Mechanism of improving hydrogenation of Mg by in-situ formation of Al* in hydriding combustion synthesis

Magnesium hydride (MgH₂) is an ideal solid hydrogen storage material and on-line hydrolysis hydrogen source. However, the synthesis of MgH₂ usually requires harsh conditions of high temperature, high pressure and long duration, bringing great challenges to production cost and safety. In this paper, MgH₂ is prepared by hydriding combustion synthesis (HCS) and the mechanism of improving Mg hydrogenation properties by the in-situ formation of Al* is revealed. Under low hydrogen pressure of 1 MPa, the hydrogenation degree of Mg can be greatly increased from 9.6% of pure Mg to 90.5% with 10 mol% Al added. In the activation stage, Al reacts with Mg to form Mg₁₇Al₁₂, which brings rich grain boundaries and provides rapid diffusion channels for hydrogen. Subsequently, in the hydrogenation stage, Mg₁₇Al₁₂ reacts with H₂ to generate Al* in-situ, which are evenly distributed and act as nucleation sites for MgH₂. Hydrogen molecules diffuse rapidly along the interface between Mg/MgH₂ and Al* and are easily adsorbed and dissociated on the surface of nano-scale Al* . The unique disproportionation hydrogenation reaction of in-situ forming highly active metallic substances as excellent catalysts provides a new idea for the modification of Mg-based hydrogen storage materials.