Protonation Prerequisite in Selective Furfural Hydrogenation to Furfuryl Alcohol: A Kinetic Isotope Effect Study.

The electrochemical hydrogenation of furfural (FF) to furfuryl alcohol (FA) offers a sustainable route for biomass valorization, but the mechanistic origins of FA selectivity─particularly the suppression of radical-mediated C-C coupling─remain insufficiently resolved. Herein, we demonstrate that enforcing a protonation-first pathway through tailored interfacial kinetics effectively suppresses undesired C-C coupling at electrified interfaces. By engineering a PdCu bimetallic catalyst with synergistic dual-active sites─electron-deficient Cuδ+ delaying premature electron transfer and oxophilic Pd-O-Cu clusters enhancing water dissociation─we prioritize protonation of the carbonyl group prior to electron transfer. This spatiotemporal decoupling is validated by a secondary inverse kinetic isotope effect (KIE = 0.56 ± 0.06), eradicating polymerization channels and achieving near-unity (99.9%) FA selectivity on the optimized catalyst, 4-fold higher than the pristine Cu catalyst. This work establishes a universal framework for controlling proton-coupled electron transfer sequences in FF electroreduction, offering a blueprint to mitigate polymerization in reactions involving electrophilic carbonyl intermediates.