SO2 Decomposition on Pt/CeO2(111) Model Catalysts: On the Reaction Mechanism and the Influence of H2 and CO

We have performed a synchrotron radiation photoelectron spectroscopy (SR-PES) study on the mechanism of SO2 decomposition on Pt/CeO2(111) model catalysts prepared on Cu(111). Resonant photoemission spectroscopy (RPES) was applied to monitor changes of the cerium oxidation state. Adsorption of SO2 at 150 K on Pt/CeO2(111)/Cu(111) yields surface sulfites (SO32-) on ceria and upright standing molecular SO2 (SO2st) on Pt along with traces of atomic sulfur. Partial decomposition and desorption of the adsorbed species in the form of SO2 occur between 150 and 300 K. In this temperature region, slow accumulation of atomic sulfur is accompanied by reduction of the ceria support. Above 300 K, all surface species completely transform into atomic sulfur; this process is accompanied by a strong reoxidation of ceria. As probed by CO adsorption, the majority of the atomic sulfur species is located on the surface of the Pt particles. Possible reaction mechanisms involving spillover of numerous sulfur-derived species are considered. Finally, reverse spillover of the sulfur from Pt to the ceria support and formation of a cerium oxysulfide phase occurs above 550 K. The spillover of atomic sulfur is readily promoted under reducing conditions, e.g., in the presence of coadsorbed hydrogen or CO.