Synthesis, Assemblies, And Photocatalytic Hydrogen Production Properties Of Novel Niobium- And Tantalum-Containing High-Nuclearity Metal-Oxygen Clusters

In view of the compositions and structures being similar to metal-oxide semiconductor photocatalysts, niobium- and tantalum-containing high-nuclearity metal-oxygen clusters have attracted great research interest. At present, compared with other polyoxometalates(POMs), such as polyoxotungstates, polyoxomolybdates, and polyoxovanadates, the development of niobium- and tantalum-containing high-nuclearity metal-oxygen clusters is still at its early stage, owing to the difficulty of controlling synthetic conditions. The aim of this thesis is to synthesize novel niobium- and tantalum-containing high-nuclearity metal-oxygen clusters under conventional reaction conditions, hydrothermal method, and steam-assisted conversion. Furthermore, the photocatalytic H2 evolution activities were observed for these compounds.1. Three novel isopolyoxoniobates(1-3) with molecular triangle, molecular square and cuboctahedral molecular cage geometry have been successfully synthesized by conventional aqueous methods,in which K7HNb6O19·13H2O was employed as precursors, and the p H value of the mixture was adjusted by Na OH(1 mol L-1).KNa2[Nb24O72H21]·38H2O(1)K2Na2[Nb32O96H28]·80H2O(2)K12[Nb24O72H21]4·107H2O(3)Compound 1 is the first isolated molecular triangle {Nb24O72} cluster. While, compound 2 is the largest isopolyoxoniobate cluster reported to date. Compound 3 is the largest solid aggregation of polyoxoniobates. To our knowledge, it is the first time these polyoxoniobate clusters are crystallized with only alkali counterions, thereby standing a chance of being redissolved in water. ESI-MS spectra indicate that compounds 1 and 2 remain structural integrity when the pure, crystalline polyanion salts are dissolved in water. The UV-Vis diffuse reflectance spectra of these powder samples indicate that the corresponding well-defined optical absorption associated with Eg can be assessed at 3.35, 3.17, and 3.34 e V, respectively, revealing the presence of an optical band gap and the nature of semiconductivity with a wide band gap. UV-light photocatalytic H2 evolution activities were observed for these compounds. The H2 evolution rates of 1-3 were 5161.4, 5312.5, 4804.1 μmol h-1 g-1, respectively.2. A new organic-inorganic hybrid 4, was synthesized from an aqueous mixture of K7HNb6O19·13H2O, Na2 Si O3·9H2O, Cu(Ac)2·3H2O, en, H2 O under hydrothermal conditions at 160 for three days.℃[Cu(en)2]3{[Cu(en)2][H6Si Nb18O54]}·22H2O(4)Compound 4 contains the crescent-shaped polyoxoanion [H6Si Nb18O54]8– and four copper-organic cations, and elemental analyses, IR spectra, thermogravimetric analyses and single-crystal X-ray diffraction were investigated. To our knowledge, 4 was the first organic-inorganic hybrid based on the crescent-shaped polyoxoanion [H6Si Nb18O54]8–.3. Three unprecedented V and Nb coexisting as mixed addenda polyoxoniobates(5-7) have been successfully synthesized by conventional aqueous methods.{[Cu6L6(H2O)3][Nb10V4O40(OH)2]}2·13H2O(5)[Na(H2en)5][VNb14O42(NO3)2]·12H2O(6)[K7Na4][VNb14O42(NO3)2]·27H2O(7)Compound 5 contains {Nb10V4O40(OH)2}12– cluster. The {[Cu6L6(H2O)3][Nb10V4O40(OH)2]}2·13H2O molecules close-pack in the solid-state to form a 3D supramolecular network via π–π stacking interaction between the neighboring 1,10-phenanthroline ligands with the distance of 3.472 ?(face to face). Interestingly, there exists one-dimensional quadrangular channels of approximately 6.9 × 10.9 ? along the [-1 1-2] direction, that are occupied by the free water molecules. The solid-state luminescence property of 5 is also investigated. 6 and 7 all constructed by classic Keggin-type {VNb12O40} capped by two novel {Nb O7} pentagonal bipyramids. To study the adsorption properties, the water, methanol and ethanol adsorption isotherms for compound 6 were measured at 298 K. The adsorption studies show that compound 6 will be a promising material for separating water from alcohols. The catalyst solution of 7 was irradiated under UV irradiation using a 150 W mercury lamp. The H2 evolution rate is 998.56 μmol h-1 g-1. The total evolved H2 during 16 h was 1597.69 μmol, and the corresponding turnover number [(moles of H2 formed)/(moles of 7)] was 48.4. Utilizing the assembly of W/Nb mixed-addendum polyoxometalates and transition-metal cations(Cr3+, Fe3+) in aqueous solution, we isolated three sandwich-like compounds with unprecedented architectures:Cs5KH[(Si2W18Nb6O78)Cr(H2O)4]·11H2O(8)Cs4[(Si2W18Nb6O78)Cr2(H2O)8]·14H2O(9)Cs4K4H[(Si2W18Nb6O78)Fe Cl2(H2O)2]·13.5H2O(10)The catalyst solution was irradiated under a 500 W Xe lamp with a 400 nm cutoff filter. The H2 evolution rates of 9 and 10 were 20.2, 7.8 μmol h-1 g-1, respectively. To the best of our knowledge, these are the first W/Nb mixed-addendum polyoxometalates photocatalysts reported with visible-light activity for H2 evolution reaction.5. A series of unprecedented polytantalotungstates(POTTs)(11-16) based on the Dawson-type {P2W15Ta3O62} and Keggin-type {Si W9Ta3O40} nanoclusters, were synthesized by steam-assisted conversion.Cs5K4{Cr3[Ta3P2W15O62]2(H2O)12}·24H2O(11)Cs4K10Na8H2{Cr4[Ta3P2W15O62]4(H2O)12}·11H2O(12)Cs16K16Na4[Ta18P12W90(OH)6(H2O)2O360]·24H2O(13)Cs26K2H2[Yb2Ta18P12W90(OH)6(H2O)16O360]·52H2O(14)Cs12.5K4.5H2[Ta13Si4W36O158]·25H2O(15)Cs12K3H6[Mn Ta18Si6W54O231]·61H2O(16)To our knowledge, compound 11 and 12 are the first transition metal-containing polytantalotungstates. Whereas compounds 13 and 14 are unprecedented hexamers of {P2W15Ta3O62} and all contain the largest {Ta18} core. The {Ta18} anion reported herein breaks the record and represents the largest polyoxotantalate cluster in POTTs so far discovered. Compounds 15 and 16 are novel tetramer and hexamer of {Si W9Ta3O40} fundamental building units containing respectively an unprecedented {Ta13} and {Mn Ta18} cluster. The catalyst solution of 12 was irradiated under a 300 W Xe lamp with a 420 nm cutoff filter. The H2 evolution rates in three runs were 91.78, 97.66, and 79.32 μmol h-1 g-1, respectively. The photocatalytic activities of 12 and 13 under UV irradiation using a 500 W mercury lamp were also studied. For compounds 12 and 13, the H2 evolution rates were 1401.3, 2244.4 μmol h-1 g-1, respectively.