Macroporous Carbon-Nitride-Supported Transition-Metal Single-Atom Catalysts for Photocatalytic Hydrogen Production from Ammonia Splitting

Ammonia (NH₃) splitting to hydrogen (H₂) is a promising route for on-site production of green hydrogen energy; however, the application is limited due to high-cost noble-metal-based catalysts and high operating temperature of the endothermic nature. Herein, we develop a series of macroporous carbon nitride-supported single-atom transition metal (TMs-MCN, TMs: Co, Mn, Fe, Ni, Cu) catalyst panels for solar light-driven photocatalytic gaseous NH₃ splitting. Under ambient reaction conditions, the optimized Ni-MCN shows an H₂ production rate of 35.6 μmol g^-1 h^-1, much superior to that of MCN and other TMs-MCN. Such enhanced photoactivity is attributed to the presence of Ni-N₄ sites, which improve the optical properties, accelerate charge carrier separation/transfer, and boost NH₃ splitting kinetics of the catalysts. Density functional theory calculations further reveal that the Ni-N₄ sites can effectively modify the electronic structure of the carbon nitride. Compared with other metal sites, the Ni-N₄ site possesses moderate NH₃ binding strength and the lowest energy barrier to facilitate the formation of key intermediates *NH + *H. These findings provide valuable guidelines for the rational design of single-atom catalysts toward energy- and cost-effective photocatalytic NH₃ splitting for H₂ production.