Synthesis and dehydrogenation of M(AlH4)2 (M = Mg, Ca)

Article abstract of DOI:10.1016/j.jallcom.2007.01.119

Mg(AlH₄)₂ was synthesized by a metathesis reaction of MgCl₂ and NaAlH₄. It was decomposed in the two-step reactions, in agreement with the previous report [1]. The first reaction is characteristic of the decomposition to form MgH₂, and the second reaction is the further decomposition of this MgH₂ into some aluminum alloy phases. The decomposition temperature of the first reaction could be reduced remarkably when it was mixed with 1-5 mol% TiCl₃. For example, in the case of Mg(AlH₄)₂ mixed with 5 mol% TiCl₃ by ball-milling for 10.8 ks, a large portion of hydrogen was released during the milling at ambient temperature. The decomposition temperature of the first reaction decreased with increasing content of TiCl₃ in the specimen. However, the re-hydrogenation reaction scarcely occurred even in the pre-decomposed TiCl₃-containing specimen. For example, only 0.3 mass% of hydrogen was absorbed in the 1 mol% TiCl₃-doped specimen by exposing it for a long period to the hydrogen atmosphere of 30 MPa at 353 K. This result indicates that Mg(AlH₄)₂ is too unstable to progress the re-hydrogenation reaction smoothly. Therefore, Ca(AlH₄)₂ was chosen next, because this seems more stable than Mg(AlH₄)₂, judging from the phase stability diagram of complex hydrides proposed by us [2]. Ca(AlH₄)₂ was synthesized and its decomposition reactions were investigated in a fundamental manner.

Full text of DOI:10.1016/j.jallcom.2007.01.119

Journal of Alloys and Compounds 446–447 (2007) 237–241
Synthesis and dehydrogenation of M(AlH4)2 (M = Mg, Ca)
K. Komiyaa,∗, N. Morisakua, Y. Shinzatoa, K. Ikedab, S. Orimob,
Y. Ohkic, K. Tatsumic, H. Yukawaa, M. Morinagaa
a Department of Materials Science and Engineering, Graduate School of Engineering,
Nagoya University, Chikusa-Ku, Nagoya 464-8603, Japan
b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
c Department of Chemistry, Graduated School of Science and Research Center for Materials Science,
Nagoya University, Chikusa-Ku, Nagoya 464-8603, Japan
Received 28 September 2006; received in revised form 22 January 2007; accepted 23 January 2007
Available online 30 January 2007
Abstract
Mg(AlH4)2 was synthesized by a metathesis reaction of MgCl2 and NaAlH4. It was decomposed in the two-step reactions, in agreement
with the previous report [1]. The first reaction is characteristic of the decomposition to form MgH2, and the second reaction is the further
decomposition of this MgH2 into some aluminum alloy phases. The decomposition temperature of the first reaction could be reduced remarkably
when it was mixed with 1–5 mol% TiCl3. For example, in the case of Mg(AlH4)2 mixed with 5 mol% TiCl3 by ball-milling for 10.8 ks, a large
portion of hydrogen was released during the milling at ambient temperature. The decomposition temperature of the first reaction decreased
with increasing content of TiCl3 in the specimen. However, the re-hydrogenation reaction scarcely occurred even in the pre-decomposed TiCl3-
containing specimen. For example, only 0.3 mass% of hydrogen was absorbed in the 1 mol% TiCl3-doped specimen by exposing it for a long
period to the hydrogen atmosphere of 30 MPa at 353 K. This result indicates that Mg(AlH4)2 is too unstable to progress the re-hydrogenation
reaction smoothly. Therefore, Ca(AlH4)2 was chosen next, because this seems more stable than Mg(AlH4)2, judging from the phase stability
diagram of complex hydrides proposed by us [2]. Ca(AlH4)2 was synthesized and its decomposition reactions were investigated in a fundamental
manner.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Complex hydride; Magnesium alanate; Hydrogen storage material; Catalyst
1. Introduction
ing, a phase stability diagram of complex hydrides has been
proposed using two parameters [2]. One is the bond energy of
XH diatomic molecules and the other is the electronegativity dif-
ference, ꢀΦX−M between M and X ions. In the diagram unstable
hydrides are located in the region of the low ꢀΦX−M and the low
bond energy of XH, so they have a large possibility of releasing
hydrogen at low temperatures.
The alanate-type complex hydrides have the lower ꢀΦX−M
and the lower XH bond energy than the amide/imide-
type hydrides such as LiNH2 and Mg(NH2)2 [6], and the
borohydride-type hydrides such as LiBH4 and NaBH4. In partic-
ular, Mg(AlH4)2 has the lowest ꢀΦX−M among the alanate-type
hydrides. Therefore, it is expected that the phase stability of
Mg(AlH4)2 will be low and hence it will release hydrogen more
easily than the other complex hydrides. Recently, the crystal
structure and the desorption process of Mg(AlH4)2 have been
investigated [1,7–10].
Recently, complex hydrides such as LiNH2 [3], NaAlH4 [4]
and LiBH4 [5] have been investigated intensively because of
their superiority in both the gravimetric and volumetric densities
of hydrogen to conventional hydrogen storage alloys. However,
for practical use of these complex hydrides, there are still several
crucial problems such as the high decomposition temperature,
poor irreversibility of hydrogenation and dehydrogenation reac-
tions, and low reaction rates.
In view of chemical bonding, complex hydrides, M(XHm)n,
are characterized by the covalent interaction between X and
H ions to form a [XHm]− ion and by the ionic interaction
between M and [XHm]− ions. On the basis of this understand-
∗
Corresponding author.
E-mail address: komiya@silky.numse.nagoya-u.ac.jp (K. Komiya).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.01.119

