Decomposition kinetics of the AlH3 polymorphs

Article abstract of DOI:10.1021/jp0546960

Aluminum hydride polymorphs (?±-AlH₃, ?2-AlH ₃, and ?3-AlH₃) were prepared by organometallic synthesis. Hydrogen capacities approaching 10 wt % at desorption temperatures less than 100 ?°C have been demonstrated with freshly prepared AlH ₃. The temperature-dependent rate constants were determined by measuring the isothermal hydrogen evolution between 60 ?°C and 140 ?°C. Fractional decomposition curves showed good fits using both the second and third-order Avrami - Erofeyev equations, indicating that the decomposition kinetics are controlled by nucleation and growth of the aluminum phase in two and three dimensions. The large activation energies measured for the AlH ₃ polymorphs suggest that the decomposition occurs via an activated complex mechanism with complexes consisting of approximately nine AlH ₃ molecules (1 - 2 unit cells for ?±-AlH₃). ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0546960

J. Phys. Chem. B 2005, 109, 22181-22185
22181
Decomposition Kinetics of the AlH3 Polymorphs
Jason Graetz* and James J. Reilly
Department of Energy Sciences and Technology, BrookhaVen National Laboratory, Upton, New York 11973
ReceiVed: August 19, 2005; In Final Form: October 1, 2005
Aluminum hydride polymorphs (R-AlH
Hydrogen capacities approaching 10 wt % at desorption temperatures less than 100 °C have been demonstrated
with freshly prepared AlH . The temperature-dependent rate constants were determined by measuring the
3
, â-AlH
3
3
, and γ-AlH ) were prepared by organometallic synthesis.
3
isothermal hydrogen evolution between 60 °C and 140 °C. Fractional decomposition curves showed good
fits using both the second and third-order Avrami-Erofeyev equations, indicating that the decomposition
kinetics are controlled by nucleation and growth of the aluminum phase in two and three dimensions. The
large activation energies measured for the AlH
activated complex mechanism with complexes consisting of approximately nine AlH
cells for R-AlH ).
3
polymorphs suggest that the decomposition occurs via an
3
molecules (1-2 unit
3
I. Introduction
synthesis, and their kinetic properties were investigated. We
demonstrate that, at a temperature <100 °C, the H2 desorption
rates of freshly prepared, undoped AlH3 are nearly sufficient
to supply a low-temperature fuel cell or an internal combustion
engine (ICE) vehicle power plant.
Aluminum hydride, AlH3, is a fascinating material that has
recently attracted attention for its potential as a hydrogen storage
medium for low-temperature fuel cells. It has a volumetric
hydrogen capacity (1.48 g/mL) greater than that of liquid
hydrogen and a gravimetric hydrogen capacity exceeding 10
wt %. AlH3 is stable at room temperature despite having an
equilibrium hydrogen pressure of between 1 and 10 kbar at 298
II. Experimental Section
Powder X-ray diffraction (XRD) patterns were acquired with
1,9
a Philips diffractometer using Cu KR radiation. AlH powders
K. The stability is generally attributed to a surface oxide layer,
which acts as a kinetic barrier to decomposition and protects
the AlH3 from the environment. As a result of the high-activation
barrier and the overall slow desorption kinetics measured for
3
were coated with a silicon-based vacuum grease and sealed
under a 7.5 µm Kapton film to prevent air contamination.
Surface area measurements were performed on the decomposed
material (Al powder) with a Quanta Chrome NOVA 1000
Brunauer Emmett and Teller (BET) surface area analyzer. The
adsorbed gas molecules were removed from the sample surfaces
prior to the measurement by heating to 200 °C under vacuum
for approximately 10 h. The high temperature of the degassing
procedure precluded any measurements on the as-prepared AlH3
material. All samples were prepared and stored in an Ar
glovebox.
2
-4
large cuboids of AlH3 prepared by the Dow Chemical Co.,
this material was long thought to be unsuitable for low-
temperature systems. Recent studies by Sandrock et al. dem-
onstrated that a dopant introduced by ball milling can alter this
surface barrier and lead to decomposition kinetics at 100 °C
that are adequate for use in H-powered vehicles.
