Hydrogenation characteristics of Mg-TiO2 (rutile) composite

Article abstract of DOI:10.1016/S0925-8388(00)01188-9

The nanostructured composite Mg-TiO₂ (rutile) was prepared by reaction ball milling (RBM). Under the combined effects of the catalyst n-TiO₂ and the mechanical driving force, Mg was hydrided into MgH₂ and γ-MgH₂ directly during RBM. The addition of TiO₂ resulted in a markedly improved hydrogenation performance of Mg, rapid kinetics, low working temperature and excellent oxidation-resistance. A hydrogenation mechanism of the composite was proposed on the basis of microstructure analysis.

Full text of DOI:10.1016/S0925-8388(00)01188-9

Journal of Alloys and Compounds 313 (2000) 218–223
L
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Hydrogenation characteristics of Mg–TiO2 (rutile) composite
*
P. Wang , A.M. Wang, H.F. Zhang, B.Z. Ding, Z.Q Hu
State Key Laboratory of RSA, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, China
Received 26 May 2000; accepted 15 August 2000
Abstract
The nanostructured composite Mg–TiO2 (rutile) was prepared by reaction ball milling (RBM). Under the combined effects of the
catalyst n-TiO2 and the mechanical driving force, Mg was hydrided into MgH2 and g-MgH2 directly during RBM. The addition of TiO2
resulted in a markedly improved hydrogenation performance of Mg, rapid kinetics, low working temperature and excellent oxidation-
resistance. A hydrogenation mechanism of the composite was proposed on the basis of microstructure analysis.
B.V. All rights reserved.
2000 Elsevier Science
Keywords: Energy storage materials; Hydrogen-absorbing materials; Mechanical alloying; Gas–solid reactions
1. Introduction
attracted much attention as an important catalyst in the
photoelectrochemical industry [17–19]. Here we focus our
attention on its ability of dissociating hydrogen molecules
and providing a rapid passageway for diffusion of hydro-
gen atoms, which imply the potential use of TiO2 as a
composite phase to improve the hydrogenation kinetics of
Mg [20]. Regarding the composite system Mg–TiO2, some
research has been carried out, but no desirable performance
has been reported which probably should be attributed to
the limitation of fabrication methods [21]. In the present
work, the reaction ball milling (RBM) method was
adopted to prepare Mg–TiO2 composite, and the hydriding
property of the composite was examined, especially at
moderate temperature.
Magnesium and Mg-based hydrogen storage materials
have been the subject of many investigations owing to
their high H-storage capacity and low price. The aim of all
these researches was to improve hydriding/dehydriding
kinetics and to lower the working temperature. Among the
various efforts, ranging from the addition of a catalytic
component, and surface treatment to the introduction of
non-conventional fabrication methods, composite forma-
tion has been generally accepted as a simple, effective
method [1–7]. The additives used in this case are metals
[7,8], nonmetals [9], intermetallics [10–13] and metal
oxides [14,15]. Depending on the nature of the additive
and the different reaction mechanism, various degrees of
enhanced activity of the magnesium with respect to
hydrogen have been achieved and reported.
2. Experimental details
The addition of some metal oxides could bring about a
favorable change in the hydrogenation performance of the
material. Iwakura et al. [15] and Cui et al. [16] found that
the discharge capacity could be greatly increased by
modification with some metal oxides in LaNi5-type and
Mg2Ni-type H-storage alloys, respectively. Khrussanova et
al. [14] reported that the hydriding/dehydriding process of
magnesium could be accelerated by 3d transition metal
oxide additions.
First, TiO2 (rutile, 200 mesh, 99%) was annealed for 1 h
at 4008C under 4.0 MPa hydrogen pressure. Then the
mixture of Mg (100 mesh, 99%)120wt.%TiO2 was
ground under high-purity hydrogen (initial pressure was
0.8 MPa) in a high-energy ball mill (Spex 8000) with a
ball-to-powder weight ratio of 20:1. At selected times,
small amounts of powder were successively taken out of
the vial in order to characterize the sample.
TiO2 possesses many interesting characteristics, and has
The structural evolution during milling was detected by
X-ray diffraction (XRD) analysis in a Rigaku X-ray
diffractometer equipped with graphite monochromator and
Cu Ka radiation. The hydrogen content in the samples
*Corresponding author.
E-mail address: amwang@imr.ac.cn (P. Wang).
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PII: S0925-8388(00)01188-9
2000 Elsevier Science B.V. All rights reserved.

