Titanium-halide catalyst-precursors in sodium aluminum hydrides

Article abstract of DOI:10.1016/S0925-8388(03)00113-0

The kinetics of absorption and desorption of hydrogen from NaAlH₄ have previously been shown to improve upon the addition of a catalyst-precursor such as TiCl₃. In this paper we demonstrate that TiCl₂, TiF₃, and TiBr₄ all effectively improve sorption kinetics. Arrhenius data indicate that the catalyst precursors behave in essentially the same manner. Evidently the valency of Ti in the catalyst-precursor is inconsequential to the role of Ti in altering the kinetic mechanism. The formation of TiAl₃ on doping with TiCl₃ has been observed. The presence of TiAl₃ appears to contribute in part to the enhanced kinetics in these systems.

Full text of DOI:10.1016/S0925-8388(03)00113-0

Journal of Alloys and Compounds 356–357 (2003) 363–367
L
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Titanium–halide catalyst-precursors in sodium aluminum hydrides
*
E.H. Majzoub , K.J. Gross
Sandia National Laboratories, Analytical Materials Science Department, P.O. Box 969, Livermore, CA 94551, USA
Received 2 September 2002; accepted 15 November 2002
Abstract
The kinetics of absorption and desorption of hydrogen from NaAlH4 have previously been shown to improve upon the addition of a
catalyst-precursor such as TiCl3. In this paper we demonstrate that TiCl2, TiF3, and TiBr4 all effectively improve sorption kinetics.
Arrhenius data indicate that the catalyst precursors behave in essentially the same manner. Evidently the valency of Ti in the
catalyst-precursor is inconsequential to the role of Ti in altering the kinetic mechanism. The formation of TiAl3 on doping with TiCl3 has
been observed. The presence of TiAl3 appears to contribute in part to the enhanced kinetics in these systems.
2003 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage materials; Mechanical alloying; X-ray diffraction
1. Introduction
Ti-precursor is of any importance, (2) if there is any
correlation between the reactivity of the Ti–halide catalyst-
precursor and the resulting rate kinetics, and (3) if TiAl3 is
responsible for enhancing kinetics.
Sodium aluminum tetrahydride decomposes in two
steps, first into the hexahydride Na3AlH6, which further
decomposes into NaH as follows
The lighter Ti–halides TiCl2 and TiF3 were also chosen
to increase the weight percent of hydrogen capacity in the
doped material. Assuming the reactions in Eqs. (1) and (2)
go to completion, there are three hydrogen atoms per
formula unit available for absorption and desorption.
Capacity as a function of catalyst loading can be easily
calculated for various Ti–halide combinations. The limit-
ing value of the reversible hydrogen weight capacity in the
NaAlH4 system is 5.55%, in the limit that the catalyst
loading approaches zero.Values for the total weight percent
of reversible hydrogen, under ideal conditions, for the
dopants used here are given in Fig. 1 for samples of
composition (1 1 xy/100)NaAlH4 1 xTi(halide)y.
1
2
3
]
]
NaAlH4á Na3AlH6 1 Al 1 H2
(1)
(2)
3
3
2
]
Na3AlH6á3NaH 1 Al 1 H2
´
This system was shown to be reversible by Bogdanovic et
al. upon doping with various transition metal catalysts [1].
Speculation about the effect of the catalyst-precursor has
led to suggestions that Ti31 plays an active role in
enhancing the kinetics of absorption and desorption [2,3].
Therefore several Ti–halide compounds of differing va-
lency were chosen as catalyst-precursors. These were
TiCl3, TiCl2, TiF3, and TiBr4. Samples were prepared for
each dopant and tested for reversible hydrogen capacity.
