Annealing Effect on Reactivity of Oxygen-Covered Au(111)

We present results of an investigation into the effect of annealing on the reactivity of atomic oxygen adsorbed on Au(111) employing reactive molecular beam scattering (RMBS) and temperature-programmed desorption (TPD) techniques. Isotopically labeled water (e.g., (H2O)-O-18 and (D2O)-O-16), carbon monoxide (CO), and oxygen-labeled carbon dioxide ((CO2)-O-18) were used as probe molecules to investigate the reactivity of adsorbed oxygen. Our results show that the reactivity of atomic oxygen-precovered Au(111) is significantly altered by annealing. The annealed surfaces were prepared by depositing atomic oxygen (O-16 or O-18) at 77 K followed by annealing to temperatures ranging from 100-420 K before dosing probe molecules ((H2O)-O-18, CO, or (CO2)-O-18) at 77 K. Without exception, annealing dramatically diminishes the reactivity of oxygen for all three probe reactions. In the case of the oxygen-water interactions, TPD indicates that annealing decreases the amount of oxygen isotope scrambling between oxygen and water. Additionally, the activity of the oxygen-precovered Au(111) surface for the CO oxidation reaction decreases monotonically as the surface is incrementally annealed to increasing temperatures. The decrease in activity is indicated both by diminishing CO2 production during reactive molecular beam scattering conducted at 77 K and by subsequent O-2 TPD following the CO RMBS experiments. A similar loss of activity due to annealing is observed for the formation and decomposition of surface carbonate on AU(111) as detected by oxygen isotope exchange between adsorbed atomic. oxygen (O-16(a)) and (CO2)-O-18. These observations are attributed to the thermally induced stabilization of metastable oxygen species, suggesting that the metastable oxygen species are responsible for greater reactivity on the unannealed surface. A plausible explanation is that, at lower temperatures, the adsorbed atomic oxygen species reside in a metastable state from which the kinetic barrier to reaction is lower than when adsorbed or annealed at higher temperatures.