Dehydrogenation kinetics of as-received and ball-milled LiAlH4

Article abstract of DOI:10.1016/j.jssc.2005.09.027

In this paper, we investigate the dehydrogenation kinetics of LiAlH4 into Li3AlH6 (reaction I) and further into LiH (reaction II). We find the apparent activation energies to be ～80 and 100 kJ/mol for reactions I and II, respectively. Furthermore, we inve

Full text of DOI:10.1016/j.jssc.2005.09.027

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Journal of Solid State Chemistry 178 (2005) 3672–3678
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Dehydrogenation kinetics of as-received and ball-milled LiAlH4
Ã
A. Andreasena,b, ,1, T. Veggea, A.S. Pedersena
aMaterials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark
bInterdisciplinary Research Center for Catalysis, Department of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark
Received 21 June 2005; received in revised form 14 September 2005; accepted 14 September 2005
Available online 19 October 2005
Abstract
In this paper, we investigate the dehydrogenation kinetics of LiAlH4 into Li3AlH6 (reaction I) and further into LiH (reaction II). We
find the apparent activation energies to be ꢀ80 and 100 kJ/mol for reactions I and II, respectively. Furthermore, we investigate the effect
of ball milling on crystallite size and the dehydrogenation kinetics of both reactions I and II. We find a clear correlation between
crystallite size and dehydrogenation kinetics of reaction I. On the other hand, we find the kinetics of reaction II to be independent of the
crystallite size. This indicates that reaction I is limited by a mass transfer process, while reaction II is limited by the intrinsic kinetics.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Hydrogen storage materials; Ball milling; X-ray diffraction; Lithium tetrahydroaluminate; Trilithium hexahydroaluminate; Gravimetry;
Dehydrogenation kinetics
1. Introduction
melt [4]). Since then, NaAlH4 has received massive
attention focusing on improved doping procedures, screen-
ing of catalytic additives, maximizing the reversible
capacity, and on obtaining a comprehension of the
catalytic effect of Ti (see Refs. [5–10] and references
therein).
The energy infrastructure is facing serious challenges in
the future, due to limited supply of oil, increasing CO2
emissions, and an expansive energy demand in the growing
Asian economies.
Hydrogen is a potential major alternative energy carrier,
although a smooth transition to a hydrogen-based society
requires a solution to several technical problems; especially
the need for a proper hydrogen storage medium for the
transport sector is a great challenge [1,2].
Until recently, complex hydrides such as NaAlH4 and
LiAlH4, with a theoretical capacity of 7.3 and 10.6 wt%
H2, respectively, have not been considered as potential
solid state hydrogen storage media due to their irreversible
dehydrogenation and slow kinetics. This perception did not
change until the 1997 discovery of Ti-catalysed reversible
solid state hydrogen storage in NaAlH4 by Bogdanovic
and Schwickardi [3] (although reversibility was already
demonstrated in 1974 by Dymova et al. for reactions in the
As a hydrogen storage medium LiAlH4 has not received
the same attention as NaAlH4, although several studies,
both theoretical and experimental, have revealed important
details about its thermal decomposition behaviour, ther-
modynamic stability, crystal structure, and the effect of
ball milling and catalytic doping on its decomposition
behaviour (see Refs. [11–31]).
Hydrogen is generally expected to desorb from LiAlH4
in a three-step decomposition, I–III [14–16], although some
controversy about the details of the mechanism exists
[17,18].
1
2
LiAlH4 ! 3Li3AlH6 þ 3Al þ H2,
(1)
(2)
(3)
13Li3AlH6 ! LiH þ 3Al þ 2H2,
1
1
Ã
Corresponding author. Materials Research Department, Risø Na-
tional Laboratory, DK-4000 Roskilde, Denmark. Fax: +45 4677 5758.
E-mail address: andr1976@gmail.com (A. Andreasen).
1
LiH ! Li þ 2H2.
1Present address: MAN B&W Diesel A/S, Basic Research and
Emission, Teglholmsgade 41, DK-2450 Copenhagen SV, Denmark.
