INFRARED REFLECTION-ABSORPTION SPECTROSCOPY OF A MODEL CATALYST IN A FLOW REACTOR - CO OXIDATION ON PT(100)

Infrared and mass spectral data from a continuous flow ultrahigh vacuum reactor of CO oxidation over Pt(100) are presented. Steady states corresponding to a high reaction rate branch and a low reaction rate branch are observed. For surface temperatures below 460 K, the adsorption of CO is detected in the high reaction rate branch as a new band at 1630 cm-1. A transition between the high reaction rate branch and the low reaction rate branch is seen at an unstable stationary point, where the 1630 cm-1 band disappears and vibrational bands corresponding to island growth of an adlayer of CO grow in. CO in the adlayer is adsorbed in top and bridged sites (bands at 2089 and 1880 cm-1) and the island growth is inferred from the lack of frequency shift with band growth. At surface temperatures of 460 K and greater, only bands from the adlayer of CO are observed and they are only detected in the low reaction rate branch. The transition from the low reaction rate branch to the high reaction rate branch goes through another unstable stationary point, where the disappearance of the CO adlayer is seen. A hysteresis in the CO2 Production rate with CO partial pressure is observed between the high and low reaction rate branches.