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·4H2O), (NH4)10[H2W12O42]·4H2O, in air to tungsten trioxide, WO3, 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, (NH4)6H4[H2W12O42]. 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, [H2W12O40]6−, which is formed during the digestion of “roasted” APT with hot water.