NMR Studies of NaH - American Chemical Society

Aug 15, 2012 - Department of Physics, South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, South Dakota. 57701, Unit...
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NMR Studies of NaH Eric G. Sorte,*,† Robert L. Corey,†,‡ Robert C. Bowman, Jr.,§ Derek Birkmire,⊥ Ragaiy Zidan,∥ and Mark S. Conradi*,† †

Department of Physics, Washington University, One Brookings Drive, Saint Louis, Missouri, 63130, United States Department of Physics, South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, South Dakota 57701, United States § RCB Hydrides, LLC, 117 Miami Avenue, Franklin, Ohio 45005, United States ⊥ Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822, United States ∥ Savannah River National Laboratory, Aiken, South Carolina 29808, United States ‡

ABSTRACT: Hydrogen and 23Na NMR were used to probe diffusive motions of the ions in several NaH powders. In three NaH samples, the H resonance is a superposition of broad and narrow components, reflecting the presence of relatively immobile and mobile H, respectively. The fraction of mobile H grows from 23 to 250 °C; this pattern has been observed previously in other ionic hydrides. By 300 °C, the formerly broad hydrogen component has itself motionally narrowed. In these samples, the observation of a smaller amount of 23Na line narrowing by 300 °C indicates that only the H− ions are mobile at 300 °C, leaving 23Na− 23Na dipole interactions unaveraged. A deep minimum in the hydrogen rotating-frame relaxation time T1ρ is observed near 325 °C, as expected from the onset of motional averaging. In a fourth sample, the entire H line is narrowed already by 150 °C. In this sample, the 23Na is also partially narrowed at this temperature and by 175−225 °C, further narrowing of the 23Na resonance indicates that now both ions are in rapid motion. In all the samples, the spin−lattice relaxation times T1 for hydrogen and sodium decrease monotonically with temperature, in qualitative accord with relaxation by physical diffusion of spin magnetization to relaxation centers.

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INTRODUCTION Hydrogen storage in solid-state systems is a promising route to address the technical challenges associated with the handling and containment of hydrogen for use as a fuel in vehicular applications.1 These solid-state hydrides are not without their own technical challenges, which continuing research is attempting to address. While the interstitial metallic hydrides are able to release and absorb hydrogen with favorable kinetics, none has a sufficiently high weight fraction of hydrogen (e.g., to meet US Department of Energy standards2). Ionic hydrides such as MgH2 address this problem by pairing the hydrogen anions with light-metal cations; however, untreated MgH2 releases hydrogen only at excessive temperatures and with poor kinetics (due in part to slow H diffusion through MgH23), leading to excessively long dehyriding and rehydriding times. Complex hydrides such as NaAlH4 and LiBH4 can exhibit higher weight fractions of hydrogen but often require high temperatures and/or hydrogen gas overpressures to effect the appropriate reactions.4 Much work has been dedicated to understanding these systems; our own group has made systematic NMR studies of the ionic hydrides MgH23 and NaMgH35 and the complex hydrides LiBH46 and Mg(BH4)2.7 Nonetheless, in all of these hydride systems more work is required to understand the fundamentals of the hydriding kinetics and the rate of atomic diffusion so that appropriate solutions to the existing challenges may be developed. © 2012 American Chemical Society

The difficulties resulting from slow diffusion in ionic and complex (ionic−covalent) hydrides can be partially addressed by ball-milling and/or confinement in nanoscaffolds such as aerogels or nanoporous carbons.8 Not only does the shorter diffusion distance improve the performance, but the rate of H diffusive hopping increases substantially for a fraction of the H. For example, in LiBH4 confined in aerogels, the hydrogen NMR resonance was found to be a superposition of a broad signal from immobile hydrogen atoms superimposed on a much sharper resonance from mobile BH4− anions undergoing motional narrowing.9 With increasing temperature, the fraction of line-narrowed signal (mobile H) increased. Because the fraction of hydrogen spins in the narrow (mobile) resonance varied approximately reciprocally with the pore size, these rapidly moving hydrogen nuclei were identified with those hydrogen (in BH4− anions) near the nanoscaffold walls. A similar pattern of superimposed broad and narrow components was found with MgH2 and NaMgH33,5 and is evident in the present study of NaH. Theoretical work and calculations have identified vacancies and interstitial ions in ionic hydrides as being key intermediates for diffusion.10−12 Not only do these include intrinsic, thermally Received: June 13, 2012 Revised: August 14, 2012 Published: August 15, 2012 18649

