Mechanisms Of Typical Oxidation And Hydrogenation Reactions On Co-N-C Single-atom Catalyst

Single-atom catalysts（SACs）with unique monodispersed metal sites and 100%metal utilization exhibit excellent activity and selectivity in various reactions.Me-N-C is a promising non-noble metal SAC with low cost and easy synthesis,which has been used in electrocatalytic reactions and organic synthesis reactions,including oxidation and hydrogenation.However,in electrochemistry,there are still some arguments on the interaction between SAC catalyst and CO.In thermocatalysis,the interaction between SAC catalyst and multiple substrates are still undefined.In this regard,it is very important to further expand the application of SAC catalysts and study the reaction mechanisms,which would lay a foundation for the rational design and application of SAC catalysts in the future.In this study,the Co-NC SAC catalyst was applied in typical oxidation and hydrogenation reactions,including the selective benzyl alcohol（BA）aerobic oxidation,the selective nitrobenzene hydrogenation and CO oxidation.We focus on the interaction between Co NC and substrates,and the reaction mechanisms involving multiple substrates activation.The research contents and main conclusions are as follows:（1）Co-NC SAC was applied in the selective aerobic oxidation of BA to benzaldehyde,exhibiting excellent activity and selectivity.A dual-site Langmuir–Hinshelwood mechanism was proposed to describe the multiple substrates activation and reaction mechanism,as revealed by kinetics experiments and theoretical calculations.Based on the mechanism,BA is activated byπ-πinteraction and O₂ adsorbs on Co atom.Strong competitive adsorption of BA to O₂was discovered.Moreover,we rationalized the poisoning effect of polar moieties on the SAC,based on which the reason of catalyst deactivation was elucidated and a regeneration method was developed.（2）Combining XAS and theoretical calculations,the active sites on Co-NC SAC were determined as Co N₄ structure.The adsorption,cleavage and spillover of hydrogen on Co-NC were studied.Homolytic cleavage and hydrogen spillover were experimentally observed,providing new insights for further hydrogenation mechanism study.The catalyst was applied in the selective nitrobenzene hydrogenation.By kinetics experiments and Hammett study,the rate-determining step and its electronic effect was identified.（3）The interaction between Co NC SAC and CO was studied by performing CO oxidation as a probing reaction.It was revealed that the CO oxidation on Co-NC SAC follows a stepwise mechanism.The reaction activity showed a complex“N”curve with increasing temperature in the range from 50 to 250 ^oC.It can be explained as the comprehensive effect between surface reaction kinetic and oxygen adsorption.Moreover,the bidentate carbonate accumulation was considered as the reason of catalyst deactivation.This research verified the prediction of DFT calculation and laid the foundation of CO oxidation on Me-N-C SACs.