Tungsten Salts Research Articles

ChemInform Abstract: Solvent‐Free Hydration of Alkynes over a Heterogeneous Silver Exchanged Silicotungstic Acid Catalyst.

AbstractA silver tungstate salt is used as an efficient and reusable heterogeneous catalyst for the conversion of terminal alkynes (Ig) into corresponding ketones (II).

WO3-modified TiO2 nanotubes for photocatalytic elimination of methylethylketone under UVA and solar light irradiation

We report on the synthesis, the characterizations and the photocatalytic activity of WO3/TiO2 nanotubes for methylethylketone (MEK) elimination under both UVA and simulated-solar light irradiations. TiO2 nanotubes (TiNTs) were synthetized using the hydrothermal method in a concentrated 10 M NaOH solution heated at 130 °C during 48 h, post-impregnated with tungstate salt and finally calcined. The photocatalytic efficiency for both UVA and solar light irradiation strongly depends on the WO3 content, ranging from 4 to 51 wt.%. The different photocatalytic activities were analysed, in terms of initial and stabilized activities and deactivation phenomena. Complementary post-tests characterizations (TGA and XPS) aiming at the establishment of correlations between WO3 content, surface composition, surface species and the photocatalytic activity and deactivation are discussed.

Use of carbon nanotubes and electrothermal atomic absorption spectrometry for the speciation of very low amounts of arsenic and antimony in waters

A procedure for the determination of inorganic arsenic (III, V) and antimony (III, V) in water samples by using a miniaturized solid-phase extraction with carbon nanotubes followed by electrothermal atomic absorption measurement is proposed. The trivalent species are first complexed with ammonium pyrrolidinedithiocarbamate, next retained in a mini-column containing nanotubes and then eluted by means of a plug of an ammonia solution. The atomizer is impregnated with a tungsten salt which acts as an effective chemical modifier during the heating cycle. Total inorganic arsenic and antimony are determined after the reduction of the pentavalent forms with an ammonium iodide solution. Pentavalent arsenic and antimony are calculated by difference. When using 50mL sample solutions, the limits of detection are 0.02 and 0.05μgL−1 for As and Sb, respectively, and the enrichment factor is 250. The relative standard deviations calculated for five determinations at the 1μgL−1 level are below 4%.

Fabrication of W–20%Cu composite powder from copper and tungsten salts

An investigation has been made to prepare homogeneous powders of CuWO4 and WO3 from ammonium paratungstate and copper nitrate to prepare nanosized W–Cu powder. Hence, a mixture of ammonium paratungstate and copper nitrate with predetermined weight proportion was made in distilled water; while the content of the beaker was being stirred at a certain speed to reach the desired composition of W–20 wt-%Cu. Mixture was heated to 80–100°C for 6 h. Also, pH range was adjusted at about 3–4. The mixture was then evaporated and dried in the air. To reach W–Cu composite powder, the precursor powders burned out at 520°C for 2 h in the air to form W–Cu oxide powder and then were ball milled and reduced in H2 atmosphere to convert it into W–Cu composite powder. The resulting powders were evaluated using scanning electron microscopy, X-ray diffraction, thermogravimetric analysis and differential thermal analysis techniques. The results showed that homogeneous powders of W–Cu with particle size of ∼100 nm and a nearly spherical shape could be obtained by this process.

Structural Distortions upon Oxidation in Heteroleptic [Cp2W(dmit)] Tungsten Dithiolene Complex: Combined Structural, Spectroscopic, and Magnetic Studies

