investigation on kinetics mechanism of hydrogen absorption in the la2mg17-based composites
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 1 9 5 1 – 1 9 5 7
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Investigation on kinetics mechanism of hydrogenabsorption in the La2Mg17-based composites
Jing Liua, Xu Zhanga, Qian Lia,*, Kuo-Chih Choua,b, Kuang-Di Xua
aShanghai Key Laboratory of Modern Metallurgy and Materials Processing, Shanghai University, Shanghai 200072, ChinabDepartment of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, PR China
a r t i c l e i n f o
Article history:
Received 19 November 2008
Received in revised form
13 December 2008
Accepted 13 December 2008
Available online 22 January 2009
Keywords:
Hydrogen storage materials
La2Mg17
Hydrogen absorption
Kinetics mechanism
Model
* Corresponding author. Tel.: þ86 21 5633404E-mail address: [email protected] (Q
0360-3199/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.12.040
a b s t r a c t
A new model has been successfully used to investigate the hydrogen absorption kinetics
mechanism of La2Mg17-based composites. The results indicate that different preparation
conditions lead to different rate-controlling steps during hydrogen absorption process. For
La2Mg17–LaNi5 composite synthesized by the method of melting, the rate-controlling step is
the surface penetration of hydrogen atoms, which does not change by addition agent
(LaNi5). However, mechanical milling can change the rate-limiting steps of hydriding
reaction in the La2Mg17–LaNi5 composite from surface penetration to diffusion of hydrogen
in the hydride layer. With the enhancement of milling intensity, the rate-controlling step
in La1.8Ca0.2Mg14Ni3 alloy changes from surface penetration to diffusion. In addition,
the activation energies of hydrogen absorption for La2Mg17�20 wt%LaNi5 and
La1.8Ca0.2Mg14Ni3 are obtained by this model.
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
reserved.
1. Introduction additions to improve the absorption properties of La2Mg17
The magnesium-rich alloys Mg–RE (RE¼ La, Ce or mischmetal)
hold a special position among hydride-forming metals and
alloys. They are characterized by the high gravimetric storage
capacity of the magnesium and the mild condition of hydride
formation owing to the catalytic effect of the rare earth
components [1,2]. However, one of their significant disad-
vantages is the very poor hydrogen absorption and desorption
kinetics, about 10 times slower than that of the well-known
hydrogen storage alloy LaNi5 [3]. An applicable hydrogen
storage material requires not only a large hydrogen capacity
but also a fast hydriding/dehydriding reaction rate. To accel-
erate the hydriding/dehydriding process, the elements in the
La2Mg17 were substituted by a small amount of calcium for
lanthanum and a 3d element for magnesium, respectively [4].
Moreover, Dutta et al. [5] selected LaNi5 and FeTi(Mn) as
5; fax: þ86 21 56338065.. Li).ational Association for H
intermetallics, since they easily become activated and possess
good kinetics at ambient condition.
In fact, it is significant to elucidate the hydriding/dehy-
driding process mechanism for improving the kinetics prop-
erties with good pertinence. Khrussanova et al. [6] used the
kinetics equations describing hydrogen chemisorption on the
alloy surface, three-dimensional hydrogen diffusion through
the product layer and chemical reaction between hydrogen
and the alloy to obtain the hydriding mechanism of La2Mg17,
La1.8Ca0.2Mg17 and La1.6Ca0.4Mg17. Their results indicated that
at low values of the reacted fraction (0< x< 0.4), hydrogen
chemisorption on the alloy surface is the rate-controlling step
and at higher x values, the hydriding process is controlled by
hydrogen diffusion through the hydride layer. But, to date
most measurements of hydrogen absorption and desorption
of La2Mg17-based composites are simply presented as plots of
ydrogen Energy. Published by Elsevier Ltd. All rights reserved.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 1 9 5 1 – 1 9 5 71952
reacted fraction or percentage of reacted amount versus time
[7–9]. It is well known that an analytical expression is benefi-
cial to discussion and analysis on the kinetics mechanism.
However, the research in experiments of the hydriding/
dehydriding mechanisms of La2Mg17-based composites has
lagged behind its theory level.
