LAUSR

The dynamic redox process of surface oxide layers on metal surfaces is of great significance for understanding the active phase in catalytic reactions. We studied the formation of surface oxide layers on Cu(111) and Cu(110) in O 2 , as well as the subsequent reduction by CO using in situ scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). By monitoring and comparing the oxidation process of Cu(111) and Cu(110) surfaces, we found a crystal-plane-dependent reaction mechanism, which also applies to the reduction of surface oxide layers on Cu surfaces. We found XPS Cu spectra cannot be used to identify the various surface oxide layer on Cu surfaces, suggesting their presence in catalytic reactions might have been overlooked. The combination of STM and XPS studies are thus advantageous in identifying surface oxide structures and pinpointing the active phases in the redox process, which paves the way for engineering the catalyst and reaction environment for optimized catalytic performances.