Pt noble metal sites anchored inside the internal channels of micropores, but also incentivized H+ spilling over to the surface of MoS2 active phases located in the external channels, which prohibited the direct contact between Pt noble metal and the sulfur-containing compounds (including H2S), consequently avoiding the poisoning of Pt active sites. Moreover, titanium element with various chemical states in the Pt@TS-1 can generate more d-electron, which is be beneficial to the creation of more S vacancies of MoS2 species. The dendritic MoS2/Pt@TD catalyst possessed the combining characteristics of noble metals of excellent hydrogenation performance at low temperatures and transition metals of low-cost. Thus, the as-prepared novel dendritic MoS2/Pt@TD catalyst showed superior activities and high sulfur-resistance stabilities for DBT and 4,6-DMDBT HDS.The synthesis procedure of Pt@TS-1 seed (molar ratios of SiO2/TiO2\u00a0=\u00a025) is as follows. 40\u00a0g of tetraethyl orthosilicate (TEOS), 2.64\u00a0g tetrabutyl titanate, and 46.72\u00a0g TPAOH (25\u00a0wt%) were added to 8.68\u00a0g of deionized water. The mixture was stirred to dissolve for 1\u00a0h in an ice water bath, then switched to a water bath at 70\u00a0\u00b0C for 3\u00a0h. 48\u00a0g of isopropanol was added to the mix and stirred for 1\u00a0h. The resulting solution was labeled as solution A. 0.10\u00a0g of NaOH and 0.12\u00a0g 3-mercaptopropyltrimethoxysilane were added to 2.0\u00a0g of deionized water. 3.08\u00a0mL of chloroplatinic acid (H2PtCl6, 100\u00a0mmol L\u22121) was added to the mixture and stirred for 20\u00a0min. The resulting solution was labeled as solution B. Then the solution B was added to the solution A slowly, and continued to stir at 70\u00a0\u00b0C for 30\u00a0min. The resulting solution C was transferred to a 200\u00a0mL autoclave at 170\u00a0\u00b0C for four days. Finally, Pt@TS-1 seed was synthesized by centrifugation, washing, and drying.0.60\u00a0g triethanolamine (TEA) was dissolved in 220\u00a0mL deionized water and stirred at 80\u00a0\u00b0C for 30\u00a0min. Then 3.34\u00a0g of cetyltrimethylammonium bromide (CTAB) and 1.48\u00a0g sodium salicylate (NaSal) were added and kept stirring for 1\u00a0h. 35.2\u00a0g TEOS (28.4\u00a0wt% SiO2) was added dropwise and agitated for 30\u00a0min, then 2.0\u00a0g of the as-synthesized Pt@TS-1 seed was blended into the mixture solution D and kept continual stirring for 1.5\u00a0h. The resulting mixture E was transferred to a 500\u00a0mL autoclave at 100\u00a0\u00b0C for 4\u00a0h. Finally, Pt@TD catalyst was prepared after washing with water and ethanol, filtrating, drying at 80\u00a0\u00b0C for 12\u00a0h and calcining at 300\u00a0\u00b0C for 2\u00a0h. Pure TS-1/DMSNs (TD) composite was prepared using the same method without H2PtCl6.The micropore of Pt@TD catalyst was modified by using the deposition\u2013precipitation method. 3.0\u00a0g of Pt@TD catalyst and a certain amount (0/0.126/0.396/0.816\u00a0g) of tetrabutyl titanate was added to 90\u00a0mL of ethanol. The mixture F was completely dissolved for 40\u00a0min by ultrasonator. Then the mixture was stirred slowly at 45\u00a0\u00b0C until all the ethanol was completely evaporated. Finally, a series of pore-modified Pt@TD catalysts were synthesized by adjusting the TiO2 amounts (0\u00a0wt%/1\u00a0wt%/3\u00a0wt%/6\u00a0wt%) incorporated into the system, and the products were noted as Pt@TD-xTi, which x represented the mass fractions of TiO2.Mo/Pt@TD-xTi catalysts with 12\u00a0wt% MoO3 and 0.1\u00a0wt% Pt were synthesized via incipient wetness impregnation method using ammonium molybdate ((NH4)6Mo7O24\u00b74H2O). The as-prepared catalysts were denoted as Mo/Pt@TD-0%Ti, Mo/Pt@TD-1%Ti, Mo/Pt@TD-3%Ti and Mo/Pt@TD-6%Ti, respectively. The corresponding MoS2/Pt@TD-xTi catalysts were obtained after presulfidation of Mo/Pt@TD-xTi catalysts using 3\u00a0wt% CS2 in hexane. The reference catalysts of MoPt/TD and Mo/TD with 12\u00a0wt% MoO3 and 0.1\u00a0wt% Pt were prepared through two-step incipient wetness impregnation of TD support using (NH4)6Mo7O24\u00b74H2O and H2PtCl6 (100\u00a0mmol L\u22121).X-ray diffraction (XRD) patterns were measured in wide-angle between 5\u00b0\u201350\u00b0 and small-angle between 1.3\u00b0\u20136\u00b0 at 40\u00a0kV and 40\u00a0mA in Cu K\u03b1 radiation. Nitrogen adsorption/desorption isotherms were acquired at \u2212196\u00a0\u00b0C on a Micromeritics TriStar \u2161 2020 instrument. Scanning electron microscopy (SEM) was taken on a Hitachi SU8010 apparatus. Transmission electron microscope (TEM) images were recorded on an FEI F20 apparatus. CO temperature-programmed desorption (CO-TPD) and H2-TPD were carried out on a quartz reactor with a quadrupole mass spectrometer (OmniSTAR TM). X-ray photoelectron spectroscopy (XPS) was collected on a Thermo Fisher K-Alpha spectrometer.The activity of MoS2/Pt@TD-xTi series catalysts and the reference catalysts were investigated within a fix-bed reactor for DBT or 4,6-DMDBT HDS. 1.0\u00a0g Mo/Pt@TD-xTi catalysts precursors were loaded in the center of the reactor. Before the HDS reaction, the precursor was presulfided at 340\u00a0\u00b0C for 4\u00a0h under the conditions of 4.0\u00a0MPa, volumetric ratio of H2/Oil of 200 (v/v) and weight hourly space velocity (WHSV) of 8.0\u00a0h\u22121. After the presulfidation was completed, fresh H2 and liquid feed (DBT or 4,6-DMDBT) were switched into the reactor. DBT or 4,6-DMDBT HDS experiments were conducted at 340\u00a0\u00b0C, 4.0\u00a0MPa, 200 (v/v) and 10\u2013100\u00a0h\u22121.Hence, the HDS conversions were determined by the following equation [38]:\n\n(4)\nHDS (%)\u00a0=\u00a0(S\n\nf\n\u00a0\u2212\u00a0S\n\np\n)/S\n\nf\n\u00a0\u00d7\u00a0100%\n\nwhere S\n\nf\n is the initial concentration of liquid feed before reaction (ppm) and S\n\np\n is the residual concentration of DBT or 4,6-DMDBT after reaction (ppm).The HDS of DBT or 4,6-DMDBT is assumed to follow the pseudo-first-order reaction kinetics, and the rate constant k\n\nHDS\n (mol\u00a0g\u22121\u00a0h\u22121) can be presented as [39]:\n\n(5)\n\n\n\nk\nHDS\n\n=\n\nF\nm\n\nln\n\n(\n\n1\n\n1\n\u2212\n\u03c4\n\n\n)\n\n\n\n\nwhere F denotes the feeding rate of DBT or 4,6-DMDBT (mol\u00a0h\u22121), m refers to the weight of catalysts (g), and \u03c4 refers to the HDS conversion of (%).Five characteristic peaks of Pt@TD-xTi series catalysts appear at the ranges of 7.8\u20138.7\u00b0 and 22\u201325\u00b0 in the XRD patterns as shown in Fig.\u00a01\n, which correspond to the (011), (020), (051), (303), (313), and (532) planes of TS-1 zeolite [40]. No discernible peak relating to Pt metal is identified, which is ascribed to the low loading and high dispersion of Pt. It is evident that peak intensities of Pt@TD-xTi series catalysts decrease in comparison of Pt@TS-1 seed, suggesting that Pt@TS-1 seeds are embedded into the skeletal structure of DMSNs.To better understand the Pt distributions in Pt@TD-6%Ti catalyst, HAADF-STEM, and EDS mappings were carried out to investigate the morphology and metal dispersion of catalysts, and the relevant photos are presented in Fig.\u00a02\n. The Pt@TD-6%Ti catalyst (Ultralow 0.1\u00a0wt% Pt content) shows no aggregation of bulk Pt particles, indicating that noble metal Pt particles are evenly distributed in the Pt@TD-6%Ti catalyst. In addition, Ti elements are also evenly distributed in the Pt@TD-6%Ti catalyst, demonstrating that Pt@TS-1 seeds are embedded into the skeletal structure of DMSNs.N2 adsorption\u2013desorption isotherms and pore size distributions of the series Mo/Pt@TD-x%Ti catalysts, MoPt/TD and pure TD composite are presented in Fig.\u00a03\n. All the isotherm curves of the series Mo/Pt@TD-x%Ti catalysts, MoPt/TD and the pure TD composite display type-IV curves with H2 hysteresis loops. The hysteresis loops of the series Mo/Pt@TD-x%Ti catalysts and MoPt/TD became narrow after the addition of Pt and Mo active metals into TD composite. Moreover, the pore size distributions of the series Mo/Pt@TD-x%Ti catalysts and MoPt/TD are relatively discrete compared with pure TD composite.The texture properties of the series Mo/Pt@TD-x%Ti catalysts, MoPt/TD and the pure TD composite are presented in Table 1\n. As shown in Table 1, the surface areas and pore volumes decrease from Mo/Pt@TD-0%Ti (407\u00a0m2\u00a0g\u22121, 1.09\u00a0cm3\u00a0g\u22121) to Mo/Pt@TD-1%Ti (399\u00a0m2\u00a0g\u22121, 1.07\u00a0cm3\u00a0g\u22121), then reduce to Mo/Pt@TD-6%Ti (367\u00a0m2\u00a0g\u22121, 0.95\u00a0cm3\u00a0g\u22121) as the TiO2 amounts increase. Notably, MoPt/TD catalyst shows the lowest surface area, pore volume and the smallest pore size among these catalysts. The sufficient surface area and pore size can provide a favorable environment for excellent dispersion of Pt and Mo active metal species\u2019 and a fast diffusion for the reactants and products.The SEM images of the series Mo/Pt@TD-x%Ti catalysts and Pt/TS-1 seed are shown in Fig.\u00a04\n. All the series Mo/Pt@TD-x%Ti catalysts display similar uniform wrinkled morphology, indicating that deposition of TiO2 has little impact on the morphology of the series Mo/Pt@TD-x%Ti catalysts. The wrinkled surfaces of the series Mo/Pt@TD-x%Ti catalysts are tremendous advantages to the enhancement of the surface area. Besides, the series Mo/Pt@TD-x%Ti catalysts present no independent phases of Pt@TS-1 seeds, manifesting that Pt@TS-1 seeds are embedded into the skeletal structure of DMSNs successfully.The TEM images of the series Mo/Pt@TD-x%Ti catalysts and the pure TD composite are shown in Fig.\u00a05\n. All the series Mo/Pt@TD-x%Ti catalysts exhibit similar open dendritic center-radial pore channels as the pure TD composite, demonstrating that the dendritic pore structures are well maintained after the additions of Pt and Mo active metals into TD composite. The open dendritic pore structure facilitates the diffusion of reactants and products in the pore channels, which finally contribute to the improvement of catalytic performance of the HDS reaction.The H2-TPD data can be used to investigate the H2 adsorption capacity of the catalyst. The H2 adsorption capacity of the catalyst increases with the increase of the adsorption peak intensity. The adsorption peak area represents the adsorption amount of H2 molecules of the catalyst. The H2-TPD curves of the series Mo/Pt@TD-x%Ti catalysts are displayed in Fig.\u00a06\n. In addition, MoPt/TD and Mo/TD catalyst were selected as reference catalysts for the H2-TPD characterization. From the H2-TPD curve of Mo/TD catalyst, it can be seen that the H2 adsorption peak of Mo/TD catalyst is very weak, which indicates that adsorption of H2 by the Mo metal and TD composite is very weak. The H2 adsorption peak of MoPt/TD catalyst is also relatively weak, indicating that Pt particles on the MoPt/TD catalyst are too large and Pt metal dispersion is not even, resulting in the exposure of less H2 adsorption site. From the H2-TPD curves of the series Mo/Pt@TD-x%Ti catalysts, it can be seen that with the increase of TiO2 modification amount, the H2 adsorption amount decreases slightly but not significantly, which indicates that the modified micropores of Mo/Pt@TD-6%Ti can still allow the migration and dissociation of small H2 molecules.CO molecule (0.369\u00a0nm) and the H2S molecule (byproduct of the HDS reaction, 0.362\u00a0nm) show the similar kinetic diameters [41], indicating that if the CO molecule cannot enter into the micropores of catalyst successfully, then H2S molecule will also be denied entry to the micropores of the catalyst, consequently no access to the active sites of Pt clusters. The CO-TPD data can be used to investigate the CO adsorption capacity of the catalyst. The CO-TPD curves of the series Mo/Pt@TD-x%Ti catalysts are displayed in Fig.\u00a07\n. In addition, MoPt/TD and Mo/TD catalysts were selected as reference catalysts for the CO-TPD characterization. From the CO-TPD curve of Mo/TD catalyst, it can be seen that the CO adsorption peak of Mo/TD catalyst is very weak, which indicates that the adsorption of CO by the Mo metal and TD composite is very weak. The CO adsorption peak of MoPt/TD catalyst is also relatively weak, which may be because Pt particles on the MoPt/TD catalyst are too large and Pt metal dispersion is not even, resulting in the exposure of less CO adsorption site. From the CO-TPD curves of the series Mo/Pt@TD-x%Ti catalysts, it can be seen that with the increase of TiO2 modification amount, the CO adsorption amount decreases significantly, which indicates that most micropores of Mo/Pt@TD-6%Ti cannot allow the migration of H2S molecules from the products to the Pt site.It can be seen from the H2-TPD and CO-TPD characterization results that the Mo/Pt@TD-6%Ti catalyst can effectively prohibit the sulfur-containing compounds transferring into the inside of micropores due to the confined small pores after the Ti modification, thus avoiding the direct contact between the sulfur-containing compounds and Pt metals. At the same time, H2 molecules can diffuse in and out of the micropore channels freely on the Mo/Pt@TD-6%Ti catalyst and hydrogen dissociation occurs on the surface of Pt sites, of which activated hydrogen spills over and transmit to MoS2 active sites.To investigate and further understand the chemical surface state of MoS2 species on the series MoS2/Pt@TD-x%Ti and MoS2\u2013Pt/TD catalysts, XPS measurements were performed and the relevant spectra are presented in Fig.\u00a08\n. The XPS fitting criterion is similar to the previous paper [42]. Table 2\n displays the Mo 3d statistical results of the series MoS2/Pt@TD-x%Ti and MoS2\u2013Pt/TD catalysts. As shown in Table\u00a02, the sulfidation degrees of the series MoS2/Pt@TD-x%Ti catalysts have a little change compared with MoS2\u2013Pt/TD catalyst, indicated that the existing state of Pt noble metal has a minor effect on the reduction and sulfidation of Mo transition metal.The HDS activities of DBT over the series MoS2/Pt@TD-x%Ti and MoS2\u2013Pt/TD catalysts with different WHSVs are presented in Fig.\u00a09\n. The DBT HDS conversions decrease with the increasing WHSVs. The DBT HDS conversions over the series catalysts follow the sequence of MoS2\u2013Pt/TD\u00a0<\u00a0MoS2/Pt@TD-0%Ti\u00a0<\u00a0MoS2/Pt@TD-1%Ti\u00a0<\u00a0MoS2/Pt@TD-3%Ti\u00a0<\u00a0MoS2/Pt@TD-6%Ti in the WHSVs ranges of 10\u2013100\u00a0h\u22121. MoS2/Pt@TD-6%Ti catalyst displays the best DBT HDS conversion than other MoS2/Pt@TD-x%Ti catalysts and MoS2\u2013Pt/TD catalyst.Further study on the product distributions shows that DBT reacts along direct desulfurization route (DDS) and hydrogenation route (HYD), and the corresponding HDS reaction mechanisms are shown in Fig.\u00a0S1 (Supporting Information). Table 3\n shows the DBT HDS product distributions of the series MoS2/Pt@TD-x%Ti and MoS2\u2013Pt/TD catalysts. The HYD/DDS ratios of DBT over the series catalysts increase as the sequence of MoS2\u2013Pt/TD (0.52)\u00a0<\u00a0MoS2/Pt@TD-0%Ti (0.69)\u00a0<\u00a0MoS2/Pt@TD-1%Ti (0.85)\u00a0<\u00a0MoS2/Pt@TD-3%Ti (1.04)\u00a0<\u00a0MoS2/Pt@TD-6%Ti (1.43), which is consistent with the trend of catalytic activity (k\nHDS). This result confirms that DBT mainly proceeds HDS via the HYD route, implying the HYD route made a considerable contribution to the total activity of DBT HDS over MoS2/Pt@TD-6%Ti catalyst.The HDS activities of 4,6-DMDBT over the series MoS2/Pt@TD-x%Ti and MoS2\u2013Pt/TD catalysts with different WHSVs are exhibited in Fig.\u00a010\n. The 4,6-DMDBT HDS conversions decrease with increasing WHSVs values. The 4,6-DMDBT HDS conversions over the series catalysts increase in the sequence of MoS2\u2013Pt/TD\u00a0<\u00a0MoS2/Pt@TD-0%Ti\u00a0<\u00a0MoS2/Pt@TD-1%Ti\u00a0<\u00a0MoS2/Pt@TD-3%Ti\u00a0<\u00a0MoS2/Pt@TD-6%Ti during the WHSVs of 10\u2013100\u00a0h\u22121. MoS2/Pt@TD-6%Ti catalyst shows the highest 4,6-DMDBT HDS conversion than other MoS2/Pt@TD-x%Ti catalysts and MoS2\u2013Pt/TD catalyst.In order to investigate the stability and sulfur-resistance performance of the series MoS2/Pt@TD-x%Ti and MoS2\u2013Pt/TD catalysts, long-period (100\u00a0h) 4,6-DMDBT HDS reactive experiments over MoS2/Pt@TD-6%Ti and MoS2\u2013Pt/TD catalysts were complemented. It can be seen from Fig.\u00a011\n, the 4,6-DMDBT HDS conversions over MoS2/Pt@TD-6%Ti catalyst were maintained at 92.1% at 100\u00a0h, In contrast, the 4,6-DMDBT HDS conversions over MoS2\u2013Pt/TD catalyst decrease significantly, indicating that the Pt-confinement MoS2/Pt@TD-6%Ti catalyst possessed better sulfur-resistance performance and 4,6-DMDBT HDS catalytic stabilities.Further study on the product distributions shows that 4,6-DMDBT reacts along DDS, HYD, and isomerization (ISO) routes, the corresponding HDS reaction mechanisms are shown in Fig.\u00a0S2 (Supporting Information). Table 4\n exhibits the 4,6-DMDBT HDS product distributions of the series MoS2/Pt@TD-x%Ti and MoS2\u2013Pt/TD catalysts. The ISO selectivities of the series catalysts increase in the sequence of MoS2\u2013Pt/TD (17%)\u00a0<\u00a0MoS2/Pt@TD-0%Ti (24%)\u00a0<\u00a0MoS2/Pt@TD-1%Ti (29%)\u00a0<\u00a0MoS2/Pt@TD-3%Ti (46%)\u00a0<\u00a0MoS2/Pt@TD-6%Ti (54%), which is in agreement with the order of catalytic activity (k\nHDS). This result confirms that 4,6-DMDBT mainly proceeds the ISO route in the HDS reaction process, proving that the ISO route contributes more to the total activity of 4,6-DMDBT HDS over MoS2/Pt@TD-6%Ti catalyst.Dendritic Pt-confinement MoS2/Pt@TD-6%Ti catalyst with excellent sulfur-resistance performance and HDS catalytic stabilities were successfully prepared. MoS2/Pt@TD-6%Ti catalyst displays much better DBT and 4,6-DMDBT HDS activities than those of other MoS2/Pt@TD-x%Ti and the MoS2\u2013Pt/TD reference catalyst. Advanced characterization and activity evaluation were conducted to interrogate the structure\u2013activity relationship and the results are discussed as follows:Firstly, the modified micropore of Mo/Pt@TD-6%Ti can still allow the migration and dissociation of small H2 molecules (Fig.\u00a06). H2 undergoes dissociation on Pt sites to produce activated hydrogen protons. Then H+ spills over to the MoS2 active sites on the mesoporous surface, which can enhance the HYD route of DBT HDS and the ISO route of 4,6-DMDBT HDS (Tables 3 and 4).Secondly, the modified micropores of Mo/Pt@TD-6%Ti catalyst possesses 0.289\u20130.362\u00a0nm in diameter, which can effectively confine Pt metal and prohibit the sulfur-containing compounds diffusing into the inside of the micropores (Fig.\u00a07), thus avoiding the direct contact between the sulfur-containing compounds and Pt metal. Thus, Mo/Pt@TD-6%Ti catalyst can effectively prevent the sulfur poisoning of noble metal Pt, thus improving the sulfur-resistance performance and HDS catalytic stabilities (Fig.\u00a011).Thirdly, the higher DBT and 4,6-DMDBT HDS activities of the MoS2/Pt@TD-6%Ti catalyst are the results of the synergistic effect from the strong H2 dissociation ability of noble metal Pt and the excellent desulfurization activity of transition metal sulfide MoS2.Fourthly, Pt noble metal\u2019s existing state shows little effect on the reduction and the sulfidation of Mo transition metal (Fig.\u00a08). The series MoS2/Pt@TD-x%Ti catalysts and MoS2\u2013Pt/TD catalyst present almost the same sulfidation degrees, indicating that the higher DBT and 4,6-DMDBT HDS activities of MoS2/Pt@TD-x%Ti catalyst were mainly due to the stronger H2 dissociation ability and the excellent sulfur-resistance performance of pore-confinement Pt metal.Fifthly, the uniform wrinkled surfaces (Fig.\u00a04) and the open dendritic pore structures (Fig.\u00a05) of Mo/Pt@TD-6%Ti catalyst can improve the accessibility of DBT and 4,6-DMDBT reactants to MoS2 active sites and reduce the diffusion resistance of reactants and products in the DBT and 4,6-DMDBT HDS reaction process.Sixthly, Ti species of Pt@TS-1 seeds can provide spillover of d-electrons for the transition metal oxide Mo species on the surface of the mesopore, which can create more sulfur vacancies and facilitate the reduction and sulfidation of the transition metal oxide Mo species.The dendritic Pt-confinement MoS2/Pt@TD-6%Ti catalyst shows excellent sulfur-resistance performance and HDS catalytic stabilities in the DBT and 4,6-DMDBT HDS reactions. The MoS2/Pt@TD-6%Ti catalyst not only exhibits excellent DBT and 4,6-DMDBT HDS activity but also reduces the production cost. This new concept of combining H2 dissociation performance of noble metal catalysts with the desulfurization ability of transition metal sulfide MoS2 may open a unique perspective of the development of noble metal catalysts for the industrial HDS process of feedstocks with high sulfur contents.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 (No. 21808079, 21878330 and 21676298), Key Research and Development Program of Shandong Province (No. 2019GSF109115), the National Science and Technology Major Project, the CNPC Key Research Project (2016E-0707), the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award (No. OSR-2019-CPF-4103.2) and the Project of National Key R&D Program of China (2019YFC1907700).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.2020.10.012.", "descript": "\n                  Metal confinement catalyst MoS2/Pt@TD-6%Ti (TD, TS-1/Dendritic mesoporous silica nanoparticles composite) in dendritic hierarchical pore structures was synthesized and showed excellent sulfur-resistance performance and stabilities in catalytic hydrodesulfurization reactions of probe sulfide molecules. The MoS2/Pt@TD-6%Ti catalyst combines the concepts of Pt-confinement effect and hydrogen spillover of Pt noble metal. The modified micropores of Mo/Pt@TD-6%Ti only allow the migration and dissociation of small H2 molecules (0.289\u00a0nm), and effectively keep the sulfur-containing compounds (e.g. H2S, 0.362\u00a0nm) outside. Thus, the MoS2/Pt@TD-6%Ti catalyst exhibits higher DBT and 4,6-DMDBT HDS activities because of the synergistic effect of the strong H2 dissociation ability of Pt and desulfurization ability of MoS2 with a lower catalyst cost. This new concept combining H2 dissociation performance of noble metal catalyst with the desulfurization ability of transition metal sulfide MoS2 can protect the noble metal catalyst avoiding deactivation and poison, and finally guarantee the higher activities for DBT and 4,6-DMDBT HDS.\n               "}
{"full_text": "The authors do not have permission to share data.Addressing climate changes and environmental sustainability while meeting the ever growing energy demand is one of the greatest challenges which our society needs [1,2]. Hydrogen has been recognized as one of the green fuels because of its high gravimetric energy density with zero emission [3,4]. To produce high quality hydrogen, electrocatalytic water splitting is one of clean and sustainable approaches [2\u20134]. Hydrogen evolution reaction (HER) is a cathodic half reaction of water splitting that reduces protons in acid or water molecules in alkaline to generate gaseous hydrogen, respectively [5,6].In particular, alkaline water electrolysis has improved stability over acidic electrolytes. Additionally, earth-abundant transition metals can be utilized as electrocatalysts which can further reduce the manufacturing cost [7]. HER mechanism in an alkaline electrolyte is generally described as following pathways of Eqs. (1)\u2013(3)\n[8,9].\n\n(1)\nH2O + e\u2212 \u2192 Had* + OH\u2212 (Volmer reaction)\n\n\n\n\n(2)\nH2O + e\u2212 + Had*\u2192 H2 + OH\u2212 (Heyrovsky reaction)\n\n\n\n\n(3)\n2Had* \u2192 H2 + OH\u2212 (Tafel reaction)\n\n\nAlkaline hydrogen evolution reaction is hindered by sluggish kinetics as a result of the accumulation of energy barriers of multiple elemental reactions [5,7,10]. Engineering the catalyst to alter the rate determined step is still a great challenge [6,7].Platinum (Pt) is known as the golden standard of HER electrocatalysts with satisfied reaction yield. However, vast commercial applications are limited by the prohibitive cost and insufficient reserves of Pt [4,11]. Pure nickel (Ni) has been considered as an alternative to Pt because of its low-cost, and abundant quantity [12]. However, Ni exhibits insufficient electrocatalytic activity and progressive deactivation toward HER which is caused by the weak ability to desorb OH- species from the surface and the formation of nickel hydride species [8,13,14]. To further enhance the HER performance, various Ni based alloys have been widely investigated [6,8,12,15]. The HER activity of Ni based binary alloys have been ranked in the following order: Ni-Mo >\u00a0Ni-Zn >\u00a0Ni-Co >\u00a0Ni-W >\u00a0Ni-Fe >\u00a0Ni-Cr [16]. Based on this trend and synergistic effects of the two components, great effort has been devoted to the design and testing of Ni-Mo alloys [6]. Ni-Mo also has higher corrosion resistance in alkaline electrolyte, good electrical conductivity, and excellent thermal stability.Until now, many works designed new electrocatalysts based on different hypotheses and theoretical calculations. However, most works failed to demonstrate the enhancement due to the complexity of electrochemical reactions on heterogeneous surfaces.In this review, we focused on the latest development of Ni-Mo alloys for alkaline hydrogen gas evolution. We first summarize the reaction mechanism with key activity descriptors affecting the HER performance of Ni-Mo alloys in terms of both thermodynamics and kinetics. Then, we highlighted various approaches to improve the HER catalytic activity and stability including ligand and strain effects. By in-depth analysis of the prior works, it is our intention to deconvolute the complex nature of HER and pave the way to further enhance the performance. Finally, we present future perspectives and challenges are presented.In order to improve the electrocatalytic properties, it is essential to understand the causes of high thermodynamic barrier and sluggish kinetic [9]. The electrochemical conversion of water into hydrogen gas involves three elementary reaction steps: i.e., 1) water adsorption, 2) water dissociation and 3) hydrogen generation where Eads (water adsorption energy), Eact (activation energy of water dissociation), \u0394GH* (hydrogen adsorption free energy which describes the binding strength of H* on the catalyst surface) are used as descriptors to evaluate each reaction step [7].The process of the adsorbed hydrogen into molecular hydrogen, i.e., the Heyrovsky or Tafel step, is the rate determining step which means that \u0394GH*, is a main descriptor [17]. \nFig. 1(a) shows that the result of the simple kinetic model plotted as \u0394GH* and \u0394EH depending on single crystals calculated as proposed by N\u00f8rskov and co-workers [18]. When \u0394GH* is close to thermoneutral, the HER electrocatalyst has well-balanced hydrogen bonding and releasing properties as well as maximum exchange current density [7,17]. Thus, Pt is well known as the best catalyst with a \u0394GH* value of \u2212\u00a00.09\u00a0eV (i.e., \u22120.33\u00a0eV of \u0394EH) [11,18]. On the other hands, Ni forms too strong hydrogen bonds in accordance with a \u0394GH* value of \u2212\u00a00.27\u00a0eV (i.e., \u22120.51\u00a0eV of \u0394EH) [19]. (Fig. 1(a)) This is related to the weak desorption ability of OH- species on Ni surface, which leads to an essentially irreversible reaction by hindering the desorption reaction and further water dissociation by forming oxides and hydroxide phases [8,13,14].The electronic synergistic effect between Ni and adjacent heteroatoms, i.e., Mo, is generally described as leading to much better surface adsorption properties. The Ni-Mo system improves HER properties due to the excellent water dissociation ability of Ni atoms and the superior adsorption properties towards hydrogen of Mo atoms, based on hydrogen spillover process in which hydrogen species adsorbed on Ni surface transfer to Mo surface (i.e., H migration) [14,15,20\u201325]. Thus, Mo is considered as the promoter of H2 dissociation by the efficient filling of the d band [26]. According to the Engel\u2013Brewer valence bond theory, the synergistic effect in HER of the material is expected when transition metals with empty or less-filled d orbitals (e.g., Mo) are alloyed with those having more-filled d orbitals (e.g., Ni) [27,28]. Highfield et al. reported the synergy effect in HER of Ni-Mo binary solid solutions formed by physical mixtures [25]. And it experimentally showed that Ni corrodes rather than evolves H2, especially in acidic electrolytes, and the Ni component associated with Mo appears more stable in HER process. It was presumed that the hydrogen trapping role of Mo protects from serious deactivation by impeding formation of Ni hydride, which influences the surface reconstruction and inhibits the further adsorption of H2O [25,26].Up to now, there have been significant efforts to synthesize Ni-Mo alloy with excellent electrocatalytic properties by varying composition to tune the electronic structure to maximize the synergy effect. In addition to the electronic structure changes, the crystal structure of Ni-Mo alloy alters with composition. For example, the addition of Mo atoms into the initial crystal structure of face centered cubic (fcc) Ni induces the local lattice expansion at low Mo content. However, the crystal structure alters to other forms when it exceeds the critical content [25]. Brewer-Engel predictions showed a multicomponent phase diagram of Mo with 3d transition series metals (i.e., Ni, Co, Fe, Mn and Cr) consisting of continuous isoelectronic curved lines (Fig. 1(b)) [20,28]. And each horizontal line between 3d metals (right side of transition series) and Mo (left side) indicate that the representative phase of alloy was changed when the atomic concentration of Mo exceeded a certain range. It has been noted the interdependence between crystal structure and electronic configuration [28]. In addition, it was predicted that the Ni3Mo phase could be the most stable alloy in the Ni-Mo system due to improved d-orbital overlap for hydrogen adherence and transference [28].The recent results accompanying the experimental evaluation of Ni-Mo electrocatalysts generally have been reported as the Mo incorporated Ni, Ni4Mo, Ni3Mo and NiMo phases and mixtures of several phases as good electrocatalysts. Pa\u0161ti et al. experimentally investigated that Ni-Mo alloys synthesized by arc melting method exhibit the electrocatalytic activities corresponding to Mo contents [29]. The result indicated that the multiphase structure composed of NiMo and Ni4Mo phases shows highest HER activity. (\nFig. 2(a) and (b)) Jak\u0161i\u0107 et al. also studies the composition effect on HER activity by synthesizing Ni-Mo alloy powders using sol-gel method [30]. The volcano shaped activity-composition relationship indicated that the best catalyst has the structure dominated by Ni4Mo. (Fig. 2(c) and (d)) In addition, Wang et al. reported that Ni-Mo alloy/MoO3\u2212x nanocomposites and Ni10Mo exhibited better HER activity than Ni4Mo and Ni3Mo phases [31]. (Fig. 2(e) and (f)) The calculated value of \u0394GH* for Ni10Mo was \u2212\u20090.27\u2009eV which is smaller than Ni4Mo (\u22120.39\u2009eV) and of Ni3Mo (\u22120.59\u2009eV). This was mainly caused by the interaction between H and the composition-dependent Ni-Mo surface, as well as the amount of MoO3 sites with increasing annealing temperature. Geng et al. reported colloidal synthesis of Ni-Mo alloy nanoparticles with various compositions where Ni0.4Mo0.6 nanoparticles exhibited the highest HER performance [32]. (Fig. 2(g) and (h)) As seen from these reported data, the optimal composition of Ni-Mo which exhibit the best HER performance significantly differed. Therefore, it is difficult to simply correlation HER activity to the crystalline features and the electronic synergistic effect, but later other additional parameters need to be considered.\u0394GH* based on volcano plot has emerged as a physical descriptor to understand HER. However, it does not capture the effect of other parameters such as interfacial interaction and interfacial reorganization at the interface between electrolyte and electrode[33]. It is worth discussing the interfacial engineering that assists water splitting into H+ and OH- with low barrier energy. Real-time spectroscopies and computation chemistry have been utilized to understand the underlying mechanism [33] where alkaline HER is less investigated than acidic HER [36].In general, the rate of HER in alkaline solution is approximately two orders of magnitude lower than in acid media due to slower the HO-H dissociation reaction [31,37,38]. The high affinity toward OH- could cause it to occupy active sites by the final products and block the consequent reaction, which can eventually affect higher HER overpotential [39]. Thus, the electron-coupled water dissociation, i.e. the Volmer step (H2O + e- \u2192 H* + OH-) has a strong influence on the HER kinetics in an alkaline solution. As important descriptors, water adsorption energy (Eads) and activation energy of water dissociation (Eact) can be used to investigate the ease of reaction at each step in the process of converting water molecules into hydrogen molecules [7].Until now, several groups studied the reaction kinetics of Ni-Mo electrocatalysts using DFT calculations followed by experimental verification. Zhang et al. synthesized Ni4Mo/MoO2 cuboids on nickel foam and investigated HER in alkaline solution [24]. The HER performance of these catalysts were comparable to that of Pt and superior to other earth-abundant electrocatalysts due to lower energy barrier for the Volmer step. The \u0394G(H2O*) on Ni4Mo is significantly decreased to 0.39\u2009eV, which is not only lower than the values of 0.91 and 0.65\u2009eV for pure Ni and Mo respectively, but even lower than Pt (i.e., 0.44\u2009eV) [24]. (\nFig. 3(a)-(d)) A smaller activation energy represents a faster water dissociation process. Shen et al. developed Ni4Mo electrocatalyst supported by graphene nanosheet. They contributed the superior HER characteristic of Ni4Mo alloy to a lower energy barrier (i.e., 0.39\u2009eV) [15]. (Fig. 3(e)-(h)) Thus, the water dissociation process is also greatly facilitated on the Ni4Mo surface, which speeds up the sluggish HER kinetics under alkaline conditions. Li et al. recently reported the potential of Ni-Mo alloys as bifunctional hydrogen oxidation reaction (HOR) and HER electrocatalysts in alkaline solution [40]. Although HOR mechanism in alkaline solution is still under debate, they suggested that Ni4Mo alloy exhibited better energy profiles for hydrogen adsorption (\u0394GH*, \u22120.09\u2009eV) and the hydroxyl adsorption (\u0394GOH*, \u22121.01\u2009eV) than pure Ni (\u22120.30\u2009eV and \u22120.09\u2009eV, respectively). And the energy barrier for water formation was calculated to be 1.03\u2009eV on Ni compared to 0.95\u2009eV on Ni4Mo alloy [40]. Based on both theoretical and experimental investigations, they showcased great potential for Ni4Mo alloy as HOR and HER electrocatalyst in alkaline media. More recently, Gao and Yu et al. reported that Ni4Mo nanoparticles exhibited notable HOR reactivity, which has higher catalytic performance than that of commercial Pt/C [41]. In addition, Ni4Mo nanoparticles exhibited remarkable tolerance against surface poisoning by carbon monoxide (CO) gas. They attributed it to lower CO adsorption energy (\u223c \u22121.45\u2009eV) compared to Pt (\u223c \u22121.68\u2009eV). Additionally, preferential OH adsorption on the surface of Ni4Mo assists the oxidation of the adsorbed CO [41].Yang et al. further discussed the HER kinetics of Ni4Mo phase in terms of interfacial engineering. Ni4Mo nanodots on MoOx nanosheets were electrochemically deposited and their performance was evaluated [34]. The results showed that the Tafel slope of Ni4Mo/MoOx (64\u2009mV/decade) is much smaller than Ni4Mo alloy (140\u2009mV/ decade). Ni4Mo/MoOx has lower charge transfer resistance (3.7\u2009\u03a9) than Ni4Mo alloy (6.9\u2009\u03a9) at an overpotential (\u03b7) of 130\u2009mV. It means that the interface consisting of metal/metal oxide affects the quicker water dissociation process by lowering the barrier and easy charge transfer. In addition, when Ni4Mo/MoOx was supported on Cu foam as a substrate, the \u03b7 was 16\u2009mV which is much lower than Ni4Mo/MoOx on Ni foam (\u223c 50\u2009mV). It showed that Cu as a substrate enabled amorphous MoOx to be more electron-rich at the interfaces, and it could facilitate water adsorption by the strong interaction with electron-deficient H atoms in water molecules [34,35]. And it is noted that the underlying layer in which Ni-Mo alloy forms an interface affects the HER performance. Ni(OH)2 is well-known as a promoter of water dissociation [42]. Yao et al. studied nanohybrid electrocatalysts consisting of NiMo alloy and Ni(OH)2 nanosheets on carbon cloth surfaces [43]. This electrode exhibited the \u03b7 of 132\u2009mV at 10\u2009mV/cm2 and Tafel slope of 134.1\u2009mV/decade, which was higher performance than NiMo alloy on carbon cloth (\u03b7\u2009=\u2009146\u2009mV and Tafel slope = 144.3\u2009mV/decade). (\nFig. 4(a) and (b)) It was assigned by the hybrid structure of NiMo alloy and Ni(OH)2 nanosheets, which resulted in enhanced HER properties by improving the water dissociation rate. On the other hand, heterogeneous structures with nanoscale interfaces have been studied to better understand the effect of HER occurring at the atomic scale rather than the micron scale hybrid structures. Markovic et al. described that the oxophilic group (i.e., M(OH)2) on the surface of Pt could lead to an easier water dissociation [35]. It experimentally showed that the nanometer-scale Ni(OH)2 clusters on the Pt surface significantly enhance the HER activity, despite a 35% reduction in Pt exposed to the electrolyte. It suggested that the water molecule absorbs the Ni(OH)2/Pt interface, and then dissociates by O atom interacting with Ni(OH)2 cluster and H atom interacting with Pt [35]. And when Li+ cations are involved in the electrolyte, the anchoring of Li+ cations to the oxophilic group leads to their strong interaction with H2O and OHad in the electrolyte. (Fig. 4(c)) Subsequently, Markovic et al. reported that even on the surface of various transition metals, Ni(OH)2 nanoclusters have an effect on enhancing HER activity. Especially, Ni(OH)2/Ni electrodes have four times higher electrocatalytic activity in alkaline electrolytes than the Ni electrode without Ni(OH)2. (Fig. 4(d)) And this approach has been proposed to be extended to Ni-based catalysts [44]. Therefore, it is meaningful to develop a heterogeneous catalyst containing Ni-Mo alloy which creates optimal interface between metal and metal oxide/hydroxide in order to lower dissociation kinetic energy barrier [10].Ni-Mo alloy as an electrocatalyst for HER has been fabricated by diverse synthetic methods. The electrochemical deposition is used to uniformly synthesize materials on the surface of a support such as Ni foam or carbon foam, and NiMo or NiMoO phases can be formed depending on process condition. An et al. designed the Ni4Mo/MoOx nanointerfaces by controlling the precursors, pH and applied current density for electrochemical deposition, and Dung et al. studied the effect of complexing agents (i.e., ammonium and citrate) depending on pH and concentration of metal ion precursors which could affect the deposit mechanism [34,45]. And the hydrothermal and solvothermal methods are utilized to synthesize radially formed nanorods with large surface area on a support. Generally, NiMoO4 nanorods were grown by the reactions, and through subsequent annealing in the reducing H2/Ar atmosphere, the Ni4Mo nanocrystals were anchored on the MoO2 or MoO3\u2212x phases. Zhang et al. showed superior electrocatalytic activity by ease electron transfer due to the high electrical conductivity of MoO2, and Chen et al. exhibited excellent catalytic performance by dual active components of oxygen-deficient MoO3\u2212x as well as Ni4Mo [24,46]. In addition, the colloidal method is useful for synthesizing NiMo alloy nanopowders with a large specific surface area. The Mo content in NiMo alloys is easily controlled by processing conditions of the colloidal method, despite the limited solubility of Mo atoms in Ni lattices [32]. And Zhang et al. showed extraordinary performance for both the HER and OER with the optimized catalyst of Mo0.6Ni0.4 nanoparticles [32]. Furthermore, arc metaling is mainly used to synthesize intermetallic compounds of NiMo. R\u00f6\u00dfner et al. fabricated well-characterized surfaces of the single-phase intermetallic compounds Ni7Mo7, Ni3Mo, and Ni4Mo to investigate their intrinsic performance [47]. Therefore, NiMo catalysts can be synthesized based on various methods depending on the purpose and direction of utilization, and it is very necessary to design materials with high activity and high durability through various strategies discussed below.The surface science on electrocatalysts have been studied for enhancing HER activity. The chemisorption properties of the alloy system can be predominantly affected by the primary two effects which are chemical composition (ligand) and physical structure (strain) [48,49]. The individual effects contribute to the shifts in the metal\u2019s d-band center which means to affect the strength of H*\u2009adsorption on the surface of a given material [11]. When the d-band center is close to the Fermi level, it tends to correspond to a strong H*\u2009adsorption [11]. Furthermore, it was suggested that it also influences the activation barrier for the bond breaking of molecules [11,19,50]. Thus, ligand and strain effects have a significant impact on thermodynamics and kinetics associated with the surface catalytic reaction of alloys, and would be considered for the rational design of electrocatalysts.Ligand effect occurs due to the formation of heteroatom bonds within first two monolayers, i.e., alloy system, and leads to the change in the electronic structure [11,51,52]. Until now, there have been few studies on the detailed HER mechanism of the Ni-Mo alloy system in relation to ligand effect excluding strain effect. Recently, Tian et al. studied the Ni-Mo alloy system of the cluster of NimMon (m + n\u2009=\u20095) based on the DFT method. It described the mechanism involving the initial adsorption of H2O and subsequent hydrogen evolution according to the clusters which are Mo, NiMo4, Ni2Mo3, Ni3Mo2, Ni4Mo and Ni [53]. It was suggested that Mo is the active site for H2O because of the electron deficiency of Mo by charge transfer from Mo to Ni, and Ni is involved in the adsorption of the split H of H2O after O-H bond breaking [53]. The adsorption energy of H2O on all designed clusters is about 20\u2009kcal/mol, slightly different depending on the NiMo ratio. (\nFig. 5(a)) Among them, the Ni4Mo cluster has the smallest reaction barrier to OH bond breaking, i.e. 3.45\u2009kcal/mol [53]. Furthermore, the reaction of Ni4Mo cluster spontaneously occurs under reduction conditions because the reaction barrier is small, i.e. \u2212\u20090.27\u2009kcal/mol. (Fig. 5(b)) It was demonstrated by the theoretical calculations how the bond formation between Ni and Mo heteroatoms affects the Volmer reaction step for HER performance, and excellent catalytic properties can be caused by lowering the high energy barrier of water activation.Strain effect occurs due to the alteration of the average length of metal-metal bonds in less than six monolayers, and potentially tunes the electronic structure. Guo et al. reviewed the strain-controlled multi-metallic electrocatalysts for the sake of achieving efficient energy conversion [54]. When the surface atoms of late transition metals (LTM, more-filled d orbitals) are subjected to tensile strain, it causes a stronger interaction with adsorbates [52,54]. For early transition metals (ETM, less-filled d orbitals) undergoing tensile strain, it lowers the adsorption energy [52,54]. On the other hand, compression strain has the opposite effect. The interaction is weaker for LTM and stronger for ETM [52,54]. In accordance with these arguments, it was attempted to induce strain effect using interfacial structure engineering (i.e. the core/shell structure, metallic overlays on substrates, de-alloyed shell) and structural defects (i.e. grain boundaries, multi-twinning) [11,51,52]. It is already known that the strain due to lattice expansion occurs as the Mo content in Ni increases, and the maximum solubility of Mo in Ni is less than 22\u201325\u2009at% [55]. If it is more than that, it can be regarded as a deformed crystalline structure by the strain energy and causes the change of crystal structure depending on Mo contents [55,56]. \u0141\u0105giewka et al. experimentally confirmed that each single phase of Ni-Mo alloys was synthesized in the range 0.5\u201323\u2009at% of Mo contents via electrochemical deposition and presented the strain values through Rietveld refinement and Williamson-Hall\u2019s method [56]. It showed that the lattice strain tended to increase with the Mo contents, and from 20\u2009at% or more, it increased relatively further. And from the alloy with 10\u2009at% Mo, the crystallite size decreased clearly. (\nFig. 6(a)) However, there has been no study on how the strain effect of Ni-Mo alloy affects HER according to the Mo content without ligand effect until now because Ni-Mo alloy system may simultaneously inherit the strain effect due to structural lattice expansion of the Ni unit cell by Mo insertion and the ligand effect due to the bonding between Ni as LTM and Mo as ETM. Thus, since the modulation of electronic structure as a dominant factor in HER activity is caused not only by strain effect but also by ligand effect, it is still a challenge to discover the mechanism by experimentally distinguishing the mutually concomitant two effects.Recently, Yang et al. studied the influence of the HER given of the elastic strain externally to Ni as a method to isolate strain effect from ligand effect [57]. It experimentally showed that compressive strain of Ni film causes higher current and lower overpotential, resulting in increased HER activity. (Fig. 6(b) and (c)) Conversely, the tensile strain leads to decreasing the catalytic performance. And based on DFT calculation, the \u0394GH of Ni(111) were \u2212\u20090.126\u2009eV at about \u2212\u20091.4% compressive strain and \u2212\u20090.141\u2009eV without strain, respectively. (Fig. 6(d)) It indicates that the increasing catalytic activity of Ni requires a weakening of H binding energy [57]. It seems that externally applied elastic strain can be considered as one of methods to understand the dominant strain effect in the Ni-Mo system.Chen et al. studied the electronic structure of Ni monolayers according to the supporting other transition metals based on DFT calculation [48]. When Ni monolayer on Pt (111) has a tensile strain (>10%), the d-band width of Ni monolayer is 1.35\u2009eV, which is narrower than 1.89\u2009eV pure Ni(111). It results in stronger adsorption energy on this surface due to the dominant strain effect [48]. Contrary to this, if the supporting metal of Ni monolayer is W(110), the d-band width is broader to 2.00\u2009eV and lowers in adsorption energy. This is because the ligand effect works predominantly, even under similar tensile strain. In the case of Ru(0001) as the supporting metal, the d-band width is 1.76\u2009eV, which is similar with Ni because of the balance between the narrowing of the d-band width due to strain effect and the broadening of it due to ligand effect [48]. It may indicate that the combined effect of the underlying support material on the Ni-Mo alloy catalyst should also be considered for controlling HER performance. Regarding the studies of Yang and Chen groups, the externally applied elastic strain and suitable underlying support materials seem to be considered one of the systematic fine-tuning methods of electronic structure on the Ni-Mo system for optimizing hydrogen adsorption energy and water dissociation energy.Although significant efforts were invested to determine the catalytic properties of Ni-Mo, limited investigations were performed regarding long term stability. For practical applications, it is essential to understand the fundamental aspects of material transformation during electrocatalysis to maximize stability. Until now, they have been reported that Mo might be dissolved in the form of highly soluble molybdate ions in the alkaline electrolyte, which will resulted in lowering electrocatalytic performance [47,58].Schalenbach et al. studied the intrinsic activity and stability of metallurgically prepared Ni-Mo alloy samples in alkaline electrolyte [58]. Bulk specimens were mechanically polished to enable clear distinctions from other factors such as interactions with the support, exposed surface area and morphology [47]. They analyzed Mo content in the electrolyte above anodic potential of \u2212\u20090.15\u2009V vs. RHE using an electrochemical flow cell coupled to a mass spectrometer. It reported that the alloys with higher Mo content resulted in greater Mo leaching and penetration depth. Selective Mo dissolution resulted in the formation of porous Ni structure such as hydroxide, oxy-hydroxides and oxide. Alternation of composition due to Mo leaching show lowering of electrocatalytic activity of Ni-Mo alloys compared to pure Ni electrode [58]. On the basis these results, they suggested that the high activity of Ni-Mo alloys might be attributed to the high surface area formed by Mo leaching, not inherent [58]. Armbr\u00fcster et al. investigated the stability of single-phase intermetallic Ni-Mo compounds using ex-situ XPS, roughness factors, electrochemically active surface area and corrosion current densities in KOH electrolytes [47]. They found that Mo in KOH electrolyte is leached from the intermetallic compound, which is more prone to form a higher Ni(OH)2 content on the near-surface region, resulting in lower surface specific activity [47]. They suggested that the crystal structure does not have effect on the catalytic activity in alkaline media and activity changes are a result of changing electroactive surface areas and the phase composition according to the degree of Mo leaching by the durability test conditions [47].Weckhuysen et al. utilized the density functional theory (DFT) to investigate the dissolution of Mo. They hypothesized that Mo is dissolved through the following reaction in alkaline medium;.\n\n(4)\nMo + 2 OH- + 2H2O \u2192 MoO4\n2- + 3H2\n\n\n\nWhich resulted in the creation of Mo vacancy to form segregated Ni phase. They also suggested that the rate of Mo leaching depends on the type of alkali cation, the reducing potential applied during electrolysis, and the substrate underlying the material [59,60]. For example, it was found that the alkali cations (i.e., K+, Na+ and Li+) infiltrate the surface of Ni-Mo alloys to form Mo vacancy where the Mo leaching rate was different due to the difference in the porosity formation through which the electrolyte infiltrated depending on the type of alkali cation [59]. Based to other cations, LiOH at pH 13 promoted the longevity of electrocatalyst by minimizing the Mo dissolution. Furthermore, it was shown that Mo leaching rate accelerated at high reduction overpotentials due to an increase in local pH. Although Mo leaching is depended on the applied potential, the leaching rate did not increased linearly with increase in applied potential [60]. Based on these observation, they hypothesized that multiple processes may occur that interfere with the oxidation of Mo to Mo6+ (in the form of MoO4\n2-). Lastly, the catalytic performance of NiMo was highly dependent on its substrate [60]. The catalytic activity decreased in the substrate sequence of Cu >\u2009Ni >\u2009Stainless Steel >Ti, whereas the stability of Mo differed from this order. The exact cause is unclear, but in terms of stability of 1\u2009M KOH, Ni-Mo deposited on a stainless steel substrate exhibited the lowest Mo leaching at low overpotential [60]. Overall, the electrocatalyst stability tends to be complicated and it is difficult to deconvolute the causes, so the Mo leaching mechanism is still under investigation. Therefore, further studies are needed to understand the destabilization mechanism and the major factors influencing the degradation of Ni-Mo electrocatalyst.Although long-term stability of electrocatalyst is essential for commercialization, only few works investigated the long-term stability of HER catalysts. For examples, Zhang et al. conducted that carbon plasma treatment improved catalyst stability by encapsulating the entire nanowires with a thin layer of graphitic carbon [61]. They explained the carbon shell effectively protects the active materials from dissolution in KOH solution through the XPS spectra of Ni and Mo before and after stability test as well as electrochemical stability test [61]. Peng et al. improved the stability of water electrolysis by coating on NiMo electrodes with chromium as a protective layer [62]. The chromium coating acted as a physical barrier to allow small molecules (e.g., H2) to pass through, while inhibiting the diffusion of oxygen by dissolved oxygen that could affect active site decomposition of the metal catalyst [62]. Recently, Zhai et al. reviewed latest progress on the long-term stability of HER electrocatalysts, and explained that dissolution of metal atoms, Osterwald ripening, agglomeration, particle detachment, active site poisoning and local corrosion could be the main reasons of catalyst deactivation [63]. And they suggested that the construction of binary transition metal (TM)-based compound materials (alloys, sulfide, phosphide, selenides, oxide, hydroxide, boride, and nitrides) is one strategy for improving electrocatalytic stability and activity.Facet engineering allows fine-tuning physicochemical properties [9]. The exposed active sites of Ni-Mo alloys for hydrogen evolution are also affected by structural and compositional engineering and cause to promote specific activity. Recently, Wang et.al described the H adsorption energies of all possible sites on Ni4Mo according to DFT calculation to approach a fundamental basis for HER activity origin [64]. The typical planes correspond to the detected Ni4Mo diffraction patterns of tetragonal structure, such as (101), (110), (121), (310), and (312), and have at least one strong preferred H adsorption position. Based on theoretical aspect, this indicates that all surface can exhibit catalytic capability and the most suitable adsorption site for H is on the facet (110) with \u2212\u20090.272\u2009eV of \u0394GH* value. Zhang et al. experimentally demonstrated that increasing the fraction of high-index (331) facets of MoNi promotes faster HER kinetics [65]. The high-index MoNi facets were formed during topological transformation of Ni4MoO to MoNi. Compared to its pure counterpart, MoNi dramatically decreased the energy barrier to 0.56\u2009eV in water dissociation step and 0.49\u2009eV in hydrogen combination step, accelerating the sluggish alkaline HER kinetics. These resulted in a remarkable electrocatalytic performance with a Tafel slope of 33\u2009mV/dec with excellent stability (i.e., upto 70 days). More recently, Lee et al. reported the H adsorption energies of Ni3Mo alloy with the low-index surfaces, which is considered to exhibit high catalytic activity. The Ni3Mo(101) showed \u0394GH* of \u2212\u20090.183\u2009eV, which is closer to that of Pt(111) [66]. And the facet of (101) has 0.504\u2009eV as the lowest energy barrier of water dissociation compared to the other facets [66] (\nFig. 7).These papers are meaningful by explaining the facet effect of the Ni-Mo alloy structure, which has been rarely addressed so far. However, the structural instability under high surface energy and the co-exposed crystal facets expressing unsatisfactory catalytic activity are still inherent problems when considering the facet engineering [9,67]. Therefore, in-depth research related to this is also a challenge.It is important to develop a material with a large catalytic surface area while facilitating electron transfer. Since the number of active sites is affected by the surface area of a given electrode, the nanostructured Ni-Mo alloys have been developed as catalysts. In this regard, the catalytic electrode must be able to promote electron transfer in order to make the most of its surface area. Gray et al. developed Ni-Mo alloy nanopowder and exhibited the overpotential of 70\u2009mV at 20\u2009mA/cm2 under a low loading of 1\u2009mg/cm2 in 2\u2009M KOH [68]. This electrode was fabricated by casting nanopowders suspended in isopropanol solvent onto a substrate without a binder, and showed a stable catalytic reaction for 100\u2009h. The current density at constant overpotential (100 and 200\u2009mV) increased with mass loading, indicating that a large amount of nanopowder could be utilized as active sites. (\nFig. 8(a)) Under a mass loading of more than a certain amount (10\u2009mg/cm2 in this study), it may be difficult to maintain HER activity because nanoparticle films have a poor adhesion to the substrate and the transport of reactant species is weakened through porous films of increasing thickness [68]. Thus, easy electron transfer and effective charge transfer into the active surface should be considered simultaneously for enhancing catalytic activity. Recently, conductive carbon support has been studied with the aim of increasing electron conductivity for facial charge-transfer process and lead to favorable adsorption of intermediates by electronic interactions with metal [8]. McKone et al. studied the effect of resistive interface on HER performance, and ameliorate activity of Ni-Mo nanopowders by thermal hydrogen annealing process or adding carbon black as a conductive support [69]. (Fig. 8(b) and (c)) It was noted that a few nanometer-thin oxide layers on the catalyst surface could have an influence on catalytic activity as a result of electrical resistivity rather than being limited by kinetics. Thus, carbon-containing Ni-Mo nanopowders lowered charge transfer resistance and showed higher HER activity than that of thermal annealing processes. Sun et al. synthesized ultrathin 2D Ni-Mo alloy nanosheets directly on a conductive substrate to ensure robust contacts, which showed HER performance with the overpotential of 35\u2009mV at the constant current of 10\u2009mA/cm2 and the Tafel slope of 45\u2009mV/decade in 1\u2009M KOH solution [70]. (Fig. 8(d)) The charge transfer resistance was very low (0.8\u2009\u03a9), which facilitated electron transfer. In addition, it exhibited smaller bubble adhesion force (\u223c 2\u2009\u03bcN) than other electrodes, meaning that the vertically aligned assemble structure could lead to much faster bubble release during HER. It showed that the excellent HER performance was attributed to fast mass transfer and easy electron transfer induced by the geometry of the catalyst grown directly on the conductive substrate. For a higher surface area for accessibility of water molecules, Ni-Mo alloys have been synthesized on the surface of 3D-structured conductive materials as the supports, including nanoform, nanorod, nanoporosity and hierarchical structure [46,71\u201377]. 3D architecture with its large surface area can help to manifest the superior catalytic properties inherent in the Ni-Mo alloy system by accelerating gas release.Interface engineering with surface decoration can modulate the adsorption/desorption energies of each species (e.g. H2O, OH-, H atom, and H2) to make more active catalysts [10]. In the kinetics section of this review, metal hydroxides on metal surfaces, in particular Ni(OH)2, have already been discussed as a promoter to lower the energy barrier of water dissociation. Jiang et al. developed the nanohybrid catalyst by integrating Ni4Mo nanoparticles with V2O3 network and observed that this surface was converted to Ni(OH)2 in the presence of water [78]. And it was evaluated in a neutral pH media for the purpose of utilizing abundant waste water or seawater sources [78]. Even in neutral electrolyte, Ni4Mo-V2O3 nanohybrids showed much better HER performance with an overpotential of 39.3\u2009mV at 10\u2009mA/cm2 and Tafel slope of 65.7\u2009mV/decade, when compared to the Ni4Mo electrode (\u03b7\u2009=\u200960.5\u2009mV at 10\u2009mA/cm2 and Tafel slope = 103\u2009mV/decade). (\nFig. 9(a)-(c)) The superior HER performance was attributed to the heterogeneous interface by the presence of oxophilic V2O3, which enhances water adsorption and promotes water dissociation into OH to form Ni(OH)2\n[78]. Based on DFT calculation, the Ni4Mo-V2O3 exhibited a higher water adsorption energy (\u22120.51\u2009eV) and a lower kinetic energy barrier for water dissociation (0.42\u2009eV) than those of Ni4Mo surface, i.e., \u2212\u20090.41\u2009eV of G(H2O*) and 0.65\u2009eV of \u0394G(H2O). Liang et al. electrochemically synthesized NiMoO4-NiO-Ni composite films and evaluated their HER and OER performance in NaCl-containing alkaline electrolyte [79]. These composite films showed excellent catalytic properties compared to its pure counterparts with exceptional durability. They found that NiO promotes initial water dissociation and facilitates the conversion from H+ to H2\n[79]. Further, the HER activities on these composite films were unaffected by the present of NaCl. Rather, the presence of Na+ and Cl- ions increased the solution conductivity which resulted in the reduction of IR drop. The NiMoO4-NiO-Ni composite films also exhibited good OER catalytic and corrosion properties. This work showcased that metal and metal oxide composite films can simultaneously promote HER and OER performance, and may offer the potential to utilize various water resources such as rainwater, seawater and wastewater as H2 feedstock. Furthermore, Qu et al. tuned the interfacial properties of NiMo nanoparticles by decorating small molecules of amine [80]. NiMo-EDA (ethylenediamine) required a much smaller overpotential of 72\u2009mV at 10\u2009mA/cm2 compared to 340\u2009mV of un-modified NiMo. And Tafel slope of NiMo-EDA was 89\u2009mV/decade, which was much lower than 135\u2009mV/decade of NiMo. (Fig. 9(d)-(f)) The diamine-modulated interface of NiMo facilitated the HER process and enabled more efficient charge transfer [80]. It may cause to accelerate HER activity by an electron-rich surface due to the electron donation from the amino group to NiMo.Non-metal elements (e.g., C, N, S, O, P, B, etc.) are attempted to be incorporated into Ni-Mo system since non-metallic elements could altered the adsorption free energy of reaction intermediates and assist the fast water dissociation [9,81,82]. Transition metal phosphides (TMPs), transition metal nitride (TMNs), transition metal chalcogenides (TMCs), and transition metal carbides have been studied to further enhance the properties [83]. Most works reported that non-mental elements incorporated metals have better HER performance in acidic electrolytes. However, these materials still have a few challenges including catalytic durability due to acid fog inhibition, corrosion, and surface reconstruction [67,83,84].There are many strategies for optimizing the catalytic performance by heterostructure engineering, phase transition and doping [67,85,86]. Ren et al. studied Ni2(1\u2212X)Mo2XP porous nanowire arrays as transition metal phosphides and showed outstanding HER activity in alkaline solution [81]. Ni2(1\u2212X)Mo2XP catalyst yielded current density of 10 and 100\u2009mA/cm2 at the overpotentials of 72 and 162\u2009mV, respectively, and exhibited a Tafel slope of 46.4\u2009mV/decade and long-term stability for over 160\u2009h. This result outperformed Ni2P nanostructure with the \u03b7 of 167 at 10\u2009mA/cm2 and a Tafel slope of 89.6\u2009mV/decade. Ni2(1\u2212X)Mo2XP catalyst obtained 1077\u2009mA/cm2 at overpotential of 300\u2009mV, which revealed that stable hydrogen generation at high current density outperformed 566\u2009mA/cm2 of commercial Pt wire. (\nFig. 10(a) and (b)) According to DFT calculations, Ni2(1\u2212X)Mo2XP catalyst possessed low H2O activation energy (0.56\u2009eV) and free energy for H adsorption (\u22120.08\u2009eV) close to Pt, which imply easy water adsorption and optimized hydrogen adsorption/desorption capability. (Fig. 10(c) and (d)) Specifically, this computational result meant that Mo exposed surface on Ni2(1\u2212X)Mo2XP catalyst has superior catalytic activity than that of Ni exposed surface on Ni2(1\u2212X)Mo2XP catalyst as well as the pure Ni2P phase. Thus, it indicates that Ni2(1\u2212X)Mo2XP catalysts adjusts electronic structure of pristine material by Mo substitution of Ni in Ni2P phase and the HER properties can be varied by the exposed atoms to the surface [81].Li et al. reported Ni-Mo-N catalyst composed metallic Ni and NiMo4N5 nanocrystals as transition metal nitrides. And it showed better HER performance of Ni-Mo-N catalyst compared to Ni-Mo catalyst in both alkaline and acid electrolytes [87]. In the alkaline electrolyte, Ni-Mo-N catalyst required \u03b7 of 43\u2009mV at current density 20\u2009mA/cm2 and Tafel slope of 40\u2009mV/decade, which superior to Ni-Mo catalyst (\u03b7\u2009=\u200986\u2009mV and Tafel slope = 74\u2009mV/decade). (Fig. 10(e)) Under the acid electrolyte, Ni-Mo-N catalyst exhibited \u03b7 of 53\u2009mV at 20\u2009mA/cm2 and the Tafel slope of 39\u2009mV/decade, outperforming Ni-Mo catalyst (\u03b7\u2009=\u200979\u2009mV and Tafel slope = 61\u2009mV/decade). (Fig. 10(f)) These excellent properties remained stable throughout the day in both electrolytes. (Fig. 10(g) and (h)) Sasaki et al. synthesized NiMoNX nanosheets on a carbon support (NiMoNX/C) and evaluated it in 0.1\u2009M HClO4 solution [88]. NiMoNX/C catalyst revealed an onset potential of \u2212\u200978\u2009mV and Tafel slope of 35.9\u2009mV/decade, which is much better catalytic performance compared to NiMo/C and MoN/C catalysts. In addition, NiMoNX/C catalyst maintained long-term durability, and the NiMo/C catalyst showed remarkable decrease in HER activity. (Fig. 10(i)-(k)) The results of the Li and Sasaki group's work provided that the introduction of nitrides into NiMo bimetallic structures improves catalytic performance and durability, which is evidence of improved corrosion resistance of Ni-Mo in acidic electrolyte. Recently, other efforts are being made to seek superior HER catalysts for neutral water splitting in order to utilize abundant water resources as H2 feedstock. The main challenges associated with HER in neutral solution is low HER kinetics similar to alkaline solution. Zhang et al. synthesized NiMoN nanowires array and investigated HER activity in neutral electrolyte with pH 6.8 consisting of 0.5\u2009M Na2SO4 +\u20090.25\u2009M KH2PO4 +\u20090.25\u2009M\u2009K2HPO4\n[89]. NiMoN nanowires exhibited the \u03b7 of 46\u2009mV at 10\u2009mA/cm2 and a Tafel slope of 78\u2009mV/dec. In addition, Ni nanoparticle decorate NiMoN nanowires showed lower \u03b7 of 37\u2009mV at 10\u2009mA/cm2 with the Tafel slope of 51\u2009mV/dec [89]. Based on these results, they calculated that the kinetic barrier for Ni/NiMoN (1.08\u2009eV) is lower than NiMoN (1.69\u2009eV) [89].Yu et al. synthesized Ni-doped Ni-Mo based sulfide (N-NiMoS) as chalcogenide systems and showed a good HER performance [90]. N-NiMoS catalyst possessed the \u03b7 of 68\u2009mV at 10\u2009mA/cm2 and a Tafel slope of 86\u2009mV/decade, which are much better performance than Ni-NiS catalyst, and displayed excellent long-term stability for 1000\u2009h. Instead of single metal sulfides, bimetal Ni-Mo based sulfides exhibited abundant interfaces derived from nickel sulfides (NiS and NiS2) and molybdenum sulfides (MoS2), which can serve as excellent active sites [90]. In addition, it was confirmed that N doping into NiMoS affects the improvement of electron transport and electronic conductivity by tuning electronic properties. Fu et al. designed heterostructure composed of amorphous NiMoS and crystalline Ni(OH)2 and demonstrated meaningful HER performance over all pH ranges [91]. According to solutions with different pH ranges (acidic, neutral, alkaline, and seawater solutions), the reported \u03b7 were 138, 198, 180 and 371\u2009mV at 10\u2009mA/cm2 and the Tafel slopes were 80, 81, 118 and 162\u2009mV/decade, respectively. It was suggested that the enhancement of HER activities in all pH ranges were mainly attributed to amorphous NiMoS, which act as more surface defective sites than its crystalline counterparts and allows for fast electron transfer [91]. And Ni(OH)2 as a promoter for H2O cleavage accelerated the water dissociation kinetics in non-acidic electrolytes, which could improve the rate-determining step for HER. Thus, it means that the TMC-based catalytic activity, which largely depends on the amount of active site and the electrical conductivity of the electrocatalyst, can be improved by interfacial engineering with the design of the bimetallic compound [40,91]. It is essential to continue in-depth study in non-metal incorporated electrocatalysts, since it may lead to the development of sustainable, non-toxic, and large-scale processable electrocatalysts.Externally field-assisted electrocatalysis such as an electric field, a magnetic field, a light field, an ultrasonic field, or an electric field are being studied as strategies to improve catalyst performance, unlike the development of catalysts themselves [92,93]. External fields can control the local environment at the interface between the catalyst and electrolyte [93]. Elias and Chitharanjan Hegde studied the effect of magnetic field-enhanced HER on Ni-W alloy [94]. It was validated by increasing magnetic field strength in the range of 0\u20130.4\u2009T, which clearly reduced overpotential and Tafel slope. (\nFig. 11(a) and (b)) The improved activity can be attributed to the magnetohydrodynamic (MHD) force-induced convection and H2 bubble release. In other words, the adhesion of hydrogen micro bubbles on the surface of catalysts can give rise to additional potential barriers for charge transport at the interface between catalyst and electrolyte. Thus, magnetic fields can accelerate electrolyte convection for easy elimination of hydrogen gas and alleviate polarization resistance [92,93]. Hourng et al. studied MHD phenomenon dependent on the electrodes by magnetic field [95]. Paramagnetic Pt, ferromagnetic Ni, and diamagnetic graphite were utilized as catalytic electrodes. When the magnetic field was applied, the current density increased by 14.6% for Ni and 10% for Pt, respectively, and the graphite changes were almost minor. (Fig. 11(c) and (d)) It showed that magnetic fields improve electrochemical hydrogen evolution and the ferromagnetic materials in particular can be preferred as good electrocatalysts under magnetic fields. Besides MHD, external fields have the possibilities to tune the inherent activity of catalysts [92]. However, the effects of various external fields on the Ni-Mo alloy system are rarely reported, a comprehensive understanding and further investigation of these strategies are required to obtain the field-boosted electrochemical reaction kinetics.For producing hydrogen as a clean fuel, intense efforts are being devoted to technological advances in water splitting in alkaline electrolytes. It is crucial to develop catalysts with essential requirements such as high performance, low cost, abundant resources and large-scale production, to replace Pt-based catalytic materials. In this regard, Ni, known as a major industrial metal, was also considered a promising candidate for alkaline HER. To compensate for Ni's insufficient catalytic activity and progressive deactivation, studies on Ni-based alloy materials are represented, of which Ni-Mo alloy is known to possess superior activity towards HER catalysis in alkaline electrolytes. The electrocatalytic performance of Ni-Mo based materials discussed in this review is summarized in \nTable 1 and simply plotted in \nFig. 12 according to substrate type, surface material, structural difference, synthesis method and electrolyte considered important in catalysis system.In this review, we summarized the progress of Ni-Mo alloys from before to recently based on theoretical calculations and experimental evaluations to understand the mechanism affecting the improvement of HER performance. The hydrogen adsorption free energy (\u0394GH*) and the activation energy for water dissociation (Eact) are presented as main activity descriptors to show the reaction pathway of Ni-Mo alloy based on thermodynamics and kinetics. The compositional feature of the Ni-Mo alloy system causes lattice expansion in the Ni unit cell, and changes in the cubic crystal structure of Ni above a certain amount. These behaviors lead to an influence on \u0394GH* and tend to vary Eact by tuning electronic structure of Ni-Mo alloy, and finally affect the HER activity of Ni-Mo alloys. In addition, the interface formation between Ni-Mo alloys and other materials as underlying layers or between Ni-Mo alloys and water dissociation promoters as overlying layers gives rise to accelerate the slow HER kinetics under alkaline conditions, leading to improved HER activity. On the other hands, Mo can be selectively dissolved in the form of highly soluble molybdate ions in an alkaline electrolyte to from a porous Ni hydroxide, which eventually causes lowering the electrocatalytic performance. To minimize the Mo leaching during electrolysis, factors such as the type of alkali cation, the reduction potential applied during electrolysis, and the substrate underlying the material need to be optimized.Furthermore, we presented effective strategies for enhancing HER performance of Ni-Mo alloys as shown in \nFig. 13, including i) optimization of ligand and strain effects, iii) stability, iv) facet engineering, v) large catalytic surface area with easy electron transfer, vi) interface engineering with surface decoration, vii) incorporation of nonmetal elements, and viii) externally field-assisted electrocatalysis. These strategies are aim at: (a) modulating adsorption/desorption energies each species, (b) control of environmental factors hampering Mo leaching, (c) tuning physicochemical properties of exposed active sites, (d) facilitating electron transfer and charge transfer into all actives sites on a large surface area, (e) leading abundant active sites and facile gas releasing, and (f) controlling the local environment at the interface between the catalyst and electrolyte. Thus, these efforts lead to enhancing intrinsic HER activity and improving stability and durability of pristine catalysts. In addition, it can provide the possibility to utilize various water resources as H2 feedstock regardless of pH ranges and expandability into bifunctional electrocatalyst. Although this review has dealt with the specific alloy as an electrocatalyst in detail, it will be helpful to provide a comprehensive understanding of how the intrinsic and extrinsic activities in other alloying catalysts change based on theoretical studies and various experimental approaches and how they relate to the final HER performance.Up to now, the remarkable progress of Ni-Mo alloyed electrocatalysts for alkaline HER has been studied. However, there are still issues to consider because the experimentally verified catalytic activities are not sufficient to satisfy the expected results based on various experimental approaches and computational science for enhancing HER properties. And the low electrochemical stability of electrocatalyst and low feasibility of large-scale production of lab-developed materials are a major constraint on commercial survival. Other catalyst materials also encountered this challenge.To begin with, it is necessary to consider how the correlation accumulated from the substrate to the surface affects the active surface of the final synthesized electrocatalyst. Although effective approaches are studied to improve HER properties, the demonstrated results are influenced by material and structure of the underlying layer of active sites. And the environment, such as strain effect and external applied field, given to the interface between the underlying layer and the electrocatalyst\u2019s active surface also affects HER performance because of modulating the electronic structure of the active surface.Second, the overlying materials on the active surface of the electrocatalyst require further investigation. The water dissociation promoters on the active surface play a major role in enhancing slow HER kinetics in alkaline electrolyte. And protecting metal catalysts from corrosion on the active surface is a crucial issue for HER stability. Until now, studies on Ni(OH)2 for promoting water dissociation and carbon materials for protecting corrosion reaction have been conducted as the overlying materials on the active surface, but more candidate materials and related studies are needed to solve the fundamental cause.Third, continuous multi-disciplinary efforts are needed to understand the direct correlation between theoretical calculations and electrochemical stability, and to uncover controversial catalytic reaction mechanisms based on in-situ analysis. It also requires an overall discussion of the inherent uncertainties in the DFT calculation is necessary. Recently, Liberto et al. raised current problems related to predicting catalytic activity, such as accuracy of calculations, ignoring important contributions in the model used, and the physical meaning of descriptors and reproducibility [102,103]. And they explained that there are no tools to predict whether it will be possible to synthesize a new catalyst, even if a catalyst is designed. Therefore, it is known that the electronic structure theory aimed at predicting the catalytic activity of a material can be an important guideline for experiments or described experiments results more logically, but the rational design of catalysts based on theoretical calculations requires in-depth discussion and attention.Fourth, there is a need for a feasible synthesis method that can produce a catalytic material developed in a laboratory on a large scale. Although interesting HER catalysts with outstanding performance have been reported so far, simple synthetic methodologies should be developed for large-scale production and ease to process.Finally, efforts on standard and systematic evaluation methods are required to accurately compare the developed electrocatalyst\u2019s performance. It is necessary to establish the essential evaluation methods that represent HER activity and the stability of electrocatalysts, especially their ability to generate stable hydrogen at high current densities for commercial viability. In addition, the criteria for factors influencing HER performance such as Pt as a reference electrode and electrolyte as reference solution during measurement should be discussed to facilitate comparative evaluation with each other.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.SHP acknowledges the support from Characterization platform for advanced materials funded by Korea Research Institute of Standards and Science (KRISS \u2013 2021 \u2013 GP2021-0011). DT and NVM acknowledge the financial support from the Hydrogen Energy Innovation Technology Development Program of the National Research Foundation of Korea (NRF-K) funded by the Korean government (Ministry of Science and ICT (MSIT)) (NRF-2019M3E6A1064020).", "descript": "\n                  Hydrogen evolution reaction (HER) from alkaline electrolytes is one of most promising methods for producing hydrogen. The remaining obstacles include the development of high performance and earth-abundant electrocatalysts which can be cost-effectively fabricated in large-scale. Ni-Mo based materials are one of potential candidates which might meet the most needs. In this review, we summarize the latest progress in Ni-Mo based HER catalysts in alkaline electrolytes from theoretical calculation to experimental results. This work also summarizes several different strategies to enhance the HER rate. Finally, the future perspective of the next generation electrocatalysts is discussed.\n               "}
{"full_text": "Sea-level rise and climate change are among the severe effects of the global energy crisis and greenhouse gas emissions. Therefore, research on renewable energy is fundamental and urgent [1\u20133]. Among new energy systems, hydrogen energy is considered a promising secondary energy source with a higher calorific value. H2 can be stored in high-pressure tanks in gas and liquid phases. It can also be reversibly absorbed and released by solid hydrogen storage materials. Moreover, as an excellent energy storage medium, it could realize high-efficiency conversion and reuse between hydrogen and electricity through water electrolysis and fuel cell technologies. Furthermore, H2 has a strong grid connection potential. It can convert wind energy, photovoltaic solar energy, and other renewable energy power, which have the characteristics of seasonal intermittent and regional limitation, into hydrogen energy, thus optimizing energy storage and transportation [4].Therefore, it has recently attracted much attention as a potential renewable energy source [5]. However, up to now, nearly all H2 is produced from fossil fuels, mainly using natural gas and coal as raw materials. To achieve the Paris Agreement's goals, H2 production should be effectively transformed from traditional fossil-fuel-based ways to the renewable-energy-based scenario [6]. In this context, H2 production by water electrolysis using renewable energy has been promising because of its environmental friendliness, abundant water resources, and high hydrogen purity (\u224899.999%) [7\u20139]. It could also integrate other renewable energies into the grid and create a substantial downstream market [6].According to electrolyte and operational temperature, water electrolysis technologies could be classified as alkaline water electrolysis, proton exchange membrane water electrolysis, anion exchange membrane water electrolysis, and solid oxide water electrolysis. Alkaline water electrolysis is considered a suitable technology for the large-scale synthesis of hydrogen because of the usage of non-precious metal catalysts [10,11]. It is well-established as the most commercially viable and applicable technology with numerous vendors [12,13].The overall water electrolysis is classified into two half slow-rate electrochemical reactions, including the HER and OER [14,15]. Compared with non-precious metal electrocatalysts, precious metals-based materials such as Ir/Ru compounds for OER and Pt for HER are generally engaged as electrocatalysts for acid\u2013media reactions to promote sluggish electrochemical activities [16,17]. However, using such relatively scarce and precious catalysts is not advisable because it significantly raises the cost of H2 manufacturing and severely impedes the expansion of productivity on an industrial scale in the future, thus necessitating the development of non-precious metal alternatives with high performance [18,19]. Currently, reported non-precious metal electrocatalysts mainly include sulfides, selenides, phosphides, carbides, nitrides, etc. [13,20\u201338] Transitional metal phosphides are potential candidates for HER due to the hydrogenase-like catalytic mechanism. However, hydrogen bonds strongly to nickel-based phosphides, which necessitates optimizing the free energy and electronic structures by rational strategies such as heteroatom-doping and heterostructure construction [37\u201341]. Transition metal selenides possess low intrinsic electrical resistivity. The 3\u00a0d orbitals of transition metal selenides bond with metal atoms because the energy level is close to 3s and 3p orbitals, with higher metallicity. The main limitation of TMSs is the deficient surface active sites. Several strategies have been proposed to increase active sites and modify catalytic activity [33,42\u201345]. Transition-metal nitrides have emerged as an attractive class of HER catalysts due to their high electrical conductivity and noble-metal-like characteristics, while the performance presents obvious variation. The mechanism for interpreting variation remains unclear, notably the modulation principle of metal centers [46,47]. Recently, great progress has been made on transition metal sulfides. Among them, layered transition metal sulfides such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) have been studied intensively due to the near-zero energy barrier for HER on edges [34\u201336,48]. However, the low density of active sites, poor electrical transport, and inefficient electrical contact may hinder further development. Among transition metal electrocatalysts, nickel sulfides, including NiS, NiS2, and Ni3S2, have been identified as ideal candidates for OER and HER catalysts due to the low price, abundant resources, simplicity of preparation, noble metal-like electrocatalytic properties, and electronic configuration [49,50]. It has been reported that Ni3S2 exhibits better HER and OER electrocatalytic performance than NiS and NiS2 [51\u201353]. Both experimental results and theoretical calculations show that the good catalytic properties of Ni3S2 could be ascribed to its inherent metallic conductivity, the abundance of active sites, and the optimal Gibbs free energy for catalyst-H\u2217 for HER. Furthermore, because of the more fraction of Ni, catalytically active species of nickel (oxy)hydroxides are formed on the surface of Ni3S2, resulting in improved OER performance [52,54\u201356].Up to now, there have been many works to review the advances of electrocatalysts for water electrolysis. However, systematic summaries of Ni3S2-based materials are few. Herein, the review of recent advances in Ni3S2-based electrocatalysts is presented. It begins by briefly introducing the mechanisms of HER and OER in alkaline solutions and the performance evaluation parameters of electrocatalysts. Then, the synthesis technologies and performance improvement strategies are summarized. The applications of Ni3S2-based electrocatalysts for industrialized alkaline water electrolysis are discussed afterward, especially for the bubble behavior control and surface wettability construction strategies. Finally, an outlook on future challenges and opportunities for Ni3S2-based electrocatalysts is presented, and potential future directions are also proposed.The HER process in alkaline solutions follows the Volmer\u2013Heyrovsky or Volmer\u2013Tafel step, as shown in equations (1)\u2013(3).Volmer step:\n\n(1)\n\n\n\nH\n2\n\nO\n+\n\ne\n\u2212\n\n\u2192\n\nOH\n\u2212\n\n+\n\nH\n\u2217\n\n\n\n\n\nHeyrovsky step:\n\n(2)\n\n\n\nH\n2\n\nO\n+\n\ne\n\u2212\n\n+\n\nH\n\u2217\n\n\u2192\n\nH\n2\n\n+\n\nOH\n\u2212\n\n\n\n\n\nTafel step:\n\n(3)\n\n\n2\n\nH\n\u2217\n\n\u2192\n\nH\n2\n\n\n\n\n\nThe Volmer step explains the splitting of water molecules and hydrogen adsorption. The subsequent stage involves the generation of hydrogen by the Heyrovsky process and/or the Tafel process. The HER mechanism comprises the Volmer process followed by a parallel Heyrovsky and Tafel step at low overpotentials, and the reaction follows the Volmer\u2013Heyrovsky pathway at high overpotentials [57,58].Compared with the HER in acidic solutions, the HER is more challenging to achieve at the low overpotential in alkaline solutions. In acidic solutions, only the hydrogen bonding energies determine the reaction rate, while in alkaline solutions, the reaction mechanism is decided by water adsorption and dissociation, hydrogen adsorption/desorption, and hydroxyl ions affinity to the electrocatalyst surface (poisoned catalysts by occupying active sites) [11,57,59\u201361].Water absorption is the first step of HER. Compared to the situation in acid media, where the number of protons is much more than that in the alkaline media, the additional water decomposition step is indispensable, leading to another energy barrier. Beak et\u00a0al. synthesized a ruthenium-based nanoparticle catalyst dispersing within a nitrogenated holey two-dimensional carbon structure (Ru@C2N) [62]. This catalyst improves the binding energy of Ru\u2013H2O significantly, which accelerates the Volmer reaction rate. Moreover, because of the shortage of protons, water dissociation is an indispensable step of HER in alkaline media, slowing down the reaction rate. According to the Sabatier Principle, a good balance between the adsorption and desorption on the electrocatalyst surface is the optimal condition for forming a suitable intermediate bond strength (not too strong and not too weak). The relations between the HER current density and metal-H (Me\u2013H) bond strength could be described using a \u201cvolcano\u201d diagram. Fig.\u00a01\ng depicts the situation of pure metals, where the bonding energy of Ni is closer to that of precious metals, indicating that Ni is a suitable candidate for water electrolysis. Owing to the intrinsic Ni\u2013Ni metal network, Ni3S2-based materials exhibit metallic-like properties, which implies that they have good electrocatalytic activity.Finally, the aqueous OH\u2212 is also essential in influencing the kinetics, even though its role is still debated. Some studies suggest that OH\u2212 and Had adsorption is competitive on one single active site, so the Volmer step's kinetics could be improved by creating dual active sites to host OH\u2212 and Had, respectively [63]. Nevertheless, others think the adsorption of OH\u2212 does not participate in the alkaline HER process, nor does it affect HER's kinetics. Recent studies suggest that the \u2018dual active sites\u2019 improve from hydrogen bonding's optimal energy bonding [64]. Therefore, those doubts can be solved after conducting further in-depth studies with a detailed consideration of intermediates' interactions.Compared with HER, the OER in alkaline solutions has a different and complex process because it involves four electrons transfer. It is regarded as the bottleneck constraining the energy efficiency of alkaline water electrolysis due to its sluggish reaction dynamics. All proposed OER mechanisms start from the same imperative step of hydroxide coordination to the active site, followed by varying proposed steps. Two broadly acknowledged OER processes include the classic absorbed evolution mechanism (AEM) and the novel lattice oxygen-mediated mechanism (LOM).In alkaline solutions, the AEM process comprises numerous electron-proton coupled steps in which OH\u2212 is oxidized into molecules of oxygen and water [65]. The reaction pathway could be characterized as follows:\n\n(4)\n\n\n\nOH\n\u2212\n\n+\n\u2217\n\u2192\n\nHO\n\u2217\n\n+\n\ne\n\u2212\n\n\n\n\n\n\n\n(5)\n\n\n\nHO\n\u2217\n\n+\n\nOH\n\u2212\n\n\u2192\n\nO\n\u2217\n\n+\n\nH\n2\n\nO\n+\n\ne\n\u2212\n\n\n\n\n\n\n\n(6)\n\n\n\nO\n\u2217\n\n+\n\nOH\n\u2212\n\n\u2192\n\nHOO\n\u2217\n\n+\n\ne\n\u2212\n\n\n\n\n\n\n\n(7)\n\n\n\nHOO\n\u2217\n\n+\n\nOH\n\u2212\n\n\u2192\n\n\u2217\n+\n\nO\n2\n\n+\n\nH\n2\n\nO\n+\n\ne\n\u2212\n\n\n\n\nwhere \u2217 denotes active sites, O\u2217, OH\u2217, and HOO\u2217 represent adsorbed intermediates on active sites [66]. Firstly, OH\u2212 originated from the electrolyte is absorbed by an active site, followed by M\u2212O generation with the removals of a water molecule. Then, an M\u2212O is converted to an M-OOH intermediate by coupling a hydroxyl anion under one-electron oxidation. After that, an M-OOH gets coupled with a hydroxyl anion, generating an O2 molecule and an initial active site under one-electron oxidation. The other approach is the linking of M\u2212O to generate O2, but this path has a higher thermodynamic barrier than that of the pathway involved in the reaction (6) and reaction (7). In addition, previous studies have also detected the existence of the M-OOH intermediate, expediting the approach by the steps from (4) to (7) as the more general OER pathway [67,68]. Significant attempts have been made to link OER activity with a particular descriptor to understand the OER active site and further forecast active OER electrocatalysts. In 1955, the M\u2212OH bonding energy was first found to be related to the OER activity, where the lowers in an almost linear relationship with the M\u2212OH bonding energy rise [69,70]. With the development of molecular orbital theory, the 3\u00a0d electron number of the B site is claimed to have a linear relationship with the catalytic ability for OER in ABO3-type perovskite oxides [71]. The enhanced catalytic ability could be attributed to decreased M\u2013OH bond strength with increasing d-band electrons. Recently, much progress has been made in correlating the OER activity with descriptors. The volcano-type relationship between eg electrons and OER activity is proposed on the perovskite oxides, which is different from the previous linear description of d band electron number [72]. Oxides with eg occupancy close to unity could attach moderately with oxygen, resulting in the ideal OER activity [73].In addition to the AEM, the LOM has been considered the new pathway for OER in alkaline solutions. It is possible that direct O\u2013O bonding with the reversible production of oxygen vacancy (LOM) may become favorable as covalency increases. This is because the capacity of metal cations to bind with oxygen will become weaker as covalency increases. Some studies have reported that the LOM is directly observed to enhance the OER activity by in-situ characterization techniques and density functional theory (DFT) calculations [74\u201377]. The lattice oxygen could participate in the OER reaction, resulting in the generation of oxygen molecules via the production of oxygen vacancies, as shown in Fig.\u00a01e. For highly covalent oxides, the OER on oxygen sites could be initiated when the Fermi level is pinned to the top of the O 2p band, resulting in electronic states near the Fermi level with significant O 2p characteristics. Furthermore, it is observed that OER activities increase with rising pH for perovskites with metal\u2013oxygen covalency, demonstrating that the activation of lattice oxygen redox processes is linked to non-constrained proton-electron transfer steps [74,78,79].Before delving into the advances in Ni3S2-based electrocatalysts in alkaline water electrolysis, it would be advantageous to go over the screening parameters and highlight what one should expect while screening HER and OER electrocatalysts. In this part, some vital performance evaluating parameters that have been well-accepted are summarized.The extra potential (beyond the thermodynamic requirement) needed to drive a reaction at a certain rate is defined as overpotential [83]. Overpotential could be presented as equation (8)\n\n\n(8)\n\n\n\nV\n\no\np\n\n\n=\n\nV\n\ne\nq\n\n\n+\n\n\u03b7\na\n\n+\n\n|\n\n\u03b7\nc\n\n|\n\n+\nI\nR\n\n\n\n\n\nV\n\nop\n is the overall potential, and V\n\neq\n is the equilibrium potential (1.23\u00a0V for water electrolysis). The \u03b7\n\na\n and \u03b7\n\nc\n are OER and HER overpotentials, respectively. IR is the overpotential caused by Ohmic impedance. The overpotential of OER serves as the primary barrier due to the four-electron transfer process. Overpotentials at specific current densities are frequently utilized as quantitative parameters for evaluating the catalytic ability of electrocatalysts [11,84\u201386]. The overpotential required under the current density of 10\u00a0mA\u00a0cm\u22122, which is the current density to match the 12.3% efficiency for photoelectrochemical water electrolysis, has been considered an important indicator to evaluate different catalysts. Still, it cannot determine the intrinsic activity [87,88]. In cases of electrocatalysts for commercial applications, higher current densities, such as 200 and 400\u00a0mA\u00a0cm\u22122 could also be taken into account [12,89]. For Ni3S2-based catalysts, recent studies have adopted the overpotential at a higher specific current density, such as 200\u00a0mA\u00a0cm\u22122, 500\u00a0mA\u00a0cm\u22122, and 1000\u00a0mA\u00a0cm\u22122, to evaluate the commercial application ability [2,90\u201392].The areal, mass, and specific activities are perspectives of the thermodynamic activity parameters. The areal activity is calculated by taking the geometrical surface area of the electrocatalysts, the mass activity is ascertained by taking the mass of the loaded catalyst and normalizing it, and the specific activity is obtained by taking either electrochemically active surface area (ECSA) or the Brunauer\u2013Emmett\u2013Teller (BET) for normalization. Mass activity is often evaluated on comparable material systems and is heavily influenced by the active area of the catalyst. Catalysts with larger surfaces frequently have higher mass activity. Due to the much lower cost of 3\u00a0d transition metals, the mass activity is less essential than the other characteristics. Among these parameters, the specific activity is the most reliable one when using ECSA rather than the BET to normalize [93]. The gas adsorption/desorption sites utilized to calculate the BET may not be electrochemically active.The Tafel slope, j0, and TOF are three kinetic activity characteristics [91]. The well-known Butler\u2013Volmer equations imply that lower Tafel slopes and higher j0 are desirable, especially for alkaline water electrolysis at high current density [94]. When the overpotential is neither very small nor not very large, it is often simplified using the Tafel equation. The Tafel equation could be expressed as equation (9)\n\n\n(9)\n\n\u03b7\u00a0=\u00a0a + b\u00a0\u00d7\u00a0log(j)\n\nWhere the b is the Tafel slope. The j0 is calculated by extending the Tafel plot's linear fit to its intersection with the corresponding logarithmic current density at the electrocatalytic study's reversible potential. The TOF measures how many products each active site in the reaction produces each second. For water electrocatalysis, TOF values are always reported as a function of the overpotential. There are several equations for TOF calculation. The most commonly used one is shown as equation (10) [95].\n\n(10)\n\n\nTOF\n=\nj\n\u00d7\n\nN\nA\n\n/\n\n(\n\nF\n\u00d7\nn\n\u00d7\n\u0393\n\n)\n\n\n\n\n\nWhere the j, NA, F, n, and \n\n\u0393\n\n indicate current density, the Avogadro constant, the Faraday constant, the number of electrons transported to generate one molecule of the product, and the precious number of active sites, respectively. The n is equal to 2 for HER and is equal to 4 for OER. Some researchers would also calculate the TOF of OER by concurrent oxygen reduction reaction (ORR) at the ring electrode utilizing a revolving ring disk electrode assembly [96].The Tafel slope is a primary evaluation perspective to evaluate the inherent kinetics and determine the mechanism of electrocatalysts [97]. The Tafel slope of 29, 38, or 116\u00a0mV dec\u22121 indicates that the HER rate-limiting step is Tafel, Heyrovsky, or Volmer step, respectively [98,99]. The kinetics occurring in the interface could be ascertained from the Tafel slope. Several approaches to acquiring the Tafel slope have been described in past studies [100,101]. The Tafel slope and j0 could provide many insights into the kinetic behavior of the electrochemical process. TOF is always tricky to measure accurately because the number of active sites participating in reactions is difficult to ascertain, and the assumption made during the calculation primarily affects the results [91,94]. Several methods used to calculate the TOF have been adopted in experiments, but they all have drawbacks. [95] Recently, a systematic path for calculation has been proposed to improve the accuracy of the calculation of TOF values. The relatively accurate calculation of TOF values should first determine the exact number of active sites and then exclude the error originating from other current normalization methods, including geometric area normalization and mass normalization. After that, errors for catalysts with FE <100% should also be excluded [102].FE is the efficiency of transferring electrons transported by the external circuit to the electroactive species to generate products [85]. Two methods are widely employed to determine the FE of HER and OER reactions. The first is based on the rotating ring disc electrode (RRDE). This instrumental technique only determines the intrinsic of OER catalysts. The equation calculating the FE is shown as equation (10)\n\n\nFE = (IRnD) / (IDnRNCL) (10)\n\n\nn\n\nR\n and n\n\nD\n represent the number of electrons transferred at the ring and disc. I\n\nR\n and I\n\nD\n represent the currents at the ring and disc, respectively. N\n\nCL\n represents the collection efficiency of the RRDE. The other method calculates the generated gas (H2 and O2) by the chronoamperometric or chronopotentiometry analysis. Subsequently, the gas amount may be determined using the water gas displacement technique, gas chromatography, and the spectroscopic technique [103]. If the calculated and collected volumes correspond, the catalyst's FE for the specific gas evolution process is 100 percent, showing that it is selective for electrochemical reactions. It assures that the supplied energy would not be wasted on side reactions. Other electrochemical events would also not contribute to the gas evolution current [89]. For Ni3S2-based electrodes, electrocatalysts are expected to have a FE of 100%. If the proportion of FE falls between 90% and 100%, the OER is deemed to be satisfactory [91,104].The Ni3S2 possesses a rhombohedral crystalline structure. Each nickel atom is in a pseudo-tetrahedral site in a sulfur lattice that is about body-centered cubic. Ni3S2 units are connected with a short distance of Ni\u2013S (2.29\u00a0\u00c5) and Ni\u2013Ni (2.53\u00a0\u00c5), which shows apparent metal\u2013metal bond interactions. According to the calculation results, the dominant interactions and contributions of Ni3S2 are Ni(d), with little contribution from S(S) orbitals. Ni3S2 exhibits metallic behavior, with the majority of its orbitals crossing the Fermi level, as shown in Fig.\u00a02\nd and e [90,105,106]. This metallic property could be explained accorded to previous reports: when all the S atoms in the Ni3S2 bulk are removed, the remaining Ni\u2013Ni structure could be regarded as a crystal, and 4 Ni\u2013Ni bond modes appear for each Ni atom in the structure. This forms a continuous Ni\u2013Ni bond network throughout the crystal. Therefore, this structure is basically more conducive to the transport of electrons, presenting good metallic properties [49,107]. Besides, its low cost makes Ni3S2-based materials more competitive as alternative electrocatalysts for alkaline water electrolysis. In addition, the high electron density around metal sites could enhance OER activity. The stability of Ni3S2 is high under alkaline solutions because of the formation of an oxide/hydroxide layer on the Ni3S2 surface. Therefore, Ni3S2-based electrodes act as good HER electrocatalysts and good OER pre-catalysts [108].Ni3S2 is considered as a pre-catalysts rather than the true catalyst during operation in alkaline, and nickel (oxy)hydroxides are always considered as the real-time catalysts during OER [109\u2013112]. This transformation process is because Ni3S2 would undergo surface reconstruction. From the point of view of solid-state chemistry, the thermodynamic stability of Ni3S2 is not very satisfactory [113]. This property makes the surface of Ni3S2 susceptible to redox reactions in the redox environment. The easy oxidation of the surface of Ni3S2 to the corresponding oxides or hydroxides is therefore expected, particularly in the oxidative conditions of OER. It is reported that this surface reconstruction process could result in a core\u2013shell structure, while other studies suggest it undergoes complete oxidation/anion exchange with hydroxide/oxide during OER [55,56,114\u2013117]. Recently, further understanding of the Ni3S2 surface self-reconstruction process has been achieved by in-situ investigation methods [115,118]. Systematic in-situ/ex-situ experimental investigations and DFT results indicate that the combination of progressive S leaching and OH\u2212 adsorption promotes the structural evolution of Ni3S2 into NiOOH/Ni3S2 and the residual SO4\n2\u2212 could benefit the OER by boosting the adsorption of intermediates [54,119].During the development of Ni3S2-based electrocatalysts, it has attracted much attention for materials preparation research. Experimentally, it is eager to synthesize Ni3S2-based materials with good performance and low cost. Especially for industrial applications, the techniques suitable for large-scale manufacture are one of the research hotspots. In this part, several methods employed to synthesize Ni3S2-based electrocatalysts and the characteristics associated with each technique are introduced.Hydrothermal and solvothermal synthesis methods are widely accepted due to low cost, simple synthesis procedure, and wide application ranges [120,121]. It could obtain electrocatalysts with high purity, good crystallinity, and regular morphology by applying these methods [122]. The core is to select suitable precursors and reaction conditions (temperature, pressure, reaction time, pH, etc.). Previous studies suggest the reaction temperature is the main factor in controlling the structure and morphology of Ni3S2-based catalysts [123]. It is a practical way to adjust catalysts by changing the ratio of reactants and reaction conditions [124].Hydrothermal and solvothermal synthesis methods are time-consuming processes despite their widespread use. Both approaches are hampered by the difficulty of managing the active chemical loading and the shape of the product. In comparison, electrodeposition is a cheap, efficient, and convenient method that synthesizes materials on a nanometer scale under ambient conditions, and it could be used for interface engineering [3,125]. It requires the applied electricity to drive the deposition on conductive substrates. Electrodeposition could also fabricate self-standing electrodes with the deposits rigidly attached to the substrate [126]. Like the hydrothermal method, almost all metal-based materials could be adjusted using varying precursor solutions and controlling the deposition time. As shown in Fig.\u00a03\na and b, electrodes with variable mass loads on nickel foam (NF) labeled NiFeS-1/NF, NiFeS-2/NF, and Ni3S2/NF are fabricated by controlling deposition time from 2.5\u00a0min to 5\u00a0min. The optimal product shows overpotentials of 231\u00a0mV for OER and 180\u00a0mV for HER and excellent stability for more than 200\u00a0h in alkaline solutions [127]. In this regard, it is highly desired to expand the electrodeposition approach's scope for developing unique micro- and nanostructured non-precious metal-based electrocatalysts [28].The sulfur powders are used as a precursor to react with the nickel substrate to form Ni3S2 directly in the deposition process at the atomic level. The direct growth of Ni3S2 produces a self-supporting electrode that does not require additional binders, which could enhance ion/electron transport and promote better electrolyte channelization [128].It is a facile method to fabricate various composite electrodes by vapor deposition. The substrate not only acts as the nickel source but could accelerate the catalyst growth [129]. Besides, it is also a practical way to create novel catalysts by combining different synthesis methods. For example, an active Ni3S2 is deposited in situ on the active material by combining hydrothermal and chemical vapor deposition techniques [130]. The synthetic process could tightly connect the NF and sulfide, which promotes electron transportation between the Ni3S2 and conductive substrates.Other methods have been applied to prepare Ni3S2-based electrocatalysts with novel nanostructures, including etching deposition and anion-exchanging strategies. A facile method to synthesize efficient catalysts for OER reactions derived from nickel-iron Prussian blue analogs (PBAs) is proposed through the comprehensive utilization of synthetic technologies, as shown in Fig.\u00a03d [131]. Ni(OH)2@NiFe PBA/NF nanoarrays are fabricated firstly by chemical etching, followed by chemical etching/anion exchange. The mass/charge transport and gas release would also be modified by the direct growth of porous nanosheets and three dimensions (3D) interconnected structures. In addition to the etch strategies, the sulfurization process is also a practical way. For example, 3D NiCo2S4 and Ni3S2 nanosheets on NF are synthesized via hydrothermal and thermal sulfurization processes, shown in Fig.\u00a03c [132]. The electrocatalyst's good catalytic performance is attributed to the massive catalytic sites, the better bonding strength, and facilitated electron transport.It is necessary to do further research to increase the electrochemical active surface area and improve the stability of Ni3S2 [49,134]. Thus, several strategies are proposed to address these issues. It generally consists of four categories: 1) electronic structure engineering, including defect engineering and heteroatoms doping; 2) adjusting surface geometry of catalysts; 3) constructing nanostructures; 4) using the conductive and 3D substrates. In the following section, the above strategies are discussed in detail.The Ni3S2 has outstanding conductivity, which makes Ni3S2 function as a critical component to improve conductivity in an electrocatalyst. To enhance the activity, generally, Ni3S2 would be combined with other materials showing high active performance to form a heterogeneous structure. Besides, some electrodes directly use Ni3S2 as an active material for OER reaction due to its unsatisfactory HER activity [135]. For Ni3S2-based materials, electronic structure engineering is always applied for active materials rather than Ni3S2. Combining these materials well to make them work synergistically is also a critical point (see Table 1\n).Constructing heterostructures is an effective way to introduce the interface effect to catalysts. The structured coupling interfaces would remarkably enhance catalytic performances because they generate electronic interactions [136].MoS2 is a promising material for electroactivity due to undercoordinated Mo\u2013S edges [137]. However, its poor conductivity hinders performance improvement. Therefore, integrating the active MoS2 and conductive Ni3S2 is a valuable method for designing effective electrocatalysts. Abundant interfaces could be created by decorating the MoS2 nanosheets on the surface of Ni3S2 nanoparticles. Interfaces between Ni3S2 and MoS2 would facilitate the HER process,\u00a0and interfaces between the in-situ formed oxidation product of Ni3S2 and MoS2 would facilitate the OER process [138].For heterostructures, synergy is the highlight of this strategy. The fast charge transfer needs to be guaranteed, and the interfaces should be manifested. As shown in Fig.\u00a03a\u2013j, a MoS2/Ni3S2 co-axial catalyst on NF (denoted MoS2/Ni3S2 NE-NF) is reported, which requires overpotentials of 182 and 200\u00a0mV at 500 and 1000\u00a0mA\u00a0cm\u22122, respectively [92]. The co-axial structure could provide great active sites and stimulate the charge transfer along the axial direction. Additionally, the electrical coupling among MoS2, Ni3S2, and NF facilitates electron transfer, thus improving the free Gibbs energy and accelerating the HER reaction's kinetics. Compared with Ni7S6 and Ni3S2\u2013NF catalysts, the better performance of MoS2/Ni3S2 suggests the heterostructure could facilitate the kinetics of the electrocatalytic reaction, which is consistent with previous reports [139\u2013142]. Besides the MoS2/Ni3S2 heterostructures, Ni3S2\u2013Co9S8 heterostructure nanowires on conductive NF could also act as an efficient catalyst for the OER, as shown in Fig.\u00a04\nk-r [124]. The defects in the metallic Co9S8 and Ni3S2 interfaces increase the number of catalytic sites and generate electrical interactions. Similarly, a novel MoS2/Co9S8/Ni3S2/Ni electrocatalyst for OER and HER reactions is reported by combining Co9S8 and MoS2, as shown in Fig.