Mg-based hydrogen storage materials: Surface segregation in Mg2Cu and related catalytic effects

Fresh surfaces of Mg₂Cu polycrystals cleaved in UHV have been investigated by AES, XPS and X-ray induced AES at room temperature. They show chemical decomposition of the surface leading to pronounced increase of surface Mg content. In contrast to the Mg the Cu remains essentially metallic, even on air exposed samples. It is suggested that this segregation prevents the formation of a compact oxide or hydroxide layer, thereby enabling dissociation of molecular hydrogen at the metallic Cu precipitations and/or the metallic Mg₂Cu subsurface. The results support comparable conclusions drawn earlier for LaNi₅, FeTi and Mg₂Ni.