Synthesis and dehydrogenation of CeAl4-doped calcium alanate

Article abstract of DOI:10.1016/j.jallcom.2010.11.044

CeAl₄-doped Ca(AlH₄)₂ was synthesized by reactive ball-milling NaH/Al and CaCl₂ with a few moles of CeAl ₄ catalyst under hydrogen atmosphere. Its synthesis mechanism and dehydriding behavior were systematically investigated by XRD, FTIR, TG/DSC and TPD analyses. These results show that the synthetic process of Ca(AlH ₄)₂ can be divided into two steps: the first step is ball-milling NaH/Al with CeAl₄ under hydrogen atmosphere to form CeAl₄-doped NaAlH₄, the second step is ball-milling the as-synthesized CeAl₄-doped NaAlH₄ and CaCl₂ to form CeAl₄-doped Ca(AlH₄)₂. The CeAl ₄-doped Ca(AlH₄)₂ can desorb more than 5.5 wt.% hydrogen below 200 °C with an acceptable reaction kinetics, and shows attractive dehydriding behavior better than that of Ca(AlH₄) ₂ directly prepared by primary NaAlH₄ and CaCl ₂.

Full text of DOI:10.1016/j.jallcom.2010.11.044

Journal of Alloys and Compounds 509S (2011) S743–S746
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis and dehydrogenation of CeAl4-doped calcium alanate
Xue-zhang Xiao, Chang-xu Li, Li-xin Chen∗, Xiu-lin Fan, Hua-qin Kou, Qi-dong Wang
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2010
Received in revised form 17 October 2010
Accepted 5 November 2010
Available online 12 November 2010
CeAl4-doped Ca(AlH4)2 was synthesized by reactive ball-milling NaH/Al and CaCl2 with a few moles
of CeAl4 catalyst under hydrogen atmosphere. Its synthesis mechanism and dehydriding behavior were
systematically investigated by XRD, FTIR, TG/DSC and TPD analyses. These results show that the synthetic
process of Ca(AlH4)2 can be divided into two steps: the first step is ball-milling NaH/Al with CeAl4 under
hydrogen atmosphere to form CeAl4-doped NaAlH4, the second step is ball-milling the as-synthesized
CeAl4-doped NaAlH4 and CaCl2 to form CeAl4-doped Ca(AlH4)2. The CeAl4-doped Ca(AlH4)2 can desorb
more than 5.5 wt.% hydrogen below 200 ◦C with an acceptable reaction kinetics, and shows attractive
dehydriding behavior better than that of Ca(AlH4)2 directly prepared by primary NaAlH4 and CaCl2.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Complex hydride
Ca(AlH4)2
Hydrogen storage
Reactive ball-milling
CeAl4-doped
1. Introduction
gen capacity of 7.8 wt.%. The decomposition of Ca(AlH4)2 can
mainly divided into four steps [9], the first two decomposition
Hydrogen is considered as an environmental friendly energy
carrier because the produces almost zero pollution emitted from
hydrogen fuel equipments. The lack of suitable high density stor-
age hydrogen material remains one of the important problems for
the development of hydrogen energy utilization [1]. Complex metal
hydrides show much promise, but their hydriding/dehydriding
kinetics need to be improved under moderate temperature and
pressure [2,3]. Since the breakthrough work by Bogdanovic´ and
Schwickardi who showed that NaAlH4 doped with Ti alkoxides
or chlorides could reversibly desorb and absorb hydrogen below
150 ◦C, sodium alanate has been investigated intensively [4–6].
