Effects of Support Composition and Pretreatment Conditions on the Structure of Vanadia Dispersed on SiO_2, AL_2O_3, TiO_2, ZrO_2, and HfO_2

摘要

Spectroscopic techniques (X-ray absorption, Raman, and UV-visible) were utilized to monitor the effect of adsorbed water, calcination temperature, and reduction in H_2 on the structure of dispersed VO_x for vanadia supported on SiO_2, Al_2O_3,TiO_2, ZrO_2, an HfO_2 prepared with VO_x surface densities ranging from 0.46 VO_xm~2 to 11.1 VO_xm~2, Supported vanadia was found to exist as monovanadated, polyvanadate, or V_2O_5 species, the distribution among these species depending on the support for a given VO_x surface density. Dehydration resulted in the appearance of monovanadate species on all supports, with the extent of these species decreasing in the order HfO-2>Al_2O_3>ZrO_2>TiO_2>SiO_2, 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 V_2O_5 present on SiO_2, a conversion of V_2O_5 into polyvanadate species on Al_2O_3, and the appearance of mixed-metal oxide phases on TiO_2, ZrO_2(ZrV_2O_7), and HfO_2(HfV_2O_7). The appearance of an interaction between vanadia and titania coincides with the transformation from anatase to rutile TiO_2. ZrV_2O_7 and HfV_2O_7 are postulated to form via the interaction of surface VO_x species with the support as the support begins to undergo a phase transition from tetragonal to monoclinic. H_2 reduction produced limited changes in the structure of dispersed vanadia except on Al_2O_3, where V_2O_5 was transformed into polyvanadate species.