The present paper firstly summarizes the experimental
kinetics data of La2Mg17–LaNi5 and La1.8Ca0.2Mg14Ni3 from the
literature [10–12]. A new kinetics model [13,14] is then applied
to study their hydrogen absorption kinetics mechanisms. The
influences of the additions, the preparation methods, and the
element substitution on the hydriding reaction kinetics
mechanisms of La2Mg17-based composites are systematically
investigated.
2. Kinetics model
The kinetics of many solid–gas reactions can be represented
by the general equation f(x)¼ kt, where x is the fraction reacted
in time t and the function f(x) is closely related to the reaction
mechanism. The equations for solid–gas reactions have been
known for decades. Most solid state reactions involving the
precipitation of a new phase can be characterized by an
equation derived by Avrami [15] and Johnson and Mehl [16].
In its simplest form, the equation can be written as
ln[�ln(1� x)]¼ ln Bþmln t [17,18], where B is a constant which
depends on the rates of nucleation and growth and m is
a constant that can vary according to the geometry of the
system. Hancock and Sharp [17] had tabulated the values of m
corresponding to 9 different reaction models, as shown in Table
1, which can be separated into three different groups: diffusion
controlled, interface controlled and nucleation and growth
models. Theoretically, the rate-controlling step can be deter-
mined by comparing their empirical results with the value of m.
However, it is not precise enough to determine which intra
group model is appropriate. Moreover, the parameters B and m
have no obvious physical significance. In addition, numerous
published articles have also reported the useful approximations
of the kinetics functions [19–27] which are also summarized in
Table 1 – Set of reaction models to describe the reaction kineti
Mechanism
One-dimensional diffusion
Two-dimensional diffusion (bidimensional particle shape)
Three-dimensional diffusion (Jander equation)
Three-dimensional diffusion (Gisling–Braunshtein equation)
First order kinetics
Two-dimensional phase-boundary reaction
Three-dimensional phase-boundary reaction
Zero order
Random nucleation and growth of nuclei (Avrami–Erofeev equation)
Random nucleation and growth of nuclei (Avrami–Erofeev equation)
Phase-boundary controlled reaction (n¼ 0, 1/2, and 2/3)
Three-dimensional diffusion (Kroger–Ziegler equation)
Three-dimensional diffusion (modified Jander relation)
Three-dimensional diffusion (Valensi–Carter model)a
a Z represents the volume of the reaction product formed per unit volum
Table 1. Some of them have the same forms as in the literature
[17]. Although many kinetics functions are available in litera-
ture, as far as most of kinetics equations are concerned, if one
treats each step rigorously, that will lead to solve a group of
differential or integral equations. The situation now is that
some treatments are so complicated that they cannot offer an
explicit analytic expression and give an intuitionistic quantita-
tive discussion. The other situation is that some equations are
often difficult to be applied owing to the lack of parameter
values [21]. From the practical point of view, we introduced
a new kinetics model (Chou model) proposed in our lab with
much calculation accuracy to analyze the hydrogen absorption
kinetics mechanism for the La2Mg17-based composites. It is
necessary to point out that Chou model has a series of simpler
and physical meaningful explicit analytic expressions.
Chou model assumes that all particles of the metal or alloy
could be regarded as spherical balls with the same density and
radius. The whole particle is a ball of radius R0. A general
process of hydriding reaction for a particle can be described as
the following seven steps: (1) Hydrogen transfer from the bulk
gas phase to the surface of metal particle; (2) Hydrogen
diffusion through the boundary layer between gas phase and
solid particle; (3) Physisorption of hydrogen molecules on the
solid surface; (4) Dissociation of hydrogen molecules and
chemisorption; (5) Surface penetration of hydrogen atoms; (6)
Diffusion of hydrogen atoms through the hydride product
layer to the hydride/metal interface; (7) Chemical reaction and
nucleus formation producing hydride.
In the most cases, the first two steps are very fast and the
steps (5) and (6) could be the rate-controlling steps. Some-
times, the process may have different controlling steps at
different stages. When the rate-controlling step is the surface
penetration, the relation of the hydriding reacted fraction
x and time t can be expressed as [13]
x ¼ 1�
0@1�
kfspK
12paK
12ca
�P
12H2� P
eq12
H2
�nmR0
t
1A
3
(1)
where ym represents the constant coefficient that depends
on the density of the storage material, kspf represents the
cs from the literatures.