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by rapid motion, leaving the line shape determined by the other, unaveraged interaction. In Table 1, we list the values of the second moments and the calculated fwhm for each NMR resonance.

activated defects, but extrinsic defects that can arise from aliovalent doping.10,12 For example, MgH2 in NaH can introduce defects quantitatively into the crystal (vacancies on the Na sites and/or hydride interstitials), the careful control of which can shed light on the identity of the point defects important in the diffusion process. Clearly, the presence of impurities is expected to have a profound effect on diffusion.13,14 This mechanism is analogous to the enhanced electrical conductivity of doped semiconductors which may greatly exceed the intrinsic conductivity. The goal of the present work is to use 1H and 23Na NMR to study the rates of H and Na diffusive motions in several NaH samples. The samples are all originally of commercial origin and of relatively low (and varied) purity. The samples include two coarsegrained powders and two ball-milled powders. Thus, ours is a study of the kinds of NaH that are similar to materials considered for storage applications, and not solids with wellknown and well-controlled impurities. Therefore, our study can not directly address the issue of aliovalent doping.

Table 1. Second Moment Contributions from Dipolar Couplings for 1H and 23Na in NaH Calculated Using Equations 2 and 3a

Na: 6170 Hz

Na: 2600 Hz

The Gaussian fwhm is calculated using eq 4.

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EXPERIMENTAL METHODS We examined four samples in this work. Two samples (SA1 and SA2) were purchased as dry powders from Sigma Aldrich with a stated purity of 95% (the identity of the impurities is not available). These samples are representative of commercially available samples and have not been ball-milled to reduce the grain size. The commercial grade samples were used without further processing. The third sample (SR) was purchased from Sigma-Aldrich as dry powder and then ball-milled in a SPEX 8000 under an argon atmosphere at Savannah River National Laboratories for 5 h. The fourth sample (UH) was from the University of Hawaii. This sample was originally purchased as a dispersion of NaH in oil. The oil was removed by washing with pentane, and the NaH was finally obtained by filtering the hydride from the pentane suspension. The NaH was dried and ball-milled in an inert atmosphere with stainless steel ball bearings for 30 min at 350 rpm. All samples were transported under nitrogen to avoid contamination and oxidation. The samples were stored in a flowing N2 glovebag. For static NMR, the samples were loaded into 5 mm outer diameter pyrex NMR tubes and flame-sealed under an argon atmosphere. Static variable temperature 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 7.8 μs (90°) excitation pulses spaced at least 5T1 apart with a receiver dead-time of 3 μs. The temperature was controlled by a stream of flowing air, regulated by a heater and resistance thermometer. The temperature was measured separately with a T-type thermocouple near the NMR sample. For 23Na the static variable-temperature NMR was performed in a 4.74 T superconducting magnet (23Na Larmor frequency 53.45 MHz).

