Enhanced hydrogen reaction kinetics of nanostructured Mg-based composites with nanoparticle metal catalysts dispersed on supports

Article abstract of DOI:10.1016/j.jallcom.2006.12.054

Hydrogen reaction kinetics of nanocrystalline MgH₂ co-catalyzed with Ba₃(Ca_1+xNb_2-x)O_9-δ (BCN) proton conductive ceramics and nanoparticle bimetallic catalyst of Ni/Pd dispersed on single wall carbon nanotubes (SWNTs) support has been investigated. The nanoparticle bimetallic catalysts of Ni/Pd supported by SWNTs were synthesized based on a novel polyol method using NiCl₂·6H₂O, PdCl₂, NaOH and ethylene glycol (EG). The nanostructured Mg composites co-catalyzed with BCN and bimetallic supported catalysts exhibited stable hydrogen desorption capacity of 6.3-6.7 wt.% H₂ and the significant enhancement of hydrogen desorption kinetics at 230-300 °C in comparison to either non-catalyzed MgH₂ or the nanocomposite of MgH₂ catalyzed with BCN. Crown Copyright

Full text of DOI:10.1016/j.jallcom.2006.12.054

Journal of Alloys and Compounds 446–447 (2007) 84–89
Enhanced hydrogen reaction kinetics of nanostructured Mg-based
composites with nanoparticle metal catalysts dispersed on supports
Yeong Yooa,∗, Mark Tuckb, Rajender Kondakindia, Chan-Yeol Seoa,
Zahir Dehouchec, Khaled Belkacemic
a Institute for Chemical Process and Environmental Technology, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada
b Department of Chemical Engineering, Dalhousie University, Halifax, Nova Scotia B3J 2X4, Canada
c Department of Soil Sciences and Agri-Food Engineering, Laval University, Quebec City, Quebec G1K 7P4, Canada
Received 29 October 2006; received in revised form 12 December 2006; accepted 13 December 2006
Available online 21 December 2006
Abstract
HydrogenreactionkineticsofnanocrystallineMgH2 co-catalyzedwithBa3(Ca1+xNb2−x)O9−δ (BCN)protonconductiveceramicsandnanoparticle
bimetallic catalyst of Ni/Pd dispersed on single wall carbon nanotubes (SWNTs) support has been investigated. The nanoparticle bimetallic catalysts
of Ni/Pd supported by SWNTs were synthesized based on a novel polyol method using NiCl2·6H2O, PdCl2, NaOH and ethylene glycol (EG).
The nanostructured Mg composites co-catalyzed with BCN and bimetallic supported catalysts exhibited stable hydrogen desorption capacity of
6.3–6.7 wt.% H2 and the significant enhancement of hydrogen desorption kinetics at 230–300 ◦C in comparison to either non-catalyzed MgH2 or
the nanocomposite of MgH2 catalyzed with BCN.
Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage materials; Polyol method; Ni/Pd nanoparticle catalyst; Magnesium hydride; Proton conductive ceramic catalyst
1. Introduction
ature is reduced significantly to less than 140 ◦C when MgH2 is
co-milled with 0.5 wt.% PdOH2O and 5 at.% or 10 at.% Ni [6].
In addition, Gutfleisch et al. have also reported a Mg–1 wt.%
Ni–0.2 wt.% Pd alloy produced via induction melting with
consequent reactive milling exhibited 41 kJ/mol for absorption
and 110 kJ/mol for the desorption reaction, indicating excellent
hydrogen absorption/desorption kinetics [7]. Therefore, nano-
Ni/Pd catalyst uniformly dispersed on nanocrystalline MgH2
can be considered to exhibit very attractive catalytic effect on
enhancing hydrogen sorption kinetics.
