SOx on ceria from adsorbed SO2

Abstract

Results from first-principles calculations present a rather clear picture of the interaction of SO(2) with unreduced and partially reduced (111) and (110) surfaces of ceria. The Ce(3+)∕Ce(4+) redox couple, together with many oxidation states of S, give rise to a multitude of SO(x) species, with oxidation states from +III to +VI. SO(2) adsorbs either as a molecule or attaches via its S-atom to one or two surface oxygens to form sulfite (SO(3)(2-)) and sulfate (SO(4)(2-)) species, forming new S-O bonds but never any S-Ce bonds. Molecular adsorption is found on the (111) surface. SO(3)(2-) structures are found on both the (111) and (110) surfaces of both stoichiometric and partially reduced ceria. SO(4)(2-) structures are observed on the (110) surface together with the formation of two reduced Ce(3+) surface cations. SO(2) can also partially heal the ceria oxygen vacancies by weakening a S-O bond, when significant electron transfer from the surface (Ce4f) into the lowest unoccupied molecular orbital of the SO(2) adsorbate takes place and oxidizes the surface Ce(3+) cations. Furthermore, we propose a mechanism that could lead to monodentate sulfate formation at the (111) surface.

The surface chemistry of cerium oxide

“…Elevated temperature, co-exposed O 2 and perhaps surface heterogeneity appear to promote sulfate formation. This is consistent with the DFT-U calculations conducted by Lu et al [197] [191] Happel et al [196] and Overbury et al [191] both observed that Ce 3+ oxidized to Ce 4+ as sulfur was reduced from S 4+ to S 2-. Using DFT+U calculations Kullgren et al concluded that SO 2 adsorption will lead to sulfide formation on reduced CeO 2-X (111) but will still lead to sulfate formation on CeO 2-X (110).…”

Atomic Scale Characterization of Defects on Oxide Surfaces

“…On the reduced oxide, the Ce 3+ ions offer more favorable adsorption sites, suggesting that CO can be used as a probe molecule to identify the reduced Ce species even on powder samples [139]. Similar results have been obtained for SO 2 and N 2 O that were found to bind as sulfate, sulfite or thionite species and often poison the surface due to their strong binding energy [137,138]. Water was found to adsorb in an associative as well as a dissociative form on ceria supports, whereby the latter pathway becomes increasingly ineffective at higher coverage [140].…”

“…According to DFT calculations, CO chemisorbs in the form of a surface carbonate on the regular surface [137,138]. On the reduced oxide, the Ce 3+ ions offer more favorable adsorption sites, suggesting that CO can be used as a probe molecule to identify the reduced Ce species even on powder samples [139].…”

Promoting Effect of Organic Ligand on the Performance of Ceria for the Selective Catalytic Reduction of NO by NH3

“…This result was further confirmed by the data of SO 2 ‐TPD analysis (Figure S7 and Table S6). Calculation presented that, for the ceria, the ability to form sulfates was different for different surfaces. The smaller amount of sulfur species formed on CeO 2 ‐B than that on CeO 2 ‐A could be attributed to its special surface microstructure.…”