Synthesis, Characterization, and In-situ Monitoring of Promoted ZnO Sorbents for Desulfurization in Fuel Cell Applications
Type of DegreePhD Dissertation
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Energy from conventional power plants and transportation vehicles usually end up as low grade waste heat and this accounts for more than 70% of the fuel energy input. Fuel cell power systems for logistic or stationary applications are far more energy efficient and environmentally benign employing primarily hydrogen gas compared to conventional energy generating systems. Natural gas, Liquefied Petroleum Gas, and recently biogas and landfill gas are being considered as hydrogen rich sources as raw materials for fuel cells. However, the major presence of sulfur compounds in these sources can cause severe corrosion of processing equipment and irreversible poisoning of fuel processor catalysts and membrane electrode assemblies in fuel cells. Compact, light and small volume unit processes addressing the fuel conversion and purification are essential, especially for on-board logistic applications. Adsorptive desulfurization employing metal oxide sorbents is a low cost and less energy intensive choice compared to hydro-desulfurization which demands high hydrogen partial pressures and elevated temperatures. More over amine based sulfur absorptions processes involve the use of amines and solvents which are typically efficient when operating at the industrial scale. Reformation is the major step for hydrocarbon conversion to hydrogen for fuel cells. Pre-reformer desulfurization typically removes high molecular weight liquid phase sulfur compounds including thiophenes and mercaptans. To protect high value membrane catalysts in fuel cells, post-reformer desulfurization is necessary to remove low molecular residual sulfur compounds including H2S and COS. This is achieved using ZnO, CuO, TiO2, and Al2O3 that serve as polishing metal oxide sorbents with capacities higher than 50mgS/g. This dissertation discusses the aspects of efficient gas phase desulfurization for fuel cell applications and is focussed on the synthesis, characterization, and in-situ monitoring of promoted ZnO adsorbents for H2S and COS removal. Highly dispersed ZnO doped (crystallites < 4nm) with transition metals on SiO2 support was found to be effective sorbents for H2S removal at ambient conditions with stable capacity over multiple adsorption regeneration cycles. CuXZn100 XO/SiO2 sorbents with a Cu concentration of 20 atomic % (X) was found to be optimized sorbent formulation for H2S removal over wide operating conditions. Enhanced reactivity at ambient temperature is due to reduced mass transfer resistance in supported ZnO during interaction with H2S. Crystal defects with oxygen vacancies in the ZnO lattice play an important role during the electronic effects in the H2S adsorption process. The oxidation environment, electronic effects and degree of dispersion of CuO and ZnO in CuXZn100-XO/SiO2 sorbents were investigated via ex-situ and in-situ characterization techniques including UV-Vis Diffuse Reflectance Spectroscopy (UV-Vis DRS), X-ray Photoelectron Spectroscopy (XPS), X ray Diffraction (XRD), and Electron Paramagnetic Resonance (EPR) Spectroscopy. It was found that Cu2+ in calcined specimen exists in a octahedral coordination environment with tetragonal distortions. In addition, Cu2+ ions created acceptor levels in the ZnO band structure causing persistent reduction in the ZnO band gap. Lower band gap means more stable valence band and enhanced ability to interact with the molecular orbitals of H2S. The absence of quantum confinement effect was confirmed since the ZnO nano-particle radius was greater than the ZnO bohr excition radius (3.2 nm). However, XPS results indicate an increase in dispersion of surface Zn atoms when promoted with Cu concentrations up to 20 atomic%. In-situ EPR spectroscopy suggests that Cu2+ ions were in isolated form and interacted with H2S that caused a reduction in oxidation state from +2 in calcined sorbents to +1 in sulfided sorbents. Further reducibility from +1 to 0 (metallic copper) was influenced by neighbouring iso-electronic Zn2+ ions. A fiber optic based UV-Vis diffuse reflectance spectroscopic system was implemented as an embedded sensor in the desulfurizer assembly to measure the utilization of adsorbent beds during H2S removal at 22 °C for logistic fuel cell applications. The efficacy of the embedded fiber optic sensor is evaluated as a function of (i) H2S concentration (ii) H2S contact time (iii) Presence of CO, CO2 and moisture in feed stream and (iv) H2S adsorption on 3/16 inch extrudates using a 6mm optical probe. It was shown that H2S adsorption proceeds via change in color and optical properties of adsorbents. When the fiber optic probe embedded horizontally at bottom of packed bed of ZnO adsorbents, the time at derivative maxima of area under UV-Vis spectrum was close to H2S breakthrough time. Adsorbent bed utilization determined from in-situ optical measurements over wide range of operating parameters is expected to provide significant savings in cost of adsorbent materials and avoid the need for over design of adsorption units during compact and small volume desulfurization operation. Promotional effect of Cu2+ ion in CUXZn100-XO/SiO2 adsorbents was assessed in terms of COS formation as a function of Cu:Zn molar ratio, moisture and temperature at constant H2S partial pressure in a model reformate containing 30%CO + 30%CO2 + 0.6% H2S and balance H2. COS concentration formed at saturation was lower in case of Cu20Zn80O/SiO2 sorbent as compared to ZnO/SiO2. Considering significant H2 concentration in the feed stream, the lower COS concentration in the case of Cu-ZnO/SiO2 was attributed to the catalytic hydrogenation of COS (at 200°C and higher) on Cu2S, formed by reduction with H2S. When experiments were carried out with 5000 ppmv moisture in feed, much lower concentration of COS at saturation was evident in case of both sorbents. Moisture had a COS inhibition effect most likely due to hydrolysis of COS, in which case ZnS is known to be a catalyst. Pure ZnO and rare earth metal oxides (La3+, Y3+ and Ce3+) promoted ZnO were synthesized using a co-precipitation method and examined for the simultaneous removal of H2S and COS from syngas at 400°C. The dual functional La2O3-ZnO bed showed a four-fold increase in sulfur saturation capacity per unit sorbent weight (176 mg/g) and three-fold increase in sulfur saturation capacity in terms of unit bed volume (56 mg/cm3) compared to a mixed bed or layered bed consisting of a COS hydrolysis catalyst and H2S adsorbent.