In another approach, transition metal oxides such as TiO2,
V2O5, CuO, Mn2O3, Cr2O3, Fe3O4, and Nb2O5, etc. have
been investigated as potential catalysts on the sorption behav-
ior of nanocrystalline Mg-based systems [3,8]. Recently, Yoo
et al. have reported the catalytic effect of proton conductive
ceramics on enhancing hydrogen desorption kinetics of nanos-
tructured magnesium hydrides [9,10]. After milling MgH2–Ba3
(Ca1+xNb2−x)O9−δ (BCN) composite, the nanostructured com-
posite showed much enhanced desorption kinetics, increased
Magnesium hydride having a high hydrogen storage capac-
ity of 7.6 wt.% is considered as one of the most interesting
alternatives for the reversible storage of hydrogen. However,
the major problem of magnesium as a rechargeable hydrogen
carrier system is its slow reaction rate and high sorption tem-
perature. To overcome this problem, metallic catalysts such as
Pd, Ni, V, Ti, Mn and Fe have been added to nanocrystalline
magnesium for a better H2-dissociation at the surface [1,2].
Recently Hanada et al. have reported the catalytic effect of
nanoparticle Ni on hydrogen storage properties of MgH2 [3,4].
The nanosized Ni doping significantly decreased the activation
energy for hydrogen desorption by surface activation, resulting
in90%ofhydrogendesorptionwithin100 minat163 ◦C. Kojima
et al. have reported that nanocomposite MgH2 catalyzed with
nano-Ni/Al2O3/C desorbed 4.9–5.8 wt.% H2 at 423–473 K [5].
Gutfleisch et al. have reported that hydrogen desorption temper-
desorption plateau pressure, and decreased hysteresis, ln(Pabs
/
∗
Pdes), indicating not only ameliorate desorption kinetics but also
Corresponding author. Tel.: +1 613 993 5331; fax: +1 613 991 2384.
lower temperature desorption in comparison to pure MgH2.
E-mail address: yeong.yoo@nrc-cnrc.gc.ca (Y. Yoo).
0925-8388/$ – see front matter. Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.12.054

Y. Yoo et al. / Journal of Alloys and Compounds 446–447 (2007) 84–89
85
Therefore, in this work, the hydrogen reaction kinetics
of nanocrystalline MgH2 co-catalyzed with Ba3(Ca1+xNb2−x
)
O9−δ (BCN) proton conductive ceramics and nanoparticle
bimetallic catalysts of Ni/Pd dispersed on single wall carbon
nanotubes (SWNTs) support has been investigated.
2. Experimental procedures
Magnesium hydride (MgH2) powder was purchased from Alfa-Aesar (98%
purity) and mechanically milled under argon by using a Spex 8000 high energy
mill for 18 h to prepare nanostructured MgH2. The ball to powder ratio was 10:1.
BCN catalyst (Ba3(Ca1+xNb2−x)O9−δ, x = 0) [11] was prepared from poly-
meric precursors by the Pechini method [12]. The starting materials of barium
carbonate dissolved in nitric acid solution, calcium carbonate dissolved in nitric
acid solution, and ammonium citric solution converted from ammonium nio-
bate oxalate hydrate were mixed with a stoichiometric amount of citric acid.
The resulting solution was stirred for about 1 h on a hot plate and the temper-
ature was stabilized at 70 ◦C. The mixture was heated to 90 ◦C, at which point
ethylene glycol was added at a mass ratio of 40:60 with respect to citric acid. The
temperature was maintained constant at 160 ◦C for resin formation and thermal
decomposition. The precursor powders were then calcined at 700 ◦C for 4 h and
again heat-treated at 1000 ◦C for 1 h as the final step for the synthesis of BCN
single phase.
The nanoparticle Ni/Pd catalysts (Ni:Pd = 10:1 and 2:1 in molar ratio) sup-
ported on SWNTs (Ni/Pd:SWNT = 1:1 weight ratio) also having catalytic effect
on enhancing kinetics of hydrogen storage materials were prepared based on
a novel polyol method developed by Bock et al. [13] for preparing Pt/Ru
nanoparticles. In this method, precursor salts of NiCl2·6H2O (Sigma–Aldrich,
PeagentPlus) and PdCl2 (Sigma–Aldrich, 99.9%) were dissolved in ethylene
glycol containing 0.2 M NaOH. The solutions were stirred for 30 min in air at
room temperature, subsequently heated under reflux to 160 ◦C for 3 h, and then
cooled in air. Appropriate aliquots of the colloidal solutions were mixed with
single wall carbon nanotubes (Microtechnano, SWNTs purity 92 wt.%, amor-
phous carbon 6 wt.%, ash 2 wt.%, surface area 640 m2/g) in a large and open
beaker for up to 24 h, resulting in the deposition of the Ni/Pd colloids on the
SWNTs. The SWNTs-supported Ni/Pd catalysts were then filtered, washed with
water, and dried at 100 ◦C in air for 1 h.
