Hydrogen NMR of LiBH4 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.

1 H and 11 B NMR measurements were performed to study BH 4 reorientations and diffusion in four samples of Mg(BH 4 ) 2 . These include the low (R) and high (β) temperature bulk solid phases, R-phase material with TiF 3 and ScCl 3 additives, and R-phase material incorporated into a carbon aerogel. All four samples show 1 H T 1 minima within our temperature range from reorientational motion of the BH 4 tetrahedra. The β-phase 1 H T 1 minimum occurs at a lower temperature than that for the R-phase, indicating a lower activation energy for reorientations. The R-phase 1 H spectra display the onset of broadening at low temperatures because of the slowing of reorientations but show no indication of line narrowing at high temperatures from translational diffusion. The β-phase 1 H spectra are similar, but here T 1D measurements were performed to determine the BH 4 hopping rate. Above 150 °C, a significant narrow component does appear in the 1 H spectra of the aerogel sample, indicating substantially increased diffusive motions in this component.

We consider small PdH x particles in equilibrium with the surrounding H2 gas. Because the hydrogen longitudinal nuclear spin relaxation rate R 1 = 1/T 1 of the gas is so much larger than that of the bulk hydride, the apparent hydride relaxation rate will be increased by rapid exchange of H between the gas and the hydride phases. Under a wide range of conditions, the apparent hydride relaxation rate R 1 will equal the rate of hydride-to-gas exchange events. This method is demonstrated for well-dispersed PdH x using commercial Pd black where the hydride and gas signals are distinguished by the Knight frequency shift. The method is an equilibrium measurement of the hydride-to-gas exchange rate and should find application in high surface area metallic hydrides for characterization of the surface reactivity toward H2.

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.