NATURE OF THE METAL SUPPORT INTERFACE IN SUPPORTED METAL-CATALYSTS - RESULTS FROM X-RAY ABSORPTION-SPECTROSCOPY

X-ray absorption spectra characterizing the metal-support interface in supported metal complexes and supported metal catalysts are summarized and evaluated. Single-metal-atom transition metal complexes on non-reducible metal oxide supports are bonded with metal-oxygen bonds with metal-oxygen distances of approximately 2.15 angstrom; the bonding distance is only weakly sensitive to thc oxidation state of thc metal. Nearly this same metal-oxygen distance is characteristic of the metal-support interface in metal-oxide-supported metal clusters following high temperature reduction in H-2 (HTR: T > 450-degrees-C The metals at the interface may be polarized sufficiently that they bond with thc oxygen of the support much as the cations in mononuclear complexes bond with it. When thc supported metals are treated in H-2 at low temperatures (LTR: T < 350-degrees-C) or are prepared under He with partially hydroxylated supports, it longer metal-support oxygen distance is observed, typically 2.5-2.7 angstrom. This distance is suggested to characterize interactions between zero-valent metals and support oxygen. Changes in the performance of supported metal catalysts resulting from differences in the temperature of pretreatment in H-2, are attributed to changes in the electronic properties and/or morphology of thc metal clusters, which arc suggested to be related to the concomitant changes in the structure of the metal-support interface.