Showing papers on "Polyoxometalate published in 2021"

Polyoxometalate-Based Metal-Organic Framework as Molecular Sieve for Highly Selective Semi-Hydrogenation of Acetylene on Isolated Single Pd Atom Sites.

Abstract: Achieving highly selective acetylene semi-hydrogenation in an ethylene-rich gas stream is of great industrial importance. Herein, we construct isolated single Pd atom in a polyoxometalate-based metal-organic framework (POMOF). The unique internal environment allows this POMOF to separate acetylene from acetylene/ethylene gas mixtures and confine it close to the single Pd atom. After semi-hydrogenation, the resulting ethylene is preferentially discharged from the pores, achieving a selectivity of 92.6 %. First-principles simulations reveal that the adsorbed acetylene/ethylene molecules form hydrogen bond networks with oxygen atoms of SiW12 O404- and create dynamic confinement regions, which preferentially release the produced ethylene. Besides, at the Pd site, the over-hydrogenation of ethylene exhibits a higher reaction energy barrier than the semi-hydrogenation of acetylene. The combined advantages of POMOF and single Pd atom provides an effective approach for the regulation of semi-hydrogenation selectivity.

Facile integration of Ni-substituted polyoxometalate catalysts into mesoporous light-responsive metal-organic framework for effective photogeneration of hydrogen

Abstract: The integration of functional homogeneous polyoxometalate (POM) complexes into highly-dispersed and recyclable heterogeneous catalysts remains a substantially challenging research topic in solar-driven water splitting area. In this work, we report a facile and broad-spectrum impregnation method to construct two POM@MOF composites, Ni3PW10@NU-1000 and Ni3P2W16@NU-1000, by respectively incorporating a tri-Ni-substituted Keggin-type (K6Na[Ni3(H2O)3PW10O39H2O], denoted as K6Na-Ni3PW10) and a Wells-Dawson-type (Na4Li5[Ni3(OH)3(H2O)3P2W16O59], denoted as Na4Li5-Ni3P2W16) polyoxometalate into a mesoporous Zr-based metal-organic framework (NU-1000). Under minimally optimized conditions, the resulting POM@MOF composites can effectively photocatalyze hydrogen production with superior long-term stability and reusability, achieving a hydrogen evolving rate of 3482 and 13051 μmol g−1 h−1 for Ni3PW10@NU-1000 and Ni3P2W16@NU-1000, respectively. Various photophysical and spectroscopic analyses elucidated the possible photocatalytic mechanism and the catalytic difference between two POM@MOF composites.

Atomically dispersed Ni on Mo2C embedded in N, P co-doped carbon derived from polyoxometalate supramolecule for high-efficiency hydrogen evolution electrocatalysis

Abstract: Herein, a novel supramolecular-confinement pyrolysis strategy was proposed to construct ultrafine Ni-Mo2C nanoparticles uniformly distributed on N,P co-doped carbon (NPC) by using polyoxometalate supramolecular aggregate as precursor. Ni-Mo2C/NPC exhibits remarkable electrocatalytic activity for hydrogen evolution reaction (HER) in both acidic and alkaline conditions, as well as superior long-term durability. Combining EXAFS with DFT calculations, we demonstrate that Ni species were atomically dispersed and anchored by Mo2C lattice and P atoms from NPC, thus defining Mo(C)–Ni–P active sites, which enhance the intrinsic catalytic activity of Mo2C by atomic-scale Ni doping and Ni–P catalyst-substrate chemical coupling. The Mo(C)–Ni–P sites have optimal hydrogen adsorption free energies and can precisely regulate and activate the neighboring C, leading to excellent HER performance. This work developed a “bottom-up” polyoxometalate-based supramolecular approach for in-situ assembly of high-active HER catalytic sites, which will provide opportunities to design and fabricate low-cost and efficient atomic-scale catalysts for clean energy conversion.

