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 ultr athin ordered CeO2-x overlayers on Pt(lll) 1 x 1 single crystals is reporte d. Resonant photoemission spectra at 120 eV excitation energy show the pres ence of an occupied Ce 4f bandgap state that is associated with Ce3+, the c oncentration of which depends on the sample preparation. Heating in oxygen (p(O2)=6 x 10(-5) mbar, T = 1000 K) yields a lower Ce 4f intensity than hea ting in vacuum (T= 800 K), while the (1.4 x 1.4) superstructure of the over layer 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 doe s not affect the Ce 4f state. Reflection-absorption infrared spectroscopy ( RAIRS) indicates that NO, dissociates into NO and atomic oxygen O-(a) at ba re Pt surface sites. O-(a) atoms remain adsorbed, even at 400 K, and oxidiz e Ce3+ to Ce4+ Surprisingly, H2O reduces CeO2-x/Pt(111) at 300 K, increasin g the Ce3+ concentration while significant concentrations of molecularly ad sorbed water or hydroxyl groups are not formed. Interestingly, CO only reac ts 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 v acuum-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). (C) 2000 Elsevier Science B.V. All rights reserved.