XPS/TPR study of the reducibility of M/CeO2 catalysts (M=Pt, Rh): Does junction effect theory apply?

The study of the reduction of a set of high surface area M/CeO2 catalysts (M = Rh, Pt and Au, S-BET ca. 90 m(2)g(-1)) prepared from several precursors, either containing or not chlorine, has been carried out using TPR/MS and XPS/Ar+-etching techniques combined with volumetric adsorption of H2. Reduction up to 773K leads in all the samples to formation of the stable CeO1.82 phase of ceria. Incorporation of hydrogen occurs in the oxygen vacancies present in this phase competing with chlorine when it is present in the samples. The results are discussed in relation to the ''junction effect'' theory proposed by Frost [1] for metal-oxide catalysts concluding that it does not apply to this type of catalysts due to high ionic mobility of the oxygen in ceria and the possibility of a levelling of the Schottky metal-oxide barriers due to hydrogen incorporation at the metal-ceria interface.