Note: Descriptions are shown in the official language in which they were submitted.
CA 02339656 2001-02-05
WO 00/07930 PCT/US99/15994
NOVEL HYDROGEN STORAGE MATERIALS AND
METHOD OF MAKING BY DRY HOMOGENATION
Field of the Invention
5 The present invention relates generally to the field of reversible hydrogen
storage. More particularly, the present invention relates to a dry homogenized
metal
hydrides, in particular aluminum hydride compounds, as a material for
reversible
hydrogen storage, and a method of making the same.
Bac round of the Invention
For decades, hydrogen has been targeted as the utopian fuel of the future due
to its abundance and environmental friendliness. A major difficulty in the
utilization
of hydrogen as a fuel is the problem of onboard hydrogen storage. High
pressure and
cryogenic hydrogen storage systems are impractical for vehicular applications
due to
safety concerns and volumetric constraints. This has prompted an extensive
effort to
develop solid hydrogen storage systems for vehicular application. Metal
hydrides,
activated charcoal, and carbon nanotubules have been investigated as hydrogen
carriers .
For example, LaNiHs has been investigated but has not proved satisfactory, due
in part
to its high cost. Unfortunately, despite decades of extensive effort,
especially in the
20 area of metal hydrides, no material has been found which has the
combination of a high
gravimetric hydrogen density, adequate hydrogen dissociation energetics, and
low cost
required for commercial vehicular applications.
It is known that the dehydrogenation of NaAlH4 is thermodynamically favorable
at moderate temperatures. Dehydrogenation is known to occur by a multistep
process
involving the reactions as shown in equations 1 and 2 below:
CA 02339656 2001-02-05
WO 00/07930 PCT/US99/15994
-2-
3 NaAlH4 ----------> Na3A1H6 + 2 A1 + 3 HZ (1)
Na3A1H6 --------- > 3 NaH + A1 + 3/2 H Z (2)
The process is characterized.by very slow kinetics and reversibility only
under severe
conditions. Thus, NaAlH4 has generally been precluded from consideration as a
potential hydrogen storage material despite having a 5.6 weight percentage of
hydrogen
which is thermodynamically available at moderate temperatures. This thinking
has been
changed by the recent finding by Bogdanovic and Schwickardi that titanium
doping of
NaAlH4 enhances the kinetics of hydrogen desorption and renders the
dehydriding
process reversible under moderate conditions. Bogdanovic found that the onset
of the
initial dehydriding was lowered by about 50°C upon titanium wet doping
by
evaporation of an ether suspension of NaAlH4 which contained 2 mol % of
titanium
tetra-n-butoxide, Ti(OBu")4. This prior art approach however, is subject to
many
limitations. For example, the temperatures are still relatively high and the
reaction
kinetics are such that it does not produce a material suitable for practical
vehicular
applications.
Thus, further development of the kinetics of the dehydriding process is
required
to produce a material which is suitable for practical vehicular applications.
It is of
interest to investigate whether further developments in the kinetics of the
reversible
dehydriding of metallic hydrides, such as NaAlH4 and the like, can be
achieved.
Further, as the aforementioned discussion demonstrates, the need exists for
safe,
plentiful, low cost, and effective materials and methods for hydrogen storage
and
release.
Summary of the Invention
The present invention provides novel reversible hydrogen storage materials and
methods of making said materials, that are readily prepared from cheap,
abundant
starting materials.
More particularly, the present invention provides a new dry doping method
comprising the steps of dry homogenizing metal hydrides by mechanical mixing,
such
as by crushing or ball milling a powder, of a metal aluminum hydride with a
transition
CA 02339656 2001-02-05
WO 00/07930 PCT/US99/15994
-3-
metal catalyst. The metal aluminum hydride is of the general formulas of: X
~A1H4,
where X1 is an alkali metal; X2(A1H 4) Z, where XZ is an alkaline earth metal;
X3(A1H
4) a, where X3 is Ti, Zr or Hf; X4AIH6, where X4 is an alkali metal;
XS(A1H6)2, where
XS is an alkaline earth metal; X ~A1H ~ 4 where X his Ti, Zr or Hf; or any
combination
of the above hydrides.
