Surface sites on Pt–CeO₂mixed oxide catalysts probed by CO adsorption: a synchrotron radiation photoelectron spectroscopy study

Abstract

By means of synchrotron radiation photoemission spectroscopy, we have investigated Pt-CeO 2 mixed oxide films prepared on CeO 2 (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-CeO 2 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 CeO 2 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-CeO 2 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 Ce 3+ cations. The development of surface species on the Pt-CeO 2 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 Pt 4+ or Pt 2+ . The onset of Pt 2+ 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 Pt 2+ in Pt-CeO 2 mixed oxide depends on Pt loading. We find excellent stability of Pt 2+ for 12% Pt content in the CeO 2 film, whereas at a Pt concentration of 25% in the CeO 2 film, a large fraction of the Pt 2+ is converted into metallic Pt particles above 300 K.

Relevance of IR Spectroscopy of Adsorbed CO for the Characterization of Heterogeneous Catalysts Containing Isolated Atoms

“…In contrast, no Pt-related band could be detected on the 800 °C-calcined sample when exposed to CO at −160 °C. Previous surface science work noted that CO adsorption on dispersed Pt cation was too weak to be observed at −160 °C . Interestingly, a symmetric band at 2090 cm –1 (fwhm = 20 cm –1 ) could be observed when both temperature and CO pressure were raised to RT and 20 mbar of CO, respectively (Figure A).…”

Adsorption and Activation of CO on Co3O4(111) Thin Films

“…Note that the formation of weakly bound surface carbonates upon CO exposure at low temperature has previously been observed on other oxide surfaces as well. 51 Based on these considerations we attribute the feature at 1750 cm -1 to a weakly bound bidentate carbonate which is formed at a defect site. A possible type of defect identified in the structural characterization (see Section 3.1) are the side facets of the grains and steps between Co 3 O 4 (111) terraces.…”

A multi-technique study of CO2 adsorption on Fe3O4 magnetite

“…S5 of the supplementary material), which corresponds to saturation of the defect peaks plus 1% of the monolayer coverage, reveal a single peak at 291.3 eV, identical to that of the physisorbed monolayer. Since carbonate species typically appear in the range 287-289 eV, 39,40 we conclude that CO 2 adsorbed at defects is also physisorbed.…”