NMR Study of Anion Dynamics in Solid KAlH4 - American Chemical

Feb 24, 2014 - NMR time scale at room temperature, the KAlH4 anions are already rotating .... 39K static NMR was performed at room temperature at 7.04...
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NMR Study of Anion Dynamics in Solid KAlH4 Eric G. Sorte,*,† Samuel B. Emery,† E. H. Majzoub,‡ Tim Ellis-Caleo,† Zayd L. Ma,§ Blake A. Hammann,§ Sophia E. Hayes,§ Robert C. Bowman, Jr.,∥ and Mark S. Conradi†,§ †

Department of Physics, Washington University, One Brookings Drive, St. Louis, Missouri 63130, United States Center for Nanoscience and Department of Physics and Astronomy, University of MissouriSt. Louis, One University Boulevard, St. Louis, Missouri 63121, United States § Department of Chemistry, Washington University, One Brookings Drive, St. Louis, Missouri 63130, United States ∥ RCB Hydrides, LLC, 117 Miami Avenue, Franklin, Ohio 45005, United States ‡

ABSTRACT: 1H and 27Al NMR is used to reveal the motions of AlH4 anions in KAlH4. Line-narrowing from rotations and from translational diffusion is observed in the NMR of both nuclei. Unlike the anions in NaAlH4 and LiAlH4 that are not rotating on the NMR time scale at room temperature, the KAlH4 anions are already rotating rapidly at 23 °C. Based on the onset of rotation-induced line narrowing, the 1H T1 minimum, and the low- temperature hydrogen T1ρ minimum associated with reorientations, the rotational activation energy Erot,act = 0.28 eV is determined. Similarly, we use the onset temperature of translational motion-induced line narrowing and the high-temperature T1ρ minimum to determine the diffusion activation energy Ediff,act = 0.70 eV. Lack of sharp structure in the first-order quadrupole pattern and the absence of second-order quadrupole structure in the 27Al NMR data suggest asymmetry (η ≠ 0) and/or variations in the anion electric field gradients from structural disorder.

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INTRODUCTION The aluminohydrides of the group 1 elements Li, Na, and K are attractive candidates for hydrogen storage. Much effort has gone into characterizing and understanding the hydrogen storage properties of LiAlH4 and NaAlH4 due to their favorable kinetics and hydrogen capacities. LiAlH4 begins to dehydride at 115 °C with a theoretical 10.5 wt % hydrogen,1−3 while NaAlH4 reversibly stores around 5.5 wt % hydrogen.4 Both compounds have been characterized by nuclear magnetic resonance (NMR),5−13 X-ray diffraction (XRD),14 neutron diffraction,15 density functional theory calculations (DFT),16 and Raman spectroscopy17,18 in pure, doped, and nanoconfined forms.18,19 NaAlH4 has found application in commercial devices intended for storing hydrogen gas in nonmobile applications.20,21 KAlH4 is much less well understood, though it has 5.7 wt % total hydrogen capacity and is related to the candidate hydrogen storage material NaAlH4. Several investigations have suggested that KAlH4 decomposes in complicated ways through various proposed intermediates, including KAlH2, AlH3, KH, or other unidentified phases.22−27 However, DFT calculations suggest decomposition occurs through simple pathways similar to the other alanates, passing through K3AlH6;28,29 this does not appear to be a settled issue. Initial confusion about the crystal structure of KAlH4 seems to have reached consensus in recent years as an orthorhombic system in space group Pnma with 4 AlH4 anions per unit cell.30,31 It has recently been shown that KAlH4 rapidly and reversibly releases hydrogen as low as 250 °C without the presence of a catalyst.32 This behavior is in contrast to NaAlH4 where © 2014 American Chemical Society