In another aspect of the present invention, a material for storing and
releasing
hydrogen is provided, comprising a dry homogenized material having transition
metal
catalytic sites on a metal aluminum hydride compound, or mixtures of metal
aluminum
hydride compounds.
The inventors have found that the homogenization method of the present
invention of metal aluminum hydrides with transition metal catalysts resulted
in a
lowering of the dehydriding temperature by as much as 7S°C and markedly
improves
the cyclable hydrogen capacities. These findings represent a breakthrough in
the
application of this class of hydrides to hydrogen storage. In particular these
findings
enable the development of practical hydrogen storage materials and methods for
the
powering of vehicles, an achievement which has not before been realized.
Brief Description of the Drawinec
These and other objects and advantages of the invention are apparent in
reading
the description herein, the appended claims, and with reference to the
figures, in
which:
Figure 1 shows a comparison of thermal desorption (2 °C/min) of
hydrogen
from undoped and wet titanium doped NaAlH4 of the prior art, and one
embodiment of
the material of the present invention, in this case dry homogenized titanium
doped
NaAlH4.
Figure 2 depicts thermal programmed desorption (2 °C/min) of
hydrogen from
samples of dry titanium doped NaAlH4 material of the present invention
prepared from
1, 2 and 4 (0.5x, x, and 2x) mol of transition metal catalyst Ti(OBu")a.
Figure 3 shows a comparison of thermal programmed desorption (2 °
C/min) of
hydrogen from undoped and wet titanium doped NaAlH4 of the prior art, and one
CA 02339656 2001-02-05
WO 00/07930 PCTNS99/15994
-4-
embodiment of the material of the present invention, in this case dry
homogenized
titanium doped NaAlH4, following one dehydrogenation/rehydrogenation cycle.
Figure 4 illustrates xhe effect of dehydriding/rehydriding cycles on thermal
programmed desorption (2°C miw') of hydrogen from an alternative
embodiment of the
material of the present invention, in this case NaAlH4 doped with zirconium.
Figure 5 shows the effect of dehydriding/rehydriding cycles on thermal
programmed desorption (2°C min-') of hydrogen from titanium doped
NaAlH4, using
the homogenization method of the present invention.
Figure 6 illustrates the thermal programmed desorption (2 °C miw') of
hydrogen
from various doped samples of NaAlH4 according to the present invention after
three
cycles of dehydriding/rehydriding.
Description of the Invention
Of significant advantage, the present invention provides novel reversible
hydrogen storage materials and methods of making said materials, that are
readily
prepared from cheap, abundant starting materials.
More particularly, the present invention provides a new dry doping method
comprising the steps of dry homogenizing metal hydrides by mechanical mixing,
such
as by crushing or ball milling a powder, of a metal aluminum hydride with a
transition
metal catalyst. The metal aluminum hydride is of the general formulas of: X
~A1H4,
where X, is an alkali metal; X2(A1H 4) 2, where XZ is an alkaline earth metal;
X3(A1H
4) 4, where X3 is Ti, Zr or Hf; X4A1H6, where X4 is an alkali metal;
X3(AIH6)2, where
XS is an alkaline earth metal; X~(A1H ~ 4, where X6 is Ti, Zr or Hf; or any
combination
of the above hydrides.
In another aspect of the present invention, a material for storing and
releasing
hydrogen is provided, consisting of a dry homogenized material having
transition metal
catalytic sites on a metal aluminum hydride compound, or mixtures of metal
aluminum
hydride compounds.
