Long term cycling behavior of titanium doped NaAlH4 prepared through solvent mediated milling of NaH and Al with titanium dopant precursors

Article abstract of DOI:10.1016/j.jallcom.2004.01.044

A simple and an efficient synthesis route, solvent mediated milling of NaH and Al with 2mol% of the dopant precursor, Ti(OBu)₄ followed by hydrogenation, has been developed and employed to synthesize Ti-doped NaAlH ₄. The long-term hydrogenation and dehydrogenation, up to 100cycles were carried out systematically. Reversibility of about 3.4wt.% hydrogen release was obtained during the first dehydrogenation (160°C) run after the initial hydrogenation of Ti-doped (NaH+Al) at 150°C; ～11.4MPa H ₂ for 12h. In the subsequent cycles, the storage capacity increased, reaching an optimum of 4.0wt.%. This capacity was retained for 40cycles with the dehydrogenation kinetic curves showing remarkable reproducibility. Comparison of the X-ray diffraction profiles of Ti-doped (NaH+Al) from initial and final stages of the cycling study reveals a growing resistance to the hydrogenation of Na₃AlH₆ to NaAlH₄.

Full text of DOI:10.1016/j.jallcom.2004.01.044

Journal of Alloys and Compounds 377 (2004) 283–289
Long term cycling behavior of titanium doped NaAlH4 prepared
through solvent mediated milling of NaH and Al
with titanium dopant precursors
Sesha S. Srinivasana, Hendrik W. Brinksb, Bjorn C. Haubackb,
Dalin Suna,1, Craig M. Jensena,∗
a
Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, HI 96822, USA
b
Department of Physics, Institute for Energy Technology, P.O. Box 40, N-2027 Kjeller, Norway
Received 6 January 2004; accepted 22 January 2004
Abstract
A simple and an efficient synthesis route, solvent mediated milling of NaH and Al with 2 mol% of the dopant precursor, Ti(OBu)4 followed
by hydrogenation, has been developed and employed to synthesize Ti-doped NaAlH4. The long-term hydrogenation and dehydrogenation, up
to 100 cycles were carried out systematically. Reversibility of about 3.4 wt.% hydrogen release was obtained during the first dehydrogenation
(160 ◦C) run after the initial hydrogenation of Ti-doped (NaH + Al) at 150 ◦C; ∼11.4 MPa H2 for 12 h. In the subsequent cycles, the storage
capacity increased, reaching an optimum of 4.0 wt.%. This capacity was retained for 40 cycles with the dehydrogenation kinetic curves
showing remarkable reproducibility. Comparison of the X-ray diffraction profiles of Ti-doped (NaH + Al) from initial and final stages of the
cycling study reveals a growing resistance to the hydrogenation of Na3AlH6 to NaAlH4.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Hydrogenation; Dehydrogenation; Cycling stability; Solvent mediated milling; Hydrogen storage capacity; Sodium aluminum hydride; Hydrogen
absorbing materials; Metal hydrides; Mechanochemical processing; Gas–solid reactions
1. Introduction
The decomposition reactions of Ti-doped NaAlH4 to
NaH + Al + 3/2H2 with intermediate stage 1/3Na3AlH6 +
2/3Al + H2 have been investigated extensively on the basis
of structural phase determination [10,16,34–37] and the
release of hydrogen [3–8,11,17–21,28,38]. However, fewer
studies have focused on the hydrogenation of mixtures
of Na/Al or NaH/Al to NaAlH4. Ashby and co-workers
[39–41], accomplished the synthesis of NaAlH4 by react-
ing sodium under high hydrogen pressure (10–35 MPa) and
temperature (140–160 ◦C) with an aluminum alkyl catalyst
in the solvent, tetrahydrofuran (THF), for several hours. Dy-
mova et al. [42] found that, in absence of solvent medium,
a temperature of at least 280 ◦C and a hydrogen pressure of
17–18 MPa were necessary to accomplish this transforma-
tion. The high temperature was required to attain a liquid
state of sodium. Bogdanovic and Schwickardi [43] success-
fully demonstrated the preparation of Ti-doped NaAlH4 by
hydrogenating (∼33 cycles) NaH powder and Al grindings
with Ti(OBu)4 as the dopant precursor. However, their syn-
thesis process involves a complicated purification procedure
Alkali and alkaline earth metal based complex aluminum
hydrides, MAlH4 [M = Na, Li, K] and Mg(AlH4)2, have
been found to have great potential as viable modes of stor-
ing hydrogen at moderate temperatures and pressures. These
hydrides have been demonstrated to have higher hydrogen
storage capacities at moderate temperatures and lower cost
than conventional intermetallic metal hydride systems such
as AB5H6, ABH2, AB2H3, and A2BH4–6 [1,2]. Among the
various alkali based complex hydrides investigated in the
recent years [3–38], titanium doped sodium aluminum hy-
dride; NaAlH4, has shown the greatest promise as a re-
versible hydrogen storage material.
∗
Corresponding author. Tel.: +1-808-956-2769;
fax: +1-808-956-5908.
E-mail address: jensen@gold.chem.hawaii.edu (C.M. Jensen).
1
Present address: Department of Materials Science, Fudan University,
220 Handan Road, Shanghai 200433, PR China.
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.01.044

