Copper-Sulfur-Nitrogen Cluster Providing a Local Proton for Efficient Carbon Dioxide Photoreduction

Atomically precise Cu clusters are highly desirable as catalysts for CO₂ reduction reaction (CO₂RR), and they provide an appropriate model platform for elaborating their structure–activity relationship. However, an efficient overall photocatalytic CO₂RR with H₂O using assembled Cu-cluster aggregates as heterogeneous photosensitive semiconductors has not been reported. Herein, we report a stable crystalline Cu–S–N cluster photocatalyst with local protonated N–H groups (denoted as Cu₆–NH). The catalyst exhibits suitable photocatalytic redox potentials, high structural stability, active catalytic species, and a narrow bandgap, which account for its outstanding photocatalytic CO₂RR performance under visible light, with ~100% selectivity for CO evolution. Remarkably, systematic isostructural Cu-cluster control experiments, in situ infrared spectroscopy, and density functional theory calculations revealed that the protonated pyrimidine N atoms in the Cu₆–NH cluster act as a proton relay station, providing a local proton during the photocatalytic CO₂RR. This efficiently lowers the energy barrier for the formation of the *COOH intermediate, which is the rate-limiting step, efficiently enhancing the photocatalytic performance. This work lays the foundation for the development of atomically precise metal-cluster-based photocatalysts.