Structure and catalytic properties of supported vanadium oxides: Support effects on oxidative dehydrogenation reactions

The effects of support (Al2O3, SiO2, HfO2, TiO2, and ZrO2) on the structure and catalytic behavior of supported vanadia in the oxidative dehydrogenati on of propane were examined over a wide range of vanadium surface densities (0.5-15.0 VOx/nm(2)). X-ray diffraction and Raman and UV-visible spectra s howed that vanadia exists as highly dispersed species at surface densities below 7 VOx/nm(2) on Al2O3, HfO2, TiO2, and ZrO2, but as large V2O5 crystal lites on SiO2. Surface structures evolve from isolated monovanadates to pol yvanadate domains and V2O5 crystallites as VOx, surface density increases. Polyvanadates appear at lower surface densities on ZrO2 and TiO2 than on Al 2O3 and HfO2. UV-visible edge energies decrease as VOx domains grow with in creasing VOx surface density on all supports. Initial propene selectivities increase with increasing VOx surface density, as monovanadate species and exposed support sites, which favor primary combustion pathways, decrease in concentration. Oxidative dehydrogenation rates per V-atom reach a maximum on VOx domains of intermediate size, which provide a balance between the ac tivity of surface VOx species and their accessibility to reactants. Interac tions with supports determine the type of VOx structures present at a given surface density, but turnover rates do not depend on the identity of the s upport when differences in VOx structure are taken into account. Oxidative dehydrogenation turnover rates are similar on polyvanadate species and on s urface VOx sites on bulk V2O5 The relative rates of oxidative dehydrogenati on to form propene and of secondary propene oxidation to COx do not depend on the identity of the support or on VOx surface density or structure. Thus , it appears that these two reactions require similar VOx surface sites and that these sites are present at similar surface densities on polyvanadate domains and small V2O5 clusters. (C) 1999 Academic Press.