SOx on ceria from adsorbed SO2

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.