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

Abstract

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.