Wide-Line Solid-State NMR Characterizations of Sodium Alanates

Jul 30, 2009 - Eric G. Sorte , Samuel B. Emery , E. H. Majzoub , Tim Ellis-Caleo , Zayd L. Ma , Blake A. Hammann , Sophia E. Hayes , Robert C. Bowman ...
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J. Phys. Chem. C 2009, 113, 15467–15472

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Wide-Line Solid-State NMR Characterizations of Sodium Alanates Margriet H. W. Verkuijlen,† P. Jan M. van Bentum,*,† Ernst R. H. van Eck,† Wiebke Lohstroh,‡ Maximilian Fichtner,‡ and Arno P. M. Kentgens*,† Institute for Molecules and Materials, Radboud UniVersity, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands, and Institut fu¨r Nanotechnologie, Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe, Germany ReceiVed: June 4, 2009; ReVised Manuscript ReceiVed: July 11, 2009

Pure NaAlH4, TiCl3-doped NaAlH4, and pure Na3AlH6 were characterized using 1H, 23Na, and 27Al solidstate NMR. The signal intensities and linewidths of 1H NMR spectra using several spin echo sequences and backprediction of a single pulse experiment were compared to find the optimal experiment to measure wide-line NMR spectra of the alanates. Second moment calculations using the Van Vleck equations compared with fits of the dipolar coupling line broadening confirm that NaAlH4 has a rigid crystal lattice. On the other hand, for Na3AlH6, a narrowing of the proton and aluminum lineshape was observed, indicating a fast rotational motion of AlH6 clusters at room temperature. A broadening of the 1H and 27 Al linewidth was observed upon lowering the temperature. This process is successfully described using thermally activated rotational jumps of AlH6 clusters assuming a fast rotational motion around one single C4 axis and a slower rotation around the other two C4 axes with an activation barrier of Ea ) 25 kJ/mol and an attempt frequency of V0 ) 4 × 1010 Hz. Introduction One of the main problems in the large scale applications of hydrogen as an energy carrier for mobile applications is to find a safe and efficient way to store the hydrogen gas. A promising method for compact storage of hydrogen is the use of metal hydrides. In this class of materials, a well-known and extensively studied material is sodium alanate, NaAlH4, which has a theoretical hydrogen storage capacity of 5.6 wt %. Hydrogen desorption takes place in two steps, first from NaAlH4 into Na3AlH6 which further decomposes into NaH with the simultaneous formation of Al metal.

3NaAlH4 T Na3AlH6 + 2Al + 3H2

(1)

Na3AlH6 T 3NaH + Al + 3/2H2

(2)

Normally, these reactions take place at relative high temperatures and are not reversible. Therefore, in 1997, a breakthrough was the discovery by Bogdanovic et al.1 that doping of NaAlH4 with a Ti-based catalyst dramatically improves the hydrogen desorption and absorption kinetics. Another way to decrease the desorption temperature of the sodium alanate was found by Balde´ et al.2 by nanostructuring of the material. However, the underlying mechanism of these methods is still not fully understood. Several NMR studies of sodium alanates have been conducted before. Bogdanovic et al.3 have studied the effect of discharging and recharging of NaAlH4 using 23Na and 27Al NMR. Majer et al.4 have measured 1H spectra of NaAlH4, Na3AlH6, and NaH. They studied the effect of different catalysts on the hydrogen dynamics by measuring the spin-lattice relaxation rate and * To whom correspondence should be addressed. E-mail: J.vanBentum@ nmr.ru.nl (P.J.M.v.B.); [email protected] (A.P.M.K.). † Radboud University. ‡ Forschungszentrum Karlsruhe.