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S.S. Srinivasan et al. / Journal of Alloys and Compounds 377 (2004) 283–289
of NaH and Al grindings and a catalyst doping mecha-
nism prior to the hydrogenation. Recently, Majzoub and
Gross demonstrated an indirect Ti-doping method, where
TiCl2/TiCl3 dopant was pre-reacted with LiH by mechani-
cal milling [44,45]. The resulting powder was then milled
with 6 mol% of (NaH + Al) mixture. This method of pro-
ducing NaAlH4 adds extra weight to the sample due to the
addition of LiH. Also, they did not report the long-term
cycling behavior of the dopant precursors. For an ideal hy-
drogen storage material, the cycling stability and the overall
weight of the sample must be evaluated, but work on these
aspects is sparsely reported. Moreover, the role of titanium
dopant on the cycling characteristics of NaAlH4 is not easily
intelligible.
We therefore have explored means of improving
Bogdanovic’s preparation of Ti-doped NaAlH4 through
simple milling of NaH and Al with Ti(OBu)4. Our approach
resembles Ashby’s simple and efficient, solvent mediated
hydrogenation of mixtures of NaH and Al, that is basis
of the commercial manufacture of NaAlH4 by Albemarle
Corp. However, our process is mediated by the presence
of only a minimal amount of solvent during milling with
the dopant precursor and does not require an alkyl alu-
minum catalyst. The dehydrogenation performance in the
first few cycles of the Ti-doped NaAlH4 prepared through
this process is markedly improved compared to the ma-
terials obtained through dry milling route. We have also
carried out the hydrogenation and dehydrogenation up to
100 cycles and the kinetic features were correlated with the
observed structural characteristics of Ti-doped (NaH + Al)
in the initial and final stages of cycling.
a syringe, was carried out and this blended mixture was
milled in a same manner described above for only 5 min.
The Ti-doped (NaH + Al) mixture thus obtained was dried
in the glove box and then loaded (0.5 g) in a stainless steel
reactor for hydrogenation and dehydrogenation studies. For
the dry milling process, (NaH + Al) mixture was blended
with Ti(OBu)4 and milled under a nitrogen atmosphere for
2 h with the same milling parameters mentioned above.
2.2. Hydrogenation and dehydrogenation cycling
measurements
The hydrogenation and dehydrogenation cycling was per-
formed using a Sieverts type volumetric apparatus [LESCA
Co., Japan], which allowed for the accurate volumetric
determination of the amount of hydrogen evolved. Rapid
heating of the sample to the desired temperature was accom-
plished by immersing the sample reactor into a pre-heated
silicon oil bath (accuracy of 1 ◦C). The cycling studies
were performed by repeated hydrogenation (150–120 ◦C,
∼11.5 MPa, 12 h) and dehydrogenation (160 ◦C, against
0.1 MPa for 3 h). The hydrogen capacity is presented in
terms of wt.% and normalized to the weight of NaAlH4
without including the weight of the catalyst.
2.3. Structural (PXD) characterization
The powder X-ray diffraction of Ti-doped (NaH +
Al) before and after 100 cycles were performed using
Swiss-Norwegian beam line (station BM1B) at the European
Synchrotron Radiation Facility (ESRF), Grenoble, France.
The sample was loaded in a 0.5 mm boron–silica–glass
capillary. The wavelength of the X-ray used was 0.50024 Å
at 22 ◦C from channel-cut Si(1 1 1) monochromator. The
measurement of intensity was done with the step scan rate
of ꢀ(2θ) = 0.003–0.005◦. Rietveld refinement of the X-ray
diffraction profiles of the Ti-doped (NaH + Al) was carried
out using Fullprof software [46].
2. Experimental details
2.1. Materials and method
Titanium butoxide Ti(OBu)4, NaH (95% dry) and Al
(99.95%, −200 mesh) were obtained from Aldrich Chemi-
cal Co. and used as received. Sodium hydride and aluminum
(1:1 mole ratio) were loaded into a chrome–nickel stain-
less steel bowl (250 ml) with balls (10 mm) under nitrogen
filled glove box. The weight ratio between the balls and
the powder was 20:1. Approximately 100 ml of distilled
pentane was injected into the bowl and the lid was cov-
ered tightly with foil. The assembly was then transferred
to Fritsch Pulverisette 6 planetary mill. The milling was
carried out at a speed of 300 rev/min with two rotations
of forward and reverse directions for 30 min each, totaling
60 min. Immediately after the completion of milling, the
bowl containing the milled sample was transferred to the
glove box and filtered to separate the (NaH + Al) mixture
from the solvent under nitrogen ambient. The yield of the
milled sample obtained was ∼95% in the form of slurry.
The successive step of adding and blending 2 mol% of
Ti(OBu)4 [called as Ti-doped hereafter] to the slurry, using
3. Results
3.1. Dehydrogenation kinetics and cycling stability
The experimental parameters, such as the pressure and
temperature of hydrogenation and dehydrogenation temper-
ature with the hydrogen capacity obtained, are listed in
Table 1.
Fig. 1 represents the dehydrogenation kinetic curves
of Ti-doped NaAlH4, prepared from the solvent mediated
milling and hydrogenation of Ti-doped (NaH + Al). These
curves were plotted by calculating the total release of hy-
drogen in terms of weight percentage with respect to the
dehydrogenation time. The release of hydrogen at 160 ◦C
under 0.1 MPa corresponds to 3.4 wt.% during the initial
dehydrogenation run. In the subsequent cycles, the hydro-

