Abstract
Despite many experimental and theoretical studies, the origin of the catalytic role of the Ti catalyst in the reversible dehydrogenation of NaAlH4 has been a controversial topic for many years. In this paper, density functional theory and phase equilibrium calculations are used to study the surface defect stabilities of NaAlH4 during dehydrogenation and rehydrogenation cycles. A Ti−H−Al cluster resulting from the Ti substitution of Al at the (001) subsurface layer of NaAlH4 is identified as the most stable surface defect. An interstitial H in this cluster enhances the Ti−Al bonds and weakens the Al−H bonds simultaneously. The large difference in the formation enthalpy of this cluster under hydrogen-poor and hydrogen-rich conditions indicates that the concentration of these Ti−H−Al clusters on the NaAlH4 (001) surface can be mediated by the hydrogenation and dehydrogenation cycles, which is responsible for the reversible catalytic behavior of the Ti catalyst.
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