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Effects of nano size mischmetal and its oxide onimproving the hydrogen sorption behaviour ofMgH2
T. Sadhasivama,b, M. Sterlin Leo Hudson a,c, Sunita K. Pandey a,Ashish Bhatnagar a, Milind K. Singh a, K. Gurunathan b, O.N. Srivastava a,*aHydrogen Storage Mission Mode MNRE Project Unit, Hydrogen Energy Centre, Department of Physics, Banaras
Hindu University, Varanasi 221005, IndiabDepartment of Nanoscience and Technology, Alagappa University, Karaikudi 630003, IndiacDepartment of Physics, Central University of Tamil Nadu, Thiruvarur 61004, India
a r t i c l e i n f o
Article history:
Received 6 October 2012
Received in revised form
5 April 2013
Accepted 8 April 2013
Available online 7 May 2013
Keywords:
Magnesium hydride
Nanoparticles
Ball-milling
Hydrogen storage
Synergistic effect
Activation energy
* Corresponding author. Tel.: þ91 0542 23684E-mail addresses: [email protected], h
0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.04.0
a b s t r a c t
Thispaper reports thecatalytic effects ofmischmetal (Mm)andmischmetal oxide (Mm-oxide)
on improving the dehydrogenation and rehydrogenation behaviour of magnesium hydride
(MgH2). It has been found that 5 wt.% is the optimum catalyst (Mm/Mm-oxide) concentration
for MgH2. The Mm and Mm-oxide catalyzed MgH2 exhibits hydrogen desorption at signifi-
cantly lower temperature and also fast rehydrogenation kinetics compared to ball-milled
MgH2 under identical conditions of temperature and pressure. The onset desorption tem-
perature for MgH2 catalyzed with Mm and Mm-oxide are 323 �C and 305 �C, respectively.
Whereas the onset desorption temperature for the ball-milledMgH2 is 381 �C. Thus, there is a
lowering of onset desorption temperature by 58 �C for Mm and by 76 �C for Mm-oxide. The
dehydrogenation activation energy of Mm-oxide catalyzed MgH2 is 66 kJ/mol. It is 35 kJ/mol
lower than ball-milled MgH2. Additionally, the Mm-oxide catalyzed dehydrogenated Mg ex-
hibits faster rehydrogenation kinetics. It has been noticed that in the first 10 min, the Mm-
oxide catalyzed Mg (dehydrogenated MgH2) has absorbed up to 4.75 wt.% H2 at 315 �C under
15atmospherehydrogenpressure.Theactivationenergydetermined for the rehydrogenation
ofMm-oxide catalyzedMg isw62 kJ/mol, whereas that for the ball-milledMg alone isw91 kJ/
mol. Thus, there is a decrease in absorptionactivationenergybyw29kJ/mol for theMm-oxide
catalyzedMg. In addition,Mm-oxide is thenativemixture of CeO2 andLa2O3whichmakes the
duoabetter catalyst thanCeO2,which is knowntobeaneffective catalyst forMgH2. This takes
place due to the synergistic effect of CeO2 and La2O3. It can thus be said that Mm-oxide is an
effective catalyst for improving the hydrogen sorption behaviour of MgH2.
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction two major issues associated with fossil fuels, the urban air
Produced from water, hydrogen burns back to water on hot
combustion (IC engine) or cold combustion (fuel cell).
Hydrogen is thus completely renewable, and it takes care of
68; fax: þ91 0542 [email protected] (O.2013, Hydrogen Energy P40
pollution and climate-change effect [1]. These properties
make hydrogen as a green fuel [2]. For harnessing hydrogen all
the major components namely production, distribution,
storage and applications need to be addressed. However, at
.N. Srivastava).ublications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 7 3 5 3e7 3 6 27354
present it is realized that hydrogen storage is the most crucial
component which cuts across production, distribution and
applications. It is generally believed that solid-state hydrogen
storage materials that are sponge-like solid capable of
absorbing (and then desorbing) significant amount of
hydrogen forms potential storage candidates. These are
metal/intermetallic solids like LaNi5, FeTi, ZrFe2, etc. The
other variety is the elemental hydrides such as AlH3, MgH2
and complex (built-in) hydrides typified by NaAlH4 and other
related compounds. Unlike, intermetallic hydrides, for built-in
hydrides, hydrogen is first desorbed and then the dehydro-
genated material is hydrogenated to regenerate the initial
hydride.
