Hydrogen-related catalytic effects of Ti and other light transition metals on NaAlH(4) surfaces.

We report a first-principles study of sodium alanate (NaAlH(4)) surfaces, both pure and with Ti atoms on them, focusing on their stability and reactivity with hydrogen. We find Ti causes the dissociation of H(2) molecules and contributes to the stability of the surface exposed to hydrogen. The Ti catalysts should thus facilitate the reaction by which NaAlH(4) (7.4 wt% of H) forms from Na(3)AlH(6) (5.9 wt% of H), which occurs under hydrogen pressure and requires H(2) dissociation to proceed. The presence of Ti also results in lower defect-formation energies, which should favour the NaAlH(4) decomposition reaction. Indeed, our calculations show that the chemical versatility of Ti brings close in energy a number of steps that presumably are relevant in the formation and decomposition of NaAlH(4), which suggests that the catalyst facilitates both processes. We also discuss the possibility of using other light transition metals (Sc, V, and Cr) as catalysts, and conclude that Ti is the best choice overall.