2005_JPCB_AlH3_deconposition kinetics
Decomposition Kinetics of the AlH 3Polymorphs
Jason Graetz*and James J.Reilly
Department of Energy Sciences and Technology,Brookha V en National Laboratory,Upton,New York 11973Recei V ed:August 19,2005;In Final Form:October 1,2005
Aluminum hydride polymorphs (R -AlH 3, -AlH 3,and γ-AlH 3)were prepared by organometallic synthesis.Hydrogen capacities approaching 10wt %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 3polymorphs suggest that the decomposition occurs via an activated complex mechanism with complexes consisting of approximately nine AlH 3molecules (1-2unit cells for R -AlH 3).
I.Introduction
Aluminum hydride,AlH 3,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 (1.48g/mL)greater than that of liquid hydrogen and a gravimetric hydrogen capacity exceeding 10wt %.AlH 3is stable at room temperature despite having an equilibrium hydrogen pressure of between 1and 10kbar at 298K.1,9The stability is generally attributed to a surface oxide layer,which acts as a kinetic barrier to decomposition and protects the AlH 3from the environment.As a result of the high-activation barrier and the overall slow desorption kinetics measured for large cuboids of AlH 3prepared by the Dow Chemical Co.,2-4this 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 AlH 3,developed by Finholt et al.,involves an ethereal reaction of LiAlH 4and AlCl 3:5
A nonsolvated form of AlH 3was initially prepared by Chizinsky et al.6and subsequently prepared by Brower et al.7by heating AlH 3in the presence of a complex metal hydride.Early studies of nonsolvated AlH 3by Brower et al.at the Dow Chemical Co.identified seven polymorphs,namely,R ,R ′, ,δ, ,γ,and .However,subsequent studies by a number of different groups,which included a structural characterization,8thermodynamic measurements,1,9and thermal and photolytic kinetic studies,2-4,9-13were all limited to the Dow material,consisting of 100μm cuboids of the R polymorph.Consequently,little is known about the other polymorphs and their properties.In this study,three crystalline AlH 3polymorphs were prepared via organometallic
synthesis,and their kinetic properties were investigated.We demonstrate that,at a temperature <100°C,the H 2desorption rates of freshly prepared,undoped AlH 3are nearly sufficient to supply a low-temperature fuel cell or an internal combustion engine (ICE)vehicle power plant.II.Experimental Section
Powder X-ray diffraction (XRD)patterns were acquired with a Philips diffractometer using Cu K R radiation.AlH 3powders were coated with a silicon-based vacuum grease and sealed under a 7.5μm Kapton film to prevent air contamination.Surface area measurements were performed on the decomposed material (Al powder)with a Quanta Chrome NOVA 1000Brunauer 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 10h.The high temperature of the degassing procedure precluded any measurements on the as-prepared AlH 3material.All samples were prepared and stored in an Ar glovebox.
Isothermal desorption measurements were performed by heating approximately 0.33g of AlH 3in an evacuated volume (～1.2L).Samples were decomposed in a stainless steel reactor of ～11mL volume,which was heated by resistive tape.The evolved H 2was collected in a calibrated reservoir of 1.222L.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.III.Synthesis
The AlH 3polymorphs were synthesized using the organo-metallic methods developed by the Dow Chemical Co.7,14,15The synthesis is extremely sensitive to the desolvating conditions (i.e.,temperature and time),and small alterations lead to the precipitation of different AlH 3polymorphs.It should be noted that freshly prepared,nonpassivated AlH 3is 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@https://www.wendangku.net/doc/d417590827.html,.