(1)

(3)

2ln 2M 2 π

H: 0 Hz

Each spin (1H or 23Na) experiences both homonuclear and heteronuclear interactions. Motion of H will time-average to zero both the H−H and H−Na interactions, leading to complete narrowing of the H resonance for sufficiently rapid H motion. However, from the 23Na viewpoint, rapid H motion only time-averages 23Na−H interactions and leaves 23Na−23Na dipole interactions unaffected. Thus, the 23Na NMR line will only be partially narrowed, regardless of the speed of the H motion. The calculated line widths appear in Table 1 for the cases of no motion (rigid lattice limit) and rapid motion of only the hydrogen ions.

where γI and γS are the gyromagnetic ratios of the nuclear spins, I and S are the spin quantum numbers of the corresponding nuclei, and rk is the distance from the central spin to the neighboring spin k. The sum over k covers all the I or S spin neighbors; we note that 1H and 23Na are both 100% abundant. We calculated the sum in eq 2 for like nuclei (1H or 23Na) in the NaH fcc lattice by using the well-known fcc lattice sum16 115.63a0−6 where a0 is the unit cell lattice parameter of the crystal (a0 = 0.489 nm for NaH).17 For the heteronuclear sum, we used the simple cubic lattice sum over all the sites15 (H as well as Na) of 8.5(a0/2)−6, and then subtracted off the 115.63a0−6 describing the homonuclear terms. Assuming a Gaussian line shape, the second moment M2 is related to the full width at half of maximum (fwhm; in units of Hertz) by15 fwhm =

H: 20,988 Hz

gaussian fwhm (rapid H motion only)

M 2Na − Na = 0.47 a

For dipole contributions to the NMR line width of the spin I, the powder-averaged homonuclear second moment MII2 , and 15 heteronuclear second moment MIS 2 are given as: 3 1 M 2II = γI 4ℏ2I(I + 1) ∑ 6 5 r (2) k k 4 2 2 2 1 γI γS ℏ S(S + 1) ∑ 6 15 k rk

M 2H − H = 19.43

M 2Na − H = 2.24

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M 2IS =

gaussian fwhm (rigid lattice)

M 2H − Na = 11.19

DIPOLAR INTERACTIONS IN NAH The dominant contribution to the NMR line width in these solids is dipolar broadening from homonuclear (23Na−23Na, 1 H−1H) and heteronuclear (23Na−1H) interactions. We note that the Na ions sit at sites of cubic symmetry, so 23Na quadrupole effects are expected to be zero (or small, from defects). The second moment MI2 representing the mean squared dipolar interaction experienced by a nuclear spin I is given by the sum of the homonuclear and heteronuclear terms: M 2I = M 2II + M 2IS

second moment (108 rad2/s2)

(4)

The Gaussian shape is a good approximation for the rigidlattice lineshapes because of the high concentration of nuclear spins;15 it also applies when one of the interactions is averaged 18650

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Figure 1. (a) 1H and (b) 23Na NMR spectra of the as-purchased Sigma Aldrich NaH sample SA2. Each spectrum is identified by its temperature (°C). The spectra are scaled to equal heights. Note the smaller extent of the 23Na narrowing, expressed as a ratio of line widths, compared to the hydrogen narrowing.

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RESULTS AND DISCUSSION The XRD and MAS NMR characterization of the NaH samples reveal the presence of the expected NaH diffraction and resonance peaks. The XRD patterns also reveal amounts of crystalline NaOH. The integrated area of the known NaOH peak of highest intensity (2θ = 38.218◦)19 was compared to the area of the known NaH peak of highest intensity (2θ = 31.667◦).17 We compared the areas of these strongest, cleanly separated reflection peaks to a calibration sample comprised of 50/50 NaH/NaOH (by mass). The SA1 and SA2 samples had 0.02−0.03 as much NaOH as NaH, on a mole basis. In the SR and UH samples, the NaOH reflections were stronger, indicating 0.15 and 0.3 as much NaOH as NaH (mol/mol). The XRD patterns of each sample are very similar with similar peak widths; the primary difference between them is the stronger NaOH reflection in the SR and UH samples. The XRD patterns were essentially unchanged after the NMR experiments (in small volume sealed tubes, with temperatures up to 350 °C). The proton decoupled MAS NMR 23Na line widths were approximately 4 ppm fwhm. Unfortunately, the NaH and NaOH chemical shifts are very similar (17.8 and 20 ppm, respectively, with respect to the aqueous NaCl reference20), making it difficult to distinguish them in the MAS NMR. We note that no other MAS NMR sodium peaks were observed from other impurities. Static variable-temperature 1H and 23Na NMR lineshapes from room temperature to 300 °C for the NaH sample SA2 are shown in Figure 1. Repeated temperature cycles showed no material effect upon the H or Na lineshapes. The hydrogen spectra in Figure 1a below about 200−250 °C are superpositions of a broad peak (≈25 kHz fwhm) and a narrow peak (≈1 kHz fwhm). As measured by integration of the resonance peaks, the percentage of spins in the narrow peak is already a nontrivial fraction of the total spins in SA2 at room temperature (about 3%). As the temperature is raised, more and more of the spin population transfers from the broad component to the narrow component. By 200 °C (250 °C), the narrowed fraction is 15% (23%). By 300 °C, all of the hydrogen nuclei are contributing to the narrow peak, indicating that the previously broad component is now itself undergoing motional narrowing.

