Oxidation of thin Ce layers on Rh(110)

Oxidation of Rh(110) covered by thin (5-9 Å) overlayers of Ce was investigated by X-ray photoelectron spectroscopy and low energy electron diffraction (LEED). The oxidation was studied for O2 exposure between 1 and 5 · 1010 L, and at a sample temperature of 500 °C. The Ce 3d core level spectrum indicated that Ce was almost completely oxidised to CeO2 after exposure of 50 L O2 with only a thin overlayer of Ce2O3. The LEED pattern observed at this exposure was diffuse, but a ring shaped structure indicated long-range order at the surface. Rhodium oxide features were only observed in the core level spectra when the sample was exposed to an O2 atmosphere of ∼ 1 · 105 Pa for 1 min (∼ 5 · 1010 L). The oxidation rate was estimated to be up to 2 times higher when the substrate was covered with a thin layer of Ce before oxidation as compared to oxidation of the clean Rh(110) substrate. The thermal stability of the rhodium and the cerium oxides was investigated by subsequently annealing the oxidized sample to temperatures between 600 and 1000 °C. Rhodium oxide disappeared completely after heating to 800 °C, while CeO2 started to deoxidize after heating to 900 °C.