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

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.