Catalysis and hydrolysis properties of perovskite hydride NaMgH3

Article abstract of DOI:10.1016/j.jallcom.2013.03.096

The addition of NaH by ball milling is shown to greatly improve the hydrogen storage properties and the hydrolysis properties of MgH₂, which is related to the formation of ternary hydride NaMgH₃ with specific perovskite structure. Th

Full text of DOI:10.1016/j.jallcom.2013.03.096

Journal of Alloys and Compounds xxx (2013) xxx–xxx
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Journal of Alloys and Compounds
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Catalysis and hydrolysis properties of perovskite hydride NaMgH3
⇑
H. Wang, J. Zhang, J.W. Liu, L.Z. Ouyang, M. Zhu
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China
Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online xxxx
The addition of NaH by ball milling is shown to greatly improve the hydrogen storage properties and the
hydrolysis properties of MgH2, which is related to the formation of ternary hydride NaMgH3 with specific
perovskite structure. The MgH2–10%NaH mixture exhibits better hydriding and dehydriding kinetics than
the MgH2–10%LiH mixture, in which the LiMgH3 with perovskite structure could not be formed. The cat-
alytic role of NaMgH3 is attributed to fast hydrogen mobility in the perovskite structure, which provides
fast hydrogen diffusion pathways for the hydriding and dehydriding of MgH2. The NaMgH3 also shows
fast hydrolysis reaction kinetics without any passivation. Our work shows that such perovskite-type
hydride demonstrates great potential as efficient catalysts for the high-capacity hydrides for whether
reversible or irreversible hydrogen storage.
Keywords:
Hydrogen storage
NaMgH3
Perovskite structure
Catalysis
Hydrolysis
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
and enhanced kinetics [21]. In addition, the reactive hydride com-
posites, such as the MgH2–2LiNH2–LiBH4 system, show improved
Hydrogen can function as energy storage medium and high
grade fuel. Moreover, the utilization of hydrogen offers a broad
range of benefits in reducing greenhouse gas emission and the oil
consumption [1]. The pursuit of hydrogen as sustainable energy
in the future society drives the research and development of so-
lid-state hydrogen storage materials to supply hydrogen for the
on-board applications [2]. To meet the requirements of high stor-
age density and fast charge/discharge rate under moderate condi-
tions, various light-weight metal hydrides, complex hydrides and
chemical hydrides have been widely attempted and investigated
to improve their reversible or irreversible hydrogen storage prop-
erties [3–9]. Despite the great achievements over past decades,
the safe and efficient hydrogen storage solution via high-capacity
hydrides remains great challenge.
Magnesium hydride shows great potential for solid-state hydro-
gen storage because of its high gravimetric density and low cost.
However, its practical application is hampered by the high desorp-
tion temperature and sluggish kinetics. Several effective strategies
have been developed to improve the hydrogen storage properties
of MgH2 by alloying, doping catalytic additives or nanostructuring
[10–19]. It is shown that the thermodynamic properties of MgH2
can be tuned by incorporating new elements or compositing with
other compounds [20–23]. For example, the dissolving of Indium
into the Mg lattice results in different hydriding/dehydriding reac-
tion route, which is characteristic of reduced desorption enthalpy
thermodynamic and kinetic performances than the constitute hy-
dride [24].
Mg-base ternary hydride NaMgH3, which can be synthesized
from the binary hydrides NaH and MgH2, demonstrates reversible
hydrogen storage properties [25–29]. NaMgH3 has a perovskite-
type structure composing of [MgH6]-octahedral and [NaH12]-
cubo-octahedral. In other words, the perovskite structure contains
two occupational sites of hydrogen, both of them are surrounded
by elemental Na and Mg. The specific hydrogen configuration
causes rather different hydrogen stability in NaMgH3 from those
in the binary hydrides NaH and MgH2. Sheppard et al. measured
the kinetic and thermodynamic data for the decomposition of
NaMgH3 into NaH and Mg. The desorption enthalpy and entropy
are 86.6 1.0 kJ/(mol H2) and 132.2 1.3 J/(mol H2 K) respec-
tively, indicating that NaMgH3 is thermodynamically more stable
than MgH2 [27]. It should be emphasized that the high rates of H
motion in the NaMgH3 was revealed by the H NMR study [28].
Wu et al. further investigated the dynamics of NaMgH3 and asso-
ciated the rapid hydrogen motion with its perovskite crystal
structure [29]. This result implies that the NaMgH3 can be used
as potential catalyst to enhance the slow hydrogenation kinetics
of some strongly bound light-metal–hydride systems, such as
MgH2.
