Monitoring the states of vanadium oxide during the transformation of TiO2 anatase-to-rutile under reactive environments: H2 reduction and oxidative dehydrogenation of ethane

The influence of vanadium oxide on the TiO2 (anatase) to rutile transformation for supported VOx/TiO2 catalysts under different environments (oxidizing, reducing and ethane oxidative dehydrogenation (ODH)) was investigated. The supported VOx/TiO2 catalysts were synthesized by incipient wetness impregnation and characterized with XPS surface analysis, Raman spectroscopy and temperature-programmed oxidation and reduction. The XPS surface analysis and Raman spectroscopy demonstrate that the supported vanadium oxide phase is initially present as surface VOx species on the titania (anatase) support below monolayer coverage (∼9 V/nm2) and that crystalline V2O5 nanoparticles are also present above monolayer coverage. The TiO2 anatase-to-rutile transformation is found to depend both on the gaseous environment and presence of vanadia. The anatase-to-rutile transformation is accelerated by the presence of vanadium oxide and the incorporation of V+4 cations into the TiO2 (rutile) lattice. The incorporation of the V+4 cations decreases the catalytic activity since the active sites are removed from the surface into the bulk, which is not accessible to the reactant gases. Although the titania anatase-to-rutile transformation readily occurs in oxidizing environments (O2 and ODH reaction), the solid-state transformation does not take place under reducing environments (H2) since large V3+ ions form prior to the transition temperature and cannot enter the titania lattice.