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

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 MN₄ site, composed of three sp² nitrogens and one bridging sp³ nitrogen, as the binding motif for both metals. While Ni and Cu occupy the same MN₄ 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.