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K. Komiya et al. / Journal of Alloys and Compounds 446–447 (2007) 237–241
The purpose of this study is to investigate the dehydrogena-
tion and re-hydrogenation reactions of Mg(AlH4)2 including
the catalytic effects of TiCl3. Furthermore, for comparison,
Ca(AlH4)2 containing about 7.9 mass% H2 was synthesized and
its decomposition reactions were investigated in a fundamental
manner.
2. Experimental
2.1. Synthesis of Mg(AlH4)2
Mg(AlH4)2 was synthesized by a metathesis reaction of MgCl2 and NaAlH4,
in a diethylether (Et2O) solution [7], and followed by purification and drying
procedures. The reactions were expressed as
2NaAlH4+MgCl2→ 2NaCl + Mg(AlH4)2·Et2O in diethylether (Et2O) (1)
Mg(AlH4)2·Et2O → Mg(AlH4)2 + Et2O
(2)
First, following the reaction (1), a mixture of MgCl2 and NaAlH4 with
diethylether was heated and stirred under reflux at about 318 K for 2 days
(86.4 ks). Then, using a Soxhlet apparatus, white crystalline Mg(AlH4)2·Et2O
was extracted. Subsequently, following the reaction (2), the solvent, Et2O, was
removed from the product under vacuum, and fine white powders of Mg(AlH4)2
were obtained. Then, the temperature gradually increased from room tempera-
ture to 363 K to dry the powders.
Fig. 1. X-ray diffraction profile of Mg(AlH4)2.
3. Results and discussion
3.1. Pure Mg(AlH4)2
The XRD profile of the final product is shown in Fig. 1. This
result agreed with the report by Fichtner et al. [1]. Any peaks
other than the magnesium alanate were not found, indicating
that this specimen was composed of a single Mg(AlH4)2 phase.
The Raman spectrum was measured at room temperature.
As shown in Fig. 2, it was characterized by the presence of the
peaks around 1803 and 1966 cm−1 due to the Al–H stretching
vibrational mode, and around 760–880 cm−1 due to the bend-
ing mode of Mg–H–Al molecules. This spectrum was also in
good agreement with the one reported by Fichtner et al. [8]. The
broad peaks around 3000 cm−1 arose from the impurities in the
specimen, which is a residual organic material, the diethylether.
The peaks were in good accordance with the reported IR spec-
trum of diethylether [11]. From these XRD and Raman spectrum
2.2. Synthesis of Ca(AlH4)2
Ca(AlH4)2 was synthesized by a metathesis reaction of CaCl2 and NaAlH4,
in a tetrahydrofuran (THF) solution, followed by purification. First, a mixture of
CaCl2 and NaAlH4 with THF was heated and stirred under reflux at about 353 K
for 1 day (43.2 ks). Then, the supernatant liquid of the mixture was decanted off
using a syringe to separate by-product NaCl from Ca(AlH4)2, and THF solvent
was dried off under vacuum. As a result, fine white powders of Ca(AlH4)2·THFx
were obtained.
2.3. Analyses of M(AlH4)2 (M = Mg, Ca)
This final product was respectively examined using X-ray diffraction (XRD)
with Ni-filtered Cu K␣ radiation.
In addition, Raman spectroscopic analysis was performed with a 532 nm
excitation laser at room temperature using a 600 grooves/mm grating, focused
at 2200 cm−1. The spectrum was measured in the range between 137 and
3822 cm−1, with an accuracy of about 12 cm−1. In a glove box, the specimen
was set in a sealed holder prior to the measurement of Raman spectra.
Also, the final product was examined using thermal desorption mass spec-
troscopy (TDS) by increasing the temperature from 298 to 873 K at a constant
heating rate of 5 or 0.5 K/min.
2.4. Synthesis of TiCl3-doped Mg(AlH4)2 and its analyses
In addition, to promote the decomposition reaction, the final product was
milled mechanically with X mol% TiCl3 (X = 1.0, 1.5, 2.0 and 5.0). The milling
was performed in a planetary ball mill apparatus (Firitsch P7) at 400 rpm for
3.6 ks under a hydrogen pressure of 1.5 MPa at ambient temperature. Then, the
TDS measurements were carried out for all the specimens in the same way as
used for pure Mg(AlH4)2. Also, the PCT measurements were conducted in the
dehydrogenation reactions, using an ordinary Sieverts-type apparatus. Here, the
measurements were recorded after 1.2 ks assuming equilibrium conditions were
reached. Moreover, re-hydrogenation reactions were investigated by exposing
the 1 mol% TiCl3-doped specimen to the hydrogen atmosphere of 30 MPa at
353 K for a long time.
All the specimens were always handled in an argon-filled glove box equipped
with a recirculation system to keep the water and oxygen concentrations below
1 ppm during operation.
Fig. 2. Raman spectrum of Mg(AlH4)2.

