Work-function-tuned electronic effect of a solute metal in the particles of copper alloys and the thin layer of surface oxides, and its influence on the catalysis on selective aerobic oxidation of benzylic alcohols

Abstract

In this work, four types of commercially available copper-based alloys, Cu-Ni-Zn, Cu-Zn, Cu-Pb-Sn and Cu-Sn were examined for their catalysis on selective aerobic oxidation of benzylic alcohols at 60 °C with N-methylimidazole (NMI) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as the co-catalysts in acetonitrile. It has been found that these solute metals exhibit various influences on the catalysis and such influences correlate to the work-functions of the solute metals and their surface oxides. The solute metals and their oxides of lower work functions (Sn and Zn) than Cu and Cu2O, respectively, can enhance the catalysis whereas Ni and NiO which possess quite larger work functions almost entirely jeopardize the catalysis. The electronic effect is conveyed through a three-ply structure, the bulky part of the alloy particles, Motty-Schottky junction and the surface oxides, of which the surface oxides affect much more profoundly the catalysis than the other two. The electronic effect is attributed to the band structure and internal electric field within the alloys caused by Fermi energy level alignment due to the difference in their work functions between Cu/Cu2O and M/MO (M = Sn, Zn, Pb and Ni). Moreover, the alloys exhibit much improved durability compared to either Cu or Cu2O, respectively.