At present, significant research activity is focused onMgH2.
This is so since it possesses a comparatively high hydrogen
storage capacity both weight wise (7.6 wt.% H2) and volume
wise (110 kg/m3) [3,4]. This is one of the closest to the required
US-DOE targets of 5.5wt.% and 40 kg/m3 [5]. Of course, the DOE
target corresponds to system. Therefore, material storage
capacities will have to be much higher. However, attempts
should also be made to make hydride container tanks with as
small weight as possible. Some efforts in this direction are
being made by us based on fabrication on carbon tubes con-
sisting of carbon nanotubes which are known to be stronger
than steel [6]. Another characteristic which makes MgH2 an
attractive candidate for hydrogen storage is that unlike other
built-in hydrides like NaAlH4, it exhibits nearly complete
reversibility. However, to make MgH2 as a viable hydrogen
storage material, two of the disadvantages associated with
MgH2 have to be overcome. One of the disadvantage is that, it
is highly stable and needs high temperature for destabiliza-
tion to release hydrogen. The other difficulty relating to MgH2
is very sluggish sorption kinetics [7].
There have been several studies in the last few years to find
effective ways to address the above said issues associated
withMgH2. Themost effectiveways are particle size reduction
[4], admixing metal [8e10], intermetallic storage alloy [11,12],
halides [13e16] and other suitable materials [17e26] to
destabilize MgH2. It is generally believed that these additives
work as a catalyst (lower the desorption temperature and
improve the hydrogen desorption kinetics). These also assist
in the opposite direction to regenerate MgH2 after desorption
(reversibility). One category of metals and their oxides which
have been most widely used as an effective catalyst for MgH2
are transition metals and their oxides [8,10,17e20,22]. The
present work describes the catalytic activity of mischmetal
(mixture of rare-earth metals, dominantly Ce and La) and its
oxide in lowering the sorption (desorption and absorption)
temperature and improving the sorption kinetics of MgH2. We
have employed Ce richmischmetal (Ce: 62.5 at.%, La: 30.0 at.%
and the negligible amounts of Pr and Nd) and its corre-
sponding oxide (Mm-oxide) as a catalyst for MgH2. The ad-
vantages of mischmetal (Mm) and its oxide as compared to
other transition metals and their oxides is that, Mm is cost-
effective and Mm-oxide gets readily prepared through low
temperature (w200 �C) oxidation of Mm [27]. In addition, the
dehydrogenation and rehydrogenation characteristics of
MgH2 employing Mm-oxide catalyst is better than some of the
known oxide catalysts, for example Al2O3, TiO2, Cr2O3, CeO2,
etc. [17,18,20,22,26].
2. Experimental section
2.1. Sample preparation
Commercial MgH2 (Alfa Aesar, 98%) and fine granules of Mm
obtained through scuffing bulk pieces of Mm (Leico, 99%) were
used in the present study. Commercially obtained bulk piece
ofMmpreserved inmineral oil was scuffed into smaller pieces
of millimetre size using a stainless steel cutting saw under
inert atmosphere. The Mm granules of millimetre size thus
obtained were ball-milled to turn these granules into fine
powder form. Mm-oxide was prepared by oxidizing the fine
Mm powder in a tube furnace at 200 �C for 60 min.
2.2. Ball-milling
Ball-milling was carried out using Retsch planetary miller (PM
400) and locally fabricated stainless steel high pressuremilling
vial (capable of retaining up to 100 atm). 2.5 g of MgH2 together
with 5 wt.% catalyst (Mm/Mm-oxide) was loaded in the 250cc
milling vial and ball-milled under hydrogen atmosphere
(w15 atm) for 25 h at an operating speed of 150 rpm. The ball to
powder ratio was kept at 40:1. Ball-milling under hydrogen
atmosphere prevents the formation of MgO and moisture
contamination. Handling of samples was done under the inert
atmosphere in an argon filled glove box (mBRAun, MB10
Compact) with H2O and O2 <1 ppm.
