Reversible Hydrogen Storage via Titanium-Catalyzed LiAlH₄ and Li₃AlH₆

Abstract

A vibrating-mill technique, which can activate the reaction system by bringing the reagents into very close contact at the preparative scale and by providing extra mechanical energy, much more effectively than the well-known ball-milling method, was used to prepare titanium(III) chloride (TiCl3·1/3AlCl3)-doped lithium tetrahydridoaluminate (LiAlH4) and lithium hexahydridoaluminate (Li3AlH6) powders with nanocrystallites. The phase structure and dehydriding/rehydriding properties were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TG), and differential scanning calorimetry (DSC). The mechanism of reversible dehydrogenation and rehydrogenation was examined by means of X-ray photoelectron spectroscopy (XPS). Thermodynamic and kinetic measurements showed a distinct change for the dehydriding/rehydriding reactions over the temperature range 25−250 °C. From the Arrhenius plot of hydrogen desorption kinetics, apparent activation energies were found to be 42.6 and 54.8 kJ/mol H2 for the hydride decompositions of LiAlH4 and Li3AlH6, respectively. The results based on the properties of reversible hydrogen storage and catalysis function indicate that both the homogeneous distribution of Ti-catalyzed nanocrystalline complex hydrides and the Ti-catalyst with a Ti0 ⇔ Ti3+ (Ti0/Ti2+/Ti3+) defect site play important roles in optimizing the reversible hydrogen storage.