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 Cu_1-4 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 Cu_3-4@C₃N₃. These results provide atomic-scale insights into Cu cluster catalysts and design principles for nitrogen-carbon-supported C-C coupling catalysts.