In Situ UV-vis-NIR Diffuse Reflectance and Raman Spectroscopic Studies of propane Oxidation over ZrO_2-Supported Vanadium Oxide Catalysts

摘要

The molecular structures and oxidation states of supported 1-5% V_2O_5/ZrO_2 catalysts during propane oxidative dehydrogenation (ODH), with varying propane/O_2 ratios, were examined by in situ UV-vis-NIR diffuse reflectance and in situ Raman spectroscopic studies. The results indicate that the reduction extent of surface V~(5+) cations to V~(3+)/V~94+) cations under steady-state reaction conditions increases with the propane/O_2 ratio. At the same propane/O_2 ratio, the relative extent of reduction of the supported V_2O_5/ZrO_2 catalysts generally increases with the surface vanadia loading, and the polymerized surface VO_4 species are more extensively reduced than the isolated surface VO_4 species during steady-state propane oxidation. The reactivity studies reveal that at the same reaction conditions, both polymerized and isolated surface V cations are active sites for propane oxidation and that the specific catalytic reactivity (as measured by turnover frequency; TOF) is independent of the surface density of the two-dimensional vanadia overlayer on the ZrO_2 support. Furthermore, the realtively constant TOF with surface vanadia coverage demonstrates that propane ODH to propylene requires only one surface VO_4 site. However, the propylene selectivity increases with increasing surface vanadia loading due to the removal of nonselective surfacce sies, possibly terminal ZrOH groups, on the ZrO_2 surface by the deposition of surface vanadia species. The propane/O_2 ratio greatly affects the selectivity of these catalysts. Highly oxygen-rich environments (e.g., propane/O_2 ratio = 1/10) give rise to the highest propylene selectivity, revealing that propylene production is favored on highly oxidized surfae vanadia (+5) sites. Small V_2O_5 crystallites above monolayer surface vanadia coverage do not contribute to propane ODH because of their low dispersion and low number of active surface sites (spectator vanadia species).