Influence of the catalyst surface chemistry on the electrochemical self-coupling of biomass-derived benzaldehyde into hydrobenzoin†

The electroreduction of biomass-derived benzaldehyde (BZH) provides a potentially cost-effective route to produce benzyl alcohol (BA). This reaction competes with the electrochemical self-coupling of BZH to hydrobenzoin (HDB), which holds significance as a biofuel. Herein, we demonstrate the selectivity towards one or the other product strongly depends on the surface chemistry of the catalyst, specifically on its ability to adsorb hydrogen, as showcased with Cu₂S electrocatalysts. We particularly analyze the effect of surface ligands, oleylamine (OAm), on the selective conversion of BZH to BA or HDB. The effect of the electrode potential, electrolyte pH, and temperature are studied. Results indicate that bare Cu₂S exhibits higher selectivity towards BA, while OAm-capped Cu₂S promotes HDB formation. This difference is explained by the competing adsorption of protons and BZH. During the BZH electrochemical conversion, electrons first transfer to the C in the CO group to form a ketyl radical. Then the radical either couples with surrounding H⁺ to form BA or self-couple to produce HDB, depending on the H⁺ availability that is affected by the electrocatalyst surface properties. The presence of OAm inhibits the H adsorption on the electrode surface therefore reducing the formation of high-energy state H_ad and its combination with ketyl radicals to form BA. Instead, the presence of OAm promotes the outer sphere reaction for obtaining HDB.