Four different cation radical salts are obtained upon electrocrystallization of [Cp(2)W(dmit)] (dmit = 1,3-dithiole-2-thione-4,5-dithiolato) in the presence of the BF(4)(-), PF(6)(-), Br(-), and [Au(CN)(2)](-) anions. In these formally d(1) cations, the WS(2)C(2) metallacycle is folded along the S···S hinge to different extents in the four salts, an illustration of the noninnocent character of the dithiolate ligand. Structural characteristics and the charge distribution on atoms, for neutral and ionized complexes with various folding angles, were calculated using DFT methods, together with the normal vibrational modes and theoretical Raman spectra. Raman spectra of neutral complex [Cp(2)W(dmit)] and its salts formed with BF(4)(-), AsF(6)(-), PF(6)(-), Br(-), and [Au(CN)(2)](-) anions were measured using the red excitation (λ = 632.8 nm). A correlation between the folding angle of the metallacycle and the Raman spectroscopic properties is analyzed. The bands attributed to the C═C and C-S stretching modes shift toward higher and lower frequencies by about 0.3-0.4 cm(-1) deg(-1), respectively. The solid state structural and magnetic properties of the three salts are analyzed and compared with those of the corresponding molybdenum complexes. Temperature dependence of the magnetic susceptibility shows the presence of one-dimensional antiferromagnetic interactions in the BF(4)(-), PF(6)(-), and [Au(CN)(2)](-) salts, while an antiferromagnetic ground state is identified in the Br(-) salt below T(Néel) = 7 K. Interactions are systematically weaker in the tungsten salts than in the isostructural molybdenum analogs, a consequence of the decreased spin density on the dithiolene ligand in the tungsten complexes.

Self-decontaminating layer-by-layer functionalized textiles based on WO 3-modified titanate nanotubes. Application to the solar photocatalytic removal of chemical warfare agents

Self-decontaminating photocatalytic textiles functionalized by the layer-by-layer approach with high aspect ratio WO 3-modified titanate nanotubes have been developed and successfully used under solar light illumination for the photocatalytic degradation of organophosphorous and organosulfide chemical weapon agents. This multilayer building method, unlike both dipping and spray techniques, has been efficiently applied to textile substrates using polyethyleneimine as counter polyelectrolyte for homogeneously functionalizing textiles by a thin layer of a dense network of entangled WO 3-modified titanate nanotubes, and preparing self-decontaminating photocatalytic textiles highly efficient toward the removal under solar light of the yperite blister live agent and of dimethylmethylphosphonate as neurotoxic agent simulant. Solar light responsive high surface WO 3-modified titanate nanotubes were synthesized by hydro-thermally treating TiO 2 powder, with tungstate salt impregnation prior to the final calcination step.

Investigation on Fabrication of W-Cu Nanocomposite via a Thermochemical Co-Precipitation Method and its Consolidation Behavior

W-25%Cu nanocomposite was produced via a thermochemical co-precipitation procedure. Copper nitrate and sodium tungstate salts were used as Cu and W containing precursors respectively. Aqueous solutions of these salts were reacted under controlled pH condition prepared by ammonia addition and the resulting precipitates were then calcined at 450oC and hydrogen reduced at 800oC. The products of each step were characterized by XRD and Electron Microscope. Using a basic medium with a pH of 13 which caused the formation of complex Cu(NH3)42+ was found to provide suitable condition for precipitation of nanosized composite powders. Cu2WO4(OH)2 and CuWO4.2H2O as raw precipitates, CuWO4-x , CuO, and WO3 as calcined powders, and W-Cu reduced composite powders, all were seen to keep nanosize dimensions through high temperature treatments of fabrication. Sintering of the reduced powders at the temperature of 1150oC led in a density of about 98% theoretical density.

Bulking Up Water Oxidation

![Figure][1] CREDIT: YIN ET AL. Storing solar energy by water oxidation, in a process akin to photosynthesis, is a promising approach for building a renewable energy infrastructure. Unfortunately, many of the most active synthetic catalysts for this process fall prey to degradation by the generated oxygen. Yin et al. (p. [342][2], published online 11 March; see the Perspective by [Hurst][3] ) used bulky polyoxometalate ligands to protect a catalytic cobalt center from this fate. The full complex was easily prepared by mixing proper ratios of inexpensive tungsten, cobalt, and phosphate salts in boiling water. After isolating and redissolving the catalyst in slightly basic aqueous solution, rapid oxygen generation was observed with a ruthenium-based oxidant. [1]: pending:yes [2]: /lookup/doi/10.1126/science.1185372 [3]: /lookup/doi/10.1126/science.1187721

Photo-electrochemical properties of nanostructured WO 3 prepared with different organic dispersing agents

