Ammonium Paratungstate Tetrahydrate Research Articles

Direct transformation of tungsten trioxide monohydrate into ammonium paratungstate tetrahydrate in ammonium salt solutions

Direct transformation of tungsten trioxide monohydrate into ammonium paratungstate tetrahydrate in ammonium salt solutions

Thermal decomposition of ammonium paratungstate tetrahydrate: New insights by a combined thermal and kinetic analysis

The thermal decomposition of ammonium paratungstate tetrahydrate has been studied in oxidising atmosphere at four heating rates employing the analytical techniques TG, DTG, DSC, and MS. In total, three endothermic and two exothermic effects have been detected. From the combined thermal and kinetic analysis based on the MS curves of water and ammonia, the decomposition path has been characterised as a system of consecutive and competing reactions. The activation energies of the five effects for water and ammonia were discussed. On the basis of the kinetic model and the quantified MS data the stoichiometry for the whole pathway of the thermal decomposition of the title compound was construed.

Surface tungsten reduction during thermal decomposition of ammonium paratungstate tetrahydrate in oxidising atmosphere: A paradox?

The interaction of ammonia, liberated during thermal decomposition of ammonium paratungstate tetrahydrate in oxidising atmosphere, with tungsten has been studied employing a conventional microbalance combined with MS (Setaram’s instrument Sensys). Applying XPS a partial reduction of tungsten at the surface with the minimal tungsten oxidation number of +5.3 for a sample generated at 293°C was detected. The balancing oxidation of ammonia to nitrogen/nitrogen oxides has been proven by MS. An amount of 0.049mole− per mol W was transferred which resulted in an ammonia conversion degree from 2.1mol% (NO2 formation) to 3.0mol% (N2 formation).

Thermal decomposition of ammonium paratungstate tetrahydrate under non-reducing conditions: Characterization by thermal analysis, X-ray diffraction and spectroscopic methods

The thermal decomposition of ammonium paratungstate tetrahydrate (APT·4H 2O), (NH 4) 10[H 2W 12O 42]·4H 2O, in air to tungsten trioxide, WO 3, was investigated under non-isothermal conditions using thermal analysis coupled on-line by a Skimmer ® system to a quadrupole mass spectrometer (TA-MS) for evolved gas analysis (EGA), with X-ray powder diffraction (XRD), with Fourier transform infrared (FT-IR) and Raman spectroscopy. The decomposition is characterized by three endothermic and one exothermic step. The first endothermic step comprises the release of crystal water resulting in the formation of crystal-water-free ammonium paratungstate. During the second endothermic step ammonia is released leaving behind ammonium hydrogen paratungstate, (NH 4) 6H 4[H 2W 12O 42]. The presence of protons in the “roasted” APT was supported by ex situ and in situ FT-IR spectroscopy (vibration bands at 1100 and 2160 cm −1). Ammonium hydrogen paratungstate is the precursor for the metatungstate anion, [H 2W 12O 40] 6−, which is formed during the digestion of “roasted” APT with hot water.

Partial thermal reduction of ammonium paratungstate tetrahydrate

Thermal decomposition of ammonium paratungstate tetrahydrate, (NH4)10[H2W12O42]·4H2O has been followed by simultaneous TG/DTA and online evolved gas analysis (TG/DTA-MS) in flowing 10% H2/Ar directly up to 900°C. Solid intermediate products have been structurally evaluated by FTIR spectroscopy and powder X-ray diffraction (XRD). A previously unexplained exothermic heat effect has been detected at 700–750°C. On the basis of TG/DTA as well as H2O and NH3 evolution curves and XRD patterns, it has been assigned to the formation and crystallization heat of γ-tungsten-oxide (WO2.72/W18O49) from β-tungsten-oxide (WO2.9/W20O58) and residual ammonium tungsten bronze.

In situ HT‐XRD Study on the Formation of Hexagonal Ammonium Tungsten Bronze by Partial Reduction of Ammonium Paratungstate Tetrahydrate

AbstractHexagonal ammonium tungsten bronze, (NH4)0.33–xWO3–y, is a possible constituent of the intermediate “tungsten blue oxide” in non‐sag (NS) tungsten production and a material of chromogenic and sensor interest. Its formation has been studied by in situ high‐temperature powder X‐ray diffraction (HT‐XRD) accompanied by semiquantitative phase analysis through the partial thermal reduction of ammonium paratungstate tetrahydrate, (NH4)10[H2W12O42]·4H2O, in flowing 10 % H2/He. The effect of the heating program on the formation of the title compound has been discussed. Highly crystalline, monophasic hexagonal ammonium tungsten bronze prepared by partial reduction of ammonium paratungstate tetrahydrate has been characterised by high‐precision powder XRD, X‐ray photoelectron spectroscopy (XPS), chemical analysis and solid‐state 1H NMR spectroscopy (1H‐MAS‐NMR). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Comparative evolved gas analyses (TG-FTIR, TG/DTA-MS) and solid state (FTIR, XRD) studies on thermal decomposition of ammonium paratungstate tetrahydrate (APT) in air

