Structural and thermal gas desorption properties of metal aluminum amides

Article abstract of DOI:10.1016/j.jallcom.2010.07.006

Metal aluminum amides M[Al(NH₂)₄]_x (M = K, Mg, and Ca; x = 1 and 2) were synthesized along with previously reported LiAl(NH₂)₄ and NaAl(NH₂)₄ by ball milling technique under liquid NH₃. The profiles of synchrotron radiation X-ray diffraction suggest that KAl(NH₂)₄, Mg[Al(NH₂)₄]₂ and Ca[Al(NH₂) ₄]₂ have been indexed with single phases, which have never been reported so far. Combination of both FT-IR and ²⁷Al MAS/3QMAS nuclear magnetic resonance suggest that they all have an anion complex unit [Al(NH₂)₄]^- as a basic component, indicating successful synthesis of the metal aluminum amides. Thermogravimetry-differential thermal analysis coupled with mass spectroscopy showed the release of NH ₃ below the temperatures of 140 °C during the thermal decomposition and a NH₃ desorption peak temperature (T_des) decreased with the increasing atomic number. Additionally, a relationship between ²⁷Al isotropic chemical shift and T_des was discussed. The present study gives an useful information that the thermal stability of the anion complex [Al(NH₂)₄]^- can be controlled by a cation M.

Full text of DOI:10.1016/j.jallcom.2010.07.006

Journal of Alloys and Compounds 506 (2010) 297–301
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Structural and thermal gas desorption properties of metal aluminum amides
Taisuke Onoa, Keiji Shimodab, Masami Tsubotab, Satoshi Hinob,
Ken-ichi Kojimac, Takayuki Ichikawaa,b,∗, Yoshitsugu Kojimaa,b
a Department of Quantum Matter, AdSM, Hiroshima University, Higashi-Hiroshima 739-8530, Japan
b Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima 739-8530, Japan
c Division of Environmental Sciences, Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 May 2010
Received in revised form 23 June 2010
Accepted 1 July 2010
Available online 8 July 2010
Metal aluminum amides M[Al(NH2)4]x (M = K, Mg, and Ca; x = 1 and 2) were synthesized along with
previously reported LiAl(NH2)4 and NaAl(NH2)4 by ball milling technique under liquid NH3. The pro-
files of synchrotron radiation X-ray diffraction suggest that KAl(NH2)4, Mg[Al(NH2)4]2 and Ca[Al(NH2)4]2
have been indexed with single phases, which have never been reported so far. Combination of both
FT-IR and 27Al MAS/3QMAS nuclear magnetic resonance suggest that they all have an anion complex
unit [Al(NH2)4]− as a basic component, indicating successful synthesis of the metal aluminum amides.
Thermogravimetry-differential thermal analysis coupled with mass spectroscopy showed the release of
NH3 below the temperatures of 140 ◦C during the thermal decomposition and a NH3 desorption peak
temperature (Tdes) decreased with the increasing atomic number. Additionally, a relationship between
27Al isotropic chemical shift and Tdes was discussed. The present study gives an useful information that
the thermal stability of the anion complex [Al(NH2)4]− can be controlled by a cation M.
Keywords:
Hydrogen storage
Ammonia
Metal aluminum amide
Mechanical milling
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
system, which can be understood by the NH3-mediated reaction
mechanism [4]. Recently, Janot et al. focused on LiAl(NH2)4 which
Global warming and the resource exhaustion are serious prob-
lems to be solved. A solution for these problems has been
extensively studied. Especially, the construction of a clean energy-
oriented society based on hydrogen and the electric power has
been paid much attention. To achieve the hydrogen energy soci-
ety, the development of a hydrogen storage system that has high
volumetric and gravimetric hydrogen capacity, high durability, and
moderate operating temperature is required. In these days, much
interest has been focused on the hydrogen storage materials com-
posed of light elements, so-called chemical hydride [1].
is more labile than LiNH2 and reported that the composite of LiH
and LiAl(NH2)4 released more than 5 mass% H2 below 130 ◦C [5].
