Hydrogenation reaction of CaH2-CaB6-Mg mixture

Abstract

Hydrogen desorption/absorption cycling properties of Ca(BH4)(2)+MgH2 mixture having 9.1 wt% of theoretical hydrogen capacity have been investigated. The hydrogenation reaction sequence starting from the dehydrogenated product CaH2+CaB6+Mg was thoroughly followed using X-ray diffraction, and that can be summarized as (i) Mg+H-2 -> MgH2, (ii) 4CaH(2)+3MgH(2 ->)Ca(4)Mg(3)H(14), (iii) Ca4Mg3H14+2CaB(6)+20H(2)-> 6Ca(BH4)(2)+3MgH(2). The steps (i) and (ii) proceeded rather fast, and the total amount of reabsorbed hydrogen was governed by the step (iii). The kinetics of the step (iii) was critically affected by the temperature of dehydrogenation/hydrogenation reaction, which we mainly attributed to the microstructure of the dehydrogenated product such as particle size. We indirectly proved our hypothesis by showing that inserting high-energy ball-milling process during hydrogenation is more effective than simply increasing the reaction time. Optimal reversibility was achieved from the sample which was dehydrogenated at 673 K and hydrogenated at 623 K. (c) 2009 Elsevier B.V. All rights reserved.