Efficient catalysis by MgCl2 in hydrogen generation via hydrolysis of Mg-based hydride prepared by hydriding combustion synthesis

Article abstract of DOI:10.1039/c2cc32353b

Magnesium chloride efficiently catalyzed the hydrolysis of Mg-based hydride prepared by hydriding combustion synthesis. Hydrogen yield of 1635 mL g ^-1 was obtained (MgH₂), i.e. with 96% conversion in 30 min at 303 K.

Full text of DOI:10.1039/c2cc32353b

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COMMUNICATION
Efficient catalysis by MgCl2 in hydrogen generation via hydrolysis of
Mg-based hydride prepared by hydriding combustion synthesisw
Zelun Zhao,z Yunfeng Zhu and Liquan Li*
Received 1st April 2012, Accepted 14th April 2012
DOI: 10.1039/c2cc32353b
Magnesium chloride efficiently catalyzed the hydrolysis of
Mg-based hydride prepared by hydriding combustion synthesis.
Hydrogen yield of 1635 mL gꢀ1 was obtained (MgH2), i.e. with
96% conversion in 30 min at 303 K.
In the present work, the Mg-based hydride (Mg and Ni in a
molar ratio of 99 : 1) prepared by hydriding combustion
synthesis (HCS)11 was chosen to generate hydrogen via hydrolysis.
Mechanical milling (MM) of Mg-based hydride was per-
formed to improve its hydrolysis property.12 As indicated in
Fig. 1 (dashed line), when the Mg-based hydride was immersed
in pure water, it released 192 mL gꢀ1 of hydrogen with a high
generation rate within initial 3 minutes. However, after that,
the rate was slowed down quickly. The low rate is mainly
attributed to the passive Mg(OH)2 layer formed on the surface
of MgH2. The solid line in Fig. 1 shows the kinetic curve of
hydrogen generation via hydrolysis of the Mg-based hydride in
0.5 mol Lꢀ1 MgCl2 solution. The yield of hydrogen reached
927 mL gꢀ1 within initial 3 min, which is about five times
larger than that in pure water. Our results revealed that
introduction of MgCl2 in solution could break the passive
layer of Mg(OH)2. The reason can be explained by a suitable
pH value of MgCl2 solution (pH = 8.91) when [Mg2+] was
0.5 mol Lꢀ1, which is lower than that of saturated Mg(OH)2
solution (pH = 10.44). After that, a high rate of hydrogen
generation kept for another 10 min, and then the hydrolysis
almost completed after nearly 30 min. The result indicated that
the hydrogen yield reached 1635 mL gꢀ1 and the hydrolysis
conversion of 96% was obtained.
To resolve the worldwide environmental and energy crisis,
hydrogen energy is considered as an ideal clean energy vector
with zero CO2 emission to replace traditional fossil energy,
especially for on-board fuel cell applications.1 However, the
production and storage technology of hydrogen is a remaining
barrier in the development of hydrogen energy economy.2
Magnesium hydride is one promising hydrogen storage material
due to its high hydrogen storage capacity (7.6 wt%), cheap
price, abundant resource and low environmental impact.3 The
major challenge in application of MgH2 as hydrogen storage
material is the poor thermodynamics in hydriding and dehydriding
reactions with the reaction temperatures usually as high as 573 K.4
Recently, hydrogen production via hydrolysis of MgH2 has
attracted more and more attention because of the theoretical
hydrogen yield, which is as high as 1703 mL gꢀ1 5
.
MgH2 shows thermodynamic instability during hydrolysis
reaction,6 while the poor kinetics does not meet practical require-
ments, which is caused by the passive Mg(OH)2 layer produced
on the surface of MgH2. The reaction kinetics could be enhanced
by reducing the pH value of the solution by adding acids7 or
acidic salts such as ammonium salts.8 But these additions corrode
the reactor and refuelling is needed during application. Mg-based
hydride composites such as Mg–La–H,9 Mg–Ca–H10 were also
used to generate hydrogen via hydrolysis with improved kinetics,
e.g., 88.3% conversion in 21 min for Mg–La–H, 80% conversion
in 30 min for Mg–Ca–H. Although these composites can
