Potential-Controlled Selectivity of Benzaldehyde Hydrogenation Pathways on a Pd Cathode

Benzaldehyde, as an important biomass-derived platform molecule, can be electrochemically reduced to produce high value-added products such as benzyl alcohol and hydrobenzoin. However, the electrochemical kinetic behavior involved in the selective reduction of benzaldehyde, as well as the evolution of key adsorbed active intermediates on the catalyst surface, has been rarely reported. We prepared a Pd/C catalyst by a Joule-heating shock method, which enables efficient reduction of benzaldehyde to benzyl alcohol at low potentials (75% selectivity at −1.3 V), while favoring the formation of the C–C coupling product hydrobenzoin at higher potentials (53% selectivity at −1.6 V). By tracking the EIS spectra, surface-adsorbed hydrogen coverage, and the quantified key intermediate benzoyl radicals at the two potentials, combined with electrochemical measurements, it is revealed that at low potentials the relatively low coverage of benzoyl radicals and high surface hydrogen coverage favor direct hydrogenation of benzaldehyde to benzyl alcohol. In contrast, at high potentials, the facilitated activation of benzaldehyde leads to the accumulation of benzoyl radicals on the catalyst surface together with a lower level of adsorbed hydrogen, so that benzaldehyde preferentially undergoes a C−C coupling pathway to form hydrobenzoin.