Density Functional Theory Evaluation of Rare-Earth Oxides for Biomass Gasification Effluent Cleanup

Biomass conversion to liquid fuels may be accomplished through gasification to syngas followed by fuel synthesis processes, enabling a renewable energy source of liquid fuels. Prior to fuel synthesis catalysts, the syngas must be cleaned of sulfur and tar species. In a Department of Energy forecast for 2012, approximately 50% of the cost to produce ethanol from biomass is involved in syngas cleanup. Mixed Rare-Earth Oxides (REOs) have shown promise in both desulfurization and hydrocarbon conversion. Our goal is to design a REO catalyst that can reform the large hydrocarbons into CO and H₂ and remove sulfur at high temperatures, thus making biomass gasification-based processes viable for sustainable liquid fuel production. Density functional theory (DFT+U) is used to generate composition-function relationships of mixed REOs for H₂S adsorption and hydrocarbon conversion. Oxygen vacancy sites in the doped oxide serve as the active site for H₂S adsorption and dissociation.  Relative rates of the initial H₂S activation step predict trends in experimental H₂S adsorption capacity over a series of dopants in ceria, suggesting surface kinetic rates impact the adsorption capacity. As an indicator of methane reforming and tar cracking activity, the C-H bond activation energy of ceria based catalysts doped with transition metals is correlated with the surface reducibility. The findings from the DFT calculations compare with experimental H₂S adsorption energy and propane reforming activity.