Grantee Research Project Results
2000 Progress 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 Period Covered by this Report: July 20, 2000 through July 19, 2001
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:
Reaction with hydroxyl (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 overall goal of this research is to determine the rates and mechanisms of OH reactions with representative oxygenated HAPs (i.e., formaldehyde, acetaldehyde, and acetone) under conditions that are representative of both atmospheric and combustion conditions. The kinetic and mechanistic studies will be used to validate comprehensive theoretical studies of these reactions.
Absolute rates for reaction 1 (formaldehyde) and reaction 3 (acetone) will be measured with previously validated PLP-LIF techniques over extended ranges of temperature and pressure. These measurements will focus on data above 550 K for formaldehyde and above 400 K for acetone. Diagnostic experiments will include kinetic isotope effect measurements using the PLP-LIF technique and product measurements using a PI-ToFMS technique. The kinetic and mechanistic studies will be used to validate comprehensive theoretical studies of these reactions. This research can then be used to provide a sound basis for understanding the mechanism of reaction of OH radicals with organic compounds containing carbonyl functional groups.
Progress Summary:
We have made significant progress on all three objectives of this study. We have completed experimental rate measurements for reaction 1 (OH + CH3COCH3). Additional rate measurements also have been performed for reaction 2 (OH + CH3CHO) at selected temperatures. These measurements have involved extended temperature measurements and measurements with deuterated samples. Theoretical studies also have examined the reaction mechanism for reaction 1. Additional detail regarding the experimental studies is given in the following paragraphs.
OH + CH3COCH3
The kinetics of the gas-phase reaction between hydroxyl (OH) radicals and acetone
(k1) and d6-acetone
(k1a) were investigated between 298 and 710
K at 740 ± 10 torr in a helium bath gas. Absolute rate measurements were
obtained using a laser photolysis/laser-induced fluorescence (LP/LIF) technique
under slow flow conditions. HONO was used as the OH radical precursor to suppress
photolysis of the substrate. An XeF excimer laser (Lamda Physik Compex Model
102) was used as the photodissociation source (351 nm). Rate coefficients for
reaction 1 (k1) exhibited non-Arrhenius behavior
as illustrated in Figure 1.
Figure 1. Arrhenius plot of kinetic data for k1. Also shown are the results of previous studies and a modified Arrhenius fit to the data from this study (p = 740 ±10 torr).
The modified Arrhenius equation best describes the data and is given by (in units of cm3 molecule-1 s-1):
Error limits are 2 values. The values for k1 between 298 and 400 K are slightly lower than previous measurements, yet within combined error limits. The higher temperature measurements, when extrapolated to combustion temperatures, are consistent with the single reported measurement at 1200 K.
Measurements with the acetone-d6 substrate (k1a) indicated a substantial kinetic isotope effect as illustrated in Figure 2.
Figure 2. Arrhenius plot of kinetic data for k1 and k1a. Also shown are modified Arrhenius fits to the data from this study (p = 740 ±10 torr).
The modified Arrhenius equation best described the data for k1a and is given by (in units of cm3 molecule-1 s-1):
The magnitude of the kinetic isotope effect is consistent with a dominant mechanism involving H-atom abstraction.
To further examine the H abstraction mechanism, the composite ab initio theory, G3, was used to calculate the activation energy. Frequencies were calculated using HF/6-31G(d) level of theory and scaled by 0.8929. Geometry was optimized using MP2(full)/6-31G(d) level of theory, and moments of inertia also were calculated with this theory. Rate constants were evaluated using conventional TST. TST rate constants were calculated from 200 to 2000 K and compared with experimental results as shown in Figure 3.
Figure 3. TST calculations of the H abstraction reaction for OH + CH3COCH3.
G3 energies obtained at MP2(full)/6-31G(d) geometries with HF ZPE yielded energy barriers at 0 K of 10.8 and 14.1 kJ mol-1 for OH + CH3COCH3 and CD3COCD3, respectively. The TST calculation is well matched with experiment for acetone. The TST calculation is slightly underestimated for d6-acetone.
OH + CH3CHO
The kinetics of the gas-phase reaction between hydroxyl (OH) radicals and CH3CHO
(k2) and CH3CDO (k2a) were investigated at selected temperatures (298, 383,
and 600 K) at 740 ± 10 torr in a helium bath gas. Absolute rate measurements
were obtained using the LP/LIF technique under slow flow conditions. HONO was
used as the OH radical precursor to suppress photolysis of the substrate at
shorter wavelengths. An XeF excimer laser (Lamda Physik Compex Model 102) was
used as the photodissociation source (351 nm). Figure 4 presents the latest
measurements for k2. Also shown are comparisons with prior measurements using
different techniques, including our previous results published in 1996 (Rahman,
et al.) using N2O/H2O as the OH photolysis source. Our most recent measurements
at room temperature are in good agreement with prior measurements and approximately
30 percent higher than our previous results. Measurements at 383 and 600 K verify
the slight negative temperature dependence reported by other researchers. The
measurement at 383 K is consistent with prior results, and the measurement at
600 K is approximately 25 percent higher than our previous results.
Figure 4. Arrhenius plot of kinetic data for k2. Also shown are the results of previous studies and the low temperature recommendation of Atkinson.
Measurements with CH3CDO (k2a) indicated a measurable kinetic isotope effect as illustrated in Figure 5. The observed kinetic isotope effect indicates that a H abstraction mechanism contributes to the overall decay observed at lower temperatures. Our previous theoretical analysis at low temperatures indicated that this mechanism was a minor channel at low temperatures. Chemically activated OH addition to the carbonyl group, followed by elimination of CH3 radicals, was suggested as the dominant reaction pathway. An alternative pathway, elimination of H atoms, previously was shown to be negligible.
Figure 5. Arrhenius plot of kinetic data for k2 and k2a (p = 740 ±10 torr)
Future Activities:
During the final year of this study, we will focus on the mechanisms of reaction for each substrate. For reaction 1, the mechanism at low temperatures may involve more than a simple H abstraction, although this channel probably accounts for at least half of the measured rate. Theoretical studies currently are investigating the possibility of a chemically activated OH addition channel. Product studies will investigate the potential formation of CH3 radicals from such a pathway. For reaction 2, theoretical studies also are examining both the low temperature and high temperature mechanisms. The kinetic isotope data suggest that H abstraction is important at both low and high temperatures. The possibility that some fraction of the OH decay at low temperatures is due to OH addition with elimination of CH3 radicals will be examined both theoretically and experimentally. For reaction 3 (OH + CH2O), studies will likely be limited to theoretical examination of a potential OH addition pathway. Previously published experimental measurements indicate that H abstraction is the dominant pathway at both low and elevated temperatures.
Journal Articles on this Report : 1 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. Rate coefficients and modeling results for reaction of OH with acetone at low to moderate temperatures. Journal of Physical Chemistry. |
R828175 (2000) |
not available |
Supplemental Keywords:
combustion chemistry, environmental chemistry, exposure, air pollution, hazardous air pollutants., 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.