Precise engineering of atomic sites on carbon nitride for carbon dioxide photoreduction from single-atom to multi-atom: a review

Solar-driven photocatalytic conversion of carbon dioxide into high-value-added chemicals offers a promising green route to achieving carbon neutrality. Polymeric carbon nitride has attracted considerable attention as a metal-free photocatalyst owing to its visible-light response, environmental compatibility, low cost, and structural tunability. However, pristine carbon nitride generally suffers from weak CO₂ adsorption and activation, rapid recombination of photogenerated charge carriers, and insufficient catalytic sites, which severely limit its conversion efficiency and product selectivity. Atomic-level engineering offers an effective strategy to overcome these intrinsic limitations by creating well-defined active centers and regulating the local electronic structure of carbon nitride. Here we review the precise engineering of atomic sites on carbon nitride for photocatalytic CO₂ reduction, with focus on the evolution from single-atom to dual-atom and multi-atom catalysts. Single-atom sites can enhance CO₂ adsorption and activation, lower the energy barriers for key hydrogenation steps, construct frustrated Lewis pairs, and promote charge separation and directional electron transfer. Single-atom-modified carbon nitride photocatalysts have achieved C₁ product yields exceeding 500 µmol g^− 1 h^− 1 and near 100% selectivity. Multi-atom configurations introduce cooperative interactions between adjacent active centers, enabling intermediate stabilization, bifunctional catalysis for CO₂ reduction and H₂O oxidation, and C–C coupling toward higher-value C₂ products. Multi-atom systems have delivered C₂ product formation rates above 150 µmol g^− 1 h^− 1 with selectivity up to 98% under reported conditions. Finally, we discuss challenges and opportunities, including scalable synthesis, precise control of atomic configurations, standardized performance evaluation, operando mechanistic characterization, and techno-economic analysis.