Dynamics of defects in alanates

Anelastic spectroscopy experiments on the dehydrogenation process of undoped and catalysed NaAlH4 revealed the formation of a highly mobile species during decomposition. This species, as indicated also by isotope effect measurements, was identified as a hydrogen containing defect complex. In particular, the most likely defect complex may be of type AlHx, in which fast local dynamics of H-vacancies can occur. The formation of such defects begins at temperatures much lower in Ti-doped than in undoped samples. The catalyst atoms decrease the energy barrier to be overcome by H to break the bond, thus enhancing the kinetics of the chemical reactions and decreasing the temperature at which the dehydrogenation processes take place. (C) 2007 Elsevier B.V. All rights reserved.