Keggin Structure Research Articles

Barrel-Shaped-Polyoxometalates Exhibiting Electrocatalytic Water Reduction at Neutral pH: A Synergy Effect.

Two copper-based barrel-shaped polyoxometalates (POMs), namely, [{H3O}4{Na6(H2O)22}][{CuI (H2O)3}2{CuII (H2O)}3{B-α-BiIIIWVI9O33}2]·7H2O (NaCu-POM) and Li4[{NH4}2{H3O}3{Li(H2O)5}][{CuII(SH)}{(CuIICuI1.5)(B-α-BiIIIWVI9O33)}2]·9H2O (LiCu-POM) have been synthesized and structurally characterized. The single-crystal X-ray diffraction analyses of NaCu-POM and LiCu-POM reveal the presence of penta- and hexa-nuclear copper wheels per formula units, respectively; these copper wheels are sandwiched between two lacunary Keggin anions {B-α-BiIIIWVI9O33}9- (BiW9) to form the barrel-shaped title POM compounds. In both the compounds NaCu-POM and LiCu-POM, the mixed-valent copper centers are present in their respective penta- and hexa-nuclear copper wheels, established by X-ray photoelectron spectroscopy (XPS) as well as by bond valence sum (BVS) calculations. Compound LiCu-POM additionally shows the presence of a sulfhydryl ligand (SH-), coordinated to one of the copper centers of its {Cu6}-wheel, that is expected to be generated from the in situ reduction of sulfate anion present in the concerned reaction mixture (lithium-ion in ammonia solution may be the reducing agent). Interestingly, the title compounds, NaCu-POM and LiCu-POM exhibit an efficient electrocatalytic hydrogen evolution reaction (HER) by reducing water at neutral pH. Detailed electrochemical studies including controlled experiments indicate that the active sites for this electrocatalysis are the W(VI) centers of the title compounds, not the copper centers. However, a relevant tri-lacunary Keggin cluster anion {PVWVI9O33}7- (devoid of copper ion) does not show comparable HER as shown by the title compounds. The intra-cluster cooperative interactions of the mixed-valent copper centers (CuII/CuI) with the tungsten centers (W6+) make the overall system electrocatalytically active toward water reduction to molecular hydrogen at neutral pH. High Faradaic efficiencies (89 and 92%) and turnover frequencies (1.598 s-1 and 1.117 s-1) make the title compounds NaCu-POM and LiCu-POM efficient catalysts toward electrochemical water reduction to molecular hydrogen.

Preparation of α-MoO3 from H3PMo12O40 precursor: synthesis of 1,2-cyclohexanediol from cyclohexene over α-MoO3-TiO2 catalyst

A series of α-MoO3-TiO2 mixed oxides were prepared by calcining a mixture of the heteropolyacid H3PMo12O40 and TiO2 at temperatures ranging from 350 °C to 600 °C. The mixed oxides thus prepared were characterized and tested for the oxidation of cyclohexene by the oxidizing mixture H2O2/CO2. FTIR and XRD characterizations showed that the Keggin structure of H3PMo12O40 was preserved for calcination temperatures below 400 °C. Above 450 °C, Keggin’s structure collapses. XRD analysis revealed that as the calcination temperature increased, more orthorhombic α-MoO3 was formed. Analysis of the reaction mixture by GC-MS showed that oxidation by the H2O2/CO2 mixture leads to 1,2-cyclohexanediol as the main product and to 2-cyclohexene-1-one and 2-cyclohexene-1-ol as minor products. Oxidation by H2O2/CO2 mixture proved to be more effective than H2O2 alone and CO2 alone. The conversion (69.4%) and the 1,2-cyclohexanediol selectivity (93.2%) obtained over α-MoO3-TiO2 mixed oxides, higher than that obtained with TiO2 monoxide and α-MoO3 monoxide, suggest a synergistic effect between TiO2 and α-MoO3. This efficient and stable catalyst after reuse can be developed for the synthesis of diols.

