Two-Peak Mystery of LiNH₂–NaH Dehydrogenation Is Solved? A Study of the Analogous Sodium Amide/Lithium Hydride System

Abstract

Alkali metal amide and imide systems especially NaNH 2 and Li 2 NH have recently been proposed as a catalyst for NH 3 cracking. This finding ignites the interest of researchers worldwide to study amide−imide/amide−hydride systems. Although these systems have shown catalytic properties toward NH 3 cracking and have also been proposed as a hydrogen storage material, the decomposition mechanism of sodium amide is still unclear due to the complexity and low melting point of NaNH 2 . Herein, pure NaNH 2 and its composite with lithium hydride as well as sodium hydride in a molar ratio 1:1 has been investigated, and a detailed mechanism associated with their decomposition has been suggested. The sodium amide−lithium hydride system is found to be analogous to lithium amide−sodium hydride. It desorbs 4 wt % H 2 through cation exchange in a temperature range of 25−200 °C followed by NH 3 mediated reaction in the temperature range 200−400 °C. The desorbed product can be rehydrogenated at around 200 °C under 2.0 MPa. An important intermediate, a double cation amide, i.e., Li 3 Na(NH 2 ) 4 , is formed during the decomposition process. The mechanism for the formation of this double cation amide and successive H 2 desorption is proposed herein. The two-peak mystery in the analogous LiNH 2 −NaH system remained unknown for almost a decade and is cracked herein during this study. We successfully propose our model based on NH 3 mediated reaction for the decomposition of sodium amide−alkali hydride composite system.

Eutectic Phenomenon of LiNH2-KH Composite in MH-NH3 Hydrogen Storage System

“…A lot of studies for M -N-H systems with various combinations of M I (NH 2 ) m - M II H n composites ( M I , M II = Li, Na, K, Ca, Mg with the valence number of M I and M II cations as m and n ) have been published before [34,35,36,37,38,39,40,41,42]. To cite an instance, an interaction between Mg(NH 2 ) 2 and LiH, known as Li-Mg-N-H system, has been regarded as one of the most practicable hydrogen storage material, because it can generate 5.5 mass% of H 2 and be re-hydrogenated under 150 °C by the following reaction [43,44,45], Mg(NH 2 ) 2 + 2LiH ↔ MgLi 2 (NH) 2 + 2H 2 …”

The Catalytic Role of D-block Elements and Their Compounds for Improving Sorption Kinetics of Hydride Materials: A Review

“…It is well known that many different materials, alloys, complex hydrides and composites have been used for the purpose of hydrogen storage [11,15,24,26,[67][68][69][70][71][72]. Even if surface oxidation can be fixed by sample activation and other related methods, it takes a long time to desorb/absorb hydrogen by hydride materials and hence this makes it almost impossible for practical usage.…”

Recent Progress and New Perspectives on Metal Amide and Imide Systems for Solid-State Hydrogen Storage

“…NaNH 2 and its composites, in particular, have attracted a lot of attentions because its considerable high amount of hydrogen, low cost and interesting thermodynamic properties [150][151][152][153][154].…”

“…Recently, two different decomposition paths have been presented. Jain et al [151], proposed that NaNH 2 , after melting at 200 • C, decomposes to sodium, ammonia and nitrogen (>400 • C), without passing for its corresponding nitride and/or imide phase as, on the other hand, shown by LiNH 2 and Mg(NH 2 ) 2 . Recently, different results have been obtained by Miyaoka and coauthors, where the thermal decomposition of sodium amide was studied under different experimental conditions [154].…”