Unraveling the Electrocatalytic Reduction Mechanism of Enols on Copper in Aqueous Media

A mechanism is proposed for the direct electrochemical deoxygenation of aldehydes to alkenes and alkanes which has implications in refining biomass-derived fuels for use as transportation fuel, as a potential green synthesis strategy for terminal olefins in aqueous, ambient conditions, and in understanding the reaction pathway for CO2 to C2 and C3 products. Here we report the electrochemical conversion of vinyl alcohol and acetaldehyde on polycrystalline Cu to ethanol, ethylene and ethane; and propenol and propionaldehyde to propanol, propene and propane. Sensitive detection was achieved using a rotating disk electrode coupled with gas chromatography-mass spectrometry (RDE-GC-MS). In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), and in situ Raman spectroscopy confirmed the adsorption of the vinyl alcohol. Calculations using canonical and grand-canonical density functional theory (DFT and GC-DFT), along with experimental findings suggest that the rate-determining step (RDS) for ethylene and ethane formation is an electron transfer step to the adsorbed vinyl alcohol. Finally, we extend our conclusions to enols resulting from higher-order soluble aldehyde and ketone.