Recording of hydrogen evolution - A way for controlling the doping process of sodium alanate by ball milling

Article abstract of DOI:10.1016/j.jallcom.2003.09.146

A simple equipment was developed which allows to monitor gas evolution or absorption occurring in the course of a ball milling process. The equipment has been applied to characterize via hydrogen evolution the reduction process of metal halides used as doping agents in the preparation of sodium alanate hydrogen storage materials. The gas flow curves resulting from the measurements (Figs. 2-4) deliver useful informations concerning the rate, progress and completion of each of the involved reduction process, so that the stoichiometry of the underlying chemical reactions could be conceived or confirmed (Eqs. (3)-(7)). In the preparation of sodium alanate hydrogen storage materials, the presented method can help to find out ball milling parameters necessary to produce an optimal catalytic activity of a dopant.

Full text of DOI:10.1016/j.jallcom.2003.09.146

Journal of Alloys and Compounds 370 (2004) 104–109
Recording of hydrogen evolution—a way for controlling the doping
process of sodium alanate by ball milling
∗
∗
J.M. Bellosta von Colbe, B. Bogdanovi c´ , M. Felderhoff, A. Pommerin, F. Schüth
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany
Received 25 June 2003; received in revised form 9 September 2003; accepted 16 September 2003
Abstract
A simple equipment was developed which allows to monitor gas evolution or absorption occurring in the course of a ball milling process.
The equipment has been applied to characterize via hydrogen evolution the reduction process of metal halides used as doping agents in
the preparation of sodium alanate hydrogen storage materials. The gas flow curves resulting from the measurements (Figs. 2–4) deliver
useful informations concerning the rate, progress and completion of each of the involved reduction process, so that the stoichiometry of
the underlying chemical reactions could be conceived or confirmed (Eqs. (3)–(7)). In the preparation of sodium alanate hydrogen storage
materials, the presented method can help to find out ball milling parameters necessary to produce an optimal catalytic activity of a dopant.
©
2003 Elsevier B.V. All rights reserved.
Keywords: Sodium alanate for hydrogen storage; Ball milling; Monitoring of gas evolution (absorption)
1
. Introduction
that the doping agent during the doping procedure is being
reduced. As will be shown in the following, the rate of
hydrogen evolution and the quantity of hydrogen evolved
can give useful informations about the ball milling doping
process. We describe here a simple set-up for performing
this kind of measurements and discuss the results thus
obtained.
The ball milling of NaAlH4, NaH + Al mixtures, or
lithium alanates together with the doping agent, e.g. TiCl3,
is at present almost generally adopted as a doping method
for the preparation of catalyzed alanates for hydrogen stor-
age [1–6]. The ball milling method has also been applied for
the preparation of earth alkali metal alanates [7]. The dis-
advantage of the ball milling as a doping method, however,
is the poorly definable nature of milling procedure, so that
equipment and milling conditions (e.g. milling apparatus,
weight of milling balls relative to milled material, duration
of the milling procedure etc.) have to be purely empirically
chosen and optimized. In order to overcome this problem,
an attempt was made to characterize the ball milling process
by recording hydrogen evolution during the process. The
prerequisite for doing this is that in the course of the doping
process the doping agent is being reduced with evolution
of hydrogen. In fact, hydrogen evolution gives an evidence
2. Experimental
2.1. Description of the set-up for measuring hydrogen
evolution during ball milling
In the jacket of the 25 ml volume milling vial (lengths
5 cm, diameter 21.8 mm) of the Retsch ball-mill (model 200
MM, 30 Hz) was welded a Swagelok gas connection sup-
plied with a filtering element (Fig. 1). The connection is
reinforced by an external steel tube welded as a corset. A
K-type thermocouple (Ni–Cr–Ni) is situated in the wall of
the milling vial. The vial is connected to a 250 ml automatic
gas burette [8,9] via a standard laboratory vacuum rubber
tube. The movements of the piston of the burette and the
temperature inside the vial during milling are recorded on a
two-channel recorder.
∗
Corresponding authors. Tel.: +49-208-306-2407;
fax: +49-208-306-2980 (B. Bogdanovi c´ ). Tel.: +49-208-306-2373;
fax: +49-208-306-2995 (F. Schüth).
E-mail addresses: bogdanovic@mpi-muelheim.mpg.de
(
B. Bogdanovi c´ ), schueth@mpi-muelheim.mpg.de (F. Schüth).
0
925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2003.09.146

