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A Study of Mechanism and Kinetics of Reversible Hydrogen Storage in Titanium Doped Sodium Aluminium Hydride

On the basis of density functional theory (DFT) and coupled cluster (CCSD(T)) calculations we propose a mechanism of dehydrogenation of titanium doped sodium aluminum hydride (NaAlH4). The mechanism is deduced from studies on the decomposition of a minimal cluster model system consisting of one Ti atom and two NaAlH4 units. Its relevance to the real-world materials is subsequently tested by periodic DFT calculations on systems created by embedding the minimal clusters into the (001) surface of the NaAlH4 crystal. It is found that the dehydrogentaion proceeds via breaking of the bridge H-Al bond and subsequent formation of a weak intermediate compound in which the H2 molecule is side-on (hapto) bonded to the transition metal centre. This means that the total barrier to the H2 release is dependant 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 optimized model structures embedded into the surface of the NaAlH4 crystal exhibit an unambiguous kinetic stability and their general geometric features remain largely unchanged. This motivated us to calculate the kinetic parametres of the de- and re-hydrogenation of the gas-phase minimal clusters making use of the variable reaction coordinate (VRC) and quasi-classical trajectories treatments. To accomplish this, we developed a model potential up to the 5-body terms treating 6 of the 13 atoms (3H, 2Al and Ti) as active. The potential as a function of 15 internuclear Morse-type coordinates is expanded in a polynomial basis that is made invariant with respect to change of the atoms of the same type. The obtained results will be compared to the experimental de- and re-hydrogenation curves.

Hydrogen Storage; Sodium Aluminum Hydride; Titanium; Mechanism; Kinetics; DFT calculations

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