Effects of catalysts on the dehydriding of alanates monitored by proton NMR

Article abstract of DOI:10.1016/j.jallcom.2004.11.105

In situ studies of the transition from NaAlH₄ to Na ₃AlH₆ are performed by proton NMR for samples doped with TiCl₃ and with Ti₁₃-nanoclusters. The local hydrogen dynamics in the different compounds is studied by the nuclear spin-lattice relaxation. For the Ti-doped NaAlH₄ samples a double-exponential recovery of the nuclear magnetization is observed, indicating two fractions of hydrogen with different mobilities. In Na₃AlH₆ a transition from hindered rotation of the AlH₆ groups to full isotropic reorientation of these groups is observed in the temperature range 200-260 K.

Full text of DOI:10.1016/j.jallcom.2004.11.105

Journal of Alloys and Compounds 404–406 (2005) 738–742
Effects of catalysts on the dehydriding of alanates monitored
by proton NMR
G. Majera,∗, E. Stanika, L.E. Valiente Banueta, F. Grinberga,
O. Kircherb, M. Fichtnerb
a
Max-Planck-Institut fu¨r Metallforschung, Heisenbergstr. 3, D-70569 Stuttgart, Germany
b
Karlsruhe Research Center, Institute of Nanotechnology, POB 3640, D-76021 Karlsruhe, Germany
Received 14 July 2004; received in revised form 29 October 2004; accepted 4 November 2004
Available online 15 July 2005
Abstract
In situ studies of the transition from NaAlH4 to Na3AlH6 are performed by proton NMR for samples doped with TiCl3 and with Ti13-
nanoclusters. The local hydrogen dynamics in the different compounds is studied by the nuclear spin-lattice relaxation. For the Ti-doped
NaAlH4 samples a double-exponential recovery of the nuclear magnetization is observed, indicating two fractions of hydrogen with different
mobilities. In Na3AlH6 a transition from hindered rotation of the AlH6 groups to full isotropic reorientation of these groups is observed in the
temperature range 200–260 K.
© 2005 Elsevier B.V. All rights reserved.
PACS: 64.70.Nd; 76.60.−k
Keywords: Complex hydrides; Hydrogen storage; Hydrogen dynamics; Nuclear magnetic resonance (NMR)
1. Introduction
however, it has been demonstrated that the hydrogen storage
properties of Ti-doped NaAlH4 could be considerably
improved by using Ti13-nanoclusters as doping agents
[5,6].
Sodium alanate NaAlH4 is a promising candidate for the
development of a reversible hydrogen storage system for
mobile applications. It decomposes in two steps, first into
the hexahydride Na3AlH6, which further decomposes into
NaH as follows:
Nuclear magnetic resonance (NMR) has seen wide ap-
plication to study the structural and dynamical properties in
classical interstitial hydrides (see e.g. [7]). However, only
very few studies have been performed on complex chemical
hydrides. The structure and hydrogen dynamics in Na3AlH6
3NaAlH4 ⇐⇒ Na3AlH6 + 2Al + 3H2
Na3AlH6 ⇐⇒ 3NaH + Al + H2
The interest in NaAlH4 for hydrogen storage increased
drastically in 1997, when it was discovered that the inverse
of reactions (1) and (2) can be catalysed by the addition
of a small amount of Ti-based complexes [1]. Until quite
recently, the addition of Ti3+ cations in the form of chlorides
was considered to hold the highest potential for rapid
release and absorption of hydrogen gas [2–4]. Very recently,
(1)
(2)
1
3
2
has been studied by H NMR [8]. First solid-state magic
angle spinning (MAS) NMR spectra of 27Al have been mea-
sured on LiAlH4 by Balema et al. [9]. Recently, 23Na and
27
´
Al MAS-NMR has been used by Bogdanovic et al. [10]
to study the effect of discharging and recharging of NaAlH4.
The present paper reports on first 1H NMR studies of the start-
ing material NaAlH4, the intermediate product Na3AlH6 and
the final product NaH. The effect of different catalysts on the
proton NMR spectra and their temperature dependent time
evolution is investigated. Furthermore, the influence of the
catalysts on the hydrogen dynamics is studied by measuring
the spin-lattice relaxation rate Γ1.
∗
Corresponding author. Tel.: +49 711 689 3686; fax: +49 711 689 3612.
E-mail address: majer@mf.mpg.de (G. Majer).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.11.105

