Temperature programmed decomposition of uranyl nitrate hexahydrate

Abstract

Temperature programmed decomposition (TPD) of uranyl nitrate hexahydrate has been studied using evolved gas analysis mass spectrometry (EGA-MS) in the temperature range 300-1400 K. Thermogravimetric (TGA) investigations were performed in the temperature range 300-1100 K. An attempt has been made to resolve the complexity of decomposition behaviour through suitable comparison of TGA and EGA-MS data. Kinetic control regimes for various decomposition stages could be deduced from EGA-MS data. The corresponding activation energies and frequency factors were also evaluated. Kinetics based on random nucleation and diffusion was found to be rate controlling. The residue left over after each decomposition stage was analysed by XRD and XPS to determine structure and composition. The ultimate product was found to be a mixture of UO₃H_1.17 and U₃O₈: the former being a topotactic hydrogen spill over compound of UO₃. Complete conversion of this residue to U₃O₈ was noticed during ion beam exposure of the residue which was performed in the course of XPS investigations.