Sodium alanate (NaAlH4) is a prototype system for storage of hydrogen in chemical form. However, a key experimental finding, that early transition metals (TMs) like Ti, Zr, and Sc are good catalysts for hydrogen release and re-uptake, while traditional hydrogenation catalysts like Pd and Pt are poor catalysts for NaAlH4, has so far gone unexplained. We have performed density functional theory calculations at the PW91 generalised gradient approximation level on Ti, Zr, Sc, Pd, and Pt interacting with the (001) surface of nanocrystalline NaAlH4, employing a cluster model of the complex metal hydride. A key difference between Ti, Zr, and Sc on the one hand, and Pd and Pt on the other hand is that exchange of the early TM atoms with a surface Na ion, whereby Na is pushed on to the surface, is energetically preferred over surface absorption in an interstitial site, as found for Pd and Pt. The theoretical findings are consistent with a crucial feature of the TM catalyst being that it can be transported with the reaction boundary as it moves into the bulk, enabling the starting material to react away while the catalyst eats its way into the bulk, and effecting a phase separation between a Na-rich and a Al-rich phase. In addition, the role of different active species such as Ti2 and TiH2 has been studied using the same model. The results imply that Ti2 and TiH2 are more stable in the subsurface region of the cluster than on the surface. Calculations were performed on the decomposition of two calcium alanates, to determine zero-point energy corrected enthalpies of dehydrogenation for these compounds, and to determine whether destabilization of LiBH4 by CaH2 might improve the performance of this material.Advanced Chemical Technologies for Sustainability (ACTS)UBL - phd migration 201
