Browsing by Author "Dr. James J. Spivey, Committee Co-Chair"

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    Direct Synthesis of Acetic Acid from Carbon Dioxide and Methane
    (2005-07-26) Wilcox, Esther Magdalene; Dr. James J. Spivey, Committee Co-Chair; Dr. H. Henry Lamb, Committee Member; Dr. Stefan Franzen, Committee Member; Dr. George Roberts, Committee Chair
    The catalytic activation of carbon dioxide in reactions with methane has been investigated. Of specific concern was the direct synthesis of acetic acid from methane and carbon dioxide using a heterogeneous catalyst: CO2 + CH4 -> CH3COOH DGr0 = + 71 kJ/mol DHr0 = + 36 kJ/mol Equilibrium thermodynamic calculations were performed on this reaction to understand its potential behavior in the laboratory. This reaction was found to be severely limited by thermodynamics at all conditions of practical interest. Equilibrium calculations were performed on other reaction systems to investigate potential methods for overcoming the thermodynamic limitations of the direct synthesis reaction. Promising approaches include the synthesis of acetic anhydride from ketene, the synthesis of vinyl acetate from ethylene and oxygen, and the synthesis of vinyl acetate from acetylene. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments definitively showed the formation of adsorbed acetates, monomeric and dimeric acetic acid from carbon dioxide and methane over solid Pt and Pd catalysts. Further reaction experiments, conducted using a micro-reactor system with an online mass spectrometer, confirmed the formation of gas phase acetic acid from carbon dioxide and methane using either a 5% Pd/alumina or 5% Pt/alumina catalyst. In addition to acetic acid, carbon monoxide, hydrogen and water were observed under some conditions. The effective catalysts for the direct synthesis reaction adsorbed both methane and carbon dioxide. Methane appeared to adsorb on the metal, and carbon dioxide on the alumina support. The effective catalysts also had small metal clusters dispersed on the support, which increases the number of adjacent carbon dioxide adsorbing and methane adsorbing sites. Preliminary experiments showed that vinyl acetate could be synthesized from carbon dioxide, methane and acetylene. The most effective catalyst appeared to be an admixture of Pt or Pd/Al2O3 and Zn-acetate/carbon. These experiments suggest that the direct synthesis of acetic acid can be driven by coupling it with a thermodynamically favorable reaction that consumes acetic acid.
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    Preferential Oxidation of Carbon Monoxide on Structured Supports
    (2004-07-05) Chin, Paul; Dr. James J. Spivey, Committee Co-Chair; Dr. H. Henry Lamb, Committee Member; Dr. George W. Roberts, Committee Chair
    Hydrocarbon fuels must be reformed in a series of steps to provide hydrogen for use in proton exchange membrane fuel cells (PEMFCs). Preferential oxidation (PROX) is one method to reduce the CO concentration to < 10 ppm in the presence of ~40% H₂, CO₂, and steam. This will prevent CO poisoning of the PEMFC anode. The PROX reaction requires an active, selective, and stable catalyst. Structured supports, such as ceramic monoliths, can be used for the PROX reaction. Metal foams offer advantages over the traditional ceramic monolith: higher thermal conductivity, radial mixing and heat transport, and a durable, low density, high strength structure. Reaction studies were conducted on catalyzed structured supports using a fixed bed adiabatic reactor with an online non-dispersive IR gas analyzer. A study on ceramic monoliths showed that higher Fe loadings promoted on 5 wt% Pt / g-Al₂O₃ increased CO and O₂ conversions and decreased CO selectivity. A study on metal foams showed that lower cell density and higher pores per inch foams exhibited higher CO conversions and selectivity. Under most operating conditions, the CO conversion and selectivity of the best 5 wt% Pt / 0.5 wt% Fe metal foam were comparable to the ceramic monolith. Comparison tests showed lower CO conversion and selectivity for the ceramic foam and the metal monolith compared to the metal foam and ceramic monolith. Two important effects limit the PROX reaction: the reverse water-gas-shift (r-WGS) reaction, and transport resistances. Under adiabatic conditions, the r-WGS reaction made it impossible to achieve low outlet CO concentrations. The metal foam showed less r-WGS activity compared to the ceramic monolith. The effects of space velocity and linear velocity were studied independently using various catalyst lengths and flow rates. The CO conversion increased at higher linear velocities, which suggested mass transfer resistance between the bulk gas and the catalyst surface. Carbon monoxide pulse chemisorption and temperature programmed desorption (TPD) were used to determine the number of active metal sites. Pulse chemisorption and TPD experiments on the blank reactor and the blank metal foam wrapped with glass wool insulation showed no CO adsorption. No CO adsorption was observed from pulse chemisorption tests on the ceramic fiber insulation, however, considerable CO desorption was observed from TPD tests. On catalyzed supports, an elongated tail in the pulse chemisorption tests was attributed to CO adsorption on either the g-Al₂O₃ washcoat or on surface impurities. The pulse chemisorption and TPD results did not describe the catalyst performance results. Higher values of the number of CO adsorbed on the catalyst surface did not correspond to higher CO conversions from the PROX reaction.