transition metal catalysts are necessary for reversible desorption/absorption of hydrogen under practical conditions, which negatively impacts the system weight percent hydrogen capacity. Some neutron diffraction30 and X-ray diffraction studies25 have been carried out, but to date, characterization work of KAlH4 by NMR has been minimal. Only one group has previously attempted an NMR study of KAlH4; they focused on measuring quadrupole coupling constants for the 27Al and 39K spins24 and on identifying the solid decomposition products23 upon heating. In this work, we report the results of a thorough investigation of AlH4 anion dynamics in KAlH4 using NMR. We show new NMR line shape measurements of KAlH4 as a function of temperature on both the 27Al and 1H resonances, revealing the line narrowing effects from rotational and translational motions of the AlH4 ions. We also show the temperature dependence of the hydrogen relaxation parameters T1 and T1ρ, which are consistent with the onset temperature of narrowing from AlH4 motions in KAlH4. From the data we determine the activation energies of rotation and diffusion. 27Al MAS NMR data show the presence of quadrupole satellites without defined singularities which diminish in intensity at higher temperatures. 39 K NMR of the cation is also reported, though the poor nuclear magnetic properties of 39K make impractical the wide Received: January 7, 2014 Revised: February 21, 2014 Published: February 24, 2014 5725

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Figure 1. Characterization of KAlH4 samples. (a) X-ray diffraction pattern of KAlH4 at room temperature. The red stick pattern shows the expected reflection positions from Pnma KAlH4. The trace was obtained from a sample annealed at 400 °C under 200 bar of H2 pressure to recrystallize the KAlH4. (b) {1H} decoupled 27Al MAS NMR at 300 MHz spinning at 9 kHz. The spectrum shows the expected KAlH4 peak at 106 ppm, along with a small aluminum metal peak Knight-shifted to 128 kHz. The main figure is a blow up for visual clarity, while the inset shows the full spectrum to illustrate the relative intensities of the KAlH4 27Al central transition, and the aluminum metal peak.

spectrometer with a Bruker HX MAS probe. 27Al background in the rotor was measured and found to be negligible. All samples were transported under nitrogen to avoid contamination by atmospheric water and oxidation. The samples were stored in a flowing N2 glovebag. For static NMR, the samples were loaded into 5 mm outer diameter glass NMR tubes and flame-sealed under an argon atmosphere. Most of the reported measurements use static variable temperature NMR (not spinning at the magic angle). 1H NMR was performed in a field of 2.0 T (1H Larmor frequency 85.03 MHz). The hydrogen signals were obtained by repeated acquisitions of free induction decays (FIDs) following 1 μs (15°) excitation pulses spaced at least T1 apart with a receiver dead time of 3 μs. For measurements above 300 K, the temperature was controlled by a stream of flowing air and regulated by a heater and resistance thermometer. For measurements below 300 K, the flowing air was replaced by a stream of cold, flowing nitrogen gas generated from the boil-off of a liquid nitrogen Dewar. The temperature was measured separately with a type-T thermocouple near the NMR sample. T1 measurements were performed by the saturation/recovery method with saturation achieved by a series of 20 π/2 pulses spaced by 500 μs (which is longer than T2). T1 values were obtained by single-exponential fits to the areas under the Fourier-transformed free induction decays as the interpulse delay was varied. T1ρ measurements were obtained using a 90x-τy-Acq pulse sequence where τy is a phase-y continuous spin-locking pulse of duration τ, at the end of which a FID signal is acquired. Some of the T1ρ decays had long components superimposed with faster initial decays, complicating the extraction of T1ρ values by simple exponential fits. Therefore, the T1ρ values were obtained by measuring the signal amplitude for each value of τ and then using a linear extrapolation (as a function of τ) of the amplitudes for short τ values to zero signal. The values of T1ρ thus obtained were similar to the values obtained by simple

temperature range of measurements achieved with aluminum and hydrogen.

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EXPERIMENTAL SECTION Potassium hydride was purchased from Sigma-Aldrich as a suspension in mineral oil, washed with pentane, and dried in an argon glovebox. KAlH4 was synthesized by the reaction22 KH + Al +

