i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 2 2 7 9 e1 2 2 8 5
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Adjustment of the decomposition path for Na2LiAlH6 by TiF3
addition
Jon Erling Fonneløp a,*, Ole Martin Løvvik a,b, Magnus H. Sørby a, Hendrik W. Brinks a,
Bjørn C. Hauback a
a
b
Institute for Energy Technology, P.O. box 40, N-2027 Kjeller, Norway
Centre for Materials Science and Nanotechnology, University of Oslo, N-0318 Oslo, Norway
article info
abstract
Article history:
The decomposition of Na2LiAlH6 is studied by in-situ synchrotron diffraction. By addition of
Received 18 April 2011
TiF3 and dehydrogenation-rehydrogenation cycling of the samples new decomposition
Received in revised form
paths are found. Na3AlH6 is formed on decomposition in the presence of TiF3. The additive
16 June 2011
brings the system closer to equilibrium, and decomposition through Na3AlH6 is demon-
Accepted 19 June 2011
strated for the first time. The results are in agreement with previously published compu-
Available online 23 July 2011
tational data. For a cycled sample with 10 mol% TiF3 Na2LiAlH6 decomposes fully into
Na3AlH6 before further decomposition to NaH and Al. This shows clear changes in the
Keywords:
kinetics of the system, and may open possibilities of tailoring the decomposition path by
Energy storage materials
the use of additives.
Hydrogen absorbing materials
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Metal hydrides
reserved.
Thermodynamic properties
X-ray diffraction
1.
Introduction
The increasing climatic problems caused by the use of fossil
fuels as well as the limited amount of these resources, raise
the need of introducing new, clean energy sources and
energy carriers. One of the most widely accepted alternatives for a versatile and reliable energy carrier of the future
is hydrogen. Efficient and safe storage of hydrogen is however a main challenge remaining before hydrogen may be
introduced as a wide-spread energy carrier. A promising
solution to this challenge is to use lighteweight complex
metal hydrides as the hydrogen storage medium, in which
hydrogen is stored chemically [1,2]. This option was intro and Schwickardi of
duced by the discovery by Bogdanovic
reversible hydrogenation of NaAlH4, Na3AlH6 and Na2LiAlH6
promoted by the addition of catalytic amounts of transition
metal based additives [3].
Among the aluminohydrides (alanates) the NaAlH4-system
has received greatest interest, having the combination of good
reversible gravimetric hydrogen capacity and relatively rapid
hydrogenation kinetics within reasonable temperature and
pressure ranges [4e6]. However, the decomposition of NaAlH4
is a two step process, which means technical challenges
related to different pressures and temperatures during the
process, as well as different kinetics in the two steps. The
decomposition of Na3AlH6, being the second step of the
NaAlH4-system, releases 3.0 wt% H2 in one reaction. The
lithium-analogue, Li3AlH6, has a higher hydrogen content of
5.6 wt%, but the plateau pressure at ambient temperature is
too high to make it reversible for mobile use [7]. The mixed
* Corresponding author. Tel.: þ47 63 80 62 09; fax: þ47 63 81 09 20.
E-mail address: j.e.fonnelop@ife.no (J.E. Fonneløp).
0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2011.06.104
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 2 2 7 9 e1 2 2 8 5
alanate Na2LiAlH6 has higher thermodynamic stability than
Na3AlH6 and Li3AlH6, and can be made reversible at moderate
conditions by addition of small amounts of transition metal or
rare earth metal compounds [3]. Recent years several mixed
alanates has been investigated as potential candidates for
hydrogen storage media [5,8e11].
