Mechanistic insights into spontaneous redispersion of ZnO onto TiO2 in water-containing environments

Abstract

Water has a profound effect on the surface structure and catalytic performance of numerous heterogeneous catalysts. Understanding the mechanism of structural evolution in water-containing reaction atmospheres is essential for the rational design of catalysts with enhanced catalytic efficiency and stability. In this work, we have observed spontaneous redispersion of physically mixed ZnO particles onto TiO₂ surfaces in water-containing environments at room temperature. Water vapor at a pressure greater than 3.2 kPa is a prerequisite for the efficient ZnO redispersion, in which a water adlayer with a thickness of about three monolayers forms on the TiO₂ surface. Raising the sample temperature to 50 °C or rendering the TiO₂ surface hydrophobic prevents the formation of the water adlayer and thereby inhibits the ZnO redispersion. Solid-state nuclear magnetic resonance spectroscopy and in situ spectroscopic analyses confirm that the surface water adlayer serves as a migration channel for ZnO species. Moreover, ZnO achieves more rapid and complete redispersion in a liquid water environment. This structural regulation strategy increases the number of exposed active sites in the ZnO–TiO₂ catalyst, leading to enhanced catalytic activity in propane dehydrogenation.