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).

CeO 2 –MnO x /ZSM-5 sorbents for H 2 S removal at high temperature

“…Thus, it was concluded that 750°C was the best for sulfidation [50]. [51], which is in agreement with the XRD results.…”

“…Bakker et al [17] reported that supporting Mn onto c-Al 2 O 3 could increase the utilization of Mn and the desulphurization performance of Mn appeared to be stable during successive sulfidation/regeneration cycles, however, it was not easy to sulfidize fully as MnAl 2 O 4 formed at reductive atmosphere until 800°C. Besides, Li et al [18] found that the intimate combination of CeO 2 and MnO x could remarkably improve the sulfur capacity and desulphurization precision of sorbents as a result of the deformation of CeO 2 lattice, however, the utilization of c-Al 2 O 3 would reduce the sulfur capacity due to Mn ions dissolution into the c-Al 2 O 3 support. In addition, the specific surface areas and pore volumes of unsupported sorbents are quite small and they are easy to be sintering during preparation and sulfidation processes.…”

Fabrication and Fractality of Fe2O3-CeO2/ZSM-5 Composites for High-Temperature Desulfurization

“…Many metal oxides have been applied for high-temperature desulfurization, such as, ZnO [6,7], CuO [8,9], MnO x [10,11], and CeO 2 [12][13][14]. However, iron oxide is the most widely used sorbent for hot coal gas desulfurization in the field of the coal chemical industry owing to its good adsorption capacity, abundant resource, lower cost, and good physical strength [15][16][17][18][19].…”

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].…”

“…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]. Dooley and coworkers recently examined the impact of partial sulfidation of an oxide on reforming catalytic chemistry on ceria [8].…”