Apart from the composite technique, the metal aluminum
amides M[Al(NH2)4]x themselves are attractive because they indi-
rectly store hydrogen in the form of amide [NH2]−. Actually,
LiAl(NH2)4 releases 35 mass% of the NH3 gas with a peak temper-
ature of about 140 ◦C [5–7]. Moreover, Aliouane et al. showed that
NaAl(NH2)4 had 30 mass% NH3 density and the NH3 desorption
peak temperature was about 100 ◦C [8] which was much lower than
that of LiAl(NH2)4 [9,10].
For chemical hydride, a technique to make a composite is quite
useful to improve gas desorption properties. This improvement is
coming from the fact that the composite takes different reaction
pathways from each constituent on heating. In fact, Chen et al.
reported that the composite of LiH and LiNH2 reversibly released
6.3 mass% H2 at around 200 ◦C [2], although LiH and LiNH2 them-
selves decomposed into Li and H2 above 650 ◦C, and into Li2NH and
NH3 at 300 ◦C, respectively [3,4]. These facts indicate that making
a composite could improve the thermodynamics in the LiH–LiNH2
In this study, we successfully prepared the metal aluminum
amides M[Al(NH2)4]x (M = K, Mg, and Ca; x = 1 and 2), along with
previously reported LiAl(NH2)4 and NaAl(NH2)4, and systemati-
cally examined the structural and gas desorption properties by
synchrotron X-ray powder diffraction, Fourier-transform infrared
spectroscopy, nuclear magnetic resonance spectroscopy, and ther-
mogravimetry coupled with mass spectroscopy.
2. Experimental
To synthesize M[Al(NH2)4]x, the starting materials, LiH (Alfa Aesar, 99.4%) and Al
(Rare metallic, 99.9%) for M = Li and NaAlH4 (Aldrich, 90%) for M = Na and K (Aldrich,
99.95%) and Al for M = K and Mg (Alfa Aesar, 99.9%) and Al for M = Mg and CaH2
(Aldrich, 99.99%) and Al for M = Ca, respectively, were put together into a Cr-steel
vessel (SDK-11, UMETOKU Co. Ltd.) with the ratio of M:Al = 1:1 or 1:2. Then, the
vessel was evacuated, bathed into a mixture of dry ice and ethanol, and kept for
a while at −79 ◦C. Gaseous NH3 (5N) was introduced into the vessel and NH3 was
∗
Corresponding author at: Institute for Advanced Materials Research, Hiroshima
University, Higashi-Hiroshima 739-8530, Japan. Tel.: +81 82 424 5744;
fax: +81 82 424 5744.
E-mail address: tichi@hiroshima-u.ac.jp (T. Ichikawa).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.07.006

298
T. Ono et al. / Journal of Alloys and Compounds 506 (2010) 297–301
Fig. 2. The FT-IR spectra (N–H vibration) for M[Al(NH2)4]x together with the data
for M(NH2)x as references; Thick and thin lines correspond to M[Al(NH2)4]x and
M(NH2)x, respectively. (a) M = Li, (b) M = Na, (c) M = K, (d) M = Mg, and (e) M = Ca.
respectively. Our results for M = Li and Na agree well with the
previous reports, indicating that our Li- and Na-samples are actu-
ally LiAl(NH2)4 and NaAl(NH2)4, respectively, where an Al atom is
coordinated by four [NH2]− units (i.e., anion complex [Al(NH2)4]−)
[6,7,9,13]. On the other hand, the crystal symmetry for our
K-sample was different from the previously reported crystal struc-
tures for ␣- and ␤-KAl(NH2)4 synthesized at −30 ◦C and at room
temperature [14–16], suggesting that KAl(NH2)4 has another struc-
ture. The crystal structure of Mg[Al(NH2)4]2 and Ca[Al(NH2)4]2 has
never been reported to the best of our knowledge. Also, it should
be noted that the Ca-sample is different from calcium aluminum
amide ammine complex Ca[Al(NH2)4]2·NH3 reported by Palvadeau
Fig. 1. The SR-XRD patterns of (a) LiAl(NH2)4, (b) NaAl(NH2)4, (c) KAl(NH2)4, (d)
Mg[Al(NH2)4]2, and (e) Ca[Al(NH2)4]2, respectively. Broad features centered at
around 2ꢀ = 11◦ are the background coming from a quartz capillary. Asterisks indi-
cate impurities.
consequently condensed into liquid. Powders were then ball milled in liquid NH3 for
10, 4, 8, 8, and 10 h for M = Li, Na, K, Mg, and Ca, respectively, at room temperature by
a rocking mill apparatus (RM-10, SEIWA GIKEN Co. Ltd.) with a frequency of 10 Hz.
