A resonant photoemission study of the Ce and Ce-oxide/Pd(111) interfaces

摘要

We have investigated the Ce 4f electronic states in the Ce/Pd(111) and Ce-oxide/Pd(111) systems, using resonant photoemission (Ce 4d → 4f transitions), and XPS to understand Pd-Ce interactions in ultra thin layers of cerium and ceria deposited on Pd(111). Cerium deposited on Pd(111) at room temperature forms surface Ce-Pd alloys with Ce rich character, while a Pd rich Ce-Pd alloy is formed by heating to 700 ℃. A modification of the chemical state of Ce can also be seen after oxygen exposure. RPES provides evidence that Ce- oxide layers deposited on Pd(111) have a CeO_2 (Ce~(4+)) character, however a net contribution of the Ce~(3+) states is also revealed. The Ce~(3+) states have surface character and are accompanied by oxygen vacancies. Heating to 600 ℃ causes Ce-oxide reduction. A significant shift of Pd 4d-derived states, induced by Pd 4d and Ce 4f hybridization, was observed. The resonant features in the valence band corresponding to Ce~(4+), Ce~(3+) and Ce~0 states have been investigated for various Pd-Ce(CeO_x) coverages.