Electrochemical Reduction of CO2 Using Copper Single-Crystal Surfaces: Effects of CO* Coverage on the Selective Formation of Ethylene

Copper oxide-derived Cu catalysts are known to exhibit enhanced energetic efficiencies and selectivities towards the reduction of carbon dioxide to commercially vital C-2 products such as ethylene (C2H4). However, the cause of this selectivity is not fully understood. In this work, we elucidated a fundamental g reason underlying the selectivity of CO2 reduction toward C-2 products by studying its reactivity on Cu(100), Cu(111), and Cu(110) single-crystal surfaces. A combination of cyclic and linear sweep voltammetries, chronoamperometry, online gas chromatography, H-1 nuclear magnetic resonance spectroscopy, and density functional theory (DFT) calculations was employed for this end. A wide range of electrochemical potentials from-0.28 to-1.25 V versus the reversible hydrogen electrode was investigated. Aqueous 0.1 M KHCO3 was used as the electrolyte. We report here two general trends on Cu2O-derived Cu and Cu single-crystal surfaces: (i) the onset potential for the formation of C2H4 always starts 300-400 mV more negative than the onset potential for CO evolution, and (ii) C2H4 was formed only after a significant amount of CO gas was produced. Among the single-crystal surfaces investigated, Cu(100) required the lowest overpotential to reduce CO, to C2H4 center dot These observations were rationalized using DFT simulations. Of the three single-crystal surfaces modeled, the dimerization of two CO* molecules on Cu(100) exhibited the lowest energy barrier, and this barrier can be further lowered with higher CO* coverages. The application of our observed experimental trends to other previously reported Cu-based systems strongly suggests that a high surface coverage of CO* is central for the selective formation of C2H4.