Bineta Keita

Polyoxometalate-Based Metal Organic Frameworks (POMOFs): Structural Trends, Energetics, and High Electrocatalytic Efficiency for Hydrogen Evolution Reaction

The grafting of the triangular 1,3,5-benzene tricarboxylate linkers (denoted trim) on tetrahedral ε-Keggin polyoxometalates (POMs) capped by Zn(II) ions, formed in situ under hydrothermal conditions, has generated three novel POM-based metal organic frameworks (POMOFs). (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)][C(6)H(3)(COO)(3)](4/3)·6H(2)O (ε(trim)(4/3)) is a 3D open-framework built of molecular Keggin units connected by trim linkers, with channels occupied by tetrabutylammonium (TBA) counterions. ε(trim)(4/3) is a novel (3,4)-connected net, named ofp for open-framework polyoxometalate, and computer simulations have been used to evaluate its relative stability in comparison with ctn- and bor-like polymorphs, showing the stability of this novel phase directly related to its greatest density. A computational study was also undertaken with the aim of locating TBA molecules, the positions of which could not be deduced from single crystal X-ray diffraction, and further rationalizes their structure directing role. In (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)][C(6)H(3)(COO)(3)] (ε(2)(trim)(2)), the building unit is not the molecular Keggin but a dimerized form of this POM. Their connection via trim linkers generates a 3D framework with channels filled by TBA cations. In (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)][C(6)H(3)(COO)(3)]·8H(2)O ([ε(trim)](∞)), zigzag chains are connected via the organic linkers, forming 2D grids. Modified electrodes were fabricated by direct adsorption of the POMOFs on glassy carbon or entrapment in carbon paste (CPE). A remarkable electrocatalytic hydrogen evolution reaction (HER) was detected with a yield greater than 95%, and a turnover number as high as 1.2 × 10(5) was obtained after 5 h. The reported POMOF-based electrodes are more active than platinum, with a roughly 260 mV anodic shift. Finally, the electrocatalytic activities of ε(trim)(4/3)/CPE electrodes in various XCl (X = Li, Na, K, Cs) media have been studied. This allowed us to detect a cation effect and propose an electrocatalytic mechanistic pathway for the HER.

Zeolitic Polyoxometalate-Based Metal−Organic Frameworks (Z-POMOFs): Computational Evaluation of Hypothetical Polymorphs and the Successful Targeted Synthesis of the Redox-Active Z-POMOF1

The targeted design and simulation of a new family of zeolitic metal-organic frameworks (MOFs) based on benzenedicarboxylate (BDC) as the ligand and epsilon-type Keggin polyoxometalates (POMs) as building units, named here Z-POMOFs, have been performed. A key feature is the use of the analogy between the connectivity of silicon in dense minerals and zeolites with that of the epsilon-type Keggin POMs capped with Zn(II) ions. Handling the epsilon-Keggin as a building block, a selection of 21 zeotype structures, together with a series of dense minerals were constructed and their relative stabilities computed. Among these Z-POMOFs, the cristobalite-like structure was predicted to be the most stable structure. This prediction has been experimentally validated by the targeted synthesis of the first experimental Z-POMOF structure, which was strikingly found to possess the cristobalite topology, with three interpenetrated networks. Crystals of [NBu(4)](3)[PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)(BDC)(2)].2H(2)O (Z-POMOF1) have been isolated under hydrothermal conditions from the reduction of ammonium heptamolybdate in the presence of phosphorous acid and Zn(II) ions. Tetrabutylammonium cations play the role of counterions and space-filling agents in this tridimensional interpenetrated framework. Moreover, the electrochemistry of the epsilon-Keggin POM is maintained and can be exploited in the insoluble Z-POMOF1 framework, as demonstrated by the electrocatalytic reduction of bromate.

Facile Synthesis of Au-Nanoparticle/Polyoxometalate/Graphene Tricomponent Nanohybrids: An Enzyme-Free Electrochemical Biosensor for Hydrogen Peroxide

A green, facile, one-pot synthesis of well-defined Au NPs@POM-GNSs tricomponent nanohybrids is reported (POM stands for polyoxometalate and GNSs for graphene nanosheets). The synthesis is convenient, rapid and environmentally friendly. The POMs serve as both reducing, encapsulating molecules, and bridging molecules; this avoids the introduction of other organic toxic molecules. Characterization using transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy analysis is performed, and the structure of the prepared nanohybrids of Au NPs@POM-GNSs is verified. Most importantly, the amperometric measurements show the Au NPs@POM-GNSs nanohybrids have high catalytic activity with good sensitivity, good long-term stability, wide linear range, low detection limit, and fast response towards H(2)O(2) detection for application as an enzyme-free biosensor. Transformation of the POMs during H(2)O(2) detection does not affect the catalytic activities of the nanohybrids. Thus, the synergistic effect of Au NPs and GNSs in the nanohybrids leads to the enhanced catalytic property.

