An XPS study of titania-supported vanadyl phosphate catalysts for the oxidative dehydrogenation of ethane

Abstract

X-ray Photoelectron Spectroscopy (XPS) has been used to investigate titania-supported vanadyl phosphates (VOPO4) with respect to thermal treatments and catalytic runs in the oxidative dehydrogenation (ODH) of ethane. The surface characterization of monolayer (9.6wt.%) and sub-monolayer (6.5wt.%) VOPO4 samples has been performed after calcination in air and after catalytic tests at different temperature (T=450 and 550°C). Vanadyl phosphates are well dispersed on the TiO2 surface. The oxidation state, surface distribution and evolution of vanadium and titanium species have been analysed by the V 2p3/2 and Ti 2p3/2 peak-fitting procedures. The interaction with the support allows the formation of different vanadium species, V5+ phosphate, V5+ and V4+ oxide species, whose relative fraction depends both on the nature of the support and on the samples treatments. The amount of tetravalent vanadium species in the samples progressively increases as effect of the temperature. Surface modifications of the titania support are evidenced by XPS after impregnation, calcination and catalytic tests.