Demonstrating the Electron–Proton-Transfer Mechanism of Aqueous Phase 4-Nitrophenol Hydrogenation Using Unbiased Electrochemical Cells

Heterogeneous thermocatalytic hydrogenation is widely believed to occur via co-adsorption of H2 and other reactants, but in aqueous phase an ionic or electrochemical mechanism was also proposed. Herein, we conduct 4-nitrophenol hydrogenation in an unbiased H-cell, where the H2 and substrate are separately supplied into two chambers connected by a proton exchange membrane, in comparison with the same reaction in a single cell in which H2 and 4-nitrophenol are co-fed. Based on the observation of the almost identical hydrogenation performance between the H-cell and the single cell, we conclude that co-adsorption of H2 and 4-nitrophenol is not a prerequisite for hydrogenation in aqueous phase in the tested pH range. Isotope experiments, scavenger test, DFT calculations, and reaction kinetics suggest that a coupled electrochemical half-reaction mechanism for 4-nitrophenol hydrogenation in acidic aqueous phase is predominant. Importantly, while H2 oxidation primarily occurs on metal sites, 4-nitrophenol reduction occurs on both metal sites and conductive support, highlighting the non-innocent role of the support if the hydrogenation reaction follows the electron–proton-transfer pathway.