Avoiding Sabatier’s conflict in bifunctional heterogeneous catalysts for the WGS reaction

•A new strategy for designing efficient heterogeneous catalysts is presented•The proposed WAP mechanism avoids the Sabatier conflict of rate-limiting step•The catalyst, Au/TiC(111), shows high efficiency and durability•A reactive substrate no longer key to a WGS catalyst The cardinal rule in catalysis Principle, believed be ubiquitous unavoidable design. This paper points out that it possible avoid inherent contradiction embedded principle find with much higher efficiencies. For illustration, we use water-gas-shift (WGS) reaction reach conclusions. A called water-assisted pathway (WAP) on two-component (metal + oxygen-covered substrate), its observed as-designed Au/TiC(111). In WAP, weakly rate-determining step, stark contrast studied highly substrates follow LH-associative or LH-redox process. bifunctional (CO H2O ? CO2 H2) presented. We conflicting tasks require ?OH ?O from dissociated water adsorb then desorb catalyst. 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Nemšák Grinter Mahapatra al.Water-promoted methane methanol CeO2-Cu2O catalyst.Science. 368: 513-517Crossref (62) WAPs never been studied. Our calculation many “substrates,” (0001) MnAXn+1 (also MAX compounds) (111) carbon-deficient MCx. compounds, M = Ti, Mo, Nb, etc.; X C, N; Si, Al, Ga, etc., lowest energy.24Zhang H.Z. S.Q. First-principles study Ti3AC2 (A Al) (001) surfaces.Acta Mater. 55: 4645-4655Crossref (75) Scholar,25Medvedeva N.I. Freeman Cleavage fracture Ti3SiC2 first-principles.Scr. 58: 671-674Crossref (31) MCx energy under conditions26Yao Zhang X. Zhou Gao W.Q. Ye Lin L. Wen Chen al.Atomic-layered clusters alpha-MoC 2017; 357: 389-393Crossref (353) Scholar,27Lin Jiang Yu Y.-W. al.Low-temperature production Pt/alpha-MoC catalysts.Nature. 544: 80-83Crossref (706) (Note S1; x 1.0 C/M number ratio). Schematic diagrams shown Figures S2 S3. terminated hexagonal array atoms, electron rich. According our calculations, bare Ti3SiC2(0001) ?-MoCx(111) surfaces, optimal coverage 75% 88%, respectively, remaining sites S4 S5). good agreement experimental observations.26Yao chose substrates. Theory atoms occupy fcc top layer, epitaxial islands three layers high. Figure S6 NAP-XPS (near ambient pressure XPS) data Au/TiCx(111)Oy(OH)z. spectra 120°C 4f core-level peak (upper panel) UHV, (middle exposure 1.5 mbar H2O, (lower gas, respectively. profiles reveal adsorbs leading side (arrow). As introduced NAP cell, reduces intensity. fact attributed scattering photoelectrons reducing collected. Factors affecting level attenuation include type pressure, temperature, kinetic travel distance photoelectrons, etc.28Nguyen Tao F.F. Tang Dou Bao X.J. Understanding during near X-ray photoelectron spectroscopy.Chem. 119: 6822-6905Crossref (66) calculated binding ?0.6 ?0.8 depending location cluster S7). small migration 0.33 cluster, indicating move favorable S8). MAX(0001) ?-MCx(111) site ?2.11 ?1.44 respectively means populates it. Calculations nomenclature, shall following convention: gold, express metal-oxidized-hydroxylated Au/Mn+1AXn(0001)Ox(OH)y Au/?-MCx(111)Oy(OH)z compounds. al.26Yao recently remarkably Au/?-MoCx(111)Oy(OH)z. member family, belonging MCx(111) group. following, explain how particularly assigned independently maximized without conflict. main text contains Au/TiCx(111)Oy(OH)z, LH-pathways. Au/TiCx(111)Oy(OH)z most stable family reaction. Earlier, Au/?-MoCx(111)Oy(OH)z conditions. Furthermore, those authors explained LH-pathways.26Yao LH 4.6 times bigger cannot measured Differences models work given Note process, one participate, relation single-water participation event, anchors gathers clusters. calculations showed had ?2 stronger far S9). Similar strengthening H-bonding Saavedra al.22Saavedra oxidation. depicted 2A, “linked” supplies ?COOH, while simultaneously depositing H Assisted additional molecules, migrates until some location, come together mix ?OH, deposition shifts equilibrium. several meV per equilibrium tenable temperature. When sufficient concentration substrate, (?H 1/2H2(g)) brings system back Gibbs free change limiting S10). Separately, encounters little resistance leave exothermic, (Figures S11 S12). before, along finding another participate 2B, case involving molecule, lower receives deposits Through chain further reduced S12, red curves versus blue curves). Au/?-MoCx(111)Oy(OH)z, two-water (rate limiting) 0.27 1.25 1.29 (Table S13). excellent apparent activation ?0.23 eV.26Yao reduction single multiple explains observation increased catalyst.26Yao proximity favors molecule. clusters, farther capped four high.26Yao CO, involves continued decrease, approaching values 0.05 0.07 S14). Although long chains barriers, probability decreases, hence contribution decreases. 3, run 423 measure percent (2% CO/6% Ar; total space velocity: 12,000 mL/[gcat?h]) samples. are: comprising mostly Au/TiCx(111)OyOHz 2 wt % loading; LH-catalysts Au/TiO2 Au/Mo2C TiCx(111)OyOHz loading. make TiC nano-crystals bounded majority facets controlling Ti:C ratio: heat mixture 50Al-43.5Ti-6.5C (wt %) 1,400°C 15 s quench oil (Methods: experiment). Scanning microscopy (SEM) image 3A) crystals near-octahedral shapes.29Jin Shen Zhan Growth self-propagating high-temperature synthesis Al? Ti? C system.Cryst. Des. 1590-1597Crossref (35) Upon loading, high-resolution transmission (HRTEM) 3B) grow epitaxially TiCx size distribution captured low-magnification scanning (STEM) 3C) energy-dispersive spectroscopy (EDX) mapping 3D). superiority sample 3E). continuous 24 h experiment, reaches steady 93%. By comparison, benchmark samples Au/TiO2, Au/Mo2C, conversions 15%. mass-specific 4.3 × 10?5 molCO/(gcat·s), among highest activities date.26Yao Scholar,30Dong Fu Mei Carbide-supported reactions: territory metal–support effect.J. 13808-13816Crossref (117) More important excels durability. After reaction, annealed air 48 and, any reactivation ran time h. remained above ?89%. time, reactivation, third ?87% did experiment subjecting condition continuously 600 found 80% initial value. Au/?-MoCx(111)Oy(OH)z,24Zhang decreased nearly 50% just 150 473 (compared carbides prone rely preparing percentage non-(111) facets. Roiaz al.,31Roiaz Monachino Dri Greiner Knop-Gericke Schlögl Comelli Vesselli Reverse methanation Ni(110)? Stable near-ambient pressure.J. 2016; 138: 4146-4154Crossref (86) stability before issue. taken HRTEM TiC(111) S15) evidence gone structural composition change. al.32Williams Kispersky V. Metallic corner rutile dominant water? 132: 14018-14020Crossref (152) al.12Shekhar increases sensitively average Thus, need assess ensure effect responsible S16, indicate TiO2 < Mo2C TiCx(111)Oy(OH)z. TiCx(111)Oy(OH)z largest growth due commensurate lattice constants Au(111). Since smaller activity, definitive proof caused varying sizes al.14Rodriguez E