Ethane and n-Butane Oxidation over Supported Vanadium Oxide Catalysts: An in Situ UV-Visible Diffuse Reflectance Spectroscopic Investigation

摘要

The coordination/oxidation states of surface vanadium oxide species on several oxide supports (Al_2O_3, ZrO_2, SiO_2) during ethane and n-butane oxidation were examined by in situ UV-vis diffuse reflectance spectroscopy (DRS). Only a small amount of the surface V(V) cations are reduced to V(IV)/V(III) cations under present steady-state reaction conditions. The extents of reduction of the surface V(V) species are a strong function of the specific oxide support, V_2O_5/ZrO_2 > V_2O_5/Al_2O_3 > V_2O_5/SiO_2, and also correlate with their reactivities (turnover frequencies) for ethane and n-butane oxidation reactions. For ZrO_2-supported samples, the polymerized surface vanadia species were found to be more easily reduced than the isolated surface vanadia species in reducing environments (i.e., ethane or n-butane in He), but no significant differences in the extents of reduction were observed under present steady-state reaction conditions (i.e., ethane/O_2He or n-butane/O_2/He). This observation is also consistent with the ethane oxidation catalytic study, which revealed that the polymerization degree, the domain size, of the surface vanadia species does not appear to significantly affect the reactivity of the supported vanadia catalysts for ethane oxidation.