The nanocrystalline composites of MgH2 containing BCN catalyst were
prepared by milling a mixture of 18 h milled MgH2 and 2 wt.% of BCN for
additional 2 h. The final co-catalyzed MgH2 nanocomposites containing the
supported Ni/Pd catalysts as well as BCN were prepared by milling a mixture of
the milled MgH2–BCN composite and the SWNTs-supported Ni/Pd catalysts
for additional 20 min.
Thermodynamics and kinetics of the nanostructured MgH2 composites were
determined by a Sievert-type sorption analyzer (Hy-Energy, PCT Pro2000). The
surface morphology, microstructure, surface oxidation state and phase of the
as-milled MgH2 and the supported catalysts were determined by using scan-
ning electron microscopy (SEM), transmission electron microscopy (TEM),
X-ray photoelectron spectroscopy (XPS) and X-ray diffraction on a Bruker’s
D8 Diffractometer. The crystallite size was also obtained by a TOPAS program
(Bruker). The dehydrogenation behavior of the nanocomposites with catalysts
was studied by differential scanning calorimetry (DSC 2920, TA instruments).
The milled nanocomposite sample of 4 mg was placed in an aluminum pan, and
a lid was crimped onto the pan in dry box filled with argon. The pan was then
placed in the sample cell of the DSC module. The temperature of the DSC mod-
ule was equilibrated at 50 ◦C and then increased at a rate of 1–20 ◦C/min under
a N2 gas purge up to 500 ◦C.
Fig. 1. X-ray diffraction patterns of the composites: (a) pure MgH2 as received,
(b) MgH2 milled for 18 h, (c) BCN catalyst and (d) MgH2–2 wt.% BCN
nanocomposite milled for additional 2 h.
sized BCN catalyst and (d) MgH2–2 wt.% BCN nanocomposite
milled for additional 2 h. As shown in Fig. 1(a) and (b), the
high energy milling of as-received MgH2 for 18 h increased
the diffraction peak line widths, indicating that the high energy
milling reduced theaveragecrystallitesizepresentintheoriginal
␤-MgH2 material and/or increased its microstructural strain, and
induced the phase transformation from ␤-MgH2 to the nanocrys-
talline mixture phases of ␤-MgH2 tetragonal phase and ␥-MgH2
orthorhombic phase. Fig. 1(c) shows a well crystallized cubic
perovskite structure of BCN single phase. As shown in Fig. 1(d),
the additional milling of the mixture of 18 h milled MgH2 and
BCN catalyst for 2 h kept the BCN phase, indicating that the
proton conductor exists on the surface of MgH2 particles and
that surface inter-diffusion between MgH2 and BCN may cause
the formation of Ba, Ca and Nb-rich reacted zone as hydroxides
or even hydrides on the surface of MgH2 as described in Ref.
[10].
SWNTs-supported Ni/Pd catalysts having two different
molar ratios of Ni/Pd = 10/1 and 2/1 were synthesized by the
polyol method using ethylene glycol (EG)–NaOH. As for a
supporting material for the deposition of metal nanocatalysts,
SWNTs was used because it was reported that SWNTs can act
as catalysts to improve the hydrogen absorption and desorption
properties of metallic catalyst-doped hydrides [14,15]. It was
also reported that the nanotube length decreases with increasing
ball milling time [16]. In order to prevent the thorough destruc-
tion of SWNTs during the high energy milling, a short milling
time of 20 min was applied for mixing the milled MgH2–BCN
composite and the SWNTs-supported Ni/Pd catalysts in this
study. However, it should be noted that the high surface area
carbon as a supporting material may provide a similar beneficial
effect on enhancing hydrogen sorption kinetics and the direct
observation of SWNTs in the milled composites as a function of
milling time has to be done to confirm the effect of mechanically
milled SWNTs in the composites.
