Adjustment of the Stability of Complex Hydrides by Anion Substitution

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J. Phys. Chem. C 2008, 112, 5658-5661

Adjustment of the Stability of Complex Hydrides by Anion Substitution Hendrik W. Brinks, Anita Fossdal, and Bjørn C. Hauback* Institute for Energy Technology, P. O. Box 40 Kjeller, NO-2027, Norway ReceiVed: October 17, 2007; In Final Form: January 26, 2008

Contrary to interstitial metal hydrides, where the thermodynamic stability of the metal hydride can be tuned by substitutions on the metal lattice, few examples of such substitutions have been reported for complex hydrides. The present paper shows a new and alternative approach to affect the stability of complex hydrides by substitution on the anion lattice. A mixed hydride-flouride has been prepared from NaF, Al, and H2. The resulting complex hydride Na3AlH6-xFx is less stable than Na3AlH6. Pressure composition isotherms indicate a plateau pressure of about 25 bar for dehydrogenation at 120 °C. The crystal structure has been determined by combined neutron diffraction and synchrotron X-ray diffraction to be isostructural with Na3AlF6 and Na3AlD6. The space group is P21/n, and x was determined to be approximately 4. No ordering of D and F was observed.

1. Introduction Aluminum-based complex hydrides (alanates) have been extensively investigated since 1997 when reversible hydrogen storage properties under moderate temperature and pressure conditions were reported for NaAlH4 with Ti additives.1 However, despite the potential for high hydrogen storage contents in several compounds in this family, such as LiAlH4 with 10.6 wt % hydrogen, the practical reversible storage capacity has so far been limited to about 4 wt % at 150 °C reported for the NaAlH4 system.2-5 The reasons for this are partly unfavorable thermodynamic stability and the complexity of the reactions, taking place in several steps. Amide/imide systems of light elements such as lithium and magnesium have shown similar reversible storage capacities but at slightly higher temperatures.6 Boron-based complex hydrides, LiBH4 in particular, have shown reversibility at around 350 °C, when destabilized by MgH2, and with a storage capacity of about 8 wt % hydrogen.7 The storage capacity of catalyzed MgH2 at 300 °C is by comparison about 6.9 wt %, and the system exhibits excellent kinetics.8 Recently it has been shown that nearly 5 wt % hydrogen can be absorbed within 15 s at room temperature under 10 bar hydrogen pressure in catalyzed MgH2.9 The stability of many interstitial metal hydrides can be tuned by adjusting the composition of the intermetallic compounds. For complex hydrides, like alanates, there are only a few examples of altered stability by substitutions on the metal lattice. It should be noted in this connection that the significant effect of Ti additives on the reversibility of NaAlH4 and other alanates is not due to substitution in the alanate lattice and is likely to be a kinetic effect.10-14 The effect on the thermodynamic stability of Na3AlH6,15,16 which has two different Na sites (NaI2NaIIAlH6),17 has been studied by substitution of K on the NaI sites in K2NaAlH6,18 as well as substitution of Li on the NaII sites in Na2LiAlH6.1,19 The composition of these phases cannot apparently be continuously changed, but they exist as compounds with well-defined compositions. In both cases substitution leads to increased stability, which for practical purposes is opposite to the desired effect, i.e., destabilization. * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +47 63 80 60 78. Fax: +47 63 81 09 20.

A different scheme of changing the stability of complex hydrides was reported by Vajo et al. for LiBH4.7 Destabilization was achieved by adding a spontaneous side reaction from Mg or MgH2 to MgB2 and thus changing the thermodynamics of the total reaction. The plateau pressure at, e.g., 350 °C increases significantly. The stability of the complex hydride phase itself has not been altered, and this approach may add more solid phases to the reaction scheme with related challenges for the kinetics. In the present paper a new and alternative approach to alter the stability of complex hydrides is presented. The changed stability of a complex hydride is brought about by substitution in the lattice of the complex hydride phase,20 but the substitution is in the hydrogen sublattice and not the metal sublattice. 2. Experimental Methods NaF (SigmaAldrich, 99.99%), KF (SigmaAldrich, 99%), Al (Aluminum Powder Co Ltd, 99.99%, mean particle size 20 µm), and TiF3 (Alfa Aesar) were used as starting materials and mixed in a Fritsch Pulverizette P7 ball mill with a ball to sample mass ratio of 20:1 and rotation speed of 700 rpm for 3h. All handling of samples was carried out in Ar atmosphere in a glove box, and transfer of samples was done in sealed vials to prevent reaction with moisture and oxygen. Synchrotron powder X-ray diffraction (SR-PXD) data at 22 °C were collected at the Swiss-Norwegian beam line (station BM01B) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The samples were kept in rotating 0.5 mm boron-silica-glass capillaries. Intensities were measured in steps of ∆(2θ) ) 0.005° at 22 °C. The wavelength 0.49957 Å was obtained from a channel-cut Si(111) monochromator. Powder neutron diffraction (PND) data at 22 °C were collected with the PUS instrument at the JEEP II reactor at Kjeller, Norway.21 Neutrons with wavelength 1.5553 Å were obtained from a Ge(511) focusing monochromator. The detector unit consists of two banks of seven position-sensitive 3He detectors, each covering 20° in 2θ (binned in steps of 0.05°). Data were collected from 10 to 130° in 2θ. The sample was

