(104a) Coulometric Titration Studies of the Redox Properties of Bulk and Supported Vanadia and Molybdena Catalysts

Selective oxidation reactions often follow a Mars-van Krevelen mechanism in which surface lattice oxygen is removed from the catalyst during one of step in the catalytic cycle and then replenished via reaction with gas-phase oxygen in a separate step. Based on this mechanism the activity and selectivity of the catalyst is expected to depend on its redox properties. For example, one might anticipate an easily reducible catalyst to have high oxidation activity. If oxygen removal is too facile, however, the catalyst may not be selective for partial oxidation products, instead producing CO or CO₂. These principles are widely recognized and many attempts to correlate catalytic activity with the redox properties can be found in the literature. The majority of these studies, however, have relied on temperature programmed reduction (TPR) to provide a measure of the relative reducibility of the catalysts. While such studies are useful, TPR measures the kinetics of reduction and it is difficult to directly relate this to oxygen binding strength or thermodynamic variables. In order to provide more quantitative relationships between redox properties and catalytic activity, we have been using a coulometric titration technique to directly measure redox isotherms, i.e. the oxygen composition of the sample as a function of P(O₂), for a range of selective oxidation catalysts and then comparing thermodynamic data extracted from these isotherms (ΔG, ΔH and ΔS) to their reactivity. In this talk we will first give an overview of the technique and then describe our coulometric titration studies of bulk and supported vanadia and molybdena catalysts. Comparisons between the measured redox properties and the reactivity of the catalysts for methanol oxidation and propane ODH will then be presented.