Reaction stoichiometry between TiCl3 and NaAlH4 in Ti-doped alanate for hydrogen storage: The fate of the titanium species

In order to understand the final state of the TiCl3 dopant during the dehydrogenation and rehydrogenation cycles of NaAlH4, we determined the reaction stoichiometry between TiCl3 and NaAlH4 by measuring the amount of hydrogen evolution from NaAlH4 with the varying TiCl3 -load. We found that: (i) TiCl3 reacted with 3 M equivalents of NaAlH4 during the doping process of ball-milling, (ii) the Ti dopant continued to react with NaAlH4 during the first dehydrogenation process until total six equivalents of NaAlH4 were consumed, and (iii) Ti fixed Al, not NaH, so that Al became insufficient during the rehydrogenation process. These findings lead to the conclusion that the reaction stoichiometry between Ti and Al is 1:6, which probably yields TiAl6 and plays a catalytic role in the hydrogen storage reactions of Ti-doped NaAlH4.