Morphology of supported metal clusters: Determination by EXAFS and chemisorption

Abstract

When small metal clusters are examined by EXAFS the apparent average coordination number is smaller than that observed in the bulk metal because of the high proportion of surface atoms. This effect is dependent on the size and shape of the metal cluster. Geometrical shape models have been derived for spheres, cubes, and disks which give the EXAFS average coordination number for first, second, and third coordination spheres as a function of cluster size. Dispersion models (via chemisorption measurements) are also presented for different cluster shapes and sizes. For a correct choice of shape these two types of observations should predict the same cluster size. Results for the models are presented with experimental data for a variety of supported catalysts. Electron microscopy is used to support the analyses where possible.