Study On Mechanisms Of Titanium On Hydrogenation/Dehydrogenation Reaction Of Sodium Alanate

NaAlH₄ is considered to be a very promising material for hydrogen storage applications. However, its practical application was limited to its irreversible dehydrogenation reaction and poor kinetic properties especially at low temperatures (below 150°C). The situation was changed after the discovery that it can reversilly release and absorb hydrogen at room temperature with acceptable kinetic behaviour under the catalysis of titanium. Thus, catalyzed hydride complexes show a great promise for the on-board hydrogen storage applications. Howerver, due to the factors that the content of additives is small and that the additives would decompose in hydrogen absorption/desorption cycles, it is hard to detect them experimentally and therefore to identify the influence of the catalysts on the reversible absorption/desorption reactions of NaAlH₄. Thus, the mechanisms that Ti catalyzing the reversiable hydrogenation/dehydrogenation reaction of NaAlH₄ are still remain unclear.The present work is therefore aimed to clarify the mechanisms of the backward reaction from Na₃AlH₆ to NaAlH₄ under the presence of AlH₃ compound and to identify the role of dopant Ti in the process of this reaction. First-Principles calculations were performed on the NaAlH₄, Na₃AlH₆ and AlH₃ with and without Ti systems. It was shown that in the Na₃AlH₆+Ti system Ti could shuffle its neighbouring H atoms, distores the [AlH₆] group, reduces the H dissociation energy from Na₃AlH₆ and may produce the AlH₃ compound, and therefore accelerates the decomposition of Na₃AlH₆. In contrast, a limited effect was found if Ti was doped into AlH₃.Based on the above researches, we further investigated the effects of Ti on the reaction of Na₃AlH₆+2AlH₃→3NaAlH₄, one of the possible pathways of the backward from Na₃AlH₆ to NaAlH₄. The main conclusions are that Ti could induce Ti-Al-H complex in this system, reduces the dissociation energy barrier of H atom from Na₃AlH₆, promotes the transition from [AlH₆] group to [AlH₄] group, and decreases to overall reaction enthalpy. Therefore the mechanism that Ti catalysis the reverse reaction from Na₃AlH₆ to NaAlH₄ is that Ti destores the stability of the [AlH₆] group amd accelerates the transition from [AlH₆] group to [AlH₄] group.