Presence Of Polyoxometalate Research Articles

Rationalizing the Role of Polyoxometalate‐Based Gel–Polymer Electrolytes to Achieve Five‐Fold Increase in the Specific Capacitance of Hard‐Carbon‐Based Supercapacitors

Boosting the charge‐transport pathways in gel–polymer electrolytes (GPEs) is important to extract their full potential for non‐Faradaic, supercapacitive energy‐storage devices. Herein, a series of polyoxometalates (POMs) ([P6W18O79]20−, [PW9O34]9− and [PW12O40]3−) as electrochemical polarization promoters is demonstrated to achieve 15‐fold enhancement in ionic conductivity of 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide‐based GPEs. Fundamentally, the three POMs offer systematically differing charge densities (200, 45, and 6.5 e− nm−3) that are linearly correlated to the observed enhancements in ionic conductivities. The charge density on the POMs contributes to the formation of additional charge‐migrative pathways across the GPE, as evidenced from facile polarization characteristics. Importantly, the presence of POM beyond a critical concentration acts as charge‐trapping centers to impede the ionic conductivity of the GPEs. The insights obtained through such detailed spectroscopic and electrochemical techniques are integrated into full‐scale supercapacitive devices with a rationally designed porous hard carbon as the electrode material. The resulting electrode–electrolyte interface synergies achieve a best‐in‐class supercapacitor exhibiting energy density of 58 Wh kg−1, power density of 14 kW kg−1, and a relaxation time constant of 0.66 s, without compromising on cycling stability for direct integration with intermittent energy‐harvesting devices. Thus, the fundamental insights presented here outline the design principles for extracting high‐performance from GPE‐based energy‐storage devices.

Comparison between Lacunary and Saturated Keggin Polyoxometalates as Steel Corrosion Inhibitors in Chloride Solution: Contribution of the Lacuna in the Inhibition Mechanism

AbstractThe effect of two lacunary Keggin polyoxometalates (K7[PW11O39].14H2O and K9[AlW11O39].15H2O) on the corrosion inhibition of XC38 steel in 0.1 M NaCl was investigated. Electrochemical methods (polarization curves and electrochemical impedance spectroscopy) showed that the presence of controlled amounts of these compounds improved protection of the steel against corrosion. These results were supported by surface analysis, where only a minor amount of rust was produced on metal surface in the presence of polyoxometalates (POMs). The lacunary POMs were more efficient compared to their saturated species, highlighting the key role played by the lacuna on the inhibition process. Efficiency of POMs (both saturated and lacunary) was explained by their ability to increase the Fe(II)/Fe(III) molar ratio in greenish protective oxide layer formed upon steel oxidation. The mechanism proposed clearly explains the higher efficiency observed with lacunary POMs, in relation to their ability to react with free Fe(III) to yield a substituted structure.

Hydrolysis of Peptide Bonds in Protein Micelles Promoted by a Zirconium(IV)-Substituted Polyoxometalate as an Artificial Protease.

The development of artificial proteases is challenging, but important for many applications in modern proteomics and biotechnology. The hydrolysis of hydrophobic or unstructured proteins is particularly difficult due to their poor solubility, which often requires the presence of surfactants. Herein, it is shown that a zirconium(IV)-substituted Keggin polyoxometalate (POM), (Et2 NH2 )10 [Zr(α-PW11 O39 )2 ] (1), is able to selectively hydrolyze β-casein, which is an intrinsically unstructured protein at pH 7.4 and 60 °C. Four surfactants (sodium dodecyl sulfate (SDS), N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (ZW3-12), 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), and polyethylene glycol tert-octylphenyl ether (TX-100)), which differ in the nature of their polar groups, were investigated for their role in influencing the selectivity and efficiency of protein hydrolysis. Under experimental conditions, β-casein forms micellar structures in which the hydrophilic part of the protein is water accessible and able to interact with 1. Identical fragmentation patterns of β-casein in the presence of 1 were observed through SDS poly(acrylamide) gel electrophoresis both in the presence and absence of surfactants, but the rate of hydrolysis varied, depending on the nature of surfactant. Whereas TX-100 surfactant, which has a neutral polar head, caused only a slight decrease in the hydrolysis rate, stronger inhibition was observed in the presence surfactants with charges in their polar heads (CHAPS, ZW3-12, SDS). These results were consistent with those of tryptophan fluorescencequenching studies, which showed that the binding between β-casein and 1 decreased with increasing repulsion between the POM and the polar heads of the surfactants. In all cases, the micellar structure of β-casein was not significantly affected by the presence of POM or surfactants, as indicated by circular dichroism spectroscopy.

