Chemisorption and reaction of sulfur dioxide with oxidized and reduced ceria surfaces

The interaction of SO2 with highly oriented ceria films is studied as a function of coverage, temperature, and degree of oxidation of the ceria. Soft X-ray photoemission (SXPS) is used to analyze the oxidation state of the ceria and the evolution of the SO2-derived species, while thermal desorption spectroscopy is used to analyze the desorption products. The principle interaction is chemisorption of SO2 at oxygen anions, which reversibly desorbs as SO2 over a broad temperature range from 200 to 600 K. This interaction is characterized as a Lewis acid-base interaction and occurs with approximately equal facility if the surface is hydroxylated or sulfided. On reduced ceria the adsorption is more heterogeneous, suggesting a variety of local bending environments, including a small, distinguishable amount of adsorption at Ce3+ sites. A significant portion of the chemisorbed SO2 converts to sulfide above 300 K, and the resulting sulfide then equilibrates between bulk and surface sites above 600 K. Adsorption at a sulfide site leads to a 1.5 eV shift in the sulfide S 2p state toward higher binding energy. Thermal conversion of chemisorbed SO2 to sulfate is not observed. The results are compared with previous studies of NO adsorption on ceria films and with similar studies of SO2 chemisorption on titania.