Chemisorption and Catalysis by Materials formed at the Metal-Semiconductor Interface: The Unique Properties of Nickel Silicides

In this talk we describe a detailed UHV study using high resolution EELS, XPS and TPD, which explores the complex properties of single crystalline nickel disilicide (NiSi2) thin films. This material is characterized by an unusual structure in which coordinately unsaturated nickel surface atoms are bonded only to silicon nearest neighbors. These studies clearly establish that the surface chemistry of nickel in NiSi2 differs substantially from that observed on the surface of bulk nickel. The trends exhibited by these differences are particularly interesting in that they simultaneously represent both a decreased and an increased reactivity of nickel. These unique chemisorption properties prompted our development of a general procedure for preparing supported, high-surface-area intermetallic materials. Using model hydrocarbon reactions, the effects of intermetallic compound formation on catalytic activity will be described.. Of particular interest is the finding that the dramatic increase in the selectivity of Group VIII intermetallic compounds for activating C-H vs. C-C bonds parallels a corresponding decrease in their activity for dissociatively chemisorbing hydrogen. In the case of nickel- silicon intermetallic compounds this selectivity can also be correlated with subtle changes in the relative surface concentration of nickel and silicon.