CO oxidation on ruthenium: The nature of the active catalytic surface

The oxidation of CO, i.e. CO + =2O2 ! CO2, over metal surfaces is one of the most studied catalytic reactions. The details of the reaction mechanism under ultrahigh vacuum (UHV) conditions have been well understood for some time [1]. Under reducing or mildly oxidizing conditions for Pt, Pd, and Rh, metals used in automotive catalytic converters, the reaction proceeds via the Langmuir–Hinshelwood (L–H) mechanism between CO molecules and O atoms, both strongly chemisorbed to the metal surface. Under such conditions, the surface is predominantly covered by adsorbed CO and the reaction rate is controlled by the rate at which desorption of CO opens adsorption/ dissociation sites for O2. The measured reaction kinetics at reducing or mildly oxidizing conditions for Pt, Pd, and Rh support this mechanistic picture showing a first order dependence on O2 pressure and a negative first-order dependence on CO pressure, with an apparent activation energy for CO2 formation on each metal approximately equal to the corresponding CO desorption energy [2]. On the other hand, CO oxidation on Ru, in particular the nature of the active catalytic surface, is controversial and the subject of this letter. Here we discuss whether the active surface at near atmospheric pressures and at typical reaction temperatures is a one monolayer oxygen-covered Ru surface or multilayer RuO2. The catalytic oxidation of CO over Ru has been the subject of considerable attention particularly because of its anomalous behavior in several respects. Notably, while