Preparation of high-purity magnesium hydride by the second hydrogenation method

When Mg powder is hydrogenated under normal conditions, an outer layer of MgH₂ forms first. This MgH₂ layer then prevents hydrogen molecules from penetrating and reacting with metallic Mg, leading to incomplete hydrogenation, even at high temperature and high hydrogen pressure. In this article, the outer MgH₂ layer was first cracked by mechanical ball milling. Then the milled mixture of Mg and MgH₂ was re-hydrogenated and high-purity MgH₂ was obtained. This method for the preparation of high-purity MgH₂ is called the second hydrogenation method. The typical synthesis conditions are: milling time 9 h, second hydrogenation time 6 h, hydrogen pressure 4.5 MPa and temperature 653 K. The hydrogen storage properties of the as-synthesized MgH₂ show that the reaction temperature for the dehydrogenation of MgH₂was reduced by 20-30 K compared with Mg powder.