Electrochemical behavior and its electrocatalytic properties of chemically modified electrode with Keggin-type [SiNi(H2O)W11O39]

A monolayer of Keggin-type heteropolyanion [SiNi(H₂O)W₁₁O₃₉]⁶⁻ was fabricated by electrodepositing [SiNi(H₂O)W₁₁O₃₉]⁶⁻ on cysteamine modified gold electrode. The monolayer of [SiNi(H₂O)W₁₁O₃₉]⁶⁻ modified gold electrode was characterized by atomic force microscopy (AFM) and electrochemical method. AFM results showed the [SiNi(H₂O)W₁₁O₃₉]⁶⁻ uniformly deposited on the electrode surface and formed a porous monolayer. Cyclic voltammetry exhibited one oxidation peak and two reduction peaks in 1.0 M H₂SO₄ in the potential range of −0.2 to 0.7 V. The constructed electrode could exist in a large pH (0-7.6) range and showed good catalytic activity towards the reduction of bromate anion (BrO₃⁻) and nitrite (NO₂⁻), and oxidation of ascorbic acid (AA) in acidic solution. The well catalytic active of the electrode was ascribed to the porous structure of the [SiNi(H₂O)W₁₁O₃₉]6⁻ monolayer.     

Solution and solid phase electrochemical behaviour of [Os(bpy)3]3[P2W18O62]

[Os(bpy)₃]₃[P₂W₁₈O₆₂] has been synthesised and characterised by elemental analysis, spectroscopic (UV-vis, IR spectroscopy) and electrochemical techniques. In 0.1 M Bu₄NPF₆ DMSO the complex shows a series of redox couples associated with the Os^3+/2+ and bipyridine ligands of the cationic [Os(bpy)₃]²⁺ moiety and the tungsten-oxo framework of the associated Dawson parent heteropolyanion, [P₂W₁₈O₆₂]⁶⁻. At this electrolyte concentration, the Os³⁺ redox form of the complex was seen to adsorb onto the electrode surface. When the electrolyte concentration is lowered to 0.01 M Bu₄NPF₆ in addition to the Os^3+/2+ redox couple, the redox process associated with the [P₂W₁₈O₆₂]^8−/7− couple also exhibited properties indicating surface based processes were present. Electroactive films of the complex were formed on carbon macroelectrodes by the redox switching of the transition metal within the complex. Voltammetric investigations into the film's behaviour in a range of buffer solutions (pH 2.0, 4.5 and 7.0) were performed. The films were found to possess better stability in acidic pH and the same pH dependence for the tungsten-oxo framework of the heteropolyanions as in solution. Solid state electrochemical measurements on mechanically attached microparticles of the complex were performed, with the effect of both the nature and concentration of the aqueous electrolyte on this behaviour being investigated. Upon redox switching between the Os^2+/3+ redox states, there is an associated insertion/expulsion of anions from/to the solution phase. Scanning electron micrographs of these solid state films were attained before and after redox cycling.     

Electrochemical characterization of self-assembly process of nano-scaled polyoxomolybdate (NH4)42[Mo72Mo60O372(CH3COO)30(H2O)72]·ca.300H2O

Cyclic voltammetry and electrochemical admittance were used to characterize the self-assembly process of nano-scaled spheres of polyoxomolybdate (NH₄)₄₂[Mo^VI₇₂Mo^V₆₀O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]·ca.300H₂O·ca.10CH₃COONH₄) or {Mo₁₃₂}. Cyclic voltammetry indicated that the self-assembly process could be detected after 8 h and completed after one day. Electrochemical admittance showed that the molybdenum species in the CH₃COONH₄-CH₃COOH buffer (pH 4.46) were totally different from those in the pure (NH₄)₆Mo₇O₂₄ solution (pH 5.54) and the (NH₄)₆Mo₇O₂₄ solution (pH 4.21) acidified by hydrochloric acid. However, the molybdate species in the latter two solutions should be mixtures of non-protonated and/or protonated [Mo₇O₂₄]⁶⁻ and [Mo₈O₂₆]⁴⁻ anions. Detailed analysis of admittance results could support the existence of the molybdate species related to the {Mo^VI₆} fragment which was one of the components of {Mo₁₃₂} clusters. Admittance results on the self-assembly process were coincident with those from cyclic voltammetry.     