Reactions I, II, and III proceed with a theoretical hydrogen
release of 5.3, 2.6, and 2.6 wt%, respectively. However, due
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.09.027

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A. Andreasen et al. / Journal of Solid State Chemistry 178 (2005) 3672–3678
3673
to the thermodynamic stability of LiH, i.e. high decom-
position temperature, only 7.9 wt% H2 is considered
accessible for practical applications.
coated aluminium lid sealed with a rubber O-ring. Three
WC balls with a diameter of ꢀ20 mm and a weight of ꢀ60 g
each were used for all ball-milling experiments. Typically
4–5 g of sample was ball milled giving a ball-to-sample
mass ratio of 36–45:1.
The kinetic parameters of reactions I and II have only
been evaluated to a limited extent [13,31]. These investiga-
tions suffer either from unclear separation of parameters
from reactions I and II [13] or limitations to reaction I only
[31]. This serves as a main motivation for studying the
dehydrogenation kinetics of both steps I and II for pure
LiAlH4. Furthermore, no systematic investigations of the
influence of ball milling, application of different milling
times, on kinetics have been reported. These investigations
are essential in order to obtain a reliable reference for
evaluation of the kinetic effects of doping with catalytic
additives in combination with ball milling.
In this paper, we investigate the kinetics of direct
dehydrogenation of solid LiAlH4 by carrying out isother-
mal experiments below the melting point of LiAlH4. In
order to extract parameters for both reactions I and II a
kinetic model, which takes both reactions into account is
formulated, and kinetic parameters are found by fitting to
experimental data. Finally, we investigate the effect of
altering ball-milling times on the dehydrogenation kinetics
of both reactions I and II. Using X-ray powder diffraction
it is possible to obtain corresponding crystallite sizes from
line broadening and relate this to the observed kinetic
effect.
Isothermal dehydrogenation of LiAlH4 was studied
using a Sartorius 4406 high-pressure balancing unit
described in detail elsewhere [32]. In the glove box,
typically 60–80 mg of sample was loaded in a stainless-
steel crucible with a weight of ꢀ800 mg. The crucible was
placed in a sealed plastic bottle in order to protect it against
air exposure during transportation from the glove box to
the instrument. However, a short period of air exposure
(o2 min.) could not be avoided when the sample was
transferred from the plastic bottle. An empty stainless-steel
crucible was used as a reference. Before the experiment was
initiated, the system was evacuated to 10ꢂ1 mbar and
refilled with Helium N47 purity from Air Liquide to 1 bar.
This was repeated three times in order to keep the level of
impurities (moisture and oxygen) at a tolerable level during
the experiment. The sample was heated to the desired
isothermal operating temperature at a heating rate of
10 ꢁC= min. The gravimetric hydrogen release from the
sample was determined on-line.
3. Results and discussion
2. Experimental
3.1. Dehydrogenation kinetics of as-received LiAlH4
Lithium aluminium hydride, LiAlH4 (purity 95% min.,
typically 97%), was obtained in powder form from Alfa
Aesar (Johnson Matthey). All materials handling was
carried in an argon filled glove box in the presence of a
drying agent.
In order to investigate the direct solid state decomposi-
tion of pure LiAlH4 [15,17], isothermal experiments at
different temperatures have been carried out well below the
melting point of LiAlH4. The results are shown in
Fig. 1(A). As seen in the figure, increasing the temperature
increases the rate of hydrogen release as would be expected
for a thermally activated process.
X-ray powder diffraction (XRPD) was performed with a
Bragg-Brantano STOE diffractometer (50 kV, 300 mA,
¯
˚
CuKa12 radiation with l ¼ 1:5418 A). Powdered samples
were pressed into 13 mm diameter pellets with a height of a
few millimetres using a pressing tool placed in the glove
box. While still in the glove box, a pellet was placed in a
specially designed air-tight sample holder with an alumi-
nium foil X-ray window. All observed peaks in the XRPD
pattern of the as-received LiAlH4 except for one could be
indexed as belonging to a monoclinic unit cell (P21=c) with
Except for the lowest applied temperature, it is evident
from the kink in the dehydrogenation curves at a hydrogen
release of ꢀ4:5 wt% that the decomposition of LiAlH4 into
LiH, Al and H2 is indeed a two-step mechanism. The
position of this transition is somewhat lower than expected
from the theoretical hydrogen release. The maximally
observed hydrogen release of ꢀ6:7 wt% is also slightly
lower than the theoretical limit even when correcting the
observed hydrogen release with the purity of the sample.