G. Majer et al. / Journal of Alloys and Compounds 404–406 (2005) 738–742
739
2. Experimental details
NaAlH4 material was purchased as powder (Chemetall,
Frankfurt) and purified by a Soxhlet extraction with THF [6].
The Na3AlH6 sample was prepared by mechanical alloying
(ball milling) of appropriate amounts of NaAlH4 and NaH
[11]. TheNaHsamplewasobtainedbyballmillingofpurified
NaAlH4 followed by thermal desorption under vacuum at
443 K.
Two types of Ti-doped samples were prepared by ball
milling of NaAlH4 with small amounts of TiCl3 and Ti13-
nanoclusters, respectively. Details about the preparation of
the small Ti13-clusters are given in [6]. The titanium con-
centration was 2.0 mol% for the TiCl3-doped sample and
1.8 mol% for the Ti-cluster-doped sample.
The proton NMR measurements were performed at
67.7 MHz and the spectra were obtained by Fourier trans-
formation of the free-induction decay (FID) following a
single 90◦ r.f.-pulse after a dead time of typically 10 µs. The
spin-lattice relaxation was measured by using an inversion-
recovery pulse sequence. In case of a single-exponential
recovery the relaxation rate Γ1 was obtained by fitting
Fig. 1. Proton NMR spectra of NaAlH4, Na3AlH6 and NaH.
M(τ) = M0[1 − 2 exp(−τ · Γ1)]
(3)
the decomposition reaction of NaAlH4 occurs very slowly.
This is evident from Fig. 2, which shows the proton NMR
spectrum of Na3AlH6 together with the spectra of the TiCl3-
doped NaAlH4 measured directly after ball-milling and after
having stored the sample in the glove box for 5 months. Even
after 5 months at room temperature the transformation from
NaAlH4 to Na3AlH6 is not yet completed. While the central
line characteristic of Na3AlH6 has already been formed,
the doublet structure of NaAlH4 is still visible as shoulders.
A faster decomposition of NaAlH4 to Na3AlH6 occurs at
higher temperatures and by using Ti-clusters as doping
to the magnetization values measured as a function of the
recovery time τ.
3. Results and discussion
A comparison of the room-temperature proton NMR
spectra of the starting material NaAlH4, the intermediate
product Na3AlH6 and the final product NaH is given in
Fig. 1. A common feature of these spectra is the existence of
the narrow central line superimposed on the much broader
component. In each case, the central line accounts only for a
few percent of the total spectrum area and may be attributed
to impurities or remaining solvent from the purification
process. The intensity of this central line is different for
NaAlH4 samples purified in different batches.
The main components of the room-temperature NMR
spectra of all three compounds are quite different with respect
to their widths and shapes. The spectrum of NaAlH4 repre-
sents a Pake-like doublet with a splitting of approximately
50 kHz. A similar splitting has been seen previously in the
NMR spectrum of AlH3 [12], which is also a non-metallic
hydride. The intermediate compound, Na3AlH6, produces
a broad unresolved line with a halfwidth of approximately
17 kHz. The spectrum of the final product, NaH, is again un-
resolved, with the halfwidth of about 35 kHz.
Although the spectra shown in Fig. 1 were measured
at a rather low resolution, the observed differences are
clear enough to distinguish between the compounds. This
property is a crucial prerequisite for an efficient in situ
study of the decomposition processes. At room temperature
Fig. 2. Proton NMR spectra of Na3AlH6 and NaAlH4 with 2.0 mol% TiCl3.
The TiCl3-catalysed NaAlH4 was measured right after the preparation as
well as after 5 months.

