Effect of preadsorbed oxygen on the adsorption of CO on Ir(110)

We report measurements of the initial adsorption probability S-0 as well as its coverage dependence S(theta(CO)) of CO on clean and oxygen-precovered Ir(110) for adsorption temperatures T-s below the onset of CO2 formation. The impact energy (1.3<E(0)24 kcal mol(-1)), the oxygen precoverage theta(O), and the surface temperature (85<T-s<200 K) were varied. The initial adsorption probability for CO on the clean surface decreases linearly with increasing impact energy E-0, and is independent of T-s. The shape of the S(theta(CO),E-0) curves points to non-activated precursor-mediated adsorption kinetics for low impact energies; for E-0 greater than or equal to 20 kcal mol(-1), direct adsorption events also contribute to the overall adsorption probability. For high impact energies (E-0>3 kcal mol(-1)), S-0(theta(0)) decreases almost linearly with increasing oxygen precoverage. However, for low impact energies (E-0<3 kcal mol(-1)), S-0(theta(0)) remains constant at 0.92+/-0.03 independent of theta(0). The results suggest a physical site blocking mechanism. No significant temperature dependence of S-0(theta(0)) was observed below the onset of CO oxidation. (C) 1997 Elsevier Science B.V.