A new type of supramolecular assembly consisting of crown ether complex cation and a polyoxometalate anion: Synthesis and crystal structure of [Na(C20H24O6)(CH3CN)2]2[Mo6O19] · 4CH3CN

A novel supramolecular assembly consisting of sodium-dibenzo-18-crown-6(DB18C6) complex cation [Na(C₂₀H₂₄O₆)(CH₃CN)₂]²⁺ and isopolyanion [Mo₆O₁₉]²⁻ has been demonstrated in the 3D structure of [Na(C₂₀H₂₄O₆)(CH₃CN)₂]₂[Mo₆O₁₉] · 4CH₃CN (1). Weak intermolecular forces (C–HO hydrogen bonds) between isopolyanion and crown ether play a significant role in the construction of supramolecular framework in the crystal structure of 1. Compound 1 has been characterized in the solid state by single crystal X-ray diffraction, IR, CHN analysis, and TGA.     

Encapsulated-polyoxometalates in surfactant/silica gel hybrid films: Electrochemical behavior and main characteristics

Immobilization of various polyoxometalates (POMs) of different sizes and charges in surfactant/silica gel hybrid films is investigated. A change in the electrochemical behavior is noticed between dissolved and immobilized POMs. This change is attributed to the microenvironment effect that POM clusters experience inside the films. The presence of surfactant Triton X-405 in the matrix is essential for these observations. Results from SEM showed rough and porous morphology for the films prepared with surfactant, while smooth and fractured film structure is observed in absence of surfactant. For POMs such as [P₂W₁₈O₆₂]⁶⁻ and [SiW₁₂O₄₀]⁴⁻, the first two one-electron redox waves merged into one two-electron redox wave. However, for other POMs such as [PW₁₂O₄₀]³⁻, [H₄AsW₁₈O₆₂]⁷⁻, [P₂W₁₅Mo₂V^IVO₆₂]⁷⁻, α₁-[P₂W₁₇O₆₁Fe^III]⁷⁻, α₂-[P₂W₁₇VO₆₂]⁸⁻ and [P₈W₄₈O₁₈₄]⁴⁰⁻, the microenvironment effect results only in a shift of the potentials towards the negative values. The magnitude of the shift in potential is shown to decrease with increasing the POM's charge. Results from Particle size analysis showed that Triton X-405 influences the POM's particle size and with increasing the POM's charge, the POM's particle size in presence of Triton X-405 decreases. The latter is attributed to formation of micelles POM-Triton X-405, which consequently create the microenvironment effect for the immobilized POMs. The main characteristics of the surfactant/silica gel immobilized POM films are explored. The influence of the scan rate demonstrates that the redox behavior of the surfactant/silica gel immobilized composites is fast and involves surface-confined electron transfer processes. The study of these films in different proton concentrations showed that they are stable at low pH, and to obtain a better electron transfer, they have to be stored in the reference medium of 0.5 M H₂SO₄.     

A novel disubstituted Lindqvist-type polyoxomolybdate [Te2Mo4O19] supporting two organic vanadyl moieties: Synthesis, characterization, and crystal structure of {[V(2,2-bipy)2]2(4,4-bipy)[Te2Mo4O19]}

The first example of disubstituted Lindqvist-type polyoxomolybdate {[V(2,2-bipy)₂]₂(4,4-bipy)[Te₂Mo₄O₁₉]} has been synthesized hydrothermally and characterized by elemental analyses, XPS, IR, TG-DTA and X-ray single crystal diffraction. The structural analysis shows that the neutral molecular unit [V(2,2-bipy)₂]₂[Te₂Mo₄O₁₉] consists of a novel Lindqvist-type polyanion [Te₂Mo₄O₁₉]⁶⁻ supporting two vanadyl moieties [V(2,2-bipy)₂]³⁺, and such neutral molecules are joined together by π − π stacking interactions between the pyridine groups to form a two-dimensional grid-like network with non-coordinating “guest” 4,4-bipys encapsulated.     

