Electron transfer rather than direct hydrogen atom transfer can drive catalytic hydrogenation on supported metal catalysts

Heterogeneous thermocatalytic hydrogenation is widely believed to occur via co-adsorption of H2 and other reactants. Herein, we test the hypothesis that hydrogenation over supported metal catalysts can proceed via two half reactions in water involving electron release from H2 and proton-electron addition to substrate. Using 4-nitrophenol as a model substrate, the core of our study is to conduct hydrogenation in an unbiased H-cell, where H2 and 4-nitrophenol are separately supplied into two electrically connected chambers separated by a proton exchange membrane. Based on the observation of the almost identical hydrogenation performance between a single cell, in which H2 and 4-nitrophenol are co-fed, and the H-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 of 0-4.5. The isotope experiments, reaction rate-pH relationship, scavenger test and DFT calculations suggest that a coupled electrochemical half-reaction mechanism for 4-nitrophenol hydrogenation in acidic aqueous phase is not just possible but is predominant. H2 oxidation primarily occurs on metal sites, while 4-nitrophenol reduction occurs on both metal sites and conductive supports.