Understanding Surface Properties in CeO2 Catalysts for the Synthesis of Dimethyl Carbonate: A Combined In Situ IR and NEXAFS Study.

In this work, we studied the surface properties of two different CeO2 catalysts, one synthesized by a modified hydrothermal method and the other obtained by a nonconventional calcination of a metal-organic framework (MOF). The first one presented a high surface area (CeO2-HSA) with accessible Ce sites located primarily on (111) planes, while the MOF-derived material (CeO2-MOF) showed coordinatively unsaturated Ce sites (CUS) located on (110) planes. In situ IR and NEXAFS spectroscopies were employed to unravel the nature of the surface intermediates and the Ce oxidation state during the reaction. Both materials show Ce reduction during the adsorption of methanol as a consequence of methoxide-to-formate decomposition, while CeO2-HSA produces a high proportion of surface HCOO-Ce3+ as a consequence of its higher surface area. However, as we reported previously, this high proportion of surface Ce3+ sites causes catalyst deactivation. In this sense, CeO2-MOF presented a high concentration of CUS sites located on (110) planes, which are beneficial for the direct synthesis of DMC from CO2 and methanol.