Na2LiAlH6 decomposes through a one step process at
around 255 C with a theoretical release of 3.5 wt% H2 [12]:
3
Na2 LiAlH6 /2NaH þ LiH þ Al þ H2
2
(1)
(Albermarle, > 99% purity), NaH (SigmaeAldrich, 95% purity)
and LiH (SigmaeAldrich, 95% purity) was milled for 3 h at
a speed of 700 rpm. After milling the material was annealed at
80 bar H2 and 150 C for about 14h. All operations and material
handling were performed under purified argon atmosphere
( < 1 ppm O2, <1 ppm H2O) in a Mbraun glove box and sealed
milling vials.
The expected reaction path was the formation of Na2LiAlH6
according to the following reaction:
NaAlH4 þ NaH þ LiH/Na2 LiAlH6 :
(2)
The binary hydrides NaH and LiH must be heated to very high
temperatures to release hydrogen, which will not be considered here. Even though Na2LiAlH6 has a lower theoretical
hydrogen density than e.g. NaAlH4, the advantage of releasing
all the hydrogen in a single-step decomposition makes it
a potential material for a one step hydrogen storage media
fulfilling many of the international requirements of hydrogen
storage materials for vehicular applications. Furthermore,
calculations by Opalka et al. show that the dehydrogenation of
NaAlH4 combined with LiH through Na2LiAlH6 to the end
products LiH and NaH gives favorable thermodynamics
compared to dehydrogenation of NaAlH4 through Na3AlH6 [13].
The mixed alanate Na2LiAlH6 was first synthesized by
Claudy et al. in 1982 by reaction of LiAlH4 with NaH either in
toluene or by a solid state reaction at elevated temperatures
and high H2-pressures [14]. Huot et al. demonstrated the
preparation of Na2LiAlH6 without solvents by mechanochemical synthesis [12]. This was done by ball milling a mixture of
NaAlH4, LiH and NaH for 40 h. Zaluski et al. reported to have
prepared Na1.7Li1.3AlH6 and Na1.5Li1.5AlH6 from the same
starting materials [15]. The deuteride Na2LiAlD6 was synthesized by Brinks et al. by ball milling LiAlD4 and 2 NaAlD4 followed by annealing under D2 [16]. Mechanochemical synthesis
of Na2LiAlH6 by ball milling has later been done through several
reaction paths; by mixing of 2 NaH þ LiAlH4 [17,18],
NaH þ LiAlH4 [19], 2NaAlH4 þ LiH [20] and by mixing of
LiAlH4 þ LiH þ NaH [21]. Na2LiAlH6 takes an ordered perovskitetype structure (space group Fm3 m) [16]. The kinetics and
thermodynamic properties of Na2LiAlH6 was investigated by
et al. [3] and Zaluski et al. [15], and several
both Bogdanovic
further studies have been reported on the system the latest
years [20e27]. All experimental studies observing dehydrogenation of Na2LiAlH6 confirm the decomposition through reaction 1. But in a theoretical study of the thermodynamics of the
NaeLieAleH system, perfomed by Opalka et al., alternative
decomposition paths of the system are discussed [13].
The present study describes the decomposition of
Na2LiAlH6 with the addition of TiF3. Samples with varying
amount of additive have been investigated, as well as a reference sample of pure Na2LiAlH6. In-situ syncrotron powder xray diffraction (SR-PXD) has been done to determine the
decomposition of the samples prior to and after de- and
rehydrogenation cycles.
The sample composition was checked by powder X-ray
diffraction (PXD) using an INEL XRG3000 diffractometer with
monochromized Cu Ka1 radiation, flat-plate geometry, and
a CPS-120 curved, position-sensitive detector that continuously
covers the 2q range from 2 to 120 (D2q ¼ 0.029 ). The sample
was covered by a thin plastic film to prevent reaction with air.
The synthesized powder was divided into four samples.
