Chemisorption and Reaction of Sulfur Dioxide with Oxidized and Reduced Ceria Surfaces

Abstract

The interaction of SO 2 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 SO 2 -derived species, while thermal desorption spectroscopy is used to analyze the desorption products. The principle interaction is chemisorption of SO 2 at oxygen anions, which reversibly desorbs as SO 2 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 bonding environments, including a small, distinguishable amount of adsorption at Ce 3+ sites. A significant portion of the chemisorbed SO 2 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 SO 2 to sulfate is not observed. The results are compared with previous studies of NO adsorption on ceria films and with similar studies of SO 2 chemisorption on titania.