performed variable temperature measurements for Na3AlH6. Herberg et al.5,6 performed NMR and X-ray diffraction experiments on titanium-doped alanates. Because of the anisotropic nature of the NMR interactions, static powder spectra in solid-state NMR are usually very broad corresponding to fast signal decay in the time domain. When the signal decay is fast compared to the receiver dead time, a significant part of the signal is not recorded and artifacts will occur in the spectrum. In order to find the optimal experiment and avoid signal loss due to complex proton magnetization dynamics, we used and compared several echo sequences to measure the static 1H spectra of alanates. The linewidths of the NMR spectra are analyzed using second moment calculations based on the Van Vleck equations to determine the line broadening of the spectra caused by dipolar couplings.7 This is used to study possible molecular motions in the crystal detected via motional averaging (ms/µs time scale) of these dipolar couplings. In addition, variable temperature measurements were done for pure Na3AlH6 to study hydrogen mobility as function of temperature. Furthermore, we have simulated the 1H and 27Al linewidths based on a computational approach proposed by Goc assuming thermal activated rotational jumps of the AlH6 clusters to shed light on the nature of the rotational dynamics in Na3AlH6.12-14 Experimental Section Sample Preparations. Commercially available NaAlH4 (Sigma-Aldrich) was purified by a Soxhlet extraction with THF and ball-milled for 30 min. The Ti-doped sample was prepared by ball-milling purified NaAlH4 with 6 mol % TiCl3 (99.999%, Sigma-Aldrich) for 30 min.15 Pure Na3AlH6 was synthesized by ball-milling pure NaAlH4 and NaH using a 1:2 molar ratio.16 The purity of the samples was checked by 23Na and 27Al magic angle spinning (MAS) NMR experiments. The chemical shift values concur with values from the literature, and no significant amounts of impurities were found.3

10.1021/jp905258x CCC: $40.75  2009 American Chemical Society Published on Web 07/30/2009

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Verkuijlen et al. TABLE 1: Comparison of Linewidths, Integrated Peak Intensities, and Lineshapes Expressed as the Fraction Lorentzian/Gaussian (Frac. L/G) for Different Spin Echo Sequencesa on NaAlH4 SPE HSH HSH MS MS MS ME ME

Figure 1. Echo sequences employed in this study: (A) the Hahn-solidHahn echo17,18 using τHSH ) 10 and 15 µs; (B) the magic echo19,20 using τME ) 20 and 30 µs; (C) the magic sandwich echo21 using τMSE ) 2.0 µs, nMSE ) 3, 4, and 5, and τ ) τMSEnMSE.

NMR Experiments. Static 1H NMR measurements at room temperature were performed on pure NaAlH4, Ti-doped NaAlH4, and pure Na3AlH6 on a Chemagnetics Infinity 300 MHz spectrometer using a home-built probe specially designed for detecting static proton signals without background signal. All NMR experiments were performed in a flow of dry nitrogen to avoid contamination of the samples. The sample was placed in a small closed quartz tube inside a 5 mm horizontal solenoid. For these 1H measurements, an RF field strength of 147 kHz and 90° pulse length of 2.10 µs were used. Several echo sequences were used and compared, including the Hahn-solidHahn echo,17,18 the magic echo,19,20 and the magic sandwich echo.21 The 1H spin-lattice relaxation time T1 was measured using a saturation recovery sequence combined with a Hahnsolid-Hahn echo. Static 23Na and 27Al NMR experiments at room temperature were performed on the purified NaAlH4 sample using a Chemagnetics Infinity 600 MHz spectrometer with a 2.5 mm MAS probe (without using magic angle spinning). The static 23 Na spectrum was recorded using an echo sequence consisting of two 90° pulses at an RF field strength of 150 kHz. The static 27 Al NMR measurement was performed using a short hard pulse of 0.20 µs at an RF field strength of 270 kHz. After these measurements at room temperature, 1H and 27Al variable temperature measurements were performed for Na3AlH6 (-130 °C to +40 °C in steps of 20 °C) using a Chemagnetics Infinity 400 MHz spectrometer and a 4 mm triple resonance MAS probe, because in our home-built probe we did not have the option of cooling. For 1H, a single 90° pulse was used at an RF field strength of 60 kHz. The free induction decay (FID) was backpredicted using a Gaussian function. For 27Al, a short single pulse of 0.2 µs at an RF field strength of 100 kHz was used. All processing of the NMR data was done using the matNMR processing package.22 For the proton spectra, the integrated peak intensities and linewidths of a single pulse and three different echo sequences, which are shown in Figure 1, were compared. First, the Hahnsolid-Hahn echo was applied using pulse delays of τHSH ) 10 and 15 µs. This echo has been used before by Schmidt-Rohr to detect the first points of the time signal of 13C-13C spin pairs without distortion. This pulse sequence combines the effects of the Hahn-echo, which refocuses resonance offsets and interactions which are linear in the spin operator, like heteronuclear dipole couplings, and the solid echo, which refocuses interac-

