Evidence for slow oxygen exchange between multiple adsorption sites at high oxygen coverages on Pt(111)

Thermal desorption (TPD) measurements using isotopically labeled oxygen were carried out on a Pt(111) surface for oxygen coverages Θ(O) between 0.25-0.7 monolayers (ML, 1 ML corresponds to 1.505×10¹⁵cm^-2). In these experiments, ¹⁸O₂ was used to produce Θ(O)=0.25 ML and then N¹⁶O₂ was used to obtain larger values of Θ(O) at the surface. We observed differences in the peak shapes and temperatures maxima of the O₂ TPD peaks from the different combinations of labeled O₂ molecules (¹⁶O₂, ¹⁶O¹⁸O, and ¹⁸O₂), and this provides direct evidence for multiple adsorption sites and slow oxygen exchange between these sites during dosing or heating in TPD. Also, we have reinvestigated the high resolution electron energy loss spectroscopy (HREELS) spectra of these surfaces. We find that there is only a 22 cm^-1 shift of the loss peak due to Pt-O stretching vibrations for Θ(O) between 0.25-0.55 ML, consistent with similar Pt-O bond energies at the different sites occupied by oxygen adatoms at these coverages on the Pt(111) surface.