The sodium lineshapes were obtained by Fourier transform of the FIDs following hard 3.7 μs pulses, again spaced at least 5T1 in time. The temperature was maintained by an oven heated with resistive thermocouple wire, wound bifilar to avoid magnetic fields generated by the heater current. T1 measurements on both nuclei were performed by the saturation/ recovery method with saturation achieved by a series of 20 π/2 pulses spaced by 500 μs (longer than T2). All signals were acquired on a home-built spectrometer and processed as described elsewhere.9,18 T1ρ was obtained using a 90x−τy−Acq where τy is a phase-y continuous spin-locking pulse, at the end of which is a FID signal. We measure the amplitude S of this signal as a function of τ, fitting to S = S0e−τ/T1ρ . We characterized our samples using powder X-ray diffraction (XRD) and 1H and 23Na magic-angle spinning (MAS) NMR. The XRD spectra were obtained using a Rigaku Geigerflex DMAX/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 25 to 70° (2θ). Analysis of spectra, 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 (PDF no. 01-076-0171 and 01-085-0733 for NaH and NaOH, respectively). Samples were loaded onto XRD slides, covered with a thin low-background plastic film that is transparent to X-rays, and transported under a nitrogen atmosphere. The MAS NMR spectra of both nuclei were acquired at 6.92 T (1H Larmor frequency 294.97 MHz, 23Na Larmor frequency 78.025 MHz) with an Apollo (Tecmag) spectrometer using 1.85 μs pulses. The samples were loaded under a nitrogen atmosphere and sealed into 4 mm Varian zirconia rotors for use in a HXY MAS solids probe at a MAS rotational frequency of 5 kHz, using nitrogen gas for spinning to minimize air exposure. Spectra generally consisted of 32 phase-cycled averages with a repetition time of 10 s. High power proton decoupling was employed during the 23Na MAS experiments. 18651

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Figure 2. (a) 1H and (b) 23Na NMR spectra of the NaH sample UH. Each spectrum is identified by its temperature (°C). The spectra are normalized to equal heights. Note the very rapid hydrogen line narrowing from 110 to 150 °C; the extent of sodium narrowing over these temperatures is much smaller.

