This short focused review shows the usefulness of the broad family of Polyoxometalates (POMs) in selected electrocatalytic processes. Reduction and oxidation capabilities exist as well, with these ...robust and versatile catalysts. Quantitative transformations were demonstrated in several examples, including hydrogen production, NO
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reduction, coenzyme oxidation. Several parameters must be taken into account in the design and study of these processes.
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Polyoxometalates (POMs) constitute a broad family of molecular oxides showing promise in several efficient and clean electrocatalytic processes, when they are handled in appropriate conditions. Reduction and oxidation possibilities exist as well. The present short focused review shows the usefulness of POMs in selected electrocatalytic processes. Quantitative transformations were demonstrated in several examples, including hydrogen production, NO
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reduction, dioxygen reduction, coenzyme oxidation… This selection leaves room for a wealth of other possibilities. It appears clearly that several parameters must be taken into account in the design and study of these processes. Among them, the stability in aqueous or non aqueous solutions is an absolute prerequisite, which excludes the use of several newly synthesised fragments solely stable in the solid state. New parameters, in addition of those highlighted here, might emerge because POMs with new atomic compositions, new structures and new reactivities, continue to be synthesised and characterised. The challenge, however, is to find new POMs or new POM-based systems to decrease overpotentials which remain high for most reactions and improve the overall kinetics.
Finally, an optimistic prediction is that POMs, owing to their versatility and possibility to accommodate nearly all the atoms of the periodic classification, will become popular to drive selected electrocatalytic processes.
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 H2O2 detection for application as an enzyme‐free biosensor. Transformation of the POMs during H2O2 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.
Well‐defined Au‐nanoparticle/polyoxometalate/graphene tricomponent nanohybrids are synthesized by a green, facile, one‐pot method. The promising application of enzyme‐free biosensor for H2O2 is selected as a preliminary test of the electrocatalytic behaviors of the prepared nanohybrids. Encouragingly, such a nanohybrid offers a remarkably improved sensitivity, selectivity, and stability for H2O2 detection.
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)3PMoV 8MoVI 4O36(OH)4Zn4C6H3(COO)34/3·6H2O (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)3PMoV 8MoVI 4O37(OH)3Zn4C6H3(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)3PMoV 8MoVI 4O37(OH)3Zn4C6H3(COO)3·8H2O (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 × 105 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.
Bimetallic Pd−Au nanostructures were synthesized in the soft templates provided by surfactant hexagonal mesophases. The nanostructures are constituted by a core rich in gold and a Pd porous shell. ...The electrocatalytic activity of these nanostructures for ethanol oxidation in basic medium was compared with that of alloyed Pd−Au nanoparticles synthesized in solution. The Pd−Au alloy is active toward the oxidation of ethanol in an alkaline medium but is not durable in realizing this process. The Pdshell−Aucore nanostructures synthesized in mesophases are promising for application in direct ethanol fuel cells as they exhibit a very good electrocatalytic activity and a high stability.
A series of carbonaceous-supported precious-metal-free polyoxometalate (POM)-based composites which can be easily synthesized on a large scale was shown to act as efficient cathode materials for the ...oxygen reduction reaction (ORR) in neutral or basic media via a four-electron mechanism with high durability. Moreover, exploiting the versatility of the considered system, its activity was optimized by the judicious choice of the 3d metals incorporated in the {(PW9)2M7} (M = Co, Ni) POM core, the POM counterions and the support (thermalized triazine-based frameworks (TTFs), fluorine-doped TTF (TTF-F), reduced graphene oxide, or carbon Vulcan XC-72. In particular, for {(PW9)2Ni7}/{Cu(ethylenediamine)2}/TTF-F, the overpotential required to drive the ORR compared well with those of Pt/C. This outstanding ORR electrocatalytic activity is linked with two synergistic effects due to the binary combination of the Cu and Ni centers and the strong interaction between the POM molecules and the porous and highly conducting TTF-F framework. To our knowledge, {(PW9)2Ni7}/{Cu(ethylenediamine)2}/TTF-F represents the first example of POM-based noble-metal-free ORR electrocatalyst possessing both comparable ORR electrocatalytic activity and much higher stability than that of Pt/C in neutral medium.
The targeted design and simulation of a new family of zeolitic metal−organic frameworks (MOFs) based on benzenedicarboxylate (BDC) as the ligand and ε-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 ε-type Keggin POMs capped with Zn(II) ions. Handling the ε-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 NBu43PMoV 8MoVI 4O36(OH)4Zn4(BDC)2·2H2O (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 ε-Keggin POM is maintained and can be exploited in the insoluble Z-POMOF1 framework, as demonstrated by the electrocatalytic reduction of bromate.
Electrochemical properties of two tri-manganese substituted Keggin-based tungstosilicates MnII3(OH)3(H2O)3(A-α-SiW9O34)7− (MnII3SiW9) andMnIII3(OH)3(H2O)3(A-α-SiW9O34)4− (MnIII3SiW9) were ...investigated. The two polyanions are isostructural, the only difference being the oxidation state of the Mn-ions. Despite their structural similarity the electrochemical behaviour is not identical. While it is well established that polyoxometalate (POM) electrochemistry is influenced by interplay between the pH of the electrolyte, the present ions and the pKa values of the complex, this is the first report that the initial oxidation state of the POM has a major influence on the electrochemistry of the transition metal within the molecule. In order to understand the influence of the initial oxidation state extensive electrochemical investigations were performed and the potential dependent adsorption behavior of the molecules on graphite was observed with atomic force microscopy. The reaction mechanism of the two POMs was determined and it was asserted that the divergent redox behavior is caused by a ligand exchange which takes place during the measurement. This influences the adsorption of the molecules on graphite which can be explained by the Born solvation model. Performing controlled potential electrolysis, a stable tri-manganese substituted Keggin ion containing MnIV3 was obtained electrochemically.
Wheely: For the first time and very unexpectedly, a rupture of the very stable {P8W48} wheel was observed in aqueous solution at pH 4 and 80 °C in the presence of FeIII, EuIII/GdIII, and H2O2. This ...inorganic ring opening is unprecedented in polyoxometalate chemistry.
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.
The five manganese-containing, Keggin-based tungstosilicates MnII 3(OH)3(H2O)3(A-α-SiW9O34)7– (1), MnIII 3(OH)3(H2O)3(A-α-SiW9O34)4– (2), MnIII 3(OH)3(H2O)3(A-β-SiW9O34)4– (3), MnIII ...3MnIVO3(CH3COO)3(A-α-SiW9O34)6– (4), and MnIII 3MnIVO3(CH3COO)3(A-β-SiW9O34)6– (5) were synthesized in aqueous medium by interaction of A-α-SiW9O3410– or A-β-SiW9O34H9– with either MnCl2 (1) or MnIII 8MnIV 4O12(CH3COO)16(H2O)4 (2–5) under carefully adjusted reaction conditions. The obtained salts of these polyanions were analyzed in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric analysis. The salts of polyanions 1, 2, and 4 were further characterized in the solid state by magnetic studies, as well as in solution by electrochemistry.
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