In situ synchrotron X-ray diffraction study on the improved dehydrogenation performance of NaAlH4-Mg(AlH4)2 mixture

Article abstract of DOI:10.1016/j.jallcom.2013.04.169

The dehydrogenation performance and mechanism of the synthesized NaAlH ₄-Mg(AlH₄)₂ powders were investigated by performing thermogravimetric analysis and in situ synchrotron X-ray diffraction analysis. NaAlH₄ not only facilitates the first step dehydrogenation of Mg(AlH₄)₂ in lowering its initial dehydrogenation temperature but also increases the total amount of H2 released. Besides, MgH2 and/or Al phases, the products of the first step dehydrogenation reaction, play a catalytic role in lowering the initial dehydrogenation temperature of NaAlH₄. The synthesized NaAlH₄-Mg(AlH₄) ₂ mixture has an initial dehydrogenation temperature as low as 120 °C, and is able to release 5.35 wt% H2 below 350 °C. The self-catalytic dehydrogenation behavior of the NaAlH₄-Mg(AlH₄) ₂ mixture was elaborated in this work with the aid of in situ synchrotron XRD.

Full text of DOI:10.1016/j.jallcom.2013.04.169

Journal of Alloys and Compounds 577 (2013) 6–10
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
In situ synchrotron X-ray diffraction study on the improved
dehydrogenation performance of NaAlH –Mg(AlH ) mixture
4
4 2
a
a
a,b,
⇑
, Bernard Haochih Liu a
Cheng-Hsien Yang , Tzu-Teng Chen , Wen-Ta Tsai
a
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan
Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan, Taiwan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The dehydrogenation performance and mechanism of the synthesized NaAlH –Mg(AlH ) powders were
4
4 2
Received 23 February 2013
Received in revised form 23 April 2013
Accepted 23 April 2013
investigated by performing thermogravimetric analysis and in situ synchrotron X-ray diffraction analysis.
NaAlH not only facilitates the first step dehydrogenation of Mg(AlH in lowering its initial dehydroge-
nation temperature but also increases the total amount of H released. Besides, MgH and/or Al phases,
the products of the first step dehydrogenation reaction, play a catalytic role in lowering the initial dehy-
4
4 2
)
2
2
Available online 2 May 2013
drogenation temperature of NaAlH
genation temperature as low as 120 °C, and is able to release 5.35 wt% H
dehydrogenation behavior of the NaAlH
in situ synchrotron XRD.
4
. The synthesized NaAlH
4
–Mg(AlH
4
)
2
mixture has an initial dehydro-
below 350 °C. The self-catalytic
Keywords:
Complex metal hydrides
2
4
–Mg(AlH
)
4 2
mixture was elaborated in this work with the aid of
NaMgH
3
Metathesis
Dehydrogenation reaction
In situ synchrotron X-ray diffraction
Ó 2013 Elsevier B.V. All rights reserved.
1
. Introduction
decrease in reaction enthalpy. Vajo et al. [27] demonstrated the ben-
eficial effects of silicon on destabilizing LiH and MgH by forming sil-
2
To date there is still no large-scale and reliable hydrogen storage
method which can operate under moderate pressure and tempera-
ture for vehicles powered by fuel cells. Magnesium aluminum hy-
icide upon thermal dehydrogenation. This work led to a number of
studies which examined thermodynamic destabilization, with a fo-
cus on mixed hydrides systems, such as LiBH
4 2
–MgH [28–33],
dride, Mg(AlH
4
2
) , is a promising solid-state hydrogen storage
LiAlH –MgH [34–38], and NaAlH –MgH [38,39].
