Theoretical and experimental investigation of the coordination effect on photocatalytic CO2 reduction efficiency of cobalt single atom catalyst

[Display omitted] •Co single atom catalysts (SACs) with diverse N coordination number were synthesized.•The N coordination number is related to photocatalytic CO2RR activity.•Increased unoccupied Co 3d electronic orbitals can improve the CO2RR activity.•This work facilitates the development of SACs designs for targeting reactions. Visible-light-driven carbon dioxide (CO2) reduction, which converts CO2 into value-added fuels, is an intriguing strategy to simultaneously mitigate the energy crisis and achieve a carbon–neutral economy. Single-atom catalysts (SACs) with homogeneously dispersed definite active sites and adjustable coordination environments show great potential in catalyzing photocatalytic CO2 reduction reaction (CO2RR). According to previously reported studies, the catalytic performance of SACs applied in various fields has a great relationship with their local coordination environment of metal centers. However, in-depth investigation of the coordination effect, which has been widely explored to regulate the electronic structure of SACs, on the activity, selectivity, and stability of CO2 photoreduction has never been reported. Herein, a series of Co single atom photocatalysts with diverse N coordination number (CoSA-Nx/C, x = 2, 3, and 4) were synthesized and their photocatalytic CO2RR performance was examined. Theoretical and experimental investigation demonstrated that the photocatalytic CO2RR activity of Co SACs can be effectively adjusted by fine-regulating the N coordination. The optimal CoSA-N2/C photocatalyst with two-coordinate N atoms with increased unoccupied Co 3d electronic orbitals possesses enhanced CO2 adsorption and activation as well as reduced energy barrier for *COOH intermediate generation, thus showing superior photoconversion of CO2-to-CO performance with 10110 μmol g−1h−1 CO evolution yield, 82.6 % CO selectivity, and long-term stability. This work promotes the future rationally design and synthesis of photocatalysts applied in targeted reactions at atomic scale.