The infrared spectrum of CO chemisorbed on alumina‐supported Rh surfaces has been studied following chemisorption at cryogenic temperatures. Major differences are observed in the distribution and spectroscopic character of chemisorbed CO species produced at low temperatures (110–170 K) compared to chemisorbed CO species produced on Rh at 295 K. It has been found that the species Rh(CO)2, formed on isolated Rh sites, is produced rapidly via an activated chemisorption process above ∼200 K. On more ’’crystalline’’ Rhxsites, containing chemisorbed CO, an activated CO adsorbate‐conversion process has been detected in which ?COdecreases by ∼50 cm−1on warming the adsorbed layer above ∼265 K. Isotopic exchange between13CO(g) and12CO(ads) has been shown to occur rapidly at low temperature (∼200 K) for Rh(CO)2species, whereas Rhx(CO) species exchange rapidly only at higher temperatures (≳250 K). These results, taken together, serve to confirm a model in whichisolatedRh sites coexist on the alumina support with crystalline Rhxsites; the two kinds of sites are separable on the basis of the spectroscopic character of the chemisorbed CO species they adsorb as well as by means of their chemical properties.
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