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Ethanol Oxidation and Its Interaction with Nitric Oxide M. U. Alzueta* and J. M. Herna´ndez Department of Chemical and Environmental Engineering, Centro Polite´ cnico Superior, Maria de Luna, 3, 50015 Zaragoza, Spain Received July 6, 2001. Revised Manuscript Received October 13, 2001
An experimental and theoretical study of the oxidation of ethanol in absence and in the presence of NO has been performed. The experiments were conducted in an isothermal quartz flow reactor at atmospheric pressure in the 700-1500 K temperature range. The influence of the temperature, oxygen concentration, and presence of NO on the concentrations of ethanol, CO, CO2, and NO has been analyzed. A reaction mechanism based on the model of Glarborg et al.1 for hydrocarbon/ NO interactions, the mechanism for methanol oxidation2, updated in relation to the ethanol reaction subset, mainly with reactions taken from Marinov3, has been used for calculations. The results show that the oxidation regime of ethanol for different air excess conditions is very similar in the absence and presence of NO, even though differences can be seen. In general, as the stoichiometry becomes leaner, the oxidation of ethanol is produced at lower temperatures, with little differences between rich and close to stoichiometric conditions. The presence of NO results in higher differences of the oxidation regimes of ethanol, in the way that NO inhibits ethanol conversion for the richest conditions, while it promotes ethanol oxidation for very lean conditions. The experimental results are analyzed in terms of detailed chemistry and the main issues are discussed.
Introduction Ethanol was tested as a fuel in the first developments of the automotive industry, but was not competitive with gasoline and fuel-oil for economical reasons. However, as environmental concerns become more and more important, the importance of alternative fuels, also for transport, gains popularity. Ethanol is significantly used as an additive or in ethanol/gasoline mixtures in the so-called “oxygenated” or “reformulated” gasolines, which include different levels of ethanol in order to minimize CO and unburned hydrocarbon emissions. Different experimental studies concerning ethanol oxidation have been reported until now. The oldest studies correspond to the 50’s, 60’s, and 70’s decades,4-6 while more recent studies which are relevant concerning ethanol oxidation include the works of Rotzol,7 Norton and Dryer,8,9 Dagaut et al.,10 and Taylor et al.11 On the basis of these studies, detailed reaction mechanisms for * Corresponding author. Fax: +34 976 761879. E-mail: uxue@ posta.unizar.es. (1) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Combust. Flame 1998, 115, 1-27. (2) Alzueta, M. U.; Bilbao, R.; Finestra, M. Energy Fuels 2001, 15, 724-729. (3) Marinov, N. M. Int. J. Chem. Kinet. 1999, 31, 183-200. (4) Smith, S. R.; Gordon, A. S. J. Phys. Chem. 1956, 60, 1059-1067. (5) Lieb, D. F.; Roblee, L. H. S., Jr. Combust. Flame 1970, 14, 286296. (6) Cooke, D. F.; Dodson, M. G.; Williams, A. Combust. Flame 1971, 16, 233-241. (7) Rotzoll, G. J. Anal. Appl. Pyrolysis 1985, 9, 43-57. (8) Norton, T. S.; Dryer, F. L. 23rd Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, PA, 1990; pp 179185. (9) Norton, T. S.; Dryer, F. L. Int. J. Chem. Kinet. 1992, 24, 319344. (10) Dagaut, P.; Cathonnet, M.; Boettner, J. C. J. Chim. Phys. 1992, 89, 867-884.