3. Results and discussion
3.1. Structural characterization of mechanically milled
MgH2 and synthesized catalysts
Ethylene glycol is an alcohol and is readily oxidized to act as
a reducing agent for the Ni and Pd precursor salts. The detailed
suggested mechanism was described in literature [13]. The addi-
tion of NaOH into the EG system accelerates the reduction
of metal ions to metal nanoparticles even at room tempera-
Fig. 1 shows the X-ray diffraction patterns of the composites:
(a) pure MgH2 as received, (b) MgH2 milled for 18 h, (c) synthe-

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Y. Yoo et al. / Journal of Alloys and Compounds 446–447 (2007) 84–89
Fig. 2. Transmission electron micrographs and electron diffraction patterns of (a) and (b) SWNT and (c) and (d) supported Ni catalysts.
to dark brown during refluxing. It was reported that the Ni2+ ions
are thought to be reduced at the surface of the Pd nanoparticles,
presumably on the most catalytically active {1 1 1} surface [20].
Fig. 2 shows the TEM images and corresponding electron
diffraction patterns of SWNTs and supported Ni catalysts. The
particles essentially were monodispersed with an average diam-
eter of 5.0–7.0 nm, consistent with the crystallite size of 5.6 nm
calculated by calculated by TOPAS program from X-ray diffrac-
tion patterns. Five fringe patterns with plane distances of 3.13,
ture without the need for external reducing agent and decreases
the particle size [17]. The formation of Ni metal nanoparti-
cles by chemical method is known to be very difficult because
nickel is easily oxidized [18]. The X-ray diffraction patterns
and XPS spectra of SWNTs-supported Ni/Pd catalysts prepared
in ethylene glycol (EG)–NaOH solution without using poly(N-
vinyl-2-pyrrolidone) (PVP) as a stabilizing agent revealed the
difficulty in preparing fully homogenous bimetallic Ni/Pd alloy
and preventing the formation of NiO on the surface of Ni
nanoparticles. Shifted Ni and pure Pd metallic phases in the X-
ray diffraction patterns and NiO related small peaks at 855.5 eV
as well as Ni and Pd at 853.7 and 335.9 eV, respectively, in XPS
spectra were observed. It was reported that even if two mono-
layers of NiO are formed on the Ni surface, the intensity of the
Ni2+ (2p) peak is greater than the Ni0 (2p) peak [19]. Therefore,
the extent of oxidation of Ni is not significant and the oxidation
may be due to oxygen associated with EG absorbed on the Ni
particles and hydroxide formation on the Ni surface during fil-
tering and washing with water. The mixing of Pd salt with Ni salt
was very effective to reduce the Ni2+ ions with ethylene glycol
because the solution showed very fast color change from yellow
˚
2.11, 1.83, 1.29, and 1.10 A could be observed from Ni nanopar-
ticle catalysts on SWNTs and four fringe patterns (nos. 2–5)
except for the first pattern related to SWNTs were consistent
with the indices (1 1 1), (2 0 0), (2 2 0), and (3 1 1) of pure face-
centered cubic (fcc) nickel as shown in Fig. 2d.
3.2. Dehydrogenation behavior of the milled
nanocrystalline composites of MgH2 containing different
catalysts
Fig. 3 shows DSC curves of the as received and as-milled
samples with or without catalysts at a heating rate of 5 ◦C/min.

Y. Yoo et al. / Journal of Alloys and Compounds 446–447 (2007) 84–89
87
Fig. 4. Kissinger plots of the hydrogen desorption reaction of (a) as-received
MgH2, (b) 18 h milled MgH2, (c) MgH2–BCN composite, (d) MgH2–BCN–
Ni/Pd(=2/1)/SWNTs composite and (e) MgH2–BCN–Ni/Pd(=10/1)/SWNTs
composite at various heating rates (β = 1, 5, 10, and 20 ◦C/min).
