High Catalytic Activities of RENi_5-xAl_x(RE = La, Er) and Low Activity of Mg₂Ni following Hydrogen Uptake: The Role of Absorbed Hydrogen

Catalysis by intermetallic compounds (IMCs) has recently received significant attention, and IMCs that are also capable of storing hydrogen, including RENi_5-xAl_x(RE = La, Er) and Mg₂Ni, have been employed as catalysts for the hydrogenation of ethylene. RENi_5-xAl_xwith absorbed hydrogen exhibits improved catalytic activity and high levels of ethylene conversion. The reaction rate for RENi_5-xAl_xwith absorbed hydrogen is higher than those for pure Ni and Cu and close to that for Pd. Pulsed gas tests have demonstrated that the presence of absorbed hydrogen is essential for high catalytic activity. In this work, the role of absorbed hydrogen in catalytic reactions was assessed by performing the trial using deuterium. It was found that absorbed deuterium in ErNi_3.75Al_1.25D_nundergoes very little reaction with ethylene. A novel activation mechanism which is different from the reaction of absorbed hydrogen directly with C₂H₄was proposed. This mechanism was investigated based on adsorption experiments and density functional theory calculations of electronic states. It was concluded that weakly adsorbed ethylene and hydrogen are more reactive on the hydride surface than on the matrix surface with no absorbed hydrogen, leading to an increase in catalytic activity. In contrast, the hydride of Mg₂Ni, Mg₂NiH₄, showed decreased activity toward the hydrogenation of ethylene, representing the direct opposite behavior to that for RENi_5-xAl_xalloys. We considered the reason for decline in catalytic activity of Mg₂NiH₄from the electronic states.