Molybdenum Sulfide within a Metal–Organic Framework for Photocatalytic Hydrogen Evolution from Water

Abstract

A representative metal-organic framework, NU-1000, was functionalized with MoS x. The previously determined crystal structure of the material, named MoS x-SIM, consists of monometallic Mo(IV) ions with two sulfhydryl ligands. The metal ions are anchored to the framework by displacing protons presented by the-OH/-OH 2 groups on the Zr 6 node. As shown previously, the MOF-supported complexes are electrocatalytic for hydrogen evolution from acidified water. The earlier electrocatalysis results, together with the nearly ideal formal potential of the Mo(IV/II) couple (i.e., nearly coincident with that of the hydrogen couple), and the physical proximity of UV-absorbing MOF linkers to the complexes, suggested to us that the linkers might behave photosensitizers for catalyst reduction, and subsequently, for H 2 evolution from water. To our surprise, MoS x-SIM, when UV-illuminated in an aqueous buffer at near-neutral pH, displays a biphasic photocatalytic response: an initially slow rate of reaction, i.e. 0.56 mmol g −1 h −1 , followed by an increase to 4 mmol g −1 h −1. Ex-situ catalyst examination revealed that nanoparticulate MoS x suspended within the reaction mixture is the actual catalyst. Thus, photo-assisted restructuring and detachment of the catalyst or pre-catalyst from the MOF node appears to be necessary for the catalyst to reduce water at neutral pH. Molecular hydrogen is a compound of undeniable global importance given its high demand as a feedstock in the industrial scale production of hydrocarbons and ammonia (i.e. Fisher-Tropsch and Haber-Bosch processes, respectively) 1-5 and as a reactant in fuel cells, 6,7 a technology of existing and increasing commercial significance. The overwhelming majority of commercially used H 2 is obtained by methane steam reforming or coal gasification 8-low-cost technologies that unfortunately are complicated by the formation of greenhouse gases, i.e. CO 2 and CO. 9-11 In this regard, development of effective photo-, 12-15 electro-, 16-18 or photoelectrochemical hydrogen evolution catalysts 19-21 (i.e., by converting water into H 2) is of utmost interest. While the in silico predicted and experimentally proven effective catalysts typically contain of noble metals, most commonly platinum, 19,22 selected transition-metal chalcogenides, namely sul-fides and selenides, have also proven functional as catalysts for the hydrogen evolution reaction (HER). 23,24 In particular, molybdenum disulfide has been extensively examined as an HER catalyst given its anomalous morphology-activity relationships. Experimental work by Jaramillo et al., 24 and computational studies by Tsai et al., 23,25 show that the catalysis primarily happens at the undercoordinated Mo sites (that form molybdenum-hydride species during the catal-ysis) solely presented at the so-called "crystallographic edge sites." Thus, amorphization of MoS x materials has been used as a strategy to increase the surface density of the edge sites and thus increase the catalytic performance compared to crystalline catalysts. 26-28 Unfortunately , the structural ambiguity of amorphous materials makes difficult the quantitative exploration of potentially mechanistically instructive structure-activity relationships. We propose crystalline metal-organic frameworks (MOFs) as an attractive and well-defined platform for satisfying the aforementioned criteria. MOFs that present a high density of labile protons, typically in the form of-OH/-OH 2 groups ligated to metal-oxo nodes, are of particular interest. We have reported elsewhere that post-synthetic exposure of MOF-node-sited aqua and hydroxo groups to highly reac-tive metal complexes (for example, metal alkyls), either in the vapor phase or inert condensed phase (e.g., heptane), results in their chemical attachment to the node-often as polynuclear clusters. Subsequent