The desorption rates were measured as a function of
temperature, and Arrhenius analysis was applied. The
Gibbs free energies of formation for the reactions of
dopant and pre-active material are evaluated with respect
to the observed desorption rate data. The primary goals of
this work are to determine (1) if the valency of the
2. Experimental details
Active NaAlH4 material was prepared from NaH
(99.99%) and 20 mm Al (99.99%) powder purchased from
Sigma–Aldrich using a direct synthesis method [4]. Typi-
cally a 7-g mixture of 1:1:0.04 molar ratio of NaH:Al:
catalyst-precursor was mechanically milled in a SPEX
8000 high energy mill using a WC milling pod and 6–9
25-g WC balls. The catalyst-precursor consisted of Ti–
halide such as TiCl3, TiF3, TiCl2, and TiBr4, purchased
*
Corresponding author. Tel.: 11-925-294-2498; fax: 11-925-294-
3410.
E-mail address: ehmajzo@sandia.gov (E.H. Majzoub).
0925-8388/03/$ – see front matter
doi:10.1016/S0925-8388(03)00113-0
2003 Elsevier B.V. All rights reserved.

364
E.H. Majzoub, K.J. Gross / Journal of Alloys and Compounds 356–357 (2003) 363–367
vacuum of 1 Torr while increasing the temperature in steps
to determine Arrhenius behaviour.
TiAl3 in the L12 structure was prepared by mechanically
milling Alpha Products 100 mesh Ti (99.9%), and Aldrich
635 mesh Al (99.9%), in a 1:3 molar ratio for 24 h.
Samples were characterized for hydride-phase content
following absorption and desorption cycles using Cu Ka
radiation on a Scintag XDS-2000 using Bragg–Brentano
geometry and airless sample holders.
3. Results and discussion
Sodium halide salts form in the doping process due to
the reactions below.
Fig. 1. Ideal hydrogen weight percent capacity with catalyst-precursor
loading for the Ti–halides used in this work.
2NaH 1 TiCl2 → 2NaCl 1 Ti 1 H2
3NaH 1 TiCl3 → 3NaCl 1 Ti 1 3/2H2
3NaH 1 TiF3 → 3NaF 1 Ti 1 3/2H2
4NaH 1 TiBr4 → 4NaBr 1 Ti 1 2H2
(3)
(4)
(5)
(6)
from Sigma–Aldrich. Sample sizes of about 2 g were
transferred to a sealed stainless-steel sample vessel and
connected to a pressure- and temperature-controlled hydro-
gen manifold. All steps for sample preparation and transfer
into the reactor vessel occurred inside an Ar glove box
with oxygen levels below 3 ppm.
Samples from earlier work were prepared from NaAlH4
which was precipitated from a 1 M solution in tetrahydro-
furan (THF) by vacuum evaporation [5]. For some com-
parisons, those samples are labeled THF-derived (THF) to
distinguish them from the direct synthesis (DS) samples in
this work.
Kinetics of absorption and desorption were determined
from pressure measurements in a calibrated fixed-volume
apparatus. Samples were placed in a 15-mm-diameter
316SS vessel of |12 cc volume with a 1-mm-diameter
internal thermocouple well for rapid response of the
material during kinetics experiments. Type K thermocou-
ples with a temperature resolution of 0.1 K were placed at
the top, middle, and bottom of the sample location on the
exterior of the containment tube, and inside the thermocou-
ple well near the center of the sample. The sample was
held in place on the top and the bottom with Fiberfax glass
fiber. Absorption measurements were made in a calibrated
volume of 120 cc at pressures around 10 MPa. For a 2 g
sample of NaAlH4, this resulted in a pressure drop of
around 0.8 MPa. Absorption pressures were measured
using a Teledyne Taber model 206 piezoelectric transducer
with a resolution of 1022 MPa. Measured values of (mol
H)/(mol Al) have a resolution of about 60.2. The volume
used for the desorption measurements was about 1040 cc,
and the pressure was measured using a Baratron capaci-
tance manometer with a resolution of 1027 MPa. The
sample temperature was regulated with an Omega model
CN76000 PID controller connected to heating tape wrap-
ped around the exterior of the SS vessel. Typical absorp-
tion parameters were 120 8C and 8.5 MPa hydrogen
overpressure. Desorptions were performed in an initial
The resultant form of the titanium is shown as metallic, but
may form a reaction product itself and will be discussed in
the next section. Bulk Ti has never been observed in our
diffraction data.
