Adsorption and dissociation of carbon dioxide on rhodium surfaces

Abstract

The adsorption and dissociation of CO 2 on Rh has been investigated using field emission microscopy and thermal desorption spectroscopy. In addition, the influence of hydrogen on the interaction of CO 2 with Rh has been examined. Two clearly distinguishable adsorption states exist, one of a chemical nature and an activation energy of desorption of 60 kJ/mol and a physically adsorbed species with an activation energy of desorption of 27 kJ/mol. The chemisorbed species desorbs to a small extent at 210 K and dissociates for the other part to CO a + O a. The heat of adsorption of chemisorbed CO 2 is particularly high on the open crystal planes like (210), (320) and (531). In dissociation the stepped planes around (111) and (100) are most active. No evidence was found for dissociation on the smooth (110), (100) and (111) surfaces. These results are discussed in relation with the disagreement in the literature concerning dissociation of CO 2 on Rh. Our results point to surface heterogeneity as a major effect explaining the controversy. In addition some investigations on the adsorption of CO and H 2 on Rh starting at 78 K are reported.