Rate constants for the reaction of oxygen atoms with some potential photosmog inhibitors

COMMUNICATION

RATE CONSTANTS FOR THE REACTION OF OXYGEN ATOMS WITH SOME POTENTIAL PHOTOSMOG INHIBITORS (First

receioed

15 June

1977 and

in final

form

1 December 1977)

are given for rate constant measurements ofground state oxygen atoms, 0(3P), with some potential photosmog inhibitors (ethylamine, triethyl~~ne, N,N’-diethylhydroxylamine, aniline and benzaldehyde) at 298 K determined in a discharge-flow reactor using electron spin resonance (ESR) detection. The O(‘P) atom reactivities are used to derive the scavenging efficiency of the photosmog inhibitors in polluted urban air. The data are discussed together with the reactivity of OH radicals with N,N’diethythydroxylamine (DEHA) in context with their proposed application for freshening polluted urban atmospheres Abstract-Results

INTRODUCTION The questioning of freshening polluted urban air by treatment with photosmog inhibitors has recently reattained topicality, largely on account of the diethylhydroxylamine (DEHA) controversy (Maugh, 1976; Tinker, 1976). Jayanti, Simonaitis and Heicklen (1974) after exhaustive qualitative studies on this and several other aliphatic amines as laboratory photosmoginhibitors suggested it as an object for field tests. Spicer et al. (1974) however, investigating other potential inhibitors observed that while reducing the NO + NO, oxidation rate some caused large increases in light scattering aerosol. Clearly it would be of interest to have information at hand enabling an easier initial choice of scavenger should their usage ever become practicable. With a view to providing preliminary information as an aid to unravelling these complex systems we report here the results of an electron spin resonance (ESR) study of the reaction ofground state oxygen atoms, O(3P), with several of the suggested inhibitors, namely diethylhydroxylamine, ethylamine, triethylamine, aniline and benzaldehyde. The latter two compounds have been suggested by Gitchell er al. (1974). Data are scarce on the reaction with hydroxylamines and amines, the only measurements being those of Kirchner et al. (1974) employing a Row system with mass spectrometric detection to measure the rate constants for the reaction of 0(3P) with some simple aliphatic amines. No rate constants are however available for the two aromatic sub stances. Data on the amines is of additional interest in connection with their U.V.deodorisation in air.

The flow rates of the carrier gas and oxygen were measured by capillary flow meters. The linear velocity amounted to cu. 36 m ‘. The high pumping speed necessary was provided by two Levbold Heraeus numas (Tvoe E 38 and DK 25). The pressure in the flow tube was ~m&ained at 4.7Torr’in all experiments. All reactants were dried and distilled prior to use. They were, after degassing by the freeze-thaw technique, maintained in a thermostatted water bath at a temperature sufficient to provide a constant pressure of 15-20 Torr for the duration of the run. Accurate monitoring into the Row system was carried out via a Hastings Mass Flowmeter (transducer principle, FALL series, calibrated against air) and a detachable jet containing 8 x 0.5 mm radially positioned holes allowing optimal mixing and negligible back diffusion of 0(3P) atoms into the reactant feed line. Calibration correction factors were taken, where available, from the manufacturers’ tables; otherwise they were theoretically derived, as recommended, from the heat capacity of the substances employed (Stull et al., 1969; Dobratz, 1941). The concentration of O(‘P) atoms was followed by ESR spectroscopy. The ESR signals for the oxygen atoms were obtained with a modulation amplitude adjusted such that the fine structure remained unresolved. In each measurement position along the flow tube the cavity was retuned. The signals with and without added reactants were obtained under identical operating conditions. The overall rates were obtained by comparing the ratio of the two signals at various positions along the flow tube. The reactions were studied in the concentration range of 10’4-10’5 molecules crnv3 inhibitor. Pseudo first order conditions were employed i.e. (inhibitor) > (0), enabling the overall rate constant to be calculated from the equation (Westenberg and de Haas, 1967):