Ammonium paratungstate tetrahydrate (NH 4) 10[H 2W 12O 42]·4H 2O (APT), a starting material of WO 3 and tungsten production, has been subjected to a complex thermoanalytical study in air. The weight loss stages and the various heat effects have been followed by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) in flowing air up to 600 °C. Meanwhile evolution of gaseous products has also been analyzed and monitored by both online coupled mass spectrometer (TG/DTA-MS) and infrared gas cell (TG-FTIR), in comparison. Besides that of the previously known H 2O and NH 3, evolution of two new gaseous products, N 2O and NO, which had not been reported earlier, has been detected and traced by both evolved gas analysis (EGA-FTIR and EGA-MS) methods. These oxides of nitrogen are considered as catalyzed oxidation products of the released ammonia in air at 260–350 and 400–450 °C. Thus, the two exothermic heat effects observed by DTA at 342 and 443 °C have been assigned to ammonia combustion. Furthermore solid intermediate products have been structurally evaluated by both FTIR spectroscopy and powder X-ray diffraction (XRD). Based on significant changes in XRD patterns, a consequent formation of hexagonal ammonium tungsten oxide bronze and monoclinic WO 3 are confirmed.

Online evolved gas analyses (EGA by TG-FTIR and TG/DTA-MS) and solid state (FTIR, XRD) studies on thermal decomposition and partial reduction of ammonium paratungstate tetrahydrate

Ammonium paratungstate tetrahydrate (NH 4) 10[H 2W 12O 42]·4H 2O (APT), a starting material in powder metallurgy of tungsten, has been subjected to a complex thermoanalytical study. The weight loss stages and the various heat effects have been followed by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) in flowing 10% H 2/Ar and 10% H 2/He atmospheres up to 600 °C. Meanwhile evolution of gaseous products has also been analyzed and monitored by both on-line coupled mass spectrometer (TG/DTA-MS) and infrared gas cell (TG-FTIR). In atmospheres containing 10% H 2, the evolution courses of H 2O and NH 3 traced by these EGA-methods and the TG/DTA curves up to 380 °C have been found quite similar to those of measured in air or in pure He. Effect of the reductive medium occurred above 380 °C, where solid products of intermediate temperatures have been structurally evaluated by both FTIR spectroscopy and powder X-ray diffraction (XRD). Based on significant changes in XRD patterns, the only small exothermic heat effect observed at 420–440 °C is assigned to the crystallization heat of a mixture of partially reduced tungsten oxide bronzes.

Processing and properties of laminated silicon nitride composites : S.Fomenko et al. (Inst. for Problems of Material Science, Kiev, Ukraine.) Science of Sintering, vol 29, no 1, 1997, 17–25

According to the proposed quantitative chemical and X-ray analyses, the composition of the ‘tungsten blue oxides’ (TBO) investigated varies over a very broad spectrum, depending on the temperature, atmosphere, and speed and also on the actual manufacturing process of the ammonium paratungstate tetrahydrate (APT·4H2O) reduction. In addition to crystalline compounds {hexagonal tunsten bronze (HTB), WO3, W20O58, and W18O49} the industrially produced TBO contains 30–55% of X-ray-amorphous fraction.APT·4H2O changes in a stream of dry hydrogen at 130°C into the phase APT·∼2H2O, observed for the first time. The intermediate X-ray-amorphous phases ranging from 250 to 350°C are the starting pool not only for HTB but also for the crystalline tungsten oxides.The HTB-containing TBO powders display a considerable potassium incorporation via the cation exchange of H+ being replaced by K+.The solid-state high-resolution 1H NMR investigation of TBO allows a distinction to be made between different proton-containing species.

Investigation of powder metallurgical production of doped tungsten with unorthodox doping specifications

An industrially produced ‘tungsten blue oxide’ (hydrogen reduction of ammonium paratungstate tetrahydrate, (NH 4) 10[H 2W 12O 42] · 4 H 2O at 880 °C, (rotary kiln) of the overall formula 0.011NH 3 · 0.013H 2O · WO 2.79 (phase composition: 20% WO 2.90, 35% WO 2.72, 5% WO 2, 40% X-ray-amorphous fraction) has been doped with 12 unorthodox doping specifications using potassium metatungstate, potassium hydroxide, 12-tungstosilicic acid, tetramethyl orthosilicate, tetraethyl orthosilicate and aluminium 12-tungstosilicate, in comparison with three conventionally doped samples and an undoped one. The sintering investigations have been carried out for H 2O/HCl/HF-washed tungsten powders both with 5-g pellets in a tungsten furnace at 2000, 2250 and 2500 °C and under manufacturing conditions with 3.5-kg compacts. The investigation of the unwashed samples has been carried out with 5-g pellets only. After studying the behaviour of the unorthodox dopants conclusions concerning the doping mechanism could be drawn. The findings of the investigations allow us to propose new doping specifications for manufacturing NS-tungsten wire.

