Effects of Support Composition and Pretreatment Conditions on the Structure of Vanadia Dispersed on SiO₂, Al₂O₃, TiO₂, ZrO₂, and HfO₂

Spectroscopic techniques (X-ray absorption, Raman, and UV−visible) were utilized to monitor the effect of adsorbed water, calcination temperature, and reduction in H2 on the structure of dispersed VOx for vanadia supported on SiO2, Al2O3, TiO2, ZrO2, and HfO2 prepared with VOx surface densities ranging from 0.46 VOx/nm2 to 11.1 VOx/nm2. Supported vanadia was found to exist as monovanadate, polyvanadate, or V2O5 species, the distribution among these species depending on the support for a given VOx surface density. Dehydration resulted in the appearance of monovanadate species on all supports, with the extent of these species decreasing in the order HfO2 > Al2O3 > ZrO2 > TiO2 > SiO2. Hydration of the samples caused a decrease in the monovanadate species and a slight increase in polyvanadate species. Oxidation at elevated temperature resulted in an increase in the crystallinity of V2O5 present on SiO2, a conversion of V2O5 into polyvanadate species on Al2O3, and the appearance of mixed-metal oxide phases on TiO2, ZrO2 (ZrV2O7), and HfO2 (HfV2O7). The appearance of an interaction between vanadia and titania coincides with the transformation from anatase to rutile TiO2. ZrV2O7 and HfV2O7 are postulated to form via the interaction of surface VOx species with the support as the support begins to undergo a phase transition from tetragonal to monoclinic. H2 reduction produced limited changes in the structure of dispersed vanadia except on Al2O3, where V2O5 was transformed into polyvanadate species.