Radiation-induced and chemical formation of gold clusters

The kinetics of the γ-radiolytical or chemical reduction of AuIII Cl4-, or of the combination of both methods, is followed as a function of the experimental conditions through the time evolution of the surface plasmon spectrum of the gold nanoparticles formed or of their sizes as observed by AFM imaging. It appears from the discussion on the mechanism that even with the strongly reducing radiolytic radicals, the low valency AuI ions are somewhat protected by the more concentrated AuIII ions from reduction, up to a ratio of AuI/AuIII=1, and are stabilized for hours, unless clusters or 2-propanol (or PVA, but more slowly) catalyze their disproportionation. The cluster concentration increases correlatively with the dose.2-Propanol or PVA are mild reducing agents and are unable to reduce AuIII directly except at the surface of clusters, previously formed, for instance, by partial radiolytic reduction. In this case, the cluster concentration remains the same but the size obtained after reduction by the alcohol increases slowly with time up to 100–500 nm, as in a development process. In order to avoid the relative extent of this development, associated with chemical reduction and even with the direct γ-reduction of AuIII, in particular the AuI disproportionation and reduction steps, high dose rate radiolysis has been used up to total reduction of the same solutions. The mechanism of reduction and growth, step-by-step, is discussed.

New aspects of the electrochemistry of heteropolyacids: Part IV. Acidity dependent cyclic voltammetric behaviour of phosphotungstic and silicotungstic heteropolyanions in water and N,N-dimethylformamide

Cyclic voltammetry has been used to study the electrochemical reduction process of three heteropolyanions, two of them, α-SiW12O4−40 and α-PW12O3−40, having the Keggin structure and the third one, α-P2W18O6−62, a Dawson structure. The study was performed in strongly acidic aqueous solutions and in dimethylformamide with stepwise addition of perchloric acid. α-PW12O3−40 is a very weak base whatever the medium and is hardly sensitive to protonation effects. For the other compounds, important morphological changes of the waves are observed in these electrolytes. In particular, their first voltammetric wave rise from a one-electron, diffusion-controlled process to an overall, apparently direct, two-electron transfer, when the acid concentration in water is raised. Due to solvent effects, much the same final results are obtained in dimethylformamide, except that it is not necessary to approach such extreme acid concentrations as in water to reach the two-electron process. It appears that exactly the same behaviour is observed for α-SiW12O4−40 and α-P2W18O6−62, albeit at different proton to hetero-polyanion ratios. A clear difference is observed between the mechanisms of formation of the two-electron wave in acidic water and in dimethylformamide. In the former case, the second wave shifts toward positive potentials until it merges with the first one, while in the latter medium, a new wave more positive than the original ones appears and grows at their expense. The observations are explained on the basis of solvent effects and allow a comparison to be made between the acido-basic properties of these heteropolyanions and their one- or two-electron reduction products.

Supramolecular Self-Assembly of Amphiphiles on Carbon Nanotubes: A Versatile Strategy for the Construction of CNT/Metal Nanohybrids, Application to Electrocatalysis

Homogeneous coating of carbon nanotubes with metallic nanoparticles was achieved using supramolecular auto-organization of amphiphilic molecules as template. The resulting Pd nanoparticles/carbon nanotube nanohybrids were then evaluated in electrocatalysis experiments, showing superior activity in ethanol oxidation compared to analogous systems.

Surface modifications with heteropoly and isopoly oxometalates: Part I. Qualitative aspects of the activation of electrode surfaces towards the hydrogen evolution reaction

Abstract It is shown that heteropoly and isopoly oxometalates and even tungstic acid can be used to modify the surfaces of materials as diverse as gold, tungsten, mercury, glassy carbon, graphite and even platinum. Except for platinum, all these electronic conductors are usually very poor surfaces on which to evolve hydrogen. Upon modification, they all appear to exhibit strikingly high exchange current densities with respect to the hydrogen evolution reaction (h.e.r.). The Keggin structure or the Dawson structure of the starting compound does not induce any specific characteristic for the modified electrode, nor does the very nature of the electrode material. In particular, mercury has been successfully derivatized. It has been ascertained by various surface analysis techniques, that the observed enhancement of catalytic activity of the chemically derivatized electrodes is not caused by traces of noble metal impurities which could accumulate during the modification process and explain the phenomena. It has also been shown that a possible increase of the microscopic surface area upon modification cannot solely and primarily account for the several orders of magnitude increase measured for the exchange current density for the h.e.r. The activation is durable and the catalyst seems perfectly suitable for use in non-purposely purified solutions. A judicious selection of the starting compounds is made to demonstrate that oxometalates of the tungstic, molybdic and the mixed (W, Mo) series can be used.

Polyoxometalate-based materials for efficient solar and visible light harvesting: application to the photocatalytic degradation of azo dyes

This report reviews the state of the art concerning the photocatalytic degradation of azo dyes using polyoxometalate-based materials. Recent developments toward polyoxometalates (POMs) and their assemblies that match multiple criteria are described. These criteria include their switchability by visible light, the possibility of multi-electronic photo-reduction of the POMs, followed by reversible electron exchange with a substrate, the possibility of heterogenization without loss of properties and finally a low cost. This strategy is recent and is shown to open up new avenues close to green chemistry achievements for studying the degradation of organic pollutants, including azo dyes.

