Regulating second-shell coordination in molybdenum single-atom catalysts for efficient photocatalytic nitrogen fixation

• A novel molybdenum single-atom catalysts with boron coordinated in the second shell was developed. • Mo SAs/BCN exhibited superior visible-light-driven photocatalytic activity for N 2 reduction to NH 3 . • The relationship between NRR performance and local coordination structure was clarified. • The N 2 reduction pathways on Mo SAs/BCN was elucidated. Photocatalytic nitrogen (N 2 ) reduction reaction (NRR) holds great potential and sustainability compared with the Haber-Bosch process for ammonia (NH 3 ) synthesis. Precise regulation of the coordination environment in the second shell of single-atom catalysts (SACs) provides an opportunity to enhance the photocatalytic NRR performance, yet a challenge. Herein, Mo SAs/BCN featured by Mo single atoms dispersed in a carbon nitride support with second-shell B coordination is developed for photocatalytic N 2 fixation. Experimental data combined with theoretical calculations demonstrate that the second-shell B coordination can not only strengthen visible light absorption and accelerate the separation and transfer of photogenerated charge carriers, but also upshift the d-band center of Mo close to the Fermi level to improve the adsorption and activation of N 2 . Thus, Mo SAs/BCN exhibits a high NH 3 yield rate of 176.4 μmol g -1 cat. h −1 under visible light irradiation, which is 5.4 times that of CN. A distal pathway may occur on Mo SAs/BCN, and the first step of *N 2 protonation is the potential-determination step. This work reveals the significance of modulating the second-shell coordination atoms of SACs for enhancing photocatalytic activity and selectivity and offers a promising strategy for the rational design of SACs.