Modification of Keggin anion structure with ion beams—A new spectroscopic insights into the effects of keV‐ and MeV‐ion beam irradiation on 12‐tungstophosphoric acid

Ion beam irradiation is a versatile tool for structural modification and engineering of new materials. In this study, 12-tungstophosphoric acid (WPA) films of different thickness were spin-coated on platinized silicon substrate and irradiated with low energy hydrogen ions (10 keV) and swift heavy ions (Bi, Xe, and V) with energies up to 710 MeV. The different energy/fluence combinations allowed controllable structural changes that were investigated in detail using Raman and Infrared spectroscopy. For 120-nm-thick WPA samples, the irradiation led to the decrease of intensity of the skeletal and W-Oc-W bands of Keggin anion in order: Bi < V < Xe (for their applied energy/fluence combination). Also, symmetry change of Keggin anion similar to the one observed in the case of Keggin anions interacting with the supports was observed. For the selected ion beam irradiation parameters, xenon ion beam induced transformation of WPA to polytungstate. For 20-μm-thick WPA samples, the irradiation with hydrogen ion beam induced changes of skeletal vibrations and increased individualistic behavior of Keggin anions. As the fluence increased, the amount of the Keggin anions partially transformed to bronze also increased. Irradiation with vanadium also caused transformation to bronze-like structure but with higher ratio of terminal W=Od bonds. The overall results show clear correlation between degree of structural modification of WPA and the calculated displacement per atom value. These results open possibilities for engineering new catalytically active structures of polyoxometalates with the help of ion beams.

Cooperative catalytic effects between aqueous acidic ionic liquid solutions and polyoxometalate-ionic liquid in the oxidative depolymerization of alkali lignin

The oxidative depolymerization of alkali lignin can be achieved by using Keggin structure polyoxometalate-ionic liquid (POM-IL) as catalyst under aerobic conditions. The effects of catalysts such as [HC4im]3PMo12O40, [HC4im]5PV2Mo10O40, [HC2im]5PV2Mo10O40, and [HC4im]4SiMo12O40 (abbreviated as POM-IL1, POM-IL2, POM-IL3, and POM-IL4, respectively), the acidity of aqueous IL solutions on lignin conversion, and aromatic product distribution were investigated. Under optimized conditions, phenolic products (mainly m (p)-cresol, veratrole, vanillin) had the highest yield and the selectivity was as high as 75.0% using POM-IL1 as the catalyst. POM-IL2 and POM-IL3 were used as the catalysts, the highest conversion of 76.0% of lignin was achieved, and the overall yield of ketone products were higher on lignin samples, in which the main products include acetovanillone, 3-methoxyphenol, and 4-methylcatechol, and the highest selectivity of ketone products was 76.8%. The results show that the lignin fragments could be successfully oxidized into platform compounds in aqueous acidic ILs (AILs) solutions catalyzed by Keggin structure POM-IL. In addition, the role of AILs and POM-ILs in the oxidative depolymerization of alkaline lignin was analyzed, and the possible reaction mechanism between AIL-water/POM-IL/lignin system was speculated.

Incorporating heterogeneous lacunary Keggin anions as efficient catalysts for solvent-free cyanosilylation of aldehydes and ketones

Polyoxometalates (POMs) as efficient catalysts can be used a wide range of chemical transformations due to their tunable Brønsted/Lewis-acidity and redox properties. Herein, we reported two hybrid and heterogeneous lacunary Keggin catalysts: (TBA)7[PW11O39] (TBA-PW11) and (TBA)8[SiW11O39]·4H2O (TBA-SiW11) (TBA+: tetrabutylammonium) in which [XW11O39]n− anions were coated by TBA+ cations. In this form, TBA+ can easily trap reactants on the surface of the catalysts and increase the catalytic reaction. Therefore, the catalytic performance of both POMs was tested in cyanosilylation of numerous compounds bearing-carbonyl group and trimethylsilyl cyanide under solvent-free conditions. TBA-PW11 is more effective than TBA-SiW11, conceivably due to the higher Lewis acidity of the P than the Si center and to the higher accessibility of the metal centers in the framework structure. Noteworthy, the recyclability and heterogeneity of both POMs catalysts were also examined, and the results confirmed that they remain active at least after three recycling procedures.

V-Containing Heteropoly Acids with Keggin Structure as Precursors of Sulfide Catalysts: Regularities of the Convertion of Dibenzothiophene and Naphthalene on Nonpromoted Mo–V Catalysts

Mixed PMoV heteropoly acids with Keggin structures H3+xPMo12–xVxO40 (x = 1–6) and modified sulfide catalysts on their basis were synthesized, their physicochemical characteristics were determined, and their performance in the reactions of hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation of naphthalene was evaluated. Modification of the Mo-containing catalysts with vanadium by impregnation with a solution of mixed heteropoly acids was found to enhance their catalytic activity in the studied reactions, as well as to increase the selectivity for the direct hydrodesulfurization route of the DBT HDS reaction.