ethanol conversion have been proposed,3,9 which cover a wide range of operating conditions. The aim of the present work is to extend the experimental database on ethanol oxidation as well as on the interaction of ethanol with nitric oxide, as well as to perform a detailed kinetic modeling study. NO may be formed in the combustion chamber and once formed, it might be partially destroyed by ethanol or its reaction products. The oxidation of ethanol is studied under flow reactor conditions in the 700-1500 K temperature range, both in the absence and the presence of nitric oxide for different air excess ratios. The results are analyzed in terms of a detailed chemical kinetic model. Experimental Section The experimental installation used in the present work is described in detail elsewhere2,12 and only a brief description is given here. A laminar quartz flow reactor following the design of Kristensen et al.13 is placed in a three-zone electrically heated oven, ensuring a uniform temperature profile ((5 K) along the reaction zone. The reactor tube has a reaction zone of 8.7 mm inside diameter and a length of 200 mm. Pure gases from gas cylinders are led to the reactor in up to four separate streams: a main flow containing nitrogen and water, and the injector tubes for the rest of reactants. Ethanol is added by saturation of a nitrogen stream in an ethanol bath at a controlled temperature. At the outlet of the reaction zone, the product gas is quenched by the addition of cooling air. The analysis of the product gas is performed by means of a FTIR (11) Taylor, P. H.; Cheng, L.; Dellinger, B. Combust. Flame 1998, 115, 561-567. (12) Alzueta, M. U.; Oliva, M.; Glarborg, P. Int. J. Chem. Kinet. 1998, 30, 683-697. (13) Kristensen, P. G.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1996, 107, 211-222.
10.1021/ef010153n CCC: $22.00 © 2002 American Chemical Society Published on Web 12/22/2001
Ethanol Oxidation and Its Interaction with Nitric Oxide (Fourier Transform Infra-Red) spectrometer and a continuous NO analyzer. The estimated uncertainty of the measurements is (5% but not less than 10 ppm.
Reaction Mechanism The experimental results are analyzed in terms of a detailed chemical kinetic model for ethanol conversion in the presence of NO. The chemical kinetic model used in the present study is based on the reaction mechanism developed by Glarborg et al.,1 to describe the interactions among C1/C2 hydrocarbons and nitric oxide, updated with the more recent mechanism of Alzueta et al.2 for the conversion of methanol, also in the presence of NO, together with the reaction subset for ethanol conversion proposed by Marinov.3 The main reactions of interest are discussed below and the full mechanism can be obtained directly from the authors. Calculations are performed using Senkin,14 which runs in conjunction with the Chemkin library.15 The reverse rate constants were obtained from the forward rate constants and thermodynamic data were taken from the same sources as the different submechanisms. As shown below, the oxidation of ethanol is mainly sensitive to the initial consumption steps, even though reactions involving the hydroxymethyl radical have also certain significance. Under the conditions of the present work, ethanol reacts mainly with the radical pool through H abstraction, particularly with OH, to produce three different isomeric forms of the C2H5O radicals:
C2H5OH + /OH/H/O/CH3/HO2 h CH3CHOH + ... C2H5OH + /OH/H/O/CH3 h CH3CH2O + ... C2H5OH + /OH/H/O/CH3 h CH2CH2OH + ... with a minor reaction path giving directly ethylene from ethanol decomposition. This is path is, however, almost negligible under the present flow reactor conditions. The more abundant C2H5O isomer under the investigated conditions is CH3CHOH, which basically reacts with molecular oxygen producing acetaldehyde, which in turns by reaction with the radical pool suffers hydrogen abstraction giving the acetyl radical. Once formed, this radical is thermally decomposed into CO and CH3 radicals. The CH3CH2O radical has two main reaction pathways, one resulting in acetaldehyde which evolves as mentioned above, and a second one through which that radical is decomposed into formaldehyde and CH3 radicals. Formaldehyde is also the main product coming from reaction of CH2CH2OH, and once formed is quickly converted into HCO and subsequently into CO. The ethylene formed from ethanol, even though in low amounts, participates in a number of reactions which result in CO, as well as in an increase of the radical pool.1 (14) Lutz, A.; Kee, R. J.; Miller, J. A. Senkin: A Fortran Program for Predicting Homogeneous Gas-Phase Chemical Kinetics with Sensitivity Analysis. Sandia National Laboratories Report SAND87-8248, 1988. (15) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin-II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics. Sandia National Laboratories Report SAND89-8009, 1989.
