Thermochemical heterolytic hydrogenation catalysis proceeds through polarization-driven hydride transfer.

Heterolytic hydrogenations, which split H₂ across a hydride acceptor and proton acceptor, comprise a key reaction class that spans the chemical value chain, including CO₂ hydrogenation to formate and NADH regeneration from nicotinamide adenine dinucleotide (NAD⁺). The dominant mechanistic models for heterogeneous catalysis of these reactions invoke classical surface reaction steps, largely ignoring the role of interfacial charge separation. Here 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. 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.