\u00a05\n [143]. Both experiments and calculation results show that the boosted activity is mainly ascribed to charge transfer at interfaces between the Co9S8 and MoS2. Unlike the MoS2 material, an advantage of Co9S8 is that it possesses better conductivity, which enhances the electrocatalytic activity of the hybrid electrocatalyst.Another effective strategy to construct a heterostructure is to combine hydroxides/(oxy)hydroxides with Ni3S2. For HER, it is reported the well-constructed Ni(OH)2/Ni3S2 nanoforests show promising catalytic performance for HER with overpotentials of 193\u00a0mV and 238\u00a0mV at the current density of 500 and 1000\u00a0mA\u00a0cm\u22122, respectively, and maintains the durability for more than 1000\u00a0h at 320\u00a0mA\u00a0cm\u22122 without obvious degradation [144]. The constructed heterointerfaces could generate numerous catalytic sites and defects [47]. In addition, by using hetero-interface engineering with the Ni(OH)2 cocatalyst, the surface atomic configuration of Ni3S2 could be changed in a way that speeds up the Volmer step and OH\u2212 adsorption during the HER without blocking the OER [144\u2013146]. For OER, the highly effective and malleable catalysts could be obtained by the combination of NiFe(OH)x and the Ni3S2 with hetero-interfaces [147]. Recently, a novel heterointerface-engineered NiFe(OH)x/Ni3S2 electrocatalyst has been fabricated by the controllable sulfurization method. The obtained catalyst with good catalytic performance shows an overpotential of 310\u00a0mV at the current density of 2000\u00a0mA\u00a0cm\u22122 with the Tafel slope of only 20.8\u00a0mV dec\u22121. In addition, this catalyst could steady work for more than 100\u00a0h without significant degradation at the current density of 1000\u00a0mA\u00a0cm\u22122. The outstanding performance could be attributed to the heterointerface synergy between NiFe(OH)x and Ni3S2. The heterointerface leads to a parallel catalytic mechanism for OER. Namely, intermediates (HO\u2217 and O\u2217) could be absorbed by NiFe(OH)x and Ni3S2, respectively, and then incorporate to HOO\u2217 at the interface. This path could break the scaling relationship of OER (\u0394GHOO\u2217\u00a0\u2212\u00a0\u0394GHO\u2217\u00a0\u223c\u00a03.2\u00a0eV), which improves the catalytic performance very well, particularly at high current densities [147,148].Among strategies for improving performance, element doping is universal and could regulate the coordination of valence states and the chemical environment. In general, the introduction of heteroatoms could increase catalytic sites and superior conductivity. Iron (Fe)-doped, zinc (Zn)-doped, tungsten (W)-doped, and nitrogen (N)-doped Ni3S2 hybrid structures have been reported, presenting the performance improvement in the HER and OER reactions.Recent studies have shown that incorporating Fe ions could significantly improve Ni3S2-based electrocatalyst performance by tuning the local electronic structure. For instance, a reported Fe-doped Ni3S2 electrocatalyst could exhibit outstanding performance under large current density [150]. However, the preparation process is complex.Recent studies have shown that incorporating Fe ions could significantly improve Ni3S2-based electrocatalyst performance by tuning the local electronic structure. For instance, a reported Fe-doped Ni3S2 electrocatalyst could exhibit outstanding performance under large current density [150]. However, the preparation process is complex.Recently, novel Fe-doped Ni3S2 electrodes conducted by simplified preparation methods have been reported [151,152]. Apparent advantages of doping Fe include: (1) The doping Fe would result in a disordered state, thereby increasing the density of states (DOS), providing more conductive paths, and facilitating electron transfer. (2) Fe could act as active sites, resulting in boosted H2O adsorption and easier O2 desorption.In addition, it is reported that Fe affects the morphology of Ni3S2-based electrocatalysts, as shown in Fig.\u00a06\na\u2013d. The experiment results suggest that adding only 2.1 (at.)% of Fe (\u2162) could make a difference to the catalytic ability and control the evolution of the morphology of Ni3S2 from nanorods to nanosheets, doubling electrochemical surface area [153]. Therefore, it is a practical and effective method to enhance the catalytic ability of Ni3S2-based catalysts, especially for the OER reaction, by doping the proper amount of Fe.Cu, as another earth\u2013abundant transition metal, possesses excellent electronic conductivity. Moreover, Cu and Ni are adjacent in the element periodic table, which means they have similar electronegativity and atomic radius. This similarity provides the basis for doping Cu into Ni3S2. Doping Cu element would optimize energy barriers of intermediate steps and enhance the conductivity. Several studies have used Cu as the doping element to improve the catalytic performance of Ni3S2-based electrocatalysts [154\u2013156]. For example, a novel Cu-doped Ni3S2-based electrocatalyst is reported for the HER reaction [157]. Copper nanodots (NDs) are deposited on the Ni3S2 surface supported by chemical reduction in carbon fibers (CFs). The introduction of Cu generates the electronic interaction between Cu and Ni3S2, which positively charges Cu NDs and negatively charges Ni3S2. Cu could promote water adsorption and the Ni3S2 could weaken S-Hads bonds, so the Volmer and Heyrovsky steps are announced, as shown in Fig.\u00a06e\u2013k. Besides, the Cu dopants could change morphology, increase active site strength, and improve intrinsic activity by optimizing the energy barrier, which is consistent with the study mentioned above [158].It is reported that V dopants could be used as a hopeful constituent to optimize the catalytic properties of Ni3S2-based catalysts for water electrolysis [159]. For the HER reaction, it is indicated that the interaction between the V and S could optimize the hydrogen adsorption Gibbs free energy, thus manifesting better catalytic activity [160,161]. Introducing V leads to a large number of free carriers around the Fermi level, which accelerates the charge transfer [160]. For the OER reaction, doping V could introduce active sites and create synergy with Ni3S2 [159]. For instance, a V-doped Ni3S2 nanorod is fabricated and decorated by NiFe-layered double hydroxide (LDH) nanosheets on the NF [162]. The Ni3S2 functions as conductive cores, and the doped V improves conductivity. The defective NiFe LDH nanosheets provide plenty of active sites and 3D core\u2013shell nanostructures. The obtained catalyst exhibits overpotentials of 209 and 286\u00a0mV at 10 and 100\u00a0mA\u00a0cm\u22122, respectively. The good performance is attributed to several strategies, including heteroatom doping, defect engineering, and constructed hierarchical nanostructure.Anion doping (F, P, and N doping, etc.) is an ideal avenue to modify the electronic structure of advanced Ni3S2-based electrocatalysts. This strategy could also increase active sites and improve conductivity [163]. Compared to transition-metal cation doping, anion doping could simplify active site identification [164,165]. A N-doped Ni3S2 material, denoted N\u2013Ni3S2/NF 3D electrode, is put forward by a one-step calcination route [16]. The electrocatalyst requires overpotentials of 330 and 110\u00a0mV for OER and HER at 100\u00a0mA\u00a0cm\u22122 and 10\u00a0mA\u00a0cm\u22122 in 1\u00a0mol L-1 KOH solutions. The decoration of N optimizes the electronic structure, changes the morphology of Ni3S2, and provides appropriate HER Gibbs free energy and water adsorption energy [166]. A similar effect could be made by rationally doping the F element in Ni3S2 because the electronegativity different between F and S is bigger than that between N and S, leading to a stronger interaction between the anion and transition-metal cation [167,168].In addition, it is reported that a phosphorus-doped Ni3S2/NF electrode needs a relatively low overpotential of 306\u00a0mV at the current density of 100\u00a0mA\u00a0cm\u22122 for OER and 137\u00a0mV at the current density of 10\u00a0mA\u00a0cm\u22122 for HER [169]. Doping P could also improve the catalytic performance of HER due to the modified electronic structure, more active sites, and improved electrical conductivity [170].Many studies suggest that W doping would modify the hydrogen adsorption energy of intermediates and improve conductivity [171\u2013173]. Inspired by this, a W-doped Ni3S2 nanoparticle catalyst that presents HER overpotentials (67\u00a0mV at 10\u00a0mA\u00a0cm\u22122 and 330\u00a0mV at 552\u00a0mA\u00a0cm\u22122) is fabricated and exhibits merely an increase of approximately 15\u00a0mV for overpotential after 40\u00a0h [174]. Moreover, the effect of doping Zn is not disappointing. A Zn-doped Ni3S2 nanosheet catalyst denoted Zn\u2013Ni3S2/NF presents a high catalytic ability and good stability for OER reaction [175]. It requires an overpotential of 330\u00a0mV at 100\u00a0mA\u00a0cm\u22122 and maintains activity for 20\u00a0h without apparent degradation.Despite many reports having applied various strategies to improve Ni3S2-based electrocatalysts, relatively few studies have focused on the Ni3S2 nanostructures with exposed active facets. The understanding of the surface structure is not deep enough, leading to the unclarity of the location of the catalytic sites that promote reactions on the surface.A stable Ni3S2 nanosheet array electrocatalyst with \n\n(\n\n2\n\n1\n\u00af\n\n0\n\n)\n\n-exposed high-index facets were reported for the first time in 2015 [49]. The catalyst exhibits excellent catalytic performance for both OER and HER reactions, which attributes to the synergistic effects between 3D nanostructure and \n\n(\n\n2\n\n1\n\u00af\n\n0\n\n)\n\n high-index facets. Compared with low-index facets, such as (001), \n\n(\n\n2\n\n1\n\u00af\n\n0\n\n)\n\n facets have more optimized surface structure for electrocatalysis, which could be substantiated by the fact that the catalytic activity of \n\n(\n\n2\n\n1\n\u00af\n\n0\n\n)\n\n-Ni3S2/NF is better than that of (001)-exposed catalysts. It is indicated that the S sites and six-coordinated Ni5 sites at the terrace of \n\n(\n\n2\n\n1\n\u00af\n\n0\n\n)\n\n facets have a less steric effect, as shown in Fig.\u00a07\na and b. In addition to exposing the \n\n(\n\n2\n\n1\n\u00af\n\n0\n\n)\n\n facets (003) facets of Ni3S2 are reported to contribute to HER activity due to active Ni3-triangle active sites, as shown in Fig.\u00a07c\u2013i. The low-coordinated Ni3-triangles are more favorable for O\u2013H bond breakage than other sites. They are the only sites for OH\u2212 adsorption [105]. Thus, the unique Ni3-triangles make (003) facets beneficial to improve water dissociation for HER reaction.Disorder engineering serves as an effective method to fabricate amorphous structures and has attracted much attention. It affects the lattice distortion of materials and thus contributes to the activity via improving ionic conductivity and increasing vacancy number.A hierarchical (Fe\u2013Ni)Cox-OH/Ni3S2 electrocatalyst for OER and HER reactions is assembled [176]. This catalyst needs overpotentials of 91 and 145\u00a0mV at 100 and 1400\u00a0mA\u00a0cm\u22122, respectively, with outstanding durability for 100\u00a0h at 200\u00a0mA\u00a0cm\u22122 in 1\u00a0mol L-1 KOH solution. The Gibbs free energy for intermediates tends to be more moderate by breaking the long-range order to form the S-vacant amorphous phase. In general, there are two types of Ni3S2-based catalysts for which unstructured engineering is applied. As mentioned above, one is the Ni3S2 material itself, and the other is materials coupled with Ni3S2. For example, the synergistic effect between the coated amorphous \u03b1-MoS2 and Ni3S2 skeleton is reported [177]. Results indicate that most compounds of Mo present long-range disordered, which leads to the distortion of the crystalline lattice of Ni3S2. The amorphous structure could generate many active sites, thus exhibiting higher catalytic activity.However, there is a paradox between the active site number and the conductivity. Excess active sites would impress the conductivity and further constrain the electrocatalytic process. An optimum electrocatalyst should exhibit a short-range order and long-range disorder structure to balance active sites and conductivity. Inspired by this, the relationship between the disorder degree and OER reaction activity is deeply explored via fabricating novel Fe-doped Ni3S2 electrocatalysts [178]. By regulating the molar concentration in hydrothermal synthesis, a series of Fe-doped Ni3S2 electrocatalysts with different disorder degrees are obtained. The best active one exhibits an overpotential of 295\u00a0mV at 10\u00a0mA\u00a0cm\u22122 in alkaline solutions. The results indicate that a great degree of disorder might restrict OER activity. It is necessary to select a suitable component ratio to obtain the facile extent of the amorphous phase.One-dimensional nanostructures have gained a lot of interest as electrocatalysts due to good mass and charge transmission and efficient bubbles removal [179\u2013181]. Thus, one-dimensional Ni3S2-based electrocatalysts have been widely investigated, allowing exquisite composition, morphology, heterostructures, and reactivity controls[139,160,182,183]. The typical structure mainly includes nanotubes, nanowires, and nanorods.Ni3S2 nanotube arrays on NF are fabricated by the catalysis of thermally reduced graphene for the HER [184]. The resulting electrode exhibits an overpotential of 157\u00a0mV at 10\u00a0mA cm-2. The average pore size is 6.9\u00a0nm, which increases the surface area and the number of active sites. Besides, the nanotube structure could bolster electrolyte diffusion, making active sites more accessible.The nanowire is also widely used in electrocatalysts [2,185]. A core/shell electrocatalyst based on Ni3S2 nanowires and N-doped carbon layers, denoted as Ni3S2@NGCLs/NF, exhibits overpotentials of 271\u00a0mV and 134\u00a0mV at 10\u00a0mA cm-2 in alkaline solutions for OER and HER reactions, respectively [183] As shown in Fig.\u00a08\nb\u2013g, one-dimensional Ni3S2 nanowires could enhance the inherent activity and provide more active sites. Moreover, the activity for HER and OER reactions could be improved by compounding other materials to facilitate charge transfer to the nanowire structure.Unlike conventional electrocatalysts with limited active sites, 2D structures could increase specific surface area to expose more active sites. For example, a 2D nanosheet heterogeneous electrocatalyst donated as Ni(OH)2/Ni3S2, as shown in Fig.\u00a08i\u2013k, exhibits the synergistic effect [146]. Due to the strong OH\u2212 adsorption ability, water could be effectively cleaved into H\u2217 and OH\u2212. Then, the increased H\u2217 intermediates would absorb on the Ni3S2 side and recombine to H2 because hydrogen generation's energy barrier is lower than Ni(OH)2 [186]. The mechanism study exhibits the significant effect of Ni(OH)2 to accelerate the water dissociation. However, the van der Waals attraction between 2D structures makes them aggregate during the preparation process, limiting the accessible regions and degrading electrochemical performance [187]. The development of 3D structures might effectively overcome this issue [188].In general, there are two ways to construct 3D Ni3S2-based electrocatalysts. One is that the active material (i.e., Ni3S2) possesses 3D morphology; the other is combining one-dimensional/two-dimensional Ni3S2 with a 3D substrate, including NF, copper foam, and so on [106,189\u2013192].Compared with the one- and two-dimensional Ni3S2 materials, 3D Ni3S2 endows a larger surface area, more active sites, plenty of pore channels, rapid gas bubbles release, and boosted electron transfer ability [188,193\u2013195]. 3D Ni3S2-based electrocatalysts have been regarded as a competitive candidate for alkaline water electrolysis. For instance, the hollow MoOx/Ni3S2 composite microsphere catalysts on NF are synthesized for OER reaction for the first time, as shown in Fig.\u00a08h-n [106]. The fabricated material possesses the hollow and 3D microporous structure with a 0.5\u20131\u00a0\u03bcm diameter. The shell thickness is determined to be\u00a0\u2248\u00a060\u00a0nm. This ultrathin nanosheet-assembled hollow architecture provides a large active area and more accessible catalytic sites, which could be verified by the fact that it has roughly 37 times the higher electrochemical active area than Ni3S2/NF. Recently, a similar Ni3S2-based electrocatalyst with a microsphere-like interconnected structure is also reported by other researchers [196]. Besides, the 3D treelike structure could reduce ion and electron transport resistance, facilitate electrolyte penetration, and expose numerous active sites [189]. If the material also has an amorphous phase, the amorphous phase would further enhance the electric conductivity.The conventional method for preparing electrodes is to drop slurry consisting of conductive binder and catalyst onto the electrodes [122,197]. However, this method has two apparent drawbacks. The first one is that the binder will decrease the contact area of the catalyst and electrolytes, thus increasing the ohmic resistance; the second is that attached catalysts tend to peel off at high current density. New electrodes are developed by growing electrocatalysts directly on the conductive substrates to overcome the abovementioned issues. This method no longer needs the binder and could accelerate the electron-transfer rate. Substrates should have good conductivity and make catalytic species expose more sites. Under these requirements, 3D substrates become the most promising candidate for preparing efficient catalysts for water electrolysis.The NF has attracted much attention among different substrates due to its good conductivity and porous 3D structure. The catalyst is evenly distributed on the porous NF, exposing more active sites and avoiding agglomeration. Besides, NF with open architectures is beneficial to release bubbles rapidly and to dispense with binder agency [154]. The NF always serves as the conductive porous substrate and forms a unique 3D coral-like electrocatalyst [32]. Besides, it can be used as the nickel source for the formation of Ni3S2. Analogously, conductive copper foam is also used as a substrate because of the 3D structure with a smooth surface [198].In addition to NF and copper foam, graphene is another promising option for porous support. The good stability and excellent conductivity of graphene could be attributed to its two-dimensional hexagonal stacked carbon nanostructure and \u03c0-bond that allows electrons to move freely [199]. Moreover, catalytic performance is also enhanced by increasing the surface area of microporous channels of graphene [200]. Besides graphene, graphene oxide (GO) is also an excellent alternative substrate. Due to many oxygen-containing functional groups, GO has good water dispersibility and is easy to assemble. The as-prepared catalyst exhibits that the GO could effectively hinder the aggregation of Ni3S2 nanoparticles [201]. Therefore, it could be an effective way to construct the composites by growing Ni3S2 active materials on carbon materials [202].Besides the abovementioned 3D substrates, a 3D NiFeCo foam is reported as the substrate to fabricate Ni3S2\u2013FeS\u2013CoS trimetallic sulfide nanosheets catalyst, shown in Fig.\u00a08l [9]. The NiFeCo foam provides the foundation for the growth of Ni3S2\u2013FeS\u2013CoS nanosheets. The catalytic performance of this catalyst is boosted by the large surface area of nanosheets grown from NiFeCo foam and the good conductivity of the substrate.Though the strategies above have been widely adopted to design active and durable Ni3S2-based catalysts at the current density level of tens mA cm\u22122, it is still imperative to further improve the performance under the high current density (see Table 2). It requires that the catalyst not exceed 300\u00a0mV at 500\u00a0mA\u00a0cm\u22122 for industrial application [203,204]. Some advances have been made in designing nanostructured Ni3S2-based electrocatalysts for industrial alkaline electrolysis. Some reported Ni3S2-based materials present good catalytic stability at 500\u00a0mA\u00a0cm\u22122 and even 1000\u00a0mA\u00a0cm\u22122. For example, the NixFe1-x alloy - oxyhydroxide nanowire arrays (donated as NixFe1-x-AHNAs) have the overpotential of merely 248 and 258\u00a0mV at 500\u00a0mA\u00a0cm\u22122 and 1000\u00a0mA\u00a0cm\u22122, respectively. Notably, the alkaline water electrolyzer using NixFe1-x-AHNAs as the anode and pure Ni as the cathode exhibits a potential of 1.76\u00a0V at 100\u00a0mA\u00a0cm\u22122 [2]. Table 3\n\n summarizes recently reported Ni3S2-based catalysts for alkaline water electrolysis at high current density. Even though impressive materials have been reported, most Ni3S2-based electrocatalysts operate inefficiently and unstably at 0.5 and 1\u00a0A\u00a0cm\u22122 [90,205]. It is essential to pay attention to electrochemical stability and bubble release behavior to enhance the performance at high current density.Stability is the most important and difficult part of maintaining the performance of Ni3S2-based electrocatalysts at high current density. An electrocatalyst with high stability should keep the composition and structure throughout the reaction, which is important because both of them affect how well the electrocatalyst works. The electrochemical composition stability is discussed in this section, and the discussion about the structure stability is provided in section 6.2.3.Electrochemical stability is a critical point in evaluating the electrocatalyst's performance. It is reported that the durability of Ni3S2 could be enhanced by increasing the Fe content [209\u2013211]. In addition to Fe, other elements, including cerium (Ce) and phosphorus (P), have been reported to be advantageous for creating durable Ni3S2-based electrocatalysts [212,213]. A reported Ce-doped Ni3S2-based electrode shows good retention of 84.7% for 24\u00a0h at 60\u00a0mA\u00a0cm\u22122, which might be because the doping Ce prevents the structure from evident evolution, as shown in Fig.\u00a09\na [214].During the OER process, it is easy to form a thin layer of amorphous oxide/peroxide on the surface, which protects the core material from being oxidized [215\u2013218]. Therefore, the stability could be boosted by building a suitable shell\u2013core structure. For example, NixCo3-xS4 coupled Ni3S2 nanosheet arrays on NF show the potential of 1.53 and 1.80\u00a0V at 10 and 100\u00a0mA\u00a0cm\u22122 with stability for >200\u00a0h for overall water electrolysis, as shown in Fig.\u00a09d [191]. The NixCo3-xS4 shell could be transformed into hydroxides during the OER process, protecting Ni3S2 nanosheets from collapsing. A similar effect has also been reported, as shown in Fig.\u00a09h [2]. The shell is densely covered on the core's surface and maintains stability under alkaline conditions, preventing the core from corrosion. In addition, the stability could also be modified by applying rational synthesis methods. It is indicated that catalytic ability and stability could be bolstered by electrodeposition because it combines interface modification and 3D porous structure construction [125,219].The alkaline water electrolysis system is constantly hampered by the adherent bubble layer resulting from dissolved gas bubbles on electrode surfaces. The coverage of bubbles would act as an electrical shield that increases the total resistance and impedes the mass transfer by reducing interfaces between electrodes and electrolytes [220,221]. In addition, the tension caused by the detachment of numerous bubbles would also damage the structure of electrodes. Thus, it is essential to understand the bubbles removal behavior to alleviate the adverse effects.The bubble release process is divided into two stages. Firstly, tiny bubbles are generated on the electrode surface. The size depends on the surface topography; secondly, tiny bubbles leave the electrode surface and combine to form large bubbles. These bubbles could be restricted within the cavity of electrode surfaces, which has adverse effects on electrode performance, especially at high current density [222]. Therefore, rapid bubble release is essential to improve electrode performance. Currently, two methods are mainly proposed to accelerate the bubbles removal process. The first one is to construct the superhydrophilic surface of electrodes. The second one introduces external fields, including magnetic, ultrasonic, and supergravity [222].\u00b7Water molecules close to the electrode surface will compete with bubbles attempting to adhere to the surface when the electrode is immersed in aqueous solutions. The surface with a bubble contact angle greater than 150\u00b0 is the superoaerophobic surface [223,224]. Equation (11) could explain the relationship between the bubble contact angle and water contact angle\uff1a\n\n(11)\n\n\u03b8\n\nb\n\u00a0=\u00a0180\u00b0 - \u03b8\n\nw\n\n\n\nWhere the \u03b8\n\nb\n represents the bubble connect angle and \u03b8\n\nw\n means the water connect angle [223]. It could be indicated that a superhydrophilic surface is superaerobic under water and vice versa. The relationship between the wettability of substrate in the atmosphere and aqueous solutions is shown in Fig.\u00a010\na [222]. It is indicated that good surface wettability favors mass transfer because it could boost the contact between electrolytes and electrode surfaces [222]. Therefore, hydrophobic and hydrophobic qualities could be concurrently gained by constructing superaerophobic surfaces, which could complement mass transfer and bubbles removal behaviors.It is indicated that rough nanostructures are investigated to modify the bubbles removal behavior of Ni3S2-based electrocatalysts [162,193,195,225\u2013229]. Rapid water entry could crowd tiny bubbles out when generated on surfaces [142,230]. Fig.\u00a010b shows that the hierarchical morphology could change the three-phase contact line (TPCL) from a continuous curve to a discontinuous line segment, which could reduce the contact area between bubbles and solid electrodes and then reduce adhesive force to accelerate the bubbles removal [225,231]. A novel CoSx-Ni3S2 nanosheet catalyst simultaneously possess apparent superhydrophobicity and superaerophobicity, which could expose more active sites and alleviate the \u201cbubble shielding effects\u201d for gas diffusion, as shown in Fig.\u00a010d\u2013j [232]. The as-prepared electrocatalyst exhibits 1.63\u00a0V at 100\u00a0mA\u00a0cm\u22122. Another MoS2/Ni3S2 catalyst with co-axial heterostructure nanowires shows a 0\u00b0 contact angle of a 1.0\u00a0M KOH droplet, demonstrating good wettability [92]. This phenomenon might be attributed to the multilayer and low-crystallinity structure of the outer MoS2 nanosheet [233,234]. It is also indicated that sulfurization time could affect surface wettability, as presented in Fig.\u00a010c [105]. Furthermore, changing chemical composition is also a method to modify surface wettability. The regulation of the amount of doping Fe is a practical way to adjust the wettability. This result again proves the important role of iron in enhancing the performance of Ni3S2-based electrocatalysts [153].Besides constructing a nanoscale surface, it has been shown that various methods, including the magnetic field, supergravity, and ultrasonic field, could accelerate bubble dissociation and release. It is reported that the external magnetic field affects the bubble releaseProcess. The bubble coverage and size are reduced under the external magnetic field because the magnetohydrodynamic (MHD) convection could expedite the bubbles removal behavior [235\u2013237]. The catalytic performance could be boosted by applying a moderate magnetic field since the external magnetic field causes some components to be magnetized into a high spin polarization state, optimizing the energy barrier of oxygen intermediates and the transfer of electrons [238]. Besides the magnetic method, it is indicated the supergravity field and ultrasonic fields could promote the bubbles removal behavior [239]. However, few studies are applying these external fields to Ni3S2-based electrocatalysts. The main limitation of using external fields is that external fields would increase the complexity and cost of the electrolysis system.The main difference between water electrolysis at low current density and high current density is the number of generated bubbles. For an electrocatalyst with good durability, both electrochemical composition and structural stability are required. In most cases, the structural stability of electrocatalysts is related to the bubble release behavior at high current density [240]. Even though the bubble release rate could be accelerated by surface engineering, the catalyst could peel-off by the strain induced by the detachment of bubbles [241]. In addition, bubbles would be made and built up in the cavity if water molecules are taken into the cavity of electrocatalysts, which would stress the structure and lead to the structure oscillation [242,243]. Therefore, the catalyst and substrate should be connected firmly to overcome the interfacial adhesion between the bubble and the catalyst [244]. Some self-stand and binder-free Ni3S2-based electrocatalysts with good structure stability have been fabricated, while the threshold value of the interaction force between the catalyst and substrate should be found [245\u2013247]. The understanding of catalyst/substrate interfacial structure that resists gas bubble strain also needs to be further deepened [240].In recent decades, it has witnessed the rapid development of non-noble metal-based electrocatalysts for alkaline water electrolysis. Among them, Ni3S2-based catalysts are considered promising candidates.Ni3S2-based catalysts could be designed and prepared by elaborate strategies consisting of electronic structure engineering, lattice strain engineering, morphology design, and usage of 3D substrates. Recent studies always adopt a combination of strategies to improve catalytic activity. Among them, constructing the heterostructure, doping heteroatom, and using 3D substrate are commonly adopted methods. For Ni3S2-based catalysts with heterostructure, the combination of MoS2 and Ni3S2 is a classic strategy and could improve the catalytic activity effectively. Besides, incorporating between (oxy)hydroxides and Ni3S2 could also enhance the performance effectively and fabricate catalysts that could meet the catalytic activity requirements of commercial applications. For heteroatom-doped Ni3S2-based catalysts, Fe, V, and anion elements (F, N, P) are widely chosen. However, most of the catalysts are doped by hydrothermal methods, which may hinder the further improvement of performance. Thus more ways of doping are expected. For Ni3S2-based catalysts with 3D substrates, the Ni foam is widely adopted due to its good conductivity and porous 3D structure.From the view of practical applications, electrode bubble release behavior, electrode surface wettability, and electrochemical stability of Ni3S2 at high current density are also vital points for industrial applications. Up to now, challenges still exist which hinder the application of Ni3S2-based catalysts. It is summarized as follows, combing with prospects.\n\n1.\nThe performance should be further optimized. Ni3S2-based electrocatalysts still have disadvantages, such as high overpotentials and inferior durability. Few of them meet the commercial requirements considering their electrochemical activity and stability, especially working at high current density (>500\u00a0mA\u00a0cm\u22122), high temperature (60\u201390\u00a0\u00b0C), and concentrated solutions (6\u201310\u00a0M KOH) [12,248,249]. The stability requirements are relatively more difficult to achieve. Most stability tests of Ni3S2-based electrocatalysts are still tested under low current densities, which obviously cannot meet the needs of future commercialization.\n\n\n2.\nThe real electrochemical active site is still unclear. For Ni3S2-based electrocatalysts, the thermodynamic instability of Ni3S2 makes electrocatalysts undergo surface reconstruction. There are still debates on whether the in-situ formed (oxy)hydroxide or the electrocatalyst itself acts as the active site. Therefore, it is necessary to clarify the active site or active phase. In addition, the reconstruction process may differ on the lab and commercial scales. To uncover the real catalytic species, in-situ characterization technologies should be further developed to observe the dynamic changes of active sites during the electrochemical reaction. Moreover, the computational simulation needs to be combined to guide further investigations.\n\n\n3.\nThe variety of nanomaterials composed of Ni3S2 should be widened. For instance, anion-doped Ni3S2-based electrocatalysts are suitable candidates and should get more attention because of the modified electronic structure, increased active sites, and better conductivity. Furthermore, exploring further viable composite catalyst material combinations based on Ni3S2 will advance the development of alkaline water electrolysis.\n\n\n4.\nResearch on bubble release behaviors needs to be carried out sincerely. The bubbles removal dynamic process is complex in alkaline water solutions, which is related to composition, microstructure, the surface wettability of electrodes and working current density, etc. It is recommended that various electrocatalysts with high intrinsic activity could be combined with substrates with the 3D structure to reduce the adverse impacts on bubbles removal, especially at high current density.\n\n\n5.\nThe synthesis method at the industrial scale needs further improvement. The synthesis method suitable for industrial application should be mature and scalable, and should be able to provide electrocatalysts with tailor-made shapes and compositions according to industrial applications. Among various catalyst preparation methods, electrodeposition could be an ideal candidate for industrial-scale preparation due to its convenient operation conditions control. However, there is a lack of comprehensive technological procedures and standards, which needs to be focused on in the following research.\n\n\nThe performance should be further optimized. Ni3S2-based electrocatalysts still have disadvantages, such as high overpotentials and inferior durability. Few of them meet the commercial requirements considering their electrochemical activity and stability, especially working at high current density (>500\u00a0mA\u00a0cm\u22122), high temperature (60\u201390\u00a0\u00b0C), and concentrated solutions (6\u201310\u00a0M KOH) [12,248,249]. The stability requirements are relatively more difficult to achieve. Most stability tests of Ni3S2-based electrocatalysts are still tested under low current densities, which obviously cannot meet the needs of future commercialization.The real electrochemical active site is still unclear. For Ni3S2-based electrocatalysts, the thermodynamic instability of Ni3S2 makes electrocatalysts undergo surface reconstruction. There are still debates on whether the in-situ formed (oxy)hydroxide or the electrocatalyst itself acts as the active site. Therefore, it is necessary to clarify the active site or active phase. In addition, the reconstruction process may differ on the lab and commercial scales. To uncover the real catalytic species, in-situ characterization technologies should be further developed to observe the dynamic changes of active sites during the electrochemical reaction. Moreover, the computational simulation needs to be combined to guide further investigations.The variety of nanomaterials composed of Ni3S2 should be widened. For instance, anion-doped Ni3S2-based electrocatalysts are suitable candidates and should get more attention because of the modified electronic structure, increased active sites, and better conductivity. Furthermore, exploring further viable composite catalyst material combinations based on Ni3S2 will advance the development of alkaline water electrolysis.Research on bubble release behaviors needs to be carried out sincerely. The bubbles removal dynamic process is complex in alkaline water solutions, which is related to composition, microstructure, the surface wettability of electrodes and working current density, etc. It is recommended that various electrocatalysts with high intrinsic activity could be combined with substrates with the 3D structure to reduce the adverse impacts on bubbles removal, especially at high current density.The synthesis method at the industrial scale needs further improvement. The synthesis method suitable for industrial application should be mature and scalable, and should be able to provide electrocatalysts with tailor-made shapes and compositions according to industrial applications. Among various catalyst preparation methods, electrodeposition could be an ideal candidate for industrial-scale preparation due to its convenient operation conditions control. However, there is a lack of comprehensive technological procedures and standards, which needs to be focused on in the following research.The authors declare no conflict of interest.This work is supported by the National Key Research and Development Program (No. 2022YFB4202200) and the Fundamental Research Funds for the Central Universities.", "descript": "\n                  Green hydrogen (H2) produced by renewable energy powered alkaline water electrolysis is a promising alternative to fossil fuels due to its high energy density with zero-carbon emissions. However, efficient and economic H2 production by alkaline water electrolysis is hindered by the sluggish hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Therefore, it is imperative to design and fabricate high-active and low-cost non-precious metal catalysts to improve the HER and OER performance, which affects the energy efficiency of alkaline water electrolysis. Ni3S2 with the heazlewoodite structure is a potential electrocatalyst with near-metal conductivity due to the Ni\u2013Ni metal network. Here, the review comprehensively presents the recent progress of Ni3S2-based electrocatalysts for alkaline water electrocatalysis. Herein, the HER and OER mechanisms, performance evaluation criteria, preparation methods, and strategies for performance improvement of Ni3S2-based electrocatalysts are discussed. The challenges and perspectives are also analyzed.\n               "}
{"full_text": "Concerns about the vital global warming and ocean acidification problems caused by CO2 excessive emission (Karl and Trenberth, 2003; Orr et\u00a0al., 2005) have triggered extensive researches on its large-scale reutilization via effective, economical, and sustainable technologies for a CO2 circular economy (Aresta et\u00a0al., 2014; Porosoff et\u00a0al., 2016). However, industrialized CO2 reutilization is just limited to the synthesis of urea and polycarbonate (occupying only 0.5% [Shima et\u00a0al., 2012; Su et al., 2017] of CO2 emissions), whereas enzymatic and electro-/photo-chemical strategies are hampered by their low CO2-conversion efficiency\u00a0(Wang et\u00a0al., 2008; Kondratenko et\u00a0al., 2013). To achieve the large-scale CO2 reutilization, CO2\u00a0hydrogenation with renewable-energy-generated H2 to CO by the reverse water-gas shift (RWGS) reaction is the most techno-economically viable candidate (Porosoff et\u00a0al., 2016; Kondratenko et\u00a0al., 2013; Xu\u00a0and Moulijn, 1996; Porosoff and Chen, 2013; Zhang et\u00a0al., 2017), thanks not only to its high efficiency, enabling to deal with vast amounts of CO2, but also to the great versatility of syngas (CO\u00a0+ H2, product gas of RWGS reaction) to produce commodity chemicals and fuels (occupying 40% CO2 emissions [Zhang et\u00a0al., 2017] via mature Fischer-Tropsch and methanol (CH3OH) syntheses [Porosoff et\u00a0al., 2016; Kondrat et\u00a0al., 2016]).The RWGS reaction is an equilibrium-limited endothermic reaction (required enthalpy of 41.17\u00a0kJ mol\u22121). According to Le Ch\u00e2telier's principle, high-temperature (about 400\u2013800\u00b0C) thermodynamically favors high CO2 conversion and high CO selectivity, but the undesired methanation also proceeds under the preferred RWGS conditions (Chen et\u00a0al., 2001; Wu et\u00a0al., 2015; Gon\u00e7alves et\u00a0al., 2017; Yang et\u00a0al., 2017). Therefore, a techno-economically available catalyst with outstanding CO2-to-syngas performance is the prerequisite for the large-scale RWGS implementation. To date, homogeneous complexes and heterogeneous solids catalysts have been extensively explored. The homogeneous catalysts show satisfactory activity and selectivity (Federsel et\u00a0al., 2010), but their difficult recovery from the reaction mixture makes them unattractive. The heterogeneous catalysts are more competitive in terms of ready catalyst-product separation and continuous processes. They mainly include the nanoparticles of precious metals (e.g., Au, Ag, Pt) (Porosoff et\u00a0al., 2016; Yang et\u00a0al., 2017) and non-precious metals (e.g., Cu, Ni) (Zhang et\u00a0al., 2017; Chen et\u00a0al., 2001; Wu et\u00a0al., 2015; Gon\u00e7alves et\u00a0al., 2017) dispersed on supports (e.g., SiO2, Al2O3, CeO2, MoCx) (Porosoff et\u00a0al., 2016; Zhang et\u00a0al., 2017; Chen et\u00a0al., 2001; Wu et\u00a0al., 2015; Gon\u00e7alves et\u00a0al., 2017; Yang et\u00a0al., 2017). Despite the excellent RWGS activity, the precious-metal catalysts suffer from their limited natural abundance. Cu and Ni catalysts are intensively studied but are not promising owing to either serious sintering (Cu) (Zhang et\u00a0al., 2017; Chen et\u00a0al., 2001) or high methanation activity (Ni) (Wu et\u00a0al., 2015; Gon\u00e7alves et\u00a0al., 2017). Given the chemical inertness of CO2 molecule (Xu and Moulijn, 1996), the heart of RWGS is to exquisitely design and tailor a groundbreaking catalytic material with both high efficiency and low cost, but this represents a grand challenge within the CO2-conversion field.Against all odds, the tantalizing progresses in nano-intermetallic catalysis (Stamenkovic et\u00a0al., 2007; Studt et\u00a0al., 2014) open an opportunity for designing and tailoring qualified RWGS catalysts because nano-intermetallic has fascinating prospects in catalysis field, with their tunable components and ratios, variable constructions, and reconfigurable electronic structures, distinctly different from their single metals (Stamenkovic et\u00a0al., 2007; Armbr\u00fcster et\u00a0al., 2012; Ji et\u00a0al., 2010). Particularly, their precise atomic ordering structure can provide rational predictions of the effects of geometry and electronic structure on their catalytic properties for required reactions (Wang et\u00a0al., 2013; Nicholson et\u00a0al., 2014; Qin et\u00a0al., 2018). One of the recent pertinent examples is the discovery of a Ni5Ga3 nano-intermetallic, which strikingly shows that the Ni, originally active for CO2 methanation, turns itself suddenly into a qualified CO2-to-CH3OH catalyst after Ga alloying (Studt et\u00a0al., 2014), because this intermetallic offers the unique Ga-rich sites for CH3OH formation. Encouraged by these big achievements toward nano-intermetallic catalysis, we believe that the nano-intermetallic can pave a road to the rational engineering of more intelligent catalysts gifted with flexibly arranged atomic structures and tailor-made catalytic properties for the RWGS reaction as well as other reactions for CO2 reutilization.Here, we present a nano-intermetallic InNi3C0.5 catalyst that is particularly active, selective, and stable for the RWGS reaction under extremely wide reaction conditions. Such nano-intermetallic is fabricated via carburizing the In-Ni nano-intermetallic in the real RWGS stream and is gifted with dual active sites (i.e., 3Ni-In and 3Ni-C) on the InNi3C0.5(111) surface. The dual sites act in synergy to facilely dissociate CO2* (adsorbed on 3Ni-In sites) into CO* (on 3Ni-C sites) and O* (on 3Ni-In sites), and the O* can favorably react with 3Ni-C offered H* to form H2O. Most notably, the CO* is mainly desorbed into gas phase at and above 400\u00b0C but can be highly selectively hydrogenated to form CH3OH below 300\u00b0C with a promising CO2-to-CH3OH capacity. Furthermore, this nano-intermetallic can fully hydrogenate dimethyl oxalate (obtainable from oxidative coupling of CO (Fenton and Steinwand, 1974), product of the RWGS) to ethylene glycol (a\u00a0commodity chemical) with high selectivity (above 96%) and favorable stability.To exquisitely tailor a groundbreaking RWGS catalyst, the elaborate choice of appropriate elements oriented by this reaction should be initially conducted but poses a great challenge because the relevant elements for this reaction traverse most of the periodic table. The first metal that mostly attracts attention is Ni, because Ni-based catalysts are typically used for the RWGS reaction despite CH4 formation (Wu et\u00a0al., 2015; Gon\u00e7alves et\u00a0al., 2017). Moreover, In is another attractive element, because In-based catalysts are burgeoning in CO2 conversion (Ye et\u00a0al., 2012; Park et\u00a0al., 2017; Larraz\u00e1bal et\u00a0al., 2016), and, for example, the intermetallic AgIn catalyst is highly efficient for electrochemical reduction of CO2 to CO (Park et\u00a0al., 2017; Larraz\u00e1bal et\u00a0al., 2016). We thus surmise that In-Ni intermetallic could reconstruct geometric-electronic structures of Ni, which might be feasible to switch Ni catalysis in CO2 reduction from CH4 formation to CO formation.A series of pure intermetallics of InNi, InNi2, and InNi3 were successfully synthesized (Figure\u00a01\nA) and then were evaluated for the RWGS reaction. Comparison with the conventional Cu-based catalysts (Zhang et\u00a0al., 2017; Chen et\u00a0al., 2001) reveals that the intermetallic In-Ni catalysts deliver exciting intrinsic RWGS performances, especially for InNi3 with a high CO formation rate of 1.96\u00a0mmol gcat\n\u22121 min\u22121 and a considerably low CH4 selectivity (Figure\u00a0S1). It is very intriguing to find that after reaction the InNi, InNi2, and InNi3 phases are in situ changed in association with a new phase formation of InNi3C0.5 (Figure\u00a01B, identified in following section). Consistently, the InNi3C0.5 formation is thermodynamically favorable with large ordering energy (such as 2.72 eV for InNi3 carburization with CO, Figure\u00a0S2), which also portends that the InNi3C0.5 is stable under the RWGS conditions. Notably, only InNi3 could be fully transformed into pure InNi3C0.5 owing to the identical stoichiometric In:Ni ratios of 1:3 and offers the highest RWGS performance, indicating that InNi3C0.5 should be responsible for the RWGS reaction.The above-mentioned results and analyses make us confident that the InNi3C0.5 intermetallic is a superior RWGS catalyst. To make it a practical catalyst, the thin-felt Al2O3/Al-fiber substrate consisting of 10 vol% 60-\u03bcm Al2O3/Al-fiber and 90 vol% voidage (Wang et\u00a0al., 2016) was used to support 9 wt% InNi3C0.5. This strategy permits the engineering of InNi3C0.5 nano-intermetallic at \u201cnano-meso-macro\u201d triple-scale levels of both porosity and structure in one step (Figures 2A\u20132C, S3A, and S3B), thereby making the catalyst development and reaction engineering (for enhanced heat/mass transfer) go hand in hand (Wang et\u00a0al., 2016; Li et\u00a0al., 2015). The InNi3C0.5/Al2O3/Al-fiber catalyst was tested for the RWGS reaction in a tubular fixed-bed reactor. As expected, this catalyst always achieves high CO2 conversions very close to the thermodynamic equilibrium values with above 97% CO selectivity under the wide reaction conditions (Figures 2D\u20132F). For example, a 53% CO2 conversion is obtainable, quite close to the equilibrium value of 54%, at 540\u00b0C and a gas hourly space velocity (GHSV) of 54,000\u00a0mL gcat\n\u22121 h\u22121. This catalyst delivers a very high intrinsic activity with a turnover frequency (TOF) of 11.0 CO per active site per second at 540\u00b0C (see detailed TOF calculation in Supplemental Information), almost one to two orders of magnitude higher than that seen with most platinum/oxide and non-noble-metal catalysts (Table S1). Furthermore, a kinetic study was carried out over the InNi3C0.5/Al2O3/Al-fiber catalyst, and the apparent activation energy was calculated with the result as shown in Figure\u00a0S3D. InNi3C0.5/Al2O3/Al-fiber provided a much lower E\na (60\u00a0kJ/mol) than Cu/ZnO-based catalysts (112\u00a0kJ/mol, Schumann et\u00a0al., 2015), further indicating that this catalyst has a high intrinsic activity. Also encouraging is the exclusive CO selectivity (above 98%) with pressure increasing from 1.0 to 4.0 MPa at 540\u00b0C (Figure\u00a02F), despite the fact that CH4 formation is much favorable at high pressure over the conventional Ni-based catalysts (Wu et\u00a0al., 2015; Gon\u00e7alves et\u00a0al., 2017; Li\u00a0et\u00a0al., 2015).Stability is a significant consideration for catalysts in practical applications. Our InNi3C0.5/Al2O3/Al-fiber catalyst is very stable with 52%\u201353% CO2 conversion and 97%\u201399% CO selectivity throughout the entire 150\u00a0h testing at a GHSV of 54,000\u00a0mL gcat\n\u22121 h\u22121 and 540\u00b0C (Figure\u00a02G). Even at a high GHSV of 300,000\u00a0mL gcat\n\u22121 h\u22121 and 600\u00b0C, the InNi3C0.5/Al2O3/Al-fiber catalyst also shows a high stability with no deactivation sign throughout 65\u00a0h testing (Figure\u00a02H). In comparison, the Cu/\u03b2-Mo2C catalyst maintains 85% of its initial activity after 40\u00a0h reaction and the Cu/ZnO/Al2O3 catalyst loses more than 60% of its initial activity within 15\u00a0h reaction under the identical reaction conditions (Zhang et\u00a0al., 2017). It is not surprising that the InNi3C0.5 crystalline phase, surface morphology, and structure of the used catalysts are preserved unchanged (Figures S3E\u2013S3H), consistent with the excellent activity/selectivity maintenance in Figures 2G and 2H. To the best of our knowledge, the InNi3C0.5 intermetallic has never been used before for any application in catalysis, and herein we discover its superior RWGS performance\u2014including CO2 conversion, CO selectivity, and especially high-temperature stability\u2014over the reported state-of-the-art catalysts (Table S1).To definitely identify the crystal structure and composition of the as-formed carbide-intermetallic from In-Ni intermetallics, such pure carbide-intermetallic was synthesized via fully carburizing InNi3, and its X-ray diffraction (XRD) pattern completely coincides with the one of InNi3C0.5 that has an anti-perovskite-type structure (Joint Committee on Powder Diffraction File No. 28-0468; Figure\u00a03\nA and Table S2). Moreover, the In:Ni:C molar ratio of the as-synthesized InNi3C0.5 was determined to be 1:2.99:0.49 (see elemental analyses in Supplemental Information), quite close to its stoichiometric ratio. Figure\u00a03B shows its structural model containing eight InNi3 units. For each unit, eight In atoms occupy the eight corners and six Ni atoms occupy the six face centers; four C atoms randomly disperse in these eight body centers, but with the most stable configuration in a regular tetrahedron (Figure\u00a0S4). The Wulff equilibrium shape of the InNi3C0.5 nanocrystal was further optimized, and its optimum shape exposes fourteen surfaces consisting of eight hexagons and six squares (Figure\u00a03C). The InNi3C0.5(111) is the most stable surface of the hexagonal shapes with the lowest surface free energy (Table S3). Interestingly, high-resolution transmission electron microscopy (TEM) also displays an approximate hexagonal morphology of the real synthetic InNi3C0.5 nanoparticles (Figures 3D, 3E, and S5), and the lattice spacing of 0.218\u00a0nm is assignable to the InNi3C0.5(111) surface.In the last decade, significant advances have been achieved in the atomistic-theoretical calculations, enabling us to computationally construct molecular and crystalline structures and to reveal the reaction pathways on the catalyst surface at atomic-molecular level (Nicholson et\u00a0al., 2014; Qin et\u00a0al., 2018; Studt et\u00a0al., 2014; Mao et\u00a0al., 2017). Therefore, the RWGS reaction mechanism on InNi3C0.5 is first investigated by the density functional theory (DFT) calculations. We selected the most stable InNi3C0.5(111) as the ideal surface and established the dual active sites (h1: Hollow(3Ni-In); h2: Hollow(3Ni-C); Figure\u00a04\nA) from nine kinds of possible active sites (see detailed results in Table S4). As shown in Figure\u00a04B, the CO2 molecule is chemically adsorbed via a bending configuration to form CO2* on h1 site, and the H2 molecule spontaneously dissociates into H* that can be adsorbed on both h1 and h2 sites. Electron density distribution for the dual active sites is richer than the others, which makes them more nucleophilic and more favorable for CO2 activation (Figure\u00a0S6). Therefore, the CO2* facilely dissociates into CO* adsorbed on h2 site and O* adsorbed on h1 site with moderate exothermicity (namely, reaction energy E\nr, \u22120.38 eV) and a low activation barrier (E\na, 0.32 eV), but with higher E\na of CO2* hydrogenation to formate (HCOO*, 0.42 eV) and to carboxyl (COOH*, 0.75 eV, Figure\u00a0S7 and Table S5). Clearly, the CO2* dissociation to CO* and O* (i.e., redox pathway) is preferred over the formate and carboxyl pathways on the InNi3C0.5(111) surface. Furthermore, the formed O* on h1 site preferably reacts with H* on the neighboring h2 site to produce an OH* group (E\na, 0.73 eV), and subsequently, two OH* groups on the dual sites are easily transformed into H2O* (E\na, 0.25 eV) that is finally desorbed into the gas phase (E\na, 0.35 eV). The dual active sites provide much lower E\na than the sole h1 sites for the above-mentioned steps (see detailed results in Table S5), probably the consequences of appropriate adsorption of reaction intermediates in terms of their adsorption strength (Table S4) and the distance between them (the dual active sites have shorter adjacent h1-h2 distance of 3.106\u00a0\u00c5 than the sole h1 sites with an adjacent h1-h1 distance of 5.345\u00a0\u00c5, Figure\u00a04A). In contrast, CO2* dissociation on Cu(111) becomes endothermic (E\nr,\u00a0+1.06 eV, thermodynamically unfavorable) and is kinetically unfavorable (E\na of 1.55 eV versus 0.32 eV on InNi3C0.5(111), Figures 4B, S8, and S9).The formed CO* either undergoes further hydrogenation to CH4 and/or CH3OH or desorbs into the gas phase. Figure\u00a04C shows that CO* desorption overcomes a slightly higher E\na of 1.36 eV at 0 K than the formation of CH4 (CH3*-to-CH4*, 1.27 eV) and CH3OH (CO*-to-HCO*, 1.05 eV), clearly exhibiting a possibility of CH3OH formation (see detailed results and discussion in Figures S10 and S11). It should be noted, however, that CO* desorption is thermodynamically more favorable at elevated temperatures (Figures S12 and S13) owing to the significant entropy contributions (Graciani et\u00a0al., 2014), and therefore CO* is preferentially desorbed into gas phase rather than hydrogenated into CH3OH at our real RWGS temperature of 420\u00b0C \u2013600\u00b0C (see experimental results in Figures 2D\u20132F).To verify the RWGS reaction pathway on InNi3C0.5 from experimental perspective, the in situ Fourier transform infrared (FTIR) spectroscopy analysis was carried out on pure InNi3C0.5 in a continuous H2/CO2/N2 (molar ratio of 66/22/12) flow at ambient pressure. As shown in Figure\u00a05\nA, the linear adsorbed CO* species are formed from CO2 dissociation even at 50\u00b0C, evidenced by infrared (IR) bands (Martin et\u00a0al., 2016) at 2132, 2107, 2094, 2077, and 2055\u00a0cm\u22121. Along with the increase in the temperature, the IR band intensity of linear adsorbed CO* becomes slightly stronger from 50\u00b0C to 175\u00b0C, remains almost unchanged from 200\u00b0C to 250\u00b0C, and then diminishes until disappearance at 325\u00b0C. In addition, two new bands at 1942 and 1824\u00a0cm\u22121 assignable to the bridge-absorbed CO* species (Dou et\u00a0al., 2017) are observed at 100\u00b0C while becoming stronger and stronger along with the temperature. Plentiful gaseous CO starts to be detected only at 300\u00b0C, and its formation is favored with the temperature. Neither CH4 (at 3013\u00a0cm\u22121) (Dou et\u00a0al., 2017) nor formate and carboxyl species (at 1281 and 1360\u20131600\u00a0cm\u22121) (Dou et\u00a0al., 2017) are detectable in the whole temperature range studied, coinciding with the DFT-suggested preferable formation of CO over CH4, formate, and carboxyl. It should be also noticed that no adsorbed CO2* species are detectable; a possible explanation is that the CO2 adsorption-dissociation is too fast to be monitored by IR, also coinciding with the DFT-indicated very low E\na of only 0.32 eV for CO2* dissociation. These IR spectra undoubtedly validate the DFT results: CO2 can be efficiently converted to CO via redox pathway rather than formate and carboxyl ones.Moreover, DFT calculations on InNi3C0.5(111) surface predict the possibility of CH3OH formation (Figures 4C and S10 and Table S5). CO* is first hydrogenated into HCO* (E\na, 1.05 eV), which is easily hydrogenated into CH2O* (E\na, 0.32 eV); CH2O* can be continuously hydrogenated into CH2OH* (E\na, 0.65 eV) or CH3O* (E\na,\u00a00.60 eV); however, CH2OH* is more favorably hydrogenated into CH3OH* (E\na, 0.88 eV) over CH3O* to CH3OH* (E\na, 1.71 eV). Therefore, we infer that CH3O* should be detectable by IR owing to its high accumulation and that CH3OH can be formed through the CO*-to-HCO*-to-CH2O*-to-CH2OH*-to-CH3OH* pathway (see detailed results and discussion in Figure\u00a0S10). Indeed, CH3O* with IR band at 1033\u00a0cm\u22121 are detectable at 200\u00b0C \u2013325\u00b0C (Figure\u00a05B), whereas CH3OH is detected by the on-line mass spectrometry (MS) at 220\u00b0C\u2013310\u00b0C accompanied by gaseous CO formation above 300\u00b0C (Figure\u00a05C). Notably, the absence of CH2O* and CH2OH* in in situ IR spectra is probably a consequence of the low residence time of these species on the surface under atmospheric conditions (Graciani et\u00a0al., 2014). These IR and MS spectra consistently display that CO* is hydrogenated into CH3OH highly selectively below 300\u00b0C, whereas it is dominantly desorbed into gas phase above 300\u00b0C.The above-mentioned DFT and FTIR results also make us confident that the InNi3C0.5 nano-intermetallic is a potential catalyst for the CO2 hydrogenation to CH3OH, which becomes more and more competing in recent years. With reaction temperature reduced from 400\u00b0C\u2013600\u00b0C (for the RWGS reaction) to 300\u00b0C and below, the InNi3C0.5/Al2O3/Al-fiber indeed turns itself suddenly into a CO2-to-CH3OH catalyst, being capable of converting 1%\u20138% CO2 into CH3OH with 60%\u201398% selectivity (corresponding to the CH3OH space time yield of 70\u2013330 gMeOH kgcat\n\u22121 h\u22121) at 200\u00b0C \u2013300\u00b0C (Table S6). The preferable CH3OH formation rather than CO formation below 300\u00b0C is attributed to the fact that low temperatures thermodynamically favor further hydrogenation of CO* to CH3OH* (Figures S12 and S13). These results exhibit an interesting temperature-dependent selectivity switching for CO2 hydrogenation.Moreover, in the light that CO2 molecule has the carbonyl property and InNi3C0.5 intermetallic can efficiently activate CO2 molecule, we wonder whether this catalyst is favorable for other carbonyl-compounds transformation, such as the hydrogenation of aldehydes/ketones/esters to corresponding alcohols. To avoid the adverse influence of acid groups on the surface of Al2O3, we directly supported the InNi3C0.5 nano-intermetallic onto a thin-sheet Ni-foam substrate with 110 pores per inch (Figure\u00a0S14, see detailed preparation in Supplemental Information). Indeed, the InNi3C0.5/Ni-foam catalyst presents the satisfying activity and high product selectivity (Tables 1\n and S7), providing the general and efficient ability to activate the C=O bond for carbonyl-to-hydroxyl transformation. Notably, ethylene glycol (EG) is an important commodity chemical, used for polyester manufacture, anti-freeze compounds, and solvents (Yue et\u00a0al., 2012), and the gas-phase hydrogenation of dimethyl oxalate (DMO) to EG (its commercialization is on the way) is an attractive alternative EG synthesis using syngas (Fenton and Steinwand, 1974) derived from non-oil resources (such as coal, natural gas, and biomass) even from CO2 through the RWGS reaction. This foam-structured catalyst is capable of completely converting DMO at a high EG selectivity of 96% with a promising stability (Table\u00a01). Moreover, the InNi3C0.5/Ni-foam also shows favorable RWGS and CO2-to-CH3OH performances that are comparable with those seen with the InNi3C0.5/Al2O3/Al-fiber (Tables S8 and S9).In summary, we have discovered an outstanding nano-intermetallic InNi3C0.5 catalyst system via RWGS-reaction-oriented pre-design combined with atomistic-theoretical calculations and experimental verifications. Practical fiber/foam-structured InNi3C0.5 nano-intermetallic catalysts engineered from nano- to macro-scale in one step have been developed, achieving unprecedented performance in the RWGS reaction and showing potential to catalyze CO2 hydrogenation to CH3OH. Most notably, such nano-intermetallic catalysts are also highly active, highly selective, and highly particularly stable for the DMO-to-EG process (EG synthesis using syngas derived from non-oil resources even from CO2 through the RWGS reaction). We anticipate our essay to be a new point closer toward the ultimate goal of catalysis, namely, designing and tailoring the catalysts atom by atom with precise structure, and our findings might lead to commercial exploitation of such kind of nano-intermetallic catalysts for applications in highly efficient reduction of CO2 to CO as well as carbonyl-to-hydroxyl transformation.The large-scale H2 production should be from the renewable solar, hydraulic, and wind energy.All methods can be found in the accompanying Transparent Methods supplemental file.We acknowledge the financial supports from the National Natural Science Foundation of China (grants 21773069, 21703069, 21703137, 21473057, U1462129, 21273075), the Key Basic Research Project (grant 18JC1412100) and Shanghai Pujiang Program (grant 17PJ1403100) from the Shanghai Municipal Science and Technology Commission, and the National Key Basic Research Program (grant 2011CB201403) from the Ministry of Science and Technology of the People's Republic of China. We thank Prof. Dr. Roel Prins from the ETH Zurich for fruitful discussion.P.C., G.Z., X.-R.S., and Y.L. conceived the idea for the project and designed the experiments; P.C., G.Z., X.-R.S., and Y.L. carried out the interpretation and wrote the manuscript; P.C., J.Z., and J.D. conducted the material synthesis, characterizations, and catalysis tests; P.C. and X.-R.S. performed the structural analysis and modeling; X.-R.S. carried out the DFT calculations; all authors discussed and commented on the manuscript; Y.L. directed the research.Y.L., P.C., J.Z., and G.Z. have a patent application related to this work filed with the Chinese Patent Office on 15 October 2017 (201710956080.1). The authors declare no competing interests.Supplemental Information can be found online at https://doi.org/10.1016/j.isci.2019.07.006.\n\n\nDocument S1. Transparent Methods, Figures S1\u2013S14, and Tables S1\u2013S11\n\n\n\n", "descript": "\n                  CO2 circular economy is urgently calling for the effective large-scale CO2 reutilization technologies. The reverse water-gas shift (RWGS) reaction is the most techno-economically viable candidate for dealing with massive-volume CO2 via downstream mature Fischer-Tropsch and methanol syntheses, but the desired groundbreaking catalyst represents a grand challenge. Here, we report the discovery of a nano-intermetallic InNi3C0.5 catalyst, for example, being particularly active, selective, and stable for the RWGS reaction. The InNi3C0.5(111) surface is dominantly exposed and gifted with dual active sites (3Ni-In and 3Ni-C), which in synergy efficiently dissociate CO2 into CO* (on 3Ni-C) and O* (on 3Ni-In). O* can facilely react with 3Ni-C-offered H* to form H2O. Interestingly, CO* is mainly desorbed at and above 400\u00b0C, whereas alternatively hydrogenated to CH3OH highly selectively below 300\u00b0C. Moreover, this nano-intermetallic can also fully hydrogenate CO-derived dimethyl oxalate to ethylene glycol (commodity chemical) with high selectivity (above 96%) and favorable stability.\n               "}
{"full_text": "Solid oxide fuel cells (SOFCs) have successfully demonstrated flexibility of the utilization of multiples of fuels ranging from syn-gas, bio-gas, natural gas and other hydrocarbons to pure hydrogen [1,2]. Conventionally, hydrocarbons are externally reformed and reformate serves as fuel for electrochemical oxidation on the cell anode [3]. Direct internal reforming in SOFC, on the other hand, allows the hydrocarbons to be simultaneously reformed and electrochemically oxidized at the anode, resulting in high conversion and efficiency for electrochemical performance improvement, cost reduction and thermal management by combining exothermic oxidations with endothermic reformation reactions [4,5]. Ni-based anode in conventional SOFC provides high electronic conductivity and electrocatalytic activity, but suffers from excessive cooling and coke formation due to rapid endothermic reforming and thermal cracking of hydrocarbons [5,6]. This leads to a steep reduction in temperature especially at the inlet of the cell stack, resulting in a non-uniform temperature distribution along the cell [7,8]. A large temperature gradient along the cell surface may cause high mechanical instability and thermal stress between the anode and the solid electrolyte, leading to inevitable cell fracture and spallation [9]. Besides changes in the mechanical properties of cell and stack components leading to failure, the role of local temperature on subsequent reforming reaction and electrochemical reaction rates, as well as ionic conductivity of the electrolyte are influences and should not be overlooked [10]. To mitigate these challenges, significant efforts have been directed towards the development of electrochemically active anode materials with uniform temperature distribution and high coking resistance [11,12].Hydrocarbons and reformate gas mixtures have been extensively used in commercial internal combustion engines and fuel cells for power generation [13]. Fig.\u00a01\n compares the energy densities of a number of sources of energy, where H2 shows the largest mass energy density, but the lowest liquid volumetric energy density due to storage and transportation challenges. Approximately 95% of H2 is currently being produced by steam reforming of methane (SRM) or partial oxidation of methane as well as gasification of coal [14,15]. Since a SOFC typically operates in the 600\u20131000\u00a0\u00b0C temperature range, relatively high operating temperature favors SRM on the cell. Additionally, the heat produced from the electrochemical reaction at the anode is used to promote the SRM reaction while the exothermic WGS reaction takes place concurrently to produce CO2 and more H2. Hence, the main advantages of utilizing internal steam reforming in SOFC include lower operational cost and higher thermal efficiency [16,17].The choice of electrocatalyst catalyst and anode configuration plays an important role on the long term stability of fuel cell operation. That is, the catalyst must exhibit high catalytic activity and stability under industrial operating conditions. The state-of-the-art catalyst for SRM utilizes precious metals such as Pt and Rh, which are considerably expensive and scarce. Ni-based catalyst is widely used owing to its comparable reforming performance to that of precious metals [19] as well as cost effectiveness and availability [20]. Despite these advantages, Ni-based catalysts deteriorate very quickly due to Ni sintering and coking [21]. At such high temperatures during SRM, carbon formation can cause rapid catalyst deactivation. The two types of carbon that can form on a catalyst surface are encapsulated and filamentous carbons [22]. The latter, although does not deactivate the catalysts, is highly responsible for mechanical failure and increase pressure drop in the reactor, especially in SOFCs. Equation (1) represents carbon formation by CH4 cracking. Subsequently, equation (2) refers to the Boudouard reaction, which is another possible route to form carbon during SRM.\n\n(1)\n\n\nC\n\nH\n4\n\n\u2192\nC\n+\n2\n\nH\n2\n\n\n(\n\n\u0394\n\nH\n\n\n298\n\u00b0\n\n\n\n=\n\n\n76\nk\nJ\n\n/\n\nm\no\nl\n\n\n\n)\n\n\n\n\n\n\n\n(2)\n\n\n2\nC\nO\n\u2192\nC\n+\nC\n\nO\n2\n\n\n(\n\n\u0394\n\nH\n\n\n298\n\u00b0\n\n\n\n=\n\n\n-\n172\nk\nJ\n\n/\n\nm\no\nl\n\n\n\n)\n\n\n\n\n\nStudies have shown thermodynamically that increasing the S/C ratio can reduce coke deposition, consequently leading to higher conversion [23,24]. It is worth nothing, however, that the introduction of excess steam may lead to higher energy demand and operating cost, as well as lower H2 yield [25].To alleviate the aforementioned challenges faced with Ni anode and reforming catalyst, our approach focused on the development of multi constituent alloys, also known as high-entropy alloys (HEAs), as anode and SRM catalysts. HEAs are promising alloys that combine five or more metal elements to improve the catalytic and mechanical properties [26,27]. One metal of consideration is cobalt (Co). Reports indicate that Co exhibits relatively high affinity for oxygen species, which is beneficial for suppressing carbon formation [28,29]. Besides an effective oxidizing catalyst, it has been observed that Co also promotes WGS reaction to produce more syngas, while simultaneously inhibit the Boudouard reaction responsible for carbon formation [30]. Copper (Cu) is another common metal additive for SRM catalysts. Huang et\u00a0al. demonstrated that the addition of Cu to Ni catalysts promotes the WGS reaction activity [31]. Despite this, Cu is known as a poor catalyst for C\u2013C and C\u2013H scission, thus slowing the rate of carbon formation [32]. DFT studies have confirmed that the incorporation of Cu results in higher activation energy barrier (E\n\nact\n) of carbon formation, while still maintaining an acceptable rate of reforming [33]. Hence, Cu is used to slow down the reforming rate, since the highly endothermic reaction could cause rapid cooling and consequently, thermal stress on the SOFC anode [5]. Besides chemical activity and stability, the SRM catalyst must also exhibit good physical stability and durability under industrial operating conditions. The use of Ni as an SOFC anode at high temperatures for long durations may undergo sintering and particle coarsening [34].One plausible approach is to add metal additives with high melting point. Fe has been shown to be thermally stable at high temperatures, which makes it candidate for SOFC anode material [5]. Huang et\u00a0al. reported that Fe possesses strong resistance against carbon formation during internal ethanol reforming in SOFC [35]. Due to the high affinity of Fe for oxygen species, the surface carbon can be easily oxidized to CO and subsequently CO2 to avoid catalyst deactivation and further promote the WGS reaction. Similarly, manganese (Mn) has shown to be a beneficial oxidation catalyst without the risk of sintering or agglomeration [36]. This is advantageous as oxygen can transfer to the carbonaceous species and frees the surface from carbon deposition. Ouaguenouni et\u00a0al. prepared a nickel-manganese oxide catalyst that exhibits good activity towards the complete oxidation of methane [37]. The ability of Mn to exist in different oxidation states makes it a good redox couple catalyst for SRM [38].\nTable 1\n summarizes the catalytic role of each metal in SRM and the corresponding drawbacks. In this study, the five metals discussed above have been consolidated in a solid solution known as the high-entropy alloy (HEA) as means to utilize the advantageous properties of each metal, while keeping thermal stress, endothermic cooling and rate of carbon formation minimal. Contrary to other fuel cell systems, the main challenge with SOFC does not concern with mass transfer or kinetics, but rather long term-stability, for which internal distributed reforming of hydrocarbon plays a key role. Long-term stable cell and stack operation require that the cell experience and possess distributed reforming and endothermic cooling as well as resistance to carbon formation. By controlling the catalytic reaction of the anode, a thermal neutral state can be achieved as a result of both the endothermic steam reforming reaction and the exothermic electrochemical oxidation reactions [39,40]. It has been shown computationally using a 3D CFD model that the reforming rate should be reduced by a factor of 0.01 relative to that of Ni-based anode for a more uniform temperature distribution along the cell [41]. With the development of advanced anode, our objective is to reduce the reforming rate without significantly lowering the electrochemical activity of the cell, so that adequate current density can still be maintained. Thin film studies performed on sputter deposited above alloy compositions indicated the formation of solid solution (R. Bhattacharya, UES Inc. Personal communication). At elevated temperatures and under the SOFC operating conditions, it is envisioned that select alloy constituent can oxidize to form respective oxide based on the local oxygen partial pressure of the fuel. For the support, gadolinium- doped ceria (GDC) was used. CeO2 is widely used as a support for SRM due to its oxygen storage capacity (OSC) to store and release oxygen species [42,43]. Additionally, Ce-based materials present high oxygen ion mobility that promotes carbon removal and hence, long-term stability of the cell [44]. The addition of Gd increases sintering resistance by enhancing the metal-support interaction [45]. HEAs with various metal contents supported on GDC were prepared and tested for SRM. Then, direct internal steam reforming in laboratory scale SOFC button cells were performed to examine the performance of HEA/GDC as a candidate anode. The reforming and electrochemical measurements, resistance to carbon formation were analyzed and compared to those of conventional Ni/YSZ and standard Ni/GDC anode.HEA was prepared using the co-precipitation method by dissolving optimized formulation of nitrate precursors obtained from Fisher Scientific (98% pure nickel (II) nitrate hexahydrate, 99% pure cobalt (II) nitrate hexahydrate, 99% pure copper (II) nitrate trihydrate, 98% iron (III) nitrate nonahydrtae and 98% manganese (II) nitrate tetrahydrate). A total of three different anode catalyst formulations were synthesized and tested for methane reforming, from which the alloy mixture with resistance to carbon formation and stable reforming was selected as the SRM catalyst for further bench-top and fuel cell studies. The resulting optimized formulation of the HEA anode is given in Table 2\n. The metal nitrates were dissolved in excess deionized (DI) water, stirred and heated to 90 OC. Then, citric acid (CA) was added as a chelating agent to the mixture using a 1.5:1 CA: metal ratio. Ammonium hydroxide (NH4OH) solution was added dropwise to the metal-chelate solution to adjust the pH value to about 7\u20139 while stirring. The solution stirred overnight to homogenize the mixture and to evaporate excess water. On the next day, the remaining solution was transferred to an alumina crucible for calcination at the rate of 5 OC/min to 500 OC and held for 6\u00a0h to burn off nitrates, organic compounds and other contaminants. The as-obtained HEA powder and commercial 10% GDC (GDC-10\u00a0M) obtained from Fuelcell Materials USA were weighed (65:35\u00a0wt%) and physically mixed in a mortar and pestle until a homogenous mixture of fine powder was obtained. Table 2 provides the metal composition of each SRM catalyst for this study.N2 adsorption/desorption analysis was conducted using a Micromeritics ASAP 2000 analyzer to determine the sample surface area, pore volume and pore distribution. Before the analysis, about 0.1\u00a0g of sample was outgassed for 12\u00a0h under vacuum in the degas port. Then, the sample was re-weighed to obtain the new moisture-free mass before starting the analysis. The measurement was carried out at 77\u00a0K under N2 flow. The Brunauer-Emmett-Teller (BET) theory was then used to calculate the surface area. H2 chemisorption was performed using the Micromeritics ASAP 2000C software to determine the metal dispersion. Powder X-ray diffraction (XRD) pattern of each sample was collected using a Bruker D8 Advance X-ray diffractometer to identify surface phases. The diffractometer was equipped with a Cu K\u03b1 radiation (\u03bb\u00a0=\u00a00.15406\u00a0\u00c5) operating at 40\u00a0kV and 40\u00a0mA. The XRD patterns were obtained in a 2\u03b8 range of 10\u201390\u00b0. The scanned XRD patterns were indexed using the ICDD (International center for Diffraction Data) database. Surface morphology and elemental composition of each sample before and after the SRM experiment were characterized using a FEI Quanta 250 FEG scanning electron microscope (SEM) coupled with energy dispersive E-ray spectroscopy (EDAX). H2 temperature-programmed reduction (TPR), oxidation (TPO) and desorption (TPD) were carried out on an Altamira Instruments AM1-200 unit. About 50\u00a0mg of sample was placed between quartz wool supports inside a U-shaped quartz tube. Prior to the TPR and TPD analyses, the sample was first pretreated in 10% O2/He gas at a flow rate of 30 SCCM from 50 to 1000\u00a0\u00b0C and heating rate of 10 OC/min. TPD was carried out under inert atmosphere in pure Ar flow. Reduction experiments were performed using 50 SCCM of 10% H2/Ar. After reduction, the gas feed was subsequently switched to 50 SCCM of 10% O2/He for TPO study. All TPR, TPO and TPD studies were analyzed using a thermal conductivity detector (TCD). Inductively coupled plasma (ICP) with an optic emission spectroscopy (ICP-EOS) was used to quantify the bulk metal loadings of each catalyst. Post-test samples were also characterized for carbon formation by a Renishaw System 2000 equipped with a 514\u00a0nm green laser.The SRM test was performed in the temperature range of 700\u2013800\u00a0\u00b0C at 1\u00a0atm and gas hourly space velocity (GHSV) of 45,000 h\u22121.100\u00a0mg of SRM catalyst was loaded into a fixed-bed quartz tube with an outside diameter (OD) of \u00bd\u201d and a length of 38\u00a0cm as shown in Fig.\u00a02\n. Both sides of the catalyst were supported by quartz wool. The reactor was then placed into a horizontal tube furnace. Prior to the test, the catalyst was first reduced in a constant 4% H2/N2 gas flow at 700 OC for 2\u00a0h. Then, the gas was switched to flow 10 SCCM of CH4 and allowed to mix with 20 SCCM of H2O inside an evaporator heated to 120\u00a0\u00b0C before entering the catalyst bed. H2O was supplied by an HPLC pump at a flow rate of 0.016\u00a0mL/min to maintain a steam-to-carbon ratio (S/C) of 2.20 SCCM of N2 was used as a carrier gas, amounting to a gas hourly space velocity (GHSV) of approximately 45,000 h\u22121. Exhaust gas was condensed, collected and analyzed by a SRI 8610 gas chromatograph with a helium ionization detector (HID). Upon completion, the reactor and gas lines were purged with N2 gas and then switched back to H2 before cooling the reactor down to room temperature. The post-test samples were carefully removed from the quartz tube and quartz wool, and saved to be analyzed under SEM and Raman spectroscopy for any carbon deposition on the catalyst. The methane conversion (X\n\nCH4\n) and hydrogen yield (Y\n\nH2\n) were determined using equations (3) and (4), respectively. The rate of CH4 consumption (r\n\nCH4\n) normalized to the active metal loading was calculated by equation (5). A time-on-stream (TOS) test was conducted at 600\u00a0\u00b0C for 30\u00a0h to investigate the stability of SRM catalysts towards carbon poisoning. Except the operating temperature, the same operating conditions for the bench-top test was adopted. The TOS post-test samples were saved and analyzed using Raman spectroscopy. Additionally, surface morphology and elemental composition of post-test samples were characterized by SEM.\n\n(3)\n\n\n\nX\n\nC\nH\n4\n\n\n\n(\n%\n)\n\n=\n\n\n\nF\n\nC\nH\n4\n,\ni\nn\n\n\n\u2212\n\nF\n\nC\nH\n4\n,\no\nu\nt\n\n\n\n\nF\n\nC\nH\n4\n,\ni\nn\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(4)\n\n\n\nY\n\nH\n2\n\n\n\n(\n%\n)\n\n=\n\n\nF\n\nH\n2\n,\no\nu\nt\n\n\n\n2\n\nF\n\nC\nH\n4\n,\ni\nn\n\n\n+\n\nF\n\nH\n2\nO\n,\ni\nn\n\n\n\n\n\u00d7\n100\n%\n\n\n\n\n\n\n(5)\n\n\n\nR\n\nCH\n4\n\n\n\n(\n\n\nmol\n\nCH\n4\n\n\n\n\nmol\nmetal\n\n\n-\n1\n\n\n\ns\n\n-\n1\n\n\n\n)\n\n=\n\n\n\nF\n\nC\nH\n4\n,\ni\nn\n\n\n\u2212\n\nF\n\nC\nH\n4\n,\no\nu\nt\n\n\n\n\nN\n\nm\ne\nt\na\nl\n\n\n\n\n\n\nWhere F\n\ni\n is the molar flow rate of species i in mol/s and N\n\nmetal\n is the amount of active metal in moles.An electrochemical button cell (HEA/GDC - Ni/ScSZ|ScSZ|LSM/YSZ) was fabricated to examine the electrochemical performance of HEA/GDC as anode material for direct internal SOFC at 750\u00a0\u00b0C. Ni/ScSZ functional layer (10% Scandia stabilized zirconia purchased from Fuelcell Materials USA) was first deposited on the anode side and then sintered at 1350\u00a0\u00b0C for 2\u00a0h. This was followed by screen-printing LSM/YSZ cathode on the opposite side of the electrolyte and sintering at 1200\u00a0\u00b0C. The final anode layer of HEA/GDC was screen-printed on top of the anode functional layer and subsequently sintered at 1000\u00a0\u00b0C. The cell performance was evaluated at 750\u00a0\u00b0C on an in-house test station shown in Fig.\u00a03\n. The cell was sealed using CeramaBond on one end of an alumina tube and the gold meshes were used as current collectors. To create a base line and reduce the anode, humidified hydrogen (9% H2\u20133% H2O\u2013N2 bal.) was supplied to the anode side at a flow rate of 100 SCCM. The corresponding I vs. T for 3\u00a0h is shown in Fig.\u00a0S1 in the supporting information. Subsequently, the gas was switched to CH4 fuel with steam (S/C\u00a0=\u00a02) before entering the catalyst bed on the anode side. Unlike the bench-top experiment, a carrier gas was not used in this case to ensure low mass transfer resistance and maximum contact between the active area of the catalyst and methane. The resulting GHSV was similar to that used in the bench-top experiment. Then, the exhaust gas was condensed, collected and analyzed by the GC-HID. Air was fed through the cathode at a flow rate of 150 SCCM. The electrochemical test, similar to the bench top test, was carried out for 30\u00a0h. Current density and the electrochemical impedance spectra (EIS) measurements were acquired at a constant voltage of 600\u00a0mV using a VMP3 Bio-Logic potentiostat/galvanostat. The frequency ranged from 10\u00a0mHz to 200\u00a0kHz, with 10\u00a0mV perturbation. For a more quantitative insight into the electrochemical phenomena, electrical equivalent circuit (EEC) R\u03a9(QRHF) (QRLF) was employed using ZSimpWin software to analyze the impedance data. The impedance of two interfaces metal/electrolyte and surface coating/electrolyte were analyzed to represent the two semicircles corresponding to the high and low-frequency arcs, respectively, which relate to gas adsorption\u2013desorption on the electrode surface followed by charge transfer and incorporation of adsorbed gas at the three phase boundary, and the gas concentration polarization loss of the electrode. To investigate a carbon-free cell operation, SEM and Raman spectroscopy on the post-test cell were conducted.In this study, the co-precipitation method was employed to synthesize the HEA anode material. To confirm if a single-phase alloy was formed, room-temperature powder XRD pattern was performed. Fig.\u00a04\n(a) presents the XRD patterns of HEA/GDC as well as those of Ni/GDC and Ni/YSZ for comparison. The intensity of the XRD peaks is directly correlated to the crystallinity of the material. As expected, the crystallinity of reduced Ni/YSZ and Ni/GDC is more pronounced than that of the HEA/GDC due to less chemical and heat treatments of the former materials, thereby preserving the integrity of the crystal. Indexing by ICDD reveals that the catalysts were successfully reduced, as evidenced by the absence of oxide peaks. For the HEA/GDC catalyst, the denoted peaks are attributed to mixed metal alloys. This confirms that HEA was successfully synthesized without additional phases of oxides. The other diffraction peaks have also been indexed and confirmed by ICDD to denote the respective metal supports. Fig.\u00a04(b) shows the N2 adsorption/desorption isotherms of the SRM catalysts and the calculated BET surface area and pore volume are tabulated in Table 3\n, along with other physicochemical properties. The linear relationship at the beginning of the isotherm, followed by a significant increase in the adsorption of N2 indicates a type II physisorption isotherm, suggesting a nonporous structure. The addition of GDC as support increased the surface area. According to Angeli et\u00a0al. the presence of CeO2 improves the surface area and active metal dispersion [3]. Subsequently, the substitution of Ni with HEA supported on GDC further increased the surface area to 35\u00a0m2/g due to enhanced pore size volume, which may enhance the catalytic properties of SRM.SEM images of as-synthesized Ni/YSZ, Ni/GDC and HEA/GDC are shown in Fig.\u00a05\n. The standard Ni/YSZ catalyst containing 45.2\u00a0wt% of Ni (Table 3) in difference resolutions is shown in Fig.\u00a05(a\u2013c). It can be seen that the Ni particles are relatively small and close to each other. H2 chemisorption reported a Ni dispersion of 0.327% with a particle size of 310\u00a0nm. For Ni/GDC, the structures of NiO and GDC powders were dissimilar and could be easily distinguished from each other as seen in Fig.\u00a05(d\u2013f). The metal dispersion was slightly lower due to the increase in crystal size to 461\u00a0nm, which suggests that the increase in surface area could be attributed to the enhanced pore volume, owing to the GDC support. Fig.\u00a05(g\u2013i) show two distinct phases on HEA/GDC, arising from the presence of HEA and the GDC support. From the morphology, it can be seen that the particles tend to sinter and form larger agglomerates. This, however, changes as the HEA/GDC catalyst was reduced at higher temperature as the oxide phase converts into the FCC cubic phase, as shown by the XRD pattern in Fig.\u00a04(a). Subjecting the catalyst to reduction may also result in higher porosity and smaller metal particles, leading to increased surface area.The coking resistance and SRM performance of a catalyst highly depends on interaction between the active metal and the support. Having a strong metal-support interaction in the catalyst suppresses metal sintering at elevated temperature and reduces coke formation, which in turn improves catalyst activity and stability [22,58]. To evaluate the chemical interaction between metal and support, the SRM catalysts were analyzed by H2-TPR as shown in Fig.\u00a06\n(a). The reduction peak centered at 375\u00a0\u00b0C was assigned the reduction peak of NiO to Ni, which resulted from a weak interaction between Ni and the support [59]. During the TPR of HEA/GDC, it was observed that a broad peak emerged at 400\u00a0\u00b0C due to the reduction of the metal alloy. The reduction peaks of Co3O4 typically appear at approximately 400\u00a0\u00b0C and 470\u00a0\u00b0C, following a two-step reduction process to Co0 [50]. Similarly, the two-step reduction of Mn2O3 to Mn3O4 and subsequently, to MnO would result in reduction peaks at 300\u00a0\u00b0C and 420\u00a0\u00b0C, respectively [60]. Finally, the reduction of Fe2O3 to Fe also follows a two-step process, although the reduction of Fe3O4 to Fe0 occurs at a much higher temperature of 835\u00a0\u00b0C [61]. The successful synthesis of HEA brings about a single-phase solid solution through which the compositions of five metals have been optimized. As a result, the properties of HEA are typically more superior than the corresponding metal counterparts. Such is the case in Fig.\u00a06(a) showing that HEA/GDC requires a higher temperature for reduction compared with Ni/YSZ and Ni/GDC. Thus, the HEA/GDC catalyst should exhibit higher sintering and coking resistances. TPO of reduced samples is displayed in Fig.\u00a06(b). Oxygen uptake appears to be minimal or non-existent, which can be explained by the rapid re-oxidation of metals and oxide supports during the switching between reducing and oxidizing gases. Nonetheless, the desorption of O2 takes places at roughly 500\u00a0\u00b0C for Ni/YSZ and Ni/GDC, and \u223c600\u00a0\u00b0C for HEA catalysts under oxidizing conditions. The higher desorption temperature suggests a stronger interaction between HEA and oxygen. Fig.\u00a06(c) displays the TPD results of samples after being subjected to oxygen pretreatment. As expected, Ni/YSZ and Ni/GDC did not show any desorption of oxygen, while HEA/GDC exhibited a TPD peak at \u223c850\u00a0\u00b0C. This suggests that some HEA constituents such as Fe2O3 or Mn2O3 possess high oxygen storage capacity (OSC) and they have been proven beneficial for carbon removal. Quantification of TPO and TPD peaks of these samples in Table 3 shows that HEA/GDC is capable of adsorbing and desorbing a higher amount of oxygen, owing to the enhanced surface oxygen mobility and oxygen uptake of HEA and the GDC support [62].The catalytic activity of SRM catalysts was examined using a fixed-bed tube reactor at varying operating temperatures of 700, 750 and 800\u00a0\u00b0C at a GHSV of 45,000 h\u22121. At each temperature, the reaction was allowed to reach equilibrium, after which methane conversion was calculated and reported in Fig.\u00a07\n(a). The main products in the exhaust were H2, CO, CO2 and CH4. Water vapor in the exhaust was condensed before being fed to the GC. All SRM catalysts showed increasing conversion with temperature, with both nickel-based catalysts (Ni/YSZ and Ni/GDC) displaying the highest methane conversion at equilibrium. This is expected as the SRM reaction is endothermic and therefore, thermodynamically favorable at higher temperature. It is also worth mentioning that while partial oxidation of CH4 may occur subsequently with steam reforming, our calculations show that the partial pressure of O2 is too low (i.e.\u00a0\u223c\u00a010\u221217\u00a0atm) for CH4 to undergo direct oxidation, consistent with other reports [63]. The lowest conversion was reported by HEA/GDC, which increased from 27% at 700\u00a0\u00b0C to 35% at 750\u00a0\u00b0C and then to 42% at 800\u00a0\u00b0C. Subsequently, the increase in conversion is accompanied by an increase in hydrogen yield with temperature, as displayed in Fig.\u00a07(b). Among the HEA catalysts, HEA/GDC reported the highest conversion at each temperature. The H2 yield was calculated by measuring the amount of H2 produced with respect to theoretical amount of H2 produced from maximum conversion of CH4 and H2O. In all cases, the H2 yield increased with temperature with both Ni/YSZ and Ni/GDC displaying slightly higher H2 yield than HEA/GDC. The rate of CH4 consumption was also calculated and compared as shown in Fig.\u00a07(c). The increased temperature enhanced the consumption rate of CH4 for all SRM catalysts. Both Ni/YSZ and Ni/GDC catalysts showed the highest rate of \u223c3.5 molCH4 molNi\n\u22121 s\u22121 by 750\u00a0\u00b0C. The high activity of Ni in SRM has been well-documented in the literature. A strong endothermic reaction may induce large temperature gradient especially when operating the catalyst on a SOFC. Furthermore, a fast reforming reaction as displayed by the Ni-based catalysts may result in thermal stresses and mechanical failures, thus lowering the cell efficiency. The HEA/GDC catalyst, however, showed a lower reforming rate of \u223c1\u00a0mol molCH4 molHEA\n\u22121 s\u22121 at 750\u00a0\u00b0C, which may provide a smaller temperature gradient during concurrent reactions of heterogeneous catalysis and electrochemistry. Experimental results show that an optimized formulation of HEA has reduced the reforming rate of methane compared with highly endothermic Ni-based catalysts. Consequently, the cell life and current distribution can be maintained. This is more advantageous than standard Ni/YSZ as an anode material since the latter has shown to experience major drawbacks especially under harsh conditions such as coking, metal agglomeration, thermal stress, mechanical failure and poor redox stability [3]. While higher localized reformation rates may imply faster and higher production of H2, the objective of the alloy anode development is largely to minimize localized cooling and carbon deposition.The catalytic stability of SRM catalysts was investigated at 600\u00a0\u00b0C and S/C ratio of 1. The TOS experiment was carried out isothermally for 30\u00a0h. Using the HSC\u00ae Chemistry 10 software, the equilibrium compositions were calculated for a temperature range of 25\u20131000\u00a0\u00b0C as shown in Fig.\u00a0S2(a) in the Supporting Information. At 600\u00a0\u00b0C, the carbon activity should be at its highest and this temperature is thermodynamically favorable for studying carbon resistance of each SRM catalyst. Fig.\u00a06(d) reports the conversion of CH4 over time. It is evident that the initial conversion rate was high for both Ni/YSZ and Ni/GDC due to enhanced catalytic activity of Ni. However, the conversion gradually decreases over time with Ni/YSZ showing the fastest degradation rate, followed by Ni/GDC. After 30\u00a0h of TOS, the final conversions were 54% and 66% for Ni/YSZ and Ni/GDC, respectively. The higher stability of Ni/GDC suggests that the GDC support plays an important role in reducing catalyst deactivation. For the HEA/GDC catalyst, the conversion rate was relatively low compared to initial conversion rates of the standard catalysts, as shown in Fig.\u00a06. Nonetheless, the catalyst maintained a stable run over 30\u00a0h of TOS between 15 and 18% conversion, revealing the ability of HEA catalysts to resist deactivation over long periods of operation. To assess the source of catalyst deactivation, post-test catalysts were saved from TOS experiments and were subjected to Raman analysis and SEM imaging.Carbon deposition has been regarded as one of the main reasons for catalyst deactivation during SRM. To identify the nature and structure of these surface carbonaceous species, post-test catalysts from the 30\u00a0h TOS test were subjected to SEM imaging. Fig.\u00a08\n(a\u2013c) show high resolution SEM images of post-test SRM catalysts after 30\u00a0h of TOS experiment. The surface of all samples appear to be free of any carbonaceous species. Post-test Ni/YSZ and Ni/GDC samples in Fig.\u00a08(a and b) did not show any dissimilarities compared to their corresponding pre-test samples. HEA/GDC was observed to be more porous with a uniform distribution of particle size after reduction at 700\u00a0\u00b0C, consistent with relatively high BET surface area reported in Table 3. The absence of surface carbon on HEA/GDC in Fig.\u00a08(c) may explain the promising stability during the 30\u00a0h of TOS. To confirm this and to further investigate the deactivation of the former two Ni-based catalysts, Raman spectroscopy was performed on all post-test samples. As shown in Fig.\u00a08(d), all SRM catalysts showed two distinct characteristic peaks at 1345 and 1595\u00a0cm\u22121. The peak at 1335\u00a0cm\u22121 can be attributed to the D band of carbonaceous species, formed by the vibrations of disordered carbon atoms (amorphous carbon for example), while the peak at 1595\u00a0cm\u22121 has been assigned the G band to represent the presence of ordered and graphitic crystalline structure caused by vibration of the in-plane sp [2]-bonded carbons [64]. Amorphous carbon has been shown to play a significant role in catalyst deactivation via encapsulation of the metal active sites [65]. On the other hand, graphitic carbon with filamentous structure may also form as a result of migration of surface carbon to the bulk metal phase, resulting in nucleation growth of carbon on the other side of the metal particle [21]. While graphitic carbon may not directly affect the activity of the catalyst, uncontrolled growth of carbon whiskers may result in reactor blockage and pressure drop [66]. Additionally, coke formation on the anode material of a SOFC can be detrimental to the long-term stability of the system and could potentially lead to mechanical failure [12]. In Fig.\u00a08(d), Ni/YSZ catalyst showed the highest amount of both amorphous and graphitic carbons on the surface, leading to catalyst deactivation during the 30\u00a0h TOS test as demonstrated in Fig.\u00a07(d). Similarly, Ni/GDC exhibited some amorphous and graphitic carbonaceous species, which explains the gradual deactivation of the catalyst. The enhanced stability of HEA/GDC during TOS was due to the high carbon resistance of the catalyst, as both post-test SEM and Raman analyses did not show signs of carbon.The electrocatalytic activity and stability of (HEA/GDC-Ni/ScSZ|ScSZ|LSM/YSZ) for direct internal SOFC have been investigated. The moderate reformation rate and high long-term stability of the anode catalyst may prove beneficial in a SOFC system by preventing mechanical failures due to rapid temperature change and carbon deposition [5,12]. In this study, the cell test was performed at 750\u00a0\u00b0C, to which the anode was subjected a constant flow of CH4 and steam (S/C\u00a0=\u00a0\u223c2) and the cathode with air. Under reduced atmosphere and internal reforming condition, the open-circuit voltage (OCV) was measured to be \u223c0.9\u00a0V at 750\u00a0\u00b0C due to favorable interfacial interaction between the HEA/GDC anode layer and the ScSz electrolyte later after high-temperature sintering. The OCV plots can be found in Fig.\u00a0S3. Upon switching the feed to CH4 fuel and imposing a bias of 0.6\u00a0V, the I-T electrochemical data was collected for 30\u00a0h of SOFC test, as shown in Fig.\u00a09\n(a). A slight dip in current density was noticed after 8\u00a0h of testing, which could be attributed to lower reformation rate, condensation of steam in cold zones of the inlet and diffusion of transition metals in the anode that could affect the ionic resistance of the cell [67]. This self-activation phenomenon has also been reported elsewhere as a result of surface modification on the anode during steam reforming [68,69]. During this time, the initial reforming rate is extremely low. However, upon reduction and activation, the reforming rate increases, leading to an increase in performance and current density. As soon as the HEA/GDC anode was fully activated after the first 15\u00a0h, the reforming rate was enhanced leading to an increase in current density to \u223c100\u00a0mA/cm2 for the next 15\u00a0h. In comparison, the current density of a Ni-based anode was reported to be \u223c250\u00a0mA/cm2 at the start of the cell operation, but quickly approached 0\u00a0mA/cm2 due to carbon formation [63]. The relatively low current density may be due to electrolyte thickness, whose role on electrochemical performance will be explored in future work. Nonetheless, the HEA/GDC anode yielded sufficient current density to maintain a stable and carbon-free operation. This confirms that controlled and distributed reforming also improved the current density distribution in the cell. A more comprehensive electrochemical study involving dual atmosphere cycling to compare the current densities in reducing atmosphere and hydrocarbon-rich atmosphere will be considered in the future. The corresponding Nyquist spectra in Fig.\u00a09(b) acquired at the different times are composed of two depressed semicircles which correspond to the polarization resistance Rp (RHF\u00a0+\u00a0RLF) while the high frequency intercept with the real impedance axis corresponds to the purely ohmic resistance (R\u03a9) of the electrolyte and current collecting wires. As demonstrated in Fig.\u00a09(c), the overall non-ohmic resistance Rp (RHF\u00a0+\u00a0RLF) decreases after 15\u00a0h indicating higher mass transfer due to increase in hydrocarbon reformation rate. The ohmic resistance remains stable during internal reforming indicating stable and carbon-free cell operation. The exhaust was simultaneously analyzed by a GC-HID and the conversion of CH4 over 30\u00a0h of testing is shown in Fig.\u00a09(a). The conversion of CH4 was stable at 20% throughout the whole electrochemical test, which is consistent with the results obtained from the bench top experiments (Fig.\u00a07). This is a good indication that the cell test is stable and scalable for future long-term testing. Once the cell test has been completed, the anode layer of the cell was analyzed using SEM microscopy and Raman spectroscopy for any carbon deposition. High-magnification SEM in Fig.\u00a09(e) shows the presence of carbon on Ni/YSZ anode with a composition of 17.3 atomic%. On the other hand, post-test HEA/GDC anode in Fig.\u00a09(f) shows a clean and carbon-free surface. After 30\u00a0h of cell test, not only did the anode layer show remarkable carbon resistance, but good and stable contact were also formed between the anode layer and electrolyte. Fig.\u00a09(d) shows the Raman spectra of two locations of the anode surface, one being the center and the other towards the edge of the anode layer. The absence of D and G bands at 1345 and 1595\u00a0cm\u22121, respectively, suggests that the HEA/GDC anode was free of both amorphous and graphitic-typed carbons. HEA/GDC also exhibits high OSC, as suggested by TPD in Fig.\u00a06(c), which can play an important role in the rapid oxidation of carbon to COx species, thus minimizing carbon poisoning. Fig.\u00a010\n shows a schematic depicting the role of coke-resistant HEA/GDC as oxygen vacancies in the anode enhance the mobility and diffusivity of oxygen ions to the anode surface to gasify any deposited carbon.The HEA/GDC catalyst displayed promising potential as an anode material for internal utilization of CH4 in SOFC. The bench top experiment suggested that HEA/GDC exhibits moderate reforming rate and excellent coking resistance under CH4 reforming conditions, owing to the optimized mixture of HEA constituents and the high OSC of the GDC support. HEA/GDC also showed superior operational stability for CH4 conversion over 30\u00a0h and post-test analysis of the catalyst did not indicate presence of carbon deposition, while both Ni/YSZ and Ni/GDC catalysts could be seen deactivating over time, despite the high initial CH4 conversion and H2 yield. The activity, stability and carbon-resistance of HEA/GDC as anode were also further investigated in a SOFC cell test. A current density of \u223c100\u00a0mA/cm2 was achieved at 750\u00a0\u00b0C. The cell performed successfully over 30\u00a0h without any sign of decay. The overall polarization (ohmic and non-ohmic resistances) of the cell was low and stable. The moderate reforming rate of HEA/GDC is important for maintaining uniform temperature distribution and high coking tolerance especially for long-term high temperature SOFC operations, without compromising the electrochemical activity of the cell. Given the promising attributes of HEA/GDC as anode material in this study, successful effort has been made to further improve HEA/GDC for the direct utilization of other hydrocarbons such as methanol and ethanol, which are more easily stored and transported.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 acknowledge financial support from Advanced Research Projects Agency - Energy under contract DE-AR0001012. Technical discussions with Dr. Mike Tucker (Lawrence Berkeley National Laboratory), Dr. Greg Tao (Chemtronergy), Dr. Abdul Jabbar Hussain (Nissan Motors) and Dr. Rabi Bhattacharya (UES Inc.) is acknowledged.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.ijhydene.2022.09.018.", "descript": "\n                  High-entropy alloy (HEA) anode and reforming catalyst, supported on gadolinium-doped ceria (GDC), have been synthesized and evaluated for the steam reforming of methane under SOFC operating conditions using a conventional fixed-bed catalytic reactor. As-synthesized HEA catalysts were subjected to various characterization techniques including N2 adsorption/desorption analysis, SEM, XRD, TPR, TPO and TPD. The catalytic performance was evaluated in a quartz tube reactor over a temperature range of 700\u2013800\u00a0\u00b0C, pressure of 1\u00a0atm, gas hourly space velocity (GHSV) of 45,000 h\u22121 and steam-to-carbon (S/C) ratio of 2. The conversion and H2 yield were calculated and compared. HEA/GDC exhibited a lower conversion rate than those of Ni/YSZ and Ni/GDC at 700\u00a0\u00b0C, but showed superior stability without any sign of carbon deposition unlike Ni base catalyst. HEA/GDC was further evaluated as an anode in a SOFC test, which showed high electrochemical stability with a comparable current density obtained on Ni electrode. The SOFC reported low and stable electrode polarization. Post-test analysis of the cell showed the absence of carbon at and within the electrode. It is suggested that HEA/GDC exhibits inherent robustness, good carbon tolerance and stable catalytic activity,` which makes it a potential anode candidate for direct utilization of hydrocarbon fuels in SOFC applications.\n               "}
