Influence of the metal oxide substrate structure on vanadium oxide monomer formation

Vanadium oxide (VO_x) molecular species supported on high surface area oxide supports are active catalysts for oxidative transformations of organic molecules. Since the reactivity of VO_x species depends on their molecular structure, the understanding and control of factors that determine their structure would be useful in surface molecular catalyst design. Reactive adsorption of vanadyl triisopropoxide (VOTP) to form monomeric VO _x species on amorphous Al₂O₃ and SrTiO ₃ (001) surfaces has been studied by X-ray photoelectron spectroscopy (XPS). Quantitative comparison of C(1s) and V(2p_3/2) peak areas has been used to determine the number of isopropoxide ligands that are replaced by V-O surface bonds. On average, three V-O surface bonds are formed during adsorption on an amorphous Al₂O₃ surface, as expected in the formation of a tridentate, VO₄ structure, typically assigned to monomeric, surface VO_x species. On the SrTiO₃ (001) surface, the number of V-O surface bonds depends on the oxygen density prior to reaction with VOTP. For adsorption on the SrTiO₃ surface cleaned and oxygen-annealed in ultrahigh vacuum, the number of V-O surface bonds is ca. 2. When the SrTiO₃ surface has been Ar-ion sputtered prior to VOTP adsorption, the number of V-O bonds is ca. 1. This study demonstrates that the atomic structure of the support can strongly influence the molecular nature of surface VO_x species.