Strong Metal–Support Interactions between Copper and Iron Oxide during the High‐Temperature Water‐Gas Shift Reaction

The commercial high‐temperature water‐gas shift (HT‐WGS) catalyst consists of CuO‐Cr₂O₃‐Fe₂O₃, where Cu functions as a chemical promoter to increase the catalytic activity, but its promotion mechanism is poorly understood. In this work, a series of iron‐based model catalysts were investigated with in situ or pseudo in situ characterization, steady‐state WGS reaction, and density function theory (DFT) calculations. For the first time, a strong metal‐support interaction (SMSI) between Cu and FeO_x was directly observed. During the WGS reaction, a thin FeO_x overlayer migrates onto the metallic Cu particles, creating a hybrid surface structure with Cu‐FeO_x interfaces. The synergistic interaction between Cu and FeO_x not only stabilizes the Cu clusters, but also provides new catalytic active sites that facilitate CO adsorption, H₂O dissociation, and WGS reaction. These new fundamental insights can potentially guide the rational design of improved iron‐based HT‐WGS catalysts.     

CO hydrogenation on stepped Cu and CuZn alloy surfaces: Competition between methanol synthesis and methanation pathways

Comprehensive fundamental understanding of CO hydrogenation reactions over Cu and ZnCu alloy surfaces is of great importance. Herein, we report a comparative DFT calculation study of elementary surface reaction network of CO hydrogenation reactions on stepped Cu(211), Cu(611), ZnCu(211) and ZnCu(611) surfaces. On ZnCu(211) and ZnCu(611) surfaces, the energetic favorable reaction path of CO hydrogenation reaction follows CO* → HCO* → H₂CO* → H₃CO* → CH₃OH* → CH₃OH with H₃CO* hydrogenation as the rate-limiting step and proceeds more facilely on ZnCu(611) surface than on ZnCu(211) surface. On Cu(211) and Cu(611) surfaces, the energetic favorable reaction path of CO hydrogenation reaction follows CO* → HCO* → HCOH* → H₂COH* → H₃COH* → CH₃* → CH₄* → CH₄ with H₂COH* hydrogenation as the rate-limiting step and proceeds more facilely on Cu(611) than on Cu(211). The key difference of CO hydrogenation reaction on ZnCu alloy surface and Cu is that the resulting CH₃OH* species desorbs to produce CH₃OH on ZnCu alloy but undergoes H*-assisted decomposition to CH₃* and eventually to CH₄ on Cu surface. These results successfully unveil elementary surface reaction networks and structure sensitivity of Cu and ZnCu alloy-catalyzed CO hydrogenation reactions.     

Simulating periodic trends in the structure and catalytic activity of coinage metal nanoribbons

We present a systematic density functional theory (DFT) study of the structure and catalytic activity of group 10 (Ni, Pd, Pt) and group 11 (Cu, Ag, Au) coinage metal nanoribbons. These infinite, periodic, quasi‐one‐dimensional structures are conceptually important as intermediates between small metal clusters and close‐packed metal surfaces, and have been shown experimentally to be practical catalysts. We find that nanoribbons have significantly higher predicted H₂ dissociation activity than close‐packed metal surfaces consistent with their lower coordination numbers. Computed periodic trends are reasonable, with late transition states and low barriers for H₂ dissociation over late group 10 nanoribbons, suggesting their promise as practical catalysts. These trends are consistent with the isolated nanoribbons' computed molecular electrostatic potentials. Calculations also predict nearly linear Brønsted–Evans–Polanyi relationships between the nanoribbons' H₂ dissociation energies and dissociation barriers. We also test new meta‐generalized gradient approximation (GGA) and hybrid DFT approximations for H₂ dissociation over these nanoribbons. These new functionals increase the (generally underestimated) dissociation barriers predicted by standard GGAs, motivating their continued application in surface chemistry. © 2015 Wiley Periodicals, Inc.     

