In situ infrared reflection-absorption spectroscopic characterization of sustained kinetic oscillations in the Pt(100)/NO+CO system

We have excited kinetic oscillations in the Pt(100)/NO + CO system at 470 K and pressure region around 2 x 10(-4) Torr. These oscillations are sustained and they arise by simply adjusting the partial pressures of NO and CO while holding the surface temperature at 470 K. However, they can only be induced if the partial pressure of CO is greater than that of NO (i.e., 1.4+/-0.05<p(CO)/p(NO)<1.7+/-0.04). Infrared reflection-absorption spectra (IRAS) collected during the oscillation showed that the periodic growth and decay of the CO band are counterphase to the reaction-rate oscillations. Infrared bands due to molecularly adsorbed NO were not found during the early stages of the temporal evolution of the system into the upper reaction-rate branch. However, during the later stages of the evolution, a single IR band due to chemisorbed NO was observed. This band eventually disappeared as the reaction rate finally reached its maximum value. With the use of the model by Fink et al., we were able to rationalize the occurrence of oscillations under the present experimental conditions. Oscillations are caused by the periodic change in the level of adsorbate coverages that controls the dissociation of NO. (C) 1998 Elsevier Science B.V. All rights reserved.