Ultrathin ordered CeO2 overlayers on Pt(111): interaction with NO2, NO, H2O and CO

The effect of gas exposure on the electronic valence band structure of ultrathin ordered CeO2−x overlayers on Pt(111) 1×1 single crystals is reported. Resonant photoemission spectra at 120 eV excitation energy show the presence of an occupied Ce 4f bandgap state that is associated with Ce3+, the concentration of which depends on the sample preparation. Heating in oxygen (pO2=6×10−5 mbar, T=1000 K) yields a lower Ce 4f intensity than heating in vacuum (T=800 K), while the (1.4×1.4) superstructure of the overlayer remains unchanged. A decrease of the Ce 4f bandgap state occurs upon even small exposures of NO2 at 300 and 400 K. At these temperatures, NO does not affect the Ce 4f state. Reflection-absorption infrared spectroscopy (RAIRS) indicates that NO2 dissociates into NO and atomic oxygen O(a) at bare Pt surface sites. O(a) atoms remain adsorbed, even at 400 K, and oxidize Ce3+ to Ce4+. Surprisingly, H2O reduces CeO2−x/Pt(111) at 300 K, increasing the Ce3+ concentration while significant concentrations of molecularly adsorbed water or hydroxyl groups are not formed. Interestingly, CO only reacts with CeO2−x/Pt(111) following a particular pretreatment of the sample. A decrease of the Ce 4f intensity upon CO exposure was observed only for a vacuum-annealed and an H2O-reduced sample, while an NO2 pre-exposed surface does not react with CO, as measured by a change of the Ce3+/Ce4+ ratio of CeO2−x/Pt(111).