Formation of ammonium paratungstate tetra- and hexa-hydrate. I: stability

The crystallization of ammonium paratungstate tetrahydrate, ((NH 4) 10OH 2W 12O 42·4H 2O, APT·4H 2O) from aqueous ammonium tungstate solutions is a key unit operation in current tungsten powder production. A literature survey showed that ammonium paratungstate hexahydrate (APT·6H 2O) can form under conditions where APT·4H 2O forms as well. Based on this information it was postulated that APT·6H 2O is metastable. Experiments were carried out to determine whether and under what conditions the hexahydrate is metastable. It was found that the hexahydrate is metastable at a solution concentration of approximately 300-230 g/kg WO 3 and from approximately 90° to 96°C. APT·6H 2O is not formed when APT·4H 2O seeds are added prior to crystallization. The experimental data are presented in a three-dimensional ternary phase diagram. It was found that the recrystallization of APT·6H 2O into APT·4H 2O is prompted by a decreasing water activity in solution during crystallization. The increase in solution temperature during isobaric crystallization also stimulates the recrystallization process. In industrial practice some residual crystals are always present in the crystallizer. Therefore, APT·6H 2O will not form during industrial crystallization.

What is behind ‘tungsten blue oxides’?

According to the proposed quantitative chemical and X-ray analyses, the composition of the ‘tungsten blue oxides’ (TBO) investigated varies over a very broad spectrum, depending on the temperature, atmosphere, and speed and also on the actual manufacturing process of the ammonium paratungstate tetrahydrate (APT·4H 2O) reduction. In addition to crystalline compounds {hexagonal tunsten bronze (HTB), WO 3, W 20O 58, and W 18O 49} the industrially produced TBO contains 30–55% of X-ray-amorphous fraction. APT·4H 2O changes in a stream of dry hydrogen at 130°C into the phase APT·∼2H 2O, observed for the first time. The intermediate X-ray-amorphous phases ranging from 250 to 350°C are the starting pool not only for HTB but also for the crystalline tungsten oxides. The HTB-containing TBO powders display a considerable potassium incorporation via the cation exchange of H + being replaced by K +. The solid-state high-resolution 1H NMR investigation of TBO allows a distinction to be made between different proton-containing species.

Kinetics of and an adsorption model for the dissolution of synthetic WO 3 in aqueous ammonia

Ammonium paratungstate tetrahydrate (APT) dissolves slowly in aqueous ammonia. The complete calcination of APT into crystalline WO 3, as a way to improve the dissolution characteristics of APT in aqueous ammonia solutions was studied. Calcination experiments were carried out in air at 550°C. The dissolution rate of this calcine was investigated as a function of various solution temperatures, ammonia concentrations, particle sizes and agitator speeds. The dissolution rate was found to be controlled by chemical reaction and has an activation energy of 73±3 kJ/mol. A model based on the adsorption of ammonia ions at the particle surface could adequate describe the asymptotic increase of the dissolution rate at higher ammonia concentrations. Complete calcinations at 650°C and 800°C resulted in a decreased dissolution rate. This could be attributed to a decreasing specific surface area of the calcine with increasing temperature. Calcination at 450°C resulted in a mixture of hexagonal WO 3 and a phase which is iso-structural with hexagonal ammonium tungsten bronze. This mixture dissolves more slowly in aqueous ammonia than does pure WO 3.

Dissolution of ammonium paratungstate tetrahydrate in aqueous ammonia before and after low temperature calcination

Ammonium paratungstate tetrahydrate, APT·4H 2O dissolves rather slowly in aqueous ammonia at room temperature. Dissolution at higher temperatures increases the rate of dissolution, but the amount of dissolved tungsten decreases due to the lower solubility of ammonia at higher temperatures which results in a lower pH of the solution. The rate of dissolution was found to be independent of the crystal size. It is therefore concluded that the rate-controlling step occurs in the solution, and is probably H 2W 12O 42 10−→2HW 6O 21 5−. Calcination of APT·4H 2O in air, nitrogen and hydrogen/nitrogen mixtures, into an X-ray amorphous product, is shown to be one way to improve the dissolution rate and to increase the dissolved amount of APT·4H 2O. The highest solubilities can be obtained by calcining APT·4H 2O in air or nitrogen at temperatures ranging from 250 to 310°C for approximately 2 h. When the temperature is too low or the residence time is too short some crystalline APT·4H 2O will still exist; WO 3 is formed when the temperature is too high or the residence time too long. It is supposed that the large H 2W 12O 42 10− structure is broken during the calcination process.

X-Ray Powder Diffraction Data and Unit Cells of Ammonium Paratungstate Tetrahydrate

AbstractX-Ray powder diffraction data and unit cell parameters of industrially produced, as well as bench scale prepared, ammonium paratungstate tetrahydrate are reported and compared with current Powder Data File (PDF) (1989) patterns. A least-squares refinement resulted in two slightly different unit cells. Both cells are monoclinic with S.G. = P21/n(14), Z = 2. The density, 4.639(2)kg/m3, calculated from one of these unit cells corresponds reasonably well with a measured value of 4.61 (2). It has, however, not been possible to determine at present why ammonium paratungstate tetrahydrate has two unit cells. No relation between the crystalline form and the method of preparation nor the exact water content could be established.