MgO-Supported Tetrairidium Clusters: Evidence of the Metal−Support Interface Structure from X-ray Absorption Spectroscopy

Abstract

Iridium clusters were prepared by chemisorption of [Ir4(CO)12] on MgO powder supports that had been calcined at 300, 500, and 700 °C to vary the degree of hydroxylation. The initially adsorbed species, identified as tetrairidium carbonyl clusters by infrared and Raman spectroscopies, were decarbonylated by treatment in He at 300 °C and then treated in H2 at 300 °C. The decarbonylated clusters at each stage were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge spectroscopy (XANES). After treatment in He, the clusters on each support nearly retained the tetrahedral metal frame of the [Ir4(CO)12] precursor. After treatment in H2, the clusters on MgO that had been calcined at 500 and at 700 °C still nearly retained this frame, whereas the clusters on MgO calcined at 300 °C underwent slight agglomeration. Ir−O interactions in the decarbonylated samples were indicated by the EXAFS data. In the family of samples treated in He but not H2, the Ir−O bonding distance (approximately 2.1 Å) decreased as the degree of dehydroxylation of the support increased. This trend, associated with an increasing loss of electron density from the iridium clusters, was also evidenced by an increase in the Ir white line area as the support dehydroxylation increased. Another Ir−O EXAFS contribution, near 2.6−2.7 Å, is attributed to a nonbonding interaction influenced by cluster and support geometry. The Ir−O bonding distances increased after treatment of the samples in H2, the increase being greatest for the MgO support that had been treated at 700 °C, from 2.07 to 2.15 Å. The Ir4/MgO clusters are among the most nearly uniform, stable, and well-characterized supported metal clusters.