Promoting Photocatalytic Activity of NH 2 -MIL-125(Ti) for H 2 Evolution Reaction through Creation of Ti III - and Co I -Based Proton Reduction Sites.

Titanium-based metal-organic framework, NH 2 -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 NH 2 -MIL-125(Ti) via impregnation of Co(NO 3 ) 2 . The resulting Co@NH 2 -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 NH 2 -MIL-125(Ti) offers a highly stable visible-light-responsive photocatalyst.