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.