4
2
4
2
material due to its relatively low dehydrogenation temperature
and high theoretical gravimetric hydrogen density of 9.3 wt% [1–
Ismail et al. [39] revealed the modified dehydrogenation steps
and improved the dehydrogenation performance of a mixed
3
]. As compared to other metal aluminum hydrides, such as NaAlH
4
NaAlH
4
–MgH
2
system as compared to that of as-milled NaAlH
4
or
and LiAlH , the weaker bond energy of Mg(AlH can be attributed
4
4
)
2
MgH alone. Thesynergistic effectsof theNaAlH
2
4
–Mg(AlH system
4 2
)
to the higher electronegativity of Mg, which causes hydride to
decompose and release hydrogen at lower temperature [4].
are of particular interest because it has similar constituents to the
mixed hydrides mentioned above. From another point of view, the
Since the first synthesis of Mg(AlH
this compound has also been produced using cations metathesis
by NaAlH and MgCl via a wet-chemical method [6–8] or a mechan-
4
)
2
by Wiberg and Bauer [5],
feasibility of NaAlH
tagesconsistof high H
and low dehydrogenation temperature of Mg(AlH
4
–Mg(AlH
4 2
) mixture with the combined advan-
2
capacityof NaAlH
4
(5.5 wt%H
2
below400 °C)
4
2
4
)
2
(beginning at
o-chemical activation synthesis [9–13]. However, further improve-
ments on the de-/re-hydrogenation properties of the synthesized
about 170 °C) is worth investigating. Srivastava et al. [40] mechan-
o-chemically synthesized a NaAlH –Mg(AlH mixture, and con-
cluded that the dehydrogenation properties of NaAlH could be
facilitated in the presence of Mg(AlH . However, the dehydrogena-
tion mechanisms and reaction steps remain unclear. In this study,
in situ synchrotron X-ray diffraction, which has been successfully
4
4 2
)
Mg(AlH
4
)
2
are necessary for more practical applications. In view of
4
the approaches applied for complex metal hydrides, both the use
of catalysts [14–16] and the nano-confinement of hydrides by a
nanoporous scaffold [17–23] have been proposed to effectively im-
4 2
)
prove the hydrogen storage performance of Mg(AlH
4
)
2
.
employed to characterize the dehydrogenation behavior of NaAlH
[41] and Mg(AlH [42,12,43], is used to explore the detailed reac-
tion steps and the synergistic effects that occur in the dehydrogena-
tion of a NaAlH –Mg(AlH mixture.
4
The concept of destabilization [24–26] has also been applied to
modify the dehydrogenation reaction by forming alternative
species, leading to the changes in the reaction pathways and a
4 2
)
4
4 2
)
2. Experimental
⇑
Corresponding author. Address: 1, Ta Hsueh Road, Tainan 701, Taiwan, ROC.
Tel.: +886 62757575x62927; fax: +886 62754395.
Mechano-chemical activation synthesis (MCAS) was employed to prepare
magnesium aluminum hydride (Mg(AlH using sodium aluminum hydride
(NaAlH , Sigma–Aldrich, 90% purity) and anhydrous magnesium chloride (MgCl
4 2
) )
E-mail address: wttsai@mail.ncku.edu.tw (W.-T. Tsai).
4
2
,
0
925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.04.169

C.-H. Yang et al. / Journal of Alloys and Compounds 577 (2013) 6–10
7
Sigma–Aldrich, P98% purity) as precursors. By properly controlling the amount of
each precursor, a mixture consisting of Mg(AlH and NaAlH with a specific mole ra-
tio could be fabricated according to the following reaction:
4
)
2
4
NaAlH4
Mg(AlH4)2
NaCl
xNaAlH
4
þ MgCl2 ! ðx ꢀ 2ÞNaAlH
4
þ MgðAlH
4
Þ2 þ 2NaCl ðx ¼ 2; 2:1; 2:5; 4; 12Þ
ð1Þ
-filled vessel
1
0 NaAlH -Mg(AlH )
4
4 2
MCAS was carried out by performing ball-milling for 0.5 h in a N
2
to avoid oxidation of the reactants and products. The details of the procedure can be
found in our previous study [43].
