Effect of Ti-catalyst content on the reversible hydrogen storage properties of the sodium alanates

The reversible hydrogen storage properties of Ti-catalyzed NaAlH4 (and associated Na3AlH6) were studied as a function of Ti-content using a dry preparation technique consisting of the ball-milling of NaAlH4 +TiCl3 mixtures (0-9 mol.% TiCl3). This process is believed to result in the in situ solid-state introduction of metallic Ti via the reduction of the TiCl3 by the Na-component of NaAlH4 (to form NaCl). Properties studied were hydriding and dehydriding rates and reversible gravimetric H-capacity as a function of starting TiCl3 content. Detailed isothermal kinetic studies were done over a wide temperature range (20-225 degreesC) and treated by Arrhenius analysis. All kinetics were found to follow the Arrhenius equation and the changes of thermal activation energies and rate constants with TiCl3-content were observed that may give valuable insights into the as-yet unknown mechanistic factors that control H-2 desorption and absorption kinetics. Ti increases both dehydriding and hydriding kinetics (and associated practical engineering rates), but at the substantial expense of H-capacity. The finite room temperature decomposition of the catalyzed NaAlH4 phase was reconfirmed and rates better quantified. Published by Elsevier Science B.V.