Experimental evidence of distinct sites for CO₂-to-CO and CO conversion on Cu in the electrochemical CO₂ reduction reaction

The electrochemical CO2 reduction reaction on Cu is widely believed to occur via two consecutive and orthogonal reaction steps, that is, CO2-to-CO and CO-to-C2+, on the same sites. Here we provide compelling experimental evidence that challenges these long-held assumptions. We show that the presence of CO2 promotes the electrochemical CO reduction reaction, and there are at least two distinct types of Cu sites, with one (Cu-CO2) more active in the CO2-to-CO conversion and the other (Cu-CO) favouring the further reduction of CO to C2+ products. CO adsorbed on Cu-CO is at least six times more active towards the formation of C2+ products than that on Cu-CO2. Isotopic labelling experiments on Cu(111) and Cu(100) surfaces indicate that Cu-CO2 and Cu-CO probably correspond to Cu(111)-like and defect sites, respectively. These insights highlight the possibility of selectivity control in the CO2 reduction reaction.