Microwave Decomposition of Solid Crystalline Ammonium Paratungstate and Ammonium Metatungstate

Pfeifer, J.; Badaljan, E.; Tekula-Buxbaum, P.; Vadasdi, K.

doi:10.1006/jssc.1993.1252

Cited by 11 publications

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“…While under oxidizing gases triclinic WO 3 is formed as the majority phase, [14,18] the decomposition under reducing gases results in the formation of partially reduced tungsten bronzes. [16] However, reduction of WO 3-x formed at 650 K under reducing gases (i.e.…”

Section: Discussion Thermal Decomposition Of Ammonium Paratungstatementioning

confidence: 99%

“…[16] It was found that the decomposition in air proceeds through three steps in the temperature ranges 300-473 K, 473-623 K and 623-773 K. The formation of WO 3 was detected at temperatures above 623 K. Besides studies in air, the thermal decomposition of APT was also investigated under hydrogen. [17,18] In the temperature range 470-520 K, APT was found to decompose into (NH 4 ) 0.33 WO 3 . This compound is stable in the temperature range 520-840 K, and is reduced to a mixture of WO 2 and tungsten metal at 870 K, while complete reduction to tungsten metal is achieved at about 1000 K. In total, although the decomposition of APT has been investigated before, several questions remain unanswered with regard to the sequence of formation of the intermediates, their characterization, and the dependence of the decomposition pathways on the reaction gases for catalytically relevant gas phases (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…[5][6][7][8][9][10][11][12][13][14][15][16][17][18] Thermal analysis studies [14] combined with in situ Fourier transform infrared (FT-IR) spectroscopy [13] identified WO 3 as the product of decomposition at temperatures above ca. 650 K. The mechanism for thermal decomposition of APT in air has been studied by X-ray diffraction, thermogravimetry, and IR and UV/Vis diffuse reflectance spectroscopy.…”

Section: Introductionmentioning

confidence: 99%

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In Situ XAS and XRD Studies on the Structural Evolution of Ammonium Paratungstate During Thermal Decomposition

et al. 2005

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Keywords:In situ studies / X-ray absorption spectroscopy / X-ray diffraction / Tungsten / Oxidation / Decomposition / Structure-activity relationships / Heterogeneous catalysisThe bulk structural evolution during the decomposition of ammonium paratungstate (APT) under various oxidizing and reducing gases was elucidated by the complementary techniques, in situ XAS, in situ XRD, and TG/DSC, combined with mass spectrometry. In the temperature range from 300 to 650 K, the decomposition of APT proceeds nearly independently of the gas employed. At higher temperatures, an oxidizing gas results in the formation of crystalline triclinic WO 3 as the majority phase at 773 K, while mildly reducing gases (propene, propene and oxygen, and helium) result in the formation of partially reduced and highly disordered tungsten bronzes. No further reduction is observed under propene or helium, and this indicates a strongly hindered oxygen mobility in the tungsten oxide lattice in the temperature range employed. The decomposition of APT under hydrogen results in the formation of WO 2 and, eventually, tungsten me-

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Section: Discussion Thermal Decomposition Of Ammonium Paratungstatementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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In Situ XAS and XRD Studies on the Structural Evolution of Ammonium Paratungstate During Thermal Decomposition

et al. 2005

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“…An unusual mode of energy input -microwave handling -results in unexpected phase transformations and morphological changes; Pfeifer et al 112 described the formation of needle crystals of ß-and y-oxides.…”

Section: The Morphology Of Tobmentioning

confidence: 99%

Chemistry of Tungsten Oxide Bronzes

Bartha

Kiss

Szalay

1995

The Chemistry of Non-Sag Tungsten

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Ammonium paratungstate (APT) is the generally accepted starting material in tungsten manufacturing. The dopants, which are responsible for the non-sag properties, are given to some tungsten oxide bronze (TOB)-type compounds which are formed by thermal reductive decomposition of APT. The doped TOB is reduced in a H 2 atmosphere to tungsten metal grains. The powder metallurgical processing and the following thermomechanical treatment of the metal powder result in the formation of the non-sag wire.The bronze forming compounds (M z x ) in this case are NH4 or H + ions, or both of them, but the degree of reduction, i.e. the oxygen deficiency, may vary in the W0 3 , therefore, the description of the possible compounds seems to be necessary.We have the general formula of M ( / ) W0 3 _ > ,+z.Jt/2, where y^zx/2 conditions are possible. We also give a phase diagram in which we have located the possible types of compounds.The preparation methods of oxide bronzes (OBs) and the general structural characterization of OBs are described with special attention to ATOB and HTOB compounds. Because many experiences prove the advantages of ionexchange doping, such properties of TOBs have also been discussed.

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“…The particle size and morphology of the metal powder are considerably influenced by the size and morphology of the oxides, which, in turn, depend on the characteristic features of APT (Basu and Sale, 1977). Thermal decomposition of APT in a batch reactor results in the formation of nonstoichiometric tungsten oxides, WO 3-x (0 < x < 0.2) (Pfeifer et al, 1993), depending on the reaction parameters. This is particularly so if the decomposition is carried out in a closed reactor.…”

Section: Introductionmentioning

confidence: 99%

A Comparative Study on the Thermal Decomposition of Ammonium p-Tungstate in Batch and Fluidized-Bed Reactors

Tripathy

Chakraborty

Sharma

et al. 1997

Ind. Eng. Chem. Res.

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Thermal decomposition of ammonium p-tungstate (APT) to tungsten trioxide (WO3) was carried out in both batch and fluidized-bed reactors. In a batch reactor, the temperature and holding time for achieving complete decomposition were found to be much higher as compared to the respective values obtained in a fluidized-bed reactor using air as the fluidizing medium. The present studies have shown that fluidized-bed decomposition of APT offers a number of distinct advantages such as lower decomposition temperature as well as time, faster kinetics, and better product morphology. The decomposition kinetics, in the fluidized-bed reactor, was found to fit well into the “Power Law” model, dα/dt = k(1/r)α1- r , where α = fraction decomposed, k = rate constant, and r (a constant) is 1/2. The activation energy of the decomposition reaction suggests the overall process to be surface controlled.

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Microwave Decomposition of Solid Crystalline Ammonium Paratungstate and Ammonium Metatungstate

Cited by 11 publications

References 0 publications

In Situ XAS and XRD Studies on the Structural Evolution of Ammonium Paratungstate During Thermal Decomposition

In Situ XAS and XRD Studies on the Structural Evolution of Ammonium Paratungstate During Thermal Decomposition

Chemistry of Tungsten Oxide Bronzes

A Comparative Study on the Thermal Decomposition of Ammonium p-Tungstate in Batch and Fluidized-Bed Reactors

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