MoO3 crystal facets modulation by doping heteroatom Fe from polyoxometalate for quasi-industrial oxygen evolution reaction

Abstract: The widespread implementation of water splitting urgently requires low-cost, effective and stable electrocatalysts for the industrial oxygen evolution reaction (OER) that can reach a large current density (>500 mA cm−2). Herein, we report a facile solvothermal strategy to fabricate a binder-free electrode of Fe-S-NiMoO4/MoO3 with nanorods array uniformly grown on a nickel-foam (NF). The Anderson-type polyoxometalate (NH4)3[FeMo6O24H6]·7H2O as the precursor has enabled an accurate and controllable Fe-doping in transition metal oxides (TMO). The combination of experiments and density functional theory (DFT) calculations reveal that Fe-doping is conductive to regulate the crystal plane of MoO3, modulate the electronic structure and improve the conductivity. Consequently, the Fe-S-NiMoO4/MoO3@NF electrode can yield 500 mA cm−2 at an overpotential of 271 mV and work steadily over 100 h under 50 °C that can meet the requirements of the industrial water electrolysis. This study provides a practical approach to design electrocatalysts for future industrial applications.

Ring-Shaped Polyoxometalate Built by {Mn4PW9} and PO4 Units for Efficient Visible-Light-Driven Hydrogen Evolution

Abstract: A novel mixed-valence, ring-shaped multinuclear Mn-containing polyoxometalate, [H2N(CH3)2]15NaH8-[MnIII3MnIV(μ3-O)3(OAc)PO4(B-α-PW9O34)]4·36H2O (1) was made and systematically characterized using v...

High Proton-Conductivity in Covalently Linked Polyoxometalate-Organoboronic Acid-Polymers.

Abstract: The controlled bottom-up design of polymers with metal oxide backbones is a grand challenge in materials design, as it could give unique control over the resulting chemical properties. Herein, we report a 1D-organo-functionalized polyoxometalate polymer featuring a purely inorganic backbone. The polymer is self-assembled from two types of monomers, inorganic Wells-Dawson-type polyoxometalates, and aromatic organo-boronates. Their covalent linkage results in 1D polymer strands, which combine an inorganic oxide backbone (based on B-O and Nb-O linkages) with functional organic side-chains. The polymer shows high bulk proton conductivity of up to 1.59×10-1 S cm-1 at 90 °C and 98 % relative humidity. This synthetic approach could lead to a new class of organic-inorganic polymers where function can be designed by controlled tuning of the monomer units.

Polyoxometalate-Based Frameworks as Adsorbents for Drug of Abuse Extraction from Hair Samples

Abstract: The linkage of molecular components into functional heterogeneous framework materials has revolutionized modern materials chemistry. Here, we use this principle to design polyoxometalate-based frameworks as high affinity adsorbents for drugs of abuse, leading to their application in solid-phase extraction analysis. The frameworks are assembled by the reaction of a Keggin-type polyanion, [SiW12O40]4-, with lanthanoids Dy(III), La(III), Nd(III), and Sm(III) and the multidentate linking ligand 1,10-phenanthroline-2,9-dicarboxylic acid (H2PDA). Their reaction leads to the formation of crystalline 1D coordination polymers. Because of the charge mismatch between the lanthanoids (+3) and the dodecasilicotungstate (-4), we observe incorporation of the PDA2- ligands into crystalline materials, leading to four polyoxometalate-based frameworks where Keggin-type heteropolyanions are linked by cationic {Lnn(PDA)n} groups (Ln = Dy (1), La (2), Nd (3), and Sm (4)). Structural analysis of the polyoxometalate-based frameworks suggested that they might be suitable for surface binding of common drugs of abuse via supramolecular interactions. To this end, they were used for the extraction and quantitative determination of four model drugs of abuse (amphetamine, methamphetamine, codeine, and morphine) by using micro-solid-phase extraction (D-μSPE) and high-performance liquid chromatography (HPLC). The method showed wide linear ranges, low limits of detection (0.1-0.3 ng mL-1), high precision, and satisfactory spiked recoveries. Our results demonstrate that polyoxometalate-based frameworks are suitable sorbents in D-μSPE for molecules containing amine functionalities. The modular design of these networks could in the future be used to expand and tune their substrate binding behavior.