Fig. 3. DSC curves of (a) as-received MgH2, (b) 18 h milled MgH2, (c)
MgH2–BCN composite, (d) MgH2–BCN–Ni/Pd(=2/1)/SWNTs composite and
(e) MgH2–BCN–Ni/Pd(=10/1)/SWNTs composite, at a heating rate of 5 ◦C/min.
gen desorption can be estimated by the Kissinger method [24]
from DSC curves using the following equation:
The endothermic peaks correspond to the decomposition of the
hydrides phases. The nanostructured MgH2 milled for 18 h has a
hydrogen desorption peak around 363 ◦C, which is much lower
than that of as received MgH2 (417.5 ◦C). Thus, the reduction
of crystallite size was very effective to decrease the decom-
position temperature. The BCN doping to milled MgH2 further
decreased the decomposition temperature and increased the heat
flow in comparison to non-catalyzed MgH2, potentially result-
ing in the increase of the enthalpies of formation. The additional
milling of the mixture of 2 wt.% of BCN and milled MgH2
may cause the formation of Ba and Ca hydrides or hydrox-
ides or (Mg, M)Hx (M = Ba, Ca) in addition to Nb–Hx at the
interface which can act as the enhanced path for hydrogen
diffusion or active sites for hydrogen dissociation and recom-
bination. However, the enthalpies of formation of BaH2 and
CaH2 are 86 and 94 kJ/mol H2, respectively, and higher than
that of MgH2 (71 kJ/mol H2) [21]. In addition, the decompo-
sition temperatures of BaH2 and CaH2 are 675 and 600 ◦C,
respectively, and much higher than that of MgH2 (327 ◦C) [22].
There are many ternary hydrides such as Ba2MgH6, BaMgH4,
Ba6Mg7H26, Ba2Mg3H10, and Ca4Mg3H14 that can be formed
at the interface and have undefined values of the decomposition
temperatureandenthalpiesofformation. Therefore, itisverydif-
ficult to determine the interfacial phases that cannot be detected
by X-ray diffraction analysis. The kinetics of hydrogen sorption
should be investigated for evaluating the effectiveness of BCN
doping and is described in the next section. The further addition
of 2 wt.% of Ni–Pd nanocatalysts on SWNTs into BCN-doped
MgH2 decreased the peak decomposition temperature to about
310 ◦CandbroadenedDSCprofileinthetemperaturerangefrom
340 to 220 ◦C. The Ni-rich nanocatalyst-added MgH2 composite
exhibited the most superior catalytic effect among all compos-
ites tested in this work and asymmetrical endothermic peaks
having the low temperature peak related to the desorption from
␥-MgH2 and some amount of ␤-MgH2 [23].
ꢀ
ꢁ
β
Tp2
Ea
ln
= −
+ α
(1)
RTp
where β is the heating rate, Tp the peak temperature, Ea the
apparent activation energy, R the gas constant, and α is the linear
constant.
From the slope of these straight lines, the activation energies
of as received MgH2 and 18 h milled MgH2 were estimated to
be 172.1 and 146.8 kJ/mol, respectively, similar to the reported
values (156 and 144 kJ/mol) [25,5]. Even though BCN cata-
lyst was very effective to enhance desorption kinetics at 300 ◦C
[9,10], Ni/Pd metallic catalysts were much effective to enhance
sorption kinetics at lower temperature according to DSC data.
Ni-rich nanocatalyst-added MgH2 composite exhibited the low
activation energy of 79.3 kJ/mol, corresponding to the reported
values (88.1 and 74 kJ/mol) [2,5].
Fig. 4 shows Kissinger plot of the hydrogen desorption reac-
tion for catalyzed MgH2 samples at various heating rates (β = 1,
5, 10, and 20 ◦C/min). The apparent activation energy for hydro-
Fig. 5. Hydrogen absorption and desorption curves of (a) 18 h milled MgH2, (b)
MgH2–BCN composite, (c) MgH2–BCN–Ni/Pd(=2/1)/SWNTs composite and
(d) MgH2–BCN–Ni/Pd(=10/1)/SWNTs composite at 300 ◦C.

88
Y. Yoo et al. / Journal of Alloys and Compounds 446–447 (2007) 84–89
Fig. 6. Hydrogen desorption curves of (a) MgH2–BCN–Ni/Pd(=10/1)/SWNTs composite, and (b) MgH2–BCN–Ni/Pd(=2/1)/SWNTs composite under an initial
hydrogen pressure of 50 mbar at different temperatures.
