ABSTRACT
Practical applications of clusters depend on their stability, and this increases on going from free to supported and passivated clusters. On the other hand, the number of low-lying excited electronic states, which are responsible for the properties unique to metals (conductivity, magnetism, versatile chemistry and catalytic activity, etc.) decreases in the order free, supported, passivated. erefore, applications that depend speci cally on metallic properties of small clusters (e.g., bimetallic catalysts) present special challenges. In molecular orbital (MO) theory, metallic properties are associated with a zero (or very small) highest occupied MO-lowest unoccupied MO (HOMO-LUMO) gap in orbital energies. In su ciently large passivated metal clusters, interior atoms do not “see” the passivating ligands, they have bulk-like electronic states with a nearly zero HOMO-LUMO gap, and have metallic properties. But the atoms at the metalligand interface have a local density of electronic states that
4.1 Introduction .............................................................................................................................4-1 4.2 Types of Bimetallic Clusters and How ey Are Made .....................................................4-2 4.3 Applications ..............................................................................................................................4-3 4.4 Free Metal Clusters ..................................................................................................................4-4
Dispersion, Surface Energy • Size Evolution of Properties • Magic Clusters 4.5 Geometric Structure of Bimetallic Clusters ........................................................................4-7 4.6 Properties of Bimetallic Clusters ...........................................................................................4-8 4.7 Structure-Energy Principles for Bimetallic Clusters .........................................................4-8 4.8 Special Bimetallic Clusters .....................................................................................................4-9 4.9 Summary and Conclusion ....................................................................................................4-10 Acknowledgments .............................................................................................................................4-10 References ...........................................................................................................................................4-10
resembles a semiconductor. In small passivated metal clusters, the HOMO-LUMO gap is large not only for surface atoms, but also interior atoms [3,4]. In that sense, small passivated metal clusters are not metallic.
