An XPS investigation of CO titration of oxygen from an Ir(111) surface

The CO oxidation reaction has been studied under UHV conditions in experiments involving the titration of pre-adsorbed oxygen from an Ir(111) surface by CO. The reaction has been carried out at crystal temperatures between 310 and 545 K and CO partial pressures between 2 × 10−9 Torr and 2 × 10−7 Torr. Time dependent surface coverages of oxygen and CO have been monitored using X-ray photoelectron spectroscopy and have been used to test proposed reaction mechanisms. Under the experimental conditions employed here, the reaction appears to occur between chemisorbed oxygen and surface CO. At temperatures below 350 K, the reaction can be described by the rate expression R CO 2 = k 0 [ O ]F CO exp ( −E a kT ) , where RCO2 is the rate of CO2 production, k0 is the pre-exponential factor of the reaction rate coefficient, [O] is the surface concentration of oxygen, FCO is the incident flux of CO, and Ea is the activation energy for the reaction. Ea and k0 are found to be 7.8 ± 1.0 kcal⧸mole and 3.2 ± 1.5 × 10−12 cm2, respectively. While this expression is of the Eley-Rideal form, which can describe a reaction mechanism in which CO reacts on impact from the ambient, the high values for the kinetic parameters obtained indicate something more complicated than an impact reaction is undoubtedly occuring at these low temperatures. An alternative, surface reaction involving weakly bound CO is suggested. At temperatures above 450 K, the dominant reaction can be described by a reaction rate expression of the form R W CO 2 = k W 0 [O](1−θ O )F CO exp ( −E W a kT ) , where (1−θO) is the fraction of empty surface sites (θO is the fractional oxygen coverage) available for CO adsorption. The superscripts are used to differentiate this reaction from that occurring at lower temperatures. EWa and kW0 are 1.2 ± 0.4 kcal⧸mole and 2.3 ± 2.5 × 10−14 cm2, respectively. At temperatures between 350 K and 450 K a model is suggested which incorporates both mechanisms.