s-

EXPERIMENTAL The overall rate constants for the reactions of the potential photosmog inhibitors were determined by following the consumption of the 0(3P) atoms by ESR spectroscopy. The complete X-band ESR instrument (20 XT, AEG) with the magnet and a TE,, ,z cylindrical cavity (Varian) is mounted on a mobile carriage so that it can be moved along the quartz reaction tube (22 mm i.d.) on rails over a 3 m range. The microwave power is drawn from a V 58 klystron (Varian). The system is essentially identical to that described previously (Kuhnes, 1974). Ground state oxygen atoms are generated by flowing oxygen in a large excess of the helium carrier gas (> 95%) through a microwave cavity (Fehsenfeld er of. (t965) powered by a 2.45 GHz Microton Mark III generator.

log ((0),/(0X)

= k’(S) d/a

where (0)s and (O), are the oxygen atom peak intensities without and with inhibitor (S) flow through the injector, respectively; d is the distance of the injector to an arbitrary zero position on the ESR cavity and u the linear flow velocity. Thus, the slopes of plots of log (O),,/(O), against (S), when plotted against d/v yield the rate constant k’. The final values have not been corrected for longitudinal diffusion of oxygen atoms in the carrier gas stream, as the effect is generally found to be negligible for all but reactions at higher temperatures. The rate constant obtained as a mean of four experiments in the case of DEHA was 6.6 + 0.14. lOi cm3 .moleeule- i s- ‘. A 20% error limit was assumed for the other reaction rates in, Table 1. 1563

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Short Communication

Table I. Rate constants for the reactions of0(3P) atoms with some potential photosmog inhibitors and reference substances. Temperature = 298 K __-Pressure Rate constant x 10” Substance (torr) (cm3 molecule-’ s-‘) Reference methylamine dimethylamine trimethylamine ethylamine

0.9-2.5 0.9-2.5 0.9-2.5 0.9-2.5 4.7 4.7

triethylamine diethylhydroxylamine (DEHA) aniline benzaldehyde

0.34 5.3 15.6 I.1 3.87 10.9

Kirchner Kirchner Kirchner Kirchner this this

6.6 0.24 0.48

et II/., 1974 et al., 1974 et al., 1974 et al., 1974 work work

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Fig. 1. (a) Reaction of O(3P) atoms with diethylhydroxylamine (DEHA) at different distances d along the flow tube at 298 K and 4.7 torr. (b) Plot of the slope kt from Fig. la vs the distance d(cm) at constant linear flow velocity I’. RESULTS AND DISCUSSION

(a) Rate constants The reaction between oxygen atoms and inhibitor vapors was studied at 4.7 Torr and 298 K. Figure la shows typical data for the decay of 0 atoms with DEHA concentration at four different reaction times, i.e. at four different positions along the flow tube, thus demonstrating the first order dependence of the reaction with respect to O(sP) atoms. The results are summarized in Fig. lb as a plot of kr (equal to _ d In (O)/dS) against r (equal to d/r), the reaction time. The slope ofcurves of the type lb yield the first order rate constant k’ uncorrected in our case for stoichiometry variations possibly associated with secondary reactions. Unfortunately, we were not equipped for stoichiometry studies in this investigation. We wish, however, to draw attention to the following points : (i) the presence of high concentrations of molecular oxygen at all times, (ii) the invariability of the pseudo first order kinetics over several half-lives, (iii) the general agreement with the stoichiometrically corrected results of Kirchner (Kirchner ef al., 1974) (see below) to support the hypothesis that 0 atom removal occurs predominantly by reaction with S with 1:l stoichiometry.

