Rehydrogenation of dehydrogenated NaAlH4 at low temperature and pressure

Article abstract of DOI:10.1021/jp030637f

NaAlH₄ is considered as a promising material for onboard storage of hydrogen. It is generally thought that high temperature (> 120?°C) and high pressure (> 15 MPa) are needed for rehydrogenation of dehydrogenated NaAlH₄. However, for practical application, reducing the rehydrogenation temperature and pressure as much as possible would be highly desirable. We have found that the dehydrogenated NaAlH₄, in either the undoped or titanium-doped state, can be rehydrogenated at temperatures of 25-120?°C and pressures of 2-12 MPa. This finding is confirmed by the subsequent dehydrogenation at 160?°C. In addition to that, after rehydrogenation under the same conditions, the amount of evolved hydrogen from the titanium-doped NaAlH₄ was much higher than that of the undoped NaAlH₄. This clearly indicates that the addition of titanium species enhances kinetically not only the dehydrogenation, but also the reverse process, the rehydrogenation reaction of the dehydrogenated NaAlH₄.

Full text of DOI:10.1021/jp030637f

10176
J. Phys. Chem. B 2003, 107, 10176-10179
Rehydrogenation of Dehydrogenated NaAlH4 at Low Temperature and Pressure
Dalin Sun,*,† Sesha S. Srinivasan,† Tetsu Kiyobayashi,‡ Nobuhiro Kuriyama,‡ and
Craig M. Jensen†
Department of Chemistry, UniVersity of Hawaii, Honolulu, Hawaii 96822, and National Institute of AdVanced
Industrial Science and Technology, 1-8-31 Midorigaoka, Osaka 563-8577, Japan
ReceiVed: May 21, 2003; In Final Form: July 11, 2003
NaAlH4 is considered as a promising material for onboard storage of hydrogen. It is generally thought that
high temperature (>120 °C) and high pressure (>15 MPa) are needed for rehydrogenation of dehydrogenated
NaAlH4. However, for practical application, reducing the rehydrogenation temperature and pressure as much
as possible would be highly desirable. We have found that the dehydrogenated NaAlH4, in either the undoped
or titanium-doped state, can be rehydrogenated at temperatures of 25-120 °C and pressures of 2-12 MPa.
This finding is confirmed by the subsequent dehydrogenation at 160 °C. In addition to that, after rehydrogenation
under the same conditions, the amount of evolved hydrogen from the titanium-doped NaAlH4 was much
higher than that of the undoped NaAlH4. This clearly indicates that the addition of titanium species enhances
kinetically not only the dehydrogenation, but also the reverse process, the rehydrogenation reaction of the
dehydrogenated NaAlH4.
Introduction
In the absence of a solvent medium, a temperature of 280 °C
and a hydrogen pressure of 17-18 MPa are necessary.23
Obviously, these conditions are too extreme to be realized for
practical applications, therefore, a reduction of rehydrogenation
temperature and pressure would be much preferable.
The dehydrogenation or the decomposition of sodium alu-
minum hydride (NaAlH4) consists of two consecutive steps that
occur at about 220 °C and 250 °C, respectively,
Recently, Bogdanovic et al.24 reported that the small addition
of the titanium species, Ti(OBu)4, to mixtures of NaH and Al
can reduce the rehydrogenation temperature to 120 °C. However,
hydrogen pressure as high as 15 MPa was still required. In our
effort toward studying the reaction mechanism, we accidentally
found that the rehydrogenation of the dehydrogenated NaAlH4
could be achieved even at temperatures as low as 25 °C. Further
studies confirmed this finding, and reaction characteristics below
120 °C and 15 MPa were systematically studied. The results
obtained on both pristine and titanium-doped NaAlH4 are
reported in this paper.
1
3
2
3
NaAlH4 f Na3AlH6 + Al + H2
(1)
and
1
3
1
3
1
2
Na3AlH6 f NaH + Al + H2
(2)
Reaction 1 gives off about 3.7 wt % hydrogen and reaction 2,
1.9 wt %, with a total hydrogen release of 5.6 wt % for the
overall reaction. In 1997, Bogdanovic et al.1 discovered that
the onset temperatures for reactions 1 and 2 were reduced to
100 °C and 140 °C, respectively, upon doping the hydride with
a few mole percent of selected transition metal complexes. This
has invoked intense attention and investigation in the area of
hydrogen storage materials.2-17 The superiority of NaAlH4 over
conventional metal hydrides (e.g., LaNi5H6, TiFeH2, and Mg2-
NiH4) is in its lightweight and high hydrogen storage capacity
per unit weight, which makes it more promising for application
in fuel cell vehicles.18
Effects of catalyst-doping on the dehydrogenation of NaAlH4
have been studied extensively in recent years.2-17,19 However,
the rehydrogenation, i.e., the reverse of reactions 1 and 2, has
not received as much attention. It is generally thought that severe
conditions, such as high temperature and high pressure, are
needed for the rehydrogenation. According to Ashby et al.,20-22
the reverse reactions of (1) and (2) could only occur at 150 °C
under a hydrogen pressure of 10-35 MPa in a solvent medium.
Experimental Section
Materials and Sample Preparation. Raw NaAlH4 was
obtained from Aldrich Chemical. Aluminum residual present
in the raw NaAlH4 was removed by filtration following
dissolution of the hydride in a minimal amount of tetrahydro-
furan (THF). Analytically pure NaAlH4 was precipitated by
addition of pentane to the THF solution. The hydride was then
isolated by filtration, washed with several portions of pentane,
and dried at 50 °C for 24 h under high vacuum. Two samples
were used in the present studysone was undoped NaAlH4, and
the other was NaAlH4 doped with 2 mole percent of titanium
butoxide (Ti(OBu)4, Aldrich Chemical), referred to as Ti-doped
NaAlH4 hereafter. The doping process was realized by ball-
milling, which has been described in detail elsewhere.19
Dehydrogenation and Rehydrogenation. Both dehydroge-
nation and rehydrogenation were carried out volumetrically with
a Sievert’s-type apparatus (LESCA Co. Japan) that was carefully
calibrated for hydrogen absorption and desorption measurements
by using LaNi5 alloy as a standard. This ensured the reliability
in the measured data for NaAlH4 in our experiments. The
* Author to whom correspondence should be addressed. E-mail: dlsun@
gold.chem.hawaii.edu.
† University of Hawaii.
‡ National Institute of Advanced Industrial Science and Technology.
10.1021/jp030637f CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/26/2003