However, utilization of such Ti-compounds as dopants gener-
ally gives birth to nonvolatile byproduct (sodium-chloride) that
occupies a significant weight percentage [4,5]. Recently, our inves-
tigations found that directly doping catalytic active species (CeAl4)
instead of catalytic precursor (CeCl3) into NaAlH4 can enhance its
kinetics without decrease of hydrogen capacity [7]. It is believed
that CeAl4 may also have similar catalyzing effects on other com-
plex aluminum hydrides (such as Li AlH4, Ca(AlH4)2).
steps (Ca(AlH4)2 first decomposes into CaAlH5, Al and H2; and
then CaAlH5 decomposes into CaH2, Al and H2) release 5.9 wt.%
H2, which could be considered for practical applications. Cur-
rently, investigations about Ca(AlH4)2 were mainly concentrated
on the synthetic method and theoretical calculation [10–13]. It was
reported that the Ca(AlH4)2 and CaAlH5 can be synthesized using
AlH3 and CaH2 as primary material [14,15]. CaAlH5 can hydro-
genate/dehydrogenate reversibly by the DSC measurement and
DFT calculation [11,16]. However, the rehydrogenation of CaAlH5
to form Ca(AlH4)2 is the endothermic reaction, indicating the
Ca(AlH4)2 system is a non-reversible hydrogen storage material.
It is well known that the metal element contents of earth crust are
7.73 wt.% for Al, 3.45 wt.% for Ca and 2.74 wt.% for Na, respectively.
So it seems that Ca(AlH4)2 is the promising off-board hydrogen
storage candidate. It was ever reported that the decompositions of
Ca(AlH4)2 and CaAlH5 are kinetically stabilized with rather high
activation energy [14], resulting in slow dehydrogenation pro-
cess. The catalytic modification and dehydriding performance of
Ca(AlH4)2 system were studied much less than those of NaAlH4
system.
So far the hydrogen storage capacity of catalyzed NaAlH4 sys-
tem was difficult to break through 5.5 wt.%. Besides NaAlH4, other
alkaline complex aluminum hydrides with higher mass and vol-
ume densities of hydrogen have also attracted attention in the
past few years [8–10]. Among these, Ca(AlH4)2 has a total hydro-
In this work, we synthesized CeAl4-doped Ca(AlH4)2 by reactive
ball-milling NaH/Al and CaCl2 with a few moles of CeAl4 catalyst.
The synthesis mechanism of CeAl4-doped Ca(AlH4)2 was investi-
gated. Moreover, for comparison, the dehydrogenation processes of
synthesized Ca(AlH4)2 doped with and without CeAl4 catalyst were
characterized and compared by simultaneous thermogravimetry,
differential scanning calorimetry. The CeAl4-doped Ca(AlH4)2 can
desorb more than 5.5 wt.% hydrogen below 200 ◦C.
∗
Corresponding author. Tel.: +86 571 8795 1152; fax: +86 571 8795 1152.
E-mail address: lxchen@zju.edu.cn (L.-x. Chen).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.11.044

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X.-z. Xiao et al. / Journal of Alloys and Compounds 509S (2011) S743–S746
Fig. 2. FTIR spectra of (a) as-synthesized 0.02CeAl4-doped NaAlH4; (b) as-
synthesized 0.04CeAl4-doped Ca(AlH4)2.
Fig. 1. XRD patterns of NaH/Al + 0.02CeAl4 samples ball-milled for (a) 0 h; (b) 30 h;
(c) 60 h; (d) 90 h; and the mixture of 0.02CeAl4-doped NaAlH4 and CaCl2 (NaAlH4
and CaCl2 with a mole ratio of 2:1) ball-milled for (e) 5 h; (f) 10 h; (g) 20 h.
ing that NaH and Al are completely hydrogenated to form Na3AlH6,
and some Na3AlH6 and Al are further hydrogenated to form NaAlH4.
When the milling-time increases to 90 h, Na3AlH6 and Al diffraction
peaks are almost disappeared, suggesting they completely trans-
form to the NaAlH4 in Fig. 1(d). The above results indicate that
the NaAlH4 can be synthesized according to reactions (1) and (2)
[6,7]. Moreover, it is remarkable to note that the CeAl4 diffraction
peaks remain unchanged in the whole ball-milling process. Here,
the CeAl4 catalyst plays a critical role in the formation of NaAlH4
from NaH and Al [7].