With respect to the irreversible hydrogen storage, the produc-
tion of hydrogen by hydrolysis reaction of hydrides (NaBH4, LiAlH4,
MgH2, etc.) recently attracts more research interest because of the
advantage of hydrogen release at ambient conditions [30–33].
Among various hydrides for hydrogen generation by hydrolysis
reaction, MgH2 is of special interest due to the high hydrogen yield,
⇑
Corresponding author at: School of Materials Science and Engineering, South
China University of Technology, Guangzhou 510640, China
E-mail address: memzhu@scut.edu.cn (M. Zhu).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.03.096
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H. Wang et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx
low cost and recycle of the byproduct. Pure hydrogen and the only
recycled remain Mg(OH)2 are obtained through the reaction:
MgH2 þ 2H2O ! MgðOHÞ2 þ 2H2
D
H ¼ ꢀ277 kJ=mol
The hydrogen yield is up to 15.2 wt% if the water for hydrolysis
reaction is recycled from the products of fuel cell. However, the
hydrolysis reaction of MgH2 is kinetically limited owing to the for-
mation of a passive magnesium hydroxide layer. Many additives
have been attempted to composite with MgH2 to improve the
hydrolysis performances, the key point is to break down the unde-
sired passive layer and expose the fresh surface of MgH2.
In this paper, small amount of NaH was doped and milled with
MgH2 to in situ form NaMgH3, and the hydrogen storage properties
of the mixture correlating with the catalytic effect of NaMgH3 were
investigated in details. In addition, we also explored the superior
hydrolysis kinetics of NaMgH3, in which the deteriorating effect
of the passivation layer was completely eliminated.
Fig. 2. Dehydriding kinetic curves for 10 h-milled MgH2–10%NaH mixture at
different temperatures.
2. Experimental procedures
The MgH2 (>98% purity, Alfa-Aesar) powders were mixed with NaH, LiH (>96%
Purity, Aladdin) with designed composition, and then milled on a vibratory mill at
1000 rpm using the stainless vial and grinding balls with the weight ratio of ball to
powder being 20:1, the milling duration is 10 h. The typical sample is denoted as
MgH2–10%NaH indicating the addition of 10% NaH by weight.
X-ray diffraction (XRD) analysis of samples were performed on Philips X’Pert X-
ray (Cu Ka radiation) diffractometer. The powder samples were mixed with liquid
paraffin to prevent exposure to air. Zeiss-Supra 40 scanning electron microscope
(SEM) was employed to observe the microstructure of as-prepared NaMgH3. Hyd-
riding and dehydriding properties of samples were measured on automatic Sie-
vert-type apparatus (AMC Co. and Setaram Co.). The initial pressures of sample
holder for the isothermal dehydrogenation and hydrogenation kinetic test are about
7 kPa and 1.7 MPa, respectively. The initial pressure is ca. 3 MPa for temperature-
programmed absorption (TPA) with the heating rate of 5 K/min. All sample han-
dlings were performed in a glove box with water and oxygen content less than
3 ppm.
The hydrolysis reactions of samples (0.25 g) with pure water were carried out in
a 250 ml Pyrex glass reactor with three openings, one for water addition, one for
hydrogen exhausting and one for inserting thermometer, as illustrate in Ref. [34].
All hydrolysis experiments were carried out at room temperature (298 K), without
external heating. The hydrogen was flowed through a drying agent in order to re-
move all water vapor before the hydrogen volume measurement.
as previously reported in Ref.[35], the MgH2–10%NaH mixture
shows greatly enhanced hydrogen sorption kinetics. As illustrated
in Fig. 2, the as-milled mixture could desorb hydrogen without any
activation. Therefore, the activation treatment of sample was per-
formed by one hydrogenation (3 MPa, 1 h) and dehydrogenation
(vacuum, 1 h) cycle at 593 K. For the hydrogenation at 553 K,
573 K and 593 K (Fig. 1), the uptake of hydrogen proceeds very
quickly, and a maximum capacity of ca. 6.0 wt% can be obtained
in less than 100 s. Even at a lower temperature of 523 K, the full
hydrogenation requires only a period of ca. 1200 s. The dehydroge-
nation curves shown in Fig. 2 indicate slower dehydriding rate rel-
ative to the hydrogenation, which could been explained by the
relatively high dehydriding activation barrier and slow diffusion
rate of H through the MgH2 lattice. The complete decomposition
of MgH2 at 573 K takes about 3600 s, whereas at this temperature
pure MgH2 hardly desorbs hydrogen. Obviously, the enhanced
hydrogen storage properties of MgH2 is related with the addition
of NaH, such catalytic role for this stable binary hydride is not pre-
viously reported.