Polyoxometalate driven dendrite-free zinc electrodes with synergistic effects of cation and anion cluster regulation

Abstract: Even rechargeable aqueous Zn-ion batteries are regarded as one of the promising secondary batteries due to their high safety and low cost, but their practical applications are still hampered by the formation of zinc dendrites. Thus, in this work, we employ a classic Anderson-type polyoxometalate for the first time, (NH4)6[Mo7O24]·4H2O (NMO), as a stable electrolyte additive during repeated Zn plating/striping to significantly suppress zinc dendrite growth. The [Mo7O24]6− anion clusters can bind with massive Zn ions to produce a Zn-rich film, thus ensuring the uniform distribution of Zn ions on the surface of the Zn anode, while the NH4+ cation can further regulate the even Zn deposition during plating via electrostatic repulsion forces. As a result, the cycle life of Zn anodes in symmetric cells has improved approximately 100% in a wide range of current densities and the Zn/NaV3O8·1.5H2O (Zn/NVO) full cells also demonstrate high capacity retention and excellent fast charge/discharge capacity. The utilization of ultra-stable polyoxometalate additives with strong complexation with metal ions provides researchers with a new conception for constructing long-term cycling life aqueous rechargeable batteries.

Multimetal-based nitrogen doped carbon nanotubes bifunctional electrocatalysts for triiodide reduction and water-splitting synthesized from polyoxometalate- intercalated layered double hydroxide pyrolysis strategy

Abstract: Over the years, transition metal-based hybrids have been advocated as one of the most promising classes of non-noble catalysts and investigated exclusively for the electrocatalytic triiodide reduction reaction (IRR) or hydrogen evolution reaction (HER). However, there is lack of effective strategy to address their site accessibility, intrinsic activity and structural stability issues, and their successful utilization as bifunctional catalysts of IRR and HER is rarely explored. Herein, presented is a multimetal-based carbon nanotube hybrid (CoFeNiMo@NCNT), synthesized from a novel polyoxometalate (POM)-intercalated layered double hydroxide (LDH) pyrolysis strategy, as an efficient catalyst for IRR and HER. During both electrochemical IRR and HER, the regulated NCNT provides a highway for electron migration and acts as a network for I3−/H+ adsorption. Benefitting from the features of porous structure, multimetal component and fast electron transfer, the CoFeNiMo@NCNT catalyst delivers a high power conversion efficiency of 6.46 % when assembled as counter electrode in dye-sensitized solar cell, superior to Pt benchmark. Furthermore, it mediates efficient HER with a relatively small overpotential of 209.9 mV at 10 mA cm-2. The strategy to achieve tunable catalytic properties via in situ pyrolysis well-defined inorganic materials paves a new way to design cost-effective and efficient bifunctional catalysts.

Temperature sensors based on europium polyoxometalate and mesoporous terbium metal–organic framework

Abstract: The first example of ratiometric luminescent thermometers based on lanthanide molecular species encapsulated in a lanthanide metal–organic framework (MOF) is reported. In EuW10@Tb-TATB, the [EuW10O36]9− polyoxometalate (POM) is incorporated into the cavities of the mesoporous terbium MOF Tb-TATB with two different POM loadings (9.1 and 19.5 wt%). Eu3+ and Tb3+ ions of the POM and MOF, respectively, act as dual emitters. Optical measurements evidence efficient Tb3+-to-Eu3+ energy transfer, suggesting that the EuW10 units are close to terbium centers. These observations are supported by computational investigations whereby a favored localisation of the EuW10 POM in the MOF's pores is found close to Tb-centers evidencing hydrogen bond type interactions between terminal oxygen atoms of the POM and Tb-coordinated water molecules. The reported materials act as temperature sensors in the physiological domain, exhibiting high relative thermal sensitivities of 2.68% K−1 and 2.37% K−1 at 300 K for the 9.1 wt% and 19.5 wt% composites, respectively, with a thermal uncertainty of 0.09 K.

A Polyoxometalate-Based Inorganic Porous Material with both Proton and Electron Conductivity by Light Actuation: Photocatalysis for Baeyer-Villiger Oxidation and Cr(VI) Reduction

