Effect of Palladium Surface Structure on the Hydrodeoxygenation of Propanoic Acid: Identification of Active Sites

Abstract

The effect of palladium surface structure on the hydrodeoxygenation of propanoic acid has been investigated by studying the mechanism over Pd(111) and Pd(211) model surfaces. We developed a microkinetic model based on parameters obtained from density functional theory and harmonic transition state theory and studied the reaction mechanism at a characteristic experimental reaction temperature of 473 K. The activity of active sites on flat surface models was found to be 3–8 times higher than the activity of stepped surface models, suggesting that the hydrodeoxygenation of propanoic acid over a palladium catalyst is not very sensitive to surface structure. Very good agreement between computations and experiments could be obtained for our Pd(111) model if we include dispersion interactions between the gas species and the metal surface approximately by using the PBE-D3 functional for adsorption/desorption processes. Our model calculations predict that on both stepped and flat surfaces, the dominant deoxygenation mechanism proceeds by a decarbonylation pathway; however, on stepped surface models, decarboxylation and decarbonylation are essentially competitive. A sensitivity analysis of our models suggests that C–OH and C–H bond cleavages control the overall rate over both Pd(111) and (211) catalyst surface models. In addition, on Pd(211) the C–C bond dissociation of propionate to CH3CH2 and CO2, a key step in the decarboxylation mechanism, is also partially rate controlling.