XRPD of the dehydrogenated samples (see list of reflec-
tions with assigned phases in Table 1) reveal that some
Li3AlH6 (and possible also LiAlH4) is left after dehydro-
genation in agreement with observations of Andrei et al.
[33]. Thus incomplete dehydrogenation of Li3AlH6 offers,
at least partially, an explanation for this difference.
˚
˚
˚
unit cell parameters a ¼ 4:83 A, b ¼ 7:83 A, c ¼ 7:92 A,
and b ¼ 112:3ꢁ in agreement with previous observations of
the crystal structure of LiAlH4=LiAlD4 [28,29]. The
unexplained peak around 2y ¼ 35ꢁ corresponds well with
the strongest reflection of LiCl, suggesting that LiCl is
present as an impurity as also suggested by Hauback et al.
[16,29]. LiCl might originate from the preparation proce-
dure [25]. We find no other impurities, e.g. hydroxides [16]
or oxides [30], suggesting that these are either absent or in
an X-ray amorphous state.
Modelling the kinetics of hydrogenation/dehydrogena-
tion is essential in order to gain a detailed comprehension
of the underlying physics and to identify rate-limiting
step(s) of the overall process. This type of kinetic modelling
has recently been performed in the case of NaAlH4 [35–37].
Ball milling was utilized with a Retsch PM 100 planetary
ball mill using a Wolfram Carbide (WC) vial with a WC-

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A. Andreasen et al. / Journal of Solid State Chemistry 178 (2005) 3672–3678
3674
provides a good fit to the experimental data
Z1
WtotðtÞ ¼ W1ð1 ꢂ expðꢂðk1tÞ ÞÞ
Z2
þ W2ð1 ꢂ expðꢂðk2tÞ ÞÞ,
ð4Þ
where Wtot is the total hydrogen release at time t, W1 and
W2 are the release of hydrogen from reactions I and II,
respectively; k1 and k2, and Z1 and Z2 are exponents for
reactions I and II, respectively. Except for the included
exponents in Eq. (4) the model is similar to the one applied
by Kiyobayashi et al. in order to model dehydrogenation of
NaAlH4 [35]. Since only dehydrogenation is considered—
and the fact that dehydrogenation is performed at constant
pressure—pressure-dependent terms, as proposed by Luo
and Gross [37] are ignored.
The kinetic expression is fitted to the isothermal
experimental data using a Levenberg–Marquandt least-
squares algorithm. A sample fit of Eq. (4) to the isothermal
dehydrogenation curve at 132 ꢁC is shown in Fig. 1(B). As
seen in the figure, the model provides an excellent
description of the dehydrogenation kinetics.
All extracted fitting parameters, and apparent activation
energies and prefactors as determined from an Arrhenius
analysis are summarized in Table 2, including w2 goodness-
of-fit values.1
Early studies on the kinetics of the isothermal decom-
position of LiAlH4 by McCarthy et al. [13] revealed an
apparent activation energy of reaction I to be ꢀ100 kJ/mol,
although it is unclear to which degree the fitted data had
been influenced by reaction II. Recent studies by Blanchard
et al. [31] shows a value of 102 kJ/mol for reaction I. The
value of the apparent activation energy determined in this
study is slightly lower (82 kJ/mol), but nevertheless in good
agreement with the previous studies. McCarthy et al. [13]
also reported an apparent activation energy of reaction II
from constant heating rate experiments of ꢀ100 kJ/mol
which is also in good agreement with the reported value of
90 kJ/mol in this work. While the work of Blanchard et al.
[31] and McCarthy et al. [13] have been limited to treating
one reaction at a time, the formulation of a two-step
kinetics model has allowed simultaneous extraction of
kinetic parameters of both reactions I and II from a single
experiment.
Fig. 1. (A) Isothermal dehydrogenation of as-received LiAlH4 investi-
gated in a high-pressure balance. (B) Fit of Eq. (4) to isothermal
dehydrogenation data at T ¼ 132 ꢁC.
