A KINETIC STUDY OF NON-ISOTHERMAL HYDROGEN DESORPTION FROM URANIUM HYDRIDE

The non-isothermal hydrogen desorption kinetics of powdered uranium hydride (UH₃) was studied by means of temperature programmed desorption - mass spectrometry (TPD-MS) at different heating rates under He flow. The measured sample was initially prepared by performing four consecutive hydrogenation-dehydrogenation cycles on a raw α-U solid. Crystal structure, particles size distribution and morphology of the thus obtained UH₃ powder were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The resulting hydrogen desorption profiles, taken at different heating rates (5-20K/min), indicate unambiguously that under non-isothermal conditions, UH₃ decomposition obeys a zero-order kinetics. Such a behavior is typically attributed, in kinetic studies of hydrogen desorption from metal hydrides, to a surface reactions controlled mechanism, i.e. transfer of hydrogen from the bulk to the surface or recombination of two chemisorbed hydrogen atoms and desorption to the gas phase [1]. The fitting procedure of the measured H₂ TPD spectra resulting from the UH₃ dehydrogenation process, yielded activation energy and pre-exponential coefficient values of 104 ± 4 kJ/mol and (3.2 ± 0.8)·10⁵ s^-1 respectively. Such activation energy fairly conforms to published values deduced from isothermal kinetic studies which also suggested zero-order kinetics [2-3].

References

[1] F. J. Castro, G. Meyer, J. Alloys Compd., 2002, 330-332, 59-63.
[2] J. B. Condon, E. A. Larson, J. Chem. Phys., 1973, 59, 855-865.
[3] D. L. Lindner, J. Less-Common Met., 1990, 157, 139-146.