Minority Sites on Cu Surfaces Drive C-C Coupling in the Electrochemical Reduction of CO₂

The electrochemical CO₂ reduction reaction (CO₂RR) on Cu has been extensively investigated as a strategy to valorize a prominent greenhouse gas; however, key mechanistic questions remain unresolved. In this work, we propose a mechanistic framework to rationalize the contradiction between CO coverages on Cu under electrochemical conditions and the reaction orders of CO₂ and CO in the formation of C₂₊ products by recognizing the site heterogeneity on Cu surfaces. OC-CO coupling occurs primarily on highly active minority sites with a strong CO affinity, while the majority of surface Cu sites are relatively inert in OC-CO coupling. Further, correlations between the structure and effectiveness of molecular inhibitors reveal that Cu_CO sites are likely located on highly curved nanostructures. This work proposes a general mechanistic framework to rationalize the Cu-catalyzed CO₂RR and provides insights into the structure of active sites.