Gas-Phase Dehydration of Glycerol into Acrolein in the Presence of Polyoxometalates

The activity of catalytic systems based on polyoxometalates in the gas-phase dehydration of glycerol into acrolein has been investigated. The catalysts synthesized have been characterized using FTIR spectroscopy, X-ray phase analysis, scanning electron microscopy, and low-temperature nitrogen adsorption/desorption (BET method). It has been shown that the nature of the surface acid sites of catalysts has a significant effect on the direction of glycerol transformation. An increase in the fraction of Bronsted acid sites in the catalyst leads to an increase in the yield of acrolein. At the same time, an increase in the fraction of Lewis acid sites in the catalyst leads to an increase to the yield of acetol, which is formed through a competing route. Among the catalytic systems considered, the best results have been obtained with the silicomolybdic acid anion supported onto alumina (SiMo/A). The main patterns of the gas-phase glycerol dehydration have been studied using SiMo/A as a catalyst, and the conditions responsible for the highest yield of acrolein have been determined.

Benefits of Organo-Aqueous Binary Solvents for Redox Supercapacitors Based on Polyoxometalates.

A novel redox electrolyte is proposed based on organo‐aqueous solvent and a polyoxometalate (POM) redox moiety. The presence of dimethyl sulfoxide (DMSO) plays multiple roles in this system. Firstly, it enhances the cathodic electrochemical stability window by shifting the H2 evolution to lower potentials with respect to pure aqueous systems; secondly, it improves the reversibility of the redox reaction of the PW12O40 3− anion at low potentials. The presence of DMSO suppresses the Al corrosion, thus enabling the use of this metal as the current collector. An activated carbon‐based supercapacitor is investigated in 1 M LiNO3/10 mM H3PW12O40 in a mixed DMSO/H2O solvent and compared with a POM‐free electrolyte. In the presence of POMs, the device achieves better stability under floating conditions at 1.8 V. At 1 kW kg−1, it delivers a specific energy of 8 Wh kg−1 vs. 4.5 Wh kg−1 delivered from the POM‐free device. The H2 evolution is further shifted by the POMs adsorbed on the activated carbon, which is one reason for the improved stability. The POM‐containing cell demonstrates a mitigated self‐discharge, owing to strong POMs adsorption into the carbon pores.

A thermodynamic model of non-ionic surfactants' micellization in the presence of polyoxometalates

Polyoxometalates (POMs) are nanometric metal−oxide anions with an unmatched range of chemical and physical properties. During the past decade, significant efforts have been made to study POM surface activity and self-assembly properties that are essential for catalysis applications and for producing organic−inorganic hybrid materials. A previous work highlighted the tungstosilicate (SiW12O404−) and tungstophosphate (PW12O403−) Keggin POM anions' spontaneous and noncovalent adsorption at the micellar surface of non-ionic surfactants. In this study, the critical micelle concentrations (CMC) of two non-ionic surfactants, the n-octyl-β-glucoside (C8G1) and the tetraethylene glycol monooctyl ether (C8E4), were measured in the presence of POMs, and we propose herein thermodynamic models to explain an increase or a decrease of the CMC depending on the choice of the POM/surfactant couple.

