Thermal Decomposition of Ammonium Paratungstate to Tungsten Trioxide Under Non-Reducing Conditions

Monoclinic ammonium paratungstate, (APT, NH4)10[H2W12O42]·4H2O ) which will be referred to as APT, is widely used as the industrial feedstock for the production of tungsten carbide, WC for the powder metallurgical production of tungsten filaments, for various tungsten heavy alloys and also as starting material for producing ammonium metatungstate (AMT), which is utilized in the catalyst industry.

There is no doubt that the decomposition of APT proceeds via different intermediates. However, the definite identification of these phases is still a matter of discussion. To investigate the thermal decomposition of ammonium paratungstate to tungsten trioxide (WO3) under non-reducing conditions.

The experimental operation procedures are as follows:

APT·4H2O from OSRAM Sylvania used in this study was characterized as shown in Table 1. TA-MS analysis was performed using a thermal analyzer STA 409C (Netzsch Gerätebau GmbH, Selb/Germany), which was coupled to a quadrupole mass spectrometer (QMG 422, Balzers). The conditions were: 70 mL min−1 air flow, 5 K min−1 heating rate, about 200 mg sample mass, corundum crucibles. Platinum crucibles were used for comparison. The overlapping individual steps have been distinguished by their minima in the DTG curve. The ion current (IC) curves for the selected ions with m/z = 15 (NH+), 17 (OH+; NH3+), and 18 (H2O+) were recorded in the multiple ion detection (MID) mode. Analogously to the heating regime of the in situ XRD measurements, stepwise isothermal TA-MS experiments (e.g. temperature holds at 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 700, and 750 °C for 50 min) were performed in air (about 70 mL min−1). Each TA-MS experiment has been repeated at least two times.

The in situ XRD data were collected using a theta–theta diffractometer (Seifert-FPM Germany) with multilayer and “quasi parallel geometry”, fixed slit, Cu Kα1 radiation, 2θ range of 5–50°, step size of 0.02°, dwell time of 1 s/step resulting in a hold time of 3.3 min at the respective temperature. The powdered sample was placed on a horizontal Pt–Rh sample holder (HDKS1 heating chamber, Bühler) and heated from 100 to 650 °C (heating rate 2.5 K min−1) in an airflow of 100 mL min−1. Prior to the measurements the sample holder was coated with gold to suppress the well-known catalytic activity of Pt–Rh for the oxidation of ammonia (see Section 3.3).

The in situ FT-IR spectra were recorded on a BRUKER IFS 66 spectrometer from a self-supporting disc (100 mg, without KBr) placed in a heatable IR cell (5 K min−1 heating rate, 50 mL min−1 air flow, KBr windows, dispersion 2 cm−1). In the course of the thermal decomposition this disc lost its mechanical stability and, hence, the maximum recorded temperature was 300 °C.

Only the in situ measurements TA-MS, XRD and FT-IR provide first-hand information about the decomposition process of APT·4H2O. All other methods applied are dealing with a situation where the substances were exposed to the ambient conditions prior to the measurement.

The ex situ XRD data were collected using a D/max-vertical diffractometer from RIGAKU, with parafocusing geometry, graphite monochromator, fixed slits (divergence slit = scatter slit = 1°), and Cu Kα radiation. Scanning conditions were 6–70° 2Θ, step size of 0.02°, and a dwell time of 2 s/step.

Conclusion

The thermal decomposition of ammonium paratungstate tetrahydrate was studied under non-reducing conditions in the temperature range 25–600 °C using thermal analysis and X-ray powder diffraction and IR spectroscopy, both techniques in ex situ and in situ modes. The APT decomposition is characterized by a sequence of three endothermic and one exothermic step. Special attention has been paid to the characterization of the products formed at these four steps.

It is confirmed that during the first endothermic step [as endo-1 (50–190 °C)] the anhydrous ammonium paratungstate, (NH4)10[H2W12O42], is formed;    During the second endothermic step, [as endo-2 (190–250 °C)] ammonia alone is released leaving behind protons under formation of ammonium hydrogen paratungstate, (NH4)6H4[H2W12O42]. The paratungstate ion remains generally unchanged during the release of NH3. In this course (NH4)6H4[H2W12O42] constitutes the precursor for the metatungstate ion, [H2W12O40]6−, which is only formed during digesting roasted APT with hot water. The subsequent crystallization from the mother liquor or spray-drying the aqueous solution results in the formation of (NH4)6[H2W12O40]·nH2O;  During the third endothermic step [as endo-3 (250–380 °C)] the main portion of gas is released as a mixture of NH3 and H2O, which requires the consumption of oxide ions from the decomposing paratungstate anion. This results in a completely X-ray-amorphous phase;  The decomposition is finished by the fourth exothermic step [as endo-1 (50–190 °C)], which is accompanied by the release of remaining ammonia/water and structural water. The exothermic heat effect of this step is caused by the formation of tungsten trioxide, WO3, detectable by XRD.

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