Different chemical routes for obtaining nanostructured WO 3 on transparent conductive oxides have been tried by considering the Santato-Augustynski recipe as a main guideline. The preparation started from tungstate salts as precursors and involved different organic dispersing agents. Calcination in air at 550 °C produced the final oxide. Aim of this work is to test the oxide materials as photoanodes for the photo-electrochemical water splitting. WO 3 is a wide bandgap semiconductor, able to transport photogenerated carriers under irradiation, provided that it has a good crystalline structure. The morphology and the crystal size of the obtained WO 3 nanoparticles have been investigated by SEM and XRD. This work shows that the particle size of the WO 3 film decreases with increase in molecular weight of the organic dispersing agents. The current–voltage characteristic curves of WO 3-based electrodes have been obtained by biasing a photo-electrochemical cell with an external voltage ramp, both in the dark and under simulated solar irradiation. Results showed that under incident irradiance a noticeable photocurrent density is obtained with a reasonably steep current–voltage slope and with a plateau current of the order of 1.6–1.8 mA/cm 2 under incident 0.14 W/cm 2 AM 1.5 irradiation.

Chemical mechanism of precipitate formation and pH effect on the morphology and thermochemical co-precipitation of W–Cu nanocomposite powders

W–25%Cu composite powder was produced via thermochemical procedure. Copper nitrate and sodium tungstate salts were used as Cu and W containing precursors, respectively. Aqueous solutions of these salts were reacted under several different pH conditions. It was found that the products of all stages involving Cu 2WO 4(OH) 2 and CuWO 4·2H 2O as raw precipitates, CuWO 4− x , CuO, and WO 3 as calcined powders, and W–Cu reduced composite powders possessed similar compositions in different pH conditions. But changing the reaction pH in the range of 3–13 was found to markedly influence the microstructure of the products. At low and medium pH precipitates were in the form of large monolithic agglomerates while in high pH smaller cotton like agglomerates were formed. At low and medium pH conditions simple ion replacement was found to be dominant mechanism of precipitation while in high pH Cu(NH 3) 4 2+ complex ion was formed and ligand exchange was the dominant mechanism of precipitation.

Selective Conversion of Fructose to 5‐Hydroxymethylfurfural Catalyzed by Tungsten Salts at Low Temperatures

Selective Conversion of Fructose to 5‐Hydroxymethylfurfural Catalyzed by Tungsten Salts at Low Temperatures

High-yield precipitation synthesis of tungsten oxide platelet particle and its ethylene gas-sensing characteristic

Hydrous tungsten oxide platelet particle was precipitated from acidic solution of an ammonium tungstate at 80 °C. By dissolving the tungstate salt in nitric acid at low temperature before being brought up to elevated temperature, the precipitating products were obtained at very high yield compared to direct mixing of the two reactants at elevated temperature. The yield increased rapidly with an increase of nitric concentration and became constant at moderate acid concentration. Higher yield was obtained from the more concentrated tungstate solution. The as-precipitated powders consisted of mixed hydrotungstite (H 2WO 4·H 2O) and tungstite (WO 3·H 2O). Electron microscopy study on the as-precipitated powder clearly revealed very fine platelet particle having average size of 60 ± 5-nm wide and 15 ± 3-nm thick loosely agglomerated to a size of 300–500 nm. A WO 3 thick film sensor made from the precipitate calcined at 500 °C consisting of 90 ± 10-nm wide and 30 ± 5-nm thick platelet particle was highly sensitive to 3–10 ppm ethylene gas at operating temperature of 300 °C.

Fixation procedures for flow cytometric analysis of environmental bacteria

Analysis of environmental bacteria on the single cell level often requires fixation to store the cells and to keep them in a state as near life-like as possible. Fixation procedures should furthermore counteract the increase of autofluorescence, cell clogging, and distortion of surface characteristics. Additionally, they should meet the specific fixation demands of both aerobically and anaerobically grown bacteria. A fixation method was developed based on metal solutions in combination with sodium azide. The fixation efficiencies of aluminium, barium, bismuth, cobalt, molybdenum, nickel, and tungsten salts were evaluated by flow cytometric measurement of the DNA contents as a bacterial population/community stability marker. Statistical equivalence testing was involved to permit highly reliable flow cytometric pattern evaluation. Investigations were carried out with pure cultures representing environmentally important metabolic and respiratory pathways as controls and with activated sludge as an example for highly diverse bacterial communities. A mixture of 5 mM barium chloride and nickel chloride, each and 10% sodium azide was found to be a suitable fixative for all tested bacteria. The described method provided good sample stability for at least 9 days.