The molybdenum storage protein forms and deposits distinct polynuclear tungsten oxygen aggregates

Some N2-fixing bacteria store Mo to maintain the formation of the vital FeMo-cofactor dependent nitrogenase under Mo depleting conditions. The Mo storage protein (MoSto), developed for this purpose, has the unique capability to compactly deposit molybdate as polyoxometalate (POM) clusters in a (αβ)3 hexameric cage; the same occurs with the physicochemically related tungstate. To explore the structural diversity of W-based POM clusters, MoSto loaded under different conditions with tungstate and two site-specifically modified MoSto variants were structurally characterized by X-ray crystallography or single-particle cryo-EM. The MoSto cage contains five major locations for POM clusters occupied among others by heptanuclear, Keggin ion and even Dawson-like species also found in bulk solvent under defined conditions. We found both lacunary derivatives of these archetypical POM clusters with missing WOx units at positions exposed to bulk solvent and expanded derivatives with additional WOx units next to protecting polypeptide segments or other POM clusters. The cryo-EM map, unexpectedly, reveals a POM cluster in the cage center anchored to the wall by a WOx linker. Interestingly, distinct POM cluster structures can originate from identical, highly occupied core fragments of three to seven WOx units that partly correspond to those found in MoSto loaded with molybdate. These core fragments are firmly bound to the complementary protein template in contrast to the more variable, less occupied residual parts of the visible POM clusters. Due to their higher stability, W-based POM clusters are, on average, larger and more diverse than their Mo-based counterparts.

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Metal-exchanged phosphotungstate nanoparticles with improved acidity as the catalyst for esterification of glycerol with acetic acid

Abstract A series of metal-exchanged phosphotungstate salts, Fe-TPA, Cr-TPA, Cu-TPA, and Ni-TPA were synthesized by sonication and tested for glycerol esterification with acetic acid to obtain biofuel additives. A systematic, chemical, structural, and morphological characterization and determination of acidity were used by various analytical techniques. XRD and FT-IR studies confirmed the Keggin structure of all metal exchanged phosphotungstate salts with no impurities coming from metal nitrate salts. The incorporation of metal cations in phosphotungstic acid leads to decreased crystallite size as seen from XRD diffractograms. N2 adsorption-desorption analysis showed that the Type II isotherms were obtained for all metal exchanged phophostungstate salts, characteristic of non-porous or macroporous materials. SEM images of metal exchanged phosphotungstate salts displayed well-shaped crystalline particles. SEM-EDX analysis showed that the calculated tungsten metal ratios are in good agreement with the theoretical ratios of those for all the metal exchanged salts, verified by XRF results. The exchange of protons by the metal cations improved the thermal stability observed in the TGA analysis. The acidity strengths, from NH3-TPD analysis, were found in the following order; TPA &gt; Fe-TPA &gt; Cr-TPA &gt; Cu-TPA. Additionally, pyridine-FTIR gives that the exchanging of transition metal into the phosphotungstic acid decreased the Brønsted acidity while increasing the Lewis acidity and this mostly affects the efficiency of the esterification reaction. The activity of metal exchanged salts was in the given order; Fe-TPA &gt; Cr-TPA &gt; Cu-TPA &gt; Ni-TPA. Among these Fe-TPA salt, having a good ratio of Brønsted and Lewis acidity, have the selectivity for di- and triacetin (68 and 10%), with the highest conversion (96%). The iron phosphotungstate salts are a promising catalyst with sustainable Bronsted acidity during the reaction, high desired product selectivity at low temperature, easy synthesizability, inexpensive, and environmentally friendly.

Rubber-Fe3O4@SiO2@H3PMo12O40 as heterogeneous catalyst for biodiesel production: Optimized by response surface methodology