Table 1
Observed reflections in XRPD of a pellet pressed from the dehydrogena-
tion product of all four as-received samples
˚
d (A)
2y (deg)
I=Imax (%)
Phase
21.99
22.55
29.68
30.12
30.42
31.70
34.92
38.54
39.97
44.79
4.0393
3.9492
3.0071
2.9646
2.9358
2.8202
2.5673
2.3344
2.2539
2.0219
1.46
1.53
0.96
1.74
1.56
1.18
1.47
100
Li3AlH6
Li3AlH6
LiAlH4
LiAlH4
LiAlH4
Li3AlH6
Li3AlH6
Al/LiH
Li3AlH6
Al/LiH
3.2. Kinetic effect of ball milling
In order to investigate the effect of ball-milling LiAlH4
on the dehydrogenation kinetics, the as-received powder
has been ball milled for different periods of time and
milling intensities. These samples have been subject to both
0.74
43.93
Phases of Li3AlH6 and LiAlH4 have been assigned according to Refs.
[28,34].
1w2 goodness-of-fit calculated for each temperature by
ꢀ
ꢁ
N
2
X
1
ðWobsðiÞ ꢂ WmodelðiÞÞ
WmodelðiÞ
w2 ¼
,
In order to model the isothermal dehydrogenation
curves in Fig. 1(A), a model with the ability to account
for both decomposition reactions should be chosen. A
standard two-step kinetic expression of the following form
N
i¼1
where N is the total number of observations during an isothermal
experiment, Wobs is the experimentally observed hydrogen release, and
Wmodel is the calculated hydrogen release using Eq. (4).

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3675
Table 2
Kinetic parameters obtained from fitting isothermal measurements of the direct decomposition of as-received LiAlH4
T ( ꢁC)
W1 (wt% H2)
W2 (wt% H2)
Z1 (dimensionless)
Z2 (dimensionless)
w2
k1 (hꢂ1
)
k2 (hꢂ1
)
115
132
140
152
4.28
4.11
4.25
4.02
1.56
2.44
2.44
2.32
0.229
0.624
1.191
1.996
0.040
0.123
0.235
0.442
1.53
1.80
1.99
2.22
1.95
1.51
1.71
1.74
0.0133
0.0030
0.0007
0.0302
2:3 ꢃ 1010
82 ꢄ 4
4:9 ꢃ 1010
90 ꢄ 4
A (hꢂ1
EA (kJ/mol)
)
Fig. 2. XRPD of as-received LiAlH4, LiAlH4 ball milled for 1 h at
150 rpm, and LiAlH4 ball milled for 1 h at 400 rpm.
Fig. 3. Isothermal dehydrogenation curves for ball-milled samples. The
isothermal temperature is ꢀ130 ꢁC. Dehydrogenation curve for un-milled
as-received LiAlH4 at 132 ꢁC is included for comparison.
XRPD analysis and isothermal measurements of the
dehydrogenation process.
unlikely. On the other hand, we noticed that the vial
heated up slightly during milling, most pronounced for the
longer milling times. Thus, the potential decomposition of
LiAlH4 may be thermally mediated.
XRPD patterns of all the ball milled samples are shown
in Fig. 2. Generally, the reflections become broader and
lose intensity as a function of milling time/intensity
suggesting that the average coherence length (crystallite
size) is reduced upon milling. After 6 h of milling an
additional reflection (marked with an asterisk) appears
around 2y ¼ 31:7ꢁ. This feature could originate from either
the ð2 2 0Þ reflection of monoclinic [34] Li3AlH6 or the
ð2 0 ꢂ 1Þ reflection of rhombohedral Li3AlH6 [20], suggest-
ing a partial decomposition of LiAlH4. In fact, after 2 h of
milling, the sample changed colour from white/greyish to
light grey indicating that a solid state reaction had taken
place. On the other hand, apparently no crystalline metallic
aluminium is formed suggesting that if any formation of
Li3AlH6 has taken place the resulting aluminium is in an
X-ray amorphous phase. Partial decomposition of LiAlH4
into Li3AlH6 can be explained in terms of a higher
thermodynamic stability of Li3AlH6 compared to LiAlH4
[11,15,19–22]. Although the ball-milling vial used in this
work was carefully cleaned, the presence of small amounts
of impurities that may act as a catalyst [26] cannot be
completely ruled out. However, we consider this as
The effect of ball milling on the dehydrogenation
kinetics is visualized by isothermal dehydrogenation curves
for all ball-milled samples in Fig. 3. It was intended to
apply the same isothermal temperature to all dehydrogena-
tion experiments for the ball-milled samples. Though, in
practice, an inter-sample temperature variance of a few
degrees around 130 ꢁC was observed, which is expected to
be within experimental uncertainty. The dehydrogenation
curve for as-received LiAlH4 at 132 ꢁC is included in Fig. 3
as a reference.
