Promoted ZnO Sorbents for Wide Temperature Range H2S and COS Removal from Reformate Streams for Applications in Fuel Cells

Abstract

High efficiency desulfurization is critical to maintain the activity of fuel processing catalysts and high-value membrane electrode assemblies in logistic fuel cell systems. On-board fuel processing of liquid hydrocarbon fuel is being investigated to supply hydrogen for fuel cell-based auxiliary power units. For such a system, if sulfur is not removed from the liquid phase, the removal of sulfur as H2S from the reformate becomes a key-step since downstream catalysts and the fuel cell itself can be poisoned by a small amount of H2S in the feed. Hydrogen sulfide is present in many high temperature gas streams during extraction and processing of fossil fuels, natural gas and geothermal brines. Steam reforming catalysts, PEM anode catalysts and also the shift catalysts are intolerant to sulfur and to ensure adequate lifetime of fuel processors the desulfurization step is very important. This dissertation presents results of R&D efforts to develop novel sorbents for efficient gas phase desulfurization. Promoted ZnO sorbents with formulation M0.05Zn0.95O (M = Mn, Fe, Co, Ni, Cu) were supported on silica and effect of support, various operating parameters and microfibrous entrapment was studied. The results of desulfurization tests on these sorbents at room temperature indicate that a copper doped ZnO (15% w/w)/MCM-41 sorbent (Cu0.05Zn0.95O/MCM-41) has the highest saturation sulfur capacity at 0.9 mol S/mol (Cu0.05Zn0.95O), which is approximately twice that of ZnO/SiO2 sorbent at similar loadings. the utilization of the reactant (M0.05Zn0.95O) toward H2S removal depended on the support employed in the order MCM-41 > MCM-48 > silica gel. This dependence was investigated in terms of the support: surface area, pore volume, and pore size; using N2 adsorption-desorption isotherms (Chapter III). The Cu-ZnO/SiO2 sorbent for ultradeep adsorptive removal of H2S from the reformate streams at room temperature was prepared, tested, and characterization of the active sites was performed. The Cu dopant significantly enhances desulfurization capacity of ZnO/SiO2 sorbent at room temperature (up to 92 % utilization of ZnO), and maintains a high sulfur uptake capacity upon multiple cycles (up to 10) of regeneration by a simple thermal oxidation in air. XRD suggests that both zinc and copper compounds of the CuO-ZnO/SiO2 sorbent are nano-dispersed. The ESR spectroscopy found that the “calcined” and “sulfided” CuO-ZnO/SiO2 sorbents contain Cu2+ in the single dispersion and coordination state and during H2S adsorption, partial reduction of Cu2+ to Cu1+ occurs (Chapter IV) The Fe- and Mn-promoted H2S sorbents Fex-Mny-Zn1-x-yO/SiO2 (x, y=0, 0.025) for the ultradeep desulfurization of model reformates at room temperature were prepared, tested and characterized. The role of Mn and Fe promoter cations in the ‘calcined’ and ‘sulfided’ forms of the FexMnyZnO (1-x-y)/SiO2 sorbent has been studied by the in-situ ESR, temperature dependent XPS. Operando ESR is used for the first time to study dynamics of reduction of Mn3+promoter sites simultaneously with measuring sulfidation dynamics of the Fex–Mny–Zn1−x−yO/SiO2 sorbent. Fe cations are believed to occupy the surface of supported ZnO nanocrystallites, while Mn cations are distributed within ZnO (Chapter V) Removal of both H2S and COS from reformate streams is critical for maintaining the activity of fuel processing catalysts. At temperatures < 250 C, COS formation is effectively inhibited, but at temperatures above 250 C, significant amount of COS is formed in presence of CO2/CO and H2S. A layered bed approach was used with layer of Al2O3/Carbon for COS hydrolysis over the followed by a layer high efficiency H2S removal over bimetallic-promoted supported ZnO sorbent (Chapter VI). The objective of our work is developing the sorbents for an efficient, cost-effective and scalable removal of H2S and COS over the broad temperature range, without significant activity loss upon multiple regeneration cycles, and understanding the mechanism of sulfur sorption by the metal oxide-promoted ZnO-based sorbents.