Investigation of the Elementary Reaction Mechanism of Surface Mediated Formation of PCDD/F from Aromatic Precursors

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Abstract CTS 96-18433 Dellinger This is a study of the mechanism of formation of polychlorinated dibenzodioxins (PCDD) polychlorinated dibenzofurans (PCDF), and related compounds (collectively PCDD/F). Previous work haq shown the process to be catalyzed by fly ash in the range of 250-400(C. Transition metals such as iron and copper also seem to catalyze the formation of these materials. A search for evidence of surface mediated reactions tests a hypothetical mechanism for the process. Samples containing various combinations of silicate and alumina glasses (surrogates for fly ash), organic precursors (chlorobenzenes, phenol, and other aromatic compounds), and chlorinating agents (chlorine, hydrogen chloride, and transition metal chlorides) are prepared using resistively heated wafers in a special adsorption cell. Chemisorption, Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), laser-induced fluorescence spectroscopy (LIF), micro-Raman spectroscopy, and other instrumental and wet chemical techniques are used to probe reaction intermediates. From these data, an elementary chemical reaction and kinetic model for the formation of PCDD/F is developed. Elimination of emissions containing polychlorinated dibenzodioxins and dibenzofurans (PCDD/F) from combustion sources is among the most pressing of environmental issues. An improved understanding of the origin and mechanism of formation of PCDD/F may lead to a strategy to avoid their formation by process modification (pollution prevention), obviating the need for post-process control measures.