An X-ray diffractometer (XRD, Rigaku MiniFlex II, Cu K
a radiation) was em-
2
NaAlH -Mg(AlH )
4 4 2
ployed to identify the crystal structure of the various synthesized powders after
preparation. In situ synchrotron X-ray diffraction (in situ XRD) was also performed
with the aid of Synchrotron Radiation Facility (beamline 01C2 in National Synchro-
tron Radiation Research Center in Hsinchu, Taiwan). In each analysis, the synthe-
sized powders were loaded in a 1-mm diameter glass capillary tube, and then
0
.5 NaAlH -Mg(AlH )
4 4 2
2
mounted on the specimen holder. Flowing N gas was introduced at one end of
the tube, and the other end was open to the atmosphere. During the diffraction
analysis, the sample was uniformly heated from room temperature to 365 °C at a
ꢀ
1
heating rate of 5 °C min by blowing hot air outside the capillary tube. The wave-
length of the synchrotron X-ray was 1.033209 Å. Every 2-D diffraction pattern was
successively collected by a Mar345 imaging plate. The 2-D diffraction pattern was
then converted to a 1-D pattern by the Fit2D software. As a result, the high temper-
ature transition of the crystal structure of the synthesized powders could be
realized.
0
.1 NaAlH -Mg(AlH )
4 4 2
Mg(AlH4)2
70
Thermogravimetric analysis (TGA) using a high-pressure microbalance (Cahn D-
10) was employed to evaluate the dehydrogenation behavior of the synthesized
powders. The amount of H released and the dehydrogenation temperature were
of particular interest. In each test, synthesized powders with an initial weight of
ca. 500 mg were loaded in a quartz crucible and transferred into the high-pressure
1
2
2
0
30
40
50
60
80
ꢀ
4
Diffraction angle (2θ)
microbalance chamber. The chamber was evacuated to 1 ꢁ 10 torr followed by
the introduction of H gas (99.999% purity) to nearly atmospheric pressure. It
2
Fig. 1. XRD patterns of the as-synthesized Mg(AlH
NaAlH –Mg(AlH (+2 NaCl) powders.
4 2
) (+2 NaCl) and (0.1, 0.5, 2, 10)
was then sealed throughout the heating process. After the microbalance system
was stable, the TGA test was executed from room temperature to 350 °C at a heat-
ing rate of 5 °C min , and the results was recorded to an accuracy of ±1 lg.
4
4 2
)
ꢀ
1
4 4 2
the as-synthesized (0.5, 2, 10) NaAlH –Mg(AlH ) mixtures exhib-
ited not only a significant decrease in the initial dehydrogenation
temperature but also a two-step weight loss characteristic as com-
pared to both the as-synthesized Mg(AlH ) and the as-received
4 2
3
. Results and discussion
.1. Preparation of NaAlH –Mg(AlH
X-ray diffraction analysis was performed to confirm the forma-
3
4
)
4 2
mixtures
NaAlH . Specifically, the initial dehydrogenation temperatures
4
were 145 °C, 125 °C, 120 °C and 132 °C for the as-synthesized
(0.1, 0.5, 2, 10) NaAlH –Mg(AlH ) mixtures, which were all lower
4
4 2
tion of Mg(AlH
The mole ratio between NaAlH
4
)
2
from NaAlH
4
and MgCl
and MgCl
2
precursors, using MCAS.
in the precursors varied
4 2
than that of the as-synthesized Mg(AlH ) (170 °C).
4
2
4 4 2
For the as-synthesized (0.5, 2, 10) NaAlH –Mg(AlH ) mixtures,
from 2 to 12. Fig. 1 shows the XRD patterns of the as-synthesized
Mg(AlH (+2 NaCl) with and without excess NaAlH . As seen in
this figure, the diffraction peaks of Mg(AlH and NaCl were iden-
the second dehydrogenation initiated at higher temperatures of
about 184 °C, 185 °C and 195 °C, respectively. Because the initial
dehydrogenation temperature of the as-synthesized Mg(AlH4)2
4
)
2
4
4 2
)
tified in all patterns, indicating that MCAS via reaction (1), men-
tioned above, was effective in preparing a sample of mixed
was lower than that of the as-received NaAlH , it is reasonable to
expect that the first and the second weight loss of the as-synthe-
4
NaAlH
For the sake of convenience, the synthesized mixtures are desig-
nated as Mg(AlH , 0.1 NaAlH –Mg(AlH , 0.5 NaAlH –Mg(AlH
and so on, with the initial precursors having mole ratios of 2, 2.1,
4
and Mg(AlH
4
)
2
powders (containing NaCl as a by-product).