τHSH ) 10 µs τHSH ) 15 µs nMSE ) 3 nMSE ) 4 nMSE ) 5 τMSE ) 20 µs τMSE ) 30 µs

intensity

FWHM (kHz)

frac. L/G

0.38 ( 0.2 0.61 ( 0.2 0.24 ( 0.2 0.96 ( 0.2 1.00 ( 0.2 0.86 ( 0.2 0.86 ( 0.2 0.64 ( 0.2

68.1 ( 1.0 43.8 ( 1.0 42.3 ( 1.0 42.0 ( 1.0 41.0 ( 1.0 41.5 ( 1.0 38.1 ( 1.0 41.7 ( 1.0

0.21 0.00 0.00 0.00 0.00 0.00 0.00 0.00

a SPE is measured by a single 90° pulse, HSH by the Hahnsolid-Hahn echo, MS by the magic sandwich echo, and ME by a magic echo.

TABLE 2: Comparison of Linewidths, Integrated Peak Intensities, and Lineshapes Expressed as the Fraction Lorentzian/Gaussian (Frac. L/G) for Different Spin Echo Sequencesa on Na3AlH6 SPE SPE HSH HSH MS MS MS ME ME

BP τHSH ) 10 µs τHSH ) 15 µs nMSE ) 3 nMSE ) 4 nMSE ) 5 τMSE ) 20 µs τMSE ) 30 µs

intensity

FWHM (kHz)

frac. L/G

0.94 ( 0.2 1.00 ( 0.2 0.79 ( 0.2 0.65 ( 0.2 0.85 ( 0.2 0.79 ( 0.2 0.74 ( 0.2 0.83 ( 0.2 0.70 ( 0.2

16.5 ( 0.4 14.2 ( 0.4 14.4 ( 0.4 14.1 ( 0.4 13.8 ( 0.4 13.9 ( 0.4 13.8 ( 0.4 13.2 ( 0.4 13.1 ( 0.4

0.24 0.00 0.06 0.00 0.01 0.01 0.02 0.00 0.00

a SPE is measured by a single 90° pulse, SPE BP by backprediction, HSH by the Hahn-solid-Hahn echo, MS by the magic sandwich echo, and ME by a magic echo.

tions bilinear in spin operator, like homonuclear dipole couplings.17,18 Second, the magic echo was applied,19,20 with time reversal pulse lengths of τME ) 20 and 30 µs. The magic echo has previously been applied in the metal hydrides YH3, LuH3, and TiH1.98 as an adequate pulse sequence to refocus the homonuclear dipolar couplings.19,20 Finally, the magic sandwich echo was used with nMSE ) 3, 4, and 5 and τMSE ) 2.0 µs. This echo was proposed by Maus et al.21 to study the crystallinity and mobile-fraction dynamics in polymers. To find the optimal experiment, the integrated peak intensities and linewidths of 1H spectra of the pure NaAlH4 using several echo sequences were compared in Table 1. All echo pulse sequences resulted in spectra with approximately the same linewidths. However, significant differences in total signal intensities were observed. For NaAlH4, the largest signal intensity was observed for the magic sandwich echo with nMSE ) 4, so this sequence was used for further analysis. We tried backprediction of the single pulse FID after recording a FID minimizing the time delay between the pulse and data acquisition for our probe. This did not give proper results for NaAlH4, as too much of the time domain signal was missing of this fast decaying signal. The same comparison of different echoes was done for Na3AlH6, and the result is shown in Table 2. The largest integrated peak intensity for Na3AlH6 was observed for the single pulse experiment with backprediction using a Gaussian function exp(-(t - µ)2/a2). In the frequency domain, this fit corresponds to a Gaussian lineshape with FWHM ) 2(ln 2)1/2/ aπ. The linewidth including backprediction was FWHM ) 14.2 ( 0.4 kHz, and µ ) 9.6 µs was found in agreement with the

