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Hydrogen Interaction in Ti-Doped LiBH4 for Hydrogen Storage: A Density Functional Analysis.

Density functional theory studies have been carried out to investigate the energetics and structures of Ti-doped LiBH4 (001), (100), and (010) surfaces. The possibilities of Ti occupying various positions at these surfaces leading to substitution, surface adsorption, and interstitial insertion are examined. Among all possible structures, a Ti atom energetically prefers to occupy interstitial positions involving three or four BH4(-) hydrides and to stay above the top layer of B atoms. The most stable species on the three surfaces have a similar local structure, described as TiB2H8-nBH4 (n = 1, 2), with varying spin states. Molecular orbital analysis for the local structures showed that the structural stability could be attributed to the symmetry-adapted orbital overlap between Ti and "inside" B-H bonds. Furthermore, the hydrogen desorption energies from many positions in these local complex structures were reduced significantly with respect to that from the clean surface. The most favorable hydrogen desorption pathways are found to lead to triplet dehydrogenation products. Therefore, the triplet TiB2H8-BH4 in (001) and TiB2H8-2BH4 in (010) can desorb hydrogen in molecular form, while the quintet TiB2H8-BH4 in (100) must first desorb hydrogen atoms, followed by the formation of a hydrogen molecule in the gas phase. The catalytic effect of Ti doped in LiBH4 has been compared with that in NaAlH4.