In another aspect of the present invention the hydrogen storage material is
us~l
to power a vehicle apparatus, and the novel method further includes the steps
of
CA 02339656 2001-02-05
WO 00/07930 PCT/US99/15994
-5-
dehydrogenating the dry homogenized hydrogen storage material to release
hydrogen,
and powering a vehicle apparatus with the released hydrogen.
The material and method of the present invention are quite different from the
prior art (in particular Bogdanovic's doped material) and exhibit markedly
improved,
and unexpected, catalytic effects. The inventors have found that the
homogenization
method of the present invention of metal aluminum hydrides with transition
metal
catalysts results in a lowering of the dehydriding temperature by as much as
about 75 °C
and markedly improves the cyclable hydrogen capacities. These findings
represent a
breakthrough in the application of this class of hydrides to hydrogen storage.
In
particular these findings enable the development of practical hydrogen storage
material s
and methods for the powering of vehicles, an achievement which has not before
been
practically realized.
As described in the Background, the dehydrogenation of certain metal aluminum
hydrides, in particular NaAI H4, are thermodynamically favorable at moderate
temperatures. It is known to occur by a mufti step process involving the
reactions
illustrated in equations 1 and 2. While this material has a relatively high
percentage
of hydrogen, the process exhibits very slow reaction kinetics and is
reversible only
under severe conditions. An example of severe conditions would be at a
pressure of
about 175 atmospheres of hydrogen at about 270 °C.
In great contrast to the prior art, the dehydrogenation kinetics of NaAIH 4
according to the present invention have been enhanced far beyond those
previously
achieved upon titanium doping of the host hydride. In one exemplary
embodiment,
homogenization of NaAlH4 with approximately 2 mole ~ of titanium catalyst, in
particular Ti (OBu")4 , under an atmosphere of argon produces a novel material
that
contains only traces of carbon. Thermal programmed desorption (TPD)
measurements
show that the dehydrogenation of this material occurs about 30 °C lower
than that
previously found for NaAlH4 doped with titanium through wet chemistry methods.
This
lowering of the temperature of dehydrogenation represents a significant
advance
towards enabling the use of the material as a hydrogen storage material for
powering
vehicles with hydrogen. The novel titanium containing material can be
completely
CA 02339656 2001-02-05
WO 00/07930 PCTNS99/15994
-6-
rehydrided under 150 atm of hydrogen pressure at 170 °C. In further
contrast to the
"wet doped" NaAlH4 material, the dehydrogenation kinetics observed for this
novel
material are undiminished over several dehydriding/hydriding cycles.
More specifically, the present invention provides for doping aluminum hydrides
with transition metal catalysts using dry homogenation. Suitable aluminum
hydrides
which may be practiced with this method are generally of the formulae: X,A1H4,
where
X1 is an alkali metal; X1 (A1H 4 ) 2 , where Xi is an alkaline earth metal; X
{A1H4 )a ,
where X3 is Ti, Zr or Hf; X4AIH6, where X4 is an alkali metal; XS (AII~ ~ ,
where XS
is an alkaline earth metal; X6(AIH 6) 4, where X6 is Ti, Zr or Hf; or any
combination
of the above hydrides. Examples of such suitable aluminum hydrides include,
but are
not limited to: sodium aluminum hydride (NaAlH3), sodium aluminum hexahydride
(Na3A1H6), magnesium aluminum hydride (Mg(AII~ ~ ), titanium aluminum hydride
(Ti(A1H 4) 4), zirconium aluminum hydride (Zr(A1H4) 4), and the like. The
transition
metal catalyst used with the present invention include titanium, zirconium,
vanadium,
iron, cobalt or nickel. Examples of transition metal and lanthanide metal
complexes
which are suitable catalyst precursors include, but are not limited to Ti(OBu)
4,
Zr(OPr)4, VO(OPri)3, Fe(acac)2, Co(acac)2, Ni(1,5-cyclooctadiene)2 ,
La(acac)3, and
mixtures thereof, where acac is acetylacetonate and Pri is isopropyl. In one
preferre d
embodiment, the hydrogen storage material of the present invention is
comprised of
NaAlH4 doped by dry homogenation with Ti(OBu"~ . In an alternative preferred
embodiment, the hydrogen storage material of the present invention is
comprised of
NaAlH4 doped with Zr(OPr)4 catalyst by dry homogenation.
