Ce–Mn Oxides for High-Temperature Gasifier Effluent Desulfurization

Abstract

We examined Ce–Mn mixed oxides as high-temperature desulfurization materials, exploring various Mn/Ce ratios and the effects of admixing other rare earth oxides. The sulfur capacities at temperatures from 900 to 1025 K with simple air regeneration were measured for repeat cycles until a stable, reversible capacity was obtained. The measured sulfur capacities with a realistic model syngas containing H2S, H2, N2, CO, H2O, and CO2 were compared to thermodynamically possible maximum sulfur capacities. The oxidized and sulfided (reduced) sorbents were characterized by X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), X-ray absorption fine structure (XAFS), temperature-programmed reduction (TPR), and Brunauer–Emmett–Teller (BET) surface area. Density functional theory calculations are used to aid in interpreting characterization data and in explaining the enhanced S adsorption capacities. There is a large synergistic effect on sulfur adsorption and reaction resulting from the intimate admixing of Mn with CeO2 and CeO2/La2O3 rare earth oxides. However, while these materials are stable at temperatures near 900 K, even using air regeneration, the observed stable sulfur capacities fall far short of predictions based on thermodynamic equilibrium. The differences are attributed to (a) inhibition by CO2 and H2O; (b) formation of some irreversible sulfates upon air regeneration; (c) inability of sulfur to diffuse into larger, sintered crystals of the mixed oxides; (d) gradual dissolution of Mn in an underlying support such as Al2O3 (when present).