Metal Clusters on Oxides
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Metal clusters on oxides play a profound role in modern-day society because such systems are the basic ingredients in catalysts used for environmental protection, refining of fossil fuels, and production of chemicals. In the automotive three-way catalytic converter, for example, chemical reactions take place on nanometer-scale metal particles of Rh, Pt, and/or Pd distributed on an oxide of a high surface area, typically alumina.
Catalysis has thus been the main motivation for research on metal clusters on oxides. The improved ability to control the size and distribution of metal clusters coupled with the development of techniques for better characterization of the structural, electronic and chemical properties of metal clusters on oxides (primarily scanning probe techniques) has spurred an increased research activity in the field. Consequently, a series of interesting new results have been obtained, which have demonstrated how, e.g., the properties of the metal clusters may change in the nanometer-scale regime due to quantum size effects.
Atomic-scale investigations of industrial catalysts are hampered by the complexity of the systems. In this article emphasis will therefore be on metal clusters on oxides prepared and investigated in laboratory research environments, i.e., under ultrahigh vacuum (UHV) conditions. The advantage of such model systems is that they lend themselves much more readily to investigations based on a large variety of surface science tools. The article is intended to give the reader an overview of the techniques for synthesis and characterization of model systems, as well as to address important issues that are relevant for metal clusters on oxides in general. For further reading, reference is given to a number of reviews on metals on oxides.