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Molybdenum sulfide clusters in zeolite resembling the nitrogenase enzyme s FeMo-cofactor
Dinuclear (Mo2S4) and tetranuclear (Mo4S4) molybdenum sulfide clusters were encapsulated in the pores of NaY zeolite using chemical vapour deposition followed by thermal treatment in reactive gases. X-ray spectroscopy showed Mo4S4 clusters are geometrically similar to the FeMo- cofactor in nitrogenase enzymes, and the obtained MoxSy/NaY phases were found to be stable hydrogenation catalysts.
Two-dimensional, layered transition metal sulfides (TMS) have been industrially applied as hydro- defunctionalisation and hydrogenation catalysts for hydrotreating of crude oil and renewable feedstock . Recently, these materials have gained increasing attention as a potential replacement for noble metals in photo- and electrocatalytic applications . In addition, TMS clusters naturally occur as building blocks for the active sites of enzymes; most prominently in the nitrogenase family, which is the only one currently known to be able to fix N2 from air. The active site of these enzymes is comprised of multiple iron/molybdenum sulfide clusters, most noticeably the so-called FeMo-cofactor .
To mimic the TMS clusters forming the active sites of enzymes, well-defined, molecular molybdenum sulfide clusters were prepared within the pore confinement of a Faujasite-type zeolite . Using Mo(CO)6 as a volatile precursor, Mo was introduced into the pores of NaY zeolite by chemical vapour deposition (CVD) and subsequently treated in H2S/H2 at 400°C to form Mo2S4 clusters. These dinuclear structures can be transformed into tetranuclear Mo4S4 clusters by reductive treatment in H2 atmosphere at 400°C (Figure 97).
The geometric arrangements and the electronic structures of both clusters in the catalysts were studied at beamline
Fig. 97: TMS clusters in nitrogenase FeMo-cofactor (left, adapted from ref ) and in Faujasite-type
zeolite (middle and right).
ID26 using extended X-ray absorption fine- structure (EXAFS) measurements (Figure 98) as well as high-energy-resolution fluorescence- detected X-ray absorption near-edge structure (HERFD-XANES) and valence-to-core (VtC) X-ray emission spectroscopy.
It was shown that di-/tetranuclear clusters can be reversibly transformed into each other by thermal treatment in sulfiding/reducing atmosphere. EXAFS data, in combination with density functional theory (DFT), made it possible to determine the structure of both clusters, revealing that the tetrameric one is structurally similar to the nitrogenase FeMo-cofactor (Figure 97). Electron paramagnetic resonance (EPR) spectroscopy in combination with DFT confirmed that both the NaY encapsulated MoxSy clusters and the FeMo- cofactor  possess unpaired electrons at their Mo centres.
Fig. 98: k3-weighted EXAFS and FT EXAFS of MoxSy/NaY-sulf (a,b), MoxSy/NaY-red (c,d), and MoxSy/NaY-resulf (e,f) catalysts.