AlCl 3+3LiAlH 4+n [(C 2H 5)2O]f
4AlH 3?1.2[(C 2H 5)2O]+3LiCl (1)22181
J.Phys.Chem.B 2005,109,22181-2218510.1021/jp0546960CCC:$30.25?2005American Chemical Society
Published on Web 11/02/2005
-AlH3.A0.8M ether solution of LiAlH4(1.52g of LiAlH4 and50mL of ether)was prepared and stirred approximately1 min.LiAlH4is moderately soluble in ether,and it is possible that the alanate did not completely dissolve.The LiAlH4solution was mixed with a0.80M ether solution of AlCl3(1.33g of AlCl3and12.5mL of ether)and stirred for approximately2 min.This produced an ether solution of LiAlH4and AlCl3in a molar ratio of4to1,which reacted immediately to form3LiCl with dissolved4AlH3and LiAlH4(reaction1).The solution was subsequently filtered to remove the LiCl precipitate.A0.6M ether solution of LiBH4(0.44g LiBH4+35mL 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 AlH3etherate(4AlH3?1.2[(C2H5)2O]),LiBH4,and excess LiAlH4.The powder was ground with a mortar and pestle and heated to around65°C in a sand bath for approximately45min.The final product, crystalline -AlH3was washed with ether to remove the excess LiBH4and LiAlH4.
r-AlH
3
.The R polymorph was synthesized by initially preparing the phase as described above,but extending the heating time to approximately3h at～65°C.In this reaction, the polymorph decomposes to the more-stable R phase.
γ-AlH3.A0.8M ether solution of LiAlH4(3.04g of LiAlH4 and100mL of ether)was mixed with a0.80M ether solution of AlCl3(1.33g of AlCl3and12.5mL of ether)to produce an ether solution of LiAlH4and AlCl3in a molar ratio of8to1, respectively.The solution was stirred for approximately2min to ensure the reaction went to completion(reaction1)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(4AlH3?1.2[(C2H5)2O])and excess LiAlH4.The powder was ground with a mortar and pestle and heated to around60°C in a sand bath for2-4h.The final product of crystalline γ-AlH3was washed with ether to remove the excess LiAlH4. IV.Kinetic Analysis
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:
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.16-20It 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 than4 orders of magnitude greater than the H2pressure.Therefore, the reaction can be considered irreversible,and eq2can be used in a more useful form,known as the Avrami-Erofeyev equation:in which k)B1/n.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-(t).21,22Hancock 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 between0.57and 0.62).22Despite the utility of this method,it is best used as a guide to the appropriate rate equation,and it is important to plot[-ln(1-R)]1/n 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≈3suggest three-dimensional growth (spheres or hemispheres),values of n≈2indicate two-dimensional growth(disks and cylinders),and n≈1is typically due to linear growth.18-20,23,24
The temperature-dependent reaction rate,k(T),can generally be described by the Arrhenius equation:
in which A is the preexponential Arrhenius parameter,E a 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(k B T/h),which yields the Polanyi-Wigner equation
in which k B and h are the Boltzmann and Planck constants, respectively.25For“normal”reactions,which obey Polanyi-Wigner kinetics,the activation energy is approximately equal to the decomposition enthalpy,and the preexponential is generally within2orders of magnitude of k B T/h.25-27For “abnormal”reactions,activated complex theory is typically used 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 eq5:
in whichσis the ratio of the partition functions for the activated complex and for the reactant.26,27
V.Results
Powder diffraction patterns from AlH3etherate(before heating),γ-AlH3, -AlH3,and R-AlH3are shown in Figure1. The solvated AlH3(4AlH3?1.2[(C2H5)2O])exhibits a few broad amorphous peaks but shows no indication of any crystalline phase at this stage of the reaction.Theγ-AlH3shows a nicely crystalline structure with a small amount of R impurity.Theγphase 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 ofγ-AlH3is unknown,but it is the focus of an ongoing investigation.Powder diffraction of -AlH3reveals a new pattern with fewer peaks,indicating a higher symmetry space group. The crystallographic parameters of -AlH3are also unknown; however,Matzek et al.suggested a rhombohedral symmetry
R)1-exp(-Bt n)(2) [-ln(1-R)]1/n)kt(3)k(T))A exp(-E a RT)(4) k(T))
k
B
T
h
exp(-E a RT)(5) k(T))
k
B
T
h
σexp(-E a RT)(6)
22182J.Phys.Chem.B,Vol.109,No.47,2005Graetz and Reilly
for this phase.15The diffraction pattern shows minor traces of γ-AlH3impurities.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 R3h c space group.8
The isothermal fractional decomposition data for R-AlH3, -AlH3,andγ-AlH3are shown in Figures2a,3a,and4a, 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
Figure1.Powder XRD of AlH3etherate([(C2H5)2O]?AlH3),R-AlH3,
-AlH3,andγ-AlH3prepared by organometallic synthesis.Markers indicate expected peak positions for each phase.7
Figure2.Decomposition of R-AlH3between60and140°C plotted as(a)fractional decomposition,R,vs t and(b)[-ln(1-R)]1/2vs t.Figure3.Decomposition of -AlH3between60and140°C plotted as(a)fractional decomposition,R,vs t and(b)[-ln(1-R)]1/2vs t. Figure4.Decomposition ofγ-AlH3between60and140°C plotted as(a)fractional decomposition,R,vs t and(b)[-ln(1-R)]1/2vs t.
