Thermo-Raman investigations on thermal decomposition of (NH4)6Mo7O24·4H2O

An extensive study by thermo-Raman spectroscopy was carried out to understand the structural changes occurring during the thermal decomposition of (NH4)6Mo7O24·4H2O in open air and under O2 and N2 flow. In addition, themogravimetry, differential thermal analysis and thermogravimetry coupled with mass spectrometry analysis were also carried out and a satisfactory decomposition mechanism has been proposed on a molecular level. The thermo-Raman investigations on the decomposition of (NH4)6Mo7O24·4H2O or 12(NH4)2O·4(MoO3)7·16H2O, since there are four formulae in a unit cell, in open air indicated the formation of a first intermediate 11(NH4)2O·4(MoO3)7 in the temperature interval from 124 to 131 °C and a second intermediate 7(NH4)2O·4(MoO3)7 from 174 to 239 °C, the coexistence of a third intermediate (mixed molybdena with residual volatile components) with MoO3 from 255 to 346 °C and the formation of MoO3 from 347 °C onwards. The present investigations indicated that during the thermal decomposition process from 25 to 250 °C, the (MoO3)7 unit in (NH4)6Mo7O24·4H2O remained intact and the collapse of (MoO3)7 occurred to form MoO3 immediately subsequent to the loss of the remaining volatile compounds at 255 °C. The thermal decomposition of (NH4)6Mo7O24·4H2O under O2 and N2 flow was also investigated in detail.