Photolysis of,and the reactions of H-atoms with,t-butanethiol

A detailed investigation of the photolysis of t-C₄HgSH has been carried out in the absence and presence of the inert gas, C₂H₆. A mechanssm consisting of three primaRy photochemical steps: t-C₄H₉SH → t-C₄H₉S + H (1), t-C₄H₉SH → t-C₄H₉ + SH (2), t-C₄H₉SH → i-C₄H₈ + H₂S (3), six hot and seven thermal reaction steps, adequately explains all the experimental observations. As in the case of hot H* atoms, both the H-atom abstraction H + t-C₄H₉SH → H₂ + t-C₄H₉S (7), and the SH-displacement reactions, H + t-C₄H₉SH → H₂S + i-C₄H₈ (8) occur with thermalized H-atoms. The Arrhenius expression of the rate constant ratio, k₇/k₈ for the latter reactions has been determined over the temperature range 25-14° C to be: ln(k₇/k₈) = (0.3 ± 0.1) + (420 ± 80)/RT.     

Effect of Halide Anions on the Electroreduction of CO2 to C2H4: A Density Functional Theory Study**

The halide anions present in the electrolyte improve the Faradaic efficiencies (FEs) of the multi-hydrocarbon (C₂₊) products for the electrochemical reduction of CO₂ over copper (Cu) catalysts. However, the mechanism behind the increased yield of C₂₊ products with the addition of halide anions remains indistinct. In this study, we analysed the mechanism by investigating the electronic structures and computing the relative free energies of intermediates formed from CO₂ to C₂H₄ on the Cu (100) facet based on density functional theory (DFT) calculations. The results show that formyl *CHO from the hydrogenation reaction of the adsorbed *CO acts as the key intermediate, and the C−C coupling reaction occurs preferentially between *CHO and *CO with the formation of a *CHO-CO intermediate. We then propose a free-energy pathway of C₂H₄ formation. We find that the presence of halide anions significantly decreases the free energy of the *CHOCH intermediate, and enhances desorption of C₂H₄ in the order of I⁻>Cl⁻>Br⁻>F⁻. Lastly, the obtained results are rationalized through Bader charge analysis.     

DFT study into the reaction mechanism of CO methanation over pure MoS2

Mo‐based catalysts are commonly used in the direct methanation of CO; however, no integrated mechanism has been proposed due to limits in characterizing the nano‐sized active structures of MoS₂. Thus, we report our investigation into the mechanism of CO methanation over pure MoS₂ through density functional theory simulations, considering that only MoS₂ edge sites exhibit catalytic activity. Simulations revealed the presence of (010) and (110) surfaces on the MoS₂ edges. Both surfaces are reconstructed by the redistribution of surface sulfur atoms upon exposure to H₂/H₂S, and after sulfur coverage redistribution, S vacancies are generated for CO hydrogenation. The reaction mechanisms on both surfaces are discussed, with the S‐edge being better suited to CO methanation than Mo‐edge on the (010) surface. The rate‐controlling step differs between surfaces, and corresponds to the initial activation reaction, which was achieved more easily on the (110) surface.     

H2S‐Mediated Thermal and Photochemical Methane Activation

Sustainable, low‐temperature methods for natural gas activation are critical in addressing current and foreseeable energy and hydrocarbon feedstock needs. Large portions of natural gas resources are still too expensive to process due to their high content of hydrogen sulfide gas (H₂S) mixed with methane, deemed altogether as sub‐quality or “sour” gas. We propose a unique method of activation to form a mixture of sulfur‐containing hydrocarbon intermediates, CH₃SH and CH₃SCH₃, and an energy carrier such as H₂. For this purpose, we investigated the H₂S‐mediated methane activation to form a reactive CH₃SH species by means of direct photolysis of sub‐quality natural gas. Photoexcitation of hydrogen sulfide in the CH₄+H₂S complex resulted in a barrierless relaxation by a conical intersection to form a ground‐state CH₃SH+H₂ complex. The resulting CH₃SH could further be coupled over acidic catalysts to form higher hydrocarbons, and the resulting H₂ used as a fuel. This process is very different from conventional thermal or radical‐based processes and can be driven photolytically at low temperatures, with enhanced control over the conditions currently used in industrial oxidative natural gas activation. Finally, the proposed process is CO₂ neutral, as opposed to the current industrial steam methane reforming (SMR).     

