The adsorption of CO onto RhI sites produced by O2 treatment of Rh crystallites supported on Al2O3, has been studied using high-sensitivity ir difference spectroscopy. The RhI(CO)2 (gem-dicarbonyl) species is found to form by means of an activated process, in agreement with previous investigations on unoxidized Rh Al2O3 surfaces. A thorough study of the infrared spectrum of the gem-dicarbonyl species as a function of surface concentration indicates that there is only one kind of RhI site involved, and that the sharp ir bands observed for RhI(CO)2 are homogeneously broadened. We have observed the reversible interconversion process RhI(CO), + CO(g) → RhI(CO)3, and have measured the ir spectrum of the RhI(CO)3 species. The presence of three carbonyl stretching modes [2120, 2078, and 2026 cm-1] suggests that the symmetry of the species is below C3v due to multicoordinated bonding of the RhI site to the Al2O3 support. The labile RhI(CO)3 species is thought to be the surface intermediate responsible for facile isotopic exchange of labeled CO into the gem-dicarbonyl species.
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