Mechanistic analyses of catalytic systems in many (if not most) cases rely on the accurate estimation of sensitivities of reaction rates to reactant/product partial pressures- sensitivities that enable the identification of possible rate expressions and plausible reaction sequences. The most common method for measuring such sensitivities is the use of low conversion (/residence time) rate data with varying partial pressures of reactants and products. The presence of product inhibitory effects-- not uncommon in heterogeneous catalytic systems of industrial interest-- 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 (/residence time) data for the oxidation of ethane over bulk nickel oxide catalysts, and the manner in which (side) product inhibition obfuscates rate parameters and mechanistic inferences derived therefrom. Product co-feed experiments generating differential beds when used to develop Mars-van Krevelen based 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. Estimation of rate parameters and mechanisms over non-stoichiometric oxygen-containing nickel oxide requires consideration of inhibitory effects of both water and CO2, unlike those over niobium-doped nickel oxides that exclusively carry lattice oxygens recalcitrant to carbonate formation, and hence strong product inhibition. A mathematical analysis of product inhibitory effects on apparent reaction orders and activation energies is also presented. The study captures the various commonly encountered pitfalls when using rate parameters from low conversion kinetic data in inferring mechanistic features for reaction systems of interest in heterogeneous catalysis applications.

Rate analysis of a curious case of (side) product inhibition

ACS Spring 2022

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