A systematic study was made of the dehydrogenation process of undoped material, using anelastic spectroscopy. Evidence was found for the formation of a highly mobile species during decomposition. This was identified in off-stoichiometric AlH6-x units, giving rise to fast H vacancy local dynamics. The formation of such stoichiometry defects began at temperatures which were much lower in Ti-doped than in undoped samples, with a concomitant change in the decomposition reaction. The catalyst atoms decreased the energy barrier which had to be surmounted by H in order to break the bond; thus enhancing the kinetics of the chemical reactions and decreasing the temperature at which the dehydrogenation processes took place. The experimental data showed that not all of the H released by the formula units during decomposition was evolved by the sample. Part of it remained in the lattice, and migrated over long ranges within the sample. The species, in this H-mobilized population, was identified which induced the fast tetragonal-to-monoclinic phase transformation which accompanied decomposition. A partial spontaneous thermally activated regression of decomposition was also observed during aging experiments. A model was proposed which accounted for the action of the Ti catalyst and for the atomistic mechanism of decomposition.

Fast H-Vacancy Dynamics During Alanate Decomposition by Anelastic Spectroscopy - Proposition of a Model for Ti-Enhanced Hydrogen Transport. O.Palumbo, A.Paolone, R.Cantelli, C.M.Jensen, M.Sulic: Journal of Physical Chemistry B, 2006, 110[18], :9105-11