Wide-Line NMR Characterizations of Sodium Alanates

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Figure 2. 1H NMR spectrum of NaAlH4 using a magic sandwich echo (nMSE ) 4) and fitted with a Gaussian (FWHM ) 41.0 ( 1.0 kHz).

experimental deadtime. The spin echo sequences gave comparable linewidths compared to the single pulse experiment with backpredition. Summarizing, for NaAlH4, it is necessary to use an echo pulse sequence because of fast signal decay. For intensity purposes, the magic sandwich echo with nMSE ) 4 is the best choice, while for all echo sequences approximately the same linewidths were observed. For Na3AlH6, having a substantially narrower proton linewidth at room temperature, a single pulse experiment with backprediction of the time domain signal using a Gaussian function gave maximum signal intensity. Again, comparing the different sequences, comparable values for the linewidths were observed. Second Moment Calculations. Second moment calculations were performed for 1H, 23Na, and 27Al to calculate the size of the dipolar line broadenings in the case of a rigid crystal lattice using the Van Vleck equations.7 The atomic positions were taken from relaxed cell parameters and atomic positions by van Setten8,9 using density functional calculations in the generalized gradient approximation, starting from the XRD data.10 All of the nuclei within a distance of 10 Å were taken into account. Inclusion of nuclei at longer distances did not give a significant difference in the calculated values for the dipolar line broadenings. Handling the Al-Al and Na-Na terms as like or semilike spins leads to only a slight difference in the overall calculated FWHM.11 For NaAlH4 and Na3AlH6, not all 27Al and 23 Na electric field gradient tensors were expected to have the same orientation within a unit cell, and therefore, all Al-Al and Na-Na terms in NaAlH4 and Na3AlH6 were approximate by handling them as semilike spin terms. The experimental values of the line broadening were obtained by using a Gaussian fit of the 1H, 23Na, and 27Al spectra. For the VT measurements, we simulated the 1H and 27Al linewidths assuming thermal activated rotational jumps of the AlH6 clusters based on the model proposed by Goc et al.12-14 to shed light on the origin of the motional averaging of the dipolar couplings. The rotations were implemented as cyclic permutations of H atoms around the C3 and C4 axes of the AlH6 clusters. The calculations were performed using a 3 × 3 × 3 unit cell of Na3AlH6. The simulation was performed in Matlab.23 The number of jumps is transformed to a temperature scale using the Arrhenius equation and assuming that averaging of the dipolar couplings takes place during the time of the recording of the FID, i.e., T2 ≈ γIM2rig1/2. As Goc13 showed before, this leads to the following equation for the temperature T as function of the number of jumps nc per rotation axis:

T(nc) )

Ea R(ln V0 - ln nc - ln γI - 0.5 ln M2rig)

(3)