Rehydrogenation of Dehydrogenated NaAlH4
J. Phys. Chem. B, Vol. 107, No. 37, 2003 10177
Figure 1. Procedure used for rehydrogenation and dehydrogenation
after the first dehydrogenation.
Figure 3. The dehydrogenation isotherms of (a) the undoped NaAlH4
at 160 °C after rehydrogenated at various temperatures (25, 40, 50,
70, 100, and 120 °C, i.e., a-f) under pressure of 12 MPa for 12 h. (b)
Redrawing of (a) using a shorter time scale. The arrows indicate the
separation point between the two dehydrogenation steps.
Figure 2. The dehydrogenation isotherms of (a) undoped NaAlH4 at
250 °C, and (b) Ti-doped NaAlH4 at 160 °C. Two distinct dehydro-
genation steps corresponding to reactions 1 and 2, respectively, were
shown.
kinetics, resulting from the addition of titanium species, was
observed. As compared to the undoped NaAlH4, the hydrogen
evolution from the Ti-doped sample was much faster, i.e.,
around 95% of the hydrogen was released from the Ti-doped
NaAlH4 in 40 min. The Ti-doped NaAlH4 released 3.8 wt %
hydrogen, about 1.7 wt % less than that of the undoped sample,
which can be attributed to a partial release of hydrogen during
the doping process. Note that the undoped NaAlH4 released 5.5
wt % hydrogen, very close to the calculated value of 5.6 wt %
from reactions 1 and 2. This confirms that the measurement
system used in this study is reliable.
Rehydrogenation and Dehydrogenation. The dehydroge-
nated samples were subjected to rehydrogenation at 25-120
°C under a pressure of 12 MPa for 12 h. Unfortunately, the
amount of hydrogen uptake with reaction time could not be
calculated accurately due to scattering in the recorded pressure
data under such a high-pressure range. Therefore, as an
alternative, the extent of hydrogen uptake was inferred from
the amount of hydrogen released in the subsequent dehydro-
genation at 160 °C in 5 h.
samples of 0.5 g were loaded into stainless steel reactors in a
nitrogen-filled glovebox (the oxygen content was 0.2 ppm, the
dew point was -76 °C), and the reactors were then connected
to the apparatus. After being degassed under vacuum at room
temperature for 2 h, the samples were first fully dehydrogenated
at 160 °C for the doped NaAlH4 and 250 °C for the undoped
NaAlH4. The amount of evolved hydrogen as a function of time
was measured by rapidly heating the samples up to the desired
temperature. This was realized by immersing the sample reactor
into an oil bath (with an accuracy of (1 °C), which was
preheated to a given temperature. After the first dehydrogena-
tion, the samples were subjected to rehydrogenation and
dehydrogenation in turn, by following the procedure as described
in Figure 1. The temperature range for rehydrogenation was
25-120 °C, and the pressure range was 2-12 MPa. Hydrogen
gas with a purity of 99.9999% was used. During these
measurements, the samples were not removed from the reactors
until they were completed.
Results and Discussion
Figure 3 a shows the evolution of hydrogen as a function of
time for the undoped NaAlH4. As the rehydrogenation temper-
ature increased from 25 °C to 120 °C, the maximum amount of
released hydrogen increased from 0.02 wt % to 0.07 wt %.
Although these values are very low, they do indicate that the
reverse reactions of (1) and/or (2) occur at 25-120 °C. This
conclusion becomes more evident when Figure 3a is redrawn
Comparison of the First Dehydrogenation. The amount of
evolved hydrogen as a function of time is shown in Figure 2,
where two distinct steps can be identified. The first dehydro-
genation step corresponds to the contribution of reaction 1, and
the second step corresponds to reaction 2. In agreement with
earlier reports,1,5,7,9 an enhancement in the dehydrogenation

