Chemisorption of CO on the Ir(111) surface: Adsorption and desorption kinetics measured with in situ vibrational spectroscopy

Abstract

The kinetics of adsorption and desorption of carbon monoxide on Ir(111) have been investigated using the well-defined and carefully calibrated IR absorption peak position of the CO intramolecular stretching vibration as a nonintrusive coverage probe. Experiments were performed with time-resolved Fourier transform infrared reflection–absorption spectroscopy (TR-IRAS), which allows high-resolution measurements over a large pressure range between 1×10−8 and 1×10−5 mbar and surface temperature range between 390 and 500 K. The rate of desorption into vacuum is compared with the rate of desorption in the presence of a finite pressure of the desorbing gas. In this case, a flux-dependent enhancement of the desorption rate has been observed previously, which, if existent, should have an important impact on heterogeneously catalyzed reactions performed under high-pressure conditions. Most of these experiments, however, were performed using thermal desorption spectroscopy, a nonequilibrium method. We have been able to measure all rate parameters that are essential to describe the desorption in situ with TR-IRAS and have, thereby, clarified the disagreements that exist in the literature.