Spectroscopic study of the interconversion of chemisorbed surface species: The reaction Rh I(CO) 2 + CO → Rh 1(CO) 3

Abstract

The adsorption of CO onto Rh I sites produced by O 2 treatment of Rh crystallites supported on Al 2O 3, has been studied using high-sensitivity ir difference spectroscopy. The Rh I(CO) 2 ( gem-dicarbonyl) species is found to form by means of an activated process, in agreement with previous investigations on unoxidized Rh Al 2O 3 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 Rh I site involved, and that the sharp ir bands observed for Rh I(CO) 2 are homogeneously broadened. We have observed the reversible interconversion process Rh I(CO), + CO(g) → Rh I(CO) 3, and have measured the ir spectrum of the Rh I(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 C 3 v due to multicoordinated bonding of the Rh I site to the Al 2O 3 support. The labile Rh I(CO) 3 species is thought to be the surface intermediate responsible for facile isotopic exchange of labeled CO into the gem-dicarbonyl species.