Another Simple AlH3 synthesizing Method

A new method of synthesizing an aluminum hydride (AlH3) has been developed.
A direct reaction#1 of aluminum with hydrogen was used for the AlH3 synthesis.
A hydrogen absorbing process and a hydrogen releasing process of the resultant AlH3 were observed by the "in-site observation"#3, which was carried out using radiation rays in the large radiation facility SPring-8"2.
Developed by:
Hiroyuki Saito, Katsutoshi Aoki, et al, in Radiation High-Density Material Science Study Group, Quantum Beam Science Directorate, Japan Atomic Energy Agency
This result was published online in Applied Physics Letters, Vol. 93, Issue 15, Oct. 17, 2008.

As known, AlH3 has attractive properties for the hydrogen storage material.
For example, its hydrogen storage density is very high, its weight is light, and its hydrogen releasing temperature is low.
To synthesize AlH3 is very difficult, however.
It has been known that aluminum can be hydrogenated in a state that hydrogen is fluid at 10,000 atm. or higher. A passive film formed on the aluminum surface hinders aluminum from being hydrogenated.
Dr. Orimo et al in Tohoku University, in cooperation with Hawaii University, succeeded in synthesizing aluminum hydrides having typical three types of crystal structures by means of the chemical synthesizing process, and elucidated basic data, including the reactivity with hydrogen. The Japan Steel Works, Ltd. (JSW)
further developed the synthesizing technology of aluminum hydrides, and confirmed that the formed AlH3 is stored for several months at normal temperature#4.

Keeping the background in mind, the researchers placed aluminum in the hydrogen fluid, which is under the conditions of 10.0 Gpa and 650 degrees of centigrade, for 24 hours.
AlH3 was produced which was about the half of pristine aluminum in volume (see microscope photo of Fig. 3).

An experiment was conducted to investigate the reaction of aluminum with hydrogen under the conditions of high temperature and high pressure.
The radiation powder x-ray diffraction was used and a test piece of aluminum was observed in site.
A multi-anvil press (high pressure generator), installed in SPring-8 BL14B1, was used. The schematic illustration of the high pressure generator was given in Fig. 1. The test piece is completely and tightly covered with super hard pistons and pressure medium when the apparatus is in operation, as seen from Fig. 1.
One cannot visually observe the test piece. To cope with this, a measurement method, called an X-ray powder diffraction method, was used. In the method, highly intensive X-rays, which are emitted from Spring-8, were made to pass through those components and irradiated the aluminum specimen. The test piece was successfully observed in site.
Powder x-ray diffraction patterns produced when the aluminum specimen is heated in a state that it is pressed at 8.9 Gpa, are shown in Fig. 2(a)#5. In Fig. 2, aluminum hydrogenation stared at positions of red dots.
When the test piece was held and heated up to 400 degrees of centigrade, no aluminum hydrogenation was observed. It would be estimated that the passive film on the aluminum surface hinders the hydrogenation of aluminum under the temperature/pressure condition.
When the specimen is heated up to 600 degrees of temperature, a peak representing aluminum hydride appeared after 20 minutes. The aluminum hydrogenation started at this peak position.
A state that AlH3 was heated and decomposed, and another state that the resultant Al was cooled and hydrogenated again are illustrated in Fig. 2(b). From those figures, it is seen that the hydrogen absorption process and the hydrogen releasing process (reverse reaction of the former) were observed in site.
The result of analyzing the test piece showed no detection of impurities.

The aluminum hydride synthesizing method, developed this time, is based on a simple direct reaction of aluminum with hydrogen. The aluminum hydride formed is much purer than that formed by the chemical process. The high purity of the aluminum hydride ensures exact investigation of the aluminum hydride. However, it is difficult for the aluminum hydride synthesizing method based on the direct hydrogenation of aluminum to produce a large amount of aluminum hydride. The direct aluminum hydrogenation method could form a new metal alloy by adding to aluminum another metal that is different from aluminum. Formation of such a metal will lead to creation of new hydrogen storage materials, which are capable of absorbing hydrogen at low pressure.
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Source: JAEA's press release

Keywords: Japan Atomic Energy Agency, direct hydrogenation of aluminum, aluminum hydride, chemical synthesizing process, hydrogen fluid, hydrogen storage material, SPring-8, passive film
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Aluminum hydride (AlH3) has excellent properties for the hydrogen storage material, as known. It is said that few cases of using AlH3 for the hydrogen storage material had been reported. Recently, the researchers (Mr. Saito and Mr. Aoki) in Japan Atomic Energy Agency (JAEA) succeeded in synthesizing AlH3. A direct hydrogenation of aluminum was used for the AlH3 synthesis. Dr. Orimo, et al. in Tohoku University have already synthesized AlH3. The chemical synthesizing process was used for synthesizing AlH3. Nihon Steel Works, Ltd. (JSW) has further developed the synthesizing process, and must have supplied it as a test material to users. (This technical news has already been described on our site.#4)The technical news of the JAEA's AlH3 synthesizing technology is slightly old but I think it needs to be reported since it is a very attractive technology for hydrogen storage.