sized (0.5, 2, 10) NaAlH –Mg(AlH ) mixtures were related to the
4
4 2
dehydrogenation of Mg(AlH4)2 and NaAlH , respectively. The sec-
4
4
)
2
4
4
)
2
4
4 2
) ,
2
.5, and so on, respectively. The diffraction peaks of NaAlH
identified for the synthesized (0.5, 2, 10) NaAlH –Mg(AlH
ders. The peak intensity of NaAlH
NaAlH /Mg(AlH mole ratio in the mixture. The disappearance
of the NaAlH peaks in the XRD pattern for the 0.1 NaAlH
Mg(AlH sample was due to its low concentration in the mixture.
4
were
) pow-
increased with increasing
1
00
4
4 2
4
4
4 2
)
4
4
–
4 2
)
9
9
6
2
3.2. Thermogravimetric analysis
NaAlH4
0 NaAlH -Mg(AlH )
1
4
4
2
2
2
2
The dehydrogenation temperatures and the corresponding
amounts of H released (excluding the weight of by-product NaCl)
from the as-received NaAlH and the as-synthesized Mg(AlH
with the various mole ratios of NaAlH were investigated by the
2
NaAlH -Mg(AlH )
4 4
2
0
0
.5 NaAlH -Mg(AlH )
4 4
4
4 2
)
.1 NaAlH -Mg(AlH )
4
4
4
Mg(AlH4)2
thermogravimetric analysis. Fig. 2 shows the weight change of
0
100
200
300
each mixture from room temperature to 350 °C (at a ramping rate
ꢀ
1
o
of 5 °C min ) under ambient H
synthesized Mg(AlH commenced to dehydrogenate at 170 °C,
while the as-received NaAlH did so at 210 °C. The as-synthesized
.1 NaAlH –Mg(AlH mixture showed a noticeable decrease in
the initial dehydrogenation temperature to 145 °C. Furthermore,
2
gas. As seen in this figure, the as-
Temperature ( C)
4 2
)
Fig. 2. Thermogravimetric analyses of the as-received NaAlH
Mg(AlH (+2 NaCl) and (0.1, 0.5, 2, 10) NaAlH –Mg(AlH
heated from room temperature to 350 °C (ramping rate: 5 °C min ) under ambient
4
, the as-synthesized
4
)
4 2
4
4
)
2
(+2 NaCl) powders
0
4
4
)
2
ꢀ1
2
H gas.

8
C.-H. Yang et al. / Journal of Alloys and Compounds 577 (2013) 6–10
ond dehydrogenation was therefore associated with the decompo-
sition of NaAlH , as will be discussed later based on the results of
4
in situ XRD analysis. The corresponding second dehydrogenation
temperatures were also lower than that of the first dehydrogena-
tion of the as-received NaAlH
NaAlH and Mg(AlH are able to catalyze the dehydrogenation
reactions of each other.
As shown in Fig. 2, the total amounts of H
synthesized Mg(AlH (excluding the weight of NaCl) and the as-
received NaAlH for temperature up to 350 °C were 6.35 wt% and
.80 wt%, respectively. In addition, the as-synthesized (0.1, 0.5, 2,
0) NaAlH –Mg(AlH mixtures decomposed and released
.35 wt%, 4.95 wt%, 4.45 wt%, and 4.20 wt% H , respectively. More-
in the mixture, more
was released from the 2nd
stage dehydrogenation reaction of this compound. Specifically,
.90 wt%, 2.40 wt%, and 3.40 wt% H was released from the second
dehydrogenation of the (0.5, 2, 10) NaAlH –Mg(AlH mixtures.
On the basis of the original mass of the precursors used, a maxi-
4
(210 °C). The results indicate that
4
4 2
)
2
released from the as-
4 2
)
4
4
1
5
4
4 2
)
2
over, with increasing amount of NaAlH
amount and higher percentage of H
4
2
0
2
4
4 2
)
(a)
mum about 69% of the theoretical amount of H
and measured.
