J. Phys. Chem. C 2007, 111, 16693-16699
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In Situ Synchrotron Powder X-ray Diffraction Studies of the Thermal Decomposition of βand γ-AlD3 Hilde Grove,* Magnus H. Sørby, Hendrik W. Brinks, and Bjørn C. Hauback Physics Department, Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Norway ReceiVed: July 4, 2007; In Final Form: August 10, 2007
Details of the thermal decomposition of β-AlD3 and γ-AlD3 have been investigated by in situ synchrotron X-ray diffraction at a constant heating rate of 1 K/min and isothermally in dynamical vacuum. The β-AlD3 transforms into R-AlD3 prior to decomposing to Al and D2. The transformation into R-AlD3 starts at about 80 °C, and the decomposition to Al and release of D2 starts at 130 °C. The transformation of γ-AlD3 into R-AlD3 starts at about 90 °C, and the sample decomposes to Al and D2 at a higher temperature (from about 130 °C). From about 110 °C, γ-AlD3 also decomposes directly into Al and D2. Detailed analyses of the changes in unit cell dimensions during the decomposition for all phases have been performed in order to propose possible routes for the transitions.
1. Introduction Aluminum hydride AlH3 (alane) has both a high hydrogen content (10.1 wt %) and volumetric density (0.148 kg H2/L) that can be released at moderate temperatures. It is therefore very attractive as a potential hydrogen storage material. AlH3 is not reversible at moderate conditions, and off-board recharging is therefore considered as the practical solution.1 AlH3 can be synthesized off-board using wet chemical methods.2 AlH3 was first synthesized in 19473 and since the work by Sandrock et al. in 20051 addressing the potential for using AlH3 for vehicle hydrogen storage, hydrogen storage in this compound has been in focus by several research groups. The first modification of AlH3 that was synthesized, the so-called R-AlH3 form, is kinetically stable at room temperature.1 It decomposes at temperatures g60 °C according to the following reaction:2,4,5
R-AlH3 f Al + 1.5H2
(R1)
Al and H2 are the thermodynamic products, and the reaction is not reversible at moderate conditions.1 H2 pressures larger than 2.5 GPa are required to reverse R1.6,7 Differential scanning calorimetry (DSC) measurements indicating a high plateau pressure are also in agreement with the low thermodynamic stability for R-AlH38,9. Sinke et al. determined in 1967 the dehydrogenation enthalpy to be ∆H ) 7.6 kJ/mol H2.10 The metastability in air is assumed to be caused by an Al2O3 layer on the surface of the particles. Six polymorphs of AlH3 were prepared by Brower et al.: R, R′, β, γ, δ, and .2 R-, R′-, β-, and γ-AlH3 have been synthesized reproducibly. The structure of R-AlH3 consists of a three-dimensional framework with corner-sharing octahedral AlH6 units.11,12 There is no interconnection between the six AlH6 octahedra that coordinate common AlH6 octahedra. A three-dimensional framework of cornersharing octahedral AlH6 units is also seen in both R′ and β, but for these structures, the octahedra connected to the same octahedron are interconnected and they contain channels.12-14 In γ-AlH3 the three-dimensional framework is formed by corner* Corresponding author. Phone: +4763806444. Fax: +4763810920. E-mail:
[email protected].
Figure 1. In situ SR-PXD measurements of β-AlD3 from 40 to 160 °C. The heating rate is 1 K/min. The presence of β-AlD3, R-AlD3, and Al are indicated in the figure.
and edge-sharing octahedral AlH6 units.15,16 R-AlH3 is the most compact polymorph. In the present work the details of the decomposition of γand β-AlD3 are investigated by in situ synchrotron X-ray diffraction. Recent work shows that γ-AlH3 transforms into R-AlH3 on heating, or partly by direct transformation to Al,17 whereas experiments for β-AlH3 transforming into R are nonconsistent.8,9 These calorimetric studies show that the decomposition enthalpy is smaller for β than for R indicating that R should be the most stable polymorphic modification. Density functional theory (DFT) calculations by Ke et al. show that the β-state is more stable than the R-phase and transformation of β into R is therefore unfeasible.14 2. Experimental Methods LiAlD4 (g98 wt % purity) and LiBH4 (g95 wt % purity) were purchased from Sigma Aldrich, diethyl ether (Et2O, certified ACS, 99.9% purity) was purchased from Fischer
10.1021/jp075212y CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007
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Figure 2. Observed intensities (circles) and calculated intensities from Rietveld refinements of β-sample at (a) 60, (b) 90, and (c) 120 °C. Positions of Bragg reflections are shown with bars for β-AlD3, γ-AlD3, R-AlD3, and Al (from top). The differences between observed and calculated intensities are shown with the bottom line.
