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Franklin, OH, United States

Shane D.T.,Washington University in St. Louis | Corey R.L.,South Dakota School of Mines and Technology | Rayhel L.H.,Washington University in St. Louis | Wellons M.,Savannah River National Laboratory | And 5 more authors.
Journal of Physical Chemistry C | Year: 2010

LiBH4 doped with 1.6 mol % well-dispersed C60 is studied with solid-state nuclear magnetic resonance (NMR). Variable-temperature hydrogen NMR shows large changes between the data upon first heating and after exposure to 300 °C. After heating, a large fraction on the order of 50% of the hydrogen signal appears in a motionally narrowed peak, similar to a previous report of LiBH4 in a porous carbon aerogel nanoscaffold. Magic-angle spinning (MAS) NMR of 13C in a 13C-enriched sample finds the C60 has reacted already in the as-mixed (unheated) material. Dehydriding and rehydriding result in further 13C spectral changes, with nearly all intensity being found in a broad peak corresponding to aromatic carbons. It thus appears that the previously reported improved dehydriding and rehydriding kinetics of this material at least partially result from in situ formation of a carbon framework. The method may offer a new route to dispersal of hydrides in carbon support structures. © 2010 American Chemical Society.

Sorte E.G.,Washington University in St. Louis | Corey R.L.,Washington University in St. Louis | Corey R.L.,South Dakota School of Mines and Technology | Bowman Jr. R.C.,RCB Hydrides LLC | And 3 more authors.
Journal of Physical Chemistry C | Year: 2012

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 T 1ρ 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 T 1 for hydrogen and sodium decrease monotonically with temperature, in qualitative accord with relaxation by physical diffusion of spin magnetization to relaxation centers. © 2012 American Chemical Society.

Shane D.T.,Washington University in St. Louis | Corey R.L.,Washington University in St. Louis | Corey R.L.,South Dakota School of Mines and Technology | McIntosh C.,Washington University in St. Louis | And 5 more authors.
Journal of Physical Chemistry C | Year: 2010

Hydrogen NMR of LiBH 4 in the pores of carbon aerogel nanoscaffolds shows the coexistence of motionally narrowed and broad components. The fraction of mobile, diffusing hydrogen, already evident at room temperature, increases continuously with temperature. Thus, a broad distribution of environments is present, as in some ball-milled hydrides. With decreasing pore size from 25 to 13 nm, the narrowed fraction increases, suggesting that the narrow resonance is from the most defective regions, the grain boundaries. The broad component eventually exhibits narrowing in the same temperature window as for bulk material, confirming the bulk-like structure of those regions. Hole-burning measurements reveal magnetization exchange between the broad and narrow resonance lines, confirming the close spatial proximity of the atoms in each line. The solid-solid transition is clearly evident in 7Li line shapes, with a 10-15 °C depression from the bulk. More rapid decay of the quadrupolar satellite signals in spin echoes, compared to the central transition, is due to lithium atoms diffusing between differently oriented nanocrystallites. Our results suggest that crystallites in neighboring pores have similar orientations but are incoherent for diffraction. Remarkably, the T 1 data of hydrogen and 7Li are continuous in the vicinity of the transition, in contrast with the bulk T 1 data, suggesting that some rapid lithium motion remains below the transition. © 2010 American Chemical Society.

Sorte E.G.,University of Washington | Emery S.B.,University of Washington | Majzoub E.H.,University of Missouri-St. Louis | Ellis-Caleo T.,University of Washington | And 5 more authors.
Journal of Physical Chemistry C | Year: 2014

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. © 2014 American Chemical Society.

Sorte E.G.,Washington University in St. Louis | Majzoub E.H.,University of Missouri-St. Louis | Ellis-Caleo T.,Washington University in St. Louis | Hammann B.A.,Washington University in St. Louis | And 4 more authors.
Journal of Physical Chemistry C | Year: 2013

The presence of approximately 10 mol % NaOH mixed with NaH powder is shown to result in much more rapid hydrogen motion in the NaH above 150 C, as indicated by the onset of hydrogen NMR line narrowing at this temperature. A similar result appears for air-exposed NaH due to the formation of hydroxide from atmospheric H2O. The NMR line narrowing is too rapid, as a function of temperature, to be described by thermal activation; rather, it is suggestive of a phase transition. Indeed, differential scanning calorimetry finds, after an initial thermal cycle, a reversible thermal anomaly indicating a phase transition near 150 C. Powder X-ray diffraction with an excess of NaOH displays a remarkable lattice expansion of the NaH in the temperature range of 100-240 C where the (200), (220), and (311) reflections indicate a volume lattice expansion of up to 11%; the expansion is reversible upon cycling. The data thus point to a reversible phase transition in which NaOH enters the NaH structure above 150 C and exits the NaH below that temperature. First-principles calculations using OH-substituted NaH supercells indicate significant solubility in the NaH lattice and find a transition temperature and enthalpy change that are in approximate agreement with differential scanning calorimetry (DSC) measurements, confirming the presence of a phase transition. © 2013 American Chemical Society.

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