Spatial Cascade Sites in Hierarchical COF‐Based Photocatalyst Enable C─C Coupling for Selective CO 2 Photoreduction to Ethylene

ABSTRACT The photoreduction of CO 2 into multi‐carbon (C 2+ ) products is a highly attractive route for CO 2 utilization; however, the yield and selectivity of C 2+ products are seriously limited by slow multi‐electron–proton transfer and sluggish C─C coupling kinetics. Herein, we construct a hierarchical tandem photocatalyst IS@COF‐Ni by growing imine‐pyridine covalent organic frameworks on non‐stoichiometric indium sulfide and introducing isolated Ni single‐atom sites at the interfacial edges. The synergistic effect between the spatially segregated sites promotes *CO dimerization, effectively lowering the kinetic barrier for high‐rate ethylene (C 2 H 4 ) generation. Thus, compared with its individual components, the IS@COF‐Ni heterojunction achieves exceptionally high C 2 H 4 productivity and selectivity in photocatalytic CO 2 reduction with water vapor in the absence of additives. In situ spectroscopic characterizations and theoretical calculations reveal that IS@COF‐Ni establishes a low‐energy pathway for electron and proton transfer, while the heterojunction interface effectively stabilizes the adsorbed CO (*CO) intermediate, facilitating C─C bond formation via coupling of adjacent *CO species to generate C 2 H 4 . This work provides a strategic approach for designing photocatalysts toward selective CO 2 ‐to‐C 2+ conversion.