In order to avoid the decomposition due to the increase in temperature during the
milling treatment, the milling process was interrupted every 15 min for 15 min. To
complete the reaction, after the milling process, the milled sample was kept in the
vessel for 7 days. Finally, liquid NH3 was slowly removed at room temperature by a
vacuum pump to collect the powder product. All the sample handlings were carried
out in a glove box (MP-P60W, Miwa MFG Co. Ltd.) with purified Ar (6N, <1 ppm O2
and H2O) to minimize sample degradation.
Structural properties were examined by the synchrotron radiation X-ray diffrac-
tion (SR-XRD) measurements at the beamline BL02B2 in SPring-8. The wavelength
used was ꢁ = 0.80245(2) Å calibrated by the lattice constant of CeO2 at room tem-
perature. The samples were enclosed in a glass capillary (quartz, ꢂ = 0.8 mm) filled
with Ar. The Fourier-transform infrared spectroscopy (FT-IR) measurements were
performed (Spectrum One, Perkin-Elmer) in a home-made glove box filled with
purified Ar. Each sample was diluted by 5 mass% with KBr. 27Al magic-angle spin-
ning (MAS) and triple-quantum MAS (3QMAS) nuclear magnetic resonance (NMR)
experiments [11] were performed using a JEOL JNM-ECA600 with the magnetic field
of 14.1 T under a spinning rate of 15 kHz. The 27Al chemical shifts were adjusted to
an aqueous solution of 1 M AlCl3 at −0.1 ppm. Sample was filled in a ZrO2 sample
rotor (ꢂ = 4 mm) under the Ar atmosphere. Z-filter sequence was used in the 3QMAS
experiments [12].
The thermal gas desorption properties were examined by thermogravimetry-
differential thermal analysis (TG-DTA; TG8120, Rigaku) installed in the Ar-filled
glove box, combined with thermal desorption quadrupole mass spectroscopy (MS;
M-QA200TS, Anelva) upon heating to 300 ◦C under He flow. The flow rate was
300 cm3/min and the heating ramp was 5 ◦C/min.
3. Results and discussion
Fig.
1 shows the SR-XRD profiles for M[Al(NH2)4]x. It is
found that they are able to be indexed with single phases with
monoclinic (a = 9.50 Å, b = 7.37 Å, c = 7.42 Å, and ˇ = 90.1◦), mono-
clinic (a = 13.24 Å, b = 6.05 Å, c = 7.34 Å, and ˇ = 94.0◦), orthorhombic
(a = 11.36 Å, b = 8.85 Å, and c = 6.15 Å), hexagonal (a = 12.10 Å and
c = 7.95 Å), and monoclinic (a = 12.27 Å, b = 6.44 Å, c = 6.43 Å, and
ˇ = 90.7◦) unit cells for the Li-, Na-, K-, Mg-, and Ca-systems,
Fig. 3. 27Al MAS NMR spectra of (a) LiAl(NH2)4, (b) NaAl(NH2)4, (c) KAl(NH2)4,
(d) Mg[Al(NH2)4]2, and (e) Ca[Al(NH2)4]2, respectively. Asterisks indicate spinning
sidebands.

T. Ono et al. / Journal of Alloys and Compounds 506 (2010) 297–301
299
vibration (3250–3450 cm−1) for LiAl(NH2)4 is higher in frequency
than that (3200–3350 cm−1) for LiNH2. Similar tendency is also
observed for NaAl(NH2)4 and NaNH2. The N–H vibrations of the K-,
Mg-, and Ca-samples are in a similar frequency region to those
of LiAl(NH2)4 and NaAl(NH2)4, strongly suggesting that they all
belong to the aluminum amide complex. Again, these metal alu-
minum amide vibrations are higher in frequency than those for
et al. [17]. The detailed crystal structures for our products will be
described elsewhere.