Equation m Ref.
x2¼ kt 0.62 [17,19]
(1� x) ln(1� x)þ x¼ kt 0.57 [17,19]
[1� (1� x)1/3]2¼ kt 0.54 [17,19,20]
1� 2x/3� (1� x)2/3¼ kt 0.57 [17,21,22]
�ln(1� x)¼ kt 1.00 [17,19]
1� (1� x)1/2¼ kt 1.11 [17,19]
1� (1� x)1/3¼ kt 1.07 [17,19]
x¼ kt 1.24 [17]
[�ln(1� x)]1/2¼ kt 2.00 [15,17]
[�ln(1� x)]1/3¼ kt 3.00 [15,17]
[1� (1� x)1�n]/(1� n)¼ kt [23]
[1� (1� x)1/3]2¼ kln t [21,24]
[(1/(1� x))1/3� 1]2¼ kt [21,25]
{Z� (Z� 1)(1� x)2/3� [1þ(Z� 1)x])2/3}/(Z� 1)¼ kt [21,26,27]
e of reactant consumed.
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Fig. 1 – Kinetics data of hydrogen absorption at 623 K and
4.0 MPa H2 for La2Mg17Lx wt%LaNi5 (x [ 5, 20 and 40) [10]
together with the fitted curves.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 1 9 5 1 – 1 9 5 7 1953
equilibrium constant for the surface penetration of hydrogen
atoms in the forward direction, Kpa and Kca are equilibrium
constants for the physisorption and chemisorption, respec-
tively, and PH2and Peq
H2represent the hydrogen partial pres-
sures in the gas phase and in equilibrium with hydride.
Define tc(sp), ‘the characteristic reaction time’ of the
hydriding reaction controlled by surface penetration of
hydrogen atoms, as
tcðspÞ ¼nmR0
kfspK
12paK
12ca
�P
12H2� P
eq12
H2
� (2)
Substituting Eq. (2) into Eq. (1), we have
x ¼ 1��
1� ttcðspÞ
�3
(3)
When the rate-controlling step is the diffusion of hydrogen
in hydride, the relation of the hydriding reacted fraction x and
time t is given by [13]
x ¼ 1� 1�
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2Db
HðCHðbÞ � CeqH ða=bÞÞ
R20nm
t
s 31A0@ (4)
where DHb represents the diffusion coefficient of hydrogen in
b phase, CH(b) is the concentration of hydrogen atoms in the
hydride phase just underneath the particle surface at radius
R0 and CHeq(a/b) represents the equilibrium composition of
hydrogen in hydride as hydrogenation reaction reaches
equilibrium.
Define
tcðdÞ ¼1
2Db
HðCHðbÞ�CeqHða=bÞÞ
R20
nm
(5)
as ‘the characteristic reaction time’ of the hydriding reaction
controlled by diffusion of hydrogen in hydride. Substituting
Eq. (5) into Eq. (4), we have
x ¼ 1�
1�ffiffiffiffiffiffiffiffi
ttcðdÞ
s !3
(6)
In addition, for the surface penetration, the relation of the
hydriding reacted fraction x and time t can also be expressed
as
x ¼ 1��1� exp
��Ev
RTv
�1Bt
t
�3
(7)
When the rate-controlling step is the diffusion of hydrogen
atoms in the solid solution, the relation of the hydriding
reacted fraction x and time t is
x ¼ 1� 1� exp
��Ev
2RTv
� ffiffiffiffiffiffiffi1Bt
t
s 3375
264 (8)
where Ev is activation energy (J/mol), t is reacted time (s), TV
is thermodynamic temperature (K), x is reacted fraction, R
is gas constant, Bt is a coefficient. If the particle radius and
temperature are fixed and the relation between tc and Bt is
tc ¼ Bt$exp
�Ev
RTv
�(9)
As mentioned above, Chou model [13] involves a series of
analytic equations for various kinds of rate-controlling steps.