The conventional procedure for synthesizing a solvated form
of AlH3, developed by Finholt et al., involves an ethereal
reaction of LiAlH4 and AlCl3:5
Isothermal desorption measurements were performed by
heating approximately 0.33 g of AlH3 in an evacuated volume
(
∼1.2 L). Samples were decomposed in a stainless steel reactor
AlCl + 3LiAlH + n[(C H ) O] f
3
4
2
5 2
of ∼11 mL volume, which was heated by resistive tape. The
evolved H2 was collected in a calibrated reservoir of 1.222 L.
The sample temperature was measured using an internal
thermocouple. Isothermal measurements were performed up to
a maximum temperature of 140 °C. Above this temperature,
the reaction proceeded rapidly, and significant decomposition
occurred before the temperature could be stabilized.
4
AlH ‚1.2[(C H ) O] + 3LiCl (1)
3
2
5 2
A nonsolvated form of AlH3 was initially prepared by Chizinsky
6
7
et al. and subsequently prepared by Brower et al. by heating
AlH3 in the presence of a complex metal hydride. Early studies
of nonsolvated AlH3 by Brower et al. at the Dow Chemical
Co. identified seven polymorphs, namely, R, R′, â, δ, ꢀ, γ, and
ú. However, subsequent studies by a number of different groups,
III. Synthesis
8
which included a structural characterization, thermodynamic
measurements,1 and thermal and photolytic kinetic studies,
,9
2-4,9-13
The AlH3 polymorphs were synthesized using the organo-
metallic methods developed by the Dow Chemical Co.7
,14,15
The
were all limited to the Dow material, consisting of 100 µm
cuboids of the R polymorph. Consequently, little is known about
the other polymorphs and their properties. In this study, three
crystalline AlH3 polymorphs were prepared via organometallic
synthesis is extremely sensitive to the desolvating conditions
i.e., temperature and time), and small alterations lead to the
(
precipitation of different AlH3 polymorphs. It should be noted
that freshly prepared, nonpassivated AlH3 is pyrophoric and
reacts violently in water and must be treated with caution. The
synthesis procedures used in this study are described below.
*
To whom correspondence should be addressed. E-mail address:
graetz@bnl.gov.
1
0.1021/jp0546960 CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/02/2005

22182 J. Phys. Chem. B, Vol. 109, No. 47, 2005
Graetz and Reilly
1
/n
â-AlH3. A 0.8 M ether solution of LiAlH4 (1.52 g of LiAlH4
in which k ) B . The rate-limiting mechanism and the
appropriate kinetic equation is ascertained from the value of n,
which can be estimated from a plot of ln[-ln(1 - R)] vs ln-
and 50 mL of ether) was prepared and stirred approximately 1
min. LiAlH4 is moderately soluble in ether, and it is possible
that the alanate did not completely dissolve. The LiAlH4 solution
was mixed with a 0.80 M ether solution of AlCl3 (1.33 g of
AlCl3 and 12.5 mL of ether) and stirred for approximately 2
min. This produced an ether solution of LiAlH4 and AlCl3 in a
molar ratio of 4 to 1, which reacted immediately to form 3LiCl
with dissolved 4AlH3 and LiAlH4 (reaction 1). The solution was
subsequently filtered to remove the LiCl precipitate. A 0.6 M
ether solution of LiBH4 (0.44 g LiBH4 + 35 mL ether) was
added to the filtrate. The ether was removed from the clear
filtrate by vaporization at room temperature. The result was a
fluffy white powder consisting of AlH3 etherate (4AlH3‚
2
1,22
(t).
