Thermal decomposition of ammonium molybdates

The thermal behavior of ammonium molybdates, i.e., (NH4)6Mo7O24·4H2O (1) and (NH4)2MoO4 (2), was studied in inert (N2) and oxidizing (air) atmospheres by TG/DTA-MS, XRD, FTIR and SEM. The thermal decomposition sequence of 2 had similarities to 1; however, there were significant differences as well. When both of them were annealed, NH3 and H2O were released parallel, and in air the as-evolved NH3 was burnt partially into NO and N2O. In both atmospheres, while 1 decomposed in four steps, the thermal decomposition of 2 involved 5 steps. In the case of 1, the intermediate products were (NH4)8Mo10O34, (NH4)2Mo4O13 and h-MoO3. In contrast, the decomposition intermediates of 2 were (NH4)2Mo3O10, (NH4)2Mo2O7, (NH4)2Mo4O13 and h-MoO3. By both 1 and 2, the final product was dominated by o-MoO3, accompanied with small amount of Mo4O11 in N2, which was absent in air. Most decomposition steps were endothermic, except for the last step around 400 °C, where crystallization from the residual amorphous phase had an exothermic heat effect. In addition, the combustion of NH3 also changed the DTA curve into exothermic in some cases. The morphology of the final products was characterized by 1–5 μm sheet-like particles, except for annealing 2 in N2, when 0.5- to 1-μm-thick and 5- to 10-μm-long needle-shaped particles were detected.