10.1021/jp7100754 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/19/2008

Adjustment of the Stability of Complex Hydrides contained in a rotating cylindrical 6 mm diameter vanadium sample holder. Rietveld refinements were carried out using the program Fullprof (version 2.80).22 X-ray form factors and neutronscattering lengths were taken from the Fullprof library. Thompson-Cox-Hastings pseudo-Voigt profile functions were used, and the backgrounds were modeled by interpolation between manually chosen points. The instrumental resolution of the SRPXD data was determined with a LaB6 standard. Pressure-composition isotherms (PCT curves) were measured in an in-house-built, fully automated Sieverts-type apparatus.23 The instrument is equipped with two pressure transducers; a Baratron MKS120 (accuracy 0.08% of reading; supplier’s calibration), which operates from 0 to 28 bar, and a Presens transducer (accuracy (4 mbar; calibrated against a deadweight tester) primarily for the range 28-100 bar. The equilibrium criterion was set to dP/dt e 8 mbar/h. The sample was kept in an SS316L sample holder. Approximately 4 mL of the autoclave was heated to the sample temperature, while the remaining system was at 40 °C in two heating cabinets with temperature stabilities of (0.02 and (0.05 °C, respectively. The total system volume was 50 mL at low pressures and 33 mL at high pressures. The H2 compressibilities of Hemmes et al. were used in the calculations.24 Recently, this Sieverts-type apparatus was used to measure accurate PCT curves for Na2LiAlH619 and K2NaAlH618, both in excellent agreement with similar measurements done by Graetz et al.14

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Figure 1. Pressure development in a sample consisting of 3NaF + Al + 0.04TiF3 after heating to 120 °C and introduction of hydrogen at a constant temperature in the PCT measurement system.

3. Results and Discussion The dehydrogenation of NaAlH4 takes place in three steps: First to 1/3Na3AlH6 + 2/3Al releasing 50% of the hydrogen and then to NaH + Al releasing 25% of the original hydrogen content. The last step to Na + Al occurs at higher temperature and is not of interest for practical applications. The hydrogen in NaH may therefore be replaced by another element, without reducing the hydrogen release per formula unit from NaAlH4 or Na3AlH6. The similarity between many hydrides and their corresponding fluorides is well-known.25 For example Na3AlH6 and Na3AlF6 take the same structure.17,26 Since it has been shown to be a limited possibility for substitution into the cation sublattices, it was attempted to partially replace hydrogen in Na3AlH6 by fluorine. This was carried out by milling NaF and Al in a 3:1 ratio with 4 mol % TiF3 additive, followed by hydrogen absorption at 120 °C and an initial H2 pressure of 57 bar. After ball milling, X-ray diffraction showed presence of only the starting materials NaF and Al. As shown in Figure 1, this sample absorbs hydrogen indicating that a mixed fluorine and hydride compound is possible. The hydrogen uptake corresponds to 0.4 H per Al atom. PCT curves measured at 100 and 120 °C, cf. Figure 2, show clearly that both the absorption and desorption isotherms are well above the plateau pressures for Na3AlH6; at 110 °C, it is about 1 bar.15,16 By extrapolation this gives about 0.7 bar at 100 °C and 1.5 bar at 120 °C for Na3AlH6. This shows that the mixed hydride/fluoride system studied here is destabilized relative to the pure hydride system. It should be added that lack of equilibrium would give a lower measured equilibrium pressure during absorption, and thus it can be concluded that the effect of destabilization is at least as large as found in the present work. Another 3 NaF + Al sample with TiF3 additive was prepared for SR-PXD and PND measurements at 120 °C and 90 bar D2 end pressure. Additional reflections in the SR-PXD diagram

Figure 2. Pressure-composition isotherms of 3NaF + Al + 4% TiF3 at 100 and 120 °C. The filled symbols show hydrogenation, and the empty symbols show subsequent dehydrogenation.

