The interaction of Al2O3 and CeO2 thin films with sulfur dioxide (2.5 mbar) or with mixtures of SO2 with O2 (5 mbar) at various temperatures (30-400 degrees C) was studied by X-ray photoelectron spectroscopy (XPS). The analysis of temperature-induced transformations of S2p spectra allowed us to identify sulfite and sulfate species and determine the conditions of their formation on the oxide surfaces. Sulfite ions, SO3(2-), which are characterized by the S2p(3/2) binding energy (BE) of approximately 167.5 eV, were shown to be formed during the interaction of the oxide films with pure SO2 at temperatures < or =200 degrees C, whereas sulfate ions, SO4(2-), with BE (S2p(3/2)) approximately 169 eV were produced at temperatures > or =300 degrees C. The formation of both the sulfite and sulfate species proceeds more efficiently in the case of CeO2. The addition of oxygen to SO2 suppresses the formation of the sulfite species on both oxides and facilitates the formation of the sulfate species. Again, this enhancement is more significant for the CeO2 film than for the Al2O3 one. The sulfation of the CeO2 film is accompanied by a reduction of Ce(IV) ions to Ce(III) ones, both in the absence and in the presence of oxygen. It has been concluded that the amount of the sulfates on the CeO2 surface treated with the SO2 + O2 mixture at > or =300 degrees C corresponds to the formation of a 3D phase of the Ce(III) sulfate. The sulfation of Al2O3 is limited by the surface of the oxide film.

The method of X-ray photoelectron spectroscopy was used to study the interaction of the model Pt/TiO 2 catalysts with NO 2 and the following reduction of the oxidized Pt nanoparticles in vacuum, hydrogen, and methane. It was shown that, while interacting with NO 2 at room temperature, the metal Pt nanoparticles transform, first, into the phase which was tentatively assigned as particles containing subsurface/dissolved oxygen [Pt-O sub ], and then, into the PtO and PtO 2 oxides. If only the first state of platinum [Pt-O sub ] is formed, it demonstrates exclusively high reactivity toward hydrogen. For the samples containing simultaneously [Pt-O sub ], PtO, and PtO 2 , the highest reaction ability was demonstrated by PtO 2 ; contrary to the other two oxidized states, it is reducing while kept in vacuum under X-ray irradiation. All three coexisting states of the oxidized platinum can be reduced when heated in vacuum as well as while interacting with hydrogen at room temperature. First, PtO 2 is reduced to PtO. PtO and [Pt-O sub ] begin being reduced after the complete consumption of PtO 2 . We propose that, when a sample contains simultaneously all three states of oxidized platinum, the supported particles have a core− shell structure with a nucleus of perturbed platinum containing oxygen atoms, which are covered with a film of Pt oxides. It was shown that none of the oxidized states of platinum react with methane at room temperature.

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Sulfur dioxide that exists in gaseous reaction media as a reagent or impurity can have various effects, both poisoning and promoting, on oxide-supported platinum catalysts. In this work, the interaction of Al2O3, Pt/Al2O3, CeO2, and Pt/CeO2 thin film model catalysts with SO2 (2.5 mbar) or a SO2 + O2 mixture (5 mbar) was studied by X-ray photoelectron spectroscopy (XPS). SO2 reacts with the bare supports to form mainly sulfite species at 200 °C and mainly sulfate species at 400 °C. For Pt/Al2O3 and Pt/CeO2, the reaction with SO2 produces additionally sulfide species on the platinum surface. Pt/Al2O3 reacts with the SO2 + O2 mixture to yield the sulfide and sulfate species in comparable amounts at ≤200 °C, whereas sulfates become the main product at higher temperatures (300–400 °C). The addition of oxygen to the Pt/CeO2 + SO2 system almost completely suppresses the formation of sulfide species and increases the yield of sulfate. The reduction of ceria accompanies the formation of sulfate; both processes proceed to a greater extent in the presence of supported platinum. To confirm the identification of surface species, the reactions of SO2 and SO2 + O2 with a Pt(111) single crystal were studied as well.

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