Comparative evolved gas analyses (TG-FTIR, TG/DTA-MS) and solid state (FTIR, XRD) studies on thermal decomposition of ammonium paratungstate tetrahydrate (APT) in air

Abstract

Ammonium paratungstate tetrahydrate (NH 4) 10[H 2W 12O 42]·4H 2O (APT), a starting material of WO 3 and tungsten production, has been subjected to a complex thermoanalytical study in air. The weight loss stages and the various heat effects have been followed by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) in flowing air up to 600 °C. Meanwhile evolution of gaseous products has also been analyzed and monitored by both online coupled mass spectrometer (TG/DTA-MS) and infrared gas cell (TG-FTIR), in comparison. Besides that of the previously known H 2O and NH 3, evolution of two new gaseous products, N 2O and NO, which had not been reported earlier, has been detected and traced by both evolved gas analysis (EGA-FTIR and EGA-MS) methods. These oxides of nitrogen are considered as catalyzed oxidation products of the released ammonia in air at 260–350 and 400–450 °C. Thus, the two exothermic heat effects observed by DTA at 342 and 443 °C have been assigned to ammonia combustion. Furthermore solid intermediate products have been structurally evaluated by both FTIR spectroscopy and powder X-ray diffraction (XRD). Based on significant changes in XRD patterns, a consequent formation of hexagonal ammonium tungsten oxide bronze and monoclinic WO 3 are confirmed.