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 AlHx 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 AlHx 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.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films