From Silylone to an Isolable Monomeric Silicon Disulfide Complex

The synthesis and characterization of the first bis‐N‐heterocyclic carbene stabilized monomeric silicon disulfide (bis‐NHC)SiS₂ 2 (bis‐NHC=H₂C[{NC(H)C(H)N(Dipp)}C:]₂, Dipp=2,6‐iPr₂C₆H₃) is reported. Compound 2 is prepared in 89 % yield from the reaction of the zero‐valent silicon complex (′silylone′) 1 [(bis‐NHC)Si] with elemental sulfur. Compound 2 can react with GaCl₃ in acetonitrile to give the corresponding (bis‐NHC)Si(S)S→GaCl₃ Lewis acid–base adduct 3 in 91 % yield. Compound 3 is also accessible through the reaction of the unprecedented silylone‐GaCl₃ adduct [(bis‐NHC)Si→GaCl₃] 4 with elemental sulfur. Compounds 2 , 3 , and 4 could be isolated and characterized by elemental analyses, HR‐MS, IR, ¹³C‐ and ²⁹Si‐NMR spectroscopy. The structures of 3 and 4 could be determined by single‐crystal X‐ray diffraction analyses. DFT‐derived bonding analyses of 2 and 3 exhibited highly polar Si S bonds with moderate p_π–p_π bonding character.     

MoP/Al2O3 as a novel catalyst for sulfur‐resistant methanation

We reported γ‐alumina supported molybdenum phosphide (MoP) catalysts as a novel catalyst for sulfur‐resistant methanation reaction. The precursors of the catalyst were prepared by impregnation method and the effect of reduction temperatures (550 °C, 600 °C, 650 °C) of the precursors for sulfur‐resistant methanation was examined. The results indicated catalyst obtained by lower reduction temperature delivered better sulfur‐resistant methanation performance. Meanwhile, the influence of H₂/CO ratios and H₂S content was also investigated. The results indicated that high H₂/CO ratio and low H₂S content was favorable for methanation of MoP catalysts. The catalysts were characterized by N₂ adsorption–desorption, XRD, XPS and TEM. The results confirmed that the MoP phase was formed on all the catalysts and the physicochemical properties of the samples influenced the performance for sulfur‐resistant methanation.     

Reactive adsorption of hydrogen sulfide by promoted sorbents Cu‐ZnO/SiO2: active sites by experiment and simulation

In the Cu_x‐Zn_(1‐x)O/SiO₂ sorbents for ultradeep adsorptive removal of H₂S from gaseous fuel reformates for fuel cells at room temperature, Cu promoter sites significantly increase sulfur uptake capacity of the sorbents. We report characterization of the family of Cu_x‐Zn_(1‐x)O/SiO₂ sorbents for reactive adsorption of H₂S using X‐ray diffraction (XRD), Brunauer‐Emmett‐Teller (BET) surface area analysis, electron spin resonance (ESR), ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy (DRS) and calculations by the density functional theory (DFT). Both the supported ZnO phase and Cu promoter sites in the Cu_x‐Zn_(1‐x)O/SiO₂ sorbents are nano‐dispersed, as shown by XRD. The Cu_x‐Zn_(1‐x)O/SiO₂ sorbents contain Cu promoter as the Cu²⁺ site of octahedral geometry, as found by the complementary ESR and UV–vis DRS. Mechanism of the promoter effect of the Cu²⁺ site in the Cu_x‐Zn_(1‐x)O/SiO₂ sorbents in reaction with H₂S is proposed based on DFT calculations. Copyright © 2012 John Wiley & Sons, Ltd.     

