ARTICLE pubs.acs.org/JPCC
Ammonia Desorption Property and Structural Changes of LiAl(NH2)4 on Thermal Decomposition Taisuke Ono,† Keiji Shimoda,‡ Masami Tsubota,#,‡ Shinji Kohara,§ Takayuki Ichikawa,*,†,‡ Ken-ichi Kojima,|| Masataka Tansho,^ Tadashi Shimizu,^ and Yoshitsugu Kojima†,‡ †
Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima 739-8530, Japan Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima 739-8530, Japan § Materials Science Division, Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan Division of Environmental Sciences, Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan ^ National Institute for Materials Science, 3-13 Sakura, Tsukuba 305-0003, Japan
)
‡
bS Supporting Information ABSTRACT: A composite material of LiAl(NH2)4 and LiH irreversibly releases H2 gas by 6.1 mass% below 130 C. The H2 desorption mechanism of the composite has been proposed, but it is still controversial because the decomposition mechanism of LiAl(NH2)4 is not well-known. We here examined the gas desorption property of LiAl(NH2)4 by using thermogravimetry and mass spectroscopy. On heating to 160 C, a large amount of NH3 release was observed, which was followed by mild gas emission until 500 C. We investigated the detailed structural changes on the NH3 gas desorption by using synchrotron X-ray total scattering, infrared spectroscopy, and nuclear magnetic resonance spectroscopy. On the basis of these structural analyses, we obtained strong evidence that the amorphous intermediate product described as LiAl(NH2)x(NH)yNz was formed during the thermal decomposition process, where the AlN4 network polymerization proceeds and the ratio of [NH2], [NH]2, and [N]3 units gradually changes with releasing NH3 gas. Finally, the crystalline Li3AlN2 and the amorphous residue, which will become AlN with longer annealing, were formed because of the complete exhaustion of hydrogen atoms by NH3 desorption.
1. INTRODUCTION Global warming and the resource exhaustion are serious problems to be solved. Nowadays, the construction of a clean energy-oriented society based on hydrogen and electric power has been paid much attention. To achieve the hydrogen energy society, the development of a hydrogen storage material which has high volumetric and gravimetric hydrogen density, high reversibility, and low operating temperature is required. In these days, much interest has been focused on the hydrogen storage materials composed of light elements.1 For chemical hydrides, a composite technique is quite useful to improve gas desorption properties. Chen et al. reported that the composite of LiH and LiNH2 reversibly released 6.3 mass% H2 at around 200 C though LiH and LiNH2 themselves decomposed into Li and H2 at 650 C and into Li2NH and NH3 at 300 C, respectively.24 These facts indicate that making a composite could improve the thermodynamics of the LiH LiNH2 system, which can be understood by the NH3 mediated reaction mechanism.5 Recently, Janot et al. focused on lithium r 2011 American Chemical Society
aluminum amide LiAl(NH2)4 which is more labile than LiNH2. LiAl(NH2)4 is attractive because it indirectly stores hydrogen in the form of amide [NH2] and releases the NH3 gas with a peak temperature of about 140 C.69 They reported that the composite of LiH and LiAl(NH2)4 released more than 5 mass % H2 below 130 C.8 A thermal decomposition pathway of the composite was also proposed. However, the composite of LiH and LiAl(NH2)4 becomes amorphous during decomposition, and the detailed reaction products are still unclear. Therefore, it is important to clarify the NH3 desorption mechanism of pristine LiAl(NH2)4 for a better understanding of the complex thermal reaction of the composite. The crystal structure and the thermal decomposition property of LiAl(NH2)4 have been well investigated.6,7,9 An X-ray diffraction study indicated that LiAl(NH2)4 transforms into an amorphous material upon NH3 Received: December 8, 2010 Revised: March 4, 2011 Published: May 02, 2011 10284
dx.doi.org/10.1021/jp111656v | J. Phys. Chem. C 2011, 115, 10284–10291
The Journal of Physical Chemistry C
ARTICLE
desorption. The amorphous feature of the reaction products makes it difficult to certify the reaction path. Several researchers deduced decomposition products at specific temperatures on the basis of the amount of desorbed NH3 molecules as the following three reaction models: Rouxel and Brec6 LiAlðNH2 Þ4 f LiAlðNHÞ2 þ 2NH3 ð160 185 CÞ f 1=3Li3 AlN2 þ 2=3AlN þ 8=3NH3 ð> 185 CÞ
ð1Þ
Jacobs et al.7 LiAlðNH2 Þ4 f LiNH2 þ 1=2Al2 ðNHÞ3 þ 3=2NH3 ð120 180 CÞ f LiNH2 þ AlN þ 2NH3 ð> 280 CÞ f 1=3Li3 AlN2 þ 2=3AlN þ 8=3NH3 ð> 400 CÞ
ð2Þ Eymery et al.
