Isotopic Mixing in CO Chemisorbed on Tungsten. A Kinetic Study

Abstract

When 12C18O and 13C16O were adsorbed together on tungsten, rapid isotopic mixing to form 12C16O and 13C18O occurred at temperatures above 850°K. These temperatures correspond to the desorption temperatures of the more strongly held adsorbed CO species (β states). No evidence was found for dissociation of CO into mobile adsorbed C and O atoms. A four-center bimolecular exchange intermediate on the surface is postulated. This is consistent with a model for molecular CO chemisorption on tungsten, which involves surface bonding through both the carbon and oxygen. The rate constant for the bimolecular surface-exchange reaction is 1.9×10−10 exp(−30 000/RT) cm2/molecule·sec.