Huaqiao Tan

Polyoxometalate-Based Nickel Clusters as Visible Light-Driven Water Oxidation Catalysts

Three new polyoxometalate(POM)-based polynuclear nickel clusters, Na24[Ni12(OH)9(CO3)3(PO4)(SiW9O34)3]·56H2O (1), Na25[Ni13(H2O)3(OH)9(PO4)4(SiW9O34)3]·50H2O (2), and Na50[Ni25(H2O)2OH)18(CO3)2(PO4)6(SiW9O34)6]·85H2O (3) were synthesized and structurally characterized. Compounds 1-3 contain {Ni12}, {Ni13} and {Ni25} core, respectively, connected by the inorganic {OH}, {PO4} and/or {CO3} linkers and encapsulated by the lacunary A-α-{SiW9O34} POM units. Compound 3 represents the currently largest POM-based Ni clusters. All three compounds contain {Ni3O3} quasi-cubane or {Ni4O4} cubane units, which are similar to the natural oxygen-evolving center {Mn4O5Ca} in photosystem II (PSII). Visible light-driven water oxidation experiments with compounds 1-3 as the homogeneous catalysts indicate that all three compounds show good photocatalytic activities. The O2 evolution amount corresponds to a high TON of 128.2 for 1, 147.6 for 2, and 204.5 for 3, respectively. Multiple experiments including dynamic light-scattering, UV-vis absorption, catalysts aged experiments, tetra-n-heptylammonium nitrate (THpANO3) toluene extraction, and capillary electrophoretic measurements results confirm that compounds 1-3 are dominant active catalysts but not Ni(2+) ions(aq) or nickel oxide under the photocatalytic conditions. The above research results indicate a new and all-inorganic polynuclear Ni-based structural model as the visible light-driven water oxidation catalysts.

N-Doped graphene-coated molybdenum carbide nanoparticles as highly efficient electrocatalysts for the hydrogen evolution reaction

In our efforts to explore promising substitutes for Pt-based electrocatalysts for the hydrogen evolution reaction (HER), a new type of molybdenum carbide nanoparticle coated with graphene shells with nitrogen dopants (abbr. MoCx@C-1) is prepared from an entangled polyoxometalate-encapsulated coordination polymer (PECP), [Zn(bimbp)2]3[PMo12O40]2·2H2O (PECP-1) (bimbp = 4,4′-bis(imidazolyl)biphenyl) via the annealing and etching processes. The synergistic effects between highly dispersive MoCx particles, graphene coatings and N dopants in MoCx@C-1 lead to remarkable HER performance in acidic media with a very positive onset potential close to that of commercial 20% Pt/C catalysts, a low Tafel slope of 56 mV dec−1, a high exchange current density of 0.27 mA cm−2, and superior long-term cycle stability. In particular, MoCx@C-1 exhibiting an overpotential of 79 mV at a current density of 10 mA cm−2 represents one of the currently best reported MoCx-based HER electrocatalysts in acidic media. Such performance is also better than that of uncoated MoCx-2 nanoparticles prepared by carburizing another PECP [Bu4N][Zn3(bimb)4Cl(MoO4)][PMoVMoVI11O40]·4H2O (PECP-2) (bimb = 1,4-bis(1-imidazolyl)benzene). This work provides a new feasible route to prepare nanostructured hybrids composed of transition metal carbides, graphene and N dopants with higher HER activity and stability.

N-Carbon coated P-W2C composite as efficient electrocatalyst for hydrogen evolution reactions over the whole pH range

Development of a highly efficient and stable hydrogen evolution reaction (HER) electrocatalyst based on non-precious-metal remains a great challenge. Herein, we report P-modified tungsten carbide encapsulated in N-doped carbon (P-W2C@NC) as an efficient and stable HER electrocatalyst over the whole pH range (0–14), prepared by annealing polyoxometalate (H3PW12O40·xH2O) and dicyandiamide at a high temperature. To achieve a current density of 10 mA cm−2, P-W2C@NC required an overpotential of 89 mV in acid, 63 mV in alkaline solution and 185 mV in neutral solution. This is the first case of a tungsten-carbide-based HER electrocatalyst operating at all pH values with high activity and stability, and might provide some new routes for the exploration of tungsten-based Pt-like electrocatalysts for the HER.