2.3. Hydrogen sorption analysis
Dehydrogenation behaviour and reabsorption kinetics of the
samples were analyzed through temperature programmed
desorption (TPD) andpressure composition isotherm (PCI) using
an automated four channel Sieverts type apparatus (Advanced
Materials Corporation, USA). About 150 mg of the sample was
loaded in the sample chamber and then inserted into the pro-
grammable solid tube furnace (Thermcraft). The TPD analysis
was carried out at the initial pressure of w10�2 Torr under
continuousheating rate of 5 �C/minwith an accuracy of�0.1 �C/min. Isothermal dehydrogenation kinetics of MgH2 samples
weredeterminedat 325 �C,337 �C,350 �Cand363 �C, respectivelyunder1atmH2pressure.Thiswascarriedoutby initiallysoaking
thesampleunder30atmH2pressureat roomtemperature.Then
the temperaturewas raisedgradually fromroomtemperature to
the desired isothermal temperature. Kinetics at 1 atmwas then
determinedby releasing thepressure from30atmto1atmusing
the release mode available in the AMC Sieverts type apparatus.
The release mode maintains the pressure constant during the
isothermal condition and monitors the amount of hydrogen
released with respect to time. Rehydrogenation kinetics of the
samplewasmeasuredbyusing the soakmodeavailable in thePCI
measurement system. The soak mode permits the desired pres-
sure into the sample chamber and periodically collects data
corresponding to the amountof gasabsorbed/adsorbed vs. time.
2.4. Characterization
Structural characterizations of the samples were carried out
through X-ray diffraction (XRD) using X’Pert PRO (PANalytical)
Fig. 1 e TEM micrograph of as synthesized Mm-oxide
nanoparticles.
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 en e r g y 3 8 ( 2 0 1 3 ) 7 3 5 3e7 3 6 2 7355
X-ray diffractometer equipped with a graphite mono-
chromator employing CuKa radiation (l ¼ 1.541 A) at room
temperature. The XRD sample holder was sealed by a fine
layer of parafilm (Pechiney plastic packing) to prevent the
sample from oxygen and moisture contamination. Particle
sizes of the ball-milled materials were characterized by using
the transmission electron microscope (TECHNAI, 20G2).
The surface morphology of pristine and ball-milled materials
were carried out using scanning electron microscope
(Quanta 200).
Fig. 2 e X-ray diffractogram of (a) pristine MgH2, (b) ball-milled M
(e) dehydrogenated Mm-oxide catalyzed MgH2 (Mg) and (e) rehy
3. Results and discussions
3.1. Structural/micro structural analysis
During oxidation of Mmparticles at 200 �C, it has been noticed
that there is a colour change of sample from dark brown to
greenish yellow. A representative optical image of ball-milled
Mm particles and Mm-oxide particles are shown in the sup-
plementary figure, Fig. S1(a) & (b), respectively. The X-ray
diffractogram of Mm particles reveals the dominant presence
of La and Ce, whereas in Mm-oxide, La2O3 and CeO2 formed
themajority phases (refer Supplementary Fig. S2(a) & (b)). TEM
analysis of Mm-oxide reveals that the particle sizes are of
<20 nm. A representative TEM micrograph is shown in Fig. 1.
The inset of Fig. 1 shows the selected area electron diffraction
pattern of Mm-oxide (native mixture of CeO2 and La2O3). This
pattern could be indexed successfully based on known lattice
structure of La2O3 (hexagonal with a ¼ b ¼ 4.060 A, c ¼ 6.410 A)
and CeO2 (fcc with a¼ b¼ c¼ 5.411 A). Thus, the oxidized form
of Mm corresponds to Mm-oxide. It also shows that Mm-oxide
particles are of nano-form. Similar TEM characterization of
the ball-milled Mm catalyst particles showed that their sizes
are also in the nano range.
Fig. 2 shows the X-ray diffractogram of (a) pristine MgH2,
(b) ball-milled MgH2, (c) Mm catalyzed MgH2 and (d) Mm-
oxide catalyzed MgH2. A noticeable feature observed in X-
ray diffractogram is that the peaks corresponding to Mm-
oxide catalyzed MgH2 are comparatively broader than that
observed for Mm catalyzed and ball-milled MgH2 under
identical conditions. Since the Mm-oxide has the higher
gH2, (c) Mm catalyzed MgH2, (d) Mm-oxide catalyzed MgH2,
drogenated Mm-oxide catalyzed MgH2.
