Supported Clusters, Structure, Reactivity and Microscopic Processes in Catalysis

Abstract

Organometallic cluster compounds with metal frameworks containing more than three metal atoms are akin to metal particles and stepped metal surfaces having neibouring multimetallic centers. These metal centers are chemically accomodated by appropriate ligand molecules such as carbonyls, phosphines, aryls and olefins. The similarity between very small metal particles and molecular clusters was firsly proposed by Muetterties(1) and Chini(2) on the basis of the chemical behavior of lignads such as CO coordinated to a cluster framework and chemisorption onto the metal surface. The adjacent metal sites in polynuclear metal clusters make available multi-metallic coordination environments(e.g., face, edge and kink) that can not be realized at a single metal atom or ionic site typical of most homogeneous metal complex catalysts. Transition metal cluster complexes are ideal objects for the study of collective behavior in stoichiometric and catalytic reactions. In this sense, metal clusters occupy an intermediate position between molecular metal complexes(homogeneous catalysts) and bulky metals such as films and crystals (classic heterogeneous catalysts) in catalysis. Heterogeneous catalysts which involve colloidal, crystalline, and supported metal particles have the advantage of being readily separable from the products, which gives a practical advantage over homogeneous metal complex catalysts even with a high selectivity and uniformity. Catalytically active metal/alloy particles of several nanometer(nm) diameters are usually dispersed on supporting materials such as metal oxides, sulfides or carbon having large surface area to prevent further metal agglomeration (“sintering”). Several examples of supported multi-metal catalysts used in industrial proccesses are Fe/K-Al2O3 for ammonia synthesis, Cu/ZnO for methanol synthesis, Ag/Re-Al2O3 for selective epoxidation of ethylene, and Pt/Re(or Ru/Cu)Al2O3 which is used for naphtha reforming. The activity and selectivity of supported metal catalysts generally depend on the state of metal dispersion(ensemble sizes), structure(shape and morphology), metal composition(geometric distribution of multi metals), and metal-support(metal-oxide/sulfide) interactions.