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X-ray absorption spectroscopy was used to characterize the metal-support interface in catalysts consistingof very small Ir clusters of nearly uniform nuclearity on the surface of MgO powder. [Ir4(CO)12] on MgO was convertedin high yield into [HIr4(CO)11]- and separately into [Ir6(CO)15l2-. Extended X-ray absorption fine structure (EXAFS)data are consistent with the hypothesis that each cluster retained its nuclearity upon decarbonylation. The averageIr-Ir coordination in Ir,/MgO is 2.6 neighbors at a distance of 2.72 A, and the structure is modeled as a mixture of40-50% tetrahedra and the remainder four-atom rafts on MgO. The average Ir-Ir coordination in Ir6/MgO is 2.7neighbors at a distance of 2.12 A, and the structure is modeled as a mixture of about 25% Ir6 octahedra and theremainder Ir rafts on MgO. Another sample was treated in H2 at 623 K to give a typical supported metal catalysthaving a distribution of Ir clusters averaging 10 8, in diameter. The Ir-support interfaces in Ir4/MgO and Irb/MgOare characterized by single Ir-Mg coordinations at 1.6 A and fourfold Ir-O coordinations at 2.6 A; the interface Itatoms are suggested to be atop Mg ions in the (100) faces of MgO. However, this structural model is simplified, andthe Ir-Mg interactions are not fully understood. The results demonstrate that the structurally simple supported Irclusters are among the simplest supported metals; the metal-oxygen distances of approximately 2.6 Aare quite generallycharacteristic of metal particles and clusters supported on metal oxides and zeolites; the long distance is attributed tometal atoms interacting with oxygen of the support with hydrogen present at the interface.

journal of the american chemical society

Structure and nature of the metal-support interface: characterization of iridium clusters on magnesium oxide by extended x-ray absorption fine structure spectroscopy