S.S. Srinivasan et al. / Journal of Alloys and Compounds 377 (2004) 283–289
285
Table 1
Total released hydrogen capacity with ongoing hydrogenation and dehydrogenation cycling of Ti-doped NaAlH4, prepared through solvent mediated
milling and subsequent hydrogenation of Ti-doped (NaH + Al)
Cycle(s) number
Hydrogenation
pressure (MPa)
Hydrogenation
temperature (◦C)
Dehydrogenation
temperature (◦C)
Hydrogen
capacity (wt.%)
1–4
5–40
41–100
∼11.50
∼11.40
∼11.35
150
120
120
160
160
160
3.74a
4.0a
3.5a
a
Average value.
gen capacity increased attaining an optimum capacity of
4.0 wt.% in the fourth dehydrogenation run. This value
corresponds to 70% of the hydrogen capacity calculated
according to Eq. (1):
This capacity was retained with dehydrogenation kinetic
curves showing remarkable reproducibility. The two-step de-
composition is indicated in Fig. 3 by a broken bifurcated line.
The amount of hydrogen discharged during the 100 cycles
of these kinetic studies of Ti-doped NaAlH4 is shown in
Fig. 4. It is to be mentioned that the storage capacity of
3.4 wt.% was obtained in the very first dehydrogenation run
and increased with increasing number of cycles. An op-
timum hydrogen capacity of about 4.0 wt.% was obtained
NaH + Al + 3/2H2 ↔ NaAlH4
(1)
Also, the release of hydrogen occurs in two stages, which is
represented by a bifurcated broken line as shown in Fig. 1.
This clearly indicates that the dehydrogenation of (Ti-doped)
NaAlH4 follows the known two-step reactions given by
Eq. (2):
2.5
4
NaAlH4 ↔ 1/3Na3AlH6 + 2/3Al + H2
3
2
↔ NaH + Al + 3/2H2
(2)
2
The release of hydrogen in the first four-dehydrogenation
runs of the Ti-doped NaAlH4, obtained from the dry milling
of Ti-doped (NaH + Al) is shown in Fig. 2. The dehydro-
genation kinetics and the total release of hydrogen from
the Ti-doped NaAlH4 obtained from the solvent mediated
milling process (see Fig. 1) was markedly improved com-
pared to the material prepared through dry milling route (see
Fig. 2).
1.5
1
1
0.5
0
The dehydrogenation kinetic curves of Ti-doped NaAlH4
at 160 ◦C between the 5th and 40th cycles are shown in
Fig. 3. It is interesting to note that the storage capacity of
about 4.0 wt.% was maintained on an average in these cycles.
0
50
100
150
200
Time, min
Fig. 2. The dehydrogenation kinetic curves of Ti-doped NaAlH4 as pre-
pared from the dry milling of Ti-doped (NaH + Al) at 160 ◦C, 0.1 MPa.
4.5
4.5
4
4
2
4
3
1
3.5
3.5
3
2.5
2
3
5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,
21,22,23,24,25,26,27,28,29,30,31,32,33,34,
35,36,37,38,39,40
2.5
2
1.5
1
1.5
1
0.5
0
0.5
0
0
50
100
150
200
0
50
100
150
200
Time, min
Time, min
Fig. 1. The dehydrogenation kinetic curves of Ti-doped NaAlH4 as pre-
pared from the solvent mediated milling of Ti-doped (NaH+Al) at 160 ◦C,
0.1 MPa.
Fig. 3. The group of dehydrogenation kinetic curves of Ti-doped NaAlH4
from 5 to 40 cycles at 160 ◦C, 0.1 MPa.