MoP/Mo2C@C: A New Combination of Electrocatalysts for Highly Efficient Hydrogen Evolution over the Entire pH Range

During the exploration of highly efficient noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER), a promising and challenging strategy is to fabricate composite nanocatalysts by finely tuning metal and/or nonmetal element components. Herein, we report a new HER electrocatalyst, which is composed of molybdenum phosphide and molybdenum carbide composite nanoparticles (NPs) coated by few-layer N-doped graphitic carbon shells (denoted as MoP/Mo2C@C). Such a new combination mode of electrocatalysts is realized by a one-step annealing route with the mixture of a Mo/P-based polyoxometalate (POM) and dicyandiamide. On the basis of this method, the simultaneous phosphorization and carbonization in a nanoscale confined space can be easily achieved by the use of POM as the molecular-element-regulating platform. MoP/Mo2C@C exhibits more remarkable HER performance over the whole pH range than those of MoP, Mo2C, and the physical mixture of MoP and Mo2C. The low overpotentials of 89, 136, and 75 mV were obtained at a current density of 10 mA cm-2 in the media of pH = 0, 7, and 14, respectively. Furthermore, MoP/Mo2C@C shows a long-term durability for 14 h over the entire pH range (0-14). Because of the protection of carbon shells, such composite electrocatalyst also possesses better transition-metal tolerance exemplified by Fe2+, Co2+, and Ni2+ than that of 20% commercial Pt/C. This work demonstrates the advantage of POM precursors in adjusting the component and properties of nanoscale composite electrocatalysts for HER, which may suggest new options for the fabrication of highly efficient composite electrocatalysts.

From Racemic Compound to Spontaneous Resolution: A Linker‐Imposed Evolution of Chiral [MnMo9O32]6−‐Based Polyoxometalate Compounds

Five compounds based on [MnMo(9)O(32)](6-): (Himi)(6)[MnMo(9)O(32)] (1) (imi=imidazole), Na(2)(Himi)(4)[MnMo(9)O(32)]2 H(2)O (2), Na(3)(Himi)(3)[MnMo(9)O(32)] (3), D-NH(4)Mn(2.5)[MnMo(9)O(32)]11 H(2)O (4 a), and L-NH(4)Mn(2.5)[MnMo(9)O(32)]11 H(2)O (4 b) were prepared and characterized. X-ray crystallographic analysis revealed that compounds 1 and 2 with imidazole molecules as linkers are racemic compounds; compound 3 is a racemic solid solution of Na(+) cations and the polyoxoanion [MnMo(9)O(32)](6-); and compounds 4 a and 4 b are enantiomers. In compound 4, the homochiral polyoxoanions [MnMo(9)O(32)](6-) are connected by Mn(2+) cations to form a unique (4(5)6)(4(7)6(8)) topology net framework. By adjusting the linkers from imidazole molecules to Na(+) and finally Mn(2+) cations, the chiral polyoxoanions [MnMo(9)O(32)](6-) were changed from a racemic compound to a conglomerate. This means that spontaneous resolution can be efficiently realized by connecting homochiral polyoxoanions into one-dimensional (1D), 2D, and 3D structures, with an emphasis on using appropriate linkers with substantial interaction strength, directionality, and enantioselectivity.

Two diphosphonate-functionalized asymmetric polyoxomolybdates with catalytic activity for oxidation of benzyl alcohol to benzaldehyde.

Two asymmetric polyoxomolybdates Na(6){Mo(2)O(5)[(Mo(2)O(6))NH(3)CH(2)CH(2)CH(2)C(O)(PO(3))(2)](2)}·16H(2)O (1) and (NH(4))(7)Na{MoO(2)[(Mo(2)O(6))NH(3)CH(2)CH(2)CH(2)C(O)(PO(3))(2)]}(4)·H(2)O (2) have been synthesized by the reactions of alendronic acid with molybdate. Structure analysis revealed that the polyoxoanions 1 and 2 can be described as dimeric and tetrameric aggregates of the {MoO(3)[(Mo(2)O(6))NH(3)CH(2)CH(2)CH(2)C(O)(PO(3))(2)]} units respectively. Their tetrabutylammonium salts show efficient selective oxidation of benzyl alcohol to benzaldehyde with 72.5% and 81.5% benzyl alcohol conversion, and 87.1% and 82.4% benzaldehyde selectivity, respectively.