Abstract: Two-dimensional (2D) crystalline porous materials with designable structures and high surface areas are currently a hot research topic in the field of proton- and electron-conducting materials, which provide great opportunities to orderly accommodate carriers in available spaces and to accurately understand the conducting path. The 2D dual-conductive inorganic framework [Co(H2O)6]2{[Co(H2O)4]4[WZn3(H2O)2(ZnW9O34)2]}·8H2O (Co6Zn5W19) is synthesized by combining [WZn3(H2O)2(ZnW9O34)2]12- (Zn5W19) and a Co(II) ion via a hydrothermal method. Due to the presence of a consecutive H-bonding network, electrostatic interactions, and packing effects between the framework and guest molecules, Co6Zn5W19 displays a high proton conductivity (3.55 × 10-4 S cm-1 under 98% RH and 358 K) by a synergistic effect of the combined components. Additionally, a photoactuated electron injection into the semiconducting materials is an important strategy for switching electronic conductivity, because it can efficiently reduce the frameworks without destroying the crystallinity. I-V curves of a tablet of Co6Zn5W19 in the reduced and oxidized states yield conductivities of 1.26 × 10-6 and 5 × 10-8 S cm-1, respectively. Moreover, Co6Zn5W19 is also successfully applied in the photocatalytic reduction of the toxic Cr(VI) metal ion by utilizing its excellent electronic storage capacity and Baeyer-Villiger (BV) oxidation in a molecular oxygen/aldehyde system.

A polyoxometalate@covalent triazine framework as a robust electrocatalyst for selective benzyl alcohol oxidation coupled with hydrogen production

Abstract: Electrocatalytic oxidation has been proven as a sustainable and promising alternative to traditional chemical transformation, but its further development is limited by the use of noble-metal electrocatalysts. Herein, a polyoxometalate-based electrode material, H5PMo10V2O40@CTF (denoted as PMo10V2@CTF), has been successfully fabricated through electrostatic assembly of a molecular polyoxometalate catalyst, PMo10V2, with a porous cationic covalent triazine framework (CTF), which, to our knowledge, represents the first combination of polyoxometalate with a cationic CTF. The resulting PMo10V2@CTF exhibits high activity for the selective electrocatalytic oxidation of alcohols to aldehydes, achieving 99% conversion of benzyl alcohol, over 99% selectivity of benzyl aldehyde, and at the same time near unity H2 production. Notably, the reported electrocatalytic system presents good atom economy, high energy conversion (96% faradaic efficiency), remarkable catalytic activity and robustness for at least eight recycles. Based on the various experimental and spectroscopic analyses, a possible catalytic mechanism was proposed, revealing that such excellent electrocatalytic performance is attributed to the versatile redox ability of PMo10V2 and the good porosity and adsorption property of the CTF in the constructed PMo10V2@CTF composite.

Two new polyoxometalate-based metal-organic complexes for the detection of trace Cr(VI) and their capacitor performance.

Abstract: Two new Keggin-type polyoxometalate (POM)-based metal-organic complexes (MOCs) H3[Cu2(4-dpye)2(PMo12O40)] (1) and H[Cu2(4-Hdpye)2(PMo12O40)(H2O)4]·2H2O (2) were constructed with a new N,N'-bis (4-pyrimidinecarboxamido)-1,2-ethane (4-H2dpye) ligand by the hydrothermal/solvothermal method. Complex 1 was a 2D layered structure constructed from 1D metal-organic chains [Cu(4-dpye)]n and Keggin-type [PMo12O40]3- polyoxoanions. Complex 2 displays a 3D supramolecular framework formed by discrete [PMo12O40]3- polyoxoanions and binuclear metal-organic loops [Cu2(4-Hdpye)2]. The electrocatalytic behaviors of carbon paste electrodes modified by complexes 1 and 2 (1-CPE and 2-CPE) were investigated. The 1-CPE and 2-CPE were used as electrochemical sensors to detect trace Cr(vi), and the low limits of detection (LOD) are 1.27 × 10-7 M for 1 and 1.71 × 10-7 M for 2, which are lower than the maximum allowable concentration of Cr(vi) in drinking water specified by the World Health Organization (WHO). In addition, the performances of complexes 1 and 2 modified carbon cloth electrodes (1-CC and 2-CC) as supercapacitor materials have also been studied. The influence of the structure on electrocatalytic and capacitor performances is discussed.

Efficient Photogeneration of Hydrogen Boosted by Long-Lived Dye-Modified Ir(III) Photosensitizers and Polyoxometalate Catalyst

Abstract: Developing efficient catalysts and photosensitizers is crucial for the construction of effective photocatalytic H2-evolving systems. Here, we report the facile preparation of Coumarin-modified Ir(I...

Pentadecanuclear Fe-Containing Polyoxometalate Catalyst for Visible-Light-Driven Generation of Hydrogen.

