The role of vanadium oxide on the titania transformation under thermal treatments and surface vanadium states

High surface area titania-supported materials prepared from V(IV) precursors and calcined at high temperatures have been characterized by Vis-UV diffuse reflectance, FT Raman, electron spin resonance, and X-ray photoelectron spectroscopies and tested in the partial oxidation of methane. Vanadium oxide loading and calcination temperature determine the structure of V2O5/TiO2 materials. Below theoretical surface monolayer coverage, V(IV) species closely interacting with the support are observed, Vanadiam oxide species anchor by reaction with titanium oxide surface hydroxyl groups. The V(IV) species are stabilized by interaction with titania support and further stabilization occurs at high calcination temperatures by their location in titania (rutile) lattice. Larger loadings of vanadium decrease the temperatures required for conversion of titania (anatase) to titania (rutile), At higher vanadium loading segregation into bulk V2O5 oxide takes place, thus decreasing interaction with titania support. This enables a larger population of V(V) species than samples with surface dispersed vanadium oxide species. Although partial oxidation of methane is nonselective on titania (anatase), partial oxidation products are observed on titania (rutile)-supported vanadium oxide catalysts, The higher selectivity to partial oxidation product formaldehyde appears to be related to the high stability of V(IV) cations located on rutile lattice and the absence of V(V) sites. (C) 1996 Academic Press, Inc.