Two other samples’ hydrogen resonances, SA1 and SR, behaved similarly as functions of temperature. We interpret the spectra in Figure 1a as implying that the broad component represents translationally immobile hydrogen spins (on the 10−5 s time scale) and the narrow peak represents hydrogen nuclei undergoing translational diffusive hops that are rapid compared to the rigid lattice spin coherence time T2 (on the order of 10 μs). A similar narrow peak that grows in intensity with increasing temperature was found in MgH2,3 NaMgH3,5 and LiBH4 in aerogel.9 For LiBH4 in aerogel, the narrow fraction also increased with smaller aerogel pores, suggesting that the mobile BH4− ions are near LiBH4−aerogel walls. This is a reasonable interpretation of the present NaH data, too, but with mobile H− ions being near grain boundaries. In Figure 1b, we present the 23Na resonance spectra for the same sample (SA2) as a function of temperature. There is substantially less narrowing visible in the sodium resonances over the same temperature range the hydrogen resonances narrow by a factor of 30 while the sodium resonances decrease in width only by a factor of 2. From the 23Na predicted line widths in Table 1, sodium atoms evidently remain motionless (compared to the 10−5 s NMR time scale) over the temperature range measured in this sample (to 300 °C). Figure 2 shows the hydrogen and sodium NMR line shape data over similar temperature ranges for the NaH sample UH. This sample shows substantially different behavior than SA2. Here at room-temperature there is only a small fraction of mobile hydrogen appearing in the H resonance, with the fraction gradually increasing until around 110 °C. The entire H line-shape is then seen to narrow dramatically between 110 and 150 °C, indicating a relatively abrupt and homogeneous change to hydrogen mobility at this temperature throughout the sample. As before, however, the sodium resonances do not exhibit this large change. In the UH sample, the hydrogen resonance narrows much more than the sodium resonance over the same temperature range (i.e., to 150 °C), indicating that at 150 °C, only the H are mobile. Figure 3 presents the measured line widths fwhm as functions of temperature for both 1H and 23Na. In this figure, the dotted lines indicate the calculated fwhm shown in Table 1 for both nuclei fixed in rigid-lattice positions, and for a fixed

Figure 3. Measured fwhm line widths of hydrogen and 23Na in NaH as functions of temperature. Solid symbols are hydrogen line widths and open symbols refer to 23Na. Data for both nuclei appear for samples SA2 and UH. For samples SA1 and SR, only the H line widths are shown. The dashed lines represent predictions from Table 1 based on calculated dipole−dipole second moments.

lattice of Na nuclei in the presence of rapidly moving H ions. For the H spectra on the samples UH, SA2, and SR, a long π/2 pulse time of 8−10 μs was used (low transmitter power). As this is a significant portion of the transverse decay time at low temperatures, the use of such pulses tends to distort the line shape; the maximum hydrogen line widths are therefore artificially broadened relative to the predicted fwhm at low temperature.18 Better H lineshapes were obtained on sample (SA1) by using 1−2 μs pulses to minimize distortion of the resonance; at 25 °C, the fwhm line width of SA1 is in better agreement with the value predicted from Table 1. Very little line shape distortion is predicted for narrower resonances.18 The fwhm widths of the hydrogen resonance plotted in Figure 3 are taken from the main, broad component of the resonance, in those cases where two components are clearly distinguishable (including SA1, SA2, and SR at temperatures ≤ 250 °C). The hydrogen narrowing of samples SA1 and SA2 occurs over the same temperature range, suggesting that this 18652

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Our value of activation energy is in agreement with the value obtained by Borgschulte et al. (1.0 ± 0.1 eV) derived from nonisothermal H/D exchange experiments.21 The apparent agreement21 of their result with that obtained by Singh and Eijt11 (who calculated 0.94 eV for the migration energy that describes neutral hydrogen vacancies V0H) is misleading, because experimental work (including ours) undoubtedly describes the total energy ED for diffusion (which is a sum of the migration and formation energies for a defect). Singh and Eijt reported 0.94 eV as a migration energy only. Hao and Sholl10 found a migration energy barrier of 0.17 eV and a formation energy of 1.01 eV for interstitial hydrogen (Hi+), giving a total diffusion activation energy ED = 1.18 eV. They note that while sodium and hydrogen vacancies (VNa− and VNa+) are the most numerous defects present in NaH (due to a slightly smaller formation energy), H diffusion is dominated by the motion of Hi+ interstitials due to the much lower migration energy of these defects. Their value of ED = 1.18 eV is somewhat higher than the 1.03 eV we measure, but we note that our value depends rather sensitively on the assumed attempt rate as discussed above. Of course, in commercial samples with unknown impurities, there may also be aliovalent doping present, so the result of Hao and Sholl should in any case be regarded as an upper limit for ED. The 1H and 23Na laboratory-frame spin−lattice relaxation times T1 also appear in Figure 4. In general, the T1 times decrease smoothly with increasing temperature. This is in qualitative accord with a model in which spin magnetization diffuses to dilute relaxation centers, a common model for many solids. In this model, the large variation in T1 between the different samples simply reflects different densities and identities of paramagnetic relaxation centers. The diffusion is spin diffusion at low temperatures and physical diffusion of H or Na at high temperatures. We note the large step decrease of more than one decade in the hydrogen T1 of sample UH between 100 and 150 °C (dashed line in Figure 4) undoubtedly associated with the rapid onset of hydrogen line narrowing for this sample, as evident in Figures 2 and 3. From the data of Figure 4, the ionic motions never become sufficiently rapid (≈109 s−1) in any of the samples to result in T1 minima.

