Catalytic influence of Ni-based additives on the dehydrogenation properties of ball milled MgH2

The catalytic influence of Ni, Zr2Ni5, and LaNi5 on the dehydrogenation properties of milled MgH2 was investigated. MgH2 milled in the presence of Ni (5 wt%) and Zr2Ni5 (5 wt%) catalysts for 2 h showed apparent activation energies, E-A, of 81 and 79 kJ/mol, respectively, corresponding to similar to 50% decrease in E-A and a moderate decrease (similar to 100 degrees C) in the decomposition temperature (T-dec). A further 27 degrees C decrease in T-dec was observed after milling with 10 wt% Ni. Based on the E-A values, the catalytic activity decreased in the following order: Ni approximate to Zr2Ni5 > LaNi5. X-ray photoelectron spectroscopy analysis of the milled and dehydrogenated states of the hydrides modified with Ni catalyst revealed that the observed reduction in E-A may be due to the ability of Ni catalyst to decrease the amount of oxygen atoms in defective positions that are capable of blocking catalytically active sites thereby enhancing the dehydrogenation kinetics. In particular, our results reveal a strong correlation between the type of oxygen species adsorbed on Ni-modified MgH2 and the E-A of the dehydrogenation reaction.