Active species on γ-alumina-supported vanadia catalysts: Nature and reducibility

The oxidation states and reducibility of γ-alumina-supported vanadium oxide catalysts with V loadings between 1.7 and 15.7 wt% were studied by means of temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) for fresh as well as used catalysts. As additional experimental techniques, X-ray diffraction (XRD), diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), and temperature-programmed desorption (TPD) were applied. For V loadings up to 6.1 wt%, the surface was found to be covered only by vanadate, while at higher loadings the formation of V2O5 crystallites was observed. The XPS and TPR data showed that under moderate oxidizing conditions, only image of V in the vanadate catalysts was in the oxidation state +5, while in the catalysts containing V2O5 crystallites it was about 80%, with the remainder being present as V(IV) in both cases. In the ambient pressure TPR experiments, all catalysts were completely reduced by hydrogen to V(III), although the vanadate catalysts were found to be more easily reducible than those containing V2O5. The V dispersion changed during redox cycles, as they appeared under working conditions. These changes were different for the exposure to ethane or hydrogen, but did not influence the maximum oxidation states. In the catalytic oxidation of ethane used as a model reaction, all catalysts were found to be in their oxidized state under steady-state conditions. Significant catalyst reduction occurred only if the surrounding gas-phase oxygen was completely consumed, leading to the loss of catalytic activity due to the disappearance of V(V) species. A novel structure for γ-alumina-supported vanadia catalysts is proposed on the basis of the experimental data.