Proton-Coupled Electron Transfer in the Catalytic Hydrogenation of Hydroxyacetone: A pH-Induced Switch from Potential Pinning to Kinetic Coupling

At metal-aqueous interfaces, the spontaneous open-circuit potential (OCP) established by H₂/H₃O⁺ quasi-equilibrium introduces electrochemical effects. This interfacial polarization can modulate the chemical potential of reaction intermediates and enable alternative proton-coupled electron transfer (PCET) pathways beyond classical Langmuir-Hinshelwood mechanisms. Herein, we demonstrate a pH-induced switch in selectivity for the catalytic conversion of biomass-derived hydroxyacetone (HA) over Pt/C under H₂. The primary C=O hydrogenation to propylene glycol (PG) exhibits an inverse volcano-shaped rate dependence on pH. Isotopic labeling experiments support the idea that solvent serves as the primary proton source, establishing PCET as the dominant mechanism. We further connect this bias-free HA hydrogenation to the half-reaction framework invoked by the mixed-potential theory. Short-circuit experiments and kinetic perturbations in H₂ pressure and HA concentration reveal a clear transition from a "potential-pinned" regime at low pH─where OCP is governed solely by H₂/H⁺ equilibrium─to a "kinetic coupled" mixed-potential regime at high pH, in which competing reactions jointly influence the catalyst potential. Both regimes are governed by the same mixed-potential framework, and the transition reflects the relative proportion of the HA reduction current to the HOR/HER exchange current. These results reflect the intrinsic complexity at the metal-aqueous interfaces and call for a more complete understanding of kinetic coupling when mixed-potential concepts are used to describe thermocatalytic reactions.