K. Komiya et al. / Journal of Alloys and Compounds 446–447 (2007) 237–241
239
(1 − 3/2x)MgH2 + 2␣(ss) → A␣ + B␤ + (1 − 3/2x)H2. Here, ␣
and ␤ are the Al–Mg solid solution and some intermetallic
phases, respectively [9]. The peak temperature for each reaction
was reduced by about 50 K for both the reactions when the
heating rate was changed from 5 to 0.5 K/min, since the kinetics
of decompositon reaction depended on the heating rate.
In the case of the heating rate of 0.5 K/min, an enlarged illus-
tration of the thermal desorption curves is shown in Fig. 3(b)
for the emission of H2 and other fragments of diethylether, such
as C2H5+, C2H5O and (C2H5)2O. As a result, the amount of
diethylether-like impurities remaining in the Mg(AlH4)2 speci-
men was found to be much lower than that of H2, and the release
of fragments was nearly completed at about 473 K.
3.2. TiCl3-doped Mg(AlH4)2
The TDS experiments were carried out to investigate the cat-
alytic effect of TiCl3 on the decomposition temperature. The
results of the TDS are shown in Fig. 4 for the TiCl3-doped
Mg(AlH4)2. It was apparent that the decomposition tempera-
tures gradually decreased as the amount of TiCl3 in the specimen
increased from 1.0 to 2.5 mol%. The peak temperature for the
hydrogen desorption was lowest in the 2.5 mol% TiCl3-doped
specimen among them. In the specimen doped with 5.0 mol%
TiCl3 by milling for 10.8 ks, most of the hydrogen in Mg(AlH4)2
appeared to be released during milling at ambient temperature,
so that remarkable release of hydrogen was not observed as
shown in Fig. 4(d). Also, a large increase in the pressure inside
the milling pot was observed after milling.
Fig. 3. TDS measurements of Mg(AlH4)2: (a) at different heating rates (5 and
0.5 K/min); (b) enlarged view of TDS peaks of H2 and other fragment ions that
are related diethyl ether (heating rate 0.5 K/min).
measurements, it was confirmed that pure Mg(AlH4)2 was syn-
thesized successfully.
The PCT curve of 1 mol% TiCl3-doped Mg(AlH4)2 was
measured in the dehydrogenation process at 333 K. As shown in
Fig. 5, the total amount of hydrogen released from the specimen
was about 3 mass%, and this value was much smaller than the
theoretical one of about 7 mass% expected from the reaction
(1 − x/2)Mg(AlH4)2 → (1 − 3/2x)MgH2 + 2␣(ss) + (3 + 3/2x)H2
[9]. One possible reason is the kinetics of the decomposition
of Mg(AlH4)2 was very slow even when a small amount of
1 mol% TiCl3 was doped. The other possible reason is that a
The thermal desorption curves for pure Mg(AlH4)2 are
shown in Fig. 3(a). Here, the heating rates used were 5 and
0.5 K/min. As is evident from Fig. 3(a), Mg(AlH4)2 appeared to
decompose in the two steps. The first and second steps occurred
around 449 and 546 K in the case of the heating rate of 0.5 K/min,
respectively. Following the recent experiment by Fossdal et
al. [9], the first reaction probably corresponds to the reaction
(1 − x/2)Mg(AlH4)2 → (1 − 3/2x)MgH2 + 2␣(ss) + (3 + 3/2x)H2, portion of hydrogen was released during milling at ambient
whereas the second reaction corresponds to the reaction temperature, so the PCT curve shown in Fig. 5 might not be
Fig. 4. TDS TiCl3-doped Mg(AlH4)2, and TiCl3 contents on: (a) 1.0 mol%; (b) 1.5 mol%; (c) 2.5 mol%; (d) 5.0 mol%.

