Theoretical Study The Performance Of The TM-Si@2D Catalyst For The Acetic Acid Synthesis By CH₄/CO₂ Via A Single Step Method

Recently,with the fast development of technology and industry,the emission of greenhouse gases increased remarkably,which results in rising global temperatures and abnormal climates.CH₄ and CO₂ are the major greenhouse gases,which account for the 70%contribution to the greenhouse effect in all greenhouse gases.Hence,converting CH₄ and CO₂into high valuable chemicals is an effective way to control climate change.Compared with the energy-intensive syngas route,it is an ideal conversion route that directly coupling CH₄ and CO₂ into C₂ or C₃oxygenated compounds.Synthesis of acetic acid by direct coupling CH₄and CO₂ is considered as a low energy cost process.This process could achieve 100%atomic economy due to all the reactant atoms end up in the desired product.However,due to the inertness property both of CH₄ and CO₂,the direct conversion of CH₄ and CO₂ to acetic acid is thermodynamically unfavorable.Therefore,studying and developing efficient catalysts are significance for achieving this reaction.In view of above statement,Single-Atom catalysts were designed by doping Ni and Si atoms into seven two dimensional（2D）materials using density functional theory（DFT）and ab initio molecular dynamics,respectively.The adsorption and dissociation of CH₄ and the coupling mechanism of the intermediates CH₃^*and CO₂ on the Ni-Si@2D surfaces were simulated by VASP software.Research suggests that the Si atom can improve the adsorption capacity of CH₄ and effectively reduce the activation energy of CH₄dissociation.The adsorption and dissociation of CH₄ mainly occur at the Ni active site forming the Ni-CH₃^*intermediate,and the detached H migrates to the Si atom forming Si-H*surface species.CO₂inserts between Ni-CH₃^*and Si-H^*by the S_E2 mechanism,acetate is formed by the C-C coupling process,which follows the Eley-Rideal mechanism.Surface species H^*of Si-H^*transferred to acetate formed acetic acid finally.Due to the lowest energy barrier of methane dissociation and C-C coupling,Ni-Si@h-BN is the best single atom catalyst among seven designed catalysts.It is believed that h-BN probably is the optimal 2D material for designing similar catalyst.Hence,another three transition metals（Mn,Fe,Co）were selected and designed TM-Si@h-BN catalysts.It is found that the Si atom still plays a positive role in the process of acetic acid synthesis.The route of synthesis acetic acid on the TM-Si@h-BN is consistent with the Ni-Si@2D,but the transition metal Ni is still the optimal active component.Through the analysis of Bader charge,it can be found that for the TM@h-BN catalyst,the transition metal is the electron donor.Nevertheless,for the TM-Si@2D catalyst,Si atom donate more electrons and the transition metal becomes electrons acceptor.This proves that Si atom could promote the reduction of transition metal to TM~0 state,which can maintain the activity of the metal.A series of different Dual Single-Atom 2D catalysts were designed and studied their catalytic property by synthesis acetic acid from the direct coupling CH₄and CO₂ with the density functional theory.Our research provides theoretical guidance and data support for the high efficient catalysts development for CH₄-CO₂ co-conversion to the high value-added chemicals.