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S.S. Srinivasan et al. / Journal of Alloys and Compounds 377 (2004) 283–289
5
4.5
4
4.5
4
4th run
3.5
100th run
3.5
3
3
4
4th
100th
2.5
2.5
2
3
2
2
1.5
1
1.5
1
1
0.5
0
0
0
20
40
60
0.5
Time, min
0
0
20
40
60
80
100
Cycle number
0
50
100
Time, min
150
200
Fig. 4. The release of hydrogen (wt.%) vs. number of cycles.
Fig. 5. Comparative kinetic curves of 4th and 100th dehydrogenation runs
of Ti-doped NaAlH4. The small scale drawing of the graph is represented
in the inset.
in the fourth dehydrogenation run. A capacity of 4.3 wt.%
was achieved in 29th cycle and an average total capacity
of 4.0 wt.% was maintained up to 40 cycles. At the end of
the 100th cycle, the hydrogen capacity was slightly reduced
from 4.0 to 3.5 wt.%. Comparative dehydrogenation kinetic
curves of 4th and 100th cycles are depicted in Fig. 5. The
junction between the two-step dehydrogenation reactions for
these extreme cycles was marked by an arrow and also shown
with smaller scale in the inset of Fig. 5.
Ti-doped (NaH + Al) before and after 100 cycles. Fig. 6
represents the PXD pattern of the Ti-doped (NaH + Al)
before undergoing hydrogenation and dehydrogenation cy-
cling experiments. The Rietveld profile fitting (refinement)
was employed and the peaks were identified as NaH and
Al. No other impurities or Ti/Ti–Al inflection peaks were
traced out. The concentration, in mol%, of each phase and
their corresponding lattice parameters obtained from the
refinement are given in Table 2.
3.2. Structural (PXD) characterization
Powder X-ray diffraction characterizations using syn-
chrotron radiation were carried out on the samples of
The PXD fitting profile of Ti-doped (NaH + Al) at the
100th dehydrogenation cycle is shown in Fig. 7. An addi-
40000
35000
30000
25000
20000
15000
10000
5000
+
NaH
Al
0
-5000
7
12
17
2 θ (degrees)
22
27
32
Fig. 6. PXD profile fitting of the Ti-doped (NaH + Al) mixture before hydrogenation–dehydrogenation cycling. The solid circle and the line are the
observed and calculated intensities, respectively. The difference pattern is shown at the bottom.