{"full_text": "Lignocellulosic biomass can be effectively used to reduce carbon footprint. As it is present in abundance, it can also be used for the sustainable production of fuels and chemicals [1]. The valorization of lignocellulose for the production of drop-in fuels and chemicals is a multi-step process that involves the steps of fractionation, depolymerization, and upgrading [2]. The fast pyrolysis can be used to produce depolymerized products from biomass. The method can be effectively used to produce biomass pyrolysis oil (or bio-oil (BO)), an essential feedstock that can potentially replace petroleum-based oil [3]. The quality of BO characterized by low heating value, high acidity, chemical instability, high viscosity, and high oxygen water content [4] can be improved by subjecting the sample to upgrading processes such as hydrotreatment and water-soluble phase separation [5,6]. It is difficult to directly use phenolic compounds and depolymerized lignin derivatives as fuels because labile radical intermediates are formed during the hemolytic cleavage [7]. These can be repolymerized to form oligomers during the recovery of liquid products [8,9]. Catalytic upgrading can be achieved following the hydrodeoxygenation (HDO) to address these issues and improve the extent of valorization of BO for the production of fuels and chemicals. Highly oxygenated compounds can be converted to highly stable products such as alcohols and alkanes during the process. The HDO has been investigated using noble metals (such as Pt, Pd, and Ru) and MoS-based catalysts as these exhibit high activities [4]. Transition metal catalysts can also be used for achieving the economically viable process [10].Among the monomeric compounds prepared by the HDO of BO, the lignin-derived cyclohexanol or alkyl cyclohexanol compounds are widely used for the development of polymers, fuels, and pharmaceuticals. Cyclohexanol is used for synthesizing cyclohexanone [11] which can be used as a precursor during the preparation of adipic acid, a monomer present in various polyamides, including nylon 6,6 [12]. Cyclohexanone is also used for the synthesis of caprolactam, a monomer of polycaprolactam such as nylon 6 [13]. Cyclohexanol is also used to produce cyclohexanolamine [14] and cyclohexyl acetate [15] that can be used in the pharmaceutical industry for the preparation of artificial sweeteners, food additives, and fragrances. Ketone\u2013alcohol oil (KA oil) is composed of cyclohexanol and cyclohexanone and is commercially produced in petrochemical refineries from cyclohexanone following the oxidation using Co and Mn-based catalysts. The product yields are low (conversion: \u2264 11%; selectivity: \u2264 85%) [16]. The conditions that can be used for the effective production of cyclohexanol from biomass has been presented (Table S1). High yields and good selectivity were observed for the products synthesized from phenolic monomers [17,18], dimers [19,20], and lignin oil [21].Supercritical methanol depolymerization and hydrodeoxygenation processes (catalyst: CuMgAlOx; pressure: 20\u00a0MPa; temperature: 300 \u00b0C) have been conducted for the production of (alkyl)cyclohexanol from lignocellulose or lignin derivatives in batch reactors [22,23] and semi-continuous flowthrough reactors. The average selectivity achieved during the production of C6-C10 cyclic alcohols from maple wood (using a semi-continuous flowthrough reactor in the presence of CuMgAlOx catalyst) [24] was 31%. Insoluble pyrolytic lignin, coke, and gases (such as H2, CO, and CO2) were also produced under severe reaction conditions that hindered the production of cyclic alcohols. The seminal work on producing cyclic alcohols from pinewood-derived pyrolysis oil was performed using Raney Ni as the catalyst at a low reaction temperature of 120 \u00b0C. The hydrogen transfer reaction was conducted in the presence of isopropanol. The one-step reaction for the production of cyclic alcohols was highly selective [25]. The results indicated a remarkable hydrogenation activity of the catalysts that could be used to saturate the aromatic rings present in lignin components. We have previously reported two-step upgrading reactions that can be followed for the conversion of sawdust pyrolysis oil [26] and furan condensates to alkane-rich upgraded oil [10] using supported noble and transition metal catalysts in a continuous flow reactor.Inspired by the reported results, we investigated if Ni-based catalysts could be used for the selective hydrodeoxygenation of lignin monomers to synthesize cyclic alcohols following the two-step upgrading reactions involving BO. While Ni catalysts can be used to conduct hydrogenation and hydrodeoxygenation reactions [10], the low affinity of Ni nanoparticles (NPs) toward phenolic monomers can hinder the selective production of deoxygenated chemicals such as phenols and cyclohexanols. It has been reported that TiO2-supported Ni catalysts can promote deoxygenation, exploiting the strong metal\u2013support interaction (SMSI) existing between the coordinatively unsaturated Ti3+ and Ni speices. This can improve the oxophilicity of Ni NPs reacting with biomass-derived oxygenates [27]. SMSI, however, can poison the active Ni sites. This can be attributed to the formation of TiO2 overlayer on the Ni NPs. The formation of the overlayer hinders the H2 dissociation. The entry of biomass-derived oxygenates into the Ni sites is hindered under these conditions [28]. As reported in the literature, SMSI can be tuned to modulate the intrinsic catalytic properties of Ni NPs to achieve the desired HDO activity.We studied the structures of active sites on the catalysts composed of Ni NPs dispersed on TiO2. The strong electrostatic adsorption (SEA) of metal on the TiO2 support and the hydrothermal (HT) treatment process led to the formation of active sites. The conventional impregnation (IM) method was also used to understand the effects of SEA-HT method on the active site forming ability. Bimetallic NiFe supported on TiO2 was fabricated and the Ni and Fe loading amounts were tuned to optimize the catalyst structures. The geometric and electronic structures of catalysts were investigated, and the formation of oxygen vacancies at the metal\u2013metal oxide interface was studied. The hydrodeoxygenation of alkyl methoxypehnols following the processes of demethoxylation and hydrogenation was performed. The investigation of structure\u2013activity indicated the presence of three active sites. The Fe-added Ni alloy NPs formed the metallic sites for the dissociative adsorption of H2 and hindered the hydrogenation of phenyl rings on the Ni active sites. The coordinatively unsaturated Fe species present on the FeOx shell layer adjacent to the NiFe sites and oxygen vacancies at the FeOx-TiO2 interface provided the oxophilic sites for the demethoxylation of guaiacylic monomers. The Ti3+ species present on the reducible TiO2 support promoted the dispersion of NiFe-FeOx core\u2013shell NPs by exploiting the FeOx-TiO2-x/TiO2 anchor sites. The optimum catalysts were identified and they were further used for the production of cyclic alcohols from a mixture of alkyl methoxyphenols prepared from BO (Fig. 1\n (B)). The high conversion and the high selectivity to cyclic alcohols were achieved during the conversion of methoxypehnols. Based on these results, we propose the strategy of producing cyclic alcohols from BO that can be used for the preparation of sustainable chemicals, such as nylon, plasticizers, and fuels [29]. Herein, the methods to synthesize improved catalysts (that can be used for HDO) consisting of non-precious metals and containing highly selective metal\u2013metal oxide interfacial sites have been presented. The rational design for the preparation of supported non-precious metal catalysts that can be used for upgrading biomass-derived oxygenates has also been presented.Nickel(II) nitrate hexahydrate (Ni(NO3)2\u00b76H2O, 99.999%), iron(III) nitrate nonahydrate (Fe(NO3)3\u00b79H2O, \u2265 99.95%), titanium(IV) oxide (TiO2, P25, \u2265 99.95%), niobium(V) pentoxide (Nb2O5, 99.9%), cyclohexane (C6H12, \u2265 99%), cyclohexanol (C6H11OH, 99%), cyclohexanone (C6H10O, 99%), 1,2-dimethoxybenzene (C6H4(OCH3)2, 99%), 2-methoxy-4-propylphenol (C10H14O2, 99%), and 2-methoxy-4-(2-propenyl)phenol (C10H12O2, 99%) were purchased from Sigma-Aldrich (St. Louis, Missouri, USA). Ammonium bicarbonate (NH4HCO3, 97%), 2-methoxyphenol (C7H8O2, 99%), n-decane (C10H22, 99%), methanol (CH3OH, 99.9%), and silicon dioxide (SiO2, extra pure) were purchased from DaeJung Chemicals (South Korea). Zirconium (IV) oxide (ZrO2, \u2265 99%), gamma-phase aluminum oxide (\u03b3-Al2O3, 99.98%), n-dodecane (C12H26, 99%), 4-methyl-2-methoxyphenol (C8H10O2, 98%), and 4-ethyl-2-methoxyphenol (C9H12O2, 98%) were purchased from Alfa Aesar (Ward Hill, Massachusetts, USA). Deionized (DI) water was obtained using the EXL\u00ae 7S Analysis Water system (Vivagen Co., Ltd., Seongnam, Korea) equipped with a filter (diameter: 0.22\u00a0\u00b5m). BO was purchased from BTG Bioliquids BV (Enschede, The Netherlands), which was prepared by the fast pyrolysis lignocellulosic biomass. N2 (99.999%), He (99.9999%), Ar (99.999%), 5% (v/v) H2/Ar, 1% (v/v) O2/He, and 5% (v/v) NH3/He, were purchased from the Sinyang Medicine (Anseong, Korea).A series of TiO2-supported Ni catalysts were prepared following the conventional wet IM (Ni/TiO2-IM) and HT synthesis methods (Ni/TiO2-HT, Fe/TiO2-HT, and NiFe/TiO2-HT). The wet IM method was used to prepare the catalyst containing 15\u00a0wt% Ni on the TiO2 support, denoted as Ni/TiO2-IM. TiO2 (P25, 1.70\u00a0g) was added to Ni(NO3)2\u00b76H2O (1.49\u00a0g) dissolved in DI water (50\u00a0mL). Following this, the mixture was stirred for 1\u00a0h at room temperature. The prepared suspension was transferred to a round bottom flask (250\u00a0mL) and the solvent was evaporated at 50\u00a0\u00b0C using a rotary evaporator. The solid powder was further dried in air at 105\u00a0\u00b0C over 16\u00a0h, following which it was calcined for 2\u00a0h under a flow of N2 at 300\u00a0\u00b0C. It was reduced under a flow of 5% (v/v) H2/Ar at 450\u00a0\u00b0C for 4\u00a0h. Following this, a gray solid catalyst (Ni/TiO2-IM) was obtained. Ni/TiO2-HT was prepared (following the HT method) by depositing Ni (15\u00a0wt%) on the surface of TiO2 support. Fe on the TiO2 support was mixed with Ni (15\u00a0wt%) to prepare NiFe(x)/TiO2-HT following the HT method. Here, x denotes Fe loading (wt%). Ni(NO3)2\u00b76H2O (1.49\u00a0g) and Fe(NO3)3\u00b79H2O (0\u20131.09\u00a0g, depending on x) were dissolved in DI water (20\u00a0mL) and transferred to a 100-mL Teflon-lined chamber. TiO2 (P25, 1.40\u20131.70\u00a0g, depending on x) was added to the mixture and the mixture was ultrasonicated for 30\u00a0min to prepare a well-distributed suspension. An aqueous solution of NH4HCO3 (1\u00a0M, 30\u00a0mL) was added drop-wise to the suspension at room temperature under conditions of continuous stirring. The Teflon-lined chamber was sealed and heated at 150\u00a0\u00b0C for 15\u00a0h. The temperature of chamber was slowly brought down to room temperature, and the prepared solid was filtered under conditions of vacuum. The solid was washed with DI water until the pH reached 7. Following this, it was further washed three times using ethanol (50\u00a0mL each). The solid powder was further dried in air at 105\u00a0\u00b0C for 16\u00a0h and calcined under a flow of N2 at 300\u00a0\u00b0C for 2\u00a0h. The sample was reduced at 450\u00a0\u00b0C over a period of 4\u00a0h under a flow of 5% (v/v) H2/Ar. Following this, a black solid catalyst (Ni/TiO2-HT or NiFe(x)/TiO2-HT) was obtained. The freshly reduced catalyst was exposed to an atmosphere of 1% (v/v) O2/N2 for 30\u00a0min (at 25\u00a0\u00b0C) to passivate the metal surface to prevent the excessive oxidation of catalyst during storage. TiO2-supported 3 and 15\u00a0wt% Fe were prepared following the HT method. The process followed was similar to the process followed during the fabrication of NiFe/TiO2-HT. The samples were denoted as Fe(x)/TiO2-HT where x indicated Fe loading (wt%). Fe(NO3)3\u00b79H2O (0.22\u00a0g and 1.09\u00a0g for 3 and 15\u00a0wt% Fe, respectively) was used as the precursor of TiO2 (P25, 1.94\u00a0g and 1.40\u00a0g for 3 and 15\u00a0wt% Fe, respectively). NiFe catalysts supported on other supports including SiO2, Al2O3, ZrO2, and Nb2O5 were prepared using the methods identical to the HT method used for NiFe(3)/TiO2-HT. Instead of TiO2, SiO2, Al2O3, ZrO2, and Nb2O5 were used to prepare NiFe(3)/SiO2-HT, NiFe(3)/Al2O3-HT, NiFe(3)/ZrO2-HT, and NiFe(3)/Nb2O5-HT, respectively.The Ni and Fe metal contents were determined using the inductively coupled plasma-optical emission spectroscopy (ICP-OES) technique (Model: 730-ES; Varian; Palo Alto, California, USA).The thermal stability of catalysts was determined using the thermogravimetry (TG) technique. The catalyst (10\u201320\u00a0mg) was dried over 10\u00a0min under a flow of N2 (30\u00a0mL/min) in the temperature range of 30\u2013200\u00a0\u00b0C prior to conducting the experiments. The sample was heated from 30\u00a0\u00b0C to 900\u00a0\u00b0C (heating rate: 10\u00a0\u00b0C/min) under a flow of air (80\u00a0mL/min). Thermal degradation was determined using a thermogravimetric analyzer (Model: Q600; TA Instruments; New Castle, Delaware, USA).The N2 physisorption behavior was studied using the MicrotracBEL (Osaka, Japan) BELSORP-mini II system. The Brunauer\u2013Emmett\u2013Teller (BET) surface area (SBET) was measured, and the pore volume (Vp) and pore size distribution were calculated following the Barret\u2013Joyner\u2013Halenda (BJH) method.The crystal structures of catalysts were determined using the Dmax2500/PC X-ray diffractometer (Rigaku, Japan) equipped with a scintillation counter and graphite monochromatic detector. The Cu K\u03b1\nave radiation (\u03bb\u00a0=\u00a00.15418\u00a0nm) was generated at 40\u00a0kV and 200\u00a0mA. The X-ray diffraction (XRD) results were recorded in the 2\u03b8 range of 3\u201390\u00b0 (scanning rate: 2\u00b0 per minute, step width: 0.02\u00b0). For the TiO2 supports, the anatase-to-rutile ratios were calculated by analyzing the intensities of peaks at 2\u03b8\u00a0=\u00a025.3\u00b0 (anatase) and 27.4\u00b0 (rutile). The Ni-based particle size (d\nNi(111)) was determined using the Scherrer equation (equation 1):\n\n(1)\n\n\n\nd\n\nN\ni\n\n\n\n111\n\n\n\n\n\n=\n\n\nK\n\u03bb\n\n\nB\nc\no\ns\n\n\n\n\u03b8\n\n\n\n\n\n\n\n\n\nwhere d\nNi(111) denotes the size of crystal domain determined by analyzing the Ni(111) peak, K is the Scherrer constant (0.94 for spherical crystals exhibiting a cubic symmetry), \u03bb denotes the wavelength (0.15418\u00a0nm for Cu K\u03b1\nave), B is the modified full width at half maximum (FWHM, rad) for the diffraction peak (calculated as ((FWHM)2 \u2013 (FWHM of bulk crystal, 0.2\u00b0 in this study)2)1/2), and \u03b8 denotes the Bragg angle corresponding to Ni (111).The electronic structures of catalysts were investigated using the high-performance X-ray photoelectron spectroscopy (XPS) technique using the VG Scientific ESCALAB 250 spectrometer (ThermoFisher Scientific Inc., Waltham, Massachusetts, USA).The atomic structure was imaged and the elemental distribution was determined using the HR-(S)TEM technique. The Talos F200X microscope (Thermo Fisher Scientific, Waltham, Massachusetts, USA) was used to record the images. A small amount of catalyst powder was dispersed in anhydrous ethanol. A significantly high dilution ratio was maintained and the dispersion was ultrasonicated for 1\u00a0h. Following this, the dispersion was mounted on a Holey carbon-coated copper grid (Electron Microscopy Sciences, Hartfield, Pennsylvania, USA; 200 square mesh; average opening size: 50 \u03bcm). The as-prepared TEM grid was further dried in air at 105\u00a0\u00b0C for 16\u00a0h. The sample was plasma etched to completely remove the residual organic layer.Temperature programmed methods and pulse chemisorption were used to characterize the catalysts using the BELCAT\u2013B system (MicrotracBEL, Osaka, Japan) equipped with a thermal conductivity detector (TCD) consisting of tungsten\u2013rhenium (W\u2013Re) filament. A quadrupole mass spectrometer (Qmass) equipped with a yttriated iridium (Y\u2013Ir) filament detector (BELMass) was also used for analyzing the samples.The H2 temperature programmed desorption (H2 TPD) results using solid samples were recorded. The catalyst (\u223c50\u00a0mg) was heated at 300\u00a0\u00b0C for 1\u00a0h under a flow of He (flow rate: 30\u00a0mL/min). Following this, the sample was reduced at 450\u00a0\u00b0C for 2\u00a0h under a flow of H2/Ar (5\u00a0vol%; flow rate: 30\u00a0mL/min), prior to bringing the temperature down to 50\u00a0\u00b0C under a flow of He (30\u00a0mL/min). The catalyst was heated from 50\u00a0\u00b0C to 900\u00a0\u00b0C (ramping rate: 10\u00a0\u00b0C/min) under a flow of He (30\u00a0mL/min). The quantity of desorbed H2 was measured using a TCD and identified using a mass spectrometer (at m/z\u00a0=\u00a02).Catalyst reducibility was determined by analyzing the H2 temperature programmed reduction (H2 TPR) results recorded for the catalysts. The as-prepared catalyst (\u223c50\u00a0mg) was heated at 300\u00a0\u00b0C for 1\u00a0h under a flow of Ar (30\u00a0mL/min). Following this, the temperature of catalyst was brought down to 50\u00a0\u00b0C. The catalyst was heated from 50\u00a0\u00b0C to 900\u00a0\u00b0C (ramping rate: 10\u00a0\u00b0C/min) under a flow of H2/Ar (5% (v/v); flow rate: 30\u00a0mL/min). The produced water was removed using a molecular sieve trap. The amount of H2 uptake per gram of the catalyst (mmolH2\u00b7gcat\n\u20131) was determined using the TCD method. This indicated the quantity of reduced sites.The temperature programmed oxidation (TPO) method was used to examine the surface/lattice oxygen vacancies present in the catalyst. The catalyst (\u223c50\u00a0mg) was heated from 50\u00a0\u00b0C to 900 \u00b0C (ramping rate: 10 \u00b0C/min) under a flow of O2/He (5% (v/v); flow rate: 30\u00a0mL/min). The quantity of desorbed O2 was measured following the TCD method. The produced water was removed using a molecular sieve trap.The results obtained using the H2 TPD and H2 TPR methods were used for determining the Ni metal dispersion (DNi, %) (equation (2)). DNi is defined as the ratio of the quantity of Ni surface atoms available for hydrogen adsorption to the quantity of Ni atoms present in the catalyst. The quantity of desorbed H2 (measured following the H2 TPD method) represents the quantity of active sites present to achieve the dissociative adsorption of H2 on the Ni NPs [30,31]. DNi can be calculated as follows:\n\n(2)\n\n\n\n\nD\n\n\nN\ni\n\n\n\n\n%\n\n\n=\n\n\n2\n\u00d7\n\n\nV\n\n\nH\n2\n\nT\nP\nD\n\n\n\u00d7\n\n\nM\n\n\nN\ni\n\n\n\u00d7\nS\nF\n\n\nm\n\u00d7\n\n\nW\n\n\nS\n\n\n\u00d7\n\n\nV\n\n\nm\n\n\n\u00d7\n\n\nd\n\n\nr\n\n\n\n\n\u00d7\n100\n\n\n\n\nwhere VH2 TPD denotes the volume of chemisorbed H2 (at STP) determined by analyzing the H2 TPD results, MNi denotes the atomic weight of Ni (58.69\u00a0g/mol), SF denotes the adsorption stoichiometric factor for the H to Ni molar ratio (H/Ni\u00a0=\u00a01 atom/atom), m represents the weight of catalyst (g), WS is the actual weight fraction of Ni in the catalyst determined using the ICP-OES technique, Vm is the molar volume of H2 (22414\u00a0mL/mol) at STP, and dr is the degree of reduction of Ni determined following the H2 TPR method. The theoretical dispersion for the Ni NPs (Dt, %) was calculated using equation (5). The size of Ni particle was determined using the TEM. A spherical model was assumed, and the value of Dt was calculated as follows:\n\n(3)\n\n\n\nD\nt\n\n\n\n\n%\n\n\n\n=\n\n\n3\n\u00d7\n\nM\n\nN\ni\n\n\n\u00d7\nS\nF\n\n\n2\n\u00d7\n\nN\nA\n\n\u00d7\n\nR\n\nN\ni\n\n\n\u00d7\n\n\u03c1\n\nN\ni\n\n\n\u00d7\n\u03c3\n\n\n\u00d7\n100\n\n\n\n\nwhere NA represents the Avogadro\u2019s number (6.02214\u00a0\u00d7\u00a01023 mol\u22121), RNi is the number-averaged radius of Ni NPs measured using the TEM, \u03c1Ni is the density of bulk Ni metal (8.908\u00a0g/mL), and \u03c3 is the atomic cross-sectional area of H atoms adsorbed on the Ni surface (0.0649\u00a0nm2). The results obtained using the H2 TPD (equation (2)) and HR-TEM (equation (3)) methods were analyzed to determine the surface coverage (\u03b8\nNi) on the Ni NPs. It was determined using equation (4) as follows:\n\n(4)\n\n\n\n\u03b8\n\nN\ni\n\n\n\n\n\n%\n\n\n\n=\n\n\n\n1\n-\n\n\n2\n\u00d7\n\u03c3\n\u00d7\n\nN\nA\n\n\u00d7\nn\n\u00d7\n\nm\n\nN\ni\n\n\n\n\n\nW\nS\n\n\u00d7\n\u03c0\n\u00d7\n\nd\n\nN\ni\n\n2\n\n\n\n\n\n\n\u00d7\n100\n\n\n\n\nwhere n is the quantity of desorbed H2 per mass of catalyst (mmol/g), mNi is the weight of a single Ni NP (g), and dNi represents the diameter of Ni NPs measured using the XRD [32\u201334]. Equation (5) was used to calculate mNi.\n\n(5)\n\n\n\nm\n\nN\ni\n\n\n=\n\n\u03c1\n\nN\ni\n\n\n\u00d7\n\n\n\n\n\n\u03c0\n\u00d7\n\nd\n\nN\ni\n\n3\n\n\n6\n\n\n\n\n\n\n\n\nThe CO diffuse reflectance infrared Fourier transform spectroscopy (CO DRIFTS) was used to analyze the samples and the experiments were performed at the UNIST Next-Generation Catalysis Center (Ulsan, Korea). A Nicolet iS10 Fourier transform infrared (FT-IR) spectrometer (ThermoFisher Scientific Inc., UK) equipped with a zinc selenide (ZnSe) DRIFT cell and mercury-cadmium-telluride detector was used to conduct the experiments. The samples were diluted with KBr and then pretreated at 100\u00a0\u00b0C (heating rate: 10\u00a0\u00b0C\u2219min\u22121) for 1\u00a0h under an atmosphere of He (flow rate: 60\u00a0mL\u2219min\u22121). CO adsorption was examined under a flow of 1% (v/v) CO/He (flow rate: 60\u00a0mL\u2219min\u22121) at room temperature. The data collection was allowed to proceed for 15\u00a0min.The HDO activities of prepared catalysts were studied using a custom-built SUS 316 batch reactor (Hanyang Precision, Gimpo, Korea). A mixture of guaiacol (2.50\u00a0g, 20.14\u00a0mmol) and the catalyst (0.25\u00a0g) was mixed in n-decane (50\u00a0mL) and the mixture was loaded into the reactor. The inner reaction system was purged with N2 (gas) three times prior to conducting the experiments. The system was pressurized (5\u00a0MPa; H2) at room temperature. The reaction was performed at the desired reaction conditions, and the reaction mixture was cooled down to 50\u00a0\u00b0C. A mixture consisting of the products and the catalyst was collected using a conical tube (50\u00a0mL). An aliquot (1\u00a0mL) of the fresh liquid product was filtered through a PTFE membrane syringe filter (pore size: 0.45\u00a0\u00b5m). Following this, the liquid was transferred to a volumetric flask (20\u00a0mL). An internal standard (n-dodecane; 1\u00a0\u03bcL) was added to the product mixture, and the mixture was further diluted using methanol for analysis using the gas chromatography (GC). The spent catalyst was recovered from the residual mixture of products following the vacuum filtration. The recovered catalyst was washed three times with acetone (25\u00a0mL each) to remove all traces of n-decane and organic compounds. The obtained solid was dried at 105\u00a0\u00b0C over 16\u00a0h, and the dried catalyst was denoted as the spent\u2013recovered catalyst (SC). The liquid products were characterized using the gas chromatography-mass spectrometry (GC\u2013MS). The GC\u2013MS system (Agilent 78900A; 5975C inert MS XLD with triple axis-detector) was equipped with an autosampler injector (Agilent 7860\u00a0N) and an HP\u20135MS capillary column (60\u00a0m\u00a0\u00d7\u00a00.25\u00a0\u00b5m\u00a0\u00d7\u00a00.25\u00a0mm ID). The liquid product was quantitatively analyzed using a gas chromatography\u2013flame ionization detector (GC\u2013FID, Hewlett 5890 Packard Series II, USA) equipped with an autosampler injector (6890 series injector) and HP\u20135MS capillary column (60\u00a0m\u00a0\u00d7\u00a00.25\u00a0\u00b5m\u00a0\u00d7\u00a00.25\u00a0mm ID). The catalytic activity was determined by analyzing the conversion (Xf\need,%), product yield (Yp\nroduct,%), and selectivity (Sp\nroduct,%). The parameters were determined as follows:\n\n(6)\n\n\n\n\nX\n\n\nf\ne\ne\nd\n\n\n\n\n\n%\n\n\n=\n\n\n\n1\n-\n\n\n\n\nn\n\n\nfeed\n\n\n\n\n\n\nn\n\n\nfeed\n\n\n0\n\n\n\n\n\n\n\n\u00d7\n100\n\n\n\n\n\n\n(7)\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\n(8)\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\nall\n\np\nr\no\nd\nu\nc\nts\n\n\n\n\n\u00d7\n100\n\n\n\n\nwhere \n\n\nn\n\nf\ne\ne\nd\n\n0\n\n\n is the number of mol of the feed compound before reaction and nfeed, \n\n\nn\n\np\nr\no\nd\nu\nc\nt\n\n\n\n, and \n\n\u2211\n\n\nn\n\n\nall\n\np\nr\no\nd\nu\nc\nts\n\n\n\n represent the number of mol of the residual feed compound, targeted product, and the total number of mol of all products after reaction, respectively.The cyclohexanol (CHNOL) production rate (rCHNOL, molCHNOL\u2219gNi\n\u22121\u2219h\u22121) was calculated as the ratio of the total number of mol of cyclohexanol produced per reaction time to the weight of Ni metal.\n\n(9)\n\n\n\n\nr\n\n\nC\nH\nN\nO\nL\n\n\n\n\n\n\nm\no\n\n\nl\n\n\nC\nH\nN\nO\nL\n\n\n\u2219\n\n\n\n\ng\n\n\nNi\n\n\n\n\n-\n1\n\n\n\u2219\n\n\nh\n\n\n-\n1\n\n\n\n\n\n=\n\n\n\n\nn\n\n\nC\nH\nN\nO\nL\n\n\n\n/\nr\ne\na\nc\nt\ni\no\nn\n\nt\ni\nm\ne\n\n\nm\n\n\u00d7\n\n\n\nW\n\n\nS\n\n\n\n\n\n\n\n\n\n\nThe activity of prepared catalyst was also described in terms of the site time yield of cyclohexanol (STYCHNOL). The mol of cyclohexanol produced per unit time was divided by the mol of active Ni species (determined by the H2 TPD and TPR methods) to determine STYCHNOL.\n\n(10)\n\n\nS\nT\n\n\nY\n\n\nC\nH\nN\nO\nL\n\n\n\n\n\n\n\nmol\n\n\nCHNOL\n\n\n\u00b7\n\n\n\n\nmol\n\n\nNi\n\n\n\n\n-\n1\n\n\n\u00b7\n\n\nh\n\n\n-\n1\n\n\n\n\n\n=\n\n\n\n\nn\n\n\nC\nH\nN\nO\nL\n\n\n\n/\nr\ne\na\nc\nt\ni\no\nn\n\nt\ni\nm\ne\n\n\n\n\n\nn\n\n\nN\ni\n\n\n\n\n=\n\n\n\n\nn\n\n\nC\nH\nN\nO\nL\n\n\n/\nr\ne\na\nc\nt\ni\no\nn\n\nt\ni\nm\ne\n\n\nm\n\n\u00d7\n\n\n\nW\n\n\nS\n\n\n\n\u00d7\n\n\n\nD\n\n\nN\ni\n\n\n\n/\n\n\n\nM\n\n\nN\ni\n\n\n\n\n\n\n\n\nA two-step reaction using BO was conducted. The first step involved the catalytic depolymerization of pyrolytic lignin fraction in the presence of 5\u00a0wt% Pd/C to obtain phenolic monomers and the second step involved the selective HDO of phenolic monomers using NiFe(3)/TiO2-HT. The second step led to the production of cyclic alcohols. The first step of reaction was performed in a batch reactor. A mixture of BO (6\u00a0g, BTG) and 5\u00a0wt% Pd/C (0.5\u00a0g) was used for the experiments. The mixture was dissolved in n-decane (50\u00a0mL) at 200\u00a0\u00b0C and the reaction was allowed to proceed for 1\u00a0h under 5\u00a0MPa (pressure generated by H2; measured at room temperature). Note that all H2 pressures depicted in this manuscript were measured at room temperature, not at the high reaction temperature. The extents of product recovery and separation achieved are depicted in Fig. S1. Four types of products (light oil (LO), water-soluble oil (WSO), heavy oil (HO), and solid residue (SR)) were recovered at the end of the first step. The fresh product containing a mixture of the liquid and solid products was collected from the reactor. Following this, the constituents were separated following the vacuum filtration. PTFE membrane filters (pore size: 0.45\u00a0\u03bcm) were used for filtration to obtain the filtrate and residue. The filtrate consisted of organic (decane-soluble) and aqueous (water-soluble) phases. The liquid\u2013liquid separation technique was used to separate the phases using a separatory funnel. The n-decane (organic) layer containing LO and the aqueous layer containing WSO formed two separate layers. These fractions were labeled BO-S1-LO and BO-S1-WSO, respectively. HO was present in the residue present in the upper part of the vacuum filtration setup. The layer could be dissolved in acetone, yielding the HO\u2013acetone soluble phase. The insoluble SR was present on the filter paper containing the catalyst and coke. The liquid filtrate appeared dark brown when washed with acetone. This indicated the presence of HO containing high-molecular-weight compounds. Acetone was removed at 40\u00a0\u00b0C using a rotary evaporator to recover oil. The recovered phase was labeled as BO-S1-HO. The solid residue, denoted as BO-S1-SR, was then dried at 105\u00a0\u00b0C over 16\u00a0h. In the second step, selective HDO was conducted to produce cyclic alcohols. In this step, BO-S1-LO (LO obtained from the first step during the conversion of BO was used in this step) was used as the reactant in the presence of the NiFe/TiO2\u00a0catalyst (0.5\u00a0g). The conditions used were similar to the conditions under which the model reaction was conducted (temperature: 270\u00a0\u00b0C; time: 1\u00a0h; pressure: 5\u00a0MPa (H2)). The method followed for product recovery and separation was similar to the method followed in the first step. The fresh LP and SR obtained were denoted as BO\u2013S2\u2013LP and BO\u2013S2\u2013SR, respectively.Ni/TiO2-based catalysts were selected for the HDO of biomass-derived compounds because they could selectively cleave C-O bonds present at the perimeter of Ni-TiOx interfacial sites [27,28]. The oxygen defects at the interface between Ni and Ti3+ have been reported to be the active centers that participate in HDO [27]. Chemical treatment (such as acid-base and redox reactions) methods can be used to generate the defects on the Ni metal, TiO2 support, or Ni-TiOx interface [35]. HDO usually necessitates the acid\u2013redox reactions [36\u201338]. Two synthetic strategies for the preparation of TiO2-supported Ni catalysts (namely IM and HT synthesis methods; Fig. 2\n(A)) were investigated to understand their effects on the strength of metal\u2013support interactions between Ni and TiO2. The strong electrostatic adsorption and HT treatment (SEA-HT) methods were combined to generate a highly dense interface between the metal and the support. The interface obtained under the conditions of the HT method was larger than that of the IM method. The adsorption of metal monolayer [39\u201341] and the ion-exchange method involving the metal cation complex precursors (on the negatively charged metal oxide surface containing defects) led to the formation of larger interace in the HT method [42]. Under these conditions, the surface chemical properties of supported metal catalysts could be tuned.To assess the accuracy of the presented hypothesis, the Ni crystal structures, their particle size, and H2 dissociation behavior were analyzed using XRD, HR-TEM, and H2 TPD. The XRD results revealed that the Ni crystal size of Ni/TiO2-HT was smaller (10.7\u00a0nm, calculated using the Scherrer equation) than that of Ni/TiO2-IM (117.2\u00a0nm) (entries 3\u20134; dNi, Table 1\n). These observations were further confirmed by the particle size distributions obtained using the HR-TEM (Fig. 3\n(A1) and (B1)). The average particle diameter of Ni NPs (14.7\u00a0nm) present in Ni/TiO2-HT was approximately 10 times smaller than that of Ni NPs present in Ni/TiO2-IM (140.1\u00a0nm). The small Ni particles in Ni/TiO2-HT were correlated with the high extent of H2 desorption determined by the H2 TPD results (0.013 mmolH2\u00b7gcat\n\u22121) and the high apparent Ni dispersion (DNi\u00a0=\u00a024.4% determined by the H2 TPD). Compared to Ni/TiO2-HT, the Ni dispersion for Ni/TiO2-IM was negligible (entries 3\u20134; DNi, Table 1).Under conditions of SEA treatment (Fig. 2(B)), the Ni precursors in the form of Ni(II) hexahydrate complex cations (Ni(H2O)6\n2+) were deposited on the surface of TiO2 support in the presence of NH4HCO3, leading to the formation of a negatively charged TiO2 surface (pH 9.0\u00a0>\u00a0PZC 6.2) [39]. It has been previously reported that metal cation hexaaqua complexes can be strongly adsorbed on the surface of TiO2, forming mono- and di-substituted complexes containing a bridging surface oxygen (inner-sphere complex). The (hydr)oxobridge connection formed the metal\u2013support interfacial region [40]. The Ti-O bonds present in the TiO2 support can be exploited to form the [Ti(OH)6]2\u2212 species under conditions of the HT method and high pH [43]. The complex can be further converted to the labile TiO6\n2\u2212 species on the TiO2 surface to generate the defects on the uppermost layer of TiO2. The NPs can be grown from the dispersed nuclei which were formed from Ni(H2O)6\n2+ (stabilized by the (hydr)oxobridges on the negatively charged TiO2 surface) [44]. The defects on the surface of TiO2 can be generated during the ion exchange between Ni(H2O)6\n2+ and surface Ti. Notably, the IM method at pH\u00a0\u223c5\u00a0<\u00a0PZC 6.2 (at room temperature and atmospheric pressure) formed the weak binding of metal precursor on the surface of TiO2 containing a smaller number of defects. The process forms metal particles that can easily aggregate during the reduction [45].The SEA-HT method was further used to synthesize the NiFe/TiO2 catalysts (exhibiting the ternary heterostructure). Under these conditions, the metal\u2013support interfacial interactions with the defect-containing support were generated [46,47]. The addition of Fe(III) precursor led to the formation of [Fe(H2O)6]3+ and Ni(H2O)6\n2+. Strong bonds were formed between the metal cation complex precursors and TiO2. It has been reported that aqua complexes containing Fe can be effectively deposited on the surface of TiO2 by forming tetradentate inner-sphere complexes [48]. In addition, the Ni2+ and Fe3+ cations can be deposited as NiFe-based hydroxide complexes. The mobile anions such as OH\u2013, HCO3\n\u2013, and CO3\n2\u2013 can be used to balance the charge under basic conditions. The deposition of bimetallic NiFe on the surface of charged and defect-containing surface of TiO2 follows a similar monolayer adsorption mechanism. The detailed mechanism of adsorption is explained in section 3.4.The pore structures and surface areas of monometallic Ni/TiO2 and bimetallic NiFe/TiO2 catalysts were examined by the N2 physisorption measurement. The adsorption\u2013desorption isotherms observed for the catalysts exhibited a weak hysteresis at p/p0\u00a0=\u00a00.9\u20131.0 (Fig. S2(A)), indicating the formation of inter-particular spaces between the TiO2 NPs. The formation of inter-particular spaces is reflected by the BJH pore size distributions (Fig. S2(B)) [49,50]. The measured BET surface areas revealed that HT treatment and the addition of metal components tuned the pore structures of TiO2. When TiO2 was subjected to the conditions of HT treatment in the absence of metals, a decrease in the BET surface area (56 to 44\u00a0m2\u22c5g\u22121) (SBET, Table 1) and an increase in the single point pore volume (0.19 to 0.44\u00a0cm3/g) were observed (Table S2). These results revealed that TiO2 (P25) was dissolved, and agglomerated TiO2 particles were formed (generating the inter-particular spaces or observed pores) under these conditions. The increase in the BET surface area (49\u201350\u00a0m2\u22c5g\u22121) and pore volume (0.48\u20130.56\u00a0cm3\u22c5g\u22121) in Ni/TiO2-HT (or NiFe/TiO2-HT) indicated that the presence of Ni and Fe precursors during the HT process influenced the nanoscopic pore structures of catalysts. The BET surface area of Ni/TiO2-IM was lower and the pore volume of Ni/TiO2-IM was higher than those measured for the HT-prepared catalysts.The crystal structures of catalysts, analyzed using the XRD, varied based on the synthetic methods followed and the Fe contents (Fig. 4\n(A)). Anatase and rutile phases of TiO2 were present in all the catalysts. The major peak corresponding to the (101) anatase TiO2 (PDF#21\u20131272) appeared at 2 \u03b8\u00a0=\u00a025.3\u00b0, and the major peak corresponding to the (110) rutile TiO2 (PDF#21\u20131276) appeared at 2 \u03b8\u00a0=\u00a027.4\u00b0. The presence of fcc Ni (PDF#04\u20130850) was observed in all the Ni-containing catalysts. The formation of fcc Ni (PDF#04\u20130850) was confirmed by the presence of distinct diffraction peaks at 2 \u03b8\u00a0=\u00a044.5\u00b0 (111) and 51.8\u00b0 (200) [51]. The results obtained using the Scherrer equation indicated that the Ni particles observed in the catalysts prepared following the HT method were smaller than those observed in the catalysts prepared following the IM method. When the monometallic Ni/TiO2-HT was analyzed, a broad Ni(111) peak was observed at 2\u03b8\u00a0=\u00a044.43\u00b0. The particle size was calculated to be 10.7\u00a0nm (entry 4; dNi, Table 1). When Ni/TiO2-IM was analyzed, sharp diffraction peaks corresponding to Ni(111) and Ni(200) were observed, indicating the presence of bulky Ni metal (entry 3; dNi, Table 1). The crystal structures of TiO2 were adjusted by the metal deposition methods (SEA-HT or IM) followed during the synthesis. The weight fraction ratio (for the anatase and rutile phases) measured for Ni/TiO2-HT (5.33 w/w) was higher than the anatase-to-rutile weight fraction ratio measured for Ni/TiO2-IM (2.65 w/w) (entries 3\u20134; WA/WR, Table S3). The anatase structure was preferentially formed at temperatures below approximately 600\u00a0\u00b0C. The acceleration in the rate of formation of anatase can be attributed to the significant reconstruction of TiO2 (in the presence of Ni precursor) observed during the HT process. Under these conditions, dissolution, ion exchange, and recrystallization of TiO2 could occur.The formation of Ni was confirmed for the bimetallic NiFe/TiO2-HT catalysts. The permeation of Fe into the bulk Ni structures (leading to the possible formation of alloys) was also validated by a decrease in the 2\u03b8 value and an increase in the d-spacing when the Fe content was increased. The addition of Fe to form NiFe/TiO2-HT led to a shift in the peak (from 2\u03b8\u00a0=\u00a044.43\u00b0 (0\u00a0wt% Fe) to 44.29\u00b0 (5\u00a0wt% Fe)) corresponding to Ni(111). The peak corresponding to Ni(200) shifted from 2\u03b8\u00a0=\u00a051.82\u00b0 (0\u00a0wt% Fe) to 51.62\u00b0 (5\u00a0wt% Fe) with an increase in the Fe content from 0\u00a0wt% to 5\u00a0wt% (entries 4\u20137; 2\u03b8, Table 1). The peaks corresponding to NiFe(111) and NiFe(110) appear at 2\u03b8\u00a0=\u00a043.7\u00b0 and 50.9\u00b0, respectively. The positions of diffraction peaks characterizing NiFe/TiO2-HT indicate the formation of a transient structure (between Ni and NiFe crystals). The atomic distribution of Fe in the Ni particles was also confirmed. The diffraction peaks corresponding to the Fe oxides in NiFe/TiO2-HT were of low intensity, indicating that the crystalline Fe oxides were present in low concentrations. It also indicated the low extent of segregation of the Fe species including Fe oxides. In Fe(15)/TiO2-HT, the formation of ilmenite FeTiO3 (PDF#29\u20130733) (alloyed Fe oxides with TiO2) was observed, and the formation could be attributed to high Fe loading. The formation of ilmenite FeTiO3 (PDF#29\u20130733) was not observed under conditions of low Fe loading (3\u00a0wt% Fe, Fig. S3(C)). The absence of FeTiO3 phase and inflated Ni structures in the NiFe/TiO2-HT catalysts indicate that the Fe species preferred to form alloys with Ni (over NiFe or Fe-Ti oxides) when subjected to the conditions of HT method [52].When the formation of supported NiFe was attempted using other oxide supports such as SiO2, \u03b3-Al2O3, and Nb2O5, the formation of Ni metals or NiFe alloys was not observed. The diffraction peaks corresponding to Ni or NiFe were not observed under these conditions (Fig. 4(B)). The segregation of Ni and Fe species on the Al2O3 support was observed using energy dispersive X-ray spectroscopy (EDS) mapping and HR-(S)TEM (Fig. S4(A)). The XPS results confirmed the absence of metallic Ni on the Al2O3 support (Ni 2p in Fig. S5(B)). These observations indicate that the formation of NiFe alloy was not favored when Al2O3 was used as the support. The XRD results observed for NiFe/ZrO2 prepared following the HT method indicated the formation of monoclinic ZrO2 and well-developed large Ni metal crystals. These results were also validated by the TEM images (Fig. S4(B)) and the XPS results (Ni 2p in Fig. S5(B)). These observations indicate that the Fe-doped Ni particles can be formed on the surface of reducible metal oxides such as TiO2 and ZrO2 rather than that of non-reducible metal oxides.The TEM images of NiFe/TiO2-HT confirmed the formation of Fe-doped Ni cores and FeOx shells on the TiO2 support (Fig. 3). Overlayers formed of TiO2 were observed on the Ni NPs, indicating the presence of strong interactions between TiO2 and Ni. The formation of the overlayers and the generation of interactions were validated by the results obtained using the HR-TEM coupled with HAADF-STEM and EDS (Fig. 3). The structural morphology and elemental distribution were also studied. Large Ni NPs with an average Ni particle size of 140.2\u00a0nm were observed (in Ni/TiO2-IM). The result agreed well with the corresponding XRD results (Fig. 4(A)). The presence of TiO2 on the surface of Ni NPs was also observed (Fig. 3(A4)). A uniform distribution of small Ni NPs (average particle size: 14.7\u00a0nm) was observed when Ni/TiO2-HT was studied (Fig. 3(B)). The HR-TEM images of Ni/TiO2-HT revealed the presence of overlayer lattices. The d\nTEM-spacings (lattice spacings measured using the TEM images) were 2.03 and 3.51\u00a0\u00c5 for the Ni NPs and anatase TiO2(101), respectively (Fig. 3(B2)). The formation of amorphous TiO2 on the surface of Ni NPs (Fig. 3(B4)) can be attributed to the formation of Ni(OH)2-Ti(OH)4 precipitates (following the HT method) on the surface of TiO2. This led to the formation of TiOx clusters on the surface of Ni. The unique structural morphology of Fe species and core\u2013shell structures consisting of NiFe cores and NiFe-FeOx shells were observed when NiFe/TiO2-HT was studied. The formation of core\u2013shell structures was proved using the EDS mapping (Fig. 3(C5, C6(iv), and (v)). The core\u2013shell structure was not observed when other supports (Nb2O5, SiO2, Al2O3, and ZrO2) were used, which can be attributed to the poor formation of Ni species (for NiFe/SiO2-HT and NiFe/Al2O3-HT as observed for the poor XRD peaks) and the permeation of Fe into Ni particles (for NiFe/Nb2O5-HT and NiFe/ZrO2-HT as observed for the shift of Ni peaks of XRD results). The average Ni particle size measured for NiFe/TiO2-HT was 19.3\u00a0nm (Fig. 3(C1)), which was larger than the average Ni particle size measured for monometallic Ni/TiO2-HT. This could be attributed to the Fe-containing shells present on the surface of Ni NPs. The formation of core\u2013shell structure with Ni cores (m/z\u00a0=\u00a059 (Ni)) and NiFe shells (m/z\u00a0=\u00a056 (Fe), 48 (Ti)) was also confirmed from the HR-STEM images (Fig. 3(C6(i and ii)). The interplanar spacings were analyzed in detail (Fig. 3(C6(iii))). The lattice spacings on the NiFe nanoparticle surface were calculated using the inverse fast Fourier transform technique. The results revealed that the d\nTEM-spacings of 0.179\u00a0nm and 0.208\u00a0nm were measured for the core area (area a) of the tetrataenite NiFe phase (PDF#47\u20131417) or the Fe-doped fcc Ni particles. A large d\nTEM-spacing of 0.239\u00a0nm was measured for the shell (area b, 1.825\u00a0nm thick) of Fe2O3 phase (PDF#39\u20131088) and non-stoichiometric FeOx. The EDS mapping technique was used to analyze NiFe/TiO2-HT, and the results indicated the presence of highly dispersed Fe species on TiO2 (Fig. 3(C3)). The lattice distance (3.53\u00a0\u00c5) measured for the TiO2 phase using the HR-TEM was slightly larger (Fig. 3(C2)) than the lattice distance measured for the monometallic Ni/TiO2-HT. This indicates that the incorporation of Fe species into TiO2 was promoted by the strong interaction present between Fe and TiO2. Ni-free Fe(3)/TiO2-HT was analyzed using the XRD (Fig. S3(C)), and the results indicated the formation of amorphous Fe2O3 at the perimeter of TiO2 support, indicating the presence of FeOx\u2013TiO2 interface at the surface of NiFe(3)/TiO2-HT.Interestingly, the presence of amorphous TiO2 clusters was not observed on the NiFe NP surface. This indicated that Fe-containing shells that wrapped the Ni NPs suppressed the generation of strong Ni-TiO2 interactions [53]. The formation of these complex structures is attributed to the formation of mixed NiFe-OH precipitate. The Ti4+ species present on the TiO2 surface favored ion-exchange reactions with Fe3+ (over Ni2+) during the HT process [42]. Improved apparent dispersion and an increase in the fraction of surface-exposed Ni atoms (in NiFe NPs) at the FeOx-TiO2 anchoring sites were observed. In addition, it was observed that the surface coverage of Ni (\u03b8\nNi, Table 1) increased from 82.6% to 89.8% when the Fe loading was increased from 1\u00a0wt% to 5\u00a0wt%. This indicated that the Ni and Fe species strongly interacted with each other. The results confirmed the formation of Fe-doped fcc Ni NPs surrounded by the FeOx shells. The addition of Fe to the Ni cores led to a slight shift in the XRD peaks corresponding to fcc Ni. The peaks indicated the formation of NiFe alloys and transient crystals of Fe-doped Ni NPs. The formation of these complex core\u2013shell structures following a simple deposition method under conditions of mild heat treatment (< 450\u00a0\u00b0C) can be achieved in the absence of structure-directing agents, organic surface modifiers [54\u201357] and carbonaceous precursors [58\u201360]. Although these have been discussed in the literature reports, papers reporting the results obtained following the process reported herein are rare. The literature reports do not discuss the results in detail, and hence the results reported herein could not be compared for effective validation.The chemical compositions and electronic states of the surface atoms present on the catalysts were observed using the XPS. Peaks corresponding to Ni 2p, Fe 2p, Ti 2p, and O 1s were observed (Fig. S5(A)). The Ni 2p3/2 peaks obtained for all the catalysts were deconvoluted into peaks corresponding to Ni0 (852.4\u00a0eV), Ni2+(856.0\u00a0eV), and satellite peaks (861.0\u00a0eV) [61,62]. The binding energy of Ni0 (852.5\u00a0eV) in Ni/TiO2-HT was comparable to that of Ni0 in Ni/TiO2-IM and the binding energy corresponding to Ni2+ in Ni/TiO2-HT (855.7\u00a0eV) was lower than the binding energy of Ni2+ in Ni/TiO2-IM (856.0\u00a0eV). This indicated that the Ni2+ ions present on the surface of Ni NPs (Ni/TiO2-HT) were partially reduced by the adjacent TiO2 species while the Ni0 ions present in the core of Ni NPs were not significantly adjusted by TiO2. The Ti 2p binding energy for Ni/TiO2-HT (458.5\u00a0eV) was lower than the Ti 2p binding energy for Ni/TiO2-IM (458.7\u00a0eV) (Fig. 5\n(A) and Table 1). This indicated the formation of Ti3+ (and not Ti4+). These observations indicate that the presence of oxygen vacancies on the TiO2 surface improved the transfer of electrons from the surface Ti4+ to Ni2+. The oxygen vacancies on the TiO2 surface maintain charge balance. The charge imbalance can be attributed to the substitution of Ti4+ by Ni2+ at the octahedral sites of TiO2 structure [63]. The absence of octahedrally coordinated O2\u2013 anions can potentially lead to a reduction in the extent of charge transfer (usually from Ni2+ to O2\u2013), leading to a decrease in the binding energy of Ni2+.O 1s peaks were adjusted by the electronic states of Ni and Ti. The O 1s spectra can be deconvoluted into three parts corresponding to three different oxygen species (lattice oxygen (OL, 529.1\u00a0eV), oxygen vacancy (OV, 530.8\u00a0eV), and chemisorbed oxygen (OC, 532.2\u00a0eV)) [64]. The spectral profile revealed that the OV/OL ratio measured for Ni/TiO2-HT (0.24) was higher than that measured for Ni/TiO2-IM (0.13). This indicated that the number of oxygen vacancies in Ni/TiO2-HT was higher than the number of oxygen vacancies in Ni/TiO2-IM. These observations further suggest that the ion-exchange reaction between Ni2+ and Ti4+ in the TiO2 crystal during the HT process led to the formation of a large number of oxygen vacancies, which in turn led to the formation of strong bonds between Ni and Ti. The interfacial charge transfer (from the electron-rich Ti\u03b4+ (\u03b4\u00a0<\u00a04)species to the Ni species (Ni\u03b4\u2013 on the Ni NP surface)) led to the formation of Ni\u03b4\u2013\u2013OV\u2013Ti\u03b4+. We also studied the effects of Fe species on the selective HDO involving NiFe/TiO2-HT. The Fe 2p results for Fe(3)/TiO2-HT (a control catalyst) revealed the presence of peaks corresponding to Fe2+, Fe3+, and Fe satellite at 709.7, 712.6, and 716.8\u00a0eV, respectively, and the peak corresponding to Fe0 was not observed. The slight shift in the binding energy of Fe2+ toward the lower binding energy region (\u223c0.6\u00a0eV) indicated that electron transfer occurred from Ti4+ to Fe2+. The charge transfer could be attributed to the strong interaction between Fe and TiO2, hinting toward the formation of Fe\u2013OV\u2013Ti (FeOx-TiO2) in Fe/TiO2-HT.The Fe 2p results of the bimetallic NiFe/TiO2-HT confirmed the electron transfer between the Ni, Fe, and Ti species. The Fe 2p3/2 results of the NiFe/TiO2-HT catalysts revealed the presence of peaks corresponding to Fe0 in the range of 705.3\u2013705.9\u00a0eV and Fe2+ in the range of 713.8\u2013715.3\u00a0eV. The peaks corresponding to Fe2+ and Fe3+ present in NiFe(1)/TiO2-HT and NiFe(3)/TiO2-HT shifted to the lower binding energy regions, indicating the partial reduction of Fe oxides (to form Fe\u03b4+, \u03b4\u00a0<\u00a03). The reduction was promoted by the electron transfer from the adjacent Ni species. This was confirmed by the shift in the binding energy (by 0.6\u00a0eV) of Ni0 (Ni/TiO2-HT) to the region characterizing Ni\u03b4+ (NiFe/TiO2-HT) (Ni 2p, Fig. 5(A)). The interaction between the Ni and Fe species can also be confirmed by the formation of NiFe alloy (at 450\u00a0\u00b0C) in the presence of NiFe(OH)x precursor under an environment of H2 (during the preparation of NiFe/TiO2-HT) [65]. The formation of FeOx shell on the surface of Ni core suppressed the generation of direct Ni-TiO2 interaction. The formation of the shell was confirmed using the HR-(S)TEM (Fig. 3(C6(iii))). It was observed that surface oxygen vacancies were present on the NiFe/TiO2-HT catalysts. The maximum OV/OL ratio of 0.32 was measured for NiFe(3)/TiO2-HT. The corresponding OV/OL ratio measured for NiFe(1)/TiO2-HT was 0.28 and that for NiFe(5)/TiO2-HT was 0.20. The fractions of surface oxygen vacancies measured for NiFe(3)/TiO2-HT catalysts were higher than that measured for monometallic Ni/TiO2-HT (OV/OL\u00a0=\u00a00.27). This could be attributed to the formation of surface oxygen vacancies at the NiFe-TiO2 interface (Fe\u2013OV\u2013Ti) and FeOx (Fe\u2013OV\u2013Fe) sites (NiFe/TiO2-HT, Fig. 5(B)). These observations further indicate that the presence of partially reduced Fe\u03b4+ (\u03b4\u00a0<\u00a03) in the bimetallic NiFe/TiO2-HT catalysts led to the formation of iron oxide (FeOx) containing coordinatively unsaturated Fe sites that can promote the selective HDO reaction. The results were confirmed using the XRD and HR-(S)TEM results.The reduction behaviors of Ni, Fe, and TiO2 were investigated using the H2 TPR method (Fig. 6\n(A)) and by analyzing H2 uptake (Table 1) to understand the metal oxide\u2013metal interaction in NiFe/TiO2 catalysts. The monometallic Ni/TiO2-IM catalyst was studied, and the presence of two primary reduction peaks was observed. One of the peaks appeared in the temperature range of 230\u2013250\u00a0\u00b0C, while the other appeared at 412\u00a0\u00b0C. A significantly low H2 uptake of 0.003 mmolH2\u00b7gcat\n\u20131 was measured. The peaks corresponded to the reduction of NiO to Ni0 weakly and strongly deposited on TiO2, respectively [32]. A strong and broad reduction peak spanning the range of 210\u00a0\u00b0C to 310\u00a0\u00b0C (representing the reduction of Ni2+ to Ni0) was observed for Ni/TiO2-HT. The peak temperature was observed to be 285\u00a0\u00b0C and the H2 uptake was 1.21 mmolH2\u00b7gcat\n\u20131. The H2 TPR results for the monometallic Fe(3)/TiO2-HT presented two-step reduction peaks at 292\u00a0\u00b0C (for the conversion of Fe2O3 to Fe3O4) and 735\u00a0\u00b0C (for the conversion of Fe3O4 to Fe) (Fig. S6) [66]. The lower reduction temperature observed for the first step (292\u00a0\u00b0C vs.\u00a0\u223c\u00a0450\u00a0\u00b0C reported in the literature) for Fe(3)/TiO2-HT indicates that H2 was consumed by the tiny Fe2O3 particles present on the surface of TiO2 support. This was reflected by the presence of peaks of low intensities in the XRD results (Figs. S3(C)). These observations also confirmed that the HT method could be followed to promote the formation of small Ni and Fe NPs that interacted strongly with the reducible TiO2 (anatase) phase [67]. The incorporation of Fe into the Ni/TiO2 structure decreased the reduction temperature (for the reduction of Ni2+ to Ni0) to 250\u00a0\u00b0C. The H2 uptake measured under these conditions (1.21 mmolH2\u2219gcat\n-1) was higher than that measured for the monometallic Ni/TiO2-HT. This indicates the presence of Ni2+ species that can be easily reduced because of the presence of a large number of oxygen vacancies on the bimetallic NiFe/TiO2-HT catalyst [68,69] allowing the effective reduction of NiO present in the neighborhood of oxygen vacancies [69].The presence of the second reduction peak (for Fe2O3) at 300\u00a0\u00b0C (or the presence of the peaks in the high-temperature region) and the low H2 uptake measured for NiFe/SiO2 and NiFe/Al2O3 indicate that the oxygen vacancies improve the efficient reduction of Ni2+. With an increase in the Fe content, the peak corresponding to Ni2+ reduction shifted to the higher temperature region. The shift can be attributed to the fact that Fe oxide present in close proximity to Ni lowers the reducibility of Ni. Interestingly, the reduction peaks appearing in the temperature range of 450\u2013700\u00a0\u00b0C were not observed for NiFe/TiO2-HT. This suggested that the reducibility of Fe oxides increased under the effect of hydrogen spillover observed in the Ni NPs located at the core\u2013shell interfaces [70]. These observations further confirmed that the NiFeOx core\u2013shells dispersed on the reducible TiO2 moieties tuned the hydrogen adsorption activity of metal.The hydrogen adsorption behavior was further studied using the H2 TPD-MS measurement (at m/z\u00a0=\u00a02) to study the activity and determine the quantity of hydrogen atoms adsorbed on the catalysts (Fig. 6(B)). Ni/TiO2-IM exhibited negligible H2 desorption, and this could be attributed to the presence of large Ni particles. Large peaks corresponding to H2 desorption appeared at 150\u00a0\u00b0C for Ni/TiO2-HT, indicating the presence of a large number of Ni active sites that could participate in the hydrogenation. These observations indicate that the HT method can be followed to increase the activity of Ni NPs during HDO. The H2 desorption peak observed for NiFe/TiO2-HT shifted to the higher temperature region (temperature\u00a0\u2264\u00a0250\u00a0\u00b0C), indicating the presence of strong bonds between hydrogen and Fe-incorporated Ni.The TPO method was also used to confirm the presence of oxygen vacancies on the catalysts (Fig. 6(C)). An intense O2 consumption peak was not present for TiO2 (P25). A peak of low intensity, corresponding to O2 consumption (at\u00a0\u2265\u00a0700\u00a0\u00b0C), was observed for TiO2-HT. The presence of the peak indicated the generation of oxygen vacancies in hydrothermally treated TiO2 (P25). The peak corresponding to O2 consumption appeared at 260\u00a0\u00b0C for Ni/TiO2-HT during the Ni oxidation. The peak position was lower than the peak position observed for Ni/TiO2-IM (450\u00a0\u00b0C). These observations indicate that the HT method can be followed to effectively activate the surface oxygen atoms at low temperatures. Under these conditions, the nearby Ni atoms were oxidized. A slight shift of the peaks corresponding to O2 consumption (for NiFe/TiO2-HT) toward the high temperatures regions indicates that the formation of NiFe alloys proceeds under conditions of thermal activation.The CO DRIFTS was used to further investigate the geometry of catalyst surface. Adsorption bands for monometallic Ni/TiO2 and bimetallic NiFe/TiO2 appeared in the regions spanning 2070\u20132020\u00a0cm\u22121 (linearly adsorbed CO) and 2000\u20131950\u00a0cm\u22121 (bridged CO) (Fig. 6(D)). The bands corresponding to the CO molecules adsorbed on the low-coordination Ni atoms appeared at 2063\u00a0cm\u22121\n[32], and the bands corresponding to the CO molecules adsorbed on the Ni tetracarbonyl species (Ni(CO)4) appeared at 2057\u00a0cm\u22121. The peak at 2050\u00a0cm\u22121 was attributed to the CO molecules linearly adsorbed on the Ni atoms (CO-Ni0), and the peak at 2035\u00a0cm\u22121 was attributed to the CO molecules adsorbed on the negatively charged Ni (CO-Ni\u03b4\u2212) [32]. Notably, the intensity of the band corresponding to CO-Ni\u03b4\u2212 appearing for Ni/TiO2-IM was lower than the intensities of the bands appearing for Ni/TiO2-HT and NiFe/TiO2-HT. This indicated that a large number of CO molecules could be adsorbed by the Ni\u03b4\u2212 present in the catalysts prepared by the HT method. Because the electron transfer from TiO2 to Ni at the Ni-TiO2 interface leads to the formation of electron-rich Ni\u03b4\u2212\n[71], the extent of back donation of electrons from Ni\u03b4\u2212 to the adsorbed CO can be improved, leading to the formation of strong Ni-CO bonds at the Ni\u2013OV\u2013Ti interfacial sites [32]. The process also leads to the generation of high-intensity bands corresponding to CO-Ni\u03b4\u2212.The bands appearing at\u00a0\u223c1995\u00a0cm\u22121 and\u00a0\u223c1983\u00a0cm\u22121 were attributed to the adsorption of bridged CO molecules on the highly coordinated Ni surface. The small intensity of the peak appearing at 1995\u00a0cm\u22121 revealed that negligible amounts of bridged-O species were adsorbed on the Ni surface present in Ni/TiO2-IM. The peak at 1995\u00a0cm\u22121, for Ni/TiO2-HT and NiFe/TiO2-HT, red-shifted toward 1993\u00a0cm\u22121. The intensity of the new peak was higher than that of the peak at 1995\u00a0cm\u22121. This indicated that a large number of CO molecules were adsorbed on the Ni\u03b4\u2212 surface present at the Ni\u2013OV\u2013Ti interfacial sites.Based on the results, it was inferred that isolated and terrace Ni sites were present in Ni/TiO2-HT. The result for NiFe/TiO2-HT contained adsorption peaks corresponding to linear and bridged CO molecules. As the Fe content in NiFe/TiO2-HT was increased, the intensity of the adsorption peaks at 1993 and 1983\u00a0cm\u22121, corresponding to bridged CO, decreased. The peak positions blue-shifted toward 1995\u00a0cm\u22121. These observations indicated that Fe encapsulation led to a reduction in the number of bridged CO adsorption sites. The process also improved the extent of electron transfer (from Ni0 to Fe). These results confirmed the presence of well-distributed FeOx species on the Ni surface. It was also hypothesized that electron transfer could potentially proceed at the Ni-Fe interface. The results agreed well with the XPS and HR-TEM results. The presence of multiple Ni geometries tuned the extent of CO adsorption achieved. This can further adjust the efficiency of HDO of the lignin-derived oxygenates.The interfacial sites present in the catalysts have been described in Fig. 5(B). The sites were analyzed based on the characterization results. It was observed that the ease of formation of the anatase form of TiO2 in Ni/TiO2-HT and NiFe/TiO2-HT was higher than the ease of formation of the anatase form of TiO2 in Ni/TiO2-IM. Compared to the rutile form, it was easier to reduce the anatase form of TiO2\n[72\u201374]. Better HDO activity was observed with the anatase form [74]. It was observed that the amorphous Fe oxides were highly dispersed on the surface of the TiO2 present in Fe(3)/TiO2-HT, which was devoid of Ni. The tiny amorphous Fe oxide clusters were composed of reduced Fe2+ species. Negligible amounts of Fe2O3 and FeTiO3 were present and a strong metal\u2013support interaction with the TiO2 support was generated (Fig. S3(B)). Two types of oxygen vacancies can be generated at the Fe-TiO2 interface: (i) Fe2+\u2013OV\u2013Ti3+ vacancies are formed during the cation exchange process (followed by the reduction step) between Ti4+ and Fe3+ during the HT method and (ii) Fe\u03b4+\u2013OV\u2013Fe\u03b4+ were formed during the reduction process [75] (Fe/TiO2-HT, Fig. 5(B)). For monometallic Ni/TiO2-HT, small Ni NPs on the surface of TiO2 form strong metal\u2013support interactions. A small amount of metallic Ni was observed on the terrace of the Ni NPs (Fig. 6(D)). Ni atoms at the edges or isolated sites were predominant in the Ni/TiO2-HT and NiFe/TiO2-HT catalysts. The Ni\u03b4\u2212\u2013OV\u2013Ti\u03b4+ oxygen vacancies (here, Ni\u03b4\u2212 species are electron-rich) (Ni/TiO2-HT, Fig. 5(B)) were formed during the cation exchange process between Ti4+ and Ni2+ when the HT method was used for fabrication. The cation exchange process was followed by the reduction process. Although the presence of amorphous TiOx was observed on the surface of the Ni NPs (Fig. 3(B4)), the formation of Ti\u03b4+\u2013OV\u2013Ti\u03b4+ was hindered by the presence of the electron-rich Ni NPs. The well-developed core\u2013shell NiFeOx NPs were dispersed on the surface of the reducible TiO2 support present in NiFe/TiO2-HT. Highly dispersed oxophilic centers were formed, which closely interacted with Ni NPs and the TiO2 support. The formation of NiFe and Fe-TiO2 interfacial sites was observed, but the formation of the Ni-TiO2 interfacial sites was not observed (NiFe/TiO2-HT, Fig. 5(B)). The generation of the surface oxygen vacancies at the NiFeOx interface can be potentially attributed to the phenomenon of hydrogen spillover. The generation of the vacancies at the Fe-TiO2 interface may help compensate for the charge imbalance attributable to the partial substitution of Ti4+ by Fe3+.To achieve the selective conversion of guaiacol to cyclohexanol, the active catalytic sites must be controlled during the demethoxylation of Ar-OCH3 and the hydrogenation of aromatic rings. NiFe(3)/TiO2-HT exhibited excellent catalytic activity with high STYCHNOL (182.7 molCHNOL\u00b7molNi\n\u22121\u00b7h\u22121). The rate of production was 0.49 molCHNOL\u00b7gcatalyst\n\u22121\u00b7h\u22121. This is the maximum rate reported to date at reaction temperature below 300\u00a0\u00b0C (Table S1). The selectivity achieved was 85% and almost all the guaiacol molecules could be converted to the desired product at 270\u00a0\u00b0C under 5\u00a0MPa of H2 (measured at room temperature) (Fig. 7\n(A); Xfeed and Sp\nroduct, Table 1). HDO of guaiacol was allowed to proceed at 270\u00a0\u00b0C under 5\u00a0MPa (H2). The reaction was allowed to proceed for 1\u00a0h using the model catalysts to understand the roles of NiFe/TiO2-HT on the selectivity of the cyclohexanol formation (demethoxylation\u2013hydrogenation) reaction.Monometallic Ni/TiO2-HT exhibited a high HDO activity (Fig. 7(A)). The STYCHNOL was 95.6 molCHNOL\u00b7molNi\n\u22121\u00b7h\u22121 and the selectivity was 60% (guaiacol conversion: 97.2%; entry 4, Table 1). When the reactions were performed using monometallic Ni/TiO2-IM, the cyclohexanol yield was 17.9% and the guaiacol conversion was 39.8% (entry 5, Table 2\n). In the absence of catalysts, negligible guaiacol conversion was observed, indicating that metals are required for effective conversion (entries 1\u20133, Table 2). The characterization results revealed that high Ni dispersion (DNi\u00a0=\u00a024.4%) and low surface coverage (\u03b8\nNi\u00a0=\u00a081.1%) could be achieved using Ni/TiO2-HT. The Ni dispersion was greater and the surface coverage achieved was lower than those achieved using the Ni/TiO2-IM catalysts. The good guaiacol conversion (to cyclohexanol) in the presence of Ni/TiO2-HT could be attributed to the presence of abundant HDO active centers in the system. The generation of a large number of active centers could be attributed to the close interaction existing between the Ni NPs (during hydrogenation) and the oxophilic centers (Ni\u03b4\u2212\u2013OV\u2013Ti\u03b4+) (during demethoxylation) present at the Ni-TiO2 interface. The low cyclohexanol yield achieved using Ni/TiO2-IM could be attributed to the presence of the inactive Ni surface covered by the TiO2 overlayers and the low density of the oxophilic centers participating in HDO.The effects of Ni loading on the efficiency of the Ni/TiO2-HT catalyst were investigated. It was observed that the cyclohexanol yield could be potentially improved by increasing Ni loading. An increase in the Ni loading helps promote the demethoxylation reaction by forming active centers for HDO at the Ni-TiO2 interface. When the Ni loading was increased (Fig. S7(A)), the cyclohexanol yield increased (yield range: 55.0\u201360.0%; at 15\u201320\u00a0wt% Ni). Under these conditions, the yield of 2-methoxycyclohexanol reached a constant (yield range: 16.6\u201320.0%; at 10\u201320\u00a0wt% Ni), but the quantity of hydrogenating Ni increased. Significant changes in the product yield and guaiacol conversion were observed when the Ni loading was increased (loading\u00a0\u2264\u00a015\u00a0wt%). When the Ni loading was increased to 20\u00a0wt%, significant changes in product selectivity were not observed. However, under these conditions, the yield of cyclohexane increased.The HDO of guaiacol was performed using Fe/TiO2-HT (in the absence of Ni) to understand the roles of Fe. The catalyst helped achieve high 1,2-dimethoxybenzene selectivity (80%) and 9.8% guaiacol conversion (Fig. 7(A); entry 4, Table 2). Other compounds such as 1,2-benzenediols (7), phenols (2), methylated phenols (8), and methanol were formed in less amounts (Fig. S8(A)). The selective formation of 1,2-dimethoxybenzene (6) indicates that transmethylation is favored in the presence of activated methyl (\u2013CH3) radicals which are formed during the demethoxylation of guaiacol.During the demethoxylation, guaiacol is converted to phenol (or catechol). During this process, \u2013CH3 radicals are adsorbed on the surface of the Fe/TiO2 catalyst (Fig. 8\nC(i) and S8(B)). The CH3 radicals present on the catalyst surface can be used for transmethylating the (i) adsorbed guaiacol molecules to form 1,2,-dimethoxybenzene (7) and (ii) aromatic rings of phenols to produce methylated phenols (9) (Fig. 1(A)). The cleavage of Ar-OCH3 can potentially take place at the oxygen vacancies present at the perimeter of the reduced FeOx site. The cleavage follows the reverse Mars\u2013van Krevelen mechanism [75]. The Fe species influence the efficiency of conversion of guaiacol. The formation of small Fe oxide particles containing reduced Fe2+ species was observed using the XRD (Fig. S3(C)) and XPS results (Fe 2p, Fig. 5). The results suggest the presence of highly dispersed oxophilic centers at the oxygen vacancies present in Fe oxides. The demethoxylation reactions conducted using the NiFe/TiO2 catalysts were promoted under these conditions.As highly dispersed oxophilic centers were present in the Fe species and the Ni/TiO2-HT catalyst (15\u00a0wt% Ni and 1\u20135\u00a0wt% Fe) exhibited good hydrogenation activity, the catalysts were used for the HDO of guaiacol. An increase in the cyclohexanol yield was observed as the Fe content was increased. The STYCHNOL (182.7 molCHNOL\u2219molNi\n\u22121\u2219h\u22121) achieved using 3\u00a0wt% Fe (1.5\u00a0wt% measured Fe content; entry 6, Table 1) was two-fold higher than that achieved using the monometallic Ni/TiO2-HT. As depicted in Fig. S7(B), the yield of 2-methoxycyclohexanol decreased (from 19.1% to 0.8%) as the Fe loading was increased (from 0\u00a0wt% to 5\u00a0wt%). The cyclohexanol yield increased (from 72.6% to 87.9%) with an increase in the Fe loading.A low HDO activity was observed for NiFe(5)/TiO2-HT (cyclohexanol yield: 72.1%; guaiacol conversion: 88.4%; entry 12, Table 2). The low activity hindered the formation of 2-methoyxcyclohexanol (following the direct hydrogenation) from guaiacol. The lower HDO activity of NiFe(5)/TiO2-HT (compared to the activity of Ni/TiO2-HT, Table 1) can be potentially attributed to the decreased Ni dispersion (from 19.73% to 17.46%), increased Ni surface coverage (from 81.1% to 89.8%), and decreased degree of reduction (from 76.1% to 29.9%). Although the HDO activity was suppressed, the STYCHNOL achieved with NiFe(5)/TiO2-HT was high (167.1 molCHNOL\u2219molNi\n\u22121\u2219h\u22121). The high STYCHNOL values confirmed that the active sites were related to the reduced Fe oxides present near the Ni NPs and TiO2. When Fe loading was further increased to 10\u00a0wt% and 15\u00a0wt%, the cyclohexanol yield and guaiacol conversion significantly decreased, indicating catalyst deactivation. The deactivation of the catalyst could be attributed to the accumulation of the inactive Fe oxides occupying the surface of Ni NPs.The results reveal that the ensembles of the NiFeOx and FeOx-TiO2 interfacial sites are selective HDO centers that promote the instantaneous demethoxylation\u2013hydrogenation reactions. The highly electron-deficient sites (oxygen vacancies) present at the FeOx-TiO2 interface (Fe2+\u2013OV\u2013Ti\u03b4+) help anchor the hydroxyl groups present in guaiacol. The oxygen vacancies present at the NiFeOx interface participate in the direct demethoxylation of guaiacol. The demethoxylation is followed by the hydrogenation of the phenyl ring (Fig. 8(C-iii)).The influence of the supports on the efficiency of HDO of guaiacol were studied using SiO2-, Al2O3-, Nb2O5-, ZrO2-, and TiO2-based supports (Fig. S7(C)). A high rate of cyclohexanol production (STYCHNOL = 160.6 molCHNOL\u00b7molNi\n\u22121\u00b7h\u22121) was achieved using NiFe/ZrO2-HT. The rate was slightly lower than the rate achieved using NiFe(3)/TiO2-HT (STYCHNOL\u00a0= 182.7 molCHNOL\u2219molNi\n\u22121\u2219h\u22121). Other than TiO2 and ZrO2, NiFe catalysts on SiO2, Al2O3, and Nb2O5 could not be used for the effective HDO of guaiacol. This could be potentially attributed to the fact that the oxygen vacancies were not present on the surfaces of these non-reducible supports.The demethoxylation\u2013hydrogenation of the phenyl rings was studied by analyzing the time-dependent HDO of guaiacol (Fig. 7(B)). The yield of cyclohexanol (produced with high selectively) increased (from 45.8% to 87.9%) with an increase in the reaction time during the first 40\u00a0min (includes the heating time of 20\u00a0min). This indicated that the formation of cyclohexanol was favored. The possible intermediates (2-methoxycyclohexanol (4) and cyclohexanone (3)) were formed in negligible amounts (Fig. 1(A)). The formation of phenol (2) was not observed under these conditions. These observations suggested that cyclohexanol (5) could be formed during the direct demethoxylation of guaiacol. The demethoxylation was followed by the rapid hydrogenation of the intermediate (2).\nFig. 7(C) (representing the pressure-dependent HDO) suggests the formation of phenol following the direct demethoxylation of guaiacol. The low-pressure reaction performed under 2\u00a0MPa (H2; measured at room temperature) led to the production of phenol (selectivity: 20.6%) and cyclohexanone (selectivity: 31.6%). These compounds rapidly hydrogenated under the high pressure of 4\u00a0MPa (H2; measured at room temperature) even when the catalyst ratio was low and the temperature was 250\u00a0\u00b0C (entries 13 and 14, Table 2). The results confirmed that HDO could be achieved following the demethoxylation of guaiacol (leading to the production of phenol) at low H2 pressure.The reaction pathway was further investigated by tuning the reaction temperature (Fig. 7(D)). The yield of cyclohexanol increased from 63.5% to 87.9% as the reaction temperature increased from 230\u00a0\u00b0C to 270\u00a0\u00b0C. Approximately 98% of guaiacol could be converted to the desired product under these conditions. When the reaction temperature was increased to 290\u00a0\u00b0C, the cyclohexanol yield decreased to 17.9%. The formation of cyclohexanol (6) was accompanied by the formation of cyclohexane (Fig. 1(A); yield: 57.2%) and methylcyclohexanol (10; yield: 5.5%). These observations indicate that the demethoxylation\u2013hydrogenation is favored when the temperature is\u00a0\u2264\u00a0270\u00a0\u00b0C although complete hydrodeoxygenation occurs at 290\u00a0\u00b0C. The apparent activation energy (Ea) was determined (assuming first-order reaction kinetics for the conversion of guaiacol to cyclohexanol) to understand the efficiency of the selective HDO reaction. The Arrhenius plots (Fig. 8(A)) were analyzed, and it was observed that the Ea for the formation of cyclohexanol from guaiacol was the lowest (69.05\u00a0kJ/mol) when NiFe(3)/TiO2-HT was used as the catalyst and the Ea was high when Ni/TiO2-HT (78.66\u00a0kJ/mol) and Ni/TiO2-IM (91.74\u00a0kJ/mol) were used as the catalysts. These observations indicate that the active sites on NiFe/TiO2-HT can effectively cleave the Ar-OCH3 bond and hydrogenate the phenyl ring present in guaiacol. The different activation energy of Ni/TiO2-IM compared to Ni/TiO2-HT can be attributed to the structure difference by the catalyst preparation methods, which was confirmed by the characterization results of TEM, XRD, and CO DRIFTS. The different structure formed the different active sites, exhibiting the different activation energies.The feed-to-catalyst ratios were also tuned and high yields of cyclohexanol were obtained when large quantities of catalysts were used (Fig. S7(D)). Incomplete conversions of guaiacol and low yields of cyclohexanol were observed under conditions of high feed-to-catalyst ratios.The results revealed that the presence of the Fe species and Ni NPs in the core\u2013shell structure of NiFe/TiO2-HT improved the generation of the oxygen vacancies at the perimeter of Fe\u03b4+, Ti\u03b4+, and Ni NPs (Fig. 5(B)). The rate of formation of cyclohexanol increase with an increase in the Fe content (till 2.5\u00a0wt%). A typical volcano-shaped curve was generated, indicating that the demethoxylation\u2013hydrogenation process was adjusted by the presence of Fe (Fig. 8(B)). When the Fe content was low (range: 0\u20131\u00a0wt%), the production of cyclohexanol was hindered as the C-O bond activation energy was high. When the Fe content was high (range: 2\u20132.5\u00a0wt%), the formation of cyclohexanol was hindered by the poisoning of the Ni surface by Fe. The maximum rate of cyclohexanol formation (STYCHNOL\u00a0= 182.7 molCHNOL\u00b7molNi\n\u22121\u00b7h\u22121) was measured when 1.5\u00a0wt% Fe was used (actual content measured for NiFe(3)/TiO2-HT).Based on the results obtained using the XPS, H2 TPR, H2 TPD, and TPO measurements, it can be inferred that the incorporation of the Fe species led to the formation of a large number of oxygen vacancies at the interfacial sites. The amount of oxygen vacancies (OV/OL) was correlated with the Fe content, and it was observed that the OV/OL ratio increased with an increase in the rate of cyclohexanol formation. The trend in the change in the number of vacancies was similar to the trend observed when the Fe content was varied (Fig. 8(B)). These observations confirmed that the demethoxylation\u2013hydrogenation activity was adjusted by the number of the surface oxygen vacancies.While the HDO of guaiacol can be achieved following the processes of demethylation, demethoxylation, hydrogenation, and hydrodeoxygenation [28,76], the formation of cyclohexanol can proceed via two major routes (Fig. 1(A)): (i) demethoxylation of guaiacol (1) to phenol (2) followed by the hydrogenation of the compound to form cyclohexanol \n(5)\n, and (ii) hydrogenation of guaiacol (1) to form 2-methoxycyclohexanol (4). The catalysts studied by us may not promote the demethoxylation of (4).The probable reaction mechanism followed (when the TiO2-supported catalysts were used) has been presented herein. When the Ni/TiO2-HT catalyst is used, the adsorption of guaiacol on the Ni surface can occur exploiting two different configurations (as inferred from the results obtained using various characterization methods): (i) The C=C bonds present in the phenyl ring in guaiacol can get adsorbed on the terrace surface, leading to ring hydrogenation in the presence of activated H atoms. This process leads to the production of 2-methoxycyclohexanol (Fig. 8(C-ii)). (ii) The methoxy or hydroxyl groups in guaiacol can get adsorbed on the defects of oxygen vacancies present at the electron-deficient Ni\u2013OV\u2013Ti interface to promote the demethoxylation reaction and not the dehydroxylation reaction (Fig. S8(C-ii)). As Ni/TiO2-HT contains Ni metal and the Ni\u2013OV\u2013Ti rich phase, routes (i) and (ii) could occur concomitantly on Ni/TiO2 producing a mixture of methoxycyclohexanol and cyclohexanol in the reaction product.For NiFe/TiO2-HT, the ternary structure of NiFe dispersed on TiO2 improved the adsorption of guaiacol. The methoxy (or hydroxyl) moieties in guaiacol improved the adsorption process. The highly dispersed FeOx species function as oxophilic centers and closely interacted with the methoxy group (Ar-OCH3) present in guaiacol. The NiFeOx (not pure Ni) present in the core\u2013shell structure of NiFe/TiO2-HT suppressed the the hydrogenation of the phenyl ring. Improvement in the extent of demethoxylation at the NiFe interface was also observed under these conditions. An increase in the Fe content in the thick FeOx outer shell layer promoted the direct demethoxylation of guaiacol followed by the hydrogenation. A high rate of cyclohexanol formation can be attributed to the fact that the highly dispersed FeOx species can help in the generation of multiple interfacial sites at the NiFeOx and TiO2 interfaces, leading to the generation of multiple active centers that participate in demethoxylation (Fig. 8(C-iii) and Fig. S8(C-ii) without significantly poisoning the hydrogenation sites.HDO of pyrolysis oil proceeded via a two-step process (Fig. S1). Pyrolysis oil dissolved in n-decane was catalytically treated with 5\u00a0wt% Pd/C in the first step (BO-S1). The product, denoted as LO, containing alkyl methoxyphenols such as guaiacol, methyl guaiacol, ethyl guaiacol, propyl guaiacol, and propyl syringol as the major components, was obtained (Fig. S9(B)). In the absence of a catalyst, BO-S1-LO (Fig. S9(A)) consisted of alkyl methoxyphenols, eugenol, isougenol, and other small oxygenates (such as acetone, methyl acetate, 2,5-dimethylfuran, cyclopentanone, and methyl cyclopentanone). The product obtained at the end of the first step was analyzed using the GC. The results revealed that the amount of alkyl methoxyphenols (696.43\u00a0\u03bcg/gbio-oil) present in the LO fraction was higher than the amount of alkyl methoxyphenols present in the fraction obtained in the absence of catalysts (173.67\u00a0\u03bcg/gbio-oil). Hydrotreatment using 5\u00a0wt% Pd/C led to a low yield of solid coke and high yields of WSO (19\u00a0wt%) and HO (49\u00a0wt%) (Fig. 9\n(A)). This indicated that recondensation reactions (C\u2013C coupling of labile monomers) were suppressed under these conditions.HDO of the LO fractions was simulated using the phenolic model compounds, such as methyl guaiacol, propyl guaiacol, and allyl guaiacol. The reaction was performed using NiFe(3)/TiO2-HT as the catalyst, and the results indicated that the conversion of the alkyl guaiacols decreased as the length of the alkyl chains were increased. This can be attributed to the steric hindrance exerted by the alkyl chains (Fig. S7(E)).Based on the results obtained using the phenolic model compounds, the HDO of the LO fraction was conducted at the second stage of the reaction involving NiFe/TiO2-HT. The solvent was not removed during the process. The yield of the cyclic alcohols was 71.7%, and 93.0% of the alkyl guaiacol reactants could be converted (Fig. S9(C)). A low yield of the cyclic alcohols (20%) and a low guaiacol conversion (54%) could be achieved using Ni/TiO2-HT (Fig. 9(A)). This confirmed that NiFe/TiO2-HT was more effective (than Ni/TiO2-HT) and could be used to achieve the selective HDO of the LO fractions.The reusability of the NiFe/TiO2-HT catalyst (for the HDO of guaiacol) was studied under the optimized reaction conditions (270\u00a0\u00b0C; 5\u00a0MPa, H2; 1\u00a0h) (Fig. 9(B)). The guaiacol conversion and the yield of cyclohexanol slightly decreased after each cycle. At the end of the 5th cycle (6th run), the guaiacol conversion was 93.6%, and the yield of cyclohexanol was 83.5%. This indicated that an excellent catalyst reusability for the NiFe(3)/TiO2-HT catalyst could be achieved. Spent NiFe(3)/TiO2-HT was characterized using the TG and XRD measurements (Fig. 9(C) and (D)). The TG results revealed that the weight of fresh catalyst (under a flow of air) increased by 2.9\u00a0wt%, and this could be attributed to the oxidation of the Ni and Fe species. It was also observed that the weight of spent catalyst after the 6th run increased by only 0.8\u00a0wt% (vs.\u00a0\u223c20\u00a0wt% assuming the complete coking of guaiacol per run [77]) under conditions of coke deposition. This indicated negligible coking under the reaction conditions and this can be attributed to the absence of coke precursors such as phenol and catechol [77]. The 6.2\u00a0wt% increase in weight can be explained by the complete oxidation of metallic Ni and Fe species in NiFe(3)/TiO2-HT.The spent catalyst obtained following the HDO of BO-S1-LO (using Pd/C) contained 1.1\u00a0wt% coke (formed during the condensation of the labile monomers). The spent NiFe(3)/TiO2-HT samples were characterized using the XRD, which revealed negligible changes in the crystal structures. A close inspection of the peaks corresponding to Ni(111) indicates the presence of large particles (18.4\u00a0nm) and low 2\u03b8 (or higher d-spacing) values. These observations indicated that the Ni-based particles were slightly sintered and were further alloyed with Fe in the presence of Fe species. The segregation (followed by agglomeration) observed for the Ni-based particles was not observed for Fe2O3. Peaks corresponding to Fe2O3 were not observed in the XRD results. Significant changes in the crystal structure of TiO2 were not observed, either. These observations indicate the highly stable nature of NiFe/TiO2-HT. The XRD results for the spent catalyst (following HDO of BO-S1-LO (using Pd/C)) revealed that the fraction of the rutile TiO2 phase was larger than the fraction obtained using the fresh catalyst. The Ni particles present in the spent catalyst were larger than the Ni particles present in the fresh catalyst. Peaks corresponding to Fe2O3 were not observed for fresh and spent NiFe/TiO2-HT catalysts. These observations indicate that NiFe/TiO2-HT was stable, and the stability was slightly adjusted by the acids present in BO-S1-LO.The NiFeOx core\u2013shell (ternary heterostructure) structure on the TiO2 support, prepared following the HT method, exhibited excellent chemoselective catalytic activity and stability during the direct demethoxylation\u2013hydrogenation process followed to produce cyclic alcohols from BO-derived alkyl methoxyphenol complex mixtures. The Ni metal active sites, tuned by the presence of highly dispersed Fe species in a core\u2013shell environment on reducible anatase TiO2, formed complexes with Fe and TiO2, leading to the generation of abundant oxygen vacancies at the NiFeOx-TiO2 interfacial sites. This led to the highly selective direct demethoxylation\u2013hydrogenation reactions that could be conducted to obtain cyclic alcohols. The results reported herein reveal the synergism between metal\u2013metal oxide support interfacial sites in NiFeOx/TiO2. The effects can be tuned to selectively form cyclic alcohols from lignin-derived phenolic oxygenates and BO.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 Changes (2020M1A2A2079798) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT of Republic of Korea.Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2022.136578.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  TiO2-supported Ni catalysts are promising candidates that can be used to achieve biomass valorization following the selective hydrodeoxygenation (HDO). Their catalytic activity can be tuned and they are characterized by strong metal\u2013support interaction (SMSI). The SMSI observed at the interfaces of Ni nanoparticles (NPs) and the TiO2 support was tuned by adding Fe and subjecting the synthesis system to hydrothermal treatment conditions. The prepared catalysts promoted the selective conversion of alkyl methoxyphenols, including lignin-derived guaiacol, to saturated cyclic alcohols. The low Fe content (\u223c1.5\u00a0wt%) in Ni/TiO2 significantly promoted the formation of cyclohexanol from guaiacol (rate: 183 molcyclohexanol\u00b7molNi\n                     \u22121\u00b7h\u22121; selectivity: 85%). Alkyl methoxyphenols present in the biomass pyrolysis oil could be effectively converted into cyclic alcohols (yield: 71%). It was observed that the interfacial sites of highly dispersed NiFe-FeOx core\u2013shell structures on the TiO2 support promote the demethoxylation of reactant to selectively produce cyclic alcohols.\n               "}
{"full_text": "The combinatorial materials chip (CMC) method [1], combined with high-throughput characterization techniques, has been proven a powerful tool for rapid material studies such as in phase diagrams [2], ferromagnetic shape memory alloys [3], and amorphous alloy [4], etc. With this approach, the magnetism of FeCo-based alloys was rapidly screened [5\u20138] as candidates for rare-earth-free permanent magnet materials. In these researches, W, Mo, Nb, and V were added to the FeCo-based alloys as dopped elements. Because of that the added refractory elements are found to have strong spin-orbit couplings, which may result in high magnetocrystalline anisotropy in FeCo-based alloys by hybridization [9\u201312].FeNi-based alloy is a class of important materials in engineering. As permalloy, it exhibits superparamagnetic properties and thus can be used for various applications related to drug delivery and magnetic sensors [13]. However, the magnetic properties of Fe-X-Ni (X stands for the third element) ternary systems were studied in a systematical way only in rare occasions [14], because it required a great deal of labor and cost under the premise of the traditional trial-and-error approach. In our previous work [15], the magnetic properties of the CMC of the Fe-Co-Ni ternary system were characterized systematically with an HT-MOKE system. Combined with the composition data by high-throughput \u03bc-XRF and structural data by high-throughput XRD, the correlation between composition, crystal structure, and the magnetic property was established and readily visualized. With such a panoramic view, the compositional regions exhibiting distinctive magnetic properties such as large coercive force, high saturation magnetization, etc. were revealed.In this work, the magnetic properties of CMCs of the Fe-X-Ni (X\u00a0=\u00a0Cr, W and V) ternary systems covering the full composition range were screened systematically by measuring the in-plane and out-of-plane hysteresis loops using an HT-MOKE system. Maps of composition-phase-magnetic properties were thus constructed very rapidly. Through these relationships, the effect of sample forms, MOKE modes, alloying elements, and heat treatment conditions on the structure and magnetic performance were systematically studied.Following a procedure described by Xing et\u00a0al. [16], the Fe-X-Ni thin-film CMCs covering the full composition range were deposited on quartz glass substrates (25\u00a0mm\u00a0\u00d7\u00a025\u00a0mm\u00a0\u00d7\u00a02\u00a0mm) at room temperature with a base vacuum pressure of 1\u00a0\u00d7\u00a010\u22127\u00a0Torr (1\u00a0Torr\u00a0=\u00a0133.322 368 4\u00a0Pa). The chip is composed of 10-cycle of Fe, X (= Cr, W, V), and Ni layers with a total thickness of about 100\u00a0nm. As shown in Fig.\u00a01\na, the Fe-Cr-Ni ternary region is an equilateral triangle of 20\u00a0mm side length in the center, surrounded by three unary regions (Fe, Cr, and Ni) and three binary regions (Fe-Cr, Fe-Ni, and Cr-Ni) separated by laser marking lines drawn after deposition. The composition C\n\ni\n (Mole fraction/%) at each point on the chip is determined by the thickness of each component [16] computed by the following equation:\n\n(1)\n\n\n\nC\ni\n\n=\n\n\n\nt\ni\n\n\n\u03c1\ni\n\n\u2215\n\nz\ni\n\n\n\n\n\u03a3\ni\n\n\nt\ni\n\n\n\u03c1\ni\n\n\u2215\n\nz\ni\n\n\n\n\n\n\n\u2211\n\ni\n\n\nt\ni\n\n=\nconst\u00a0\n\n\n\nwhere \n\n\nt\ni\n\n\n is the thickness, \n\n\n\u03c1\ni\n\n\n the density, and \n\n\nz\ni\n\n\n the atomic mass of the ith component. The chips were annealed isothermally under dynamic pumping to maintain the vacuum at 1\u00a0\u00d7\u00a010\u22127\u00a0Torr followed by rapid cooling after deposition.Time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling was conducted to evaluate the degree of interdiffusion between layers. The TOF-SIMS (TOF-SIMS 5\u2013100, IONTOF GmbH, Germany) was operated in in-depth profile mode under the base pressure of the analysis chamber below 1.1\u00a0\u00d7\u00a010\u22129\u00a0mbar (1\u00a0bar\u00a0=\u00a00.1\u00a0MPa). The layer-by-layer pealing was performed with an ion beam raster on 300 \u03bcm\u00a0\u00d7\u00a0300\u00a0\u03bcm areas using 1\u00a0keV O (69.27\u201382.74\u00a0nA current). As shown in Fig.\u00a01b, TOF-SIMS elemental depth profiles of Fe, Cr, and Ni are quite flat in the majority of the depths except for the very top cycle, indicating that interdiffusion is essentially complete in the thickness direction of the Fe-Cr-Ni system. Microbeam X-ray fluorescence (\u03bc-XRF) spectroscopy (M4 TORNADO, Bruker Co., Germany) was employed to calibrate the composition distribution of the chip with a step size of 50\u00a0\u03bcm. A linear distribution of each element over the chip surface was confirmed in Supplementary Materials.For structural characterization, part of the samples was mapped by a combined XRD\u00a0+\u00a0XRF device on BL09B at the Shanghai Synchrotron Radiation Facility (SSRF), China. Micro-beam X-ray with an energy of 12\u00a0keV (\u03bb\u00a0=\u00a00.103\u20093\u00a0nm) was focused by a pair of KB mirrors to 24\u00a0\u00d7\u00a035\u00a0\u03bcm2. The incident angle of X-ray to sample surface was set to 15\u00b0 and diffraction pattern was collected by a Dectris Pilatus 3S 2M detector covering a 2\u03b8 range from 16\u00b0 to 52\u00b0. The XRF signal was collected by an energy-dispersive silicon drift detector simultaneously. The rest of the samples were characterized by a laboratory micro-beam X-ray diffractometer (Bruker D8 advance) with a rotating anode (Cu K\n\u03b1\n radiation) and area detector, whose beam diameter is 300\u00a0\u03bcm.The characterization of the magnetic hysteresis loops of the CMCs was conducted on a home-built HT-MOKE system, where a He-Ne laser with a wavelength of 632.8\u00a0nm is used as the light source. The beam was focused to 300\u00a0\u03bcm diameter and linearly polarized by a polarizer placed at an angle of 0\u00b0 (parallel to the sample surface). After being reflected by the sample, the elliptically polarized light passes through a photoelastic modulator (PEM) (II/FS50LR, Hinds Co.) with a modulation frequency of 50\u00a0kHz, and a phase delay parameter A\n0\u00a0=\u00a02.405\u00a0rad. After modulation, the light passes through the analyzer set at 45\u00b0 and then is received by the detector. A lock-in amplifier is employed to improve the signal-to-noise ratio. The mathematical derivation process of the Kerr signal can be seen in the Supplementary Materials.The magnetic field is imposed by a 4-pole electromagnet composed of two pairs of electromagnets arranged at 90\u00b0 from each other. By controlling the intensity ratio of the two electromagnet pairs, the resultant magnetic field vector relative to the sample is adjustable so that the measurement can be switched between the transverse (in-plane) and polar (out-of-plane) mode. Typically, polar geometry has a much larger Kerr effect [17,18] and is commonly used in MOKE experiments. The magnetic field is scanned between\u00a0\u22120.5\u00a0T and\u00a0+0.5\u00a0T during hysteresis loop measurement.MOKE magnetic hysteresis loops are mapped automatically using a custom-designed sample stage system. The combinatorial chip is mounted on a sample holder with a 20\u00a0cm long aluminum shaft and positioned in the center of the 4-pole electromagnet for measurement. The shaft is attached to a three-dimensional stage system (Zolix TSA50-B stage) located on the side of the electromagnet, with a horizontal movement in the X-Y plane and vertical movement in the Z-axis. The sample can rotate around the x-axis or y-axis to change sample orientations. The whole process was controlled by a LabVIEW program with each data point taking about 60\u00a0s to measure. Python programs were written to automate the visualization of hysteresis maps and data analysis.A coordinate transform system (CTS) capable of correlating multiple instruments is required to ensure measurements are performed in the same position. To establish the correlation between two coordination, the three vertices of the ternary region triangle were first marked using a laser marking machine as the reference point. Considering the coordinate value of any point (x, y, z) in the rectangular Cartesians coordinate system O\n1\n-x\n1\ny\n1\nz\n1 (sample coordinate system) and O\n2\n-x\n2\ny\n2\nz\n2 (equipment coordinate system) are noted as coordinate sets U\n\ni\n and X\n\ni\n, respectively. The conversion between the two systems is expressed as\n\n(2)\n\n\n\nX\ni\n\n=\nR\n\u2217\n\nU\ni\n\n+\nT\n\n\n\nin which \nR\n is the rotation matrix and \nT\n the translation matrix. Using the coordination data for the three reference points, \nR\n and \nT\n can be obtained mathematically based on the Singular Value Decomposition (SVD) method [19]. This relationship is extendable to any two coordinates.For comparison purposes, bulk samples of 5 selected compositions were prepared by arc melting and drop-casting under a pure Ar atmosphere. Each bulk sample weighed about 20\u00a0g, and the purity of each of the raw materials was at least 99.9%. The as-cast ingots were homogenized at 1\u2009200\u00a0\u00b0C for 2\u00a0h followed by cold rolling at room temperature with 75\u00a0% thickness reduction and subsequent recrystallization annealing at 1\u2009000\u00a0\u00b0C for 0.5\u00a0h to refine the grain size. Optical microscopy (OM) was used for microstructure observation on specimens metallurgically polished and then etched with aqua regia (HCl: HNO3: C3H8O3\u00a0=\u00a03:1:1) for 10\u201330\u00a0s. Magnetic properties were measured on a magnetic performance measurement system (Quantum Design, MPMS3) in fields up to 2\u00a0T along the long axis of samples having a dimension of 2\u00a0mm\u00d7\u00a02\u00a0mm\u00d7\u00a04\u00a0mm. Phase constitution of the bulk samples was identified by XRD (Rigaku Ultima IV) using Cu K\n\u03b1\n radiation with a scanning speed of 5 (\u00b0)/min.According to the direction of the magnetization and the plane of incidence, MOKE measurement can be conducted in longitudinal, transverse, and polar modes. As shown in Fig.\u00a02\na, in the transverse, and longitudinal geometries, the in-plane signal is obtained, in which the magnetization is perpendicular (transverse) or parallel (longitudinal) to the plane of incidence. In the polar geometry, the out-of-plane signal is detected. The magnetization is parallel to the light propagation direction and perpendicular to the surface of the sample. The polar mode usually shows the largest Kerr effect followed by the longitudinal mode and the transverse mode. In this study, due to the restriction of the geometric setting, the longitudinal mode cannot achieve high-throughput measurement. Therefore, we use the transverse (T-MOKE) and polar (P-MOKE) modes to obtain the in-plane and out-of-plane Kerr signals.As shown in Fig.\u00a03\na, 210 hysteresis loops in transverse mode (in-plane), color-coded with the crystal structure, are displayed for the Fe-Cr-Ni CMC sample annealed at 600\u00a0\u00b0C for 2\u00a0h. 5050 points of crystal structure data were characterized by synchrotron radiation at SSRF. With the help of CTS, phase structure data were integrated with the hysteresis loops in the map (Fig.\u00a03a). The Fe-Cr-Ni ternary system is divided into three-phase regions including fcc and bcc two single-phase regions and fcc\u00a0+\u00a0bcc one two-phase region. The fcc single-phase region (color-coded red) resides on the Ni-rich portion and the bcc single-phase region (color-coded black) mainly distributes along the Fe-Cr edge with the fcc\u00a0+\u00a0bcc two-phase region (color-coded blue) in between (Fig.\u00a03a).Based on the shapes of hysteresis loops, ferromagnetism is mainly found in the fcc single-phase region and the other regions exhibit weak ferromagnetic or paramagnetic characteristics. Fig.\u00a03b shows that the largest saturation magnetization appears on the Ni-side along the Fe-Ni edge where the Fe concentration ranges between 10% and 40%. The saturation magnetization decreases with increasing Cr content because Cr is an antiferromagnetic element. The coercivity distribution displays a certain trade-off with the saturation magnetization even though the largest coercivity is \u223c36\u00a0mT, which appears at the boundary between the bcc\u00a0+\u00a0fcc two-phase region and fcc single-phase region (Fig.\u00a03c).To verify the results of thin-films by MOKE, the magnetic performance of some selected bulk alloys was compared with the CMC. Five compositions were selected from the fcc single-phase region in Fig.\u00a03a, labeled as #1-#5 with an order of increasing Cr content, namely Fe38.7Cr5.8Ni55.5, Fe56.6Cr6.7Ni36.7, Fe41.7Cr11.7Ni46.6, Fe26.7Cr11.7Ni61.6, and Fe41.6Cr21.7Ni36.7. After recrystallization annealing, all the five alloys are fcc single phase and the grain size is \u223c50\u00a0\u03bcm (Fig.\u00a04a and b). The hysteresis loops by VSM show that four of the five alloys are ferromagnetic while one alloy with the highest Cr content is non-magnetic (Fig.\u00a04c). The saturation magnetizations of the four alloys decrease with increasing Cr content which agrees with the CMC results. The saturation magnetizations of the #1 (Fe38.7Cr5\u00b78Ni55.5) and #2 (Fe56\u00b76Cr6\u00b77Ni36.7) alloys are similar, \u223c0.9\u00a0T despite their slightly different Cr content. The saturation magnetizations of the #3 and #4 alloys are \u223c0.7\u00a0T and \u223c0.6\u00a0T respectively due to the increased Cr content. Compared with the hysteresis loops of the corresponding compositions measured by T-MOKE (Fig.\u00a04d), there is an agreement with the magnetization in general. However, the signals by MOKE measurement are much noisier, which makes a detailed comparison of magnetization as a function of composition impossible. In addition, the measured coercivities of the bulk alloys are almost neglectable while the MOKE measurements showed a coercivity of \u223c20\u00a0mT for all the compositions. This can be attributed to the grain size effect because the grain size in the CMC of \u223c100\u00a0nm thick is at the nano-meter scale and is at the micro-meter scale in the bulk form.The hysteresis loops measured in the polar mode (out-of-plane) on the same Fe-Cr-Ni CMC display certain differences from those in the transverse mode (in-plane) (Fig.\u00a05\na). On the one hand, the signal-to-noise ratio of the polar mode is higher than the transverse mode. The saturation magnetization decreases with increasing Cr content (Fig.\u00a05c) and the highest saturation magnetization resides in a similar region as measured by the transverse mode. On the other hand, relatively large coercivity whose maximum is \u223c400\u00a0mT is found in the region with Ni concentration between 25% and 75% and Cr concentration of \u223c20% (Fig.\u00a05d). Fig.\u00a05b shows the out-of-plane hysteresis loops of the same composition points #1\u2013#5 as in Fig.\u00a04d. The signal-to-noise ratio is \u223c10 times higher than in Fig.\u00a04d, which makes the plots more recognizable. Among them, #1 and #4 compositions have the maximum saturation magnetization followed by #2 and #3, and #5 with the highest Cr content shows the lowest value. This is roughly in agreement with Fig.\u00a04d whereas the sequence of the saturation magnetizations of #1\u2013#4 compositions is different from the bulk alloys (Fig.\u00a04c). Unlike the bulk alloys and the hysteresis loops measured in the transverse mode, the five compositions show a coercivity from 70\u00a0mT to 310\u00a0mT, which is in reverse relationship with their saturation magnetization. The relatively large coercivity observed in the polar mode is probably due to the size effect because the film thickness is \u223c100\u00a0nm and the beam size of MOKE measurement is \u223c300\u00a0\u03bcm. Fackler [20] also showed that the out-of-plane coercivity is greater than the in-plane one either in the MOKE test or VSM test for thin-film. In the thin-film, the shape anisotropy effect is particularly important, where the shape of the magnet is essentially an infinite plane.The out-of-plane hysteresis loop maps of the Fe-W-Ni and Fe-V-Ni systems show a decrease in saturation magnetization with increasing W or V content like the Fe-Cr-Ni system as a general trend (Fig.\u00a06\n). The compositions with obvious ferromagnetism were mainly distributed in the fcc single-phase region. The largest saturation magnetization values are distributed along the Fe-Ni edge where the distributed range of the region is slightly different between the two systems (Fig.\u00a06b and e). A relatively large coercivity of \u223c300\u00a0mT was found in an area where W is less than 20\u00a0% and Fe is between 20% and 70% in the Fe-W-Ni system. The overall coercivity of the Fe-V-Ni system was extremely weak and distributed evenly except for several abnormal points at the Fe-rich and Ni-rich ends. As can be seen from the contour map Fig.\u00a06c, the shape of the area with a large coercivity (more than 150\u00a0mT) was similar to that of the Fe-Cr-Ni system (Fig.\u00a05d) with a maximum value of about 300\u00a0mT.It is well-understood that while FeNi bulk alloys are soft magnetic and Cr, W and V are non-magnetic elements, when a small amount of them is added to FeNi-based alloys, the magnetic moment of ferromagnetic materials would be diluted, leading to a decreased saturation magnetization. This agrees with both in-plane and out-of-plane MOKE measurements of the CMCs. However, relatively large coercivity was found in the Fe-Cr-Ni and Fe-W-Ni systems in the out-of-plane measurements. Similarly, a scenario was observed in the Fe-Co-Ni CMC in our previous work [15]. The characteristic of magnetism is a strong function of both composition and crystal structure. The saturation magnetization is mainly composition-dependent while the coercivity can be sensitive to microstructure. This on the one hand can be explained by the film thickness effect (shape anisotropy) in the out-of-plane measurement. On the other hand, there is a microstructure effect in the corresponding CMCs. Fig.\u00a07\n compares the 2D diffraction patterns in the fcc single-phase region in the three systems heat-treated under the same conditions. The #1 composition corresponds to Fe38.7\nX\n5.8Ni55.5, #3 Fe41.7\nX\n11.7Ni46.6, and #5 Fe41.6\nX\n21.7Ni36.72 where X\u00a0=\u00a0Cr, W, and Ni. In the Fe-Cr-Ni system (Fig.\u00a07a-c), all three compositions showed strong (111) texture. In the Fe-W-Ni system (Fig.\u00a07d-f), the intensity (111) diffraction distributed evenly in #1 indicating weak texture while texture became strong in #3 and #5 with increasing W content. In comparison, the textures of the three compositions were all weak in the Fe-V-Ni system (Fig.\u00a07g-i), and the diffraction intensity decreased with the increase of V content. Correlating the composition and structural characteristics with magnetic properties, it appears that the coercivity is closely related to crystal orientation and crystallinity. Because the (111) plane in fcc structure is a closely packed plane with low surface energy, the thin film tends to form the (111) texture and such a tendency may vary depending on the type of and the content of the alloying element.The Fe-Co-X (X\u00a0=\u00a0W [5], Mo [6], Nb [7], and V [8]) systems have been surveyed by a similar method trying to design a magnetic crystal with sufficient magnetization and a large magnetic anisotropy without the use of rare-earth elements. Similar to the Fe-X-Ni systems in this study, Fe-Co-X ferromagnetic materials are distributed in the region with low X element concentrations. Among these systems, the coercivity of 230\u00a0mT in the Fe-Co-W system and 260\u00a0mT in the Fe-Co-V system were obtained in the out-of-plane direction, which is in the same order of magnitude as what was found in this work. It should be noted that the annealing time of the Fe-Co-X systems was not always clear. For instance, The Fe-Co-Nb system was annealed at 700\u00a0\u00b0C for 1\u00a0h, the Fe-Co-Mo system, at 700\u00a0\u00b0C for 45\u00a0min and the Fe-Co-W and Fe-Co-V systems, at 600\u00a0\u00b0C or/and 700\u00a0\u00b0C [5\u20138]. In fact, heat treatment condition is closely related to magnetic properties, which is to be discussed in the next section. In the Fe-Co-W system, the vertically standing platelet-like grain structure was ascribed to the enhanced coercivity. The increase of coercivity in the Fe-Co-V system was explained by the shape anisotropy from column-shaped grains. A new noncubic ferromagnetic phase with a hexagonal crystal structure (C36) embedded in a FeCo-based matrix was identified in the Fe-Co-Nb system. These suggest that the coercivity is sensitive to the local microstructure of the corresponding system, including the crystal orientation and crystallinity.To study the effect of heat treatment conditions on magnetic properties, the #3 composition in the Fe-Cr-Ni system was chosen to compare systematically under different heat treatment conditions (600\u00a0\u00b0C, 700\u00a0\u00b0C and 800\u00a0\u00b0C for 1\u00a0h and 700\u00a0\u00b0C for 4\u00a0h, respectively). The hysteresis loops were collected in polar mode due to their high signal-to-noise ratio. As shown in Fig.\u00a08\na, when the heat treatment time is fixed for 1\u00a0h, the magnetic properties of all samples are roughly similar despite the annealing temperature increasing from 600\u00a0\u00b0C to 800\u00a0\u00b0C. The coercivity is increased from 230\u00a0mT for annealing at 600\u00a0\u00b0C to 350\u00a0mT at 700\u00a0\u00b0C but remains almost unchanged at 800\u00a0\u00b0C, or for an extended time. For the remanence, an average of 6 was obtained for all samples annealing at 600\u2013800\u00a0\u00b0C for 1\u00a0h. However, the value of remanence rose to 19 with annealing at 700\u00a0\u00b0C for 4\u00a0h. In addition, under 600\u2013800\u00a0\u00b0C for 1\u00a0h heat treatment, the hysteresis loops of samples are barely smooth, and the shape is less regular at 600\u00a0\u00b0C, for 1\u00a0h treatment. Both the shape of the hysteresis loop and the squareness are better when sample annealing at 700\u00a0\u00b0C for 4\u00a0h. Meanwhile, it can be seen from the #3 hysteresis loop in Fig.\u00a05b that the coercivity at the same composition point after annealing at 600\u00a0\u00b0C for 2\u00a0h is 240\u00a0mT and the remanence is 14 The results indicate that coercivity has a stronger dependency on temperature. However, the remanence or saturation magnetization is more closely related to the duration of heat treatment but is not sensitive to the heating temperature.To find out the structural difference between the materials under different heat treatment conditions, the XRD spots (Fig.\u00a08b-d) are compared. Two peaks corresponding to (111) and (200) of the fcc phase, respectively, exist for the samples annealed at 600\u2013800\u00a0\u00b0C for 1\u00a0h. Obviously, the fcc (200) spot for the sample annealed at 600\u00a0\u00b0C for 1\u00a0h showed a wider 2\u03b8 range than that at 700\u00a0\u00b0C and 800\u00a0\u00b0C for 1\u00a0h, indicating more residual stress for annealing at 600\u00a0\u00b0C for 1\u00a0h, which caused that the hysteresis loop at 600\u00a0\u00b0C looked more irregular (Fig.\u00a07b, c & d). However, the (200) peak disappeared for the sample annealed at 700\u00a0\u00b0C for 4\u00a0h (Fig.\u00a08e). Meanwhile, the remaining ring of (111) peak is more aggregated than that of the samples annealed at 600\u2013800\u00a0\u00b0C for 1\u00a0h. This indicates that the grains grow preferentially under this extended heat treatment. In general, grains grow along with a given orientation due to interface and surface energy. Hence, the whole film will move towards its thermodynamically most stable state by orienting all grains with the plane of lowest surface tension parallel to the substrate. As shown in Fig.\u00a02b and c, for the fcc crystal structure, the close-packed (111) surface energy is the lowest energy surface among all indices. Thus, the films with fcc structure show a strong [111] preferred orientation perpendicular to the surface of the sample. However, the easy axis [111] of the fcc phase is parallel to the magnetization direction of the P-MOKE. Meanwhile, the degree of crystallization of the material is positively correlated with the heat treatment time. The higher the crystallinity, the greater the coercivity [21]. Therefore, materials annealed at 700\u00a0\u00b0C for 4\u00a0h show better magnetic properties than that of 600\u2013800\u00a0\u00b0C for 1\u00a0h may be attributed to the texture and crystallinity of materials. This also explains why in Section 3.1, the saturation magnetization of the film sample is slightly higher than that of the bulk, especially #5, due to the texture of the film.A series of Fe-X-Ni (X\u00a0=\u00a0Cr, W and V) CMC samples were fabricated, covering the full range of component elements. After heat treatment with different conditions, the phase structure was characterized by XRD, and the magnetic properties were characterized by a self-built HT-MOKE system. The composition-phase-magnetic properties relationship maps were established for the Fe-X-Ni (X\u00a0=\u00a0Cr, W and V) systems. Based on these relationships, the effects of sample forms, MOKE modes, alloying elements, and heat treatment conditions on magnetic properties were systematically studied.The results showed that the saturation magnetization of all systems has a strong dependence on alloying elements, and generally decreases with increasing Cr, W, and V content. This is due to the magnetic moment of ferromagnetic materials being diluted by these added non-magnetic elements. The largest saturation magnetization appeared in the fcc single-phase region and is on the Ni-rich side near the Fe-Ni edge in all three systems. A large difference in coercivity between the in-plane (transverse mode) and out-of-plane (polar mode) hysteresis loops was noticed due to the shape anisotropy. At Fe37Cr21Ni42, the maximum coercivity was found. The value is \u223c36\u00a0mT by the in-plane measurements and it increases to \u223c400\u00a0mT by the out-of-plane measurements. The trend of magnetic properties in CMC samples was not totally repeated on 5 bulk samples with selected compositions surrounding Fe37Cr21Ni42. Although the trend of saturation magnetization in bulk is in good agreement with that from CMC, all bulk samples show almost no coercivity, attributable to the much smaller grain size, grain shape anisotropy, and stronger texture in CMC samples. Comparing the Fe-X-Ni systems under a similar condition, Cr alloying obtained the largest coercivity followed by W alloying and then V alloying. The maximum out-of-plane coercivities in the Fe-W-Ni and Fe-V-Ni systems were \u223c300\u00a0mT and \u223c200\u00a0mT, respectively. We suggest that alloying with different elements leads to the diverse orientation and crystallinity of the fcc phase resulting in different magnetic properties.Meanwhile, the effect of heat treatment on magnetic properties was evaluated in the Fe-Cr-Ni system. The materials annealed at 700\u00a0\u00b0C for 4\u00a0h show better saturation magnetization than that of 600\u00a0\u00b0C, 700\u00a0\u00b0C and 800\u00a0\u00b0C for 1\u00a0h, indicating that coercivity has a stronger dependency on temperature. However, the remanence or saturation magnetization is more closely related to the duration of heat treatment but is not sensitive to the heating temperature. This can be explained by the highly crystallized fcc structure and the strong (111) texture formed.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 for the financial support from the National Key Research and Development Program of China (Grant Nos. 2021YFB3702102 and 2017YFB0701900) and the Major Science and Technology Project of Yunnan Province \u201cGenome Engineering of Rare and Precious Metal Materials in Yunnan Province (Phase One 2020)\" (Grant No. 202002AB080001-1). Boyue Instruments (Shanghai) Co., Ltd for support of \u03bc-XRF is also acknowledged.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.jmat.2022.07.006.", "descript": "\n                  Fe-X-Ni (X\u00a0=\u00a0Cr, W and V) combinatorial thin-film (\u223c100\u00a0nm thick) materials chips covering the full composition range of ternary systems were fabricated. The crystal structure distribution was mapped by micro-beam X-ray diffractometers (XRD) and the magnetic hysteresis loops over the chip were characterized by a high-throughput magneto-optical Kerr effect (HT-MOKE) system to establish the composition-phase-magnetic properties relationships. The results showed that saturation magnetization for all systems has a strong dependency on alloying composition, and decreases with increasing dopped elements content as a general trend. Although the trend of saturation magnetization in bulk is in good agreement with that from thin films, all bulk samples show almost no coercivity, attributable to the much smaller grain size, and stronger texture in thin-film samples. Comparing the Fe-X-Ni systems under a similar condition, in the out-of-plane, Cr alloying obtained the largest coercivity (\u223c400\u00a0mT) followed by W alloying (\u223c300\u00a0mT) and then V alloying (\u223c200\u00a0mT). We suggest that alloying with different elements leads to the diverse orientation and crystallinity of the fcc phase resulting in different magnetic properties. Meanwhile, the effect of heat treatment on magnetic properties indicates that saturation magnetization is more closely related to the duration of heat treatment.\n               "}