These equations describe the reacted fraction (x) of hydriding/
dehydriding reaction as an explicit function of time (t),
temperature (T ), hydrogen partial pressure (PH2) and particle
radius (R0). Among them, a very important and useful
parameter is the new concept of ‘‘characteristic reaction time
tc’’, the physical meaning of which is the required time for the
particle to be completely hydrogenated. The smaller the tc, the
faster the hydriding reaction rate. It indicates the hydriding/
dehydriding kinetics performance of hydrogen storage mate-
rials is quantified with accuracy using the characteristic
reaction time directly. Since this method gives the possible
formulae for calculating the reacted fraction x at the different
reaction times corresponding to different kinds of controlling
steps, the rate-controlling step for any hydriding reaction can
be determined by comparing the experimental results with
the theoretical modeling results. If we know the value of tc, the
kinetics of the hydriding/dehydriding reaction can be
described conveniently. However, in most cases, tc is not easy
to be measured accurately. In this study, we use linear
regression method to obtain tc. Moreover, the activation
energy Ev in the two rate-controlling steps can be attained by
non-linear fitting. The detail derivation of Chou model was
given in the Ref. [13].
3. Results and discussion
3.1. The effect of the amount of addition agent onhydriding kinetics of La2Mg17
Pal [10] had investigated the hydriding property of La2Mg17�x
wt% LaNi5 composite, which was synthesized by melting the
La2Mg17 and LaNi5 with the proper stoichiometric ratios in
a radiofrequency induction furnace. Fig. 1 shows the hydrogen
absorption kinetics of La2Mg17�x wt%LaNi5 (x¼ 5, 20 and 40) at
623 K and at a pressure of 4.0 MPa H2.
The relationship between x and t is modeled by fitting
the experimental data with the rate equations of each step
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Table 2 – The linear regression results of La2Mg17Lx wt%LaNi5 (x [ 5, 20 and 40) at 623 K.
rate-controlling step Parameters x¼ 5 x¼ 20 x¼ 40
Surface penetration R2 0.9983 0.9992 0.9967
tc(sp)/s 5137 4017 1493
Diffusion R2 0.7533 0.6872 0.7331
tc(d)/s 10,057 9703 3406
Fig. 3 – Kinetics data for hydrogen absorption of samples
a [11], b [10] and c [11] together with the fitted curves
(solid curves: surface penetration step; dotted curves:
diffusion step).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 1 9 5 1 – 1 9 5 71954
(Eqs. (3) and (6)) using the least squares and the results are
listed in Table 2. For the surface penetration step, the corre-
sponding R2 of the linear regression equations are 0.9983,
0.9992 and 0.9967 for x¼ 5, 20 and 40, respectively. On the
contrary, these experimental data cannot be fitted by the
model with the rate-controlling step of diffusion. From these
results, the surface penetration can be considered as the rate-
controlling step.
Based on Eq. (3) and Table 2, a series of fitted curves were
also drawn in Fig. 1 together with the experimental data. It can
be seen that the fitted results are in good agreement with the
experimental data. Table 2 shows that tc(sp)5> tc(sp)20> tc(sp)40,
where tc(sp)5, tc(sp)20 and tc(sp)40 are the characteristic reaction
time at 623 K for La2Mg17�x wt%LaNi5 (x¼ 5, 20 and 40),
respectively.
Fig. 2 shows the hydrogen absorption kinetics of
La2Mg17�20 wt%LaNi5 at 623 K, 648 K, and 673 K. The param-
eters Ev and Bt can be evaluated using Eq. (7) by non-linear
fitting method. The corresponding R2 is 0.9915, which indi-
cates that the experimental data can be fitted with good
accuracy by Chou model and the surface penetration is
the rate-controlling step for this system from 623 K to 673 K.
The calculated activation energy is 61.79� 0.67 kJ/mol H2.
By the same method, Chou model was also used to analyze the
hydriding kinetics of La2Mg17�x wt%LaNi5 (x¼ 5, 40) at 623 K,
648 K and 673 K, and the activation energy can be calculated as
64.83� 1.25 and 69.70� 0.95 kJ/mol H2, respectively.