S.S. Srinivasan et al. / Journal of Alloys and Compounds 377 (2004) 283–289
287
Table 2
The phase composition in mol% and corresponding lattice constant values determined using synchrotron powder X-ray diffraction and Rietveld profile
refinement
Sample
Concentration (mol%)
Lattice constants (Å)
a
NaH
Al
Na3AlH6
8.7
NaCl
1.3
NaH
Al
Na3AlH6
NaCl
Ti-doped NaH + Al before cycling
Ti-doped NaH + Al after 100 cycling
57.1
35.3
42.9
54.6
4.88136
4.88003
4.04991
4.04904
5.41014
5.53615
7.75615
5.64154
a
Unit cell volume of Na3AlH6: 232.306 Å3.
46500
NaH
Al
Na3AlH6
+
NaCl
36500
26500
16500
6500
-3500
9.6
14.6
19.6
2θ (degrees)
24.6
29.6
Fig. 7. PXD pattern of Ti-doped (NaH + Al) after the completion of 100 cycles.
tional reflection corresponds to Na3AlH6 and NaCl was ob-
served in addition to NaH and Al. The amount of NaCl or
NaCl-related structure was very small (less than 2 mol%),
and hence it is negligible from the present discussion.
release estimated was 3.4 wt.% (Fig. 1). In the consecutive
hydrogenation and dehydrogenation cycles, the capacity was
increased and attained an optimum of 4.0 wt.% in the 40th
cycle. The contribution of the second reaction as per Eq. (2),
i.e. Na3AlH6 to NaH, to the total effective hydrogen capac-
ity was found in these cycles (Fig. 3). The storage capacity
decreased from 4.0 to 3.5 wt.%, between the 40th and 100th
cycle (Fig. 4). This results from the partial reversibility of the
two-step reaction, which is in accordance with the hypothe-
sis proposed by Kiyobayashi et al. [38]. According to their
theory, the delocalization of titanium catalyst may be the
cause for the poor dehydrogenation rate of Na3AlH6, which
in turn affects the overall hydrogen capacity. The compara-
tive dehydrogenation curves of Fig. 5 reveal the following
features: (i) the kinetics of the first step of dehydrogenation
reaction has been reduced; (ii) the second dehydrogenation
reaction proceeds to a significantly reduced extent. The PXD
4. Discussion
The long term cycling studies of Ti-doped NaAlH4 and
its structural correlation were discussed in this section. The
PXD pattern of the elemental mixture Ti-doped (NaH + Al)
shows no extra reflections of impurities and ensures the cor-
rect stoichiometric ratio of NaH:Al (see Table 2). This mix-
ture was hydrogenated under high-pressure and the cycling
experiments were further carried out. The initial dehydro-
genation of Ti-doped NaAlH4 at 160 ◦C exhibits the fast ki-
netics of the first and second steps and the total hydrogen

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S.S. Srinivasan et al. / Journal of Alloys and Compounds 377 (2004) 283–289
studies of the Ti-doped (NaH + Al) after 100 cycles show
the presence of an additional phase related to the Na3AlH6
structure besides the starting mixture (NaH +Al). The pres-
ence of Na3AlH6 indicates that the reaction of Eq. (2) is not
complete and also supports the argument of Majzoub and
Gross [44]. However, formation of crystalline Ti or TiAl3
intermetallic phase was not observed in this 100-cycle mea-
surement. This is in agreement with the recent finding that
after 7 cycles, ∼ 1/3 of the Ti is present as Al0.93Ti0.07 and
that the remainder of the Ti is apparently present in a highly
dispersed fashion in the host lattice [47].
nologies of the US Department of Energy and the New En-
ergy and Industrial Technology Development Organization
(NEDO) as a part of the International Clean Energy Net-
work using Hydrogen Conversion (WE-NET) program with
funding from the Ministry of Economy, Trade and industry
of Japan.
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A simple and efficient synthesis method, solvent medi-
ated milling and hydrogenation, was developed and em-
ployed in order to prepare Ti-doped NaAlH4 from the basic
materials, NaH and Al in the presence of Ti(OBu)4. Unlike
Bogdanovic’s approach [43], this method does not involve
complicated purification of NaH/Al and doping procedures
prior to hydrogenation. Also our synthesis requires no
precursor formation, which was earlier adopted by Gross
et al. [45]. The dehydrogenation kinetic measurements of
Ti-doped NaAlH4, obtained from solvent mediated milling
and subsequent hydrogenation of Ti-doped (NaH + Al)
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of hydrogen capacity was obtained in the very first de-
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increased and optimum hydrogen storage of about 4.0 wt.%
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