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[en] Graphical abstract: The mechanism for dimethyl carbonate (DMC) formation by oxidation carbonylation of methanol on a single-atom Cu1/graphene catalyst and the rate-limiting steps (RDS) on Cu/MG and Cu/DG catalyst surfaces are depicted in this picture. The RDS for the DMC sythesis on these two catalysts surfaces are CH3O + CO → CH3OCO for the Cu/MG catalyst, CH3OCO + CH3O → DMC for the Cu/DG catalyst, with maximum energy barriers of 73.5 kJ mol−1 and 190.9 kJ mol−1, respectively. Compared with the energy barrier of these rate-determining steps, the active performance of the Cu/MG catalyst is superior to the Cu/DG catalyst because of the monovacancy in graphene sheet. Therefore, Cu/MG can be considered as the promising catalyst for DMC formation. - Highlights: • Mechanism for DMC synthesis was explored on a single-atom Cu1/graphene catalyst. • Carbon vacancy is beneficial to the stability of the Cu1/graphene catalyst. • CO prefers to insert to methoxide species rather than dimethoxide species. • The productivity for DMC can be significantly improved by Cu/MG catalyst. - Abstract: The mechanism for dimethyl carbonate (DMC) synthesis by oxidation carbonylation of methanol on a single-atom Cu1/graphene catalyst was investigated by density-functional theory calculations. Carbon vacancies in graphene can significantly enhance the interaction between Cu atoms and graphene supports, and provide an increased transfer of electrons from Cu atoms to the graphene sheet. Compared with Cu-doped divacancy graphene (Cu/DG), Cu-doped monovacancy graphene (Cu/MG) provides a stronger interaction between adsorbents and the catalyst surface. Among the reaction processes over Cu1/graphene catalysts, CO insertion into methoxide was more favorable than dimethoxide. The rate-limiting step on the Cu/DG surface is the carbomethoxide reaction with methoxide, which is exothermic by 164.6 kJ mol−1 and has an activation barrier of 190.9 kJ mol−1 energy. Compared with that on the Cu crystal surface, Cu4 and Cu3Rh clusters, and the Cu2O(111) surface, the rate-determining step for DMC formation on Cu/MG, which is CO insertion into methoxide, needs to overcome the lowest barrier of 73.5 kJ mol−1 and is exothermic by 44.6 kJ mol−1. Therefore, Cu/MG was beneficial to the formation of DMC as a single-atom catalyst.