“Cu–Nx” 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.