Energy & Fuels, Vol. 16, No. 1, 2002 167 Table 1. Experimental Conditionsa expt
C2H5OH (ppm)
O2 (ppm)
H2O (%)
NO (ppm)
λ
res. time (s)
Set 1 Set 2 Set 3 Set 4 Set 5 Set 6
700 720 600 735 725 800
1531 1365 2038 2187 76300 75664
1.01 0.96 0.80 0.64 0.70 0.61
0 520 0 576 0 490
0.73 0.63 1.13 0.99 35.1 31.5
220/T(K) 210/T(K) 210/T(K) 200/T(K) 230/T(K) 230/T(K)
a The experiments are conducted at a constant flow rate, and thereby the residence time is dependent on the reaction temperature, as listed.
Figure 1. Concentration of C2H5OH, CO, and CO2 as a function of temperature for the conditions of set 3 in Table 1. Comparison between experimental data (symbols) and model predictions (lines).
It should be mentioned that the subset for ethanol conversion used in the present work, taken from Marinov,3 is based on RRKM calculations. The reactions included in it are very similar to the mechanism proposed by Norton and Dryer,9 which was built using data from literature and including some estimations for the branching ratios of the H abstraction reactions of ethanol. We have also made a number of calculations with the Norton and Dryer subset and the fitting to the present experimental conditions was a bit worse compared to the calculations shown. However, the main trends were also identified by the Norton and Dryer submechanism. Results and Discussion A study of the oxidation of ethanol at atmospheric pressure in the temperature range 700-1500 K has been carried out. The experiments were conducted keeping a constant flow rate and varying the reaction temperature, resulting in a variable residence time as a function of temperature. In addition to temperature, the influence of other variables such as the air excess ratio, λ, and the NO concentration has been analyzed. Table 1 lists the conditions of selected experiments, all performed under highly diluted conditions. Reactants used are C2H5OH, O2, H2O, NO, and nitrogen to balance. Figure 1 shows an example of the results of ethanol, CO, and CO2 as function of temperature for the conditions of set 3, i.e., almost stoichiometric conditions, in the absence of NO. Symbols denote experimental results and lines model calculations. The model matches well
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Figure 2. Conversion of C2H5OH as a function of temperature for different air excess ratios. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 1, 3, and 5 in Table 1.
Figure 4. CO2 concentration profiles during ethanol oxidation as a function of temperature for different air excess ratios. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 1, 3, and 5 in Table 1.
Figure 3. CO concentration profiles during ethanol oxidation as a function of temperature for different air excess ratios. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 1, 3, and 5 in Table 1.
Figure 5. Reaction path diagram for ethanol oxidation.
the experimental data. For the conditions of Figure 1, the conversion of ethanol starts slightly above 900 K. The onset of CO formation coincides with the decrease in ethanol concentration, and CO peaks at around 1100 K. As the temperature is increased, ethanol is completely converted into CO2. Apart from ethanol, CO, and CO2, small amounts of methanol and ethylene were detected in the FTIR spectra, but those are not shown in the figures because the amount detected lies within the uncertainty of the measurements, i.e., (10 ppm. Figures 2 to 4 show the results of ethanol, CO, and CO2 as a function of temperature for different air excess ratios, rich, almost stoichiometric and very lean conditions, in the absence of NO. As seen in Figure 2, the oxygen concentration does not influence significantly the onset of ethanol conversion for the richest and almost stoichiometric conditions, even though both experimentally and theoretically, the results obtained under richest conditions are slightly shifted toward higher temperatures, about 20 K. At the leanest conditions studied, the conversion of ethanol is produced at approximately 100 K lower compared to the rest of operating conditions. The model agrees reasonably well
