Understanding the Decomposition Mechanisms of LiNH₂, Mg(NH₂)₂, and NaNH₂: A Joint Experimental and Theoretical Study

Abstract

Metal amides are promising candidates for hydrogen storage, hydrogen production, NH3 synthesis and cracking, and so on. However, the decomposition behaviors and mechanisms of metal amides remain unclear. In this study, the decomposition properties of three metal amides, including LiNH2, Mg­(NH2)2, and NaNH2, are studied by thermogravimetry, mass spectroscopy, and in situ X-ray diffraction techniques combined with density functional theory (DFT) calculations. It is found that Mg­(NH2)2, LiNH2, and NaNH2 exhibit very different metal–N and N–H bond strengths, which precipitate various formations energies of different kinds of vacancies. As a result, LiNH2 releases a major amount of NH3, with a small amount of N2 at a temperature as high as 350 °C. Mg­(NH2)2 releases NH3 and N2 synchronously at a temperature range of 300–400 °C without the emission of H2. NaNH2 synchronously releases H2, NH3, and a small amount of N2, at a narrow temperature range of 275–290 °C. Using DFT calculations, the decomposition behaviors and the corresponding decomposition mechanisms for LiNH2, Mg­(NH2)2, and NaNH2 have been well understood.

A Review of High Density Solid Hydrogen Storage Materials by Pyrolysis for Promising Mobile Applications

“…However, N 2 was recently detected as a main decomposition product of Mg­(NH 2 ) 2 . A synchronous release of NH 3 and N 2 was observed at 300–400 °C without the release of H 2 as shown in Figure …”

“…The temperature was detected by a thermocouple placed slightly above the sample . It is worth noting that NH 3 was found to be easily released during the dehydrogenation processes of Li–N–H composites when the temperature was over 350 °C …”

“…Nitrogen hydrides, such as Mg­(NH 2 ) 2 , are also popular materials for hydrogen storage. In the Mg­(NH 2 ) 2 structure, each N atom is bonded to two H atoms and forms an amidogen radical NH 2 , as shown in Figure b . The H vacancy formation is much more energetically favored than the N vacancy ones …”

Dehydrogenation properties of hydride‐hydroxide systems containing potassium

“…Various catalysts and the catalytic mechanism have been studied to develop hydrogen storage materials. [12][13][14][15][16] Among them, KH have attracted attention in M-N-H, M-B-H, and M-O-H systems. 15,16 It is reported that LiH-LiNH 2 system desorbs hydrogen in lower temperature with addition of KH.…”

“…Various catalysts and the catalytic mechanism have been studied to develop hydrogen storage materials 12‐16 . Among them, KH have attracted attention in M‐N‐H, M‐B‐H, and M‐O‐H systems 15,16 .…”