Elucidating the Role of Surface Ce(4+) and Oxygen Vacancies of CeO(2) in the Direct Synthesis of Dimethyl Carbonate from CO(2) and Methanol

Cerium dioxide (CeO(2)) was pretreated with reduction and reoxidation under different conditions in order to elucidate the role of surface Ce(4+) and oxygen vacancies in the catalytic activity for direct synthesis of dimethyl carbonate (DMC) from CO(2) and methanol. The corresponding catalysts were comprehensively characterized using N(2) physisorption, XRD, TEM, XPS, TPD, and CO(2)-FTIR. The results indicated that reduction treatment promotes the conversion of Ce(4+) to Ce(3+) and improves the concentration of surface oxygen vacancies, while reoxidation treatment facilitates the conversion of Ce(3+) to Ce(4+) and decreases the concentration of surface oxygen vacancies. The catalytic activity was linear with the number of moderate acidic/basic sites. The surface Ce(4+) rather than oxygen vacancies, as Lewis acid sites, promoted the adsorption of CO(2) and the formation of active bidentate carbonates. The number of moderate basic sites and the catalytic activity were positively correlated with the surface concentration of Ce(4+) but negatively correlated with the surface concentration of oxygen vacancies. The surface Ce(4+) and lattice oxygen were active Lewis acid and base sites respectively for CeO(2) catalyst, while surface oxygen vacancy and lattice oxygen were active Lewis acid and base sites, respectively, for metal-doped CeO(2) catalysts. This may result from the different natures of oxygen vacancies in CeO(2) and metal-doped CeO(2) catalysts.