3 H 2 → KAlH4 2

heating the KH powder with a stoichiometric amount of Al powder to 275 °C under 200 bar of H2 for 72 h in a glass-lined stainless steel vessel. Powder X-ray diffraction (XRD) patterns were obtained using a Rigaku Geigerflex D-MAX/A diffractometer with Cu Kα radiation at 35 kV and 35 mA. The instrument was calibrated using the silicon (111) reflection at 28.443° (2θ). The XRD pattern was obtained at room temperature from 20° to 50° (2θ). Analysis of diffraction data, including background removal, was completed using Jade Plus software. The PDF-4 and American Mineralogist powder diffraction databases were used for the matching of the reference patterns. Samples were loaded onto XRD slides, covered with a thin film that is nearly transparent to X-rays, and transported under a nitrogen atmosphere. MAS NMR spectra of both nuclei were acquired at room temperature at 7.04 T (1H frequency 299.87 MHz, 27Al frequency 78.085 MHz) with a home-built spectrometer using small angle (1 μs) rf excitation pulses for improved spectral coverage of the broad lines. The samples were loaded under a nitrogen atmosphere and sealed into 5 mm zirconia rotors for use in a Chemagnetics HX MAS solids probe at a MAS rotational frequency of up to 9 kHz. Spectra generally consisted of 100 averages with a repetition time of 1 s. Some 1H NMR MAS spectra were acquired at 589.88 MHz in a 2.5 mm rotor spinning at a maximum of 35 kHz using a Redstone (Tecmag) 5726

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exponential fits for the data that decayed to zero but gave more reliable results for those values (at high temperature) where the magnetization had not decayed fully at the longest values of τ. For 27Al measurements the static variable-temperature NMR was performed in a 7.04 T superconducting magnet (27Al Larmor frequency 78.085 MHz). The aluminum line shapes were obtained by Fourier transformation of the FIDs following 3.5 μs “effective 90°” pulses,33,34 spaced at least 5 × T1 in time. Here the pulses nutate primarily the 27Al central transition, resulting in 90° pulses in the “fictitious spin-1/2” formalism. The temperature was maintained by an oven heated with resistive thermocouple wire, wound bifilar to avoid magnetic fields generated by the heater current. The 27Al NMR spectra were referenced to 0.1 M Al(NO3)3 in water. 39 K static NMR was performed at room temperature at 7.04 T (39K frequency 13.983 MHz) and referenced to 0.1 M KBr aqueous solution. Hahn echo pulse excitation (90°-τ-180°ACQ) with an “effective” 90° pulse width of 6 μs (excitation primarily of the central transition) was used with a repetition time of 1s for 64 800 averages with various values of τ. All signals were acquired on a home-built spectrometer and processed as described elsewhere.35

Figure 2. Full width at half-maximum of the 1H (solid, blue) and 27Al (hollow, black) static NMR resonances of KAlH4 as a function of temperature. At the lowest temperatures, the widths of both nuclear resonances approach their predicted values for motionless rigid-lattice dipole broadening; these values are shown by short blue (1H) and short black (27Al) dotted lines. At the first temperature plateau the AlH4 anion is rapidly rotating, and the line widths of both nuclei approach those of the calculated dipole broadening34,37,38 for isotropic rotations of the anion; these values are shown by long blue (1H) and long black (27Al) dash-dotted lines. 27Al spectra represented here are those observed on second and subsequent heatings.

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RESULTS AND DISCUSSION Successful synthesis of the Alanate was confirmed with XRD and 27Al MAS NMR, which showed that the majority of the sample was KAlH4 in the Pnma (orthorhombic) space group (see Figure 1a). The initial XRD traces on the KAlH4 immediately after synthesis had poor signal-to-noise and showed several peaks that do not correspond to Pnma KAlH4. These peaks could be due to unknown impurities or to other crystal phases of KAlH4. We subjected some KAlH4 to an annealing process at 400 °C under 200 bar of H2 in order to improve its crystallinity. XRD traces from the resulting material are shown in Figure 1a, where the Pnma KAlH4 is clearly dominant. Figure 1b shows the 27Al magic-angle spinning (MAS) NMR on an unannealed sample. The expected KAlH4 peak at 106 ppm is evident, along with some aluminum metal, Knight-shifted to 1640 ppm = 128 kHz. No other aluminumbearing impurities are evident. The inset of Figure 1b shows the relative intensities of the aluminum metal and KAlH4 27Al intensities. The KAlH4 peak near 0 kHz under represents the number of aluminum atoms in this species by a factor of 9/35 (because it is central transition only) compared to the Al metal peak (all transitions present). Hence, the ratio of Al atoms in Al metal to KAlH4 is approximately 3%. We also performed 1H MAS NMR and Raman spectroscopy (data not shown) which confirmed the product was principally KAlH4.36 NMR Overview. Figure 2 shows the full width at halfmaximum (fwhm) static NMR line widths as a function of temperature for both the 1H and the 27Al nuclei. At the lowest temperatures, the line widths of both nuclei approach the values predicted by rigid-lattice second moment calculations for dipole broadening, shown as short dashed lines for both nuclei (blue for 1H and black for 27Al). Beginning at 170 K, the resonances narrow to a plateau that is maintained from 190 K to around 430 K. This partial narrowing indicates the effect of rotational motion of the AlH4 groups as the intramolecular dipole interactions (Al−H and H−H within each AlH4 unit) are motionally averaged. The long dashed-dotted lines in Figure 2 (blue for 1H and black for 27Al) show the expected line widths for rotating AlH4 units, calculated as described below. Reorientations of the AlH4 anions average the intramolecular

