Rate determining step in the absorption and desorption of hydrogen by magnesium

The kinetics of hydrogen absorption and desorption by magnesium has been investigated by a volumetric technique. Experimental data have been analysed in order to find the rate determining step for both the absorption and desorption processes. It is shown that a nucleation and growth (NG) mechanism, with exponent values n=2 for desorption and n=0.5 to n=1 for absorption provides suitable equations in order to fit the experimental data. The influence of hydrogen pressure and temperature on the process rate has been studied to obtain an expression for the driving force of the reaction and its activation energy. The driving force for desorption seems to follow a parabolic law though the experimental data are also compatible with a linear law. According to our data, the rate determining step in the desorption of hydrogen by magnesium, and probably also in the absorption process, seems to be the hydrogen diffusion through the beta phase. An activation energy for such a diffusion process of 100+/-10 kJ mol(-1)H has been obtained from the desorption data. (C) 2002 Elsevier Science B.V. All rights reserved.