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要旨:
Models of supported metal catalysts have been prepared by deposition of transition metal vapor onto thin, well-ordered oxide layers, particularly alumina. The average sizes of the metal particles and their size distributions are determined by nucleation and growth. Here, we review the morphology and structure of the oxide layers and the metal particles dispersed on them, as measured with a variety of methods including low-energy electron diffraction and scanning tunneling microscopy. Electronic and magnetic structure as a function of particle size, adsorption properties, as well as reactivity with varying particle size are reviewed. The electronic structure of supported palladium particles has been studied and indicates that a nonmetal to metal transition occurs for particles exceeding 70–80 atoms per aggregate. Adsorption of CO has been studied in detail by Fourier transform infrared spectroscopy. Interesting variations in the spectra when very small particles consisting of only a few metal atoms are investigated compared with larger particles and single-crystal surfaces are observed. CO dissociation has been studied on rhodium aggregates and a maximal dissociation rate has been found for aggregates containing several hundred metal atoms. The presence of defects on the particles is deduced to be origin of this behavior.