3. Results and discussion
In addition to the isothermal kinetic properties, a temperature-
programmed absorption test was performed to determine the min-
imum hydrogen absorption temperature of the MgH2–10%NaH
mixture, and the result is shown in Fig. 3. The pressure variation
with soaking time originates from the increase of temperature as
well as the hydrogen uptake of Mg. The TPA curve consists of three
distinct stages: at the low temperature region, the linear rise of
pressure is just assigned to the increasing temperature. Then the
pressure goes to the reverse at elevated temperature, indicating
the predominant decrease of pressure due to hydrogen uptake.
The rise again of pressure at ca. 550 K indicates that the hydriding
reaction of Mg is finished. Therefore, the differentiating plot (Fig. 3)
showing the derivative of hydrogen pressure with respect to the
soaking time is characteristic of a hydrogenation peak, by which
the onset hydrogen absorption temperature is estimated to be ca.
420 K, and the complete hydrogenation of MgH2–10%NaH mixture
requires ca. 25 min.
3.1. Hydrogen storage properties of the MgH2–10%NaH mixture
Figs. 1 and 2 shows the isothermal hydriding and dehydriding
properties of MgH2–10%NaH mixture at different temperatures,
respectively. In comparison with pure MgH2 milled for longer time
XRD analysis was performed to reveal the nature and mecha-
nism of catalyst in the MgH2–10%NaH mixture, and the results
are shown in Fig. 4. In comparison with the original mixture
(Fig. 4a), a new phase is identified to be NaMgH3 with perov-
skite-type structure present in the milled mixture (Fig. 4b), which
is coupled with the absence of NaH. It is thus deduced that the
added NaH readily reacted with MgH2 to form NaMgH3 in the mill-
ing process, which is consistent with Ikeda et al.’s work on the syn-
Fig. 1. Hydriding kinetic curves for 10 h-milled MgH2–10%NaH mixture at different
temperatures.
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H. Wang et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx
3
Fig. 5. PCI desorption curves for the MgH2–10%NaH mixture.
Fig. 3. The TPA curve for the hydrogenation process of MgH2–10%NaH sample
showing the pressure dependence on the temperature with heating rate of 5 K/min.
The differentiation curve (dP/dt) shows the onset hydrogen absorption
temperature.
Fig. 6. Hydriding and dehydriding kinetic curves at 573 K of MgH2–10%LiH mixture.
Fig. 4. XRD patterns of MgH2–10%NaH mixture at different states. (a) Unmilled;
(b) milled for 10 h; (c) after dehydrogenation at 573 K.
role of NaMgH3. Furthermore, Wu et al.’s work on the structure
refinement of isomorphic deuteride NaMgD3 indicated that there
exists D vacancies in the perovskite structure, because the occu-
pancies of two-types of D atoms were less than 1 [29], more D
vacancies with increasing temperature. The hydrogen vacancies
in the NaMgH3 are similar with O vacancies in the perovskite oxi-
des [37], which are commonly observed at high temperatures. It is
thereby believed that the presence of H vacancies would enhance
the hydrogen motion in the perovskite structure. Therefore, the
catalytic role of NaMgH3 could be explained that this perovskite
hydride acts as active sites for the adsorption and dissociation of
hydrogen, and offers fast diffusion pathways for hydrogen into
the adjacent MgH2 grains. Similar catalytic role of NaMgH3 was
also found by Nwakwuo et al. in the 3NaH + MgB2 + 4H2 -
M NaBH4 + NaMgH3 reactive composite [38], which presented bet-
thesis of pure NaMgH3 from the MgH2 and NaH mixture [26]. In
our study, the NaMgH3 could be synthesized with shorter milling
time and the absence of hydrogen atmosphere. Therefore, it is pro-
posed that NaMgH3 other than NaH plays as real catalyst to im-
prove the hydrogen storage properties of MgH2. The formation of
NaMgH3 also well explains the enhanced activation property for
the milled MgH2–10%NaH mixture, because this combination reac-
tion results in large amount of freshly exposed surface of MgH2
particles.
As shown in Fig. 4c, the NaMgH3 remains after the full dehydro-
genation of the MgH2–10%NaH mixture at 573 K. Fig. 5 further
shows the high stability of NaMgH3 by the pressure-composition
isotherms for the dehydrogenation of the MgH2–10%NaH mixture.
At 673 K, the increased desorption capacity is ascribed to the
decomposition of NaMgH3, which is featured with the relatively
lower pressure plateau in comparison with MgH2. At 623 K, the
decomposition of NaMgH3 could not be observed. Therefore, at
all testing temperatures for kinetic measurements, the NaMgH3 re-
mains stable. Despite this, the NaMgH3 shows great catalytic role
to the hydriding and dehydriding of MgH2.
ter
absorption
and
desorption
properties
than
the
2NaH + MgB2 + 4H2 M NaBH4 + MgH2 reactive composite due to
the formation of NaMgH3.
