Quasi-elastic neutron scattering studies of hydrogen dynamics for nanoconfined NaAlH₄

The hydrogen dynamics of nanoconfined sodium Alanate (NaAlH₄) has been studied using quasi-elastic neutron scattering (QENS). Results indicate thermodynamic destabilization is responsible for reduced desorption temperatures of NaAlH₄ upon confinement within the nanopores of a metal organic framework (MOF). The quasi-elastic broadening in the nano-NaAlH₄ indicates that there are two dynamic states of hydrogen, which can be tracked by fitting the QENS signal to Lorentzian functions. The fastest hydrogen dynamics show some limited amount and range of long-range diffusion (i.e., not spatially confined motion) as indicated by a weakly varying Q-dependent fwhm on the broad Lorentzian quasi-elastic broadening data. These data trend toward zero at Q = 0 A^-1. Slower hydrogen dynamics, described by a narrow Lorentzian function, are present in the nanoconfined sample and can be attributed to reorientation and localized motion of H around AlH_x tetrahedra. Both the as-purchased NaAlH₄ (hereafter called "bulk" or "micro" NaAlH₄) and the nanoconfined NaAlH₄ data were fitted to reorientation models which yielded corresponding percent mobile hydrogen and jump lengths. The jump lengths calculated from the nano-NaAlH₄ were ≈2.5 Å and in conformity with those jump lengths determined for bulk NaAlH₄ of ≈2.3 Å. As much as 18% of the hydrogen atoms were estimated to be mobile in the nano-NaAlH₄ sample even at relatively low temperatures of 350 K. In contrast, bulk NaAlH₄ shows less than 7% mobile H atoms even at higher temperatures of ≈450 K. The hydrogen motion in the nanoconfined samples are fitted to a "high temperature (HT)" reorientation model in which a motion occurs by "tumbling" reorientation of AlH_x tetrahedra. The model assumes 3 of the 4 H atoms in the AlH₄ tetrahedra to be continuously exchanging their coplanar positions plus taking turns to exchange position with the fourth axial H atom. The microscale sample was fitted to a convoluted 2-site/3-site model, which can be viewed as three-dimensional jumps requiring the reorientation of the AlH₄ tetrahedra. The activation energy is 3.1 meV (at 125-320 K) and the attempt frequency (or energy) is 4.7 meV for this motion.