Molecular oxygen has been observed to quench the consumption of atoms by secondary radicals, presumably by adduct formation, thus maintaining close to stoichiometric conditions in oxygen-rich systems (Hand and Obenauf, 1972). Table 1 shows the final values of k’ obtained in these experiments. For comparison the values recently obtained by Kirchner et al. (1974) for some related amines are included. Clearly, except for the cases discussed below, there is a good order of magnitude agreement between measurements employing different methods and pressure range. Very evident is the large increase in k’ on progressively substituting the amino group, resulting in increases by factors ofca. 50and 2-8 on tri-methylating or triethylating, respectively. The lower increase in the ethyl case may result from increasing protection against attack by bulky groups around the reaction centre. The lower values for the aromatic substituted (aniline) compared to the aliphatic substituted amines (methyl, ethyl) is expected on the basis of the lower electron density at nitrogen in the former (Gould, 1969). Oxygen (sP) atoms have demonstrably electrophihc properties (Cvetanovic, 1963). The reason for the large difference in the values for the ethylamine is not clear though may be due to an as yet uninvestigated pressure dependence or uncertainties in the stoichiometry. As far as the authors are aware data are not available on the products of these reactions under atmospheric conditions. On the basis of the known electrophiticity of both radicals, O(3P) atoms and OH radicals, we feel justified in compar-

Short Communication ing the former with properties of the latter in solution. In air saturated aqueous solution reactions of OH radicals with neutral primary and secondary alkyl amines are known to be governed by the reaction sequence (Getoff and SchwSrer, 1973; Clay and Rashid, 1971). RNH, + OH +RNH + H,O%RNHO,+RCHO. Acetaldehyde is generally formed via dehydration of the a carbon atom of the oxygen adduct. In the case of aniline RNH’ radicals also participate (Neta and Fessenden, 1974). It is, however, difficult to predict the secondary reactions of these systems under atmospheric conditions. (b) Consequences for air pollution control It has been suggested that efficiency of photosmog inhibitors is based on the scavenging of the OH radicals responsible for carrying the long chain mechanism effecting the NO + NO, conversion (Gitchell et al.. 19741.It can not however be a priori excluded that they al& bloci the O(‘P)hydrocarbon reactions in polluted air. Thus, the relative magnitude of the product k(Inhibitor) vs k(Hydrocarbon) is important in estimating the theoretical efficiency of photosmog inhibition. Taking our rate constant for the 0 atom reaction with DEHA (see Table 1)and the recentlv- nublished eas ohase rate constant for the OH radical (Gorse et al., 1977) in comparison to the reaction rates for both species with the reactive hydrocarbon propylene (Graedel et al., 1976),we see that only the competition of DEHA with OH would allow photosmog inhibition in steady state concentrations in the sub-ppm range (Gorse et al., 1977). To suppress the reactions of O(‘P) atoms with aliphatic amines (see Table 1) in polluted air concentrations in the ppm range would be necessary. The gas phase OH rate constant for DEHA (1.0 x lo-“‘cm3 molecule-’ s- I) seems unusually high for what appears to be a hydrogen abstraction reaction. Additionally, it is reversed in magnitude with respect to the liquid phase (kp_ > kliquid) whereas the reverse is suggested by theory (kliquid- 20. kp_) (Benson, 1960). If the confirmed liquid phase rate constant for OH with DEHA (- 2.2 x lo-” cm3 molecule-' s-‘) of Gorse er 01. (1977) were correct and the gas phase value in error, then estimation of the latter forces the conclusion that scavenging reactions involving OH radicals will also be unimportant in polluted urban air unless one would increase the atmospheric concentration of DEHA to over 1 ppm, which comes close to the limit for human exposure for aliphatic amines (S-lOppm, Handbook of Chemistry and Physics. 1976-77). DEHA’s odor threshold level is 0.5 ppm (Pitts, Jr. et al., 1977). The observed diminution and retardation ofmaximal NO, concentrations in smog chambers by photosmog inhibitors such as DEHA, aniline and benzaldehyde (Jayanti et al., 1974; Spicer et al., 1974) must have more complex origins, whose secondary effects may lead to problems worse than those it is wished to suppress (Pitts, Jr. et al., 1977). The participation of benzaldehyde in the formation of the eye irritant perbenzoyl nitrate (PBZN) in irradiated auto exhaust is already documented (Dimitriades and Wesson, 1972; Kuntz et al., 1973), while health hazards are also associated with environments containing amine emissions (Walker et al., 1976). a

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