10178 J. Phys. Chem. B, Vol. 107, No. 37, 2003
Sun et al.
Figure 5. The dehydrogenation isotherms of (a) the Ti-doped NaAlH4
at 160 °C after rehydrogenated at 70 °C under various pressures (2, 4,
6, 8, and 10 MPa, i.e., a-e) for 12 h. (b) Redrawing of (a) using a
shorter time scale. The arrows indicate the separation point between
the two dehydrogenation steps.
the Ti-doped sample, two dehydrogenation steps can be seen
after rehydrogenation at all of the temperatures observed (see
Figure 4a,b). Moreover, the hydrogen released was about 1.0-
3.6 wt %, 50 times as much as that of the undoped NaAlH4. It
should be stated here that an additional experiment showed that
at 25 °C when the rehydrogenation time was prolonged from
12 to 100 h, the amount of released hydrogen increased up to
2 wt % (see Figure 4 c). Figure 5 represents the dehydrogenation
kinetic curves of the Ti-doped NaAlH4 after rehydrogenation
at 70 °C under various hydrogen pressures. It can be seen that,
as the rehydrogenation pressure decreased from 10 to 2 MPa,
the amount of evolved hydrogen gradually decreased, but
remained in the range of 4-10 MPa. The total amount of
released hydrogen was around 2-2.5 wt %. When the pressure
was reduced to 2 MPa, the released hydrogen was dramatically
decreased to 1 wt %.
On the basis of the results obtained above on the pristine
and the Ti-doped NaAlH4, it is concluded that titanium doping
enhances both the dehydrogenation and rehydrogenation of
NaAlH4. The nature of the catalytic effect of titanium species
on the rehydrogenation is presently unclear. One of the most
likely mechanisms is that the formation of [AlH6]3- and [AlH4]-
is facilitated by the presence of titanium species.
Figure 4. The dehydrogenation isotherms of (a) the Ti-doped NaAlH4
at 160 °C after rehydrogenated at various temperatures (25, 40, 50,
70, 100, and 120 °C, i.e., a-f) under pressure of 12 MPa for 12 h. (b)
Redrawing of (a) using a shorter time scale. The arrows indicate the
separation point between the two dehydrogenation steps. (c) The
dehydrogenation curve at 160 °C after rehydrogenation at 25 °C under
a pressure of 12 MPa for 100 h.
at a shorter time scale in Figure 3b. As shown in Figure 3, two
dehydrogenation steps were also observed after rehydrogenation
at 70, 100, and 120 °C. The separation points between the two
steps were indicated by arrows in the figure. When the
rehydrogenation temperature was lower than 70 °C, only one
step was observed, which means that at 25, 40, and 50 °C, the
reverse reaction of (2) occurs, whereas the reverse reaction of
(1) does not. This was further confirmed by infrared spectro-
scopic analysis on these rehydrogenated samples, showing that
the absorption band, corresponding to Na3AlH6, existed in the
range of 1320-1340 cm-1 but the typical absorption band at
∼1650 cm-1 from NaAlH4 was not observed. In the case of
Conclusions
In summary, we have demonstrated for the first time that the
reverse of reactions (1) and/or (2) occur at temperatures of 25-

Rehydrogenation of Dehydrogenated NaAlH4
J. Phys. Chem. B, Vol. 107, No. 37, 2003 10179
(5) Gross, K. J.; Guthrie, S.; Takara, S.; Thomas, G. J. Alloys Comp.
120 °C and pressures of 2-12 MPa for the dehydrogenated
NaAlH4 both in the undoped and in the Ti-doped forms. This
finding is confirmed by the observation of two distinct dehy-
drogenation steps after rehydrogenation. Under the same rehy-
drogenation conditions, the amount of evolved hydrogen from
the Ti-doped NaAlH4 is about 50 times as much as that of the
undoped NaAlH4. According to the present study, rehydroge-
nation may be possible at moderate conditions by optimizing
the doping materials, temperature, and hydrogen pressure.
Another important conclusion drawn from this study is that the
addition of titanium species enhances kinetically not only the
dehydrogenation process, but also the reverse rehydrogenation
reactions of (1) and (2). Further investigations are needed to
establish the underlying mechanism of why titanium species
enhance the rehydrogenation.
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