Thermochemical transformations in 2 MNH 2–3MgH 2 systems ( M = Li or Na)

Abstract

Thermochemical reactions between alkali metal amides and magnesium hydride taken in 2:3 molar ratios have been investigated using pressure-composition-temperature, X-ray powder diffraction and residual gas analysis measurements. The thermally induced reactions in both title systems are stoichiometric and proceed as a following solid state transformation: 2 MNH 2 + 3MgH 2 → Mg 3N 2 + 2 MH + 4H 2↑. A total of 6.45 wt.% of hydrogen is released by the 2LiNH 2–3MgH 2 system beginning at 186 °C, and a total of 5.1 wt.% H 2 is released by the 2NaNH 2–3MgH 2 system starting at 130 °C. Combined structure/property investigations revealed that the transformation in the lithium containing system proceeds in two steps. In the first step, lithium amide reacts with MgH 2 to form Li 2Mg(NH) 2 and hydrogen. In the second step, reaction between Li 2Mg(NH) and MgH 2 leads to the formation of the Mg 3N 2 nitride, lithium hydride and additional gaseous hydrogen. The transformation in the sodium containing system appears to proceed through a series of competing solid state processes with formation of Mg(NH 2) 2 and NaMgH 3 intermediates. Partial rehydrogenation in 190 bar hydrogen pressure leading to formation of the MgNH imide was observed in the dehydrogenated 2NaNH 2–3MgH 2 system at 395 °C.