The composite material based on Dawson-type polyoxometalate and activated carbon as the supercapacitor electrode

A supercapacitor electrode assembled from activated carbon (AC) and (NH₄)₆[P₂Mo₁₈O₆₂]·14.2H₂O (P₂Mo₁₈) was fabricated for the first time, and showed remarkable electrochemical performance ascribed to the synergy of the double layer capacitance of AC and the pseudocapacitance of P₂Mo₁₈. The investigations indicate that the AC/P₂Mo₁₈ electrode exhibits a specific capacitance of 275 F g^− 1 at a high current density of 6 A g^− 1, which is substantially larger than the 182 F g^− 1 of the AC electrode. In addition, the AC/P₂Mo₁₈ electrode possesses a remarkable rate capability (89%) when the current density is increased from 2 to 6 A g^− 1.     

[As8W48O184], a new crown-shaped heteropolyanion: Electrochemistry and electrocatalytic properties towards reduction of nitrite

The electrochemical behavior of the new crown-shaped heteropolyanion [As₈W₄₈O₁₈₄]⁴⁰⁻ was investigated. Compared to the analogous complex [P₈W₄₈O₁₈₄]⁴⁰⁻, [As₈W₄₈O₁₈₄]⁴⁰⁻ displayed a close similar behavior. As previously reported for [P₈W₄₈O₁₈₄]⁴⁰⁻, [As₈W₄₈O₁₈₄]⁴⁰⁻ is found to be stable in a large range of pH, roughly from 0 to 8 and found to be slightly more soluble than [P₈W₄₈O₁₈₄]⁴⁰⁻. Comparison between the cyclic voltammetry curves of the two complexes reveals that the potentials of the redox waves of [As₈W₄₈O₁₈₄]⁴⁰⁻ are slightly shifted towards the positive values. Contrary to the high dependence of [P₈W₄₈O₁₈₄]⁴⁰⁻ to buffer capacity of the supporting electrolyte, interestingly, [As₈W₄₈O₁₈₄]⁴⁰⁻ displays less sensitivity. The numerous possibilities for chemical applications opened up by the electrochemistry of this new complex are exemplified here by the good linear relationship between the peak potentials of the redox waves as a function of pH and, by the electrocatalytic reduction of nitrite.     

Chitosan-assisted fabrication and electrocatalytic activity of the composite film electrode of heteropolytungstate/carbon nanotubes

A novel chemically modified electrode based on the immobilization of heteropolytungstate in carbon nanotubes-chitosan (CNTs-chitosan) composite film by one step self-assembly technique was fabricated. Chitosan was chosen as a natural polymer for dispersing CNTs to form a stable CNTs-chitosan composite, which provided an interface containing amino groups for assembling [P₂W₁₈O₆₂]⁶⁻ (abbr. P₂W₁₈) by electrostatic interactions. The CNTs-chitosan composite film was investigated and characterized by infrared spectroscopy (IR), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CNT-chitosan film provides a high active area and good conduction pathway, and enhances the adsorptive capability of P₂W₁₈ molecules on the film. The electrochemical behavior of the P₂W₁₈/CNTs-chitosan/GCE was studied in detail, including scan rate, pH dependence, and stability. The CV and amperometry studies demonstrate that the P₂W₁₈/CNTs-chitosan/GCE has fast response and good electrocatalytic activity for the reduction of S₂O₈²⁻ and IO₃⁻ anions in acidic aqueous solution.     