J.M. Bellosta von Colbe et al. / Journal of Alloys and Compounds 370 (2004) 104–109
105
Fig. 1. Experimental set-up for recording hydrogen (gas) evolution or absorption during ball milling.
2
.2. Carrying out of the measurement
added. After adding the milling ball(s) the vial is closed and
connected to set-up (Fig. 1). Good results with the above
mentioned vial were obtained for example using one stain-
less steel ball of Ø = 20 mm (32 g). During ball milling
hydrogen evolution is recorded on the paper strip of the
recorder. According to our experience, optimal results in the
In a typical experiment, 2–3 g of NaAlH4 (or of a NaH
+ Al mixture) are weighed in a glove box in the vial of the
mill, and the desired (weighed) amount of the doping agent
typically a transition metal halide or a mixture thereof)
(

1
06
J.M. Bellosta von Colbe et al. / Journal of Alloys and Compounds 370 (2004) 104–109
subsequent cycle tests are obtained when the milling time
is prolonged well above the time required for the reduction
of the dopant (see discussion). As a possible reason for er-
ror in measurements, it can happen that the gas outlet tube
becomes clogged with the finely dispersed milled material,
thus impairing of even stopping the hydrogen flow. If a doubt
concerning this exists, after the measurement the outlet tube
should be inspected and the measurement repeated.
the reaction Eq. (3) has been proposed [11].
b.m.
xNaAlH + TiCl −→ (x − 3) NaAlH + Ti
4
3
4
x≈50
+
3Al + 3NaCl + 6H2 ↑
(3)
In the present work, the described set-up (Fig. 1) has been
applied to record the hydrogen evolution during b.m. of
NaAlH4 with some titanium, zirconium and iron halides as
doping agents (Table 1).
B.m. of NaAlH4 with 2 mole% of TiCl3 (Fig. 2 (᭹))
yielded in the course of 4 h, in accordance with Eq. (3), al-
most exactly 6 mole H2/mole TiCl3. The hydrogen evolution
curve for the case of b.m. of NaAlH4 with 4 mole% of TiCl3
3
. Results and discussion
The stoichiometries of the doping reactions of Ti(OBu)4,
Bu = n-C4H9, and of Ti(OBu)4/Fe(OEt)2 combinations
with NaAlH4 has been earlier investigated in toluene via
measurement of hydrogen evolution and IR-spectra of the
reaction products. On the basis of results thus obtained, it
was concluded that the doping reactions proceed accord-
ing to Eqs. (1) and (2) [10]. At this place should be men-
tioned that Eqs. (1) and (2) as well as the following ones
(᭿) is characterized by a relatively high evolution rate dur-
ing the first 4 h, and, after a bend in the curve, by a slower
hydrogen evolution in the remaining 12 h of milling time.
The amount of hydrogen evolved until the bend in the curve
is again in good agreement with that calculated for 6 mole
H2/mole TiCl3 (Eq. (3)). The steeper part of the hydrogen
evolution curve represents apparently the process of reduc-
tion of TiCl3 to metallic Ti, while the subsequent slower hy-
drogen evolution is the result of hydrogen liberation from
NaAlH4, catalyzed by the reduced doping agent. In a par-
allel experiment (4 mole% TiCl3, ᭜) the milling ball used
was exchanged by a considerably smaller (lighter) one with
the result that only ∼5 mole H2/mole TiCl3 were released
after 10 h of milling time and that the milled sample was
inferior in the subsequent hydrogen dis- and recharging test
than the precedent sample [12]. This indicates that record-
ing of hydrogen evolution can give valuable hints concern-
ing optimization of the milling procedure, as well as toward
an improving of hydrogen storage properties of the milled
samples.
(Eqs. (3)–(7)) represent only the overall stoichiometry of the
reduction processes, without referring to the post-reduction
changes of finely dispersed metals, e.g. formation of alloys
[6,10].
toluene
xNaAlH4 + Ti(OBu)4 −− −→ (x − 1) NaAlH4 + Ti
+
NaAl(OBu)4+2H2 ↑ (1)
xNaAlH4 + Ti(OBu)4 + Fe(OC2H )2
5
x≈50
toluene
−
− −→ (x − 1.5) NaAlH4 + Ti + Fe
+
1.5 NaAl(OC2H )2OBu + 3H2 ↑
(2)
5
The so called “direct synthesis” of doped sodium alanate
for the purpose of hydrogen storage [13] is usually car-
ried out by b.m. of a NaH/Al mixture with a doping agent,
For the ball milling (b.m.) of NaAlH4 with TiCl3, based on
reflections of NaCl found in the XRD of the milled product,
Table 1
a
Monitoring of hydrogen evolution during ball milling of NaAlH4 + Al or of a NaH + Al mixture, with doping agents
Doping agent
Mole% relating to NaAlH4
Milling time (h)
Figure number
H2(mole)/ dopant (mole)
TiCl3
TiCl3
TiCl3
TiCl3
TiCl2
TiCl4
2
4
4
4
4
3
4
2
2
5
16
10
8
13
10
10
21
5.5
2.5
2.5
2.5
Fig. 2
Fig. 2
Fig. 2
Fig. 2
Fig. 3
Fig. 3
Fig. 3
Fig. 3
Fig. 4
Fig. 4
Fig. 4
Fig. 4
6
6
b
∼5
∼1
3.6
8
c
d
TiCl3·1/3 AlCl3
8
TiF3
1.3
6.2
2.5
2.7
4.5
ZrCl4
FeCl2
FeCl2
FeCl3
2.6
4
2
a
If not otherwise noted, a milling ball of ∅ = 20 mm, 32 g was used.
Milling ball of ∅ = 11.6 mm, 6.3 g.
b
c
d
Direct synthesis of doped NaAlH4 [9].
In this case hydrogen evolution took place without b.m., by stirring the NaAlH4/TiCl4 mixture in a closed system (see text).