The aim of this research is to develop magnetically recoverable Heteropoly acid (HPA) based catalyst (Rubber-Fe3O4@SiO2@H3PMo12O40) for biodiesel production. Magnetic Rubber powder with high dispensability, porosity and magnetism were prepared by co-precipitation method and covered by silica using TEOS to form Rubber-Fe3O4@SiO2. Finally, nanostructured catalyst was prepared by impregnation of phosphomolybdic acid (PMA). The proposed catalyst was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Scanning Electron Microscopy (SEM) equipped with Dispersive Energy X-ray spectroscopy (EDS), Thermogravimetric analysis (TGA), The Brunauer-Emmett-Teller (BET) surface area analysis and vibrating sample magnetometer (VSM). The results showed the Keggin structure of the catalyst. The performance of the proposed core-shell nanocomposite to catalyze transesterification reaction was evaluated and optimized by response surface methodology (RSM). According to the results, 3 w/w %, 3 h, 12:1 and 69 °C were found as optimum values for catalyst dosage, reaction time, molar ratio of methanol to oil and reaction temperature, respectively. The production yield was higher than 85% for four runs. At the fifth try, the catalyst performance was decreased slightly to 78.87%.

Silver-Templated γ-Keggin Alkyltin-Oxo Cluster: Electronic Structure and Optical Limiting Effect.

As one of the most representative polyoxometalate (POM) structures, Keggin clusters have attracted considerable attention. Nevertheless, the noble-metal-templated Keggin structure has not been reported to date. In this work, for the first time, a Ag atom was successfully incorporated to template the formation of a γ-Keggin alkytin-oxo cluster. Moreover, the central Ag atom has brought a significant heavy atom effect, showing the important influence on the electronic structure and optical properties. Theoretical calculations demonstrate that the Ag atom affects the frontier molecular orbitals and excited states of the AgSn12 cluster, and also the process of electron transfer. The solid structure of the AgSn12 cluster exhibits a significant third-order nonlinear optical (NLO) response, and an excellent optical limiting effect has been experimentally verified. The success of this work opens the way for the construction and optical properties modulation of noble metal templated Keggin structures.

Silver‐Templated γ‐Keggin Alkyltin‐Oxo Cluster: Electronic Structure and Optical Limiting Effect

AbstractAs one of the most representative polyoxometalate (POM) structures, Keggin clusters have attracted considerable attention. Nevertheless, the noble‐metal‐templated Keggin structure has not been reported to date. In this work, for the first time, a Ag atom was successfully incorporated to template the formation of a γ‐Keggin alkytin‐oxo cluster. Moreover, the central Ag atom has brought a significant heavy atom effect, showing the important influence on the electronic structure and optical properties. Theoretical calculations demonstrate that the Ag atom affects the frontier molecular orbitals and excited states of the AgSn12 cluster, and also the process of electron transfer. The solid structure of the AgSn12 cluster exhibits a significant third‐order nonlinear optical (NLO) response, and an excellent optical limiting effect has been experimentally verified. The success of this work opens the way for the construction and optical properties modulation of noble metal templated Keggin structures.

Preparation, characterization and photocatalytic performance of K8[Co(H2O)W11CdO39]/PANI/V2O5 ternary composite

A new ternary composite, K8[Co(H2O)W11CdO39]/PANI/V2O5, was synthesized by electrostatic self-assembly; K8[Co(H2O)W11CdO39] was prepared by direct synthesis method, PANI/V2O5 was prepared by coating polyaniline (PANI) on the surface of V2O5 particles using chemical oxidation method and (NH4)2S2O8 as the oxidant. First, the catalyst was characterized by infrared (IR) spectroscopy, ultraviolet-visible spectroscopy, X-ray diffraction, scanning electron microscopy and N2 adsorption-desorption. Then, its photocatalytic performance was discussed by degradation of Congo Red as an organic pollutant. The results showed that K8[Co(H2O)W11CdO39]/PANI/V2O5 was successfully composited and its Keggin structure still remained; its decolorization of Congo Red was up to 93.32% with initial concentration of Congo Red of 5 mg/L, pH = 2 and the catalyst was 5 mg. After recycling three times, the ternary composite catalyst maintains good catalytic activity. Therefore, this new ternary composite catalyst has excellent application potential.

