A DFT Study Of Heteroatoms-doped Au Single-atom Catalysts For Acetylene Hydrochlorination Reaction

Vinyl chloride monomer?VCM?is a significant chemical intermediate for the polymerization of polyvinyl chloride,which is one of the most widely used engineering plastics in various applications nowadays.In China,the primary approach to manufacturing VCM is by the acetylene hydrochlorination,and this reaction uses mercuric chloride catalysts.However,the toxic mercuric chloride is harmful to the environment and human health.Gold catalysts have been considered to be the optimum substitute of no-mercury for this reaction.Single-atom catalysts?SACs?can not only take full advantage of every metal atom in order to maximize their catalytic efficiency,but also enhance the selectivity,activity,and stability,for the supported single metal atoms as the active sites of a chemical reaction.In this work,density function theory?DFT?was used to study the acetylene hydrochlorination reaction.We studied the adsorption characteristics of HCl and C₂H₂on pure and heteroatomsbasedAuG-SACs and simulated the corresponding reaction mechanism,and the mechanism of the acetylene hydrochlorination on heteroatoms-doped graphene catalyst and Au-based/graphene supports catalyst.So as to provide theoretical support for the experiment,provide reference for the selection and design of the catalyst.The results show that C₂H₂ is preferentially adsorbed on the Au-embedded graphene single atom catalyst?AuG-SAC?compared to HCl;Au atom is the only active site of the reaction,and acetylene hydrochlorination requires two energy barriers on Au G-SAC.The energy barrier which from co-adsorption to the first transition state is 31.12kcal/mol,indicating that the dissociation of HCl is the rate control step of the reaction,and AuG-SAC has a good catalytic effect on the hydrochlorination of acetylene.The heteroatom B doping on AuG-SAC can effectively increase the adsorption capacity of HC1 on the catalyst.The adsorption capacity of C₂H₂ does not change much,and the adsorption capacity of the product C₂H₃Cl increases,which is not conducive to the removal of the product.However,the AuG-SACs still preferentially adsorbed C₂H₂ and then adsorbed HCl to form the co-adsorbed structure.The doping of B atom can reduce the activation energy of the reaction,and the corresponding activation energy of the best doping site is26.72kcal/mol.The doping of N atoms can improve the adsorption capacity of HCl on AuG-SACs.The adsorption capacity of C₂H₂ decreases slightly,but C₂H₂ is still preferentially adsorbed.The optimum activation energy for doped N sites is25.43kcal/mol.The co-doping of B and N leads to a further reduction in the activation energy of the reaction,and the synergistic effect is related to the number of C atoms separated by B and N doping.The closer the distance,the more favorable it is to reduce the activation energy of the reaction.The activation energy corresponds to best co-doping site is 24.58kcal/mol.According to the analysis of orbital frontier molecular theory?FMO?,the doping of heteroatoms changes the electron cloud density distribution on the frontier orbit,thereby changing the single adsorption capacity of the adsorbate.