TABLE 1: Selected Crystallographic Values for the β-Sample T β-AlD3 wt % space group a (Å) V (Å3) V (Å3)/Z′ R-AlD3 wt % space group a (Å) c (Å) V (Å3) V (Å3)/Z′ γ-AlD3 wt % space group a (Å) b (Å) c (Å) V (Å3) V (Å3)/Z′ Al wt % space group a (Å) V (Å3) Rwp
60 °C
80 °C
100 °C
120 °C
140 °C
160 °C
88.3(7)
86.9(7)
22.1(3)
2.4(2)
9.00442(13) 730.08(2) 45.63
9.00512(14) 730.25(2) 45.64
9.0061(2) 730.48(3) 45.66
1.0(2)
2.8(2)
71.5(6)
92.9(9)
75.0(6)
4.435(2) 11.787(11) 200.8(2) 33.47
4.4390(8) 11.787(4) 201.15(9) 33.53
4.44016(9) 11.7849(3) 201.213(8) 33.54
4.44248(9) 11.7858(3) 201.437(8) 33.57
4.44465(8) 11.7866(3) 201.648(7) 33.61
10.3(5)
9.9(5)
6.1(5)
2.6(5)
0.2(4)
7.342(3) 5.364(2) 5.7565(11) 226.71(14) 37.79
7.343(3) 5.364(2) 5.7572(12) 226.77(15) 37.80
7.340(5) 5.370(4) 5.758(2) 227.0(2) 37.83
0.37(11)
0.37(12)
0.25(15)
2.1(2)
24.8(3)
100
4.0586(4) 66.85(1)
4.06035(8) 66.941(2)
4.06318(6) 67.081(2)
6.43
5.65
5.36
Fd-3m
R-3c
Pnnm
Fm-3m
5.84
5.94
Scientific, and AlCl3 (g99 wt % purity) was purchased from Alfa. The LiAlD4 was purified with Soxhlet extraction. The synthesis of β- and γ-AlD3 were previously described in detail in refs 13 and 15. The two polymorphs were synthesized using wet chemistry methods. Similar routes were applied, both mixing LiAlD4 and AlCl3 dissolved in Et2O in a ratio 4:1, the only main difference being that LiBH4 is added in the synthesis of β-AlD3. Deuterated samples were used to allow neutron diffraction studies published elsewhere.13,15 Time-resolved in situ synchrotron powder X-ray diffraction (SR-PXD) data were collected at the Swiss-Norwegian beam line (BM01A) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The samples were kept in 0.8 mm
8.03
boron-silica-glass capillaries, kept in place by a glass rod, and mounted in a Swagelok fitting. The capillary was then evacuated with a rotary pump and kept under dynamic vacuum. A hot-air blower was used to heat the capillary at constant heating rate of 1 K/min, from about 40 to 160 °C or at a constant temperature of 83 °C for the isothermal experiment. Twodimensional powder data were collected using an imaging plate system (MAR345) with an exposure time of 30 s. The capillaries were rotated 30° during the exposure. A time of 90 s was needed for data readout and erasing; thus, a complete data set was collected every second minute. The wavelength was 0.7111 Å for the β sample and 0.8000 Å for the γ sample. The two-dimensional data were converted into one-dimensional
Thermal Decomposition of β- and γ-AlD3
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Figure 3. Relative amount of β-AlD3 (black squares), R-AlD3 (light gray triangles), γ-AlD3 (dark gray circles), and Al (gray inverted triangles) calculated from the in situ SR-PXD measurements of the β sample plotted against temperature. The heating rate is 1 K/min.
