Recycling process of sodium metaborate to sodium borohydride

https://doi.org/10.1016/S0360-3199(02)00173-8Get rights and content

Abstract

Sodium borohydride (NaBH4) was synthesized by reacting sodium metaborate (NaBO2) with magnesium hydride (MgH2) or magnesium silicide (Mg2Si) by annealing (350–750°C) under high H2 pressure (0.1–7MPa) for 2–4h. As the temperature and the pressure increased, the yield increased to have a maximum value (97–98%) at 550°C under 7MPa, but the value was independent of time. A concept for converting NaBO2 back to NaBH4 using coke or methane is described.

Introduction

A fuel cell is a battery that is actuated with a gas, hydrogen. The energy obtained upon a reaction of hydrogen and oxygen is directly converted into electric energy. Since such a fuel cell has an efficiency much higher than that of conventional combustion engines, a fuel cell vehicle (FCV) is expected to have a high efficiency [1], [2].

Hydrogen can be stored in tanks of compressed [2], [3] or liquefied H2 [3], or by adsorption on activated carbon [4], carbon nanotubes [3], [5] and graphite nanofiber [6], [7], or in a hydrogen-storing alloy [8]. Among these methods, the hydrogen-storing alloy is considered to play an important role in FCV. For the hydrogen-storing alloy, however, there are many problems to overcome: heaviness (small amount of storage per unit weight) due to its nature as an alloy, deterioration (the alloy turning into finer particles or changing its structure) upon repeated storage and release, and the problem of securing metallic resources when the alloy includes rare metals.

Therefore, attention has been given to the hydrolysis of chemical hydride such as NaBH4 [9], [10], [11], [12], [13]. The reaction of sodium hydride with boric oxide has been used for commercial production of NaBH4 [13]:4NaH+2B2O3NaBH4+3NaBO2.

In the previous paper, we found that Pt–LiCoO2 worked as an excellent catalyst for releasing hydrogen by the hydrolysis of NaBH4 [14]:NaBH4+2H2ONaBO2+4H2.The byproduct obtained by the reaction was NaBO2.

NaBH4 is a nonreversible chemical hydride for the one-time hydrolysis generation of H2. Therefore, it is attractive to find a procedure for recycling NaBO2 back to NaBH4. In this paper, we synthesize NaBH4 by annealing NaBO2 and MgH2, or NaBO2 and Mg2Si under high H2 pressure. Recycling of NaBO2 to NaBH4 using coke or methane is also discussed.

Section snippets

Materials

Sodium metaborate (NaBO2, Kojundo Chemical Laboratory Co., Ltd. Japan), magnesium hydride (MgH2, assay 90%, Sigma-Aldrich), magnesium (average particle size <180μm, Kojundo Chemical Laboratory) and silicon (average particle size <5μm, Kojundo Chemical Laboratory) were used for the reaction of NaBH4. NaBO2(66mg) was mixed with MgH2(59mg) or Mg(49mg). The specimen was packed into a stainless-steel autoclave (internal volume: 4cm3). After closing the autoclave, the specimen was compressed at

Synthesis of NaBH4 using MgH2

The reaction of NaBO2 and MgH2 is represented as follows:NaBO2+2MgH2NaBH4+2MgO.The standard-free-energy change ΔGo for the reaction is −270.22kJ, because the standard-free-energy changes of the compounds compared with the elements ΔGfo are −920.7kJ(NaBO2),−71.8kJ(2MgH2),−123.86kJ(NaBH4) and −1138.86kJ(2MgO) [15].

Therefore, the reaction can proceed spontaneously. Thus, the reaction of NaBO2 and MgH2 was carried out. Fig. 1 shows the temperature desorption scan of the product reacted at 550°C

Acknowledgements

We are greatly indebted to Mr. Y. Kawai, Mr. K. Suzuki and Mr. H. Hayashi of the Toyota Central Research & Development Laboratories, Inc. for their help. We also thank Mr. S. Matsumoto, Mr. M. Kimbara and Dr. H. Nakanishi of Toyota Motor Corporation for the discussions.

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