Unraveling Electroreductive Mechanisms of Biomass-Derived Aldehydes via Tailoring Interfacial Environments

Electrochemical reduction of biomass-derived feedstocks holds great promise to produce value-added chemicals or fuels driven by renewable electricity. However, mechanistic understanding of the aldehyde reduction toward valuable products at the molecular level within the interfacial regions is still lacking. Herein, through tailoring the local environments, including H/D composition and local H3O+ and H2O content, we studied the furfural reduction on Pb electrodes under acid conditions and elucidated the pathways toward three key products: furfuryl alcohol (FA), 2-methylfuran (MF), and hydrofuroin. By combining isotopic labeling and incorporation studies, we revealed that the source of protons (H2O and H3O+) plays a critical role in the hydrogenation and hydrogenolysis pathways toward FA and MF, respectively. In particular, the product-selective kinetic isotopic effect of H/D and the surface-property-dependent hydrogenation/deuteration pathway strongly impacted the generation of FA but not MF, owing to their different rate-determining steps. Electrokinetic studies further suggested Langmuir–Hinshelwood and Eley–Rideal pathways in the formation of FA and MF, respectively. Through modifying the double layer by cations with large radii, we further correlated the product selectivity (FA and MF) with interfacial environments (local H3O+ and H2O contents, interfacial electric field, and differential capacitances). Finally, experimental and computational investigations suggested competitive pathways toward hydrofuroin and FA: hydrofuroin is favorably produced in the electrolyte through the self-coupling of ketyl radicals, which are formed from outer-sphere, single-electron transfer, while FA is generated from hydrogenation of the adsorbed furfural/ketyl radical on the electrode surface.