On the surface-dependent oxidation of Cu2O during CO oxidation: Cu2+ is more active than Cu+

Abstract

Solid materials can exhibit very different surface catalytic properties depending on the exposed surface. Different from the previous finding by Bao et al. (Angew. Chem. Int. Ed. 50 (2011) 12294–12298) that Cu2O octahedra are more active than Cu2O cubes in CO oxidation, we found that Cu2O cubes exposing {100} surface showed significantly higher activity than Cu2O octahedra exposing {111} surface in the reaction. The Cu2O(100) surface was found to be oxidized during CO oxidation, leading to the formation of a CuO surface layer. On the other hand, Cu2O(111) strongly resisted such surface oxidation under CO oxidation conditions. Density functional theory calculations demonstrate that CO oxidation follows a Mars-Van Krevelen mechanism on Cu2O(111) with a high activation barrier of 1.36 eV. In contrast, the overall activation barrier is only 0.58 eV for CuO(111). These differences explain well the higher activity of oxidized Cu2O cubes.