The high capacity and controllable hydrolysis rate of Mg3La hydride

Article abstract of DOI:10.1016/j.jallcom.2013.03.153

This paper reports the synthesis of Mg₃La alloys with different hydrogenation degree through controlling the hydrogen amount introduced and reveals the hydrogenation mechanism. The hydrolysis rate and hydrogen yield of Mg₃La hydride

Full text of DOI:10.1016/j.jallcom.2013.03.153

Journal of Alloys and Compounds 580 (2013) S317–S319
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
The high capacity and controllable hydrolysis rate of Mg3La hydride
L.Z. Ouyang a,d, , J.M. Huang a,d, C.J. Fang a,d, H. Wang a,d, J.W. Liu a,d, Q.A. Zhang c, D.L. Sun b,d, M. Zhu a,d
⇑
a School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China
b Department of Materials Science, Fudan University, Shanghai 200433, People’s Republic of China
c School of Materials Science and Engineering, Anhui University of Technology, Maanshan, Anhui 243002, People’s Republic of China
d Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 26 March 2013
This paper reports the synthesis of Mg3La alloys with different hydrogenation degree through controlling
the hydrogen amount introduced and reveals the hydrogenation mechanism. The hydrolysis rate and
hydrogen yield of Mg3La hydride could be controllable through adjusting the ratio of LaH3 to Mg or
MgH2. The fully hydrogenated Mg3La alloy can generate 873.24 ml gꢀ1 in 66 min (7.80 wt.%). The results
show that the hydrolysis performance of MgH2 in Mg3La hydrides could be significantly accelerated by
LaH3 while that of Mg was affected limitedly by LaH3.
Keywords:
Energy storage materials
Hydrogen absorbing materials
Metals and alloys
Rare earth alloys and compounds
Metal hydrides
Ó 2013 Elsevier B.V. All rights reserved.
Kinetics
1. Introduction
[17–19], acid solution [20] or using ultrasonic irradiation [9] to im-
prove the hydrolysis properties, but they would pollute the envi-
As the development of economy and expansion of population,
the global energy supplies issues face the social, economic and
environment problems [1]. Hydrogen is considered as a promising
energy carrier because it is high energy density (142 MJ kgꢀ1),
lightweight (0.0899 g lꢀ1) and environmentally benign product of
oxidation (water) [2,3]. The feasible method for hydrogen produc-
tion is especially important for the realization of hydrogen
economy.
Hydrolysis is an efficient and convenient hydrogen generation
method for low operation temperature, pure hydrogen, high
hydrogen yield and simple reaction equipment [4]. Hydrolysis
materials can be divided into metal [5–7], metal hydride [8–10],
chemical hydride [11–13] and so on. Hydrolysis of magnesium hy-
dride with hydrogen yield 15.2 wt.% is cheap, available raw mate-
rial and environment friendly process which only produce
Mg(OH)2 besides pure hydrogen gas [14,15]. Therefore, Hydrolysis
of magnesium hydride as hydrogen resource is a promising meth-
od for on-board vehicular or portable application. However, the
hydrolysis of MgH2 is easy to be rapidly interrupted because the
formation of a low water solubility magnesium hydroxide layer
to inactive material [16]. Hence, ball-milled magnesium hydride
composites have to be performed in aqueous/alcoholic solution
ronment, corrode the equipment or had to design extra
ultrasonic equipment which needs extra energy supplied. The
author had found that LaH3 could significantly improve the hydro-
lysis of MgH2 [21] and the LaMg12 hydrides with controllable
hydrolysis rate had also been explored by adjusting the different
hydrogenation degree of LaMg12 alloys [10]. But the whole hydro-
lysis rate of LaMg12 hydrides is relatively slow. Due to LaH3 play
the key role in the hydrolysis of MgH2, the hydrolysis performance
of MgH2 can be further improved by increasing the amount of LaH3
on the base of LaMg12 hydrides to meeting the hydrogen genera-
tion rate for commercial utilization. Hence, in this work, the con-
trollable and effective hydrolysis materials, Mg3La alloys with
different hydrogenation degree, were prepared through controlling
the relative ratio of the in situ formed LaH3 to Mg/MgH2 phases.
