Microkinetic modeling of the decarboxylation and decarbonylation of propanoic acid over Pd(1 1 1) model surfaces based on parameters obtained from first principles

Abstract

We have studied the reaction mechanism of deoxygenation of propanoic acid to alkanes over Pd(111) model surfaces by a combination of microkinetic modeling and density functional theory calculations. Approximate, coverage-dependent adsorption energies of CO and H have been implemented in a microkinetic model that shows that the decarbonylation mechanism is slightly preferred over the decarboxylation mechanism at various H2 partial pressures. The most significant decarbonylation pathway proceeds via dehydrogenation of the acid to yield CH3CHCOOH which illustrates the important role of α-carbon dehydrogenation steps, followed by dehydroxylation to yield CH3CHCO which further dehydrogenates to CHCHCO. Finally, facile C–CO bond scission occurs to yield CO and acetylene which gets hydrogenated to ethane. Overall, the dehydroxylation of CH3CHCOOH and to a lower degree the removal of the hydrocarbon pool from the surface and the dehydrogenation of the α-carbon of the reactant are found to be the rate-controlling steps.