H−H and Al−H dipole couplings to zero, leaving the smaller intermolecular couplings incompletely averaged. Translational diffusion then averages away these remaining interactions at higher temperatures. The resonances begin to narrow further starting around 430 K, indicating that translational diffusive motion of the AlH4 anions becomes sufficiently rapid at that temperature. In detail, the motions (rotation or translation) will narrow the NMR signal when the motion rate exceeds the line width in the absence of motion, approximately 105 s−1. Below, we discuss the features of the resonance of each nucleus in more detail. Proton NMR. Static, variable-temperature 1H NMR was used to investigate the proton behavior in KAlH4 as a function of temperature. Representative spectra from the line width data in Figure 2 are shown in Figure 3. As mentioned above, the proton resonance narrows in two steps, representing the onsets of line narrowing from rotational motion (at low temperature) and translational motion (at high temperature) of the AlH4 anion. The spectra show a uniform narrowing of the entire

Figure 3. Static variable-temperature 1H NMR of KAlH4 at 85 MHz. All curves have been normalized to the room-temperature peak amplitude. 5727

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this implies 1/τ at the T1 minimum is 0.53 × 109 s−1. The T1ρ minimum is a low-field relaxation measure, occurring when the molecular rotations are comparable to the nutation frequency at B1 (B1 is the strength of the rotating-frame rf field). For our experiments, ω1/2π = γB1/2π = 32 kHz. According to Torrey’s isotropic random-walk model for self-diffusion on a crystal lattice,41−43 ω1τ = 0.869 at the T1ρ minimum, leading to a mean rotation rate 1/τ = 2.5 × 105 s−1 = 105.4 s−1. The temperatures for the hydrogen T1 minimum (50 °C) and the low temperature T1ρ minimum (−90 °C) are plotted in Figure 5 for the rotational motion, along with the temperature

proton resonance at both narrowing regions, indicating homogeneous dynamics of all the alanate ions in the sample. This is in contrast to many hydride systems such as ball-milled MgH239 or nanoconfined LiBH440 where a narrow resonance is observed superimposed on a broad solid resonance as the sample temperature is raised, indicating regional differences in hydrogen mobility. At low temperature, second moment calculations that assume a static alanate ion predict a hydrogen fwhm line width of 40.4 kHz (short blue dotted line in Figure 2). For these calculations, we used the atomic positions measured by neutron diffraction to perform the lattice sums.30 The computed second moment was used to calculate the fwhm line width by assuming a Gaussian shape, generally a very good approximation for solids. NMR line narrowing occurs when the motion of the resonant nuclei approaches the 105 s−1 rigid lattice FID decay rate. At 190 K, the rotational motions begin to approach this rate (105 rotations/s), evident as the first narrowing of the hydrogen resonance to approximately 12 kHz fwhm. Second moment calculations that assume uncorrelated, isotropic alanate ion rotations predict a line width of 11.75 kHz (blue dash-dotted line in Figure 2). These calculations place all of the AlH4 nuclear spins at the center (Al) site and calculate only dipole interactions between different AlH4 units.35 This line width is maintained until 430 K, at which temperature the translational motion of the AlH4 unit begins to approach 105 jumps/s, which further narrows the resonance line width. The resonance width then becomes constant at about 500 Hz at temperatures at and above 540 K. To further investigate the rotational and translational diffusion of the AlH4 anions (and determine activation energies), we performed hydrogen spin relaxation measurements that are sensitive to motions at different rates. Figure 4 shows the spin−lattice relaxation time T1 and the rotatingframe spin−lattice relaxation time T1ρ plotted with the inverse line width of the 1H NMR resonance.

