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. ► Transition metal halides show different reactivity during high-energy ball milling with NaAlH4. ► ScCl3 was the most active dopant in all hydrogen desorption and reabsorption steps. ► Co-doping did not show any advantage for the systems investigated. ► Influence of the temperature was significantly stronger for desorption than for absorption. ► The hydrogen back pressure significantly influences the 2nd desorption step.