Hydrogenation Mechanisms of Mg during Reaction Ball Milling

Magnesium hydrides are considered as the potential hydrogen storage materials. However, high work temperature, slow reaction kinetics and hard activation process limit the practical application of Mg-based hydrides system. Recently, the reaction ball milling (RBM) technique has been successfully introduced to prepare hydrogen storage materials [1, 2]. It combines the courses of sample preparation, activation and hydrogenation into one step. Pure Mg milled under the hydrogen atmosphere was studied by Gennari [3]. In his study, the metastable-MgH 2 was generated by the RBM process. A reduction of particle and crystallite size and an increase of specific surface were observed. Unfortunately, only half of Mg was transformed into hydrides and the crystallite size was still very big. Moreover, it took many hours to finish the hydriding process. Some researchers tried to improve the reaction efficiency between Mg and H 2 by addition of some second phases, such as FeTi 1.2 [4], TiO 2 [5], and amorphous TiMn 1.5 [6]. Due to the excellent kinetic properties, the reaction efficiency was greatly improved. However, few investigations have been directed to the mechanisms of hydriding process in the RBM process, especially with the existence of catalyst. Further investigation is much needed. 2 In this work, Ti 37.5 V 25 Cr 37.5 (bcc structure) alloy was used as catalyst due to good hydriding properties and high hydrogen storage capacity [7]. Ti 37.5 V 25 Cr 37.5 alloy was prepared using arc melting with electromagnetic stirring (EMS) in an argon atmosphere. The starting materials were of a purity of 99.9% and the alloy was turned and melted for four times to improve the homogeneity. Then the as-cast alloy was crushed into powder, which passed through a 0.5mm screen. Finally, the powder of the as-cast alloy (30wt. %) was mixed with Mg (purity>99%, 100 mesh) and then mechanically milled in a SPEX8000 machine. A cylindrical stainless steel vial was used with the stainless steel balls and the ball to powder ratio is 20:1. The vial was sealed by an O-ring and connected to a gas reservoir under an initial pressure of 1 MPa, as shown in Fig. 1 [8, 9]. The total volume of the system was 260 cm 3 and about 2.7 g material could be produced. The milling was carried out for the duration of 5 h. During the milling, small changes in the hydrogen pressure representing absorption of hydrogen by the sample were monitored …