Thermochemical transformations in 2MNH2–3MgH2 systems (M = Li or Na)

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: 2MNH2 + 3MgH2 → Mg3N2 + 2MH + 4H2↑. A total of 6.45 wt.% of hydrogen is released by the 2LiNH2–3MgH2 system beginning at 186 °C, and a total of 5.1 wt.% H2 is released by the 2NaNH2–3MgH2 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 MgH2 to form Li2Mg(NH)2 and hydrogen. In the second step, reaction between Li2Mg(NH) and MgH2 leads to the formation of the Mg3N2 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(NH2)2 and NaMgH3 intermediates. Partial rehydrogenation in 190 bar hydrogen pressure leading to formation of the MgNH imide was observed in the dehydrogenated 2NaNH2–3MgH2 system at 395 °C.