Ionic and electronic transport in stabilized β-La2Mo2O9 electrolytes

Increasing temperature from 973 to 1173 K leads to a substantial increase of the electronic contribution to the total conductivity of undoped lanthanum molybdate and La₂Mo₂O₉-based solid electrolytes, including La₂Mo_1.7W_0.3O₉, La₂Mo_1.95V_0.05O₉ and La_1.7Bi_0.3Mo₂O₉, where the stabilization of β-La₂Mo₂O₉ down to room temperature was confirmed by high-resolution X-ray diffraction (XRD) and differential scanning calorimetry (DSC) data. In air, the ion transference numbers determined by the modified Faradaic efficiency (FE) technique, decrease from 0.991-0.997 at 973-1023 K down to 0.977-0.984 at 1173 K. Reducing oxygen partial pressure also increases electronic conduction evaluated by the emf and oxygen permeability (OP) measurements, which indicates that the electronic transport is n-type, resulting from decreasing oxygen content in the molybdate lattice. The level of n-type electronic conductivity in air is quite similar for all La₂Mo₂O₉-based ceramics. The results show that these materials can be used as solid electrolytes only under oxidizing conditions and only at temperatures below 1073 K. Their practical applications may also be complicated due to relatively high thermal expansion coefficients (CTEs), (14.1-14.8)×10⁻⁶ K⁻¹ at 300-700 K and (16.4-22.5)×10⁻⁶ K⁻¹ at 850-1070 K, which are close to those of stabilized δ-Bi₂O₃ and γ-Bi₂VO_5.5 electrolytes.     

Hexacyanoferrate ion adsorbed on propylpyridiniumsilsesquioxane polymer film-coated SiO2/Al2O3: use in an electrochemical oxidation study of cysteine

Hexacyanoferrate ion, [Fe(CN)₆]⁴⁻, was immobilized by an ion-exchange reaction on the propylpyridiniumsilsesquioxane chloride polymer thin-film-coated SiO₂/Al₂O₃ surface. The amount of [Fe(CN)₆]⁴⁻ immobilized was 0.22 mmol g⁻¹ with a surface coverage of 9.6×10⁻⁶ mmol cm⁻². A carbon paste electrode made with this material was prepared and its electrochemical properties studied. The electrode presented two well-defined redox peaks with midpoint potentials, E_m, of 0.152 V vs SCE. This potential was not significantly affected by pH changes between 2 and 9.5. The electrode showed much reproducible responses and was successfully used to study the electrochemical oxidation of cysteine.     

Hydrothermal synthesis,structure and properties of two new phosphomolybdates based on Strandberg-type {P2Mo5O23}6 − building units

Two inorganic–organic hybrid compounds based on Strandberg-type phosphomolybdates (abbreviated as {P₂Mo₅O₂₃}^6 −) complexes [H₃O]₂[Cu₂(Pyim)₂(H₂O)₃][P₂Mo₅O₂₃] · 6H₂O (1) and [Cu₃(Pyim)₃(H₂O)₄][P₂Mo₅O₂₃] · 7H₂O (2) were successfully synthesized at different pH values under hydrothermal conditions. Both 1 and 2 are based on the {P₂Mo₅O₂₃}^6 − clusters connected by copper complexes to generate one dimensional chain. The magnetic analyses of compounds 1 and 2 indicate antiferromagnetic interactions between copper ions.     

Surface immobilisation of transition metal substituted Krebs type polyoxometalates, [X2W20M2O70(H2O)6] (X = Bi or Sb, M = Co or Cu), by the layer by layer technique

A series of transition metal (i.e. Cu²⁺ and Co²⁺) substituted Krebs type polyoxometalates (POMs), of the general formula [X₂W₂₀M₂O₇₀(H₂O)₆]ⁿ⁻, X = Sb or Bi, M = Co(II) or Cu(II), have been successfully immobilised onto carbon electrode surfaces through the employment of the layer-by-layer (LBL) technique. This involved the construction of alternating anionic POM, [X₂W₂₀M₂O₇₀(H₂O)₆]ⁿ⁻, layers and the cationic metallodendrimer, Ru(II)-metallodendrimer as the cationic layers, in addition to a [poly(diallyldimethylammonium chloride)] PDDA base layer. Stable multielectron redox couples associated with the W–O framework, for the Krebs type POMs, and the Ru(III/II) for the metallodendrimer, were clearly observed upon layer construction and redox switching within the pH domain of 2–6.5. The constructed multilayer assemblies exhibited pH dependent redox activity and thin layer behaviour up to 100 mV s⁻¹. The porosity and permeability of the individual multilayer assemblies towards an anionic probe were determined by AC impedance and cyclic voltammetry. The surface morphology of each multilayer was also determined by Atomic Force Microscopy (AFM).     

