The Role of Radical-Surface and Molecule - Surface Reactions in the Extended Precursor Pathway to PCDD/F

摘要

The "fast" de novo and extended precursor mechanisms for formation of combustion-generated polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) suggest that flame-generated species interact with carbonaceous particles to form key reaction intermediates and/or PCDD/F precursors. Other steps in the pathway to PCDD/F formation imply chemisorption of precursors and condensation reactions. Using model systems and laser photolysis techniques, we have examined the role of hydroxyl radicals in the formation of these intermediates. The formation of methane, ethylene, acetylene and propylene ( ppb levels) were observed from the interaction of ArF laser-generated hydroxyl radicals with carbon pellets in the temperature range of 200-500 C. The formation of the olefins and acetylene reach maximum value between 430 and 450 C. These species may then be catalytically chlorinated by transition metals. These small, chlorinated hydrocarbons may then undergo molecular growth reactions to form chlorinated aromatic species. Using FTIR spectroscopy, we have shown that chlorobenzenes, chlorophenols, and other related compounds chemisorb on fly ashes. The chemisorption results in a favorable orientation for condensation reactions that eventually lead to formation of PCDD/F.