CHEMISORPTION OF CO ON THE IR(111) SURFACE - ADSORPTION AND DESORPTION-KINETICS MEASURED WITH IN-SITU VIBRATIONAL SPECTROSCOPY

The kinetics of adsorption and desorption of carbon monoxide on Ir(111 ) have been investigated using the well-defined and carefully calibrat ed IR absorption peak position of the CO intramolecular stretching vib ration as a nonintrusive coverage probe. Experiments were performed wi th time-resolved Fourier transform infrared reflection-absorption spec troscopy (TR-IRAS), which allows high-resolution measurements over a l arge pressure range between 1X10(-8) and 1X10(-5) mbar and surface tem perature range between 390 and 500 K. The rate of desorption into vacu um is compared with the rate of desorption in the presence of a finite pressure of the desorbing gas. In this case, a flux-dependent enhance ment of the desorption rate has been observed previously, which, if ex istent, should have an important impact on heterogeneously catalyzed r eactions performed under high-pressure conditions. Most of these exper iments, however, were performed using thermal desorption spectroscopy, a nonequilibrium method. We have been able to measure all rate parame ters that are essential to describe the desorption in situ with TR-IRA S and have, thereby, clarified the disagreements that exist in the lit erature. (C) 1997 American Vacuum Society.