Thermochemical Heterolytic Hydrogenation Catalysis Proceeds Through Polarization-Driven Hydride Transfer

Heterolytic hydrogenations, which split H2 across a hydride acceptor and proton acceptor, are a key class of reactions that span the chemical value chain, including CO2 hydrogenation to formate and NADH regeneration from NAD+. The dominant mechanistic models for heterogeneous catalysis of these reactions invoke classical surface mechanisms that ignore the role of interfacial charge separation. Herein, we quantify the electrochemical potential of the catalyst during turnover and uncover evidence supporting an interfacial electrochemical hydride transfer mechanism for this overall thermochemical reaction class. We find that the proton acceptor induces spontaneous electrochemical polarization of the metal catalyst surface, thereby controlling the thermodynamic hydricity of the surface M–H intermediates and driving rate-determining electrochemical hydride transfer to the hydride acceptor substrate. Overall, this model invokes that heterolytic hydrogenations proceed via the coupling of two electrochemical half-reactions, the hydrogen reduction reaction (HRR) and hydrogen oxidation reaction (HOR), which convert H2 to hydride and proton, respectively. This mechanistic framework, which applies across diverse reaction media and for the hydrogenation of CO2 to formate and NAD+ to NADH, enables the determination of intrinsic reaction kinetics and exposes design principles for the future development of sustainable hydrogenation reactivity.