3.3. Hydrogen sorption properties
are shown in Fig. 7. The various catalyst-containing MgH2
composites exhibited high maximum hydrogen absorption
capacities, (H/M)max in the range of 6.3–6.7 wt.%. The equi-
librium pressures of H-desorption and reversible hydrogen
absorption–desorption capacities for catalyzed MgH2 increased
significantly in comparison to non-catalyzed MgH2. The hys-
teresis, ln(Pabs/Pdes), of catalyzed MgH2 during hydrogen
absorption and desorption was much smaller than non-catalyzed
MgH2 due to the decreased mechanical stress between the metal
and the hydride phases or the reduced kinetic constraints during
the measurement [27].
Fig. 5 shows the hydrogen absorption and desorption curves
of the composites. The Ni-rich (Ni/Pd = 10/1) nanocatalyst-
added MgH2 composite exhibited the best absorption and
desorptionkineticsat300 ◦C, correspondingtotheapparentacti-
vation energies of hydrogen desorption determined by DSC. Ni
has a comparatively high 3d orbital occupancy. Therefore, occu-
pied state is dominant with respect to the 3d orbital. This largely
contributes to the back-donation of the electrons to the antibond-
ing orbitals of MgH2, resulting in the easier Mg–H dissociation
[26].
Fig. 6 shows the hydrogen desorption curves of (a)
MgH2–BCN–Ni/Pd(=10/1)/SWNTs composite, and (b) MgH2–
BCN–Ni/Pd(=2/1)/SWNTs composite under an initial hydrogen
pressure of 50 mbar at 230, 250 and 300 ◦C. It can be seen that
both of catalysts are not sufficient to provide fast kinetics and
high hydrogen capacity at 230 and 250 ◦C, even if DSC data of
catalyzed MgH2 are quite promising to enhance sorption behav-
ior at low temperature. Therefore, the effect of the concentration
of catalysts on enhancing desorption kinetics at low temperature
should be further investigated.
4. Conclusions
Nanocrystalline MgH2 composites co-catalyzed with
Ba3(Ca1+xNb2−x)O9−δ (BCN) proton conductive ceramics and
nanoparticle bimetallic catalyst of Ni/Pd dispersed on single
wall carbon nanotubes (SWNTs) support have been successfully
prepared by a consecutive combination of Pechini process,
novel polyol method, and three step high energy milling. The
apparent activation energies for the hydrogen desorption of
MgH2 nanocomposites with or without catalysts were deter-
mined by the Kissinger method from DSC curves. The Ni-rich
(Ni/Pd = 10/1) nanocatalyst-added MgH2 composite exhibited
the fastest absorption and desorption kinetics at 300 ◦C, but
2 wt.% addition of SWNTs-supported Ni/Pd catalysts was not
enough to provide fast kinetics and high hydrogen capacity at
230 and 250 ◦C. The maximum hydrogen absorption capacities
of nanostructured MgH2 composites co-catalyzed with BCN
and bimetallic supported catalysts, (H/M)max in the range of
6.3–6.7 wt.% were obtained by PCI (pressure–composition-
isotherms) curves at 300 ◦C. The nanostructured Mg composites
co-catalyzed with BCN and bimetallic supported catalysts
exhibited the significant enhancement of hydrogen desorption
kinetics at 230–300 ◦C in comparison to either non-catalyzed
MgH2 or the nanocomposite of MgH2 catalyzed with BCN.
The PCI (pressure–composition-isotherms) curves at 300 ◦C
for nanostructured MgH2 composites with different catalysts
Acknowledgements
Fig. 7. PCI curves at 300 ◦C for nanostructured MgH2 composites with different
catalysts.
This work was supported by Program of Energy Research
and Development (PERD) Technology and Innovation (T&I)

Y. Yoo et al. / Journal of Alloys and Compounds 446–447 (2007) 84–89
89
in Canada. The authors gratefully acknowledge the help of Mr.
DashanWangforTEM, Mr. DavidKingstonforXPS, Mr. Nguon
Lim for DSC and Dr. Patrick Mercier for XRD data analysis.
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