Dual Metal Active Sites in an Ir1/FeOx Single‐Atom Catalyst: A Redox Mechanism for the Water‐Gas Shift Reaction

Herein, we report a theoretical and experimental study of the water‐gas shift (WGS) reaction on Ir₁/FeO_x single‐atom catalysts. Water dissociates to OH* on the Ir₁ single atom and H* on the first‐neighbour O atom bonded with a Fe site. The adsorbed CO on Ir₁ reacts with another adjacent O atom to produce CO₂, yielding an oxygen vacancy (O_vac). Then, the formation of H₂ becomes feasible due to migration of H from adsorbed OH* toward Ir₁ and its subsequent reaction with another H*. The interaction of Ir₁ and the second‐neighbouring Fe species demonstrates a new WGS pathway featured by electron transfer at the active site from Fe³⁺?O???Ir²⁺?O_vac to Fe²⁺?O_vac???Ir³⁺?O with the involvement of O_vac. The redox mechanism for WGS reaction through a dual metal active site (DMAS) is different from the conventional associative mechanism with the formation of formate or carboxyl intermediates. The proposed new reaction mechanism is corroborated by the experimental results with Ir₁/FeO_x for sequential production of CO₂ and H₂.     

Density Functional Study of Catalytic Activity of Cu<Subscript>12</Subscript>TM for Water Gas Shift Reaction

Based on density functional theory calculations, we have systematically studied the WGS reaction on various nanosized Cu₁₂TM of Co, Ni, Cu (from the 3d row), Rh, Pd, Ag (from the 4d row), Ir, Pt, Au (from the 5d row). The reaction mechanism proposed by Langmuir–Hinshelwood has been followed, which corresponds to \({\text{CO* + OH* }} \to {\text{COOH*}} \to {\text{CO}}_{2} {\text{ + H*}}\). The comparison of the Gibbs free energy profiles of carboxyl mechanism on different Cu₁₂TM systems concludes that WGS reaction is determined by the steps of H₂ forming and OH* reacting with CO* to form COOH*. BEP relationship between activation barriers (Ea) and reaction energies (ΔH) on a series of Cu₁₂TM clusters is very good. What’s more, the activation barrier of rate-determining step of Cu₁₂Au is the smallest. TOF, with the aid of An Energetic Span Model (ESM), is used to estimate the efficiency of the different Cu₁₂TM clusters. The results show that the values of TOFs in doping Cu₁₂Rh, Cu₁₂Ir and Cu₁₂Pt are smaller than that in pure Cu. Moreover, the values of TOFs in doping Cu₁₂Co, Cu₁₂Ni, Cu₁₂Pd, Cu₁₂Ag, and Cu₁₂Au are higher than that in Cu₁₃. The higher value of TOF, the more favorable catalysts they are. This results shoud be helpful in developing efficient catalysts for WGS reaction. Finally, d-band center is used to explain the binding energy of CO and H₂O. It shows that there is a good liner relationship between d-band center and binding energy of CO but not for H₂O.     

Transition Metal Nitrides as Promising Catalyst Supports for Tuning CO/H2 Syngas Production from Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO₂RR) to produce synthesis gas (syngas) with tunable CO/H₂ ratios has been studied by supporting Pd catalysts on transition metal nitride (TMN) substrates. Combining experimental measurements and density functional theory (DFT) calculations, Pd‐modified niobium nitride (Pd/NbN) is found to generate much higher CO and H₂ partial current densities and greater CO Faradaic efficiency than Pd‐modified vanadium nitride (Pd/VN) and commercial Pd/C catalysts. In‐situ X‐ray diffraction identifies the formation of PdH in Pd/NbN and Pd/C under CO₂RR conditions, whereas the Pd in Pd/VN is not fully transformed into the active PdH phase. DFT calculations show that the stabilized *HOCO and weakened *CO intermediates on PdH/NbN are critical to achieving higher CO₂RR activity. This work suggests that NbN is a promising substrate to modify Pd, resulting in an enhanced electrochemical conversion of CO₂ to syngas with a potential reduction in precious metal loading.     