where Ea is the activation energy, R the molar gas constant, V0 the attempt frequency of the rotational jumps, γI the gyromagnetic ratio, and M2rig the rigid lattice value of the second moment.12,13 The activation energy Ea and the attempt frequency V0 were varied to find the correct fit. The calculated absolute linewidths were compared to the experimental data for different rotation axes because the structure of Na3AlH6 is noncubic, and therefore, the values for Ea and V0 may be not equal for different rotation axes. The following models for reorientations of the AlH6 clusters were compared: • C4Z, C4XY1, C4XY2: Rotation around only one of the three single C4 axes. With each iteration n, one of the 54 AlH6 clusters in the 3 × 3 × 3 unit cells of Na3AlH6 was randomly selected and rotated, giving a number of jumps per rotation axis nc ) n/54. • C3: Rotation around one single C3 axis. The other C3 axes give the same result because of symmetry. • C4All: Rotation around any C4 axes assuming they have the same energy barrier. With each iteration n, one of the 54 AlH6 clusters is randomly rotated around one of its three C4 axes, giving a number of jumps per rotation axis nc ) n/(3 × 54). • C3All: Rotation around any C3 axes, giving a number of jumps per rotation axis nc ) n/(4 × 54). • C4Zfast + C4XY1slow + C4XY2slow: A combined rotation assuming a fast rotation around the C4Z axis and a slow rotation around the C4XY1 and C4XY2 axes. This was implemented in the model by rotating for each iteration all clusters around the C4Z axis (fast rotation) in a random direction. For every 256th iteration, to simulate a large enough difference in rotation speed, one of the 54 clusters in the 3 × 3 × 3 unit cells was rotated around one of the two other C4 axes (slow rotation) in a random direction, giving a number of jumps per rotation axis for C4XY1 and C4XY2, nc ) n/(256 × 54 × 2). For higher temperatures, the dipolar couplings because of rotation around the C4Z axis are already fully averaged, so it was sufficient to use 16 iterations instead of 256 (for C4XY1 and C4XY2, nc ) n/(16 × 54 × 2)) saving calculation time. Results and Discussion The 1H spectrum of NaAlH4 recorded using a magic sandwich echo is shown in Figure 2. A lineshape close to Gaussian is observed. When a sufficiently large number of spins is present, proton lineshapes generally tend to a Gaussian lineshape, caused by the homonuclear dipolar couplings. The FWHM of the observed lineshape is 41.0 ( 1.0 kHz. Second moment calculations predict a line broadening of 41.5 kHz, which is very close to the experimental value. The same measurement on the commercially available nonpurified NaAlH4 sample showed a comparable lineshape but with an additional narrow peak in the center. This is most likely an impurity. A remarkably large difference in T1 values between the purified and ball-milled NaAlH4 (∼88 s) and commercially available nonpurified NaAlH4 (∼1100 s) was observed. The same effect of strongly enhanced relaxation rate after ball-milling was observed in MgH2-based hydrides by Skripov et al.24,25 For the nonpurified NaAlH4, the estimated particle sizes are several µm, whereas, for the ball-milled materials, typical particle sizes are around 50-100 nm. Therefore, the difference in relaxation times could be explained by a difference in particle size and/or crystallite grain size. This would imply that relaxation takes place at the particle surfaces with spin-diffusion to the surface being the rate-limiting step. The results of static 27Al and 23Na measure-

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Figure 3. 23Na NMR spectrum of NaAlH4 using a solid echo. The fit parameters used were CQ ) 154 ( 3 kHz, ηQ ) 0.10 ( 0.05, and FWHM ) 7.5 ( 0.2 kHz.

Figure 5. 1H NMR spectrum of Ti-doped NaAlH4 using a Hahn-solidHahn echo. Fractions of NaAlH4 (FWHM ) 42.5 ( 1.0 kHz, T1 ∼ 20 s, and FWHM ) 13.2 ( 0.4 kHz, T1 ∼ 8 s) are observed.

Figure 4. 27Al NMR spectrum of NaAlH4 using a short single pulse. The fit parameters used were: CQ ) 3.10 ( 0.05 MHz, ηQ ) 0.00 ( 0.05, and FWHM ) 15.7 ( 0.2 kHz.

Figure 6. 1H NMR spectrum of Na3AlH6 using a magic sandwich echo (nMSE ) 3) and fitted with a Gaussian (FWHM ) 13.8 ( 0.4 kHz).

