Boosting the catalytic activity of Cu2O via forming composite with both Cu and metal oxides MO2 (M = Mo and W) by tuning its band structure and Lewis acidity

Abstract

In this work, composite materials (Cu-Cu2O/MO2, M = Mo and W) and their thermally treated form were prepared and characterized by using a variety of spectroscopic techniques (XRD, SEM, HRTEM, XPS and UPS) and cyclic voltammetry. Their catalytic performance on aerobic oxidation of various alcohols under mild conditions was investigated with N-methylimidazole (NMI) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as the co-catalysts. Of the materials, the thermally treated MoO2-based material exhibited the best performance and durability. Compared to Cu-Cu2O, the involvement of MO2 significantly enhances the catalytic activity on the oxidation. Our results suggest that the presence of MO2 not only raises the energy level of VBM of Cu2O which is beneficial to O2 activation and generates internal electric field pointing to the catalytic center (Cu2O) which would accelerate the electron transfer from the substrate to the catalyst through forming heterojunction with both Cu and MO2, but also disperses the catalytic center. Further, the Lewis acidity of MO2 with appropriate strength can bring the substrate closer to the catalytic center via adsorption. But appropriate acidity is essential as easy on–off adsorption is desired in the catalysis. Thermal treatment generates oxidation state VI species of stronger acidity which offsets the effect of the raised VBM energy level.