Sulfidation of Ceria Surfaces from Sulfur and Sulfur Diffusion

Even very low levels of sulfur contaminants can degrade the catalytic performance of cerium oxide. Here, the interaction of atomic sulfur with the ceria (111) and (110) surfaces has been studied using first-principles methods. Two sulfoxy species are identified: oxido-sulfate(2-) species (SO2-) on both the CeO2(111) and (110) surfaces and hyposuffite (SO22-) on the (110) surface. Sulfide (S2-) is formed when a surface or a subsurface oxygen atoms is replaced by sulfur. These sulfide species are more stable at the surface. Furthermore, sulfite (SO32-) structures are found when sulfur is made to replace one Ce in the ceria (111) and (110) surfaces. The calculated sulfur diffusion barriers are larger than 1.4 eV for both surfaces, and thus sulfur is essentially immobile, providing a possible explanation for the sulfidation phenomena of the ceria-based catalyst. Thus, we find three different species from interaction of S with ceria which are all, due to their strong binding, capable of poisoning the surface, reduced or unreduced. Our results suggest that under reducing conditions sulfur is likely to be found in the (111) surface (replacing oxygen) but on the (110) surface (as SO22-).