Influence of transition metal dopants and temperature on the dehydrogenation and rehydrogenation kinetics of NaAlH₄

NaAlH4 has been doped with ScCl3, TiCl3, CeCl3, and combinations of these additives by high-energy ball milling. The phase composition of the samples after milling was analyzed by X-ray diffraction. Microstructure and catalyst distribution were investigated by scanning electron microscopy. The effect of the additives and their combination on the two dehydrogenation and rehydrogenation steps of suchlike doped NaAlH4 has been studied under isothermal and isobaric conditions by thermogravimetry under a H2 back pressure of 1 bar. From these studies it turned out that ScCl3-doped NaAlH4 was superior to all other dopants and combinations investigated, both for dehydrogenation and rehydrogenation. For this dopant, the influence of the temperature on the kinetics of each single dehydrogenation and rehydrogenation step and the dehydrogenation kinetics in dependence on the H2 back pressure were studied in detail.