Abstract
Classification of polyoxometalates (POMs) is based on their chemical composition, basically represented by two general formulae: a) [MmOy]p− b) [XxMmOy]q−, where M is the main transition metal, O is the oxygen atom and X can be a non-metal atom such as Si. Additionally, in the most cases, the structure of the polyoxometalates is derived from a combination of octahedral units MO6 with a central metal atom M and the oxygen atoms placed at their corners. In such octahedra, oxygen atoms allow the condensation between two octahedral units, while one oxygen atom (or max. two atoms) makes double bond with the central metal atom and is not shared with other metal atoms within the complex (terminal oxygens). On the other hand, niobium-containing mesoporous silicates contain mainly MO4 tetrahedra and reveal superior activity in heterogeneous catalysis. Thus, the proper coordination of niobium is crucial for the catalytic activity and will be deeply discussed. The similarity in the catalytic behavior of niobium-polyoxometalates and heterogeneous niobium single-site catalysts in selective oxidations will be demonstrated.
Highlights
The family of polyoxometalates (POMs) consist of different anionic polynuclear metal-oxygen clusters which comprise the edge-sharing and corner-sharing pseudo-octahedral MO6 units that form an ionic core and cover mainly early transition metals (Gumerova and Rompel, 2018)
668 cm−1, that could be attributed to bridging Nb–Ob vibrations from Lindqvist hexaniobate structure. These results suggests that the framework of the catalyst was subjected to the partial decomposition and peroxidation by H2O2 (Li et al, 2017)
The aminografted samples based on supports with high niobium-content revealed much higher activity in catalytic reactions (Knoevenagel condensation and Michael addition) than their counterparts with no incorporated niobium species (Blasco-Jiménez et al, 2009)
Summary
The family of polyoxometalates (POMs) consist of different anionic polynuclear metal-oxygen clusters which comprise the edge-sharing and corner-sharing pseudo-octahedral MO6 units that form an ionic core and cover mainly early transition metals (Gumerova and Rompel, 2018). This compound may be viewed as four {SiW9O34} units supported on {Nb16O30} cluster and it demonstrated high photocatalytic activity for H2 evolution in the presence of Pt species as a co-catalyst (Yang et al, 2014).
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