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