that the average composition of the first coordination shell of atoms (nearest neighbors) surrounding a platinum atom is different from that surrounding an iridium atom. The catalyst appears to exhibit platinum-rich and iridium-rich regions.
One might visualize a distribution of metal clusters with different compositions, some of which are platinum-rich and others of which are iridium-rich. Both the platinum-rich and iridium-rich clusters would contain substantial amounts of the minor component on the basis of the distances derived from the EXAFS data. Alternatively, one can visualize platinum-rich and iridium-rich regions within a given metal cluster. This possibility seems reasonable on the basis of surface energy considerations. According to this view, the platinum-rich region would be present at the surface, since platinum would be expected to have a lower surface energy than iridium. In support of this expectation, recent work on platinum-iridium films indicates that platinum concentrates in the surface.46
When the ratio of surface atoms to total atoms is equal to 0.5 for clusters containing 50 percent each of platinum and iridium, one can visualize a situation in which essentially all of the platinum is present in the surface and all the iridium in the interior. There would then be a close resemblance to the ruthenium-copper clusters considered earlier. When the ratio of surface atoms to total atoms approaches unity, the notion of complete or nearly