Other Metal Hydrides Containing Mg

An offset of the research on the RE-Mg compounds for hydrogen storage resulted in an interest in the MgjRE-type compounds, where RE = Y, La, Ce, Pr, Sm, Gd, Tb, and Dy. Using very high pressures on the order of a few gigapascals. 

Kamegawa et al. [156] synthesized MgjLaH, MgjCeHj, and MgjPrH, from powders of elemental metals. These hydrides decomposed into Mg and RE-hydride at about 300°C with an endothermic reaction. Obviously, because of the high pressures involved in their synthesis, the hydrides are irreversible. 

Mgj Alj + 9H2i6MgH3 + 4Mg3Al2 (2.40wt.%H3) 4Mg3Al3 H- 8H3 iiSMgH -t 12A1 (2.40wt.%H3) 

In practice, about 3.2 wt.%H2 is absorbed at 250°C under 5.3 MPa and slightly more at 350°C. Desorption requires at least 250°C. 

From the Van t Hoff plot the decomposition enthalpy change AH) and entropy change (AS) for the first and second step were calculated to be AH j = 93.9 kJ/mol-Hj and AS, = 116.2 J/mol-H K, and AH = 102.2 kJ/mol-H and = 125.9 1/ mol-HjK. Taking into account the very high desorption temperature of this compound, much higher than that of a catalyzed MgH, and the fact that MgH is used in its synthesis by ball milling, the compound is not at all competitive to MgH.  

No desorption occurred below 400°C. Automatically, these compounds can be eliminated from any consideration as viable storage materials. 

Subsequently, Herley et al. [182-184] studied the thermal and photolytic (UV irradiated) decomposition kinetics of a-AlH3. In general, decomposition was quite slow. At 145°C, the length of time to obtain a full desorption was about 100 min and it was correspondingly longer at lower temperatures. Nearly no desorption was observed at 100°C. Slightly faster kinetics were obtained in the isothermal coirradiated decomposition, but still too slow for any practical application as a storage material. 

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The azides of the alkali and alkaline-earth metals are colourless crystalline salts which can almost be melted without decomposition taking place. X-ray studies have been made of several ionic azides (Li, Na, K, Sr, Ba ) they contain linear symmetrical ions with N—N close to 1-18 A. (A number of azides MN3 are isostructural with the difluorides MHF2.) Other azides which have been prepared include B(N3)3, A1(N3)3, and Ga(N3)3 from the hydrides, Be(N3)2 and Mg(N3)2 from M(CH3)2 and HN3 in ether solution, and Sn(N3)4 from NaN3 and SnCl4. 

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