Hydroxyl Radical Initiated Oxidation of Olefin-Nitric Oxide Mixtures
The Journal of Physical Chemistry, Vol. 82,No. 2, 1978
135
Mechanism for Hydroxyl Radical Initiated Oxidation of Olefin-Nitric Oxide Mixtures in Parts per Million Concentrations H. Niki," P. D. Maker, C. M. Savage, and L.
P. Breitenbach
Research Staff, Ford Motor Company, Dearborn, Michigan 48 121 (Received August 22, 1977) Publication costs assisted by the Ford Motor Company
The photolysis of HONO-NO-olefin mixtures in ppm concentration was studied using the long-path Fourier transform IR spectroscopicmethod. The stoichiometry for the HO radical chain oxidation of CzH4, C3H6, and trans-2-C& was determined as follows: (HO) + C2H4+ 2N0 + 2 0 2 2CH20 + 2N02 + (HO); (HO) + C3Hs + 2N0 + 202 CH2O CHBCHO + 2N02 + (HO); (HO) + trans-2-C& + 2N0 + 202 2CH3CHO + 2N02 (HO). These results are compared with photochemical smog mechanisms proposed previously.
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Introduction Conversion of NO to NO2 in the HO radical initiated oxidation of hydrocarbons is responsible, in large part, for the formation of O3 in photochemical smog. However, the detailed mechanism is not well established because of the lack of information about the relevant reaction products. This is particularly true for HO-olefin reactions. There have been numerous conflicting views on the relative importance of thermochemically possible channels involving HO addition to the double bond and H-atom abstra~tion.l-~ These uncertainities are reflected in the existing chemical models for mechanisms of smog formation. For instance, Niki, Daby, and Weinstock (NDW)' adopted HO addition reactions exclusively, while Demerjian, Kerr, and Calvert (DKQg assumed a comparable importance of both addition and abstraction. Only very recently, conclusive evidence for the predominant occurrence of addition reactions has been obtained by CvetanoviE's group." The results reported herein pertain to the oxidation of the HO-olefin adduct in the presence of NO. The flow diagrams given in Schemes I and I1 depict the essential features of two different schemes postulated by NDW and by DKC. Namely, for the HO-C2H4 reaction (Scheme I), the two schemes differ with respect to the fate of the CHzOH radical, i.e., step 5a for NDW and step 5b for DKC. Thus, the predicted product yields per C2H4 reacted are 2CHz0 + 2N02 for NDW and HCOOH + CH20 + 3NO2for DKC. Step 7c postulated by Heicklen, Westberg, and Cohenll appears to be too endothermic to be of importance (AH= 23 kcal/mol).12 For the HO-truns-2-C4H8 reaction, the two schemes in Scheme I1 differ with respect to the reaction of the CH3CH(OH) radical. The corresponding product yields are 2CH3CH0 + 2N02 (NDW) and HCOOH + CH20 + CH3CH0 + 4N02 (DKC). Thus, these two schemes lead to distinctly different products and their yields. In the present study, the product yields were determined for CzH4, C3H6,and trans-2-C4H8using Fourier transform IR spectrometry. Experimental Section Mixtures of C2H4,C3H6, or trans-2-C4H8with HONO and NO up to 50 ppm each in 700 Torr of air (N2:02= 4:l) were irradiated with fluorescent lamps (G.E., F40 BLB) in a long-path IR absorption cell (1-m long, 40-pass, 70-L Pyrex cylinder), and the ensuing products were analyzed by the Fourier-transform spectrometric method. Details of the facility have been described previously.13-15 Gaseous HONO was prepared from the reaction of H2S04with a dilute solution of NaN02 as described by 0022-3654/78/2082-0 135$0 1.OO/O
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Scheme I: Reaction Schemes for the HO Radical Initiated Oxidation of Ethylene-NO System CH,=CH,
(1)
LHO
CH,(OH)CH,
\lo,
(2)
CH,(OH)CH,O~
lNO + JNO,I
(3)
CH,(OH)CH,O
(4) CH,(OH)
HO,
+ ICH,O
I
+
Hot
H 6 t
Nash16 and by Approximately 4 ppm each of NO and NO2, and 25% relative humidity were present in the HONO sample at 20 ppm. The observed decay rate of HONO in the IR cell was
