T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY Registered in U.S. Patent Office 0 Copyright, 1976, by the American Chemical Society
VOLUME 80, NUMBER 1 JANUARY 1,1976
Reactions of
Hopwith NO and NO2 and of OH with NO
R. Simonaitls and Julian Helcklen" Department of Chemistry and Ionosphere Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received July 7, 1975)
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The reaction of HO2 with NO was studied a t room temperature (-23OC) in competition with the reaction 2H02 H2Oz 0 2 (3) and the competition for HO2 between NO and NO2 was studied by monitoring NO removal via the chemiluminescent detection of NO. The HO2 radicals were produced either by N2O or NO photolysis at 213.9 nm in the presence of 0 2 and excess H2. Two possible paths for the H02-NO reaction are HO2 NO HO NO2 (la) and HO2 NO HONO2 (lb). If 123 is taken as 3.3 X cm3/sec, cm3/sec. If an H02NO intermediate is formed in cm3/sec and k l b < 2 X then hl, = (1.0 f 0.2) X reaction 1, its lifetime is 2 X cm3 sec-' (based on k3 = 3.3 X cm3 sec-l) and kla/k2, = 7 f 1; therefore h l , > 1.4 X 10-l2 cm3 sec-l. Recently COX,^ using a flow system at atmospheric pressure and monitoring NO, found kl, = 1.2 X cm3 sec-l and kza = 1.2 X cm3 sec-l in reasonable agreement with our lower limits. In order to obtain precise values of the rate coefficients for reactions l a and 2a and to resolve the discrepancy between the measurements from the several laboratories, we have undertaken further studies of these systems. As in the earlier studies, the reactions of HO2 with NO are studied by competitive methods with reaction 3 and with the reactions of HOP with NO2. The HOz radicals are generated by photolysis of N20 a t 213.9 nm in the presence of Ha and 02,3,4or by photolysis of NO a t 213.9 nm also in the presence of HZ and 0 2 . The reaction scheme assumed for the latter system is the following: NO
+ hu(213.9 nm) +
-
NO(Q+)
02
N0(2Z+) H2 +2H02 (effective) 1
2
R. Simonaitis and Julian Heicklen
The reactions are monitored by the measurement of the NO removal rate using chemiluminescent detection of NO. By proper adjustment of reaction conditions, the reaction of OH radicals with NO could also be studied, OH NO (+MI HONO (+M) (4) There is good agreement in the literature for the rate coefficient for reaction 4 at low pressures, but very little work has been done near the high-pressure limita6Results for the rate coefficient of reaction 4 at -100 f 5 Torr and -730 f 40 Torr total pressure (-95% H2) are also presented in this paper.
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Experimental Section
Apparatus and Procedure. A conventional high-vacuum line utilizing Teflon stopcocks with Viton 0 rings was used. The reaction vessel was a 2-1. Pyrex bulb provided with two quartz windows opposite each other for the passage of 213.9-nm radiation obtained from two Phillips Zn resonance lamps (TYP 931063). Two lamps were used to obtain uniform illumination of the reaction vessel, Experiments performed with one lamp (under conditions where the quantum yields depend on the absorbed intensity, I,) gave essentially the same results as with two lamps indicating that any remaining nonuniformity in illumination does not affect the results. The contents of the reaction vessel could be sampled into the chemiluminescent detector either continuously or intermittently by a capillary bleed from the center of the vessel. The flow rate through the capillary was -30 cm3/min. The chemiluminescent analyzer was similar to the one described by Stedman et aL7 The analyzer was a cylindrical -500-cm3 vessel equipped with a quartz window for viewing the red emission, and with two nozzles for sample gas containing NO (from the reaction vessel) and 0 3 injection. The analyzer was pumped with a Welch (No. 1402) highvelocity pump. Typically the analyzer pressures ranged from -1-3 Torr. Ozonized 0 2 was prepared by a high-voltage discharge through 0 2 . The 02-03flow rate was -130 cm3/min. The red emission was viewed with an EM1 photomultiplier Model No. (9558B) operated a t 1600 V. The photomultiplier current was measured with a Keithly Picoammeter Model 410A, and the signal could be monitored continuously with a strip chart recorder. The detector was calibrated with known pressures of NO. The lower detectability limit was about 10 ppb. The detector was linear in the measured range of 10 ppb to 200 ppm. The time constant of the electronics and the pumping system was always much less than that of the reaction. In experiments in which the time of reaction was short, continuous sampling of the reaction mixture was done. However, for long time experiments the reaction mixture was sampled intermittently in order to maintain a constant pressure in the reaction vessel. Actinometry for the NzO and NO photolysis experiments was done by measuring the rate of NO production from the photolysis of pure N20. For this system @(NO]= 1.2.8 To determine the absorbed light intensity, I,, in the NO photolysis system, the relative absorption coefficients €(NO}/ t{NzO)= 8 f O.@ were used. When NO2 was present in the NO photolysis experiments, an appreciable amount of light was absorbed by the Nos. T o compute I , under these conditions, @(O(lD))= 0.5 was taken for the photolysis of NO2 a t 213.9 nm,9 and the absorption cross section for NO2 at 213.9 nm was taken to be 5.3 X 1019cm2.lo The Journal of Physical Chemistry, Vol. 80, No. 1, 1976
Materials. All gases except H2 were from the Matheson Co. N2O was purified by degassing a t -196OC. Cylinder Hz was purified by passage over traps maintained a t -196°C. 0 2 was used directly from the cylinder. NO was purified by distillation from a trap maintained at -186°C to a trap a t -196°C; the color of the frozen NO was white indicating it to be free of NO2. Cylinder NO2 was stored by mixing with excess 0 2 to oxidize any NO. Analysis showed a negligible amount of NO.
Results N2O Photolysis. Photolysis of N20-H2-02-NO mixtures at 213.9 nm and 23°C leads to the consumption of NO. There is no measurable induction period (
