Coverage-Induced Conformational Effects on Activity and Selectivity: Hydrogenation and Decarbonylation of Furfural on Pd(111)

Adsorption, hydrogenation, and decarbonylation of furfural on hydrogen-covered Pd(111) was investigated using density functional theory calculations. It was found that both the energy and the conformation of adsorbed furfural vary with increasing coverage of hydrogen or furfural. Furfural lies flat at low coverage but becomes tilted on crowded surfaces. The energy profiles of hydrogenation and decarbonylation reactions on a hydrogen-covered Pd(111) change profoundly compared to those on bare Pd(111). The energy span theory shows that the furfural hydrogenation and decarbonylation effective barriers exhibit a maximum with increasing hydrogen coverage. In contrast, the selectivity to hydrogenation toward furfuryl alcohol over decarbonylation is favored with increasing hydrogen coverage. Microkinetic modeling suggests that the conformation change with increasing H coverage has a significant effect on reaction rates (up to orders of magnitude) and induces a selectivity reversal from furan as the main product (low-H coverage limit) to furfuryl alcohol (high-H coverage limit). Our results may rationalize different selectivity trends seen experimentally under typical reactor and UHV conditions. Importantly, this study underscores the potential importance of operating conditions on hydrodeoxygenation activity and selectivity due to conformational changes of multifunctional biomass derivatives.