Kinetic study of gas-surface molecular exchange between CO/CO on metals

Isotopic switching technique has been used to study the exchange of gaseous CO* with chemisorbed CO molecules on metal surface. The kinetic curves observed on Re(0001) exhibit a nonlinear dependence of desorption rate on the surface coverage of CO. However, the desorption rate for CO on Rh(111) in the same temperature range is strictly linear with the coverage of CO. A concerted adsorption-desorption mechanism is postulated which emphasizes that the desorption of CO in enhanced by the pressure of gas phase CO* on the one hand, and may be retarded by the screening effect of the CO* molecule chemisorbed on the surface on the other hand. It is pointed out that the adsorption of CO* (or CO) and the desorption of CO (or CO*) on a surface at high coverage, i.e. the molecular exchange processes, are primarily taking place simultaneously and concertedly.