Fabrication of carbon paste electrode containing [PFeW11O39] polyoxoanion supported on modified amorphous silica gel and its electrocatalytic activity for H2O2 reduction

[PFeW₁₁O₃₉]⁴⁻ (PFeW₁₁) supported on the surface of 3-aminopropyl(triethoxy)silane modified silica gel was synthesized and used as a bulk modifier to fabricate a renewable three-dimensional chemically modified electrode. The electrochemical behavior of the modified electrode was investigated. Cyclic voltammetry studies showed that the PFeW₁₁ on the electrode surface sustained the same electrochemical properties as that of the PFeW₁₁ in solution. The preparation of chemically modified electrode is simple and quiet reproducible using inexpensive material. The modified electrode had high electrocatalytic activity toward H₂O₂ reduction and it was successfully applied as an electrochemical detector to monitor H₂O₂ in flow injection analysis (FIA). The electrocatalytic peak current was found to be linear with the H₂O₂ concentration in the range 10-200 μmol L⁻¹ with a correlation coefficient of 0.998 and a detection limit (3σ) of 7.4 μmol L⁻¹ H₂O₂. The electrode has the remarkable advantage of surface renewal owing to bulk modification, as well as simple preparation, good mechanical and chemical stability and reproducibility.     

A multifunctional organic-inorganic multilayer film based on tris(1,10-phenanthroline)ruthenium and polyoxometalate

The novel multifunctional thin films composed of transition metal complex tris(1,10-phenanthroline)ruthenium(II) Ru(phen)₃Cl₂ (abbr Ru(phen)₃, phen = 1,10-phenanthroline), and Dawson-type polyoxometalate [P₂Mo₁₈O₆₂]⁶⁻ (abbr P₂Mo₁₈) were fabricated on quartz, silicon and ITO substrates by layer-by-layer (LBL) method. The LBL films deposition were found to be linearly related to the number of bilayers as monitored by UV-vis spectroscopy. And the compositions of the films were measured by X-ray photoelectron spectra (XPS). The result of atomic force microscopy (AFM) revealed a relatively uniform surface morphology of the multilayer films. In particular, the films exhibited the photo-luminescence arising from π^*-t_2g ligand-to-metal transition of Ru(phen)₃ and catalytic activity to the reduction of NO₂⁻ attributing to molybdenum-centered redox processes of P₂Mo₁₈.     

Electrochemical properties and structural characterization of layered Li[Ni0.5Mn0.5]O2 cathode materials

Layered Li[Ni_0.5Mn_0.5]O₂ materials with high homogeneity and crystallinity were prepared using high speed ball milling. The Li[Ni_0.5Mn_0.5]O₂ electrode delivered a high discharge capacity of 152 mA h g⁻¹ between 2.8 and 4.3 V with excellent cycleability. The TEM analysis showed that the Li[Ni_0.5Mn_0.5]O₂ electrode went through a considerable morphological change without altering its initial layered structure while the electrode retained its initial discharge capacity even after 50 cycles.     

Fabrication of CeO2 nanoparticles modified glassy carbon electrode and its application for electrochemical determination of UA and AA simultaneously

A glassy carbon electrode modified with CeO₂ nanoparticles was constructed and was characterized by electrochemical impedance spectrum (EIS) and cyclic voltammetry (CV). The resulting CeO₂ nanoparticles modified glassy carbon electrode (CeO₂ NP/GC electrode) was used to detect uric acid (UA) and ascorbic acid (AA) simultaneously in mixture. This modified electrode exhibits potent and persistent electron-mediating behavior followed by well-separated oxidation peaks towards UA and AA with activation overpotential. For UA and AA in mixture, one can well separate from the other with a potential difference of 273 mV, which was large enough to allow the determination of one in presence of the other. The DPV peak currents obtained in mixture increased linearly on the UA and AA in the range of 5.0 × 10⁻⁶ to 1.0 × 10⁻³ mol/L and 1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ mol/L, with the detection limit (signal-to-noise ratio was 3) for UA and AA were 2.0 × 10⁻⁷ and 5.0 × 10⁻⁶ mol/L, respectively. The proposed method showed excellent selectivity and stability, and the determination of UA and AA simultaneously in serum was satisfactory.     