To further verify the catalytic role of perovskite structure, a
comparative study was carried out with MgH2–10%LiH mixture
prepared under identical experimental conditions. Fig. 6 shows
both the isothermal hydriding and dehydriding curves of MgH2–
10%LiH mixture at 573 K. One can see that the duration for approx-
imately complete hydrogenation and dehydrogenation at this tem-
Recent work on the H NMR and first-principle calculations re-
vealed the fast hydrogen mobility in the perovskite structure of
NaMgH3 [28,36], which seems to be responsible for the catalytic
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H. Wang et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx
Fig. 7. XRD pattern of MgH2–10%LiH mixture after dehydrogenation.
perature are ca. 1200 s and 14400 s, respectively. Both the hydrid-
ing and dehydriding rates are only quarter of those for the MgH2–
10%NaH mixture. It should be noted that the activation treatment
of the MgH2–10%LiH mixture was performed at 663 K overnight.
Therefore, the LiH addition shows negligible catalytic effect on
the hydrogen storage properties of MgH2. The huge difference on
the kinetic performances results from different phase composition.
Fig. 7 shows the XRD pattern of MgH2–10%LiH mixture after dehy-
drogenation at 573 K. The LiH other than the LiMgH3 phase is
found in the dehydrogenated mixture, indicating that the LiMgH3
with similar perovskite structure could not be formed. Our result
agrees well with the previous work by Ikeda et al. [39], who pre-
dicted the formation abilities of perovskite-type structure in the
Mg-based ternary hydride from the viewpoint of geometric restric-
tion on ions that were described by the Goldschmidt tolerance fac-
tors. The conclusion indicated that the NaMgH3, KMgH3, and
RbMgH3, but not the LiMgH3 could be formed. This comparison
definitely confirms the catalytic role of perovskite structure on
the hydrogen storage properties of MgH2.
Fig. 9. SEM image showing the morphology of as-synthesized NaMgH3.
small amount of MgO. The SEM image (Fig. 9) shows the uniform
microstructure composing of micro-sized NaMgH3 crystallites.
Fig. 10 shows the hydrogen evolution curves for the hydrolysis
of NaMgH3 at room temperature, the hydrolysis properties of 1 h-
milled pure MgH2 and the MgH2–NaH mixture with equal molar
ratio are also compared. As previously reported, the hydrolysis
reaction of pure MgH2 with water proceeds at fast rate in the
beginning, however, the reaction rate declines quickly due to the
formation of passivation layer Mg(OH)2. The hydrolysis reaction
of pure MgH2 is fully suspended in 20 min, generating a total of
760 ml/g of the hydrogen. In contrast, the NaMgH3 show extremely
fast hydrolysis rate, 1360 ml/g hydrogen is generated in seconds,
equivalent to 12.14 wt% hydrogen release capacity. This strong
hydrolysis reaction of NaMgH3 is obviously related with its compo-
sition and phase structure. Fig. 8b shows the XRD pattern for the
hydrolysis products of NaMgH3, the absence of MgH2 indicates
the complete hydrolysis of NaMgH3. The presence of NaOHꢁH2O
and Mg(OH)2 suggests the following hydrolysis reaction formula:
NaMgH3 + 3H2O ? NaOH + Mg(OH)2 + 3H2. The NaOHꢁH2O com-
pound should be formed during the baking of hydrolysis products.
It is deduced that the readily reaction of the component NaH
with water would be in favor of the decomposition of NaMgH3
and initialize the hydrolysis of the component MgH2 by giving
3.2. Hydrolysis properties of NaMgH3
The NaMgH3 for hydrolysis reaction measurements was synthe-
sized by milling the mixture of NaH and MgH2 with equal molar ra-
tio for 40 h, the milled product was further hydrogenated in a
reaction cell filled with 5 MPa hydrogen under 623 K for 5 h to ob-
tain highly pure NaMgH3. Fig. 8a shows the XRD pattern of as-
hydrogenated mixture, consisting of the main phase NaMgH3 and
Fig. 8. XRD patterns of as-synthesized NaMgH3 (a) and the hydrolysis products of
Fig. 10. Kinetic curves of hydrogen evolution for the NaMgH3, as-milled pure MgH2,
NaMgH3 (b).
and the mixture of NaH–MgH2 with 1:1 molar ratio at room temperature.
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H. Wang et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx
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We report the addition of NaH and its improving effect on the
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