Grantee Research Project Results
Final Report: Toward the Development of a Detailed Mechanism of Transition Metal Catalyzed Formation of PCDD/F from Combustion Generated Hydrocarbons
EPA Grant Number: R828191Title: Toward the Development of a Detailed Mechanism of Transition Metal Catalyzed Formation of PCDD/F from Combustion Generated Hydrocarbons
Investigators: Dellinger, Barry , Lomnicki, Slawomir
Institution: Louisiana State University - Baton Rouge
EPA Project Officer: Hahn, Intaek
Project Period: July 1, 2000 through June 30, 2003
Project Amount: $345,000
RFA: Combustion Emissions (1999) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
Formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/PCDF) in combustion sources is a significant environmental issue. Field studies strongly suggest that they are formed in the post-combustion, cool zone of combustors by surface-mediated/catalytic pathways. Laboratory studies have demonstrated that some transition metals incorporated into silica-based fly ash can catalyze dioxin formation in the 250 to 500°C range. The exact mechanism, however, has not been determined yet.
The objective of this research project was to reveal the mechanism of the formation of PCDD/PCDF over metal oxide surfaces based upon two assumptions:
- (1) dioxin can be formed by surface condensation of the dioxin precursor compounds
(such as chlorophenols and chlorobenzenes). (2) light hydrocarbons present
in the exhaust stream can undergo metal-catalyzed growth and aromatization
to form dioxin precursors.
Summary/Accomplishments (Outputs/Outcomes):
We focused on the first of these hypotheses. We studied the reaction over copper oxide deposited on silica, as the presence of the former in the fly ash is widely reported.
For the copper oxide-mediated reaction, dibenzo-p-dioxin (DD), 1-monochloro dibenzo-p-dioxin (MCDD), and dichlorodibenzofuran (DCDF) were found to be formed from 2-chlorophenol. The highest conversion was found to be around 400 to 450° C. For the gas phase reaction, the maximum of dioxin formation was reported to be at around 550 to 600° C. This indicates that the surface of the catalyst is involved in the formation process. Also, the yields of formation are significant: for each 10,000 molecules of chlorophenol introduced into the system, 28 are converted into DD, MCDD, or DCDF. The highest overall conversion was recorded for DD. At low temperatures (i.e., 200 to 350° C), however, the DCDF yield was the highest.
The sharp decline of the dioxin formation is attributed to the oxidation of PCDDs/PCDFs or their surface precursors. In fact, during the process more than 9 percent of chlorophenol was completely oxidized to CO/CO 2 at temperatures above 350° C. Among higher chlorinated compounds, only higher chlorinated PCDDs congeners were detected, and no PCDF congeners other than DCDF were detected. In the case of PCDDs, almost all congeners were detected, from mono- to octa-substituted dibenzo-dioxin. When reaction temperature reached 350°C, however, only mono- and dichlorinated PCDD were formed.
A correlation between the amount of DD formed and the amount of highly chlorinated dioxins can be seen. At 350°C, where PCDDs are no longer observed, the concentration of DD suddenly increases. This indicates that highly chlorinated dioxins result from the chlorination of DD.
The reaction of 2-chlorophenol was also studied for oxidative conditions. In the presence of
3 mg of catalyst, no products were detected at temperatures above 250°C. For temperatures below this value, the only product detected was MCDD; however, its maximum yield was only 0.003 percent (250°C) and was at the detection limit.
The results obtained for 1 mg revealed a significant difference versus the results obtained in pyrolytic conditions. In this case, the highest yield was recorded for MCDD formation, one order of magnitude higher (~ 0.28%) than pyrolytic conditions. The maximum yield is at 350 to 375°C. In contrast, the yield of DCDF seems to be unaffected by the presence of oxygen. The lack of influence of oxygen on the DCDF yield indicates that oxygen is not involved or is not a limiting factor in its formation.
DD and MCDD exhibit a similar reaction rate of 0.6 and 0.7, respectively. The fractional rate orders for the reaction are typical for the surface mediated processes, where the adsorption factor is present in the denominator of the rate expression. A slightly higher value (closer to 1) observed for MCDD compared to DD indicates stronger adsorption factor. Indeed, the strong adsorption of DD precursor on the surface is supported by the fact that its yield seems to drop and the maximum formation is shifted significantly from 450 to 400°C when changing from pyrolytic to oxidative. At the same time, significantly more greater chlorinated dioxins are formed in oxygen rich conditions compared to pyrolysis.
It was concluded that the presence of oxygen enhances the chlorination process by surface hypochlorite species formation. As a result, a decrease in DD formation is observed in exchange for increased yields of PCDDs. This indicates that oxidation of DD increases significantly with the presence of oxygen in the stream. Although the yield of DD appears to decrease, it may have actually increased but the chlorination rate increased significantly and resulted in a real decrease in DD yields. On the other hand, the increased yield of MCDD under these conditions results from the fact that MCDD is desorbed from the surface during the process of its formation and thus is subject to extensive surface oxidation.
In contrast, DCDF formation exhibits a negative rate order. Taking into account the rate order for DCDF and that dioxins and furans are formed by condensation reaction of two chlorophenol molecules, DCDF is most probably formed by a Langmuir-Hinshelwood mechanism, whereas DD and MCDD are formed according to an Ely-Rideal mechanism.
These experimental results have been used to develop a detailed mechanism of the formation of PCDD/Fs over the catalytic surfaces.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 8 publications | 3 publications in selected types | All 3 journal articles |
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Type | Citation | ||
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Lomnicki S, Dellinger B. Formation of PCDD/F from the pyrolysis of 2-chlorophenol on the surface of dispersed copper oxide particles. Proceedings of the Combustion Institute 2002;29(2):2463-2468. |
R828191 (Final) |
Exit Exit |
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Lomnicki S, Dellinger B. A detailed mechanism of the surface-mediated formation of PCDD/F from the oxidation of 2-chlorophenol on CuO/silica surface. Journal of Physical Chemistry A 2003;107(22):4387-4395. |
R828191 (Final) R827719 (Final) |
Exit Exit Exit |
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Lomnicki S, Dellinger B. Development of supported iron oxide catalyst for destruction of PCDD/F. Environmental Science & Technology 2003;37(18):4254-4260. |
R828191 (Final) R827719 (Final) |
Exit Exit Exit |
Supplemental Keywords:
dioxin-like compounds, incineration, toxics, waste, environmental chemistry, health risk assessment, incineration/combustion, endocrine disruptors, pesticides, chemical mixtures, chlorinated phenols, combustion byproducts, dioxins, furans, PCDD, PCDF, fly ash, metal chlorides, surface mediated reactions,, RFA, Health, Scientific Discipline, Toxics, Waste, Health Risk Assessment, Environmental Chemistry, pesticides, Endocrine Disruptors - Environmental Exposure & Risk, HAPS, endocrine disruptors, Analytical Chemistry, Atmospheric Sciences, Incineration/Combustion, Endocrine Disruptors - Human Health, 33/50, combustion byproducts, dioxin, surface mediated reactions, hydrocarbon, fly ash, metal chlorides, endocrine disrupting chemicals, Tetrachloroethylene, air pollution, chemical mixtures, Trichloroethylene, complex combustion effluents, hydrocarbons, dioxins, chlorinated phenols, incineration, homogenous reaction studiesProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.