Heterogeneous combustion of benzene on Rh(111): Kinetics and dynamics of CO and CO2 production

The heterogeneous combustion of benzene on Rh(lll) has been examined using molecular beams in an ultrahigh vacuum environment. For the reaction condit ions studied, CO is the dominant carbon-containing product. CO2 is a minor component, accounting for a maximum of 10% of the carbon species at 650 K a nd dropping to 2% when the temperature is raised to 1000 K. The relative yi elds of CO and CO2, as well as the reaction rate for CO production, are str ongly influenced by surface oxygen concentration, controlled through the re lative ratio of oxygen and benzene flu:res, with the fastest rate of CO pro duction and the greatest proportion of CO2 occurring under the most oxidizi ng conditions. Because the catalytic decomposition of benzene is rapid on t he rhodium surface, the kinetics of CO and CO2 evolution are dominated by t he reaction of atomic carbon and oxygen species on the surface. We calculat e an activation energy for the reaction C(a) + O(a) --> CO(a) of 130 kJ/mol . CO2 is produced by the further reaction of CO with adsorbed O, with the e xtent of reaction being substantially influenced by reaction at defect site s. CO is evolved with a thermal kinetic energy distribution, while CO2 deso rbs hyperthermally. (C) 2000 Academic Press.