According to the present invention, dry homogenation is performed to dope the
aluminum hydride with the transition metal catalyst. Homogenation is performed
by
mechanical methods; such as for example by manual grinding in a mortar and
pestle,
preferably for about 15 minutes; by mechanically blending in a mixer-grinder
mill,
preferably for a time in the range of about 5 to 10 minutes; or by balling,
preferably
for a time in the range of about 5 to 20 minutes. The homogenatinn nrnr.P~~ ;~
considered "dry" because the process takes place in the absence of a solvent
or any
aqueous medium. Preferably, the homogenation process is performed in an inert
CA 02339656 2001-02-05
WO 00/07930 PCTNS99/15994
_'7_
atmosphere, such as argon, and the like.
The amount of transition metal catalyst used in the dry homogenation process
of the present invention is not particularly limited, and is generally
selected as that
amount useful for providing the desired catalytic activity. For example, to
attain a
catalytic effect when using a titanium catalyst, at least 0.2 mol % of the
titanium
precursor is used in the doping of the hydride. The maximum catalytic effect
is
observed at about 2.0 mol % of the titanium precursor, and the catalytic
effect is not
improved by doping with greater than 2.0 mol % of the titanium precursor. An
illustrated preferred range when doping the hydride with a titanium catalyst
is in the
range of about 0.5 to about 1 mol % Ti catalyst to aluminum hydride. An
illustrated
preferred range when doping the hydride with a zirconium catalyst is in the
range of
about 0.5 to about 1 mol % Zr catalyst to aluminum hydride.
In an exemplary embodiment, NaAlH4 is doped with Ti (OBun~ in an inert
atmosphere according to the method of the present invention to produce an
inventive
1 S material for storing and releasing hydrogen. The novel titanium containing
(dry doped)
materials were prepared by adding prescribed amounts of Ti (OBu°) 4 to
freshly
recrystallized NaAlH4 under an atmosphere of argon. The originally colorless
mixtures
were homogenized using a mortar and pestle until they became red-violet. This
color
change suggests that at least some of the Ti4+ was reduced to Ti3+, The
resulting paste
was visually very distinct from the brown powders obtained through
Bogdanovic's
procedure for producing titanium containing (wet doped) material. Elemental
analysis
showed that only trace amounts of carbon are present in the dry homogenized
material of
the present invention. Evidently, beta-hydride elimination from the allcoxy
ligands leads
dissociation of the organic groups as butanal from the titanium center and the
deposition
of a hydrido titanium species on the NaAlH4 host material. The presence of
nonmetallic
titanium on the surface of the novel material has been confirmed through
surface x-ray
studies. The dry homogenizing method thus creates titanium catalytic sites on
the
NaAlH4 fresh crushed crystals. Of significant advantage these dry homogenized
materials repeatedly store and release hydrogen at obtainable temperatures and
modest
pressures. In one exemplary embodiment, the dry homogenized method of the
present
CA 02339656 2001-02-05
WO 00/07930 PCT/US99/15994
_g_
invention results in the release of about 4 to 5.5 weight percent of hydrog en
with rapid
discharge of hydrogen occurring at a temperature in the range of about 80 to
120 °C.
In another exemplary embodiment, the inventors have investigated the
dehydriding/rehydriding behavior of NaAlH4 in which a zirconium catalyst was
introduced according to the dry homogenadon doping method of the present
invention.
While zirconium was found to enhance the dehydriding kinetics of NaAlH4, the
catalytic action is seen to be different than that of titanium. Furthermore,
the inventors
have found that the differing catalytic effects of titanium and zirconium can
be carried
out in concert.
