Energetics and Mechanism for H₂S Adsorption by Ceria-Lanthanide Mixed Oxides: Implications for the Desulfurization of Biomass Gasifier Effluents

Abstract

Density functional theory (DFT) and ab initio thermodynamics are used to calculate the free energies of H2S adsorption and dissociation on CeO2(111) and CeO2(111) doped with La or Tb. Experimental sulfur capacities are reported for La- and Tb-doped CeO2 adsorbents for comparison with computed energetics. The DFT-based free energies of H2S 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 H2S adsorption and dissociation over substoichiometric oxygen vacancies, and is rate limited by a strongly endergonic molecular adsorption of H2S. 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 H2S 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 H2S 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 H2S dissociation.

ZrO2 Acting as a Redox Catalyst

“…These results demonstrate that the thermodynamics of dissociation depends strongly on the nature of the site. The fact that the dissociation is energetically more favorable on oxygen deficient ZrO 2 is in line with recent DFT calculations on ZnO [29] and CeO 2 (111) [28] surfaces.…”

“…Moreover, due to the weak binding, molecular H 2 S cannot be significantly present on the surface at temperatures where increased reactivity has been observed. Weak interaction between H 2 S and ZrO 2 is also in line with experimental findings [21,23,27] and calculations on H 2 S adsorption on CeO 2 [28] and on YSZ (111) [25]. Dissociative adsorption of H 2 S has been suggested to take place on the strong Lewis acid-base pair site [22,23].…”

The analysis of electronic structures, adsorption properties, NBO, QTAIM and NMR parameters of the adsorbed hydrogen sulfide on various sites of the outer surface of aluminum phosphide nanotube: a DFT study

“…H 2 S is also present as a contaminant in gas feed streams to be a by-product in the electro welding process. Several researchers have concentrated on the production of hydrogen sulfide adsorbents which are capable to adsorb more H 2 S and can remove it from gas streams [3,7,8]. Over the past decade, nanotube surfaces have been the subject of extraordinary activity and survey for using in gas sensors, hydrogen storage devices or for removal of toxic gases [9][10][11].…”

Alkane reforming on partially sulfided CeO2 (1 1 1) surfaces

“…A single material capable of performing both cleanup steps at gasifier effluent temperatures would aid process integration [1,2]. MO x -CeO 2 materials can be used to adsorb sulfur [3][4][5][6][7][8] as well as reform hydrocarbons [8][9][10][11][12][13]. These two processes have mostly been studied independently from one another on oxides, though reforming in the presence of sulfur species has been examined on metal catalysts [14,15].…”

“…There are several studies (both experimental and computational) examining hydrocarbon reforming over ceria, with most of them focused on methane [9][10][11][12][13][16][17][18]. The overall mechanism to reform methane over a ceria surface is established [4,11,19]. The adsorption of sulfur using M-doped ceria has also been examined using both experimental and computational techniques [3][4][5][6][7].…”