2
was thus released
Mg Al
2
3
3.3. In situ synchrotron XRD analysis
Srivastava et al. [40] presented the synergistic benefits of
Mg(AlH on lowering the dehydrogenation temperature and
accelerating the H desorption rate of NaAlH . Our previous study
43] reported the effect of the minor and residual NaAlH on facil-
itating the decomposition of Mg(AlH and lowering the temper-
4 2
)
2
4
Al
[
4
4 2
)
ature needed for the first step dehydrogenation temperature.
However, the detailed mechanisms underlying the synergistic ef-
4 4 2
fects between NaAlH and Mg(AlH ) are still unclear. Therefore,
in situ synchrotron X-ray diffraction (in situ synchrotron XRD) tech-
nique was applied to characterize the dehydrogenation reactions
and mechanisms involved.
MgH2
During in situ synchrotron X-ray diffraction analysis, the pow-
ꢀ1
ders were heated at a rate of 5 °C min from room temperature
up to 365 °C to induce the dehydrogenation reactions. The diffrac-
tion patterns were then collected and compiled every 20 °C. Fig. 3
Mg(AlH4)2
shows the in situ synchrotron XRD patterns of the 0.1 NaAlH
Mg(AlH powder mixture heated from room temperature to
65 °C. As shown in this figure, Mg(AlH and NaCl were stable,
and no significant phase transformation was observed below
25 °C. However, when increasing the temperature to 145 °C, the
MgH and Al peaks appeared, while the intensity of Mg(AlH
4
–
4 2
)
3
4 2
)
1
0
100
200
300
400
2
4
)
2
o
Temperature( C)
peaks decreased, indicating the occurrence of the first step dehy-
drogenation reaction:
(
b)
MgðAlH
4
Þ ! MgH þ 2Al þ 3H
2
ð2Þ
2
2
4 4 2
Fig. 3. (a) In situ synchrotron XRD patterns of the 0.1 NaAlH –Mg(AlH ) (+2 NaCl)
heated from room temperature to 365 °C and (b) variation of the strongest peak
intensity with temperature for the major species appeared during thermal
dehydrogenation.
The temperature-dependent X-ray diffraction peak intensity, de-
rived from Fig. 3a, is summarized in Fig. 3b. The results clearly show
that the initial decomposition occurred at a temperature between
the as-synthesized Mg(AlH
although the initial dehydrogenation temperature changed with
the presence of NaAlH . In the 0.1 NaAlH –Mg(AlH powder mix-
ture, the decomposition reaction of NaAlH at low concentration
was not significantly affected by the presence of Mg(AlH
Fig. 4a shows the in situ synchrotron XRD patterns of the 0.5
NaAlH –Mg(AlH powder mixture heated from room temperature
to 365 °C. As seen in this figure, NaAlH and Mg(AlH were stable at
temperatures lower than 125 °C. As the temperature increased to
45 °C, the MgH and Al peaks indicated the commencement of
the first step dehydrogenation reaction of Mg(AlH ) in accordance
4 2
) (shown in our previous study [43]),
1
25and 145 °C, consistentwith thatmeasured by TGAresults shown
in Fig. 2. The initial dehydrogenation temperature of Mg(AlH was
significantly lowered because of the presence of NaAlH . The cata-
lytic effect of NaAlH on lowering the initial dehydrogenation tem-
perature of Mg(AlH was confirmed by in situ XRD analysis. The
Mg Al peaks, along with the gradually decreasing intensity of the
MgH and Al peaks, were observed as the temperaturewas increased
4 2
)
4
4
4 2
)
4
4
4
4 2
) .