TPD measurements were made on the following samples: the dry doped material
("sample 1 ") of the present invention; the wet doped material of the prior
art ("sample
2"); and undoped NaAlH4 ("sample 3") of the prior art. Excellent agreement was
found
among samples which were prepared at different times. The data obtained for
sample 2
was consistent with Bogdanovic's findings. The TPD measurements were made on
samples of the three different materials. The plot of the hydrogen weight
percentage
desorbed as a function of temperature seen in Figure 1 is based on the
integrated TPD
data. While the catalytic effect of titanium is evident for both samples 1 and
2, of
significant advantage the dehydrogenation temperature of sample 1 is seen to
be about °30
C lower than that of sample 2.
A priori, it seemed possible that the differences observed in the dehydriding
behavior of samples 1 and 2 were simply due to variation in the level of
titanium loading
in the two materials. In order to probe this possibility, independent TPD
measurements
were made on samples of 1 and 2 that were prepared using 1.0, 2.0, and 4.0 mol
% Ti
(OBu") 4, shown as curves 1 l,12 and 13, respectively. As seen in Figure 2,
variations
in the amount of Ti (OBu") 4 used in the preparation showed little effect on
the
dehydrogenation temperature. However, increasing the titanium content of the
material
does show the gravimetric effect of lowering the H/M weight percent {i.e. the
weight of
hydrogen evolved per unit weight of the metal aluminum hydride). These results
indicate that only a fraction amount of titanium introduced into the materials
is
catalytically active. Furthermore, there is clearly a significantly larger
amount of
CA 02339656 2001-02-05
WO 00/07930 PCT/US99/15994
_g_
catalytically active titanium in sample 1 than sample 2. It appears that the
dry doping
method is more effective than the wet doping method for the generation of the
active
titanium sites. It is possible. that the action of wet doping method is
restricted to the
surface of the hydride while the dry doping method introduces active titanium
sites in the
bulk of the material.
Since reversibility is an important requirement for most hydrogen storage
applications, the behavior of the samples during repeated cycling was
investigated. The
sample materials of the present invention were rehydrided under 1600 psi of
hydrogen
pressure at 200 °C. Only about 40% of the hydrogen in the original
material is replaced
at the moderate hydrogen pressure. TPD measurements were then made on the
rehydrided
samples. Figure 3 shows the percentage of hydrogen desorbed from the samples
as a
function of temperature, considering the desorbed hydrogen at the first cycle
to be 100%.
The uptake is clearly less than found in the original sample, showing that
only partial
hydrogenation could be obtained under these conditions. The second dehydriding
cycle
of sample 1 occurs at nearly the same temperature observed for the first. This
sharply
contrasts the dehydriding behavior of sample 2 (prior art) for which
dehydriding occurs
at a significantly higher temperature, closer to that of sample 1, in the
second cycle.
The inventors have found that the enhancement of the dehydrogenation kinetics
of NaAlH4 upon introduction of titanium to the material is highly sensitive to
the doping
method. The novel dry doping method of the present invention is much more
effective
for the generation of catalytically active titanium sites than the wet doping
method
previously reported. It is also significant that unlike the wet doped
material, the kinetic
enhancement of the dry doped material is undiminished over several
dehydriding/
hydriding cycles. The results also indicate that the catalytic effect in the
titanium doped
material is due to that only a fraction amount of titanium is introduced into
the host
hydride.
In an alternative exemplary embodiment of the present invention, zirconium
doped NaAlH4 was prepared by homogenizing freshly recrystallized hydride with
Zr(OPr)4 under an atmosphere of argon. Hydrogen evolution from samples of the
zirconium doped hydride was studied by thermal programmed desorption (TPD).
