Electrochemical reduction of CO2, as well as a key intermediate CO, to valuable chemicals is recognized as a promising strategy to enable a sustainable carbon cycle. Since both CO2 and CO only contain one carbon atom, C-C coupling chemistry in the CO2 and CO reduction reactions (CO2RR and CORR, respectively) is essential for upgrading them to multi-carbon fuels. Therefore, understanding the C-C coupling mechanism in CO2RR/CORR is urgently needed. Up to now, few studies have focused on the coupling step leading to C3 products, e.g., 1-propanol (PrOH), owing to the difficulty in isolating such steps experimentally. An informative method is to introduce proposed reaction intermediates in the CO2RR/CORR system to probe or verify their roles in the reaction network. For example, acetaldehyde (AcH) is identified as an intermediate for ethanol (EtOH) production in CORR on Cu catalysts, which could be used as a potential additive in CORR for mechanism investigation. We present a combined isotopic labeling and in-situ spectroscopic investigation of the C-C coupling of CO with AcH in CORR. The detailed analysis of NMR spectra conclusively demonstrate that CO attacks the carbonyl carbon of AcH in the C-C coupling reaction, and the carbon in CO ends up in the hydroxymethyl group (−CH2OH) of the produced PrOH. Quantitative analysis indicates that only 36% of the produced PrOH originates from the C-C coupling between CO and the added 40 mM AcH, with the rest being formed in CORR through the self-coupling among CO even with a low CO saturation concentration of ~1 mM. Combined with insights gained from in-situ surface enhanced spectroscopic techniques, we conclude that AcH is unlikely the species that directly couples with CO to form PrOH. Instead, we propose that a small fraction of AcH dehydrogenatively adsorbs on the surface to form a methylcarbonyl species, which is the reaction intermediate in CORR where the reaction pathway bifurcates to C2 and C3 products. The methylcarbonyl species could either be hydrogenated to AcH and EtOH or couple with CO leading to the formation of PrOH. This work provides important insights into the mechanism of C-C coupling and helps guide the catalyst design to favor the formation of C3+ products in CORR.

Mechanistic insights into electroreductive C–C coupling between CO and acetaldehyde into multi-carbon products

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.