Non-Faradaic Promotion of Pd-Catalyzed Benzaldehyde Hydrogenation Induced by Cathodic Potential.

Pd-catalyzed hydrogenation of benzaldehyde to benzyl alcohol in the aqueous phase is significantly enhanced relative to open-circuit conditions when H2 pressure and an external cathodic potential are applied simultaneously. This dual-mode operation affords hydrogenation rates up to 6-fold higher than those observed under open-circuit conditions. The rate enhancement exceeds the expected charge-transfer contribution by at least a factor of 2 at all investigated potentials, indicating a non-Faradaic promotion effect. Combined experimental and theoretical analyses suggest that cathodic polarization increases the quasi-equilibrium surface coverage of a hydroxy intermediate formed via a proton-coupled electron transfer step preceding the rate-determining addition of the second hydrogen atom. At more negative potentials, where this coverage saturates, further rate acceleration likely arises from modulation of the standard free energies of intermediates and transition states along the rate-determining pathway, thereby lowering the apparent standard activation free energy.