Date of Award

1-1-2009

Degree Name

Master of Science

Department

Chemistry

First Advisor

Ge, Qingfeng

Abstract

In this thesis, two types of complex hydrides doped with transition metals as potential hydrogen storage materials were investigated using density functional theory (DFT) calculations. TiAl3 cluster as well as its interaction with hydrogen was characterized. Our calculation showed that Ti atom and three Al atoms form a tetrahedron in the most stable structure of TiAl3Hx for x=0~8. Starting from TiAl3H9, the Ti atom and three Al atoms form a planar structure. Our results also showed that a TiAl3 cluster can hold up to twelve hydrogen atoms, corresponding to dissociation of six hydrogen molecules. Further adding hydrogen to the clusters beyond TiAl3H12, hydrogen becomes molecularly adsorbed. Moreover, the formation energy, the hydrogen removal energy, HOMO-LUMO gap, and electron affinity of TiAl3Hx clusters were also investigated. The formation energy of TiAl3Hx varies almost linearly up to x = 8. Beyond 8, the energy gain with increasing x slows down. Correspondingly, the hydrogen removal energy decreases, indicating smaller energy cost for hydrogen release. The exploration of electronic charge on hydrogen activation indicates that both tetrahedral and planar TiAl3Hx- cluster can break hydrogen molecular bond. Combing with the previous analysis of hydrogen desorption from Ti-doped NaAlH4, we demonstrated that TiAl3Hx plays an important role in hydrogenation and dehydrogenation of Ti-doped NaAlH4 system. Hydrogen adsorption on Pt4, Ni4, and Pd4 doped Li3N were also studied. Since the transition metals play the catalytic role to dissociate H2, with the higher diffusivity and activity of hydrogen atom to spillover from metal sites to Li3N, the storage capacity and kinetic can be improved. Our calculation results showed that Pd is a better catalyst for hydrogen spillover than Pt and Ni. The calculated barrier for the diffusion of hydrogen from Pd4 to the N site is 25kJ/mol. It is in good agreement with the value from experimental analysis. These results indicated that the kinetic barrier for the hydrogenation process of Pd-doped Li3N is located at the migration of hydrogen from Pd to N sites. This work suggests that catalytic metals doped in hydrogen storing complex metals can ease the H-H bond cleavage, and the spillover of the resulting hydrogen can lead to the hydrogenation of complex metals via a kinetically more favorable pathway.

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