Figure 5. Rotational and translational activation energies Erot,act and Ediff,act determined from the slopes of 1H Arrhenius plots. The points show the locations of the onsets of narrowing, T1ρ minima, and the T1 minimum (for rotations onlysee Figure 4). Both Arrhenius plots assume an attempt frequency ω0 of 1013 s−1 (at infinite temperature).

for the onset of line narrowing by rotations (−100 °C) at 1/τ = 105 s−1. In Figure 5, the hollow shapes correspond to rates associated with rotational motion while the solid shapes correspond to rates associated with translational motion. Assuming an attempt frequency ωA of 1013 s−1 for both motions and using the measured temperatures of the relaxation parameters above, we can use the Arrhenius relation ⎛ −E ⎞ 1 = ωA exp⎜ act ⎟ τ ⎝ kBT ⎠

(1)

to extract the rotational activation energy Erot,act = 0.28 eV. Changing the attempt frequency over the reasonable range between 1012 and 1014 s−1 would cause changes in the slope corresponding to a ±12% change in the rotational activation energy. Using the onset of translational diffusion at 160 °C, the high-temperature T1ρ minimum at 200 °C, and an attempt frequency of 1013 s−1, we can also use eq 1 to calculate a diffusion activation energy of Ediff,act = 0.70 eV. Changing the attempt frequency over the range between 1012 and 1014 s−1 would cause a ±14% change in the determined activation energy. The quoted error obtained by changing ωA over 2 decades, though small, is likely overstated in the rotational case. The wide range of frequency and temperature data measured here in the case rotational motion make the determination of the activation energy more precise.44 27 Al NMR. Static, variable-temperature 27Al NMR was used to investigate the Al behavior in KAlH4 as a function of temperature. In Figure 2, the 27Al resonance line widths are shown as a function of temperature. Representative spectra from the line width data in Figure 2 are shown in Figure 6,

Figure 4. Relaxation parameters T1 and T1ρ of the 1H NMR resonance in KAlH4 as a function of temperature along with the reciprocal line width, 1/fwhm.

The 1H T1 minimum occurs when τω0 is approximately 1.0, i.e. when the rate 1/τ of rotational molecular motions is comparable to the angular resonance frequency ω0.41 The exact value of τω0 at the T1 minimum depends weakly on the model of molecular reorientation and on whether like-spin or unlikespin interactions dominate. For protons at ω0/2π = 85 MHz, 5728

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8.3 kHz (107 ppm) all the way to 350 °C (right inset in Figure 6). As discussed above, the peak just below 8 kHz seen in the left inset in Figure 6 is large on first heating. Its intensity is too large to be explained by impurities. We know from 1H MAS NMR and Raman spectroscopy that there is only a trace amount of residual mineral oil from the KH shipping suspension and some oxides on the order of a few percent of the sample. The only other non-KAlH4 constituents in our samples revealed in Raman spectroscopy, NMR, and XRD are small amounts of Al metal and K3AlH6. We speculate that the resonance bifurcation behavior observed here between 225 and 300 °C in previously unheated KAlH4 samples is due to a second crystallographic phase of KAlH4, possibly stabilized by impurities. We note that unknown phases that appear between 225 and 330 °C (labeled there as compounds 2 and 3) were recently observed by one group using XRD;25 other groups have found similar unknown phases using in situ XRD, TGA, and DSC.26,27 Arnbjerg et al. found several unknown KAlH4 phases around 250−300 °C with crystal structures different than the Pnma.25 We note that first temperature cycles have been shown to exhibit significantly different effects than subsequent temperature cycles on the NMR resonances of other hydrides (e.g., NaH and NaAlH4).7,37 As in those studies, we focus on the cycled material’s consistent behavior as the most relevant from a practical standpoint, and these are the data represented in Figure 2. Quadrupole Effects. For a static sample (not MAS), the first-order quadrupole frequency shift of the m to m + 1 spin transition is45