9
LiAlðNH2 Þ4 f LiAlðNHÞ2 þ 2NH3 ð> 130 CÞ f LiNH2 þ AlN þ 2NH3 ð> 130 CÞ
ð3Þ
f 1=2Li2 NH þ AlN þ 5=2NH3 ð> 400 CÞ
However, the proposed intermediate products, LiAl(NH)2 and Al2(NH)3, are tentative compounds, which could decompose into LiNH2 and AlN. Thus, the decomposition pathway of LiAl(NH2)4 is still controversial. In the present study, we have investigated the detailed structural properties of the decomposition products by using synchrotron X-ray total scattering, in-situ infrared spectroscopy, and solid-state nuclear magnetic resonance spectroscopy as well as the thermal gas desorption property by thermogravimetry-mass spectroscopy and, then, have reexamined the thermal decomposition pathway of LiAl(NH2)4.
2. EXPERIMENTAL PROCEDURES 2.1. Sample Preparation. The sample preparation of LiAl(NH2)4 was the same as in the previous publication.10 The starting materials, LiH (99.4%, Alfa Aesar) and Al (99.9%, Rare metallic), were put together with the ratio of LiH:Al = 1:1 into a vessel with the inner volume of 30 cm3 (SDK-11 steel) under Ar atmosphere (6N purity). Then, the vessel was bathed in a mixture of dry ice and ethanol where it was kept for a while to cool down to 79 C. Gaseous NH3 (5N purity) was introduced into the vessel, and NH3 was consequently condensed into liquid. Powders were then ball milled (8 mm in diameter, ZrO2 ball) in liquid NH3 for 10 h at room temperature by a vibration milling apparatus (RM-10, SEIWA GIKEN Co. Ltd.) with a frequency of 10 Hz. To avoid the decomposition due to an increase in temperature during milling, the milling process was interrupted every 15 min for 15 min. For a complete progress of the reaction, after the milling process, the milled sample was kept statically for 7 days in the vessel. Finally, liquid NH3 was slowly removed at the room temperature by evacuation, and white powder was obtained. 15N-Enriched LiAl(NH2)4 samples were synthesized accordingly from the 15N-enriched NH3 (20% enriched, HydroLabo Inc.) for 15N NMR measurement. 15 N-Enriched LiNH2 was prepared by the mechanochemical reaction of LiH and 15N-enriched NH3 according to the previous report.11 15N-Enriched Li2NH was synthesized by the heat treatment (350 C, 20 h) of 15N-enriched LiNH2 under vacuum. 15 N-Enriched Li3AlN2 was synthesized by the heat treatment (750 C, 30 h) of the composite of 15N-enriched AlN (40%
enriched, HydroLabo Inc.) and Li3N (g99.9%, Aldrich) under Ar atmosphere. The identification of these products was carried out by X-ray diffraction (XRD). All the sample handlings were carried out on a glovebox (MP-P60W, Miwa MFG Co. Ltd.) with purified Ar (