Fig. 3 e SEM micrographs of (a) pristine MgH2, (b) ball-milled MgH2, (c) ball-milled MgH2 D 5 wt.% Mm and (d) ball-milled
MgH2 D 5 wt.% Mm-oxide.
Fig. 4 e TEM micrograph of Mm-oxide catalyzed MgH2
(inset image shows the SAED pattern).
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 7 3 5 3e7 3 6 27356
degree of hardness than MgH2, the most likely reason is that
MgH2 gets pulverized by Mm-oxide during ball-milling. This
helps to form homogeneous distribution of catalysts and fine
particles of MgH2. Fig. 2(e) shows the XRD of dehydrogenated
Mm-oxide catalyzed Mg. As it can be seen that the peaks of
Mg and catalyst are discernible, indicating a complete dehy-
drogenation of MgH2. Fig. 2(f) shows the XRD pattern of
rehydrogenated Mm-oxide catalyzed MgH2. It has been
observed that upon rehydrogenation (315 �C and 15 atm H2
pressure), the crystallite size has increased due to clustering
and segregation of macroscopic phases. Furthermore, a
fraction of Mg remains un-reacted upon rehydrogenation.
This clearly suggests agglomeration of Mg after dehydroge-
nation making the rehydrogenation difficult. The approxi-
mate particle size of MgH2 samples determined by using
Scherrer formula are, 30.65 nm, 21.54 nm, 18.90 nm and
14.19 nm for the pristine, ball-milled, Mm and Mm-oxide
catalyzed MgH2, respectively. It has been found that with
the addition of Mm-oxide, the particle size of MgH2 gets
decreased to w15 nm.
Representative scanning electron micrograph (secondary
electron image) of pristine MgH2, ball-milled MgH2, Mm cata-
lyzedMgH2 andMm-oxide catalyzedMgH2 are shown in Fig. 3.
As it can be seen in Fig. 3(a), the particle size of pristine MgH2
is in the range ofw50 to 100 mm. After ball-milling, the particle
size of MgH2 gets decreased to w2 to 5 mm (Fig. 3(b)). As it can
be seen in Fig. 3(d), the particle sizes of Mm-oxide catalyzed
MgH2 are in the range of <1 mm. Moreover, the distribution of
particles here, unlike in Fig. 3(a, b and c) are nearly uniform. It
is expected that upon the ball-milling MgH2 gets pulverized by
the Mm-oxide which has higher hardness than MgH2.
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 en e r g y 3 8 ( 2 0 1 3 ) 7 3 5 3e7 3 6 2 7357
Fig. 4 brings out the typical transmission electron micro-
graph (TEM) of ball-milled Mm-oxide catalyzed MgH2. The
inset image shows the selected area electron diffraction
(SAED) pattern. The bright spots in this pattern could be
indexed based on the lattice structure of MgH2 (tetragonal,
primitive with a ¼ b ¼ 4.517 A, c ¼ 3.020 A). On the other hand,
the continuous ring was explicable based on the lattice
structure of CeO2 (fccwith a¼ b¼ c¼ 5.411 A). As it can be seen
in the TEM micrograph, the Mm-oxide nanoparticles typically
of the sizes w15 to 20 nm are embedded in the large MgH2
(w400 nm) particle.
3.2. Hydrogen desorption/absorption studies
It may be pointed out that in the present studies, the con-
centration of Mm and Mm-oxide catalysts on MgH2 have been
varied from 2 to 8 wt.% (2, 5 and 8 wt.%). However, the opti-
mum results in regard to catalytic effect leading to the
improvement in sorption behaviour of MgH2 was obtained for
Fig. 5 e (a) TPD curves of pristine, ball-milled (BM), Mm and
Mm-oxide catalyzedMgH2 at the heating rate of 5 �C/min, (b)
TPDrateprofilecharacteristic curvesofMgH2at5 �C/minwith
respect to peak desorption temperature (derived from (a)).
5 wt.% catalyst. Therefore, we have used this catalyst con-
centration for all our studies reported in this communication.
