Effect of Water Vapor on Oxidation Processes of the Cu(111) Surface and Sublayer

Copper-based catalysts have different catalytic properties depending on the oxidation states of Cu. We report operando observations of the Cu(111) oxidation processes using near-ambient pressure scanning tunneling microscopy (NAP-STM) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The Cu(111) surface was chemically inactive to water vapor, but only physisorption of water molecules was observed by NAP-STM. Under O₂ environments, dry oxidation started at the step edges and proceeded to the terraces as a Cu₂O phase. Humid oxidation of the H₂O/O₂ gas mixture was also promoted at the step edges to the terraces. After the Cu₂O covered the surface under humid conditions, hydroxides and adsorbed water layers formed. NAP-STM observations showed that Cu₂O was generated at lower steps in dry oxidation with independent terrace oxidations, whereas Cu₂O was generated at upper steps in humid oxidation. The difference in the oxidation mechanisms was caused by water molecules. When the surface was entirely oxidized, the diffusion of Cu and O atoms with a reconstruction of the Cu₂O structures induced additional subsurface oxidation. NAP-XPS measurements showed that the Cu₂O thickness in dry oxidation was greater than that in humid oxidation under all pressure conditions.