Dynamic Ionization Equilibrium-Induced “Oxygen Exchange” in CO Electroreduction

Abstract

The oxygen source of oxygenates is the fundamental issue for CO₂/CO electroreduction, which was firmly believed to originate from the gas feed (O_gas) for a long time. However, recent experiments have confirmed that most O atoms of the generated alcohols via CO reduction arise from the solvent (O_aq), 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 CO_gas oxidation by surface *O_aqH, which opens a pathway for oxygenate production. The generated *CO_gasO_aqH can react with another *CO to form *COCO_gasO_aqH, which leads to the formation of a series of carboxyl-containing intermediates (RCO_gasO_aqH) in subsequent steps. Due to the dynamic ionization equilibrium, H⁺ moves rapidly between O_gas and O_aq via reversible “inner” proton transfer (*RCO_gasO_aqH ⇌ *R-CO_gasO_aq^– + H⁺ ⇌ *RCO_aqO_gasH). The oxygen exchange completes when *RCO_aq forms via the dehydroxylation of a certain *RCO_aqO_gasH. The completed reaction pathways were further explored by using CO_gas reduction into C₂H₅O_aqH 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.