Structure and catalytic properties of supported vanadium oxides: Support effects on oxidative dehydrogenation reactions
- Lawrence Berkeley National Lab., CA (United States)
The effects of support (Al{sub 2}O{sub 3}, SiO{sub 2}, HfO{sub 2}, TiO{sub 2}, and ZrO{sub 2}) on the structure and catalytic behavior of supported vanadia in the oxidative dehydrogenation of propane were examined over a wide range of vanadium surface densities (0.5--15.0 VO{sub x}/nm{sup 2}). X-ray diffraction and Raman and UV-visible spectra showed that vanadia exists as highly dispersed species at surface densities below 7 VO{sub x}/nm{sup 2} on Al{sub 2}O{sub 3}, HfO{sub 2}, TiO{sub 2}, and ZrO{sub 2}, , but as large V{sub 2}O{sub 3} crystallites on SiO{sub 2}. Surface structures evolve from isolated monovanadates to polyvanadate domains and V{sub 2}O{sub 3} and HfO{sub 2} crystallites as VO{sub x} surface density increases. Polyvanadates appear at lower surface densities on ZrO{sub 2} and TiO{sub 2} than on Al{sub 2}O{sub 3} and HfO{sub 2}. UV-visible edge energies decrease as VO{sub x} domains grow with increasing VO{sub x} surface density on all supports. Initial propene selectivities increase with increasing VO{sub x} 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 VO{sub x} domains of intermediate size, which provide a balance between the activity of surface VO{sub x} species and their accessibility to reactants. Interactions with supports determine the type of VO{sub x} structures present at a given surface density, but turnover rates do not depend on the identity of the support when differences in VO{sub x} structure are taken into account. Oxidative dehydrogenation turnover rates are similar on polyvanadate species and on surface VO{sub x} sites on bulk V{sub 2}O{sub 5}. The relative rates of oxidative dehydrogenation to form propene and of secondary propene oxidation to CO{sub x} do not depend on the identify of the support or on VO{sub x} surface density or structure. Thus, it appears that these two reactions require similar VO{sub x} surface sites and that these sites are present at similar surface densities on polyvanadate domains and small V{sub 2}O{sub 5} clusters.
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 316345
- Journal Information:
- Journal of Catalysis, Vol. 181, Issue 2; Other Information: PBD: 25 Jan 1999
- Country of Publication:
- United States
- Language:
- English
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