Oxygen Involvement in the Partial Oxidation of Methane on Supported and Unsupported V2O5

Abstract

O exchange of O2 with V2O5/SiO2, V2O5/Al2O3, V2O5, SiO2, and Al2O3 was studied using steady-state isotopic transient kinetic analysis. It was found that bulk V2O5, V2O5/SiO2, V2O5/Al2O3, and Al2O3 exhibited some oxygen exchange capability, whereas SiO2 exhibited negligible oxygen exchange capability under the conditions used. Oxygen exchange reactions were also studied over supported vanadia catalysts under steady-state oxidation reaction conditions. O exchange of O2 with V2O5/SiO2 increased significantly in the presence of methane. This is attributed to a redox reaction making the surface more active for O exchange in the presence of the methane. The involvement of catalyst oxygen in the formation of the products HCHO, CO, and CO2 was demonstrated. However, an estimation of the contribution of the lattice oxygen in the formation of the products is complicated by the presence of secondary O exchange. The total amount of 16O exchanged with the feed O2 and the products indicates that the oxygen associated with silica or the vanadia-silica interface is also involved in the exchange process. In general, O exchange behavior of various product species with V2O5/Al2O3 was found to be similar to that with V2O5/SiO2. However, unlike V2O5/SiO2, the O exchange of O2 with V2O5/Al2O3 did not increase significantly in the presence of methane. This was attributed to the differences in the interactions of vanadia with silica and alumina. It is speculated that the O associated with highly dispersed tetrahedral surface vanadia is involved in a primary oxidation reaction, whereas O associated with the bulk-like vanadia and with the support is involved either in secondary oxidation reactions of HCHO or CO or in secondary O exchange of various O containing product species. Caution must be taken in making any conclusions about the source of reactive oxygen during partial oxidation on oxide catalysts based on isotopic oxygen studies due to the ease of secondary oxygen exchange.