Quasi-Elastic Neutron Scattering Studies of Hydrogen Dynamics for Nanoconfined NaAlH4
Shathabish NaraseGowda, Craig Brown, Madhu Sudan Tyagi, Tabbetha A. Dobbins
The hydrogen dynamics of nano-confined 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 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 re-orientation and localized motion of H around AlHx tetrahedra. Both the bulk (microscale) NaAlH4 and the nanoconfined NaAlH4 data were fitted to re-orientation models which yielded corresponding percent mobile hydrogen and jump lengths. The jump lengths calculated from the nano-NaAlH4 were approximately equal} 2.5 A, and in conformity with those jump lengths determined for bulk NaAlH4 of approximately equal} 2.3 A. As much as 18% of the hydrogen atoms were estimated to be mobile in the nano-NaAlH4 shows less than 7% mobile H-atoms even at higher temperatures of approximately equal}450 K. The hydrogen motion in the nanoconfined samples are fitted to a "high temperature (HT)" re-orientation model in which a motion occurs by "tumbling" re-orientation of AlHx tetrahedra. The model assumes 3 of the four H-atoms in the AlH4 tetrahedra to be continuously exchanging their co-planar 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 re-orientation of the AlH4 tetrahedra. The activation energy is 3.1meV and the attempt frequency (or energy) is 4.7meV for this motion.
, Brown, C.
, Tyagi, M.
and Dobbins, T.
Quasi-Elastic Neutron Scattering Studies of Hydrogen Dynamics for Nanoconfined NaAlH<sub>4</sub>, Journal of Physical Chemistry C, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=921134
(Accessed December 3, 2023)