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

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 N-2 at a temperature as high as 350 degrees C. Mg(NH2)(2) releases NH3 and N-2 synchronously at a temperature range of 300-400 degrees C without the emission of H-2. NaNH2 synchronously releases H-2, NH3, and a small amount of N-2, at a narrow temperature range of 275-290 degrees C. Using DFT calculations, the decomposition behaviors and the corresponding decomposition mechanisms for LiNH2, Mg(NH2)(2), and NaNH2 have been well understood.