Atomic-Scale View of the Oxidation and Reduction of Supported Ultrathin FeO Islands

By means of scanning tunneling microscopy (STM) measurements, we studied in situ the oxidation and reduction of FeO bilayer islands on Au(111) by oxygen (O₂) and hydrogen (H₂), respectively. The FeO islands respond very dynamically toward O₂, with the coordinatively unsaturated ferrous (CUF) sites at the island edges being essential for O₂ dissociation and O atom incorporation. An STM movie obtained during oxidation reveals how further O₂ molecules can dissociate after the consumption of all initially existing CUF sites through the formation of new CUF sites. In contrast, we found that H₂ molecules only dissociate when vibrationally excited through the ion gauge and only at the basal plane of FeO islands, implying that the CUF sites are not relevant for H₂ dissociation. Our STM results reveal how excess O atoms are incorporated and released in O₂ and H₂ and thus shed light onto the stability of inverse catalysts during a catalyzed reaction.