dc.description.abstract | This dissertation describes a quest to develop a low-temperature (<200 °C), cost-effective, and energy-saving technology for the destruction of chlorobenzene (CB) and dioxins [polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs)]. The catalytic oxidation of gaseous CB and PCDD/Fs with ozone [O3 catalytic oxidation (OZCO)] over transition metal oxides (iron oxide and manganese oxide) was investigated at temperatures of 60–210 °C, while monitoring the effects of operating temperature, O3 concentration, space velocity (SV), and water vapor content on conversion. Moreover, the catalyst stability and the kinetics of the conversion processes were also studied.
CB was tested initially as a model dioxin compound to evaluate the oxidation behavior. Substituting O3 for O2 during the course of catalytic oxidation of CB decreased both the operating temperature and the activation energy. In the absence of O3, the iron oxide and manganese oxide catalysts were almost inactive toward CB oxidation at temperatures up to 200 °C in 20% of O2. In contrast, high conversions of CB (ca. 90%) were obtained in the presence of 1,200 ppm of O3 at a relatively low temperature (150 °C) and a short gas residence time (GHSV = 300,000 h–1). CO and CO2 were the only carbon-containing products detected in the effluent gas stream; their concentrations and the carbon recovery all increased upon increasing the concentration of O3 from 0 to 1,200 ppm. The iron oxide catalyst provided a stable conversion during testing for more than 96 h. In contrast, a 3% reduction of the CB conversion occurred over manganese oxide and O3 because (i) small amounts of partially oxidized byproducts (e.g., carboxylic acids) deposited on the surface of the catalyst and (ii) deactivating MnCl2 species were formed during the oxidation process. Complete regeneration of the manganese oxide catalyst was possible at temperatures higher than 400 °C in 20% of O2. The Langmuir–Hinshelwood model adequately described the kinetic behavior for the oxidation of CB over iron oxide and O3, suggesting that the atomic oxygen species that formed on the surfaces of the catalyst upon the decomposition of O3 played important roles in the catalytic oxidation of CB.
In the second part of this study, total oxidation of gaseous PCDD/Fs was investigated using the OZCO process. The addition of O3 greatly enhanced the catalytic activity of the iron oxide catalyst toward the oxidation of gaseous PCDD/Fs. At 180 °C, the destruction efficiencies of gaseous PCDD/Fs over iron oxide and 100 ppm O3 exceeded 90%. At a relatively low temperature of 120 °C and in the absence of O3, the destruction efficiencies of all PCDD/F congeners were less than 20% and decreased upon increasing the degree of chlorination of the dioxin congener. At 180 °C in the presence of O3, however, the destruction efficiencies were greater than 80% for all of the PCDD/F congeners over iron oxide. Moreover, in the presence of 5% water vapor, the destruction efficiencies of PCDD/Fs were greater than 90%, even at a relatively low operating temperature of 150 °C. Thus, the presence of an appropriate amount of water vapor enhanced the catalytic activity for the decomposition of gas-phase PCDD/Fs, presumably because (i) highly reactive OH radicals were formed to oxidize PCDD/Fs and (ii) water vapor facilitated the removal of Cl– ions from the catalyst surfaces.
Overall, this OZCO method—using eco-friendly, cost-effective iron oxide and manganese oxide catalysts and small amounts of O3—is a novel, feasible, and economical approach for the removal of low-concentrations of CB and dioxins from gas streams. It can be employed directly to treat the flue gas at the outlet of a baghouse or scrubber in field application.
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