Abstract
The formation of MgH 2 directly from magnesium and molecular hydrogen is extremely slow; however, reasonable hydrogenation kinetics have been reported in compounds and alloys of magnesium with nickel, copper and rare earth metals. Using X-ray photoemission spectroscopy and Auger electron spectros-copy, we analysed the chemical states and the concentrations of magnesium, nickel and copper at the surface of the eutectic alloy MgMg 2Ni and of the intermetallic compounds Mg 2Ni and Mg 2Cu. Air-exposed samples are greatly magnesium enriched on their surface, the magnesium being oxidized. Surfaces cleaned in ultrahigh vacuum show segregation of magnesium under the influence of oxygen even at room temperature. The segregated magnesium oxidizes and keeps the nickel and copper in the metallic state. Molecular hydrogen can be dissociated at the metallic nickel or copper particles or at the metallic subsurface. The catalytic activity of Mg 2Ni and Mg 2Cu in the hydrogenation of magnesium observed by Reilly and Wiswall is therefore due to a surface decomposition which continues the supply of atomic hydrogen.
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