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 (CuCO2) more active in the CO2-to-CO conversion and the other (CuCO) favouring the further reduction of CO to C2+ products. CO adsorbed on CuCO is at least six times more active towards the formation of C2+ products than that on CuCO2. Isotopic labelling experiments on Cu(111) and Cu(100) surfaces indicate that CuCO2 and CuCO probably correspond to Cu(111)-like and defect sites, respectively. These insights highlight the possibility of selectivity control in the CO2 reduction reaction. Despite decades of intensive research, the precise mechanism and active sites involved in CO2 electroreduction on copper catalysts remains unclear. Now, a combination of experimental techniques reveals distinct sites for CO2-to-CO and CO conversion and shows that the presence of CO2 promotes CO reduction.