2. Experiment section
2.1. Sample preparation
The high purity Mg (99.9%) and La (99.9%) were using to prepare Mg3La alloy by
induction melting at pure Ar atmosphere. The obtained alloy was ball-milled on a
QM-SP1 planetary ball mill rotating at 350 rpm for 1 h with the ball-to-powder
mass ratio of 20:1. All samples were performed in an Ar filled glove box in order
to prevent them from oxidation or hydrolysis.
The samples of different hydrogenation degree were carried out on a gas reac-
tion controller of AMC (Advanced Materials Corporation). Each sample was hydro-
genated about 2.5 h under the pattern of Auto soak (small) with the different initial
hydrogen pressures. The hydrogenated alloys reacted with pure water in a 250 ml
Pyrex glass reactor following the previous method [21].
⇑
Corresponding author at: School of Materials Science and Engineering, South
China University of Technology, Guangzhou 510641, People’s Republic of China.
Tel.: +86 20 87114253; fax: +86 20 87112762.
E-mail address: meouyang@scut.edu.cn (L.Z. Ouyang).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.03.153

S318
L.Z. Ouyang et al. / Journal of Alloys and Compounds 580 (2013) S317–S319
2.2. Samples characterization
Mg3La þ 6=2H2 ! LaH3 þ 3=2MgH2 þ 3=2Mg
Mg3La þ 9=2H2 ! LaH3 þ 3MgH2
Judging from the corresponding reactions as shown in Eqs. (1)–
(3), the hydrogenation steps of Mg3La were that La first reacted
with hydrogen to form LaH3 and then Mg reacted with H to form
MgH2 after all La had reacted with H.
ð2Þ
ð3Þ
The phase structures of samples were obtained on Philips X’Pert diffractometer
using graphite monochromatized Cu K
a radiation as the light source. A scanning
rate of 0.02°sꢀ1 was applied to record the patterns in the 2h range of 15–90°. The
powder samples were covered with paraffin oil to prevent oxidation during mea-
surements. The XRD profiles were analyzed by the Rietveld method with the pro-
gram of X’Pert HighScore Plus to determine the strain and grain size [10].
3. Results and discussion
3.2. Hydrolysis properties of Mg3LaH3, Mg3LaH6 and Mg3LaH9
3.1. Preparation of Mg3LaH3, Mg3LaH6 and Mg3LaH9 by controlling the
hydrogenation degree
Fig. 2 shows the hydrogen evolution curves of Mg3LaH3, Mg3-
LaH6 and Mg3LaH9 alloys at 298 K, respectively. As shown in
Fig. 2a, Mg3LaH3 produced 443.39 ml gꢀ1 hydrogen in 3 min and
up to 452.83 ml gꢀ1 in 59 min in the hydrolysis process. For the
hydrolysis of Mg3LaH6 alloy, as presented in Fig. 2b, the conversion
yield was 455.45 ml gꢀ1 in 2 min and released to 618.81 ml gꢀ1 in
73 min. But the hydrolysis rate of Mg3LaH9 alloy had a great
change. Mg3LaH9 generated 586.85 ml gꢀ1 of hydrogen in 3 min
and quickly up to 685.45 ml gꢀ1 in 22 min then continued to gen-
erate 873.24 ml gꢀ1 in 66 min (Fig 2c). The final hydrogen yield of
hydrolysis reaction of Mg3LaH3, Mg3LaH6 and Mg3LaH9 were
4.04 wt.%, 5.52 wt.% and 7.80 wt.%, respectively. Conversion into
equivalent mole amount, 4.34 mol H2, 6.02 mol H2 and 8.61 mol
H2 can be generated by hydrolysis of per mole of Mg3LaH3, Mg3-
LaH6 and Mg3LaH9, respectively. The hydrogen produced was
greater than that in H–Mg3La itself, indicating that this extra part
of hydrogen generated from that in water. The results illustrated
that the hydrolysis of metal hydride was an effective method for
hydrogen generation compared with that of metals and/or alloys.
