STRUCTURE AND NATURE OF THE METAL-SUPPORT INTERFACE - CHARACTERIZATION OF IRIDIUM CLUSTERS ON MAGNESIUM-OXIDE BY EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY

X-ray absorption spectroscopy was used to characterize the metal-support interface in catalysts consisting of very small Ir clusters of nearly uniform nuclearity on the surface of MgO powder. [Ir4(CO)12] on MgO was converted in high yield into [HIr4(CO)11]- and separately into [Ir6(CO)15]2-. Extended X-ray absorption fine structure (EXAFS) data are consistent with the hypothesis that each cluster retained its nuclearity upon decarbonylation. The average Ir-Ir coordination in Ir4/MgO is 2.6 neighbors at a distance of 2.72 angstrom, and the structure is modeled as a mixture of 40-50% tetrahedra and the remainder four-atom rafts on MgO. The average Ir-Ir coordination in Ir6/MgO is 2.7 neighbors at a distance of 2.72 angstrom, and the structure is modeled as a mixture of about 25% Ir6 octahedra and the remainder Ir rafts on MgO. Another sample was treated in H-2 at 623 K to give a typical supported metal catalyst having a distribution of Ir clusters averaging 10 angstrom in diameter. The Ir-support interfaces in Ir4/MgO and Ir6/MgO are characterized by single Ir-Mg coordinations at 1.6 angstrom and fourfold Ir-O coordinations at 2.6 angstrom; the interface Ir atoms are suggested to be atop Mg ions in the (100) faces of MgO. However, this structural model is simplified, and the Ir-Mg interactions are not fully understood. The results demonstrate that the structurally simple supported Ir clusters are among the simplest supported metals; the metal-oxygen distances of approximately 2.6 angstrom are quite generally characteristic of metal particles and clusters supported on metal oxides and zeolites; the long distance is attributed to metal atoms interacting with oxygen of the support with hydrogen present at the interface.