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.