Hancock and Sharp demonstrated that this method not
only applies to the Avrami-Erofeyev equations (n ) 1, 2, or
3), but also to nonintegral values of n (e.g., in diffusion-
controlled decomposition, n is typically between 0.57 and
2
2
0.62). Despite the utility of this method, it is best used as a
guide to the appropriate rate equation, and it is important to
1
/n
plot [-ln(1 - R)] vs t to confirm that the predicted model
produces a linear plot. In the nucleation and growth model, n
is a function of the nucleation barrier and the geometry of the
growth. Values of n ≈ 3 suggest three-dimensional growth
(spheres or hemispheres), values of n ≈ 2 indicate two-
dimensional growth (disks and cylinders), and n ≈ 1 is typically
1
.2[(C2H5)2O]), LiBH4, and excess LiAlH4. The powder was
1
8-20,23,24
ground with a mortar and pestle and heated to around 65 °C in
a sand bath for approximately 45 min. The final product,
crystalline â-AlH3 was washed with ether to remove the excess
LiBH4 and LiAlH4.
due to linear growth.
The temperature-dependent reaction rate, k(T), can generally
be described by the Arrhenius equation:
-
RT
Ea
r-AlH3. The R polymorph was synthesized by initially
preparing the â phase as described above, but extending the
heating time to approximately 3 h at ∼65 °C. In this reaction,
the â polymorph decomposes to the more-stable R phase.
γ-AlH3. A 0.8 M ether solution of LiAlH4 (3.04 g of LiAlH4
and 100 mL of ether) was mixed with a 0.80 M ether solution
of AlCl3 (1.33 g of AlCl3 and 12.5 mL of ether) to produce an
ether solution of LiAlH4 and AlCl3 in a molar ratio of 8 to 1,
respectively. The solution was stirred for approximately 2 min
to ensure the reaction went to completion (reaction 1) and was
subsequently filtered to remove the LiCl precipitate. The ether
was removed from the clear filtrate by vaporization at room
temperature, producing a fluffy white powder consisting of AlH3
etherate (4AlH 3‚1.2[(C2H5)2O]) and excess LiAlH4. The powder
was ground with a mortar and pestle and heated to around 60
k(T) ) A exp(
(4)
)
in which A is the preexponential Arrhenius parameter, Ea is the
activation energy, and R is the universal gas constant. The
activation energy and Arrhenius parameter are determined from
the slope and intercept of an Arrhenius plot. In many cases, the
preexponential in the Arrhenius equation is replaced with an
atomic frequency factor (kBT/h), which yields the Polanyi-
Wigner equation
k T
-Ea
RT
B
k(T) )
exp
(5)
(
)
h
in which kB and h are the Boltzmann and Planck constants,
respectively. For “normal” reactions, which obey Polanyi-
Wigner kinetics, the activation energy is approximately equal
2
5
°
C in a sand bath for 2-4 h. The final product of crystalline
γ-AlH3 was washed with ether to remove the excess LiAlH4.
to the decomposition enthalpy, and the preexponential is
generally within 2 orders of magnitude of kBT/h.2
5-27
For
IV. Kinetic Analysis
“
abnormal” reactions, activated complex theory is typically used
Decomposition reactions can be complex, with a number of
different parameters influencing the kinetics (e.g., grain bound-
aries, defects, surface area, nucleation sites, and thermal
conductivity). Generally, a single mechanism will dominate, and
this rate-limiting step can be determined from isothermal
decomposition experiments. Typical analysis methods produce
a linear function in which the slope reveals information
regarding the reaction mechanism. For isothermal solid-state
reactions, the fractional decomposition, R, can generally be
expressed by the following time-dependent (t) kinetic equation:
to explain the large activation energy. In this case, the
decomposition occurs via a molecular complex rather than via
an individual molecule. For these reactions, a partition function,
σ, is typically included in the preexponential of eq 5:
k T
-Ea
RT
B
k(T) )
σ exp
(6)
(
)
h
in which σ is the ratio of the partition functions for the activated
26,27
complex and for the reactant.
n
V. Results
R ) 1 - exp(-Bt )
(2)
Powder diffraction patterns from AlH3 etherate (before
heating), γ-AlH3, â-AlH3, and R-AlH3 are shown in Figure 1.
