Kinetics of hydrogen evolution from MgH2: Experimental studies, mechanism and modelling

Decomposition of magnesium hydride was investigated by means of Thermal Desorption Spectroscopy (TDS) in its barometric modification. TDS and microscopy data revealed that hydrogen evolution from the stoichiometric α-MgH2 proceeds in two steps: (i) formation of the nuclei of the metal phase, followed by (ii) hydrogen evolution through the surface of the formed metallic islets. The latter stage is a limiting step of the overall process and determines the hydrogen desorption rate. For the partially hydrogenated magnesium, the first stage was absent, supporting the proposed mechanism of the desorption process. The influence of the other reactions on the overall outgassing rate is substantially lower than the effect of the desorption from the surface of Mg.To describe the decomposition process, we have elaborated a mathematical model, which takes into account relative rates of hydrogen desorption, chemical transformation on the MgH2–Mg interface and size distribution of the powder particles. From numerical fitting of the experimental data we have estimated an activation energy and pre-exponential factor for the associative desorption from the surface of Mg metal.A physically justified, well-developed model of hydrogen desorption from the hydrogenated magnesium has obvious advantages over the conventionally used Avrami–Erofeev approach. These advantages are discussed in the paper.