Grantee Research Project Results
Final Report: Oxidative Transformation of Model Oxygenated Hazardous Air Pollutants
EPA Grant Number: R828175Title: Oxidative Transformation of Model Oxygenated Hazardous Air Pollutants
Investigators: Taylor, Philip H. , Marshall, Paul
Institution: University of Dayton , University of North Texas
EPA Project Officer: Hahn, Intaek
Project Period: July 20, 2000 through July 19, 2002
Project Amount: $215,900
RFA: Exploratory Research - Engineering, Chemistry, and Physics) (1999) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Water , Land and Waste Management , Air , Safer Chemicals
Objective:
The overall objective of this research project was to determine the rates and mechanisms of hydroxyl (OH) reactions with representative oxygenated hazardous air pollutants (i.e., formaldehyde, acetaldehyde, and acetone) under conditions that are representative of both atmospheric and combustion conditions. Reaction with OH radicals is an important step in the oxidation of organic compounds in the atmosphere and in combustion systems. Formaldehyde (CH2O) and acetaldehyde (CH3CHO) are hazardous air pollutants (HAPs) regulated under Title III of the Clean Air Act Amendments. The kinetic and mechanistic studies were used to validate comprehensive theoretical studies of these reactions.
Specific objectives of this research project were to conduct: (1) direct measurement of the OH + CH3COCH3 reaction over an extended temperature range; (2) experimental validation of the mechanism of the OH + CH3CHO and OH + CH3COCH3 reaction under both low temperature atmospheric conditions and higher temperature, post-combustion conditions; and (3) theoretical evaluation of the mechanism of the OH + CH2O, OH + CH3CHO and OH + CH3COCH3 reaction of both low temperature atmospheric conditions and higher temperature, post-combustion conditions.
Summary/Accomplishments (Outputs/Outcomes):
We have performed absolute rate measurements and/or theoretical analysis of the following reactions:
- OH + CH3COCH3 → Products
OH + CH3CHO → Products
OH + HCHO → Products
OH + CH3COCH3. The pulsed laser photolysis/pulsed laser-induced fluorescence (PLP/PLIF) technique has been applied to obtain rate coefficients for OH + CH3C(O)CH3 and CD3C(O)CD3 of kH(298-832 K) = (3.99 ± 0.40) x 10-24 T4.00 except (453 ± 44)/T and kD(298-710 K) = (1.94 ± 0.31) x 10-21 T3.17 except (-529 ± 68)/T cm3 molecule-1 s-1, respectively. New measurements bridge the gap between shock-tube and lower temperature data, and data for the deuterated analog have been obtained at elevated temperatures. Three pathways were characterized via the CBS-QB3 ab initio method to obtain complete basis set limits for coupled-cluster theory. This addition slowly forms CH3C(O)(OH)CH3, followed by dissociation to CH3 + CH3C(O)OH. Variational transition state theory reveals that the dominant products are CH3C(O)CH2 + H2O, formed by direct abstraction at higher temperatures, and via a hydrogen-bonded complex below about 450 K. Inclusion of tunneling gives excellent accord with the kinetic isotope effect.
OH + CH3CHO. The kinetics of the gas-phase reaction between OH radicals, CH3CHO, CH3CDO, and CD3CHO were investigated at selected temperatures (297, 383, 600, and 860 K) at 740 ± 10 torr in a helium bath gas. Absolute rate measurements were obtained using the PLP/PLIF technique under slow flow conditions. Primary kinetic isotope effect measurements for the OH + CH3CHO reaction at temperatures of 297, 383, 600, and 860 K indicate that H abstraction from the acetyl group dominates that of the methyl group at low to modest temperature (≤ 600 K), and H abstraction from the methyl group dominates that from the acetyl group at higher temperatures (860 K). A high-level theoretical analysis of the primary kinetic isotope effect (KIE) of the different pathways supported these general conclusions. The calculated KIE ratio for the low temperature OH attack on the acetyl group was remarkably consistent with experimental results, whereas the KIE ratio for the OH attack on the methyl group needs further investigation. The theoretical analysis also indicated that OH addition to the carbonyl group, previously identified as a potentially important reaction channel at low temperatures, had the highest energy barrier of the reactions identified and would be a negligible reaction at all but the very highest temperatures.
OH + HCHO. Two pathways were identified from theoretical studies. The first involved OH addition to CH2O for an initial hydrogen-bonded adduct bound by 13.0 kJ mol-1. The second involved dissociation of this adduct to H2O + HCO, overall ΔH0 = -126.1 kJ mol-1. This is H abstraction via a bound intermediate and is the dominant path. The other path is OH addition to the C= O π bond over a barrier of about 11 kJ mol-1, and through a tight TS, to yield CH2(OH)O, with a predicted overall ΔH0 of -91 kJ mol-1. This initially energized radical will decompose rapidly through H-atom elimination to yield formic acid. There also is a high barrier path (+24 kJ mol-1 relative to OH + CH2O) for elimination of H2O, which is a second channel to yield H2O + HCO. Work is continuing to reconcile the large deviation between the calculated and experimental rate coefficients for pathway 1 between 250 and 500 K. We have reevaluated the energies using the more demanding but more accurate coupled cluster approach, combined with extrapolation to the complete basis set limit. CCSD(T) results with the basis sets aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ are employed. An interesting aspect of the reaction coordinate for abstraction (i.e., elimination of H2O from the H-bonded adduct) is the shape of the barrier. It is very broad, which makes tunneling negligible. It also is poorly described by standard 1-D potentials (such as the Eckart formalism), which makes earlier literature analysis of tunneling in this system suspect. We hope to eventually check this idea experimentally, via the kinetic isotope effect, which is sensitive to the degree of tunneling.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 8 publications | 4 publications in selected types | All 4 journal articles |
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Type | Citation | ||
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Yamada T, Taylor PH, Gourmi A, Marshall P. The reaction of OH with acetone and acetone-d6 from 298 to 832 K: rate coefficients and reaction mechanism. Journal of Chemical Physics 2003;119(20):10600-10606. |
R828175 (Final) |
not available |
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Yamada T, Taylor PH, Gourmi A, Marshall P. A reexamination of the mechanism of the reaction of OH with acetaldehyde and deuterated acetaldehyde. Journal of Physical Chemistry A. |
R828175 (Final) |
not available |
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Yamada T, Taylor PH, Gourmi A, Marshall P. The mechanism of the reaction of OH with formaldehyde. Journal of Physical Chemistry A. |
R828175 (Final) |
not available |
Supplemental Keywords:
air, exposure, hazardous air pollutants, HAPs, combustion chemistry, atmospheric chemistry, engineering., RFA, Scientific Discipline, Air, Waste, Environmental Chemistry, air toxics, tropospheric ozone, Engineering, Chemistry, & Physics, Incineration/Combustion, hydroxyl radical, risk assessment, combustion systems, hydrocarbon, oxidation, stratospheric ozone, Acetaldehyde, VOCs, hazardous air pollutants, HAPS, Clean Air Act , hydrocarbon oxidation, kinetic models, laser induced flouresence studies, chemical kinetics, combustion, acetone, hazardous air pollutants (HAPs), hydrocarbons, Volatile Organic Compounds (VOCs)Progress 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.