Plots
CA 02339656 2001-02-05
WO 00/07930 PCT/US99/15994
-10-
of the desorbed hydrogen weight percentage as a function of temperature are
illustrated
in Figure 4. The discontinuity in the desorption curves reflects the
difference in
activation energies of the dehydriding reactions as seen in equations 1 and 2.
In
contrast to the titanium doped material, the catalytic effect is most
pronounced for the
dehydriding of Na3A1H6 to NaH and A1 (equation 2) rather than the dehydriding
of
NaAlH4 to Na3A1H6 and A1 (equation 1). In view of the closely related
chemistry of
titanium and zirconium, it is surprising that their primary catalytic effects
are exerted
on different reactions in the dehydriding process.
The rehydriding is also catalyzed by zirconium doping. As observed for
titanium doped NaAlH4, recharging of the dehydrided materials can be achieved
at
170°C and 150 atm of hydrogen pressure.
An important aspect of a hydrogen storage material is its ability to perform
afte r
repeated dehydriding/rehydriding cycles. Of particular advantage, after the
preliminary
cycle of dehydriding/rehydriding, the TPD spectra of the zirconium containing
materials showed excellent reproducibility. As shown in Figure 4, the
temperature
required for dehydriding is consistently 20°C lower than for the first
cycle. Similar
behavior was observed in a parallel study of materials doped with 2 mol %
titanium
through homogenization method of the present invention. As further illustrated
in
Figure 5, the temperature required for the dehydriding reactions is lowered by
20 ° C
after the preliminary dehydriding/rehydriding cycle. The onset of rapid
dehydrogenation at 100°C in the titanium doped material is noteworthy
as it suggests
the application of these materials as hydrogen carriers for onboard fuel
cells.
The hydrogen capacity of these materials drops to 4.5 wt % in the second cyc
le
but is also stabilized by the third cycle. We previously noted similar
stabilization of
the hydrogen storage capacity in titanium doped NaAlH4 which was prepared
through
the inventive homogenization technique.
The chain of advancement in the development of metal catalyzed NaAlH4 is
illustratal by comparison of the TPD spectra of the third dehydriding cycle of
variety
of doped materials. As shown in Figure 6, hydride which was doped with
titanium
through the method of Bogdanovic has a cyclable hydrogen capacity of 3.2 wt %
.
CA 02339656 2001-02-05
WO 00/07930 PCT/US99/15994
-11-
Titanium doping through the homogenization method of the present invention
significantly enhances the kinetics of the first dehydriding reaction and
improves the
cyclable hydrogen capacity to 4.0 wt % . The zirconium doped material shows
enhancement of the kinetics of the second dehydriding reaction and a further
improved
cyclable hydrogen capacity of 4.5 wt % . However, the kinetics of the first
dehydriding
reaction in the Zr doped material are inferior to those of the titanium doped
material
of the present invention.
In order to determine the compatibility of the catalytic action of zirconium
and
titanium according to the present invention, a sample was prepared in which
NaAlH4
homogenized with 1 mol % of both Zr(OPr)4 and Ti(OBu°~ . The sample was
then
stabilized by three dehydriding/rehydriding cycles. With further reference to
Fig. 6,
it is shown that the TPD spectrum of the titanium/zirconium doped material is
a virt ual
superposition of the first segment of the curve for the titanium doped
material and the
second segment of the zirconium doped material. Thus, titanium and zirconium
can act
in concert to optimize the dehydriding/rehydriding behavior of NaAlH4.
The inventors have found that the dehydriding kinetics of NaAlH4 are
significantly enhanced through zirconium doping. While zirconium is inferior
to
titanium as a catalyst for the dehydriding of NaAlH4 to Na3A1H6 and Al, it is
a superior
catalyst for the dehydriding of Na3A1H6 to NaH and Al. The benefit of both
catalytic
effects can be realized in materials containing a combination of both titanium
and
zirconium catalysts. After the initial dehydriding/rehydriding cycle, NaAlH4
which is
doped with titanium and/or zirconium is stabilized with a greater than 4 wt %
cyclable
hydrogen. Finally, the occurrence of rapid dehydriding in the titanium
containing
materials at temperatures below 100°C suggests their application as
hydrogen carriers
for onboard fuel cells.