step size of 0.02°. Temperature calibration was performed based on measurements of the thermal expansion of a Ag sample. Quantitative phase analyses (QPA) using the Rietveld method were carried out with the program Fullprof (version 3.20).19 The X-ray form factor coefficients were taken from the Fullprof library. Pseudo-Voigt profile function was used to describe the peak shapes, and the background was described by a cubic spline interpolation between manually selected points. Structural parameters were taken from the published structural data of the individual phases and only refined when appropriate.12,13,15 3. Results and Discussion
Figure 4. Relative amount of β-AlD3 (black squares), R-AlD3 (light gray triangles), γ-AlD3 (dark gray circles), and Al (gray inverted triangles) calculated from the in situ SR-PXD measurements of a β-sample kept at 83 °C plotted against time.
Figure 5. In situ SR-PXD measurements of γ-AlD3 from 40 to 160 °C. The heating rate is 1 K/min. The presence of γ-AlD3, R-AlD3, and Al are indicated in the figure.
powder diffraction patterns with the program Fit2d.18 Data were collected in the 2θ range of 1-34.5° and were rebinned with a
It is well-known that AlH3 decomposes to aluminum and hydrogen on heating. The present in situ SR-PXD data show that neither γ-AlD3 nor β-AlD3 decompose directly into Al and D2 when heated from about 40 to 160 °C at a heating rate of 1 K/min. Figure 1 shows that β-AlD3 transforms to an intermediate state, R-AlD3, before decomposing to Al and D2. Three additional phases were included in the Rietveld refinements of the β-AlD3 sample: γ-AlD3, R-AlD3, and aluminum. At 60 °C the sample contained about 88.3 wt % β-, 1.0 wt % R-, 10.3 wt % γ-AlD3, and 0.4% Al according to the Rietveld refinement. QPA results for selected temperatures are given in Table 1, and Figure 2 shows the obtained fits of the Rietveld refinements at 60, 90, and 120 °C, respectively. The relative amounts of β-, R-, and γ-AlD3 and Al in the β-sample in the temperature range of 40-160 °C are shown in Figure 3. The uncertainties in the wt % of each phase are within the plotted data points. The γ-AlD3 is present as an impurity phase from the synthesis. At about 80 °C the β-phase starts to transform into the R-phase, and this transformation continues until all of the β-phase is transformed into the R-phase at about 125 °C. At the same time the minor γ-phase is also transformed into R. The amount of Al phase starts to increase from about 110 °C and increases rapidly from about 130 °C as the R-phase starts to decay. The R-phase is decomposing into Al and D2. The inset in Figure 3 shows the details of the decomposition of the minority γ-phase (see below). The reaction pathway for the decomposition of β-AlD3 is 80 °C
130 °C
β-AlD3 98 R-AlD3 98 Al + 1.5D2
(R2)
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Figure 6. Observed intensities (circles) and calculated intensities from Rietveld refinements of γ-sample at (a) 80 °C before the decomposition, (b) 110 °C when the transformation to R has started, and (c) 120 °C as the R phase is at its maximum. Positions of Bragg reflections are shown with bars for γ-AlD3, R-AlD3, Al, and LiCl (from top). The differences between observed and calculated intensities are shown with the bottom line.