Figure 6. Static variable-temperature 27Al NMR of KAlH4 at 78.085 MHz. All curves have been normalized to the room-temperature peak amplitude and referenced to 0.1 M Al(NO3)3. The insets show the highest temperature resonances on an expanded frequency scale to illustrate their structure. The left inset shows the line shapes on a first heating cycle, while the right inset shows the line shapes for subsequent heating cycles.

where the main panel shows the 27Al line shapes of the KAlH4 at several temperatures. In the insets of Figure 6, an expanded frequency scale shows the highest temperature spectra. Hightemperature spectra from a first heating cycle are shown in the left inset, while high temperature spectra from subsequent heating cycles are shown in the right inset. Not shown at 1640 ppm is the Knight-shifted Al metal resonance from a slight excess of Al powder in the KAlH4 synthesis. The low temperature 27Al resonance width at −140 °C is 13.2 kHz fwhm. Rigid lattice second moment dipole−dipole calculations predict a value of 15.5 kHz fwhm; the data are in reasonable agreement with this rigid lattice limit. As the temperature is raised, rotational motion begins to narrow the resonance line just above −100 °C (compare to the −75 °C spectrum in Figure 6), as expected from the behavior of the hydrogen resonance discussed above. With reorientations of the tetrahedral AlH4 units, the 27Al−1H intramolecular interactions will be averaged to zero, reducing both the 1H and 27Al line widths. Until the onset of translational motional narrowing near 200 °C, the 27Al resonance displays a plateau in its line widths due to intermolecular dipole coupling, similar to the hydrogen line width. The 27Al resonance line shapes narrow homogeneously as a function of temperature with the exception of a small shoulder at around −2.98 kHz (−38 ppm), attributable to K3AlH6.23 During a first heating in our samples (left inset, Figure 6), a bifurcation in the resonances with peak separation of 6 ppm appears around 225 °C and then disappears as the temperature continues to rise. When the sample was cooled after heating to a maximum of 375 °C, its resonance line shape returned to its initial room-temperature shape, indicating that the sample has not decomposed or converted to other products. Subsequent heat cycles on the same sample of KAlH4 do not show the 27Al resonance splitting at high temperature. Rather, on second and subsequent heatings the line shape remains a single peak near

⎧ 3 cos2 β − 1 ⎫⎛ η 1⎞ vm , m + 1 − vL = vQ ⎨ + sin 2 β cos 2α ⎬⎜m + ⎟ ⎝ 2 2 2⎠ ⎩ ⎭

(2)

where m = −1/2 corresponds to the central transition and the quadrupole frequency is vQ =

3 CQ 2I(2I − 1)

(3)

where CQ is the quadrupole coupling constant. Here, I = 5/2 and νL = 78.085 MHz is the Larmor frequency. The Euler angles α and β are described in ref 45, and η is the asymmetry parameter describing the deviation of the electric field gradient tensor from axial symmetry. The previous NMR study24,31 reported three quadrupole coupling constants and three asymmetry parameters associated with the AlH4 anions: CQ = 0.58 MHz and η = 0.1, CQ = 2.07 MHz and η = 0.0, and CQ = 2.8 MHz and η = 0.1 for “monodentate, bidentate, and tridentate states”, respectively. The axial symmetry of the η = 0 case would lead to the sharpest and therefore most readily observable cusps. If there are other anionic states with lower symmetry (η > 0), their effects on the 27Al powder satellite pattern would be less visible as their features would not be as sharp and prominent.45 We note that the fast reorientations of the AlH4 anions at room temperature and the energetic equivalence of all four AlH4 units in the Pnma unit cell are not in accord with the reported multiple environments of the AlH4 in KAlH4. The observation of a single line-narrowing event associated with anion reorientations (see Figure 2) underscores the equivalence of all the AlH4 anions. It is possible that different crystal structures could be simultaneously present, which would lead to different quadrupole coupling constants coexisting in the KAlH4. Indeed, DFT calculations suggest the 5729

