PNNL

Abstract

Pd-catalyzed hydrogenation of benzaldehyde to benzyl alcohol in 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 sixfold higher than those observed under open-circuit conditions. The rate enhancement exceeds the expected charge-transfer contribution by at least a factor of two 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 nega-tive 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 appar-ent standard activation free energy.