Thermally Selective Formation of Subsurface Oxygen in Ag(111) and Consequent Surface Structures


A long-standing challenge in the study of heterogeneously catalyzed reactions on silver surfaces has been the determination of what oxygen species are of greatest chemical importance. This is due to the coexistence of several different surface reconstructions on oxidized silver surfaces. A further complication is subsurface oxygen (Osub). Osub are O atoms absorbed into the near surface region of a metal, and are expected to alter the surface in terms of chemistry and structure; however, these effects have yet to be well characterized. We studied oxidized Ag(111) surfaces after exposure to gas-phase O atoms to determine how Osub is formed and how its presence alters the surface structure. Using a combination of surface science techniques to quantify Osub formation and the resultant surface structure, we observed that once 0.1 ML of Osub formed, the surface was dramatically, and uniformly, reconstructed to striped structures at the expense of all other surface structures. Furthermore, Osub formation was hindered at temperatures above 500 K. The thermal dependence for Osub formation suggests that, under the industrial catalytic conditions of 475–500 K for the epoxidation of ethylene to ethylene oxide, Osub would be present and is a factor in the subsequent reactivity of the catalysts. These findings point to the need for the incorporation of Osub into catalytic models, as well as further theoretical investigation of the resultant structure observed in the presence of Osub.

ACS Catalysis