with the experimental results, but the calculated profile seems to be sharper compared to the experimental one. The oxygen availability has a slightly more pronounced effect on the concentrations of CO and CO2, keeping the same tendency: the onset of CO and CO2 formation is shifted to lower temperatures as the stoichiometry becomes leaner. This is more evident when shifting from stoichiometric to very lean conditions. Again, calculations agree well with experimental data. Compared to the results obtained in a previous methanol oxidation study by our group,2 a significant amount of CO is found for the richest conditions. Despite the presence of oxygen in the ethanol molecule which favors a major extent of its oxidation, the ratio C/O in such a molecule is 2 and therefore a complete oxidation cannot be expected as it happened for methanol, even under rich conditions. Figure 5 shows a reaction path diagram for the oxidation of ethanol under the present conditions. The oxidation of ethanol is initiated mainly by reaction with OH radicals, even though the interaction with H and CH3 radicals is also significant. Only for the leanest conditions, the interaction of ethanol with O radicals becomes appreciable. The interaction of ethanol with the radical pool results in three different isomers of the C2H5O radical, being the CH3CHOH radical the dominant under all the conditions of the present work. A certain amount of ethylene is also formed by thermal
Ethanol Oxidation and Its Interaction with Nitric Oxide
Energy & Fuels, Vol. 16, No. 1, 2002 169
Table 2. Linear Sensitivity Coefficients for CO at Selected Temperatures for the Experiments of Table 1a reaction C2H5OH(+M) h CH2OH + CH3(+M) C2H5OH + OH h CH2CH2OH + H2O C2H5OH + OH h CH3CHOH + H2O C2H5OH + OH h CH3CH2O + H2O C2H5OH + H h CH3CH2O + H2 C2H5OH + CH3 h CH3CHOH + CH4 C2H5OH + CH3 h CH3CH2O + CH4 C2H5OH + HO2 h CH3CH2O + H2O2 CH3CH2O + M h CH3HCO + H + M CH3CH2O + M h CH3 + CH2O + M O + OH h O2 + H H + HO2 h H2 + O2 H + HO2 h OH + OH OH + HO2 h H2O + O2 H2O2 + M h OH + OH + M CH2O + OH h HCO + H2O HCO + M h H + CO + M CH4 + O2 h CH3 + HO2 CH3 + HO2 h CH3O + OH CH3 + CH3(+M) h C2H6(+M) NO2 + H h NO + OH HCO + NO h CO + HNO CH3 + NO2 h CH3O + NO
Set 1 1000 K
Set 2 975 K
Set 3 1000 K
Set 4 950 K
Set 5 950 K
Set 6 855 K
0.29 0.24 0.13 -0.49 -0.10 0.18 0.29 0.69 0.37 -0.37 0.54 0.33 0.11 -0.17 0.16 0.36 0.02 -0.20 0.64 -0.38
0.28 0.15 0.64 -0.93 -0.18 0.12 0.34 0.01 0.61 -0.61 1.09 0.00 -0.00 -0.00 0.00 0.31 0.21 -0.00 0.00 -0.44 -0.66 -0.34 0.74
0.32 0.25 0.08 -0.47 -0.11 0.18 0.26 0.69 0.46 -0.46 0.80 -0.36 -0.13 -0.21 0.19 0.41 0.04 -0.22 0.81 -0.42
0.30 0.16 0.99 -1.27 -0.28 0.16 0.50 0.01 1.12 -1.12 2.01 -0.01 -0.00 -0.00 0.00 0.37 0.24 -0.00 0.00 -0.66 -0.94 -0.44 1.28
0.65 0.08 -1.04 1.34 -0.03 0.07 -0.15 1.73 1.38 -1.38 3.05 -0.10 -0.04 -0.44 2.06 0.09 0.07 -0.38 2.22 -0.64
7.27 0.34 8.78 -0.30 -0.73 3.97 12.28 0.02 34.49 -34.53 35.28 -0.00 -0.00 -0.01 0.02 0.80 0.01 -0.01 0.04 -6.12 -0.55 -0.01 17.48
a The sensitivity coefficients are given as A δY /Y δA , where A is the preexponential constant for reaction i and Y is the mass fraction i j j i i j of the jth species. Therefore, the sensitivity coefficients listed can be interpreted as the relative change in predicted concentration for the species j caused by increasing the rate constant for reaction i by a factor of 2.