{"full_text": "Chemical reactions are always around our lives, and more importantly, they affect and even change our lives.\n1\n Almost 80% of synthetic chemical reactions are dependent on the efficiency of catalysts.\n2\n Compared with traditional trial-and-error methods, the rational design of catalysts based on efficient descriptors for catalytic activity could improve the reaction efficiency,\n3\n decrease the cost of catalysts, and result in significant economic benefit.\n4\n In the long history of the development of catalysts, single-atom catalysts (SACs) have shown the most potential as the altar of catalysts because of their high active atom utilization, high catalytic activity, and high catalytic selectivity.\n5\u20137\n\nExcept for the difficult synthesis of SACs with high loading and high stability, the optimal SAC for a specific reaction is fairly random and unpredictable,\n8\n which hinders the development of SACs in the field of catalysis.\n9\n\n,\n\n10\n Some effective strategies have been proposed to address this issue; however, these designed SACs are still mainly based on traditional trial-and-error methods.\n11\n\n,\n\n12\n Even though some insights on active-metal atoms have been developed,\n13\n\n,\n\n14\n the activation mechanism of reactants for SACs remains obscure. The catalytic activity of SACs is not only dependent on the active atoms but also heavily influenced by the substrates and metal-substrate interactions.\n15\n\n,\n\n16\n\nA number of theories and descriptors have been proposed for predicting the adsorption energy and for further determining the catalytic activity of corresponding reactions.\n13\n\n,\n\n17\u201320\n For example, the d band center model has been widely applied to metal-based catalysts.\n21\n Unfortunately, the descriptor cannot be effectively applied to SACs because the d band structures of active-metal atoms are drastically influenced by the substrate.\n19\n\n,\n\n22\n Moreover, Gao et\u00a0al. proposed a model with a new electronic descriptor of \u03c8 and the generalized coordination number (CN) of active sites for quantitatively predicting the adsorption energies of small molecules on metallic materials and oxides.\n17\n For SACs, Xu et\u00a0al. presented a universal design principle for a rational design of graphene-based SACs using the coordination number, the electronegativity of active center, and the electronegativity of the nearest neighbor atoms.\n13\n These efforts have made a great contribution for rational catalyst design from the two aspects of atomic and electronic structures. However, identifying the real atomic structures of coordination environments is too complex, especially during the catalytic reaction process.Chemical reactions include the breakage of chemical bonds in the reactants and the formation of new chemical bonds in the products, which are essentially a process of electronic redistribution. The role of the catalyst is to guide the redistribution of electrons purposefully. Thus, the catalytic activity of SACs is expected to be predicted only through the intrinsic electronic characteristics of single metal atoms and the substrates without consideration of the complex coordination environment, where these electronic characteristic parameters are easily available through publicly available databases.In this study, we constructed 126 SACs with 9 two-dimensional (2D) substrates and 14 metal atoms for developing a machine learning (ML) model based on density functional theory (DFT) calculation. The nitrogen reduction reaction (NRR) is taken as the model reaction because the product of NH3 is vital for many important chemicals, particularly fertilizers and potential hydrogen storage materials.\n23\n\n,\n\n24\n The developed ML model takes intrinsic electronic characteristics of single metal atoms and the substrates as input and shows a high accuracy for predicting the catalytic activity of SACs for NRR, which is verified by available experimental data and independent DFT computations. More importantly, a new bidirectional activation mechanism is proposed for thoroughly analyzing the activation of N2 by considering the number of isolated electrons in the d band (N\nie-d) of active-metal atoms. Our work not only gives insights on the relationship between the electronic structure of SACs and their catalytic performance but also provides a guided direction for the rational design of SACs.We chose nine 2D materials (C3N4, graphdiyne, C2N, InSe, black phosphorus, BN, MoS2, WSe2, and Mo2C) as the substrates for SACs. As shown in Figure\u00a01\n, these nine 2D materials include single atomic layers, multiple atomic layers, single elements, double elements, flat surfaces, furrowed surfaces, and porous structures, which represent almost all types of 2D materials. Moreover, 14 single metal atoms with d electrons (3d: Mn, Fe, Co, Ni, Cu; 4d: Mo, Ru, Rh, Pd, Ag; 5d: W, Ir, Pt, and Au) were selected as active atoms of SACs. The single metal atoms are adsorbed on the energetically favorable sites of different 2D materials, which are determined by their electronic structures and high-symmetry sites. The most stable adsorption configurations are shown in Figure\u00a0S1 and are consistent with the previous DFT studies.\n25\u201327\n The binding energy of single metal atoms could be used for evaluating the thermodynamic stability of SACs, where the strong binding energy indicates high thermodynamic stability. From the viewpoint of the stability of SACs, C2N, C3N4, Mo2C, and InSe have more potential than others as substrates for SACs because of their stronger binding energy values (Figure\u00a0S2). For example, experimentally, C3N4 has been proved to have high stability for supporting Ru SAC even at high temperatures.\n28\n Note that although the binding energies of single metal atoms on GDY are medium among all 2D materials, GDY endows a huge potential as the substrate for SACs because of the confinement effect of the uniform distribution of pore configurations, which has been demonstrated by recent theoretical and experimental results.\n29\u201331\n On the basis of these 126 SACs, we further explore the activation mechanism of N2 and predict the catalytic activity of NRR through ML.The reaction mechanisms of NRR are very complex and include distal, alternating, enzymatic, and dissociative mechanisms. Except for the dissociative mechanism, the first protonation of N2\u2217 exists in all mechanisms, which is usually the potential limiting step (PLS) of NRR, especially for SACs.\n32\n\n,\n\n33\n The dissociative mechanism is almost impossible on SACs given that the single metal atom does not have enough activity to break the strong N\u2261N triple bond with a bond energy of 9.75 eV. Therefore, the most common distal mechanism on SACs is the only one considered for all systems in this work, as shown in Figure\u00a02\nA, which includes six protonations:\n\nN2\u2217\u00a0+ H+\u00a0+ e\u2212 \u2192 NNH\u2217 (R1)\n\n\n\n\nNNH\u2217\u00a0+ H+\u00a0+ e\u2212 \u2192 NNH2\u2217 (R2)\n\n\n\n\nNNH2\u2217\u00a0+ H+\u00a0+ e\u2212 \u2192 N\u2217\u00a0+ NH3 (R3)\n\n\n\n\nN\u2217\u00a0+ H+\u00a0+ e\u2212 \u2192 NH\u2217 (R4)\n\n\n\n\nNH\u2217\u00a0+ H+\u00a0+ e\u2212 \u2192 NH2\u2217 (R5)\n\n\n\n\nNH2\u2217\u00a0+ H+\u00a0+ e\u2212 \u2192 NH3\u2217 (R6)\n\n\nAmong all catalyst systems (126 SACs), only the PLS of nine SACs (W/MoS2, W/WSe2, Au/WSe2, W/BN, Mo/Mo2C, W/Mo2C, Ru/Mo2C, Cu/Mo2C, and Ag/Mo2C) are not the first protonations of N2\u2217, as shown in Table S1. Note that about half of these nine SACs are W/SACs, and the reaction free-energy values of R1 (\u0394G\nR1) on all W/SACs are very low, as shown in Figure\u00a0S3 and Table S2, demonstrating that the W single atom has enough activity to activate the N2 molecule, which is mainly due to its outmost 5d orbitals with four isolated electrons, as discussed in detail below. Moreover, more than half of these nine SACs are based on the substrate of Mo2C. As shown in Figure\u00a0S2, the strong binding energy of the single metal atom (\u0394E\nb-M) on Mo2C indicates the strong interaction between single metal atoms and Mo2C, which could lead to the different reaction mechanisms, where the PLS changes from R1 to other steps, as shown in Table S1.\nFigure\u00a02B demonstrates the catalytic activity of all SACs with the \u0394G\nPLS for NRR. About 34% of all designed SACs have better catalytic activity than the state-of-the-art Re(111) surface.\n34\n In addition, considering the 100% utilization rate of active atoms, SACs have been proved to have great potential for NRR. These 14 metals are divided into three regions according to their periods in the periodic table, as shown in Figure\u00a02B. Apparently, in the same period, the \u0394G\nPLS increases from left to right on the periodic table, where the VIB group (Mo and W) SACs show excellent catalytic activity for NRR, whereas the IB group (Cu, Ag, Au) and its nearest VIII group (Ni, Pd, Pt) SACs are almost inert for activating N2 molecule. This interesting phenomenon can be illustrated by the following activation mechanism.The activation process of N2 includes two continuous steps, as shown in Figure\u00a03\nA: the lone-pair electrons of N2 in 2\u03c3 bonding orbitals first transfer to the d orbitals of active-metal atoms, and then the d electrons of active-metal atoms feed back to the 1\u03c0\u2217 antibonding orbital of N2. However, researchers tend to focus on electron transfer from the d band of metal atoms to the antibonding orbital of N2 while ignoring electron transfer from the lone-pair electrons of N2 to the d orbitals of metal atoms.\n35\n In fact, the initial electron transfer is more important because this process is also an activation process with reduced bond order due to the electron transfer from the bonding orbitals of N2 to active-metal atoms. Subsequently, the d electrons of active-metal atoms feed back to the antibonding orbital of N2, which further reduces the bond order of N2. As shown in the molecule orbitals of N2 (Figure\u00a03A), before activation, the bond order of N2 is as large as 3, indicating a stable N2 molecule. After activation, the bond order of N2 reduces to 3\u00a0\u2212 (a\u00a0+ b)/2, where a and b represent the number of electrons transferred in both directions. We define this bidirectional electron transfer for activating molecules as the \u201cbidirectional activation mechanism.\u201dThe nature of the bidirectional activation mechanism indicates that the optimal catalyst should accept electrons from the bonding orbitals and provide electrons to antibonding orbitals of pre-activated molecules. Therefore, a new descriptor of isolated electron number in d orbitals (N\nie-d) for metal catalysts is proposed for evaluating bidirectional activation mechanism. Figure\u00a03B describes the relationship between the catalytic activity and N\nie-d in SACs on C2N; this substrate has strong binding energies for SACs and uniform distribution of pore configurations. In the same period, the larger N\nie-d shows a smaller \u0394G\nPLS, indicating a better catalytic activity for NRR. This trend can be understood by the electron configuration rule where the d orbitals with isolated electrons could gain electrons and lose electrons, while the d orbitals with an electron pair could not gain electrons and the empty d orbitals could not lose electrons. The electron configurations of Mo and W are shown in Figure\u00a03C. Mo and W SACs with high catalytic activity have high N\nie-d values of 5 and 4, respectively, indicating a strong ability for both gaining and losing electrons. Because of the high N\nie-d of Fe (4) and Mo (5) based on the bidirectional activation mechanism, we are able to better understand the high catalytic activity of Fe- and Mo-based catalysts in experiments, which endow high faradic efficiencies and high yield rates for NRR.\n36\u201338\n\nNote that the bidirectional activation mechanism is directly related to the catalytic activity rather than the adsorption energy. More importantly, this proposed mechanism can be applied not only to the activation of nitrogen but also to the activation of other molecules with large bond order, such as O2, CO, and CO2. When these molecules (O2, CO, and CO2) are absorbed on active atoms of catalysts, the first step is always electron transfer from the bonding orbitals of molecules to the active atoms and then electron transfer from the active atoms to the antibonding orbitals of molecules. This means that the bidirectional activation mechanism in this work can be extended to other catalytic reactions, such as the oxygen reduction reaction, CO2 reduction reaction, and CO reduction reaction, which is worthy of further study in the field of catalysis.To further elucidate the bidirectional activation mechanism, we investigated the local and partial density of state (LDOS and PDOS, respectively), as demonstrated in Figure\u00a04\n. The sixth-period elements with the best catalytic activity of the W element are used as examples for analyzing their electronic structure characteristics on the substrate of C2N. As shown in Figure\u00a04A, the d band centers of metal atoms are distant from the Fermi level from W to Au in the same period, indicating that the d electrons of W are more active and thus transfer to the antibonding orbitals of N2 more readily than others. Moreover, there are more vacant d orbitals in the W/C2N system. These half-filled d orbitals could give more room for the lone-pair electrons of N2, which is consistent with our previous analysis.The PDOS of W/C2N, N2 adsorbed W/C2N (N2-W/C2N), and N2 molecule are depicted in Figure\u00a04B. A strong hybridization between W-dxy, W-dz2 orbitals, and the bonding orbitals of N2-\u03c3 is found, where the electrons transfer to W-d orbitals from the N2-\u03c3 orbitals through W-dxy and W-dz2 orbitals. This means that the number of electrons in the bonding orbitals of N2 decreases, indicating the reduced bond order of N2. Moreover, all the W-d orbitals have hybridization with antibonding orbitals of N2-\u03c0\u2217, leading the N2-\u03c0\u2217 orbitals to be broadened and half filled by the electrons from the W-d orbitals. In particular, there is a strong hybridization between W-dyz, W-dxz, and N2-\u03c0\u2217 orbitals, denoting that the transfer of electrons from W-d orbitals to N2-\u03c0\u2217 orbitals is mainly through W-dyz and W-dxz orbitals. The electrons occupied in N2-\u03c0\u2217 orbitals further reduce the bond order of N2. Therefore, both the loss of electrons in bonding orbitals and the acquisition of electrons in antibonding orbitals could achieve activation of the N2 molecule, indicating the bidirectional activation mechanism. This insight into the catalytic nature of N2 on SACs plays a decisive role in the rational design of SACs for NRR.To quantify the effect of the bidirectional activation mechanism on SAC\u2019s catalytic properties, here we leverage the predicting power of ML models trained on DFT-calculated data. In addition, ML models can address the challenge presented by the vast chemical space of 2D material-supported SAC given that it is time consuming to exhaustively characterize all possible materials either theoretically or experimentally.ML models are developed to simultaneously predict three crucial physical quantities to NRR performance: \u0394G\nPLS for NRR, the adsorption free energy of hydrogen (\u0394G\nH\u2217) for the hydrogen evolution reaction (HER), and \u0394E\nb-M. \u0394G\nH\u2217 is an important factor in determining NRR selectivity because HER is always a competing side reaction in NRR.\n39\n Moreover, \u0394E\nb-M is directly related to the stability of the SAC, a high value of which is desired to resist sintering.\n40\n This is the first time that the outputs include the activity, selectivity, and stability of SACs for NRR rather than one aspect of NRR performance.The ML prediction is made on the basis of a total of seven input features, three intrinsic features of which characterize the 2D materials (Fermi energy, electrostatic potential, and the work function of pure 2D materials) and four intrinsic features of\u00a0which characterize the single atoms (electronegativity, electronic affinity, ionization energy, and N\nie-d of the single atom). Note that all features are only related to\u00a0the electronic structures rather than complex coordination environments. Moreover, these seven input features can be obtained directly from some databases and even the periodic table. The electronic properties of each SAC are not required in our ML models, and their acquisition is quite time consuming and laborious. Therefore, the simple input features are a huge advantage of our ML models.The developed ML models are based on the boosted-regression-tree ensemble method, which utilizes an ensemble of regression trees such that each regression tree makes its own prediction and the final prediction is the collective prediction over all individual regression trees. One ML model is built for each output variable (\u0394G\nPLS, \u0394G\nH\u2217, and \u0394E\nb- M), and three models are built as a result. All models take the same input features and are subject to the same hyperparameters and evaluation criteria.The developed ML models have high accuracy as demonstrated by their parity plots shown in Figures 5\nA and S5. The predicted values match those obtained from DFT calculations both in the training set and in the testing set for all three quantities predicted. The developed ML models show a testing-set mean absolute error (MAE) of 0.21, 0.24, and 0.43 eV and a root-mean-square error (RMSE) of 0.28, 0.31, and 0.52 eV for \u0394G\nPLS, \u0394G\nH\u2217, and \u0394E\nb- M, respectively, with a testing-set percentage of 20%. These ML performance metrics are reasonably high given the moderate dataset size, drastic value ranges of the outputs, and the complexity involved in the electronic structure of a metal-substrate combination. The models are only moderately overfit, as indicated by the slightly lower MAE values on the training sets corresponding to each of the three output variables. To compare more directly with the experimental data, our ML model also predicted an overpotential (\u03b7) for NRR, which is a direct indicator of catalytic reactivity in experiments, and a smaller \u03b7 value represents a faster NRR.\n41\n The ML model still has high accuracy with an MAE of 0.21\u00a0V and an RMSE of 0.28 V, as shown in Figure\u00a0S4. More importantly, the ML model also possesses high generalizability given that the ML-predicted \u03b7 values match those obtained from the previous theoretical and experimental works with reasonable accuracy (Figure\u00a05B).\n19\n\n,\n\n27\n\n,\n\n42\u201344\n This implies that the ML models are generalizable to a broad range of literature. For instance, according to the ML models, the predicted \u03b7 value of Ru/N4-C is 0.55\u00a0V versus a reversible hydrogen electrode (RHE), indicating an excellent catalytic performance for NRR, which is consistent with the experimental work of Geng et\u00a0al., where Ru/N4-C achieved a high yield rate of 120.9\u00a0\u03bcgNH3 mg\u22121\ncat. h\u22121 for NRR.\n44\n\nTo understand how important each input feature is to the output prediction, we analyzed feature importance on the prediction of the catalytic performance of NRR and HER. The relative importance of input features is shown in Figure\u00a05C (for HER) and in Figure\u00a05D (for NRR). Interestingly, the overall feature importance is different for HER and NRR such that HER is mostly influenced by the Fermi energy of the 2D material and the electronegativity and electronic affinity of the single metal atom and NRR is mostly influenced by N\nie-d. The high feature importance in N\nie-d for NRR, as opposed to HER, is in excellent agreement with the important role that directional activation mechanism plays in activating nitrogen molecules, as highlighted in the earlier discussion. Since the adsorption of H does not involve bidirectional electron transfer, the new descriptor of N\nie-d shows low feature importance in HER. Meanwhile, the different feature importance is conducive to regulating the catalytic activity of NRR and HER from different features. This means that it is easier to design SACs with high catalytic activity of NRR but low catalytic activity of HER, which is crucial for the selectivity during the NRR process.The accuracy and generalizability of the developed ML models could potentially act as a tool for mass-scale screening of combinations of 2D materials and single metal atoms. These ML models highlight the holistic approach toward rational SAC design by their ability to simultaneously predict NRR activity, selectivity, and SAC stability. It is important to note, on the other hand, that because of the nature of tree-based ML models, it might be difficult to accurately extrapolate our ML model for new types of 2D materials or new types of single metal atoms. However, for a new type of 2D material, the developed model requires the calculation of only a few single-atom metal types to predict for all other single-atom metal types, which was demonstrated in the randomly shuffled training-testing experiment above. This can still provide much time savings in comparison with calculating all combinations of 2D materials and single atoms via DFT.In conclusion, we investigated the catalytic performance of a series of SACs for electrocatalytic NRR on the basis of DFT calculations and developed a ML model to represent the structure-activity relations. Based on the bidirectional activation mechanism proposed, a new descriptor of N\nie-d is closely related to the catalytic activity for NRR. The large N\nie-d indicates high catalytic activity, which is further revealed by electronic structure analysis, where the electrons in \u03c3 orbitals of N2 transfer to the d orbitals of active atoms through dxy and dxz orbitals and the electrons go back to \u03c0\u2217 of N2 mainly by dyz and dz2 orbitals, resulting in the activation of N2. The insights achieved in this work on the bidirectional activation mechanism will further promote the rapid development of metal catalysts. More importantly, we can predict the catalytic activity of SACs for NRR through ML, which is further validated by both DFT calculations and experimental works. We hope that our strategy and the ML model can be extended to other catalytic systems.Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Chandra Veer Singh (chandraveer.singh@utoronto.ca).All unique reagents generated in this study will be made available upon request.There is no dataset or code associated with this paper.All computations were performed with the spin-polarized DFT calculations using the Vienna Ab initio Simulation Package.\n45\n The Projector-augmented wave pseudopotential and Perdew-Burke-Ernzerhof functional of the generalized gradient approximation were utilized to describe the interactions between valence electrons and ionic cores, as well as the exchange-correlation effects.\n46\n\n,\n\n47\n The kinetic energy cutoff for the wave-function calculations was set to 550 eV. A smearing width of 0.1 eV was applied for the Fermi smearing function. The supercells of all systems were calculated with a 4\u00a0\u00d7 4\u00a0\u00d71 Monkhorst-Pack grid of k-points, and a vacuum gap of about 15\u00a0\u00c5 was used to avoid interactions between the system and its mirror images. The Tkatchenko-Scheffler method was applied to describe the van der Waals interactions,\n48\n which has been successfully used in 2D materials such as boron nitride,\n49\n MoS2,\n50\n and boron monolayer.\n51\n The geometric relaxation was stopped when the incremental changes in total energy and forces were smaller than 1\u00a0\u00d7 10\u22124 eV and 0.02 eV/\u00c5, respectively.To evaluate the electrochemical catalytic reactions with the transfer of proton-electron pairs, we used the computational hydrogen electrode model to obtain the free energies,\n34\n\n,\n\n52\n where we set the reference potential to be RHE and determined the chemical potential of the proton-electron pair by one-half of the chemical potential of H2. Reaction free energy (\u0394G) was determined by\n\n(Equation\u00a01)\n\u0394G\u00a0= \u0394E\u00a0+ \u0394ZPE \u2013 T\u0394S,\n\nwhere \u0394E, \u0394ZPE, T, and \u0394S denoted the reaction energy, zero-point energy change, temperature, and entropy change, respectively. The overpotential (\u03b7) was a good indicator of catalytic activity, where a smaller \u03b7 value indicated a better catalytic activity for NRR. The \u03b7 value was determined by,\n\n(Equation\u00a02)\n\n\u03b7\u00a0= U\nequilibrium \u2212 U\nlimiting\n\n\nwhere U\nequilibrium and U\nlimiting denoted the equilibrium potential (U\nequilibrium\u00a0= \u22120.16\u00a0V versus RHE for N2\u00a0+ 6 H+\u00a0+ 6e\n\u2212 \u2192 2NH3) and the applied potential required for PLS (U\nlimiting\u00a0= \u2212\u0394G/e). We determined the stability of SACs by computing the binding energy of metal atoms on 2D materials (\u0394E\nb) as shown below,\n\n(Equation\u00a03)\n\u0394E\nb\u00a0= E\nSACs \u2013 E\n2D \u2013 E\nM\n\n\nwhere E\nSACs, E\n2D, and E\nM are the total energies of the SACs, the substrate of 2D materials, and metal atoms, respectively.The ML models were developed on the basis of the boosted-regression-tree ensemble method, which utilized a collection of individual regression trees. Each regression tree made its own prediction, and the final prediction was the collective prediction over all decision trees. The tree-based method was chosen primarily because each 2D material possessed distinct characteristics, making the tree-based method suitable. The algorithm was implemented in MATLAB with its built-in function, \u201cfitrensemble.\u201d The boosting algorithm was chosen to be least-squares boosting (\u201cLSBoost\u201d). All data were first shuffled randomly, and training and testing data were then split into a 80%/20% ratio. Based on Bayesian hyperparameter optimization from preliminary investigations, a learning rate of 0.05, number of learning cycles of 180, minimum leaf size of 3, and number of sampled variables of 10 were used as the hyperparameters throughout training. To analyze the feature importance, we used MATLAB\u2019s built-in function \u201cpredictorImportance,\u201d which determined the importance of each feature by summing feature importance values from all decision trees in the ensemble.We acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada, the Hart Professorship, and the University of Toronto. We also acknowledge Compute Canada for providing computing resources at the SciNet, CalculQuebec and the Westgrid consortia.Z.W.C., Z.L., and L.X.C. contributed equally to this work. C.V.S. and Z.W.C. conceived and designed the study. Z.W.C., Z.L., and L.X.C. carried out the DFT simulations and machine-learning model development. Z.W.C., Z.L., L.X.C., and C.V.S. wrote the paper. All authors discussed and revised the manuscript.The authors declare no competing financial interests.Supplemental information can be found online at https://doi.org/10.1016/j.checat.2021.03.003.\n\n\nDocument S1. Figures S1\u2013S5 and Tables S1 and S2\n\n\n\n\n\nDocument S2. Article plus supplemental information\n\n\n\n", "descript": "\n                  Single-atom catalysts (SACs) have provided new impetus to the field of catalysis because of their high activity, high selectivity, and theoretically full utilization of active atoms. However, the ambiguous activation mechanism prevents a clear understanding of the structure-activity relationship and results in a great challenge of rational design of SACs. Herein, by combining density functional theory (DFT) calculations with machine learning (ML), we explore 126 SACs to analyze and develop the structure-activity relationship for the electrocatalytic nitrogen reduction reaction (NRR). We first propose a bidirectional activation mechanism with a new descriptor for catalytic activity, which provides new insights for the rational design of SACs. More importantly, we establish a ML model for predicting the catalytic performance of NRR, validated by both DFT calculations and experimental works. The successful ML prediction in this work helps with the accelerated design and discovery of new catalysts by computational screening with high practical significance.\n               "}
{"full_text": "Substituting the traditional fossil fuels by high-energy-density hydrogen with zero-environmental impact promises a carbon-free era in fuel utilization [1\u20133]. Water electrolysis driven by renewable energy is an effective way for hydrogen production with little environmental impact [4,5], and therefore has drawn increasing attention in a world-wide scale [6\u20138]. Exploring non-noble electrocatalysts with high activity, long-term stability and easy deployment is of importance for practical applications.Many non-precious electrocatalysts in a powdery state have been designed [8\u201310]. The indispensable inert organic binders used to paste the powdery catalysts onto the conductive substrates tend to block active sites [11]. In addition, powdery catalyst film is prone to peel off from the substrate during long-term electrolysis, especially at large current densities in practical applications [12]. Moreover, fleeing-away of hydrogen bubbles from the organic binder-containing surfaces is impeded due to the deterioration of wettability [11,12], resulting in increased overpotential and energy consumption.Some non-precious metal catalysts such as Ni, Cu, Mo, and their alloys show appreciable catalytic activity for HER [3,5,8,13\u201315]. Importantly, these metals with high ductility can be easily processed and manufactured into various shapes according to the practical requirement, serving as both substrates for conducting current and catalysts for boosting HER [12,15]. Therefore, the above drawbacks of powdery catalysts can be avoided, enabling long-term stability and large-current-density electrolysis. For example, Ni is the most common HER catalysts for industrial water electrolysis in alkaline solutions [15,16]. However, the activity of these metal catalysts needs to be essentially improved.Constructing interfaces between metal catalysts and oxide promoters is an effective strategy to improve the activity of metal catalysts [13,15\u201318]. For example, addition of alkaline oxides into Fe-based catalysts can promote the Fischer-Tropsch synthesis reactions [18]. Enhancements of other catalytic reactions including CO oxidation, CO2 hydrogenation, and methane reforming are also reported by delicately designed metal/oxide interfaces [15,16,19,20]. Recently, HER is also greatly boosted by a metal/oxide interface-induced synergistic effect according to a \u201cchimney effect\u201d [14]. Improving the activity of metal catalysts for HER by constructing metal/oxide interfaces is an appealing strategy.Herein, Ni-V2O3 interfaces are one-pot constructed by electrochemical reduction of NaVO3 on porous nickel cathode in NaCl molten salt. The high-temperature molten salt enables the tightly adherence of electrodeposited V2O3 on porous nickel. The high solubility of NaVO3 in molten salt contributes to an ultra-fast homogeneous deposition of V2O3, contributing to binder-free construction of V2O3 coatings on nickel substrates within only several minutes. Specially, the nanostructured strip-like V2O3 is perpendicularly anchored on the surfaces of nickel substrate, promising abundant Ni-V2O3 interfaces and fully surface exposure of both nickel and V2O3. The V2O3-modified nickel results in interface-induced synergy between Ni and V2O3, contributing to enhanced activation of H2O and improved reduction of H* due to efficient electron transfer between substrate and reactive intermediates. Resultantly, V2O3-modified nickel shows much improved activity toward HER in alkaline solution when compared with the bare nickel counterpart.All chemical reagents were of analytical purity and used as received without further purification. NaCl (anhydrous, 99% purity) was provided by Shanghai Titan Scientific Co., Ltd. NaVO3 (anhydrous, 99.9% purity) was purchased from the Aladdin (Shanghai, China). Porous nickel (99.9% purity) was provided by Dongguan Zehui New Material Technology Co., LTD. Before electrolysis, NaCl was well mixed with 5 wt% NaVO3 and contained in an alumina crucible (inner diameter, 70\u00a0mm; height, 150\u00a0mm). The alumina containing the salts was then transferred into an alumina tube reactor, which was continuously flushed by an argon flow (150\u00a0mL min\u22121) to keep an inert atmosphere. The physically adsorbed water by the salts was removed by keeping the temperature at 400\u00a0\u00b0C for 12\u00a0h (h). Then the temperature is increased to 850\u00a0\u00b0C and maintained for half an hour to enable the complete melting of the salts. After that, a graphite anode (15\u00a0mm in diameter and 99.9% in purity) and porous nickel cathode were immersed into the molten salt and the electrolysis was then initiated. All the electrolysis temperature was kept at 850\u00a0\u00b0C, with the cell voltage being 2.5 or 2.8\u00a0V. The electrolysis time was varied from 1 to 60\u00a0min. After electrolysis, the cooled nickel cathode was immersed in de-ionized water for more than 12\u00a0h to remove the entrained salts and then dried at 60\u00a0\u00b0C in air overnight.X-ray diffraction (XRD) spectra were acquired on Rigaku Miniflex600 at a scan rate of 4\u00b0 min\u22121 with Ni filtered Cu K\n\u03b1 radiation (\u03bb\u00a0=\u00a01.5406\u00a0\u00c5). The field-emission scanning electron microscope (FESEM, Zeiss SIGMA) and transmission electron microscopy (TEM, Titan G 2 60\u2013300) were used to probe the morphology of prepared samples. The composition and elemental mapping images of the samples were analyzed by energy dispersive X-ray spectroscopy (EDS, GENESIS 7000 and OXFORD IET 200) attached to TEM apparatus. X-ray photoelectron spectra (XPS) were collected on X-ray photoelectron spectrometer (ESCALAB250Xi, Thermo Fisher Scientific). All the XPS results were calibrated by C 1\u00a0s at 284.8\u00a0eV.All the electrochemical texts were conducted in a 1\u00a0M KOH aqueous solution, using a standard three-electrode configurations programmed by a CHI 760 electrochemical workstation. Before test, the newly prepared solution was continuously bubbled for 1\u00a0h by a N2 flow at 100\u00a0mL min\u22121. A carbon rod (10\u00a0mm in diameter and 99.999% in purity) and a home-made reversible hydrogen electrode (RHE) were used as the counter electrode and reference electrode, respectively [11]. The porous nickel deposited with V2O3 (Ni-V2O3) was cut into a square piece (1\u00a0cm\u00a0\u00d7\u00a01\u00a0cm) and directly used as the working electrode, without addition of any organic binders or conducting agents. For comparison, performance of bare porous nickel (Ni) with the same dimension was also tested. In another case, a part of V2O3 powders were scraped off from the porous nickel substrate by sonication in de-ionized water at 60\u00a0\u00b0C for more than 12\u00a0h. After centrifugation and being dried at 60\u00a0\u00b0C overnight, solid V2O3 powder was obtained, which was then made into ink by addition of 2\u00a0mg as-prepared V2O3 powder into 400\u00a0\u03bcL ethanol and sonication for 1\u00a0h. The V2O3 working electrode (denoted as V2O3) was prepared by dropping 17.5\u00a0\u03bcL ink onto the surface of a glassy carbon rotating disk electrode (RDE, Pine Research Instrumentation, 5\u00a0mm in diameter), with further addition of ~2\u00a0\u03bcL Dupont Nafion 117 solution (5\u00a0wt%). The Pt/C working electrode was prepared by the same way, replacing the catalysts with the Pt/C powder (20\u00a0wt% Pt, Sigma-Aldrich) instead. The mass loading for Pt/C and V2O3 sample are both 0.5\u00a0mg cm\u22122, which are the same with that in Ni-V2O3 obtained by electrolysis at 2.8\u00a0V for 5\u00a0min. For all the test of the V2O3 and Pt/C working electrodes, the rotating speed was fixed at 1600 rounds per minute. All the linear scanning voltammogram (LSV) curves were recorded at 5\u00a0mV s\u2212\n1. All polarization curves were iR-corrected unless noted. The electrochemical impedance spectra (EIS) were acquired in the frequency range of 0.01\u2013105\u00a0Hz, with the amplitude and overpotential being fixed at 0.01\u00a0V and 230\u00a0mV (Solartron 1470E).The density-functional-theory (DFT) calculations were performed by using Vienna Abinitio Simulation Package (VASP) with Projector Augmented Wave (PAW) method [21\u201324]. Perdew-Burke-Ernzerhof (PBE) functional for the exchange-correlation term was used with the projector augmented wave method [25,26], with the kinetic energy cutoff of electron wave functions being set as 400\u00a0eV. The convergence of energy and forces were set to be 1\u00a0\u00d7\u00a010\u22124\u00a0eV and 0.05\u00a0eV \u00c5\u22121, respectively [27\u201330]. The crystal surfaces with the highest XRD peak intensities of Ni and V2O3, namely Ni(111) and V2O3(104), were used to construct the geometry structures [31].The solubility of NaVO3 in NaCl molten salt is high up to 8 wt% [32,33], which means that the NaVO3 and NaCl can form a homogenous system in a molten state at the electrolysis temperature [34\u201336]. When the cell voltage is applied, the soluble VO3\n\u2212 on the surface of porous nickel cathode can be easily electroreduced to V2O3 (2VO3\n2\u2212\u00a0+\u00a04e\u2212 =V2O3\u00a0+\u00a03O2\u2212) [7,32,33], which can be in situ adhered onto the porous nickel to form V2O3-coated nickel surfaces (denoted as Ni-V2O3), as illustrated in Fig.\u00a01\n.Deposition of V2O3 on the porous nickel cathode is strongly validated by the XRD and XPS results after electrolysis (Ni-after, Fig.\u00a02\na\u2013c). In addition to Ni (JCPDS No. 01\u20131258), extra diffraction peaks relating to V2O3 (JCPDS No. 84\u20130318) are observed in the XRD pattern of the sample Ni-after (Fig.\u00a02a). The appearance of V 2p signals in the XPS survey spectrum of the sample Ni-after (Fig.\u00a02b) also reveals the existence of vanadium element on the surface of porous nickel, which is further manifested by the comparison of high-resolution V 2p spectra between the samples of Ni-before and Ni-after.The morphology characterizations further uncover the adherence of nanostructured V2O3 on the surface of porous nickel cathode. After deposition of V2O3, the smooth surface of bare porous nickel (Fig.\u00a02d) becomes furry (Fig.\u00a02e), with deposition of closely anchored strip-like V2O3 nanoarrays (Fig.\u00a02f). The length and width of single V2O3 strips are ~1\u00a0\u03bcm and 100\u2013200\u00a0nm, respectively (Fig.\u00a02g), with the thickness being several nanometers (Fig.\u00a02f). Specially, the V2O3 strips are perpendicularly attached to the surface of nickel (Fig.\u00a02f), which is further verified by the TEM images in Fig.\u00a02(g). Such a configuration contains Ni/V2O3 interfaces and maximize the surface exposure of both Ni and V2O3. The Ni/V2O3 interface is further manifested by the high-resolution TEM image in Fig.\u00a02(h), as evidenced by the clear boundary between (200) plane of Ni and (104) plane of V2O3. Fig.\u00a02(i) shows the high-resolution TEM image of region 3 in Fig.\u00a02(g), in which the (104) plane of V2O3 is also observed. The results also reveal that the strips are V2O3, as further manifested by the homogenous distribution of V and O in one detached strip (Fig.\u00a02j\u2013m).Ni-V2O3 shows much more superior HER performance in alkaline solution than the Ni and V2O3 counterparts. The lowest onset potential (20\u00a0mV) occurs in Ni-V2O3 among the samples of Ni-V2O3, Ni, and V2O3 (Fig.\u00a03\na), which is further validated by the overpotential comparison at 10\u00a0mA cm\u22122 (\u03b7@10\u00a0mA cm\u22122) for various samples (Fig.\u00a03b). The optimal electrolysis time for electrodeposition of V2O3 is 5\u00a0min, as evidenced by both the \u03b7@10\u00a0mA cm\u22122 (Fig.\u00a03b) and LSV curves (Fig. S1). Too short electrolysis time can hardly provide enough V2O3 strips to construct abundant Ni/V2O3 interfaces (Fig. S2) while too long electrolysis time leads to overgrowth of V2O3 for blocking the surfaces of nickel (Fig. S3). Electrolysis of 5\u00a0min. can realize the balance between the as-mentioned two aspects. A \u03b7@10\u00a0mA cm\u22122 value as low as 136\u00a0mV (Fig.\u00a03b) hence appears in the optimal sample, much smaller than that of Ni (256\u00a0mV, Fig.\u00a03b) and V2O3 (632\u00a0mV, Fig.\u00a03b). Of note, this strategy with such a short time (5\u00a0min) and extremely easy post-processing of samples (just leaching in de-ionized water) for constructing the well-defined metal/oxides interfaces is much more convenient when compared with previously reported methods [15\u201317]. Compared with electrolysis time, the influence of electrolysis voltage on the performance of Ni-V2O3 samples is minor (Fig. S4).The activity enhancement of porous Ni by deposited V2O3 is further manifested by the comparisons of Tafel slopes (Fig.\u00a03c), electrochemical surface areas (Fig.\u00a03d), and EIS spectra (Fig.\u00a03e). Compared with Ni and V2O3, Ni-V2O3 presents the lowest Tafel slop (125\u00a0mV dec\u22121), meaning much improved reaction kinetics for HER after deposition of V2O3 on nickel. By extrapolation of the Tafel curves, the exchange current density (j\n0) can be obtained, which is an indicator for the intrinsic activity of electrocatalysts. As shown in Fig. S5, deposition of V2O3 on porous nickel increases the exchange current density from 0.34\u00a0mA cm\u22122 (Ni) to 0.53\u00a0mA cm\u22122 (Ni-V2O3), which are also higher than that of V2O3 (0.04\u00a0mA cm\u22122). According to the CV curves at different scan rates recorded in the non-faradic regions (Fig. S6), the electrochemical surface area (ECSA) is measured based on the double-layer capacitance (C\ndl). The C\ndl of Ni-V2O3 (0.95 mF cm\u22122) is much higher than that of Ni (0.21 mF cm\u22122) and V2O3 (0.38 mF cm\u22122), even close to that of Pt/C (1.09 mF cm\u22122). The results also reveal that deposition of V2O3 on nickel can increase the reactivity sites for HER. The electrochemical impedance spectra (EIS) reveal the lowest charge transfer resistance of Ni-V2O3 among the three tested samples (Fig.\u00a03e), indicating the constructed Ni/V2O3 interfaces can facilitate the electrons transfer between substrate and reactive intermediates. Such a synergy also endows the Ni-V2O3 electrode a good long-term durability, as evidenced by very small overpotential increases after electrolysis for 20\u00a0h at various current densities (Fig.\u00a03f). The performance of Ni-V2O3 is comparable/superior to reported Ni-based electrocatalysts or other HER catalysts (Table S1). Surface modification of Ni substrate by forming a phosphide or carbide layer is a potential route for further enhancing the performance [3,5,8,13\u201315].The synergy between Ni and V2O3 induces enhanced electron transfer between the Ni/V2O3 interfaces. Fig.\u00a04\n(a) shows the XPS spectra of Ni 2p before and after deposition of V2O3. Shift of Ni0 peak toward a higher binding energy is observed in the XPS spectra after deposition of V2O3 on Ni surfaces, revealing a tendency for electron transfer from Ni to V2O3\n[13]. Such a phenomenon is also observed by other researchers [15,16]. Ni2+ in Fig.\u00a04(a) should be attributed to the surface oxidation of Ni substrate [15,16]. Compared with bare Ni, the increase in the peak intensity of Ni2+ in Ni-V2O3 indicates the tendency for the electron transfer from Ni surface to V2O3. Such results are in concert with the decreased peak intensity of Ni0 after decorating V2O3 on Ni, further validating the electron transfer from Ni to V2O3. Of note, over-oxidation of Ni surface can decrease the surface conductivity of Ni substrate, which is one reason for limiting the further enhancement of HER performance. The accumulation of electrons around V2O3 in the Ni/V2O3 interfaces is further uncovered by the local charge density difference (\u0394\u03c1, Fig.\u00a04b). From a closer observation of O and Ni atoms at positions close to the interfaces, positive \u0394\u03c1 near O (denoted as ONearby in Fig.\u00a04b) is found, indicating the increase of electron density. The increase of electron density in ONearby is in concert with the decrease of electron density in NiNearby, as evidenced by the negative \u0394\u03c1 value of Ni atoms at the interfaces [14,38]. Such results mean that electrons are transferred from Ni atoms of nickel sheet to O atoms of V2O3 in the Ni/V2O3 interfaces, being consistent with the XPS results [38].The induced electron transfer from Ni to surface V2O3 promotes the activation of H2O and combination of H* to generate H2. Two steps are involved for hydrogen evolution reaction (HER) in alkaline solution, namely prior water dissociation to form H* intermediates (Volmer step) and subsequent combination of H* to form H2 (Tafel step or Heyrovsky step) [17,39\u201341]. The activated water adsorption energy (\u0394G\nH2O) is applied as the activity descriptor for the former step while the binding free energy of H* (\u0394G\nH*) is used as the activity descriptor for the latter step [17,42,43]. Details for calculating the \u0394G\nH2O and \u0394G\nH\n\u204e are provided in the supporting information (see Text 1), with the corresponding results being presented in Fig.\u00a04(c) and optimized structures being shown in Figs. S7 and S8. The construction of Ni/V2O3 interfaces decreases the value of \u0394G\nH\n\u204e from \u22120.549\u00a0eV for Ni to \u22120.092\u00a0eV for Ni-V2O3 (Fig.\u00a04c), approaching to the optimal value (\u0394G\nH*=0) [17]. Therefore, the Ni/V2O3 interfaces promote the H2 generation. More specifically, accumulation of electrons around the interface-positioned O atoms stimulates the combination of H* toward generation of H2. Although \u0394G\nH\n\u204e (\u22120.143\u00a0eV, Fig.\u00a04c) for single V2O3 is also close to the optimal value (0\u00a0eV), the high \u0394G\nH2O (0.269\u00a0eV) for single V2O3 retards the dissociation of water to generate H*, leading to sluggish HER kinetics. The same case is observed for single Ni, as evidenced a large value of \u0394G\nH2O for Ni (0.525\u00a0eV, Fig.\u00a04c). Interestingly, the lowest \u0394G\nH2O value (0.073\u00a0eV) is observed for Ni-V2O3, implying that the V2O3 on the surfaces of nickel accelerates the cleavage of HOH [17]. Hence, V2O3 strips attached onto the Ni surface can act as a water dissociation promoter to generate hydrogen intermediates, which are then combined to generate H2 on the O sites of V2O3.The deposited V2O3 strips with cuspidal ends are perpendicularly anchored on the surface of nickel, creating abundant Ni/V2O3 interfaces (as illustrated in Fig.\u00a05\na). The V2O3 strips attached onto the surface of Ni promote the dissociation of H2O to generate hydrogen intermediates (denoted as step \u2460 in Fig.\u00a05b). The Ni substrate with high conductivity swiftly guides the electrons from external circuit to the surfaces (denoted as step \u2461 in Fig.\u00a05b). Importantly, the synergy between Ni and V2O3 pumps the electrons on surfaces of Ni to surroundings of O atoms in V2O3, promoting the combination of H* for generating H2 (denoted as \u2462 in Fig.\u00a05b). Therefore, the high-efficiency construction of Ni/V2O3 interfaces by molten salt electrodeposition of V2O3 from highly soluble NaVO3 provides a facile and swift way for improving the activity of Ni toward HER. Other metallic substrates can be also used for fabricating free-standing metal-oxide electrodes by this method. When a carbon cloth is used as the substrate, formation of free-standing carbides is highly possible during molten salt electrolysis [33,36,37]. Of note, NaVO3 is the primary intermediate product during processing of vanadium slag, which is a typical industrial waste from metallurgy of vanadium-containing ores [33,36,37]. Therefore, the strategy provided herein can hopefully be also applied for the deep processing of vanadium slags.V2O3-modified Ni with superior HER activity in alkaline solution is fabricated by molten salt electrodeposition of V2O3 on porous nickel. The high solubility of NaVO3 in molten salts and high-temperature medium enables the swift deposition and tightly adhesion of V2O3 strips on Ni, creating abundant Ni-V2O3 interfaces with a special perpendicular-anchoring configuration. Such Ni-V2O3 interfaces induce a synergy between Ni and V2O3, promoting the activation of H2O and combination of H* to form H2. Resultantly, the V2O3-modified Ni shows a \u03b7@10\u00a0mA cm\u22122 value as low as 136\u00a0mV and a Tafel slope of only 125\u00a0mV dec\u22121 as well as a long-term stability for HER in alkaline solution. The strategy herein can provide for not only the activity improvement of Ni towards HER but also value-added processing of vanadium slag wastes.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 funding support from the National Natural Science Foundation of China (51722404, 51674177, 51804221 and 91845113), the National Key R&D Program of China (2018YFE0201703), the China Postdoctoral Science Foundation (2018M642906 and 2019T120684), the Fundamental Research Funds for the Central Universities (2042017kf0200), and the Hubei Provincial Natural Science Foundation of China (2019CFA065).Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2020.03.048.\n\n\nImage, application 1\n\n\n\n", "descript": "\n                  Implementation of non-precious electrocatalysts is key-enabling for water electrolysis to relieve challenges in energy and environmental sustainability. Self-supporting Ni-V2O3 electrodes consisting of nanostrip-like V2O3 perpendicularly anchored on Ni meshes are herein constructed via the electrochemical reduction of soluble NaVO3 in molten salts for enhanced electrocatalytic hydrogen evolution. Such a special configuration in morphology and composition creates a well confined interface between Ni and V2O3. Experimental and Density-Functional-Theory results confirm that the synergy between Ni and V2O3 accelerates the dissociation of H2O for forming hydrogen intermediates and enhances the combination of H* for generating H2.\n               "}
{"full_text": "Intense research on the water-oxidation catalyst (WOC) center in photosystem II (PSII) over the last decades has revealed deep insights on the mechanisms by which nature liberates electrons and protons from H2O, two critical ingredients for downstream reactions such as CO2 reduction and N2 fixation.\n1\n\n,\n\n2\n This knowledge has propelled research on using molecular catalysts to oxidize water, and impressive progress has been made in terms of catalyst performance as measured by turn-over frequencies (TOFs) and turn-over numbers.\n3\n\n,\n\n4\n From a technological development perspective, there is a strong incentive to perform the reaction on heterogeneous catalysts, especially on those of low-cost and outstanding stability. Indeed, recent years have witnessed a surge of such research activities.\n5\u201311\n Despite the apparent variety of these catalysts, they share important commonalities in the chemical mechanisms. For instance, it is generally believed that the reaction proceeds through a series of proton-coupled electron transfer steps that lead to the formation of M=O (where M represents an active metal center) intermediates.\n12\n\n,\n\n13\n It is also agreed upon that the subsequent O\u2013O bond formation is of critical importance to the overall reaction.\n14\n However, the details of the O\u2013O formation and the subsequent steps have been the subject of diverging views. At least two possible pathways have been proposed and supported.\n15\u201318\n One involves direct nucleophilic attack of water, followed by O2 release and regeneration of the catalyst. In the literature, this mechanism is referred to as water nucleophilic attack (WNA) (Figure\u00a01\n, right pathway).\n4\n\n,\n\n15\n The other involves the coupling of two metal-oxo intermediates followed by O2 release, which is referred to as intramolecular oxygen coupling (IMOC) (Figure\u00a01, left pathway).\n15\n\nFor Ir- and Ru-based molecular catalysts, density-functional theory (DFT) calculations predicted that the IMOC pathway dominates at low overpotentials, whereas the WNA pathway becomes accessible at higher overpotentials.\n17\n\n,\n\n19\n The two pathways were also predicted to be competitive on a heterogenized dinuclear Ir oxide cluster.\n17\n With optical pump-probe spectroscopy, Cuk et\u00a0al.\n18\n monitored the microsecond decay of oxyl (Ti\u2013O\u22c5) and bridge (Ti\u2013O\u22c5\u2013Ti) intermediates on SrTiO3 photoelectrodes. They found that the two species decay with distinct reaction rates on a microsecond timescale. It was suggested that Ti\u2013O\u22c5\u2019s convert to Ti\u2013O\u2013O\u2013Ti by dimerization (IMOC pathway) and Ti\u2013O\u22c5\u2013Ti converts to Ti\u2013OOH by nucleophilic attack of water (WNA pathway). Furthermore, it was found that the relative predominance of the two pathways was controlled by the ionic strength of the electrolyte, with the WNA pathway dominating at low ionic strength. However, how the relative predominance of these mechanisms depends on the applied electrode potential has not been investigated in experiments. Herein, we address this central question.Inspiration on how to further this understanding could be drawn from progress made in molecular WOC-based studies. To discern different pathways for the water-oxidation reaction by molecular catalysts, researchers have resorted to a strategy of correlating the reaction rate with the catalyst concentrations.\n4\n With the help of additional experiments such as isotope labeling, significant knowledge has been gained.\n20\u201322\n However, similar approaches are challenging to implement for heterogeneous catalysts, because the active sites, including their structures and densities, are often poorly defined on a heterogeneous catalyst. The challenge could be circumvented using clever experimental designs. For instance, Durrant et\u00a0al.\n23\n have identified a change of reaction orders relative to the hole concentration from the first to the third order on Fe2O3 using photoinduced absorption spectroscopy. Frei et\u00a0al.\n13\n have succeeded in observing both the metal-oxo and superoxo species, using an infrared spectroscopy (IR) technique. In both studies, different reaction mechanisms were proposed for different light intensities. Nevertheless, owing to the lack of detailed information on the active centers, particularly their density under different conditions, it remains difficult to directly corroborate these early observations for an unambiguous understanding of water oxidation on heterogeneous catalysts. Although it is possible to address this challenge by synthesizing heterogeneous catalysts with well-defined active centers, as has been demonstrated recently by others and us,\n24\n\n,\n\n25\n the catalyst library remains limited, and significant work is needed before the values of such catalysts can be materialized. An alternative approach is to study how the reaction kinetics changes as a function of water activity, which is the main strategy for this present work.To appreciate the significance of this strategy, it helps to examine the proposed WNA and IMOC pathways on a heterogeneous Co phosphate (Co\u2013Pi) catalyst (Figure\u00a01). Previous studies have suggested that the initial electron/proton transfer steps (vertical arrow in the center) are fast in comparison with the O\u2013O formation. Therefore, these steps are quasi-equilibrated, whereas O\u2013O formation limits the rate of the reaction. From the oxidized state of the catalyst shown on the bottom of the scheme, the water-oxidation process can proceed through two distinct pathways: the WNA pathway involves a water molecule within the electric double layer in the rate-determining O\u2013O forming step (right arrow). By contrast, the IMOC pathway only involves surface species in the rate-determining step (RDS) (left arrow). On the basis of this simplified mechanistic picture, the water-oxidation reaction is expected to be (pseudo) first order in the water activity when proceeding through the WNA pathway, whereas it is (pseudo) zeroth order when proceeding through the IMOC pathway. This simplified view assumes that the change in the water activity does not significantly affect the positions of the quasi-equilibria before the presumed RDS of O\u2013O bond formation, as discussed later. Therefore, it is possible to discern the reaction mechanisms even without detailed knowledge of the active centers by altering the water activity, which has not been investigated previously.The problem is now reduced to how to alter water activity in a water-oxidation reaction. Indeed, most previous studies on this subject have treated water as a substrate of invariant activity, such that it was excluded in most kinetic considerations.\n26\n\n,\n\n27\n Only recently did we see advances where the water activity could be suppressed significantly in aqueous solutions.\n28\u201330\n The so-called \u201cwater-in-salt\u201d electrolyte containing high concentrations of salts (e.g., up to 21\u00a0m [mole per kg of H2O]) represents one such system. The strong solvation effect of the high-concentration ions renders its H2O behaviors drastically different from those in bulk H2O. It becomes possible to perform water-oxidation reactions in an aqueous system where the water activity is no longer unity. Therefore, we are offered an opportunity to test the hypothesis proposed in the previous paragraph. That is, we expect a different sensitivity of the kinetics on the water activity for different mechanisms.To prove this concept, we have chosen Co-oxide-based catalysts as a study platform because they represent a class of most studied heterogeneous WOCs, with Co\u2013Pi receiving arguably the most attention. A broad knowledge base has already been generated.\n15\n\n,\n\n27\n\n,\n\n31\u201336\n For example, the coordination environment of Co has been identified by a suite of spectroscopic techniques.\n34\n That the O\u2013O formation is the RDS has been supported by numerous studies.\n15\n\n,\n\n27\n\n,\n\n31\n\n,\n\n32\n\n,\n\n35\n\n,\n\n36\n Both WNA and IMOC mechanisms have been proposed and supported by either experimental or computational studies for this catalyst.\n15\n\n,\n\n32\n\n,\n\n35\u201339\n Herein, we report the new observation of a switch from the IMOC pathway at low applied potentials to the WNA mechanism at high applied potentials.Previous studies have shown that various implementations of infrared and surface-enhanced Raman spectroscopies are powerful probes of water-oxidation intermediates.\n12\n\n,\n\n13\n\n,\n\n18\n\n,\n\n40\u201348\n To examine the mechanistic proposal (Figure\u00a01), we employed surface-enhanced infrared absorption spectroscopy (SEIRAS) in the attenuated total reflection (ATR) geometry. In SEIRAS-ATR, the surface plasmon resonance of rough metal films locally enhances the evanescent field, rendering the technique sensitive to sub-monolayers of species adsorbed on the electrode.\n49\n With this work, we establish SEIRAS-ATR in the Kretschmann configuration as a tool for probing water-oxidation intermediates on metal oxide catalysts. For this purpose, we first electrochemically deposited a thin layer of CoOx(OH)y\n\n31\n onto a chemically deposited Au thin film (CoOx(OH)y-Au)\n50\n on a micro-machined Si-ATR crystal,\n51\n which affords high infrared transparency below 1,200\u00a0cm\u22121. A scheme of the setup is shown in Figure\u00a0S1 in the supplemental information. For SEIRAS-ATR, CoOx(OH)y instead of Co\u2013Pi was employed as the prototypical catalyst because the latter would greatly complicate the interpretation of the IR spectra in the region around approximately 1,000\u00a0cm\u22121 owing to the phosphate anion and its response to the applied potentials. As will be discussed in detail later in this work, the electrochemical behaviors of CoOx(OH)y are comparable with Co\u2013Pi. It also features structurally similar active sites and the same cobalt oxidation states under water-oxidation conditions as Co\u2013Pi.\n31\n\n,\n\n52\n The CoOx(OH)y-Au film exhibits a large activity for water oxidation in comparison with the Au substrate (Figure\u00a0S2).\nFigure\u00a02\n shows the steady-state spectra of the CoOx(OH)y-Au electrode in 0.1\u00a0M potassium phosphate (KPi) in D2O, H2O, and H2\n18O. The absorbance was calculated according to absorbance\u00a0= \u2212log(S/R), where S and R refer to the sample and reference spectra, respectively, taken at 2.21 and 1.61 V. Unless otherwise noted, all potentials in this work are relative to the reversible hydrogen electrode. The spectrum in the D2O-based electrolyte exhibited a band centered at 1,014\u00a0cm\u22121 (at 2.21 V) (Figure\u00a02A). The intensity of this band increased with increasing applied potential (Figure\u00a0S3), suggesting that it is caused by a water-oxidation intermediate. To assign the band to a water-oxidation intermediate, we performed the following control experiments: First, to exclude the possibility that the band (1,014\u00a0cm\u22121) arises from a phosphate species in solution, we confirmed that the band also appears when the electrolyte is 0.1\u00a0M KCl in D2O and in H2O (Figure\u00a0S4). Second, the band is absent on an Au electrode without the CoOx(OH)y film (Figure\u00a0S4).These observations strongly suggest that the band centered at 1,014\u00a0cm\u22121 is a water-oxidation intermediate on CoOx(OH)y-Au. According to the proposed mechanism, this spectral feature can be associated with either Co\u2013O\u2013O\u22c5\u2013Co from the IMOC pathway or Co\u2013O\u2013O\u22c5 or Co\u2013O\u2013OH from the WNA pathway (Figure\u00a01). To further assign this band, we conducted isotopic labeling experiments with H2O and H2\n18O. The lack of an isotopic shift when the solvent was switched from D2O to H2O implies that the vibrational mode of the species does not involve a hydrogen atom (Figure\u00a02B). Upon switching to the H2\n18O electrolyte, this band shifts to 966\u00a0cm\u22121 (Figure\u00a02C). The 48\u00a0cm\u22121 difference (from 1,014 to 966\u00a0cm\u22121) indicates that the intermediate involves an O-containing motif. These experimental observations support the conclusion that the 1,014\u00a0cm\u22121 band is associated with the superoxide intermediates (Co\u2013O\u2013O\u2022\u2013Co or Co\u2013O\u2013O\u2022).\n13\n\n,\n\n43\n\n,\n\n53\n The other possible water-oxidation intermediate, hydroperoxide (Co\u2013O\u2013OH), would feature characteristic bands in the 740\u2013920\u00a0cm\u22121 region.\n42\n\n,\n\n44\n\n,\n\n54\n\n,\n\n55\n Owing to the absorption by the H2O librational mode, the signals were too weak to be discernable in that spectral range. The other bands in the spectra in Figures 2A\u20132C are due to the enrichment and depletion of electrolyte phosphate species at the interface with changes in applied potential. The magnitude of those spectral changes depends on the characteristics of a specific electrode, such as film thickness and homogeneity, and the electrolyte system. The negative band at \u223c1,050\u00a0cm\u22121 in Figures 2A and 2B is likely due to a surface-adsorbed phosphate species.\n56\n\n,\n\n57\n The spectrum of a bulk KPi solution is shown in Figure\u00a02D. Duplicate experiments confirm the reproducibility of the spectroscopic results (Figure\u00a0S5). Taken together, this set of experiments demonstrates the utility of the SEIRAS-ATR technique for the detection of water-oxidation intermediates under operating conditions. Importantly, the result confirms the presence of a superoxo species, consistent with the mechanistic proposal (Figure\u00a01). Future research should be directed to further distinguish between Co\u2013O\u2013O\u22c5\u2013Co and Co\u2013O\u2013O\u22c5.To further probe the mechanisms as shown in Figure\u00a01, we monitored the electrochemical water oxidation current as a function of electrode potential in water-in-salt electrolytes of varying water activities. As noted earlier, different reaction orders with respect to water activity are expected from the two competing mechanisms: a (pseudo) first-order dependence on H2O activity (a\n\nw\n) is expected for the WNA route, whereas a (pseudo) zeroth-order dependence on a\n\nw\n is expected for the IMOC pathway. In a practical electrochemical system, the dependence of the kinetics on a\n\nw\n is likely more complicated because of a number of other factors, including the participation of H2O as a solvent; these potential complications notwithstanding, the value of quantitatively analyzing the reaction rates as a function of water activity becomes obvious.\nFigure\u00a03\nA compares the steady-state electrochemical current densities due to the oxidation of water on Co\u2013Pi in contact with 0.1\u00a0M KPi containing 0, 2, 4, and 7\u00a0m NaNO3. The corresponding water activities are shown in the legend and were calculated on the basis of empirical equations.\n58\n These values describe the activity of bulk water in these water-in-salt electrolytes. We caution that the activity of water at the electrocatalytic interface may be different from those values. Nevertheless, the activities of interfacial water are expected to qualitatively follow the same trend with increasing water-in-salt concentration. All electrolytes were at neutral pH and were stirred during measurements, which were performed on electrodeposited Co\u2013Pi on fluorine-doped tin oxide (FTO) substrates in a single-compartment electrochemical cell. The potential window was carefully chosen so as to avoid mass transport limitations (i.e., >1.71 V) or large experimental errors due to low current densities (i.e., <1.62 V). Details of the data collection protocol are given in the supplemental information, and a representative dataset is shown in Figure\u00a0S6. As shown, with increasing molality of NaNO3 and, hence, decreasing a\n\nw\n, the current density of water oxidation is increasingly suppressed. A similar trend was observed for CoOx(OH)y (Figure\u00a0S7), suggesting that the observed trend is a more general feature of cobalt oxide-based catalysts. This finding further corroborates our assertion made earlier that CoOx(OH)y is an appropriate alternative model system for Co\u2013Pi.The observed suppression of the water-oxidation reaction could arise from a number of different physical phenomena. First, to test whether the catalyst undergoes irreversible structural changes in the different electrolytes, we recorded the cyclic voltammograms (CVs) of the same Co\u2013Pi electrode in 0.1\u00a0M KPi before and after collection of 3 cycles of CVs in the four electrolytes (of molalities 0, 2, 4, and 7 m). As shown in Figure\u00a0S8, the CVs in 0.1\u00a0M KPi before and after catalysis in the water-in-salt electrolytes overlap. These data suggest that no irreversible changes in catalytic activity occur during water oxidation in the water-in-salt electrolytes.Second, to test whether the mass transport of water to the electrode limits the reaction rate at high salt concentrations, we collected the steady-state electrochemical current densities of a Co\u2013Pi-coated Pt rotating disk electrode (RDE) at rotation rates of 2,000\u00a0rpm (Figure\u00a0S9) and 0\u00a0rpm (Figure\u00a0S10). Comparison of the two figures reveals that the recorded current densities on the RDE exhibit the same trend with increasing salt concentration, irrespective of the rotation rate. Moreover, as demonstrated in Table S1, the increase in the thickness of the stagnant layer with electrolyte concentration is expected to be small. Collectively, these results suggest that the suppression of the water-oxidation reaction is not caused by limited mass transport of water to the electrode.Third, at high concentrations of NaNO3, nitrate anions are expected to limit the enrichment of phosphate anions in the electric double layer with increasing potential. As a result, the pH buffer capacity at the electrocatalytic interface might decrease with increasing NaNO3 concentration. Changes in the pH in the vicinity of the electrode (local pH) could impact the reaction rate and mechanism.\n27\n\n,\n\n59\n To exclude local pH effects as a possible reason for the reactivity trends with increasing NaNO3 concentration, we performed three different control experiments: (1) we monitored the electrochemical current density as a function of solution pH at a fixed (absolute) electrode potential. As shown in Figures S11\u2013S13, the pH dependence of the current density was independent of the rotation rate of the RDE. (2) We performed galvanostatic titration experiments. The potential shows an approximately Nernstian shift of 60\u00a0mV/pH for all electrolytes (Figures S11\u2013S13). (3) We varied the concentration of KPi in the electrolytes containing 4 and 7\u00a0m NaNO3. As shown in Figure\u00a0S14, the potential dependence of the reaction rate is insensitive to the concentration of KPi. Taken together, these control experiments suggest that the local pH does not significantly depend on the concentration of NaNO3.Fourth, to test whether nitrate anions block catalytic sites, we recorded the electrochemical current density as a function of potential in 7\u00a0m NaClO4. Perchlorate typically does not chemisorb on electrodes.\n60\n As shown in Figure\u00a0S15, the impact of 7\u00a0m NaClO4 on the current density is similar to that of 7\u00a0m NaNO3. This result indicates that nitrate anions do not block catalytic sites of Co\u2013Pi.Fifth, alkali metal cations are known to influence the rate of the water-oxidation reaction on various electrocatalysts.\n61\u201365\n In the case of Ni oxyhydroxides, intercalated electrolyte cations have been proposed to stabilize reaction intermediates.\n62\n\n,\n\n64\n To test whether the catalytic activity is affected by the identity of the cation, we conducted additional control experiments in 2\u00a0m KNO3. As shown in Figure\u00a0S16, the current modulation ratio virtually overlaps with the one obtained in 2\u00a0m NaNO3 (higher concentrations of KNO3 could not be tested because of the lower solubility of that salt relative to NaNO3). This result is consistent with earlier work\n66\n showing that the substitution of K+ in Co\u2013Pi by Na+ has no significant impact on the catalytic activity of this catalyst. On the basis of this finding and our observation that the catalytic activity of Co\u2013Pi is retained after a sequence of CVs in three water-in-salt electrolytes (Figure\u00a0S8), we can exclude the incorporation of Na+ into the Co\u2013Pi film as the origin of the change in catalytic activity with increasing electrolyte concentration. Cations can also influence an electrocatalytic process by altering the properties of the electric double layer in a number of distinct ways,\n67\n which are not fully understood to date. One of the principal ways in which cations can impact the catalytic activity is by altering the structure and dynamics of water at the interface.\n65\n\n,\n\n67\n This possibility is included in our interpretation of these results in terms of the decreasing activity of water with increasing concentration of the water-in-salt electrolytes.Sixth, to exclude the possibility that impurities, for example, Fe, incorporate into the catalyst\n68\n with increasing salt concentration, we performed CV tests in electrolytes with reagent grade and trace metal grade salts. As shown in Figure\u00a0S17, the same water-oxidation activity was observed in both electrolytes.Finally, to test whether the electrochemical currents arise from the oxidation of water to molecular oxygen, we conducted gas chromatography measurements. Figure\u00a0S18 shows that O2 is produced with near-unity faradaic efficiency. This measurement demonstrates that: (1) other possible oxidation products (such as H2O2) are not produced in appreciable amounts and (2) parasitic chemical reactions (such as the oxidation of nitrate) do not occur.Taken as a whole, this set of results indicates that the observed suppression of the water-oxidation reaction is most likely caused by the decrease of water activity (a\n\nw\n) from 1 to 0.83 as the concentration of NaNO3 increases from 0 to 7 m.To further analyze the data shown in Figure\u00a03A, we plotted the ratio of the current density at a\n\nw\n\u00a0= 1 over that at a\n\nw\n\u00a0= 0.83 at different potentials (Figure\u00a03B). This ratio quantifies the extent to which the reaction rate is modulated by the water activity. It is clear that the impact of the water activity strongly depends on the electrode potential: at 1.71 V, the rate is suppressed by a factor of \u22484.3. By contrast, at a potential of 1.615 V, the modulation factor is only \u22481.2, indicating that the rate of the reaction is less sensitive to the change in water activity at that potential. Identical trends were obvious for the other a\n\nw\n\u2032s (i.e., 0.94 and 0.89), albeit with different magnitudes.That the reaction rate is suppressed by up to a factor of 4.3 by an a\n\nw\n change from 1 to 0.83 at 1.71\u00a0V strongly suggests that H2O is involved in the RDS at that potential. Conversely, for the same a\n\nw\n, the modulation is close to unity at 1.615 V, indicating that H2O involvement in the RDS is less likely. Taken as a whole, the data suggest that a mechanistic switch occurs between 1.615 and 1.71 V. A possible mechanistic switch that is consistent with our observations is the transition from the IMOC pathway ([pseudo] zeroth order in a\n\nw\n) to the WNA route ([pseudo] first order in a\n\nw\n) as the electrode potential is increased from 1.615 to 1.71 V.To corroborate further this assertion, we measured the steady-state current density on the FTO-supported Co\u2013Pi electrode in 0.1\u00a0M KPi in heavy water (D2O) as a function of electrode potential. The ratio of the current density of the corresponding measurement in light water over that in heavy water is the apparent kinetic isotope effect (KIE). The apparent KIE is close to 2 at 1.625\u00a0V and increases to \u22484.2 as the potential is tuned to 1.71 V. Because the IMOC pathway does not involve water in the RDS, we expect the rate of the reaction to be insensitive to H/D substitution. By contrast, the WNA involves a water molecule in the RDS. Therefore, a dependence of the rate on the isotope of hydrogen is expected. Collectively, the KIE measurements further corroborate our notion that the mechanism switches from the IMOC route to the WNA pathway with increasing potential.We note that the interpretation of KIE effects can be highly complex. For example, a similar KIE dependence on potential might be explained by a switch of the oxidized substrates from OH\u2212 to H2O, as has been reported by Zhao et\u00a0al. on Fe2O3.\n69\n However, that mechanism is not applicable to the Co\u2013Pi catalyst because OH\u2212 is unlikely to be the oxidized substrate at pH 7. Furthermore, Hammes-Schiffer et\u00a0al. demonstrated that the relative contributions that specific reactant/product vibronic states make to the current density are dependent on the isotope.\n70\n They showed that this effect could give rise to a potential dependence of the KIE. Although we cannot fully rule out that such effects contribute to the potential dependence of the KIE in the present case, the corroboration between the KIE data and the potential-dependent impact of the water activity on the reaction rate supports the conclusion of a potential-induced switch from the IMOC mechanism to the WNA pathway with increasing potential. A KIE on the WNA pathway was also reported by Cuk et\u00a0al. during the photocatalytic oxidation of water on SrTiO3.\n18\n\nAs far as the KIE effect is concerned, it is noted that Dau and co-workers also found a suppression of the water-oxidation reaction in D2O relative to that in H2O.\n32\n Their electrokinetic results were similar to those reported herein. However, they interpreted these data differently. In particular, the authors found that the redox potential of the pre-equilibrium [CoIII\u2013OH] \u21cb [CoIV\u2013O]\u00a0+ H+\u00a0+ e\u2212 shifts by approximately 60\u00a0mV in the anodic direction upon switching the solvent from H2O to D2O. Because galvanostatic measurements for water oxidation in H2O and D2O show a similar shift, they suggested that the suppression of the water-oxidation reaction is due to the shift in this pre-equilibrium (rather than a KIE on the RDS of the water-oxidation reaction). This pre-equilibrium is a critical factor determining the activity of Co-oxide-based catalysts.\n27\n\n,\n\n31\n\n,\n\n71\n This alternative interpretation could also account for the observed suppression of the water oxidation in D2O. However, we note that on the basis of the CVs of Co\u2013Pi in H2O and D2O (Figure\u00a0S19), we estimated a shift of \u224828\u00a0mV in the Co(II)/Co(III) redox half-wave potential. The relatively small shift in the pre-equilibrium suggests that it may not be the sole reason for the observed dependence of the rate of the water oxidation on the H/D isotope. Most importantly, this interpretation cannot account for the suppression of the current with increasing salt concentration (Figure\u00a03). As discussed earlier, our control experiments in which we varied the rotation rate of the RDE (Figures S9 and S10), the pH of the electrolyte (Figures S11\u2013S13), and the concentration of KPi (Figure\u00a0S14) confirm that the buffer capacity is sufficient to maintain the [CoIII\u2013OH] \u21cb [CoIV\u2013O]\u00a0+ H+\u00a0+ e\u2212 equilibrium in the water-in-salt electrolytes. To further corroborate this notion, we analyzed the Co(II)/Co(III) redox equilibrium of Co\u2013Pi in contact with the water-in-salt electrolytes with cyclic voltammetry. As shown in Figure\u00a0S20, the Co(II)/Co(III) redox half-wave potential is shifted by only 10\u201320\u00a0mV in the cathodic direction with increasing salt concentration. This small shift indicates the pre-equilibrium is not significantly affected by the presence of water-in-salt electrolytes. Therefore, when the isotope effect results are viewed in the context of the electrokinetic results for the water-in-salt electrolytes, our interpretation provides a cohesive, self-consistent picture, whereas the hypothesis of the shift in the pre-equilibrium can only partly explain the collective results. Although the shift may be a contributing factor, we conclude that it is not the dominating effect.In the following section, we discuss two possible molecular origins for our proposed potential-induced mechanistic switch. First, we show that the interfacial electric field at the electrocatalyst/electrolyte contact may affect the relative activation barriers of the two pathways and, thus, the relative weight of each route as the potential is altered. Second, we performed a DFT study of the two routes. These calculations show that only at high potentials does the WNA mechanism become thermodynamically accessible. In a practical system, the two effects may synergistically combine to favor the WNA pathway at high electrode potentials. Next, we discuss the impact of the interfacial electric field on the activation barriers; then we describe the insights derived from the DFT modeling.The key distinction between the IMOC and WNA pathways is the involvement of water in the RDS of the latter one (Figure\u00a01). On the basis of this observation, we expect the energetics of the two pathways to exhibit distinct sensitivity to the interfacial electric field. The magnitude of the interfacial electric field of the electric double layer increases as the potential of the electrode is increased. It is well established that electric fields can profoundly impact the rates and selectivity of chemical reactions.\n72\u201376\n Reaction intermediates with sufficiently large dipole moments and polarizabilities can interact with the electric fields. As a result of this interaction, the free energy profile of the reaction processes can be altered.\n72\n\n,\n\n73\n N\u00f8rskov et\u00a0al. have shown that the impact of electric fields on surface-bound water-oxidation intermediates (M\u2013OOH, M\u2013OH, M=O) is typically very small because these species have small dipole moments and polarizabilities.\n75\n On the basis of these findings, it is likely that the interfacial electric field has a negligible impact on the IMOC pathway. Because the rate-determining O\u2013O bond-formation step is a chemical step, we expect the principal activation barrier of the IMOC pathway to be independent of the electrode potential. By contrast, because water has a relatively large dipole moment and polarizability, the orientation and dynamics of water molecules at electrified interfaces may strongly depend on the electrode potential.\n76\u201378\n It has been suggested that the water dynamics and structure at interfaces affect the rates of various electrocatalytic processes, such as water oxidation and reduction.\n65\n\n,\n\n76\n Therefore, even though O\u2013O coupling in the WNA as hypothesized in Figure\u00a01 is a chemical step, we expect the activation barrier of this process to depend on the electrode potential: \n\n\u0394\n\n\nG\n\u00af\n\n\nW\nN\nA\n\n\u2260\n\n=\n\u0394\n\nG\n\nW\nN\nA\n\n\u2260\n\n\u2212\n\u0394\n\n\u03bc\n\u2192\n\n\u22c5\n\nE\n\u2192\n\n\n, where \n\n\u0394\n\nG\n\nW\nN\nA\n\n\u2260\n\n\n is the standard chemical free energy of activation in the absence of an electric field; \n\n\u0394\n\n\u03bc\n\u2192\n\n\n represents the change in the surface dipole when going from the reactant to the activated complex state; and \n\n\nE\n\u2192\n\n\n is the interfacial electric field, which depends on the electrode potential. These qualitative considerations show that because of the participation of water in the rate-determining chemical step of O\u2013O bond formation for the WNA mechanism, the activation barrier of this step is a function of electrode potential. Nevertheless, without knowledge of the molecular-level structure of the electrocatalyst/electrolyte interface at the present time, our considerations must remain qualitative at the present\u00a0stage. Irrespective, this model describes one possible origin of the observed mechanistic switch from the IMOC route to the WNA pathway with increasing potential.To explore further other possible causes of the potential-induced switch, we studied the energetics of the two pathways with DFT. All calculations were performed with the B3LYP functional and def2-SV(P) and def2-TZVP basis set implemented in the Gaussian 16 software package. Further computational details are provided in the supplemental information. We constructed atomic models on the basis of previous EXAFS\n34\n and X-ray pair distribution function analysis.\n52\n The Co7O24H27 cluster has a Co ion surrounded by 6 Co ions at the edge that are connected to the center Co ion by \u03bc3-O bridges (Figure\u00a0S21). The energetics of the water-oxidation reaction is sensitive to the protonation state of the cluster.\n35\n\n,\n\n36\n We considered different protonation states and found that the lowest energy protonation state is a highly symmetric cluster with one side of the \u03bc3-O being protonated and each pair of edge Co ions having strong hydrogen bonds between nearby hydroxide and water ligands (Figure\u00a0S22). The protonation of the hydroxide ligand of the edge Co atoms is energetically unfavorable because it destroys the strong hydrogen bond interaction between OH\u2212 and nearby H2O. However, the edge OH\u2212 group can be protonated by reducing the corresponding edge Co(III) to Co(II) (Figure\u00a0S23).On the basis of this structural model, we investigated the water-oxidation mechanism (Figure\u00a04\n) starting from the H2O\u2013Co(II)\u2013(\u03bc-\u039f)2\u2013Co(III)\u2013OH2 intermediates (I). We note that our computational method overestimates the potential for oxidation potential of Co(III) to Co(IV) by \u223c0.3\u00a0V (Figure\u00a0S24). All potentials quoted herein are not corrected for this systematic error. The oxidation of Co(II) to Co(III) requires 0.95 V, which is much lower than the applied potential during catalysis. The second oxidation requires 1.98\u00a0V to generate intermediate III with one Co oxidized to Co(IV). This oxidation is a metal-center oxidation, consistent with X-ray absorption near edge structure results of the Co\u2013Pi catalyst under catalytic conditions, which suggest a valence of Co greater than 3.\n34\n When the overestimation of the redox potential is accounted for, this intermediate is predicted to be prevalent under water-oxidation conditions. Consistent with the prediction, the resting state of the catalyst has been assigned to intermediate III in previous reports.\n15\n\n,\n\n27\n\n,\n\n31\n\n,\n\n32\n\n,\n\n59\n\n,\n\n79\n The hydroxide coordinated to the Co(IV) center in intermediate III has a partial radical character as indicated by a Mulliken spin population of 0.21 (Figure\u00a0S25). Therefore, the two hydroxides can couple to form hydroperoxide through the IMOC mechanism. Thermodynamically, this pathway is favored over the WNA pathway under low applied potentials. The following two oxidations require low potentials. Therefore, it is fairly easy to form intermediate VI. The release of O2 and binding of two water molecules complete the catalytic cycle.Under high applied potential, intermediate III can be further oxidized to form intermediate IV\u2032 with two nearby Co being oxidized to Co(IV). The terminal O atom coordinated to Co(IV) is best described as an oxyl radical because the Mulliken spin population on the O atom is 0.89 (Figure\u00a0S25), close to 1 for a perfect radical. The intermediate IV\u2032 can react with a water molecule from the solution to form intermediate V\u2032 through the WNA mechanism. The incoming H2O forms hydroperoxide with the oxyl radical and releases a proton to the nearby OH\u2212 group. Intermediate V\u2032 can be further oxidized to intermediate VI\u2032, which releases O2 and adsorbs a water molecule to complete the catalytic cycle.We note that both IMOC and WNA mechanisms feature a superoxo intermediate (VI and VI\u2032, respectively). This prediction is consistent with our spectroscopic results, which indicate the presence of a superoxo species. On the basis of the simulated O\u2013O vibrational frequencies (Figure\u00a0S26) alone, we cannot identify which of the two species gives rise to the vibrational band at 1,014\u00a0cm\u22121 (Figure\u00a02). We reserve a more detailed assignment for future investigations.Although alternative reaction pathways may be available,\n36\n\n,\n\n80\n the DFT computations show that: (1) the IMOC and WNA pathways are feasible from a thermodynamic perspective and (2) their energetics are consistent with the proposed mechanistic framework (Figure\u00a01) and the interpretation of our electrokinetic results (Figure\u00a03); at low overpotential, the IMOC pathway predominates, whereas the WNA pathway becomes accessible at high overpotential. Finally, it is noted that, in line with previous precedence, we only considered the thermodynamics of the pathways.\n19\n\n,\n\n80\n The calculation of the activation barriers is complicated by spin-state changes during the conversion of intermediate III to IV. Furthermore, the activation barriers are sensitive to the protonation state of the catalyst, which is a complex function of applied electrode potential and reaction conditions. Fully accounting for these complications will require additional research that is beyond of the scope of the current work.Taken as a whole, the thermodynamic description of the two pathways and the qualitative considerations of the impact of the interfacial field on the relative magnitude of activation barriers of the O\u2013O bond-forming steps provide strong support for the conclusion of a potential-dependent mechanistic switch. The DFT modeling predicts that a certain threshold potential for the WNA pathway needs to be surpassed before this pathway becomes thermodynamically feasible. In addition, the involvement of water in the RDS may further lower the activation barrier for the O\u2013O bond-formation step for the WNA route, leading to a further acceleration of the reaction rate. Our conclusions are graphically summarized in Figure\u00a05\n.Previous research on homogeneous water-oxidation mechanisms has revealed that the 4-proton, 4-electron process of water oxidation can take place on a mononuclear or a dinuclear catalyst. Whether WNA or oxygen coupling is the preferred mechanism has been at the center of intense studies for the natural PSII and for molecular catalysts. In testing the various hypotheses for the reaction mechanisms, researchers mainly relied on kinetic models that depend on the information of key species, such as the concentration of the catalyst and the TOFs. In principle, the same methodology could be applied for the establishment of a similar knowledge base for heterogeneous water-oxidation reactions. However, the lack of knowledge on the detailed information of the catalytically active centers creates a critical challenge for such an approach. Our strategy of probing the kinetics of heterogeneous water oxidation by varying water activities is new. It generates information that permits the verification of hypotheses that are difficult or impossible to obtain by other methods. How the water-oxidation reaction proceeds is sensitive to a number of factors, including the local catalytic environment (e.g., the availability of mononuclear, dinuclear, or multinuclear active centers), substrate concentration, and the driving forces (see, e.g., a recent report on how varying applied potentials change water-oxidation products on a copper porphyrin in acidic solutions.\n81\n) Although our studies suggest that the WNA mechanism is favored at high driving force, we are inspired to understand that in a practical water-oxidation system (such as the oxygen evolution catalyst in PSII or in an electrolyzer), both mechanisms may co-exist. The fact that this switch is observed on Co\u2013Pi and CoOx(OH)y (Figures S7 and S27) suggests that the potential-induced changes in pathway may be a more general phenomenon of Co-oxide-based electrocatalysts. The dynamic switch of the mechanisms could help to explain how nature ensures the most efficient route for the utilization of solar energy to liberate electrons and protons; it also implies that future designs and optimization of heterogeneous catalysts for large-scale engineering implementations of water oxidation should consider the facile switch of reaction mechanisms. It is noted that the WNA mechanism could proceed through a mononuclear site or a dinuclear site depending on the catalytic conditions.\n13\n\n,\n\n82\n\n,\n\n83\n However, it likely makes only a minor contribution to our study because of the narrow and low overpotential regime investigated and the equivalent involvement of a water molecule in the RDS on both sites. Finally, although we envision that studying water oxidation by varying water activities indeed adds a new dimension to the tool kit, it faces limitations for systems at highly alkaline conditions where OH\u2212 but not H2O is being oxidized.In conclusion, this work introduced two key innovations. Using SEIRAS-ATR, we detected a key intermediate corresponding to O\u2013O bond formation in Co-based water oxidation. This information lends support to the proposed mechanisms. By varying the water activity, we established a kinetic model that allowed us to verify the two competing mechanisms of water oxidation. We found that the dinuclear route (i.e., IMOC) is favored at relatively low driving forces, whereas the mononuclear route (i.e., WNA) is preferred at relatively high driving forces. The results contribute significantly to the understanding of water oxidation by heterogeneous catalysts.Further information and requests for resources should be directed to and will be fulfilled by the Lead Contact, Dunwei Wang (dunwei.wang@bc.edu).This study did not generate new unique reagents.This study did not generate any datasets.The work at Boston College is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division (DE-SC0020261). Work at Yale University was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division (DE-FG02-07ER15909). V.S.B. acknowledges the computer time from the National Energy Research Scientific Computing Center (NERSC) and Yale Center for Research Computing (YCRC).C.L. and Y.W. performed the electrochemical experiments; J.L. performed the SEIRAS-ATR experiments; K.R.Y. conducted computational studies; J.E.T., Q.D., D.H., and Y.Z. contributed experimentally; all authors participated in discussions and the writing of the manuscript; V.S.B., M.M.W., and D.W. co-directed the project.The authors declare no competing interests.Supplemental information can be found online at https://doi.org/10.1016/j.chempr.2021.03.015.\n\n\nDocument S1. Figures S1\u2013S27, Table S1, supplemental experimental procedures, and supplemental references\n\n\n\n\n\nDocument S2. Article plus supplemental information\n\n\n\n", "descript": "\n                  O\u2013O bond formation is a key elementary step of the water-oxidation reaction. However, it is still unclear how the mechanism of O\u2013O coupling depends on the applied electrode potential. Herein, using water-in-salt electrolytes, we systematically altered the water activity, which enabled us to probe the O\u2013O bond-forming mechanism on heterogeneous Co-based catalysts as a function of applied potential. We discovered that the water-oxidation mechanism is sensitive to the applied potential: At relatively low driving force, the reaction proceeds through an intramolecular oxygen coupling mechanism, whereas the water nucleophilic attack mechanism prevails at high driving force. The observed mechanistic switch has major implications for the understanding and control of the water-oxidation reaction on heterogeneous catalysts.\n               "}
{"full_text": "Magnetic nanoparticles are highly exploited in various fields like ferrofluids, magnetic separations, magnetic drug delivery, magnetic data storage systems, magnetic resonance imaging (\nCao\u00a0et\u00a0al., 2014\n\n;\n\nSong\u00a0et\u00a0al., 2015\n\n;\n\nXuan\u00a0et\u00a0al., 2011\n) recording, absorbents (\n\nAndjelkovic et\u00a0al. 2018\n\n;\nJacek,\u00a01984\n;\nMazen\u00a0and Abu-Elsaad,\u00a02015\n;\n\nSaha\u00a0et\u00a0al., 2018\n\n), telecommunications, transformers, etc. (\nAmor\u00a0et\u00a0al., 2019\n). Their properties differ in their methods of preparation and hence the oxide content (\n\nKarimi\u00a0et\u00a0al., 2015\n\n;\nMohamed\u00a0and Abu-Dief,\u00a02020). They are highly sensitive to the extrinsic elements used as dopants (\n\nEl\u00a0Foulani et\u00a0al. 2019\n\n). The doping can be done in a controlled way that tunes their properties and performance easily (\n\nSupriya\u00a0et\u00a0al., 2017\n\n). The quality of the ferrites depends on their electromagnetic properties (\n\nAhmed\u00a0et\u00a0al., 2021\n\n). The most common fabrication methods for such ferrites are Auto combustion, hydrothermal, citrate gel, microemulsion, ceramic, co-precipitation, etc. (\n\nChandramouli\u00a0et\u00a0al., 2021\n\n;\n\nHimakar\u00a0et\u00a0al., 2021\n\n;\n\nHussain\u00a0et\u00a0al., 2021\n\n;\nJesus\u00a0Mercy et\u00a0al., 2020; \nMulushoa\u00a0et\u00a0al., 2018\n\n;\nParajuli et\u00a0al., 2021b; Parajuli and Samatha, 2021b\n;\nSubrahmanya\u00a0Sarma et\u00a0al., 2022). The external elements can either be added or substituted for the betterment of their electrical and magnetic properties. The most commercially used mixed ferrites are cobalt-cupper, manganese-zinc, nickel-zinc, magnesium-manganese ferrites, etc. (\n\nJasrotia\u00a0et\u00a0al., 2020\n\n). The content of dopant is another parameter for the resulting property of such ferrites (\n\nFarea\u00a0et\u00a0al., 2008\n\n). Recently, F. Matloubi Moghaddam et\u00a0al. studied the CoCuFe2O4 ferrites and found a good catalyst for the cyanation of amines (\n\nRamakrishna\u00a0et\u00a0al., 2018\n). Hadi et\u00a0al. studied Cu-doped Co-Zn ferrites and their applicability in multilayer inductor chip applications in \nMoghaddam\u00a0et\u00a0al. (2021\n\n). Sabih et\u00a0al. investigated Ce and Zn substituted Co-Cu ferrites and found them to be of soft magnetic behavior with their applicability in water treatment, recording, and memory devices (Qamar\u00a0et\u00a0al., 2020). We have studied vigorously different ferrites materials based on which this work is initiated (\n\nHadi\u00a0et\u00a0al., 2021\n\n;\n\nParajuli\u00a0and Samatha 2021a\n\n,\n\nParajuli\u00a0and Samata 2021b\n\n;\nParajuli\u00a0et\u00a0al., 2022a\n;\nParajuli\u00a0et\u00a0al., 2021a).In addition, \nAjeesha\u00a0et\u00a0al. (2022\n\n) studied the antibacterial activity of Ca1-xCuxFe2O4; x\u00a0=\u00a00, 0.2, 0.4, 0.6, 0.8, 1) incorporating BET analysis and found 96% degradation in 180\u00a0min using methylene blue dye. \nAlmessiere\u00a0et\u00a0al. (2019\n\n), have studied CoNdxCexFe2\u20132xO (x\u00a0=\u00a00.00, 0.03, 0.05, 0.09, 0.10, 0.15, and 0.2) and found their excellent anticancer activity. Ameerah and her group investigated hydrothermally prepared Co0.5Ni0.5BixFe2-xO4 (x\u00a0=\u00a00.00\u20130.10), found a single domain structure through its squareness ratio of less than 0.5 and are sensible and efficient enough for spin reversal (Ameerah\u00a0et\u00a0al., 2022\n). Recently, the rare earth elements, like , Thulium substituted CoTmxFe2-xO4 (x\u00a0=\u00a00.00, 0.04 and 0.08)(\u00dcnal\u00a0et\u00a0al., 2020), Dysprosium substituted Mn0.5Zn0.5Dy0.03Fe2\u20130.03O4 (x\u00a0=\u00a00.005, 0.01, 0.015, 0.02, 0.025, and 0.03) for sensor applications. In the last couple of years, K. M. Jadhav et\u00a0al. have intensively studied the interesting properties of different types of ferrites with the substitution of various elements. The researches were focused on surface modification (Somvanshi\u00a0et\u00a0al., 2020a), RE metal Dy substituted yttrium iron garnet (Y3-xDyxFe5O12) nanoparticles for advanced high frequency devices (\n\nBhosalea et\u00a0al. 2020\n\n), RE metal Gd substituted Zn-Mg ferrite for biomedical applications (Somvanshi\u00a0et\u00a0al., 2020b), thermal conductivity (\nKharat\u00a0et\u00a0al., 2020\n), Viral RNA-extraction protocol for COVID detection (Somvanshi\u00a0et\u00a0al., 2021), green method to prepare CoFe2-\n\nx\n Al\nx\nO4 to enhance the dielectric parameters (\n\nChavan\u00a0et\u00a0al., 2021\n\n), reviewed ferrites materials for biomedical applications (\n\nKharat\u00a0et\u00a0al., 2020\n\n), Ni-Zn ferrites incorporated with rhodamine B for photocatalytic purposes (\n\nJadhav\u00a0et\u00a0al., 2020\n\n), Zn Fe2O4 for chalcones preparation which gives pigment to flowers in nature (\n\nBorade\u00a0et\u00a0al., 2020\n\n), surface functionalized CoFe2O4 in hyperthermia for cancer and other treatments (\n\nEivazzadeh-Keihan\u00a0et\u00a0al., 2021\n\n;\n\nEivazzadeh-Keihan\u00a0et\u00a0al., 2020\n\n;\n\nKharat\u00a0et\u00a0al., 2020\n\n), NiFe2O4 for toxic dye removal (\n\nJadhav\u00a0et\u00a0al., 2021\n\n), thermoacoustics (\n\nKharat\u00a0et\u00a0al., 2019\n\n), Mg substituted ZnFe2O4 for absorbents (\n\nSomvanshi\u00a0et\u00a0al., 2020\n\n), hyperfine interaction study for identifying ferrimagnetic nature (\n\nHumbe\u00a0et\u00a0al., 2020\n\n), green vs ceramic synthesis comparative study of multifunctional Mg-Zn ferrites nanoparticles for electronic and biomedical treatments (\n\nKhirade\u00a0et\u00a0al., 2020\n\n). Similarly, many bionanocomposites of ferrites nanoparticles are used to prepare Pyranopyrazoles for analgesic agents in medical treatments (\n\nKamalzare\u00a0et\u00a0al., 2021\n\n) and various others for the recyclable nanocatalysts, biosensors and so on (\n\nBahrami\u00a0et\u00a0al., 2020\n\n;\n\nKamalzare\u00a0et\u00a0al., 2020\n\n;\n\nMaleki\u00a0et\u00a0al., 2020\n;Maleki\u00a0et\u00a0al., 2017).We have used the sol-gel auto combustion method in this work to prepare Ni, Zn, and Mg substituted Co-Cu ferrite nanoparticles and studied their structural, dc electric resistivity, and magnetic properties. The reason for choosing this method was due to its simple process, easily controlled stoichiometry, cheap starting materials, and producing ultrafine particles in a short time at very low temperatures (\nParajuli\u00a0et\u00a0al., 2022\n\n).The highly analytical grade (99.9% purity) nitrates of cobalt, copper, magnesium, nickel, zinc, and iron were mixed with citric acid in a 1:1 molar ratio. Citric acid acts as a fuel for auto-combustion and makes the precipitation homogeneous at relatively low temperatures. The dropwise addition of ammonium hydroxide makes the solution neutral. The electronegative ions due to oxygen attract the metal ions with electropositive nature and get dissolved. The solution is stirred at 150\u00a0\u00b0C until a gel is formed and dried. It is sintered at 1000\u00a0\u00b0C for 3\u00a0h. The product is then grounded finely using agate mortar and pestle. Finally, they are pressed with 5 tons hydraulic system to get pallets. The two flat surfaces of the pallets can be coated with silver to form two electrodes which can be used for the determination of their electric and other properties. The chemical name and molar mass of the chemical used are listed in Table\u00a01\n.In this study, a Rigaku Miniflex II X-ray diffractometer incorporated with CuK\u03b1 radiation (wavelength\u00a0=\u00a01.5406\u00a0\u00c5) system was used for their structural properties. TESCAN, MIRA II LMH microscope with attached EDX, Inca Oxford was used for morphological and compositional properties respectively. Their functional groups were confirmed with the FTIR study. The magnetic properties and DC resistivities were studied with the help of the EZ VSM model and a two-probe DC resistivity system.The XRD pattern of Mg, Ni, and Zn substituted Co-Cu nano spinel ferrite is shown in Fig.\u00a01\n. The lattice parameter is affected by several factors such as the atomic size, interatomic forces, particle or grain size (\nRamakrishna\u00a0et\u00a0al., 2018\n\n), etc. X-ray diffraction measurements validated the mono-phase formation and nanocrystalline character of the produced samples. The peaks (111), (311), (222), (400), (422), (511), and (522); obtain after the indexing confirms the cubic structure of the samples. The lattice parameters obtained were 8.474\u00a0\u00c5, 8.374\u00a0\u00c5, and 8.393\u00a0\u00c5 for synthesized ferrites respectively. The Zn substituted sample has a higher lattice constant than the other two. The ionic radii of Cu2+, Zn2+ and Mg2+ ions were 0.73\u00a0\u00c5, 0.74\u00a0\u00c5, and 0.72\u00a0\u00c5 respectively higher than that of Ni2+ (0.69\u00a0\u00c5) (Anand et\u00a0al., 2017).The crystallite sizes of the samples were obtained with the help of the following relation (\n\nAmar\u00a0et\u00a0al., 2019\n\n) considering the highest peak of the (311) plane;\n\n(1)\n\n\na\n=\nd\n\n\n\nh\n2\n\n+\n\nk\n2\n\n+\n\nl\n2\n\n\n\n\n\n\nwhere d is interplanar spacing which can be calculated from Bragg's law. The accuracy of the lattice constant is done by the Nelson-Riley extrapolation function given by (\n\nBatoo et\u00a0al. 2016\n\n),\n\n(2)\n\n\nF\n\n(\n\u03b8\n)\n\n=\n\n1\n2\n\n\n[\n\n\n(\n\n\nc\no\n\ns\n2\n\n\u03b8\n\n\nsin\n\u03b8\n\n\n)\n\n+\n\n(\n\n\nc\no\n\ns\n\n2\n\n\n\n\u03b8\n\n\u03b8\n\n)\n\n\n]\n\n\u00b1\n0.002\n\u00c5\n\n\n\n\nThe data obtained were listed in Table\u00a02\n which agrees well with the previous (\n\nYadav\u00a0et\u00a0al., 2018\n\n).The XRD patterns show the single-phase spinel ferrite matching with a JCPDS-card no. 22\u20131086 corresponding to (311) peak (\n\nJnaneshwara\u00a0et\u00a0al., 2014\n\n). The nano range of the average crystallite sizes as indicated by the sharpness of the peaks is connected by Debye Sherer's equation. The crystallite sizes were found at 47.19\u00a0nm, 40.15\u00a0nm, and 29.01\u00a0nm, respectively while sintering at 1000\u00a0\u00b0C gives. The crystallite sizes (below 100\u00a0nm) confirmed the nanocrystalline nature of the sample prepared (\n\nBhosale\u00a0et\u00a0al., 2020\n\n).\nThe average crystallite size is obtained by the use of Scherer's formula for (311) peak (\n\nAkhtar\u00a0et\u00a0al., 2019\n\n),\n\n(3)\n\n\n\n\nD\n\n(\n311\n)\n\n\n=\n\n\n0.9\n\u03bb\n\n\n\u03b2\nc\no\ns\n\u03b8\n\n\n\n\n\nWhere \u03bb is equal to 1.5406\u00a0\u00c5, \u03b2 and \u03b8 are full width half maximum (FWHM) of (311) peak and angle of diffraction respectively.The X-ray density is given by (\n\nRouhani\u00a0et\u00a0al., 2018\n\n),\n\n(4)\n\n\n\nd\nx\n\n=\n\n\n\n8\n\nM\n\u2032\n\n\n\nN\n\na\n3\n\n\n\n\n\n\n\nM' is the prepared samples' molecular weight, a is the lattice constant, and N is Avagadro's number.Bulk density is given by (\n\nChaudhari\u00a0et\u00a0al., 2015\n\n),\n\n(5)\n\n\n\nd\nb\n\n=\n\n\n\nW\n1\n\n\n\nW\n1\n\n\u2212\n\nW\n2\n\n\n\n\n\n\n\nW1 is the weight of the sample in air, and W2 is the sample's weight in water.Porosity gives the empty spaces or voids in a material with the help of the relation (Yoon\u00a0and Raju,\u00a02016)\n\n(6)\n\n\np\n=\n1\n\u2212\n\n\n\nd\nb\n\n\nd\nx\n\n\n%\n\n\n\nwhere db and dx are the bulk and X-ray densities, respectively. The plot between lattice constant and crystallite size is displayed in Fig.\u00a02\n. Fig.\u00a03\n shows the X-ray density and porosity variation of the synthesized ferrite nanoparticles. Here, the X-ray density and porosity values have been increasing in the order of Mg, Ni and Zn substituted Co-Cu nano ferrites samples.The volume of the unit cell is given by,\n\n(7)\n\n\nV\n=\n\na\n3\n\n\n\n\u00c5\n\n3\n\n\n\n\nshowing the same trend as that of the lattice parameter since it is directly related with it.The ionic radii can be calculated with the help of two relations (Somvanshi\u00a0et\u00a0al., 2020a),\n\n\n(8)\n\n\n\nr\nA\n\n=\n\n(\n\nu\n\u2212\n\n1\n4\n\n\n)\n\na\n\n3\n\n\u2212\nr\n\n(\n\nO\n\n2\n\u2212\n\n\n)\n\n\u00c5\n\n\n\n\n\n\n(9)\n\n\n\nr\nB\n\n=\n\n(\n\n\n5\n8\n\n\u2212\nu\n\n)\n\na\n\u2212\nr\n\n(\n\nO\n\n2\n\u2212\n\n\n)\n\n\u00c5\n\n\n\n\nFESEM images of the Mg, Ni, and Zn substituted Co-Cu nano spinel ferrite are as shown in Fig.\u00a04\n(a) \u2013 (c), offering their spherical grains with particle sizes 40\u00a0nm, 50\u00a0nm \u2013 74\u00a0nm, in agreement with XRD graphs. The figure shows clear and inhomogeneous crystal grains. Nevertheless, the density is reduced due to Cu with higher atomic weights (Nikmanesh\u00a0and Eshraghi,\u00a02019).The EDS analysis was used to determine the identity of metals in the synthesized Mg, Ni, and Zn substituted Co-Cu nano spinel ferrites are shown in Fig.\u00a05\n(a) \u2013 (c). From the EDS images, it is clear that Co, Cu, Mg, Ni, Zn, Fe, and O ions are visible. The histograms of compositions are shown in Fig.\u00a06\n, and their elemental composition has weight and atomic values, which are listed in Table\u00a03\n. The results are in good agreement with the standard stoichiometric ratio.The FTIR spectrum of M substituted (M\u00a0=\u00a0Mg, Ni, and Zn) Co-Cu nano spinel ferrite is shown in Fig.\u00a07\n. Two absorption peaks below 600\u00a0cm\u22121 of all ferrites indicate their spinel structures. The spectra of all the considerable series of powders show the presence of two primary absorption bands in which one is intense in the wavenumber around 300\u00a0cm\u22121 while the other band is around 600\u00a0cm\u22121. The higher band (\u028b1) is generally observed in the range 579\u2013592\u00a0cm\u22121 Mtetra\u2194O. is caused by the stretching vibrations of the tetrahedral metal-oxygen band. The extension of FT-IR bands might be a result of the cation integration in the crystal layers. The lowest band (\u028b2) commonly seen in the range 357\u2013397\u00a0cm\u22121 is caused by the metal-oxygen vibrations in the octahedral site Mocta \u2194O. Such an IR wave number absorption difference can be expected because of the difference in bond lengths (Me-O) at the two sites.The two prominent peaks observed in 550\u00a0cm\u22121 and 453\u00a0cm\u22121 are due to the presence of the M\u2013M bond and M\u2013O stretching frequencies, respectively (\n\nShahbaz et\u00a0al. 2012\n\n). The two peaks at 1603\u00a0cm\u22121 and 1115\u00a0cm\u22121 are associated with C=O and CO2 stretching vibrations respectively (\n\nBatoo et\u00a0al. 2016\n\n). The Mtetra\u2194O stretching bond has with higher frequency (\u028b1), and Mocta\u2194O has a lower frequency (\u028b2) with respective force constants K\nt and K\no at tetrahedral and octahedral sites (Kumar\u00a0and Shirage,\u00a02017) as listed in Table\u00a04\n.The hysteresis loops of Mg, Ni, and Zn substituted Co-Cu nano spinel ferrite at room temperature are shown in Fig.\u00a08\n. They show the showing ferromagnetic nature. The saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc), experimental and theoretical Bohr magnetron, and anisotropy values of the samples were obtained from these hysteresis loops (\n\nDraack\u00a0et\u00a0al., 2019\n\n). Likewise, the variations of saturation magnetization (Ms) and magnetic moment (nB) of the samples are shown in Fig.\u00a09\n. From the figure, the Mg, Ni, and Zn substituted Co-Cu nano spinel ferrites have the saturation magnetizations (Ms) 42.95, 49.5, and 41.3 emu/g; remnant magnetizations (Mr) 12.4, 14.8, and 11.8 emu/g; magnetic moments (nB) 1.79, 2.09 and 1.7\u00a0\u00b5B; and the coercivities (Hc) 264.14, 566 and 409.5\u00a0Oe respectively. The FESEM micrograph shows the agglomeration which decreases the surface defect of the particle (\n\nZubair\u00a0et\u00a0al., 2017\n\n). We have observed that the values of the Ms, Mr, nB, and Hc are higher for the Ni2+ substituted Co-Cu than that of Mg2+ and Zn2+ substituted ones. The addition of nonmagnetic Cu2+ ions decreases the magnetic saturation as the divalent Cu2+ ion occupies tetrahedral sites, thereby replacing magnetic Fe3+ with octahedral (\n\nLi\u00a0et\u00a0al., 2016\n\n). It gives the inverse spinel structure (\n\nZubair\u00a0et\u00a0al., 2017\n\n) according to Neel's sublattice model (\n\nAti\u00a0et\u00a0al., 2021\n\n). The ferrites have A-A, B-B, and A-B exchange interactions, out of which A-B exchange interactions are stronger. The Co2+, Cu2+, Mg2+, Ni2+, and Zn2+ ions occupy the octahedral site (B-site) thereby reducing magnetization as tetrahedral (A-site) has constant sublattice. Among these three interactions, the A\u2013B interaction is (antiferromagnetic and) the strongest, and it predominates over AA and BB (ferromagnetic) interactions in determining the magnetic moment of the spinel ferrite compounds. The net magnetic moment of the lattice is therefore the difference between the magnetic moments of B and A sub-lattices, i. e., \u00b5B\u00a0=\u00a0\u00b5B(B)\u00a0-\u00a0\u00b5B(A)\nand its magnitude is given by \u00b5B\u00a0=\u00a0|\u00b5B(B)\u00a0-\u00a0\u00b5B(A)|. The occupancy of ions on tetrahedral and octahedral can be explained [Co0.5Cu0.2Fe]A [Co0.5M0.3Fe]BO4 where M\u00a0=\u00a0Mg, Ni, and Zn. M replaces Cu, the Co2+ cations do not change their occupancy. The reason seems to be clear that M2+ and Cu2+ occupy their preferred sites, i.e., tetrahedral and octahedral sites, respectively.The value of the remnant ratio (R\u00a0=\u00a0M\nR\n/M\nS) below 0.5 is isotropic with a single domain ferrimagnetic nature (\n\nMandal\u00a0et\u00a0al., 2017\n\n). Magnetic recording media need high M\ns\n(\n\nNayeem\u00a0et\u00a0al., 2017\n\n). The magnetic parameters are listed in Table\u00a05\n. The coercivity (HC) variation vs. magneto-crystalline anisotropy constant (K1) plots for Mg, Ni, and Zn doping Co-Cu nano spinel ferrite samples are shown in Fig.\u00a010\n.The coercivity (HC) is obtained from (\n\nHumbe\u00a0et\u00a0al., 2018\n\n),\n\n(10)\n\n\n\nH\nC\n\n=\n\n\n2\n\nK\n1\n\n\n\n\n\u03bc\no\n\n\nM\ns\n\n\n\n\n\n\n\nThe anisotropy constant (K1) and aspect ratio (MR/MS) can be obtained using (\n\nMandal\u00a0et\u00a0al., 2017\n\n),\n\n(11)\n\n\nAspe\nct\n\nRatio\n=\n\n\nM\nR\n\n\nM\nS\n\n\n\n\n\n\n\n\n(12)\n\n\n\nK\n\n1\n\n\n\n=\n\n\n\n\nH\nC\n\n\u00d7\n\n\n\nM\nS\n\n\n\n0.96\n\n\n\n\n\nwhere MS, MR\n, and \u03bco are the saturation magnetization , the remnant magnetization, and the free space permeability respectively.The magnetic moment (nB) of synthesized nano ferrites can also be calculated by the relation as specified below (\n\nChuang\u00a0et\u00a0al., 2015\n\n),\n\n(13)\n\n\n\n\u03bc\n\nB\n\n\n\n=\n\n\n\n\nM\n\ns\n\n\n\n\u00d7\n\n\nMW\n\n5585\n\n\n\n\nwhere MW denotes the molecular mass of each synthesized specimen.There might be three reasons for the decrease in saturation magnetization in Mg and Ni substituted compounds: (1) single domain to multi-domain phase transition with increasing size (2) combined surface effect and its surface anisotropy (3) inert surface layer. The variation in magnetic saturation of impurity less and single-phase homogeneous crystal without defects is due to particle size variation of the samples. This behavior was already reported for different ferrites (\n\nRen\u00a0and Xu 2014\n\n).The observed resistivity variation for the Mg, Ni, and Zn substituted Co-Cu nano spinel ferrites is according to the hopping mechanism (\n\nSaini\u00a0et\u00a0al., 2016\n\n) which was reported by Verwey and his co-workers. During the sintering process, there is a possibility of forming Fe2+ and Fe3+ ions in both divalent and trivalent iron ions in octahedral sites between which their hopping takes place. For higher sintering temperatures, the formation of Fe2+ ions can also be from the evaporation of some elements. Co2+ \u21d4 Co3+ and Cu+ \u21d4 Cu2+ hopping also take place in the present system, mainly in octahedral sites (\n\nStergiou\u00a0and Litsardakis 2014\n\n).In Mg, Ni, and Zn substituted Co-Cu ferrite samples; the resistivity is expected to get modified with the increase in Zn compound in place of Co ions through the following mechanisms:\n\n1\nFe2+ \u21d4 Fe3+ ions; electron hopping between iron ions in different valence states is supposed to be the dominant conduction mechanism thereby reducing resistivity considerably.\n\n\n2\nCu+ \u21d4 Cu2+ ions; the probability of forming Cu+ ions may be small. Due to this, conduction through this mechanism is also expected to be relatively low thereby resulting in low resistivity.\n\n\n3\nCo2+\u21d4 Co3+ ions; there is some probability of this kind of conduction mechanism in which there is the possibility of the formation of Co3+ ions in B-sites thereby lowering the resistivity.\n\n\n4\nFe2+\u00a0+\u00a0Co3+ \u21d4 Fe3+\u00a0+\u00a0Co2+ ions; the simultaneous presence of iron in cobalt ions in different valence states may often get locked in this mechanism thereby increasing the resistivity.\n\n\n5\nFe2+\u00a0+\u00a0Cu2+ \u21d4 Fe3+\u00a0+\u00a0Cu+ ions; the simultaneous presence of iron in copper ions in different valence states may also often get locked in this mechanism. But the increase in resistivity is small.\n\n\n6\nThe presence of Zn ions between the iron ions increases the bond length thereby reducing electron hopping or increasing the resistivity.\n\n\n7\nFine-grained microstructures with relatively more grain boundaries; since grains contribute to conduction and their boundaries in insulation, the fine-grained microstructures often increase resistivity.\n\n\nFe2+ \u21d4 Fe3+ ions; electron hopping between iron ions in different valence states is supposed to be the dominant conduction mechanism thereby reducing resistivity considerably.Cu+ \u21d4 Cu2+ ions; the probability of forming Cu+ ions may be small. Due to this, conduction through this mechanism is also expected to be relatively low thereby resulting in low resistivity.Co2+\u21d4 Co3+ ions; there is some probability of this kind of conduction mechanism in which there is the possibility of the formation of Co3+ ions in B-sites thereby lowering the resistivity.Fe2+\u00a0+\u00a0Co3+ \u21d4 Fe3+\u00a0+\u00a0Co2+ ions; the simultaneous presence of iron in cobalt ions in different valence states may often get locked in this mechanism thereby increasing the resistivity.Fe2+\u00a0+\u00a0Cu2+ \u21d4 Fe3+\u00a0+\u00a0Cu+ ions; the simultaneous presence of iron in copper ions in different valence states may also often get locked in this mechanism. But the increase in resistivity is small.The presence of Zn ions between the iron ions increases the bond length thereby reducing electron hopping or increasing the resistivity.Fine-grained microstructures with relatively more grain boundaries; since grains contribute to conduction and their boundaries in insulation, the fine-grained microstructures often increase resistivity.Mechanisms 1 and 2 are dominant in our present study of Zn-like element substitution where the resistivity is decreased significantly first and further decrease is controlled by 3, 4, 5, 6, and 7 mechanisms.\nFig.\u00a011\n shows the plot of temperature vs. dc electrical resistivity (log \u03c1 vs. 1000/T) of Mg, Ni, and Zn substituted Co-Cu nano spinel ferrite. From the figure, it is clear that the resistivity is decreased with the increase in temperature of all the synthesized nano spinel ferrite showing their semiconducting nature (\n\nHossain\u00a0et\u00a0al., 2021\n\n). The impurities increase conductivity at a lower temperature. The energy needed for hoping of the electrons or activation energies are obtained from the fitted curves of the Arrhenius equation (Roy\u00a0and Bera,\u00a02006):\n\n(14)\n\n\n\u03c1\n=\n\n\u03c1\no\n\n\ne\n\n\u2212\n\n(\n\n\n\n\u0394\n\nE\n\n\nK\nT\n\n\n)\n\n\n\n\n\n\nwhere \u03c1 is the room temperature dc electrical resistivity at temperature T, \u03c1o\n is the pre-exponential factor\n\n,\n\n\n\u0394\n\nE\n\n is the activation energy, K is the Boltzmann constant, and T is the absolute temperature. The activation energy based on hopping is found in 0.58, 0.51, and 0.583\u00a0eV. Above this range, the hopping is called polaron hopping. The hopping can be of either electron or hole. In the case of polaron hopping, there is p-type or hole hopping in which there is hardly a quantum mechanical effect due to the larger grain size. The graph of the activation energies for Mg-Cu, Ni-Cu, and Zn-Cu cobalt nano spinel ferrites is shown in Fig.\u00a012\n.The activation energy needed for Ni substituted Co-Cu ferrite is the lowest indicating its preference for the system for higher conductivity. Further, these semiconducting ferrites systems have isotropic ferromagnetic nature. As a result, they behave as magnetic semiconductors as defined by Chuwang et\u00a0al. in 2015 and used for spin transistors. The calculated values of activation energies of the Mg, Zn, and Ni substituted Co-Cu ferrite nanoparticles ferrite samples are shown in Fig.\u00a012. The resistivity variation for the synthesized ferrite nanoparticles was according to Verwey and de Boer's hopping mechanism. Here, electron hopping occurs between Fe2+\n\n\u2194\n Fe3+ ions during the sintering of the ferrites. If the ferrite's sintering temperature is higher, more Fe2+ ions are produced, thereby accelerating the hopping process. The hopping process is possible in M2+\n\n\u2194\nM3+ and Cu3+\n\n\u2194\nCu2+ Existing together in a system, where M\u00a0=\u00a0Ni, Mg, and Zn.The Mg, Ni, and Zn substituted Co-Cu nano spinel ferrite were successfully synthesized with the sol-gel auto combustion method with the use of 1000 \u00b0C sintering temperature. X-ray diffraction measurements validated the mono-phase formation and nanocrystalline character of the produced samples. It possesses a monophasic cubic-spinel lattice structure with an average crystallite size ranging from 29.01 to 47.17\u00a0nm. The lattice constant determined by XRD analysis falls between 8.374 and 8.474\u00a0\u00c5. FESEM micrographs showed grain sizes of 40, 50, and 74\u00a0nm. EDS has validated the composition of the resulting ferrite nanoparticles. The magnetic saturation (MS), magnetic moment (MR), and coercivity (Hc) are found to be the highest for the Ni substituted Co-Cu ferrite system. So, the Ni substituted Co-Cu nano ferrite is preferable for electromagnets and recording purposes than Mg and Zn substituted Co-Cu nano ferrites. The aspect ratio between Mr and Ms is 0.5 for all showing their ferromagnetic isotropic nature. The results of the DC resistivity of nanoparticles were increased with increasing temperature indicating their semiconducting nature. So, these ferrites are magnetic semiconductors. We can fabricate dilute magnetic semiconductors using transitional elements rather than electronic active elements.\nD. Parajuli: Conception and design, or analysis and interpretation of the data; drafting the article or revising it critically for important intellectual content; and Approval of the final version. Correspondence, Yonatan Mulushoa S.: Data curation, Investigation, Validation, Venkateswara Rao: Data curation, Investigation, Validation, N. Murali: Data curation, Investigation, Validation, Correspondence, K. Samatha: Approval of the final versionThe 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                  The sol-gel auto-combustion procedure was used to synthesize M (M\u00a0=\u00a0Mg, Ni, and Zn) substituted Co0.5Cu0.2M0.3Fe2O4 micro ferrites. X-ray diffraction measurements validated the mono-phase formation and nanocrystalline character of the produced samples. It possesses a monophasic cubic-spinel lattice structure with an average crystallite size ranging from 29.01 to 47.17\u00a0nm. The lattice constant determined by XRD analysis falls between 8.374 and 8.474\u00a0\u00c5. The morphological characteristics were shown using FESEM analysis, which revealed the almost spherical form of grains with sizes ranging from 40, 50, and 74\u00a0nm. The compositional verification was undertaken with the help of the EDS analysis which ensured the presence of elements in desired proportions. IR spectra confirm their spinel spectra with the help of the two absorption bands lying between 1200 and 400\u00a0cm\u22121 resembling their two sites: tetrahedral and octahedral. The magnetic saturation (MS), magnetic moment (M)R, and coercivity (Hc) are found to be the highest for the Ni substituted Co-Cu ferrite system, i.e., MS\u00a0=\u00a049.5 emu/g. The ratio MR/MS for all ferrites is less than 0.5 indicating their ferromagnetic isotropic nature thereby acting as magnetic semiconductors. The electrical properties were measured using the \u2018two probe technique\u2019 with varying temperatures which indicates their semiconducting nature. Ni substituted Co-Cu nanoferrite is preferable for electromagnets and recording purposes the magnetic and electrical behavior of Ni substituted Co-Cu nanoferrite shows its relevance for recording and electromagnetic applications.\n               "}
