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 uti lized to monitor the effect of adsorbed water, calcination temperature, and reduction in H-2 on the structure of dispersed VOx for vanadia supported o n SiO2, Al2O3. TiO2, ZrO2, and HfO2 prepared with VOx surface densities ran ging from 0.46 VOx/nm(2) to 11.1 VOx/nm(2). Supported vanadia was found to exist as monovanadate. polyvanadate, or V2O5 species. the distribution amon g these species depending on the support for a given VOx surface density. D ehydration resulted in the appearance of monovanadate species on all suppor ts, with the extent of these species decreasing in the order HfO2 > Al2O3 > ZrO2 > TiO2 > SiO2. Hydration of the samples caused a decrease in the mono vanadate species and a slight increase in polyvanadate species. Oxidation a t 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 (HfV(2)O7). The appearance of an interaction between vanadia and titan ia coincides with the transformation from anatase to rutile TiO2. ZrV2O7 an d 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 t etragonal to monoclinic. H-2 reduction produced limited changes in the stru cture of dispersed vanadia except on Al2O3, where V2O5 was transformed into polyvanadate species.