The effect of oxygen vacancies in ZnO at an Au/ZnO interface on its catalytic selective oxidation of glycerol

In catalytic reactions, the nature of the support has a major effect on the formation of active sites, especially in the case of catalysts with strong metal–support interactions. Two types of ZnO with different concentrations of oxygen vacancies (ZnO-U, produced by a hydrothermal procedure using urea, and ZnO-C, produced by a precipitation method using sodium carbonate) have been prepared and employed as supports for Au catalysts. The results of O1s X-ray photoelectron spectroscopy and positron annihilation spectroscopy showed that ZnO-U has fewer oxygen vacancies than ZnO-C. After the materials were loaded with Au, the formation of an Au/ZnO interface was demonstrated by high-resolution transmission electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy. Electron paramagnetic resonance spectroscopy and Au4f X-ray photoelectron spectroscopy showed that the formation of the materials involved electron transfer from Au to the ZnO support, resulting in the formation of positively charged Au species. A close correlation between the formation of the interface and the level of oxygen vacancies in the ZnO support was observed: low oxygen-vacancy concentrations result in an increase in the work function of ZnO, which facilitates electron transfer and makes the formation of the Au/ZnO interface more thermodynamically favorable. When they are used as catalysts in glycerol oxidation, the TOF of Au/ZnO-U (1159 h−1) was 1.47 times higher than that of Au/ZnO-C (786 h−1). The higher activity of Au/ZnO-U can be attributed to the abundance of positively charged Au sites, which strengthen the surface coverage of OH* and then promote H abstraction from an O–H bond in glycerol.