2. Experimental
The starting materials of NaH powder (purity 95%), Al powder (purity 99%),
NaAlH4 (purity >95%) and CaCl2 (purity ≥96%) which purchased from Sigma–Aldrich
Corp. were used as received. CeAl4 catalyst (purity 99%) was prepared by induc-
tion melting stoichiometric mixtures of pure Ce (purity 99%) and Al (purity 99.9%)
metals, which detail can be found in our previous paper [7]. The samples were han-
dled and stored in a glove box with a dry argon atmosphere to prevent any contact
with oxygen or moisture. The CeAl4-doped Ca(AlH4)2 was synthesized by two-step
process. First, about 2.5 g mixture of NaH, Al and CeAl4 in a mole ratio of 1:1:0.02
was milled in the Planetary mill for 0–90 h under a hydrogen pressure of 3 MPa.
The CeAl4-doped NaAlH4 was synthesized when the milling time increases to 90 h.
Second, mixture of CeAl4-doped NaAlH4 and CaCl2 (NaAlH4 and CaCl2 with a mole
ratio of 2:1) according to the reaction: 2NaAlH4 + CaCl2 → Ca(AlH4)2 + 2NaCl with a
total amount of 2.08 g was milled in the Planetary mill for 0–20 h under a hydrogen
pressure of 2 MPa. The ball-to-powder weight ratio was around 40:1.
The structural characteristics of the as-synthesized samples were determined
by means of by X-ray diffractometer (XRD, X’Pert Pro) with Cu K␣ radiation at 40 kV
and 40 mA. A special plastic tape was used to prevent the XRD sample from con-
tacting with water air and moisture. A–H vibrations in the samples were identified
using a Fourier infrared spectrometer (FTIR, Bruker Tensor27). The powder sample
was mixed uniformly with KBr powder at a weight ratio of about 1:30 and then
the mixture was cold pressed into a pellet of 13 mm diameter for testing. The heat
decomposition features of samples were also measured by a synchronous thermal
analyses thermogravimetry and differential scanning calorimetry (TG/DSC, Netzsch
STA 449F3). The heating rate was 3–10 ◦C/min and the flow rate of argon protec-
tive gas was 40 ml/min. Thermal programmed desorption (TPD) of CeAl4-doped
Ca(AlH4)2 sample was carried out on a Sievert’s type apparatus and heated to the
assigned temperature at a rate of 2 ◦C/min. The hydrogen capacity was calculated in
weight percent and normalized to the weight of Ca(AlH4)2 with CeAl4 catalyst, but
the weight of NaCl was excluded.
3NaH + Al + 1.5H2 ↔ Na3AlH6
Na3AlH6 + 2Al + 3H2 ↔ 3NaAlH4
(1)
(2)
The XRD patterns of mixtures of as-synthesized CeAl4-doped
NaAlH4 and CaCl2 (NaAlH4 and CaCl2 with a mole ratio of 2:1)
ball-milled for different milling-times under 2.0 MPa initial hydro-
gen pressure are also shown in Fig. 1. After ball-milled for 5 h,
the sample consists of NaAlH4 and CaCl2 main phases together
with CeAl4 minor phase. Except the CaCl2·(H2O)4 comes from the
impure phase of CaCl2 reactant, no new phase has been found in
Fig. 1(e). Some NaAlH4 and CaCl2 diffraction peaks are disappeared
as the milling-time increases to 10 h, while the Ca(AlH4)2 and NaCl
diffraction peaks are present in the sample. When the milling-time
increases to 20 h, the NaAlH4 and CaCl2 phases are disappeared
completely, and the Ca(AlH4)2 phase can be detected obviously
in Fig. 1(g). The CeAl4 phase is almost unchanged in the whole
synthetic process. These results suggest that the Ca(AlH4)2 can be
synthesized according to reaction (3) [12,17]:
2(0.02CeAl4-doped NaAlH4)
3. Results and discussions
+ CaCl2 → 0.04CeAl4-doped Ca(AlH4)2 + 2NaCl
(3)
Ca(AlH4)2 was generally synthesized by a metathesis reaction of
NaAlH4 and CaCl2 [12,17]. However, as a raw material of hydride,
NaH and Al are cheap than NaAlH4. So we chose NaH/Al and
CaCl2 instead of NaAlH4 and CaCl2 as raw materials to synthe-
size Ca(AlH4)2. To investigate the mechanics of synthesis, the XRD
patterns of NaH/Al + 0.02 CeAl4 ball-milled under 3.0 MPa initial
hydrogen pressure for different milling-times are shown in Fig. 1. It
can be found that the NaH and Al diffraction peaks become weaker
and broader when milling-time increases to 30 h, indicating the
average crystallite size is strongly reduced. At the same time, the
Na3AlH6 diffraction peaks are detected in Fig. 1(b). After ball-milled
for 60 h, some Na3AlH6 and broader Al diffraction peaks together
with NaAlH4 diffraction peaks can be detected in Fig. 1(c), indicat-
The synthetic process of Ca(AlH4)2 can be mainly divided into
two steps: the first step was ball milling NaH/Al with CeAl4 under
hydrogen to form CeAl4-doped NaAlH4, and the second step was
ball milling the as-synthesized CeAl4-NaAlH4 and CaCl2 to form
CeAl4-doped Ca(AlH4)2.
