CO Adsorption on the Model Catalyst Pd/CeO2-x(111)/Rh(111)

X-ray photoelectron spectroscopy (XPS) has been used to investigate the stoichiometry of CeO2-x(111) ultrathin films grown on Rh(111), as well as the modification of the average Ce oxidation state by Pd and CO at room temperature. The stoichiometry of three-layer CeO2(111) has been varied between CeO1.91 and CeO1.61 by altering the oxygen pressure during Ce deposition. Pd deposition induces a change in the Cc oxidation state that is highly dependent on the initial stoichiometry of the CeO2-x films. The difference in behavior Of CeO2-x thin films with different oxidation states is ascribed to the presence of a Ce2O3(0001) termination of the more reduced thin films. CO adsorbs only in the presence of Pd, independent of the stoichiometry of the original film, without changing the oxidation state of the CeO2-x thin film.