Fig. 5(a) shows the temperature programmed desorption
(TPD) curves of pristine MgH2, ball-milled MgH2, Mm andMm-
oxide catalyzed MgH2 at the heating rate of 5 �C/min. The
degree of improvement in dehydogenation for MgH2 priority
wise is Mm-oxide catalyzed MgH2 > Mm catalyzed
MgH2 > ball-milled MgH2 > pristine MgH2. Thus for the heat-
ing rate of 5 �C/min, the onset desorption temperature ofMgH2
(Fig. 5(a)) has been lowered from 410 �C to 305 �C upon ball-
milling MgH2 with 5 wt.% Mm-oxide.
As it can be seen in Fig. 5(a), the hydrogen desorption from
pristineMgH2 begins at 410 �C and that for ball-milledMgH2 at
381 �C. However, Mm and its oxide facilitate hydrogen
desorption fromMgH2 at lower temperatures than that of ball-
milled MgH2. Thus for 5 wt.% Mm and Mm-oxide catalyzed
MgH2, the hydrogen desorption starts at 323 �C and 305 �C,respectively. Therefore, the onset hydrogen desorption tem-
perature of pristine MgH2 gets lowered significantly by 105 �Cupon admixing 5 wt.% Mm-oxide. This is lower by 76 �C when
compared with ball-milled MgH2. This decrease is found to be
higher than that obtained from Mm catalyzed MgH2, where
the onset desorption temperature is 323 �C.Fig. 5(b) shows the TPD profile characteristic curves at the
heating rate of 5 �C/min in terms of peak desorption temper-
ature. As it is clear from Fig. 5(b), the Mm and Mm-oxide
catalyzed MgH2 shows hydrogen desorption at lower tem-
peratures. However, Mm-oxide catalyzed MgH2 is found to be
superior than Mm catalyzed MgH2. The Mm-oxide catalyzed
MgH2 exhibits hydrogen desorption in two steps. The first step
is observed at 310 �C and the second step at 351 �C. These two
desorption temperatures presumably correspond to the cat-
alytic effects of CeO2 and La2O3, the two constituents of Mm-
oxide. The desorption temperature of w310 �C is one of the
lowest obtained so far for MgH2 and its catalyzed versions.
Rehydrogenation kinetics determined for Mg samples
(ball-milled, Mm and Mm-oxide catalyzed Mg) is shown in
Fig. 6. Hydrogen absorption kinetics was measured at 315 �C
Fig. 6 e Rehydrogenation kinetics of dehydrogenated Mg
catalyzed with Mm and Mm-oxide.
Fig. 7 e Dehydrogenation kinetics of (a) ball-milled MgH2, (b) 5 wt.% Mm catalyzed MgH2, (c) 5 wt.% Mm-oxide catalyzed
MgH2 at different temperatures under 1 atm H2 and (d). Arrhenius plot (ln(k) vs 1000/T ) of MgH2 samples.
Table 1 e Comparison of dehydrogenation and rehydrogenation activation energy values from literature with presentinvestigations.
Materials Dehydrogenationactivation
energy, Ea (kJ/mol)
Rehydrogenationactivation
energy, Ea (kJ/mol)
Ref.
MgH2 þ 50 wt.% Al2O3 80 * [31]
MgH2 þ 30 wt.% LaNi5 80 * [32]
Mg þ 40 wt.% ZrFe1.4Cr0.6 <94 * [33]
Mg þ 4 wt.% Nano Fe * 56 � 3 [37]
MgH2 þ 5 wt.% Nano Ni * 60 [38]
MgH2 þ 2 wt.% CNS * 66 [25]
MgH2 þ 7 wt.% ZrF4 82 * [16]
MgH2 þ 7 wt.% NbF5 70 *
MgH2 þ 7 wt.% TaT5 95 *
MgH2 þ 7 wt.% TiCl3 79 *
MgH2 þ 7 wt.% VCl3 96 *
MgH2 þ 1 mol% Nb2O5 71 � 3 * [19]
MgH2 þ 0.2 mole% Nb2O5 62 * [34]
MgH2 þ 1 mol.% BaRuO3 90 * [21]
MgH2 þ 20 wt.% TiO2 72 � 3 * [35]
MgH2 þ 5 wt.% SWNT 96 * [36]
MgH2 (ball milled) 101 91 Present study
MgH2 þ 5 wt.% Mm 81 70
MgH2 þ 5 wt.% Mm-Oxide 66 62
* Implies that results on these are not available.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 7 3 5 3e7 3 6 27358
Fig. 8 e Rehydrogenation kinetic curves determined at
different temperature (a). Ball-milled Mg, (b) 5 wt.% Mm
catalyzed Mg and (c) 5 wt.% Mm-oxide catalyzed Mg.