In addition to the experiment at a constant heating rate described above, an experiment at isothermal condition was performed at 83 °C for the same sample. Diffraction patterns were recorded every second minute. Figure 4 shows the relative amount of the four phases (β-, R-, γ-AlD3, and Al) plotted as a function of the time. The amount of β-AlD3 decreases while the R phase increases accordingly. The amount of the γ-phase is decreasing very slowly, while the Al content is negligible. This result supports the conclusion from the first experiment and clearly shows that β-AlD3 transforms directly into R-AlD3. This is in agreement with the results of Graetz and Reilly and Orimo et al.,8,9 but disprove the DFT calculations of Ke et al., stating that β-AlH3 is more stable than R-AlH3.14 Figure 5 shows the decomposition of γ-AlD3. The intermediate phase R-AlD3 is present prior to the decomposition into Al and D2. Four different phases were included in the refinements for the γ-sample: γ-AlD3, R-AlD3, LiCl, and Al. The amount of LiCl is constant (≈20 wt %) during heating and is therefore excluded from the calculation of the relative wt %. At 60 °C the sample contained about 90 wt % γ-AlD3 and 10 wt % R-AlD3. The fits of the Rietveld refinements for the γ-sample at selected temperatures are shown in Figure 6: (Figure 6a) at 60 °C before the composition, (Figure 6b) at 80 °C when the transformation to R has started, and (Figure 6c) at 120 °C when the amount of R-AlD3 is at a maximum. Table 2 shows refined parameters at selected temperatures. The relative amounts (in wt %) of γ-AlD3, R-AlD3, and Al phases in the γ-sample are plotted in Figure 7 in the temperature range of 40-160 °C. At about 90 °C the γ-phase starts to transform into the R-phase. The rate of the transformation continues to increase until about 105 °C; then the transformation from γ-AlD3 to R-AlD3 slows down. At about 110 °C the decomposition rate of γ-AlD3 increases again and accompanied by formation Al. This can be
explained by a transformation of the γ-phase directly to Al. In the temperature range of 110-130 °C the amount of the R-phase is changed very little compared to the change in the amount of Al and γ-phase (see the inset Figure 7). At about 120 °C the formation rate of Al is reduced; this is consistent with the disappearance of the γ state. At 125 °C the amount of Al starts to increase faster again, taking place at the same time as the amount of R-AlD3 starts to decrease. Thus above 125 °C the only process is the decomposition of R-AlD3 into Al and D2. This indicates that the R-phase is kinetically the stable product at lower temperatures, whereas the thermodynamically stable products, Al and D2, are formed directly at higher temperature. The transformations are 90 °C
130 °C
γ-AID3 98 R-AID3 98 A1 + 1.5D2 110 °C
γ-AID3 98 A1 + 1.5D2
(R3) (R4)
The transformation of the 10 wt % γ-AlD3 in the β-sample shows the same features as for the γ-sample (compare the insets in Figures 3 and 7). Figure 8 shows connection of the AlD6 octahedra in the three different phases, R-AlD3, β-AlD3, and γ-AlD3, respectively. The three structures crystallize in three different space groups: R-AlD3 in the trigonal R-3c;11,12 β-AlD3 in the cubic Fd-3m;13 γ-AlD3 in the orthorhombic Pnnm.15 Possible pathways for the transition of β-AlD3 to R-AlD3 and for γ-AlD3 to R-AlD3 were considered. It is difficult to see similarities in the β- and R-AlD3 structures that can indicate the phase transition pathways. Both structures consist of three-dimensional frameworks of cornersharing tilted octahedra. The β-phase has interconnection between octahedra connected to the same octahedron, but such
Thermal Decomposition of β- and γ-AlD3
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TABLE 2: Selected Crystallographic Values for the γ-Samplea T γ-AlD3 wt % space group a (Å) b (Å) c (Å) V (Å3) V (Å3)/Z′ R-AlD3 wt % space group a (Å) c (Å) V (Å3) V (Å3)/Z′ Al wt % space group a (Å) V (Å3) Rwp a
60 °C
80 °C
100 °C
120 °C
140 °C
160 °C
89.9(7)
89.9(7)
80.6(6)
8.8(2)
7.3346(4) 5.3632(3) 5.7534(2) 226.33(2) 37.72
7.3349(4) 5.3628(3) 5.7541(2) 226.34(2) 37.72
7.3358(4) 5.3632(3) 5.7554(2) 226.44(2) 37.74
7.330(16) 5.367(1) 5.7551(5) 226.41(7) 37.74
10.1(3)
10.1(3)
19.4(3)
59.8(2)
31.9(2)
0.2(2)
4.4309(2) 11.7779(9) 200.26(2) 33.38
4.4330(2) 11.7788(9) 200.46(2) 33.41
4.43757(14) 11.7810(5) 200.91(1) 33.49
4.44161(5) 11.7829(2) 201.310(4) 33.55
4.44317(7) 11.7831(3) 201.453(7) 33.58
31.3(2)
68.1(2)
99.8(2)
4.05694(5) 66.772(1)
4.05892(6) 66.870(2)
4.06047(6) 66.947(2)
6.46
6.52
3.85
Pnnm
R-3c
Fm-3m
12.0
12.2
11.1
The amount of LiCl is constant at about 20 wt % and excluded from the calculations of the relative wt %.
