Hydride Transfer at the GaP(110)/Solution Interface: Mechanistic Implications for CO₂ Reduction Catalyzed by Pyridine

Surface hydrides (H^-∗s) play a crucial role in one of the heterogeneous mechanisms proposed for pyridine-catalyzed CO₂ reduction on p-GaP electrodes. In this mechanism, H^-∗ is transferred to adsorbed pyridine (Py∗) concomitant with aqueous proton addition to form the active catalyst adsorbed dihydropyridine (DHP∗), which in turn transfers hydride to CO₂ and leads to its reduction. In this contribution, we test the validity of these hypothesized hydride transfers, determining whether or not H^-∗ can participate in the mechanism of CO₂ reduction on p-GaP electrodes. To this end, we use our previously developed cluster models with hybrid density functional theory and a mixed implicit-explicit solvation approach to calculate the thermodynamic hydricity of relevant species involved in the proposed mechanism. Overall, the proposed heterogeneous mechanism is supported by the computed thermodynamic hydricities. However, computed reaction and activation energies for H^-∗ transfer from the surface reveal that H^-∗ cannot participate in CO₂ reduction on p-GaP electrodes because of a high kinetic barrier to both formation of DHP∗ and direct CO₂ reduction via H^-∗ transfer. We thus conclude that an intermediate whose formation does not require H^-∗ transfer must play the role of the active catalyst in this system. Specifically, our computed thermodynamic hydricities suggest that a recently proposed 2-PyH^-∗ intermediate, formed via two-electron reduction and protonation of Py∗, is a plausible candidate for the active catalyst in this system. (Chemical Equation Presented).