Surface sites on Pt–CeO2 mixed oxide catalysts probed by CO adsorption: a synchrotron radiation photoelectron spectroscopy study

Abstract

By means of synchrotron radiation photoemission spectroscopy, we have investigated Pt–CeO₂ mixed oxide films prepared on CeO₂(111)/Cu(111). Using CO molecules as a probe, we associate the corresponding surface species with specific surface sites. This allows us to identify the changes in the composition and morphology of Pt–CeO₂ mixed oxide films caused by annealing in an ultrahigh vacuum. Specifically, two peaks in C 1s spectra at 289.4 and 291.2 eV, associated with tridentate and bidentate carbonate species, are formed on the nanostructured stoichiometric CeO₂ film. The peak at 290.5–291.0 eV in the C 1s spectra indicates the onset of restructuring, i.e. coarsening, of the Pt–CeO₂ film. This peak is associated with a carbonate species formed near an oxygen vacancy. The onset of cerium oxide reduction is indicated by the peak at 287.8–288.0 eV associated with carbonite species formed near Ce³⁺ cations. The development of surface species on the Pt–CeO₂ mixed oxides suggests that restructuring of the films occurs above 300 K irrespective of Pt loadings. We do not find any adsorbed CO species associated with Pt⁴⁺ or Pt²⁺. The onset of Pt²⁺ reduction is indicated by the peak at 286.9 eV in the C 1s spectra due to CO adsorption on metallic Pt particles. The thermal stability of Pt²⁺ in Pt–CeO₂ mixed oxide depends on Pt loading. We find excellent stability of Pt²⁺ for 12% Pt content in the CeO₂ film, whereas at a Pt concentration of 25% in the CeO₂ film, a large fraction of the Pt²⁺ is converted into metallic Pt particles above 300 K.