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 en e r g y 3 8 ( 2 0 1 3 ) 7 3 5 3e7 3 6 2 7359
and 15 atm hydrogen pressure. A noticeable feature in Fig. 6 is
that the Mm-oxide catalyzed material shows significant
improvement in the rehydrogenation behaviour. The Mm-
oxide catalyzed dehydrogenated Mg reabsorbs 4.75 wt.% H2
within 10 min (5.5 wt.% H2 within 40 min). Whereas, Mm
catalyzed and ball-milled Mg under identical conditions
reabsorb only 4.32 and 3.25 wt.% H2 respectively. The wt.% H2
described here is with respect to the total weight of the ma-
terial (MgH2 together with the catalyst).
3.3. Determination of activation energy
To determine the effect of the catalysts Mm andMm-oxide on
the sorption characteristics of MgH2, we have evaluated
the activation energy (Ea) for dehydrogenation and rehy-
drogenation of Mm-oxide catalyzed MgH2, Mm catalyzed
MgH2 and ball-milled MgH2 by using JohnsoneMehleAvrami
[28] coupled with Arrhenius equation. The rate constant k
has been determined from the kinetic model for nucleation
and crystal growth in solids formulated by John-
soneMehleAvrami. This is given in the following equation:
[�ln(1 � a)]1/n ¼ kt (1)
where, ‘a’ is the extent of the reaction which can be iden-
tifiedwith a normalized hydrogenwt.% (range: from 0 to 1), t is
the time, k and n are constants (at constant temperature).
Arrhenius equation representing the dependence of rate
constant ‘k’ of the reaction, gas constant R and the absolute
temperature T is given by
k ¼ Aexp(eEa/RT ) (2)
Logarithm of Eq. (2) corresponds to a straight line and Ea can
be determined from the slope.
3.3.1. Dehydrogenation kineticsFig. 7 shows the isothermal dehydrogenation (desorption) ki-
netics determined at different temperatures (325 �C, 337 �C,350 �C and 363 �C) under 1 atm H2 pressure for (a) ball-milled
MgH2, (b) Mm catalyzed MgH2 and (c) Mm-oxide catalyzed
MgH2. It has been noticed that at 325 �C under 1 atmhydrogen,
the Mm-oxide catalyzed MgH2 has desorbed 5 wt.% H2 within
10 min, but Mm catalyzed and ball-milled MgH2 desorbed
5wt.%H2 in 20min and 50min, respectively. It may be pointed
out that the increase in desorption kinetics is only due to the
effect of catalyst (which remains intact and does not react
with MgH2) and not due to any additives to MgH2. A repre-
sentative figure comparing the kinetics of ball-milled MgH2
and MgH2 catalyzed with Mm and Mm-oxide is given in sup-
plementary figure (Fig. S3). It clearly suggests that the kinetics
of Mm-oxide catalyzed MgH2 is improved significantly than
both ball-milled and Mm catalyzed MgH2. Among various
metal oxides (Cr2O3, TiO2, Fe3O4, Fe2O3, In2O3 and ZnO) cata-
lyzed MgH2, Polanski et al. [17] reported that Cr2O3 and TiO2
catalyzed MgH2 exhibit superior dehydrogenation kinetics at
325 �C under 1 atm H2 pressure. In the present study, it has
been observed that the dehydrogenation kinetics of Mm-oxide
catalyzed MgH2 is similar to that reported for MgH2 catalyzed
Table 2 e Summary of rehydrogenation kinetic data obtained in the present investigation.
