Influence of transition metal dopants and temperature on the dehydrogenation and rehydrogenation kinetics of NaAlH_4

摘要

NaAlH_4 has been doped with ScCl_3, TiCl_3, CeCl_3, 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 scan-ning electron microscopy. The effect of the additives and their combination on the two dehydrogenation and rehydrogenation steps of suchlike doped NaAlH_4 has been studied under isothermal and isobaric conditions by thermogravimetry under a H_2 back pressure of 1 bar. From these studies it turned out that ScCl_3-doped NaAlH_4 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 dehydro-genation and rehydrogenation step and the dehydrogenation kinetics in dependence on the H_2 back pressure were studied in detail.