Activation and isomerization of hydrocarbons over WO 3/ZrO 2 catalysts : II. Influence of tungsten loading on catalytic activity: Mechanistic studies and correlation with surface reducibility and tungsten surface species

We studied the correlation among the catalytic behavior of WO 3/ZrO 2 samples toward unsaturated and saturated hydrocarbons transformation, tungsten surface species oxidation states, and the crystallographic structure of the zirconia support. Different tungsten-loaded catalysts were studied, from 9 wt% (near-monolayer coverage) to 30 wt%. The resulting WO 3/ZrO 2 materials were obtained by impregnation of a tungsten salt on either a commercially available monoclinic zirconia or an amorphous hydroxide, ZrO x (OH) 4−2 x , followed by a calcination step (according to the Hino and Arata procedure), leading to a tetragonal structure. In contrast to previous works, here we demonstrate that the crystallographic structure of zirconia has no influence on catalytic properties. Correlations with XPS analyses revealed two aspects of catalytic behavior that depend strongly on the catalyst reducibility and thus on the W surface species oxidation states. First, on hardly reducible (tungsten loadings <15 wt%) or slightly reduced (below 423 K) surfaces, a purely acidic monomolecular mechanism for both isomerization (largely predominant) and cracking reactions, associated with W 6+ and W 5+ surface species, was demonstrated. Second, on easily reducible (tungsten loadings >15 wt%) or deeply reduced (over 723 K) surfaces, a bifunctional mechanism associating dehydrogenating/hydrogenating properties occurring on metallic tungsten and acidic isomerization and cracking on W 5+ and W 6+ surface species was observed. However, in this last case, we could not exclude the participation of a purely metallic isomerization mechanism occurring through σ-alkyl adsorbed species on the β-W metallic phase. A more pronounced reduction then led to an increase in the extensive hydrogenolysis mechanism, causing catalyst deactivation.

Removal of Ammonia from Air on Molybdenum and Tungsten Oxide Modified Activated Carbons

Microporous coconut-based activated carbon was impregnated with solutions of ammonium metatungstate or ammonium molybdate and then calcined in air in order to convert the salts into their corresponding oxides. The surface of those materials was characterized using adsorption of nitrogen, potentiometric titration, Fourier-transform infrared spectroscopy, X-ray diffraction, and thermal analysis. The results indicated a significant increase in surface acidity related to the presence of tungsten or molybdenum oxides. On the materials obtained, adsorption of ammonia from either dry or moist air was carried out. The oxides distributed on the surface provided Lewis and/or Brønsted centers for interactions with ammonia molecules or ammonium ions. Water on the surface of carbon or in the gas phase increased the amount of ammonia adsorbed via involvement of Brønsted-type interactions and/or by leading to the formation of molybdate or tungstate salts on the surface. Although the amount of ammonia adsorbed is closely related to the number of moles of oxides and their acidic centers, the carbon surface also contributes to the adsorption via providing small pores where ammonia can be dissolved in the water film.

Synthesis of Half‐Sandwich Tungsten Chlorogermyl and Chlorostannyl Complexes

AbstractThe reaction of the dichlorogermane Me2GeCl2 and stannanes R2SnCl2 (R = Me, Bu) with the alkali tungsten salts M[(η5‐C5R′5)L(OC)2W] (L = CO, PPh3; M = Li, Na; R′ = H, Me) yields via alkali salt elimination a range of novel chlorostannyl‐ and chlorogermyl‐functionalized tungsten complexes of the general formula [(η5‐C5R′5)L(OC)2W(ER2Cl)] (R′ = H, Me; L = CO, PPh3; E = Ge, Sn; R = Me, Bu) (3a–e). The use of the phosphane‐substituted tungsten anion Na[(η5‐C5Me5)(Ph3P)(OC)2W] (1c) and Me2SnCl2 leads to a mixture consisting of the mono‐ (3e) as well as the di‐substituted stannyl compound [{(η5‐C5H5)(Ph3P)(OC)2W}2(SnMe2)] (4). Access to 3a is also given by reaction of ClMe2Sn–C5H5 (5) with [(MeCN)3W(CO)3] which includes chemoselective tin shift from the cyclopentadiene to the tungsten atom. In addition, a successive chlorination of [(η5‐C5H5)(OC)3W(SnMe3)] (6) via [(η5‐C5H5)(OC)3W(SnMe2Cl)] (3a) and [(η5‐C5H5)(OC)3W(SnMeCl2)] (7) to [(η5‐C5H5)(OC)3W(SnCl3)] (8) is presented. All new compounds have been fully characterized by elemental analyses and in solution by IR and multinuclear NMR spectroscopy. The crystal structures of complexes 3b–d and 8 were presented and discussed, showing bond lengths of 2.6272(4) Å for Ge–W, 2.7959(5) (3b), 2.7866(3) (3d) and 2.7244(3) (8) Å for the Sn–W bonds.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2007)

