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., H ₂ ¹⁸O and D ₂ ¹⁶O), carbon monoxide (CO), and oxygenlabeled carbon dioxide (C ¹⁸O ₂) 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 ( ¹⁶O or ¹⁸O) at 77 K followed by annealing to temperatures ranging from 100-420 K before dosing probe molecules (H ₂ ¹⁸O, CO, or C ¹⁸O ₂) 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 CO ₂ 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 ( ¹⁶Oa) and C ¹⁸O ₂. 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.