Temperature(�C)
Mm-oxide catalyzed Mg (wt.% of H) Mm catalyzed Mg (wt.% of H) Ball-milled Mg (wt.% of H)
10 min 20 min 40 min 120 min 10 min 20 min 40 min 120 min 10 min 20 min 40 min 120 min
315 4.75 5.18 5.43 5.57 4.32 4.50 4.62 4.70 3.25 3.84 4.15 4.38
292 4.12 4.6 4.8 4.91 4.08 4.19 4.26 4.46 3.09 3.62 3.90 4.25
255 2.8 3.54 4.2 4.88 3.10 3.42 3.81 4.20 2.20 3.00 3.61 4.10
236 1.65 2.48 3.36 4.09 1.60 2.20 2.66 3.23 1.27 1.73 2.16 2.73
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 7 3 5 3e7 3 6 27360
with Cr2O3 and TiO2. It may be pointed out, Mm-oxide cata-
lyzed MgH2 exhibit better dehydrogenation kinetics than
some other known catalysts [21,29,30].
JohnsoneMehleAvrami and Arrhenius equations were
used to determine the dehydrogenation activation energy for
catalyzed MgH2. As discussed earlier the activation energy for
dehydrogenation of MgH2 has been calculated from the slope
of Arrhenius plot (Fig. 7(d)). It has been determined that the
dehydrogenation activation energy (Ea) for ball-milled MgH2
and MgH2 catalyzed with Mm andMm-oxide arew101 kJ/mol,
w81 kJ/mol and w66 kJ/mol, respectively. Table 1 compares
the activation energy value reported for catalyzed MgH2.
The activation energy corresponding to Mm-oxide catalyzed
MgH2 is comparable with that of MgH2 catalyzed with
Nb2O5, which is thought to be a very effective catalyst for
MgH2.
3.3.2. Rehydrogenation kineticsIn order to determine activation energy for rehydrogenation of
Mg, we have evaluated the kinetics of dehydrogenated sam-
ples at different temperatures (315 �C, 292 �C, 255 �C and
236 �C) under 15 atm hydrogen pressure. Fig. 8 shows the
rehydrogenation (absorption) kinetic curves of dehydro-
genated (a) ball-milled Mg (b) Mm catalyzed Mg and (c) Mm-
oxide catalyzed Mg. Higher reabsorption capacity was ach-
ieved at 315 �C. However, as the temperature decreases, the
rehydrogenation capacity gets lower and the kinetics become
slower. The hydrogen reabsorption kinetic data obtained at
different temperatures for the ball-milled, Mm and Mm-oxide
catalyzed Mg are summarized in Table 2.
The JohnsoneMehleAvrami equation has been used to plot
a graph between [�ln(1 � a)] and ln(t) in which isothermal
experimental values are linear (Fig. 9(a)). The logarithmic
Fig. 9 e (a) Avrami plot (ln(Lln(1 L a)) vs. ln(t)) and (b). Arrheni
transforms of the equation have been calculated from hy-
drogenation curves as shown in Fig. 8(c). The John-
soneMehleAvrami plot of ln(�ln(1 � a)) vs ln (t) gives a
straight line and from the slope of that line, the rate constant
(k) can be determined. Using the Arrhenius equation, the
activation energy for the rehydrogenation of Mg has been
determined from the slope of Arrhenius plot (Fig. 9(b)).
The representative JohnsoneMehleAvrami plot and
the respective Arrhenius plot for Mm-oxide catalyzed Mg
is shown in Fig. 9(a) & (b). The corresponding John-
soneMehleAvrami and Arrhenius plots for ball-milledMg and
Mm catalyzedMg are given in supplementary figures, Fig. S4(a
& b) and Fig. S5(a & b). It has been determined that the
hydrogen absorption activation energy (Ea) for Mm and Mm-
oxide catalyzed Mg are w70 kJ/mol and w62 kJ/mol, respec-
tively. For ball-milled Mg it is w91 kJ/mol. Table 1 gives the
comparative activation energy values of catalyzed MgH2 re-
ported in literature. The absorption activation energy deter-
mined for Mm-oxide catalyzed Mg is 62 kJ/mol. This is very
much comparable to that reported for Mg catalyzed with nano
Fe (56 � 3 kJ/mol) [37] and nano Ni (60 kJ/mol) [38]. Our earlier
studies reveal that the activation energy corresponding to the
hydrogenation kinetics of effective carbon nanostructures
catalyzed Mg is 66 kJ/mol [25]. Thus, the present study unveil
that the hydrogenation activation energy determined for Mm-
oxide catalyzed Mg is lower than that of Mm and carbon
nanostructure catalyzed Mg.