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K. Komiya et al. / Journal of Alloys and Compounds 446–447 (2007) 237–241
Fig. 6. XRD profile of Ca(AlH4)2·THFx.
Fig. 5. PCT measurement of 1 mol% Ti-doped Mg(AlH4)2 at 333 K.
measured in the equilibrium condition. Judging from the PCT
curve shown in Fig. 5, a plateau pressure of hydrogen will be
higher than 10 MPa. This result indicates that Mg(AlH4)2 is a
more unstable hydride than the other alanate hydrides such as
NaAlH4 and KAlH4, in agreement with the prediction from the
phase stability diagram [2].
The re-hydrogenation reactions were investigated for the
1 mol% TiCl3-doped specimen by exposing it for a long time
to the hydrogen atmosphere of 30 MPa at 353 K. Consequently,
only 0.3 mass% of hydrogen was absorbed in the 1 mol% TiCl3-
doped specimen. This result indicates that Mg(AlH4)2 is too
unstable so that the re-hydrogenation reaction can not take place
smoothly in this condition. Therefore, it is important to make
Mg(AlH4)2 more stable by alloying according to the phase sta-
bility diagram.
Fig. 7. Raman spectrum of Ca(AlH4)2·THFx.
come from a residual organic material, THF, whose peaks were
in accordance with the reported IR spectrum of THF [11].
From these XRD, SEM–EDX and Raman measurements, it was
thought that Ca(AlH4)2·THFx was synthesized.
The thermal desorption curves are shown in Fig. 8 for
Ca(AlH4)2·THFx. Here, the heating rate used was 0.5 K/min.
3.3. Ca(AlH4)2·THFx
The XRD profile of the final product is shown in Fig. 6. New
peaks different from that of NaAlH4 or CaCl2 or NaCl appeared.
While the crystal structure of Ca(AlH4)2 was not revealed yet,
this experimental profile was compared with the other reported
one of Ca(AlH4)2 [12,13]. As a result, this profile was not in
accordance with the other data because of the difference of the
total amount of THF. According to SEM–EDX analysis, the
content was determined to be 31 mol% Ca and 62 mol% Al,
the value of which is close to the theoretical composition of
Ca(AlH4)2.
The Raman spectrum measured at room temperature is shown
in Fig. 7. It was characterized by the presence of strong peaks
at 1810 and 1896 cm−1 due to the Al–H stretching vibrational
mode, and around 770–918 cm−1 due to the bending mode
of Ca–H–Al molecules. The sharp peaks around 3000 cm−1
Fig. 8. TDS measurement of Ca(AlH4)2·THFx.

K. Komiya et al. / Journal of Alloys and Compounds 446–447 (2007) 237–241
241
As is evident from Fig. 8, Ca(AlH4)2·THFx appeared to decom-
Acknowledgments
pose in the three steps. The first, second and third steps occurred
around 446, 514 and 550 K, respectively. However, each dehy-
drogenation reaction was not revealed yet. Compared with the
results of Mg(AlH4)2 shown in Fig. 3(a), it was found that
Ca(AlH4)2·THFx is more stable than Mg(AlH4)2. Further exper-
iments are now under way.
This study was supported by a Grant-Aid for Science
Research from the Ministry of Education, Culture, Sports, Sci-
ence and Technology of Japan, by the Japan Society for the
Promotion of Science, and also by the 21st Century COE
Program, “Nature-Guided Materials Processing” in Nagoya
University.
4. Summary
References
Mg(AlH4)2 was synthesized by a metathesis reaction of
MgCl2 and NaAlH4. According to the thermal desorption
analysis, Mg(AlH4)2 was decomposed in the two steps. The
first reaction is characteristic of the decomposition to form
MgH2, and the second reaction is the further decomposition of
this MgH2 into some aluminum alloy phases. However, in the
case of Mg(AlH4)2 doped with 1–5 mol% TiCl3 by ball-milling,
the onset temperatures of the first and second steps was reduced
significantly. For Mg(AlH4)2 doped with 5 mol% TiCl3 by
ball-milling for 10.8 ks, most of hydrogen was released during
milling at ambient temperature. Also, for Mg(AlH4)2 doped
with 1 mol% TiCl3, about 3.0 mass% of hydrogen was released
at 333 K.
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