(582b) A DFT Study of Desulfurization and Tar Cracking of Gasifier Effluents Over Ceria-Based Rare-Earth Oxides

Cleanup of biomass gasifier effluents involves both the removal of sulfur-containing compounds such as H₂S and the cracking of tars. Mixed rare earth oxides of ceria containing lanthanides or transition metal additives are effective H₂S sorbents and tar cracking catalysts. We report density functional theory (DFT+U) results used to evaluate the H₂S adsorption capacity and tar cracking activity of pure ceria and ceria doped with a series of lanthanides and group VIIB-VIIIB transition metals. We use ab initio thermodynamics to construct phase diagrams for the surface layers of these rare earth oxides as well their corresponding sulfides and oxysulfides as a function of H₂S and O₂ partial pressures. Our results illustrate the relationship between oxide reducibility and H₂S sorbent capacity, and quantify changes in the surface activity for sulfur adsorption as a result of doping. We also evaluate the effects of transition metal additives on C-H and C-C bond activation, and discuss our results in the context of designing improved rare earth oxide catalysts for simultaneous desulfurization and tar cracking of gasifier effluents.