SO_x on ceria from adsorbed SO_2

摘要

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.