Experimental
The following experiments are provided for illustration purposes only and are
not
intended to limit the present invention in any way.
CA 02339656 2001-02-05
WO 00/07930 PCTNS99/15994
-12-
Experiments in General
All reactions and operations were performed under argon in a glove box or
using
standard Schlenk techniques with oxygen and water free solvents. Sodium
aluminum
hydride, NaAlH4, was purchased from Aldrich Chemical Inc. and recrystallized
from
THF/pentane before use. Ti (OBu ") 4 was used as purchased from Strem Chemical
Inc.
"Wet" titanium doped NaAlH4 was prepared from the evaporation of an ether
suspension
of NaAlH4 that contained 2 mol of Ti (OBu ") 4 as previously described by
Bogdanovic.
The elemental analysis was performed by Oneida Research Services Inc.,
Whitesboro,
NY.
The titanium doped material
In a glove box, NaAlH4 (0.54 g, 10 moI) was combined with Ti (OBu ")4 (0 .26
mL, 0.76 mol). The mixture was homogenized a using mortar and pestle for 15
minutes
until a red-violet paste -was produced. Elemental analysis of the resulting
material showed
its composition to be C, 0.25%; H, 7.01%. Samples were also prepared through
this
procedure using 0.13 mL (0.38 mol) and 0.52 mL (1.52 mol) of Ti (OBu") a.
A thermovolumetric analyzer (TVA), based on a modified Sievert's type
apparatus, was used to characterize the gas-solid interaction between hydrogen
and the
sodium aluminum hydride systems. The TVA consisted of two high pressure
stainless
steel Parr reactors (Model 452HC-T316), one used to hold the sample and the
other as a
gas reservoir, between which very small precisely measured volumes of hydrogen
may
be transferred. The sample vessel contained an aluminum insert with two narrow
cylindrical cavities. A K-type thermocouple was placed inside each of the
cavities. One
of the cavities contained the sample and the other was used as a temperature
reference.
The sample cavity was designed to insure intimate contact between the aluminum
insert
and the sample. This, together with the high thermal conductivity of the
insert served to
minimize temperature fluctuations within the sample resulting from the heat of
reaction
or rapid pressure change. The entire sample vessel could be heated and cooled
using a PID
programmable controller unit that allows sample temperatures to be controlled
and
programmed to change between 196 and 673 °K. In order to reach 196
°K, the entire
CA 02339656 2001-02-05
WO 00/07930 PCT/US99/15994
-13-
sample vessel was placed inside a container surrounded by a mixture of dry ice
and
acetone.
Hydrogen pressures inside the vessels were measured using high precision
pressure transducers. Different size aluminum inserts were available to adjust
the dead
volume above the sample, allowing total pressure and pressure changes to be
maintained
within the range and precision of our instrumentation as sample size and
hydrogen
loading varied. The volumes of the sample vessel and gas reservoir and the gas
flows
between them were calibrated using hydrogen and argon.
The gas system was constructed using high purity regulators, a VCR sealed
manifold capable of operating under vacuum or at elevated pressure, diaphragm
type shut-
off valves, and micro valves to control gas flows between reactors. The gas
lines and
vessels were tested on regular basis for inboard or outboard gas leaks. System
temperatures and pressures were recorded using a high data acquisition system
together
with a software developed for this task.
The rates of hydrogen desorption for each of the three samples were measured
using a thermal programmed desorption (TPD) technique. A sample of about 0.5
grams
was weighted, and loaded into the high pressure reactor under argon
atmosphere. The
samples were then heated from room temperature to 280 °C at a rate of 2
°C per minute
while maintaining low hydrogen over pressure in the sealed reactor. The rate
of hydrogen
desorption was measured as a function of temperature. On selected samples, the
TPD
measurements were repeated to insure the reproducibility of the samples and
the
measurements.
