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

Photocatalytic H 2 O 2 production from H 2 O and O 2 is an energy-efficient and environmentally friendly process. While single-atom catalysts (SACs) offer superior selectivity over nanoparticle-based systems, their H 2 O 2 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 TiO 2, leading to a significant enhancement in photocatalytic performance through axial coordination engineering. The optimized IPr 0.5 @Ag 1 /TiO 2 catalyst achieves a 5-fold increase in the H 2 O 2 production rate (15.06 mmol g –1 h –1 ) compared to pristine Ag 1 /TiO 2 (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 O 2 activation to selectively generate key O 2 – 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.