The halide salts produced in these reactions add addi-
tional weight of inactive material, resulting in two effects.
(1) It causes a significant reduction in the overall revers-
ible hydrogen capacity of the material. (2) It necessitates
the comparison of capacities on an ‘active material’ basis.
This makes it desirable to plot capacities of the post-doped
material as (mol H)/(mol Na) versus the at.% doping level,
where the moles of Na are those not bound in Na–halide.
This eliminates the weight of the inactive material com-
ponent. We represent this quantity as H/Naa (representing
the ‘active’ Na available) which should have a maximum
value of 3. This allows us to determine the extent of the
reversibility of the reactions. Values for H/Naa are shown
for second desorptions for several samples in Fig. 2.
The reference NaAlH4 sample (only desorbed once)
labeled ‘no catalyst’ was prepared from THF and me-
chanically milled, resulting in a small loss of capacity.
Clearly, the Ti-doped samples are not entirely reversible,
whether they were prepared from THF alanate or by the
direct synthesis method, which generally show lower H/
Naa values. This loss in reversibility is perhaps due to
phase separation induced during cycling and the inability
of aluminum to diffuse quickly enough during absorptions,
thus leading to deficient desorptions. Crystalline aluminum
can be seen in the X-ray of a post fourth absorption of
TiF3-doped material as shown in Fig. 3. In situ studies
have demonstrated that aluminum precipitates to form
crystallites during desorption [6,7].
Arrhenius data for the rates of decomposition of the

E.H. Majzoub, K.J. Gross / Journal of Alloys and Compounds 356–357 (2003) 363–367
365
Fig. 2. Capacity of Ti–halide doped material for 2nd desorptions. The
maximum value of 3 represents the total number of hydrogens per
formula unit which should participate in the absorption/desorption
reactions.
Fig. 4. Arrhenius plot of the rates of desorption of tetrahydride for
samples using Ti–halide catalyst-precursors.
reaction, produces samples with nearly no improvement in
kinetics over two cycles. The system behaves almost
undoped, and it appears that the halide component is
necessary for obtaining active samples. Thus the very
exothermic reaction with the Ti–halides seems to atomical-
ly disperse the Ti, since it is apparently unfavorable,
energetically, to remove it from bulk Ti when doping with
this material. Ti doping with other non-halide materials
can, however, show enhancement of kinetics [8]. Assum-
ing that the titanium is atomically dispersed as a substitute
ion in the bulk of NaH in the desorbed state, or in NaAlH4
in the absorbed state, its charge state is expectedly
independent of its precursor valency and thus all halide
dopants show similar kinetic effects as indicated in Fig. 4.
tetrahydride are shown in Fig. 4 for each of the Ti–halide
catalyst precursors. These data indicate that all the Ti–
halides behave in essentially the same manner with the
desorption rates in the order of highest to lowest corre-
sponding to the dopants TiCl2, TiCl3, TiF3, TiBr4. Note
that the TiBr4 sample had a lower doping level (1.4 at.%)
and a slightly lower desorption rate, consistent with results
obtained by Sandrock et al. [5]. The ‘no catalyst’ sample
prepared from THF is shown as a comparison.
The Gibbs free energies of the reactions (3)–(6) are
shown in Table 1 and indicate that the order of the most
exothermic to least exothermic reaction is in the order
TiCl3, TiBr4, TiCl2, TiF3. Yet the TiCl2-doped sample
yields the best kinetics. Therefore, the free energy of
reaction does not seem to be correlated with the enhance-
ment of sorption kinetics. However, doping with Ti
powder alone, in which there is no large enthalpy of
3.1. Titanium compounds
The product titanium in Eqs. (3)–(6) may, however,
form a hydride or Ti–Al alloys. The free energy of
formation, DGf8, for Eq. (4) is 2398 kJ/mol, if the
resultant Ti is metallic. If we assume the Ti forms TiH2 or
TiAl3, the resulting reactions are
3NaH 1 TiCl3 → 3NaCl 1 1/2H2 1 TiH2
(7)
(8)
3NaH 1 3Al 1 TiCl3 → 3NaCl 1 TiAl3 1 3/2H2
with free energies of formation of 2484 kJ/mol, and
Table 1
Gibbs free energies of formation for the reactions in Eqs. (3)–(6)
Compound
Equation
DGf8 (kJ/mol)
TiCl2
TiCl3
TiF3
(3)
(4)
(5)
(6)
2236.8
2398.3
2176.5
2323.5
Fig. 3. X-ray powder diffraction of post 4th absorption of a sample
prepared from NaH, Al, TiF3. The presence of hexahydride and free
aluminum indicate that the reverse reaction (Eq. (2)) is incomplete.