Figure 3. Observed SR-PXD diagram of Na3AlD6-xFx, compared to simulated diagrams for Na3AlD6 and Na3AlF6, normalized to the strongest reflection. Na3AlD6-xFx is an intermediate between Na3AlD6 and Na3AlF6.

compared to simulated diagram for Na3AlH6 are evident in Figure 3. These reflections can be indexed by the unit cell of Na3AlH6 (P21/n), but in Na3AlH6, these reflection do not have significant intensity. However, in the isostructural Na3AlF6 these

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Figure 5. Crystal structure of Na3AlD6-xFx with Al(D,F)6 octahedra in yellow and Na(D,F)6 octahedra in red. Red atoms are Na2 and black small atoms D/F.

Figure 4. Observed intensities (circles) and calculated intensities from Rietveld refinements (upper line) of Na3AlD6-xFx at 295 K for: (a) PXD (BM01B, ESRF) and (b) PND (PUS, Kjeller) data. Positions of Bragg reflections are shown with bars for Na3AlD6-xFx, Al, Al1-yTiy, and NaF (from top). The differences between observed and calculated intensities are shown with the bottom line.

reflections do have intensity. The present sample therefore shows strong indication for being an intermediate between Na3AlH6 and Na3AlF6. Rietveld refinements with combined PND and SR-PXD data result in very satisfactory fit, cf. Figure 4. The unit-cell dimensions are a ) 5.4145(1) Å, b ) 5.5741(1) Å, c ) 7.7676(2) Å, and β ) 90.015(4)°. The unit-cell volume is 234.433(10) Å3. This is closer to the unit cell volumes of Na3AlF6 (234.48 Å3) than Na3AlD6 (229.59 Å3). In the refinements with the present data no significant improvement was achieved by refining the positions of D and F independently. Furthermore, the positions of the atoms did not change much, and the uncertainty in the positional parameters increased considerably by distinguishing between the positions of D and F. Hence, for the anion sublattice, there are no indications of ordering of D and F, and the final refinement was carried out with disordered D and F, giving an occupation of 32(1)% D. Very similar results were obtained by Rietveld refinements of PND and SR-PXD independently. This result is furthermore in line with the pressure-composition isotherms (cf. Figure 2). Diffraction techniques give an average over every unit cell of the sample, and in the present refinements it is found that on aVerage about 1/3 of the D/F positions are occupied by D and 2/3 by F. This means that the exact coordination number of D and F around the Al atoms cannot be determined by X-ray and neutron diffraction.

The structural model from the Rietveld refinement is presented in Table 1, and selected interatomic distances and angles are shown in Table 2. The phase composition of the sample was determined to be 33 wt % Na3AlD6-xFx, 10 wt % Al/Al1-yTiy, and 57 wt % NaF. The estimated crystallite size, based on isotropic broadening of the reflections, were refined to 91 nm for Na3AlD6-xFx, 74 nm for Al, and 61 nm for NaF. The crystal structure is illustrated in Figure 5. The crystal structure of Na3AlD6-xFx consists of a 3D framework of alternating Al(D,F)6 octahedra and Na1(D,F)6 octahedra, with each anion shared between two different types of octahedra. The structure is similar to Na3AlD6 and Na3AlF6. In Na3AlD6-xFx, the Al-D/F distances are 1.777-1.815 Å, compared to 1.748-1.769 Å for Na3AlD6 and 1.800-1.808 Å for Na3AlF6. In the Na1(D,F)6 octahedra the Na1-D/F distances are similar for all three compounds 2.232-2.266, 2.242-2.278, and 2.237-2.291 Å in Na3AlD6, Na3AlD6-xFx, and Na3AlF6, respectively. The shortest distance between Al atoms for Na3AlD6-xFx is 5.4145(1) Å. The similar distance for Na3AlD6 is 5.390 Å, and for Na3AlF6 it is 5.414 Å. This distance is determined both by the Al-F/H distance, the Na1-H distance, and the rotation of the octahedra. The magnitude of the rotations can be expressed in the Al-D/F-Na1 angles which for Na3AlD6-xFx are 143.2150.2°, for Na3AlD6 145.8-150.7°, and for Na3AlF6 143.4149.7°. In summary, the crystal structure of Na3AlD6-xFx is isostructural with Na3AlD6 and Na3AlF6, and the unit cell dimensions are very similar to Na3AlF6. In a similar way, K3AlD6-xFx was synthesized from 3 KF and Al. SR-PXD indicated a structure similar to K3AlD6,27 but with 4.2% larger unit-cell volume. This shows that mixed hydrides-fluorides are not limited to sodium hexa-alanate but should be possible for a number of complex hydrides. For intermetallic metal hydrides it has earlier been found that hydrogenation of ThI2 is possible and that the plateau pressure is different from metal hydride prepared from Th metal.28 For complex hydrides nothing similar has been reported, even though the compounds K3AlH4F2 and NaMgHxFx have been reported to exist.29,17 It has earlier been reported anomalies in differential scanning calorimety results for TiF3-doped Na3AlH6 that are in line with the present work.30 Furthermore, very recent

