Chemisorption site retention during thermal desorption - A study of O2 chemisorption on Pt(112)

Three molecular states of chemisorbed O2 are observed in thermal desorption from a stepped Pt(112) surface containing three atom wide (111) terraces separated by (100) steps. Isotopic studies of 16O2 and 18O2 desorption have shown that site exchange between these three molecular O2 states does not occur at the temperature of desorption (∼100-200 K) from the molecular O2 states. The results imply that the three molecular states may be associated with the three kinds of adsorption sites on the Pt(111) terraces which exhibit Pt-Pt coordination numbers in the range 10-7. Migration of O2(a) species from site to site is slow compared to desorption. It has been shown that site retention is observed at saturation O2 coverages as well as at partial initial O2 coverages.