Therefore, for the La2Mg17�x wt%LaNi5 materials synthe-
sized by melting, the hydrogen absorption rates are improved
by increasing the amount of addition agent (LaNi5) and rising
Fig. 2 – Kinetics data of hydrogen absorption at different
temperatures (623 K, 648 K and 673 K) and under 4.0 MPa H2
for La2Mg17L20 wt%LaNi5 [10] together with the fitted curves.
temperature and the reaction rate can be compared quanti-
tatively. The hydrogen absorption mechanism is surface
penetration of hydrogen atoms and cannot be changed by
addition agent or increased temperature.
3.2. The effect of mechanical milling on hydridingkinetics of La2Mg17–LaNi5
Gross et al. [11] had investigated the hydriding property of
La2Mg17–LaNi5 composite formed by mechanical milling. They
had measured the hydriding kinetics at 523 K for the samples
of pure La2Mg17 and La2Mg17�50 wt%LaNi5, which had been
milled for 25 min. Fig. 3 gives the plot of x versus t for the
different samples mentioned above. To compare the effect of
mechanical milling on hydriding kinetics, the kinetics data at
623 K of un-milled La2Mg17�40 wt%LaNi5 prepared by Pal [10]
were included in Fig. 3. The milled pure La2Mg17 was labeled as
‘a’, the un-milled La2Mg17�40 wt%LaNi5 was labeled as ‘b’ and
the milled La2Mg17�50 wt%LaNi5 was labeled as ‘c’. The rela-
tionship between x and t is modeled by fitting the experi-
mental data with the rate equations of each step (Eqs. (3) and
(6)) using the least squares and the results are listed in Table 3.
It can be seen from Table 3 that the R2 of sample a is 0.9920
for surface penetration model. For sample b, the quality of
fitting is also better for the surface penetration model than
for diffusion model. On the contrary, for sample c, the
Table 3 – The linear regression results of the samples a,b and c.
rate-controllingstep
Parameters Samplesa
Samplesb
Samplesc
Surface penetration R2 0.9920 0.9967 0.7916
tc(sp)/s 2306 2692 38.66
Diffusion R2 0.9674 0.7134 0.9873
tc(d)/s 23,750 5605 65.33
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Fig. 4 – Hydriding data at 473 K and under 4.0 MPa H2 for
La1.8Ca0.2Mg14Ni3 alloys milled for different times [12]
together with the fitted curves (solid curves: surface
penetration step; dotted curves: diffusion step).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 1 9 5 1 – 1 9 5 7 1955
corresponding R2 is 0.9873 for the diffusion model, which is
much higher than that for the surface penetration model.
Fig. 3 shows the hydrogen absorption curves of samples a,
b and c fitted by models. It can be seen that the surface
penetration model can represent the experimental curves of
the samples a and b and the diffusion model fits the experi-
mental data of sample c very well. From Section 3.1, we know
that the hydrogen absorption rate-limiting step is surface
penetration of hydrogen atoms and cannot be changed with
the increase of LaNi5. So it can be concluded that mechanical
milling combined with the catalyzer not only to improve the
hydriding kinetics (tc for samples a and b are much longer
than that for sample c), but also change the hydrogen
absorption rate-limiting step of La2Mg17-based material from
surface penetration to the diffusion of hydrogen atoms
through the hydride layer. The increase in the specific surface
area and the introduction of defects by the intensive milling
contribute to the improvement of kinetics [28].
3.3. The effect of combination of mechanical milling andelement substitution on kinetics of La2Mg17
Gao et al. [12] studied the hydriding behaviors of
La1.8Ca0.2Mg14Ni3 alloy modified by ball-milling under argon.
Fig. 4 shows the initial hydriding data at 473 K and 4.0 MPa H2
of the alloy milled for different periods of time. To better
Table 4 – The linear regression results of the La1.8Ca0.2Mg14Ni3
rate-controlling step Parameters Milled 0 h
Surface penetration R2 0.9909
tc(sp)/s 3374
Diffusion R2 0.7387
tc(d)/s 11,130
interpret the data, the term ‘‘percentage of reacted amount’’ is
used to describe the hydriding kinetics of the alloy. The rela-
tion between the ‘‘reacted fraction’’ x and the ‘‘percentage of
reacted amount’’ is
x ¼ DmDmmax
¼Dmm0
Dmmaxm0
(10)
where m is the reacted amount, m0 is the initial weight of the
sample, m/m0 is the percentage of reacted amount and mmax
represents the maximum reacted amount.
