Promoting Photocatalytic Activity of NH2-MIL-125(Ti) for H2 Evolution Reaction through Creation of TiIII- and CoI-Based Proton Reduction Sites

Titanium-based metal-organic framework, NH2-MIL-125(Ti), has been widely investigated for photocatalytic applications but has low activity in the hydrogen evolution reaction (HER). In this work, we show a one-step low-cost postmodification of NH2-MIL-125(Ti) via impregnation of Co(NO3)(2). The resulting Co@NH2-MIL-125(Ti) with embedded single-site Co-II species, confirmed by XPS and XAS measurements, shows enhanced activity under visible light exposure. The increased H-2 production is likely triggered by the presence of active Co-I transient sites detected upon collection of pump-flow-probe XANES spectra. Furthermore, both photocatalysts demonstrated a drastic increase in HER performance after consecutive reuse while maintaining their structural integrity and consistent H-2 production. Via thorough characterization, we revealed two mechanisms for the formation of highly active proton reduction sites: nondestructive linker elimination resulting in coordinatively unsaturated Ti sites and restructuring of single Co-II sites. Overall, this straightforward manner of confinement of Co-II cocatalysts within NH2-MIL-125(Ti) offers a highly stable visible-light-responsive photocatalyst.