Structure of bimetallic clusters. Extended x-ray absorption fine structure (EXAFS) studies of Pt–Ir clusters

Abstract

The extended x‐ray absorption fine structures (EXAFS) associated with the L_III absorption edges of platinum and iridium were investigated for several catalysts consisting of metal clusters of these elements dispersed on silica or alumina. Previous x‐ray diffraction studies on silica supported platinum–iridium catalysts containing 5%–20% by weight of platinum plus iridium indicated that these elements were present in the form of bimetallic clusters with sizes in the approximate range of 25–50 Å. The present EXAFS investigation has provided more detailed structural information on such clusters and has extended the scope of the research to include more highly dispersed platinum–iridium catalysts which cannot be investigated satisfactorily by x‐ray diffraction. In this way, it has been possible to extend the structural evidence for platinum–iridium bimetallic clusters to the case in which the ratio of surface atoms to total atoms approaches unity.