Abstract: The structurally new, carbon-free pentadecanuclear Fe-containing polyoxometalate, Na21[NaFe15(OH)12(PO4)4(A-α-SiW9O34)4]·85H2O (Na21-Fe15P4(SiW9)4), was synthesized using a facile one-pot, solution-based synthetic approach and systematically characterized by various spectroscopic techniques. Single-crystal X-ray diffraction reveals that the title complex is composed of two [Fe4(A-α-SiW9O34)] fragments and two [Fe3.5(A-α-SiW9O34)] fragments stabilized by four PO4 linkers in a tetrameric style with idealized Td point group symmetry. When coupling with (4,4'-ditert-butyl-2,2'-dipyridyl)-bis(coumarin)-iridium(III) hexafluorophosphate ([Ir(coumarin)2(dtbbpy)][PF6]) photosensitizer and triethanolamine (TEOA) sacrificial electron donor, polyoxoanion Fe15P4(SiW9)4 effectively catalyzed hydrogen production with a minimally optimized TON of 986, which represents, to our knowledge, one of the highest values among known Fe-substituted POM-catalyzed hydrogen production systems. Both a mercury-poisoning test and FT-IR characterizations proved the structural stability of Fe15P4(SiW9)4 catalyst under photocatalytic conditions. The photocatalytic mechanism of the present hydrogen-evolving system was investigated by time-solved luminescence and static emission quenching measurements.

Hierarchically Ordered Macro-Microporous Polyoxometalate-Based Metal-Organic Framework Single Crystals.

Abstract: Facile construction of ordered macroporous polyoxometalate-based metal-organic frameworks (POM@MOFs) to break the intrinsic microporous restriction is significant but remains challenging. On one hand, the POMs introduced improve the structural stability and modify the pores of MOFs, e.g., introducing functional catalytic and adsorptive units. Meanwhile, the acidic POMs severely affect the nucleation and growth of the POM@MOFs, resulting in complicated synthesis and difficult assembly control. Herein, a general approach has been developed to fabricate ordered macroporous POM@MOF single crystals, involving close-packed polystyrene (PS) nanosphere templates. The artificially selected polar solvents exerting strong solvent effect with POMs weaken the affinity between POMs and metal ions, thereby effectively stabilizing the precursors from assembly before filling into the PS template interstices. The weak alkaline carboxylate used regulates the in situ nucleation and growth of POM@MOFs through deprotonation of the ligands as well as coordinating modulation, affording a series of hierarchically cuboctahedral POM@MOF single crystals with ordered macropores (ca. 180 nm) and intrinsic micropores after template removal. The ordered macroporous structure and thinned microporous skeleton markedly improve mass diffusion properties, while the integral single-crystal lattice retains superior stability.

A Copper-Containing Polyoxometalate-Based Metal-Organic Framework as an Efficient Catalyst for Selective Catalytic Oxidation of Alkylbenzenes.

Abstract: A copper-containing polyoxometalate-based metal-organic framework (POMOF), CuI12Cl2(trz)8[HPW12O40] (HENU-7, HENU = Henan University; trz = 1,2,4-triazole), has been successfully synthesized and well-characterized. In addition, the excellent catalytic ability of HENU-7 has been proved by the selective oxidation of diphenylmethane. Under the optimal conditions, the diphenylmethane conversion obtained over HENU-7 is 96%, while the selectivity to benzophenone is 99%, which outperforms most noble-metal-free POM-based catalysts. Moreover, HENU-7 is stable to reuse for five runs without an obvious loss in activity and also can catalyze the oxidation of different benzylic C-H with satisfactory conversions and selectivities, which implied the significant catalytic activity and recyclability.

Cu/Ag Complex Modified Keggin-Type Coordination Polymers for Improved Electrochemical Capacitance, Dual-Function Electrocatalysis, and Sensing Performance.