One was kept unchanged, as a reference (sample 1). TiF3 was
added to the remaining powder by ball milling in a Fritsch
Pulverisette 7 planetary mill for 20 min at 350 rpm in different
amounts; 2 mol% TiF3 (sample 2), and 10 mol% (samples 3 and
4), respectively. The difference between samples 3 and 4 was
the treatment after milling: sample 3 was kept without further
treatment (“as milled”), while sample 4 was dehydrogenated
and rehydrogenated (cycled) once. Sample 2 was also cycled
once. The cycling was performed by keeping the samples at
a hydrogen pressure below 1 bar for 2e5 h during dehydrogenation, and reloading with hydrogen at an initial pressure of
80 bar for 16 h.
Time-resolved in-situ SR-PXD-data were collected at the
Swiss-Norwegian Beam Line (station BM01A) at the European
Synchrotron Radiation Facility (ESRF) in Grenoble, France. The
wavelength of 0.8000 Å was obtained from a channel-cut
Si(111) monochromator. The samples were contained in
boron-silica glass capillaries of 0.5 mm diameter mounted in
a Swagelok fitting. The capillaries were connected to
a vacuum pump and heated from 50 to 400 C at a constant
rate of 1 C/min (sample 2) or 2 C/min (samples 1, 3 and 4)
using a hot air blower. Two-dimensional data were collected
using a MAR345 image plate detector with an exposure time of
30 s. The capillary was rotated approximately 30 during the
exposure. Data readout and erasing required 90 s, thus a full
diffractogram was collected every second minute. The twodimensional data were converted into one-dimensional
powder diffraction patterns with the program Fit2d [28].
Diffraction data were collected in the range 2q ¼ 0.03 e36.4
and were rebinned in steps of D(2q) ¼ 0.02 .
The obtained SR-PXD data were analyzed using the Rietveld refinement program Fullprof (version 3.40) [29]. X-ray
form factors were taken from the Fullprof library. PseudoVoigt profiles were used, and the backgrounds were modeled
by linear interpolation between manually chosen points.
2.
3.
Experimental method
Na2LiAlH6 was mechanochemically synthesized in a Fritsch
Pulverisette 7 planetary mill. A 1:1 mixture of NaAlH4
Results
The synthesized Na2LiAlH6 starting material was checked by
laboratory x-ray. It was confirmed that the composition was
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 2 2 7 9 e1 2 2 8 5
nearly single phase, with only a small amount of Na3AlH6 d
the latter was most probably formed by direct reaction
between NaAlH4 and NaH in the milling process. Small
amounts of remaining LiH are therefore also expected in the
mixture, but this is not easily detected by laboratory x-ray due
to the weak scattering of LiH.
Quantitative phase analysis based on Rietveld refinements was performed on the in-situ SR-PXD data in order to
determine the amount of each crystalline phase in the
samples during the thermal decomposition. The analysis
was done for selected data sets from each series of SR-PXDdata for the 4 samples. For all samples the diffraction data
showed reaction with the capillary giving unknown phases
and peaks at temperatures above 300 C. Therefore the
quantitative analysis is presented only for temperatures
below 300 C. Structural data for the different phases were
taken from the ICSD database [30]. During the refinements
the scale factors, unit cell dimensions, profile- and
displacement parametres were allowed to vary to obtain
a best possible fit to the observed diffraction data. The unit
cell dimensions increased with increasing temperature
according to thermal expansion of the unit cell. A typical
refinement pattern is shown in Fig. 1.
To visualize the evolution of the reactions in the samples
the diffraction patterns are shown as 3-dimensional graphs
in Figs. 2 and 3 for sample 1 and 3 respectively. Sample 1 is
shown for reference, displaying the behaviour according to
reaction 1. For samples 3 it is clearly seen that the decomposition follow a different route. The phase compositions
during heating found from quantitative phase analysis are
shown in Fig. 4. Standard deviations taken from the Rietveld refinement are within the points in the graph. It is clear
that both the addition of TiF3 and cycling have dramatic
effects on the dehydrogenation process. The decomposition
routes for the different samples are presented in the
following.
Fig. 1 e SR-PXD of sample 4, Na2LiAlH6 D 10 mol% TiF3.