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G. Majer et al. / Journal of Alloys and Compounds 404–406 (2005) 738–742
total relaxation rate Γ1 consists in general of the sum of two
contributions
Γ1 = Γ1,para + Γ1,dip
(4)
where Γ1,para results from the dynamic interaction of the pro-
ton spins with fluctuating magnetic fields arising from param-
agnetic impurities. The dipolar relaxation Γ1,dip is due to the
motion-induced fluctuations of the magnetic dipole–dipole
interaction of a given proton with neighboring protons and
with the sodium and aluminum nuclei.
In pure NaAlH4 the nuclear magnetization was found
to recover single exponentially according to Eq. (3) with a
rather long relaxation time of T1 = (Γ1)−1 ≈ 3200 s at room
temperature. Shorter relaxation times and deviations from
a single-exponential recovery of the nuclear magnetization
were observed in less pure samples, purified in a different
batch. We assume that Γ1 in undoped NaAlH4 is mainly due
to the impurity-related paramagnetic relaxation Γ1,para with
negligible contributions from the motion-induced dipolar re-
Fig. 3. Temperature dependence of the proton NMR spectrum of Na3AlH6.
agents. The proton NMR spectrum of NaAlH4 with 1.8 mol%
Ti-clusters measured after having heated the sample for 1 h
to 360 K already closely resembles that of Na3AlH6. This
allows us to conclude that the first decomposition stage
of the Ti-cluster-doped NaAlH4 is completed after 1 h at
360 K.
laxation Γ1,dip
.
The overall relaxation in the Ti-catalysed samples is much
faster than in the untreated samples. Furthermore, rather
pronounced deviations from single-exponential recovery of
the nuclear magnetization are observed in the doped samples.
The data can be reasonably well described by a double-
exponential magnetization recovery, as shown in Fig. 4
for NaAlH4 with 1.8 mol% Ti-clusters. The time constants
obtained directly after activating the samples are about 4 and
45 s for doping with TiCl3 and about 0.7 and 15 s for doping
with Ti-clusters. The fast relaxation in the Ti-doped samples
may in principle be attributed to an increase of both Γ1,dip
and Γ1,para. It is reasonable to assume that the hydrogen dy-
namics is enhanced due to the presence of catalysts, resulting
in an increase in Γ1,dip. Besides this effect the catalysts may
act as paramagnetic centers of relaxation and thus contribute
to Γ1,para. However, preliminar NMR experiments show
that ball milling of pure NaAlH4 reduces T1 even without
catalysts. Moreover, the recovery of the nuclear magnetiza-
tion of hydrogen in Ti-doped samples is not exceeding that
measured on Na3AlH6 (cf. Fig. 4). The Na3AlH6 sample
was prepared by ball milling of NaAlH4 and NaH without
catalysts, and the proton relaxation of Na3AlH6 can be
consistently described in terms of hydrogen dynamics (see
below). Finally, we found that adding the same amount of
catalyst to a paraffin sample is not affecting the proton Γ1.
Therefore, we assume that the strong increase in the relax-
ation rate in the case of NaAlH4 is mainly due to an enhanced
hydrogen dynamics caused by the catalysts. The processes
of hydrogen dynamics in NaAlH4 are most likely related
to the rotation of the AlH4 tetrahedra. The different time
constants observed in the samples with Ti-based catalysts
may be ascribed to AlH4 tetrahedra at different positions,
e.g. in the bulk and at the surface of the crystalline grains.
It is evident from Fig. 4 that after 6 weeks the recovery
of the nuclear magnetization in Ti-cluster-doped NaAlH4
The proton NMR spectrum of NaAlH4 suggests that even
at room temperature the so-called rigid lattice condition
is fulfilled, i.e. the characteristic rate of hydrogen motion
νH is much lower than the width of the NMR spectrum
ꢀν ≈ 50 kHz. For Na3AlH6, the proton NMR spectrum is
more narrow at room temperature, but it changes substan-
tially at lower temperatures. Fig. 3 shows a comparison of the
spectra measured on Na3AlH6 at three different temperatures
between 146 and 283 K. The 146 K spectrum of Na3AlH6
consists of a small central line superimposed on a doublet
structure with a splitting of about 50 kHz, quite similar to
the room-temperature spectrum of NaAlH4. The intensity of
the central line grows with increasing temperature at cost
of the doublet structure. Already at 225 K the doublet struc-
ture has essentially disappeared with only two shoulders left.
The structure of Na3AlH6 suggests that this phenomenon is
related to hindered rotation of the AlH6 groups, which form
nearly regular octahedra with a four-fold symmetry [13]. The
phenomena of the thermally activated rotation of the AlH6
octahedra in Na3AlH6 have been studied previously by pro-
ton NMR [8]. The rotation of the AlH6 octahedra allows
a given proton spin to sample different local fields, result-
ing in a partial averaging of the resonance frequencies. In
NaAlH4 the (local) mobility of hydrogen seems to be much
lower than in Na3AlH6, since the proton NMR spectra of
NaAlH4 indicate no effect of motional averaging. This re-
sult is in good agreement with the relaxation data discussed
below.
In order to gain further insight into the hydrogen dynamics
in the different compounds the proton spin-lattice relaxation
has been measured on the same samples. In these systems the

