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The oxygen source of oxygenates is the fundamental issue for CO2/CO electroreduction, which was firmly believed to originate from the gas feed (Ogas) for a long time. However, recent experiments have confirmed that most O atoms of the generated alcohols via CO reduction arise from the solvent (Oaq), indicating the existence of a rather mysterious "oxygen exchange" process. In this work, we solved this mechanistic puzzle using comprehensive computations. Our results revealed that high CO pressure enables COgas oxidation by surface *OaqH, which opens a pathway for oxygenate production. The generated *COgasOaqH can react with another *CO to form *COCOgasOaqH, which leads to the formation of a series of carboxyl-containing intermediates (RCOgasOaqH) in subsequent steps. Due to the dynamic ionization equilibrium, H+ moves rapidly between Ogas and Oaq via reversible "inner" proton transfer (*RCOgasOaqH ⇌ *R-COgasOaq– + H+ ⇌ *RCOaqOgasH). The oxygen exchange completes when *RCOaq forms via the dehydroxylation of a certain *RCOaqOgasH. The completed reaction pathways were further explored by using COgas reduction into C2H5OaqH as an example, which explains related experiments. Therefore, these results refresh the insights into CO2/CO electroreduction and give specific guidelines for the optimization of catalytic performance.
Zhang et al. (Wed,) studied this question.