High Catalytic Activities of RENi5–xAlx (RE = La, Er) and Low Activity of Mg2Ni Following Hydrogen Uptake: The Role of Absorbed Hydrogen

Abstract

Catalysis by intermetallic compounds (IMCs) has recently received significant attention, and IMCs that are also capable of storing hydrogen, including RENi5–xAlx (RE = La, Er) and Mg2Ni, have been employed as catalysts for the hydrogenation of ethylene. RENi5–xAlx with absorbed hydrogen exhibits improved catalytic activity and high levels of ethylene conversion. The reaction rate for RENi5–xAlx with 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 ErNi3.75Al1.25Dn undergoes very little reaction with ethylene. A novel activation mechanism which is different from the reaction of absorbed hydrogen directly with C2H4 was 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 RENi5–xAlx alloys. We considered the reason for decline in catalytic activity of Mg2NiH4 from the electronic states. The reason can be the low ability to break multiple bonds in hydrocarbons or single bonds in hydrogen molecules due to a lack of an electron supply from the non-metallic Mg2NiH4.