From the dehydrogenation curves in Fig. 3 it is observed
that ball milling effectively leads to faster kinetics for
reaction I (first and steepest part of the curves up to
3–4.5 wt%), while reaction II seems to be more insensitive
to the ball-milling. Applying longer ball milling times, the
smooth transition between reactions I and II as observed in
the un-milled sample become more abrupt. The initial rate
of dehydrogenation for the sample ball milled for 10 h
seems to deviate somewhat from the others. This is due to
the fact that dehydrogenation initiated during the heating

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Table 3
Kinetic parameters obtained from fitting isothermal measurements of the direct decomposition of ball-milled LiAlH4
w2
k1 (hꢂ1
)
k2 (hꢂ1
)
Time (h)
Intensity (rpm)
W1 (wt% H2)
W2 (wt% H2)
Z1 (dimensionless)
Z2 (dimensionless)
1
1
150
400
400
400
400
3.85
4.12
3.57
3.39
2.81
2.17
2.07
3.26
2.04
1.97
0.751
1.567
1.305
3.272
3.817
0.180
0.168
0.190
0.216
0.163
2.48
2.78
3.66
2.95
2.26
1.18
1.41
0.80
1.60
1.63
0.0002
0.0047
0.0020
0.0095
0.0113
2
6
10
period. It is also observed from Fig. 3 that longer ball-
milling times results in reduced hydrogen release mainly
from LiAlH4.
In order to quantify the effect of ball milling on the
dehydrogenation kinetics of LiAlH4, kinetic parameters are
extracted from the dehydrogenation curves by fitting Eq.
(4) to the experimental dehydrogenation curves of the ball-
milled samples; the results are summarized in Table 3. In
order to fit the dehydrogenation curve of the sample ball
milled for 10 h it was necessary to exclude the first part of
the curve corresponding to the non-isothermal hydrogen
release from the fitting procedure. As seen from the fitted
parameters in Table 3 the faster kinetics of reaction I seen
in Fig. 3 is confirmed quantitatively. It is interesting to see
that ball milling has virtually no effect on the rate constant
of reaction II.
Fig. 4. Rate constants for reaction I as a function of crystallite size
obtained from XRPD line broadening.
From Table 3 it is clearly observed that the parameter
W1 decreases as a function of milling time, especially for
the longest periods of milling. This suggests some partial
decomposition of LiAlH4 during milling as discussed
previously, and also in agreement with Fig. 3. This requires
a careful application of the kinetic model. However, we
find no reasons to believe that the fitting results are biased
by the initial presence of Li3AlH6 in the ball-milled
samples. First, the decomposition of Li3AlH6 is slow
compared to the decomposition of LiAlH4 as visualized by
both Figs. 1(A) and 3. This is even more pronounced for
the samples ball milled for the longest times. According to
Fig. 3, the dehydrogenation of LiAlH4 becomes so fast for
the longest periods of milling, that an induction period in
the decomposition of Li3AlH6 is revealed. Thus for the
samples containing Li3AlH6 from the beginning, effectively
no decomposition of Li3AlH6 takes place before all LiAlH4
is completely transformed into Li3AlH6. Second, the fitting
parameter W2 shown in Table 3 is more or less constant
even despite the initial presence of Li3AlH6. This is a
natural consequence of the first point and it gives
confidence in the fact that the two dehydrogenation steps
described by the model are treated as separate reactions
without any intermixing of kinetic parameters, i.e. the
kinetic parameters of step I are independent of the kinetic
parameters of step II and vice versa.
crystallite sizes of the ball-milled samples before dehydro-
genation including the as-received sample by applying the
Scherrer equation
l
b ¼
,
(5)
B cos y
where b is the crystallite size, l is the X-ray wavelength and
B is the FWHM (corrected for instrumental broadening,
which is assumed to be 0:10ꢁ). In order to get a reliable
picture of the average crystallite size instead of an average
coherence length in certain crystallographic directions, the
average line broadening of several reflections up to 35ꢁ in
2y have been used.
