Mechanistic insights into the oxidation of catalytically relevant AgCu near‐surface alloy interfaces

Abstract

Recent heterogeneous catalysis studies have demonstrated that synergy between Ag and Cu can lead to more selective partial oxidation chemistries. We performed a series of scanning tunneling microscope experiments to gain a better understanding of the AgCu system under oxidative conditions. These experiments were carried out by exposing sub-monolayer coverages of Ag on Cu(111), in the form of a near-surface alloy (NSA), to range of oxygen exposures and temperatures. This enabled us to study the initial stages of oxidation of well-defined Ag/Cu interfaces with atomic resolution and thereby understand the dynamic response of the AgCu NSA to oxygen environments. At low oxygen exposures, oxidation was observed on exposed Cu terraces and at the interface between the AgCu NSA and Cu(111). Higher oxygen exposure led to the segregation of Cu atoms up through the Ag layer and the appearance of surface adsorbed oxygen. Significant phase segregation of Cu was then observed at higher oxygen exposures at elevated temperatures, evidenced by the formation of Cu oxide patches within and on the top of the Ag layer. These results provide a more detailed picture of how AgCu NSAs interact with, and restructure in response to, oxygen.

Selective Epoxidation of 1,3-Butadiene on AgCu Near-Surface Alloys

“…These TPRs were recorded after exposure of different coverages of Ag on Cu(111) to 500 L of oxygen at 300 K and 0.1 L of butadiene at 90 K. The 500 L exposure to oxygen was selected to ensure that enough oxygen would be present at the surface to form a measurable amount of product while limiting the formation of Cu oxide patches in the Ag layer, which can occur at higher exposures (effects of oxygen exposure on epoxybutene yield are shown in Figure S4). This oxygen exposure was also demonstrated previously to be in a regime where small amounts of Cu reverse segregate to the surface of the Ag monolayer where it significantly enhances the O 2 dissociation probability . The Ag coverage varied from pure Cu(111) (black trace) to the 1 ML AgCu NSA (top trace) with the intermediate coverages of Ag corresponding to 0.20, 0.42, and 0.87 ML.…”