Axial Ligand Engineering of Silver Single-Atom Catalysts with N -Heterocyclic Carbenes Unlocks Efficient Photocatalytic H 2 O 2 Production

Photocatalytic H₂O₂ production from H₂O and O₂ is an energy-efficient and environmentally friendly process. While single-atom catalysts (SACs) offer superior selectivity over nanoparticle-based systems, their H₂O₂ production efficiency is often limited by the electron deficiency of isolated metal sites. Herein, N-heterocyclic carbene (NHC) ligands are used to tailor the electronic structure of single Ag atoms on TiO₂, leading to a significant enhancement in photocatalytic performance through axial coordination engineering. The optimized IPr_0.5@Ag₁/TiO₂ catalyst achieves a 5-fold increase in the H₂O₂ production rate (15.06 mmol g^-1 h^-1) compared to pristine Ag₁/TiO₂ (2.64 mmol g^-1 h^-1), outperforming most semiconductor-based photocatalysts. In situ and time-resolved characterizations, along with theoretical calculations, reveal that the electron transfer from NHC ligands to single Ag atoms promotes efficient charge separation and facilitates O₂ activation to selectively generate key O₂^- intermediates, thus accelerating overall reaction kinetics. This work establishes axial NHC coordination as an effective strategy for modulating the electronic structure of SACs, offering a promising approach for the development of high-performance photocatalytic systems for sustainable chemical synthesis.