To further identify the chemical reactions occurring during the
ball milling process, the FTIR spectra of the as-synthesized CeAl4-
doped NaAlH4 and Ca(AlH4)2 are shown in Fig. 2. It is well known
that the characteristic band of Al–H bond changes with differ-
ent chemical environment. The as-synthesized 0.02CeAl4-doped
NaAlH4 exhibits two infrared active vibration peaks in 1640 and

X.-z. Xiao et al. / Journal of Alloys and Compounds 509S (2011) S743–S746
S745
Fig. 4. DSC curves and Kissinger plots of as-synthesized 0.04CeAl4-doped Ca(AlH4)2
at different heating rates: (a) DSC curves; (b) Kissinger plots.
compared to that of primary Ca(AlH4)2. The dehydriding tempera-
ture of CeAl4-doped Ca(AlH4)2 is much lower than that of primary
Ca(AlH4)2. It indicates that the CeAl4 catalyst can improve the dehy-
drogenation performance of Ca(AlH4)2 effectively, which catalysis
may be similar to that in the NaAlH4 system [7].
In order to gain the insight of the improved dehydrogenation of
CeAl4-doped Ca(AlH4)2, the reaction kinetics parameters (activa-
tion energy Ea and pre-exponential factor A) were measured by the
Kissinger relation of reaction (4) [14,20]:
ꢀ
ꢁ
ꢂ
ꢃ
ˇ
Tp2
Ea
AR
Ea
ln
= −
+ ln
(4)
RTp
where ˇ is the heating rate, Tp is the transformation peak temper-
ature that changed with different heating rate, Ea is the activation
energy, R is the gas constant, and A is the pre-exponential factor.
A series of DSC measurements were conducted at different heating
rates of 3, 5, 8 and 10 ◦C/min for the as-synthesized 0.04CeAl4-
doped Ca(AlH4)2 in Fig. 4(a). With increasing the heating rate, Tp
shifts towards the higher temperature side [14]. It is important to
note that, the first weak exothermic peak between 72 and 96 ◦C
can be attributed to the decomposition of Ca(AlH4)2 into CaAlH5,
Al and H2, the second endothermic peak between 198 and 217 ◦C
can be attributed to the decomposition of CaAlH5 into CaH2, Al and
H2 [9]. The relation of ln(ˇ/Tp2) vs. 1/Tp was plotted in Fig. 4(b),
the activation energy Ea and pre-exponential factor A can be cal-
culated from the slope −Ea/R and the intercept ln(AR/Ea) according
to reaction (4). The values of Ea and A of the primary Ca(AlH4)2
were also determined and compared in Table 1 for comparison. It
is found that the Ea values of Ca(AlH4)2 and CaAlH5 are reduced,
and their A values are enhanced obviously after doped with CeAl4
catalyst. The changes of Ea and A also agree with the TG/DSC results
in Fig. 3. The reduction of activation energy is associated with the
change in the rate-limiting step [21], which finally improves the
dehydriding kinetics of CeAl4-doped Ca(AlH4)2. However, further
investigation on the reaction path of dehydrogenation and its rate-
Fig. 3. TG/DSC profiles of dehydrogenation from (a) as-synthesized 0.02CeAl4-
doped NaAlH4; (b) primary Ca(AlH4)2; (c) as-synthesized 0.04CeAl4-doped
Ca(AlH4)2 (heating rate is 5 ◦C/min).