may reflect intrinsic NaH behavior. In SA2, the sodium line width had decreased by only a factor of ≈2 by 300 °C, in accord with the picture of rapid H motion yet little or no motion of sodium. The hydrogen line of sample SR narrows at slightly lower temperatures, a shift of perhaps 30 °C; we recall it was ball-milled, and this may increase the H diffusion rate. The UH hydrogen resonance narrows at a much lower temperature than the other materials, as made clear by Figure 3. In the UH material, the 23Na signal narrows by a factor of about 2 by 150 °C (at which temperature the H line width has decreased by more than an order of magnitude). Thus, at 150 °C, the sodium ions are not yet diffusing on the NMR time scale. But the sodium line narrows further starting at 175 °C, indicating that now sodium ions are starting to diffuse. The line shape data for sample SA2 (hydrogen and sodium) were reproducible upon repeated temperature cycling (to 300 °C). But the 23Na lines in sample UH narrowed at a higher temperature (between 150 and 200 °C) on first heating, compared to subsequent heatings. All of the data from UH presented in Figures 2 and 3 are from second and subsequent heatings. Presented in Figure 4 are the longitudinal relaxation times of the samples, including the hydrogen rotating-frame longitudinal

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Figure 4. Longitudinal relaxation times T1 of the NaH samples, together with hydrogen rotating-frame relaxation times T1ρ. Note the large step-like decrease in hydrogen T1 of sample UH, shown by a dashed line. The other lines are guides for the eyes.

CONCLUSIONS The hydrogen NMR of samples SA1, SA2, and SR shows two components, a broad line from immobile H (immobile on the 10−5 s time scale) and a much narrower line from rapidly diffusing H. The intensity of the narrow component grows with rising temperature, similar to the behavior of other ionic hydrides. In these NaH samples, by 300 °C even the formerly broad component is motionally narrowed. Observation of a minimum in the rotating-frame relaxation time T1ρ at 325 °C confirms this picture. At 300 °C, the 23Na line width of SA2 is only reduced by a factor of 2, indicating that the H atoms are mobile on the 10−5 s time scale, but not the Na atoms. In the UH sample, the entire H resonance narrows from 110 to 160 °C, demonstrating the much greater diffusion of hydrogen in this material. At 175 °C, the UH 23Na line width also begins to narrow below the line width calculated for the case of hydrogen motion only, indicating that rapid Na motion has begun. The greatly enhanced diffusion kinetics of the UH sample of NaH is almost certainly due to some impurities; we note that its ball-milling was not particularly aggressive. From XRD, the UH sample had substantially more NaOH impurity than SA1 and SA2, and this may be relevant to the enhanced diffusion in