Fig. 2 gives the FT-IR spectra in the frequency region of N–H
stretching mode for M[Al(NH2)4]x together with those for M(NH2)x
as references. The FT-IR spectra of our Li- and Na-samples are very
similar to those of LiAl(NH2)4 and NaAl(NH2)4 reported by Jacobs et
al. [7] and Lutz et al. [13]. It should be noted that the N–H stretching
Fig. 4. 27Al 3QMAS NMR spectra of (a) LiAl(NH2)4, (b) NaAl(NH2)4, (c) KAl(NH2)4, (d) Mg[Al(NH2)4]2, and (e) Ca[Al(NH2)4]2, respectively. Asterisks indicate spinning sidebands.
The other small peaks aligned along F1 dimension are due to the artificial ripples coming from the finite FT truncation.

300
T. Ono et al. / Journal of Alloys and Compounds 506 (2010) 297–301
Table 1
The 27Al isotropic chemical shifts ıiso and quadrupolar products Pq estimated from
the 3QMAS spectra.
Samples
ıiso (ppm)
Pq (MHz)
LiAl(NH2)4
NaAl(NH2)4
KAl(NH2)4
Mg[Al(NH2)4]2
Ca[Al(NH2)4]2
123
121
116
121
119
2.7
3.4
4.9
5.4
3.1
their corresponding metal amides, KNH2, Mg(NH2)2, and Ca(NH2)2.
This may indicate that Al plays an important role on the N–H bond
strength in the aluminum amide complex [Al(NH2)4]−.
27Al MAS and 3QMAS NMR spectra for our Li-, Na-, K-, Mg-, and
Ca-samples are shown in Figs. 3 and 4, respectively. In the MAS
spectra, all the products have a peak in the chemical shift range of
100–130 ppm. We observed a single sharp signal for the Li-sample,
being consistent with a single Al crystallographic site in LiAl(NH2)4
[6,7]. Taking account of the fact that NaAl(NH2)4 also has a single Al
site [9,13], it is found that a small peak splitting observed for the Na-
sample is coming from the second-order quadrupolar interaction as
is evidenced in Fig. 4(b). The K- and Mg-samples have complex peak
shape, but the Ca-sample showed a single sharp peak. The isotropic
projections (lateral spectra in Fig. 4) of the 27Al 3QMAS spectra of
the K- and Mg-samples unambiguously indicate that they also have
a single Al site. The 27Al isotropic chemical shifts ıiso and quadrupo-
lar products Pq estimated from the 3QMAS spectra are listed in
Table 1. The obtained ıiso of 116–123 ppm indicates tetrahedral Al
sites in our products. Also, the large Pq values of K- and Mg-samples
imply significant distortions on the tetrahedral Al sites. Combined
with the SR-XRD and FT-IR results, therefore, we conclude that
we successfully synthesized metal aluminum amides KAl(NH2)4,
Mg[Al(NH2)4]2, and Ca[Al(NH2)4]2, along with the already reported
LiAl(NH2)4 and NaAl(NH2)4, where an Al atom is covalently coor-
dinated by four amide units [NH2]− and cations M+ or M2+ play a
charge-balancing role.
Fig. 5 shows the TG-DTA curves and the NH3 desorption pro-
files (mass number = 17) for the M[Al(NH2)4]x samples. The H2 gas
desorption was not observed above the detection limit. The sample
weights suddenly started to decrease with releasing NH3 gas at low
temperatures of 100, 85, 49, 116, and 82 ◦C and the NH3 desorption
peak temperatures Tdes in the MS profiles were 136, 92, 58, 132,
and 107 ◦C for M = Li, Na, K, Mg, and Ca, respectively. DTA curves
showed that M[Al(NH2)4]x released NH3 by endothermic reaction
at Tdes. It is noteworthy that Tdes of M[Al(NH2)4]x are much lower
than those of M(NH2)x [4,18–20]. The released NH3 molecules esti-
mated from the weight losses are listed in Table 2. This indicates
that a considerable amount of NH3 is able to be released above
the peak temperatures. Also, the gradient of the TG curve changed
drastically above the peak temperature. These results indicate that
Fig. 5. The TG-DTA-MS profiles for (a) LiAl(NH2)4, (b) NaAl(NH2)4, (c) KAl(NH2)4,
(d) Mg[Al(NH2)4]2, and (e) Ca[Al(NH2)4]2. Thick and thin solid lines correspond to
the NH3 desorption profile (M/z, mass number = 17) and DTA curves, respectively.