Decomposition Kinetics of AlH3Polymorphs J.Phys.Chem.B,Vol.109,No.47,2005
22183
The fraction reacted plotted for R-AlH3, -AlH3,andγ-AlH3 as[-ln(1-R)]1/2are shown in Figures2b,3b,and4b, respectively.These plots demonstrate good least-squares fits over the range of0.04e R e0.95with a linearity constant of R≈0.99,with a few exceptions for theγphase,which are discussed in more detail below.This supports the use of the Avrami-Erofeyev equation(eq3).The rate constants deter-mined from the slope of the least-squares fit are displayed in Table1.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 and the corresponding kinetic function kt)[-ln(1-R)1/2]. 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 Figure5.
Arrhenius plots for R-AlH3, -AlH3,andγ-AlH3over a temperature range of60e T e138°C are displayed in Figure 6along with data from Herley et al.for Dow-synthesized R-AlH
3
.4The activation energies and preexponential constants (A using eq4andσusing eq6)were determined from Figure 6and are shown in Table 2.Surface area measurements performed on the decomposed material(Al powder)and the calculated crystallite size based on isolated spherical particles are also shown in Table2.The molar volume of R-AlH3is approximately twice that of Al metal.Assuming this volumetric ratio applies to the other polymorphs,the particle diameters of the hydrides are approximately27%larger than the values listed in Table2.XRD after isothermal analyses confirmed that,in
each case,the final products were fcc aluminum powder. VI.Discussion
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
nucleation and“slow”growth.3Since little decomposition
accompanies the formation of a nucleation site,this region
exhibits slow H2evolution.This model is supported by
decomposition experiments in which the reaction was stopped
during the acceleratory period by reducing T(Figure5).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 )2or3,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
TABLE1:Isothermal Decomposition Rate Constants(s-1)for r-, -,andγ-AlH3a
polymorph k(60°C)k(80°C)k(99°C)k(120°C)k(138°C)
R-AlH
31.35×10-69.14×10-6 4.21×10-5 2.80×10-4 1.39×10-3
-AlH3 4.41×10-6 1.36×10-5 5.99×10-5 4.88×10-4 2.46×10-3γ-AlH3 3.97×10-6 1.18×10-5 3.94×10-5 2.71×10-47.98×10-4
a The sample temperatures are typically within(1°C of the value
listed.
Figure 5.Temperature and fractional decomposition of R-AlH3
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.Figure6.Arrhenius plot for large crystallites of R-AlH3(Dow)4and small crystallites of R-AlH3, -AlH3,andγ-AlH3.Reaction rates for the large crystallites of R-AlH3were measured at135°C e T e160°C and are extrapolated down to T～60°C.
