Ceriumoxide (CeO2, ceria) is a material with interesting properties as an additive in catalytic materials. It is used in the automotive catalyst, and in a number of other catalytic systems the presence of ceria has been found beneficial for the catalytic performance. In order to study the surface chemistry of cerium oxide, we prepared a thin CeOx film (~5 nm) with the (111) orientation on a Ru(0001) substrate [1]. Depending on the preparation conditions we were able to control the amounts of Ce3+ and Ce4+ present in the cerium oxide layer [1,2]. Qualitative LEED measurements showed that we could prepare a well-ordered CeOx layer with the CeO2 (111) orientation, irrespective of the Ce3+ content. Photoemission experiments revealed the chemical composition of our ceria layer, and allowed us to estimate the thickness of the layer. The Ce4f signal is indicative for the presence of Ce3+ ([Xe]4f1). When measured on-resonant (120 eV photon energy coincides with a 4d-4f transition in Ce) the sensitivity for Ce3+ is significantly enhanced, and small changes in the amount of Ce3+ can be detected [3].
The CeO2 structure is stable up to 1000 K, but around 1200 K the CeO2 layer looses oxygen, as evidenced by the appearance of the signal from Ce3+ [3]. LEED shows that the surface structure is influenced by the thermal treatment. It was found that both CO and CO2 adsorb on CeOx at low temperature (~100 K), presumably forming carbonates. Above 400 K these species decompose into and CO2 (g) and O.
In an attempt to create a model catalyst, Au was evaporated on the ceriumoxide layer. The Au/CeOx system was then investigated using core level spectroscopy to obtain information on changes in the electronic structure of both the metal and the substrate. Furthermore, the interaction of both CO2 and CO with this model catalyst was studied.
[1] D.R. Mullins, P.V. Radulovic and S.H. Overbury, Surf. Sci. 429, 186 (1999)
[2] D.R. Mullins, S.H. Overbury and D.R. Huntley, Surf. Sci. 409, 307 (1998)
[3] S. Eck, C. Castellarin-Cudia, S. Surnev, M.G. Ramsey and F.P. Netzer, Surf. Sci. 520, 173 (2002) |