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hydrogen absorption/desorption.48,49 Recent work has found that the presence of hydroxides (e.g., NaOH) in NaAlH4 leads to the creation of a mobile, Al bearing species named S105 which appears as a narrow resonance near 105 ppm in the 27Al NMR.6,7 The mobile species is created during heat treatment and remains upon return to room temperature if hydrogen gas (at pressures above the reaction equilibrium pressure) is applied to the sample. Suggestions have been made that such a metal-bearing mobile species should be present in all alanates to explain the reversibility of the hydrogen reactions, transporting the alkali and Al metal atoms back into stoichiometric contact upon rehydriding (during dehydriding, these segregate as alkali hydride and Al metal).6,7,50−52 We attempted to create the analogous mobile species in KAlH4 by following a similar prescription as was used in NaAlH4; i.e., heat a mixture of the alanate with the corresponding hydroxide under hydrogen gas pressures above the reaction equilibrium pressure. For NaAlH4 with NaOH, the NaAlH4 melts at 183 °C with an equilibrium pressure of 140 bar of H2. Melting the alanate is thought to promote thorough mixing of the alanate and hydroxide. KAlH4 with 40 mol % KOH was mixed by mortar and pestle and heated to 420 °C (above the 406 °C melting temperature of KOH) for 20 min under 100 bar of H2 pressure, more than enough pressure to prevent the dehydriding/decomposition of the KAlH4 (equilibrium pressure of 10 bar at 355 °C).27 While we observed significant changes in the mixture after the heat and pressure treatment, no narrow Al-bearing species reminiscent of the S105 found in NaAlH4 was observed in the KAlH4. 39 K NMR. Potassium NMR on the KAlH4 cation is somewhat less informative than the 27Al and 1H NMR due to the poor magnetic properties of the 39K nucleus (spin 3/2, γK = 0.046γH). However, the magnetically active isotope has high abundance, and NMR is therefore feasible. We used a spin− echo pulse sequence 90°-τ-180° with τ = 200 μs to refocus 64 800 echoes with a repetition time of 1 s (18 h). The pulses used here were again “effective” 90° and 180° pulses. The Fourier transform of the apodized echo, left-shifted to the echo peak, shown in Figure 7 was acquired at 7.04 T (13.98 MHz) and room temperature.

existence of at least one other crystallographically distinct structure (tetragonal) that is energetically very near the accepted orthorhombic KAlH4 phase;46 other calculations have found other structures in the orthorhombic space group for KAlH4 as well.47 Closely competing structures could result in disorder and result in broadening of the quadrupole spectral features. Room temperature 27Al MAS NMR was performed at 7.04 T 27 ( Al frequency 78.085 MHz) at variable speeds up to 9 kHz to investigate the quadrupole behavior. The static 2.5 kHz room temperature line width (see Figures 2 and 6) narrows at 2 kHz spinning speed to 300 Hz fwhm and is thereafter independent of spinning speed up to 9 kHz. The 27Al chemical shift of the KAlH4 was found to be 106 ppm in both our static and MAS experiments, consistent with past studies.24 In the MAS experiments, the first-order quadrupole powder pattern is evident as a forest of spinning sidebands (see Figure 1b), but there are no detectable sharp singularities in the first-order quadrupole spectrum’s envelope. We performed quadrupole echoes (θx-τ−θy-Acq) without sample spinning as a function of temperature to investigate the temperature behavior of the quadrupole satellites of the 27Al resonance. For these experiments, we found the optimum echo response using 30° pulses (as calibrated from an Al3+ solution) and an interpulse delay time τ = 50 μs. The Fourier transform of the resulting quadrupole echo showed only the broad powder pattern features; no singularities were apparent. This result suggests again that the quadrupolar singularities were broadened by some disorder, resulting in a distribution of the parameters CQ and η. We observed the amplitude of the narrow quadrupole echo (narrow in the time domain) decrease as a function of temperature from room temperature to 250 °C. At 250 °C, the quadrupole echo was unobservable. KAlH4 has Z = 4 (4 anions per unit cell), and these AlH4 have two different orientations, so translational motion should begin to average the EFGs once the hopping rate becomes sufficiently fast. This would lead to a loss of the quadrupole echo signals around the temperature of line narrowing from translational diffusion (about 160 °C). The loss of the quadrupole echo signal is due to time dependence of the quadrupole shift of a given AlH4 unit, so that the second rf pulse produces incomplete rephasing of the quadrupole satellites. The initial echo amplitude was recovered upon cooling, again showing that no permanent change had occurred in the KAlH4 up to 350 °C. No second-order quadrupole splitting was evident in either the MAS or in the high temperature static resonances, though both methods achieved similar resonance line widths of around 300 Hz (about 4 ppm). Decoupling the hydrogen with 32 kHz continuous wave decoupling during the 27Al MAS acquisition did not lead to any further narrowing of the 27Al line widths at spinning speeds above 5 kHz. The second-order quadrupole splitting for the quadrupole coupling constant CQ = 2.07 MHz and η = 0 is expected to be 1.7 kHz (22 ppm) when static and 395 Hz (5 ppm) under MAS.45 Our observed (single) resonance of 4 ppm width therefore has approximately the same width as the expected second-order splitting in the 27Al MAS resonance. The lack of a resolved second-order quadrupole splitting in the high temperature static spectra may reflect the motional averaging of the electric field gradient by rapid translational motion. Mobile Species. For LiAlH4 and NaAlH4, it is generally agreed that metal atom transport is the rate-limiting step for

