AuCu/ZnO heterogeneous photocatalysts: Photodeposited AuCu alloy effect on product selectivity in alkene epoxidation

Abstract

AuCu metal alloy nanoparticles were photodeposited on ZnO nanorods (ZnO_NRs) which proved to be effective photocatalysts for alkene epoxidation. The alloy nanoparticles were photodeposited onto ZnO nanorods with controlled ratios of gold and copper, with the deposition monitored in situ by UV-Vis spectroscopy. The alloy catalyst hybrids were tested for activity toward styrene epoxidation and HMF oxidative esterification, where the photoreactions were both optimized for time, temperature, and metal ratio content of the catalyst. The Au0.54Cu1/ZnO_NR alloy catalysts showed excellent photocatalytic activity and were most effective for conversion of styrene to styrene epoxide, where the product selectivity could be controlled by varying the metal ratio. Cu content in the alloy NP was essential to this process, as shown by the extrema in terms of metal content, using Au/ZnO only, where 100% benzaldehyde was obtained as the product. Au/ZnO evidenced best photocatalytic activity for HMF esterification, with conversion rapidly diminishing upon alloying of Au with Cu. A detailed XPS study was carried out to investigate reaction mechanism based on these studies, in particular, mechanisms are proposed for styrene epoxidation and oxidation cycles using the AuCu/ZnO_NR photocatalysts.