TABLE2:Morphological and Kinetic Values for the AlH3 Polymorphs a
polymorph
surface
area
(m2/g)
particle
diameter
(nm)E a(kJ/mol)Aσ
R-AlH
3
10.9204102.2(3.2 1.2×1010 1.9×10-3 -AlH314.615292.3(8.68.8×108 1.4×10-4γ-AlH316.413679.3(5.18.5×106 1.4×10-6
a Particle diameters were calculated from the surface area using a spherical geometry.The preexponential constants A andσwere determined from eqs4and6,respectively.
22184J.Phys.Chem.B,Vol.109,No.47,2005Graetz and Reilly
to note that the kinetics are not limited by diffusion through a
surface oxide as previously expected.Diffusion-controlled
decomposition reactions typically yield values of0.54e n e
0.6227and can be easily differentiated from nucleation and
growth reactions.Finally,the decay period is attributed to the
disappearance of the unreacted AlH3phase.
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.26,27The decomposition kinetics of AlH3clearly do not
obey the Polanyi-Wigner relation(equation5),since the
preexponential factors for the three polymorphs are more than
2orders of magnitude lower than k B T/h at298K(k B T/h)6.2×1012),and the activation energies(Table2)are much greater than the decomposition enthalpy measured for R-AlH3(?H)
7.6kJ/mol H21).However,Shannon27and others28demonstrated
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 AlH3may 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-AlH
3
,these complexes consist of approximately nine AlH3
molecules,or1-2unit cells.Although this is only one possible
decomposition mechanism,it is reasonable to suggest that the
conversion of R-AlH3to Al occurs in increments of whole unit
cells.Values ofσfrom eq6are listed in Table2(in whichσ
)hA/k
B
T)and are in agreement with the expected values for a
solid complex with only a few degrees of freedom(low
mobility).
The preexponential and activation energy for R-AlH3(Table
2)are considerably smaller than the values measured by Herley
et al.(A)3.5×1016and E a)150.3(10.0kJ/mol).4The
hydrides synthesized for this study demonstrate reaction rates
that are an order of magnitude greater at60°C than those
measured for the Dow material.This is attributed to a smaller
particle size and a reduced surface oxide in the freshly prepared
material.In addition to particle size and surface coatings,the
rate constants are also sensitive to the polymorph structure as
shown in Table2(the small variations in particle size are not
believed to have a large affect on the kinetics).The decrease in
activation energy of the andγphases with respect to that of
the R phase is attributed,in part,to a smaller decomposition
enthalpy.Preliminary calorimetry results suggest that R is the
most stable,followed by and ultimately theγphase.This is
also supported by the order in which these phases appear during
the synthesis.γ-and -AlH3precipitate early and quickly
decompose to the more-stable R phase.The transition to the R
phase is exothermic,supplying an energetic boost to the
decomposition.The total energy required is reduced,and
therefore there is a larger driving force toward decomposition.
This may also explain the odd shape of the decomposition curve
forγ-AlH3at60°C,which has an acceleratory region that
exhibits rapid H2evolution at short times(t<80×103s).It
is likely that,during the region of rapid hydrogen evolution,
the material is decomposing while simultaneously transforming
to the R phase.When the phase transition is complete,the
decomposition continues at a slightly slower rate because of
the greater stability of R-AlH3.VII.Conclusion
The kinetics of the aluminum hydride polymorphs(R-AlH3, -AlH3,andγ-AlH3)are controlled by nucleation and growth in two and three dimensions and are not limited by H2diffusion through a surface oxide.The decomposition of AlH3occurs in complexes of approximately nine molecules,or1-2unit cells for R-AlH3.Decomposition of the R phase was slower than that of theγand phases because of its greater stability.In general, the rapid low-temperature kinetics and high-energy density make AlH3an unusual and promising hydrogen storage medium for a number of applications.However,the conventional organo-metallic synthesis is a costly procedure,and AlH3is not a reversible hydride at moderate H2pressures.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 AlH3from the spent Al powder in a cost-effective and energetically efficient manner.
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-AC02-98CH1-886with the U.S.Department of Energy.
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Decomposition Kinetics of AlH3Polymorphs J.Phys.Chem.B,Vol.109,No.47,200522185