Fig. 3 shows the grain sizes and strains of LaH3, MgH2 and Mg in
Mg3LaHx. The sizes of LaH3, MgH2, Mg phases in the different hy-
drides of Mg3LaH3, Mg3LaH6 and Mg3LaH9 were in the same order
of 50 10 nm, respectively. Also, the strains of LaH3, MgH2, Mg
phases in the different hydrides were similar, which were in the
same order of 0.2 0.1%. Hence, it can be concluded that the
improvements of hydrolysis performances had rarely relation with
the sizes and strains of LaH3, MgH2, Mg phases.
Fig. 1 shows the XRD patterns of the ball-milled and three kinds
of materials with the rations being 3, 6 and 9 of H to Mg3La named
Mg3LaH3, Mg3LaH6 and Mg3LaH9. Fig. 1a shows the X-ray diffrac-
tion pattern of Mg3La alloy after ball milling. All diffraction peaks
can be attributed to Mg3La phase (JCPDS Card, No. 65-0919), which
can be indexed as D03 structure. This result showed that no other
phase existed in the alloy. Fig. 1b shows the X-ray diffraction pat-
tern of Mg3LaH3, in which the peaks can be indexed as LaH3 phase
with a face-centered cubic structure and Mg phase with close-
packed hexagonal structure. No peak corresponding to the MgH2
phase in the XRD patterns confirmed that La reacted with hydro-
gen prior to Mg and form LaH3 due to that La had more affinity
with hydrogen than that of Mg. And no peak in accordance with
Mg3La phase indicated that all the Mg3La phase had transited to
LaH3 and Mg phases during the hydrogenation process. The hydro-
genation reaction can be described as Eq. (1):
Mg3La þ 3=2H2 ! LaH3 þ 3Mg
ð1Þ
Mg3LaH6 and Mg3LaH9 were also prepared by gradually increas-
ing the amount of introduced hydrogen. Fig. 1c and d show the X-
ray diffraction patterns of Mg3LaH6 and Mg3LaH9, respectively. The
peaks of the Mg3LaH6 alloy in XRD patterns were attributed as
LaH3, MgH2 and Mg phase. Apparently, part of Mg started to absorb
hydrogen and form MgH2 while LaH3 still kept the same fashion.
The existence of Mg peaks as shown in Fig. 1b and c indicated that
both Mg3LaH3 and Mg3LaH6 alloys were only partially hydroge-
nated. The diffraction peaks of Mg3LaH9 can be indexed as LaH3
and MgH2 phase. No existence peaks of Mg phase as shown in
Fig. 1d indicated that Mg3La alloy had fully reacted with hydrogen
and formed LaH3 and MgH2 phases. The corresponding reactions
are shown in Eqs. (2) and (3), respectively.
In order to reveal the hydrolysis mechanism of Mg3LaHx, the
hydrolysis products of Mg3La hydrides can be analyzed by XRD,
as shown in Fig. 4. Fig. 4a shows the XRD diffraction pattern of
hydrolysis product of Mg3LaH3. The new appeared peaks can be de-
tected as Mg(OH)2 and La(OH)3 while the peaks of the original LaH3
Fig. 2. The hydrogen evolution curves of the hydrolysis for (a) Mg3LaH3, (b)
Mg3LaH6 and (c) Mg3LaH9 at 298 K. Hydrolyses of Mg3LaH3, Mg3LaH6 and Mg3LaH9
generated 452.83 ml gꢀ1, 618.81 ml gꢀ1 and 873.24 ml gꢀ1 in the end, with total
hydrogen yields of 4.04 wt.%, 5.52 wt.% and 7.80 wt.%, respectively.
Fig. 1. The XRD patterns of ball milled Mg3La alloys (a), partially hydrogenated
Mg3LaH3 (b), Mg3LaH6 (c) and fully hydrogenated Mg3LaH9 (d) at 573 K.