Figure 7. Relative amount of γ-AlD3 (dark gray circles), R-AlD3 (light gray triangles), and Al (gray inverted triangles) calculated from the in situ SR-PXD measurements of the γ-sample plotted against temperature. The heating rate is 1 K/min. The LiCl content is constant and excluded in this plot.
Figure 8. First coordination sphere for AlD6 units in (a) R-AlD3, (b) β-AlD3, and (c) γ-AlD3. Medium gray is used for octahedra with only corner sharing, while dark gray is used for octahedra with edge sharing.
interconnections are not present in the R-phase. Also, the density of the relatively compact structure of R-AlD3, 1.643 g/cm3, is different from β-AlD3 that has channels of 3.9 Å running through the structure, thus lowering the density to 1.202 g/cm3.
Major rearrangements of the AlD6 octahedra in the β-phase are needed for a transformation to the R-phase. Compressing the unit cell to eliminate the channels will cause a reduction of the space group symmetry, and therefore, R-AlD3 has lower
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Figure 9. (a) Structure of γ-AlD3, viewing along the c-axis. (b) Schematic drawing of the transformation from γ-AlD3 to R-AlD3 in an ab-layer; the arrows indicate the sliding of octahedra. Grey squares indicate AlD6 units in R-AlD3, while squares with a brick pattern indicate AlD6 units in γ-AlD3.
Figure 10. a-axis in β-AlD3 in the β-sample plotted against temperature. The errors bars are calculated by the Rietveld refinements.
symmetry than β-AlD3. A close inspection of the diffraction data did not reveal splitting of any reflections close to the transition temperature, indicating that the transition is instant. The transition from γ-AlD3 to R-AlD3 is also difficult to interpret, since the γ-AlD3 in addition to corner-sharing octahedra also has edge-sharing octahedra (see Figure 8). γ-AlD3 has cavities and a density of 1.453 g/cm3. The octahedra are arranged so that the shared edges are parallel to the ab-plane (Figure 9a). Sliding the octahedra as shown in Figure 9b will give a structure similar to the R-phase. This will cause a reduction of cell volume of 12% and accordingly increase the density. Figures 10-12 show the changes in the unit cell parameters with temperature for the different phases in the different samples. Figure 10 shows the changes in the a-axis for β-AlD3. The unit cell expansion is almost linear with temperature. It is only a slight decrease in rate at around 80 °C where the transition from β- to R-ΑlD3 starts. The changes in the unit cell axes for γ-AlD3 in the γ-sample are shown in Figure 11. The a- and b-axes are not increasing as much as expected from thermal expansion. The b-axis is actually decreasing up to about 95 °C, while the a-axis is only slightly increasing up to about 100 °C. This is in agreement with shrinkage in the cavities in the ab-plane. Along the c-axis no shrinkage is necessary for the transition from γ- to R-AlD3 since no cavities are present along this axis. This is in agreement with the significantly increased c-axis with temperature Figure 12 shows the changes in the R-AlD3 for both the βand γ-samples. The changes in the a-axis are significantly larger than the changes in the c-axis. These changes are also significantly larger than what is found in this work for the β-
Figure 11. a-axis (top), c-axis (middle), and b-axis (lower) for γ-AlD3 in the γ-sample plotted against temperature. The errors bars are calculated by the Rietveld refinements.
Figure 12. a- and c-axes in R-AlD3 for the β- and γ-samples, plotted against temperature. Inverted triangles are for R-AlD3 in the β-sample, and squares are for R-AlD3 in the γ-sample. The errors bars are calculated by the Rietveld refinements.
