Novel Pd-loaded urchin-like (NH4)xWO3/WO3 as an efficient visible-light-driven photocatalyst for partial conversion of benzyl alcohol

Abstract

Photocatalytic oxidation of organic compounds by solar light is a promising strategy for environmentally benign conversion processes. Herein, we first report an urchin-like Pd/(NH4)xWO3/WO3 with three-dimensional hierarchical microstructure prepared through a thiourea-assisted solvothermal method followed by calcination under the reductive atmosphere. The Pd/(NH4)xWO3/WO3 exhibits higher selectivity in comparison to Pd/WO3 nanorods in the partial conversion of aqueous benzyl alcohol into benzaldehyde (e.g., 80% selectivity for benzaldehyde production with ca. 84% conversion of benzyl alcohol) under visible light irradiation. The preferential generation of hydroxyl radicals from water on the (NH4)xWO3/WO3 surface in the initial reaction stage was responsible for the high selectivity for benzaldehyde production because hydroxyl radicals react with benzyl alcohol much more efficiently than benzaldehyde. DFT calculations demonstrate that the energy barrier of the reaction between benzyl alcohol and hydroxyl radicals largely decreased due to the photo-excited triplet states of (NH4)xWO3/WO3, while such a trend in the energy barrier was not observed for other photocatalysts. On the other hand, with extended irradiation time, a large amount of hydrogen peroxide, which was produced by the multi-electron reduction of oxygen molecules, accumulates on the Pd/(NH4)xWO3/WO3 due to the low activity of the Pd cocatalyst. The generated hydrogen peroxide preferentially eliminated the photoexcited holes when the concentrations of benzaldehyde increased, thus inhibiting the peroxidation of benzaldehyde by holes. The Pd/(NH4)xWO3/WO3 may provide a feasible strategy for the photocatalytic oxidation of various other organic compounds due to its unique reactivity shown in this study.