Abstract: Graphitic carbon nitride (g-C₃N₄), as an efficient photocatalytic material, has garnered significant attention in CO₂ reduction. However, its practical application is hindered by the high recombination rate of photogenerated charge carriers and insufficient product selectivity. In this study, an indium (In)-modified polymeric carbon nitride (In-PCN) catalyst was successfully synthesized via calcination of MIL-68(In) precursor with urea, aiming to systematically investigate its regulatory mechanism in photocatalytic CO₂ reduction. Experimental characterizations revealed that trace amounts of In were uniformly incorporated into the PCN framework through In-N bonds, preserving its layered porous structure while significantly enhancing charge carrier separation efficiency and reducing interfacial charge transfer resistance. Under visible-light irradiation, the In-PCN exhibited a CO production rate of 19.37 µmol g^-1 h^-1 with 91.5 % selectivity, representing a 2.2-fold enhancement compared to pristine PCN, and maintained stable activity over 16 h of cyclic operation. In situ Fourier transform infrared (in-FTIR) spectroscopy and density functional theory (DFT) calculations demonstrated that In sites stabilize the critical intermediate *COOH (∆G decreased from +2.02 eV to +1.03 eV) and optimize electron transfer pathways, thereby significantly lowering the activation energy barrier for CO₂ reduction. Furthermore, the incorporation of In suppresses the generation of H₂ and increases the reduction efficiency of CO₂ by preventing the dissociation of H₂O molecules on the catalyst surface. Band structure analysis further revealed that In doping reconstructs the electronic distribution of PCN, enhancing surface charge density to promote CO₂ adsorption and selective reduction. This work provides theoretical insights and experimental validation for the rational design of metal-modified PCN catalysts with a chemical coordination environment, advancing their application in efficient and selective photocatalytic CO₂ conversion.

Key words: Polymeric carbon nitride, In modification, *COOH intermediate, Photocatalytic reduction of CO₂, In-N bond, Active site