P. Wang et al. / Journal of Alloys and Compounds 313 (2000) 218–223
219
ground for different periods was characterized by thermo-
gravimetric analysis (TGA), which was carried out by
Perkin-Elmer TGA-7 under a flowing argon atmosphere in
a heating rate of 10 K/min. The hydriding properties of
composite were measured at different temperatures or
under different hydrogen pressures using a conventional
volumetric method. The topography and morphology of
the composite particles were characterized by using TEM
(Philips EM420).
tensity milling. Then under the action of the catalyst TiO2
and the mechanical driving force, the hydrided phase
began to form. The diffraction pattern corresponding to
MgH2 became visible for the sample ground for 0.5 h, and
the pattern of g-MgH2 after 4 h. The MgH2 and g-MgH2
formed was all nanocrystalline. The reason for the forma-
tion of the high-pressure phase g-MgH2 has been reported
in our previous work [24].
To further investigate the hydriding course of Mg during
RBM, TGA was carried out for the samples ground for
different periods. Fig. 2 shows the relationship between the
hydrogen content in the samples and milling time. Because
a dynamic hydrogenation equilibrium exists in the RBM
process, the hydrogen content in the sample almost satu-
rated with grinding times longer than 5 h, and finally
reached up to about 3.0 wt.%.
3. Results
3.1. Direct hydrogenation of Mg during RBM
Rutile is a stoichiometric oxide of the type TinO2n21
(where n→`) [22]. Annealing TiO2 in vacuum or in a
hydrogen atmosphere at high enough temperatures will
produce semiconducting n-type rutile [19]. Hydrogen
atoms, and even hydrogen molecules can diffuse along the
c-axis in n-TiO2, and the former can reach a rather rapid
rate: D5231028 cm2 s21 at 4008C for the tritium isotope
[23]. For the above-mentioned reason, TiO2 was annealed
at 4008C under hydrogen atmosphere before RBM.
In Fig. 1 the XRD patterns of the Mg–TiO2 powders
milled for different periods are shown. High-intensity
milling resulted in the rapid formation of nanostructured
TiO2. For Mg, the reduced peak intensity and peak
broadening should be attributed to grain refinement, con-
sumption of Mg in the Mg–H reaction and the introduction
of micro-strains. It could be reasonably deduced that the
composite structure was achieved rapidly under high-in-
3.2. Microstructure characteristics
Fig. 3 shows the TEM result of the composite particles
ground for 6 h. During TEM observation, the action of the
electron beam resulted in the decomposition of the formed
hydride. The homogeneous dispersion of TiO2 with an
average grain size of 10 nm and the nanostructured Mg
with an average grain size of about 20 nm can be directly
observed from Fig. 3c and 3d, respectively.
3.3. Hydrogenation characteristics
After being dehydrided at 3508C, the as-prepared sample
needs no activation for rapid hydrogen absorption. The
composite possessed superior kinetics and a high H-capaci-
ty. Under the condition of 3508C and 2.0 MPa H2, the
Fig. 1. XRD patterns of Mg–TiO2(rutile) ground under hydrogen atmosphere for various times.

220
P. Wang et al. / Journal of Alloys and Compounds 313 (2000) 218–223
Fig. 2. Hydrogen content as a function of milling time for the composite Mg–TiO2 under hydrogen atmosphere.
Fig. 3. TEM morphology and electron diffraction pattern of the composite Mg–TiO2 bright field image electron diffraction pattern dark field image
corresponding to TiO2 dark field image corresponding to Mg.

