Unraveling the Size-Dependent Effect of Cu Cluster on CO₂ Reduction Insight from Theory

Atomically dispersed and cluster copper (Cu) catalysts anchored on nitrogenated holey carbon frameworks have garnered increasing attention as promising platforms for the electrochemical CO2 reduction reaction (CO2RR), yet the structural evolution and mechanistic origin of selectivity and C–C coupling on Cu clusters remain elusive. Herein, we performed a comprehensive first-principles study of Cu1–4 clusters supported on C3N3 substrates to elucidate their structural stability, electronic properties, and catalytic behavior toward C1–C3 products. The Cu atoms stably bind to N sites, evolving from planar to 3D configurations with an increasing cluster size. We demonstrated that the Cu–N interfacial synergy in Cu1@C3N3 promoted CH4 selectivity via enabling dynamic interconversion of the key intermediate *CHO between Cu and N sites. Moreover, we revealed that a dynamic *CO generation-migration-regeneration cycle, driven by apex-basal site synergy in Cu4@C3N3, facilitates multi-*CO accumulation, essential for C–C coupling. Elevated *CO coverage lowers the hydrogenation barrier, enabling efficient *CHO-*CHO coupling (0.34 and 0.41 eV, respectively) and promoting alkane-type C2+ product (such as C2H6 and C3H8) formation on Cu3–4@C3N3. These results provide atomic-scale insights into Cu cluster catalysts and design principles for nitrogen–carbon-supported C–C coupling catalysts.