Molecular structure and reactivity of vanadia-based catalysts for propane oxidative dehydrogenation studied by in situ Raman spectroscopy and catalytic activity measurments

The effect of support (ZrO2 and TiO2) and vanadia loading (1.2-10 wt%) on the molecular structure and the catalytic performance in propane oxidative dehydrogenation (ODH) were investigated using in situ Raman spectroscopy and catalytic activity measurements. In situ Raman spectra under oxidizing, reducing, and steady-state ODH reaction atmospheres were obtained for the studied catalysts. The main differences between the structure of VOx entities in the two supporting materials derive from variations in the vanadia dispersion, which appears to be better in the case of the V2O5/TiO2 compared to that Of V2O5/ZrO2 at the same VOx density. In situ Raman spectra recorded in the presence of water vapor revealed that VOx species are stable at high temperature while at low temperature the surface of the catalysts becomes hydrated. Significant perturbations of all kinds of V-O bonds were observed under reduction with propane and reaction conditions at 500 degreesC. The reactivity studies revealed that under the same reaction conditions oxidative dehydrogenation rates expressed per V atom are influenced by the type of the specific support and are functions of the VOx density. The ability of vanadium to activate the C-H bond weakens with increasing VOx density, whereby the number of V-O-M (M = support metal atom) bonds per V is reduced due to incorporation of vanadium in formation of V-O-V bridges. The combination of the results obtained from the in situ Raman spectra and the catalytic activity measurements points to a significance of V-O-support bonds in the kinetically significant reaction steps. (C) 2003 Elsevier Inc.