Influence of temperature and gas atmosphere on the thermal decomposition of (NH4)6Mo7O24.4H2O

Abstract

This study investigates the effects of gas atmosphere and synthesis temperature on the thermal decomposition of (NH4)6Mo7O24.4H2O. Samples were synthesized in a CVD furnace at temperatures of 450, 550, and 650 °C, using different gases: H2, O2, and Ar, individually. The resulting samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), vibrating sample magnetometry (VSM), and optical absorbance. XRD results revealed the formation of MoO3 in all samples produced in an O2 atmosphere. However, in samples synthesized in an inert atmosphere (Ar), a mixture of MoO3 and Mo4O11 phases was detected. The same phase mixture was observed in samples produced at 450 and 550 °C in an H2 atmosphere. However, in samples synthesized in H2 at 650 °C, MoO2 mixed with Mo4O11 was formed. Raman and FTIR spectroscopy confirmed the results obtained by XRD, and these two techniques were also used to compare samples synthesized at 550 °C in different environments. The VSM results indicate a paramagnetic response in the analyzed samples, and there is also an increase in magnetization as the percentage of the Mo4O11 phase increases. Absorbance measurements were also performed and further confirm expected changes in sample composition.