A Planar {Mn19(OH)12}26+ Unit Incorporated in a 60‐Tungsto‐6‐Silicate Polyanion

Polyoxometalates (POMs) are discrete metal–oxo anions of early transition-metals in high oxidation states (e.g. W, Mo, V) and they are usually synthesized in aqueous, acidic medium. Most classical POMs are based on edgeand corner-shared MO6 octahedra. However, the recently discovered POM subclass of noble metalates comprises linked square-planar MO4 units (M = Pd , Au). Lacunary (vacant) POMs can be considered as inorganic, multidentate ligands, and hence they are good candidates for the encapsulation of large, multinuclear dand f-block metal–oxo fragments, sometimes resulting in compounds with interesting magnetic properties. A pioneering result in this area was the synthesis of [Mn12(CH3COO)16(H2O)4O12] (Mn12) by Lis in 1980, which was shown to exhibit single-molecule magnet (SMM) behavior by Gatteschi s group 13 years later. During the past two decades many high-nuclearity, transition-metal based, coordination complexes with interesting electronic and magnetic properties have been prepared. High-nuclearity manganese complexes have been amongst the most studied in this class, and there are examples containing up to 84 manganese ions. 6] To date there are only a few high-nuclearity manganese– oxo-containing POMs, such as {[XW9O34]2[Mn III 4Mn II 2O4(H2O)4]} 12 (X = Si, Ge) and [Mn13Mn O12(PO4)4(PW9O34)4] 31 . Herein we report the synthesis and structure as well as the magnetic and electrochemical properties of a 19 manganese(II) center containing 60-tungsto-6-silicate, [Mn19(OH)12(SiW10O37)6] 34 (1), which was isolated as a hydrated sodium salt, Na34[Mn19(OH)12(SiW10O37)6]·115H2O (Na-1). Single-crystal X-ray diffraction revealed that polyanion 1 consists of a cationic {Mn19(OH)12} 26+ (Mn19) assembly stabilized by six dilacunary [a-SiW10O37] 10 units resulting in a structure with S6 point-group symmetry (Figure 1, top). To the best of our knowledge, 1 is the highest nuclearity manganesecontaining POM known to date. All 19 Mn ions lie in the same plane forming a hexagonal structure based on edgeshared MnO6 octahedra. The Mn II ions in Mn19 are connected by a total of twelve m3-hydroxo bridges, as determined by bond valence sum (BVS) calculations (Supporting Information, Table S1). The discrete Mn19 nanosheet (Figure 1, bottom) is held in place by six dilacunary [a-SiW10O37] 10

Nucleation Process in the Cavity of a 48‐Tungstophosphate Wheel Resulting in a 16‐Metal‐Centre Iron Oxide Nanocluster

The 16-Fe(III)-containing 48-tungsto-8-phosphate [P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)](20-) (1) has been synthesised and characterised by IR and ESR spectroscopy, TGA, elemental analyses, electrochemistry and susceptibility measurements. Single-crystal X-ray analyses were carried out on Li(4)K(16)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)]66 H(2)O2 KCl (LiK-1, orthorhombic space group Pnnm, a=36.3777(9) A, b=13.9708(3) A, c=26.9140(7) A, and Z=2) and on the corresponding mixed sodium-potassium salt Na(9)K(11)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)].100 H(2)O (NaK-1, monoclinic space group C2/c, a=46.552(4) A, b=20.8239(18) A, c=27.826(2) A, beta=97.141(2) degrees and Z=4). Polyanion 1 contains--in the form of a cyclic arrangement--the unprecedented {Fe(16)(OH)(28)(H(2)O)(4)}(20+) nanocluster, with 16 edge- and corner-sharing FeO(6) octahedra, grafted on the inner surface of the crown-shaped [H(7)P(8)W(48)O(184)](33-) (P(8)W(48)) precursor. The synthesis of 1 was accomplished by reaction of different iron species containing Fe(II) (in presence of O(2)) or Fe(III) ions with the P(8)W(48) anion in aqueous, acidic medium (pH approximately 4), which can be regarded as an assembly process under confined geometries. One fascinating aspect is the possibility to model the uptake and release of iron in ferritin. The electrochemical study of 1, which is stable from pH 1 through 7, offers an interesting example of a highly iron-rich cluster. The reduction wave associated with the Fe(III) centres could not be split in distinct steps independent of the potential scan rate from 2 to 1000 mV s(-1); this is in full agreement with the structure showing that all 16 iron centres are equivalent. Polyanion 1 proved to be efficient for the electrocatalytic reduction of NO(x), including nitrate. Magnetic and variable frequency EPR measurements on 1 suggest that the Fe(III) ions are strongly antiferromagnetically coupled and that the ground state is tentatively spin S=2.