Showing papers on "Tungstate published in 2007"
TL;DR: In this paper, the 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.
145 citations
TL;DR: In this paper, nearly monodisperse microspheres of tetragonal tungstate (MWO4, M = Pb, Ca) composed of subunits with different shapes have been found to be sensitive to reaction conditions and could be tuned by controlling the reaction conditions.
Abstract: Nearly monodisperse PbWO4 and CaWO4 microspheres have been synthesized in large scale by a surfactant-assisted solution route, in which either sodium dodecyl benzenesulfonate (SDS) or cetyltrimethyl ammonium bromide (CTAB) is used. By controlling the solution reaction conditions, such as temperature, surfactant, and pH value, we can synthesize nearly monodisperse microspheres of tetragonal tungstate (MWO4, M = Pb, Ca) composed of subunits with different shapes. The diameters of these microspheres have been found to be sensitive to the reaction conditions and could be tuned by controlling the reaction conditions. The growth process of these nearly monodisperse microspheres has been examined. The optical properties of the tungstate monodisperse microspheres in the temperature range of 20−270 K were studied.
120 citations
TL;DR: The optimal temperature for the preparation of pure crystals with well defined shape was found to be near 773 K as mentioned in this paper, which is contrary to what can be expected from the known data on their structure and the relative electronegativity of W and Mo ions.
93 citations
TL;DR: Strontium tungstate nanoparticles with diameters of 40−50 nm, nanopeanuts with diameter of 100−150 nm and nanorods with a rough surface were controllably synthesized by a solvothermal-mediated mi....
Abstract: Strontium tungstate nanoparticles with diameters of 40−50 nm, nanopeanuts with diameters of 100−150 nm, and nanorods with a rough surface were controllably synthesized by a solvothermal-mediated mi...
82 citations
TL;DR: In this paper, the effects of flux, concentration of molybdate, and resulting crystal structure on the PL properties was discussed, which indicated the formation of a new crystal structure, rather than solid solution formation.
70 citations
TL;DR: In this paper, the pitting corrosion of carbon steel in carbonate-formation water solution in the presence of chloride ions and the effect of addition WO 4 2−, MoO 42−, and NO 2 − anions on the Pitting corrosion were studied using cyclic voltammetry and potentiostatic current-time measurements and complemented by scan electron microscope (SEM), energy dispersive X-ray (EDX) and Xray photoelectron spectroscopy (XPS) investigations.
69 citations
TL;DR: In this paper, the 24-electron reduction of ammonium metatungstate ((NH4)6[H2W12O40]) yields a material with electrocatalytic activity towards the oxidation of hydrogen in acid electrolyte which approaches 25% that of platinum.
65 citations
TL;DR: WOx/TiO2 catalysts were synthesized by impregnating aqueous (NH4)2WO4 on hydrous titania nanotubes as discussed by the authors.
Abstract: WOx/TiO2 catalysts were synthesized by impregnating aqueous (NH4)2WO4 on hydrous titania nanotubes. The materials were annealed in air at 500 °C and characterized by X-ray diffraction (XRD), Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and X-ray photoelectron spectroscopy (XPS); their catalytic activity was evaluated in the oxidation reaction of dibenzothiophene (DBT). After annealing at 500 °C, the structure of the support transformed from orthorhombic, with nanotubular morphology, to tetragonal, yielding anatase nanoparticles decorated by tungsten nanoparticles on its surface. During this transformation, the nanotubes released residual Na+ ions from the interlayer space, which reacted with tungstate species to change the W coordination from octahedral to tetrahedral, yielding ≤1 nm Nax(WO4) nanoparticles on the surface of anatase TiO2. These nanoparticles were highly active for the DBT ...
61 citations
TL;DR: In this paper, a novel method for preparation of WO3-SnO2 nanocomposite was described, which resulted in uniform mixing of ∼35nm wO3 and ∼100nm SnO2 nanoparticles as revealed by direct observation using a transmission electron microscope and by performing elemental dot mapping.
Abstract: A novel method for preparation of WO3–SnO2 nanocomposite was described. 0.3–10.0 wt.% WO3 was incorporated into the SnO2 powder by precipitation of hydrated WO3 from ammonium tungstate precursor solution containing SnO2 powder. This preparation method resulted to uniform mixing of ∼35 nm WO3 and ∼100 nm SnO2 nanoparticles as revealed by direct observation using a transmission electron microscope, and by performing elemental dot mapping. Upon calcination to 500 °C, both cassiterite SnO2 and monoclinic WO3 were stable, and no other crystalline phase was observed. To evaluate gas sensing performance, the powders were formed into thick-film sensors using a screen printing technique. It was found that the gas sensitivity of the SnO2 base was enhanced when very small amount of the WO3 (0.3 wt.%) was incorporated. The 0.3 wt.% WO3–SnO2 sensor was very sensitive to 2–8 ppm ethylene at optimum operating temperature of 300 °C. The sensitivity decreased with increasing amount of the WO3.
