1 Risø National Laboratory for Sustainable Energy, Technical University of Denmark2 Department of Energy Conversion and Storage, Technical University of Denmark3 Atomic scale modelling and materials, Department of Energy Conversion and Storage, Technical University of Denmark4 Department of Physics, Technical University of Denmark5 Theoretical Atomic-scale Physics, Department of Physics, Technical University of Denmark6 Imaging and Structural Analysis, Department of Energy Conversion and Storage, Technical University of Denmark7 Department of Chemistry, Technical University of Denmark8 Ceramic Engineering & Science, Department of Energy Conversion and Storage, Technical University of Denmark9 Aarhus University10 unknown11 Department of Chemistry, Technical University of Denmark12 Haldor Topsoe AS13 Haldor Topsoe AS
The dehydrogenation kinetics of pure and nickel (Ni)-doped (2w/w%) magnesium hydride (MgH2) have been investigated by in situ time-resolved powder X-ray diffraction (PXD). Deactivated samples, i.e. air exposed, are investigated in order to focus on the effect of magnesium oxide (MgO) surface layers, which might be unavoidable for magnesium (Mg)-based storage media for mobile applications. A curved position-sensitive detector covering 120 degrees in 20 and a rotating anode X-ray source provide a time resolution of 45 s and up to 90 powder pattems collected during an experiment under isothermal conditions. A quartz capillary cell allowed the in situ study of gas/solid reactions. Three phases were identified: Mg, MgH2 and MgO and their phase fractions were extracted by Rietveld refinement or integration of selected reflections from each phase. Dehydrogenation curves were constructed and analysed by the Johnson-Mehi-Avrami formalism in order to derive rate constants at different temperatures. The apparent activation energies for dehydrogenation of pure and Ni-doped magnesium hydride were E-A approximate to 300 and 250 kJ/mol, respectively. Differential scanning calorimetry gave, E-A = 270 kJ/mol for dehydrogenation of the Ni-doped sample. The relatively high activation energies are due to MgO surface layers, retarding the diffusion of hydrogen (H-2) out of MgH2/Mg. The observed difference in E-A of ca. 50 kJ/moI is likely due to the catalytic effect of Ni on the recombination of H atoms to H-2 molecules verified by theoretical considerations.
International Journal of Hydrogen Energy, 2006, Vol 31, Issue 14, p. 2052-2062