Cavity‐Confined Indium Single‐Atom Catalysts Enable Localized Electron Enrichment for Highly Efficient and Selective Photocatalytic CO ₂ Reduction

ABSTRACT Single‐atom catalysts are attractive for CO 2 photoreduction due to their maximized atomic utilization and tunable electronic structures. In this study, indium (In) single atoms were incorporated into graphitic carbon nitride (CN) through an impregnation‐assisted thermal polycondensation strategy. The optimal 3 wt% In sample exhibited a CO formation rate of 54.66 μmol g −1 h −1 with a CO selectivity of 94.60%. Structural analysis confirmed that In atoms are atomically dispersed and stably coordinated within the intrinsic cavities of CN, generating a cavity‐confined microenvironment. Charge analysis revealed significant local electron accumulation near the In sites, which aids CO 2 activation. Under nitrogen coordination constraints, In predominantly exists in the +3‐oxidation state and exhibits hard acid characteristics according to the hard and soft acid–base (HSAB) theory. This leads to preferential adsorption of *COOH while maintaining weak affinity for *CO, thereby promoting efficient CO desorption. Experimental results combined with DFT calculations show that *COOH forms rapidly under illumination and precedes *CO formation, indicating strong adsorption and stabilization of *COOH. Furthermore, the energy barrier for *COOH‐to‐*CO conversion is reduced, accelerating the key step of CO release. These findings highlight the synergistic roles of cavity confinement and affinity regulation based on the HSAB theory in optimizing CO 2 activation and intermediate transformation, offering valuable insights for the design of high‐performance single‐atom photocatalysts.