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Abstract

Rh deposits in a wide range of metal exposures have been grown on a thin, well-ordered alumina film under ultra-high vacuum conditions. Their morphologies range from individual Rh particles with sizes below ten atoms to closed metal films, as determined by scanning tunneling microscopy (STM). CO chemisorption on these deposits has been studied by infrared reflection absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS). At low metal coverages, the formation of isolated Rh carbonyl species associated with surface defects is observed. Changes in the infrared spectra of CO adsorbed on annealed Rh aggregates are attributed to a thermally induced ordering of the Rh particle surface on a local scale, leading to a reduction in the number of low coordinated metal atoms. This is accompanied by a marked decrease of the CO chemisorption capacity. Adsorbed CO reduces the extent of this ordering process, while the adlayer itself reorganizes.