TiBr4

366
E.H. Majzoub, K.J. Gross / Journal of Alloys and Compounds 356–357 (2003) 363–367
2535 kJ/mol, respectively. This difference results from
the energy of formation for TiH2 (286 kJ/mol [9]), or
TiAl3 (2136 kJ/mol [10]). The value of DGf8 for TiAl is
272 kJ/mol, which is lower in magnitude than that for
TiH2 and is ignored. This suggests that the formation of
TiAl3 is preferred. Due to the small volume fraction of the
phases that would be present, X-ray diffraction of the
doped material was inconclusive on the presence of TiH2
or TiAl3 in these (DS) materials. However, favorable
stoichiometry is achieved for TiAl3 formation with the use
of TiCl3 as a dopant and its presence is indicated in the
LiAlH4 system, where micro-crystalline TiAl3 is thought
to act as a heterogeneous dehydrogenation catalyst after
reacting with TiCl4 [11].
amount of TiAl3 was prepared by mechanical alloying and
added without Ti–halide to the direct synthesis material.
The doping procedure was identical to that of the Ti–
halides. The desorption kinetics of 8 mol.% L12-doped
material are an order of magnitude below those of Ti–
halide-doped samples (see Fig. 4). This suggests that if
formation of TiAl3 occurs in the 4 mol.% doped Ti–halide
samples, it is not the only mechanism responsible for
enhancing the sorption kinetics. It may be possible that
during ball milling Ti is removed from TiAl3 and is
available for incorporation into the alanate. This would be
consistent with recent results on doping with TiH2, which
indicate that Ti can be removed from this relatively stable
compound and participate in kinetics enhancement [8].
There is also the small possibility that the addition of
TiAl3 may act only as a grinding aid, similar to the results
obtained by using carbon as a milling ingredient [12].
Therefore, if the Ti is not in TiAl3 and the sorption
kinetics are significantly enhanced, the high absolute
activity of the Ti resulting from the strongly exothermic
reactions (3)–(6), and the relatively low mol.% doping
level suggest that the titanium is left as a substitute ion in
the system.
To investigate the formation of TiAl3 in the NaAlH4
system, a mechanically milled 3:1 mixture of NaAlH4 and
TiCl3 was prepared. X-ray diffraction indicates the forma-
tion of NaCl and TiAl3 in the L12 structure, as shown in
Fig. 5 and is consistent with the following reaction
TiCl3 1 3NaAlH4 → 3NaCl 1 TiAl3 1 6H2
(9)
The presence of TiAl3 contributes to the broad peak widths
of the reacted sample peaks at 45, 65, and 80 degrees 2u.
Scherrer analysis of the L12 phase peak widths shown in
Fig. 5 for the pure TiAl3 pattern place an upper limit on
the coherence length of about 7 nm. The NaCl peaks which
are not overlapping with TiAl3, at 56 and 75 degrees 2u,
have peak widths corresponding to a coherence length of
about 25 nm. The 3:1 mixture of NaAlH4 and TiCl3
corresponds to a doping level of 33 mol.%. At doping
levels of 4 mol.%, it is difficult to observe the presence of
the L12 phase in X-ray diffraction due to the large peak
widths and small volume fraction of the phase. To
investigate the kinetic effect of doping with TiAl3, a small
4. Conclusions
The enhanced sorption kinetics due to Ti–halide catalyst
precursors is independent of the valence state of the Ti–
halide, and the data indicate that the more exothermic
halides do not result in faster rate kinetics. The free
energies of formation suggest that TiAl3 is the most likely
reaction product. The presence of TiAl3 alone, however,
cannot explain the sorption kinetics of doped sodium
aluminum hydride. This suggests that the Ti is atomically
dispersed in the system, both as a substitute ion and
perhaps in TiAl3, by the very exothermic formation of
Na–halide. This conclusion is consistent with recent X-ray
diffraction studies suggesting that Ti may be substituting
into bulk NaAlH4 [3]. Because the sorption kinetics are
similar for the Ti–halide dopants, if the titanium is
substituted in the NaAlH4, or Na3AlH6, it is reasonable
that its resulting charge state will be independent of the
catalyst-precursor used.
