Studies on self-sustained reaction-rate oscillations II - The role of carbon and oxides in the oscillatory oxidation of carbon monoxide on platinum.

CO oxidation on a platinum foil was studied in both a high pressure flow cell (10(2)-10(5) Pa) and a UHV chamber (10(- 8) to 5x10(4) Pa) both interfaced to a surface infrared spectrometer. Real-time surface infrared and calorimetry experiments performed in the cell during oscillatory oxidation indicated a slow periodic variation (~~ 40%) in the number of active sites, the period of which was commensurate with that of the reaction-rate oscillations. Surface carbon was found to account quantitatively for the surface deactivation, as evidenced by the inverse correlation of the number of surface sites active towards CO adsorption with the surface carbon concentration and by the demonstration that, at the oscillation temperatures, carbon can diffuse from the bulk to the surface, oxygen can remove surface carbon and CO can block carbon diffusion.