Abstract: Different metal-organic units were introduced into the {PMo12} polyoxometalate (POM) system to yield three porous coordination polymers with distinct characteristics, {Cu(pra)2}[{Cu(pra)2}3{PMo11VIMoVO40}] (1), [{Ag5(pz)6(H2O)0.5Cl}{PMo11VIMoVO40}] (2), and [{Cu3(bpz)5(H2O)}{PMo12O40}] (3) (pra = pyrazole; pz = pyrazine; bpz = benzopyrazine), via an in situ hydrothermal method. In comparison with the maternal Keggin cluster and most reported POM electrode materials, compounds 1-3 exhibit larger specific capacitances (672.2, 782.1, and 765.2 F g-1 at a current density of 2.4 A g-1, respectively), superior cyclic stability (91.5%, 89.3%, and 87.8% of cycle efficiency after 5000 cycles, respectively), and boosted conductivity, which may be attributed to the introduction of metal-organic units. The result indicates that metal-organic units can effectively enhance the capacitance performance of POMs. This may be due to the fact that they provide additional redox centers, induce the formation of stable porous structures, and improve ion/electron transfer efficiency. Compounds 1-3 present excellent electrocatalytic activity in reducing peroxide (H2O2) and oxidizing ascorbic acid (AA). In addition, compound 2 shows an outstanding sensing performance detection of AA and H2O2.

Notched-Polyoxometalate Strategy to Fabricate Atomically Dispersed Ru Catalysts for Biomass Conversion

Abstract: The development of the synthesis methods of single-atom catalysts (SACs) is of great significance for the study of the specificity of SACs. Herein, we report a strategy to immobilize Ru atom using ...

Function-Targeted Lanthanide-Anchored Polyoxometalate–Cyclodextrin Assembly: Discriminative Sensing of Inorganic Phosphate and Organophosphate

Abstract: Polyoxometalate (POM) clusters containing lanthanide ion (LnPOM) possess excellent luminescence features, but the envisaged applications are hindered by the challenges in integration into functional architectures. Herein, a novel cross-linked cyclodextrin (CL-CD) and LnPOM composite is developed and applied for discriminative detection of inorganic and organic phosphate phases. For inorganic phosphates, a ratiometric fluorescence response is demonstrated with excellent selectivity, and anti-interference ability in complex analyte mixtures. The outstanding performance is attributed to the high affinity of POM and distinct interactions between La3+ and Eu3+ with the phosphate. For organophosphates, a “signal-off” fluorescence response for p-nitrophenyl-substituted organophosphates is discovered due to the encapsulation of nitrophenyl group into the hydrophobic cavity of CD that enhances the interactions between POM and p-nitrophenyl phosphate. The discriminative responses of CL-CD–LnPOM to inorganic phosphates and organophosphates bring new insights into POM-based fluorescence probes for the detection of inorganic and organic phases based on the intrinsic structural difference between the phosphate analogs.

Anderson-type polyoxometalate-based complexes constructed from a new ‘V’-like bis-pyridine–bis-amide ligand for selective adsorption of organic dyes and detection of Cr(VI) and Fe(III) ions

Abstract: By introducing a new ‘V’-like bis-pyridine–bis-amide ligand 4,4′-bis(4-pyridinecarboxamide)phenylmethane (L) into the reaction system based on Anderson-type polyoxometalates [XMo6(OH)6O18]3− (X = Al or Cr), four metal–organic complexes (MOCs) 1–4, [Zn(HL)(H2O)2(XMo6(OH)6O18)]·5H2O (1: X = Al; 2: X = Cr), [Co(HL)(H2O)2(XMo6(OH)6O18)]·5H2O (3: X = Al; 4: X = Cr), have been prepared under solvothermal conditions, which have been characterized by single crystal X-ray diffraction, IR spectra, and PXRD. All structures of 1–4 involved a 2D layer that originated from XMo6 polyoxoanions and metal ions, on both sides of which the L ligands were hanged. Complexes 1–4 showed not only outstanding selective adsorption capacities for organic dyes crystal violet, methylene blue and neutral red, but also electrochemical sensing behaviors toward Cr(VI) ions and Fe(III) ions with well-pleasing limits of detection of 0.606 nM (6.24 × 10−5 ppm) and 0.0192 μM (1.08 × 10−3 ppm), suggesting their potential application as multifunctional materials.

AgBr/Polyoxometalate/Graphene Oxide Ternary Composites for Visible Light-Driven Photocatalytic Hydrogen Production

Abstract: The exploration of high efficient and stable visible light-driven photocatalysts for water splitting into green energy hydrogen has been considered as one of the currently most challenging topics f...