Abstract
Abstract Highly dispersed metal catalysts containing supported clusters of only several metal atoms each, exemplified by Ir 4 and Ir 6 , were prepared by removal of CO ligands from supported precursors, for example, [Ir 4 (CO) 12 ] and [Ir 6 (CO) 16 ]. Transmission electron microscopy (TEM), extended X-ray absorption fine structure spectroscopy and density functional theory indicate the metal–support-oxygen coordination numbers and distances, which identify the supports as multidentate oxygen-donor ligands. Theory indicates that Ir 4 clusters in zeolite NaX are neutral or slightly negatively charged and that cluster-support bonding induces a polarization of the cluster that could affect reactivity and catalysis. Supported catalysts prepared from precursors with noble metal-oxophilic metal bonds are modeled as clusters of a few atoms of the noble metal ‘nested’ in a supported cluster of the oxophilic metal oxide, which helps to anchor and stabilize the noble metal clusters. Changes in the oxide support have only modest effects on the catalytic activities of supported metal clusters for toluene hydrogenation, but the catalytic activity of γ-Al 2 O 3 -supported Ir clusters per exposed Ir atom increases with increasing cluster size, and this observation remains to be explained.
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