Observed intensities (dots) and calculated intensities (line).
Positions for Bragg reflections are shown as vertical bars
for the phases (from top): Na2LiAlH6, Al, NaH and Na3AlH6.
The difference between observed and calculated values is
shown in the bottom line.
12281
Fig. 2 e In-situ SR-PXD data for sample 1, as milled
Na2LiAlH6, heated at 2 C/min. The decomposition of the
sample follows reaction 1.
3.1.
Sample 1
The as milled Na2LiAlH6 sample behaves as expected, see
Fig. 2; the amount of Na2LiAlH6 starts to decrease at 215 C, in
parallel with an increase of Al and NaH. This is consistent with
reaction 1. According to this reaction LiH should also be
present. However, the coherent scattering from LiH is weak,
and in addition its peaks overlap with the stronger reflections
from Al. LiH is therefore not visible by PXD in this case, but its
presence is assumed. The reaction is finished at 250 C. The
next reaction starts at around 300 C, where NaH starts to
disappear. In addition, a rise in the background is observed.
This indicates amorphous or liquid phases, possibly due to
decomposition of NaH to Na(l) and H2 gas above 300 C. This
Fig. 3 e In-situ SR-PXD data for sample 3, Na2LiAlH6 D 10 mol
% TiF3, heated at 2 C/min. Na3AlH6 is formed and increases,
but rapidly disappears before the decomposition of
Na2LiAlH6.
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a
b
c
d
Fig. 4 e The changes in phase composition during heating of the four samples: (a) Sample 1 - Na2LiAlH6 - as milled, (b) Sample 2
- Na2LiAlH6 D 2 mol% TiF3 and cycled, (c) Sample 3 - Na2LiAlH6 D 10 mol% TiF3 and (d) Sample 4 - Na2LiAlH6 D 10 mol% TiF3 and
cycled. The present phases are shown with the following symbols: -: Na2LiAlH6, :: Al, C: Na3AlH6, ;: NaH, +: Li2O, >:
AleTi. The high proportion of Al may originate partly from LiH, due to overlapping peaks in the two phases.
reaction is completed at around 325 C. The only remaining
phase detected in the diffraction data at this temperature is
Al, but the presence of LiH is assumed also here. A similar
reaction was seen for the other samples, though at slightly
lower temperatures.
3.2.
Sample 2
The decomposition of sample 2 (2 mol% TiF3 and cycled once)
is quite similar to that of sample 1, with some notable differences. The first is that the relative amount of Na2LiAlH6 has
decreased to around 80%, while a substantial amount of NaH
(around 16%) is present after cycling, due to incomplete
rehydrogenation. Contrary to sample 1, where no reaction is
seen before 150e200 C, a change is observed in sample 2
already from 50 C. As the temperature is raised, the amount
of NaH decreases, accompanied by an increase of Na3AlH6.
This formation will be further examined in sample 4, where it
is more clear. The relative amount of Na2LiAlH6 also decreases
slightly up to around 200 C. The onset temperature for the
main reaction (200 C) is slightly lower than for sample 1
(215 C). However the reaction is quite similar, and it follows
reaction 1.
3.3.
Sample 3
When the amount of TiF3 is increased to 10 mol% (sample 3),
the changes relative to sample 1 are more pronounced. During
ball milling significant amounts of Na3AlH6 and Al have been
formed. The decomposition of Na2LiAlH6 starts already at
50 C, the content of Na2LiAlH6 decreases to 60 mol% without
any formation of NaH, and is accompanied by an increase of
Na3AlH6 to 30 mol%. As the temperature is increased to 200 C,
the relative amount of Na3AlH6 increases further, while the
amount of Na2LiAlH6 has decreased to 50 mol%. Both
Na2LiAlH6 and Na3AlH6 disappear above 200 C, with corresponding increase of NaH and Al.