Intercluster porous nanocomposites from combination of [AlO4Al12(OH)24(H2O)12]7+ cation and Anderson-type [Al1? x Cr x Mo6O24H6]3? anion: XRD, TG-DTA, SEM-EDAX and EPR studies

The synthesis of intercluster porous nanocomposites obtained from polyoxometallate compounds such as the [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ cation (named Al₁₃) and the Anderson-type [Al_1−x Cr _x Mo₆O₂₄H₆]³⁻ anion (named Al_1−x Cr _x Mo₆) has been performed in order to study the interaction between the two cluster ions, the stability of the XMo₆ planar configuration, the products obtained after thermal treatment, the structure and the local symmetry of the Cr³⁺ species. Chemical, thermal, structural and spectroscopic characterizations of the original and thermally treated phases have been followed by different techniques such as TG-DTA, XRD, SEM-EDAX, and mainly by EPR. All the results have shown that the structure of the intercluster nanocomposites (Al₁₃)(Al_1−x Cr _x Mo₆)₂ precursors is similar to that reported by Son et al. for the chromium-free (Al₁₃)(AlMo₆)₂ intercluster nanocomposite [Son et al., J. Am. Chem. Soc. 122 (2000) 7432]. After thermal treatment in air at several temperatures of the (Al₁₃)(Al_1−x Cr _x Mo₆)₂ nanocomposites the following phases have been observed and characterised: (i) at 400 °C an amorphous phase containing dispersed Cr³⁺ ions; (ii) at 700 °C a crystalline phase corresponding to Cr₂(MoO₄)₃/Al₂(MoO₄)₃ solid solutions; (iii) at 950 °C α-Al₂O₃/Cr₂O₃ solid solutions with a random dispersion of the Cr³⁺ ions.     

Novozym 435 catalyzed regioselective acylation of ethane-1,2-diol in the presence of ionic liquids

The acylation of ethane-1,2-diol catalyzed by lipase (Novozym 435) with ethyl acetate in 1-butyl-3-methyl imidazolium hexaflorophosphate [BMIM][PF₆] and 1-butyl-3-methyl imidazolium tetrafloroborate [BMIM][BF₄] was studied. It was demonstrated that the activity of the enzyme in [BMIM][BF₄] was very low for this reaction, while it was very active in [BMIM][PF₆]. Both the equilibrium conversion and the selectivity to ethane-1,2-diol monoacetate (EDMA) in [BMIM][PF₆] could be considerably higher than that at solvent-free condition. The IL and enzyme have synergetic effect to prevent thermal deactivation of the enzyme. Addition of small amount of water in [BMIM][PF₆] could enhance the conversion and selectivity to EDMA.     

Synthesis, crystal structure and NMR of [Na(DB18C6)(CH3CN)]3[α-AsM12O40] (M = Mo/W)

Two super-molecular complexes [Na(DB18C6)(CH₃CN)]₃[α-AsM₁₂O₄₀] (M = Mo/W) were obtained by solvothermal reaction and characterized by IR,¹H ¹³C and gUMBC NMR, X-ray. The result reveals that the complex consists of a [α-AsM₁₂O₄₀]³⁻ (M = Mo/W) anion with α-Keggin structure, and three complex [Na (DB18C6)(CH₃CN)]⁺ cations in which every sodium ion is located in the cavity of dibenzo-18-crown-6 with 6 Na–O bonds and coordinated with one of the terminal O atom of [α-AsM₁₂O₄₀]³⁻ (M = Mo/W) and the N atom of CH₃CN from two sides of the distorted DB18C6 plane, respectively. The three terminal O atoms linked with sodium ion are from a single M₃O₁₃ (M = Mo/W) triplet of the α-Keggin metalatoarsenate anion, and M–O_b (M = Mo/W) bonds exhibit alternating single–double bond character.