4 2
)
2
3
4
4 2
)
2
4
4 2
)
to 305 °C, indicatingthe occurrenceof the second step dehydrogena-
tion associated with the following reaction:
1
2
2
MgH þ 3Al ! Mg Al
3
þ 2H
2
ð3Þ
2
2
4
2
with reaction (2). The temperature-dependent diffraction peak
intensity shown in Fig. 4b indicates that the decomposition of
NaAlH took place according to the following reaction:
4
Nevertheless, the TGA results shown in Fig. 2 indicate that less H
released in the second step dehydrogenation than in the first. The
dehydrogenation reactions (2) and (3) were the same as those of
2
was

C.-H. Yang et al. / Journal of Alloys and Compounds 577 (2013) 6–10
9
3
NaAlH
4
! Na
3
AlH
6
þ 2Al þ 3H
2
ð4Þ
This reaction started to occur at 185 °C, which was lower than
that of pure NaAlH . This result suggests that MgH and/or Al,
the products of the first step dehydrogenation reaction of
Mg(AlH , play a catalytic role in lowering the initial dehydroge-
nation temperature of NaAlH . This finding is in good agreement
with the TGA results shown in Fig. 2.
The diminishing intensity of Na AlH
4
2
4 2
)
4
3
6
peaks from 285 to 325 °C
might be associated with the following third step dehydrogenation
reaction of the mixture:
2
Na
3
AlH
6
! 6NaH þ 2Al þ 3H
2
ð5Þ
(
a)
Mg Al
2
3
NaH
Al
NaMgH3
MgH2
(
a)
Mg Al
2
3
Mg(AlH4)2
Na AlH
Al
3
6
MgH2
NaAlH4
Mg(AlH4)2
Na AlH
0
100
200
300
400
o
Temperature ( C)
3
6
(
b)
NaAlH4
Fig. 5. (a) In situ synchrotron XRD patterns of the 2 NaAlH
4
–Mg(AlH )
4 2
(+2 NaCl)
heated from room temperature to 365 °C and (b) variation of the strongest peak
intensity with temperature for the major species appeared during thermal
dehydrogenation.
0
100
200
300
400
o
This phenomenon was more pronounced for NaAlH
-rich mix-
pow-
Tempera ature ( C)
4
tures, as will be discussed later. As the 0.5 NaAlH –Mg(AlH )
4
4 2
der mixture continued to heat up to 305 °C, the fourth step
dehydrogenation reaction accompanied by the formation of Mg2-
(
b)
Al
3
via reaction (3) was found.
Fig. 4. (a) In situ synchrotron XRD patterns of the 0.5 NaAlH
4
–Mg(AlH )
4 2
(+2 NaCl)
heated from room temperature to 365 °C and (b) variation of the strongest peak
intensity with temperature for the major species appeared during thermal
dehydrogenation.
Fig. 5a shows the in situ synchrotron XRD patterns of the
2 NaAlH –Mg(AlH ) powder mixture heated from room tempera-
4 4 2

1
0
C.-H. Yang et al. / Journal of Alloys and Compounds 577 (2013) 6–10
ture to 365 °C. Basically, the results were similar to those for the
.5 NaAlH –Mg(AlH powders at temperatures below 185 °C.
Nonetheless, Fig. 5b shows a noticeable NaMgH peak at tempera-
tures above 225 °C, according to the following reaction:
[4] Y. Nakamori, H.W. Li, M. Matsuo, K. Miwa, S. Towata, S. Orimo, J. Phys. Chem.
Solids 69 (2008) 2292–2296.
0
4
4 2
)
[
[
5] E. Wiberg, R. Bauer, Z. Naturforsch 5b (1950) 396–397.
6] M. Fichtner, O. Fuhr, J. Alloys Comp. 345 (2002) 286–296.
3
[7] M. Fichtner, J. Engel, O. Fuhr, A. Gloss, O. Rubner, R. Ahlrichs, Inorg. Chem. 42
2003) 7060–7066.
(
2
Na AlH þ 6MgH ! 6NaMgH þ 2Al þ 3H
Previous studies [39,44–46] reported that NaMgH
result of reacting NaAlH with MgH
absent at 225 °C, as revealed in Fig. 5b, reaction (6) was more likely
responsible for the formation of NaMgH . Fig. 5b shows that NaH
appeared at the expense of Na AlH above 245 °C via reaction
5). The formation of Mg Al via reaction (3) was again identified
above 345 °C. Moreover, the following reaction:
NaMgH þ 3Al ! Mg Al þ 2NaH þ 2H
3
6
2
ð6Þ
[8] K. Komiya, N. Morisaku, Y. Shinzato, K. Ikeda, S. Orimo, Y. Ohki, K. Tatsumi, H.