Sulfur Poisoning and Self-Recovery of Single-Site Rh1/Porous Organic Polymer Catalysts for Olefin Hydroformylation

Sulfur poisoning and regeneration are global challenges for metal catalysts even at the ppm level. The sulfur poisoning of single-metal-site catalysts and their regeneration is worthy of further study. Herein, sulfur poisoning and self-recovery are first presented on an industrialized single-Rh-site catalyst (Rh₁/POPs). A decreased turnover frequency of Rh₁/POPs from 4317 h⁻¹ to 318 h⁻¹ was observed in a 1000 ppm H₂S co-feed for ethylene hydroformylation, but it self-recovered to 4527 h⁻¹ after withdrawal of H₂S, whereas the rhodium nanoparticles demonstrated poor activity and self-recovery ability. H₂S reduced the charge density of the single Rh atom and lowered its Gibbs free energy with the formation of inactive (SH)Rh(CO)(PPh₃-frame)₂, which could be regenerated to active HRh(CO)(PPh₃-frame)₂ after withdrawing H₂S. The mechanism and the sulfur-related structure–activity relationship were highlighted. This work provides an understanding of heterogeneous ethylene hydroformylation and sulfur-poisoned regeneration in the science of single-atom catalysts.     

Pt/CexZr1-xO2 催化剂在含硫合成气中催化水煤气变换反应活性

 采用浸渍法制备了 Pt/Ce_xZr_1-xO₂ 催化剂, 通过 X 射线衍射和程序升温还原等方法对催化剂进行了表征, 并在固定床反应器中评价了催化剂在合成气和含硫合成气中的水煤气变换活性. 结果表明, 铈锆固溶体的氧化还原能力强于 CeO₂, Zr 的掺杂明显改善了 CeO₂ 的孔道结构, 其担载的 Pt 催化剂具有更高的金属分散度, 因而活性更高. 两种催化剂在含硫合成气中的催化活性较无硫合成气中的均有所降低, 且 H₂S 浓度越大, 催化剂活性下降越多, 但这种因吸附硫而引起的失活是可逆的, 即催化剂重新暴露在无 H₂S 重整气的还原性气氛下活性能基本恢复.     

A Reduced 2Fe2S Cluster Probe of Sulfur–Hydrogen versus Sulfur–Gold Interactions

The Ph₃PAu⁺ cation, renowned as an isolobal analogue of H⁺, was found to serve as a proton surrogate and form a stable Au₂Fe₂ complex, [(μ‐SAuPPh₃)₂{Fe(CO)₃}₂], analogous to the highly reactive dihydrosulfide [(μ‐SH)₂{Fe(CO)₃}₂]. Solid‐state X‐ray diffraction analysis found the two SAuPPh₃ and SH bridges in anti configurations. VT NMR studies, supported by DFT computations, confirmed substantial barriers of approximately 25 kcal mol⁻¹ to intramolecular interconversion between the three stereoisomers of [(μ‐SH)₂{Fe(CO)₃}₂]. In contrast, the largely dative S Au bond in μ‐SAuPPh₃ facilitates inversion at S and accounts for the facile equilibration of the SAuPPh₃ units, with an energy barrier half that of the SH analogue. The reactivity of the gold‐protected sulfur atoms of [(μ‐SAuPPh₃)₂{Fe(CO)₃}₂] was accessed by release of the gold ligand with a strong acid to generate the [(μ‐SH)₂{Fe(CO)₃}₂] precursor of the [FeFe]H₂ase‐active‐site biomimetic [(μ₂‐SCH₂(NR)CH₂S){Fe(CO)₃}₂].     