decomposition, which contributes to understanding the detection of this compound which was observed in the FTIR spectra. The CH3CHOH radical reacts almost exclusively with molecular oxygen giving acetaldehyde, which in turns reacts with the radical pool suffering hydrogen abstraction and producing the acetyl radical, that decomposes into CH3 radicals and CO. The second C2H5O isomer formed, CH3CH2O, is converted either into CH3HCO or decomposed to formaldehyde and methyl radicals. The third isomer, CH2CH2OH, is almost quantitatively converted into formaldehyde which by reaction with the radical pool produces HCO and subsequently CO and CO2. As mentioned above, little differences are seen among the different stoichiometric conditions studied in this work with respect to reaction pathways, and the most important differences are found in the production of the three isomers from ethanol. Table 2 shows a first-order sensitivity analysis for CO corresponding to the initiation conditions of the experiments corresponding to experiments of Table 1. Sensitivity coefficients for ethanol oxidation in the absence of NO are those corresponding to sets 1,3, and 5, while coefficients in the presence of NO correspond to sets 2, 4, and 6. In general, the sensitivity analysis confirms that the predicted onset of CO formation is very sensitive to the ethanol + radical reactions, in particular OH, H, CH3, and HO2. It is interesting to note the importance of the reaction between ethanol and the hydroperoxy radical, in the absence of NO when appreciable amounts of HO2 can be present. In the present work, the influence of NO presence has also been considered. The use of ethanol itself or in fuel mixtures may result in the generation of nitrogen oxides, under combustion conditions, through the thermal NOx formation mechanism. Once formed, nitrogen oxides may interact with ethanol or its derivatives. NO may be reduced in reburning type reactions under subestequiometric conditions16 or may favor the oxida-
Figure 6. Conversion of C2H5OH as a function of temperature for different air excess ratios in the presence of NO. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 2, 4, and 6 in Table 1.
tion of ethanol in a mutually sensitized oxidation process.11,17 Figures 6 to 8 show, respectively, the results of ethanol, CO, and CO2, obtained during the oxidation of ethanol in the presence of NO, corresponding to the experiments of sets 2, 4, and 6 in Table 1. Model calculations agree quite well with the experimental findings. The results of ethanol as a function of temperature for different air excess ratios are shown in Figure 6. Compared to Figure 2 (ethanol conversion in the absence of NO), the presence of NO implies a higher difference in the temperature regime for which ethanol conversion is produced under different stoichiometric (16) Alzueta, M. U.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1997, 109, 25-36. (17) Bromly, J. H.; Barnes, F. J.; Muris, S.; You, X.; Haynes, B. S. Combust. Sci. Technol. 1996, 115, 259-296.
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Figure 7. CO concentration profiles during ethanol oxidation as a function of temperature for different air excess ratios in the presence of NO. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 2, 4, and 6 in Table 1.
Alzueta and Herna´ ndez
Figure 9. NO concentration profiles during ethanol oxidation as a function of temperature for different air excess ratios. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 2, 4, and 6 in Table 1.
with the subsequent conversion of NO2 to NO, and under rich conditions through the interconversion NO/ HNO:
NO + HCO h HNO + CO HNO + H h NO + H2 NO2 + OH h NO + HO2
Figure 8. CO2 concentration profiles during ethanol oxidation as a function of temperature for different air excess ratios in the presence of NO. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 2, 4, and 6 in Table 1.
conditions. In particular, the results obtained under very lean conditions and in the presence of NO are shifted about 200 K to lower temperatures than the results obtained in the absence of NO. Again, the results shown in Figure 6 indicate that the influence of oxygen concentration is not very important in the stoichiometry range of rich to close to stoichiometric conditions. However, the comparison of Figures 2 and 6, for the rich and stoichiometric conditions of the present work, indicates that the presence of NO inhibits slightly the conversion of ethanol under the mentioned conditions. This inhibition is attributed to radical recombination catalyzed by NO,17,18 which under all the conditions studied proceeds through the NO/NO2 interconversion, but also under very lean conditions through
NO + OH h HONO HONO + OH h NO2 + H2O (18) Glarborg, P.; Kubel, D.; Kristensen, P. G.; Hansen, J.; DamJohansen, K. Combust. Sci. Technol. 1995, 111, 461-485.
