COADSORPTION-INDUCED EFFECTS AT SURFACES - THERMAL-DESORPTION SPECTROSCOPY AND X-RAY PHOTOELECTRON-SPECTROSCOPY STUDY OF OXYGEN COMPRESSION BY AU ON RU(001)

Coadsorption of gold on an oxygen saturated ruthenium surface causes dramatic changes in the energetics and kinetics of oxygen desorption and the selectivity of surface processes. The activation energy for desorption of oxygen (E(A)) is lowered, it is coverage independent, and the process is governed by first-order kinetics. The lower E(A) prevents oxygen bulk diffusion, which otherwise competes with desorption from a pure oxygen layer, thus leading to a twofold increase in the amount of desorbing oxygen. A complete analysis of desorption data yields E(A) = 81 +/- 4 kcal/mol, prefactor v(1) = (4 +/- 2) x 10(14) S-1, and reaction order n = 0.94. There is no direct interaction between Au and O, as inferred from gold desorption and x-ray photoelectron spectroscopy data. We propose that changes in E(A), v, and n arise as three-dimensional gold islands compress the oxygen overlayer causing a local increase in oxygen coverage. Desorption of oxygen occurs before the onset of Au desorption and as oxygen leaves the Ru surface, gold adatoms spread over Ru keeping the local oxygen coverage constant.