Preparation of silver nanoparticles in the presence of polyoxometalates

The mechanisms of reduction of silver ions and subsequently oxidation of silver atoms in the presence of polyoxometalates (POMs) are discussed. A step-by-step room temperature electron reduction of silver ions and its subsequent reactions has been used in this work to monitor oxidation of silver atoms and its clusters. The silver atoms can transfer electron to POMs is revealed by decrease in the yield of silver clusters and increase in their decay rates. The results of continuous γ-irradiation are compared using UV–visible (UV–Vis) absorption spectra, underlying the effect of silver atoms accumulation in the absence of POMs. Silver nanoparticles (Ag NPs) prepared by reduced silicotungstic acid (STA) were used as Raman substrate and also for antibacterial studies against panel of human pathogenic bacteria. The Ag NPs exhibited antibacterial activity against both Gram-positive and Gram-negative bacterial pathogens evaluated by well diffusion assay. The inhibition zones were within the range of 10 to 14 mm. We have also explored the surface enhanced Raman scattering (SERS) activity of the Ag NPs substrates using 1.0 × 10−7 M solution of crystal violet (CV) as Raman probe molecule. It was possible to detect SERS spectral pattern of CV on Ag NPs substrate with a high signal-to-noise ratio. Both SERS and antibacterial studies show that this simple, low cost, and greener method for synthesizing Ag NPs may be valuable in future studies about SERS sensor development and bio-applications.

Molecular Scale Insights into Activity of Polyoxometalate as Membrane-Targeting Nanomedicine from Single-Molecule Observations

Polyoxometalate (POM) is rapidly emerging as an attractive antimicrobial inorganic cluster that exhibits its antimicrobial activity by attacking the cell membrane. Precise understanding and control of the antimicrobial activity of POM can allow us to design novel functional nanomedicines with high stability, high selectivity, and low cost. Therefore, in this study, we investigated the interaction between POM and a model cell membrane through single-molecule observation and found that the presence of POM causes macroscopic morphological changes in the microbial membrane, and these changes were detectable as modulations in the diffusivity of the membrane. Numerical analyses based on mean square displacement and diffusion length histogram revealed the reduction in the fluidity of the membrane in the presence of POM. Further analysis from single-molecule tracking revealed the formation of pores in the membrane, along with the formation of POM–lipid assemblies. The pores were found to act as diffusion barriers and diffusion trap sites and thus contributed to the reduction in the fluidity of the membrane. Furthermore, pore formation also led to the loss of important functions of the cell membrane. Based on this ability of POM to induce pore formation and form assemblies with membrane lipids, we believe that POM is a promising candidate for use as a membrane-targeting bioactive nanomedicine.

Polyoxometalate-Surfactant Hybrids Directed Assembly of Ni3S2 into Hollow Microsphere as Pt-Comparable Electrocatalyst for Hydrogen Evolution Reaction in Alkaline Medium.

A large surface area of catalytic sites and low electric resistance are desirable properties for electrocatalysts that lower the overpotential required for electrochemical reactions, such as the hydrogen evolution reaction (HER), in this study. In the presence of polyoxometalate (POM) and triblock copolymer pluronic (P123) as a hybrid soft template, the hydrothermal sulfurization of nickel foam leads to the formation of a hollow microsphere, assembled from the Ni3S2 motif. Sonication for preparing POM+P123 hybrids, while adjusting the POM content, is an effective strategy for synthesizing a Pt-like electrocatalyst with excellent hydrogen evolution efficiency. The reason is that both sonication and optimal POM content can simultaneously enhance the electroactive surface area and electron transfer of the Ni3S2 electrocatalyst. Adopting the optimal conditions, the three-dimensional porous network, composed of Ni3S2 hollow microspheres on the nickel foam, shows that the HER in an alkaline medium requires only 77 mV overpotential for a current density of 10 mA cm-2, with a robust long-term stability during a 25 h test. The performance at a large current density outperforms the current benchmark electrocatalyst (Pt) for HER.

One-pot synthesis of Pd@Pt core–shell nanocrystals for electrocatalysis: control of crystal morphology with polyoxometalate

Pt-based bimetallic nanocrystals (NCs) are an important class of catalytic materials especially in the area of energy conversion due to their outstanding catalytic performance. The catalytic function of Pt-based bimetallic NCs toward a specific catalysis reaction can be optimized by tailoring their surface structure, which can be realized through the precise control of the NC morphology. In the present work, we developed a facile one-pot polyoxometalate (POM)-mediated synthesis approach for the synthesis of Pt-based bimetallic NCs with controlled morphologies and fine crystal structures, and investigated the influence of their morphology on their catalytic function. Pd@Pt core–shell NCs with well-defined morphologies and controlled surface structures could be prepared by the simultaneous reduction of Pt and Pd precursors in the presence of a typical Keggin-type POM (H3PMo12O40) and ascorbic acid (AA). During the formation of NCs, the POMs served not only as a stabilizing agent but also a reducing agent. Notably, the presence of the POMs as well as the relative concentration of AA to metal precursors could afford fine control over the growth mode of the Pt shells on the Pd cores, and thus the final morphology of the Pd@Pt NCs. The prepared POM-passivated Pd@Pt NCs outperformed Pd@Pt NCs synthesized without the POMs as well as a commercial Pt/C catalyst for the electrooxidation of methanol, and their catalytic activity and stability distinctly depended on their Pt shell structure. We envision that this strategy will pave the way for the rational design of NC-based catalyst systems.

