"Cu-N_x" Site-Driven Selectivity Switch for Electrocatalytic CO₂ Reduction

The comprehensive understanding of the effect of the chemical environment surrounding active sites on the pathway for the electrochemical carbon dioxide reduction reaction (eCO₂RR) is essential for the development of advanced catalysts for large-scale applications. Based on a series of model catalysts engineered by the coordination of copper ions with various isomers of phenylenediamine [i.e., o-phenylenediamine (oPD), m-phenylenediamine (mPD), and p-phenylenediamine (pPD)] featuring two amino groups in ortho-, meta-, and para-positions, the steric effects could significantly govern the selectivity of the "Cu-N" sites for eCO₂RR. It was found the steric distance between adjacent copper and nitrogen active sites in Cu-oPD enhanced the C-C coupling of the *COOH intermediate, thereby resulting in increased selectivity for C₂H₄ production. In contrast, the weak van der Waals interactions arising from steric electrostatic effects surrounding the *CHO intermediate on Cu-pPD facilitated subsequent hydrogenation, leading to the preferential synthesis of CH₄. However, Cu-mPD exhibited diminished eCO₂RR activity due to a higher free energy associated with the rate-determining step, which primarily led to the formation of H₂. This study underscores the significant role of a steric effect-driven selectivity switch for eCO₂RR.