Density Functional Theory Study of the TiH₂ Interaction with a NaAlH₄ Cluster

To understand the role of titanium in catalyzing the de/rehydrogenation reactions in NaAlH 4, it is necessary to determine the composition and structure of the catalytically active species that is (are) formed during ball milling and de/rehydrogenation reactions. One of the species that is thought to be catalytically active is TiH 2 . We have performed a density functional theory study of TiH 2 interacting with a NaAlH 4 cluster. First, a TiH 2 molecule was adsorbed on the (001) surface of NaAlH 4 in different sites. Next, the TiH 2 molecule or its Ti atom was moved inside the cluster either by exchanging the whole TiH 2 molecule with Na or Al or by exchanging only the Ti atom with Na or Al and leaving the two hydrogen atoms on the surface together with the exchanged atom. Our calculations suggest that, when restricting the possible outcomes to adsorption, TiH 2 adsorbs on the surface above a Na site, pushing the Na atom subsurface. However, exchanging the whole TiH 2 unit with the subsurface displaced Na atom is favorable compared to leaving the TiH 2 on the surface. All other investigated exchanges were found to be less stable than TiH 2 adsorbed on the surface. The results are consistent with a zipper model that we recently proposed.