On the Structure and Redox Behavior of Ni and Cu Single Atoms Supported on Carbon Nitride

ABSTRACT Single‐atom catalysts (SACs) offer molecular‐level control in heterogeneous catalysis, but their activity depends on whether the support can sustain metal‐centered redox cycling. Here, Ni and Cu single atoms on carbon nitride (CN x ) are compared to determine how coordination geometry governs redox reversibility and photocatalytic performance. EPR/ENDOR spectroscopy, x‐ray absorption, and DFT identify a unique edge MN 4 site, composed of three sp 2 nitrogens and one bridging sp 3 nitrogen, as the binding motif for both metals. While Ni and Cu occupy the same MN 4 site in the oxidized state, their redox behavior diverges. Ni preserves a distorted square–planar geometry and undergoes fully reversible Ni 2 + /Ni + cycling. In contrast, Cu collapses upon reduction to a low‐coordinate Cu + species that cannot be re‐oxidized. This structural mismatch suppresses catalytic turnover. Accordingly, Ni@CN x efficiently promotes photoredox C─N, C─O, and C─S couplings, whereas Cu@CN x remains inactive. Catalytic performance thus depends on redox compatibility within a rigid binding pocket, rather than on metal identity alone.