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

Abstract

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.