Role of Metal-Oxide Interfaces in Methanol Decomposition: Reaction of Methanol on CeO2/Ag(111) Inverse Model Catalysts

Abstract

Metal-oxide interfaces play a critical role in catalytic processes, such as methanol adsorption and decomposition reactions. In this work, we investigated methanol reactions on the inverse model CeO₂/Ag(111) catalyst surfaces, i.e., submonolayer CeO₂ films on Ag(111), under ultrahigh vacuum (UHV) conditions to specially address the role of CeO₂–Ag interface in the catalytic methanol decomposition reactions. Using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and synchrotron radiation photoemission spectroscopy (SRPES), we found that, at the submonolayer ceria coverages, the CeO₂ nanoislands exhibit a hexagonal CeO₂(111) lattice with fully oxidized Ce⁴⁺ on Ag(111). At higher ceria coverages, multilayer ceria nanoislands form on the Ag(111) surface instead of a well-ordered film. A combination of temperature-programmed desorption (TPD) and SRPES reveals that methanol adsorbs dissociatively on the CeO₂/Ag(111) surfaces at 110 K, resulting in the formation of methoxy groups. These methoxy groups subsequently decompose via two pathways: (i) interaction with lattice oxygen to produce formate species at 230 K, which then decompose to CO, and (ii) direct dehydrogenation of methoxy to formaldehyde. Notably, the surface with submonolayer CeO₂ film on Ag(111) demonstrates low-temperature reactivity (440 K) for methoxy dehydrogenation to formaldehyde, which occurs at a much lower temperature, compared to the surface of multilayer CeO₂ on Ag(111) surface (530 K). This finding emphasizes that the CeO₂–Ag(111) interfaces provide unique active sites for methoxy dehydrogenation reactions.