and γ-samples. This anisotropy is possibly related to the higher density of Al atoms along the a-axis compared to the c-axis. It is also seen from Figure 12 that for the γ-sample, with some R-ΑlD3 present at the as-synthesized sample, the unit cell is smaller than what is found in a pure sample.12 When reaching the setpoint for the formation of R-AlD3 from γ-AlD3 at about
Thermal Decomposition of β- and γ-AlD3 90 °C, both the a- and the c-axis in R-AlD3 increase more with temperature up to about 105 °C than both before and after this temperature region. This can possibly be explained by interface interaction between the surfaces of γ-AlD3 and R-AlD3 affecting the unit cell of R-AlD3 with only a small amount present. 4. Conclusions The stability of β-AlD3 and γ-AlD3 has been studied. β-AlD3 transforms into R-AlD3 at 80 °C, and at about 130 °C R-AlD3 decomposes to Al and D2. The QPA of the SR-PXD data shows that β-AlD3 is transformed to R-AlD3 prior to decomposition to Al and D2. γ-AlD3 transforms into R-AlD3 at about 90 °C, and R-AlD3 decomposes to Al and D2 at about 125 °C. From about 110 °C γ-AlD3 also decomposes directly to Al. The transitions from β- and γ-AlD3 to R-AlD3 are accompanied by more compact crystal packing, and major rearrangement of the AlD6 tetrahedra is needed. Acknowledgment. We acknowledge the skillful help and assistance from the project team at the Swiss-Norwegian Beam Line, ESRF, Grenoble, France. The Research Council of Norway is acknowledged for financial support through the NANOMAT project (H.G.). W. Langley and C. Brown are thanked for the preparation of samples. References and Notes (1) Sandrock, G.; Reilly, J. J.; Graetz, J.; Zhou, W. M.; Johnson, J.; Wegrzyn, J. J. Appl. Phys. A 2005, 80, 687.
J. Phys. Chem. C, Vol. 111, No. 44, 2007 16699 (2) Brower, F. M.; Matzek, N. E.; Reigler, P. F.; Rinn, H. W.; Roberts, C. B.; Schmidt, D. L. J. Am. Chem. Soc. 1976, 98, 2450. (3) Finholt, A.; Bond, A.; Schlesinger, H. J. Am. Chem. Soc. 1947, 69, 1199. (4) Herley, P. J.; Christofferson, O. J. Phys. Chem. 1981, 85, 1882. (5) Graetz, J.; Reilly, J. J. J. Phys. Chem. B 2005, 109, 22181. (6) Baranowski, B.; Tkacz, M. Z. Phys. Chem. Neue Folge 1983, 135, 27. (7) Konovalov, S. K.; Bulychev, B. M. Inorg. Chem. 1995, 34, 172. (8) Orimo, S.; Nakamori, Y.; Kato, T.; Brown, C.; Jensen, C. M. Appl. Phys. A 2006, 83, 5. (9) Graetz, J.; Reilly, J. J. J. Alloys Compd. 2006, 424, 262. (10) Sinke, G. C.; Walker, L. C.; Oetting, F. L.; Stull, D. R. J. Phys. Chem. 1967, 47, 2759. (11) Turley, J. W.; Rinn, H. W. Inorg. Chem. 1969, 8, 18. (12) Brinks, H. W.; Istad-Lem, A.; Hauback, B. C. J. Phys. Chem. B 2006, 110, 25833. (13) Brinks, H. W.; Langley, W.; Jensen, C. M.; Graetz, J.; Reilly, J. J.; Hauback, B. C. J. Alloys Compd. 2007, 433, 180. (14) Ke, X.; Kuwabara, A.; Tanaka, I. Phys. ReV. B 2005, 71, 184107. (15) Brinks, H. W.; Brown, C.; Jensen, C. M.; Graetz, J.; Reilly, J. J.; Hauback, B. C. J. Alloys Compd. 2007, 441, 364. (16) Yartys, V. A.; Denys, R. V.; Maehlen, J. P.; Frommen, C.; Fichtner, M.; Bulychev, B. M.; Emerich, H. Inorg. Chem. 2007, 46, 1051. (17) Maehlen, J. P.; Yartyes, V. A.; Denys, R. V.; Fichtner, M.; Frommen, C.; Bulychev, B. M.; Pattison, P.; Emerich, H.; Filinchuk, Y. E.; Chernyshov, D. J. Alloys Compd. [Online early access]. DOI:10.1016/ j.jallcom.2006.11.199; available online January 12, 2007; dx.doi.org. (18) Hammersley, A. P. FIT2D V12.077; Internal Report ESRF-97HA02T, 1997; Internal Report ESRF-98-HA01T; European Synchrotron Radiation Facility, Grenoble, France, 1998. (19) Rodrı´guez-Carvajal, J. Physica B 1993, 192, 55.
