Electrocatalytic conversion of benzaldehyde on Cu in alkaline media

Aqueous phase electrochemical hydrogenation (ECH) of benzaldehyde (BZ) on Cu/C in alkaline electrolytes ($\mathit{pH}$$\mathit{pH}$ varying between 8.6 and 12.3) forms both benzyl alcohol (BA), the C=O hydrogenation product, and hydrobenzoin (HB), the carbon–carbon coupling product, with high Faradaic selectivity towards C–C coupling (>84 %) and high Faradaic efficiency. The rate-determining step for BA formation is the second H addition to the radical α-C of the surface hydroxy intermediate, while that for HB formation is the first H addition to the carbonyl O of an adsorbed BZ molecule. The subsequent C–C bond formation and second H addition (for HB formation) are fast. In the absence of BZ (i.e., in pure electrolyte), the rate-determining step for H₂ evolution on Cu/C in alkaline conditions is the dissociation of H₂O on the electrode’s surface to form surface H* (i.e., the Volmer step). The H addition occurs primarily via a proton-coupled electron-transfer (PCET)-type mechanism wherein H₂O molecules act as the proton source, forming OH^– in the process. The high selectivity towards C–C coupling in alkaline media, compared to acidic media, is attributed to the slow H addition kinetics caused by the weaker protonation ability of H₂O compared to H₃O⁺. Increasing electrolyte $\mathit{pH}$$\mathit{pH}$, concentration of Na⁺ cations, or the applied external overpotential positively influences the BZ ECH rates while maintaining high C–C coupling selectivity.