Crystal Structure Analysis in the Dehydrogenation Process of Mg(NH 2 ) 2 -LiH System
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0971-Z07-05
Crystal Structure Analysis in the Dehydrogenation Process of Mg(NH2) 2-LiH System Tatsuo Noritake1, Masakazu Aoki1, Shin-ichi Towata1, Yuko Nakamori2, and Shin-ichi Orimo2 1 Materials Dept., Toyota Central R&D Labs., Inc., Aichi, 480-1192, Japan 2 Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan
ABSTRACT Mg(NH2)2-LiH system which have the properties of reversible hydrogenation and dehydrogenation is one of the promising candidates for new hydrogen storage materials. For understanding of the reversible reaction mechanism, we investigated the crystal structure changes in 3Mg(NH2)2-12LiH system using the pressure-composition (p-c) isotherm measurement and synchrotron X-ray diffraction. The sample was prepared by the hydrogenation of Mg3N2 + 4Li3N. At the several dehydrogenation stages of the p-c isotherm measurement at temperature 523 K, the sample was taken out and X-ray diffraction measurement was performed. By the amount of desorbed hydrogen, the reaction was expressed as the following formula,
Mg(NH2)2 + 4LiH → LixMg(NH2)2-x(NH)x + (4-x)LiH + xH2 (x = 0~2). The crystal structures of LixMg(NH2)2-x(NH)x, similar to CaF2-type one, formed during the dehydrogenation reaction were determined by Rietveld analysis. As a result, it is considered that the dehydrogenation process might relate to the diffusion of Li+ ion in cation sites of Mg(NH2)2.
INTRODUCTION Complex hydrides, which consist of light elements, such as LiBH4 and LiNH2, are attracting a great deal of attention as new hydrogen storage materials because of their high gravimetric hydrogen density [1-6]. Reversible hydrogen storage reaction of these complex hydrides is generally difficult. However, it was recently reported that reversible hydrogen storage is possible in a NaAlH4 system with a Ti catalyst [7] and in a LiNH2+LiH mixture system [8]. Furthermore, it was found that the mixture of Mg3N2+4Li3N possesses reversible hydrogen storage functions in which 9.1 mass% of hydrogen can be stored [9]. In this system, the intermediate phases such as Li2Mg(NH)2 are created in the reaction process according to the following formula. 3Mg(NH2)2+12LiH
↔ 3Li Mg(NH) +6LiH+6H ↔ Mg N +4Li N+12H 2
2
2
3
2
3
2
As for the Mg(NH2)2-LiH system, the investigations of the different composition or the different starting materials were reported [10-12]. Because of the high hydrogen density and the reversibility of reaction, it is currently considered that this system is one of the most promising candidates for new hydrogen storage materials. However, the reversible reaction mechanism under hydrogen pressure has not yet been clarified. The intermediate phases are novel hydrides and their structures are unknown. Therefore, these crystal structure analyses have been performed in order to understand the reaction mechanism. The diffraction intensities from
hydrogen and lithium atoms are very weak, so the highly brilliant X-ray source of synchrotron radiation (SPring-8) was used for the diffraction measurement. EXPERIMENT Starting materi
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