Oxidation of Rh(110) covered with thin (5-9 Å) vapour deposited overlayers of Ce is investigated by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). The oxidation is studied for O2 exposure between 1 L to 5e10 L, and at a sample temperature of 500 °C.
After exposure to low O2 doses (1-200 L), the XPS core level spectra indicate oxidation of the Ce overlayer, while the underlying Rh(110) substrate exhibits no indication of oxidation. The Ce 3d core level spectrum indicates that Ce was completely oxidised to CeO2 after exposed to 50 L O2 The LEED pattern observed after an exposure of 50 L O2 is diffuse, but it shows a ring formed or hexagonal structure, indicating some kind of long-range order at the surface.
Oxidation of the Rh(110) substrate is not observed before exposed to an O2 atmosphere of ~1 bar for a few minutes. In this condition, both the clean Rh(110) substrate and the Ce/Rh(110) system exhibit clear oxidation features in the Rh 3d XP spectrum. The ratio between the rhodium oxide features and the clean rhodium features is enhanced with a factor of 3.7 when the substrate is covered with a thin layer of Ce before oxidation. Dependent of the stoichiometry of the overlayer, this indicates that the rate of the oxidation increases with a factor of 2-4 when the Rh(110) surface is pre-covered with Ce. Catalytic effect of Ce on oxidation of metals has earlier been observed on Nb [1], Ta [2], Al [2] and Si [3], but then oxidation was observed for samples exposed to much lower O2 doses.
The thermal stability of the rhodium and the cerium oxide is investigated by subsequently annealing the oxidised sample to temperatures between 600 and 1000 °C. The rhodium oxide disappear completely after heating to 800 °C, while CeO2 starts to deoxidise after heating to 800 °C.
References:
[1] E.-E. Latta, M. Ronay, Phys. Rev. Lett. 53 (1984) 948.
[2] N.A. Braaten, J.K. Grepstad, S. Raaen, Surf. Sci. 222 (1989) 499.
[3] F. U. Hillebrecht, M. Ronay, D. Rieger, F. J. Himpsel, Phys. Rev. B (1986) 5377.
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