CO and Hydrocarbon Oxidation on PD/Ceria-Zirconia/Alumina Three-Way Catalysts
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The optimization of combustion engine aftertreatment systems is critical in reducing the negative health and environmental impact of automotive emissions. Over the years, three-way catalysts (TWCs) have been developed and widely implemented to control and reduce tailpipe emissions of carbon monoxide (CO), hydrocarbons (HCs), and nitrogen oxides (NOx) by converting these harmful and undesirable species to CO2, H2O, and N2. The oxidation behavior of CO and model hydrocarbons (propylene, acetylene, and ethylene) and performance of model TWCs is investigated in this dissertation to systematically evaluate trends such as self- and mutual inhibition within reaction and catalytic systems with the intent of improving low-temperature combustion. The use of alumina as a conventional washcoat support material was compared to that of ceria-zirconia (CZO) as an alternate support with oxygen storage capacity in experimental studies. CO, propylene, and acetylene oxidation were enhanced using Pd/CZO compared to Pd/Al2O3, as observed in light-off performance tests. Differences in reaction mechanisms utilizing the two catalysts were further investigated via steady state kinetic experiments. Ethylene oxidation was studied using Pd/CZO and exhibited behavior deviating from self-inhibition observed during CO, propylene, and acetylene oxidation. Combinations of reactants in binary and ternary mixtures for simultaneous oxidation were studied, revealing competitive mutual inhibition effects and particularly strong inhibition by acetylene. The use of ceria was found to significantly enhance co-oxidation and mitigate inhibition effects. A low-dimensional reactor model was developed and employed to verify experimental results for CO and propylene oxidation on Pd/Al2O3 and predict nonisothermal and heat and mass transfer effects within the monolith catalyst. Refined global and microkinetic models ably captured mechanisms for CO and propylene oxidation and provided key insights into inhibition within three-way catalyst systems.