Kinetic study and determination of the enthalpies of activation of the dehydrogenation of titanium- and zirconium-doped NaAlH4 and Na3AlH6

The rates of the dehydrogenation of the sodium alanates NaAlH4 and Na3AlH6 doped with 2 mol % Ti or Zr have been measured over the temperature range 363-423 K. NaAlH4 and Na3AlH6 undergo dehydrogenation at equal rates upon direct doping with titanium. However, Na3AlH6 arising from the dehydrogenation of Ti-doped NaAlH4 undergoes dehydrogenation at much slower rates. Rate constants were determined from the slopes of the dehydrogenation profiles. On the basis of Eyring theory, the enthalpies of activation, DeltaH(double dagger), for the dehydrogenation reactions were determined to be similar to100 kJ(.)mol(-1) for both Ti-doped NaAlH4 and Na3AlH6 and similar to135 kJ(.)mol(-1) for both Zr-doped NaAlH4 and Na3AlH6. These results suggest that the dehydrogenation reaction pathways are highly sensitive to the nature and distribution of the dopant but not to differences in the AI-H bonding interactions in [AlH4](-) and [AlH6](3-) complex anions. Furthermore, we conclude that the kinetics are probably influenced by processes such as nucleation and growth and/or range atomic transport phenomenon.