Comparative evolved gas analyses (TG-FTIR, TG/DTA-MS) and solid state (FTIR5 XRD) studies on thermal decomposition of ammonium paratungstate tetrahydrate (APT) in air

Ammonium paratungstate tetrahydrate (NH4)(10)[H2W12O42](.)4H(2)O (APT), a starting material Of WO3 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 600degreesC. 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 H2O and NH3, evolution of two new gaseous products, NO 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 400-450degreesC. Thus, the two exothermic heat effects observed by DTA at 342 and 443degreesC 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 WO3 are confirmed. (C) 2004 Elsevier B.V. All rights reserved.