  We report a first-principles study of the energetics of hydrogen absorption
and desorption (i.e. H-vacancy formation) on pure and Ti-doped sodium alanate
(NaAlH4) surfaces. We find that the Ti atom facilitates the dissociation of H2
molecules as well as the adsorption of H atoms. In addition, the dopant makes
it energetically more favorable to creat H vacancies by saturating Al dangling
bonds. Interestingly, our results show that the Ti dopant brings close in
energy all the steps presumably involved in the absorption and desorption of
hydrogen, thus facilitating both and enhancing the reaction kinetics of the
alanates. We also discuss the possibility of using other light transition
metals (Sc, V, and Cr) as dopants.
