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

Abstract

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.

Mechanistic Insights into the Formation of Hydroxyacetone, Acetone, and 1,2-Propanediol from Electrochemical CO2 Reduction on Copper

“…This approach is a useful strategy to experimentally identify potential intermediates, although it does not rule out the possibility of alternative C 2 intermediates participating in the reaction pathway. The reaction pathway for 1-propanol will not be discussed here as it has been discussed in detail elsewhere. − Additionally, a small amount of hydroxyacetone (FE < 0.05%) was detected, which was also observed by Kuhl et al However, unlike 1-propanol, the reaction mechanism for hydroxyacetone formation has not yet been investigated. Interestingly, based on the reduction of hydroxycarbonyls, as shown in Figure S7, as hydroxyacetone was detected, it is also expected to form 1,2-propanediol and acetone since its reduction leads to these molecules at −1.0 V. Nonetheless, it is possible that these products were formed, but their productivity was below the chromatograph’s detection limit.…”

“…For C 3 compounds, 1-propanol is the predominant product. 1-Propanol is known to be produced through the coupling of adsorbed methylcarbonyl and CO, which leads to the formation of propionaldehyde and subsequently reduced to 1-propanol. − The experimental insights suggesting methylcarbonyl as a probable intermediate leading to 1-propanol were obtained by introducing a stable C 2 compound, namely, acetaldehyde, into the electrolyte and observing the resulting products after electrolysis. This approach is a useful strategy to experimentally identify potential intermediates, although it does not rule out the possibility of alternative C 2 intermediates participating in the reaction pathway.…”

“…However, the production is still rather low even when the electrolyte contains 50 mM of the presumed intermediate. We faced a similar problem when we used acetaldehyde as an intermediate to enhance 1-propanol formation . Acetaldehyde is not the actual intermediate for 1-propanol synthesis but dehydrogenated acetaldehyde (or adsorbed methylcarbonyl).…”

“…For example, {100} facets have been shown to have a better ability to promote CO–CO coupling, − which is a key step in the ethylene pathway. Therefore, the use of Cu nanocubes which have {100} nanofacets is a promising strategy for ethylene synthesis, as demonstrated by multiple studies. − In the case of 1-propanol, the CO–methylcarbonyl (adsorbed acetaldehyde) coupling is considered a key step for the C 3 formation. − Therefore, developing systems that promote the formation of methylcarbonyl on the electrode surface could lead to a higher faradaic efficiency for 1-propanol.…”

Ab initio study for late steps of CO2 and CO electroreduction: from CHCO* toward C2 products on Cu and CuZn nanoclusters

“…9 This means that adjusting the adsorption strength of C 2 intermediates would affect the faradaic efficiency (FE) related to C 3 products, competing with the production of ethanol or ethylene. In this regard, previous studies have suggested that the concentration of acetaldehyde is crucial to improving n -propanol production by coupling with CO. 17…”