Defective Mesocrystal ZnO-Supported Gold Catalysts: Facilitating CO Oxidation via Vacancy Defects in ZnO

We present a strategy to prepare a highly active Au/ZnO catalyst for CO oxidation by introducing abundant Zn- and O-vacancy defects into a ZnO support of mesocrystal form. Two different ZnO supports were chosen for comparison; almost defect-free ZnO nanorods (NR-ZnO) and twin-brush-like ZnO mesocrystals (TB-ZnO) with rich Zn/O-vacancy defects gave Au/NR-ZnO and Au/TB-ZnO upon deposition of gold nanoparticles. The catalytic test of CO oxidation over Au/TB-ZnO catalyst showed an enhanced catalytic activity that was 153 times greater than the activity of Au/NR-ZnO. The dramatic enhancement in CO oxidation is attributed to a room-temperature Mars-van Krevelen (MvK) mechanism on the surface of the Au/TB-ZnO catalyst, which was promoted by extensive vacancy defects in TB-ZnO. To elucidate the increase in activity, the vacancy ratio (i.e., [VO^•]/[VZn^•]) of TB-ZnO was systematically modulated by adjusting calcination conditions. The defective ZnO support altered the tendency in the variation of size, valence state, and activity of gold correlated to an increased vacancy ratio. Combining experimental results and theoretical modeling, it is concluded that the higher vacancy ratio [VO^•]/[VZn^•] in support endows defective ZnO with accommodation of plenty of "Au-O-Au_Zn" linkages (Au_Zn denotes Au substitution at a Zn site) around gold nanoparticles. The O atom extraction from "Au-O-Au_Zn" linkages formed by gold doping in ZnO lattice is energetically more favorable than typical "Au-O-Zn" linkages at the perimeter of gold, facilitating CO oxidation via MvK mechanism. Systematic manipulation of defects density in the support provides a method in improving catalytic properties of supported gold catalysts.