Mechanism of Electrocatalytic H2 Evolution, Carbonyl Hydrogenation, and Carbon–Carbon Coupling on Cu

Aqueous-phase electrocatalytic hydrogenation of benzaldehyde on Cu leads not only to benzyl alcohol (the carbonyl hydrogenation product), but Cu also catalyzes carbon–carbon coupling to hydrobenzoin. In the absence of an organic substrate, H2 evolution proceeds via the Volmer–Tafel mechanism on Cu/C, with the Tafel step being rate-determining. In the presence of benzaldehyde, the catalyst surface is primarily covered with the organic substrate, while H* coverage is low. Mechanistically, the first H addition to the carbonyl O of an adsorbed benzaldehyde molecule leads to a surface-bound hydroxy intermediate. The hydroxy intermediate then undergoes a second and rate-determining H addition to its α-C to form benzyl alcohol. The H additions occur predominantly via the proton-coupled electron transfer mechanism. In a parallel reaction, the radical α-C of the hydroxy intermediate attacks the electrophilic carbonyl C of a physisorbed benzaldehyde molecule to form the C–C bond, which is rate-determining. The C–C coupling is accompanied by the protonation of the formed alkoxy radical intermediate, coupled with electron transfer from the surface of Cu, to form hydrobenzoin.