Surface-Dependent Role of Oxygen Vacancies in Dimethyl Carbonate Synthesis from CO2 and Methanol over CeO2 Catalysts

The conversion of CO₂ into value-added commodity chemicals, such as dimethyl carbonate (DMC), represents an environmentally friendly approach to CO₂ utilization. This study exhaustively investigates the influence of oxygen vacancies (O_v) on CeO₂ catalysts and, in particular, the role of surface structure. By integrating density functional theory calculations with experimental synthesis, we analyze the complex reaction mechanisms involved in DMC synthesis over both oxidized (Sto-(111), Sto-(110), and Sto-(100)) and nonoxidized (O_v^sub-(111), O_v^sur-(110), and O_v^sur-(100)) CeO₂ catalysts. Our findings indicate that O_v on the (111) surface inhibits DMC formation, whereas O_v on the (110) and (100) surfaces promotes it. This differential behavior is primarily attributed to O_v's modulation of the microscopic coordination environment on distinct surfaces, which impacts the rate-limiting step of C-O bond formation: CO₂ + OCH₃ → CH₃OCOO (monodentate methyl carbonate, MMC) and CH₃OCO + OCH₃ → DMC. Additionally, analysis of the highly active Sto-(111) and O_v^sur-(110) catalysts shows that their unique surface coordination microenvironments mitigate steric hindrance and facilitate an optimal arrangement of Lewis acid sites in proximity to Lewis base sites, thereby enhancing the DMC activity. This work underscores the pivotal role of surface structure in determining the effects of O_v, paving the way for the rational design of CeO₂-based catalysts for the direct synthesis of DMC from CO₂ and methanol.