Insights into the CO2 Reduction Pathway through the Electrolysis of Aldehydes on Copper

Investigating the electrochemical reduction of aldehydes to alcohols provides insights into the mechanistic pathways of converting CO2 to alcohols electrochemically. In this work, both acetaldehyde and propionaldehyde were electrochemically reduced on a Cu catalyst to illustrate that it is a viable pathway to ethanol and 1-propanol, respectively, supporting the mechanistic route previously proposed in the literature. Also noteworthy is that the electroreduction of acetaldehyde to ethanol is selective on Cu and does not occur on a catalyst such as Au. 13C and 1H NMR analysis on isotopically labeled acetaldehyde was utilized to trace the reduction process. Experimental observations were also supported with DFT calculations, indicating a higher-energy reaction intermediate on Au(111) over Cu(100). In an aqueous solution, acetaldehyde is at equilibrium with ethanediol, and propionaldehyde with propanediol. DFT calculations suggest that acetaldehyde, not ethanediol, is more likely to be reduced to ethanol. The dissociation of adsorbed ethanediol to acetaldehyde and water was also found to be favorable on both Cu and Au surfaces. In summary, the results from this study support previously proposed mechanisms and provide a framework for testing other stable CO2 reaction intermediates to gain insights into the overall CO2 reaction pathway.