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

[Image: see text] 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.