Similar trends are seen for CO and CO2 concentrations, shown, respectively, in Figures 7 and 8. The CO and CO2 profiles do follow the same shape in as the experiments in the absence of NO, but similar shifts with respect to the onset for CO and CO2 formation as a function of temperature compared to the profiles of ethanol in Figure 6 are found. The reaction pathway analysis performed shows that the main reaction paths for ethanol conversion in the presence of NO are equal to those obtained in the absence of NO which are shown in Figure 5. Therefore, the only effect of NO on ethanol oxidation comes from its capacity of modifying the composition of the radical pool. Also, the sensitivity analysis shown in Table 2 indicates that ethanol conversion in the presence of NO is sensitive to almost the same reactions as in the absence of this compound. Also, the influence of NOx, both NO2 and NO, is noticeable with the appearance of significant sensitivity coefficients for CO when NO is added to the experiments. Figure 9 shows the results of NO as a function of temperature for the different experiments of ethanol oxidation in the presence of NO. The results show that the temperature regime for which appreciable facts are seen coincide with the temperature range for the main conversion of ethanol. Thus, under rich and stoichiometric conditions, no effect in NO concentration is seen up to a temperature of about 1100 K, while for lean conditions, a minimum in NO concentration is found at a temperature slightly lower than 900 K. Model calculations show that the evolution of NO during the oxidation of ethanol is as shown in Figure 10. Solid lines indicate the reaction pathways occurring for all the conditions studied, dashed lines correspond to those reactions
Ethanol Oxidation and Its Interaction with Nitric Oxide
Figure 10. Reaction path diagram for the conversion of NO.
active only for rich and stoichiometric conditions, while dotted lines indicate the pathways happening exclusively under very lean conditions. As seen in Figure 10, the main reaction involving NO is the conversion into NO2 by reaction with HO2 radicals, which is dominant for all the studied conditions. Under rich and stoichiometric conditions, there is also one important path, which implies the conversion of NO into HNO by reaction with H radicals. In both cases, NO2 and HNO are recycled back to NO by reaction with a number of radicals Under the very lean conditions studied, there is a new reaction path which starts to be relevant, implying the interaction of NO with OH radicals to give HONO which further evolves to NO2 by reaction also with OH radicals. Also, for the richest conditions of the present work, additional reaction pathways (not shown in Figure 10) are active, but they represent overall a contribution lower than 10%. Those pathways correspond to the interaction of NO with intermediate nitrogen species, such as NH2 and NCO, to produce N2 or N2O which is further converted to N2. It is important to note that under the conditions studied, calculations do not show any reduction by reburn-type reactions, not even under rich conditions and at the highest temperatures studied. Conclusions The oxidation of ethanol has been studied in a laminar quartz flow reactor at atmospheric pressure and
Energy & Fuels, Vol. 16, No. 1, 2002 171
temperatures from 700 to 1500 K. The stoichiometry has ranged from rich to lean conditions and the effect of the addition of nitric oxide has been investigated. The results have been analyzed in terms of a detailed chemical kinetic model. In the absence of nitric oxide, oxidation of ethanol is initiated slightly below 1000 K for rich and stoichiometric conditions, while the onset for ethanol conversion is shifted toward lower temperatures, about 100 K, under very lean conditions. Little influence of the oxygen level is seen for rich and stoichiometric conditions. CO and CO2, formed from ethanol, follow similar temperature regimes compared to ethanol, with CO peaking at a temperature coinciding with approximately 90% of the ethanol conversion, and CO2 increasing monotonically as the temperature increases. The conversion of ethanol is initiated basically by reaction with the radical pool producing three different C2H5O isomers, with a minor path producing C2H4 directly. Those isomers react mainly with molecular oxygen or are decomposed resulting into actaldehyde and/or formaldehyde, which evolve subsequently to CO and CO2. The kinetic modeling predictions agree quite well with the experimental data. The addition of NO in the ethanol oxidation experiments results, for very lean conditions, in a significant promotion of the onset of ethanol conversion, which is shifted to lower temperatures compared to the results in the absence of NO. However, under rich and stoichiometric conditions the presence of NO inhibits slightly the conversion of ethanol. NO is mainly converted to NO2, and recycled back to NO, even though other minor reaction pathways are also active. Acknowledgment. The authors express their gratitude to project DGA-P061/99-T for financial support. EF010153N
