(91c) CO Dissociation Induced by Adsorbed Oxygen and Water On Ir(111)

It is well-known that CO dissociation is a critical step for Fischer-Tropsch synthesis (FTS) which has been widely investigated theoretically and experimentally. In the FTS process, carbon monoxide reacts with hydrogen to produce hydrocarbon alternatives to petroleum. Due to the fundamental importance of C-O bond cleavage for the FTS process, we have studied dissociation of CO on the Ir(111) single crystal surface under ultra high vacuum (UHV) conditions by employing isotopically labeled reactants and temperature programmed desorption (TPD) measurements. Firstly, our investigations indicated that dissociation of CO is immeasurable by our techniques on clean Ir(111). However, we found that co-adsorbed atomic oxygen induces CO dissociation on the Ir(111) surface [e.g., 13C16O was detected desorbing from 13C18O and 16O co-adsorbed on Ir(111)]. We observed similar phenomenon on CO covered Ir(111) when water alone or both water and oxygen were co-adsorbed, likely indicative of the importance of the formed hydroxyl groups since we have found a ~2% dissociation probability of water and a strong interaction between adsorbed atomic oxygen and water on Ir(111). When H218O and 16Oa were co-adsorbed on Ir(111), the TPD spectra show the production of H216O, 16O18O, and 18O2, as well as the occurrence of a new desorption feature at 235 K indicating dissociation of water in which adsorbed hydroxyl groups (·OH) are likely intermediates. To our best knowledge, this is the first observation of hydroxyl group and oxygen atom induced CO dissociation on metal surfaces. These findings provide deeper insight into reaction mechanisms involving co-adsorbed carbon monoxide and oxygen (e.g., CO oxidation) on metal-based catalysts.