Figure 7. 39K static NMR on KAlH4 at room temperature and 7.04 T (13.98 MHz). fwhm is 800 Hz, 57 ppm. The peak is at −10.9 ppm relative to aqueous KBr. Hahn echo pulse separation is 200 μs.

The potassium resonance peak of KAlH4 was measured at −10.9 ± 1 ppm relative to 0.1 M KBr aqueous solution. The line width of the aqueous KBr reference due to magnetic field inhomogeneities is around 30 Hz. We note that the only previous measurement of 39K in KAlH4 quotes an isotropic chemical shift value of −49 ppm for KAlH4, which was 5730

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calculated based on a resolved second-order quadrupole splitting of 1.1 kHz.24 Those data have a center-of-mass (uncorrected for second-order quadrupole effects) near −10 to −20 ppm, consistent with our measurement. The previous measurements were done at the same field and same temperature as our experiments. Figure 7 does not show a resolved second-order quadrupole splitting as did the previous study.24,31 Multiple experiments were performed with varying interpulse delay times τ from 100 to 200 μs, all with results similar to that of Figure 7. Unlike in the 27Al NMR, the convolution here (in the frequency domain) of our 57 ppm fwhm peak with the expected 79 ppm second-order quadrupole splitting may obfuscate the structure. As in the 27Al NMR case, however, the lack of a resolved second-order quadrupole splitting is suggestive that there may be other crystal structures present in our KAlH4 or that used in the previous study.24,31 The spectrum in Figure 7 here and that of ref 24 are not compatible with their being from the same species. As a check of our methodology, we measured the 39K NMR of cubic KH under the same conditions and measured a resonance peak at +63 ppm with 700 Hz fwhm, which agrees with previous measurements.24

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CONCLUSIONS H and 27Al NMR measurements were performed as a function of temperature to investigate the rotational and translational motion of the AlH4 anion of KAlH4. The hydrogen relaxation parameters T1 and T1ρ were also measured as a function of temperature, and the location of the minima together with the onset temperatures of line narrowing gives activation energies for both the rotations (Erot,act = 0.28 eV) and diffusion (Ediff,act = 0.70 eV). First-order quadrupole patterns are observed in the 27 Al NMR data, from both MAS and quadrupole echoes, with no sharp singularities. This suggests a distribution of quadrupole coupling constants due to structural disorder and/or asymmetry (η ≠ 0) in the electric field gradients experienced by the aluminum nuclei. The 39K NMR is different from prior studies on KAlH4 which suggests multiple crystal polymorphs may be present in various KAlH4 samples. 1

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (E.G.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the US Department of Energy, Basic Energy Science, through grant DE-FG02-ER46256, and the NSF through grant DMR1206447. We also thank Steve Holmes at University of Missouri−St. Louis for cleaning and drying the KH used in the KAlH4 synthesis.

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REFERENCES

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