Redox Isotherms for Vanadia Supported on Zirconia

Abstract

Redox isotherms were measured for zirconia-supported vanadia between 10⁻² and 10⁻²⁸ atm at 748 K for two vanadia loadings, 2.9 and 5.8 V/nm², corresponding to isolated VO₄ species and monolayer, polymeric vanadates. The catalyst with isolated VO₄ species, which is expected to have predominantly V–O–Zr linkages, had a redox isotherm that showed a well-defined step corresponding to one oxygen per V. By contrast, the redox isotherm for the catalyst with polymeric vanadates changed more gradually with $\hbox{P}_{{\rm O}_2}$ and the change in the oxygen stoichiometry corresponded to 0.85 O/V. Comparison of these results to the redox isotherms for bulk vanadates suggests that oxidation of the isolated vanadates proceeds by a direct transition from V⁺³ ↔ V⁺⁵, while transitions from V⁺³ ↔ V⁺⁴ and V⁺⁴ ↔ V⁺⁵ are possible with the polyvanadates. Rate measurements for methanol and propane oxidation over the two supported vanadia catalysts and several bulk vanadates showed that specific rates for each reaction were similar on all of the samples, suggesting that that the V–O bond strength does not affect the rate determining step of these reactions.