Enhanced Water Splitting at Thin Film WO3 Photoanodes Modified with the Electro-Catalysts

An effective solar light-driven water splitting in photo-electrochemical devices requires use of materials which combine high photo-conversion efficiency and long term stability. For the latter reason the choice of the suitable systems is restricted to the semiconducting oxides that, in most cases, do not undergo photo-corrosion in aqueous solutions and are able to absorb efficiently the visible light. However, although the nanostructured scaffold of the semiconducting photo-electrodes contributes generally to decrease the bulk charge recombination by reducing the minority charge carriers pathway to reach the solution species, at the same time, it tends to enhance the recombination of charges at the surface due to the increased contact area with the electrolyte. Common approach to reduce recombination losses, involves deposition at the photo-electrode surface of electrocatalysts allowing to overcome slow kinetics of oxygen evolution. There are only few, precious metal (Pt, IrO2), oxygen-evolution electrocatalysts that can be used in pH acidc in which WO3 operates effectively. In this regard, we will present an increase of water photo-oxidation currents at WO3 photo-anodes, caused by presence of polyoxometalates (POMs), such as phosphomolybdate (PMoO40 3-) Keggin type ions. This class of compounds is known to undergo reversible, stepwise multi-electron transfer reactions that are combined with their excellent proton conductivity. What is more, the POMs might act as a buffer layer when form a cover for a metallic nano - particles largely used as a regular catalysts. The POMs may act either as relays for photogenerated electrons or as catalysts directly involved in the hole transfer reactions. In particular, they may affect the pathway of water oxidation by modifying the stability of the intermediates (e.g., peroxides) involved in this processes. Ongoing efforts devoted to minimize the bias voltage required to perform visible light-driven photo-oxidation of water at NS WO3 photoanodes also include the use of dopants or incorporation of plasmonic metal nanostructures allowing to enhance the light capture by the WO3 films1. 1. R. Solarska, K. Bienkowski, S. Zoladek, A. Majcher, T. Stefaniuk, P. J. Kulesza and J. Augustynski, Angew. Chem. Int. Ed. 2014, 53: DOI: 10.1002/anie.201408374

Polyoxometalate‐assisted Metal Nanomaterials Synthesis#

In this review, we present the polyoxometalate (POM)‐mediated synthesis of nanomaterials and their performance as catalysts. Various nanomaterials have been successfully synthesized in a facile process using POMs as a reducing and stabilizing agent. The reduction of metal ions was effected by the reduced POMs, which converted their oxidation state without structural change. The presence of POMs on the nanomaterials was confirmed through a variety of analytical methods. The alteration of the size and morphology of nanomaterials was observed to be controlled by the molar ratio of metal salt to the POMs. The synthesized nanomaterials showed good catalytic activity, which could be used as multifunctional catalysts.

Photocatalytic degradation of water taste and odour compounds in the presence of polyoxometalates and TiO2: Intermediates and degradation pathways

Abstract Geosmin (GSM) and 2-methylisoborneol (MIB) are produced by several species of cyanobacteria and actinomycetes. These compounds can taint water and fish causing undesirable taste and odours. Studies have shown that GSM/MIB are resistant in standard water treatments. Polyoxometalates (POM) are efficient photocatalysts in the degradation and mineralization of a great variety of organic pollutants, presenting similar behaviour with the widely published titanium dioxide (TiO 2 ). Photocatalytic degradation of GSM and MIB under UV-A light in the presence of a characteristic POM photocatalyst, SiW 12 O 40 4− , in aqueous solution has been studied and compared with the photodegradation by TiO 2 suspensions. GSM and MIB are effectively degraded in the presence of both photocatalysts. Addition of OH radical scavengers (KBr and tertiary butyl alcohol, TBA) retards the photodegradation rates of both compounds, suggesting that photodegradation mechanism takes place via OH radicals. Intermediates identified using GC–MS in the case of GSM and MIB, are mainly identical in the presence of both photocatalysts, also suggesting a common reaction mechanism. Possible photocatalytic degradation pathway for both GSM and MIB is proposed.

