(222g) 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.