{"full_text": "Data will be made available on request.Persistent Organic Pollutants.Metal-organic frameworks.persistent, bio-accumulative, and toxic.dichlorodiphenyltrichloroethane.polybrominated diphenyl ethers.Organochlorine.Persulfate.polychlorinated biphenyls.hexachlorocyclohexane.polyfluoroalkyl substances.brominated flame retardants.pharmaceutical and personal care products.United Nations Environmental Program.Canadian Environmental Protection Agency.advanced oxidation processes.Reactive Oxygen Species.metal-to-ligand charge transfer.ligand-to-ligand charge transfer.Water quality is of universal interest, and the UN Member States have acknowledged its importance in sustainable development in the Agenda for Sustainable Development. To be delivered by 2030, the Sustainable Development Goal (no.6) seeks to \u201cimprove water quality by reducing pollution, eliminating dumping and minimizing release of hazardous chemicals and materials, halving the proportion of untreated wastewater, and substantially increasing recycling and safe reuse globally\u201d (Nations (UN), 2015). Water reclamation and reuse have equally gained widespread attention in recent years due to water scarcity caused by climate change and inadequate water resource management. Likewise, constant water quality degradation, resulting from ineffective wastewater management and treatment, has been observed to limit water provision in growing population countries and be an ever-increasing issue in an expanding global economy (Zendehbad et al., 2019). In response to water quality issues and associated limitations to clean water accessibility, one of the attractive solutions is wastewater reuse and reclamation to ensure sustainable water development and management. However, decades of industrial and agricultural products with growing urbanization and anthropogenic activities have resulted in multiple water pollutants, including persistent organic pollutants. POPs arise concerns that can be challenging as they could still be present in treated water.POPs are carbon-based chemical compounds resistant to environmental degradation and have been continually discharged into the environment. Due to their poor biodegradability, POPs can cause severe harm to wildlife and human beings. On closer inspection, POPs inhibit the immune system's natural reaction while also lowering the body's viral resistance (S. Zhang et al., 2015; L. Zhang et al., 2015). Besides, several studies have revealed that organisms exposed to POPs might lead to congenital malformations and reproductive disorders (Lee et al., 2014; Nadal et al., 2015; Tartu et al., 2015). Several POPs have been identified to be carcinogenic and potential endocrine-disrupting substances (F\u00e5ng et al., 2015; Ong et al., 2018). Therefore, to ensure that water is devoid of POPs, advanced treatment solutions are crucial.Historical interest in the construction of organic/inorganic hybrid compounds dates back to the 1830\u00a0s, with a report on the first organometallic platinum species by Zeise (1830)). In Zeise\u2019s report, the challenges with both synthesis and characterization of the later-called \u201cZeise\u2019s Salt\u201d (K[PtCl3(C2H4)]H2O) were detailed. This Pt complex indeed marked a remarkable triumph in experimental characterization; it initiated a whole new field of organometallic chemistry and, more broadly, the interest in reactivity occurring at the metal-organic interface.Over the following two centuries, various breakthroughs in chemical physics, quantum mechanics, and optics allowed advanced complex analytical methods that developed synthetic curiosities beyond compositional and structural elucidation to targeted application and function. However, similar to Zeise\u2019s salt, most of the 1900s cutting-edge chemistries were zero-dimensional (or molecular), homogeneous compounds. Driven by industrial motives for heterogeneous materials and academic interests in forming more sophisticated multidimensional compounds, researchers started to investigate physical characteristics that could only appear by extending chemical connectivity into higher dimensions (1D, 2D, and 3D) in both amorphous and crystalline structures (Ozin, 1992).Intrinsic porosity emerged as both a goal and a result of greater dimensionality. Despite being purely inorganic, siliceous zeolite appeared to be a milestone that exhibited how novel chemical characteristics could be experienced by harnessing both the porosity and the capacity to attach heterogeneous catalytic sites into the scaffold (Breck and Breck, 1973; Davis, 1993). Still, the chemical compositions of the zeolitic structure type materials were chiefly limited to aluminosilicates, which are capable of accommodating only minor quantities of transition metals, mainly as defects (Hunger et al., 1987; Yamagishi et al., 1991). Progression to bulkier chalcogenides, bigger organic anions, and metal replacements beyond group IV elements resulted in novel and isostructural topologies with, until now, unseen chemical connectivity. With the later inclusion of organic bridging ligands, the multidimensional porous coordination complexes shaped their own distinctive family: metal-organic frameworks (MOFs).As with any artificial material, synthetic methods and compositions are vital for targeting specific properties (e.g., selectivity, pore aperture). Numerous attempts have been devoted to the synthesis of MOFs and applying MOFs for water treatment. MOF materials provide several paths to targeting different pollutants in aqueous environments (\nFig. 1). In this review, we explore the application of MOFs and MOF-based materials in removing POPs from water, focusing on the most recent advances.This review looks into the properties, impacts, and degradation methods of POPs found in water/wastewater. The focus of this study is adsorption and photocatalytic degradation. Aside from adsorption, which is one of the most essential methods for water purification by MOFs, improved oxidation techniques may be used to remove contaminants, converting these compounds into H2 and harvesting the chemical energy trapped in their bonds. Each method's merits, limitations, and process improvements are discussed in detail. To detect relevant findings and articles, key search terms such as persistent organic pollutants (POPs), metal-organic frameworks (MOFs), adsorptive removal of environmental contaminants, and photocatalytic degradation were utilized in several resource banks such as SciFinder, Elsevier, Springer Link, and Google Scholar.POPs are a group of natural or synthetic chemical compounds which enter the nature cycle either intentionally or unintentionally. These compounds have become a prevalent contaminant and are worldwide pollution due to their ability to travel large distances via air circulation (wind) and ocean currents (water), as well as stored in snow and ice (cryosphere), plants, sediments, and soil. Numerous documents have been accumulated to illustrate that regions far from the predominant POPs sources, like the Arctic and high mountains (the so-called grasshopper effect), have been widely contaminated by POPs (Ma et al., 2016). The rate at which POP particles disperse into the atmosphere is affected by temperature. When the temperature drops, POPs accumulate on the soil surface, while rising temperatures lead them to evaporate into the atmosphere. (X. C. Wang et al., 2016; T. Wang et al., 2016; X. Wang et al., 2016). POPs do not easily degrade into less harmful forms due to their stability and can thus persist in the environment for decades or centuries. Therefore, over time they are expected to accumulate, e.g. in fat-rich tissue and bio-concentrates (Ashraf, 2017). Exposure to POPs has been linked to endocrine disruption, cancer, neurobehavioral problems, reproductive and immunological malfunction, according to several studies. Rachel Carson established POPs as the cause of reproductive failure in birds exposed to dichlorodiphenyltrichloroethane for the first time (DDT) (Carson and Neill, 1962). Studies have shown that POPs may change female and male reproductive processes in mammals, birds, reptiles, fish, and snails and lead to deterrent strength to generate viable oocytes and create and retain a pregnancy (Buck Louis et al., 2011). For instance, clinical studies in humans have shown that polybrominated diphenyl ethers PBDEs may decrease semen quality (Abdelouahab et al., 2011), change reproductive testosterone and progesterone metabolism in both genders in humans (Gao et al., 2016; Makey et al., 2016; Meeker et al., 2009).To reduce the impact of POPs on living beings and the environment, a global treaty was signed in 2008 in Stockholm, Sweden, by more than 90 countries. The Stockholm Convention introduces 12 POPs as a \u2018dirty dozen\u2019 chemical that has destructive effects on the environment (B. J. Sun et al., 2020; B. Sun et al., 2020). Presently, there are 26 POPs; however, with urbanization and industrialization developing, the POPs are rapidly produced (Jeong et al., 2020; Mouly and Toms, 2016). These contaminant compounds have been identified into four categories listed in \nTable 1 (Ma et al., 2016), containing various compounds. First: agricultural POPs, including pesticides, insecticides, and herbicides such as Aldrin, Dieldrin, Chlordane, DDT, Endrin, Heptachlor, Mirex, Toxaphene, etc. However, Organochlorine (OC) pesticides, introduced in 1940\u20131950 and banned later, are still used in some countries to aid agricultural practice and industrial sectors (Wagner et al., 2021). For instance, many countries use DDT due to its low cost and high efficiency in anti-malarial activity (C. Wang et al., 2016; T. Wang et al., 2016; X. Wang et al., 2016). Second: industrial chemical and unintentional productions, including polychlorinated biphenyls (PCBs), hexachlorocyclohexane (HCB) which was used as a fungicide in the past, polyfluoroalkyl substances (PFASs), brominated flame retardants (BFRs), dibenzodioxins, and dibenzofurans. However, it is speculated that more will be evaluated in the future (Praetorius et al., 2012). Third, pharmaceutical and personal care products (PPCPs) like norfloxacin, carbamazepine, diclofenac, and ibuprofen. The last group contains organic dyes such as methylene blue and methyl orange.Persistence and half-life features in the individual medium, such as air, water, soil, and sediment, were identified as the crucial parameters to introduce POPs chemicals in the Stockholm Convention. For example, the United Nations Environmental Program (UNEP) and the Canadian Environmental Protection Agency (CEPA) identified 60 and 180 days of half-lives for chemical persistence in water and soil, respectively. The transformation half-life of a compound in the environment can involve direct or indirect photolysis, redox reactions, biodegradation, and hydrolysis processes that are strongly influenced by environmental factors such as temperature, salt, redox status, microorganism activity, and sunlight exposure. Thus, based on ecological or physiological conditions, the half-life of POPs may individually vary from a few weeks to many years (Bu et al., 2016). Since most POPs have been used for a variety of purposes, including agriculture and medicine, it is critical to investigate appropriate methods that are highly efficient and cost-effective for monitoring and controlling POPs in environments for degradation and preventing them from accumulating in the natural cycle. The food chain is a global concern.Literature reports various methods for removing POPs from the environment, including chlorination, ion exchange, neutralization, oxidation, filtration, membrane filtration, activated carbon, etc. However, these approaches are limited in their application by low effectiveness, high pesticide concentration requirements, high prices, and the formation of harmful by-products, which further complicates their removal (Adithya et al., 2021). The most prominent technologies are divided into three categories: physical, chemical, and biological (Ghoreishi and Haghighi, 2003) (\nFig. 2). The biological process, which includes bioremediation, is one of the most prevalent ways of POPs degradation and is frequently used to remove POPs due to the number of microorganisms and their capacity to function in severe environments. Many of these wastewater treatment technologies have not been widely used in industry due to their high cost and disposal issues. Most research has focused on chemical and mixed chemical\u2013biological treatments to promote dye biodegradation and reduce sludge generation. As toxicity requirements grow more widespread, new approaches for reducing the content of POPs and their breakdown products in wastewater are required. These issues have prompted research into novel solutions.A very promising solution is the combination of photocatalysis and adsorption in water/wastewater treatment. Photocatalytic remediation of POPs offers a highly effective degradation process, simultaneous production of hydrogen and possibly other compounds as renewable solar fuels, while adsorption synergistically complements this with low cost, energy effectiveness, environmental friendliness, and ease of regenerating adsorbents.Any materials utilized in these procedures must meet specified requirements, such as stability of the adsorbents in reaction media, which is a critical issue in photocatalytic applications. Other aspects include thermal and mechanical stability, as well as their toxicity. Traditional materials for water purification, such as carbon compounds, zeolites, aluminosilicates, mesoporous materials, and others, have limited utility due to their low surface area, unsuitable electronic structure (either metallic or insulating), and limited options for chemical modifications (Pi et al., 2018). Most importantly, the materials need to have semiconducting properties with suitable band gaps to both ensure optimum light absorption and efficient redox activity of excited charge carriers (D\u00f6rr et al., 2018). So far, the most important photocatalysts for POPs removal from aqueous solutions include transition metal oxide (e.g. TiO2, Fe2O3) and porous organic polymer composite (Doll and Frimmel, 2005).While organic catalysts suffer from low stability and degradation, metal oxides are often limited by their relatively large band gap. For example, TiO2 has a band gap of 3.2\u2009eV for anatase (Eder et al., 2010, 2009, p. 2) that limits light absorption to near UV range. It also suffers from fast e-/h+ recombination. Nevertheless, while at the moment, TiO2 still represents the most used semiconductor for photocatalytic applications, we need to develop catalysts that combine suitable photocatalytic and adsorption properties under solar light irradiation.Photocatalysts of first-generation are made up of single-component materials (e.g., WO3 and ZnS), while second-generation photocatalysts consist of multiple components in a suspension (e.g., RGO/CdS and g-C3N4/BiOI). Among the most promising candidates are metal-organic frameworks (MOFs). Photocatalysis based on MOFs can be considered third-generation photocatalysts immobilized on solid substrates (Hayati et al., 2021).MOFs are a class of porous crystalline material consisting of metal ions or clusters and organic linkers (Kitagawa, 2014). Yaghi and Li, (1995) were the first to synthesis these compounds, which were then studied for a variety of uses as well as wastewater treatment. MOFs are made up of metal ions or metal-oxo clusters (called secondary building blocks, SBUs) that are connected by organic linkers into highly porous crystalline networks (\nFig. 3). A variety of pore sizes and shape flexibility, tunable functionality, decent thermal stability, and record surface areas render MOFs ideal candidates for the removal of a variety of pollutants. MOFs mainly synthesis via solvothermal routes. This synthesis method became the most popular way to obtain grams of MOFs in laboratories. In this process, inorganic salt and organic linker solutions are mixed in a sealed reactor vessel and heated to stimulate the formation of insoluble frameworks that precipitate as fine crystals. Moreover, synthesis methods such as non-solvothermal processes include: conventional electric (CE) heating, microwave (MW) heating, electrochemistry (EC), mechanochemistry (MC), and ultrasonic (US) techniques are also of interest in the synthesis of some MOFs (Stock and Biswas, 2012).\nThe possible compositions and structures of MOFs are nearly infinite. MOFs can be functionalized at the organic or inorganic linker, or catalytic units can be accommodated in their pore space to generate catalytic activity (\nFig. 4). MOFs large surface area and pore volume enable active guest species to be introduced into the pores/cages/channels and facilitate access of substrates to the internal active sites. The active sites of MOF catalysts can be categorized as follows: (i) coordinatively unsaturated metal (CUM) centers and functional linkers, (ii) functional groups attached to the linkers and/or metal centers by direct synthesis or by post-synthesis modification (PSM), and (iii) active guest species such as metal nanoparticles (MNPs), complexes, and polyoxometallates (POMs) encapsulated in the pores (Y.-B. J. Huang et al., 2017; Y.-B. Huang et al., 2017; Gascon, 2013).Photocatalysis involves redox reactions induced by the light energy that typically occurs on the surface of a semiconductor (Zango et al., 2020b). Among the various reactions, advanced oxidation processes (AOPs) or pollutants have been intensely investigated in the last decades. The Key is the utilization of photoexcited holes that can directly oxidize organic compounds. Alternatively, these holes, but also photoexcited electrons can transform water into reactive hydroxyl radicals and superoxide species that, in turn, can drive the desired degradation reaction. In addition to faster degradations, the advantages of photocatalysis in the removal of pollutants also include operation under ambient conditions, tunable selectivity towards the desired oxidation products, and utilization of non-reactive reactants, such as water and CO2. (Russo et al., 2020). The catalytic performance of a semiconductor is guided by an optimum band gap that allows to absorb of a large portion of light photons and provides long-living free charge carriers with suitable potential energies (defined by the HOMO-LUMO levels for holes and electrons, respectively) to drive the desired reactions. Effective exciton separation and enhanced charge transport properties, as well as direct reactant access to a large number of active sites with suitable adsorption chemistries without kinetic limitations (e.g. by reactant diffusion) are also crucial to maximize photocatalytic efficiency.MOFs have the potential to fulfill all those requirements. In addition to an unparalleled number of accessible active sites, the tunability of both SBUs and organic linkers, offers a powerful tool to adjust the chemistry of active sites (e.g. Lewis acidity) as well as the optoelectronic properties (i.e. band gap, charge absorption, and transfer pathways) (Choi et al., 2009; L. Zhang et al., 2015; S. Zhang et al., 2015).With the development of water/acid-resistant MOF materials in recent years (T. Wang et al., 2016; X. Wang et al., 2016; C. Wang et al., 2016), an increasing number of light-responsive MOFs for photocatalytic pollution removal have been reported.In the last years, reviews have been published on various aspects of MOFs in photocatalytic environmental applications (Wang et al., 2020). The majority of photoactive MOFs were used to degrade pollutants in water using UV light. Thus, the stability of MOFs in both aqueous solutions and UV irradiation becomes a crucial parameter and has to be considered for each reaction and set of process conditions.Chemical stability refers to MOFs\u2019 durability under various chemical conditions, including moisture, solvents, acidic, basic, or aqueous solutions containing coordinating anions. On the other hand, thermal and mechanical stability refers to MOFs' capacity to maintain structural integrity when exposed to heat, pressure, or vacuum treatment conditions. MOFs generally have good thermal stability, and some MOFs can even be stable at 500\u2009\u00b0C (Cavka et al., 2008). The majority of known MOFs, on the contrary, have low stability in aqueous environments. This has become a significant disadvantage that has hampered the actual application of MOFs (Ding et al., 2019). Water could break the coordination and displace the attached ligand, block the binding site, prevent other target molecules from adsorbing, and potentially lead to a collapse of the MOF structure (K. F. Tan et al., 2015; K. Tan et al., 2015). Based on Low et al. research (Low et al., 2009), there are two possibilities for degradation mechanisms of MOFs in water vapor and liquid water: (1) ligand substitution by water and (2) hydrolysis. These mechanisms were established by computer simulations. Some of the most water-stable MOFs reported are listed in (\nFig. 5). It appears that the stability of most of the MOFs remains limited to just a few hours to days at most.In the majority of cases, water molecules break up the coordination between SBUs and the organic linkers. At first, water substitutes the organic linker and forms aqua ligands that often undergo H-bond formation with the former linkers. (Eq. (1)).\n\n(1)\nM\u2212L + H2O\u2192M\u2212(H2O)\u22efL\n\n\nThis may be followed by deprotonation to yield hydroxo ligands without interactions with the former linkers, thus resulting in a structural collapse (F. Tan et al., 2015; K. Tan et al., 2015). (Eq. (2)).\n\n(2)\nM\u2212L + H2O\u2192M\u2212(OH) + LH\n\n\nExperimental studies have confirmed that adding hydrophobic functional groups to the organic ligands can improve the water stability of MOFs. For example, by combining several water-repellent functional groups in the frameworks, Wu et al. were able to create a novel MOF with water resistance in boiling water for up to one week(Wu et al., 2010). The downside of this process, however, is that this modification also changed the adsorption chemistry as well as the optoelectronic properties.Another need for an effective photocatalyst, as previously noted, is stability under irradiation conditions since otherwise, owing to degradation of the photocatalytic sites, its activity would diminish with time. Several publications indicate that MOFs utilized as photocatalysts, particularly those based on terephthalic acid (BDC), are stable under irradiation conditions (Dhakshinamoorthy et al., 2018; Jing et al., 2017; P. W. Wu et al., 2017; P. Wu et al., 2017). However, this conclusion is mainly based on relatively short time experiments, typically only a few hours. However, some of the most effective photocatalytic MOFs, such as UiO-66, MIL-101, and MIL-125, include aromatic carboxylates as linkers, and several of these compounds have been demonstrated to be photolabile (Mateo et al., 2019).The Photocatalysis process enables the degradation of refractory organic compounds into by-products by the in-situ generation of reactive oxygen species (ROS), such as superoxide (\u2022O2\n-), hydroperoxyl (HO2\u2022), alkoxyl (RO\u2022), sulfate (SO4\u2022\u2212) and chlorine (Cl\u2022) radicals (depending on the catalyst or the oxidant used). Hydroxy radicals (\u2022OH), which are the most appealing among the others, are usually generated from reactions involving oxidants such as hydrogen peroxide, ozone, or catalysts including metal ions and semiconductors under UV-Vis irradiation or other sources of energy. Depending on the catalysts phase, different AOP methods can be categorized as heterogeneous and homogenous photocatalysis (Antonopoulou et al., 2021; Bedia et al., 2019; Russo et al., 2020; Zango et al., 2020b).In heterogeneous photocatalysis, ROS are produced after charge separation caused by semiconductor irradiation (photocatalyst) (Bedia et al., 2019). Due to their existence in solid form, heterogeneous catalysts are very easy to separate from solution. However, the initial rate (turnover frequency) in heterogeneous photocatalysis is very low as the active sites are not well-defined. Hence product conversion is not uniform and leads to secondary intermediates, products. In comparison, homogenous processes have limitations in terms of catalyst loss and non-recovery (Lu et al., 2021). In addition, the catalyst deactivation is quite fast. Therefore, compared to homogeneous processes, heterogeneous photodegradation is an interesting option for wastewater treatment since the catalyst can be separated from the reaction media and reused, reducing costs and environmental issues (Bedia et al., 2019). However, the accessibility of reactive sites is an important factor. Diffusion of reactants and products to and from the catalytically active sites is often the rate-limiting step (Gascon, 2013).An essential criterion in the choice of MOFs for photocatalytic applications is the ability of the MOFs to harvest and channel light energy (Zango et al., 2020b). The high flexibility of the MOFs framework allows elaborate design and tailoring of their structure to enhance photocatalytic activity (Y. X. Li et al., 2016; Y. Li et al., 2016; Wen et al., 2021a, 2021b; Wu et al., 2019; Younis et al., 2020). Some MOFs containing Fe, Cr, Zr, and Ti metal ions can harvest and channel solar energy. They usually possess a small bandgap that enables visible light excitation; hence, they are considered promising candidates for photocatalytic degradation of organic pollutants (Pi et al., 2018; Zango et al., 2020b). The presence of organic linkers in MOFs allows them to have a tunable absorption spectrum and an efficient charge separation enabling lifetimes in the microsecond range. However, a few MOFs have been reported showing photocatalytic activities under visible light. Heterogeneous photocatalysis principal is illustrated in \nFig. 6.A photodegradation reaction is initiated when a catalyst absorbs energy equal to its band gap energy (Bedia et al., 2019; Russo et al., 2020; Younis et al., 2020). After photoexcitation, and generation of electron (e-)-hole (h+), (Eq. (3)), electron transfer takes place from the catalyst surface to the adsorbed acceptor (A) molecules and from the adsorbed donor (D) molecules to the catalyst, (Eqs. (4) and (5)):\n\n(3)\n\n\nPhotoexcitation\n:\nM\nO\nF\n\n\u27f6\n\nh\n\u03c5\n\n\n\n\ne\n\n\u2212\n\n+\n\n\nh\n\n+\n\n\n\n\n\n\n\n(4)\n\n\nReduction\n\nof\n\nacceptor\n:\nA\n+\n\n\ne\n\n\u2212\n\n\u2192\n\n\nA\n\n\u2212\n\n\n\n\n\n\n\n(5)\n\n\nOxidation\n\nof\n\ndonor\n:\nD\n+\n\n\nh\n\n+\n\n\u2192\n\n\nD\n\n+\n\n\n\n\n\nThese chain reactions continue until final oxidation products are formed, while recombination of electron-hole leads to photoelectric energy dispersion (Eq. (6)).\n\n(6)\n\n\nTermination\n\nreaction\n:\n\n\ne\n\n\u2212\n\n+\n\n\nh\n\n\u2212\n\n\u2192\nN\n+\nE\n\n\n\nwhere E is the energy released in the form of heat or light and N is the neutral center resulting in a reduction of the photoexcitation process efficiency (Russo et al., 2020).The electron e--h+ charge transition can be described by four mechanisms (\nFig. 7): (1) ligand-to-metal charge transfer (LMCT), (2) metal-to-ligand charge transfer (MLCT), (3) metal-to-metal charge transfer, node-localized excitation, (MMCT), (4) ligand-to-ligand charge transfer, linker-localized excitation, (LLCT) (Wu et al., 2019). The LMCT is the most effective charge transfer mechanism to prevent e--h+ recombination because MOFs can achieve better e--h+ separation via ligand-to-metal charge transfer (Y. X. Li et al., 2016; Y. Li et al., 2016; Wen et al., 2021a, 2021b; Wu et al., 2019; Younis et al., 2020). The LLCT is generally rare because the linkers in the MOF structure are separated by metal clusters (Wen et al., 2021a). The LMCT mechanism's efficiency depends on the energy required to transfer photo-generated electrons from the LUMO of ligands to the LUMO of metal nodes, ELMCT (Y. X. Li et al., 2016; Y. Li et al., 2016; Wen et al., 2021a, 2021b; Wu et al., 2019; Younis et al., 2020). Also, Eg and Eabs are other fundamental parameters that affect LMCT. Eg is the band gap between HOMO and LUMO of ligands. Eabs is absorption energy, the energy required to excite the semiconductor (Wen et al., 2021b). The Eabs is the sum of Eg and ELMCT. Therefore, for a constant Eg, the zero or negative values of ELMCT lead to the smaller values of Eabs that favor the absorption of visible light. MOFs composed of ligands with high-energy lone pairs and metals with low-lying empty orbitals are more desirable due to the favorable ELMCT (Y. X. Li et al., 2016; Y. Li et al., 2016; Wen et al., 2021a, 2021b; Wu et al., 2019; Younis et al., 2020).\nAlvaro et al. (2007) reported on the semiconducting characteristics of MOF-5 as an early example of MOFs as photocatalysts. They synthesized MOF-5 by precipitating a mixture of two DMF solutions with triethylamine at room temperature. MOF-5 material contains clusters of Zn4O located at the corners of the structure that are connected orthogonally to six units of terephthalate. According to the author's findings, absorption of solar irradiation (from 350 to 400\u2009nm) by the MOF-5 framework will produce charge separation and generate photoexcited electrons. The author tested the activity of MOF-5 in the photodegradation reaction of phenol in comparison with Degussa P25 (TiO2 nanoparticles) and another semiconductor, ZnO. The result revealed that MOF-5 behaves as a semiconductor and undergoes charge separation (electrons and holes). The remarkable photocatalytic activity of MOF-5, compared with that of ZnO and TiO2, also confirmed that MOF-5 degraded under catalytic conditions, forming ZnO. Hence, these ZnO nanoparticles could be responsible for the catalytic process (Alvaro et al., 2007).When dyes are released into the environment, they frequently pose severe ecological hazards, such as harming aquatic life, impeding plant growth, and posing various forms of toxicity to humans, such as genotoxicity, reproductive toxicity neurotoxicity, and other forms of the disease (Zango et al., 2020b).The majority of photocatalytic degradation studies using MOFs were focused on the photodegradation of organic dyes. Numerous MOFs have been used for the photodegradation of dyes, and some relevant studies are summarized in \nTable 2. As can be concluded from the literature review, Methylene blue (MB, cationic dye), Methyl orange (MO, anionic dye), and Rhodamine B (RhB) have been studied as target pollutants in most articles mainly because they are easy to analyze. However, the number of researchers who examined sunlight for dye removal is rare. The sunlight absorption capacity of pure MOFs is low; therefore, a lower rate of conversion and removal can be achieved when using sunlight in comparison with UV light sources. The studies focused on the synthesis and application of MOF composites instead of pure MOFs to enhance light harvest capacity. The results of the literature review revealed that the mechanism for photodegradation of organic dyes is the formation of electron (e-)-hole (h+) pair in MOF structure under a light source as the initial step of the photocatalytic dye degradation process. After absorption of energy by the MOF, the e- was excited from the valence band (VB) and entered into the conduction band (CB), leaving the h+ in the VB. Cd (II)-imidazole MOFs, used under UV light for the MB and MO degradation demonstrated that the Cd (II) complexes formed with the linkers were excited by UV light, generating the photogenerated charges that are required for dyes degradation (B. Liu et al., 2014; L. Liu et al., 2014). Zn (II)-imidazolate MOF, named ZIF-8, also revealed a high activity for MB degradation under UV light due to the excitation of the Zn-complexes (Zhang et al., 2016). Different Fe-based MOFs have been tested for the photodegradation of dyes under visible light. Among them, MIL-88B (based on terephthalate linkers) showed higher activity for R6G degradation than other MIL-100 and MIL-101 MOFs, which was associated with the visible light absorption of the Fe-oxo-clusters that formed the structure so as the photogenerated electron is transferred to Fe3+ from O2\n- (Laurier et al., 2013).The transformation of traditional into modern agricultural practice requires insecticides, herbicides, pesticides, and fertilizers. The discharge of increasing agrochemicals to water sources poses potential hazards to the ecosystem (Mon et al., 2018; Zango et al., 2020b; Wen et al., 2021a; Zendehbad et al., 2022). Pesticides contain various chemicals such as herbicides, insecticides, and fungicides and are commonly applied in agriculture (Khodkar et al., 2019; Wen et al., 2021a). Herbicides are primarily produced to inhibit weeds that compete with the plant\u2019s growth (Oladipo, 2018; Zango et al., 2020b). At the same time, insecticides are aimed at repelling or mitigating insects and other pests from attacking agricultural products (Zango et al., 2020b). Herbicides account for about 62% of the total pesticides used in agriculture in the United States. During the past two decades, the five most-used herbicides were glyphosate, atrazine, metolachlor-(S), 2,4-D, and acetochlor (Wen et al., 2021a). Traditional materials such as TiO2, WO3 and their modified derivatives have shown good performance on the photocatalytic degradation of pesticides like diazinon, acephate, omethoate, (4-nitrophenol (4-NP) and methyl parathion herbicides like isoproturon and triazine, etc. (Pi et al., 2018). However, the number of literatures on photocatalytic degradation of these agricultural materials using MOFs is rare. The most recent literature related to herbicides and pesticides is reviewed here.The photodegradation of 4-nitrophenol (4-NP) was investigated by (Samuel et al., 2018) using [Zn(BDC)(DMF)] crystal which was synthesized via ultrasonic irradiation and solvothermal method. The synthesized MOF [Zn(BDC)(DMF)] exhibited high photocatalytic activity in the presence of NaBH4 under natural sunlight irradiation, and the reduction of 4-NP to 4-aminophenol (4-AP) was completed within 10\u2009min. The catalyst showed higher catalytic activity even after ten cycles, with an efficiency above 95%. The 4-NP degradation reaction data were fitted to the first-order kinetic plot with a rate constant of 0.6008\u2009min\u22121. After irradiation and generation of electron-hole pairs, electron transfer takes place from BH4\n- ion to 4-NP. In the presence of MOF[Zn(BDC)(DMF)], BH4\n- ion gets adsorbed on its surface, and discharge of electron from BH4\n- ion takes place through the metal oxide to the acceptor 4-NP. The aqueous medium consists of H+ ions for complete reduction of 4-NP to 4-AP.Photocatalytic removal of 4-nitrophenol (4-NP) under visible LED light irradiation was investigated by using a MOF/CuWO4 composite system which was synthesized by (Ramezanalizadeh and Manteghi, 2018). [CoNi(\u03bc3-tp)2(\u03bc2-pyz)2] MOF was prepared with the utilization of hydrothermal approaches. The comparison between PL spectrum of the pure CuWO4, pure MOF, and MOF/CuWO4 composite structures demonstrates that the MOF/CuWO4 composite has the minimum recombination of electron-hole pairs. This behavior leads to a high lifetime for the charge carrier\u2019s species and increases the photocatalytic performance. Photocatalytic destruction in the presence of pure MOF was found to be 24%, while photodegradation efficiency was enhanced to 81% in the presence of MOF/CuWO4 (1:1) composite. There was no data on the kinetic of photodegradation 4-NP using the as-synthesized composite.Motivated by the phenomenon of utilizing two types of the organic linker and assisting in extending the lifetime of charge carriers, (Surib et al., 2018) constructed a Cu-MOF photocatalyst They employed H4btec (1,2,4,5-benzenetetracarboxylic acid) and 1,4-bimb (1,4-bis(imidazole-1- methylbenzene)) as ligands, and the framework was constructed through a facile and simple hydrothermal route. Photocatalytic degradation of 2-chlorophenol was examined by undertaking solar light. After four hours of irradiation, 100% photodegradation for this pollutant was achieved and photodegradation of 2-chlorophenol under solar light was fitted to the first-order kinetic. The light activation excited the electron from HOMO to LUMO. The excited electrons at LUMO are utilized by the surface adsorbed O2 and converted to reactive \u2022O2\n- state and in turn to \u2022OH. Meanwhile, the removal of one e- from HOMO has altered the stability of the molecule that, forces the HOMO to react with the same surface adsorbed water molecule and convert it into active \u2022OH. This chain reaction continues until the irradiation is terminated, resulting in the generation of a higher concentration of ROS. These reactive species continuous the reaction further with the available substrate (2-CP) to its simplest nontoxic form.\nRamezanalizadeh and Manteghi (2017) prepared a photocatalyst by immobilization of mixed cobalt/nickel metal-organic framework on a magnetic BiFeO3. The 2.18\u2009eV band gap of BiFeO3 makes it attractive to visible light harvesting. However, due to the fast electron-hole pairs regeneration, it needs to be combined in the composite form with a foreign high surface area structure. The photocatalytic performance of the prepared samples was evaluated for the photodegradation of 4-NP. The visible light source was a 5\u2009W LED lamp (\u03bb\u2009=\u2009440\u2009nm). With the aid of visible-light irradiation, MOF and BiFeO3 absorb the photons and produce the electron-hole pairs. Afterward, the photogenerated charge carriers are separated under the influence of internal electrostatic fields in the heterojunction areas. As a result, the regeneration of the electron-hole pairs in the MOF/BiFeO3 (1:1) effectively decreased, and this trend leads to higher photocatalytic performance for this system. The degradation of about 70% was obtained by MOF/BiFeO3 (1:1) as shown in \nFig. 8. The data on kinetic of photodegradation 4-NP using the as-synthesized composite was not available in their study.Removal of diazinon and atrazine pesticides under visible light illumination with a high-pressure mercury-vapor lamp (400\u2009W and \u03bb\u2009=\u2009546.8\u2009nm) was studied by Fakhri et al. (2020). They synthesized a core-shell structured magnetic graphene oxide@MIL-101(Fe) which acts as a photo-Fenton catalyst to degrade the mentioned pesticides. After 105\u2009min irradiation, the photodegradation efficiency was about 100% and 81% for diazinon and atrazine, respectively. However, TOC analysis reveals that the mineralization of diazinon and atrazine was approximately 84.0% and 62.0%, respectively. The difference between mineralization and photocatalytic efficiency is due to the presence of intermediates during photodegradation, which allows the ongoing process to mineralize for a longer time. According to this study, the Fenton reaction was initiated through the photo-reduction of Fe3+ to Fe2+ in the MIL-101 (Fe) component. Thus, the photo-produced electrons transfer to the cores of Fe3O4 through graphene oxide (GO) layers. GO assists in increasing the transformation of Fe3+ to Fe2+, which is ideal for producing more OH radicals. The photodegradation process occurs by the attack of these active radicals on pesticides\u2019 structure and subsequent transformation into H2O and CO2.Recently, Hayati et al. (2021) prepared a novel Ag (I) coordination complex [Ag(p-OH-C6H4COO)2(NO3)]n (1) using the laying as well as sonochemical irradiation methods (Hayati et al., 2021). The synthesized catalyst was used for simultaneous photodegradation of 2,4-D and MCPA under sunlight irradiation. These herbicides belong to the phenoxy acid herbicide group. The decline in TOC (total organic content) of irradiated samples confirms the mineralization of the herbicides to CO2 and H2O. The TOC was reduced from 2.20 to 0.10\u2009mg/L with the irradiation of sunlight. The synthesized photocatalyst's FOM (figure of merit) was the best among other photocatalysts, such as TiO2, BiVO4/Ag3VO4, FTO/BiOBr, and Steel/TiO2\u2013WO3. The mechanism of photodegradation of 2,4-D is not discussed in their study.\nKhodkar et al., (2018) prepared a magnetic photocatalyst \u03b1-Fe2O3 @MIL-101 (Cr)@TiO2 via the sol-gel method to remove paraquat from an aqueous solution . A 125\u2009W medium UVC lamp was used as a light source. The response surface methodology, RMS, was applied to optimize the effective parameters such as contact time, pH, catalyst dosage, and herbicide concentration. They utilized potassium dichromate analysis for measuring COD to determine the number of organic compounds before and after photodegradation. The catalyst dosage and paraquat concentration significantly affected photocatalytic degradation, while pH and contact times were not significant factors. The maximum photocatalytic degradation and COD reduction were achieved at 87.46% and 90.09% at optimal conditions with paraquat concentration of 20\u2009mg/L, catalyst dosage of 0.2 gL\u22121, pH 7, and contact time of 45\u2009min. The major role of the improved photocatalytic elimination of paraquat was due to an increase in the total active surface area of the catalyst. Their data for photodegradation was well fitted with pseudo-second-order kinetic (R2 = 0.9986 and k\u2009=\u20090.0053\u2009g\u2009mg\u22121 min\u22121). The photodegradation mechanism was not discussed in their research.\nMei et al. (2019) modified MIL-53 (Fe) by regulating the electronegativity and BET surface area of Fe-O clusters in MIL-53 (Fe) (Mei et al., 2019). The synthesized MOF was used for photocatalytic removal of Thiamethoxam (TMX) under the LED light source. They confirmed that the excellent photo-Fenton-like catalytic activity could be obtained through modifying MOF by Xylitol and D-sorbitol (\nFig. 9). Their findings revealed that the Fe2 + and photogenerated electrons in the system respectively reduce persulfate (PS) and O2 to produce the SO4\u2022\u2212 radicals and the \u2022O2\n- radicals, similar to those observed by homogeneous Fe-containing complexes by Fenton-like reaction. Activation of as-synthesized MOF by visible light initiates charge separation, in which electrons are captured by Fe3+ to form \u2022O2\n- radicals and Fe2+. The as-formed Fe2+ and the original Fe2+ can reduce PS to produce SO4\u2022\u2212 radicals, and the Fe2+ is oxidized back to Fe3+ by PS. The more electrons are consumed by Fe3+, accelerating the separation of electrons (e-) and holes (h+) pairs effectively, the more holes (h+) corresponding to it are in a free state. And the free holes (h+) and other radicals (\u2022O2\n-, SO4\u2022\u2212, \u2022OH) are united together, contributing to the degradation of TMX.Taking advantage of the catalytic properties of MOFs and fast carrier mobility of 2D materials, MIL-125 (Ti)/BP as the first 2D/MOF photocatalyst for diazinon removal from water under UV light was synthesized by (Hlophe and Dlamini, 2021). The removal mechanism of diazinon was based on Ti3+-Ti4+ intervalence electron transfer (Wang et al., 2021), which generates electron-hole pairs as shown in \nFig. 10. The composites were prepared by varying the weight loadings of MIL-125 (Ti) while keeping the amount of FLBP constant and denoted 4%BpMIL, 6%BpMIL, and 12%BpMIL (m/m). According to their findings, the 4%BpMIL at neutral pH had the best removal efficiency (96%) after 30\u2009min irradiation.Other studies on the photodegradation of agricultural products and their main results have been summarized in \nTable 3. As can be concluded, the number of agricultural products studied has been limited to several special chemicals. Also, the single component photocatalyst suffers from quick charge recombination and limited activity. Therefore, the research in recent years focused on constructing heterostructure MOF composites.Zr-based MOFs (i.e. UiO-66) and other MOFs such as MIL-53 (Fe) and MIL-100 (Fe) have high chemical and thermal stability in water (Ahmad et al., 2019). Fe-based MOFs are found to be a better choice compared to Zr-based MOFs for efficient photo-Fenton activity owing to their Fe-metal clusters. However, weak visible light response remains an unsolved issue when using MOFs as photocatalysts. Therefore, a combination of MOFs with materials such as BiOBr (Xue et al., 2018) and WO3/graphene oxide (Fakhri and Bagheri, 2020) is possibly an effective way to enhance photocatalysis efficiency. Synergistic effect of these materials in combination with MOF improves the photocatalytic activity. One example is the nanocomposite WO3/graphene oxide, where the conductive band (CB) of UiO-66 (\u22120.53\u2009eV vs. NHE) is more negative than the CB of the W sample (+0.4\u2009eV vs. NHE). It appears that after visible light irradiation, the photoexcited electron move from UiO-66 to CB of W, and holes move from the valence band (VB) of UiO-66 (+3.5\u2009eV vs. NHE) to VB of W (+3.2\u2009eV vs. NHE). Consequently, graphene oxide acts as an acceptor for photogenerated electrons, thereby limiting recombination of charge carriers. Furthermore, the \u2022O2\n- and \u2022OH radicals can be easily formed by the photo-excited electrons and holes. These active radicals subsequently attack pollutant molecules that are adsorbed on the photocatalyst surface, and oxidize them to H2O and CO2.The different materials are widely used in making useful industrial products such as petrochemicals and plastics. Material such as phenol and its derivatives are used as a precursor in pharmaceuticals, dyes, herbicides, pesticides, detergents, epoxy resins, and polycarbonate plastics.Polycyclic aromatic hydrocarbons (PAHs) comprise benzene ring molecules, forming highly stable structures (Zango et al., 2020b, 2020a, 2020c). They can originate from natural processes (e.g., volcanic eruptions, bush fires) (Zango et al., 2020c) and more from oil exploration, exportation, effluents discharged from petroleum refineries and petrochemical industries, oil spillage, etc. (Zango et al., 2020b, 2020a, 2020c).Perfluorinated compounds (PFCs), which are called \u2018\u2019forever chemicals\u2019\u2019 are another group of POPs that have been widely used for the production of industrial (e.g., surfactants) and consumer products (e.g., non-stick coatings) (Zango et al., 2020b). These chemicals are composed of a fluorinated carbon backbone terminated by a carboxylate functional group (Chen et al., 2016). The most toxic of these groups in the perfluoroalkyl carboxylates (PFCAs) and perfluoroalkyl sulfonates (PFAS) (Zango et al., 2020b).In recent years MOF based materials have gained attention as an excellent candidate for treating industrial residues. However, to the best of our knowledge, there is no report on the application of MOFs in the photocatalytic removal of PAHs and PFASs. Few papers have been published on the adsorption removal of these chemicals. Because of their lipophilic character, they resist photo- and bio-degradation (Zango et al., 2020d). Most studies related to industrial pollutant removal are limited to photocatalytic removal of phenol and its derivatives which are discussed in this section.\nAhmad et al. (2019) synthesized MIL-100 (Fe) loaded with ZnO nanoparticles using the solvothermal technique (Ahmad et al., 2019). The photo-Fenton activity of as-prepared M.MIL-100 (Fe)/ZnO NS hybrid has been evaluated degradation of phenol and bisphenol-A under LSH-500\u2009W Xe arc lamp irradiation. A certain amount of H2O2 was added to the mixture of catalyst and pollutants to increase photo-Fenton activity. The photodegradation pollutants under various reaction conditions like initial pH value and H2O2 concentration were investigated. Their analysis confirms ligands\u2013to-clusters charge transfer (LCCT) mechanism, suggesting the strong bonding of ligands oxygen with Fe (III) atoms. The synthesized photocatalyst shows much lower photoluminescence (PL) intensities compared to MIL-100 (Fe), and MIL-100 (Fe), which are associated with recombination inhibition and a longer lifetime of photoinduced charge carriers.Taking advantage of the sulfate radical (SO4\u2022\u2212)-based AOPs (SR-AOPs), Lv et al. (2020) prepared two novel MOFs/COFs hybrid materials with nitrogen-rich building blocks for the first time (Lv et al., 2020). The pristine MOFS were MIL-101-NH2 and UiO-66-NH2. The photocatalyst was fabricated by a feasible step-by-step assembling method and used for the photodegradation of BPA (bisphenol-A). The possible photocatalytic mechanism was explored through radical species quenching experiments. Their results also reveal that the composite photocatalyst displayed a lower PL intensity, implying the recombination of electron-hole pairs was efficiently inhibited. In the case of MIL-101-NH2, results suggested that the h+ and \u2022OH were responsible for the degradation process of BPA, while h+ was involved in photocatalytic degradation over UiO-66-NH2.Another study by Lin et al. (2020) confirmed the synergic effect between MIL-88 (Fe) and PS for the photodegradation of BPA under visible light (Lin et al., 2020). The results revealed efficient charge carrier separation ability in the presence of PS, which could scavenge conduction band electrons and prevent the recombination of electrons and photo holes. The reaction of photogenerated electrons with PS can result in abundant production of SO4\n- as shown in \nFig. 11. In addition, the excitation of PS could also produce other active species, such as O2\n- and OH radicals.To enhance the Fenton-like performance of MIL-88B-Fe, (S. Zhang et al., 2019; X. Zhang et al., 2019; Y. Zhang et al., 2019; H. Zhang et al., 2019) studied the incorporation of carbon nanotubes (CNTs) into the MOF structure. CNTs, with electron-rich oxygen-functional groups on the surface, facilitate the reduction of Fe (III) to Fe (II) to initiate the Fenton reaction. Phenol, BPA, and 2,4-dichlorophenol were entirely removed within 30\u2009min using this photocatalyst. Also, in the presence of H2O2, the coordinatively unsaturated metal sites in the MIL-88B-Fe, which behaved as Lewis acid sites, are prone to adsorb the H2O2 molecules (a Lewis base). As a result, the generated Fe (II) directly reacted with H2O2 to form \u2022OH, attacking pollutant molecules.\nC. Wang et al. (2019); Q. Wang et al. (2019); H. Wang et al. (2019) investigated the coupling of NH2-MIL-125 (Ti) with CdS to photo-degrade organic pollutants such as BPA under visible light (H. C. Wang et al., 2019; Q. Wang et al., 2019; H. Wang et al., 2019). The synthesized composites showed higher photo-degradation activities compared with pristine CdS and NH2-MIL-125 (Ti) nanocrystals. Following other research, the results confirmed a lower recombination rate of photo-generated electrons and holes in synthesized photocatalysts rather than pristine nanocrystals.The NH2-MIL-125 (Ti)/Bi2MoO6 core-shell heterojunction was synthesized by H. Zhang et al. (2019); X. Zhang et al. (2019); Y. Zhang et al. (2019); S. Zhang et al. (2019) to accelerate the transformation of photogenerated electrons and to improve the photocatalytic performance of pure Bi2MoO6 and NH2-MIL-125 (Ti) (S. H. Zhang et al., 2019; X. Zhang et al., 2019; Y. Zhang et al., 2019; S. Zhang et al., 2019). The electrons are transferred from the organic linkers and Bi2MoO6 to the Ti-oxo clusters of NH2-MIL-125 (Ti). Generally, they suggest that three features can improve the photocatalytic performance of the as-synthesized structure. The formation of heterojunctions leads to the rapid transfer of electrons and holes. The pores of NH2-MIL-125 (Ti) inside the heterojunction can provide a channel for electron transfer and extend the lifetime of an electron and help to scatter and reflect the light. Also, the presence of surface defects and smaller contact angles can effectively enhance photocatalytic ability.The increasing consumption of pharmaceutical drugs, cosmetics, and household chemicals in modern societies releases enormous amounts into the ecosystem (Mon et al., 2018). Photocatalytic removal of antibiotics which belong to pharmaceutical groups has been widely investigated. MOFs of the MIL family, such as MIL-100 (Fe), MIL-101 (Fe), and MIL-53 (Fe), are used as photocatalysts for tetracycline (TC) removal. (Jiang et al., 2019) prepared a MIL-53 (Fe) using a solvothermal method. Photocatalytic activity of as-synthesized photocatalyst was evaluated by TC removal under visible light. A 300\u2009W Xe lamp was used as a light source. They also investigated the effect of the HCl modulator on the functional group of MIL-53. Their findings reveal that more uncoordinated ligands are present in the MOF structure due to the absence of the metal clusters dissolved in HCl. Compared with MIL-53, the photocatalytic activity of acid-regulated MIL-53 increased by 1.5 times due to the easier separation of photogenerated carriers.Photocatalytic activity of three Fe-based MOFs, including Fe-MIL-101, Fe-MIL-100, and Fe-MIL-53, was studied by Wang et al. (2018) through TC removal under visible light (Wang et al., 2018). Their results show that Fe-MIL-101 has much higher activity than the other two kinds of Fe-MILs under visible light exposure. The degradation rate was not very fast in the first hour under irradiation. However, 96.6% of the total TC was removed after three hours of irradiation. Fe-MIL-101 also exhibited high stability and recyclability for the photocatalytic degradation of tetracycline. Extensive iron oxo clusters cross-linked by ligands make this MOF responsive to visible light. As per their results, the photo-excited electrons on the valence band (VB) of Fe-MILs move to their conduction bands (CB) under visible-light irradiation. Consequently, the formed active species such as \u2022O2\n-, \u2022OH, and H+ could effectively degrade the tetracycline into small intermediates or end products directly.In another study, Rasheed et al. (2018) investigated the effect of using carbon aerogel (CA) in MIL-Fe structure through TC removal (Rasheed et al., 2018). The synthesized MIL-100 (Fe) was combined with Fe3O4 and CA. The performance of ternary MIL-100 (Fe)@Fe3O4/CA photocatalysts was compared to the CA, Fe3O4, Fe3O4/CA, and MIL-100 (Fe)/CA. According to their results, 85% removal was achieved by MIL-100 (Fe)@Fe3O4/CA, which confirms the highest performance for ternary systems. The coupling of CA considerably accelerated the transfer of photo-generated charge carriers and enhanced the performance of MIL-100 (Fe)/Fe3O4.Fe-based MOFs MIL-53 (Fe) as an integrated photocatalytic adsorbent was synthesized by (Gao et al., 2017), and the photocatalytic activity of MIL-53 (Fe) was tested by the photocatalytic decomposition of clofibric acid (CA)and carbamazepine (CBZ) with visible light illumination According to their findings, the MIL-53 (Fe) became highly active when a small amount of H2O2 was added in acid conditions (\nFig. 12). The conversion efficiencies of CBZ in MIL-53 (Fe)/Vis, MIL-53 (Fe)/H2O2, and MIL-53 (Fe)/H2O2/Vis system were 6.8%, 4.9%, and 90.1%, respectively, while for CA in MIL-53 (Fe)/vis, MIL-53 (Fe)/H2O2, and MIL-53 (Fe)/H2O2/vis system were 19.8%, 2.2%, and 98.2%, respectively. In this system, Fe (III) on the defect sites of MIL-53 (Fe) was exposed and could catalyze the decomposition of H2O2 to produce \u2022OH by the Fenton-like reaction. On the other hand, since H2O2 can serve as an efficient scavenger that captures the photo-induced electrons in the excited MIL-53 (Fe), more \u2022OH can be formed, further enhancing photocatalytic activity.The photocatalytic activity of the Pd@MIL-100 (Fe) has been firstly evaluated through the degradation of theophylline and ibuprofen under visible light irradiation by Liang et al. (2015) (Liang et al., 2015b). The results show that under visible light irradiation, Pd@MIL-100 (Fe) was able to degrade about 7.3% of theophylline after 150\u2009min (without H2O2), which may contribute to the direct holes oxidation pathway. Pd@MIL-100 (Fe) becomes highly active by adding a certain amount of H2O2. Moreover, after visible light illumination for 150\u2009min, the Pd@MIL-100 (Fe) exhibits much higher activity than MIL-100 (Fe). The Pd@MIL-100 (Fe) exhibits higher photoactivity than original-MIL-100 (Fe) in ibuprofen degradation. According to their results, with the decrease of the pH value, the degradation efficiency of theophylline was increased. However, an opposite tendency was observed for the Pd@MIL-100 (Fe)/ibuprofen system.\nS. Li et al. (2019); N. Li et al. (2019) synthesized Fe3O4/MIL-100 (Fe), via microwave in 30\u2009min, for photocatalytic removal of Diclofenac sodium (DCF) (N. Li et al., 2019; S. Li et al., 2019). As the author claimed, it was the first in-situ fabricated Fe3O4 @MIL-100 (Fe) nanostructures using a microwave (MW) assisted synthesis method. In this catalytic system, Fe-O clusters in Fe3O4 @MIL-100 (Fe) were activated by visible light, and an electron-hole pair on its surface was formed. The electrons reacted with H2O2 and O2 to generate \u2022OH and \u2022O2\n-radicals. The holes reacted with H2O to produce hydroxyl radicals which could directly oxidize DCF molecules. Based on their results, more than 99.4% of DCF was removed at the Fe3O4 @MIL-100 (Fe) dosage of 0.1\u2009g/L. Their finding revealed the positive effect of the MW synthesis method on photodegradation of DCF, as shown in \nFig. 13. The MW synthesis can be an effective way in MOFs synthesis because of its unique superiorities such as fast crystallization and narrower particle size distribution.MOFs such as MIL-88 (Fe), MIL-125\u2009, and UiO-66 are also used for photocatalytic removal of PPCPs. \nTable 4 summarizes the results of applying these MOFs for the removal of pharmaceuticals.To promote photocatalytic performance, strategies such as functionalization, deposition of metal nanoparticles, combination with semiconductors in different types of heterostructures, and sensitization with dyes are adopted. MOF functionalization can be carried out by in-situ functionalization or post-synthetic modification (PSM) (Bedia et al., 2019; X. Li et al., 2016; Y. Li et al., 2016; Subudhi et al., 2018; Younis et al., 2020).In-situ functionalization is based on using functionalized organic linkers to synthesize MOF (Bedia et al., 2019; Subudhi et al., 2018). Metal nodes, organic linkers, and external functionalized groups can act as active photocatalytic sites (Subudhi et al., 2018). Various ligand functionalized groups (e.g., \u2014CH3, \u2014NH2, \u2014NO2, \u2014OH, Br-, Cl-, and \u2014SH) have been applied to shift the photon absorption from the UV to the visible region. In particular, amine functional (electron-donating) groups have been broadly employed due to their interactions with \u03c0\u2217\u2014 orbitals of the ligand benzene ring, which increase electron density around the antibonding orbitals (formation of sp2) in the aromatic carbon ring for enhanced visible photon absorption (Younis et al., 2020).Post synthetic methods consist of functionalizing parent MOF by guest molecules (Bedia et al., 2019; C. Liu et al., 2019; W. Liu et al., 2019; N. Liu et al., 2019; Younis et al., 2020). Some of these strategies are shown in \nFig. 14. Metal ion doping is an effective tool to enhance photocatalytic activity. Partial substitution of the metal centers can improve their semiconducting properties by electron-hole separation (Bedia et al., 2019; Younis et al., 2020). The design of bimetallic MOFs with targeted band gap structures is another PMS strategy for photocatalytic performance (Younis et al., 2020). In general, Au, Ag with strong plasmonic absorption properties and Pt, Pd, Ni, and Cu nanoparticle (MNPs) with excellent catalytic activity have commonly been used as active co-catalysts to improve electron transfer (Guo et al., 2021). These metal nanoparticles are expected to increase electron-hole pair separations and enhance photocatalytic activity (Wen et al., 2021b; Younis et al., 2020). Co-doped UiO-66 nanoparticles and Cu NPs MOF-based materials are among the less expensive alternatives than Pt NPs MOF-based photocatalysts (Bedia et al., 2019). There are different strategies for the synthesis of MNP/MOFs. Adsorption of MNPs by MOF substrates simply by mixing them in one of these methods. In this method, MNPs and MOFs are synthesized separately. The adsorption strategy is simple and easily scalable for practical industrial production, but the interactions between MNPs and MOFs need to be strengthened by adding binders to generate stable performances in long-term photocatalytic applications. MNPs also can be introduced into the matrix of preformed MOF substrates. One-pot, bottle around-ship, and sandwich structure strategies are among the other methods for synthesis of MNP/MOFs photocatalysts. Among all, the one-pot strategy is obviously the simplest and most straightforward one to obtain MNP/MOF composites, but the success of this strategy is far away from research satisfaction due to the very limited selections of specific metal precursors and MOF ligands as well as rigorous synthesis conditions including solvent selection, temperature programming, reaction time and etc. Until now, different MNP/MOFs structure such as Au/MIL-125, Pd/MIL-125, Pt/MIL-125, Pt/NH2-MIL-125, Au@ZIF-8, Au@\u2009UiO-66 (NH2) has been synthesized (Guo et al., 2021). MNP/MOF composites not only combine both advantages of individual MNPs and MOFs also take advantage of the synergistic effect arising from the strong interaction between different components.However, there are unsolved issues regarding MNP/MOF development. Energy efficiencies obtained by MNP/MOF composites are still far lower than the basic criteria required by industry applications. A comprehensive understanding of mechanism of photocatalytic process requires advanced characterizing tools. Other challenging issue is constructing MNP/MOF composite with multifunctionality to expand their photocatalysis reaction scope (Guo et al., 2021) significantly.In recent years, heterojunction photocatalysts (MOF-based composite materials) have been introduced as the most common strategy for improving MOF activity under visible and solar light (Bedia et al., 2019; Wen et al., 2021b; Younis et al., 2020). Various semiconductors such as TiO2, BiVO4, and In2S3, Bi2MoO6 have been incorporated into MOFs to improve the charge transfer (Bedia et al., 2019; Wen et al., 2021b; Younis et al., 2020). Also, the synthesis of ternary systems to take advantage of the synergy between different heterojunctions has received significant attention (Bedia et al., 2019). Other hybrid structures such as MIL-100 (Fe)/Fe3O4/CA (Rasheed et al., 2018), Ag/AgCL/MIL-88A (Fe) (W. J. Huang et al., 2018; L. Huang et al., 2018; W. Huang et al., 2018), Ag/Ag3PO4/HKUST1 (Shen et al., 2013), Pd/GO/MIL-101 (Cr) (Wu et al., 2015), etc., are prepared, and their photocatalytic performance was examined through the removal of organic pollutants.Despite the accelerating progress in applying effective strategies to improve MOFs' photoactivity, a complete analysis is necessary till MOFs can offer remarkable opportunities for natural waste water purification. Contaminant concentrations varied widely from study to study, but it is unclear how relevant the contaminant concentrations are to a specific wastewater stream. Another aspect is the limited diversity of target contaminants (mostly organic dyes) despite the wide range of existing pollutants. Also, in most studies, synthetic waste samples (i.e. simple matrix) are considered. A complete discussion of photocatalysis mechanism is still needed. Furthermore, identifying possible decomposition products and byproducts (other than CO2 and H2O) needs to be studied.Adsorption techniques have been widely applied as an alternative wastewater remediation process and presented as a solution to the challenging task of incomplete extraction of pollutants during wastewater processing. Organic contaminants, in particular, tend to be more resistant to various types of water treatment due to their hydrophobicity and lower molecular weight. In the process of adsorption, pollutant molecules are drawn onto the adsorbent materials' surfaces through the process of diffusion from the solution bulk to the adsorbents' active pores (Kumar et al., 2019; Siipola et al., 2020). Generally, the mechanism occurs due to intermolecular forces of attraction, such as physisorption (e.g., van der Waals, \u03c0\u2013\u03c0 interactions and hydrogen bonding) and chemisorption (e.g., ionic interactions) (Fu et al., 2019; Lv et al., 2019; Ahmed et al., 2022). The adsorption process has been highlighted by the adsorbent materials' unique properties, such as large specific Brunner Emmett Teller (BET) surface area, high porosity, water, and thermal stabilities, high selectivity for the contaminant of interest, low cost and availability, ease of regeneration, etc. (Xin et al., 2017; Zhan et al., 2020).The flexibility to synthesize a variety of frameworks from numerous clusters of metal ions with organic linkers allows for an unlimited range of crystalline MOFs with microporous or mesoporous structures. Furthermore, diverse functional groups in the metal node and organic linkers serve such as adsorption centers for different organic pollutants (Hasan and Jhung, 2015). In general, the adsorption of POPs on MOF-based nanomaterials (Shan and Tong, 2013; Song and Jhung, 2017) is mainly determined by the materials' chemical properties and physical structure, as the pore structure, specific surface area, and surface functional groups might affect the capacity of adsorption directly. In general, the process of adsorption can occur through physical or chemical interaction. Determination factors are molecular size, solubility, chemical composition, surface charge, reactivity, and hydrophobicity. The interaction between contaminants and adsorbent surfaces, commonly, may occur through electrostatic interaction, complexation/coordination, electrostatic interaction, ion exchange, oxidation. Some of the mechanisms governing the adsorption of MOFs in aqueous solutions are illustrated in \nFig. 15.Parameters influencing adsorption behavior of MOFs, including adsorbent dosage, adsorbate concentration, pH, and contact time (\nFig. 16). It is essential for identifying optimized conditions leading to study these parameters for each type of MOFs in order to maxize adsorption performance and to better understand the mechanisms of interaction between adsorbates and adsorbents. The following section of this review paper will discuss the reports on using MOF-based adsorbents in the removal of persistent organic pollutants (POPs) from water and wastewater.The broadly investigated absorbent materials, so far, mainly involve activated carbon (AC), organo-zeolites, alkylsilane modified silica, polymeric resins, etc. (Liu et al., 2013; Lemi\u0107 et al., 2006; Westerhoff et al., 2005; Groisman et al., 2004; Bagheri and Mohammadi, 2003; Masqu\u00e9 et al., 1998b, 1998a) have recently been explored as promising adsorbents for the removal of organic molecules from aqueous solutions. MOFs diverse chemistry, good chemical and physical stability, and tunable pore structure possibility of introducing functional groups make them very attractive candidates for the adsorption of POPs such as pesticides (Feng et al., 2018; Ghanbari et al., 2020; Li et al., 2018).Most MOF-based water purification researches focus on the adsorption capacity and regeneration. MOFs\u2019 adsorption performance (e.g. selectivity) is governed and influenced by the interactions between MOFs\u2019 active sites (or functional groups) and targeted contaminants.To remove pesticides from aqueous solutions, common MOFs like Chromium-based MIL-53 and MIL-101; Zinc-based ZIF-8 and Cobalt-based ZIF-67; Zirconium-based UiO-67 and UiO-67/GO; and Cu-BTC have shown to be particularly efficient. MIL-53 (Cr) was employed for the first time in a study by Jung et al. (2013) to adsorb 2,4-D weed killer (2,4-dichlorophenoxyacetic acid), a commonly used herbicide in agriculture (Jung et al., 2013). Compared with AC or USY zeolite, Cr-based MIL-53 exhibited a much larger adsorption capacity and substantially faster adsorption (within 1\u2009h). The adsorption of 2,4-D weed killer was highly effective, particularly at low 2,4-D concentrations in solution. The adsorption process was greatly favored by \u03c0\u2009\u2212\u2009\u03c0 stackings and electrostatic interactions between the Cr-based MIL-53 and 2,4-D weed killer. After four cycles of use, the MOFs were still effective. Smedt et al. (2015) also demonstrated that Fe-based MOF-235 had an adsorption advantage over zeolite and AC (De Smedt et al., 2015). The researchers compared the adsorptive performance of Fe-based MOF-235 with that of AC and zeolite in the removal of clopyralid, bentazon, and isoproturon from water. In terms of kinetics and capacity, MOF-235 was the most effective at removing these three pesticides. The MOF-235, on the other hand, had poor reusability due to its low water stability. Mirsoleimani-azizi et al. (2018) studied the mesoporous Cr-based MIL-101 as an adsorbent in a fixed-bed system for the constant removal of diazinon (Mirsoleimani-azizi et al., 2018). At 150\u2009mg/L diazinon concentration, the MIL-101 was reported to have 92.5% diazinon removal from aqueous solutions. In another study for the adsorptive removal of pesticide 14C-ethion, which is considered carcinogenic and toxic, a Cu-based MOF, Cu-BTC, was used and exhibited reliable removal efficiency (Abdelhameed et al., 2018). The adsorption capacity of Cu-BTC at 150\u2009min was calculated to be approximately 122\u2009mg/g for a 14\u2009C-ethion concentration of 75\u2009mg/L, and the MOF demonstrated stability for up to six cycles. The adsorption followed first-order kinetics, confirming the chemisorption process through the coordination of 14\u2009C-ethion to the Cu (II) atom of Cu-BTC MOF. Abdelhamid et al. (2019) also conducted a study comparing the adsorption of two widely-used pesticides (ethion and prothiofos) on Co-based ZIF-67 and Zn-based ZIF-8 (Abdelhameed et al., 2019). The maximum adsorption capacity of Co-based ZIF-67 for ethion and prothiofons was calculated to be 211 and 261\u2009mg/g, respectively, while Zn-based ZIF-8 was 279 and 367\u2009mg/g, respectively. The different calculated capacity of adsorption was reported to be attributable to the weaker coordination of ethion and prothiofos to the Co metal ions compared with that to the metal ions of Zn. Despite this, in the adsorption of ethion and prothiofos onto both MOFs, hydrogen bonding (HB) played a significant role.Zr-based MOFs have been widely explored in removing different pesticides via adsorption due to their high-water stability, high surface area, optimal pore size, and Zr-OH moiety presence. In this sense, to remove methylchlorophenoxypropionic acid (MCPP) from water, a Zr-BDC, Zr-based UiO-66, was studied. The results showed that the UiO-66's adsorption capacity was \u223c7.5 times, and the kinetic constant was \u223c 30 times that of AC (Seo et al., 2015). In another study to remove glyphosate (GP) and glufosinate (GF), a Zr-based UiO-67 - which appeared to have high stability, suitable pore size, and strong affinity - was investigated, and results showed a large adsorption capacity for GF (360\u2009mg/g) and GP (537\u2009mg/g) (Zhu et al., 2015, p. 67). Pankajakshan et al. suggested that, since the metal nodes are the same in both MOFs, it is likely the larger pore diameter of the NU-1000 (Zr), which makes its adsorption more effective than UiO-67 (Zr). They also observed a higher adsorption energy for GP (\u221237.63\u2009kJ/mol) for NU-1000 (Zr) compared to UiO-67 (\u221217.37\u2009kJ/mol). The more negative adsorption energy indicates a stronger interaction of GP with the Zr clusters of the MOFs. This is in line with the shorter Zr\u00b7\u00b7\u00b7O\u2013P interatomic distance during the polar interaction in the case of NU-1000 (4.2\u2009\u00c5) compared to UiO-67 (4.6\u2009\u00c5). (Pankajakshan et al., 2018). In a comparative study by Akpinar and Yazaydin (2018), Zr-based MOFs of UiO-66 and UiO-67 and Zn-based ZIF-8 were applied for the atrazine (ATZ) removal from aqueous solution and compared with commercial AC (F400) (Akpinar and Yazaydin, 2018). UiO-67 (Zr), ZIF-8 (Zn), and F400 were able to remove the ATZ up to 98%; however, UiO-66 appeared ineffective. Due to its larger pores, UiO-67 displayed stronger and interestingly faster adsorption performances (removing \u223c98% ATZ within 2\u2009min, while UiO-66's hydrophilic characteristics rendered it ineffective for the ATZ adsorption even through surface interactions. ZIF-8 was also revealed to be an efficient ATZ adsorbent due to its hydrophobic characteristics, which inhibited considerable amounts of water adsorption, thus favoring the adsorption of ATZ.Another study revealed that only Ze-based NU-1000 out of eight Zr6-based MOFs exhibited a 100% ATZ removal effectiveness within 5\u2009min (Akpinar et al., 2019). This study also suggested that the linker structure might significantly influence the adsorption characteristics of the MOFs, as their performance improved with the number of carboxylic acid groups and aromatic rings in the linker. The presence of a pyrene-based linker in the MOF structure, which provides enough sites for \u03c0\u2013\u03c0 interactions, might explain the significant adsorption of NU-1000 (Zr). Additionally, a negligible efficacy loss was reported after three cycles of regeneration in the adsorbent. Jamali et al. (2019) (Jamali et al., 2019) used Zr-based MOFs UiO-66 and UiO-67 for the adsorptive removal of metrifonate and dichlorvos (OPP) from an aqueous solution in another investigation. The UiO-67 outperformed the UiO-66 in OPP removal, with greater removal rates of 99.8% versus 97.8%, respectively. UiO-67 (Zr) adsorption capacity for the metrifonate and dichlorvos removal was also measured to be 379 and 571\u2009mg/g, respectively.Due to their high porosity and sufficient adsorption sites, pure MOFs are particularly efficient in pesticide removal from aqueous solutions. The findings of the adsorptive removal of pesticides using pristine MOFs are summarized in \nTable 5.Abdelhameed et al. have validated the efficiency of Al-based MIL-53 in eliminating organophosphorus (OPP) pesticide dimethoate from an aqueous solution (2021) (Abdelhameed et al., 2021b). The surface area of MIL-53 was increased as a result of functionalization by Al, resulting in a rise in the number of adsorption sites and a higher absorption capacity.To remove herbicides such as alachlor (ALA), diuron (DUR), gramoxone, and tebuthiuron, a Cr-based MIL-101 was recently modified with thiophene or furan. The resulting Cr-MIL-101 (Cr)-C(1\u22125) samples showed enhanced adsorption efficiency, which was ascribed to HB interactions and \u03c0\u2013\u03c0 stacking (Y. Q. Yang et al., 2019; Y. Yang et al., 2019). In another study for GP adsorption, Feng and Xia (2018) used a modified Cr-based MIL-101 loaded with a urea group or amino and studied the effects of ionic strength and pH on the adsorption process (Feng and Xia, 2018). Due to ESI between the MOFs' surface with positive charges and the GP anions, the urea- and amino-functionalized MOFs demonstrated the highest adsorption capability at pH >\u20096 and pH =\u20093, respectively. The researchers witnessed lower adsorption performance of urea-functionalized MIL-101 (Cr) compared to NH2-MIL-101 (Cr) due to steric hindrance. In a recent study by Wu et al. (2020), UiO-66 (Zr)-NMe3+, a MOF functionalized with cationic sites, was tested for the removal of 2,4-D, exhibiting a significant capacity of adsorption (279\u2009mg/g), which may be attributed to the existence of \u03c0\u2013\u03c0 conjugation and electrostatic interactions (ESI). Furthermore, the modified MOF demonstrated high reusability and an acceptable adsorption rate within 2\u2009h of contact time (Wu et al., 2020).Development and use of MOF-based composites in different applications are gaining momentum amongst researchers of other disciplines. In this section, several classes of MOF-based composites that are being used for POPs removal will be discussed.In a recent study, a Cu-BTC@cellulose acetate composite was prepared by Abdelhameed et al. (2021) and used for the removal of dimethoate (Abdelhameed et al., 2021a). An increase in the cellulose acetate's adsorption performance was observed when adding 40% Cu-BTC. The coordination between the composite's active site and dimethoate molecules, HB, and ESI might explain the adsorption of dimethoate via Cu-BTC@cellulose acetate composite in this study. In another study by the same research team, a Cu-BTC@cotton composite extracted ethion (Abdelhameed et al., 2016). By removing 97% ethion from water, the adsorption capacity was calculated to be 182\u2009mg/g. Langmuir model was found to be the best-fitted adsorption model, and the adsorption process remained almost unchanged for up to five cycles of regeneration. The composite's adsorption performance was explained by the interaction of the sulfur atoms of ethion with the copper atom in the MOF through a coordination bond and the availability of cellulose functional groups for HB. Also, several other pesticides, including 4-chlorophenoxyacetic acid, dicamba, 2,4-D, and 2-(2,4-dichlorophenoxy) propionic acid, were preferentially absorbed by using the composite adsorbent Cotton@UiO-66 (Zr) in a packed column, in a study by Su et al. (2020) (Su et al., 2020).By depositing Zr-based UiO-66 on ionic liquid-modified chitosan (ILCS), Huang et al. (2020) prepared ILCS/U-X, a powder adsorbent, for the removal of organic herbicides from an aqueous solution (Huang et al., 2020). High maximum capacity for the adsorption of 2,4-D (893\u2009mg/g) was reported for the contact time of 1\u2009h, attributable to the presence of O-containing groups in Zr-based UiO-66 and through ESI and HB.To remove GP, Y. Yang et al. (2017); Q. Yang et al. (2017) prepared a highly effective nanocomposite constructed from Zr-based UiO-67 combining graphene oxide (GO) (Q. Y. Yang et al., 2017; Q. Yang et al., 2017). Compared with the other GO-based adsorbents, a higher capacity of adsorption (483\u2009mg/g) was reported for the produced composite, which was attributable to the interaction of the GP molecule with the Zr-OH functionality UiO-67 through the formation of Zr\u2013O\u2013P bonds.\nLiang et al. (2021) used a composite made up of Zr-based MOFs (ZIF-8 or UiO-66\u2013NH2) on a carbon nanotube aerogel (MPCA) to remove herbicides (chipton and alachlor) (Liang et al., 2021). The resultant UiO-66\u2013NH2 @MPCA composite demonstrated significant chipton adsorption capacity (227.3\u2009mg/g) and was reported to be reusable for up to 5 cycles. The exposed active site explained the chipton and alachlor adsorption onto the MOF@MPCA composite. The MPCA's micron-size pores expanded the interaction between pesticides and MOF nanoparticles and enhanced the adsorption of the pesticides. The influence of pH was also investigated, and it was discovered that UiO-66\u2013NH2 @MPCA displayed the highest chipton adsorption capability at pH=\u20094 since chipton exists mainly in anionic forms at pH>\u20093.1. The surface charge of UiO-66-NH2 is positive at pH\uff1c5.1. As a result, the adsorption of chipton could be due to electrostatic interaction. However, despite occurring charge repulsion at pH>\u20095.1, the adsorption capacity was good, revealing that there might also be other interaction forces like \u03c0\u2013\u03c0 stacking and HB interactions. Furthermore, for the nonionic pesticide alachlor, UiO-66\u2013NH2 @MPCA also showed good adsorption capabilities, implying that \u03c0\u2013\u03c0 or HB interactions were the significant contributors to adsorption.Amongst various MOF-based composites, magnetic derivatives have equally been confirmed to enhance the MOFs' adsorptive performances due to their magnetic properties (Kumar Gupta et al., 2017; Wang et al., 2014). For instance, using iron oxide-GO-\u03b2-cyclodextrin (Fe4O3-GO-\u03b2-CD) and a Cu-based MOF (resulting in a rise in the BET surface area and thus boosting its adsorptive performance), Liu et al. (2017) produced a magnetic nanocomposite (M-MOF). Acting as magnetic cores, the M-MOF extracted neonicotinoid insecticides (i.e., imidacloprid, thiamethoxam, nitenpyram, acetamiprid, dinotefuran, thiacloprid, and clothianidin) from water (Liu et al., 2017). Such results were observed due to the M-MOF's large surface area and hydrophobic inner pore. The pesticides' adsorption mechanisms were also described by HB, ESI, hydrophobic interactions (HPI), and \u03c0\u2013\u03c0 stacking interactions attributable to hydrophobic and N-containing groups delocalized \u03c0 electrons from the benzene rings in the molecules of the adsorbate. Through coordination polymerization, Liu et al. (2018) prepared a functional magnetic hybrid composite (M\u2212M\u2212ZIF\u22128) by depositing particles of Zn-based ZIF-8 on magnetic multiwalled carbon nanotubes (Liu et al., 2018, p. 8). The provided composite was found to be an effective adsorbent for the removal of eight different OPPs (i.e., diazinon, triazophos, phosalone, methidathion, profenofos, ethoprop, isazofos, and sulfotep) from water and soil. Additionally, the Freundlich bimolecular adsorption model was reported to be best fitted, and the OPPs' adsorption proceeded by electron exchange between the adsorbents' vacant active sites and molecules of OPPs. For the detection and adsorption of glyphosate, Yang et al. (2018) applied a layer-by-layer assembly method to prepare magnetic hybrid materials Fe3O4 @SiO2 @UiO-67 (Zr) (Yang et al., 2018). The authors reported a high adsorption capacity (257\u2009mg/g) and good regeneration functionality, attributable to the Zr\u2013OH moieties' effective interaction with the phosphate group and Fe3O4 magnetic core. A double-layer magnetic MOF (M-ZIF-8 (Zn)@ZIF-67 (Zr)) prepared by B. Li et al. (2020); Z. Li et al. (2020); T. Li et al. (2020) exhibited excellent capability in removing fipronil and its derivatives from aqueous solution (T. B. Li et al., 2020; Z. Li et al., 2020; T. Li et al., 2020). The established large pores in the composite's bilayer structure allowed for 70.9\u201399.7% fipronil removal from water and cucumber.Providing sufficient adsorption sites, MOF composites and functionalized MOFs have proved to be highly efficient in eliminating pesticides from water. Notably, for removing anionic/cationic pesticides, MOF fabricated with anionic/cationic (respectively) active sites can be favorable because of electrostatic interactions. Composite MOFs have also been shown to efficiently remove nonionic pesticides due to active sites for coordination, \u03c0\u2013\u03c0 stacking, and HB interactions. MOFs incorporating magnetic materials have also shown excellent reusability since used adsorbents may be easily removed from an aqueous solution using a magnetic field. The application of magnetic strategy to the field of MOFs is justified for a number of reasons. On the one hand, the potential for incorporating porosity into these magnetic coordination polymers presents an appealing way to create multifunctional materials in which the magnetism can be tuned by the presence of molecules in the pores. The chemical pressure created by the guest molecules, the intermolecular interactions between the guests and the framework, or changes in the MOF's electronic properties can all be attributed to these systems as potential explanations for the magnetic behavior. Due to their magnetic response, these characteristics might be useful for detecting the molecular species trapped in the pores. On the other hand, the presence of magnetic centers within a crystalline MOF's nodes or pores makes it possible to organize these magnetic centers into nanostructures while maintaining their spatial segregation. As quantum technologies require a controlled disposition of magnetic moieties in the space, such a feature might be of potential interest (Espallargas and Coronado, 2018). \nTable 6 summarizes the results of the adsorptive removal of pesticides from aqueous media using MOF-based composites and functionalized MOFs.Due to their nanoporous structure, high surface area, and unique physical/chemical properties; MOF-derived carbonaceous materials have received enormous attention in a variety of applications, including catalysis, adsorption, and energy conversion and storage (Borchardt et al., 2017; Islamoglu et al., 2020; Duan et al., 2020; Mondol and Jhung, 2021).Prepared by carbonization of Fe (III)-modified Zn-based MOF-5, a MOF-derived magnetic porous carbon-based sorbent (MPC) was employed to remove ATZ from an aqueous solution (Chen et al., 2017). The enhanced adsorption capacity for ATZ removal was reported, attributed mainly to adequate hydrogen bonding between MPC and ATZ with the best-fitted isotherm model of Dubinin-Ashtakhov for this investigation. The synthesis of a magnetic nanoporous carbon (MNPC) via carbonization of Co-based ZIF-67 MOF was performed to extract multiple neonicotinoid insecticides (i.e., acetamiprid, thiacloprid, thiamethoxam, and imidacloprid) from an aqueous solution (Hao et al., 2014). Exhibiting 15 cycles of regeneration, the Co-MNPC confirmed an excellent adsorption efficacy for removing the compounds. Bhadra et al. (2020) synthesized a Zn-based MOF-74-derived porous carbon (CDM-74) to investigate its removal performance of DEET insecticide from an aqueous solution (Bhadra et al., 2020). Due to the high acid site density and mesopore volume, the CDM-74 demonstrated a high adsorption capacity (340\u2009mg/g). Additionally, the adsorbent performance remained unchanged for up to 4 cycles of DEET removal, confirming the derived MOFs' efficacy in removing pesticides from water. Through the carbonization of a multifunctional \u03b2-cyclodextrin MOF, another carbonaceous material, \u03b2-CD MOF-NPC, was prepared and investigated to remove amide herbicides (i.e., Pretilachlor, Acetochlor, Alachlor, Metolachlor) (C. N. Liu et al., 2019; W. Liu et al., 2019; C. Liu et al., 2019). The results exhibited that the resultant microporous structure with a high potassium content and large surface area effectively removes amide herbicides via ESI, HB, and \u03c0\u2013\u03c0 interactions.The sufficient adsorption sites and high porosity of MOF-derived carbonaceous materials have been reported as highly effective factors in removing pesticides from water. \nTable 7 summarizes the research findings on using carbonaceous materials produced from MOFs or MOF composites in the adsorptive removal of pesticides.Acting as intermediates for industrial production, phenols (a class of organic compounds) are broadly applied by chemical and allied industries in manufacturing valuable products, such as petrochemicals, plastics, pesticides, herbicides, dyes, cosmetics, pharmaceuticals, epoxies, detergents, among others. Non-feasible and non-economical conventional treatment methods that failed to address the persistent problems and limited applicability of photocatalytic degradation due to the semi-conductive properties of the materials have turned the attention towards the adsorptive removal of phenolic compounds as an effective removal technique.To date, there have been several reports on the adsorptive removal of phenolic compounds using MOFs and MOF-based composites. Park et al. (2013) extracted bisphenol A (BPA) from water employing Cr-based MIL-53, which outperformed AC and ultra-stable Y zeolite in terms of the maximum adsorption capacity (Park et al., 2013). The \u03c0\u2013\u03c0 interactions and HB between the hydroxyl groups of MIL-53 (Cr) and BPA were identified as the particular interactions that favored adsorption in MOFs. Qin et al. (2014) also successfully used MIL-100 (Fe) and MIL-101 (Cr) to remove BPA from an aqueous solution (Qin et al., 2015). MIL-101 (Cr) absorbed BPA more significantly than MIL-100 (Fe) and faster than AC. These findings were ascribed to MIL-101(Cr)'s favorable textural characteristics, including a higher surface area and bigger pore apertures. Furthermore, the interactions between BPA and MIL-101 (Cr) have been proposed to be \u03c0\u2013\u03c0 interactions and HB. In another work, MIL-100 (Cr), MIL-100 (Fe), NH2-MIL-101 (Al), and an AC were used to remove phenol and p-nitrophenol (PNP) from aqueous solutions (B. L. Liu et al., 2014; B. Liu et al., 2014). In terms of adsorption of phenol, the AC samples outperformed the MOF materials. The poor performance was attributed to MIL-100 (Cr) and MIL-100 (Fe) having high binding energies with water. In terms of PNP adsorption, the performance order was as follows: NH2-MIL-101 (Al) >\u2009AC >\u2009MIL-100 (Fe) \u223c MIL-100 (Cr). According to the authors, the impact of metal sites in MOFs was insufficient to explain the PNP and phenol adsorption from water. Instead, HB between the PNP nitro groups and the MOF amine groups was ascribed to the enhanced adsorption of PNP by NH2-MIL-101 (Al). To remove BPA from the aqueous solution, two Al-based MOFs, MIL-53 (Al) and MIL-53 (Al)-F127, were synthesized, and the results showed a fast BPA removal (90\u2009min for MIL-53 (Al) and 30\u2009min for MIL-53 (Al)-F127) (Zhou et al., 2013). The maximum removal on MIL-53 (Al) and MIL-53 (Al)-F127 was measured at 329.2, and 472.7\u2009mg/g, respectively, and the BPA sorption kinetics data followed the pseudo-second-order model. \u03c0\u2013\u03c0 Bonds and HB explained the BPA sorption. The well-known Co-based MOF, HKUST-1, was investigated for the removal of p-nitrophenol (PNP) and demonstrated a high adsorption capacity of 400\u2009mg/g, with maximum removal accomplished within 40\u2009min (Andrew Lin and Hsieh, 2015). The HKUST-1 high adsorption capacity for the PNP was attributable to the MOF's high interactions with the NO2 compounds of the PNP via HB. In another study, Cu- and Zr-based MOFs, MOF-199, and ZIF-8 were used to remove phenol and PNP (Giraldo et al., 2017). The adsorption capacity for phenol and PNP on MOF-199 was higher (Phenol: 79.55% and PNP: 89.3%), while ZIF-8 was measured to be 65.5% for phenol and 77.0% for PNP. Adsorption of phenols followed the Langmuir isotherm model, and kinetics was fit to pseudo-second-order. A hexagonal MOF of NH2-MIL-88B was prepared to remove 2,4,6-trinitrophenol (TNP), and the maximum capacity of adsorption based on the Langmuir isotherm was reported to be 163.66\u2009mg/g (Guo et al., 2018). The NH2-MIL-88B adsorption mechanism for TNP was attributed to HB interaction and complexation between unsaturated Fe (III) on the surface of NH2-MIL-88B and -OH in TNP. The crystalline and water-stable Zr-based MOF, NH2-UiO-66, was also examined for the adsorptions of 2,4-dinitrophenol, 2,4,6-trinitrophenol, 2,4-dinitrotoluene, and 2,4,6-trinitrotoluene in an aqueous solution (Xu et al., 2017). Higher equilibrium adsorption capacities were achieved with the formation of HB between the atoms of NH2-UiO-66 (Zr) and the pollutants. In a study for removing bisphenol A, Luo et al. (2019) observed an enhanced removal performance by employing a sodium alginate-chitosan-based aluminum MOF composite, Al-MOF/SA (Luo et al., 2019). The capacity of adsorption was calculated to be 136\u2009mg/g. The experimental findings demonstrated greater adsorptive performance when compared to the counterparts of Al-MOF/SA. The most important processes involved in adsorption mechanisms were reported to be cation-interaction, HB, and \u03c0\u2013\u03c0 stacking. After five consecutive batch cycles, as-synthesized beads were recycled and regenerated with over 95% adsorption efficiency. Abazari and Mahjoub (2018) prepared a Zn (II)-based MOF, [Zn(TDC)(4-BPMH)]n\u00b7n(H2O), and investigated its performance for the removal of 2, 4-dichloropheno from wastewater (Abazari and Mahjoub, 2018). With a 60-ppm concentration, the 2, 4-dichloropheno removal efficiency (after 180\u2009min) was observed up to 94.5%. The removal of 2, 4-dichloropheno over this MOF follows the first-order reaction kinetics.The SiO2 @MIL-68 (Al) composites were prepared, and their adsorption efficiency was tested for the removal of aniline (Han et al., 2016). The results demonstrated a high adsorption capacity of 532\u2009mg/g of the composite towards aniline. The aniline adsorption on SiO2 @MIL-68 (Al) obeyed the Langmuir model. The adsorption equilibrium was reached in only 40\u2009s with good reusability up to 5 cycles. HB interaction between \u03bc\n2-O in adsorbent and \u2212NH2 in aniline and the \u03c0\u2013\u03c0 interaction between the framework and the benzene rings of aniline ascribed to the process of adsorption. A carbon nanotube (CNT)@MIL-68 (Al) composite was synthesized and compared with pure MIL-68 (Al) to investigate their performances in phenol removal from an aqueous solution (Han et al., 2015). CNT@MIL-68 (Al) showed the highest capacity of adsorption (257\u2009mg/g), 119% higher than that of pristine MIL-68 (Al) (117.6\u2009mg/g), possibly attributable to the expansion of small micropores introduced by the incorporation of CNTs. The high adsorption capacity is caused by the HB between \u2013OH in phenol and \u2013COO\u2212 in adsorbent and the \u03c0\u2013\u03c0 interactions between the benzene rings of phenol and the composites. Another comparison between pristine MOF and its composite was made in a study to remove PNP using MIL-68 (Al) and MIL-68 (Al)/GO (Wu et al., 2016). The adsorption capacity of PNP calculated from the Langmuir model was 332\u2009mg/g for MIL\u201368 (Al)/GO, which was higher than the MIL-68 (Al) (271\u2009mg/g) due to the higher surface area of the composite. The adsorption process fitted well with the pseudo-second-order model. This performance is attributed to the HB and \u03c0\u2013\u03c0 dispersion interaction between the composite and PNP. Cu-BDC MOFs decorated over GO and CNT hybrid nanocomposites, namely Cu-BDC@GO and Cu-BDC@CNT, were synthesized by Ahsan et al. (2019) for BPA removal from water (Ahsan et al., 2019). The hybrid nanomaterials demonstrated a high adsorption capacity of 182 and 164\u2009mg/g toward the BPA removal for BDC@GO and Cu-BDC@CNT, respectively, which was much higher than that of Cu-BDC MOF itself (60.2\u2009mg/g). The results confirmed that the Freundlich model describes the experimental data best, and the kinetics data were best fitted to the pseudo-second-order kinetic model. The \u03c0\u2013\u03c0 interactions between the nanomaterials and BPA played a crucial role in the BPA adsorption process. A biocomposite of laccase@HKUST-1 was also prepared by Zhang et al. (2020) (Zhang et al., 2020). The adsorption performance of the material for BPA was tested, and the BPA removal after four h by laccase@HKUST-1 was reposted to be 74.2%.Combining the unique surface activity of MOFs with the excellent carry nature of the porous polymers can result in unexpected adsorption performances. A MOFs/polymer composite membrane, MIL-68 (Al)/PVDF, was fabricated and tested to remove PNP from an aqueous solution (Tan et al., 2019). The maximum adsorption capacity of PNP on MIL-6/PVDF reported 183.49\u2009\u03bcg/cm2 (94%), and the material was reusable for up to 6 cycles. The Langmuir isotherm model characterized the adsorption process. The coordination bonding formed between Al (III) of MIL-68 (Al) and NO2 of PNP was responsible for the PNP removal.Pharmaceuticals and personal care (PPCPs) items are other significant areas of emerging pollutants. They comprise various chemicals such as medications, cosmetics, and veterinary treatments; all considered necessary life elements. Unfortunately, conventional wastewater treatment procedures such as coagulation, sedimentation, and flocculation, frequently followed by chlorination, are not optimally suited to remove these potentially hazardous PPCPs efficiently. Many conventional treatment approaches (e.g., adsorption, photocatalytic degradation, separation, thermal decomposition, hydroxylation, biodegradation, coagulation, flocculation, sedimentation, ozonation, and advanced oxidation processes) account for partial contribution to the removal of PPCPs (Westerhoff, 2003; Li et al., 2015; Wang and Wang, 2016; Jin et al., 2020; Reyes et al., 2021). Among the methods described above, the adsorptive elimination of PPCPs is one of the most effective approaches.\nZhao et al. (2019) used UiO-66 MOFs for the removal of three broadly applied PPCPs: 2,4-Dichlorophenoxyacetic acid (2,4-D), diclofenac sodium (DCF), an anti-inflammatory drug, and clofibric acid (CLA), an extensively employed herbicide (Zhao et al., 2019). The results showed that UiO-66-NH2 had the highest capacity for absorption of these PPCPs, which was 3\u20134\u2009times larger than the UiO-66-COOH counterpart, which had the lowest absorption capacity. The structure-function analysis revealed that HB, ESI, and interfaces between MOFs and PPCP molecules significantly impacted the adsorption process. Despite UiO-66's high PPCPs removal capacity, its usage has been limited by difficult separation and recovery from aqueous environments.In a study on the adsorptive removal of tetracycline, a method was proposed to make hierarchical pore-structured UiO-66 (Yuxi Jie Zhang et al., 2018; Ying Zhang et al., 2018; Yuxi Zhang et al., 2018). These mesostructured MOFs demonstrated a significant boost (up to 430%) in adsorption capacities compared with microporous UiO-66 (i.e., 667 and 126\u2009mg/g, respectively). The pseudo-second-order kinetic model and Langmuir isotherm model could well describe the adsorption of tetracycline on H-UiO-66 MOFs. Authors attributed the high tetracycline removal capacity of H-UiO-66 MOFs to the \u03c0\u2013\u03c0 stacking interaction between the aromatic ring of tetracycline and the benzene ring of H-UiO-66 (\nFig. 17). The enhanced adsorption capability of UiO-66 with hierarchical pores demonstrates that adsorbate and adsorbent size matching is critical.A new mechanism for the adsorption of p-arsanilic acid was proposed in another study using amine-modified UiO-67 (C. H. Tian et al., 2018; C. Tian et al., 2018). The synthesized MOFs were comprised of UiO-67-NH2 (1) (the ligand has one amino group) and UiO-67-NH2 (2) (the ligand has two amino groups), and pristine UiO-67. The order of maximum adsorption capacity was UiO-67\u2009>\u2009UiO-67-NH2 (1) >\u2009UiO-67-NH2 (2); however, the order was reversed following normalization by surface area. Three adsorption mechanisms were discovered using DFT calculations: \u03c0\u2013\u03c0 stacking, As-O-Zr coordination, and NH\u2219\u2219\u2219O HB, which is a novel characteristic in the adsorption of p-arsanilic acid. Both UiO-67\u2013NH2 and UiO-67 maintained the adsorption performances up to 4 cycles of adsorption.\nPeng et al. (2019) also used MIL-101 (Cr) to remove sulfonamide-containing antibiotics such as sulfamonomethoxine, sulfadimethoxine, and sulfachlorpyridazine (Peng et al., 2019). The authors reported high removal efficiency of the pharmaceuticals over the MIL-101 (Cr) (196.08, 588.24, and 142.86\u2009mg/g, respectively). The adsorption proceeded according to the pseudo-second-order kinetic model. The key mechanisms for three sulfonamides adsorption on MIL-101 were found to be ESI and HB.\nChai et al. (2019) tested the application of MIL-101 (Cr)\u2013SO3H on the efficient adsorption of moxifloxacin and gemifloxacin (Chai et al., 2019). MIL-101 (Cr)\u2013SO3H showed maximal adsorption capacities of 493 and 535\u2009mg/g for the respective fluoroquinolones. These results are significantly greater than those for MIL-101 (Cr) adsorption of gemifloxacin and moxifloxacin. The MOFs were able to be regenerated for up to 4 cycles. The adsorption behavior of MOF was shown to follow the Langmuir isotherm and pseudo-second-order models.\nB. Li et al. (2020); T. Li et al. (2020); Z. Li et al. (2020) also used the same MOF material (MIL-101 (Cr)\u2013SO3H) to remove ciprofloxacin, a frequently used antibiotic from the fluoroquinolone family (Z. B. Li et al., 2020; T. Li et al., 2020; Z. Li et al., 2020). The adsorption kinetics were best fitted to the pseudo-second-order model. Furthermore, the equilibrium adsorption data followed the Langmuir model. The maximum capacity of adsorption (564.9\u2009mg/g) was found to be much higher than that of pure MIL-101 (Cr) (113.2\u2009mg/g).Same as other fluoroquinolones, ciprofloxacin adsorption was facilitated by the ESI of SO3\u2212. MIL-101 (Cr) was also used by Shadmehr et al. (2019) to eliminate diazinon from aqueous media (Shadmehr et al., 2019). This chemical is a thiophosphoric acid ester insecticide designed to replace DDT; however, it is harmful to humans and animals. The maximum adsorption capacity was 75.04\u2009mg/g (96.1%), higher than activated bentonite and activated carbon. The adsorbent was reusable for up to 5 successive cycles for adsorption of diazinon. The Langmuir model and pseudo-second-order model were found to be more consistent with the experimental data.Gao et al. (2019) prepared MIL-53 (Al), MIL-53 (Cr), and MIL-53 (Fe) with narrow and large pore sizes (Gao et al., 2019a). The large pore conformation of MIL-53 (Al) and MIL-53 (Cr) exhibit a high removal efficiency of sulfamethoxazole (451 and 469\u2009mg/g, respectively), while the narrow pore-sized MIL-53 (Fe) was found to be inefficient for the adsorptive removal of sulfamethoxazole's large molecules.In a study to capture nitroimidazole antibiotics, MIL-53 (Al) demonstrated high performance in the adsorption of dimetridazole (DMZ) (Peng et al., 2018). The adsorption capacity was 467.3\u2009mg/g, with a fast adsorption rate of 10\u2009min. The strong van der Waals interactions and the HB between \u2013NO2 of DMZ and \u03bc\n2-OH of the MOF were suggested to be the interaction mechanism between DMZ and MIL-53 (Al). The data were best fitted by the Langmuir isotherm model and pseudo-second-order model. After four cycles, MIL-53 (Al) retained over 98% of its initial DMZ adsorption capacity.An amino-functionalized In-based MOF, MIL-68 (In)\u2013NH2, was also prepared to capture p-arsanilic acid in an aqueous media and compared with pristine MIL-68 (In) in terms of the capacity of adsorption (Lv et al., 2018). Accordingly, the adsorbents exhibited a maximum capacity of 402\u2009mg/g and 340\u2009mg/g, respectively. The adsorption obeyed the pseudo-second-order model and was better characterized by the Langmuir model than the Freundlich model. The removal performance of MIL-68 (In)\u2013NH2 was attributed to \u03c0\u2013\u03c0 interaction, ESI, and HB between the functional groups of p-arsanilic acid and organic linkers of the adsorbents.\nS. Li et al. (2019); N. Li et al. (2019) used ZIF-8 to remove two common antibiotics, oxytetracycline hydrochloride (OTC) and tetracycline (TC), simultaneously (N. S. Li et al., 2019; N. Li et al., 2019). Resulting of the synergistic interaction of TC and OTC, the removal efficiency of a mixture of the two antibiotics was higher than that of single pollutants. The maximum capacities for adsorption of OTC and TC were reported to be 312 (89%) and 303\u2009mg/g (95%), respectively. The adsorption of both pollutants followed pseudo-second-order kinetics and was better explained by the Langmuir adsorption model. \u03c0\u2013\u03c0 interaction of multiple phenolic hydroxyl groups and benzene ring structures of TC and OTC with the imidazolate rings ZIF-8 attributed to the high adsorption capacities.ZIF-67 was also used by Dehghan et al. (2019) for the adsorption of tetracycline from aqueous solutions (Dehghan et al., 2019). ZIF-67 synthesized by cobalt acetate (ZIF-67-OAC) showed the highest capacity of adsorption for tetracycline (447\u2009mg/g; with a removal efficiency of 93.7%) among other synthesized MOFs using different cobalt sources (e.g., sulfate, chloride, nitrate, and acetate). At the end of cycle 4, the tetracycline removal performance of the regenerated adsorbent was maintained relatively unchanged. Freundlich model and pseudo-second-order kinetic model could better explain the adsorption by ZIF-67-OAC. The adsorption could be attributed to Van der Waals forces attraction and HB between polar tetracycline groups and groups on the surface of ZIF-67-OAC. Large organic contaminants can be challenging to remove from the aqueous media via microporous MOFs.Antibiotic chloramphenicol is commonly prescribed for treating a variety of bacterial illnesses. The widespread usage of such a beneficial medication, on the other hand, can result in significant water contamination, causing aplastic anemia and bone marrow depression in humans. To remove chloramphenicol, PCN-222 (Zr porphyrinic MOF) was successfully used (Zhao et al., 2018). Compared to various other porous materials, including carbon nanotubes, mesoporous sol-gels, and ordered mesoporous carbon, PCN-222 showed quicker sorption and a significantly higher adsorption capacity of 370\u2009mg/g. Two interactions between the adsorbate and the adsorbent were reported, in which the structural characteristics of PCN-222 play a key role in the high chloramphenicol removal efficiency; ESI, which was facilitated by the charge differences between chloramphenicol (negatively charged) and PCN-222 (positively charged), and HB via hydroxyl groups provided by the Zr cluster of PCN-222 and different organic groups in the molecules of chloramphenicol (e.g., \u2013OH, \u2013NO2, and \u2013CO). When compared to varying MOFs like MIL-101 (Cr), MIL-68 (Al), and MIL-53 (Al), PCN-222's large surface area and pores resulted in a high diffusion rate and chloramphenicol removal performance. As coexisting cations and anions, various common inorganic salts (e.g., KCl, NaCl, Na2SO4, and NaNO3) were used to test PCN-222's adsorption ability in actual water. PCN-222 showed an adsorption capability of 250\u2009mg/g in solution with coexisting salts.Gao et al. (2019), in a study for degradation and adsorption of diclofenac, prepared PCN-134 (a mixed-ligand Zr-MOF), comprised of TCPP, (tetrakis(4-carboxyphenyl)porphyrin), as pillar ligand and BTB (benzene-1,3,5-tribenzoate) ligands in the 2D layer (Gao et al., 2019b). The adsorptive removal studies showed a maximum adsorption capacity of 604\u2009mg/g for diclofenac via PCN-134, considerably more significant than that of MIL-101 (Cr), activated carbon, and anion exchange resin Amberlite IRA 67.\nSun et al. (2019) synthesized composite microspheres of calcium alginate/MOFs and tested its adsorption performance toward levofloxacin, a widely prescribed antibiotic, from water (Sun et al., 2019). The observed adsorption capacity (86.43\u2009mg/g) was found to be considerably higher than that of calcium alginate or individual UiO-66. The reusability testing, following five cycles of levofloxacin adsorption, revealed more than 70% levofloxacin adsorption.In an investigation by Jun et al. (2019), a new MOF named Basolite A100 outperformed commercial activated carbon in removing ibuprofen and CBZ fitting pseudo-second-order kinetics (Jun et al., 2019). The potential adsorption processes of MOFs were ascribed mostly to hydrophobic interactions, with contributions from HB and ESI. Furthermore, the recycling and regeneration properties of the MOF were examined for four continuous cycles to confirm its feasibility in wastewater remediation.\nC. Liu et al. (2019); N. Liu et al. (2019); W. Liu et al. (2019) used a three-dimensional porous and water-stable Cu (II)-based MOF to investigate the adsorption ability of three common personal care items and pharmaceutically active medicines, including chlorpromazine hydrochloride (CLF), amodiaquine dihydrochloride (ADQ), and diclofenac sodium (DCF) (W. C. Liu et al., 2019; N. Liu et al., 2019; W. Liu et al., 2019). As reported by the authors, the Cu (II)-based MOF effectively eliminated 650\u2009mg/g DCF from the water sample. In comparison, only 67 and 72\u2009mg/g of adsorption was reported for CLF and ADQ, respectively. The small-sized DCF molecules enable it to enter the Cu (BTTA) pores, where it interacts weakly with the open metal sites of Cu2 +\u2009and the N-atoms of the triazole ring. The adsorption of diclofenac sodium over the MOF followed the Freundlich model and pseudo-first-order kinetics.Synthetic dye pollution is becoming a rising environmental concern, as many dyes are hazardous to humans and aquatic life. More than 10,000 tons of dyes are used by textile companies globally each year, with around 5000 tons of these dyes and 3600 tons of various wastes containing high concentrations of dyes being discharged into water streams (Murugesan et al., 2021; Renita et al., 2021; Yagub et al., 2012).Many dyes present in industrial wastewater are poisonous, carcinogenic, and teratogenic (Liang et al., 2018).Various processes have been used to remove dyes from industrial effluents, including adsorption, coagulation, advanced oxidation, and membrane separation (Wong et al., 2020).Along with its ease of use and great efficiency, adsorption is regarded as one of the most influential modern wastewater treatments for the removal of toxic organic contaminants such as dye in effluents (Abhinaya et al., 2021; Akpinar and Yazaydin, 2017, p. 67; Sharma et al., 2021; Ullah et al., 2021). (\nFig. 18).MOFs have been used as adsorbents for dye contaminants in liquid-phase extractions (Ahmed et al., 2017; Khan et al., 2018; Uddin et al., 2021; Yu et al., 2018; Y. H. Zhang et al., 2019; S. Zhang et al., 2019; X. Zhang et al., 2019; Y. Zhang et al., 2019), solid-phase extractions (Liu et al., 2021), solid-phase micro-extractions (Gonz\u00e1lez-Hern\u00e1ndez et al., 2021), and high-performance liquid chromatography (Aqel et al., 2021). Due to a large number of recent studies on the use of metal-organic frameworks for dye adsorption, this review refrains of deep-diving into the MOF-based dye adsorption processes. \nTable 8, however, summarizes some of the works that have been reported on using MOFs for the removal of different.This review has explored recent advances and discussed the potential of using MOFs and MOF-based composites for the removal of POPs from contaminated water and wastewater streams. From the literature discussed in this review, it can be concluded that MOFs, due to the tunability of their structural and electrical properties, are considered promising materials for the removal of POPs. They can be used not only as effective adsorbents but also as very efficient catalysts for removing and degrading POPs in contaminated water. The pore size of MOF can be tuned to accommodate targeted contaminant molecules. MOFs can also be functionalized to improve electrostatic, acid-base, \u03c0\u2013\u03c0 interactions, or hydrogen bonding. They can be coupled with metals, inorganic semiconductors, or organic linkers to increase their photoexcitation rate and decrease electron-hole recombination, resulting in composites with high photocatalytic efficiency.Photocatalysts hold the promise of low-cost, environmentally friendly pollution control that required little to no energy as they do not release harmful residues or change organic contaminants from one phase to another. However, photocatalysts are still limited by their inherently low chemical activity, which is not always offset by a large number of active sites. This limitation has two significant consequences: photocatalysts may produce undesirable byproducts due to an incomplete reaction or during degradation processes, in addition to having a limited ability to remove pollution.Although studies have documented that photocatalytic oxidation does not always completely oxidize organics to CO2 and H2O, only a few studies have looked at the byproducts. According to (Selishchev et al., 2017) research, CO was produced during the photocatalytic oxidation of VOCs in the presence of CO2 and H2O. Mo et al. (2009) discovered that humidity and VOC content affected the production and concentration of byproducts in a study of toluene degradation. S-methyl-methanethiosulfonate and methane thiol were synthesized from sulfur-containing precursors by Yao and Feilberg (2015). Wang et al. (2011) investigated the decomposition of DMS on TiO2, identifying the products using chromatography without mentioning CH2O concentrations. Formaldehyde synthesis from S-doped TiO2 photocatalysts was observed, but no yield was reported. It must be noted that some byproducts of photocatalytic degradation of POPs can be harmful chemicals. Therefore, the toxicity of the (by)products should constantly be tested, if possible. Given the significance of the subject matter, it is obvious that more future research should study byproducts produced by the catalytic photodegradation of POPs (Yu et al., 2021).MOFs for photocatalysis:Based on the literature reviewed in this study, MOFs show great promise for photocatalysis. In general:\n\ni.\nMOFs are great but sometimes limited by low activity despite having a large surface area/a large number of sites.\n\n\nii.\nAbsorption can be tuned by ligand; One of the benefits is the use of ligands to render the MOFs active under visible light, however it seems that most literature mainly focused on UV irradiation. Moreover, the performance of MOFs still subpar than most inorganic semiconductors because of their low photo responsivity, low efficiency of visible light utilization, and fast recombination of photo-generated electron-hole pairs.\n\n\niii.\nLow charge recombination and low chemical activity can be addressed by adding MNPs as guest compounds and by designing MOF composites\n\n\nMOFs are great but sometimes limited by low activity despite having a large surface area/a large number of sites.Absorption can be tuned by ligand; One of the benefits is the use of ligands to render the MOFs active under visible light, however it seems that most literature mainly focused on UV irradiation. Moreover, the performance of MOFs still subpar than most inorganic semiconductors because of their low photo responsivity, low efficiency of visible light utilization, and fast recombination of photo-generated electron-hole pairs.Low charge recombination and low chemical activity can be addressed by adding MNPs as guest compounds and by designing MOF compositesTherefore, MOF-based hybrid (composite) materials have gained more attention in recent years. Efforts have been made to increase MOF photocatalytic activity and solar light-harvesting capacity through different methods, including combing MOFs with other semiconductors (heterojunction photocatalysts). Moreover, MOFs unrivaled adaptability provides a variety of ways to alter and regulate pure MOFs for improved photocatalytic activity in visible light. Integration of MOFs with light-harvesting semiconductor materials such as CdS and Fe3O4, gCN and In2S3 to form hybrid materials has been recognized as an efficient way for the production of effective photocatalyst with good light absorption and photocatalytic efficiency. Compared with MOFs themselves, the hybrids display significant benefits because of their synergistic effect (Ding et al., 2017; Huo et al., 2019; H. Zhang et al., 2019; S. Zhang et al., 2019; Y. Zhang et al., 2019; X. Zhang et al., 2019).MOFs for adsorption:According to the literature reviewed in this paper, MOF-based adsorptive processes for the removal of POPs are very promising, particularly because our knowledge of making water-resistant MOFs with high adsorption capacity is advancing quickly. So far, the MOF-based adsorption technique is very promising in treating small to medium volumes of water with low contamination levels. That said, however, given the cost associated with manufacturing such MOFS is still high, there is still a long path toward commercializing such technologies. While working toward the development of inexpensive yet efficient MOFs should be the focus of future research in this field, researchers should work on the development of facile regeneration techniques to recover and reuse saturated MOF-based adsorbents. It is noteworthy that poor selectivity of some MOF-based adsorbents for removal of some organic contaminants is a challenge that needs additional research. The suitability of the applications in a batch reactor or fixed-bed column reactor, reusability, lifespan of materials, the cost of regeneration of spent materials, the ease of post-treatment, and the environmental impact of exhausted/used adsorbents/catalysts should be considered in the design of larger-scale treatment processes (Li et al., 2022).Despite the fact that scientists have had significant advancements in the manufacturing and modification of MOF-based materials as adsorbents and catalysis, however, synthesis of many highly efficient MOFs are still a very energy-intensive and expensive process. Therefore, more research is needed on optimizing the manufacturing processes of MOF-based compounds using inexpensive materials and methods. In addition, strong hydrodynamics, water turbulence, and water flow scouring, especially for unshaped powder-like MOFs, could cause leakage of nano-sized MOFs currently in use. Similar to the well-known high toxicity of nanoparticles (such as silver nanoparticles), nano-sized MOFs may be harmful to humans and other living things (Li et al., 2022).This report also discussed MOFs as adsorbents that allow easy separation and constant recycling in water remediation. Although the adsorption technique has numerous benefits over photocatalysis, theoretically, photodegradation is a better approach as it results in the entire pollutant elimination and the need for subsequent treatment.Challenge for both applications:\n\ni.\nKinetic restrictions by reactant diffusion can be managed by introducing mesoporous, but ligand chemistry is also important, as hydrophobic/hydrophilic ligands will affect access of POPs to the active sites. Strategies such as selective ligand removal (Naghdi et al., 2022), (e.g. via selective oxidation, thermal degradation or dissolution) to design novel hierarchical microporous-mesoporous MOFs that facilitate reactant diffusion as well as induce uncoordinated centers as potential catalytic sites and new adsorbent sites for water purification can provide a new tool for the purposeful engineering of hierarchical MOFs with advanced applicability in liquid media.\n\n\nii.\nLow stability can be solved by mixed-ligand approach (refer to Section 2.1.1).\n\n\nKinetic restrictions by reactant diffusion can be managed by introducing mesoporous, but ligand chemistry is also important, as hydrophobic/hydrophilic ligands will affect access of POPs to the active sites. Strategies such as selective ligand removal (Naghdi et al., 2022), (e.g. via selective oxidation, thermal degradation or dissolution) to design novel hierarchical microporous-mesoporous MOFs that facilitate reactant diffusion as well as induce uncoordinated centers as potential catalytic sites and new adsorbent sites for water purification can provide a new tool for the purposeful engineering of hierarchical MOFs with advanced applicability in liquid media.Low stability can be solved by mixed-ligand approach (refer to Section 2.1.1).Despite the MOFs' promising prospects, various concerns must be addressed, especially in large-scale and real-life practical situations such as developing strategies to use thermodynamically water-stable MOFs with a stronger coordination bond or kinetically water-stable MOFs with a better hydrophobic coat, MOFs with redox-active metals and/or organic functionalized ligands, MOFs for adsorptive removal of gaseous pollutants, understanding of the mechanism of adsorptive reduction of POPs, the influence of pH, temperature, and solute ions, the MOFs effectiveness against a wide range of organic pollutants found in real-life applications, focus on low-bandgap MOFs or employing different techniques (e.g., doping, nanocomposite formation) to improve the bandgap of the MOF to make it more appropriate for absorbing visible light. Although there are several publications on water-resistant MOFs, such as the UiO-66, MIL-125, MIL-101, and ZIF series, the stability under extreme circumstances (strong acidic and alkaline pH) is still lacking.Lastly, another challenge comes from the vast number of available MOFs. Which makes it difficult to wisely choose the most promising ones for the respective POP and removal process.To conclude, innovative material design, quality control, and environmental considerations are essential with an ever-expanding potential in MOFs\u2019 applications. Consequently, interdisciplinary research among scientists from different disciplines such as chemistry and chemical engineering, environmental science engineering, and computer science (modeling) is undoubtedly significant to developing MOF science and technology and commercializing MOF-based processes.POPs are among the most hazardous materials released, intentionally or unintentionally, to water sources due to human activities and resist environmental degradation. Due to harmful impact of POPs on wildlife and human beings it is crucial to explore and develop effective strategies for POP removal from water/wastewater. MOFs possess unique features which make them a proper potential for this purpose. Photocatalytic degradation and adsorption of POPs by MOFs has drawn scientific attraction as a potential solution. Resealing scientific results discussing the advances, challenges and improvement strategies in this field could open up a new door for future researches.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.D. Eder and S. Naghdi acknowledge the support of the Austrian Science Fund (FWF, I 5413-N) H. Kazemian and H. Djahaniani acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) through Discovery Grant [RGPIN-2019-06304].", "descript": "\n                  The presence of persistent organic pollutants (POPs) in the aquatic environment is causing widespread concern due to their bioaccumulation, toxicity, and possible environmental risk. These contaminants are produced daily in large quantities and released into water bodies. Traditional wastewater treatment plants are ineffective at degrading these pollutants. As a result, the development of long-term and effective POP removal techniques is critical. In water, adsorption removal and photocatalytic degradation of POPs have been identified as energy and cost-efficient solutions. Both technologies have received a lot of attention for their efforts to treat the world's wastewater. Photocatalytic removal of POPs is a promising, effective, and long-lasting method, while adsorption removal of persistent POPs represents a simple, practical method, particularly in decentralized systems and isolated areas. It is critical to develop new adsorbents/photocatalysts with the desired structure, tunable chemistry, and maximum adsorption sites for highly efficient removal of POPs. As a class of recently created multifunctional porous materials, Metal-organic frameworks (MOFs) offer tremendous prospects in adsorptive removal and photocatalytic degradation of POPs for water remediation. This review defines POPs and discusses current research on adsorptive and photocatalytic POP removal using emerging MOFs for each type of POPs.\n               "}
