Photochemical Activation of CO₂ on RhI(CO)₂/Al₂O₃−CO₂ Dissociation and Oxygen Atom Exchange

The ultraviolet (3.8 eV) photolysis of atomically dispersed Rh I ( 13 C 18 O) 2 species supported on an Al 2 O 3 surface in the presence of CO 2 at 256 K has been studied by infrared spectroscopy (FTIR). Carbon dioxide is activated on photochemically produced Rh I ( 13 C 18 O) sites to produce various isotopically labeled rhodium gem -dicarbonyl species. The two major products of CO 2 activation exhibit infrared bands at 2077 and 1958 cm - 1 assigned to Rh I ( 12 C 16 O)( 13 C 18 O) species and bands at 2036 cm - 1 and near 1958 cm - 1 assigned to Rh I ( 13 C 16 O)( 13 C 18 O). The infrared band assignments for the isotopic rhodium gem -dicarbonyl species are supported by a thermally driven isotopic exchange experiment with 13 C 16 O(g) and by a comparison to the frequencies predicted with frequency calculations based on known force constants. Two dissociation pathways are proposed for CO 2 coordinated on Rh I ( 13 C 18 O) sites. Path I involves direct dissociation of the C−O bond in CO 2 and Path II involves oxygen atom exchange between CO 2 and CO ligands, followed by C−O bond dissociation in the CO 2 ligand which was produced by the O exchange process. The lack of formation of oxidized rhodium carbonyl species during CO 2 activation suggests that CO 2 temporarily coordinates to Rh I (CO) with η 1 bonding. This work demonstrates the first steps of a potential low-temperature route for the conversion of CO 2 into other molecules on catalyst sites using ultraviolet light as an energy source.