Enhanced catalytic reduction of 4-nitrophenol over titania nanotube supported gold nanoparticles by weak ultraviolet light irradiation: Role of gold surface charge

Supported gold nanoparticles (AuNPs) have recently attracted significant interest in catalysis, with much emphasis on optimizing the catalytic activity of AuNPs. We herein reported the promotion effect of weak ultraviolet (UV) light on the activity of titania nanotube (TNT) supported AuNPs (Au/TNT) for catalytic reduction of 4-nitrophenol by sodium borohydride. The presence of low-intensity UV irradiation increased the turnover frequency of Au/TNT for the reaction by a factor of 2.56–4.97 (increase factor depending on the Au loading amount). The photonic efficiency of the reaction for Au/TNT was found to be far more higher than the literature values for titanium oxide-based catalysts, suggesting that the reaction was not primarily implemented via a photocatalytic pathway. We attributed the pronounced promotion effects to the negative surface charge on AuNPs that formed from the charge separation between AuNPs and TNT. The negative surface charge assisted the formation of active Au-H intermediates and the activation of 4-nitrophenol molecules, giving rise to markedly enhanced catalytic activity for 4-nitrophenol reduction under low-intensity UV irradiation. The findings imply that weak UV irradiation is a very efficient and cost-effective approach to tune the surface charge and hence catalytic activity of semiconductor supported AuNPs.