Abstract
Hydrogenation mechanism of magnesium-based hydrides has attracted considerable interest but many details still remained unclear or controversial. This work aimed to investigate the isothermal hydrogenation kinetics for a number of un-catalyzed and catalyzed MgH2 samples prepared by high-energy ball milling. The effects of processing parameters on the hydrogenation kinetics of magnesium hydrides were studied. X-ray diffraction and transmission electron microscopy were carried out in order to relate MgH2 crystalline size and catalytic effect to the kinetics of hydrogenation. Different kinetic models were employed to further understand the mechanism of hydrogenation. The best kinetic model to describe the hydrogenation behavior of the ball-milled Mg/MgH2 in this work was found to be the JMA (Johnson-Mehl-Arrami) model. In addition, hydrogen diffusion can be a controlling step when the kinetic rates are slow. Adding catalysts contributes to lower hydrogenation activation energy in two ways: reducing crystalline size of the as-milled MgH2 powder, as well as facilitating the hydrogen diffusion in Mg/MgH2.
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