Dashed line indicates the weight loss (the values at 300 ◦C are shown).
Table 2
The released NH3 molecules at the end of peak temperatures.
Samples
Released NH3 (moleculeperM[Al(NH2)4]x)
Te
300 ◦
C
LiAl(NH2)4
NaAl(NH2)4
KAl(NH2)4
Mg[Al(NH2)4]2
Ca[Al(NH2)4]2
1.4 (160 ◦C)
0.8 (108 ◦C)
0.5 (72 ◦C)
1.5 (144 ◦C)
1.9 (129 ◦C)
2.1
2.0
1.5
2.6
3.4
the NH3 desorption in M[Al(NH2)4]x proceeds in at least two-step
processes.
We found that Tdes decreased with their increasing atomic num-
ber on the respective alkali and alkaline-earth aluminum amides,
i.e., Tdes(Li) > Tdes(Na) > Tdes(K) and Tdes(Mg) > Tdes(Ca). This sug-
gests that the kind of cation M has an influence on the NH3
desorption temperature, in other words, on the thermal stability
of M[Al(NH2)4]x. Here, there appears to be a positive correlation
between Tdes, and 27Al isotropic chemical shift and the Pauling’s
electronegativity ꢃp [21] of a cation M as shown in Fig. 6. When
M = Li is replaced by Na and K, ꢃp decreases from 0.98 (for Li) to
Fig. 6. Relationships (a) between electronegativity of cation M and peak temperature of NH3 desorption, and (b) between electronegativity and 27Al isotropic chemical shift.

T. Ono et al. / Journal of Alloys and Compounds 506 (2010) 297–301
301
0.93 (for Na) and 0.82 (for K), Tdes decreases from 136 ◦C to 92 and
58 ◦C, respectively. 27Al isotropic chemical shift similarly decreases
from 123 ppm to 121 and 116 ppm. This tendency is also observed
for the alkali-earth system, M = Mg and Ca. It is interesting to note
that the Tdes–ꢃp relationship for M[Al(NH2)4]x shows an opposite
tendency to that for metal borohydries M[BH4]n (M = Mg, Ca–Mn,
Zn, Al, Y, Zr and Hf; n = 2–4), where Tdes decreases as a function of
ꢃp of M [22]. We here try to rationalize the relationship between
Tdes and magnetic shielding on the Al site. When the atomic num-
ber of the cation is increased (i.e., ꢃp decreases), the difference of
ꢃp between the cation and nitrogen in [Al(NH2)4]− becomes larger.
Increasing the electron from the cation to nitrogen will reduce the
electron donated from Al to nitrogen. That causes Al having higher
charge density, which increases magnetic shielding effect on the
Al nucleus and gives a negative frequency shift (Table 1). Reducing
the electron donation from Al to nitrogen suggests weakening the
Al–N bonds to lower the NH3 desorption temperature. This indi-
cates that the thermal stability of M[Al(NH2)4]x can be driven by
the choice of the cation M. In addition, the Tdes–ꢃp relationship for
M[Al(NH2)4]x shows different tendencies in the groups of IA and
IIA, which should be quite different from the tendency reported on
M[BH4]n by Nakamori et al. [22].
tion temperature, which may suggest that the weakening of Al–N
bonds involves the NH3 gas release. This means that the NH3 des-
orption temperature can be controlled by replacing the cation, and
thus could provide a new insight for hydrogen storage application
when the MH–Mꢀ[Al(NH2)4]x composite materials are prepared.
Acknowledgement
This work was partially supported by the NEDO project
“Advanced Fundamental Research Project on Hydrogen Storage
Materials”. The SR-XRD experiments were performed at the SPring-
8 with the approval of the Japan Synchrotron Radiation Research
Institute (JASRI) (Proposal No. 2008B2101).
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