Since the decomposition of Na2LiAlH6 below 180 C occurs
simultaneously as an increase of Na3AlH6, it is reasonable to
assume that the reaction up to around 180 C is:
2
1
1
Na2 LiAlH6 / Na3 AlH6 þ LiH þ Al þ H2 ðgÞ:ðw50 180 + CÞ
3
3
2
(3)
However the increase in Al is lower than predicted from this
reaction, and no other phases accounting for the lacking Al
were found. Based on reaction 3 LiH should also be present,
but as previously mentioned this is not easily detected by PXD,
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 2 2 7 9 e1 2 2 8 5
especially in the presence of Al. The main part of the
decomposition of the sample at temperatures above 180 C is
a mixture of the direct decomposition of Na2LiAlH6 (reaction 1)
and the decomposition of Na3AlH6 according to:
3
Na3 AlH6 /3NaH þ Al þ H2 ðgÞ ðw180 225 + CÞ:
2
(4)
These reactions start at a significantly lower temperature
than the decomposition in sample 1; the amount of NaH
already starts to increase somewhat at 150 C. However, the
rapid part of the reaction(s) takes place from 200 C d around
the same temperature as for sample 2. For both samples
Na2LiAlH6 is still present when all Na3AlH6 is decomposed. In
sample 3 Na3AlH6 is completely decomposed at 210 C, while
Na2LiAlH6 remains until 225 C. This means that, as the
temperature increases, reaction 4 is more rapid than the
combination of reactions 1 and 3 for these samples.
alanates mixed with Ti-additives has earlier been confirmed,
and formation of both a solid solution Al1exTix and a crystalline Al3Ti have been reported [24,31e33]. The Al1exTix-phase
(x z 0.15) with unit cell dimensions a ¼ 4.038 Å has been found
after cycling in several samples, but Al3Ti (a ¼ 3.987 Å) is only
seen after prolonged cycling for 7 weeks.
In the present study the the lattice constant of the found
phase is a ¼ 3.98 Å in all three samples, indicating formation of
Al3Ti even in the first dehydrogenation cycle. The 100 reflection of Al3Ti was not seen, and the 110-peak is hidden under
the stronger 111-reflection of NaH, therefore the solid solution
Al1exTix can not completely be ruled out, but based on the unit
cell dimensions the phase is identified as Al3Ti. The content of
Al3Ti remains constant during the in-situ PXD measurement
of sample 4.
4.
3.4.
12283
Discussion
Sample 4
Sample 4 is similar to sample 3 (10 mol% TiF3 added), but has
been through a dehydrogenation-rehydrogenation cycle
before the in-situ SR-PXD measurement. This has led to some
significant changes. At the start of the measurement the
cycled sample contains 40 mol% Na2LiAlH6 and 30 mol% NaH,
in addition to appreciable quantities of Al and Al1exTix (see
below). Between 70 and 150 C, the amount of Na3AlH6
increases, while Na2LiAlH6 and NaH decrease. In the range
from 70 to 150 C weak peaks or shoulders are observed
around the 111 and 200 diffraction peaks of Al. This could
origin from NaOH at the low angle side and LiF at the highangle side of the Al peaks. But the content could not be precicely determined due to strong overlap with Al peaks. An
impurity phase, Li2O, has also emerged at 125 C. As previously
mentioned, the capillaries were strongly corroded after the
measurements. The presence of Li2O only in sample 4, may
show that the corrosion occurs earlier in this sample due to
the high content of TiF3 and the dehydrogenationrehydrogenation cycle.
The fact that the amount of NaH present in the sample is
decreasing instead of increasing up to about 180 C (as
expected from reaction 1) indicates that Na3AlH6 is formed in
a reaction involving both NaH and Na2LiAlH6 in this temperature range. Thus, most of the decomposition of Na2LiAlH6
takes place due to the presence of NaH without the release of
hydrogen through the following reaction:
Na2 LiAlH6 þ NaH/Na3 AlH6 þ LiH:ðw70 180 + CÞ
(5)
At 180 C there is no more Na2LiAlH6 left. The main part of
the dehydrogenation, starting at around 180 C, thus follows
reaction 4.
