Catalytic influence of Ni-based additives on the dehydrogenation properties of ball milled MgH_2

摘要

The catalytic influence of Ni, Zr_2Ni_5, and LaNi_5 on the dehydrogenation properties of milled MgH_2 was investigated. MgH_2 milled in the presence of Ni (5 wt%) and Zr_25Ni_5 (5 wt%) catalysts for 2 h showed apparent activation energies, E_A, of 81 and 79 kJ/mol, respectively, corresponding to -50% decrease in E_A and a moderate decrease (～100 ℃) in the decomposition temperature (T_(dec)). A further 27 ℃ 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 ≈ Zr_2Ni_5 > LaNi_5. 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 MgH_2 and the E_A of the dehydrogenation reaction.