J.M. Bellosta von Colbe et al. / Journal of Alloys and Compounds 370 (2004) 104–109
107
9
8
7
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9
10
time [h]
Fig. 2. Hydrogen evolution curves in the case of NaAlH4 (NaH + Al)/TiCl3 ball milling (if not otherwise noted, a milling ball of Ø = 20 mm, 32 g
was used). (᭹) NaAlH4/2 mole% TiCl3; (᭿) NaAlH4/4 mole% TiCl3; (᭜) NaAlH4/4 mole% TiCl3 (milling ball of Ø = 11.6 mm, 6.3 g); (᭡) NaH
+
Al/4 mole% TiCl3.
e.g. TiCl3, followed by hydrogenation under pressure. In
this case the monitoring of hydrogen evolution during b.m.
brought a surprising result. As can be seen in Fig. 2 (᭡), an
initial hydrogen evolution of a ∼1 mole H2/mole TiCl3 is
10
(
reproducibly) followed by absorption of roughly 0.5 mole
H2/mole Ti, so that after the hydrogen flow stops, a net hy-
drogen uptake of ∼0.5 mole H2/mole Ti results. Whether
and what kind of relation this phenomenon has to the course
3 3
TiCl *1/3 AlCl
8
(mechanism) of the direct synthesis of doped sodium alanate
TiCl
4
remains yet to be explored. Provisionally, it may be as-
sumed that simultaneously with reduction of TiCl3 to Ti(0)
(Eq. (4)), the highly reactive Ti(0) particles react with the
hydrogen evolved with formation of titanium hydride TiH2.
Alternatively, there might be a Ti-catalyzed hydrogenation
of the present NaH/Al mixture.
6
b.m.
xNaH + xAl + TiCl3 −→ (x − 3) NaH + x Al + 3NaCl
x≈25
4
+
Ti + 1.5H2 ↑
(4)
TiCl
2
B.m. of NaAlH4 with TiCl2 (4 mole%) as a doping agent
Fig. 3) led to the evolution of 4 mole H2/mole TiCl2, in ac-
(
cordance with the Eq. (5). On the other hand, using the set-up
of Fig. 1 it was not possible to get reproducible hydrogen
evolution curves for the NaAlH4/TiCl4 ball milling, since a
rapid hydrogen evolution starts immediately upon dropwise
addition of the liquid doping agent TiCl4 to NaAlH4 in the
vial. This determination was therefore carried out by adding
TiCl4 to NaAlH4 in closed system by means of a syringe. In
this case hydrogen evolution took place even without b.m.,
by merely stirring the mixture at ambient temperature. The
recorded hydrogen evolution curve (Fig. 3) is in good agree-
ment with the expected 8 mole H2 /mole TiCl4 (Eq. (6)).
2
TiF
3
0
0
1
2
3
4
5
time [h]
6
7
8
9
10
Fig. 3. Hydrogen evolution during ball milling, or stirring (TiCl4), of
NaAlH4 with titanium halides.

