Competition of CO and Acetaldehyde Adsorption and Reduction on Copper Electrodes and Its Impact on n‑Propanol Formation

Selective synthesis of n-propanol from electrocatalytic CO2/CO reduction on copper remains challenging and the impact of the local interfacial effects on the production of n-propanol is not yet fully understood. Here, we investigate the competition between CO and acetaldehyde adsorption and reduction on copper electrodes and how it affects the n-propanol formation. We show that n-propanol formation can be effectively enhanced by modulating the CO partial pressure or acetaldehyde concentration in solution. Upon successive additions of acetaldehyde in CO-saturated phosphate buffer electrolytes, n-propanol formation was increased. Oppositely, n-propanol formation was the most active at lower CO flow rates in a 50 mM acetaldehyde phosphate buffer electrolyte. In a conventional carbon monoxide reduction reaction (CORR) test in KOH, we show that, in the absence of acetaldehyde in solution, an optimum ratio of n-propanol/ethylene formation is found at intermediate CO partial pressure. From these observations, we can assume that the highest n-propanol formation rate from CO2RR is reached when a suitable ratio of CO and acetaldehyde intermediates is adsorbed. An optimum ratio was also found for n-propanol/ethanol formation but with a clear decrease in the formation rate for ethanol at this optimum, while the n-propanol formation rate was the highest. As this trend was not observed for ethylene formation, this finding suggests that adsorbed methylcarbonyl (adsorbed dehydrogenated acetaldehyde) is an intermediate for the formation of ethanol and n-propanol but not for ethylene. Finally, this work may explain why it is challenging to reach high faradaic efficiencies for n-propanol, as CO and the intermediates for n-propanol synthesis (like adsorbed methylcarbonyl) compete for active sites on the surface, where CO adsorption is favored.