Facet‐Dependent Catalytic Activity of Cu2O Nanocrystals in the One‐Pot Synthesis of 1,2,3‐Triazoles by Multicomponent Click Reactions

We report the highly facet‐dependent catalytic activity of Cu₂O nanocubes, octahedra, and rhombic dodecahedra for the multicomponent direct synthesis of 1,2,3‐triazoles from the reaction of alkynes, organic halides, and NaN₃. The catalytic activities of clean surfactant‐removed Cu₂O nanocrystals with the same total surface area were compared. Rhombic dodecahedral Cu₂O nanocrystals bounded by {110} facets were much more catalytically active than Cu₂O octahedra exposing {111} facets, whereas Cu₂O nanocubes displayed the slowest catalytic activity. The superior catalytic activity of Cu₂O rhombic dodecahedra is attributed to the fully exposed surface Cu atoms on the {110} facet. A large series of 1,4‐disubstituted 1,2,3‐triazoles have been synthesized in excellent yields with high regioselectivity under green conditions by using these rhombic dodecahedral Cu₂O catalysts, including the synthesis of rufinamide, an antiepileptic drug, demonstrating the potential of these nanocrystals as promising heterogeneous catalysts for other important coupling reactions.     

H2S在立方ZrO2(110)面的吸附与解离

采用量子化学的密度泛甬理论方法,探讨了H2S、HS和S在立方ZrO2(110)面上不同吸附位的吸附情况.构型优化的结果表明:在bridge位H2S以垂直底物平面H原子向上、垂直底物平面H原子向下、平行底物平面和hollow位H2S平行底物平面模式吸附在ZrO2(110)面发生解离吸附.SH和S的最佳吸附位分别为桥位和顶位.Mulliken布局和态密度分析显示S原子的p轨道与Zr原子的d轨道发生相互作用.通过计算解离反应的能垒,表明H2S分子在立方ZrO2(110)面发生两步解离.     

Metalloradicals Supported by a meta‐Carborane Ligand

In this work, a pincer‐type complex [Cp*Ir‐(SNPh)(SNHPh)(C₂B₁₀H₉)] ( 2 ) was synthesized and its reactivity studied in detail. Interestingly, molecular hydrogen can induce the transformation between the metalloradical [Cp*Ir‐(SNPh)₂(C₂B₁₀H₉)] ( 5 ^.) and 2 . A mixed‐valence complex, [(Cp*Ir)₂‐(SNPh)₂(C₂B₁₀H₈)] ( 7 ^.+), was also synthesized by one‐electron oxidation. Studies show that 7 ^.+ is fully delocalized, possessing a four‐centered‐one‐electron (S‐Ir‐Ir‐S) bonding interaction. DFT calculations were also in good agreement with the experimental results.     

铜物种对Cu/Fe2O3水煤气变换反应催化剂性能的影响

采用共沉淀法制备了系列铜负载量不同的Cu/Fe₂O₃水煤气变换(WGS)催化剂,并考察了铜负载量对催化剂结构和水煤气变换反应性能的影响. 结果表明,Cu/Fe₂O₃催化剂呈现出良好的水煤气反应性能,当CuO质量分数为20%时,催化剂的WGS性能最优,250 ℃时CO转化率高达97.2%,同时热稳定性也最好. 运用X射线粉末衍射(XRD)、N₂物理吸脱附和H₂程序升温还原(H₂-TPR)等手段对Cu/Fe₂O₃催化剂的物相、织构特征及还原性能进行了表征,结果表明,CuFe₂O₄物种的存在极大地改善了催化剂的还原性能和WGS反应活性. 这是由于CuFe₂O₄特殊的尖晶石结构有利于Cu微晶的稳定;同时,CuFe₂O₄在低温下即被还原为单质铜,有利于促进催化剂体系中电子的转移. 此外,通过(NH₄)₂CO₃溶液处理,研究了独立相CuO对Cu/Fe₂O₃催化剂WGS反应性能的影响,结果发现,独立相CuO的存在,有利于H原子在各组分传递,从而促进催化剂的CuFe₂O₄的还原,改善Cu/Fe₂O₃催化剂的WGS反应性能.