(196f) Mixed Rare Earth Oxides for Desulfurization and Tar Removal From Gasifier Effluents

The removal of H₂S and tars from syngas generated in biomass or coal/biomass gasification plays an important role in syngas cleanup. Rare earth oxides (REOs, e.g., Ce/LaO_x) mixed with transition metals (e.g., Mn, Fe, Ni) are promising materials for both sulfur removal and for tar reforming in the presence of some sulfur.  We have synthesized such materials by a variety of methods, including templated sol-gel, hydrothermal, incipient wetness coimpregnation, and evaporation-induced self assembly (EISA). These materials have been investigated for either H₂S adsorption or tar reforming at high temperatures (~630-750ºC), using model syngas mixtures, a goal being to improve heat integration throughout the syngas process. Successive desulfurization-regeneration cycles using Mn/Ce/LaO_x demonstrated good sulfur capacities can be obtained even in the presence of water and CO₂, using just air for the regenerations. The limits of Mn incorporation into the REOs without bulk phase separation have been studied, and the sulfur adsorption has been modeled using DFT.  Naphthalene was used as a model compound for tar reforming/cracking, and the effects of H₂S, CO, water and CO₂ on its reactions have been examined. While H₂S, water and CO₂ are inhibitors, CO is not. Transition metal-doped REOs are more effective than the pure REOs, suggesting an important role of the transition metal in increasing the number of oxygen vacancies. We have also examined propane reforming in model syngas, because its surface behavior is easier to model by DFT.  Fresh, used and regenerated sorbents and catalysts have been characterized by BET, TPR, XRD, XANES and EXAFS.