Towards understanding a mechanism for reversible hydrogenstorage: theoretical study of transition metal catalysed dehydrogenation of sodium alanate

On the basis of density functional theory and coupled-cluster CCSD(T) calculations we propose a mechanism of the dehydrogenation of transition metal doped sodium alanate. Insertion of two early 3d-transition metals, scandium and titanium, both of which are promising catalysts for reversible hydrogen storage in light metal hydrides, is compared. The mechanism is deduced from studies on the decomposition of a model system consisting of one transition metal atom and two NaAlH4 units. Subsequently, the significance of such minimal cluster model systems to the real materials is tested by embedding the systems into the surface of the NaAlH4 crystal. It is found that the dehydrogenation proceeds via breaking of the bridge H–Al bond and consequent formation of intermediate coordination compounds in which the H2 molecule is side-on (η2-) bonded to the transition metal centre. The total barrier to the H2 release is thus dependent upon both the strength of the Al–H bond to be broken and the depth of the coordinative potential. The analogous mechanism applies for the recognized three successive dehydrogenation steps. The gas-phase model structures embedded into the surface of the NaAlH4 crystal exhibit an unambiguous kinetic stability and their general geometric features remain largely unchanged.