Yukawa, M. Morinaga, J. Alloys Comp. 446–447 (2007) 237–241.
[9] R.A. Varin, Ch. Chiu, T. Czujko, Z. Wronski, Nanotechnology 16 (2005) 2261–
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3
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forms as the
. However, since NaAlH was
2 4
2274.
4
[
[
10] R.A. Varin, Ch. Chiu, T. Czujko, Z. Wronski, J. Alloys Comp. 439 (2007) 302–311.
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Schüth, C. Weidenthaler, J. Alloys Comp. 407 (2006) 78–86.
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[15] P.A. Berseth, A.G. Harter, R. Zidan, A. Blomqvist, C.M. Araujo, R.H. Scheicher, R.
Ahuja, P. Jena, Nano Lett. 9 (2009) 1501–1505.
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[17] F. Schüth, B. Bogdanovi c´ , A. Taguchi (inventors), MPI Mühlheim, Germany,
assignee, Materials encapsulated in porous matrices for the reversible storage
of hydrogen, Patent application WO 2005014469, 2003.
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B. Schmid, W. Shaw, M. Gutowski, T. Autrey, Angew. Chem. Int. Ed. 44 (2005)
3
[
[
[
3
6
(
2
3
2
3
2
ð7Þ
3
2
also contributed to the formation of the intermetallic compound.
The results shown in Figs. 4 and 5 indicated that the mole ratio be-
[
tween NaAlH
tion behavior of each compound. NaAlH
decrease of the initial dehydrogenation temperature of Mg(AlH
while the dehydrogenated products of Mg(AlH , specifically
MgH and/or Al, facilitated the dehydrogenation of NaAlH
a lower temperature. With increasing amount of Mg(AlH
crease of the dehydrogenation temperature of NaAlH
significant. Besides, the composition of the powder mixtures can
also contribute to a notable difference in the formation of the inter-
mediate reaction products during heating as described in reactions
4
and Mg(AlH
4
)
2
can greatly affect the dehydrogena-
4
in the mixture caused a
[
4 2
) ,
3
578–3582.
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4 2
)
[
2
4
towards
4 2
) , the de-
(
(
4
became more
[
[
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[
[
2
4
. Conclusions
(
4 4 2
The mixture of NaAlH and Mg(AlH ) exhibited multiple-step
[
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dehydrogenation processes during heating from room temperature
to 350 °C, with at least five steps being identified. The first step
dehydrogenation was attributed to the decomposition of
Mg(AlH
NaAlH . A maximum about 69% of the theoretical H
from the mixture with a NaAlH :Mg(AlH mole ratio of 10:1.
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[
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4
2
) , while the second step was mainly associated with
(
4
2
was released
[
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Int. J. Hydrogen Energy 36 (2011) 1200–1205.
4
4 2
)
Each complex metal hydride acted as a catalyst in lowering the ini-
tial dehydrogenation temperature of the other in the mixture. The
[33] M.Q. Fan, L.X. Sun, Y. Zhang, F. Xu, J. Zhang, H.L. Chu, Int. J. Hydrogen Energy 33
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(
[
[
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initial dehydrogenation reaction temperature for Mg(AlH
lowered from 170 °C to 120 °C with the presence of NaAlH
that of the latter decreased from 210 °C to 184 °C. The mutual cat-
alytic roles of NaAlH and Mg(AlH in the mixture were con-
4
)
2
was
4
, while
2
339.
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3–87.
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[
4
4 2
)
8
firmed by TGA and in situ synchrotron XRD analyses.
5
[
[
[
Acknowledgements
2
9
The authors gratefully acknowledge the financial support from
the National Science Council of the Republic of China (Taiwan) un-
der Grant NSC 100-2221-E-006-245-MY2 and the Research Center
for Energy Technology and Strategy, National Cheng Kung Univer-
sity under Grant D100-23003.
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Energy 32 (2007) 4933–4938.
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2
[
[
(
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