Theoretical Study On Catalytic Hydrogenation,Reverse Water Gas Shift Reaction And Methanol Synthesis Of M₁/W₆S₈ Single-Atom Catalyst

The finite resources of fossil fuels along with environmental concerns havestimulated extensive and in-depth research on alternative energy sources.To decrease the dependence on crude oil and adhere to strict global environmental stipulations,the hydrogenation of CO is an extremely important reaction in quite a few contexts.The reverse water gas shift?RWGS?reaction could be used to produce CO,the first step in producing various fuels?methane,gasoline,diesel,and methanol?by CO₂ hydrogenation.It is generally accepted that CO is the key intermediate to formation of CH₃OH.Methanol is attractive both as a feedstock for making commodity chemicals and as fuel additives.Fischer-Tropsch?FTS?,which generates hydrocarbons through CO and H₂ becomes a mounting number of important route for the producing chemical feedstocks or motor fuels without the production of the environmentally harmful compounds.CO₂ hydrogenation to produce useful chemicals?such as CO,CH₄,CH₃OH,C₂H₄ and C₂H₆?plays a pivotal role in future energy conversion and storage.Therefore,this paper studied in detail the M₁/W₆S₈?M=Co,Ni,Fe,Ru,Os and Rh?single-atom catalysts for the catalytic hydrogenation of CO and CO₂,reverse water gas shift reaction and methanol synthesis.The main contents are:1.Comprehensive density functional theory?DFT?calculation of methanol synthesis and C₂ hydrocarbons formation in Fischer-Tropsch synthesis?FTS?on M₁/W₆S₈?M=Co and Ni?Single-Atom Catalysts have been carried out.The activation barriers and reaction energies for CO dissociation,CH_x hydrogenation,CH_x+CH_y coupling and CO/HCO insertion into CH_x,and CH_xCH_y-O bond scission involved in C₂ hydrocarbons formation have been examined.Methanol production?CO*+4H*?CHO*+3H*?CH₂O*+3H*?CH₃O*+H*?CH₃OH*?is highly favored by Ni₁/W₆S₈ catalysts.In contrast,Co₁/W₆S₈catalysts strongly favor C₂ hydrocarbons production.The rate constant of the step for the subsequent transformation of CH₃O*species on the Co₁/W₆S₈ and Ni₁/W₆S₈ was determined with the harmonic transition state theory?TST?.Simultaneously,we use the d-band center to prove the correctness of the previous steps,the d-band center value of Co₁/W₆S₈ is closer to E_F than that of Ni₁/W₆S₈;the result indicates that the catalytic activity of Co₁/W₆S₈ is the best.The present study provides the basis to understand and develop novel M₁/W₆S₈?M=Co and Ni?single-atom catalysts for C₂ hydrocarbons and methanol synthesis.2.We first performed density functional theory calculations to investigate the mechanism for the CO₂ hydrogenation to CO through the RWGS?CO2?g?+H2?g??CO?g?+H2O?g??over M₁/W₆S₈?M=Fe,Ru,and Os?single-atom catalysts.The results showed that formic acid mechanism?mechanism C?on the Fe₁/W₆S₈ single-atom catalyst is the most suitable pathway for RWGS with 30.1 kcal/mol rate-determining energy barrier.Additionally,on the basis of the energetic span model?ESM?and d-band center position,it is demonstrated that Fe₁/W₆S₈ is effective catalysts for the reaction.Afterward,we chose the most higher catalytic activity catalysts Fe₁/W₆S₈ for CO hydrogenation.CH₄*is formed via CO*?COH*?HCOH*?CH*?CH₂*?CH₃*?CH₄*,the effective barrier for CH₄*formation is 24.8 kcal/mol.CH₃OH*is formed via CO*?COH*?HCOH*?H₂COH*?CH₃OH*,the effective barrier for CH₃OH*formation is 26.0 kcal/mol.On Fe₁/W₆S₈,the CH*species is the most favorable monomeric CHx*species for production of C₂H₄ and C₂H₆,whose formation goes through a path of CO*?COH*?HCOH*?CH*.Once CH*is produced,it will be more selective to C₂H₂ via C-C coupling of CH*,rather than its hydrogenation to CH₄ due to the higher hydrogenation barrier of CH₂ species relative to the barriers for CH*+CH*coupling.Ultimately,C₂H₆ can be produced from further hydrogenation of C₂H₄ with moderate barriers.3.Our work clearly shows that in comparison with the W₆S₈ or Rh₁/Mo₆S₈,the Rh₁/W₆S₈ has higher catalytic activity and highest TOF value.DFT calculations demonstrate that Rh₁/W₆S₈ promote the CO₂ hydrogenation to CH₃OH via the reverse water-gas-shift?RWGS?reaction to produce CO followed by its hydrogenation to CH₃OH through the formation of methoxy?CH₃O*?as a reaction intermediate.