Fig. 4 shows the rate constants as a function of crystallite
size. Fitting the data to a power-law yields an excellent fit
(dashed line) with an 1=b2:3 dependence. The strong
dependence of k1 on crystallite size may suggest a mass
transfer process to be limiting the kinetics, e.g. long-range
atomic diffusion of Al as proposed by Kiyobayashi et al.
[35] and Sandrock et al. [7]. The exact order of the
exponent can be used to obtain detailed physical informa-
tion about the underlying rate-limiting process(es).
In the Nabarro–Herring model [38,39], a 1=b2 relation
corresponds to a process controlled by lattice (intergra-
nular) diffusion as observed for e.g. hydrogen diffusion in
Co90Zr10 [40], whereas a process controlled by grain
boundary diffusion displays a 1=b3 relation according to
Coble [41]; as observed for e.g. hydrogen enhanced
3.3. Diffusion limited kinetics?
In order to investigate a possible correlation between
crystallite size and rate constants we have extracted

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A. Andreasen et al. / Journal of Solid State Chemistry 178 (2005) 3672–3678
3677
diffusional creep in Pd [42]. Considering Fig. 4 displays a
1=bꢀ2 relation, reaction I is likely limited by a lattice
diffusional process, although grain boundary diffusion
cannot be excluded.
dehydrogenation of LiAlH4 to Li3AlH6 (reaction I) and the
subsequent dehydrogenation of Li3AlH6 to LiH (reaction
II), respectively.
Investigations of ball-milling LiAlH4 for different
periods of time and subsequent kinetic investigations at
isothermal conditions at ꢀ130 ꢁC clearly show improved
kinetics of the dehydrogenation reaction I as a function of
milling time, whereas reaction II seems more or less
insensitive to the ball-milling process. Detailed kinetic
fitting allowing extraction of rate constants for both
reactions I and II shows a clear correlation between the
rate constant of reaction I and crystallite size determined
from XRPD line broadening with higher rate constant
corresponding to smaller crystallite sizes (longer ball-
milling times). In contrast, the rate constant of reaction
II is more or less unchanged despite the drastic reduction in
crystallite size of LiAlH4 going from 140 to 46 nm. This
suggests reaction I is limited by mass transfer, whereas
reaction II is limited by the intrinsic kinetics. Thus a
suitable catalyst is needed in order to improve the kinetics,
not only for the reversibility of reactions I and II, but
certainly also the kinetics of reaction II at temperatures
suitable for practical applications.
The fact that k2 is more or less insensitive to the large
variations in crystallite size indicates that reaction II, in
contrast to reaction I, is not limited by mass transfer.
Instead, the intrinsic kinetics seems to be limiting at the
relatively low temperature applied. Previous DTA results
[26] show that the kinetics of reaction II at elevated
temperatures is indeed improved by ball milling as seen by
a lowering of the decomposition temperature from ꢀ250 to
225 ꢁC after ball milling for only 10 min. This suggests that
at the elevated temperatures in the DTA experiment, the
intrinsic kinetics is improved sufficiently in order not to be
rate limiting for the overall kinetics. Instead, the crystallite
size seems to become important. However, for practical
applications the temperature cannot be increased above
200 ꢁC in order to obtain improved kinetics. Thus, ball
milling alone is not sufficient to improve the kinetics of
reaction II. This example very clearly illustrates the need
for a suitable catalytic additive, not only to the make
reaction I+II reversible [21] but also to improve the
kinetics—especially of reaction II.
Acknowledgements
3.4. Influence on oxygen contamination on kinetics
This work has received financial support from The
Danish Technical Research Council through the Center of
Excellence Towards a hydrogen-based society.
Although no oxygen contamination is observed in any of
the XRPD results shown in this study, it seems unques-
tionable that the samples contain a certain amount of
oxides. This is supported by the studies of Andrei et al. [30]
in which aluminium oxide was detected by EELS, even
though the samples have been carefully transferred from
the glove box to the instrument in a special vacuum
transfer device including a removable glove bag mounted
on the instrument.
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