TABLE 3: Quadrupolar Parameters of NaAlH4 23 27

Na Al

CQ

ηQ

FWHM

154 ( 3 kHz 3.10 ( 0.05 MHz

0.10 ( 0.05 0.00 ( 0.05

7.5 ( 0.2 kHz 15.7 ( 0.2 kHz

ments are summarized in Figures 3 and 4, respectively. They both show a typical quadrupolar powder pattern, and fitting gives the quadrupolar coupling constant CQ ) e2qQ/p and quadrupolar asymmetry parameter ηQ. These are shown in Table 3. The observed values for CQ are in agreement with previous studies by Tarasov et al.28,29 On the basis of the symmetry of the crystal structure and a simple point charge model using Na+Al3+H4to calculate the electric field gradients, values for the quadrupolar asymmetry parameter of ηQ = 0 are indeed expected.26,27 Both lineshapes are broadened by a Gaussian broadening with a FWHM of 15.7 ( 0.2 and 7.5 ( 0.2 kHz for 27Al and 23Na, respectively. The second moment calculations for 23Na and 27Al give values for the FWHM of 15.8 and 7.7 kHz, in agreement with the experimental values. These broadenings are mostly due to the dipolar interaction with the neighboring protons. The agreement of the calculated line broadenings and the measured linewidths shows that the NMR experiments are consistent with the known crystal structure from diffraction experiments and confirms a rigid crystal lattice for NaAlH4 because no motional averaging of the dipolar coupling is observed. A static 1H spectrum of the Ti-doped NaAlH4 sample measured using the Hahn-solid-Hahn echo with τHSH ) 15 µs is shown in Figure 5. Clearly, two fractions are present with different T1 values, which were measured using a saturation recovery sequence combined with the Hahn-solid-Hahn echo. Comparison with previous NMR results by Majer et al.4 show that the broad component with a FWHM of 42.5 ( 1.0 kHz (T1 ∼ 20 s) corresponds to NaAlH4 and the narrow component with FWHM 13.2 ( 0.4 kHz (T1 ∼ 8 s) originates from Na3AlH6. For NaAlH4, the T1 is shorter than that for the pure material,

which was also observed by Majer et al.4 This difference in T1 can be explained by a smaller particle size for Ti-doped NaAlH4 due to milling effects which occur when two materials of different brittleness are mixed. 23 Na and 27Al MAS NMR experiments of this sample confirmed the existence of Na3AlH6 and showed that NaH was not present in this sample. The presence of Na3AlH6 is explained by H2 release of the NaAlH4. The sample was stored at room temperature, and the experiment was performed approximately 2 months after the actual manufacturing of the sample. Sandrock et al.30 showed that, even under this mild temperature condition, hydrogen desorption already takes place. Further hydrogen desorption of Na3AlH6 to NaH was not observed, most likely because of the lower hydrogen desorption rate constant and higher activation energy30 and reaction enthalpy31 of the second step of the hydrogen desorption reaction. The fact that the NaAlH4 and Na3AlH6 1H lineshapes have the same linewidth both in the pure samples and in the Ti-doped sample leads to the conclusion that Ti doping in our sample does not influence proton mobility at the NMR time scale (∼ms/µs) at room temperature. This also means that titanium has no observable effect on the dipolar coupling strength, indicating that in our Ti-doped sample the internuclear distances and crystal structure are the same as those for the undoped materials. Usually, magic angle spinning is required to achieve resolved spectra. The clear difference in linewidths between NaAlH4 and Na3AlH6 makes it possible to distinguish between these two fractions without MAS. To further explore the strongly reduced 1H linewidth in Na3AlH6, a pure sample was investigated using various echo sequences. The static proton spectrum using a magic sandwich echo (nMSE ) 3) is shown in Figure 6. Fitting using a Gaussian lineshape results in a linewidth of 13.8 ( 0.4 kHz, which confirms the assignment of the two peaks in Ti-doped NaAlH4. Second moment calculations give an average linewidth of 38.9

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nucleus 1

H 23 Na 27 Al 1 H 23 Na1b 23 Na2 27 Al

exp. value (kHz)

rigid

fast rotating AlH6

41.0 ( 1.0 7.5 ( 0.2 15.7 ( 0.2 13.8 ( 0.4 4.7 ( 0.2 5.3 ( 0.2 2.8 ( 0.1

41.5 7.7 15.8 38.9 8.0 7.6 14.9

n/a n/a n/a 14.1 4.5 5.3 2.7

a For Na3AlH6, the rigid lattice value and the line broadening assuming isotropic rotating AlH6 clusters are given. b The peaks corresponding to the two different sodium sites (intensity 1:2) in Na3AlH6 were overlapping. Therefore, the peak positions from the MAS data were used for this fit.