The solvated AlH3 (4AlH 3‚1.2[(C2H5)2O]) exhibits a few broad
amorphous peaks but shows no indication of any crystalline
phase at this stage of the reaction. The γ-AlH3 shows a nicely
crystalline structure with a small amount of R impurity. The γ
phase is unstable with respect to its decomposition to R-AlH3,
which makes the desolvation process a challenge without
forming a small amount of the R phase. The crystallographic
structure of γ-AlH3 is unknown, but it is the focus of an ongoing
investigation. Powder diffraction of â-AlH3 reveals a new pattern
with fewer peaks, indicating a higher symmetry space group.
The crystallographic parameters of â-AlH3 are also unknown;
however, Matzek et al. suggested a rhombohedral symmetry
in which the rate constant, B, is typically a function of the
nucleation frequency and growth rate, while the constant, n, is
dependent upon the geometry of growth.1
6-20
It should be noted
that, for a reversible hydride, the rate equation should be
modified by the driving force for the back reaction (i.e., H2 +
Al f AlH3). In this case, the driving force for this reaction is
essentially zero since the equilibrium pressure is more than 4
orders of magnitude greater than the H2 pressure. Therefore,
the reaction can be considered irreversible, and eq 2 can be
used in a more useful form, known as the Avrami-Erofeyev
equation:
1/n
[
-ln(1 - R)] ) kt
(3)

Decomposition Kinetics of AlH3 Polymorphs
J. Phys. Chem. B, Vol. 109, No. 47, 2005 22183
Figure 1. Powder XRD of AlH
â-AlH , and γ-AlH prepared by organometallic synthesis. Markers
indicate expected peak positions for each phase.
3
etherate ([(C
2
H
5
)
2
O]‚AlH
3
3
), R-AlH ,
3
3
7
Figure 3. Decomposition of â-AlH
3
between 60 and 140 °C plotted
as (a) fractional decomposition, R, vs t and (b) [-ln(1 - R)] vs t.
1
/2
3
Figure 2. Decomposition of R-AlH between 60 and 140 °C plotted
3
Figure 4. Decomposition of γ-AlH between 60 and 140 °C plotted
as (a) fractional decomposition, R, vs t and (b) [-ln(1 - R)]1 vs t.
/2
1/2
as (a) fractional decomposition, R, vs t and (b) [-ln(1 - R)] vs t.
for this phase.15 The diffraction pattern shows minor traces of
γ-AlH3 impurities. The XRD pattern from the R polymorph
suggests that the material is pure R-AlH3, which has a hexagonal
unit cell and crystallizes in the R 3h c space group.8
The isothermal fractional decomposition data for R-AlH3,
â-AlH3, and γ-AlH3 are shown in Figures 2a, 3a, and 4a,
respectively. The decomposition curves are isothermal ((1 °C),
and the data acquired during the initial sample heating are not
included. All three polymorphs exhibit a similar set of decom-
position curves with a short induction period (R < 0.04),
followed by an acceleratory period (0.04 < R < 0.7) in which
the rate of H2 evolution increases rapidly, and, finally, a decay
period (R > 0.7). The data were plotted as ln[-ln(1 - R)] versus
ln(t) (not shown) to measure the constant, n, and determine the
most suitable kinetic equation. The average values of n were
approximately 2, suggesting the second-order Avrami-Erofeyev
equation. The acceleratory period was fit using eq 3 with n )
3 and n ) 2, which showed similar results. Although both values
are suitable, n ) 2 was chosen for this analysis because it
provided a slightly better fit.

2
2184 J. Phys. Chem. B, Vol. 109, No. 47, 2005
TABLE 1: Isothermal Decomposition Rate Constants (s-1) for r-, â-, and γ-AlH a
polymorph k(60 °C) k(80 °C) k(99 °C)
Graetz and Reilly
3
k(120 °C)
k(138 °C)
-
6
6
6
-6
-5
-4
-3
R-AlH
3
1.35 × 10
4.41 × 10
3.97 × 10
9.14 × 10
1.36 × 10
1.18 × 10
4.21 × 10
5.99 × 10
3.94 × 10
2.80 × 10
4.88 × 10
2.71 × 10
1.39 × 10
-
-
-5
-5
-5
-5
-4
-4
-3
â-AlH
γ-AlH
3
2.46 × 10
-4
3
7.98 × 10
a
The sample temperatures are typically within ( 1 °C of the value listed.