1675 cm−1, the as-synthesized 0.04CeAl4-doped Ca(AlH4)2 also
exhibits two vibration peaks in 1640 and 1797 cm−1. The vibration
peak at 1640 cm−1 can be associated to water bending vibration
visible [18], it may be due to the moisture present in test system
during the characterization of the as-synthesized samples. It can be
found that, the band at 1675 cm−1 corresponds to a Al–H stretch-
ing vibration for NaAlH4, and the band at 1797 cm−1 corresponds to
a Al–H stretching vibration for Ca(AlH4)2 [19]. This result suggests
further that the as-synthesized NaAlH4 and CaCl2 have transformed
completely to Ca(AlH4)2 according to reaction (3).
Fig. 3 presents the TG/DSC profiles of the as-synthesized
0.02CeAl4-doped NaAlH4, primary Ca(AlH4)2 and as-synthesized
0.04CeAl4-doped Ca(AlH4)2. The primary Ca(AlH4)2 was directly
synthesized from primary NaAlH4 and CaCl2 without doping cata-
lyst. It can be found that two weight loss signals about 2.56 wt.% at
the first endothermic peak of 172 ◦C, and 1.68 wt.% at the second
endothermic peak of 200 ◦C appear in the TG curve of 0.02CeAl4-
doped NaAlH4, which are in agreement with previous reports [6].
Moreover, 0.04CeAl4-doped Ca(AlH4)2 desorbs 2.46 wt.% hydro-
gen at the first exothermic peak of 76.9 ◦C, and desorbs 2.36 wt.%
hydrogen at the second endothermic peak of around 205 ◦C, which
total dehydriding capacities in first two stages are high than that
of 0.02CeAl4-doped NaAlH4. This result indicates that Ca(AlH4)2
system is a potential candidate for off-board hydrogen storage
material with large capacity. Moreover, although the 0.04CeAl4-
doped Ca(AlH4)2 shows a slight decrease of dehydriding capacity,
Table 1
Activation energy (Ea) and pre-exponential factor (A) calculated from Kissinger rela-
tion for the decompositions of Ca(AlH4)2 and CaAlH5.
Complex hydride
Primary sample
Ea (kJ/mol)
A (min−1
)
Ca(AlH4)2
CaAlH5
62.8
154.9
4.58 × 105
1.64 × 1014
Ca(AlH4)2
CaAlH5
43.7
136.1
6.32 × 105
2.68 × 1014
CeAl4-doped sample

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X.-z. Xiao et al. / Journal of Alloys and Compounds 509S (2011) S743–S746
Acknowledgments
This work was financially supported by the National Basic
Research Program of China under Grant No. 2007CB209701 and
2010CB631300, the National Natural Science Foundation of China
under Grant No. 51001090, 50871099 and 50631020, the China
Postdoctoral Science Foundation under Grant No. 20080440196
and 200902622, and the University Doctoral Foundation of the
Ministry of Education under Grant No. 20090101110050.
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Fig. 5 is the TPD curve of as-synthesized 0.04CeAl4-doped
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200 ◦C. In the temperature ranges of 70–140 ◦C and 140–200 ◦C,
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As a summary, the CeAl4-doped Ca(AlH4)2 was successfully syn-
thesized by ball-milling NaH/Al and CaCl2 instead of NaAlH4 and
CaCl2. Introducing a few moles of CeAl4 catalyst can accelerate the
synthetic process of Ca(AlH4)2 and improve its dehydrogenation
performance. The activation energies of Ca(AlH4)2 and CaAlH5 are
reduced after doped with CeAl4 catalyst, resulting in the enhance-
ment of dehydriding kinetics of calcium alanate hydride. The
dehydrogenation of CeAl4-doped Ca(AlH4)2 starts at 77 ◦C, and des-
orb more than 5.5 wt.% H2 below 200 ◦C.