Density functional theory analysis of Ti-catalyzed hydrogen storage reaction on surfaces and nanoparticles of NaAlH₄ and its intermediates

NaAlH₄ doped with ∼ 2% titanium is a promising hydrogen storage material. Density Functional Theory was used to understand the multi-step hydrogen absorption-desorption cycle. The critical issue that remains unanswered from the experimental results is how Ti catalyzes hydrogen storage. Different existing experimental results were tested on two model surfaces: one, by replacing sodium by titanium on the NaH surface and the other, by forming a surface Ti-Al alloy. Large-scale computations using the RPBE functional were performed to follow the hydrogen loading reaction starting from the depleted material (NaH and Al) to the fully hydrogen enriched NaAlH₄. NaH {001} surface doped with Ti promoted exothermic dissociative absorption of molecular hydrogen, explaining the reported use of nanometric NaH doped with Ti as hydrogenation catalyst. Ways of removing the kinetic bottlenecks in the intermediate perovskite phase (Na₃AlH₆) are also suggested to facilitate faster cycling of hydrogen. This is an abstract of a paper presented at the 228th ACS National Meeting (Philadelphia, PA, 8/22-26/2004).