For all the three samples containing TiF3 there are small
peaks or shoulders on the high-angle side of the strongest Al
reflections, 111 and 200. These peaks are identified as an
Al1exTix-phase, and are most clearly seen in sample 4. In
samples 2 and 3 the phase is formed upon heating and is
observed from 300 C to 250 C, respectively. In sample 4 this
phase is present at the start of the measurement, showing
that it has been formed during the initial dehydrogenationrehydrogenation process. The formation of Al-Ti-phases in
All earlier publications on the syntesis and decomposition of
Na2LiAlH6 report the decomposition through reaction 1. Small
amounts of Na3AlH6 are often seen after synthesis of
Na2LiAlH6 by ball milling [16,17,20,23,24], but no deviation
from the expected decomposition path is reported. Mamatha
et al. observe co-existence of Na2LiAlH6 and Na3AlH6 in
a sample studied by in-situ PXD [23]. Na2LiAlH6 is found to
decompose before Na3AlH6, but no additional formation or
participation of Na3AlH6 in the decomposition is discussed.
Thus the present study represents new findings, and the
observed results must originate from the use of additive and
cycling.
In all the samples with added TiF3, formation of Na3AlH6
was observed, both in the as milled sample and after cycling.
Opalka et al. have calculated the phase diagram of the
NaeLieAleH system from first principles, and concluded that
below 87 C reaction 3 is thermodynamically favorable
compared to reaction 1 [13]. Thus thermodynamically
Na2LiAlH6 should decompose to Na3AlH6 at low decomposition temperatures, while the direct decomposition of
Na2LiAlH6 to binary hydrides, Al and H (reaction 1) should only
occur at higher decomposition temperatures. This is in
agreement with the present observation of decomposition
from Na2LiAlH6 through Na3AlH6 at lowered decomposition
temperature obtained by additives. Based on the calculations
by Opalka et al. reaction 3 should be seen in all the samples,
including the decomposition of pure Na2LiAlH6 (sample 1), if
the system was in equilibrium. As this is not the case the
reaction must be limited by kinetic barriers. Thus introduction
of a suited catalyst would bring the system closer to equilibrium and facilitate the decomposition through reaction 3. A
recent study reports a similar effect for the combined system
Na2LiAlH6eMg(NH2)2 [34]. Varying the ball milling conditions
results in new decomposition paths, showing that the stability
of the system is sensitive to the sample treatment.
The presence of Na3AlH6 in the as milled sample 3 shows
that the decomposition of Na2LiAlH6 to Na3AlH6 occurs
already during ball milling when TiF3 is added. This shows
that the increased energy and temperature during milling is
sufficient to start reaction 3. The decomposition of the sample
does not proceed further during milling, since no NaH is seen.
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NaH is seen at the start of the measurement in both the cycled
samples 2 and 4. This shows that the cycling has not been
complete; meaning that the reverse reaction 1 has not been
completely finished. This is particularly pronounced in
sample 4, where more than 30 mol% remains as NaH in the
rehydrogenated sample.
The onset temperatures for the decomposition reactions of
Na2LiAlH6 decrease when TiF3 is added. This is most readily
explained as a catalytic effect. The lower heating rate for
sample 2 compared to the other samples might play a role, but
as the change in decomposition temperature is much more
pronounced in sample 3 and 4, the contribution from a change
in heating rate is regarded as a minor effect.