60 citations
TL;DR: In this paper, the formation process of novel tungstate-based inorganic−organic hybrid nanobelts/nanotubes with lamellar mesostructures has been investigated, with an emphasis on monitoring the morphological and microstructural changes of the products during the reactions of H2W2O7·xH2O (x = 3.49) with n-alkylamines (CmH2m+1NH2, 4 ≤ m ≤ 14).
Abstract: The formation process of novel tungstate-based inorganic−organic hybrid nanobelts/nanotubes with lamellar mesostructures has been investigated, with an emphasis on monitoring the morphological and microstructural changes of the products during the reactions of H2W2O7·xH2O (x = 3.49) with n-alkylamines (CmH2m+1NH2, 4 ≤ m ≤ 14) in a system of heptane/n-alkylamine/H2W2O7·xH2O (n-alkylamine:H2W2O7·xH2O molar ratio of about 30) under ambient conditions. The results indicate that normal intercalation occurrs in the early stage to form intercalation compounds with double-octahedral W−O layers, which are then dissolved in the highly alkaline aqueous solutions confined in the reverse-micelle-like media, where the dissolved species recrystallize to form hybrid nanobelts/nanotubes with single-octahedral W−O layers. Both the intercalation compounds obtained after a short reaction time (e.g., 30 min) and the hybrid nanobelts/nanotubers formed after a long reaction time (e.g., 5 days) possess a bilayered arrangement of...
57 citations
TL;DR: In this article, a series of WO3/Al2O3 /HY catalysts were prepared by thermal spread method for the metathesis reaction between ethene and 2-butene to propene.
Abstract: A series of WO3/Al2O3/HY catalysts were prepared by thermal spread method (i.e., calcination of physically mixed WO3, Al2O3 and HY zeolite) for the metathesis reaction between ethene and 2-butene to propene. The transformation of tungsten oxide species was studied by the characterization techniques of XRD, H2-TPR, UV-Vis, UV Resonance Raman, NH3-TPD and N2 adsorption–desorption techniques. The thermal spread effect during calcination results in the dispersion of bulk WO3 phase into microcrystallites on γ-Al2O3 surface, followed by a chemical transformation to the monomeric surface tungstate species via the involvement of Bronsted acid sites of HY zeolite. These monomeric surface tungstate species, identified by a characteristic Raman band at 970 cm−1, are crucial for the metathesis activity. We suggest such activity related species are tetrahedrally coordinated. Moreover, it is found that there is an optimal calcination time for the formation of these WO42− species and further calcination causes the condensation to the polytungstate clusters, which are less active for the metathesis reaction. The optimized catalyst shows superior catalytic activity of metathesis reaction between ethene and 2-butene to propene at 453 K with 2-butene conversion close to thermodynamic equilibrium value (∼64%).
TL;DR: In this paper, spherical particles of bismuth tungstate (Bi2WO6) with hierarchical architecture were prepared by facile hydrothermal reaction without using any surfactants and polymers as structu...
Abstract: Micrometer-sized spherical particles of bismuth tungstate (Bi2WO6) with hierarchical architecture were prepared by facile hydrothermal reaction without using any surfactants and polymers as structu...
TL;DR: Anion exchange chromatography has been coupled to inductively coupled plasma mass spectrometry to determine tungstate, molybdate, and phosphate species at the sub-microg l (-1) and microg l(-1) levels, providing quantitative determination of these species in about 10 min with the capability to simultaneously determine other oxyanion species.
TL;DR: In this article, the microstructure and morphology of nanocomposites were characterized by X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) respectively, and the resistance and sensitivities to poisonous volatile organic compounds (PVOCS) were investigated on the static testing chamber.
Abstract: Zn–W–O nanocomposite ceramics with the whole concentration range (W at.%=0%, 1%, 5%, 25%, 50%, 75%, 95%, 99%, and 100%) have been fabricated by the sol–gel method, starting from zinc acetate dihydrate, ammonium tungstate, monoethanolamine (MEA), and anhydrous ethanol. Nanocomposites, that were sintered at 650°C in a conventional oven for 120 min were ultrasonically emulsified and coated on alumina ceramic tubes by dip coating. The microstructure and morphology of nanocomposites were characterized by X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM), respectively. The resistance and sensitivities to poisonous volatile organic compounds (PVOCS) were investigated on the static testing chamber. The XRD analysis of nanocomposites revealed the appearance of ZnWO4 with a monoclinic structure in the whole concentration range (1–99 at.%). The FESEM results showed that as-prepared nanocomposites were composed of almost spherical particles with size range from 200 to 300 nm. Gas-sensing properties of Zn–W–O nanocomposites ceramics, to formaldehyde, benzene, and xylene, were detected. The results showed that the ZnWO4 phase with high resistance almost did not respond to PVOCS. The appropriate amounts of ZnWO4 in ZnO-rich samples and in WO3-rich samples lend themselves to enhancement of gas-sensing properties to PVOCS. The effect of ZnO, WO3, and ZnWO4 with a scheelite structure as solid acidic/alkaline catalysts on gas-sensing property is also discussed.
