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((CO)-C-13-O-18)(2) species supported on an Al2O3 surface in the presence of CO2 at 256 K has been studied by infrared spectroscopy (FTIR). Carbon dioxide is activated on photochemically produced Rh-I((CO)-C-13-O-18) sites to produce various isotopically labeled rhodium gem-dicarbonyl species. The two major products of CO2 activation exhibit infrared bands at 2077 and 1958 cm(-1) assigned to Rh-I((CO)-C-12-O-16)((CO)-C-13-O-18) species and bands at 2036 cm(-1) and near 1958 cm(-1) assigned to Rh-I((CO)-C-13-O-16)((CO)-C-13-O-18). Th, infrared band assignments for the isotopic rhodium gem-dicarbonyl species are supported by a thermally driven isotopic exchange experiment with (CO)-C-13-O-16(g) and by a comparison to the frequencies predicted with frequency calculations based on known force constants. Two dissociation pathways are proposed for CO2 coordinated on Rh-I((CO)-C-13-O-18) sites. Path I involves direct dissociation of the C-O bond in CO2 and Path II involves oxygen atom exchange between CO2 and CO ligands, followed by C-O bond dissociation in the CO2 ligand which was produced by the O exchange process. The lack of formation of oxidized rhodium carbonyl species during CO2 activation suggests that CO2 temporarily coordinates to Rh-I-(CO) with eta(1) bonding. This work demonstrates the first steps of a potential low-temperature route for the conversion of CO2 into other molecules on catalyst sites using ultraviolet light as an energy source.