relaxation times T1ρ. The T1ρ minima in the NaH samples SA1 and SR occur at 300−325 °C and confirm the observation of line narrowing (of the main, broad component) near 300 °C. For the rf field strength employed here (f1 = γB1/2π = 32 kHz), the T1ρ minimum is expected for 2πf1τhop = 0.86916, so a hopping time τhop = 4.3 × 10−6 s. We note that the value of 0.869 is specific to an fcc lattice of like spins, so this is only a good approximation for the present system of two spins (H, Na). The above value of τhop is only slightly smaller than the 10−5 s describing the onset of line narrowing, so the appearance of the T1ρ minimum just above the temperature of line narrowing is as expected. This hopping time translates into a hopping rate ωhop of 2.3 × 105 s−1. Treating the hopping rate as thermally activated and assuming a typical attempt frequency of 1014 s−1, we calculate the diffusion activation energy ED to be 1.03 eV. An error of 20 °C in the temperature of the T1ρ minimum would result in an error of 3% in ED. A change in the attempt (prefactor) rate by a factor of 10 would change ED by about 11%. 18653

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(12) Mott, N. F.; Gurney, R. W.; Electronic Processes in Ionic Crystals; Oxford University Press: Oxford, 1940. (13) Oei, Y. S.; Richtering, H.; Wiemhofer, H. D. Ber. Bunsenges. Phys. Chem. 1979, 83, 463−470. (14) Michel, K. J.; Ozoliňs, V. Native Defect Concentrations in NaAlH4 and Na3AlH6. J. Phys. Chem. C. 2011, 115, 21443−21453. (15) Abragam, A. Principles of Nuclear Magnetism; Oxford University Press: Oxford, 1961. (16) Sherwood, J. N.; Boden, N. The Plastically Crystalline State; John Wiley and Sons: Great Britian, 1979. (17) Shull, C. G.; Wollan, E. O.; Morton, G. A.; Davidson, W. L. Neutron Diffraction Studies of NaH and NaD. Phys. Rev. 1948, 73, 842−847. (18) Brady, S. K.; Conradi, M. S.; Majer, G.; Barnes, R. G. Proton Magnetic Resonance Spectra of YH3 and LuH3. Phys. Rev. B 2005, 72, 214111. (19) Stehr, H. Neubestimmung der Kristallstrukturen des dimorphen Natriumhydroxids, NaOH, bei verschiedenen Temperaturen mit Rö ntgenstrahl- und Neutronenbeugung Kristallogr. Kristallphysik 1967, 125, 332−359. (20) Terskikh, V. V.; Lapina, O. B.; Bondareva, V. M. Sodiummodified V2O5-TiO2 catalysts: 23Na and 51V solid-state NMR study. Phys. Chem. Chem. Phys. 2000, 2, 2441−2448. (21) Borgschulte, A.; Pendolino, F.; Gremaud, R.; Zuttel, A. Hydrogen Diffusion in NaH as Derived from Isotope Exchange Experiments. App. Phys. Lett. 2009, 94, 111907.

the UH material. However, we note that the SR sample which displayed similar H line narrowing as SA1 and SA2 also had greater NaOH content according to X-ray diffraction. We do not understand the difference but note that the NaOH in polycrystalline form (which appears as diffraction peaks) is not likely to influence directly the NaH behavior. Rather, impurities in the NaH phase would be crucial, and these would not show the characteristic X-ray reflections of solid NaOH. Of course, while NaOH from inadvertent exposure to oxygen or water is the likeliest impurity, there may be others that we have not considered. The onset of atomic motion in the UH sample at significantly lower temperatures than in the other samples of NaH holds open the prospect that the reaction kinetics of other ionic and complex hydrides may be similarly improved.

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AUTHOR INFORMATION

Corresponding Author

*Electronic address: [email protected] (E.G.S.); msc@ wustl.edu (M.S.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank S. E. Hayes and K. M. Wentz for help with the MAS NMR and for helpful discussions. The authors gratefully acknowledge support from DOE Basic Energy Sciences grant DE- FG02-05ER46256.

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REFERENCES

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