TL;DR: In this article, the photochromic effect can be erased by thermal treatment at 200 °C for a few minutes, and the effect was investigated by several techniques such as, UV-visible absorption, Raman and XANES at the L1 and L3 tungsten absorption edges.
TL;DR: Ammonium paratungstate was prepared from tungsten alloy swarf employing anodic dissolution in sodium hydroxide using a packed bed configuration followed by chemical conversion into ammonium tungstate through tungstic acid.
TL;DR: The precursor of the mixed metal oxide catalyst composed of H3PO4-WO3-Nb2O5, which exhibits excellent activity in Friedel-Crafts alkylations, was identified with 31P NMR.
TL;DR: In this article, the reaction of GdIII(NO)3·6H2O and (Bu3NH)3[WV(CN)8] (Bu 3N = tributylamine) in N,N-dimethylacetamide (DMA) yielded two types of gd-W bimetal assemblies with non-centrosymmetric crystal structures.
Abstract: Reaction of GdIII(NO)3·6H2O and (Bu3NH)3[WV(CN)8] (Bu3N = tributylamine) in N,N-dimethylacetamide (DMA) yielded two types of Gd–W bimetal assemblies with noncentrosymmetric crystal structures: GdII...
TL;DR: In this article, the preparation of Cobalt tungstate (CoWO4) thin film by spray pyrolysis with ammonical solution as a precursor is presented.
Abstract: Preparation of Cobalt tungstate (CoWO4) thin film by spray pyrolysis with ammonical solution as a precursor is presented. The phase and surface morphology characterizations have been carried out by XRD and SEM analysis. The study of optical absorption spectrum in the wavelength range 350 – 850 nm shows direct as well as indirect optical transitions in the thin film material. The d. c. electrical conductivity measurements in the temperature range 310–500 K indicate semiconducting behavior of the thin film. The thin films deposited on fluorine doped tin oxide (FTO) coated conducting glass substrates were used as a photoanode in photovoltaic electrochemical (PVEC) cell with configuration: CoWO4 | Ce4+, Ce3+ | Pt; 0.1 M in 0.1 N H2SO4. The PVEC characterization reveals the fill factor and power conversion efficiency to be 0.36 and 0.62%, respectively. The flat band potential is found to be −0.18 V (SCE).
TL;DR: In this paper, a Raman confocal spectromicroscopic system was used to study in situ phase composition and surface morphology in amorphous and nanocrystalline tungsten oxide and tungstate thin films, prepared on silicon and glass substrates by dc magnetron co-sputtering technique.
Abstract: A Raman confocal spectromicroscopic system was used to study in situ phase composition and surface morphology in amorphous and nanocrystalline tungsten oxide and tungstate thin films, prepared on silicon and glass substrates by dc magnetron co-sputtering technique. The possible use of these films for the phase-change optical recording was demonstrated using 442 nm He–Cd laser with a variable power of up to 50 mW. The formation of nanocrystalline tungsten trioxide or tungstate phases was observed under the laser irradiation. These nanocrystalline phases show relatively strong Raman activity, which can be used for information reading purposes. A multilayer structure composed of several tungstate films with different chemical composition is proposed as potential write-once optical recording media.
TL;DR: In this paper, a new crystalline cation-exchanger aluminium tungstate was synthesized by mixing solutions of 0.1 m aluminium nitrate and 0. 1 m sodium tungstane at pH 1.2.
TL;DR: In this article, a commercial anionic resin has been tested to evaluate its suitability for tungsten recovery from aqueous solutions, showing a higher exchange capacity at acid pH.
Abstract: A commercial anionic resin has been tested to evaluate its suitability for tungsten recovery from aqueous solutions. Equilibrium runs at fixed temperature have been carried out at different pH levels, showing a higher exchange capacity at acid pH. Kinetics studies consist of the analysis of the breakthrough curves at different tungsten concentrations, flow rates, pHs, and bed depths. At basic pH, the breakpoint and the saturation times of the system are almost constant with pH whatever the flow rates and the tungsten concentrations are. Moreover, the length of the mass transfer zone remains almost unchanged with bed depth, feed flow rate and concentration. At acid pH a critical working condition appears, as the breakpoint time progressively decreases and the saturation time strongly increases. The different behaviors of the exchange resin at acid and basic pHs can be connected to the different forms of tungsten present in solution.