Tungsten based electrocatalyst for fuel cell applications

A barrier to the widespread use of fuel cells is their reliance on expensive and scarce platinum and other precious metal catalysts. We present a catalyst for hydrogen oxidation, prepared electrochemically from high-purity aqueous tungstate salt precursors. The 24-electron reduction of ammonium metatungstate ((NH 4) 6[H 2W 12O 40]) yields a material with electrocatalytic activity towards the oxidation of hydrogen in acid electrolyte which approaches 25% that of platinum. Moreover, the tungstate catalyst is unusually tolerant to CO and H 2S contaminants in the fuel stream.

Synthesis of tungsten oxide nanoparticles by acid precipitation method

Tungsten oxide (WO 3) nanoparticle having average size of 30 nm, and nanoplate having average dimension of 190 ± 15 nm wide and 50 ± 5 nm thick were prepared by controlling a precipitation of ammonium tungstate para pentahydrate with nitric acid. The two nanostructured microstructures were obtained by varying the stirring time during the precipitation reaction while the other synthesis parameters were identical. Phase transition of the WO 3 precipitate was investigated by means of thermal analysis and X-ray diffraction, while the microstructure was observed by using a scanning electron microscope and a transmission electron microscope. The effect of the concentrations of the tungstate salt and nitric acid on the rate of precipitation and the percent yield of the precipitating product is also described.

Phosphine-Substituted Diborane(4)yl Complexes of Tungsten

The reaction of the 1,2-dihalodiborane(4) B2(NMe2)2Cl2 with the lithium tungsten salts Li[(η5- C5H5)(R3P)(OC)2W] [R = Me (3a), Ph (3b)] yields via alkali salt elimination the phosphinesubstituted diborane(4)yl tungsten complexes [(η5-C5H5)(R3P)(OC)2W-{B(NMe2)-B(NMe2)Cl}] [R = Me (4a), Ph (4b)]. Both compounds have been fully characterized in solution by NMR and IR spectroscopy and 4a additionally by X-ray diffraction analysis.

New Method To Minimize High-Temperature Corrosion Resulting from Alkali Sulfate and Chloride Deposition in Combustion Systems. II. Molybdenum Salts

The use of fuels other than natural gas in gas turbine generators still is fraught with blade corrosion problems that result from the formation of sodium sulfate or potassium sulfate, in the presence of chlorine. The present work illustrates that the addition of molybdenum salts to synthetic fuels (synfuels) modifies this deposition process and benign protective coatings of alkali polymolybdates are produced instead. This study is a follow-up to research published earlier in this journal that showed closely similar behavior with tungsten salt additives. In the case of molybdenum, a new preferential ranking of product formation on the surface is established that is closely related to the thermodynamic stabilities, namely, Na2Mo2O7 > Na2SO4 > Na2MoO4 > NaCl and K2Mo4O13 > K2Mo3O10 > K2Mo2O7 > K2SO4 > K2MoO4 > KCl. This chemistry exists under fuel-lean conditions and is otherwise not sensitive to the fuel, combustion conditions, surface temperature, or material. The required additive trace levels of molybdenum salt are on the order of twice that of the gaseous alkali, on an atomic basis. Consequently, when used as a final pre-gas turbine polishing technique and following other fuel precleaning methods, the approach offers a very practical and inexpensive solution.