Comparisons of the decomposition of the four investigated
samples clearly show that there are at least three reactions
involved in the decomposition of Na2LiAlH6; reactions 1, 3 and
5, and that they are sensitive to the altered conditions by
addition of TiF3 and cycling. Sample 1 decomposes solely by
reaction 1. In samples 2 and 3 the decomposition starts by
reaction 3 and switches to reaction 1 as the temperature is
increased. This is consistent with the modeling results of
Opalka et al. Up to around 200 C, the amount of Na3AlH6
increases at the expense of Na2LiAlH6, but above 200 C the
desorption proceeds through direct decomposition of
Na2LiAlH6. At the same time a direct decomposition of
Na3AlH6 through reaction 4 is seen. Na3AlH6 decomposes
faster than Na2LiAlH6, and the decomposition is completed at
a lower temperature. In sample 4 all the Na2LiAlH6 has
decomposed through reactions 3 and 5 before the decomposition path changes to reaction 4. The observed temperature
of the switch between the two reactions 3 and 1 is higher than
what was predicted by modeling, but this may be due to
different kinetic barriers for the various possible reactions.
Also, the uncertainty of the transition temperature predicted
by modeling is quite large.
The fact that the contribution from reaction 3 increases
first by addition of TiF3 in sample 2 and then a further increase
by larger amount of additive in sample 3 strongly indicates
a catalytic effect of the additive. Several reports show an
enhanced decomposition rate of Na2LiAlH6 by addition of Ti
species, even at low fractions of the additive [3,20e22,24].
From this kinetic enhancement by cycling the sample with
additives the same results should be expected in both sample
2 and 4. But sample 4 seems to decompose by the suggested
reaction 5 by combination of Na2LiAlH6 and NaH to form
Na3AlH6. This indicates that when NaH is present, reaction 5 is
favorable and happens faster than reaction 3.
One mechanism that may promote the production of
Na3AlH6 from Na2LiAlH6 (reaction 3 and 5) is growth without
nucleation. Since there is already Na3AlH6 present in all
samples 2e4, growth of this phase may take place without the
need for nucleation, which may be a rate-limiting step. This
effect has been reported by Blanchard et al. for the LiAlD4system [35]. It is shown that the decomposition rate increases
significantly when some crystallites of the end product are
present.
Finally, partial substitution of the added species could be
a source of enhanced kinetics. Fluoride anion substitution has
shown to give enhanced kinetics in doped NaAlH4-systems
[36e38]. The diffraction data of the present study was
therefore investigated for substitution, but no sign of substitution of neither Fe or Ti3þ in Na2LiAlH6 was found. Thus it is
concluded that the changed decomposition routes origin from
a catalytic effect of the additive and cycling.
5.
Conclusion
The addition of TiF3 and cycling changes the dehydrogenation
reactions of Na2LiAlH6. The temperature of the decomposition
of Na2LiAlH6 is lowered by the addition of TiF3, and new
decomposition paths are found. The additive shows a catalytic
effect on the system, and three competing decomposition
reactions are observed. Decomposition of Na2LiAlH6 through
Na3AlH6 is shown for the first time, in accordance with
previously published theoretical studies predicting this to be
thermodynamically favorable for the system. This decomposition is not observed in pure Na2LiAlH6-samples, thus the
present study shows that the catalytic effect of the additive
brings the system closer to equilibrium. Na2LiAlH6 partially
decomposes to Na3AlH6 before the expected decomposition to
NaH, LiH an Al. Prior to cycling Na2LiAlH6 decomposes directly
to Na3AlH6. However in a cycled sample containing NaH the
initial Na2LiAlH6 combines with NaH to form Na3AlH6 in
parallel with the direct decomposition from Na2LiAlH6 to
Na3AlH6. Better understanding of this effect could give the
possibility of changing and tailoring the decomposition route
of the major phases by selective use of additives.
Acknowledgments
Financial support from the RENERGI and SYNKROTRON programmes in The Research Council of Norway is acknowledged. The skillful assistance from the project team at the
Swiss-Norwegian Beam Line, ESRF, is gratefully acknowledged. Dr. Anita Fossdal is acknowledged for preparation of
the samples at IFE.
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