Energetics and mechanism for H ₂S adsorption by ceria-lanthanide mixed oxides: Implications for the desulfurization of biomass gasifier effluents

Density functional theory (DFT) and ab initio thermodynamics are used to calculate the free energies of H ₂S adsorption and dissociation on CeO ₂(111) and CeO ₂(111) doped with La or Tb. Experimental sulfur capacities are reported for La-and Tb-doped CeO ₂ adsorbents for comparison with computed energetics. The DFT-based free energies of H ₂S adsorption, dissociation, and oxygen vacancy formation are evaluated at the operating conditions for the high temperature desulfurization of biomass gasifier effluents. DFT results indicate that the sulfur adsorption process occurs via H ₂S adsorption and dissociation over substoichiometric oxygen vacancies, and is rate limited by a strongly endergonic molecular adsorption of H ₂S. Sulfur incorporation is only favorable if multiple adjacent oxygen vacancies are present to provide the flexibility required to accommodate the larger coordination shell of sulfur atoms. The thermodynamics of the overall H ₂S adsorption process do not correlate with oxygen vacancy formation energy, implying that the optimization of ceria-based sulfur sorbents cannot be achieved by tailoring composition to maximize the exergonicity of vacancy formation. The rate-limiting step for H ₂S adsorption and dissociation involves reaching the transition state for dissociation of the first S-H bond to form SH* + H* (>2.23 eV over each surface), and this rate is highest over ceria-lanthana (>ceria-terbia > ceria). This is the same order as the experimental sulfur capacities, if these capacities are compared on a similar molar (Ce/La vs Ce/Tb) basis. Agreement with the experimental capacities suggests that the actual sulfur capacities of ceria-based mixed oxides are largely determined by the kinetics of the H ₂S dissociation.