Catalytic delignification of cellulose fibers by hydrogen peroxide in presence of polyoxometalates

Peroxide delignification of cellulose fibers in acid medium in presence of polyoxometalates as catalysts was studied. The effects of the hydrogen peroxide concentration, the catalyst nature, and the addition of the alkali processing stage on the process of delignification of leafy and coniferous celluloses were investigated.

Keggin‐Type Polyoxometalate ([PMo12O40]3−) in Radical Initiating Systems: Application to Radical and Cationic Photopolymerization Reactions

AbstractNew photoinitiating systems based on a polyoxometalate (POM) are proposed: phosphomolybdic acid in combination with silane, germane, or iodonium salt can be used to generate silyl, germyl, or phenyl radicals as well as silylium cations. The photochemical mechanisms are studied by steady‐state photolysis and electron spin resonance. The phosphomolybdic acid/silane/iodonium salt system can initiate either the radical photopolymerization of acrylates or the cationic photopolymerization of epoxides. The synthesis of interpenetrated polymer networks can also be carried out. The mechanical properties of the synthesized polymers are affected by the presence of POM in the matrix, as shown by their dynamic mechanical analysis (DMA).magnified image

Heterogeneous acid catalytic esterification by porous polyoxometalate-tantalum pentoxide nanocomposites

Polyoxometalates (POMs) are useful catalysts in both redox and acid catalysis reactions since its catalytic features can be controlled at a molecular level. We investigated POM (POM = H3PW12O40) on tantalum pentoxide (Ta2O5) via hydrolysis of tantalum pentachloride (TaCl5) in the presence of POM using the sol-gel method for acid catalytic reaction. All obtained amorphous materials have been characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), Fourier transform infrared (FT-IR) spectra, Raman scattering spectroscopy, 31P magic-angle spinning (MAS) NMR, field emission scanning electron microscopy (FESEM), and nitrogen adsorption/desorption analysis, in order to confirm the structure integrity of the Keggin unit in as-prepared composites and investigate the morphology and surface textural property of the composites. Catalytic activity of the catalysts has been evaluated for esterification of acetic acid with ethanol as a model reaction with different catalytic reaction parameters such as POM loading, reaction time, catalyst dose, molar ratio of the reactants, etc. The conversion of ethanol reached 87.4 % and the turnover frequency (TOF) was 2.9×103.Meanwhile, it is easy to separate and recover POM/Ta2O5 from the reaction system.

High performance of polyoxometalate/PtPd nanoparticles/carbon nanotubes electrocatalysts for the methanol electrooxidation

A novel method was put forward to synthesize PtPd nanoparticles deposited on multiwalled carbon nanotubes (MWCNTs) in the presence of polyoxometalate (PMo). In the synthesis of PMo/PtPd/MWCNT electrocatalysts, PMo plays an important role in the formation and distribution of PtPd nanoparticles on the surface of MWCNTs. It is founded that PtPd nanoparticles are deposited uniformly on the surface of MWCNTs in high density by the assistant of PMo; most of PtPd nanoparticles are aggregated, and their dispersion is uneven without PMo in synthesis. The composition of PtPd nanoparticles is optimized by tuning the Pt/Pd ratio. Among these Pt-based catalysts, it is founded that PMo/Pt4Pd1/MWCNT catalysts have smallest PtPd nanoparticles with an average diameter of 1.9 nm. The smallest size of PtPd nanoparticles enables PMo/Pt4Pd1/MWCNT catalysts to possess a highest electrochemical surface area, and thus a largest current density in methanol oxidation. In long-term methanol oxidation, PMo/Pt4Pd1/MWCNT catalysts show higher stability than Pt4Pd1/MWCNT catalysts. The current density of PMo/Pt4Pd1/MWCNT catalysts remain 203 mg−1 even after 3000 s continuous electrocatalysis at 0.6 V, being 12 times higher than that of Pt4Pd1/MWCNT catalysts. CO-stripping voltammetry reveals PMo is beneficial to oxidative removal of adsorbed CO at Pt active site. On the other hand, Pd content also has important effect on CO tolerance of Pt catalysts. It can reduce the overpotential of CO oxidation at these Pt catalysts, gives a high CO removal efficiency.