Adjustment of the Stability of Complex Hydrides

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TABLE 1: Refined Crystal Structure Parameters for Na3AlD6-xFxa atom

Wyckoff position

x

y

z

Biso (Å2)

occupation

Na1 Na2 Al D1 D2 D3 F1 F2 F3

2b 4e 2a 4e 4e 4e 4e 4e 4e

0 -0.0085(14) 0 0.1009(10) 0.2324(11) 0.1685(11) 0.1009(10) 0.2324(11) 0.1685(11)

0 0.4547(8) 0 0.0433(10) 0.3267(11) 0.2659(11) 0.0433(10) 0.3267(11) 0.2659(11)

1/2 0.2506(11) 0 0.2172(9) 0.5455(9) 0.9339(9) 0.2172(9) 0.5455(9) 0.9339(9)

1.71(9) 1.71(9) 1.4(2) 3.3(7) 3.3(7) 3.3(7) 1.3(2) 1.3(2) 1.3(2)

1 1 1 0.324(10) 0.324(10) 0.324(10) 0.676(10) 0.676(10) 0.676(10)

a The space group is P21/n, Z ) 4, and the unit cell dimensions are a ) 5.4145(1) Å, b ) 5.5741(1) Å, c ) 7.7676(2) Å, and β ) 90.015(4) °. Reliability factors are Rwp ) 5.74% and χ2 ) 1.48 for the SR-PXD data and Rwp ) 4.23% and χ2 ) 1.76 for the PND data. Estimated standard deviations are in parentheses.

TABLE 2: Selected Interatomic Distances (Angstroms) and Angles (Degrees) in the Crystal Structure of Na3AlD6-xFx (Estimated Standard Deviations in Parentheses) atoms

distance

Al-D1/F1 (2x) Al-D2/F2 (2x) Al-D3/F3 (2x) Na1-D1/F1 (2x) Na1-D2/F2 (2x) Na1-D3/F3 (2x) Na2-D1/F1 (1x) Na2-D2/F2 (1x) Na2-D3/F3 (1x) Al-Al

1.790(7) 1.777(6) 1.815(6) 2.276(6) 2.242(6) 2.278(6) 2.275(9) 2.337(10) 2.287(9) 5.4145(1) 2x

atoms

angles

Al-D1/F1-Na1 Al-D2/F2-Na1 Al-D3/F3-Na1 D-Al-D

145.3(4) 150.2(3) 143.2(3) 88.7(3) 90.7(3)

2.383(7) 5.4937(2) 8x

89.3(3) 91.3(3)

5.5741(1) 2x

89.8(3) 180.0(-)

90.2(3)

DFT calculations for TiF3 doped Na3AlH6 are in agreement with the present work.31 The reversible storage capacity for Na3AlH2F4 present here is moderate, and Na3AlH2F4 was not completely dehydrogenated under the temperature and pressure conditions of the PCT measurements. Further work is in progress to clarify this. However, this is to our knowledge the first systematic attempt resulting in adjustments of the thermodynamics by anion substitution for complex hydrides. For Na3AlH6 this leads to destabilization, which is desirable, but it is possible that substitution of halogen in other cases may lead to stabilization. Still further work has to be carried out in order to clarify whether a gradual adjustment of the thermodynamic stability can be achieved by using different amounts of halogen substitution. The present approach may give new possibilities to tune the stability of other complex metal hydrides with high storage capacities, such as, e.g., LiBH4 and AlH3, that is at present not suitable for practical applications. Acknowledgment. The authors gratefully acknowledge Dr. Arnulf Maeland for encouragement and discussions. The skillful assistance from the project team at the Swiss-Norwegian Beam Line, ESRF, is gratefully acknowledged. Partial funding by the European Commission DG Research Contract SES6-200651827/NESSHY) is gratefully acknowledged.

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