Ti EELS standards for identification of catalytic species in NaAlH_4 hydrogen storage materials

摘要

Titanium-catalyzed sodium-aluminum hydride has been studied by using in situ TEM heating experiments in conjunction with electron energy loss spectroscopy and X-ray energy dispersive spectroscopy. Electron energy loss spectroscopy (EELS) standards of Ti, TiO_2, Ti_3Al, cubic Al_3Ti, orthorhombic Al_3Ti, TiH_2, and TiCl_3 were prepared and analyzed to enable identification of the form of Ti and how it changes as a function of the number of hydrogenation-dehydrogenation cycles. The TiCl_3 precursor in NaAlH_4 when cycled at 3 wt. percent H_2 capacity remains after 5 cycles but is eliminated by the 10th cycle. The dominant form of Ti is then cubic Al_3Ti. The number of cycles required to achieve this depends on the cycled capacity, with partially cycled requiring more cycles than fully cycled material. The form and location of the Ti responsible for the enhanced performance was not discovered which has implications for the effective amount of Ti. The effects of the electron beam and exposure to air on the particles are shown and discussed.