(102a) Roles of Catalysts on Hydrogen Desorption of Mixtures of LiBH4 and MgH2 and Its Reversibility

Effects of catalysts (Ti, TiO₂, and TiCl₃) on hydrogen desorption of a 2:1 molar ratio of LiBH4:MgH2 and its reversibility were investigated. 3 mol% of a Ti-compound were added as a catalyst in the hydride mixture using mechanical ball-milling for one hour as a mixing means. Hydrogen desorption was carried out under atmospheric pressure with the temperature range from 25 to 500°C and a heating rate of 2°C min^-1 while hydrogen re-absorption took place at 5 MPa hydrogen pressure and 350°C. The hydrogen desorption results of the uncatalyzed (LiBH₄-MgH₂) and catalyzed mixtures occurred in two steps. It was found that the uncatalyzed and catalyzed mixtures started to release hydrogen at the same temperature of 150°C for the first step and 400°C for the second step. In the presence of a catalyst, the mixtures released hydrogen at a higher amount (6.5 wt%) than that of LiBH₄-MgH₂ (3 wt%) in the first step. When the temperature reached 500°C, the hydrogen desorption of both mixtures were approximately the same, 7 wt%. To understand roles of the metal hydrides and the catalysts in each step of the hydrogen desorption, all mixtures were characterized by XRD. The results identified that the hydrogen desorption from the first step was contributed by a reaction between LiBH₄ and MgH₂ while that from the second step was from a reaction between LiBH₄ and Mg. Furthermore, the catalysts affected the decomposition of MgH₂ in a greater extent than LiBH₄. Both LiBH₄-MgH₂ and that with any tested catalyst possessed better reversibility than each hydride alone. In addition, the hydrogen desorption and re-absorption using difference types of the catalysts will also be discussed.