Mixed adsorption of carbon monoxide and oxygen on tungsten (100), (110), and (111) single crystal faces

Flash desorption spectra of C16O and C18O were measured for C16O dosed on tungsten (100), (110), and (111) at 300 K with varying 18O2 predoses and postdoses. Heavy doses of C16O resulted in flash desorption spectra characterized by a single α peak and two β peaks on each of W(100) and W(110) crystal faces, and single sharp α and β peaks on the W(111) crystal face. Predoses or postdoses of 18O2 resulted in extensive isotope exchange in all β peaks, and slight exchange in all α peaks. Postdosing of a C16O monolayer by 18O2 resulted in no change in total CO on the surface, although in cases where two β peaks occurred, that state desorbing at lower temperature was enhanced at the expense of that desorbing at higher temperature. With increasing predose of 18O2 the total β-CO resulting from a fixed C16O dose steadily decreased, and the amount of α-CO at first increased, then decreased. The results are broadly in accord with the Goymour--King model, according to which β states arise from dissociatively chemisorbed CO, α states from CO associatively adsorbed on isolated single sites. Isotopic exchange at low C16O doses followed by heavy 18O2 doses appears best interpreted in terms of random selection of oxygen from immediate neighbors in an immobile monolayer, rather than in terms of random mixing of oxygen isotopes over the entire monolayer