Metal-organic frameworks (MOFs) provideatunable platform for hierarchically integrating multiple components to effect synergistic functions that cannot be achieved in solution. Here we report the encapsulation of aN i-containing polyoxometalate (POM) [Ni 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 10À (Ni 4 P 2 ) into two highly stable and porous phosphorescent MOFs. The proximity of Ni 4 P 2 to multiple photosensitizers in Ni 4 P 2 @MOF allows for facile multi-electron transfer to enable efficient visible-light-driven hydrogen evolution reaction (HER) with turnover numbers as high as 1476. Photophysical and electrochemical studies established the oxidative quenching of the excited photosensitizer by Ni 4 P 2 as the initiating step of HER and explained the drastic catalytic activity difference of the two POM@MOFs.O ur work shows that POM@MOF assemblies not only provide at unable platform for designing highly effective photocatalytic HER catalysts but also facilitate detailed mechanistic understanding of HER processes.Metal-organic frameworks (MOFs) have attracted considerable attention for their unprecedentedly high surface areas and well-defined pore structures that can be readily tuned via judicious selection of bridging ligands and metal-connecting nodes. [1,2] In particular, potential applications of MOFs in gas storage and separations have been extensively studied in the past decade. [2, 3] By introducing catalytically competent moieties via either the bridging ligands or the metal-connecting nodes,MOFs have also emerged as anew class of recyclable and reusable single-site solid catalysts for ab road range of organic transformations. [4, 5] Encapsulation of metal and metal-oxide (such as Au,P d, Pt, ZnO) nanoparticles in MOF cavities has recently been demonstrated to be another effective strategy for preparing heterogeneous catalysts with high activities. [6,7] Photocatalytic hydrogen evolution reaction (HER) is an essential half reaction of water splitting, [8, 9] in which effective generation of charge-separated excited states (electron-hole pairs) is followed by facile transfer of the electrons to HER centers to reduce proton to hydrogen.[10] Recent studies have shown the feasibility of integrating the two essential components-the photosensitizer and the HER catalyst-into MOFs to enable light-driven proton reduction. [11][12][13] For example,b yl oading noble metal Pt nanoparticles into the cavities of ap hotosensitizing MOF,w es howed that the Pt@MOF system facilitated electron injection from the MOF framework to the encapsulated Pt nanoparticles to enable photocatalytic proton reduction with ahigh turnover number of ca. 7000. [13] As al arge family of nano-sized inorganic clusters with oxygen-rich surfaces,p olyoxometalates (POMs) can readily undergo multi-electron reduction and oxidation processes, thus representing excellent candidates as HER and water oxidation catalysts. [14,15] In order to transition to HER catalysts based on earth-abundant elements,w er ecently used the noble-metal-free Wells-D...
