Doctor of Philosophy (PhD)
Formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) which occurs inexorably from most thermal and combustion processes constitutes a major toxic component of environmental pollutants. Generally, it is well established that transition metal-mediated reactions account for the majority of PCDD/F emissions from combustion sources. Specifically, both copper and iron ions, which occur abundantly in combustion generated particulate matter, are considered probably the most active in promoting PCDD/F formation typically in the low temperature post-combustion zone and flue gas pollution control devices. It has also been demonstrated that chlorinated phenols are key intermediates in essentially all pathways of PCDD/F formation. Chlorinated benzenes have been presumed to be potent precursors that form PCDD/Fs and are among the most abundant aromatic compounds in incinerator exhaust. Notably, numerous studies have been reported in regard to surface-mediated processes of PCDD/F formation via de novo synthesis and transition metal-mediated processes from reactions of chlorinated phenols. However, few experimental studies have been conducted on chlorinated benzenes. In addition, even though iron oxide is present at 2-50 times higher concentrations than copper oxide, virtually no studies of the iron oxide mediated formation of PCDD/Fs have been reported in the literature. For this study, PCDD/F formation over iron oxide and copper oxide surfaces were investigated using the reactants 2-MCP and 1,2-DCBz in pure and mixture form. The surface-mediated reactions were studied under pyrolytic and oxidative conditions over a temperature range of 200 to 550 oC. For the entire study, simplified model surfaces of 5% copper (II) oxide on silica and 5% iron (III) oxide on silica were used in order to facilitate comparison with previous data from similar experiments performed with pure samples of 2-MCP and 1,2-DCBz. Precursor 2-MCP is useful as a model chlorinated phenol, while 1,2-DCBz was selected because it has been found to be present in high concentration in relation to other congeners of polychlorinated benzenes in combustion exhaust and is nearly isoeletronic with 2-MCP, that provides a basis for comparison of product distributions and yields. Reaction pathways of PCDD/F products as well as the intermediates involved are comprehensively analyzed and discussed.
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Nganai, Shadrack Kirwa, "Iron (III) Oxide and Copper (II) Oxide Mediated Formation of PCDD/Fs from Thermal Degradation of 2-MCP and 1,2-DCBz" (2010). LSU Doctoral Dissertations. 381.