Reagents
All reactions and operations were performed under argon in a glove box.
NaAlH4 and zirconium tetra-n-propoxide, Zr(OPr~ (70 wt. 9& in propanol
solution)
were purchased from Aldrich Chemical Inc. NaAlH4 was recrystallized from
THF/pentane using standard Schlenk techniques with oxygen and water free
solvents.
Ti(OBu°)4 was used as purchased from Strem Chemical Inc.
CA 02339656 2001-02-05
WO 00/07930 PCT/US99/I5994
-14-
Zirconium and titanium doped materials
In a glove box, NaAlH4 (0.54 g, 10 mmol) was combined with 94 ~L of a 70
wt ~ solution of Zr(OPr)4 in propanol. Homogenized samples were prepared by
first
manual mixing with a mortar and pestle for 5 minutes and then mechanical
blending
with a Wig-L-Bug electric grinder/mixer for 15 minutes. Titanium doped samples
were
similarly prepared using Ti(OBu")4 (70 ~,L 0.20 mmol). Titanium/zirconiurn
doped
hydride was homogenized with 0.047 mL of a 70 wt % solution of Zr(OPr) 4 and
Ti(OBu")4 (35 ~,L, 0.10 mmol).
Thermal programmed desorption (TPD) measurements
The gas-solid interaction between hydrogen and the sodium aluminum hydride
systems were characterized using a thermovolumetric analyzer (TVA), based on a
modified Sievert's type apparatus. The TVA system contained a high pressure
reactor
vessel with a PID programmable temperature controller unit. Hydrogen pressures
inside the vessels were measured using high precision pressure transducers.
Different
size aluminum inserts were available to adjust the dead volume above the
sample,
allowing total pressures and pressure changes to be maintained within the
range and
precision of our instrumentation as sample size and hydrogen loading varied.
The
volumes of the sample vessel and gas reservoir and the gas flows between them
were
calibrated using hydrogen and argon. The gas system was constructed using high
purity
regulators, a VCR sealed manifold capable of operating under vacuum or at
elevated
pressure, diaphragm type shut-off valves, and micro-valves to control gas
flows
between reactors. The gas lines and vessels were tested on a regular basis for
gas
leaks. System temperatures and pressures were recorded using a high precision
16-bit
National Instruments data acquisition system together with software developed
for this
task.
The hydrogen desorption behavior of the samples was monitored as a function
of temperature using a thermal programmed desorption (TPD) spectrum technique.
Samples (= 0.5 grams) were loaded into the high pressure reactor under argon
atmosphere and heated from room temperature to 280°C at a rate of
2°C per minute
CA 02339656 2001-02-05
WO 00/07930 PCT/US99/15994
-15-
while maintaining low hydrogen overpressure in the sealed reactor. On selected
samples, the TPD measurements were repeated to insure the reproducibility of
the
samples and the measurements.
In summary the foregoing description and figures demonstrate the development
of new and advanced materials useful as reversible hydrogen storage materials
and
methods of making the same. Of significant advantage, the dehyriding and
rehydridin g
of such materials occur under conditions that enable their use as hydrogen
storage
materials in vehicular applications. Other features and advantages of the
present
invention may be apparent to a person of skill in the art who studies the
present
invention disclosure. The foregoing description of specific embodiments and
examples
of the invention have been presented for the purpose of illustration and
description, an d
although the invention has been illustrated by certain of the preceding
examples, it is
not to be construed as being limited thereby. They are not intended to be
exhaustive
or to limit the invention to the precise forms disclosed, and obviously many
modifications, embodiments, and variations are possible in light of the above
teaching.
It is intended that the scope of the invention encompass the generic area as
herein
disclosed, and by the claims appended hereto and their equivalents.