Figure 5. Temperature and fractional decomposition of R-AlH
3
demonstrating that the reaction rate goes to zero as the sample
temperature is reduced to ∼23 °C. The inset shows an expanded view
of the decomposition curve and illustrates that the rates before and
after the temperature change are equivalent.
4
Figure 6. Arrhenius plot for large crystallites of R-AlH
small crystallites of R-AlH , â-AlH , and γ-AlH
the large crystallites of R-AlH were measured at 135 °C e T e 160
C and are extrapolated down to T ∼ 60 °C.
3
(Dow) and
3
3
3
. Reaction rates for
3
°
The fraction reacted plotted for R-AlH3, â-AlH3, and γ-AlH3
TABLE 2: Morphological and Kinetic Values for the AlH
3
1
/2
Polymorphsa
as [-ln(1 - R)] are shown in Figures 2b, 3b, and 4b,
respectively. These plots demonstrate good least-squares fits
over the range of 0.04 eR e 0.95 with a linearity constant of
R ≈ 0.99, with a few exceptions for the γ phase, which are
discussed in more detail below. This supports the use of the
Avrami-Erofeyev equation (eq 3). The rate constants deter-
mined from the slope of the least-squares fit are displayed in
Table 1. Analysis of the isothermal decomposition curves gives
consistent values of n ≈ 2. This value indicates that the kinetics
are limited by random nucleation and growth. The same
Avrami-Erofeyev equation applies to decomposition over a
wide range in temperature (60-138 °C), suggesting a common
decomposition pathway. This consistency is crucial for this
analysis and to assign any physical meaning to the constant n
surface particle
area diameter
2
polymorph (m /g)
(nm)
E
a
(kJ/mol)
A
σ
10
-3
-4
-6
R-AlH
3
10.9
14.6
16.4
204
152
136
102.2 ( 3.2 1.2 × 10 1.9 × 10
8
â-AlH3
92.3 ( 8.6 8.8 × 10 1.4 × 10
6
γ-AlH
3
79.3 ( 5.1 8.5 × 10 1.4 × 10
a
Particle diameters were calculated from the surface area using a
spherical geometry. The preexponential constants A and σ were
determined from eqs 4 and 6, respectively.
in Table 2. XRD after isothermal analyses confirmed that, in
each case, the final products were fcc aluminum powder.
1
/2
VI. Discussion
and the corresponding kinetic function kt ) [-ln(1 - R) ].
The reaction rate can be lowered and even stopped by
decreasing the sample temperature, as shown in Figure 5.
Decreasing the sample temperature to ∼23 °C during the
acceleratory period completely stops the evolution of H2. Upon
subsequent heating, no induction period is observed, and the
decomposition reaction rate returns directly to the rate prior to
reducing T, as shown in the inset of Figure 5.
