Dual-Site Photocatalysis With Titanium-Oxo Clusters and Single Metal Atoms for H₂ Evolution

Rational engineering of photocatalysts with spatially separated photophysical and catalytic functions is an effective strategy to alleviate charge recombination and kinetic bottlenecks in solar hydrogen evolution. Herein, we developed a family of atomically engineered metal-organic frameworks (MOFs)-based photocatalysts, denoted as 125-TNQ-M (M = Co, Ni, Cu), constructed by Schiff-base modification of NH₂-MIL-125(Ti) with thionaphthenquinone (TNQ), followed by covalent anchoring of Lewis-acidic single metal atoms onto its framework. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), synchrotron-based x-ray absorption spectroscopy (XAS), and x-ray photoelectron spectroscopy (XPS) analyses confirm the atomic dispersion of M sites and reveal their unique coordination environments. Among them, 125-TNQ-Ni demonstrates the highest hydrogen evolution rate of 13.54 mmol g^-1 h^-1 under visible-light irradiation, far surpassing pristine MOFs. Mechanistic studies combining electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations reveal that Ni single atoms function as efficient electron extraction and proton-reduction centers, while the Ti-oxo clusters act as photoactive charge-generation units. The enhanced activity originates from metal-induced reorganization of the hydrogen evolution pathway and improved charge separation, rather than direct dual-metal synergy. This work establishes a structurally well-defined MOF-based platform for decoupling light harvesting and catalytic functions at the atomic scale.