{"full_text": "Massive dearth for industrialization, globalization and civilization has brought annihilation of natural resources to liberal extents. Fresh water, fossil fuels, atmosphere and forests encounter rapid pollution due to man-made actions. Discharge of untreated/poorly treated textile wastewater into water bodies is one such event that causes catastrophic alarms. More than 15% of the dye produced globally is found to be lost in the dyeing processes and escape through the drains [1]. In addition to these unfixed dyes, the effluents are also concentrated with huge amounts of inorganic salts, surfactants and alkalis [2]. Their contrary impact on chemical and biological oxygen demand (COD and BOD), total organic carbon (TOC), salinity, turbidity, suspended solids, color intensity, toxicity and pH pose severe threat not only to mammalian and aquatic lives but to the entire ecosystem [3]. Myrna Sol\u00eds et\u00a0al. [4] professed that every year nearly 280,000 tons textile dyes are liquidated in the form of industrial wastewater. This marked the textile industry to be one of the foremost contributor of enviro-pollution.Commercially used synthetic dyes are categorized into three types (i) Cationic (basic dyes), (ii) Anionic (direct, disperse, metal complex, acid & reactive dyes) and (iii) Non-ionic dyes. They can also be classified in terms of their chemical structure as azo, anthraquinone, triaryl methane, sulfur linkages etc [5,6]. Among these the azo dyes containing azo (-NN) and sulphonic (-SO3-) linkages have fascinated the textile industries with its excellent structural stability, amplified conjugation, stronger color intensity and high aromatic nature. Consequently, 60\u201370% dyes manufactured around the world are only azo dyes. On the contradictory side the wastewater discharge from azo dyeing processes are found to have negligible biodegradability owing to its high molecular weight and complex structure [7] thus leading to certain ecological conflicts. They increase the turbidity of water and obstruct the sunlight from entering into the water bodies thereby interrupting the growth of flora and fauna. This deficiency of sunlight induces the micro-organisms and hydrophytes to decompose and dissipate foul smell to the atmosphere. Also, heavy metals like Pb, Cu, Cd, Cr, As and Ni dispersed in the contaminated water bio accumulate and absorbed by aquatic life which then directly affect humans over consumption. Carcinogenic and mutagenic effluent-contaminated water triggers several health issues over persistent exposure and ingestion. Some major conditions are cancer, kidney failure, metabolic stress, heart diseases, hepatocellular carcinoma, dermatitis, splenic sarcoma, emphysema, dysfunction of central nervous system and reproductive system etc [8,9].Since industrial dyes are tailored to withstand harsh environments like sunlight, humidity, chemical corrosion, wear and tear resistance etc they cannot be removed or degraded into non-toxic fragments naturally. Along decades there are three different methods of approach for the removal of dye from the effluent wastewater: (i) Biological techniques (algae degradation, enzyme degradation, aerobic & anaerobic remediation, fungal decolorization microbial cultures etc), (ii) Physical techniques (adsorption, reverse osmosis, membrane filtration, coagulation & flocculation, nano & ultra-filtration etc) and (iii) Chemical techniques (ozonation, Fenton's oxidation, electrochemical destruction, electro-kinetic coagulation, photochemical etc) [10,11]. Amidst the various techniques, photocatalytic degradation takes a crucial advantage over the other techniques by its efficient, sludge-free, chemical-free and ecofriendly process for the removal of toxic dyes from industrial effluents. Photocatalytic degradation technique works on the basis of photo-inducing reactive oxygen species (ROS) such as superoxide radicals, singlet oxygen and hydroxide radicals by means of a photocatalyst to strike the organic dye moiety and break it to harmless end-products like water and carbon dioxide [12].Over the years semiconductor photocatalyst materials have gained augmented attention in enviro-remediation. Some frequently used photocatalysts are TiO2, WO3, NiO, SnO2, CuO, Cu2O, ZnO, Bi2O3, Al2O3 and Fe2O3 [13]. Munir Ahmad et\u00a0al. [14] fabricated ZnO and gold decorated ZnO nanoparticles via green synthesis involving pecan nut leaf extract as the reducing agent. The enhanced Au-decorated ZnO nanoparticles showed 95% degradation against rhodamine B. Hematite (\u03b1-Fe2O3) nanoparticles were created in two forms (nanorods & nanocubes) and enhanced with gold decoration by Emre Alp et\u00a0al. [15]. These nanoparticles exhibited almost complete degradation of rhodamine B in addition to its lethal effects on E.coli bacteria. Owing to their non-toxic nature, tunable bandgap, chemical stability, versatile structure and cost effectiveness TiO2 has proved to be a proficient candidate for photocatalysis. But due to its narrow absorption in the visible light region the need for doping it with other metal oxides is a necessity to extend its applicability in sunlight [16,17]. Researchers are engaging consistent efforts in tuning TiO2 to widen its bandgap range and enhance the photocatalytic activity. Ravi Kumar Mulpuri et\u00a0al. [18] co-doped zinc and boron to TiO2 nanocatalyst via sol\u2013gel route and incorporated it for the photodegradation of acid red 6A (AR6A). Zhongming Liu et\u00a0al. [19] put forth the template-assisted synthesis of xylan/PVA/TiO2 composite and its efficiency in photocatalytic degradation of ethyl violet and astrazon brilliant red 4G dyes with 94% degradation rates.This work emphases in embracing a unique solvent system acknowledged as the first-ever deep eutectic solvent (DES) discovered by A. P. Abbott et\u00a0al. [20], choline chloride:urea in 1:2 ratio. DES's are well-known for their biodegradability, high polarity and dielectric constant and exclusively for assisting material synthesis. We report a conventional solid-state synthesis mediated with DES to prepare calcium-based TiO2 composite ceramics and further doping it with lanthanum to study the effect of rare-earth dopant on photocatalytic activity. Furthermore, adaptation of the ceramics as photocatalyst under simulated solar radiation against reactive black 5 (RB5), reactive red 198 (RR198) and reactive yellow 145 (RY145) will be reconnoitered.Choline chloride, urea, calcium oxide, lanthanum oxide and titanium dioxide were all purchased and used without further purification from Sigma. Industrial dyes reactive black 5, reactive red 198 and reactive yellow 145 were also procured from Sigma and their general data is listed in Table 1\n. Doubly distilled water with neutral pH was used for making up and diluting the dye solutions.Firstly, the DES solvent medium was prepared by mixing 1:2 ratio of choline chloride and urea at 80\u00a0\u00b0C. Then, a finely ground mixture of the metal oxide precursors titanium dioxide and calcium oxide was added to the DES solution under simultaneous stirring. The resultant homogenous mixture was calcined at 800\u00a0\u00b0C for an hour to achieve calcium titanate. For the lanthanum doped photocatalyst, lanthanum oxide precursor was added to the above mixture in adequate amount. Finally the obtained pure and La-doped calcium titanate photocatalysts (LaxCa1-xTiO3, x\u00a0=\u00a00.0 & 0.5) were labelled as pure CTO and La-doped CTO followed by vacuum storage.The diffraction patterns of pure and La-doped CTO photocatalysts were studied using X-ray diffraction and are as shown in Fig.\u00a01\n. The pattern obtained for pure CTO was indexed to the orthorhombic crystal phase of CaTiO3 which matched well with the standard JCPDS 78\u20131013. Major diffraction peaks occurred at 2\u03b8 23.3\u00b0, 33.2\u00b0, 47.6\u00b0, 59.5\u00b0 and 69.7\u00b0 were expressive of the planes (101), (100), (050), (042) and (242). Whereas, pattern of the La-doped CTO exposed the advent of some peaks in addition to that of the pure CTO peaks referring to the fusion of lanthanum in the CTO lattice. These peaks were characteristic to the presence of monoclinic La2Ti2O7 in accord with JCPDS 27\u20131182. Some prominent peaks of La-doped CTO were 2\u03b8 27.6\u00b0, 37.8\u00b0, 38.7\u00b0, 40.7\u00b0 and 48.1\u00b0 pertaining to (400), (031), (220), (022) and (\u2212104). The peak at 2\u03b8 47.6\u00b0 (pure CTO) has partially split and gave rise to two peaks at 2\u03b8 47.4\u00b0 and 48.1\u00b0 (La-doped CTO), which showed the successful incorporation and formation of lanthanum in the pure CTO lattice. Both the patterns had sharp intense peaks exhibiting their crystalline nature. Also, the presence of precursor materials like CaO, TiO2 and La2O3 were not detected on both the photocatalysts showing its high degree of phase purity. Based on the Debye\u2013Scherrer's formulations the crystallite size of pure and La-doped CTO photocatalysts were found as 45 and 22\u00a0nm. The inclusion of lanthanum ions has restricted the grain growth and also led to the increment of lattice strain from 0.00216 to 0.00252 [21]. This parallel behaviour of decreasing crystallite size and increasing lattice strain demonstrates the dopant's presence in the lattice.FT-IR spectrum exposed the type of functional groups and bonds present in pure and La-doped CTO photocatalysts and are displayed in Fig.\u00a02\n. The peaks at 490 and 643\u00a0cm\u22121 were indicative of the Ti\u2013O and O\u2013Ti\u2013O bonds present in pure CTO photocatalyst which were also expressed at 478, 584 and 740\u00a0cm\u22121 for the La-doped CTO photocatalyst [22]. The act of doping has transformed the broad peaks into refined peaks when compared to the pure photocatalyst. Likewise, both the photocatalyst had a peak at 3420\u00a0cm\u22121 pertaining to O\u2013H stretching of water molecules adsorbed on the surface of the photocatalyst. Notably, none of the peaks related to organic residues from the DES solvent medium were detected ensuring the photocatalyst purity.Absorption edge and optical response of the photocatalysts were studied and are presented in Fig.\u00a03\n. Pure CTO had an optical absorption at 310\u00a0nm while that of the La-doped photocatalyst was around 360\u00a0nm. They simultaneously extended their edges into the visible wavelength region. This showcases the ability of the photocatalyst to absorb incident light not only in UV region but also in visible region and hence approves their applicability on a wide spread spectrum. Bandgap (Eg) was calculated using the Kubelka\u2013Munk function modified Tauc relation which then gave the direct (n\u00a0=\u00a01/2) and indirect (n\u00a0=\u00a02) bandgap values of the photocatalysts [23] (Fig.\u00a04\n). The bandgaps of pure CTO (3.2 & 3.1eV) had decreased to (2.95 & 2.8eV) illustrating the incidence of Burstein-Moss shift [24]. This decline in Eg values of both the direct and indirect bandgaps depicted the increased generation of electron\u2013hole pairs thereby the greater photocatalytic activity of La-doped photocatalyst.Photoluminescence emission, electron\u2013hole recombination rate and optical properties of pure and La-doped CTO were studied using photoluminescence spectroscopy technique. The studies were conducted at an excitation wavelength of 320\u00a0nm and is exhibited in Fig.\u00a05\n. It was clearly observed that the emission spectra of pure CTO was elevated in comparison to the La-doped CTO photocatalyst. This lowered photoluminescence emission intensity of the La-doped CTO directly indicates the depressed electron\u2013hole recombination rate of the photocatalytic material which occurs due to the instance of doping lanthanum. The low recombination rate results in increased mobility of the electrons which attacks dye molecules through the formation of hydroxyl radicals. Hence La-doped CTO photocatalyst can effectively ease the process of dye degradation.Surface characteristics of the photocatalyst were analyzed using BET analysis technique. Their adsorption\u2013desorption isotherms are as exposed in Fig.\u00a06\n. Based on the Brunauer's classification of surface properties pure and La-doped CTO are categorized under the type II and type III isotherms [25]. Surface area of pure CTO was 6.28\u00a0m2g-1 while that of the La-doped photocatalyst was 32.84\u00a0m2g-1. Whereas the pore size decreased from 4.035\u00a0nm to 1.564\u00a0nm and exposed the transition from macroporous to non-porous nature [26]. This inverse proportionality of the surface area and pore size is a rare phenomenon where the non-porous La-doped CTO photocatalyst compensates its low porosity with its higher surface area and oxygen vacancies. Thus La-doped CTO photocatalyst proves to be a superior candidate for photocatalytic activity.Transmission electron micrographs showed the absolute morphological representation of the photocatalysts (Fig.\u00a07\n). Pure CTO was found to form pillar-like structures whereas the La-doped CTO had broken pillar-like morphologies. This clearly explained the advent of doping a larger ion (La3+) with ionic radius 1.032\u00a0\u00c5 in the A-site has brought about destruction in the structural level causing an increase in the photocatalytic active surface.Pure and La-doped CTO were examined for photocatalytic activity towards 50\u00a0ppm RB5 solution with 1\u00a0mg catalyst/ml loading. Solar light imitated in a photoreactor aided as the UV\u2013Vis light source to trigger the photocatalytic activity of the catalysts. A time-dependent spectrum was taken to analyze the degradation of RB5 when kept in contact with the photocatalysts. In a continuous experiment of 120\u00a0min duration aliquots were collected at an interval of 10\u00a0min and were centrifuged to get rid of any micro particles present. The absorbance spectra displayed in Fig.\u00a08\n pictured the superior degradation of RB5 in La-doped CTO catalytic surface in comparison to that of pure CTO. This can be attributed to the increased surface area, porosity and surface defects which arose due to the La3+ infusion in the lattice sites of the La-doped CTO. As a result La-doped CTO is evidenced to be enhanced and effective among the two catalysts and was selected for further studies.When exposed to UV\u2013Vis light an electron (e\n-\n) from the valence band of the photocatalyst is excited to the conduction band leaving a positive hole (h+) behind. Consequently, the credible mechanism of photocatalytic degradation (Fig.\u00a09\n) can take place in two ways: photo-induced splitting of water molecules to form hydroxyl radicals (OH\n-\n) by means of h+ and reduction of surface adsorbed oxygen molecules to form superoxide anions (O2\n\n.-\n) which in turn forms hydroxyl radicals by means of e\n-\n. However, the highly reactive hydroxyl radicals formed either way exclusively attacks the organic dye moiety forcing it to breakdown into smaller intermediates and finally to non-hazardous carbon dioxide (CO2) and water (H2O) [27]. Also, it is worth mentioning that due to the absence of chemical stimulants like hydrogen peroxide, sodium hypochlorite, ozone etc the possibility of chemical oxidation is completely eliminated. Thus, UV\u2013Vis light stands as the only external source to trigger the photocatalytic degradation of the dyes.La-doped CTO catalyst was tested for its photocatalytic activity on RB5 in a wide range of dye concentration to find out its effectiveness over highly concentrated real-time textile effluents. Degradation efficiency was estimated using the formula: % Degradation efficiency= (C0\u2013C/C0) x100 where C0 is the absorbance at time\u00a0=\u00a00 and C is the absorbance at time\u00a0=\u00a0t. Figure\u00a010\n showcased the gradual decrement of photocatalytic efficiency from 85.6 to 59.4% when the dye solution was concentrated from 25 to 100\u00a0ppm. This occurrence at higher concentrations is due to the increasing hindrance of dye molecules which obstruct the photons from reaching the catalytic surface and therefore depressing the generation of hydroxyl radicals. Since only fewer hydroxyl radicals are generated to breakdown more number of dye molecules a drop in efficiency takes place [28]. However, La-doped CTO had a moderate efficiency (72.6%) over 50\u00a0ppm RB5 and was preferred for forthcoming scrutiny with dyes of similar properties.The photocatalytic efficiency of La-doped CTO photocatalyst was efficaciously examined with 50\u00a0ppm RB5 and was also evaluated with two more reactive anionic dyes, RR198 and RY145. From Fig.\u00a011\n it is clear that the degradation efficiency increased in the order of RB5\u02c2RR198\u02c2RY145. Additionally, the efficiency didn't reach a plateau which resembled that the catalyst is capable of progressing towards complete destruction of the dye provided extra duration. The catalytic surface expressed 86.2% efficiency in RR198 whereas 97.8% efficiency in RY145 solution. Exclusively, the catalyst was able to degrade almost complete RY145 in just 60\u00a0min in spite of its higher molecular weight. This confirms that La-doped CTO is highly favored for RY145 remediation from the environment.The order of the degradation process and its kinetic parameters were found by applying the relation: ln (C0/C)\u00a0=\u00a0k\u2217t where C0 is the initial concentration, C is the concentration at time t of the dye and k\u2217 is the rate constant of the reaction. The linear fit to the plot of ln (C0/C) verses time (Fig.\u00a012\n) resulted in a linear graph proving that the reaction kinetics falls under the pseudo first order kinetics category. The rate constant, correlation coefficient (R2) and the calculated concentration of dye at equilibrium time (Ceq cal) found from the graph are charted in Table 2\n. Since La-doped CTO catalyst has the ability of degradation beyond 120\u00a0min a stable saturation level has not been attained in all the three dye degradations. Hence estimating the experimental concentration of dye at equilibrium time (Ceq exp) is not feasible. But the Ceq cal values attest that while attaining equilibrium the concentration of the dyes would be very negligible. Thus complete degradation of dye is once more substantiated strongly.pH of the dye solution plays a chief pivotal role in the photocatalytic degradation process. It can either serve as a driving force or an impeding force towards the degradation mechanism. Degradation taking place at neutral pH varies drastically when the pH of the medium is altered towards acidic or basic scale. Influence of pH over dye molecules mainly depend on functional groups, molecular weight and the type of dye being degraded besides the nature of the catalyst. Also it is noteworthy that the three anionic dyes are weak acids containing number of sulphonate functional groups (-SO3\n\n-\n) which favors the dye molecules to be negatively charged [29]. The variation in degradation efficiency of RB5, RR198 and RY145 with respect to the pH of the solution is disclosed in Fig.\u00a013\n. Overall, in all the three dyes it is noticeable that in comparison with the neutral pH the efficiency is amplified in acidic medium and declined in basic medium. As La-doped CTO is a titanium dioxide based material it can have amphoteric nature. Due to this fact, at acidic pH the catalyst is positively charged and has greater electrostatic affinity towards the anionic dye species while at basic pH the catalyst becomes negatively charged leading to repulsion in contact with the anionic dye. This explicates the change in efficiencies at different pH [30].The reusability of a catalyst after several number of photocatalytic cycles is a crucial property of a proficient catalyst. It reinforces the existence of ample number of surface active sites in the catalyst and the possibility of rapid seizing/relinquishing dye molecules in the porous cavities of the catalytic surface. Due to such significance, stability and recyclability of La-doped CTO catalyst was verified with RB5, RR198 and RY145 solutions (Fig.\u00a014\n). Degradation efficiencies remained almost constant up to five cycles for all three dye degradations. The photocatalyst showed 82.4, 90.6 and 92.6% average efficiencies for RB5, RR198 and RY145 respectively ensuring its active usage for more cycles.Collectively in summary, pure and La-doped CTO were synthesized using a conventional solid-state technique which was mediated by a DES medium (choline chloride:urea). The synthesized materials were applied as photocatalysts for aiding the photocatalytic degradation of RB5, RR198 and RR198. The deviation of degradation efficiencies were noted in terms of type of catalyst, concentration of dye, type of dye and pH. Kinetics of the reaction order and the recycling capacity of the catalyst were also examined to strengthen the potential activity of the photocatalyst.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 Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah Saudi Arabia has funded this project, under grant number (KEP-40-130-42). This work is sustained by the support rendered by the DST-FIST facility (CRIST), Stella Maris College, Chennai - 600086. The author Ayyar Manikandan is thankful to Tamil Nadu State Council for Science and Technology (TNSCST), DOTE Campus, Chennai for the financial support (S&T Project: TNSCST/ STP-PRG/AR/2018-2019/9307). The authors are also thankful to Ms. Jeena N Baby and Dr. Raghu Subashchandrabose for their research assistance and support.", "descript": "\n                  Water serves as a key component in the basis of life to function and evolve unceasingly. Excessive deterioration of fresh water resources through discarding enormous amounts of dye effluents is becoming a problem of concern. In this work we report a conventional solid-state technique improved with choline chloride: urea DES medium in 1:2 ratio for synthesizing pure calcium titanate (CTO) and lanthanum doped calcium titanate (LaxCa1-xTiO3; x\u00a0=\u00a00.0 and 0.5). Both the materials were subjected to photocatalytically degrade reactive dyes RB5, RR198 and RY145. La-doped calcium titanate showed effective degradation efficiency when compared to the pure calcium titanate. On increasing the concentration of the dye, the efficiency of degradation declined gradually. Also, the efficiency was assessed to be greatest for RY145 followed by RR198 and lastly RB5. Efficiency dependence on different pH exposed the enhanced degradation in acidic medium and comparatively lowered pH in basic medium. Recyclability was highly satisfying and promising to validate the real-time applicability of the catalyst.\n               "}
{"full_text": "Zeolites are crystalline and microporous materials built mainly of silicon, aluminum, and oxygen. The occurrence of Br\u00f8nsted acid sites inside micropores allows to use this group of aluminosilicates as catalysts in numerous industrial processes [1,2]. However, due to a limited accessibility of the active sites and slow transport of reactants in micropores, which can lead to rapid deactivation of catalyst, the application of this groups of minerals as catalysts is not fully satisfying [3-5]. The formation of mesopores via alkaline treatment of zeolites seems to resolve this problem [6-8].One of the routes in the synthesis of mesoporous materials is the application of ultrasonic irradiation. This technique allows to reduce the duration procedure, the use of milder pressure and temperature conditions and may cause the limitation (or even elimination) of the utilization of expensive and toxic reagents, which can also result in the decrease of synthesis costs. The phenomenon of the replacement of conventional desilication of microporous zeolites (in alkaline media) with ultrasonic technique is based on the cavitation mechanism of ultrasounds propagation and the formation of local \u201cspots\u201d of ultrahigh temperature (5000\u00a0K) and pressure (1000\u00a0bar). The cavitation leads to collisions of particles moving at high velocities, promoting the formation of radicals triggering sonochemical reactions [9,10].The application of ultrasounds indicated beneficial effects in syntheses of zeolites: CHA [11], A [12], NaP [13], bilikalite [14], MCM-22 [15] and RHO-type zeolites [16]. Generally, ultrasonic-assisted synthesis of zeolites caused the generation of products in a shorter time, which presented an improved crystallinity degree and smaller crystal sizes.Ultrasonic irradiation was also used in the preparation of single metal oxides, such as: ZrO2\n[17], TiO2\n[18], MnO2\n[19], Cu2O [20], CeO2\n[21] and ZnO [22]. Furthermore, ultrasonic-assisted procedure of the synthesis of mixed oxides was reported for: BayZr3-yTiO3\n[23], NiO/Al2O3\n[24], Ce0.5Zr0.5O2\n[25], CuO/ZnO/ZrO2/Al2O3\n[26] and Ni-Co/Al2O3-ZrO2\n[27].Ultrasonic-assisted modification of zeolites was also reported in literature. Hosseini et al. [28] performed dealumination of zeolite Y in ethanol-acetylacetone solution as a chelating agent both in the absence and presence of ultrasounds. It was shown that the sonochemical-assisted modification of zeolite samples resulted in a higher aluminum extraction from zeolite framework than for the materials prepared conventionally.Zhang et al. [29] obtained mesoporous FAU-type zeolites via chemical dealumination (using citric acid and H4EDTA aqueous solutions) and a subsequent ultrasonic-assisted alkaline treatment in aqueous NaOH solutions. The use of ultrasounds accelerated the formation of mesoporosity in respect to the analogues prepared traditionally. Similar observations were reported by Oruji et al.\n[30], who synthesized mesoporous FAU-type zeolites in sodium form. Based on porosity studies, it was shown that the rising duration of base-wash procedure led to gradual increase of mesoporosity with a simultaneous decrease of crystallinity.In turn, Kuterasi\u0144ski et al. [31] investigated the effect of ultrasonic-assisted desilication of commercial FAU-type zeolite. The prepared samples have been used as catalysts for the decarbonylation of furfural into furan. It was shown that the application of high-frequency ultrasounds during alkaline treatment procedure caused higher mesoporosity and enhanced catalytic properties in respect to the catalysts modified under conventional desilication conditions.Khoshbin and Karimzadeh [32] prepared mesoporous ZSM-5 zeolite via ultrasonic-assisted desilication of parent zeolite, which was synthesized using a rice husk ash as a silica source and various contents of carbon nanotubes (between 0 and 30\u00a0wt% of CNTs) playing a role of hard template. It was evidenced that increasing amount of carbon nanotubes in precursor mixture caused the increase of both external surface area and mesopore volume of such prepared ZSM-5-type zeolite product.In this study, we present an ultrasound-assisted desilication of MFI-type zeolite, which can be used as a catalyst for the dehydration of ethanol into diethyl ether (DEE) and ethylene. DEE can be used in pharmaceutics, explosives and in petrochemistry [33-35]. In turn, ethylene undergoing polymerisation, oligomerisation, hydrogenation, halogenation, oxidation and a lot of other reactions, has also a great meaning in many industrial processes [36-40].In this study, we applied commercial zeolite of MFI type structure (Si/Al\u00a0=\u00a040) \u2013 MFI-40. The physicochemical and catalytic properties of the zeolite-based catalysts prepared in the presence of ultrasounds were compared with those obtained under conventional treatment. Presented research results constitute a precious enrichment of the knowledge concerning the synthesis of hierarchical materials.The parent MFI-type zeolite (Si/Al\u00a0=\u00a040) from Zeolyst (CBV 8014) was used as a reference sample. Ultrasonic-assisted desilication was performed using 6\u00a0g of zeolite and 200\u00a0ml of 0.2\u00a0M aqueous solutions of the pure sodium hydroxide (NaOH) or mixture of NaOH and tetrabutylammonium (TBAOH) hydroxide, which contained 10 or 70\u00a0mol% of TBAOH (i.e. 0.18\u00a0M of NaOH and 0.02\u00a0M of TBAOH or 0.06\u00a0M of NaOH and 0.14\u00a0M of TBAOH, respectively) at the same pH values (13.8).Alkaline treatment was performed for 30\u00a0min. Whole reaction system (alkaline solution, zeolite and the sonicator probe) was placed in an ice bath, which ensured low temperature. QSonica Q700 sonicator with power of 600\u00a0W and frequency of 20\u00a0kHz was used as a generator of ultrasounds. The device was equipped with a \u201c1\u201d diameter horn (Church Hill Rd, Newtown, CT, USA). For comparison, the conventional desilication (ultrasonic-free procedure) was carried out for 30\u00a0min also in the ice bath using alkaline solution at the same chemical compositions as above. In order to investigate a direct influence of ultrasounds on the demineralization intensity, we changed only one parameter, namely, we introduced ultrasonic irradiation into \u201czeolite-alkaline solution\u201d at the same temperature of ice bath, chemical composition of alkaline solution, mass ratio of zeolite to mixture and the duration of procedure as for conventional demineralization. After desilication procedure, the suspension was fourfold centrifuged at 4000 RPM and dried overnight at 80\u00a0\u00b0C.Subsequently, desilicated zeolites were calcined in air with a flow rate of 50\u00a0ml/min for 10\u00a0h at 525\u00a0\u00b0C and with a temperature ramp of 1.5\u00a0\u00b0C/min.Afterwards, fivefold Na+\u2192NH4\n+ ion-exchange of desilicated zeolites with 500\u00a0ml of 0.5\u00a0M aqueous NH4NO3 solution was performed at 80\u00a0\u00b0C for 2\u00a0h. In next step, the zeolite samples in ammonium form were centrifuged at 4000 RPM, washed, and dried at 80\u00a0\u00b0C again. Finally, the samples were calcined in 50\u00a0ml/min air flow for 8\u00a0h at 450\u00a0\u00b0C and with temperature ramp of 2\u00a0\u00b0C/min.The catalysts prepared conventionally or sonochemically were designated by the index \u201cc\u201d or \u201cs\u201d, respectively. Depending on the molar content of TBAOH in 0.2\u00a0M NaOH/TBAOH of desilication agent (0 or 10 or 70\u00a0mol% of TBAOH), the samples were named as M\u22120c or M\u22120\u00a0s, M\u221210c or M\u221210\u00a0s and M\u221270c or M\u221270\u00a0s, respectively. The parent MFI-type zeolite was denoted as M.ICP-OES chemical analysis of zeolites was carried out by the dissolution of ca. 100\u00a0mg of powder in a HF/HCl mixture in a Teflon vessel for one day. In next step, the liquid was diluted to 250\u00a0ml and both Si and Al quantitative analyses were performed using Optima 2100DV - PerkinElmer instrument.In order to determine the zeolite crystallinity, X-ray diffraction (XRD) experiments were performed using a PANalytical X\u2019Pert PRO MPD diffractometer (40\u00a0kV and 30\u00a0mA), equipped with CuK\u03b1 generator (\u03bb\u00a0=\u00a01.5418\u00a0\u00c5). 2\u03b8 angle was at 5\u201350\u00b0 with a 0.033\u00b0 step. The zeolite samples were in the form of powder and were placed in holders. The calculations of the average size of crystallites were performed using PANanalytical\u00a0X\u00a0Pert Data Viewer software connected with the diffractometer and were based on Scherrer equation (1).\n\n(1)\n\n\nL\n=\n\n\n\u03bb\nK\n\n/\n\n\u03b2\ncos\n\u03b8\n\n\n\n\n\nWhere: \u028e corresponds to X-ray wavelength value (1.5418\u00a0\u00c5); K is a dimensionless shape factor (0.9); \u03b2 means FWHM, i.e. full width at half maximum and \u03b8 is the Bragg angle.The status of Al in the investigated samples was determined by the solid-state 27Al MAS NMR method using a Bruker Advance III 500\u00a0MHz WB spectrometer operating at 11\u00a0T of magnetic field. 27Al MAS NMR spectra were recorded at 130.3\u00a0MHz of the basic resonance frequency and at 10\u00a0kHz of a spinning rate (in zirconia rotors) with a short pulse length of 0.2\u00a0\u03bcs (\u03c0/12) and a recycle delay of 0.1\u00a0s. 1\u00a0M aqueous Al(NO3)3 was used as a reference for 27Al MAS NMR chemical shifts. Prior to NMR experiments, the samples were fully hydrated at ambient temperature in the presence of vapor-saturated Mg(NO3)2 solution.The solid-state 29Si MAS NMR spectroscopy was used in order to determine the status of silicon. 29Si MAS NMR spectra were recorded using a Bruker Advance III 500\u00a0MHz WB spectrometer operating at a magnetic field of 11.7\u00a0T and at the basic resonance frequency of 99.4\u00a0MHz, a spinning rate of 8\u00a0kHz (in zirconia rotors) with high-power proton decoupling (SPINAL64), at 5.8\u00a0\u03bcs (\u03c0/3) pulses and repetition time of 20\u00a0s. The chemical shifts of 29Si MAS NMR spectra were externally referenced to Tetramethylsilane (TMS; >99%).The porosity was determined by the low temperature sorption of nitrogen at \u2212196\u00a0\u00b0C using Autosorb-1 Quantachrome. Specific surface area (SBET) was determined by BET model, external surface area (Sext) and volume of mesopores (Vmeso) were estimated by the application of Barrett-Joyner-Halenda (BJH) model on the adsorption branch of the isotherm. Micropore volume (Vmicro) was calculated using t-plot method. Prior to each measurement, the sample was outgassed for 20\u00a0h at 250\u00a0\u00b0C in a vacuum.The morphology of the prepared samples was investigated with a JEOL JSM \u2013 7500F Field Emission Scanning Electron Microscope (SEM). Prior to the SEM analysis, the samples were dried for 24\u00a0h without the covering of specimens with the coating in order to enable the detailed observation of the surface of the studied materials.Transmission electron microscopy analysis (TEM) of chosen samples was performed using JEOL JEM 2100 HT LaB6 (JEOL USA, Inc., Peabody, MA, USA), with accelerated voltage of 80\u00a0kV and the spot size of 1\u00a0nm. Prior to TEM analyses, the studied materials were sprayed onto formvar film coated copper grids.The FT-IR measurements were conducted with NICOLET iS10 spectrometer (supplied by Thermo Scientific) equipped with a MCT detector. The IR spectra were recorded at 4000\u2013650\u00a0\u00b1\u00a04\u00a0cm\u22121 with 128 scans per each spectrum. FT-IR measurements were preceded by the activation of samples (in the form of self- supporting wafers of ca. 70\u00a0mg) for 1\u00a0h at 400\u00a0\u00b0C with a temperature ramp of 5\u00a0\u00b0C/min under vacuum conditions.Quantitative analysis of acidity was performed by the IR studies of the sorption of ammonia (Air Products, 99.95%) at 120\u00a0\u00b0C and calculated based on the intensities and the extinction coefficients of the bands assigned to ammonia interacting with Br\u00f8nsted and Lewis acid sites. The bands of 1450\u00a0cm\u22121 are attributed to ammonium ions and are characterized by the extinction coefficient of 0.12\u00a0cm2/\u03bcmol, while the maxima at 1620\u00a0cm\u22121 correspond to ammonia interacting with Lewis sites with the extinction coefficient of 0.026\u00a0cm2/\u03bcmol [31].The acid strength of Si-OH-Al groups was determined via CO sorption at \u2212100\u00a0\u00b0C, followed by the comparison of the values of frequency shifts between the maxima of free acidic OH groups and OH interacting with CO by hydrogen bonding, according to the procedure described in [41].The dehydration of ethanol into diethyl ether and ethylene as a testing reaction was investigated at 150\u2013290\u00a0\u00b0C (with a 50\u00a0\u00b0C step) in a fixed-bed glass microreactor coupled on-line with a gas chromatograph. Analysis of the products obtained in the reaction was carried out in a Perkin Elmer Clarus 580 equipped with Elite-Plot Qcapillary column (with length of 30\u00a0m and inner diameter of 0.53\u00a0mm) and TCD detector. Prior to each experiment, 100\u00a0mg of catalyst (with the granulation of 190\u2013260\u00a0\u03bcm) was placed on a quartz wool plug in the reactor and exposed to a pure helium flow of 35\u00a0ml/min (Air Products, 5.0) at 300\u00a0\u00b0C for 30\u00a0min. Subsequently, He was passed through the liquid ethanol, generating the ethanol in helium flow with a concentration of 2.24\u00a0mmol/ml. The total reaction gas mixture stream was 35\u00a0ml/min. The weight hourly space velocity was kept at 2.0 gethanol/(gcatalyst\u00b7h).Turnover frequency (TOF, s\u22121) was defined, as follows:\n\n(2)\n\n\nTOF\n=\n\n\nn\n\nE\nt\n\n\n\nOH\n\n\n\n2\n\n\n/\n\n\n\nm\n\u00b7\n\nc\n\nBAS\n\n\n\n\n\n\n\n\n\nWhere: nEt(OH)2 corresponds to the number of transformed molecules of substrate in one second (\u00b5mol/s); m is the catalyst mass (g), and cBAS is the protonic acidity concentration (\u00b5mol/g). Experimental error was not higher than 5%.Analysis of the ICP-OES results (summarized in Table 1\n) indicated that the treatment of MFI-40 with 0.2\u00a0M of NaOH solution in the absence of ultrasounds resulted in a small leaching of both Si (0.5%) and Al (0.1%) from zeolite structure. The addition of ultrasounds caused a significant acceleration in the extraction of both silicon (15.3%) and aluminium (2.6%) from MFI framework, leading to the decrease of Si/Al ratio from 37.7 to 32.4.In case of the application of demineralizing agent containing TBAOH, the introduction of ultrasonic irradiation into system \u201czeolite - alkaline mixture\u201d also caused elevated removal of Si and Al from MFI-type zeolite framework. For instance, the use of 0.2\u00a0M NaOH/TBAOH solution containing 10% of TBAOH led to the increase of Si and Al extraction from 1.0% to 9.4% and from 1.0% to 5.9%, respectively. Analogous situation was found for the NaOH/TBAOH demineralizing mixture including 70% of TBAOH. The leaching of silicon was 2.3% vs. 10.8%, meanwhile the aluminium extraction was 2.3% vs. 8.6% in the absence vs. presence of ultrasonic irradiation, respectively. The alkaline treatment of MFI structure type zeolites with solutions containing TBAOH led to a slight decrease of Si/Al ratio from 37.7 to 36.2\u201337.5 due to simultaneous extraction of both elements (Si and Al), although the desilication took place more intensively than dealumination process.At first sight, it seems that the alkalinity of 0.2\u00a0M NaOH should be stronger than 0.2\u00a0M (NaOH/TBAOH) due to the presence of TBAOH playing the role of a protective layer on the zeolitic external surface and being a moderator in the desilication process [42-44]. However, it was found that TBAOH is able to remove aluminum from zeolite framework, which was in line with Sadowska [45] and Abello [46]. This effect rose with the TBAOH mol% content in NaOH/TBAOH demineralizing agent regardless of the way of demineralization procedure (conventional vs. ultrasonic), however, the Al leaching was more intensive under sonochemical conditions. Elevated extraction of aluminum from zeolite framework could lead to the removal of vicinal silicon atoms, which led to the production of holes, followed by the formation of \u201cswiss cheese\u201d type zeolite grains (see TEM analysis, Section 3.4). This effect can be observed only at low temperature (in our case: ice bath conditions). The application of much higher temperature of alkaline treatment always leads to a more intensive desilication, which was reported for the zeolites with MFI [45,47\u201350], MTW [50], BEA [51] and FER [52] of similar Si/Al ratio (30\u201350), for which removal of silicon exceeded 50%. So far, however, a direct introduction of the ultrasonic irradiation into high-temperature desilication performed under identical conditions was not found probably due to technical problems, such as the overheating of sonicator.In order to avoid the overheating of the source of ultrasounds and investigate a direct influence of ultrasounds on the demineralization intensity, we just introduced ultrasonic irradiation into \u201czeolite-alkaline solution\u201d at the same (ice bath) temperature, chemical composition of alkaline solution, mass ratio of zeolite to mixture and the duration of procedure like in the case of conventional treatment.From our ICP-OES results obtained for the desilicated MFI zeolite (Si/Al\u00a0=\u00a040), it may be concluded that our zeolite was weakly prone to desilication in comparison with FAU-type zeolite of Si/Al\u00a0=\u00a031 [31]. For faujasite ultrasonically treated with alkaline solutions of various TBAOH content under similar conditions, the percentage amount of Si extracted from framework was 20\u201340% (for MFI-zeolite was ca 15%). Observed differences in the extraction degree of Si between two types of zeolite structure (FAU vs. MFI) can be explained by a higher stability of MFI zeolite framework than FAU zeolite of elevated Si/Al of 31 (being not natural for the zeolite of this topology, obtained commercially via dealumination of pristine faujasite). Hence, FAU31 zeolite was more sensitive for any modifications (including interaction with aqueous alkaline solutions).The XRD patterns of the investigated catalysts are presented in Fig. 1\n. For all samples, the occurrence of MFI-type zeolite phase was detected [53]. Analysis of the XRD reflexes of the studied samples leads to the conclusion that neither ultrasonic irradiation nor chemical composition of NaOH/TBAOH alkaline mixture had a significant impact on the crystallinity of the prepared materials. In all cases, the crystallinity was preserved, which well corresponds to the ICP-OES results (Table 1). It may be explained by a limited extraction of both Si and Al from zeolite MFI, which resulted in minor changes of Si/Al of parent material. Hence crystalline structure of modified zeolites did not undergo a collapse.Based on the analysis of crystallite sizes calculations (Table 1), it can be concluded that the modification of M\u221240 zeolite with alkaline solutions caused some drops in the crystallite sizes. The average size of crystallites decreased from 501 to 341\u2013448\u00a0\u00c5. Registered changes in the size of crystallites did not reveal apparent relationship with the conditions of alkaline treatment procedure.Abello et al. [46], Schmidt et al. [47], Groen et al. [54], Rutkowska et al. [55-57] and Ahmadpour et al. [58] also did not observed significant changes in crystalline structure of desilicated MFI-type zeolites in respect to parent samples of similar Si/Al ratio.\n27Al MAS NMR spectra (Fig. 2\n) illustrate the status of Al in the prepared catalysts. For the reference sample (M), the occurrence of the Al signal at ca 57\u00a0ppm is attributed to zeolite framework tetrahedral aluminum [45,50]. Simultaneously, very weak signal at 0\u00a0ppm originating from extra-framework octahedral Al species was found. The status of Al depended slightly on both the application of ultrasounds during desilication and the chemical composition of 0.2\u00a0M NaOH/TBAOH aqueous solution. In the absence of ultrasounds, aluminum did not go from tetrahedral to octahedral positions. According to Sadowska et al. [45], the treatment of zeolite MFI with NaOH did not lead to the formation of extra-framework aluminum species, what means that mild desilication allows to stay all Al atoms in zeolite framework. The lack of the growth of signal at 0\u00a0ppm can be also explained by the reinsertion of Al into the zeolite framework (known as realumination), which was previously reported for zeolites with FAU [41,59,60] or MFI-type structure [45,54,61,62]. In case of ultrasonic-assisted procedure of demineralization of MFI-40 zeolite with alkaline mixture containing TBAOH, the removal of aluminum from tetrahedral coordination is accompanied by a slight formation of extra-framework Al species.\nFig. 3\n shows the 29Si MAS NMR spectra of the prepared samples. In all cases, the occurrence of the Si signals at \u2212112 and at \u2212108\u00a0ppm originates from Si(4Si) and Si(3Si), respectively [45,50]. The Si(3Si) signal correspond mainly to Si(1Al) surroundings. The Si(4Si) signal was dominating for all investigated catalysts. For all studied samples, the 29Si MAS NMR spectra given in Fig. 3 and data obtained from the deconvolution of 29Si MAS NMR spectra (Table S1) are similar. Generally, the status of silicon in the studied samples was independent of the route of demineralization (conventional vs. ultrasonic) and chemical composition of the alkaline mixture applied during the procedure of modification. Slight differences in the intensity of Si(4Si) signals correspond to the leaching of Si (and to a lesser content of Al), resulting in slight changes of Si/Al ratios (Table 1). In both series (conventional vs. ultrasonic-assisted), the most intensive Si(4Si) signals were found for the zeolites modified with the NaOH/TBAOH with TBAOH content of 70\u00a0mol% due to the highest Al extraction (and Si/Al ratio). Minor changes in the status of either Si and Al also agree with the crystallinity of the prepared samples (Fig. 1).The results of the porosity of the prepared materials are summarized in Table 2\n and Figure S1. The parent MFI zeolite is characterized by the presence of both micropores (0.162\u00a0cm3/g) and intercrystalline mesopores (0.185\u00a0cm3/g) with the average pore diameter of 30.3\u00a0\u00c5. Hence, the percentage contribution of mesopores volume was 53.3%.The way of the alkaline treatment influenced the porous structure of the prepared catalysts. The modification of MFI-40 with 0.2\u00a0M NaOH under conventional conditions led to a decrease of the total volume of pores from 0.347 to 0.215\u00a0cm3/g with simultaneous growth of the mesoporosity to 61.9%. That caused growth of the average pore diameter from 30.3 to 37.3\u00a0\u00c5.The use of the ultrasonic-assisted technique led to more significant production of mesoporosity in relation with the conventional method of modification of MFI-40. The volume of micropores decreased from 0.162 to 0.110\u00a0cm3/g and the volume of mesopores and average pore diameter rose from 0.185 to 0.230\u00a0cm3/g and from 30.3 to 42.1\u00a0\u00c5, respectively. That led to the increase of percentage mesoporosity contribution up to 67.6%.The appearance and further rising of TBAOH in the demineralising agent strengthened the formation of mesoporosity in prepared catalysts. Furthermore, all zeolite samples modified sonochemically revealed higher mesoporosity than counterparts prepared classically. The use of 0.2\u00a0M NaOH/TBAOH (10\u00a0mol% of TBAOH) in the absence or presence of ultrasounds caused the increase of mesopores volume participation from 53.3% to 62.8% vs. 64.8% and the increase of average pore diameter from 30.3 to 37.5 vs. 37.7\u00a0\u00c5, respectively. In the case of the utilization of the NaOH/TBAOH alkaline solution containing 70\u00a0mol% of TBAOH, a minimal increase of mesoporosity from 53.3% to 63.0% vs. 66.7% with a simultaneous growth of an average pore diameter from 30.3 to 38.9 vs. 41.7\u00a0\u00c5 were found.It also was found that an alkaline treatment of MFI-40 zeolite resulted in a minimal growth of percentage Sext/SBET ratio from 16.6% to 17.4\u201322.4%. However, all catalysts prepared ultrasonically demonstrated higher Sext/SBET values (20.1\u201322.4%) in comparison with the samples modified conventionally (17.4\u201319.3%).For all studied samples, the appearance of hysteresis loop of IV type can be attributed to the presence of both intercrystalline pores between the MFI crystals (particularly for parent sample \u201cM\u201d) and the formation of mesoporosity.According to the XRD patterns illustrated in Fig. 1 as well as ICP-OES data summarized in Table 1, small changes in the crystallinity are in line with minor changes in porous structure of the studied samples.Our results are in good agreement with, Zhang et al. [29], Oruji et al. [30], Khoshbin et al. [32] as well as with our previous studies [31]. It was evidenced that a sonochemical demineralization procedure enhanced the production of higher mesoporosity in respect to the conventional alkaline-treatment technique. Additionally, it was indicated that the use of ultrasounds during modification resulted in preserved microporosity. On the other hand, observed changes in the porous structure of our MFI-type zeolites are significantly smaller than for MFI-type analogues desilicated by Sadowska et al. [45,49], Abello et al. [46], Shmidt et al. [47], Gil et al. [50] and Groen et al. [62] due to the use of much milder conditions in our experiments (relatively short duration of the procedure, ice bath temperature, and respectively low concentration of desilicating agents) in comparison with the research quoted above.The analysis of the morphology of the prepared MFI-40-based catalysts are illustrated in Fig. 4\n (magnification to 50,000x). The appearance of SEM images leads to the conclusion that the particles of all modified samples are of irregular shape with dimensions ranging from 300 to 1000\u00a0nm. For the samples treated under conventional conditions (Fig. 4a-c), alkaline treatment led to some fragmentation of grains, which slightly influenced the porosity of this series of samples.In case of the catalysts prepared via ultrasonic-assisted procedure (Fig. 4d-f), treatment of the parent zeolite (M) led to the appearance of cracks, cavities and strengthened the fragmentation of grains in comparison with the analogues modified in the absence of ultrasonic irradiation. Observed changes in microscopy well correlate with crystallite sizes and textural properties given in Table 2. Nevertheless, the registered changes in the morphology of the investigated MFI-type zeolite materials are quite small, particularly when we compare our current samples (MFI-40) with the materials based on FAU-31 zeolite, which we reported in [31]. Relatively stable morphology of our MFI-type catalysts is in line with their preserved crystallinity (Fig. 1).Furthermore, from analysis of the SEM pictures, it may be concluded that the chemical composition of demineralizing mixture (meant as NaOH/TBAOH ratio) both with and without ultrasounds had no apparent impact on the size and the shape of zeolite crystals.For comparison, the TEM images (50,000\u00d7) of the variously prepared catalysts are illustrated in Fig. 5\n. Analysis of the appearance of crystalline grains led to the conclusion that alkaline treatment of the parent M\u221240 zeolite caused the production of holes and the formation of \u201cswiss cheese\u201d-type zeolite grains followed by the changes in porous structure of the prepared MFI-based samples. Observed perforation seems to be more apparent in the case of zeolites desilicated in the presence of ultrasounds (Fig. 5 C and E). Nevertheless, observed changes in the appearance of zeolite grains were not very sharp, which was in line with crystallinity, structure, morphology and subtle changes in porosity of the studied samples.The characterization of the acidity of the prepared MFI-type samples was illustrated in Fig. 6\n and Table 1. For the parent sample (M), the IR spectrum of the OH groups region indicated the presence of the bands at: 3740\u00a0cm\u22121 attributed to external Si-OH groups with a shoulder at 3730\u00a0cm\u22121 of silanols in the defects, 3670\u00a0cm\u22121 assigned to Al-OH, 3620\u00a0cm\u22121 originating from acidic Si-OH-Al groups and at 3490\u00a0cm\u22121 coming from silanol nests [45,49,50,62].Alkaline treatment with 0.2\u00a0M NaOH or NaOH/TBAOH aqueous solutions led to significant changes in the appearance of IR spectra. The disappearance of the band at 3490\u00a0cm\u22121 as well as an apparent decrease of the signals at 3740 and 3620\u00a0cm\u22121 demonstrated the removal of a significant part of OH groups during desilication procedure. Another effect was a slight increase of the band at 3670\u00a0cm\u22121, which suggests that the interaction of MFI-40 zeolite with alkaline mixtures caused the production of Al-OH groups.Information on the strength of the acidic OH groups (in Si-OH-Al) was given in Table 1. The data obtained from CO sorption showed that the modification of parent MFI-40 zeolite with NaOH or NaOH/TBAOH mixtures both in the absence or presence of ultrasounds did not influence the acid strength of Si-OH-Al (\u0394\u03bd3620OH\u2026CO\u00a0=\u00a0ca 310\u00a0cm\u22121). In case of M\u221240 zeolite treated with NaOH solution under conventional conditions (M\u22120c), the broad band with the maximum at 3670\u00a0cm\u22121 probably overlapped with the band at 3620\u00a0cm\u22121 of Si-OH-Al, making the latter signal undetectable, therefore the strength of protonic acid sites was not determined for M\u22120c.Our results obtained in the current study are in contrast to other research reported for zeolites with MFI [49] and BEA [63], for which desilication caused the decrease of the acid strength of Si-OH-Al by ca. 10\u201320\u00a0cm\u22121, which derived from the occurrence of the more and less acidic OH groups. This contradiction may be explained by the fact that our MFI-40 zeolite was modified under milder conditions in comparison with the majority of experimental procedures reported in the available literature. Furthermore, taking into account relative high amounts of Al extracted from zeolite structure in respect to Si, it could be possible that the process of demineralization of our MFI-40 zeolite caused the destruction of Si-OH-Al groups of the highest and the lowest acid strength. Alternative situation might refer to the removal of Si and Al responsible for Si-OH-Al of medium acid strength. Hence in both suggested cases, an average acid strength of the remaining protonic acid sites after MFI-40 treatment under presented conditions did not undergo apparent changes.The results on the quantitative analysis of the acid sites present in the prepared catalysts are given in Table 1. In all cases, the modification of the parent MFI-40 zeolite with NaOH or NaOH/TBAOH mixtures caused a strong decrease of both Br\u00f8nsted and Lewis acid sites from 228 to 34\u201370\u00a0\u03bcmol and from 309 to 53\u2013105\u00a0\u03bcmol/g, respectively. The observed tendency implies a simultaneous removal of silicon and aluminum from modified MFI-type zeolite. It also was found that the technique of alkaline treatment slightly influenced concentrations of both types of acid sites. Generally, the application of ultrasonic-assisted procedure of MFI-40 modification with the alkaline mixture at concrete chemical composition resulted in a little higher concentrations of acid sites than for counterparts prepared conventionally. Obtained data agrees with the results of ICP-OES analysis, which showed that the presence of ultrasounds during modification procedure of MFI-40 zeolite shifted demineralization towards Si extraction from zeolite framework. For comparison, the interaction between MFI-40 zeolite and alkaline mixture under ultrasonic-free conditions caused practically equal Si and Al removal from zeolite structure (Table 1).Catalytic activity of variously prepared MFI-type zeolite - based catalysts in the dehydration of ethanol reaction was depicted in Fig. 7\nA.For all studied catalysts, It was found that catalytic activity rose with the temperature of experiment in a whole range. In the case of a reference catalyst (M), at 150\u00a0\u00b0C, 170\u00a0\u00b0C, 190\u00a0\u00b0C, 210\u00a0\u00b0C, 230\u00a0\u00b0C, 250\u00a0\u00b0C 270\u00a0\u00b0C and 290\u00a0\u00b0C, the conversion of ethanol formed the sequence, as follows: 2%, 9%, 23%, 40%, 57%, 70%, 82% and 100%, respectively. It also was indicated that an alkaline treatment of MFI-40 zeolite (independently of the procedure conditions) led to the enhancement of the catalytic properties of the prepared materials. For all zeolites treated with basic solutions, the conversion of ethanol was 100% at 270\u2013290\u00a0\u00b0C. The best results were found for the catalyst modified under ultrasonic-assisted conditions with NaOH/TBAOH solution (containing 70\u00a0mol% of TBAOH (M\u221270\u00a0s), for which the conversion of ethanol was 8%, 26%, 49%, 68%, 76%, and 84% at 150\u00a0\u00b0C, 170\u00a0\u00b0C, 190\u00a0\u00b0C, 210\u00a0\u00b0C, 230\u00a0\u00b0C and 250\u00a0\u00b0C, respectively. For comparison, some worse conversion of ethanol was found for the counterpart prepared in the absence of ultrasounds (M\u221270c), which was 5%, 20%, 42%, 60%, 72% and 78% at the same temperature range, as above.The presence of ultrasonic irradiation during alkaline treatment of MFI-40 type zeolites also improved catalytic activity in the case of the NaOH/TBAOH solutions containing 10\u00a0mol% of TBAOH. At 150\u00a0\u00b0C, 170\u00a0\u00b0C, 190\u00a0\u00b0C, 210\u00a0\u00b0C, 230\u00a0\u00b0C and 250\u00a0\u00b0C, the conversion of ethanol was 0% vs. 5%, 6% vs. 20%, 22% vs. 39%, 47% vs. 59%, 65% vs. 71%, 80% vs. 85% for the catalysts prepared conventionally and sonochemically, respectively.Much weaker effect of the application of ultrasounds during demineralization procedure of zeolites on their catalytic activity was found for the samples modified with NaOH solution. At 150\u2013190\u00a0\u00b0C, the conversion of ethanol was higher for the catalysts prepared classically (14%-44% for M\u22120c vs. 6%-41% for M\u22120\u00a0s), while the zeolites treated ultrasonically revealed higher catalytic activity at 210\u2013250\u00a0\u00b0C (56%-74% for M\u22120c vs. 63%-80% for M\u22120\u00a0s).The observed effects correspond to the degree of demineralization of zeolites and the production of mesopores, which facilitated transport of the reagents within porous structure of investigated catalysts.Alkaline treatment of MFI-40 zeolite also influenced the reaction rates in the prepared catalysts (Fig. 7B). Analysis of turnover frequency (TOF) results led to the conclusion that the modification of the parent zeolite (M) with alkaline solutions raised TOF values, which agreed with the changes in either porous structure or acidity of the prepared catalysts (Tables 1 and 2) and with the results reported by Verboekend and P\u00e9rez-Ram\u00edrez [42].It also was found that the application of ultrasonic irradiation during preparation procedure of catalysts (M\u22120\u00a0s and M\u221270\u00a0s) resulted in lower TOF values than for the analogues prepared conventionally (M\u22120c and M\u221270c). An opposite tendency was indicated for M\u221210c and M\u221210\u00a0s. Observed correlations between TOF values implied from catalytic activity as well as from concentrations of protonic acid sites (responsible for this reaction). Actually, the catalysts prepared ultrasonically revealed generally higher conversion of ethanol, but the concentrations of protonic acid sites also were higher for this group of materials (with one exception for M\u221210\u00a0s) in comparison with the catalysts modified conventionally. Direct comparison of the changes in the concentration of Br\u00f8nsted acid sites (Table 1) and catalytic activity (Fig. 7A) allows us to claim that a quantitative analysis of active centres has a stronger impact of TOF values than the conversion of ethanol at given temperatures.Analysis of the results obtained from catalytic performance illustrated in Fig. 8\nA and 8B indicated clearly that both the type and amount of a concrete product is determined by the temperature range. Up to 210\u00a0\u00b0C, it was possible to manufacture practically pure diethyl ether: for all studied catalysts, selectivity was 93\u2013100%. At higher temperatures, the appearance of ethylene was registered. It is worth to underline that no pure ethylene was produced at 210\u2013290\u00a0\u00b0C due to both co-existence of diethyl ether (at 210\u2013270\u00a0\u00b0C) and the coking occurring at the highest temperatures of experiment.Significant selectivity to ethylene was found for all studied catalysts at 250\u2013290\u00a0\u00b0C. For parent zeolite (M) it was 17\u201380%. Alkaline treatment of MFI-type zeolites resulted in a visible enhancement of selectivity to ethylene, however, this effect was generally stronger for the samples after ultrasonic irradiation. For instance, in case of MFI-type zeolite sonochemically modified with NaOH/TBAOH alkaline solution containing 10\u00a0mol% of TBAOH (M\u221210\u00a0s), selectivity to ethylene was 55\u201384%, meanwhile for zeolite modified conventionally (M\u221210c), selectivity to ethylene was 41\u201380%. Other by-products like acetaldehyde were not detected.The best catalyst was M-70s, for which the highest conversion of ethanol was found. Additionally, M-70s demonstrated very high selectivity to diethyl ether (94-100%) at 150-210 \u00b0C and the highest selectivity to ethylene among investigated catalysts (21%, 66% and 84%) at 230 \u00b0C, 250 \u00b0C and 270 \u00b0C.Similar observations were reported by Oliveira [64], who investigated the ethanol dehydration on Cu- and Fe-ZSM-5 catalysts. The production of diethyl ether (70\u2013100%) was found in the temperature range of 180\u2013200\u00a0\u00b0C on Cu-ZSM-5, while ethylene was formed mainly at temperatures exceeding 200\u00a0\u00b0C over both pure and Fe containing ZSM-5 catalysts (20\u2013100% depending on the type of catalyst). For comparison, Zhan et al. [65] reported yield of DEE reaching 67% using 2% PHZSM-5 catalyst. In turn, Jinfa et al. [66] obtained yield of DEE exceeding 70% at 180\u00a0\u00b0C over ZSM-5.Our catalytic results are also in line with Phung, Chiang et al. [33,67\u201369], who studied the ethanol dehydration on commercial H-FER, H-MFI, H-MOR, H-BEA, H-Y and H-USY zeolites For the investigated systems, diethyl ether was mainly produced at low temperatures, while the production of ethylene was predominant at high temperatures, which was independent of the type of acid sites of the studied catalysts. It was found that at 180\u00a0\u00b0C, the selectivity to DEE was higher for H-MFI and H-BEA (exceeding 70%) than for other zeolites. On the other hand, at high temperature, almost full selectivity to ethylene was registered for H-FER, H-Y and H-USY, while co-production of higher hydrocarbons took place in the case of H-MFI, H-BEA and H-MOR, which was in line with the data reported by Stepanov et al. [70] and Wang et al. [71]. Rising reaction temperature led to the formation of coke, particularly over H-MOR and H-BEA. The strength of protonic acid sites was found as similar for all studied zeolites, which was in agreement with Xu et al. [72] and did not correlate with catalytic activity and selectivity.It was also shown [33,67\u201369] that porous structure and morphology of the investigated catalysts influenced the catalytic properties in given reaction. Medium pore zeolites, such as H-MFI, H-BEA and H-MOR demonstrated the highest selectivity to diethyl ether (98%) at moderate temperatures (180\u00a0\u00b0C). It may be concluded that zeolites of medium size channels are the most suitable for this reaction than counterparts possessing either larger or smaller cavities (faujasites or ferrierite, respectively). That implies from a confinement effect favouring the production of diethyl ether at lower temperatures and the shape selectivity enabling the formation of ethylene at higher temperatures. It may be also explained by the different kinetic behaviour of these two reactions, namely, in higher activation energy and lower ethanol reaction order towards ethylene formation in relation to DEE.Almost full selectivity to ethylene and yield were obtained at high temperature over small-pore H-FER and on large-pore H-Y and H-USY [69]. In the case of medium-pore zeolites like H-MFI, H-MOR and H-BEA, the selectivity to ethylene was limited by the production of higher hydrocarbons and coke.Detailed mechanism of ethanol dehydration on protonic form of zeolites with MOR, MFI and FER type structures was described by Phung, Chiang et al. [33,67]. It was explained that a selective production of diethyl ether from ethanol occurs at lower temperature by the reaction of ethoxy groups with undissociated ethanol. At higher temperatures, the formation of ethylene is going via decomposition of ethoxy groups on catalysts containing active acid sites.Osuga et al. [73] also investigated ethanol dehydration over MFI- and MOR-type zeolites and under different contact time conditions. DEE was found as one of the initial reaction products, influencing the catalytic activity. At the same catalyst weight / ethanol flow ratio (24.3\u00a0g\u00b7h/mol), for MFI zeolite, DEE was detected as dominating product with selectivity reaching 100%, whereas ethene was produced over mordenite with selectivity reaching 60%. Therefore, it may be concluded that the dehydration of ethanol over zeolites with two different structure types can occur via different reaction routes.According to Madeira et al. [74], both the acidity and porous structure of zeolites determined their catalytic behaviour in the ethanol conversion into hydrocarbons possessing three carbon atoms or more (C3+). It was indicated that high pore size H-FAU and H-BEA type zeolites were characterized by a high yield of ethylene and diethyl ether as well as low contribution of C3+ hydrocarbons in comparison with medium pore zeolite HZSM-5. This observation can be explained by a faster deactivation of large pore zeolites, which was caused by the formation of coke, eliminating protonic acidity, responsible for the transformation of ethylene into higher hydrocarbons. For H-ZSM-5 zeolite, after 16\u00a0h of reaction, practically full conversion of ethanol towards C3+ hydrocarbons (including butenes, parafins and aromatics) was found. In turn, in the case of large pore zeolites (H-BEA, H-FAU), high amounts of by-products like more condensed aromatics were detected. Furthermore, it was found that for H-ZSM-5 zeolite, the deactivation was slower and the production of C3+ hydrocarbons was found even under the saturation of the catalyst with coke molecules, thus it may be supposed that for this zeolite, reaction could take place at the pore mouth of the channel.According to D\u00e4umer et al.[75], the formation of C3+ hydrocarbons is favoured in the zeolites of relatively narrow cavities, thus the acid sites strength present in micropores plays a supporting role in the production of higher hydrocarbons. It was also reported that the formation of C\u2013C bonds was strongly dependent on the presence of highly strong Br\u00f8nsted acid sites. In turn, the intermediate species stability in zeolite framework depended on interaction between fragments of confined hydrocarbon and zeolite framework [76]. The size of cavities influences the stability of the confined species defined by an electrostatic and van der Waals interaction between fragments of hydrocarbons and zeolite framework.Our findings are also in line with Go\u0142\u0105bek et al. [77], who investigated the role of pore arrangement of 10-ring zeolites (ZSM-5, TNU-9 and IM-5) on their catalytic properties in ethanol transformation. From obtained data, it was concluded that all studied catalysts were active at 150\u2013300\u00a0\u00b0C and at atmospheric pressure, leading to the production of diethyl ether (DEE) and ethylene as the products. It was also shown that the conversion of ethanol increased with reaction temperature. ZSM-5-based catalysts did not undergo deactivation and small and uniform ZSM-5 crystals did not affect catalytic lifetime.Based on available literature, a beneficial impact of the application of ultrasonic irradiation in the synthesis of zeolite-based materials on their physicochemical and catalytic properties has been reported. For instance, Oruji et al. [30] performed ultrasonic-assisted desilication of NaY zeolite in order to prepare hierarchical materials with elevated mesoporosity and higher crystallinity than zeolites treated conventionally. Independently of the used technique, the mesoporosity of desilicated FAU-type zeolite gradually increased with the duration of procedure and was higher for the samples modified sonochemically. In the reaction of catalytic cracking of middle distillate at 550\u00a0\u00b0C, the it was revealed that all sonochemically treated samples demonstrated higher catalytic activity with their high liquid and low gas yields (78\u201386% and 14\u201322%, respectively). Coking was practically absent. Furthermore, it was shown that the catalyst lifetime for sonochemically prepared materials was higher than for the samples treated conventionally due to more apparent destruction of microporosity with simultaneous more noticeable generation of mesoporosity in zeolite.Another example of beneficial influence of ultrasounds during catalysts preparation is an ultrasonic-assisted deposition of active phase on zeolite carrier, which was reported in our previous papers [78-80]. Jod\u0142owski et al. [78] reported that sonochemically prepared structured reactors with a deposited copper on ZSM-5 and USY zeolite revealed a full NO conversion and almost constant 100% selectivity to nitrogen in SCR- DeNOx reaction. In turn, Chlebda et al. [79] indicated that ultrasonic procedure of iron containing ZSM-5-based catalysts preparation enhanced catalytic activity in the DeNOx process, with almost full selectivity to N2. Sobu\u015b et al. [80] reported that Cu- and Co containing BEA zeolites revealed the best catalytic properties in the Selective Conversion of Lactic Acid into Acrylic Acid reaction with the selectivity to Acrylic Acid exceeding 60%.In this research, we investigated an ultrasonic-assisted desilication of zeolite with MFI type structure using aqueous NaOH/TBAOH alkaline solutions of various chemical compositions. Subsequently, we compared the physicochemical and catalytic properties of such prepared samples with counterparts desilicated in the absence of ultrasounds.It was shown that the application of ultrasounds during demineralization procedure caused higher both silicon and aluminum extraction in comparison with analogues treated classically. The Si and Al contents leached from MFI-type zeolite framework were in the range of 0.5\u201315.3% and 0.1\u20138.6%, respectively. Si/Al ratio of the modified MFI-based samples was in the range of 32.4\u201337.6 and was only slightly lower in respect to the reference sample (37.7). Alkaline treatment of MFI-type zeolite resulted in the formation of \u201cswiss cheese\u201d - type zeolite grains containing numerous holes inside zeolite crystallites, which was more visible for the zeolites modified under ultrasonic-assisted procedure.Surprisingly, we observed that the conducting of demineralization procedure (independently of the presence/absence of ultrasonic irradiation) did not alter the crystallinity, structure and morphology of the modified materials. Nevertheless, the application of ultrasounds during alkaline treatment procedure led to the production of higher mesoporosity, which enabled better mass transfer of reagents in porous structure and therefore caused enhanced catalytic properties in the reaction of the dehydration of ethanol in relation to the catalysts obtained under conventional demineralization conditions.Furthermore, it was indicated that independently of the alkaline treatment technique (conventional vs. ultrasonic), a notable decrease of both protonic and Lewis acidity corresponded to a simultaneous leaching of Si and Al from the structure of MFI-type zeolite.The analysis of the results presented above led to the conclusion that ultrasonic assisted demineralization of MFI-type zeolite led to the production of attractive catalysts with easily accessible active sites for the ethanol processing.\n\u0141. Kuterasi\u0144ski: Conceptualization, Data curation, Formal analysis, Methodology, Software, Validation, Supervision, Visualization, Writing - original draft, Writing - review & editing. U. Filek: Formal analysis, Investigation, Methodology. M. Gackowski: Formal analysis, Investigation, Methodology, Visualization, Writing - review & editing. M. Zimowska: Formal analysis, Investigation, Methodology. M. Ruggiero-Miko\u0142ajczyk: Formal analysis, Investigation, Methodology. P.J. Jod\u0142owski: Formal analysis, Investigation, Methodology, Visualization, 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 financed by the statutory funds from the Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences.Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2021.105581.The following are the Supplementary data to this article:\n\nSupplementary data 1\n\n\n\n", "descript": "\n                  In this paper, the ultrasonic-assisted desilication technique was reported as an attractive and efficient way for the preparation of hierarchical zeolites with MFI structure type. The prepared materials were used as active catalysts for the dehydration of ethanol into diethyl ether and ethylene. For all catalysts, the selectivity to diethyl ether was ca 95% or higher up to 210\u00a0\u00b0C, with catalytic activity in the range of 40\u201368%. In case of desilicated zeolites, at 270\u2013290\u00a0\u00b0C, the conversion of ethanol was full with selectivity to ethylene ca 80%. MFI-type commercial zeolite was treated with a sodium and/or tetrabutylammonium hydroxide aqueous solutions (NaOH or NaOH/TBAOH) for 30\u00a0min. In the case of the application of ultrasounds, a QSonica Q700 sonicator (60\u00a0W and 20\u00a0kHz) equipped with a \u201c1\u201d diameter horn was used. In all cases, desilication was performed in an ice bath in order to keep the procedure conditions at low temperature.\n                  It was indicated that the use of ultrasounds during desilication procedure caused higher extraction of silicon and aluminum, which was connected with an elevated mesoporosity in relation to the samples modified in the absence of ultrasounds. Ultrasonic-assisted treatment of MFI-type zeolite caused also an apparent formation of numerous holes inside zeolite grains, resembling the look of \u201cswiss cheese\u201d. Furthermore, it was indicated that the samples prepared using ultrasonic irradiation exhibited enhanced catalytic properties in the dehydration of ethanol. For instance, MFI-type zeolite treated with NaOH/TBAOH alkaline mixture containing 10\u00a0mol% of TBAOH in the presence of ultrasounds (M\u221210\u00a0s) demonstrated higher both conversion of ethanol (59% vs. 47%) and selectivity to diethyl ether (95% vs. 93%) in comparison with zeolite modified conventionally (M\u221210c).\n                  The best catalyst was zeolite ultrasonically desilicated with NaOH/TBAOH solution of 70 mol% of TBAOH (M-70s). Generally, this catalyst indicated the highest conversion of ethanol, very high selectivity to diethyl ether (94-100%) at 150-210 \u00a0\u00b0C and the highest selectivity to ethylene among investigated catalysts (21%, 66% and 84%) at 230 \u00a0\u00b0C, 250 oC and 270 \u00a0\u00b0C.\n               "}
{"full_text": "The C\u2013C coupling reactions have attracted significant attention since their discovery and have been one of the most important research areas [1]. These reactions typically require the use of precious metal catalysts (e.g., Pd [2,3], Au [4] and Ru [5]) and sometimes significant excess of reductants. In this regard, the economical and less toxic Ni catalysts have drawn increasing attention [6\u201310]. The design of catalysts and related techniques to overcome these drawbacks is thus of utmost importance. Electrochemical transformations have a high potential to address these issues [11\u201315]. Several remarkable electrochemical coupling reactions for the C\u2013C [16\u201320], C\u2013O [21,22], C\u2013N [23\u201325], C\u2013S [26], N\u2013S [27] and N\u2013P [28] bond formation have been developed, but to the best of our knowledge, less attention has been paid to the homocoupling reactions using cyclic voltammetry methods [29\u201333].Different pathways are possible for metal-mediated radical formation from aryl and benzyl halides. Among them, single electron transfer pathways, either outer-sphere or inner-sphere mechanism, have been proposed [34\u201336]. Electron transfer proceeds from metal to the aryl/benzyl halides to form radical, followed by C-X bond cleavage [10,37,38]. Recent DFT calculations on several Ni0 and NiI systems propose the concerted halogen-atom-abstraction pathway and most of the transition-metal catalyzed coupling reactions reported so far require metal center reduction to its formal zero valent state [39\u201341]. In the case of Ni-based systems, the commonly found reduced metal species is Ni0. In very few systems, the coupling goes through a formal NiI species, probably because it is difficult to control the reduction by using metal powders. On the other hand, while low-valent NiI species is difficult to isolate due to its inherent instability, such type of species can be generated by electrochemical methods and employed in coupling reactions.We have developed various PN3(P) pincer ligand architectures and have demonstrated that their metal complexes often offer some unique reactivities because of their different kinetic and thermodynamic properties compared to those of analogs with CH2 spacers [42\u201349]. Herein, we report a PN3P\u2013Ni complex (1) (Fig.\u00a01\n) for benzyl halide homocoupling reactions under electrochemical conditions. We present electrochemically, for the first time, that two different formal oxidation states (0 and\u00a0\u200b+1) are catalytically active in the presence of benzyl chloride and bromide, respectively.[Ni(LPN3P) (Cl)]Cl (1) was synthesized by mixing the PN3P ligand and NiCl2 in THF and recrystallized in MeOH (Fig.\u00a01) [48]. The purity was confirmed by NMR, HRMS, elemental analysis, and UV\u2013vis (Figs.\u00a0S1-S5, Tables\u00a0S1 and S2 in Supporting information). The crystal structure shows that the NiII center is bound to the N atom (pyridine ring), two P atoms of two di-tert-butylphosphine arms and one chloride anion (the other noncoordinating chloride anion resides outside the metallo-ligand to balance charge) to give a slightly distorted square planar geometry, where the metal center has no deviation from the plane (d\nNi\u00a0\u200b=\u00a0\u200b0\u00a0\u200b\u00c5; d\nplane\u00a0\u200b=\u00a0\u200b0\u00a0\u200b\u00c5). As we have previously showed that POCOP\u2013Ni complexes could catalyze the homocoupling reaction of a series of benzyl halide derivatives by using zinc dust as a reductant at 115\u00a0\u200b\u00b0C (Scheme 1\n) [50], complex 2 was chosen for benchmarking the activity under electrochemical conditions.Cyclic voltammograms (CV) of complexes 1 and 2 using glassy carbon as a working electrode displayed a quasi-reversible reduction wave at \u22121.49\u00a0\u200bV (\u0394E\np\u00a0\u200b=\u00a0\u200b66\u00a0\u200bmV) and \u22121.33\u00a0\u200bV (\u0394E\np\u00a0\u200b=\u00a0\u200b80\u00a0\u200bmV) vs. Fc+/0 in DMF, respectively (Fig.\u00a02\nA), which corresponded to a NiII/I redox couple, consistent with values reported for other Ni complexes [51\u201354]. Additionally, a quasi-reversible reduction wave was observed at a more negative potential at \u22122.06\u00a0\u200bV (\u0394E\np\u00a0\u200b=\u00a0\u200b110\u00a0\u200bmV) vs. Fc+/0 for 1 [53,55]. No additional wave was observed for complex 2 when further scanning was done to more cathodic potentials, indicating that the associated redox process is kinetically hindered (Fig.\u00a0S6 in Supporting information). The event observed at a more negative potential of complex 1 can be attributed to a NiI/0 redox couple, suggesting the PN3P ligand may stabilize the Ni0 oxidation state. Coulometric study of the reduction of 1 confirmed that the first reduction wave corresponds to a one-electron reduction process. The CV of the NiII/I process was obtained at different scan rates to reveal a linear relationship, confirming a diffusion controlled process with the diffusion coefficient determined to be 1.5\u00a0\u200b\u00d7\u00a0\u200b10\u22127\u00a0\u200bcm2/s by the Randles-Sevcik equation (Fig.\u00a0S7 in Supporting information).Spectroelectrochemical data supported the generation of a single-step one-electron transfer at \u22121.60\u00a0\u200bV, and the cathodic potential induced a gradual decrease (by ~25%) in the LMCT band at 318\u00a0\u200bnm, while the bands around 459\u00a0\u200bnm shifted to 474\u00a0\u200bnm (Fig.\u00a0S8 in Supporting information). The depletion of the optical band at 318\u00a0\u200bnm, along with a decrease of the 274\u00a0\u200bnm band and the formation of a new band at 355\u00a0\u200bnm\u00a0\u200bat the same solution, suggest a possible one-electron reduction of 1, attributed to the reduction of NiII to NiI.In order to elucidate this reduction process and to address the limited solubility of complex 1, a counterion exchange reaction with NaBPh4 was conducted to afford complex 3 (~95%) (Scheme 2\n and Fig.\u00a03\n, Figs.\u00a0S9-S12 and Tables\u00a0S1 and S2 in Supporting information). Complex 3 showed similar redox behaviors in CV that observed for 1 in DMF (Fig.\u00a02A, violet line). The observed similar NiII/I and NiI/0 redox couples suggest that there is no substantial effect of BPh4 to the NiII center. Chemical reduction of 3 with 1 equiv. of cobaltocene in dry and degassed THF led to the formation of complex 4 (Figs.\u00a0S13 and S14 in Supporting information). The optical spectrum observed after stoichiometric reduction of complex 3 by cobaltocene is analogous to that of reduced 1. The EPR spectrum of complex 4 in acetone was essentially identical to that obtained by electrochemical reduction of 3 in acetonitrile. The X-band EPR spectrum of this species exhibit a rhombic signal, with g\n\nx\n\u00a0\u200b=\u00a0\u200b2.21; g\n\ny\n\u00a0\u200b=\u00a0\u200b2.11; and g\n\nz\n\u00a0\u200b=\u00a0\u200b2.02, which confirms an S\u00a0\u200b=\u00a0\u200b\u00bd ground state at 100\u00a0\u200bK (Fig.\u00a02B, red line). The spectrum shows distinct splitting from the pincer ligand donor atoms as well as the chlorine [56]. The g-values and splitting constants for complex 4 is summarized in Table\u00a01\n. The deviation of the g-values from g\ne\u00a0\u200b=\u00a0\u200b2.002\u00a0\u200bat low temperature concludes that the singly occupied molecular orbital (SOMO) is principally Ni-based. Moreover, the computational analysis of the spin density at DFT level with the wB97X-D functional indicates the radical localized at the Ni center (Fig.\u00a02C), more details are deposited in Supporting information).Our further electrochemical investigations on catalysts 1 and 2 in the presence of benzyl halides, showed clear indication of catalysis. Addition of benzyl chloride (BnCl) in the electrolytic solution containing 1.0\u00a0\u200bmmol/L of catalyst 1 showed a gradual increase of catalytic current (up to 20.0\u00a0\u200bmmol/L of substrate addition). The onset potential in this case (\u22122.07\u00a0\u200bV vs. Fc+/0) overlays with the NiI/0 process (Fig.\u00a04\nA), strongly suggesting that the Ni0 state could catalyze the homocoupling of BnCl, independent of the NiII/I process. Very interestingly, in the presence of benzyl bromide (BnBr), a large electrocatalytic current was observed with an earlier onset potential at \u22121.38\u00a0\u200bV vs. Fc+/0 (Fig.\u00a04B), indicating that NiI state is the active species to catalyze the homocoupling of BnBr, in sharp contrast to that for BnCl. In this context, it is worthy to mention that, the difference in onset reduction potentials (0.69\u00a0\u200bV vs. Fc+/Fc) between BnCl and BnBr is comparable to that of direct electrochemical reduction of these substrates at a GC electrode in MeCN (0.45\u00a0\u200bV vs. SCE) [57]. The catalytic current (i\ncat) shows a linear correlation with the concentration of catalyst (Fig.\u00a0S15 in Supporting information), which confirms that the catalytic process is mononuclear.Complex 2 exhibits a large electrocatalytic current in the presence of BnBr with an onset potential at \u22121.48\u00a0\u200bV vs. Fc+/0 (Fig.\u00a04D), close to the redox couple NiII/I. However, in the presence of BnCl, no enhancement of current was observed with further scanning of reduction potential (Fig.\u00a04C). These observations imply that Ni0 oxidation state is essential to catalyze the homocoupling reaction of BnCl. The catalysis is absent for blank GC electrode at 10\u00a0\u200bmmol/L substrate in electrolytic solution without catalyst (Fig.\u00a04, black line).The catalytic rate to form the homocoupling product with Ni0 or NiI states for benzyl halides can be determined if i\ncat/i\np are analyzed at a fixed substrate concentration (20.0\u00a0\u200bmmol/L), where the catalytic current plateaus. The pseudo-first order rate of the catalysis is determined by the equation: i\ncat/i\np\u00a0\u200b=\u00a0\u200b0.72 (k[substrate]/\u03bd)1/2 (see Supporting information for details) [58], where i\ncat is the peak current of catalysis in the presence of the substrate, i\np is the peak current of NiI and Ni0 species without the substrate obtained from the CV data, \u03bd is the scan rate in V/s, and n\ncat\u00a0\u200b=\u00a0\u200b2 is the number of electrons transferred in each catalytic cycle (Table\u00a02\n). The TOF for BnCl to C\u2013C coupling product was determined from the scan rate dependence study and was found to be 0.43 s\u22121 (Fig.\u00a05\nA and Fig.\u00a0S16 in Supporting information) with a rate constant of 17.0\u00a0\u200bL\u00a0\u200bmol\u22121 s\u22121 (Figs.\u00a0S17 and S18 in Supporting information). Changing the substrate to BnBr resulted in an increase in TOF to 19.5 s\u22121 (Fig.\u00a05B and Fig.\u00a0S19 in Supporting information) with a rate constant of 1642\u00a0\u200bL\u00a0\u200bmol\u22121 s\u22121 (Figs.\u00a0S20 and S21 in Supporting information) for catalyst 1. Similar method is applied for catalyst 2 (Figs.\u00a0S22-S24 in Supporting information).Controlled potential electrolysis at \u22121.1\u00a0\u200bV vs\n. Ag/AgCl on a graphite electrode was examined with 1.0\u00a0\u200bmmol/L catalyst loading along with BnBr (10.0\u00a0\u200bmmol/L, 0.13\u00a0\u200bmmol) (Fig.\u00a0S25 in Supporting information). After 3\u00a0\u200bh of electrolysis, the current slightly decreased but maintained a steady state value, consistent with the consumption of the substrate. Bibenzyl formed in 81% isolated yield after bulk-electrolysis (confirmed by 1H NMR, Fig.\u00a0S26 in Supporting information), with a Faradaic yield of 98%. For BnCl and derivatives, the electrolysis was carried out at \u22121.7\u00a0\u200bV vs. Ag/AgCl for 3\u00a0\u200bh (Fig.\u00a0S27 in Supporting information). The stability of our Ni catalyst was checked before and after the controlled potential electrolysis by 31P NMR, HR-MS and UV\u2013vis spectroscopy (Figs.\u00a0S28-S30 in Supporting information). Electrodes including glassy carbon (GC) after multiple CVs gave no catalytic response in a fresh, catalyst-free electrolyte in DMF, suggesting no catalyst deposition on the surface of GC under these electrochemical conditions. After 3\u00a0\u200bh of electrolysis with graphite rod electrode, no catalyst decay was observed, indicating a robust nature of the catalyst. Indeed, even with the increasing initial substrate concentration (50.0\u00a0\u200bmmol/L), no obvious decay was detected after 14\u00a0\u200bh of electrolysis. However, the optical data shows a significant decomposition of 2 after 3\u00a0\u200bh of electrolysis in the presence of substrate (Fig.\u00a0S31 in Supporting information). Accordingly, a series of substrates were further studied. A significant improvement in yields was observed when the substrate was changed from BnCl to BnBr under similar conditions (Table\u00a0S3 in Supporting information). After electrolysis for 15\u00a0\u200bh in the presence of BnCl, the yield of coupling product reached 90%, consistent with experimental results with zinc dust.From the detailed analysis of the cyclic voltammogram and chronoamperometry of 1, evidently PN3P ligand must stabilize the low-valent Ni species, which is necessary for the C-X bond activation of substrate. Based on these observations, a catalytic cycle for the coupling reaction was proposed (Fig.\u00a06\nA). The low-valent Ni species (Ni0 or NiI for BnCl and BnBr, respectively) is generated at the cathode at a respective potential which reacts with benzyl halides and abstract a halogen atom from the benzyl halide molecule to cleave the C-X bond to afford a NiI or NiII intermediate along with a benzyl radical which dimerizes to form bibenzyl [59]. The resulting high valent Ni species immediately take one electron from the electrode with loss of a halide ion to regenerate the low-valent catalyst to continue the catalytic cycle. Additionally stoichiometric treatment of 4 with BnBr at room temperature results in complete conversion to bibenzyl within 5\u00a0\u200bmin. In sharp contrast, performing the reaction with BnCl did not generate the coupling product. The proposed mechanism is also supported by the trapping experiment of the benzyl radical with TEMPO (Fig.\u00a06B). No bibenzyl product was observed, but TEMPO-Bn (Fig.\u00a0S32 in Supporting information) [60].In conclusion, we have unambiguously elucidated the role of NiI and Ni0 for successive homocoupling of benzyl bromides and benzyl chlorides. For the first time, two different formal oxidation states of the catalytically active Ni species (0 and\u00a0\u200b+1) were determined for BnCl and BnBr, presumably due to different bond strengths of C-X bonds. Alternatively, it is also possible that in the case of BnCl, a more reduced Ni species (Ni0) is required to facilitate electron transfer in order to cleave C\u2013Cl bond. Accordingly, higher yields of the homo-coupling products from BnBr and derivatives were achieved when the reaction time of electrolysis was restricted to 3\u00a0\u200bh. Our findings show that catalyst 1 containing PN3P-ligand is an incredibly effective catalyst for the homocoupling reactions. The onset potential for the coupling reaction of catalyst 1, in the presence of BnBr, is less negative than catalyst 2. CV experiments confirmed that NiI/0 redox couple was not observed under electrochemical condition for catalyst 2, not catalyze the coupling reaction of BnCl. Above all observations of the PN3P system indicate its distinct kinetic and thermodynamic properties compared to their POCOP analogs. We have further demonstrated the reaction kinetics and mechanism. Our finding reveals a strong ligand effect on the reactivity and selectivity of the reaction. Investigations on the potential applications for other classes of substrates are currently ongoing and will be reported in due course.All experiments with metal complexes and phosphine ligands were performed under an argon atmosphere in a glovebox or using standard Schlenk techniques. All the solvents were purified before use. For C\u2013C coupling reaction, all the substrate were degassed before use. Column chromatography purifications were performed by flash chromatography using Merck silica gel 60. All other reagents were used as received. 1H, 13C, and 31P NMR spectra were recorded using Bruker AVIII 400, AVIII 500 or AVIII 600 spectrometers. EPR spectra was recorded using X-band continuous wave Bruker EMX PLUS spectrometer (BrukerBioSpin, Rheinstetten, Germany) equipped with standard resonator for high sensitivity CW-EPR operating with a microwave frequency. A typical Evans measurement was done in a coaxial tube containing the solvent and the internal standard. The absorption spectra were measured in an Agilent 8453 UV\u2013visible spectrophotometer. Chemical shifts in 1H NMR and 13C NMR were reported in parts per million (ppm). The residual solvent peak was used as an internal reference: 1H NMR (DMSO-d\n5 in DMSO\u2011d6\n, \u03b4 2.54, chloroform in CDCl3 7.26) and 13C NMR (DMSO\u2011d6\n, \u03b4 40.45). 31P NMR chemical shifts are reported in parts per million downfield from H3PO4 and referenced to an external 85% solution of phosphoric acid in D2O. Multiplicity was indicated as follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), dd (doublet), br s (broad singlet). Coupling constants were reported in hertz (Hz). Elemental analyses were performed at the KAUST analytical core lab using a Flash 2000d Thermo Scientific CHNS Analyzer. PN3P ligand was synthesized using reported procedure [61].The PN3P ligand (1.00\u00a0\u200bg, 2.50\u00a0\u200bmmol) was weighed in a Schlenk flask along with NiCl2 (596\u00a0\u200bmg, 2.50\u00a0\u200bmmol), 15.0\u00a0\u200bmL of dry THF was added, and the solution was stirred overnight. Orange product insoluble in THF was obtained after drying off the solvent under vacuum. It was then washed with THF. Crystallization was done by dissolving orange product in MeOH. Orange solid; 70% yield. 1H NMR (600\u00a0\u200bMHz, DMSO\u2011d6\n): 7.60 (t, 1H, J\u00a0\u200b=\u00a0\u200b12\u00a0\u200bHz), 7.24 (m, 2H), 6.44 (d, 2H), 3.60 (t, 3H, J\u00a0\u200b=\u00a0\u200b12\u00a0\u200bHz), 3.36 (s, 12H), 2.30 (s, 1H), 1.51 (t, 36H, J\u00a0\u200b=\u00a0\u200b12\u00a0\u200bHz); 13C NMR (151\u00a0\u200bMHz, DMSO\u2011d6\n): 162.6 (t, NC\u2013NH), 141.7 (s, pyridine ring carbon), 97.9 (d, pyridine ring carbon), 34.1 (d, (CH3)3\u2013C), 28.3 (d, (CH3)3\u2013C); 31P NMR (242.93\u00a0\u200bMHz, DMSO\u2011d6\n): 99.03 (s); HRMS (ESI): calcd. for C21H41N3P2Cl1Ni, m/z 491.18, found 491.189. Anal. Calcd. for C21H41N3P2Cl2Ni: C, 47.85; H, 7.84; N, 7.97. Found: C, 47.80; H, 7.81; N, 7.95. UV\u2013vis (THF, [1\u00a0\u200b\u00d7\u00a0\u200b10\u22124]), \u03bb\nmax, nm (\u03b5, L mol\u22121 cm\u22121): 277 (3568), 324 (12,144), 357 (3896), 464 (914).To a solution of 1 (50\u00a0\u200bmg, 0.094\u00a0\u200bmmol) in 10\u00a0\u200bmL of dry THF in glovebox at room temperature was added sodium tetraphenylborate (32.5\u00a0\u200bmg, 1.00 equiv.), and the solution was stirred for 1\u00a0\u200bh. Then, the solution was filtered through a small Celite plug and concentrated under reduced pressure to afford orange solid (73\u00a0\u200bmg, 95%). Complex show broadened signals for \nt\nBu groups of the ligand arm in the 1H NMR spectra due to hindered rotation. Orange solid; 70% yield. 1H NMR (400\u00a0\u200bMHz, CD3OD): 7.32 (br), 6.98 (br), 6.83 (br), 6.30 (br), 3.75 (br), 3.33 (br), 1.88 (br), 1.56 (br); 13C NMR (125.75\u00a0\u200bMHz, CD3OD): 163.80 (m, BPh4), 142.46 (s, NC\u2013NH), 135.39 (s, pyridine ring carbon), 121.37 (s, BPh4), 125.15 (s, BPh4), 97.95 (s, pyridine ring carbon), 27.39 (s, (CH3)3\u2013C), 24.99 (s, (CH3)3\u2013C); 31P NMR (202.45\u00a0\u200bMHz, CD3OD): 98.72 (s); Anal. Calcd. for C45H61N3P2Cl1B1Ni: C, 66.65; H, 7.58; N, 5.18. Found: C, 66.60; H, 7.50; N, 5.10. UV\u2013vis (THF, [1\u00a0\u200b\u00d7\u00a0\u200b10\u22124]), \u03bb\nmax, nm (\u03b5, L mol\u22121 cm\u22121): 267 (sh, 10,626), 274 (sh, 8023), 291 (sh, 5292) 317 (13,077), 353 (5278), 467 (1530).Method 1: Chemical reduction using CoCp2. To a solution of 3 (20\u00a0\u200bmg, 0.024\u00a0\u200bmmol) in dry THF was added cobaltocene (5\u00a0\u200bmg, 1.00 equiv.) at room temperature. The solution turns red, and after 2\u00a0\u200bh of stirring, the mixture was filtered through a Celite plug. The THF was evaporated to afford a red solid (8.9\u00a0\u200bmg, 80% yield). Complex in dry acetone was characterized by EPR spectroscopy (see main text). 1H NMR (600\u00a0\u200bMHz, C6D6): 8.09 (br s), 5.96 (br s), 1.12 (br t), \u221250.83 (br s). UV\u2013vis (THF, [1\u00a0\u200b\u00d7\u00a0\u200b10\u22124]), \u03bb\nmax, nm (\u03b5, L mol\u22121 cm\u22121): 275 (sh, 15,755), 321 (sh, 11,199), 342 (13,410), 397 (2638), 470 (1112); Method 2: Generation of NiI complex by electrochemical reduction. NiI complex was prepared via controlled potential electrolysis at \u22121.55\u00a0\u200bV vs. Fc+/0 in acetonitrile. A constant voltage was applied using graphite rod working electrode, a platinum wire counter electrode and an Ag/AgCl/1\u00a0\u200bmol/L KCl reference electrode to orange solution of 3 (1.0\u00a0\u200bmmol/L of complex using 0.1\u00a0\u200bmol/L \nn\nBu4NPF6 as the electrolyte in dry and degassed CH3CN). The voltage was applied for half an hour until the current measures was less than 1% of the starting current. At this point, the solution had become red. The nature of the product formed was confirmed by comparing its low-temperature EPR spectra and g tensor values with those from the chemically reduced 4.The change in the oxidation states of complex 1 in organic media was investigated via spectroelectrochemical technique. Chronocoulometric experiments were performed on compound 1, where it was reduced electrochemically at \u22121.60\u00a0\u200bV vs. Fc+/0 over time, and the gradual change in its optical spectra was monitored. The electrochemical reductions were performed at a slightly cathodic direction compared to the reduction peak maxima to ensure the complete reduction during chronocoulometry.The bulk electrolysis experiments of complexes 1 and 2 were performed in a four-neck glass vessel (volume 10\u00a0\u200bmL including the headspace) where three of the necks were occupied with a coiled Pt wire as counter electrode, Ag/AgCl (in saturated KCl) as a reference electrode, and a carbon rod (surface area 1.7\u00a0\u200bcm2) as working electrode, respectively. The last of the necks was connected with argon flow tubing. During the experiment, 10\u00a0\u200bmL of complexes (1.0\u00a0\u200bmmol/L catalyst, 10.0\u00a0\u200bmmol/L substrate) were added in the vessel, all the electrodes (along with a magnetic bead) were inserted along with a rubber septum cap (in a gastight manner), and the solution was purged with argon for 30\u00a0\u200bmin. Then, the purging was stopped and the chronocoulometric experiment was started at corresponding catalytic potentials (\u22121.1\u00a0\u200bV vs. Ag/AgCl for complex 1 in the presence of BnBr). The solution was continuously stirred with a magnetic stirrer during the experiment.We acknowledge the generous financial support from King Abdullah University of Science and Technology (KAUST).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.gresc.2020.10.001.", "descript": "\n                  We present the mechanistic understanding of an electrochemically-driven nickel-catalyzed coupling reaction. Computational analysis reveals that the spin density is mostly residing on the nickel (Ni) center when NiII is reduced to NiI. Ni-mediated halogen atom abstraction through outer-sphere electron-transfer pathway to yield coupling products under mild conditions is demonstrated. Importantly, we have elucidated the role of NiI and Ni0 for successive coupling of benzyl bromide and benzyl chloride derivatives, respectively, to corresponding bibenzyl products. The Ni-catalyst bearing a PN3P-ligand is an effective catalyst, producing a strong ligand effect on the reactivity and selectivity for the homocoupling reactions.\n               "}
