Reactivity of Epitaxial Vanadia on TiO₂: Are Support Interactions Required for Reactivity?

The reactivities of vanadium oxide epitaxial thin films were studied using temperature-programmed desorption (TPD). The vanadia films were grown on rutile TiO 2 (110) using oxygen-plasma-assisted molecular beam epitaxy and were shown to exhibit the (1 × 1) rutile structure using reflection high-energy electron diffraction, low-energy electron diffraction, and scanning tunneling microscopy. Meanwhile X-ray and ultraviolet photoelectron spectroscopy showed that monolayer films contained V 5+, while V 4+ predominated in the bulk of multilayer films. Two reaction channels, at 400 K and above 500 K, were detected for submonolayer vanadia coverages for 1-propanol oxidation to propionaldehyde. The reaction channel at 400 K persisted through multilayer films and thus was attributed to the dehydrogenation of intermediates attached to V surface cations; meanwhile, the sensitivity of 500 K reaction channel to vanadium oxidation states as well as a comparison of the branching ratio between these two reaction channels suggested that the reaction above 500 K involves intermediates around the periphery of vanadia islands where surface oxygens are bound to both V 5+ and Ti 4+ . It was also found that the activation energy of the lower temperature channel on the submonolayer film is unaffected by reduction and reoxidaton, while the activation energy starts to increase for the higher temperature channel upon reduction. The persistence of the same lower temperature reaction channel at 400 K throughout multilayer films where a direct chemical interaction with the titania support is absent, indicates that multilayer epitaxial films retain dehydrogenation reactivity, in contrast to prior studies where disordered vanadia films were reported to be unreactive. A comparison of the branching ratio for aldehyde desorption versus unreacted alcohol desorption for submonolayer and multilayer films revealed that a higher fraction of the alcohol dehydrogenates on submonolayer films. Together the results indicate that the TiO 2 support increases the vanadia reactivity by stabilizing reactive surface structures and by aiding the initial deprotonation of adsorbed alcohols.