Thermal decomposition of ammonium paratungstate hydrate in air and inert gas atmospheres

In this study, the thermal decomposition of ammonium paratungstate hydrate was investigated in dynamic air and inert gas (nitrogen and argon) atmospheres by simultaneous thermo-gravimetric analysis (TGA), differential thermal analysis (DTA) and mass spectroscopy (MS). The intermediate and end products obtained during thermal decomposition of ammonium paratungstate hydrate were characterized by X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR).It was found that the kind of gas was not effective on the thermal decomposition mechanism of ammonium paratungstate hydrate below 665 K. In the first decomposition step, anhydrous ammonium paratungstate was obtained by dehydration. In the second decomposition step, only deammoniation occurred and ammonium hydrogen tungstate was obtained. During the third decomposition step, dehydration, deammoniation and reduction of some tungsten atoms to lower valences were observed. At this stage, first, ammonium hydrogen tungsten oxide bronze and later ammonium tungsten oxide bronze were formed by two serial reactions. In the fourth step which occurs after 665 K, ammonium tungsten oxide bronze formed stoichiometric tungsten oxide in air atmosphere and tungsten suboxide in inert gas atmosphere.

► Below 665 K, decomposition of APT2.25 is not affected from gas atmosphere. ► In the first step, APT2.25 releases its all crystal water and becomes dehydrated. ► In the second step, APT loses a part of its NH3 molecules from NH4. ► At the end of the third step, ammonium tungsten oxide bronze was formed. ► In the fourth step, in air WO3 and in inert gas tungsten suboxide were formed.