L.Z. Ouyang et al. / Journal of Alloys and Compounds 580 (2013) S317–S319
S319
Mg3LaH9 was quickest among Mg3LaH3, Mg3LaH6 and Mg3LaH9.
As the hydrogenation degree rose, the content of MgH2 increased
and Mg content decreased. The hydrolysis performance of MgH2
could be significantly accelerated by LaH3 but that of Mg was af-
fected by LaH3 limitedly. For the fully hydrogenated Mg3LaH9 sam-
ple, the coexisted LaH3 and MgH2 promoted each other which
resulted in complete hydrolysis. Hence, the hydrolysis rate and
yield of Mg3LaHx hydride could be controlled through adjusting
the ratio of LaH3 to Mg or MgH2. Compared to the hydrogenated
LaMg12 [10], the fully hydrogenated Mg3La had the higher hydroly-
sis rate due to the higher ratio of LaH3 to MgH2. So Choosing the
hydride with suitable ratio of La:Mg is also important to get the op-
tional hydrolysis rate for commercial application.
4. Conclusion
Control the amount of hydrogen involved the reactions can ob-
tain different hydrogenated degree Mg3La alloys. La first adsorbed
hydrogen and transformed into LaH3, after that Mg element re-
acted with hydrogen and transformed into MgH2. The hydrolysis
performance of MgH2 could be significantly accelerated by LaH3
but Mg was affected by LaH3 limitedly. The fully hydrogenated
Mg3La alloy can generate 873.24 ml gꢀ1 H2 in 66 min (7.80 wt.%).
The hydrolysis rate and yield of Mg3La hydride could be controlla-
ble through adjusting the ratio of LaH3 to Mg or MgH2.
Fig. 3. The sizes and strains of LaH3, MgH2 and Mg in Mg3LaH3 (X = 3), Mg3LaH6
(X = 6) and Mg3LaH9 (X = 9), respectively. The sizes/strains of LaH3 in Mg3LaH3,
Mg3LaH6, and Mg3LaH9 are 39 nm/0.23%, 60 nm/0.23% and 43 nm/0.32%, respec-
tively. The sizes/strains of MgH2 in Mg3LaH6 and Mg3LaH9 are 52 nm/0.14% and
46 nm/0.17%, respectively. And the sizes/strains of Mg in Mg3LaH3 and Mg3LaH6 are
51 nm/0.18% and 48 nm/0.21%, respectively.
Acknowledgements
This work was financially supported by the Ministry of Science
and Technology of China (No. 2010CB631302), the National Natu-
ral Science Foundation of China (Nos. U1201241, 51271078 and
50925102) and KLGHEI(KLB11003).
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Fig. 4. The XRD patterns of the hydrolysis production of (a) Mg3LaH3, (b) Mg3LaH6
and (c) Mg3LaH9.
and Mg phases still existed, which indicated that the Mg3LaH3 only
partially hydrolyzed. As we know, pure LaH3 can react with water
completely. However, only partial LaH3 could react with water in
Mg3LaH3 indicated that the coexisted Mg could partially prevent
the LaH3 from being hydrolyzed in water. It can be also indicated
that hydrolysis performance of Mg was affected limitedly by
LaH3. For the hydrolysis of Mg3LaH6, the newly appeared peaks
in the XRD pattern could also be indexed as Mg(OH)2 and La(OH)3
phases. All the LaH3, MgH2 and Mg phases in Mg3LaH6 had disap-
peared. Likewise, both LaH3 and MgH2 phases disappeared and
the newly peaks can be attributed as Mg(OH)2 and La(OH)3 phases
after the hydrolysis of Mg3LaH9 as shown in Fig. 4c. In the case of
hydrolysis of Mg3LaH6 and Mg3LaH9, it can be illuminated that
LaH3 can significantly accelerate the hydrolysis of MgH2.
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Based on that the LaH3 content was the same but the MgH2 and
Mg were various in different hydrides, the hydrolysis rate of