Influence of Water Molecule Adsorption and H2 Reduction Treatment on Acid Sites of WO3 with the (002) Crystal Facet Studied by Solid-State NMR Spectroscopy

Abstract

In-depth investigations of the influence of water molecule adsorption and H2 reduction treatment on surface acid sites of metal oxides are vital to elucidate the mechanism of catalytic reaction and to optimize the design of catalysts. By introduction of different amounts of water and reduction treatment by H2 at different temperatures, solid-state 31P magic angle spinning (MAS) NMR spectroscopy with trimethylphosphine as the probe molecule and 1H MAS NMR spectroscopy have been employed to study the surface acidity of WO3 with the (002) crystal facet. The results show that after adsorption of different amounts of water molecules, new Brønsted acid sites appear on the surface of WO3 due to the formation of surface bridging hydroxyl groups. In addition, after H2 reduction treatment, new Lewis acid sites appear due to the dehydroxylation of the terminal hydroxyl groups on the surface. Based on the NMR results, the models of the Brønsted and Lewis acid sites on the surface of WO3 are established, which provide insight into understanding surface acidity and the nature of its evolution of metal oxides.