Tetrabutyl Ammonium Salts of Keggin-Type Vanadium-Substituted Phosphomolybdates and Phosphotungstates for Selective Aerobic Catalytic Oxidation of Benzyl Alcohol

A series of tetrabutyl ammonium (TBA) salts of V-included Keggin-type polyoxoanions with W (TBA4PW11V1O40 and TBA5PW10V2O40) and Mo (TBA4PMo11V1O40 and TBA5PMo10V2O40) as addenda atoms were prepared using a hydrothermal method. These synthesized materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), UV-Vis diffuse reflectance (DRS UV-Vis), thermogravimetric analysis (TGA), CHN elemental analysis (EA), inductively coupled plasma spectrometry (ICP-MS), and N2 physisorption techniques to assess their physicochemical/textural properties and correlate them with their catalytic performances. According to FT-IR and DRS UV-Vis, (PVXW(Mo)12−XO40)(3+X)− anions are the main species present in the TBA salts. Additionally, CHN-EA and ICP-MS revealed that the desired stoichiometry was obtained. Their catalytic activities in the liquid-phase aerobic oxidation of benzyl alcohol to benzaldehyde were studied at 5 bar of O2 at 170 °C. Independently of the addenda atom nature, the catalytic activity increased with the number of V in the Keggin anion structure. For both series of catalysts, TBA salts of polyoxometalates with the highest V-substitution degree (TBA5PMo10V2O40 and TBA5PW10V2O40) showed higher activity. The maximum benzyl alcohol conversions obtained were 93% and 97% using (TBA)5PMo10V2O40 and (TBA)5PW10V2O40 as catalysts, respectively. In all the cases, the selectivity toward benzaldehyde was higher than 99%.

Polyvinyl Alcohol Incorporated with Mesoporous Phosphotungstic Acid for Direct Methanol Fuel Cell Application

The development of potential and novel proton exchange membrane (PEMs) is imperative for further commercialization of Direct Methanol Fuel Cell (DMFC). In this study, flat sheet hybrid membrane as an alternative PEM made up of polyvinyl alcohol (PVA) and mesoporous phosphotungstic acid-Silica (mPTA-Si) at various loadings of 0.5, 1.0, 1.5 and 2.0 wt% was prepared to examine the desirable properties for DMFC application. The characteristic test such as water uptake and proton conductivity were conducted. As the mPTA-Si loading increases, the proton conductivity of PVA/mPTA-Si membrane increased up to 1.81 x 10-3 Scm-1 with 68% of water uptakes. The obtained results could reflect the enhance performance of mPTA-Si membrane and high potential alternative membrane for DMFC. Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR) results proved a good dispersion and the existence of Keggin anion from mPTA-Si particles formed a strong interfacial bonding between fillers and PVA matrix. Owing to these superior characteristics of the mPTA-Si, PVA/ mPTA-Si membrane is considerably promising materials as PEM for DMFC application.

Catalytic oxidative desulfurization of light gas oil over Keggin-type phosphomolybdic acid supported on TUD-1 metallosilicates

A facile one-pot synthesis was developed to produce a series of metal-substituted TUD-1 mesoporous (MTUD-1) supports. The synthesized supports, with nominal Si/M molar ratio of 40 (where M = Al, Zr, Fe, Cu, or Ce), were impregnated with 20 wt% heteropoly phosphomolybdic acid (HPMo) and applied for the catalytic oxidation desulfurization of light gas oil (LGO). Physicochemical properties of the supports and catalysts were characterized using techniques such as N2-sorption, TPD-NH3, FT-IR, XRD, XPS, and XANES. Powder XRD analyses of the catalysts confirmed the highly dispersed Mo Keggin ion particles on metal substituted TUD-1 supports that that of pristine TUD-1 support. The substitution of heteroatoms in the TUD-1 matrix was verified by FTIR and XPS. The heterogenized molybdenum heteropoly acid was found to retain its Keggin structure on TUD-1 supports. Catalytic oxidation desulfurization performances of the catalysts were evaluated with LGO in a stirred tank batch reactor. Among the catalysts, cerium substituted TUD-1 supported catalyst showed the highest oxidation activity to sulfur compounds of LGO. The oxidation activity of HPMo/CeTUD-1catalyst was optimized by varying the process conditions. Besides a catalyst, ODS of LGO required an oxidant. The efficiency of oxidants for ODS of LGO over HPMo/CeTUD-1 followed the order: cumene hydroperoxide > tert-butyl hydroperoxide > hydrogen peroxide > molecular oxygen. The ODS activity of heterogenized molybdenum heteropoly acid is promoted by the Lewis acidity of CeTUD-1.

