Abstract
Abstract Organometallic chemistry and heterogeneous catalysis are developing into common territory, illustrated by catalysts incorporating structures ranging from mononuclear metal complexes to metal aggregates (crystallites) on metal oxide supports. Reactions of precursors such as metal carbonyls and metal allyls with metal oxide surfaces have been understood on the basis of solution organometallic chemistry combined with functional group chemistry of the supports. The supported species are transformed by processes including reductive carbonylation (whereby metal complexes and salts are converted into metal clusters) and oxidative fragmentation (whereby metal clusters and aggregates are converted into mononuclear metal complexes). Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used in conjunction with other techniques to characterize bonds between mononuclear metal complexes and metal oxide surfaces and also to provide structural information about metal-support interfaces in highly dispersed supported metal aggregates. EXAFS results are also helping to define the limitations of standard methods for estimating surface areas of highly dispersed metals by measurements of chemisorption of H 2 and CO. Some supported ‘molecular’ organometallics are structurally and catalytically novel, indicating possibilities for design of surface catalytic sites. Ensembles consisting of three Re complexes on MgO are active for the structure-sensitive cyclopropane hydrogenolysis, whereas a catalyst consisting of the same complexes (evidently isolated on the support surface) is inactive. ‘Molecular’ osmium carbonyl clusters stabilized on the basic MgO surface and in basic zeolites catalyze CO hydrogenation, and 10-atom aggregates of osmium, formed by removal of CO ligands from a ‘molecular’ precursor, are active for n-butane hydrogenolysis. These structurally well-defined catalysts offer good opportunities for understanding metal-support interactions and structure sensitivity in metal catalysis.
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