Kersting, Alexandra Louise (2012)
Ph.D. thesis, University of Birmingham.
The thermal decomposition of mixed cation amides, Li\(_3\)Na(NH\(_2\))4 and LiNa\(_2\)(NH\(_2\))\(_3\), with light metal hydrides, lithium hydride, sodium hydride and magnesium hydride, was investigated and hydrogen gas was identified as the major desorption product in all cases. Minimal ammonia was detected and therefore the mixed cation amides could be considered as hydrogen storage materials. The reactions were found to be similar to previously studied light metal amide-hydride systems like lithium amide/lithium hydride and lithium amide/magnesium hydride. Magnesium hydride caused the hydrogen desorption from the mixed cation amides to occur at a lower temperature than when they were heated with lithium hydride. The hydrogen desorption was also at a lower temperature than LiNH\(_2\) + LiH and 2LiNH\(_2\) + MgH\(_2\). Although hydrogen was desorbed when the mixed cation amides were heated with NaH, the amount was much smaller than for LiH and MgH\(_2\), therefore making it less suitable as a hydrogen storage material.
Reactions in various ratios between NaNH\(_2\) and MgH\(_2\) were investigated to intermediate temperatures up to 350 °C. Thermal decomposition, Raman spectroscopy and thermogravimetric analysis were all employed to explore the decomposition and reaction pathways of these reactions. It was found that the products were analogous to those formed
by lithium amide heated with magnesium hydride in similar ratios. The more hydride included in the reaction, the greater the hydrogen loss for the products. Three new phases
were identified and each was attempted to be made pure in order to characterise them.
Evidence for the formation of a mixed Na-Mg amide, Na-Mg imide and Na-Mg nitride, under different temperatures and reaction conditions, was obtained. Attempts to rehydrogenate the Na-Mg imide were successful, reforming magnesium amide and sodium hydride. Sodium amide(NaNH\(_2\)) has been relatively neglected in the literature and so an examination of its decomposition products and reaction with sodium hydride, as a comparison to LiNH\(_2\)-LiH, were carried out. Using Raman and thermal decomposition techniques, it was proposed that a sodium imide may be formed, possibly with the same variable stoichiometry as observed for LiNH\(_2\)-Li\(_2\)NH. Rehydrogenation was attempted on this phase, but was unsuccessful.
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