Two Coordination Polymers@Graphene Hybrid Electrodes for High‐Performance Supercapacitors with Enhanced Rate Capability and Specific Capacitance

AbstractHybrids of polyoxometalate(POM)/transition‐metal complexes/graphene‐based electrode materials not only integrate the advantages of the individual components completely but also ameliorate the shortcomings to some extent, providing an efficient approach to fabricate high‐performance supercapacitors. In this work, we synthesized two novel isostructural compounds (Co(itmb)4[H2VW12O40] ⋅ 4H2O (1) and Zn(itmb)4[H2VW12O40] ⋅ 3H2O (2)) based on the Keggin ions via a hydrothermal method (itmb=1‐(imidazo‐1‐ly)‐4‐(1,2,4‐triazol‐1‐ylmethyl)benzene). The specific capacitance of 1@50 %graphene (405.3 F g−1) and 2@50 %graphene (339.2 F g−1) were 1.33 and 1.19 times of 1 and 2, respectively. In addition, the 1@50 %graphene (93.5 %) and 2@50 %graphene (96 %) possess higher rate performance than 1 (90 %) and 2 (91 %), suggesting a significant enhancement of the electrical performance.

Interaction of Ru(II) polypyridyl complex cations and Keggin anions [α-SiW12O40]4− in self-assembled organic-inorganic networks

Some Keggin polyoxometalates (POMs) have shown interesting photocatalytic properties. However, because most POMs primarily absorb ultraviolet light, their ability to use solar energy is limited. In this report, we have shown that photosensitizers such as ruthenium (II) polypyridyl complexes can be linked to POMs to form hybrid solids: ([Ru(bpy)3]2[SiW12O40] (1) (bpy = 2,2′-bypyridine) and [Ru(bpy)2(MeCN)2]2[SiW12O40]·4.5H2O (2)) with broad absorption band in the UV–visible range. X-ray crystallographic studies showed that both compounds contain the [α-SiW12O40]4− units and the ruthenium (II) polypyridyl units, which form three-dimensional frameworks through noncovalent bonds such as CH⋯O and anion-π interactions. These solids were also studied using thermogravimetric analysis, FT-IR, FL, and UV–vis spectroscopy. Additionally, compound 1 shows strong luminescence at 450 nm in its solid state at room temperature.

How the Cobalt Position in the Keggin Anion Impacts the Activity of Tungstate Catalysts in the Furfural Acetalization with Alkyl Alcohols

AbstractKeggin heteropolyacids (HPAs) comprise a class of versatile catalysts, which have properties that can be easily adjusted through modification in their structure, such as the protons exchange, the removal or one or more MO unity (i. e., W or Mo) followed by doping with transition metals, or even less frequently, the substitution of heteroatom (i. e., P or Si). In this work, these effects were assessed by modifying the structure of silicotungstic acid (i. e., H4SiW12O40) with Co2+ cations. A series of Cobalt‐polyoxometalates (i. e., Co2SiW12O40, K6CoW12O40, K8SiW11O39, K6SiCoW11O39, K8CoW11CoO39) were synthesized and catalytically evaluated in furfural acetalization with alkyl alcohols. Spectroscopy analyses (i. e., UV‐Vis, FT‐IR, EDS), powder X‐ray diffraction patterns, scan electronic microscopy images, surface area, porosity, and thermal analyses provided the main physicochemical properties of the synthesized salts. Acidity properties were assessed by potentiometric titration and adsorbed Py‐FT‐IR spectroscopy. The impact of temperature, nature, and catalyst loading, and type of alcohol were investigated. The K6CoW12O40 salt was the most active and selective catalyst. We paid special attention to correlating the catalyst activity with its composition and properties.

Promoting the effect of cationic substitution on thermal stability and redox properties of new synthesized Keggin type lacunary polyoxometalates (L-POMs) Ni2.5PMo11 M(H2O)O39 (M = Co, Fe, Cu, Zn)

Abstract New lacunary Keggin type polyoxometalate salts (Ni2.5PMo11 M(H2O)O39/M = Co, Fe, Cu, Zn), were synthesized using the inorganic condensation method. X-ray diffraction and infrared spectroscopy results showed that the Keggin structure of the salts was preserved. The thermal stability was checked by differential scanning calorimetry coupled with thermogravimetric analysis showing that the thermal resistance was significantly improved by the incorporation of the metals into polyoxometalates. Proton and molybdenum substitution respectively by nickel and cobalt, iron, copper and zinc metals induced a positive effect on the crystallographic configuration of the prepared salts allowing them a better thermal resistance (up to 580 °C). Cyclic voltammetry revealed an interesting electrochemical property of synthesized samples, especially for Ni2.5PMo11Zn(H2O)O39 sample in which the presence of zinc greatly boosted the redox behavior.