Acknowledgements
This work was funded by the US Department of Energy,
Office of Power Technologies, Hydrogen Program Office
under contract no. DE-AC36-83CH10093. The authors
thank Vidvuds Ozolins for useful discussions. We thank
Scott Spangler for performing the X-ray diffraction and
Fig. 5. X-ray diffraction indicating TiAl3 production in the NaAlH4
system when mechanically milled in a 3:1 ratio with TiCl3.
´
Andres Orozco for sample preparation.

E.H. Majzoub, K.J. Gross / Journal of Alloys and Compounds 356–357 (2003) 363–367
367
[6] K.J. Gross, G. Sandrock, G.J. Thomas, Dynamic in situ X-ray
diffraction of catalyzed alanates, J. Alloys Comp. 330–332 (2002)
691–695.
[7] K.J. Gross, S. Guthrie, S. Takara, G. Thomas, In-situ X-ray
diffraction study of the decomposition of NaAlH4, J. Alloys Comp.
297 (1–2) (2000) 270–281.
[8] K.J. Gross, E.H. Majzoub, S.W. Spangler, Hydrogen absorption
properties of alanates: from principle to practice. International
Conference on Metal Hydrogen Systems, Annecy, France (2002)
submitted.
[9] W.M. Mueller, J.P. Blackledge, G.G. Libowitz, Metal Hydrides,
Academic Press, New York, 1968.
[10] C.J. Smithells, G.B. Brook, E.A. Brandes (Eds.), 7th Edition,
Smithells Metals Reference Book, Butterworth–Heinemann, 1998.
[11] V.P. Balema, J.W. Wiench, K.W. Dennis, M. Pruski, V. Pecharsky,
Titanium catalyzed solid-state transformations in LiAlH4 during
high-energy ball-milling, J. Alloys Comp. 329 (2001) 108–114.
[12] A. Zaluska, L. Zaluski, J.O. Strom-Olsen, Sodium alanates for
reversible hydrogen storage, J. Alloys Comp. 298 (1–2) (2000)
125–134.
References
´
[1] B. Bogdanovic, R.A. Brand, A. Marjanovic, M. Schwickardi, J.
Tolle, Metal-doped sodium aluminium hydrides as potential new
hydrogen storage materials, J. Alloys Comp. 302 (1–2) (2000)
36–58.
[2] C.M. Jensen, R. Zidan, N. Mariels, A. Hee, C. Hagen, Advanced
titanium doping of sodium aluminum hydride: segue to a practical
hydrogen storage material?, Int. J. Hydrogen Energy 24 (5) (1999)
461–465.
[3] D. Sun, T. Kiyobayashi, H.T. Takeshita, N. Kuriyama, C.M. Jensen,
X-ray diffraction studies of titanium and zirconium doped NaAlH4:
elucidation of doping induced structural changes and their relation-
ship to enhanced hydrogen storage properties, J. Alloys Comp. 337
(2002) L8–L11.
[4] K.J. Gross, E.H. Majzoub, Direct synthesis of light-weight catalyzed
hydride for hydrogen storage, United States patent application
submitted 2002.
[5] G. Sandrock, K. Gross, G. Thomas, C. Jensen, D. Meeker, S.
Takara, Engineering considerations in the use of catalyzed sodium
alanates for hydrogen storage, J. Alloys Comp. 330–332 (2002)
696–701.