3.4. Synergistic effect arising due to the presence ofLa2O3 together with CeO2
The improvement in sorption behaviour of catalyzed MgH2
(i.e, ball-milling together with catalyst) will increase the
us plot (ln(k) vs 1000/T ) of 5 wt.% Mm-oxide catalyzed Mg.
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 en e r g y 3 8 ( 2 0 1 3 ) 7 3 5 3e7 3 6 2 7361
surface area and defect densities in MgH2, creating an easy
path for hydrogen atoms to diffuse and hence enhance the
hydrogen sorption kinetics. Improvement in desorption
behaviour of MgH2 might be due to the electronic exchange
reaction (reduction/oxidation) between the catalyst nano-
particles and MgH2 which are responsible for weakening the
bond between Mg and H. It is known that the reducible oxides
such as TiO2 and CeO2 contain cations of multiple valance,
which can easily undergo reduction to become TiO2�x and
CeO2�x. These oxides can undergo reduction and oxidation,
and hence they are known to be effective catalysts [39,40]. In
the present case, during sorption process, the CeO2 of native
Mm-oxide catalyst is expected to undergo reduction and
oxidation leading to the electronic exchange between the
catalysts and MgH2. Additionally, there is yet another oxide,
i.e, La2O3 which is present in Mm-oxide. This oxide is not
expected to get easily reduced. However, the presence of La2O3
can enhance the catalytic effect of CeO2. This is so, since La2O3
[41,42] has hardness of 8 Mohs as against 6 Mohs for CeO2 [43]
and 4 Mohs for MgH2 [44,45]. Thus, La2O3 will work like a
dispersing and cracking agent for MgH2. This will imply that
MgH2 would obtain smaller particle size in lower milling time
than that required in the absence of La2O3. The lowering of
desorption temperature and improvement in absorption ki-
netics observed in the present study for Mm-oxide catalyzed
MgH2 is more than that reported while using CeO2 alone as a
catalyst [26]. In view of the above, it can be said that the sig-
nificant improvement in hydrogen sorption from MgH2 cata-
lyzed by Mm-oxide is due to synergistic effect [46e48]
produced by the combination of CeO2 and La2O3.
4. Conclusions
Based on the present investigations, the following conclusions
can be drawn:
(i) Mm (Ce and La are the dominant components) and its
oxide exhibits superior catalytic effect for improving the
hydrogen sorption from MgH2. Thus, for the heating rate
of 5 �C/min the onset desorption temperature corre-
sponding to 5 wt.% Mm-oxide catalyzed MgH2 has been
lowered from 381 �C (ball-milled) to 305 �C and that for
MgH2 catalyzed with Mm, it is lowered to 323 �C. Thisdecrease in desorption temperature is due to the catalytic
effect of Mm and Mm-oxide. Thus, the lowering of
desorption temperature for MgH2 when catalyzed by
Mm-oxide and Mm are 76 �C and 58 �C, respectively.(ii) The dehydrogenation activation energy is 101 kJ/mol for
ball-milled, 81 kJ/mol for Mm catalyzed and 66 kJ/mol for
Mm-oxide catalyzed MgH2. The activation energy deter-
mined for the rehydrogenation of ball-milled, Mm and
Mm-oxide catalyzed Mg (dehydrogenated MgH2) in the
present investigations are 91, 70 and 62 kJ/mol.
(iii) The improvement in sorption kinetics of Mm-oxide
catalyzed MgH2 is due to the synergistic effect produced
by the combination of CeO2 and La2O3 in Mm-oxide.
(iv) Based on the above, the cost-effective Mm-oxide ore can
be taken as an effective catalyst for improving hydrogen
sorption from MgH2.
Acknowledgements
Financial support from the Ministry of New and Renewable
Energy (Missionmode project onHydrogen Storage), DST, UGC
and DAE are thankfully acknowledged. Thanks are that due to
Prof. B. Vishwanathan and Prof. S. Srinivasa Murthy for
helpful discussions.
Appendix A. Supplementary data
Supplementary data related to this article can be found, in the
online version at http://dx.doi.org/10.1016/j.ijhydene.2013.04.
040.
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