Facet dependence of electrocatalytic furfural hydrogenation on palladium nanocrystals

Electrocatalytic hydrogenation (ECH) offers a sustainable route for the conversion of biomass-derived feedstocks under ambient conditions; however, an atomic-level understanding of the catalytic mechanism based on heterogeneous electrodes is lacking. To gain insights into the relation between electrocatalysis and the catalyst surface configuration, herein, the facet dependence of the ECH of furfural (FAL) is investigated on models of nanostructured Pd cubes, rhombic dodecahedrons, and octahedrons, which are predominantly enclosed by {100}, {110}, and {111} facets, respectively. The facet-dependent specific activity to afford furfuryl alcohol (FOL) follows the order of {111}>{100}>{110}. Experimental and theoretical kinetic analyses confirmed the occurrence of a competitive adsorption Langmuir-Hinshelwood mechanism on Pd, in which the ECH activity can be correlated with the difference between the binding energies of chemisorbed H (*H) and FAL (*FAL) based on density functional theoretical (DFT) calculations. Among the three facets, Pd{111} exhibiting the strongest *H but the weakest *FAL showed the copresence of the *H and *FAL intermediates on the Pd surface for subsequent hydrogenation, experimentally confirming its high ECH activity and Faradaic efficiency. The free energies determined using DFT calculations indicated that *H addition to the carbonyl of FAL on Pd{111} was thermodynamically preferred over desorption to gaseous H2, contributing to efficient ECH to afford FOL at the expense of H2 evolution. The obtained insights into the facet-dependent ECH underline that surface bindings assist ECH or H2 evolution considering their competitiveness. These findings are expected to deepen the fundamental understanding of electrochemical refinery and broaden the scope of electrocatalyst exploration.