The relationship between x and t is modeled by fitting the
experimental data with the rate equations of each step (Eqs.
(3) and (6)) using least squares and the results are listed in
Table 4. It can be seen that the R2 of the linear regression
equation is strongly affected by milling time. For the un-
milled sample, the R2 is 0.9909 for surface penetration model
but only 0.7387 for diffusion model. On the contrary, for the
samples milled for 15 h and 20 h, the corresponding R2 for
diffusion model are 0.9786 and 0.9855, which are higher than
that for surface penetration model. For the sample milled for
10 h, the situation is more complex. It is possible that more
than one controlling step in whole process. There is a trend
that the R2 for diffusion model becomes higher and higher
with the increase of milling time, which indicates that the
rate-controlling step of the alloy has been changed from
surface penetration to the diffusion of hydrogen in the
hydride layer with increases of milling time.
Fig. 4 shows the fitted curves for the La1.8Ca0.2Mg14Ni3 alloy
modified by ball-milling. For the alloy milled for 10 h, it looks
like that at the beginning of hydriding reaction (t< 700 s), the
controlling step is the surface penetration and at a later stage
(t> 700 s), the diffusion of hydrogen in the hydride changes
into the controlling step due to a longer diffusion path.
Fig. 5 shows the hydrogen absorption kinetics at 300 K,
373 K, and 473 K and under a hydrogen pressure of 4.0 MPa
for La1.8Ca0.2Mg14Ni3 alloys milled for 20 h. The parameters
Ev and Bt can be evaluated using Eq. (8) by non-linear fitting
method. The corresponding R2 is 0.9908, which indicates
that the experimental data can be fitted with good accuracy
by Chou model and the diffusion of hydrogen in the hydride
is the rate-controlling step for this system from 300 K to
373 K. The calculated activation energy is 28.62� 0.63 kJ/mol
H2, which is much lower than that of La2Mg17�x wt%LaNi5.
The result was probably due to the effect of mechanical
milling as mentioned in Section 3.2. Moreover, the element
substitution also plays an important role in the hydroge-
nation process [29].
From the mathematical treatment and comparison of
model prediction with experimental data in the La2Mg17-based
alloy milled for different times.
Milled 10 h Milled 15 h Milled 20 h
0.9764 0.9223 0.9211
1550 1437 948.2
0.9095 0.9786 0.9855
3510 3780 1890
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Fig. 5 – Hydriding data at different temperatures and under
4.0 MPa H2 for La1.8Ca0.2Mg14Ni3 alloys milled for 20 h [12]
together with the fitted curves.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 1 9 5 1 – 1 9 5 71956
intermetallics system, we can see that the rate-limiting
step of hydrogen absorption changes with preparation
conditions.
4. Conclusion
Chou method has been used to investigate the hydrogen
absorption kinetics mechanism of La2Mg17-based composites.
For La2Mg17–LaNi5 composite synthesized by the method of
melting, the rate-controlling step is the surface penetration
and cannot be changed by addition agent (LaNi5). However,
mechanical milling combined with the catalyzer not only
improves the hydrogenation kinetics, but changes the reac-
tion rate-controlling step from surface penetration to diffu-
sion. Similarly, mechanical milling can also change the
reaction rate-limiting step of La1.8Ca0.2Mg14Ni3 alloy. There is
a trend that the rate-controlling steps change from surface
penetration to diffusion with the enhancement of milling
intensity. From Chou model and the non-liner regression
method, the activation energies of hydrogen absorption for
La2Mg17�20 wt%LaNi5 and for La1.8Ca0.2Mg14Ni3 can be calcu-
lated as 61.79� 0.67 kJ/mol H2 and 28.62� 0.63 kJ/mol H2,
respectively.
Acknowledgement
The authors gratefully acknowledge the financial supports
from the National High Technology Research and Develop-
ment Program of China (2007AA05Z118), the National Natural
Science Foundation of China (50804029), a Foundation for the
Author of National Excellent Doctoral Dissertation of P.R.
China (200746), the Program for Changjiang Scholars and
Innovative Research Team in University (IRT0739) and the
Innovation Fund for Graduate Student of Shanghai
University.
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