Effect of molybdenum and tungsten polyoxometalates on the composition of surface layers and electrochemical behavior of stainless steel in sulfuric acid

The composition and thickness of the surface layers formed on ferrite steel at various applied potentials in pure 1 N H2SO4 and in the presence of polyoxometalate (POM) anions ( in cationic composition are observed near the passivation potential. At anodic potentials the surface oxide layer is enriched by chromium oxide. The thickness of this layer does not exceed 2-3 nm. If polymolybdate or polytungstate anions are added to sulfuric acid, they are located mostly in the uppermost oxide layer. The anodic currents decrease notably with an increase in POM concentration. At the passivation potential the oxide layer thickness exceeds 3-4 nm and it is substantially enriched by molybdenum. On the samples kept for 1 h in 1 N H2SO4, a protective layer is formed in the presence of POM and an after-affect is achieved. This layer remains stable upon transfer into pure 1 N H2SO4, especially in the case of

Photochemical in situ elaboration of polyoxometalate (α-[SiMo12O40]4−)/polymer hybrid materials

α-Silicon polyoxomolybdates in combination with an iodonium salt and/or a silane are used to generate phenyl and/or silyl radicals as well as silylium cations upon UV light irradiation. These species can initiate the radical photopolymerization of acrylates, the cationic photopolymerization of epoxides and the photopolymerization of epoxy/acrylate blends (i.e. for the synthesis of interpenetrated polymer networks) thereby leading to the formation of polyoxometalate/polymer hybrid materials. The mechanical properties of e.g. the polyether films are affected by the presence of polyoxometalate in the matrix as shown by their dynamic mechanical analysis (DMA). The photochemical mechanisms are studied by steady state photolysis, cyclic voltammetry, and electron spin resonance techniques.

Novel MoO2/carbon hierarchical nano/microcomposites: synthesis, characterization, solid state transformations and thiophene HDS activity

Novel MoO(2)/C nano/microcomposites were prepared via a bottom-up approach by hydrothermal carbonization of a solution of glucose as a carbon precursor in the presence of polyoxometalates (POMs: phosphomolybdic acid [H(3)PMo(12)O(40)] and ammonium heptamolybdate tetrahydrate [(NH(4))(6)Mo(7)O(24)]·4H(2)O). The structural characterization by FT-IR, XRPD, SEM and TEM analyses revealed the controlled formation of hierarchical MoO(2)/C composites with different morphologies: strawberry-like, based on carbon microspheres decorated with MoO(2) nanoparticles; MoO(2)/C core-shell composites; and irregular aggregates in combination with ring-like microstructures bearing amorphous Mo species. These composites can be fine-tuned by varying reaction time, glucose/POM ratio and type of POM precursor. Subsequent transformations in the solid state through calcinations of MoO(2)/C core-shell composites in air lead to hollow nanostructured molybdenum trioxide microspheres together with nanorods and plate microcrystals or cauliflower-like composites (MoO(2)/C). In addition, the MoO(2)/C composite undergoes a morphology evolution to urchin-like composites when it is calcined under nitrogen atmosphere (MoO(2)/C-N(2)). The MoO(2)/C strawberry-like and MoO(2)/C-N(2) composites were transformed into Mo carbide and nitride supported on carbon microspheres (Mo(2)C/C, MoN/C, and MoN/C-N(2)). These phases were tested as precursors in thiophene hydrodesulphurization (HDS) at 400 °C, observing the following trend in relation to the thiophene steady-state conversion: MoN/C-N(2) > MoN/C > Mo(2)C/C > MoO(2)/C-N(2) > MoO(2)/C. According to these conversion values, a direct correlation was observed between higher HDS activity and decreasing crystal size as estimated from the Scherrer equation. These results suggest that such composites represent interesting and promising precursors for HDS catalysts, where the activity and stability can be modified either by chemical or structural changes of the composites under different conditions.