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 conditions 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 and dropping to 2% when the temperature is raised to 1000 K. The relative yields of CO and CO2, as well as the reaction rate for CO production, are strongly influenced by surface oxygen concentration, controlled through the relative ratio of oxygen and benzene flu:res, with the fastest rate of CO production and the greatest proportion of CO2 occurring under the most oxidizing conditions. Because the catalytic decomposition of benzene is rapid on the rhodium surface, the kinetics of CO and CO2 evolution are dominated by the reaction of atomic carbon and oxygen species on the surface. We calculate 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 extent of reaction being substantially influenced by reaction at defect sites. CO is evolved with a thermal kinetic energy distribution, while CO2 desorbs hyperthermally. (C) 2000 Academic Press.