The dynamic behaviour of CH3OH and NO2 adsorbed on CeO2(111) studied by noncontact atomic force microscopy

The adsorption and reaction of methanol (CH3OH) and nitrogen dioxide (NO2) on CeO2 (111) surfaces in different oxidation states were studied by noncontact atomic force microscopy (NC-AFM). After CH3OH exposure, surface oxygen defects without reconstruction of the surrounding oxygen atoms were healed by methoxy species (CH3O). Under a CH3OH atmosphere, a heterogeneous feature in the reactivity of surface oxygen atoms to CH3OH was observed by successive NC-AFM observation: new line defects of oxygen were produced as a result of a chemical event. Successive NC-AFM measurements also revealed that oxygen vacancies on a slightly reduced CeO2 (111) surface were healed by exposure to NO2. The contrast of healed oxygen defects in NC-AFM images was similar to that of other surface oxygen atoms. This indicates that dissociative adsorption of NO2 occurred at the defects. Nitrogen trioxide (NO3) was also observed on a slightly reduced CeO2 (111) surface after NO2 exposure. On a CeO2 (111) surface without surface oxygen defects, neither adsorption of NO2 nor formation of NO3 was observed. The reaction mechanisms for the formation of line defects and NO3 are discussed briefly.