Theoretical Investigation Into The Reaction Mechanism Of Propylene Epoxidation Over Cu₂O-based Single-Atom Catalyst

Single-atom catalyst（SAC）has received extensive attention due to its well-defined structure and good catalytic selectivity.Cuprous oxide is a nice catalyst for the epoxidation of propylene to propylene oxide and can be used as a substrate for SAC.Therefore,the study of cuprous oxide-based SACs plays an important role in the activity and selectivity of heterogeneous catalytic reactions such as propylene epoxidation.This dissertation is mainly devoted to study the stability of cuprous oxide-based SACs and the mechanism of propylene epoxidation on SAC Rh-Cu₂O.In chapter one,the development and research progress of SACs have been briefly introduced.Then,the status of propylene epoxidation to prepare propylene oxide has been summarized;these research provide a basic rule for designing new Cu₂O-based SAC of propylene epoxidation.Finally,the objective of this dissertation is determined:Stability of Cu₂O-based SAC and the mechanism of propylene epoxidation on SAC Rh-Cu₂O.In chapter two,the theoretical methods based on DFT have been reviewed and various software based on DFT have been briefly summarized.In chapter three,the stability of SACs formed by TMs replacing copper atoms on the surface of Cu₂O and single-atom clusters（SCCs）formed by replacing oxygen atoms has been studied by DFT+U+vdw.Computational results show that among the 174 novel catalysts studied,there are 60 novel SACs and SCCs stabilized catalysts.Among them,SACs are formed by TMs replacing the second layer of coordination saturated copper atoms on the surface of Cu₂O（111）,while SCCs are formed by replacing the second layer of oxygen atoms on the surface of Cu₂O（110）.The study shows that the binding and formation energies of SACs and SCCs are bimodal,which is caused by the occupation of the d orbitals of transition metal single atoms.The TMs on the three low-index surfaces of Cu₂O are more favorable for the formation of SACs rather than SCCs.Doping 3d TMs on Cu₂O surfaces has higher stability than 4d and 5d TMs,and the variation of binding energy and bond length of SACs and SCCs formed by doping 4d and 5d TMs on Cu₂O surface can be explained by lanthanide shrinkage.The results show that the charge transfer decreases from left to right along the periodic table with increasing electronegativity,while the spin states of SACs and SCCs on the Cu₂O surface show a periodic trend along the d-sequence.The research results provide basic knowledge for the doping of TM atoms on the surface of Cu₂O and provide a theoretical basis for the design of novel catalysts for SACs and SCCs.In chapter four,the epoxidation mechanism of propylene on the SAC Rh-Cu₂O（111）has been researched.It was found that Rh in the catalyst had the ability to promote the activation of O-O bond,which was beneficial to the epoxidation of propylene.When propylene reacts with O₂^*（O^*）,both dehydrogenation（AHS）and epoxidation（OOMMP）processes are endothermic,indicating that monatomic Rh is not conducive to dehydrogenation and epoxidation of reactants.While the epoxidation（OOMMP）process of the reactants is exothermic when O₂^*（O^*）is bound to the monocoordinate surface copper(Cu_CUS)atom linked to the Rh atom indicating that the surface copper(Cu_CUS)atom promotes the epoxidation process of the reaction,It has been found that the origin of high activity is electronic effect of Rh atom leads to the high catalytic activity of the Cu_CUS site linked to Rh and provided a theoretical basis for the further study of high performance SAC catalysts.The research results confirmed the promoting effect of single atom SACs on the epoxidation of propylene,and provided a theoretical basis for the further study of high performance SAC catalysts.