The role of lattice oxygen in the oxidative dehydrogenation of ethane on alumina-supported vanadium oxide

The oxidative dehydrogenation (ODH) of ethane on alumina-supported vanadia was investigated with the aim of understanding the effects of lattice oxygen and vanadium oxidation state on the catalyst ODH activity and ethene selectivity. Transient-response experiments were carried out with both a fully oxidized sample of 10 wt% VOx/Al2O3 (7 V nm−2) and a sample that had been partially reduced in H2. The experimental results were analyzed to determine the rate coefficients for ethane ODH, k1, and ethene combustion, k3. The rate of ODH was found to depend solely on the concentration of reactive oxygen in the catalyst, but not on the means by which this oxygen concentration was attained (i.e., by H2versus C2H6 reduction). On the other hand, the ethene selectivity observed at a given concentration of active oxygen was found to depend on the composition of the reducing agent, higher ethene selectivities being observed when H2, rather than C2H6, was used as the reducing agent. It is proposed that the higher ethene selectivity achieved by H2versus C2H6 reduction might be due to a lower ratio of V4+ to V3+ cations attained upon reduction in H2 for a given extent of V5+ reduction. This interpretation is based on the hypothesis that ethene combustion is initiated by C2H4 adsorption on Vn+ cations present at the catalyst surface and that the strength of adsorption decreases in the order V5+ > V4+ > V3+ consistent with the decreasing Lewis acidity of the cations.