1
08
J.M. Bellosta von Colbe et al. / Journal of Alloys and Compounds 370 (2004) 104–109
B.m. of NaAlH4 with TiCl3·1/3 AlCl3 as a dopant (Fig. 3)
proceeded also with evolution of 8 mole H2/mole dopant, in
accordance with the Eq. (7).
For the b.m. of NaAlH4 with 2.6 or 4.0 mole% of FeCl2
(Fig. 4), in both cases the hydrogen evolution stopped af-
ter release of ∼2.5 mole H2/mole FeCl2. On the other hand,
the b.m. of NaAlH4 with 2 mole% of FeCl3 (Fig. 4) resulted
in 4.5 mole H2/mole FeCl3. Both for FeCl2 and FeCl3 the
hydrogen evolution is thus found to be by the amount of
b.m.
xNaAlH4 + TiCl2 −→ (x − 2) NaAlH4 + Ti + 2Al
+
2NaCl + 4H2 ↑
(5)
(6)
∼
1.5 mole H2/mole Fe lower than the amount expected for
the reduction of Fe chlorides to the zerovalent stage (4 and
stirring
xNaAlH4 + TiCl4 −−−→ (x − 4) NaAlH4 + Ti + 4Al
◦
6 mole H /mole Fe). Since in all three experiments with Fe
at 25 C
2
+
4NaCl + 8H2 ↑
chlorides the hydrogen evolution after 2–3 h stops, a par-
tial reduction of FeCl2 or FeCl3 in the presence of excess
NaAlH seems improbable. The “lack” of 1.5 mole H /mole
b.m.
1
1
3
4
2
xNaAlH4 + (TiCl3)3 −→ (x − 4) NaAlH4 + Ti + 4 Al
Fe upon b.m. of NaAlH4 with Fe chlorides could be a hint for
a possible formation of a hydride (Fe-Al hydride?), which
is worth of further investigation.
+
4NaCl + 8H2 ↑
(7)
From these results it is apparent that, contrary to other
findings [14], independently of the oxidation state of
Ti-chloride dopants 2, 3 or 4, the reduction with NaAlH4
always proceeds up to the zerovalent stage of titanium.
In comparison to TiCl3 (Fig. 2), the rate of hydrogen evo-
lution upon b.m. of TiF3 with NaAlH4 (Fig. 3) is found to be
extremely low. If one assumes that also in this case reduc-
tion of TiF3 proceeds till the zerovalent stage of titanium,
then after 10 h of milling time only a small fraction of TiF3
could have been reduced. The incomplete reduction may ex-
plain the low catalytic activity of TiF3 as a dopant in previ-
ous cycle tests [12]. In order to achieve better results with
TiF3 as a doping agent, evidently the milling time and/or
the milling temperature must be increased.
4. Conclusion
From the preliminary results presented, it appears that
for processes taking place with gas evolution or absorption
during b.m., the monitoring of gas flow can be used as a way
for characterization of b.m. processes. In particular, such
measurements can deliver following informations.
• The rate of gas evolution (absorption) and the total amount
of gas evolved (absorbed) can give direct evidence about
an underlying chemical reaction, so that such a reaction
can be conceived or confirmed.
• The recorded gas flow curves (Figs. 2–4) inform about the
rate, progress and completion of a b.m. process giving rise
to gas evolution. Thus, they can be used as a graphical rep-
resentation of a specific performed b.m. experiment and
also for controlling reproducibility of such experiments.
The b.m. of NaAlH4 with 2 mole% of ZrCl4 (Fig. 4) de-
livered after 5.5 h of b.m. 6.2 mole H2/mole ZrCl4, which is
below the amount expected for the reduction of ZrCl4 to Zr
metal (8 mole H2/mole ZrCl4). In order to complete the re-
duction process, also in this case an increase of milling time
and/or temperature appear to be necessary.
8
ZrCl4
6
FeCl3
4
FeCl (4 mole %)
2
FeCl (2.6 mole %)
2
2
0
0
1
2
3
4
5
6
time [h]
Fig. 4. Hydrogen evolution curves resulting from ball milling of NaAlH4 with zirconium and iron chlorides.

