Interaction of SO2 with Pt Model Supported Catalysts Studied by XPS

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.