Isotopic Exchange Between Chemisorbed CO¹⁸ and Gaseous CO¹⁶ on the Pt(111) Surface

The isotopic exchange rate between a CO¹⁸ saturated Pt(111) surface and 10^-8 to 10^-6 torr of CO¹⁶ gas has been studied using temperature programmed desorption over the 298 to 343 K temperature range. The exchange rate is first order in CO¹⁸ coverage and 0.43 order in CO¹⁶ pressure. The activation energy for exchange is only about one half the heat of adsorption for a vacuum saturated surface (ΔH_ex~14kcal/mole). The exchange mechanism appears to invole adsorption of gas phase CO¹⁶ into a weakly adsorbed high coverage intermediate state. All the adsorbed CO molecules in this high coverage state are equivalent (rapid exchange occurs). Desorption of CO¹⁸ from the high coverage intermediate state completes the overall exchange reaction. The desorption step appears to be rate limiting and results in the first order behavior and observed activation energy.