J.M. Bellosta von Colbe et al. / Journal of Alloys and Compounds 370 (2004) 104–109
109
•
•
•
Furthermore, gas flow curves can help in choice of milling
References
parameters (milling time, milling balls and their weight
relative to the milled material etc.) and milling equipment.
Monitoring of the gas flow during b.m. can also be used to
compare the efficiency of different high energy ball mills,
as f.i. in up-scaling of a b.m. process.
[1] B. Bogdanovi c´ , G. Sandrock, MRS Bull. 27 (2002) 712. Review.
[2] K.J. Gross, E.H. Majzoub, S.W. Spangler, J. Alloys Comp. 356
(2003) 423.
[3] E.H. Majzoub, K.J. Gross, J. Alloys Comp. 356 (2003) 363.
[4] A. Zaluska, L. Zaluski, J.O. Ström-Olsen, Appl. Phys. A. 72 (2001)
In the use of the b.m. method for the preparation
of doped sodium alanate hydrogen storage materials
157.
[5] J. Chen, N. Kuriyama, Q. Xu, H. Takeshita, T. Sakai, J. Phys. Chem.
B. 105 (2001) 11214.
[1–3,10,11,13], the recording of hydrogen evolution dur-
[6] V.P. Balema, J.W. Wiench, K.W. Dennis, M. Pruski, V.K. Pecharsky,
J. Alloys Comp. 329 (2001) 108.
ing b.m. can be used to find out milling parameters
necessary to achieve the optimal catalytic activity of a
dopant (c.f. above, TiF3 as dopant).
Occasionally, measurements of gas flow during b.m. can
bring about to trace out the presence of a known or un-
known metal hydride.
[7] N.N. Maltseva, A.I. Golovanova, T.N. Dymova, D.P. Aleksandrov,
Russ. J. Inorg. Chem. 46 (2001) 1793.
[8] B. Bogdanovi c´ , B. Spliethoff, Chem. Ing. Tech. 55 (1983) 156.
9] B. Bogdanovi c´ , M. Schwickardi, J. Alloys Comp. 253–254 (1997)
[
1.
[
10] B. Bogdanovi c´ , R.A. Brand, A. Marjanovi c´ , M. Schwickardi, J. Tölle,
J. Alloys Comp. 302 (2000) 36.
[11] G. Sandrock, K. Gross, G. Thomas, J. Alloys Comp. 339 (2002)
2
99.
Acknowledgements
[
[
12] The results of the cycling tests will be presented in a later publication.
13] B. Bogdanovi c´ , M. Schwickardi, Appl. Phys. A. 72 (2001)
This work was partially supported by General Motors Fuel
Cell Activities, in addition to the basic funding by the Max
Planck Gesellschaft.
221.
[14] D. Sun, T. Kiyobayashi, H. Takeshita, N. Kuriyama, C. Jensen, J.
Alloys Comp. 337 (2002) 8.