Spontaneous resolution of a new diphosphonate-functionalized polyoxomolybdate

A new diphosphonate-functionalized polyoxomolybdate L,D-(NH4)6{MoV2O4[(MoVI2O6)NH3CH2CH2CH2C(O)(PO3)2]2}·10H2O (1) has been synthesized by the reaction of alendronic acid with {Mo2O4(H2O)6}2+ fragments. In the structure, a restriction of “folding” alkyl induced chiral polyoxometalate has been observed.

Resolution of chiral polyoxoanion [P2Mo18O62]6− with histidine

The classical Dawson-type racemic polyoxoanions [P2Mo18O62]6− have been separated in aqueous solution with L,D-histidine, respectively. Two enantiomerically pure compounds: H3(L-HC6H9N3O2)3[P2Mo18O62]·20H2O (1a), H3(D-HC6H9N3O2)3[P2Mo18O62]·20H2O (1b), have been synthesized.

Polyoxometalate-based metal–organic coordination networks for heterogeneous catalytic desulfurization

Three new polyoxometalate (POM)-based metal–organic coordination networks (MOCNs) with chemical formulae of [Co(BBTZ)1.5(HBBTZ)(H2O)2][PW12O40]·H2O (1), [Co2.5(BBTZ)4(H2O)2] [BW12O40]·4H2O (2) and [Cu(BBTZ)2]5[BW12O40]2·4H2O (3) (BBTZ = 1,4-bis-(1,2,4-triazol-1-ylmethyl)benzene) were hydrothermally synthesized in a reaction system containing Keggin-type POMs, transition metal salts (cobalt salts and copper salts) and BBTZ ligands. All compounds were characterized by elemental analyses, IR, powder X-ray diffraction, TG analyses and single-crystal X-ray diffraction analyses. Compound 1 exhibits a POM-encapsulated 3-D supramolecular network, while compounds 2 and 3 display POM-supported 3-D coordination networks. Using the oxidative desulfurization of dibenzothiophene (DBT) as the model, the catalytic activities of compounds 1–3 are investigated. All three compounds show efficient catalytic activity for the oxidation of DBT with the order of 2 > 3 > 1. It is found that the POM species of compounds 1–3 play the main role in the catalytic oxidative desulfurization process, while the TM ions, the loading amounts of POMs, and the structural features of these POM-based MOCNs are also necessary factors that affect the catalytic activities. Furthermore, a surfactant-assisted hydrothermal synthesis method has been developed to prepare nanocrystal 2. SEM reveals that the as-synthesized nanocrystalline 2 is about 245 nm in diameter. The catalytic oxidative desulfurization experiments show that nanocrystal 2 possesses much higher catalytic activities than those of the large single-crystal products of 2.

Keggin‐Type Polyoxometalate‐Based Metal–Organic Networks for Photocatalytic Dye Degradation

The reaction of Keggin-type polyoxometalate (POM) units, transition-metal (TM) ions, and a rigid bis(imidazole) ligand (1,4-bis(1-imidazolyl)benzene (bimb)) in a hydrothermal environment led to the isolation of four new POM-based metal-organic networks, [H2 L][CuL][SiW12 O40 ]⋅2 H2 O (1), [H2 L]2 [Co(H2 O)3 L][SiW11 CoO39 ]⋅6 H2 O (2), KH[CuL]2 [SiW11 CoO39 (H2 O)]⋅2 H2 O (3), and [CuL]4 [GeW12 O40 ]⋅H2 O (4; L=bimb). All four compounds were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. Compounds 1 and 3 are new 3D networks with 1D channels. Compounds 2 and 4 contain 2D networks, which further stack into 3D supramolecular networks. The contributions of pH value, the negative charge of the POM, and the TM coordination modes to the construction of 3D networks were elucidated by comparing the synthetic conditions and structures of compounds 1-4. The photocatalytic properties of compounds 1-4 were investigated using methylene blue (MB) degradation under UV light. All compounds showed good catalytic activity and structural stability. The possible catalytic mechanism was discussed on the basis of active-species trapping experiments. The different photocatalytic activities of compounds 1-4 were explained by comparison of the band gaps of different POM species and different packing modes of POM units in these hybrid compounds.