Synthesis and decomposition reactions of metal amides in metal–N–H hydrogen storage system

doi:10.1016/j.jpowsour.2005.03.228

Journal of Power Sources

Volume 156, Issue 2, 1 June 2006, Pages 166-170

https://doi.org/10.1016/j.jpowsour.2005.03.228 Get rights and content

Abstract

The synthesis and decomposition properties of some metal amides M(NH₂)_x such as LiNH₂, NaNH₂, Mg(NH₂)₂ and Ca(NH₂)₂ were investigated, which play important roles for designing a new family of metal–N–H hydrogen storage systems. Both the gas chromatographic examination and X-ray diffraction measurement indicated that the reaction between alkali or alkaline earth metal hydride MH_x (such as LiH, NaH, MgH₂ and CaH₂) and gaseous NH₃ could quickly proceed at room temperature by ball milling and the corresponding metal amides were easily synthesized in high quality. The kinetics of these kind of reactions is faster in the order of NaH > LiH > CaH₂ > MgH₂, which is consistent with the inverse order of electronegativity of those metals, i.e. Na < Li = Ca < Mg. The thermal decomposition properties indicated that both Mg(NH₂)₂ and Ca(NH₂)₂ decomposed and emitted NH₃ at lower temperature than LiNH₂.

Introduction

As early as 1910, Dafert and Miklauz [1] had reported that the reaction between lithium nitride Li₃N and hydrogen H₂ had generated Li₃NH₄, which was later proved to be a mixture of lithium amide LiNH₂ (1 mol) and lithium hydride LiH (2 mol) by Ruff and Goeres [2]. After almost 1 century, Chen et al. [3] and Hu and Ruckenstein [4] have investigated the system Li₃N as one of the hydrogen storage materials, where the hydrogenation and dehydrogenation of Li₃N were reversibly performed by the following two-step reversible reactions [3]:Li₃N + 2H₂ ↔ Li₂NH + LiH + H₂ ↔ LiNH₂ + 2LiH

According to the two-step reactions in reaction (1), Li₃N can theoretically store 10.4 wt.% hydrogen. However, the standard enthalpy change (∼148 kJ mol⁻¹ H₂ [5]) of the desorption reaction at the first step is so high that a very high temperature over 430 °C [4] is required for the complete recovery of Li₃N from the hydrogenated state. On the other hand, since the second step reaction has much lower standard enthalpy change (44.5 kJ mol⁻¹ H₂) of desorption and has still large amount of hydrogen capacity (6.5 wt.%), it is worthy to be studied as one of the rechargeable hydrogen storage system, which is expressed as follows:LiNH₂ + LiH ↔ Li₂NH + H₂

Ichikawa et al. [6] reported that the mixture of LiNH₂ and LiH doped with 1 mol% TiCl₃ by ball milling method reversibly desorbed and absorbed a large amount of hydrogen (∼5.5 wt.%) in the temperature range from 150 to 200 °C. Furthermore, the mechanism of desorption reaction (2) was experimentally examined in detail [5], [7], and the desorption has been clarified to proceed by the following two elementary reactions:2LiNH₂ → Li₂NH + NH₃LiH + NH₃ → LiNH₂ + H₂The reaction (3) is endothermic while the reaction (4) is exothermic which has been proved to be ultra-fast in the hydrogen desorption reaction (2) [7].

Some elements, especially those in groups of I–IV in the periodic table, can form their nitride, hydride and amide/imides. Among them, it still is possible to design plenty of metal–N–H systems with similar reactions to Eqs. (1) and (2), which are expected to be candidates for hydrogen storage. Quite recently, some new metal–N–H systems for hydrogen storage have been developed, such as the system composed of magnesium amide Mg(NH₂)₂ and LiH [8], [9], [10], [11], the system composed of Mg(NH₂)₂ and MgH₂ [11], [12], the system of Li–Ca–N–H [9] and so on. The hydrogen storage properties of those systems are quite different among them. For instance, the system composed of Mg(NH₂)₂ and LiH has much higher equilibrium pressure for hydrogen desorption than the system of LiNH₂ and LiH [9]. In order to understand the differences among them and to further develop the metal–N–H system as a new family of H-storage materials, the investigations on the reactions between metal hydrides and NH₃ and on the decomposition behaviors of the metal amides are very important and indispensable.

In this work, we investigated the novel reaction between MH_x and NH₃ at room temperature by mechanically milling, by which the corresponding metal amides could be effectively produced in high purity. Then, we examined the thermal decomposition behaviors of these metal amides.

Section snippets

Experimental

The starting materials were purchased from Sigma–Aldrich. LiH and NaH with 95% purity and MgH₂ with 90% purity (most of the impurities is no-hydrogenated Mg) and CaH₂ with 99.99% purity. All the material handlings (including weighing and loading) were performed in a glove-box filled with purified argon to keep a low water vapor concentration and a low oxygen concentration (less than 2 ppm for both) during operation using a gas recycling purification system (MP-P60W, Miwa MFG Co., Ltd.).

Reaction between metal hydride and NH₃ by ball milling

Motivated by the reaction (4) between LiH and NH₃, we expected that the reactions between the alkali and alkaline earth metal hydrides and gaseous NH₃ would proceed by ball milling at room temperature according to the following reaction:MH_x + xNH₃ → M(NH₂)_x + xH₂where M represents alkali or alkaline earth metals like M = Li, Na, Mg or Ca. Fig. 1 shows the kinetic properties of the reactions between MH_x and NH₃. The time for the completion of the reaction (5) was estimated to be 1, 2, 8 and 13 h for NaH,

Conclusions

The reaction between MH_x and gaseous NH₃ was confirmed to proceed quickly at room temperature by ball milling and the resultant product is the corresponding metal amide M(NH₂)_x (M = Na, Li, Mg or Ca), because the milling treatment leads to the acceleration of the reaction between the metal hydrides and gaseous NH₃ by continuous creation of fresh reactive surfaces between metal hydrides and NH₃. The kinetics of the reaction between MH_x and NH₃ by ball milling is faster in the order of NaH > LiH > CaH₂ >

Acknowledgements

This work was supported by the Grant-in-Aid of Japan Society for the Promotion of Science, by the project “Development for Safe Utilization and Infrastructure of Hydrogen Industrial Technology” of the New Energy and Industrial Technology Development Organization (NEDO), and by COE Research program (No. 13CE2002) of the Ministry of Education, Culture, Sports, Sciences and Technology of Japan. The authors gratefully acknowledge Miss E. Gomibuchi, Mr. K. Nabeta, Mr. K. Kimura, and Mr. T. Nakagawa

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Synthesis and decomposition reactions of metal amides in metal–N–H hydrogen storage system

Abstract

Introduction

Section snippets

Experimental

Reaction between metal hydride and NH3 by ball milling

Conclusions

Acknowledgements

J. Alloys Compd.

J. Power Sources

Monatsh. Chem.

Chem. Ber.

Nature

Ind. Eng. Chem. Res.

J. Phys. Chem. B

J. Phys. Chem. A

Reaction between metal hydride and NH₃ by ball milling