Studies of covalent amides for hydrogen storage systems: Structures and bonding of the MAl(NH2)4 phases with M=Li, Na and K

Metal aluminum amides for hydrogen storage – Crystal structure studies

“…They can thus be regarded as metal alanates where the hydrogen atoms have been substituted by NH 2 . The compounds desorb NH 3 during thermal decomposition and the NH 3 desorption properties have been investigated with temperature-programmed-desorption coupled with a mass spectrometer (TPD-MS), and it reveals that the main NH 3 release occurs in the range 50 -150 °C for M[Al(NH 2 ) 4 ] x (M = Li, Na, K, Mg and Ca) [4,5]. Analogously to the amideimide system [6], NH 3 desorption in metal aluminum amides can be replaced by H 2 desorption by making composites with alkali metal hydrides, therefore the compounds have recently been studied as hydrogen storage materials.…”

“…Analogously to the amideimide system [6], NH 3 desorption in metal aluminum amides can be replaced by H 2 desorption by making composites with alkali metal hydrides, therefore the compounds have recently been studied as hydrogen storage materials. The lithium amide-imide system desorbs ~6.5 mass% H 2 at 255 °C through the following reaction: [6] LiNH 2 + LiH → Li 2 NH + H 2 For more unstable amides the H 2 desorption temperature is decreased, e.g., the composite LiHLiAl(NH 2 ) 4 can desorb up to 6.2 mass% hydrogen at 130 °C [7].…”

“…The crystal structures of LiAl(NH 2 ) 4 , NaAl(NH 2 ) 4 , α-KAl(NH 2 ) 4 , RbAl(NH 2 ) 4 and CsAl(NH 2 ) 4 have been reported from single crystal X-ray diffraction measurements [11][12][13][14]. The structure models included hydrogen positions, but they are regarded as uncertain due to their low X-ray scattering power.…”

“…The structure models included hydrogen positions, but they are regarded as uncertain due to their low X-ray scattering power. The space groups and unit cell parameters of MAl(NH 2 ) 4 Table 1 In the preparation of the metal deuterides, Na or K were annealed under D 2 atmosphere (1 MPa) at 650 °C for 20 h and Ca at 300 °C for 8 h. AlD 3 was prepared from the reaction between LiAlD 4 and AlCl 3 by wet chemistry [20]. The starting materials for each metal aluminum amide listed in Table 2 were loaded into Cr-steel vials (SDK-11, UMETOKU Co. Ltd.) with ZrO 2 balls and milled with sequences of 15 min mill followed by 15min pause under liquid ND 3 or gaseous argon by using a rocking mill apparatus (RM-10, SEIWA GIKEN Co. Ltd.) with a frequency of 10 Hz.…”

“…The spectrometer was installed in a glove box filled with purified Ar. [4,5] with the expected shift due to the isotope effect when replacing H with D [34]. Amide ions have symmetric and asymmetric stretching modes, which provide two absorption bands in the IR spectra.…”