{"full_text": "Furfural, obtained via acid-catalyzed dehydration of pentose such as xylose and arabinose or via fast pyrolysis of biomass, is one of the most promising biomass-derived platforms as a building block in the bio-refinery approach [1,2]. Selective hydrogenation of furfural has gained much attention for the production of furfuryl alcohol (FA), an important intermediate for the manufacturing of lysine, ascorbic acid (vitamin C), plasticizers, dispersing agent, and lubricants and is mainly used in the manufacture of resins, synthetic fibers, and agrochemicals [3]. In furan industrial process, copper chromite has traditionally been used as the catalyst for hydrogenation of furfural to FA [2,3]. However, concern of the environmental toxic impact due to the chromium presence in the catalysts led to the development of chromium free catalysts.Platinum-based catalysts are known as highly efficient catalysts used in the hydrogenation of furfural to FA because of their ability to hydrogenate the CO under mild conditions [3\u20135]. Pt nanoparticles in the range of 3\u20137\u00a0nm were found to favor the hydrogenation of furfural to FA whereas Pt particle size less than 3\u00a0nm promoted the decarbonylation of furfural to furan [3,6]. However, modification of Pt catalysts by alloying [7,8], adding promoters [7,9], and by using strong metal-support interaction to induce electronic effects [8,10] show significant impact on both activity and selectivity towards FA of the catalysts in furfural hydrogenation. For examples, The metal-support interaction on Pt/TiO2 induced hydrogen spillover, leading to the formation of furfuryl-oxy intermediate on the titania support [11,12]. Synergetic effect between Pt and Co on carbon obtained by co-impregnation was reported to be beneficial in furfural hydrogenation and product selectivity can be adjusted depending on the weight ratio of Pt and Co [13,14].In this study, monometallic Pt/TiO2 and bimetallic Pt-Co/TiO2 were prepared by flame spray pyrolysis (FSP) and studied in the liquid-phase selective hydrogenation of furfural to FA under mild conditions. The characteristics of Pt-Co/TiO2 formed by one-step FSP were quite different from those obtained by co-impregnation as illustrated by various characterization techniques such as CO pulse chemisorption, H2-temperature-programmed reduction (H2-TPR), transmission electron spectroscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy of adsorbed CO. A structural-activity relationship of these FSP-PtCo/TiO2 catalysts was proposed.The Pt/TiO2 (0.7\u00a0wt% Pt) and Pt-Co/TiO2 (0.7\u00a0wt% Pt and 0\u20130.4\u00a0wt% Co) catalysts were prepared by FSP method according to the procedure described in Ref. [15] using platinum (II) acetylacetonate (Pt(C5H7O2)2, 99.99%, Aldrich), cobalt naphthenate (CoC22H14O4, 6\u00a0wt% in mineral spirits, Aldrich), and titanium (IV) butoxide (Ti(OCH2CH2CH2CH3)4, 97%, Aldrich) as Pt, Co, and TiO2 precursors, respectively. The catalysts prepared by FSP are denoted as (F) Pt/TiO2 and (F) Pt-Co/TiO2, respectively. The TiO2 supported Pt and PtCo catalysts were also prepared by incipient wetness impregnation using platinum (II) acetylacetonate (Pt(C5H7O2)2, 99.99%, Aldrich), cobalt naphthenate (CoC22H14O4, 6\u00a0wt% in mineral spirits, Aldrich) as Pt and Co precursors and P-25 TiO2 as the support. The catalysts were dried in an oven at 100\u00a0\u00b0C overnight, and calcined in air at 500\u00a0\u00b0C for 2\u00a0h. The impregnated catalysts are denoted as (I) Pt/TiO2 and (I) Pt-Co/TiO2, respectively.The XRD patterns were recorded using a Bruker D8 Advance X-ray diffractometer with Ni-filter CuK\u03b1 radiation. The actual amount of Pt and Co loadings in the samples were analyzed by AAS. The N2 physisorption was conducted by using a Micrometrics ASAP 2020 instrument with the Brunauer\u2013Emmett\u2013Teller (BET) method. The TEM observations were performed with a JEOL JEM 2010 transmission electron microscope operated at 200\u00a0kV and equipped with LaB6 thermoionic electron-gun, an UHR pole piece (point resolution 0.196\u00a0nm) and a Pentafet Link-INCA EDX spectrometer (Oxford Instruments). The percentages of Pt dispersion were measured by CO pulse chemisorption using a Micromeritics ChemiSorb 2750 equipped with ChemiSoft TPx software. Prior to chemisorption, the catalyst was reduced with hydrogen (25\u00a0cm3/min) at 500\u00a0\u00b0C for 2\u00a0h and then cooled down to the room temperature under helium flow (25\u00a0cm3/min). The H2-TPR was carried out on a Micromeritics ChemiSorb 2750 with ChemiSoft TPx software. The sample was pretreated at 150\u00a0\u00b0C under N2 flow (25\u00a0cm3/min) for 1\u00a0h. The FT-IR spectra of adsorbed CO were collected using FTIR-620 spectrometer (JASCO) with a MCT detector at a wavenumber resolution of 2\u00a0cm\u22121. The sample was heated to 300\u00a0\u00b0C and reduced by H2 for 30\u00a0min.Prior to the reaction test, the catalyst was reduced with hydrogen (25\u00a0cm3/min) at 500\u00a0\u00b0C for 2\u00a0h. Approximately 0.05\u00a0g of catalyst was dispersed into the reactant mixture of 50\u00a0\u03bcL furfural (99%, Aldrich) and 10.0\u00a0mL methanol (98%, Aldrich) in 100\u00a0cm3 stainless steel autoclave reactor (JASCO, Tokyo, Japan). The selective hydrogenation reaction was carried out at temperature of 50\u00a0\u00b0C and H2 pressure of 2\u00a0MPa for 2\u00a0h. After that, the liquid product was collected and analyzed by a gas chromatography equipped with a flame ionization detector (FID) and Rtx-5 capillary column.Based on the XRD patterns of the Pt/TiO2 and Pt-Co/TiO2 catalysts (Fig. 1\n), the crystalline phases of TiO2 consisting of anatase phase at 2\u03b8\u00a0=\u00a025\u00b0 (major), 37\u00b0, 48\u00b0, 55\u00b0, 56\u00b0, 62\u00b0, 71\u00b0, and 75\u00b0 and rutile phase at 2\u03b8\u00a0=\u00a028\u00b0 (major), 36\u00b0, 42\u00b0, and 57\u00b0 [16] were observed. The characteristic peaks for Pt metal/Pt oxides and Co metal/Co oxides could not be detected due probably to the low amount of metal (Pt and Co) loading and/or high dispersion of these metals on the TiO2 support. As shown in Table 1\n\n, the TiO2 anatase phase composition of (I) Pt/TiO2 (85.8%) was slightly lower than that of (F) PT/TiO2 (89.6%). Adding Co by different methods led to different effects on the TiO2 phase composition. When Co was co-impregnated with 0.7\u00a0wt% Pt on the TiO2 support, the anatase phase composition increased from 85.8 to 90.5% with increasing Co loading from 0.04 to 0.2\u00a0wt%. Because the ionic radius of Co2+ (0.075\u00a0nm) is not much different from Ti4+ (0.061\u00a0nm), it has been postulated that Co2+ ions could enter into TiO2 anatase structure and inhibit the phase transformation from anatase to rutile [17]. Upon substitutional dopants, the level of oxygen vacancies decreased, inhibiting the rutile phase transformation [18,19]. On the contrary, the TiO2 anatase phase composition of the FSP-derived bimetallic PtCo catalysts was significantly decreased to ~70% comparing to the monometallic (F) PT/TiO2. The ionic substitution of Ti4+ with metal cationic ions could occur during the particle formation step during FSP. The replacement of Ti4+ with metal cationic ions having lower valences such as Co2+ would generate more oxygen vacancies, promoting phase transformation from anatase to rutile along the flame. Simultaneous formation of TiO2 support and Co metal by using polymeric precursor [20] or by sol-gel technique [21] also showed similar effect of TiO2 phase transformation as in the FSP method. The average crystallite size of anatase phase TiO2 for the (F) PT/TiO2 remained unaltered at 12\u00a0nm upon Co loading by FSP method whereas those obtained by co-impregnation led to a slight increase of anatase phase TiO2 crystallite size from 12 to 16\u00a0nm.The BET surface areas of the Pt-based catalysts prepared by impregnation were in the range of 52\u201360\u00a0m2/g, and were slightly larger than those prepared by FSP (43\u201350\u00a0m2/g). Regardless of the preparation method used, Co addition as a second metal led to the increment of the BET surface area. For a similar amount of metal loading, Pt dispersion of the Pt-based catalysts prepared by impregnation was higher than that prepared by FSP. During FSP synthesis, suppression of CO chemisorption on surface Pt sites may be due to partial coverage of metal surface by the support matrix [22,23]. However, addition of Co as a second metal resulted in higher Pt dispersion regardless of the preparation method used. The interaction between Pt and Co was reported to compensate the electron deficiency of Pt sites by d-electron rehybridization, thus enhancing the adsorption ability of CO [24].The H2-TPR profiles of the catalysts are shown in Fig. 2\n. All the synthesized Pt-based catalysts showed three major reduction peaks at 94 - 105\u00a0\u00b0C, 313\u2013380\u00a0\u00b0C, and 509\u2013687\u00a0\u00b0C, which were attributed to the reduction of PtOx particles to metallic Pt0 particles, the reduction of Pt species interacting with the TiO2 support in the form of Pt-TiOx interface sites, and the reduction of surface capping oxygen of TiO2 support, respectively [25]. It is obviously seen that the reduction peak of the Pt-TiOx interface sites for the (F) Pt/TiO2 catalyst shifted towards higher temperature and became broader compared to the (I) Pt/TiO2, indicating the stronger metal-support interaction induced by FSP method. For both cases, Co addition as a second metal in the Pt-based catalysts further shifted the reduction temperature of the Pt-TiOx species, due probably to the PtCo interaction and/or the migration of Co particles onto the Pt surface (decoration effect) [26]. Moreover, the reduction of surface capping oxygen of TiO2 drastically shifted from 545\u00a0\u00b0C for the monometallic (I) Pt/TiO2 to 615\u2013687\u00a0\u00b0C for the bimetallic (I) Pt-Co/TiO2 while those of the FSP made catalysts remained unchanged at 510\u00a0\u00b0C. On the other hand, for bimetallic Pt-Co/TiO2, the Pt-TiOx reduction peak of (I) Pt-Co/TiO2 were located at higher temperatures compared to (F) Pt-Co/TiO2, which could be attributed to stronger metal-support interaction upon Co addition on the impregnated catalysts. As a consequence, the (I) Pt-0.2Co/TiO2 showed higher conversion and selectivity of FA compared to (F) Pt-0.2Co/TiO2, which was consistent with the metal-support interaction from H2-TPR. Nevertheless, it can be suggested that Co addition on FSP-made catalyst was not necessary for improvement of the interaction between metals and support in order to enhance the selectivity to FA.As observed from the TEM images (Fig. 3\n\n), the monometallic (I) Pt/TiO2 catalyst was significantly composed of very small and narrowly distributed nanoparticles (\u2264 2\u00a0nm). Very rarely large (10\u2013100\u00a0nm) Pt particles could be observed. (F) Pt/TiO2 catalyst showed three types of nanoparticle sizes. First, a collection of very small nanoparticle (\u2264 2\u00a0nm) similar to the one found on (I) Pt/TiO2, then a collection of nanoparticles between 3 and 10\u00a0nm, and finally some very large spherical nanoparticles with sizes of several tens to several hundreds nm. It is suggested that (I) Pt/TiO2 essentially showed very small Pt nanoparticle sizes (\u2264 2\u00a0nm) and the selectivity of FA was significant lower than the (F) Pt/TiO2. The selectivity of FA may depend on the Pt particle size. It has been report that Pt nanoparticles of sizes less than 2\u00a0nm supported on the silica are highly selective towards decarbonylation of furfural to main product of furan over vapor phase furfural hydrogenation [6].Concerning the Co addition as a second metal, (I) Pt-0.2Co/TiO2 was essentially consisted of very small nanoparticles as (I) Pt/TiO2. The average particle size of these nanoparticles slightly increased with the addition of Co (1.7\u00a0nm for (I) Pt-0.2Co/TiO2 and 1.3\u00a0nm for (I) Pt/TiO2) and the particle size distribution was substantially broader. There are also very rare of larger particles. The (F) Pt-0.2Co/TiO2 catalyst also showed characteristics similar to those of (F) Pt/TiO2. Two types of nanoparticles and some large spherical ones. The smaller nanoparticles also slightly increase in size of 1.9\u00a0nm for (F) Pt-0.2Co/TiO2 compared to 1.5\u00a0nm for (F) Pt/TiO2. The intermediate nanoparticles were not vary too much in size (6.8\u00a0nm for (F) Pt-0.2Co/TiO2 compared to 6.2\u00a0nm for (F) Pt/TiO2). The large spherical particles have several hundreds nm. The larger spherical particles that originated by FSP method were much larger than the size of the grains of titania and thus their interaction was poor, resulting to reduce the catalytic activity. The IR spectra of adsorbed CO on the different catalysts reduced at 500\u00a0\u00b0C are shown in Fig. 4\n. The bands at 2188\u20132185\u00a0cm\u22121 represented to CO formed with Ti4+ cations on the surface [27]. The band at 2086\u20132078\u00a0cm\u22121 was assigned to CO linearly adsorbed on low - coordination Pt (Pt0-CO) or Co (Co0-CO) atoms on edge sites, while the band at 1850\u00a0cm\u22121 was assigned to CO bridged between two Pt metal atoms (Pt0-CO-Pt0). It has been reported that linear-type adsorbed CO dominated on small Pt particles while bridge-type adsorbed CO formed mainly on larger Pt particles [28,29]. The ratio of Pt on linearly CO adsorbed (Pt0-CO) to bridged CO adsorbed (Pt0-CO-Pt0) atom was high due to the presence of very small Pt particles size (\u2264 2\u00a0nm), which was consistent with the average particle size observed from TEM and further increase selectivity of FA with the Co addition. The Pt0-CO band center of (F) Pt/TiO2 shifted to lower frequency with lower intensity compared to (I) Pt/TiO2. The lower shift and increase in intensity of the Pt0-CO band were also observed in bimetallic (F) Pt-0.2Co/TiO2 compared to (I) Pt-0.2Co/TiO2.In this study, FA was the only desired product being formed and 2-furaldehyde dimethyl acetal was the solvent product (SP) which can be generated upon furfural acetalization when using methanol as the solvent [3] in the presence of metal catalysts. The catalytic performances of all the Pt-based catalysts in the liquid-phase selective hydrogenation of furfural to FA are summarized in Table 2\n. The furfural conversion and selectivity to FA over (I) Pt/TiO2 catalyst were 84.6% and 71.5%, respectively. Modification of (I) Pt/TiO2 by Co addition, furfural conversion and selectivity to FA increased with increasing Co loading up to 0.2\u00a0wt% in which furfural was fully converted within 2\u00a0h of reaction time and high selectivity to FA up to 97.5% was achieved. Improvement in furfural conversion upon Co addition by co-impregnation was correlated with high Pt dispersion and high TiO2 anatase phase composition. The subsurface oxygen vacancies on the reduced anatase TiO2-supported metal catalysts were found to be favorable adsorption sites for H atoms [30]. On the other hand, the hydrogen would weakly interact with the rutile TiO2-supported ones [31]. In addition, increase in the selectivity to FA could be attributed to the formation of Pt-TiOx interface sites, in which oxygen atom in the CO group of furfural could be coordinated via a lone pair of electrons, promoting the selective activation of carbonyl bonds in furfural [32]. In the perspective of electronic effect, the stronger interaction between Pt and TiO2 support indicated more electron transfer from support to metals, thus leading to the formation of electron-rich metal particles. The electronegativity of metal catalysts has been reported to affect the CO activation in furfural hydrogenation [32]. Stronger electronegativity of metal particles (or electron-rich metals) could activate the CO bond towards the furfural hydrogenation to FA. Similar results have been reported over Cu/TiO2-SiO2, Au/Al2O3, Fe-promoted NiB amorphous alloy catalyst, Au/ZrO2, and Au/TiO2.Furfural conversion of the monometallic (F) Pt/TiO2 was not much different from the (I) Pt/TiO2 catalyst but the selectivity to FA was found to be largely improved. This improvement was attributed to the stronger metal-support interaction upon FSP. However, Co addition upon FSP slightly improved the selectivity to FA but decreased furfural conversion. It is suggested that the presence of higher rutile TiO2 phase diminish the furfural conversion despite its high Pt dispersion based on CO chemisorption ability. Nevertheless, it could be said that Co addition into the Pt-based catalysts upon FSP was unnecessary for modification of the interaction between metals and support in order to enhance the selectivity to FA. Comparing the bimetallic Pt-0.2Co/TiO2 catalysts prepared by impregnation and flame spray pyrolysis, the (I) Pt-0.2Co/TiO2 catalyst showed higher furfural conversion and selectivity to FA than the (F) Pt-0.2Co/TiO2 catalyst because of higher Pt dispersion, TiO2 anatase phase composition, and the strong interaction between metals and support. Among all Pt-based catalysts in this study, the (I) Pt-0.2Co/TiO2 exhibited the highest FA yield at 97.5%. The activity of 1st cycle and 2nd cycle for (F) Pt-0.2Co/TiO2 catalyst were 71.1% and 67.4%, respectively. It can be observed that the activity decreased by 5.2% after 2nd reaction. Table 3\n shows a comparison between the Pt-based catalysts in this study and those reported in the literature. It is notable that one of the best results with furfural conversion (100%) and FA selectivity (95.7%) could be obtained over 2\u00a0wt% Pt-1\u00a0wt% Re/TiO2-ZrO2 [10], however more severe conditions (130\u00a0\u00b0C, 5\u00a0MPa of H2) and long reaction time (8\u00a0h) were used in the presence of ethanol as a solvent. The catalysts in the present studies achieved similar conversion/selectivity under milder reaction conditions.While Co addition via co-impregnation on Pt/TiO2 catalysts improved both furfural conversion and selectivity to FA due to higher TiO2 anatase phase composition, high Pt dispersion, and stronger interaction between metals and support, formation of Pt-Co/TiO2 by one-step FSP resulted in a decreased furfural conversion despite its high Pt dispersion due to the acceleration of rutile phase transformation of the TiO2. However, a largely improvement of FA selectivity on the FSP-made catalysts was attributed to the stronger metal-support interaction upon FSP as revealed by the higher reduction temperature of the Pt-TiOx interface sites. Thus, modification of the Pt/TiO2 by Co addition was unnecessary for catalyst improvement when using the FSP Pt/TiO2 catalysts.Weerachon Tolek: Conducting a research and investigation process, specifically performing the experiments, or data/evidence collection, Writing first draft.Kitima Khruechao: Conducting a research and investigation process, specifically performing the experiments, or data/evidence collection.Boontida Pongthawornsakun: Data analyzing and discussion.Okorn Mekasuwandumrong: Data analyzing and discussion.Francisco Jos\u00e9 Cadete Santos Aires: Data analyzing and discussion.Patcharaporn Weerachawanasak: Conducting a research and investigation process, specifically performing the experiments, or data/evidence collection.Joongjai Panpranot: Acquisition of the financial support for the project leading to this publication, 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.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:The financial supports from the Rachadapisek Sompote Endowment Fund for the Postdoctoral Fellowship, Chulalongkorn University for W.T. and the Thailand Science Research and Innovation for the Basic Research grant BRG6180001, IRN62W0001, and CAT-REAC Industrial Project RDG6250033 are gratefully acknowledged. The authors also would like to thank the Research Team Promotion grant from the National Research Council of Thailand (NRCT).", "descript": "\n                  Flame spray-synthesized Pt/TiO2 and PtCo/TiO2 catalysts with 0.7\u00a0wt% Pt and 0\u20130.4\u00a0wt% Co were studied in the hydrogenation of furfural to furfuryl alcohol (FA) at 50\u00a0\u00b0C and 2\u00a0MPa H2. Particle formation under high temperature flame facilitated high Pt dispersion and formation of Pt-TiOx interface sites, which were beneficial for furfural conversion to FA. Modifying with Co accelerated rutile phase TiO2 formation, which strongly diminished hydrogenation activity on the (FSP)-PtCo/TiO2. On the other hand, (I)-PtCo/TiO2 prepared by conventional impregnation, anatase phase TiO2 was preserved (> 85%) and both furfural conversion and FA selectivity increased upon increasing Co loading.\n               "}
{"full_text": "Hydrogen gas (H2) is eco-friendly, sustainable, efficient, and has renewable properties. Moreover, it is a clean, efficient energy supporter for fuel cells and an auspicious alternative to control air pollution, crises of future fossil fuel, and problems of renewable resources [1\u20133]. The electro-splitting of water is assumed as a sustainable technique of H2 generation among the diverse methods of hydrogen generation [4,5]. It is confirmed that the electrolysis of alkaline water links to the small efficiency and high exhaustion of energy. While the utilization of acid solutions supplies a probable alternative to that matter [6]. Where HER in acidic aqueous solution necessitates lesser applied overpotential, which is more effective and economic than the reaction in alkaline aqueous electrolytes. Although, alkaline media is still utilized because of its possibility for prevailing the overall water electrochemical splitting reaction accompanied by the production of pure hydrogen gas accumulated at the negatively polarized cathode and oxygen gas at the positively polarized anode department concurrently [7,8].Noble metals including platinum are particularized as the most appropriate electrochemical catalysts for HER. This is owing to their low electro-reduction overpotentials and fast electron transfer kinetics [9\u201311]. On the contrary, their application in the industry of large-scale production of hydrogen is obstructed by the high-cost and rarity [12\u201314]. Subsequently, finding earth-abundant metals as alternatives to the rare and costly noble metal electrocatalysts is an indispensable request to consolidate the growth of the hydrogen production economy [15\u201318].Among electrocatalysts containing non-precious metals, nickel and nickel-based materials are strongly recommended as premium candidates to substitute Pt-based materials as electrocatalysts for HER in acidic aqueous media. This is attributed to their high stability and electrocatalytic activity [6,19,20]. The incorporation of Ni with different elements to yield alloys such as NiFe [21], NiCo [22], NiSe [23], Ni2P [24], NiCu [25], and NiMo [26] is a noteworthy protocol to fabricate electrochemical catalysts with enhanced performance of HER. Particularly, Ni\u2013Co systems were previously examined and characterized to behave as efficacious electrocatalysts for HER. This is due to their intrinsic electrocatalytic validation and increases the corrosion resistance in comparison to the pure Ni in acidic media [27\u201329]. Nevertheless, Ni\u2013Co system up to this time display considerably application in HER. This is due to their stability compared to noble metals in acidic media. For obtaining Ni\u2013Co alloy of highly catalytic performance for the HER, two general strategies should be adopted. One of them is preparing catalysts of high surface area to dramatically raise the number of active centers, and the second is preparing catalysts of porous structures. This leads to enhance the electrical conductivity of the catalyst and increase the mass transport process for the species under the influence of the polarization of the electrode [30\u201333].Ni\u2013Co alloys have been produced by various techniques including hydrothermal/solvothermal [34\u201337], polyol method [38,39], double composite template approach [40], chemical vapor deposition [41], mechanical alloying [42], non-aqueous ethylene glycol refluxing [43] and electrodeposition [44\u201346]. The electrodeposition technique has numerous benefits including low cost, facile, reproducible, and highly efficient. For that, it can extensively be used for fabricating thin-film coating [47,48]. Moreover, the electrochemical deposition technique does not demand hard conditions. Furthermore, the particle sizes and the structure can be readily controlled by simply adjusting the parameters of electrodeposition, such as time, concentration, temperature, or applied potential [48]. In this work, NiCo alloy flower-like structure loaded on C-steel, have been successfully fabricated via electrodeposition methods. The as-synthesized NiCo/steel inherit significant HER catalytic properties in HCl solution for the first time in that system. The constructed NiCo/steel catalyst possesses excellent electrocatalytic performance for HER. Furthermore, the obtained catalyst has higher anti-corrosion and excellent long-term stability in the aggressive acidic medium of HCl.Cobalt chloride hexahydrate (CoCl2\u00b76H2O), nickel chloride hexahydrate (NiCl2\u00b76H2O), and potassium chloride (KCl) were purchased from Merck. All electrolytes were prepared utilizing bi-distilled water. The bath of electrochemical deposition contains 0.02M CoCl2\u00b76H2O, 0.05M NiCl2\u00b76H2O, and 0.1M KCl. All the electrochemical experiments were implemented using a 3-electrode cell contains a Pt sheet and Ag/AgCl as a counter and reference electrodes, respectively. Cu sheet (1cm2) and carbon steel rode (4.55cm2) were used as working electrodes. All electrochemical experiments were accomplished by using a computerized Potentiostat/Galvanostat of VersaSTAT4. Potentiostatic electrodeposition method of Ni\u2013Co alloys on carbon steel rods was applied at various potentials of \u22120.8, \u22120.9, \u22121, \u22121.1, and \u22121.2V for 1h.The chemical composition and morphology of the electrodeposited Ni\u2013Co alloys were investigated using the scanning electron microscope of JEOL JSM-5500 LV, from JEOL company in Japan, supplemented with Microanalyzer of Energy Dispersive X-ray (EDX) driven via a system with Link Isis @ Software of model 6587 from Oxford, England. The electrodeposited films of Ni\u2013Co alloy have been introduced for the measurements of XRD. The formed phases of the Ni\u2013Co alloy deposits were detected via a phaser of Bruker Dron-2 table-top model using Cu K\u03b1 radiation source of wavelength 1.5418\u00c5.Experiments of the electrochemical corrosion were achieved at 30\u00b0C in 1.0M HCl. The PDP for the bare and surface covered carbon steel rods were performed after 1h of dipping in the test electrolyte, by applying a potential ranged from \u22120.25 to +0.25V versus the open circuit potential (OPC, E\ncorr) at a potential scan rate of 0.001Vs\u22121. The experiments of EIS were accomplished using a DC potential amplitude of 0.01V by applying a frequency ranged from 100,000 to 0.5Hz. EIS data has been fitted utilizing Z-view by selecting the most appropriate equivalent circuit.The HER effectiveness of the as-prepared catalysts was examined with linear sweep voltammetry (LSV) at a potential sweep rate of 0.005V/s in the test solution of 0.1M HCl.The deposition potential and other optimized conditions for the electrodeposition of NiCo alloys on the steel rod surface have been accurately determined via cyclic voltammetry, and via the variation the concentration of the electrodeposition bath components. The potentiostatic method (chronoamperometric) has been used for the achievement of the electrochemical deposition process of the alloys in demand. At ambient temperature and pH=6.5, suitable deposition potentials (chosen with the aid of cyclic voltammetry, near from the peak potential of reduction of Ni and Co ions) of \u22120.8, \u22120.9, \u22121.0, \u22121.1, \u22121.2V were applied for detecting the best potential at which the alloy is deposited as well, shown in Fig. 1\n.It seems from Fig. 1 that the onset of the curves of current\u2013time is distinguished by an abrupt convert up or down, depending on the applied potential. This may be attributed to the presence of a double layer between the negatively charged electrode surface and the cations of the electrolyte under consideration. Accordingly, instantaneous nucleation occurred for all investigated alloys particles as exhibited in Fig. 1. Then, the current is accompanied by a slight increase. This means that the electrodeposited layer still grows at this current which should lead to an increase in the electroactive surface area, during the deposition process. But there is a sudden increase of current (more negative) value in the case of the sample (e) at \u22121.2V. This can be explained as at the higher potential, the rate of electrodeposition increases and hence the nucleation also increases. This led to an accumulation of the loaded electrodeposited particles which cause a limitation of the electro-reduction reaction progress.\nFig. 2\n exhibits the XRD analysis of the electrochemical deposited Ni\u2013Co alloys via the potentiostatic technique. The alloys have been electrodeposited at various deposition potentials as follows: a) \u2212800, b) \u2212900, c) \u22121000, d) \u22121100, and e) \u22121200mV versus Ag/AgCl, at pH=6.5, and room temperature on Cu substrate. The data of XRD demonstrate that crystalline Ni\u2013Co alloy has been obtained as illustrated in Fig. 2.From the figure, it can be noticed that all samples contain four lines of Ni\u2013Co appeared at 2\u03b8\n=75.85, 51.14, 41.46, and 43.80 corresponding to Ni\u2013Co FCC (220), Ni\u2013Co FCC (200), Ni\u2013Co FCC (111), and Ni\u2013Co HCP (100), respectively. Also, it can be noticed that all samples contain nickel metal except sample (a). Where, at sample (a) the deposition potential is \u22120.8V. This potential is not sufficient for the deposition of nickel metal at these conditions. Furthermore, it can be observed that sample (c) has the highest percent of the phases Ni\u2013Co FCC (200) and Ni\u2013Co FCC (220) compared with the other samples. This will cause a difference in the measurable physical properties of this sample compared with the other samples as shown in the next sections.The surface morphology of the electrochemically deposited Ni\u2013Co alloys at the optimized conditions was investigated using SEM, as shown in Fig. 3\n. Image (a) shows the bare copper substrate which is chemically treated and etched (cleaned) with the Piranha solution before the electrodeposition process. As shown in Fig. 3, the surface morphology of the electrodeposited alloys heavily is controlled by the potential of the electrochemical deposition. It is noticed that the etched area of the bare Cu substrate has been covered with the electrodeposits of Ni\u2013Co alloy, as shown in image (b). Also, it contains some cracked parts and large crystals. The shape of the electrodeposited Ni\u2013Co has been changed by increasing the electrodeposition potential E\nd to \u22120.9V to be spongy like structure, as shown in image (c). A very interesting flowered like structure has been obtained at E\nd \u22121.0V which of course has a high surface area compared with other samples. When the E\nd was increased to \u22121.1V, smooth carrying small spherical particles and some large particles of structure like sticks were observed (image (e)). At the applied potential of \u22121.2V, the obtained structure looks like that obtained in the case of E\nd of \u22121.1V, but with larger spherical particles and sticks. These differences in morphology, of course, affect the physical properties of the obtained samples.Furthermore, the value of electrochemical deposition potential has a significant impact on the composition and stoichiometry of electrodeposited Ni\u2013Co alloy, as shown in Fig. 4\n. It can be observed that by increasing the applied deposition potential the percent of Ni in the electrodeposited alloy increases from 61.63% to 75.46% at the applied range of E\nd (from \u22120.8 to \u22121.2V). In contrast, by increasing the applied deposition potential the percent of Co in the electrodeposited alloy decreases from 38.37% to 24.54%. This may be interpreted that Ni ions are firstly electro-reduced due to that the charge on Ni ions is larger than that of Co ions and hence the Ni ions can be attracted towards the opposite polarized electrode faster than that of Co ions. Besides, the hydrogen evolution at high negative potentials can affect the completion of the electrodeposition of Co more than Ni.The data of potentiodynamic polarization for the uncoated and coated mild steel with the electrodeposited Ni\u2013Co alloys have been examined (Fig. 5\n). Some important data including the current density of corrosion (I\ncorr), potential of corrosion (E\ncorr), and slopes of Tafel's plots (\u03b2\nc, \u03b2\na) have been determined using curves of the polarization (Table 1\n). The efficiency of protection (\u03b7%) was intended via the values of current density of corrosion (I\ncorr) using the following equation [49]:\n\n(1)\n\n\n\u03b7\n%\n=\n\n\n\n\n\nI\n\ncorr\n(\nB\n)\n\n\n\u2212\n\nI\n\ncorr\n(\nD\n)\n\n\n\n\n\nI\n\ncorr\n(\nB\n)\n\n\n\n\n\n\n\u00d7\n100\n\n\n\nwhere I\ncorr(B) and I\ncorr(D) are the current densities of corrosion for the uncoated and the coated steel samples, respectively. Based on the data in Table 1, the C-steel sample had a corrosion current density (I\ncorr) of 3.9mAcm\u22122 attributed to the highly corroded of metal in the corrosive medium. The I\ncorr of Ni\u2013Co coated alloys decreased to starts from 0.490 and reaches to 0.026mAcm\u22122 for the electrodeposited Ni\u2013Co alloys deposited at \u22120.8 and \u22121.0V, respectively. Then, I\ncorr was increased to 0.065mAcm\u22122 for the Ni\u2013Co alloy electrodeposited at E\nd\n=\u22121.2V. The decrease of I\ncorr in case of coated steel samples indicating a protection impact for these coatings. This was ascribed to the existence of the deposited Ni\u2013Co layer on the surface of C-steel, which acts as a protective layer (barrier) and reduces the contact between HCl and metal surface. Ni\u2013Co alloy, deposited at E\nd\n=\u22121.0V, exhibits the highest protection among the studied alloy coatings. This due to the full covered flowered structure of the deposited film for that alloy, which prevents the reach of Cl\u2212 ions to the metal surface more than the other alloys and also the percent ratio of Ni and Co in the deposited alloy.It is observed from Fig. 5 and Table 1 that the corrosion potential (E\ncorr) for all coated samples is more positive than the uncoated mild steel, which implied that samples coated with Ni\u2013Co alloys enhance the barrier effect and protect the samples from corrosion. According to Table 1, \u03b2\na and \u03b2\nc values for coated samples are slightly changed indicating the coating barriers are blocked the anodic sites at the same corrosion pathway [50]. It is noteworthy to emphasize that the good corrosion resistance of the prepared coated films can be partly related to their structure and denseness (i.e., lack of porosity). The porosity (P\no) of the coating films of the electrodeposited Ni\u2013Co alloys can be determined from the PDP measurements via the following Eq. [51]:\n\n(2)\n\n\n\nP\no\n\n=\n\n\n\nR\ns\n\n\n\n\nR\nd\n\n\n\n\u00d7\n\n\n10\n\n\n\u2212\n(\n\n\u0394\n\n\nE\n\ncorr\n\n\n/\n\n\u03b2\na\n\n)\n\n\n\n\n\nwhere R\nd and R\ns are the polarization resistances of the deposited film and substrate, respectively, \u0394E\ncorr the potential difference between the coated substrate and the uncoated mild steel, and \u03b2\na is the anodic slope of Tafel's plot for the substrate. The obtained P\no values are in the range between 1.4% and 4.6% (Table 1). The higher porosity value 4.6% was obtained for the sample coated with Ni\u2013Co (deposited at E\nd\n=\u22120.8V), which would facilitate Cl\u2212 anions diffusion towards the layer/bare interface.As mentioned above, that all the corrosion currents values for the Ni\u2013Co coatings were smaller than the C-steel substrate. This displays that the critical coating thickness for the steel substrate protection was accomplished. On the other hand, the lowest corrosion current and highest protection capacity (\u03b7%) were obtained for the Ni\u2013Co (3) sample at deposition potential \u22121.0V (Table 1). For the coatings at deposition potential \u22121.1 and \u22121.2V, the corrosion currents were found to be increased and \u03b7% decrease. The efficiency of corrosion protection of electrodeposited Ni\u2013Co alloys can be returned to many important factors such as roughness and surface morphology, defects (porosity), phase composition, chemical composition, and grain size of the coating layer [52].To confirm the obtained results from the PDP experiments, the protection effect of electrodeposited Ni\u2013Co films in the solution of 1.0M HCl was examined using EIS. The obtained data by EIS are shown in Fig. 6\n (Nyquist plot) and Fig. 7\n (Bode and Phase angle plots). Fig. 6 exhibits impedances spectroscopic behavior of the electrodeposited Ni\u2013Co alloys that are uncommonly greater than that of the uncoated steel. Ni\u2013Co films exhibit significant protective properties with increasing deposition potential until \u22121.0V after that the protective properties were decreased.The comparison between EIS experimental results and the fitted data for coated and uncoated C-steel substrates immersed in HCl was presented in Fig. 6b and c. A remarked fitting was obtained for the proposed model that utilized for all resulted data. In the case of the uncoated sample, a one-time constant only is observed; the proposed model of EIS is exhibited as in the inset of Fig. 6b. The proposed model can be identified as a simple equivalent circuit of Randel [53,54], comprises of parameters namely R\ns is the solution resistance which connected in series with the double layer capacitance (C\ndl) and series with the charge transfer resistance (R\nct). Equivalent circuit characterizing the behavior of C-steel coated by electrodeposited Ni\u2013Co samples is frequently suggested for the modified substrate surface by the electrodeposited alloys (inset of Fig. 6c). As in the exhibited circuit, C\ncoat represents pseudo-capacitance of coating and R\npore is the electrical resistance of pore against the ionic current that directed to the pores. Similar equivalent circuits have been used by many researchers to simulate EIS data for coated systems, for example, Ullal and Hegde in case of electrodeposited multilayer nanocomposite of Zn\u2013Ni\u2013SiO2 on mild steel [55], and by Cheraghi et al. [49] in the case of AISI 316L stainless steel covered by TiO2\u2013NiO nanocomposite thin films in a solution of 3.5% NaCl.\nFig. 7a and b exhibits the Bode and phase plots for C-steel substrate and Ni\u2013Co deposits on C-steel substrate in the test solution of HCl after immersion for 30min at the corrosion potential E\ncorr and temperature of 30\u00b0C. From Fig. 7a C-steel displays one-time constancy, it was accomplished that only one mechanism was proper for the corrosion process. However, after coating with electrodeposited Ni\u2013Co, the curve shows two-time constancy. The first at higher frequencies (HF) can be connected with the coating's response; the second at low frequencies (LF) can be attributed to the process of corrosion. It could be elucidated that the electrochemical corrosion of samples in HCl is predominantly depending on the process of charge transfer. In general, the greater Z modulus LF manifests the best resistance against corrosion [56]. The obtained data in this study exhibited the highest Z modulus for Ni\u2013Co coatings as shown in Fig. 7a. The bode phase plots of all the samples in Fig. 7b shows only one-time constancy in the case of the uncoated C-steel substrate, while in the case of coated samples showed two-time constancy. The electrochemical parameters related to EIS measurements were measured from curve fitting method and these are exhibited as in Table 2\n. The obtained R\ns values are very low around 0.22\u20130.48 due to the higher conductivity of HCl solution (Table 2). By comparing the R\nct between the coating samples which deposited at different deposition potentials, it is observed that the sample 3 (at deposition potential \u22121000mV) has the highest R\nct and thus corrosion protection. The reason for this corrosion protection could be the higher amount of Ni in the film, which promotes corrosion resistance.Indeed, from Table 1, it is revealed that the R\nct increased and the protection efficiencies improved by increasing the potential of the electrochemical deposition until \u22121000mV, then decreased. The highest P% of 88.72% was obtained for the films that electrochemically deposited at \u22121000mV.The high R\nct values of deposited coatings in comparison to C-steel bare indicated that the Co\u2013Ni deposited films can efficiently suppress the diffusion of corrosive electrolyte into the metal surface by forming a strong hurdle between the interface of the metal and the media of corrosion (HCl solution). Coating resistance (R\nc) is an apparent index that reflects the protective effect of the coating film, the higher R\nc value can lead to better corrosion protection. It is worth noting that EIS measurements and Tafel plots are consistent with each other. As a result, the coated substrates have better corrosion protection compared to the uncoated specimens. However, samples coated with Co\u2013Ni deposited at \u22121000mV manifested the highest protection from corrosion in a test solution of HCl.The electrocatalytic activities for HER of the electrochemically deposited NiCo on C-steel substrate were performed in a solution of 0.1M HCl. Fig. 8a exhibits the LSV curves for the electrodeposited alloys of NiCo, by the side of those the blank C-steel electrode has been used for comparison. As observed, all the NiCo/steel electrodes exhibited better electrocatalytic activity than the blank C-steel indicating that NiCo/steel can provide as a high-performance cathode for electrocatalysis of HER. The high-performance of the electrocatalytic HER for NiCo/steel may be related to the synergistic catalytic activity of the alloyed Ni with Co [57,58]. One can be elucidated that aside from noble metals, the Ni and Co metal system manifested a significant HER activity among transition elements [59]. Moreover, it has proved that the NiCo alloy can elevate the splitting of the attached water on the electrode surface by gradually destroying the O\u2013H bond. While the produced hydrogen as intermediates maybe get adsorbed on the surface of NiCo alloy [60]. Accordingly, it provides extra active sites on the surface and then accelerates the rate of HER.Among the NiCo catalysts, the Ni\u2013Co (3) alloy exhibits considerable HER activity at the smallest start of overpotential, and an instantaneous increase of cathodic current under more negative potentials occurred. Moreover, the appropriated overpotential to generate a current density of \u221210mAcm\u22122 for the Ni\u2013Co (3) alloy is solely \u2212535mV. This value is lower than that of other NiCo alloys and the blank C-steel electrocatalysts as seen in Fig. 8b.The reaction mechanism of HER, generally, can be deduced from the plot of Tafel (\u019e vs log I). Moreover, the rate-determining step for the reaction of hydrogen evolution can be recognized. The linear cathodic part of the Tafels\u2019 curves (Fig. 5) has been fitted according to the Tafel's equation (\u019e\n=\na\n+\n\u03b2\nc log I; where \u201c\u03b2\nc\u201d is the Tafel slope of the cathodic line and \u201ca\u201d logarithm of the exchange current) giving cathodic values of Tafel slope for all NiCo/steel and blank C-steel catalysts as seen in Table 1. During the reaction of the evolution of hydrogen in the acidic medium, three possible intermediate steps can occur [61\u201363]. Form the Tafel slope, it can be elucidated that the first comprises proton-coupled electron transfer at the surface of catalyst producing adsorbed hydrogen, this step is a discharge one, recognized as the Volmer reaction:\n\n(3)\nH3O+\n+e\u2212\n=Hads\n+H2O\n(Volmer reaction, \u03b2\nc\n=120mV/dec).\n\n\nThe following process is desorption which occurs by either mechanism of Heyrovsky, at which the adsorbed hydrogen combines with a hydrogen ion (proton) from the working solution,\n\n(4)\nHads\n+H3O+\n+e\u2212\n=H2O+H2\n\n(reaction of Heyrovsky, \u03b2\nc\n=40mV/dec).\n\nor by combining two hydrogen atoms that are adsorbed on the electrode surface, this known as the Tafel reaction:\n\n(5)\nHads\n+Hads\n=H2\n\n(reaction of Tafel, \u03b2\nc\n=30mV/dec).\n\n\nDespite that, it is a complicated matter to visualize a precise mechanism to diverse performance of electrocatalysts towards HER, the slope of Tafel is considered to be a perfect indication of the rate-determining reaction step. Regarding this work, the slopes of Tafel for the electrochemically deposited Ni\u2013Co alloys are within the range of 40\u2013120mV/dec. Accordingly, the mechanism of the hydrogen evolution for these modified electrodes can be ascribed to the Volmer\u2013Heyrovsky mechanism, in which the adsorption of the hydrogen atom on the surface is the slow step that determines the reaction (r.d.s.). Where little slopes of the Tafel plot means a more catalyzed rate for HER can take place. One could be noted that the slope of Tafel for Ni\u2013Co (3) alloy is found to be 81mV/dec, which is the smallest value compared with all values of the alloys under investigation, as listed in Table 1. This reveals that the HER at Ni\u2013Co (3) catalyst is kinetically fast. It should be noted that Ni\u2013Co (3) catalyst is the best HER catalyst among all investigated catalysts. Aside from the activity towards HER, the durability of the electrocatalyst is a significant property for the investigation of actual electrocatalytic efficiency.To investigate the stability for the long-term of all the NiCo/steel electrocatalysts and blank C-steel, chronoamperometry at fixed potential of \u2212500mV versus Ag/AgCl for an hour in 0.1M HCl solution at room temperature should be carried out, as exhibited in Fig. 8c. Fig. 8c exhibits that, a higher current density of Ni\u2013Co (3) than that of all other catalysts at the same overpotential (\u2212500mV) which proves the high electrocatalytic activity of Ni\u2013Co (3) catalyst for HER. An apparent change of current density can be also noticed after 1h at all investigated catalysts except for Ni\u2013Co (3). Also, a long-term chronoamperometric test was performed for the most stable alloy (Ni\u2013Co (3)) which exhibits and confirms the stability of the alloy form a long time (20h). No obvious current density change with time proves the excellent electrochemical stability of Ni\u2013Co (3) compared to all of the electrochemically deposited Ni\u2013Co catalysts, and blank C-steel in acidic media. Furthermore, it seems to be there is no change in the SEM and XRD data for all samples after chronoamperometry and HER experiments. Its expected results because they exhibited high corrosion resistance in this media and gave a considerable HER. Besides, they exhibited a reproducibility and repeatability.Potentiostatic route was applied for the synthesis of Ni\u2013Co alloys at various deposition potentials, neutral media, and room temperature and without any additives. SEM images show different structures by varying the deposition potential and flowered like structure was obtained at a constant potential of \u22121000mV. This alloy gives a chemical composition of Ni70Co30 under EDX analysis. The increase of deposition potentials increases Ni content and decreases Co content in the electrodeposited alloys. The results of XRD confirm a good agreement with the crystalline phases (fcc) and (hcp) for the electrodeposited Ni\u2013Co alloys. According to potentiodynamic polarization and EIS measurements, it has been concluded that all the rate values of the electrochemical corrosion for the Ni\u2013Co coatings were smaller than that of the carbon steel substrate. Furthermore, the lowest corrosion current and highest protection efficiency were obtained for Ni\u2013Co (3) sample at deposition potential \u22121.0V. This could be correlated to several substantial factors including phase composition, chemical composition, porosity, and surface morphology. It can be concluded that this alloy of unique properties is qualified and promising to be a stable and efficient electrocatalyst material for HER in HCl solution.None declared.", "descript": "\n                  Ni\u2013Co alloy of flowered like structure is successfully electrodeposited from neutral aqueous solutions of NiCl2 and CoCl2 salts as precursors at room temperature. The prepared alloy at our optimized conditions has a high protection effect from the electrochemical corrosion of steel in the hydrochloric acid solution. The electrochemically deposited alloys are examined via X-ray diffraction (XRD) giving two main phases of (fcc) and (hcp). The morphology is investigated via scanning electron microscope (SEM) showing flowered like structure for the alloy that is electrodeposited at \u22121.0V only. The obtained data of the potentiodynamic polarization (PDP) and spectroscopy of electrochemical impedance (EIS) demonstrates that Ni\u2013Co coatings have high stability in the highly corrosive media, especially the alloy of the flowered like structure. This alloy of unique properties is subjected to be an efficacious electrocatalyst for the reaction of hydrogen evolution (HER) in HCl solution.\n               "}
{"full_text": "Multi-metallic formulations have raised a lot of interest in catalysis and electrochemistry because of the synergies that may be achieved by combining different elements [1\u20134]. The possibility to dilute expensive noble metals into base metals, while maintaining acceptable or even improving catalytic properties, is also clearly valuable. The catalytic activity in terms of reactant conversion rates and selectivity to various reaction pathways will directly depend on the surface composition and local geometric arrangements of atoms [5]. The bulk or surface composition or structure of nanoparticles will often be modified under reaction conditions [6,7], making the use of in situ and operando methods a requirement to obtain meaningful structure-activity relationships.\nin situ and operando IR spectroscopies applied to catalysis have been primarily used to determine the nature and reactivity of adsorbates present under reaction conditions [8\u201312]. This is because many vibrational modes associated with adsorbates bonds are highly IR sensitive and lead to sharp bands enabling detection at the nanomole levels using only milligrams of catalysts [13]. In contrast, the vibrational bands of catalysts are difficult to investigate by IR because the corresponding lattice vibrations lead to very broad absorption bands typically located in the far-IR region.IR can yet lead to indirect information about the structure of nanoparticles under reaction conditions if an adsorbed intermediate leads to a measurable signal. This can be the case for reactions involving strongly IR-sensitive molecules such as carbon oxides and unsaturated hydrocarbons. The examples dealt within the present review focus on the interpretation of IR spectra of CO adsorbed on various bimetallic catalysts. A particular interest is given to Au, Pt, Pd and Sn-containing bimetallics, as these elements commonly appear in catalytic formulations.IR spectra analysis is yet not straightforward, as exemplified by recent misinterpretations [14\u201317]. Even the case of monometallic samples is complex as carbonyl band position will depend on several parameters such as surface coverage, CO coordination mode, coordination number and the oxidation state of the metallic site. The case of monometallic Pt-based catalysts was recently reviewed in details and is therefore not recalled here [18,19]. The interpretation of spectra obtained over Au monometallic materials is examined in the first section and its impact on the interpretation of spectra obtained on some bimetallic materials will then be explored. This contribution discusses data obtained from transmission IR, diffuse reflectance FT-IR (DRIFTS) and reflection absorption IR (RAIRS) spectroscopies.Gold can lead to highly active CO oxidation catalysts at low temperatures [20] and has been used for many other reactions [21]. There have been numerous characterisations of CO adsorption on Au-based samples. Bands in the range 2120\u22122080\u2009cm\u22121 have typically been assigned to CO adsorbed at steps or defects on metallic gold [22,23], while those located within 2080\u22122000\u2009cm\u22121 have been assigned to CO adsorbed on negatively charged Au clusters [24\u201327]. Bands located at 2140\u22122120 and above 2140\u2009cm\u22121 have been assigned to CO adsorbed on positively charged Au\u03b4\n+ with low (\u03b4\u2009<\u20091) and high (1 or 3) charges, respectively. [28\u201331]The assignment of Au\u2212CO bands is in fact not straightforward, especially below 2080\u2009cm\u22121. Some of us reported on the growth of a band at 2070\u22121950\u2009cm-1 derived from neutral Au\u00b0 (2098\u2009cm-1) and Au\u03b4\n+ (2125\u2009cm-1) species during an operando investigation of the water-gas shift reaction over CeO2-supported Au catalysts (Fig. 1\n.A) [29]. The two latter species were actually quantitatively interconverted into the former, as an isosbestic point was observed (red circle in Fig. 1.A). It was suggested that the 2070\u22121950\u2009cm-1 band could possibly be associated with negatively charged gold particles that was gradually building on CeO2-x oxygen vacancies.Behm and co-workers had reported a similar interconversion of a band at 2110\u2009cm\u22121 into one at 2060\u2009cm\u22121 in the case of TiO2-supported Au32. These authors stressed that a CO pressure higher than 1\u2009kPa was necessary for the interconversion to occur and that a spreading of the Au also took place, deduced from Au(4f)/Ti(2p) XPS signal ratios. The 2060 cm\u22121 band was not formed in the presence of O2 and was removed when O2 was introduced after CO. Behm et al. concluded that the 2060\u2009cm\u22121 band was due to the formation of negatively charged Au clusters located at TiO2-x oxygen vacancies.Bianchi and co-workers have observed a similar interconversion over Au supported on alumina, which importantly is a non-reducible support. A band at 2095\u2009cm\u22121 (Au\u00b0) was transformed into a broader one between 2070\u2009cm-1 (Fig. 1.B) [33]. The 2070\u2009cm\u22121 band was assigned to an Au phase restructured by CO. The restructuring happened faster with increasing CO pressures. The heat of CO adsorption at zero coverage on the restructured 2070\u2009cm-1 species (100\u2009kJ mol\u22121) was significantly higher than that on the species at 2095\u2009cm\u22121 (62\u2009kJ mol\u22121), thereby providing the driving force for the restructuration.The presence of an Au\u2212CO band at a wavenumber as low as 2060\u2009cm\u22121 has been reported by Lee and Schwank on Au supported on SiO2, another non-reducible support [34]. We were able to observe the interconversion at 50\u2009\u00b0C of a band at 2113 cm\u22121 (Au\u00b0) into one located at 2076\u2009cm\u22121 over an Au/SiO2 pre-reduced with H2 (Fig. 2\n.A). Isosbestic points were observed in most cases (red circles in Figs. 1.A, 1.B and 2.A), signaling the quantitative interconversion of one species or site into the other one.The carbonyl bands located between 2080\u22122060\u2009cm\u22121 formed upon exposure to CO of Au supported on non-reducible supports such as alumina33 and silica34 are unlikely to be related to the formation of negatively charged Au species, since no oxygen vacancies can be created under the corresponding conditions on these supports. Therefore, another explanation has to be put forward to account for the evolution of the spectra and the origin of these low frequency bands.The propensity of Au surfaces to reconstruct has long been reported. Theoretical work has even proposed that the shape of the whole nanoparticle may vary upon CO adsorption [36]. The tendency for steps on surfaces vicinal to Au(111) to exhibit a much higher kink density when exposed to CO than those on other metals has been stressed [37,38]. Piccolo et al. [39] observed a roughening of the step edges near Au(111) terraces by scanning tunnel microscopy (STM) at room temperature and CO pressures as low as 1 torr. A plethora of rough islands was then noted at 100 torr. The corresponding reflection absorption IR spectroscopy (RAIRS) signal of adsorbed CO was too low to be observed on the initial surface at low CO pressures, related to the weak adsorption of CO on Au(111) terraces [40,41]. Yet, the reconstructed surface exhibited a clearly visible IR band at 2060\u2009cm\u22121. DFT calculations indicated that such wavenumber was consistent with on-top CO adsorption on kinks such as those found on Au(874) surfaces [39].RAIRS experiments over Au(110) [22] and Au(332) [42], which are stepped surfaces that do not exhibit kinks in the absence of reconstruction, revealed bands at 2115 and 2125\u22122110\u2009cm\u22121, respectively. These bands can be primarily assigned to CO adsorbed on step edges rather terraces, since CO adsorption on the latter is much weaker. Nakamura et al. [43] reported similar experiments over Au(311), Au(100) and Au(111). The low index surfaces Au(100) and Au(111) yielded IR bands at 2076 and 2081\u2009cm\u22121, respectively. These bands were only measurable above 0.1 torr of CO and, though not mentioned by Nakamura et al., probably corresponded to kinks of a reconstructed surface (i.e. no longer Au(100) and Au(111)) as discussed above.The case of the Au(311) surface reported by Nakamura et al. is puzzling43. A band at 2117\u2009cm\u22121 was initially observed, which was then gradually replaced at higher pressure (and incidentally time) with one at 2071\u2009cm\u22121. The authors concluded that the 2117\u2009cm\u22121 was due to CO adsorbed on step edges, which is agreement with the above-discussion. Surprisingly, these authors proposed that the 2071\u2009cm\u22121 was due to atop adsorption on terraces at 273\u2009K. This assignment is unlikely to be correct in view of (i) the negligible adsorption of CO on Au(111) terrace sites at this temperature [40\u201342], (ii) recent surface science work [41] carried out at 30\u2009K showed that the band of CO on Au(111) terrace was located at 2130 cm\u22121 and (iii) the simultaneous unexplained disappearance of the 2117\u2009cm\u22121 steps. An alternative interpretation of the data of Nakamura et al. was that the Au(311) surface gradually restructured to a kink-rich surface (band at 2071\u2009cm\u22121), leading to a disappearance of the regular step edges and the corresponding band at 2117\u2009cm\u22121.In conclusions, Au terrace sites lead to bands at around 2130\u2009cm\u22121, but are not occupied above 70\u2009K [41]. At room temperature, non-restructured edges and corners of nanoparticles lead to bands in the range 2115\u22122095\u2009cm\u22121] (Fig. 2.B) [22,28,33,42]. Yet, Au\u00b0 surfaces readily evolve (within minutes) under CO exposure, already at low temperatures. The band in the region 2080\u22122060\u2009cm\u22121 can be assigned to CO adsorbed on Au\u00b0 at low coordination sites such as kinks and roughened edges [39] (Fig. 2.B). Bands in this region can also potentially be assigned to atop CO on negatively charged Au\u03b4- species, whenever Au is supported on a reducible oxide [24,28,32]. These conclusions stress the challenges existing in band assignment on Au-based materials, which will of course be even greater when bimetallics are considered.Au-Pt alloys have been investigated as catalysts for naphta reforming [44] and electrocatalysts [45,46]. The characterization of Pt-Au bimetallic nanoparticles (size ca. 4\u2009nm) supported on silica by IR of CO adsorption at room temperature has been reported by Mott et al. (Fig. 3\n) [47]. The monometallic Au/SiO2 exhibited a single band located at 2115\u2009cm\u22121, typical of CO adsorbed on edges of Au\u00b0 nanoparticles [22,42]. No other band was observed, particularly between 2080\u22122060\u2009cm\u22121 region (delimited by the red dotted box), indicating that Au\u00b0 reconstruction did not occur over the duration of the experiment for this sample. The monometallic Pt/SiO2exhibited a single band located at 2096\u2009cm\u22121, typical of CO adsorbed on Pt\u00b0 nanoparticles [18,19].The bimetallic formulations exhibited markedly different IR spectra, particularly in the 96\u221265 Au at.% range (Fig. 3, curve b\u2013e). New large bands between 2080 and 2050\u2009cm\u22121 were observed. The authors appeared to assign this band only to atop CO on Pt in an alloyed phase. This is partly sensible, since diluted Pt should lead to a lower CO wavenumber as compared to plain Pt because of reduced dipole coupling [48]. An electronic transfer from Au to Pt could also explain this shift, as proposed by Thomas and co-workers [49].However, Au has a significantly lower surface tension than Pt (i.e. 1333 and 2203\u2009mJ m\u22122, respectively [50]) and should therefore preferentially occupy surface sites with low coordination numbers such as corners, equivalent to kinks on a stepped surface. Such sites were shown to exhibit bands in the 2080\u22122060\u2009cm-1 range in the previous section. Surface reconstruction of Au-rich nanoparticles could potentially occur at a faster rate as compared to the case of plain Au. Therefore, it is possible that part of the signal in the 2080\u22122060\u2009cm-1 region could be assigned to Au\u2212CO species.A signal of Au\u2212CO in the region 2080\u22122060\u2009cm\u22121 would also explain the surprisingly low signal observed above 2100\u2009cm-1 (typical of monometallic Au edges, Fig. 3, spectrum a) for the samples still exhibiting high Au contents (Fig. 3, spectra d\u2013h). The lack of Au at the surface could yet be explained by surface segregation, CO preferentially pulling Pt towards the surface because of the stronger bond Pt\u2212CO strength49, though this possibility was not discussed by the authors. Surface segregation effects induced by CO adsorption can possibly be observed with time resolved analysis (vide infra in the case of Au-Pd and Pt-Sn) and were reported by Thomas and co-workers on Pt-Au materials [49].The desorption of CO from Au-Pt bimetallics supported on silica [51] and zirconia [52] has been investigated. A fast desorption of the bands associated with Au (above 2100\u2009cm\u22121) could be observed at room temperature, while those associated with Pt (below 2100\u2009cm\u22121) were stable. None of these groups could yet provide a clear evidence of the presence of alloyed nanoparticles, which would probably exhibit an intermediate behavior, since Au could serve as a porthole for CO desorption after surface diffusing from Pt sites.In conclusion, the assignment of band in the region 2080\u22122050\u2009cm\u22121 solely to Pt\u2212CO species is not a priori warranted in the case of Au-Pt bimetallics and more attention should be given to surface segregation dynamics induced by the addition/removal of CO. To our view, no conclusive evidence for the formation of an alloyed phase has yet emerged solely on the basis of IR analyses of CO adsorption on Au-Pt bimetallics.Au\u2013Pd bimetallic systems are more active and selective than monometallic Pd catalysts in the direct synthesis of H2O2 and have therefore raised of lot of interest [53]. Guesmi and co-workers reported DRIFTS of CO adsorption on alumina-supported Au, Pd and Au-Pd nanoparticles between 2 and 3\u2009nm in size (Fig. 4\n.A) [54]. In contrast to the case of Pt, Pd usually lead to a large signal of carbonyl bands below 2000\u2009cm\u22121 associated with bridged and multi-bonded CO (Fig. 4.A, spectrum b) [55\u201357]. These bands are crucial to ascertain the presence of Pd at the surface of alloy particles in the case of Au-Pd bimetallics, like those at 1950 and 1927\u2009cm\u22121 in spectrum c, since the other bands around 2077\u2009cm\u22121 could in theory also be assigned to Au\u2212CO species.Interestingly, the spectrum of the bimetallic sample evolved with time under CO (Fig. 4.B). The intensity of the Au\u2212CO band at 2109\u2009cm\u22121 declined, while those at 2077, 1950 and 1927\u2009cm\u22121 increased in concert. This observation indicates that the latter bands were associated in most part to Pd and that the surface was getting richer in Pd with time under CO. DFT calculations have quantified the various energy gains of various nanoalloy configurations associated with pulling Pd to the surface due to the stronger bonding between CO and Pd in comparison to that of Au and CO [58].A similar surface segregation of Pd was observed over alumina-supported Au-Pd particles (size between 2 and 6\u2009nm) by combined X-ray absorption and DRIFTS spectroscopies during CO oxidation [59]. The authors proposed that no Au remained at the nanoparticle surface after cooling the catalyst back to room temperature, based on the absence of the 2110\u2009cm\u22121 band typical of Au\u2212CO. The latter statement should yet be taken with caution, as the previous section showed that restructured Au can lead to bands in the 2080\u22122060\u2009cm\u22121 region (actually similar to that of Pd) instead of that around 2110\u2009cm\u22121. In addition, Au atoms on terrace sites would not adsorb CO at room temperature [41].Titania-supported Au-Pd and Au-Rh nanoparticles were investigated by Piccolo and co-Workers [60]. Again, the authors suggested the absence of Au at the surface of a reduced Au-Rh sample based on the absence of a band at 2100\u2009cm\u22121. This somewhat surprising conclusion was explained by a CO-induced surface segregation, which overcame the effect of the higher surface tension of Rh (2325\u2009mJ m-2) [50] in comparison to that of Au (1333\u2009mJ m-2). The same caution as noted above applies here, since restructured Au does not lead to Au\u2212CO bands above 2080\u2009cm\u22121 and thus the absence of bands above 2080\u2009cm\u22121 cannot be taken as a proof of the absence of Au at the surface. The same observation can be made again for the DRIFTS data reported on SiO2-supported Au-Pd by Mou and co-workers [61].In conclusion, the assignment of band in the region 2080\u22122050\u2009cm\u22121 solely to Pd\u2212CO species is not a priori warranted in the case of Au-Pd bimetallics, similarly to the case of Au-Pt sample. Yet, surface segregation dynamics induced by the addition/removal of CO and the presence of strong bands below 2000\u2009cm\u22121 on Pd can provide clear and direct evidence of the presence of alloyed phases, as convincingly reported by Guesmi and co-workers [54].Some of us recently reported an operando DRIFTS investigation of CO oxidation over Au-Ag embedded on silicalite-1 (a zeotype with MFI structure) [27]. The formation of an alloyed phase was conclusively demonstrated from (i) the strongly shifted bands of Ag\u03b4\n+\u2212CO in the case of the alloy (ca. 2160\u2009cm\u22121) as compared to the case of the plain Ag sample (2189\u2009+\u20092172\u2009cm\u22121) and (ii) the reduction by half of the Au\u03b4\n+\u2212CO signal at 2134\u2009cm\u22121, while Au loading and particle size remained the same, due to the preferential presence of Ag on low coordination surface sites (Fig. 5\n.A). Ag is in fact one of the few catalytic metals that presents a surface tension lower than that of Au (1086\u2009mJ m\u22122 for Ag against 1333\u2009mJ m\u22122 for Au) [50] and is thus expected to be preferentially segregated at the surface of the nanoparticles on low coordination sites such as edges and corners (Fig. 5.B).This section discusses bimetallics based on Sn and Zn. These two metals commonly form intermetallic compounds (i.e. ordered alloys) with catalytic metals and present the particular feature of not adsorbing CO under standard conditions [62]. The latter aspect facilitates markedly spectral analysis, since no direct contribution is expected from Sn and Zn.CO adsorption on Pd was noted above (Fig. 4) as usually exhibiting strong bands below 2000\u2009cm\u22121 due to the presence of bridged and multi-bonded carbonyls. Another example of spectrum obtained over an alumina-supported Pd sample is shown in Fig. 6\n.A, in which linearly adsorbed (atop) CO is located at ca. 2090\u2009cm\u22121 and two bridged species are present at 1987 and 1954\u2009cm\u22121. A bimetallic Pd-Sn sample prepared with the same wt.% of Sn and Pd exhibited only the linear species at ca. 2075\u2009cm-1 (Fig. 6.B). This observation indicates that there was no Pd-Pd pairs available at the surface of the bimetallic sample and therefore the IR analysis proved the formation of an intermetallic compound, at least superficially.The IR-based technique (so-called Adsorption Equilibrium IR, AEIR) was used to determine the heat of adsorption at low coverages of the various CO species observed. The linear CO displayed an adsorption heat of around 90\u2009kJ mol\u22121 both in the Pd and Pd-Sn samples. These values are much lower than that of bridged species measured over the Pd sample, which was higher than 165\u2009kJ mol\u22121. Overall, the modifications induced by Sn were found to be essentially geometric and not electronic.Pd-Zn bimetallics are highly active and selective for the selective hydrogenation of acetylene into ethylene [57,63]. The Pd-Zn bimetallics discussed here were prepared by reducing Pd and Zn nitrates impregnated over CeO2[57]. The effect of Zn over Pd in terms of CO adsorption was found to be rather similar to that of Sn described in the previous section. The bridged species (and hence Pd-Pd pairs) were completely eliminated in the case of Zn-containing formulations as compared to the corresponding Zn-free material (Fig. 7\n.A). The heat of adsorption of the linear CO at low coverage determined by the AEIR method was ca. 115\u2009kJ mol\u22121 over Pd-Zn/CeO2 (Fig. 7.B), somewhat higher than that measured over the Pd-Sn sample (90\u2009kJ mol\u22121), suggesting a more significant electronic effect of Zn, besides the obvious geometric effect.It must be noted that the absence of Pd-Pd pairs was not sufficient to achieve the highest selectivity. The samples reduced at 673\u2009K and 773\u2009K both showed no evidence of bridged CO, but the former was significantly less selective than the latter [57]. The linear CO signal was actually different, with maxima at 2054 and 2039\u2009cm\u22121 in the case of the lowest and highest reduction temperatures, respectively (Fig. 7.A). This shift and difference in band shape probably corresponded to differences in the nature of the terminal crystallographic planes exposed by the Pd-Zn nanoparticles, which may be of crucial importance to selectivity.Cobalt-based catalysts typically produce linear alkanes and short alkenes during CO hydrogenation with H2 (Fischer-Tropsch synthesis) [64\u201366]. The IR spectra obtained under reaction conditions are quite complex (Fig. 8\n.A). The CO(ads) IR signal between 2100 and 1950\u2009cm\u22121 can be decomposed into several bands that can be assigned to various linear CO, mostly relating to (111) and (100) planes (Fig. 8.A and B) [64,67\u201372]. A broader band is also observed at ca. 1850\u2009cm\u22121, typical of multi-bonded CO (noted Hollow\u2212CO). The latter band appeared to correlate with the sample activity during an investigation of catalyst resistance to chlorine [70].Co and Sn form various intermetallic compounds (Co3Sn2, CoSn, CoSn2), but in view of the lower surface tension of Sn (675\u2009mJ m\u22122) as compared to that of cobalt (2550\u2009mJ m\u22122), this element is expected to segregate at the surface at low loadings. Small proportions of Sn was added to alumina-supported cobalt (molar ratios Sn:Co\u2009=\u20091:120 up to 1:30) and appeared to selectively titrate the Hollow\u2212CO sites associated with multi-bonded CO (Fig. 9\n.A) [11].Methane and propene were the main reaction products under these reaction conditions (atmospheric pressure). Interestingly, the rates of methane and propene formation appeared to be proportional to the fraction of Hollow\u2212CO (Fig. 9.B). This observation suggested that these Hollow\u2212CO were potential reaction intermediates in the formation of hydrocarbons.A full quantitative analysis of adsorbate concentration was carried out to estimate specific decomposition rates of the Hollow\u2212CO and relate it to product formation rates. The specific rates of decomposition of these Hollow\u2212CO appeared to be about twice as high as the rates of production of methane, the main product formed (Fig. 10\n). These Hollow\u2212CO were thus proposed as being part of the main reaction pathway into hydrocarbon formation, taking into account that methane selectivity is usually around 40\u201350% under these conditions [11]. In contrast, the linear CO(ads) species were shown to be unimportant for the reaction.Catalysts based on bimetallic Pt-Sn have been investigated for many reactions such as the selective hydrogenation of unsaturated compounds [74,75], the dehydrogenation of light alkanes to alkenes [76,77],and the low temperature oxidation of CO without [78] or with an excess of H2 (preferential oxidation of CO, PROX), [7,79].Pt and Sn are fully miscible and form numerous intermetallic compounds. Sn does not bind CO under standard conditions. In a recent combined DRIFTS - DFT study [80], a gradual decay of the linear Pt\u2212CO wavenumber from plain Pt (2075\u2009cm\u22121), through a Sn-poor Pt-Sn (2054\u2009cm\u22121) and to a Sn-rich Pt-Sn (2039\u2009cm\u22121) were reported. The heat of CO adsorption also decreased in the same order [80,81], explaining the improved resistance of Pt-Sn electrodes against CO poisoning as compared to plain Pt [82]. The heat of adsorption of CO over plain Pt [81]was about 185\u2009kJ mol\u22121, while that over a Sn-rich alloy [80] was only 85\u2009kJ mol\u22121.The difference of affinity for CO between Sn and Pt explained the gradual increase of the Pt\u2212CO band around 2030\u2009cm\u22121 corresponding to the gradual Pt-enrichment at the surface of the nanoparticles in a Sn-rich sample following the introduction of CO (Fig. 11\n.A) [80]. DFT calculations confirmed the relevance of this surface segregation, as the segregation energy for Pt in a Sn(100) surface was dramatically lowered in the presence of CO (Fig. 11.B).A different type of segregation was observed in the presence of CO at lower temperature. A phase segregation occurred in which metallic Pt-Sn nanoparticles were converted into Pt\u2009+\u2009SnOx following CO dissociation83. This phase segregation was apparent through the gradual shift of the carbonyl band from 2046\u2009cm\u22121 (corresponding to a Pt-Sn alloy) to higher wavenumber towards plain Pt phases (> 2060 cm\u22121) following H2 removal (Fig. 12\n.A). This phenomenon could be reversed upon heating the sample in a CO + H2 feed up to 300\u2009\u00b0C (Fig. 12.B). The presence of both Pt and Pt-Sn phases was obvious as the Pt\u2212CO and (Pt-Sn)\u2212CO bands could be resolved during the conversion of the Pt phase into Pt-Sn.Somorjai and co-workers have long ago evidenced CO dissociation and the related carbon deposition on Pt steps at room temperature [84] and also on Pt(100) at 498\u2009K [85]. Thus, observing CO dissociation over Pt-Sn nanoparticles is not totally unexpected. The formation of graphitic carbon deposits derived from CO dissociation over the Pt-Sn nanoalloys was confirmed by an in situ XPS analysis [83]. One driving force of the process was the formation of a stable SnOx phase. Thermodynamic calculations (realised on bulk phases) showed that the reaction between CO and Sn to give graphitic carbon\u2009+\u2009SnO was as favourable as the well-known Boudouard reaction (i.e. 2 CO \u21d2 C\u2009+\u2009CO2) (Fig. 13\n). The formation of graphitic carbon\u2009+\u2009SnO2 was even more favourable.In the case of CO oxidation with O2, Sn oxidation and the formation of metallic Pt intimately mixed with SnOx domains has been observed whether H2 is present [7] or not [78]. The segregation of Pt from the Pt-Sn alloy was clearly observed by operando DRIFTS at 225\u2009\u00b0C (Fig. 14\n.A) [7]. The alloy was present before O2 admission, exhibiting a band at 2050\u2009cm\u22121. The band shifted to higher wavenumbers typical of plain Pt (2067\u2009cm\u22121) as soon as O2 was introduced. The higher activity over the Sn-containing catalysts was therefore not due to the presence of an alloyed Pt-Sn phase, but instead to the intimate dispersion of Pt into a SnOx matrix that participated into the oxidation of CO by supplying O to CO adsorbed on Pt (Fig. 14.B). This was reflected in a lower reaction order in O2 (i.e.\u2009+\u20090.2) over the Pt-SnOx sample, as compared to an order of +1 for the plain Pt.An ideal IR probe molecule should not react or degrade the sample under investigation during the IR analysis. Ni forms highly volatile Ni(CO)4 at temperatures as low as 85\u2009K and the erosion of Ni-based catalysts under CO has been documented [86]. The effect of Ni(CO)4, formed in situ or over equipment not suited for use with CO, is often unaccounted for and can lead to gross data misterpretation [14]. Thus, Ni-based catalysts should not be investigated with CO below 200\u2009\u00b0C, to prevent the formation of Ni(CO)4 and the resulting erosion and dispersion of Ni in the apparatus. CO reactivity at low temperatures has also been reported over cobalt-based samples with the formation of surface carbides leading to bands in the 2060\u22122050\u2009cm\u22121 region, instead of 2030\u22121990\u2009cm\u22121 measured over metallic surface [68,69]. Cobalt is highly active for CO dissociation and is thus commonly used for CO hydrogenation in Fischer-Tropsch reactions.CO dissociation at room temperature has even been reported on elements less known for their CO hydrogenation abilities, e.g. Pt steps [84] and, even more surprisingly, on Au(110) surfaces [38]. Marie and co-workers [87] also reported that CO was able to oxidize Ag nanoparticles at room temperature, with the concomitant formation of elemental carbon through Boudouard reaction. These observations stress that the occurrence of CO reactivity as observed on our Pt-Sn (Fig. 12) [81,83] is more common than usually thought and probably was overlooked in many instances.A strategy to limit or avoid carbon deposition is to carry out the adsorption of CO in the presence of H2, though the level of CO conversion (e.g. to CH4) should thus be checked as to not modify significantly the partial pressure of CO used [81,57]. The same \u201ccleaning\u201d effect of H2 was observed in the case of the Pt-Sn materials, where hydrogen helped removing adsorbed O that would otherwise oxidize Sn and lead to phase segregation (Fig. 12) [83].The use of IR spectroscopy of CO adsorption to investigate the surface of bimetallics or alloys is challenging and not conclusive in many instances. Firstly, this results from the complexity of the spectra exhibited by individual metals that depends on many structural parameters. Secondly, CO adsorption often results in structural modification of the surface composition of nanoalloys via reconstruction (as in the case of monometallic surfaces) or surface segregation when one metal displays a higher affinity for CO. The dynamics of these modifications may actually be used to ascertain the presence (or absence) of alloyed surfaces.The addition of catalytically inactive elements such as Sn and Zn can be used to improve selectivity (via a dilution geometrical effect) of otherwise highly active but unselective metals, as in the case of Pd used for acetylene hydrogenation. Conveniently, these inactive elements (e.g. Sn and Zn) do not chemisorb CO under standards conditions and can be used to poison selectively surface sites and facilitate the understanding of surface structure and reactivity. Sn added to cobalt nanoparticles was shown to specifically poison the surface active sites associated with the hydrogenation of multi-bonded CO into hydrocarbons.CO was also shown to dissociate on Pt-Sn surfaces, albeit at a slow rate over timescale of minutes around room temperature. CO dissociation has also been reported for Pt, Ag and Au-based surfaces under similar conditions and should be a reminder that CO is not always inert under these conditions, even on metals not efficient in CO methanation-like reactions.All the authors participated to the writing of the review and approved its final version.The authors declare no competing interests.R.A. acknowledges a PhD scholarship from the Ministry of higher Education and Research of France at the University of Lyon. T.E. acknowledges a PhD scholarship from the ANR, project DECOMPNOx (ANR-18-CE07-0002-01).", "descript": "\n                  This contribution reviews some of the structural features of supported bimetallic catalysts that could be unravelled using IR spectroscopy of CO adsorption. The few examples presented are focussed on active metals such as Au, Pd, Pt and Co and modifications by inert elements such as Sn or Zn. The difficulty in interpreting the IR spectra during CO adsorption over Au-Pt and Au-Pd is underlined, because of Au restructuring that leads to bands typically in the range associated with Pt and Pd carbonyls. Crucial aspects of metal dispersion and surface segregation can yet be obtained, such as active metal site isolation by the disappearance of bridged or multi-bonded CO. The use of IR spectroscopy of CO adsorption to investigate the surface of bimetallics or alloys remains yet challenging and not conclusive in many instances. Firstly, this results from the complexity of the spectra obtained (even over single metals) that depends on many parameters. Secondly, CO adsorption often results in modifications of both surface structure and composition of nanoalloys via reconstruction, as in the case of monometallic surfaces, or surface segregation when one of the metals exhibits a greater affinity for CO. CO dissociation near room temperature has also been documented on many metals, including cobalt and more surprisingly Pt, Ag and Au-based materials.\n               "}
{"full_text": "In so many aspects, the irruption of Metal-Organic Framework (MOF) materials revolutionized the applications of the porous materials [1\u20133]. A good example could be found with Fe-based MOFs, taking advantage of some favorable properties of iron amongst the possible metals forming MOFs, such as its low price, high abundance, low toxicity, redox behavior, etc. As a consequence, Fe-carboxylate MOFs are nowadays very promising materials in applications as diverse as biomedicine [4], redox and/or acid catalysis [5\u201311], supports of enzymes [12\u201315] or photocatalysis [13,16], amongst others.On the other hand, like other trivalent metals such as Cr [17,18], Sc [19,20] or V [21,22], iron is able to form the pair of closely related MOFs MIL-100(Fe) [23] (iron trimesate) and MIL-101(Fe) [24] (iron terephthalate) which were (in their chromium form) the first reported MOFs having mesocavities. Moreover, unlike the above mentioned trivalent metals, iron can also form the semicrystalline Fe-BTC material, which is structurally close to MIL-100(Fe) [7,25]. In spite of its semiamorphous nature, it sometimes surpasses the catalytic activity of MIL-100(Fe), especially when Lewis acid sites are demanded by the reaction [26]. The interest in Fe-BTC material is such that it was one of the first MOFs to be commercialized, specifically as so-called Basolite F300 by BASF/Aldrich. As a consequence, it was used in different applications [27,28], particularly in catalysis [5,26], before its indirect [29] or direct [7,30] syntheses were reported. Since then, it continues being applied in some other different fields [31\u201345], again with special emphasis on heterogeneous catalysis [12,13,16,46\u201352], although its structure continues being unknown in spite of an important recent progress [53].The relationship between MIL-100(Fe) and Fe-BTC is very singular in so many aspects. Firstly, although it has been shown that both materials possess many similarities (composition [25], thermal stability [54], nature/structure of metal clusters [55], a common mesocavity [7], the possibility of being prepared at room temperature and in water [7,54] or similar catalytic applications [26]), they do not form a nanocrystalline-microcrystalline pair. They are rather a semicrystalline-full crystalline pair of materials. To the best of our knowledge such relationship is unique in the huge family of MOFs. In this sense, it is quite remarkable the difference in their preparation at room temperature: starting from Fe(III) source, Fe-BTC is formed instantaneously [7], whereas if Fe(II) source is used in otherwise equal synthesis, crystalline MIL-100(Fe) is formed more slowly, in parallel with the oxidation of Fe(II) to the more stable Fe(III) in aqueous solution [54]. Secondly, as it was mentioned above, the semiamorphous Me-BTC has been only described when the metal Me is Fe but not for other trivalent counterparts like Cr, Al, V or Sc, which suggests that Fe possesses singular trend to form this kind of semiamorphous MOFs.Furthermore, apart from Fe-BTC, MIL-100(Fe) has another highly related MOF material: MIL-101(Fe). Although both MIL materials are prepared with different linkers (trimesate and terephthalate, respectively), they have: (i) the same metal clusters [Fe3O(X)(H2O)2]6+ (X\u00a0=\u00a0F- or OH-); (ii) the same topology MTN [56]; (iii) the possibility of being easily extended or functionalized [56], and (iii) two different but highly-related mesocavities (with diameters of 25 and 29\u00a0\u00c5, and 29 and 34\u00a0\u00c5, respectively), all having microporous entrances [17,18]. Moreover, MIL-101(Fe) has in principle two key advantages over MIL-100(Fe): the lower price of its organic linker and its much higher textural properties (surface area, pore volume, diameter of its cavities and the entrance to these cavities, etc.). With this background, one might wonder if MIL-101(Fe) could also have a semiamorphous counterpart yet to be discovered: Fe-BDC, the missing link amongst the Fe3O-clustered carboxylate-based MOFs.This work describes systematic approaches to afford the semiamorphous Fe-BDC at room temperature. Although it was not achieved in water as solvent, the use of iron(II) acetate as Fe source in either N,N-dimethylformamide or ethanol as solvents led to a relatively porous semiamorphous Fe-BDC material, with evident similarities to both the crystalline Fe-BDC MOF MIL-101(Fe) and the semiamorphous Fe-BTC. The catalytic performance of this Fe-BDC was tested in the aerobic oxidation of cyclohexene. It at least equals the catalytic activity of the commercial Fe-BTC Basolite F300.A typical synthesis of the samples FeBDC starts with the preparation of two solutions/suspensions. Solution 1 is formed by dissolving 1.5\u00a0mmol of iron(II) acetate (Fe(OAc)2, supplied by Sigma-Aldrich) in 5\u00a0mL of one of the following solvents: DMF, ethanol (EtOH), methanol (MeOH) or H2O. Similarly, solution/suspension 2 is prepared by adding 1.5\u00a0mmol of linker H2BDC in 5\u00a0mL of the same solvent; actually it only becomes a solution when the linker is added to DMF, as such amount of linker is not soluble in any of the other three solvents. Next, solution 1 was added dropwise (for 5\u00a0min) over the solution/suspension 2 under stirring, what provokes the immediate appearance of a brownish orange precipitate. The resultant mixture was kept at room temperature for 18\u00a0h under stirring, and then the suspension was filtered, and the solid were washed 2 times with the synthesis solvent and 3 more times with ethanol. Then, the material was immersed in ethanol for 6 days changing the washing solvent by fresh ethanol every 2 days. Finally, the samples were dried at 80 \u00baC for 2\u00a0h, and were labeled as Fe-BDC-DMF, Fe-BDC-EtOH, Fe-BDC-MeOH and Fe-BDC-H2O according to the solvent used in their synthesis.The same methodology was used for the preparation of the samples denoted as Fe-BDC-DMF-75 and Fe-BDC-DMF-100, except the temperature at which the mixture was treated during the 18\u00a0h after the addition of solution 1 over solution/suspension 2, which was 75 and 100 \u00baC, respectively, instead of the room temperature (and stirring) used for the preparation of the sample Fe-BDC-DMF.For comparative purposes, a commercial Fe-BTC (Basolite F300) was purchased from Sigma-Aldrich.X-ray diffraction (XRD) patterns were collected with a Philips X\u2019PERT diffractometer having a X\u2019Celerator detector and using Cu K\u03b1 radiation. Nitrogen adsorption/desorption isotherms were measured at \u2212\u00a0196\u00a0\u00b0C in a Micromeritics ASAP 2420 device; the samples were previously degassed at 150\u00a0\u00b0C under high vacuum for at least 16\u00a0h; Specific surface areas were estimated by BET method, external/micropore surface area by t-plot method, and pore size distribution (PSD) by BJH method. Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer TGA7 instrument, with a heating rate of 20\u00a0\u00b0C/min under air flow. Scanning electron microscopy (SEM) studies were carried out in an ultrahigh resolution FEI-NOVA NanoSEM 230 FESEM instrument. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra of samples were recorded using a Thermo Nicolet Nexus 670 FTIR spectrometer equipped with a SensIR Technologies DurasamplIR horizontal ATR accessory and a liquid nitrogen-cooled MCT detector. Fe content of the samples was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) in a PlasmaQuant PQ 9000 after the samples were dissolved by acidic treatment in a microwave-assisted oven.In a typical catalytic reaction, 50\u00a0mg of the catalyst were activated at 100\u00a0\u00b0C overnight under N2 flow (20\u00a0mLmin-1) within a round bottom flash of 25\u00a0mL. 8\u00a0mL (80\u00a0mmol) of cyclohexene and 0.85\u00a0mL (5.23\u00a0mmol) of octane (chromatographic internal standard), were added under a flow of O2 (20\u00a0mLmin-1). The flask containing the reaction mixture was submerged within a silicon bath and connected to a refrigerant where water at 5 \u00baC (to avoid evaporation of the volatiles reactants and products) recirculated. Once the reaction temperature is reached (60 \u00baC inside the flask), oxygen was slowly bubbled into the reaction mixture. Aliquots of 0.1\u00a0mL diluted in 1\u00a0mL of acetonitrile were taken for quantification in GC Varian 430 with a 15\u00a0m\u00a0x\u00a00.25\u00a0mm diameter column and a FID detector.As explained in the introduction, the semiamorphous Fe-BTC and the crystalline MIL-100(Fe) possess both structural similarities and structural differences. Such similarities/differences are manifested in the XRD characterization (\nFig. 1A) in such a way that the broad XRD reflections characteristic of Fe-BTC are reproduced in corresponding groups of sharp XRD peaks in the pattern of the MIL-100(Fe) [7,25] (pointed by arrows in Fig. 1A). The XRD patterns of the samples Fe-BDC prepared at room temperature in different solvents are plotted in Fig. 1B. Just like it occurs in Fig. 1A, the broad XRD reflections of the pattern of the samples Fe-BDC prepared in either ethanol or DMF media, match very well with different set of reflections found in the simulated pattern of MIL-101(Cr). (The simulated pattern of MIL-101(Fe) is not plotted instead of that of MIL-101(Cr) because, to the best of our knowledge, the cif file of this material is not available in the literature and/or crystallographic databases. Nevertheless, the patterns of MIL-101(Fe) and MIL-101(Cr) should be practically equal, just like the patterns of the also isostructural materials MIL-100(Fe) and MIL-100(Cr) are [23,57]). It strongly suggests that these two samples are related to MIL-101(Fe) in a way very similar to how MIL-100(Fe) and the semi amophous Fe-BTC are related. In other words, it seems that the Fe-BDC and MIL-101(Fe) materials would form a semiamorphous / crystallized pair of MOFs, just like Fe-BTC and MIL-100(Fe) materials do.On the other hand, the number of arrows indicating the structural similarities between XRD patterns of Fe-BDC and MIL-101(Cr) (Fig. 1B) is lower than that indicating the same for Fe-BTC and MIL-100(Fe) (Fig. 1A). It is particularly remarkable the absence of any reflection bands in the 2\u03b8 region of 4\u20136.5\u00ba in the XRD patterns of both samples Fe-BDC (that is normally associated to relatively large microporosity within the material), whereas the XRD pattern of the MIL-101 materials has a group of quite intense bands precisely in such 2\u03b8 region (\nFigs. 1B and 2). The equivalent reflections in the simulated XRD pattern of the material MIL-100(Fe) (in this case slightly shifted towards the 2\u03b8 range of 5.5\u20138\u00ba) found their corresponding XRD band in the XRD pattern of Fe-BTC (Fig. 1A). All this suggests that Fe-BTC somehow have higher construction degree than the Fe-BDC prepared at room temperature, that is, Fe-BTC is a bit closer of becoming a MIL-100 material than the here-presented Fe-BDC of becoming a MIL-101 material.However, the XRD patterns of the samples Fe-BDC prepared in either water or methanol at room temperature are clearly different to these of the samples Fe-BDC prepared in ethanol or DMF (Fig. 1B). The former are dominated by three intense and sharp reflections at ca. 17.5, 25.4 and 28.1\u00ba (marked by asterisks in Fig. 1B) which are typical of the XRD pattern of the unreacted protonated linker H2BDC [58]. Some other relatively sharp XRD reflections, which does not match with the simulated pattern of MIL-101, were detected at 2\u03b8 below 13\u00ba (and then potentially related to the existence of certain microporous features) in the diffractogram of the sample prepared in methanol but not in the one prepared in water. It must be noted that the solubility of terephthalic acid, which is essential to get a BDC-based MOF under the studied conditions, increases as a function of the solvent nature in the following order: water (practically insoluble) <\u2009methanol <\u2009ethanol <\u2009DMF (very soluble). Obviously, unlike the samples Fe-BDC-DMF and Fe-BDC-EtOH, the samples Fe-BDC-MeOH and Fe-BDC-H2O cannot be considered a semiamorphous Fe-BDC.Trying to certify that our samples Fe-BDC-DMF and Fe-BDC-EtOH prepared at room temperature really possess strong structural likenesses with MIL-101(Fe), the Fe-BDC synthesis mixture prepared in DMF was heated up different temperatures for 18\u2009h, since, to the best of our knowledge, the synthesis of MIL-101(Fe) has not been described at room temperature and then heating could be an essential stimulus to the formation of this crystalline phase. The low angle XRD patterns of the corresponding samples compared to the simulated pattern of MIL-101(Cr), are shown in Fig. 2 whereas the high-angle XRDs are plotted in Fig. S1. The low-angle diffractogram of the sample Fe-BDC-DMF prepared at room temperature does not practically show any indication of reflection at 2\u03b8 below ca. 7\u00b0 (Fig. 2), suggesting that this sample lacks spatially ordered mesocavities. Contrasting with this fact, Fe-BTC possesses the smallest ordered mesocavity of the two found in MIL-100(Fe) [7]. As the synthesis temperature rises, some low angle reflections start to appear. These reflections, which are very broad after a thermal treatment at 75\u2009\u00baC for 18\u2009h, become unequivocally demarcated when the sample results from treating the synthesis mixture at 100\u2009\u00baC. These peaks match very well with the XRD pattern of the MIL-101(Cr) at low angle. It indicates that the structural units responsible for the reflections in the XRD pattern of the sample Fe-BDC-DMF prepared at room temperature are capable of recombining to give rise to MIL-101(Fe)-like material with the unique stimulus of temperature. Therefore, one would expect that at least some of the structural building units forming the MIL-101(Fe) structure are already present in the sample Fe-BDC-DMF. It is important to remark that the transformation of such structural units into MIL-101(Fe) is not a crystal growth phenomenon (which would transform a nanocrystalline material into a microcrystalline one) but the construction of an ordered structure from its already existing \u2018bricks\u2019. The absence of low angle XRD reflections in the pattern of the room-temperature prepared sample Fe-BDC-DMF as well as the generation of new reflections (rather than a simple narrowing of the existing ones) as a consequence of the increase of synthesis temperature, support this hypothesis.Unlike the long-range information given by XRD, IR spectroscopy region of ca. 650\u22121800\u2009cm\u22121 (\nFig. 3) is rather sensitive to the conformational and/or local environment of organic molecules (short-range information). That IR region can be taken as a fingerprint of the materials nature, especially when they contain organic entities like MOFs [59,60]. Therefore, this technique can somehow complement the limited structural long-range information provided by XRD on the semiamorphous materials. It is remarkable the substantial difference of this spectra in comparison with that of the protonated linker H2BDC (Fig. 3), in spite of the FTIR features of this spectra region are due to almost the same chemical species: terephthalate linked to either iron metal ions or protons. The spectra of the four sample Fe-BDC are practically identical. Only the broad bands at 1100 and 1651\u2009cm-1 in the spectra of all three samples Fe-BDC-DMF, attributed to CO bond strength of DMF, differentiate the FTIR spectra of the samples Fe-BDC. The most intense bands were assigned as follows: (i) 746\u2009cm-1, out-of-plane bending vibration of C\u2013H in aromatic rings (750\u2009cm-1\n[61] or 749\u2009cm-1\n[62] reported for MIL-101(Fe)); (ii) 1381\u20131384\u2009cm-1, symmetric stretching of carboxyl groups of the aromatic carbon C\u2013C vibrational mode in terephthalate (~\u20091400\u2009cm-1\n[61] and 1394\u2009cm-1\n[63] reported for MIL-101(Fe); (iii) 1504\u2009cm-1 asymmetric stretching of CO bonding carboxyl groups in terephthalate (~\u20091500\u2009cm-1 reported for MIL-101(Fe) [61]); (iv) 1567\u20131571\u2009cm-1, CO bonding in the carboxylates (1584\u2009cm-1 reported for MIL-101(Fe) [63]). (It must be noted that the FTIR-ATR spectra presented in this study has not been corrected, so the frequency of the bands should be shifted to a few higher wavenumbers [64] to make them comparable to reported transmission/absorbance FTIR values, which would actually lead to a better agreement between the frequencies found in this work and these of the reported ones). In summary, all the main bands found in the FTIR spectra of the semiamorphous samples Fe-BDC were also found in the FTIR spectra of MIL-101(Fe). It suggests that terephthalate anions within Fe-BDC materials adopt practically equal conformation/environment to that given within MIL-101(Fe).\n\nFig. 4 plots the TGA profiles of the samples Fe-BDC prepared in different solvents and at different temperatures. The temperature at which linker decomposes (event normally related to the thermal stability of MOFs), is practically the same in all Fe-BDC samples, at ca. 360\u2009\u00baC, under the studied analysis conditions. The weight loss of linker is taken place in one relatively quick step. Terephthalate in MIL-101 (Fe) has been reported to begin to decompose at 350\u2009\u00b0C [65] (in two steps) and at 345\u2009\u00baC [57] (in practically one step). Although the analysis conditions could slightly alter the temperature of the weight losses, the similar values in linker decomposition suggest that terephthalate is in very similar coordination to iron metal clusters in MIL-101(Fe) and in our semiamorphous Fe-BDC samples. Moreover, the linker decomposition temperature is slightly higher than the linker decomposition temperature found in Fe-BTC materials (341\u2013343 \u00baC either commercial one or prepared in the laboratory) under the same TGA conditions [7]. It has to be noted that both materials, Fe-BDC and Fe-BTC, are based on different organic linkers, so differences in thermal stability of such order were expected.The textural properties of the different Fe-BDC samples prepared in DMF were evaluated by means of N2 sorption isotherms at \u2212\u2009196\u2009\u00baC (\nFig. 5 and S2 and \nTable 1). All the samples Fe-BDC are quite porous, having BET specific surface areas above 500\u2009m2g-1, in particular, 536, 675 and 926\u2009m2g-1 for the samples prepared at room temperature, at 75\u2009\u00baC and at 100\u2009\u00baC, respectively. As expected according to the XRD discussion, the samples Fe-BDC prepared at temperatures above room temperature (Fe-BDC-DMF-75 and -100), which possesses some XRD features of the material MIL-101(Fe), widely surpass the textural properties of the sample Fe-BDC-DMF prepared at room temperature. Fig. 5A makes clear that the difference in the textural properties of the three samples is specially given at small relative pressures p/p0 (in the 0.01\u20130.3 range). It means that the porosity of these samples are mainly differentiated in relatively small mesoporosity, precisely in the isotherm region at which the mesocavities of the MIL-101(Fe) material, which are absent in the sample Fe-BDC-DMF but present in the samples Fe-BDC-DMF-75 and -100, should manifest. This fact is even more evident in the PSD curves presented in Fig. 5B, which does not show any small mesoporosity for the sample Fe-BDC-DMF, whereas the other two samples unequivocally present relatively sharp PSD maxima at ca. 25.1 and 30.7\u2009\u00c5, which are more abundant and well-defined at higher synthesis temperature, in good agreement with the intensity of the low-angle XRD peaks (Fig. 2). We ascribed these PSD maxima to the mesocavities of the MIL-101(Fe) material, having diameters of 29 and 34\u2009\u00c5 (for MIL-101(Cr) [17]. The slight deviation of the here-estimated diameter cage (of ca. 3.3\u20133.9\u2009\u00c5) must be due to the well-known underestimation in pore size of mesoporous by the BJH method [7,54,66].It must be noted that the BET surface areas of these samples Fe-BDC (in the range 536\u2013926\u2009m2g-1) are significant even when compared with reported textural properties of MIL-101(Fe). In principle, one could expect that MIL-101(Fe) would reach similar BET specific surface areas close to that of 4100\u2009m2g-1 reported for MIL-101(Cr). However, the reported surface areas of MIL-101(Fe) are quite lower than this expected value: 101 [67], 125 [68], 560 [69], 1018 [57], 1312 [70] or 1642 [71] m2g-1. According to a recent publication, the reason behind so poor textural properties is the co-crystallization of MOF-235 and MIL-101(Fe) in different proportions as a function of synthesis conditions [72].In addition, the isotherms of the three samples Fe-BDC-DMF show high N2 adsorption at high relative pressure p/p0 (Fig. 5A), which should be ascribed to large interparticles mesopores (above 30\u2009nm of average diameter, Fig. S2), probably as a consequence of the agglomeration/aggregation of the particles formed by an instantaneous and massive precipitation. To shed light on the morphology of the samples, \nFig. 6 shows some representative FE-SEM images of the sample Fe-BDC-DMF, which would be the \u2018real\u2019 semiamorphous Fe-BDC. As forecasted by the isotherms from Fig. 5A, the sample is indeed formed by large particles that are formed by a huge number of much smaller aggregated nanoparticles, leaving some meso-/macro-porosity in their aggregation. It could explain the high adsorption of N2 at high relative pressure p/p0 (above 0.8). The morphology of this sample resembles that of the Basolite F300 and the lab-made Fe-BTC [7], the latter also obtained by immediate precipitation as soon as iron and linker sources made contact.The catalytic potential of some of these Fe-BDC materials was tested in the solvent-free aerobic oxidation of cyclohexene. Cyclohexene can be oxidized by two different mechanisms, either through epoxidation or allylic oxidation, leading to different products (Scheme S1). \nFig. 7 shows the cyclohexene conversion using the following catalyst: (i) the most semiamorphous sample Fe-BDC-DMF (prepared at room temperature), (ii) the sample Fe-BDC structurally closest to MIL-101(Fe) (prepared at 100\u2009\u00baC), (iii) the commercial Fe-BTC Basolite F300, (iv) a Zn-MOF-74 prepared according to literature [60] as an example of MOF-based catalyst having open metal sites but free of any redox-active metal center, and (v) no catalyst (the blank experiment). The first remarkable aspect from Fig. 7 is that a MOF that lacks redox metals like Zn-MOF-74 is not active in the reaction, even becoming an inhibitor as the reaction goes at some extent in the absence of any catalyst (blank experiment) but not in the presence of Zn-MOF-74. On the contrary, all Fe-MOF-based catalysts gave higher conversion than the blank. Therefore, the semiamorphous Fe-BDC is an active catalyst in oxidation reactions.The catalytic performance of the semiamorphous Fe-BDC prepared at room temperature is below that given by the commercial Fe-BTC Basolite F300 after short reaction times, due to a longer induction time by the former (it does not show any activity until 4\u2009h). It could be related to a limited accessibility of the reactants to the metal centers, as the samples Fe-BDC-DMF does not have any mesocavities (Figs. 1B and 5B), whereas Fe-BTC does[7]. Supporting this interpretation about the catalytic activity delay of Fe-BDC-DMF, the induction time is also shorter when the catalyst is Fe-BDC-DMF-100, which is also a semiamorphous Fe-BDC but having further structural similarities with MIL-101(Fe), including certain amount of mesocavities. It must be noted that, once the induction period has passed, the kinetics of the cyclohexene conversion follows the same slope for both samples Fe-BDC-DMF and Fe-BDC-DMF-100, indicating the same intrinsic catalytic activity of their metal centers. Moreover, at longer reaction times, the catalytic performances of the samples Fe-BDC-DMF equals or even slightly exceeds that of the Basolite F300, which has been previously proved to be a good heterogeneous catalyst in the cyclohexene oxidation [7]. For strict comparison, the concentration of the active Fe centers should be also taken into account. In good agreement with what was expected from the supposed composition, Fe-BTC contains slightly lower iron concentration (23.0\u2009wt% Fe in the dry sample, according to ICP-OES) than Fe-BDC-DMF (26.6\u2009wt% Fe) and Fe-BDC-DMF-100 (24.5\u2009wt%).\n\nFig. 8 separates the yield of products obtained by epoxidation and by radical mechanisms. For all Fe-based MOF catalysts, the products via radical mechanism dominates over the epoxidation products. Nevertheless, the epoxidation proportion is much higher with this series of catalysts than with M-MOF-74 catalysts (with M being a redox metal like Cu, Co, Mn or Ni) under similar conditions [73\u201375]. Although the reason behind such radical/epoxidation ratio is not clear at this moment, it could be related to the higher accessibility of reactants to the active sites (framework iron) in these MOFs materials, as the epoxidation requires that both substrate (cyclohexene) and oxidant (molecular oxygen) become coordinated to a given active center.On the other hand, the structure of the sample Fe-BDC-DMF is basically maintained after reaction (Fig. S3), whereas Fe-BTC seems to suffer severe degradation under similar reaction conditions.This work presents the discovery of the semiamorphous MOF material Fe-BDC. Just like the widely-used catalysts Fe-BTC and MIL-100(Fe) form an unprecedented semiamorphous/full-crystallized pair of MOFs, the new material Fe-BDC is highly related to MIL-101(Fe) in a similar way. The semiamorphous Fe-BDC can be prepared in ethanol or in N,N-dimethlyformamide at room temperature by simply contacting both iron and terephthalic acid sources, with no extra chemical species (modulator, deprotonating agent, etc.). However, it could not be prepared in water or in methanol probably due to the negligible solubility of the organic linker in these solvents. The semiamorphous Fe-BDC and MIL-101(Fe) possess many key properties in common such as the same metal clusters, almost equal linker environments and conformations and similar thermal stability. However, the semiamorphous Fe-BDC, despite its acceptable microporosity, basically lacks both the crystalline nature and the mesoporosity of MIL-101(Fe). The similarities are accentuated when Fe-BDC is synthesized at higher temperatures (for instance, 100 \u00baC), which potentially allows to prepare on demand Fe-BDC-based materials having the same building units and different long-range order and porosity. These materials were catalytically tested in the solvent-free aerobic oxidation of cyclohexene, showing catalytic performance at least of the same order than that given by the commercial Fe-BTC catalyst. Such catalytic performance becomes higher when the semiamorphous Fe-BDC is prepared at high temperature.All authors have read and agree to the published version of the manuscript. J. Gabriel Flores Aguilar: Experimental work and design, Results discussion, Figures for the first draft of the manuscript. Rafael Delgado-Garc\u00eda: Experimental work and design, Results discussion. Manuel S\u00e1nchez-S\u00e1nchez: Conceptualization, Resources, Writing \u2013 original draft, 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.Authors thanks Dr. Carlos Marquez-\u00c1lvarez for his help in registering the FTIR-ATR spectra. This work has been partially financed by CSIC through the programs: (i) CVCSIC-AEPP-Ayudas Extraordinarias para preparaci\u00f3n de proyectos 2019 (2019AEP076) and (ii) i-COOP-2018 (COOPA20271). This work has been also financed by the CONACyT project A1\u20135-30646. J.G.F. acknowledges a Ph.D. CONACyT grant (687839). R.D.G. thanks the JAE-Intro CSIC grant (JAEINT18_EX_0043).Supplementary data associated with this article can be found in the online version at doi:10.1016/j.cattod.2021.11.004.\n\n\nFigure S1\n\nSupplementary material\n\n\n\n.", "descript": "\n                  Porous Fe carboxylates are amongst the most promising MOF-based materials due to their low price, low toxicity, metal environments (including open metal sites) and remarkable (meso)porosity variety. Fe-MOFs based on the cluster [Fe3O(X)(solvent)2]6+, that is, MIL-101(Fe), MIL-100(Fe) and semiamorphous Fe-BTC, are of particular interest. These three materials are quite related each other: (i) MIL-100(Fe) and MIL-101(Fe) have the same zeolitic topology MTN and two types of mesocavities, whereas (ii) Fe-BTC and MIL-100(Fe) form an unprecedented semiamorphous / fully-crystallized pair, having in common the metal cluster, the composition, one mesocavity, etc but without becoming a nano- / micro-crystalline pair. This work describes the room-temperature synthesis, characterization and catalytic performance in the aerobic cyclohexene oxidation of the semiamorphous Fe-BDC, which together with MIL-101(Fe) would form the second semiamorphous / crystallized pair in MOFs. Unfortunately, Fe-BDC could not be prepared in water as solvent, but in either ethanol or in N,N-dimethylformamide. It possesses relatively high textural properties (above 500\u00a0m2g-1) and key common features with MIL-101(Fe): XRD reflections at the same 2\u03b8 positions, similar thermal stability, almost equal linker conformations, etc. Fe-BDC became quite active in the solvent-free aerobic oxidation of cyclohexene under mild conditions, surpassing the activity performance of the well-known commercial Fe-BTC catalyst in the same reaction under the same mild conditions.\n               "}
