(19f) Noble Metal Catalyzed Ketone Hydrogenation with Microkinetic Modeling

Vapor-phase hydrogenation of acetone was studied over supported Pt catalysts in a packed bed reactor. To fundamentally understand both the mechanism and rate controlling steps during carbonyl hydrogenation, we performed a comprehensive macro- and micro-kinetic analysis of acetone hydrogenation. We report apparent reaction orders in acetone and hydrogen, activation energies, and observed kinetic isotope effects over a large range of temperatures and species partial pressures. Generally, ketone hydrogenations on metal surfaces follow a Horiuti-Polanyi mechanism, wherein hydrogen atoms add sequentially to the adsorbed ketone. Overall, our experimental observations are best described by assuming atomic hydrogen and surface-bound hydrocarbons do not compete for adsorption at the same active sites; this suggests that, under reaction conditions, there is a sub-set of surface sites that are accessible to hydrogen atoms, but not larger hydrocarbon fragments. We further conclude that two sequential surface reactions, respectively involving the formation of H-C and an H-O bonds, each exert partial rate control under most reaction conditions. The degree of rate control from each is ultimately determined by a combination of intrinsic kinetic parameters (e.g., elementary activation barriers) and coverages of reacting species under experimental conditions conditions. Despite our conclusion that H-X bond formation steps are rate controlling, we observe a very minor kinetic isotope effect upon switching between H₂ and D₂. We reconcile these observations by considering both kinetic and thermodynamic impacts of isotopic switching, as well as partial rate control from multiple elementary steps. Finally, we report an optimized set of elementary kinetic parameters that, when coupled with our microkinetic model, allow one to predict rates of acetone hydrogenation over a large range of temperatures and pressures.

References:

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