P. Wang et al. / Journal of Alloys and Compounds 313 (2000) 218–223
221
Fig. 4. Hydrogen absorption kinetics for the composite Mg–TiO2 at the temperature range of 140–2008C under 2.0 MPa hydrogen pressure.
H-capacity reached up to 4.7 wt.%, and only 2 min were
needed for the accomplishment of 80% H-absorption. Even
in the moderate temperature range 140–2008C, the compo-
site still absorbed significant amounts of hydrogen at a
relatively high rate (shown in Fig. 4). To further investi-
gate the hydriding characteristics of the composite at
moderate temperature, H-absorption measurements were
carried out at 1608C under different hydrogen pressures.
As shown in Fig. 5, hydriding kinetics showed no obvious
dependence on hydrogen pressure, especially when the
pressure was higher than 1.0 MPa. On the desorption side,
the composite also exhibited rather high rate. As shown in
Fig. 6, H-desorption to 90% of the steady state could be
accomplished within about 10 min in the temperature
range 330–3508C.
The composite also possessed excellent oxidation-resist-
ance. After exposure in air for 1 week, the composite could
absorb hydrogen rapidly at 2008C even from the first cycle.
3.4. Hydrogenation mechanism analysis
The addition of TiO2 resulted in a markedly improved
hydrogenation performance of Mg. As regards the reaction
mechanism, Khrussanova et al. [21] once suggested that
Fig. 5. Hydrogen absorption kinetics for the composite Mg–TiO2 at 1608C under different hydrogen pressures.

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P. Wang et al. / Journal of Alloys and Compounds 313 (2000) 218–223
Fig. 6. Hydrogen desorption kinetics for the composite Mg–TiO2 at the temperature range of 320–3508C under 0.1 MPa hydrogen pressure.
during the processes of mechanical grinding and hydriding,
a partial reduction of the added oxide leads to the
formation of the corresponding transition metal clusters,
that facilitate the hydrogenation reaction of Mg. But in the
present work, no experimental evidence was detected for
the formation of metallic Ti clusters during the RBM
process and hydriding cycling. The existence of n-TiO2,
and concretely the above-mentioned characteristics of n-
TiO2 was considered to be the main reason for the
favorable performance of the composite.
grain boundary regions, associated with the enhanced
solubility in the nanocrystalline microstructure, facilitates
the diffusion of hydrogen atoms in the Mg matrix and
accelerates the hydrogenation reaction. Additionally, ball
milling creates an ideal microstructure suitable for hydro-
genation. The large surface-to-volume ratio, as well as
significant amounts of defects and imperfections in the
surface and bulk of the composite enhance the accessibility
for hydrogen penetration.
Based on previous research, we believe that the achieve-
ment of an ideal catalytic function is not only due to the
characteristics of the catalyst itself, but is also due to its
distribution state. Generally, the high ductility of mag-
nesium will impede formation of a desirable composite
structure. In the present work, the use of the RBM method
has overcome the problem effectively by forming the
brittle Mg hydride, thus ensuring a high-degree dispersion
of the catalyst TiO2. A high density of TiO2 islands on the
surface of the particles would accelerate the initial hydro-
genation of Mg. The dissociated and dissolved hydrogen
atoms can diffuse through TiO2 to the Mg–TiO2 interface
and enter the Mg matrix. In the following stage, TiO2
particles (dispersing homogeneously in the bulk of the Mg
particles) would provide the rapid diffusion passageway for
hydrogen atoms, thus avoiding the diffusion of hydrogen
atoms through the already formed hydrided layer which is
generally accepted to be the most difficult step in the
hydrogenation of Mg.
4. Conclusion
RBM is an effective method for the preparation of
Mg–TiO2 (rutile) composite. During RBM, Mg was hy-
drided into MgH2 and g-MgH2 directly under the com-
bined effects of the catalyst TiO2 and the mechanical
driving force. The formation of the hydrided phase ensures
a high-degree dispersion of the TiO2. The favorable
catalytic characteristics of TiO2 and the ideal microstruc-
ture of the composite determine the superior hydriding/
dehydriding performance. The composite Mg–TiO2 (rutile)
possesses a high H-storage capacity, shows rapid kinetics,
and requires only a low working temperature. This compo-
site has furthermore excellent oxidation-resistance.
Acknowledgements
Another important reason probably lies in the nano-
structure of Mg. The broad energy distribution of the
available sites for hydrogen atoms in the disordered nano-
The authors acknowledge gratefully the financial support
of the National Natural Science Foundation of China.

P. Wang et al. / Journal of Alloys and Compounds 313 (2000) 218–223
223
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