Oscillatory CO oxidation on Pd(100) studied with in situ scanning tunneling microscopy

We have studied the catalytic oxidation of CO on Pd(1 0 0) at atmospheric pressures, using a combined flow reactor and scanning tunneling microscope (STM). By switching from a CO-rich flow to an O-2-rich flow, and vice versa, we could reversibly oxidize and reduce the surface, as we have observed with the STM. Both during the initial oxidation and during the catalytic reaction on the oxide, the surface became rough. Coinciding with the formation of the palladium oxide there was a step up in the CO oxidation rate. Whereas the reaction rate on the metallic surface followed traditional Langmuir-Hinshelwood kinetics, the reaction rate on the oxide was proportional to the CO-pressure and did not depend on the O-2-pressure. We suggest that the high reaction rate on the oxide resulted from the low stability of the oxide, making it highly reactive, and that the reaction followed the Mars-Van Krevelen mechanism, in which the oxide is continually 'consumed' and reformed. The switching between the metallic surface and the oxide showed significant hysteresis in the CO-pressure for O-2-rich conditions. This resulted in a CO-pressure window where reaction was stable both on the metal and on the oxide. We have observed that the reaction switched spontaneously between these branches, leading to oscillations in the reaction rate. Our interpretation is in strong disagreement with well-established models for CO oxidation on Pt-group metal surfaces. (C) 2004 Elsevier B.V. All rights reserved.