Figure 7. Experimental and simulated NMR linewidths of 1H and 27Al for different rotation axes (Ea ) 24 kJ/mol, V0 ) 4 × 1010 Hz).

Figure 8. Experimental and simulated 1H and 27Al linewidths for Na3AlH6. A calculation (solid lines) is shown representing a fast rotation around one C4 axis and rotations around the other two C4 axes with an activation barrier of Ea ) 25.5 kJ/mol (protons) and Ea ) 24.5 kJ/mol (aluminum) and an attempt frequency of V0 ) 4 × 1010 Hz.

kHz for the three different proton sites in Na3AlH6. Compared to the experimental value, the calculated linewidth is significantly broader, indicating some form of mobility in the crystal that leads to narrowing of the dipolar broadened Na3AlH6 lineshape. To quantify this line narrowing effect, we have calculated the second moments for 1H assuming AlH6 fast rotating clusters with an isotropic distribution of orientations and reorientations giving equal probabilities for all angles between the internuclear vectors with respect to the crystal axes.32 In this model, the intragroup dipolar interactions are averaged to zero. The intergroup dipolar interactions are partly averaged giving a residual linewidth of 14.1 kHz, which is close to the experimental value of 13.8 ( 0.4 kHz. For aluminum and sodium, the same comparison was done. The experimental values of 2.8 ( 0.1 kHz and 4.7 ( 0.2 kHz/5.3 ( 0.2 kHz for 27Al and 23Na, respectively, were in agreement with calculated values of 2.7 kHz and 4.5 kHz/5.3 kHz, indeed indicating isotropic rotating AlH6 clusters. After this analysis of Na3AlH6 spectra at room temperature, as a next step, we investigated the line broadening of 1H and 27 Al as a function of temperature. These variable temperature measurements were performed in a 4 mm MAS probe (static measurements) at 400 MHz using a single pulse experiment

with backprediction, giving a broadening at low temperatures for both 1H and 27Al in Na3AlH6, as shown in Figure 7. Previous VT 1H measurements on Na3AlH6 were done by Senegas et al., which showed a similar low temperature effect.33 Using second moment calculations and a model describing the effect of motion of atoms on the dipolar couplings, we calculated the linewidths as a function of the number of individual jumps which can be converted to temperature using eq 3. A qualitative comparison of the temperature dependence of the linewidths for different rotation axes using eq 3 (V0 ) 4 × 1010 Hz and Ea ) 24 kJ/mol) is shown in Figure 7. Using the same model for rotation, the aluminum and hydrogen curves can show a different behavior, because of their different position within the crystal lattice and symmetry. Our calculations exclude that the motional averaging of the dipolar couplings as a function of temperature is a single-step process consisting of a rotation around a single C3 or C4 axis. For higher temperatures, the curve is very well described assuming an isotropic rotation around all C4 or C3 axes. For low temperatures around 150 K, a plateau is observed giving linewidths that do not correspond to the rigid lattice linewidths of 38.9 and 14.9 kHz as calculated for 1H and 27Al, respectively. This might correspond to a transition from a rigid crystal lattice to a rotation around the single C4Z axis. To obtain a full model of hydrogen motion in Na3AlH6, including values for Ea and V0 for the rotation around the C4Z axis, measurements at lower T are needed. On the basis of these observations, a novel set of calculations was done assuming a fast rotation around the C4Z axis and a slower rotation around the C4XY1 and C4XY2 axes. This combined rotation resulted in the curve in Figure 8. For the rotation of the C4XY1 and C4XY2 axes, fitting using eq 3 gave a barrier of Ea ) 25.5 kJ/mol (protons) and Ea ) 24.5 kJ/mol (aluminum), with an average value of 25 kJ/mol. The attempt frequency associated with this motion equals V0 ) 4 × 1010 Hz. It should be noted that these values are obtained from fitting the linewidth change as a function of temperature. The “smooth” S-curve observed experimentally is best represented by the values given here. A higher attempt frequency can be partially compensated by a higher activation barrier Ea, but the end result is a steeper transition. Acceptable values for the attempt frequency and energy barrier were estimated between V0 ) 4 × 109 Hz, Ea ) 20 kJ/mol, and V0 ) 4 × 1011 Hz, Ea ) 30 kJ/mol. Above T > 300 K, the rotational motion of the AlH6 clusters is well described assuming fully isotropic tumbling, because all dipolar couplings within the clusters are averaged above this temperature. Since combined rotations around all C3 axes show approximately the same isotropic averaging effect as that for

