(675a) Rate Analysis Implications of (Side) Product Inhibition in Mars-Van Krevelen Redox Cycles

Mechanistic analyses of catalytic systems in many cases rely on the accurate estimation of sensitivities of reaction rates to reactant/product partial pressures - sensitivities that enable identification of possible rate expressions and plausible reaction sequences. The most common method for measuring such sensitivities is the use of low conversion (or residence time) rate data with varying partial pressures of reactants and products. The presence of product inhibitory effects – not uncommon in catalytic partial oxidation reactions – makes it highly challenging to rigorously measure rates, even at very low conversions. This talk will discuss some of the challenges encountered in interpreting low-conversion data for the oxidation of ethane over bulk nickel oxide catalysts, and the manner in which (side-)product inhibition obfuscates both rate parameters, as well as mechanistic inferences derived therefrom.

Product co-feed experiments generating differential beds when used to develop Mars-van Krevelen kinetic models for ethane partial and total oxidation are insufficient to explain integral data at low residence times that probe lower ranges of hydroxyl/carbonate coverages than those prevalent in differential co-feed experiments. Measured (average) rates deviate from true ones even at conversions as low as 0.1%, with apparent rates being more sensitive to residence time at low, rather than high conversions (Figure 1). Estimation of rate parameters and mechanisms over excess oxygen-containing nickel oxide requires consideration of inhibitory effects of both water and CO₂, unlike those over niobium-containing nickel oxides that carry exclusively lattice oxygens recalcitrant to carbonate formation. In this talk, we will describe how the quasi-equilibrated nature of recombinative water desorption steps in Mars-van Krevelen cycles result in an amplification of rate inhibitory features that are critical to consider during kinetic analysis of alkane oxidation over bulk oxides more generally.