G. Majer et al. / Journal of Alloys and Compounds 404–406 (2005) 738–742
741
average values obtained by fitting Eq. (3) to the experimental
data. Below 200 K the recovery of the nuclear magnetization
shows, however, again a single-exponential behavior. These
low-temperature Γ1 data indicate a much lower activation en-
thalpy, of the order of Ha ≈ 0.02 eV. The overall temperature
dependence of the spin-lattice relaxation can be explained
consistently within the framework of the rotation of AlH6 oc-
tahedra in Na3AlH6. Starting below 200 K, hindered rotation
of the AlH6 groups occurs, most likely around an axis with
a four-fold symmetry [8]. At higher temperatures a phase
change from hindered rotation to full isotropic tumbling
of the AlH6 groups appears. The temperature dependence
of Γ1 leads to activation enthalpies of about 0.02 eV for
the hindered rotation and 0.23 eV for the full isotropic
exchange.
Previous proton NMR studies on polycrystalline Na3AlH6
indicated also a transition between these two types of rota-
tional motion of the AlH6 octahedra [8]. However, the activa-
tion enthalpies reported in [8] for polycrystalline samples are
substantially greater than the values of the present work. This
result is consistent with the picture that in nanocrystalline
samples, obtained by mechanical milling of the material, the
local hydrogen mobility is enhanced, resulting in a reduction
of the hydrogen desorption temperature.
Fig. 4. Recovery of the nuclear magnetization of protons in Ti-cluster-
catalysed NaAlH4. The measurements were performed at room temperature
right after activating the sample and after storing it for 6 weeks in an argon-
filled glove box. The dotted line represents the room-temperature recovery
of the nuclear magnetization of protons in Na3AlH6.
occurs distinctly faster than right after activating the sample.
The corresponding time constants are about 0.3 and 3 s. This
result may be ascribed to the more advanced decomposition
of NaAlH4 into Na3AlH6, in which a fast rotational motion
of the AlH6 groups occurs. If the decomposition into
Na3AlH6 is completed, the relaxation at room temperature
is single exponential with a rather short relaxation time of
T1 = 0.3 s (cf. Fig. 4).
4. Summary
Fig. 5 shows the Γ1 values determined for hydrogen
in Na3AlH6 between 130 and 380 K. Above about 260 K
the nuclear relaxation is single exponential according to
Eq. (3). The temperature dependence of the corresponding
Γ1 data yields an activation enthalpy of Ha ≈ 0.23 eV. At
intermediate temperatures, between about 200 and 260 K,
strong deviations from a single exponential relaxation occur.
The Γ1 data included in Fig. 5 in this temperature range are
NaAlH4, Na3AlH6 and NaH are well characterized by
their proton NMR spectra. Thus, the NMR permits an in
situ monitoring of the decomposition reactions in these
complex metal-hydrides. The hydrogen dynamics in the
different compounds is investigated by means of the proton
spin-lattice relaxation. In Ti-doped NaAlH4 deviations from
a single-exponential recovery of the nuclear magnetization
are observed, indicating at least two fractions of hydrogen
with different mobilities. A rather fast local mobility is found
for the AlH6 groups in Na3AlH6. Below about 200 K, the
AlH6 groups rotate around an axis with four-fold symmetry,
while above about 260 K, a full isotropic reorientation of the
AlH6 groups occurs.
Acknowledgement
This work has been supported by the Deutsche Forschu-
ngsgemeinschaft through the graduate college “Modern
Methods of Magnetic Resonance in Materials Science” and
through Grant No. MA 1382/6.
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Fig. 5. Temperature dependence of the spin-lattice relaxation of hydrogen
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