TL;DR: In this article, the nonlinear refraction and nonlinear absorption in barium tungstate crystal which is widely used in Raman lasers is studied using the single-beam z-scan technique with 1 ps laser pulses at 790 and 395 nm.
TL;DR: In this paper, the authors examined the effect of temperature, pH, Ca/WO3 molar ratio and nature of precipitated calcium tungstate in the recovery of tungsten from alkaline leach solutions.
TL;DR: In this paper, an attempt was made to isolate bacterial strains capable of biologically removing tungstate (WO 4672 2−676 /l ) from water samples collected from various areas of Anzali lagoon, Iran.
Abstract: An attempt was made to isolate bacterial strains capable of biologically removing tungstate (WO
4
2−
). Thirty-eight water samples were collected from various areas of Anzali lagoon, Iran. Initial screening of a total of 100 bacterial isolates at pH 5, resulted in the selection of one isolate with maximum adsorption capacity of 65.4 mg tungstate/g dry weight. It was tentatively identified as Bacillus sp. according to morphological and biochemical properties and named strain MGG-83. Tungsten concentration was measured spectrophotometrically using the dithiol method. Higher adsorption capacity was observed in the acidic pH ranging from 1 to 3. At pH 2, the strain removed 274.4 mg tungstate/g dry weight within 5 min from the solution with 300 mg WO
4
2−
/l initial concentration and thereafter adsorption rate decreased remarkably. The applicability of the Freundlich isotherm for representation of the experimental data was investigated. Using 1 mM sodium azide and 10 mM 2,4−dinitrophenol, it was shown that only 20% reduction occurred in adsorption and steam sterilization of the bacterial cells resulted in 11% decrease in tungstate uptake. Temperature variations (20–40°C) had no significant effect on tungstate uptake. Pretreatment with the cations had no effect in uptake but pretreatment with anions decreased the tungstate uptake as indicated: sulfate > chromate > nitrate > molybdate > selenate > rhenate. Tungstate was removed from metal-laden biomass after desorption treatments by addition of different desorbing solutions with the results sodium acetate > EDTA > NaCl > KOH > H2SO4.
TL;DR: In this paper, the preparation, optical absorption and photoluminescence properties of luminescent materials consisting of Ln 2− x Tb x (WO 4 ) 3 [where Ln = Gd(III) or La(III)] incorporated into silica xerogel was studied by the luminescence spectra.
TL;DR: In this paper, the Czochralski method was used to grow pure and Nd 3+ -doped NaGd(WO 4 ) 2 (NGW) single crystals, up to ∼85mm long and ∼30mm in diameter, using X-ray diffraction at room temperature.
TL;DR: Ba11W4O23 was synthesized at 1300°C, followed by quenching with liquid nitrogen as discussed by the authors, and the crystal structure was known to be cryolite-related but has remained unclear.
TL;DR: In this article, a multilayer structure of alternating thin titanium and tungsten oxide layers having dimensions of ∼20 nm have been fabricated from titanium alkoxide and various tungstate precursor solutions using the dip coating technique.
Abstract: Multilayer structures of alternating thin titanium and tungsten oxide layers having dimensions of ∼20 nm have been fabricated from titanium alkoxide and various tungstate precursor solutions using the dip coating technique. Single, double, and triple layer titanate and tungstate thin films were deposited on silicon substrates, and these films were initially annealed at 400 °C. Structural and microstructural aspects of the films were investigated using a variety of techniques, including X-ray reflectometry, grazing incidence X-ray absorption spectroscopy (GIXAS), cross-sectional transmission electron microscopy (TEM), and secondary ion mass spectrometry. The dimensions of the films and the character of the interfaces were principally gauged by cross-sectional TEM and X-ray reflectometery. All films were continuous on a local scale and had relatively low surface roughness. At the treatment temperature of 400 °C, only the tungsten oxide component showed appreciable crystallinity. The multilayer films had rel...
TL;DR: In this paper, the authors demonstrate that plasma electrolytic processing of aluminum and titanium in room-temperature aqueous solutions containing tungstate iso-and heteropolyoxoanions is a viable approach to producing Al/Al2O3/WO3 and Ti/TiO2/O3 structures.
Abstract: Experimental data are presented which demonstrate that plasma electrolytic processing of aluminum and titanium in room-temperature aqueous solutions containing tungstate iso-and heteropolyoxoanions is a viable approach to producing Al/Al2O3/WO3 and Ti/TiO2/WO3 structures. Tungsten trioxide can be obtained both in the form of a continuous layer and surface islands.