The oxidation of zinc vapor and non-stoichiometric ceria by water and carbon dioxide to produce hydrogen and carbon monoxide.
2012-06
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The oxidation of zinc vapor and non-stoichiometric ceria by water and carbon dioxide to produce hydrogen and carbon monoxide.
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2012-06
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Experimental studies of two pathways for solar thermochemical metal oxide cycles to split water and carbon dioxide are presented. The heterogeneous oxidation of Zn(g) is investigated in Part I, and the oxidation of porous ceria is investigated in Part II.
The heterogeneous oxidation of Zn(g) is proposed as an improved approach for rapid and complete oxidation of Zn. Reaction rates are measured gravimetrically in a quartz tube flow reactor at atmospheric pressure for conditions in which Zn is the limiting reactant, at temperatures between 800 and 1150 K, and for Zn(g), H<sub>2</sub>O(g), and CO<sub>2</sub> partial pressures between 10<super>-5</super> and 0.25 atm. The rate of Zn(g) oxidation by CO<sub>2</sub> is between 0.3×10<super>-8</super> and 6.5×10<super>-6</super> mol cm<super>-2</super> s<super>-1</super>, permitting conversions of Zn to ZnO greater than 84% in one second. The rate of Zn(g) oxidation by H<sub>2</sub>O is between 0.8×10<super>-7</super> and 1.5×10<super>-5</super
mol cm<super>-2</super> s<super>-1</super> permitting conversions greater than ~80% in one second. A finite volume based numerical model decouples mass transfer and surface kinetics from the reaction rate data. The CO<sub>2</sub>-splitting kinetics are second-order, proportional to the Zn(g) and CO<sub>2</sub> concentrations. The kinetic parameter is expressed in Arrhenius form, and the activation energy and pre-exponential factor are 44±3 kJ mol<super>-1</super> and 92±6 mol m<super>-2</super> s<super>-1</super> atm<super>-2</super>, respectively. When expressed in second-order form, the apparent activation energy and pre-exponential factor of H<sub>2</sub>O-splitting are -110 kJ mol<super>-1</super> and 1.8×10<super>-5</super> mol m<super>-2</super> s<super>-1</super> atm<super>-2</super> between 800 and 1050 K. At 1100 K, the activation energy becomes positive. A precursor mechanism, where the apparent activation energy is the sum of the heat of adsorption of H<sub>2</sub>O and the activation energy of the rate-limiting kinetic step is postulated to explain this behavior. The benefit of completely converting Zn via the heterogeneous oxidation of Zn(g) is an increase in the Zn/ZnO cycle efficiency from ~6% for polydisperse aerosol reactors, which have been limited to Zn conversions of 20% for reaction times on the order of a minute, to 27% and 31% for H<sub>2</sub>O- and CO<sub>2</sub>-splitting, respectively.
In Part II, the effect of material morphology on the reduction and oxidation of ceria is investigated. The oxidation by H<sub>2</sub>O and CO<sub>2</sub> of three-dimensionally ordered macroporous ceria (3DOM CeO<sub>2</sub>), which features an interconnected, ordered pore network, solid feature sizes between 80 and 200 nm, and a moderate specific surface area of 10 m<super>2</super> g<super>-1</super>, is compared to the oxidation of non-ordered mesoporous ceria and sintered, low porosity ceria at 1100 K in 6 isothermal chemical cycles. The 3DOM CeO<sub>2</sub> increases the maximum H<sub>2</sub> and CO production rates over the low porosity CeO<sub>2</sub> by 125 and 260%, and increases the maximum H<sub>2</sub> and CO production rates over the non-ordered mesoporous CeO<sub>2</sub> by 75 and 175%. 3DOM CeO<sub>2</sub>, non-ordered macroporous ceria (NOM CeO<sub>2</sub>), and aggregates of ceria nanoparticles are also cyclically reduced at ~1500 K under p<sub>O2</sub> = 10<super>-5</super> atm and oxidized at ~1100 K by 25 mol% CO<sub>2</sub>. The 3DOM and NOM CeO<sub>2</sub> retain an interconnected, disordered pore network and achieve maximum CO production rates of 6.4 and 4.0 mL min<super>-1</super> g<super>-1</super>, respectively, an order of magnitude increase over the ~0.1 mL min<super>-1</super> g<super>-1</super> rate of CO production of the sintered ceria nanoparticles and low porosity ceria. The present study demonstrates the importance of engineering ceria with interconnected porosity and solid feature sizes on the order of hundreds of nm.
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University of Minnesota Ph.D. dissertation. June 2012. Major: Mechanical Engineering. advisor:Dr.Jane H. Davidson. 1 computer file (PDF); xiv, 193 pages, appendices A-B.
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Venstrom, Luke J.. (2012). The oxidation of zinc vapor and non-stoichiometric ceria by water and carbon dioxide to produce hydrogen and carbon monoxide.. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/132018.
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