The chemisorption and catalytic properties of nickel intermetallic compounds: Studies of single crystalline and high surface area, supported materials

We summarize here our detailed studies on the complex chemisorption properties of the single crystalline (111) surface of nickel disilicide (NiSi₂). 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 the bulk metal. 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, such as the competitive dehydrogenation and hydrogenolysis of cyclohexane, the effects of intermetallic compound formation on catalytic activity are described. Of particular interest is the finding that the dramatic increase in the selectivity of Group VIII intermetallic compounds for activating C-H versus C-C bonds parallel a corresponding decrease in their activity for dissociatively chemisorbing hydrogen.