improve the kinetics, it is still not satisfactory because of the
lower theoretical hydrogen yield of these additives (474 mL gꢀ1
for LaH3, 1065 mL gꢀ1 for CaH2). Additionally, some impurities
in the by-product, such as La(OH)3 and Ca(OH)2, are not easy
to separate in the recycling process.
Fig. 2 shows the cyclic curve of hydrogen generation via
hydrolysis of Mg-based hydride with the MgCl2 catalyst. After
each hydrolysis reaction cycle, the reaction solution with
College of Materials Science and Engineering, Nanjing University of
Technology, 5 Xinmofan Road, Nanjing, Jiangsu, P.R. China.
E-mail: lilq@njut.edu.cn; Tel: +86-25-83587255
w Electronic supplementary information (ESI) available: Experimental
details, characterization of samples, Scheme S1, Fig. S1–S3. See DOI:
10.1039/c2cc32353b
Fig. 1 Kinetic curves of hydrogen generation via hydrolysis of
Mg-based hydride prepared by HCS + MM: solid line, in 0.5 mol Lꢀ1
MgCl2; dashed line, in pure water at 303 K.
z Graduate student.
c
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the pH value in MgCl2 solution decreased to 8.95 at about
30 min, which was in accordance with that of MgCl2 solution
before reaction. After that, the pH value kept near 8.95, which
agreed with the hydrogen yield curve after 30 min as shown
in Fig. 3.
The pH value was decreased according to eqn (1) when
MgCl2 was added into the solution, which promotes the
dissolution of Mg(OH)2su and Mg(OH)2aq (Mg(OH)2su and
Mg(OH)2aq denote Mg(OH)2 formed on the surface of MgH2
and that formed in the solution, respectively). The dissolution
of Mg(OH)2su results in the formation of the fresh surface of
MgH2 exposed to the solution, which then reacts with water
and HCl according to eqn (2). Thus two moles of hydrogen
are produced for every mole of MgH2 reacted. Now once
the passive film of Mg(OH)2su formed and slowed down the
hydrogen release, HCl will destroy the formed passive layer
according to eqn (3). At the same time, the Mg2+ from
Mg(OH)2su and MgH2 complements the Mg2+ consumed in
the formation of Mg(OH)2aq to keep the low pH value of the
solution.
Fig. 2 Cyclic curve of hydrogen generation via hydrolysis of Mg-based
hydride with the MgCl2 catalyst (the hydrogen yield data collected
within 30 min in each cycle).
the catalyst was separated from the deposited solid by-product.
Then the collected solution was reused for the next reaction
cycle without adding MgCl2. After nine cycles, the yield of
hydrogen generation reached 1600 mL gꢀ1 (94% conversion) in
30 min. As compared to the first cycle, the decrease in hydrogen
yield was about 35 mL gꢀ1 (2% decrease in conversion). The
result indicates that MgCl2 in solution has a good catalytic
effect in the hydrolysis of Mg-based hydride and could be
recycled with little degradation.
MgCl2 + 2H2O - Mg(OH)2aqk + 2HCl
MgH2 + 2HCl - MgCl2 + 2H2m
(1)
(2)
(3)
Mg(OH)2suk + 2HCl - MgCl2 + 2H2O
MgCl2 is a standard solution found in any corrosion
laboratory and is used to determine the susceptibility against
stress corrosion cracking (standard ASTM method), because it
attacks the grain boundary of the oxide or the passive layer of
the metal under investigation. Thus MgCl2 attacks the grain
boundary of Mg(OH)2 formed at the surface and disintegrates the
formed layer.13 Fig. 3 shows the hydrogen yields via hydrolysis
of Mg-based hydride prepared by HCS in 1.0 mol Lꢀ1 NaCl
and 0.5 mol Lꢀ1 MgCl2 solutions with the same molarities
of Clꢀ. The large hydrogen yield difference indicates that
the magnesium ion has an important role in our hydrolysis
system.
This research was supported by National Natural Science
Foundation of China (Grant No. 51171079 and 51071085),
Specialized Research Fund for the Doctoral Program of High
Education (Grant No. 20093221110008) and the Priority
Academic Program Development of Jiangsu Higher Education
Institutions (PAPD).
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