{"full_text": "With the increasing number of motor vehicles, the harmful automotive exhaust becomes a major source of pollution in the urban environment all over the world. During combustion of fuel, in which sulfur-containing compounds are presented with trace amount, SO\nx\n is inevitably produced, leading to that 1) air pollution, and 2) irreversible poisoning of the three-way catalysts that are used in the emission after-treatment system. Therefore, more stringent environmental legislations have been introduced worldwide, which are especially crucial for the oil refinery industry. In Euro V and Tier III legislations, sulfur content in gasoline is required to be less than 10\u00a0\u03bcg g\u22121 [1]. The National V standard, implemented in China, also requires the sulfur content in gasoline to be lower than 10\u00a0\u03bcg\u00a0g\u22121. Hydrodesulfurization (HDS) is the prevailing approach for sulfur removal in hydrocarbons, where H2 acts as the major reductant [2]. The HDS process using Co-Mo/Al2O3 or Ni-Mo/Al2O3 catalysts [3\u20136] is highly efficient in removing thiols, sulfides, and disulfides, but less effective for aromatic thiophenes and thiophene derivatives. In order to achieve overall high HDS efficiency, it requires high H2 pressure, simultaneously leading to the saturation of olefins. Such hydrogenation of unsaturated hydrocarbons will reduce the octane number of fluid catalytic cracking (FCC) gasoline.As the catalytic hydrodesulfurization (HDS) process itself is difficult to compromise the requirement of ultra-deep desulfurization and retaining high octane number [7\u20139], many new approaches, such as catalytic oxidative desulfurization [10], adsorption desulfurization [9], reactive adsorption desulfurization [1,11\u201313] ionic liquid extraction desulfurization [14,15], biocatalytic treatment [16], have been developed for sulfur removal from hydrocarbons. The reactive adsorption desulfurization (RADS) is one of the promising alternative approaches for deep desulfurization. It combines the advantages of HDS and adsorptive desulfurization, enabling the rapid removal of formed H2S in order to 1) improve the desulfurization efficiency, and 2) lower the H2 partial pressure thus beneficiary for retaining high octane number of resulted hydrocarbons.The S Zorb process of SINOPEC Corp has been proved to be effective for the production of low-sulfur gasoline via RADS by using a solid material [14,16\u201318]. The process was carried out in a fluidized-bed reactor at 375\u2013450\u00a0\u00b0C, under 1.0\u20133.0\u00a0MPa H2 pressure. In the S Zorb process, Ni/ZnO is the major body of catalyst, where Ni functions as hydrodesulfurization site, while ZnO traps sulfur and transforms into ZnS [18]. During the process, the S atom is retained on the solid material, while the hydrocarbon with reduced sulfur content is released back into the reaction stream. The unique reaction pathway does not generate gas-phase H2S, and, therefore, prevents the recombination of H2S and olefins, which mainly produces mercaptans. In addition, it shifts the equilibrium of the desulfurization reaction and achieves ultra-deep desulfurization under low H2 pressures, thus limiting the olefin saturation [19,20].Many efforts have been made to unravel the mechanism of RADS. In a model reaction mimicking RADS, where thiophene is used as representative sulfur-containing molecule, the whole process comprises 5 steps, i.e. 1) localization and adsorption of thiophene molecule, 2) the interaction of S atoms with Ni followed by the removal of sulfur from thiophene molecule, 3) the formation of nickel sulfide species, 4) the S-transfer from Ni to ZnO forming ZnS on the catalyst surface, 5) the diffusion of ZnS from surface to bulk. Above 5 steps can be further classified into two categories, i.e. the catalytic HDS steps (1, 2 and 3) and S-transfer steps (4 and 5). Above network was generally accepted in a series of reports by Bezverkhyy and co-workers [21\u201324].Regarding the HDS steps, Robert and Angelici [25] summarized configurations of 8-coordinated organometallic complexes formed by thiophene interaction with transition metal, and classified those complexes into three major types, i.e. \u03c0-complexation mode of thiophene, nickel sulfurthiophene with SM (\u03c3) bond mode, and a combination of the former two adsorption modes. Some theoretical studies were performed on the thiophene desulfurization mechanism. Mittendorfer et\u00a0al. [26\u201328] studied the preferential adsorption state of thiophene molecule on the surface of Ni (100). It was pointed out that the adsorption of thiophene molecule was preferred to be adsorbed as \u03c0-complexation mode. As soon as the C-S bond breaks, the S atom will fill the Ni atomic gap, forming NiS. Gao et\u00a0al. [29] argued that both \u03c0-complexation mode and direct sulfur metal (S\u2013M) bonds mode were possible and likely to be presented simultaneously. Following the thiophene adsorption, based on the previous studies, two basic hydrodesulfurization (HDS) reaction routes were proposed for the sulfur removal: direct desulfurization (DDS) and hydrogenative sulfur removal (HYD) [30]. The major difference between HYD and DDS lies on whether H2 is involved in the elementary steps for C-S bond cleavage. Despite numerous experimental works [31\u201335] on the HDS mechanism, it has, however, not been possible to clarify some fundamental arguments, due to the current limitations of the characterization techniques. Thus, the mechanism of thiophene transformation on Ni surface is still under debate.Regarding the S-transfer steps, Li et\u00a0al. [36] studied the effect of ZnO particle size on the desulfurization performance. It was found that the apparent activation energy of sulfur removal on Ni/ZnO was much lower for smaller ZnO particles. After the ZnO particle shell was sulfided, the smaller particle size of ZnO could effectively decrease the energy barrier of sulfur transport in ZnO, leading to high sulfur removal efficiency in the latter stage, where sulfur transport is the rate limiting step. Moreover, the ZnO with smaller particle sizes could result in higher dispersion of Ni species on the ZnO surface, beneficiary for the overall activity enhancement. However, the direct evidence for the S-transfer has still not been reported, limiting a thorough understanding of the process. Zhang et\u00a0al. [37] studied the desulfurization mechanism of thiophene via reactive adsorption over a Zn3NiO4 cluster by DFT calculation and found that thiophene was first decomposed on the Ni site of Zn3NiO4 bimetallic oxide catalyst to form nickel sulfide, followed by the reduction of the nickel sulfide with two reaction pathways. However, above theoretical model was apart from the practical case, because the oxidation state and dispersion were not taken into consideration.In this work, the RADS mechanism of thiophene on Ni/ZnO sample was studied by a combination of DFT calculation and in-situ experiments. The detailed HYD and DDS pathways of thiophene over Ni (111) were simulated by DFT models. The phase transition during desulfurization process on Ni/ZnO was studied by in-situ XRD and STEM-EDS technology, in order to acquire direct evidence of S-transfer phenomenon between Ni and ZnO. As a result, a comprehensive overview of RADS mechanism, initiating with hydrogenative desulfurization followed by sulfur absorption and transfer, was proposed. It provides fundamental understanding for the site requirement of the S Zorb materials, which were demonstrated at industrial scale.The DFT calculations were performed with the program package of DMol3 in the Materials Studio of Accelrys, Inc. The exchange correlation energy was calculated within the generalized gradient approximation (GGA) using the form of the functional proposed by Perdew and Wang, usually referred as Perdew-Wang 91 [38]. The density functional semi core pseudo potential method was employed for the Ni atoms, and the carbon, sulfur, and hydrogen atoms were treated with an all-electron basis set. The valence electron functions were expanded into a set of numerical atomic orbitals by a double numerical basis with polarization functions (DNP). Fermi smearing of 0.136\u00a0eV and real-space cut off of 4.5\u00a0\u00c5 were used to improve the computational performance. All computations were performed with spin polarization.The configurations of the adsorption complex were optimized by relaxing all atoms of the thiophene and two uppermost layers of the surface. The spin-polarized calculations were performed for the adsorption on Ni (111). Relative adsorption energies of all the configurations were calculated as given in Eq. (1).\n\n(1)\n\nE\nad\u00a0=\u00a0E\nth/surf\u00a0\u2212\u00a0E\nsurf\u00a0\u2212\u00a0E\nth\n\n\n\nIn Eq. (1), the adsorption energy was defined as E\nad. The energy notation with other subscripts, i.e. \u201csurf\u201d, \u201cth\u201d and \u201cth/surf\u201d, represent the total energy of clean surface, of the thiophene molecule and of the thiophene adsorbed on the surface, respectively. A negative value of E\nad indicates that the adsorbed system was energetically favored, as compared to the ground state.Transition state (TS) searches were performed with the complete LST / QST (linear synchronous transit maximization / quadratic synchronous transit maximization) method. In this method, the LST was first performed, followed by an energy minimization in directions conjugating to the reaction pathway to obtain approximated TS. After the approximated TS was roughly determined, QST was thereof performed, followed by another conjugated gradient minimization. The cycle was repeated until a stationary point was achieved. The convergence criterion for the TS searches was set to 0.272\u00a0eV\u00a0\u00c5\u22121 for the root-mean-square of atomic forces. Vibrational frequencies were calculated for the initial / final states (IS and FS, respectively) and the TS, which were obtained from the Hessian matrix based on the harmonic approximation. The zero-point energy (ZPE) was accordingly calculated from the above-mentioned vibrational frequencies.All chemicals used for catalyst preparation were of analytical grade. ZnO was purchased from Simopharm Chemical Reagent Co., Ltd. Ni(NO3)2 was purchased from Alfa Aesar. ZnO powders, calcined at 650\u00a0\u00b0C for 4\u00a0h before use, were impregnated in aqueous solution of Ni(NO3)2 with 20\u00a0wt% of NiO loading. The resulted samples were dried at 120\u00a0\u00b0C for 4\u00a0h and calcined at 550\u00a0\u00b0C for 4\u00a0h in static air, denoted as NiO/ZnO. Further reduction was conducted at T\u00a0=\u00a0400\u00a0\u00b0C, under H2 flow with p\u00a0=\u00a01.38\u00a0MPa and 80\u00a0mL\u00a0min\u22121 flow rate for 2\u00a0h.X-ray diffraction (XRD) was used to characterize the crystal structure. In this work, XRD patterns were obtained by using a Siemens D-500 X-ray diffractometer equipped with Ni-filtrated Cu-K\u03b1 radiation (40\u00a0kV / 100\u00a0mA). The 2\u03b8 scanning angle range was 10\u201370\u00b0 with a step of 0.02\u00b0 s\u22121. The average crystal size was estimated from the line broadening of the most intense XRD diffraction peak by the Scherrer equation. The in-situ experiments were performed by coupling an in-situ cell (XRK-900) to the diffractometer under N2, H2 or S-containing hydrocarbons. The detailed conditions are shown together with the results later in the manuscript.TEM analysis was conducted on an aberration-corrected microscope (JEM-ARM200F) working at an acceleration voltage of 200\u00a0kV. STEM EDS-mapping was acquired from selected areas of the HAADF-STEM images with continuous drift correction.As evidenced by XRD, among all the facets of metallic Ni, the major exposed facet of reduced Ni/ZnO is Ni (111) (SI, Fig.\u00a0S1). Therefore, theoretical studies on the energy and structure of thiophene adsorption were performed on the surface of metallic Ni (111) for both \u03c0-complexation mode and S-M adsorption mode (Fig.\u00a01\n), and the calculated thermodynamic parameters are listed in Table 1\n.It can be seen from the table that the adsorption enthalpy values of \u03c0-complexation mode and S-M mode are \u2212156\u00a0kJ\u00a0mol\u22121 and \u2212129\u00a0kJ\u00a0mol\u22121, respectively. Compared to SM adsorption mode, the more negative adsorption enthalpy of \u03c0-complexation mode indicates that \u03c0-complexation is the preferred adsorption mode of thiophene on the Ni surface. Note that the above model is based on the low coverage assumption, i.e. the adsorption is without the influence of neighboring adsorbed thiophene or other molecules, in line with the practical condition of hydrocarbon desulfurization (the sulfur content is at hundreds of \u03bcg g\u22121 level). The preference of thiophene's \u03c0-complexation mode is further supported by our observation from IR spectroscopy (SI, Section 2).After the thiophene molecule is adsorbed on the metal Ni surface with the \u03c0-complexation mode, the C\u2013S bond length of thiophene molecule increases from 0.178 to 0.183\u00a0nm, indicating that the C\u2013S bond is weakened. On the other hand, the S atom begins to approach the Ni atom, and the distance between the Ni and S atom is shortened from initial 0.231\u00a0nm\u20130.217\u00a0nm, close to the NiS bond length (0.220\u00a0\u00b1\u00a00.003\u00a0nm) [39]. This observation was in good agreement with previous experimental studies of XAFS [40,41], XPS [42] and TPD [43].In order to further distinguish the two desulfurization pathways, i.e. DDS and HYD, the elementary steps involved in the two pathways are considered. It is worth mentioning that the activation barrier of H2 splitting is very low, so it is rational that the major species of H2 is H radicals over the Ni surface in the steady state kinetic model.In the DDS pathway, from the adsorbed thiophene of \u03c0-complexation form, the reaction is initiated with the formation of NiS bond and the first CS bond cleavage without involving H radicals, forming C4H4S as the final state of the first elementary step. As for the second step, the hydrogenation of the intermediate C4H4S takes place on the C radical formed via the CS cleavage. Simultaneously, the Ni-S bond has been weakened, leading to a total exothermicity of the second step. The third and fourth steps follow the same order, i.e. CS bond cleavage before hydrogenation. Noteworthy, the energy requirement for the last two steps is much less than the first two steps, and the formation of S-free hydrocarbon molecule is highly exothermic (thermodynamically favored). Fig.\u00a02\n shows the corresponding structures of IS, TS and FS of different elementary steps in the DDS pathway.In this scenario, the first CS bond cleavage (Fig.2a) leads to a downward movement of the S atom to form bond with a surface Ni atom. By forming the first TS (A-TS1), the CS bond is elongated to 0.219\u00a0nm from 0.183\u00a0nm as in the IS. An intermediate C4H4S is then formed (A-FS1) on the surface. The Ni atom is included in the adsorption structure and thus forms a 6-member ring analog. This is similar to the organometallic reaction of thiophene, in which one metal atom inserts into CeS bond [44]. The activation barrier and reaction enthalpy change are calculated as 78.1\u00a0kJ\u00a0mol\u22121 and 28.5\u00a0kJ\u00a0mol\u22121, respectively. The second transition state (A\u2013TS2) is encountered upon the hydrogenation of the intermediate C4H4S. The activation barrier of forming A-TS2 (35.6\u00a0kJ\u00a0mol\u22121) is much lower than of forming A-TS1 (78.1\u00a0kJ\u00a0mol\u22121). The second CS bond cleavage is activated with assistance of the CS bond stretching. After that, the intermediate C4H5S reacts with the H radical, forming adsorbed butadiene molecule and the S atom removed from thiophene is bonded to the Ni (111) surface. The activation barrier of these two steps are 50.5\u00a0kJ\u00a0mol\u22121 and 14.9\u00a0kJ\u00a0mol\u22121, respectively. The results indicate that the first step of CS cleavage is the rate-determining step in the sulfur removal via DDS pathway.As for the HYD pathway, in contrast to the DDS, hydrogen is involved in the CS bond cleavage. It is still rational that the H radicals are presented on the surface with high coverage. The CS bond is only weakened upon the interaction between H radical and the neighboring C atom, forming an unstable 5-coordinated carbon (Step 1). The CS bond is subsequently split, leading to the formation of surface C4H5S intermediate (Step 2). Following the first CS bond cleavage, the second H radical hydrogenates another C atom besides the S atom, weakening another CS bond (Step 3). Finally, the second CS bond is split (Step 4) and the reaction sequence is then closed. The intermediate estimation and energy profile is shown in Fig.\u00a03\n.By now, the overall reaction network with different pathways, i.e. DDS and HYD of thiophene, has been thoroughly simulated by DFT modeling. The energy profile is compared for the HYD and DDS pathways, as shown in Fig.\u00a04\n.The activation barrier of \u201cpre\u201d-hydrogenation step in the HYD pathway is much higher than that of CS bond scission in DDS pathway (117.2 vs 78.1\u00a0kJ\u00a0mol\u22121). Therefore, it can be confidently concluded that thiophene desulfurization on Ni (111) proceeds along the DDS pathway preferentially, thus the first half of sulfur trail has been revealed.Previous works suggested a general scenario that the removed sulfur undergoes a sulfur-transfer process from Ni to ZnO, and finally be absorbed by ZnO. However, there are still no direct evidences of the sulfur-transfer and the phase transformation via sulfur contact. In this section, in-situ XRD and STEM with EDS mapping were applied to deliver the information of key intermediates during the whole sulfur adsorption process, in order to provide convincing evidences of above-mentioned S-transfer scenario.The XRD patterns of the reduced fresh and used Ni/ZnO samples are shown in Fig.\u00a05\n. The reduced sample shows the characteristic diffraction peaks at 44.1\u00b0 and 51.3\u00b0, which are correspondent to the Ni (111) and (200) planes of metallic Ni (JCPDS 65\u20132865), while the characteristic peaks for NiO phase (37.3\u00b0, 43.3\u00b0 and 62.9\u00b0, JCPDS 65\u20135745) were not observed. Therefore, upon reduction in H2 for 2\u00a0h, NiO is completely reduced to metallic Ni. Furthermore, a weak diffraction peak appeared at 42.9\u00b0 which was attributed to a metallic NiZn alloy and indicated that the reduction treatment of the sample may lead to a partial reduction of ZnO and the solid\u2013state reaction between Zn and Ni, in line with what has been noticed by Bezverkhyy et\u00a0al. [23] on the reduced Ni/ZnO.After RADS for 12\u00a0h, peaks at 21.8\u00b0, 31.1\u00b0and 49.7\u00b0 that correspond to rhombohedral Ni3S2 were observed and the intensity of Ni3S2 peaks increased significantly with increasing time on stream. Simultaneously, peaks at 26.9\u00b0 and 28.6\u00b0 that correspond to cubic ZnS were also observed on the used sample, and the intensity of ZnS peak increased gradually with the time on stream. By far, it has been shown that Ni3S2 and ZnS are the major new phases formed during the sulfur exposure to the initial Ni/ZnO materials. However, there is still not enough evidence to demonstrate the sulfur transfer from Ni3S2 to ZnO. Wang and coworkers [45,46] reported a similar case of sulfur transfer on a desulfurization of dibenzothiophene (DBT) using ZnO-based sorbent. It was suggested that the Ni was responsible for sulfur removal from hydrocarbon, while the residual sulfur was transferred to the ZnO phase in the presence of H2. The previous study showed comprehensive evidences, e.g. XRD, XAS and XANES, on the transformation of Ni and ZnO phases during the reaction. Unfortunately, the characterization was focused on each phase, but the overall scenario of both phases was missing.Moreover, the mechanism was based on DBT desulfurization in the above study. DBT is the major sulfur-containing molecule in the gas oil, which is usually used for diesel production. It is well known that the DBT desulfurization reactivity is very low [47], so that the desulfurization step occurring on the Ni site is naturally considered as the rate limiting step. However, the RADS technology is currently applied for the raw gasoline refinery, where the major sulfur comes from thiophene with much higher reactivity. In such cases, whether the similar sulfur transfer mechanism is valid remains vague, so it is eventually worth revisiting with further insight.In order to further confirm the occurrence of S-transfer, a mechanical mixture of Ni3S2 and ZnO was prepared with the same Ni/Zn ratio as in the Ni/ZnO sample reported in the current work. 200\u00a0mg of the Ni3S2/ZnO mixture were exposed to hydrogen and nitrogen under 1.0\u00a0MPa for 24\u00a0h\u00a0at different temperatures. The results of in-situ XRD are shown in Fig.\u00a06\n.The characteristic diffraction peaks of ZnO, ZnS and Ni3S2 did not change substantially after nitrogen treatment. In contrast, in the XRD patterns of the physical mixture Ni3S2/ZnO before and after H2 exposure, the intensity of Ni3S2 and ZnO peak decreases slightly, while that of ZnS peak increases slightly, indicating that sulfur is gradually transferred from Ni3S2 to ZnO to form ZnS in the presence of hydrogen. It provides direct evidence of the sulfur-transfer between Ni3S2 and ZnO under H2 atmosphere, implying that in the S Zorb process, the presence H2 is more essential for the sulfur-transfer than for the desulfurization over Ni.In addition to the above observation, there has still been one open issue to unravel. In previous work, the proposed mechanism was lack of direct evidence at the interface between Ni and ZnO, which is the key feature of the adsorptive desulfurization process. Thus, in order to more visibly describe the sulfur transfer process in the adsorptive desulfurization, and to further confirm the spatial distribution of sulfur content on the sample, the samples after 24\u00a0h of desulfurization reaction were analyzed by STEM-EDS.In combination with the EDS surface scanning element distribution mapping (Fig.\u00a07\n), the spatial distribution of active components in Ni/ZnO samples after desulfurization can be further determined. It was further confirmed that Ni3S2 was formed mainly after the desulfurization reaction, which is consistent with in-situ XRD results. After desulfurization, S elements are mainly distributed in the outer surface of ZnO (Fig.\u00a07e), while ZnS and Ni3S2 mainly appear at the interface of Ni and ZnO (Fig.\u00a07g). In order to realize S transfer efficiently, the suitable accessible distance can effectively reduce the hindrance / energy barrier of mass transport of S to ZnO, so that the sample has higher desulfurization activity. If the Ni and ZnO phases are not with close vicinity, the mass transport becomes difficult, and ZnO will partially not be able to interact with Ni3S2, resulting in incomplete utilization of sorption materials. The results are consistent with experimental observation that the desulfurization activity can be enhanced by using ZnO with small particle size [36]. Similarly, it is also reasonable to propose that the mass transport sulfur transfer can be reduced by using highly dispersed Ni.Based on above experimental investigation, it is possible to complete a desulfurization scenario evidently.First, thiophene is adsorbed onto the metallic Ni sites of the reduced samples. Thiophene molecules move freely in the gas phase and approach to the surface of the sample, mainly as \u03c0-complexation adsorption. During the adsorption of thiophene, the S atom gradually approaches the nickel atom, and the electron transfer occurs. Ni weakens the CS bond of thiophene ring, the CS bond length increases and tends to be split while the Ni-S bond length shortens. In parallel, under the present of metallic Ni, the hydrogen molecules adsorbed on the surface of nickel dissociate to form active hydrogen atoms.After twice of CS bond cleavage and following hydrogenation, the adsorbed thiophene ruptures to form Ni3S2 and C4 hydrocarbons. The sulfur cannot be retained with Ni, but gradually transferred from Ni3S2 to ZnO, forming ZnS in the presence of H2. The metallic Ni is therefore regenerated during the sulfur-transfer process. After that, the regenerated Ni sites can participate in the adsorption of thiophene again. The possible reaction paths are shown in Fig.\u00a08\n.As a concluding remark for the site requirement, in order to achieve efficient sulfur transfer, the catalyst active components Ni and ZnO is preferred to have close vicinity, or Ni needs to be highly dispersed on ZnO with small particle size. The ideal sample is that Ni is highly dispersed on the surface of ZnO, and the contact surface between nickel and ZnO is enlarged, which promotes S transfer. With the formation of ZnS, the concentration of ZnO nucleus decreases. The nickel active center can be recovered until ZnO is completely transformed to ZnS. When the ZnO phase reaches the saturation of sulfur content in the bulk, the Ni/ZnO material is required to be regenerated.The adsorptive desulfurization process of thiophene on Ni/ZnO was studied, and the desulfurization mechanism was elucidated. Thiophene molecule is preferentially adsorbed on metallic Ni as \u03c0-complexation mode. The desulfurization follows a DDS pathway. Namely, under the reaction conditions, the CS bond is weakened and split with Ni-S bond formation, and H2 is spontaneously split into H radicals, leading to the hydrogenation of C atom besides S. This is evidenced by DFT study. After twice of above CS bond cleavage and following hydrogenation, the S-free C4 olefin is formed and can be further hydrogenated, while the sulfur is retained on the Ni surface, forming Ni3S2 phase. The resulted Ni3S2 phase after desulfurization, which is in close vicinity with ZnO phase in a typical S Zorb catalyst, undergoes a sulfur-transfer towards ZnO. As a consequence, the Ni active site is recovered and the sulfur moves towards ZnO, forming ZnS phase. Efficient sulfur-transfer is achieved at the interface of Ni/ZnO materials with the presence of H2. This is demonstrated by a multi-technique characterization of in-situ XRD, STEM and EDS mapping. For the S Zorb process which has been widely applied in large scale, the current study is specifically meaningful, as it provides convincing evidences of the essential materials transformation during practical operation.There are no conflicts to declare.The authors are grateful to Prof. Dr. Mingyuan He for fruitful discussion on the scientific scope of the manuscript. Technical supports of STEM experiments from Yanjuan Xiang (Sinopec RIPP) are highly appreciated. This work has been financially supported by research grant from Sinopec (Fund No. 118016-8).The following is the supplementary data related to this article:\n\nMultimedia component 1\nMultimedia component 1\n\n\n\nSupplementary data related to this article can be found at https://doi.org/10.1016/j.gee.2020.05.010.", "descript": "\n                  The reactive adsorption behavior of thiophene on the reduced Ni/ZnO sample was investigated by a combination of theoretical and experimental study. It is widely accepted that Ni is responsible for the sulfur-removal of thiophene to release S-free hydrocarbons. Such surface reaction was simulated by DFT method. It is demonstrated that thiophene is mainly adsorbed as \u03c0-complexation mode over metallic Ni. During desulfurization, the SNi bond is formed and the CS bond is thus split without pre-hydrogenation, resulting in the formation of Ni3S2 phase and S-free C4 olefin which can be further saturated in the presence of H2. The S-transfer between Ni3S2 and ZnO was monitored by in-situ XRD and STEM with EDS mapping. Two essential features were identified for efficient S-transfer, namely, 1) the H2 atmosphere, and 2) the two phases are presented with close contact. Based on the acquired information, a general scenario of sulfur trail has been proposed for the desulfurization of thiophene on Ni/ZnO.\n               "}
{"full_text": "Diesel oxidation catalysts (DOCs) are used in the automotive industry to oxidize hydrocarbons and CO and convert NO to NO2, which is critical to oxidize soot collected on DPFs and improve SCR efficiency. Nowadays, the emphasis on lowering real world driving emissions requires active catalysts for CO and hydrocarbon oxidation at temperatures significantly lower than the current state of the art to meet future pollutant emission regulations, specially associated with city driving [1]. In this regard, the U.S. Department of Energy roadmap has set the goal to achieve greater than 90 % conversion of criteria pollutants at 150\u202f\u00b0C or lower for the full useful life of the vehicle [2]. To tackle this challenge, new catalyst formulations are being developed in order to achieve advances in low-temperature DOCs.In search of low-temperature CO oxidation formulations, single atom catalysts (SACs) have reported high reactivity, which also provide efficient utilization of platinum group metals (PGM) [3\u20135]. For instance, Datye et al. have reported high performance of Pt single atom catalysts over ceria support for low-temperature CO oxidation, reaching 90 % of CO conversion at 64\u202f\u00b0C [6]. Nevertheless, noble metal-based catalysts can still be considered expensive and scarce, continuously leading to the scientific community to the search for low cost and with good performance alternatives such as inexpensive transition metals based-catalysts (i.e. Fe, Mn, Cu, Co or Ni) [7\u20139]. Actually, Kim et al. reported in a recent review [10], that when active metals are loaded on the CeO2 surface, many active sites could be acquired by increasing the dispersion, and the catalytic activity can be dramatically improved by newly introducing the interfacial sites between the metals and the CeO2 support. Among these active metals, one of the most promising candidates for the CO oxidation reaction is the CuO-CeO2 system [11]. The reason lies in the fact that exists an important synergistic effect between copper and ceria to generate its exceptional catalytic activity [12].Nowadays, many progress has been made in order to understand the origin of these synergies that can be generated in copper-ceria systems [10,13\u201321]. More specifically, the key features of the CuO-CeO2 system which contributes to the denominated synergetic effect are: i) the facilitation of oxygen vacancies formation; ii) the redox interplay between copper and cerium pairs (Cu2+ + Ce3+\u2194 Cu+ + Ce4+); iii) the superior interfacial sites with enhanced reactivity; iv) the higher reducibility; and v) the enhanced oxygen mobility. Moreover, although the use of the undoped ceria as a support is well documented for copper-ceria binary systems, ceria-zirconia mixed oxide can be considered as a better alternative due to its enhanced thermal resistance and superior ability to promote the creation of oxygen vacancies and, thereby, enhancing oxygen mobility [22,23].In light of the above aspects, the peculiar reactivity of copper/ceria-based materials is due to many different physical-chemical contributions, which results in a much rather complex system in practice. In fact, the modulation of metal-support interactions has been deeply investigated by employing different: i) ceria-based support morphologies [24\u201326]; ii) synthesis routes [27\u201329]; iii) metal oxides precursors (for copper and cerium) [15,25,26]; and even iv) inverse configurations [30,31]. In that way, significant changes have been achieved in the composition, the shape, the size, and the electronic state of these copper/ceria-based systems, resulting in different catalytic performances. Given the current requirements for highly active, efficient and selective catalysts at very low temperatures, it is imperative to keep on investigating the nature of these synergistic effects on copper/ceria-based catalysts.This research aims to conduct a systematic study of CO oxidation reaction catalyzed by several copper/ceria-zirconia samples, evaluating factors affecting catalytic activity under different preparation routes to incorporate copper (co-precipitation, incipient wetness impregnation and physical mixing methods) and different copper contents (from 0.5\u20136\u202fwt.%) for the synthesized catalysts. Focus will be put in this case linking CO oxidation catalytic results in parallel with CO-temperature programmed reduction profiles and selected characterization parameters in order to find out the correlation among catalyst\u2019 properties/reducibility and catalytic behaviors, especially those corresponding to the nature and roles of the different CuOx species over ceria-based support on catalytic activity.The whole study will allow us not only to provide some insight into the nature and type/s of active site/s determining the CO oxidation, mainly at low temperatures, but also if the magnitude and extent of these active sites could be modulated or controlled by choosing a preparation route and a certain copper content. This approach is of paramount importance for the effective and rational design of last-generation catalysts.The Ce0.8Zr0.2O2 mixed oxide (denoted as CZ) has been synthesised by the co-precipitation method in alkaline medium, by using the cerium and zirconium precursors (NH4)2Ce(NO3)6 (supplied by Panreac with 99.0 % purity) and ZrO(NO3)2\u00b7xH2O (supplied by Sigma\u2013Aldrich, with x \u2248 6, technical grade), respectively. The appropriate amounts of these precursors were dissolved in distilled water. The corresponding hydroxides of cerium and zirconium were co-precipitated by drop wise addition of a 10 % ammonia solution in water until pH\u202f=\u202f9, under constant stirring. The solid obtained was then filtered under vacuum and the yellowish precipitate was washed with distilled water until neutral pH. Finally, it was dried overnight at 110\u202f\u00b0C and calcined in air in a muffle at 500\u202f\u00b0C for 1\u202fh, with a heating rate of 10\u202f\u00b0C/min. The composition of this mixed oxide (Ce0.8Zr0.2O2) was chosen in terms of optimal thermal resistance towards sinterisation and good response towards other catalytic reactions studied by some of these authors [32,33] if compared with bare ceria and other Ce/Zr compositions analyzed.Ce0.8Zr0.2O2-supported copper catalysts with Cu wt.% of 0.5, 1, 2, 4 and 6 (denoted as Cu0.5CZ-IWI, Cu1CZ-IWI, Cu2CZ-IWI, Cu4CZ-IWI and Cu6CZ-IWI, respectively) were prepared by incipient wetness impregnation with Cu(NO3)2\u00b73H2O (supplied by Panreac with 99.0 % purity) solutions of different concentration. After impregnation, the samples were dried overnight in an oven at 110\u202f\u00b0C and thereafter calcined under air at 500\u202f\u00b0C for 1\u202fh, with a heating rate of 10\u202f\u00b0C/min. Due to the limitations of the Cu(NO3)2\u00b73H2O solubility in water, the Cu 0.5 %, Cu 1% and Cu 2% catalysts were impregnated in a single step, while Cu 4% and Cu 6% were prepared by successive impregnations with the solution used to prepare Cu 2% (two and three consecutive impregnations, respectively).In order to study the influence of copper entities in contact with ceria-zirconia, which eventually yield synergistic catalytic effects, several samples with different preparation routes have been prepared with a same copper loading (2%).The Ce0.76Zr0.19Cu0.05O2 sample (named as Cu2CZ-CP) was synthesized by the co-precipitation method in alkaline medium, by using the following cerium, zirconium and copper precursors: (NH4)2Ce(NO3)6 (supplied by Panreac with 99.0 % purity), ZrO(NO3)2\u00b7xH2O (supplied by Sigma-Aldrich, with x \u2248 6, technical grade) and Cu(NO3)2\u00b73H2O (supplied by Panreac with 99.0 % purity), respectively. The same amounts of precursors than those used for the Cu2CZ-IWI synthesis were dissolved in distilled water, and after jointly co-precipitation, a 2\u202fwt.% of copper was achieved. This co-precipitation procedure continues exactly as described above for the CZ catalyst.Bulk CuO was obtained by calcination of Cu(NO3)2\u00b73H2O in air, at 500\u202f\u00b0C during 1\u202fh with a heating rate of 10\u202f\u00b0C/min. It was used to prepare the physically-mixed samples, and also as a catalyst itself.A physically-mixed sample of bulk CuO with ceria-zirconia was also prepared. Physical mixing between CuO and CZ have been conducted by tight contact mode in an agate mortar with pestle, and consisted of an intimate mixture of the proper amount of CuO and CZ, during 5\u22126\u202fmin, to yield a 2\u202fwt.% of copper. Afterwards, it was calcined in air at 500\u202f\u00b0C for 1\u202fh, with a heating rate of 10\u202f\u00b0C/min. The sample obtained after this treatment is denoted as Cu2CZ-PM.The commercial 1%Pt/Al2O3 catalyst was supplied by Sigma-Aldrich (with BET surface area of 160\u202fm2/g).Catalytic tests for CO oxidation were carried out in a U-shaped quartz reactor (16\u202fmm inner diameter), loaded with 50\u202fmg of catalyst and 100\u202fmg of silicon carbide. The total flow rate of the feed gas (1000\u202fppm CO and 10 % O2 in He) was 100\u202fmL/min, corresponding to GHSV of 90000 h\u22121. The catalytic tests consisted of Temperature-Programmed Reactions, where the temperature was increased from room temperature up to 300\u202f\u00b0C at 5\u202f\u00b0C/min under the reactive atmosphere. Previously, the samples were pretreated in situ at 500\u202f\u00b0C under a flow of 5% O2/He (100\u202fmL/min) for 30\u202fmin. The outlet gases were analyzed using a gas chromatograph (HP model 6890 Plus Series) equipped with two columns: Porapak Q 80/100 for CO2 separation and Molecular Sieve 13X for O2 and CO separation, coupled to a thermal conductivity detector (TCD). The CO conversion (XCO, %) was calculated as follows (1):\n\n(1)\n\n\nX\n\nC\nO\n\n\n\n%\n\n=\n\n\n\n\n[\nC\nO\n]\n\n\ni\nn\n\n\n-\n\n\n[\nC\nO\n]\n\n\no\nu\nt\n\n\n\n\n\n\n[\nC\nO\n]\n\n\ni\nn\n\n\n\n\nx\n\u2009\n100\n\n\nwhere [CO]in and [CO]out are the CO concentration (ppm) in the inlet and outlet gas streams, respectively.Reaction rates were estimated as \u03bcmol CO2 produced/gCu\u00b7s, at certain temperatures, and apparent activation energies (Ea) were calculated assuming differential conditions (CO conversions \u2264 20 %). Repeatability of the catalytic measurements considering different batches was quite good.Temperature-programmed reduction (TPR) measurements employing CO as a reductant were carried out using the same experimental setup than that employed for the catalytic tests for CO oxidation. The total flow rate of the feed gas (5% CO in He) was 35\u202fmL/min. The temperature was increased from room temperature up to 650\u202f\u00b0C at 5\u202f\u00b0C/min under the reactive atmosphere. Prior to every run, the catalysts were pretreated in situ at 500\u202f\u00b0C under flow of 5% O2/He for 30\u202fmin, and then, the catalysts were cooled down to room temperature in the gas flow and purged under inert gas. The whole details of the procedure are described elsewhere [34].A very complete description of the classical characterization techniques, used for the investigation of the physico-chemical features (surface, textural and structural properties), of the fresh catalysts used in this work is described in depth elsewhere [35,36]. The protocol for determining the copper dispersion data in selected catalysts, by means of H2 volumetric studies.Catalytic testing was performed to understand the effect of the amount and nature of copper species present on the catalysts on activity. CO oxidation was performed first on IWI-samples with various copper loadings, including the undoped Ce0.8Zr0.2O2 sample and a reference CuO sample calcined at the same temperature (Fig. 1\n). Complete conversion of CO is achieved for all the catalysts, at temperatures lower than 300\u202f\u00b0C, with the exception of the undoped ceria-zirconia (CZ). The amount of copper significantly affects the ignition, that occurs at temperatures as low as 50\u202f\u00b0C for the best catalysts, and the oxidation rate, that gradually increases with temperature. The slope of the light-off curve seems to be influenced by the copper contents onto the catalysts, the higher the copper loading, the more pronounced the slope. The shape of the curves, in general, differs from that of the 1%Pt/Al2O3 commercial catalyst (chosen as an effective benchmark, due to its high activity towards other oxidation reactions and high BET surface area), which is characterized by an abrupt ignition between 110\u202f\u00b0C and 140\u202f\u00b0C. This characteristic rapid increase from low to high conversion can be explained due to the first-step coverage of the active sites with CO at low temperatures, inhibiting initially the CO oxidation reaction [37,38]. Conversely, all the synthesised ceria-based catalysts are active at temperatures as lower at 75\u202f\u00b0C, including the support. Interestingly, the bulk CuO seems to be quite active, if compared with the uncatalyzed reaction, and around 200\u202f\u00b0C even outperforms the support\u2019s activity. The fact that CuO entities (like CuO bulk) can be active towards CO oxidation under these experimental conditions, could partially account for the non-gradual increasing trend in activity with the highest copper contents of the catalysts.\nFig. 2\n depicts the effect of copper content on the CO oxidation activity expressed at the temperatures at which several CO conversions are reached: 10 % (T10), 50 % (T50) and 90 % (T90), respectively. As the copper loading increases, (in the range among 0.5 and 2%), those parameters are lowered indicating that the catalyst activity becomes better. However, the profile of these curves exhibits minor decrements of T10, T50 and T90 temperatures with the highest copper contents (4 and 6%). Zhu et al. have studied the CO oxidation behavior of related formulations, where copper was incorporated by incipient wetness impregnation over a Ce0.5Zr0.5O2 support, and have reported an optimal copper oxide loading around 5.25 % [39].Following with the ideas of presenting reliable comparisons with an effective benchmark, (the commercial platinum\u2019s catalyst), Table 1\n compiles the list of the intersection temperatures of the copper-containing catalysts\u2019 curves with regard to the Pt\u2019s curve. The fact that copper/ceria-zirconia catalysts prepared from IWI method outperforms Pt\u2019s activity in a wide range of low temperatures, can point to high degrees of CuOx entities\u2019 dispersion, and consequently, quite small average crystallite sizes, since will be discussed later. The more copper content on the catalysts, the higher the temperature should be reached by the platinum\u2019s catalyst to achieve the same value of conversion than those of the corresponding copper catalysts.In our work, the copper-containing catalysts exhibit decrease of BET surface area with increase in metal loading due to partial blocking of porosity as a consequence of the IWI method, but, on the other hand, the incorporation of copper seems to be critical to define the activity. To ascertain and split the influence of both effects, Fig. 3\n shows specific rates for this set of catalysts normalized to catalyst surface area. Surface-area normalized values were derived from the rate of CO2 production per second and per square meter of solids\u2019 surface areas. By comparing Fig. 1 with Fig. 3, it can be said that the differences are becoming larger for the normalised parameters with the copper content. By comparing the lowest copper contents catalysts (0.5, 1 and 2% with regard to CZ), the values seem to increase proportionally with the copper content. This trend is quite interesting suggesting that the concentration of active sites per m2 increases in a gradual way with the copper loading, thus pointing out the goodness of the preparation method under low copper contents (providing this interesting trend). This tendency is also consistent with the extremely low (and similar) copper crystallite sizes (1.7\u202fnm for Cu0.5CZ-IWI, 1.7\u202fnm for Cu1CZ-IWI and 1.8\u202fnm for Cu2CZ-IWI, respectively, see Table S2 on SI) which provides certain evidences of very well-spread CuOx entities able to create a relatively high population of active interfaces. When increasing the contents up to 4 and 6%, the increase is not gradual anymore. This is reflected in Fig. 4\n for three representative temperatures of reaction (80, 100 and 120\u202f\u00b0C, respectively). These trends could suggest the co-existence of several active sites of different nature/relevance for this catalytic process. It should be reminded that: i) both support and CuO bulk are active for the process and ii) the IWI process brings differences in the steps conducted, because for samples Cu4CZ-IWI and Cu6CZ-IWI successive impregnation steps were needed to incorporate the desired copper contents (two and three consecutive impregnations, respectively). Despite this consideration involved in the preparation method, Cu crystallite size is still very low for Cu4CZ-IWI (2.3\u202fnm) and Cu6CZ-IWI (2.7\u202fnm), which justifies that the CO oxidation activity remains high.It should be noted that crystallite size determined by H2 chemisorption method is a measure of metallic copper particle size. However, CuOx entities are the supposed active species towards CO oxidation. CuO particle size can be calculated from metallic Cu particle size by assuming that CuO will adopt a spherical or near-spherical shape when they are dispersed over ceria-zirconia. Therefore, the size of CuO crystallite is only slightly larger than that of metallic copper particle (Table S2).It is relevant in this context, as well, to compare the reactivity per total metal content on the catalysts. Therefore, the representation of reaction rate (\u03bcmol CO2/gCu\u00b7s) versus the inverse of reaction temperature is illustrated on Fig. 5\n. Consistently with the information shown on Fig. 4, by increasing the copper contents in the range of 0.5, 1 up to 2%, the representation of the reaction rate expressed per gram of copper versus the inverse of temperature is almost identical for these three catalysts indicating that the number of active sites participating in the process seem to grow in a gradual way when adding more copper contents in this range. However, the representation of 4 and 6% Cu contents falls below the trend traced by the mentioned three catalysts, thus suggesting a lowest effectivity for copper metal atom into these two catalysts. All these evidences are in general agreement with the trend in copper dispersion data.The composition of 2% of copper loading was selected to analyse the relevance of the preparation method thus unraveling if CuOx entities of different nature can exist and which is the relationship among their type and amount with their own catalytic activity. Indeed, given the complexity of the copper/ceria-zirconia system, it is challenging to shed light on the preliminary identification of the active sites and their correlation with catalytic activity. For this purpose, three different preparation routes (very different among them, regarding their physico-chemical fundamentals) were approached. It is supposed that different procedures of copper incorporation onto the catalyst will yield different degrees of contact/distribution/nature of copper species onto the ceria-zirconia catalyst.In line with these ideas, three catalysts with the same 2% copper content were synthesized. The corresponding nomenclature is: Cu2CZ-IWI, Cu2CZ-CP and Cu2CZ-PM. The second solid was prepared by a combined co-precipitation procedure of the metallic precursors and the third solid was obtained by an intimate physical mixture of the support and the copper precursor and, subsequent calcination a 500\u202f\u00b0C (see Section 2 for additional details). Besides, the BET surface areas obtained for the three catalysts are highly similar (71, 72 and 70\u202fm2/g, respectively), thus allowing us a reliable comparison of the influence of the different nature of the copper species, since both parameters (copper content and exposed surface areas) are practically identical in the three samples considered. The corresponding CO conversion curves are depicted on Fig. 6\n. These three copper/ceria-zirconia catalysts were found to be more active than unsupported ceria-zirconia and bulk CuO. It is relevant to point out that the route of procedure significantly affects the catalytic activity, both the onset reaction temperature and the slope of the curve. The order of activity is as follows:\n\nCu2CZ-PM\u202f<\u202fCu2CZ-CP < < Cu2CZ-IWI\n\n\nIn accordance with the exposed ideas, reaction rates against the reciprocal of temperature for these three catalysts show the highest values for the catalyst prepared by the IWI method on Fig. 7\n. To better understand the differences in reactivity provided with the different variables studied (copper content and type of preparation procedure), CO-TPR experiments are presented below, with the aim to determine the distinctions among all the catalysts in the reaction between CO and oxygen from the surface/lattice of the catalysts. The combined analysis of all the experimental data (catalytic tests, CO-TPR results and characterization of the catalysts) will allow us to ascertain if the nature and type of copper species can play a role during the promotion/activation of oxygen from the catalysts, taking part in the catalytic reaction.CO-TPR has been widely used to characterize reducibility of the CuO-CeO2 systems [40]. Additionally, with the general purpose to infer some arguments which can assist the understanding of the trends in the catalytic activity towards CO oxidation of the different sets of catalysts, the use of CO is preferred as a probe molecule instead of H2 [41]. Nevertheless, the discussion of the influence of copper contents and preparation procedure on the H2-TPR profiles for the catalysts studied was presented elsewhere [35,36] and the corresponding data and additional interpretation for selected catalysts is reported on the next section.CO-TPR experiments have been performed in order to investigate the influence of copper entities of different possible nature and interaction degree with the support on the catalysts\u2019 reducibility [42]. In fact, if several CuOx species co-exist onto these samples, they could play different roles during the activation of surface/sub-surface oxygen from the ceria-based support, which could produce, eventually, characteristic CO2 emission profiles for this type of copper/ceria-zirconia systems [13,40,42,43]. Besides, by comparing the corresponding profiles of the different catalysts prepared with those of bulk CuO and the bare support, possible synergetic effects among CuOx species well spread onto the catalysts\u2019 surface and the cerium centers in close vicinity with them (thus facilitating ceria\u2019s reduction due to close interfacial interactions, well commented on literature [18,20]) could be evidenced.\nFig. 8\nA and B shows CO2 emission profiles during CO-TPR experiments for the two sets of copper/ceria-zirconia catalysts analyzed in this work. The reduction profile of pure CuO, is characterized by a single and broad peak in a temperature range from 120 to 260\u202f\u00b0C, indicating the temperature window where the unsupported CuO tenorite-like species reduction is expected to take place. In this case, CuO sample\u2019s profile is presented on the Fig. 8 like a calculated curve corresponding to 2% of bulk CuO profile, for comparative purpose. Considering the conditions employed, this broad peak can be ascribed to direct reduction to metallic copper, in agreement with previous studies [13,44,45]. The undoped ceria-zirconia profile exhibits an asymmetric first broad peak starting at 80\u202f\u00b0C, which can be also considered a shoulder of a second one centered at around 430\u202f\u00b0C. Unfortunately, the interpretation of its CO-TPR profiles becomes extraordinarily complex due to the simultaneous occurrence of the water-gas shift (WGS) reaction and Boudouard reaction, according to these Eqs. ((2) and (3), respectively):\n\n(2)\nCO\u202f+\u202fOH\u2212 \u2192 CO2 + 1/2 H2\n\n\n\n\n\n(3)\n2CO \u2192 CO2 + C\n\nthus providing a constant CO2 emission level at high temperatures during the experiments, which makes that the profile does not return back completely to the baseline. Many authors have revealed the contribution of these side reactions catalyzed by ceria-based catalysts, under CO-TPR experiments [13,46\u201348], mainly at medium temperature range (> 275\u202f\u00b0C) for WGS reaction and high temperature range (> 400\u202f\u00b0C) for Boudouard reaction. For the sake of a reliable comparison and taking into account that the most interesting/representative peaks or contributions appear on the corresponding patterns of copper/ceria-zirconia catalysts at low/medium temperatures, the subtraction of the whole profile of the bare support to those corresponding to the catalysts prepared, was conducted in an attempt to remove the side effects of the WGS and Boudouard reactions (whatever the extension at which both reactions take place). Additionally, this subtraction highlights in an adequate manner: i) the CuOx entities\u2019 reduction patterns and ii) possible synergetic effects arisen from the copper entities in close contact with ceria-zirconia\u2019s surface, consisting of \u201cextra\u201d cerium centers\u2019 reduction as a consequence of promoted interfacial interactions.In line with these exposed ideas, Fig. 9\nA and B depicts the \u201ctreated\u201d profiles after conducting these subtractions. Assuming that the Boudouard reaction occurs in similar extent for all the catalysts, Fig. 9A shows two different low-temperature peaks or contributions, that can be ascribed to the CuOx species reduction or to the mentioned interfacial interactions which could yield to additional cerium centers reduction. Cu2CZ-IWI and Cu2CZ-CP presents a very low temperature broad contribution, centered at around 80\u202f\u00b0C and completely absent both in the catalyst prepared by physical mixing method and in the CuO bulk sample. Some authors reported in the literature that this first contribution could be ascribed to very finely CuO dispersed onto ceria-based materials or to the reduction of copper species strongly interacting with ceria [40]. The experimental fact that this \u03b1 contribution arises at lower temperature than those reported in other papers is consistent with the very high activity of these catalysts towards CO oxidation (even outperforming that of Pt\u2019s catalyst) [40]. This comparison also suggests that the physical mixture method yields a poorer copper/support contact or a lack of these very finely dispersed CuOx species and for this reason this catalyst presents much lower activity in the range of low temperatures compared with their counterparts having the same copper loading. This behavior is consistent with the high copper crystallite size for Cu2CZ-PM (32.7\u202fnm) and low dispersion data (3%), estimated from H2 adsorption volumetric studies.Conversely, the second contribution (\u03b2) appears as a clear peak (specially for Cu2CZ-IWI and for Cu2CZ-PM) in the temperature window of the unsupported CuO. Interestingly, by comparing the relative areas of the CuO bulk (2% of the whole profile) against these catalysts, the inferior area of the reference CuO can be clearly seen whatever the catalyst considered, but specially for Cu2CZ-IWI. Moreover, the profiles are negative at medium temperatures. A reasonable explanation for this observation is that a concomitant reduction of cerium centers is taking place in this temperature range (occurring at higher temperatures in the bare support). In order to tentatively distinguish if it is motivated by an \u201canticipated\u201d reduction of the support or if, additionally, \u201cextra\u201d reduction of cerium centers, in close vicinity with the several active CuOx species, takes place, additional quantifications were estimated and compiled on Table 2\n, as commented on below.Regarding the effect of the copper content for IWI-catalysts (Fig. 9B), three contributions (\u03b1, \u03b2 and \u03b3) are presented in different extensions and relative importance, being \u03b3 specially relevant for the catalyst prepared with 6% of copper loading (this peak appears like a very low and broad contribution or is absent for the rest of catalysts). All the IWI catalysts show the contribution at low temperatures (\u03b1), which seems to reach maximum values for Cu2CZ-IWI and Cu4CZ-IWI. For these two catalysts, the second contribution (\u03b2), emerging in the temperature window of the unsupported CuO tenorite-like species, presents the highest values of the series as well. Conversely, the Cu6CZ-IWI catalyst exhibits less intensity for these two first contributions (\u03b1 and \u03b2) in favor of a very prominent and sharp peak at higher temperature (\u03b3). It is worth reminding that this catalyst was prepared by three successive impregnations of the copper precursor solution.As advanced, Table 2 compiles additional quantifications estimated from Fig. 9A and B. The data on the second column correspond to the integrated amounts of CO2 emitted by each catalyst after subtraction of the CO2 emission corresponding to the ceria-zirconia\u2019 support. The third column collects these values after subtraction of the theoretical CO2 emissions corresponding to the stoichiometric reduction of the CuO molar contents of the different catalysts, according to this global stoichiometry (4), (by assuming a general formula of CuO for all the CuOx entities):\n\n(4)\nCuO\u202f+\u202fCO \u2192 Cu\u202f+\u202fCO2\n\n\n\nAssuming the validity of these subtractions, the \u201cextra\u201d amounts obtained as \u03bcmol CO2/g of every catalyst (Table 2) suggest that whatever the method of preparation of the catalyst or the amount of copper introduced by the IWI method, \u201cadditional\u201d or \u201cextra\u201d cerium centers reduction can be measured, thus suggesting a good degree of interaction copper/ceria-zirconia and an excellent reducibility under CO in the systems prepared. Following with this discussion and in an attempt to rationalize these \u201cextra\u201d amounts along the sets of catalysts, the CO2 \u03bcmols/g estimated values were referred to the CuO molar content of the different catalysts. Assuming the general stoichiometry of cerium cations\u2019 reduction on ceria\u2019s surface (5):\n\n(5)\n2CeO2 + CO \u2192 Ce2O3 + CO2\n\n\n\nthe last column on Table 2 compiles the potential number of cerium centers surrounding the CuOx entities which are supposed to suffer reduction according to these quantifications. It is clearly demonstrated that more cerium centres are affected due to improved synergetic interactions according to this order in terms of the method of preparation:\n\nCu2CZ-PM\u202f<\u202fCu2CZ-CP\u202f<\u202fCu2CZ-IWI\n\n\nThis was the order observed for the catalytic activity, and as well, the order also agrees with the contribution of the \u03b1 peak (at the lowest temperature), which is more prominent for the IWI-catalyst.Regarding the effect of copper content, generally speaking, the approximate calculations report a trend consisting of more cerium centers affected by CO reduction in the vicinity of CuOx entities as the copper loading decreases. However, this seems not to be a linear effect with the copper loading. This observation can be tentatively explained by the variety of CuOx entities which seem to coexist on this set of catalysts.In recent years, copper/ceria-based catalysts have attracted considerable attention due to their low cost and excellent catalytic performance in many oxidation reactions [49]. Nevertheless, due to their inherent reactivity, the use of classical methods to determine copper entities\u2019 dispersion by means of traditional chemisorption procedures is very problematic due to the existence of large spillover phenomena, (in the case of H2 chemisorption methods), or the possibility of concomitant oxidation of cerium centers if other probe molecules (CO and N2O, as examples) are used for chemisorption studies after a previous reduction step. In this context, H2 adsorption isotherms at sub-ambient temperatures or sophisticated imaging analysis techniques such as HAADF-STEM might be applied to determine reliable copper dispersion data in recent years (but with certain amount of difficulties) [50].Since it is widely recognized that the dispersion states, redox properties and catalytic performances of CuO/ceria-based catalysts are critically dependent on the preparation methods [49], this section will be devoted to a comparative analysis considering the characterization\u2019s features of some representative catalysts of this study. On this basis, implications will be intended to be extracted concerning the nature and roles of the different CuOx species on catalytic activity. By a combined discussion considering the whole characterization data obtained from catalysts prepared by different methods, CO-TPR results and CO oxidation activity data, an interesting interpretation of the correlation among catalyst\u2019 properties and catalytic behaviors can be obtained.By having a look at the rate of CO2 production during the catalytic tests of CO oxidation at a representative temperature (e.g. 80\u202f\u00b0C), it can be noted that the catalyst prepared by physical mixture presents a rate of CO2 production of 0.020\u202f\u03bcmol/g\u00b7s (being an intermediate value among that of the bare support -0.011- and that of the lowest copper content catalyst prepared by IWI method (0.037), but quite far from that of the catalyst with a same copper content, (2%), prepared by IWI method as well (0.290). On the other hand, the coprecipitation method yields a catalyst whose rate of CO2 production (0.094) is much more similar to that of the 0.5 % Cu (0.037) and far from that of its counterpart 2% copper content (0.290). These data provide some signs about the relevance of the preparation method on the type, amount and nature of active sites on these complex systems.In order to have in mind the complete picture, Fig. 10\nA and B illustrates the representations of the CO oxidation activities and the CO2 production profiles (obtained from the CO-TPR profiles) for selected catalysts. It is interesting to note that the nature and type of CuOx species generated onto the catalysts, modulated by the different methods, seem to exhibit more influence on catalytic activity values and catalysts\u2019 reducibility than the whole content of CuO onto them. According to these results, Cu0.5CZ-IWI (prepared by incipient impregnation method) presents the same activity and similar redox properties than Cu2CZ-CP, prepared by a coprecipitated method of the three metal precursors, in spite of having 4 times less copper content.In order to shed light on these experimental observations, comparative characterizations results will be presented now. As commented earlier, the whole characterization results, dealing separately the effect of copper content and the influence of preparation method on the activity towards diesel soot combustion, were presented elsewhere [35,36]. For the discussion presented in this section, only representative characterization results of selected catalyst will be shown.First of all, XPS analyses will be presented. Cu-2p3/2 photoelectron spectra of selected catalysts (Cu0.5CZ-IWI, Cu2CZ-CP and Cu2CZ-IWI) were depicted on Fig. 11\n. As illustrated, Cu0.5CZ-IWI with Cu2CZ-CP present much more similarities on their XPS spectra than Cu2CZ-CP with Cu2CZ-IWI (its counterpart with the same copper content). To deepen into the mode in which copper distribution takes place among the surface with regard to the bulk of the catalysts in terms of the synthesis route and the amount of copper for representative catalysts, Table 3\n collects the corresponding Cu/(Cu\u202f+\u202fCe\u202f+\u202fZr) surface atomic ratios (designated as Cu/(Cu\u202f+\u202fCe\u202f+\u202fZr)sur) and the estimated bulk atomic ratios (designated as Cu/(Cu\u202f+\u202fCe\u202f+\u202fZr)nom). The catalysts prepared by incipient wetness impregnation present higher Cu surface atomic ratios than the theoretical bulk values, which means that copper is finely dispersed onto the support\u2019s surface, partially blocking porosity, as inferred from previous publications [35,36]. Cu2CZ-CP has a lower surface atomic ratio than the theoretical bulk value, thus indicating that copper has been (at least in part) incorporated into the ceria-zirconia lattice or subsurface due to the combined coprecipitated method of preparation used. This makes that the fraction of Cu atoms exposed for Cu2CZ-CP remains much lower than that obtained by incipient wetness impregnation method, and yields values close to those shown by Cu0.5CZ-IWI.To approach the discussion about reducibility of the catalysts and trying to complement the data obtained by means of CO-TPR, results obtained by H2-TPR for selected catalysts are presented on Fig. 12\n, whose detailed discussion was presented elsewhere [35,36]. Again, a relevant similarity can be found by comparing the profiles of Cu0.5CZ-IWI and Cu2CZ-CP (in terms of intensity and shape of the patterns), however, the catalyst prepared by incipient wetness impregnation presents the profile a little bit moved forward to lower temperatures, indicating a promoted reducibility at lower temperatures. Anyway, both profiles are considerably lower than that of the Cu2CZ-IWI, in agreement with the rest of catalytic and characterization data presented so far.With the aim of supporting the discussion regarding the extension of the reduction process occurring onto the copper-containing catalysts (if only \u201canticipated\u201d cerium centers reduction, compared with the reduction of bare support, takes place, or if, additionally, \u201cextra\u201d or \u201cnew\u201d cerium centers, not susceptible to be reduced in the case of the bare support, can be affected by the reduction process due to an excellent quality of the interphase copper species/ceria-zirconia, the same estimations, based on subtractions conducted for CO-TPR profiles, were carried out, now, with the data obtained from H2-TPR profiles, considering the following stoichiometries ((6) and (7)):\n\n(6)\nCuO + H2 \u2192 Cu + H2O\n\n\n\n\n(7)\n2CeO2 + H2 \u2192 Ce2O3 + H2O\n\n\nIn this case, the results (not shown for the sake of brevity), yield that, approximately, the same amount of cerium centers that suffer reduction onto the bare support are reduced \u201cin advance\u201d, but not \u201cextra\u201d cerium centers reductions are evidenced after the corresponding subtractions. These discrepancies can be motivated by two reasons: i) the estimations are subjected to several source of errors and, as a consequence, they need to be viewed as \u201capproximate results\u201d and ii) CO can be considered a compound with a more pronounced reducing character than H2 towards these catalysts (actually, having been conducted under the same experimental conditions, CO-TPR profiles are moved toward lower temperatures with regard to H2-TPR ones).This study has revealed that very high but different ranges of catalytic performances can be reached by means of different CuO/ceria-zirconia catalysts and that CO-TPR curves seem to be very sensitive to the presence of several types of CuOx entities onto this support, thus displaying several CO2 contributions/peaks due to the optimal interaction of CO molecules with the oxygen from the own reservoir of every catalyst. Both CuOx entities and ceria-zirconia were more readily reduced than, at least, the corresponding independent components. In addition, it is suggested that \u201cextra\u201d cerium cations could be reduced in variable amounts due to the excellent qualities of the interfaces created. Even though the whole estimations should be considered taking into account that some assumptions or oversimplifications were taken, attempts were tried to estimate the \u201cextra\u201d cerium centers affected by the reduction under CO, which is a primary trial of \u201ctitrating\u201d the quality and extension of the interphase CuOx/ceria-zirconia, responsible of improvements of redox properties and catalytic responses in these systems with regard to CuOx well-dispersed onto different nature supports. These values are suggested to be influenced by copper content, (linked to dispersion of CuOx species) but more importantly to the method of preparation.To clarify the connection or coupling between the profiles obtained by CO oxidation (CO2 formed by reacting 1000\u202fppm of CO with 10 % of O2) and by CO-TPR patterns (CO2 originated by reacting 5% of CO with the oxygen coming from the own catalysts), a combined representation of both profiles expressed as \u03bcmol CO2/gcat\u00b7s is presented versus temperature for selected samples on Fig. 13\n. Dotted lines correspond to the rate profiles from CO-TPR and solid lines to those of CO oxidation in presence of O2. The catalysts prepared by \u201cchemical\u201d routes (co-precipitation or incipient wetness impregnation) exhibit a remarked ability to activate and oxidize CO molecules with their own oxygen (coming from CuOx entities or surrounding ceria-zirconia\u2019s surface). Conversely, the sample prepared by physical mixing between copper precursor and ceria-zirconia (and subsequent calcination) does not possess the \u201cactive sites\u201d responsible of oxidizing CO at very low temperatures (\u03b1 contribution is absent for this catalyst) and, accordingly, the CO2 production curve is delayed with regard to the rest of the catalysts. From the combined analysis illustrated on Fig. 13, the best catalytic response shown by Cu2CZ-IWI can be attributed to a joint presence of a relevant \u03b1 contribution (probably CuOx entities very well-dispersed onto ceria-zirconia, proved by extremely low size of copper crystallite) and accessible CuO-like tenorite entities, (\u03b2 contribution) similar to those presented by bulk CuO and Cu2CZ-PM (with high average crystallite size). These entities are probably at a surface level and very accessible and are able to promote reduction of cerium centers in close vicinity, as inferred for the respective comparison with bulk CuO area presented above. Conversely, the sample prepared by co-precipitation, shows much lower \u03b2 and a slightly lower \u03b1 contribution than its counterpart prepared by IWI. Part of its CuO species seem to be more \u201cburied\u201d onto the catalyst\u2019s sub-surface, and as a consequence of this different copper distribution/accessibility, the resultant catalytic behavior reaches the same level than that of a catalyst prepared with much lower copper content, but more accessible on surface (Cu0.5CZ-IWI), as previously pointed out.It is assumed that oxidation of CO onto these catalysts proceeds though a Mars-van Krevelen mechanism, which has been previously invoked by ceria-based materials with very positive effect of facile oxygen transfer from the catalysts due to a favorable formation of oxygen vacancies. This is clearly promoted by the presence of CuOx species, because the relevant step during CO oxidation is the reaction between adsorbed CO and oxygen from the own reservoir of the catalyst. In this sense, CO-TPR studies were approached in order to analyze possible differences in the catalysts with the different amount and nature of CuOx species. Nevertheless, whatever the copper-containing catalyst studied, CO conversion is coupled with CO2 production during CO-TPR, evidencing a fast and facile transfer of oxygen from the catalyst (CuOx species/ceria-zirconia\u2019surface) towards CO molecules in the absence of O2-gas phase, which could, eventually replenish the own reservoir of oxygen under oxidant atmosphere.These experimental evidences are congruent with the values of apparent activation energies (Ea) estimated from CO oxidation catalytic tests. The corresponding values are presented on Table 4\n for all the catalysts studied. For most of the cases, CZ presents a higher apparent activation energy (68\u202fkJ/mol) than that of copper-containing catalyst. It is worth noting that the couple of catalysts previously discussed because of similarities among physico-chemical features and CO-TPR profiles, (Cu0.5CZ-IWI and Cu2CZ-CP) are characterized by presenting a same value of this parameter (52\u202fkJ/mol, lower than that of the bare support), supporting the idea that a similar number and type of active sites are present on these catalysts. On the contrary, Cu2CZ-IWI, showing a highlighted ability to transfer oxygen to CO molecules (prominent \u03b1 and \u03b2 peaks) presents the lowest Ea values (38/42\u202fkJ/mol) in agreement with the idea of a higher number of active sites on this catalyst. This Ea value is even lower than that estimated for 1%Pt/Al2O3 catalyst, in agreement with a poorer reducibility of this catalyst. In this sense, the CO oxidation mechanism of alumina-supported platinum catalyst is known to take place via a single-site competitive Langmuir\u2013Hinshelwood mechanism (suprafacial mechanism) [37]. The non-participation of oxygen lattice from the support in this mechanism provokes the low reducibility of 1%Pt/Al2O3 under CO-TPR conditions [51]. This contrasts with copper-containing catalysts, in which the high participation of the lattice oxygen atoms from these catalysts seen under CO-TPR conditions is indicative for the dominating Mars-van Krevelen mechanism (intrafacial mechanism). However, Cu2CZ-PM presents even a higher value of Ea (73\u202fkJ/mol) than 1%Pt/Al2O3 and, accordingly, this catalyst is characterized by a reduced capacity to transfer oxygen to CO molecules, since the \u03b1 contribution is completely absent for this catalyst.This research has been dedicated to the preparation and study of the catalytic activity of different copper/ceria-zirconia catalysts in order to understand the nature of the active sites and the generated synergies for CO oxidation reaction at low temperature. The general conclusions that have been drawn are the following:\n\n-\nAll the catalysts obtained by incipient wetness impregnation method are more catalytically active towards CO oxidation than 1%Pt/Al2O3 at low temperatures (T\u202f<\u202f130\u202f\u00b0C), due to the high synergies created among CuOx species and the ceria-zirconia support, even at copper loading as low as 0.5 %. This seems to be connected with a very high copper\u2019s dispersion degree reached with this procedure, yielding significantly low copper crystallite sizes.\n\n\n-\nRegarding the IWI-catalysts, the CO oxidation activity increases linearly with Cu loading up to 2\u202fwt.%. When increasing the contents up to 4 and 6\u202fwt.%, respectively the increase is not gradual anymore.\n\n\n-\nThe synthesis method significantly affects the CO oxidation activity. The catalytic activity increases along the series: Cu2CZ-PM\u202f<\u202fCu2CZ-CP << Cu2CZ-IWI.\n\n\n-\nDifferent CO-TPR peaks/contributions were observed in different extensions and relative importance for copper/ceria-zirconia catalysts. Remarkably, Cu2CZ-IWI and Cu4CZ-IWI exhibit the most intense low-temperature contribution (\u03b1 peak), as well as \u03b2 contribution.\n\n\n-\nQuite similar CO oxidation activities can be reached from different synthesis methods and different copper contents (Cu0.5CZ-IWI and Cu2CZ-CP), evidencing the importance of the nature and the type of CuOx species generated over the catalysts\u2019 surface, which can be modulated by the synthesis procedure. These features seem to be more relevant than the own CuO content on catalysts.\n\n\n-\nFrom the combined study of CO-TPR and H2-TPR profiles, it can be seen that CuOx entities are reduced at low temperatures (very dependent on the preparation method) onto ceria-zirconia, but, importantly all the cerium centers susceptible to be reduced under CO and/or H2 in the support, are reduced in an anticipated way, with regard to the interval of temperatures where this reduction process takes place in the bare support. The detailed investigation of the several contributions and peaks that appear on the CO-TPR profiles, compared with the bare support and with the CuO bulk profile has been revealed as very useful for a first approach into the understanding of the synergies created on this system. Finally, the evidences provided by the reducibility and catalytic profiles of the catalysts prepared by the different procedures can contribute, interestingly, to the clues about the features which should be determinant for these catalyst to present very high catalytic activity towards CO oxidation, mainly at low temperatures.\n\n\nAll the catalysts obtained by incipient wetness impregnation method are more catalytically active towards CO oxidation than 1%Pt/Al2O3 at low temperatures (T\u202f<\u202f130\u202f\u00b0C), due to the high synergies created among CuOx species and the ceria-zirconia support, even at copper loading as low as 0.5 %. This seems to be connected with a very high copper\u2019s dispersion degree reached with this procedure, yielding significantly low copper crystallite sizes.Regarding the IWI-catalysts, the CO oxidation activity increases linearly with Cu loading up to 2\u202fwt.%. When increasing the contents up to 4 and 6\u202fwt.%, respectively the increase is not gradual anymore.The synthesis method significantly affects the CO oxidation activity. The catalytic activity increases along the series: Cu2CZ-PM\u202f<\u202fCu2CZ-CP << Cu2CZ-IWI.Different CO-TPR peaks/contributions were observed in different extensions and relative importance for copper/ceria-zirconia catalysts. Remarkably, Cu2CZ-IWI and Cu4CZ-IWI exhibit the most intense low-temperature contribution (\u03b1 peak), as well as \u03b2 contribution.Quite similar CO oxidation activities can be reached from different synthesis methods and different copper contents (Cu0.5CZ-IWI and Cu2CZ-CP), evidencing the importance of the nature and the type of CuOx species generated over the catalysts\u2019 surface, which can be modulated by the synthesis procedure. These features seem to be more relevant than the own CuO content on catalysts.From the combined study of CO-TPR and H2-TPR profiles, it can be seen that CuOx entities are reduced at low temperatures (very dependent on the preparation method) onto ceria-zirconia, but, importantly all the cerium centers susceptible to be reduced under CO and/or H2 in the support, are reduced in an anticipated way, with regard to the interval of temperatures where this reduction process takes place in the bare support. The detailed investigation of the several contributions and peaks that appear on the CO-TPR profiles, compared with the bare support and with the CuO bulk profile has been revealed as very useful for a first approach into the understanding of the synergies created on this system. Finally, the evidences provided by the reducibility and catalytic profiles of the catalysts prepared by the different procedures can contribute, interestingly, to the clues about the features which should be determinant for these catalyst to present very high catalytic activity towards CO oxidation, mainly at low temperatures.\nJ.C. Mart\u00ednez-Munuera: Conceptualization, Methodology, Investigation, Validation, Visualization, Writing - original draft. V.M. Serrano-Mart\u00ednez: Investigation. J. Gim\u00e9nez-Ma\u00f1ogil: Investigation. M.P. Yeste: Investigation, Validation, Visualization. A. Garc\u00eda-Garc\u00eda: Conceptualization, Methodology, Writing - original draft, 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.The authors gratefully acknowledge the financial support of Generalitat Valenciana (PROMETEO/2018/076 project) and the Spanish Ministry of Science and Innovation (PID2019-105542RB-I00 project) and the UE-FEDER funding. Mart\u00ednez-Munuera also acknowledges Spanish Ministry of Science and Innovation for the financial support through a FPU grant (FPU17/00603).", "descript": "\n                  The aim of this research is an attempt to shed some light on the understanding of the nature of the active sites and the generated synergies in the copper/ceria-zirconia formulations for low temperature CO oxidation by means of the creation of copper entities with different physico-chemical nature. For this reason, several CuOx/ceria-zirconia catalysts, with different Cu contents and different methods to incorporate copper species, were synthesized. Focus was specially put in this case trying to link the results of CO oxidation catalytic tests with the CO-temperature programmed reduction profiles/approximate estimations and selected characterization parameters in order to find out correlations among catalysts\u2019 properties/reducibility and catalytic behaviors, especially those corresponding to the nature and roles of the different CuOx species in contact with ceria-based support on catalytic activity.\n                  Results reveal a significant improvement in CO conversion compared to the ceria-zirconia support by adding a small amount of copper loading (as low as 0.5 %), emphasizing the paramount role of copper incorporated by the method of IWI. From 0.5 up to 2% of copper loading, an interesting increase gradual trend in activity and reducibility can be noted. It should be mentioned that all the catalysts obtained by this procedure are more catalytically active towards CO oxidation than 1%Pt/Al2O3 at low temperatures (T\u202f<\u202f130\u202f\u00b0C). CO-TPR results show that the reducibility of these catalysts is in line with their CO oxidation activity. The method of preparation has been revealed as a critical variable in the catalytic performance, and quite similar catalytic activities can be reached from different synthesis methods and different copper contents, due to the similar nature and type of CuOx species generated over the catalysts\u2019 surface, identified by the CO-TPR profiles and the rest of characterization data. Finally, IWI method seems to be the best one among those tested, thus combining superior areas of both \u03b1 and \u03b2 contributions assigned on CO-TPR profiles, which seem to be critical in the interpretation of the catalytic behaviors.\n               "}
{"full_text": "Al\u2013Ni reactive materials, including composite particles and multilayered foils, are a class of energetic materials (EMs) with high-energy content [1\u20133]. These materials can undergo intermetallic reaction with a significant amount of heat release and forming composites with high mechanical strength [4,5]. Due to this promising feature, Al\u2013Ni reactive materials have been widely used in various energetic applications such as reactive fragments for warhead [6], reactive shaped charge liner [7], insensitive penetrator based on nano-structured EMs [8,9], However, Al\u2013Ni mixtures are not able to be readily ignited especially when their sizes are in micron scale. The relatively low exothermicity of reactions between metallic reactants may results in a high energy barrier for reliable ignition. In addition, the naturally formed oxide shell on the surface of Al particles is likely to lead to the lowered reactivity between the Al and Ni, thereby limiting its broader application [10\u201312]. In order to improve the ignition and energy release efficiency of Al\u2013Ni, the novel design and relevant advanced preparation of Al\u2013Ni materials have shown to be promising according to recent literature summarized as follows.Various methods used for the preparation of Al\u2013Ni reactive materials with lower ignition threshold and higher combustion performance have been proposed [13\u201318]. For instance, Hadjiafxenti et\u00a0al. produced Al\u2013Ni nanocomposite powders by low energy ball milling (LEBM), which exhibit a lower ignition temperature below 600\u00a0K [19\u201321]. The increased reactivity of Al\u2013Ni nanocomposites produced by ball milling is likely due to the fact that the nano-Ni was embedded in Al matrix without formation of oxide barriers or intermediate layers [1,21,22]. Except for composite particles, Al\u2013Ni foils with varied bilayer thicknesses could be fabricated at different atomic ratios by using sputtering method. The onset temperatures of those multilayered foils are below 800\u00a0K, higher than its nanoparticle style [23]. Mukasyan et\u00a0al. proposed that the combustion wave in the Ni/Al nano-foil appears to be a sequential two-stage process, which involves the chemical and physical exothermic transformations [24]. Gunduz et\u00a0al. reported another two-stage reaction in their experimental and modeling work, which includes the flame front propagates near the reverse peritectic transformation temperature of Ni2Al3 into NiAl and melts at 1406\u00a0K. The reaction continues with the growth of NiAl until the melting temperature of 1911 K [25\u201327]. The above-mentioned preparation methods may provide Al and Ni with more intimate and high surface area contacts, which are critical to the self-propagation combustion of the solid-state reactions. In this way, reactions that normally require high heat input for initiation can be realized at lower temperatures.In addition to the preparation methods that were used to reduce ignition threshold and improve the combustion performance of Al\u2013Ni by increasing the intimate contact, the additives can also be used to enhance the reactivity of Al\u2013Ni by accelerating of the reaction rate and reducing the agglomeration. Researchers have managed to use metallic additives to prepare Al\u2013Ni/M (M: molybdenum, copper and magnesium). The initial temperatures of Al\u2013Ni were found to be increased to various levels depending on the type of metals used. Among them, Cu shows the most significant effect on the combustion process of Al\u2013Ni with a remarkable increase in the flame temperature from 2000 K to 3000\u00a0K [28]. For fluoropolymers, the addition of PTFE to Al\u2013Ni may decrease the critical shock pressure for initiation of shock-induced chemical reaction, due to a lowered the apparent activation energy and increased the chemical reaction efficiency of Al\u2013Ni in the presence of fluorine as a highly oxidative element. In particular, with the addition of PTFE, the pre-ignition reaction (PIR) occurs between Al2O3 and PTFE, so that the heat release from PIR reaction plays a positive role in the promotion of intermetallic reaction between Al and Ni [11]. Besides, the agglomeration was greatly reduced due to generation of gaseous product (AlF3) during PIR, so that the reaction efficiency may also be enhanced. In addition to PTFE, other types of fluorine-containing polymers could also react with Al2O3 passivation layer that facilitates the exposure of the active Al, thereby promoting the reactivity [29\u201331].Besides fluoropolymers, the same effect can be observed for the transition metals used as coating agents. A thin Ni coating layer on the surface of Al particle was shown that the agglomeration of the CCPs would be prevented. In this way, the ignition temperature of Al was reduced to 760\u2013950\u00a0K and the front velocity was increased by a factor of 4 as compared to the unmodified ones [2,32]. The ignition mechanism was found to be correlated directly with intermetallic exothermic reactions between Al and Ni [2,33,34]. It was explained that the improvement in combustion performance is a result of the cracking Al2O3 shell due to thermal stress that promotes ignition of Al. In order to further improve the combustion performances of Al\u2013Ni, it would be desirable if the coating agents can react with the passivation layer of Al and forming gaseous products. Except for fluoropolymer, the coating of halogen-containing EMs on the surface of Al\u2013Ni has the great potential to meet both requirements.It has been shown that coating modification with halogen-containing oxidants may lead to a significant enhancement of ignition and combustion by improving the reactivity of Al [35\u201338]. In addition, the coating technique has a certain positive effect on improvement of the mechanical properties of Al-based composites, which is, however, not in the scope of this study. Our group have conducted various investigations on the coating of Si with optimized ECs. As typical examples, the Si@PVDF/CL-20 (PVDF/CL-20 with the mass ratio of 1:6), Si@AP/NC, (AP/NC with the mass ratio of 2:1) composites with a core-shell structure have been successfully synthesized by using spray-drying technology [39,40]. The results showed that Si@ECs can undergo a more complete reaction between Si and the decomposition products of ECs during the combustion process. The ECs have relatively lower ignition threshold, higher reactivity and better stability. Those ECs could be an appropriate candidate for enhancing the combustion performance of Al\u2013Ni. Therefore, it is of great interest to investigate the effect of energetic composites AP/NC and PVDF/CL-20 on the combustion performance of Al\u2013Ni.In this work, the Al\u2013Ni@ECs at various Al\u2013Ni atomic ratios have been prepared by arrested high energy ball milling (AHEBM) followed with spray-drying technique. The morphologies and compositions of prepared composites and the CCPs were characterized by scanning electron microscopy (SEM) and X-ray diffraction technique, respectively. The thermal reactivity and combustion performances of Al\u2013Ni@ECs composites were evaluated by thermal analyses and customized combustion diagnostic system. The thermodynamic calculation of the full-range chemical equilibrium of the Al\u2013Ni@ECs was conducted by HSC software as a supporting information to elucidate the mechanisms of the enhanced combustion.The micro-spherical aluminum powder (\u03bc-Al) with an average diameter of 1\u00a0\u03bcm, nano-spherical nickel particles (n-Ni) with a mean diameter of 100\u00a0nm, acetone, and dimethylformamide (DMF) were purchased from Sigma-Aldrich company. NC with the nitrogen content of 12.6\u00a0wt%, AP (\u226599.5%), CL-20 (\u226599.5%), PVDF (\u226599.9%) were supplied by Xi\u2019an Modern Chemistry Research Institute.In order to obtain the maximized energy output, the ECs including AP/NC (NA) and PVDF/CL-20 (PC) were optimized at mass ratio of 2:1 and 1:6, respectively. The maximum energy release of NA and PC were 5919\u00a0J/g and 3536\u00a0J/g, respectively [39,40]. Coating of ECs on Al\u2013Ni particles were prepared by spray-drying technique. For the practical applications, it is beneficial to use the least mass content of ECs in the reaction mixture, which is capable of yielding a sufficiently low ignition temperature and initiating the rapid self-propagating combustion. To achieve a thermo-chemical activation mode, the smallest quantity of ECs used for the activation of Al\u2013Ni in this system that was experimentally obtained is 10\u00a0wt%. The process for preparation of Al\u2013Ni@ECs composites is illustrated in Fig.\u00a01\n.Ball milling time is the major controlled factor variable in preparation of Al/Ni and Al/3Ni composites at molar ratios of 1:1 and 1:3, respectively. It has been shown that the heat release of Al/Ni starts to decrease when the milling time increases up to 2\u00a0h, whereas it is 6\u00a0h for Al/3Ni (more details are shown in Section 3.1). Therefore, the milling times used for preparation of Al/Ni and Al/3Ni are fixed to be 2\u00a0h and 6\u00a0h, respectively. The preparation of Al/3Ni reactive composite is briefly introduced as follows: taking Al/Ni as an example, 3.15\u00a0g Al and 6.85\u00a0g Ni powders are milled in a 250\u00a0mL stainless steel jar by using a planetary ball milling facility (XQM-2-DW, China) for 2\u00a0h. The rotation speed was 300\u00a0rpm, and the diameter of stainless steel ball is 5\u00a0mm. The mass ratio of ball to powder was 10:1. In this process, 20\u00a0mL mixed solution of DMF and acetone with the volume ratio of 4:1 was used as the processing media. It is also the case for preparation of Al/3Ni, but the only difference is that the milling time was increased to 6\u00a0h.Afterwards, 10\u00a0wt% of ECs as the coating agents are introduced to two typical Al\u2013Ni composites as shown in Table\u00a01\n. The uniformly ball milled Al\u2013Ni reactive materials are firstly collected into a beaker, and then 10\u00a0wt% of ECs with 50\u00a0mL mixed solution are added. The resulted precursor solution was stirred for 2\u00a0h to ensure sufficient dispersion of the Al\u2013Ni powders. Finally, the precursor solution was spray-dried to obtain the final composite coated products. The parameters for the spray-drying process are as follows: diameter of feed well is 1\u00a0mm, and fluid \ufb02ow rate is 3\u00a0mL/min. The inlet and outlet temperature are kept at 170\u00a0\u00b0C and 110\u00a0\u00b0C, respectively.The spray-dried powders are enclosed in a cylindrical mould with the internal dimension \u0424 10\u00a0\u00d7\u00a045\u00a0mm2 and uni-axially pressed at a pressure of 2\u00a0MPa for 5\u00a0min to make a dense sample. It was then placed in the sealed chamber with pressurized Ar for the combustion diagnosis. The CCPs were collected from DSC experiments and then characterized in terms of phase compositions and morphologies by using XRD and SEM techniques. The details of such characterizations are provided in the Supporting Information.The DSC experiments for all involved Al\u2013Ni composites with milling time have been conducted, and the resulted heat flow curves as a function of temperature are shown in Fig.\u00a02\n.As shown in Fig.\u00a02(a), there are two exothermic peaks at 579.9\u00a0\u00b0C and 618.1\u00a0\u00b0C for Al/Ni composite, when it was prepared by ball milling of 0.5\u00a0h. However, the second exothermic peak disappeared when the ball milling time was in the range of 1\u00a0\u20136\u00a0h. Interestingly, the second exothermic peak appears again for the composite after ball milling of 9\u00a0h. Moreover, the peak temperature of the exotherm for Al/Ni composite decreases from 579.9 to 562.6\u00a0\u00b0C as the ball milling time increases. However, when the milling time further increases (e.g. to 9\u00a0h), the exothermic peak shifts to a higher temperature, and it is also the case for Al/3Ni (shown in Fig.\u00a02(b)).The detailed DSC parameters obtained for Al/Ni and Al/3Ni composites are summarized in Table\u00a02\n. It can be noticed that the measured heat flows (Q) increase first and then decrease with the increase of ball-milling time for both composites. There is an appropriate milling time, when the Al and Ni is homogeneously mixed and well contacted, so that the initial reaction temperature reach the lowest point. It also suggests that excessive ball milling leads to the partial intermetallic reaction between Al and Ni, so that the reactivity would be decreased and the measured heat of reaction becomes lower. According to the heat of reaction, the appropriate ball-milling time for Al/Ni should be about 2\u00a0h (e.g. 832.0\u00a0J/g). In comparison, the best ball-milling time for Al/3Ni could be around 6\u00a0h, where the maximum heat release was 598.8\u00a0J/g, lower than that of Al/Ni.The morphologies of Al, Ni, Al\u2013Ni and Al/Ni@ECs are shown in Fig.\u00a03\n. For Al and Ni as the starting materials (Fig.\u00a03(a) and Fig.\u00a03(b)), their surfaces are neat and smooth. In case of the ball-milled Al\u2013Ni composites, the surface of Al particle (Fig.\u00a03(c) and Fig.\u00a03(d)) becomes uneven, where the n-Ni particles are randomly distributed. The surfaces of Al/Ni@NA and Al/Ni@PC composites show a relatively rough morphology (Fig.\u00a03(e) and Fig.\u00a03(f)). The initial particle size of Al is about 2\u00a0\u03bcm, and it increases a little once Ni is covered. Once ECs is included, the Al/3Ni or Al/Ni seem aggregated to about 3\u20135\u00a0\u03bcm (Fig.\u00a03(e) and Fig.\u00a03(f)). Meantime, the surface of these large particles is defected with lots of pores, showing increased specific surface areas.The obtained element mapping results for Al/Ni@NA and Al/Ni@PC are shown in Fig.\u00a03(g) and Fig.\u00a03(h), where the distributions of Cl/N/O and F/N/O illustrate the AP/NC and PVDF/CL-20 are uniformly coated on the surface of assembled aggregated Al/Ni particles.The DSC experiments have been performed to investigate the thermal behaviors of Al\u2013Ni@ECs composites and the corresponding heat flow curves are plotted in Fig.\u00a04\n.As shown in Fig.\u00a04(a), the first exothermic peak of AP/NC at 213.2\u00a0\u00b0C is mainly due to the decomposition of NC, and the followed endothermic peak at 246.8\u00a0\u00b0C is due to the polymorphic transition of AP from orthorhombic to cubic form [41,42]. The second exothermic peak of AP/NC is due to low temperature decomposition of AP with release of NH3 and HClO4 intermediates, but this peak temperature is slightly lower than that of AP (299.3 vs. 312.9\u00a0\u00b0C). In addition, the third exothermic peak of AP/NC at 386.3\u00a0\u00b0C could be a result of the high-temperature decomposition of AP, where the oxidation of absorbed NH3 or NH4\n+ by\u00b7ClO3 radical [42]. Obviously, this decomposition process of AP is accelerated, likely attributed to the catalytic effect by the condensed decomposition products of NC (e.g., hydrocarbon chains) [43]. For PVDF/CL-20, the first endothermic peak representing the melting process of PVDF becomes very weak at about 202.1\u00a0\u00b0C due to relative lower content, as compared to that of pure PVDF at 164.6\u00a0\u00b0C. It means that there might be interaction between F and NO2 group, which may stabilize both components. Therefore, the first exothermic peak of PVDF/CL-20 at 238.2\u00a0\u00b0C is due to the decomposition of CL-20, which is slightly higher than that of pure CL-20. The second exothermic peak caused by decomposition of PVDF is shifted from 503.4\u00a0to 496.6\u00a0\u00b0C, due to the catalytic effect by condensed decomposition product of CL-20 as the case for NC towards AP shown above [43].An endothermic peak at 661.1\u00a0\u00b0C for pure Al can be clearly seen in Fig.\u00a04(b), which is corresponding to the melting of Al [43]. Exothermic peaks displayed at 572.2\u00a0\u00b0C and 601.9\u00a0\u00b0C are observed for Al/Ni and Al/3Ni, respectively. It reveals that ECs decomposes much earlier than the intermetallic reaction. Those two exothermic peaks are attributed to formation of AlNi and AlNi3, respectively [17,19]. The detailed reaction mechanisms of Al\u2013Ni are discussed in the following Section 3.3.2.As the coating layer of Al/Ni, the first exothermic peak of AP/NC appears approximately at 205.4\u00a0\u00b0C, which is surely caused by the decomposition of NC [44] and it is 7.8\u00a0\u00b0C lower than that of pristine AP/NC. The following exothermic peak at 303.6\u00a0\u00b0C is due to decomposition of AP. In presence of Al/Ni, the two decomposition peaks of AP are merged into one, and the peak temperature is 9.3\u00a0\u00b0C lower than that of the second T\np of AP. The third exothermic step associated with a small peak at 506.7\u00a0\u00b0C may be caused by the reaction between the acidic condensed products of AP and the Al2O3 passivation layer, with heat release of 116.9\u00a0J/g [45]. The final exothermic process should be attributed to the intermetallic reaction between Al and Ni, which has a peak at around 573.9\u00a0\u00b0C, the heat release is 771.0\u00a0J/g and the condensed alloy product was confirmed by XRD spectrum (see in Section 3.3.2). The total energy release of thermit and intermetalllic reactions of Al is 887.9\u00a0J/g, which is improved by 6.7% in comparison with pure Al/Ni. A very similar exothermic reaction process is shown for Al/3Ni@NA composite, the energy release of the Al-related reaction was 860.8\u00a0J/g, which was 28.8\u00a0J/g higher than Al/Ni. Obviously, the energy release of Al\u2013Ni was increased when it is coated with ECs, the increased energy is due to the preheating and coupling effects of ECs thermolysis. In comparison, for Al/Ni@PC, the first exothermic peak at around 237.9\u00a0\u00b0C can be assigned to CL-20 thermolysis, which is only 0.3\u00a0\u00b0C lower than that of pristine PC. The second small exothermic peak at around 418.8\u00a0\u00b0C was due to the pre-ignition reaction (PIR) between PVDF and Al2O3 passivation layer [46]. It is followed by the main intermetallic exothermic reaction with a peak at 596.5\u00a0\u00b0C. It is also the case for Al/3Ni@PC, where the first two exothermic peaks at 233.4\u00a0\u00b0C and 465.4\u00a0\u00b0C are due to the decomposition of CL-20 and the PIR reaction, respectively. The third and last reaction steps are partially overlapped with a final peak at 628.5\u00a0\u00b0C, respectively. Such a two-step exothermic pattern is covered with a heat release of 507.0\u00a0J/g, which is 353.8\u00a0J/g lower than that of Al/Ni@PC. The energy release of Al-related reaction of Al/3Ni@ECs is lower than that of Al/3Ni, which maybe due to the reduced reaction rate between Al/3Ni and the condensed products of ECs, so that part of the heat release is covered by the baseline and overlooked.The above results imply that the improved reactivity of Al\u2013Ni@ECs could be due to a synergistic effect, where Al\u2013Ni catalyzes the decomposition of ECs and the heat release of Al\u2013Ni is promoted in presence of condensed thermolysis products of ECs, especially when the products are acidic and could easily react with the oxide layer of Al and Ni.In order to understand the condensed phase reaction mechanisms of Al\u2013Ni@ECs composites, the XRD was implemented to identify all possible intermediate products quenched at different temperatures from DSC experiments. The obtained XRD spectra are shown in Fig.\u00a05\n. As expected, a strong diffraction peak of intermetallic compound is detected for the product collected at 800\u00a0\u00b0C, but the diffraction patterns are different depending on the types of ECs and atomic ratio of Al\u2013Ni.As shown in Fig.\u00a05(a), only the diffraction peaks of Al and Ni can be observed for the Al/Ni at room temperature. At 620\u00a0\u00b0C, the diffraction peaks of AlNi, Al3Ni2, Al0.9Ni4.22, and unreacted Al and Ni are shown together. At further elevated temperature of 800\u00a0\u00b0C, the diffraction peaks of unreacted Al and Ni disappear and the relative diffraction intensities of AlNi and Al0.9Ni4.22 are increased as well. Additionally, a new diffraction peak of Al3Ni is observed at this temperature. The results indicate that the intermetallic exothermic reactions between Al and Ni occurs in between 620\u00a0\u00b0C and 800\u00a0\u00b0C for Al/Ni.For Al/Ni@NA (Fig.\u00a05(a) and Fig.\u00a0S1), the diffraction peaks of Al, Ni, and AP can be clearly seen in the XRD pattern at room temperature. At 520\u00a0\u00b0C, only the diffractions of Al and Ni are observed. It indicates that AP completely decomposed at this temperature, which is consistent with the DSC results. At 800\u00a0\u00b0C, the diffraction peaks are dominated by the intermetallic phase of AlNi, suggesting the intermetallic reaction between Al and Ni dominates at this temperature. For Al/Ni@PC, the XRD patterns are almost identical to those of Al/Ni@NA at each attempted temperature.For Al/3Ni (Fig.\u00a05(b)), when the temperature elevates to 800\u00a0\u00b0C, the phase compositions of the final condensed product are dominated by AlNi3 and which is contaminated with small amounts of Al3Ni2 and AlNi. With the addition of ECs, a new phase of AlN is formed at 800\u00a0\u00b0C for Al/3Ni@ECs implying that the reaction takes place between N element from ECs and Al. This reaction partially consumes the Al, which is supposed to participate in the intermetallic reaction later, and thereby resulting in decreased practical reacting ratio between Al and Ni.The maximum energy releases of Al\u2013Ni@ECs during the combustion process have been measured by using a bomb calorimeter. The heats of reaction are shown in Fig.\u00a06\n and summarized in Table\u00a0S1.The variation in the measured heat of reaction for Al\u2013Ni@ECs composites is shown in Fig.\u00a06. The highest energy generation is achieved by Al/Ni@NA, indicating that a higher combustion efficiency of Al/Ni could be realized by a minor use of NA. Compared with Al/Ni, the reaction heat of Al/Ni@NA and Al/Ni@PC are increased by 108% and 53%, respectively. It is strange that pure Al/3Ni cannot be easily ignited under 3\u00a0MPa of Ar. However, with the introducing of ECs, the ignition of Al/3Ni@ECs composites can be easily achieved. Herein, the theoretical energy release value of Al/3Ni (1230\u00a0J/g) is used as reported by Fischer et\u00a0al. [47]. to assess the effect of coating ECs on Al/3Ni. Compared with the theoretical energy release value of pure Al/3Ni, the reaction heat of Al/3Ni@NA was increased by 42%, whereas that of Al/3Ni@PC was increased by 22%. Obviously, the energy release of Al\u2013Ni was greatly improved with the inclusion of ECs.Meantime, the reaction heat of Al/Ni@ECs is higher than that of Al/3Ni@ECs, suggesting that Al\u2013Ni with the atomic ratio of 1:1 has higher energy content than that of 1:3. The results indicate that both the atomic ratio of Al\u2013Ni and the type of ECs have significant effects on the energy content and release rate of Al\u2013Ni. The Al\u2013Ni with the inclusion of PC seem to be less reactive as compared to NA. The possible reason could be related to the relative low exothermicity from the interfacial reaction between PC and Al\u2013Ni. In fact, the reactivity of Al/CuO nanothermite composites with fluoropolymers has been measured with similar findings [48]. They showed that hydrogen fluoride (HF) released from PVDF may react with Al/CuO, resulting in less energy due to higher heat of formation of fluorides than oxides. This less exothermic reaction may be responsible for the lower combustion rate and less heat release. Therefore, it can be concluded that the exothermicity of the reactions between ECs and Al\u2013Ni plays a critical role in tunning energy contents and heat release rates of Al\u2013Ni the involved composites.The combustion behaviors of Al\u2013Ni@ECs composites have been studied using our customized combustion diagnostic system. The sequential snapshots of all samples burning in Ar have been done using a high-speed camera through a transparent window (Fig.\u00a07\n and Fig.\u00a0S2). All the samples except Al/3Ni were successfully ignited and proceeded to a self-sustainable combustion. The low ignitability of Al/3Ni is likely due to high chemical stability of Ni, especially when the fraction of Ni in the Al/3Ni composite is surpassing a certain threshold. The difficulty in ignition suggests that this composite at atomic ratio of 1:3 is insensitive to heat. Thus, the reactivity of the Al/3Ni@ECs may also be relatively lower compared to that of Al/Ni@ECs samples.For Al/Ni, the light emission of the burned sample lasts for \u223c2\u00a0s, which mainly involves the intermetallic reaction between Al and Ni. When the intermetallic reaction was completed, the brightness of the burned Al/Ni gradually reduces during cooling process. When it was coated with NA, the flame propagation process of Al\u2013Ni@NA presents significant visible spots as well as irregular cracks on the surface of burned samples as shown in Fig.\u00a07(b) and d. At the same time, an axial elongation of Al\u2013Ni@NA increases with the increase of burn time. The same phenomenon was shown in the case of Al/3Ni@NA. Such a behavior could be attributed to the effect of a large number of gaseous products (e.g., HF, NO and CH4) released from ECs, which were ejected from the sample surface and generating porous structures in the residues. Furthermore, the violent combustion reactions were observed for Al\u2013Ni@NA, indicating that the reactivity of intermetallic reaction between Al and Ni has been greatly enhanced with the inclusion of NA. The flame front of Al/Ni@NA takes about \u223c450\u00a0ms to reach the bottom of charge, since the flame propagation rate of NA is much faster than PVDF/CL-20. Moreover, the self-sustained combustion rate of Al/Ni@PC is smaller than that of Al/3Ni@PC, which is probably caused by the less exothermic reaction between the condensed phase products decomposed from PC and Al\u2013Ni.To further investigate the combustion behaviors of the prepared composites, the flame propagation rates have been calculated based on the recorded images by using high-speed camera in Fig.\u00a0S2, and the results are summarized in Table\u00a03\n.From Table\u00a03, it can be seen that the flame propagation rate of Al\u2013Ni was increased with the inclusion of ECs. The flame propagation velocity of Al/Ni is 15.8\u00a0mm/s. In case of Al/Ni@NA, the flame propagation rate was increased by 30.0% from 15.82\u00a0mm/s to 20.6\u00a0mm/s under the effect of the NA coating. The same positive effect of PC coating on Al/Ni is obtained, where the flame propagation rate of Al/Ni@PC was 11.6% higher than pristine Al/Ni. For Al/3Ni@ECs composites, the propagation rate is nevertheless reduced with the increase of Ni. It suggests that despite Ni contributes to catalytic effect on ECs, excessive Ni would reduce the overall energy content in comparison to Al, thereby reduce the flame propagation rate.Combustion wave temperature usually demonstrate the efficiency of the heat generation and the heat capacity of the combustion products. The temperature distribution and its dependence on burn time have been obtained by using the high-speed infrared camera. The recorded thermal images are displayed in Fig.\u00a0S3, that allows one to preliminarily judge the relative difference in surface temperature distribution for the involved samples, the actual temperature could be much different, since the radiation parameter is very difficult to be accurately obtained. Anyway, these values are meaningful for a systematically comparing the effect of ECs on the improving the reactivity of intermetallic reaction of Al\u2013Ni system.As shown in Fig.\u00a0S3, the maximum flame temperature of Al/Ni@ECs is about 1800\u00a0K, which is \u223c500\u00a0K higher than that of Al/3Ni@ECs with less energy content. The bright red and blue vapors indicate that large amounts of gases are produced by ECs during their combustion processes. It is clear that Al/Ni@ECs composites release more gaseous products than Al/3Ni@ECs. The fast heat generation by gaseous production of ECs could be absorbed by Al\u2013Ni for preheating, which has positive affect on the combustion efficiency/rate of Al\u2013Ni. Furthermore, the secondary reaction of Al/Ni@ECs as depicted in Fig.\u00a0S3(b) and Fig.\u00a0S3(c) during the combustion process is featured with a secondary temperature rise of the burned sample, which was not observed for Al/3Ni@ECs. The secondary heat release process of Al/Ni@ECs is mainly dominated by the intermetallic reaction, which is greatly enhanced by the presence of the decomposed gaseous products by ECs as the catalysts or reactive sites, where porous structure has been formed. The absence of this phenomenon of Al/3Ni@ECs might be responsible for the relatively low reaction temperature change. It further confirms that an enhanced reactivity and higher combustion efficiency can be obtained for Al/Ni with an atomic ratio of 1:1.For a better comparative analysis, the average combustion wave temperature of Al\u2013Ni@ECs have been evaluated. The corresponding temperature vs. time profiles and its derivatives are shown in Fig.\u00a08\n(a) and Fig.\u00a0S3(b), respectively. All the curves are shifted manually along the X-axis in order to avoid overlapping. In general, the average temperatures of the composites with ECs coating are several hundred Celsius higher than that of the reference sample of Al/Ni. Moreover, an average maximum combustion temperature (T\nmax) obtained for both Al/Ni@ECs composites are higher than that of Al/3Ni counterparts.The temperature rise rate (\u03b3\nt), serves as the parameter indicating the intensity and efficiency of self-sustained combustion. It is affected by the gases production rate, thermite reaction rate and intermetallic reaction rate during the combustion process. Fig.\u00a08(b) shows the temperature rise rate derived for Al\u2013Ni@ECs composites, where \u03b3\nt is basically increased with the addition of ECs regardless of their types. For the best scenario observed, it is over 11 times higher than that of the reference sample. Furthermore, the temperature rise rate of Al/3Ni@ECs is relatively lower than that of Al/Ni@ECs, which can also be supported by the fact of the lower exothermicity of Al/3Ni.The morphology and composition of the CCPs were characterized aiming to reveal the combustion reaction mechanisms. The CCPs of Al\u2013Ni@ECs composites were collected for SEM, EDS and XRD analyses and the results are shown in Fig.\u00a09, Fig.\u00a0S4 and Fig.\u00a010\n\n. Compared with Al/Ni shown in Fig.\u00a0S4(a), the holes of the CCPs from Al\u2013Ni@ECs (Fig.\u00a09, Fig.\u00a0S4(b) and Fig.\u00a0S4(c)) are increased. It can be obtained that the gaseous products of Al\u2013Ni@ECs are increased in comparison to that of Al/Ni without the ECs coating. The gaseous products on the interface layer can exclude the sintering and form lots of pores as literature reported [37]. Those pores also provide new channels for the further reaction between Al\u2013Ni and the condensed products of ECs.The surface of the CCPs from Al/Ni@PC contains many hollow spheres with varied diameters ranging from 5 to 200\u00a0\u03bcm as shown in Fig.\u00a09(a). According to the previous studies, the hollow structure provides the penetrating channels for better heat and mass transfer, which leads to an enhanced intermetallic reaction rate of Al/Ni. The CCPs of Al/Ni@PC were examined by powder XRD techniques, where AlNi and a small amount of Al5O6N (Fig.\u00a010) were discovered, indicating that the reaction between the gaseous products of ECs and Al occurred before intermetallic reaction of Al and Ni.For Al/3Ni@PC, a plenty of cubic crystals with smooth surfaces can be easily observed in their CCPs as shown in Fig.\u00a09. The EDS results show that the element of fluorine is dominated in the crystals, suggesting that they are F-containing compounds. As mentioned above, the reactivity of Al/3Ni@PC is relatively lower, which is responsible for the low reaction temperature as well as low burning rate of Al/3Ni@PC. Thus, the lowered reaction temperature and reduced burning rate may collectively be in favor of the growth of this F-containing crystal. However, such a crystal is unknown in the chemical database, and the specific formation mechanism needs to be further studied. Moreover, the CCPs of Al/3Ni@PC display a less porous structure in comparison to that of Al/Ni@PC. Since the formation of F-containing crystals would reduce the availability of F, so that the gaseous products are consequently reduced. The reduced gaseous products have a detrimental effect on the formation of porous structure, and thereby it significantly decreases the reaction rate of Al/3Ni. The CCPs of Al/3Ni@PC mainly contain AlNi3, a small amount of AlN and Al5O6N (Fig.\u00a010). The presence of some other unknown diffraction peaks in the CCPs of Al/3Ni@PC may be associated with the F-containing cubic crystals.In order to further understand the reaction process, the equilibrium compositions of Al/Ni@NA at different temperatures were calculated by using HSC software. Fig.\u00a011\n shows the equilibrated species of Al/Ni@NA vs. temperature. The quantities of HCl and AlNi decrease with the increase of temperature due to the formation of AlCl, suggesting that the reaction between HCl released from AP and shell of Al has occurred, which is beneficial to improve the combustion performance of the composite [49]. With the increase of AlCl content, the AlO appears at higher temperature. The AlO is a primary combustion intermediate of Al, indicating that the reaction between ECs and Al occurs after the etching of Al2O3 shell. Furthermore, the formation of Al5O6N determined by XRD analysis further confirms that the reaction is most likely to occur between gaseous products of ECs and Al. In addition to the CCPs of AlNi, the calculation also shows that gaseous species such as NH3, Ni, and AlCl are generated during the combustion process. The formation of gaseous substances would greatly change the reaction pathways, by shifting it from a solid-solid to solid-gas or even liquid-gas modes. The combustion performance was significantly improved under the combined combustion mode [50].According to the above results, the overall reaction processes of Al\u2013Ni@ECs are proposed as follows. First, ECs decompose and ignited with combustion products uniformly cover the particle of Al\u2013Ni, and then the products or intermediates of ECs would react with preheated Al. The fast heating by ECs combustion can promote the flame propagation rate of Al\u2013Ni. Additionally, the acidic products such as HCl and HF generated from ECs may etch the Al2O3 shell, so that the inner active Al to be exposed and easily react with Ni. Therefore, more efficient energetic systems can be obtained by introducing these halogen containing ECs.In this work, two types of ECs (NA and PC) were used to coat Al\u2013Ni reactive materials by ball milling followed with spray-drying technique. The effects of ECs on the heat release, combustion characteristics of Al\u2013Ni reactive materials, morphologies and compositions of the corresponding combustion products have been comprehensively studied. It has been shown that the combustion performances of those composites are greatly affected by introducing different types of halogen-containing ECs as coating agents. Particularly, the combustion performance of Al\u2013Ni can be significantly improved by coating of NA. The flame propagation rate was increased from 15.8\u00a0mm/s to 20.6\u00a0mm/s, which was 30.0% higher than that of the reference. In addition, the combustion wave temperature of the corresponding surface modified composites was \u223c500\u00a0K higher than that of the reference without surface modifications. The acidic gaseous products decomposed from halogen-containing energetic composites can react with Al2O3 passivation layer, which make the inner active Al to be exposed and easily react with Ni. Therefore, the intermetallic reaction between Al and Ni was greatly enhanced.These results presented in this paper demonstrate that the halogen-containing energetic composites are the promising candidate for tuning the reactivity and combustion characteristics of the reactive intermetallic materials. Further efforts can be made on the clarification of the detailed combustion mechanisms of such composites with advanced techniques such as Time of Flight/Mass Spectroscopy (TOFMS).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 Nation Natural Science Foundation of China (Grant No. 51776176) and the Fundamental Research Funds for the Central Universities, China (Grant No. G2017KY0301). This paper was also partially funded by NSAF project (Grant No.2030202) and sponsored by Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (Grant No. CX2021048).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.dt.2022.01.007.", "descript": "\n                  In this paper, various core-shell structured Al\u2013Ni@ECs composites have been prepared by a spray-drying technique. The involved ECs refer to the energetic composites (ECs) of ammonium perchlorate/nitrocellulose (AP/NC, NA) and polyvinylidene fluoride/hexanitrohexaazaisowurtzitane (PVDF/CL-20, PC). Two Al\u2013Ni mixtures were prepared at atomic ratios of 1:1 and 1:3 and named as Al/Ni and Al/3Ni, respectively. The thermal reactivity and combustion behaviors of Al\u2013Ni@ECs composites have been comprehensively investigated. Results showed that the reactivity and combustion performance of Al\u2013Ni could be enhanced by introducing both NA and PC energetic composites. Among which the Al/Ni@NA composite exhibited higher reactivity and improved combustion performance. The measured flame propagation rate (v\u00a0=\u00a020.6\u00a0mm/s), average combustion wave temperature (T\n                     max\u00a0=\u00a01567.0\u00a0\u00b0C) and maximum temperature rise rate (\u03b3t\u00a0=\u00a01633.6\u00a0\u00b0C/s) of Al/Ni@NA are higher than that of the Al/Ni (v\u00a0=\u00a015.8\u00a0mm/s, T\n                     max\u00a0=\u00a0858.0\u00a0\u00b0C, and \u03b3t\u00a0=\u00a0143.5\u00a0\u00b0C/s). The enhancement in combustion properties could be due to presence of the acidic gaseous products from ECs, which could etch the Al2O3 shell on the surface of Al particles, and make the inner active Al to be easier transported, so that an intimate and faster intermetallic reaction between Al and Ni would be realized. Furthermore, the morphologies and chemical compositions of the condensed combustion products (CCPs) of Al\u2013Ni@ECs composites were found to be different depending on the types of ECs. The compositions of CCPs are dominated with the Al\u2013Ni intermetallics, combining with a trace amount of Al5O6N and Al2O3.\n               "}