The CO oxidation reaction on the Ir(111) surface

Abstract

Carbon monoxide oxidation over the (111)iridium surface was found to obey a Langmuir-Hinshelwood reaction rate expression indicative of a surface reaction between a chemisorbed carbon monoxide molecule and a chemisorbed oxygen atom. An extremely small value of the preexponential factor of the rate coefficient (approximately 10 −11 cm 2/sec) was determined, while the activation energy to the surface reaction was found to be 10.5 kcal/mole. The small preexponential factor may well be due to the presence of large, independent islands of each adsorbed reactant on the surface. Since the reaction would be restricted to the region of the boundaries of the islands, this would result in an apparently small preexponential factor of the rate coefficient. The primary difference between (111)iridium and various surfaces of platinum and palladium insofar as the reaction between adsorbed oxygen and adsorbed carbon monoxide is concerned is that only at elevated temperatures does the iridium surface exhibit the reaction rates displayed by platinum and palladium at room temperature. Moreover, extensive oxidation of the (111)iridium surface (e.g., 2 × 10 −7 Torr oxygen for 10 min at 700 °K) was found to induce, under certain conditions, marked changes in the oxidative properties of the surface. These changes are related to the formation of a nonstoichiometric oxide in the near-surface region although the oxygen atoms in this oxide do not participate in the carbon monoxide oxidation reaction.