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the C4 axes, we cannot exclude or distinguish between rotations around C3 axes at high temperatures.

Verkuijlen et al. Wijs (Radboud University, Nijmegen), and Bernard Dam (VU University, Amsterdam) for helpful discussions. References and Notes

Conclusions Static solid-state NMR spectra are usually very broad. To solve the receiver deadtime problem, the Hahn-solid-Hahn echo, the magic echo, the magic sandwich echo, and a single pulse experiment with backprediction were used and compared for NaAlH4 and Na3AlH6 at room temperature in combination with a specially designed NMR probe to measure spectra without proton background signals. The magic sandwich echo gave the maximum signal intensity of the echoes used for NaAlH4. For Na3AlH6, the single pulse experiment with Gaussian backprediction was the best choice. Using these pulse sequences to compare the line broadening of the static 23Na, 27Al, and 1H spectra with second moment calculations indicates a rigid crystal lattice for NaAlH4. However, for Na3AlH6, the calculated values for the second moment are significantly smaller than the experimental values. This indicates hydrogen mobility in the crystal, which averages the dipolar couplings. The main mobility is most likely caused by rotating AlH6 groups. In a Ti-doped sample, approximately the same linewidths were observed, an indication that Ti doping has no large scale influence on the mobility of hydrogen on the NMR time scale at room temperature. Different T1 values for commercially available, purified, and Ti-doped NaAlH4 were observed, which is explained by a difference in crystallite grain size, indicating that spin-diffusion to the surface is the ratelimiting step. The hydrogen mobilities in Na3AlH6 are temperature dependent, because VT studies of both 1H and 27Al for Na3AlH6 showed a low temperature broadening. This is fitted with a model assuming thermally activated rotational jumps of AlH6 clusters. The origin of the motion is probably a transition from rotation around the C4Z axis to rotation around all C4 axes. Fitting using eq 3 gave an activation energy of Ea ) 25 kJ/mol and an attempt frequency of V0 ) 4 × 1010 Hz for the rotation around the C4XY1 and C4XY2 axes. Measurements at lower temperature are necessary to fully understand the hydrogen dynamics in Na3AlH6 and quantify the activation barrier and attempt frequency for these low temperature hydrogen mobilities. All NMR experiments described in this paper were done statically, so without magic angle spinning, which is a common technique in solid-state NMR to average anisotropic interactions. Despite the lower resolution of static wide-line spectra compared to MAS spectra, it is possible to obtain useful information. This is important in the development of NMR methods to study thin film metal hydride materials because, due to practical limitations, it is impossible to apply magic angle spinning in a thin film setup.34-36 Acknowledgment. This work was financially supported by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO-ECHO). The authors thank Hans Janssen, Gerrit Janssen, Adri Klaassen, and Jan van Os for their technical support and Michiel van Setten (Forschungszentrum Karlsruhe), Gilles de

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