Dehydriding reaction of Mg(NH 2) 2–LiH system under hydrogen pressure

Abstract

The dehydriding and structural properties of the 3Mg(NH 2) 2 + 12LiH system under hydrogen pressure were investigated using the pressure-composition ( p- c) isotherm measurement and X-ray diffraction (XRD) analysis. Two distinct regions, a plateau region and a sloping region, can be seen on the p- c isotherms and the amount of the desorbed hydrogen at 523 K was 4.9 mass%. The enthalpy of hydrogenation calculated using a van’t Hoff plot was −46 kJ/mol H 2. The dehydriding reaction was proposed for the 3Mg(NH 2) 2 + 12LiH system based on the obtained p- c isotherms and XRD profiles and chemical valences of Li, Mg, N, and H. In the plateau region on the p- c isotherm, Mg(NH 2) 2, Li 4Mg 3(NH 2) 2(NH) 4 (tetragonal), and LiH phases coexist and the molar ratio of the Li 4Mg 3(NH 2) 2(NH) 4 phase increases (while those of Mg(NH 2) 2 and LiH phases decrease) with the amount of the desorbed hydrogen. On the other hand, the mixture of Li 4+ x Mg 3(NH 2) 2− x (NH) 4+ x + (8− x)LiH (0 ≤ x ≤ 2) is formed and the lattice volume of the Li 4+ x Mg 3(NH 2) 2− x (NH) 4+ x phase continuously increases with the amount of the desorbed hydrogen in the sloping region on the p- c isotherm.