The sigmoid shape of the fractional decomposition curve is
indicative of an autocatalytic reaction and is typical of solid-
state decomposition. The induction period is attributed to
3
nucleation and “slow” growth. Since little decomposition
accompanies the formation of a nucleation site, this region
exhibits slow H2 evolution. This model is supported by
decomposition experiments in which the reaction was stopped
during the acceleratory period by reducing T (Figure 5). At this
stage, many nucleation sites are present, but the growth is
inhibited by the low sample temperature. Upon subsequent
heating, the growth continues, and the reaction rate returns
directly to the acceleratory period (no induction period). During
the acceleratory stage of the reaction, the kinetics are controlled
by random nucleation and rapid growth. The decomposition
curves demonstrate the best fits with a geometric constant of n
) 2 or 3, suggesting that the growth of the Al phase occurs in
two or three-dimensions. The average value of n is insensitive
to the polymorph structure, indicating that a common decom-
position mechanism exists for the three phases. It is interesting
Arrhenius plots for R-AlH3, â-AlH3, and γ-AlH3 over a
temperature range of 60 e T e 138 °C are displayed in Figure
6
along with data from Herley et al. for Dow-synthesized
4
R-AlH3. The activation energies and preexponential constants
A using eq 4 and σ using eq 6) were determined from Figure
and are shown in Table 2. Surface area measurements
(
6
performed on the decomposed material (Al powder) and the
calculated crystallite size based on isolated spherical particles
are also shown in Table 2. The molar volume of R-AlH3 is
approximately twice that of Al metal. Assuming this volumetric
ratio applies to the other polymorphs, the particle diameters of
the hydrides are approximately 27% larger than the values listed

Decomposition Kinetics of AlH3 Polymorphs
J. Phys. Chem. B, Vol. 109, No. 47, 2005 22185
VII. Conclusion
to note that the kinetics are not limited by diffusion through a
surface oxide as previously expected. Diffusion-controlled
decomposition reactions typically yield values of 0.54 e n e
The kinetics of the aluminum hydride polymorphs (R-AlH3,
â-AlH3, and γ-AlH3) are controlled by nucleation and growth
in two and three dimensions and are not limited by H2 diffusion
through a surface oxide. The decomposition of AlH3 occurs in
complexes of approximately nine molecules, or 1-2 unit cells
for R-AlH3. Decomposition of the R phase was slower than that
of the γ and â phases because of its greater stability. In general,
the rapid low-temperature kinetics and high-energy density make
AlH3 an unusual and promising hydrogen storage medium for
a number of applications. However, the conventional organo-
metallic synthesis is a costly procedure, and AlH3 is not a
reversible hydride at moderate H2 pressures. Incorporating
dopants or catalytic additives is not likely to produce the large
thermodynamic changes required to substantially reduce the
equilibrium pressure. Therefore, the utility of this material will
depend on the development of new techniques to regenerate
AlH3 from the spent Al powder in a cost-effective and
energetically efficient manner.
2
7
0
.62 and can be easily differentiated from nucleation and
growth reactions. Finally, the decay period is attributed to the
disappearance of the unreacted AlH3 phase.
In most reactions, the value of the activation energy is
independent of the enthalpy. However, in certain cases, such
as evaporation and thermal decomposition, the activation energy
can be related to the enthalpy through activated complex
theory.2
6,27
The decomposition kinetics of AlH3 clearly do not
obey the Polanyi-Wigner relation (equation 5), since the
preexponential factors for the three polymorphs are more than
2
orders of magnitude lower than kBT/h at 298 K (kBT/h ) 6.2
12
×
10 ), and the activation energies (Table 2) are much greater
than the decomposition enthalpy measured for R-AlH3 (∆H )
1
27
28
7.6 kJ/mol H2 ). However, Shannon and others demonstrated
that activated complex theory can be used to predict the thermal
decomposition rates of solids and other reactions that do not
obey the Polanyi-Wigner relation. Therefore, the large activa-
tion energies measured for AlH3 may be attributed to a
decomposition process involving activated complexes, rather
than individual molecules. Upon the basis of the measured
activation energy and the known dissociation enthalpy for
R-AlH3, these complexes consist of approximately nine AlH3
molecules, or 1-2 unit cells. Although this is only one possible
decomposition mechanism, it is reasonable to suggest that the
conversion of R-AlH3 to Al occurs in increments of whole unit
cells. Values of σ from eq 6 are listed in Table 2 (in which σ
Acknowledgment. The authors gratefully acknowledge Gary
Sandrock for his insight on hydride kinetics and his encourage-
ment to investigate aluminum hydride. This work was supported
by the Department of Energy’s Office of Energy Efficiency and
Renewable Energy. This manuscript was authored by Brookhaven
Science Associates, LLC under Contract No. DE-AC02-98CH1-
8
86 with the U.S. Department of Energy.
References and Notes
)
hA/kBT) and are in agreement with the expected values for a
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(
(
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4
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3
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(
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