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Electronic structure and the total energy of the Mg(NH(2))(2) were calculated using first principle theory. The bonding characteristics and decomposition mechanism of the Mg(NH(2))(2) were clarified based on the electronic structure and the total energies. The bonding interactions of the Mg atoms with the two [NH(2)] ligands are slightly different, while it shows a significant difference in the bonding interactions between the N and the H atoms within the [NH(2)] ligands. The weakest bond is the N(2)-H(2) bond in the [NH(2))(2) ligand. A decomposition mechanism of the Mg(NH(2))(2) was proposed based on the bonding characteristics. The decomposition of the Mg(NH(2))(2) is performed by two steps. First H(+) cations decompose from the [NH(2)] ligands due to their weaker bonds with the matrix, and then [NH(2)](-) anions decompose. The H(+) cations and [NH(2)](-) anions therefore react each other to generate NH(3). For the Mg(NH(2))(2) + LiH systems, it is most likely that the Mg(NH(2)) decomposes to MgNH, H(+)cation, and [NH(2)](-) anion first, and then the released H(+) cation and [NH(2)]- anion either react each other to form NH(3) and then reacts with LiH, or directly react with Li(+) cation and H(-) anion if LiH is decomposed. Both of the reactions generate the LiNH(2) and the H(2). And the LiNH(2) further mixes with MgNH to form the LiMgN(2)H(3). The is the first step of a multi-step dehydrogenation process of the Mg(NH(2))(2)-LiH system [Isobe S, Ichikawa T, Leng H, Fujii H, Kojima Y. Hydrogen desorption processes in Li-Mg-N-H systems. J Phys Chem Solids 2008;69:22234.]. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.