Oxygen induced adsorption and reaction of H 2, H 2O, CO and CO 2 on single crystal Ag(110)

Abstract

The adsorption and reaction of H 2, H 2O, CO, and CO 2 were examined on clean and oxygen-dosed Ag(110) surfaces. Limited adsorption with no reaction of these species was observed at surface temperatures down to 160 K on the clean surface. Although no adsorption of H 2 was observed on the oxygen-dosed surface, the adsorption and reaction of the remaining species were strongly affected by the presence of oxygen. The sticking probability and desorption rate measurements for oxygen were in reasonable agreement with the results of other workers. For sequential exposure of the surface to 16O 2 and 18O 2, random isotopic mixing of the O 2 desorbed from the surface was observed, indicating appreciable mobility within the adlayer. At 160 K ten H 2O molecules could be adsorbed for each oxygen atom initially present on the surface; experiments with isotopically labelled oxygen revealed that complete isotopic mixing of oxygen between adsorbed H 2O and the surface had occurred suggesting the formation of adsorbed OH groups. During the adsorption of CO on the oxygen-dosed Ag(110) surface, CO was rapidly oxidized via a chemisorbed state to CO 2. The negative apparent activation energy for this process is shown to be the difference between the activation energies for the reaction step and for the desorption of CO from the surface. In the presence of surface oxygen CO 2 was adsorbed to form a surface carbonate species which decomposed at 485 K to yield CO 2 and O(a).