Atomic-Scale Site Characterization of Cu-Zn Exchange on Cu(111)

An accurate understanding of the physicochemical properties of bimetallic heterogeneous catalysts relies on atomic scale knowledge of the surface morphology and the atomic distribution. Alloys of Cu and Zn created during catalyst operation are frequently studied and debated in relation to a description of the active phase of Cu/ZnO/Al2O3 methanol synthesis catalysts. This makes it relevant to build a better understanding of Zn dissolution pathways in Cu surfaces and the resulting surface morphology. Herein, we use scanning tunneling microscopy to investigate surface morphology and the distinct atom site configurations of Zn and Cu on Cu(111) resulting from room-temperature Zn exchange from a Zn monolayer into the topmost layer of Cu(111). A gradual dissolution of Zn islands induces an extensive element intermixing at room temperature, resulting in Zn alloying at Cu terrace lattice sites. In addition, we observe and address an interlayer element exchange between the Zn submonolayers in direct contact with Cu. The exchange process is driven by lattice strain and is strongly facilitated at the perimeter of Zn edges. The STM contrast associated with the resulting intermixed sites is reported together with the simulation of these sites based on density functional theory, showing that imaging of isolated Zn sites in Cu(111) is sensitive to the STM tip state. The findings provide new insight into the atomic-scale exchange for Zn/Cu bimetallic surfaces, which may be used onward for understanding the debated surface morphology that develops during reductive activation and alloy formation in the Cu/ ZnO/Al2O3 methanol synthesis catalyst.